= Zirconium =

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= Zirconium =
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Introduction
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Zirconium is a chemical element; it has symbol Zr and atomic number
40. First identified in 1789, isolated in impure form in 1824, and
manufactured at scale by 1925, pure zirconium is a lustrous transition
metal with a greyish-white color that closely resembles hafnium and,
to a lesser extent, titanium. It is solid at room temperature,
ductile, malleable and corrosion-resistant. The name 'zirconium' is
derived from the name of the mineral zircon, the most important source
of zirconium. The word is related to Persian 'zargun' (zircon;
'zar-gun', "gold-like" or "as gold"). Besides zircon, zirconium occurs
in over 140 other minerals, including baddeleyite and eudialyte; most
zirconium is produced as a byproduct of minerals mined for titanium
and tin.

Zirconium forms a variety of inorganic compounds, such as zirconium
dioxide, and organometallic compounds, such as zirconocene dichloride.
Five isotopes occur naturally, four of which are stable. The metal and
its alloys are mainly used as a refractory and opacifier; zirconium
alloys are used to clad nuclear fuel rods due to their low neutron
absorption and strong resistance to corrosion, and in space vehicles
and turbine blades where high heat resistance is necessary. Zirconium
also finds uses in flashbulbs, biomedical applications such as dental
implants and prosthetics, deodorant, and water purification systems.

Zirconium compounds have no known biological role, though the element
is widely distributed in nature and appears in small quantities in
biological systems without adverse effects. There is no indication of
zirconium as a carcinogen. The main hazards posed by zirconium are
flammability in powder form and irritation of the eyes.

Characteristics
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Zirconium rod
Zirconium is a lustrous, greyish-white, soft, ductile, malleable
metal that is solid at room temperature, though it is hard and brittle
at lesser purities. In powder form, zirconium is highly flammable, but
the solid form is much less prone to ignition. Zirconium is highly
resistant to corrosion by alkalis, acids, salt water and other agents.
However, it will dissolve in hydrochloric and sulfuric acid,
especially when fluorine is present. Alloys with zinc are magnetic at
less than 35 K.

The melting point of zirconium is 1855 °C (3371 °F), and the boiling
point is 4409 °C (7968 °F). Zirconium has an electronegativity of 1.33
on the Pauling scale. Of the elements within the d-block with known
electronegativities, zirconium has the fourth lowest electronegativity
after hafnium, yttrium, and lutetium.

At room temperature zirconium exhibits a hexagonally close-packed
crystal structure, α-Zr, which changes to β-Zr, a body-centered cubic
crystal structure, at 863 °C. Zirconium exists in the β-phase until
the melting point.

Isotopes
==========
Naturally occurring zirconium is composed of five isotopes. 90Zr,
91Zr, 92Zr and 94Zr are stable, although 94Zr is predicted to undergo
double beta decay (not observed experimentally) with a half-life of
more than 1.10×1017 years. 96Zr has a half-life of 2.34×1019 years,
and is the longest-lived radioisotope of zirconium. Of these natural
isotopes, 90Zr is the most common, making up 51.45% of all zirconium.
96Zr is the least common, comprising only 2.80% of zirconium.

Thirty-three artificial isotopes of zirconium have been synthesized,
ranging in atomic mass from 77 to 114. 93Zr is the longest-lived
artificial isotope, with a half-life of 1.61 million years.
Radioactive isotopes at or above mass number 93 decay by electron
emission, whereas those at or below 89 decay by positron emission. The
only exception is 88Zr, which decays by electron capture.

Thirteen isotopes of zirconium also exist as metastable isomers:
83m1Zr, 83m2Zr, 85mZr, 87mZr, 88mZr, 89mZr, 90m1Zr, 90m2Zr, 91mZr,
97mZr, 98mZr, 99mZr, and 108mZr. Of these, 97mZr has the shortest
half-life at 104.8 nanoseconds. 89mZr is the longest lived with a
half-life of 4.161 minutes.

