pyrites, blende, and other sulphides. Deposits in sedimentary rocks are almost always poorer in silver than fissure vein deposits associated with metamorphic or eruptive rocks.

Galena occurs comparatively free from both blende and pyrites at Engis (Belgium), Mechernich (Rhen.-Pruss.), and in the upper parts of the limestone deposits (Lower Silurian) of Wisconsin and Illinois. Galena associated with blende alone is found at Raibl and Bleiberg (Carinthia) and at Joplin (Mo.), while in the mines of St. Joe and La Motte (Mo.) it is associated with pyrites to the exclusion of blende. In all these cases the galena is almost free from silver, the ratios of that metal to lead varying from 1 dwt. Ag for 70 per cent. Pb (Bleiberg, Carinthia), to 11 ozs. Ag for 80 per cent. Pb (Granby, Mo.), and 4 ozs. Ag for 60 per cent. Pb at Mechernich, according to a useful little table given by Hofman.* All the above are distinctly sedimentary deposits, either in sandstone, as at Mechernich, or in limestone, as at the remaining localities cited. The Spanish galena deposits in limestone (Linares, Sierra Almagrera, Almeria, &c.), which are also comparatively free from other metallic sulphides, are of an intermediate character, averaging, as prepared for smelting, from 6 to 10 ozs. Ag for 75 per cent. Pb.

When galena is found most intermingled with other sulphides (especially with complex sulphides and sulphantimonides like fahlerz), and more particularly when in contact or in connection with crystalline, metamorphic, or eruptive rocks, as in Bohemia, Saxony, Hungary, the Harz and the Rocky Mountains, it is usually richest in silver. As already pointed out, however, a large part of the silver contents of these complex sulphide mixtures does not exist as a constituent of the galena itself, but rather in distinctly silver minerals, such as argentite, stephanite, and fahlerz, either finely disseminated through the cleavage planes of the galena, or through the matrix. Even the blende and pyrites in these mixed ores usually carry appreciable quantities of silver; in some parts of the great Broken Hill Proprietary Mine the blende carries nearly as much silver as the galena. In one interesting case described by Freeland, † some ore from the Minnie Mine at Leadville was separated as nearly as possible into its component parts of galena, blende, and pyrites, and the blende turned out to be much richer than the galena. The following analyses are by Boggs.

This is, of course, rather an exceptional case, and by no means the rule in the Leadville district; thus, Taylor and Brunton report a similar series of analyses from the Col. Sellars Mine, in which the galena contains 20 ozs., the pyrites 5 ozs., and the blende only 3 ozs. The series of analyses mentioned Metallurgy of Lead, N. Y., 1893, p. 26.

*

"Iron Hill sulphide deposits," Trans. A. I. M. E., vol. xiv., p. 181. Eng. and M. J., 8th May, 1886.

has been quoted in full merely as a warning against the too common assumption that in dealing with low grade complex ores the only thing to aim at is a successful concentration of the lead. It may frequently happen that in spite of a fairly good saving of this metal, what with the losses of silver in blende and pyrites tailings, and what with the loss of finely divided true silver minerals in the slimes, the lead concentrates do not contain more than one-half of the silver value of the original ore. It is, therefore, in many cases preferable to smelt the whole mass of a complex ore body rather than to make any attempt at concentration. Analyses of smelting ores will be given in the sections dealing with the various processes of treatment.

SECTION II-LEAD SMELTING.

INTRODUCTORY.

PRACTICALLY speaking, only smelting methods are applicable to the extraction of lead from its ores. Wet methods, based on the leaching out of PbCl, or of PbSO, by means of suitable solvents and precipitation with iron, have been at various times suggested, but none have proved sufficiently promising to be worked on a commercial scale.

As already seen, by far the commonest lead ore is galena; comparatively small proportions only of carbonate, sulphate, and other ores are now accessible,* and, as a rule, only as incidental accompaniments of large bodies of galena. The treatment of these oxidised ores, moreover, offers no special difficulty and requires no special appliances; they may be treated generally as roasted sulphide ores, oxidation having been effected slowly by atmospheric agencies instead of quickly in a furnace. The smelting of these oxidised ores in Australia, Mexico, and elsewhere will be described in its proper place, and the treatment of lead ores may be considered generally as referring only to galenas.

Classification of Methods.-Three principal methods of treatment may be distinguished:

1. The Roast and Reaction Method, in which the galena is first roasted so as to convert part of it into PhO and PbSO4; the temperature is then raised so that these compounds react upon the unaltered sulphide with production of metallic lead and SO†

2. The Roast and Reduction Method, in which the galena is roasted pretty completely to lead oxide, which is then reduced to metal with coke or charcoal. A small proportion of sulphate formed in the roasting is either transformed into silicate at the end of the roasting process, and then reduced in the smelting furnace with the help of iron oxide, or it may be reduced to sulphide in the smelting furnace, and so form a "matte" from which part of the metallic lead is again set free by iron. In any case nearly the whole of the lead is recovered direct in a metallic condition.

