Solid–Liquid Equilibria of the Quaternary Water–Salt System K+, Mg2+, Ca2+// SO42––H2O at 323.2 K

In order to develop and utilize polyhalite (K2SO4·MgSO4·2CaSO4·2H2O), solid–liquid phase equilibria of the quaternary system K+, Mg2+, Ca2+// SO42––H2O and the ternary systems K+, Ca2+// SO42––H2O and Mg2+, Ca2+// SO42––H2O were studied by the isothermal dissolution method at 323.2 K. The results show that only one double salt, K2SO4·CaSO4·H2O, is formed in the ternary system K+, Ca2+// SO42––H2O at 323.2 K; the ternary system of Mg2+, Ca2+// SO42––H2O has no double salt formation at 323.2 K; the solid–liquid phase diagram of the quaternary system K+, Mg2+, Ca2+// SO42––H2O (323.2 K) consists of five invariant points, ten univariable curves, and six crystal regions (two single salts K2SO4, MgSO4·6H2O, and CaSO4·2H2O and three double salts K2SO4·CaSO4·H2O, K2SO4·MgSO4·4H2O, and K2SO4·MgSO4·2CaSO4·2H2O). The results comparing the quaternary system at 308.2 and 323.2 K are as follows: when the temperature increases, the crystal region of the double salt K2SO4·CaSO4·H2O shrinks, while the double salts K2SO4·MgSO4·2CaSO4·2H2O and K2SO4·MgSO4·4H2O increase; and the double salt K2SO4·MgSO4·6H2O transforms into the double salt K2SO4·MgSO4·4H2O. This implies that increasing temperature is conducive to the crystallization of polyhalite.