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Zinc

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Zinc (from German zink) is a chemical element in the periodic table that has the symbol Zn and atomic number 30.

Contents

Notable characteristics

Zinc is a moderately reactive bluish-white metal that tarnishes in moist air producing a layer of carbonate. It reacts with acids and 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.

Applications

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

Popular misconceptions

The highly characteristic metal counters of traditional French bars are often referred to as zinc bars or vaguely zinc, but actually zinc has never been used for this purpose and the counters are really made of an alloy of lead and tin.

History

Zinc
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Zinc

In ancient India the production of Zinc metal was very common.Many mine sites of Zawarmaala were active even during 1300-1000 BC.There are references of medicinal uses of zinc in the Charaka Samhita 300 BC.The Rasa Ratna 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 [Ref]. Zinc alloys have been used for centuries, as brass goods dating to 1000–1400 BC have been found in Israel and zinc objects with 87% zinc have been found in prehistoric Transylvania. 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 Romans 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 being accomplished as early as 1000 AD in India and China.In the West, impure zinc as a remnant in melting ovens was known since Antiquity, but usually thrown away as worthless. Strabo mentions it as pseudo-arguros "mock silver". The Berne Zinc tablet is a votive plaque dating to Roman Gaul, probably made from such zinc remnants. The discovery of pure metallic zinc is most often credited to the German Andreas Marggraf, in the year 1746, though the whole story is disputed.

In 1597 AD , Libavius , a metallurgist in England received some quantity of Zinc metal in its pure form which was unknown to the west till then.The proof of the ignorance of this metal can be gauged from the fact that till 1751 AD Postlewayt's Universal Dictionary (the most authentic source of all technological information in Europe) did not mention anything on Zinc.Libavius named it as Indian / Malabar Lead.Paracelsus (1616) was credited to have given the name "Zinc" for this "Malabar / Indian" lead .

In 1738, William Champion is credited with patenting in Britain a process to extract zinc from calamine in a smelter, a technology he acquired after visiting Zawar Zinc mines in Rajasthan. His first patent was rejected by the patent court on grounds of plagiarisng the technology common in India.However he was granted the patent on his second submission of patent approval.

Before the discovery of the zinc sulfide flotation technique, calamine was the mineral source of zinc metal.

Foods and spices that contain the essential mineral zinc
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Foods and spices that contain the essential mineral zinc

Biological role

Zinc is an essential element, necessary for sustaining all life. It is estimated that 3000 of the hundreds of thousands of proteins in the human body contain zinc prosthetic groups. 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. Intriguingly, brain cells in the mammalian forebrain are one type of cell that secretes zinc, along with its other neuronal messenger substances. Cells in the salivary gland, prostate, immune system and intestine are other types that secrete zinc.

Food sources

Zinc is found in oysters, and to a far lesser degree in most animal proteins, beans, nuts, whole grains, pumpkin seeds and sunflower seeds. 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, but the effect is extremely small and not likely to be clinically important. Significant dietry 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.

Zinc deficiency

Zinc deficiency results from inadequate intake of zinc, or inadequate absorption of zinc into the body. Signs of zinc deficiency includes hair loss, skin lesions, diarrhea, wasting of body tissues, and, eventually, death. Eyesight, taste, smell and memory are also connected with zinc and a deficiency in zinc can cause malfunctions of these organs and functions. Congenital abnormalities causing Zinc deficiency may lead to a disease called Acrodermatitis enteropathica.

Obtaining a sufficient zinc intake during pregnancy and in young children is a very real problem, especially among those who cannot afford a good and varied diet. Brain development is stunted by zinc insufficiency in utero and in youth. Teenage boys require 11 milligrams of zinc.

Zinc toxicity

Even though zinc is an essential requirement for a healthy body, too much zinc can be harmful. Excessive absorption of zinc can also suppress copper and iron absorption. On the other hand, 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 of 6 micromolar killing 93% of all daphnia in water is in Muyssen et al., (Aquat Toxicol. 2006). Swallowing an American one cent piece (98% zinc) can also cause damage to the stomach lining due to the high solubility of the zinc ion in the acidic stomach (Bothwell and Mair, PEDIATRICS 2003) whereas the British penny is made from copper-plated steel. 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 (Stowe CM, Nelson R, Werdin R, et al: Zinc phosphide poisoning in dogs. JAVMA 173:270, 1978)

Immune system

Zinc salts are effective against pathogens in direct application. Gastrointestinal infections are also 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 absorption of the zinc and re-release from immune cells (all granulocytes secrete zinc) or both.

The direct effect of zinc (as in lozenges) on bacteria and viruses is also well-established, and has been used since at least 2000 BC, from when zinc salts in palliative salves are documented. However, exactly how to deliver zinc salts against pathogens without injuring one's own tissues is still being investigated.

Abundance

''See also:
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).

Zinc production

There are zinc mines throughout the world, with the largest producers being Australia, Canada, China, Peru and the U.S.A. Mines in Europe include Vieille Montagne in Belgium, Tara, Galmoy and Lisheen in Ireland, and Zinkgruvan in Sweden. Zinc metal is produced using extractive metallurgy. Zinc sulfide (sphalerite) minerals are concentrated using the froth flotation method and then usually roasted using pyrometallurgy to oxidise the zinc sulfide to zinc oxide. The zinc oxide is leached in several stages of increasingly stronger sulfuric acid. Iron is usually rejected as Jarosite or goethite, removing other impurities at the same time. The final purification uses zinc dust to remove copper, cadmium and cobalt. The metal is then extracted from the solution by electrowinning as cathodic deposits. Zinc cathodes can be directly cast or alloyed with aluminium.

Another process to produce zinc is flash smelting, a pyrometallurgical process. Then zinc oxide is obtained, usually producing zinc of lesser quality than the hydrometallurgical process. Zinc oxide treatment has much fewer applications, but high grade deposits have been successful in producing zinc from zinc oxides and zinc carbonates using hydrometallurgy.

Compounds

''See also:
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 sulfide (in luminescent paints), and zinc methyl or zincdiethyl in the organic laboratory. Roughly one quarter of all zinc output is consumed in the form of zinc compounds.

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). 21 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.

Precautions

Metallic zinc is not considered to be toxic, but free zinc ions in solution (like copper or iron ions) are highly toxic. There is also a condition called zinc shakes or zinc chills (see metal fume fever) that can be induced by the inhalation of freshly formed zinc oxide formed during the welding of galvanized materials. Excessive intake of zinc can promote deficiency in other dietary minerals.

References

External links

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