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ZINC
Chemical symbol: Zn
Atomic number: 30
Atomic mass: 65.409(4) g/mol
Melting point: 419.53°C
Boiling point: 907°C
Density: 7.14 g.cm-3
Crystal structure: hexagonal
Hardness: 2.5 Moh’s scale
Lustre: metallic
Magnetic ordering: diamagnetic
ID marks: bluish white silvery colour
 
Sphalerite, the purplish ore mineral of zinc
 
 

Centuries before zinc was recognized as a distinct element, zinc ores were used for making brass. A prehistoric statuette containing 87.5% zinc was found in a Dacian archaeological site in Transylvania (modern Romania). Palestinian brass from the 14th to 10th centuries BC contains 23% zinc. The primitive alloys with less than 28 per cent zinc were prevalent in many parts of the world before India. Brass in Taxashila has been dated from third century BC to fifth century AD. A vase from Taxashila is of particular interest because of its 34.34 percent zinc content and has been dated to the third century BC. Recently two brass bangles belonging to the Kushana period are discovered from Senuwar (U.P.), which also shows 35 percent zinc.
The Indians were the first to know about metal zinc, the bluish-white, lustrous metal and smelt on a large scale. The testimony to it is the presence of huge numbers of retorts and ruins spread over a large area in old Zawar village in southeastern Rajasthan. References to medicinal uses of zinc are in the Charaka Samhita, which is believed to have been written as early as 300 BC in India.
Zinc was rediscovered in Europe by Marggraf in 1746. It got its name after German word ‘zinke’ for this metal. In English and French it became ‘zinc’, in German and Dutch ‘zink’, in Spanish ‘cinc’ and in Welsh ‘sinc’ (pronounced "shink"). The Greek word for zinc is ‘pseudargyros’, literally meaning "pseudo-silver" for its silvery lustre. In Russian, it is ‘tsink’. In India it is known as ‘Yashad’, Jasta, Jast, Naag in Hindi/ Sanskrit, Tunga in Tamil and Naagam in Malyalam.
Zinc is a bluish-white, lustrous, diamagnetic metal. In nonscientific contexts zinc is known as ‘spelter’. It is covered by a protective transparent layer of basic carbonate, hence, show a dull finish. Zinc makes up about 75 ppm (0.007%) of the Earth's crust. It is the 24th most abundant element in the Earth's crust. Soil contains 5 to 770 ppm of zinc with an average of 64 ppm. Seawater has only 30 ppb zinc. It has five stable isotopes. After iron, aluminum and copper, zinc is the fourth-most used metal, competing with lead. We probably use it every day. A sheet of zinc looks like an aluminum sheet but it is more than twice as heavy. The hardness of zinc at Mohs scale is 2.5. Zinc is brittle at ordinary temperatures but malleable at 100 to 150°C. Above 210°C, it becomes brittle again and can be pulverized by beating. Zinc is a fair conductor of electricity. For a metal, zinc has relatively low melting (419.53°C) and boiling points (907°C). Zinc is not very ductile or malleable, especially when pure. It is less dense than iron and has a hexagonal close-packed crystal structure. Pure zinc almost does not react with water or dilute acids, because of the formation of a thin layer of hydrogen gas on its surface, or "polarization”. It is relatively resistant to corrosion in air or water, and therefore, is widely used as a protective layer on iron products to protect from rusting. Five isotopes of zinc occur in nature.
Zinc is recovered from a number of different zinc ores. The types of zinc ores include sulfide, carbonate, silicate and oxide. Most significant of these ores are zinc sulphide or sphalerite i.e. (Zn,Fe)S, zinc carbonate or smithsonite (ZnCO3), zinc silicate or willemite (Zn2SiO4) and zinc oxide or zincite (ZnO). Ores of lead, zinc, cadmium and silver often occur together.
