Removal and recovery of fluoride from wastewater via mixed calcium salts by fluidized bed induced crystallization technology

Abstract

To overcome the disadvantages of high sludge production and the inability to collect calcium fluoride caused by chemical precipitation for defluoridation, the performance of fluidized bed induced crystallization (FBC) with fluorite particles as crystal seeds was investigated for the removal and resource recovery of fluoride (F^-) from wastewater. The effects of parametric factors were explored, and the optimal operating conditions were: [Ca²⁺]:[F^-] molar ratio of 0.5, fluoride load of influent of 4 kg/(m²·h), up-flow velocity of 6.5 m/h and initial bed height of 0.6 m. The pilot-scale experiment showed that the mixed calcium salts scheme and the automated system could overcome the adverse impacts caused by influent pH fluctuations. Fluoride removal and recovery were achieved through the heterogeneous crystallization of Ca²⁺ and F^- on the surface of seeds to form CaF₂. XRD and EDX Mapping results confirmed that the main ingredient of the product was calcium fluoride (CaF₂) with a purity of 89 %, which met the standard for ceramic grade fluorspar. SEM indicated that the crystal seeds transformed from fine lumpy powder to spherical particles. The fluidized bed induced crystallization technology recovered 55 % F^- as fluorspar and reduced wastewater treatment cost by 3.0 ∼ 3.5 RMB/m³, making it a more environmentally friendly and more economical technique compared with chemical precipitation.