Study on phase equilibrium of the Mg2+, NH4+//Cl−, SO42−−H2O quaternary system at 363.15 K and its application

Abstract

The stable phase equilibrium data of Mg2+, NH4+//Cl−, SO42−−H2O 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 MgSO4·(NH4)2SO4·6H2O, MgSO4·6H2O, MgSO4·4H2O, MgCl2·6H2O, (NH4)2SO4, MgCl2·NH4Cl·6H2O and NH4Cl, respectively. MgSO4·(NH4)2SO4·6H2O has the largest crystalline region among these crystalline regions, which indicates that the double salt MgSO4·(NH4)2SO4·6H2O 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 (MgSO4·(NH4)2SO4·6H2O) at 298.15 K and crystallizing NH4Cl 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.