Method and Process for Fractional Crystallization to Extract High-Purity Sodium Sulfate and Sodium Chloride from High-Salt Wastewater for Resource Uti

Method and Process for Fractional Crystallization to Extract High-Purity Sodium Sulfate and Sodium Chloride from High-Salt Wastewater for Resource Utilization

High-salt wastewater is widely generated in industries such as coal, power generation, petroleum refining, chemicals, metallurgy, papermaking, pesticides, and others. With the advancement of modern industrial technologies, the volume of high-salt wastewater has rapidly increased, posing significant challenges to current wastewater treatment and recycling technologies. High-salt wastewater contains various substances, including organic matter, inorganic salts, oils, organic heavy metals, and radioactive materials. Due to its high salinity, which inhibits microbial survival, it is difficult to treat via biochemical methods. Consequently, such wastewater is internationally recognized as one of the most challenging types to manage and is more environmentally hazardous compared to ordinary wastewater.

This invention provides a method for recovering sodium sulfate and sodium chloride from high-salt wastewater containing these salts. The method enables low-cost, high-efficiency recovery and utilization of sodium sulfate and sodium chloride from high-salt wastewater, alleviating environmental pressure on industries like coal chemical production, reducing wastewater treatment costs, and achieving comprehensive resource utilization of wastewater.

Technical Solution
The method involves pretreating high-salt wastewater with activated carbon for decolorization, followed by sequential concentration using electrodialysis and mechanical vapor recompression (MVR). Cooling crystallization is employed to recover sodium sulfate from the concentrated solution. The desalinated mother liquor is then subjected to a two-stage evaporation crystallization process to recover sodium chloride. A portion of the mother liquor after the second-stage evaporation is recycled by mixing with incoming wastewater, while the remainder is directed to a mixed-salt evaporation crystallizer.

Detailed Steps

1. ​Pretreatment: Activated carbon is used for decolorization and removal of insoluble impurities, soluble calcium (Ca²⁺), magnesium (Mg²⁺), and silicate ions.

   • Activated carbon dosage: 7–9 g/L of wastewater.
   • COD removal rate: Approximately 75–85%.
   • Feed wastewater parameters: Total sodium chloride (NaCl) and sodium sulfate (Na₂SO₄) content in the range of 3–10%, with a NaCl-to-Na₂SO₄ mass ratio of 1–3.2.

2. ​Concentration:

   • Electrodialysis concentrates the feed wastewater to a total salt content of 18–20%.
   • Further concentration via MVR achieves final NaCl and Na₂SO₄ mass fractions of 6.5–20.58% and 6.0–9.0%, respectively, approaching saturation.

3. ​Cooling Crystallization:

   • The concentrated solution is cooled to -5–0°C (optimally -5°C) in a crystallizer to precipitate Glauber’s salt (Na₂SO₄·10H₂O).
   • The Glauber’s salt is separated, washed, dried, and processed into anhydrous sodium sulfate (purity ≥98.5%).
   • Post-crystallization mother liquor contains 0.8–1.0% Na₂SO₄.

4. ​Two-Stage Evaporation Crystallization:

   • ​First Stage: Conducted at 65–70°C under an absolute pressure of 0.05–0.06 MPa. Evaporated water accounts for 37–83% (optimally 37.8–82.3%) of the feed flow.
   • ​Second Stage: Similar conditions, with evaporated water representing 4.0–48% (optimally 4.0–47.7%) of the feed flow.
   • Sodium chloride is recovered, washed, dried, and meets industrial standards (purity ≥98.5%).
   • The mass ratio of NaCl product to feed flow in crystallizers is 0.9–1.1.

5. ​Mother Liquor Recycling:

   • 38–42% of the desalinated mother liquor from the second-stage evaporation is recycled by mixing with incoming wastewater. The remainder is processed in a mixed-salt crystallizer.

Advantages
This method delivers a high-efficiency, low-cost, and stable fractional crystallization process for high-salt wastewater. It enables separate recovery of sodium sulfate and sodium chloride from coal chemical and related industrial wastewaters. The sodium sulfate product meets the Class II Grade 1 standard (GB/T 6009-2014) with ≥98.5% purity, while the sodium chloride product complies with the Refined Industrial Salt Grade 1 standard (GB/T 5462-2015) with ≥98.5% purity. Recovery rates for both salts exceed 90%, achieving comprehensive resource utilization. The process is simple, stable, and generates no secondary pollutants beyond water evaporation.