Sphalerite or zinc blende is the most important zinc ore as it contains 64.06% zinc. It occurs mostly as veins. It shows brown, yellow, red, green and black colour, has uneven fracture and occur as colloform, euhedral crystals and granular masses. It has 3.5 to 4 hardness, brownish white streak, 3.9 to 4.2 density and adamantine lustre. It is fluorescent and triboluminescent. It occurs in isometric-hextetrahedral crystal system, which is analogous to diamond. Sphalerite is a polymorph (many shapes but same chemistry) and has two minerals i.e. wurtzite and matraite. It is sometimes difficult to identify sphalerite due to its variable colour, lustre and crystal habit. The smithsonite, zinc carbonate, commonly shows various shades of green and purple; has a silky to pearly lustre; and occurs in trigonal crystal system and botryoidal form. It shows white streak, hardness of 4 to 4.5 and specific gravity of 4.4. The willemite is somewhat rare zinc mineral. It has vitreous to resinous lustre, hardness of 5.5; specific gravity from 3.9 to 4.2, trigonal crystal system, conchoidal to uneven fracture and white streak. It fluoresces a bright green colour under ultra-violet light. Some willemite specimens even show phosphorescence. Phosphorescence is the ability of a mineral to glow after the initial light is removed. The zincite is also rare zinc mineral, which is orange-yellow to deep brown or red in colour. It shows orange-yellow streak, conchoidal fracture, hardness of 4 and specific gravity between 5.4 and 5.7. It occurs in hexagonal crystal system forming small crystals and masses.
Zinc ores occur in a variety of geological environment Principal lead-zinc deposit types include carbonate-hosted ores, sandstone-hosted ores, shale-hosted (or SEDEX) deposits and volcanogenic (VMS) deposits. Zinc mines are throughout the world with the largest producers being Australia, Canada, China, Peru and United States. Most zinc mines are underground (80%) but some are of the open pit type (8%). China produced 2,600,000 tonnes i.e. one-fourth of the global zinc output in 2006 while India produced 420,000 tonnes.
The raw material for the production of zinc is zinc concentrate, which is the result of a flotation process after the ore is mined and milled. Zinc ore contains 1-15% zinc whereas the concentrate contains about 55% zinc, 6.5% iron and 32% sulphur and other elements. The process begins with roasting of the concentrate. At a temperature of 950°C, oxidization of zinc, iron and sulphur occurs. Sulphur is collected as SO2 for making sulphuric acid. Iron and zinc oxides, reduced to powder form, are leached with dilute sulphuric acid. The refined solution is electrolyzed in a cell fitted with lead anode and aluminum cathode. The cathode is immersed in the solution for 48 hours during which zinc is deposited on the cathode from which it is stripped. The final process takes place in the foundry where zinc takes its final form as a metal with a purity of 99.95%. The zinc is then cast into various forms such as ingots or plates for marketing.
In ancient India, production of zinc metal was common. It was mainly used in making brass whereas the oxide was used in medicines. There are references of medicinal uses of zinc in the ‘Charaka Samhita’ (300 BC). Process of the production of metallic zinc has been described in several Sanskrit works e.g. ‘Rasarnavam Rastantram’ (500-100 BC), ‘Rasratnakar’ (2nd century AD), ‘Rasprakash Sudhakar’ (12th century AD) and ‘Rasaratna Samuchchaya’ (13th century AD). The ingredients for zinc production mentioned in the last book include lac, bark of ‘pipal’ tree, ‘harad’, ‘haldi’, resin, salt, borax and ‘kharpar’ (zinc ore); and grinding them with cow milk and purified butter to prepare balls to put into the retorts.
A team of scientists from British Museum (London), HZL (Udaipur) and Baroda University (Gujarat) unearthed this ancient Indian technique of zinc smelting at old Zawar, Udaipur district, Rajasthan, which is now considered to be the oldest site of industrial zinc production in the world. During the process, typical slag is not produced because of small-sized retorts. Ore was roasted to convert zinc sulphide to zinc oxide; mixed with reducing agents and fluxes (dolomite/ salt) and filled-in cylindrical retorts (brinjal-shaped, tapering at one end). The retorts are about 20 to 35 cm long and 8 to 12 cm in diameter with about 1 cm thick walls. A small diameter tube was fixed onto the open end of the retort for condensation of zinc vapours through this. The retorts are fitted upside down in perforated plate, to be fired in-situ, in a closed furnace. By controlled-firing the zinc gets evaporated, condensed and collected at the bottom. Assuming 20-25% losses of zinc, the quantity obtained was about 300 to 400 g of zinc metal per retort or 10 to 15 kg per smelt of the furnace of 36 retorts. Over 130,000 tons of residues remain at Zawar which indicates the extraction of the equivalent of 1,000,000 tons of metallic zinc and zinc oxide. Radiocarbon age determinations of launder wood from the old lead-zinc mines of Zawar Mala yielded an age of 2180+/- 35 years (Hindzinc Tech, HZL, V.1, No.1, 1989).
