and proposes to treat galena ores by melting and blowing in a kind of Bessemer converter to obtain (1) a small quantity of lead rich in silver; (2) molten litharge; (3) fume which by excess of air is converted into lead sulphate. Huntington* regards the volatilisation of PbS in N, CO2, SO2, and other gases as probably a case of mere solution in a gas; and with regard to the proposed "new metallurgy of lead," points out the impracticability of the process, especially as regards the condensation of the enormous amount of lead fume formed, and the difficulty of finding any furnace lining to withstand the action of molten litharge at the temperature of molten PbS. Lodin† also confirms the reactions heretofore universally recognised as being correct at the temperatures prevailing in ordinary lead furnaces. There is no doubt that Hannay's work is interesting, as showing that the volatility of PbS is greater than it is generally supposed to be, and that it is increased in the presence of certain gases; but his explanation of the reactions taking place in lead furnaces must be regarded as untenable, and his proposed "new metallurgy of lead" impracticable. Jenkins and Smith, while entirely confirming the old reactions, prove that lead is attacked by sulphur dioxide. Lead Carbonate, PbCO,.-Lead carbonate occurs as the mineral cerussite (sp. gr. 6·46). At 200° it is decomposed into PbO and CO2, and at the same temperature is reduced to metallic lead by CO, charcoal only acting upon it at a somewhat higher temperature. The white lead of commerce is a basic salt, the composition of which is 2PbCO. PbH2O2.

Lead Silicates.-Lead oxide and silica begin to combine at the softening temperature of the oxide-i.e., below 800° C.; and if both substances are finely divided, and the heat maintained at this temperature, combination will be effected without the mixture becoming fluid. According to Percy, all the silicates of lead up to the trisilicate, 2PbO. 3SiO2, are readily fusible at low temperatures, the monosilicate in particular, 2PbO. Sio, being as fluid as water. Higher silicates are successively less fusible up to 2PbO. 9SiO2, which only melts to a substance resembling porcelain, while PbO. 18SiO, can barely be fritted at the highest temperature obtainable. All the fused silicates of lead are yellow, the presence of other metallic oxides altering the colour, as seen in the scorification process of silver assaying.

Silicates of lead are not readily decomposed by carbon or sulphur, but are decomposed by carbon in combination with lime, or some other base which combines with the liberated silica. Iron in excess effects a complete decomposition of all lead silicates at a bright red heat, forming monosilicate, 2FeO. SiO.

* Ibid., p. 217.

+ Comptes Rendus, 1895, vol. exx., pp. 1164-1167; and abstract in Journ. Soc. Chem. Ind., vol. xiv., 9, p. 807.

Journ. Chem. Soc., 1898.

Sulphides of iron, baryta, and lime all decompose basic or subsilicates of lead at comparatively low temperatures, forming silicate of the base, sulphurous acid and metallic lead. reaction in the case of iron sulphide is as follows::

4PbO. SiO2 + 2FeS

=

2FeO. SiO2 + PbS+ 3Pb + SO2.

The

The monosilicate is only partially decomposed, and requires for complete decomposition a higher temperature and the addition of a free base, such as lime :

3(2PbO. SiO) + 4CaO + 2CaS = 3(2CaO. SiO2) + 3Pb + 2SO.

The bisilicate and lower silicates are readily decomposed by nitric acid, but decomposition is not complete in the case of the trisilicate, and higher silicates are attacked with increasing difficulty.

Lead Borates.-Lead oxide and boric trioxide melt together in all proportions, and the compounds formed are more fusible than the corresponding silicates. Advantage is taken of this fact in lead and silver assaying.

The haloid compounds of lead are very volatile, and form volatile chloro-sulphides and chloro-phosphates. They are readily reduced by contact with carbon and metallic bases at a red heat.

CHAPTER III.

LEAD ORES.

