Kinetics of dehydration of Ca-montmorillonite

Isothermal thermogravimetric experiments have been carried out to determine the reaction kinetics of the dehydration processes in fuller's earth, a natural Ca-montmorillonite. Dehydration in swelling clays is a complex reaction, and analysis of the thermogravimetric data using empirical rate equations and time-transformation analysis reveals that the nature of the rate controlling mechanism is dependent upon both the temperature regime of the sample as well as the extent of reaction. For fuller's earth, we find that the dehydration kinetics are dominated by a nucleation and growth mechanism at low temperatures and fractions transformed (stage I), but above 90 degrees C the last stages of the reaction are diffusion controlled (stage II). The activation energy for dehydration during stage I is around 35 kJ.mol(-1), whereas the removal of water during stage II requires an activation energy of around 50 kJ.mol(-1). These two stages of dehydration are associated with primary collapse of the interlayer (stage I) and movement of water that is hydrated to cations within the interlayer (stage II).