Solid-liquid equilibria of the CO(NH2)2-MgSO4-(NH4)2SO4-H2O system at 0 °C and 25 °C

Solubilities of the quaternary system CO(NH2)2-MgSO4-(NH4)2SO4-H2O at 0 °C and 25 °C were studied by isothermal solution saturation method. Under the experimental data, phase diagrams were constructed. This quaternary system at 25 °C contains five crystallization regions corresponding to MgSO4·7H2O, MgSO4·(NH4)2SO4·6H2O, CO(NH2)2, (NH4)2SO4 and MgSO4·CO(NH2)2·2H2O. Compared with the phase diagram at 25 °C, there was no crystallization region of MgSO4·CO(NH2)2·2H2O in the phase diagram at 0 °C·MgSO4·(NH4)2SO4·6H2O has the largest crystallization area among the salts, indicating MgSO4·(NH4)2SO4·6H2O is the easiest to crystallize out. Based on the characteristic of the phase diagrams, a new technology to separate (NH4)2SO4 and CO(NH2)2 from urea waste water is proposed. This process shows the advantages of recovering desirable substances and making urea waste water discharged up to standard.