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 line k d n o of fig. 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 at e ; its entire length in a straight line is 43 feet beyond the dotted line e n , before the bend, which is an extension of this conduit. Upon the slope a b of a hillock a b c , lumps r of iron pyrites are piled upon the pieces of wood i i for roasting. A conduit d f e forms the continuation of the space denoted by r , which is covered by stone slabs so far as f ; and from this point to the chamber h it is constructed in boards. At the beginning of this conduit, there is a recipient g . The chamber h is divided into five chambers by horizontal partitions, which permit the circulation of the vapours from one compartment to another. The ores r being distributed upon the billets of wood i i , whenever these are fairly kindled, they are covered with small ore, and then with rammed earth l l . Towards the point m , 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 nece ary. The liquid sulphur runs into the recipient g , whence it is laded out from time to time. The sublimed sulphur pa es into the conduit f e and the chamber h , 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, . 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, ., promise a succe much surer, a roasting far better executed, than by any other proce . It is known, besides, that flame mingled with much undecomposed air i uing 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 proce es. 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, ., 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 nece ary 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 pa age; 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 pa 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 le 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), ., 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. IMG:4147767755307473660_illo0823a.png:Reverberatory furnace IMG:4147767755307473660_illo0823b.png:Reverberatory 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. In fig. 686. we see that the two parts A B , B C , are absolutely like, the two furnaces being built in one body of brickwork. Fig. 687. is the plan of the furnace B C , taken at the level E F of fig. 686. Fig. 688. is a vertical section of the similar furnace A B , taken in the prolongation of the line G H in fig. 687. a is the fire-place of the furnace, its grate and ash-pit. b is the conduit of vaporization, which communicates with the chambers c ; c , chambers into which the vaporized substances are deposited; d , chimney for the escape of the smoke of the fire-place a , after it has gone through the space b 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 in fig. 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 channels m , m′ . m is the channel for carrying towards the chimney d , the vapours which escape by the door e′ . n is a walled-up door, which is opened from time to time, to take out of the chambers c , 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 altar h . The period of this operation is from 6 to 12 hours, according as the schlich may be more or le dry, more or le rich in lead, or more or le charged with blende. When the latter substance is abundant, the proce 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. See Arsenic . Figs. 689 , 690 , 691. represent the reverberatory furnace generally employed in the Hartz, in the district of Mansfeldt, Saxony, Hungary, ., for the treatment of black copper, and for refining rose copper upon the great scale. An analogous furnace is used at Andreas berg for the liquefaction or purification of the mattes, and for workable lead when it is much loaded with arsenic. IMG:4147767755307473660_illo0823c.png:Reverberatory furnace IMG:4147767755307473660_illo0824a.jpg:Reverberatory furnace Fig. 689. presents the elevation of the furnace parallel to the line I K , of the plan fig. 690. ; which plan is taken at the level of the tuyère n , of fig. 691. ; fig. 691. is a vertical section in the line L M , fig. 690. k represents 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 at Cupellation of Silver ; 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 nece ary to run off the melted matter into either of the basins h ; x , door through which the slags or cinders floating upon the surface of 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. IMG:4147767755307473660_illo0824b.png: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 proce in action. Fig. 693. is the plan, in the line F , G , of fig. 692. A liquation cake is composed of — Black copper holding at least 5 or 6 loths (2 1 ⁄ 2 or 3 oz.) of silver per cwt., and weighing 90 to 96 lbs. Lead obtained from litharge, 2 cwts. Litharge, 1 ⁄ 2 cwt. From 30 to 32 cakes are succe ively 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 po ible. 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 1 1 ⁄ 2 quintals 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. See fig. 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. See Silver . IMG:4147767755307473660_illo0825.png:Water blowing engine The trompe , 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 the trompe b b , set upright, called the funnels, which terminate above in a water cistern a , and below in a close basin under c , called the tub or drum . The conical part p , of the funnel has been called etranguillon , being strangled , 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 this narrow 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 the drum upon the pedestal d . An aperture l , 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 i ues is received in a chest l m o n , divided into two parts by a vertical slide-plate between m 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 pre ure. The superfluous water then flows off by the hole o . The air-pipe e f , fig. 696. , is fitted to the upper part of the drum ; it is divided, at the point f , into three tubes, of which the principal one is destined for the furnace of cupellation, whilst the other two g g , serve for different melting furnaces. Each of these tubes ends in a leather pocket, and an iron nose-pipe k , adjusted in the tuyère of the furnace. At Pesey, and in the whole of Savoy, a floodgate is fitted into the upper cistern a , to regulate the admi ion of water into the trome; but in Carniola, the funnel p is closed with a wooden plug, suspended to a cord, which goes round a pulley mounted upon a horizontal axis, as shewn in fig. 697. By the plug a being raised more or le , merely the quantity of water required for the operation is admitted. The plug is pierced lengthwise with an oblique hole c c , in which the small tube c is 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 trompe p 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 (33 1 ⁄ 3 feet) from the melting furnace. This is the case at the smelting works of Jauerberg, in Upper Carniola. OF THE ASSAY OF ORES. A ays ought to occupy an important place in metallurgic instructions, and there is reason to believe that the knowledge of a aying is not sufficiently diffused, since its practice is so often neglected in smelting houses. Not only ought the a ays 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 a ays, 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 proce es, and afford a clear indication of the quantities of precious metal which they ought to produce. By the title A ays , 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 a ays 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 a ays. Three kinds of a ays may be practised in different circumstances, and with more or le advantage upon different ores. 1. The mechanical a ay; 2. the a ay by the dry way; 3. the a ay by the humid way. 1. Of mechanical a ays. —These kinds of a ays 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 a sebilla . After pulverizing with more or le pains the matters to be a ayed by this proce , 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 a schlich of greater or le purity will be obtained, which may afford the means of judging by its quality of the richne of the a ayed ores, and which may thereafter be subjected to a ays of another kind, whereby the whole metal may be insulated. Washing, as an a ay, is practised on auriferous sands; on all ores from the stamps , and even on schlichs already 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 di eminated in much earthy gangue, are well adapted to this species of a ay, because the tin oxide is very dense. The mechanical a ay may also be employed in reference to the ores whose metallic portion presents an uniform composition, provided it also po e es considerable specific gravity. Thus the ores of sulphuret of lead (galena) being susceptible of becoming almost pure sulphurets (within 1 or 2 per cent. ) by mere washing skilfully conducted, the richne 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 a ay, 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 a ay by washing is also had recourse to for ascertaining if the scoriæ or other products of the furnaces contain some metallic grains which might be extracted from them by stamping and washing on the great scale; a proce employed considerably with the scoriæ of tin and copper works.