Conversion of hazardous fluorine-containing sludge into monodisperse, highly crystalline, and acid-grade CaF2 nanomaterials via digestive ripening under acid-enhanced hydrothermal treatment

Abstract

Fluorine-containing sludge (FCS), an industrial hazardous waste generated during processes such as fluorine chemical production and solar cell manufacturing, is highly toxic and challenging to degrade, posing significant environmental threats. In this study, we developed a promising new method to convert FCS into monodisperse, highly crystalline, and high-purity CaF₂ nanomaterials using low-concentration inorganic or organic acids as regulators under hydrothermal conditions. This method involved regulating the dissolution–deposition behaviours of the CaF₂ matrix, controlling impurity transformation, promoting the growth of CaF₂ crystals and achieving deep removal of impurities. Under optimized conditions (0.5 M hydrofluoric acid, liquid-to-solid ratio of 10 : 1, 180 °C, and 24 h), the produced CaF₂ exhibited 98% purity, 99% crystallinity and excellent monodispersity. The purification and impurity separation mechanism of CaF₂ in FCS was studied by analysing the dissolution–deposition process, the crystal growth behavior of CaF₂, and the migration and transformation of impurities under the treatment. It was revealed that hydrofluoric acid, as a regulator, plays the following important roles: (i) targeted conversion of calcium in sludge into CaF₂ and enhancement of its purity, (ii) regulation of the digestive ripening process of fine CaF₂ particles and enhancement of the dispersity and crystallinity of the CaF₂ product, and (iii) release of encapsulated and lattice-doped impurities and dissolution of other solid impurities into the liquid phase. Ultimately, this process yielded monodisperse, high-crystallinity, acid-grade CaF₂ nanomaterials. This study provided critical insights into the nano-structure control and transformation of hazardous FCS, thereby developing a sustainable approach to fluorine resource recovery.