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.