Experimental determination and modeling of gypsum and insoluble anhydrite solubility in the system CaSO4–H2SO4–H2O

Abstract

The solubility isotherms of gypsum and insoluble anhydrite in the ternary system CaSO4–H2SO4–H2O were determined at T=(298.1, 323.1, 348.1, and 363.1) K using the isothermal method. The kinetics of the transformation between gypsum and insoluble anhydrite in H2SO4 aqueous solutions at 298.1 and 363.1K were also studied. Our measured solubility isotherms for gypsum are generally in good agreement with the literature solubility data. However, the solubilities of anhydrite measured in this work are much lower than those reported by Zdanovskii and Vlasov (1968b) [Zdanovskii, A.B., Vlasov, G.A., 1968b. Russ. J. Inorg. Chem. 13, 1418–1420.]. Kinetic experiments showed that the complete transformation from gypsum to anhydrite at 363.1K takes at least 120h in 0.5molkg−1H2SO4 aqueous solution and over 6h for higher concentrations of H2SO4. Furthermore, much more time is needed for the Ca2+ concentration in the solutions to equilibrate with the end solid phase “insoluble anhydrite” than for the complete transformation of gypsum to insoluble anhydrite. These kinetic results were used to identify why the insoluble anhydrite solubilities in H2SO4 aqueous solution in the literature were higher than those in our results. A Pitzer thermodynamic model was chosen to simulate and predict the solubility isotherms of gypsum and insoluble anhydrite in this ternary system, and the good agreement between the experimental results and the model supports the reliability of the experimental solubility data obtained in this work. Finally, the stable fields for gypsum and insoluble anhydrite as a function of temperature and H2SO4 concentration were constructed by the thermodynamic model.