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mvmm111 process machinery

Grinding Mills

li'N:·f·I§i·]l// grinding mills

Table of Contents Introduction Nordberg grinding mill types Grinding circuits Charge volume Calculating motor and mill size Selection of optimum size grinding media Nordberg rod , ball and pebble mills Nordberg mill design technology Nordberg mil l drives Nordberg mill heads Process machinery mineral research and test center Nordberg ball mill dimensions and weights Nordberg rod mill d imensions and weights Warranty Other Rexnord process machinery

Page 3 4 6 7 8 14 16 18 20 22 23 24 26 27 27

Introduction Grinding is one phase of comminution or size reduction , crushing is another. There is no definite boundary between the two. In many cases, either method may be used. The term " grinding," as used in this bulletin, refers to reducing the size of material by tumbling it, together with suitable media, in a horizontal rotating cylinder. The media may be steel rods, steel balls, or pebbles of flint, ceramic, alumina or rock. Keeping the mill and media in motion requires energy; part of this energy is used to perform the usefu I work of breaking the material which surrounds the media. Goal: Maximum Profits This method of size reduction is statistical in nature. The feed material will have a particle size distribution which normally follows a certain pattern. When this material is fed to a m iII, there is a probability that any given piece will be broken up. In designing grinding circuits for Nordberg mills, Rexno rd engineers select conditions which increase the probabilities of breaking particular sized particles to produce the desired product size distribution. This must be accomplished at an optimum combination of capital and operating costs. These costs, together with expected productive availability and total service life, comprise the factors which determine grinding machinery's contribution to profit. Rexnord's design and manufacturing goals are to produce grinding mills which will return maximum profittothe customers' operations.

Meeting Your Changing Needs Nordberg process machinery has been known for quality since 1896. Nordberg grinding mills are designed and manufactured with skills developed through years of experience, to meet specified operating conditions in the manufacture of cement, and in reduction of metallic and non-metallic minerals, and other materials. No one is better equipped than Rexnord to produce grinding mills of any size, to increase production most profitably. Modern computeraided design techniques, our own advanced foundry, plus streamlined fabrication and machining facilities, all insure the same high quality in today's giant Nordberg mills as in yesterday's smaller models.

Pre-Tested Answers In order to help users select the optimum type and size of grinding machinery for particular applications, Rexnord maintains the Rexnord Process Machinery Mineral Research and Test Center. These facilities, and the abilities and know-how of a professional research staff, are available to every potential purchaser of Rexnord Process Machinery (see page 23).

3

lt't.Jt·/.I&:·111/ grinding mill types

Feed and desired product size distribution are usually the main criteria in selecting the type of grinding mill for a particular application.

Rod Mills Rod mills can accept feeds up to approximately 2", depending on material hard ness. In most cases, an ideal rod mill feed will be minus %" with about 80% passing %". Rod mills are used to produce products with maximum sizes from 4 mesh to as fine as 35 mesh. Since the rods in the mill are in line contact, they tend to grind largest particles first. Thus the product has a relatively narrow range of size distribution, with minimum tramp oversize or extreme fines. Figure 1 shows the various types of rod mill discharge arrangements. Overflow discharge is the most common and is used extensively in mining applications. The grinding is done wet and the mill is used to reduce a crushing plant product to a size suitable for ball mill feed. End peripheral discharge is used primarily for dry grinding when a relatively fine products is required. It can be used for wet grinding but it offers no advantage, as the product size is approximately the same as an overflow mill. Peripheral discharge gives a high gradient and good flow rate with dry material. Center peripheral discharge is used for wet or dry grinding when a coarse product is desired. The mill is fed from both ends, resulting in a short grinding length and high gradient. The discharge is very rapid, reducing the production of extreme fines. Th is type of mill is used principally in the aggregate industry for the production of sand.

4

Rod mill design is subject to several limitations. Rod lengths should be at least 20% greater than the mill's inside diameter. Otherwise, mill rotation may cause the rods to upend and become tangled. Operating speed of the mill should be no more than 70% of its critical speed* and preferably in the 60% to 68% range. The ideal speed allows the rods to roll on the downrunning side of the charge rather than being thrown across the mill.

