Direct smelting and alternative processes for the production of iron and steel

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<ul><li><p>INTERNATIONAL JOURNAL OF ENERGY RESEARCH, VOL. 20,1103-1128 (1996) </p><p>DIRECT SMELTING AND ALTERNATIVE PROCESSES FOR THE PRODUCTION OF IRON AND STEEL </p><p>T. ZERVAS, J. T. MCMULLAN AND B. C. WILLIAMS </p><p>Energy Research Centre, Uniuersiy of Ulster, Coleraine, Co. Londondeny, BT 52 1SA U.K. </p><p>SUMMARY </p><p>Processes for the direct smelting of iron and steel are presented together with a discussion of alternatives based on plasmas and the direct production of iron carbide. The advantages and disadvantages of each technology are presented and their energy efficiencies are discussed. </p><p>KEY WORDS: iron production; direct reduction; direct smelting; energy in steel production; iron carbide production; plasma technology </p><p>1. INTRODUCTION Over the past century, considerable research and development effort has been expended in developing methods for producing iron that could substitute for pig iron from blast furnaces (Strassburger, 1969). Much of this work was stimulated by a need to use lower-grade iron ores and non-coking coals that are unsuitable for the coke oven unit in an integrated coke oven and blast furnace plant. This has led to renewed interest in direct reduction and direct smelting processes. Direct reduction processes are those which produce iron by the reduction of iron ore at temperatures below the melting point of the product; the product is referred to as direct reduced iron. Processes which produce a molten product similar to blast-furnace hot metal directly from the ore are defined as direct smelting processes. If the objective is to produce liquid steel directly from the ore, the term direct smelting processes. If the objective is to produce liquid steel directly from the ore, the term direct steelmaking is often used. These definitions are clearly based on the characteristics of the product, though further treatment steps may be added to produce special grades of steel. As has been discussed by Zervas et al. (1996a, 1966b and 1966~1, the coke oven and blast furnace </p><p>integrated plant is expected to continue to be the worlds main source of raw material (pig iron) for steelmaking production, as long as supplies last of suitable coke for the coke ovens, and are available at a competitive price. Currently, direct reduction iron contributes a small portion of total world iron-mak- ing capacity with about 30 million tonnes per year of direct reduction iron capacity having been installed by June 1994 (Scott, 1994). </p><p>The direct reduction iron process has not been commercialized at the expected rate. In 1976 it was estimated that direct reduction iron capacity would reach 45 million tonnes per year by 1985. Instead, plant capacity had only reached 30 Mtonne per year by 1994, as mentioned above. The main reasons have been the lower-than-expected demand for iron in the developing world and the consequent reductions in plant capacity and utilization. The world-wide utilization of direct reduction iron plant has been about 60%. </p><p>Other research and development efforts in the reduction of iron ore have concentrated on the development of the direct smelting of iron. The main effort here is to develop a molten iron-making process that does not depend on metallurgical coal. Many processes are under development in the U.S.A., Australia, Japan and Europe (Fruehan, 1993). In Austrialia, CRA Ltd and Midrex are cooperat- ing in the development of HIsmelt process in which non-coking coal and pre-reduced iron ore are </p><p>CCC 0363-907X/96/121103-26 0 1996 by John Wiley &amp; Sons, Ltd. </p><p>Received 11 December 1995 </p></li><li><p>1104 T. ZERVAS, J. T. McMULLAN AND B. C. WILLIAMS </p><p>injected into a smelting reactor, and post-combustion of the off-gas with air is possible. In Japan, an association of the steel industry and public organizations are collaborating in the direct iron ore smelting (DIOS) process. In the U.S.A. the steel industry is responding to this challenge through a collaboration in research and development for a process sponsored by the America1 Iron and Steel Institute (AISI) and the U.S. Department of Energy (DOE) which involves a number of U.S.A. and Canadian steel firms and academic research centres. </p><p>So far, only two technologies have come to full commerical application (see Table 1). These are the Plasmadust variation of the Plasmasmelt process developed by SKF in Sweden, where it is used to process 70ktonne per year of allowed steelworks dust, and the Corex process which was developed in South Africa by Korf Engineering and Voest Alpine with a capacity fo 300 ktonne of hot metal per year. The ideal concept of a direct-smelting reduction process to convert iron ore and coal to steel has not yet been realized. </p><p>The generalized iron and steel metallurgical complex is shown in Figure 1 and the underlying chemical reactions are given in equations (1)-(12) (Davis et al., 1982; Lankford et al., 1985; Stephenson 1980). </p><p>3Fe,O3 + H, = 2Fe30 + H,O (1) 3Fe,03 + CO = 2Fe30, + CO, (2) </p><p>Fe,O, + H, = 3FeO + H,O (3) Fe30, + CO = 3FeO + CO, (4) </p><p>FeO + H, = Fe + H,O ( 5 ) FeO + CO = Fe + CO, (6) </p><p>3Fe + CO + H, = Fe,C + H,O (7) 3Fe + 2CO = Fe,C + CO, (8) </p><p>CO, + C = 