Occurrence
============
Zirconium has a concentration of about 130 mg/kg within the Earth's
crust and about 0.026 μg/L in sea water. It is the 18th most abundant
element in the crust. It is not found in nature as a native metal,
reflecting its intrinsic instability with respect to water. The
principal commercial source of zirconium is zircon (ZrSiO4), a
silicate mineral, which is found primarily in Australia, Brazil,
India, Russia, South Africa and the United States, as well as in
smaller deposits around the world. As of 2013, two-thirds of zircon
mining occurs in Australia and South Africa. Zircon resources exceed
60 million tonnes worldwide and annual worldwide zirconium production
is approximately 900,000 tonnes. Zirconium also occurs in more than
140 other minerals, including the commercially useful ores baddeleyite
and eudialyte.

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

EPR spectroscopy has been used in investigations of the unusual 3+
valence state of zirconium. The EPR spectrum of Zr3+, which has been
initially observed as a parasitic signal in Fe‐doped single crystals
of ScPO4, was definitively identified by preparing single crystals of
ScPO4 doped with isotopically enriched (94.6%)91Zr. Single crystals of
LuPO4 and YPO4 doped with both naturally abundant and isotopically
enriched Zr have also been grown and investigated.

Zirconium is a by-product formed after mining and processing of the
titanium minerals ilmenite and rutile, as well as tin mining. From
2003 to 2007, while prices for the mineral zircon steadily increased
from $360 to $840 per tonne, the price for unwrought zirconium metal
decreased from $39,900 to $22,700 per ton. Zirconium metal is much
more expensive than zircon because the reduction processes are costly.

Collected from coastal waters, zircon-bearing sand is purified by
spiral concentrators to separate lighter materials, which are then
returned to the water because they are natural components of beach
sand. Using magnetic separation, the titanium ores ilmenite and rutile
are removed.

Most zircon is used directly in commercial applications, but a small
percentage is converted to the metal. Most Zr metal is produced by the
reduction of the zirconium(IV) chloride with magnesium metal in the
Kroll process. The resulting metal is sintered until sufficiently
ductile for metalworking.

Separation of zirconium and hafnium
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Commercial zirconium metal typically contains 1-3% of hafnium, which
is usually not problematic because the chemical properties of hafnium
and zirconium are very similar. Their neutron-absorbing properties
differ strongly, however, necessitating the separation of hafnium from
zirconium for nuclear reactors. Several separation schemes are in use.
The liquid-liquid extraction of the thiocyanate-oxide derivatives
exploits the fact that the hafnium derivative is slightly more soluble
in methyl isobutyl ketone than in water. This method accounts for
roughly two-thirds of pure zirconium production, though other methods
are being researched; for instance, in India, a TBP-nitrate solvent
extraction process is used for the separation of zirconium from other
metals. Zr and Hf can also be separated by fractional crystallization
of potassium hexafluorozirconate (K2ZrF6), which is less soluble in
water than the analogous hafnium derivative. Fractional distillation
of the tetrachlorides, also called extractive distillation, is also
used.

Vacuum arc melting, combined with the use of hot extruding techniques
and supercooled copper hearths, is capable of producing zirconium that
has been purified of oxygen, nitrogen, and carbon.

Hafnium must be removed from zirconium for nuclear applications
because hafnium has a neutron absorption cross-section 600 times
greater than zirconium. The separated hafnium can be used for reactor
control rods.

Compounds
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Like other transition metals, zirconium forms a wide range of
inorganic compounds and coordination complexes. In general, these
compounds are colourless diamagnetic solids wherein zirconium has the
oxidation state +4. Some organometallic compounds are considered to
have Zr(II) oxidation state. Non-equilibrium oxidation states between
0 and 4 have been detected during zirconium oxidation.

