Fluoride-containing wastewater from chip fabs is mainly generated from hydrofluoric acid processes including wafer wet cleaning, wet etching and photoresist stripping. Characterized by highly fluctuating water volume, wide fluoride concentration range and complex water composition, it is typical refractory industrial wastewater in chip manufacturing.
The core treatment principle lies in converting dissolved fluoride ions in water into poorly soluble calcium fluoride precipitates. Multi-stage solid-liquid separation combined with advanced adsorption polishing treatment is adopted to achieve stable discharge of effluent with fluoride ions meeting emission standards.
Conventional single lime precipitation process suffers from low chemical utilization, large sludge output, poor settling performance of microcrystals and unstable effluent quality, failing to satisfy current stringent environmental regulations.
At present, the mainstream process adopted in the industry is the combined two-stage precipitation plus advanced adsorption technology. It features low sludge generation, stable operation and high treatment accuracy, which ensures long-term stable compliance of wastewater discharge.
Ⅰ.Standardized Standard-Compliant Treatment Process
Primary Treatment: Calcium Salt Precipitation Pretreatment
Fluoride-containing wastewater first flows into the equalization tank for homogenization and flow balancing to mitigate fluctuations in water quality and flow rate, stabilizing chemical dosing and reaction conditions in subsequent units.
Equalized wastewater enters the primary reaction tank, where calcium chloride solution is dosed for main fluoride removal. Compared with lime, calcium chloride features better solubility and more stable reactions while generating less sludge. Calcium ions react with free fluoride ions in water to form microcrystalline calcium fluoride precipitates.
The pH value must be strictly controlled within 8.0–9.0. Too low pH inhibits the formation of calcium fluoride precipitates and leads to incomplete fluoride removal; excessively high pH causes hydroxide ions to compete for calcium ions, increasing chemical consumption and sludge output and reducing fluoride removal efficiency.
Over 90% of free fluoride ions can be removed in this section, greatly lowering the load of subsequent treatment. Restricted by the slight solubility of calcium fluoride at ambient temperature, single-stage precipitation cannot achieve ultra-low discharge limits. Subsequent coagulation-sedimentation and advanced adsorption processes are therefore required.
Secondary Treatment:Coagulation-Sedimentation for Fine Fluoride Removal
The effluent from primary treatment still contains ultrafine calcium fluoride microcrystals and trace fluoride ions, which require further purification via secondary coagulation and sedimentation.
Polyaluminum chloride (PAC) coagulant and polyacrylamide (PAM) flocculant are sequentially added to realize fine fluoride removal and high-efficiency solid-liquid separation. PAC hydrolyzes to form aluminum hydroxide flocs with high specific surface area, which adsorb and capture ultrafine calcium fluoride microcrystals. Meanwhile, residual fluoride ions combine to form poorly soluble hydroxyaluminum fluoride complexes. Via polymer bridging, PAM aggregates tiny flocs into dense alum flocs to accelerate sedimentation and improve separation performance.
After secondary sedimentation, fluoride ion concentration in effluent can be steadily reduced below 5 mg/L, significantly easing the burden on advanced treatment.
Tertiary Advanced Treatment: Adsorption Filtration Polishing
To avoid risks caused by fluctuations in upstream processes and meet ultra-low emission requirements, an advanced adsorption polishing unit is installed as the final safeguard for qualified effluent.
Secondary effluent enters the advanced treatment unit, where modified activated alumina or fluoride-specific adsorbent resin is applied for selective adsorption. Such adsorbents exhibit strong resistance to impurity interference and can precisely eliminate trace fluorides in water, stabilizing effluent fluoride concentration below 1 mg/L. In addition, adsorbents can be regenerated and reused, ensuring high-standard effluent quality while effectively cutting down system operating costs.
Ⅱ.Key Process Control Points
Stable compliance of fluoride-containing wastewater treatment for chip fabs relies on refined process management and control. The core control points are listed below:
1.Accurate Automatic pH Regulation
pH is the key parameter for fluoride removal reactions and shall be steadily maintained within the optimal range of 8.0–9.0. Equipped with online pH sensors linked to the PLC automatic control system, the system dynamically adjusts the dosing of acid and alkali chemicals to eliminate errors from manual regulation. This maintains a constant reaction environment and stabilizes fluoride removal performance.
2.Precise Control of Calcium Salt Dosage
Based on the solubility product characteristics of calcium fluoride, the calcium chloride dosage should be 10%–20% in excess of the theoretical value. Insufficient dosing leads to incomplete fluoride removal and substandard effluent. Excessive dosing results in chemical waste, higher sludge output and increased hazardous waste disposal costs. The dosing ratio must be precisely adjusted according to actual operating conditions.
3.Enhanced High-precision Solid-Liquid Separation
Conventional sedimentation tanks cannot effectively intercept ultrafine calcium fluoride microcrystals, which easily escape and cause fluctuations in effluent quality. By optimizing hydraulic parameters of sedimentation tanks, implementing regular sludge discharge, and combining tubular or submerged ultrafiltration equipment, the process achieves high-precision solid-liquid separation to prevent microcrystal loss and ensure clear and stable effluent.
4.Standardized Disposal of Fluoride-Containing Sludge
Fluoride-containing sludge is classified as hazardous waste and must be disposed of in compliance with regulations. After thickening for volume reduction, the sludge is dewatered by plate-and-frame filter press or centrifuge to reduce moisture content below 60%. Following solidification and stabilization treatment, the sludge shall be transferred to qualified institutions for centralized disposal to avoid secondary pollution.
5.Periodic Calibration and Maintenance of Online Instruments
Online pH meters and fluoride ion electrodes serve as core monitoring equipment of the automatic control system and require routine maintenance and calibration. pH meters shall be calibrated weekly, and fluoride ion electrodes shall be cleaned and verified regularly to guarantee accurate monitoring data and provide reliable support for automatic process control and water quality management.
6.Separate Collection and Treatment of High and Low Concentration Wastewater
Fluoride wastewater from chip manufacturing consists of low-volume high-concentration waste acid mother liquor and high-volume low-concentration cleaning wastewater. Mixed discharge may shock the treatment system and cause effluent exceeding limits. The separate pretreatment mode for wastewater of different concentrations is adopted. High-concentration waste liquid receives independent pretreatment before flowing into the main system to ensure stable overall operating conditions.
Ⅲ.Process Summary
This process adopts an integrated treatment flow of equalization homogenization – primary calcium salt precipitation – coagulation separation – adsorption polishing to remove fluorides in water step by step. It adapts to the characteristics of wastewater from chip manufacturing processes, including large fluctuations in flow rate and complex water quality.
The keys to stable system operation lie in accurate automatic pH control, scientific chemical dosing ratio, high-efficiency solid-liquid separation, routine maintenance of instruments, and separate treatment of high-concentration and low-concentration wastewater.
Under standardized operation and maintenance, the effluent fluoride ion concentration can be steadily controlled within 0.5~1.5 mg/L, meeting strict environmental discharge standards for the semiconductor industry and ensuring compliant and stable production of enterprises.
