NaOH-HF stepwise extraction method for extracting high-purity calcium fluoride from fluoride-containing sludge: A sustainable approach

Abstract

Fluoride-containing sludge (FCS), a hazardous waste, poses significant environmental challenges, while the growing shortage of fluorite minerals makes the recovery of CaF₂ from FCS an attractive solution. This study presents a novel two-step extraction process for the purification and recovery of CaF₂. After optimizing reagent formulations, the process involves an initial reaction with 6 % NaOH, followed by a second reaction with 3 % HF. Under optimal conditions—3h reaction time, 40 °C, solid-to-liquid ratio of 1:2.5, and stirring speed of 250 r/min for the first stage, and 1 h reaction time with identical conditions for the second stage—a CaF₂ product with 70.30 % purity and 90.3 % recovery was achieved. X-ray diffraction (XRD) analysis confirmed that the reagents transformed impurities in the sludge into more soluble phases. Thermodynamic analysis further validated the feasibility of the chemical extraction process. The cost analysis showed that the total extraction cost was 758.125 CNY/t, yielding a profit of 686.675 CNY/t. In conclusion, this process is an efficient and cost-effective method for defluorination and CaF₂ recovery in the fluoride chemical industry, providing both theoretical and practical support for FCS purification and impurity removal.