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Highlights • Solubilities of the K+, NH 4 +//Cl−, SO 4 2− - H 2 O system were measured by isothermal method. • Completed determination of solid solution composition based on the linear rule. • A technological process to produce chlorine-free K-N compound fertilizers and by-product potassium ammonium chloride. • The proposed process was verified by experiments. Abstract In order to produce chloride-free K-N compound fertilizers by the reaction of KCl (carnallite) and (NH 4) 2 SO 4 as raw materials, the phase equilibrium of the mutual quaternary system K+, NH 4 +//Cl−, SO 4 2− - H 2 O and its two ternary subsystems KCl-NH 4 Cl-H 2 O, K 2 SO 4 -(NH 4) 2 SO 4 -H 2 O were measured by isothermal method at 273.15 K. And the corresponding phase diagrams were plotted and analysed. The equilibrium phase diagram of the quaternary system K+, NH 4 +//Cl−, SO 4 2− - H 2 O shows that there is one saturation point and three solid phase fields of crystallization. Three fields of crystallization represent solid solution salts corresponding to (K 1−t , (NH 4) t) 2 SO 4, (K 1−m , (NH 4) m)Cl and ((NH 4) n , K 1−n)Cl respectively. The phase analysis shows that field of (K 1−t , (NH 4) t) 2 SO 4 is larger than other crystalline fields, and there is no crystalline field of pure potassium sulfate, which indicates that the K 1-t, (NH 4) t) 2 SO 4 is easy to crystallize out from solution. By analysing and calculating the phase diagrams of K+, NH 4 +//Cl−, SO 4 2− - H 2 O at 273.15 K and 323.15 K, chloride-free K-N compound fertilizers (K 1−t, (NH 4) t) 2 SO 4) can be obtained using KCl and (NH 4) 2 SO 4 as raw materials at 273.15 K, and ((NH 4) t , K 1−t)Cl can crystallize out at 323.15 K. The new technology shows the advantages of effective utilization of carnallite resources. The research on this quaternary phase diagram is the foundation of the chloride-free potash production by the reaction of carnallite and ammonium sulfate. [ABSTRACT FROM AUTHOR]

• Solubilities of quaternary system NaCl-MgCl 2 -NH 4 Cl-H 2 O at 348.15 K. • The extended Pitzer model was used to predict the phase equilibrium data. • The ion interaction parameters θ and ψ at 348.15 K were regressed and calculated. • An optimized process to produce ammonium carnallite. Based on the requirement of the new technology to achieve comprehensive utilization of sodium and magnesium resources by using the mother solution from the preparation of potassium salt as raw material, the phase equilibrium data of the quaternary system NH 4 Cl-MgCl 2 -NaCl-H 2 O as well as its ternary subsystems MgCl 2 -NH 4 Cl-H 2 O and NaCl-NH 4 Cl-H 2 O were investigated by an isothermal equilibrium method under atmospheric pressure at 348.15 K, and the dry-salt phase diagram and water-phase diagram were drawn. Two invariant points and four crystallization fields including two single salts NH 4 Cl and NaCl, one hydrate salt MgCl 2 ·6H 2 O, and one double salt MgCl 2 ·NH 4 Cl·6H 2 O were determined in this quaternary system. Combining our results with available experimental data in the literature, an optimized process to produce ammonium carnallite and NaCl was proposed. Using the Pitzer electrolyte solution model, the experimental results were regressed and calculated. The calculated values agree well with the experimental values. [ABSTRACT FROM AUTHOR]

• Solubilities of NaCl-MgCl 2 -NH 4 Cl-H 2 O and its ternary subsystem NaCl-MgCl 2 -H 2 O were investigated. • The extended Pitzer model was applied to the phase equilibrium calculation. • The ion interaction parameters θ and ψ were correlated. • A technological process to produce ammonium carnallite and NaCl via mixed solution after extracting potassium chloride. Based on the requirement of the new technology for producing ammonium carnallite and NaCl via the solution left after the preparation of potassium salt as raw materials, the solubilities of the quaternary system NaCl-MgCl 2 -NH 4 Cl-H 2 O and ternary subsystem NaCl-MgCl 2 -H 2 O at 333.15 K were measured using the isothermal dissolution method, and the corresponding phase diagrams were plotted. The phase diagram of the ternary system consists of three fields of crystallization, which correspond to NaCl, MgCl 2 ·6H 2 O and their co-existing fields respectively. There are four fields of crystallization in the quaternary system, which correspond to MgCl 2 ·6H 2 O, NH 4 Cl, NaCl and the double salt MgCl 2 ·NH 4 Cl·6H 2 O respectively. By analyzing and calculating the phase diagrams, a technological route was proposed, which indicated that the Na-Mg resources were fully utilized. The extended Pitzer model was applied to the phase equilibrium calculation and the results showed that the calculated data were consistent with experimental values well. [ABSTRACT FROM AUTHOR]