Gunnar Nordberg

Occurrence and Uses

Zinc (Zn) is widely distributed in nature in quantities which amount to approximately 0.02% of the earth’s crust. It is found in nature as the sulphide (sphalerite), carbonate, oxide or silicate (calamine) in combination with many minerals. Sphalerite, the principal zinc mineral and the source of at least 90% of metallic zinc, contains iron and cadmium as impurities. It is almost always accompanied by galena, the sulphide of lead, and occasionally is found in association with ores containing copper or other base metal sulphides.

On exposure to air, zinc becomes covered with a tenacious film of oxide which protects the metal from further oxidation. This resistance to atmospheric corrosion forms the basis for one of the most common uses of the metal, the protection of steelwork by galvanizing. Zinc’s ability to protect ferrous metals against corrosion is reinforced by electrolytic action. It acts as an anode with respect to iron and other structural metals, except aluminium and magnesium, and is thus preferentially attacked by corrosive agents. This property is used in many other important applications of zinc—for example, in the use of zinc plates as anodes for cathodic protection of ships’ hulls, underground tanks and so on. Zinc metal is die cast for components in the automobile industry, electrical equipment industry, and in the light machine tool, hardware, toys and fancy goods industries. It is rolled into sheets in rolling mills for the manufacture of roofing, weather stripping, cases for dry batteries, printing plates and so on. Zinc is also alloyed with copper, nickel, aluminium and magnesium. When it is alloyed with copper, it forms the important groups of alloys known as the brasses.

Zinc oxide (ZnO), or zinc white (flowers of zinc) is produced by the oxidation of vaporized pure zinc or by the roasting of zinc oxide ore. It is used as a pigment in paints, lacquers and varnishes, as well as a filler for plastics and rubber. Zinc oxide is found in cosmetics, quick-setting cements, and in pharmaceuticals. It is useful in the manufacture of glass, automobile tyres, matches, white glue and printing inks. Zinc oxide is also used as a semiconductor in the electronics industry.

Zinc chromate (ZnCrO₄), or zinc yellow, is produced by the action of chromic acid on slurries of zinc oxide, or on zinc hydroxide. It is used in pigments, paints, varnishes and lacquers, and in the manufacture of linoleum. Zinc chromate acts as a corrosion inhibitor for metals and epoxy laminates.

Zinc cyanide (Zn(CN)₂) is produced by precipitation of a solution of zinc sulphate or chloride with potassium cyanide. It is used for metal plating and for gold extraction. Zinc cyanide acts as a chemical reagent and as a pesticide. Zinc sulphate (ZnSO₄·7H₂O), or white vitriol, is produced by roasting zinc blende or by the action of sulphuric acid on zinc or zinc oxide. It is used as an astringent, a preservative for hides and wood, a bleach for paper, a pesticide adjuvant and a fungicide. Zinc sulphate also serves as a fireproofing agent and as a depressant in froth flotation. It is used in water treatment and in textile dyeing and printing. Zinc sulphide is used as a pigment for paints, oilcloths, linoleum, leather, inks, lacquers, and cosmetics. Zinc phosphide (Zn₃P₂) is produced by passing phosphine through a solution of zinc sulphate. It is used mainly as a rodenticide.

Zinc chloride (ZnCl₂), or butter of zinc, has numerous uses in the textile industry, including dyeing, printing, sizing and weighting fabrics. It is a component of cement for metals, dentifrices, and soldering fluxes. It is used alone or with phenol and other antiseptics for preserving railway ties. Zinc chloride is useful for glass etching and for the manufacture of asphalt. It is a vulcanizing agent for rubber, a flame retardant for wood, and a corrosion inhibitor in water treatment.

Hazards

Zinc is an essential nutrient. It is a constituent of metalloenzymes, which play an important role in nucleic acid metabolism and protein synthesis. Zinc is not stored in the body, and a minimum daily intake of zinc is recommended by nutritional experts. Absorption of zinc takes place more readily from animal protein sources than from plant products. The phytate content of plants binds zinc, rendering it unavailable for absorption. Zinc deficiency states have been reported from countries where cereals are the major source of protein consumed by the population. Some of the recognized clinical manifestations of chronic zinc deficiency in humans are growth retardation, hypogonadism in males, skin changes, poor appetite, mental lethargy and delayed wound healing.

In general, zinc salts are astringent, hygroscopic, corrosive and antiseptic. Their precipitating action on proteins forms the basis of their astringent and antiseptic effects, and they are absorbed relatively easily through the skin. The taste threshold for zinc salts is approximately 15 ppm; water containing 30 ppm of soluble zinc salts has a milky appearance, and a metallic taste when the concentration reaches 40 ppm. Zinc salts are irritating to the gastrointestinal tract, and the emetic concentrations for zinc salts in water range from 675 to 2,280 ppm.

