is contained in considerable quantity in lead which has been desilverised by the Parkes process, and must be got rid of by oxidation before the desilverised lead is marketable.

The process of desilverisation adopted must, therefore, be preceded by a process of oxidation to remove impurities, which is called "softening," or, sometimes in England, "improving"; while the Parkes process is followed by another similar oxidation to remove zinc, which is usually called "refining," or, sometimes, "dezincing." Both operations require similar plant, and may be conveniently described together in a single chapter. The concentration of precious metals in rich lead by the Pattinson and Parkes processes follows in two separate chapters, and, lastly, the cupellation of the rich lead and refining of the silver.

As an alternative to the combined softening and desilverisation by ordinary methods, Rösing* experimented at Tarnowitz (Silesia) on a Bessemerising process in a converter, with basic lining, taking charges of 6 tons. By this means lead containing 12.4 ozs. silver was almost entirely converted into a fume containing 75 per cent. lead and 2-4 ozs. silver per ton, the residual rich lead carrying 1863 ozs. to the ton. The author has not succeeded in learning any later particulars as to the practical working of this process, but the losses of lead and silver consequent on the re-treatment of so large a quantity of flue dust would seem to be a fatal objection.

CHAPTER XV.

SOFTENING AND REFINING FOR MARKET.

THE softening of lead as a preliminary to desilverisation may take place in cast-iron kettles or in reverberatory furnaces. The former are suitable for softening lead practically free from antimony and arsenic, and containing only small proportions of copper. The latter are much more commonly employed, since they are adapted to the softening of more impure lead, containing larger quantities of copper, arsenic, and antimony. When lead contains per cent. of copper and upwards, as at Freiberg and Przibram, it is advisable to preface the ordinary softening process by liquation in a special liquation furnace with inclined bed, the lead gradually draining out into a kettle, from which it can be tapped or syphoned into the softening furnace. In any case, the rapidity with which softening proceeds depends on the exposure of fresh surfaces to oxidation.

* E. and M. J., April, 16, 1892.

*

33-----

Refining, like softening, may be carried out either in kettles or in reverberatories. In the former, steam is the active agent (Cordurie's process); in the latter, atmospheric oxidation is exclusively relied upon, the process being slower and, perhaps, less thorough, but on the whole less costly.

Softening in Kettles.-At Lautenthal (Upper Harz)* the

Schnabel, Handbuch der Metallhüttenkunde, vol. i., pp. 451, et seq.

Fig. 110.

Fig.

109.

softening, desilverisation, and subsequent refining are all conducted successively in the same kettle. Analyses of the worklead treated are given in Table XXXIII.; it is comparatively free from arsenic and copper, and not very high in antimony. The kettles used are 6 feet 2 inches (1.90 m.) diameter by 2 feet 11 inches (0.90 m.) deep, and hold about 15 tons of lead each; their arrangement is shown in Figs. 109 and 110. The lower part of the kettle is heated directly by the flame, which then rises through the opening shown by the arrow and circulates round the upper part of the kettle in the flue, ƒ, before passing, by a downcast, to the principal chimney flue. The hood and steam apparatus shown in Fig. 109 is not used for softening, but only for refining, for which purpose also a removable cast-iron pipe, p, hangs in the kettle and can be connected at will by means of a valve with the steam pipe, s.

Softening. The charge of 15 tons of lead (containing 0-28 per cent. Cu and 0.57 per cent. Sb) is slowly melted down at a low temperature, taking six hours, at the end of which time twothirds of the copper, together with all the sulphur and one-half of the arsenic, iron, and nickel will have risen to the surface as a pasty "dross," which is skimmed off, leaving, however, practically all the antimony behind alloyed with the lead. A sample of the lead at this point poured into a mould shows, on cooling, a bright white crystalline spot in the centre of its dull greyish white surface, which is an infallible indication of the presence of antimony or arsenic.

The desilverisation with zinc follows, lasting fifteen hours and removing most of the remaining copper, without, however, affecting the antimony.

