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zinc

zinc

zinc, metallic chemical element; symbol Zn; at. no. 30; at. wt. 65.38; m.p. 419.58°C;; b.p. 907°C;; sp. gr. 7.133 at 25°C;; valence +2. Zinc is a lustrous bluish-white metal. It is found in Group 12 of the periodic table. It is brittle and crystalline at ordinary temperatures, but when heated to between 110°C; and 150°C; it becomes ductile and malleable; it can then be rolled into sheets. It is a fairly reactive metal. Although it is not abundant in nature, it is of great commercial importance. It is used principally for galvanizing iron, but is also important in the preparation of certain alloys, e.g., Babbitt metal, brass, German silver, and sometimes bronze. It is used for the negative plates in certain electric batteries and for roofing and gutters in building construction. Since the metal reacts with dilute mineral acid to liberate hydrogen, it is often used for this purpose in the laboratory. Zinc compounds are numerous and are widely used. Perhaps most important is zinc oxide, or zinc white, a versatile compound with many uses. Other zinc compounds include zinc chloride, used as a wood preservative, in soldering fluxes, as a mordant in dyeing textiles, and in adhesives and cements; and zinc sulfide, used in making lithopone as well as television screens and X-ray apparatus. The chromate, zinc yellow, serves as a pigment; sodium zincate, as a water softener and as a flocculating agent in water purification. The crystalline sulfate is known commonly as white vitriol. Zinc is essential to the growth of many kinds of organisms, both plant and animal. It is a constituent of insulin, which is used in the treatment of diabetes. Chief sources of zinc are the sulfide ore, zinc blende, or sphalerite (called also blende or "black Jack"); zincite, an oxide; calamine, a silicate; and smithsonite, the zinc carbonate. Zinc ores are widely and abundantly distributed throughout the world. The United States is the leading producer. The metallurgy of zinc depends upon the ore used. The sulfide ore is roasted to the oxide, then mixed with coal and heated to 1,200°C;. The zinc vaporizes and is condensed outside the reaction chamber and cast into blocks called spelter. In another method the ore is processed by flotation, filtering, roasting, and leaching; the resulting solution is filtered and the zinc removed by electrolysis.
or blende or zincblende

Sphalerite from Baxter Springs, Kan.

Zinc sulfide (ZnS), the chief ore mineral of zinc. It is found associated with galena in most important lead-zinc deposits. The most important deposits are in the Mississippi River valley (U.S.), Poland, Belgium, and North Africa. Seealso sulfide mineral.

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Metallic chemical element, chemical symbol Zn, atomic number 30. Zinc is a bluish silver metal, ductile when very pure but brittle otherwise. It forms brass (with copper) and many other alloys. Its major use is in galvanizing iron, steel, and other metals. Zinc is an essential trace element, particularly in red blood cells; in snails, it corresponds to iron in the blood of vertebrates. Zinc oxide is used as a pigment, ultraviolet light absorber (to prevent sunburn), dietary supplement and seed treatment, and photoconductor. Zinc's many other compounds (in which it has valence 2 or, rarely, 1) are used in industrial and consumer applications, including as pesticides, pigments, mordants (see dye), fluxes, and wood preservatives.

Learn more about zinc with a free trial on Britannica.com.

Zinc (from Zink) is a metallic chemical element with the symbol Zn and atomic number 30. In nonscientific context it is sometimes called spelter. Zinc plating of steel is the major application for zinc other applications are in batteries and in alloys, for example brass. Sphalerite is the most important zinc ore. Zinc production includes roasting, leaching and at the end electrowinning. Commercially pure zinc is known as Special High Grade, often abbreviated SHG, and is 99.995% pure. Zinc is an essential mineral, necessary for sustaining all life, but at higher concentrations zinc poisoning can occur.

Characteristics

Zinc is a moderately reactive bluish grey metal that tarnishes in moist air and burns in air with a bright bluish-green flame, giving off fumes of zinc oxide. It reacts with acids, alkalis and other non-metals. If not completely pure, zinc reacts with dilute acids to release hydrogen. The one common oxidation state of zinc is +2.

From 100 °C to 210 °C to zinc metal is malleable and can easily be beaten into various shapes. Above , the metal becomes brittle and will be pulverized by beating. Zinc is nonmagnetic.

Isotopes

Naturally occurring zinc is composed of the 5 stable isotopes 64Zn, 66Zn, 67Zn, 68Zn, and 70Zn with 64Zn being the most abundant (48.6% natural abundance). Twenty-one radioisotopes have been characterised with the most abundant and stable being 65Zn with a half-life of 244.26 days, and 72Zn with a half-life of 46.5 hours. All of the remaining radioactive isotopes have half-lives that are less than 14 hours and the majority of these have half lives that are less than 1 second. This element also has 4 meta states.

