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Zinc

Metal used primarily in the construction of larger organ pipes. Zinc is the third most used nonferrous metal (after aluminum and copper), of which the U.S. consumes more than one million metric tons annually. The average person will use 730 pounds of zinc in his or her lifetime, according to the U.S. Bureau of Mines. Zinc (from German Zink and may be from Old Persian) is a chemical element in the periodic table that has the symbol Zn and atomic number 30.

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.

Applications

     Zinc is the fourth most common metal in use, trailing only iron, aluminium, and copper in annual production. Zinc is used to galvanize metals such as steel to prevent their corrosion.  Zinc is used in alloys such as brass, nickel silver, typewriter metal, various solder formulas, German silver, etc. Brass, in turn, has wide application because of its strength and corrosion resistance. Since 1982, zinc has been the primary metal used in making American pennies. Zinc is used in die castings, especially by the automobile industry.  Rolled zinc is used as part of the containers of batteries. Zinc oxide is used as a white pigment in watercolors and 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 and nose to prevent dehydration and thereby 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 protects 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 as a wood preservative. Zinc sulfide is used in luminescent pigments, for making 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 proprietary over-the-counter daily vitamin and mineral preparations. Along with some other metals, it is believed by some 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 taken in lozenge form as a remedy for the common cold.

A popular misconception

     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.

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 AD 1000 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.

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.

Food Sources

     The best and most abundant natural food source of zinc is oysters. 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 Deficiency

     Zinc deficiency in humans is often a result of inadequate intake of zinc, or inadequate absorption of zinc into the body. Signs of Zinc deficiency include 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 develepment is stunted by zinc insufficiency in utero and in youth.
There is zinc in semen. As much as half a milligram of zinc will be found in 1 mL of seminal fluid, which explains why males require a higher intake of zinc than do females.  Zinc deficiency in males can lead to reduced sperm count and sex drive, but this is probably just a by-product of the general malaise of advanced malnutrition. There is no evidence that zinc supplements enhance male sexuality in males who have adequate general nutrition.

Zinc Toxicity

     Even though zinc is almost an essential requirement for a healthy body, too much zinc can be harmful to the human body. Excessive absorption of zinc into the human body can lead to reduced iron function, and impair the immune system. Experiments have also been conducted where the excessive intake of zinc led to nausea and vomiting within 30 minutes of ingestion.

Psoriasis

     Since Zinc deficiency can cause hair loss, many naturopathic health products and topical creams used to treat psoriasis contain zinc as one of the ingredients.

Immune System

     According to some sources, taking zinc tablets may provide some immunity against colds and flu, although this is disputed. The biological basis of this is that it has been established that the immune system is impaired by zinc deficiency and so will respond to zinc supplements. 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 (germs) 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 tend to contain roughly 10% iron as well as 40-50% zinc. Minerals from which zinc is extracted include sphalerite, zinc blende, smithsonite, calamine, and franklinite.

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 aluminum.

     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

     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.
 
Isotopes

     Naturally occurring zinc is composed of the 4 stable isotopes Zn-64, Zn-66, Zn-67, and Zn-68 with 64 being the most abundant (48.6% natural abundance). 22 radioisotopes have been characterized 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.

     Centuries before zinc was discovered in the metallic form, its ores were used for making brass and zinc compounds were used for healing wounds and sore eyes. Although the word brass frequently occurs in the Old Testament, there is little evidence that an alloy of zinc and copper was known in early times. The word translated "brass" might equally well be rendered bronze or copper, both of which were in common use.

     In the latter part of the thirteenth century, Marco Polo described the manufacture of zinc oxide in Persia and how the Persians prepared tutia (a solution of zinc vitriol) for healing sore eyes.

     The Roman writer Strabo (66 B.C. - 24 A.D.) mentioned in his writings that only the Cyprian ore contained "the cadmian stones, copper vitriol, and tutty," that is to say, the constituents from which brass can be made. It is believed that the Romans first made brass in the time of Augustus (20 B.C. to 14 A.D.) by heating a mixture of powdered calamine, charcoal and granules of copper. Roman writers observed that coins made from orichalcum were undistinguished from gold.

