• Author / Uploaded
• Victor Alvarez

• Categories
• Alto horno
• Hierro
• Coca-Cola (Combustible)
• Acero
• Silicio

Citation preview

Iron Production$ FT Mahi, Trinity College Dublin, Dublin, Ireland CL Nassaralla, Michigan Technological University, Houghton, MI, USA r 2016 Elsevier Inc. All rights reserved.

Many materials have been developed and used by society; however, none is more versatile and cost effective than iron. Annually, steel, an iron–carbon alloy, accounts for 93% of the total metal production in the world, which is about five times the total production of composites, plastics, and all other metals. Steel is extensively used in industry because it is inexpensive and has good mechanical properties (such as strength, ductility, high melting point, appealing esthetic appearance, etc.). The low cost associated with the production of iron is mainly due to the following factors:

• • •

iron abundance in the earth’s crust (4.9%); relatively easy reduction to its metallic state; and well-developed technology associated with its production.

Approximately 57.4% of the raw steel produced in the world comes from hot metal produced in blast furnaces; the other 42.6% comes from steel scrap. Because most of the iron produced in the world is in the form of steel, this article will focus on the iron production processes used by the steel industry, i.e., the blast furnace, direct reduction, and novel smelting–reduction processes. Yilmaz et al. (2015) performed a streamlined LCA (raw material transformation to production of ductile iron casting piece) to evaluate the impact of eleven best available techniques (BATs). The life cycle inventory was created using industrial average data for European iron casting industry from the literature. The avoided burdens originating from reduced raw material consumption and reuse of process residues were considered. Since economic factors largely influence methods to be implemented, the most preferable strategies toward cleaner production in iron foundries was determined using a combination of achieved benefits, environmental impacts, and cost of implementation.

Blast Furnace Process The blast furnace is a tall, vertical shaft furnace which has the purpose of heating and reducing iron oxides (hematite and magnetite) into hot metal which is basically a carbon-saturated silicon and manganese iron alloy with residual amounts of sulfur and phosphorus. A schematic representation of a blast furnace and its charged raw materials, products, and by-products is shown in Figure 1.

Raw Materials The raw materials added to a blast furnace can be divided into solids, liquids, and gases. The solid raw materials are essentially iron oxides, metallurgical coke, and flux. The size range of the solid raw materials is strictly controlled to optimize the gas flow and iron oxide reduction rate inside the blast furnace. There are three major iron sources: pellets, sinter, and iron ore. Metallurgical coke is used as the fuel and reducing agent in the blast furnace process. Coke is the product of carbonization (or distillation) of mainly bituminous coal. Flux is also added to the blast furnace charge to absorb impurities present in the iron sources and coke ash. Flux is essentially limestone (or calcium carbonate) and dolomite (calcium and magnesium carbonates). In addition, some steel producers use powder injection coal (PCI) or other sources of hydrocarbon (such as used motor oil) at the tuyeres with the preheated air. The purpose of PCI and hydrocarbon additions is to decrease the coke rate which is defined as the amount of coke needed to produce a ton of hot metal. The reduction of coke rate is desirable because of the hazardous emissions (such as sulfur dioxide and carcinogenic organic compounds) associated with the coke-making process. Preheated air is also injected through the tuyeres to combust the coke and generate a hot and highly reducing gas. This reducing gas acts to reduce and melt the iron oxides. Besides hot metal, the blast furnace generates slag, dust, and blast furnace gas. The slag is sold as aggregate for the concrete industry, as a raw material for the cement industry, and is also used as aggregate for road construction. The dust is recycled in the sinter plant as a source of iron. The blast furnace gas is used to heat boilers or stoves within the steel plant.

Process Description The blast furnace process has auxiliary equipment associated with it. This auxiliary equipment and the gaseous pathways inside a blast furnace process are shown in Figure 2. The blast furnace process can be briefly described as follows. Iron oxides, flux agents, ☆

Change History: July 2015. F.T. Mahi added Abstract and Keywords; expanded text with additional review materials, and updated the list of references.

