Phase diagrams of Na2SO4–MgSO4–CO(NH2)2–H2O system at 60 °C and their application

• Determination of the phase equilibrium of system Na2SO4–MgSO4–CO(NH2)2–H2O and its sub-systems.
• Phase diagrams of these systems were investigated and analyzed.
• Separation of Na2SO4 and MgSO4 by salting out method with CO(NH2)2.
• Calculation of the phase equilibrium of Na2SO4–MgSO4–CO(NH2)2–H2O system by Pitzer model.

The solubilities of ternary system Na2SO4–CO(NH2)2–H2O and MgSO4–CO(NH2)2–H2O and quaternary system Na2SO4–MgSO4–CO(NH2)2–H2O at 60 °C were measured by isothermal method, and the corresponding phase diagrams were studied. There are five crystalline fields in the quaternary system Na2SO4–MgSO4–CO(NH2)2–H2O at 60 °C, which correspond to MgSO4·6H2O, Na2SO4·MgSO4·4H2O, Na2SO4, CO(NH2)2 and MgSO4·CO(NH2)2·4H2O respectively. On the basis of the analyses of the phase diagram of the quaternary system Na2SO4–MgSO4–CO(NH2)2–H2O at 60 °C and 25 °C, Na2SO4 and Mg–N compound fertilizers can be produced using CO(NH2)2 as salting-out agent directly. On the basis of the extended Pitzer model of electrolyte solution theory, the solubilities of Na2SO4–MgSO4–CO(NH2)2–H2O system at 60 °C were calculated. The results showed that calculated values were well consistent with experimental results.

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