MERCURY, BICHLORIDE OF;Corrosive sublimate; (Deutochlorure de mercure, Fr.;Aetzendes quecksilber sublimat, Germ.) is made by subliming a mixture of equal parts of persulphate of mercury, prepared as above described, and sea-salt, in a stone-ware cucurbit. The sublimate rises in vapour, and encrusts the globular glass capital with a white mass of small prismatic needles. Its specific gravity is 5·14. Its taste is acrid, stypto-metallic, and exceedingly unpleasant. It is soluble in 20 parts of water, at the ordinary temperature, and in its own weight of boiling water. It dissolves in 21⁄2times its weight of cold alcohol. It is a very deadly poison. Raw white of egg swallowed in profusion, is the best antidote. A solution of corrosive sublimate has been long employed for preserving soft anatomical preparations. By this means the corpse of Colonel Morland was embalmed, in order to be brought from the seat of war to Paris. His features remained unaltered, only his skin was brown, and his body was so hard as to sound like a piece of wood when struck with a hammer.In the valuable work upon the dry rot, published by Mr. Knowles, secretary of the committee of inspectors of the navy, in 1821, corrosive sublimate is enumerated among the chemical substances which had been prescribed for preventing the dry rot in timber; and it is well known that Sir H. Davy had, several years before that date, used and recommended to the Admiralty and Navy Board, corrosive sublimate as an anti-dry rot application. It has been since extensively employed by a joint-stock company for the same purpose, under the title of Kyan’s patent.
MERCURY, BICHLORIDE OF;Corrosive sublimate; (Deutochlorure de mercure, Fr.;Aetzendes quecksilber sublimat, Germ.) is made by subliming a mixture of equal parts of persulphate of mercury, prepared as above described, and sea-salt, in a stone-ware cucurbit. The sublimate rises in vapour, and encrusts the globular glass capital with a white mass of small prismatic needles. Its specific gravity is 5·14. Its taste is acrid, stypto-metallic, and exceedingly unpleasant. It is soluble in 20 parts of water, at the ordinary temperature, and in its own weight of boiling water. It dissolves in 21⁄2times its weight of cold alcohol. It is a very deadly poison. Raw white of egg swallowed in profusion, is the best antidote. A solution of corrosive sublimate has been long employed for preserving soft anatomical preparations. By this means the corpse of Colonel Morland was embalmed, in order to be brought from the seat of war to Paris. His features remained unaltered, only his skin was brown, and his body was so hard as to sound like a piece of wood when struck with a hammer.
In the valuable work upon the dry rot, published by Mr. Knowles, secretary of the committee of inspectors of the navy, in 1821, corrosive sublimate is enumerated among the chemical substances which had been prescribed for preventing the dry rot in timber; and it is well known that Sir H. Davy had, several years before that date, used and recommended to the Admiralty and Navy Board, corrosive sublimate as an anti-dry rot application. It has been since extensively employed by a joint-stock company for the same purpose, under the title of Kyan’s patent.
MERCURY, PROTOCHLORIDE OF;Calomel; (Protochlorure de mercure, Fr.,Versüsstes quecksilber, Germ.) This compound, so much used and abused by medical practitioners, is commonly prepared by triturating four parts of corrosive sublimate along with three parts of running quicksilver in a marble mortar, till the metallic globules entirely disappear, with the production of a black powder, which is to be put into a glass balloon, and exposed to a subliming heat in a sand bath. The calomel, which rises in vapour, and attaches itself in a crystalline crust to the upper hemisphere of the balloon, is to be detached, reduced to a fine powder, or levigated and elutriated. 200 lbs. of mercury yield 236 of calomel and 272 of corrosive sublimate.The following more economical process is that adopted at the Apothecaries’ Hall, London. 140 pounds of concentrated sulphuric acid are boiled in a cast iron pot upon 100 pounds of mercury, till a dry persulphate is obtained. Of this salt, 124 pounds are triturated with 81 pounds of mercury, till the globules disappear, and till a protosulphate be formed. This is to be intimately mixed with 68 pounds of sea-salt, and the mixture, being put into a large stone-ware cucurbit, is to be submitted to a subliming heat. SeeCalomel.From 190 to 200 pounds of calomel rise in a crystalline cake, as in the former process, into the capital; while sulphate of soda remains at the bottom of the alembic. The calomel must be ground to an impalpable powder, and elutriated. The vapours, instead of being condensed into a cake within the top of the globe or in a capital, may be allowed to diffuse themselves into a close vessel, containing water in a state of ebullition, whereby the calomel is obtained at once in the form of a washed impalpable powder. Calomel is tasteless and insoluble in water. Its specific gravity is 7·176.For the compound of mercury with fulminic acid, seeFulminate.Periodide of mercuryis a bright but fugitive red pigment. It is easily prepared by dropping a solutionof iodide of potassium into a solution of corrosive sublimate, as long as any precipitation takes place, decanting off the supernatant muriate of potash, washing and drying the precipitate.
MERCURY, PROTOCHLORIDE OF;Calomel; (Protochlorure de mercure, Fr.,Versüsstes quecksilber, Germ.) This compound, so much used and abused by medical practitioners, is commonly prepared by triturating four parts of corrosive sublimate along with three parts of running quicksilver in a marble mortar, till the metallic globules entirely disappear, with the production of a black powder, which is to be put into a glass balloon, and exposed to a subliming heat in a sand bath. The calomel, which rises in vapour, and attaches itself in a crystalline crust to the upper hemisphere of the balloon, is to be detached, reduced to a fine powder, or levigated and elutriated. 200 lbs. of mercury yield 236 of calomel and 272 of corrosive sublimate.
The following more economical process is that adopted at the Apothecaries’ Hall, London. 140 pounds of concentrated sulphuric acid are boiled in a cast iron pot upon 100 pounds of mercury, till a dry persulphate is obtained. Of this salt, 124 pounds are triturated with 81 pounds of mercury, till the globules disappear, and till a protosulphate be formed. This is to be intimately mixed with 68 pounds of sea-salt, and the mixture, being put into a large stone-ware cucurbit, is to be submitted to a subliming heat. SeeCalomel.
From 190 to 200 pounds of calomel rise in a crystalline cake, as in the former process, into the capital; while sulphate of soda remains at the bottom of the alembic. The calomel must be ground to an impalpable powder, and elutriated. The vapours, instead of being condensed into a cake within the top of the globe or in a capital, may be allowed to diffuse themselves into a close vessel, containing water in a state of ebullition, whereby the calomel is obtained at once in the form of a washed impalpable powder. Calomel is tasteless and insoluble in water. Its specific gravity is 7·176.
For the compound of mercury with fulminic acid, seeFulminate.Periodide of mercuryis a bright but fugitive red pigment. It is easily prepared by dropping a solutionof iodide of potassium into a solution of corrosive sublimate, as long as any precipitation takes place, decanting off the supernatant muriate of potash, washing and drying the precipitate.
METALLURGY (Erzkunde, Germ.) is the art of extracting metals from their ores. This art, which supplies industry with the instruments most essential to its wants, is alike dependent upon the sciences of chemistry and mechanics; upon the former, as directing the smelting processes, best adapted to disentangle each metal from its mineralizer; upon the latter, as furnishing the means of grinding the ores, and separating the light stony parts from the rich metallic matter.Notwithstanding the striking analogy which exists between common chemical and metallurgic operations, since both are employed to insulate certain bodies from others, there are essential differences which should be carefully noted. In the first place, the quantity of materials being always very great in metallurgy, requires corresponding adaptations of apparatus, and often produces peculiar phenomena; in the second place, the agents to be employed for treating great masses, must be selected with a view to economy, as well as to chemical action. In analytical chemistry, the main object being exactness of result, and purity of product, little attention is bestowed upon the value of the reagents, on account of the small quantity required for any particular process. But in smelting metals upon the great scale, profit being the sole object, cheap materials and easy operations alone are admissible.The metallic ores as presented by nature, are almost always mixed with a considerable number of foreign substances; and could not therefore be advantageously submitted to metallurgic operations, till they are purified and concentrated to a certain degree by various methods.OF THE PREPARATION OF ORES FOR THE SMELTING HOUSE.There are two kinds ofpreparation; the one termed mechanical, from the means employed, and the results obtained, consists in processes for breaking and grinding the ores, and for washing them so as to separate the vein-stones, gangues, or other mixed earthy matters, in order to insulate or concentrate the metallic parts.Another kind of preparation, called chemical, has for its object to separate, by means of fire, various volatile substances combined in the ores, and which it is requisite to clear away, at least in a certain degree, before trying to extract the metals they may contain.Lastly, an indispensable operation in several circumstances, is to discover, by simple and cheap methods, calledassays, the quantity of metal contained in the different species of ores to be treated.This head of our subject, therefore, falls under three subdivisions:—§ 1. The mechanical preparation of ores, includingpicking,stamping, and different modes of washing.§ 2. The chemical preparation, consisting especially in the roasting or calcination of the ores.§ 3. The assay of ores, comprehending the mechanical part: that is, by washing; the chemical part, or assays by thedry way; and the assays by themoist way.Section1.Of the mechanical preparation or dressing of ores.—I. The first picking or sorting takes place in the interior, or underground, workings, and consists in separating the fragments of rocks, that apparently contain no metallic matter, from those that contain more or less of it. The external aspect guides this separation; as also the feeling of density in the hand.The substances when turned out to the day, undergo anothersorting, with greater or less care, according to the value of the included metal. This operation consists in breaking the lumps of ore with the hammer, into fragments of greater or less size, usually as large as the fist, whereby all the pieces may be picked out and thrown away that contain no metal, and even such as contain too little to be smelted with advantage. There is for the most part, a building erected near the output of the mine, in which the breaking and picking of the ores are performed. In a covered gallery, or under a shed, banks of earth are thrown up, and divided into separate beds, on each of which a thick plate of cast iron is laid. On this plate, elderly workmen, women, and children, break the ores with hand hammers, then pick and sort them piece by piece. The matters so treated, are usually separated into three parts; 1. the rock or sterile gangue, which is thrown away; 2. the ore for the stamping mill, which presents too intimate a mixture of rock and metallic substance to admit of separation by breaking and picking; and 3. the pure ore, or at least the very rich portion, called thesorted mineor thefat ore. On the sorting floors there remains much small rubbish, which might form a fourth subdivision of ore, since it is treated in a peculiar manner, by sifting, as will be presently mentioned.The distribution of fragments more or less rich, in one class or another, is relative to the value of the included metal, taking into account the expenses necessary for its extraction.Thus in certain lead mines, pieces of the gangues are thrown away, which judged by the eye may contain 3 per cent. of galena, because it is known that the greater portion of this would be lost in the washings required for separating the 97 parts of the gangue, and that the remainder would not pay the expenses.II. The very simple operations ofpickingare common to almost all ores; but there are other operations requiring more skill, care, and expense, which are employed in their final state of perfection only upon ores of metals possessing a certain value, as those of lead, silver, &c. We allude to thewashingof ores.The most simple and economical washings are those that certain iron ores, particularly the alluvial, are subjected to, as they are found near the surface of the ground agglutinated in great or little pieces. It is often useful to clean these pieces, in order to pick out the earthy lumps, which would be altogether injurious in the furnaces.This crude washing is performed sometimes by men stirring in the midst of a stream of water, with iron rakes or shovels, the lumps of ore placed in large chests, or basins of wood or iron.In other situations, this washing is executed more economically by a machine called abuddleor dolly-tub by our miners. A trough of wood or iron, with a concave bottom, is filled with the ore to be washed. Within the tub or trough, arms or iron handles are moved round about, being attached to the arbor of a hydraulic wheel. The trough is kept always full of water, which as it is renewed carries off the earthy matters, diffused through it by the motion of the machine, and the friction among the pieces of the ore. When the washing is finished, a door in one of the sides of the trough is opened, and the current removes the ore into a more spacious basin, where it is subjected to a kind of picking. It is frequently indeed passed through sieves in different modes. SeeLeadandTin, for figures ofbuddlesanddollies.Stamping millIII.Stamping.Before describing the refined methods of washing the more valuable ores of copper, silver, lead, &c., it is proper to point out the means of reducing them into a powder of greater or less fineness, bystamping, so called from the namestampsof the pestles employed for that purpose. Its usefulness is not restricted to preparing the ores; for it is employed in almost every smelting house for pounding clays, charcoal, scoriæ, &c. A stamping mill or pounding machine,fig.670., consists of several movable pillars of woodl l l, placed vertically, and supported in this position between frames of carpentryK K K. These pieces are each armed at their under end with a mass of ironm. An arbor or axlea a, moved by water, and turning horizontally, tosses up these wooden pestles, by means of wipers or cams, which lay hold of the shoulders of the pestles atl l l. These are raised in succession, and fall into an oblong trough belowm m, scooped out in the ground, having its bottom covered either with plates of iron or hard stones. In this trough, beneath these pestles, the ore to be stamped is allowed to fall from a hopper above, which is kept constantly full.The trough is closed in at the sides by two partitions, and includes three or four pestles; which the French miners call a battery. They are so disposed that their ascent and descent take place at equal intervals of time.Usually a stamping machine is composed of several batteries (two, three, or four), and the arrangement of the wipers on the arbor of the hydraulic wheel is such that there is constantly a like number of pestles lifted at a time; a circumstance important for maintaining the uniform going of the machine.The matters that are not to be exposed to subsequent washing are stamped dry, that is without leading water into the trough; and the same thing is sometimes done with the rich ores, whose lighter parts might otherwise be lost.Most usually, especially for ores of lead, silver, copper, &c., the trough of the stamper is placed in the middle of a current of water, of greater or less force; which, sweeping off the pounded substances, deposits them at a greater or less distance onwards, in the order of the size and richness of the grain; constituting a first washing, as they escape from beneath the pestles.In the dry stamping, the fineness of the powder depends on the weight of the pestles, the height of their fall, and the period of their action upon the ore; but in the stampers exposed to a stream of water, the retention of the matters in the trough is longer or shorter, according to the facility given for their escape. Sometimes these matters flow out of the chest over its edges, and the height of the line they must surmount has an influence on the size of the grain; at other times, the water and the pounded matterwhich it carries off, are made to pass through a grating, causing a kind of sifting at the same time. There are, however, some differences in the results of these two methods. Lastly, the quantity of water that traverses the trough, as well as its velocity, has an influence on the discharge of the pounded matters, and consequently on the products of the stampers.The size of the particles of the pounded ore being different, according to the variable hardness of the matters which compose them, suggests the means of classing them, and distributing them nearly in the order of their size and specific gravity, by making the water, as it escapes from the stamping trough, circulate in a system of canals called alabyrinth, where it deposits successively, in proportion as it loses its velocity, the earthy and metallic matters it had floated along. These metalliferous portions, especially when they have a great specific gravity like galena, would be deposited in the first passages, were it not that from their hardness being inferior to that of thegangue, they are reduced to a much finer powder, or into thin plates, which seem to adhere to both the watery and earthy particles; whence they have to be sought for among the finest portions of the pulverised gangue, called slime,schlich, orschlamme.There are several methods of conducting the stamps; in reference to the size of the grains wished to be