Reference Module in Materials Science and Materials Engineering

doi:10.1016/B978-0-12-803581-8.03590-6

1

2

Iron Production

Figure 1 Materials balance for a blast furnace.

Figure 2 Schematic cross-section of a blast furnace and auxiliary equipment.

and metallurgical coke are added to a skip car at the stock house. The skip car ascends through a skip bridge to the top of the blast furnace where it is emptied. A system of double bells is used to charge the blast furnace to minimize the escape of blast furnace gases and dust to the environment. As this charge descends through the blast furnace it meets highly reducing and hot gases. The transfer of energy from the hot gases to the solid charge, the reduction of the iron ore into hot metal, and the formation of a slag

Iron Production

3

Figure 3 (a) Physical zones and (b) chemical zones of a blast furnace.

phase (i.e., complex liquid oxide phase) take place. The hot and highly reducing gases ascending through the blast furnace come from the reaction of preheated air with incandescent coke at the tuyere zone (near the base of the furnace). The liquid hot metal and slag are tapped from the blast furnace intermittently or continuously depending on the size of the furnace. Hot metal is tapped into a torpedo car which is transported by rail to the hot metal pretreatment station, and the blast furnace slag is tapped into a slag ladle which is transported by trucks to a slag pit. At the slag pit the slag is water cooled, processed, and sold as aggregate material. The reducing gases leaving the blast furnace are cleaned by passing them through electrostatic precipitators which capture the dust. The cleaned blast furnace gases are used to preheat stoves and boilers within the steel plant. A stove is a heat exchange vessel. Stoves absorb heat from the combustion of blast furnace gases with air and transfer this heat to the air blast that will be injected into the blast furnace tuyeres.

Physical and Chemical Zones of a Blast Furnace A blast furnace can be structurally divided into five regions: shaft, belly, bosh, tuyeres, and hearth as shown in Figure 3(a). However, more important than the physical division of a blast furnace is its chemical division (or chemical zones) and the reactions taking place in each of them. In the 1970s blast furnace dissection studies conducted in Japan unveiled the complexity of its chemical reactions. The blast furnace can be divided in five chemical zones as shown in Figure 3(b).

(a) Stack zone The raw materials are charged as alternate layers of ore and coke into the blast furnace. Therefore, this layered arrangement is retained as these layers move downward, countercurrent to the flow of gases. At this stage the raw materials are preheated, and iron oxides are prereduced. The stack zone can be subdivided as follows: (i) To600 1C. Preheating and indirect reduction of hematite: 3Fe2 O3 þ COðgÞ ¼ 2Fe3 O4 þ CO2 ðgÞ

3Fe2 O3 þ H2 ðgÞ ¼ 2Fe3 O4 þ H2 OðgÞ (ii) 600 1C o T o 950 1C. Indirect reduction of magnetite:

½1

½2

4

Iron Production Fe3 O4 þ COðgÞ ¼ 3FeO þ CO2 ðgÞ

Fe3 O4 þ H2 ðgÞ ¼ 3FeO þ H2 OðgÞ

½3

½4

(iii) 950 1C o T o Tsoftening. Direct reduction of wustite and regeneration of carbon dioxide: FeO þ COðgÞ ¼ Fe þ CO2 ðgÞ

CO2 ðgÞ þ CðcokeÞ ¼ 2COðgÞ

½5

½6

(iv) Also in this region, the decomposition of limestone and dolomite will take place: CaCO3 ¼ CaO þ CO2 ðgÞ

½7

MgCO3 ¼ MgO þ CO2 ðgÞ

½8

(b) Cohesive (or softening) zone (i) The formation of a partially liquid primary slag (fayalite): FeO ¼ FeOðlÞ

2FeOðlÞ þ SiO2 ¼ 2FeO:SiO2 ðfayaliteÞ

½9

½10

(ii) Reduction of wustite and phosphorus oxide (present in the iron ore): FeOðlÞ þ COðgÞ ¼ FeðlÞ þ CO2 ðgÞ

½11

P2 O5 þ 5COðgÞ ¼ 2P þ 5CO2 ðgÞ

½12

CO2 ðgÞ þ CðcokeÞ ¼ 2COðgÞ

½13

Here, and in what follows, underlined elements are dissolved in the hot metal. Because of the softening of the charge the reducing gases can only escape this zone by passing through the coke layers. Therefore, it is essential to use coke in a blast furnace because coke has the porosity and strength necessary to offer as little resistance as possible to the gas flow.