THE principal minerals containing lead are the following:Galena, PbS; 86.6 per cent. Pb.-This, the commonest of lead minerals, is frequently found in groups of cubical or octahedral crystals; also massive, filling cavities. It is found in every country in the world, and in almost every variety of deposit, from true fissure-veins to impregnations in certain beds of limestone, dolomite and sandstone. In most cases it is so mixed with other vein, or rock matter, as to require some process of dressing" to fit it for treatment by the smelter; and very often the admixture with other metallic sulphides is so intimate as to give rise to great loss in these preliminary mechanical operations. The subject of ore-dressing scarcely appertains to metallurgy, and the student may consult with advantage the very complete compendium given by Dr. C. le Neve Foster in his Ore and Stone Mining.*

* Reference should also be made to Kunhardt, Practice of Ore Dressing in Europe, New York, 1892; to Bellom, "On the Mechanical Preparation of Ores in Germany," in Ann. des Mines, 1891, vol. xx., p. 1; and to two papers in the Proc. Civ. Eng., viz., Curtis on "Gold Quartz Reduction," and Commans on the " 'Concentration and Sizing of Crushed Minerals " (Proc. Civ. Eng., vol. cviii., p. 97, and vol. exvi., p. 3, respectively).

Galena is almost always argentiferous, and its silver contents may be present either as isomorphous sulphide, Ag2S, or as some distinctly silver-bearing mineral (e.g., argentite, stephanite, polybasite, fahlerz) finely disseminated through the cleavage planes, and this difference is important as regards the dressing operations. In the former case the loss of silver in dressing is proportionate to the loss of lead; in the latter, and by far the commoner, case, the loss of silver in dressing greatly exceeds the loss of lead, as the silver-bearing mineral, besides being finer to begin with, is also both lighter and much more brittle, so that it is driven into the finest slime, which floats away in and upon the surface of the dressing water. A notable case of this kind is to be found in the Zeehan and Dundas field of Tasmania, where, according to a series of assays of ores from different mines and workings given by Thomæ,* the handpicked ore contains on an average 94 ozs. silver per ton for 66 per cent. of lead, while the dressed ore carries only 76 ozs. silver for 70 per cent. lead, a clear loss of 25 per cent. of the silver in the fine slimes carried off by the dressing water. The poorest galena as regards silver contents is that from Bleiberg (Carinthia), which contains only 1 dwt. per ton of ore; the richest known is that from Schemnitz (Hungary), a specimen of which, according to Beudant, gave 7 per cent. of silver, or over 2200 ozs. per ton.f

It is a common belief among miners that the grain of galena is influenced by its richness in silver, the general idea being that a coarse-grained galena is likely to be poor in silver, and a fine-grained variety rich. No general rule of the kind can be formulated; nevertheless, as regards certain districts, some connection is observable between the grain of the galena and its silver contents. For instance, at Broken Hill, N.S. W., the coarse-grained galenas are said to be generally richer in silver than the fine grained; while, according to the author's experience in various parts of Mexico, the contrary rule prevails as regards the ores of each particular mine, though the coarse-grained galenas from one mineral deposit may be richer than the finestgrained varieties from another.

Cerussite, PbCO; 775 per cent. Pb.-This mineral is seldom found pure in quantity, except when occurring, as at Leadville and Broken Hill, in secondary crystals distributed through the "gozzan" of large complex ore bodies. In these cases, though itself comparatively free from silver, it is found associated with native silver and its haloid compounds, especially the chloride and chloro-bromide. It is nearly always mixed with inore or less anglesite, and with oxide of iron and other * Trans. Inst. Min. Met., vol. iv., p. 60.

+ Phillips, Elements of Metallurgy, 1891, p. 617.
v. Percy, Metallurgy of Lead, p. 97.

vein material. Important deposits at Laurium (Greece), Eureka (Nev.), and Leadville (Colo.), are now practically worked out, but other important localities which still yield large quantities of this ore are Carthagena and Linares (Spain), Sierra Mojada, St. Eulalia, and Velardeña (Mexico), and Broken Hill (N.S.W.).

Anglesite, PbSO4; 68.3 per cent. Pb.-Like the preceding, this mineral is invariably a secondary or decomposition product of galena, and it is found mixed with the carbonate everywhere, generally, however, in small proportion. When crystallised, it is, like the carbonate, nearly free from silver.

Pyromorphite, 3Pb(PO4)2 + PbCl with 69 5 per cent. Pb, and Mimetesite, 3Pb(AsO4)2 + PbCl, with 76-2 per cent. Pb.These two minerals are mentioned together, for, although the formulæ of pure specimens are as above, the former frequently contains As, and the latter large quantities of PO; while both may contain lime replacing the lead and fluorine replacing the chlorine. Both are comparatively free from silver, and occur associated with cerussite in the upper decomposed portions of lead lodes, mimetesite being, perhaps, on the whole, the commoner of the two.

