The American Cyclopædia (1879)/Metallurgy
|←Metalloids||The American Cyclopædia
|Edition of 1879. Written by Thomas M. Drown and Rossiter W. Raymond (Ore Dressing). See also Metallurgy on Wikipedia, and the disclaimer.|
METALLURGY (Gr. μεταλλουργός, working metals), the science which treats primarily of the separation and isolation of the metals contained in their natural combinations or associations, known as ores, and secondarily of the manipulation of the metals and the production of metallic compounds or alloys. The modes of occurrence of metals in nature are: 1, native, either pure or alloyed; 2, sulphides and combinations of sulphides and arsenides; 3, oxides and combinations of oxides with silicic and carbonic acids. More rarely arsenides, chlorides, tellurides, &c., are met with, and also compounds of oxides with other acids than those mentioned, as phosphoric, sulphuric, &c. The metallurgical treatment of an ore depends first on the physical characters of the minerals and accompanying rocks, and secondly on their chemical composition. It may therefore be divided into mechanical and chemical, the former dealing with the separation of native metals or metallic combinations from enclosing rock and gangue, and with the separation of associated minerals from each other according to their relative specific gravities; and the latter with the resolution of the chemical combinations of the metals with the non-metallic elements and with each other. The separation of the associated minerals and of minerals and metals from the gangue is usually effected by purely mechanical contrivances known as ore-dressing and ore-concentrating machines. Sometimes the metal or mineral is isolated by a process of liquation, heat being used (bismuth, sulphide of antimony). I. Ore Dressing. The dressing of ores is the separation by mechanical means, preliminary to further treatment, of the worthless portions of the material obtained in mining. This art is usually referred to the province of the mining engineer, rather than the metallurgist, because in most cases, where the mines and reduction works belong to separate proprietors, the former are expected to deliver ores to the latter in a suitable condition for treatment. But strictly speaking ore dressing is a metallurgical process. Gillon classifies it as “mechanical metallurgy.” Every ore has a valuable and a worthless portion, and there may be also an injurious portion, which causes loss in the treatment. To remove the worthless portion (gangue) and the actively injurious portion, by mechanical means, is the object of ore dressing. In a few instances, such as the washing of coal and the simple panning or sluicing of gold without amalgamation, the separation furnishes, without further treatment, the marketable commodity desired. But these operations, though involving the same principles, are not usually classed as ore dressing; and in most instances the mechanical preparation merely precedes the actual process of reduction. Whether any given ore should be subjected to this preliminary treatment is a question of economy, involving local conditions of expense and the unavoidable loss of valuable material attendant upon the additional manipulation required. To decide this question, for instance, with regard to an ore that is to be treated by smelting, it must be determined whether the expense of smelting the whole mass of the ore mined would be greater than that of first separating its worthless or injurious contents, and then smelting with better results the smaller quantity of concentrated materials; and also whether in the mechanical separation the loss of valuable material would be so great as to counterbalance the saving in smelting expenses, and the possible gain in purity of product and in completeness of extraction. It is also necessary to determine specially in every case how far the process of preliminary treatment shall be carried. It would not be desirable, even if practicable, to remove every trace of gangue, since some earthy material is requisite for the formation of slag to protect the metal in the hearth of the furnace from oxidation. Simple concentration removes a portion of the worthless gangue, and divides the ore into two parts, usually called “headings” and “tailings.” Headings may be made pure only by a loss of valuable material in the tailings; and, vice versa, the tailings can be made entirely worthless only by leaving considerable gangue in the headings. The best systems therefore involve the formation of one or more classes of so-called “middlings,” or intermediate grades, which may be subsequently separated again. This principle is also important in the treatment of a material containing different ores which cannot be advantageously smelted together. Thus zinc blende, which is very commonly associated with silver-bearing galena, is often poor in silver, and moreover seriously embarrasses the lead-smelting, not only by requiring extra fuel for its own volatilization or fusion in the slag, but also by taking up and removing in its vapors or slags a larger portion of silver than it originally contained. Yet this mineral, if separated, can under favorable circumstances be profitably treated by itself. — The mechanical separation of minerals depends either upon their magnetic properties or their specific gravity. The former principle has been employed to a limited extent in the