• Author / Uploaded
• Ana Marcia Ladeira

• Categories
• Papel de pulpa)
• Celulosa
• Lignina
• Procesos industriales
• Química

Citation preview

Pulp Bleaching

The Association assumes no liability or responsibility in connection with the use of this information or data, including, but not limited to, any liability or responsibility under patent, copyright, or trade secret laws. The user is responsible for determining that this document is the most recent edition published.

-

Within the context of this work, the author(s) may use as examples specific manufacturers of equipment. This does not imply that these manufacturers are the only or best sources of the equipment or that T APPI endorses them in any way. The presentation of such material by T APPI should not be construed as an endorsement of or suggestion for any agreed upon

Principles and Practice

course of conduct or concerted Copyright

...

CarltonW.

Science and Forestry, Retired State University of New York Syracuse, NY, U.SA.

Permission of T APPI is granted to photocopy items for internal or personal use of clients, for libraries or other users provided that the copying organization pays the ofSl.OO U.S. per ropy, plus $.50 U.S. per page directly to the Copyright Clearance 222 Rosewood Drive, Danvers, MA 01923, U.S.A. (ISBN 0-89852-063-0) $1.00 + ISBN 0-89852-063-0 TP 0102B061 Printed in the United States of America

and

Douglas W. Reeve

Library of Congress

Department of Chemical Engineering and AppUed Chemistry, Pulp & Paper Centre, University of Toronto Toronto, Ontario, Canada

AtJanta, Georgia @ 1996

U.S.A.

All rights reserved

College of Environmental

TAPPI

«;:)1996 by:

TAPPIPRESS Technology Park/Atlanta P.O. Box 105113 Atlanta, GA 30348-5113

Dence

action.

Editors

Pulp bleaching: principles and Douglas W. Reeve p. cm.

Cataloging-in-Publication

Data

and practice / edited by Carlton W. Dence

Includes bibliographical references. ISBN 0-89852-063-0 I. Wood-pulp--Bleaching. I. Dence, Carlton W. II. Reeve, Douglas W., 1945. III. Technical Association of the Pulp and Paper Industry. TS1176.6.B6P85 1996 676'.12--dc20 96-4084 CIP

~ TAPPI PRF.SS it

specific base fee Center, $.50 pp.

TABLE OF CONTENTS

Section V:The Technology of Mechanical Pulp Bleaching

Acknowledgements

v

Preface

vii

Section I: Introduction 11:Introductionto the Principlesand Practiceof PulpBleaching

1

Section II: Raw Materials 111:Production of Unbleached Pulp

25

II 2: Bleaching Chemicals: Chlorine Dioxide

59

II 3: Bleaching Chemicals: Hydrogen Peroxide,

71

Chlorine, Sodium Hydroxide, Peroxy Acids, ..oxygen, and Ozone

Section III:The Chemistry of Bleaching 1111: ChemicalStructureOf

and Brightness

PulpComponents

Reversion 91

III 2: Reaction Principles in Pulp Bleaching

113

11I3:Chemistry of Chemical Pulp Bleaching

125

11I4:Chemistry of Mechanical Pulp Bleaching

161

III 5: Chemistry of Brightness Reversion and its Control

183

Section IV:The Technology of Chemical Pulp Bleaching IV 1: Oxygen Delignification

213

IV 2: Chlorination

241

IV 3: Chlorine Dioxide in Delignification

261

IV 4: (Oxidative) Alkali Extraction

291

IV 5: Ozone Delignification

321

IV 6: Hydrogen Peroxide as a Delignifying Agent

347

IV 7: Enzyme Treatments of Pulp

363

IV 8: ChlorineDioxide in Bleaching Stages

379

IV 9: Hypochlorite and Hypochlorous Acid Bleaching

395

IV 10:HydrogenPeroxideBleaching

411

IV 11: Bleaching Shives and Dirt

443

Iii

V 1: Peroxide Bleaching of (Chemi)mechanical Pulps

457

V 2: Hydrosulfite (Dithionite) Bleaching

491

Section VI:Bleach Plant Operations. Equipment and Engineering V11: Pulp Pumping and Hydraulics

513

VI 2: Mixing and Mixers

537

V13: Washing and Washers

569

V14: Towers and Reactors

597

VIS: Sensorsand ProcessControl

625

VI 6: Water Reuseand Recycle

647

Section VII:The Properties

of Bleached Pulp

VII 1: Bleached Pulp Composition and Its Determination

675

VII 2: Brightness: Basic Principles and Measurement

695

VII 3: Strength Properties and Characteristics of Bleached Chemical and (Chemi)mechanical Pulps

717

Section

VIII: Pulp Bleaching

and The Environment

VIII1: Effluent Characteristics and Composition

749

VIII 2: Assessing the Potential Impacts of Pulping and Bleaching Operations on the Aquatic Environment

767

VIII 3: Dioxins

and Furansin Effluent,Pulp,and SolidWaste

799

VIII 4: Bleach Plant Air Emissions

821

VIII 5: Environmental

835

Regulations

Subject Index

847

iv

Acknowledgments As is usually the case in any major undertaking, many individuals worked behind the scenes in a variety of capacities to help create this book. In particular, we acknowledge with thanks the efforts of those who contributed a substantial amount of time in serving as referees for specific chapter manuscripts: Terry Adams, Terry N. Adams Consulting; Chad Bennington, PAPRICAN; Dennis Borton, NCASI; Bruce Fleming, Boise Cascade; Raymond Francis, SUNY College of Environmental Science and Forestry; Maurice Hache, Morton International, Inc.; Bruno Marcoccia, Kamyr, Inc. ;Thomas McDonough, Institute of Paper Science and Technology;Anil Mislankar, University of Toronto; Esa Vilen, University of Toronto, and Paul Wollwage, Weyerhaeuser Company.We also wish to express our appreciation to the many companies and institutions for the "loan" of their employees for the purpose of contributing to this book and to TAPPI PRESS for their patience and support in this project. As editors and contributing authors we individually wish to acknowledge those who have been involved with us on a more personal level in the production of this book.

Doug Reeve: First and foremost, I am very grateful to my partner in this project, Carlton Dence. He has been his usual self: capable, energetic, insightful, meticulous, thorough, reliable, and feisty through these many, many long months.A better partner I could not have wished for. . I am grateful to my colleagues at the Pulp & Paper Centre for their steady and capable support: in particular, Mimee Cheung, David Goring, Bruce McKague, Kate Reeve, linda Staats, Cindy Tam, Esa Vilen, and KathyWeishar. Finally, I wish to express my heartfelt thanks to my family for their support and encouragement and most especially to my wife, Melanie, who tolerated my occupation of the dining room well beyond the originally agreed-upon six-month period, often with fiery forbearance.

Carlton Dence: The scope and detail of the material found in this book required the participation of a co-editor whose educational background and experience complemented my own. By providing the chemical engineering expertise and an extensive practical knowledge of commercial bleaching operations, Doug Reeve filled that requirement to perfection and I sincerely acknowledge his pivotal role in determining the content and quality of this book. I am also indebted to Elizabeth Elkins and James Williamson of the Moon Memorial library staff at the SUNY College of Environmental Science and Forestry for providing me with copies of journal articles and for corroborating reference citations. Thanks also are due Sarah Remon for her careful work in transferring the manuscripts for my chapters to diskette. Financial support from TAPPI as reimbursement for out-of-pocket expenses incurred in connection with this book is noted with gratitude. In conclusion, I wish to acknowledge the efforts of my wife, Frances, whose vicarious suffering in this literary odyssey took the form of frequent manuscript-burdened trips to Sinclair Office Supply, Mail Boxes Ine. and local post offices in the vicinity of Hendersonville, N.C. on my behalf.

I'

Pulp Bleaching Principles and Practice Preface

-

This edition of theTAPPI Bleaching Monograph is the fourth in a series of such books published in 1953, 1963, and 1979, all projects of the TAPPI Pulping Bleaching committee. In a radical departure from the format of previous monographs, the contents of this edition have been organized under eight major headings (designated by Roman numerals) to highlight what we perceive to be the main subject areas for any discussion of contemporary pulp bleaching. By this device, we have sought in particular to give subjects relating to bleach plant engineering, bleaching chemistry, and environmental issues a more distinct identity than

in the past.

.

As can be inferred from the title of the book, we have sought to stress the principles undergirding the selection, design, and implementation of a successful bleaching operation. The decision to follow this approach was prompted by our belief that commercial bleaching technology has progressed so rapidly and has become so multifaceted that fundamental chemical and engineering principles, properly understood and applied, are critical to the ongoing quest for new and improved bleaching techniques. Our decision to emphasize bleaching principles was also motivated by a desire to encourage the use of the book by the academic and research communities while still retaining its traditional usefulness to bleach plant designers, managers, and operating personnel. In the preface to the 1953 edition of the TAPPI Bleaching Monograph, the editor, Howard Rapson, took note of the "virtual revolution in pulp bleaching" that had occurred over the preceding ten years. The term "revolution" applies equally well to the period since the last bleaching monograph

was published. For example, the present monograph documents the continuing trend away from the use of elemental chlorine to ECF and TCF bleaching sequences that favor the use of chlorine dioxide, oxygen, ozone, and hydrogen peroxide. Moreover, since the previous monograph appeared, oxygen and peroxide have found new and significant uses as de lignification chemicals in alkaline extraction stages. Also the application of enzymes in bleaching, a subject not included in the 1979 monograph, is an integral part of this book portending continuing interest in the use of biological systems in bleaching. Although external factors have been mainly responsible for shaping the content of this book, each edition of this series, including this one, reflects to some degree the personal viewpoints and tastes of its editors and authors. As examples of the latter, we cite the tWo-column page format (unique to this edition) and its red cover which we hope will make it visible from anywhere in a room where it is found. Who says erstWhile and active college professors can't be flamboyant when properly inspired? To our regret, the planning and production of this book has taken much longer than we expected; we first discussed the possibility of collaborating on the project in the summer of 1992. It seems almost inevitable that delays will occur when the project is as large as this and when a multiauthor format is used and when many authors must find time outside of their normal work responsibilities. Nevertheless, the authors of this book ultimately fulfilled their commitment and we are very grateful for their efforts in making Pulp Bleaching: Principles and Practice a reality. Carlton Dence Hendersonville,

NC, USA

Doug Reeve Toronto, ON, Canada February, 1996

vii

SEcnON I: Introduction Chapter 1: Introduction to the Principles and Practice of Pulp Bleaching Douglas W. Reeve Department of Chemical Engineering and Applied Chemistry and Pulp & Paper Centre University of Toronto Toronto, Ontario

1. What is bleaching? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 A general description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Objectives and critical parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7. Trends

in pulp bleaching.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..

23

Chapter I I: Introduction to Principles & Practice of Pulp Bleaching

Chapter I 1: Introduction to the Principles and Practice of Pulp Bleaching Preface Since the first bleaching monograph in texts 1953 (1) two other comprehensive have been published on pulp bleaching, in 1%3 (2) and in 1979 (]). Thorough treatments of pulp bleaching also appear as chapters in other texts (4,5) and in the annual TAPPI Bleach Plant Operations Short Course Notes. In this chapter, the foundation concepts of pulp bleaching are introduced including the objectives, economics, history, chemistry, and engineering of pulp bleaching systems. In the other chapters in this text, these concepts are described in greater detail, and more penetrating explanations and fuller descriptions are provided. There are many places in the introductory chapter where reference might have been made to more detail elsewhere in the text. This has not been attempted; however, the reader is encouraged to seek greater detail through the use of the table of contents and the index.

1. What is bleaching? 1.1 A general description Bleaching is a chemical process applied to cellulosic materials to increase their brightness. Brightness is the reflectance of visible light from cellulosic cloth or pulp fibers formed into sheets. Bleaching processes are applied to cellulosic textiles, woven articles made from conon or linen, and cellulosic pulp, in the form of aqueous suspensions of individual fibers separated from wood or non-woody materials such as straw, reed, jute, sugarcane (bagasse), and bamboo. Bleaching increases the capacity of paper for accepting printed or written images and so increases its usefulness. It also is a means of purifying pulp, thereby extending its application, increasing its sta-

3

billty,and enhancing some of its properties. Bleaching is also effective in removing unwanted particles that contaminate pulp fibers. The absorbance of visible light by wood pulp fibers is caused mainly by the presence of lignin, one of the principal constituents of wood. lignin in native wood is colored slightly, and residual lignin remaining after some chemical (e.g., alkaline) pulping processes is highly colored. In addition, lignin darkens with age. Bleaching processes increase brightness by lignin removal or lignin decolorization. In the manufacture of chemical pulps, most of the lignin is removed during pulping; when these pulps are later bleached, lignin removal is continued. In the manufacture of mechanical and (chemi)mechanical pulps, wood is fiberized with linle or no lignin removal, and bleaching of these pulps takes place solely by the decolorization of lignin. Bleaching produces improvements other than enhanced brightness ofthe final product. For example, lignin-removing bleaching not only increases the brightness, but the brightness stability of the product as well. Treatment of pulp with some bleaching chemicals is particularly effective in bleaching contaminating particles such as shives (incompletely fiberized slivers of wood). In the manufacture of pulp for reconstituted cellulose such as rayon and for cellulose derivatives such as cellulose acetate, all wood components other than cellulose must be removed. In this situation, bleaching is an effective purification process for removing hemicelluloses and wood extractives as well as lignin. To achieve some of these product improvements, it is often necessary to bleach to high brightness. Thus, high brightness may in fact be a secondary characteristic of the final product and not the primary benefit. It is therefore simplistic to suggest that bleaching to lower brightness should be practiced based on the reasoning that not all products require high brightness. The chemicals commonly used for pulp bleaching include oxidants (e.g., chlorine, chlorine dioxide, oxygen, ozone, and hydrogen peroxide), alkali (namely, sodium hy-

4

Pulp Bleaching - Principles and Practice

droxide), and for mechanical pulp bleaching only, a reducing agent, sodium hydrosulfite (sodium dithionite). These chemicals are mixed with pulp suspensions and the mixture is retained at prescribed pH, temperature, and concentration conditions for a specified time period. The bleaching reactions that occur are highly complex due to the complexity of lignin and the wide variety of reactive bleaching species present. The progress of bleaching reactions is monitored by measuring pulp lignin content, pulp brightness, and residual chemical. Bleaching chemicals are frequently applied sequentially with intermediate washing between treatments (stages), because it is not possible to achieve sufficient removal or decolorization of lignin by the action of anyone chemical in a single treatment or stage. To carry out these reactions, appropriate process equipment is required for mixing steam with the pulp to control the temperature, mixing chemicals with the pulp, pumping or otherwise conveying the pulp, and washing the pulp after the reaction is complete. Reaction times for bleaching chemicals are generally in the range of a few minutes to several hours, requiring the construction of large towers (reactors) to provide an adequate retention (t. e., reaction) time. Bleaching processes are monitored by on-line sensors and process control algorithms devised to achieve product quality targets with efficient use of chemicals and energy. 1.2 Objectives and critical parameters The prinicipal objective of pulp bleaching is to achieve a high brightness with secondary objectives, for particular end uses, being high brightness stability, high pulp cleanliness (freedom from colored particles), and a high cellulose content. These objectives must be achieved without compromising the strength of the final product; cellulose degradation during bleaching can lead to significant loss of strength in product paper sheets. Bleaching costs must be appropriate to the value added by the process. Variable

costs include those of chemicals, steam, and electric power. Capital costs associated with the bleach plant must also be taken into consideration, namely, the initial investment cost. depreciation, and ongoing maintenance. Bleaching reactions can lead to significant dissolution of substance from the pulp, decreasing the yield of final product and affecting production cost. The use of bleaching chemicals must be considered in the context of the health and safety of workers and inhabitants of the neighboring environment and in the context of responsible environmental management. Some bleaching chemicals are highly reactive so that proper equipment design, operations management, materials of construction, and safeguards are required. In the manufacture of bleached kraft pulp, for example, effluents from bleaching operations usually constitute a significant fraction of substances whose discharge to receiving waters is regulated according to parameters such as BOD (Biochemical Oxygen Demand) and color. When chlorine is used in pulp bleaching, as it has been for manufacture of bleached chemical pulp for many years, chlorinated organic matter is found in the effluent. Since the late 1980s, environmental concern has led to regulation of

o o

20

40

60

CI02 Sulmitution,

80

100

%

Fig. 8. Effect of cbiorllle dioxide substitution on tbe chlorine an4 metboxyl contents of lipi. (M. w'>1(00) in tbe bleachi.g effJuent (70).

Chapter ill 3: Chemistry of Chemical Pulp Bleaching Neutralization of acidic groups The most important reaction of sodium hydroxide with the residual lignin during the extraction stage of a multi-stage bleaching sequence is the conversion of partially degraded acidic fragments to their corresponding anionic fonns (i.e., neutralization). The principal effect in such reactions is the accompanying increase in the solubiliry of the lignin fragments to which the acidic groups are attached. Typical acidic groups in native and oxidized lignin and their neutralization with alkali (i.e., sodium hydroxide) are shown in Fig. 9. Carboxylic acid groups arise mainly from the oxidative rupture and later fragmentation of aromatic rings in the initial bleaching stage as is described in Sects. 2.1,2.2,2.4, and 2.5. A spectrometric iIIvestigation of a high molecular weight lignin-derived material recovered from the effluent of a chlorinated and alkaIi-extracted kraft pulp has provided ~ldence suggesting that the carboxyl groups are attached to saturated carbons and to a-substituted

or ethylenic carbons

(37, 70, 71).

RCOOH Carboxylic (R =

Aryl

or

Carboxylate

acid

anion

Alkyl)

\ Phenol

I

C-QH II He

Phenolate

-

I

C-O-

HO-

He I

Enol f).

Enolate anion

NeuIrfllizllHoll tioruIl

+ H2O

II

I

Fig.

anion

grtnI/JS

of lignill-tIerlreti i. prebk4ebetl

addle jilrreJItIlps.

Besides the phenolic groups initially present in unbleached pulps, more such structures are formed in the first bleaching stage especially when the bleaching agent is chlorine. Phenols of the catechol-type

139

(i.e., 1,2-dihydroxybenzenes) appear to be dominant in the extraction effluent of a chlorinated pulp but, in the corresponding effluents of oxygen- and chlorine dioxidepretreated pulps, the guaiacyl-type prevails (70). Enol groups arise from carbonyl groups adjacent to carbon atoms containing an ahydrogen. Although this structural requirement is met by lignin in unbleached and prebleached pulps, the contribution of enol groups to lignin solubilization in alkaline treatments has not been established. Base-catalyzed hydrolysis of organically bound chlorine A large fraction (ca. 60-70"10for kraft pulp) of the organically bound chlorine in chlorinated pulp is removed in the ensuing extraction stage (72-74). In a study of the base-CI'\

0

0

~,

1

1

b=a

~, I I

"

1-~

"

~C>CI'\

j

«~ ~II

C>CI'\

HO-

--~-"'-l~

~--~~ l-oow

OH

Reduction (Proton donor)

~,HO.,orM+ Oxidation

"

~C>CI'\

--~ 0

Fig. 10. FomJIItkm of'l,dtUmOiIl cbronwpbore arulleru:ocbrrnrropbore systems by reactiorr witb diff-t ffucleopbiles arul electropbiles.

174

Pulp Bleaching - Principles and Practice

sequence are a nucleophilic attack of a hydroperoxide ion on the «-carbonyl group and a following series of rearrangements of the hydroperoxide adduct leading finally to the formation of a methoxyhydroquinone structure. Detection of methoxyhydroquinone itself in the effluent of a peroxidebleachedTMP (67) indicates the relevancy of this reaction in the bleaching process. Methoxyhydroquinones are the leuco (colorless) forms of the corresponding pbenzoquinones into which they are readily converted by an one of several one-electron transfer oxidants, for example, molecular oxygen (45), present in the bleaching system (Fig. 10). In aqueous alkaline media, the quinone may be reconverted (t.e., reduced) to the hydroquinone (colorless) form (68) to complete the redox cycle as illustrated in the figure. In an analogous series of reactions, catechol units can be oxidized to their o-benzoquinone counterparts as shown in the right-hand sequence of Fig. 10. The latter structure, in turn, may be reduced to catechol units in the presence of suitable proton donors (69). It is important to note that the Dakin and Dakin-like reactions are restricted to phenolic units and that, in both instances, the side chain is displaced from the aromatic ring. The formation of chromophore systems by base-catalyzed elirnination reactions is illustrated by the two sequences in Fig. 11. In each instance, the alkalinity and temperature of a conventional peroxide bleaching system is sufficient to cause the reactions to occur. In the left-hand sequence, the 1,3-ketol unit comprising the side chain is dehydrated under the prevailing alkaline conditions to a pheny1-conjugated enone chromophore system (45,46). In the righthand sequence of the figure, a 1,2-diarylpropane unit undergoes loss of water or formaldehyde and a proton to produce a stilbene-type structure (70,71). This leucochromophore can then be oxidized to a stilbene quinone-type chromophore as shown in the figure. With the foregoing as a basis, the peroxide bleaching of (chemi)mechanical pulps

may be said to consist of the partial or total destruction of chromophore systems initially present in the lignin and the formation of new chromophore structures. With this concept as a foundation, the typical bleaching rate plot (see Fig. 13, Chap. V 1) can be interpreted as follows: the rapid initial brightness increase may be assigned principally to the removal of the original chromophore systems; the phase exhibiting a reduced bleaching rate may reflect the offsetting effect of chromophore creation (49,50,52, 64, 65, 72, 73). The structural characteristics of the chromophore systems formed during peroxide bleaching are of particular interest because of their presumed influence in determining the upper limits to which (chemi)mechanical pulps can be realistically bleached. In all probability, the types of chromophore structures formed from lignin during peroxide bleaching are similar or are closely related to those pictured in Fig. 10. Although, in general, quinonoid structures are readily degraded to colorless fragments by hydroperoxide ions, typified by biphenyl (a, b, c), ~-5 (d), and diaryl ether (e) units (Fig. 12) linked to methoxy-pbenzoquinone groups (R) formed during bleaching are reportedly unreactive (74). If, in fact, degradation-resistant quinonoid or related chromophore systems are present during pulp bleaching, their practical effect would be to increase the amount of peroxide required to reach a given brightness level (64,65).

2.3 Reactions of alkaline hydrogen peroxide with carbohydrates and extractives Unlike the reactions of alkaline hydrogen peroxide with lignin, the participation of carbohydrates and extractives in peroxide bleaching has received only limited attention as would be expected from their generally recognIzed mInor effect on the brightness of most (chemi)mechanical pulps. Nevertheless, the reactions of alkaline peroxide and its decomposition prod-

Chapter III 4: Chemistry of Mechanical Pulp Bleaching R

= H,

Aryl,

175

=

;r

\

\

,t, I

/

/ I

-Principles

and Practice

40

/,...,--

E

CL

193

I

I I

C)

Na2S03 Charge, % on wood

-1 ---

~ ....... C\I

E

4

20

'.L VP), used to improve the whiteness and gloss in paper coatings, as an additive in inks to prevent paper slippage, and to improve the dimensional stability of wood, also reduces light-induced yellowing (79). The monomer, n-vinyl-2pyrrolidone, and the lowest molecular weight (molar mass 104) PVP are the most efficient. It is has been proposed (79) that the phenolic hydroxyl forms strong hydrogen bonds with the amide group ofPVP and prevents its oxidation by alkoxyl and peroxyl free radicals.

=

(Fig.

-Principles

17). Addition of a~or.

and Practice

50 .

Free radical scavengers All five light-induced yellowing pathways illustrated earlier in Schemes 8 and 9, involve a series of free radical reactions leading to a phenoxy free radical (27, 28, 35, 46, 61). Also, as illustrated in Scheme 6, the phenoxy free radical reacts with peroxyl radicals to form chromophore structures (43). The most successful methods for inhibiting light-induced yellowing of lignincontaining papers are those that scavenge

of

Pulp Bleaching

it to an o-quinonoid

Ascorbic acid and ascorbates The inhibition oflight-induced yellowing of lignin-containing papers by ascorbic acid and sodium ascorbate was first reported 25 years ago (73). The ability of ascorbates to inhibit light-induced yellowing may be understood by considering the relationship of this biological antioxidant to (X-tocopherol. Niki et al. (80) observed that an (X-tocopherol radical, generated by an alkoxyl radical, rapidly disappeared when mixed with ascorbic acid, suggesting that ascorbic acid reduces the phenoxy radical of (X-tocopherol. Similarly, ascorbic acid reduces phenoxy radicals in Iignin-containing paper produced by exposure to near-UV light. BleachedTMP irradiated with near-UV light exhibits a large ESR signal attributed to a phenoxy radical which decreases after the addition of ascorbic acid (81). The ESR signal attributed to phenoxy radicals was replaced by a new persistent signal assigned to the ascorbyl radical. When applied to a paper surface, ascorbic acid inhibits light-induced yellowing for a finite time, (1-2 h) after irradiation with near-UV light (34). With longer irradiation, yellowing proceeds at the same rate as that for the untreated paper (34, 75) as shown in Fig. 16. This limitation has been attributed in part to photooxidation of the ascorbic acid. Besides undergoing air oxidation, ascorbic acid is oxidized by photochemically produced peroxyl radicals, superoxide radical anions, and singlet oxygen (82, 83). If ascorbic acid is to be an effective inhibitor of light-induced yellowing, its usefullifetime must be significantly increased. Agnemo (84) has studied the effect of bleached aspen CfMP treatment with ascorbic acid and sodium sulfite. From this work it appears that, relative to the initial specific absorption, sodium sulfite and ascorbic acid addition to paper decreases light-induced yellowing no more than ascor-

Sebeme 10. Ligbt-ttUl.eeli t..tomertztltto" Z-byi/lYJxy~.

206

40

!if N" E

w 5'

15

0.22

11 ~~3Q°C o 40.C

. 50.C

rn

0.24

£

[J 60°C

Il 70.C V BO.C

5 5

0.16 22

24

26

28

30

C-stage brightness,

32

34

Fig. 7. The re14timlsblp betrrem C stage IJrlgbmas and CE "I("

,""nber,

Ing of a softwood

In tbB 14bomtory

ImIft Jnllp.

36

o

20

40

60

80

100

120

bleaeb-

Fig. 9. Effect of liqruJr carryover on moleentar cblorine mnmple required to obtain a constant C-stage Jnllp IJrlgbtness.

6 7 8 9 CI2 applied, % on pulp

I I I I 0.160 0.192 0.224 0.256 Kappa factor

COD, Ibiton of pulp

OJ.

4.2 Temperature and Time Chlorination stages may be conducted at various temperatures ranging from ambient to 70°C. As is shown in Fig.ll, viscosity loss is independent of temperature; for the most part, given sufficient reaction time, the extent oflignin removal also is independent of C-stage temperature. However, temperature does affect reaction rate. Increased efforts to close the system and conserve water have led to the recycle of chlorination and chlorine dioxide stage filtrate and use of paper machine or similar process waters instead of fresh water for dilution of unbleached pulp. The result of such filtrate recycle is an increase in C-stage temperature. The use of recycled filtrate may be limited by the material of construction in pumps, piping, and unbleached pulp storage chests. As is discussed later, it is important to maintain a constant temperature in this stage to facilitate control strategies and reduce variability in the post-extraction K

30

::;;

2.5

were generated by allowing pulp to react to zero residual chemical after increasing amounts of chlorine were added together with a small amount of chlorine dioxide to help preserve viscosity. Viscosity loss has been shown to be independent of temperature but proportional to chlorine consumption (35).

IIonsfor MrloIISbleaebhlg~.

26

30 25 Kappa number

\

Fig. 10. IIetnrmItks of optirnhitlf

28

20

\

0.10

0.2 '3 u CI> 0 0.18

o

\

,-....... /

30

15

:2 u

(,)3.5

[5]

32 W CJ ~0 u; In .. C E C> 'C .Q .. C> as 7ij

'C 0

4

=

\

A number of studies (29-31) have indicated that a molecular chlorine multiple of 0.15-0.17 or less is sufficient to avoid formation of chlorinated dioxin and furans as trace by-products. Although the molecular chlorine multiple concept does take into account changes in the incoming kappa number, it has been shown that using a constant molecular chlorine multiple does not result in a uniform C stage pulp brightness or CE K number (Fig. 8) (32). Variable levels of black liquor carryover with unbleached pulp also affect the required chlorine multiple (Fig. 9) (33).

0.3 '3 0.28 E .. 0.26 C

o

multiple

Pulp Bleaching - Principles and Practice

Cl2 applied, % on pulp Kappa number

.. 15.

3

250

charge, expressed as percent on pulp, divided by the unbleached pulp kappa number:

regulations created pressure to decrease the unbleached pulp kappa number.

~ W

249

Fig.

11.

Bffect

of cblorine

charge on Jnllp vIscostty.

10

I I 0.288 0.320

Chapter IV 2: Chlorination number. Knowledge of expected temperatures is critical in choosing sites for mounting on-line brightness and residual chlorine sensors. Most chlorine is consumed within the first few minutes of reaction, even at room temperature. Most delignification occurs in this same period as was noted earlier. Even so, allowing extra time for the slower phase of the chlorination reactions to be completed significantly improves the economics for attaining a high final brightness. Pulp quality, as measured by pulp viscosity, does not suffer as retention time increases (36). Additional retention time also provides flexibility for handling upset conditions and production rate changes. 4.3 Consistency Most pulp chlorination stages are conducted at relatively low (2.5-4.0"16) consistency. Several mills now operate mediumconsistency (8-15%) chlorination stages, utilizing recently develQped medium-consistency pumping and mixing equipment. There are limitations in the volume of gas that can be mixed into a medium-consistency slurry (37, 38). Laboratory studies have shown some benefits in selectivity and efficiency of delignification accruing from medium-consistency chlorination (39,40). However, because of the higher concentration of chlorine, more chlorinated organics would be expected to be generated at medium consistency (17,41). The operation of in-line sensors may also be adversely affected by changing from low to medium consistency.

4.4 pH The pH of the pulp suspension influences chlorination reactions by controlling the relative amounts of chlorine or hypochlorous acid present as discussed previously. Organic acids and hydrochloric acid are generated as by-products as chlorine reacts with lignin causing the pH to drop as the chlorination proceeds. The bleach plant operator can control the pH in only a few ways. Recycle of chlorination washer (or chlorine dioxide washer) filtrate for major or minor dilution of unbleached pulp

251

252

Pulp Bleaching - Principles and Practice

32

co

30

28 (fJ a,

~24

~..~20

:> 16

22

~

18

o o(fJ

234 567 End pH in the first stage

'S; W U

I'll. 12. 1JfIeel 01 cillorllllllitnl JIll 011fII8co8IIy .per ~

will lower the pH. In some cases, paper machine white water is used for such dilution and buffering components may be present that affect the pH. It has been demonstrated in laboratory studies (42) that a lower pH favors reaction of chlorine with lignin (Fig. 12). When relatively little chlorine is applied, such as to oxygen-delignified hardwood, pulp filtrate recycle may be necessary to obtain a sufficiently low pH. This practice has an added advantage itl that any carryover of organics with the unbleached pulp may be eliminated by reaction with residual chlorine in the recycled filtrate. 4.5 Chlorine dioxide substitution It is well known that small amounts of chlorine dioxide added to the chlorination stage will protect pulp viscosity. This is an important consideration in some mills, particularly for market pulp producers. A 1988 EPA survey showed that many mills in the United States did not require chlorine dioxide substitution to meet pulp quality targets (43). Environmentally driven modifications in the early 1990s have led to significant increases in chlorine dioxide substitution for all NorthAmerican mills. Chlorine dioxide also is an effective oxidant for lignin and can replace part of the chlorine charge in the first stage. Because of the higher unit cost of chlorine dioxide and the fact that it must be generated onsite, its use in this manner has found limo ited acceptance until relatively recently. Many studies have been published dealing with the substitution of chlorine

points

26

Q.. E 'ii)

12

Optimum CIO2 addition-

30%

~

14

30°C

10

I

I 1/ I I" 10 5 I

300

..-

60 3020

Seconds before

5. Process equipment Chlorinatlon equipment varies considerably at different mills, but many of the system components are common. The diagram in Fig. 1 shows the following components: Brown stock pulp dilution Blending and consistency control Chemical injection and dispersion

Sensors and control Reaction

mechanisms

vessels for retention

Pulp washing.

