Engineering Application of Fluoride-Containing Wastewater Treatment Process Design Scheme for the Electronics Industry

I. Engineering Application Background and Design Scope

1.1 Application Scenarios and Water Quality Characteristics

This scheme is primarily applied to the treatment engineering of fluoride-containing wastewater generated during the production processes of electronic components such as semiconductor manufacturing, integrated circuits (ICs), flat panel displays (FPDs), and photovoltaic (PV) cells.

This type of wastewater mainly originates from processes like chip etching, cleaning, chemical mechanical polishing (CMP), and glass surface treatment. Its core characteristics are as follows:

• High Fluoride Ion Concentration: The total fluoride concentration typically ranges from 100 to 2,000 mg/L, or even higher, far exceeding the discharge limit (8 mg/L) stipulated in the Emission Standard of Water Pollutants for Electronic Industry(GB 39731-2020).

• Complex Composition: Besides fluorides, it often contains acidic substances such as nitric acid, phosphoric acid, acetic acid, as well as trace metal ions (e.g., aluminum, calcium), suspended solids (e.g., silicon dioxide), and some organic pollutants.

• Significant Fluctuations in Water Quality and Quantity: Closely related to production process batches, product types, and maintenance cycles.

• High Treatment Requirements: Needs to stably meet national and local discharge standards, with stringent requirements for system reliability and automation level.

1.2 Design Objectives and Principles

• Treatment Objective: Effluent fluoride concentration stably ≤ 8 mg/L (or meeting stricter local standards at the project site), with other indicators (e.g., pH, COD, SS) concurrently meeting standards.

• Design Principles:

  1. Stable and Reliable: Mature process, strong resistance to shock loads, ensuring continuous and stable operation.

  2. Deep Removal: Employs multi-stage synergistic processes tailored to the high fluoride removal requirements of the electronics industry.

  3. Sludge Reduction: Optimizes the process to reduce chemical sludge production and achieve effective sludge dewatering.

  4. Intelligent Control: Achieves automated operation, reduces manual intervention, with online monitoring and feedback control of key parameters.

  5. Economical and Applicable: Optimizes capital and operating costs while ensuring treatment effectiveness.

II. Core Treatment Process Flow Design

Addressing the characteristics of electronics industry fluoride-containing wastewater, this project recommends the main process route of "pH Adjustment & Primary Calcium Salt Precipitation + Secondary Enhanced Coagulation-Sedimentation (or Combined Defluoridation) + Polishing/Safeguard Filtration," with the optional addition of a "Deep Adsorption" unit based on water quality conditions. This route has been successfully applied in multiple large-scale electronics factories.

2.1 Complete Process Flow Diagram

2.2 Key Points for Engineering Design of Critical Units

(1) Pretreatment and Primary Calcium Salt Precipitation Unit

• Function: Remove the majority of fluoride ions, reducing the load on subsequent stages.

• Engineering Application Details:

  • Reaction pH Control: Precisely control the reaction tank pH within the 10-11 range via pH online analyzers linked to metering pumps. Under these conditions, calcium fluoride (CaF₂) has its minimum solubility, yielding the highest theoretical defluoridation efficiency.

  • Calcium Salt Selection and Dosing: Prioritize Calcium Hydroxide (Lime Slurry), serving the dual function of pH adjustment and providing a calcium source. A lime slurry preparation and automatic dosing system is required. The calcium salt dosage is typically 1.1-1.3 times the theoretical value (calculated based on fluoride concentration, Ca:F ≈ 1.0-1.2:1).

  • Reaction and Sedimentation Design: Employ mechanically stirred reaction tanks to ensure sufficient reaction time (≥20 minutes). Subsequent sedimentation tanks should adopt a lower surface loading rate (0.6-0.8 m³/(m²·h)) to ensure the settling of fine CaF₂ particles.

(2) Secondary Enhanced Coagulation-Sedimentation Unit

• Function: Deep removal of residual fluoride ions and colloidal particles to ensure stable, compliant effluent.

• Engineering Application Details:

  • pH Fine-Tuning: Adjust the pH of the primary effluent to 7.5-8.5, which is optimal for the effectiveness of aluminum or iron salt coagulants.

  • Coagulant Selection: Dose Calcium Chloride (CaCl₂) to supplement calcium ions, combined with Polyaluminum Chloride (PAC) or Polyferric Sulfate (PFS). PAC/PFS forms abundant aluminum/iron hydroxide flocs, which further entrap and remove residual fluoride ions and suspended solids through adsorption, sweep coagulation, and coprecipitation.

  • Flocculant Aid: Dose a small amount of Polyacrylamide (PAM) to accelerate the formation of large, dense flocs, improving settling velocity.

