zinc
zinc
zinc
[ziŋk]zinc
ZINK
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Zinc
Zn, a chemical element of Group II of Mendeleev’s periodic system. Atomic number, 30; atomic weight, 65.38. A bluish white metal, zinc occurs in the form of five stable isotopes, with mass numbers 64, 66, 67, 68, and 70. 64Zn is the most widespread isotope, accounting for 48.89 percent. Nine artificial zinc radioisotopes have been obtained, of which 65Zn, with a half-life of 245 days, is the longest lived and is used as an isotopic indicator.
History. Brass, an alloy of zinc and copper, was known to the ancient Greeks and Egyptians. Pure zinc could not be isolated for a long time. In 1746, A. S. Marggraf developed a method for the production of metallic zinc that involved heating a mixture of zinc oxide and coal without air in clay refractory retorts, followed by condensation of the zinc vapor. The industrial production of zinc was begun in the 19th century.
Distribution in nature. The average content of zinc in the earth’s crust (clarke) is 8.3 × 10–3 percent by weight. The zinc content in basic igneous rocks is somewhat higher (1.3 x 10–2 percent) than in acid rocks (6 × 10–3 percent). A total of 66 minerals of zinc are known, the most important of which are zincite, sphalerite, willemite, calamine, smithsonite, and franklinite, ZnFe2O4) (seeZINCITE; SPHALERITE; WILLEMITE; CALAMINE; and SMITHSONITE).
Zinc is an energetic water migrant. Its migration in thermal waters together with lead is especially characteristic. Zinc sulfides, which have great industrial significance, precipitate from these waters (seeCOMPLEX ORE). Zinc also actively migrates in surface and underground waters. The major precipitant of zinc is H2S, while sorption by clays plays a lesser role.
Zinc is an important biogenic element; its average content in living matter is 5 × 10–4 percent, although there exist some organisms that concentrate zinc, such as some violets.
Physical and chemical properties. Zinc is a metal of medium hardness. When cold, it is brittle; however, at temperatures of 100°–150°C, it is extremely ductile and readily rolled into sheets and foil hundredths of a millimeter thick. At 250°C, it again becomes brittle.
Zinc does not have polymorphic modifications. It crystallizes jn a hexagonal lattice, with parameters a = 2.6594 angstroms (Å) and c = 4.9370 Å. The atomic radius is 1.37 Å, and the ionic radius of Zn2+ is 0.83 Å. The density of solid zinc is 7.133 g/cm3 (at 20°C), while the density of liquid zinc is 6.66 g/cm3 (at 419.5°C). Zinc has a melting point of 419.5°C and a boiling point of 906°C. The linear coefficient of thermal expansion is 39.7 × 10–6 (between 20° and 250°C), the thermal conductivity is 110.950 W/(m.°K), or 0.265 cal/cm·sec·°C (at 20°C), the electrical resistivity is 5.9 × 10–6 ohm· cm (at 20°C), and the heat capacity is 25.433 kilojoules/(kg–°K), or (6.07 cal/(g–°C). The tensile strength is 200–250 meganewtons/m2, or 2,000–2,500 kilograms-force/cm2, the specific elongation is 40–50 percent, and the Brinell hardness is 400–500 meganewtons/m2, or 4,000–5,000 kilograms-force/cm2. Zinc is diamagnetic, with a magnetic susceptibility of –0.175 × 10–6.
The electronic configuration of the Zn atom’s outer shell is 3d104s2. Zinc exhibits an oxidation number of + 2 in all of its compounds. Its standard oxidation-reduction potential, equal to 0.76 volt, characterizes zinc as a reactive metal and active reducing agent. Zinc rapidly tarnishes in the presence of air at temperatures up to 100°C and becomes covered by a surface layer of basic carbonates. In moist air, especially in the presence of CO2, metallic zinc is decomposed even at ordinary temperatures. Upon strong heating in the presence of air or oxygen, zinc actively burns with a bluish flame, with the formation of a white smoke of zinc oxide, ZnO. Dry fluorine, chlorine, and bromine do not react with zinc at cold temperatures, but in the presence of water vapor, zinc may ignite, forming, for example, ZnCl2. A heated mixture of zinc powder and sulfur yields zinc sulfide, ZnS. Zinc sulfide precipitates upon the action of hydrogen sulfide on weakly acidic or ammonium aqueous solutions of zinc salts. The hydride ZnH2 is obtained in the reaction of LiAlH4 with Zn (CH3)2 and other zinc compounds; a metal-like substance, it decomposes upon heating into its elements. The nitride Zn3N2, a black powder, is formed upon heating zinc to 600°C in a stream of ammonia; it is stable in the air at temperatures to 750°C and is not decomposed by water. Zinc carbide, ZnC2, is produced by heating zinc in a stream of acetylene.
