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Preparation of Metallic Tantalum

For the industrial preparation of tantalum a tantalite is used which contains at least 60 per cent, of tantalum pentoxide and the minimum quantity of titanium. The finely pulverised mineral is fused with either caustic potash or potassium hydrogen sulphate. In the former case the aqueous extract contains potassium tantalate and potassium niobate, which undergo hydrolysis and precipitate mixed tantalic and niobic acids when boiled with sulphuric or nitric acid; in the latter case the mixed tantalic and niobic acids are left in the insoluble residue after extraction with water. The niobium is removed by dissolving the mixed acids in hydrofluoric acid; sufficient potassium fluoride is then added to produce the double fluorides, K2TaF7. and K2NbOF5.H2O, which are then separated by repeated fractional crystallisation. Considerable secrecy has been maintained concerning the details of the large-scale processes employed to convert potassium tantalum fluoride, or the tantalum pentoxide obtained from it by hydrolysis, into metallic tantalum which possesses good mechanical properties. It is understood, however, that reduction is effected by one of the following methods: (1) The double fluoride is heated with potassium or sodium at a high temperature; the product is washed with water and mineral acid, pressed into rods and fused in an electrically heated vacuum furnace, whereby all traces of impurities are removed and a ductile material is obtained. (2) The molten double fluoride is electrolysed between an anode of impure tantalum and a cathode of pure tantalum, or of other conductive and inert material, in a vessel made of tantalum pentoxide, magnesia, or other refractory substance. The product is fused in a vacuum furnace as in the previous process.

Tantalum can also be prepared in small quantities by passing an alternating electric current through rods of tantalum dioxide sealed into a glass bulb in which a good vacuum is maintained during the heating. This is not, however, an industrial process.

The malleability and ductility of tantalum are destroyed by the presence of even traces of foreign bodies; 0.1 per cent, of carbon, for instance, renders the material brittle. Older laboratory reactions which gave rise to more or less pure tantalum deficient in mechanical properties consisted in reducing tantalum pentoxide with "mixed metal" or with carbon in the electric furnace; the equilibrium conditions of the reduction of tantalum pentoxide by carbon at high temperatures have been investigated by Slade and Higson. The thermite process yields an alloy of tantalum and aluminium.

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