obtained, and which is previously determined agreeably to the nature of the ore, and of the gangue; its richness, &c. The height of the slit that lets the pounded matters escape, or the diameters of the holes in the grating, their distance, the quantity of water flowing in, its velocity, &c., modify the result of the stamping operation.When it is requisite to obtain powder of an extreme fineness, as for ores that are to be subjected to the process of amalgamation, they are passed under millstones, as in common corn mills; and after grinding, they are bolted so as to form a species of flour; or they are crushed between rolls. SeeLeadandTin.Washing of ores.IV. The ores pounded under the stamps are next exposed to very delicate operations, both tedious and costly, which are called thewashings. Their purpose is to separate mechanically the earthy matters from the metallic portion, which must therefore have a much higher specific gravity; for otherwise, the washing would be impracticable.The medium employed to diminish the difference of specific gravity, and to move along the lightest matters, is water; which is made to flow with greater or less velocity and abundance over the schlich or pasty mud spread on a table of various inclination.But as this operation always occasions, not only considerable expense, but a certain loss of metal, it is right to calculate what is the degree of richness below which washing is unprofitable; and on the other hand, what is the degree of purification of theschlichat which it is proper to stop, because too much metal would be lost comparatively with the expense of fusing a small additional quantity of gangue. There cannot, indeed, be any fixed rule in this respect, since the elements of these calculations vary for every work.Before describing the different modes of washing, we must treat of the sifting or riddling, whose purpose, like that of the labyrinth succeeding the stamps, is to distribute and to separate the ores (which have not passed through the water stamps) in the order of the coarseness of grain. This operation is practised particularly upon the debris of the mine, and the rubbish produced in breaking the ores. These substances are put into a riddle, or species of round or square sieve, whose bottom is formed of a grating instead of a plate of metal pierced with holes. This riddle is plunged suddenly and repeatedly into a tub or cistern filled with water. This liquid enters through the bottom, raises up the mineral particles, separates them and keeps them suspended for an instant, after which they fall down in nearly the order of their specific gravities, and are thus classed with a certain degree of regularity. The sieve is sometimes dipped by the immediate effort of the washer; sometimes it is suspended to a swing which the workman moves; in order that the riddling may be rightly done, the sieve should receive but a single movement from below upwards; in this case the ore is separated from the gangue, and if there be different specific gravities, there is formed in the sieve as many distinct strata, which the workman can easily take out with aspatula, throwing the upper part away when it is too poor to be re-sifted. This operation by the hand-sieve, is calledriddling in the tub, or riddling by deposit.We may observe, that during the sifting, the particles which can pass across the holes of the bottom, fall into the tub and settle down there; whence they are afterwards gathered out, and exposed to washing when they are worth the trouble.Sometimes, as at Poullaouen, the sieves are conical, and held by means of two handles by a workman; and instead of receiving a single movement, as in the preceding method, the sifter himself gives them a variety of dexterous movements in succession. His object is to separate the poor portions of the ore from the richer; in order to subject the former to the stamp mill.Among the siftings and washings which ores are made to undergo, we must noticeamong the most useful and ingenious, those practised byiron gratings, called on the Continentgrilles anglaises, and thestep-washingsof Hungary,laveries à gradins. These methods of freeing the ores from the pulverulent earthy matters, consist in placing them, at their out-put from the mine, upon gratings, and bringing over them a stream of water, which merely takes down through the bars the small fragments, but carries off the pulverulent portions. The latter are received in cisterns, where they are allowed to rest long enough to settle to the bottom. The washing by steps is an extension of the preceding plan. To form an idea, let us imagine a series of grates placed successively at different levels, so that the water, arriving on the highest, where the ore for washing lies, carries off a portion of it, through this first grate upon a second closer in its bars, thence to a third, &c., and finally into labyrinths or cisterns of deposition.Grilles anglaisesFig. 671 enlarged(103 kB)Thegrilles anglaisesare similar to thesleeping tablesused at Idria. The system of theseen gradinsis represented infig.671.There are 5 such systems in the works at Idria, for the sorting of the small morsels of quicksilver ore, intended for the stamping mill. These fragments are but moderately rich in metal, and are picked up at random, of various sizes, from that of the fist to a grain of dust.These ores are placed in the chesta, below the level of which 7 grates are distributed, so that the fragments which pass through the firstb, proceed by an inclined conduit on to the second gratec, and so in succession. (See the conduitsl,o,p). In front, and on a level with each of the gratesb,c,d, &c., a child is stationed on one of the floors, 1, 2, 3, to 7.A current of water, which falls into the chesta, carries the fragments of ore upon the grates. The pieces which remain upon the two gratesbandc, are thrown on the adjoining tablev, where they undergo a sorting by hand; there the pieces are classified, 1. into gangue to be thrown away; 2. into ore for the stamp mill; 3. into ore to be sent directly to the furnace. The pieces which remain on each of the succeeding grates,d,e,f,g,h, are deposited on those of the floors 3 to 7, in front of each. Before every one of these shelves a deposit-sieve is established, (seet,u,) and the workmen in charge of it stand in one of the corresponding boxes, marked 8 to 12. The sieve is represented only in front of the chesth, for the sake of clearness.Each of the workmen placed in 8, 9, 10, 11, 12, operates on the heap before him; the upper layer of the deposit formed in his sieve, is sent to the stamping house, and the inferior layer directly to the furnace.As to the grains which, after traversing the five grates, have arrived at the chestx, they are washed in the two chestsy, which are analogous to the German chests to be presently described. The upper layer of what is deposited inyis sent to the furnace; the rest is treated anew on three tables of percussion, similar to the English brake-sieves, also to be presently described.After several successive manipulations on these tables, an upper stratum ofschlichis obtained fit for the furnace; an intermediate stratum, which is washed anew by the same process; and an inferior stratum, that is sent to the system of stamps,fig.672.Stamping millFig. 672 enlarged(128 kB)This figure represents the general ground plan of a stamping and washing mill. The stampsFare composed of two batteries similar tofig.670.The ore passes in succession under three pestles of cast iron, each of which is heavier the nearer it is to the sieve through which thesandor pounded matter escapes.In the upper part of the figure we see issuing from the stamps, two conduits destined to receive the water and the metalliferous sand with which it is loaded. The first, markedF,S,w, is used only when a certain quality of ore isstamped, richer in metal than isusually treated by means of the second conduit, the first being closed. The second conduit, or that employed for ordinary manipulation when the other is shut, is indicated byF, 0·7,B; then by 0·58 and 0·29. These numbers express the depth of the corresponding portions of this conduit. FromFtoB, the conduit or water-course is divided into three portions much shallower, called therich conduit, themiddle conduit, and theinferior. Beyond the basinB, the conduit takes the name of labyrinth. There the muddy sediments of ore are deposited; being the finer the further they are from the stampsF. Darts indicate the direction of the stream in the labyrinth. On theGerman chests, placed at 3, the sand derived from the rich and middle conduits is treated, in order to obtain three distinct qualities ofschlich, as already mentioned.Pis a cloth-covered table, for treating the deposit of the German chests at 3.M Nare two sweep tables (à balai), for treating the ore collected in the lower conduit, which precedes the midmost of the three German chests. Upon the three similar tablesR T V, are treated in like manner the muddy deposits of the labyrinth, which forms suite to three parallel German chests situated at 3, not shown for want of room in the figure, but connected in three rectangular zigzags with each other, as well as by a transverse branch to the points 0·7 andP. At the upper part of these five sweep tables, the materials which are to undergo washing are agitated in two boxesO O, by small paddle-wheels.We shall now describe thepercussion-tablesused in the Hartz, for treating the sand of ore obtained from the conduits represented above.Percussion tablePercussion tablePercussion tableFigs.673,674.and675.exhibit a plan, a vertical section, and elevation, of one of these tables, taken in the direction of its length. Thearboror great shaft in prolongation from the stamps mill, is shown in section perpendicularly to its axis, atA. Thecamsor wipers are shown round its circumference, one of them having just acted onn.These cams, by the revolution of the arbor, cause the alternating movements of a horizontal bar of woodo,u, which strikes at the pointuagainst a tabled,b,c,u. This table is suspended by two chainst, at its superior end, and by two rods at its lower end. After having been pushed by the pieceo,u, it rebounds to strike against a block or bracketB. A leverp,q, serves to adjust the inclination of the movable table, the pivotsqbeing points of suspension.The ore-sand to be washed, is placed in the chesta, into which a current of water runs. The ore floated onwards by the water, is carried through a sieve on a sloping small tablex, under which is concealed the higher end of the movable tabled,b,c,u; and it thence falls on this table, diffusing itself uniformly over its surface. The particles deposited on this table form an oblongtalus(slope) upon it; the successive percussions that it receives, determine the weightier matters, and consequently those richest in metal, to accumulate towards its upper end atu. Now the workman by means of the leverp, raises the lower endda little in order to preserve the samedegree of inclination to the surface on which the deposit is strewed. According as the substances are swept along by the water, he is careful to remove them from the middle of the table towards the top, by means of a wooden roller. With this intent, he walks on the tabled b c u, where the sandy sediment has sufficient consistence to bear him. When the table is abundantly charged with the washed ore, the deposit is divided into three bands or segmentsd b,b c,c u. Each of these bands is removed separately and thrown into the particular heap assigned to it. Every one of the heaps thus formed becomes afterwards the object of a separate manipulation on a percussion table, but always according to the same procedure. It is sufficient in general to pass twice over this table the matters contained in the heap, proceeding from the superior bandc u, in order to obtain a pureschlich; but the heap preceding from the intermediate beltb c, requires always a greater number of manipulations, and the lower bandd bstill more. These successive manipulations are so associated that eventually each heap furnishes pureschlich, which is obtained from the superior bandc u. As to the lightest particles which the water sweeps away beyond the lower end of the percussion table, they fall into conduits; whence they are lifted to undergo a new manipulation.Hartz jolterFig.676.is a profile of a plan which has been advantageously substituted, in the Hartz, for that part of the preceding apparatus which causes the jolt of the pieceo uagainst the tabled b c u. By means of this plan, it is easy to vary, according to the circumstances of a manipulation always delicate, the force of percussion which a barx y, ought to communicate by its extremityy. With this view, a slender piece of wooduis made to slide in an upright piece,v x, adjusted upon an axis atv. To the pieceua rod of iron is connected, by means of a hingez; this rod is capable of entering more or less into a case or sheath in the middle of the piecev x, and of being stopped at the proper point, by a thumb-screw which presses against this piece. If it be wished to increase the force of percussion, we must lower the pointz; if to diminish it, we must raise it. In the first case, the extremity of the pieceu, advances so much further under the cam of the driving shaftt; in the second, it goes so much less forwards; whereby the adjustment is produced.Sleeping tablesFigs.677.and678.represent a complete system ofsleeping tables,tables dormantes; such as are mounted in Idria.Fig.678.is the plan, andfig.677.a vertical section. The mercurial ores, reduced to a sand by stamps like those offig.672., pass into a series of conduitsa a,b b,c c, which form three successive floors below the level of the floor of the works. The sand taken out of these conduits is thrown into the cellsq; whence they are transferred into the troughe, and water is run upon them by turning two stopcocks for each trough. The sand thus diffused upon each table, runs off with the water by a groovef, comes upon a sieveh, spreads itself upon the boardg, and thence falls into the slanting chest, or sleeping tablei k. The under surfacekof this chest is pierced with holes, which may be stopped at pleasure with wooden plugs. There is a conduitm, at the lower end of each table, to catch the light particles carried off by the water out of the chesti k, through the holes properly opened, while the denser parts are deposited upon the bottom of this chest. A general conduitnpasses across at the foot of all the chestsi k; it receives the refuse of the washing operations.Stamping and washing worksFig. 679 enlarged(83 kB)Fig.679.is a set of stamping and washing works for the ores of argentiferous galena, as mounted atBockwiese, in the district of Zellerfeldt, in the Hartz.Ais the stamp mill and its subsidiary parts; among which area, the driving ormain shaft;b, the overshot water-wheel;c c, six strong rings or hoops of cast iron, for receiving each a cam or tappet;g, the brake of the machine;k,k,k, the three standards of the stamps;l l, &c. six pestles of pine wood, shod with lumps of cast iron. There are two chests, out of which the ore to be ground falls spontaneously into the two troughs of the stamps. Of late years, however, the ore is mostly supplied by hand; the watercourse terminates a short distance above the middle of the wheelb. There is a stream of water for the service of the stamps, and conduits proceeding from it, to lead the water into the two stamp troughs; the conduit of discharge is common to the two batteries or sets of stamps through which the water carries off the sand or stamped ore. There is a movable table of separation, mounted with two sieves. The sands pass immediately into the conduit placed upon a level with the floor, and separated into two compartments, the first of which empties its water into the second. There are two boards of separation, or tables, laid upon the ground, with a very slight slope of only 15 inches from their top to their bottom. Each of these boards is divided into four cases with edges; the whole being arranged so that it is possible, by means of a flood-gate or sluice, to cause the superfluous water of the case to pass into the following ones. Thus the work can go on without interruption, and alternately upon the two boards. There are winding canals in the labyrinth,N,N,N, in which are deposited the particles carried along by the water which has passed upon the boards. The depth of these canals gradually increases from 12 to 20 inches, to give a suitable descent for maintaining the water-flow. AtD, two percussion tables are placed.F Gare two German chests.H Jare two percussion tables, which are driven by the camsz z, fixed upon the main shaftx y.K K′ are two sloping sweep tables (à balai).TheGerman chestsare rectangular, being about 3 yards long, half a yard broad, with edges half a yard high; and their inclination is such that the lower end is about 15 inches beneath the level of the upper. At their upper end, usually called the bolster, a kind of trough or box, without any edge at the side next the chest, is placed, containing the ore to be washed. The water is allowed to fall upon the bolster in a thin sheet.Thesleeping tableshave upright edges; they are from 4 to 5 yards long, nearly 2 yards wide, and have fully a yard of inclination.The preceding tables are sometimes covered with cloth, particularly in treating ores that contain gold, on a supposition that the woollen or linen fibres would retain more surely the metallic particles; but this method appears on trial to merit no confidence, for it produces a very impureschlich.Swing-sieveFig.680.is a swing-sieve employed in the Hartz, for sifting the small fragments of the ore of argentiferous lead. Such an apparatus is usually set up in the outside of a stamp, and washing mill; its place being denoted by the letterA,fig.672.The two movable chests or boxesA B, of the sieve, are connected together, at their lower ends, with an upright rod, which terminates at one of the arms of a small balance beam, mounted between the driving shaft of the stamps and the sieve, perpendicularly to the length of both. The opposite arm of this beam carries another upright rod, which ears (cams ormentonnets), placed on purpose upon the driving shaft, may push down. During this movement the two lower endsA,B, are raised; and when the peg-cam of the shaft quits the rod which it had depressed, the swing chests fall by their own weight. Thus theyare made to vibrate alternately upon their axes. The small ore is put into the upper part of the chestA, over which a stream of water falls from an adjoining