Preparation of Niobium Metal Powder by Two-Stage Magnesium Vapor Reduction of Niobium Pentoxide

Magnesium vapor reduction of niobium pentoxide was studied using a laboratory system. Niobium powder was prepared by the magnesium vapor reduction at 1123？K for 5 hours and it contained about 8 mass % oxygen. However, the oxygen concentration could be decreased to 0.65% when it was prepared by double-step reduction by magnesium vapor and a chemical treatment. Controlled and diluted supply of magnesium vapor to the reaction front has averted excess heat generation at the reaction front and thereby fine particles were produced. Effects of various factors on the vapor reduction process were studied and discussed. 1. Introduction Extractive metallurgy of niobium has attained immense academic interest [1–14], mainly due to its application in the emerging niobium and its niobium oxide electrolytic capacitors and also in the superconducting radio frequency cavities for particle accelerators, also used in mint metal [1]. Reductants such as aluminum and calcium are used for the preparation of niobium metal from its oxide [2–5]. Niobium metal and its alloys such as ferro niobium and nickel niobium alloys are being commercially produced by aluminothermic reduction process. High purity niobium metal is prepared by electron beam melting and refining of the niobium metal which is obtained from the aluminothermic reduction of its oxide [2]. Carbothermic process can be considered as another process exploited for its commercial production [6]. Awasti prepared niobium by silicothermic reduction of niobium in ultrahigh vacuum [7]. These processes result in the formation of chunk lets and coarsened powders and hence may not be useful in the preparation of fine niobium metal powder. However, requirement of niobium metal powder with stringent purity specifications for its possible use in electrolytic capacitor has necessitated the development of new techniques. Niobium metal powder is generally prepared by hydriding, crushing of niobium hydride, and dehydriding [8]. It is reported that niobium metal powder is prepared from its oxide by calcium using electron-mediated reduction process [9]. Similar to tantalum, niobium fine powder is also prepared by liquid-liquid reduction of K2NbF7 with sodium [10, 11]. Niobium metal powders can be also prepared by using magnesiothermic reduction process [12–14]. Using a cyclone separator assembly, niobium is prepared by the reaction between the magnesium vapor and niobium oxide [12]. Okabe reported a preforms reduced process (PRP) for niobium metal using magnesiothermic reduction process [13]. There are a number of factors to be considered