Seconds

60

300

after.......

Timing of chlorine dioxide addition

dioxide for chlorine to achieve viscosity protection and delignification. The impact of chlorine dioxide substitution on environmental parameters has also been reported in recent years (44). The timing of chlorine dioxide addition relative to that of chlorine is an important consideration for pulp quallty as shown in Fig. 13. When used in relatively small quantities in softwood kraft pulp bleaching, chlorine dioxide should be added with or slightly after the chlorine (45). Chlorine dioxide may be entirely consumed if added too far ahead of the chlorine, resulting in poor viscosity protection. Similar curves have been developed for hardwood (46).

Mixing

5 10 2030

Chlorine added

5.1 Stock dilution Following brown stock washing and screening, pulp usually is stored at medium consistency before bleaching to decouple the pulping and bleaching operations. Water is added at the base of the storage tank to dilute the pulp for pumping. The volume of water used for dilution can be con. siderable, particularly when, for example, pulp is stored at medium consistency (12%) and the C-stage is operated at low consistency (3%). In this case, a mill producing 1000 metric tons per day would add 0.27 cubic meter per second (4340 gal/min) of dilution water. This water can be recycled chlorinationor chlorine dioxide-stage filtrate, fresh water, paper machine or pulp dryer white water, or other process streams. There are many operational considerations, depending on the choice of dilution water: Corrosion due to incompatible materials of construction Chlorine demand of dissolved organics in the dilution water Formation of highly chlorinated by.products from recycle of chlorinated organics C-stage pH, temperature Overall mill water usage Waste treatment plant hydraulic capacity,

--Chapter IV 2: Chlorination Fillers, clays, other additives which that may interfere with optical sensors Inorganic compounds which buffer pH and create scale or deposits The choice of the dilution water source must involve these and perhaps other considerations. For low-consistency chlorination stages, the C-stage water balance is the highest volume filtrate/effluent system in the bleach plant. A purge from this system to the wastewater treatment plant is required because corrosiveness due to the presence of chloride ions and the low pH preclude countercurrent use of this filtrate for screening or brown stock washing. 5.2 Blending and consistency control Stock is diluted to a fixed consistency in the 2.5-4.0% range and pumped out ofstOrage to an agitated blend chest. This step helps smooth out fluctuations in the lignin content of the unbleached pulp and consistency. Elimination of short-term variations in these two variables facilitates control of chemical addition and improves the uniformity of chlorinated pulp. This procedure has taken on added significance with the recent concerns raised about formation of chlorinated organics that can occur in cases of excess chlorine application. Dilution is generally controlled at two points: out of storage (major dilution) and out of the blend chest (minor dilution). 5.3 Chemical injection and dispersion The goal is to treat each fiber in the pulp suspension uniformly with the proper chemical charge. Injection techniques and mixing devices are used to disperse the chlorine gas because more chlorine is generally required than can practically be dissolved in the water, throughout the slurry. In low consistency applications, this goal generally is achieved by dispersing the gas in water in the form of small bubbles using venturi devices or static mixers. High-shear mixers are used in low- and medium-consistency chlorination systems, because of the importance of good mixing for lowering the formation of chlorinated by-products (39). The effects of water temperature

253

on chlorine gas dispersion have been studied in the laboratory (47). Hot water was shown to facilitate coalescence of small bubbles into larger ones in less than 0.25 second. These findings have practical significance for the design of chlorine injection systems: the chlorine gas dispersion should be formed and injected into the slurry with as little "dead time" in the pipeline as possible and should be mixed with the pulp slurry immediately following injection. Maintaining an adequate water flow to the dispersion device has also been shown to be important. At least 60 kg of water per kg of chlorine is required to form a stable dispersion and prevent plug flow (47). Adding chlorine gas without prior dispersion can be practiced on low- or mediumconsistency systems when high-shear mechanical mixers are used. Because no injection water is used, check valves are recommended to prevent plugging chlorine addition ports by pulp during interruptions in the chlorine flow. Chlorine dioxide solution should be added together with the chlorine dispersion, upstream or downstream depending on chlorine dioxide substitution (Fig. 13). Because chlorine dioxide is added as a solution, the mixing requirements are less rigorous and static mixers are often used. 5.4 Mixing Diffusion of chlorine in pulp slurries has been shown to be slow relative to the rate of reaction of chlorine with pulp (47). Good mixing is therefore important to avoid pockets of over-chlorinated and under-chlorinated pulp. Proper performance of in-line sensors depends on uniform mixing of chemicals with pulp. Symptoms of poor mixing may include dirty pulp (shives, bark particles) after the chlorination stage, loss of pulp viscosity, and varying levels of residual chemicals in the washer vat. Local over-chlorination may result in formation of highly chlorinated by-products even at relatively low chlorine dosages. Several general types of mixers currently are used in chlorination stages, each having advantages and disadvantages.

254

Pulp Bleaching - Principles and Practice

In-line static mixers are also used in low consistency chlorination stages. A 1982 survey found that these mixers dominated NorthAmerican bleach plants (48). In this equipment, mixing elements are built into a special section of the stock line to repeatedly subdivide the flow and induce turbulence and radial mixing. One advantage provided by this type of mixer is that it does not require much space because it is part of the stock line. Because there are no moving parts, maintenance and operating costs are low. A potential disadvantage is that these mixers must be sized properly to perform well. Changes in production rate may cause problems with inadequate mixing at low flow rates and high pressure drops at high flow rates. "High-shear" mechanical mixers also are used to disperse chlorine. These mixers, introduced in the early 19808, operate with low/medium consistency stock. Chlorine gas can be injected directly without prior dispersion in water, and some results indicate improved performance compared to static mixers. These mixers are relatively insensitive to production rate changes. Depending on the manufacturer, the pressure drop across these devices may be slightly negative (pumping action) or 1(}'15 psi positive. Low-consistency stock can be pumped through some of these mixers even when the motor is off. The rotating elements operate at 6O(}.1500 rpm which implies high maintenance requirements compared with static mixers.

6. Process control The objective of a chlorination stage control strategy is to produce a uniform pulp exiting the C-stage washer regardless of incoming pulp variations in kappa number, black liquor carryover, consistency, temperature, or flow rate. In part, this implies maintaining a constant ratio of chlorine consumed (not applied) to mass flow of lignin through the pipe line. In practice, this goal can be achieved by good feedback control of chemical addition based on in-line sensor measurements of the degree of chlorination at a point downstream. Sensors can detect variations in pulp brightness (reflec-

tance), temperature, pH, and level of residual chemical(s). Control room personnel and the computer control system can then use this information, together with consistency and flow measurements, to adjust chemical flows in response to detected variations. Sensors can be located a short distance after chlorine/chlorine dioxide addition points, after the retention tower, in the washer vat, or in all of these locations (49). Another technique for controlling chemical addition to the C stage is to measure Cstage washer pulp brightness and extracted pulp K number. These tests are still widely used today as a longer term set point to adjust pre-tower sensor set points. Other manual tests that are commonly performed are chlorine residual in the washer vat and vat pH. Reflectance of light by the chlorinated pulp slurry is an indication of lignin content. Several optical sensors use this principle to estimate the degree of chlorination by illuminating the flowing stock at a specified wavelength. As noted earlier, paper machine additives such as titanium dioxide can give false indications of delignification progress by increasing the reflectance of the stock slurry. Aiso, the relationship between chlorinated pulp brightness and post-extraction K number shown in Fig. 7 changes with changes in chlorine dioxide substitution level. Practically, this means that brightness setpoints must be adjusted or sensors must be tuned as the degree of substitution changes. Two basic types of sensors are used to measure chemical residuals (49): one is based on measuring the "ORP" (oxidationreduction potential) and the other type depends on polarography. The signal from a polarographic sensor indicates the voltage required to reduce chlorine at an electrode which is directly proportional to the residual chlorine concentration. The ORP sensor measures the potential difference between a platinum and reference electrode immersed in the solution. The signal is markedly non-linear, however, and rises sharply as the chlorine residual increases from zero but levels off at higher residuals.

Chapter IV 2: Chlorination

7. Effect of chlorination on shives and dirt removal The chlorination stage has played an important role in the bleaching of coarse particles derived from the woody starting material, for example fiber bundles (shives), bark specks, and particles from ground up knots. Bleaching has relatively little impact on other extraneous particles, such as sand, pitch particles, and rust that may be present. Removal of these materials, collectively known as "dirt; is an important goal of bleaching, in addition to brightness development. The presence of dirt in the final paper or board products can cause a variety of quality defects. A significant observation is that particle reduction during bleaching is a statistical operation; ev~n though the percent reduction may be acceptably large, when more particles enter the bleach plant, more leave the bleaching operation intact (50, 51). According to extensive laboratory studies conducted by AxegaId and co-workers, bleaching eliminates most of the shives, leaving the bark and knot particles and inorganic materials relatively enriched among the remaining particles. Given that the relative rate of chlorine bleaching of dirt particles is much lower than that for fibers, several principles have been proposed for improving the cleanliness of the final pulp (52). These include maintaining high concentrations of chlorine and operating the chlorination stage at lower temperatures to ensure a chemical residual for a longer period of time. These same principles apply to chlorine dioxide and hypochlorite bleaching, but not to oxygen and ozone bleaching. In these latter cases, the kinetics of bleaching particles and fibers are similar so that little can be done by way of process modification to enhance dirt removal dirt. Even with partial chlorine dioxide substitution, higher chlorine charges lead to improved dirt bleaching (50). Clearly,with the trend toward decreasing use of chlorine for bleaching, it becomes increasingly advantageous to take steps to prevent dirt particles from entering the bleach plant.

255

256

Pulp Bleaching - Principles and Practice

These steps could include improved chip quality (fewer oversize, over-thick chips), better liquor penetration in the pulping process, improved debarking, and greater efficiency in brown stock screening. Removal of shives and dirt is covered in more detail in Chap. IV 11.

8.

Environmental

aspects of

chlorination Lignin represents about 4.5% of the dry weight of unbleached softwood kraft pulp at 30 kappa number. Perfectly selective delignification would therefore lead to a 4.5% yield loss across the bleach plant. Actual values are estimated at about 7%, or 70 kg/ton of unbleached pulp, depending on the bleaching process and the final brightness target. Because most of the residuallignin is degraded and removed in the chlorination and extraction stages, the chlorination-stage filtrate is a major contributor of organic loading to the bleach plant effluent stream. Because, in most cases, chlorination is conducted at low consistency, the volume of filtrate in the overall water balance is large in comparison with other bleaching stages operated at medium consistency unless recycled filtrate is used for dilution. In most existing bleach plants, a large purge to the waste treattnent plant is required because of the corrosiveness of chlorination filtrate. The degree of recycle of this filtrate plays an important role in determining the temperature and pH of the chlorination stage and affects the quantity and type of chlorinated organics that are formed. The lignin remaining in the fiber after pulping still contains aromatic structures, but differs from "native" lignin in that it contains more carbon-carbon inter-unit linkages and fewer labile inter-unit ether linkages (53). Reactions in the chlorination stage lead to substantial depolymerization oflignin; carboxylic acid groups are formed and chlorine atoms are introduced. Most of the aromatic rings are destroyed by oxidative cleavage reactions. Some of the degraded material is dissolved and removed in the acidic chlorination stage filtrate, but

t 5% Mw< 1000

30% Mw < 25000

C-stage Jlig.14.

Mo'-"'"

mgllt

E-stage

MstrlIndimt of ",.,erllIl 4isMJlfletl

most of the degraded lignin and carbohydrate fragments are solubilized in the following hot alkaline extraction stage. It has been estimated that of the 70 kg/ton that is removed during bleaching: about 50 kg originates from lignin, 19 kg from carbohydrates, and 1 kg from extractives (54). About 90% of the chlorine used for bleaching is converted to chloride ion, which contributes to salinity and corrosivity but has no harmful effect on the receiving streams at the concentrations encountered. The balance, about 10%,is bound to organic molecules or high molecular weight material. The organically bound fraction is measured asAOX (adsorbable organic halogen). AOX is used world-wide as a basis for estimating the discharge of chlorinated organics, and has in some cases been used in establishing effluent discharge limits for pulp bleaching operations. Details on the significance of this sum parameter for characterizing effluents are covered in Chap. Vlll, Sects.I and 2. Dence andAnnergren initially compiled much of the relevant information that has been generated regarding the chemical composition of the chlorination filtrate (55). Kringstad and Lindstrom provided an update in 1984 on the nature and identity of compounds in the C- and E-stage filtrates (56). Through the use of membrane separation techniques, it has been shown that about 70% of the organically bound chlorine in the chlorination stage filtrate is present as high molecular weight material

If blMdmIg stIIges.

C "'

"""

(greater that about 1000 g/mole, based on a polymer standard). The remaining 30"AJ of the weight of material passes through the 1000 molecular weight cut-off membrane. In contrast, about 95% of the organically bound chlorine in the E-stage filtrate is in the high molecular weight fraction (Fig. 14) (57). In the low molecular weight fraction, various classes of monomeric compounds have been identified including carboxylic acids, carbohydrates (present as oligomeric fragments), neutrals (including methanol, chloroform, and acetone), and chlorinated phenols. Some of these compounds contribute to the observed toxicity of the untreated chlorination stage filtrate. This topic is discussed in greater detail in Chap. VIII, Sects. 1-3. Material comprising the high molecular weight fraction is considered to be of less significance from a biological standpoint because this material is too large to pass through cell wall membranes. Nevertheless, the material is environmentally significant because it contains chromophoric structures that interfere with the passage of light through the receiving water. The high molecular weight fraction consists primarily of chlorine-substituted polycarboxylic acid polymers originating from oxidative degradation of the aromatic ring system in the residual lignin. The aromatic nuclei content has been shown in several studies to be low (58-61).

Chapter IV 2: Chlorination

Degraded material from the chlorination stage represents a biological oxygen demand (BOD) for the wastewater treatment system and contributes to the dark color of the effluent as noted above. The chlorination stage ffitrate is Initially light straw-colored but darkens with time and also as the pH is increased following mixing with other waste streams. The relative amounts of some of the more prominent materials in chlorination ffitrate are noted in Table 1 (62). Methanol,which arises from demethylation of methoxyl groups by chlorine, is by far the most abundant single component (20, 21). This compound is easily degraded in biological treatment systems.

Component

Amount kg/ton bleached pulp

Methanol

4.6

Formic Acid

1.2

Acetic Acid

0.1

Non-volatile acids

0.3

Carbohydrates'

0.4

Chlorophenols

2.3 x 10'3

'present as polymers Some organically bound chlorine remains with the pulp and cannot be removed even by exhaustive extraction with water or organic solvents (16). The ecological significance of this type of organically bound chlorine has been compared with that of polyvinyl chloride (PVC), a common plastic, in which the chlorine is part of the structure of the polymer itself and quite inert. Chloroform is one of the volatile by-products of pulp chlorination which is proposed for regulation in bleach plant ffitrates in the United States. A number of structures found in pulp can react with chlorine to yield chloroform, including carbohydrates and lignin (63). Although chloroform is found in largest amounts in hypochlorite bleaching stages, it is also detected in C, E, and D stages (14,64-67). The source of the chlo-

257

roform in D stages may be from chlorine in the chlorine dioxide solution but may also arise from reactions with the intermediately formed hypochlorous acid (63). Factors affecting the formation of chloroform in the chlorination stages have been studied and reported recently (68). Under certain conditions, chlorinated dioxin and furans, most notably 2,3,7,8tetrachlorodibenzo-JHlioxin (2378- TCDD) and 2,3,7 ,8-tetrachlorodibenzofuran (2378TCDF) may also be formed in the chlorination stage. These compounds are detected in only trace amounts but are highly stable and persistent in the environment and are classified as extremely toxic. Detection of the foregoing compounds in bleaching effluent has prompted the development of new technology to prevent their inadvertent formation. This subject is covered in more detail in Chap.VIll 3.

References 1. Dence, C.W and Annergren, G. E., in Tbe Bleaching of Pulp (R. P. Singh, Ed.) 300 eOO.,TAPPI PRESS,Atlanta, 1979, Chap. 3. 2. Loras,V., in Pulp and Paper Chemistry and Chemical Technology Q. P. Casey, Ed.) 3rd eOO., Wtley, New York, 1980, Vol. I, p. 670. 3. Smook, G.A., Handbook for Pulp & Paper Technologists (M.}. Kocurek, Ed.), TAPPI PRESS,Atlanta, 1982, p. 160. 4. Singh, R. P., Handbook of Pulp and Paper Technology, 2nd eOO., Reinhold, New York, 1970, p. 249.

258

Pulp Bleaching - Principles and Practice

11. Dence, C.W, in Ugnins: Occurrence, Formatton, Structure, and Reactions, (K. V. Sarkenen and C. H. Ludwig, Eds.) WileyInterscience, New York, 1971, p. 373. 12. Gess,}. M. and Dence, 54(7):1114 (1971).

C. W., Tappi

31. Berry, R. M., Pulp Pap. Can. 90(8):T279 (1989).

14. Kringstad, K. P. and Lindstrom, K., Environ. Set. Tech. 18(8):236A (1984).

32. Hise, R. G., "Chlorination of Pulp," 1992 Tappi Bleach Plant Operations Short Course Notes,TAPPI PRESS,Atlanta, p. 69.

15. Kempf, A. W. and 53(5):864 (1970).

Dence,

C. W., Tappi

16. Reeve, D. W and Weishar, K. M.,] Paper Sci. 16(4):}118 (1990). 17. Hise, R. G., TapPiJ.

72(12):121

Pulp

(1989). (1992).

19. Van Buren,}. B. and Dence, 53(12):2246 (1970).

C. W, Tappi

21. Van Heiningen, A. R. P.,j Pulp 16(3):}83 (1990).

38. Reeve, D. W, Pu, C. M.,Ashinowo, Pap. 59(3): 172 (1985).

Pap. Set.

22. Ni, Y., Kubes, G.}., van Heiningen, A., "Demethylation Kinetics of Kraft Pulp Chlorination; 1989Tappi Pulping Conference Proceedings, TAPPI PRESS, Atlanta, Book 1, p. 23. 23. Russell, N.A., Tappi 49(9):418

(1966).

B.

6. Rydholm, Intersdence,

26. Berry, R. M., and Fleming, 69(3):226 (1987).

9. Kraft, E, The Pulping of Wood, (R. G. MacDonald, Ed.), 2nd edn.,McGraw Hill, New York, 1969, p. 629. 10. Rydholm, S. A., pulptng Processes, Interscience, New York, 1965, p. 921.

R., Pulp

36. Reeve, D. W, "Chlorination Stage Conditions," CPPA Annual Meeting Preprints, Tech. Sect., CPPA, Montreal, 1992, p. 139A.

25. Berry, R. M. and Fleming, HolzfiJrschung 41(3):177 (1987).

S. A., pulping Processes, New York, 1965, p. 916.

35. Histed,}. A. and Vega Canovas, Pap. Can. 88(1):T22 (1987).

20. Van Heiningen,A. R. P., "The Characteristics of Pulp Demethylation During Chlorine Dioxide Bleaching," 1991 Tappi Pulping Conference Proceedings, TAPPI PRESS,Atlanta, Book 2, P 657.

Tappi

8. Rydholm, Interscience,

33. Streisel, R. c., Hise, R. G., BilIs,A. M., "The Effect of Brownstock Washing on Bleach Plant Effluents," 1991 Thppi Brown Stock Washing Short Course Notes, TAPPI PRESS, Atlanta, Section 20. 34. Annergren, G., lindblad, P.-0., Norden, S., Svensk Pappersttdn. 90(12):29 (1987).

18. Hise, R. G., Tappi j 75(2):57

5. Parsons,}. 1., Handbook of Pulp and Paper Technology, (K.W Britt, Ed.) 1st eOO., Reinhold, New York, 1964, p. 267.

7. Kraft, E, The Pulping of Wood, (R. G. MacDonald, Ed.), 2nd eOO., McGraw Hill, New York, 1969, p. 629.

30. Kringstad, K. P., Fleming, B. I., Voss, R. H., Luthe, C. E., 1988Tappi International Pulp Bleaching Conference Proceedings,TAPPI PRESS, Atlanta, p. 63.

13. Voss, R. H., Wearing,}. T., Mortimer, R. D., Kovacs, T., Wong,A., Pap. Puu 62(12):809 (1980).

24. Pugliese, S. C. and McDonough,T.}., j 72(3);159 (1989).

S. A., pulping Processes New York, 1965, p. 916.

29. Axegard,P., 1988Tappi International Pulp Bleaching Conference Proceedings,TAPPI PRESS, Atlanta, p. 69.

27. Grangaard.

I.,

B.I., Pap. Puu

D. H., Tappi 39(5):270

(1956).

28. "Molecular Chlorine Multiple" has been proposed for use by the Tappi Bleaching Committee. Related terms still in common use include "Chlorine Factor" (synonymous) and "Kappa Factor." The latter

is more accurately called" Active Chlorine Multiple" because it includes chlorine and chlorine dioxide, expressed as % equivalent chlorine on pulp, divided by kappa number.

37. Bennington,P.}.,

Tappij.

76(7);77

(1993). T., Pulp

39. Reeve, D. W, Earl, P. E, Gullichsen,}., Pu, C. M., Magued, A., Rapson, W H., Pulp Pap. Can. 89(6):T202 (1988). 40. Tibbling, P.,"Medium Consistency Chlorination: Studies in a High Intensity laboratory Mixer," 1988 Tappi International Pulp Bleaching Conference Proceedings, TAPPI PRESS,Atlanta, p. 127. 41. Berry, R. M., Fleming, B. I., Voss, R. H., Luthe, C. E., Wrist, P. E., Pulp Pap. Can. 89(12):151 (1988). 42. Rapson, W H., Tappi 61(10):97

(1978).

43. US EPA Document,"Summary ofTechnologies for the Control and Reduction of Chlorinated Organics from the Bleached Chemical Pulping Subcategories of the Pulp and Paper Industry," draft dated April 27, 1990. 44. Pryke, D. c., "The Impact of Chlorine Dioxide Delignification on Pulp Manufacturing and Effluent Characteristics at Grande Prairie,Alberta; Non Chlorine Bleaching Conference Proceedings, Miller Freeman, Inc., San Francisco, 1993, Paper #11.

Chapter IV 2: Chlorination 45.

du Manoir,]. R., Trans. Tech. Sect. CPPA 6(2):TR25 (1980).

46.

Macas, T. S. and tion Strategy: Kraft Softwood ing Preprints, Montreal, 1985,

Gowan, G. A., "ChlorinaKraft Hardwood versus Pulp," CPPAAnnual MeetTech. Sect. (CPPA), Book A, p. 205.

47. Reeve, D.W, Pu, C. M., Oshinowo,T., Pap. 59(3):172 (1985). 48. Reeve, tion

D. W and Davis,].

Practice

in North

Pulp

c., "Chlorina-

America

-

Part

I:

Process Conditions, Chlorine Dispersion, and Mixing," 1982 Tappi Pulping Conference Proceedings, TAPPI PRESS, Atlanta, p.347. 49. Baker,]., "Bleach Plant Dual Sensor Control," 1991 Tappi Bleach Plant Operations Short Course Notes,TAPPI PRESS,Atlanta, p.205.

259

57. lindstrOm, K., Nordin,]., Osterberg, E, in Advances in tbe Identification and Analysis of Organic PoUutants in Water, (L. H. Keith, Ed.) Ann Arbor Science,Ann Arbor, MI, 1981. 58. Hardell, H.-L. and de Sousa, Papperstidn. 80:110 (1977).

E, Svensk.

59. Hardell, H.-L. and de Sousa, Papperstidn.8O:201 (1977).

F., Svensk.

60. Lindstrom, Holzforscbung

K.

and Osterberg, 39(3):149 (1985).

F.,

61. Erickson, M. and Dence, C. W, Svensk. Papperstidn. 74:316 (1976). 62. Hardell, H.-L. and Lindgren, B. 0., SSVL Project No.7, "Chloride in the Recovery System," Report No. 21, Stockholm, 1976. 63. Hrutfiord, 73(6):219

B. E and Negri, A. R., Tappi ]. (1990).

50. Axegard, P., Lindblad, P.-D., Popke, I., Puukko, M., "The Matrix for Softwood Pulp Quality and Effluent Kraft Pulp Load," 1991 Tappi Bleach Plant Operations Short Course Notes,TAPPI PRESS,Atlanta, p.21.

64. NCASI Technical Bulletin 515, "Results of Laboratory Studies of Bleaching Parameters Affecting Chloroform Production from Kraft Pulps; National Council of the Paper Industry for Air and Stream Improvement, New York, 1987.

51. Annergren, G. E. and Lindblad, P.0., Tappi 59(11):95 (1976).

65. NCASI Technical Bulletin 558, "Results of Field Measurement of Chloroform Formation and Release from Pulp Bleaching; National Council of the Paper Industry for Air and Stream lmprovement, New York, 1988.

-

52. Axegard,

P. and Bergnor,

Shives and Dirt

E., "Bleaching

- An Overview,"

1991

Tappi Bleach Plant Operations Short Course Notes, TAPPI PRESS,Atlanta, p. 31. 53. Marton,]., in Lignins:Occummce, Formation, Structure, and Reactions, (K. V. Sarkenen and C. H. Ludwig, Eds.), WdeyInterscience, 1971, p. 639. 54. Annergren,

G.E., unpublished

results.

55. Dence, C. W, in The Bleacbing of Pulp, (R. P. Singh, Ed.), 3rd edn., TAPPI PRESS, Atlanta, 1979, pp. 69-71. 56. Kringstad, K. P. and Lindstrom, K., Environ. Sd. Technol. 18(8):236A (1984).

66. Dallons, V. J., Hoy, D. R., Messmer, R. A., Crawford, R.]., TapPiJ. 73(6):91 (1990). 67. Crawford, R.]., Dallons, V. J., Jain, A. K., Jett, S.W, TapPiJ 74(4):159 (1991). 68. DaIIons, V. ]. and Crawford, R. J., "Chloroform Formation in Bleaching; 1990Tappi Pulping Conference Proceedings, TAPPI PRESS,Atlanta, p. 195.

SECTIONIV: The Technology of Chemical Pulp Bleaching Chapter 3: Chlorine Dioxide in Delignification Douglas W. Reeve Department of Chemical Engineering and Applied Chemistry Pulp & Paper Centre University of Toronto Toronto, Ontario, Canada 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 2. DeUgnificationchemistryandkinetics 2.1 Definitions.. . . . . . . . . . . . . . . . '. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Reactions during chlorine dioxide delignification ..... 2.3 Delignification kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. ~tlonefEkiency.. ... 3.1 Degree ofsubstinItion and mode of addition. 3.2 Alkali required for extraction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Brightness development. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Chlorate formation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Process flowsheet and delignification conditions . . . . . . . . . . . . . . . . . . . . . . .. 4.1 Flowsheets and equipment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Chemical charge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. 3 Timing of chlorine dioxide addition. 4.4 Time and temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6 Chloride ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7 Consistency 4.8 Carryover from brown stock washing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

263 263 263 264 265 266 266 268 269 271 273 273 273 274 275 276 278 278

5. Pulp quality .. .. .. .. . .. .. .. .. .. .. . .. .. .. . .. .. .. .. . .. .. .. . .. .. .. .. 5.1 Brightness stability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 VIScosity and strength. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Cleanliness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Extractives and organochlorine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Dioxins in pulp. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Effluent properties. . . . . . . . . .. . . . . . . . .. .. . . . . . .. . . . . . . . .. . .. . . .. . . . . ..

278 278 279 280 281 282 282

6.1 Dioxins

278

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282

6.2 AOX and EOX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282 6,3 Chlorophcnols 6.4 Biological 6.5 Other 7.

Economic

and other effects.

parameters: factors.

organochlorine

compound5

..... .......,....

. . ~61

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 color, chlorate,

BOD, COD.

. . . . . . . . . . . . . . . . . . . . . . . . . 286

.................................................

261

286 "

Chapter IV 3: Chlorine Dioxide in Delignification

Chapter IV 3: Chlorine Dioxide in Delignification 1. Introduction "Euchlorine," a mixture of chlorine dioxide and chlorine generated from the reduction of potassium chlorate by hydrochloric acid, was first described by Davy in 1811 who noted that it destroyed the color of vegetable dyes (1). Watt and Burgess patented the bleaching of soda wood pulp with chlorine, euchlorine, and sodium hydroxide in 1854 (2). In the 1920s,Schmidt and co-workers demonstrated that chlorine.dioxide does not react with carbohydrates (3). In the 1930s, the Mathieson Chemical Corp. introduced a process for bleaching in which sodium chlorite was added to a pulp suspension and ~ous chlorine was added to "activate" the. chlorite, that is, to generate chlorine dioxide. The first industrial processes for continuous generation and use of chlorine dioxide for pulp bleaching began in 1946 with almost simultaneous, though independent, developments in Canada and Sweden (4, 5). In one early application, chlorine dioxide was used as the sole bleaching agent, replacing chlorine for delignification and, in later stages for bleaching, to facilitate resin removal and for the preparation of pure alpha cellulose from sulfite pulp (6). However, from 1946 on into the 1980s, chlorine dioxide was not used extensively for delignification but was mainly used for bleaching in the final stages. Chlorine dioxide was more expensive than chlorine or calcium hypochlorite, but it enabled production of high brightness kraft pulp without loss of pulp strength. Indeed, it was these advances in pulp bleaching technology and chlorine dioxide generation which stimulated the growth ofthe bleached kraft industry. Industrial use of chlorine dioxide to replace chlorine was at first only a means of protecting the strength of the pulp; 5-10%

263

of the chlorine was replaced with equivalent chlorine dioxide (7). However,in 1963 it was reported that substantial substitution of chlorine by chlorine dioxide resulted in a synergistic effect so that delignification efficiency was greatly improved (6). Further improvement was achieved when the chlorine dioxide and chlorine were added sequentially, chlorine dioxide first, with 50% substitution having the greatest efficiency (8). This technique did not gain wide acceptance until the late 1980s when environmental concerns about chlorinated dioxins and other chlorinated organic materialled the industry to decrease chlorine use and adopt substantial, and later complete, replacement with chlorine dioxide. In this chapter, the subjects discussed are: process chemistry, flowsheets, and variables; pulp qualities such as brightness, strength, and cleanliness; dioxins in pulp and effluent; effluent qualities such as organochlorine content, toxicity, and color; and finally, economics. There have been several reviews on chlorine dioxide delignification in which these issues are discussed (9-14).

