(zərkō'nēəm), metallic chemical element; symbol Zr; at. no. 40; at. wt. 91.224; m.p. about 1,852 degrees Celsius; b.p. 4,377 degrees Celsius; sp. gr. 6.5 at 20 degrees Celsius; 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 table. 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 reactors. It is employed in tubes for cladding uranium 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.
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