Supersaturation mechanisms and regulation strategies in nucleation-induced crystallization for high-purity fluoride recovery from photovoltaic wastewater

Photovoltaic wastewater, characterized by high and fluctuating fluoride (F⁻) concentrations and low pH, poses challenges for conventional precipitation methods that overuse chemicals and coagulants, leading to excessive sludge accumulation and fluoride loss. To address these limitations, this study introduces a supersaturation-guided nucleation-induced crystallization reflux process (NCRP) for efficient fluoride recovery. The application of a high initial reflux ratio enhanced the crystallization efficiency to over 90 %. A pilot test was further conducted using photovoltaic wastewater (fluoride concentration: 450–2500 mg/L; pH: 2.1–9.4) obtained from a photovoltaic cell manufacturing facility in Yunnan Province, China. Through dynamic supersaturation regulation and co-dosing of NaOH and CaCl₂ (Ca/F molar ratio: 0.45–0.6), the effluent fluoride concentration consistently remained below 10 mg/L, with F⁻ removal efficiency above 98 %. Stable dosage control and pH adjustment in the range of 6–8 facilitated the formation of crystalline particles with a median diameter (D50) of 1.62 mm within 25 days. The resulting high-purity calcium fluoride (CaF₂ content >85 % and surface silicon content <2 %) met industrial reuse standards. Reactor performance was validated through characterization analyses, which provided valuable insights into the reaction mechanism. The XPS, XRD, Raman, and zeta potential analyses revealed that lower supersaturation levels in the reaction zone promoted calcium fluoride crystallization, thereby improving recovery efficiency. The NCRP-based technology effectively removes fluoride, recovers resources from photovoltaic wastewater, and reduces treatment costs by over 1.5 RMB/m³. These findings provide technical guidance for the industrial-scale implementation of the technology.