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What is Zinc ?

Author: Date:10/25/2012 12:45:35 AM
Introduction of Zinc
What is Zinc ?
Zinc was named by the Swiss alchemist Theophrastus Bombastus von Hohenheim (Paracelsus, 1493-1541), who coined the new Latin word zincum from antecedents that are not clear. A name was necessary for the newly-prepared metal, although its alloy, brass, had been known since ancient times. In English and French, this became zinc, in German and Dutch zink, in Spanish cinc, in Welsh sinc (pronounced "shink"), in Greekpseudargyros ("false silver") or tsigkos, pronounced "tsingos." In Russian, it is tsink. The kitchen sink has nothing to do with zinc, unless it happens to be made from it.
After iron, aluminium and copper, zinc is usually the fourth-most used metal, competing with lead. We probably see it every day, use it nearly as often as a source of electrical power, and handle small parts made mostly of it. This would be enough to make the metal interesting, but it also gives us valuable insights in physics and chemistry, and gives us an excuse to discuss them in this article. 
Uses of Zinc 
The largest use of zinc is as a protective coating for iron. The process is called galvanizing with reference to the cathodic protection the zinc offers to the iron. The name was coined by Sorel, who patented the process in 1836. Some sources say that galvanizing began at Swansea, at the early zinc smelter, in 1740. It could not have been called "galvanizing" then, of course. It is usually carried out by dipping carefully cleaned iron or steel in molten zinc, which gives the rather thick, robust coating required. Tin, another familiar protective coating, is now usually applied electrolytically, since it is easier to make a reliable thin coating this way. Tin is much more costly than zinc, which is an incentive to thin coats. Galvanized steel cannot be used in cans for food preservation as tin is, however, because the zinc coating is attacked by food acids to which tin is immune. Zinc, however, gives excellent protection against the weather and moisture, so it is preferred where this is important, since it is cheaper. Zinc protects the iron by cathodic protection, since it is higher on the electrochemical scale than iron and will sacrifice itself to protect the iron, reducing it to the metal and eliminating rust. This phenomenon was noted by Faraday in 1829. If it sacrifices too much, however, the iron is exposed to oxidation, as is sometimes seen with old or damaged galvanized iron.
The life of galvanized steel depends on the thickness of the coating and the environment. The life is approximately proportional to the thickness of the coating, however it was applied. A coating of 1 ounce per square foot, giving a film 0.0018" thick, has a life of about 25 years in rural locations, 10-15 years in an urban environment. Note that the coating is on both sides of a sheet, so the total zinc use will be 2 oz./sqft in this case. Zinc will not give cathodic protection if it becomes passivated, or covered by a closely adherent layer of hydroxide, since then the necessary currents cannot flow. However, the layer will protect the zinc from corrosion, also protecting the underlying metal. Zn(OH)2 is insoluble for pH between 6 and 13, and in this range the hydroxide will protect the zinc under water. Aluminium and chromium are protected and passivated by the oxides, lead by lead sulphate.
Iron or steel articles can be mixed with zinc dust and tumbled in a steel drum at about 370°C, below the melting point of zinc. The articles must first be thoroughly cleaned, by pickling in 50% HCl, or a similar process. The zinc alloys with the surface to form a thin but very adherent layer that protects the iron underneath, and will not clog fine details such as threads. About 15 mg/cm2 of zinc is used. This process is called Sherardizing, after Sherard Cowper-Coles, who patented it in 1901. Iron can be coated with chromium or aluminium by similar processes. These procedures are known in general as cementation, in which an alloy is formed without melting.
The next use in tonnage is as wrought zinc, that is, as plate, tubing, and other forms made by rolling or other shaping processes. Sheet zinc was once used for roofing, where it has a very long life in this application destructive to most metals. In contact with the atmosphere, which contains H2O and CO2, a closely-adhering layer of basic zinc carbonate, Zn(OH)2·ZnCO3, forms and protects the metal. Sheet zinc is now used mainly for battery anodes, in which it is effectively "burned" to produce electrical energy. Batteries are little fuel cells burning zinc, and are a more practical device in their field of application than fuel cells burning alcohol or hydrogen and using the oxygen of the air, at least so far.
Closely following this use, and perhaps exceeding it at times, is the use of zinc to make die castings. The most common process is pressure die casting, in which the molten zinc is forced into steel dies that make the mold. Zinc expands on cooling, so it fills the mold exactly, like type metal, and can make precision castings requiring very little machining. The low melting point of zinc gives long die life. Automobiles are full of die castings, from brake cylinders and fuel pumps to door handles, many of which are plated to give them a shiny finish. Carburetors at one time were made up nearly completely of die castings. In mass production, die castings are much cheaper than machined parts, since the large cost of the dies can be amortized over the numerous products. Die castings required the production of extremely pure zinc, since the usual impurities caused the castings to swell and "crystallize" in a short time. They didn't actually crystallize, of course, but the impurities migrated to the crystal boundaries and caused embrittlement. A typical die-casting alloy is Zn 94.9, Al 4.1, Cu 1.0, called "Zamak." Lead, cadmium and tin impurities must be kept to a very low level in the zinc used for this purpose.