Except, perhaps, in Mexico.

+For equations representing these reactions . Chap. ii.

3. The Precipitation Method in which galena is decomposed at a high temperature by the agency of metallic iron, forming metallic lead and a mixture of iron and lead sulphides, the so-called "lead matte."

These methods are rarely employed quite independently of each other, for the residues of the first method are almost always worked up by the second or third methods, and the second method is almost always combined with the third, and often with the first also.

1

The choice of method depends partly upon the quantity and nature of the impurities in the ore, partly upon local conditions, such as the kind of fuel available, and the price of labour.

The Roast and Reaction Method may be carried on in hearths or in reverberatories, and requires raw fuel only, but is suited only to high grade galenas containing 60 to 70 per cent. lead and upwards, since large quantities of other substances interfere with the reactions of PbO and PbSO4 on PbS. It is quite inapplicable to ores containing much quartz or silicates, which would form a fusible lead silicate, and so put a stop to the reaction. The advantages of this method for dealing with rich ores are:-(1) Yield of a very pure lead; (2) use of cheap forms of fuel; (3) no fluxes required; (4) comparatively small volatilisation loss. Its disadvantages are:-(1) Production of a very rich slag which must be re-treated; (2) heavy labour cost; (3) large quantity of fuel required.

The Precipitation Method, formerly carried out in reverberatories, but now only in blast furnaces, and that to a very limited extent, requires coked fuel and much ferruginous flux. It is only applicable to rich lead ores containing comparatively small quantities of antimony, arsenic, &c., because these metals are also reduced by iron, rendering the lead very impure. Silica and other non-metallic minerals do not interfere with the process, and copper ores are well suited to it because the whole of that metal. even if existing in very small proportions, becomes concentrated together with part of the silver in the matte, from which both can be recovered. The advantages of this process are that it dispenses with a preliminary ore roasting, and gives ultimately a good extraction of lead with comparatively low volatilisation losses of both lead and silver. It has, however, many grave disadvantages, especially the high consumption of coked fuel, and the slow, because only partial and gradual, recovery of the lead, together with a complicated matte treatment which makes the method a very costly one for labour. As a separate and distinct process, therefore, the method is practically obsolete, surviving only in the Upper Harz at Clausthal under exceptional local conditions; it is, however, frequently combined to some extent with the following :

The Roast and Reduction Method is applicable to all kinds

of ore, but especially to the poor siliceous, pyritous, and blendiferous ores which are so common, and which cannot be treated by the foregoing methods. It comprises two independent processes carried on in separate furnaces. The roasting is conducted in the various ways to be described, and the smelting of the roasted ore takes place in blast furnaces, the construction of which shows great variation in detail, though the principle is always the same.

The roast and reaction method may be carried out either in reverberatories or in hearths, the residues in either case being run down in blast furnaces. Whatever the appliance employed there is always considerable loss of lead by volatilisation, which becomes of greater importance in proportion as the ores are richer in silver. In all cases, therefore, the furnace flues are connected with a more or less complete system of flues and condensing chambers for recovering the flue dust. A separate chapter is devoted to this matter, which need not, therefore, be further dealt with here.

CHAPTER IV.

LEAD SMELTING IN REVERBERATORIES.

Preliminary. The ore for this process must be crushed sufficiently fine to pass a 4 or 5-mesh sieve, an operation which is usually performed as part of the "dressing" or "concentration" almost always required to produce ore of sufficiently high grade for the process before the ore reaches the smelting works. In all cases the process comprises two stages, which follow each other in the same furnace, and which are rapidly alternated several times.

1st Stage or Roasting.-The ore is spread in a layer of from 3 to 4 inches over the hearth of the furnace, and gradually brought to a low red heat, say 500° to 600° C., at which it is kept with free access of air, being continually raked to expose new surfaces and to prevent clotting, which would suspend the process of oxidation. The roasting is not allowed to complete itself, sufficient undecomposed sulphide being left to bring about the reactions described in Chap. ii. The rabbling in of small quantities of lead carbonate and sulphate ores or of litharge is of great advantage, as it shortens the period of roasting. Could a sufficient amount of these oxidised substances be obtained to mix with the galena in proper proportion, roasting might, in fact, be dispensed with altogether, and the reaction period entered upon at once in a hot furnace, but this is rarely, if ever, possible.