separation of magnetic iron ores, in a finely divided state, from their accompanying gangue. Both permanent magnets and electro-magnets have been employed; but the process can scarcely be said to have proved an economical success, or to be now in practical operation outside of the laboratory. — Separation by specific gravity is performed in air and water. So-called dry concentrators are chiefly used in localities where water is scarce. The simplest form is that of a bowl or hide, in which auriferous dirt is placed and tossed in the wind, the lighter earthy particles being blown away, while the heavier sands containing the gold return to the vessel. Other air concentrators are winnowing channels, in which the material is separated by a draught of air, the heavier particles falling first to the floor, and the lighter ones being carried for longer distances; the result is a distribution of the materials on the floor of the channels in the order of their specific gravity. The most complete ore-dressing machines employing air are those in which the current of air is replaced by impulses, somewhat after the manner of the water jigs to be mentioned presently. The apparatus usually employed in dressing ores involves the use of water as a medium, and depends upon the relative periods occupied by bodies of different size, shape, and gravity in falling through water. The most favorable condition for separating ores would be the employment of a liquid exceeding in specific gravity one portion, and exceeded by the other portion, of the materials to be treated. The former would then float, while the latter would sink to the bottom, as in the preparation of fine porcelain clays. But in almost all cases the minerals to be separated sink in water, and a separation must be based on their different rates of sinking, dependent upon variations of specific gravity in different materials, and of size and shape of particles in the same material. In many machines for the separation of minerals by water, those particles which actually fall at equal rates will be brought together; hence, if it is desired to bring together particles of the same mineral (that is, of the same specific gravity) only, the disturbing effect of difference in size must be avoided. In other words, a careful sizing of the material must precede its separation according to gravity. A given quartz sphere will be 4 times as large in diameter, 68 times in volume, and 23 times in weight, as a galena sphere that falls through water at the same rate; and if such a piece of quartz were present with such a piece of galena in the material under treatment, they would not be separated. The separation is not usually effected in still water. In some machines the ore is made to fall against a rising current, or against impulses given by the motion of pistons; in others, the action of a stream of water passing through a trough or over an inclined plane is employed. The sizing is frequently effected beforehand by sieves, but the inclined plane itself may produce a separation according to the size of the particles, by reason of the greater rapidity of the upper surface of the stream, and the greater effect produced by the current upon larger particles. The shape has here an important influence, as determining the retardation of the particle by rolling or sliding friction. The apparatus in which still water is employed includes various kinds of settling tanks. The upward current and impulse is characteristic of the machines known as jigs; while the thin stream of water passing over an inclined surface is a feature of the buddle, the plane table, the rotary table, and the percussion table. The most universally serviceable machine for ore dressing is the “jig” or “jigger.” This was originally a simple improvement upon the treatment of ore by hand on a sieve under water. By plunging the sieve down suddenly under water, and allowing the particles to come again to rest upon it, a separation is effected; and if the stuff has been sized, and the operation has been repeated often enough, the denser particles are found in strata under the less dense. By scraping off the upper layers horizontally, ore and gangue may be separated. The first improvement was that of imparting motion by machinery to the sieve, but it was afterward found more convenient and effective to employ a submerged stationary sieve, and impart a vertical oscillatory motion to the water. This is done by using pistons or elastic diaphragms placed in the sides of the box, or on the top of a lower chamber full of water, and communicating with a lower box through the sieve. An additional feature of recent mechanical jigs is the continuous discharge, by means of which the different classes of separated material are removed without interrupting the operation. It was formerly supposed that coarsely crushed ores only could be effectively treated by jigging; and since ores so crushed usually contain a large proportion of fragments composed of adhering gangue and ore, and therefore possessing a specific gravity different from both gangue and ore, it was supposed that the jig could not be employed for the most delicate separation. But the improvements which have been made in this apparatus permit the treatment by it of much finer material than was formerly practicable, and at the same time have greatly reduced the hand labor and consequently the expense involved in