Zinc has been proposed as a "salting" material for nuclear weapons (cobalt is another, better-known salting material). A jacket of isotopically enriched 64Zn, irradiated by the intense high-energy neutron flux from an exploding thermonuclear weapon, would transmute into the radioactive isotope Zn-65 with a half-life of 244 days and produce approximately 2.27 MeV of gamma radiation, significantly increasing the radioactivity of the weapon's fallout for several days. Such a weapon is not known to have ever been built, tested, or used.

History

The name of the metal zinc is unusual and, while vague in origin, was probably first used by Paracelsus, a Swiss-born German chemist, who referred to the metal as "Zincum", in the 16th century. These words in German apparently mean "tooth-like, pointed or jagged part" and, as metallic zinc crystals are needle-like, the derivation appears plausible.

Zinc mines of Zawar, near Udaipur, Rajasthan, India were active during 400 BC. There are references of medicinal uses of zinc in the Charaka Samhita (300 BC). The Rasaratna Samuccaya (800 AD) explains the existence of two types of ores for zinc metal, one of which is ideal for metal extraction while the other is used for medicinal purpose. Because of the low boiling point and high chemical reactivity of this metal (isolated zinc would tend to go up the chimney rather than be captured), the true nature of this metal was not understood in ancient times.

The manufacture of brass was known to the Ebi by about 30 BC, using a technique where calamine and copper were heated together in a crucible. The zinc oxides in calamine were reduced, and the free zinc metal was trapped by the copper, forming an alloy. The resulting calamine brass was either cast or hammered into shape.

Smelting and extraction of impure forms of zinc was accomplished around 1200 AD in India. China did not learn of the technique until 17th Century AD. In the West, impure zinc as a remnant in melting ovens was known since antiquity, but usually discarded as worthless. Strabo mentions it as pseudo-argyros — "mock silver". The Berne zinc tablet is a votive plaque dating to Roman Gaul, probably made from such zinc remnants.

Pure zinc in the West

The metallurgist Andreas Libavius received in 1597 a quantity of zinc metal in its pure form, which was unknown in the West before then. Libavius called it Indian/Malabar lead. It was regularly imported to Europe from the orient in the 17th and first half of the 18th century, but was at times very expensive.

The isolation of metallic zinc in the West may have been achieved independently by several people:

  • Traders from the Orient were bringing zinc to England in the early 1700s. It is suggested that they also brought the secret of its smelting, but evidence of this is lacking.
  • Dr. John Lane is said to have carried out experiments, probably at Landore, prior to his bankruptcy in 1726. Postlewayt's Universal Dictionary, a contemporary source giving technological information in Europe, did not mention zinc before 1751.
  • In 1738, William Champion patented in Great Britain a process to extract zinc from calamine in a vertical retort style smelter, using a technology somewhat similar to that used at Zawar zinc mines in Rajasthan. However, there is no evidence that he visited the orient. Champion's process was used through 1851.
  • In 1742, the Swedish chemist Anton von Swab distilled zinc from calamine.
  • The discovery of pure metallic zinc is often credited to the German Andreas Marggraf in 1746.

In 1758, William's brother, John, developed a new process for calcining zinc sulfide into an oxide for use in the retort process. Prior to this only calamine could be used to produce zinc. This process was then used into the 20th century. In 1798, Johann Ruberg built the first horizontal retort smelter in Upper Silesia. This was much more fuel efficient and less labor intensive than the vertical retort process. Jean-Jacques Daniel Dony built a different kind of horizontal zinc smelter in Belgium, which processes more.

Occurrence

See also: Zinc minerals

Zinc is the 23rd most abundant element in the Earth's crust. The most heavily mined ores (sphalerite) tend to contain roughly 10% iron as well as 40–50% zinc. Minerals from which zinc is extracted include sphalerite (zinc sulfide), smithsonite (zinc carbonate), hemimorphite (zinc silicate), and franklinite (a zinc spinel).

The earth has been estimated to have 46 years supply of zinc. A chemist estimated in 2007 that at the current rate of usage, the world's supply of zinc would be exhausted by about the year 2037.

Zinc mining and processing

Zinc is the fourth most common metal in use, trailing only iron, aluminium, and copper in annual production.

There are zinc mines throughout the world, with the largest producers being China, Australia and Peru. In 2005, China produced almost one-fourth of the global zinc output, reports the British Geological Survey. Zinc mines and refineries in Europe include Tara, Galmoy and Lisheen in Ireland and Zinkgruvan in Sweden.

Zinc metal is produced using extractive metallurgy.

