Solid–Liquid Equilibria of the Quaternary Water–Salt System Li ⁺ , Na ⁺ , Rb ⁺ //SO ₄ ²– –H ₂ O at 273.2 K

For efficient lithium and rubidium extraction from Qaidam Basin salt lake brines, the phase equilibria of the quaternary system Li +, Na +, Rb + //SO 4 2– –H 2 O and its ternary subsystem Na +, Rb + //SO 4 2– –H 2 O were investigated at 273.2 K using the isothermal dissolution equilibrium method. The ternary subsystem Na +, Rb + //SO 4 2– –H 2 O at 273.2 K comprises one invariant point, two univariant curves, and two crystallization regions (one single salt Rb 2 SO 4 and one hydrated salt Na 2 SO 4 ·10H 2 O). A comparison of the ternary phase diagrams reveals that with increasing temperature, the crystallization region of Na 2 SO 4 ·10H 2 O shrinks while that of Rb 2 SO 4 expands. At 323.2 K, the crystalline form of sodium sulfate transforms from Na 2 SO 4 ·10H 2 O into Na 2 SO 4 . The quaternary system Li +, Na +, Rb + //SO 4 2– –H 2 O at 273.2 K comprises four invariant points, nine univariant curves, and six crystallization regions (one single salt Rb 2 SO 4, two hydrated salts Li 2 SO 4 ·H 2 O and Na 2 SO 4 ·10H 2 O, and three double salts 3Na 2 SO 4 ·Li 2 SO 4 ·12H 2 O, 3Li 2 SO 4 ·Rb 2 SO 4 ·2H 2 O, and Li 2 SO 4 ·Rb 2 SO 4 ). Comparison of the phase diagrams at 273.2, 298.2, and 323.2 K reveals that with decreasing temperature, the Rb 2 SO 4 crystallization region expands and the Li 2 SO 4 ·H 2 O region contracts, indicating that lower temperatures favor Rb 2 SO 4 crystallization while inhibiting Li 2 SO 4 ·H 2 O.