Solid–Liquid Equilibria of the Quaternary Water–Salt System Li+, Na+, Rb+//SO42––H2O at 273.2 K

Abstract

For efficient lithium and rubidium extraction from Qaidam Basin salt lake brines, the phase equilibria of the quaternary system Li⁺, Na⁺, Rb⁺//SO₄^2––H₂O and its ternary subsystem Na⁺, Rb⁺//SO₄^2––H₂O were investigated at 273.2 K using the isothermal dissolution equilibrium method. The ternary subsystem Na⁺, Rb⁺//SO₄^2––H₂O at 273.2 K comprises one invariant point, two univariant curves, and two crystallization regions (one single salt Rb₂SO₄ and one hydrated salt Na₂SO₄·10H₂O). A comparison of the ternary phase diagrams reveals that with increasing temperature, the crystallization region of Na₂SO₄·10H₂O shrinks while that of Rb₂SO₄ expands. At 323.2 K, the crystalline form of sodium sulfate transforms from Na₂SO₄·10H₂O into Na₂SO₄. The quaternary system Li⁺, Na⁺, Rb⁺//SO₄^2––H₂O at 273.2 K comprises four invariant points, nine univariant curves, and six crystallization regions (one single salt Rb₂SO₄, two hydrated salts Li₂SO₄·H₂O and Na₂SO₄·10H₂O, and three double salts 3Na₂SO₄·Li₂SO₄·12H₂O, 3Li₂SO₄·Rb₂SO₄·2H₂O, and Li₂SO₄·Rb₂SO₄). Comparison of the phase diagrams at 273.2, 298.2, and 323.2 K reveals that with decreasing temperature, the Rb₂SO₄ crystallization region expands and the Li₂SO₄·H₂O region contracts, indicating that lower temperatures favor Rb₂SO₄ crystallization while inhibiting Li₂SO₄·H₂O.