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(NH 2 ) 2 -MgSO 4 -(NH 4 ) 2 SO 4 -H 2 O 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 MgSO 4 ·7H 2 O, MgSO 4 ·(NH 4 ) 2 SO 4 ·6H 2 O, CO(NH 2 ) 2 , (NH 4 ) 2 SO 4 and MgSO 4 ·CO(NH 2 ) 2 ·2H 2 O. Compared with the phase diagram at 25 °C, there was no crystallization region of MgSO 4 ·CO(NH 2 ) 2 ·2H 2 O in the phase diagram at 0 °C·MgSO 4 ·(NH 4 ) 2 SO 4 ·6H 2 O has the largest crystallization area among the salts, indicating MgSO 4 ·(NH 4 ) 2 SO 4 ·6H 2 O is the easiest to crystallize out. Based on the characteristic of the phase diagrams, a new technology to separate (NH 4 ) 2 SO 4 and CO(NH 2 ) 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. [ABSTRACT FROM AUTHOR]