Ball Mills Ball mills are normally used for fine grinding, where product topsize ranges from 35 mesh to 10 micrometers. In the mining industry the most common flow-sheet- often called conventional grinding - is made up of crushers, rod mills and then final grinding in ball mills. In the cement industry, and in some mining applications, ball mills do primary grinding of crusher products. As a general rule, hard ores should not be coarser than 80% passing %", and soft ores or cement clinker should not be coarser than 80% passing 1", when fed to a ball mill. Larger feed is undesirable because the larger balls required to break the topsize cause high wear and loss of efficiency. Figure2 illustrates the various ball mill discharge arrangements. As with rod mills, the most common discharge arrangement is trunnion overflow. This type of mill operates wet. Its major advantages are simplicity of design and easy access for inspection and replacement of liners. Low or intermediate level diaphragm discharge can be applied to either wet or dry grinding. This type of discharge results in a higher gradient than overflow discharge, which enables dry material to flow through

the mill. The discharge lifters (pans) raise the ground material and direct it to a center cone and into the discharge trunnion. In effect, the mat&rial is pumped out of the mill. A ball charge level between 45% and 50% of mill volume can be obtained without danger of ball spillage. Multiple compartment ball mills are used when the application calls for very high reduction ratios. This allows use of one very long mill rather than multiple units, which require extra floor space and auxiliary equipment. The mill is divided into two or more compartments so that the ball charge for each compartment can be sized to achieve the most efficient grinding based on the size of material entering the compartment.

Ball mills are extremely flexible in both geometry and operating speeds. Mills with lengths between three and five times their diameter are usually selected for applications where surface area of the product is critical and a high recycling load is not desirable. On the other hand, mills with lengths between one and two times their diameter are usually selected for applications where it is desirable to have the product size _ predominantly in a narrow intermediate range, such as is required for liberation of mineral grains from gangue. In this instance, the classifier is expected to remove finished material as soon as possible, and a recycling load of 200% to 300% of the new feed rate is desirable. Ball mills have been successfully run at speeds between 60% and 90% of critical speed*, but most mills operate between 65% and 75% of critical speed.

Pebble Mills Pebble mills, or secondary autogenous mills, are used predominantly for secondary grinding in installations where low media costs and liner wear are of prime consideration, and the material being ground is capable of forming a sufficient quantity of suitable pebbles. Pebble mills are similar to diaphragm discharge ball mills in nearly all aspects of design. They are usually run at speeds between 75% and 85% of critical speed. Pebble mills are considerably larger than ball mills for

Rod Mills

the same horsepower input. The power drawn by any mill is proportional to the bulk density of the charge, and since pebbles have a lower bulk density than steel balls, the grinding chamber of a pebble mill must be proportionally larger.

*Critical Speed: A grinding mill's critical speed is the lowest rpm which will cause an infinitely small particle on the shell liner to centrifuge. This is expressed by the following equation: Critical Speed (in rpm)

=

D

=

Internal diameter of mill in feet, measured inside shell liners

Example: a mill measuring 11 '0" diameter inside of new shell liners operates at 17.3 rpm. Critical speed is: C.S.

76.63

=

j

=

23.1 rpm

11

Mill speed expressed as a percent of critical speed is:

76.63

17.3 x 100 23 .1

= 75% Critical Speed

J1)

Ball Mills

Overflow Discharge

Overflow Discharge

End Peripheral Discharge

Intermediate Level Diaphragm Discharge

Center Peripheral Discharge

Low Level Diaphragm Discharge

Figure 1

Figure 2 5

Ut.J:.l.XM·111/ grinding circuits

The majority of materials must be ground when either wet enough to form a slurry, or else completely dry. Grinding in the moist state is difficult, and requ ires extremely high power. Wet grinding is widely used in the mining industry because most subsequent processes such as flotation, leaching or magnetic separation are done wet. In areas where water is scarce or where a dry process follows, grinding is done dry. In cement plants, all finish (clinker) grinding is done dry because of the nature of the material, but both dry and wet processes are used for grind ing the raw mix. Some considerations:

Wet grinding requires - less power per ton of material than dry grinding; - less space than dry grinding, and in general installation costs are less for a closed circuit operation; - no elaborate dust control equipment; - large quantities of water, and pump maintenance can be high.

