Phase Equilibria and Phase Diagram of the Quaternary System (Na⁺, K⁺, Ca²⁺//Cl^– -H₂O) and Its Subsystems at 338.15 K

The Qaidam Basin has potassium- and calcium-rich oil field brine resources at high temperatures. Since the phase diagram is a powerful tool to guide the separation and purification of inorganic chemical engineering for the development of potassium resources in oil field brine, the quaternary oil field brine system (Na+, K+, Ca2+//Cl– -H2O) and its two subsystems ((Na+, Ca2+//Cl– -H2O) and (K+, Ca2+, //Cl– -H2O)) at 338.15 K were reported for the first time by the isothermal dissolution equilibrium method. The compositions of the equilibrium solid phases were analyzed and identified by combined chemical analyzing and X-ray diffraction. The quaternary system has four crystallized regions corresponding to NaCl, KCl, KCl·CaCl2, and CaCl2·2H2O at 338.15 K, and the incongruent double salt KCl·CaCl2 occurred at 338.15 K. The study facilitates the enrichment of thermodynamic data and exploits brine-based potassium resources.