THE K INET ICS OF DEHYDROXYLAT ION OF KAOL IN ITE

ABSTRACT: The dehydroxylation of kaolinite has been investigated by isothermal thermogravimetry. Kinetic analysis using the Avrami equation shows that a combination of atomic mechanisms operates throughout the temperature ange 734 K to 890 K. An empirical activation energy of 222 kJ mo1-1 was calculated from the Arrhenius relationship using rate constants based on diffusion and homogeneous models. The activation energy (Ea) was calculated for a series of degrees of dehydroxylation bythe time to a given fraction method, showing an increase in E a during the early stages of the reaction. The isothermal p ots indicate that OH is retained in the final stages of the reaction. The observations are explained in terms of a reaction mechanism inwhich kaolinite grains dehydroxylate from the edges inwards, parallel to (001). The dehydroxylation f kaolinite is one of many such reactions that occur within the clay mineral group. In this topotactic transformation, a metastable intermediate phase called metakaolin is formed, this persisting over a temperature range of several hundred degrees. The structure of kaolinite can be thought of as an extended sheet consisting of two components: (i) a sheet of SiO4 tetrahedra in hexagonal array with bases roughly coplana