Study on the Effect of pH, Ionic Strength and Pressure on the Solubility of CaSO4-NaCl-H2O System

Abstract

The present study investigates the solubilities of calcium sulphate (CaSO4) in NaCl-H2O solutions at 5 and 10 Kg/cm2 pressures respectively under boiling water temperature. As no previous work has been carried out to account for the effect of very low concentrations of NaCl the solubility of on calcium sulphate over a wide acidic pH range, various concentrations of NaCl (0.001-0.1 M) were used and the experiments were performed at different pH values (3.0 – 6.0) under hydrothermal conditions using an autoclave. Dissolved sulphates were determined by the turbidity measurement. Dissolved sulphates in each experiment were calculated from the respective absorbance datum at 420 nm wavelength recordedfrom a UV-Visible spectroscopy using the standard calibration curve. Highest soluble sulphate (14.58 x 10-3 moles L-1) is observed at pH 5.0 when the concentration of NaCl is 0.05 M at 5 Kg/cm2 pressure. At NaCl concentration of 0.01 M, highest soluble sulphates (12.50 x 10-3, 13.33 x 10-3 and 13.33 x 10-3 molesL-1 at pH 3.0, 3.5 and 4.0), respectively has been found at lower pH range. Upon increasing the concentration of NaCl to 0.05 M, the sulphates with the highest solublities, i.e., 13.33 x 10-3, 14.58 x 10-3 and13.33 x 10-3 moles L-1 appeared at pH values (4.0, 5.0 and 5.5, respectively). A further increase of the concentration of NaCl to 0.1 M, the highest sulphates solublities (13.33 x 10-3 and 14.37 x 10-3 moles L-1) have been found at pH values of 4.5 and 6.0, respectively. However, almost similar solubility values (12.91-13.33 x 10-3 moles L-1) at different NaCl concentrations have been observed at pH 4.0. Comparatively the solutions with less soluble sulphate  containing ionic strengths of NaCl at different pH values, were obtained when the experiments were performed under 10 Kg/cm2 pressure. In this case, the highest soluble sulphate (12.08 x 10-3 moles L-1) was obtained at 0.05 M NaCl solution at pH 4.0. The values of soluble sulphate are lower at lower pH values (3.0, 3.5 and 4.0) in all the concentrations of NaCl, whereas above the pH of 4.0, the values of soluble sulphate are insignificant. The results of this study are valuable in the assessment of calcium sulphate solubilities over a broad pH range and low concentrations of NaCl under boiling water temperature conditions at two different pressures.