(c) Active coke zone As the liquid metal percolates through this zone it absorbs carbon, silicon, and sulfur: FeOðslagÞ þ COðgÞ ¼ FeðlÞ þ CO2 ðgÞ

½14

Iron Production

5

SiO2 ðcoke ashÞ þ CðcokeÞ ¼ SiOðgÞ þ COðgÞ

½15

SiOðgÞ þ C ¼ Si þ COðgÞ

½16

SiOðgÞ þ SðcokeÞ þ CðcokeÞ ¼ SiSðgÞ þ COðgÞ

½17

SiSðgÞ ¼ Si þ S

½18

The reaction of eqn [18] occurs at the surface of liquid metal droplets.

(d) Raceway (or tuyere) zone The hot blast air enters the furnace through the tuyeres around the top part of the hearth zone. The number of tuyeres can vary from 15 to 40 depending on the size of the blast furnace. The oxygen in the air reacts with the incandescent coke to form carbon monoxide, a very exothermic reaction. Therefore, the temperature at the raceway zone is approximately 2000 1C. The formation of silicon monoxide and silicon sulfide is made possible by the extremely high temperatures in the zone. Silicon and sulfur are transferred to the metal phase via gaseous silicon sulfide and silicon monoxide. Therefore, the higher the temperature at the raceway zone, the higher the silicon and sulfur content of the hot metal: 2CðcokeÞ þ O2 ðairÞ ¼ 2COðgÞ þ ash

½19

SiO2 ðcoke ashÞ þ CðcokeÞ ¼ SiOðgÞ þ COðgÞ

½20

SiOðgÞ þ SðcokeÞ þ CðcokeÞ ¼ SiSðgÞ þ COðgÞ

½21

(e) Hearth zone The hearth is the container for liquid hot metal, liquid slag, and coke. The liquid slag floats at the top of the liquid hot metal because of its lower density. Therefore, as the metal droplets pass through the slag layer some of the silicon picked up earlier by the metal droplets is oxidized by iron oxide and/or manganese oxide present in the slag: Si þ 2MnOðslagÞ ¼ SiO2 ðslagÞ þ 2Mn

½22

Si þ 2FeOðslagÞ ¼ SiO2 ðslagÞ þ 2FeðlÞ

½23

The sulfur content of the metal droplets decreases as they pass through the slag layer according to the following reaction: C þ S þ CaOðslagÞ ¼ CaSðslagÞ þ COðgÞ

½24

These five zones are present in all blast furnaces. However, the size, shape, and location of each zone is determined by the overall thermal state of the furnace, gas, burden distribution, and quality of the raw materials used. The maximum daily blast furnace production is approximately 12 000 t.

Hot Metal Pretreatment The hot metal is usually pretreated before being added to a steel converter to be transformed into steel. The hot metal pretreatment has several benefits:

• • • •

it it it it

is the ideal place in the process to remove sulfur; minimizes the amount of slag formation in the steel converter; decreases refractory wear in the converter; and increases the converter productivity.

There are two types of hot metal pretreatment: (a) removal of silicon, sulfur, and phosphorus, and (b) removal of sulfur. The type of pretreatment to be used will depend on the amount of steel scrap to be added to the steel converter. For example, in the USA the steel scrap charge in the converter varies from 20 to 30% of the total metallic charge. Consequently, US hot metal

6

Iron Production

pretreatment is limited to sulfur removal because phosphorus cannot be removed unless the silicon level in steel is less than 0.15%. The oxidation of silicon to silica during the steel production process provides the heat to melt the steel scrap in the charge. However, in Japan only 5% of the iron units charged to a steel converter is in the form of steel scrap, thus the amount of silicon present in the hot metal is irrelevant. Consequently, Japanese hot metal pretreatment involves the removal of silicon, sulfur, and phosphorus.

