Kinetics and thermodynamics of calcium carbonate and calcium sulfate at salinities up to 1.5 M

This paper examines the effect of salinity on kinetics and thermodynamics of precipitaion for CaCO3 and CaSO4 when they exist in isolation as well as together. Batch tests under isothermal (30°C) conditions were carved out for salinity values ranging between 0.5 to 1.5 M of NaCl, calcium sulfate in the range of 0.06–0.15 M, calcium carbonate in the range of 0.007–0.02 M and sulfate to carbonate ratios of 2–8. Thermodynamic solubility constants were determined using the Pitzer model for determination of the activity coefficients of ionic species. As expected, the thermodynamic solubility constant (Ksp) of the pure salts was not affected by different salinity levels; however, salinity level affected Ksp in mixed salt systems. This is believed to be due to the effect of salinity on polymorphism and crystal habit. Images of the pure and mixed system were produced using Scanning Electron Microscopy (SEM), demonstrating that the co-precipitation in the mixed system changed the scale morphology of the crystals. Also, the deposit tenacity was affected by co-precipitation. Further work is under way to assess the effect of salinity on polymorphism with X-Ray Diffraction (XRD) analysis. Kinetics of pure CaSO4 precipitation was found to be strongly affected by the level of salinity; however, salinity level had no significant effect on kinetics of pure CaCO3 precipitation. For kinetics analysis of the mixed system, total initial Ca2+ of 0.06 M of CaSO4 was compared to that of the mixed system. Qualitatively, the reaction rate of pure CaSO4 is found to be faster compared to the mixed system. These findings prove that the conventional means of assessing the behaviour of salts in pure systems are no longer extendable and applicable to mixed salt systems.