Page:The American Cyclopædia (1879) Volume XV.djvu/792

 '62 TIN ring the dry season of eight months, the rainy season being devoted to smelting the ore. The workings are open pits and cuts ; and the ma- terial is conveyed away to be washed, water being collected by means of dams and reser- voirs. After the washing the ore is calcined, leached in water (to remove sulphates of iron and copper), smelted in shaft furnaces with charcoal, drawn into a purifying receptacle, and poled. The resulting tin is the best in the market. The government furnishes engi- neers, superintendents, and furnaces; all the rest is supplied by the workmen (Chinese), who receive about $5 09 for each 100 Ibs. of cast tin. The "Straits" tin comes from the British settlement of Malacca, and from vari- ous points on the Malay peninsula and the islands between it and Java. Drought and troubles with the Malays have temporarily re- duced the supply from this source. Its qual- ity varies according to the locality of the mines and the skill of the metallurgical treat- ment ; but it is usually less pure than Banca tin. Metallurgy of Tin. The tin ore found in drift or alluvium is usually purer than that in veins, because the arsenides, sulphides, and metallic salts are decomposed and carried away by the action of water. The veinstuff as mined is usually rock or gangue containing dissemi- nated ore (sometimes as little as -J- of 1 per cent, of tin), and requires a careful preliminary concentration, the difficulty of which is en- hanced by the presence of heavy minerals (wolfram, bismuth, &c.), which must be re- moved to secure a pure metal as the result of smelting. Connected with the mechanical concentration there is usually a calcination, to convert heavy sulphides into oxides, which can be more easily washed away. The apparatus of concentration comprises launders, plane tables, buddies, percussion tables, jigs, &c. (See METALLURGY.) The theory of the reduc- tion of tin ore is simple. The stannic oxide must be deprived of its oxygen by contact with carbon at high temperature, and reduced to metallic form in fusion, while the earths and metallic oxides accompanying it must be col- lected in the slag. In practice the operation is embarrassed by several difficulties. One of these arises from the high temperature neces- sary for the reduction of the stannic oxide, at which temperature other metallic oxides, which should pass into the slag, are also partially re- duced and enter the metallic bath, or cause u salamanders " or " scaffolds " by chilling in the furnace. Hence the necessity of removing lead, bismuth, copper, antimony, arsenic, zinc, iron, tungsten, molybdenum, &c., as far as practicable, before smelting. There is also danger that the stannic oxide, which plays the part of an acid toward many bases, and of a base toward acids, may pass partly into the slag as ferrous or calcic stannate, or stannic silicate. The oxidability and volatility of tin are also sources of loss, to avoid which the shaft furnace is so constructed as to remove the metal, once reduced, as soon as practicable from the influence of the heat and blast. The earthy ingredients of the ore, in which usually silica predominates, tend to form u stiff" (not easily fusible) slags ; and, rather than add fluxes to counteract this evil, at the cost of an increase of the amount of zinc carried into the slag, it is common to smelt with little or no extra flux, producing a scarcely fused slag, in which more tin is mechanically caught and retained than is chemically combined with silica or the bases. This slag may be remelted or treated by me- chanical concentration, to extract the tin which it contains, in tine metallic grains. Wolfram, which cannot be completely washed out, either with or without preliminary roasting, and which if present in the smelting charge goes partly into the slag and partly as tungsten into the metallic tin, is sometimes removed by a preliminary smelting of the ore with sodic car- bonate or sulphate (Glauber's salt), by which a soluble tungstate of soda is formed, which can be leached out. Muriatic acid will leach out from roasted tin ore the chlorides of iron, copper, and bismuth. The melting of the con- centrated and purified tin ore may take place in a reverberatory or in a cupola furnace. The former is advantageously employed where coal is cheap and good. It loses less tin by oxida- tion than the shaft furnace, in which the blast acts more or less on the tin, and it requires less fuel for the production of a given amount of tin. Zirkel says the reverberatory consumes for each part of tin produced If part of coal and loses 5 per cent, of tin, while the shaft furnace consumes 3 parts of coal and loses 15 per cent, of tin. But when the ore is impure, the reverberatory furnishes an inferior tin. The greater product is due to the better oppor- tunity afforded for the grains of tin to settle from the slag into the bath, which in the shaft furnace must be quickly removed to prevent oxidation from the blast. But this oxidizing blast, on the other hand, removes more com- pletely arsenic, bismuth, &c. The principal ingredients added in the reverberatory are reducing agents (carbon), and sometimes, to counteract predominant silica in the ore, small quantities of slacked lime and fluor spar. This furnace is used in England, and also in Aus- tralia. In the cupola furnaoe, which is em- ployed on the continent of Europe and in the Indies, the additions, aside from the fuel, are chiefly stanniferous slags and residues from the same process, which serve to prevent the fine dressed ore from packing too closely in the furnace to permit the passage of the blast. The cupola furnaces are made comparatively small in section, and contracted near the tu- yeres, in order to secure the necessary tem- perature ; and to prevent the reduction of iron oxides, they are made low (in Saxony, T88 to 2-82 metres; in Banca, 1'26 to 2-82* metres). The hearth slopes at the bottom from the rear wall toward the breast, and the fused material, flowing down this slope, passes con-