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 (Fe0·934O) 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.