Kinetic study of the formation of cation-excess magnetite

Abstract

A cation-excess magnetite Fe_3+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→ 3Fe_3+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.