Fluorine recovery from low-concentration fluorine wastewater by flow-electrode capacitive deionization and fluid bed crystallization (FCDI-FBC): Preconcentration and high-quality fluorite pellets formation.

Fluorine (F), critical for various industries, faces resource scarcity due to limited reserves of its primary source, fluorite (CaF₂). While fluorine-containing wastewater from industrial processes represents a valuable potential resource, recovering fluorine from low-concentration wastewater remains challenging. This study introduces a cyclic "preconcentration + recovery" system combining flow-electrode capacitive deionization (FCDI) and fluidized bed crystallization (FBC) to address this gap. FCDI preconcentrates fluorine ions into high-concentration brine, and FBC facilitates the formation of high-purity fluorite crystals. Experimental parameters influencing FCDI efficiency - such as influent fluoride concentration, electrode solution composition, and flow rate - were systematically evaluated. Additionally, the cyclic operation was modeled to enhance the whole recovery rate across multiple cycles. The experimental results demonstrated that FCDI achieves an 83.90% fluoride removal rate under optimal conditions with energy-efficient operation. FBC produces fluorite crystals of up to 97.20% purity, classified as acid-grade. The integrated FCDI-FBC system achieves a fluoride recovery rate of 64.40% in single operation mode, with further improvements in cyclic mode. The proposed system offers a sustainable and economically feasible solution to fluorine recovery from low-concentration wastewater, representing a significant step toward the sustainable utilization of non-renewable fluorite resources.