Oxides, nitrides, and carbides
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The most common oxide is zirconium dioxide, ZrO2, also known as
'zirconia'. This clear to white-coloured solid has exceptional
fracture toughness (for a ceramic) and chemical resistance, especially
in its cubic form. These properties make zirconia useful as a thermal
barrier coating, although it is also a common diamond substitute.
Zirconium monoxide, ZrO, is also known and S-type stars are recognised
by detection of its emission lines.

Zirconium tungstate has the unusual property of shrinking in all
dimensions when heated, whereas most other substances expand when
heated. Zirconyl chloride is one of the few water-soluble zirconium
complexes, with the formula [Zr4(OH)12(H2O)16]Cl8.

Zirconium carbide and zirconium nitride are refractory solids. Both
are highly corrosion-resistant and find uses in high-temperature
resistant coatings and cutting tools. Zirconium hydride phases are
known to form when zirconium alloys are exposed to large quantities of
hydrogen over time; due to the brittleness of zirconium hydrides
relative to zirconium alloys, the mitigation of zirconium hydride
formation was highly studied during the development of the first
commercial nuclear reactors, in which zirconium carbide was a
frequently used material.

Lead zirconate titanate (PZT) is the most commonly used piezoelectric
material, being used as transducers and actuators in medical and
microelectromechanical systems applications.

Halides and pseudohalides
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All four common halides are known, ZrF4, ZrCl4, ZrBr4, and ZrI4. All
have polymeric structures and are far less volatile than the
corresponding titanium tetrahalides; they find applications in the
formation of organic complexes such as zirconocene dichloride. All
tend to hydrolyse to give the so-called oxyhalides and dioxides.

Fusion of the tetrahalides with additional metal gives lower zirconium
halides (e.g. ZrCl3). These adopt a layered structure, conducting
within the layers but not perpendicular thereto.

The corresponding tetraalkoxides are also known. Unlike the halides,
the alkoxides dissolve in nonpolar solvents. Dihydrogen
hexafluorozirconate is used in the metal finishing industry as an
etching agent to promote paint adhesion.

Organic derivatives
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Organozirconium chemistry is key to Ziegler-Natta catalysts, used to
produce polypropylene. This application exploits the ability of
zirconium to reversibly form bonds to carbon. Zirconocene dibromide
((C5H5)2ZrBr2), reported in 1952 by Birmingham and Wilkinson, was the
first organozirconium compound. Schwartz's reagent, prepared in 1970
by P. C. Wailes and H. Weigold, is a metallocene used in organic
synthesis for transformations of alkenes and alkynes.

Many complexes of Zr(II) are derivatives of zirconocene, one example
being (C5Me5)2Zr(CO)2.

History
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The zirconium-containing mineral zircon and related minerals (jargoon,
jacinth, or hyacinth, ligure) were mentioned in biblical writings. The
mineral was not known to contain a new element until 1789, when
Klaproth analyzed a jargoon from the island of Ceylon (now Sri Lanka).
He named the new element Zirkonerde (zirconia), related to the Persian
'zargun' (zircon; 'zar-gun', "gold-like" or "as gold"). Humphry Davy
attempted to isolate this new element in 1808 through electrolysis,
but failed. Zirconium metal was first obtained in an impure form in
1824 by Berzelius by heating a mixture of potassium and potassium
zirconium fluoride in an iron tube.

The 'crystal bar process' (also known as the '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
metallic zirconium. It involves the formation and subsequent thermal
decomposition of zirconium tetraiodide (), and was superseded in 1945
by the much cheaper Kroll process developed by William Justin Kroll,
in which zirconium tetrachloride () is reduced by magnesium:

:

Applications
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Approximately 900,000 tonnes of zirconium ores were mined in 1995,
mostly as zircon.

Most zircon is used directly in high-temperature applications. Because
it is refractory, hard, and resistant to chemical attack, zircon finds
many applications. Its main use is as an opacifier, conferring a
white, opaque appearance to ceramic materials. Because of its chemical
resistance, zircon is also used in aggressive environments, such as
moulds for molten metals.

