CHAPTER VI.

MATERIALS AND PRODUCTS OF METALLURGICAL PROCESSES.

Ores. This term is applied by the metallurgist only to those minerals from which, on a large scale, metals may be obtained with profit. The ores must be supplied to the works in a suitable condition for smelting, the preliminary washing and dressing operations being carried out at the mine. Gangue, vein-stuff, or matrix is the extraneous earthy matter associated with the ore.

Ores contain the metals-(1) in the native or metallic state (examples-gold, silver, copper, mercury); (2) in combination with oxygen as oxides (for example, ferric oxide, tin oxide); (3) as oxides in combination with water (limonite, Fe,H,O,); (4) in combination with halogens (silver chloride); (5) in combination with sulphur, arsenic, and antimony (galena, PbS); (6) in combination with acids as salts (anglesite, PbSO); ores also occur in nature in a state of mixture; (7) as various combinations of the same metal (for example, azurite, 2CuCO,+ Cu(HO),); (8) as various combinations of more than one metal in one mineral species (for example, pyrargyrite, 3Ag,S,Sb,S,); lastly, (9) as several mineral species occurring together in the ore-deposit, galena and blende, spathic iron ore and iron pyrites.

The value of an ore depends upon the nature of the metal it contains and the difficulty with which its extraction is attended. Thus, iron ores containing less than 30 per cent. of metal are rarely smelted. Ores of iron, lead, or zinc are not considered rich unless they contain 50 per cent. of metal. Copper ores are rich when they contain 25 per cent. of metal, whilst ores yielding a few ounces of gold per ton are extremely valuable.

Fluxes. In order to separate the extraneous matter usually contained in a furnace charge of ore and reducing agent, certain materials must be added to form slag. These materials are known as fluxes.

In the smelting processes earthy, alkaline, and metallic sub

stances are used as fluxes. The alkaline fluxes are expensive, and are rarely used except in refining processes. The following are the principal earthy fluxes in use:

1. Lime, which acts as a powerful base for removing silica. Lime is used in a pure state or as carbonate. As a rule, limestone is used, especially if the furnace is large. Limestone is largely used in iron smelting, as most iron ores contain unsaturated silica. Dolomite, the carbonate of lime and magnesia, is specially useful.

2. Fluorspar (calcium fluoride), which is a useful flux for ores containing silica, barytes, or gypsum. With the two latter it easily fuses. It has also been used for removing phosphorus from iron ores in the blast furnace, but on account of its expense its employment, though advantageous under all conditions, is limited. The greater portion of the fluorspar is found unchanged in the slag; it facilitates the fusion, and increases the fluidity of the mass.

3. Barytes, which acts as a powerful base. It is a good sulphurising agent, for example, in the concentration of nickel speise, the copper present with the speise being removed and a regulus formed.

4. Alumina-bearing rocks, such as clay-slate, are used in smelting ores very rich in lime. As a rule, however, the charge is so mixed that argillaceous and calcareous ores are present in suitable proportions.

5. Siliceous materials, such as quartz, natural silicates, siliceous slags, are used when the ores contain an excess of basic materials that have to be removed.

Metallurgical Agents.-The metallurgical agents chiefly used are-1. Metals, either for decomposing the combinations of other metals, or as a means of concentration. Thus, iron is used for decomposing galena, zinc for desilverising argentiferous lead, and mercury for concentrating gold.

2. Metallic oxides are used for purifying the raw metal in refining processes. Thus, iron ore or hammer-scale is used for refining iron, and as a flux in lead smelting.

3. Slags serve to increase the fluidity of the furnace charge; or mixtures of slags may be used for recovering the metals they contain, and lastly for preventing "scaffolding" or choking of the furnace when pulverised ores are smelted. Their chemical action varies with their composition; basic slags are able to take up a large proportion of silica, while acid slags readily absorb bases. Alkaline slags are rarely used except in crucible processes, as in assaying.

4. Iron pyrites (FeS,) and magnetic pyrites (Fe,S) exert a reduc

ing action on oxides by giving up sulphur and forming sulphurous anhydride. Certain sulphides, such as zinc sulphide, infusible alone, may be rendered fusible by the addition of iron pyrites.

In the roasting of ores in furnaces or in piles the following chemicals are used:-(1) Salt for the formation of silver chloride in the treatment of silver ores; (2) Lime for the absorption or separation of the acids formed on roasting copper regulus, &c. ; (3) Ferrous sulphate in the treatment of silver ores containing a little iron pyrites.

Selection of Fluxes. In the choice of fluxes, when the gangue is siliceous, at least two bases must be added in order to form a fusible silicate. When the gangue is argillaceous, a single base is sufficient to form a double silicate. When the gangue is basic, that is, containing lime, magnesia, alumina, or iron, a siliceous flux must be added. Quartz alone is sufficient if the gangue already contains two or more bases, but, if there is but one base, clay or some other silicate must be added with the quartz. Siliceous slags may be advantageously used for the purpose.