(a) Silicon removal (desiliconization) Hot metal desiliconization is accomplished by adding sinter or scale plus lime to the hot metal in the blast furnace runner or in the torpedo car. Silicon dissolved in the hot metal reacts with the iron oxide in the sinter (or scale) and is oxidized to silica according to the reaction: 2FeOðsinter or scaleÞ þ Siðhot metalÞ þ CaO ¼ CaO:SiO2 ðslagÞ þ 2Feðhot metalÞ

½25

After this process the slag is removed and the hot metal is ready for the sulfur and phosphorus removal operations.

(b) Sulfur and phosphorus removal (desulfurization and dephosphorization) The hot metal is dephosphorized and desulfurized with the injection of a moderated oxidizing and highly basic flux mixture. The oxidizing portion of the flux mixture will enhance the phosphorus removal, and the highly basic portion of the slag will enhance both sulfur and phosphorus removal. Examples of typical desulfurizing and dephosphorizing agents are:

• • •

mixture of CaO þ scale (FeO) þ CaF2 þ CaCl2; Na2CO3; and mixture of CaO þ CaF2 þ scale.

After the dephosphorization and desulfurization step is completed, the sulfur- and phosphorus-rich slag from this operation is removed. The pretreated hot metal is sent to a steel production converter for further refining.

Advantages and Disadvantages of the Blast Furnace Process The blast furnace is a well-established technology, a highly productive process, and widely used in the steel production industry. However, the blast furnace process has some disadvantages:

• • •

it requires a high capital investment; the process is inflexible because it takes about eight hours for it to respond to any changes in charge input; and coke is an essential ingredient for its operation.

The coke requirement is one of the biggest drawbacks because of the hazardous emissions associated with the coke production process. The aging coke plants of the USA and stringent environmental regulations have exacerbated the problem. For example, the US Environmental Protection Agency (EPA) ‘2000 Zero Emissions’ has speeded the closure of some of the older coke producing facilities that could not comply with the regulations without a massive investment in their facilities. Many integrated steel facilities in the USA are dependent on imported coke mainly from Germany and Japan. These companies feel uneasy about their dependency and have been trying over the years to find an alternative iron production process to replace the blast furnace process. There are, however, alternative ways to produce iron, such as direct reduction processes, as well as smelting processes such as COREX, AISI/DOE Direct Steelmaking process, DIOS, and HIsmelt.

Direct Reduction Processes Direct reduction (DR) is defined as any process in which metallic iron is produced by the reduction of iron ore at temperatures below the iron melting point. The product of a DR process is called direct reduced iron (DRI). The major consumers of DRI are the smaller steel mills which use DRI to dilute the residual elements present in the steel scrap. The DR processes can be classified into three major categories: (I) those using gaseous reductants in a granular bed, such as Midrex and Hylsa processes; (II) those using gaseous reductants in a fluidized bed, such as the iron carbide process; and (III) those using solid reductants in a granular bed, such as the Fastmet process. Zhang et al. (2014) investigated that the industrial application of fluidized bed direct reduction (DR) process for fine iron ore is hampered by the sticking of direct reduction iron (DRI) particles. The carbon precipitation reaction is coupled with the reduction reaction of fine iron ore to modify the cohesive force among DRI particles. The competition between the reduction reaction of fine iron ore and the carbon precipitation reaction leads to three types of fluidization behaviors: fluidization, unstable fluidization, and

