Combined treatment of polishing wastewater and fluoride-containing wastewater from a semiconductor manufacturer

Collection and treatment of chemical mechanical polishing (CMP) wastewater and fluoride-containing wastewater in semiconductor manufacturing are conventionally separated. The combined treatment of these two streams of wastewater was studied. Polishing wastewater containing silica was mixed at different ratio with synthetic fluoride-containing (640 mg/L) wastewater. Calcium chloride (CaCl₂) was then added at neutral pH to induce calcium fluoride (CaF₂) precipitates. Fluoride removal efficiency increased when the mixing ratio (v/v) of CMP wastewater to fluoride-containing wastewater increased from 1:9 to 2:3 at molar ratio, ([Ca]:[F]), of 0.75 at neutral pH. Size of CaF₂ precipitates increased when at higher mixing ratio as well. The subsequent solid-liquid separation was very effective by the use of anionic flocculant, polyacrylic acid (PAA) of lower molecular weight (90,000 and 450,000) at dosage of 10 mg/L and at pH 7. Lower residual turbidity was obtained for mixed wastewater when flocculated by PAA. Results indicated that the combined treatment both polishing and fluoride-containing wastewater is beneficial. It is proposed that silica in CMP wastewater become adsorbed on the surface of positively charged CaF₂ particles, and enhances flocculation since repulsive force of CaF₂ particles is decreased. In addition, silica may act as nuclei for precipitation of CaF₂ as well and facilitate agglomeration. Potential advantages include lower chemical dosage, better control, and smaller footprint.