ously by hindering the fusion of the charge, at all events up to amounts of 10 or 12 per cent. When not present in the ore, lime is frequently added in small quantities during the process with the object of stiffening the charge and preventing fusion.

Barytes and fluorspar remain inert during the process, but when present together tend to increase the fusibility of the charge. Chalybite often occurs as a gangue of galena ores, but can be mostly removed by dressing. Small quantities which remain with the ore lose their CO, during the roasting and become converted into magnetic oxide of iron, which assists the reaction by helping to stiffen the softened charge and, perhaps, also, by acting as an oxygen carrier.

Pyrites in small quantity has a decidedly favourable influence on both the roasting and reaction periods. During the former it becomes oxidised to iron sulphates, which subsequently give off SO, and so directly assist the formation of lead sulphate. During the latter, the ferric oxide formed tends to stiffen the charge and probably also to act as an oxygen carrier. In quantities above 10 or 12 per cent., however, ferrous sulphide tends to form and run down prematurely as a matte or "regulus," retaining, moreover, lead sulphide in the residues; and with 35 per cent. pyrites the roast and reaction process becomes not only uneconomical but impossible.*

Chalcopyrite behaves like pyrites, except that some copper is reduced with the lead.

Blende is partly converted into oxide and sulphate during the roasting and partly remains unaltered. Up to 4 or 5 per cent., the presence of blende assists the roasting process; but 10 or 12 per cent. retards the reactions very much; and 35 per cent., as in the case of pyrites, stops them altogether.

Antimony (stibnite, Jamesonite, Bournonite, zinkenite) is the most deleterious of all foreign elements, even in quantities of only 2 to 3 per cent. In the first place, both its oxide and sulphide are volatile, causing losses both of lead and silver. In the second place, they react upon each other, like the corresponding lead compounds, forming metallic antimony, part of which alloys with the lead, making it hard; and part volatilises again, carrying lead with it. Moreover, both oxide and sulphide are readily fusible, causing the ore to cake even during the roasting process, while the former combines with lead oxide to form lead antimoniate, which remains in the residues and causes them to yield a very hard lead when re-smelted.

Arsenic (mispickel, &c.) like antimony, is very detrimental, causing losses by volatilisation and by combination with lead oxide to form arseniate. A portion also combines with the lead

and contaminates it.

Silver and gold in the ores follow the lead reduced to metal, * Bouhy, Ann, des Mines, 1870, vol. xvii., p. 178.

very little being left in the oxidised residue; the lead reduced first is by far the richest in precious metals, so much so that it frequently contains four or five times as much as the last lead, and seven to ten times as much as that in the residues.

Classification of Processes.-Until recently three main varieties of process have been distinguished, namely, the Carinthian, English, and Silesian. The first, however, is only a survival, and the last is but a trifling modification of the second. Besides these there was formerly in existence a distinct Cornish process (of which the so-called Breton process was a modification) now no longer in use.

The Carinthian process was only suited to pure rich ores, but from such extracted a large quantity of pure lead with a low loss, its disadvantages being the very high cost for fuel and labour. It was characterised by small furnaces and charges, and by a low temperature throughout, with the object of producing as nearly as possible 1 mol. of PbSO4, or 2 mols. of PbO for each mol. of unaltered PbS, so as to get the theoretical reactions ::PbS+ PbSO4 = 2Pb+2SO PbS + 2PbO 3Pb+ SO2

=

Coal was added to the residues in the same furnace so as to get out as much lead as possible.

*

This method was in use at Bleiberg, Carinthia, and also in Belgium, but has now given way to a modified Silesian process, followed by smelting the residues in Pilz furnaces. There is, however, an American form of the process, which still exists to some small extent at isolated points in the Mississippi valley.

The English (Flintshire) process is conducted in large furnaces and at a high temperature so as to get out as much lead as possible in the shortest time and with a small consumption of fuel; the volatilisation loss is, however, rather high. Owing to the high temperature, reaction is at first chiefly between Pho and PbS, but little PbSO, being formed, except towards the end of the process.

The residues are treated in small blast furnaces (cupolas). The Silesian process is simply an improvement on the ordinary Flintshire method, with the object of saving labour and securing lower volatilisation losses. It is carried on in large furnaces, and at as low a temperature as possible, in consequence of which the proportion of lead obtained at first is smaller and the residues for subsequent cupola treatment are much richer.

