Zirconium

Background

Zirconium, symbol Zr on the Periodic Table, is a metal most often found in and extracted from the silicate mineral zirconium silicate and the oxide mineral baddeleyite. In its various compound forms, the grayish-white zirconium is the nineteenth most plentiful element in the earth's crust, where it is far more abundant than copper and lead. It belongs to the titanium family of metals, a group that also includes titanium and hafnium and that is favored in industry for its members' good electrical conductivity as well as their tendency to form metallic salts. Because it is stable in many electron configurations and physical states, zirconium can be made into many products. However, since the 1940s, its most significant applications have been in various structural components of nuclear reactors.

Zirconium was discovered by German chemist Martin Heinrich Klaproth, who first isolated an oxide of the mineral zircon in 1789. The first metallic powder was produced in 1824 by a Swedish Chemist, Jons J. Berzelius. The forms of the metal that could be isolated during the nineteenth century, however, were impure and thus very brittle. The earliest method of purifying useable quantities of the metal was developed in 1925 by Dutch chemists Anton E. van Arkel and J. H. de Boer, who invented a thermal iodide process by which they thermally decomposed zirconium tetraiodide. The drawback with van Arkel and de Boer's method was its cost, but twenty years later William Justin Kroll of Luxembourg invented a cheaper process, using magnesium to break down zirconium tetrachloride. Relatively inexpensive, this process produced zirconium in quantities large and pure enough for industrial use.

Since Kroll's breakthrough, zirconium has become an important element in several industries: steel, iron, and nuclear power. It is used in the steel industry to remove nitrogen and sulfur from iron, thereby enhancing the metallurgical quality of the steel. When added to iron to create an alloy, zirconium improves iron's machinability, toughness, and ductility. Other common industrial applications of zirconium include the manufacture of photoflash bulbs and surgical equipment, and the tanning of leather.

Despite its ability to be used for many different industrial applications, most of the zirconium produced today is used in water-cooled nuclear reactors. Zirconium has strong corrosion-resistance properties as well as the ability to confine fission fragments and neutrons so that thermal or slow neutrons are not absorbed and wasted, thus improving the efficiency of the nuclear reactor. In fact, about 90 percent of the zirconium produced in 1989 was used in nuclear reactors, either in fuel containers or nuclear product casings.

Raw Materials

Of the two mineral forms in which zirconium occurs, zircon is by far the more important source. Found mainly in igneous rock, zircon also appears in the gravel and sand produced as igneous rock erodes. In this form, it is often mixed with silica, ilmenite, and rutile. The vast majority of the zircon used in industry today originates in these sand and gravel deposits, from which the purest zircon is extracted and refined to be used as zirconium metals. Less pure deposits are used in the form of stabilized zirconia for refractories and ceramic products. The world's largest zircon mines are in Australia, South Africa, and the United States, but rich beds also exist in Brazil, China, India, Russia, Italy, Norway, Thailand, Madagascar, and Canada. Like zircon, baddeleyite is extracted from sand and gravel deposits. Unlike zircon, commercially viable baddeleyite deposits contain relatively high concentrations of zirconium oxide, and baddeleyite can thus be used without refining. The mineral is, however, much more scarce than zircon, with significant amounts occurring only in Brazil and Florida.

Extraction and Refining

Extracting zircon

• 1 The sand and gravel that contain zircon mixed with silicate, ilmenite, and rutile are typically collected from coastal waters by a floating dredge, a large steam shovel fitted on a floating barge. After the shovel has scooped up the gravel and sand, they are purified by means of spiral concentrators, which separate on the basis of density. The ilmenite and rutile are then removed by magnetic and electrostatic separators. The purest concentrates of zircon are shipped to end-product manufacturers to be used in metal production, while less pure concentrations are used for refractories.

Refining zircon

• 2 End-product manufacturers of zircon further refine the nearly pure zircon into zirconium by using a reducing agent (usually chlorine) to purify the metal and then sintering (heating) it until it becomes sufficiently ductile—workable—for industrial use. For small-scale laboratory use, zirconium metal may be produced by means of a chemical reaction in which chloride is used to reduce the zircon.
• 3 The less-pure zircon is made into zirconia, an oxide of zirconium, by fusing the zircon with coke, iron borings, and lime until the silica is reduced to silicon that alloys with the iron. The zirconia is then stabilized by heating it to about 3,095 degrees Fahrenheit (1,700 degrees Celsius), with additions of lime and magnesia totalling about five percent.

Refining baddeleyite

• 4 As mentioned above, baddeleyite contains relatively high, pure concentrations of zirconium oxide that can be used without filtering or cleansing. The only refining process used on baddeleyite involves grinding the gravel or sand to a powder and sizing the powder with different sized sieves. All zirconium oxide that comes from baddeleyite is used for refractories and, increasingly, advanced ceramics.