Strong mineral acids vigorously dissolve zinc, especially upon heating, with the formation of the corresponding salts. Upon reaction with dilute HCl and H2SO4, H2 is liberated while with HNO3, NO, NO2, and NH3 are also liberated. Zinc reacts with concentrated HCl, H2SO4, and HNO3, liberating H2, SO2, NO. and NO2. Solutions and melts of alkalies oxidize zinc with the liberation of H2 and the formation of water-soluble zincates. (seeZINC ATE).
The intensity of the action of acids and alkalies on zinc depends on the presence of impurities. Pure zinc is less reactive toward these reagents owing to the hydrogen overstress on it. In water, zinc salts are hydrolyzed upon heating, forming a white precipitate of zinc hydroxide, Zn(OH)2. Complexes containing zinc, such as [Zn(NH3)4]SO4, are known.
Preparation. Zinc is obtained from complex ores containing 1–4 percent zinc in the form of a sulfide, as well as Cu, Pb, Ag, Au, Cd, and Bi. These ores are concentrated by selective flotation, yielding zinc concentrates containing 50–60 percent zinc, as well as lead, copper, and sometimes pyrite concentrates. Zinc concentrates are roasted in a fluidized-bed furnace, in the course of which zinc sulfide is converted into zinc oxide, ZnO; the sulfur dioxide, SO2, formed in this process is used in the production of sulfuric acid.
There are two ways of obtaining zinc from zinc oxide. In the pyrometallurgical, or distillation, method, which has long been known, the roasted concentrate is subjected to sintering to impart granularity and gas permeability and is then reduced by coal or coke at 1200°–1300°C: ZnO + C = Zn + CO. The metallic zinc vapor produced in this process is condensed and poured into ingot molds. Initially, the reduction was conducted manually only in retorts made of fire clay. Later, mechanized vertical retorts made of Carborundum came into use, and then shaft and arc furnaces. Zinc is produced from lead-zinc concentrates in shaft furnaces with hot blasting. Although the efficiency of these processes gradually increased, the zinc produced still contained as much as 3 percent impurities, including valuable cadmium. Zinc obtained by distillation is purified by liquation, in which the liquid metal is separated from the iron and some of the lead at 500°C; this method of purification yields zinc that is 98.7 percent pure. Rectification, a more complex and expensive method of purification, is used occasionally; it yields metallic zinc that is 99.995 percent pure and makes it possible to extract cadmium.
The major method for the production of zinc is the electrolytic, or hydrometallurgical, method. Roasted zinc concentrates are treated with sulfuric acid. The sulfate solution obtained is purified to eliminate impurities (by precipitation using zinc dust) and subjected to electrolysis in tanks thickly lined with lead or polyvinyl chloride plastic. Zinc is deposited on aluminum cathodes, from which it is hand-stripped daily and then smelted in induction furnaces. Electrolytic zinc is 99.95 percent pure, and its recovery from concentrates (taking into account processed wastes) is 93–94 percent. Zinc vitriol (zinc sulfate), Pb, Cu, Cd, Au, and Ag are obtained from zinc production wastes, as well as, occasionally, In, Ga, Ge, and Tl.
Uses. About one-half of the zinc produced is used to protect steel from corrosion (see).
Since zinc precedes iron in the electromotive series, it undergoes decomposition when galvanized iron is introduced into a corrosive medium. As a result of its good casting properties and low melting point, zinc is used for the pressure casting of many small parts of airplanes and other machines. Alloys of zinc with copper—brass and nickel silver—are commonly used in industry, as are alloys of zinc with lead and other metals (see). Zinc yields intermetallic compounds with gold and silver, which are insoluble in molten lead, and thus zinc is used in the refining of lead to remove noble metals. In powdered form, zinc is used as a reducing agent in many industrial chemical processes: in the production of hydrosulfite, in the precipitation of gold from industrial cyanide solutions, and in the precipitation of copper and cadmium in the purification of zinc vitriol solutions.
Many zinc compounds are luminophores, for example, the three basic colors on the kinescope screen depend on ZnS·Ag (blue color), ZnSe·Ag (green color), and Zn3(P04)2·Mn (red color). Type AIIBVI zinc compounds, such as ZnS, ZnSe, ZnTe, and ZnO, are important semiconductor materials. Soviet brands MN and NN magnetically soft ferrites are manganese-zinc and nickel-zinc spinels, respectively.
The most common chemical sources of electric current (for example, the Leclanché cell and mercury cell) use zinc as the negative electrode.