2. Delignification chemistry and kinetics 2.1 Definitions Chlorine dioxide is an oxidant which accepts five electrons per molecule in being reduced to chloride ion: CI02 + 4 W + 5 e- --+ CI- + 2 HP

[1]

The molecular weight of chlorine dioxide is 67.5 and so its equivalent weight is 13.5 (67.5 + 5). Chlorine accepts two electrons when reduced to chloride ion, has a molecular weight of 71, and so has an equivalent weight of 35.5 (71 + 2): CI2 + 2 e- --+ 2 Cl"

264

Pulp Bleaching - Principles and Practice

CI02 Bleaching Reactions Oxidation states 5

C103-

4

CI02 _

3

--

~

C102:

1

I

1 2

8 9 10 J

L_-t~HCIO

1

3--.

o

Cb ~l~

CI- .,--4

-1

)

1

6 4 5 7

J

~

~

~

Organochlorine

of chlorine dioxide is equal to 2.63 weight units of "equivalent chlorine." Chlorine dioxide substitution, expressed as a percent, is also a very common unit and is based on equivalent chlorine. Equivalent chlorine applied on pulp is sometimes expressed as "kappa factor," also known as (equivalent or active) chlorine multiple: chlorine + chlorine dioxide appliedl as % equivalent chlorine on pulp [3] fkappal funbleached PulP x

[

Lfactor

J

Lkappa

]

number

J

=

As an example, a 25 kappa pulp treated with chlorine and chlorine dioxide corresponding to a kappa factor of 0.20 at 50";6substitution would be treated with: 25 x 0.20

[2]

Therefore, in substituting chlorine dioxide for chlorine to provide equivalent electron transfer, one weight unit of chlorine dioxide can replace for 2.63 weight units (35.5 + 13.5) of chlorine. "Equivalent chlorine," also known as active chlorine, is a common unit of oxidant in bleaching technology; one weight unit

Pulp Reactions

"\.0 X0."\0 = and 5.0 x 0.50

/ 2.63

5.0";6equivalent chlorine on pulp 50 kg equivalent chlorine per metric ton of pulp 100 Ib equivalent chlorine per short ton of pulp which would be made up of 2."j% chlorine on pulp

= 0.95%

chlorine

on pulp.

dioxide

2.2 Reactions occurring during chlorine dioxide delignification Chlorine dioxide is reduced through a series of steps involving several intertnediates before chloride ion is produced. Hypochlorous acid (HOCl) and chlorine (CI2) are among these intertnediates, and their presence can lead to the fortnation of chlorinated organic matter. Another undesirable by-product is chlorate ion (CIO;) which is not reactive and so its formation' means loss of bleaching efficiency. Figure 1 illustrates the changes in oxidation state and reaction pathways of the reactive intermediates during chlorine dioxide bleaching of pulp (15). Chlorine dioxide reacts with pulp, transferring, in the process, one electron to produce chlorite ion (CI02") by reaction {I} (Fig. 1). Chlorite ion does not react directly with pulp. Chlorine dioxide also reacts with pulp 12} to fortn HOCl, which is, in part, converted to CI2by hydrolysis 13}. Hypochlorous acid and chlorine react with pulp producing chloride ion and chlorinated organic matter {4 to 7}. Chlorine reacts with chlorite to regenerate chlorine dioxide {l0}, while hypochlorous acid reacts with chlorite to

265

Chapter IV 3: Chlorine Dioxide in Delignification

.'

100

....

as 35 40 60 CI02 Substitution,

20

Fig. 14. fte brlgbtua ofJnllPufler three tIrIdfive sltlges ofbletu:Wag of sojlwJood lmIfI pulp ","b eblorirle tIrId eblorirle dIo:xIiIe.

50

S40

20

o

Cia,

~

o

70

34.8

15

.10 Kappa number

o

Fig. 13. fte post-t:#flStie-uIrYI£tIort brlgblrless of JIIIlps .fIn' tlellpljletllltm U1IIfbI",,""tb tUnu of eblorirle dIo:xIiIe tIrId eblorlu.

tion, increases with degree of delignification with the exception of the initial phases of chlorine-only delignification; the brightness increase is greater with increasing degree of chlorine dioxide substitution (39). Equimolar concentrations of chlorine dioxide and chlorine were used for the (D+C)E case. The discussion of delignification in the previous section is predicated on the assumption that the kappa number is a true measure of progress toward fully bleached pulp and, in particular, that it is an accurate measure of the amount of bleaching agent required in later stages. It has been argued that, because chlorine dioxide oxidizes (and bleaches) lignin to a greater degree than does chlorine, an oxidative permanganate test, for example, kappa number, gives a false measure of lignin content. However, it has been clearly shown that the post-extraction kappa number is linearly correlated

with chlorine dioxide required in later stages to achieve the desired final brightness for a wide range of degree of substitution (4043). TI1is subject is discussed in greater detail in the chapter on chlorine dioxide bleaching (125). The synergistic effect of chlorine dioxide in combination with chlorine in the first stage is apparent from the brightness reached in later stages. Figure 14 shows the brightness advantage gained, before and after thermal aging, in the third and fifth stages of a (D+C)EDED sequence when mixtures,O+C, are used in the first stage (44). Figure 15 shows the even more dramatic advantage obtained when sequential treatment, DC, is used in the first stage of a (DC) ElI sequence as compared to mixtures (8). Assessment of the impact of chlorine dioxide use in delignification on the achievement of high brightness is complicated by the variety of chemicals and sequences used and the range of conditions applied that can be used for bleaching partially delignified pulp and by the diversity of criteria available by which to judge a bleaching sequence. Germg3.r'd and co-workers have optimized (DC)EDED sequences by adjusting the kappa factor in the first stage to give the lowest overall equivalent chlorine use. The results are given in Table 1 for several pulps (4043, 45). The minimum overall

chemical requirement is achieved with 3050% chlorine dioxide substitution. Optimization for lowest chemical cost was also studied and was usually close to the optimal conditions for lowest overall equivalent chlorine. For unbleached softwood pulp, the lowest chemical cost was at 30% chlorine dioxide substitution. Increasing substitution to 70-90% required a significant increase in chemical and led to a significant increase in cost. The results of a similiar study on optimization of (OC)(E+O)O bleaching of oxygen-delignified eucalyptus are summarized inTable 2 (33). To decrease the cost at 70-90% chlorine dioxide substitution, for bleaching softwood kraft, consistency was increased from 3-8%, chloride ion was added, and the interval between 0 and C was increased from 2 to 4 minutes; a 5% decrease in cost was reported (40). These changes in process conditions had no influence on costs at 3050",,(,chlorine dioxide. Another optimization study, for (OC)E (HO) bleaching to 86 ISO brightness,found minimum costs at 30% substitution independent of kappa factor in the range 0.20 to 0.27 (46). At 90% substitution the lowest cost was at 0.20-0.24 kappa factor; costs increased at 0.27 kappa factor. At 50% chlorine dioxide for the same sequence bleaching to 90 ISO brightness, cost was minImized at 0.16-0.18 (47).

Chapter IV 3: Chlorine Dioxide in Delignification Tllble 1. optImIzetI (DC)EDED6lMe1mtf ollmlft /NIlp 10 ~

271

272

ISO MI,,,,-.

Pulp Bleaching - Principles and Practice 8

80

o

ClO, in first bleaching stage, % of total equivalent chlorine

Optimal charge in first bleaching stage, kappa factor

Kappa number after extraction

Total equivalent chlorine to reach 90% ISO brightness, kappa factor

A. 29.8 Kappa number softwood (40) 10.

0.20

30 50 70 90

0.20 0.18 0.17 0.14

6.4 4.8 5.1 5.6 8.7

B. 26.8 Kappa number softwood from modified continuous cooking (41) 10. 0.18 5.9 30 50 70 90

0.17 0.17 0.17 0.14

6.1 4.9 5.8 8.1

0.29 0.27 0.25 0.28 0.30

0.17 0.17 0.17 0.16 0.16

4.3 4.1 3.7 4.4 5.4

40 60 CI~ Substitution, %

100

80

10 10 30 Chlorine dioxide applied, kwtonne FI,. 17. C61ort1tfJ.for'mH

m, ~

frmrt e61or1u

.,...

tI'-Itle

lrfJ"'-Ia.ftnm4lt1

VGrl-

OIlS sllulla.

0.28

1.0

0.27 0.25 0.28 0.31

o :to.8 *-

C. 18.2 Kappa number oXygen-delignified softwood (45) 10' 30 50 70 90

10

0.27 0.26 0.26 0.27 0.31

Pulp J'

"i' 0.6 E ..2 0.4 S ~ 0.1 U

~

Optical and residual sensor 10

40 60 Cia, Substitution, %

80

100

Mix tank

Chlorination

tower

PI,. 16. C61on11e m I. .IIpiJbtiort 01 .., tll./fernt e6wsojlrlJoHI"""" Inwft ""p rl1N-e61or1u~ IIIItIItImtmotles.

FI,. 18. FlmDs6efJllor

Iow-etnlSlsIf!fU:J

(DC) .11181.

By changing the process, particularly the mode of chlorine/chlorine dioxide addition, pH, chloride ion concentration, and consistency, chlorate formation can be decreased. Chlorate formation decreases oxidizing chemical available for delignification; however, there are other factors that also contribute to delignification efficiency. When chlorine dioxide is added first in sequential treatment, (DC), the chlorate

formed is decreased compared to mixtures or to (CD), in part accounting for the increased efficiency of this sequential treatment. Figure 16 illustrates this statement (30); similiar results have been reported by others (42, 50-52). In a (DC) treatment,in the interval before chlorine addition, chlorite undergoes acidic decomposition to give chlorine dioxide and chloride but no chlorate (16). Conversely, in a (CD) treatment,

./b1Um.

D. 17.2 Kappa number birch (42) 10' 30 50 70 90

0.37 0.35 0.34 0.36 0.42

0.24 0.17 0.17 0.24 0.24

E. 16.0 Kappa number eucalyptus (43) 10' 30 50 70 90

0.29 0.24 0.26 0.28 0.32

0.17 0.17 0.15 0.15 0.20

'Sequence (C+D)EDED Some mills using only chlorine dioxide in the first stage have had difficulty in reaching high brightness (90 ISO); the addition of peroxide to the second extraction stage has helped to overcome this problem (48, 49).

TIIbk

2. optImIzetI

(DC)(B+O)D

CIO, in first bleaching stage, % of total equivalent chlorine

6lMe1mtf

0112.0

bpJJtI

tI_'-

Optimal charge in first bleaching stage, kappa stage, kappa factor

0xyffJtI-6IfJGC6fJtI_lyptus

""p

3.4 Chlorate formation

10" 30

0.15 0.15

0.22 0.21

M described in Sect. 2.2, the chlorate ion formed when chlorine dioxide is used in bleaching represents a loss of active bleaching chemical. Chlorate is formed by reaction of chlorite ion with hypochlorous acid.

50 70 ~

OJ7 0.17 QB

O,~l 0.25 Q~

'Sequence (C+D)EDED

(33).

Total equivalent chlorine to reach 90",{,ISO brightness kappa factor

Chapter IV 3: CWorine Dioxide in Delignification hypocWorous add is present when chlorite is formed by the reduction of chlorine dioxide and this leads to chlorate formation. CWorate formed is a function of chlorine dioxide applied. All the data for (DC) treatment from the reports dted above have been plotted in Fig. 17. Although chlorate formation is strongly affected by process conditions, it can be seen that cWorate formation, expressed as a percentage of chlorine dioxide consumed, is approximately 20% on a weight basis (13% on a mole basis). The percentage cWorate formation is higher for chlorine dioxide bleaching stages as discussed in Chap. IV 8 (105).

4. Process flowsheets and conditions 4.1 Flowsheets and equipment At this time, most delignification stages using chlorine dioxide in combination with chlorine or alone are modified chlorination stages. Unbleached pulp is diluted to a low consistency, typically 34%, and chlorine dioxide solution is added using a mixer; the mixed suspension passes into an upflow tower and then to a washer. When chlorine is also added and it is added after the chlorine dioxide, there are two mixers separated by a section of pipe, or a modest flowthrough tank, to provide retention time. The flowsheet must provide suitable process conditions for effective chlorine dioxide delignification. Typically, those conditions fall in the following ranges: Total chemical charge:

0.15-0.25 kappa factor

Chlorine dioxide charge:

25-100% of the total

Temperature:

30-60°C nil up to 5 min.

Time between D and C: Total time:

20-60 min.

End pH:

1.5-3

Consistency:

3-4%

Reports of mill implementation of chlorine dioxide delignification provide many

273

examples of flowsheets. In a 1967, trial in which hardwood pulp was delignified with chlorine dioxide/chlorine mixtures, chlorine dioxide was added to the dispersion of chlorine gas in water immediately downstream of the chlorine disperser (53). In a 1973 report on sequential (DC) treatment of softwood, one mill employed a 9-minute, 10% consistency retention tower for the chlorine dioxide reaction, a dilution step, and chlorine addition followed by a 60minute, 3.5% consistency tower; a second mill employed two in-line mixers separated by a 3-minute section of pipeline followed by a 45-minute tower, all at 3.5% consistency (54). Several recent mill reports provide other examples of low consistency flowsheets (55-57) and another report of successful mill trials provides five examples of process flowsheets, an example of which is given in Fig. 18 (58, 59). This flowsheet is appropriate for 30-700A,substitution with a small portion of the cWorine dioxide (5l00A,)being added with the cWorine to pre-

vent viscosity loss. . Medium-consistency (10-15%) cWorine dioxide delignification is not widely practiced at present but is being adopted more widely. An example of a medium-consistency flowsheet is shown in Fig. 19 (58). 4.2 Chemical

charge

The issue of how much chemical to add in a delignification stage using chlorine dioxide has already been discussed, to a certain extent, in Sect. 3 on delignification effidency. Germgard has shown that, as delignification is extended, the incremental chemical charge required to achieve an incremental kappa number decrease becomes greater, that is, delignification effidency decreases (see Fig. 10). This effect can also be seen by plotting extracted kappa number against chemical consumed as shown in Fig. 20 (60). The softwood pulp used had a kappa number of 26; (C+D) was at 15% substitution and (DC) was at 50% substitution. As more chemical is applied, the curve flattens, that is, the chemical becomes less and less effective and delignification effidency de-

274

Pulp Bleaching

-Principles

and Practice 10

Chlorine gas Pulp

26 kappa softwood kraft

.~ ~~:::--~:

t Pulpto tower

DE (C~+D,JE

~

MC pump

CIO,

Optical and residual sensor

JIIg.If). ~tftn'mftiam~(DC)de. IIpljlaltlml. creases. There is merit in limiting the charge of chemical to the range in which it is most effident, that is, an amount corresponding to the steep part of the curve. As the cWorine dioxide in the first stage increases above 500A"the curve flattens at a higher extracted kappa number. Optimal chemical charge must be determined in the context of the overall bleach plant as discussed in Section 3.3. 4.3 Timing of chlorine dioxide addition Chlorine dioxide, added in advance of the cWorine, reacts with the pulp before cWorine is added and is more effident than when cWorine and chlorine dioxide mixtures are used. The logical extension of this finding is therefore that delignification effidency is maximized when all the chlorine dioxide is consumed before the chlorine is added. The time required for complete reaction of the cWorine dioxide is a function of temperature, consistency, and kappa factor. Early laboratory wode:, by Hatton (8), on the timing of sequential delignification was done only at 200C with hand mixing and chlorine dioxide was limited to 50% substitution. It was found that a 3-minute chlorine dioxide reaction time at this temperature was insuffident to give maximum benefit; 5 minutes gave greater delignification and higher brightness (8). Hatton also used a 5-minute chlorine dioxide reaction time at 200C; delignification was not increased at 10 minutes and, in fact, was poorer at 15 minutes' reaction (27).

o

i

O-{D"C.,JE

0.20 kappa factor

2

4 Chemical consumed,

Fig. 20. DellgrdJ'ktlUmt

cblorlu.

wltb

6 % equiv. CI, on pulp cblorine

dWxUle """

.

With effident laboratory mixing, incorporating a stirred tank reactor at low consistency, it was shown that at 300C and with 30% substitution, 30 seconds was suffident to obtain almost maximum delignification (61). However, in the same study, it was shown that if 60 seconds or more were used, pulp viscosity suffered because all the cWorine dioxide had disappeared before it could be useful in protecting the pulp against cWorine. Further discussion of this topic is found in Chap. III 2 on chlorination (125). To avoid viscosity loss, where the combination of time, temperature, and amount of added chlorine dioxide lead to complete consumption of protective chlorine dioxide,it is recommended that a small amount of chlorine dioxide (5-10% of the total charge of equivalent chlorine) be added with the chlorine. In another study, it was found that 60 seconds was required to obtain maximum delignification at 400C 18

D(C+D)

Delignification . 2 min between D and (C+D)

10

. D fully reacted before C + D

o

20

40 Clo,

60 Substitution,

J

80

100

')(,

Fig. 21. Tbe effed of cblorlu dw:dlle subtitrltklll .rultbe "me to _me tbe cblorlu diox. ide 011(DC) deUpljlaltlml.

Chapter IV 3: Chlorine Dioxide in Delignification

CI02 in first stage, % of total equivalent chlorine

275

Temperature, °C 20

35

50

Tune to 95% consumption, minutes 10 30 70 aDelignification

14 11 11 conditions:

kappa

8 6 5

factor 0.155; consistency

for 30-50% substitution (62). In the same study it was found that when the consistency was increased to 10%, the chlorine dioxide reaction time required for maximum delignification decreased to 20 se~onds for the same conditions. Increased delignification results if the chlorine dioxide is completely consumed before the chlorine is added. This is clearly shown in Fig. 21 for delignification of 31 kappa number softwood kraft pulp at 300c with a kappa factor of 0.19 (47). The high delignification efficiency is mainly due to the high efficiency of that phase of sequential delignification occurring immediately after chlorine is added (63).

4.4 Time and temperature Tune and temperature are critical factors in determining the extent of consumption of chemical in delignification using chlorine dioxide. If the time is too short or the temperature too low, the chemical applied is not totally consumed and residual chemical remains at the end of the stage. Usually the principal objective of the chlorine dioxide/chlorine delignification stage is to achieve a particular degree of delignification by consumption of a specified amount of chemical. Most of the discussion in this section concerns this objective; however, it is important to note that if residual is consumed too rapidly shive removal in this stage may be adversely affected. The rate of deligniflcation controls the consumption rate as discussed earlier in the section on kinetics. A higher delignification rate occurs with pulp having a higher kappa

5 4 2 1.5%

number, with application of a higher concentration of chlorine dioxide and chlorine, at higher temperature, and with chlorine dioxide/chlorine mixtures in the range 3060% chlorine dioxide as compared to chlorine or chlorine dioxide alone. For each lOoC increase in temperature, the delignification rate doubles. In many cases, mills delignifying with chlorine dioxide, with or without chlorine, have fixed volume towers built originally for low consistency chlorination. The retention time available in such a stage would typically have been designed for 30-60 minutes, but, with production rate increases over and above design as is often the case in mills, the retention time is much lower in operation than originally designed. If a longer retention time is required, a slight increase can be achieved by increasing consistency although this strategy is very much limited by the "pumpability" of the pulp. The principal means of overcoming limitations in retention time is to increase the temperature. As shown in Table 3, the time required to achieve 95% consumption of chemical decreases rapidly as temperature is increased (64). Temperature is controlled by regulating the temperature of the incoming unbleached pulp, usually by adjusting wash water temperature in the last stage of brown stock washing. In modem mills, no dilution water other than recycled first-stage flttrate is used for dilution of the pulp leading into the delignification stage. However, where other sources of water are used, temperature control is possible by adjusting the

276

Pulp Bleaching

-Principles

and Practice

temperature of the dilution water. Some delignification stages are subjected to wide fluctuations in temperature due to seasonal variations which provide large changes in the raw water temperature. In Canada, the raw water temperature may change from 2 or 3°C in the winter to 20 or 25°C in the summer. Chlorine dioxide solution is typically applied at 5-lOoC and is added to the system in sufficient volume to decrease the temperature several degrees. To offset this negative effect on temperature, heat exchangers may be used to heat the chlorine dioxide solution. Controversy exists about the impact of higher temperature on delignification efficiency. A thorough study by Germgard (35), discussed previously in the section on delignification efficiency, showed the absence of a temperature effect on delignification of unbleached softwood kraft and oxygen-de lignified softwood kraft using chlorine dioxide alone or a 50/50 mixture of chlorine dioxide and chlorine (9). Another study failed to demonstrate any effect on delignification efficiency for substitution in the 30-70% range when the temperature was increased from 20 to 50°C at a 0.20 kappa factor (64). However, this same study showed that, when the kappa factor was 0.155, delignification efficiency decreased as the temperature increased. In yet another study, it was found that delignification with mixtures of chlorine and 1IIbk 4. Effect o/~ mft jHltp. D-stage temperature, °C

chlorine dioxide at 5()-6(}oA,substitution was significantly less effective at 60°C than at 30°C (61). Another example of decreased efficiency as chlorine dioxide delignification temperature increases is shown in Table 4 (65). It should be noted that the time to reach zero residual for a 100% chlorine dioxide treatment is much greater than the time given in Table 3 for sequential addition of chlorine dioxide and chlorine. There is no evidence that temperature alteration has any impact on the quality of pulp produced by chlorine dioxide delignification. As a final note, neither pulp viscosity nor kappa number is adversely affected by extending the retention time well beyond the time needed to exhaust the residual in either chlorine dioxide (66) or chlorine delignification (67). 4.5 pH pH has a significant impact on most bleaching stages. There is some controversy about the optimal pH for chlorine dioxide delignification. Rapson studied the effect of pH on chlorine dioxide/chlorine stages and found that delignification was slightly influenced by pH with delignification being greatest, as evidenced by the kappa number after extraction, near an end pH of 2 for both chlorine dioxide alone and a 70/ 30 chlorine dioxide/chlorine mixture (68). The kappa number after the extraction stage

OftelJlorlIIedioxide dellplftetlliml e./ftderleyfor" 31.0 luIpJMsoJt-04 Extracted kappa number

Tune to zero residual, min

E

(EO)

(EP)' 5.8 5.5 6.4 6.2

Kappa factor 0.190 30 40 50 60 Kappa factor 0.155

180 82 34 19

7.0 6.8 7.9 7.6

5.9 6.1 6.7 6.8

30 60

155 16

8.3 9.6

7.5 8.2

'0.5 % HP2 on pulp

Chapter IV 3: Chlorine Dioxide in Delignification 3.5 _

3.0

Pulp Bleaching - Principles and Practice

12

4.6 Chloride ion 10

8] E 6

"

:;; Q. Q.

~

w

o

'* 0.5

o 5 pH in D stage Fig. 22. De IffI/HId of JIll 011tlellp'.fktlt"m

.,."" cbIurl1Ie ~

278

End pH in the range 2-4 has no affect on pulp viscosity (70, 72).

14

.!j-Q. "§ 5:E u c: 0 2.5 CIi "t:J cra .5 (5 2.0 .!j~"5 ..Qc 1.5 CIi --"u!: ~ 'S 1.0 "t:JCT Q,) "in &!

277

~

increased slightly as the pH increased to 4 and then increased dramatically with further increase in pH. However, delignification efficiency was not directly related to the formation of non-reactive oxidant (CI03- and CIO;) as shown in Fig. 22 for chlorine dioxide alone. At a pH below 2, sodium chlorate formation was significant but thereafter decreased sharply up to pH 4. Above pH 4,chlorite formation increased very dramatically. The minimum chemical loss in the form of chlorate and chlorite occurred at a pH between 3 and 4, in an identical manner to that found to occur in chlorine dioxide bleaching. However, this minimum did not correspond precisely with the optimal pH for delignification which was 2. Several studies by various authors have shown chlorine dioxide delignification to be insensitive to pH from an end pH ofless than 2 to an end pH of 4,for 28 kappa pine kraft pulp and 17 kappa oxygen-delignified pine kraft pulp (69),33 kappa number softwood kraft (66), 16 kappa number eucalyptus (43), 13.8 kappa number oxygendelignified eucalyptus pulp (70), and 24 kappa number southern pine 01). Two studies reported the optimal pH to be in the range of approximately 3-4. In one study, treatment of oxygen-delignified

a4

jomItItitm

of35/u1pJ111 __

9 of dllorl~

a4

dllmwle

SDftrDootllmlft JHIlp.

kraft pulp of 22.4 kappa number with chlorine dioxide indicated a steadily decreasing DED brightness with decreasing initial pH in the first D stage from 8.3 "natural pH" to 1.5; this range corresponded to end pH values ranging from approximately 2.5-4 to 1.5 respectively (72). Another study provided supporting evidence indicating that the maximum delignification efficiency and DE brightness occurred at a pH slightly below 4, a value consistent with the minimum loss of chemical in the form of chlorate and chlorite (73). The kinetics of chlorine dioxide/chlorine delignification are affected by pH in a manner complicated by the chloride ion concentration. With chlorine dioxide alone, increasing the pH from 2 leads to a steady increase in delignification rate. In the presence of 0.1 M sodium chloride (5.8 g/L), the delignification rate is higher at all pH levels but a distinct minimum appears at pH 3 (18). When mixtures of chlorine dioxide and chlorine are used, the delignification rate is again lowest at low chloride ion concentration but increases substantially with increasing chloride ion concentrations up to 0.50 M at all pH levels. The delignification rate decreases from pH 2 to pH in the " from absence of chloride ion but increases pH 2 to pH 4 in the presence of chloride ion (20).

Given the number of reactions in which chloride ion takes part as shown in Fig. I, it is not surprising that chloride ion concentration has a significant influence on delignification kinetics and efficiency. In the chloride ion concentration range 0.020.5 M, delignification efficiency is unchanged but, when the chloride ion concentration is less than 0.001 M, delignification efficiency is decreased by almost 50% (35). In another study it was found that only 0.01 M chloride was required in a chlorine dioxide delignification stage to increase final brightness in a D(EO)D bleaching sequence by 1% ISO. Separate studies on the delignification of hardwood pulp using chlorine dioxide revealed that addition of chloride ion decreased the kappa number after extraction and decreased chlorate formation (43, 66). 4.7 Consistency In the 1970s, delignification studies of high-consistency pulp (>35%) using gaseous chlorine dioxide and chlorine showed that an increase in consistency led to a dramatic increase in delignification efficiency (74). However, another early study provided evidence showing that increasing the pulp consistency from 0.4%-10% in delignification of oxygen-prebleached softwood kraft pulp with chlorine dioxide had no impact on delignification efficiency (35). Independent studies of hardwood pulps have shown the significant benefit resulting from an increase in consistency from low (3-4%) to medium (1(}14%) in delignification efficiency for eucalyptus (33, 43), aspen (66), and oxygen-delignified kraft pulp of kappa 13.1 (72). Another report provides numerous examples of superior delignification obtained at medium consistency as compared to low consistency for sequential treatment of oxygen-delignified kraft pulp with 10,30, and 50% chlorine dioxide substitution (62). Recent studies show that increases in consistency from low to medium to high

increase delignification efficiency of 30-35 kappa softwood kraft and decreases chlorate formation (75). This is explained by reaction pathways and reaction intermediates which provide a sound theoretical basis for these empirical observations.

4.8 Carryover of dissolved organic material It is well-known that residual dissolved organic maner retained by unbleached or, oxygen-delignified pulp because of incomplete washing, consumes bleaching chemical in a chlorination stage. However, relatively linle information has been published for the case where chlorine dioxide or chlorine dioxide/chlorine is used in the first stage. An early report of mill trials provides evidence of the increased chemical consumption that may result from carryover (54). Carryover can be determined by total organic carbon (fOe) measurement or by oxidant consumption, for example, permanganate consumed per volume of illtrate.The traditional measurement, "saltcake losses; based on sodium or conductivity measurement, is not appropriate in systems employing illtrate recycle. Recycled illtrate from the chlorine dioxide/chlorine stage can also consume chemical in a closed system. Per unit of organic carbon, extraction stage illtrate consumes more chemical than does recycled first stage illtrate and black liquor consumes more chemical than does extraction stage illtrate (76). Oxygen and 100% chlorine dioxide delignification both decrease the chemical consumed by illtrate carryover in illtrate recycle systems using little water (77).

5. Pulp quality 5.1 Brightness stability Bleached pulp darkens with age. This "brightness reversion" is a function of chemical structures in the pulp. It is possible to accelerate aging by elevating the temperature and, for light-sensitive structures, by exposing the pulp to light. Until recently, there has been no standard procedure for measurement of brightness stabil-

Chapter N 3: Chlorine Dioxide in Delignification 40

5 w

U 35 'ifci '"

-: 30

6-

~ .~ 25 5 20 o

20

40 60 ClO, Substitution. ')(,

Fig.23. 1IoeImJNIdofcblorlu ~

- so~

80

100

~

-/I JnIlJI "'-11].

ity, that is, for accelerated aging; a range of temperatures, exposure times, and humidities have been used. Although mechanical pulps are light sensitive, accelerated aging of chemical pulps is principally accomplished by increasing the temperature. The use of chlorine dioxide in place of chlorine for delignification has been reported to increase brightness stability. It has been well-known for many years that substantial replacement of chlorine by chlorine dioxide increases the brightness stability of sulfite pulp (25). This finding has been confirmed more recently both in the laboratory (78) and on a mill scale (79). Poor brightness stability has been shown to be directly related to the high extractives content of chlorine-bleached sulfite pulp. A significant decrease in extractives content can be achieved when chlorine dioxide replaces chlorine (80). Similarly, hardwood pulps have been found to be more stable when a high degree of chlorine dioxide substitution is used in the bleaching of mixed Canadian hardwood species (25, 26, 78) and birch (42). Once again, a decrease in extractives Ttlble5. hIfl-

of cbltJrlu ~

1125 1135 1115

of cblorl_ ~

and Practice

sUsllhItItm ~ tilestrefIgtbof so~

ing chemicals attack the cellulose fraction in pulp, the cellulose degree of polymerization may decrease and with it viscosity. Beyond a critical point a decrease in viscosity leads to a decrease in sheet strength. Because chlorine dioxide is known to be less damaging to cellulose than chlorine, increasing substitution of chlorine dioxide for chlorine logically should reduce degradation of the cellulose. A comparison of pulp strength data, particularly those in early studies, is complicated by the lack of standard testing procedures. It is now generally accepted that strength comparisons can best be made by comparing tear strength at a given tensile strength. Data listed in Table 6 show that, although oxygen delignification leads to a lower tear at a given tensile, chlorine dioxide substitution in the 1009Q"Ai range has no influence on tear (82). It should also be noted that oxygen delignification increases the beating time requlred to achieve a given tensile strength, but chlorine dioxide substitution has no effect (82). Other laboratory ,studies have shown that 1009Q"Ai chlorine dioxide substitution has no effect on strength, that is, on tear at a given tensile (31, 83). Two mill studies have shown that significant increases in tear occur at a given ten-

of so~

1nW/IJnllp bleacbed ", "

Starting Pulp (oxygen-delignified) 16.0

10.7

Viscosity, dm3fkg 10 50 90

-Principles

5.2 Viscosity and pulp strength Pulp viscosity increases dramatically with only 5-10% chlorine dioxide substitution and then decreases slightly as percent substitution increases further at a fixed chemical charge (Fig. 23) (25). The decrease experienced in the mid-range of percent substitution is likely related to the greater delignification achieved and the comparatively lower post-extraction kappa number that results. Pulp viscosity is also sensitive to the mode of addition of chlorine dioxide and chlorine. Addition of chlorine dioxide in advance of chlorine gives the highest viscosity at a given kappa number (31). On the other hand, the viscosity of fully bleached pulp has been found to be unchanged as chlorine dioxide substitution increases from 10 to 50 to 90% as shown in Table 5 (82). The relationship between viscosity and strength is complex as discussed in other chapters of this book (125). When bleach-

Starting Pulp (unbleached) 33.3

Pulp Bleaching

Ttlble6. r-fl-

SllbstUrllloll~ tile "'-II]

Kappa number:

280

content is associated with this increase in brightness stability. For softwood pulps the situation is less clear. Several authors have claimed that softwood pulps are more heat stable when delignified with more chlorine dioxide and less chlorine (6, 25, 81). However, there are also data which show that the brightness stability of softwood pulp does not change with increasing chlorine dioxide substitution (32, 51, 78). Even though there is some doubt arising from the lack of a standard test method for reversion, it is likely that chlorine dioxide substitution has little impact on brightness stability of softwood pulps (12).

(C+D)EDEDset[fImU.

00,,%

279

920 935 950

740 745 745

-/I Jnllpbleacbed",

(C+D)EDEDset[fImU.

