Recovery of (NH ₄ ) ₂ SO ₄ , NaCl, and NaBr from Hydrobromic Acid-Containing Wastewater by Cascade Evaporative Crystallization

To achieve the resource recovery from acidic wastewater containing H+, Br–, SO42–, and Cl–, this work proposes a cascade evaporation crystallization process. This process involves neutralization with aqueous ammonia (NH3(aq)) to convert wastewater into the quaternary system NH4Cl–NH4Br–(NH4)2SO4–H2O, followed by crystallization of (NH4)2SO4 and solid solution NH4(Br, Cl). Subsequent addition of NaOH converts NH4(Br, Cl) to the solid solution Na(Br, Cl), which is further separated into NaCl and NaBr via crystallization. To enable this targeted separation, the phase equilibrium solubility data of the quaternary system were determined at 298.15 and 323.15 K. Interaction parameters for the Pitzer model were fitted and used to calculate solubilities, showing good agreement with experimental data. Based on the constructed phase diagrams, a separation process was designed. Theoretical calculations indicated single-pass recovery rates of 100% for (NH4)2SO4, 67.92% for NaCl, and 70.19% for NaBr. After three cycles, the recovery rates for NaCl and NaBr increased to 94.52% and 97.55%, respectively. Experimental validation was conducted at the laboratory scale with 500 g of raw material, confirming the process feasibility with all products exceeding 99.00% purity. This work presents a scalable and sustainable strategy for valorizing hydrobromic acid-containing wastewater, bridging resource circularity, and environmental remediation in wastewater treatment.