*

Other lead minerals are Crocoisite, PhCrO4; Wulfenite, PbMoO; Stolzite, PbWO,; Mendipite, PbC1,2PbO; Vanadinite, 3Pb(VO4)2 + PbCl; Lanarkite, Phosgenite, Clausthalite, &c., as well as sulphantimonides, such as Jamesonite, Bournonite, &c. Seeing, however, that none of these minerals occur in large quantities, or otherwise than associated with other ores of lead, they cannot be said to be of any metallurgical impor

tance.

Commercial Lead Ores.-For metallurgical purposes the ores of lead are divided into "sulphide ores," consisting in the main of galena, with greater or less admixture of blende, pyrites, chalcopyrite, mispickel, fahlerz, &c., and oxidised ores," called in America "carbonates," under which general term is included not only cerussite itself, but all its admixtures with anglesite, pyromorphite, &c.

Oxidised Ores.-It will be understood that the oxidised ores are not essentially a product of different districts from those producing sulphides, but that they are merely the surface gozzans, into which large masses of the latter have been decomposed by atmospheric agencies; anglesite representing an intermediate stage in the transformation of galena into cerussite. The change from oxidised ores to sulphides may take place gradually, as at Catorce and some other Mexican mining districts, or suddenly, as on Iron Hill, Leadville.† The depth at which the change

* For details, works on mineralogy should be consulted, especially Dana, System of Mineralogy, New York.

+ Blow, Trans. A.I.M. E., vol. xviii., p. 170.

takes place also varies considerably; thus at Broken Hill (N.S.W.) large masses of unoxidised sulphides are found at only 200 feet from the surface, while at Eureka (Nev.) oxidised ores continued down to 1300 feet.* The principal factors which determine the depth to which oxidation extends are-(1) Depth below surface of permanent water level of country; (2) permeability of the rocks; and (3) amount of rainfall. By atmospheric oxidation the pyrites of the original mixed sulphide ore becomes iron oxide in the gozzan, chalcopyrite forms oxide of iron and carbonate of copper, blende is partly leached away as zinc sulphate, but a part remains as calamine and smithsonite. The silver contents of the original sulphides partly remain as sulphide or sulphantimonite, partly become native silver, but more generally form haloid compounds with the small quantities of haloid elements contained in the air and meteoric water. In this way the silver of the Leadville "carbonates" was chiefly found as chloride, while at Broken Hill and in Chili the bromide and iodide are more common. Carbonate ores are generally richer in silver than the deep-seated sulphides, to the oxidation of which they owe their origin. This may be due to several causes, but the principal one is undoubtedly the much greater solubility of lead sulphate and carbonate, than of silver chloride and sulphide. Small nodules of galena are frequently found in carbonate ores, which are very much richer in silver than the surrounding matrix, but these are of secondary origin in most cases. Any gold contained in the original sulphides remains in the carbonates in the native condition.

Carbonate and other oxidised ores can be but seldom concentrated to advantage, owing to the comparatively low specific gravity of the oxidised lead minerals compared with galena, and to their great friability compared with the quartz, limonite, and other gangue. The loss of silver in the fine slimes is usually more serious than the loss of lead. Attempts have been made to leach out part of the silver contents with a hyposulphate solution before concentrating for lead, also to separate most of the lead by a dry concentration process prior to working up the tailings by amalgamation or lixiviation processes; but neither attempt can be said to have been very successful.

Sulphide Ores. With reference to these ores, it may be said that, as a general rule, those deposits in which the galena occurs most free from other metallic sulphides will be poorer in silver than those in which it is intimately associated with

Curtis, "Silver Lead Deposits of Eureka," Monograph VII. U.S. Geol.

Survey.

For further information on this interesting subject the student is referred to a paper by Halse on "Deep Mining in Mexico," Trans. Inst. Min. Met., vol. iii., Part III., p. 418.

Concentration was, however, successfully practised at Tombstone (Ariz.), v. Church, Trans. A.I.M.E., vol. xv., p. 601.