Kinetic study of the formation of cation-excess magnetite

A cation-excess magnetite Fe3+δO4, has been found to form in the range 0.00 < δ < 0.09, in the course of the H2-reduction of magnetite into α-Fe at 300 °C. The formation of the cation-excess magnetite described by (3 +δ)Fe3O4+ 4δH2→ 3Fe3+δO4+ 4δH2O was studied kinetically. In the temperature range 250–350 °C, the rate constants, k1 and k2, were determined for the forward and reverse reactions, respectively. The rate of formation of the cation-excess magnetite depended on the partial pressure of H2 gas. The rate constant, k1, was estimated to be 10–13.8 at 300 °C and the reaction order was 1.11 with respect to the partial pressure of H2O gas. The rate of reverse reaction depended on the partial pressure of H2O vapour and the excess of cations in magnetite, δ. The reaction rate constant k2 was estimated to be 10–2.45 at 300 °C, and the reaction orders were 0.45 and 0.95 with respect to the partial pressure of H2O vapour and the δ value, respectively. The activation energies of the forward and reverse reactions were evaluated to be 63.8 kJ mol–1 and 25.5 kJ mol–1 from the Arrhenius plots.