The phase equilibria of ternary systems Li2SO4 + Na2SO4 + H2O and Na2SO4 + K2SO4 + H2O at 303.2 K were investigated by using the isothermal dissolution method. The solubility, density, and refractive index of the systems were measured. There are two ternary invariant points and three crystallization regions corresponding to Li2SO4·3Na2SO4·12H2O, Li2SO4·H2O, and Na2SO4·10H2O in the system Li2SO4 + Na2SO4 + H2O at 303.2 K. There are two ternary invariant points and three crystallization regions corresponding to Na2SO4·10H2O, Na2SO4·3K2SO4, and K2SO4 in the system Na2SO4 + K2SO4 + H2O at 303.2 K. By comparing the phase diagrams of the ternary system Li2SO4 + Na2SO4 + H2O at different temperatures, one can find that the existence form of double salt changes from Li2SO4·3Na2SO4·12H2O to Li2SO4·Na2SO4. By comparing the phase diagrams of the ternary system Na2SO4 + K2SO4 + H2O at different temperatures, the double salt Na2SO4·3K2SO4 was not found at 273.2 K, while it was formed at 303.2 and 313.2 K. Meanwhile, the thermodynamic data of these two systems at 303.2 K were fitted by the Pitzer–Simonson–Clegg model, and the calculated values agree well with the experimental values.
The phase equilibria of the quaternary systems NH 4 Cl−SrCl 2 −AlCl 3 −H 2 O and NH 4 Cl−MgCl 2 −AlCl 3 −H 2 O at 298.2 K were investigated by using the isothermal dissolution method. The solubility, density, and refractive index of the systems were measured experimentally, and the related phase diagrams, density/refractive index versus composition diagrams, are plotted. There is one quaternary invariant point and three crystallization regions corresponding to single salts NH 4 Cl, AlCl 3 •6H 2 O, and SrCl 2 •6H 2 O in the phase diagram of the system NH 4 Cl−SrCl 2 −AlCl 3 −H 2 O at 298.2 K, which belongs to a simple system. In addition to the single salts NH 4 Cl, AlCl 3 •6H 2 O, and MgCl 2 •6H 2 O, the double salt NH 4 Cl•MgCl 2 •6H 2 O is formed in the quaternary system NH 4 Cl−MgCl 2 −AlCl 3 −H 2 O at 298.2 K, which indicates that the addition of magnesium makes the interaction relationship between Al 3+ , NH 4+ , and Mg 2+ more complex and increases the difficulty of separation of magnesium. The crystallization region of NH 4 Cl is the largest in both the systems, which indicates that NH 4 Cl is much easier to separate than the other salts from the two systems.
The phase equilibria of aqueous ternary systems NH4Cl + CaCl2 + H2O and NH4Cl + MgCl2 + H2O at 308.2 K were measured by the isothermal dissolution method, and the solubility, density, and refractive index were determined experimentally. The stable phase diagrams and the diagrams of density/refractive index versus composition of these two ternary systems are plotted. The results show that both these systems at 308.2 K are complex systems, with the formation of double salts 2NH4Cl·CaCl2·3H2O and NH4Cl·MgCl2·6H2O, respectively. The multitemperature phase diagrams of the ternary systems NH4Cl + CaCl2 + H2O at (273.2, 298.2, 308.2, 323.2, and 348.2 K) and NH4Cl + MgCl2 + H2O at (273.2, 298.2, 308.2, and 323.2 K) are compared, respectively. The results show that within the temperature range discussed, the double salt NH4Cl·MgCl2·6H2O is formed, and the double salt 2NH4Cl·CaCl2·3H2O is only formed at 308.2, 323.2, and 348.2 K. The crystallization areas of 2NH4Cl·CaCl2·3H2O and NH4Cl·MgCl2·6H2O gradually increase with increasing temperature. Thus, the increasing temperature is conducive to the separation of calcium or magnesium from these two systems in the form of 2NH4Cl·CaCl2·3H2O or NH4Cl·MgCl2·6H2O. The interaction relationship between the ions of the system NH4Cl + CaCl2 + H2O becomes simpler at 273.2–298.2 K, which is more beneficial to the precipitation of single salts. When the temperature reaches 323.2 K, NH4Cl·MgCl2·6H2O precipitates into a more stable salt of commensurate type. Meanwhile, the thermodynamic calculations of these two systems at 308.2 K were fitted by the Pitzer–Simonson–Clegg model, and the predicted values agree well with the experimental values.
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.
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