Editing Hafnium (section)

==Production==
[[File:Hafnium ebeam remelted.jpg|thumb|left|Melted tip of a hafnium consumable electrode used in an [[Electron-beam additive manufacturing|electron beam]] [[Electron-beam furnace|remelting furnace]], a 1&nbsp;cm cube, and an oxidized hafnium electron beam-remelted ingot (left to right)]]

The heavy mineral sands ore deposits of the [[titanium]] ores [[ilmenite]] and [[rutile]] yield most of the mined zirconium, and therefore also most of the hafnium.<ref>{{cite web|title = 2008 Minerals Yearbook: Zirconium and Hafnium|first = Joseph|last = Gambogi|publisher=[[United States Geological Survey]]|date=2010|access-date=2021-11-11|url = https://www.usgs.gov/centers/nmic/zirconium-and-hafnium-statistics-and-information}}</ref>

Zirconium is a good nuclear fuel-rod cladding metal, with the desirable properties of a very low neutron capture cross section and good chemical stability at high temperatures. However, because of hafnium's neutron-absorbing properties, hafnium impurities in zirconium would cause it to be far less useful for nuclear reactor applications. Thus, a nearly complete separation of zirconium and hafnium is necessary for their use in nuclear power. The production of hafnium-free zirconium is the main source of hafnium.<ref name="ASTM">{{cite book|url = https://books.google.com/books?id=dI_LssydVeYC|title = ASTM Manual on Zirconium and Hafnium|first = J. H.|last = Schemel|publisher = [[ASTM]]|date = 1977|isbn=978-0-8031-0505-8|pages=1–5|location =Philadelphia|volume=STP 639}}</ref>
[[File:Hafnium pellets with a thin oxide layer.jpg|thumb|right|Hafnium oxidized ingots which exhibit [[thin-film optics|thin-film optical]] effects]]

The chemical properties of hafnium and zirconium are nearly identical, which makes the two difficult to separate.<ref name="Larsen">{{cite journal|title = Concentration of Hafnium. Preparation of Hafnium-Free Zirconia|first1 = Edwin M.|last1 = Larsen|last2 = Fernelius |first2=W. Conard |last3=Quill|first3=Laurence |journal = [[Ind. Eng. Chem. Anal. Ed.]]|date=1943|volume=15|pages=512–515|doi =10.1021/i560120a015|issue = 8|url=https://docecity.com/concentration-of-hafnium-preparation-of-hafnium-free-zirconi-5f1098025158d.html}}</ref> The methods first used—[[Fractional crystallization (chemistry)|fractional crystallization]] of ammonium fluoride salts<ref name="Ark1924a" /> or the fractional distillation of the chloride<ref name="Ark1924b" />—have not proven suitable for an industrial-scale production. After zirconium was chosen as a material for nuclear reactor programs in the 1940s, a separation method had to be developed. [[Liquid–liquid extraction]] processes with a wide variety of solvents were developed and are still used for producing hafnium.<ref name="Hend" /> About half of all hafnium metal manufactured is produced as a by-product of zirconium refinement. The end product of the separation is [[Hafnium tetrachloride|hafnium(IV) chloride]].<ref name="USGS1952">{{cite book|publisher = The first production plants Bureau of Mines|title = Minerals yearbook metals and minerals (except fuels)|date = 1952|chapter-url = http://digicoll.library.wisc.edu/cgi-bin/EcoNatRes/EcoNatRes-idx?type=turn&entity=EcoNatRes.MinYB1952v1.p1172&isize=M|last = Griffith|first = Robert F.|chapter =Zirconium and hafnium|pages=1162–1171}}</ref> The purified hafnium(IV) chloride is converted to the metal by reduction  with [[magnesium]] or [[sodium]], as in the [[Kroll process]].<ref name="Gilb">{{cite journal|title = Preliminary Investigation of Hafnium Metal by the Kroll Process|first = H. L.|last = Gilbert|author2=Barr, M. M.|journal = Journal of the Electrochemical Society|date =1955|volume =102|page=243|doi = 10.1149/1.2430037|issue = 5}}</ref>
: <chem>HfCl4{} + 2 Mg ->[1100~^\circ\text{C}] Hf{} + 2 MgCl2</chem>

Further purification is effected by a [[chemical transport reaction]] developed by [[Crystal bar process|Arkel and de Boer]]: In a closed vessel, hafnium reacts with [[iodine]] at temperatures of {{convert|500|°C|sigfig=1}}, forming [[hafnium(IV) iodide]]; at a tungsten filament of {{convert|1700|°C|sigfig=2}} the reverse reaction happens preferentially, and the chemically bound iodine and hafnium dissociate into the native elements. The hafnium forms a solid coating at the tungsten filament, and the iodine can react with additional hafnium, resulting in a steady iodine turnover and ensuring the [[chemical equilibrium]] remains in favor of hafnium production.<ref name="Holl" /><ref name="Ark1925" />
: <chem>Hf{} + 2 I2 ->[500~^\circ\text{C}] HfI4</chem>
: <chem>HfI4 ->[1700~^\circ\text{C}] Hf{} + 2 I2</chem>