Isothermal thermogravimetric experiments have been carried out to determine
the reaction kinetics of the dehydration processes in fuller's earth, a na
tural Ca-montmorillonite. Dehydration in swelling clays is a complex reacti
on, and analysis of the thermogravimetric data using empirical rate equatio
ns and time-transformation analysis reveals that the nature of the rate con
trolling mechanism is dependent upon both the temperature regime of the sam
ple 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 wit
h primary collapse of the interlayer (stage I) and movement of water that i
s hydrated to cations within the interlayer (stage II).