Isothermal Kinetics of the Pentlandite Exsolution from mss/Pyrrhotite Using Model-Free Method

The pentlandite exsolution from monosulfide solid solution (mss)/pyrrhotite exsolution is a complex multi-step process, including nucleation, new phase growth and atomic diffusion, and lamellae coarsening. Some of these steps occur in sequence, others simultaneously. These make its kinetic analysis difficult, as the mechanisms cannot be elucidated in detail. In mineral reactions of this type, the true functional form of the reaction model is almost never known, and the Arrhenius parameters determined by the classic Avrami method are skewed to compensate for errors in the model. The model-free kinetics allows a universal determination of activation energy. Kinetic study of pentlandite exsolution from mss/pyrrhotite was performed over the temperature range 200 to 300°C. For mss/pyrrhotite with bulk composition (Fe0.77Ni0.19)S, activation energy of pentlandite exsolution, Ea, varies from 49.6 kJ · mol−1 at the beginning of reaction (nucleation is dominant) to 20.7 kJ · mol−1 at the end (crystal growth is dominant). In general, the activation energy varies during the course of solid reaction with the extent of reaction. The surrounding environment of reactant atoms affects the atom's activity and more or less accounts for changes of activation energy Ea.