conduit. The fragments which cannot pass through a cast-iron grid in the bottom of that chest, are sorted by hand upon a table in front ofA, and they are classed by the workman, either among the ores to be stamped, whether dry or wet, or among the rubbish to be thrown away, or among the copper ores to be smelted by themselves. As to the small particles which fall through the grid upon the chestB, supplied also with a stream of water, they descend successively upon two other brass wire sieves, and also through the iron wirer, in the bottom ofB.In certain mines of the Hartz, tables calledà balais, orsweeping tables, are employed. The whole of the process consists in letting flow, over the sloping table, in successive currents, water charged with the ore, which is deposited at a less or greater distance, as also pure water for the purpose of washing the deposited ore, afterwards carried off by means of this sweeping operation.At the upper end of thesesweep-tables, the matters for washing are agitated in a chest, by a small wheel with vanes, or flap-boards. The conduit of the muddy waters opens above a little table or shelf; the conduit of pure water, which adjoins the preceding, opens below it. At the lower part of each of these tables, there is a transverse slit, covered by a small door with hinges, opening outwardly, by falling back towards the foot of the table. The water spreading over the table, may at pleasure be let into this slit, by raising a bit of leather which is nailed to the table, so as to cover the small door when it is in the shut position; but when this is opened, the piece of leather then hangs down into it. Otherwise the water may be allowed to pass freely above the leather, when the door is shut. The same thing may be done with a similar opening placed above the conduit. By means of these two slits, two distinct qualities ofschlichmay be obtained, which are deposited into two distinct conduits or canals. The refuse of the operation is turned into another conduit, and afterwards into ulterior reservoirs, whence it is lifted out to undergo a new washing.In the percussion tables, the water for washing the ores is sometimes spread in slender streamlets, sometimes in a full body, so as to let two cubic feet escape per minute. The number of shocks communicated per minute, varies from 15 to 36; and the table may be pushed out of its settled position at one time, three quarters of an inch, at another nearly 8 inches. The coarse ore-sand requires in general less water, and less slope of table, than the fine and pasty sand.Themechanicaloperations which ores undergo, take place commonly at their out-put from the mine, and without any intermediate operation. Sometimes, however, the hardness of certaingangues(vein-stones), and of certain iron-ores, is diminished by subjecting them to calcination previously to the breaking and stamping processes.When it is intended to wash certain ores, an operation founded on the difference of their specific gravities, it may happen that by slightly changing the chemical state of the substances that compose the ore, the earthy parts may become more easily separable, as also the other foreign matters. With this view, the ores of tin are subjected to a roasting, which by separating the arsenic, and oxidizing the copper which are intermixed, furnishes the means of obtaining, by the subsequent washing, an oxide of tin much purer than could be otherwise procured. In general, however, these are rare cases; so that the washing almost always immediately succeeds the picking and stamping; and the roasting comes next, when it needs to be employed.The operation of roasting is in general executed by various processes, relatively to the nature of the ores, the quality of the fuel, and to the object in view. The greatest economy ought to be studied in the fuel, as well as the labour; two most important circumstances, on account of the great masses operated upon.Three principal methods may be distinguished; 1. the roasting in a heap in the open air, the most simple of the whole; 2. the roasting executed between little walls, and which may be called case-roasting (rost-stadeln, in German); and 3. roasting in furnaces.We may remark, as to the description about to be given of these different processes, that in the first two, the fuel is always in immediate contact with the ore to be roasted, whilst in furnaces, this contact may or may not take place.1. The roasting in the open air, and in heaps more or less considerable, is practised upon iron ores, and such as are pyritous or bituminous. The operation consists in general in spreading over a plane area, often bottomed with beaten clay, billets of wood arranged like the bars of a gridiron, and sometimes laid crosswise over one another, so as to form a uniform flat bed. Sometimes wood charcoal is scattered in, so as to fill up the interstices, and to prevent the ore from falling between the other pieces of the fuel. Coal is also employed in moderately small lumps; and even occasionally, turf. The ore either simply broken into pieces, or even sometimes under the form ofschlich, is piled up over the fuel; most usually alternate beds of fuel and ore are formed.The fire, kindled in general at the lower part, but sometimes, however, at the middle chimney, spreads from spot to spot, putting the operation in train. The combustion must be so conducted as to be slow and suffocated, to prolong the ustulation, and let the whole mass be equably penetrated with heat. The means employed to direct the fire, are to cover outwardly with earth the portions where too much activity is displayed, and to pierce with holes or to give air to those where it is imperfectly developed. Rains, winds, variable seasons, and especially good primary arrangements of a calcination, have much influence on this process, which requires, besides, an almost incessant inspection at the beginning.Nothing in general can be said as to the consumption of fuel, because it varies with its quality, as well as with the ores and the purpose in view. But it may be laid down as a good rule, to employ no more fuel than is strictly necessary for the kind of calcination in hand, and for supporting the combustion; for an excess of fuel would produce, besides an expense uselessly incurred, the inconvenience, at times very serious, of such a heat as may melt or vitrify the ores; a result entirely the reverse of a well-conducted ustulation.Roasting moundFigs.681,682,683.represent the roasting in mounds, as practised near Goslar in the Hartz, and at Chessy in the department of the Rhone.Fig.681.is a vertical section in the lineh coffigs.682.and683.Infig.682.there is shown in plan, only a little more than one half of the quadrangular truncated pyramid, which constitutes the heap.Fig.683.shows a little more than one fourth of a bed of wood, arranged at the bottom of the pyramid, as shown bya a,fig.681., andc g h,fig.683.Cis a wooden chimney, formed within the heap of ore, at whose bottomcthere is a little parcel of charcoal,d dare large lumps of ore distributed upon the wooden pilea a;e eare smaller fragments, to cover the larger;f fis rubbish and clay laid smoothly in a slope over the whole.g,fig.683., a passage for air left under the bed of billets; of which there is a similar one in each of the four sides of the basea a, so that two principal currents of air cross under the upright axisCc, of the truncated pyramid indicated infig.681.The kindling is thrown in by the chimneyC. The charcoalc, and the wooda a, take fire; the sulphureous oresd e fare heated to such a high temperature as to vaporize the sulphur. In the Lower Hartz, a heap of this kind continues roasting during four months.2. The second method. The difficulty of managing the fire in the roasting of substances containing little sulphur, with the greater difficulty of arranging and supporting in their place theschlichsto be roasted, and last of all, the necessity of giving successive fires to the same ores, or to inconsiderable quantities at a time, have led to the contrivance of surrounding the area on which the roasting takes place with three little walls, or with four, leaving a door in the one in front. This is what is called awalled area, and sometimes, improperly enough, a roasting furnace. Inside of these little walls, about 3 feet high, there are often vertical conduits or chimneys made to correspond with an opening on the ground level, in order to excite a draught of air in the adjacent parts. When the roasting is once set agoing, these chimneys can be opened or shut at their upper ends, according to the necessities of the process.Several such furnaces are usually erected in connexion with each other by their lateral walls, and all terminated by a common wall, which forms their posterior part; sometimes they are covered with a shed supported partly by the back wall, built sufficiently high for this purpose. These dispositions are suitable for the roasting ofschlichs, and in general of all matters which are to have several fires; a circumstance often indispensable to a due separation of the sulphur, arsenic, &c.3. The furnaces employed for roasting the ores and themattesdiffer much, accordingto the nature of the ores, and the size of the lumps. We shall content ourselves with referring to the principal forms.When iron ores are to be roasted, which require but a simple calcination to disengage the combined water and carbonic acid, egg-shaped furnaces, similar to those in which limestone is burned in contact with fuel, may be conveniently employed; and they present the advantage of an operation which is continuous with a never-cooling apparatus. The analogy in the effects to be produced is so perfect, that the same furnace may be used for either object. Greater dimensions may, however, be given to those destined for the calcination of iron ores. But it must be remembered that this process is applicable only to ores broken into lumps, and not to ores in grains or powder.It has been attempted to employ the same method a little modified, for the roasting of ores of sulphuret of copper and pyrites, with the view of extracting a part of the sulphur. More or less success has ensued, but without ever surmounting all the obstacles arising from the great fusibility of the sulphuret of iron. For sometimes it runs into one mass, or at least into lumps agglutinated together in certain parts of the furnace, and the operation is either stopped altogether, or becomes more or less languid; the air not being able to penetrate into all the parts, the roasting becomes consequently imperfect. This inconvenience is even more serious than might at first sight appear; for, as the ill-roasted ores now contain too little sulphur to support their combustion, and as they sometimes fall into small fragments in the cooling, they cannot be passed again through the same furnace, and it becomes necessary to finish the roasting in a reverberatory hearth, which is much more expensive.In the Pyrenees, the roasting of iron ores is executed in a circular furnace, so disposed that the fuel is contained and burned in a kind of interior oven, above which lie the pieces of ore to be calcined. Sometimes the vault of this oven which sustains the ore, is formed of bricks, leaving between them openings for the passage of the flame and the smoke, and the apparatus then resembles certain pottery kilns; at other times the vault is formed of large lumps of ore, carefully arranged as to the intervals requisite to be left for draught over the arch. The broken ore is then distributed above this arch, care being taken to place the larger pieces undermost. This process is simple in the construction of the furnace, and economical, as branches of trees, without value in the forests, may be employed in the roasting. SeeLime-kilnfigures.In some other countries, the ores are roasted in furnaces very like those in which porcelain is baked; that is to say, the fuel is placed exteriorly to the body of the furnace in a kind of brick shafts, and the flame traverses the broken ore with which the furnace is filled. In such an apparatus the calcination is continuous.When it is proposed to extract the sulphur from the iron pyrites, or from pyritous minerals, different furnaces may be employed, among which that used in Hungary deserves notice. It is a rectangular parallelopiped of four walls, each of them being perforated with holes and vertical conduits which lead into chambers of condensation, where the sulphur is collected. The ore placed between the four walls on billets of wood arranged as infigs.681,682,683., for the great roastings in the open air, is calcined with the disengagement of much sulphur, which finds more facility in escaping by the lateral conduits in the walls, than up through the whole mass, or across the upper surface covered over with earth; whence it passes into the chambers of condensation. In this way upwards of a thousand tons of pyrites may be roasted at once, and a large quantity of sulphur obtained. SeeCopper.Swedish furnaceRoasting of Pyrites.—Figs.684,685.represent a furnace which has been long employed at Fahlun in Sweden, and several other parts of that kingdom, for roasting iron pyrites in order to obtain sulphur. This apparatus was constructed by the celebrated Gahn.Fig.684.is a vertical section, in the linek d n ooffig.685., which is a plan of the furnace; the top being supposed to be taken off. In both figures the conduit may be imagined to to be broken off ate; its entire length in a straight line is 43 feet beyond the dotted linee n, before the bend, which is an extension of this conduit. Upon the slopea bof a hillocka b c, lumpsrof iron pyrites are piled upon the pieces of woodi ifor roasting. A conduitd f eforms the continuation of the space denoted byr, which is covered by stone slabs so far asf; and from this point to the chamberhit is constructed in boards. At the beginning of this conduit, there is a recipientg. The chamberhis divided into five chambers by horizontal partitions, which permit the circulation of thevapours from one compartment to another. The oresrbeing distributed upon the billets of woodi i, whenever these are fairly kindled, they are covered with small ore, and then with rammed earthl l. Towards the pointm, for a space of a foot square, the ores are covered with movable stone slabs, by means of which the fire may be regulated, by the displacement of one or more, as may be deemed necessary. The liquid sulphur runs into the recipientg, whence it is laded out from time to time. The sublimed sulphur passes into the conduitf eand the chamberh, from which it is taken out, and washed with water, to free it from sulphuric acid with which it is somewhat impregnated; it is afterwards distilled in cast-iron retorts. The residuum of the pyrites is turned to account in Sweden, for the preparation of a common red colour much used as a pigment for wooden buildings.The reverberatory furnace affords one of the best means of ustulation, where it is requisite to employ the simultaneous action of heat and atmospherical air to destroy certain combinations, and to decompose the sulphurets, arseniurets, &c. It is likewise evident that the facility thus offered of stirring the matters spread out on the sole, in order to renew the surfaces, of observing their appearances, of augmenting or diminishing the degree of heat, &c., promise a success much surer, a roasting far better executed, than by any other process. It is known, besides, that flame mingled with much undecomposed air issuing from the furnace, is highly oxidizing, and is very fit for burning away the sulphur, and oxidizing the metals. Finally, this is almost the only method of rightly roasting ores which are in a very fine powder. If it be not employed constantly and for every kind of ore, it is just because more economy is found in practising calcination in heaps, or on areas enclosed by walls; besides, in certain mines, a very great number of these furnaces, and many workmen, would be required to roast the considerable body of ores that must be daily smelted. Hence there would result from the construction of such apparatus and its maintenance a very notable outlay, which is saved in the other processes.But in every case where it is desired to have a very perfect roasting, as for blende from which zinc is to be extracted, for sulphuret of antimony, &c., or even for ores reduced to a very fine powder, and destined for amalgamation, it is proper to perform the operation in a reverberatory furnace. When very fusible sulphurous ores are treated, the workman charged with the calcination must employ much care and experience, chiefly in the management of the fire. It will sometimes, indeed, happen, that the ore partially fuses; when it becomes necessary to withdraw the materials from the furnace, to let them cool and grind them anew, in order to recommence the operation. The construction of these furnaces demands no other attention than to give to the sole or laboratory the suitable size, and so to proportion to this the grate and the chimney that the heating may be effected with the greatest economy.The reverberatory furnace is always employed to roast the ores of precious metals, and especially those for amalgamation; as the latter often contain arsenic, antimony, and other volatile substances, they must be disposed of in a peculiar manner.The sole, usually very spacious, is divided into two parts, of which the one farthest off from the furnace is a little higher than the other. Above the vault there is a space or chamber in which the ore is deposited, and which communicates with the laboratory by a vertical passage; which serves to allow the ore to be pushed down, when it is dried and a little heated. The flame and the smoke which escape from the sole or laboratory pass into condensing chambers, before entering into the chimney of draught, so as to deposit in them the oxide of arsenic and other substances. When the ore on the part of the sole farthest from the grate has suffered so much heat as to begin to be roasted, has became less fusible, and when the roasting of that in the nearer part of the sole is completed, the former is raked towards the fire-bridge, and its ustulation is finished by stirring it over frequently with a paddle, skilfully worked, through one of the doors left in the side for this purpose. The operation is considered to be finished when the vapours and the smell have almost wholly ceased; its duration depending obviously on the nature of the ores.When this furnace is employed to roast very arsenical ores, as the tin ores of Schlackenwald in Bohemia, and at Ehrenfriedersdorf