ClO" % Kappa number:

Starting Pulp (unbleached) 33.3

"

Starting Pulp ( oxygen-delignified) 16.0

Tear index,Nm'/kg

10.7

at tensile index 90 kNm/kg

10

14.6

13.4

11.0

50 90

13.4

13.0

11.5

13.4

13.3

11.5

Beating revolutions to tensile index 90 kNm/kg IO

900

2000

2500

50 90

1000

2000

2300

1000

2000

2400

sile with increasing chlorine dioxide substitution. An increase in substitution from 10% to 30,40, or 50% increased the tear strength of softwood kraft pulp by

10% (55). Increasing the chlorine dioxide substitution from 30 to 100 % and decreasing the chlorination factor led to a similar increase in tear at a given tensile index for an oxygen-delignified softwood kraft pulp (84). 5.3 Cleanliness Removal of contaminating particles is an important function of bleaching. A rate equation for particle removal during chlorine dioxide delignification is given below (85): -d(partlcles)

= k [ClO,]07

[particles]l

dt where

= shives,

particles k

knots, and bark constant (varies according to particle).

= rate

The particle concentration decreases linearly with time, and so this is a first-order equation. This is distinctly different from the concentration of1ignin which decreases very rapidly initially and then more slowly. Thus, particle removal is favored by prolonged retention at a given concentration of chlorine dioxide. The Arrhenius activation energy for chlorine dioxide reaction with paniclc~, jj :t j kJ/mol ({J5), compares to a value of 60 kJ/mol for reaction with lignin as measured by kappa number decrease. Thus, lower temperature in a

Chapter IV 3: Chlorine Dioxide in Delignification chlorine dioxide delignification stage decreases the rate of delignification and the rate of chlorine dioxide disappearance while the rate of particle elimination is reduced to a much lesser degree. Lower temperatures therefore favor particle elimination. Particle removal in bleaching is dependent on many factors, the most important of which may be the extent of bleaching in a particular stage or sequence. The first chemical applied in a bleaching stage is consumed mainly by lignin rather than by particles and so extended bleaching with greater addition of chemical leads to improved particle elimination. Further, particles may not be immediately eliminated in a particular stage but may be made more susceptible to removal in a later stage. Chlorine dioxide alone has been found in severallaboratory studies to be highly effective in removing particles (85, 86). As shown in Fig. 24, chlorine dioxide alone is uniformly superior to chlorine; alone in the first stage of a five-stage sequence over a range of targeted values. In Fig. 25, a similar result is seen for a given consumption of bleaching chemical. Figure 24 shows that mixtures of chlorine dioxide and chlorine are not as effective as chlorine dioxide by itself. Another study showed that 30, 50, and 70% substitution was decidedly inferior for wood dirt removal compared with 10 and 90% substitution (46). To complement information in the literature about the effectiveness of combinations of chlorine and chlorine dioxide in removing particles,

281

several recent mill studies have shown that no change in pulp cleanliness occurs when chlorine dioxide substitution is altered over a wide range (13, 55-57). 5.4 Extractives and organochlorine It has long been known that the bleaching of pulp having a high extractives content (e.g., sulfite softwood or kraft birch) with chlorine dioxide rather than chlorine leads to decreases in the extractives content of the final product. Indeed, the bleaching of sulfite pulp to produce a dissolving grade with low extractive content was one of the first uses of chlorine dioxide in commercial bleaching. The substantial impact of chlorine dioxide substitution on extractives in sulfite pulp has been reported frequently (25, 79, 80, 87). An increase in chlorine dioxide substitution from 0 to 100% decreased the extractives content of a sulfite pulp by 50% (from 1.5% on pulp to 0.75% on pulp) after CEDED bleaching (25). The extractives content of fully bleached birch kraft pulp decreased from 0.5 to 0.4% on increasing the chlorine dioxide. substitution from 0 to 100",1,and the chlorine content of the extractives decreased from 25% to approximately 1% (42). The organochlorine content of pulp can be subdivided into water extractable and carbon adsorbable (AOX), solvent extractable, and inextractable fractions (88, 89). Substitution of chlorine dioxide for chlorine decreases the organochlorine content in pulp (80) as shown inTable 7 (90). 400

400 Substitution. o 0

jb

Substitution, o 0

.. SO 70

300

300

. 90 o 100

~c: -" 100

.o 100 90

lJ oc:

%

.. 50 70

100 o

o 75

80 85 Brightness, % ISO

Fig. 24. '111eImpad of clllorl_ dloxiile on dirt bleaclling capability against briglltness.

90

Sllbstitutton measured

70

80 90 100 110 Chemical consumed. kg equiv. CI, /tonne

Fig. 25. 1'be Impact of cI1lorl_ diOJdde Sllbstitutton on dirt bleacblng capability measured against cllemkal etmSfImption.

282

Pulp Bleaching - Principles and Practice

5.5 Dioxins in pulp Following the discovery, in the mid1980s, of the formation of chlorinated dioxins and furans during pulp bleaching, extensive research was undertaken to discover means of avoiding their formation. This subject is covered in considerable detail in the chapter on dioxins (Chap.VIII 3) (125). Substitution of chlorine dioxide for chlorine was found to have a profound affect on dioxin formation. As chlorine was replaced by chlorine dioxide and the chlorine (CIZ>multiple was reduced to below approximately 0.15, the formation of 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD) and its furan analog (TCDF) was eliminated (91). Similar studies have shown that a combination of low kappa factor and high percent substitution can be used to maintain the application of chlorine below the threshold which leads to TCDD formation (92). Numerous recent mill studies of dioxin control techniques have been reported. In one such study involving the bleaching of softwood kraft in an O(DC)(EO)HD sequence, chlorine dioxide substitution was increased incrementally until, at 52% substitution, TCDD and TCDF became non-detectable in the pulp (55). In another mill study,TCDD andTCDF became non-detectable at 85% substitution (57).

6. Effluent properties 6.1 Dioxins Substitution of chlorine dioxide for chlorine has a profound affect on the formation of chlorinated dioxins as is discussed in more detail in Chap. VII 3 (125). It was established in 1989 that increasett chlorine dioxide substitution is one means by which the formation of chlorinated dioxins and furans can be reduced; at substitution greater than 50-70%, TCDF and TCDD are no longer detectable in pulp (91-93). Because of their very low concentration and limited solubility in bleaching effluent, chlorinated dioxins and furans are difficult to detect and measure with the result that little information about them has been forthcoming from laboratory studies. However, several mill studies have shown that,as chlo-

Table 7. Effeel of Increased clllorl_ dloxiile sullstitution 018tbe org_IIlorlne content of fully bleacbed commerdill pulps. Organochlorine, Total

mg/kg

Nonextractable

Birch kraft O(C+D)(EO)DED OD(EO)DED

650 120

250 100

Softwood kraft O(C+D)(EO)D(E+P)D OD(EOP)D(E+P)D

270 110

230 100

rine dioxide substitution increases, TCDD and TCDF concentrations in the effluent decrease. Non-detectable concentrations have been achieved at several different chlorine dioxide substitution levels: 57% (55), 85% (57), and 70% (94). Another study showed that, as chlorine dioxide substitution increased to 100%, the concentration of dioxins and furans decreased to less than 5 mgTEQ per kg in whitefish fillets (95). 6.2 AOX and EOX The first measurement of the impact of chlorine dioxide substitution on formation of chlorinated organic material was made by Rapson in 1966: 6% of the chlorine (CI) charged was found as organochlorine. This figure decreased to zero at 70% substitution (25). In 1983 Germg:ird showed that it was possible to estimate the organochlorine formed by multiplying the amount of chlorine (CI) in all the chlorine bleaching agents used in a sequence by a factor (96). Numerous reports indicate that, as chlorine dioxide is substituted for chlorine, the amount of chlorinated organic matter in the effluent decreases. In earlier publications, chlorinated organic matter was measured asTOCI (total organochlorine) (34). However, this method is not as accurate and much more complex than theAOX (adsorbable organic halogen) technique. Early research showed TOCI to be much less than AOX (91), but a more recent comparison showed contrary results (97).

Chapter IV 3: Chlorine Dioxide in Delignification

.

&

283

o

200

Kappa no. Unbl. 30.0

o (E+O) 23.5 t>. (E+O+ P) 20.5 00 16.6

Pulp Bleaching- Principles and Practice

284

6.3 Chlorophenols

150

.c E ~c:

as a. a. as

x:

I(E~)I

8 6 4 2 0

1 2 3 4 NaOH charge, % on o.d. pulp

ng. 15. E./fectof f»r1P"-mtljtm:tNllll1ull_ ~ Inwft}1tllp 1h ""'_

(IIIpJM nlllber, ", plltntbesa

5

0tI tbe IuIjJJM___

vIseosIty of" softr«NNl

32; tJIseosIty, 2BmP"os; chlorllllltell with 6.M>""" Cl, 0.1% CIO on PUlP). IIt'e 0.5% CED IIIseoslty of tbe pulp II.ftn' extrtu:tlon """ 'ti ",p"os (J3).

sequences where most of the bleaching work is done in oxidative exttaction stages (37-40) and where the initial acidic-oxidant stage can be considered a lignin activator. At the limit of this scenario, the use of nitrogen dioxide (41), chlorine, chlorine dioxide, or ozone (42) before oxygen delignification can confer on the oxygen delignification process some of the characteristics of a fortified exttaction stage. 7.2 Oxygen Oxygen-reinforced alkaline extraction decreases the kappa number of the pulp compared with a non-oxidative exttaction without affecting pulp viscosity to any significant degree (Fig. 15) (43). This lack of reaction with the carbohydrates in the pulp is also confirmed by evaluation of pulp strength (43). Most frequently, the decreased kappa number is used to decrease the amount of the more expensive oxidant, chlorine dioxide, used in the following stage (fable 4) (30, 43). Under constant extraction conditions, the magnitude of this kappa number decrease is determined by the amount of oxidation occurring in the previous stage as measured by the incoming kappa number. Applying chlorine at a low active chlorine multiple in the first bleaching stage allows a greater benefit to be ob-

tained from the oxygen applied in the exttaction stage than when chlorine dioxide is applied at a high active chlorine multiple (44). In some cases, the degree of oxidation of the first stage pulp can be so high, for example when using ozone, that a negligible benefit is obtained from the application of oxygen (45) .These results show that not only the stage but the sequence needs to be optimized to ensure the maximum benefit from the use of the lowest cost chemical. In general, both softwood and hardwood furnishes and pulps produced by the kraft and sulfite processes all respond well to the application of oxygen in the extraction stage (fable 5) (43). Thekappanumberdecreaseob~Nedin oxygen-reinforced alkaline extraction does not accurately represent a corresponding Ttlble4. Chlorltle IIlonu Sllvltlgs 'Itrough usltlg oxygetl- or JH!t"fJXlU-mtiforcetl "lI"dltle

extrtlctton (30). Stage (EO)

(EP)

Cost of Oxidant, $ per metric ton of pulp 5 kg 02 in E1 saves

0.75

3 kg C102 in D.

3.00

1.5 kg Hz02 in E2 saves 3 kg CI02 in D.

1.65 3.00

Net saving 2.25 Net saving 1.35

Chapter IV 4: (Oxidative) Alkaline Extraction Tllbk 5. ~""""fUIIIIber~by" .pp11allUmof.. ~-rri1Ifon:«l IIne~(43). Pulp Type

"",..

Post Post &kappa (EO}kappa

1st Stage

Softwood kraft

C90+D10 D7OC30

5.8 4.6

3.4 3.3

Hardwood kraft

C90+D10 D7OC30

3.4 2.7

2.2 2.1

Softwood sulfite

C90+DI0

2.6

1.2

Unbleached Softwood Hardwood Softwood

pulps: kraft; kappa

number,

kraft; kappa sulfite;

kappa

C stage: 0.22 x kappa 3.0% consistency

33.9

number,

17.7

number,

30.5

number;

20 min at sooC;

E stage: 0.12 x kappa number; 60 min at 60°C; 10% consistency (EO) stage: 60 min at 60°C; 10% consistency; 0, gauge pressure, 0.14 MPa applied for 10 min

decrease in the lignin content of the extracted pulp because the remaining lignin is more oxidized and therefore consumes less potassium permanganate in the kappa number test than a conventionally extracted pulp (Fig. 16) (9). However, kappa number is still the critical indicator of the chlo-

307

cine dioxide consumption required in the ensuing D stage to achieve a particular brightness (Fig. 17) (12).The reactions occurring with oxygen in this stage have not been studied, but it is likely that phenolic structures liberated in the extraction process are the point of attack for the oxygen as discussed in earlier chapters. Pressure and time-at-pressure distinguish the oxygen-reinforced alkaline extraction stage from oxygen delignification.The minimum conditions necessary to obtain most of the kappa number decrease are an oxygen pressure of O.14 MPa held for 3 minutes (Fig. 18). For mills equipped with an upflow extraction tower, these conditions can be easily achieved; the head of pulp in the tower provides the necessary pressure over the requisite retention time. However, because the majority of mills in North America use downflow towers, an upflow pre-retention tube is required to provide the necessary pressure and time. When pressurization is provided by the head of pulp in the tower, the pressure decreases as the pulp progresses through the tower. If the starting gauge pressure at the bottom of the tower is 0.14 MPa and attnospheric at the top, then 10 minutes' retention is required to obtain the same results

308

Pulp Bleaching

-Principles

and Practice

50 (C90+D10)EDED

[J

cD "C 'x

~

(D3OC70)EDED

(D5OCSO)EDED

40

o

Q)

(D7OC30)modEDED

.

.£~

(D70C30)convEDED

. .

30 ~~ (..)E

(D9OC10)modEDED

--

"C Q)

§

rn c: o U

(D9OC10)convEDED

20

10 2

3

4 567 8 E-stage kappa number

Q) 6 .D E :J c: 5 as a. a. as 4 27.0

"C Q)

U

7

as

3

9

10

Pressure02 I-....~ PSIG MPa 10 0.07

~(211.8) · ~2b 20

0.14 ~50 0.50

(27.2)

~UJ

2

6

o 1 2 3 4 5 6 7 B 9 10

60

02 treatment time from beginning of stage, min

2 0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Total lignin. % on o.d. pulp Fig. 16. RellltWnsbtp betwetm aJ1/Hl..mber .forced ,"aline extr'tlcUmu (9).

KlIIstm lipi. "'"'

t:tmIe8i i. CMI_Uo1uIl

~-ret.. "'"'

that are obtained when a gauge pressure of 0.14 MPa is applied constantly over 3 minutes (Fig. 19) (46).This time can be shortened significantly by increasing the pressure at the bottom of the tower; for example, at 0.35 MPa, a retention time of only S minutes is required. In some installations, however, the retention time may be too short to provide the necessary pressure and time-

at-pressure conditions. In these installations a flow restriction valve may be placed a~ the outlet of the upflow tower to ensure that the minimum required pressure exists at the top of the tower. Pressure application is the first factor in ensuring good oxygen-reinforced alkaline extraction. The second is good mixing. Oxygen is a sparingly soluble gas and must

Chapter IV 4: (Oxidative) Alkaline Extraction

309

co a.. 0.4 ~cD .... ::J en 0.3 en Q)

"..................

\',

....

J

c.. Q) 0) 0.2 ::J CO 0) c: Q) 0.1 0)

>. x 0

.

\ \ .".

\'

g-J.................

\

\

. : I

\

1

\: \:

oftbe

..

~: \~~, :' \

0

Kappa number shown in circles.

..................

' , ........

\

o Fig. If). ComptIrlstm

............ ..........

.\

.

I I

\:

':\

",

..

..........

....

.........

"\

.........

........ ............

I

2 3 4 5 6 7 8 9 Oxygen treatment time, min effect ofgrw4lullly

Mctwsmg

the tMYfM Jna$8refrom

10 0.35 MP. tMYfM Jna$8re

to .tmospberk ad tbe effect of tlUd"tldttUrg. wutllflt Jna$8re of 0.14 MP. lit VlJrWusm,refl Ire"",,",t times. Tbe reSJIltiltg1uIJIPII bers lire cIrr:W Tbe Iiotmlilfle reJIrWefItstbe expected to tbllt obtllifled Iry~g . wutll"t Ire"",,",t (5 ",.tes) rel/fdred to glfJII. result t:OIIIJNII'fIble Jna$8re of 0.14MP. _ 3 ",I""tes (46). be well distributed throughout the pulp mass for uniform reaction to occur. The advent of high-intensity mixers allowed the successful application of (EO) (36). Other less expensive mixer designs (for example, static mixers) may be used, (47, 48) but the decreased rate of reaction of pulp with oxygen because of poorer gas distribution must be compensated for by using higher pressure and extending the reaction time at pressure. This type of installation represents an option for those mills using upflow extraction towers. Most of the operating conditions in oxygen-reinforced alkaline extraction are similar to those of conventional extraction.Time and temperature are generally in the same ranges. The kinetic models described earlier in Equations [1] to [3] are still relevant in the context of oxygen-reinforced alkaline extraction. It is perhaps significant that there is a correspondence between the time required for pressurization in (EO) and the time span of the rapid phase described in the model. This correlation suppons the conclusion that the rapid phase is the period during extraction when most of the chemical reactions occur with the pulp.

-Principles

and Practice

7.3 Peroxide

.............

I

Pulp Bleaching

trained in the pulp, pulp washing becomes more difficult. Ensuring full reaction without excess gas entrainment may be one of the benefits of good oxygen-addition control.

c~..... \~\ "\, \ \

310

One operating condition that does change is the sodium hydroxide charge which, for oxygen-reinforced extraction, must be 0.5% on pulp higher than in conventional extraction if the stage is controlled to the same final pH (9). The reason for this increase, as stated earlier, is the increased formation of sodium carbonate occurring during oxygen-reinforced alkaline extraction. The oxygen charge applied is commonly 0.5% on pulp, most of which is consumed. Generally, the amount required is determined by the point at which no further benefit is obtained in terms of kappa number reduction or cWorine dioxide savings. Some analyses of the residual gases have been made (49); however, mills do not routinely measure the oxygen content of the off gases. For the limited amount of oxygen added, it is judged not worthwhile to control the oxygen addition more closely. Although some carbon monoxide and hydrocarbons are produced in oxygen-reinforced alkaline extraction stages, the concentrations are not high enough to cre. ate an explosion or fire hazard (50). One negative aspect of oxygen-alkali extraction is that when residual gas is en-

Peroxide use in the extraction stages of multi-stage bleaching has a long history. In the past, peroxide cost has generally militated against its widespread use, but this is no longer true. In Scandinavia, even when (EO) technology was available, peroxide instead of oxygen was the preferred chemical for use in the first extraction stage, partly because of the capital costs associated with implementing oxygen-reinforced alkaline extraction (51). Today, with the need to minimize the use of cWorine-containing oxidants in the bleaching sequence, both oxygen and peroxide are used extensively in the first extraction stage. Because peroxide bleaching is dealt with extensively in Chap. V 1, only peroxide addition to oxygen-reinforced alkaline extraction is described here. Peroxide addition to the first extraction stage can be used to decrease the amount of cWorine-containing oxidant in the first bleaching stage. When concerns about cWorinated dioxins and furans arose, and

in the absence of the necessary cWorine dioxide generator capacity to provide for the alternative strategy of increasing the cWorine dioxide substitution, this approach allowed some mills to promptly decrease the fIrst-stage cWorine multiple (52). Alternatively, by applying peroxide in either or both of the extraction stages, chlorine dioxide could be made available to increase the degree of cWorine dioxide substitution in the first bleaching stage (53). As in oxygen-reinforced alkaIine extraction, the effect of the oxidant, in this case peroxide or both peroxide and oxygen, is superimposed on the effect of the alkali on the pulp. Figure 20 shows the change occurring in the extracted kappa number when different oxidants or combination of oxidants are applied to a softwood kraft pulp (54). There is much debate, and no definitive answer,about how to achieve the maximum bleaching effect from the peroxide addition, particularly when oxygen is also being applied to the extraction step. One configuration commonly being used is shown in Fig. 21 (39). However, it is difficult to obtain the full value from the peroxide in an (EOP) stage. If the full value of the oxygen is obtained,llitrate-peroxide reactions tend to dominate. If, on the other hand, the full value of the peroxide is ob-

0.26

~ 0.24 c.. :;::::: '3 0.22

E

~ .;:: o

0.2

:c 0.18 o ~ 0.16 13

2

750 0.4

0.5 0.6 Ozone consumption

0.7 0.8 in Z, % on O.D. pulp

0.9

2

2.5

Fig. 6. 1jff«t of t~ OfITuIJI/M1I8Ml1er1lllll1Jiseosi1y i" metH_ pi_ AS.4MoxygM-tkllg,dfW jnIlp (58).

to be responsible for cellulose degradation (78,79). Elimination of metal ions before an ozone treatment (68,69,71,80) and increased ozone stability (78) or solubility (53) in organic solvents and in acids at low pH are also claimed to improve selectivity. The improvement in ozone delignification purported to result from the addition of acetic acid (68,77,78,81, 82),fortnic acid (78, 82, 83), and oxalic acid (65,78, 82) remains contradictory, especially considering the excess of acid required to detect an advantage over sulphuric acid (65,77). In general, the particular acid used to ensure efficient delignification is not as important as maintaining a pH of less than 3 during ozonation (53). Some organic additives, specifically methanol (42,76,83), urea-methanol (76), and dimethylformamide (76) help to retain viscosity to varying degrees during an ozone treatment, but the quantities required exceed practical limitations. For example, Jacobsen et al. (42) have reported that 50% replacement of water by methanol increased the viscosity of a softwood kraft pulp by only 100 viscosity units (dm3fkg). Methanol has received attention because of its reported ability to act as a radical scavenger (84), thereby preventing free radical species from contacting the carbohydrate fraction. Ni et aL (37), however, have reported a viscosity increase of only 2-3 mPa.s units when a kraft pulp acidified to pH 1.8 with sulphuric acid was impregnated with a 5% methanol solution before ozonation.

3 3.5 Kappa number

etmSistlm9

4 4.5 after OZ

5

OZOM blMcW"g of II

Magnesium compounds, which are used effectively as cellulose protectors during oxygen delignification fail to protect cellulose during ozone delignification (76). The addition of small amounts of iron and copper salts (37, 69, 76) increases cellulose degradation slightly. The presence of cobalt (C02+) and iron (pe2+ and Fe3+)enhances ozone decomposition in sulphuric acid at pH 3, but considerably less so than in acetic acid at pH 3 (71). Transition metals must be removed before a peroxide stage, but whether this is necessary before an ozone stage remains unclear. Some authors claim that ozonation at medium consistency is insensitive to metal ions (24, 37,85). The improved effect of operating an ozone stage after an acid wash (80, 81) or after chelation with EDTA (38) or DTPA (68) has been attributed to the removal of metal ions_ A detailed investigation aimed at isolating the effects of process variables and metal ions in pulp is necessary to clarify their effect on ozonolysis reactions. A xylanase treatment (I.e., enzyme) designated as X, provides a brightness increase of 2-4 points when it precedes an ozone stage in OXZP (86), XQP(ZE)P (24), and OXZED (87) bleaching sequences. The observed lack of viscosity improvement indicates that the preceding xylanase treatment does not suppress carbohydrate degradation in the ozone stage. Combining an ozone and chlorine dioxide in a single stage to form the (DZ) (3,

Chapter IV 5: Ozone Delignification Tule 2. eo-m.l

/JIu Ittwft pIIlJt' bletriN"

(% on pulp')

Kappa Number

no prior treatment

2.25 2.25 1.3 1.3

16.9 4.9 2.7 0.5 3.0 0.7

1045 1120 645 630 695 675

Z-mge

Sequence 0 (CD)(EO) OZ OZD OZE OZED

332

Pulp Bleaching - Principles and Practice

.. OZ, OZE, om IIfI4 OZEDsequences (Z5).

Viscosity (dm' Ig)

03Applied in

331

NaBH. reducedd

775 680 675

Tear index (mNm'/g) at

ISO tensile index eNmllz) of Brightness 70 90 (%) 35.2 42.4 70.2 88.1 58.8 84.1

14.2 15.5 14.0 13.0 14.8 14.0

10.2 12.1 10.3 9.5 10.5 10.3

'Unbleached kappa number, 34.0; viscosity, 1280 dm'/kg bZ-stage: 1% consistency, pH 3; E stage, 2% NaOH, 70°C, 60 miD; D, 1.0% CIO ,,70°C 180 miD , 10% consistency ' 'All charges are expressed as % on pulp, oven-dry basis "Pulp reduced with 3% NaBH. for 24 hr at 2% consistency before viscosity test

31, 88-90) or the (ZD) (3, 91) configuration is proving to be an attractive solution to decrease the chiorine dioxide requirement and, subsequently, AOX formation. Several mills have incorporated this stage in their ECF bleaching sequence (3). Dillner and TibbIing (88) has shown that 6 kg of ozone applied 5-20 seconds after chiorine dioxide at pH 4 in a medium-consistency (DZ) stage of the (DZ)(EOP)D(ED) sequence replaced 9.7 kg of chlorine dioxide in the D(EOP)D(ED) sequence.Whether the (DZ) or (ZD) combination has a synergistic effect on delignification is unclear.

3.6 Alkaline extraction after ozone stages There are conflicting reports about the merits of extracting a pulp with alkali after an ozone stage. The general consensus is that an alkali extraction stage decreases the kappa number of an ozone-delignified pulp by about 1.5-5 units and is beneficial to the bleaching process by reducing the need for the application of more costly bleaching chemicals in later oxidizing stages (9, 27, 39, 59, 92-98). The effect of alkali extraction on viscosity and strength is, however, a subject of controversy. Numerous reports have provided data showing that the high alkalinity of the viscosity test contributes to a viscosity decrease in which the values are lower than those attributable only to the

ozone stage. TI1is effect is eliminated by treating ozonated pulp with sodium borohydride as a separate stage (27, 39, 69, 96, 99, 100) or as part ofthe following extraction stage (96). The high charges of borohydride required (as much as 5%) (39), combined with the long retention time (up to 24 hours) (92), cast doubt on the commercial viability of this approach. The response of Z-treated pulp to alkaline and non-alkaline conditions can be evaluated in ZD and ZED sequences in which the D stage is conducted at a pH of less than 4. Comparisons of strength properties after Z, ZE, ZD, and ZED sequences indicate that the negative viscosity effects arising from alkaline extraction are not necessarily reflected in the strength properties of the pulp (25).These results are summarized in Table 2. Unlike after the D stage in an OZED sequence, a borohydride treatment preceding a viscosity measurement of the Z- and the D-stage pulps in an OZD sequence increases the viscosity value. However, pulp strength, as measured by the tear index (mNm'/g) extrapolated at 90 Nm/g tensile index, is lower after an OZD sequence than after an OZED sequence, despIte the borohydnde-reduced vIscosIty levels of each pulp being the same. Without a borohydride treatment, the viscosity after OZD is lower than that after OZED. A

Pulp Source Kappa number Viscosity, rnPa.s Sequence

Western Canadian Softwood Kraft

Black Spruce Kraft

33.3 (19.0 after 0) 28.6 (22.3 after 0)

42.0 (29.7 after 0)

OZD

OZED

30.6 (15.1 after 0) OZP

0.5 1.0 1.0

03, % in Z' NaOH, % in E' CIO" % in D' % in pa HP" Kappa number

OZEP 0.5 1.5

2.0 6.8

5.7

6.8

5.8

ISO Brightness, % DorP

83.4

89.2

76.1

85.1

Viscosity, rnPa.s ZorZE DorP

18.9 16.7

17.2 15.4

23.4 21.6

24.8 22.8

ZorZE

-z stage: 40 % consistency; ambient temperature; 2.5 end pH; E, D and P stages, E: 60 or 70°C, 90 min; P: 10 min, 80°C; D stage, 180 min, 70°C.

viscosity measurement after a borohydride treatment therefore may not give a more accurate indication of the strength profile. Also, an alkaline treatment does not necessarily cause strength deterioration of ozonetreated pulp. Because the comparisons in Table 2 are for pulps having the same kappa number entering the D stage, the ISO brightness after OZD bleaching (88.1 %) is higher than that after OZED bleaching (84.1%) as a result of the brightness of the OZ pulp (70.2%) being higher than that of the OZE pulp (58.8%). Comparison at a constant kappa number after the Z stage indicates that, after an OZED sequence, the brightness is 89.2% but only 83.4% after an OZD seq~oce~wle~.G~taandEckert~~ have postulated that, not only does the removal of chromophores by the extraction stage improve the response to chlorine dioxide, but the alkali also improves the accessibility and uniformity of the pulp following chlorine dioxide treatment. Inclusion of an extraction stage between the Z and P stages in the OZEP sequence also improves the brightness compared with

10% consistency;

that obtained in the OZP sequence ~able 3) (9,24). Elimination of the washing step between the Z and E stages decreases the kappa numberby 1 point (9, J01);however,additional alkali is required to achieve a pH adequate for an extraction stage.

4. Role of ozone in a bleaching sequence 4.1

Ozone delignification

as a replace-

ment for chlorination The main role advocated for ozone in a bleaching sequence is that of a replacement for chlorine (or chlorine dioxide) as a delignification agent. A chlorination and extraction sequence (CE or DE) decreases the kappa number of a softwood kraft pulp from 30 to 5 with relative ease, without adversely affecting viscosity (Fig. 7). Achieving this level of delignification with ozone is difficult unless a high charge is used or the pulp entering the ozone stage has a low lignin content. To achieve a low lignin content, some form of delignification process is required, either extended pulping or oxygen delignification. An illustration of

Chapter IV 5: Ozone Delignification

333

334

Pulp Bleaching

-Principles

and Practice

40 35 (/) cis

30

a...

E

;i'0

0 0 (/)

:>

25

PO)

20 15 10 5 0

5

15

10

20

25

30

35

Kappa number Fig. 7. ~IecUlJity

of"

bigb-cmrsIsteru:y

oz_

~

COfIIIeIIIIoul (D+C)E trNIrrIe18I (34). "

the effect of an ozone treatment applied to a kraft pulp and its oxygen-de1ignified counterpart is shown in Fig. 8. An ozone charge of 1.4% on a softwood kraft pulp having a kappa number of 22.4 entering the Z stage is required to produce a pUlp of kappa number 10.9 (43). A kappa number of 5 is achieved in an ozone stage with a 1.2% ozone application provided the entering pulp has a kappa number of 15.8. Half of this ozone charge is required when the pulp entering the ozone stage is first delignified to a kappa number of 11.7 in an oxygen stage. Apart from being a costly approach. the relatively high ozone doses required to delignify kraft pulp from a kappa number Q)

of 30 to as low as 5. which is suitable for a pulp destined for further bleaching. invariably impair fiber quality. Consequently. it is preferable to include an oxygen delignification stage before an ozone stage to minimize the amount of ozone usage required to reach a given kappa number (41, 43.99). Replacement of cWorine or cWorine dioxide in the first bleaching stage therefore requires both oxygen and ozone delignification stages.

4.2 Placement of ozone in a bleaching sequence Ozone is currently being applied in both elemental cWorine-free (ECF) and total cWo-

rine-free (fCF) processes (fable 4). Considerable variation in the sequence configuration is apparent. Medium-consistency ozone stages are included in 9 out of the 13 installations reported in 1995. An example of how an ozone stage is integrated into a TCF bleaching sequence is shown in Fig. 9. In all commercial sequences, the ozone stage is placed after an oxygen delignification treatment. Some sequences are arranged with an ozone stage even later in the sequence, following a peroxide stage. Final brightening is accomplished with peroxide (fCF). cWorine dioxide (ECF), or both. Chelation is appropriately situated in the sequence to improve the bleaching efficiency of the ozone or peroxide stage.

