Influence of doping with CaO and/or MgO on stepwise reduction of pure hematite compacts

Abstract

Fired compacts made from pure Fe₂O₃ and Fe₂O₃ doped with 1·0%CaO and/or 1·0%MgO were isothermally reduced in a stepwise manner at 1173–1473 K. At each temperature, hematite was first reduced to magnetite in 10CO–90CO₂, then to wustite (Fe_0·934O) in the appropriate mixture of CO–CO₂, and finally to iron in pure CO. The influence of doping with CaO and/or MgO on structural changes and the reduction kinetics and mechanism was intensively studied. The reduction rate of Fe₂O₃ to Fe₃O₄ was increased in the order CaO–MgO–Fe₂O₃ > MgO–Fe₂O₃ > CaO–Fe₂O₃ > Fe₂O₃, and this transformation step was controlled by gaseous diffusion in the early stages and by interfacial chemical reaction at later stages. The reduction of magnetite to wustite was controlled by a combined gaseous diffusion and interfacial chemical reaction mechanism. The reduction of wustite was also promoted, in the early stages, in the presence of CaO and/or MgO. At a level of ≥40%, the presence of MgO retarded reduction, resulting in a slowing down of the rate at lower temperatures. The wustite–iron step was controlled by a mixed reaction in the early stages. At later stages, the reduction of CaO–FeO specimens was controlled by an interfacial chemical reaction, while solid state diffusion had a pronounced effect on the reduction of pure and MgO containing compacts.