Alteration of kinetics and thermodynamics of thermal decomposition of alkaline-earth carbonates induced by grinding

Abstract

The effect of grinding on the kinetics and thermodynamics of thermal decomposition reactions of alkaline-earth carbonates has been studied. Grinding of MgCO3, SrCO3 and BaCO3 leads to a decrease in the activation energy of the reaction which becomes greater as the lattice energy of the salt decreases. These results can be attributed to the mechanically produced lattice disorder that is facilitated with decreasing lattice energy. A decrease in the enthalpy of decomposition has also been noted in the ground samples of SrCO3.By contrast, the enthalpy and activation energy of thermal decomposition of CaCO3 do not undergo significant alterations. Noticeable changes occur only after quite a long grinding time. This behaviour is probably related to an elimination of the microstrains created during grinding as a result of the phase transformations calcite → aragonite and aragonite → calcite, induced in this salt by grinding.A compensation effect between the enthalpy and entropy of decomposition and also between the activation energy and frequency factor is observed. These correlations seem to indicate that the reaction takes place through the same activated complex, its thermodynamic constants depending on the nature of the cation. Thus, the decrease in the frequency factor of the reaction as the grinding of the salt continues, might be related to an increase in the carbonate ion entropy.