Optimizing the recovery process of ceramic grade calcium fluoride from hydrofluoric/hexafluorosilicic acid wastewater

Chemical precipitation using calcium salts has been widely applied in hydrofluoric acid wastewater treatment due to its simplicity, flexibility and economy. Coagulants/flocculants are used to improve solid-liquid separation in a fine precipitate of calcium fluoride (CaF₂). As a result, a large amount of sludge is generated by CaF₂ precipitation process. In order to move towards a circular economy, the current study developed an environment-friendly and cost-effective approach to treat concentrated hydrofluoric/hexafluorosilicic acid wastewater (∼117 g F⁻/L) obtained from an optoelectronic industry in Taiwan and to recover fluoride as the high purity CaF₂, as well as to reduce the amount of final waste disposal. The influence of three independent variables, including calcium-to-fluoride molar ratio ([Ca²⁺/F⁻], silicon concentration ([Si⁴⁺]), and polymer dosage applied, on the purity of recovered CaF₂ products, was investigated and optimized by combining response surface methodology (RSM) with central composite design (CCD). The ceramic grade CaF₂ of 85.6–87.9% was achieved under the optimum conditions as [Ca²⁺/F⁻] of 0.618–0.645, [Si⁴⁺] of 1.48 g/L, and polymer concentration of 17.2–17.8 mg/L. The final turbidity, residual fluoride concentration in the supernatants and fluoride removal efficiency were 14.8–16.2 NTU, 62.2–67.6 mg/L and greater than 99%, respectively. The value of R² = 0.9498 and adjusted R² = 0.9319 indicated an adequate correlation between experimental results and predictions by the quadratic model. In addition, a new insight into the CaF₂ purification method has been proposed based on the idea of pH control.