LEAD. This metal was known to the ancients, and is mentioned in the Old Testament. The Romans used it largely, as it is still used, for the making of water pipes, and soldered these with an alloy of lead and tin. Pliny treats of these two metals as plumbum nigrum and plumbτιm album respectively, which seems to show that at his time they were looked upon as being only two varieties of the same species. In regard to the ancients’ knowledge of lead compounds, we may state that the substance described by Dioscorides as gολvβ^αlvα was undoubtedly litharge, that Pliny uses the word minium in its present sense of red lead, and that white lead was well known to Geber in the 8th century.
Galena, the principal lead ore of the Old World, is a dark-coloured metallic-looking compact solid of 7 ’ 3 to 7 ’ 7 specific gravity and 2 o, 5 hardness, crystallizing in cubes or other forms of the regular system, but often presenting itself in non-crystallized granular masses. All galena is contaminated with sulphide of silver,—the proportion of noble metal varying from about 0Ό1 or less to 0 ’ 3 per cent., and in rare cases coming up to ∣ or 1 per cent. Galena occurs in veins in the Cambrian clay-slate, accompanied by copper and iron pyrites, zinc-blende, quartz, calc-spar, iron-spar, &c.; also in beds or nests within sandstones and rudimentary limestones, and in a great many other geological formations. It is pretty widely diffused throughout the earth’s crust. The principal English lead mines are in Derbyshire; but there are also mines at Allandale and other parts of western Northumberland, at Alston Moor and other parts of Cumberland, in the western parts of Durham, in Swaledale and Arkendale and other parts of Yorkshire, in Salop, in Cornwall, in the Mendip Hills in Somersetshire, and in the Isle of Man. The Welsh mines are chiefly in Flint, Cardigan, and Montgomery shires; the Scotch in Dumfries, Lanark, and Argyll; and the Irish in Wicklow, Waterford, and Down. Of Continental mines we may mention those in Saxony and in the Harz, Germany; those of Carinthia, Austria; and especially those of the southern provinces of Spain, from which country large quantities of lead are now imported into Great Britain.
The native carbonate occasionally presents itself in the form of pure crystals of the compound PbCO 3 (“cerusite ”), but more frequently in a state of intimate intermixture with clay (“Bleierde ”), limestone, oxide of iron, &c. (as in the ores of Nevada and Colorado), and sometimes also with coal (“black lead ore ”). All native carbonate of lead seems to be derived from what was originally galena, which, in fact, is always present in it as an admixture. This ore, metallurgically, was not reckoned of much value, until immense quantities of it were discovered in Nevada and in Colorado (U.S.). The Nevada mines are mostly grouped around the city of Eureka, about 200 leagues from San Francisco. The ore there occurs in “pockets” disseminated at random through limestone. The dimensions of these pockets are very variable; one is quoted measuring 300 by 60 by 180 feet. The crude ore contains about 30 per cent, of lead and 0 ’ 2 to 0 - 3 per cent, of silver. The Colorado lead district is situated pretty high up in the Rocky Mountains, a few miles from the source of the Arkansas river. The ore was discovered as late as 1877 by a mining engineer, Stephens. It forms gigantic deposits of almost constant thickness, embedded between a floor of limestone and a roof of porphyry. Stephens’s discovery was the making of the city of Leadville, which, in 1878, within a year of its birth, had over 10,000 inhabitants. The Leadville ore contains from 24 to 42 per cent, of lead and 0 ’ 1 to 2 per cent, of silver. In Nevada and Colorado the ore is worked chiefly for the sake of the silver; but this industry, especially since 1878, has developed at such a rate as to seriously affect the price of lead even in Europe. Of other American lead districts the most important are those of Utah, of Missouri, and of the Upper Mississippi, where the ore consists substantially of galena.
