1902 Encyclopedia > Metallurgy

Metallurgy




METALLURGY, a branch of applied science whose object is to describe and scientifically criticize the methods used industrially for the extraction of metals from their ores. Of the large number of metals enumerated in the handbooks of chemistry, the vast majority, of course, lie outside its range; but it is perhaps as well for us to point out that in metallurgic discussions even the term " metallic," as applied to compounds, has a restricted meaning, being exclusive of all the light metals, although one of these, namely aluminium, is being manufactured industrially. The following table enumerates in the order of their importance the metals which our subject at present is understood to include; the second column in each case gives the chemical characters of the native compounds utilized, italics indicating ores of subordinate importance. The term " oxide " must be understood to include carbon-ate, hydrate, and occasionally (when marked in the table with *) silicate.

Metal. Character of Ores.
Iron Oxides, sulphide.
p \ Complex sulphides, also oxides,
Pi \ metal.
t Sulphide and reguline metal, buverJ chloride.
Gold Reguline metal.
T , \ Sulphide and basic-carbonate, sul-
L,eaa\ phale, &c.
Zinc Sulphide, oxide.*
Tin Oxide.
Mercury Sulphide, reguline metal.
Antimony Sulphide.
Bismuth Reguline metal.
Nickel and cobalt Arsenides.
Platinum and platinum metals... Reguline.
Aluminium Oxide,* sodio-fluoride.

"We have separated the last two from the rest because the methods used for their preparation are more of the character of laboratory operations, and because we do not mean to include these in our general exposition of metal-lurgic principles. The history of metallurgy, up to the most recent times, is obscure. It is only since about the beginning of this century that the art has come to be at all scientifically criticized; and in the case of the most important processes all that science has been able to do has been merely to put her stamp upon what experience has long found to be right. Great and brilliantly successful scientific efforts in the synthetic line are not wanting, but they all belong to recent times. Science, by its very nature, aims at publicity; empiricism at all times has done the reverse; hence a history of the development of the art of metallurgy does not and cannot exist. A few historical notes on the discovery of certain of the useful metals are given in the introduction to METALS (q.v.).

General Sequence of Operations.—Occasionally metallic ores present themselves in the shape of practically pure compact masses, from which the accompanying matrix or "gangue" can be detached by hand and hammer. But this is a rare exception. In most cases the " ore," as it comes out of the mine, is simply a mixture of ore proper and gangue, in which the latter not unfrequently predomi-nates so much that it is not the gangue but the ore that really occupies the position of what the chemist would call the impurity. Hence, in general, it is necessary, or at least expedient, to purify the ore as such before the libera-tion of the metal is attempted. Most metallic ores are specifically heavier than the impurities accompanying them, and their purification may be (and generally is) effected by reducing the crude ore to a fine enough powder to detach the metallic from the earthy part, and then washing away the latter by a current of water, as far as possible. In the case of a "reguline" ore, such as auriferous quartz, for instance, the ore thus concentrated may consist substan-tially of the metal itself, and require only to be melted down and cast into ingots to be ready for the market. This, however, is a rare case, the vast majority of ores being chemical compounds, which for the extraction of their metals demand chemical treatment. The chemical operations involved may be classified as follows:—

1. Fiery Operations.—The ore, along in general with some kind of "flux," is exposed to the direct action of a powerful fire. The fire in most cases has a chemical, in addition to its obvious physical function. It is intended either to burn away certain components of the ore—in which case it must be so regulated as to contain a sufficient excess of unburned oxygen; or it is meant to deoxidize ("reduce") the ore, when the draught must be restricted so as to keep the ore constantly wrapped up in combustible flame gases (carbonic oxide, hydrogen, marsh-gas, &c). The vast majority of the chemical operations of metallurgy fall into this category, and in these processes other metal-reducing agents than those naturally contained in the fire (or wind) are only exceptionally employed.

