Treatment of semiconductor wastewater by dissolved air flotation

Journal of Environmental Engineering, Vol. 128, No. 10, October 1, 2002. - pp. 974-980Treatment 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 (SOl) 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/cm²) 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 SOl was needed. An ionic strength higher than 0.1 M did not affect the removal of CaF², but rendered residual fluoride concentrations higher.