ZIRCONIUM, a metallic chemical element, so-called from the identification of its oxide, zirconia, by Klaproth (1789) in the mineral zircon (symbol Zr, atomic number 4o, atomic weight 91.2). It was first isolated by J. J. Berzelius in 1824, who heated potassium zirconifluoride, with potassium and obtained an iron grey powder, which was, however, impure.
Preparation.—Preparation of the pure metal is difficult, owing to the facility with which zirconium combines with hydrogen, oxygen, nitrogen, boron, carbon and silicon and forms alloys with such metals as aluminium and magnesium. Specimens containing more than 99.6% of zirconium were obtained by pressing the impure metal into rods which could be used as electrodes. When an electric arc was struck across such electrodes in an atmosphere of hydrogen or ammonia under I I mm. pressure, the metal of the positive electrode fused and fell in iron grey drops on the negative electrode. This procedure is allied to the method for obtaining pure zirconium by heating its hydride or nitride in vacuo. By electrical heating of zirconium tetrachloride with sodium in an evacuated bomb, ductile metal was produced which when fashioned into rods gave a shiny surface on burnishing and a similar product was obtained in the reduction of zirconia by metallic calcium. Pure zirconium is deposited on a heated tungsten filament when the vapour of zirconium iodide is passed over the filament. Colloidal zirconium has been obtained by dialysis of the reduction product of potassium zirconifluoride with potassium. The melting point of zirconium has been variously given as 1,530° C and 1,7oo° C. Its specific gravity is 6.4.
Ferrozirconium (2o% Zr), employed as a scavenger in steel cast ings, is made by heating zirconia and ferric oxide with aluminium in graphite crucibles. The addition of about 0.34% of zirconium to armour plating steel containing 3% of nickel adds to the tensile strength of the metal.
Hydrofluoric acid, even when dilute, hot concentrated sulphuric acid and aqua regia attack zirconium energetically, whereas hydro chloric and nitric acids dissolve it very slightly. It is oxidized on fusion with alkali hydroxides or nitre. At red heat zirconium reduces boric oxide, silica, titanic and metallic oxides such as chromic oxide. Its great affinity for oxygen explains the employ ment of zirconium and its alloys as cleansing agents (scavengers for oxygen and nitrogen) in metallurgy. The commercially im portant zirconium minerals are (r) baddeleyite (brazilite), the native zirconia found in large quantities in Brazil (Sao Paulo and Minas Geraes) ; (2) zirkelite, a mixture of brazilite with zircon silicates, also found in Brazil; (3) zircon, ZrSiO4, mined in Colorado. Following on their discovery of hafnium (q.v.)
Coster and Hevesy report (1923) that most zirconium ores con tain very appreciable quantities of this new element, the amount rising in certain instances to io or 2o%.
Crystallised zirconia, Zr02, is obtained in colourless quadratic crystals by heating zirconium tetrafluoride with boric oxide. Owing to its refractory character and high melting point (given as 2,653±10° C or as 3,000° C) it has been recommended for furnace linings, crucibles, muffles and pyrometer tubes. It has also been used in enamels and in porcelain and opaque glasses. The first incandescent mantles made by A. v. Welsbach consisted principally of zirconia but this oxide is now superseded by thoria and ceria. In the Nernst lamp the incandescent body is chiefly zirconia (85%) with yttrium oxides (15%). Zirconium tetra fluoride, ZrF4, obtained by heating zirconia with ammonium fluoride in highly refracting crystals, dissolves in water to form the hydrate, and combines with alkali fluorides to yield several types of double fluorides employed in the isolation of the metal by reduction with potassium or sodium. The com monest type is exemplified by the potassium salt, Zirconium tetrachloride, prepared by passing chlorine saturated with the vapour of carbon tetrachloride over zirconia heated in a silica tube at 600° 'C, is a white crystalline sublimate volatilis ing at 300° C and hydrolysed by water to zirconyl chloride, which forms a crystalline octahydrate. Zirconium sul phate, Zr(SO4)2,4H20, forms colourless crystals prepared by dissolving zirconia in concentrated sulphuric acid and diluting slightly with water. It is also regarded as a zirconyl acid sulphate, Other Salts.—Many other zirconyl salts are known including the nitrate which crystallises from aqueous solutions as Zirconium tetra-acetylacetone, Zr CH(CO• CH3)214,1oH20, obtained by the interaction of zirconyl nitrate, acetylacetone and aqueous sodium carbonate, crystallises from alcohol or acetylacetone in anhydrous colourless prisms melting at 193-195° C. When zirconium tetrachloride reacts with acetyl acetone, benzoylacetone or dibenzoylmethane in anhydrous media, compounds are obtained of the general formula Cl, where Dk is the univalent diketone radical (G. T. Morgan and A. R. Bowen, Zirconium carbide, ZrC, a hard metallic substance, produced from zirconium and carbon in the electric furnace (H. Moissan and Lengfeld, 1896) is used as an abrasive and for cutting glass. A mixture of this carbide (9o%) with ruthenium (RD%) has been suggested as a filament for incandescent lamps.