Iron Production

7

defluidization. The carbon precipitation reaction is dominant at the temperature below 600–675 1C, and the presence of H2 could retard the growth of iron whiskers and promote carbon precipitation. A new type of DRI particle covered with carbon shell is therefore constructed and named as DRIc particle. The growth of iron and carbon gasification can destroy the carbon shell, and lead to the increase of stickiness; however, the presence of CO can retard or prevent the destroying. The C/Fe mass ratio on the surface has significant influence on the stickiness and also the fluidization behavior of DRIc particles. The lower limit of C/Fe mass ratio, below which defluidization occurs, increases sharply with increasing temperature. Based on these findings, a two-step fluidized bed DR process for fine iron ore is proposed and proved feasible, and the operating lines of the fluidization zones are indicated as maps. Koo et al. (2015) conducted a research on Ca-promoted Ni/MgAl2O4 catalysts prepared by co-impregnation method with different Ca/Ni ratios of 0.0–1.0, were applied to syngas production for direct reduced iron via the combined H2O and CO2 reforming (CSCR) of coke oven gas (COG). The physicochemical properties of prepared catalysts were characterized by XRD, BET, H2-chemisorption, TPR, and CO2-TPD. The SEM, TGA and TEM analysis was carried out to observe the coke deposition and agglomeration of Ni particle in used catalysts. It was confirmed that Ca addition improved the Ni dispersion, strong metal-support interaction (SMSI), and CO2 adsorption of catalyst. The CSCR of COG was carried out under the reaction conditions of CH4:H2O: CO2:H2:CO:N2 ¼ 1:1.2:0.4:2:0.3:0.3, 700–900 1C, 5 atm. The Ca-promoted 10Ni/MgAl2O4 catalysts show higher CH4 conversion and coke resistance than 10Ni/MgAl2O4 catalyst without Ca addition. In particular, 10Ni–5Ca/MgAl2O4 catalyst with Ca/Ni ratio of 0.5 showed a good catalytic activity and sinter-stability in CSCR of COG at high temperature of 900 1C due to high Ni dispersion and improved SMSI.

Midrex Process Midrex is the most successful gas-based DR process; it is a continuous process. It is basically a countercurrent process where a hot and highly reducing gas (95 vol% of this gas mixture being hydrogen and carbon monoxide with a ratio of H2:CO varying from 1.5 to 1.6) reduces lump iron ore or pellets to metallic iron as the metallic charge descends through the top portion of the vessel. At the bottom third portion of the vessel, the metallic iron is cooled by an inert gas to a temperature below 50 1C before it is discharged from the furnace. The DRI product is unstable in air due to its higher surface area, thus it must be briquetted to decrease its surface area and make it more stable.

Hylsa Process Hylsa is a batch process in which four reactors operate simultaneously, each one at a different stage in the reduction cycle at a given time.

• • • •

Stage 1: cooling stage. DRI is cooled by transferring its heat to a cold reducing gas (72% H2 and 17% CO). The reducing gas leaving the vessel is regenerated by quenching and scrubbing to remove humidity, CO2, and dust. The regenerated gas is then injected into stage 2. Stage 2: primary stage. Wustite (FeO) is reduced to metallic iron. The reducing gas leaving the vessel is once more regenerated by quenching and scrubbing to remove humidity, CO2, and dust. The regenerated gas is then injected into stage 3. Stage 3: secondary stage. Hematite (Fe2O3) is reduced to wustite. The reducing gas leaving the vessel is regenerated by quenching and scrubbing to remove humidity, CO2, and dust. The regenerated gas is then reformed to its initial reducing strength of 72% H2 and 17% CO. Stage 4: charging and discharging stage. The reactor is discharged from the bottom and then charged through the ore loading nozzle on the top of the reactor.

During reduction the lump iron ore or pellets are stationary in the reactor and therefore the risk of sticking is high. A DRI removal tool is used to break the superficial bonds among DRI pieces. The DRI product must also be briquetted to avoid reoxidation.

Iron Carbide Process Iron carbide (Fe3C) is the product of the iron carbide process. It is a stable and nonpyrophoric product, thus it is not required to be briquetted to avoid reoxidation. Iron carbide production exploits the lower cost of ore fines compared to lump ore and pellets. The process consists of feeding preheated iron ore fines into a fluid bed reactor. A gas mixture (CH4, CO, H2, and small amounts of CO2 and H2O) which is at 600 1C and 1.8 atm is forced up through the iron ore fines bed reducing it to iron carbide. The reduction reaction of the process is: 3Fe2 O3 þ 5H2 þ 2CH4 ¼ 2Fe3 C þ 9H2 O

½26

The used gas mixture is quenched, scrubbed, and reformed to be reused in the process. Iron carbide is magnetic so it can be upgraded with magnetic separation.

