Solid–Liquid Phase Equilibria of Quaternary System Li⁺, NH₄⁺, Ca²⁺//Cl^––H₂O at 323.2 and 348.2 K

The solid–liquid phase equilibria of the quaternary system Li+, NH4+, Ca2+//Cl––H2O at 323.2 and 348.2 K was investigated using isothermal dissolution method, targeting deep brines rich in ammonium and calcium. Equilibrium liquid-phase composition and density were determined. The solid solution (NH4Cl)x(LiCl·H2O)1–x was confirmed by XRD and TG-DSC. The results show that the quaternary system Li+, NH4+, Ca2+//Cl––H2O at 323.2 and 348.2 K exhibits complex behavior involving the formation of solid solution (NH4Cl)x(LiCl·H2O)1–x and double salt (2NH4Cl·CaCl2·3H2O). The phase diagram of the system at 323.2 K consists of three invariant points, seven univariate curves, and five crystalline phase regions (NH4Cl, LiCl·H2O, 2NH4Cl·CaCl2·3H2O, (NH4Cl)x(LiCl·H2O)1–x, and CaCl2·2H2O). At 348.2 K, it has two invariant points, five univariant curves, and four crystalline regions (NH4Cl, 2NH4Cl·CaCl2·3H2O, (NH4Cl)x(LiCl·H2O)1–x, and CaCl2·2H2O). Comparative analysis of multitemperature phase diagrams of the quaternary system Li+, NH4+, Ca2+//Cl––H2O at 298.2, 323.2, and 348.2 K demonstrates that the crystalline phase region of LiCl·H2O decreases with the increase of temperature. Notably, at 348.2 K, lithium chloride exclusively exists within the (NH4Cl)x(LiCl·H2O)1–x. This thermal behavior suggests that cooling crystallization could be effectively employed for lithium extraction from such brines.