Phase Equilibria in the Aqueous Ternary System (NH 4 ) 2 SO 4 + Na 2 SO 4 + H 2 O at T = (303.15 and 313.15) K and p = 0.1 MPa

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The solubility for the aqueous ternary system (NH 4 ) 2 SO 4 + Na 2 SO 4 + H 2 O at T = (303.15 and 313.15) K and p = 0.1 MPa was investigated experimentally with the method of isothermal dissolution equilibrium. In the phase diagram of the ternary system at 303.15 K, there are eight crystallization regions, four univariant curves, and three invariant points corresponding to (Na 2 SO 4 ·10H 2 O + Na 2 SO 4 ), (Na 2 SO 4 + Na 2 SO 4 ·(NH 4 ) 2 SO 4 ·4H 2 O), and (Na 2 SO 4 ·(NH 4 ) 2 SO 4 ·4H 2 O + (NH 4 ) 2 SO 4 ). However, in the phase diagram of the same ternary system at 313.15 K, the crystallization region of Na 2 SO 4 ·10H 2 O is disappeared, and there are three single salt crystallization regions corresponding to Na 2 SO 4, Na 2 SO 4 ·(NH 4 ) 2 SO 4 ·4H 2 O, and (NH 4 ) 2 SO 4 . A comparison of the phase diagrams for this ternary system at (273.15, 288.15, 298.15, 303.15, 313.15, and 333.15) K shows that the area of Na 2 SO 4 ·(NH 4 ) 2 SO 4 ·4H 2 O and (NH 4 ) 2 SO 4 is increased obviously, whereas the area of Na 2 SO 4 (or Na 2 SO 4 ·10H 2 O) is decreased significantly and no solid solution was found. The experimental phase equilibria and phase diagrams can provide a fundamental basis for salt recovery in industrial wastewaters.