Tin Production

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.

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 -

1. Purification of the ore.
2. Reduction of stannic oxide.
3. Recovery of tin from slags.
4. Refining of tin.

Purification of the Ore

Tinstone 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.

1. The dressing of the ore secures the removal of a large proportion of siliceous gangue. The ore is crushed and washed to remove the lighter particles of gangue, this being easily effected owing to the high density of the stannic oxide.
2. The roasting process converts sulphur into sulphur dioxide which escapes, and arsenic into arsenious oxide, which is condensed, though some of this element remains as ferric arsenate, which needs to be specially reduced. The ore, after roasting, contains besides stannic oxide, oxides of iron, copper, and bismuth, sulphates of copper and iron, tungsten compounds, and small quantities of unaltered sulphides and arsenides.
   
   

   Furnace for tin ores, with fixed roasting chamber (Saxony). A. Opening through which the ore enters the chamber after being roasted on the roof. B Roasting chamber. C. Fireplace. D. Chimney. E. Flue by which arsenious oxide passes to condensing chambers.

   The roasting process is carried out in furnaces provided with fixed or movable roasting chambers.
   
   A furnace with a fixed roasting chamber, such as is employed in Saxony, is shown in Fig. The ore is dried on the roof of the furnace and then dropped through the opening A into the roasting chamber B. A sliding door leading to the chimney D can be opened or closed at will, and the same applies to the damper leading to the flue E. By this flue the arsenious oxide vapour passes to chambers where it is condensed.
   
   An important form of movable roaster is Oxland and Hocking's revolving calciner, which is used in Cornwall for ores rich in sulphur and arsenic.
   
   

   Oxland and Hockings revolving calciner (metallurgy of tin). A Hopper. B Revolving cylinder down which the ore slides. C Fireplace. D Condensing chambers for arsenious oxide. E Chamber into which roasted ore falls.

   The calciner, shown in Fig., consists of a cylinder, B, 30 to 40 feet long and from 4 to 6 feet in diameter; it is built of boiler-plate and lined with firebrick. The inclination of this cylinder depends upon the rate at which it is desirable to roast the ore; it makes from 3 to 8 revolutions per minute. The ore, which is dried on the roof of the condensing chamber, is fed in through the hopper A, and is distributed within the revolving cylinder B by means of four longitudinal ridges. Hot gases from the fire at C are drawn with air up the cylinder, and the arsenious oxide formed by the volatilisation and oxidation of the arsenic in the ore is condensed and deposited in the series of chambers, D, shown at the right of the figure, while the calcined ore drops through the opening in the arch fixed at the lower end of the cylinder into the chamber E. From 20 to 25 tons of ore can be roasted per diem in this calciner, 1 cwt. of coal being required per ton of ore.
The roasted ore is next lixiviated to remove soluble oxidation products. If copper sulphate is present it is washed out with water, after which hydrochloric acid is employed to extract oxides of copper, iron, and bismuth. The copper is precipitated from solution by iron, and the bismuth by water, as oxychloride.

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 Oxide

The 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.

Reverberatory furnace for tin-smelting. I. Elevation, II. Plan. A Hearth or bed of furnace. B. Fireplace. C. Door for introducing charge D. Door for working charge. E. Kettle for receiving tin. F. Kettle for receiving refined tin after liquation.

A reverberatory furnace, such as is used in Cornwall, consists of a shallow elliptical bed, which may be 14 to 18 ft. long and 8 to 12 ft. wide (A, Fig.). The bed is made of firebrick and lasts about three months; it slopes towards the middle and to the taphole. The charge is introduced by a ]door C opposite the taphole and worked through another door opposite the firebridge. The whole process lasts from six to twelve hours, and the impure tin is run off into a kettle or float placed to receive it. The slag produced is variable in composition. Some is so poor in tin that it may be thrown away; other contains " prills " of metal, or is rich enough in stannic oxide to merit further treatment.

Recovery of Tin from Slags

The 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 Tin

Crude 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 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.