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

Abstract Bloedite (Na 2 SO 4 ·MgSO 4 ·4H 2 O) 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 Na 2 SO 4 and Mg–N compound fertilizers by a (NH 4 ) 2 SO 4 salting-out method to separate bloedite was proposed, the phase equilibrium of the quaternary system Na 2 SO 4 –MgSO 4 –(NH 4 ) 2 SO 4 –H 2 O at 25 °C was studied. The solubilities of the quaternary system Na 2 SO 4 –MgSO 4 –(NH 4 ) 2 SO 4 –H 2 O 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 MgSO 4 ·7H 2 O, MgSO 4 ·Na 2 SO 4 ·4H 2 O, Na 2 SO 4 ·10H 2 O, Na 2 SO 4 ·(NH 4 ) 2 SO 4 ·4H 2 O, (NH 4 ) 2 SO 4 and MgSO 4 ·(NH 4 ) 2 SO 4 ·6H 2 O respectively. MgSO 4 ·(NH 4 ) 2 SO 4 ·6H 2 O has the largest crystalline zone among these crystalline zones, which indicates that the double salt MgSO 4 ·(NH 4 ) 2 SO 4 ·6H 2 O 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 Na 2 SO 4 ·10H 2 O using bloedite and (NH 4 ) 2 SO 4 as raw materials shows the advantage of non-evaporation of water and low energy consumption.