Sulfuric acid breakdown of fluorite in the presence of silica

We study the reaction of CaF₂ with concentrated sulfuric acid in the presence of silica at 120 and 140°C using fluorite mineral, fluorite concentrate, and mica-fluorite ore samples. When steam is fed to the reaction mixture, fluorosilicic acid is formed and then distilled off, materially influencing the fluoride ion recovery. Addition of quartz makes the reaction to proceed in the kinetic area and increases the fluoride ion recovery, while the presence of mica-bound SiO₂ displaces the reaction to the diffusion area and decreases the fluoride ion recovery. We have found that rare alkali metals are concentrated as hexafluorosilicates in the insoluble residue during the sulfuric acid breakdown of fluorite in the presence of SiO₂.