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Highlights: [•] Determination of phase equilibrium of Na2SO4–MgSO4–(NH4)2SO4–H2O system at 25°C. [•] Phase diagram of this system was investigated and analyzed. [•] A new technology to produce Mg–N compound fertilizers was proposed. [•] The advantage of recycling mother liquid, steady production and energy saving. [ABSTRACT FROM AUTHOR]

Based on the requirement of the new technology for producing Mg–N compound fertilizer by the reaction of MgCl 2 and (NH 4 ) 2 SO 4 , the phase equilibrium of the quaternary system Mg 2+ , NH 4 + //Cl − , SO 4 2− –H 2 O at 25 °C was studied. The solubilities of the quaternary system Mg 2+ , NH 4 + //Cl − , SO 4 2− –H 2 O were measured using isothermal method, and the phase diagram of this system was investigated. According to the phase diagram, there are nine solid phase crystalline zones, which correspond to MgSO 4 ·7H 2 O, MgSO 4 ·6H 2 O, MgSO 4 ·5H 2 O, MgSO 4 ·4H 2 O, (NH 4 ) 2 SO 4 , MgSO 4 ·(NH 4 ) 2 SO 4 ·6H 2 O, MgCl 2 ·6H 2 O, NH 4 Cl and MgCl 2 ·NH 4 Cl·6H 2 O respectively. MgSO 4 ·(NH 4 ) 2 SO 4 ·6H 2 O has the largest crystalline zone among these crystalline zones, which indicates that the double salt MgSO 4 ·(NH 4 ) 2 SO 4 ·6H 2 O is the easiest to crystallize out. Based on the analysis and calculation of the phase diagrams, the suitable conditions for the new technology were determined. The extended Pitzer model was derived and applied to the phase equilibrium calculation of system Mg 2+ , NH 4+ //Cl − , SO 4 2 –H 2 O and its sub-systems at 25 °C. The results showed that the calculated values were consistent with experimental results well. [ABSTRACT FROM AUTHOR]

In order to improve the technology for producing Mg–N compound fertilizer and NH 4 Cl based upon the analysis of phase diagram of Mg 2+ , NH 4 + //Cl − , SO 4 2 − –H 2 O system at 25 °C, the phase equilibrium of this quaternary system at 0 °C was studied. The solubilities of the quaternary system Mg 2+ , NH 4 + //Cl − , SO 4 2 − –H 2 O at 0 °C were measured by isothermal method, and the corresponding phase diagram was plotted. In this phase diagram, there are six solid phase crystalline zones, which correspond to MgCl 2 ·6H 2 O, NH 4 Cl, MgCl 2 ·NH 4 Cl·6H 2 O (NH 4 ) 2 SO 4 , MgSO 4 ·7H 2 O and MgSO 4 ·(NH 4 ) 2 SO 4 ·6H 2 O respectively. The crystalline zones of MgSO 4 ·(NH 4 ) 2 SO 4 ·6H 2 O and NH 4 Cl are larger than the others, which indicates that MgSO 4 ·(NH 4 ) 2 SO 4 ·6H 2 O and NH 4 Cl can crystallize out easily. According to the analysis and calculation of the phase diagrams of Mg 2+ , NH 4 + //Cl − , SO 4 2 − –H 2 O system at 0 °C and 25 °C, the appropriate conditions for this technology were obtained. In the process, the Mg–N compound fertilizer can be prepared at 0 °C and the NH 4 Cl can crystallize out at 25 °C. The solubilities of the studied system were calculated on the basis of the extended Pitzer model, and the results showed that the calculated values agrees well with the experimental results. [ABSTRACT FROM AUTHOR]

The solubilities of the quaternary system K 2 SO 4 –MgSO 4 –(NH 4 ) 2 SO 4 –H 2 O were measured by isothermal method at 25 °C and their dry salt phase diagrams were plotted. The results indicated that, at 25 °C, the solubility phase diagram of this quaternary system consists of seven crystallization regions, those are single-salt crystallization region MgSO 4 ·7H 2 O, (NH 4 ) 2 SO 4 , and K 2 SO 4, complex salt crystallization region MgSO 4 ·(NH 4 ) 2 SO 4 ·6H 2 O, MgSO 4 ·K 2 SO 4 ·6H 2 O, solid solution crystallization region (K 1−n , NH 4n ) 2 SO 4 (n:0.143–0.893) and (K 1−n , NH 4n ) 2 SO 4 ·MgSO 4 ·6H 2 O (n:0.552–0.973), respectively. The extended Pitzer model was derived and applied to the phase equilibrium calculation of system K 2 SO 4 –MgSO 4 –(NH 4 ) 2 SO 4 –H 2 O and its sub-systems at 25 °C. The results showed that the calculated values were consistent with experimental results well. The study on the quaternary system K 2 SO 4 –MgSO 4 –(NH 4 ) 2 SO 4 –H 2 O provides a foundation for the N–Mg–K–S compound fertilizers preparation. [ABSTRACT FROM AUTHOR]

The stable phase equilibrium data of Mg2+, NH 4 +//Cl−, SO 4 2−−H 2 O quaternary interaction system and its four ternary subsystems at 363.15 K were determined by isothermal dissolution equilibrium method, and the corresponding stable phase diagrams of the above systems were plotted. According to the phase diagram, there are seven solid phase crystalline regions, which correspond to MgSO 4 ·(NH 4) 2 SO 4 ·6H 2 O, MgSO 4 ·6H 2 O, MgSO 4 ·4H 2 O, MgCl 2 ·6H 2 O, (NH 4) 2 SO 4 , MgCl 2 ·NH 4 Cl·6H 2 O and NH 4 Cl, respectively. MgSO 4 ·(NH 4) 2 SO 4 ·6H 2 O has the largest crystalline region among these crystalline regions, which indicates that the double salt MgSO 4 ·(NH 4) 2 SO 4 ·6H 2 O is the easiest to crystallize out. Based on the extended Pitzer model, the solubility data of the ternary subsystems and the quaternary system were calculated. The results showed that the calculated values were close to the experimental values. According to the comparison between the stable phase diagram of the quaternary system at 298.15 K and 363.15 K, a new process of preparing N–Mg compound fertilizer (MgSO 4 ·(NH 4) 2 SO 4 ·6H 2 O) at 298.15 K and crystallizing NH 4 Cl at 363.15 K by using ammonium sulfate and magnesium chloride as raw materials was designed in this paper and the process route was verified by experiment. [ABSTRACT FROM AUTHOR]