On the modeling of calcium sulfate solubility in aqueous solutions

The electrolyte–NRTL model used in a previous work (B. Messnaoui, T. Bounahmidi, Fluid Phase Equilibr. 237(1–2) (2005) 77–85), was extended for modeling of α-hemihydrate and gypsum solubilities in the complex system Ca2+–H+–SO42−–HSO4−–H2PO4−–H3PO4–H2O, at wide range of temperature and P2O5 concentration. The chemical equilibrium constant was evaluated as function of temperature according to the Gibbs–Helmholtz equation. The temperature dependence was taken into account in the expression of the standard state heat capacity of ionic, molecular and cristalline salts species. The standard-state heat capacity of Ca2+at 298.15 K is calculated to be 27.30 J mol−1 K−1. It is also shown that the experimental data agree with the predicted values of gypsum and anhydrite solubilities in water, at high temperature, by using only the values of parameters τH2O−(Ca2+,SO42−)τH2O−(Ca2+,SO42−), τ(Ca2+,SO42−)−H2Oτ(Ca2+,SO42−)−H2O which were calculated, at 298.15 K from data of gypsum solubilities in phosphoric acid solutions.