Solid–Liquid Phase Equilibria of the Quaternary System Na⁺, K⁺, Mg²⁺(Ca²⁺)//SO₄^2–-H₂O at 363.2 K

The solid–liquid phase equilibria of the quaternary systems Na+, K+, and Mg2+//SO42–-H2O and Na+, K+, and Ca2+//SO42–-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+)//SO42–-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+//SO42–-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+//SO42–-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.