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

To meet the urgent demand for potash fertilizer in agriculture and accomplish the efficient utilization of polyhalite (K 2 SO 4 ·MgSO 4 ·2CaSO 4 ·2H 2 O) resources, the quaternary system K +, Mg 2+, Ca 2+ //SO 4 2– –H 2 O and its ternary subsystem Mg 2+, Ca 2+ //SO 4 2– –H 2 O were studied by the isothermal dissolution method at 363.2 K. The experimental results show that the system Mg 2+, Ca 2+ //SO 4 2– –H 2 O is of a simple system at 363.2 K, and the two crystalline regions are corresponding to CaSO 4 ·2H 2 O and MgSO 4 ·H 2 O, without double salts or solid solution formed. The interactions among the ions in K +, Mg 2+, Ca 2+, and SO 4 2– 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 MgSO 4 within the range of 0.70%–24.67%, CaSO 4 within the range of 0.01%–0.06%, and a minimum concentration of K 2 SO 4 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.