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The isothermal dissolution equilibrium method was used to determine the solid−liquid phase equilibria of the quaternary system NaBr + KBr + MgBr 2 + H 2 O at 273.2 K. The equilibrium liquid phase and the equilibrium solid phase composition at 273.2 K were analyzed by the chemical analysis method and the X-ray powder crystal diffraction method, respectively. The isothermal phase diagram of the quaternary system listed above was drawn. The results show that the phase diagram of the quaternary system at 273.2 K has two invariant points, five isothermal solubility curves, and four solid phase crystallization zones, with double salt KBr•MgBr 2 •6H 2 O, hydrates MgBr 2 •6H 2 O and NaBr•2H 2 O, and anhydrous single salt KBr. Due to the salting-out effect of MgBr 2 on NaBr and KBr, the solid phase crystal region of KBr is the largest area in the phase diagram, followed by NaBr•2H 2 O and KBr•MgBr 2 •6H 2 O. Furthermore, the solubilities of salts in the quaternary system are also theoretically predicted by the Pitzer model, and the calculated phase diagram of the quaternary system is also plotted. The theoretical calculation solubilities are compared with the experimental data. The calculated results are in agreement with the experimental results.

To efficiently exploit and utilize bromine-rich brine resources of China, solid–liquid phase equilibria of the quinary system NaBr–KBr–MgBr2–SrBr2–H2O (saturated with KBr) and the quaternary system NaBr–MgBr2–SrBr2–H2O at 308.2 K were studied in detail by the isothermal dissolution equilibrium method. The dry salt phase diagram of the quinary system NaBr–KBr–MgBr2–SrBr2–H2O (saturated with KBr) includes one invariant point, three isothermal dissolution curves, and three crystallization regions (NaBr·2H2O, KBr·MgBr2·6H2O, and SrBr2·6H2O) under the condition of KBr saturation. And the crystallization region of KBr·MgBr2·6H2O is the smallest, so KBr·MgBr2·6H2O is difficult to crystallize and precipitate. Similarly, The dry salt phase diagram of the quaternary system NaBr–MgBr2–SrBr2–H2O is composed of two invariant points, five isothermal solubility curves, and four crystallization fields, which are hydrate salts NaBr·2H2O, MgBr2·6H2O, and SrBr2·6H2O and anhydrous simple salt NaBr. The crystallization region of MgBr2·6H2O is the smallest, the solubility of magnesium bromide is the largest, and it is difficult to crystallize and precipitate in this saturated solution. Furthermore, the mixing ion-interaction parameter ΨNa, Sr, Br of Pitzer’s model is fitted, which can be used for the subsequent prediction of phase equilibria containing strontium system. In addition, the solubilities of salts in the quinary system and the quaternary subsystem were theoretically predicted, and the experimental value was compared with the calculated value. The calculated value was in good agreement with the experimental value.

Sichuan Basin of China is a typical underground brine enrichment area. In order to develop and utilize the brine resource, it is necessary to study the phase equilibria and phase diagram of brine systems. In this work, the phase equilibria of the quinary system NaCl + KCl + MgCl2 + SrCl2 + H2O (saturated with NaCl) at 308.2 K of the underground brines in Sichuan Basin are measured experimentally by isothermal dissolution equilibrium method. The phase diagram, water content diagram, and sodium chloride content change diagram of the quinary system with NaCl saturated are also drawn, respectively. The results show that the phase diagram of the quinary system contains three invariant points, seven isothermal solubility curves, and five solid crystal zones at 308.2 K. In the phase diagram, the KCl crystallization area is the largest, followed by SrCl2·6H2O, KCl·MgCl2·6H2O, and SrCl2·2H2O, while MgCl2·6H2O has the smallest crystallization area. Magnesium chloride in the quinary system has the largest solubility and has strong salting‐out effect on other salts. Furthermore, the unreported mixed ion interaction parameters θMg, Sr and ΨMg, Sr, Cl of Pitzer model are fitted, respectively. Consequently, the solubilities of the quinary system NaCl + KCl + MgCl2 + SrCl2 + H2O at 308.2 K are predicted using the Pitzer model, and the calculated phase diagram at 308.2 K is also plotted in detail. The predicted solubilities of the quinary system were in good agreement with the experimental solubilities. Finally, the study on the phase equilibrium of the quinary system can provide phase equilibrium data for the development and utilization of liquid mineral resources.