Development of an Improved Chemical Model for the Estimation of CaSO₄ Solubilities in the HCl−CaCl₂−H₂O System up to 100 °C

A self-consistent chemical model based on a single set of model parameters for all three CaSO 4 modifications namely, dihydrate, hemihydrate, and anhydrite was developed and described in this work. The model was successfully tested for the estimation of CaSO 4 solubilities in concentrated (up to 20 m) mixed HCl−CaCl 2 −H 2 O systems up to 100 °C. The new model makes use of the OLI Systems software platform. Via regression of experimental solubility data, new Bromley−Zemaitis model parameters were determined for the Ca 2+ −SO 4 2- and Ca 2+ −HSO 4 - ion pairs. Moreover, for the first time, the new model incorporates data for hemihydrate modification (CaSO 4 · 1 / 2 H 2 O). After validation, the model was calibrated by determining new temperature-dependent parameters of the solubility product constants of the hemihydrate and anhydrite. With the aid of the newly developed model, the bell-shaped solubility curves for dihydrate and anhydrite, as a function of HCl concentration, were successfully explained, based on the bisulfate ion formation and ion-activity coefficient.