5t/h sodium chloride triple-effect evaporation crystallization system for pigment chemical enterprises

In the pigment chemical industry, the treatment of saline wastewater has always been a crucial aspect of environmental protection and resource recycling for enterprises. A well-known pigment chemical company, specializing in the production of high-end organic pigments such as azo and phthalocyanine pigments, has an annual production capacity of 30,000 tons. During its production process, it generates a large amount of sodium chloride wastewater daily, with concentrations fluctuating between 8% and 15%. This wastewater contains trace amounts of organic impurities such as pigments and additives, exhibiting high color and a CODcr value exceeding 2000 mg/L. Direct discharge would severely pollute water bodies and waste valuable salt resources.

I. Project Background

Faced with increasingly stringent environmental regulations and the company's own need for sustainable development, this enterprise urgently required a highly efficient, energy-saving, and environmentally friendly sodium chloride evaporation and crystallization system to achieve zero wastewater discharge and resource recovery of sodium chloride. After market research and technology comparison, the 5th sodium chloride triple-effect evaporation and crystallization system was ultimately selected. This system won the client's favor due to its excellent energy-saving effect, stable operating performance, and highly automated operation.

II. Process Route

1. Pretreatment Stage

Decolorization and Degradation: Raw water first enters the equalization tank. After homogenization and equalization, it is pumped to the decolorization reactor. A special decolorizing agent (Fenton's reagent) is added, and an oxidation reaction is carried out under acidic conditions (pH=3-4) to break down the large pigment molecules and decolorize them. The reaction time is 2 hours, with a decolorization rate exceeding 95% and CODcr reduced to below 500 mg/L.

Neutralization and Softening: After decolorization, the wastewater enters the neutralization tank. Sodium hydroxide is added to adjust the pH to 8-9, and sodium carbonate is added simultaneously to remove calcium and magnesium ions and prevent scaling in the subsequent evaporator. After softening, the water hardness (calculated as CaCO₃) is reduced to below 50 mg/L.

Filtration and Preheating: Softened water is filtered through a plate and frame filter press to remove suspended solids. The filtrate enters the preheater, where it is preheated to 80℃ using secondary steam generated by the evaporation system, preparing for the evaporation and crystallization stage.

2. Triple-Effect Evaporation and Crystallization Stage

First-Effect Evaporator: After preheating, wastewater enters the first-effect falling film evaporator. Fresh steam is introduced into the shell side, while the wastewater evaporates in the tube side under a vacuum of -50 kPa at an evaporation temperature of 85°C. This generates secondary steam, which serves as the heat source for the second effect. The wastewater concentration increases to 15%, and sodium chloride begins to precipitate. The crystal slurry is pumped through a circulation pump into the first-effect separator, where sodium chloride crystals are initially separated. The mother liquor is returned to the first-effect circulating evaporator.

Second-Effect Evaporator: The secondary steam from the first effect enters the second-effect forced circulation evaporator, further concentrating the wastewater to 25% at an evaporation temperature of 70°C and a vacuum of -70 kPa. A large amount of sodium chloride crystals precipitate. The crystal slurry is pumped through the second-effect discharge pump to a thickener. After thickening, it enters a centrifuge for dehydration, reducing the wet salt moisture content to below 3%. The mother liquor is returned to the second effect for further evaporation.

Triple-Effect Evaporator: Secondary steam from the second effect serves as the heat source for the third effect, concentrating the wastewater to near saturation. The evaporation temperature is 55℃, and the vacuum degree is -85kPa. Residual sodium chloride crystals precipitate, and the slurry is pumped to a centrifuge via the triple-effect discharge pump. The mother liquor enters a mother liquor tank and is periodically returned to the pretreatment system to ensure salt balance and impurity removal.

3. Post-Treatment Stage

1. Centrifugal Dehydration: The slurry from the first and second effects is dehydrated by a centrifuge to obtain wet salt. The wet salt enters a fluidized bed dryer, where it is heated to 100℃ using steam and dried for 30 minutes to obtain refined sodium chloride with a moisture content ≤0.5% and a purity ≥99%, meeting the standards for refined industrial salt. It can be sold directly as a by-product.

2. Condensate Recovery: The secondary steam generated during evaporation is condensed into water in a condenser. The condensate is pure, with a conductivity ≤50μS/cm, and is entirely reused in the production process, achieving water resource recycling and saving up to 15,000 tons of fresh water annually.

III. Key Equipment and Materials

1. Evaporator: The single-effect falling film evaporator uses titanium heat exchange tubes, which are corrosion-resistant and have a high heat transfer coefficient; the second and third-effect forced circulation evaporators are made of duplex stainless steel 2205, which has strong resistance to pitting and crevice corrosion and a long service life.

2. Centrifuge: Equipped with a horizontal screw discharge centrifuge, it has a large processing capacity, high dehydration efficiency, and a high degree of automation, reducing manual intervention.

3. Vacuum System: A water ring vacuum pump is used to provide a stable vacuum environment for the evaporator, ensuring efficient evaporation.

IV. Operating Data

The system operates stably, with a processing capacity of 5t/h, a unit steam consumption of 0.35t/t water, a comprehensive energy consumption of 35kWh/t water, a sodium chloride recovery rate of over 90%, and an annual recovery of 3000 tons of refined salt, generating economic benefits of approximately 1.5 million yuan, while reducing wastewater discharge by 18,000 tons and reducing production costs by 2 million yuan. 

V. Technical Highlights

1. High Efficiency and Energy Saving: The triple-effect evaporation system utilizes secondary steam in stages, saving over 40% more energy than single-effect evaporation, significantly reducing steam consumption and operating costs.

2. High Degree of Automation: Full DCS control enables real-time monitoring and automatic adjustment of key parameters, achieving unattended operation and reducing labor costs and operational error risks.

3. Resource Recycling: Sodium chloride is recycled, and condensate is reused, achieving zero wastewater discharge and maximizing resource utilization, aligning with green production principles.

VI. Customer Value

By introducing the 5th-stage sodium chloride triple-effect evaporation crystallization system, this company not only solved the problem of saline wastewater treatment, achieving environmental compliance and clean production, but also gained additional refined salt byproducts, creating new economic growth points. The system operates stably and reliably, providing a solid environmental guarantee for the company's pigment production capacity expansion, helping the company continue to lead in the pigment chemical industry and setting a new environmental benchmark for the industry.