Purification of CaF2 from Fluorine-Containing Sludge: Optimization via Calcium Ion Coexistence Effect

The rapid development of the photovoltaic industry has significantly increased fluorine-containing sludge production. Calcium fluoride (CaF2), a vital non-renewable raw material used in optics, metallurgy, and chemical synthesis, holds immense significance for ensuring the sustainable supply of fluoride resources. This study focuses on purifying CaF2 from fluorine-containing sludge using a systematic approach. Through characterization techniques such as XRF, SEM-EDS, XRD, FT-IR, and laser granulometry, the sludge’s composition was thoroughly analyzed. An acid-leaching–alkali-leaching method was proposed and validated for CaF2 purification. A key finding during acid leaching was the “calcium ion coexistence effect”, where the dissolution of other calcium salts influences CaF2 dissolution equilibrium, reducing its loss. Leveraging this phenomenon, an optimized strategy was developed by increasing acid concentration while reducing acid volume. This approach effectively addresses two common challenges in traditional acid-leaching processes: high CaF2 dissolution loss and difficulties in impurity removal. Experimental results revealed that under optimized acid-leaching conditions, the purity of CaF2 increased significantly from an initial 36.7 wt% to 76.1 wt% after acid-leaching–alkali-leaching. This study demonstrates a successful method for purifying CaF2 from fluorine-containing sludge, providing a sustainable solution for fluoride resource recovery.