Phase Equilibria Of The Quaternary Systems Containing Lithium Magnesium And Borate

The composition of brine is simplified to the system Li⁺,Mg²⁺//Cl^-,SO₄^2-,B₄O₇^2-H₂O after extracting potassium.The phase equilibrium of this complex system and subsystems containing lithium,magnesium and boron was researched by isothermal dissolution method.The solubility and the physico-chemical properties including density,refractive index,pH value were analyzed and the solid phase was identified.On the basis of the experimental data,the phase diagrams and the diagrams of physico-chemical properties versus composition were respectively plotted.The phase equilibrium of the ternary system Li⁺,Mg²⁺//borate-H₂O at 288.15 K belongs to Hydrate I Type System,and neither double salt nor solid solution was generated.In the phase diagram there was one invariant point,two univariant curves,two crystallization regions corresponding to Li₂B₄O₇·3H₂ O and Mg₂B₆O₁1·15H₂O.The phase diagrams at 298.15 K and 308.15 K were analogous to that at 288.15 K.In all of the phase diagrams,no double salt or solid solution was generated and belongs to Hydrate I Type System.The concentration of MgB₄O₇ decreased with the increase of the content of Li₂B₄O₇ in liquid phase,which indicates that Li₂B₄O₇ had a strong salting-out effect on MgB₄O₇·9H₂O.It was also found that MgB₄O₇·9H₂O is unstable,and can easily react with H₂O and then converts into Mg₂B₆O₁1·15H₂O.It also indicated that the physicochemical properties including density,refractive index,and pH value are in regular changes with the concentration of Li₂B₄O₇.Density and refractive index achieve the maximum values at the invariant point with the increase of the concentration of Li₂B₄O₇.Using the empirical equations,the calculated values of density and refractive index highly matched the experimental values.Comparing with the three phase diagrams at different temperature,with the raise of temperature both the crystallization were the same while the area of crystalline fields of Li₂B₄O₇·3H₂O and Mg₂B₆O₁1·15H₂O gradually decreased and their solubilities increased.In the dry-salt phase diagram of the quaternary system Li⁺,Mg²⁺//SO₄^2-,B₄O₇^2--H₂ O at 298.15 K,there were two invariant points?F₁: Li₂SO₄·H₂O+MgSO₄·7H₂O+MgB₄O₇·9H₂O,F₂: Li₂SO₄·H₂O+Li₂B₄O₇·3H₂O+MgB₄O₇·9H₂O?,five univariant solubility isothermal curves and four crystallization regions.What is different from the ternary system was the conversion from hungchsaoite to inderite can be retarded if existence of MgSO₄ and Li₂SO₄ in the solution.The physico-chemical properties of the quaternary system Li⁺,Mg²⁺//SO₄^2-,B₄O₇^2--H₂O at 298.15 K changed regularly with the composition.With SO₄^2-content increasing,the refractive index and density increases obviously and because of the increase of B₄O₇^2-content,the pH value reduced.Compared with phase diagram at 288.15 K,no new solid phase was formed,and the area of crystalline fields of Li₂B₄O₇·3H₂O reduced while that of Mg₂B₄O₇·7H₂O increased.In the dry-salt phase diagram of the quaternary system Li⁺,Mg²⁺//Cl^-,B₄O₇^2--H₂ O at298.15 K there were three invariant points?F₁: LiCl·H₂O+MgB₄O₇·9H₂O+Li₂B₄O₇·3H₂O,F₂: LiCl·H₂O+LiCl·MgCl₂·7H₂O+MgB₄O₇·9H₂O,F₃: MgCl₂·6H₂O+LiCIlMgCl₂·7H₂O+MgB₄O₇·9H₂O?,five crystallization regions and seven univariant solubility curves.The conversion from hungchsaoite to inderite can be blocked for the existence of high concentration of LiCl and MgCl₂,but it can convert into macallisterite.It was found that LiCl and MgCl₂ had strong salting-out effect on borate.With Cl^-content increasing,the refractive index and density increases obviously and because of the increase of B₄O₇^2-content,the pH value reduced.Compared with phase diagram at 288.15 K,no new solid phase was formed,and the area of crystalline fields of Li₂B₄O₇·3H₂O reduced while that of Mg₂B₄O₇·9H₂O and MgCl₂·7H₂ O increased for the salting-out effect of LiCl.