Production and principal reactions
H
2SiF
6 is mainly produced as a by-product from the production of phosphoric acid from
apa e and
fluorapa e. In the U.S. about 85% of
fluorspar is used to produce hydrofluorosilic acid.
[3] The phosphoric and hydrofluoric acids are liberated from the mineral by the action of
sulfuric acid. Some of the HF in turn reacts with silicate minerals, which are an unavoidable cons uent of the mineral feedstock, to give silicon tetrafluoride. Thus formed, the silicon tetrafluoride reacts further with HF. The net process can be described as:
[4]
SiO
2 + 6 HF → H
2SiF
6 + 2 H
2O Hexafluorosilicic acid can also be produced by treating silicon tetrafluoride and hydrofluoric acid.
Neutralization of solutions of hexafluorosilicic acid with alkali metal bases produces the corresponding alkali metal fluorosilicate salts:
H
2SiF
6 + 2 NaOH → Na
2SiF
6 + 2 H
2O The resulting salt Na
2SiF
6 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:
Na
2SiF
6 + 4 NaOH → 6 NaF + SiO
2 + 2 H
2O
[edit] Uses
Hexafluorosilic acid is the feedstock for "virtually all organic and inorganic fluorine-bearing chemicals".
[3] The majority of the hexafluorosilicic acid is converted to
aluminium fluoride and
cryolite.
[4] These materials are central to the conversion of aluminium ore into
aluminium metal.
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