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.
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.