Zinc has many uses for mankind. It’s most common use is as an anti-corrosion agent by way of Galvanization, a process of its coating on iron to protect it against corrosion. Zinc is coated electrochemically or as molten zinc by hot-dip galvanizing or spraying. A protective surface layer of oxide and carbonate is formed as zinc corrodes. With an electrochemical potential of -0.7618 volts, zinc makes a good material for anode in batteries. Powdered zinc is used in alkaline batteries. Another important use of zinc is in making different alloys e.g. brass, nickel silver, typewriter metal, bronze, German silver and aluminum solder. Brass is an attractive golden-coloured alloy of copper and zinc (3% to 45%). It is more ductile and strong and has better resistance to corrosion, which makes it very useful in communication, hardware and musical instruments etc. Zinc oxide is used as white pigment in paints. Zinc sulfide is used in making luminous dials, fluorescent lights and TV screens.
Lotions made of calamine, a mix of Zn-(hydroxy-)carbonates and silicates, are used in treating treat skin. Zinc is believed to possess anti-oxidant property, which protect against premature aging of the skin and muscles. Brain cells, salivary gland, prostate, immune system and intestine secrete zinc. Effect of this metal (zinc gluconate glycine and zinc acetate in lozenges) on bacteria and viruses is well-known. Signs of zinc deficiency include hair loss, skin lesions, diarrhea and wasting of body tissues.
India has zinc ore resources (UNFC) of 522580 thousand tonnes as on 1.4.2005 located in different States. Important zinc-lead deposits, mostly localized within Precambrian peninsular shield, include Zangamrajupalle and Gollapalle in A.P.; Amjhore within Vindhyan rocks in Bihar; Amba Mata in Delhi Supergroup of rocks in Gujarat; Kolari within Sakoli Group of rocks in Maharashtra; Agucha within Banded Gneissic Complex; Deri and Kayar-Ghugra within Delhi Supergroup; Rajpura-Dariba, Devpura, Samodi, Sindesar and Tiranga within Pur-Banera belt of Pre-Aravalli; and Paduna and Zawar within Aravalli Supergroup of rocks in Rajasthan; Mamandur within Peninsular Archaean Complex of rocks in Tamil Nadu; and Gorubathan within Extra-peninsular Daling Formation (Pre-Cambrian - Cambrian) in West Bengal.
Zinc production in India initially was done by government through a public sector company i.e. Hindustan Zinc Limited. It was the biggest company in India, which took care of mining to extraction of zinc. In April 2002, HZL was privatized. Vedanta Group (still named HZL) is now conducting mining and production zinc and associated metals in the country. Reserves and resources of 232.3 Mt of ore containing 27.5 Mt of zinc-lead metal are present as on 31.3.08. HZL’s operations include three lead-zinc mines (Agucha in Bhilwara district, Zawar in Udaipur district and Rajpura-Dariba in Rajsamand district in Rajasthan), three zinc smelters (Debari and Chanderiya in Rajasthan; Vizag in A.P.). HZL’s zinc production increased from 283,698 tonnes in 2006 to 426,323 tonnes in 2008. The expansions at the Sindesar Khurd and Kayar mines will be completed in phases by early 2012. A zinc smelter of Binani is located at Alwaye in Kerala.

 
Map showing lead-zinc deposits and zinc smelters in India
 
Large open cast lead-zinc mine of Agucha
 
A general view of the ancient zinc smelting site of old Zawar
 
Large heap of ancient retort-refuge at old Zawar
 
Wall of a house built by spent retorts at old Zawar - a noble example of re-using the industrial waste by ancient Indians
 
Peacock - an artifact made of zinc metal
 
Ancient zinc smelting furnace unearthed at Zawar A spent retort found at old Zawar
Compiled by: A.K. GROVER, GSI, WR

 
 
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