Dry grinding requires -

feed material low in moisture content or artificially dried; less media and liner material per ton of material ground than wet grinding; no costly filtering and drying of the product; economical, simple product storage; however, in some cases the product must be cooled before further handling.

Open Circuit In open c ircuit grinding, the mill receives feed and grinds to the desired product size in one pass. Only rod mills offer intrinsic control of product top size in open circuit grinding. Depending upon feed and product sizes, open circuit grinding requires more power than closed circuit grinding (see page 9) . Open circuit grinding is usually applied to: 1. Most rod mills. 2. Wet cement raw mix. 3. Grinds where sizing costs would make a closed circuit process uneconomical. 4. Operations where tramp oversize is permitted and extreme fines are tolerable.

6

Closed Circuit In closed circuit grinding the mill discharge is fed to a sizing device and the overs ized material is recycled to the mill. The particles make several trips through the mill before being ground to final size. The recycled material is termed circulating load and is referred to in percent of new feed. Closed circuit grinding usually requires the least horsepower except for rod mill applications. The product size is controlled by the sizing device; the mill does not control the circu it. Since there is a controlling device in a closed circuit operation, this system lends itself to instrumentation and automation. This circuit requires less operator skill to give a constant product size analysis. Closed circuit grinding is applicable to most grinding systems, except as noted under open circuit grinding.

charge volume

The charge volume (l evel) of a grinding mil l is usually expressed as a percentage of the volume with in the liners that is filled with grinding media. When the mill is stationary, the c harge volume can be obtained by measuring the diameter inside the liners and the d istance from the top of the mill inside the liners to the top of the charge. Measurements should be made at several poin ts down the mill length to obtain the average charge volume. The percentage load ing or charge volume can then be read off the graph in Figure 3, or can be approximated from the following equation: % Loading= 113- 126 H

o

where H is the distance from top of mill inside the liners to top of charge and 0 is the mi ll diameter inside the liners. Figure 3 can also be used to determine the volume of the charge. The area of charge can be determined from the curve ND 2 or the following equation:

mill or req uired to charge the mill can easily be ca lculated from the bul k density of the med ia.

In rod mil ls, the charge is swollen by particles of feed which separate the rods. If the mill is shut down immed iately after the feed is shut off, the charge level will be greater than if the mill had been "ground out" prior to shutdown. Because of thi s, rod mills are normally operated with 32% to 40% charge by volu me. In operation, this becom es a 40% to 50% charge. However, the c harge bulk density is considerably lower than that of stacked rods.

If the bu lk density of the grindi ng med ia is to be determined experimentally, care should be taken to insure that the smallest di mension of the container is at least 25 times larger than the largest p iece of medi a. Using too small a container will cause errors due to inc reased voids at the sides of the container. A mil l's maximum power d raw is obtained when the charge occupi es 50% of the mil l volume. However, when the charge vo lume is inc reased from 45% to 50% the power draw increase is approximately 1%. Therefore, in most cases, the in crease in media consumption would be greater than the production in crease. As a result, mills are seldom run with charge levels greater than 45%.

Ball mil l charges become measurably swol len only wh en there is a bui ldup of large unground material in the bal l mil l or wh en the density of the pu lp in a wet mill is extre mely high. Althoug h these conditions are sel dom encountered, it is recommended th at ball mills be ground out prior to shutdown for measu rement of the c harge level.

Fig. J

AI D' C/ 0

HI D

r:4 0

2

ND = % loading x 11 400

CENT ER OF G RAVITY

I--..

. 00

.35

Multiplying the area by the length will give the cubic feet of mill volume occupied by the charge. The weight of the grinding med ia in the

·'

Read value on curve C/ 0 multiply by 0 Ellample: Loadmg 40",;,. 0 =- 14.5 Ft _(15'·0 I.D Shell) Curve C/ 0 reads .255 x 14.5= 3.69751t.=C

........._

t--, ........... "-... ['..

5

""

.30

.2 0

...........

