Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Patent
1991-09-26
1993-04-13
Andrews, Melvyn J.
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
75443, 75623, 75707, C22B 3432
Patent
active
052019403
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
The present invention relates to a process for pre-heating and pre-reducing a metal oxide.
BACKGROUND OF THE INVENTION
Off-gases from smelt-reduction furnaces are of high temperature and may contain significant quantities of reducing gases such as carbon monoxide and hydrogen. It would clearly be of economic advantage to recover at least some of the sensible heat and make use of some of the reducing potential of the gases.
A number of prior art processes are known which provide either pre-heating or pre-reduction of metal oxide ores. Two processes, one by the Kawasaki Steel company and the other by the Nippon Kokan company, combine both features.
Generally speaking, the known processes entail one or other of the following disadvantages, and sometimes more than one: residence time or low pre-heat temperature achieved.
If the temperature to which the oxide ore is exposed is too high, the particles may soften, leading to accretion on the apparatus and/or agglomeration of particles.
In the case of chromite ore, the pre-heating and/or pre-reduction processes described in the prior art are generally speaking limited to temperatures of around 1200.degree. C., at which temperature reduction is very slow.
The specification of U.S. Pat. No. 4,566,904 discloses a process for using exhaust gases from a melting crucible to pre-reduce iron ore. The exhaust gases are first reduced and cooled with a reductant such as natural gas. The cooled reducing gas is then used to pre-reduce the iron ore in a shaft furnace, a circulating fluidized layer or a fluidized bed. The optimum temperature of the cooled reducing gas if 900.degree. C. which is considerably less than the temperature of the exhaust gas. Consequently this process involves a considerable loss of sensible heat which is used to increase the concentration of reductants in the exhaust gas.
The specification of U.S. Pat. No. 4,629,506 describes the production of ferrochromium from a ferriforous chrome ore. The ore is heated in a rotary kiln for 20 minutes to 2 hours to a temperature in the range from 1480.degree. C. to 1580.degree. C. An atmosphere containing carbon monoxide is maintained inside the kiln to reduce the ore. The resultant plastic mass is cooled, crushed and magnetically separated into a coal rich fraction and a metal rich fraction. Preferably, the metal rich fraction is separated by dry density separation into a metal poor slag fraction and a metal rich alloy fraction, the slag rich fraction is crushed and a metal rich slag fraction is extracted by magnetic separation. The metal rich slag fraction is added to the metal rich fraction and both are then melted in a crucible for further processing.
The process described in U.S. Pat. No. 4,629,506 involves a number of process steps thereby resulting in additional capital and operating expenditure when compared with a process requiring less steps. Furthermore, the exhaust gases from the crucible are preferably used as carrier gases for blowing coal and ore into the crucible or for low temperature coking of the coal.
U.S. Pat. No. 4,851,040 describes a process for producing iron from fine grained iron ore by direct reduction. The process involves inserting sponge iron and coal fines or a low temperature carbonized coal into an iron bath and injecting oxygen to produce a reducing gas and iron. The reducing gas is used to reduce a pre-heated carbon coated fine grained iron ore in a "fluidized bed" at a temperature in the range from 700.degree. C. to 1100.degree. C. Spent reducing gas is used to preheat the fine grained ore to a temperature in the range from 450.degree. C. to 700.degree. C. as well as coat the grains of ore with fine grains of carbon. The fine grains of carbon are deposited on the grains of ore by decomposition of carbon monoxide. The carbon layer on the grains of ore prevents the grains from sticking during the reduction phase.
A process developed by Kawasaki Steel KK uses fine, unagglomerated ore; see JP 59080706. Pre-heating and pre-reduction is performed in a fluidized be
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Batterham Robin J.
Grant Roderick M.
Happ James V.
Thiele Glenn A.
Andrews Melvyn J.
CRA Services Limited
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