Quantitative Thermal Analysis of Clay Minerals

Vold's mathematical analysis of the d.t.a, curve has been applied to Murray's data on the dehydration of clays. Owing to experimental uncertainties, values for the heats of dehydration are lower than those determined by other methods. The first ruder law is obeyed and the activation energies of dehydration are of the same order as those derived from isothermal data; rates of dehydration are, however, higher. Furthermore, Smith's constant heat flow method has been used to determine specific heats and heats of dehydration of clays, and the effect of sample porosity upon the results has been determined. The thermal data has also been used to estimate the amount of clay mineral present. The kinetics of the dehydration process, evaluated from the apparent specific heat-temperature curves, follow a first order reaction curve. The activation energies obtained are, however, higher than those derived from other data. INTRODUCTION Attempts have been made to determine the heats of dehydration of clays by two methods, namely, (1) by applying Vold's analysis (1949) to normal differential thermal analysis curves, and (2) by employing the constant heat flow method developed by Smith (1940). Since powdered clay samples were used throughout this work giving rise to thermal gradients within the specimen, part of the work has been concerned with a study of the effect of these gradients upon the thermal data obtained. In addition, an attempt has been made to derive the kinetics of the dehydration process from the thermal data for comparison with isothermal data. APPLICATION OF VOLD'S ANALYSIS TO THE DEHYDRATION OF CLAYS The shape of a differential thermal curve depends upon several factors, namely, the kinetics of the reaction, the magnitude of the heat effect within the sample and the thermal diffusivity of the materials and containers. Furthermore, even when the reaction is complete, there is a finite differential temperature. Thus there are two problems to be solved : (a) At what point on the d.t.a, curve does the reaction cease? (b) To what extent is tbe curve influenced by thermal diffusivity? Vold has shown that these problems can be solved by applying the analysis outlined below. The rate of flow of heat into the sample and reference materials can be expressed by the equations : dqs/dt=Ks(Tw Ts) + ~(Tr Ts) + ~zs(To Ts) (1) and dqr/dt=Kr(Tw -Tr ) + a(Ts T r ) + err(To T r ) (2)