Quality Control

The quality control methods implemented in the production of zirconium metal are typical Statistical Process Control (SPC) methods used in most metal production. These involve tracking and controlling specific variables determined by the end product requirements. Stringent government quality control is applied to all zirconium metal produced for nuclear applications. These controls assure that the zirconium produced for use in a nuclear plant has been processed correctly and also allow for accountability: processing is tracked so that it can be traced back to each individual step and location.

Quality control methods for zirconium used in refractory applications also focus on SPC. However, in the refractory industries, it is also necessary to ascertain the beach (and even what part of the beach) from which the zirconium mineral was extracted. Manufacturers need to know exactly where the zirconium came from because each source contains slightly different trace elements, and different trace elements can affect the end product.

Byproducts/Waste

Silicate, ilmenite, and rutile—all byproducts of the zircon refining process—are typically dumped back in the water at the extraction site. These elements compose typical beach sand and are in no way detrimental to the environment. Magnesium chloride, the only other notable byproduct of zirconium manufacturing, results from the reduction of the zircon with chlorine in the refining process and is typically sold to magnesium refineries. No byproducts or waste result from baddeleyite refining.

The Future

Many believe that the future of zirconium lies in its use as an advanced ceramic. Advanced ceramics—also called "fine," "new," "high-tech," or "high-performance" ceramics—are generally used as components in processing equipment, devices, or machines because they can perform many functions better than competing metals or polymers. Zirconium is fairly hard, doesn't conduct heat well, and is relatively inert (i.e., it doesn't react readily with other elements), all excellent qualities for advanced ceramics. Zirconium oxide, manufactured as a ceramic, can be used to make crucibles for melting metals, gas turbines, liners for jet and rocket motor tubes, resistance furnaces, ultra-high frequency furnaces, and refractories such as the facing of a high-temperature furnace wall.

Where To Learn More

Books

Heuer, A. H., ed. Science and Technology of Zirconia. American Ceramic Society, 1981.

Specifications for Zirconium and Zirconium Alloy Welding Electrodes and Rods. American Welding Society, 1990.

Zirconium and Hafnium. Gordon Press Publishers, 1993.

Periodicals

Burke, Marshall A. "Ceramics Enter the Foundry," Design News. June 16,1986, p. 56.

"Fuel Cell's Future Gets a Boost," Design News. August 18, 1986, p. 38.

"Zirconium," Machine Design. April 14, 1988, pp. 234-35.

"Zirconium Holds Down Costs of Making Zirconium," Metal Progress. November, 1983, pp. 11-12.

"Adding Strength to Glassy Ceramics," Science News. September 13, 1986, p. 170.

—Alicia Haley and

Blaine Danley

ZIRCONIUM (REVISED)

Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.

Overview

Compounds of zirconium have been known for centuries. Yet, the element itself was not recognized until 1789. In that year, German chemist Martin Heinrich Klaproth (1743-1817) discovered the element in a stone brought to him from the island of Ceylon (now Sri Lanka).

Zirconium is one of the transition metals. The transition metals are the elements found in Rows 4 through 7 and between Groups 2 and 13 in the periodic table. The periodic table is a chart that shows how chemical elements are related to each other. Zirconium is located below titanium, which it resembles, in the periodic table. Below zirconium is hafnium, a chemical twin of zirconium.

An important use of zirconium is in nuclear power plants. Its most important compound is zircon, which has a number of industrial applications. Zircon can also be obtained in gemstone quality. A gemstone is a mineral that can be cut and polished and used in jewelry or art.

SYMBOL
Zr

ATOMIC NUMBER
40

ATOMIC MASS
91.22

FAMILY
Group 4 (IVB)
Transition metal

PRONUNCIATION
zir-KO-nee-um

Discovery and naming

Naturally occurring compounds of zirconium have been used by humans since before the birth of Christ. For example, St. John talks about the jacinth (or hyacinth) stone. He says it was one of the jewels found in the walls surrounding Jerusalem. The jacinth stone was the same mineral referred to by the Persians as zargun, meaning "gold-like" in Persian.

Early chemists did not study the jacinth stone very carefully. They thought it was another form of alumina (aluminum oxide). Alumina was a well-known mineral at the time. In fact, it was not until Klaproth undertook the study of the jacinth stone that he realized it contained a new element. Klaproth at first referred to the stone as Jargon of Ceylon. When he knew that he had found a new element, he suggested the name zirconium for it.

