Reduction kinetics of hematite to magnetite under hydrothermal treatments

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.