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Zinc
Gist
Zinc is found in cells throughout the body. It is needed for the body's defensive (immune) system to properly work. It plays a role in cell division, cell growth, wound healing, and the breakdown of carbohydrates. Zinc is also needed for the senses of smell and taste.
Summary
The metallic element zinc is commonly used to coat buckets, rainspouts, and other iron or steel objects to prevent rusting. There are, however, many other uses for this bluish white and fairly soft metal. All brass contains zinc. Zinc is an ingredient in some medicines. In the human body, zinc is an essential trace element. In the red blood cells it helps to metabolize carbon dioxide, and the zinc present in the pancreas may aid in the storage of insulin.
Element Properties
Symbol : Zn
Atomic number : 30
Atomic weight : 65.39
Group in periodic table : 12 (IIb)
Boiling point : 1,665 °F (907 °C)
Melting point : 788 °F (420 °C)
Specific gravity : 7.13
The first people to recognize zinc as a distinct metal were apparently the Hindus in about the 13th century. In the West, commercial zinc production got under way in England by the middle of the 18th century.
A large share of the zinc produced today is used for galvanizing iron and steel—that is, coating them with zinc to make them rustproof. Brass making generally ranks second in the use of zinc. The different types of brass contain from 5 to 45 percent zinc. For many purposes, zinc is simply flattened into sheets called rolled zinc. These sheets are used in the manufacture of many roofing products, refrigerator linings, and printing plates.
The compounds of zinc have numerous uses. Because of its high heat conductivity, zinc oxide is used in rubber as a heat dissipater. Zinc sulfate is used in weed killers. Zinc sulfide has been used in X-ray screens and in luminous dials for clocks and watches. Zinc compounds are also used in medicines and toiletries. Metallic zinc is distilled from crushed ore by heating it with carbon (usually coal), and condensing the zinc vapor. Zinc is also separated from the ore by electrolysis.
Details
Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic table. In some respects, zinc is chemically similar to magnesium: both elements exhibit only one normal oxidation state (+2), and the Zn2+ and Mg2+ ions are of similar size. Zinc is the 24th most abundant element in Earth's crust and has five stable isotopes. The most common zinc ore is sphalerite (zinc blende), a zinc sulfide mineral. The largest workable lodes are in Australia, Asia, and the United States. Zinc is refined by froth flotation of the ore, roasting, and final extraction using electricity (electrowinning).
Zinc is an essential trace element for humans, animals, plants and for microorganisms and is necessary for prenatal and postnatal development. It is the second most abundant trace metal in humans after iron, an important cofactor for many enzymes, and the only metal which appears in all enzyme classes. Zinc is also an essential nutrient element for coral growth.
Zinc deficiency affects about two billion people in the developing world and is associated with many diseases. In children, deficiency causes growth retardation, delayed sexual maturation, infection susceptibility, and diarrhea. Enzymes with a zinc atom in the reactive center are widespread in biochemistry, such as alcohol dehydrogenase in humans. Consumption of excess zinc may cause ataxia, lethargy, and copper deficiency. In marine biomes, notably within polar regions, a deficit of zinc can compromise the vitality of primary algal communities, potentially destabilizing the intricate marine trophic structures and consequently impacting biodiversity.
Brass, an alloy of copper and zinc in various proportions, was used as early as the third millennium BC in the Aegean area and the region which currently includes Iraq, the United Arab Emirates, Kalmykia, Turkmenistan and Georgia. In the second millennium BC it was used in the regions currently including West India, Uzbekistan, Iran, Syria, Iraq, and Israel. Zinc metal was not produced on a large scale until the 12th century in India, though it was known to the ancient Romans and Greeks. The mines of Rajasthan have given definite evidence of zinc production going back to the 6th century BC.[23] The oldest evidence of pure zinc comes from Zawar, in Rajasthan, as early as the 9th century AD when a distillation process was employed to make pure zinc.[24] Alchemists burned zinc in air to form what they called "philosopher's wool" or "white snow".
The element was probably named by the alchemist Paracelsus after the German word Zinke (prong, tooth). German chemist Andreas Sigismund Marggraf is credited with discovering pure metallic zinc in 1746. Work by Luigi Galvani and Alessandro Volta uncovered the electrochemical properties of zinc by 1800.
