Electrocoagulation-crystallization synergistic regulation for efficient fluoride removal and cryolite recovery from...

This study addresses the pollution challenges posed by fluoride-containing wastewater in the photovoltaic industry and explores the potential for fluorine resource recovery. It proposes a seed-induced electrocoagulation-crystallization (EC-C) strategy to achieve efficient fluoride removal and high-value recovery of cryolite (Na₃AlF₆). The key operating parameters for electrocoagulation (EC) were systematically optimized, with optimal conditions determined as: electrode area of 150 cm², electrode spacing of 1.2 cm, 0.05 M NaCl + NaHCO₃ composite electrolyte, and initial fluoride concentration of 3000 mg/L. Under these conditions, fluoride removal efficiency reached 94.4% with minimal energy consumption and anode loss, conforming to pseudo-first-order kinetics. To regulate crystallization pathways and product quality, seed-assisted crystallization was introduced: Commercial Na₃AlF₆ seed crystals (CNSC) outperformed Autocatalysed seeds crystals (ASC), with an optimal dosage of 0.75 g administered via stepwise addition over 0-20 min, significantly enhancing product phase content. This yielded high-content cryolite with 97.5% content. FTIR/XRD analysis revealed that fluorinated aluminum intermediates undergo progressive dehydration and transformation into a cryolite crystal phase dominated by Al-F bonds. This study established an integrated pathway for efficient fluoride removal and high-value recovery through coupled regulation of parameter windows and seeding timing, providing theoretical and process foundations for the high-value utilization of fluorinated photovoltaic wastewater.