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

Masao Morita, Giuseppe Granata, Chiharu Tokoro

    Research output: Contribution to journalArticle


    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-.

    Original languageEnglish
    Pages (from-to)290-296
    Number of pages7
    JournalMaterials Transactions
    Issue number2
    Publication statusPublished - 2018 Jan 1


    • Calcium fluoride
    • Fluorine
    • Mechanism
    • Neutralization
    • Recovery

    ASJC Scopus subject areas

    • Materials Science(all)
    • Condensed Matter Physics
    • Mechanics of Materials
    • Mechanical Engineering

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