2. Amalgamation.—The ore by itself (if it happens to be a reguline one), or the ore plus certain reagents (if it does not), is worked up with mercury so that the metal is obtained ultimately as an amalgam, which can be separated mechanically from the dross. The purified amalgam is subjected to distillation, when the mercury is recovered as a distillate while the metal remains.

3. Wet Processes.—Strictly speaking, certain amalgama-tion methods fall under this head; but, in its ordinary acceptance, the term refers to processes in which the metal is extracted either from the natural ore, or from the ore as it is after roasting or some other preliminary treatment, by means of an aqueous acid or salt solution, and from this solution precipitated—generally in the reguline form—by some suitable reagent.

Few methods of metal extraction at once yield a pure product. What as a rule is obtained is a more or less impure metal, which requires to be " refined" to become fit for the market. We now pass to the individual con-sideration of the several steps referred to.

Comminution of Ores. —Assuming the ore to be given in the shape of large lumps, these must first be broken up into small stones (of about the size of those used for macadamizing a road) before they can go to the grinding-mill. This formerly used to be done by hand-work; nowadays it is preferably effected by means of an American invention called the stone-breaker (fig. 1). This consists essentially of two substantial verticaliron plates; one is fixed, the other is connected with an excentric worked by an engine so as to alternately dash against and recede from the former. The lumps of ore, in passing through this jaw-like contrivance, are broken up into smaller fragments fit for

FIG. 1.—American Stone-Breaker.

the mill. For the production of a coarse powder revolving cylinders are often employed. Two cylinders" of equal diameter and length, made of iron, steel, or stone, are suspended by parallel axes in close proximity to each other. The width of the slit between them can be made to vary according to the requirements of the case. The cylinders are made to revolve in opposite directions, so that the stones when run into the groove formed by their upper halves are drawn between them and are crushed into bits of a size depending on the least distance between the two surfaces. Exceptionally hard stones might bring the machine to a standstill or cause breakages ; hence only one of the two axes of rotation is absolutely fixed ; the cushions of the other are only held in relatively fixed positions, each between a couple of guiding rails, by means of powerful springs at their backs. The springs are made of alternate disks of india-rubber and sheet-iron, and yield appreciably only to very strong pressures. "When an exceptionally hard stone comes on, they yield and allow it to pass through uncrushed. Sometimes two sets of cylinders are arranged one above the other, so that the grit from the upper falls into the jaws of the lower set to receive further com-minution. The diameter of the cylinders is from a foot to a yard, their length from 9 inches to a yard, the velocity of a point on the periphery a foot to a yard per second. The quantity of ore reduced per hour per horse-power is about 5 cubic feet for quartz or other liard minerals, and about 14 cubic feet for minerals of moderate hardness.

For the production of a relatively fine powder the pounding-mill is frequently used, which, in its action, is analogous to a mortar and pestle. The mortar is a rectangular trough, while the pestle is replaced by a parallel set of heavy metal or metal-shod beams, which (by means of a revolving cylinder with cogs catching pro-jections on the beams) are lifted up in succession and then let fall by their own weight so as to pound up the ore in the trough. The ore is supplied from a prismatic reservoir with a sloping bottom leading into a canal through which the stones slide into the trough. A current of water, which constantly flows into the trough from below, lifts up the finer particles and carries them away over the edge of the trough into a settling tank.

The object pursued in powdering an ore is to prepare it for being purified by washing. But the velocity with which a solid particle tails through water depends on its size as well as on its specific gravity—an increase in either accelerating the fall; hence, where the difference in specific gravity between the things to be separated is small, the washing must be preceded by a separation of the ore-powder into portions of approximately equal fineness. This is often effected by passing the ore through a system of sieves of different width of mesh superposed over one another, the coarser sieve always occupying the higher position. Sometimes the sieves are made to "go dry," sometimes they are aided in their action by a current of water which, more effectually thau mere shaking, pre-vents adherence of dust to coarser parts.

