Recovery of calcium fluoride from highly contaminated fluoric/hexafluorosilicic acid wastewater

We investigated the recovery of calcium fluoride (CaF₂) from highly concentrated hexafluorosilicic acid wastewater by neutralization- purification. Neutralization was achieved by dosing calcium hydroxide, whereas purification was carried out by alkaline leaching with sodium hydroxide. X-ray diffraction, X-ray absorption fine structure, mineral liberation analyzer, Fourier transform infrared spectroscopy and Inductively-coupled plasma atomic emission spectroscopy were used to quantify the neutralization and leaching performance and to elucidate their mechanisms. The precipitation behavior was strongly dependent on the calcium (Ca)/silicon (Si) molar ratio. For a Ca/Si ratio of 1.12, approximately 25% of the total fluorine was precipitated selectively as CaF₂. By increasing the Ca/Si ratio to 3.91, the recovery yield increased to 100% because of the precipitation of CaSiF₆ and the hydrolytic decomposition of hexafluorosilicate ion (SiF₆ ^2-) to CaF₂. The hydrolytic decomposition of SiF₆ ^2- resulted in the precipitation of silicon dioxide on the surface of the previously formed CaF₂. Alkaline leaching by sodium hydroxide at 70°C resulted in an efficient removal of the silicon phase from the neutralized sludge with the formation of a CaF₂ product with a grade above 90%. Leaching parameters, such as the kinetic constant and the activation energy, were determined by assuming first-order kinetics. The residual silicon phase in the final product could not be dissolved because of the formation of non-leachable SiO₃^2-.