The extraction of the metal from pure (or nearly pure) galena is the simplest of all metallurgical operations. The ore is roasted (ι.e., heated in the presence of atmospheric oxygen) until all the sulphur is burned away and the lead left. This simple statement, however, correctly formulates only the final result. The first effect of the roasting is the elimination of sulphur as sulphurous acid, with formation of oxide and sulphate of lead. In practice this oxidation process is continued until the whole of the oxygen is as nearly as possible equal in weight to the sulphur present as sulphide or as sulphate. The heat is then raised in (relative) absence of air, when the two elements named unite into sulphurous acid (SO 2 ), while a regulus of molten lead remains. In Wales and the south of England the process is conducted in reverberatory furnaces of the form shown in fig. 1. The sole of the furnace is paved with slags from previous operations, and has a depression in the middle where the metal formed collects to be let off by a tap-hole T. The dressed ore, 12 to 24 cwts., is introduced through the “hopper” H at the top, and exposed to a moderate oxidizing flame until a certain proportion of ore is oxidized, the openings Ο, O at the side enabling the workmen to stir up the ore so as to constantly renew the surface exposed to the air. At this stage as a rule some rich slags of a former operation are added and a quantity of quicklime is incorporated, the chief object of which is to diminish the fluidity of the mass in the next stage, which consists in this, that, with closed air-holes, the heat is raised so as to cause the oxide and sulphate on the one hand and the sulphide on the other to reduce each other to metal. The lead produced runs into the hollow and is tapped off. The roasting process is then resumed, to be followed by another reduction, and so on.
A similar process is used in Carinthia; only the furnaces are smaller (adapted to a charge of only 420 lb) and of a somewhat different form. They are long and narrow; the sole is plane, but slopes from the fire-bridge towards the flue, so that the metal runs to the latter end to collect in pots placed outside the furnace. In Carinthia the oxidizing process from the first is pushed on so far that metallic lead begins to show, and the oxygen introduced predominates over the sulphur left. The mass is then stirred to liberate the lead, which is removed as “Rührblei.” Charcoal is now added, and the heat urged on to obtain “Pressblei,” an inferior metal formed partly by the action of the charcoal on the oxide of lead. The fuel used is fir-wood.
In Cumberland, Northumberland, and Durham the reverberatory furnace is used only for roasting the ore, and the oxidized ore is then reduced by fusion in a low square blast furnace (a “Scottish hearth furnace”) as depicted in figs. 2 and 3. The rectangular cavity C is lined with cast-iron, as is also the inclined sole-plate which is made to project beyond the furnace, the outside portion W (the “work-stone”) being provided with grooves g guiding any molten metal that may be placed on the “stone’’into the cast-iron pot P; t is the “tuyere” for the introduction of the wind.
As a preliminary to the melting process, the ‘ ‘ browse” left in the preceding operation (half-fused and imperfectly reduced ore) is introduced with some peat and coal, and heated with the help of the blast. It is then raked out on the work-stone and divided into a very poor “grey” slag which is put aside and a richer portion which goes back into the furnace. Some of the roasted ore is strewed upon it, and, after a quarter of an hour’s working, the whole is taken out on the work-stone, where the lead produced runs off. The “browse,” after removal of the “grey” slag, is reintroduced, ore added, and, after a quarter of an hour’s heating, the mass again placed on the work-stone, &c.
In any form of the lead-smelting process one of the conditions of complete success is the absence of silica, because this when present unites with a certain proportion of the oxide of lead into a fusible silicate (slag). Practically the formation of a plumbiferous slag cannot be altogether avoided in any case, and such slag accordingly must be worked up. At Alston Moor, Cumberland, this is effected by means of a hearth (blast) furnace similar to the one just described. The slags (oxide, sulphate, and silicate of lead) are introduced with coal-ashes, furnace bottoms, and other residues, and melted down, this leading to the formation of lead and of a poorer slag. The lead is run off as much as possible; the slag is run into water, which disintegrates it so that the particles of metal shut up within it are set free and become recoverable by elutriation.
Lead being very appreciably volatile at a red heat, lead-smelting generally, but more especially the Scottish-hearth process, and pre-eminently the slag-recovery process, involve the production of large quantities of “lead-smoke” (finely divided highly impure oxide and sulphate of lead), which, for sanitary and economic reasons, must be condensed and recovered. At Alston Moor the smoke for this purpose is led through a very long succession of flues, ascending the slope of a hill, into a chamber at the top which communicates with a chimney. The chamber, by a number of screens going alternately from the floor to near the top, and vice versa, is divided into compartments charged with such a quantity of water that the smoke, which is propelled by means of a fan, is compelled to bubble repeatedly through the water, where most of what has failed to come down in the flues is precipitated. The smoke deposit is collected, dried, and worked up for lead.
