Metastable Phase Equilibrium in the Aqueous Quaternary System (Li₂SO₄+ Na₂SO₄+ Li₂B₄O₇+ Na₂B₄O₇+ H₂O) at 273.15 K

Abstract

The quaternary system (Li2SO4 + Na2SO4 + Li2B4O7 + Na2B4O7 + H2O) is one of the most important and basic subsystems of the brines located in the area of the Qinghai-Xizang (Tibet) Plateau. To make certain the solubility of the lithium, sodium, and borate changes with the temperature, the metastable equilibrium of the quaternary system (Li2SO4 + Na2SO4 + Li2B4O7 + Na2B4O7 + H2O) was investigated at 273.15 K using an isothermal evaporation method. The solubilities of the equilibrated solution were measured using chemical analytical methods. The crystalloid forms of the solid phase were determined using an X-ray diffraction method. This system is of a complex type, with a double salt of sodium and lithium sulfate (3Na2SO4·Li2SO4·12H2O) formed at 273.15 K. The phase diagram of this system consists of three invariant points, seven univariant curves, and five crystallization fields. Comparisons between the phase diagrams of this system at 273.15 K and at 288.15 K show that the crystallization forms and the size of crystallization field of salt have changed. The solubility of salt sodium borate changes with temperature more obviously than that of salt lithium borate. Thus, the crystallization area of Na2B4O7·10H2O becomes bigger obviously, whereas the crystallization area of salt LiBO2·8H2O becomes smaller at 273.15 K. This difference is beneficial to separate sodium and lithium using an evaporation method, and a rise in temperature is necessary to obtain more lithium borate from the solution.