Phase diagrams of the quaternary system Mg2+, NH4+//Cl−, SO42− in water at 25 °C and their application

• Determination of phase equilibrium of Mg2+, NH4+//Cl−, SO42−–H2O system at 25 °C.
• Phase diagram of this system has been investigated and analyzed.
• A new technology to produce Mg–N compound fertilizers has been proposed.
• Calculation of the phase equilibrium concerning this system by Pitzer model.
• Consistence between calculated results and experimental data has been presented.

Based on the requirement of the new technology for producing Mg–N compound fertilizer by the reaction of MgCl2 and (NH4)2SO4, the phase equilibrium of the quaternary system Mg2+, NH4+//Cl−, SO42−–H2O at 25 °C was studied. The solubilities of the quaternary system Mg2+, NH4+//Cl−, SO42−–H2O 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 MgSO4·7H2O, MgSO4·6H2O, MgSO4·5H2O, MgSO4·4H2O, (NH4)2SO4, MgSO4·(NH4)2SO4·6H2O, MgCl2·6H2O, NH4Cl and MgCl2·NH4Cl·6H2O respectively. MgSO4·(NH4)2SO4·6H2O has the largest crystalline zone among these crystalline zones, which indicates that the double salt MgSO4·(NH4)2SO4·6H2O 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 Mg2+, NH4+//Cl−, SO42–H2O and its sub-systems at 25 °C. The results showed that the calculated values were consistent with experimental results well.