The material discovered by Klaproth was not a pure element. Instead, it was a compound of zirconium and oxygen, zirconium oxide (ZrO₂). The pure metal was not produced until 1824 when Swedish chemist Jons Jakob Berzelius (1779-1848) made fairly pure zirconium. He made the metal by heating a mixture of potassium and potassium zirconium fluoride (ZrK₂F₆):

Physical properties

Zirconium is a hard, grayish-white, shiny metal. Its surface often has a flaky-like appearance. It also occurs in the form of a black or bluish-black powder. It has a melting point of 1,857°C (3,375°F) and a boiling point of 3,577°C (6,471°F. Its density is 6.5 grams per cubic centimeter.

Zirconium has one physical property of special importance: It is transparent to neutrons. Neutrons are tiny particles with no charge in the nucleus (center) of almost all atoms. Industrially, they are used to make nuclear fission reactions occur. Nuclear fission is the process in which large atoms break apart. Large amounts of energy and smaller atoms are produced during fission. Fission reactions are used to provide the power behind nuclear weapons (such as the atomic bomb). They are also used to produce energy in a nuclear power plant.

One of the difficult problems in building a nuclear power plant is selecting the right materials. Many metals capture neutrons that pass through them. The neutrons become part of the metal atoms and are no longer available to make fission reactions occur. An engineer needs to use materials in a power plant that are transparent to neutrons—that is, that allow neutrons to pass through them.

Zirconium is one of the best of these metals. If zirconium is used to make the parts in a nuclear power plant, it will not remove neutrons from the fission reaction going on inside the plant.

A special alloy of zirconium has been developed just for this purpose. It is called Zircaloy. The manufacture of Zircaloy accounts for 90 percent of the zirconium metal used in the world today.

Chemical properties

Zirconium is a fairly inactive element. When exposed to air, it reacts with oxygen to form a thin film of zirconium oxide (ZrO₂). This film protects the metal from further corrosion (rusting). Zirconium does not react with most cold acids or with water. It does react with some acids that are very hot, however.

Occurrence in nature

Zirconium is a fairly common element in the Earth's crust. Its abundance is estimated to be 150 to 230 parts per million.

That places it just below carbon and sulfur among elements occurring in the Earth's crust.

The two most common ores of zirconium are zircon, or zirconium silicate (ZrSiO₄); and baddeleyite, or zirconia or zirconium oxide (ZrO₂). The amount of zirconium produced in the United States is not reported. That information is regarded as a trade secret. The largest suppliers of zirconium minerals in the world are Australia and South Africa. These two countries produce about 85 percent of the world's zirconium.

Isotopes

There are five naturally occurring isotopes of zirconium: zirconium-90, zirconium-91, zirconium-92, zirconium-94, and zirconium-96. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

About a dozen radioactive isotopes of zirconium are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.

No radioactive isotope of zirconium has any important practical application.

Extraction

Zirconium ores are first converted to zirconium tetrachloride (ZrCl₄). This compound is then mixed with magnesium metal at high temperature:

Naturally occurring zircon is in demand as a gemstone. It is polished, cut, and used for jewelry and art.

Uses

Many zirconium alloys are available. They are used to make flash bulbs, rayon spinnerets (the nozzles from which liquid rayon is released), lamp filaments, precision tools, and surgical instruments. These uses make up only a small amount of the metal produced, however, compared to its application in nuclear power plants.

Compounds

About 95 percent of all zirconium produced is converted into a compound before being used. The two most common compounds made are zircon (zirconium silicate) and zirconia (zirconium oxide).

Naturally occurring zircon is in demand as a gemstone. It is polished, cut, and used for jewelry and art. Natural zircon often includes uranium, thorium, and other radioactive elements. The presence of these elements often gives a zircon a special brilliance and fire-like quality, resembling fine diamonds.

Zircon has other properties that make it desirable in industrial applications. For example, it is an excellent refractory material. A refractory is a material that does not conduct heat well. It is able to withstand very high temperatures without cracking or breaking down.

Zircon is used to make the foundry molds used to make metal pieces of all shapes. Molten metal is poured into the mold. When it cools, it is removed from the mold. The use of zircon in a refractory mold produces a smooth surface on the metal.

Zircon is also used to make bricks in high-temperature furnaces and ovens. These furnaces and ovens are used to work with molten metals. Zircon bricks are ideal for such ovens because they reflect heat and are not destroyed by high temperatures.

Both zircon and zirconia are used as abrasives. An abrasive is a powdery material used to grind or polish other materials. Another important use of zircon and zirconia is in making objects opaque. Opaque means that light is not able to pass through. Suppose a person wants to make a glaze for pottery that looks completely white. The glaze must reflect all light that strikes it and not let any light through. Adding zircon or zirconia to the glaze will achieve this result.