in Saxony, the arsenical pyrites of Geyer (in Saxony), &c., the chambers of condensation for the arsenious acid are much more extensive than in the furnaces commonly used for roasting galena, copper, or even silver ores.Reverberatory furnaceReverberatory furnaceFigs.686,687,688.represent a reverberatory furnace employed in the smelting works of Lautenthal, in the Hartz, for roasting the schlichs of lead ores, which contain much blende or sulphuret of zinc. Infig.686.we see that the two partsA B,B C, are absolutely like, the two furnaces being built in one body of brickwork.Fig.687.is the plan of the furnaceB C, taken at the levelE Foffig.686.Fig.688.is a vertical section of the similar furnaceA B, taken in the prolongation of the lineG Hinfig.687.ais the fire-place of the furnace, its grate and ash-pit.bis the conduit of vaporization, which communicates with the chambersc;c, chambers into which the vaporized substances are deposited;d, chimney for the escape of the smoke of the fire-placea, after it has gone through the spaceb c c;e′, is the charging door, with a hook hanging in front to rest the long iron rake upon, with which the materials are turned over;f, chamber containing a quantity of schlich destined for roasting; this chamber communicates with the vaulted corridor (gallery)D, seen infig.686.;g, orifice through which the schlich is thrown into the furnace;h, area or hearth of the reverberatory furnace, of which the roof is certainly much too high;i, channels for the escape of the watery vapours;k l, front arcade, between which and the furnace, properly speaking, are the two orifices of the conduits, which terminate at the channelsm,m′.mis the channel for carrying towards the chimneyd, the vapours which escape by the doore′.nis a walled-up door, which is opened from time to time, to take out of the chambersc,c, the substances that may be deposited in them.At the smelting works of Lautenthal, in such a roasting furnace, from 6 to 9 quintals (cwts.) of schlich are treated at a time, and it is stirred frequently with an iron rake upon the altarh. The period of this operation is from 6 to 12 hours, according as the schlich may be more or less dry, more or less rich in lead, or more or less charged with blende. When the latter substance is abundant, the process requires 12 hours, with about 60 cubic feet of cleft billets for fuel.In such furnaces are roasted the cobalt ores of Schneeberg in Saxony, the tin ores of Schlackenwald in Bohemia, of Ehrenfriedersdorf in Saxony, and elsewhere; as also the arsenical pyrites at Geyer in Saxony. But there are poison towers and extensive condensing chambers attached in the latter case. SeeArsenic.Figs.689,690,691.represent the reverberatory furnace generally employed in the Hartz, in the district of Mansfeldt, Saxony, Hungary, &c., for the treatment of black copper, and for refining rose copper upon the great scale. An analogous furnace is used at Andreasberg for the liquefaction or purification of the mattes, and for workable lead when it is much loaded with arsenic.Reverberatory furnaceReverberatory furnaceFig.689.presents the elevation of the furnace parallel to the lineI K, of the planfig.690.; which plan is taken at the level of the tuyèren, offig.691.;fig.691.is a vertical section in the lineL M,fig.690.krepresents one of two basins of reception, brasqued with clay and charcoal;n,n, two tuyères, through which enters the blast of two pairs of bellows, like those shown atCupellation ofSilver;q, door by which the matter to be melted is laid upon the sole of the furnace;v,v, two points where the sole is perforated, when necessary to run off the melted matter into either of the basinsh;x, door through which the slags or cinders floating upon the surfaceof the melted metal are raked out;y, door of the fire-place. The fuel is laid upon a grate above an ash-pit, and below the arch of a reverberatory which is contiguous to the dome or cap of the furnace properly so called. In the section,fig.691., the following parts may be noted: 1, 2, 3, mason-work of the foundation; 4, vapour channels or conduits, for the escape of the humidity; 5, bed of clay; 6, brasque composed of clay and charcoal, which forms the concavity of the hearth.Liquation furnaceFigs.692,693,694., show the furnace employed for liquation in one of the principal smelting works of the Hartz.Fig.694.exhibits the working area charged with the liquation cakes and charcoal, supported by sheets of wrought iron; being an image of the process in action.Fig.693.is the plan, in the lineF,G, offig.692.A liquation cake is composed of—Black copper holding at least 5 or 6loths(21⁄2or 3 oz.) of silver per cwt., and weighing 90 to 96 lbs.Lead obtained from litharge, 2 cwts. Litharge,1⁄2cwt.From 30 to 32 cakes are successively worked in one operation, which lasts about 5 hours; the furnace is brought into action, as usual, with the aid of slags; then a little litharge is added; when the lead begins to flow, the copper is introduced, and when the copper flows, lead is added, so that the mixture of the metals may be effected in the best way possible.From 8 to 16 of these cakes (pains) are usually placed in the liquation furnace,figs.692,693,694.The operation lasts 3 or 4 hours, in which time about 11⁄2quintals of charcoal are consumed. The cakes are covered with burning charcoal, supported, as I have said, by the iron plates. The workable lead obtained flows off towards the basin in front of the furnace; whence it is laded out into moulds set alongside. Seefig.693.If the lead thus obtained be not sufficiently rich in silver to be worth cupellation, it is employed to form new liquation cakes. When it contains from 5 to 6 loths of silver per cwt., it is submitted to cupellation in the said smelting works. SeeSilver.Water blowing engineThetrompe, or water-blowing engine,figs.695,696,697.Fig.695.is the elevation;fig.696.is a vertical section, made at right angles to the elevation. The machine is formed of two cylindrical pipes, the bodies of thetrompeb b, set upright, called the funnels, which terminate above in a water cisterna, and below in a close basin underc, called thetubordrum. The conical partp, of the funnel has been calledetranguillon, beingstrangled, as it were, in order that the water discharged into the body of the trompe shall not fill the pipe in falling, but be divided into many streamlets. Below thisnarrow part, eight holes,q q, are perforated obliquely through the substance of the trompe, called the vent-holes or nostrils, for admitting the air, which the water carries with it in its descent. The air afterwards parts from the water, by dashing upon a cast-iron slab, placed in thedrumupon the pedestald. An aperturel, at the bottom of the drum, allows the water to flow away after its fall; but, to prevent the air from escaping along with it, the water as it issues is received in a chestl m o n, divided into two parts by a vertical slide-plate betweenm n. By raising or lowering this plate, the water may be maintained at any desired level within the drum, so as to give the included air any determinate degree of pressure. The superfluous water then flows off by the holeo.The air-pipee f,fig.696., is fitted to the upper part of thedrum; it is divided, at the pointf, into three tubes, of which the principal one is destined for the furnace of cupellation, whilst the other twog g, serve for different melting furnaces. Each of these tubes ends in a leather pocket, and an iron nose-pipek, adjusted in the tuyère of the furnace. At Pesey, and in the whole of Savoy, a floodgate is fitted into the upper cisterna, to regulate the admission of water into the trome; but in Carniola, the funnelpis closed with a wooden plug, suspended to a cord, which goes round a pulley mounted upon a horizontal axis, as shewn infig.697.By the plugabeing raised more or less, merely the quantity of water required for the operation is admitted. The plug is pierced lengthwise with an oblique holec c, in which the small tubecis inserted, with its top some way above the water level, through which air may be admitted into the heart of the column descending into the trompep q.The ordinary height of the trompe apparatus is about 26 or 27 feet to the upper level of the water cistern; its total length is 11 mètres (36 feet 6 inches), and its width 2 feet, to give room for the drums. It is situated 10 mètres (331⁄3feet) from the melting furnace. This is the case at the smelting works of Jauerberg, in Upper Carniola.OF THE ASSAY OF ORES.Assays ought to occupy an important place in metallurgic instructions, and there isreason to believe that the knowledge of assaying is not sufficiently diffused, since its practice is so often neglected in smelting houses. Not only ought the assays of the ores under treatment, to be frequently repeated, because their nature is subject to vary; but the different products of the furnaces should be subjected to reiterated assays, at the several periods of the operations. When silver or gold ores are in question, the docimastic operations, then indispensable, exercise a salutary controul over the metallurgic processes, and afford a clear indication of the quantities of precious metal which they ought to produce.By the titleAssays, in a metallurgic point of view, is meant the method of ascertaining for any substance whatever, not only the presence and the nature of a metal, but its proportional quantity. Hence the operations which do not lead to a precise determination of the metal in question, are not to be arranged among the assays now under consideration. Experiments made with the blow-pipe, although capable of yielding most useful indications, are like the touchstone in regard to gold, and do not constitute genuine assays.Three kinds of assays may be practised in different circumstances, and with more or less advantage upon different ores. 1. The mechanical assay; 2. the assay by the dry way; 3. the assay by the humid way.1.Of mechanical assays.—These kinds of assays consist in the separation of the substances mechanically mixed in the ores, and are performed by a hand-washing, in a small trough of an oblong shape, called asebilla. After pulverizing with more or less pains the matters to be assayed by this process, a determinate weight of them is put into this wooden bowl with a little water; and by means of certain movements and some precautions, to be learned only by practice, the lightest substances may be pretty exactly separated, namely, the earthy gangues from the denser matter or metallic particles, without losing any sensible portion of them. Thus aschlichof greater or less purity will be obtained, which may afford the means of judging by its quality of the richness of the assayed ores, and which may thereafter be subjected to assays of another kind, whereby the whole metal may be insulated.Washing, as an assay, is practised on auriferous sands; on all ores from thestamps, and even onschlichsalready washed upon the great scale, to appreciate more nicely the degree of purity they have acquired. The ores of tin in which the oxide is often disseminated in much earthy gangue, are well adapted to this species of assay, because the tin oxide is very dense. The mechanical assay may also be employed in reference to the ores whose metallic portion presents an uniform composition, provided it also possesses considerable specific gravity. Thus the ores of sulphuret of lead (galena) being susceptible of becoming almost pure sulphurets (within 1 or 2per cent.) by mere washing skilfully conducted, the richness of that ore in pure galena, and consequently in lead, may be at once concluded; since 120 of galena contain 104 of lead, and 16 of sulphur. The sulphuret of antimony mingled with its gangue may be subjected to the same mode of assay, and the result will be still more direct, since the crude antimony is brought into the market after being freed from its gangue by a simple fusion.The assay by washing is also had recourse to for ascertaining if thescoriæor other products of the furnaces contain some metallic grains which might be extracted from them by stamping and washing on the great scale; a process employed considerably with thescoriæof tin and copper works.Of assays by the dry way.—The assay by the dry way has for its object, to show the nature and proportion of the metals contained in a mineral substance. To make a good assay, however, it is indispensably necessary to know what is the metal associated with it, and even within certain limits, the quantity of the foreign bodies. Only one metal is commonly looked after; unless in the case of certain argentiferous ores. The mineralogical examination of the substances under treatment, is most commonly sufficient to afford data in these respects; but the assays may always be varied with different views, before stopping at a definite result; and in every instance, only such assays can be confided in, as have been verified by a double operation.This mode of assaying requires only a little experience, with a simple apparatus; and is of such a nature as to be practised currently in the smelting works. The air furnace and crucibles employed are described in all good elementary chemical books. These assays are usually performed with the addition of a flux to the ore, or some agent for separating the earthy from the metallic substances; and they possess a peculiar advantage relative to the smelting operations, because they offer many analogies between results on the great scale and experiments on the small. This may even enable us often to deduce, from the manner in which the assay has succeeded with a certain flux, and at a certain degree of heat, valuable indications as to the treatment of the ore in the great way. SeeFurnace.In the smelting houses which purchase the ore, as in Germany, it is necessary to bestow much attention upon the assays, because they serve to regulate the quality andthe price of the schlichs to be delivered. These assays are not by any means free from difficulties, especially when ores containing several useful metals are treated, and which are to be dosed or proportioned; ores, for example, including a notable quantity of lead, copper, and silver, mixed together.In the central works of the Hartz, as well as in those of Saxony, theschlichsas delivered are subjected to docimastic assays, which are verified three times, and by three different persons, one of whom is engaged for the interests of the mining partners, another for that of the smelting house, and a third as arbiter in case of a difference. If the first two results of assaying differ by1⁄2loth(or1⁄4ounce) of silver per cwt. ofschlich, the operations must be resumed; but this rarely happens. When out of the three assays, the one differs from the two others by no more than1⁄4loth of silver per cwt., but by more in one, and by less in another, the mean result is adopted. As to the contents of theschlichin lead, the mean results of the assays must be taken. The differences allowed, are three pounds for theschlich, when it contains from 12 to 30 per cent. of lead, increasing to six pounds forschlich, when it contains less than 55 per cent. of that metal.Assaying forms in great establishments, an important object in reference to time and expense. Thus, in the single work of Franckenscharn, in the Hartz, no less than 300 assays have to be made in a threefold way, every Monday, without taking into account the several assays of the smelting products which take place every Thursday. Formerly fluxes more or less compound were employed for these purposes, and every assay cost about fifteen pence. At present all these assays are made more simply, by much cheaper methods, and cost a penny farthing each upon an average.Of the assays by the humid way.—The assays by the humid way, not reducible to very simple processes, are true chemical analyses, which may in fact be applied with much advantage, either to ores, or to the products of the furnace; but which cannot be expected to be practised in smelting-houses, on account of the complication of apparatus and reagents they require. Moreover, an expert chemist is necessary to obtain results that can be depended on. The directors of smelting-houses, however, should never neglect any opportunities that may occur of submitting the materials operated upon, as well as their products, to a more thorough examination than the dry way alone can effect. One of the great advantages of similar researches is, to discover and appreciate the minute quantities of injurious substances which impair the malleability of the metals, which give them several bad qualities, about whose nature and cause, more or less error and uncertainty prevail. Chemical analysis, rightly applied to metallurgy, cannot fail to introduce remarkable improvements into the processes.—See the different metals, in their alphabetical places.For assays in the dry way, both of stony and metallic minerals, the process of Dr. Abich deserves recommendation. In consists in mixing the pulverized mineral with 4 or 6 times its weight of carbonate of baryta in powder, fusing the mixture at a white heat, and then dissolving it after it cools, in dilute muriatic acid. The most refractory minerals, even corundum, cyanite, staurolite, zircon, and felspar, yield readily to this treatment. This process may be employed with advantage upon poor refractory ores. The platinum crucible, into which the mixed materials are put for fusion, should be placed in a Hessian crucible, and surrounded with good coak.The following tabular view of the metallic produce of the British mines, is given by two very skilful observers, in a work published in 1827, entitledVoyage Metallurgique en Angleterre, par MM. Dufrénoy et Elie de Beaumont:—Tons.Tons.TinCornwall alone3,160Copper-Cornwall9,331-11,469Devonshire537Staffordshire38Anglesey738Wales55Cumberland21Ireland738Scotland11Lead-Wales7,500-31,900Scotland2,800Cornwall and Devonshire800Shropshire800Derbyshire1,000Cumberland19,000Cast Ironabout600,000[32][32]I have converted the weights of lead and cast iron, given in kilogrammes, into tons, at the rate of 1000 kilogrammes per ton; which is sufficiently near.The manganese raised in England exceeds 2000 tons.M. Heron de Villefosse inserted in the last number of theAnnales des Minesfor 1827, the following statistical view of the metallic products of France:—Tons.Lead in pigs (saumons)103Litharge513Sulphuret of lead, ground galena (alquifoux)112Black copper164Antimony91Manganese765Crude cast iron25,606Bar iron127,643Steel3,500Silver in ingots11⁄6The total value of which is estimated at 80 millions of francs; or about 3,400,000 pounds sterling.
