Gunnar Nordberg
Occurrence and Uses
It has been estimated that zirconium (Zr) constitutes about 0.017% of the lithosphere. Because of its very high chemical activity at temperatures only slightly above normal atmospheric temperature, the element occurs only in combined states. The most common ores are zircon (ZrO2) and baddeleyite (ZrSiO4). Zirconium is found in all animal tissues.
Hafnium (Hf) is found associated with zirconium in all its terrestrial occurrences. The amount of hafnium varies but averages about 2% of the total zirconium plus hafnium. In only one ore, low in both elements, has hafnium been found in greater quantity than zirconium. Spectrographic evidence indicates that the distribution is also about 2% hafnium in the total zirconium-plus-hafnium in the universe. These two elements are more closely identical in their chemical properties than are any other pair in the periodic table. The similarity is so great that no qualitative differences have yet been found which would permit their separation. For this reason, it can be assumed that most of the zirconium which has been used, and on the basis of which physiological effects have been reported, has contained 0.5 to 2% hafnium.
Zircon has been valued since the earliest times as a gem stone, since it occurs quite commonly in large single crystals; however, most of the commercially useful deposits of zirconium ore are in beach sands or other places where the relatively heavy and chemically inert zirconium minerals have been deposited while the lighter portions of the rocks in which they occurred have been disintegrated and washed away by the action of water. Substantial deposits of such beach sands are known in India, Malaya, Australia and the United States. Baddeleyite in commercially useful deposits was first observed in Brazil, and has since been found in a number of other locations including Sweden, India and Italy. Some zirconium ores have also been mined commercially in Madagascar, Nigeria, Senegal and South Africa.
Zircon is used as a foundry sand, an abrasive, and as a component of zircon and zirconia refractory compositions for laboratory crucibles. It is found in ceramic compositions where it acts as an opacifier in glazes and enamels. Zircon and zirconia bricks are used as linings for glass furnaces. Zirconia forms are also used as dies for extrusion of both ferrous and non-ferrous metals and as spout linings for pouring metals, particularly for continuous casting.
More than 90% of zirconium metal is now used in nuclear power generation because zirconium has a low absorption cross-section for neutrons and a high resistance to corrosion inside atomic reactors, provided it is free of hafnium. Zirconium is also used in the manufacture of cast iron, steel and surgical appliances. It is employed in arc lamps, pyrotechnics, in special welding fluxes, and as a pigment in plastics.
Powdered zirconium metal is used as a “getter” in thermionic tubes to absorb the last traces of gas after pumping and out-gassing of the tube elements. In the form of fine ribbon or wool, the metal is also used as the filter in photographic flash-bulbs. The massive metal is used either pure or in alloy form for the lining of reaction vessels. It is also used as a lining for pumps and piping systems for chemical processes. An excellent super-conducting alloy of zirconium and columbium has been used in a magnet with a field of 6.7 T.
Zirconium carbide and zirconium diboride are both hard, refractory, metallic compounds which have been used in cutting tools for metals. The diboride has also been used as a thermocouple jacket in open-hearth furnaces, providing very long-lived thermocouples. Zirconium tetrachloride is used in organic synthesis and in water repellents for textiles. It is also useful as a tanning agent.
Hafnium metal has been used as a cladding on tantalum for rocket engine parts which must operate in very high-temperature, erosive conditions. Because of its high thermal-neutron cross-section, it is also used as a control rod material for nuclear reactors. In addition, hafnium is used in the manufacture of electrodes and light-bulb filaments.
Hazards
It is inaccurate to state that zirconium compounds are physiologically inert, but the tolerance of most organisms to zirconium appears to be great in comparison to the tolerance for most heavy metals. Zirconium salts have been used in the treatment of plutonium poisoning to displace the plutonium (and yttrium) from its deposition in the skeleton and to prevent the deposition when treatment was started early. In the course of this study, it was determined that the diet of rats could contain as much as 20% of zirconia for comparatively long periods without harmful effects, and that the intravenous LD50 of sodium zirconium citrate for rats is about 171 mg/kg body weight. Other investigators have found an intraperitoneal LD50 of 0.67 g/kg for zirconium lactate and 0.42 g/kg for barium zirconate in rats and 51 mg/kg of sodium zirconium lactate in mice.
Zirconium compounds have been recommended and used for the topical treatment of Rhus (poison ivy) dermatitis and for body deodorants. Some compounds which have been used are carbonated hydrous zirconia, hydrous zirconia and sodium zirconium lactate. There have been a number of reports of the production of persistent granulomatous conditions of the skin as the result of these applications.
Of more direct interest in connection with occupational exposures is the effect of inhalation of zirconium compounds, and this has been less extensively investigated than the other routes of administration. There have, however, been several experiments and at least one report of human exposure. In this instance, a chemical engineer with seven years’ exposure in a zirconium and hafnium processing plant was found to have a granulomatous lung condition. Since examination of all the other employees revealed no comparable lesions, it was concluded that the condition was most probably to be attributed to a relatively heavy beryllium exposure prior to zirconium exposure.
Exposure of experimental animals to zirconium compounds showed that zirconium lactate and barium zirconate both produced severe, persistent, chronic interstitial pneumonitis at atmospheric zirconium concentrations of about 5 mg/m3. Much higher atmospheric sodium zirconium lactate concentrations of 0.049 mg/cm3 for shorter exposures have been found to produce peribronchial abscesses, peribronchiolar granulomas and lobular pneumonia. Although documentation of zirconium pneumoconiosis in humans has been lacking, authors of one study conclude that zirconium should be considered a likely cause of pneumoconiosis, and recommend taking appropriate precautions in the workplace.
The small number of investigations on the toxicity of hafnium compounds has indicated an acute toxicity slightly higher than that of zirconium salts. Hafnium and its compounds cause liver damage. Hafnyl chloride at 10 mg/kg produced cardiovascular collapse and respiratory arrest in a cat in the same manner as soluble zirconium salts; the intraperitoneal LD50 of 112 mg/kg for hafnium is not much smaller than that for zirconium.
Safety and Health Measures
Fire and explosion. Zirconium metal in the form of a fine powder burns in air, nitrogen or carbon dioxide. The powders are explosive in air in the range of 45 to 300 mg/l, and are self-igniting if disturbed, probably because of static electricity generated by separation of the grains.
The powdered metals should be transported and handled in the wet state; water is usually used for wetting. When the powder is dried prior to use, the quantities employed should be kept as small as possible and operations should be carried out in separate cubicles to prevent propagation in the event of an explosion. All sources of ignition, including static electric charges, should be eliminated from areas in which the powder is to be handled.
All surfaces in the area should be impervious and seamless so that they can be washed down with water and kept completely free from dust. Any spilled powder should be cleaned up immediately with water so that it has no chance to dry in place. Used papers and cloths which have become contaminated with the powders should be kept wet in covered containers until they are removed to be burned, which should be done at least daily. The dried powders should be disturbed and handled as little as possible, and then only with non-sparking tools. Rubber or plastic aprons, if worn over work clothes, should be treated with an anti-static compound. Work clothing should be made from non-synthetic fibres unless effectively treated with antistatic materials.
All processes using zirconium and or hafnium should be designed and ventilated to keep airborne contamination below the exposure limits.