Tantalum carbothermic reduction

The carbothermic reduction processes outlined so far apply to relatively unstable oxides of those metals which do not react with the carbon used as the reductant to form stable carbides. There are several metal oxides which are intermediate in stability. These oxides are less stable than carbon monoxide at temperatures above 1000 °C, but the metals form stable carbides. Examples are metals such as vanadium, chromium, niobium, and tantalum. Carbothermic reduction becomes complicated in such cases and was not preferred as a method of metal production earlier. However, the scenario changed when vacuum began to be used along with high temperatures for metal reduction. Carbothermic reduction under pyrovacuum conditions (high temperature and vacuum) emerged as a very useful commercial process for the production of the refractory metals, as for example, niobium and tantalum, and to a very limited extent, of vanadium. 

The principles of tantalum metal formation by the carbothermic reduction of tantalum pentoxide and the technology of tantalum metal production by this method are similar to those pertaining to niobium metal production by carbothermy. 

Tantalum obtained by carbothermic reduction at 2000 °C and 10-4 torr is more than 99.8% pure. The levels of the principal impurities, carbon and oxygen, are less than 0.1% each. 

The carbothermic reduction processes are usually strongly endothermic and require high temperatures. For example, carbothermic reduction of uranium (U), boron (B), zirconium (Zr), niobium (Nb), and tantalum (Ta) from their oxides requires 2000 000 K and, therefore, application of thermal plasma. In most plasma-chemical carbothermic reduction processes, an arc electrode is prepared from well-mixed and pressed oxide and carbon particles. The arc provides heating of the mixture, stimulating the reduction process on the electrode. Carbon oxides leave the electrode, finalizing the reduction process. 

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