Zirconium

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. 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

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, and the boiling point is 4409 °C. 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, ⁹⁴Zr, and ⁹⁶Zr. The first four are stable, while ⁹⁶ is observed to decay with a half-life of 2.34×10¹⁹ years by double beta emission; this is theoretically possible also for ⁹⁴Zr. Of these natural isotopes, ⁹⁰Zr is the most common, making up 51.45% of all zirconium, and ⁹⁶Zr is the least common, comprising only 2.80%.
The artificial radioisotopes of zirconium known range from ⁷⁷Zr to ¹¹⁴Zr, and 13 nuclear isomers are also listed. The most stable among them is ⁹³Zr, a long-lived fission product, with a half-life of 1.61 million years. Radioactive isotopes at or above mass number 93 decay by electron emission resulting in niobium isotopes, whereas those at or below 89 decay by positron emission or electron capture, resulting in yttrium isotopes.

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, 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 2023, Australia and South Africa are the leading producers, together accounting for approximately half of global zircon production. 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 ⁹¹Zr. Single crystals of LuPO₄ and YPO₄ doped with both naturally abundant and isotopically enriched Zr have also been grown and investigated.

Production

Occurrence

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 chloride with magnesium metal in the Kroll process. The resulting metal is sintered until sufficiently ductile for metalworking.

Separation of zirconium and hafnium

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, 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

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 oxidation state. Non-equilibrium oxidation states between 0 and 4 have been detected during zirconium oxidation.

Oxides, nitrides, and carbides

The most common oxide is zirconium dioxide, ZrO₂, also known as zirconia. This clear to white-coloured solid has exceptional fracture toughness 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 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 is the most commonly used piezoelectric material, being used as transducers and actuators in medical and microelectromechanical systems applications.

Halides and pseudohalides

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. 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.