25

25

89.9

90

0>

1;;20 '" N iD

0

~§

(/) CD 70

Initial

~

Initial

Krait pulp 22.4

10

c as Q. 51- 5 ~

o

77.9

~80

uj

ti 15

~ Krait pulp 22.4 After 0 15.8

After 0 AlterO

o

0.5

1.5

Ozone charge. % on 0.0. pulp Fig. 8. Effect of ""ryt..g oz_ oxygen tkUpijle4timt

The implications of where the ozone stage is positioned within a bleaching sequence have been studied extensively (10). The OZ sequence is preferred over ZO because a given kappa number can be achieved with less ozone consumption and viscosity loss (102). Earlier studies have shown that a multistage ozone treatment requires 20-50"/0 less ozone than does a single stage, provided a washing step is placed between the ozone stages (74,78,103). The improved delignification of the multi-stage system was attributed to the intermediary removal of degraded lignin products which, when generated in a single-stage treatment, consume part of the ozone. A sequence that includes

15.8 11.7

AlterO

o

0.5

11.7

1.5

c:

.s= C)

60

'C:

.c 0

P2 Z

P2

P1

-

P1

P1

50 ~40

83.9 P2

-

Z

Ozone charge, % on 0.0. pulp

pulps witb 111IIIflJitbollt cIMrge Oft tlH IuIppa ..mnber 111IIIvisrosity of ""'ft (43).

OQPP FII. 10. Brlgbtrum of uo.tt-H

OZQPP

Inwft JIIII/Jbktrebetl flJitb oxygen, oz_,

OQPZP 111III ~

(110).

Chapter IV 5: Ozone Delignification

Tok 4. 0z0M

335

blMelml6 ,lIStIIlilltUms'.

Company

Process

Z-Stage

Sequence

Consistency

Ozone Process Supplier

Owne Generator Manufacturer

LenzingAG Hwdb sulfite

MCC

(EOP)ZP

Kvaerner

Schmidding

Union Camp Franklin, VA, USA

Swdb kraft ECF

HCC

OZ(EO)D

Soods

Ozonia

SOdraCell Monsteras, Sweden

Swd and hwd rCF kraft

Lenzing, Austria

MC

OQZP

MC

O(DZ)(EOP)D(EP)D 01JQ(EOP)P

Kvaemer

Ozonia

MC

OQPW OQPZP

AhlstromKamyr

Ozonia

Kvaemer

rrailigaz

Stora Billerud

Skogha1l, Sweden MoDo Husum, Sweden WJSafurest Pietarsaari,

Swd and hwd

Finland

ECF,rCF kraft

Swd ECF, rCF kraft Hwd ECF, rCF kraft

Metsii-Botnia Kaskinen,

Swd and hwd ECF, rCF kraft

Finland Peterson Seffle Siiffle, Sweden

Sulfite Swd and hwd rCF kraft Swd and hwd ECF kraft Recycled fiber Swd kraft rCF

SCA Ostrand

rinui, Sweden SAPPI Ngodwana, SA Ponderosa~rs Memphis, Metsii-Rauma Rauma, Finland Consolidated Papers Wisconsin Rapids

Swd and hwd rCF kraft

WI,USA

.

O(ZD)(EO)(ZD)(EP)D

MC'

OQ1JQ(EOp)ZP O(ZD)(EOP)(ZD)(EP)D

MC

OQZlQ(EOP)ZP

AhlstromKamyr

rrailigaz Schmidding

AhlstromKamyr

Ozonia

MC

ZEP

Kvaemer

Schmidding

HC

OPZEP

Ozoma

HC

O(W)(EO)D

MC

ZPZP

MC

OZPZP

Soods AnOOtz Sunds AhlstromKamyr AhlstromKamyr

Schmidding

HC

OZEDD

Sunds

Ozonia

Ozoma Ozonia

List compiled in 1995

b

,

Hwd

= hardwood;

MC = medium

Swd

= softwood

consistency;

HC

= high

consistency

ozone in several positions, for example OZEZP and OPZEZ, can provide an 88-89 brightness pulp (104), but a final Z stage causes the bleached pulp to undergo considerable brightness reversion, unlike a pulp bleached in a sequence consisting of a peroxide stage at the end of the sequence (102). Although ozone is commonly applied as part of an OZEP sequence (8,9,15), moving the Z stage toward the end of the sequence increases brightness and decreases the ozone requirement in the bleaching

process (24, 47, 102, 105-109). Earl (105) and Soteland (111) have reported that insertion of the ozone stage between two peroxide stages "reactivates. the pulp so that it responds better to the second peroxide stage. This is illustrated by a comparison of OQPp, OZQPP, and OQPZP sequences (Fig. 10) which yielded kraft pulps having brightness values of 77.9%, 83.9%, and 89.9%, respectively (110). An evaluation of the delignification and brightness progression within the stages of each sequence indicates that a peroxide stage de lignifies and espe-

336

Pulp Bleaching

-Principles

and Practice

.. IIrlfImras ofllOfttr004 T"bk 5. KiIJIPII ."""- blMeW", TCF~ lnwft"uJl' COItpi III'.'", (1',1!',flll1lll sIIIgtIS(110). Kappa number

Brightness, % OSO)

OZ OQP

9.9 7.1

46.7 71.1

OQPP OZQP OQPZ OZQPP OQPZP

5.9 3.6 2.7

77.9 77.5 77.6

2.8 nde

83.9 89.9

Sequence

'Kappa number after 0, 15.0 bZ: 0.5% 10% consistency OJ' 'Q: EDTA chelation at pH 6 "p: 2.5% H,o, (0.9-1.9% consumed), final pH 11 'nd: not determined

cially bleaches an oxygen-delignified kraft pulp more extensively than does an ozone stage (fable 5). The extent of brightening and lignin removal (as measured by kappa number) depends on the placement of ozone and peroxide in a sequence. The OQPP, OZQP, and OQPZ sequences produce pulps having identical brightness (77.5-77.9%) but with markedly different kappa numbers: 5.9 after OQPp, 3.6 after OZQP, and 2.7 after OQPZ (Table 5). Malinen et al. (61) have compared OQZPZP and OQPZP sequences in which a 0.55% ozone charge was split between the two ozone stages in the first sequence; in this instance, the OQPZP sequence produced a pulp having a slightly superior brightness and viscosity. The combination of ozone and cWorine dioxide sequentially in a single stage as a (ZD) or (DZ) configuration early or later in a sequence, is emerging in several mills (]) (fable 4).

5. Process equipment Process equipment for ozone delignification of pulp is designed to achieve the desired degree of delignification and to maximize selectivity and pulp uniformity while minimizing ozone usage. The process of ozone bleaching of pulp comprises several fundamental steps: pulp washing to minimize carryover of organic material to

the ozone stage, pulp pretreatment for acidi. fication, ozone-fiber contact and reaction, separation of residual ozone gas and pulp, and washing of dissolved organic matter from the pulp. In a high- or a medium-consistency process, the residual gas from the ozone/pulp reactor is treated to destroy any remaining ozone. The gas, mainly oxygen, can be recycled back for use in ozone generation or diverted for use in other parts of the mill. Residual gas can be recycled back to the ozone generator only if it is freed of any contaminants that may be detrimental to the ozone generator. Residual ozone can be destroyed thermally or catalytically. Hydrocarbons are removed by catalytic oxidation to carbon dioxide and, finally, the gas is dried to achieve the low dew point essential for ozone generation. Ozone-pulp mixing is the most critical unit operation because selectivity and complete ozone consumption depend on ozone-fiber contact. Equipment for ozone bleaching has been developed and tested on pilot and commercial scales at high(12,18,19,112), medium- (2,14,16,17,113) and low- (16,114) consistency levels. The first commercial ozone stage was installed at Lenzing AG,Austria, with a medium-consistency Z stage in an (EOP)ZP sequence (17). In North America, the first ozone bleaching stage was started up as a highconsistency process by Union Camp at Franklin, Virginia, as part of an OZ(EO)D sequence (18). 5.1 High-consistency ozonation High-consistency ozone bleaching occurs in a gas-phase reactor when the pulp fibers are dispersed in an ozone/oxygen gas phase. Ozone gas can be supplied at low concentration and pressure. An example of a high-consistency ozone bleaching stage is depicted in Fig. 11. Pulp preparation for high-consistency ozone bleaching consists of thorough washing to reduce the carryover of organic material into the Z stage,acidification ofthe pulp to pH 2-3 at low consistency, thickening to the high-consistency range (30-40%), and fluffing. Lowering the pulp consistency

Chapter IV 5: Ozone Delignification Feeder

Pulp in

337

338

Pulp Bleaching

- Principles

and Practice

(EO) filtrate

Flutter

Preheater

~

Chilled water in Scrubber r:

l

1Liiiiii]}

,

Pulp to (EO) stage

:

;:

r

Pulp in at medium consistency

1

I

d

~ 1

Discharger

L

l

t

jlJ-~

L1iJ

Exhaust

Destructor

1

II I

19

Pulp out

'-I

Acid

Ozone reactor Pump Pump

Filtrateto! O-stagewasher Fig.

11. Flowsbeetfor

I

t

! m

Wgb-coruisteru:y 0_

m

Z filtrate

mm

bletIebhtg systMrt

m..........

i

-.. Ozonein

(lhIw.

CimIp

Corp.)

(112).

{

=

~

1 I

Compressor

with dilution water during acidification also serves to reduce the pulp temperature. During the pressing step, transition metal cations are removed together with the effluent. At the press flischarge, the pulp mat is shredded and fluffed so that the pulp floes are decreased in size to increase the gassolid interfacial specific surface area. Two reactor designs have been proposed to provide good contact between ozone and pulp fibers: a vertical static bed-type reactor (57) and a horizontal dynamic reactor (60).A narrow pulp residence time distribution approaching plug flow is required for uniform treatment of pulp with ozone to prevent under- or over- bleaching which leads to pulp strength loss. In the bed-type reactor, ozone is usually added at the fluffer co-currently with the pulp flow. The fiber floes are exposed to the ozone-rich gas phase as they fall to the bottom of the reactor and form a bed. The retention time is in the order of minutes. At the bottom of the vessel the pulp is diluted, neutralized, and pumped to the next processing step.

The horizontal dynamic reactor improves the ozone-pulp fiber contact and narrows the pulp residence time distribution which leads to improved uniformity (60). A specially designed paddle conveyor reactor disperses the pulp flocs into the gas phase while conveying the pulp forward. Ozone is added counter-currently to the pulp flow. 5.2 Medium-consistency ozonation Recent developments in ozone generation and in bleach plant equipment design have made medium-consistency ozone delignification a commercial reality (15. 17, 20). Medium-consistency processes for ozone bleaching have, so far, predominated in mill-scale installations because of lower capital cost and ease of implementation compared with high-consistency processes (Table 4). The main processing steps in mediumconsistency ozone bleaching, as shown in Fig. 12, are acidification of pulp, transfer of pulp under pressure to a high-intensity mixer, compression of ozone gas, mixing of ozone gas with the pulp suspension, back

Fig. 12. J1IorIJsbeetformdI_-~

0%_ bleJu;W.g syst_ (Abhtrom-~

pressure control at the reactor to maintain high pressure, separation of the gas from the pulp, destruction of any residual ozone gas, and pulp washing. A liquid-ring compressor is used to compress the ozone/oxygen gas to a pressure of 7-12 atm. Increasing the partial pressure of ozone in the ozone/oxygen gas mixture, which facilitates treatment of pulp with the required amount of ozone, is done in two ways: by compressing the gas to reduce the volume (48, 115) and by increasing the ozone concentration (2,3). Ozone generators routinely produce ozone at 12% concentration, decreasing substantially the volume of gas to be mixed with pulp in ozone bleaching stages (2, 3). High-intensity mixers, originally designed to mix a fluidized suspension of pulp with oxygen gas, have proved adequate for low ozone charses in a medium-consistency stage (2, 15,20,58). These mixers are limited by the amount of gas that can be sufficiently dispersed and mixed effectively with pulp (62,63). Pilot-scale investigations of

I,".) (116).

mixing efficiency at various phase ratios, that is, the ratio of gas phase volume to pulp suspension volume, indicate that mixing efficiency decreases at a phase ratio greater than 0.43 (63). At these phase ratios, the gas bubbles become too large to be dispersed by the mixer into micro-bubbles. As the ozone concentration increases, however, more ozone can be charged to the pulp at a given pressure. Two mixers in series provide a further improvement in ozone consumption at a given charge (116). 5.3 Low-consistency ozonation Before the development of medium-consistency bleaching technology, the low-consistency approach was investigated (16, 114). At low consistency, the ozone gas must be dissolved in water before it can react with the pulp fiber. The low solubility of ozone and the large volumes of water required to achieve the desired ozone charge make low-consistency ozone processes unattractive for commercial implementation.

Chapter IV 5: Ozone Delignification

(3). Generally, the metallurgy recommended for wetted parts of the fiber line and ozone gas is 316 stainless steel (3, 117119). Piping connections should be welded or flanged with fluorocarbon gaskets. Overall, the ozone bleaching process causes less corrosion problems than does chlorine and chlorine dioxide bleaching.

The VAl process, which was piloted at the OZF facilities in Gratkorn,Austria (16), makes use of a high-concentration aqueous ozone solution prepared by stripping ozone from the oxygen carrier gas by recycling bleaching filtrate to an absorption column under high pressure (-7 atm). Using this approach, wet oxygen from the absorption column can be dried and recycled back to the ozone generator. The ozone concentration in the gas is increased by recycling a portion of the aqueous ozone solution back to an expansion tank. The ozone gas evolves from the solution and increases the concentration of the gas entering the absorption column. The aqueous ozone solution is added to medium-consistency pulp under pressure. Depending on the concentratioQ of the ozone solution and the desired ozone charge on pulp, the pulp consistency drops to 1-5%. Major drawbacks in this process are carryover organic material in the filtrate which reacts with the ozone and the increasing temperature of the filtrate recycled to the absorption column. (With increasing temperature, ozone solubility in the filtrate decreases.)

6. Carryover to the ozone stage The use of ozone technology in a bleaching sequence offers more possibilities than does chlorine-based technology for recycling the filtrates rather than discharging them to a receiving stream. This approach serves to decrease the water usage and effluent discharge which is the essence of systems closure. The waste materials in the filtrates contribute to carryover in the pulp suspension, which affects the bleaching process.The sources of waste in the filtrates are carried forward with the pulp suspension from the preceding stage into the ozone stage or carried back from the ozone stage itself and used for tower dilution, consistency adjustment, or washing. As COD (chemical oxygen demand) carryover from an oxygen stage increases, so does the ozone consumption per unit decrease in kappa number (63, 120), as indicated in Fig.H. Published results on the

5.4 Materials of construction Information on the corrosive effect of ozone gas at high concentrations is limited 1

3.5

.

3

c: o '5. EO.as 2.5 ~o. II) as 2 c:.>o:

8~

C')~ 1.5 O"M .!:!0 ~CI

1

&!.>o:

0. en

o

Fig.

13.

EjJeet

o

2

4

6

8

10

COD carryover from oxygen stage, kg! O. D. metric ton of pulp

of CtI1TJOfJ6f"frmrt oxygm

340

Pulp Bleaching - Principles and Practice

TIIbk 6. Effl'"'" f'II'lUy of')(1'J(, I1rl,6'-lnwft

In", (112). Effluent Loading kg/ODTP"

Bleaching Sequence

kg/ADTBP"

Color

BOD

COD

0.25 0.8 0.6 0

15 12 12 6

10 6 6 3

33 17 14 12

1.7 2.4 1.6 0.3 0

53 69 57 35 27

13 16 15 13 12

52 35 34 32 30

Hardwood kraft" D,oo(EO)DED

O(D50+C~O)DED 0(070 +C~O)DED OZ(EO)PY Softwood kraft" D,oo(EO)DED

0(050 +C~O)DED 0(070 +C~O)DED OD,oo(EO)DED

OZ(EO)PY 'Air-dry metric ton of bleached "Oven-dry metric ton of pulp 'Unbleached "Unbleached

pulp

kappa number, 15.8 kappa number, 30.6

effect of COD carryover on ozone bleaching show a high degree of variability (57). For example, a COD carryover of 10 kg/ton of pulp from the oxygen stage was reported to increase the specific ozone consumption by 20% (113), 47% (59), and as much as 100",1,(63). Ozone bleaching is more sensitive to carryover from an oxygen stage than to carryback from Z-stage filtrate (59). Carryback from the Z stage has been shown to improve selectivity (113, 120). The effect of the carryover to the ozone stage is less sensitive at high than at medium consistency because six times more water is removed from the pulp ozonated at high consistency (18, 59,121).

7. Environmental considerations

.

0.5

339

sliJge 011 speclJk

-

corurImJJtIorI

12 (63).

14

An advantage of using an ozone stage in a sequence is that, contrary to bleaching with chlorine chemicals, its filtrate can be recycled to existing chemical recovery processes. However, because many mills are limited in their recovery capacity, processing this filtrate may not be possible. There-

fore, the properties of effluents from an ozone bleaching stage need to be characterized. The properties of ECF and TCF bleaching effluents are summarized in Table 6. Effluent from the oxygen stage is excluded because it is sent to the recovery process. OD1OO(EO)DEDand OZ(EO)PY bleaching of softwood kraft pulp produce effluents with similar BOD (biological oxygen demand) and COD characteristics (122). Bleaching processes containing an ozone stage generated effluents having 25% less color than did effluents from ECF sequences. Comparable results have been obtained for a hardwood kraft pulp (122). The relative merits of ECF and TCF bleaching on toxicological properties of effluents are inconsistent (123). For example, biotests conducted in the same laboratory showed that chronic effects of effluents from TCF bleaching could be greater than (Flg.14) (J 24) or less that £CP effluents (123). Collectively, the results indicate that effluents from ECF or TCF bleaching have the potential to cause bio-

Chapter N 5: Ozone Delignification

0 .n C\I S:2

100

Stage

C12:C102

80

(C+D) (DC)

90:10 50:50 0:100

~D

341

60

40

Pulp Bleaching - Principles and Practice

16. Schwarzl, K., "Pulp Bleaching Using Ozone,"Workshop on Emerging Pulping and BleachingTechnologies Proceedings, North Carolina State University. Raleigh, NC, 1991.

32. Gierer,}., and Zhang, Y, "The Role of Hydroxyl Radicals in Ozone Bleaching Processes," Seventh International Symposium of Wood and Pulping Chemistry Notes, CTAPI, Beijing, 1993, p.951.

17. Peter,W, "First Experience With Mill-Scale Ozone Bleaching," Non-Chlorine Bleaching Conference Proceedings, Miller Freeman, San Francisco, 1993, Session 22.

33. Balousek, P.}., McDonough,T.}., McKelvey, R.D.,Johnson, D.C., Svensk Papperstidn. 84(9) R49 (1981).

18. Nutt, W.E., Griggs, B.F, Eachus, S.W., PikuIin,M.A., TapPiJ 76(3):115 (1993).

20 0

342

19. Gotlieb, P.M., Nutt, W.E., Miller, S.R., Macas,TS., TapPiJ 77(6):117 (1994).

(DC)E

DE

Bleaching

logical effects only at high concentrations not likely encountered in'receiving waters. The advances being made with systems closure, however, may make these contradictions inconsequential.

References 1. Rice, R.C. and Netzer, A., Handbook of Ozone Tecbnology and Application, Vol. 1, Ann Arbor Science Publishlrs, Ann Arbor, MI, 1982. 2. Berry, R.M., Barclay, H., Prins,}., Sacciadis, G., Skothos,A. ,Ayala, v., Magnotta, v., Breed, D., Rounsaville,}., Shackford, L., Pulp Pap. Can. 96(9):T324 (1995). 3. Homer, G., Muguet, M., Epiney, M., Johnson, S., "Oxygen, Ozone, and Chlorine Dioxide," 82ndAnnuai Meeting,Tech. Sect., CPPA, Montreal, 19%, p. A297. 4. Sax, N.I. and Lewis, R.}. Sr., Dangerous Properties on Industrial Materials,Vol. ill, 7th eOO.,Van Nostrand Reinhold,NewYork, 1989. ), Drin,A. and Drin, L.Q., U.S. Pat. No. 396,3Z5 Ouly 17.1889) 6. Brabender, G.}., Bard, }.W, Daily, }.M., U.S. Pat. No. 2,466,633 (April 5, 1949).

ODE

OQP

20. Helander, R., Nilsson, B., Bohman, G., "Development and Progress in Ozone Bleaching at the Skoghall Mill", International Pulp Bleaching Conference Paper Session Proceedings, Tech. Sect. CPPA, Montreal, 1994, p.289.

OZEP

sequence

7. Gellman, I. "Delignification and Bleaching of Chemical Pulps With Ozone: A literature and Patent Review," NCASI Technical Bulletin No. 619, National Council of the Paper Industry for Air and Stream Improvement, New York, 1991. 8. Uebergott, N., van lierop, TapPiJ 75(1):145 (1992).

8., Skothos, A.,

9. liebergott, N., van lierop, Tappi J 75(2): 117 (1992).

B., Skothos,A.,

10. Byrd, M.V., Gratzl,}.S., 75(3):207 (1992).

Singh, R.P., Tappi]

13. Mokfienski,A. and Demuner, 77(11):95 (1994).

Pap.

Mag.

Can.

22. Sote1and, 75(4):T153

Pap.

Mag.

Can.

N., Pulp (1974).

23. Karnishima, H., Fujii, T.,Akanatsu, pan Tappi 30(7):381 (1976).

25. lindholm, C.-A., Nord. Pulp 5(1):22 (1990).

B.}.. Tappi J

14. Funk, E.. Szopinski, R., Munro, F, Vtlpponen,A.,"Espanola Ozone Bleaching Pilot Plant: Status Report", 1992 TAPPI Pulping Conference Proceeding~, TMPI PRESS, Atlanta, p.1091. 15. Sixta, H., Otzinger, G., Schrittwieser, Hendel, P.,PaPier 45(10):610 (1991).

A.,

Pap. Res.]

44(6):405

(1991).

27. Gupta, M.K. and Eckert, R.C., "OZ Prebleaching: Influence on VISCosity and Sheet Strength," 1984 TAPPI Oxygen DeIignification Symposium Notes, TAPPI PRESS,Atlanta, p.133. 28. Gierer,}.,Holz/orscbung

(1982).

I.,ja-

24. Colodette,}.L., Ghosh,A.K., Dhasmana, B., Singh, U.P., Gomide, }.L., Singh, R.P., "Bleaching Processes for Market Grade TCF Pulps", International Non-Chlorine Bleaching Conference Proceedings,Miller Freeman, San Francisco, 1994, Session 7-2.

26. Allison, R.W, Appita

11. liebergott, N. and van lierop, 8. ,"The Use of Ozone in Bleaching and Brightening Wood Pulps, Part I. Chemical Pulps," 1978 TAPPI Oxygen/OzonelPeroxide Pulp Bleaching Seminar Notes, TAPPI PRESS, Atlanta, p.90. 12. Singh, R.P., Tappi 65(2):45

21. Procter, A.R., Pulp 75(6):T21O (1974).

36(2):43

(1982).

29. Mansson, P. and Oster, R., Nord. Pulp Pap. Res.] 3(2):75 (1988). 30. Sun, Y. and Argyropoulos, forscbung 50(2): 175 (1996).

D.S., Holz-

31. Lachenal, D., Taverdet, M.T., Muguet, M., "Improvement in the Ozone Bleaching of Kraft Pulps," International Pulp Bleaching Conference Proceedings, SPCI, Stockholm, 1991, p.33.

34. Godsay, M.P., "Ozone-cellulose Studies: Physico-chemical Properties of Ozone Oxidized Cellulosic and lignocellulosic InMaterials" , PhD Thesis, Polytechnique stitute of New York. 1985. 35. Ek, M., Gierer,}., T, Holz/orscbung,

}ansbo, K., Reitberger, 43(6):391 (1989).

36. Eriksson, T and Gierer,}.,J Tecbnol., 5(1):53 (1985).

Wood

Cbern.

37. Ni, Y, Kang, G., van Heiningen,A.R.P."Are Hydroxyl Radicals Responsible for Degradation of Carbohydrates During Ozone Bleaching of Chemical Pulp?," International Pulp Bleaching Conference Poster Session Proceedings, Tech. Sect. CPPA, Montreal, 1994 p.19. 38. Chirat, C.,VJ.radin,M.T., Lachenal, D.,"Protection of Cellulose During Ozone Bleaching," 1992 TAPPI Pulping Conference Proceedings, TAPPI PRESS, Atlanta, p. 1055. 39. Hartler, N., Lidstrom, E.-B., TubekLindblom, A., Cel/ul. Cbern. Tecbnol., 21:387 (1987). 40. Godsay, M.P. and Pearce, E.M., "Physicochemical Properties of Ozone Oxidized Kraft Pulps," 1984 TAPPI Oxygen DeUgnification Symposium Notes,TAPPI PRESS, Atlanta, p.55. 4l.lindholm, 3(1):44

C-A., Nord. (1988).

Pulp

Pap. Res.]

42.}acobson, B., lindblad, P., Nilverant, N., "Lignin Reactions Affect the Attack of Ozone on Carbohydrates," International Pulp Bleaching Conference Proceedings, Vol. 2, SPCI, Stockholm, 1991, p.45. 43. Sonnenberg, L.8. and Poll, K.M., "Studies on High Consistency Ozone Delignification of Oxygen Bleached Pulps," inter-

national

Pulp DIeaching

ceedings, Tech. 1994, p.lOl.

Sect.,

Conference Pr0CPPA, Montreal,

Chapter IV 5: Ozone Delignification 44. Chint, C., Lachenal, D., Coste, C., Zumbrunn,].P., "Effect of Ozone on Cellulose and Pulp," Seventh International Symposium on Wood and Pulping Chemistry Proceedings, crAPI, Beijing, 1993, p.368. 45. Chandra, S. and GratzI, ].S., "Kinetics of Carbohydrate and Ugnin Degradation and Formation of Carbonyl and Carboxyl Groups in Low Consistency Ozonation of Softwood Pulps," International Pulp Bleaching Conference Proceedings, Quebec City, Tech. Sect., CPPA, Montreal, 1985, p.27. 46. Lindholm, C-A., "Effect of Pulp Consistency and pH in Ozone Bleaching, Part 2, Lignin Removal and Carbohydrate Degradation," I987TAPPI International Oxygen Delignification Conference Proceedings, . TAPPI PRESS,Atlanta, p.155. 47. Kassebi, A., GratzI,].S., Chen, CL., Singh, R., "Non-conventional Kraft Pulp Bleaching - The Role of Ozone," 1982 TAPPI Pulping Conference Proceedings, TAPPI PRESS, Atlanta, p.327. 48. taxen, T., Ryynanen, H., Henricson, Pap. Puu 72(5):504 (1990).

K.,

49. Allison, R.W, Chandler, N.G., Ellis, M.]., "Effect of Pulp Consistency of Ozone Bleaching in the Laboratory," 47th Annual General Conference Proceedings, APPITA, Parkville, Victoria, Australia, 1993, p.207. 50. Lindholm, (1987).

C-A., Pap.

Puu

69(3):211

51. Suren, A., Hsieh,]., Su, W, "The Effect of Pretreatment on Ozone Bleaching at Medium Consistency Conditions," 1993 TAPPI Pulping Conference Proceedings, TAPPI PRESS, Atlanta, p.1103. 52.0sawa, 46(2):79

Z. and (1963).

Schuerch,

c.,

Tappi

53. Bouchard,]., Nugent, H.M., Berry, R.M., Tappt] 78(1):74 (1995). 54. Reeve, D.W., in "Pulp and Paper Manufacture," Vol. 5,Alkaltne Pulptng,Third edn., Joint Textbook Comminee of CPPA and TAPPI,Montreal and Atlanta, 1989,p.425. 55. Griffin,

R., Ni, Y, van Heiningen,

A.R.P.,

"The Development of Delignification and Lignin-ceUulose Selectivity During Ozone Bleaching," 81st Annual Meeting, Tech. Sect., CPPA, Montreal, 1995, p.AI17.

56. Reeve, D.W and Earl, P.E, Tappt] (1986).

343 69(7):84

344

Pulp Bleaching - Principles and Practice

72. Fujii, T., Kamishima, pan Tappi 40(5):477

H., Akamatsu, (1986).

57. Kappel, J., Brauer, P., Kinel, EP., Tappt] 77(6):109 (1994).

73. Dillner, B. and 74(9):720 (1992).

58. Oltmann, E., Gause, E., Kordsachia, Pan, R.,Papter 46(7):341 (1992).

0.,

74. Kobayashi, T., Hosokawa, K., Kubo, T., Kimura,YJapan Tappi 30(3): 159 (1976).

59. Gause, E., Oltmann, E., Kordsachia, Pan, R. papter 47(7):331 (1993).

0.,

60. White, D.E., Gandek, T.P., Pikulin, M.A., Friend, WH., Pulp Pap. Can. 94(9):T242 (1993). 61. Malinen, R., Rantanen, T., Rautonen, R., Toikkanen, L., "TCF Bleaching to High Brightness

-Bleaching

Sequences

and Pulp

Properties," International Pulp Bleaching Conference Proceedings, Tech. Sect. CPPA, Montrea1, 1994, p.187. 62. Greenwood, B.E, and Szopinski, R., "Ozone-bleaching Technology," International Non-Otl.orine Bleaching Conference Proceedings, Miller Freeman, San Francisco, 1992, Session V. 63. Funk, E., Dekourt, T., Vilpponen, A., Munro, E, "Espanola Ozone Bleaching Pilot Plant: Progress Update," 47th Annual General Conference, APPITA, Parkville, Victoria, Austra1ia, 1993, p.217. 64. Hosokawa,]., Kimura,Y,]apan

Kobayashi, T., Kubo, T., Tappt 30(4):226(1976).

65. Lindholm, C.-A., Cellul. 23:307 (1989). 66. Wang, D.L.K. and 38(6):245 (1984).

Chem.

Patt,

Technol.

R.,

Papier

67. Sreeram, C., Sundaram,V.S.M., Jameel, H., Chang, H., Tappi] 77(10):161 (1994). 68. Al1ison, R.W, "Effects of Temperature and Chemical Pretreatment of Pulp Bleaching with Ozone; International Pulp Bleaching Conference Proceedings, Tech. Sect., CPPA, Montreal, 1985, p.47. 69. Chirat, C. and Lachenal, Set., 21(9): J316 (1995).

D.,]

Pulp

Pap.

70. Parthasarathy, V.R. and Peterson, R.S., "Ozone Bleaching, Part I.The Decomposition of Ozone inAqueous Solution - Influence of pH, Temperature, and Transition Metals on the Rate Kinetics of Ozone Decomposition," 1990 TAPPI Oxygen Delignification Symposium Proceedings, TAPPI PRESS,Atlanta, p.23. 71. Pan, G., Chen, C., Chang, H., GratzI, ].S., ] Wood Chem. Tecbnol. 4(3):367 (1984).

Peter,

W., Pap.

!.,jaPuu

92. Lindholm, 16(6):]190

C.-A., (1990).

J

Pulp

Pap.

Set.

93. Lindholm, C.-A., Nord. Pulp 7(2):95 (1992).

Pap. Res.]

94. Lindholm, (1992).

C.-A., Pap. Puu

74(3):224

75. Kobayashi, T., Hosokawa, K., Kubo, T., Matuo, R., Kimura, Y., Japan Tappt 31(12):S07 (1977).

95. Lindholm, (1992).

C.-A., Pap.

74(9):738

76. Kamishima, H., Fujii, T., Akamatsu, pan Tappt 31(9):664 (1977).

96. Lindholm, 19(3):JI08

!.,ja-

77. Brolin, A, Gierer, J., Zhang, Y, Wood Sci. Technol. 27:115 (1993). 78. Mbachu, 64(1):67

R.A.D. and Manley, R.St.]., Tappi (1981).

79. Zhang, Y., Kang, G., Ni, Y, van Heiningen, A.R.P., "Degradation of Carbohydrate Model Compounds During Ozone Treatment," Workshop on Lignocellosics and Pulp, The Royal Institute of Technology and STFI, Stockholm, 1994, p.112. SO. Bergnor, E., Ek, M., Johansson, E., "The Role of Metal Ions inTCF-bleaching,'Workshop on Lignocellosics and Pulp, The Royal Institute of Technology and STFI, Stockholm, 1994, p.284. 81. Lachenal, D. and Bokstrom, Pap. Set. 12(2):J50 (1986).