the process. The result has been a great extension of the application of the jig, and the gradual abandonment to a large extent of the more cumbrous buddles and tables, which were formerly considered necessary for the treatment of the finest sands. The “dolly tub” is a very simple machine for separating the particles of crushed ore. It consists of a cylindrical vessel filled with water, in which the ore is rotated by means of revolving arms. When rotation has been maintained long enough, according to the quality and size of the material, the ore is allowed to settle, while the workman jars the table by blows upon the side of the tub with an iron bar, to prevent adhesion upon the inner surface. This machine has been elaborated by Hund, Rittinger, and others, and provided with a continuous discharge. In the machines already described, the material to be worked must previously be carefully sized, but sometimes it is already too fine to be accurately or rapidly sized in sieves or “trommels,” and for this reason resort is had to a different treatment. The material is first separated into “equal-falling” portions, grains constituting each portion being of such relative size and specific gravity that they will sink through water in equal times. Each of these portions is then treated alone upon a machine capable of separating the particles according to their specific gravity. For classifying the equal-falling particles, various machines are employed, in which use is made either of a horizontal stream of water of decreasing rapidity and of considerable depth, or a comparatively shallow, smooth stream, or a vertically ascending column with decreasing rapidity. The Spitzkasten or pointed box employs the first of these agencies. These boxes are hopper-shaped, and several of them of different sizes are connected. The water carrying the ore flows into and over the first box, and the heavier particles settle, while the lighter flow on to the second box, and so on. The rapidity of the current is diminished by varying the breadth of the boxes. The vertically ascending column of water is employed in the so-called Spitzlutten, a system of conical boxes, in which the water does not flow over as a covering current, but enters at the bottom. Both the flowing current and the ascending current have been combined in some recent forms of pointed boxes. The riffle, so frequently used in placer gold mining, involves the same principle. — The material, having been classified by any of the machines just mentioned into portions of equal-falling particles, must be treated further in order to separate each of these portions according to specific gravity; and for this purpose machines must be employed in which the particles will be affected more in proportion to size than in proportion to weight. Of equal-falling grains, the smallest are of course the densest; hence, the smaller will be mainly ore, the larger mainly gangue. A very thin, smooth stream of water, passing over a plane surface, exerts different forces upon large and small grains lying in the current. The friction on the layer of water next to the bottom is much greater than on the layer above. Hence large grains, the tops of which protrude into the layer above, will be acted upon by a much more rapid current, and will be moved forward, while the smaller grains lying in the lowest layer are unaffected. This is incidentally also a separation according to gravity, since the large grains are specifically lightest. It is essential that the stream should be thin; a deep stream acts upon all points very nearly alike. Another requisite condition is a proper velocity, which depends upon the inclination of the plane. If too nearly horizontal, the current will not move even the coarser particles, and if too steeply inclined it will be so violent as to sweep away fine and coarse alike. The amount of material held in suspension must also be regulated; if the water is too muddy, it will not be free to act on the separate grains, and the grains will act on each other. Keeping the water perfectly clear will effect the most complete sizing, but this condition is unfavorable to the quantity of work performed. The economical medium is found by practical experiment. Among the machines employed for this purpose are the plane table, the buddle, and the percussion table. The first requires little description. It is an inclined plane, near the head of which the ore is deposited in the form of slime, and acted upon by a stream of water distributed uniformly over the board. To prevent the cutting of furrows or channels in the ore bed, the workman continually smooths and consolidates with a wire brush or piece of plank the layer of concentrated ore deposited near the head of the table. When the table is full of accumulated ore, the water is shut off by means of the riffle, and the layer is divided into four parts, or zones, parallel with the head of the table. The upper zone is concentrated ore; the second zone is usually about as rich as the original material; the third is poor, but still rich enough to pay for reworking; the fourth is too poor for further treatment, and is rejected as tailings. Minor subdivisions may sometimes be made to advantage. The manual labor involved in this process has caused it to be more or less superseded by mechanical contrivances. One of these is the buddle, which may be considered as consisting of