Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/72

Rh 62 METALLURGY largely used for the separation of silver or gold from base metallic oxides. Metallic lead is to metals generally what oxide of lead is to metallic oxides. It accordingly is available as a solvent for so to say licking up small particles of metal diffused throughout a mass of slag or other dross, and uniting them into one regulus. This naturally leads us to consider the process of &quot; cupellation,&quot; which discounts the solvent powers of both metallic lead and its oxide. This process serves for the extraction of gold and silver from their alloys with base metals such as copper, antimony, &c. The first step is to fuse up the alloy with a certain proportion of lead, which is determined by the weight of base metal to be eliminated, and is always sufficient to produce a lead-alloy of low fusing point. This alloy is heated on a shallow dish-shaped bed of bone earth to red ness, and at this temperature subjected to the action of air. The base metals (copper, &c. ) are oxidized away, the first portions as an infusible scum containing little oxide of lead, the latter in the form of a solution in molten litharge. Lead is, in general, less oxidiz- able than the other base metals; hence the last instalment of liquid litharge which runs off is pure, and the ultimately remaining regu lus consists of silver and gold only. These latter may be separated by nitric acid or boiling oil of vitriol, which converts the silver into soluble salts and leaves the gold. Oxide of iron, and also binoxide of manganese, are used for the decarburation of pig-iron. The oxygen of the reagent burns the carbon of the pig into carbonic acid, while the metal of the reagent becomes iron and FeO or MnO respectively, the oxides uniting with the silica added as such, or formed by the oxidation of the silicon of the pig, into a fusible slag. Iron pyrites, FeS 2, is employed for the preliminary concentration of traces of gold diffused throughout slags or base ores. The reagent, through the action of the heat, gives up one-half of its sulphur, which reduces part of tlie metallic oxides present. The gold and silver unite with what is left of protosulphide of iron (FeS) into a mat, which is then worked up for the noble metals. Fluxes. Practically speaking, all ores are contaminated with more or less of gangue, which in general consists of infusible matter, and consequently, if left unheeded in the reduction of the metallic part of the ore, would retain more or less of the metal disseminated through it, or at best foul the furnace. To avoid this, the ore as it goes into the furnace is mixed with &quot;fluxes&quot; so selected as to convert the gangue into a fusible &quot;slag,&quot; which readily runs down through the fuel with the regulus and separates from the latter. The quality and proportion of flux should, if pos sible, be so chosen that the formation of the slag sets in only after the metal has been reduced and molten ; or else part of the basic oxide of the metal to be extracted maybe dissolved by the slag and its reduction thus be prevented or retarded. Slags are not, as one might be inclined to think, a necessary evil; if an ore were free from gangue we should add gangue and flux from without to producea slag, because one of its functions is to form a layer on the regulus which protects it against the further action of the blast or furnace gases. Fluxes may be arranged under the three heads of (1) fluor-spar (which is sui generis], (2) basic fluxes, and (3) acid fluxes. Fluor-spar owes its name to the facility with which it fuses up at a red heat with silica, sulphates of lime and barium, and a few other infusible substances into homogeneous masses. It shows little tendency to dissolve basic oxides, such as lime, &c. One part of fluor-spar liquefies about half a part of silica, four parts of sulphate of lime, and one and a half parts of sulphate of baryta. Upon these facts its wide application in metallurgy is founded. Carbonate of soda (or potash) may be said to be the most power ful of basic fluxes. It dissolves silica and all silicates into fusible glasses. On the other hand, borax may be taken as a type for the acid fluxes. At a red heat, when it forms a viscid fluid, it readily dissolves up all basic oxides into fusible complex borates. Now the gangue of an ore in general consists either of some basic material such as carbonate of lime (or magnesia), ferric oxide, alumina, &c., or of silica (quartz) or some more or less acid silicate, or else of a mixture of the two classes of bodies. So any kind of gangue might be liquefied by means of borax or by means of alkaline carbonate ; but neither of the two is used otherwise than for assay ing ; what the practical metal-smelter does is to add to a basic gangue the proportion of silica, and to an acid ore the proportion of lime, or, indirectly, of ferrous or perhaps manganous oxide, which it may need for the formation of a slag of the proper qualities. The slag must possess the proper degree of saturation. In other words, taking Si0 2 + ?iMeO (where MeO means an equivalent of base) as a formula for the potential slag, n must have the proper value. If n is too small, i.e., if the slug is too acid, it may dissolve up part of the metal to be brought out as a silicate ; if n is