Phase Equilibria Of The Quinary System (Li⁺//Cl^-,SO₄^2-,BO₂^-,B₄O₇^2- （?） H₂O) And Its Subsystems At 288.15 And 308.15 K

It is famous for the rich boron and lithium resources in the Qinghai Tibet Plateau of our country.Therefore,the research on the phase equilibria of the borate-containing systems is of great significance to guide the separation and extraction of boron resources and their application in the production process.In this thesis,the isothermal dissolution equilibrium method was used to study the phase equilibria and phase diagrams of the quinary system(Li⁺//Cl^-,SO₄^2-,BO₂^-,B₄O₇^2-（?）H₂O)and its subsystems at 288.15 and 308.15 K.Combined with Pitzer ion interaction theory,the solubility of the corresponding system is predicted,and the following main conclusions are obtained:Neither solid solution nor double salt in the ternary system(Li⁺//BO₂^-,B₄O₇^2-（?）H₂O)was found at 288.15 and 308.15 K.The phase diagram of the system contains one invariant point（Li BO₂·8H₂O+Li₂B₄O₇·3H₂O）,two solid-phase crystallization zones（Li BO₂·8H₂O and Li₂B₄O₇·3H₂O）and two univariate solubility curves at 288.15 and 308.15 K,respectively.The physicochemical properties（refractive index,density and p H）of the equilibrium liquid phase of the systems change regularly with the increase of Li BO₂ content at 288.15 and308.15 K.The empirical equations of density and refractive index are used to calculate the density and refractive index theoretically.A comparison shows the computed values of refractive index and density agree well with the experimental values.The phase diagram of the quaternary system(Li⁺//SO₄^2-,BO₂^-,B₄O₇^2-（?）H₂O)at 288.15K contains one invariant point（Li₂SO₄·H₂O+Li BO₂·8H₂O+Li₂B₄O₇·3H₂O）,three solid crystalline regions（Li₂SO₄·H₂O,Li BO₂·8H₂O and Li₂B₄O₇·3H₂O）and three univariate solubility curves.The phase diagram of the system also contains one invariant point,three univariate solubility curves and three crystallization regions at 308.15 K.The area of crystallization region of Li₂B₄O₇·3H₂O is the largest and the solubility is the smallest,and the area of crystallization region of Li₂SO₄·H₂O is the smallest and the solubility is the largest.The diagram of the quaternary system(Li⁺//Cl^-,BO₂^-,B₄O₇^2-（?）H₂O)has two invariant points（Li Cl·H₂O+Li BO₂·2H₂O+Li₂B₄O₇·3H₂O,Li BO₂·2H₂O+Li BO₂·8H₂O+Li₂B₄O₇·3H₂O）,five univariate solubility curves and four crystallization regions,namely Li Cl·H₂O,Li BO₂·2H₂O,Li BO₂·8H₂O and Li₂B₄O₇·3H₂O in the dry salt phase diagram at308.15 K.The physicochemical properties of the equilibrium solutions in the quaternary system(Li⁺//Cl^-,BO₂^-,B₄O₇^2-（?）H₂O)change regularly with the change of Li BO₂concentration.The phase diagram of the quinary system(Li⁺//Cl^-,SO₄^2-,BO₂^-,B₄O₇^2-（?）H₂O)saturated with lithium sulfate at 288.15 K has two invariant points（Li₂SO₄·H₂O+Li Cl·H₂O+Li BO₂·2H₂O+Li₂B₄O₇·3H₂O and Li₂SO₄·H₂O+Li BO₂·2H₂O+Li BO₂·8H₂O+Li₂B₄O₇·3H₂O）,five univariate solubility curves and four crystalline phase regions are Li Cl·H₂O,Li BO₂·2H₂O,Li BO₂·8H₂O and Li₂B₄O₇·3H₂O,respectively.The crystallization area of Li Cl·H₂O region is the smallest,and the solubility is the largest;Li₂B₄O₇·3H₂O has the largest crystallization area and the smallest solubility.The quinary system also has two invariable points,five univariate solubility curves and four crystalline phase regions at308.15 K.The physicochemical properties（refractive index,density and p H）,water-phase diagram and lithium-phase diagram change regularly with the change of Li BO₂ content at two temperatures.The Pitzer model predicted the phase diagram of the ternary system(Li⁺//BO₂^-,B₄O₇^2-（?）H₂O)at 308.15 K.It is concluded that the calculated results are in good agreement with the experimental values.Similarly,combined with the Pitzer binary parameters obtained in the literature,the missing Pitzer mixing parameters in the quaternary system are fitted.The solubilities of the quaternary system(Li⁺//SO₄^2-,BO₂^-,B₄O₇^2-（?）H₂O)and(Li⁺//Cl^-,BO₂^-,B₄O₇^2-（?）H₂O)at 308.15 K are calculated.The calculated values deviate significantly from the experimental values.Still,the changing trend of the calculated solubility curve is consistent with the experimental results,and the main reason is that it is assumed that there are only two forms of boron species i.e.metaborate and tetraborate in the solution,indicating that there are other forms of boron species in the system solution.