Small amounts of zinc are used for other purposes. Zinc has occasionally been used in coins, such as the Albanian 1/2 Leku and the United States cent. Zinc chloride, ZnCl2 is used as a soldering flux for soldering iron with tin-lead solders in a water solution. It hydrolyzes to give an acid reaction, ZnCl2 + 2HOH → Zn(OH)2 + 2H+ + 2Cl-, which is not as corrosive as pure hydrochloric acid would be. Zinc is used in aluminium solders, such as 75 Zn, 20 Cd, 5 Al, which is Bureau of Standards aluminium solder ZN1. Zinc is also used in silver solders, such as 52 Cu, 38 Zn, 10 Ag, which melts at 820°C. This is actually a silver-bearing brass, which gives the name "brazing" to the process. Zinc oxide, ZnO, is a white pigment, and "blue powder," a colloidal dust of small spheres coated with oxide, is used in paints for protecting ships. Zinc oxide, ZnO, with 0.5% ferric oxide, Fe2O3 is the active ingredient in calamine lotion, which soothes irritated skin. Zinc appears in very small amounts in the mineral supplements favored by those who think eating it will improve the health. Zinc is required in the diet in such small amounts that it is always present in a normal diet without the need for supplementation. Zinc appears to be rather non-poisonous, though in large quantities it is carcinogenic. It is used in several lotions applied topically. If zinc oxide is breathed, the strange nervous malady "oxide shakes" seems to result.
Physical Properties of Zinc
Zinc is a bluish-grey metal covered by a protective transparent layer of basic carbonate in air. A sheet of zinc looks very much like a sheet of aluminium, but it is more than twice as heavy, and does not bend easily. Zinc is not very ductile or malleable, especially when pure. Its atomic number is 30, atomic weight 65.38. Its naturally occurring isotopes are 64 (49%), 66 (28%), 67 (4%), 68 (19%) and 70 (0.6%). Its density is 7.14 g/cc, electrical resistivity 6.16 μΩ-cm, heat capacity 0.0925 cal/g-K, and heat conductivity 0.268 cal/cm-s-K. Its coefficient of linear expansion is 40.0 x 10-6 per K. Zinc melts at 419.5°C and boils at 907°C. The heat of fusion is 24.09 cal/g. In the cast form, its tensile strength is only 4-12 ksi, but the cold work of rolling gives 28-36 ksi. Hard-drawn zinc has a strength of about 10 ksi. The Young's modulus is 12.4 x 106 psi. Zinc, at Mohs 2.5, is harder than tin or cadmium. Its crystal form is hexagonal close packed, with a = 0.266 nm, c = 0.494 nm. The ionic radius of Zn++ is 0.074 nm. The ionization potentials of zinc are 9.36V and 17.89V.
Chemistry of Zinc
The electron configuration of zinc is 1s22s22p63s23p63d104s2.It shows a valence +2 in their compounds. Pure zinc shows almost no reaction with water or dilute acids, because of the formation of a thin layer of hydrogen gas on its surface, or "polarization." Impure zinc, or zinc in the presence of copper or platinum, reacts readily with the evolution of hydrogen. Arsenical zinc is used for making hydrogen in the chemical laboratory. The reduction potential of zinc is -0.76V, which is relatively reactive.
Zinc forms a hydroxide, Zn(OH)2 that can dehydrate to form the anhydrous oxide ZnO, form Zn++ salts in acid solution, or zincates, ZnO2 --, in alkaline solution. In the former case, an acid strips off the OH- to form water, and in the latter case the alkali strips off the H+ with the same end in mind. Therefore, zinc is amphoteric, like aluminium. ZnO will not dissolve in water to form the hydroxide, but will dissolve in acids to form zinc salts. In the presence of ammonium ion, zinc forms a tetrammino complex, Zn(NH3)4--.
Zinc chloride is hydrolyzed in solution, but does not give off HCl on evaporation. Its use as a soldering flux has already been mentioned. It is a drying agent and a catalyst, and is used for preserving wood. It is not as good as creosote for this purpose, and is easily leached out. If zinc chloride solution is swallowed, the whites of egg are an antidote. Zinc salts are poisonous, if not violently so. Zinc sulphate is less toxic, and is used in medicine as an astringent and antiseptic, and also as a mordant, because gelatinous zinc hydroxide is produced when it is hydrolyzed. Zinc sulphide, remarkably, is white, unlike all other sulphides. When it is found in nature as sphalerite, however, impurities darken it and it is even called Black Jack. When pure, it has a large band gap and there are few charge carriers. Impurity centers phosphoresce easily, and ZnS is one of the most useful phosphors, also responding to alpha particle bombardment as well as to electron bombardment and ultraviolet. As with most phosphors, the ZnS is merely the vehicle that holds the fluorescence centers provided by impurities. Zinc cyanide, Zn(CN)2, is only very slightly soluble, a fact used in the cyanide process for the production of gold and silver. The nitrate, carbonate, acetate, borate and stearate also have their uses.
