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.