The solubility of zinc in weakly acidic solutions, in the presence of iron, has led to accidental ingestion of large quantities of zinc salts when acid foods such as fruit drinks were prepared in worn galvanized iron vessels. Fever, nausea, vomiting, stomach cramps and diarrhoea occurred in 20 minutes to 10 hours following ingestion.

A number of zinc salts may enter the body by inhalation, through the skin or by ingestion and produce intoxication. Zinc chloride has been found to cause skin ulcers. A number of zinc compounds present fire and explosion hazards. The electrolytic manufacturing of zinc can produce mists containing sulphuric acid and zinc sulphate that can irritate the respiratory or digestive systems and lead to dental erosion. Metallurgic processes involving zinc can lead to arsenic, cadmium, manganese, lead and possibly chromium and silver exposures, with their associated hazards. Since arsenic is frequently present in zinc, it can be a source of exposure to highly toxic arsine gas whenever zinc is dissolved in acids or alkalis.

In zinc metallurgy and manufacturing, welding and cutting of galvanized or zinc-coated metal, or melting and casting of brass or bronze, the most frequently encountered hazard from zinc and its compounds is exposure to zinc oxide fumes, which cause metal-fume fever. Symptoms of metal-fume fever include shivering attacks, irregular fever, profuse sweating, nausea, thirst, headache, pains in the limbs and a feeling of exhaustion. Attacks are of short duration (most cases are on the way to complete recovery within 24 hours of the onset of symptoms), and tolerance seems to be acquired. A significant increase in free erythrocyte protoporphyrin has been reported in zinc oxide packing operations.

Zinc chloride fumes are irritating to the eyes and mucous membranes. In an accident involving smoke generators, 70 exposed persons experienced varying degrees of irritation of the eyes, nose, throat and lungs. Of the 10 fatalities, some died within a few hours with pulmonary oedema, and others died later of bronchopneumonia. On another occasion, two firemen were exposed to zinc chloride fumes from a smoke generator during a firefighting demonstration, one briefly, the other for several minutes. The former recovered rapidly while the latter died after 18 days, due to respiratory failure. There was a rapid rise of temperature and marked upper respiratory tract inflammation soon after exposure. Diffuse pulmonary infiltrations were seen on the chest radiograph, and autopsy revealed active fibroblastic proliferation and cor pulmonale.

In an experiment primarily designed to evaluate carcinogenesis, groups of 24 mice received 1,250 to 5,000 ppm of zinc sulphate in drinking water for one year. Apart from severe anaemia in animals receiving 5,000 ppm, there were no adverse effects from zinc. Tumour incidence was not significantly different from that seen in the controls.

Zinc phosphide, which is used as a rodenticide, is toxic to humans whether swallowed, inhaled or injected, and, together with zinc chloride, is the most dangerous of the zinc salts; these two substances have been responsible for the only deaths definitely due to zinc poisoning.

Skin effects. Zinc chromate in primer paints used by car-body builders, tinsmiths and steel cupboard makers has been reported to cause nasal ulceration and dermatitis in exposed workers. Zinc chloride has a caustic action, which may result in ulceration of the fingers, hands and forearms of those who handle timber impregnated with it or use it as a flux in soldering. It has been reported that zinc oxide dust may block the ducts of the sebaceous glands and give rise to a papular, pustular eczema in humans packaging this compound.

Safety and Health Measures

Fire and explosion. Finely divided zinc powder, and other zinc compounds, can be fire and explosion hazards if stored in damp places, sources of spontaneous combustion. Residues from reduction reactions may ignite combustible materials. Zinc ammonium nitrate, zinc bromate, zinc chlorate, zinc ethyl, zinc nitrate, zinc permanganate and zinc picrate are all dangerous fire and explosion hazards. In addition, zinc ethyl will ignite spontaneously in contact with air. It should, therefore, be stored in a cool, dry, well-ventilated place away from acute fire risks, open flames and powerful oxidizing agents.

In all cases where zinc is heated to the point where fumes are produced, it is most important to ensure that adequate ventilation is provided. Individual protection is best ensured by education of the worker concerning metal-fume fever and the provision of local exhaust ventilation, or, in some situations, by wearing of a supplied-air hood or mask.

Workers who are none the less exposed to zinc chloride fumes should wear personal protective equipment including protective clothing, chemical eye and face protection and appropriate respiratory protective equipment. Exposure to zinc chloride fumes should be treated by copious irrigation of the exposed areas.

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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.

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