2008 Volume 48 Issue 10 Pages 1359-1367
Pure Fe2O3 and Fe2O3 doped with either 2.5–7.5 mass% SiO2, 6.0 mass% MnO2 and (6.0%MnO2+7.5%SiO2) compacts annealed at 1473 K for 6 h were prepared. The different phases were identified by X-ray diffraction (XRD) and their structures were examined by optical and scanning electron microscopes. The magnetic properties were measured with Vibrating Sample Magnetometer (VSM). Total porosity and pore size distribution were determined and their external volume was also measured. Unlike in SiO2 doped samples where no new phases were detected, manganese ferrite (MnFe2O4) was identified in MnO2-containing samples. Annealed compacts were isothermally reduced with CO at 1073–1373 K and the O2-weight loss was continuously recorded. It was found that the reduction rate of SiO2–Fe2O3 samples increases at the early stages with SiO2 mass% due to the increase in their original porosity. At final stages, the rate was retarded due to the formation of hardly reducible fayalite (Fe2SiO4). In MnO2-containing samples, the reduction rate was retarded at initial stages due to the presence of manganese ferrite. At later stages, the formation of fayalite–manganoan [(Fe, Mn)2SiO4] greatly hindered the reduction process. A catastrophic swelling (ΔV=405%) was recorded in 6%MnO2–Fe2O3 compacts and was greatly diminished to 55.95% in presence of 7.5% SiO2 due to the decrease in size and number of metallic iron whiskers and plates. The reduction mechanism of pure and doped samples was predicted from the correlations between reduction kinetics and the microscopic examinations of partially and completely reduced samples.