Kinetics and mechanism of three stages of thermal transformation of kaolinite to metakaolinite

The formation of metakaolinite during the thermal treatment of kaolinite consists of three processes which include the destruction of kaolinite sheet structure, the dehydroxylation and the recombination of silica and alumina into the structure of metakaolinite. The sequence of these steps is significantly affected by heating rate. The kinetics, mechanism and the influence of heating rate on the course of these processes were investigated by the methods of thermal analysis (DTG and TDA) and high-temperature X-ray diffraction analysis (HT-XRD) using the Kissinger kinetic equation. The activation energy required for the delamination, dehydroxylation and recombination of silica and alumina tetrahedron into metakaolinite is 538, 195 and 143 kJ mol− 1, respectively. The thermodynamic data of the activated complex, calculated from the Wertera and Zenera law, enable to explain experimentally the observed influence of heating rate on the thermal transformation of kaolinite during heating.