Phase Diagram Determination Of Alkali Metal-Carbonation (Bicarbonate) Brine System And Study On Separation Of Lithium,potassium And Boron Single Salt With CO₂ Carbonation

Salt lakes are widely distributed in China,which are rich in lithium,potassium and boron resources.In the later stage of potassium extraction process,it is difficult to separate potassium from double salt of potassium and sodium.The extraction process of lithium and boron has problems such as high cost,long time,and unstable process flow.Therefore,the alkali metal carbonate(bicarbonate)brine was studied in this paper.A high pressure CO₂carbonation process was taken as the core process to change the original carbonate system to bicarbonate system.Evaporation,extraction,thermal decomposition steps were also used in this art.The efficiency separation of lithium,potassium was realized,and the comprehensive utilization of the brine resources was greatly improved.In view of the difficulty of separation of potassium and sodium double salt,isothermal evaporation method was used to study the metastable phase equilibrium of the quaternary system Na⁺,K⁺//Cl^-,CO₃^2--H₂O at 298 K.The solubility were measured at each system point,and the metastable phase diagram was drawn.It was found that there are 6 invariant points,7 univariate curves,4 single salt crystallization regions and 1 double salt crystallization region.The HW equation was used to calculate the solubility of this system under the condition of thermaldynamic stability,which was close to the result of previous experiment on stable phase equilibrium.Compared with the stable phase diagram,the area of Na KCO₃·6H₂O and K₂CO₃·1.5H₂O crystallization zone decreased and increased respectivetly,indicating that the potassium extraction process guided by the metastable phase diagram could make it more easily to cross the Na KCO₃·6H₂O crystallization zone.The process parameter was explored for potassium salt separation with CO₂ carbonation,and the application range was determined in the phase diagram.It was found that the separation of KHCO₃ single salt could be effectively achieved when the value of[Na⁺/K⁺]in brine was lower than 0.44 with a maximum recovery of 59.88%,and the purity of the product could reach the standard of food-grade potassium bicarbonate.In view of the problems of high cost,long time,and unstable process flow during extraction process of lithium and boron,isothermal evaporation method was used to study the metastable phase equilibrium of Li⁺,K⁺//B₄O₇^2-,HCO₃^--H₂O at 298 K.The positions of two invariant points in the phase diagram was determined,and the data of solubility,density and p H value of the liquid phase points were given.Compared with the previous research on the metastable phase diagram of Li⁺,K⁺//B₄O₇^2-,CO₃^2--H₂O at 298 K,it is found that the area of Li HCO₃ crystallization zone decreased.High-pressure CO₂ carbonation could play a role in the solubility of lithium salt,and achieve the purpose of enrichment of lithium.The one-step and two-steps processes were explored,and the physical and chemical properties of brine at each stage were given.The flowchart of one-step process was plotted.After one-step process,the extraction rate of boron was 87.6%,the recovery rate of potassium was74%,the recovery rate of lithium was 31%,and the purity of lithium salt was 89%.The two-steps process can achieve boron extraction rate of 73.58%,potassium recovery of 67%,lithium recovery of 14%,and lithium purity of 68%.Finally,the reasons for the efficiency difference between one-step and two-steps processes were explored.