Phase Equilibria of Aqueous Ternary Systems NH₄Cl + CaCl₂ + H₂O and NH₄Cl + MgCl₂ + H₂O at 308.2 K: Measurement and Calculation

Abstract

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 the Pb2+, Ca2+, Mg2+//Cl––H2O Quaternary System and Its Subsystems at 303.2 K

“…30 The ternary phase diagram consists primarily of one ternary invariant point (E 1 ), two solubility curves (A 1 E 1 , E 1 B 1 ), and three crystalline regions. 25 As seen in Figure 1b, when the CaCl 2 content in the liquid phase is below 6 wt %, the solubility of PbCl 2 decreases with increasing CaCl 2 content. This is primarily due to the dominant co-ion effect between PbCl 2 and CaCl 2 , which is most pronounced at lower Cl − concentrations.…”

“…Particularly, there is no independent crystallization region for anhydrous CaCl 2 in the phase diagram. Due to the high hygroscopicity of anhydrous CaCl 2 , it does not precipitate from the solution by CaCl 2 saturation crystallization …”

“…Due to the high hygroscopicity of anhydrous CaCl 2 , it does not precipitate from the solution by CaCl 2 saturation crystallization. 25 …”

“…Subsequently, another new salt was introduced and gradually added to the mixed solution covering different mass fractions ranging from 0 to saturation. 25 − 27 All samples were then placed in a constant-temperature bath and stirred for a minimum of 2 weeks. The mixed solution system was considered to reach equilibrium when the adjacent sampling analyses of the same sample were within 0.3%.…”

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

“…Commonly, the chemical type of underground brine belongs to chloride type, and the main compositions of the underground brine can be simplified as the complex seven-component system Li + , Na + , K + , NH 4 + , Mg 2+ , Ca 2+ //Cl – –H 2 O. So far, a series related to the ammonium-containing system can be found in the open literature as follows: NH 4 + , Mg 2+ //Cl – –H 2 O at 273.2–323.2 K; − Li + , NH 4 + //Cl – –H 2 O at 273.2–323.2 K; NH 4 + , Ca 2+ //Cl – –H 2 O at 298.2–348.2 K; − Na + , NH 4 + //Cl – –H 2 O at 303.2 K; K + , NH 4 + //Cl – –H 2 O at 298.2 K; Li + , K + , NH 4 + //Cl – –H 2 O at 298.2 K; K + , NH 4 + , Ca 2+ //Cl – –H 2 O at 293.2 K; K + , NH 4 + , Mg 2+ //Cl – –H 2 O at 333.2 K; and Na + , NH 4 + , Mg 2+ //Cl – –H 2 O at 333.2 and 348.2 K. , According to the above-mentioned literature, it can be seen that temperature and the coexisting ions are important factors for the phase equilibria. (1) Solid solutions of (NH 4 , K)­Cl, (K, NH 4 )­Cl, and (Li, NH 4 )Cl are found in the chloride system containing lithium/potassium and ammonium; (2) whether the presence of double salt 2NH 4 Cl·CaCl 2 ·3H 2 O in the chloride system containing calcium and ammonium is related to temperature, there is no double salt at 298.2 K, while it is found at 323.2 K; and (3) the crystallization regions of the solid solution and double salt change with changing temperature.…”