Another contrivance is the "Drum " (fig. 2). A long perforated circular cylinder made of sheet-iron, open at both ends, is suspended, in a sloping position, by a revolving shaft passing through its axis. The size of the perforations is generally made to increase in passing from the upper to the lower belts of the cylinder. While the drum

pHir - - \ I a

FIG. 2.—Drum.

is revolving, the ore, suspended in water, flows in at the upper end, and in travelling down it casts off first its finest and then its coarser parts, the coarsest only reaching the exit at the lower end. The several grades of powder produced fall each into a separate division of the collecting tank.

so that the narrow end of No. 1 projects into the wider end of No. 2, and No. 2 similarly into No. 3. The drums are not perforated, but are armed inside with screw-threads formed of strips of sheet metal

The drum, of course, is subject to endless modifications. A very ingenious combination is H. E. Taylor's ' ' Drum Dressing Machine" (fig. 3). It consists of three truncated cone-shaped drums D, fixed co-axially to the same horizontal revolving shaft, fixed edgeways to the drum, The ore grit to be dressed is placed in a hopper A, and from it, by a worm B fixed to the revolving shaft, is being screwed forward into a short fixed truncated cone C projecting into the revolving drum No. 1, into which it flows in a constant current. The rotary motion of the drum tends to convey the ore along the spiral path prescribed by the screw-thread towards the other end, and from it into drum No. 2, and so on. But the ore in each drum meets with a jet of water E impelling it the opposite way, and the effect is that, in each drum, the lighter parts follow the water, and with it run off over the entrance edge to be collected in a special tank, while the coarser parts roll down the spiral path toward the next drum to undergo further parting. The tank or pit for drum 1 receives the finest and lightest parts, that of drum 2 a heavier, that of drum 3 a still heavier portion, while only the very heaviest matter finds its way out of the exit eud of No. 3 into a fourth receptacle.

Of the large number of other ore-dressers, only two need be men-tioned here.

The "Clausthal Turn-Table" consists of a circular table, the sur-face of which rises from the periphery towards the centre so as to form a very flat cone of about 170°, which is fixed co-axially to a ver-tical rotary shaft. At the apex of the table, surrounding the shaft, but independent of its motion, there is a circular trough of sheet zinc, divided into two compartments; one receives a stream of water carrying the ore, the other a supply of pure water. A large annu-lar trough of sheet zinc is placed below the periphery of the table, so as to receive whatever may fall over the edge. It also is divided into compartments, as shall be explained further on. Supposing the table to be at rest, a sector of about 60° of it would be constantly run over by the ore-mud out of the first compartment of the upper trough. This mud current would suffer partial separation into heavier and lighter parts,—rich ore resting in the higher and poorer in the lower latitudes, and a still poorer ore falling over the periphery into the lower trough. The same happens with the moving table; only each sector of such partially analysed ore under-goes further purification by passing through about 90° of water-shower. After passing this, it meets with a perforated fixed water-pipe going up radially to about half the radius of the table. This pipe also carries sweeping brushes, so that the belt of ore from the lower latitudes of the table is swept off into the corresponding section of the receiving trough. What of ore remains on the higher latitudes subsequently meets with a similar arrangement which sweeps it off into its compartment. If the table turns from the left to the right, and we follow the process, beginning at the left edge of the ore-mud compartment, it will be seen that a first sector of the receiving trough gathers the light dross, a succeeding one an intermediate product, a third the most highly purified ore. The "intermediate" is generally run into the ore-mud trough of a second table to be further analysed.

The crude ore-mud goes into action of the pump alternately tosses the particles up into the water and allows them to fall; the heaviest naturally come clown first, but what is most striking is that nothing will pass through the bed of galena but what is at least as heavy as galena itself. In a similar manner No. 2 and No. 3

