Since the mid-1970s a considerable change has taken place in worldwide steelmaking, namely the growth of direct-reduction processes. Direct reduction processes have been developed using either natural gas or coal to reduce iron ore in the form of lump, pellets or fines to an iron rich charge material for electric arc furnaces. Two processes predominate: Midrex and Hyl, both of which use lump ore or pellets and gas as a reductant. Iron carbide is a new charge material made from iron ore fines, using natural gas. Commercial scale production has only recently started. There are a number of other processes currently under development to use iron ore fines. Gas based processes include Circored and Finmet and coal based processes include Circofer and Fastmet.
Fig. 1: Alternative iron and steelmaking routes.
Current status
Alternative iron and steelmaking routes are showed in Fig. 1. In general today direct reduction processes are not seen as an alternative to the blast furnace, because they produce solid sponge iron, but as a competition or supplement to the use of scrap in electric steel production especially in the case of the production of high quality steel made via the electric steelmaking route. The development of the worldwide capacity and production of sponge iron is shown in Fig. 2. It can be seen that a steady growth in capacity and production occurred year by year. The utilization of existing capacity increased remarkably over the past three years. In 1992 the plants were only utilized up to 60% with a production of 20.7 Mt of sponge iron and a capacity of around 34 Mt, whilst in 1995 utilization increased to 83% with a sponge iron production of 30.7 Mt and a capacity of 36.8 Mt. In 1995 7.7 Mt of sponge iron corresponding to 25% of the total production was not processed at the plants directly but traded on the market.
Fig. 2: DRI capacity, production and trade [2]
Fig. 3 shows the share of the different direct reduction processes in world sponge iron production for 1995. There are two major group of processes: Gas based and coal based processes. Gas based reduction accounts for 93%. The two most popular direct reduction methods have been well described and are known as the Midrex process and the HyL process, the latter being named after Hojalata y Lamina in Mexico, where the process originated. Of the many reviews written about direct reduction that by Small, an elementary account, and a later global review by Kalla et al. particularly can be recommended.
Fig. 3 Share of direct reduction processes 1995 [2]
These new developments have taken place despite the efficiency of the blast furnace-oxygen converter steelmaking route because:
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Metallurgical coke is an expensive form of energy -any country (usually in the 'third world') having cheap natural gas available can produce iron much more cheaply than via the blast furnace
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The blast furnace is only economically viable if large production tonnages (5-10kt/d) are envisaged. This entails very high capital expenditure so reducing the flexibility of production.
The direct reduction units can be of especial use in the "third world" since
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They allow small flexible units of iron production, coupled with electric arc steelmaking. The material reserve problems are much reduced compared with the blast furnace.
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Scrap imports are eliminated.
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Sponge iron can be exposed.
The processes
The Midrex development is shown in Figure 4. A mixture of methane and recycled waste gases is reformed by heating at 1000°C in the presence of a catalyst
Fig. 4: The Midex normal flow sheet [1]
The resulting gas CO plus hydrogen, from the reaction
CH4 + H2O -> CO + 3H2
passes into a vertical shaft into which it mixture of iron- pellets and lump iron ore is fed. The reducing gas is introduced into the shaft at about its mid-point. Reduction takes place in the upper stack at about 760 C The resulting lion is cooled in the lower stack by recycled cooling gas.
The HyL process also uses reformed natural gas to reduce a lump ore/pellet mixture in a series of retorts, which operate independently, and comprise in turn:
- Loading.
- Initial reduction.
- Final reduction.
- Cooling.
In "initial reduction", recycled waste gases partially reduce the charge Then a
"raw" CO and H, mixture completes the reduction in the third stage In the last stage, cool reformed natural gas cools the charge and adds carbon to the material in controlled amounts
Other processes, less popular than those above, use a three-stage retort, or coal in a rotary kiln to reduce ore. Table 1 shows the distribution (in 1980) of D R plants between the processes.
Table 1: Distribution of DR plants between processes (as of 1980)
The Midrex process
Of the two major direct reduction processes, the Midrex process has probably been best described in the scientific,literature. A shaft process. the Purofer process developed by Huttenwerk O berhausen, is similar to the Midrex, and has been discussed in detail by yon Bogdandy and Engell
Chemical reactions in the shaft
The heat required to raise the burden to the required reaction temperature is contained in the input reformed gas. The quantity, composition and temperature of the reformed methane is therefore of great importance and must he carefully controlled.
The reformed gas has been following typical composition:
This gas reduces Fe2O3 to Fe, via Fe3O4 and FeO as follows:
The residence time of those materials in the reduction zone is around 6h. In fact the last reaction (Wustite to iron) takes longest, While the other two reactions, haematitc to magnetite and magnetite to Wustite, occur very near the top of the shaft, quite rapidly. The chemistry of the reform methane is important. Fe3C can also be produced by methane. Too much methane will cool the bed and the Fe3C content (which is undesirable) will be increased. Too much CC) in the input gas will also produce an undesirable level of Fe3C.
General feature of the Midrex process
These have been well discussed by Kalla et al, who give the following data:
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