Elucidation of the thermo-kinetics of the thermal decomposition of cameroonian kaolin: mechanism, thermodynamic study and identification of its by-products

Abstract

This work elucidates the thermo-kinetics of the thermal conversion of cameroonian kaolin to metakaolin as the main product. The thermokinetical parameters (activation energy E_a and pre-exponential factor A) for the kaolin conversion were calculated using model-free methods, i.e. the Kissinger–Akahira–Sunrose (KAS) and the Flynn–Wall–Ozawa (FWO) method, and differential methods (Kissinger and Ozawa) additionally including iterative procedures for KAS and FWO methods (KAS-Ir; FWO-Ir). The cameroonian kaolin was heat-treated using three different heating rates, i.e. 5, 20 and 40 K min⁻¹, leading to metakaolin samples named MK-(5), MK-(20) and MK-(40). The TGA analysis showed a total mass loss of ∼12.5% in two steps related to the dehydration (step 1) and dehydroxylation (step 2). The E_a of the two steps were most accurately determined using the iterative procedures KAS-Ir and FWO-Ir. The average E_a values were 88.44/88.58 kJ mol⁻¹ for step 1 and 261.85/261.91 kJ mol⁻¹ for step 2, for the KAS-Ir and FWO-Ir models, respectively. The most probable mechanism function was determined by the multiple heating rate method (MHR) and the Coats-Redfern method. The kinetic analyses showed that the dehydroxylation of kaolin is controlled by a random nucleation and subsequent growth mechanism (G₄) and a second order chemical reaction (F₂). Thermodynamic parameters, namely the entropy ΔS^≠, the enthalpy ΔH^≠ and Gibbs free energy ΔG^≠, were evaluated. The average values of ΔS^≠, ΔH^≠ and ΔG^≠ using both the KAS-Ir and FWO-Ir models exhibited less than 5% deviation. The obtained metakaolin samples were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and Fourier-transform infrared spectroscopy (FT-IR).