Study On The Phase Equilibrium For The System Na⁺, K⁺, Mg²⁺//Cl^-, SO₄^2-, NO₃^--H₂O And Its Sub-systems At 298.16K

In this work, the phase equilibrium of K⁺, Mg²⁺// SO₄^2-, NO₃--H₂O and Na⁺, K⁺, Mg²⁺//NO₃--H₂O, which are subsystems of the system Na⁺, K⁺, Mg²⁺// Cl-, SO₄^2-, NO₃--H₂O and have not been reported yet, and the evaporation trends of two five-salt invariant points were investigated by isothermal method at 298.16K. Based on the solubilities data attained, the phase diagrams of each system were respectively plotted, including the dry salt diagrams, the corresponding water diagrams, the phase equilibrium diagrams. These works will become the basis for studying the liquid-solid phase equilibrium of the system Na⁺, K⁺, Mg²⁺// Cl-, SO₄^2-, NO₃--H₂O.The results show:The dry salt phase diagram of K⁺, Mg²⁺ // NO₃-, SO₄^2--H₂O system consists of five crystallization fields(KNO₃, K₂SO₄, Mg(NO₃)2·6H₂O, MgSO₄·7H₂O and double salt K₂SO₄·MgSO₄·6H₂O), seven univariant curves and three invariant points corresponding to KNO₃+K₂SO₄+K₂SO₄·MgSO₄·6H₂O, KNO₃+MgSO₄·7H₂O+K₂SO₄·MgSO₄·6H₂O and Mg(NO₃)2·6H₂O+MgSO₄·7H₂O+KNO₃. One of univariant curves is reaction curve. One of invariant points is co-crystallization point, the others are reaction points.The dry salt phase diagram of Na⁺, K⁺, Mg²⁺//NO₃--H₂O system consists of three crystallization fields (NaNO₃, KNO₃ and Mg(NO₃)2·6H₂O), three univariant curves and one invariant point corresponding to NaNO₃+KNO₃+Mg(NO₃)2·6H₂O. No complex salt are found in this system. All univariant curves are reaction curves saturated with two salts.During evaporation process, the liquid composition of one of invariant points, which is saturated with five salts (KNO₃, NaNO₃, NaCl, Na7K₃Mg₂(SO₄)6(NO₃)2·6H₂O and Na₂SO₄·MgSO₄·4H₂O), will move to another five-salt invariant point(saturated with Na₂SO₄·MgSO₄·4H₂O,Na7K₃Mg₂(SO₄)6(NO₃)2·6H₂O,MgSO₄·7H₂O,NaNO₃å’ŒNaCl) if the mole ratio of Mg²⁺/SO₄^2- is higher in the brine; or move to another five-salt invariant point(saturated with Mg(NO₃)2·6H₂O, KNO₃, NaNO₃, Na7K₃Mg₂(SO₄)6(NO₃)2·6H₂O and NaCl) if the mole ratio of Mg²⁺/SO₄^2- is lower in the brine. During evaporation process, the liquid composition of one of invariant point, saturated with five salts(KCl, 3K₂SO₄·Na₂SO₄, KNO₃, Na7K₃Mg₂(SO₄)6(NO₃)2·6H₂O and NaCl) will move to another five-salt invariant point(saturated with 3K₂SO₄·Na₂SO₄, KCl, K₂Mg(SO₄)2·6H₂O, Na7K₃Mg₂(SO₄)6(NO₃)2·6H₂O and NaCl) if the concentration of NO₃- is higher in the brine; or move to another five-salt invariant point(saturated with KNO₃, KCl, KCl·MgCl·6H₂O, Na7K₃Mg₂(SO₄)6(NO₃)2·6H₂O and NaCl) if the concentration of NO₃- is lower in the brine.