Enhancement mechanism of ultrasonic flotation separation for calcium fluoride from photovoltaic sludge

Photovoltaic sludge, a solid waste generated from the flocculation–filter press treatment of fluoride-containing wastewater, is rich in calcium fluoride with high recovery potential. However, because the sludge particles form complex floc structures under the action of flocculants and thus exhibit poor dispersibility, it is difficult to achieve efficient separation of calcium fluoride by conventional flotation. This study introduces an ultrasonic pretreatment to enhance sludge dispersion and promote CaF₂ flotation recovery through cavitation effects. The particle dispersion behavior, reagent adsorption, surface charge, and microstructural evolution before and after ultrasonic treatment were systematically analyzed using a laser particle size analyzer, UV–visible spectrophotometer, zeta potential analyzer, and FSEM–EDS. Results indicate that ultrasound effectively disintegrates sludge aggregates, breaks floc structures, and significantly improves particle dispersion, facilitating subsequent flotation. Cavitation-induced microjets remove SiO₂ and other surface impurities, while the notable change in surface potential confirms impurity detachment. Ultrasonic pretreatment enhances sodium oleate adsorption, thereby improving CaF₂ floatability. Under optimal conditions （450 W, 3 min）, the CaF₂ grade in the concentrate increased from 63.50% to 67.62%, and the recovery rate rose from 53.24% to 72.37%. This study provides a green and effective approach for the efficient recovery of calcium fluoride from photovoltaic sludge, offering valuable insights into the reduction, harmless treatment, and resource utilization of solid waste in the photovoltaic industry.