Significance of kinetic theories on the recrystallization of kaolinite

Mathematical methods have been extensively used for the analysis of data obtained from non-isothermal thermal analysis of kaolinite or other clay minerals and Kissinger description is frequently considered. It is based on an Avrami-type transformation and an Arrhenian dependence of the reaction rate. In general, the calculation of the activation energy uses an incorrect neglect of the temperature dependence of the transformation rate. For kaolinite, Kissinger method applied to dehydroxylation gives an activation energy close to the measured enthalpy change for the reaction, but for recrystallization the activation energy exceeds the enthalpy change by a factor as high as 30. It is related to the complex character of recrystallization, since nucleation and crystal growth simultaneously occur, with the existence of both spinel and mullite phases. Consequently, the temperature dependence of recrystallization cannot be assumed to be Arrhenian. An interpretation is also found with Polanyi–Wigner equation applied to kaolinite transformations. Using thermodynamic data of entropy variation, the recrystallization rate should attain very high values, ranging from 5 to 10 order of magnitude over standard values for solid transformations. These observations cast some doubts on calculated activation energy of kaolinite recrystallization obtained from Kissinger kinetic method.