50t/d Sodium Thiocyanate Evaporation and Concentration System

Process Scheme

1. Material Characteristics

• Boiling point elevation Δb = 9℃, 50% NaSCN, 80kPa·A

• Critical supersaturation ΔCmax = 12g/L, highly prone to nucleation in the 50~55℃ range

• Contains small amounts of HCN and H₂S; the system requires a closed system and a two-stage alkaline scrubbing tail gas treatment.

2. Process Summary

• Raw material → Preheating

• Condensate + Secondary steam → Falling film + forced circulation triple-effect MVR

• First effect: Falling film evaporator, 48℃, concentrated to 18%

• Second effect: Forced circulation FC, 58℃, concentrated to 35%

• Third effect: Forced circulation FC + salt leg, 65℃, concentrated to 48% → Continuous centrifugation → Fluidized bed drying → Automatic packaging

• Secondary steam experiences a temperature rise of 18℃ after passing through a two-stage titanium compressor, compression ratio 1.7, power consumption 28kWh/t water.

3. Key Innovations

• Separate Effect Temperature Control: Avoiding the ΔCmax range, the first effect uses low-temperature falling film to inhibit nucleation, while the second and third effects use high-shear forced circulation to ensure crystal growth.

• Salt Leg + Washing Structure: Crystal residence time ≥45 min, C.V. ≤8%, moisture content after centrifugation ≤3%.

• Online Backflushing: Heating tubes are backflushed with secondary steam every 30 min to maintain a K value ≥1200 W/(m²·K).

• Mother liquor reflux ratio adjustable from 0~40%, ensuring the system can withstand Na₂S₂O₃ enrichment for ≥30 days.

Equipment Configuration

Evaporator: 2205 duplex steel + 2205/titanium composite tube sheet; falling film distributor uses detachable spiral nozzles to ensure film uniformity ≥95%.

Compressor: Titanium impeller, variable frequency drive, maximum linear velocity ≤230 m/s, vibration value <2.8 mm/s, design life 80,000 h.

Centrifuge: Horizontal spiral screen centrifuge, drum L/D=2.2, differential speed frequency conversion control, washing section with secondary NaSCN saturated liquid spray to reduce Na₂S₂O₃ entrainment.

Drying: Closed-loop circulating fluidized bed, nitrogen protection, oxygen content <5%, product temperature ≤55℃ to prevent thermal decomposition.

Automation and Safety: 

DCS+SIS dual redundancy, key parameters with two-out-of-three voting; more than 20 interlocks are set up for compressor surge, high liquid level, high vacuum, and excessive oxygen content.

The system has two-stage alkaline washing + activated carbon adsorption, with HCN <0.3mg/m³ and H₂S <0.5mg/m³ at the discharge port.

Explosion-proof zoning: The compressor area, motor, and instruments are all explosion-proof certified.

Operating Data (Performance Testing, October 2024)

• Evaporation Rate: 2100 kg/h Design Value: 2083 kg/h

• Live Steam: 0.18 t/t water Design Value: 0.20

• Power Consumption: 28 kWh/t water Compressor + Circulating Pump + Vacuum Pump

• Cooling Water: 2 t/t water 32℃→38℃ NaSCN yield: 97.8%

• Crystal particle size d50: 0.85mm, purity 98.7%, whiteness ≥85%, Cl⁻≤200ppm, SO₄²⁻≤300ppm

Economic and Environmental Benefits

Annual steam savings of 42,000 tons, equivalent to 4,800 tons of standard coal, and carbon reduction of 12,000 tons.

External sales price of high-purity NaSCN increased from 3,800 yuan/ton to 5,500 yuan/ton, generating approximately 18 million yuan in additional annual revenue.

Zero wastewater discharge, eliminating the previous outsourced disposal fee of 45 yuan for high-salt mother liquor from multi-effect evaporation. 0 RMB/year Investment Payback Period: 2.1 years

Customer Feedback

"The system has operated for nine months without a single unplanned shutdown. The status monitoring data for core equipment such as the compressor, centrifuge, and circulating pump are excellent, and the product purity consistently meets the owner's requirements. The project truly achieves a win-win situation of high-salinity wastewater treatment and value chain extension."

Conclusion

Kang Jinghui's 50t/d sodium thiocyanate evaporation and concentration system fully validates the reliability and economy of the "MVR + fractional-effect temperature control + salt leg washing" combined process in treating heat-sensitive, high-boiling-point elevated salt solutions. In the future, we will continue to optimize compressor efficiency and mother liquor resource utilization routes to provide more chemical, coking, and pharmaceutical companies with low-consumption, high-value, zero-discharge overall wastewater solutions.