Thermochemistry and kinetics of the attack of magnesium-carbonate co-substituted fluorapatites by hydrochloric acid at different temperatures (25-55) °C

The thermochemical and kinetic studies of dissolution of magnesium-carbonate co-substituted fluorapatites in 3 wt% hydrochloric acid solution were performed using microcalorimetry at a temperature between 25 and 55 °C. The results were processed iteratively in order to deduce the thermochemical and kinetic parameters. Increasing the temperature leads to an increase in the apparent rate constant and an increase in the enthalpy of dissolution (between −230 and −140 kJ mol−1). A kinetic scheme has been proposed with an order close to 2 with respect to the apatite. Simultaneous incorporation of CO3 and Mg ions into the fluorapatite generates a decrease in the activation energy from 20 to 6 kJ mol−1. These values suggest a diffusion controlled phenomenon. The isoconversional model showed that the dissolution occurs in a single step and led to activation energies values close to that of the kinetic model ones.