M., ] Pulp

82. Xu, J., Cogo, E., Briois, L., Duprat, S., Molinier, J., Coste, c., Klack, Ph., Pulp Pap. Can., 96(9):49 (1995). 83. Al1ison, R.W, Appita 84. Reitberger,T.and 42:351 (1988).

36(1):42

(1982).

Gierer,J.,Holzforschung

85. Henricson, K. and Lindholm, Puu 75(3):133 (1993).

C.-A., Pap.

86. Yang, J.L. , Sacon, V.M., Law, S.E., Eriksson, K.-E., Tappi] 76(7):91 (1993). 87. Brown,]., Cheek, M.C., Jameel, H., Joyce, T., Tappt] 77(11):105 (1994). 88. Dillner, B. and Tibbling, P. "Use of Ozone at Medium Consistency for Fully Bleached Pulp. Process Concept and Effluent Characteristics." International Pulp Bleaching Conference, Vol. 2, SPCI, Stockholm, 1991, p.59. 89. Tsai, T.Y, U.S. Pat. No. 4,959,124 25, 1990).

(Sept.

C.-A., (1993).

J.

Puu Pulp

Pap.

Set.

97. Liebergon,N., van Lierop,B.,Garner,B.C., Kubes, G.]., Tappi] 67(8):76(1984). 98. Sundin,]., and Hartler, N., Nord. Pulp Pap. Res.] 9(3):140 (1994). 99. Hartler, N., Granlund,V., Sundin,].,ThbekLindbolm,A., "Ozone Bleaching of Chemical Pulps," International Pulp Bleaching Conference Proceedings, Vol. 2, SPCI, Stockholm, 1991, p.75. 100. Kordsachia, 0., Oltmann, D.L.K., Patt, R., Wochenbl. 7:251 (1990).

E., Wang, Papierfabr.

101. Liebergott, N., "Peroxyacids used in Bleaching Sequences," 1995 TAPPI Workshop on Emerging Pulping and Bleaching Technologies, TAPPI PRESS, Atlanta, Section 3-1. 102. Lachenal, D. and Nguyen-Thi, N.B., "TCF Bleaching - Which Sequence to Choose?," 1993 TAPPI Pulping Conference Proceedings, TAPPI PRESS,Atlanta, p.799. 103. Rothenberg, Johnsonbaugh, (1975).

S., Robinson, D.H., D.K., Tappi 58(8):182

104. Phillips, R.B., Kempf,A.W, Eckert, R.C.. U.S. Pat. No. 4,372,812 (Feb. 8, 1983). 105. Earl, P.E and Nguyen, X.T., "High Brightness Chlorine-free Bleaching Without Oxygen Delignification; International Non-Ghlorine Bleaching Conference Pr0ceedings, Miller Freeman, San Francisco, 1993, Session 3-2. 106. Dillner, B. and Tibbling,P., "Isothermal Cooking to Low Kappa Numbers Facilitates TCF Bleaching to Full Brightness,"

90. Lachenal, D. and Muguet, M., Nord. Pulp Pap. Res.], 7(1):25 (1992).

International Non-Chlorine Bleaching Conference Proceedings, Miller Freeman,

91. Larsson, P., and Samuelson, 0., Nord. Pulp Pap. Res.], 5(4):180 (1990).

San Francisco, 1993, Session 3-7.

Chapter IV 5: Ozone Delignification

345

107. Dillner, B. and Tibbling, P.,"Optimum Use of Peroxide and Ozone inTCF Bleaching; International Pulp Bleaching Conference Proceedings,Tech. Sect., CPPA, Montreal, 1994, p.319.

116. Henricson, K., "MC Ozone Bleaching: Machinery and Pulp Reactor; International Non-Chlorine Bleaching Conference Proceedings, Miller Freeman, San Francisco, 1994, Session 4-1.

108. Dahllof, H. ,"Medium Consistency Ozone Bleaching, an Established Technology in Mill Scale; International Non-Chlorine Bleaching Conference Proceedings, Miller Freeman, San Francisco, 1994, Session 4-2.

117. Byrd, M.V. and Knoernschild, J 75(5):101 (1992).

109. Stromberg, B. and Szopinski, R.,"Pressurized Hydrogen Peroxide Bleaching for Improved TCF Bleaching; International Pulp Bleaching Conference Proceedings, Tech. Sect.,CPPA,Montreal, 1994,p.199. 110. van lierop, B., Berry, R., Roy, B., "High Brightness Bleaching of Softwood Kraft Pulps with Oxygen, Ozone and Peroxide; 82nd Annual Meeting, Tech. Sect. CPPA, Montreal, 1996, p.A247. 111. Soteland, (1978).

N., Norsk

Skogtnd.

32(9): 199

112. Nutt, WE., "One Year's Experience of High Consistency Ozone Bleaching" ,Air Liquide/Ozonia Ozone Symposium Notes,Air liquid, Paris, 1993. 113. Munro, E, "Espanola Ozone Pilot Plant Results, Delignification and Pulp Quality, Hardwood and Softwood; Air liquide/ Ozonia Ozone Symposium Notes, Air liquid, Paris, 1993. 114. Bentvelzen,].M., Bogart, S.L., Gupta, M. K., McKean, W.T., Meredith, M.D., Torregrossa, L.O., U.S. Pat. No. 4,216,054 (Aug. 5, 1980). 115. Sixta, H., Gotzinger, G., Hog/inger, A., Hendel, P., Riicld, W, Peter, W, Kurz, E, SchrittWieser, A., Schneeweisz, M. U.S. Pat. No. 5,346,588 (Sept. 13, 1994).

K.]., Tappi

118. Bardsley, D.E.,"Compatable Meta1lurgies for Today's New Bleach Washing Processes; Eighth International Symposium on Corrosion in the Pulp and Paper industry Notes, STFl and Swedish Corrosion Institute, Stockholm, 1995, p. 75. 119. Klarin,A. and Pehkonen,A., "Materials in Ozone Bleaching; Eighth International Symposium on Corrosion in the Pulp and Paper Industry Notes, STFI and Swedish Corrosion Institute, Stockholm, 1995, p. 96. 120. Halinen, E., lindholm., C.-A., Gullichsen, ]., Henricson, K., "Effect of Dissolved Organic Material From Various Sources on the Efficiency and Selectivity of MC Ozone Bleaching; International Pulp Bleaching Conference Proceedings,Tech. Sect., CPPA, Montreal, 1994, p.l. 121. Neubauer, G., Kappel, J., Brauer, P., "Pr0cess Parameters of the High Consistency Ozone Stage; International Pulp Bleaching Conference Poster Proceedings,Tech. Sect., CPPA, Montreal, 1994, p.41. 122.liebergott, N., van lierop, B., Fleming, B., Pulp Pap. Can. 94(11):T344 (1993). 123. Kovacs, T.G., Tana,]., Lehtinen, K.-]., Sangfars, O.,"A Comparison of the Environmental Quality of Elemental ChlorineFree (ECF) and Totally Chlorine-Free (TCF) Hardwood Bleach Plant Effluents; International Non-Chlorine Bleaching Conference Proceedings, Miller Freeman, San Francisco, 1995, Session 5-3. 124. O'Connor, B.I., Kovacs, T.G., Voss, R.H., Martel, P.H., van Lierop, B., Pulp Pap. Can. 95(3):47 (1994).

SECfION IV: The Technology of Chemical Pulp Bleaching Chapter 6: Hydrogen Peroxide as a Delignifying Agent D. Lachenal CfP Grenoble, France 1. The dual role of hydrogen peroxide in delignifying and bleaching . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . .. . . .. . .. 349 2. Hydrogen

peroxide

delignification

Processes

. . . . . . . . . . . . . . . . . . . . . . . . . .. 351

3. Variables in hydrogen peroxide delignHication 3.1 Metals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 3.2 Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . " 3.3Time .. "~' 3.4 Chemical charge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 3.5 Consistency 4. Modified peroxide

delignification

treatments

353 353 355 355 355 356

. . . . . . . . . . . . . . . . . . . . . . . . . .. 356

5. Process equipment and Oowsheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 357 6. Process control 7. Characteristics

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. of peroxide

-

delignHied pulp.

8.En~nmentdf.actors

358

. . . . . . . . . . . . . . . . . . . . . . . .. 358 359

347

Chapter IV 6: Hydrogen Peroxide as a Delignifying Agent

Chapter IV 6: Hydrogen Peroxide as a Delignifying Agent 1. The dual role of hydrogen peroxide in delignifying and bleaching Alkaline hydrogen peroxide is widely used in the pulp industry to bleach ligninrich pulps to brightness levels of 80-83% ISO without any substantial dissolution of lignin. The bleaching effect of hydrogen peroxide has been attributed to its ability to react with various colored carbonyl-containing structures in lignin. Its use in chemical pulp bleaching has long been limited to the final stages to improve not only brightness but also brightness stability (1). The commercial use of hydrogen peroxide as a delignifying agent for kraft pulp was reported in the late 19705 (2). More recently, some Swedish, Spanish, and Canadian mills (3, 4) started the commercial production of bleached kraft pulps using sequences containing at least one extensive peroxide delignification stage. A similar development has been observed in the sulfite pulp industry, especially in Germany (5). The advantages of delignification with hydrogen peroxide are low investment cost and the accompanying strong bleaching effect. The action of alkaline peroxide as a bleaching agent has been explained through the reactions of the hydroperoxide anion, HO~, formed in an alkaline medium according to the equilibrium: HP2

+ OH- +-+ HO;

where the pKa

= 11.6

+ HP

[1]

at 25°C.

This anion is believed to be the principal active species involved in the elimination of chromophores in lignin structures,

particularly conjugated ca.rbonyl structures that are prone to react with the hydroperoxide anion (6) (See also Chap. 1lI 4, section 2.2.1.) The bleaching of mechanical

349

pulp by hydrogen peroxide has been essentially explained by this mechanism even though it has been claImed recently that peroxide decomposition products such as the hydroxyl and superoxide anion radicals ('OH and 0>" respectively) may participate in the bleaching mechanism, at least to a small extent (7) .Whatever their form, these bleaching reactions proceed with only minor amounts of material being dissolved and cannot explain the delignification observed with hydrogen peroxide in the processes described below. It has been shown that, under alkaline conditions, hydrogen peroxide is unable to attack phenols of the type present in lignin (B).This finding was confirmed recently in a study on the modification of the lignin in a mechanical pulp during peroxide bleaching (9,10). No degradation of the phenolic rings was observed. These considerations indicate that no significant degradation of the residual lignin in kraft pulp by hydrogen peroxide should be expected. Indeed, there is no apparent reason why residuailignin would be more reactive than the original lignin. On the contrary,residual kraft lignin is more condensed (11) and does not contain easily cleavable ether bonds. Despite this, delignification does in fact take place when a kraft pulp is treated with alkaline hydrogen peroxide (12). A temperature of 90°C is usually required to produce a substantial decrease in kappa number. Higher temperatures (e.g., 110°C) lead to increased delignification, comparable to that occurring in a conventional oxygen delignification process (13), Studies on the behavior of lignin in kraft pulp during hydrogen peroxide delignification at 90°C indicate that some depolymerization of lignin occurs (11) and carboxyl groups are created (11,14). A comparison of peroxide and conventional oxygen bleaching shows that the brightness is higher (13,15) and the resulting viscosity is lower for a peroxidebleached pulp than for a corresponding

oxygen.IJleached mt't pulp of the EIlC li~nin content; that is, the selectivity is lower for peroxide delignification. The selectivities of chlorine, oxygen, and hydrogen

350

Pulp Bleaching

-Principles

and Practice

1600

40

E

Fe

Mn

'",

/0 :&1400 CE o

(0)

b 30 u;S> .£.20 ~

:a a; " ~1200

p

C

'E

~

~/ 1000 5

10

15 20 Kappa number

25

30

o'" ~"

a:

8DTPA

added

10

0

0.285 0.150 0.213 0.285 Metal ion concentration, mM

Fig.2. lJ«:tmIposItItmof by4rogM ~ IIb_ IIIUl~e of trwutltm Itms (M") (17). peroxide are compared in Fig. 1 for a softwood kraft pulp (9). The decomposition products of hydrogen peroxide in alkaline media are thought to cause the oxidation of lignin structures which leads to the introduction of hydrophilic (carboxyl) groups, the cleavage of some interunit bonds and, eventually, the dissolution of lignin (12,16,17). Alkaline hydrogen peroxide decomposes to hydroxyl radicals and superoxide anions in the presence as well as in the absence of transition metals (17): H202 + H02 0;

+ .OH

+

.OH + 0>, + H20

+ 02 + OM"

M(n+I)++ 'OH + OHH202 + Mn+ + HO; + M(n+,)+ + Mn+ + 02: + H+ The radicals formed may react further with each other or hydrogen peroxide and give rise to oxygen and water as the final products. Some transition metals accelerate peroxide decomposition (Fig. 2). Formation of hydroxyl radicals can also result from the thermal homolysis of hydrogen peroxide. This decomposition pathway is substantial at a temperature above 100°C. The contribution of peroxide decomposition products to lignin degradation is supported by several studies which clearly show a close relationship between the rate of oxidation of lignin model compounds and the rate of decomposition of hydrogen peroxide (17). As an example, oxidation

,,, tile _t,d

of some phenolic models was found to be maximum at pH 11.5 where the decomposition of hydrogen peroxide is also near its maximum. The primary reaction of residual lignin with hydroxyl radicals is believed to be the formation of phenoxy radicals from phenolate ions by one-electron abstraction. Hydroxyl radicals and oxygen are known to give rise to such a reaction. On the contrary, it has been shown that superoxide ions do not react with phenolate ions (17). Superoxide ions can, however, react with the phenoxy radical intermediate to form an organic hydroperoxide which is subsequently degraded to low molecular weight compounds (1B). Similarly, oxygen also reacts with phenoxy radicals. The observed bleaching effect is most likely a result of the action of undecomposed hydroperoxide anions on carbonykontaining chromophores. Cellulose may undergo depolymerization by reaction with hydroxyl radicals (19). If transition metals are present in the cellulose matrix, formation of hydroxyl radicals may take place in close enough proximity to the cellulose chains to react with them. In oxygen bleaching, peroxides are formed during the reaction of oxygen with lignin and therefore the hydroxyl radicals are generated at or near a lignin site. This difference may explain why the selectivity is less in hydrogen peroxide delignification than in oxygen delignification (Fig. I). Moreover, a higher yield of hydroxyl radicals in peroxide delignification may also be expected because of the higher peroxide charge (20).

Chapter IV 6: Hydrogen Peroxide as a Delignifying Agent

351

352

Pulp Bleaching

-Principles

and Practice

To evaporation plant and recovery plant

Sequence

Pulp type

PC ED ED

Hardwood and softwood and soda AQ

OPDED PPDED Op,OPP' OPD PDp,PDPD PHDH PCHH (PO) (PO)

(EOP) P

.

PPacP OPZ OPZP (EOP) Z P

b

Reference

-

Softwood

kraft

Softwood

kraft

Softwood

and hardwood

Hardwood

kraft

Softwood

sulfite

Softwood

sulfite

Softwood

sulfite kraft

Softwood

and hardwood

Softwood

sulfite

Hardwood

kraft

Hardwood

2,12,42 13,18,29,30

Softwood

Softwood

kraft

39 4,57 9 24 5,25 5 31 52 58 9 59 52

kraft

sulfite

kraft and softwood

sulfite

AQ, anthraquinone; Pac, peracid; Z, ozone . Semi-bleached (70-80% ISO for softwood and 75-85% ISO for hardwood) b

Low-brightness pulp (50.60 % ISO)

To summarize, in the bleaching of chemical pulp hydrogen peroxide may function as: 1. A true bleachiDi aRent. Because its specific and efficient action on carbonyl and conjugated carbonyl groups, the hydroperoxide anion can destroy many of the chromophore groups present in pulp, including those created by the other reagents applied in previous bleaching stages. Chlorine, chlorine dioxide, and oxygen are known to form quinone groups (21-23). To eliminate such structures hydrogen peroxide is added in an alkaline extraction stage or at the end of the bleaching sequence.A temperature of 50 to 70°C is sufficient for this purpose. 2. A deli2nifyinll a2ent. At a temperature higher than 80°C, hydrogen peroxide starts to have a marked effect on unbleached lignin or oxygen-bleached lignin. Several options are available for introducing hydrogen peroxide in the bleaching process, among them the application of hydrogen peroxide after oxygen bleaching. An OP sequence offers the advantage of extending the chlorine-free part of a bleaching

.

process at a relatively moderate investment cost. Simultaneous addition of oxygen and peroxide is an alternative, although less efficient for brightness development (9). As indicated previously, peroxide delignification theoretically can replace oxygen delignification. However, oxygen is more cost effective for the same degree of delignification. This is why this last option is currently restricted to the prebleaching of some sulfite pulps. Indeed, the easily solubilized lignin in sulfite pulps responds particularly well to hydrogen peroxide delignification (5, 24-27). Several bleaching sequences, including hydrogen peroxide delignification, that have been proposed for commercial use are listed in Table 1.

2. Hydrogen peroxide delignification processes Three hydrogen peroxide delignification proce55e5 havebeen de5Cribed.In the fir5t, the peroxide is applied directly to the unbleached pulp.This process has been implemented on kraft pulp at Cellulose des Ardennes in Belgium since 1979under the

,

,.------...

To evaporation plant and recovery boiler

,

_~ l-_-:_~

:Brownstock:

washing , : J

.------. I

,

I

I

: Pulping :

:I Pulping :,

L:

:-~~~~!~t=k:

,

washing

-...

~--

:

J

:

0

: Stage:

O2 +

NaOH

'--+--'

:Washing 1 I

:

I

---'

r-------...

I I : Bleaching: I L

I J

FI,.3. ~toftlle~~.lIpijbIIoII (MI_J prot:ns. commercial name MINOX (for mini - QXygen) (2). The peroxide stage is integrated into brown stock washing in a way similar to that for an oxygen bleaching stage (Fig. 3). Low charges of hydrogen peroxide and sodium hydroxide (NaOH) are used (less than 0.5% on pulp) which explains why delignification is limited to 20%. The second process is characterized by a first step that alters the trace metal profile of the pulp by pretreatment with a complexing agent (Q stage) and by a second step in which the alkaline hydrogen peroxide bleaching solution is applied (2830). Commercialized under the name of UGNOX (for lignin oxidation), this process has so far been used to complement oxygen bleaching. The hydrogen peroxide and sodium hydroxide charges are generally high (up to 3-4% on pulp for HP ) and delignification can be as high as 5o%.This process theoretically can be integrated with oxygen bleaching into brown stock washing. However, the acidic nature of the effluent from the sequestering stage and its relatively high metal content introduces a

Water

Water

difficulty that has not yet been overcome. The flowsheet for this process is shown in Fig. 4. The third process uses both oxygen and hydrogen peroxide to provide a hydrogen peroxide-reinforced oxygen delignification (J1).When hydrogen peroxide is added in a conventional oxygen bleaching process, delignification is extended.This effect is substantial even for peroxide additions as low as 0.2 -0.5% (31). The viscosity is increased because of the lower alkali charge and the lower temperature required in the process

Chapter IV 6: Hydrogen Peroxide as a Delignifying Agent

353

the rate of peroxide decomposition influence the extent of delignification cellulose degradation.

also and

To evaporation plant and recovery boiler

,

,

: Pulping : iuTuJ t____..

:Brown stock: washing

I

I

J

'

,-------...

I

: Bleaching: I I

I I J

Fig.5. FIort1sbHtoftlle~osygM-~ peroxIik dellg,dJbtloft

process.

(32).A kappa number reduction of 60% is easily achieved in an oxygen stage reinforced by hydrogen peroxide. The schematic flowsheet presented in Fig. 5 is the same as that of an oxygen delignification process. For practical reasons, hydrogen peroxide is added before oxygen injection. No commercial implementation of this process has been described so far. Addition of hydrogen peroxide in a non-pressurized (E+O)-type of delignification is possible if a full-fledged oxygen delignification system is not available.The (E+O+P) stage may lead to a 30-35% kappa number reduction when 0.5% HP2 is added (4, 33, 34).The (E+O+P) stage is even more attractive when applied to sulfite pulps (35).

3. Variables in hydrogen peroxide delignification As noted previously hydrogen peroxide delignification results partly at least from the oxidation of lignin by the decomposition products of the hydroperoxide anion, H02~Therefore, the parameters that affect

3.1 Metals Decomposition of hydrogen peroxide under alkaIine conditions is greatly influenced by the presence of specific inorganic substances that act as catalysts or stabilizers. Among the catalysts, transition metals such as Fe, Cu, and Mn have been studied extensively (28, 36, 37). On the other hand, magnesium salts and sodium silicate are well-known peroxide stabilizers According to some studies, the key to optimum hydrogen peroxide delignification is to control the metal profile in the pulp before hydrogen peroxide treatment in such a way that the concentration of transition metals is as low as possible and that of alkalineearth metals (essentially Mg) is sufficient (28). In other studies (3637), it has been shown that not all transition metals are detrimental. For instance, addition of manganese has been found to be effective in retarding carbohydrate degradation (36). Although the ideal metal profile is not known, a beneficial effect is observed when a pretreatment with a chelating agent is used. For example ethylenediaminetetraacetic acid (EDTA) is best used at pH 5-7, as shown in Figs. 6 and 7 (28). Other preferred conditions in the pretreatment are 90°C for 60 minutes at a consistency of 10%. A charge of 2 kg EDTA per ton of pulp is sufficient. After EDTA pretreatment, not only is hydrogen peroxide delignification improved, but bleaching is enhanced and cellulose degradation is less. At the same time, peroxide consumption is lower (9). It is probable that other complexing agents applied under similar or different conditions lead to the same improvements.Thus,DTPA in the presence of sodium sulfite provides about the same benefits (38)Acid pretreatment with sulfur dioxide or sulfuric acid improves peroxide delignification (11, 39, 40) but not to the same extent as complexing agents (41). Therefore, it appears essential to limit the effect of metal catalysts even though delignification involves the decomposition

354

Pulp Bleaching

- Principles

and Practice 70

.

A

.

o

400 E Q. Q. Ci ~ 200

en ~60 :Z CD C -§,5O 'C m

3

5

7

9

11

3

5

pH

7

9

11

7

9

11

9

11

pH

80 E Q. Q. c: ~40

o

o 1

7

3

9

11

3

pH

5 pH

20 950 C>

E Q.

~925 E '0

A

': 10 CD LL

. , .EDTA

~900 o

o

~

>875

pretreated

o 1

3

I 7

5

850

I 11

9

1

pH

Plf. 6. Ejfed of JIll 018tile ~

3

5

7 pH

of-Ws

rIIIIrMtM .. ED1'A-JlretrMtM sojlrt1ootl "' prdJts (18). nft

D

j c:

products of hydrogen peroxide. This suggests that the radicals and oxygen have to be formed slowly to actively participate in delignification process without any detrimental effect on cellulose. The effect of stabilizers on delignification has been investigated. Sodium silicate, organophosphonates, and polylactones do not lead to any significant improvement when added in a peroxide stage (5,12,39, 42).Although addition of magnesium sulfate does not seem to improve delignification either, its effect on cellulose degradation is favorable (4, 13).

~E 15

~

.,::> c

8

N ,1, used two stages; 74% of chlorine dioxide bleaching stages were in kraft mills and 20% were in sulfite mills; 58% of the chlorine dioxide generators were Mathieson and 34% were Solvay generators. From that time, chlorine dioxide bleaching stages grew to be universal in kraft mi1ls producing bleached market-grade kraft pulp. There are a number of comprehensive references on chlorine dioxide bleaching beginning in 1953 (2-6). In this chapter, the use of chlorine dioxide to brighten pulp in the latter stages of bleaching sequences is discussed, including chemistry and kinetics, process flow sheets and process conditions, bleaching se-

Chlorine dioxide gas is unstable; when the concentration in the gas phase exceeds 100 mm Hg partial pressure, it spontaneously decomposes to chlorine and oxygen, sometimes explosively. Chlorine dioxide is therefore generated at the pulp mill site, absorbed into chilled water to form a dilute solution, generally 8-12 grams per liter at 5-1O"C. Depending on the generator process, some chlorine may be present in the solution. When cool chlorine dioxide solution is added to hot pulp, chlorine dioxide vapor can be released. To prevent such release, chlorine dioxide bleaching towers are designed to provide a significant hydrostatic head at the point of chemical addition which maintains sufficient pressure to keep the chlorine dioxide in solution until it reacts with pulp. Chlorine dioxide bleaching towers are therefore upflow towers or upflow-downflow towers. Chlorine dioxide reacts with lignin fragments responsible for light absorption in the visible region. The chemistry of these reactions is discussed in detail in Chap. III 3. Some lignin fragments remain in the pulp and some are dissolved. Chlorine dioxide reacts only to a minor extent with carbohydrates; aldehyde groups are oxidized to carboxyl groups. As shown in Fig. 1 of Chap. IV 3, when chlorine dioxide reacts with pulp, a number of intermediates containing chlorine are formed before chloride ion, the ultimate reaction product, is produced. The first step is the formation of chlorite ion (CIO;). Chlorite ion then reacts with the pulp forming hypochlorous acid (HCIO) which in turn reacts with the pulp to form chloride ion (Ct"). Hypochlorous acid can form chlorinated organic structures by reaction with pulp. It is important to note,as is discussed in more detail later in Chap. IV 3, that chlorite ion does not react when the pH is above

quences, engineering considerations, pulp

4 and, therefore, the bleaching potential of

quality and, finally, environmental factors. It is important to note that the use of chlorine dioxide as a delignification agent has been described in Chap. IV 3.

the chemical added is lost. Further, the intermediates formed by reaction with the pulp react with one another to form chlorate ion (CI03'). Chlorate ion does not react

1. Introduction CWorine dioxide is one of the most im-

382

Pulp Bleaching - Principles and Practice

with the pulp and also represents a loss of bleaching potential. Further information on the intermediates and reaction mechanisms in chlorine dioxide bleaching can be found in Chap. IV 4 and Chap. III 3 (7-12).

CL ],

0.15

--E0 ""

pH 2

CWorate Formed (mole % of Cl02 added) pH

pH 3

E 0.10 11' E .E S 0.05 oi! :2 u 0.10 0.2 0.3 0.4 ClO, consumed, mmoll g pulp

2

4

6

38

28

17

(Ref. 17)

28

15

5

(Ref. 19)

Formation of chlorate ion decreases the efficiency of chlorine dioxide bleaching (13-17). Chlorate formation is proportional to chlorine dioxide consumption and strongly dependent on the bleaching pH as is illustrated in Table 1 (15, 17). Low-. 0 c: Q) ::J 10 c:r

\./ \¥ \

I I I

68°C \

I I I I

~u...

l

5 0 52

56

60 Brightness,

50-60" '£: CC

Pulp Bleaching - Principles and Practice 88

90 ~~88

86

~ c: 1: C)

82

22

80

20 '(jj 0 u I/)

80

18 :> 78 76

78

84

86

88

Initial brightness,

90

92

a. u

;i.

82

'£:

III

"1. B.Rellllimublp be'sojhtItJN itwft ~

24

w ~ cf?84 u)

cii c: 86 'en as ... Q) 84 =as I/) I/) Q) c:

408

16 2.0

3.0

% ISO

4.0

5.0

6.0

pH

tbe IIIUW.. T-ZfiOfftIe1'tM""'lbl8tJss'" tbe blHd1i"l of(CD)H.'ipiftetl ,,, (1JIlVD) (HID).,.~. "'"'

after some six-stage bleacheries used the H stage as a water soak and second wash. In this example, the carryover of EI dissolved solids into the DI stage d(ove the cWorine dioxide application to the asymptotic limit. The uppermost curve in Fig. 7 illustrates the asymptotic approach of brightness to its maximum value with increasing chemical application. A hypocWorite stage can be used to control the beating properties of bleached kraft pulps (6). Less refining energy is requln:d to achieve a given freeness when a hypochlorite stage is part of the bleaching sequence. This is a particularly attractive feature for paper mills lacking sufficient refining capacity. A normal hypocWorite stage in a bleach sequence has no adverse effect on the yield of fully bleached pulp. However, yield may decrease if severe cellulose degradation occurs as a result of bleaching in the 6-8 pH range.

10. Environmental aspects The United States government became concerned about cWoroform in the environment following its detection in cWorinated drinking water (12). This finding led ulti. mately to the identification of chloroform in bleach plant effluents (13). In 1985 the Environmental Protection Agency reported

that pulp bleaching was the single largest source of chloroform emission to the atmosphere in the United States (14). Primarily because of cWoroform formation, the hypocWorite stage is gradua1ly being phased out of pulp mills in the United States. The hypocWorite stage is by far the largest source of cWoroform in a multi-stage bleaching sequence, accounting for 5-15 times the total amount produced by the C,E, and D stages (15). HypocWorite consumption is the dominant factor affecting the amount of cWoroform produced in the hypochlorite stage (15). CWoroform formation is a1sO indirectly a function of the kappa number of the pulp entering the hypochlorite stage (16) because the CE kappa number usually determines the size of the hypocWorite charge. For a specified hypocWorite charge,hardwoods produce much less chloroform than do softwoods (15). An investigation of the mechanism of cWoroform formation (17) has led to the conclusion that most of the chloroform is derived from aromatic ring carbon atoms in residual lignin.

11.Chlorinemonoxidebleaching of pulp 11.1 Background Chlorine monoxide (CIP) is a reddish yellow gas which was first synthesized in

1834 by the French chemist Belard. A comprehensive review of the chemistry of this compound has been published (18). Uebergott and Bolker (19, 20) used chlorine monoxide to delignify unbleached kraft pulp in the first stage of a high PGW > CfMP > TMP > RMP

5.3 Metal impurities 5.3.1 Metal sources in peroxide bleaching systems The main source of metal ion contamination in (chemi)mechanical pulp bleaching is the pulp itself. Trees (plants in general)

assimilate metal ions and nutrients present in the soil in which they are grown (11). The amounts and types of metal ions present depend on the species and growing location. Trees grown on the mineralrich soils in Ontario and Quebec (the Canadian shield formation) have a different

464

Pulp Bleaching - Principles and Practice

metal ion content than do trees grown in the Pacific Northwest. Coniferous trees (e.g., hemlock) tend to have a higher metal content than do hardwoods such as aspen. The metal ion contents of four pulp samples are listed in Table 4. Besides their effect on bleach response and their deleterious effect on hydrogen peroxide stability, metals in pulp can form colored complexes with lignin macromolecules (12). Also of concern is metal contamination from the chemicais used to make bleach liquor. Iron is a contaminant in technical grade sodium hydroxide or in bulk sodium silicate. Other non-wood sources of metal contamination in the pulp include process water and processing equipment (through wear and chemical attack). 1iIbk 4. 'J'yJIIad-W

etmImt of tIJOOII sJI«ks (13).

