Chemical Modelling of Calcium Sulphate Phase Equilibria in Multicomponent Electrolyte Solutions

A new database was developed to successfully model the solid and aqueous phase equilibria in a number of hydrometallurgical processes using the Mixed Solvent Electrolyte (MSE) model of the OLI Systems software. In order to validate the predictability of the model developed, a number of experimental measurements were conducted in multicomponent solutions containing NiSO4, H2SO4, MnSO4, HCl, etc. from 25 to 90oC. The developed model accurately predicts the chemistry of calcium sulphate solid formation in the systems studied in this work as well as in multicomponent sulphate-chloride solutions in nickel hydrometallurgical processes. Since it is not practical to measure solubility data under all possible conditions, because of the large number of components involved, the developed model is a valuable tool for assessing calcium sulphate process chemistry for a wide variety of complex aqueous processing solutions. Introduction Scaling or precipitation fouling is the formation of a solid layer on equipment surfaces or piping systems. It is a persistent problem encountered in many industrial processes such as oil and gas production, desalination operations, steam generation plants, heat transfer systems, water supply systems, and hydrometallurgical operations. As the scale layer becomes increasingly thicker, it reduces the production capacity and process efficiency because of increased heat transfer resistance, reduction of material flow, corrosion and wearing out of construction materials. Calcium sulphate occurs widely in nature and is encountered in many industrial processes such as brine evaporation and phosphate fertilizer industries as well as in mineral processing industries using aqueous chemistry (hydrometallurgy). Calcium sulphate scales are deposited almost anywhere calcium and sulphate are together in aqueous solutions due to their relative insolubility and results in fouled reactor walls, impellers and pumps, as well as clogged pipes. The resulting scale even forms at low pH and can be effectively removed only mechanically. In a recent study, the costs for maintenance and calcium sulphate scales removal were estimated between $6 and $10 million per year. Calcium sulphate occurs in three forms: dihydrate (or gypsum) (CaSO4.2H2O), hemihydrate (or bassanite) (CaSO4.0.5H2O) and anhydrite (CaSO4), depending on the temperature, pH and formation conditions. Attempting to theoretically model calcium solubility in pure water and in multicomponent electrolyte solutions has been the topic of many previous studies. The solubility of calcium sulphate hydrates is equal to the sum of the molalities of the free calcium ion, Ca, and the associated calcium sulphate neutral species, CaSO4(aq). Therefore, the solubility of calcium sulphate hydrates is governed by following equilibria: O nH SO Ca O nH CaSO s 2 2 4 2 ) ( 2 4. + + = − + (1) ) ( 4 2 4 2 aq CaSO SO Ca = + − + (2)