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 (K 2 SO 4 ·MgSO 4 ·2CaSO 4 ·2H 2 O), solid–liquid phase equilibria of the quaternary system K +, Mg 2+, Ca 2+ // SO 4 2– –H 2 O and the ternary systems K +, Ca 2+ // SO 4 2– –H 2 O and Mg 2+, Ca 2+ // SO 4 2– –H 2 O were studied by the isothermal dissolution method at 323.2 K. The results show that only one double salt, K 2 SO 4 ·CaSO 4 ·H 2 O, is formed in the ternary system K +, Ca 2+ // SO 4 2– –H 2 O at 323.2 K; the ternary system of Mg 2+, Ca 2+ // SO 4 2– –H 2 O has no double salt formation at 323.2 K; the solid–liquid phase diagram of the quaternary system K +, Mg 2+, Ca 2+ // SO 4 2– –H 2 O (323.2 K) consists of five invariant points, ten univariable curves, and six crystal regions (two single salts K 2 SO 4, MgSO 4 ·6H 2 O, and CaSO 4 ·2H 2 O and three double salts K 2 SO 4 ·CaSO 4 ·H 2 O, K 2 SO 4 ·MgSO 4 ·4H 2 O, and K 2 SO 4 ·MgSO 4 ·2CaSO 4 ·2H 2 O). 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 K 2 SO 4 ·CaSO 4 ·H 2 O shrinks, while the double salts K 2 SO 4 ·MgSO 4 ·2CaSO 4 ·2H 2 O and K 2 SO 4 ·MgSO 4 ·4H 2 O increase; and the double salt K 2 SO 4 ·MgSO 4 ·6H 2 O transforms into the double salt K 2 SO 4 ·MgSO 4 ·4H 2 O. This implies that increasing temperature is conducive to the crystallization of polyhalite.