Wood Species

Metal Content, Iron Manganese

Hemlock, western Hemlock, eastern Aspen, midwest Spruce, eastern

115 20 55 17

105 110 4 136

ppm Copper

14 1 5 1

5.3.2 Control of hydrogen peroxide decom. position As described in the section on peroxide decomposition (Sect. 2.2, eqns. [8J and [9]), metais, particularly transition metals, act as catalytic decomposition agents when in contact with hydrogen peroxide. The most common metals routinely encountered in the peroxide bleaching of (chemi)mechanical pulps, in order of decreasing decomposition activity, are manganese, iron, copper, chromium, and nickel. Of these, the most active decomposition element is manganese. Other transition metais (e.g., cobalt) also cause peroxide decomposition but are usually not present in the pulp or bleaching chemicals in amounts significant enough to have an appreciable effect. The first step in successful hydrogen peroxide bleaching is to minimize the occurrence of catalytic decomposition. Two approaches, commonly used together, are

used to achieve this goal: pretreatment of the pulp before bleaching and stabilization of the bleach liquor before addition to the pulp. Bleach liquor stabilization is covered in detail in a later section. 5.3.2.1 Pretreatment of (chemi)mechanical pulps with organic chelants The purpose of pretreating (chemi)mechanical pulps is to complex and wash out most of the transition metals present in the pulp before the addition of bleach liquor. Pretreatment is commonly carried out using an organic chelant which forms a organo-metallic complex with the free metal. Typically, the pentasodium salt of diethylenetriaminepentaacetic acid (Na5DTPA) is used in this role. Na,DTPA fortnS a very strong complex with manganese, the most active of the peroxide decomposition agents. The pretreatment is usually carried out at low consistency (e.g. ,3-5%) at a pH of 4.0-6.0, after refining or grinding (e.g., in a latency chest following refining).The amount ofDTPA added is usually in the range of 1 to 6 kg/ton of pulp and is added on an "as received" basis. Before being bleached, the pulp is thickened to moderate or medium consistency «).1 0%) on a decker or disc filter or to high consistency (20-35%) on a belt (twin wire) press or twin roll press. This thickening step is important because the chelated (complexed) metals are washed from the pulp in the process. When a thickening step before bleaching is not possible, the addition of the chelant still decreases the amount of peroxide decomposition which occurs during bleaching, though not as significantly. The addition of organic stabilizers to bleach liquors is discussed in a later section. There are two easily measured parameters to detertnine the effect of a pretreatment and define the optimum application. These are brightness and hydrogen peroxide consumption, which is determined by measuring the residual peroxide on the pulp at the discharge point of the bleach tower. In the example shown in Fig. 4, the effect of a DTPA pretreatment on CTMP 1 was to increase brightness by at least 4 points and reduce peroxide consumption

Chapter VI: Peroxide Bleaching of (Chemi)mechanical by 35%. This reduction

in peroxide

con-

sumption can be important in situations where bleach liquor is reclaimed at the end of the bleaching treatment and recycled to a previous bleaching stage.The response of the PGW sample to pretreatment was much less pronounced, suggesting the need to determine if pretreatment was necessary in this case. Figure 4 also provides an example of a pulp (CfMP 2) where DTPA pretreatment had no significant effect on brightness response or peroxide consumption. In the

Pulps

465

case of CfMP 2, no pretreatment is recommended. It should be noted that aluminum, although not usually considered a peroxide decomposition cata1yst, can affect the chelation of other metals, notably manganese. This effect is usually seen where aluminum concentrations in the process stream reach 500 ppm or more, although an effect on pretreatment requirements has been seen at levels as low as 100 ppm.This occurs as a result of the preferential chelation of alu-

78

466

-

Pulp Bleaching Principles and Practice 73.0

o -a. Q) 72.0 ....

iIi cfl 71.0 en (/)

Bleaching conditions: 2% H202, 1.4% NaOH 3% silicate 68aC. 120 min. 20% consistency

Q) c: E 70.0 OJ .;:: III

69.0 0.0

76

0.6

0.8

1.0

1.2

DTPA, % on pulp

0

PGW

~74

Fig.5. Effectof DTPA tU/dltimt taining a bigb al"",l,",m

~0 en 72 III Q) c: .E 70 OJ .;:: CD 68 0

0.2

0.4 DTP A, % on pulp

0.6

0.8

0.6

0.8

100 ¥ c..

a. as 90

0 ~0

80

r:: 0

70

EE

60

CTMP1

:J III c: 0 ()

50

C\I

40

0 C\I :I:

CTMP2

30 0

0.2

0.4 DTP A, % on pulp

Spruce:

CTMP:

60°C. 120 min. 15% consistency

3% H202.

Fig. 4. Effect of DTPA pretreatment ofsjmu:e

0.4

Cl'MP (8).

1.5% NaOH.

2% silicate OIl brlgbtllt!ss

development

PGW: 2% H202. 2.5% NaOH, 3% silicate

and peroxide

OOtISUmptitm

In the bleaebing

OIl the brlgbmas

respmue of a peroxide

bleached SOfItbent }rill/! TMP con-

content (8).

minum over manganese in the pH range 4.0 to 6.0 (normal pretreatment pH). It is a concern in mills which pulp resinous woods and use alum for pitch control or when paper machine whitewater (aluminum-rich where alum is being used for pH control) is recycled back to the bleach plant. Under these conditions. both the use of alum in pulping and the recycle of paper machine white water should be minimized to reduce the aluminum loading in the bleach plant. Figure 5 shows the effect of high aluminum content on bleach response. The pulp sample contained 630 ppmAl, 59 ppm Mo, 1.7 ppm Cu, and 34 ppm Fe. The bleach response curve in Fig. 5 indicates that even after a pretreatment consisting of 1.2% DTPA the bleaching response is still not optimized. It also suggests that a further improvement may result with even greater DTPA applications. In this case, a metal management strategy focuses on the alum usage and encompassing both the pulp and paper mills is necessary to ensure a successful pretreatment and efficient hydrogen peroxide bleaching stage.

5.U.2 Stabilization of peroxide bleach liquor with sodium silicate The second approach to minimizing catalytic peroxide decomposition is bleach Ii-

quor stabilization.This is routinely done by adding sodium silicate (41 °Be), hereafter referred to as "silicate; to the alkaline peroxide bleach liquor. Silicate is a cost-effective stabilizer for alkaline peroxide bleaching and produces two strong benefits: it significantly reduces peroxide decomposition occurring during bleaching, and it improves the internal stability of the bleach liquor solution itself. Silicate applications in modem (chemi)mechanical pulp bleach plants are routinely 3% or less on pulp. In cases where closed water loops promote silicate buildup or where scaling problems lead to downtime for equipment de-scaling. applications of 1-2% are common. With good pretreatment or for pulps having low levels of metal contamination (e.g., aspen), the level of silicate required for good bleaching can sometimes be reduced even further. The effect of consistency on metal and bleach liquor concentrations is discussed in later sections. In systems where silicate use is not practical (e.g., in refiner bleaching), other means of stabilization must be employed.The most common non-silicate alternative for bleach liquor stabilization is an organic stabilizer, notably organo-phosphonates. Organophosphonates are preferred over aminocarboxylic acid derivatives such as DTPA,

Chapter VI: Peroxide Bleaching of (Chemi)mechanical

Pulps

467

20 c

16

'c :J

14

CI) CI) Q)

12

-

~OJ

.;:: III

-Principles

and Practice

For a given set of bleaching conditions and furnish, there is a threshold beyond which increased peroxide dosage has a minimal effect on brightness. This observation is consistent with industry experience with many types of bleaching chemicals.

5.4.1 Peroxide charge Brightness response in the peroxide bleaching of (chemi)mechanical pulp is directly related to peroxide application. As shown in Fig. 8, increased peroxide dosage leads to increased brightness. Response to increased peroxide application is dictated, in part, by bleaching conditions such as initial pH, time, temperature, consistency, and furnish type. The impact of each of these conditions on peroxide bleaching performance is discussed in the following subsections.

CI)

c

Pulp Bleaching

5.4 Process variables Chelated Unchelated

18

'(ij OJ

468

10 8 6 0.0

0.5

1.0

1.5

2.0

2.5

3.0

35

Total alkali, % on pulp

30

c '(ij whose use is favored in pretreatment applications.The use of organic stabilizers is not, however, common beca,use of both performance limitations and economics: It is difficult to fmd a more cost-effective stabilizer than silicate. 5.3.2.2.1 Mechanism of sodium silicate stabilization in the peroxide bleach ing of (chemi)mechanical pulp Several theories about the role of silicate in the peroxide bleaching of (chemi)mechanical pulp have been suggested. These roles include silicate acting as a peroxide stabilizer, metal ion sequestrant, buffering agent, and metal surface passivator. For stabilization, metal sequestration and metal surface passivation are two important silicate functions. Indications of silicate activity as a sequestrant or metal complexing agent have been reported (15). Experience has also shown that, even with inclusion of a pretreatment to eliminate some of the metals, the addition of silicate to the bleach liquor leads to a higher brightness for the same peroxide application.This is illustrated in Fig. 6 where the bleaching response at varing silicate applications for chelated and unchelated groundwood pulp is shown. Peroxide residual is also increased by silicate addition (15), suggesting that peroxide decomposition is reduced, probably

through metal control. There is also evidence indicating that the amount of silicate applied should be increased with higher peroxide applications (14) as illustrated in Fig. 7 (B). The brightness response at varying silicate addition levels is somewhat flat at low peroxide applications «1%). At intertnediate peroxide applications (3-5%), the brightness response levels off at 3% silicate addition and, at elevated peroxide application levels (8%), the addition of high sillcate charges further improves the brightness response. The inclusion of silicate also allows the use of some types of mild steel equipment in peroxide bleaching. Mild steel is not normally used in peroxide service and any equipment used for bleach liquor storage or transport of bleach liquor must be made of compatible materials such as stainless steel or aluminum. Silicate, however, coats equipment surfaces, presenting a non-reactive surface for the alkaline peroxide bleach liquor to contact. This allows the safe and efficient use of mild steel equipment such as stock mixers and pumps.

5.4.2 Total alkalinity (pH) After peroxide stabilization concerns are addressed, the most important relationship for the proper control of alkaline peroxide (chemi)mechanical pulp bleaching is that between peroxide and alkalinity. H the alkali charge is too low, inefficient bleaching is likely to result; too high an alkali charge

-

CI CI)

25

'c :J

20

0

0-

CI) CI)

CD 15 C

CI -;:: CD

[}.-

--- .;::

2

c:

'(ii

C) !/)

o o

c: '0 0..

,/

2

c:: .t;j C> J9 c:: '0 0.. !/)

!/) CD

6

".""

". /' ,.,',' ,I,,"

4

C> 'C:

on brigbtness

tlellelopmertt

pH= 6.0

pH=5.0 pH=4.0

."

2

0.2 Hydrosulfite,

of ,,,""'1/,,11, bytlroSfllftte (4).

Consistency: pH: 5.0

Consistency: Temperature:

4% 60°C

0.4

0.6

0.8

% on pulp

0.6

m tbe blellebl"g

of _them

JIll Oft brightness tlellelopment I" tbe bletU:blng of _them

pine TMP wltb

4%

0.8

Hydrosulfite, % on pulp Fig. 3. E.I1«t of temperature Stllftte (3).

-',-'

I,,'/" I""

c::

E

Fig.5. E.I1«t

0.4

0.2

70

time, min

_-".,,,._.>..,' .,' ".".,,,,,,'"".",."

8

0

0 0

60

50

0

I

a:I

40

50°C

/ ,,' " I ,I' 1',1' ,I," , /,,'

4

!/) !/) CD

c: .c: C) .;::

./ ,,,' "

",,'-

30

10

.---------------------.

./''/

20

Retention

./,- 60°C ,./

10

Fig. 4. E.I1«t of retention tbne on brigbtness tlellelopment I" tbe bletU:blngof smahem pine TMPflJUb bytlroSfllftte (4)

-----------.

6

0.5% 60°C 4%

a:I

m 8

and Practice

pine

TMP wltb bytlro-

The pH of the pulp mixture decreases during hydro sulfite bleaching as a result of pulp bleaching and decomposition reactions. As shown in Fig. 5, decreasing the pH in the 4-6 range leads to decreased pulp brightness. This may be because hydrosulfite decomposition is enhanced under conditions of increasing acidity. For this reason, commercial hydrosulfite blends containing buffers are available to ensure that the appropriate pH is uniformly main-

tained throughout the pulp over the course of bleaching. If pH buffers are not included with the hydrosulfite solution, they must be added separately. Consistency Within the 2-5% range, consistency

has a

minimal effect on pulp bleachability. However, when bleaching is performed at a medium-consistency level (6-10%), a brightness gain 1 or 2 points higher than that cor-

Chapter V 2: Hydrosulfite (Dithionite) Bleaching

499

500

Pulp Bleaching - Principles and Practice

12 c: '(ij C> II)

"E '0 a.

8 7

10 10% consistency

8

-

4

J:: C> 'C: m

2

II)

c: '0 a.

Temperature: 60°C pH: 5.5

,

II) II) Q)

-

"

,, ,,,

J:: C> 'C: m

"

0.2

0.4

0.6

0.8

Hydrosulfite, 011 brl,,,,-

1.0

1.2

TMP bytIro. ""'"

-

Goodmixing

II)

8

c: '0 a.

6

--- --

c:

J:: C> 'C: m

4

""

2

-- --

"

,I"

"

0

o

0.2

0.4

Hydrosulfite,

FI,.7.If/feetof~

Temperature: 60°C Consistency: 4% pH: 5.5

"

/"

FI,. 8. Blfeetof ."",i1IM

0.2 0.4 0.6 Hydrosulfite, % on pulp

",,. 011brl,,,,-

~loJtrt-I'.

-1fUe (4).

discussed

_-------------------

,,/'--,

II)

II) Q)

,,"

2

tion is increased. Mixing principles and technology, as they apply to bleaching, are

10

effldaey

~1fUe(4).

011brlt"'-

leveloJtrt-l

responding to low-consistency bleaching can be obtained over a wide range of by. drosu1fite applications (Fig. 6). The benefit accruing from higher-consistency hydrosulfite bleaching offers mills the option of reaching the same brightness in less time with smaller bleaching towers.

3.2 Mixing efficiency Goo

-

6

CD c:

c:

//---/_.

II) II)

_---

#'------------------------

in Chap.VI

2.

Hydrosulfite bleaching solutions are usual1y introduced through the suction side of a stock pump in which the impeller serves as a mixing device. Pump malfunctioning, often indicated by noise and vibration, may imply that poor mixing is occurring. In such instances, a brightness loss may be observed. This problem can be resolved by the institution of a preventative pump main. tenance program. Some mills use in.line mixers following the addition of the bleaching solution. In this case, there is concern that dewatering may actual1y negate anticipated gains from their use. The recommended solution is to use a pump which is designed to accom. modate the addition of chemicals into the pulp. Such a pump would fluidize the pulp and enhance the dispersion of the added chemical introduced into it.

3.3 Air (oxygen) entrainment As indicated in Sect. 2, oxygen has a det. rimental effect on brightness in bydrosulfite bleaching because it reacts with and con. sumes the bleaching chemical. In addition, oxygen can react with the reduced chromophores in the pulp and re-convert (re. oxidize) them to their corresponding

tN "luc6lltg

of_tlJer.

0.8

pirie TMP

""t"

IIytlr'O.

colored forms thereby lowering the bright. ness. The plots in Fig. 8 indicate that this brightness loss can be as much as 2 points. Because the amount of air entrained in pulp increases with increasing consistency, the adverse effect of air entrainment can be offset to some extent by bleaching at lower consistency levels. Although the theory is unproven to date, lower bleaching temperatures may have a negative ef. fect on brightness because the resulting increased solubility of oxygen in the bleach. ing solution leading to increased hydrosulfite decomposition. 3.4 Metal impurities The oxidized form of some metals in pulp can lead to a decreased bleaching response by catalyzing the decomposition reaction [6) of the hydrosulfite and the re-oxidation (brightness reversion) of bleached pulp. Iron and manganese are generally viewed as the worst offenders in each instance (7. 9). Analysis of thermo mechanical pulping systems has shown that as much as 10 ppm iron and 150 ppm manganese can be contributed by the wood. One of the most manageable sources of iron contamination is the alum used for pitch or deposit control. Iron is present in varying concentrations throughout the ra11geof commercial1y available alum products. When attempting to maximize brightness, it is advisable to

Chapter V 2: Hydrosulfite (Dithionite) Bleaching

501

66 64

o ~

62

............ ...... ---- ...... ~

Ethylenediaminetetraacetic

'::::.---------------- ---

?f. U; rJ) Q) 60 c: .C) c:

-

'C CD

acid

------- ~"'.......

Diethylenetriaminepentaacetic

-------acid

............. ...

58 Sodium

tripolyphosphate

56 100

200

300

Iron added,

Fig.,. Eff«Iof lUff-I ebe"",,,gagerttstml1rlgllhtnS . ~ofi"", (9). use only low iron-containing or iron-free alum. Iron in its reduced state does not affect brightness. However, when deposited on a paper sheet, ferrous iron is subject to oxidation. When this occurs, the resulting black form of iron oxides reduces brightness significantly. Most commercial hydrosulfite blends and some liquld formulations contain sufficient amounts of chelating agents to control the adverse effect of small amounts of iron, copper, manganese, aluminum, and chromiumonbrightne~.Howeve~whenmetal contamination is especially high or when a mill uses hydrosulfite generated on-site, the application of additional amounts of chelating agents may be required. The effect of different chelating agents on brightness response in hydrosulfite bleaching is illustrated in Fig. 9. Normally, 0.1 to 1.0",6(2-6 lb. per ton of pulp) of chelant perton of pulp provides adequate control. Ethylenediaminetetraacetic acid (EDTA) is generally used in a pH range below 7.0. Sodium tripolyphosphates also control metals and are viewed more as sequestering agents than as chelants. Diethylenetriaminepentaacetic acid roTPA) is used in alkaline systems. It should be noted that excessive amounts of chelating agent may adversely affect brightness.

500

400

ppm on pulp ~

of _Ibent

~u

TMP CtnI"'''''''g

fMJryiIIg

Regardless of the choice of chelating agent, once metals enter the system it is best to reduce them to their non-colored state, complex them with a chelating or sequestering agent, and remove them from the pulp before it is sent to the paper machine. The most satisfactory way of dealing with metal contamination is to exclude them from the process.

4, Effect of hydrosulfite properties

502

Pulp Bleaching - Principles and Practice

it forms a dihydrate that reacts almost instantaneously with oxygen. The solubility of sodium hydrosulfite in water is about 18% at 20"C. A 1% solution of commercial sodium hydrosulfite has a pH of 6.5 +/- 1. Sodium hydrosulfite can decompose when exposed to air, moisture, heat, or oxidizing agents. Its decomposition produces both heat and oxygen; the heat generated can be sufficient to cause a fire. Solutions of sodium hydrosulfite are clear liquids at varying concentration levels. For long-term stability purposes, the liquid products are cooled to about 7"C, stabilized with alkali, and maintained in an oxygenfree environment. The aerobic and anaerobi~ecomposition of hydrosulfite solutions is described by reactions [8] and [9] (Sect. 2.2). Although the aerobic decomposition of sodium hydrosulfite occurs in a relatively short

time, anaerobic decomposition begins only after several days. The latter reaction is governed by pH and temperature. Hydrosulfite solutions, maintained at low temperatures (-7"

In general, the following can be stated regarding the effect of hydro sulfite bleaching on pulps: Ugnin and lignin containing compounds are bleached (whitened) but not removed, there is little effect on hemicelluloses, extractives are bleached or removed, the degree of polymerization of cellulose is essentially unchanged, and the amounts of shives and fiber bundles are decreased (10). To date, little has been reported in the literature about pulp strength as it relates to pulp bleaching with sodium hydrosulfite.

2XIK:

3X2 -

5, Commercial hydrosulfite bleaching operations

1*

5.1 Properties of sodium hydrosulfite Sodium hydrosulfite is a free-flowing, white crystalline solid. When moistened,

2.truck unloedlng

lJnloedll1!J""-'"P

Fig.10.St:bemIIIkIUagrwM for ~

"-"

ptlmpo

aU slorwgeof

l1y4msulfltesol.1imI

at" mill sile.

Toproceso

Chapter V 2: Hydrosulfite (Dithionite) Bleaching hydrosulfite is supplied to the industry in a number of different fOnDs. It can be purchased as a liquid or powder or made onsite from the addition of sulfur dioxide or sodium bisulfite solution to caustic soda (sodium hydroxide) and sodium borohydride. When delivered as a solution, hydrosulfite is pumped from the delivery tank truck into a storage tank at the mill (Fig. 10). A typical installation uses a 37,85460,566 L (l0,OOO-16,OOO gallon) insulated fiberglass-reinforced plastic tank equipped with a nitrogen padding system to keep exposure to air at a minimum. Depending on usage rates, refrigeration on the storage tank may be necessary. If approximately three truck loads per week (56,781 liters or 15,000 gallons) are used refrigeration wilt most probably not be needed. The newly arriving material (supplied at 7"C) assists in maintaining a suitable storage temperature.

503

When delivered as a powder in 113 kg or 1000 kg (250 or 4,400 Ib) returnable containers, the sodium hydrosulfite product is dissolved in water and added directly to the pulp slurry (Fig. 11). On-site generation of hydrosulfite is accomplished through the use of a skidmounted reaction unit into which is added sulfur dioxide or sodium bisulfite solution, caustic soda (sodium hydroxide), and s0dium borohydride (Fig. 12). If sodium bisulfite solution is used, sulfuric acid is also added to neutralize excess sodium hydroxide in the borohydride solution (reaction [10)). 10 this case, the added caustic soda is added only for post pH adjustment of the resulting sodium hydrosulfite solution. NaBH. + 3.2 NaOH + 8 NaHS03 + 1.6

-

H2SO. -+ 4 Na2Sp.

+ NaB02 + 6 H20 [10]

504

Pulp Bleaching - Principles and Practice

In alternative procedure (reaction [II)), sodium hydroxide is used for both the generation of sodium hydrosulfite and for postpH adjustment of the manufactured hydrosulfite solution. NaBH. + 3.2 NaOH + 8 S02 + 4.8 NaOH -+

NaBH. + 8 NaHS03 -+

~

4 Na2Sp. + NaB02 + 6 HP

[11]

The borohydride is supplied as a solution containing 12% sodium borohydride, 40% sodium hydroxide, and 48% water. Regardless of which of the above reactions [10 and 11] is used, the reaction of sodium bisulfite and sodium borohydride produces sodium hydrosulfite and the by-product, sodium borate. Hydrogen gas is also produced by a simultaneous competing hydrolysis reaction: NaBH. + 2 HP

-+ NaB02 + H2 (gas) [12]

The sodium borate remains in the hydrosulfite solution, but the hydrogen gas must be carefully vented to the atmosphere.

5.3 Hydrosulfite bleaching systems The choice of hydrosulfite bleach addition point(s) depends on the amount of brightness gain required. On average, a mill can expect the following results: Points Brightness Gain

Addition Point

4-6 8-10 2-6 9-14

Refiner (single-stage) Tower (single-stage) Stock chest (single-stage) Refiner and Tower (two-stage)

Refiner bleaching A process for bleaching lignocellulose material (pulp) with a reducing bleaching agent in multi-stage refiners was patented

Sodium borohydrlde solution BIn

BIn Vent

Water

'" Oeuas

IXlJ

Sodllm blsumt.

-

To

t t

)(()(IJI

lank

(jJ

t t

Static mixer

Sulfuric acid

I L

I I I I J

1L Caustic soda solution DlnoMng tank

i ' L

m

~ lie

,nrr

pHI

pll".'

Fig. 11. Sebem4tk

tUtlgrwm of II maUap

systtlm

for

soM_

~lftle

bktldmtg

sol.tItms.

Fig. 11. Seberrrstk tUtlgrwm of OIl-sUe bytlros,Ilflle

Demand from process

gnreraU18g sysltlm.

To storaaelProcess

Chapter V 2: Hydrosulfite (Dithionite) Bleaching

in 1969 by Crown Zellerbach Corpora-

(11) tion. The addition of sodium hydrosulfite bleaching products into the refining zone of a TMP refiner was commercially undertaken in North America in the early 1970s by Bowater Corporation to increase pulp brightness. With respect to the bleaching effect of hydrosulfite, the following factors are critical: pulp temperature, pH, alkalinity, bleaching chemical charge, and effective chelation (12). Refiner bleaching provides mills with the opportunity to obtain incremental brightness gains without having to rely on a singlestage hydrosulfite addition step to attain the brightness objective. In refiner bleaching, 50-95% of the total brightness gain required is typically obtained. Refiner bleaching, combined with a following second-stage bleach treatment in an upflow tower, can provide the desired brightness at reduced cost under more easily controlled bleaching condition than does tower bleaching alone. An operator needs only to reduce hydrosulfite addition in the tower to reach the target brightness.

Hydrosulfite

505

Operationally, all that is required for the implementation of an on-ref mer hydrosulfite bleaching treatment is extension of the bleach solution line (including addition of suitable control valves) to the dilution water line at the refiner (Fig. 13). Bleach dosage can be controlled automatically or by manual adjustment. Sodium hydrosulfite addition to pulp in aTMP refiner is successful because it allows several very important bleaching conditions to be accentuated. First, the temperature in a refiner zone is very high, forcing the bleaching reactions to completion in a shorter time. Second, the design of the refiner itself represents one of the best pulp mixing environments in a mill. Even though the residence time of the pulp in the refiner is short, the mixing intensity is extremely high thus ensuring a uniform distribution of bleaching chemical in the pulp fibers. Coupled with the high bleaching chemical-to-fiber ratio created in the high-consistency refining zone, refiner bleaching has proven to be a very effective initial treatment in combination with further hydrosulfite bleaching in a TMP mill.

506

Pulp Bleaching - Principles and Practice

The addition of excessively high amounts of hydrosulfite bleaching solution to the refiner should be avoided. Because of the high temperature in the refining zone, excess hydrosulfite may decompose to sodium bisulfite or sodium sulfate which can undergo thermal decomposition to gaseous sulfur dioxide. Specially formulated hydrosulfite blends are available to alleviate this problem. A portion of the brightness gained in refiner bleaching may be lost in ensuing pulp processing steps because of reversion. However, by paying proper attention to the factors that cause reversion (metals, oxygen, heat), a 4- to 6-point brightness gain can be maintained. Tower bleaching Hydrosulfite bleaching in towers evolved from the need for a defined amount of retention time following the addition of bleaching chemical to the pulp. The bleaching solution is added to the pulp on the discharge side of a medium-consistency pump (the point where maximum fluidization of the pulp occurs) or on the suction side of a Unbleached stock storage

Bleaching

Solution

centrifugal stock pump preceding an upflow bleaching tower (Fig. 14). The pulp moves through the upflow tower in a plugflow condition and is not mixed or aerated. The tower is sized according to the consistency of the pulp used and the time required to complete the bleaching reaction. Most towers are tile-lined and provide a retention time of 30 to 40 minutes. Brightness gains from the use of hydrosulfite in tower bleaching typically range from 8 to 10 points. Stock chest bleaching Bleaching in a stock chest is similar to tower bleaching but the bleaching response is not as favorable. Stock storage chests do not possess the upflow character of bleaching towers; therefore, it is not possible to properly regulate the retention time or to ensure that the bleaching chemicals react uniformly with the pulp. Because of their design characteristics, pulp storage chests usually do not prevent air from contacting the pulp. As a result, air entrained in the pulp can consume any unreacted hydrosulfite and cause oxidative brightness reversion of the pulp. Hydrosulfite bleaching

Steam

Upflow tower

Temperature r:::J o confrol

I 1-2 houre retention time

Pulp I

I

PrInwy

SecondBry

Ratio control Ter1I8y

+

+

Level control

Refiners

.......

Latency Chest Fig. 13. Flowsheet

for refiner

bletu:btng

wltb bytlrrJSIIlflte.

nleached stoc~ to

Level chest Fig. 14. Flowsbeet

for bytlrosfIlflte

blellCbtng

paper

In an upflow

tower.

machine

507

Chapter V 2: Hydrosulfite (Dithionite) Bleaching

solution is introduced into the stock chest in much the same manner as in a bleaching tower (see Fig. 14). Brightness increases of from 2 to 6 points are typical.

finer bleaching is followed by tower bleaching as outlined in the process fIowsheet shown in Fig. 15, a combined brightness increase of 14 points is obtainable (Fig. 16).

Sequential refiner and tower bleaching As stated in Sect. 5.3.1, a 4- to 6-point brightness increase can be achieved by hydrosulfite bleaching in a refiner. When re-

Sequential peroxide-hydrosulfrte bleaching Sequential oxidative-reductive bleaching (i.e., peroxide-hydrosulfite) is used byTMP mills to obtain large brightness increases

~

gl

508

Pulp Bleaching

- Principles

and Practice

(15-25 points) at lower total bleaching cost compared with that for single-stage peroxide bleaching (Fig. 17). It is the preferred method for obtaining a brightness gain which surpasses the limit achieved with single- or multiple-stage hydrosulfite additions (13).

The fIowsheet for a sequential peroxidehydrosulfite bleaching sequence is shown in Fig. 18. The two-stage sequence consists of the following steps: 1) A first-stage peroxide bleach performed under conditions described in Chap. V 1

25

r ,--........

c 'Cij 20 C> UJ

---

Pulp

-

--1-'

-

C '0Q. UJ UJ Q) C

-

..._ CIw8t

hydrosulfite addition:

10

" .-,

5

o

Fig.17. Brlgblrlns ~

o

1 2 Hydrogen peroxide, % on pulp of,."

blHdJed I.." two-Stllp ~-bydrosulftte

3

system (5).

Up.Iow

,,-Pulp 10 Blond

O.1_8-~~~:-_.

"""".......................

15

.J::.

C> .;:: II)

Second-stage

BIo8ch

Tower

Cho8I

16 14 C 'Cij C> 12 UJ

-

C '0Q. UJ UJ Q) C

E

C> 'C II)

Refiner

/

10 S 6

,.-- - -- --

/

---------

addition

=

1%

FeraKIde Bleac:hlng System

-----------------------------.

"""""'0.5%

::',/,

R_

will appropriate resldU81 neulnllzatlon

4 2 0

o

0.2 Hydrosulfite

Fig. 16. Brlgblrlns resptmse of_them bydrosulftte (5).

0.4 applied

0.6

0.8

in tower, % on pulp

pille TMPblellCbl..g I.. seqrumUtlI reft_-upjl_

Hydrosulfite Bleach Solution

tower system tdtb

Pulp to Paper

Machine

Blind Chili

Chapter V 2: Hydrosulfite (Dithionite) Bleaching

509

2) Reduction (neutralization) of residual peroxide with sulfur dioxide or sodium bisulfite 3) Adjustment of pH, as required, to a level suitable for optimal hydrosulfite bleaching 4) Application of a second-stage hydrosulfite treatment in an upflow tower

both requirements are met by the addition of a single, inexpensive chemical, sulfurous acid. Care must be exercised when storing and handling peroxide and hydrosulfites. These two chemicals are not compatible: peroxide is an oxidizing agent and hydrosulfite is a reducing agent. (14)

Because peroxide bleaching is terminated before the peroxide is totally consumed, the residual oxidant is first destroyed so that it does not consume hydrosulfite in the following stage. Sulfurous acid (82S03)' formed by dissolving sulfur dioxide in water, or sodium bisulfite (NaHSO,) is generally used for this purpose. The neutralizing agent is automatically added as called for by in-line pH sensors. Sulfurous acid both reduces residual peroxide and lowers the pH from the alkaline levels used for peroxide bleaching to the slightly acidic levels required for optimized hydrosulfite bleaching. .If sulfuric acid is added to make this pH adjustment, as is sometimes done, a separate reducing agent must be added to destroy the residual peroxide. Although the addition of these two chemicals may achieve the desired effect, this approach is more costly than when

4

'0 a. U)

3

U) Q)

-

c:

ofslllgle-color}Hl/ler

StI1IIJIles to sodium

35.3 83.4

92.2 94.6

-13.4 -0.8

46.2 2.5

Blue

None Hydrosulfite

75.0 82.0

83.1 93.4

-4.6 -2.1

-10.6 1.6

6. Application of hydrosulfite to secondary (recycled)fibers

Red

None Hydrosulfite

42.5 84.7

73.3 94.6

32.3 0.0

3.2 1.6

Sodium hydrosulfite plays two roles in the processing of secondary fibers and deinked pulps. First, because of its high reducing potential, sodium hydrosulfite is an excellent additive for the removal of color introduced into the pulp in the form of dyes or other coloring agents (see Fig. 19). Second, deinked pulps containing mechanical pulps can be bleached with sodium hydrosulfite to regain brightness which may have been lost through reversion.

