Abstract

The attack of fluorapatite (Fap) by phosphoric acid solution between 25 and 65°C was followed by microcalorimetry. The molar enthalpy decreases until a temperature close to 45°C, and then increases. This behaviour was attributed to a change of mechanism. Iteration of the results enables us to propose a two or one step mechanism, depending on the temperature range. Kinetic and thermochemical parameters corresponding to each step were also deduced. For temperature lower than 45°C, the activation energy, E_a, equals about 20 kJ mol^-1, suggesting a diffusion phenomenon control, whereas for higher temperature the attack seems to be controlled by a chemical process (E_a = 101 kJ mol^-1).

Key words: Kinetics, microcalorimetry, fluorapatite, phosphoric acid, temperature, dissolution.