Treatment of semiconductor wastewater by dissolved air flotation

Treatment of fluoride-containing wastewater from the semiconductor industry by dissolved air flotation was investigated. Calcium chloride was added to the wastewater to form calcium fluoride precipitate (CaF2), which was then removed from the aqueous solution by flotation. When the molar ratio, [Ca2+]/[F-], was equal to 0.5, residual fluoride concentration was lower than 15 mg/L. All flotation reactions were completed in 5 min. Experimental results indicated that the sodium oleate (SOI) dosage was the major factor in determining the removal efficiency of CaF2. Effective flotation efficiency (>95%) of CaF2 was found in the pH range of 5.5-9.5. Flotation efficiency increased slightly with increasing saturation pressure (3 to 5 kg/cm(2)) and recycle ratio (20-40%). The presence of sulfate and phosphate resulted in higher residual fluoride concentration and lower flotation efficiency. The interference could be overcome by increasing [Ca2+][F-] to 1.0. However, a higher dosage of SOI was needed, An ionic strength higher than 0.1 M did not affect the removal of CaF2, but rendered residual fluoride concentrations higher.