a large number of plane tables placed radially round a central point. They may be arranged with their heads together, the ore being fed in the centre and discharged on the circumference, in which case the buddle forms the frustum of a low cone, and is called a convex buddle; or they may be grouped with the tails to ward the cen- tre, the feed being on the circumference and the discharge in the centre, constituting a concave buddle. In these machines the tedious operation of maintaining an even surface to the ore layer is performed by revolving arms carrying brushes or scrapers. A concave buddle is usually preferred, by reason of its discharge in the centre, where the working surface is smallest. This secures a maximum force of current for carrying away the worthless material; whereas on the convex buddle the current is strongest in the centre, and is most likely at that point to carry past the proper zone particles of rich ore; while on the circumference, where the discharge takes place, the current is so much spread out as to have lost the power of carrying away the worthless portions of the material. The percussion table is another improvement on the plane table, in which the smoothing and consolidating of the surface of the ore layer is effected by means of a periodical jar communicated to the table itself. This jar is ordinarily given by suspending the table, swinging it from its position of equilibrium, and allowing its backward swing to be stopped by striking against a stationary block. The consolidation of the ore by brushes on the buddle, already alluded to, is not effective for very small particles. The finest slimes, when treated in buddles, remain too loosely on the surface, and permit the formation of furrows or channels; but on the percussion table the shock imparted to the particles thoroughly shakes them together, and consolidates the mass. The percussion tables until recently employed were stopped at intervals and cleaned up by hand, like the ordinary plane table; and the buddles were treated in the same way, the zones of classified material in the latter case being of course annular. But the most recent practice has given rise to continuously working buddles and percussion tables. The former are known as rotating tables, and are substantially buddles which revolve slowly under feeding spouts, and from which the dressed ore, instead of being allowed to accumulate on the table, is washed off by clean water as soon as the separation of the grains has been effected. These machines, as well as the continuous percussion tables, were perfected by the late Herr von Rittinger of Austria. The rotating tables have been found somewhat complicated and wasteful of water, and require very careful and skilful management; but the continuous percussion tables are pronounced both cheaper and more convenient than the similar machines of the intermittent type. These continuous tables receive their shock sidewise instead of endwise, and the result is a distribution of the ore in peculiar curved zones upon the table. The stuff to be washed is delivered upon the tables at an upper corner. The clear water is furnished by distributors. The tendency of the pulp is to flow down the slope in a direct line; but by means of the lateral percussion the path of the heavier particles is changed, and they are gradually thrown toward the side receiving the shock. The combination of this motion, at right angles to the current of water, with the downward motion of the current gives to the particles, according to their size and weight, a more or less curved path, and gradually separates the heavier and richer particles from the poor stuff. By the time they have reached the foot of the table the richest portions have been transferred to the corner diagonally opposite to that upon which they entered, while the middlings and tailings are discharged along the lower edge of the table, in the order of their concentration. By placing compartments below the edge of the table, to receive the different discharges, the products of the classification are conducted away separately. The best authorities on this subject are the elaborate treatises in German by Rittinger and Gätzschmann. II. Extraction of Metals from their Ores. The chemical processes employed for this purpose depend, 1, on the affinity of carbon for oxygen; 2, on the mutual reaction of an oxide with the sulphide of a metal; and 3, on the replacement of one metal in combination by another. The reactions in 1 and 2 take place only at high temperatures, while those in 3 may be effected either by fusion or in solution. — 1. Metals reduced from the state of Oxide by Carbon. The affinity of carbon for oxygen at high temperatures is sufficient to decompose most of the metallic oxides. Even the alkaline metals may be thus obtained. A few of the oxides (alkaline earths, earths, &c.) cannot be reduced by carbon, but their chlorides are reduced by the alkaline metals. In ordinary metallurgical practice the naturally occurring oxides that are reduced by carbon are iron, tin, zinc, and lead. The ores of iron and tin are exclusively oxides. Zinc occurs both in oxidized condition as carbonate or silicate, and also as sulphide. Lead exists mainly as sulphide, but occasionally as carbonate, phosphate, &c. Metallic zinc is always 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 according to the process employed the sulphide is either treated as such (see below) or oxidized by roasting. The methods employed for reduction by carbon consist either in heating the oxide in direct contact with coal or in exposing it to the action of heated carbonic oxide gas. Iron ore is smelted in a high-shaft furnace, and is reduced entirely by carbonic oxide generated by the partial combustion of the fuel in the lower part of the furnace, where the metallic 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 heated in clay 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 reduction. 