ZIRCONIUM [symbol Zr, atomic weight 90.6 (O = 16)], a metallic chemical element. Klaproth in 1789 analysed the mineral zircon or hyacinth and found it to contain a new earth, which he called “zirconia.” The metal was obtained by Berzelius as an iron-grey powder by heating potassium zirconofluoride with metallic potassium. The amorphous metal also results when the chloride is heated with sodium; the oxide reduced with magnesium; or when fused potassium zirconofluoride is electrolysed (Wedekind, Zeit. Elektrochem., 1904, 10, p. 331). Troost produced crystaUized zirconium by fusing the double fluoride with al-uminium in a graphite crucible at the temperature of melting iron, and extracting the aluminium from the melt with hydrochloric acid. It is more conveniently prepared by heating the oxide with carbon in the electric furnace. The crystals look like antimony, and are brittle, and so hard as to scratch glass and rubies; their specific gravity is 4.25. The powdery metal burns readily in air, the crystalline metal requires to be heated in an oxyhydrogen flame before it catches fire. Mineral acids generally attack the crystallized metal very little even in the heat; aqua regia, however, dissolves it readily, and so does hydrofluoric acid. In its chemical affinities zirconium resembles titanium, cerium and thorium; it occurs in company with these elements, and is tetravalent in its more important salts.
Zirconium oxide or zirconia, ZrO2, has become important since its application to the manufacture of mantles for incandescent gas-lighting. For its extraction from zircon the mineral is heated and quenched in water to render it brittle, and then reduced to a fine powder, which is fused with three to four parts of acid potassium fluoride in a platinum crucible. When the mass is quietly fusing, the crucible is heated for two hours in a wind-furnace. The porcelain-like melt is powdered, boiled with water, and acidified with hydrofluoric acid, and the residual potassium fluosilicate is filtered off. The filtrate on cooling deposits crystals of potassium zirconofluoride, K2ZrF6, which are purified by crystallization from hot water. The double fluoride is decomposed with hot concentrated sulphuric acid; the mixed sulphate is dissolved in water; and the zirconia is precipitated with ammonia in the cold. The precipitate, being difficult to wash, is (after a preliminary washing) re-dissolved in hydrochloric acid and re-precipitated with ammonia. Zirconium hydroxide, Zr(OH)4, as thus obtained, is quite appreciably soluble in water and easily in mineral acids, with formation of zirconium salts, e.g. ZrCl4. But, if the hydroxide is precipitated in the heat, it demands concentrated acids for its solution. The hydroxide readily loses its water at a dull red heat and passes into an hydride with vivid incandescence. Zirconia can be obtained crystalline, in a form isomorphous with cassiterite and rutile, by fusing the amorphous modification with borax, and dissolving out with sulphuric acid. The anhydrous oxide is with difficulty soluble even in hydrofluoric acid; but a mixture of two parts of concentrated sulphuric acid and one of water dissolves it on continued heating as the sulphate, Zr(SO4)2. Zirconia, when heated to whiteness, remains unfused, and radiates a fine white light, which suggested its utilization for making incandescent gas mantles; and, in the form of disks, as a substitute for the lime-cylinders ordinarily employed in “limelight.” Zirconia, like stannic and titanic oxides, unites not only with acids but also with basic oxides. For instance, if it be fused with sodium carbonate, sodium zirconate, Na2ZrO3, is formed. If the carbonate be in excess, the salt Na4ZrO4 results, which when treated with water gives Na2Zr8O17•12H2O, which crystallizes in hexagonal plates. When heated in a loosely covered crucible with magnesium the nitride Zr2N3 is formed (Wedekind, Zeit. anorg. Chem., 1905, 45, p. 385).
Zirconium hydride, ZrH2, is supposed to be formed when zirconia is heated with magnesium in an atmosphere of hydrogen. Zirconium fluoride, ZrF4, is obtained as glittering monoclinic tables (with 3H2O) by heating zirconia with acid ammonium fluoride. It forms double salts, named zircono-fluorides, which are isomorphous with the stanni- and titani-fluorides. Zirconium chloride, ZrCl4, is prepared as a white sublimate by igniting a mixture of zirconia and charcoal in a current of chlorine. It has the exact vapour-density corresponding to the formula. It dissolves in water with evolution of heat; on evaporation a basic salt, ZrOCl2•8H2O, separates out in star-shaped acicular aggregates. Zirconium bromide, ZrBr4, is formed similarly to the chloride. Water gives the oxybromide ZrOBr2. Zirconium iodide, ZrI4, was obtained as a yellow, microcrystalline solid by acting with hydriodic acid on heated zirconium (Wedekind, Ber., 1904, 37, p. 1135). It fumes in air; with water it gives ZrOI2•8H2O; and with alcohol ethyl iodide and zirconium hydroxide are formed. The iodide combines with liquid ammonia to form ZrI4•8NH3; and with ether to give ZrI4•4(C2H5)2O. Zirconium combines with sulphur to form a sulphide, and with carbon to form several carbides. The sulphate, Zr(SO4)2, is a white mass obtained by dissolving the oxide or hydroxide in sulphuric acid, evaporating and heating the mass to nearly a red heat. Since it forms a series of double sulphates, Ruer (Zeit. anorg. Chem., 1904, 42, p. 87) regards it as a dibasic acid, ZrOS04•S04H2, and that the crystalline sulphate is ZrOSO4•SO4H2•3H2O (not Zr(SO4)2•4H20). Zirconium also forms double sulphates of the type Zr2O3(SO4M)2•nH2O, where M=K, Rb, Cs, and n=8 for K, 15 for Rb, 11 for Cs (Rosenheim and Frank, Ber., 1905, 38, p. 812). The atomic weight was determined by Marignac to be 90.03; Bailey (Proc. Roy. Soc., 1890, 46, p. 74) deduced the value 89.95.