Zinc in India

     The production of metallic zinc was described in the Hindu book Rasarnava which was written around 1200 A.D. The fourteenth century Hindu work Rasaratnassamuchchaya describes how the new "tin-like" metal was made by indirectly heating calamine with organic matter in a covered crucible fitted with a condenser. Zinc vapour was evolved and the vapour was air cooled in the condenser located below the refractory crucible (Figure 1). By 1374, the Hindus had recognized that zinc was a new metal, the eighth known to man at that time, and a limited amount of commercial zinc production was underway.

     At Zawar, in Rajasthan, great heaps of small retorts bear testimony to extensive zinc production from the twelfth to the sixteenth centuries. The tubular retorts are about 25 cm long and 15 cm in diameter with walls about 1 cm thick. A small diameter tube was sealed onto the open end and the zinc vapours likely condensed in this. The retorts were closely spaced in a furnace which was probably heated with charcoal fanned by bellows. Both zinc metal and zinc oxide were produced. Zinc was used to make brass whereas the oxide was used medicinally. Over 130,000 tons of residue remain at Zawar and this represents the extraction of the equivalent of 1,000,000 tons of metallic zinc and zinc oxide.

Zinc in China

      From India, zinc manufacture moved to China where it developed as an industry to supply the needs of brass manufacture. The Chinese apparently learned about zinc production sometime around 1600 A.D. An encyclopedia issued in the latter half of the sixteenth century makes no mention of zinc, but the book Tien-kong-kai-ou published early in the 17th century related a procedure for zinc manufacture. Calamine ore, mixed with powdered charcoal, was placed in clay jars and heated to evolve zinc vapour. The crucibles are piled up in a pyramid with lump coal between them, and, after being brought to redness, are cooled and broken. The metal is found in the center in the form of a round regulus. Zinc production expanded and metal began to be exported.

Zinc in Europe

     Albertus Magnus (ca. 1248) described how either calamine or furnace tutty might be used to colour copper gold. He suggested that a more golden lustre might be obtained by sprinkling crushed glass on top of the mixture in the crucible to form a slag which would help prevent the escape of the zinc vapour; in other words, increase the zinc content of the brass.  Biringuccio (ca. 1540) has the next most complete description of brass making. He described how either calamine or furnace tutty could be mixed with broken up pieces of copper and sprinkled with a layer of powdered glass, then heated in a closed crucible for 24 hours. Agricola (Figure 4) in 1546 reported that a white metal was condensed and scraped off the walls of the furnace when Rammelsberg ore was smelted in the Harz Mountains to obtain lead and silver to which he gave the name "contrefey" because it was used to imitate gold. This often consisted to metallic zinc, although he did not recognize it as such. He observed, furthermore, that a similar metal called "zincum" was being produced under similar circumstances in Silesia by the local people. Paracelsus (1493-1541) (Figure 5) was the first European to state clearly that "zincum" was a new metal and that it had properties distinct from other known metals.

     Thus, by about 1600, European scientists were aware of the existence of zinc. All the metal they had examined, however, had likely been imported from the East by Portuguese, Dutch and Arab traders. However, there was a profusion of names quite unrelated to the local names for zinc ores. These included tutenag (derived from the Persian tutiya, calamine,which became the English tutty, zinc oxide) and spelter (likely from the similar coloured lead-tin alloy, pewter, or the Dutch equivalent, spiauter or Indian tin which the British scientist Robert Boyle latinised to speltrum in 1690 from which originates spelter, the commercial term for zinc. The word tutia, an old name for zinc oxide, is derived from a Persian word that means smoke and refers to the fact that zinc oxide is evolved as white smoke when zinc ores are roasted with charcoal.

     In Renaissance times, latten (or laten, laton, lattyn) became the common English word for brass, akin to the French laiton (= brass) and Italian latta (= sheet brass), and probably based on the Latin latte or lathe (= sheet). The origins of the German word for brass, Messing, may be related to the Latin massa (= lump of metal). The modern English brass may be related to the French braser (= braze or solder). The word "zinc" may be derived from the Persian word sing meaning stone. In Arabic, zinc is known as kharseen, i.e. Khar from Al-Ghar = mine, seen from Al-Seen = China, hence kharseen, the metal from Chinese mines. The spelter trade with the East flourished throughout the seventeenth and first half of the eighteenth centuries, although there seem to be no records concerning the tonnages involved.

