The Phase Equilibrium Of K⁺,NH₄⁺,Mg²⁺//SO₄^2-,Cl^-- H₂O And Its Subsystems At 0?

Aiming to realize the comprehensive exploitation and utilization of magnesium and potassium resources of salt lakes,a new process that carnallite and ammonium sulfate directly react to produce chloride-free potash fertilizer and ammonium chloride has been proposed.The phase diagram of K⁺,NH₄⁺,Mg²⁺//SO₄^2-,Cl^--H₂O plays a vital role in exploiting brine resources.The solid-liquid equilibrium could offer theoretic foundation for the production of chloride-free potash fertilizer.The solubilities of the quinary system and its two subsystems were measured by isothermal solubility saturation method.Corresponding phase diagrams of dry salt were plotted.The quaternary phase diagram of?NH₄?₂SO₄-MgSO₄-K₂SO₄-H₂O was divided into six crystallization fields corresponding to MgSO₄·7H₂O,K₂SO₄,?NH₄?₂SO₄·MgSO₄·6H₂O,K₂SO₄·MgSO₄·6H₂O,(NH_4n,K_1-n)₂SO₄ and(NH_4n,K_1-n)₂SO₄·MgSO₄·6H₂O.The contrast of the phase diagrams between 25?and 0?suggests low temperature is useful for getting K₂SO₄·MgSO₄·6H₂O crystal while exerts negative influence on the?NH₄?₂SO₄crystallization.There exists seven invariant points and eight crystallization fields in the quaternary system of NH₄Cl-MgCl₂-KCl-H₂O:MgCl₂·6H₂O,KCl,NH₄Cl,KCl·MgCl₂·6H₂O,NH₄Cl·MgCl₂·6H₂O,(K_1-m,NH_4m)Cl,(NH_4m,K_1-m)Cl and(NH_4m,K_1-m)Cl·MgCl₂·6H₂O.The salt addition could considerably enlarge the crystallization of NH₄Cl·MgCl₂·6H₂O and KCl.Compared with the phase diagrams of this system at 60?,crystalline regions of NH₄Cl·MgCl₂·6H₂O and KCl are weakened.It indicates that the crystallization of NH₄Cl·MgCl₂·6H₂O and KCl become easily as temperatures go up.Accordingly,a new technology to separate NH₄Cl·MgCl₂·6H₂O and KCl was proposed.Through modification of operating condition,potassium chloride and ammonium carnallite can be obtained simultaneously.In this case,the comprehensive exploitation and utilization of magnesium chloride and potassium chloride in the salt lakes can be realized.There are eight crystallization fields?saturated with K₂SO₄?in the quinary system of K⁺,NH₄⁺,Mg²⁺//SO₄^2-,Cl^--H₂O:(NH_4n,K_1-n)₂SO₄,(K_1-m,NH_4m)Cl,(NH_4m,K_1-m)Cl,?NH₄?₂SO₄·MgSO₄·6H₂O,(NH_4n,K_1-n)₂SO₄·MgSO₄·6H₂O,KCl,K₂SO₄·MgSO₄·6H₂O and(NH_4m,K_1-m)Cl·MgCl₂·6H₂O.The solubility of ternary subsystems can be acquired in previous literature.As a result,the binary and ternary parameters of interionic interaction can be correlated.On the basis of Pitzer model,solubilities of the quaternary system can be predicted.Calculated values showed well agreement with experimental values.Accordingly,the Pitzer model is feasible to predict the solid-liquid equilibrium of these two quaternary systems.