Study On The Multi-temperature Metastable Phase Equilibrium Of The Quaternary System Na⁺, Mg²⁺, K⁺ // Cl^-- H₂O And Its Subsystems

Based on the characteristics of the potassium and magnesium resources in the brine of West Taijinar Salt Lake, the method of isothermal evaporation was used to study the metastable phase equilibrium of the ternary system Na+, K+ // Cl-- H2 O at 273 K and 288 K, and the metastable phase equilibrium of K+, Mg2+ // Cl-- H2 O at multi-temperature of 273 K, 288 K, 308 K and 318 K in this research. The metastable phase diagram and the density versus composition diagram were constructed on the basis of the experimental data. The metastable phase equilibria of the quaternary system Na+, Mg2+, K+ // Cl-- H2 O at multi-temperature 273 K、288 K、308 K and 318 K were also studied and the metastable phase diagrams, density versus composition diagrams and the water content diagrams under each temperature were made according to the experiment data.The ternary system Na+, K+ // Cl-- H2 O at 273 K and 288 K is both of a simple eutectic type, no double salt or solid solution formed. The metastable phase diagram is composed of one invariant point, two single variable lines and the crystallization field of two single salts KCl and and Na Cl. Comparisions between the two metastable phase diagrams show that under different temperature, the crystallization forms of salt are the same, whereas the size of crystallization phase region are changed. With the increase of temperature, there is no obvious changed in Na Cl crystallization region, whereas there is a slight decrease in KCl crystallization region. So decreasing the temperature helps the separation and purification of the single salt KCl.The ternary systems K+, Mg2+ // Cl-- H2 O at 273 K、288 K、308 K and 318 K are all of complex ternary systems, with potassium carnallite KCl·Mg Cl2·6H2O formed. The metastable phase diagram is composed of two invariant points, three single variable lines, and three crystallization area corresponding to two single salt KCl, Mg Cl2·6H2O and one double salt for potassium carnallie KCl·Mg Cl2·6H2O. Comparing between the metastable phase diagrams at 273 K, 288 K, 308 K and 318 K show that the crystalline form of salt is not changed under different temperature. When the temperature increase, the crystallization area of Mg Cl2·6H2O is barely changed, while the crystallization area of KCl·Mg Cl2·6H2O become bigger and the crystallization area of single salt KCl become smaller. Therefore, decreasing the temperature is beneficial for the separation and purification of salt KCl from the brines.The quaternary systems Na+, Mg2+, K+ // Cl-- H2 O at 273 K, 288 K, 308 K and 318 K are of complex quaternary systems, with double salt potassium carnallite formed. The metastable phase diagram is composed of two invariant point, five single variable lines and four crystallization fields which corresponding to the double salt potassium carnallite KCl·Mg Cl2·6H2O and three single salt KCl, Mg Cl2·6H2O and Na Cl. After comparing the quaternary metastable phase diagrams, it could be seen that the crystallization form of salt does not change with the temperature, but the size of crystallization region changed. With the increase of the temperature, there is no obvious change in crystallization region of single salt Mg Cl2·6H2O, while the size of crystallization regions of double salt potassium carnallite KCl·Mg Cl2·6H2O and single salt Na Cl become bigger while the size of crystallization region of single salt KCl become smaller. In a word, decreasing temperature is helpful to the separation and purification of salt KCl from the brines.