     In an extensive research "On the method of extracting zinc from its true mineral, calamine", Andreas Marggraf (Figure 6) in 1746 reduced calamine from Poland, England, Breslau and Hungary with carbon in closed retorts and obtained metallic zinc from all of them. He described his method in detail, thereby establishing the basic theory of zinc production. Marggraf also showed that the lead ores from Rammelsberg contained zinc and that zinc can be prepared from sphalerite. Marggraf was probably unaware that in 1742, the Swedish chemist Anton von Swab (1703-1768) had distilled zinc from calamine and that, two years later, he had even prepared it from blende. Since the vapors rose to the top of the alembic before passing into the receiver, this process was called distillation per ascendum. In 1752 Swab and another Swedish chemist Axel Fredrik Cronstedt (1722-1765) developed at government expense the use of Swedish zinc ores for the manufacture of brass, to avoid the necessity of importing calamine.

     The knowledge of deliberate zinc smelting in a retort was acquired by an Englishman on a visit to China just prior to 1740. A vertical retort procedure was developed by William Champion (1709-1789) and by 1743 a zinc smelter had been established at Bristol in the United Kingdom. A charge of calamine and carbon was sealed into a clay crucible having a hole in the bottom. This was luted onto an iron tube extending below the crucible furnace into a cool chamber below. The closed end of the iron tube sat in a tub of water and it was here that the metallic zinc was collected (Figure 7). The distillation took a total of about 70 hours to yield 400 kg of metal from all 6 crucibles positioned in the furnace. An annual production rate of 200 tons has been suggested for the works at that time.

     This type of apparatus continued to be employed until 1851 although it was fuel inefficient, consuming 24 tons of coal for every ton of spelter produced. In 1758, William's brother, John, patented the calcination of zinc sulfide to oxide for use in the retort process, thereby laying the foundation for the commercial zinc practice which continued well into the twentieth century. The English zinc industry was concentrated in Bristol and Swansea.

     The Welsh process was a batch operation which required withdrawing the crucible and retort after each cycle. It was labour intensive and fuel inefficient. A major technological improvement came with the development of the German process by Johann Ruberg (1751-1807) who built the first zinc smelting works in Wessola in Upper Silesia in 1798 which used the horizontal retort process developed by him. The principal advantage of this technique is that the retorts were fixed horizontally into the furnace allowing them to be charged and discharged without cooling. By placing the retorts in large banks, fuel efficiency was greatly increased. The raw material initially used was zinc galmei (calamine), a by-product of lead and silver production. Later, it became possible to produce zinc directly from smithsonite, an easily smelted ore. This was shortly followed by the use of zinc blende, which had first to be converted into the oxide by roasting. After this development, other smelting works were soon erected in Silesia near the deposits, in the areas around Lige in Belgium, in Aachen, in the Rhineland and Ruhr regions in Germany.

     The first Belgian plant was built by Jean-Jacques Daniel Dony (1759-1819) in 1805 and also used horizontal retorts but of slightly different design. A larger plant was built in 1810. This was the predecessor of the Societ de la Vieille Montagne which a few years later became the largest zinc producing company in the world.

     Zinc production in the United States started in 1850 using the Belgium process and soon became the largest in the world. In 1907, world production was 737,500 tons of which the USA contributed 31%, Germany 28%, Belgium 21%, United Kingdom 8%, and all other countries 12%.

     The excellent resistance of zinc towards atmospheric corrosion soon led to its use in sheet production. The possibility of rolling zinc at 100-150C was discovered as early as 1805 and the first rolling mill was built in Belgium in 1812. More such mills were built in Silesia from 1821 onwards. Hot-dip galvanizing, the oldest anticorrosion process, was introduced in 1836 in France. This became possible on an industrial scale only after the development of effective processes for cleaning iron and steel surfaces. At first, only small workpieces were zinc coated. Continuous hot-dip galvanizing of semi-finished products and wire came later. In the United States, the rich ore deposits led to rapid growth in zinc production in 1840, so that by 1907, Germany, which had for long been the world's leading producer of zinc, was left behind.

     Zinc was produced for about 500 years from its oxide ores which are far less abundant than the sulfides, before the sulfides became the major source of supply. The technology of zinc production changed gradually during the centuries towards a more pyrometallurgical route. However, this tendency underwent a radical change during World War I when the roasting-leaching-electrowinning process was introduced and in the 1980's, when pressure leaching-electrowinning offered another practical route to zinc production.

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