Silesian Process at Tarnowitz (Upper Silesia).*-The process at the Friedrichshütte, Tarnowitz, is a little peculiar in that a portion of the ore, specially rich in blende, is roasted as thoroughly as possible in a long reverberatory roaster and then added to the rest of the charge in the lead furnace, together with flue dust, litharge, and other oxidised products from the refinery, all of which assist in the reaction upon undecomposed lead sulphide.

The ores treated contain about 70 per cent. lead as galena, with 5 per cent. zinc as blende and trifling admixtures of pyrites, chalybite, calcite, dolomite, and calamine. They are practically free from copper and arsenic, contain very little antimony, and only about 5 ozs. of silver per ton.

The

The construction of the furnace used is shown in the following Figs. 11 to 14. In general form it somewhat resembles the English furnaces already described, especially the Coueron modification of the Flintshire furnace, for the hearth slopes gradually towards the sump, and not suddenly as in the old Flintshire furnace, and the well is at the farther end door. It is rectangular in form, with 4 working doors on each side, the well being at the coolest end of the furnace below the door next the flue, the opening into which is divided into 4 by means of brick partitions. The bridge is hollow, and contains a cast-iron trough, cooled by a current of air which is drawn through it by means of a hole in the roof, as shown on the plan, Fig. 12. iron-grate bars are placed transversely in the firebox, and not longitudinally as usual, firing takes place from both sides, ashes being removed at the end of the furnace. The roof is horizontal from the firebox to the centre of the hearth, where the charging opening is situated, falling thence in a straight line to the flue (longitudinal section, Fig. 11). The furnace is completely cased in iron plates strongly bound by iron-rail buckstays and tie-rods. The hearth is built of red brick on a thick sand bed sloping towards the flue at about the right angle for the final hearth, as shown in Fig. 13. Over this is formed the trough-like brasque bottom, composed of clay 1 part, coke 3 parts, which slopes towards the well, and varies in thickness from 4 inches to 12 inches. The actual working bottom is a layer 4 inches thick of slag having a high melting point, and spread evenly over the whole area, following the surface of the brasque bottom already rammed to the proper curve. This working hearth used to be made of tap cinder, but ordinary gray slag mixed with lime is now used instead, exactly as in English furnaces.

Mode of Working. The charge weighing 2500 kilos. (say 21 tons English), and crushed to pass a 5-mesh screen, is dropped into the furnace, which is already at a dark red heat (500° to 600° C.),

*Zeitschrift für Berg-Hütten.- und Salinen-Wesen, vol. xxxii., p. 94, and vol. xxxiv., p. 292; also Schnabel, Handbuch der Metallhüttenkunde, vol. i., p. 305.

16'

Fig. 11.-Silesian Furnace (Longitudinal Section).

and spread out so as to form a layer 3 inches to 4 inches thick. The fire is fed with cinders only, so as to keep the temperature low; the galena begins to decrepitate, and the charge gradually acquires the temperature of the furnace after about an hour. The doors are now opened, and roasting begins at the surface of the charge, a white crust of oxide and sulphate being formed. About every twenty minutes the charge is turned over by means of paddles, or stirred with rabbles, so as to expose new surfaces to the action of the air and prevent clotting. After four hours the oxidised additions are made, consisting of a portion of the same ore previously roasted, with flue dust and carbonate ore when available. The total weight of these additions is about 10 to 12 cwts., and they are well stirred in; at the same time the grates are cleaned and fed with good coal, and the damper is fully opened so as to raise the temperature. The reaction period begins, lead runs down, and the ore softens so much as the temperature rises that lime has to be thrown on the charge and well rabbled in to stiffen it, after which the whole charge is turned over. The working doors being open during these operations, the furnace cools off so much that the flow of lead ceases, while more of the undecomposed sulphide oxidises. On shutting the furnace doors the temperature soon rises, reaction begins again and more lead flows out. This alternate oxidation and heating of the charge takes place at intervals of about one and a-half to two hours throughout the whole period of reaction. About three hours from the start the well is tapped into the kettle, poled there, drossed, and ladled into moulds, the dross being set aside for subsequent treatment. This first kettleful of "virgin lead" carries from 50 to 60 ozs. of silver per ton. About 10 cwts. of oxidised skimmings from the steam refining of lead desilverised by the Parkes process, and containing 75 to 80 per cent. PbO and 18 to 20 per cent. ZnO, are now added and the doors closed, while the fire is fed with small coal to ensure a smoky flame. This reduces lead sulphate to sulphide, and lead oxide to metal, and the flow of lead begins again. Further tappings are obtained each hour and a-half, steadily decreasing, however, in amount and in silver contents. Before the last tapping the kettle drosses obtained by poling the previous kettle leads are thrown into the furnace and melted down with the last tapping. In all, the reaction period lasts seven hours; after which the residues are raked out, the hearth repaired where necessary and sprinkled with lime, and a new charge added.

