Selective separation of phosphate and fluoride from semiconductor wastewater

Hydrofluoric acid (HF) and phosphoric acid (H₃PO₄) are widely used in semiconductor industry for etching and rinsing purposes. Consequently, significant amount of wastewater containing phosphate and fluoride is generated. Selective separation of phosphate and fluoride from the semiconductor wastewater, containing 936 mg/L of fluoride, 118 mg/L of phosphate, 640 mg/L of sulfate, and 26.7 mg/L of ammonia, was studied. Chemical precipitation and flotation reactions were utilized in the two-stage treatment processes. The first-stage reaction involved the addition of magnesium chloride (MgCl₂) to induce selective precipitation of magnesium phosphate. The optimal condition was pH 10 and molar ratio, [Mg²⁺]/[(PO ₄^3-)], of 3:1, and 66.2% of phosphate was removed and recovered as bobierrite (Mg₃(PO₄)₂· 8H₂O). No reaction was found between MgCl₂ and fluoride. Calcium chloride (CaCl₂) was used in the second-stage reaction to induce precipitation of calcium fluoride and calcium phosphate. The optimum molar ratio, [Ca²⁺]/[F^-], was 0.7 at pH 10, and residual fluoride concentration of 10.7 mg/L and phosphate concentration of lower than 0.5 mg/L was obtained. Thermodynamic equilibrium was modeled with PHREEQC and compared with experimental results. Sodium dodecylsulfate (SDS) was an effective collector for subsequent solid-liquid removal via dispersed air flotation (DiAF). The study demonstrated that phosphate can be selectively recovered from the wastewater. Potential benefits include recovery of phosphate for reuse, lower required dosage of calcium for fluoride removal, and less amount of CaF₂ sludge.