Hf - Hafnium
|Chemical Element||Hafnium||Melting Point °C||2230|
|Chemical Symbol||Hf||Boiling Point °C||4602|
|Atomic Number||72||Density g/cm3||13.3|
Hafnium is a lustrous, silvery metal that is also extremely ductile. It is a member of Group 4 of the Periodic Table and is the 45th most abundant element within the Earth’s crust. Chemically, hafnium is nearly identical to its Group 4 neighbour, zirconium, and preparation of either element is nearly impossible without having traces of the other, due to the two elements having the same sized atoms. Normally, as you go down a group of the periodic table the atoms get larger, but for hafnium and zirconium this general rule does not hold. The extra electrons in hafnium occupy an inner orbit.
Hafnium is extremely resistant to corrosion, as when it is exposed to air, hafnium produces a tough, impenetrable oxide layer on its surface. However, powdered hafnium will burn in air. The metal is unaffected by acids and alkalis, except hydrofluoric acid.
Hafnium was discovered in 1923 by the Dutch and Hungarian physicists, Dirk Coster and George Karl von Hevesy. It was one of the last elements to be discovered. Numerous, scientists had previously predicted the existence of this element. The first prediction came from Georges Urbain, who reported a ‘missing element’ below zirconium in Group 4. Whilst working with a young Englishman, Henry Mosely, Urbain’s prediction was shown to be wrong. Two years later, Mosely made one of the greatest discoveries within the field of Chemistry. He discovered the fundamental property of Atomic Number, deducing it from the x-ray spectra of the elements and was then able to make a prediction of his own. Mosely predicted elements with atomic number 43, 61, 72 and 75, 72 being what was believed to be Urbain’s new element. Using Mosely’s x-ray analysis technique and the new quantum theory of electron shells and sub-shells developed by Niels Bohr, Coster and von Hevesy were able to finally find the missing element number 72 whilst analysing a zirconium ore, zircon. Hafnium eluded chemists for so long, due to the fact that it is so hard to distinguish from zirconium. The two scientists named this new element after the Latin word (Hafnia) for Copenhagen the city in which it was found.
Zircon sand (ZrO2), from Australia, South Africa, Brazil and others is the main source of hafnium, where it arises at the rate of 1:50 versus the zirconium with which it is found. In order to reach metallic form, zircon sand is reduced via a solvent process using magnesium and chlorine.
Hafnium is nowhere produced for itself but only as a result of the purifying of zirconium for use in nuclear plant and equipment, where the lowest possible level of hafnium is required. Zirconium, being a low neutron absorber, is used for the fuel rods containing uranium, while hafnium’s obverse properties of blocking neutrons gives rise to its role for the control rods.
Due to the strategic nature of the nuclear industry, very few hafnium producers exist – the two most notable being Wah Chang in USA (circa 40mt/yr) and Cezus in France (circa 30 mt/yr), owned by Areva, the largest producer of nuclear power stations in the world. Until recently, Toshiba Westinghouse of the USA was also a producer of hafnium, but has now ceased. The Ukraine also used to produce hafnium under the Soviet system, but is thought to have ceased also, although some plans exist to re-start production in Russia. China is at an early stage of hafnium production.
Non-nuclear use of Hafnium in high temperature Nickel base single crystal alloys for aero and industrial gas turbine blades is the largest use today. The leading hafnium-bearing alloy, MAR-M-247, contains Hf 1.5% and consumes 20-30mt mt per year. A further 10 mt per year is used in plasma-cutting equipment where hafnium’s properties have revolutionised this field. Hafnium has also made a surprise appearance in the current generation of Intel’s Pentium™ processors, where temperature resistance is a factor. Hafnium oxide is used in blue lasers for DVD readers, and also in CVD/PVD for thin films.
Competing nuclear/non-nuclear demands underpin the hafnium market, with the nuclear field likely to obtain priority if any shortage occurs. Looking ahead, both China and Russia will produce hafnium in the future, which could work to ease this pressure.
Hafnium is classed as a ‘dual use’ metal under the ‘Non-Proliferation of Nuclear Weapons Treaty’. Import/export licences and end user statements will be required in a number of instances.