Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Patent
1994-07-18
1995-08-29
Rosenberg, Peter D.
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
75447, C21B 1302
Patent
active
054456673
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to a method for reducing material containing metal oxide in solid phase in a circulating fluidized bed reactor.
The present invention is particularly suited for reduction of iron ore to metallic iron with carbon, i.e. with a mixture of CO and CO.sub.2. The invention can advantageously be used for pre-reducing iron ore before the smelting stage in a direct smelting reduction process.
The reduction of iron oxide is an endothermic process and requires supply of energy. In a reduction process in which coal or coke in solid form is supplied, the energy required for the reaction can easily be supplied by partial combustion of the coal. Depending on the temperature, a certain content of CO.sub.2 in the gas can be permitted, preferably however so that the CO.sub.2 /CO+CO.sub.2 ratio does not exceed 0.2. This implies a certain degree of oxidation of the coal or the coke beyond the CO stage, but requires then preheating of the ore concentrate as well as the air, if air and not oxygen is used.
The reaction kinetics of the reduction fluidized bed reactors. At temperatures of about 800.degree. C., reaction times of several minutes, possibly tens of minutes, are required, depending on the particle size and the desired degree of reduction. The subsequent reaction according to appropriate gas composition.
The reduction of iron ore to metallic iron in the fluidized bed is impeded by the tendency of the particles in the bed to sinter. Higher temperatures, which would give higher, and therefore more favourable, reaction kinetics for the reduction process, lead to a higher tendency to sinter. The risk of sintering has considerably limited the use of fluidized bed technique for pre-reduction of iron ore.
Sintering is believed to be caused in part by the sticky iron ore particles in which the iron is completely or partly in metallic form. FeO appears as a molten layer on the surface of the pre-reduced ore, which causes sintering of small particles into larger particles and aggregates. Sintering of the particles in the reactor renders it diffucult or impossible to bring about fluidization in the reactor.
Sintering can, in addition to a molten iron layer on the particles, be caused by crystallization of metallic iron as dendrites on the ore particles, whereby particles are formed that very easily become attached to and grow into each other. Sintering is also believed to be caused by a particularly active layer of metallic iron surrounding the larger ore particles, the active layer having a certain adhesion force and attracting smaller particles.
Sintering can be avoided by carrying out the reduction at very low temperatures, which however would result in unfavourable reaction kinetics and, at lower temperatures, in formation of carbides instead of metallic iron.
To avoid sintering in reduction in a fluidised bed at higher temperatures, coal or coke has been mixed in, which has been believed to prevent sintering, either in form of indvidual particles in the bed or in form of a protecting coke layer on the bed particles. Injection of oil in the hot bed has also been believed to contribute to the formation of a layer of coke on the iron particles, which would prevent sintering.
Addition of coke has, however, proved to cause segregation, particularly in conventional fluidized beds, so that the iron particles concentrate in the lower part of the reactor and the coke particles in the upper part of the reactor. This has had a negative effect on the reduction process.
It is an object of the present invention to provide a method for reducing material containing metal oxide in which the above mentioned drawbacks, i.e. segregation and sintering, can be avoided.
The present invention has in a surprisingly simple manner solved the problems of the reduction processes described earlier by carrying out the reduction in an circulating fluidized bed (CFB) reactor so that oxide, and gas containing oxygen gas is introduced in the fluidization chamber of the reactor so as to bring
REFERENCES:
patent: 2894831 (1959-07-01), Old
patent: 2978315 (1961-04-01), Schenck
A. Ahlstrom Corporation
Rosenberg Peter D.
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