Phase diagram of Mg2+, NH4+NH4+//Cl−, SO42−SO42−–H2O system at 0 °C and their application

• Determination of phase equilibrium of Mg2+, NH4+NH4+//Cl−, SO42−SO42−–H2O system at 0 °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.

In order to improve the technology for producing Mg–N compound fertilizer and NH4Cl based upon the analysis of phase diagram of Mg2+, NH4+NH4+//Cl−, SO42−SO42−–H2O system at 25 °C, the phase equilibrium of this quaternary system at 0 °C was studied. The solubilities of the quaternary system Mg2+, NH4+NH4+//Cl−, SO42−SO42−–H2O 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 MgCl2·6H2O, NH4Cl, MgCl2·NH4Cl·6H2O (NH4)2SO4, MgSO4·7H2O and MgSO4·(NH4)2SO4·6H2O respectively. The crystalline zones of MgSO4·(NH4)2SO4·6H2O and NH4Cl are larger than the others, which indicates that MgSO4·(NH4)2SO4·6H2O and NH4Cl can crystallize out easily. According to the analysis and calculation of the phase diagrams of Mg2+, NH4+NH4+//Cl−, SO42−SO42−–H2O 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 NH4Cl 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.