In tho "Continuous Wash-Pumps" (Continuirliche Setzpumpe) of the Harz, three funnel-shaped vessels (one of which is shown in fig. 4) are set in a frame beside one another, hut at different levels, so that any overflow from No. 1 runs into No. 2 and thence into No. 3. Each funnel communicates below with its own compart-ment of a common cistern. Into each funnel a riddle with narrow meshes is in-serted somewhere near the upper end, while, beside the riddle, there is a pump of short range, which, by means of an excentric, is worked so that the piston alternately goes rapidly down and slowdy up. The mode of working is best explained by an example. At Breinigerberg in Rhenish Prussia the apparatus serves to separate a complex ore into the following four parts, which we enumerate in the order of their specific gravities—(1) galena (the heaviest), (2) pyrites, (3) blende, (4) dross. Sieve No. 1 is charged with granules of galena, just large enough not to slip through the meshes, No. 2 similarly with granules of pyrites, No. 3 with those of blende, sieve 1 ; the jerkin

funnels sift out the pyrites and the blende respectively, so that almost nothing but dross runs off ultimately. The apparatus is said to do its work with a wonderful degree of precision, and of course is susceptible of wider application, but it ceases to work when the raw material is a slime so fine that the particles fall too slowly.

Modes of Producing High Temperatures. —Most of what is to be said on this topic has already been anticipated in the articles FUEL, FURNACE, and BELLOWS ; but a few notes may be added on specially metallurgic points.

Furnace Materials.—In a metallurgic furnace the working parts at least must be made of special materials capable of withstanding the very high temperatures to which they are exposed and the action of the fluxes which may be used. No practically available material fully meets both requirements, but there is no lack of merely fire-proof substances.
Of native stones, a pure quartzose sandstone, free from, marl, may be named as being well adapted for the generality of structures; but such sandstone, or indeed any kind of fire-proof stone, is not always at hand. What is more readily procured, and consequently more widely used, is refractory brick, made from "fire-clay." The characteristic chemical feature of fire-clays is that in them the clay proper (always some kind of hydrated silicate of alumina) is associated with only small proportions of lime, magnesia, ferrous oxide, or other protoxides. If the percentage of these goes beyond certain limits, the bricks, when strongly heated, melt down into a slag. The presence of free silica, on the other hand, adds to their refrac-toriness. In fact the best fire-bricks in existence are the so-called Dinas bricks, which consist substantially of silica, contaminated only with just enough of bases to cause it to frit together on bein g baked. Dinas bricks, however, on account of their high price, are reserved for special cases involving exceptionally high temperatures. Amongst ordinary fire-bricks those from Stourbridge enjoy the highest reputation. It follows from what has just been said that, in a metallurgic furnace, lime-mortar cannot be used as a cement, but must be replaced by fire-clay paste.





In the construction of cupels, reverberator}- furnaces, &c, only the general groundwork is, as a rule, made of built bricks, and this groundwork is coated over with some kind of special fire-proof and flux-proof material, such as bone-ash, a mixture of baked fire-clay and cokes or graphite, or of quartz and very highly silicated slags, &c. These beddings are put on in a loose powdery form, and then stamped fast. They offer the advantage that, when wTorn out, they are easily removed and renewed. The powerful draught which a metallurgic fire needs can be produced by a chimney, where the fuel forms a relatively shallow layer spread over a large grating; but, when closely-packed deep masses of fuel or fuel and ore have to be kept ablaze, a blast becomes indispensable.

Chimneys.—The efficiency of a chimney is measured by the velocity V with which the air ascends through it, multiplied by its section ; and the former is in roughly approximate accordance with the formula
V hsjlgh-y T.;-T...
where h stands for the height of the chimney, g for the acceleration of gravity (32'2 feet per second), and T and T0 for the absolute temperatures (meaning the temperatures counted from - 273° C.) of the air within and the air without the chimney respectively, while i is a factor meant to account for the resistances which the air, in its progress through the furnace, &c, has to overcome. In practice T is taken as the mean temperature of the chimney gases, which theoretically is not unobjectionable ; but the weakest point in the formula is the smallness and utter inconstancy of the factor Jc, which, according to Peclet, generally assumes some value of the power \, f, &c. Yet the formula is of some use as enabling one to see the way in which V depends on h and (T - T0)/T0 con-jointly,—to see, for instance, how deficient chimney height may be compensated for by an increase of temperature in the chimney gases, and vice versa.

