5m³/H Flame Retardant Industry Mixed Salt Triple-Effect Evaporation Crystallization System

I.Project Background

This bromine-phosphorus flame retardant manufacturer produces 23,000 tons of tetrabromobisphenol A and decabromodiphenyl ethane annually. The production process generates 5 m³/h of highly concentrated saline mother liquor with a complex composition: NaCl 12%, Na₂SO₄ 8%, sodium bromide 3%, zinc borate 1%, organophosphorus 0.5%, COD 20,000 mg/L, pH 1–2. The raw water is classified as HW18 hazardous waste, with an outsourced disposal cost of 2,800 RMB/ton and an annual cost of nearly 120 million RMB. The company urgently needs a comprehensive solution for "reduction of mixed salts + resource utilization".

II.Core Process–"Triple-Effect Countercurrent Forced Circulation+Refrigeration Coupling"

1.Pretreatment: Lime neutralization → Zinc removal (≤200 mg/L) → Ozone catalytic oxidation (COD reduced to 3000 mg/L) → Plate and frame filtration, resulting in clear and transparent effluent with a hardness <50 mg/L, preventing subsequent scaling.

2.Triple-Effect Countercurrent Evaporation

First Effect: Live steam 0.5MPa, evaporation temperature 110℃, concentrated to 18% salt content;

Second Effect: Secondary steam from the first effect, 90℃;

Third Effect: Secondary steam from the second effect, 75℃, vacuum degree –80kPa, final concentration 38%, NaCl precipitates first.

Forced circulation pump flow rate 400m³/h, duplex stainless steel 2205, resistant to bromide pitting; online CIP + ball brush, continuous operation cycle ≥60 days.

3.Freeze-Crystallization

The mother liquor from the triple-effect is cooled to 2℃, resulting in a large amount of Na₂SO₄·10H₂O precipitating. After centrifugation, it is hot-melted back to the front end, achieving a sodium sulfate recovery rate >95%.

4.Mother Liquor Drying

The high-concentration mother liquor at the end (0.3m³/h) enters a rake dryer. The residue has a water content <10%, and is disposed of according to hazardous waste standards, reducing the total amount of impurities by 92%. 5. Condensate Reuse: Secondary steam, after passing through a surface condenser, has a COD < 30 mg/L and conductivity < 80 μS/cm. All of this steam is reused in the bromine chemical section, saving 36,000 tons of primary water annually.

III.Key Equipment Parameters (5t/h Line)

IV.Operating Data (Performance Assessment - April 2025)

1.Processing Capacity: 5.0 m³/h (100% load)

2.Steam Consumption: 0.28 t/t water, 72% steam saving compared to single-effect operation

Electricity Consumption: 12 kWh/t water

Salt Recovery: NaCl 1.1 t/d, purity 98.8%, meeting GB/T5462 Grade 1 refined industrial wet salt; Na₂SO₄ 0.7 t/d, purity 99.1%.

Total Mixed Salt: Reduced from 12 kg/t water to 0.9 kg/t water, a reduction of 92%.

Operating Cost: 22 RMB/t water (steam priced at 180 RMB/t), saving 2750 RMB/t compared to outsourced treatment, with an annual economic benefit of 32 million RMB.

V.Technical Highlights

1.For flame retardant zinc borate sensitive systems, online monitoring of Zn²⁺ prevents sodium sulfate crystal distortion, ensuring product quality.

2.Counter-current triple-effect + forced circulation provides a high heat transfer coefficient, strong anti-scaling ability, and an evaporation rate of 18 kg/m²·h.

3.Modular design with a reserved interface for an MVR steam compressor, allowing for future upgrades to zero-steam operation due to electricity price advantages.

4.Full-process DCS + APC intelligent control with automatic adjustment of key parameters enables unmanned on-site operation.

VI. Customer Value

After project commissioning, the company will completely solve the problem of high-salt mother liquor disposal, achieving "reduction, resource utilization, and harmlessness of mixed salts": Annual reduction of hazardous waste emissions by 42,000 tons, saving disposal costs of 117 million yuan; Recovery of 6,600 tons of refined salt, generating additional sales of 4.8 million yuan;