Solid–Liquid Phase Equilibria Determination of Quaternary System NH4+, Mg2+, Ca2+//Cl––H2O at T = 298.2 and 323.2 K and p = 94.77 kPa

Solid–liquid phase equilibria of quaternary system NH4+, Mg2+, Ca2+//Cl––H2O are investigated at T = 298.2 and 323.2 K and p = 94.77 kPa by the isothermal dissolution equilibrium method, and the corresponding stable phase diagram, density vs composition, and water content diagram are plotted at 298.2 and 323.2 K. Results show that when the temperature is 298.2 K, there are three quaternary invariant points and five crystallization fields corresponding to two double salts (2MgCl2·CaCl2·12H2O, NH4Cl·MgCl2·6H2O), two hydrate salts (MgCl2·6H2O, CaCl2·6H2O), and one single salt (NH4Cl) in the phase diagram. When the temperature is 323.2 K, the phase diagram structure of the system and the crystalline form of the salt have changed, and there are four quaternary invariant points and six crystallization regions including the four crystallization forms existing at 298.2 K. The salt change is mainly reflected when CaCl2·6H2O is dehydrated into CaCl2·2H2O at 323.2 K and a new double salt (2NH4Cl·CaCl2·3H2O) is formed at 323.2 K. The composition of double salt 2NH4Cl·CaCl2·3H2O is determined by chemical analysis and thermogravimetric analysis.