AREA OF CHA RGE Read value on curve A/ fY. mulliply by 0 ' Example: Loadmg 40~. 0 -= 145 Ft (15 -a ~ 1.0 Shell]

r--....

r--... r---..

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

~

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

.........

.25

~

!'--..

.2 5

20 .3

or--

" .3 5 1 - -

1--

Curve

~'

""""

.........

A/ 0~

1t

14

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

700

r--... ............

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

800

23 Sq Fl = A

750

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

H';;; .........

5 ~= 66

............ ~0

A/ 0~

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

............ r......

"'

650

['...... ............

r-.._

TOP OF" C HARGE TO TOP OF MILl Read value o n curve H / 0 , multiply by D

E•ample: Loadmg 40,_, 0 = 14.5 Ft.

Charge

reads .315

Curve! H/ 0 rets 5 7~ r 14,i = 8 39

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

........

""

r-=H

.4 0

600

...........

I'- ........ !'--..

"-,.

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

5 50

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

.................. 500 14

16

18

20

22

24

26

28

30

32

34

36

38

40

42

44

46

48

50

% MI LL VOLUME

Figure 3 7

IN·Wf.lat·il/1 calculating motor and mill size

-----------------------------------------------------------~ Determining Motor Horsepower Requirement The power required to grind a material from a given feed size to a given product size can be estimated from the follow ing equation: W= 1

.34(

1 0 Wi

10 Wi )

~~

Where W = power consumption in hp-hrs/ short ton for a wet-grinding, closed circuit operation with P greater than 70 micrometers. Multiplying W by the new feed rate (in STPH) will give the power requirement at the mill pinion shaft including bearing and gear losses. Motor losses and other drive component losses, such as reducer and clutches, are not included.

Wi = work index, which is the power in kwh/ short ton required to reduce a material from theoretically infinite size to 80% passing 1 00 micrometers. Figure 4 is a table of average work ind ices for various materials.

P =s ize in micrometers of the screen opening which 80% of the circuit product will pass. See Figures 6A and 6B for typical open and closed circuit product analyses. F = size in micrometers of the screen opening which 80% of the feed will pass. See Figure 5 for typical cone crusher product analyses.

Average Work Indices

Bauxite

s

Material

2.4

13.t

3.1

Coal

11 .3

1.6

Work Index Wi

Specific Gravity

s

Lead Ore

11 .3

3.4

Lead-Z inc Ore

11 .4

3.4

Lignite (CoaQ

13.4

1.4

Limestone

10.3

2.7

Molybdenum Ore

12.8

2.7

N1ckel Ore

12.0

3.3 2.7

Coke

21 .0

1.5

Coke Petroleum

74 .5

1.7

Copper Ore

13.0

3.0

Fluorspar

10.0

3.0

Phosphate Rock

10.5

Gold Ore

14.8

2.9

Silver Ore

17.0

2.7

Iron Ore

13.5

3.6

Tin Ore

11 .2

3.9

Hematite

12.9

3.8

Titanium Ore

11 .9

4.2

Magnetite

11 .5

3.9

Uranium Ore

17.0

2.7

Taconite

15.2

3.5

Zinc Ore

12.4

3.7

NOTE: Average Work Indices should on ly be used for init1al feasibility and est1mates. Tests must be performed on representative samples of the material prior to final equipment selection as the work 1ndices for a specifiC material have a w1de vanat1on.

Figure4

Approximate Mill Feed Size 80% Passing From Symons Cone Crusher Topsize of Mill Feed Inches

Open Circuit Crush ing 80% Passing Size Micrometers*

A. The power consumption per short ton, W, will only be correct for the specified size reduction when wet grinding in closed circuit. If the grinding circuit is changed then the power consumption also changes. Letting G =the gross hp-hrs. per short ton, the power consumption for other circuits becomes:

2

32 .300

1:V.

28.400

1!1,

24.000

1Y.

20.200

1

16.300

Closed Circuit Crush ing 80% Passing Size Micrometers*

Ys

14.250

:v.

12.200

12.200

Y.

10.150

10.150

v, y,

8.100

8.100

.-:-

6.100

Y.

