Isothermal reduction behaviour of MnO2 doped Fe2O3 compacts with H2 at 1073–1373 K

Abstract

Pure Fe2O3 and Fe2O3 doped with 2, 4, and 6 mass% of MnO2 (>99%) compacts annealed at 1473 K for 6 h were isothermally reduced with H2 at 1073–1373 K. The O2 weight loss resulted from the reduction of compacts was continuously recorded as a function of time using thermogravimetric analysis (TGA). High pressure mercury porosimeter, optical and scanning electron microscopes, X-ray phase analysis and vibrating sample magnetometer were used to characterise both the annealed and reduced samples. In MnO2 containing samples, manganese ferrite (MnFe2O4) was identified. The rate of reduction of pure and doped compacts increased with temperature and decreased with the increase in MnO2 content. Unlike in pure compacts, the reduction of MnO2 containing samples was not completed and stopped at different extents depending on MnO2 (mass%). At initial reduction stages, the decrease in the rate was due to the presence of poorly reducible manganese ferrite (MnFe2O4) phase which was partially reduced to iron manganese oxide (FeO0.899, MnO0.101) at the final stages. The reduction mechanism was predicted from the correlation between the reduction kinetics and the structure of partially reduced samples at different temperatures. The reduction of pure and doped samples was controlled by a combined effect of interfacial chemical reaction and gaseous diffusion mechanism at their initial stages. At final stages, the interfacial chemical reaction was the rate controlling mechanism.