Zirconium can cause skin irritation. Deodorant products containing zirconium have been found to produce skin rashes.

Health effects

Zirconium is regarded as relatively safe. Some studies have shown that it can cause skin irritation, however. Deodorant products containing zirconium have been found to produce skin rashes.

Zirconium

melting point: 185°C
boiling point: 4,375°C
density: 6.4 g/cm ³
most common ions: Zr ⁴⁺

Zirconium was discovered by the German chemist Martin H. Klaproth in 1789. The principal ore of zirconium is zircon, which is widely distributed in nature as beach sands, particularly in Australia and India. Zirconium is the nineteenth most abundant element in the earth's crust (at approximately 0.03%). Zircon is a silicate of formula ZrSiO₄ and occurs as the gemstones hyacinth and zirconite. Synthetic gemstones are prepared from zircon and from the oxide ZrO₂. Zirconium metal is difficult to produce. Its production requires treatment of the tetrachloride ZrCl₄ with magnesium metal. Zirconium metal is silvery-gray, ductile , and malleable.

Zirconium's major use is as cladding for nuclear reactors. It is ideal for this use, as it has a limited ability to capture neutrons, strength at elevated temperatures, considerable corrosion resistance, and satisfactory neutron damage resistance. Almost all ores of zirconium contain about 2 percent of zirconium's sister element, hafnium (Hf). Hafnium readily absorbs neutrons and therefore must be completely separated and removed from zirconium before either element can be used in nuclear reactors. A major task of the Manhattan Project was the separation of hafnium from zirconium. The elements are the two most chemically similar in the Periodic Table. The recovered hafnium metal is used to make the control rods of nuclear reactors, as the metal readily absorbs neutrons.

Zirconium oxide, or zirconia, occurs as the mineral baddeleyite, but zirconium oxide is obtained commercially mainly via its recovery from zircon. Zircon is treated with molten sodium hydroxide to dissolve the silica. Zirconia is used as a ceramic, but it must be doped with about 10 percent CaO or Y₂O₃ to stabilize it in its face-centered cubic form. Zirconia is monoclinic , meaning that it has one oblique intersection of crystallographic axes, but it undergoes a phase change at about 1,100°C (2,012°F), its crystal structure becoming tetragonal, and above 2,300°C (4,172°F) it becomes cubic. To

prevent expansion and shrinkage across the 1,100°C phase change (which produces cracking), the stabilized form of zirconia is used, as it does not change phase until very high temperatures are reached. Stabilized zirconia is used to regulate the air-fuel mixtures in automobiles, as it is an oxide ion conductor . It generates an electric potential based on the amount of oxygen in the fuel and can adjust the mixture by electrical control of valves to ensure proper fuel burning.

The most important soluble compound of zirconium is zirconyl chloride, ZrOCl₂·8H₂O, but this compound does not contain the zirconyl ion ZrO²⁺. It is a tetramer of composition [Zr(OH)₂·4H₂O]₄⁸⁺, prone to polymerize to larger species as pH increases, forming hydrous zirconia, ZrO_x (OH)_y ·n H₂O. Other important soluble salts are the sulfate, Zr(SO₄)₂·4H₂O, and the nitrate, Zr(NO₃)₄·5H₂O, isolated from strongly acidic solutions. Both zirconium and titanium form organometallic compounds that are important catalysts in the conversion of ethylene to polyethylene.

see also Hafnium; Nuclear Chemistry; Manhattan Project; Organometallic Compounds.

Abraham Clearfield

Bibliography

Greenwood, N. N., and Earnshaw, A. (1985). Chemistry of the Elements. New York: Pergamon Press.

Stevens, R. (1986). Zirconia and Zirconia Ceramics, 2nd edition. Twickenham: Magnesium Elektron, Inc.

zirconium (symbol Zr) Greyish-white, metallic element, one of the transition elements. Zirconium was first discovered (1789) by the German chemist Martin Klaproth, and its chief source is zircon. Lunar rocks collected during the Apollo space missions show a higher content of zirconium than Earth ones, and zirconium exists in meteorites and stars, including the Sun. Chemically similar to titanium, it is used in ceramics, and in alloys for wire and absorption of neutrons in nuclear reactors. Properties: at.no. 40; r.a.m. 91.22; r.d. 6.51; m.p. 1852°C (3366°F); b.p. 4377°C (7911°F); most common isotope Zr⁹⁰ (51.46%).

zir·co·ni·um / ˌzərˈkōnēəm/ • n. the chemical element of atomic number 40, a hard silver-gray metal of the transition series. (Symbol: Zr)