Measurement and chemical modeling of calcium sulfate solubilities in concentrated chloride media

Measurement and chemical modeling of CaSO4 solubilities in HCl-containing multi-component aqueous chloride solutions have been thoroughly investigated. Solubilities of calcium sulfate dehydrate, hemihydrate and anhydrite in concentrated HCl (up to 12 mol·dm-3), CaCl2 (up to 3.5 mol·dm-3) and their mixed aqueous solutions were experimentally determined by using the classic isothermal dissolution method at the temperature range from (283 to 353) K. A self-consistent chemical model based on a single set of model parameters for all three CaSO4 modifications was developed with the aid of OLI Systems software platform. This was accomplished via regression of experimental solubility data that led to the determination of new Bromley-Zemaitis model parameters for the Ca2+---SO42- and Ca2+ --HSO4- ion pairs. The model was successfully tested for the estimation of CaSO4 solubilities in concentrated (up to 20 mol·kg-1) HCl-CaCl2-H2O systems up to 373 K. In addition this newly developed OLI-based chemical model was successfully applied to construct the CaSO4 phase transition diagrams in the HCl-CaCl2-H2O system. The effect of various chloride salts (NaCl, MgCl2, FeCl2, AlCl 3 and FeCl3) on the solubility of CaSO4 phases in aqueous HCl or HCl + CaCl2 solutions up to 353 K was further investigated by experiment and modeling. This led to the development of a truly global model of CaSO4 solubility in the H + Na + Ca + Mg + Al + Fe(II) + Cl + SO4+ H2O system. The new model makes use of new Bromley-Zemaitis activity coefficient model parameters for many ion pairs consisting of cations (Na+, Mg2+, Fe2+, and Al3+) and anions (SO4 -, HSO4-, and Al(SO4)2 -), as well as new empirical dissociation constant parameters for the species, MgSO4(aq), AlSO4+, and Al(SO 4)2-. The new model was shown to successfully predict the solubility of calcium sulfate phases in multi-component systems not used in model parameterization.