Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/71

Rh METALLURGY 61 gives the highest temperature. Wood or coal is indicated when a voluminous flame is one of the requisites. Obviously fuel of the same kind and quality gives a higher calorific intensity when, before use, it is deprived by drying of its moisture, or when it is used in conjunction with a hot instead of a cold blast. This latter prin ciple, as every one knows, is largely discounted in the manufacture of pig-iron, where nowadays coal, with the help of the hot blast, is made to do what formerly could only be efl ected with charcoal or coke. For further information see FUEL and IRON. Chemical Operations. In regard to processes of amalgamation and to wet-way processes, we have nothing to add to what was given in a previous paragraph ; J we therefore here confine ourselves, in the main, to pyro-chemical operations. The method to be adapted for the extraction of a metal from its ore is determined chiefly, though not entirely, by the nature of the non-metallic component with which the metal is combined. The simplest case is that of the reguline ores where there is no non- Tiietallic element. The important cases are those of GOLD, BISMUTH, and MERCURY (q.v.). Oxides, Hydrates, Carbonates, and Silicates. All iron and tin ores proper fall under this heading, which, besides, comprises certain ores of copper, of lead, and of zinc. In any case the first step consists in subjecting the crude ore to a roasting process, the object of which is to remove the water and carbonic acid, and burn away, to some extent at least, what there may be of sulphur, arsenic, or organic matter. The residue consists of an impure (perhaps a very impure) oxide of the respective metal, which in all cases is reduced by treat ment with fuel at a high temperature. Should the metal be present in the silicate form, lime must be added in the smelting to remove the silica and liberate the oxide. In the case of zinc the temperature required fur the reduction lies above the boiling point of the metal ; hence the mixture of ore and reducing agent (charcoal is generally used) must be heated in a retort combined with the necessary condensing apparatus. In all the other cases the reduction is effected in the fire itself, a tower- shaped blast furnace being preferably used. The furnace is charged with alternate layers of fuel and ore (or rather ore and flax, see be low), and the whole kindled from below. The metallic oxide, partly by the direct action of the carbon with which it is in contact, but principally by that of the carbonic oxide produced in the lower strata from the oxygen of the blast and the hot carbon there, is re duced to the metallic state ; the metal fuses and runs down, with the slag, to the bottom of the furnace, whence both are withdrawn by the periodic opening of plug-holes provided for the purpose. Sulphides. Iron, copper, lead, zinc, mercury, silver, and anti mony very frequently present themselves in this state of combin ation, as components of a very numerous family of ores which may be divided into two sections : (1) such as substantially consist of simple sulphides, as iron pyrites (FeS 2 ), galena (PbS), zinc blende (ZnS), cinnabar (HgS) ; and (2) complex sulphides, such as the various kinds of sulphureous copper ores (all substantially com pounds or mixtures of sulphides of copper and iron) ; bournonite, a complex sulphide of lead, antimony, and copper ; rothgiltigerz, sulphide of silver, antimony, and arsenic ; fahlerz, sulphides of arsenic and antimony, combined with sulphides of copper, silver, iron, zinc, mercury, silver ; and mixtures of these and other sul phides with one another. In the treatment of a sulphureous ore, the first step as a rule is to subject it to oxidation by roasting it in a reverberatory or other furnace, which, in the first instance, leads to the burning away of at least part of the arsenic and part of the sulphur. The effect on the several individual metallic sulphides (supposing only one of these to be present) is as follows : 1. Those of silver (Ag 2 S) and mercury (HgS) yield sulphurous acid gas and metal ; in the case of silver, sulphate is formed as an intermediate product, at low temperatures. Metallic mercury, in the circumstances, goes off as a vapour, which is collected and con densed ; silver remains as a rcgulus, but pure sulphide of silver is hardly ever worked. 2. Sulphides of iron and zinc yield the oxides Fe 2 3 and ZnO as final products, some basic sulphate being formed at the earlier stages, more especially in the case of zinc. The oxides can be reduced by carbon. 3. The sulphides of lead and copper yield, the former a mixture of oxide and normal sulphate, the latter one of oxide and basic sulphate. Sulphate of lead is stable at a red heat ; sulphate of copper breaks up into oxide, sulphurous acid, and oxygen. _ In practice, neither oxidation process is ever pushed to the end ; it is stopped as soon as the mixture of roasting-product and unchanged sulphide contains oxygen and sulphur in the ratio of 2 : S. The access of air is then stopped and the whole heated to a higher temperature, when the potential S0. 