Potassium heptafluorotantalate
chemical compound
Potassium heptafluorotantalate is an inorganic compound with the formula K2[TaF7]. It is the potassium salt of the heptafluorotantalate anion [TaF7]2−. This white, water-soluble solid is an intermediate in the purification of tantalum from its ores and is the precursor to the metal.
Preparation
Industrial
Potassium heptafluorotantalate is an intermediate in the industrial production of metallic tantalum. Its production involves leaching tantalum ores, such as columbite and tantalite, with hydrofluoric acid and sulfuric acid to produce the water-soluble hydrogen heptafluorotantalate.
Ta2O5 + 14 HF → 2 H2[TaF7] + 5 H2O
This solution is subjected to a number of liquid-liquid extraction steps to remove metallic impurities (most importantly niobium) before being treated with potassium fluoride to produce K2[TaF7]
H2[TaF7] + 2 KF → K2[TaF7] + 2 HF
Lab-scale
Hydrofluoric acid is both corrosive and toxic, making it unappealing to work with; as such a number of alternative processes have been developed for small-scale syntheses. Potassium heptafluorotantalate can be produced by both anhydrous and wet methods. The anhydrous method involves the reaction of tantalum oxide with potassium bifluoride or ammonium bifluoride according to the following equation:
Ta2O5 + 4 KHF2 + 6 HF → 2 K2[TaF7] + 5 H2O
The method was originally reported by Berzelius.
K2[TaF7] can also be precipitated from solutions in hydrofluoric acid provided that the concentration of HF is below about 42%. Solutions having higher concentrations of HF yield potassium hexafluorotantalate [KTaF6].
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