Carbonate and oxide of lead are easily reduced by charcoal or coal. In Leadville and Eureka (U.S.) the carbonate is smelted with charcoal in small blast furnaces, about 8 feet high, and rectangular section of 31 by 47 inches, worked with charges of about three tons of ore. There are five tuyeres, two at each of the longer sides, and one at the end opposite the outlet-hole. The “crucible” is quite surrounded by hollow wrought-iron plates, kept cool by circulating water.
Complex lead ores of course demand a complex treatment. The famous Frankenscharner Hütte near Klausthal in the Harz, where a very complex ore is worked up with a wonderful degree of exhaustiveness and precision, may serve as an example. The ore in this case consists of argentiferous galena associated with copper pyrites, fahl-ore, bournonite, zinc blende, and a gangue consisting of silica, limestone, and heavy spar. After the copper pyrites has been, as far as possible, picked out by hand, the remainder is assorted so as to produce an average of about 55 per cent, of lead. One hundred parts of such ore are mixed with 11 of hearth-mass and litharge, 90 parts of a variety of slags from previous operations, and 11 parts of metallic iron (or the equivalent of some rich iron ore plus charcoal), and melted down in blast furnaces similar to those used for iron-smelting, but only 22 feet high. The furnace is charged with alternate layers of ore mixture and charcoal. The smelting takes fourteen hours, and yields per charge of 100 parts of ore (containing in all about 74 parts of lead) 25 parts of metallic lead, and 18 , 4 parts of a “stein” consisting of an alloy of sulphides of lead, iron, copper, zinc, silver, antimony, intimately mixed with particles of metallic and (? subsulphide of) lead—apart from the slags formed, which contain 4 to 8 per cent, of lead and a trace of silver. The “stein” is subjected to a protracted series of roast-ings, and then melted down with iron and selected slags. There result a ferruginous slag, a certain proportion of metallic lead, and a “stein” of the second order, which of course is richer in copper than the original one was. This “stein” is again roasted, melted down with iron, &c., until the whole of the lead is extracted, and the copper concentrated in a mass sufficiently rich and pure to be wrought as a copper “stein.”
Refining.— The lead obtained by any of the above processes is as a rule contaminated with more or less of iron, antimony, zinc, arsenic, and silver, which must be removed,—the base foreign metals because they deteriorate the lead, the silver on account of its high commercial value. The base metals are easily eliminated by subjecting the crude metal to oxidizing fusion in a shallow castiron dish inserted into a reverberatory furnace; the foreign metals, being more oxidizable than lead, go to the top as an oxide-scum, which is constantly removed until pure litharge, instead of the foreign oxides, makes its appearance.
The extraction of the silver is easily effected by means of the process of cupellation, one of the oldest metallurgical operations, which dates back to a time beyond that of Pliny. The metal is placed on a shallow kind of dish made of compressed bone-ash powder and forming the sole of a reverberatory furnace, and therein kept at a red heat in the presence of an abundant supply of air. The lead (and with it the foreign base metals) is oxidized into “litharge” (PbO), which, at the temperature prevailing, melts into a thin liquid, and is made to run off through a slit or hole made in the side of the “cupel” (or “test ”); the silver remains unchanged, so that the regulus becomes richer and richer as the process proceeds. The foreign base metals, as will readily be understood, go off as oxides along with the first portion of litharge, and accordingly can be removed without contaminating the bulk of the latter product. When the percentage of silver has increased to about 8 per cent., the regulus, as a rule, is transferred to a fresh cupel, and thereon treated in the same way as before, until the last trace of litharge is seen to go off as a thin film on the regulus, presenting, on account of its thinness, in the glow of the fire, the magnificent appearance of a soap-bubble in sunlight. The silver then is “fine,” i.e., almost pure, and ready for the market. The lead, however, is all obtained in the shape of oxide, and consequently, if not saleable as such, must be reduced with charcoal or coal. The process accordingly is expensive, and generally does not pay with a raw lead containing less than 1 ⅛ per cent, of the noble metal.
