Salt-nitrate separation process in zero-discharge industrial wastewater treatment equipment

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　For the separation of mixed solutions containing sodium chloride and sodium sulfate, under different operating conditions, the following treatment processes are generally employed: 1. Obtaining a mixed salt of sodium chloride and sodium sulfate through direct evaporation crystallization. 　　2. Cryogenic denitration yields sodium sulfate decahydrate; the mother liquor containing large amounts of sodium chloride and small amounts of sodium sulfate is used for electrolysis to produce caustic soda, while mirabilite, after being melted and evaporated, gives rise to anhydrous sodium sulfate of higher purity. 　　3. Co-production of salt and nitrate: sodium sulfate is obtained through high-temperature evaporation, while sodium chloride is obtained from the mother liquor via low-temperature evaporation; part of the mother liquor is discharged. Either sodium chloride is obtained through high-temperature treatment, while anhydrous sodium sulfate is obtained by evaporating the mother liquor at low temperature, with part of this mother liquor being discharged; or anhydrous sodium sulfate is obtained through high-temperature evaporation, after which mirabilite and a double salt of sodium chloride dihydrate crystallize out upon freezing. This double salt is melted and then returned to the original solution tank where it is evaporated again to yield anhydrous sodium sulfate. Sodium chloride crystals of higher purity are obtained by evaporating the mother liquor containing sodium chloride, with part of the mother liquor after centrifugation being discharged. 　　4. Nanofiltration membrane method: Utilizing the high retention rate of nanofiltration membranes for sulfate ions, the mixed solution is separated by these membranes to yield a retentate with a high concentration of sodium sulfate and a permeate with high purity of sodium chloride. The permeate can be subjected to further evaporation and crystallization to produce sodium chloride crystals, while the retentate, which mainly contains sodium sulfate, can either be evaporated and crystallized to recover sodium sulfate or returned to the preliminary treatment stage for removing calcium hardness.