Kinetics of the oxidation of magnetite using simultaneous TG/DSC

Abstract

Kinetics of the oxidation of magnetite (Fe₃O₄) to hematite (a-Fe₂O₃) are studied in air using simultaneous TG/DSC. The mechanism is complex and the differences between the kinetic conclusions and Arrhenius parameters based on either TG or DSC are discussed. As in our previous work on CaCO₃ [1], the determination of a satisfactory baseline for the DSC results adds considerable uncertainty to those kinetic results. Consequently the calculations based on the TG data are considered superior. Solid state reactivity varies from one source of material to another and the results are compared for two different commercial samples of magnetite, both presumably prepared by wet chemical methods. These materials are much more reactive than the material studied previously [2], which had been coarsened and refined at high temperatures. In that earlier study, the metastable spinel, g-Fe₂O₃, was formed as an intermediate in the oxidation to the final stable form, a-Fe₂O₃. The exothermic reaction of the gamma to alpha form of the product during the oxidation process destroys the direct comparison between the TG and DSC results, since the former only detects the change in mass of the sample and not the crystallographic transformation. The TG results, however, represent the true oxidation process without superposition of the structural aspects.