Applications

Metal and alloys

Most of the produced zinc is used to galvanize or Parkerize steel and iron products to prevent corrosion. For example in 2006 in the United States 56% or 773,000 tonnes of the zinc metal was used for this purpose. Similar corrosion resistance can be acchieved by by plating with tin or cadmium. Zinc bassed alloys for die casting especially for the automobile industry is another major application. Several widely used alloys contain zinc. Brass is an alloy of copper and zinc and is used because of its machining properties and the decorative golden color. Nickel silver and German silver, which are used for coins and jewlery, are alloys of zinc and nickel or nickel, copper and zinc. Solder alloys and typewriter metal sometimes contain zinc. Zinc is used in contemporary pipe organs as a substitute for the traditional lead/tin alloy in pipes.

Zinc is the primary metal used in making American one cent coins since 1982. The zinc core is coated with a thin layer of copper to give the impression of a copper coin. In 1994 33,200 tons of zinc were used to produce 13,6 billion pennies.

The most widely used alloy of zinc is brass, in which copper is alloyed with anywhere from 9% to 45% zinc, depending upon the type of brass, along with much smaller amounts of lead and tin. Alloys of 85–88% zinc, 4–10% copper, and 2–8% aluminium find limited use in certain types of machine bearings. Alloys of primarily zinc with small amounts of copper, aluminium, and magnesium are useful in die casting as well as spin casting. An example of this is zinc aluminium. Similar alloys with the addition of a small amount of lead can be cold-rolled into sheets. An alloy of 96% zinc and 4% aluminium is used to make stamping dies for low production run applications where ferrous metal dies would be too expensive.

Batteries and sacrificial anode

Electrochemical properties of zinc make it a good material for anode materials. Zinc is used as part of batteries. The most widespread such use is as the anode in alkaline batteries Zinc is used as the anode or fuel of the zinc-air battery/fuel cell providing the basis of the theorised zinc economy Zinc is used as a sacrificial anode on boats and ships that use cathodic protection to prevent corrosion of metals that are exposed to sea water.

Other Use

  • Zinc sheet metal is used in making contemporary zinc bars.
  • Zinc powder is sometimes used as a propellant in solid-fuel rockets. When a compressed mixture of 70% zinc and 30% sulfur powder is ignited there is a violent chemical reaction. This produces zinc sulfide, together with large amounts of hot gas, heat, and light.

Compounds

See also: Zinc compounds
Zinc oxide is perhaps the best known and most widely used zinc compound, as it makes a good base for white pigments in paint. It also finds industrial use in the rubber industry, and is sold as opaque sunscreen. A variety of other zinc compounds find use industrially, such as zinc chloride (in deodorants), zinc pyrithione (anti-dandruff shampoos), zinc sulfide (in luminescent paints), and zinc methyl or zinc diethyl in the organic laboratory. Roughly one quarter of all zinc output is consumed in the form of zinc compounds.

  • Zinc oxide is used as a white pigment in watercolours or paints, and as an activator in the rubber industry. As an over-the-counter ointment, it is applied as a thin coating on the exposed skin of the face or nose to prevent dehydration of the area of skin. It can protect against sunburn in the summer and windburn in the winter. Applied thinly to a baby's diaper area (perineum) with each diaper change, it can protect against rash. As determined in the Age-Related Eye Disease Study, it is part of an effective treatment for age-related macular degeneration in some cases
  • Zinc chloride is used as a deodorant and can also be used as a wood preservative
  • Zinc sulfide is used in luminescent pigments such as on the hands of clocks and other items that glow in the dark.
  • Zinc methyl (Zn(CH3)2) is used in a number of organic syntheses since its discovery by Edward Frankland in 1849.
  • Zinc stearate is a lubricative plastic additive.
  • Lotions made of calamine, a mix of Zn-(hydroxy-)carbonates and silicates, are used to treat skin rash.
  • Zinc is included in most single tablet over-the-counter daily vitamin and mineral supplements. It is believed to possess anti-oxidant properties, which protect against premature aging of the skin and muscles of the body. In larger amounts, taken as zinc alone in other proprietaries, it is believed by some to speed up the healing process after an injury. Preparations include zinc acetate and zinc gluconate.
  • Zinc gluconate glycine and zinc acetate are also used in throat lozenges or tablets to reduce the duration and the severity of cold symptoms.
  • Zinc lactate is used in toothpaste to prevent malodour.
  • Zinc pyrithione is widely applied in shampoos because of its anti-dandruff function.

Biological role

Zinc is an essential mineral, necessary for sustaining all life. It is a key factor in prostate gland function and reproductive organ growth. It is estimated that 3,000 of the hundreds of thousands of proteins in the human body contain zinc prosthetic groups, one type of which is the so-called zinc finger. In addition, there are over a dozen types of cells in the human body that secrete zinc ions, and the roles of these secreted zinc signals in medicine and health are now being actively studied. Zinc ions are now considered to be neurotransmitters. Cells in the salivary gland, prostate, immune system and intestine use zinc signalling.

