Abstract

Previous work on the structural and thermal properties of various types of kaolinite have led to different conclusions, rendering comparison of analytical results difficult. The objectives of the present study were to investigate the thermal behaviour of kaolinite and to carry out a kinetic analysis of the decomposition of kaolinite at high temperatures. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and thermogravimetry-differential scanning calorimetry (TG-DSC) were used to study the mechanism of the thermal decomposition. The modified Coats–Redfern, Friedman, Flynn–Wall–Ozawa and Kissinger decomposition models were used to determine the decomposition mechanism of the kaolinite sample. The dehydroxylation of kaolinite occurred at ~600°C with the formation of metakaolin, which then transformed into either γ-alumina or aluminium-silicon spinel together with amorphous silica. The results of the XRD and FTIR analyses indicated that the γ-alumina, or aluminium-silicon spinel and amorphous silica phases, transformed into mullite and α-cristobalite, respectively, after decomposition at 900°C. Good linearity was observed with the modified Coats–Redfern, Flynn–Wall–Ozawa and Kissinger models from room temperature to 1400°C and the range of the activation energy determined was 120–180 kJ/mol.

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