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

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  • INTERNATIONAL JOURNAL OF ENERGY RESEARCH, VOL. 20,1103-1128 (1996)

    DIRECT SMELTING AND ALTERNATIVE PROCESSES FOR THE PRODUCTION OF IRON AND STEEL

    T. ZERVAS, J. T. MCMULLAN AND B. C. WILLIAMS

    Energy Research Centre, Uniuersiy of Ulster, Coleraine, Co. Londondeny, BT 52 1SA U.K.

    SUMMARY

    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.

    KEY WORDS: iron production; direct reduction; direct smelting; energy in steel production; iron carbide production; plasma technology

    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

    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).

    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%.

    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

    CCC 0363-907X/96/121103-26 0 1996 by John Wiley & Sons, Ltd.

    Received 11 December 1995

  • 1104 T. ZERVAS, J. T. McMULLAN AND B. C. WILLIAMS

    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.

    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.

    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).

    3Fe,O3 + H, = 2Fe30 + H,O (1) 3Fe,03 + CO = 2Fe30, + CO, (2)

    Fe,O, + H, = 3FeO + H,O (3) Fe30, + CO = 3FeO + CO, (4)

    FeO + H, = Fe + H,O ( 5 ) FeO + CO = Fe + CO, (6)

    3Fe + CO + H, = Fe,C + H,O (7) 3Fe + 2CO = Fe,C + CO, (8)

    CO, + C = 2CO (Boudouard reaction) (9) H,O + C = CO + H , (10) FeO + C = Fe + CO 3Fe + C = Fe,C

    (11)

    (12)

    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.

    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). The main chemical reactions involved in gasification are as follows.

    Corn bustion

    C + O , = AH= -394kJmol-' (13)

  • Tabl

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    1972

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    1 1 1 1 1 1 1 1 2 2 1 1 1 1 2 4 2 1 3 2 5 I 1

    G

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