  • High-Efficiency Sedimentation Equipment: Inclined Plate Settlers, High-Rate Clarifiers (e.g., DensaDeg®), or Integrated Coagulation-Sedimentation Units are recommended for their small footprint and high settling efficiency.

(3) Polishing and Safeguard Unit

• Function: Serves as the final water quality safeguard to handle occasional fluctuations.

• Engineering Application Details:

  • Filtration: Install Multi-Media Filters (quartz sand, anthracite) or Cartridge Filters (5-10 µm) to remove residual trace suspended solids and ensure effluent turbidity.

  • Deep Adsorption (Backup/Optional): Activated when influent fluoride concentration is extremely high or ultra-low discharge is required. Activated Alumina Adsorption Vessels or Special Defluoridation Resin Columns can be used. This unit acts as a "polishing" step, capable of stably reducing fluoride ions below 2 mg/L, but requires periodic regeneration, and the resulting spent regenerant needs to be returned to the front end for treatment.

(4) Sludge Treatment Unit

• Engineering Application Details:

  • Sludge Characteristics: Primarily chemical sludge containing calcium fluoride and metal hydroxides, with good settleability but corrosive properties.

  • Dewatering Equipment Selection: Prioritize High-Pressure Plate & Frame Filter Presses, which can reduce sludge moisture content to below 55%, significantly decreasing sludge volume and disposal costs. Centrifugal dewaterers are also an option.

  • Filtrate Return: The filtrate generated during dewatering may still have high fluoride content and must be collected and returned to the equalization tank for retreatment.

III. Automation Control and Operation Management

3.1 Core Control Loops

1. Precise pH Control Loop: pH control in the primary and secondary reaction tanks is core, using online monitoring and automatic acid/alkali dosing; control accuracy is recommended at ±0.2.

2. Chemical Dosing Interlock Control: Automatically adjusts calcium salt and coagulant dosing rates based on influent flow and signals from online fluoride/calcium ion monitors (optional), according to set ratios.

3. Sludge Discharge and Dewatering Interlock: Automatically controls sludge discharge valves and the start/stop of the sludge dewatering system based on signals from sludge level indicators in the sedimentation tanks.

3.2 Online Monitoring Instrumentation Configuration

• Essential Instruments: pH meters, flow meters, sludge level indicators.

• Recommended Instruments: Online fluoride ion analyzers (for effluent monitoring and early warning), turbidity meters.

3.3 Key Operation Management Points

• Regular Calibration and Maintenance: All online instruments must be regularly calibrated and cleaned to ensure data accuracy.

• Chemical Quality Management: Ensure the active ingredient content of chemicals like lime and PAC to avoid treatment failure due to poor chemical quality.

• Timely Sludge Disposal: Dewatered sludge cake is classified as hazardous waste and must be disposed of safely by a licensed entity, with proper records maintained.

IV. Engineering Economic Analysis

• Capital Cost Estimate (Example for 100 m³/h treatment capacity): Approximately 4 - 8 million RMB. Major cost components include: civil works for reaction/sedimentation tanks, high-efficiency sedimentation/filtration equipment, sludge dewatering system, automation control system, and corrosion protection works (for acidic wastewater).

• Operating Cost: 4 - 9 RMB per ton of water.

  • Chemical Costs: 2-5 RMB/ton (major cost, depends on fluoride concentration and chemical types).

  • Power Costs: 0.5-1.5 RMB/ton.

  • Sludge Disposal Costs: 1-2 RMB/ton.

  • Labor and Maintenance: 0.5-1 RMB/ton.

• Benefits: Ensures corporate environmental compliance, avoiding high fines; partial water reuse after treatment saves freshwater costs; automation reduces labor costs.

V. Conclusion and Recommendations

This "Two-Stage Chemical Precipitation + Filtration Safeguard" process scheme is currently the most technologically mature, widely applied, and stably operated engineering solution for treating fluoride-containing wastewater in the electronics industry.

Engineering Implementation Recommendations:

1. Pilot Testing is Essential: Conduct continuous pilot testing for the specific project wastewater to determine the optimal pH setpoint, chemical dosing ratios, and costs.

2. Emphasize Corrosion Protection Design: All tanks, pipelines, and equipment (especially agitators) in contact with acidic wastewater must use corrosion-resistant materials such as PPH, FRP, or 316L stainless steel.

3. Design Sufficient Buffering Capacity: The equalization tank volume must fully consider the irregularity of production discharges, providing adequate retention time (typically ≥8 hours).

4. Reserve Upgrade Interfaces: Reserve space and connection points for future polishing unit upgrades to accommodate potentially stricter discharge standards.

Through rigorous engineering design, high-quality equipment selection, and intelligent operation management, this scheme can ensure the long-term, stable, and economical operation of fluoride-containing wastewater treatment facilities in the electronics industry.