Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/394

376 LEAD

duced 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.

Fig. 2. – Vertical Section of Furnace. Fig. 3. – Horizontal Section.

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 lith arge, 90 parts of a variety of slags from previous opera tions, 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 alter nate 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 cast-iron 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 in creased 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 trice 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