Experimental Kinetic Data on the Formation of Mullite in some Kaolin Clays

Abstract

The rate of formation of mullite has been studied for three different kaolin clays. Two of these contained appreciable amounts of mica as bimineral. The mica present influences and increases the rate of mullite formation. The experimental data show that the formation of the mullite phase at 1 100–1 200° C is not a recrystallization. It is shown that after the exothermic effect at 950° C, and before the formation of the mullite phase as revealed by X-ray diffraction, not less than 80 per cent of the total available A1₂O₃ is present as γ-Al₂O₃. Thus no mullite phase, either totally or partially, can be formed in the strongly exothermic reaction at 950° C.

The kinetic course is treated in two different ways. The first method presumes that the rate limiting step is diffusion-controlled. The course is represented by an empirical time function and a characteristic time constant defined as 50 per cent conversion time. The temperature dependence of the later time constant is given by an Arrhenius' relation with an activation energy of 200 kcal/mole. The second method does not correspond to any reaction mechanism but shows that the course can be satisfactorily described as a first order reaction if the concentration variable is suitably chosen.