METALLURGY (Erzkunde, Germ.) is the art of extracting metals from their ores. This art, which supplies industry with the instruments most essential to its wants, is alike dependent upon the sciences of chemistry and mechanics; upon the former, as directing the smelting processes, best adapted to disentangle each metal from its mineralizer; upon the latter, as furnishing the means of grinding the ores, and separating the light stony parts from the rich metallic matter.
Notwithstanding the striking analogy which exists between common chemical and metallurgic operations, since both are employed to insulate certain bodies from others, there are essential differences which should be carefully noted. In the first place, the quantity of materials being always very great in metallurgy, requires corresponding adaptations of apparatus, and often produces peculiar phenomena; in the second place, the agents to be employed for treating great masses, must be selected with a view to economy, as well as to chemical action. In analytical chemistry, the main object being exactness of result, and purity of product, little attention is bestowed upon the value of the reagents, on account of the small quantity required for any particular process. But in smelting metals upon the great scale, profit being the sole object, cheap materials and easy operations alone are admissible.
The metallic ores as presented by nature, are almost always mixed with a considerable number of foreign substances; and could not therefore be advantageously submitted to metallurgic operations, till they are purified and concentrated to a certain degree by various methods.
OF THE PREPARATION OF ORES FOR THE SMELTING HOUSE.
There are two kinds ofpreparation; the one termed mechanical, from the means employed, and the results obtained, consists in processes for breaking and grinding the ores, and for washing them so as to separate the vein-stones, gangues, or other mixed earthy matters, in order to insulate or concentrate the metallic parts.
Another kind of preparation, called chemical, has for its object to separate, by means of fire, various volatile substances combined in the ores, and which it is requisite to clear away, at least in a certain degree, before trying to extract the metals they may contain.
Lastly, an indispensable operation in several circumstances, is to discover, by simple and cheap methods, calledassays, the quantity of metal contained in the different species of ores to be treated.
This head of our subject, therefore, falls under three subdivisions:—
§ 1. The mechanical preparation of ores, includingpicking,stamping, and different modes of washing.
§ 2. The chemical preparation, consisting especially in the roasting or calcination of the ores.
§ 3. The assay of ores, comprehending the mechanical part: that is, by washing; the chemical part, or assays by thedry way; and the assays by themoist way.
Section1.Of the mechanical preparation or dressing of ores.—I. The first picking or sorting takes place in the interior, or underground, workings, and consists in separating the fragments of rocks, that apparently contain no metallic matter, from those that contain more or less of it. The external aspect guides this separation; as also the feeling of density in the hand.
The substances when turned out to the day, undergo anothersorting, with greater or less care, according to the value of the included metal. This operation consists in breaking the lumps of ore with the hammer, into fragments of greater or less size, usually as large as the fist, whereby all the pieces may be picked out and thrown away that contain no metal, and even such as contain too little to be smelted with advantage. There is for the most part, a building erected near the output of the mine, in which the breaking and picking of the ores are performed. In a covered gallery, or under a shed, banks of earth are thrown up, and divided into separate beds, on each of which a thick plate of cast iron is laid. On this plate, elderly workmen, women, and children, break the ores with hand hammers, then pick and sort them piece by piece. The matters so treated, are usually separated into three parts; 1. the rock or sterile gangue, which is thrown away; 2. the ore for the stamping mill, which presents too intimate a mixture of rock and metallic substance to admit of separation by breaking and picking; and 3. the pure ore, or at least the very rich portion, called thesorted mineor thefat ore. On the sorting floors there remains much small rubbish, which might form a fourth subdivision of ore, since it is treated in a peculiar manner, by sifting, as will be presently mentioned.
The distribution of fragments more or less rich, in one class or another, is relative to the value of the included metal, taking into account the expenses necessary for its extraction.Thus in certain lead mines, pieces of the gangues are thrown away, which judged by the eye may contain 3 per cent. of galena, because it is known that the greater portion of this would be lost in the washings required for separating the 97 parts of the gangue, and that the remainder would not pay the expenses.
II. The very simple operations ofpickingare common to almost all ores; but there are other operations requiring more skill, care, and expense, which are employed in their final state of perfection only upon ores of metals possessing a certain value, as those of lead, silver, &c. We allude to thewashingof ores.
The most simple and economical washings are those that certain iron ores, particularly the alluvial, are subjected to, as they are found near the surface of the ground agglutinated in great or little pieces. It is often useful to clean these pieces, in order to pick out the earthy lumps, which would be altogether injurious in the furnaces.
This crude washing is performed sometimes by men stirring in the midst of a stream of water, with iron rakes or shovels, the lumps of ore placed in large chests, or basins of wood or iron.
In other situations, this washing is executed more economically by a machine called abuddleor dolly-tub by our miners. A trough of wood or iron, with a concave bottom, is filled with the ore to be washed. Within the tub or trough, arms or iron handles are moved round about, being attached to the arbor of a hydraulic wheel. The trough is kept always full of water, which as it is renewed carries off the earthy matters, diffused through it by the motion of the machine, and the friction among the pieces of the ore. When the washing is finished, a door in one of the sides of the trough is opened, and the current removes the ore into a more spacious basin, where it is subjected to a kind of picking. It is frequently indeed passed through sieves in different modes. SeeLeadandTin, for figures ofbuddlesanddollies.
Stamping mill
III.Stamping.Before describing the refined methods of washing the more valuable ores of copper, silver, lead, &c., it is proper to point out the means of reducing them into a powder of greater or less fineness, bystamping, so called from the namestampsof the pestles employed for that purpose. Its usefulness is not restricted to preparing the ores; for it is employed in almost every smelting house for pounding clays, charcoal, scoriæ, &c. A stamping mill or pounding machine,fig.670., consists of several movable pillars of woodl l l, placed vertically, and supported in this position between frames of carpentryK K K. These pieces are each armed at their under end with a mass of ironm. An arbor or axlea a, moved by water, and turning horizontally, tosses up these wooden pestles, by means of wipers or cams, which lay hold of the shoulders of the pestles atl l l. These are raised in succession, and fall into an oblong trough belowm m, scooped out in the ground, having its bottom covered either with plates of iron or hard stones. In this trough, beneath these pestles, the ore to be stamped is allowed to fall from a hopper above, which is kept constantly full.
The trough is closed in at the sides by two partitions, and includes three or four pestles; which the French miners call a battery. They are so disposed that their ascent and descent take place at equal intervals of time.
Usually a stamping machine is composed of several batteries (two, three, or four), and the arrangement of the wipers on the arbor of the hydraulic wheel is such that there is constantly a like number of pestles lifted at a time; a circumstance important for maintaining the uniform going of the machine.
The matters that are not to be exposed to subsequent washing are stamped dry, that is without leading water into the trough; and the same thing is sometimes done with the rich ores, whose lighter parts might otherwise be lost.
Most usually, especially for ores of lead, silver, copper, &c., the trough of the stamper is placed in the middle of a current of water, of greater or less force; which, sweeping off the pounded substances, deposits them at a greater or less distance onwards, in the order of the size and richness of the grain; constituting a first washing, as they escape from beneath the pestles.
In the dry stamping, the fineness of the powder depends on the weight of the pestles, the height of their fall, and the period of their action upon the ore; but in the stampers exposed to a stream of water, the retention of the matters in the trough is longer or shorter, according to the facility given for their escape. Sometimes these matters flow out of the chest over its edges, and the height of the line they must surmount has an influence on the size of the grain; at other times, the water and the pounded matterwhich it carries off, are made to pass through a grating, causing a kind of sifting at the same time. There are, however, some differences in the results of these two methods. Lastly, the quantity of water that traverses the trough, as well as its velocity, has an influence on the discharge of the pounded matters, and consequently on the products of the stampers.
The size of the particles of the pounded ore being different, according to the variable hardness of the matters which compose them, suggests the means of classing them, and distributing them nearly in the order of their size and specific gravity, by making the water, as it escapes from the stamping trough, circulate in a system of canals called alabyrinth, where it deposits successively, in proportion as it loses its velocity, the earthy and metallic matters it had floated along. These metalliferous portions, especially when they have a great specific gravity like galena, would be deposited in the first passages, were it not that from their hardness being inferior to that of thegangue, they are reduced to a much finer powder, or into thin plates, which seem to adhere to both the watery and earthy particles; whence they have to be sought for among the finest portions of the pulverised gangue, called slime,schlich, orschlamme.
There are several methods of conducting the stamps; in reference to the size of the grains wished to be obtained, and which is previously determined agreeably to the nature of the ore, and of the gangue; its richness, &c. The height of the slit that lets the pounded matters escape, or the diameters of the holes in the grating, their distance, the quantity of water flowing in, its velocity, &c., modify the result of the stamping operation.
When it is requisite to obtain powder of an extreme fineness, as for ores that are to be subjected to the process of amalgamation, they are passed under millstones, as in common corn mills; and after grinding, they are bolted so as to form a species of flour; or they are crushed between rolls. SeeLeadandTin.
Washing of ores.
IV. The ores pounded under the stamps are next exposed to very delicate operations, both tedious and costly, which are called thewashings. Their purpose is to separate mechanically the earthy matters from the metallic portion, which must therefore have a much higher specific gravity; for otherwise, the washing would be impracticable.
The medium employed to diminish the difference of specific gravity, and to move along the lightest matters, is water; which is made to flow with greater or less velocity and abundance over the schlich or pasty mud spread on a table of various inclination.
But as this operation always occasions, not only considerable expense, but a certain loss of metal, it is right to calculate what is the degree of richness below which washing is unprofitable; and on the other hand, what is the degree of purification of theschlichat which it is proper to stop, because too much metal would be lost comparatively with the expense of fusing a small additional quantity of gangue. There cannot, indeed, be any fixed rule in this respect, since the elements of these calculations vary for every work.
Before describing the different modes of washing, we must treat of the sifting or riddling, whose purpose, like that of the labyrinth succeeding the stamps, is to distribute and to separate the ores (which have not passed through the water stamps) in the order of the coarseness of grain. This operation is practised particularly upon the debris of the mine, and the rubbish produced in breaking the ores. These substances are put into a riddle, or species of round or square sieve, whose bottom is formed of a grating instead of a plate of metal pierced with holes. This riddle is plunged suddenly and repeatedly into a tub or cistern filled with water. This liquid enters through the bottom, raises up the mineral particles, separates them and keeps them suspended for an instant, after which they fall down in nearly the order of their specific gravities, and are thus classed with a certain degree of regularity. The sieve is sometimes dipped by the immediate effort of the washer; sometimes it is suspended to a swing which the workman moves; in order that the riddling may be rightly done, the sieve should receive but a single movement from below upwards; in this case the ore is separated from the gangue, and if there be different specific gravities, there is formed in the sieve as many distinct strata, which the workman can easily take out with aspatula, throwing the upper part away when it is too poor to be re-sifted. This operation by the hand-sieve, is calledriddling in the tub, or riddling by deposit.
