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 $\left({\mathrm{CaF}}_2\right)\text{,}$ which was then removed from the aqueous solution by flotation. When the molar ratio, $\left[{\mathrm{Ca}}^{2+}\right]/\left[F^{-}\right]\text{,}$ 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 ${\mathrm{CaF}}_2\text{.}$ Effective flotation efficiency (>95%) of ${\mathrm{CaF}}_2$ was found in the pH range of 5.5–9.5. Flotation efficiency increased slightly with increasing saturation pressure (3 to 5 $\mathrm{kg}/{\mathrm{cm}}^2)$ 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 $\left[{\mathrm{Ca}}^{2+}\right]\left[F^{-}\right]$ 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 ${\mathrm{CaF}}_2\text{,}$ but rendered residual fluoride concentrations higher.