Kinetic aspects of the thermal decomposition of zinc carbonate

Abstract

The thermal decomposition of zinc carbonate has been studied isothermally at various temperatures between 200 and 260°C. Rising temperature experiments, both thermogravimetry (TG) and differential thermal analysis (DTA), support the formulation of the material studied as ZnCO 3 · 2 ZnO · 2 H 2O. The decomposition proceeds through the loss of water and then the loss of carbon dioxide in two overlapping stages. The method of utilizing the concept of reduced time indicates only approximate isokinetic character to the decomposition over the temperature range studied. Initial rates of decomposition may be indicative of the rate of loss of water whilst later rates of decomposition may be taken as indicative of the rate of loss of carbon dioxide. The character of the kinetics is deceleratory, but in the region beyond 40% decomposition is approximately linear and due to the loss of carbon dioxide. Ancillary supporting evidence on the mode of decomposition is drawn from surface area measurements and these indicate a loss of area during dehydration and a rise in surface area in the region where carbon dioxide was evolved. The activation energy and pre-exponential term are calculated and possible models for the activated complex considered using the treatment outlined by Shannon. Possible short-comings in the utilization of this data are considered.