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zirconium(zərkō`nēəm), metallic chemical element; symbol Zr; at. no. 40; at. wt. 91.224; m.p. about 1,852°C;; b.p. 4,377°C;; sp. gr. 6.5 at 20°C;; valence +2, +3, or +4.
Zirconium is a very strong, malleable, ductile, lustrous silver-gray metal. At ordinary temperatures it has a hexagonal close-packed crystalline structure. Its chemical and physical properties are similar to those of titanium, the element above it in Group 4 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
..... Click the link for more information. . Zirconium is extremely resistant to heat and corrosion. It forms a number of compounds, among them zirconate (ZrO3−2) and zirconyl (ZrO+2) salts.
The most important compound is the oxide zirconia (ZrO2), used extensively as a refractory material in furnaces and crucibles, in ceramic glazes, and, formerly, in gas mantles. It occurs in nature as the silicate (ZrSiO4) and is used as a gemstone; it may be clear or colored, and is usually called zircon or hyacinth. Zirconium compounds also have minor uses as catalysts, in the dye, textile, plastics, and paint industries, and in pharmaceuticals such as poison ivy lotions.
The metal also has many other uses, among them in photographic flashbulbs and surgical instruments, in the removal of residual gases from electronic vacuum tubes, and as a hardening agent in alloys, especially steel. A major use of the metal is in nuclear reactorsnuclear reactor,
device for producing controlled release of nuclear energy. Reactors can be used for research or for power production. A research reactor is designed to produce various beams of radiation for experimental application; the heat produced is a waste product and is
..... Click the link for more information. . It is employed in tubes for cladding uraniumuranium
, radioactive metallic chemical element; symbol U; at. no. 92; mass number of most stable isotope 238; m.p. 1,132°C;; b.p. 3,818°C;; sp. gr. 19.1 at 25°C;; valence +3, +4, +5, or +6.
..... Click the link for more information. oxide fuel. It is well suited for this purpose because it is corrosion resistant and does not readily absorb thermal neutrons. It is specially purified to remove hafnium, which absorbs neutrons much more readily. It is usually alloyed with other metals to make it more corrosion resistant for these uses.
Zirconium is a fairly abundant element and is widely distributed in minerals, but it is never found uncombined in nature. It always occurs with hafnium, which has almost identical chemical properties. The chief ore is zircon (the silicate); baddeleyite (the oxide) also has some importance. Zircon is recovered (along with monazite, ilmenite, and rutile) from certain beach sands in New South Wales, Australia, and near Jacksonville, Fla. The metal is produced by the Kroll process. The zircon is treated with carbon in an electric furnace to form a cyanonitride, which is in turn treated with chlorine gas to form the volatile tetrachloride. The tetrachloride is carefully purified by sublimation in an inert atmosphere and is then chemically reduced to metal sponge by reaction with molten magnesium. The spongy metal is cleaned and further processed into ingots.
Special care is taken to exclude hydrogen, nitrogen, and oxygen, which make the metal brittle. If the metal is too brittle to be worked, it can be further purified by the Van Arkel–de Boer process, in which the crude metal is reacted with iodine to form volatile iodides that are thermally decomposed on a hot wire, resulting in pure crystalline zirconium. The commercial metal usually contains between 1% and 3% hafnium; for nuclear reactor use the hafnium is usually removed by solvent extraction from the tetrachloride. Zirconium was discovered as the oxide zirconia in the mineral zircon by M. H. Klaproth in 1789 and was first isolated in impure form by J. J. Berzelius in 1824.
Zr, a chemical element of Group IV of Mendeleev’s periodic system. Atomic number, 40; atomic weight, 91.22. A silvery white metal with a characteristic luster, zirconium occurs in nature as five isotopes: 90Zr (51.46 percent), 90Zr (11.23 percent), 92Zr (17.11 percent), 94Zr (17.4 percent), and 96Zr (2.8 percent). Of the artificial radioisotopes of zirconium, the most important is 95Zr, with a half-life of 65 days, which is used as an isotope tracer.
History. In 1789 the German chemist M. H. Klaproth isolated zirconium oxide while analyzing the mineral zircon. Powdered zirconium was first obtained in 1824 by J. Berzelius, while ductile zirconium was first obtained in 1925 by the Dutch scientists A. van Arkel and J. de Boer in the course of the thermal dissociation of zirconium iodides.
Distribution in nature. The average content of zirconium in the earth’s crust (clarke) is 1.7 × 10–2 percent by weight; the average content in granites, sandstones, and clays (2 × 10–2 percent) is somewhat greater than in basic rocks (1.3 × 10–2 percent). The maximum concentration of zirconium occurs in alkali rocks (5 × 10–2 percent). Zirconium weakly participates in water and biogenic migration. Seawater contains 0.00005 mg/l zirconium.
