Study on the metastable phase equilibria of the quaternary system Na⁺//NO₃^-, Cl^-, SO₄^2--H₂O at 25°C and 50°C

The metastable liquid-solid phase equilibria of the quaternary system Na⁺//NO^3-, Cl^-, SO₄²⁺-H₂O, which is the composition of the nitrate deposits in Xinjiang of China, were studied at 25°C and 50°C by isothermal evaporation method. Based on the solubility data obtained, the metastable liquid-solid phase diagrams of the system at above two temperatures were plotted respectively. The results show that, in the metastable liquid-solid phase diagrams of the system at different temperatures, there are five univariant curves, two invariant points and four crystallization fields of single salt corresponding separately to NaCl, NaNO₃, Na₂SO₄ and double salt NaNO₃·Na₂SO₄ ·H₂O. One of the invariant points is an incongruent point saturated with NaCl, Na₂SO₄ and NaNO₃·Na₂ SO₄·H₂O, and the other is a congruent point saturated with NaCl, NaNO₃ and NaNO₃·Na₂ SO₄·H₂O. Compared the metastable liquid-solid phase diagrams obtained with equilibrium phase diagrams of the same system at 25°C and 50°C, both the metastable phase diagrams obtained have the same variation trend: the NaCl crystallization regions in the metastable liquid-solid phase diagrams are bigger, the NaNO₃·Na₂SO₄ ·H₂O crystallization regions are smaller and those of NaNO₃ do not change obviously.