Étude cinétique et thermodynamique de l’attaque d’une fluorapatite par des mélanges d’acides sulfurique et phosphorique À 25 °C

A calorimetric study of the kinetics and thermodynamics of the attack of synthetic fluorapatite by mixtures of sulfuric and phosphoric acids is undertaken at 25 °C. The global enthalpy of the attack equals −407.0 kJ/mol. Interpretation of the calorimeteric results by Avrami model shows the existence of two steps. The first is attributed to the dissolution of the solid, and the second to precipitation of gypsum. The deconvoluted curves were analysed iteratively to obtain the kinetic parameters and the enthalpy of precipitation. The later quantity does not differ from the experimental value by more than 2.3% and the reaction order equals 2 and 0.5 for calcium and sulphate ions respectively.