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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