The removal of the residues was formerly effected by raking them out of the last working door opposite the well into a box filled with water, but the fumes were very injurious to the workmen employed, and dangerous explosions sometimes occurred. In the latest furnace, the residues are raked through an

opening in the furnace hearth, which is covered by an iron plate during the working of the charge, into a wheeled iron slag pot underneath. This improvement, not shown in the figure, has had a great effect on the health of the workmen.

About 60 to 65 per cent. only of the lead contents of the ores are extracted as work-lead, containing on the average 6 to 7 ozs. silver per ton; the remainder is all found in the residues (amounting to 30 to 35 per cent. of the weight of the charge and containing 40 to 55 per cent. metal), except about 4.45 per cent. Of this, most is recovered in the form of flue-dust, which chiefly consists of PbSO4, so that the real loss in the process unaccounted for is only 1 per cent.*-calculated on the commercial dry assay of the ore. The residues are treated in a cupola furnace.

The total length of an operation is eleven to twelve hours, 2 men working on each 12-hour shift. The consumption of fuel varies between 40 and 50 per cent. of the weight of the charge, and that of lime amounts to about 1 per cent.

At Bleiberg near Montzen (Belgium)† the furnace in use is similar, except that it has a fireplace at each end, the flue opening being in the middle of the arch. The calcination period on a 2-ton charge lasts six hours, and the period of reactions six hours. Further details are given in Tables VIII. and IX.

The Flowing or Cornish Process.-In this process, now extinct, the operations of calcination and of reaction were performed in separate furnaces, and a larger quantity of PbO and PbSO was formed in the first than was required for reaction upon PbS in the second. The hearth of the calcining furnace was rhomb-shaped, the firebridge and flue door being at the ends of the longer axis, and the working doors at the ends of the transverse axis. The charge was from 1 to 3 tons, according to size, and the calcination took fifteen to eighteen hours with a coal consumption of 30 per cent. by weight.

The flowing furnace is rectangular and longer, and contains four working doors on opposite sides, besides the flue door at the end opposite the firebridge. The hearth is saucer-shaped in longitudinal section and wholly inclined sideways towards the second door on the front, beneath which is the usual cast-iron pot. The whole of the hearth is carried on iron bars stretched transversely across the foundation walls and arranged roughly parallel to the contour of the working bottom.

The charge in the Cornish process was 2 tons of calcined ore, which was spread on the hearth and run down into the well in about three hours, when the lead was tapped out, the residues thickened with lime and cut in and "set up.' Some 5 per cent. of scrap iron was then thrown into the well, and the charge * Schnabel, Handbuch der Metallhüttenkunde, vol. i., p. 307. + Schnabel, op. cit., p. 308.

again melted down, the lead being tapped out together with the matte, or "slurry," formed, which carried nearly all the copper present in the ores. The whole time required to work off a charge was about eight hours, and the coal consumption 40 per cent. The slags were usually poor enough to throw away and the process was specially adapted to the smelting of coppery ores containing only 60 to 70 per cent. lead.

The whole operation in the flowing furnace usually took about eight hours, and on account of the higher temperature required to melt the larger quantity of refractory material, the volatilisation loss and consumption of fuel were both greater than in the Flintshire furnace, which more than offset the advantage of cleaner slags. The flowing furnace has been, however, very largely used in Wales and elsewhere for reducing the grey slags of the Flintshire furnace, as well as for treating rich silver ores, lead residues, jeweller's "sweep," &c., and is still in use to a considerable extent for such and similar purposes.

TABLE VIII.-DETAILS OF REVERBERATORY LEAD FURNACES.

Comparison of the Various Processes.-Comparative data have been thrown into tabular form to facilitate reference. Table VIII. gives constructional details of various lead furnaces, and Table IX. details of the operations carried out in them.