Phase Diagrams of the Na2SO4–MgSO4–(NH4)2SO4–H2O System at 60 °C and Their Application

Abstract

The solubilities of quaternary system Na2SO4–MgSO4–(NH4)2SO4–H2O at 60 °C were measured by the isothermal method, and the corresponding phase diagram was studied. The analysis of the phase diagram shows that there are five crystalline fields, MgSO4·6H2O, Na2SO4·MgSO4·4H2O, Na2SO4, (NH4)2SO4, and MgSO4·(NH4)2SO4·6H2O, respectively. MgSO4·(NH4)2SO4·6H2O, and Na2SO4 crystalline fields are larger than other crystalline fields, which indicates that MgSO4·(NH4)2SO4·6H2O and Na2SO4 are crystallized out easily. On the basis of the analysis and calculation of the phase diagrams of the quaternary system Na2SO4–MgSO4–(NH4)2SO4–H2O and ternary system Na2SO4–(NH4)2SO4–H2O at 60 °C, a technology was designed to produce Mg–N compound fertilizers and Na2SO4 using (NH4)2SO4 as a salting-out agent to separate bloedite, which is easy to operate, fast to reach phases equilibrium, and can gain anhydrous Na2SO4 directly. On the basis of the Pitzer model of electrolyte solution theory, the solubilities of Na2SO4–MgSO4–(NH4)2SO4–H2O system at 60 °C were calculated. The results showed that calculated values were well consistent with experimental results.