Preparation of carbon incorporated Nb-Al alloy and its subsequent conversion to pure niobium by electron beam melting

Aluminothermic reduction of niobium pentoxide coupled with either direct electron beam melt refining of thermit niobium or a combination of pyrovacuum treatment of thermit niobium followed by electron beam melt refining of niobium sponge are the most widely adopted processes for the production of high purity (greater than 99.9 wt.%) niobium metal. Prior to pyrovacuum treatment, the thermit niobium ingot is crushed to smaller size of the order of 6 to 8 mm. Owing to very high hardness of thermit ingot, the crushing has been found to be quite difficult and energy intensive. The effectiveness of electron beam melt refining of thermit niobium is directly dependent on residual aluminum and oxygen concentration in the feed, which are inversely related. In the present studies, it has bean found possible to eliminate problems encountered during crushing of thermit niobium as well as the pyrovacuum step by incorporating a judicious amount of carbon in the thermit niobium. A modified charge composed of niobium pentoxide, aluminium reductant corresponding to 10 wt.% excess over the stoichiometric amount and carbon amounting to 1 wt.% of Nb2O5 when reacted, resulted in a product alloy of Nb-Al-C with higher niobium recovery (equal to or greater than 99 wt.%) and low residual aluminium concentration (less than or equal to 2 wt.%). It has been observed that the presence of carbon has not affected the overall yield. Carbon in the form of NbC preferentially precipitated at the grain boundaries of thermit alloy and rendered it easily crushable. Crushed alloy product could be subjected to direct electron beam melt refining without undergoing a pyrovacuum treatment step. No operational problems have been encountered, and in fact residual carbon assisted deoxidation in the final stage of electron beam melt refining. The electron beam melted button exhibited a hardness value of 65-70 VPN and a total impurity concentration of less than 0.05 wt.%.