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Atomistry » Tin » Production | ||||||||
Atomistry » Tin » Production » |
Tin ProductionProduction
0.01% Tin in placers and 0.1% in ores make the processing commercially useful; usually several percents of tin in ores are satellites of tungsten, zirconium, caesium, rubidium, rare-earth elements tantalum, niobium and other commercially valuable elements. Fresh raw materials are enriched: placers gravitationally, ores - by flotation only or gravity flotation. Concentrates with 50-70% tin are roasted for sulphur removing and treated by HCl for getting rid of iron. Such impurities as volframite (manganese tungstate (Fe, Mn) WO4) and scheelite CaWO4 are treated by HCl; then WO3×H2O is separated by NH4OH. Pig tin (94-98% Sn), which contains Copper, Lead, Iron, Arsenic, Antimony, Bismuth, is extracted by fusion with carbon in electrical or flame furnaces. On furnace outlet the pig tin is filtered at 500-600°C through coke or centrifuge for iron separation. The rest of iron and copper are removed by mixing molten metal with sulphur; solid sulphides of impurities float up, and are removed from the surface. Refining from arsenic and antimony are processed by the same method mixing with aluminium, and lead is removed by SnCl2. Sometimes Bismuth and Lead evaporate in vacuum. Electrolytic refining and floating-zone refining are used relatively seldom for obtaining high-pure tin.
Metallurgy of Tin
The essential reaction for obtaining tin from tinstone in the dry way is very simple. It consists in the reduction of the oxide with carbon thus,
SnO2 + 2C = Sn + 2CO. Owing, however, to the large proportion of foreign matter in the ore, considerable preliminary treatment is necessary, as well as the refining of the liberated metal. Processes are also in use for extracting the metal in the wet way and by electrometallurgy. Extraction of Tin in the Dry Way. - This process involves the following operations -
Purification of the OreTinstone may contain originally as little as between 1 and 2 per cent, of the metal; by the processes of purification this proportion is raised to between 50 and 70 per cent. The impurities in the ore have not simply a negative influence; they are positively injurious for two reasons - (i) The oxides of heavy metals, especially ferric oxide, would be reduced at the same time as the tin and the metals alloy with it. (ii) Silica and alkalis combine with stannic oxide to form respectively silicate of tin and alkali stannate, and in both cases tin would enter the slag. Moreover, tin ore frequently contains wolfram, (Fe,Mn)WO4, and scheelite, CaWO4, which must be removed, as well as arsenic and sulphur in the form of arsenical and common pyrites, which may be oxidised and vaporised by roasting. The processes by which the ore is purified are threefold (1) dressing, (2) roasting, (3) lixiviation.
Tungsten compounds are removed from the ore at this stage by means of a magnetic separator, which attracts the wolfram, or by an oxidising fusion with sodium sulphate, which converts the tungsten into sodium tungstate. Reduction of Stannic OxideThe reduction of stannic oxide to metal, by smelting it with charcoal or anthracite, is carried out either in shaft furnaces, constructed on the principle of the blast-furnace used in iron-smelting, or in reverberatory furnaces. The former method is adopted in Saxony, Bohemia, Finland, Bolivia, Burma, Siam and the Malay Peninsula, Banca, Billiton, South China, and Japan; the latter in England, France, Germany, Spain, Mexico, California, and the Malay Peninsula.A shaft furnace, as used in Saxony, is about 10 ft. high, is built of granite or gneiss, and is lined with granite. It is trapezoidal in cross-section and tapers towards the base. The blast is conveyed by two tuyeres which enter the furnace at the back. The molten products pass through the "eye" at the base of the furnace into the forehearth, where the slag and tin separate from each other. The metal obtained in this process contains about 97 per cent, of tin, the rest being copper and iron; the slag consists chiefly of silica, ferrous oxide, and stannic oxide, and may contain as much as 30 per cent, of the latter substance.