2CO (Boudouard reaction) (9) H,O + C = CO + H , (10) FeO + C = Fe + CO 3Fe + C = Fe,C </p><p>(11) </p><p>(12) </p><p>At temperatures below about 1000" C, the dominant reactions are given by equations (1)-(6). Metallic iron will also absorb carbon according to equations (7) and (8). At temperatures above 1000"C, the additional reaction of carbon, CO, and H,O begins to play a part as shown in equations (9) and (10). CO and H, are produced, so increasing the reducing potential of the gas. The resultant net reaction of equation (11) is important for processes that produce DRI directly from solid coal without prior gasification of its fixed carbon. Above about 1200" C, metallic iron absorbs any carbon present, according to equation (121, leading to fusing or melting of the solid even though the melting point of pure iron is 1530" C. Thus, 1200" C represents the watershed between direct reduction and direct smelting processes. In practice, direct smeling processes operate above 1300" C to ensure that carbon is absorbed rapidly and liquid hot metal is produced. </p><p>Regardless of the reduction mechanism as discussed above, a necessary precursor to any iron oxide reduction process is the generation of a suitable reductant gas (H,,CO). This can be generated in a one-step process within the reductant reactor or it can be produced externally in a two-step process (Meyers, 1984). 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The water gas shift reaction takes place in the gasifier and helps to adjust the hydrogen-to-carbon monoxide ratio. It is also catalytically induced in a separate methanation step to form SNG. </p><p>2. REACTORS </p><p>A reactor is a unit of equipment designed to carry out certain reaction processes. As the majority of the reaction processes in metallurgy are heterogeneous, the main purpose of the reactor is to bring the reacting phases together under favourable conditions of pressure and temperature, to supply the necessary energy and to allow the separation of the product phases. </p></li><li><p>1108 T. ZERVAS, J. T. McMULLAN A N D B. C. WILLIAMS </p><p>Although the reactors are similar, there are differences and the reactors can be classified as continuous or discontinuous, series or parallel, etc. (Rosenqvist, 1983). </p><p>2.1 Vertical shaft reactor The vertical shaft reactor (Stephenson and Smaller, 1980) is a moving bed reactor in which iron ore moves under gravity in counter current to a reductant gas stream. The shaft reactor has been always a favoured reactor for the production of iron from iron ore and it has been commercially adopted as the preferred reduction unit in a number of direct reduction processes. As iron oxide requires higher concentrations of reductant to drive the reactions, a counter current </p><p>reactor is paticularly suited to the reduction of iron oxide. It also permits efficient heat transfer between the gas and solids. As the vertical shaft furnace is simple in construction, and existing and proven designs can be used </p><p>with a minimum of modification, it is attractive for the application of direct reduction processes. The disadvantage of the shaft reactor is that it is not possible to use ore fines without agglomeration </p><p>because of the need to ensure adequate space within the bed to permit access and passage of the reductant gas. </p><p>The development of mechanical devices to discharge the direct reduction iron from the shaft reactor is essential for its successful application in the reduction processes. </p><p>The shaft is usually fitted with slowly oscillating charge feeders situated just below the reductant gas, which helps to maintain the inflow of material into the reduction zone. The inflow of the charge and the uniform distribution of reductant gas across the vertical shaft reactor are essential to assure uniform metallization in the product. The vertical shaft reactor is designed to operate at ambient pressure so that the feed and discharge legs can be sealed with a dynamic gas seal using neutral gas. This avoids the capital costs associated with the mechanical sealing systems required for high pressure operation. The high pressure process operates at 4-5atm and therefore requires sealing valve systems at the top and bottom of the shaft, involving considerable complexity. Some shafts have no mechanical feeders but depend on furnace geometry and good temperature control to avoid charge sticking and to maintain the inflow. </p><p>2.2 Fluidized bed reactors The main advantage of fluidized bed reactors is their ability to accept iron ore fines as the charge material without agglomeration. Another advantage is that the rate of reaction is higher. These advantages over the vertical shaft reactor are, however, offset by some disadvantages. </p><p>Pellets Lump ore ""r1 </p><p>.rc-------- Reducing gas I + </p><p>Sponge iron </p><p>Figure 2. Simplified diagram of the vertical shaft reactor (shaft furnace) </p></li><li><p>ALTERNATIVE IRON /STEEL PRODUCTION PROCESSES 1109 </p><p>As the main principle of operation of a fluidized bed reactor is good mixing of the charge, it is necessary to employ two or more beds in series to obtain an approximation of counter current operation, with associated problems of charge transfer between the beds (Stephenson and Smaller, 1980). </p><p>Th

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