We may observe, that during the sifting, the particles which can pass across the holes of the bottom, fall into the tub and settle down there; whence they are afterwards gathered out, and exposed to washing when they are worth the trouble.
Sometimes, as at Poullaouen, the sieves are conical, and held by means of two handles by a workman; and instead of receiving a single movement, as in the preceding method, the sifter himself gives them a variety of dexterous movements in succession. His object is to separate the poor portions of the ore from the richer; in order to subject the former to the stamp mill.
Among the siftings and washings which ores are made to undergo, we must noticeamong the most useful and ingenious, those practised byiron gratings, called on the Continentgrilles anglaises, and thestep-washingsof Hungary,laveries à gradins. These methods of freeing the ores from the pulverulent earthy matters, consist in placing them, at their out-put from the mine, upon gratings, and bringing over them a stream of water, which merely takes down through the bars the small fragments, but carries off the pulverulent portions. The latter are received in cisterns, where they are allowed to rest long enough to settle to the bottom. The washing by steps is an extension of the preceding plan. To form an idea, let us imagine a series of grates placed successively at different levels, so that the water, arriving on the highest, where the ore for washing lies, carries off a portion of it, through this first grate upon a second closer in its bars, thence to a third, &c., and finally into labyrinths or cisterns of deposition.
Grilles anglaisesFig. 671 enlarged(103 kB)
Fig. 671 enlarged(103 kB)
Thegrilles anglaisesare similar to thesleeping tablesused at Idria. The system of theseen gradinsis represented infig.671.There are 5 such systems in the works at Idria, for the sorting of the small morsels of quicksilver ore, intended for the stamping mill. These fragments are but moderately rich in metal, and are picked up at random, of various sizes, from that of the fist to a grain of dust.
These ores are placed in the chesta, below the level of which 7 grates are distributed, so that the fragments which pass through the firstb, proceed by an inclined conduit on to the second gratec, and so in succession. (See the conduitsl,o,p). In front, and on a level with each of the gratesb,c,d, &c., a child is stationed on one of the floors, 1, 2, 3, to 7.
A current of water, which falls into the chesta, carries the fragments of ore upon the grates. The pieces which remain upon the two gratesbandc, are thrown on the adjoining tablev, where they undergo a sorting by hand; there the pieces are classified, 1. into gangue to be thrown away; 2. into ore for the stamp mill; 3. into ore to be sent directly to the furnace. The pieces which remain on each of the succeeding grates,d,e,f,g,h, are deposited on those of the floors 3 to 7, in front of each. Before every one of these shelves a deposit-sieve is established, (seet,u,) and the workmen in charge of it stand in one of the corresponding boxes, marked 8 to 12. The sieve is represented only in front of the chesth, for the sake of clearness.
Each of the workmen placed in 8, 9, 10, 11, 12, operates on the heap before him; the upper layer of the deposit formed in his sieve, is sent to the stamping house, and the inferior layer directly to the furnace.
As to the grains which, after traversing the five grates, have arrived at the chestx, they are washed in the two chestsy, which are analogous to the German chests to be presently described. The upper layer of what is deposited inyis sent to the furnace; the rest is treated anew on three tables of percussion, similar to the English brake-sieves, also to be presently described.
After several successive manipulations on these tables, an upper stratum ofschlichis obtained fit for the furnace; an intermediate stratum, which is washed anew by the same process; and an inferior stratum, that is sent to the system of stamps,fig.672.
Stamping millFig. 672 enlarged(128 kB)
Fig. 672 enlarged(128 kB)
This figure represents the general ground plan of a stamping and washing mill. The stampsFare composed of two batteries similar tofig.670.The ore passes in succession under three pestles of cast iron, each of which is heavier the nearer it is to the sieve through which thesandor pounded matter escapes.
In the upper part of the figure we see issuing from the stamps, two conduits destined to receive the water and the metalliferous sand with which it is loaded. The first, markedF,S,w, is used only when a certain quality of ore isstamped, richer in metal than isusually treated by means of the second conduit, the first being closed. The second conduit, or that employed for ordinary manipulation when the other is shut, is indicated byF, 0·7,B; then by 0·58 and 0·29. These numbers express the depth of the corresponding portions of this conduit. FromFtoB, the conduit or water-course is divided into three portions much shallower, called therich conduit, themiddle conduit, and theinferior. Beyond the basinB, the conduit takes the name of labyrinth. There the muddy sediments of ore are deposited; being the finer the further they are from the stampsF. Darts indicate the direction of the stream in the labyrinth. On theGerman chests, placed at 3, the sand derived from the rich and middle conduits is treated, in order to obtain three distinct qualities ofschlich, as already mentioned.Pis a cloth-covered table, for treating the deposit of the German chests at 3.M Nare two sweep tables (à balai), for treating the ore collected in the lower conduit, which precedes the midmost of the three German chests. Upon the three similar tablesR T V, are treated in like manner the muddy deposits of the labyrinth, which forms suite to three parallel German chests situated at 3, not shown for want of room in the figure, but connected in three rectangular zigzags with each other, as well as by a transverse branch to the points 0·7 andP. At the upper part of these five sweep tables, the materials which are to undergo washing are agitated in two boxesO O, by small paddle-wheels.
We shall now describe thepercussion-tablesused in the Hartz, for treating the sand of ore obtained from the conduits represented above.
Percussion table
Percussion table
Percussion table
Figs.673,674.and675.exhibit a plan, a vertical section, and elevation, of one of these tables, taken in the direction of its length. Thearboror great shaft in prolongation from the stamps mill, is shown in section perpendicularly to its axis, atA. Thecamsor wipers are shown round its circumference, one of them having just acted onn.
These cams, by the revolution of the arbor, cause the alternating movements of a horizontal bar of woodo,u, which strikes at the pointuagainst a tabled,b,c,u. This table is suspended by two chainst, at its superior end, and by two rods at its lower end. After having been pushed by the pieceo,u, it rebounds to strike against a block or bracketB. A leverp,q, serves to adjust the inclination of the movable table, the pivotsqbeing points of suspension.
The ore-sand to be washed, is placed in the chesta, into which a current of water runs. The ore floated onwards by the water, is carried through a sieve on a sloping small tablex, under which is concealed the higher end of the movable tabled,b,c,u; and it thence falls on this table, diffusing itself uniformly over its surface. The particles deposited on this table form an oblongtalus(slope) upon it; the successive percussions that it receives, determine the weightier matters, and consequently those richest in metal, to accumulate towards its upper end atu. Now the workman by means of the leverp, raises the lower endda little in order to preserve the samedegree of inclination to the surface on which the deposit is strewed. According as the substances are swept along by the water, he is careful to remove them from the middle of the table towards the top, by means of a wooden roller. With this intent, he walks on the tabled b c u, where the sandy sediment has sufficient consistence to bear him. When the table is abundantly charged with the washed ore, the deposit is divided into three bands or segmentsd b,b c,c u. Each of these bands is removed separately and thrown into the particular heap assigned to it. Every one of the heaps thus formed becomes afterwards the object of a separate manipulation on a percussion table, but always according to the same procedure. It is sufficient in general to pass twice over this table the matters contained in the heap, proceeding from the superior bandc u, in order to obtain a pureschlich; but the heap preceding from the intermediate beltb c, requires always a greater number of manipulations, and the lower bandd bstill more. These successive manipulations are so associated that eventually each heap furnishes pureschlich, which is obtained from the superior bandc u. As to the lightest particles which the water sweeps away beyond the lower end of the percussion table, they fall into conduits; whence they are lifted to undergo a new manipulation.
Hartz jolter
Fig.676.is a profile of a plan which has been advantageously substituted, in the Hartz, for that part of the preceding apparatus which causes the jolt of the pieceo uagainst the tabled b c u. By means of this plan, it is easy to vary, according to the circumstances of a manipulation always delicate, the force of percussion which a barx y, ought to communicate by its extremityy. With this view, a slender piece of wooduis made to slide in an upright piece,v x, adjusted upon an axis atv. To the pieceua rod of iron is connected, by means of a hingez; this rod is capable of entering more or less into a case or sheath in the middle of the piecev x, and of being stopped at the proper point, by a thumb-screw which presses against this piece. If it be wished to increase the force of percussion, we must lower the pointz; if to diminish it, we must raise it. In the first case, the extremity of the pieceu, advances so much further under the cam of the driving shaftt; in the second, it goes so much less forwards; whereby the adjustment is produced.
Sleeping tables
Figs.677.and678.represent a complete system ofsleeping tables,tables dormantes; such as are mounted in Idria.Fig.678.is the plan, andfig.677.a vertical section. The mercurial ores, reduced to a sand by stamps like those offig.672., pass into a series of conduitsa a,b b,c c, which form three successive floors below the level of the floor of the works. The sand taken out of these conduits is thrown into the cellsq; whence they are transferred into the troughe, and water is run upon them by turning two stopcocks for each trough. The sand thus diffused upon each table, runs off with the water by a groovef, comes upon a sieveh, spreads itself upon the boardg, and thence falls into the slanting chest, or sleeping tablei k. The under surfacekof this chest is pierced with holes, which may be stopped at pleasure with wooden plugs. There is a conduitm, at the lower end of each table, to catch the light particles carried off by the water out of the chesti k, through the holes properly opened, while the denser parts are deposited upon the bottom of this chest. A general conduitnpasses across at the foot of all the chestsi k; it receives the refuse of the washing operations.
Stamping and washing worksFig. 679 enlarged(83 kB)
Fig. 679 enlarged(83 kB)
Fig.679.is a set of stamping and washing works for the ores of argentiferous galena, as mounted atBockwiese, in the district of Zellerfeldt, in the Hartz.
Ais the stamp mill and its subsidiary parts; among which area, the driving ormain shaft;b, the overshot water-wheel;c c, six strong rings or hoops of cast iron, for receiving each a cam or tappet;g, the brake of the machine;k,k,k, the three standards of the stamps;l l, &c. six pestles of pine wood, shod with lumps of cast iron. There are two chests, out of which the ore to be ground falls spontaneously into the two troughs of the stamps. Of late years, however, the ore is mostly supplied by hand; the watercourse terminates a short distance above the middle of the wheelb. There is a stream of water for the service of the stamps, and conduits proceeding from it, to lead the water into the two stamp troughs; the conduit of discharge is common to the two batteries or sets of stamps through which the water carries off the sand or stamped ore. There is a movable table of separation, mounted with two sieves. The sands pass immediately into the conduit placed upon a level with the floor, and separated into two compartments, the first of which empties its water into the second. There are two boards of separation, or tables, laid upon the ground, with a very slight slope of only 15 inches from their top to their bottom. Each of these boards is divided into four cases with edges; the whole being arranged so that it is possible, by means of a flood-gate or sluice, to cause the superfluous water of the case to pass into the following ones. Thus the work can go on without interruption, and alternately upon the two boards. There are winding canals in the labyrinth,N,N,N, in which are deposited the particles carried along by the water which has passed upon the boards. The depth of these canals gradually increases from 12 to 20 inches, to give a suitable descent for maintaining the water-flow. AtD, two percussion tables are placed.F Gare two German chests.H Jare two percussion tables, which are driven by the camsz z, fixed upon the main shaftx y.K K′ are two sloping sweep tables (à balai).
TheGerman chestsare rectangular, being about 3 yards long, half a yard broad, with edges half a yard high; and their inclination is such that the lower end is about 15 inches beneath the level of the upper. At their upper end, usually called the bolster, a kind of trough or box, without any edge at the side next the chest, is placed, containing the ore to be washed. The water is allowed to fall upon the bolster in a thin sheet.
Thesleeping tableshave upright edges; they are from 4 to 5 yards long, nearly 2 yards wide, and have fully a yard of inclination.
The preceding tables are sometimes covered with cloth, particularly in treating ores that contain gold, on a supposition that the woollen or linen fibres would retain more surely the metallic particles; but this method appears on trial to merit no confidence, for it produces a very impureschlich.
Swing-sieve
Fig.680.is a swing-sieve employed in the Hartz, for sifting the small fragments of the ore of argentiferous lead. Such an apparatus is usually set up in the outside of a stamp, and washing mill; its place being denoted by the letterA,fig.672.The two movable chests or boxesA B, of the sieve, are connected together, at their lower ends, with an upright rod, which terminates at one of the arms of a small balance beam, mounted between the driving shaft of the stamps and the sieve, perpendicularly to the length of both. The opposite arm of this beam carries another upright rod, which ears (cams ormentonnets), placed on purpose upon the driving shaft, may push down. During this movement the two lower endsA,B, are raised; and when the peg-cam of the shaft quits the rod which it had depressed, the swing chests fall by their own weight. Thus theyare made to vibrate alternately upon their axes. The small ore is put into the upper part of the chestA, over which a stream of water falls from an adjoining conduit. The fragments which cannot pass through a cast-iron grid in the bottom of that chest, are sorted by hand upon a table in front ofA, and they are classed by the workman, either among the ores to be stamped, whether dry or wet, or among the rubbish to be thrown away, or among the copper ores to be smelted by themselves. As to the small particles which fall through the grid upon the chestB, supplied also with a stream of water, they descend successively upon two other brass wire sieves, and also through the iron wirer, in the bottom ofB.
In certain mines of the Hartz, tables calledà balais, orsweeping tables, are employed. The whole of the process consists in letting flow, over the sloping table, in successive currents, water charged with the ore, which is deposited at a less or greater distance, as also pure water for the purpose of washing the deposited ore, afterwards carried off by means of this sweeping operation.
At the upper end of thesesweep-tables, the matters for washing are agitated in a chest, by a small wheel with vanes, or flap-boards. The conduit of the muddy waters opens above a little table or shelf; the conduit of pure water, which adjoins the preceding, opens below it. At the lower part of each of these tables, there is a transverse slit, covered by a small door with hinges, opening outwardly, by falling back towards the foot of the table. The water spreading over the table, may at pleasure be let into this slit, by raising a bit of leather which is nailed to the table, so as to cover the small door when it is in the shut position; but when this is opened, the piece of leather then hangs down into it. Otherwise the water may be allowed to pass freely above the leather, when the door is shut. The same thing may be done with a similar opening placed above the conduit. By means of these two slits, two distinct qualities ofschlichmay be obtained, which are deposited into two distinct conduits or canals. The refuse of the operation is turned into another conduit, and afterwards into ulterior reservoirs, whence it is lifted out to undergo a new washing.
