Nucleation and anisotropic growth model for isothermal kaolinite dehydroxylation under controlled water vapour pressure

Abstract : A new kinetic model of kaolinite dehydroxylation is proposed in order to take into account data obtained by gravimetry at 450°C under controlled water vapour pressure ranging from 2.5 to 10 mbar. This model involves a process of random nucleation and anisotropic growth of nuclei. Under the above experimental conditions, the growth of nuclei on the surface of the particles is rapid, whereas the growth towards the inside of the particles is limited by two-dimensional diffusion. This model allowed us to evaluate the 'areic frequency of nucleation' and the 'areic reactivity of growth' for our experiments. The variation of the latter quantity with the water vapour pressure was also obtained by a method based upon a sudden pressure change during isothermal experiments. It is this variation which allows us to conclude that the growth of nuclei into the particle is limited, under our experimental conditions, by the diffusion of hydroxyl groups from the internal interface to the external one.