Reduction kinetics of hematite to magnetite under hydrothermal treatments

Abstract

The phase transformation of α-Fe₂O₃ to Fe₃O₄ was observed by the hydrothermal treatment of a ferric solution at 160–220 °C with the addition of both KOH and EDA into the reaction system. The reactions began with the formation of α-Fe₂O₃ hexagonal plates followed by the phase transformation involving the dissolution of the α-Fe₂O₃ hexagonal platelets, the reduction of Fe³⁺ to Fe²⁺, and the nucleation and growth of new Fe₃O₄ polyhedral particles. The activation energies for the phase transformation of α-Fe₂O₃ to Fe₃O₄ under hydrothermal conditions are estimated to be 96.4 ± 11, 113.1 ± 5, and 118.3 ± 11 kJ mol⁻¹ for the addition of 0.5, 1 and 1.5 ml of EDA, respectively, which are about the same for the typical phase transformation of α-Fe₂O₃ to Fe₃O₄ in a hydrogen atmosphere.