Zinc: the essentials
Zinc-deficient animals require 50% more food to gain the same weight of an animal supplied with adequate amounts of zinc. Zinc is not particularly toxic and is an essential element in the growth of all animals and plants.
Plating thin layers of zinc on to iron or steel is known as galvanising and helps to protect the iron from corrosion.
Zinc: historical information
Centuries before zinc was recognized as a distinct element, zinc ores were used for making brass (a mixture of copper and zinc). A brass dating from between 1400-1000 BC has been found in Palestine. An alloy containing 87% zinc was found in prehistoric ruins in Transylvania. The smelting of zinc ores with copper was apparently discovered in Cyprus and was used later by the Romans. Metallic zinc was produced in the 13th century in India by reducing calamine (zinc carbonate, ZnCO3) with organic substances such as wool.
The metal was rediscovered later in Europe. William Champion set up a zinc industry in Bristol (England) in the 1740s. Other plants were established a little later in Belgium and Silesia.
Sometime prior to the autumn of 1803, the Englishman John Dalton was able to explain the results of some of his studies by assuming that matter is composed of atoms and that all samples of any given compound consist of the same combination of these atoms. Dalton also noted that in series of compounds, the ratios of the masses of the second element that combine with a given weight of the first element can be reduced to small whole numbers (the law of multiple proportions). This was further evidence for atoms. Dalton's theory of atoms was published by Thomas Thomson in the 3rd edition of his System of Chemistry in 1807 and in a paper about strontium oxalates published in the Philosophical Transactions. Dalton published these ideas himself in the following year in the New System of Chemical Philosophy. The symbol used by Dalton for zinc is shown below. [See History of Chemistry, Sir Edward Thorpe, volume 1, Watts & Co, London, 1914.]
Zinc: physical properties
Zinc: orbital properties
Isolation: zinc metal is readily available commercially so it is not normally necesary to make it in the laboratory. Most zinc production is based upon sulphide ores. These are roasted in industrial plants to form zinc oxide, ZnO. This may be reduced with carbon to form zinc metal, but in practice ingenious technology is required to ensure that the resulting zinc does not contain oxide impurities.
ZnO + C → Zn + CO
ZnO + CO → Zn + CO2
CO2 + C → 2CO
The other type of extraction is electrolytic. Dissolution of crude zinc oxide, ZnO, in sulphuric acid gives zinc sulphate, ZnSO4 in solution. Cadmium is an impurity and this is removed as a precipitate of cadmium sulphate by the addition of zinc dust. Electrolysis of the ZnSO4 solution using aluminium cathodes and lead alloyed with silver anodes gives pure zinc metal coated on the aluminium. Oxygen gas is liberated at the anode.
Very pure zinc may be formed from crude zinc by zone refining and single crystals can be grown with purities of better than 99.9999%.
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