Precipitation removal of fluoride from semiconductor wastewater

Chemical precipitation method utilizing calcium salts has been extensively used to treat fluoride-containing wastewater from semiconductor manufacturers. The major objective of this study was to investigate coagulation-flocculation of calcium fluoride precipitates (CaF2). Residual fluoride concentration of lower than 15 mg/L, the effluent standard, was found when molar ratio of calcium to fluoride, [Ca2+/F-], was 0.5 and pH ranged from 6.5 to 8.5. The fine precipitates of CaF2 were further flocculated by polyaluminum chloride (PAC) and anionic polyelectrolyte, polyacrylic acid (PAA) with different molecular weight, to facilitate better solid-liquid separation. Experimental results indicated that both PAC and PAA with low to medium molecular weight could remove CaF2 precipitates effectively. Satisfactory removal efficiency was achieved at lower dosage when PAC and PAA were used in combination. Recycling of CaF2 precipitates or CaF2 sludge did not enhance removal of fluoride or CaF2 precipitates. Reaction mechanisms were discussed based on zeta potential and particle size distribution.