3.800

4 Mesh

3.300

6 Mesh

2.400

NOTE: These average table values will vary with the method of feeding, selection of crusher cavity, the weigh' cleanliness and moisture content of the material and its fracture pattern. Accurate values should be established by actual testing.

Figure 5 8

9.5

Specific Gravity

Cement Clinker

In using the above equation, the following points must be considered:

1. *Wet grinding, open circui~ product topsize not limited: G = W. (Most rod mill applications fall into this category) .

Work Index Wi

Material

'1 1

10' Meter 25.400 Micrometers

M i crometer~ Inch~

'

2. *Wet grind ing, open circuit, product topsize li mited: G = W to 1.25 w. 3. Dry grind ing, closed circu it: G 1.30 w.

Typical Grinding Mill Product Analysis Closed Circuit Operation

=

Sieve Opening in Tyler Mesh 0 0

ln

4. Dry grinding, open circuit, product tops ize not limited: G = 1.30 w.

100

"':

B. The values of P and F must be based on materials having a natural particle size distribution. C. The work index, Wi, should be obtained from plant data or test results, where the feed and product size distributions are as close as possib le to those under study.

I vj J1/

so

VI v Vj

40

w

~

30

~ 20 ~

~ 10

o.n

0 0 .-

1D

00 V

lll M

CD N

0 N

0 -

" 2 ' -6%" 10' -0"

R

3 ' -11 " 2 ' -8W' 4' -1 "

14' -6" 1' -7" 2 ' -11 W ' 5 ' -4"

1'-1 0W' 3 ' -7"

2 '- 11 Y4'

2 ' -1 "

3 ' -11" 2 ' -8 112" 4 '-1"

15'-0" 1' -7" 2'-1H'a" 6 ' -0"

3 ' -1V4'

2'-1 "

3 ' -11 " 2 '-8 W' 4' -1 "

15' -6" 1' -7" 2 ' -11lfa" 6 ' -0"

2'-4"

4 ' -0W' 2 '-8 W ' 4 ' -3"

15' -6" 1 ' -7" 3 ' -2lfa"

s

7'-3" 5' -11 "

460-615

2 ' -8W' 3 '-7"

15' -0" 1'-7" 2 ' -1Hi{ 5'-5"

2 '- 1

HP CD Range

Gear Drive

N

ZJ:

M

2 ' -8W' 3 ' -7"

22.6

ow

L

1' -10 112" 3'-7"

12' -6"

cu..+

K

1' -10W' 3 ' -7"

11 ' -6"

1-:::jiD

J

2 ' -3W'

12' -0"

f.-

H

2 ' -8 Y4'

13' -0"

W.J

G

2' -8Vi' 2 ' -7o/s" 13' -0" 1' -7" 2 ' -1Hil" 5 ' -4"

7'-1 "

Wt/Ft @ Comp. Length

Wt.of @ Rod Chg. Per Ft. of Rod Length

Lbs./Ft.

Lbs.

Lbs./Ft.

A

9,205

220,000

10' -0''

5870

Approx. Mill Wgt. for Max. Length

1.0. Shell

10'-2" 4 ' -0"

2 ' -9W' 10'-6"

7'-6" 5 ' -11 "

640-880

7010

11 ,245

272 ,000

11 ' -0"

10'-2" 4'-2W'

3 '-2v.'' 11 '-0"

7'-6" 5 '-11"

860-980

8050

13,490

318,000

12'-0"

10' -8" 4'-3"

3 ' -2V4'' 11 ' -0"

7'-6" 5 ' -11 "

980-1165

8050

13,490

340,000

12' -0"

10'-8" 4 ' -6"

3 ' -3W' 11 '-6"

8 '-7" 5 '-1 1"

1130-1360

8730

15,935

361 ,000

13' -0"

11 '-2" 4 '-7W'

3'-3W' 11 ' -6"

8 ' -9" 5 ' -11 "

1360-1580

8730

15,935

393,000

13' -0"

9'-5" 6 ' -0"

1465-1740

9860

18,590

459,000

14' -0"

11 ' -2" 4 ' -10Y>" 3 '-5"

12'-0"