Zinc is also involved in olfaction: the olfactory receptors contain zinc binding sites and a deficiency in zinc causes anosmia.

Zinc is an activator of certain enzymes, such as carbonic anhydrase. Carbonic anhydrase is important in the transport of carbon dioxide in vertebrate blood. It is also required in plants for leaf formation, the synthesis of indole acetic acid (auxin) and anaerobic respiration (alcoholic fermentation).

Zinc is a good lewis acid, making it a useful catalytic agent in hydroxylation and other enzymatic reactions. Also Zinc has a flexible coordination geometry, allowing enzymes using Zinc to rapidly shift conformations and perform biological reactions .

Food sources

Zinc is found in oysters, and to a far lesser degree in most animal proteins, beans, nuts, almonds, whole grains, pumpkin seeds and sunflower seeds. A turkey's neck and beef's chuck or shank also contain significant amounts of zinc. Phytates, which are found in whole grain breads, cereals, legumes and other products, have been known to decrease zinc absorption. Clinical studies have found that zinc, combined with antioxidants, may delay progression of age-related macular degeneration. Significant dietary intake of zinc has also recently been shown to impede the onset of flu. Soil conservation analyzes the vegetative uptake of naturally occurring zinc in many soil types.

The US recommended dietary allowance of zinc from puberty on is 11mg for males and 8mg for females, with higher amounts recommended during pregnancy and lactation.

Zinc deficiency

Zinc deficiency occurs where insufficient zinc is available for metabolic needs. It is usually nutritional, but can also be associated with malabsorption, acrodermatitis enteropathica, chronic liver disease, chronic renal disease, sickle cell disease, diabetes, malignancy, and other chronic illnesses.

Immune system

''See also: Zinc gluconate
Zinc salts are effective against pathogens in direct application. Gastroenteritis is strongly attenuated by ingestion of zinc, and this effect could be due to direct antimicrobial action of the zinc ions in the GI tract, or to the absorption of the zinc and re-release from immune cells (all granulocytes secrete zinc), or both.

In clinical trials, both zinc gluconate and zinc gluconate glycine (the formulation used in lozenges) have been shown to shorten the duration of symptoms of the common cold. The amount of glycine can vary from two to twenty moles per mole of zinc gluconate. It should be known that there have been clinical trials that both support the use of zinc for the common cold, and are inconclusive of its effectiveness. All clinical trials have their critics, including the dosage amount used, and the highly subjective format of patient self-reporting the results of their trials.

Precaution

Zinc toxicity

Even though zinc is an essential requirement for a healthy body, excess zinc can be harmful. Excessive absorption of zinc can also suppress copper and iron absorption. The free zinc ion in solution is highly toxic to plants, invertebrates, and even vertebrate fish. The Free Ion Activity Model (FIAM) is well-established in the literature, and shows that just micromolar amounts of the free ion kills some organisms. A recent example showed 6 micromolar killing 93% of all Daphnia in water.

The free zinc ion is also a powerful Lewis acid up to the point of being corrosive. Stomach acid contains hydrochloric acid, in which metallic zinc dissolves readily to give corrosive zinc chloride. Swallowing a post-1982 American one cent piece (97.5% zinc) can cause damage to the stomach lining due to the high solubility of the zinc ion in the acidic stomach. Zinc toxicity, mostly in the form of the ingestion of US pennies minted after 1982, is commonly fatal in dogs where it causes a severe hemolytic anemia. In pet parrots zinc is highly toxic and poisoning can often be fatal.

There is evidence of induced copper deficiency at low intakes of 100–300 mg Zn/d. The USDA RDA is 15 mg Zn/d. Even lower levels, closer to the RDA, may interfere with the utilization of copper and iron or to adversely affect cholesterol..

There is also a condition called the zinc shakes or "zinc chills" that can be induced by the inhalation of freshly formed zinc oxide formed during the welding of galvanized materials.

Zinc poisoning

In 1983, the United States Mint began minting pennies made primarily of zinc and only coated in copper. Pennies and other small coins are sometimes ingested by dogs resulting in the need for medical treatment to remove the foreign body. With the new zinc pennies, there is the added potential for zinc toxicosis which can be fatal. This can result in Hemolysis, liver or kidney damage, vomiting and diarrhea.

There are also cases reported in that humans suffered zinc intoxication by the ingestion of zinc coins.

Notes

References

  • Craddock, P.T. et al. (1983). Zinc production in medieval India, World Archaeology, vol. 15, no. 2, Industrial Archaeology.

External links

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