General Information

Zircorium(Zr), chemical element, metal of Group IVb of the periodic table, used as a structural material for nuclear reactors.

Zirconium occurs widely in the Earth's crust, but not in concentrated deposits. The mineral zircon, ZrSiO4 (zirconium orthosiliCATe), which is generally found in alluvial deposits in stream beds, ocean beaches, or old lake beds, is the only commercial source of zirconium. Baddeleyite, which is essentially pure zirconium(IV) oxide, ZrO2, is the only other important zirconium mineral. These zirconium minerals generally have a hafnium content that varies from a few tenths of 1 percent to several percent. 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 atomic 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. Pure hafnium in fact is also used in some atomic reactors as a control element material because of its high neutron-capture cross section.

Separation of hafnium and zirconium is generally accomplished by a liquid-liquid countercurrent-extraction procedure. In the procedure, crude zirconium(IV) chloride 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(IV) chloride is preferentially extracted.

Either of the metals, or a mixture of the two, is produced by the same process as that described in the section Titanium (see above). Both the metals have extremely high melting points: zirconium 1,850 C and hafnium 2,230 C. Hafnium and zirconium are fairly resistant to acids and are best dissolved in hydrofluoric acid, in which procedure the formation of anionic fluoro complexes is important in stabilizing the solution. At normal temperatures neither metal is particularly reactive, but both become quite reactive with a variety of nonmetals at elevated temperatures.

The atomic radii of zirconium and hafnium are 1.45 and 1.44 Å, respectively, while the radii of the ions are Zr4+, 0.74 Å, and Hf4+, 0.75 Å. The virtual identity of atomic and ionic sizes, resulting from the lanthanide contraction, has the effect of making the chemical behaviour of these two elements more similar than for any other pair of elements known. Although the chemistry of hafnium has been studied less than that of zirconium, the two are so similar that only very small quantitative differences--for example, in solubilities and volatilities of compounds--would be expected in cases that have not actually been investigated.

The most important respect in which these two elements differ from titanium is that lower oxidation states are of minor importance; there are relatively few compounds of hafnium or zirconium in other than their tetravalent states. The increased size of the atoms makes the oxides more basic and the aqueous chemistry somewhat more extensive, and permits the attainment of coordination numbers 7 and, quite frequently, 8 in a number of zirconium and hafnium compounds.

Occurrence, uses, and properties.

Zirconium, obscure before the late 1940s, became a significant engineering material for nuclear energy appliCATions because it is highly transparent to neutrons. The element was identified (1789) in zircon from its oxide by the German chemist Martin Heinrich Klaproth, and the metal was isolated (1824) in impure form by the Swedish chemist Jöns Jacob Berzelius. The impure metal, even when 99 percent pure, is hard and brittle. The white, soft, malleable, and ductile metal of higher purity was first produced in quantity (1925) by the Dutch chemists Anton E. van Arkel and J.H. de Boer by the thermal decomposition of zirconium tetraiodide, ZrI4. In the early 1940s, William Justin Kroll of Luxembourg developed his cheaper process of making the metal based on the reduction of zirconium tetrachloride, ZrCl4, by magnesium. It is relatively abundant in the Earth's crust and is characteristically observed in S-type stars. Zirconium is commercially obtained principally from the minerals zircon and baddeleyite.

The most important use of zirconium is in nuclear reactors for cladding fuel rods, for alloying with uranium, and for reactor-core structures because of its unique combination of properties. Zirconium has good strength at elevated temperatures, resists corrosion from the rapidly circulating coolants, does not form highly radioactive isotopes, and withstands mechanical damage from neutron bombardment. Hafnium, chemically similar to zirconium and present in all zirconium ores, must be scrupulously removed from the metal intended for reactor uses because hafnium strongly absorbs thermal neutrons.

Zirconium absorbs oxygen, nitrogen, and hydrogen in astonishing amounts. At about 800 C it combines chemically with oxygen to yield the oxide, ZrO2. Zirconium reduces such refractory crucible materials as the oxides of magnesium, beryllium, and thorium. This strong affinity for oxygen and other gases accounts for its use as a getter for removing residual gases in electron tubes. At normal temperatures in air, zirconium is passive because of the formation of a protective film of oxide or nitride. Even without this film, the metal is resistant to the action of weak acids and acidic salts. Because of its high corrosion resistance, zirconium has found widespread use in the fabriCATion of pumps, valves, and heat exchangers. Zirconium is also used as an alloying agent in the production of some magnesium alloys and as an additive in the manufacture of certain steels.

Natural zirconium is a mixture of five stable isotopes: zirconium-90 (51.46 percent), zirconium-91 (11.23 percent), zirconium-92 (17.11 percent), zirconium-94 (17.40 percent), zirconium-96 (2.80 percent). Two allotropes exist: below 862 C (1,584 F) a hexagonal close-packed structure, above that temperature a body-centred cubic.

History

Chemical compounds