Thermal transformation kinetics of a kaolinitic clay

Abstract

The objective of the present study was to use the zero-order reaction rate (ZORR) assumption and transition state theory (TST) to examine the kinetics of the thermal changes in solids. The method was firstly applied to the thermal transformation of spinel–mullite in a kaolinitic clay. The selected clay contains kaolinite (K), illite (I), and quartz (Q) minerals. The cylindrical compacts of the clay powder with a diameter of 40.22 mm were prepared under 150 MPa by uniaxial pressing using water (7 % by mass) as binder. The compacts after dried at 100 °C for 4 h were fired at various temperatures in the interval of 900–1050 °C by changing the time between 20 and 120 min. Besides the bulk density measurements, phase transformation in the compacts depending on the firing temperature was examined by X-ray diffraction and scanning electron microscopy techniques. Time derivative of the bulk densities for each firing temperature was taken as the transformation rate constant (k) according to the ZORR assumption. Arrhenius equation for the transformation was found as k = A exp(−E#/RT) = (136 kg m−3 s−1) exp(−12,022 J mol−1/RT) where A is the proexponential factor, E# is the activation energy, R is the universal gas constant and T is the absolute temperature of firing. The most general thermodynamic relation of the activation for the transformation was obtained from the TST with the SI units as ΔG# = −RT ln K# = ΔH# − TΔS# = 89,367 + 224T where ΔG#, K#, ΔH#, and ΔS# are the Gibbs energy, equilibrium constant, enthalpy, and entropy for the activation of the thermal transformation, respectively. The positive value of the ΔG# between 900 and 1050 °C indicated that the activation complex for the transformation does not form spontaneously.