Solid–liquid Phase Equilibria of the Aqueous Quaternary System K ⁺ , Mg ²⁺ , Ca ²⁺ //SO ₄ ^2– –H ₂ O at <i>T</i> = 363.2 K

To meet the urgent demand for potash fertilizer in agriculture and accomplish the efficient utilization of polyhalite (K2SO4·MgSO4·2CaSO4·2H2O) resources, the quaternary system K+, Mg2+, Ca2+//SO42––H2O and its ternary subsystem Mg2+, Ca2+//SO42––H2O were studied by the isothermal dissolution method at 363.2 K. The experimental results show that the system Mg2+, Ca2+//SO42––H2O is of a simple system at 363.2 K, and the two crystalline regions are corresponding to CaSO4·2H2O and MgSO4·H2O, without double salts or solid solution formed. The interactions among the ions in K+, Mg2+, Ca2+, and SO42– coexisting systems are more complex; besides three single salts, five double salts are formed, and thus the phase diagram of the quaternary system at 363.2 K contains seven invariant points, 14 univariate curves, and eight crystalline regions. With the concentration of MgSO4 within the range of 0.70%–24.67%, CaSO4 within the range of 0.01%–0.06%, and a minimum concentration of K2SO4 at least 2.92%, polyhalite can be formed at 363.2 K. Comparing the phase diagrams of the quaternary system at 323.2 and 363.2 K, it shows that within this temperature range, increasing temperature is not conducive to the dissolution of polyhalite.