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The second part of the apparatus consists of a wooden frame, cut out of a solid board 1 inch thick, and supplied with two large iron handles. This framo is the same length as the box holding the zinc plates, but 3 inches narrower. It contains on each side, parallel to the direction of the zinc plates, twelve slits 1 inch long, which hold silver bands 4 inch broad, and the thickness of a threepenny-piece. These silver bands aro passed through the slits in the board, so as to form on cach side of it six loops, 114 inches in length, and 1 inch wide. The six loops on one side are exactly opposite to those on the other side of the board, at a distance of about 9 inches. They are intended to hold the slabs of argentic chloride, which are 12 inches long, 10 inches high, and abont & inch thick, and are put through these loops lengthwise, projecting on each end about i inch beyond the silver bands.
The whole frame holds, as before stated, six of these slabs of argentic chloride, which are placed between the six spaces formed by the seven zinc plates, from which latter they are about 4 inch apart on each side.
The projecting horizontal strips of silver jammed into the sides of the lower frame are then connected with the ends of the silver forming the loops in which the argentic, chloride is suspended; and the whole apparatus thus charged is placed in a tub filled with water. After a short time, galvanic action is discernible; the liquid gets gradually warmer, and a strong galvanic current is observed. After about twentyfour hours, the action has nearly ceased, and the whole argentic chloride is found to bo completely reduced to metallic silver, which retains in the silver loops the same shape, and outwardly also, nearly the same appearance as when tirst introduced as argentic chloride. The latter contains always more or less chloride of copper, (eliminated, together with the silver during the operation of retinage by chlorine,) which is reduced together with the chloride of silver; in fact, this soluble chloride of copper helps to act as an exciting liquor for the battery. In the first experiments, a weak solution of salt (chloride of solium) was used as exciting liquor; but it was found that this could be dispensed with, and only common water used, (the action, however, is, in this case, a little retarded and does not become powerful until about two hours after the battery is set.). By using a part of the resulting liquor from a previous reduction of argentic chloride, and which contains chloride of zinc, it has been found that the galvanic action sets in very rapidly, and accelerates thereby the completion of the reduction.
No acid is used; and, therefore, the amount of zinc used in each reduction has invariably been found to be almost the theoretical qnantity required to combine the chlorine of the argentic chloride treated with the metallic zinc, in order to form chloride of zinc.
The qnantity of metallic zinc thus used was always from 24 to 25 per cent. of the weight of the argentic chloride reduced.
The reduced silver is boiled out in acidulated water, in order to remove the basic and oxy-chlorides, and finally in pure water, wbile still suspended in the silver loops. As soon as it is taken off the last boiling, it is immediately ready for the melting pot, since the heat from the boiling water dries the porous mass of silver sufficiently to allow of its immediato melting. The seven zinc plates, when first used, weiglı about 140 pounds avoirdupois; the six slabs of argentic chloride, of the dimensions already given, weigh about 1,400 ounces troy.
The zinc plates are used over again, until too thin for that purpose, when they are remelted, and cast into new plates. It has been found that tho quantity of zinc used is little, if at all, increased by prolonging the time of connection with the silver plates after the reduction is completed; tho whole apparatus, when once set in operation, can therefore bo left to itself until it is found convenient to melt the reduced silver.
While this apparatus reduces the argentic chlorido much quicker than if the latter is simply placed in contact with zinc or iron plates, it obviates any handling of tho argentic chloride from the time the latter has been placed in the silver loops until tho reduced silver is ready for the melting-pot-advantages which have been fully appreciated by those who formerly had to resort to tedious and disagreeable manipulations.
SMELTING SILVER ORES.
The base metals used for the extraction of silver from its ores by smelting are lead and copper, and the different methods employed may be accordingly classified under the general headings of "extraction by means of lead," and "extraction by means of copper.” The latter is as yet not in operation in the United States, and I therefore pass over this subject for the present. The extraction of silver from its ores by means of lead has, on the contrary, assumed such proportions in the West during the last year, that a discussion of this business at the present time seems important. Although it is here impracticable to go into the details of all the different processes in use in various parts of the world, I may still hope to do some good by dwelling especially on those evidently suited best for the conditions under which the extraction of silver by smelting may be most economically carried on in our western mining districts.
Silver extraction by means of lead is classified according to the shape of the furnaces used for the purpose. Thus we have:
I. Smelting in the open hearth;
All these processes have one common purpose, the reduction of the lead to the metallic state and the concentration of the metallic silver in it; but the chemical reactions by which this is accomplished often differ greatly, and the efficiency of each method varies with local circumstances. To know therefore the reactions, and to weigh the circumstances in their economical bearings, is the first duty of those who wish to select a process for a particular locality.
