8t/h sodium sulfate and sodium chloride dual-effect MVR + single-effect MVR evaporation crystallization system

I. Project Background

This agrochemical company is constructing a new 30,000-ton/year pyraclostrobin project, producing 8 tons/hour of saline wastewater as a byproduct. The wastewater composition is: Na₂SO₄ 15%, NaCl 12%, CODcr 800 mg/L. The company requires:

1. Zero wastewater discharge from the entire plant;

2. Sodium sulfate ≥99.0%, sodium chloride ≥98.5% (refined industrial wet salt standard);

3. Comprehensive energy consumption per ton of water for the evaporation system ≤42 kWh;

4. All crystalline salt will be sold externally, generating annual revenue ≥3.2 million RMB.

II. Process Route – “Dual-Effect MVR High-Temperature Nitrification + Single-Effect MVR Low-Temperature Salting + Refrigeration Coupling”

1.Pretreatment: High-hardness, high-COD wastewater first undergoes “ozone catalytic oxidation + chemical softening + tubular microfiltration”, resulting in effluent hardness ≤50 mg/L and COD ≤200 mg/L, preheated to 95℃.

2.Double-Effect MVR Concentration and Nitrate Extraction Section: The first-effect falling film evaporator (Ti heat exchanger tubes) evaporates at 95℃, with a compressor temperature rise of 18℃. The discharge density is 1.28 t/m³, and the Na₂SO₄ supersaturation is 1.15. Crystals are continuously discharged to the thickener. The second-effect forced circulation evaporator utilizes the secondary steam from the first effect, operating at 78℃, to further concentrate Na₂SO₄ to near saturation. The mother liquor is returned to the first effect. Sodium sulfate crystals are centrifuged, dehydrated, and then sent to a flash dryer. The product purity is 99.3%, whiteness is 82, and free water is ≤0.2%.

3.Deep Nitrate Extraction via Freeze-Crystallization: The mother liquor from the double-effect system is cooled to 2℃, resulting in a large precipitation of sodium sulfate decahydrate (Na₂SO₄·10H₂O). This precipitate is then returned to the double-effect system via hot melting, ensuring a total Na₂SO₄ recovery rate of ≥97%. 

4.Single-Effect MVR Salt Settling Section: The cryogenic supernatant (NaCl ~18%, Na₂SO₄ ≤2%) enters the single-effect forced circulation MVR, operating at 55℃ (vacuum -87kPa), and is concentrated until NaCl is supersaturated. The crystal slurry is discharged into a centrifuge to obtain refined NaCl with a purity of 98.8% and free water ≤0.3%. A small amount of impurities in the mother liquor is dried by drum drying and sold as a de-icing agent, achieving full utilization.

5.Condensate and Heat Recovery: Secondary steam is mechanically compressed and fully reused. Condensate is treated by a cation exchange bed + mixed bed, with a conductivity ≤20μS/cm, and is fully reused as makeup water for circulating cooling water. The system's waste heat is used to preheat raw water, achieving a comprehensive thermal efficiency of 92%.

III. Key Equipment and Materials

1.Double-Effect MVR: Evaporation capacity 6t/h (4t/h for the first effect, 2t/h for the second effect). The compressor is a magnetic levitation centrifugal type with a power of 560kW, a compression ratio of 1.7, and a COP value of 7.8. 

2.Single-effect MVR: Evaporation capacity 2t/h, compressor power 160kW, titanium heat exchange tubes, resistant to Cl⁻ corrosion.

3.Refrigeration unit: Ethylene glycol indirect cooling, cooling capacity 850kW, frequency converter control, temperature fluctuation ±0.5℃.

4.Wet-flow components: Duplex stainless steel 2205+Ti composite plate, extending equipment life to over 12 years.

IV. Operational Data (Performance Assessment in March 2024)

1.Processing Capacity: 8.0 t/h (100% load)

2.Comprehensive Evaporation Energy Consumption: 39.8 kWh/t water (including 8.2 kWh for chilling)

3.Steam Consumption: 0.03 t/t water (preheating only)

4.Sodium Sulfate Production: 1.28 t/h, Annual Revenue: RMB 1.65 million

5.Sodium Chloride Production: 0.95 t/h, Annual Revenue: RMB 1.55 million

6.Impurity Salt Rate: 1.8% (significantly lower than the industry average of 5%)

V. Technical Highlights

1."Double-effect + Single-effect" MVR series connection, zero steam replenishment, saving 58% more energy than traditional triple-effect evaporation.

2.High-temperature nitrate precipitation - low-temperature salt precipitation - chilling coupling, salt and nitrate separation in one step, crystallized salt purity above 98%.

3. Full-process DCS + APC intelligent control, online closed-loop adjustment of evaporation endpoint density and crystal particle size, achieving unattended operation. 4. Modular skid-mounted design, the main equipment occupies only 480m², with a construction period of 4.5 months, 30% shorter than similar projects.

VI.Customer

Value After the project is put into operation, the company will reduce hazardous waste disposal costs by 4.2 million yuan annually, generate 3.2 million yuan in by-product salt sales revenue, and achieve a net profit of 7.4 million yuan; simultaneously, it will achieve zero discharge of high-salinity wastewater, providing a solid environmental performance foundation for its subsequent IPO.