N. N. SEVRIUKOV
Zinc in the organism. As a biogenic element, zinc is always present in the tissues of plants and animals. The average content of zinc in most terrestrial and marine organisms is on the order of thousandths of a percent. Fungi, especially poisonous ones, are rich in zinc, as are lichens, coniferous plants, and certain marine invertebrates, such as oysters (0.4 percent dry weight). Calamine plants, which accumulate zinc, are encountered in zones of rocks with high zinc content. Plants obtain zinc from the soil and water, while animals obtain it through food. In man, the daily requirement of adults for zinc is 5–20 mg and is met by ingestion of bread products, meat, milk, and vegetables; in infants, the zinc requirement of 4–6 mg is met by breast milk.
The biological role of zinc is related to its participation in enzymatic reactions that occur in cells. Zinc is a constituent of a number of important enzymes, such as carbonic anhydrase and various dehydrogenases and phosphatases, that are involved in respiration and other physiological processes and of proteinases and peptidases, which participate in protein metabolism. It is also a constituent of enzymes of nucleic-acid metabolism, such as RNA-polymerase and DNA polymerase. Zinc plays an important role in the synthesis of messenger RNA on the corresponding DNA segments (transcription) and in the stabilization of ribosomes and biopolymers (RNA, DNA, and some proteins).
In plants, in addition to participation in respiration, protein metabolism, and nucleic-acid metabolism, zinc regulates growth, affects the formation of the amino acid tryptophan, and increases the content of gibberellins. Zinc stabilizes various macromolecules in biological membranes and may be an integral component of these membranes; it also affects ion transport and participates in the supermolecular organization of cellular organelles. In the presence of zinc, a larger number of mitochondria is formed in a culture of Ustilago sphaerogena; in the case of insufficient zinc, the ribosomes disappear in Euglena gracilis. Zinc is necessary for the development of plant egg cells and buds (seeds are not formed in the absence of zinc). Zinc increases the resistance of plants to drought, heat, and cold. A lack of zinc leads to disturbances in cell division and to various functional diseases, such as the whitening of corn tops and rosette.
In animals, in addition to participation in respiration and nucleic-acid metabolism, zinc increases the activity of the sex glands and affects the formation of the fetal skeleton. It has been shown that a zinc deficiency in nursing rats leads to a decrease in RNA content and protein synthesis in the brain and to slower brain development. A zinc-containing protein has been isolated from the saliva of the human parotid gland. It is believed that this protein stimulates the regeneration of the cells of the taste buds and aids the function of the taste buds. Zinc protects the organism from the effects of cadmium.
M. IA. SHKOL’NIK
Medical importance. A zinc deficiency in the body leads to dwarfism and retardation of sexual development. According to experimental data, excess zinc in the body can result in carcinogenic phenomena and can also adversely affect the heart, blood, and gonads. Certain occupational injuries may be related to the deleterious effects of both metallic zinc and zinc compounds. Cases of metal fume fever are possible during the melting of zinccontaining alloys.
Zinc preparations in the form of solutions (zinc sulfate) and as a component of powders, pastes, ointments, and suppositories (zinc oxide) are used in medicine as astringents and antiseptics.
A. A. KASPAROV and G. N. KRASOVSKII
REFERENCES
Kratkaia khimicheskaia entsiklopediia, vol. 5. Moscow, 1967.Lakernik, M. M., and G. N. Pakhomova. Metallurgiia tsinka i kadmiia. Moscow, 1969.
Sevriukov, N. N., B. A. Kuz’min, and E. V. Chelishchev. Obshchaia metallurgiia. Moscow, 1976.
Paribok, T. A. “O roli tsinka v metabolizme.” In the collection Biologicheskaia rol’mikroelementov i ikh primenenie v sel’skom khoziaistve i meditsine. Moscow, 1974.
Koval’skii, V. V. Geokhimicheskaia ekologiia. Moscow, 1974.
Shkol’nik, M. Ia. Mikroelementy v zhizni rastenii. Leningrad, 1974.
Peive, Ia. V. “Mikroelementy i fermenty.” In the collection Fiziologicheskaia rol’ i prakticheskoe primenenie mikroelementov. Riga, 1976.
Bowen, H. J. M. Trace Elements in Biochemistry. London-New York, 1966.
Dvizhkov, P. P. “Soedineniia tsinka.” In Mnogotomnoe rukovodstvo po patologicheskoi anatomii, vol. 8, book 1. Edited by A. I. Strukov. Moscow, 1962.
Vrednye veshchestva v promyshlennosti, [vol.] 2. Edited by N. V. Lazarev. Moscow-Leningrad, 1965.