Corrosion-resistant zinc plating of iron (hot-dip galvanizing) is the major application for zinc. Other applications are in electrical batteries, small non-structural castings, and alloys such as brass. A variety of zinc compounds are commonly used, such as zinc carbonate and zinc gluconate (as dietary supplements), zinc chloride (in deodorants), zinc pyrithione (anti-dandruff shampoos), zinc sulfide (in luminescent paints), and dimethylzinc or diethylzinc in the organic laboratory.
Additional Information:
Occurrence, uses, and properties
A little more abundant than copper, zinc makes up an average of 65 grams (2.3 ounces) of every ton of Earth’s crust. The chief zinc mineral is the sulfide sphalerite (zinc blende), which, together with its oxidation products smithsonite and hemimorphite, constitute nearly all of the world’s zinc ore. Native zinc has been reported from Australia, New Zealand, and the United States, and the leading early 21st-century producers of zinc are China, Australia and Peru. For zinc’s mineralogical properties, see native element.
Zinc is an essential trace element in the human body, where it is found in high concentration in the red blood cells as an essential part of the enzyme carbonic anhydrase, which promotes many reactions relating to carbon dioxide metabolism. The zinc present in the pancreas may aid in the storage of insulin. Zinc is a component of some enzymes that digest protein in the gastrointestinal tract. Zinc deficiency in nut-bearing and fruit trees causes such diseases as pecan rosette, little leaf, and mottle leaf. Zinc functions in the hemosycotypsin of snails’ blood to transport oxygen in a way analogous to iron in the hemoglobin of human blood.
Metallic zinc is produced by roasting the sulfide ores and then either leaching the oxidized product in sulfuric acid or smelting it in a blast furnace. Zinc is won from the leach solution by electrolysis or is condensed from the blast furnace gas and then distilled of impurities.
The major uses of zinc metal are in galvanizing iron and steel against corrosion and in making brasses and alloys for die-casting. Zinc itself forms an impervious coating of its oxide on exposure to the atmosphere, and hence the metal is more resistant to ordinary atmospheres than iron and corrodes at a much lower rate. In addition, because zinc tends to oxidize in preference to iron, some protection is afforded the steel surface even if some of it is exposed through cracks. The zinc coating is formed either by hot-dip galvanizing or electrogalvanizing.
Hot-dip galvanizing is the most common procedure for coating steel with zinc. This may be a batch process known as general galvanizing or a continuous coating of coils of steel strip. In general galvanizing, steel is pickled in acid, treated with fluxing agents, and then dipped in a bath of molten zinc at about 450 °C (840 °F). Layers of iron-zinc alloy are formed on the surface and are topped with an outer layer of zinc. Objects so treated range from small nuts and bolts to steel window frames and large girders used in construction. An ordinary grade of zinc containing up to 1.5 percent lead is normally used in this process.
In electrogalvanizing, zinc is deposited on a steel strop in as many as 20 consecutive electrolytic coating cells. There are several successful cell designs; the simple vertical cell is discussed here to explain the principle. The strip, connected to the negative side of a direct current through large-diameter conductor rolls located above and between two cells, is dipped into a tank of electrolyte by a submerged sink roll. Partially submerged anodes, opposing the strip, are connected to the positive side of the electric current by heavy bus bars. Zinc cations (i.e., positively charged zinc atoms) present in the electrolyte are converted by the current into regular zinc atoms, which deposit on the strip. The bath is supplied with zinc cations either by zinc anodes, which are continuously dissolved by the direct current, or by zinc compounds continuously added to the electrolyte. In the latter case the anodes are made of insoluble materials, such as titanium coated with iridium oxide. The electrolyte is an acidic solution of zinc sulfide or zinc chloride with other bath additions to improve the quality of the coating and the current efficiency. Coating thickness is easier to control than in the hot-dip process because of the good relationship between electrical current and deposited zinc.
The negative electrode (outside can) in one common type of electric dry cell is composed of zinc. Another important series of alloys are those formed by the addition of 4 to 5 percent aluminum to zinc; these have a relatively low melting point but possess good mechanical properties and can be cast under pressure in steel dies. Considerable quantities of zinc in the rolled form are used for roofing, particularly in Europe; small additions of copper and titanium improve creep resistance—i.e., resistance to gradual deformation.
Freshly cast zinc has a bluish silver surface but slowly oxidizes in air to form a grayish protective oxide film. Highly pure zinc (99.99 percent) is ductile; the so-called prime western grade (99.8 percent pure) is brittle when cold but above 100 °C (212 °F) can be rolled into sheets that remain flexible. Zinc crystallizes in the hexagonal close-packed structure. When iron and zinc together are exposed to a corrosive medium, they constitute an electrolytic cell, and the zinc is attacked (oxidized to the Zn2+ ion) preferentially because of its higher electrode potential. This so-called sacrificial protection, coupled with the much greater corrosion resistance of zinc under atmospheric conditions, is the basis for galvanizing.
