Controlled Rate Thermal Analysis of kaolinite dehydroxylation:: effect of water vapour pressure on the mechanism

The present kinetic study is focused on one aspect of kaolinite dehydroxylation, namely the influence of water vapour pressure in the 10(-3) to 5 hPa range and in the presence of crystalline defects. The experimental problem of keeping, throughout the dehydroxylation, the pressure gradients negligible around and within the sample is solved by means of Controlled Rate Evolved Gas Detection (CR-EGD). The dehydroxylation rate selected is as low as 0.014 h(-1) (which corresponds to a duration of 70 h for the whole experiment). Moreover, more than 20 independent measurements of the apparent Arrhenius energy of activation are carried out all along the dehydroxylation, with help of the rate-jump method, and therefore, without any assumption about the rate law of the determining step. In these conditions, the apparent Arrhenius energy of activation measured during the dehydroxylation of a poorly crystallised kaolinite is shown to be constant in the range 0.02 < alpha < 0.84 (under 10(-3) hPa) and in the range 0.18 < alpha < 0.80 (under 5 hPa), indicating that the rate law obeys the Arrhenius law in this range of extent of reaction. The corresponding activation energies obtained are (233 +/- 15) kJ/mol under 10(-3) hPa and only (188 +/- 10) kJ/mol under 5 hPa. Although this decrease is in contradiction with previously published results, it can be interpreted by considering that, under 10(-3) hPa, diffusion is the limiting step whereas, under 5 hPa, the part of water desorption probably becomes predominant. (C) 2002 Elsevier Science B.V. All rights reserved.