LEAD. 737 tated by an excess of carbonate of ammonium, which, if left to stand for a few hours, dissolves the copper, but leaves the cadmium in the form of carbonate. The separation of copper from bismuth may be effected by means of carbonate of ammonium, as directed for cadmium. The other metals of the sixth group are separated by precipitating them with the copper as sulphides, and then digesting the mixed sulphides with a solution of sulphide of potassium (sulphide of ammonium dissolves traces of copper); the sulphide of copper alone remains undissolved. § II. LEAD: Pb"=207, or Pb=103.5. Sp. Gr. 11'36; (889) Almost all the lead of commerce is obtained from galena, the native sulphide of lead. It occurs, mixed with quartz, blende, pyrites, sulphate of barium, and fluor-spar, in veins traversing the primitive rocks, and particularly in the clay-slate in Cornwall, and mountain limestone in Cumberland. Small quantities of carbonate and phosphate of lead are frequently met with, but they are unimportant as ores of the metal. Galena always contains a small proportion of sulphide of silver; when the mineral is found in bold, well-characterized cubes, it is usually nearly pure. The proportion of silver in galena is liable to considerable variation; a mineral yielding 120 ounces of silver to the ton, or o 36 per cent., is considered to be extremely rich. England and Spain afford the principal supply of this metal, about 65,000 tons of lead being annually raised in England, which furnish on the average 560,000 ounces of silver. (890) Extraction.-After the lead ore has been raised to the surface, it undergoes a careful mechanical preparation, conducted upon the principles already explained (529); and having been thus freed to a great extent from its earthy impurities, it is ready for smelting. If the galena be tolerably free from siliceous gangue, this operation is sufficiently simple. About 1 ton of the dressed ore is mixed with from a fortieth to a twentieth of its weight of lime, and is heated to dull redness in a reverberatory furnace, through which a strong current of air is passing. Fig. 353 exhibits a section of the reducing furnace employed in Derbyshire. A is the fire-grate, the bridge, h the hopper by which the charge is introduced; c c, the bed on which the ore is placed, sloping downwards towards a gutter in the centre, by which the melted metal 3 B II. 738 EXTRACTION OF LEAD. is drawn off; d, d, d, are doors for working the charge and for admitting air, the draught of the furnace being completely under control by a damper placed in the flue f. During the roasting a large quantity of the sulphur burns off as sulphurous anhydride, and a portion of oxide of lead is formed: another portion of the sulphide of lead is converted into sulphate of lead, and much of the ore still remains undecomposed. In the course of the operation, the mass is frequently stirred, and care is taken not to allow the temperature to rise sufficiently high to fuse it. When it is considered that the roasting has been carried far enough, the materials on the bed of the furnace are thoroughly mixed together, the furnace doors are closed, and the heat is suddenly raised. The oxide and the sulphate of lead then react upon the undecomposed sulphide of the metal; a large quantity of sul phurous anhydride is evolved, whilst metallic lead runs copiously from the mass. The successive stages of this operation may traced as follows: : = 4 be Two atoms of sulphide of lead, by combining with 6 of oxygen, furnish 2 atoms of oxide of lead and 2 of sulphurous anhydride, as is exhibited by the equation: 2 PbS+3 0,2 Pb0+2 S0, If one atom of galena unite with 4 atoms of oxygen, I atom of sulphate of lead is formed, PbS+20,-PbSO. Both oxide of lead and sulphate of lead, when heated with fresh sulphide of lead, are decomposed, metallic lead and sulphurous anhydride being in each case the result of the reaction. Two atoms of oxide of lead, and I of galena furnish 3 of lead and I of sulphurous anhydride: 2 PbO+PbS=3 Pb+SO. One atom of sulphate of lead, when heated with 1 of galena, yields 2 atoms of lead and 2 of sul phurous anhydride: thus PbS+PbS0,=2 Pb+2 SO,. During the roasting a portion of subsulphide of lead, Pb,S, is also produced. This substance forms a fusible matt, which flows from the furnace with the metallic lead, constituting a stratum which floats above the melted metal. This subsulphide of lead is again returned to the furnace and roasted with fresh ore. After the melted mass has been drawn off into cast-iron basins placed for its reception, a few spadefuls of lime to which a quantity of fluor-spar is sometimes added, are thrown into the furnace, with a view to act upon the scoriæ which remain behind in considerable quantity: the lime decomposes the fusible silicate of lead, liberates oxide of lead, and forms a less fusible silicate of calcium, and the fluor-spar forms a fusible compound with the sulphate of calcium, or sulphate of barium, if either of them is present. The scoriæ usually contain an excess of oxide and of sulphate of lead; they are therefore mixed with coke or charcoal, and exposed to heat on the bed of the furnace, after the doors have been carefully closed; the oxide of lead then becomes reduced by the carbon. Refining of Lead.