Refining. The cooled desilverised lead, now containing 0.7 per cent. of zinc as well as the original 0:57 per cent. Sb, is then heated to a dull red by four hours' strong firing, the hood lowered to prevent access of air as far as possible, and steam at 30 to 40 lbs. pressure blown through the bath to oxidise the zinc. It is impossible to prevent the introduction of air along with the steam, and therefore some lead and antimony are oxidised together with the zinc. The expulsion of zinc generally takes about two hours, at the end of which time the hood is raised and the oxides skimmed off. The hood is again lowered, its four working doors are opened and steam is blown through with free access of air to oxidise the antimony, with formation of a thick semi-fluid mass of lead antimoniate. This also takes about two hours, when fluid yellow feather-litharge begins to form showing the antimony to be practically all expelled; after cooling, the lead is then cast into moulds by means of the Rösing lead pump.* The yellow powder of zinc and lead oxides is separated by hand buddling from the shots of lead, and the most impalpable * Berg- und Hüttenmännische Zeitung, 1889, p. 262.

*

part of it, which floats away, forms, after drying, a good reddishyellow paint containing 60 to 67 per cent. ZnO, the remainder being practically all PbO. In order to avoid so much oxidation of lead, Rössler* has forced in, together with the steam, CO2, which yields a nearly pure zinc oxide; also a mixture of CO and nitrogen, which yields a considerable proportion of the zinc as "blue powder"-i.e., in the metallic condition. The improvement has not, however, come into general use.

The great drawback of kettle refining is the wear and tear of the kettles, which are rapidly corroded away at the surface of the metallic bath by the lead antimoniate. At Lautenthal, castiron kettles only last for 20 to 40 charges; at Tarnowitz, with a lead almost free from antimony, cast-iron kettles last for 90 charges; while those of open hearth steel last 120 to 150 charges; and those of crucible steel 250 to 270 charges. Whatever their material, kettles for these other purposes should always be cast bottom downwards, though the other way up is easier for the foundry.

At Hoboken (Antwerp) Spanish lead is treated by this process, no attempt being made to separate the zinc and antimony, which are recovered together in one product for further treat

ment.

Liquation of Cupriferous Lead.-At Freiberg † the work-lead containing 0.9 per cent. copper and other impurities (v. analysis in Table XXXIII.) is liquated, together with all the skimmings. from the subsequent Pattinsonisation, on the hearth of a special furnace very similar to that shown in Fig. 131 (Chap. xvii.). The pigs are piled on an inclined hearth of marl, and gently heated by means of the flame from a closed grate supplied with blast under the bars so as to burn inferior slack coal. The lead trickles out and collects in a pot, from which it is tapped from time to time through a spout. In twelve hours 12 tons of base work-lead are slowly melted down in the above furnace with a consumption of about 2 tons of slack coal; and from 4 to 5 per cent. of residues remain upon the hearth containing practically all the sulphur, 93 per cent. of the copper, 96 per cent. of the nickel and cobalt, 77 per cent. of the iron, and 25 per cent. of the arsenic of the original lead, which, however, retains all its original bismuth, all its tin but 1 per cent., and all its antimony save about 5 per cent. The dross melted down and allowed to cool separates into three separate products-viz., lead, speiss, and matte. The comparative absence of iron from the two latter, however, proves that they are not original furnace products held in suspension by the lead, but that the natural affinities of S for Cu and of As for Ni, Cu, and Pb come into operation to form compounds less fusible than the lead, which * Ibid., 1890, p. 248.

+ Schertel, Berg- u. Hüttenmännische Zeitung, 1882, p. 293.

+ Ibid.

dissolved them in the smelting furnace at a much higher temperature.

Neither at Freiberg nor at Przibram is the liquated lead sufficiently pure to be passed on to the Pattinson process without further softening, most modern works, however, treat lead less cupriferous than that mentioned above, so that it is possible to purify it sufficiently for desilverisation in a single furnace with great saving of time and fuel. Still, at the Broken Hill Works, it has been found very advantageous to employ a system of two separate, but similar, furnaces for drossing and softening, as will be described hereafter.

TABLE XXXIII.-ANALYSES OF WORK-LEAD BEFORE
AND AFTER LIQUATION.

References.-1 and 2. Hampe, Zeitschr. f. B. H. u. S. W. im Preussen, vol. xviii., p. 203. 3 and 5. Schertel, B- u. H. Zeitung, 1882, p. 293. 4. By difference from 3 and 5.

Softening in Reverberatories.-The operation consists of two stages-viz., drossing and oxidation; the former is a liquation which takes place during melting down, the dross which rises to the surface containing most of the copper. The temperature is then raised to cherry red, and with free access of air tin, arsenic, and antimony oxidise successively, and are removed by skimming.

In Germany and at a majority of old works in England, softening furnaces are still used, in which the hearth is only