Mineralogy and Production of Zinc
The source of most zinc is the sulphide, sphalerite, ZnS. Sphalerite is closely associated with galena, PbS, pyrites, FeS2 and other sulphides in hypogene ore deposits, and must be separated from them in smelting. Sphalerite is also called blende, which means "blind" or "deceiving," because although resembling galena and occurring with it, yields no lead. Since many minerals may do this, it may specifically be called Zinc Blende. Sphalerite, indeed, is from the Greek for "treacherous." Another term is Black Jack, from its typical dark appearance, though the pure substance is white. Good crystals can be transparent, with a resinous lustre, and make attractive gems. Transparent sphalerite is often green. Sphalerite is cubic in crystal structure. ZnS crystallized in hexagonal crystals instead is called wurtzite.
The usual process for the extraction of zinc from sulphide ores is as follows. First, the ore must be roasted in air to burn off the sulphur as SO2. When this gas is released to the atmosphere, it devastates the vegetation over a considerable area. However, it can be recovered and made into sulphuric acid, which can be used in the smelting process. A good deal of acid is left over for sale to improve the economics. The oxide from the roasting is then leached with the acid produced from the sulphur to produce zinc sulphate, which is then electrolyzed to the metal. This produces a good, pure zinc and is the currently favored process. 
Alternatively, the oxide from roasting can be mixed with carbon, from anthracite coal for example, and sintered if desired. When this is heated in a retort to about 1100°C, the zinc vapor is driven off and condensed in molten zinc. Under certain conditions, "blue powder" can be formed, that is a valuable product in its own right. It consists of colloidal zinc particles covered with ZnO, produced by the reaction of Zn with CO2. It will not melt to zinc metal. The condensed zinc from the retort is cast into ingots, and the material is called spelter. Before metallic zinc was produced in Europe around 1600, it was imported from India (and China, but was produced mainly in India) and called "spiauter" or something like that, which gave us spelter.
Spelter can be refined by liquation, where impurities are skimmed from the top, and the liquids separated into one containing lead and cadmium, and the other purified zinc. Better purification can be obtained by fractionaldistillation, since zinc vaporizes at a conveniently low temperature. There is also electrolysis, but zinc produced by this method is quite pure anyway. All purification costs money and loses zinc, so where less purity is satisfactory, it is allowed. Lead, copper and tin, for example, do not have a bad effect on brass. For die casting material, on the other hand, it is essential to reduce lead, copper and tin to very small amounts to avoid swelling and embrittlement.
The History of Zinc 
The history of zinc is peculiar. It was widely used before the metal was discovered. By adding zinc carbonate to copper ores that were being smelted, brass was the product. This, as we shall see, was a very desirable metal that had a color like gold, was easy to work, and corrosion-resistant. Brass coinage was issued in the time of Augustus, and is well attested. Brass was just another variety of aes, like bronze and copper itself. It was just a flavor, as lead and tin were flavors of lead, black and white. If zinc metal had been known then (and it might have been) it would be considered just another flavor of lead, which had so many, as copper did. In fact, brass was called aurichalcum, "gold-copper."
We saw above that the critical process in making zinc was reduction in a closed retort, away from the air, and the condensation of the resulting vapors. This was not a customary process in classical metallurgy, which was a matter of heating things in an open hearth and adding all kinds of magical ingredients. There was no way that zinc would run as a shining silvery stream from any such hearth, as lead, tin, copper, silver and gold did. It would be found as ZnO dust in furnace cracks, as impure blue powder, but never as a recognizable metal. The example of mercury, which was condensed from vapor, might serve as a hint, but zinc, unlike mercury, was chemically active and combined with oxygen. So, it is not impossible, merely unlikely, that anyone would have been inquisitive enough to try to keep the air away and condense zinc metal. If it had been done, the zinc would not even be attractive enough to make acceptable trinkets.
The retort was apparently hit upon in India, and zinc metal produced, which was traded with China, and may have been used in alloys. Although we have credited Paracelsus with the name, he was certainly not the discoverer, merely one of the workers at the frontiers of science. Apparently Agricola had heard of zinc too, and mentions it in his book, which was published before Paracelsus's was. There was, as usual, great confusion in the names of compounds in the absence of certain knowledgy of their composition. Cadmia furnacis was one variety of deposits of furnace dust that contained zinc, as well as the cadmium discovered much later. Cadmia fossilis was, apparently, Smithsonite or something similar, that made brass. The recognition of zinc as a particular metal occurred about 1600 in Europe, but its first use was some two thousand years earlier.
The early centers of zinc smelting in modern Europe were at Swansea (1720) and Bristol (1740) in England. Zinc ores were typically transported large distances for smelting. Today, when the SO2 is recovered, this allows the sulphuric acid to be produced near its markets, instead of at the distant mines, improving the overall profitability of the operation. Roasting of sulphide ores was an environmental disaster, it is a serious topic since most of countries all over the world care too much for its safety and ask for more high technology to make environmental friendly zinc productions !