Solid–Liquid Equilibria (SLE) of Aqueous Ternary System M, Ca ²⁺ //SO ₄ ^2– -H ₂ O (M = Na ⁺ , K ⁺ , Mg ²⁺ ) at 288.2 K

To understand the interrelationships within the multi-ion coexisting sulfate system during the solid–liquid transformation of polyhalite in the Bieletan area, phase equilibria of ternary systems M, Ca2+//SO42–-H2O (M = Na+, K+, Mg2+) at 288.2 K were studied by the isothermal dissolution equilibrium method. The solubility and density of the equilibrium liquid phases of the system were measured experimentally, and the composition of the equilibrium solid phase at the ternary invariant point was determined by X-ray powder diffractometry. It is found that the ternary system K+, Ca2+//SO42–-H2O at 288.2 K forms the double salt syngenite (K2SO4·CaSO4·H2O), while the systems Na+, Ca2+//SO42–-H2O and Mg2+, Ca2+//SO42–-H2O at 288.2 K only form with the formation of CaSO4·2H2O and Na2SO4·10H2O (MgSO4·7H2O). Multitemperature comparison of the M, Ca2+//SO42–-H2O (M = Na+, K+, Mg2+) systems shows that the phase diagrams of Mg2+, Ca2+//SO42–-H2O are consistent at the selected temperature, only containing CaSO4·2H2O and MgSO4·nH2O (n = 1, 6, 7). While K+, Ca2+//SO42–-H2O and Na+, Ca2+//SO42–-H2O change from 288.2 and 298.2 to 348.2 K, the phase diagram forms double salt(s) K2SO4·5CaSO4·H2O (Na2SO4·5CaSO4·3H2O and Na2SO4·CaSO4).