I. SMELTING IN THE OPEN HEARTH.
This method is the oldest and simplest; and up to the present day very few improvements bave been made in its original features. It has been and is still employed principally in the United States, and in Scotland and the north of England. The process as practiced in the American hearth is distinguished from the method followed in England and Scotland, chiefly by the employment of hot blast in smelting very pure raw ores. The ores smelted in the Scotch hearth must likewise be free from silica, but not necessarily from other gangue. They are prepared for smelting by roasting in reverberatories, and the blast employed in smelting is cold. In both processes, inferior kinds of fuel, such as wood, peat, &c., can be used. The first condition of the economical use of the hearth'in smelting lead ores or a mixture of these with silver ores is therefore purity of ore, especially absence of silica and of foreign sulphureted metals. The ore ought to be in the form of pieces, not crushed. If brought to the smelting.works in the latter condition, it ought to be agglomerated in reverberatories before it is smelted in the hearth, but if this bas to be done it would be really more economical to finish the smelting process also in the reverberatory.
The above conditions being primarily requisite for successful smelting in the hearth, and a large loss of both lead and silver by volatilization being certain, unless a very extensive and costly system of condensing
chambers or canals is connected with the work, it is evident that, for these reasons alone, (though others might be adduced,) this method cannot come into use economically in the western mining districts. The ores there, though often rich in silver, are rarely free from siliceous gangue, foreign sulphurets, and antimoniurets; and dressing is prevented in some localities by the scarcity of water, and in nearly all of them at present by the high price of labor. Besides, it is extremely difficult to dress rich silver ores without incurring an enormous loss of the precious metal.
It is therefore useless at the present time to dwell upon the process of smelting argentiferous lead ores in the open hearth; and I refer those who may wish to inform themselves more fully on this head to the excellent metallurgy of Professor Kerl, which has of late been made accessible to American readers by the translation of Messrs. Crookes and Roehrig
II. SMELTING IN REVERBERATORY FURNACES. The application of the reverberatory furnace to lead smelting is limited by many conditions similar to those enumerated in the preceding paragraph.
There are two processes in use, which are executed in the reverberatory: the roasting and reducing, and the roasting and precipitating process. Foremost as a condition for the economical employment of the roasting and reducing process is the absence, to a certain extent, of siliceous or argillaceous gangue. Whenever the ore contains more than 4 per cent of these substances, or less than 58 per cent. of lead, this process cannot be executed satisfactorily any longer, because silicate of lead is formed, which is hard to reduce. Moreover the process permits the presence of lime, heavy spar, zincblende, and other foreign sulphurets in small quantities only.
An important drawback in the employment of the reverberatory processes is also the proportionately large quantity of fuel required, and in this country the item of labor, which is larger in proportion to the production than in shaft-furnace smelting. The loss of copper and the deterioration of the lead by the same metal is another objection.
As mentioned above, there are two reverberatory processes in use, the roasting and reducing, and the roasting and precipitating process. These are again carried out in various localities in a somewhat different manner, the deviations consisting principally in a slower or quicker roasting and reducing, or the employment of a lower or higher temperature.
Roasting and reducing processes. Carinthian process. It is the object of this process* to accomplish, at the lowest possible temperature, the reduction of a maximum percentage of very pure lead and the formation of a poor slag, which may be thrown away; but this is only possible with very pure ores, and involves, moreover, a small production, as well as a great expenditure of time, fuel, and labor. Success is, therefore, the more probable, the purer the ores and the cheaper fuel and labor. The process cousists in a roasting of the galena at a gradually increasing temperature. During the first period oxide of lead and sulphate of lead are formed in suicient quantity to make a reduction of the larger part of the lead to the metallic state possible by their action on undecomposed galena in the
* Kerl’s IIüttenkunde, vol. ii, p. 51. H. Ex. 10-28
second period. To facilitate the second reaction, the temperature is in. creased and the ore is frequently turned. The following are the realtions: PbS+ 2 PbO = 3 Pb + S02 and PbS + PbO,SO3 = 2 Pb + 250, Part of the galena is changed to sub-sulphide of lead, Pb,S, wbich is also reduced to metallic lead by the oxides formed in the furnace.* To lessen the loss of lead by volatilization, the metal reduced first at low temperature is allowed to run immediately down the inclined floor, and out of the furnace. It is of great purity. By a continued stirring and turning of the charge the opportunity to oxidize is given to nei particles of galena, and by the reaction of the oxides on the sulphides more metal is continually reduced. Thus the galena is more and more decomposed until finally a point is reached, when the charge consists, for the greatest part, of oxide and sulphate of lead, together with sinall quantities of oxy-sulphuret of lead, a mixture, from which no more metal is reduced. Then the third period commences, that of the " leadpressing," i. e., the working of the remaining doughy mass at a higher temperature after mixing small coal with it. Hereby the free oxide of lead, and that contained in the oxy-sulphuret, are reduced, and the suiphide of lead freed from the latter, as well as that now formed from the sulphate, is changed by an excess of oxide into metallic lead, so that at last a proportionally small quantity of slag, poor in lead, remains in the furnace. When, however, galena rich in silver is worked, the slags retain a great deal of that metal on account of the sulphur contained in them, and the great aflinity of silver for it. It is thus clear that only ores containing little silver should be worked by this process.
