Study On Metastable Equilibrium Of System Na⁺//Cl^-, SO₄^2-, NO₃^--H₂O At 25℃ And Its Applications

In this paper, the liquid-solid metastable phase of quaternary system Na⁺//Cl^–, SO₄^2–, NO₃^–—H₂O, to which the nitrate deposits in Xinjiang belong, was studied by isothermal-evaporation method at 25℃. According to the corresponding data obtained, metastable phase diagram, dry salt diagram, the relevant water diagram and the crystallization-region separation diagram were drawn.On these bases, the comparison of metastable phase diagram of quaternary system Na⁺//Cl^–, SO₄^2–, NO₃^–—H₂O at 25℃with that at 50℃and the equilibrium phase diagram of quaternary system Na⁺//Cl^–, SO₄^2–, NO₃^–—H₂O at 25℃, have been carried out. The results show:(1)In the metastable dry salt diagram of quaternary system Na⁺//Cl^–, SO₄^2–, NO₃^–—H₂O at 25℃, there are four crystallization fields with one salt corresponding to Na₂SO₄,NaCl,NaNO₃ and NaNO₃·Na₂SO₄·H₂O, the crystallization fields of Na₂SO₄·10H₂O can't be found; five univariant curves with two salts corresponding to NaNO₃+NaNO₃·Na₂SO₄·H₂O,Na₂SO₄+NaCl,NaCl+NaNO₃,NaNO₃·Na₂SO₄·H₂O +NaCl,NaNO₃·Na₂SO₄·H₂O+Na₂SO₄; two invariant points with three salts corresponding to Na₂SO₄+NaCl+NaNO₃·Na₂SO₄·H₂O,NaNO₃+NaNO₃·Na₂SO₄·H₂O +NaCl.(2) Compared with the metastable system at 50℃, the crystallization area of NaNO₃·Na₂SO₄·H₂O expands about by four times, that of NaNO₃ about one time, and that of NaCl enlarges a little. The quaternary metastable system at 25℃contains more water, less salt, and the corresponding solubility is smaller.(3) Compared with equilibrium phase diagram at 25℃, the crystallization region of NaCl in metastable liquid-solid phase diagram is bigger, that of NaNO₃·Na₂SO₄·H₂O is smaller, and that of NaNO₃ does not change obviously.