Kinetics of reduction of iron oxides by H2: Part II. Low temperature reduction of magnetite

This study deals with the reduction of Fe3O4 by H2 in the temperature range of 210–950 °C. Two samples of Fe3O4 produced at 600 and 1200 °C, designated as Fe3O4(600) and Fe3O4(1200), have been used as starting material. Reduction of Fe3O4(600) by H2 is characterized by an apparent activation energy ‘Ea’ of 200, 71 and 44 kJ/mol at T < 250 °C, 250 °C < T < 390 °C and T > 390 °C, respectively. The important change of Ea at 250 °C could be attributed to the removal of hydroxyl group and/or point defects of magnetite. This is confirmed during the reduction of Fe3O4(1200). While transition at T ≈ 390 °C is probably due to sintering of the reaction products as revealed by SEM. In situ X-rays diffraction reduction experiments confirm the formation of stoichiometric FeO between 390 and 570 °C. At higher temperatures, non-stoichiometric wüstite is the intermediate product of the reduction of Fe3O4 to Fe. The physical and chemical modifications of the reduction products at about 400 °C, had been confirmed by the reduction of Fe3O4(600) by CO and that of Fe3O4(1200) by H2. A minimum reaction rate had been observed during the reduction of Fe3O4(1200) at about 760 °C. Mathematical modeling of experimental data suggests that the reaction rate is controlled by diffusion and SEM observations confirm the sintering of the reaction products. Finally, one may underline that the rate of reduction of Fe3O4 with H2 is systematically higher than that obtained by CO in the explored temperature range.