Mechanism and kinetics of thermal decomposition of carbonates

A physical approach to the interpretation of the mechanisms and kinetics of thermal decomposition of solids has been applied to the investigation of decomposition mechanisms of Ag, Cd, Zn, Mg, CaMg, Ca, Sr and Ba carbonates. The method consists in comparing experimental literature data on the kinetic parameters with their theoretical values calculated on the basis of the physical approach. Two parameters were used: the E parameter and the initial temperature of decomposition, Tin, defined as the temperature of decomposition which corresponds to the fixed partial pressure, Pin, of CO2 evolved. The results of examination of the available data supported the general mechanism of decomposition which includes as a primary stage the congruent dissociative evaporation of reactants. For all the carbonates, except of BaCO3, the transfer to the reactant one-half of the energy released in the course of the condensation of low-volatility product has been taken into account in the calculations. The effect of self-cooling on the results of experimental determination of both parameters has been examined, and several important conclusions has been deduced. In particular, in the decomposition of CaCO3 in the presence of CO2 (10−6–10−4 bar), the self-cooling effect is responsible for the lack of expected hyperbolic dependence of decomposition rate on CO2 pressure and the appearance of the Topley–Smith effect. Several quantitative criteria of the validity of measured E values has been proposed. On this basis, the values of E parameters reported in the literature were critically analysed.