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 the 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 temperatures above their points of fusion, and are obtained in a molten state, while zinc is only reduced above its boiling point, and is obtained as vapor, which is condensed to a liquid. — 2. Metals produced by the mutual Reaction of an Oxide and Sulphide. This reaction results, in the case of a few metals, in the formation of sulphurous acid gas and the separation of the metal, according to the following general formula: MS+2MO=M3+SO2. 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 another. The chemical affinity which the metals possess for the non-metallic elements differs greatly, and those possessing the strongest 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. Antimony is reduced from its sulphide in the same way. This replacement of one metal by another 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 processes, in contradistinction to the dry or smelting processes, are employed. Silver is produced in both ways. When associated with lead it is smelted by either of the lead processes given above, and is obtained alloyed with the lead. But when the ore or smelting product from which the silver is to be extracted 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 sodium 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 considerable extent by wet processes. It is rendered soluble by dissolving the native oxides or carbonates, or the oxide produced by roasting 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 mercury. Metallic 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 production of mercury from its principal ore (sulphide 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 copper ores which may contain 5 per cent. or less of metal are worked by a process known as matte smelting, whereby the copper is raised to 70 or 80 per cent. by the removal of earthy ingredients and iron. In this process the copper in the ore is collected in a matte which is mainly a combination of iron, copper, and sulphur. The iron is gradually removed in successive operations, until only copper and sulphur are left. From this enriched product the copper is separated by the roasting-reaction process. (See Copper.) Nickel ores are likewise first smelted to a matte. — The furnaces employed for metallurgical operations may be divided into the shaft or blast furnace, the gas or reverberatory furnace, and the crucible. In the shaft furnace the ore or other metallic compound is charged alternately with the fuel, and a reducing atmosphere, that is, one in which carbonic oxide predominates, always exists. In a reverberatory furnace the fuel does not come in contact with the hearth of the furnace, on which the substance to be heated is placed. The atmosphere here is generally oxidizing, owing to the practical impossibility of obtaining a high temperature without the admission of a slight access of air. In gas furnaces the access of air can be better regulated than with the ordinary reverberatory, but it must not be overlooked that carbonic acid, the product of combustion, gives off oxygen to some metals (as iron) at a red heat. Where a low temperature only is required, a more or less reducing atmosphere may be maintained by limiting the supply of air. But when an active reducing action is required on the hearth of a reverberatory furnace, the substance to be treated is intimately mixed with coal. A covered crucible or retort heated from without is entirely independent of the source and character of the heat, and consequently the nature of crucible smelting depends solely on the substances employed. — The earthy matters associated with the metals in ores are removed in ordinary smelting operations in the form of a fusible compound, which when solidified is generally hard and stony and is called slag or cinder. In iron smelting the slag consists mainly of silica, lime, and alumina. It is rarely the case that an iron ore contains these substances in the proper proportion to form a fusible cinder, and consequently the substance which is deficient must be added. As most iron ores are silicious, limestone is usually added as flux. In copper and lead smelting, the cinder usually contains, besides the earthy bases, a considerable amount of iron in the form of ferrous oxide. The cinder from copper matte smelting is almost exclusively a ferrous silicate. Mill cinder produced in the operation of puddling pig iron is also a ferrous silicate, but the iron contained in it is the incidental and unavoidable result of the oxidizing atmosphere in the furnace necessary for the removal of the silicon and carbon; it does not therefore represent an enriched product, but a waste of iron. — In order to facilitate the extraction of the metal, it is often necessary or desirable to change