Solid–Liquid Phase Equilibria in the Quinary System NaCl-KCl-MgCl2-SrCl2-H2O at 273 K

Abstract

To realize the comprehensive utilization of underground brines, this work focuses on the research of phase equilibria in salt–water systems containing potassium, sodium, magnesium, strontium, and chlorine. The solid–liquid phase equilibria in the quinary system NaCl-KCl-MgCl2-SrCl2-H2O at 273 K were determined by the isothermal dissolution equilibrium method. X-ray powder diffraction was used to identify the equilibrium solids in invariant points, and the XRD diffraction patterns of equilibrium solids in invariant points were obtained. Based on the experimental data, the three-dimensional equilibrium phase diagram was constructed. The results show that no solid solution is formed in the quinary system, while the double salt KCl·MgCl2·6H2O and corresponding hydrates SrCl2·6H2O and MgCl2·6H2O are formed. The three-dimensional equilibrium phase diagram of this quinary system at 273 K contains two invariant points, seven isothermal solubility curves, and five single salt crystallization regions corresponding to NaCl, KCl, MgCl2·6H2O, SrCl2·6H2O, and KCl·MgCl2·6H2O. In addition, two-dimensional projection phase diagrams with the saturated surfaces of NaCl, KCl, and SrCl2·6H2O were also analyzed, and the dry-salt phase diagrams were also obtained. When NaCl is saturated, there are two invariant points, five univariant curves, and four crystallization regions (KCl, MgCl2·6H2O, SrCl2·6H2O, and KCl·MgCl2·6H2O) in the dry-salt phase diagram. When KCl is saturated, the dry-salt phase diagram consists of only one invariant point, three univariant curves, and three crystallization regions (NaCl, SrCl2·6H2O, and KCl·MgCl2·6H2O). When SrCl2·6H2O is saturated, the dry-salt phase diagram contains two invariant points, five univariant curves, and four crystallization regions corresponding to NaCl, KCl, MgCl2·6H2O, and KCl·MgCl2·6H2O.