Tantalum

TANTALUM, a metallic chemical element belonging to a family of three metals which includes vanadium (q.v.) and colum bium (q.v.). It occurs in certain rare but widely distributed min erals usually associated with columbium. Columbite (Massachu setts), the native ferrous columbate, and tantalite, ferrous tantalate, frequently contain both columbium and tantalum with manganese partially replacing iron. Ferguson ite is a complex columbo-tantalate of iron and the rare earth metals, samarskite has an even more complicated composition since it also contains uranium. It is accordingly not surprising that the history of tantalum is closely intermingled with that of colum bium, some confusion having arisen formerly in regard to the separate identities of these closely allied metals. Struverite, a titaniferous tantalum ore (Ti0246.0%, 36.0%) found exten sively in Malaya, is a valuable source of both tantalum and titan ium.

The first successful application of tantalum was as a filament for the incandescent electric lamp, but for this purpose it has now been superseded by tungsten.

Separation and Extraction.

Tantalum is extracted from the foregoing minerals by one of the following processes :—(i.) Fusion with potassium fluoride and extraction of the melt with dilute hydrofluoric acid. On evaporating the solution, potassium tantalifluoride, separates first, as one part of this salt is soluble only in 155 parts of cold water, whereas potassium columbium oxyfluoride, being soluble in 12-13 parts of water, is obtained on further concentration. (ii.) Alkali fusion with sodium peroxide, caustic potash or potassium carbon ate and acidification of the melt with dilute hydrochloric acid, when on boiling, the oxides and are precipitated. (iii.) Fusion with potassium bisulphate, and extraction with acidified water, removal of tin and tungsten with ammonium sul phide and of silica and titania with hydrofluoric and sulphuric acids. Final neutralisation with ammonia and boiling with salicylic acid leads to precipitation of the two pentoxides.

Metallic tantalum was first obtained by Berzelius (1820) on heating potassium tantalifluoride with potassium ; a more modern procedure is to reduce it in an evacuated electric furnace with either potassium or sodium. The product compressed and fused in the electric furnace with exclusion of air led to the pure metal (W. von Bolton, 1905). Moissan reduced the pentoxide with car bon in the electric furnace (1902). Other methods of obtaining the metal include electrolysis of the fused or of tantalum compounds in dilute sulphuric acid.

Tantalum (symbol Ta, atomic number 73, atomic weight 181.5) is a white metal; its melting point is 2,850° C and its specific gravity 16.6. It is very ductile and malleable with considerable toughness. It may be drawn into very thin wires. Its tensile strength is greater than that of steel or platinum; its coefficient of expansion is less than that of the latter metal so that it may be sealed into glass. On heating, it absorbs large volumes of hydrogen

or nitrogen and these gases are only removed by heating to fusion in a vacuum. It resists all single acids except hydrofluoric and hot concentrated sulphuric acids.

Applications.

Because of its resistance to corrosive agents tantalum may be used instead of platinum in standard weights and crucibles. Chemical apparatus in ductile tantalum, which is much cheaper than platinum, excels in its degree of resistance to acid corrosion. Metals may be deposited electrolytically on tantalum cathodes and since tantalum is not attacked by aqua regia the metallic deposit may be removed completely. Tantalum is claimed to surpass steel for surgical instruments and special tools, since these do not rust. It is used extensively as an electrolytic valve since it has the property shared with certain other metals of allow ing electric current to pass in one direction only when immersed in acid. But unlike most metals, it is not subject to acid corrosion and hence it makes an ideal material for rectifiers and under the trade name of "Balkite" it forms the essential constituent of many rectifying units.

Compounds of Tantalum.

The pentoxide, is a white infusible mass obtained by heating the metal in oxygen or by ignit ing tantalic acid.

Tantalic acid, is a gelatinous precipitate obtained on adding water to tantalum pentachloride ; it dissolves in aqueous alkalis to yield solutions of alkali tantalates, and is soluble in hydrofluoric acid to form tantalum pentafluoride (colourless prisms, m.p. 97°, b.p. 229° C). Addition of potassium fluoride to the pentafluoride solution gives potassium tantalifluoride which crystallises in needles. Solutions of tantalates furnish insoluble yellow tantalum ferrocyanide. Tantalum pentachloride, white hygroscopic needles melting at C, and boiling at 241.6° C, is produced by heating the pentoxide to redness in a stream of chlorine and carbon tetrachloride. A suboxide (hypotantalic oxide) (Ta02), is a grey mass produced by heating the pentoxide with carbon, or magnesium; the corresponding sulphide, TaS2, results from the action of hydrogen sulphide and carbon disulphide on the heated pentoxide.

Tritantalum hexachloride ("tantalum dichloride") Ta3C16, formed by heating the pentachloride with lead at 60o° C, is ex tracted with hydrochloric acid and left as a green precipitate on evaporating in vacuo. Saturation of the extract with hydrogen chloride yields the green crystalline tritantalochloric acid [Ta3C17,H20]1131120. Acids of mixed type [Ta3C1613r,H20]H,31120 and [Ta3C16SO4]}12, are obtained by the use of hydrogen bromide and dilute sulphuric acid respectively. In these compounds tantalum is bivalent, and analogous substances have been prepared containing bivalent molybdenum and tungsten (K. Lindner and others,