Non-isothermal kinetic analysis of thermal decomposition of the Ca-bentonite from Santai, China

Abstract

The thermal decompositions of Ca-bentonites (CaB) from Santai, Shichuan Province, China, over the temperature range of 30–1,100 °C were investigated by simultaneous thermal analyzer. Non-isothermal kinetic analysis was employed to study the thermal decomposition mechanism by using Netzsch Thermokinetics software. Flynn-Wall-Ozawa and Friedman isoconversional methods were used to calculate the activation energy and analyze the reaction steps. The probable mechanism and the corresponding kinetic parameters were determined by multivariate non-linear regression program. The results show that the thermal decomposition process of CaB over the temperature range of 30–800 °C is a kind of six-step, competitive reaction (F ₁ D ₃ F _n C _1E F _n F _n model). The dehydration reaction is controlled by two consecutive mechanisms, nucleation and growth, followed by a diffusion-controlled reaction (F ₁ D ₃ model), the first step: E = 61.68 kJ mol⁻¹, logA = 6.75 s⁻¹; the second step: E = 50.73 kJ mol⁻¹, logA = 3.11 s⁻¹. The dehydroxylation reaction is controlled by three-step competitive mechanisms, an autocatalytically activated, initial reaction followed by n-order competitive reaction (C _1E F _n F _n model), the first step: E = 124.74 kJ mol⁻¹, logA = 5.67 s⁻¹; the second step: E = 245.29 kJ mol⁻¹, logA = 11.69 s⁻¹; the third step : E = 261.73 kJ mol⁻¹, logA = 11.23 s⁻¹. A combination reaction of the dehydration and dehydroxylation is observed, and controlled by one n-order reaction (F _n model), E = 8.99 kJ mol⁻¹, logA = ⁻1.91 s⁻¹.