Crystallization of spinel phase from metakaoline: The nonisothermal thermodilatometric CRH study

The crystallization of the Al–Si spinel phase (Si3Al4O12) during thermal treatment of the middle ordered kaolinite was investigated by the non-isothermal thermodilatometric analysis (TDA) under load. The disc of compacted kaolinite powder was heated with constant rate of heating (CRH) from 0.25 to 10 °C·min− 1 up to 1400 °C. The parameters of kinetic triplet were evaluated via model-free Kissinger equation and mechanisms fitting method based on the KJMA equation. It was found that the mechanism of the process is sensitive to the heating rate. The formation of Al–Si spinel phase includes the nucleation and subsequent diffusion controlled growth of nuclei that is changed to the process controlled by the rate of nucleation on the interface if heating rate ≥ 3 °C·min− 1 is used. This change significantly reduces the value of apparent activation energy of the process from 954 kJ·mol− 1 to 562 kJ·mol− 1. The reconstruction based on the assessed values of apparent activation energy, frequency factor and mechanism is in the good agreement with the experimental data. The thermodynamic parameters of spinel phase were evaluated using Eyring laws.