The process, in its direct application to the lead, is now almost extinct, being superseded by the following two methods of “concentration,” which offer the advantage of desilverizing at least the bulk of the lead without depriving it of its metallicity.
1. Pattinson's Process (invented about forty years ago) is founded upon the fact that, when molten argentiferous lead is allowed to cool slowly, a relatively silver-free lead crystallizes out while a richer metal remains as a motherliquor. It will be readily understood that, by a persistent systematic application of this method of partial separation to the primary products and again to their derivatives, it is possible to, so to say, split the original material into a very poor portion containing most of the lead, and a “rich” one containing almost all the silver. Practical smelters are generally satisfied when the proportion of silver in the former is reduced to from the one to the three millionth of the weight of the lead, and the latter enriched to the extent of 0 - 5 to 1 ’ 5 per cent, of silver, although it is possible to bring up the percentage to 2 ’ 5. A lead containing as little as half an ounce of silver per ton can be “Pattinsonized” with a profit.
2. Karsten's Process is still more perfect. It has long been known that lead refuses to alloy itself with more than traces of zinc. In 1842 the eminent metallurgist Karsten made the important discovery that, when argentiferous lead is mixed with 1 per cent, or more of zinc (at a temperature insuring liquidity to even the latter metal), about ⅜ per cent, of zinc remains dissolved in the lead, while the rest rises to the top as a scum, and, besides a deal of lead, takes almost the whole of the silver with it. Parkes subsequently brought the process into a workable form, for which he took a patent in England in 1850. The argentiferous lead is molten in large cast-iron pots, intimately mixed with about 30 parts of zinc per unit of silver present, the mixture allowed to rest, and the argentiferous scum removed by means of perforated ladles. The scum, when subjected to “liquation” (partial fusion) on an inclined sole, lets off a quantity of rich lead, which goes to the cupel. From the residue the bulk of the zinc can be withdrawn by distillation, the non-volatile part being fit for cupellation. The desilverized lead is freed from its zinc and the other base impurities it may contain by “refining” (see above). The Parkes process seems to be on a fair way of being superseded by a far more perfect form of the Karsten method which was patented by Cordurié for France in 1866 (October 18, No. 73,167), and of which the most characteristic feature is that the removal of the zinc from the scum and the refining of the desilverized lead are both effected by means of superheated steam. The treatment with zinc is effected in a deep upright half-egg-shaped castiron pan (standing on an upper floor), which is provided with a vertical shaft bearing horizontal paddles, and at its lowest point a perforated cast-iron box, which serves to accommodate the zinc; 1 kilogramme per 100 kilos of crude lead containing 04 kilo of silver, or up to twice the proportion for richer leads. The argentiferous lead—10 tons at a time —is melted down in the pan, and the paddle-shaft with the zinc introduced and made to revolve until all the zinc has become incorporated with the mass. The shaft is then withdrawn, the mixture allowed to rest for a time at a lower temperature, the scum removed, and the zinc treatment repeated once or twice to eliminate the whole of the silver. The desilverized lead runs direct from the pan into another pan standing on the ground floor, which has no tap-hole, but is provided with a wrought-iron hood communicating by means of a pipe with a condensation chamber. In this pan the metal is heated to redness, and a current of superheated steam is blown through it for two or three hours. The zinc and the rest of the impurities are thereby converted into oxides which mostly remain on the surface of the metal, the rest being carried into the chamber and deposited there. The silver scums, after extraction from them of argentiferous lead by liquation, are collected, and, when a sufficient quantity has accumulated, worked with superheated steam like the zinciferous lead,—to produce a richly argentiferous regulus, adapted for cupelling, and an oxide-mixture intimately intermixed with particles of the former and containing even some silver oxide. The working of this bye-product seems to have given the inventor a deal of trouble. Passing over his method, we will mention the one introduced in Lautenthal since 1869. There they dispose of the argentiferous oxides by adding them to the rich lead during its cupellation; the silver is sucked in by the regulus, the base oxides amalgamate with the litharge. The “poor” lead resulting from this form of the Karsten process contains only 5 or 6 grammes...
