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.