Metastable phase equilibria in the aqueous ternary system (Na2SO4 + Li2SO4 + H2O) at 308.15 and 348.15 K

• Isothermal evaporation simulating device was established successfully.
• Two double salts are found in the metastable ternary system at 308.15 K.
• Only one double salt is formed in the metastable ternary system at 348.15 K.
• The density of the solution changes regularly with lithium sulfate content.

The solubility and density of the metastable phase equilibria in the aqueous ternary system (Na2SO4 + Li2SO4 + H2O) at 308.15 and 348.15 K were determined using the isothermal evaporation method. On the basis of the experimental data, the metastable phase diagrams and the diagrams of density property versus composition in the system at 308.15 and 348.15 K were plotted. In the metastable phase diagram of the ternary system at 308.15 K, there is four crystallization regions corresponding to thenardite (Na2SO4, Th), double salt 1 (Li2SO4·3Na2SO4·12H2O, Db1), double salt 2 (Li2SO4·Na2SO4, Db2), and lithium sulfate monohydrate (Li2SO4·H2O, Ls), four univariant curves, and three invariant points of the ternary system corresponding to (Th + Db1), (Db1 + Db2) and (Db2 + Ls). However, in the metastable phase diagram of the same ternary system at 348.15 K, the crystallization region of Db1 is disappeared, and there is only three crystallization regions corresponding to thenardite Th, Db2 and Ls. A comparison of the metastable phase diagrams for the ternary system at 308.15 and 348.15 K shows that the area of Th is increased whereas the area of Ls is decreased obviously and no solid solution was found. The solution density of the metastable ternary system at two temperatures changes regularly with the increasing of lithium sulfate concentration.