Page:The American Cyclopædia (1879) Volume XI.djvu/437

 tfETALLURGY 425 ed by car- by the al- occurring oxides that are are iron, tin, zinc, and lead, d tin are exclusively oxides. . oxidized condition as car- md also as sulphide. Lead carbonate, phosphate, Ac. Metallic zinc is al- ways produced from the oxide; the sulphide must therefore first be converted into oxide before it can be treated for the reduction of the metal. Lead may be prepared both from the oxide and the sulphide, and accord- ing to the process employed the sulphide is either treated as such (see below) or oxidized by roasting. The methods employed for re- duction by carbon consist either in heating the oxide in direct contact with coal or in expo- sing it to the action of heated carbonic oxide gas. Iron ore is smelted in a high-shaft fur- nace, and is reduced entirely by carbonic oxide generated by the partial combustion of the fuel in the lower part of the furnace, where the metaffic iron.is melted. Tin ore is either smelted in a low-shaft furnace or on the hearth of a reverberatory furnace in contact with fueL Oxidized lead ore is likewise treated in shaft furnaces. Zinc ore is mixed with fuel and heat- ed in day retorts. Since the reduction of iron ore is effected at a temperature below the point of fusion of the metal, the latter may be obtained in the solid state having the form of the pieces of ore used (iron sponge); but as ordinarily smelted the metal is fused after In this fusion it combines with carbon and silicon and forms cast iron. This product, although containing only about 93 per cent, of iron, has manifold applications in tike arts. In the preparation of wrought iron, which is nearly pure, cast iron is submitted to an oxidizing smelting to remove the carbon and silicon. Tin and lead are reduced at tem- peratures above their points of fusion, and are obtained in a molten state, while zinc is only reduced above its boiling point, and is obtained , which is condensed to a liquid. 2. rwbM* fty ike m*tmml Reaction of am, xf and SmlpAide. This reaction results, in tibe case of a few metals, in the formation of sulphurous acid gas and the separation of the metal, according to the following general for- mula: MS+2MO=M,+SO^ The principal examples of this mode of smelting are lead and copper. The sulphides of these metals are partially oxidized, and the oxide thus formed is intimately mixed at a high temperature with the unaltered sulphide, with the result given above. 3. The Replacement of one Metal by emotker. The chemical affinity which the met- ak possess for the non-metallic elements dif -
 * .-: .-:.::; :.: :i ~ : ~ :ir ?:r:r.,--

est affinity are capable, in many instances, of driving the weaker out of combination. Advantage is taken of this in the separation of many of the metals. When sulphide of lead is heated with metallic iron, metallic lead is produced with sulphide of iron, owing to the stronger affinity of iron for sulphur. Anti- mony is reduced from its sulphide in the same way. This replacement of one metal by an- j other is still more readily accomplished when the metal to be separated is in solution. It is only for the more valuable metals, as gold, silver, and copper, that the so-called wet pro- cesses, in contradistinction to the dry or smelt- i ing processes, are employed. Silver is pro- j duced in both ways. When associated with lead it is smelted by either of the lead pro- ceases given above, and is obtained alloyed with the lead. But when the ore or smelting product from which the silver is to be ex- tracted is free from lead or nearly so, the silver may be converted into sulphate and dissolved out by water, or into chloride and dissolved either by a solution of chloride of so- dium or hyposulphite of soda or Erne. From these solutions the silver may be precipitated by iron or copper, or it may be thrown down as sulphide and this decomposed by iron. Rich silver ores (sulphides) are sometimes added directly to the molten lead in the process of cupellation, in which case the silver is reduced by lead. Copper is also produced to a con- siderable extent by wet processes. It is ren- dered soluble by dissolving the native oxides or carbonates, or the oxide produced by roast- ing the sulphide by acid, and is precipitated by metallic iron. In gold and silver extraction processes, mercury is largely used to collect the finely divided metals, since it combines with them readily, forming an amalgam. When the metals occur native in the ore, they may be directly extracted with mercury. Some natural compounds of silver are decomposed by simple trituration in an iron pan or mortar. If mercury is present, the silver is taken up as soon as set free. In the more refractory ores the silver is converted into oxide or chloride by roasting before treating with iron and mer- cury. Metaffic gold may be rendered soluble and extracted from its ores by means of chlorine. From the solution thus obtained the gold is precipitated by iron (iron vitriol). The pro- duction of mercury from its principal ore (sul- phide or cinnabar) does not depend on any of the three processes given above. The ore is simply heated with access of air, when the sulphur is oxidized to sulphurous acid and the metal liberated in the form of vapor. (For detailed accounts of processes see the articles on the different metals.) Not unfrequently an ore after dressing may contain several metals, and the processes involved in the extraction and separation may be very complicated and include the operations of all three classes given above, (See Freiberg smelting process, under LEAD.) The amount of a metal in an ore is often so small that it is necessary to subject the ore to a concentrating process before it is treated for the extraction of the metaL Thus