Zinc: Encyclopedia - Zinc

Zinc

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

Zinc - Notable characteristics

Zinc is a moderately reactive metal that will combine with oxygen and other non-metals, and will react with dilute acids to release hydrogen. The one common oxidation state of zinc is +2.

Zinc - Applications

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

• Zinc is used to galvanise metals such as steel to prevent corrosion.
• Zinc is used in alloys such as brass, nickelled silver, typewriter metal, various soldering formulas and German silver.
• Zinc is the primary metal used in making American pennies since 1982.
• Zinc is used in die casting noteably in the automobile industry.
• Zinc is used as part of the containers of batteries.
• 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's part of an effective treatment for age-related macular degeneration in some cases.
• Zinc chloride is used as a deodorant and can 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.
• 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 metal 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 is used as a lozenge in an attempt to remedy the common cold.

Zinc - Popular misconceptions

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

In Argentina some people wrongly believe that zinc is a poison, and some of them are avoiding food which is known to include zinc. In 1997 a municipality north of the centre of Buenos Aires posted advertisements in popular magazines explaining the usefulness of zinc in the human body.

Zinc - History

Zinc alloys have been used for centuries, as brass goods dating to 1000-1400 BC have been found in Palestine 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. By the end of the 14th century, the Hindus were aware of the existence of zinc as a metal separate from the seven known to the ancients. 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 considerably more involved.

Descriptions of brass manufacture are found in Western Europe in the writings of Albertus Magnus, c. 1248, and by the 16th century, the understanding and awareness of the new metal broadened considerably. Georg Agricola observed, in 1546, that a white metal could be condensed and scraped off the walls of a furnace when zinc ores were smelted. He added in his notes that a similar metal called "zincum" was being produced in Silesia. Paracelsus (died 1541) was the first in the West to say that "zincum" was a new metal and that it had a separate set of chemical properties from other known metals.

The upshot is that zinc was known by the time Marggraf made his discoveries and in fact zinc had been isolated two years earlier by another chemist, Anton von Swab. However, Marggraf's reports were exhaustive and methodical and the quality of his research cemented his reputation as the discoverer of zinc.

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

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.

Zinc - Food Sources

The best and most abundant natural food source of zinc is oysters, although these bottom scavengers also accumulate toxic metals. Zinc is found in most animal proteins such as beef, pork and poultry. Other food sources of zinc include 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. This, coupled with the fact that the human body absorbs zinc more easily from animal protein than from plant protein means that vegetarians are required to eat many more food sources containing zinc than non-vegetarians.

Zinc - 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.

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

There is zinc in semen. As much as 0.25 milligram of zinc will be found in 1 mL of seminal fluid.

Zinc - Zinc Toxicity

Even though zinc is almost an essential requirement for a healthy body, too much zinc can be harmful. Excessive absorption of zinc can also suppress copper and iron absorption.

Zinc - Psoriasis

Ionic zinc is a potent antimicrobial, used since 2500 BC in topical creams. Calomine lotion, diaper creams, and dandruff treatments are just some of the common antimicrobial applications. At low concentrations, zinc ions promote wound healing. Zinc ions also directly stimulate zinc receptors on skin cells, promoting wound healing.

Zinc - 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.

Zinc - Abundance

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 - 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 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 sulfuric acid and the resulting solution is purified using zinc dust. The metal is then extracted 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.

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

Zinc - Isotopes

Naturally occurring zinc is composed of the 5 stable isotopes Zn-64, Zn-66, Zn-67, Zn-68, and Zn-70 with 64 being the most abundant (48.6% natural abundance). 21 radioisotopes have been characterised with the most {abundant and/or stable} being Zn-65 with a half-life of 244.26 days, and Zn-72 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 - 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 that can be induced by the inhalation of freshly formed zinc oxide. Excessive intake of zinc can promote deficiency in other dietary minerals.

Adapted from the Wikipedia article "Zinc", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki