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perhaps from zinke point, barb, prong, from Old High German zinko; akin to Old High German zint point, spike, tine; from its forming jags under certain temperatures
Minerals; Inorganic; Zinc-Group
It was proved by Hahnemann and his associates. Allen's Encyclop. Mat. Med. Vol. X, 176.
Description of the substance
History The separation of metallic zinc from its ores by pyrometallurgy is much more difficult than with other common metals, such as copper, lead, and iron, because the reduction of zinc oxide by carbon (C) proceeds spontaneously only above the zinc boiling point of 907° C (1,665° F). Efficient methods of condensing the vapour to liquid metal were not discovered until the 14th century AD. As an alloy constituent, however, zinc was in use well before that time. Brass, an alloy of copper and zinc, was produced by the Romans as early as 200 BC by heating copper, zinc oxide (ZnO), and carbon together. The zinc formed by the reduction of its oxide was absorbed into the copper and did not appear as a separate phase. Evidence suggests that zinc was first produced in quantity in India and China. At Zawar in Rajasthan, India, the remains of a smelting industry dating from the 14th century have been found. Although no written record exists, the process appears to have involved large numbers of small clay retorts, which were charged with zinc oxide and charcoal, placed in a setting, and heated. The exact method of condensing and collecting the zinc can only be surmised. Subsequent commercial procedures for zinc production all involved retort processes, the key overall reaction being initiated by external heat and involving the reduction of ZnO to zinc vapour by carbon, which was itself oxidized to carbon monoxide (CO). Important advances were made by William Champion in Bristol, Eng., in the mid-18th century, by Johann Ruberg in Silesia in the late 18th century, and by Jean-Jacques-Daniel Dony in Liège, Belg., in the early 19th century. Belgian-type horizontal retorts were operated in Britain as the main zinc-producing process for about 100 years starting in the mid-19th century. The daily output of each retort was about 40 kilograms (90 pounds), and several hundred retorts were banked together and fired by gas. The process was physically arduous in the extreme and suffered all the disadvantages of small-scale batch operation with high energy and labour costs. In the late 1920s a continuous vertical-retort process was developed in the United States. The retort was constructed of silicon carbide brick for high heat conductivity, with a rectangular cross section of two metres (six feet) by one-third metre and a height of 11 metres. The charge of roasted sulfide concentrate and anthracite coal was sized, briquetted, and preheated in a coking furnace prior to charging to the heated retort. Zinc vapour, removed with CO at the top of the retort, was condensed in a stirred molten-zinc bath. The output of each retort was about eight tons per day, and a typical plant operated about 20 retorts. A variant of the vertical retort, known as the electrothermic furnace, was also developed in the United States at about the same time. In this process, heat was supplied through the direct electrical-resistance heating of the coke in the charge. The most serious disadvantage of the improved retort processes was that they were restricted to ore concentrates with a low iron content, because high iron content in the feed caused plates of iron to form in the retorts. For this reason, zinc production by this means is now obsolete. Early attempts to devise a blast-furnace process for zinc production failed because of the difficulty of condensing zinc vapour from a gas containing substantial quantities of carbon dioxide. This difficulty was finally overcome in the mid-20th century by the development of the lead-splash condenser, a means of shock-cooling furnace gases and absorbing zinc vapour into solution in molten lead. This allowed the zinc blast furnace to become the main pyrometallurgical means of producing zinc. The zinc blast furnace should actually be referred to as the zinc-lead blast furnace, since, beginning with the first successful recycling of lead drosses from the condenser, blast-furnace operations evolved to the handling mixed zinc-lead feed materials up to a ratio of 2:1 zinc to lead. The major zinc-recovery process, electrolysis, made steady progress after commercial operation commenced around 1915-18. Prior to this, numerous attempts had been made, without success, following a patented method of sulfate electrolysis by the Frenchman Léon Letrange in 1881. The discovery that a high-purity sulfate electrolyte was required led to the eventual success of the process.Ores Zinc ores are widely distributed throughout the world, although more than 40 percent of the world's output originates in North America and Australia. The common zinc-containing minerals are the zinc sulfide known as zinc blende or sphalerite (ZnS), a ferrous form of zinc blende known as marmatite [(ZnFe)S], and a zinc carbonate known as calamine or smithsonite (ZnCO3). The geology of zinc deposits is complex. In most cases, hydrothermal mechanisms have occurred in which aqueous solutions were forced through porous strata at high temperatures and pressures to dissolve zinc, lead, and other minerals, which were finally precipitated as sulfides. The zinc content of mined ore is usually between 3 and 10 percent. Almost all ores contain the lead sulfide mineral galena and small quantities of cadmium sulfide. Chalcopyrite, and copper-iron sulfide, is often present. The most common gangue constituents are calcite, dolomite and quartz.
Zinc ores are recovered by many mining techniques, ranging from open-pit mining (mainly in the case of oxidized ore bodies, which are located closer to the Earth's surface) to the normal underground methods (used for the more deeply located sulfide ores). The most common underground method of ore extraction is cut-and-fill stoping, in which tunnels are dug to moderate depths, branching away from the mine portals. The small fraction of zinc sulfide minerals present in the ore makes beneficiation necessary in order to produce a concentrate suitable for treatment. The most common method for accomplishing this concentration is to isolate the sulfide mineral from the impure constituents, or gangue, by flotation separation. In this process, the ore initially is crushed to about 1.9 centimetres (0.75 inch), combined with water, and ground to less than 0.1 millimetre in a ball mill. The finely ground particles and water form a slurry that flows from the mill to flotation cells or tanks, where, in the presence of selected chemical reagents that create a suspension of air bubbles, the slurry is agitated by beaters. The mineral particles cling to the bubbles and float to the surface, forming an oily froth that is constantly skimmed, while the gangue is wetted by the action of the chemicals and sinks in the cell. The proper choice of frothing agents makes it possible to separate each constituent mineral of complex lead and zinc sulfides in a concentrated form.