The lead obtained in the last period of the process, at a high temperature, is less pure than that reduced in the earlier period, because other oxides of metals, which are usually present, are easier reduced in a high temperature. In order to refine this impure lead it is remelted at a low temperature in the reverberatory.
The following remarks on the Carinthian process are from an article by Professor M. L. Gruner, of Paris, republished in the Berg und Hüttenmännische Zeitung:
Ores containing little blende and carbonate of lime are treated at a slowly-increasing temperature, and a very pure lead is obtained, while a maximum yield is secured; but the amount of fuel used is very large. The lower the percentage of metal in the ore the larger the loss of lead. While with ores containing 82 per cent. the smelters are permitteil to lose 2 per cent., the loss from ores containing 58 per cent. is often 14 per cent, of the metal.
The Carinthian process is especially characterized by its long duration, (210 kilograms, 462.96 pounds avoirdupois, of ore are smelted in ten to twelve hours,) which is principally caused by the feature that the air can only get in contact with the ore by passing through the grate. For this reason the use of wood is more favorable than that of coal. In other smelting-works, as, for instance, at Nouvelle Montagne, near Engis, in Belgium, where stone-coal is used as fuel, air is therefore permitted to enter the smelting-room through canals lying in the fire-bridge along its entire length. It would be a decided improvement on the Carinthian process if the so-called period of pressing were shortened to two or three operations, and the remaining slag were then smelted in shaft-furnaces This is done at Nouvelle Montagne. Charges of from 550 to 600 kilograms, equal 1212.5 to 1322.7 pounds avoirdupois, are treated in a reverberatory heated with stone-coal, which has a sump under the flue for the
* Plattner, Berg und Hüttenmännische Zeitung, 1854, p. 22.
reception of the lead. At the end of the stirring” period some stonecoal is mixed with the rich remaining ore; its contents in lead are then reduced to 25 to 30 per cent., and the reduction of the remainder is effected in a shaft-furnace. The ore is spread in a reverberatory in a deeper layer than in other works, and by virtue of the higher temperature the process is finished in less time. To save fuel, double furnaces, (with two bearths, one above the other,) have been tried in Carinthia, but they have not been permanently introduced, because the work was inconvenient, and repairs became frequently necessary. It would have been better to build the hearths side by side on different levels, and to connect the lower one, to which the grate is attached, with the upper one opposite the grate by a side-canal, opposite the entrance of which the upper one should have its working-door. On the upper hearth, which should have a slightly-inclined floor, the roasting would be done, while the lower one would be destined for the periods of " stirring” and “pressing." The transfer of the ore from the upper to the lower hearth would have to be effected by a door, which could be closed at will, located opposite the connecting canal on the npper hearth. Such an arrangement would have been preferable to the return to the old furnace, especially if the rich remaining slag taken out after a shortened “pressing” period, had been smelted in a shaft.furnace.
The French process. This process* is based on the fact that galena, when roasted for a long time at a low and gradually-increasing temperature, is principally changed into sulphate of lead and less into oxide; and that if at a certain period the roasting is interrupted while there is yet some undecomposed galena present with a preponderating mass of sulphate, and the temperature is then increased, without reaching the smelting point, the constituents of the charge decompose each other in such a way that, with but a trifling reduction of metal, oxide of lead is principally formed, while sulphurous acid escapes. The oxide of lead is then easily reduced by treating it with charcoal, Pbs +3Pb0,30= 4Pb 0 + 4802
, () 4Pb 0 +4C = 4Pb + 4CO. If, besides the oxide of lead, sulphate should be present it also will be decomposed by the charcoal. According to Gay-Lussac t, when an excess of carbon is present, the sulphate is changed into sulphide of lead while carbonic acid escapes. When equal equivalents of sulphate and carbon are present at a low temperature, carbonic acid is developed and orily half of the sulphate of lead is changed into sulphide (2 PbO,SO3 +20= Pb0,803 + Pb S+2002). When the temperature is increased to a glowing heat, the sulphate and sulphide of lead mutually react so that metallic lead and sulphurous acid result. When two equivalents of carbon are brought together with four equivalents of sulphate of lead one equivalent of sulphide is first originated at a moderate glowing heat, (4 Pb0,803+20=3 Pb0,803 + Pb S +2C02,) which at an increased temperature is changed by the action of the three equivalents of sulphate of lead into sulphurous acid and oxide of lead, (3 Pbo, SO,+ PbS=4 Pb0+4 SO2).
This process was originated and for some time operated at Albertsville and Poullaouen, in Brittany, and is at present yet in use in several other European establishinents. By it ores can be worked which contain some silica, because at the low temperature used in roasting, a silicate of lead is not so easily formed. But this is no longer true when the contents of silica exceed five per cent. In that case much lead passes
* Kerl's Hüttenkunde, vol. ii, page 78. Erdmann's Journal für praktische Chemie, xi, 68.