Green

None Hydrosulfite

51.6 68.5

81.3 93.7

-22.3 -6.0

7.8 13.0

6.1 Bleaching of deinked pulps Bleaching conditions for deinked pulps are very similar to those used for TMP except that the pH is considerably higher. TMP

bleaching is usual1y performed at pH 4.56.0; Deinked pulps are bleached at pH 5.57.5.

50°C 3.5%

2

Color

Chemical

Yellow

'CIELAB or STARLAB values

L. = Yaxis a*= Xaxis

b. =

Z axis

6.2 Color stripping When color removal is the goal, operators of deinking mills are advised to consider the use of hydrosulfite to strip dyes. Many dye colors are eliminated more effectively by sodium hydrosulfite than by an oxident If the furnish contains mainly chemical fibers such as a mixed office waste furnish, decoloration occurs under conditions slightly different from those used for a furnish containing mostly mechanical fibers. Higher temperatures, 8Q-l00"C, and a pH of7.0 and higher improve the dye-stripping efficiency of sodium hydrosulfite. The textile industry successfully uses large quantities of hydrosulfite in a higher pH range (9.5-11.5) to reduce dyes. Table 1 shows the bleaching and decoloration responses of single-color paper samples to sodium hydrosulfite using 1% addition of sodium hydrosulfite at 4% consistency, pH 7.0, 80"C, and a 6O-minute retention time.

.L: C)

7. Typicalhydrosulfite bleaching responses for representative pulp types

'C: CD 0

0.2

0.4 Hydrosutfite,

Fig. 19. Brlgbmess

resptmse

of dellIied

old -sJIrlllt

(4).

None Hydrosulfite

Temperature: Consistency: pH: 6.5

0

bydrosuljite

a.'

5

-c:

Table 1. Resptmse

L"'

'6

C) U)

Pulp Bleaching - Principles and Practice

Brightness (% ISO)

7

c: '(ij

510

0.6

0.8

1.0

% on pulp (ONP) pulp

bletlCbed wltb bydrosuljite

(3).

Wood species vary greatly from region to region. They contain different levels of metals, may be cut at different ages of

growth, and may be subjected to different handling practices. All these conditions affect unbleached brightness and, therefore, final brightness. It is difficult to assign typical brightness response values to groundwood or mechanical pulps without regard to their history. Nevertheless, the information in Table 2 provides as approximate indication of the differences in response of different (chemi)mechanical pulps and secondary fibers to hydrosulfite bleaching.

8. Safety and environmental aspects of sodium hydrosulfite bleaching 8.1 Chemicals in hydrosulfite bleaching

solutions

Powder forms of sodium hydrosulfite are classified by the United States Department of Transportation as being spontaneously combustible. Sodium hydrosulfite is a strong reducing agent and care must be exercised to ensure that it does not contact oxidizing agents, acids, or water. Depending on their pH, solutions of sodium hydrosulfite mayor may not be classified as corrosive. When delivered to the mill in solution form, these products are slightly alkaline; when prepared as a solution from the powder form, these solutions are near neutral to slightly acidic,

Chapter V 2: Hydrosulfite (Dithionite) Bleaching

Pulp Description TMP

Spruce Aspen Douglas-fir Hemlock Balsam Pine Hardwoods

+++ +++ + ++ +++ +++ ++

Northern Western

Cottonwood Pine Spruce Hemlock

Northeast Northwest

SprucelFir ?

++ +++ +++ ++ ++b ++b

Secondary Fiber

Mechanical Pulp-Containing Mechanical Pulp-Fn:e

Northern Western

Southern Groundwood

CTMP Other

Bleaching Response

Southern

Kenaf

=

=

511

.

++ + +

=

Brightness points gain: + 3-5;++ 4-7; +++ ~10 Information provided by Morton International, Inc.

512

Pulp Bleaching - Principles and Practice

free kraft pulp demonstrated that (under o~ timized conditions) as many as 7 points of brightness can be gained(6). The use of secondary fiber will continue to grow until availability or market conditions limit its use. This will lead to incn:ased use of sodium hydrosulfite. Addition of hydrosulfite to pulp will not only be in bleach towers but will encompass din:ct addition into dispersion units, and other new equi~ ment designed to ease the burden of deinking on washers and flotation cells. Sodium hydrosulfite will even be applied to n:cycled corrugated for some form of decoloring so that the base sheet will have a more uniform appearance making coating operations mon: consistent from batch to batch and from mill to mill.

References 1. Cotton, EA. and Wi1kinson, G., Adt'anced Inorganic Chemistry, 4th edn. WileyInterscience, New York, 1980, p.535. 2. Atkins, P.W., Horsfield,A.,

8.2 Bleaching reaction products Proposed United States and worldwide environmental regulations are currently being designed to limit air and water pollutants including, but not limited to, chloroform, adsorbable organic halides (AOX ), dioxin, and furans. In contrast to kraft bleaching, (chemi)mechanical pulp bleaching with sodium hydro sulfite does not generate chloro-organic compounds and other

pollutants. Biological oxygen demand (BOD) and chemical oxygen demand (COD) incn:ases caused by lignin decomposition an: not seen because sodium hydrosulfite modifies but does not degrade or solubilize lignin in its pulp bleaching process.

9. Future trends in hydrosulfite bleaching Because of its ease of use and n:latively low cost, sodium hydrosulfite should continue to be the chemical of choice in mechanical pulp bleaching and in secondary fiber bleaching and decoloring. With the continuing need for chlorinefree bleaching systems and mon: environmentally compatible processes, sodium hydrosulfite may find a new application in the bleaching of kraft pulp. Work has already begun in this an:a and severallaboratory, pilot plant, and even mills trials have been completed. The resulting pulp will almost certainly have different physical properties, including the possibility of gn:ater stn:ngth and a lower lignin content. Laboratory bleaching of a totally chlorine-

M.R.C.,

3. Ellis, M.E.. "Deinking of Secondary Fibers,"Seminar Proceedings, Western Michigan University, Kalamazoo, MI. 1990, p.277. 4. Fluet, A., "The Brightening of Mechanical Pulp with Sodium Hydrosulfite;TECH '94, CPPA Mechanical Pulping Course Notes, Tech. Sect., CPPA, Montreal, 1992, p. 1. 5. Unpublished results, Corp., Charlotte, NC.

Hoechst

Celanese

6. Ducharme, N.R. and Nye, J.E, "Optimum Conditions for Bleaching Totally Chlorine Fn:e Kraft Pulps with Hydrosulfite; 1993 TAPPI Pulping Conference Proceedings, TAPPI PRESS,Atlanta, p. 777. 7. Hart, R., Pulp Pap. 8. Hart, R., Tappi 9. Ganguli,

When hydrosulfite is prepared on-site from sodium borohydride and sodium bisulfite and sulfuric acid or sulfur dioxide and sodium hydroxide (caustic soda), can: must be taken in handling these chemicals. Sodium borohydride is supplied as a solution containing 12% sodium borohydride, 40% sodium hydroxide, and 48% water. This solution is highly alkaline and should not be allowed to contact the skin. Sulfur dioxide is a Class B poison and must be handled accordingly. As with all chemicals, the appropriate Material Safety Data Sheet (MSDS) should be consulted befon: working with the material.

Symons,

J Chern. Soc. 1%4:5220.

55:138

(1981).

64 (3):43 (1981).

K.K., Pulp Pap.

54:108

(1980).

10. Voelker,M.H.,The Bleaching of Pulp CR.P. Singh, Ed.), TAPPI PRESS, Atlanta, 1979, p.342. 11. West, W B., US Pat. No. 3,467,574 16, 1%9).

(Sept.

12. Ellis, M. E., "Pulp Bleaching with Sodium Hydrosulfite in a TMP Refiner System:Important Parameters,» 1989TAPPI Pulping Conference Proceedings, TAPPI PRESS, Atlanta, p. 191. 13. Ellis, M.E., Pulp Pap. 61:129

(1987).

14. Kise, M.A., The Bleaching of Pulp CR.P. Singh, Ed.), TAPPI PRESS, Atlanta, 1979, pp. 255-273.

SECfION VI: Bleach Plant Operations, Equipment and Engineering Chapter 1: Pulp Pumping and Hydraulics Jeffrey lindsay Institute of Paper Science and Technology Atlanta, GA (Currently at Kimberly-Clark Corp., Neenah,WI) Johan Gullichsen Department of Forest Products Technology Helsinki University of Technology Espoo, Finland 1. Overview of pumping

needs and trends . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . .. 515

2. Pulp suspension behavior. . . . . .., . .. . . . . . . . . . .. . . . . . . . . . . .. . . . . . . . . . .. 515 2.1 Low-consistency flows. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515 .2.2 Medium-consistency flows. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 516 3.Basicpumpingconcepm 518 3.1 Centrifugal pumps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 518 3.2 Positive-displacement pumps 520 4. Pumping of pulp 4.1 Low-consistency pulp. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Medium-consistency centrifugal pumps. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4.3 Positive-displacement pumps ..... 4.4 Design of medium-consistency systems 5. General considerations for design of pumping and piping systems . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 5.1 Power requirements and pipe size 5.2 Sizing pumps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Materials for pumps and pipes 5.4 Pressure measurement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Partial flow. . . . . . . . . . . . . . : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6 Elbows, valves, flanges, supports. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7 Pump drive trains. . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8 Seals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.9 Potential of sealless pumps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.10 Filtrate air. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S.11 5.12 5.13 5.14

Maintenance ........................................ Control and stability issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Startup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flow metering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513

521 521 522 526 528 529 530 530 530 530 531 531 531 532 532 532 532 533 533 534

Chapter VI I: Pulp Pumping and Hydraulics

Chapter VI 1: Pulp Pumping and Hydraulics 1. Overview of pumping needs and trends No other non-Newtonian fluid is pumped in larger volumes than wood fiber suspensions,and yet these suspensions remain one of the least understood industrial flows. In the past, pulp operations at low-consistency (less than 6%) were commonplace. In the bleach plant, chemical addition and mixing, pulp discharge from storage towers, and dilution and washing historically were done at low-consistency. Conventional centrifugal pumping strategies were usually adequate.Where pulps of higber consistencies had to be pumped, positive displacement pumps were the standard, but significant pumping at elevated conSistencies was generally avoided. Now, however, there are strong incentives to operate at higher-consistency wherever possible. The advantages of higher-consistency include lower energy costs to pump and heat water, less capital equipment when diluting and thickening operations are eliminated, more compact equipment and facilities, chemical savings when less diluting water is present during bleach addition, and decreased effluent volume (a major environmental and economic consideration). Since the late 19505, many bleaching and transfer systems have employed mediumto high-consistency (10-18%) pumping to decrease water use, energy use, and pulp storage space. These "thick stock" systems accept the various sized lumps of wet stock and air and displace them into a pipe for mixing, heating, bleaching, refining, or transport to storage. In the past decade, a need has developed to process the higherconsistency pulps at pressures up to 1.4 MPa (200 psi) and with significantly less air content as newer bleaching processes replace cWorine bleaching.

515

Recently, significant gains have been made in pumps for medium-consistency pulp slurries, but feedback from the industry suggests that even greater gains are needed. It also appears that a fundamental understanding of medium- and high-consistency flows does not yet exist to adequately guide equipment development. While medium-consistency technology will grow in importance, low-consistency pumping operations will continue to be significant. Both processes are discussed below. The behavior of pulp suspensions relevant to pumping and pipelines is first reviewed, and pumping strategies are then discussed.

2. Pulp suspension behavior 2.1 Low-consistency flows The unique characteristics of pulp suspensions in pipe flow have been reported by many authors (1-9), and will only be touched upon here. The ability of fibers to entangle and form a network dominates the physics of pulp suspension flow. The fibrous network causes high head losses at low velocities,sometimes leads to plugging especially in contracting channels or small passages, and entrains air bubbles.The behavior of the pulp suspension depends strongly on consistency and flow rate within a given pipe. Loosely following Duffy et al. (2), several common effects are discussed in terms of Fig. 1, which is a typicallogarithmic head loss-velocity curve for a low-consistency pulp suspension. In the region fromA to B, plug flow of the fibrous

-

B

Pulpcurve

516

Pulp Bleaching

-Principles

network occurs. Near or slightly beyond B, at a higher velocity, a clear annulus of water with laminar flow may form around the plug; the annulus tends to be thin, typically less than a fiber length. (In some short-fibered or mechanical pulps, the maximum at point B may be suppressed.) Near C, turbulence in the annulus is apparent, and the fibers still form a plug in the center. The plug is increasingly disrupted and begins to shrink at some point between C andE. At point D, the pressure drop in the suspension is the same as in pure water at the same liquid velocity. This marks the onset of drag reduction for, at higher velocities, the friction losses are less than for pure water in spite of the higher apparent viscosity of the suspension.The point of maximum drag reduction occurs at point E. Increases in velocity continue to disrupt the plug until the flow is fully turbulent, perhaps at point E Drag reduction still occurs although the degree of drag reduction tends to decrease as velocity increases further. Details of the head loss curve for pulp suspensions can vary widely depending on fiber properties, slurry concentration, and even configuration of the flow loop used in the measurements. The behavior of a pulp suspension is closely related to the network strength of the flocs. A useful parameter is 'td,the wall shear stress at the point where the pulp frictionallosses and the water frictional losses are equal (point D in Fig. I). This factor is a measure of the stress required to disrupt the network. Moller (10) found that different sets of data for a given pulp type can be collapsed onto a single curve if the data are plotted in terms of a dimensionless pressure loss term,

(~)D c

I

I I I I Water curve (turbulent

flow)

Log velocity Fig. 1. ~of'-"IossCllroesftwWtllertmil II /IfIlp SfU/IeIISUm.

and Practice

[1]

4'td compared

with a dimensionless

( Y'PZI1 )

velocity tenn,

average velocity, p is the liquid density, and Jl is the viscosity. Methods for estimating friction losses and for optimizing pipeline design are welldescribed in several recent references (J 1-16). Pumping operations in the bleach plant tend to fall in the range of regimes A to B, though flows in static mixers or in centrifugal pump impeller zones may be in the C to E regime. Details of flow behavior are related to characteristics of the pulp fibers themselves, meaning that flow properties vary with species (especially between hardwoods and softwoods), pulping method (e.g., kraft compared with mechanical pulping), the degree of refining, and the bleaching process (if any). In designing a hydraulic system using a centrifugal pump, data are rarely available for the exact pulp-type and flow conditions to be used. Typically, friction data for a similar pulp-type are used (similar origin, similar pulp processing and treatment), and correction terms are used to account for differences in temperarure,refming,consisteney, f1l1er, and other factors. Published data are usually obtained for flow in long, straight runs of pipe, but the actual application includes elbows, valves, and other departures from fully developed flow conditions. There is a lack of basic data on the effects of these perturbations, and general rules used to estimate their effect on pressure loss may be significantly in error. For example, elbows increase the pressure drop in Newtonian fluids; but, in some cases of pulp flow, elbows may decrease the pressure drop by disrupting part of the plug when plug flow is occurring. In general, predictions of pressure losses and power requirements in the pulp flow systems in a bleach plant may be in error by 25% or more. The calculations made by various vendors may offer widely different predictions about the power and even pump size required for a system. 2.2 Medium-consistency

1/6

[2]

't3dD

where .1PIL is the pressure drop per unit distance, D is the pipe diameter, V is the

flows

The frictional resistance of pulp flowing at a given velocity increases strongly with consistency. In the medium-consistency range (8-18%), the strength of the fibrous

Chapter VI I: Pulp Pumping and Hydraulics NPSH

=Z + (h, -h..) -(ht, + h)

[4]

where Z is static head in the suction line h is the pressure above the li q uid level 'vp h ' i~ the vapor pressure of the liquid, hIs is the friction loss in the suction line, and hi is the head loss at the pump inlet, all expressed in equivalent height of fluid. This value must be greater than the pump manufacturer's specified net positive suction head required (NPSHR) or, for positive displacement pumps, the net positive inlet pressure required (NPIPR). NPSHR is a function of the pump type, pump speed, and the fluid properties. If the NPSH is less than the NPSHR when a pure fluid is pumped, cavitation occurs in the pump, lowering the pump capacity, possibly damaging the pump, and causing excessive vibration'in the pipeline. For air-containing stock (dissolved air as well as air bubbles), cavitation in the classic sense (sudden vapor collapse creating shock waves) is reduced or may not occur (23). Instead,-Iarge air and vapor bubbles fill the impeller and cause flow reduction or air binding. The presence of air greatly affects the NPSHR. Manufacturer recommendations are typically based on trials with pure water and may not apply well to pulp suspensions. NPSHR is determined empirically by the manufacturer. The Hydraulic Institute has established useful standards for the determination of NPSHR (24). Operating above the NPSHR typically does not mean that cavitation is completely absent: it may take 2-20 times the head given by the NPSHR to completely suppress cavitation bubbles, but the degree of cavitation occurring for heads above the NPSHR is usually harmless and does not reduce the available pump head by more than 5% (25). However, for some high-energy pumps, maximum cavitation damage may occur at suction heads of two to three times the NPSHR. Performance curves Performance curves for centrifugal pumps can show the relationships between pump capacity and efficiency, horsepower, NPSHR, and other factors. Performance curves are typically determined for a spe-

519

cific pump speed, impeller diameter and width, and fluid viscosity. Typical performance curves for one pump speed are shown in Fig. 5.A given piping system also has a characteristic curve of head loss against flow rate.This system curve can only be modified by changing system head loss (adjusting valves) or by changing the consistency within the system (e.g., by dilution). The intersection of the system head loss curve with the pump head curve gives the flow rate and head loss through the system. Pump suppliers may provide performance curves that show the effect of impeller size and pump speed to assist in the choice of pump operating conditions.

520

I

Flow rate RI.5. '/)JIIaIlmfIrl./ilpl JHItIIP~~. Centrifugal pumps tend to be inexpensive, reliable, and easily repaired. They are the mainstay of most processes in the chemical process industries, including the pulp and paper industry. Centrifugal pumps cannot perform well when large amounts of gas are in the system unless some strategy is applied to remove the gas. likewise, startup requires that the pump be primed with liquid to create suction to pull in fluid into the impeller. Self-priming pumps, which must be IDled with fluid before the first operation, are available.

-Principles

and Practice

3.2 Positive-displacement pumps Positive-displacementpumps mechanically open a volume to suction, increase the volume to take in flow, seal off the volume, then displace it to the discharge. Pressure is created as a system response to the motion of the discharged flow and any static head on the discharge. Two types of positive-displacement pumps have been used in pulp handling. The reciprocating plunger pump was used for many years for pulp up to 6% consistency. These pumps have valves and were thus limited to low-consistency application where valve plugging was less likely. The other type, rotary positive-displacement

(a)

'U as Q)

Pulp Bleaching

pumps, are still widely used. These include lobe, gear, single-screw, and two-screw types. Figure 6 shows examples of twinscrew and gear pumps that can be used for pumping medium-consistency pulp. Standards and definitions for rotary positive displacement pumps have been provided by the Hydraulic Institute (26). Positive displacement pumps can operate with an open suction (no head of stock at the inlet) or with a suction head. Normally, pumps are run at a fixed speed or volumetric displacement rate slightly above the average rate of flow from the source to allow for normal fluctuations. The self-priming action of the pump displaces whatever

Chapter VI I: Pulp Pumping and Hydraulics 50

E ~20

8

~ 10 o o

2

4

6

8 10 12 14 Consistency, %

16

18

20

Fig. Z. '1'ypkiI1 ,,'r C08tM1s of pulp.

network imparts a high-yield stress to the pulp and allows large volumes of air to be entrained (17). The high-flow resistance and the increased air content prohibit the use of conventional centrifugal pumps f9r medium-

222- Trichloroethanol

6-Chlorovanillin

°"

1.1.Dichlorodimethyt

Chloroform

a

xx

oyya

al#o~a sulfone

Type of Chlorinated

eomptnmdsformed

in bleaebtng.

Number Reported

Compound

Acids, esters, anhydrides,

furanones,

2.3.7.8- Telrachlorodibenzo-p.dioxin

Aldehydes

and ketones

75 25 20

Hydrocarbons Alcohols Dioxins and furans Miscellaneous

Total few compounds, for example, 1, l-dichlorodimethyl sulfone, do not appear to be markedly affected by the change from conventional to ECF bleaching (32,89). Only trace quantities of a few polychlorinated (> 2 chlorine atoms) compounds are found in ECF effluent. The level of chloroform drops from > 300 g/t of pulp in conventional bleaching effluent to < 10 g/t in ECF effluent (104). As stated in the introduction, individual compounds have not been identified in TCF effluent.

Process for 4. Cook, R.A., "A Bleaching Mlnimising AOX Discharges," 1992 TAPPI Environmental Conference Proceedings, TAPPI PRESS, Atlanta, p.l 055. 5. Hise, R.G., Streisel, R.C., Bills, A.M., "The Effect of Brownstock Washing, Split Addition of Chlorine, and pH Control in the CStage on Formation of AOX and Chlorophenols during Bleaching," 1992 TAPPI

Environmental

Conference

Proceedings,

TAPPI PRESS, Atlanta, p.1135. Fig. 3. Examples

of various

types of eblorluted

eom}HnInds formed

in bleael1tng.

77 52 66

pyrones

Phenols and phenol ethers

3. Allison, WA., McFarlane, P.N., Judd, M.C., Pap. Puu 75:234 (1993).

.y CHO °

Tule 10. TyJ1esof eblorluted

2. NCASI,Effects of Chlorine Dioxide Substitution on Bleach Plant Effluent BOD and Color, National Council of the Paper Industry for Air and Stream Improvement, New York, Tech. Bull. No. 630, 1992.

3-ChI0r0-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone

II ~CHSC~

Pulp Bleaching - Principles and Practice

1. Kutney, G.W, Holton, H.H. ,Andrews, D.H., du Manoir, JR., Donnini, G.P., Pulp Pap. Can. 85(5):195 (1984).

c,~k

CI3CCOOH

762

References

.yZ. Tnchloroacetic

761

6. Liebergott, N., van Lierop, B., Kovacs, 1., Nolin,A., TappiJ 73 (10):207 (1990).

15 330

7. Senior, D.T. and Hamilton, J, Tappi J 76(8):200 (1993) and references therein. 8. Liebergott, N., van Lierop, B., Fleming, B.I., "LoweringAOX Levels in the Bleach Plant," 1992TAPPI Environmental Conference Proceedings,TAPPI PRESS,Atlanta, p.l065, and references therein. 9. Harden, H-L., and de Sousa, Papperstidn. 80:201 (1977).

E, Svensk

10. Howard, 1.E., and Walden, c.c., Pulp Pap. Mag. Can. 72(1):73 (1971). 11. McCubbin Eng,N., Tbe Basic Tecbnology of the Pulp and Paper Industry and Its Environmental Protection Practices, Beauregard Press, Ottawa, EPS 6-EP-83-1, 1983. 12. Liebergott, N., van Lierop,B., Garner,B.c., Kubes, G.J, Tappi J 67(8):76 (1984). 13. Liebergott, N., van Lierop, B., Nolin, A., Faubert, M., Laflamme,J, Pulp Pap. Can. 92(3):T70 (1991). 14. Pfister, K. and Sjostrom, Papperstidn. 81:195 (1978).

E., Svensk

15. Liebergott, N., and van Lierop, Pap. Can. 87(8):T3oo (1986).

B., Pulp

16. Wong, A., Le Bourhis, M., Wostradowski, R., Prahacs, S., Pulp Pap. Can. 79(7):T235 (1978). 17. Germgard, D., Karlsson, R-M., Kringstad, K., de Sousa, E, Stromberg, L., Svensk Papperstidn. 88:R1l3 (1985) and references therein. 18. Shroyer, N.C. and Trimble, D.S., in NCASI Technical Workshop - "Effects of Alternative Pulping and Bleaching Processes on Production and Biotreatability of Chlorinated Organics," National Council of the Paper Industry for Air and Stream Improvement, New York, Special Rep. No. 94-01, 1994.

Chapter VIlli: Effluent Characteristics and Composition 19.

Graves,lw.,Joyce,T.W,Jameel, ) 76(7):153 (1993).

20.

Schwantes, T.A. and McDonough, T.A., "CharacteriZation of Effluent Fractions From ClO, and CI, Bleaching of Unbleached and 0, Bleached Softwood Kraft Pulps; 1993 TAPPI Pulping Conference Proceedings,TAPPI PRESS,Atlanta,p.17.

21.

du Manoir, J.R., 83(2):T58 (1982).

22.

Rapson, H., Anderson, C. 8., Reeve, Pulp Pap. Can. 78(6):T137 (1977).

23.

Fasten H.,"TCF Pulp atASPA - History and Future; 1993 Non-chlorine Bleaching Conference Proceedings, Miller Freeman, San Francisco, 1993, presentation No. 18.

24.

Pulp

H., Tappi

Pap.

38.

Renberg, L., "The Use of Cost-effective Chemical and Biological Tests for the Estimation of the Environmental Impact of Bleaching Plant Effluents; 1992 TAPPI Environmental Conference Proceedings, TAPPI PRESS,Atlanta, p. 317.

73.

Ekman, K.H. and Lindberg,J.J.,Pap.Prm 48:241 (1966).

54. Jokela, lK. and Salkinoja-Salonen, M., Environ. Sd. Tee/mol. 26:1190 (1992). 55.

Roy-Arcand, L. and Archibald, Res. 27(5):873 (1993).

ES., Wat.

56.

Sameshima, K., Simson, B., Dence, C.W, Svensk Paperstidn. 82:162 (1979).

Stuthridge,T.R., Campin, D.N., Langdon, A.G., Mackie, K.L., McFarlane, P.N., Wilkins, A.L., Wat. Set. Teebnol. 24(3/ 4):309 (1991).

74. Jokela,lK., Laine, M., Ek. M., SalkinojaSalonen, M., Environ. Set. Tecbnol. 27:547 (1993). 75.

Dahlman, 0., Reimann, A., Stromberg, L.M., Morek, R., "On the Nature of High MolecularWeight Effluent Materials from Modem ECF- and TCF-Bleaching; 1994 International Pulp Bleaching Conference Preprints. Tech. Section, CPPA, Montreal, p.123.

58.

Osterberg, Holzforscbung

59.

Pfister, K. and Sjostrom, E., Svensk Papperstidn. 81:195 (1978).

76.

60.

Pfister, K. and Sjostrom, 61:619 (1979).

Gierer,}.. Wood Set. Teebnol. 20:1 (1986) and references therein.

77.

61.

Hardell, H.-L. and de Sousa, Papperstidn. 80:110 (1977).

Morek, R., Yoshida, H., Kringstad, K.P., Hatakeyama, H., Holzforsebtmg 40:51 (1986).

62.

Hardell, H.-L. and de Sousa, E, Svensk Papperstidn. 80:201 (1977).

78.

Manson. P. and Oster, R., Nord. Pulp Pap. Res.) 3:75 (1988).

63.

Dorica,lJ.PuIpPap.Set.18:J231

79.

64.

Wilson, R., Swaney,}.. Pryke, D.C., Luthe, C.E., O'Connor, B.I., Pulp Pap. Can. 93(10):T275 (1992).

Kempf, A.W. and 53(5):864 (1970).

65.

McKague, B. and Reeve, D.W, Environ. Set. TecbnoL 28:573 (1994).

Gelierstedt,G.and Lindfors,E.-L.,"On the Structure and Reactivity of Residual lignin in Kraft Pulp Fibres; 1991 international Pulp Bleaching Conference, SPCI, Stockholm, p. 73.

42.

Connors,WJ., Holzforschung

66.

McKague, A.B., Kang, G., Reeve, D.W, Nord. Pulp Pap. Res.) 9:84(1994).

80.

43.

Concin, R., Burtscher, Cbromatogr. 198:131

67.

McKague, A.B., Reeve, D.W., Xi, F.,Nord. Pulp Pap. Res.) 10:114 (1995).

44.

Lundquist, K. and Wesslen, Cbem. Scand. 25:1921 (1971).

68.

Sagfors, Tecbnol.

McKague, A.B., Jarl, M., Kringstad, K.P., "An Up-to-date list of Compounds Identified in Bleaching Effluent as of January 1989; SSVL Miljo 90, Project Bleaching, Stockholm, 1988. Report available from AF-IPK, Box 8309, 104-20, Stockholm, Sweden.

45.

Kristersson, P., Lundquist, R. Simonson, R., Tingsvik, K., Holzforscbung 37:51 (1981).

69.

PeIIinen, l and Salkinoja-Salonen Cbromatogr. 322:129 (1985).

81.

Suntio, L.R., Shiu, W.Y., Mackay, Cbemospbere 17:1249 (1988).

46.

Stenlund, (1976).

70.

82.

Kuokkanen, T. and Koistinen,J., sPhere 19:727 (1989).

Cbemo-

0 Connor, B.I., Kovacs, T.G., Voss, R.H., Martel, P.H., van lierop, B. Pulp Pap. Can. 95(3):T99 (1994).

47.

Sarkanen, McCarthy, (1981).

Ym, C.E,Joyce, T.W, Chang, H-m., "CharacteriZation and Biological Treatment of Bleach Plant Effluent; 1989 Industrial Waste Conference Proceedings, Lewis Pub\., Chelsea, MI, p. 747.

83.

Rantio,T.,

(1992).

84.

Hall, E.R., Cornacchio, 9O(11):T421

Fraser, J., Garden, S., L-A., Pulp Pap. Can. (1989).

48.

Lindberg,J.J.,

Stuthridge, T.R., Wilkins, A.L., Langdon, A.G., Mackie, K.L., McFarlane, P.N., Environ. Sd. Tecbnol. 24:903 (1990).

49.

Gross, S.K., Sadamen, K., Schuereh, Anal. Chem. 30:518 (1958).

85.

Lunde. A., Skramstad,}.. Carlberg, Pap. Puu 73:522 (1991).

Loutfi, H., "Mill Experience with the Optimization of the D ,ooEopStages; 1993 Non-chlorine Bleaching Conference Pr0ceedings, Miller Freeman, San Francisco,

50.

Lindstrom, T., Colloid. 257:277 (1979).

86.

51.

Kirk, T.K., Brown, W., Cowling, Biopolymers 7:135 (1%9).

E.B.,

Rosenberg, C., Aalto, T., Tornaeus, J.. Hesso, A., Jappinen, P., Vainio, H.,) Cbromatog. 552:265 (1991).

87.

PeIIinen,l and Salkinoja - Salonen, Cbromatogr. 328:299 (1985).

M.,]

Petersen-Bjergaard, S., Asp, T.N., Vedde. l, Carlberg, G.E., Greibrook,T., Cbromatograpbia 35:193 (1993).

Gall, R.J. and Thompson, 56(11):72 (1973).

EH.,

Tappi

McCubbin, N., Edde, H. , Barnes, E., Folke, J., Bergman, E., Owen, D., Best Available Tecbnology.for tbe Ontario Pulp and Paper Industry, Queen's Printer for Ontario, Report PffiS 1847,1992.

28.

Axegard, (1986).

29.

Axegird, (1989).

P., Pulp Pap. Can. 90(5):T183

30.

Axegird,

P., Tappi)

31.

Marwah, N., Joyce, T.W., Chen, c.-L., Gratzl, J.S., Tappi) 75(7):167 (1992) and references therein.

P.,)

presentation 35.

Tappi)

53.

Pulp Bleaching - Principles and Practice

Lindstrom, K. and Osterberg, HolzjOrsebung 38:201 (1984).

Rush, R.l and Shannon, E.E., Review of Colour Removal Tecbnology in tbe Pulp and Paper Industry, Minister of Supply and Services Canada, Ottawa, Report EPS 3-WP-76-5,1976.

34.

D.W.,

764

57.

26.

33.

Earl, P.F. and Reeve, 72(10):183 (1989).

D.,

Voss, R., Wearing, J.T., Wong, A., Pulp Pap. Can. 81(12):T367 (1980).

32.

OECD, "OECD Guidelines for Testing of Chemicals; Organisation for Economic C