Hafnium is a chemical element with the atomic number of 72, and it's a transition metal from Group 4 (IVb). It is a ductile metal with a brilliant silvery lustre. The Dutch physicist Dirk Coster and the Hungarian Swedish chemist George Charles von Hevesy in 1923 in Norwegian and Greenland zircons by analyzing their X-ray spectra. They named the new element for Copenhagen (in New Latin, Hafnia), the city in which it was discovered. Hafnium is dispersed in Earth’s crust to the extent of three parts per million and is invariably found in zirconium minerals up to a few percent compared with zirconium. For example, the minerals zircon, ZrSiO4 (zirconium orthosilicate), and baddeleyite, which is essentially pure zirconium dioxide, ZrO2, generally have a hafnium content that varies from a few tenths of 1 percent to several percent.

Ion-exchange and solvent-extraction techniques have supplanted fractional crystallization and distillation as the preferred methods of separating hafnium from zirconium. In the procedure, crude zirconium tetrachloride is dissolved in an aqueous solution of ammonium thiocyanate, and methyl isobutyl ketone is passed countercurrent to the aqueous mixture, with the result that the hafnium tetrachloride is preferentially extracted. The metal itself is prepared by magnesium reduction of hafnium tetrachloride (Kroll process, which is also used for titanium) and by the thermal decomposition of tetraiodide (de Boer–van Arkel process).

For some purposes separation of the two elements is not important; zirconium containing about 1 percent of hafnium is as acceptable as pure zirconium. In the case of the largest single use of zirconium, however, namely, as a structural and cladding material in nuclear reactors, it is essential that the zirconium be essentially free of hafnium, because the usefulness of zirconium in reactors is based on its extremely low absorption cross section for neutrons. Hafnium, on the other hand, has an exceptionally high cross section, and accordingly even slight hafnium contamination nullifies the intrinsic advantage of the zirconium. Because of its high neutron-capture cross section and its excellent mechanical properties, hafnium is used for fabricating nuclear-control rods.


Hafnium | Britannica