Natural zinc is a mixture of five stable isotopes: 64Zn (48.6 percent), 66Zn (27.9 percent), 67Zn (4.1 percent), 68Zn (18.8 percent), and 70Zn (0.6 percent).
History
Metallic zinc appeared much later in history than the other common metals. Copper, lead, tin, and iron can be obtained as the molten metals by heating their oxide ores with charcoal (carbon), a process called reduction, in shaft furnaces, which were developed quite early in history. Zinc oxide, however, cannot be reduced by carbon until temperatures are reached well above the relatively low boiling point of the metal (907 °C). Thus, the furnaces developed to smelt the other metals could not produce zinc. Small quantities of metallic zinc can sometimes be found in the flues of lead blast furnaces.
There is some evidence that the Greeks knew of the existence of zinc and called it pseudargyras, or “false silver,” but they had no method of producing it in quantity. The Romans as early as 200 bce produced considerable quantities of brass, an alloy of zinc and copper, by heating in crucibles a mixture of zinc oxide and charcoal covered with lumps of metallic copper. The zinc oxide was reduced in the lower part of the crucible. Zinc vapour was formed and dissolved in the copper to form brass. At the end of the process the temperature was raised to melt the brass for casting into ingots. Brass production was the Romans’ only use of zinc.
The realization that to make zinc it was necessary to produce the metal as a vapour and then condense it seems first to have been reached in India in the 13th or 14th century. The metallurgists of China had achieved large-scale production of zinc by the 16th century. In the West this principle was first applied in England in 1743 under the leadership of William Champion. At the end of the 18th century in Belgium and Poland improvements were made in the furnace, and the process remained unchanged until an electrolytic process was developed in 1917. At the end of the 1920s a radical advance was made in the United States by developing a continuous retort process, and during the 1930s an electrothermic process was designed for producing zinc continuously. A development of the 1960s was the zinc-lead blast furnace, in which rapid quenching of the gases is a key principle. Zinc production processes are treated in detail in zinc processing.
Compounds
In chemical compounds, zinc exhibits almost exclusively a +2 oxidation state. A few compounds of zinc in the +1 state have been reported, but never any compounds of zinc in the +3 state or higher.
Zinc oxide, ZnO, is one of the most important zinc compounds. It can be prepared in a state of high purity and in a variety of crystal shapes and sizes by burning zinc vapour in air. Because of its high heat conductivity and capacity, zinc oxide is frequently incorporated into rubber as a heat dissipater. In the crystal of zinc oxide, the lattice (i.e., the orderly structure formed by the ions) is an open one in which the zinc and oxygen ions occupy only 44 percent of the volume. Defects can be created in the lattice by specific treatments such as the introduction of foreign atoms or of zinc atoms in the vacancies of the lattice. Such treatment of zinc oxide crystals produces various electrical, photoelectrical, and catalytic properties. As a result, zinc oxide is used as a semiconductor in the production of phosphors for television tubes and fluorescent lamps. Its effects on the reactivity of many compounds make it useful as a catalyst in such operations as the manufacture of synthetic rubber and methanol. It is also used in paints, cosmetics, plastics, pharmaceuticals, and printing inks. Because under the influence of light the electrical conductivity of zinc oxide can be increased many times, it is employed in certain photocopying processes.
Zinc sulfate, ZnSO4, is an intermediate compound in the production of zinc from its ores by the electrolytic process. It is used as a weed killer, in the manufacture of viscose rayon, and in dyeing, in which it functions as a mordant. Zinc chloride, ZnCl2, can be prepared by a direct reaction or by evaporating the aqueous solution formed in various reactions. It is strongly deliquescent (water-absorbing) and is utilized as a drying agent and as a flux. In aqueous solution it is used as a wood preservative. Zinc sulfide, ZnS, occurs in nature as the mineral sphalerite and may be prepared by treating solutions of zinc salts with hydrogen sulfide. It was long used as a white pigment but has been gradually replaced by titanium dioxide. Zinc sulfide has luminescent properties when activated by the addition of small quantities of copper, manganese, silver, and so has been used in X-ray screens, in luminous dials for clocks and watches, and in fluorescent lights.
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