Study on the Phase Equilibrium of the Na ₂ SO ₄ -(NH ₄ ) ₂ SO ₄ –NH ₄ Cl-NaCl-CH ₃ OH-H ₂ O System at 333.15 K and Its Application

To develop a novel process for separating high-salinity wastewater containing Na +, NH 4 +, Cl –, SO 4 2–, this study determines the phase equilibrium of the Na +, NH 4 + //Cl –, SO 4 2– -CH 3 OH-H 2 O system at 333.15 K. Data for the system and its subsystems were determined, and the corresponding phase diagrams were constructed. A new double salt, 4Na 2 SO 4 ·(NH 4 ) 2 SO 4 ·H 2 O (denoted as D), was identified in the Na 2 SO 4 -(NH 4 ) 2 SO 4 –CH 3 OH-H 2 O subsystem, differing from the Na 2 SO 4 ·(NH 4 ) 2 SO 4 ·4H 2 O found in the pure aqueous system. The full system exhibits four single-salt crystallization regions (Na 2 SO 4, (NH 4 ) 2 SO 4, NaCl, NH 4 Cl) and one double-salt region (D). Using the extended Pitzer model, the relevant ion-interaction parameters were regressed, enabling accurate prediction of system solubility. The calculated values showed good agreement with experimental data. Based on the phase diagram, a novel separation process was designed for the recovery of salts from complex wastewater containing Na +, NH 4 +, Cl – and SO 4 2– .