Blowing-Machines.—Of the several kinds of blowers described under BELLOWS (q.v.), the " fans " arc the best means for producing large volumes of wind of relatively small but steady pressure : "bellows" are indicated in the case of work on a relatively small scale requiring moderate wind pressure; wdiile the "cylinder blast" comes in where large masses of high-pressure wind are required. Two highly interesting blowing-machines, howrever, are omitted in that article, which may be shortly described here.

The "Water Blast" (Wassertrommelgebla.se) is interesting historically, having been used metallurgically in Hungary for many centuries. A mass of water, stored up in a reservoir, is made to fall down continuously through a high narrow vertical shaft having air-holes at its upper end. The vertical column of water sucks in air through these holes and carries it down with it into a kind of inverted tub standing in a reservoir kept at a constant level. Air and water there separate, the former flowing away through a pipe into a wind-box, from which it is led to its destina-tion.

The "Cagniardelle" (figs. 5,6), so called fromits inventor Cagniard Latour, also utilizes water to carry air, but in quite another way. By means of a round shaft passing through its axis, a cylindrical drum of sheet-metal is suspended slantingly in a mass of water, so that the lower end is fully immersed, while of the upper end the segment above the upper side of the shaft is uncovered. The space between shaft and drum is converted into a very wide screw-shaped canal by a band of sheet-metal hermetically fixed edgeways to the two. Both the top and the bottom end of the drum are partially closed by flat


|V
M
_ Hi

!
FIG. 5.—Cagniardelle.

bottoms soldered or riveted to the respective edges ; the lower one leaves a ring-shaped opening between its edge and the shaft, which serves for the introduction of a fixed air-pipe bent so as to reach up to near the top of the drum's air-space ; in the upper bottom three quadrants are closed, the fourth is open. Supposing the screw-canal, traced from below, to go from the left to the right, the drum is made to revolve in the same sense, and the effect is that, in each revolution, the screw-canal at its top end swallows a certain volume of air which, by the succeeding entrance of the water—which, of course, moves relatively to the screw —is pushed towards and ultimately into the air-space at the bottom end. The Cagniardelle yields a perfectly continu-ous blast, and, as it is not encumbered with any dead resistances except the friction of the shaft against its bearings (which can be reduced to very little) and the very slight friction of the water against the screw-canal, it utilizes a very large percentage of the energy spent on it. This percentage, accord-ing to experiments by Schwamkrug, amounts to from 75 to 84'5; in the case of the cylinder-blast it is 60 to 65 per cent. ; with bellows, about 40 per cent.; with the " Wassertrommelgeblase" 10 to 15 per cent. Hence the " Wassertrommelgeblase " stands last in relative efficiency ; but we must not forget that it alone directly utilizes native energy, while, in the cylinder blast, for example, 100 nnits of work done by the steam-engine involve a vastly greater energy spent on the engine as heat.

To maintain a desired temperature in a given furnace charged in a certain manner, the introduction of a certain volume of air per unit of time is necessary. But this quantity, in a given blowing-machine, is determined by the over-pressure of the wind, as measured by a manometer, the velocity of the wind being approxi-mately proportional to «yM/(B + M), where M stands for the height of the mercury-manometer, and B for that of the barometer. Hence the practical metallurgist, in adjusting his blast, has nothing to do but to see that the manometer shows the reading which, by previous trials, has been proved to yield an adequate supply of wind.

Fuel.—In some isolated cases the ore itself, by its combustion, supplies the necessary heat for the operation to be performed upon it. Thus, for instance, the roasting of blackband iron-stone is effected by simply piling up the ore and setting fire to it, so that the ore is at the same time its own furnace and fuel; in the Bessemer process of steel-making, the burning carbon of the pig-iron supplies the heat necessary for its own combustion; and a similar process has been tried experimentally, and not without success, for the working up of certain kinds of pyrites. But, as a rule, the high temperatures required for the working of ores are pro-duced by the combustion of extraneous fuel, such as wood, wood-charcoal, coal, coke. Of these four, wood-charcoal is of the widest applicability, but not much used in Britain on account of its high price." High-class coke or pure anthracite, volume for volume, 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 effected 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 ;1 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 regulino ores where there is no non-metallic 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 for 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 flux, 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 (FeSs), 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 (Ag2S) 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 regulus, but pure sulphide of silver is hardly ever worked.