2 actually goes off as sulphurous- acid gas and the whole of the metal is eliminated as such. This method is largely utilized in the smelting of lead (from galena) and of copper from copper pyrites. In the latter case, however, the 1 Examples are given in GOLD and COPPER. See also SILVER. sulphide Cu 2 S has first to be produced from the ore, which is done substantially as follows. The ore is roasted with silica until a certain proportion of the sulphur is burned away as SO* while a corresponding proportion of oxygen has gone to the metal part of the ore. Now it so happens that copper has a far greater affinity for sulphur than iron has ; hence any locally produced oxide of copper as long as sufficient sulphide of iron is left, is sure to be reconverted into sulphide, and the final result is that, while a large quantity of oxidized iron passes into the slag, all the copper and part of the iron separate out as a mixed regulus of Cu 2 S and FeS (&quot; mat&quot;). This regulus, by being fused up repeatedly with oxidized copper ores or rich copper slags (virtually with CuO and silica), gradually yields up the whole of its iron, so that ultimately a regulus of pure subsnl- phide of copper, Cu 2 S (&quot;fine mat&quot;), is obtained, which is worked up for metal as above explained. 4. Sulphide of antimony, when roasted in air, is converted into a kind of alloy of sulphide and oxide ; the same holds for iron, only its oxysulphide is quite readily converted into the pure oxide Fe 2 3 by further roasting. Oxysulphide of antimony, by suitable processes, can be reduced to metal, but these processes are rarely used, because the same end is far more easily obtained by &quot;precipitation,&quot; i.e., withdrawing the sulphur by fusion with metallic iron, forming metallic antimony and sulphide of iron. Both products fuse, but readily part, because fused antimony is far heavier than fused sulphide of iron is. A precisely similar method is used occasionally for the reduction of lead from galena. Sulphide of lead when fused together with metallic iron in the proportion of 2Fe : IPbS yields a regulus ( = lPb) and a &quot;mat&quot; Fe 2 S, which, however, on cooling, decomposes into FeS parts of ordinary sulphide and Fe parts of finely divided iron. What we have just been explaining are only two special cases of a more general metallurgic proposition. According to Fournet, any one of the metals copper, iron, tin, zinc, lead, silver, antimony, arsenic, in general, is capable of desulphurizing or precipitating (at least partially) any of the others that follows it in the series just given, and it does so the more readily and completely the greater the number of intervening terms. Hence, supposing a complete mix ture of these metals to be melted down under circumstances admit ting of only a partial sulphuration of the whole, the copper has the best chance of passing into the &quot; mat,&quot; while the arsenic is the first to be eliminated as such, or, in the presence of oxidants, ns oxide. Arsenides. Although arsenides are amongst the commonest impurities of ores generally, ores consisting essentially of arsenides are comparatively rare. The most important of them are certain double arsenides of cobalt and nickel, which in practice, however, are always contaminated with the arsenides or other compounds of foreign metals, such as iron, manganese, &c. The general mode of working these ores is as follows. The ore is first roasted by itself, when a part of the arsenic goes off as such and as oxide (both volatile), while a complex of lower arsenides remains. This residue is now subjected to careful oxidizing fusion in the presence of glass or some other fusible solvent for metallic bases. The effect is that the several metals are oxidized away and pass into the slag (as silicates) in the following order, first the manganese, secondly the iron, thirdly the cobalt, lastly (and very slowly) the nickel ; and at any stage the as yet unoxidized residue of arsenide assumes the form of a fused regulus, which sinks down through the slag as a &quot; speis.&quot; (This term, as will readily be understood, has the snme meaning in reference to arsenidesas &quot;mat&quot; has inregard to sulphides. ) By stopping the process at the right moment, we can produce a speis which contains only cobalt and nickel, and if at this stage also the flux is renewed we can further produce a speis which con tains only nickel and a slag which substantially is one of cobalt only. The composition of the speises generally varies from AsMe 3/2 to AsMe 2, where &quot; Me&quot; means one atomic weight of metal in toto, so that in general lMe = o?Fe + 2/Co + zNi, where x + y + z=.^ The siliceous cobalt is utilized as a blue pigment called &quot;smalte&quot;; the nickel-speis is worked up for metal, preferably by wet processes. Minor Reagents. Besides the oxidizing and reducing agents natu rally present in the fire, and the &quot; fluxes &quot; added for the production of slags, there are various minor reagents, of which the more im portant may be noticed here. One namely, metallic iron as a desulphurizer has already been referred to. Oxide of lead, PbO (litharge), is largely used as an oxidizing agent. At a red heat, when it melts, it readily attacks all metals, except silver and gold, the general result being the formation of a mixed oxide and of a mixed regulus, a distribution, in other words, of both the lead and the metal acted on between slag and regulus. More important and more largely utilized is its action on metallic sulphides, which, in general, results in the formation of three things besides sulphurous acid gas, viz., a mixed oxide sing includ ing the excess of litharge, a regulus of lead (which may include bismuth and other more readily reducible metals), and, if the litharge is not sufficient for a complete oxidation, a &quot;mat&quot; comprising the more readily sulphurizaMe metals. Oxide of lead, being a most powerful solvent for metallic oxides generally, is also