There are 27 known minerals of zirconium, of which baddeleyite, ZrO2, and zircon are of commercial importance (see). The major types of zirconium deposits are alkali rocks with malacon and cyrtolite; magnetite-forsterite-apatite rocks and carbonatites with baddeleyite; and littoral-marine and eluvial-deluvial deposits.
Physical and chemical properties. Zirconium exists in two crystal modifications: the α-phase, with a hexagonal close-packed lattice (a = 3.228 angstroms and c = 5.120 angstroms), and the β-phase, with a cubic body-centered lattice (a = 3.61 angstroms). The transition from the α-phase to the β-phase occurs at 862°C. The density of α-zirconium is 6.45 g/cm3 (at 20°C). Zirconium has a melting point of 1825 ± 10°C and a boiling point of 3580–3700°C. The heat capacity (in the range 25°–100°C) is 0.291 kilojoule/(kg·°K), or 0.0693 cal/(g·°C), the thermal conductivity at 50°C is 20.96 W/(m• °K), or 0.050 cal/(cm• sec• °C), and the coefficient of linear thermal expansion is (20°–400°C) 6.9 × 10–6. The electrical resistivity of high-purity zirconium is 44.1 µohm·cm (at 20°C). The superconducting transition temperature is 0.7°K.
Zirconium is paramagnetic, with the magnetic susceptibility increasing upon heating, from 1.28 × 10–6 at –73°C to 1.41 × 10–6 at 327°C. The thermal neutron absorption cross section of zirconium is (0.18±0.004) × 10–28 m2; hafnium impurities increase this value.
Pure zirconium is ductile and readily susceptible to cold and hot working by rolling, forging, and stamping. The presence of small amounts of oxygen, nitrogen, hydrogen, and carbon (or compounds of these elements with zirconium) dissolved in the metal cause the brittleness of zirconium. The modulus of elasticity of zirconium is 97 giganewtons/m2, or 9,700 kilograms-force/mm2 (at 20°C), the tensile strength is 253 meganewtons/m2, or 25.3 kilograms-force/mm2, and the Brinell hardness is 640–670 meganewtons/m2, or 64–67 kilograms-force/mm2. The oxygen content greatly affects hardness: at oxygen concentrations of greater than 0.2 percent, zirconium is not susceptible to cold pressure treatment.
The electronic configuration of the outer shell of the Zr atom is 4d25s2. Zirconium usually exhibits an oxidation number +4. It exhibits the lower oxidation numbers of +2 and +3 only in its compounds with chlorine, bromine, and iodine. Compact zirconium begins to oxidize slowly in the range of 200° to 400°C and becomes covered by a film of zirconium oxide (or zirconium dioxide), ZrO2; above 800°C, it reacts energetically with atmospheric oxygen. Zirconium metal powder is pyrophoric—it may ignite in the presence of air at ordinary temperatures.
Zirconium actively absorbs hydrogen even at 300°C, forming a solid solution and the hydrides ZrH and ZrH2; these hydrides dissociate in a vacuum at 1200°–1300°C, and all the hydrogen may be removed from the metal. With nitrogen, zirconium forms zirconium nitride, ZrN, at 700°–800°C, and it reacts with carbon above 900°C, forming zirconium carbide, ZrC. Zirconium carbide and zirconium nitride are solid high-melting compounds. Zirconium carbide is an intermediate product in the production of ZrCl4. Zirconium reacts with fluorine at ordinary temperatures and with chlorine, bromine, and iodine above 200°C, forming higher halides, ZrX4, where X is a halogen.
Zirconium is stable in water and water vapor up to 300°C and does not react with hydrochloric acid and sulfuric acid (up to 50 percent) or with alkaline solutions. Zirconium is the only metal that is stable in alkalies containing ammonia. Zirconium reacts with nitric acid and aqua regia above 100°C. It dissolves in hydrofluoric acid and hot concentrated (above 50 percent) sulfuric acid. Zirconium salts of the corresponding acids of various compositions may be isolated from acid solutions, depending on the acid concentration. Thus, the crystal hydrate Zr(SO4)2 · 4H2O precipitates from concentrated sulfuric-acid solutions of zirconium, and basic sulfates with the general formula xZrO2 · ySO3 · zH2O (where x : y > 1) precipitate from dilute solutions. Zirconium sulfates decompose completely at 800°–900°C, with the formation of zirconium oxide. Zirconium in the form of Zr(NO3)4 · 5H2O or ZrO(NO3)2 · xH2O (where x = 2–6) crystallizes from nitric-acid solutions, while ZrOCl2·8H2O, which undergoes dehydration at 180°–200°C, precipitates from hydrochloric-acid solutions.