-Lead which contains antimony or tin is harder than the pure metal, and is subjected to a further operation, termed improving, in order to refine it. This consists simply in melting the lead, and heating it for a period, longer or shorter, as may be necessary, in a shallow cast-iron pan set in the bed of a reverberatory furnace; the antimony and tin being more oxidi zable than the lead, are thus removed in the pellicle of oxide which is continually being formed. From time to time the workman takes out a small sample of the metal to examine the appearance which it presents on cooling. As soon as it exhibits a peculiar flaky crystalline appearance on the surface, the oxidation has been carried far enough; the metal is then run off and cast into pigs. (891) Concentration of Silver in Lead by Pattinson's Process.Silver may be profitably extracted from lead, even when the quantity does not exceed from three to four ounces of silver to the ton, by a process introduced by Mr. Pattinson, of Newcastle. This gentleman observed that if melted argentiferous lead be briskly stirred during slow cooling, a portion of the metal solidifies first, in the form of crystalline grains, which sink to the bottom of the portion which remains melted. These crystals consist of lead nearly free from silver, the fusing-point of the argentiferous alloy occurring at a lower temperature than that of pure lead. This observation is turned to account in the following simple manner :Eight or nine cast-iron pots, each capable of containing about 740 EXTRACTION OF SILVER FROM LEAD five tons of melted lead, are arranged in a row, set in brickwork, and each provided with a separate fireplace underneath. A quantity of lead is introduced into the middle pot, and melted; the fire is then withdrawn, and the metal is briskly stirred by the workman whilst it cools: the crystals of lead subside as they form, and are removed at intervals by means of a large perforated iron ladle, and transferred to the next pot on the right hand. When about four-fifths of the metal have been thus removed in grains, the concentrated argentiferous alloy is ladled out into the next pot on the left-hand side, and the empty pot is charged with a fresh portion of lead, which is subjected to a similar treatment. When the pot to the right and to the left has in this manner received a sufficient quantity either of poor or of argentiferous lead, it is subjected to a similar operation; the concentrated argentiferous portion being passed off continually to the next pot on the left, whilst the crystalline or poorer portion is handed over to the next pot on the right-hand side. The last pot to the left thus at length becomes filled with lead which may contain 300 ounces of silver to the ton; it is not found advantageous to concentrate it beyond this point the lead which accumulates in the last pot on the righthand side does not contain more than half an ounce of silver in the ton. This poor lead is much improved in quality by the operations which it has undergone, and is at once cast into pigs for the market. (892) Extraction of Silver from Lead by Cupellation.—The rich argentiferous lead is now subjected to cupellation. This process is founded upon the circumstance that lead, if exposed at a high temperature to a current of air, absorbs oxygen rapidly, and is converted into a fusible oxide, whilst silver does not become oxidized, but is left behind in the metallic state. The litharge or oxide of lead melts at a high temperature, and flows off the convex in the plan of the furnace (Fig. 354): it consists of a shallow oval basin, c, composed of a mixture of bone ash with fern or wood ashes; this mixture is slightly moistened, and beaten into an iron ring of about 4 feet in its long diameter, and 2 feet in the shorter the cupel is introduced into the furnace from beneath, and is supported by bricks, so that it can be readily removed and renewed,—an operation which is generally required once a week. When dry, the fire is cautiously lighted, and the lead introduced; a continual blast of air from a tuyère, т, is made to play over the surface of the melted metal; litharge is formed abundantly, and runs off through a gutter, g, into an iron pot, p, placed beneath the furnace for its reception; in front is a hood, h, for carrying off the fumes of oxide of lead which would otherwise escape and injure the workmen. Fresh lead is added from time to time to supply the place of that which is oxidized; until at length a quantity of lead, originally amounting to about 5 tons, is reduced to between 2 and 3 cwt. This melted metal is withdrawn by making a hole through the bottom of the cupel; the aperture is afterwards closed with fresh bone ash, and another charge is proceeded with. When a quantity of rich lead sufficient to yield from 3000 to 5000 ounces of silver has thus been obtained, it is again placed in a cupel, and the last portions of lead are removed. It is found advantageous to effect this final purification of the concentrated silver-lead separately, because in the last stages of the operation the litharge carries a good deal of silver down with it: these portions of litharge, therefore, on being reduced, are again subjected to the desilvering process. The litharge from the first fusion is either sold as oxide of lead, or it is reduced in a small reverberatory furnace with anthracite, or powdered coal. The porous cupels absorb a large quantity of litharge, and they likewise are passed through the furnace in order to extract the metal. A very beautiful phenomenon, known as the fulguration of the metal, attends the removal of the last portions of lead from the silver. During the earlier stages of the process the film of oxide of lead, which is constantly forming over the surface of the melted mass, is renewed as rapidly as it is removed; but when the lead has all been oxidized, the film of litharge upon the silver becomes thinner and thinner as it flows off; it then exhibits a succession of the beautiful iridescent tints of Newton's rings; and at length the film of oxide suddenly disappears, and reveals the brilliant surface of the metallic silver beneath. In the Hartz the hearth of the cupellation furnace is fixed, and is made of brick, covered with marl, which is renewed after each operation, but the cover of the furnace is moveable. Karsten |