Efficient conversion mechanism of calcium fluoride in fluoride-containing sludge by calcination

Abstract

Fluorine-containing sludge (FCS) generated by the semiconductor and photovoltaic industries is hazardous waste, and its treatment and resource utilization are both urgent and critically important. To address the issues of lengthy process flows and low efficiency in the current resource recovery of CaF₂ in FCS, a novel method for converting CaF₂ through low-temperature calcination using Mg(NO₃)₂·6H₂O was proposed. The effect of roasting parameters on the conversion efficiency of CaF₂ was explored and optimized, and the reaction kinetics were clarified. Under the optimal conditions, 98.12% of CaF₂ in FCS was converted to MgF₂. The roasting of CaF₂ follows a chemical diffusion-controlled mechanism with an activation energy of 23.28 kJ/mol. Ultimately, a MgF₂ product with a purity of 98.30% was obtained. Density functional theory calculations showed that Mg(NO₃)₂·6H₂O remained molten at low temperatures, reducing the chemical energy barrier for breaking the Ca-F bond and promoting the reaction. This method offers a straightforward, quick, and effective way to treat FCS and recover fluorine resources, laying a foundation for producing other fluorides and opening a new path for future FCS treatment strategies.

Graphical abstract

Keywords

Fluoride-containing sludge / CaF₂ / MgF₂ / Conversion mechanism

Highlight

	● Conversion mechanism of CaF2 clarified by low-temperature roasting with Mg(NO3)2·6H2O.
	● 98.12% of the CaF2 in the FCS was converted, yielding MgF2 with a purity of 98.30%.
	● Magnesium salts are superior to calcium salts in treating fluoridated wastewater.

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