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 (FeO0.899, MnO0.101). 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.