Hexafluorosilicic Acid - Production and Principal Reactions

Production and Principal Reactions

The commodity chemical hydrogen fluoride is produced from fluorspar by treatment with sulfuric acid. As a by product, approximately 50 kg of H₂SiF₆ is produced per tonne of HF owing to reactions involving silica-containing mineral impurities. H₂SiF₆ is also produced as a by-product from the production of phosphoric acid from apatite and fluorapatite. Again, some of the HF in turn reacts with silicate minerals, which are an unavoidable constituent of the mineral feedstock, to give silicon tetrafluoride. Thus formed, the silicon tetrafluoride reacts further with HF. The net process can be described as:

SiO₂ + 6 HF → H₂SiF₆ + 2 H₂O

Hexafluorosilicic acid can also be produced by treating silicon tetrafluoride with hydrofluoric acid.

Neutralization of solutions of hexafluorosilicic acid with alkali metal bases produces the corresponding alkali metal fluorosilicate salts:

H₂SiF₆ + 2 NaOH → Na₂SiF₆ + 2 H₂O

The resulting salt Na₂SiF₆ is mainly used in water fluoridation. Related ammonium and barium salts are produced similarly for other applications. With excess base, the hexafluorosilicate undergoes hydrolysis, so the neutralization of the hexafluorosilicic acid must guard against this easy hydrolysis reaction:

Na₂SiF₆ + 4 NaOH → 6 NaF + SiO₂ + 2 H₂O