the physical or chemical constitution of the ores. This is effected by roasting or calcination. Roasting in its simplest form consists merely in the exposure of the ore to heat, in order to render it friable and porous and thereby more readily reducible. Compact iron ores are often thus treated. Again, an ore may contain volatile ingredients which can be driven off by heat. It becomes thereby enriched, besides being rendered porous. The spathic ores of iron (carbonates) and the brown hematites (hydrates) when heated part with their carbonic acid and water, and are converted, the former into porous magnetic oxide, and the latter into red oxide. This kind of roasting, generally called calcination, is usually effected in low-shaft furnaces or kilns, the heat being generated by fuel charged with the ore, or by the use of gas. The sulphides and arsenides of the heavier metals are often roasted in order to break up existing combinations and to form others which are more susceptible to treatment. This roasting may be either oxidizing or chloridizing. Oxidizing roasting consists in subjecting the ore, or other metallic combination, as matte, in the form of lumps or powder, to the action of heat, with free access of air. It is sometimes conducted in heaps in the open air by piling up lumps of ore or matte with layers of fuel. When sufficient sulphur is present, the pile when once ignited continues to burn without the aid of fuel. This method is always tedious, and generally imperfect. The ordinary method of furnace roasting consists in exposing the ore in the form of powder to the action of heat and air on the hearth of a reverberatory furnace. The ore must be frequently turned and rabbled, so that the oxidation shall proceed uniformly. It is also necessary to avoid a temperature which would sinter or fuse the mass, and thus hinder the complete exposure of the small particles to the air. To obviate the necessity of hand labor in turning the charge, and also to hasten the roasting, mechanical appliances have been employed, such as revolving chambers and hearths. It has also been found that showering the ore or matte in fine powder into a heated chamber or stack is a very expeditious method of roasting. Gerstenhöfer was the first to introduce this practice. His furnaces are rectangular chambers provided with iron bars of triangular section arranged at regular intervals, base uppermost. The time of exposure in falling is thus somewhat prolonged. The sulphides of the different metals behave very differently when roasted. In general the metal and sulphur are both oxidized, part of the latter passes off as sulphurous acid, which under favorable conditions can be utilized in the production of oil of vitriol; and part is converted into sulphuric acid, which combines with the metallic oxide. At a high temperature this sulphuric acid may be driven off, either wholly or in part, and the oxide left. When arsenic and antimony are present, the reactions become much more complicated and complete roasting more difficult. Gold, mercury, and silver may be reduced to metal by roasting, owing to their feeble affinity for oxygen at high temperatures. Some sulphides are difficult to roast, owing to their fusibility (sulphides of lead, bismuth, antimony, &c.). Chloridizing roasting has for its object the conversion of silver in ore or matte into the condition of chloride, in which form it may be dissolved, or directly treated with metallic iron to separate the silver, as mentioned above. The chlorination of silver is effected by the addition of common salt to the charge to be roasted, which is decomposed by the sulphuric acid generated by the oxidation of the sulphur with the liberation of hydrochloric acid. The methods employed are in general the same as those used in oxidizing roasting. The revolving cylinder (Brückner's) and the showering furnace (Stetefeldt's, on the principle of Gerstenhöfer) are largely used, and are found to give good results. — In many smelting operations, especially where ores of complex composition are used, an alloy of several metals is frequently produced. Thus the furnace or work lead from many ores contains copper, antimony, silver, gold, and other metals. The separation of the metals from each other is based on their relative oxidability, on the solvent action of metals and metallic oxides on each other, on difference in fusing point, on crystallization, and on solubility in acids. Examples of these methods will be found in the accounts of the different metals. It will suffice here to mention briefly the separation of lead from silver by the oxidation of the former, and the removal of copper and other metals at the same time, by the solvent action of litharge on their oxides (cupellation); the separation of silver from copper by alloying the latter with lead, and subsequently removing the lead with the silver by heat (liquation); the removal of silver from lead by zinc (Parkes's process) and by crystallization (Pattinson's process); and the separation of silver from gold by acids. — Metals occur in the arts either cast as ingots or in finished forms, or wrought by hammering, rolling, and drawing into sheets, rails, wire, &c. These mechanical processes are intimately connected with and dependent upon the physical properties of the different metals and on their purity. — See “Elements of Metallurgy,” by J. Arthur Phillips (London, 1874).