Cyclic thermogravimetrjc methods for the study of the decomposition of carbonates—CaCO₃

Abstract

The equilibrium decomposition pressure of metal carbonates is difficult to determine accurately using static equilibration techniques, particularly at low temperatures where the time required to reach equilibrium may be excessively long. Furthermore, it has been reported that static methods are subject to kinetic difficulties; thus true equilibrium may never be reached. To overcome these problems, a dynamic approach has been developed. In this study, two different cyclic thermogravimetric methods were used to measure the equilibrium CO₂ decomposition pressure of a mixture of calcium carbonate and calcium oxide as a function of temperature. Calcium carbonate was chosen to test the validity of these two cyclic techniques because it is a well-characterized compound. The measurements involved repeated cycling of either temperature or CO₂ partial pressure while holding the other parameter constant. The response of the sample to this cycling was a periodic mass variation. Equilibrium was established when a symmetric mass signal was observed with no net mass change. Using these techniques, reliable measurements of the equilibrium CO₂ partial pressure were obtained in a relatively short period of time as compared to other methods. The experimental results obtained using the two cyclic methods of analysis agree with currently accepted values. A †H°_298.15 value of 179.55 ± 0.16 kJ/mol was calculated for the decomposition reaction of calcium carbonate from the thermal cycling data, and a value of 178.98 ± 0.45 kJ/mol was determined from the partial pressure cycling results.