Slags. The silicates formed in metallurgical processes by the combination of silica with the earths and metallic oxides are termed slags. As a rule, slags are smelters' refuse. In the refining of metals, however, slags are frequently formed by the oxidation of the metallic impurities. Such slags consist largely of metallic oxides, and are smelted again in order to recover the large proportion of metals they contain. These slags may be distinguished as cinder or scoria. The quantities of these slags obtained are not very great, and they may therefore be more conveniently regarded as ores.

The bases that occur in silicate-slags, mostly combined with the silica, are-lime, alumina, magnesia, rarely ferric and manganic oxides, ferrous oxide, manganous oxide, zinc oxide, more rarely baryta, and alkalies in slags of all blast-furnaces using charcoal as fuel, and, in addition to these, small amounts of the metals that are being smelted are invariably present, partly in chemical combination with the silica, and partly in other combinations mechanically mixed with the slag.

Some oxides and earths, such as zinc oxide and alumina, not only interfere with the fluidity of the slag, but also cause a scum to form which is difficult to separate from the slag. In many slags, calcium fluoride is found. This is frequently the case in the slag from cupola furnaces, as fluorspar is often used as a flux in re-melting pig-iron. The calcium fluoride melts without decomposition, and is merely in a state of mechanical mixture

with the slag. This is also the case with the calcium sulphide frequently met with in blast-furnace slags.

The chemical composition of slags is variable. If a slag is distinctly crystallised, it may generally be assumed to have a definite chemical composition, and a formula may be calculated for it. If, however, it is not crystallised, nor even crystalline, it must be regarded either as a mixture of several silicates or as a solution of one silicate in another.

Silica consists of one atom of silicon and two atoms of oxygen, so that the various silicates have the compositions given below. 2RO+ SiO, is the monosilicate or neutral silicate of the metallurgist. It contains in acid and base equal amounts of oxygen, whilst the neutral silicate of the chemist, which has the formula RO+ SiO,, is termed a bisilicate by the metallurgist, because the acid contains twice as much oxygen as the base. Proceeding from the monosilicate, the following series of formulæ are obtained for silicates:

For bases having the composition R,O, the formulæ are—

3

=

=

R.Si O Sesquisilicate

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The quantities of oxygen in the bases bear the following proportions to those in the acids in the various silicates:

10

1:3

2:3

Every sesquisilicate may be resolved into a monosilicate and a bisilicate; thus R,SiO is equal to R,SiO,+2(RSIO,) and R.Si,O is equal to R.Si,O,,+ 2(R,Si,O,). ́If a silicate contains but one base, it is termed a monobasic or simple silicate, but if two or more bases are present, it is termed a double silicate or a

30

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multibasic silicate, and in the formula the silicates of the various bases are united by a plus sign.

A classification of the silicates as ortho- and meta-silicates, based upon their analogies to phosphates, was suggested by Odling; but it has not come into use amongst metallurgists.

*

The fusibility of slags is dependent on the amounts of silica and bases they contain. The subsilicates are very fusible, forming a limpid liquid; they consolidate very rapidly and split up in so doing. They have usually a dark colour and, on account of the large proportion of bases they contain, a high specific gravity. The monosilicates are less fusible, and do not form so limpid a liquid. The bisilicates are less fusible still; they form a viscous mass which may be drawn out into threads. They are usually glassy and consolidate slowly. The trisilicates also flow very slowly, consolidate slowly, and require a very high temperature for their formation.

The most fusible silicates are those of the alkalies, then follow those of lead, iron, manganese, and copper, and finally, the earthy silicates. The simple silicates are always less fusible than the double silicates.

Economic Application of Slags.-The slags obtained in smelting metals other than iron are largely used as fluxes in various metallurgical processes for taking up either the silica or the bases, or, when they are ferruginous, for precipitating purposes. Occasionally the slags are moulded into bricks, and used for walls or other constructions in which no great weight has to be sustained.

The slags obtained in iron smelting are used, if not too glassy, for macadamising roads. For the ballast of railway lines, glassy slags may advantageously be used, as the mass is very permeable and keeps the sleepers dry. In the form of large blocks, they may be used for road making. They may be moulded into bricks, the best results being obtained when the moulded bricks are kept at a white heat for several hours in a tightly closed space, whereby they are devitrified and become considerably harder if they are allowed to cool slowly under a cover of coal dust and ashes. Good bricks may be made of granulated slag mixed with lime, the proportions being usually 10 parts of slag-sand to I part of lime. Blast furnace slag, if not too acid, may be burnt in a state of powder with lime, and gives an hydraulic cement almost equal to Portland cement.

The manufacture of slag cement has of recent years acquired

* Phil. Mag., vol. xviii. (1859), p. 368.