Production. In the USSR, the mineral zircon, ZrSiO4, is the major industrial source for the production of zirconium. Zirconium ores are concentrated by gravity methods, with subsequent purification of the concentrates by magnetic and electrostatic separation. Zirconium metal is obtained from zirconium compounds. In order to obtain these compounds, the concentrate is initially decomposed by (1) chlorination in the presence of coal at 900°–1000°C (sometimes, with prior carbidization at 1700°–1800°C to remove most of the silicon present as highly volatile SiO), which yields ZrCl4, which is volatilized and collected; (2) melting with caustic soda (sodium hydroxide) at 500°–600°C or with sodium carbonate at 1100°C (ZrSiO4 + 2Na2CO 3= Na2ZrO3 + Na2SiO3 + 2CO2); (3) roasting with lime or calcium carbonate in the presence of CaCl2 at 1100°–1200°C (ZrSi0 4+ 3CaO = CaZrO3 + Ca2SiO4); and (4) melting with potassium fluorosilicate at 900°C (ZrSiO4 + K2SiF6 = K2ZrF6 + 2SiO2). Silicon compounds are removed from the sintered mass or melt obtained in the case of alkaline treatment (methods 2 and 3) by first leaching with water or dilute hydrochloric acid and then decomposing the residue by hydrochloric or sulfuric acid. In this process, zirconium oxychloride and zirconium sulfates are formed, respectively. The fluorozirconate cake (method 4) is treated with acidified water by heating; in this case, potassium fluorozirconate enters the solution, 75–90 percent of which is separated upon cooling the solution.
The following methods are used for separating zirconium compounds from acid solutions: (1) crystallization of zirconium oxychloride, ZrOCl2·8H2O, upon evaporation of hydrochloric-acid solutions, (2) hydrolytic precipitation of basic zirconium sulfates, xZrO2· ySO3 · zH2O, from sulfuric-acid or hydrochloric-acid solutions, and (3) crystallization of zirconium sulfate, Zr(SO4)2, upon the addition of concentrated sulfuric acid or upon evaporation of sulfuric-acid solutions. Zirconium oxide, ZrO2, is obtained as a result of the calcination of zirconium sulfates and zirconium chlorides.
Zirconium compounds obtained from ores always contain hafnium as an impurity. The hafnium is removed by fractional crystallization of K2ZrF6, by extraction from acid solutions by organic solvents (for example, by tributyl phosphate), by ionexchange methods, and by selective reduction of the tetrachlorides ZrCl4 and HfCl4.
Zirconium is obtained in the form of a powder or sponge by the thermal reduction of ZrCl4, K2ZrF6, or ZrO2. Zirconium tetrachloride is reduced by magnesium or sodium, potassium zirconium fluoride is reduced by sodium, and zirconium oxide is reduced by calcium or calcium hydride. Electrolytic zirconium powder is obtained from a melt consisting of a mixture of zirconium halides and the chlorides of alkali metals. Compact malleable zirconium is produced by melting in vacuum arc furnaces compressed zirconium powder or sponge, which usually serve as consumable electrodes. High-purity zirconium is produced by electron-beam smelting of ingots obtained in arc furnaces or rods after iodide refining.
Uses. Alloys based on zirconium that has been purified to remove the hafnium are used primarily as structural materials in nuclear reactors because of zirconium’s low thermal neutron absorption cross section (see). Zirconium is also a component of a series of alloys based on magnesium, titanium, nickel, molybdenum, niobium, and other metals used as structural materials, for example, for rockets and other aircraft. The coils of superconductor magnets are made from alloys of zirconium with niobium. In foundry work, zirconium refractories are used. The members of the lead zirconate-titanate group (for example, TsTS-23) are among the most common piezoceramic materials (piezoceramics). Zirconium is a metallic component of metalloceramic materials (cermets), while zirconium oxide is their ceramic component. Zirconium wire serves as a getter in the production of transmitting tubes.
Zirconium is used as a corrosion-resistant material in the production of equipment for the chemical industry. Zirconium additives are used for the deoxidation of steel and the removal of nitrogen and sulfur from steel. Zirconium powder is used in the production of fireworks and ammunition. Zirconium sulfate is a tanning agent in the leather industry.
REFERENCESSpravochnikpo redkim metallam. Edited by C. A. Hampel. Moscow, 1965. (Translated from English).
Osnovy metallurgii, vol. 4. Moscow, 1967.
Zelikman, A. N., and G. A. Meerson. Metallurgiia redkikh metallov. Moscow, 1973.
O. E. KREIN