Reduction of Iron Oxide Fines to Wustite with CO/CO 2 Gas of Low Reducing Potential
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DUCTION
EXTENSIVE research efforts have been devoted to the reduction of iron oxides for conditions pertinent to blast furnace and direct reduction processes. Considering the gaseous reduction of porous hematite to iron, attention has been focused mainly on the later stages of the reduction process, which involve reactions to metallic iron as this is the overall slowest reduction step.[1,2] In these conditions, gases of high reducing potential are employed such that the early stages of reduction (from Fe2O3 to FeO) are fast and often influenced by mass transport mechanisms. Limited information is available for the initial stages of the reduction process that involve iron oxide fines under highly oxidized atmospheres and moderate temperatures in which Fe2O3 or Fe3O4 can be reduced to FeO but not to metallic iron. These conditions are relevant to bath smelting processes using coal directly to produce hot metal such as HIsmelt, DIOS, and AISI in which the process off-gas (primarily CO–CO2–H2–H2O) may be used for prereduction of iron ore. They also are relevant to a newly proposed smelting process in which coal-char is employed, and an off-gas consisting mainly of CO-CO2 is generated.[3] The coal-char is obtained in a R. CORBARI, formerly Graduate Student, with Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213; is now Senior Process Engineer, with Vallourec & Mannesmann, 2669 Martin Luther King Jr. Blvd., Youngstown, OH 44510. Contact e-mail: Rodrigo. [email protected] R.J. FRUEHAN, Professor, is with Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213. Manuscript submitted March 8, 2009. Article published online December 22, 2009. 318—VOLUME 41B, APRIL 2010
separate process in which the hydrogen from coal is captured. The CO–CO2 smelter off-gas is used conceptually to preheat and partially reduce iron oxide fines in a prereduction unit. The smelter off-gas composition or the off-gas reducing potential is determined by the oxygen source (O2, or air) and the extent of conversion of CO to CO2 in the smelter, i.e., the post combustion degree (PCD), defined by Eq. [1] as follows: PCD ¼
pct CO2 pct CO2 þ pct CO
½1
Post combustion is an important smelter parameter because it has the potential to reduce coal consumption in the process.[4] Generally, higher PCDs may be achieved for smelters operating with coal-char compared with coal.[3,5] An ideal situation is a PCD of about 50 pct, which allows for an adequate supply of energy to the smelter while preserving limited reducing power in the off-gas to convert Fe2O3 to FeO.[4] This reduction degree corresponds to the removal of about one third of the oxygen in Fe2O3 and addresses a use of the chemical and sensible energy available in the smelter off-gas. The objective of this study was to investigate the kinetics and mechanisms of reduction of porous iron oxide fines to FeO at moderate temperatures with a simulated smelter off-gas of low reducing potential. In this study, the reaction gas is limited to an equimolar CO–CO2 mixture, which would be
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