Zirconium dioxide (ZrO2) is used in laboratory crucibles, in
metallurgical furnaces, and as a refractory material Because it is
mechanically strong and flexible, it can be sintered into ceramic
knives and other blades. Zircon (ZrSiO4) and cubic zirconia (ZrO2) are
cut into gemstones for use in jewelry. Zirconium dioxide is a
component in some abrasives, such as grinding wheels and sandpaper.
Zircon is also used in dating of rocks from about the time of the
Earth's formation through the measurement of its inherent
radioisotopes, most often uranium and lead.

A small fraction of the zircon is converted to the metal, which finds
various niche applications. Because of zirconium's excellent
resistance to corrosion, it is often used as an alloying agent in
materials that are exposed to aggressive environments, such as
surgical appliances, light filaments, and watch cases. The high
reactivity of zirconium with oxygen at high temperatures is exploited
in some specialised applications such as explosive primers and as
getters in vacuum tubes. Zirconium powder is used as a degassing agent
in electron tubes, while zirconium wire and sheets are utilized for
grid and anode supports. Burning zirconium was used as a light source
in some photographic flashbulbs. Zirconium powder with a mesh size
from 10 to 80 is occasionally used in pyrotechnic compositions to
generate sparks. The high reactivity of zirconium leads to bright
white sparks.

Nuclear applications
======================
Cladding for nuclear reactor fuels consumes about 1% of the zirconium
supply, mainly in the form of zircaloys. The desired properties of
these alloys are a low neutron-capture cross-section and resistance to
corrosion under normal service conditions. Efficient methods for
removing the hafnium impurities were developed to serve this purpose.

One disadvantage of zirconium alloys is the reactivity with water,
producing hydrogen, leading to degradation of the fuel rod cladding:

:

Hydrolysis is very slow below 100 °C, but rapid at temperature above
900 °C. Most metals undergo similar reactions. The redox reaction is
relevant to the instability of fuel assemblies at high temperatures.
This reaction occurred in the reactors 1, 2 and 3 of the Fukushima I
Nuclear Power Plant (Japan) after the reactor cooling was interrupted
by the earthquake and tsunami disaster of March 11, 2011, leading to
the Fukushima I nuclear accidents. After venting the hydrogen in the
maintenance hall of those three reactors, the mixture of hydrogen with
atmospheric oxygen exploded, severely damaging the installations and
at least one of the containment buildings.

Zirconium is a constituent of uranium zirconium hydrides, nuclear
fuels used in research reactors.

Space and aeronautic industries
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Materials fabricated from zirconium metal and ZrO2 are used in space
vehicles where resistance to heat is needed.

High temperature parts such as combustors, blades, and vanes in jet
engines and stationary gas turbines are increasingly being protected
by thin ceramic layers and/or paintable coatings, usually composed of
a mixture of zirconia and yttria.

Zirconium is also used as a material of first choice for hydrogen
peroxide () tanks, propellant lines, valves, and thrusters, in
propulsion space systems such as these equipping the Sierra Space's
Dream Chaser spaceplane where the thrust is provided by the combustion
of kerosene and hydrogen peroxide, a powerful, but unstable, oxidizer.
The reason is that zirconium has an excellent corrosion resistance to
and, above all, do not catalyse its spontaneous self-decomposition as
the ions of many transition metals do.

Medical uses
==============
Zirconium-bearing compounds are used in many biomedical applications,
including dental implants and crowns, knee and hip replacements,
middle-ear ossicular chain reconstruction, and other restorative and
prosthetic devices.

