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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 consists 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 sufficient 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 Hüttenkunde, vol. ii, p. 51.
H. Ex. 10- -28
second period. To facilitate the second reaction, the temperature is increased and the ore is frequently turned. The following are the reac tions: PbS+2 PbO = 3 Pb + SO2 and PbS+ PbO,SO, = 2 Pb + 2SO2. Part of the galena is changed to sub-sulphide of lead, Pb2S, which 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 new 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 small 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 sulphide 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 affinity 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 permitted 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 hearths, 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 upper 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+3PbO,SO, 4Pb O + 4SO2
4Pb O +4C 4Pb + 4CO.
If, besides the oxide of lead, sulphate should be present it also will be decomposed by the charcoal. According to Gay-Lussact, when an excess of carbon is present, the sulphate is changed into sulphide of lead while carbonic acid escapes. When equal equivalents of sulphate aud carbon are present at a low temperature, carbonic acid is developed and only half of the sulphate of lead is changed into sulphide (2 PbO,SO, +2CPbO,SO3+Pb S+2CO2). 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 PbO,SO3+2C3 PbO,SO, + Pb S+2CO2,) 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, SO3+PbS4 PbO+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 establishments. 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.
into the slag. A percentage of zinc-blende in the ore acts favorably because the zinc forms with the silica a stiff slag which prevents sinter ing. As in the Carinthian process, the ore is exhausted as much as possible in the last or hottest period; but this occasions a loss of lead, as well as an impurer product, so that smelting of the residue in the shaft-furnace would be more advantageous. Comparing the capacity of charge of the Carinthian furnace with that of the French, shows the latter to have a smaller production, a charge of 1,200 to 1,300 kilograms, (2645.5 to 2866 pounds avoirdupois,) in a hearth with sump from 4 to 5 meters square, consuming fifteen to sixteen hours. The reverberatory being larger than the Carinthian, somewhat less fuel is used, but this advantage disappears if double furnaces are employed in the latter process.
The English process.-The principal object of the English modification of the reverberatory process is to reach the greatest possible production; and therefore larger furnaces with three working-doors on each side, stone-coal as fuel, and higher temperatures are employed. This causes a copious formation of oxysulphuret of lead, and especially if silica is present, even in very small quantities, much rich slag remains in the last period, from which the lead and silver have to be extracted in a subsequent smelting in a shaft-furnace. On account of the high temperature employed in the reverberatory, a great deal of lead and silver is volatilized and much of this is lost, although very extensive systems of condensation chambers are attached to the furnaces. If carbonate of lime is present in ores rich in silver, they may be smelted by this process without serious loss, as the presence of the lime prevents the passage of silver into the slags by decomposing the sulphide, and permitting the metallic lead to take up the silver. If it is not present, burned lime is added during the process. Professor Kerl (Hüttenkunde, vol. 2, page 62,) gives the following theory in regard to the reactions in this process:
The normal proceeding consists in roasting considerable quantities of galena at a quickly-increasing temperature, in less time than in the Carinthian process, so that in proportion to the undecomposed galena the quantity of oxides formed is less than in the method just mentioned. If the charge, when in this condition, is exposed to a still higher temperature, metallic lead and subsulphide of lead are formed by the action of the oxides on the sulphides: 2PbS+PbO,SO3=Pb+Pb2S+2SO3. If the temperature is decreased when no more lead results, the subsulphide of lead, (containing 92.8 per cent. of lead, and 7.2 per cent. of sulphur,) which is homogeneous at a high temperature, but not when at a heat when the mass becomes doughy, is decomposed into sulphide of lead and metallic lead. If, during this cooling, air is permitted to enter the furnace, a part of the sulphide of lead is oxidized, and in a subsequent higher temperature more metallic lead is reduced. The process aims to extract as much lead as possible by repeating these reactions several times; but a point is finally reached where the increased temperature causes the whole mass to melt, and then oxysulphuret is formed, which requires a repeated addition of lime and access of air to make its decomposition possible. When by this last resort no more lead can be extracted, the residue must be smelted in a shaft-furnace.
The consumption of fuel in the English process is very large, surpassing even that in the Carinthian furnace; the cost of labor is proportionally less, but there is a larger loss of lead, by reason of the high temperature employed. The process would be far more economical if the first roasting were conducted slowly and at a very low heat.
After comparing the three roasting and reducing reverberatory processes above described, Professor M. L. Gruner gives the following resumé:
It is clear that the form of the reverberatory has less influence on the success of the process than the mode of working. If, in any of the furnaces, the work is conducted quickly and at too high a temperature, the losses are large; but if the roasting is conducted slowly, at a low red-heat, and the temperature is increased in the second period, just when the correct proportion of sulphuretted and oxidized matter has been reached, the most complete extraction of the lead and silver is secured; and this can be accomplished in any of the reverberatories in use in either of the three processes. It is most advisable to extract the lead from the ore in the reverberatory to within 35 to 40 per cent., and for this purpose the large English reverberatories are more economical than the small Carinthian. A number of working-doors on the two long sides of the furnace is advantageous; but the arrangement of two fireplaces, as in the Belgian modification of the English furnace, is not advisable, because the temperature will thereby too easily transgress the proper limits. A sump for the reception of the lead in side the furnace is best placed in the coolest place. The different furnaces can work up the following quantities of ore, containing from 70 to 80 per cent. of lead, during the year, (three hundred working days:)
Common single Carinthian furnace....
Single Carinthian furnace with air-canals in the firebridge....
French double furnace.
350 to 400
1,000 to 1, 200 1,000 to 1, 400
2. Roasting and precipitating furnaces.-There are two processes of this kind. The one, as practiced in Vienne, France, is based on the fact that iron and galena, when mixed and exposed quickly in a reverberatory to such a heat that the mass begins to sinter, and kept for some time at that temperature with continual stirring, react, so that the iron gradually disappears by taking up the sulphur from the galena and forming a matte, while the greater part of the lead remains in a metallic state. If this mass is then smelted by increasing the heat, and permitted to run into a sump, the different constituents separate themselves by their specific gravity, the lead going to the bottom, while the matte, which contains, besides the iron, other metals and some lead, remains on top of the lead and below the slag. The lead contains, nearly always, some metallic iron, which must be removed by a slow remelting. The matte and slag, if not argentiferous, are thrown away, as the first generally contains not more than from 6 to 12, and the latter from 4 to 6 per cent. of lead, which would not pay for extraction by a separate process. If silver ores are smelted by this process, by far the most of the silver will follow the sulphur into the matte, from which it will have to be extracted by further and expensive treatment. For this reason the method is not adapted for the western mining districts, to say nothing of the impracticability of obtaining metallic iron, of which often 35 per cent. is required. It is true that very pure iron ores, together with charcoal, may replace the metallic iron, but even these cannot be cheaply obtained in the necessary purity in most localities. In the second process it is intended to save metallic iron; and the charge is therefore first roasted alone at a low temperature, so that the galena is as perfectly as possible changed into oxide and sulphate of lead. One half per cent. of coal dust and 12 per cent. of quartz are then