Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/393

LEAD 375

per cent. of metal) and white lead ore or cerusite, PbOC0 2 (77 5 per cent), might almost be said to be the only ones which come into consideration as lead ores. Occa sionally, however, the following also are utilized: – lead- vitriol or anglesite, PbOS0 3 (68 3 per cent.), and the pyro- morphite group, 3(P 2 or As 2 )0 5 .3PbO + PbCL, (76 to 69 per cent.). Bournonite, CuPbSbS, may also be named, although, containing 13 per cent. of copper besides 42 3 per cent. of lead, it is rather a copper than a lead ore.

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 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 0 01 or less to 0 3 per cent., and in rare cases coming up to ½ or 1 per cent. Galena occurs in veins in the Cambrian clay-slate, accom panied 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 Arkeudale 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 im ported into Great Britain.

The native carbonate occasionally presents itself in the form of pure crystals of the compound PbC0 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" dissemin ated 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 2 to 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 im portant are those of Utah, of Missouri, and of the Upper Mississippi, where the ore consists substantially of galena.

Fig. 1. – Reverberatory Furnace. C, chimney; D, opening for feeding the fire.

The extraction of the metal from pure (or nearly pure) galena is the simplest of all metallurgical operations. The ore is roasted (i.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 (rela tive) 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 O, 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 quick lime 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.^[1]

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 intro-

1. In England coal is employed everywhere, sometimes along with peat.