Niobium

Niobium is a very soft (HV 40), malleable metal that is air-stable up to approx. 250°C, forms a thin protective oxide film and has a body-centered cubic crystal structure. Niobium reacts with oxygen, nitrogen, hydrogen and carbon at temperatures over 250°C, so any processing above that temperature must be performed in a vacuum or under inert gas. It is highly resistant to corrosion by all acids, except for fuming sulfuric acid (oleum) and hydrofluoric acid, but will react with alkaline solutions. It occurs with valences of -3, -1, 0, +2, +3, +4 and +5 in compounds, although pentavalent niobium is most common. Niobium is superconductive at temperatures below 9.46 K. Compounds such as NbTi and Nb₃Sn are also valuable superconductors.

The largest deposits are found in Brazil (approx. 85%), Canada (approx. 10%), Africa and Australia. Niobium occurs primarily in mineral ores (over 60 different types) as a mixed oxide (most significant: columbite) or alkali fluoride (most significant: pyrochlore), and is always found together with tantalum. For that reason, niobium must be separated from the tantalum during production by first dissolving the ore concentrate, then performing a time-consuming liquid-liquid extraction using MIBK or distillation of the chlorides. The niobium pentoxide (Nb₂O₅) derived from that process can be further reduced with Na, C or Al (most common process) to recover Nb metal. The metal is then refined by repeated melting under vacuum in an electron beam furnace.

Approximately 85% of the annual production worldwide (about 55,000 tons in 2006) is used as ferro-niobium in the production of construction and high-temperature steels. A further 10% is processed with cobalt to create superalloys for jet engines and gas turbines. Only approx. 3–4 % is utilized for ceramics, alloys or as high-purity metal. Heraeus produces high-purity niobium for elementary particle accelerators, NbZr 1% for high-pressure vapor lamps, and a diverse array of semifinished goods for the dental and semiconductor industries. NbTi and Nb₃Sn are used to make superconductive wires for high performance magnets. Niobium/lead alloys are used for Piezo applications and capacitors, while Nb₂O₅ is used in optical lenses due to its high refractive index.