Solid–Liquid Phase Equilibria of the Quaternary System Na+, K+, Mg2+(Ca2+)//SO42–-H2O at 363.2 K

The solid–liquid phase equilibria of the quaternary systems Na⁺, K⁺, and Mg²⁺//SO₄^2–-H₂O and Na⁺, K⁺, and Ca²⁺//SO₄^2–-H₂O at 363.2 K were investigated by the isothermal dissolution method. The solubility and density of these systems were determined, and the corresponding phase diagrams were constructed based on the experimental data. The experimental results show that quaternary systems Na⁺, K⁺, and Mg²⁺(Ca²⁺)//SO₄^2–-H₂O are complex systems with double salts formed. Their phase diagrams both have 6 invariant points, 13 isothermal solubility curves, and 8 crystalline areas. Comparing quaternary system Na⁺, K⁺, Mg²⁺//SO₄^2–-H₂O at 298.2, 323.2, 348.2, and 363.2 K, it has been found that the crystalline areas of double salts 3Na₂SO₄·MgSO₄, Na₂SO₄·MgSO₄·4H₂O and 6Na₂SO₄·7MgSO₄·15H₂O increased significantly with increase of temperature, while the crystalline area of K₂SO₄ decreased, indicating that Na⁺ and Mg²⁺ are easier to be separated from the system, which is beneficial to the concentration and enrichment of K⁺. In addition, the interaction between Na⁺, K⁺, and Ca²⁺ is complex in system Na⁺, K⁺, Ca²⁺//SO₄^2–-H₂O at 363.2 K, and it is easy to form double salts K₂SO₄·5CaSO₄·H₂O, K₂SO₄·CaSO₄·H₂O, Na₂SO₄·5CaSO₄·3H₂O, Na₂SO₄·3K₂SO₄, and Na₂SO₄·CaSO₄.