8

Iron Production

Fastmet Process The Fastmet is a continuous process which basically consists of a rotary hearth furnace where one or two layers of self-reducing iron ore pellets are placed. These self-reducing pellets are made from a mixture of iron ore concentrate, reductor (coal or coke), and binder. Unlike the other processes previously described, the Fastmet process uses a solid instead of a gas to reduce the iron oxide. The pellets travel through the rotary hearth furnace and are heated to 1250–1350 1C by burners placed throughout the length of the furnace. The rapid reduction rate of 12 minutes is attributed to the high reduction temperatures and the close contact of the reductor and the iron oxide particles. DRI produced in this process is also unstable in air and must be briquetted to avoid reoxidation.

Alternative Iron Production Processes These processes focus on eliminating the dependency on coke, increasing process flexibility by decreasing response time, and decreasing capital investment. Of the alternative iron production processes, only the COREX process is in commercial operation. The DIOS, AISI/DOE Direct Steelmaking, and HIsmelt processes have only been tested at a pilot plant scale.

COREX Process COREX is a coal-based hot metal process. The process basically consists of two reactors: a reduction shaft and a melter/gasifier reactor. Lump iron ore, sinter, and/or pellets are reduced by first being metallized in a reduction shaft by off-gases coming from the melter/gasifier reactor. The product consists of approximately 90% reduced iron. The reduced iron is discharged into a melter/ gasifier reactor where residual reduction and melting take place by a highly reducing gas which is obtained from the gasification of coal with oxygen. Hot metal and slag are products of the melter/gasifier reactor.

Other Alternative Processes The other alternative iron production processes, such as DIOS, AISI/DOE Direct Steelmaking, and HIsmelt, are similar when considering that they are divided into two stages: prereduction and smelting. The prereduction of iron ore can be done in a shaft reactor for the Direct Steelmaking and HIsmelt processes, or in a fluidized bed for the DIOS process. In the case of the smelting stage, the DIOS and Direct Steelmaking processes use a reactor similar to a steel production converter. The HIsmelt process uses a horizontal vessel where the coal fines and natural gas are injected into the smelter vessel through tuyeres, and not from the top. The products of these smelting processes are hot metal and slag. The off-gases of the smelt reactor are used to prereduce the iron ore in the reduction stage. Iron demand is, and will continue to be, high. However, the preferred process for producing iron will gradually shift from the blast furnace to a cleaner, cheaper, and more flexible way, such as DR processes and the novel smelting processes.

References Yilmaz, O., Anctil, A., Karanfil, T., 2015. LCA as a decision support tool for evaluation of best available techniques (BATs) for cleaner production of iron casting. Journal of Cleaner Production 105, 337–347. Koo, K.Y., Lee, J.H., Jung, U.H., Kim, S.H., Yoon, W.L., 2015. Combined H2O and CO2 reforming of coke oven gas over Ca-promoted Ni/MgAl2O4 catalyst for direct reduced iron production. Fuel 153, 303−309. Zhang, T., Lei, C., Zhu, Q., 2014. Reduction of fine iron ore via a two-step fluidized bed direct reduction process. Powder Technology 254, 1–11.

Further Reading Bodsworth, C., 1994. The Extraction and Refining of Metals. Boca Raton, FL: CRC Press. Feinman, J., MacRae, D.R., 1999. Direct Reduced Iron: Technology and Economics of Production and Use. Warrendale, PA: Iron and Steel Society. Fenton, M., 1996. Iron and steel Minerals Yearbook Metals and Minerals, vol. 1. Washington, DC: US Geological Survey, pp. 433−446. Peacey, J.G., Davenport, W.G., 1979. The Iron Blast Furnace. London: Pergamon. Poveromo, J.J., 1979. Blast furnace burden and gas distribution. Iron Steelmak 7, 22–34. Turkdogan, E.T., 1996. Fundamentals of Steelmaking. Cambridge: Institute of Materials. Wakelin, D.H., 1999. The Making, Shaping and Treating of Steel: Ironmaking Volume, eleventh ed. Pittsburgh, PA: AISE Steel Foundation.