INCEPTION OF TRANSFORMATION OF HEMATITE TO MAGNETITE DURING MECHANICAL ACTIVATION: A THERMODYNAMICAL APPROACH

In this research the thermodynamic of mechanochemical activation by high energy milling of hematite has been studied. It has been shown that transformation reaction due to ball milling of α-Fe2O3 has been started after 15 hours milling. As the sum of amorphization and dislocation energies is a suitable approximation of total stored energy, the contribution of these two energies has been identified. The contribution of amorphization and dislocation energies after 15 hours milling have been calculated at about 76 and 9 kJ/mol, respectively. Thus, the increase in molar enthalpy of milled hematite is about 85 kJ/mol. The total Gibbs free energy of mechanochemical transformation of hematite to magnetite after 15 hours milling has also been determined. The temperature for transformation of activated hematite to magnetite has been calculated at about 300 K. The bulk temperature of powder after 15 hours milling has been measured at about 325 K. Therefore, at this temperature hematite cannot be stable and will transform to magnetite during milling.