Stepwise Reduction of CaO and/or MgO Doped-Fe_2O_3 Compacts to Magnetite Then Subsequently to Iron at 1173-1473 K
Compacts of pure Fe<sub>2</sub>O<sub>3</sub> and Fe<sub>2</sub>O<sub>3</sub>-doped with either of 1.0 % CaO and/or 1.0 % MgO, sintered at 1473 K for 20 h, were isothermally reduced at 1173-1473 K with 10%CO-90%CO<sub>2</sub> to magnetite then to metallic iron with purified CO. The oxygen weight-loss resulted from Fe<sub>2</sub>O<sub>3</sub>-Fe<sub>3</sub>O<sub>4</sub> or Fe<sub>3</sub>O<sub>4</sub>-Fe reduction steps was continuously recorded as a function of time. Chemical and X-ray analyses, microscope examination and pore size analyzer were used to characterize the fired and reduced compacts. The influence of CaO and/or MgO on the reduction behaviour of Fe<sub>2</sub>O<sub>3</sub> and Fe<sub>3</sub>O<sub>4</sub> was intensively studied. The reduction mechanisms predicted from both of apparent activation energy values and heterogeneous gas-solid mathematical models were correlated with the microstructures of partially reduced samples. The results obtained showed that the doping of these fluxing oxides promoted the reduction of Fe<sub>2</sub>O<sub>3</sub> at 1173-1473 K. The reduction of pure and doped Fe<sub>2</sub>O<sub>3</sub> compacts is controlled by gaseous diffusion at early stages and by interfacial chemical reaction at later stages. In Fe<sub>3</sub>O<sub>4</sub>-Fe reduction step, the doping of CaO and/or MgO enhanced the reduction at early stages which is temperature and compact composition dependent. With progress in reduction, the presence of MgO retarded the reduction of Fe<sub>3</sub>O<sub>4</sub> at ≤1273 K resulting a slowing down in the rate at latter stages. This was attributed to formation of entrapped lower oxide relics which hindered gaseous diffusion. At early stages, the reduction of pure and doped Fe<sub>3</sub>O<sub>4</sub> compacts is controlled by mixed control reaction mechanism. At latter stages, interfacial chemical reaction is the rate determining step for pure and CaO-containing samples, whereas solid-state diffusion is the rate controlling step for MgO-doped compacts.
- ISIJ international
ISIJ international 37(9), 844-853, 1997-09-15
The Iron and Steel Institute of Japan