Reduction Kinetics and Catastrophic Swelling of MnO₂-doped Fe₂O₃ Compacts with CO at 1073–1373 K

Abstract

Pure Fe₂O₃ and Fe₂O₃ doped with either 2, 4, or 6 mass% MnO₂ annealed at 1473 K for 6 h were isothermally reduced with carbon monoxide at 1073–1373 K. The oxygen weight loss resulted from the reduction at a given temperature was continuously recorded as a function of time. Reflected and scanning electron microscopes were used to characterize the annealed and reduced samples whereas the different phases developed were identified by X-ray phase analysis technique. The external volume of partially and completely reduced samples was measured by displacement method and the volume change (ΔV%) was calculated. At a given temperature, the influence of MnO₂ mass% on the reduction behaviour and volume change of Fe₂O₃ compacts was investigated. The doping of MnO₂ showed different effects during the reduction of Fe₂O₃ which is temperature dependant. At <1198 K, the rate of reduction decreased at early stages with the increase in MnO₂ mass% due to the presence of hardly reducible manganese ferrite phase (MnFe₂O₄). At final reduction stages the retardation effect was attributed to the formation of dense iron manganese oxide (FeO_0.899, MnO_0.101). At ≥1198 K, the presence of MnO₂ promoted the reduction of Fe₂O₃ and the catastrophic swelling resulted from the formation of both metallic iron plates and whiskers was observed. Maximum swelling (ΔV%) was measured at 1198 and at 1248 K for pure Fe₂O₃ and MnO₂-doped compacts respectively and it increased with the increase in MnO₂ mass% resulting about 405% for 6 mass% MnO₂-doped samples. The reduction mechanism was predicted from the correlation between the apparent activation energy values, testing of different mathematical formulations derived from gas-solid reaction model and the microscopic examination of partially reduced compacts.