2. Sulphides of iron and zinc yield the oxides Fe203 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-produet and unchanged sulphide contains oxygen and sulphur in the ratio of 02: 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 ease,, however, the

Examples are given in GOLD and COPPER. See also SILVER, sulphide Cu2S 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 S02, 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 Cu2S and FeS (" mat"). 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 subsul-phide of copper, Cu2S ("fine mat"), 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 Fe203 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 "precipitation," 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 : lPbS yields a regulus ( = lPb) and a "mat" Fe2S, 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 auc 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 "mat," while the arsenic is the first to be eliminated as such, or, in the presence of oxidants, as 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 " speis." (This term, as will readily be understood, has the same meaning in reference to arsenidesas "mat" has in regard 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 AsMe3/2 to AsMe2, where " Me" means one atomic weight of metal in toto, so that in general 1 Me=aFe + j/Co +zNi, where x + y + z = l. The siliceous cobalt is utilized as a blue pigment called " smalte"; the niekel-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 " fluxes " 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 slag 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 "mat" comprising the more readily sulphurizable metals. Oxide of lead, being a most powerful solvent for metallic oxides generally, is also largely used for tlie 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 " cupellation," which discounts the solvent powers of both metallic lead and its oxide. ! This process serves for the extraction of gold and silver from their I alloys with base metals such as copper, antimony, &c. The first I 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 sufficieut 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-irou. 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,, is employed for the preliminary concentration i 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 the metallic oxides present. The gold and ] silver unite with what is left of protosulphide of iron (FeS) into a j mat, which is then worked up for the noble metals.

Muxes.—Practically speaking, all ores are contaminated with more or less of gaugue, 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 "fluxes" so selected as to convert the gangue into a fusible "slag," which i 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 may be 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 au 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, &e. One part of fluor-spar liquefies about half a part of silica, four parts of sulphate j of lime, and one and a half parts of sulphate of baryta. Upon these I facts its wide application in metallurgy is founded.

Carbonate of soda (or potash) may be said to be the most power- I 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 j acid fluxes. At a red heat, when it forms a viscid fluid, it readily j dissolves up all basic oxides into fusible complex borates. Now i the gangue of an ore in general consists either of some basic : material such as carbonate of lime (or magnesia), ferric oxide, I 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 j may need for the formation of a slag of the proper qualities. The | slag must possess the proper degree of saturation. In other words, j taking Si02 + ?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 slag 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 oxide of iron Fe203 is to FeO, the latter more readily to FeO than j

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 seen, 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 H2C12 of muriatic acid or the quantity H2S04 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 H2 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 K20 of potash, Na20 of soda, CaO of lime, MgO of magnesia, FeO of ferrous oxide, JA~1203( = alO) of alumina, gFe203( = feO) of ferric oxide, as representing each " one equivalent " of base also in reference to silica, although silica has a characteristically indefinite basicity. Most slags are alloys or com-pounds of silicates of A1203 or Fe203, and of silicates of protoxides (CaO, &c.), hence their general composition is

n(B.0 + a;Si02) + m[(fe or al)0 + zSi02] .-

This introduction will enable the reader to understand the following mode of classifying and naming composition in silicates.

== TABLE ==

The names are the metallurgic ones ; scientific chemists designate Class I. as orthosilicatcs, Class II. as metasilicates, Class III. as sesqui-silicates. In the formulae M stands for K2, Ca, Fe, &c, or for al = §A1, 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 — ,andofthenatureoftheindividualbases m
that make up the RO and (fe or al) 0 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 A1203.3CaO, 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 (Cu20) silicates, the bi-silicate is violet, and melts pretty easily; the singulo-silicate is red, dense, and rather refractor}'.

