Thermodynamic modeling and experimental measurement of calcium sulfate in complex aqueous solutions

A newly developed database for the Mixed Solvent Electrolyte (MSE) model of the OLI Systems software was employed to model the solid and aqueous phase equilibria of calcium sulfate hydrates in electrolyte solutions containing NiSO 4, H 2SO 4, MgSO 4, Fe 2(SO 4) 3, LiCl and HCl from 25 to 90 °C. The MSE model is a variant of an excess Gibbs free energy model for Mixed Solvent Electrolyte systems which takes into account long-range electrostatic interactions from a Pitzer–Debye–Hückel equation, middle-range interactions from a second virial coefficient-type equation and short-range interactions from the UNIQUAC model. The effect of cations with similar anions on the solubility was investigated and it was found that the solubility depends mainly on the anion type while the cation has a minor effect. The effect of acid addition and the acid type was also studied. The addition of both HCl and H 2SO 4 increases the solubility; however H 2SO 4 has a less pronounced effect due to the common ion effect. Furthermore, the effect of phase transitions between different calcium sulfate hydrates was studied. The transformation of CaSO 4 dihydrate to anhydrite results in a significant decrease in the solubility, which complicates the chemistry of the system. Since it is not practical to measure solubility data under all conditions of interest, the model developed and the experimental results obtained serves the purpose of assessing the calcium sulfate scaling potential for a wide variety of complex aqueous industrial streams.