Recovery of Tin from SlagsThe tin contained in slags is either in the form of mechanically enclosed metal, or as stannic oxide combined with silica. The free metal may be recovered by crushing the slag and washing it, or by the settling out of the metal in the molten condition. The metal is obtained from the combined oxide either by reduction or precipitation. Reduction consists in resmelting with coal after the addition of a stronger base, such as lime, which liberates the stannic oxide from the silica; precipitation is smelting with iron, which displaces tin, forming ferrous silicate.Refining of TinCrude or raw tin from the shaft or reverberatory furnace contains small quantities of iron, lead, antimony, and arsenic, which are removed by the process of refining. This consists of liquation and boiling and tossing.Liquation is the gradual heating of the metal till the pure and more fusible tin melts and runs away, leaving behind the less fusible liquation-dross. It is carried out in Saxony and Bohemia directly the metal has been tapped from the forehearth of the shaft furnace, upon the liquation-hearth, which is simply an inclined plate with ridges upon it, down which the purified molten metal runs. In the English process the tin is remelted and liquated in either the same or a specially constructed reverberatory furnace, and then boiled in the refining pot or kettle. This boiling consists in stirring with a bundle of green twigs the molten metal kept hot by a fire underneath the pot. The dry distillation of the wood generates gas which keeps up a bubbling action within the molten metal and secures its exposure to the air. The dross or boil-scum is removed from the surface of the metal. Tossing consists in removing the liquid metal from the pot in a ladle and pouring it back again from a height. This likewise secures aeration and oxidation of impurities, which are similarly removed. For the extraction of tin in the wet way various chemical methods have been devised, but they have been applied chiefly to the recovery of tin from tin-plate, and result in the preparation of tin salts rather than the metal itself. Obsolete methods consisted in dissolving the tin from the plate in acid or alkali and then precipitating the metal from solution with iron or zinc, or preparing the oxide and smelting it with coal. Among the methods now in use is that of attacking the plate with chlorine gas and obtaining a solid hydrate of stannic chloride. An electrolytic method consists in packing loosened chips of tin-plate in wire baskets immersed in hot caustic soda solution. These baskets serve as anodes, whilst the iron vessel containing the soda is made the cathode. The tin is obtained as a spongiform mass on passing a current of 1.5 volt. Electrometallurgical Processes
Electrometallurgical Processes have been applied to the extraction of tin from its ores and slags, but without much success; the metal is recovered from tin-plate, however, and is also refined successfully by this means. Both acid and basic electrolytes, i.e. solutions of tin salts and of stannates or thiostannates respectively, have been used, but the latter solutions yield the better results. A solution proposed by Borchers contains 3 to 5 per cent, of sodium stannate and 12 to 15 per cent, of common salt. The addition of the salt confers high conductivity on the solution, and enables the tin to be obtained as a spongy metallic deposit when a current of 50 to 150 amperes at 2 to 3 volts is employed, with the solution at a temperature of 40° C. to 50° C. Tin has been refined electrolytically through the medium of a solution of sodium thiostannate.
Commercial tin reaches a high state of purity; it seldom contains less than 99 per cent, of the metal. Although all tin has been refined, the best quality is known as refined tin. It is made form the purest ores and submitted to a lengthened process of refining. The ordinary quality of tin is cast into moulds and known as block tin. The purity of tin may be judged by melting it and casting it in an ingot mould. The ingot should be smooth, bright, and rounded; small impurities will sharpen its edges and cause it to " frost " over on solidifying, whilst much impurity will give the metal a yellow or purple tinge. Grain tin is produced from refined tin by heating it to a temperature a little below its melting-point to induce crystallisation and make it brittle. It is then broken by a hammer or dropped from a height. The masses thus obtained show a columnar structure. The London Metal Exchange recognises two classes of tin - Class A containing not less than 99.75 per cent, and Class B not less than 99 per cent, of the metal; and these official brands include the following - Straits, Australian, Banca, Billiton, English, German, and Chinese. |
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