Cupric silicates, as slags, hardly exist,—the CuO being always reduced to at least Cu20.
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 Fe203 being reduced in the fire to lFeO, although Fe203 occasionally replaces part of the A1203 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 3 CaO. 1A1203 according to Sefstrbm, 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 cither of the latter two. Thus for instance most magnesia-lime silicates fuse,—the bi-silicate combination (Mg, Ca)OSi02 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 A1203 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 guidance, to rely on the very numerous analyses which have been made of slags actually produced (by the rule of thumb) in successful metallurgical operations. For some of such slags also Plattner has determined the fusing points. He found for (1) Freiberg lead slag, 9EO, 3alO, 8Si02 ; oxygen-ratio, 3 : 4 ; melting-point at 1317° G. ; (2) Freiberg crude slag, 15RO, 3alO, 18Si03 ; oxygen-ratio, 1:1 ; melting-point at 1331° C. ; (3) Freiberg black-copper slag, 24FeO, A1203, 15Si02 ; oxygen-ratio, 9 :10 ; melting-point at 1338° C. ; (4) fligh-furnaco slag, 6CaO, 3alO, 9Si02 ; oxygen-ratio, 1:1 ; melting-point at 1431° C.

Metallurgic Assaying.—To assay an ore originally meant to execute a set of tentative experiments on a small scale in order to find out the proper mode of working it practically. But nowadays the term is always used in the sense of an analysis carried out to determine the money-value of an ore. For this purpose, in many cases it is sufficient to determine the percentages of the metals for which the ore is meant to be w7orked. But sometimes nothing short of a complete analysis will do. This holds more especially of ores of iron. As this metal is cheaj), the value of an ore containing it depends as much on the nature and relative quantities of the im-purities as on the percentage of metal. The proved absence of sulphur and phosphorus may be worth more than an additional 5 per cent, of iron, which latter again would perhaps not compensate for the proved presence of a large percentage of uncombined silica.

An assay to be of any value must start with a fair sample of the object of sale. The fulfilment of this condition in all cases is difficult. The general method is, from say a given ship load of ore, to take out (say) half a ton of ore from a large number of different places and to crush this large sample into small fragments of uniform size, which are well shovelled up together. From different parts of this ore-heap a sample of the second order—amounting to, say, 20 lb—is then drawn, and rendered more homogeneous by finer powder-ing and mixing. From this sample of the second (or perhaps from one of the third) order quantities of 1 or 2 Tb are bottled up for assaying. At the same time the moisture of the ore is determined, on a large scale, by some conventional method, such as the drying of 1 or 2 lb in an open basin at 100° C., and weighing of the residue as dry ore. This is done at the sampling place by the firms concerned. The assayer further pounds up and mixes his sample, and then pro-ceeds to determine the percentages of moisture and metal in his own way. He has always the choice between two methods, the dry and the wet. For the majority of gold or silver ores, and for cobalt and nickel ores almost as a rule, certain dry-process tests are preferred as the most exact analytically. In almost all other cases it may be said that the wet method is susceptible of the higher degree of pre-cision, yet even in some of these cases the old dry-process tests are preferred to the present day. For instance, all oopper ores in the British Isles are sold by the result of the Swansea assay, a kind of imitation of the process of sulphureous copper-ore smelting; and this, singularly, is adhered to even in the case of such cupriferous materials as are worked by the wet way, although the Swansea assay is well known to lose about 1 per cent, of the copper present. A copper-smelter therefore had better buy 5 per cent, than 10 per cent, copper-pyrites cinders, because in the first case he pays only for four-fifths, while in the latter he must pay for nine-tenths of the copper present. To compensate for this anomaly, empirical methods have been contrived for calculating prices. ( W. D. )


Footnotes

Few slags contain more than traces of alkalies.

For further information on slags, see Berthier, Traité des essais par la voie sèche ; Winkler, Erfahrungssätze über die Bildung der Schlacken, Freiberg, 1827 ; Plattner, Vorlesungen über allgemeine Hüttenkunde, i. 28 sq.; Percy, Metallurgy.




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