Hafnium is one of the last naturally-occuring elements to be discovered. By 1915, Dmitri Mendeleev's periodic table of the elements had been rearranged according to atomic number (number of protons). Seven gaps remained, one of them at element 72. The search was on.
In the reorganized table, element 72 likely fell below zirconium and therefore would have similar properties. Indeed, in 1923, Dutch physicist Dirk Coster and Hungarian Georg von Hevesy detected this element in zirconium ore from Norway. The work was done in Copenhagen, and the element was named "hafnium" from the Latin Hafnia for that city. As it happens, zircon crystals and other zirconium minerals all contain some hafnium. Coster and von Hevesy found this to be true in all of the minerals and commercial zirconium they examined.
Mining and Production
All hafnium comes from the mining of zircon, which is itself usually found with the titanium minerals ilmenite and rutile. The amount of hafnium is 1-4% of the zirconium. The two elements have very similar chemistries but different nuclear behavior. Therefore, zirconium used in nuclear fuel rods must be purified of its hafnium, and that process is the source of most commercial hafnium. A liquid-liquid separation technique using chemical solvents was developed after World War II and is still the best method.
Properties and Uses
Hafnium is chemically almost identical to zirconium. It is a silvery ductile metal. In air, it forms an oxide coating that prevents further erosion. The density of hafnium is double that of zirconium, which results in quite different melting points and boiling points. This density difference is large enough that the published density of zirconium had to be corrected after the discovery of contaminating hafnium.
In almost all uses, hafnium can be replaced with zirconium. Hafnium is a strong neutron absorber, however, which is the opposite of zirconium. Hafnium is used in nuclear control rods.