too great, i.e., the slag too basic, it may refuse to dissolve, for instance, the ferrous oxide which is meant to go into it, and this oxide will then be reduced, and its metal (iron in our example) contaminate the regulus. In reference to the problem under discussion, it is worth noting that oxides of lead and copper are more readily reduced to metals than oxido of iron Fe 2 3 is to FeO, the latter more readily to FeO than FeO itself to metal, and FeO more readily to metal than manganous oxide is. Oxide of calcium (lime) is not reducible at all. The order of basicity in the oxides (their readiness to go into the slag) is precisely the reverse. Most slags being, as we have si en, complex silicates, it is a most important problem of scientific metallurgy to determine the relations in this class of bodies between chemical composition on the one hand and fusibility and solvent power for certain oxides (CaO, FeO, SiO, &c.) on the other. Now the composition of a silicate can be stated in an infinite number of ways ; but there must be one mode of formulation which reduces the law to its simplest terms. The mode adapted by metallurgists is something like the following. If we start with the quantity H 2 C1 2 of muriatic acid or the quantity H 2 S0 4 of sulphuric acid, it is clear that to convert either into a normal salt we require such a quantity of base as will convert the H 2 of the acid completely into water; but the quantity of base that does so is that containing one atomic weight of oxygen. Hence it is reasonable to define the quantities K 2 of potash, 1 Na 2 of soda, 1 CaO of lime, MgO of magnesia, FeO of ferrous oxide, ^Al 2 3 ( = alO) of alumina, JFe 2 3 ( =feO) of ferric oxide, as representing each &quot; one equivalent &quot; of base also in reference to silica, although silica has a characteristically indefinite basicity. Most slags are alloys or com pounds of silicates of A1 2 3 or Fe 2 3, and of silicates of protoxides (CaO, &c. ), hence their general composition is ?i(RO + zSi0 2 ) + m[(fe or al)0 + a;Si0 2 ]. This introduction will enable the reader to understand the following mode of classifying and naming composition in silicates. Name. Formula. Oxygen Ratio. r iSiO 2 +!MO Base. Acid. 1 : 1 i !SiO,+lMO 1 : 2 1 III. Tri-silicates iSi0 2 4-lMO 1 : 3 5 The names are the metal lurgic ones ; scientific chemists designate Class I. as orthosilicates, Class II. as metasilicates, Class III. assesqui- silicates. In the formulae M stands for K 2, Ca, Fe, &c., or for al = fAl, fe = Fe, &c. ; or, shortly, MO for one equivalent of base as above defined. It should be possible to represent each quality of a silicate as a function of x, ~. and of the nature of the individual bases m that make up the RO and (fe or al) respectively. Our actual knowledge falls far short of this possibility. The problem, in fact, is a very tough one, the more so as it is complicated by the existence of aluminates, compounds such as Al 2 3 .3CaO, in which the alumina plays the part of acid, and the occasional existence of compounds of fluorides and silicates in certain slags. The following notes on the fusibility of simple silicates are taken from Plattner s researches. Of the lime silicates, the tri-silicate melts at 2100 C., the bi- silicate at 2150. Magnesia silicates are most refractory. The bi-silicate and tri- silicate melt in the oxyhydrogen flame at 2250. Of manganous silicates, the easily fusible bi-silicate is yellow or red ; the tri-silicate is more refractory. Of cuprous (Cu 2 0) silicates, the bi-silicate is violet, and melts pretty easily; the singulo-silicate is red, dense, and rather refractory. Cupric silicates, as slags, hardly exist, the CuO being always reduced to at least Cu 2 0. Lead silicates all melt readily into yellowish transparent glasses. But they have no standing as slags. As regards the ferrous silicates, the singulo-silicate (orthosilicate) fuses at 1790 (this is about the composition of iron-puddling slag) ; the bi-silicate is less readily fusible. Ferric silicates (unmixed) do not exist as slags, the Fe./)., being reduced in the fire to IFeO, although Fe 2 3 occasionally replaces part of the A1 2 3 in complex silicates. Alumina silicates are all infusible in even the hottest furnace fires. They begin to soften in the oxyhydrogen flame at about 2400. But certain aluminates, for instance the salt 3Ca0.1Al 2 3 according to Sefstrom, melt at furnace heats. The fusing points of mixtures of two simple silicates cannot be calculated from those of the components. In many cases it is lower than either of the latter two. Thus for instance most magnesia- lime silicates fuse, the bi-silicate combination (Mg, Ca)OSi0 2 most readily. Alumina silicates become fusible by addition of a sufficient pro portion of silicate of lime at about 1918. The singulo-silicate and bi-silicate combinations melt into grey glasses. Magnesia acts like lime, and so, in a more limited sense, do ferrous and manganous oxides ; but their double compounds with A1 2 3 and silica are more viscid when fused. Plattner s work is a bold attempt to deal synthetically with the problem here presented, but it does not go the length of even an approximate solution. No one seems to have done much to con tinue it ; hence in the meantime the metallurgist has, for his 1 Few slags contain more than traces of alkalies.