Preparation of Metallic Niobium

The preparation of metallic niobium is not carried out industrially, as there is no demand for the metal. Its laboratory preparation depends on the reduction of the pentoxide. This is effected with difficulty, firstly, because of the tendency for partially reduced products to be formed, and secondly, because of the tendency of the reducing agent, for example carbon or aluminium, to combine or alloy with any niobium formed.

One method of preparation consists in a modification of the Goldschmidt process. Niobium pentoxide is mixed with an alloy of the rare earths, called mixed metal, obtained in the manufacture of thorium nitrate, and consisting roughly of 45 per cent, of cerium, 20 per cent, of lanthanum, 15 per cent, of " didymium," and about 20 per cent, of other rare-earth metals. The reaction is carried out in a magnesia-lined crucible, and is started with a firing mixture of barium peroxide, potassium chlorate, and aluminium powder. Considerable evolution of heat takes place and the reduction is extremely rapid; a button of niobium is obtained which, however, is not pure.

Carbon and aluminium were successfully employed as the reducing agents in the following special manner. Pure niobium pentoxide was moulded into filaments about half a millimetre in diameter with the aid of a little paraffin, and these were reduced to the tetroxide by heating to whiteness for four or five hours in carbon powder. The filaments so obtained were then heated to whiteness in a vacuum by means of an alternating current, whereupon rapid reduction to the pure metal took place. In order to prepare larger quantities of niobium the pentoxide is first reduced with aluminium powder, and the product, which contains about 3 per cent, of aluminium and some unchanged oxide, is heated in the electric arc in a vacuum until all the impurities are vaporised (a current of 185 amperes at 40 volts for fifteen hours is required for 20 grams of metal). Perfectly pure niobium obtained in this manner has been used for the investigation of the properties of the metal.

Niobium pentoxide has been reduced to the metal by means of hydrogen at 7 atmospheres pressure and at 1910° C., and a fairly pure sample has been obtained by the action of hydrogen on niobium pentachloride at a red heat.