The extinct Cornish process was adapted to the treatment of 60 to 70 per cent. lead ores containing some copper and other impurities. The roasting and reaction stages were carried on in separate furnaces, and a larger quantity of PbO and PbSO was formed in the first stage than was required for reaction

Oesterr. Zeitschr. f. Berg-, und Hüttenresen, June 3rd, 1893, p. 283.

upon PbS in the second stage. This excess of PbO was reduced, partly by addition of "culm "* and partly by scrap iron, the latter of which also served to collect copper and any remaining sulphur into a "regulus," which cleaned the slag so effectually that it could be thrown away. The loss by volatilisation was, however, high, owing to the high temperature employed, and the process was more expensive than the ordinary English process. The Carinthian Process.-This process was, until recently, used at Bleiberg in Carinthia for treating galena concentrates of from 60 to 75 per cent. lead. A good description, with illustrations of the furnace adopted, is given by Percy. The charges in this furnace were only 3 to 4 cwts., and the slag from two successive charges was reduced in the same furnace with addition of charcoal.

An almost exactly similar process was largely in use in Missouri and Wisconsin for treating the rich lead ores of the

Figs. 4 and 5.-Carinthian Furnace, American Variety
(Vertical and Horizontal Section).

Mississippi Valley, and still lingers in some places. A good
description is given by Hofman. The furnace employed is in
both cases essentially the same, and the American variety (which
is the simpler) is shown in plan and section in Figs. 4 and 5.
+ Metallurgy of Lead, 1870, pp. 261-271.
+ Metallurgy of Lead, 1893, p. 89.

*Anthracite slack.

The charge in this furnace was about 1500 lbs. of galena, which was worked off in twelve hours, extracting an unknown proportion of the lead present, and leaving a "grey slag" which contained 55 per cent. Pb, chiefly as oxide, but partly also as silicate and sulphate. The Carinthian furnace is now, practically speaking, extinct, except in a few isolated localities, having been replaced by other methods less costly for labour and less wasteful of fuel.

The English Method. As already mentioned, the characteristics of this method are the use of large furnaces and moderately heavy charges, which are worked off quickly at a high temperature, the direct yield of lead being high and the volatilisation loss also rather high. Two main varieties of the method may be described according to the style of furnace employed-viz., the Flintshire, and the Spanish reverberatory furnace or "boliche."

The Flintshire Furnace.-The ordinary form of this furnace is built upon an air vault, which serves to cool the hearth. In some modified furnaces, instead of this vault, the whole furnace is open underneath, and is supported on iron rails or bars, and in any case it must be very strongly bound with cast-iron plates, buckstaves, and tie-rods. Common red brick is employed for all the outside work for the lower bottom, and for all parts of the furnace not exposed to a high heat, but the interior part of the furnace is built of firebrick. The true working bottom of the

furnace varies from 6 to 12 inches in thickness, and is composed of the grey slag produced in the smelting process, about 5 tons being required for the formation of the bottom, which is patched as required with slags produced in the furnace itself. Fig. 6*

Fig. 7.--Smelting Furnace, Coueron; Horizontal Section on C.D. shows the ordinary form of Flintshire furnace, in which the centres of the fireplace and flue are approximately in the longitudinal axis of the hearth, and the cast-iron lead well is situated in front of the middle working door at the front of the furnace. Figs. 7, 8, and 9+ show Hutchison's modification introduced at Coueron, in which the line joining the grate and flue is set still further to one side of the longitudinal axis of the hearth, and the lead pot is placed in front of the third front door in order to withdraw the lead as far as possible from the influence of heat. This furnace also shows the construction on iron girders, instead of the usual "air vault"; but these details are, of course, not essential. The firebridge is always air cooled, and it will be noticed that the exit flue is divided so as to permit of the flame being more spread towards the back of the hearth (i.e., the side opposite the well). The cast iron pot is usually provided with a separate fireplace in order to keep the lead molten while it is being ladled. The door frames and skew backs are of cast iron, inch thick.

*From Percy, Metallurgy of Lead, 1870, p. 225.

+From Phillips, Elements of Metallurgy, 1891, pp. 643, 644.