Zinc is a moderately reactive, bluish-white metal that tarnishes in moist air
and burns in air with a bright bluish-green flame, giving off plumes 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 (212 °F to 410 °F) zinc metal is malleable and can
easily be beaten into various shapes. Above 210 °C (410
°F), the metal becomes brittle and will be pulverized by beating.
History
In ancient India the production of zinc metal was very common. Many mine
sites of Zawar Mines, near Udaipur, Rajasthan;-Zawarmaala were active even
during 1300–1000 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. Zinc alloys
have been used for centuries, as brass goods dating to 1400–1000 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 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 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 discarded 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.
Metallic zinc in the West
The English metallurgist Libavius received in 1597 a quantity of zinc metal
in its pure form, which was unknown in the West before then. Libavius identified
it as Indian/Malabar lead. Paracelsus (1516) was credited for the name
zinc. It was regularly imported to Europe from the orient in the 17th
century, but was at times very expensive.
The isolation of metallic zinc in the West may have been achieved
independently by several people:
- Dr John Lane is said to have carried out experiments, probably at Landore,
prior to his bankruptcy in 1726. Postlewayt's Universal Dictionary, the most
authentic source of all 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 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.
- The discovery of pure metallic zinc is also often credited to the German
Andreas Marggraf, in 1746, though the whole story is disputed.
Before the discovery of the zinc sulfide flotation technique, calamine was
the mineral source of zinc metal.
Biological
Role
Zinc is an essential element, necessary for sustaining all life. 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
neurotransmitters. Cells in the salivary gland, prostate, immune system and
intestine are other types that secrete zinc.
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).
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. 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.
Foods and spices that contain
the essential mineral zinc
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 results from inadequate intake of zinc, or inadequate
absorption of zinc into the body. Signs of zinc deficiency include hair loss,
skin lesions, diarrhea, and wasting of body tissues. Eyesight, taste, smell and
memory are also connected with zinc. 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.
It is widely recognised that lack of zinc can contribute to acne.
Leukonychia, white spots on the fingernails, are often seen as an indication of
zinc deficiency.
Zinc deficiency as a cause of anorexia
nervos
Zinc deficiency causes a decrease in appetite -- which could degenerate in
anorexia nervosa (AN). Appetite disorders, in turn, cause malnutrition and,
notably, inadequate zinc nutriture. The use of zinc in the treatment of anorexia
nervosa has been advocated since 1979 by Bakan. At least 5 trials showed that
zinc improved weight gain in anorexia. A 1994 randomized, double-blind,
placebo-controlled trial showed that zinc (14 mg per day) doubled the rate of
body mass increase in the treatment of anorexia nervosa (AN). Deficiency of
other nutrients such as tyrosine and tryptophan (precursors of the monoamine
neurotransmitters norepinephrine and serotonin, respectively), as well as
vitamin B1 (thiamine) could contribute to this phenomenon of
malnutrition-induced malnutrition.
- Inadequate zinc nutriture, however, is not only involved in such an extreme
appetite dysregulation as AN. Growth, appetite and zinc status have been shown
to be correlated.
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. 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. 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. 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.
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
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.
Zinc mining and
processing
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. Mines and refineries in Europe
include Umicore 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 to zinc sulfate (ZnSO4) in several stages
of increasingly stronger sulfuric acid (H2SO4). 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
in two to three different stages. The metal is then extracted from the purified
zinc sulfate solution by electrowinning as cathodic deposits over aluminium
sheets. Zinc cathodes can be directly cast or alloyed with aluminium.
Electrolyte zinc sulfate solutions must be very pure for electrowinning to be
at all efficient. Impurities can change the decomposition voltage enough to
where the electrolysis cell produces largely hydrogen gas rather than zinc
metal.[13]
There are two common processes for electrowinning the metal: the low
current density process, and the Tainton high current density
process. The former uses a 10% sulfuric acid solution as the electolyte, with
current density of 270–325 amperes per square meter. The latter uses 22–28%
sulfuric acid solution as the electrolyte with a current density of about 1,000
amperes per square meter. The latter gives better purity and has higher
production capacity per volume of electrolyte, but has the disadvantage of
running hotter and being more corrosive to the vessel in which it is done. In
either of the electrolytic processes, each metric ton of zinc production expends
about 3900 kW·h (14 MJ) of electric power.
There are also several pyrometallurgical processes that reduce zinc oxide
using carbon, then distill the metallic zinc from the resulting mix in an
atmosphere of carbon monoxide. These include the Belgian-type
horizontal-retort process, the New Jersey Zinc continuous
vertical-retort process, and the St. Joseph Lead Company's
electrothermal process. The Belgian process requires redistillation to
remove impurities of lead, cadmium, iron, copper, and arsenic. The New Jersey
process employs a fractionating column, which is absent in the Belgian process,
that separates the individual impurities, where they can be sold as byproducts.
The St. Joseph Lead Company process heats the zinc oxide/coke mixture by passing
an electric current through it rather than by coal or gas fire.
Another pyrometallurgical process is flash smelting. 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
Alloys
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%
aluminum find limited use in certain types of machine bearings. Alloys of
primarily zinc with small amounts of copper, aluminum, and magnesium are useful
in die casting. An example of this is zinc aluminum. Similar alloys with the
addition of a small amount of lead can be cold-rolled into sheets. An alloy of
96% zinc and 4% aluminum is used to make stamping dies for low production run
applications where ferrous metal dies would be too expensive.
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.
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.
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.
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