Phase diagrams of Na2SO4–MgSO4–(NH4)2SO4–H2O system at 25 °C and their application

• 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.

Bloedite (Na2SO4·MgSO4·4H2O) is a kind of abundant natural resources. So far, it has not been developed and utilized effectively. In order to process and develop bloedite, a new technology to produce Na2SO4 and Mg–N compound fertilizers by a (NH4)2SO4 salting-out method to separate bloedite was proposed, the phase equilibrium of the quaternary system Na2SO4–MgSO4–(NH4)2SO4–H2O at 25 °C was studied. The solubilities of the quaternary system Na2SO4–MgSO4–(NH4)2SO4–H2O were measured using isothermal method, and the phase diagram of this system was investigated. According to this phase diagram, there are six solid phases crystalline zones, which correspond to MgSO4·7H2O, MgSO4·Na2SO4·4H2O, Na2SO4·10H2O, Na2SO4·(NH4)2SO4·4H2O, (NH4)2SO4 and MgSO4·(NH4)2SO4·6H2O 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 most easily to crystallize out. Based on the analysis and calculation of the phase diagrams, a new technology to produce Mg–N compound fertilizers and Na2SO4·10H2O using bloedite and (NH4)2SO4 as raw materials shows the advantage of non-evaporation of water and low energy consumption.