Studies On Phase Equilibria In The Quinary System Li⁺,K⁺,Mg²⁺//Cl^-,B₄O₇^2--H₂O And Partial Related Sub?systems Containing Sulfate At 273 K

Located in the northern margin of the Qinghai?Tibet Plateau,the Qaidam Basin is highly regarded for its abundant all kinds of salt resources.Rational exploitation and utilization of these salt resources can effectively promote the development of Chinese industry and reduce the dependence on foreign imports of some special resources such as lithium and potassium.The comprehensive utilization and production process of salt lake resources cannot be separated from the guidance of phase diagrams.In this paper,phase equilibrium experiments were carried out with quinary system Li⁺,K⁺,Mg²⁺//Cl^?,B₄O₇^2??H₂O containing lithium and some ternary and quaternary subsystems containing potassium or magnesium in the salt lake of Qaidam Basin.The specific experimental content is the isothermal evaporation method for the ternary systems MgB₄O₇?MgCl₂?H₂O,KCl?MgCl₂?H₂O and K₂SO₄?MgSO₄?H₂O,quaternary systems Li₂B₄O₇?K₂B₄O₇?MgB₄O₇?H₂O,KCl?K₂B₄O₇?MgCl₂?MgB₄O₇?H₂O and K₂SO₄?K₂B₄O₇?MgSO₄?MgB₄O₇?H₂O were studied at metastable phase equilibria of 273 K.The isothermal equilibrium method were used to study the stable phase equilibria of the quaternary system Li₂SO₄?Na₂SO₄?MgSO₄?H₂O and the quinary system Li⁺,K⁺,Mg²⁺//Cl^?,B₄O₇^2??H₂O at 273 K.The solubility data and density data about each component of each system were measured by analytical experiments,and the isothermal phase diagrams and density composition diagrams of each system were plotted.For the identification of the invariant points'equilibrium solid phases,XRD characterization was used,and the partial ternary systems were also assisted by the wet solid phase method.Furthermore,the invariant points'data of ternary and quaternary systems with the shared data at the different temperatures that have been reported were compared and analyzed.Some contrast phase diagrams were also drawn and the phase transition laws at different temperatures were analyzed and summarized.The experimental results show that the metastable phase equilibrium of the ternary system MgB₄O₇?MgCl₂?H₂O at 273 K is co?saturated type.Its phase diagram consists of one invariant point,two univariate curves and two single salt crystallization zones.It can be seen from the phase diagram of the ternary system that MgCl₂ has a certain salt?dissolving effect on MgB₄O₇ under metastable state.The metastable phase equilibria of the ternary systems KCl?MgCl₂?H₂O and K₂SO₄?MgSO₄?H₂O at 273 K have double salt formation,which are KCl·MgCl₂·6H₂O and K₂SO₄·MgSO₄·6H₂O,respectively.The phase diagrams have two invariant points,three univariate curves and three solid phase crystallization regions.It can be seen a similar rule,when compared of the ternary systems KCl?MgCl₂?H₂O and K₂SO₄?MgSO₄?H₂O with the metastable phase diagrams at different temperatures.As the temperature increases,the corresponding crystallization region of each component becomes smaller and solubility is increasing.The metastable phase equilibrium of the quaternary system Li₂B₄O₇?K₂B₄O₇?MgB₄O₇?H₂O at 273 K is simple saturation,and its phase diagram includes a invariant point,three solubility curves and three single salt crystal phase regions.The metastable phase equilibrium of the interactive quaternary systems KCl?K₂B₄O₇?MgCl₂?MgB₄O₇?H₂O and K₂SO₄?K₂B₄O₇?MgSO₄?MgB₄O₇?H₂O at 273 K have a double salt formation.The double salts are KCl·MgCl₂·6H₂O and K₂SO₄·MgSO₄·6H₂O,respectively.In addition,the metastable dry salt phase diagrams of these two systems both have three invariant points,seven univariate curves and five solid phase crystallization regions.Comparing the three ternary system metastable phase diagrams,it can be found that due to the small solubility of MgB₄O₇,the crystalline phase region occupies a larger area in the metastable phase diagrams of the three systems containing MgB₄O₇.From the comparison data of the invariant points in the three quaternary metastable and stable systems at different temperatures,it is easy to reveal that compared with the stable state,the solubility of the B₄O₇^2?increases significantly under the metastable state.It indicates that the borate has obvious supersaturation under the metastable state.The stable phase equilibrium of the quaternary system Li₂SO₄?Na₂SO₄?MgSO₄?H₂O at 273 K has double salt 3Na₂SO₄·Li₂SO₄·12H₂O formation.The dry salt phase diagram contains two invariant point,five univariate curves and four crystallization regions.In the phase diagram of the quaternary system shows that the double salt formed between Li₂SO₄ and Na₂SO₄ occupies a small area,indicating that it has a large solubility.The projection surface stable phase diagram of the quinary system Li⁺,K⁺,Mg²⁺//Cl^?,B₄O₇^2??H₂O at 273 K was drawn by the saturation of MgB₄O₇.The simplified dry salt diagram has double salt carnallite?KCl·MgCl₂·6H₂O?and lithium carnallite?K₂SO₄·MgSO₄·6H₂O?projection,which contains eleven univariate solubility curves,five invariant points and seven solid phase crystallization regions.By comparing the area of each crystalline phase zone,it can be judged that the solubility of each crystalline salt decreases in the order of LiCl·2H₂O>LiCl·MgCl₂·7H₂O>MgCl₂·6H₂O>KCl·MgCl₂·6H₂O>KCl>K₂B₄O₇·4H₂O>Li₂B₄O₇·3H₂O.