Niobium 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. 

Among the metals considered for carbothermic reduction under reduced pressure, an important example is the reduction of niobium oxides. The possibilities and the conditions for the carbothermic reduction of niobium oxides can be examined on the basis of the Pourbaix-... 

Having established the feasibility of niobium metal production by the carbothermic reduction of niobium pentoxide under temperature and pressure conditions readily attainable in the laboratory and in industry, the principles of efficient process execution may now be examined. In a high-temperature vacuum furnace operation, the quantity of gas that is to be pumped off can influence the choice of the vacuum process. When the reduction of niobium pentoxide with either carbon or niobium carbide is attempted according to the following overall equations ... 

Oxygen and carbon have substantial solid solubilities in niobium at the temperatures normally required for reduction. As the activity coefficients of both carbon and oxygen in niobium are low, their retention in the niobium metal produced by the carbothermic reduction of niobium oxide is expected. It is, however, possible (as explained later) to remove these residual impurities by extending the pyrovacuum treatment to still higher temperatures and lower pressures. 

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. 

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. 

Production of Niobium by Carbothermic Piasma-Chemicai Reduction of Niobium Oxides... 

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