Experiment and Calculation of Solubilities and Densities in the Ternary System Na2SO4−ZnSO4−H2O at 323 K and 0.1 MPa

In the treatment process of sulfur-containing wastewater from oil fields, sulfides and metal salts are used to generate precipitation to remove sulfides from oil and gas field wastewater. As an excellent precipitant, soluble sulfate is necessary to be investigated in depth for its dissolution and crystallization in aqueous solution, which can provide theoretical guidance for the optimization of the oilfield water treatment process. In this paper, the result of the stable solid-liquid equilibria in the ternary system sodium sulfate–zinc sulfate–water at 323 K and 0.1 MPa were presented based on the method of isothermal dissolution equilibrium. In the experiment, saturated liquid phase and humid residue composition for a series of synthetic brine were determined. At the same time, the corresponding equilibrium solid phases were identified by using the X-ray powder diffraction combined with Schreinemakers’ method. Since the foundation of phase diagram theory concerning water salt system, the phase diagram of the system was constructed. The result show that the ternary system Na2SO4−ZnSO4−H2O at 323 K is considered to be the double salt type owing to the existence of double salt Na2SO4⋅ZnSO4⋅4H2O. And the phase diagram of the ternary system Na2SO4−ZnSO4−H2O at 323 K includes two invariant points, three univariant solubility curves and three solid crystalline phase regions corresponding to Na2SO4, Na2SO4⋅ZnSO4⋅4H2O, and ZnSO4⋅6H2O. The Pitzer equation was selected to calculate the solubility data in the ternary system Na2SO4−ZnSO4−H2O at 323 K. The solubility modeling approach achieved a very good agreement with experimental solubility data at 323.15 K. The experimental densities of the equilibrium solutions were determined by the density bottle method, and the theoretical densities of each saturated solution were calculated based on the Laliberte model. The results show that the theoretical densities are basically in agreement with the experimental densities, and the model can reasonably predict the densities of this ternary system.