Low‐temperature dehydration of vermiculite I. Rate of isothermal dehydration

Abstract

Abstract The kinetics of the isothermal dehydration of sheet and powdered vermiculite were studied both in vacuo and in the presence of a constant water vapour pressure. Dehydration of the sheet form followed a first‐order mechanism both in vacuo and in water vapour. The extent of dehydration under constant vapour pressure was greater than in vacuo , and this is believed to be due to inhibition of the collapse of the fine structure of the dehydration product in a certain direction, thus accelerating the extent of dehydration. The energy of activation was calculated to be comparable to the latent heat of vaporisation of water, and it is suggested that this would measure the ‘heat of desorption’ of water from the surface of the dehydration product. For the dehydration of powdered vermiculite in vacuo and in water vapour, the kinetics may be discussed in conjunction with Mampel's theory of the decomposition of powders. The mechanism is interfacial, and the energy of activation is much lower than for the sheets. Diffusion plays a dominant role for powder dehydration, and in the present case it is an ‘activated diffusion’, being mainly ‘surface diffusion’ along the grain boundaries of the dehydration product. Water vapour also increases the extent of dehydration, and this could be interpreted in the same manner as for the sheet form.