250t/d Rubber Additives CBC Water Triple-Effect Evaporation Crystallization System

I. Project Background 

The mother liquor and wash water discharged during the production of CBC (N-cyclohexyl-2-(4-morpholine thio)benzothiazole), a rubber vulcanization accelerator, exhibit four high characteristics: high salt content (Na₂SO₄ 12-16%, NaCl 3-5%), high COD (30000-40000 mg/L), high toxicity (morpholine, benzothiazole derivatives), and high color (platinum-cobalt color 3000). The traditional "dilution + biochemical treatment + evaporation pond" process can no longer meet the region's requirements for the resource utilization of miscellaneous salts. After comparing multiple options, the company ultimately decided to adopt the route of "pretreatment for hardening and COD removal → triple-effect forced circulation evaporation and crystallization → freeze-fractional salt extraction → mother liquor drying," constructing a 250t/d triple-effect evaporation and crystallization system to achieve zero wastewater discharge and sodium sulfate resource utilization.

II. Design Water Quality and Objectives

III. Process Route

1. Pretreatment

"Lime-Soda Ash Softening + High-Efficiency Air Flotation + Ozone Catalytic Oxidation" coupling, hardness reduced to 50mg/L, COD removal rate 45%, color reduction 80%, ensuring no risk of calcium carbonate and calcium sulfate scaling in subsequent evaporation.

2. Triple-Effect Forced Circulation Evaporation Crystallization

The process employs a triple-effect mode of "co-current + external heating + forced circulation": The first effect heats at 108℃, utilizing 0.5MPa saturated live steam from the plant area; the second effect heats at 92℃, and the third effect at 72℃. Secondary steam from the final effect enters the surface condenser, maintaining a vacuum of -0.088MPa; the circulation pumps in each effect have a flow rate of 5500 m³/h, with a flow velocity of 3.2 m/s within the pipes, resulting in low crystal shear and large particle size; the Oslo DTB crystallizer is located in the third effect, producing a slurry solid content of 25-30%, an average particle size of 0.50 mm, and a water content ≤3% after centrifugation.

3. Freezing and Separation Salt Extraction

The centrifuged mother liquor enters a freezing crystallizer (-5℃), where Na₂SO₄ precipitates as Na₂SO₄·10H₂O. This precipitate is then returned to the front section via hot melting, achieving dual quality standards for sodium sulfate and sodium chloride.

4. Mother Liquor Drying: The frozen mother liquor is re-entered into a scraped-film evaporator for drying. The dried solids contain ≤5% water, producing 0.8 t/d of mixed salts, which are then sent to the industrial park's hazardous waste center. The system has zero mother liquor discharge.

5. Condensate Treatment: The mixed condensate, after passing through an equalization tank + anaerobic bioreactor + aerobic MBR + ozone BAF process, has a COD ≤60 mg/L and conductivity ≤300 μS/cm. All of this condensate is reused in the cooling tower makeup water and salt treatment processes.

IV. Key Equipment Configuration

V. Operating Performance (365-day average from October 2023 to September 2024)

• Capacity: 255t/d (load) (Efficiency 102%)

• Steam consumption: 0.31t/t water (including drying)

• Comprehensive power consumption: 19kWh/t water

• Sodium sulfate production: 31.5t/d, purity 97.5%, whiteness 84, sold to the glass/detergent industry, price 360 yuan/t

• Sodium chloride production: 7.8t/d, purity 98.2%, sold to chlor-alkali plants

• Condensate reuse rate: 93%, annual water saving 82,000t

• System operating rate: 97%. 1%, 3 unplanned shutdowns, each <6 hours. Cleaning cycle: 110 days for the first effect, 110 days for the second effect, 95 days for the third effect, 3 times longer than traditional solutions. Operating cost: 39 RMB/t wastewater (including steam, electricity, chemicals, and labor). Investment payback period: 1.7 years. 

Technological Innovation

1. High salt and high COD synergy: Ozone catalytic oxidation pretreatment + three-effect staged crystallization, total COD removal rate ≥99.5%, avoiding high-temperature coking.

2. Titanium/2205 composite material: Resistant to Cl⁻ + SO₄²⁻ mixed acid corrosion, equipment design life 10 years, reducing annual corrosion maintenance costs by 85% compared to the 316L solution.

3. Co-current + forced circulation + controllable DTB particle size: Average particle size increased by 40%, centrifugal dehydration power consumption decreased by 18%, salt whiteness increased by 20%. 

4. Freezing Separation: Utilizing the Na₂SO₄-NaCl-H₂O ternary phase diagram, low-temperature nitrate precipitation and hot-melt recovery increase sodium sulfate recovery rate by 12%.

5. AI Anti-Scale Module: Online monitoring of temperature difference, vibration, and conductivity predicts scaling trends and automatically triggers the "water washing + acid washing" program, reducing annual cleaning costs by 60%.

Environmental and Economic Benefits: Annual COD emissions reduced by 3650t, hazardous waste and miscellaneous salts reduced by 70%, successfully passing the Shandong Provincial Department of Ecology and Environment's "Zero Discharge" performance level A certification.

Annual savings of 82,000t of primary water and 23,000t of live steam, reducing hazardous waste disposal costs by 10.5 million yuan, with a comprehensive benefit of 15.8 million yuan.

Conclusion: The 250t/d rubber additive CBC water triple-effect evaporation crystallization system, with "pretreatment-triple-effect crystallization-freezing separation-mother liquor drying" as its core, successfully achieves zero discharge of high-salt, high-COD wastewater and dual resource recovery of sodium sulfate and sodium chloride. The project validated the reliability, economy, and replicability of the integrated technology of "titanium corrosion protection + forced circulation + DTB particle size control + AI scale prevention," providing a mature and scalable paradigm for the treatment of high-salt organic wastewater in the rubber additives industry. In the future, we will continue to iterate on cutting-edge technologies such as vapor compression-multi-effect coupling, high-value utilization of miscellaneous salts, and digital operation and maintenance to help more chemical companies achieve green and low-carbon transformation and high-quality development.