11 ' -8" 4 ' -11 "

3 '-5Y•" 12' -0"

9 ' -4" 6'-7V>"

174Q-2015

9860

18,590

498,000

14' -0"

12' -2" 5 ' -0V•"

3 ' -8"

9' -4" 6 '-7¥2"

1970-2300

10,570

21 ,445

502,000

15' -0"

12' -2" 5 ' -4Y>"

3 ' -9v.'' 12' -6" 10' -8" 6 '-1 0W '

2300-2625

11 ,060

21 ,445

540,000

15' -0''

12' -6"

CD Based on 65% Critical Speed & 34% Rod Charge. @ Shown to Enable Wt. Calculation of Various Lengths. E.G. 12' -0" x 16' -6" Net Wt. @ Based on 34% of Mill Volume .

= 340 ,000 - (8050) (19-16.5) = 319,875 Lbs.

The abo v e dimen sio ns ar e a p prox im at ion s Rex n o rd w ill furni s h d e tail e d dimens1on e d draw ing s f o r co n s tr uc ti on p u rp ose s

26

Safety Although safety engineering is an important aspect of all Rexnord products, compliance with safety standards, including OSHA and other Federal, State and local codes or regulations , is the responsibility of the user.Piacement of guards and other safety equipment is often dependent upon the area and circumstances of use. A safety study should be made of the application, and additional guards and warning signs should be installed wherever appropriate. Warranty Rexnord warrants that all products of Rexnord manufacture will be of good merchantable quality, free from defects in material and workmanship and will possess the characteristics represented in writing by Rexnord. In addition, Rexnord certifies that the products will comply with OSHA

Standards in effect at time of order acceptance by Rexnord that relate solely to the physical characteristics and not to the circumstances of use (including noise) of the products. Claim for breach of the above warranty must be made within six (6) months from the date of readiness for operation but not more than twelve (12) months from the date products are ready for shipment, whichever is sooner; however, with respect to any item not manufactured by Rexnord, the warranty shall be limited to that extended to Rexnord by the supplier. Upon satisfactory proof of claim, Rexnord will, within a reasonable time, make any necessary repairs, additions or corrections, or at the option of Rexnord, supply replacement parts free of charge. Purchaser labor costs or other charges for correcting defects or making additions will not be allowed, nor will Rexnord accept products returned for credit unless the return or cor-

rection is authorized by Rexnord in writing. THE FOREGOING IS IN LIEU OF ALL OTHER WARRANTIES, EXPRESS OR IMPLIED, INCLUDING ANY WARRANTIES THAT EXTEND BEYOND THE DESCRIPTION OF THE PRODUCTS. This warranty statement (together with the LIABILITY LIMITATIONS stated herein) sets forth the extent of the liability of Rexnord for breach of any warranty or deficiency in connection with the sale or use of Rexnord products.

Liability Limitations Under no circumstances shall Rexnord be liable for consequential damages of any nature (whether based on contract or tort) including, but not limited to, loss of profits, loss of production, delays or expense, and the liability of Rexnord shall not, under any other circumstances, exceed the purchase price of the products furnished.

STANDARD CONE CRUSHER

FINE ORE STORAGE

PRODUCT TO CONCENTRATOR

27

Rexnord /III/IIIII

Process Machinery Division

Bulletin 463

Rexnord Inc. Process Machinery Division 3073 South Chase Avenue P.O. Box 383 Milwaukee, Wisconsin 53201

Ateliers Bergeaud Macon 41 , Rue de Ia Republique Boite Postale 505 Macon, France 71009

Rexnord Inc. Process Machinery Division Clifton House, 83-89 Uxbridge Road Ealing, London, W.5 , 5TD England

Nordberg Mfg. Co. (SA) (Pty.) Ltd. Process Machinery Division P.O. Box 2253 Johannesburg, South Africa

Printed in U.S.A.

Rexnord Canada Ltd. Process Machinery Division P.O. Box 1330 700 Woodlawn Road West Guelph, Ontario, Canada N1 H 6N8 Nordberg Industrial Ltda. Av. Distrito Industrial SINo ... Vespasiano M.G., Brazil

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