Solid-liquid Phase Equilibria in the Process of Producing Lithium Hydroxide from Lithium Sulfate

The solid-liquid phase equilibria in the process of producing lithium hydroxide from lithium sulfate at 283.15, 298.15 and 323.15 K were studied via the isothermal solubility equilibrium method, and the solid species, phase diagrams, solution densities and pH were determined. The results show that four solid species of Na₂SO₄·10H₂O, Na₂SO₄·2.5H₂O, Na₂SO₄ and LiOH·H₂O occurred in the system, among them, Na₂SO₄·2.5H₂O was a new solid species not reported in the open literature, which was determined via chemical analysis, Karl Fischer water titration, X-ray diffraction(XRD), thermogravimetic analysis(TG) and differential scanning calorimetry(DSC) testing. Based on the SLE data, one systemic process to produce LiOH·H₂O from Li₂SO₄·H₂O was proposed including two crystallization steps at lower and higher temperatures.