In the percussion tables, the water for washing the ores is sometimes spread in slender streamlets, sometimes in a full body, so as to let two cubic feet escape per minute. The number of shocks communicated per minute, varies from 15 to 36; and the table may be pushed out of its settled position at one time, three quarters of an inch, at another nearly 8 inches. The coarse ore-sand requires in general less water, and less slope of table, than the fine and pasty sand.
Themechanicaloperations which ores undergo, take place commonly at their out-put from the mine, and without any intermediate operation. Sometimes, however, the hardness of certaingangues(vein-stones), and of certain iron-ores, is diminished by subjecting them to calcination previously to the breaking and stamping processes.
When it is intended to wash certain ores, an operation founded on the difference of their specific gravities, it may happen that by slightly changing the chemical state of the substances that compose the ore, the earthy parts may become more easily separable, as also the other foreign matters. With this view, the ores of tin are subjected to a roasting, which by separating the arsenic, and oxidizing the copper which are intermixed, furnishes the means of obtaining, by the subsequent washing, an oxide of tin much purer than could be otherwise procured. In general, however, these are rare cases; so that the washing almost always immediately succeeds the picking and stamping; and the roasting comes next, when it needs to be employed.
The operation of roasting is in general executed by various processes, relatively to the nature of the ores, the quality of the fuel, and to the object in view. The greatest economy ought to be studied in the fuel, as well as the labour; two most important circumstances, on account of the great masses operated upon.
Three principal methods may be distinguished; 1. the roasting in a heap in the open air, the most simple of the whole; 2. the roasting executed between little walls, and which may be called case-roasting (rost-stadeln, in German); and 3. roasting in furnaces.
We may remark, as to the description about to be given of these different processes, that in the first two, the fuel is always in immediate contact with the ore to be roasted, whilst in furnaces, this contact may or may not take place.
1. The roasting in the open air, and in heaps more or less considerable, is practised upon iron ores, and such as are pyritous or bituminous. The operation consists in general in spreading over a plane area, often bottomed with beaten clay, billets of wood arranged like the bars of a gridiron, and sometimes laid crosswise over one another, so as to form a uniform flat bed. Sometimes wood charcoal is scattered in, so as to fill up the interstices, and to prevent the ore from falling between the other pieces of the fuel. Coal is also employed in moderately small lumps; and even occasionally, turf. The ore either simply broken into pieces, or even sometimes under the form ofschlich, is piled up over the fuel; most usually alternate beds of fuel and ore are formed.
The fire, kindled in general at the lower part, but sometimes, however, at the middle chimney, spreads from spot to spot, putting the operation in train. The combustion must be so conducted as to be slow and suffocated, to prolong the ustulation, and let the whole mass be equably penetrated with heat. The means employed to direct the fire, are to cover outwardly with earth the portions where too much activity is displayed, and to pierce with holes or to give air to those where it is imperfectly developed. Rains, winds, variable seasons, and especially good primary arrangements of a calcination, have much influence on this process, which requires, besides, an almost incessant inspection at the beginning.
Nothing in general can be said as to the consumption of fuel, because it varies with its quality, as well as with the ores and the purpose in view. But it may be laid down as a good rule, to employ no more fuel than is strictly necessary for the kind of calcination in hand, and for supporting the combustion; for an excess of fuel would produce, besides an expense uselessly incurred, the inconvenience, at times very serious, of such a heat as may melt or vitrify the ores; a result entirely the reverse of a well-conducted ustulation.
Roasting mound
Figs.681,682,683.represent the roasting in mounds, as practised near Goslar in the Hartz, and at Chessy in the department of the Rhone.Fig.681.is a vertical section in the lineh coffigs.682.and683.Infig.682.there is shown in plan, only a little more than one half of the quadrangular truncated pyramid, which constitutes the heap.Fig.683.shows a little more than one fourth of a bed of wood, arranged at the bottom of the pyramid, as shown bya a,fig.681., andc g h,fig.683.Cis a wooden chimney, formed within the heap of ore, at whose bottomcthere is a little parcel of charcoal,d dare large lumps of ore distributed upon the wooden pilea a;e eare smaller fragments, to cover the larger;f fis rubbish and clay laid smoothly in a slope over the whole.g,fig.683., a passage for air left under the bed of billets; of which there is a similar one in each of the four sides of the basea a, so that two principal currents of air cross under the upright axisCc, of the truncated pyramid indicated infig.681.
The kindling is thrown in by the chimneyC. The charcoalc, and the wooda a, take fire; the sulphureous oresd e fare heated to such a high temperature as to vaporize the sulphur. In the Lower Hartz, a heap of this kind continues roasting during four months.
2. The second method. The difficulty of managing the fire in the roasting of substances containing little sulphur, with the greater difficulty of arranging and supporting in their place theschlichsto be roasted, and last of all, the necessity of giving successive fires to the same ores, or to inconsiderable quantities at a time, have led to the contrivance of surrounding the area on which the roasting takes place with three little walls, or with four, leaving a door in the one in front. This is what is called awalled area, and sometimes, improperly enough, a roasting furnace. Inside of these little walls, about 3 feet high, there are often vertical conduits or chimneys made to correspond with an opening on the ground level, in order to excite a draught of air in the adjacent parts. When the roasting is once set agoing, these chimneys can be opened or shut at their upper ends, according to the necessities of the process.
Several such furnaces are usually erected in connexion with each other by their lateral walls, and all terminated by a common wall, which forms their posterior part; sometimes they are covered with a shed supported partly by the back wall, built sufficiently high for this purpose. These dispositions are suitable for the roasting ofschlichs, and in general of all matters which are to have several fires; a circumstance often indispensable to a due separation of the sulphur, arsenic, &c.
3. The furnaces employed for roasting the ores and themattesdiffer much, accordingto the nature of the ores, and the size of the lumps. We shall content ourselves with referring to the principal forms.
When iron ores are to be roasted, which require but a simple calcination to disengage the combined water and carbonic acid, egg-shaped furnaces, similar to those in which limestone is burned in contact with fuel, may be conveniently employed; and they present the advantage of an operation which is continuous with a never-cooling apparatus. The analogy in the effects to be produced is so perfect, that the same furnace may be used for either object. Greater dimensions may, however, be given to those destined for the calcination of iron ores. But it must be remembered that this process is applicable only to ores broken into lumps, and not to ores in grains or powder.
It has been attempted to employ the same method a little modified, for the roasting of ores of sulphuret of copper and pyrites, with the view of extracting a part of the sulphur. More or less success has ensued, but without ever surmounting all the obstacles arising from the great fusibility of the sulphuret of iron. For sometimes it runs into one mass, or at least into lumps agglutinated together in certain parts of the furnace, and the operation is either stopped altogether, or becomes more or less languid; the air not being able to penetrate into all the parts, the roasting becomes consequently imperfect. This inconvenience is even more serious than might at first sight appear; for, as the ill-roasted ores now contain too little sulphur to support their combustion, and as they sometimes fall into small fragments in the cooling, they cannot be passed again through the same furnace, and it becomes necessary to finish the roasting in a reverberatory hearth, which is much more expensive.
In the Pyrenees, the roasting of iron ores is executed in a circular furnace, so disposed that the fuel is contained and burned in a kind of interior oven, above which lie the pieces of ore to be calcined. Sometimes the vault of this oven which sustains the ore, is formed of bricks, leaving between them openings for the passage of the flame and the smoke, and the apparatus then resembles certain pottery kilns; at other times the vault is formed of large lumps of ore, carefully arranged as to the intervals requisite to be left for draught over the arch. The broken ore is then distributed above this arch, care being taken to place the larger pieces undermost. This process is simple in the construction of the furnace, and economical, as branches of trees, without value in the forests, may be employed in the roasting. SeeLime-kilnfigures.
In some other countries, the ores are roasted in furnaces very like those in which porcelain is baked; that is to say, the fuel is placed exteriorly to the body of the furnace in a kind of brick shafts, and the flame traverses the broken ore with which the furnace is filled. In such an apparatus the calcination is continuous.
When it is proposed to extract the sulphur from the iron pyrites, or from pyritous minerals, different furnaces may be employed, among which that used in Hungary deserves notice. It is a rectangular parallelopiped of four walls, each of them being perforated with holes and vertical conduits which lead into chambers of condensation, where the sulphur is collected. The ore placed between the four walls on billets of wood arranged as infigs.681,682,683., for the great roastings in the open air, is calcined with the disengagement of much sulphur, which finds more facility in escaping by the lateral conduits in the walls, than up through the whole mass, or across the upper surface covered over with earth; whence it passes into the chambers of condensation. In this way upwards of a thousand tons of pyrites may be roasted at once, and a large quantity of sulphur obtained. SeeCopper.
Swedish furnace
Roasting of Pyrites.—Figs.684,685.represent a furnace which has been long employed at Fahlun in Sweden, and several other parts of that kingdom, for roasting iron pyrites in order to obtain sulphur. This apparatus was constructed by the celebrated Gahn.Fig.684.is a vertical section, in the linek d n ooffig.685., which is a plan of the furnace; the top being supposed to be taken off. In both figures the conduit may be imagined to to be broken off ate; its entire length in a straight line is 43 feet beyond the dotted linee n, before the bend, which is an extension of this conduit. Upon the slopea bof a hillocka b c, lumpsrof iron pyrites are piled upon the pieces of woodi ifor roasting. A conduitd f eforms the continuation of the space denoted byr, which is covered by stone slabs so far asf; and from this point to the chamberhit is constructed in boards. At the beginning of this conduit, there is a recipientg. The chamberhis divided into five chambers by horizontal partitions, which permit the circulation of thevapours from one compartment to another. The oresrbeing distributed upon the billets of woodi i, whenever these are fairly kindled, they are covered with small ore, and then with rammed earthl l. Towards the pointm, for a space of a foot square, the ores are covered with movable stone slabs, by means of which the fire may be regulated, by the displacement of one or more, as may be deemed necessary. The liquid sulphur runs into the recipientg, whence it is laded out from time to time. The sublimed sulphur passes into the conduitf eand the chamberh, from which it is taken out, and washed with water, to free it from sulphuric acid with which it is somewhat impregnated; it is afterwards distilled in cast-iron retorts. The residuum of the pyrites is turned to account in Sweden, for the preparation of a common red colour much used as a pigment for wooden buildings.
The reverberatory furnace affords one of the best means of ustulation, where it is requisite to employ the simultaneous action of heat and atmospherical air to destroy certain combinations, and to decompose the sulphurets, arseniurets, &c. It is likewise evident that the facility thus offered of stirring the matters spread out on the sole, in order to renew the surfaces, of observing their appearances, of augmenting or diminishing the degree of heat, &c., promise a success much surer, a roasting far better executed, than by any other process. It is known, besides, that flame mingled with much undecomposed air issuing from the furnace, is highly oxidizing, and is very fit for burning away the sulphur, and oxidizing the metals. Finally, this is almost the only method of rightly roasting ores which are in a very fine powder. If it be not employed constantly and for every kind of ore, it is just because more economy is found in practising calcination in heaps, or on areas enclosed by walls; besides, in certain mines, a very great number of these furnaces, and many workmen, would be required to roast the considerable body of ores that must be daily smelted. Hence there would result from the construction of such apparatus and its maintenance a very notable outlay, which is saved in the other processes.
But in every case where it is desired to have a very perfect roasting, as for blende from which zinc is to be extracted, for sulphuret of antimony, &c., or even for ores reduced to a very fine powder, and destined for amalgamation, it is proper to perform the operation in a reverberatory furnace. When very fusible sulphurous ores are treated, the workman charged with the calcination must employ much care and experience, chiefly in the management of the fire. It will sometimes, indeed, happen, that the ore partially fuses; when it becomes necessary to withdraw the materials from the furnace, to let them cool and grind them anew, in order to recommence the operation. The construction of these furnaces demands no other attention than to give to the sole or laboratory the suitable size, and so to proportion to this the grate and the chimney that the heating may be effected with the greatest economy.
The reverberatory furnace is always employed to roast the ores of precious metals, and especially those for amalgamation; as the latter often contain arsenic, antimony, and other volatile substances, they must be disposed of in a peculiar manner.
The sole, usually very spacious, is divided into two parts, of which the one farthest off from the furnace is a little higher than the other. Above the vault there is a space or chamber in which the ore is deposited, and which communicates with the laboratory by a vertical passage; which serves to allow the ore to be pushed down, when it is dried and a little heated. The flame and the smoke which escape from the sole or laboratory pass into condensing chambers, before entering into the chimney of draught, so as to deposit in them the oxide of arsenic and other substances. When the ore on the part of the sole farthest from the grate has suffered so much heat as to begin to be roasted, has became less fusible, and when the roasting of that in the nearer part of the sole is completed, the former is raked towards the fire-bridge, and its ustulation is finished by stirring it over frequently with a paddle, skilfully worked, through one of the doors left in the side for this purpose. The operation is considered to be finished when the vapours and the smell have almost wholly ceased; its duration depending obviously on the nature of the ores.
When this furnace is employed to roast very arsenical ores, as the tin ores of Schlackenwald in Bohemia, and at Ehrenfriedersdorf in Saxony, the arsenical pyrites of Geyer (in Saxony), &c., the chambers of condensation for the arsenious acid are much more extensive than in the furnaces commonly used for roasting galena, copper, or even silver ores.
