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

Masao Morita, Giuseppe Granata, Chiharu Tokoro*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

4 Citations (Scopus)


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 SiO32-.

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


  • 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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