Zirconium

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Zirconium

Zirconium is a chemical element in the modern periodic table that is assigned the symbol Zr and has the atomic number 40. A lustrous gray-white, strong transition metal that resembles titanium, zirconium is obtained chiefly from zircon and is very corrosion resistant. Zirconium is primarily used in nuclear reactors due to its resistance to corrosion and low neutron cross-section.

Notable characteristics

It is a grayish-white metal, lustrous and exceptionally corrosion resistant. Zirconium is lighter than steel and its hardness is similar to copper. When it is finely divided, the metal can spontaneously ignite in air, especially at high temperatures (it is much more difficult to ignite the solid metal). Zirconium zinc alloy becomes magnetic at temperatures below 35 K. Oxidation states of zirconium include +4, although +3 and +2 can be obtained.

Applications

The major end uses of zircon (ZrSi O₄) are refractories, foundry sands, and is also used in high strength lacrosse sticks (including investment casting), and ceramic opacification. Zircon is also marketed as a natural gemstone used in jewelry, and its oxide is processed to produce the diamond simulant, cubic zirconia. Other uses:

Zirconium has a low absorption cross section for thermal neutrons, which makes it ideal for nuclear energy uses, such as cladding fuel elements. More than 90% of zirconium metal production is consumed by commercial nuclear power generation. Modern commercial scale reactors can use as much as a 150,000 meters of zirconium alloy (Zircaloy) tubing. Reactor-grade zirconium has to be purified of hafnium, which has 600 times higher neutron cross-section; a hafnium-free zirconium can be 10 times more expensive than zirconium with naturally occurring 1-5% of hafnium.
Extensively used by the chemical industry for piping in corrosive environments.
Zirconium is pyrophoric (flammable) and has been used in military incendiaries such as Dragon's Breath. It is also planned for use in the baseline variant of the AGM-154 Joint Standoff Weapon for incendiary effects.
Its carbonate was used in poison-ivy lotions until it was evident that many people are allergic. (allergies greatly vary between people)
Impure zirconium oxide, Zirconia, is used to make laboratory crucibles that can withstand heat shock, for linings of metallurgical furnaces, and by the ceramic and glass industries as a refractory material.
Human tissues can easily tolerate this metal which makes it suitable for biocompatible implants, eg. some artificial joints and limbs.
Also used in heat exchangers, as a "getter" in vacuum tubes, in lamp filaments and various specialty alloys.
When alloyed with niobium, zirconium becomes superconductive at low temperatures and is used to make superconductive magnets with possible large-scale electrical power uses.
Zirconium Diamide-Diamine complexes can be used to catalyse the polymerisation of alkenes, especially ethene, when activated with Trityl-BArF.
Zirconium nitride has been used more recently as an alternative to titanium nitride for coating drill bits. Both coatings are supposed to keep the bit sharper and cooler during cutting.
Bis(cyclopentadienyl)zirconium(IV) chloride hydride (Schwartz's Reagent) is a commercially available metallocene used in the hydrozirconation of alkenes and alkynes.

Hafnium-free zirconium

Reactor-grade zirconium alloys must be made of purified zirconium free of hafnium contamination, as hafnium has very high neutron absorption cross-section, 600 times higher than zirconium. Commercial zirconium naturally contains 1-5% of hafnium which has to be removed. This removal process is difficult (zirconium and hafnium are two of the most difficult elements to separate). Two main process are in use: liquid-liquid extraction, exploiting the difference of solubility of metal thiocyanates in methyl isobutyl ketone, used mainly in United States, and extractive distillation, used primarily in Europe. The resulting reactor-grade zirconium is about 10 times as expensive as the hafnium-contaminated commercial grade. The separated hafnium is used for control rods. The zirconium is used mostly almost pure, in the form of low alloys, most often from the zircaloy group.

History

Zirconium (Arabic zarkûn from Persian zargûn meaning "gold like") was discovered in 1789 by Martin Heinrich Klaproth and isolated in 1824 by Jöns Jakob Berzelius.

The zirconium-containing mineral zircon, or its variations (jargon, hyacinth, jacinth, or ligure), were mentioned in biblical writings. The mineral was not known to contain a new element until Klaproth analyzed a jargon from Ceylon in the Indian Ocean. He named the new element Zirkonertz (zirconia). The impure metal was isolated first by Berzelius by heating a mixture of potassium and potassium zirconium fluoride in a small decomposition process conducted in an iron tube. Pure zirconium wasn't prepared until 1914.

The crystal bar process (or Iodide process), discovered by Anton Eduard van Arkel and Jan Hendrik de Boer in 1925, was the first industrial process for the commercial production of pure ductile metallic zirconium. It was superseded by the Kroll process.

Occurrence

Zirconium is never found in nature as a free metal. The principal economic source of zirconium is the zirconium silicate mineral, zircon (ZrSiO₄), which is found in deposits located in Australia, Brazil, India, Russia, and the United States. (It is extracted as a dark sooty powder, or as a gray metallic crystalline substance). Zirconium and hafnium are contained in zircon at a ratio of about 50 to 1 and are difficult to separate. Zircon is a coproduct or byproduct of the mining and processing of heavy-mineral sands for the titanium minerals, ilmenite and rutile, or tin minerals. Zirconium is also in 30 other recognized mineral species including baddeleyite. This metal is commercially produced by reduction of the Zirconium(IV) chloride with magnesium in the Kroll process, and through other methods. Commercial-quality zirconium still has a content of 1 to 3% hafnium.

This element is also abundant in S-type stars and has been detected in the sun and meteorites. Lunar rock samples brought back from several Apollo program missions to the moon have a very high zirconium oxide content relative to terrestrial rocks.

See also Zirconium minerals.

Isotopes

Naturally occurring zirconium is composed of four stable isotopes and one extremely long-lived radioisotope (Zr-96). The second most stable radioisotope is Zr-93 which has a half life of 1.53 million years. Eighteen other radioisotopes have been characterized. Most of these have half lives that are less than a day except Zr-95 (64.02 days), Zr-88 (63.4 days), and Zr-89 (78.41 hours). The primary decay mode is electron capture before Zr-92 and the primary mode after is beta decay.

Precautions

Compounds containing zirconium are not noted for toxicity. The metal dust can ignite in air and should be regarded as a major fire and explosion hazard. Zirconium has no biological role.

References

Los Alamos National Laboratory – Zirconium

External links

WebElements.com – Zirconium
Zirconium Producers in the United States:
- Wah Chang, part of Allegheny Technologies
- Westinghouse

Chemical elements | Transition metals

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