Reverberatory furnaceReverberatory furnace
Figs.686,687,688.represent a reverberatory furnace employed in the smelting works of Lautenthal, in the Hartz, for roasting the schlichs of lead ores, which contain much blende or sulphuret of zinc. Infig.686.we see that the two partsA B,B C, are absolutely like, the two furnaces being built in one body of brickwork.Fig.687.is the plan of the furnaceB C, taken at the levelE Foffig.686.Fig.688.is a vertical section of the similar furnaceA B, taken in the prolongation of the lineG Hinfig.687.
ais the fire-place of the furnace, its grate and ash-pit.bis the conduit of vaporization, which communicates with the chambersc;c, chambers into which the vaporized substances are deposited;d, chimney for the escape of the smoke of the fire-placea, after it has gone through the spaceb c c;e′, is the charging door, with a hook hanging in front to rest the long iron rake upon, with which the materials are turned over;f, chamber containing a quantity of schlich destined for roasting; this chamber communicates with the vaulted corridor (gallery)D, seen infig.686.;g, orifice through which the schlich is thrown into the furnace;h, area or hearth of the reverberatory furnace, of which the roof is certainly much too high;i, channels for the escape of the watery vapours;k l, front arcade, between which and the furnace, properly speaking, are the two orifices of the conduits, which terminate at the channelsm,m′.mis the channel for carrying towards the chimneyd, the vapours which escape by the doore′.nis a walled-up door, which is opened from time to time, to take out of the chambersc,c, the substances that may be deposited in them.
At the smelting works of Lautenthal, in such a roasting furnace, from 6 to 9 quintals (cwts.) of schlich are treated at a time, and it is stirred frequently with an iron rake upon the altarh. The period of this operation is from 6 to 12 hours, according as the schlich may be more or less dry, more or less rich in lead, or more or less charged with blende. When the latter substance is abundant, the process requires 12 hours, with about 60 cubic feet of cleft billets for fuel.
In such furnaces are roasted the cobalt ores of Schneeberg in Saxony, the tin ores of Schlackenwald in Bohemia, of Ehrenfriedersdorf in Saxony, and elsewhere; as also the arsenical pyrites at Geyer in Saxony. But there are poison towers and extensive condensing chambers attached in the latter case. SeeArsenic.
Figs.689,690,691.represent the reverberatory furnace generally employed in the Hartz, in the district of Mansfeldt, Saxony, Hungary, &c., for the treatment of black copper, and for refining rose copper upon the great scale. An analogous furnace is used at Andreasberg for the liquefaction or purification of the mattes, and for workable lead when it is much loaded with arsenic.
Reverberatory furnaceReverberatory furnace
Fig.689.presents the elevation of the furnace parallel to the lineI K, of the planfig.690.; which plan is taken at the level of the tuyèren, offig.691.;fig.691.is a vertical section in the lineL M,fig.690.krepresents one of two basins of reception, brasqued with clay and charcoal;n,n, two tuyères, through which enters the blast of two pairs of bellows, like those shown atCupellation ofSilver;q, door by which the matter to be melted is laid upon the sole of the furnace;v,v, two points where the sole is perforated, when necessary to run off the melted matter into either of the basinsh;x, door through which the slags or cinders floating upon the surfaceof the melted metal are raked out;y, door of the fire-place. The fuel is laid upon a grate above an ash-pit, and below the arch of a reverberatory which is contiguous to the dome or cap of the furnace properly so called. In the section,fig.691., the following parts may be noted: 1, 2, 3, mason-work of the foundation; 4, vapour channels or conduits, for the escape of the humidity; 5, bed of clay; 6, brasque composed of clay and charcoal, which forms the concavity of the hearth.
Liquation furnace
Figs.692,693,694., show the furnace employed for liquation in one of the principal smelting works of the Hartz.Fig.694.exhibits the working area charged with the liquation cakes and charcoal, supported by sheets of wrought iron; being an image of the process in action.Fig.693.is the plan, in the lineF,G, offig.692.
A liquation cake is composed of—
Black copper holding at least 5 or 6loths(21⁄2or 3 oz.) of silver per cwt., and weighing 90 to 96 lbs.
Lead obtained from litharge, 2 cwts. Litharge,1⁄2cwt.
From 30 to 32 cakes are successively worked in one operation, which lasts about 5 hours; the furnace is brought into action, as usual, with the aid of slags; then a little litharge is added; when the lead begins to flow, the copper is introduced, and when the copper flows, lead is added, so that the mixture of the metals may be effected in the best way possible.
From 8 to 16 of these cakes (pains) are usually placed in the liquation furnace,figs.692,693,694.The operation lasts 3 or 4 hours, in which time about 11⁄2quintals of charcoal are consumed. The cakes are covered with burning charcoal, supported, as I have said, by the iron plates. The workable lead obtained flows off towards the basin in front of the furnace; whence it is laded out into moulds set alongside. Seefig.693.If the lead thus obtained be not sufficiently rich in silver to be worth cupellation, it is employed to form new liquation cakes. When it contains from 5 to 6 loths of silver per cwt., it is submitted to cupellation in the said smelting works. SeeSilver.
Water blowing engine
Thetrompe, or water-blowing engine,figs.695,696,697.Fig.695.is the elevation;fig.696.is a vertical section, made at right angles to the elevation. The machine is formed of two cylindrical pipes, the bodies of thetrompeb b, set upright, called the funnels, which terminate above in a water cisterna, and below in a close basin underc, called thetubordrum. The conical partp, of the funnel has been calledetranguillon, beingstrangled, as it were, in order that the water discharged into the body of the trompe shall not fill the pipe in falling, but be divided into many streamlets. Below thisnarrow part, eight holes,q q, are perforated obliquely through the substance of the trompe, called the vent-holes or nostrils, for admitting the air, which the water carries with it in its descent. The air afterwards parts from the water, by dashing upon a cast-iron slab, placed in thedrumupon the pedestald. An aperturel, at the bottom of the drum, allows the water to flow away after its fall; but, to prevent the air from escaping along with it, the water as it issues is received in a chestl m o n, divided into two parts by a vertical slide-plate betweenm n. By raising or lowering this plate, the water may be maintained at any desired level within the drum, so as to give the included air any determinate degree of pressure. The superfluous water then flows off by the holeo.
The air-pipee f,fig.696., is fitted to the upper part of thedrum; it is divided, at the pointf, into three tubes, of which the principal one is destined for the furnace of cupellation, whilst the other twog g, serve for different melting furnaces. Each of these tubes ends in a leather pocket, and an iron nose-pipek, adjusted in the tuyère of the furnace. At Pesey, and in the whole of Savoy, a floodgate is fitted into the upper cisterna, to regulate the admission of water into the trome; but in Carniola, the funnelpis closed with a wooden plug, suspended to a cord, which goes round a pulley mounted upon a horizontal axis, as shewn infig.697.By the plugabeing raised more or less, merely the quantity of water required for the operation is admitted. The plug is pierced lengthwise with an oblique holec c, in which the small tubecis inserted, with its top some way above the water level, through which air may be admitted into the heart of the column descending into the trompep q.
The ordinary height of the trompe apparatus is about 26 or 27 feet to the upper level of the water cistern; its total length is 11 mètres (36 feet 6 inches), and its width 2 feet, to give room for the drums. It is situated 10 mètres (331⁄3feet) from the melting furnace. This is the case at the smelting works of Jauerberg, in Upper Carniola.
OF THE ASSAY OF ORES.
Assays ought to occupy an important place in metallurgic instructions, and there isreason to believe that the knowledge of assaying is not sufficiently diffused, since its practice is so often neglected in smelting houses. Not only ought the assays of the ores under treatment, to be frequently repeated, because their nature is subject to vary; but the different products of the furnaces should be subjected to reiterated assays, at the several periods of the operations. When silver or gold ores are in question, the docimastic operations, then indispensable, exercise a salutary controul over the metallurgic processes, and afford a clear indication of the quantities of precious metal which they ought to produce.
By the titleAssays, in a metallurgic point of view, is meant the method of ascertaining for any substance whatever, not only the presence and the nature of a metal, but its proportional quantity. Hence the operations which do not lead to a precise determination of the metal in question, are not to be arranged among the assays now under consideration. Experiments made with the blow-pipe, although capable of yielding most useful indications, are like the touchstone in regard to gold, and do not constitute genuine assays.
Three kinds of assays may be practised in different circumstances, and with more or less advantage upon different ores. 1. The mechanical assay; 2. the assay by the dry way; 3. the assay by the humid way.
1.Of mechanical assays.—These kinds of assays consist in the separation of the substances mechanically mixed in the ores, and are performed by a hand-washing, in a small trough of an oblong shape, called asebilla. After pulverizing with more or less pains the matters to be assayed by this process, a determinate weight of them is put into this wooden bowl with a little water; and by means of certain movements and some precautions, to be learned only by practice, the lightest substances may be pretty exactly separated, namely, the earthy gangues from the denser matter or metallic particles, without losing any sensible portion of them. Thus aschlichof greater or less purity will be obtained, which may afford the means of judging by its quality of the richness of the assayed ores, and which may thereafter be subjected to assays of another kind, whereby the whole metal may be insulated.
Washing, as an assay, is practised on auriferous sands; on all ores from thestamps, and even onschlichsalready washed upon the great scale, to appreciate more nicely the degree of purity they have acquired. The ores of tin in which the oxide is often disseminated in much earthy gangue, are well adapted to this species of assay, because the tin oxide is very dense. The mechanical assay may also be employed in reference to the ores whose metallic portion presents an uniform composition, provided it also possesses considerable specific gravity. Thus the ores of sulphuret of lead (galena) being susceptible of becoming almost pure sulphurets (within 1 or 2per cent.) by mere washing skilfully conducted, the richness of that ore in pure galena, and consequently in lead, may be at once concluded; since 120 of galena contain 104 of lead, and 16 of sulphur. The sulphuret of antimony mingled with its gangue may be subjected to the same mode of assay, and the result will be still more direct, since the crude antimony is brought into the market after being freed from its gangue by a simple fusion.
The assay by washing is also had recourse to for ascertaining if thescoriæor other products of the furnaces contain some metallic grains which might be extracted from them by stamping and washing on the great scale; a process employed considerably with thescoriæof tin and copper works.
Of assays by the dry way.—The assay by the dry way has for its object, to show the nature and proportion of the metals contained in a mineral substance. To make a good assay, however, it is indispensably necessary to know what is the metal associated with it, and even within certain limits, the quantity of the foreign bodies. Only one metal is commonly looked after; unless in the case of certain argentiferous ores. The mineralogical examination of the substances under treatment, is most commonly sufficient to afford data in these respects; but the assays may always be varied with different views, before stopping at a definite result; and in every instance, only such assays can be confided in, as have been verified by a double operation.
This mode of assaying requires only a little experience, with a simple apparatus; and is of such a nature as to be practised currently in the smelting works. The air furnace and crucibles employed are described in all good elementary chemical books. These assays are usually performed with the addition of a flux to the ore, or some agent for separating the earthy from the metallic substances; and they possess a peculiar advantage relative to the smelting operations, because they offer many analogies between results on the great scale and experiments on the small. This may even enable us often to deduce, from the manner in which the assay has succeeded with a certain flux, and at a certain degree of heat, valuable indications as to the treatment of the ore in the great way. SeeFurnace.
In the smelting houses which purchase the ore, as in Germany, it is necessary to bestow much attention upon the assays, because they serve to regulate the quality andthe price of the schlichs to be delivered. These assays are not by any means free from difficulties, especially when ores containing several useful metals are treated, and which are to be dosed or proportioned; ores, for example, including a notable quantity of lead, copper, and silver, mixed together.
In the central works of the Hartz, as well as in those of Saxony, theschlichsas delivered are subjected to docimastic assays, which are verified three times, and by three different persons, one of whom is engaged for the interests of the mining partners, another for that of the smelting house, and a third as arbiter in case of a difference. If the first two results of assaying differ by1⁄2loth(or1⁄4ounce) of silver per cwt. ofschlich, the operations must be resumed; but this rarely happens. When out of the three assays, the one differs from the two others by no more than1⁄4loth of silver per cwt., but by more in one, and by less in another, the mean result is adopted. As to the contents of theschlichin lead, the mean results of the assays must be taken. The differences allowed, are three pounds for theschlich, when it contains from 12 to 30 per cent. of lead, increasing to six pounds forschlich, when it contains less than 55 per cent. of that metal.
Assaying forms in great establishments, an important object in reference to time and expense. Thus, in the single work of Franckenscharn, in the Hartz, no less than 300 assays have to be made in a threefold way, every Monday, without taking into account the several assays of the smelting products which take place every Thursday. Formerly fluxes more or less compound were employed for these purposes, and every assay cost about fifteen pence. At present all these assays are made more simply, by much cheaper methods, and cost a penny farthing each upon an average.
Of the assays by the humid way.—The assays by the humid way, not reducible to very simple processes, are true chemical analyses, which may in fact be applied with much advantage, either to ores, or to the products of the furnace; but which cannot be expected to be practised in smelting-houses, on account of the complication of apparatus and reagents they require. Moreover, an expert chemist is necessary to obtain results that can be depended on. The directors of smelting-houses, however, should never neglect any opportunities that may occur of submitting the materials operated upon, as well as their products, to a more thorough examination than the dry way alone can effect. One of the great advantages of similar researches is, to discover and appreciate the minute quantities of injurious substances which impair the malleability of the metals, which give them several bad qualities, about whose nature and cause, more or less error and uncertainty prevail. Chemical analysis, rightly applied to metallurgy, cannot fail to introduce remarkable improvements into the processes.—See the different metals, in their alphabetical places.
For assays in the dry way, both of stony and metallic minerals, the process of Dr. Abich deserves recommendation. In consists in mixing the pulverized mineral with 4 or 6 times its weight of carbonate of baryta in powder, fusing the mixture at a white heat, and then dissolving it after it cools, in dilute muriatic acid. The most refractory minerals, even corundum, cyanite, staurolite, zircon, and felspar, yield readily to this treatment. This process may be employed with advantage upon poor refractory ores. The platinum crucible, into which the mixed materials are put for fusion, should be placed in a Hessian crucible, and surrounded with good coak.
The following tabular view of the metallic produce of the British mines, is given by two very skilful observers, in a work published in 1827, entitledVoyage Metallurgique en Angleterre, par MM. Dufrénoy et Elie de Beaumont:—
[32]I have converted the weights of lead and cast iron, given in kilogrammes, into tons, at the rate of 1000 kilogrammes per ton; which is sufficiently near.
[32]I have converted the weights of lead and cast iron, given in kilogrammes, into tons, at the rate of 1000 kilogrammes per ton; which is sufficiently near.
The manganese raised in England exceeds 2000 tons.
M. Heron de Villefosse inserted in the last number of theAnnales des Minesfor 1827, the following statistical view of the metallic products of France:—
The total value of which is estimated at 80 millions of francs; or about 3,400,000 pounds sterling.