Zirconium binds urea, a property that has been utilized extensively to
the benefit of patients with chronic kidney disease. For example,
zirconium is a primary component of the sorbent column dependent
dialysate regeneration and recirculation system known as the REDY
system, which was first introduced in 1973. More than 2,000,000
dialysis treatments have been performed using the sorbent column in
the REDY system. Although the REDY system was superseded in the 1990s
by less expensive alternatives, new sorbent-based dialysis systems are
being evaluated and approved by the U.S. Food and Drug Administration
(FDA). Renal Solutions developed the DIALISORB technology, a portable,
low water dialysis system. Also, developmental versions of a Wearable
Artificial Kidney have incorporated sorbent-based technologies.

Sodium zirconium cyclosilicate is used by mouth in the treatment of
hyperkalemia. It is a selective sorbent designed to trap potassium
ions in preference to other ions throughout the gastrointestinal
tract.

Mixtures of monomeric and polymeric Zr4+ and Al3+ complexes with
hydroxide, chloride and glycine, called aluminium zirconium glycine
salts, are used in a preparation as an antiperspirant in many
deodorant products. It has been used since the early 1960s, as it was
determined more efficacious as an antiperspirant than contemporary
active ingredients such as aluminium chlorohydrate.

Defunct applications
======================
Zirconium carbonate (3ZrO2·CO2·H2O) was used in lotions to treat
poison ivy but was discontinued because it occasionally caused skin
reactions.

Safety
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Although zirconium has no known biological role, the human body
contains, on average, 250 milligrams of zirconium, and daily intake is
approximately 4.15 milligrams (3.5 milligrams from food and 0.65
milligrams from water), depending on dietary habits.
Zirconium is widely distributed in nature and is found in all
biological systems, for example: 2.86 μg/g in whole wheat, 3.09 μg/g
in brown rice, 0.55 μg/g in spinach, 1.23 μg/g in eggs, and 0.86 μg/g
in ground beef. Further, zirconium is commonly used in commercial
products (e.g. deodorant sticks, aerosol antiperspirants) and also in
water purification (e.g. control of phosphorus pollution, bacteria-
and pyrogen-contaminated water).

Short-term exposure to zirconium powder can cause irritation, but only
contact with the eyes requires medical attention. Persistent exposure
to zirconium tetrachloride results in increased mortality in rats and
guinea pigs and a decrease of blood hemoglobin and red blood cells in
dogs. However, in a study of 20 rats given a standard diet containing
~4% zirconium oxide, there were no adverse effects on growth rate,
blood and urine parameters, or mortality. The U.S. Occupational Safety
and Health Administration (OSHA) legal limit (permissible exposure
limit) for zirconium exposure is 5 mg/m3 over an 8-hour workday. The
National Institute for Occupational Safety and Health (NIOSH)
recommended exposure limit (REL) is 5 mg/m3 over an 8-hour workday and
a short term limit of 10 mg/m3. At levels of 25 mg/m3, zirconium is
immediately dangerous to life and health. However, zirconium is not
considered an industrial health hazard. Furthermore, reports of
zirconium-related adverse reactions are rare and, in general, rigorous
cause-and-effect relationships have not been established. No evidence
has been validated that zirconium is carcinogenic or genotoxic.

Among the numerous radioactive isotopes of zirconium, 93Zr is among
the most common. It is released as a product of nuclear fission of
235U and 239Pu, mainly in nuclear power plants and during nuclear
weapons tests in the 1950s and 1960s. It has a very long half-life
(1.53 million years), its decay emits only low energy radiations, and
it is not considered particularly hazardous.

See also
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* Zirconium alloys
* Zirconia light

External links
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* [http://www.rsc.org/chemistryworld/podcast/element.asp Chemistry in
its element podcast] (MP3) from the Royal Society of Chemistry's
Chemistry World:
[http://www.rsc.org/images/CIIE_zirconium_remix2_48k_tcm18-117340.mp3
Zirconium]
* [http://www.periodicvideos.com/videos/040.htm Zirconium] at 'The
Periodic Table of Videos' (University of Nottingham)

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Original Article: http://en.wikipedia.org/wiki/Zirconium