Study On Phase Equilibrium Of K⁺,Mg²⁺,NH₄⁺//Cl^-,SO₄^2--H₂O And MgCl₂-NaCl-NH₄Cl-H₂O And Application

The production of chloride-free potassium fertilizer by the reaction of carnallite,a salt lake product from solar ponds,and ammonium sulfate,an industrial by-product,is a new idea for the comprehensive utilization and development of inorganic resources such as potassium and magnesium in the salt lake.Therefore,the phase equilibrium of K⁺,Mg²⁺,NH₄⁺//Cl^-,SO₄^2--H₂O quinary system and some quaternary systems was studied.In addition,in order to comprehensively utilize the mother solution after extracting KCl from carnallite,the phase equilibrium of the NaCl-MgCl₂-NH₄Cl-H₂O quaternary system at 60℃and 75℃was investigated in this paper,and a process route for producing ammonium carnallite was proposed.Phase equilibrium data of K⁺,NH₄⁺//Cl^-,SO₄^2--H₂O quaternary system and KCl-NH₄Cl-H₂O and K₂SO₄-（NH₄）₂SO₄-H₂O ternary subsystems at 0℃were determined by isothermal dissolution method,and the phase diagrams were plotted.It can be seen from the dry-salt phase diagram that there is one saturation point and three solid phase fields of crystallization in this quaternary phase diagram,which correspond to(K_1-t,（NH₄）_t)₂SO₄,(K_1-m,（NH₄）_m)Cl,and(（NH₄）_n,K_1-n)Cl.Field of(K_1-t,（NH₄）_t)₂SO₄is larger than other crystalline fields,which indicates that it is easy to crystallize out from solution.By analyzing and calculating the phase diagrams at 0℃and 50℃,chloride-free K-N compound fertilizer(K_1-t,（NH₄）_t)₂SO₄and by-product(（NH₄）_n,K_1-n)Cl can be produced via KCl and（NH₄）₂SO₄as raw materials.Theoretical calculations were accomplished for this process.Phase equilibrium of the quinary system K⁺,Mg²⁺,NH₄⁺//Cl^-,SO₄^2--H₂O at 0℃and25℃were studied using the isothermal dissolution equilibrium method,and the corresponding phase diagrams were plotted.According to the diagrams,there are seven saturation points and eight solid phase crystalline fields in this system.Based on the difference in the size of the crystallization zone,a process was proposed using carnallite and ammonium sulfate as raw materials to produce K-Mg compound fertilizer and K-NH₄compound fertilizer,and the corresponding mother solution was recycled.Theoretical calculations were accomplished.Phase equilibrium data of the quaternary NaCl-MgCl₂-NH₄Cl-H₂O quaternary system and its related ternary subsystems were measured by the isothermal dissolution method at60℃and 75℃,and the phase diagrams were plotted.The quaternary system has two saturation points and four crystalline regions,which corresponds to MgCl₂·6H₂O,NH₄Cl,NaCl,and double salt MgCl₂·NH₄Cl·6H₂O.No solid solution is formed.According to dry salt phase diagram of the quaternary system at high and low temperatures,a new process for producing ammonium carnallite from the mother solution after extracting KCl was proposed.In addition,based on the of Pitzer electrolyte theory,the phase equilibrium data of the quaternary system and three ternary subsystems were calculated by constructing a model of the quaternary system.The results show that the calculated values agree with the experimental values.The effect of the metastable conditions on the（NH₄）₂SO₄-Mg SO₄-K₂SO₄-H₂O quaternary system was studied.The results show that,compared with the stable phase diagram,the saturation point and solid phase crystalline field of the system under metastable conditions show significant differences:the crystalline region of K₂SO₄·Mg SO₄·6H₂O decreases,and the crystalline region of（NH₄）₂SO₄·Mg SO₄·6H₂O increases.The metastable phase diagram has one more crystalline region of solid phase Mg SO₄·6H₂O than the stable phase diagram.Based on this metastable phenomenon,a new process for producing N-Mg compound fertilizer using shoenite as raw material was proposed,and the mother solution was recycled.Theoretical calculations were accomplished for this process.