At Mansfeld, the refining furnace in use is shown by Figs. 92 and 93. It is provided with an apparatus, b, for generating FIG. 93. k

gas from hard charcoal, which is charged by a; the gas enters by the horizontal channel, c, into the vertical channel, d, and from thence by the openings, e and f, into the channel g, where it is mixed with atmospheric air, which has access through several flues-h, is the fire-bridge; k, the arched roof; 1, the flue; m, m are condensation chambers. The smelting hearth is formed of washed wood ash with 12 per cent of lime.

After the hearth has been dried for 12 hours, and heated for 6 hours longer with about 7 cubic feet of coal, the apparatus for the generation of gas is heated with wood until the furnace has become hot enough to ignite by its own heat the oxide of carbon produced; when this does not take place, explosions may occur in the condensation chambers; the apparatus is afterwards charged with charcoal.

The precipitated silver obtained by Ziervogel's process, and which is contaminated with gypsum, metallic copper, and sulphates, is charged in lots of about ton, two or three times in succession; each time when the previous charge is melted, the silver from the residues of the last process is added; the smelting is performed in 6 or 7 hours, the doors being closed. The frothy slag formed is now skimmed off, a strong fire is made up for some time, and then the generation of gas is interrupted, the working door is opened and the dross

removed from the silver till it has the appearance and quality of fine silver. It now has a fineness of 98'0 to 98*3 per cent. It is removed in iron ladles which have been previously warmed and coated with clay; the ingots formed weigh from 30 to 40 lbs.

The dross is once more put into the hot furnace to melt out the silver mechanically contained in it; the residues and the hearth are pounded and sifted to separate the enclosed silver grains; they are then mixed with 12 per cent of sulphuric acid, and exposed for two or three days to a temperature of from 60 to 80° C., and then slightly roasted in a reverberatory furnace for 1 or 1 hours till sulphate of silver is formed. After cooling it is again treated with 6 per cent of sulphuric acid, and then a third time in the same way, when it will be found that the residues only contain 8 or 9 ozs. of silver per cwt., which are worked up together with the copper matt.

1,000 lbs. of precipitated silver consume in 12 hours about 7 cubic feet of charcoal and 120 bundles of wood for warming, and 68 cubic feet of charcoal for refining. 5 tons of precipitated silver of 86-87 per cent fineness yield 3 tons of fine silver of 98 to 98.3 per cent.

Refining in Crucibles.

This mode consumes less fuel than the refining process under the muffle, and causes less loss of silver, and also yields a more uniform fine silver, but it only permits a small production, and it causes loss by breakage of the crucibles; for this reason it is only used in a few smelting works. It has been in use at Pontgibaud since 1849. The brightened silver is melted in plumbago crucibles, o'3 metre high, and respectively o'18 and o'09 metre wide at the top and bottom, with an addition of saltpetre and quartz. The air furnace is o ̊5 metre in section, o'6 metre deep, from the grate to the mouth; the chimney is 5 metres high, and o'5 by o'15 metre in section.

The crucible surrounded by glowing coals and coke, is

B. u. h. Ztg., 1851, p. 379.

gradually charged with 38 or 40 lbs. of silver, with an addition of 1 lb. of saltpetre and quartz; the mass will be fused in about 1 hour, and the process finished in 1 hours. The dross which is formed is removed, and an addition of saltpetre and quartz will show whether new dross (oxide and slags) will be formed; if such is not the case, the contents of the crucibles are poured out into cylindrical moulds; the resulting silver is of o'997 fineness.

Of late the silver has been melted, without any addition, and as quickly as possible, in order to render it more homogeneous.

A similar process is carried on in Vialas* with brightened silver which has been brought to a moderate fineness in the cupelling furnace; this causes a greater loss of silver, but the subsequent melting with quartz is cheaper.

At Arany-Idka (Upper Hungary), the silver resulting from the amalgamation process is melted in cast iron crucibles, 24 inches high and 28 inches wide, furnished with a clay lid; carbonate of potash, saltpetre, and sulphate of soda being added.

The Extraction of Silver from Rich Lead by means of Zinc.

According to Parkes,† argentiferous lead may be desilverised by fusing it with zinc; when the metals are well stirred together, and afterwards allowed to remain quiet for some time, the zinc will rise to the surface, and will be found to contain nearly all the silver. The argentiferous zinc is now skimmed off, or allowed to become solid and form a crust; this may be raised and separated from the lead, and then treated for its silver, which may be obtained in a pure state. The separation of the zinc from the silver is effected either by distillation or by a treatment with muriatic or sulphuric acids. The argentiferous residue may be purified by cupellation. The desilverised lead may be purified from zinc by refining in reverberatory furnaces.

Though this mode permits a complete desilverising of the

* Ann d. min., iv., 121.

+DINGL. polyt. Journ., Bd..119, p. 466. Bgwksfd., xv., 653.

lead, it was relinquished on account of the difficulty in treating the alloy of lead and zinc.

Very rich lead cannot be completely desilverised in one operation by this method, and a combination of Parkes' and Pattinson's processes might perhaps be suitable for it. The former process will extract a great part of the silver, and the latter will at the same time enrich and purify the poor lead.

According to Nevill, who has compared both methods, a much greater quantity is extracted by Parkes' process, and he states that the loss of lead amounts to I per cent, and that of zinc to 3-5ths of the quantity employed.

The quantity of zinc to be added depends on the amount of silver contained in the lead, and also on the foreign impurities present (sulphur, arsenic, antimony, &c.), which then combine in great part with the zinc.

This process was carried out at the works of Messrs. Nevill,+ at Llanelly, as follows:

About 6 tons of lead, containing about 14 ozs. of silver per ton, are fused in an iron pan like that used in Pattinson's process; into this about 1 per cent of fused zinc is poured, and four men stir the fluid metal for one or two hours. The fire is then lowered, and the mixture allowed to rest for some time until a scum forms; this scum is removed with a ladle pierced with a number of holes.

When the crust ceases to form, the lead is run off into a reverberatory furnace, and heated in a flat pan, to allow the zinc that may have remained in it to volatilise or oxidise; when this treatment has been sufficiently prolonged, it is run into an iron pot, stirred with a stick of green wood to bring about a still more perfect oxidation, and then cast into moulds for sale.

The scum taken from the pot is partly purified from the lead with which it was mixed, by heating in an inclined iron retort. The lead that runs out of this zinc contains about 1,000 ozs. of silver to the ton, and is ready for cupellation; the residue in the retort is transferred to clay pots, and the zinc distilled from it. The residue consists of silver mixed

* ERDMANN'S Journ. f. pr. Ch., lxvii., 257.

+ B. u. h. Ztg., 1852, p. 1. ERDMANN'S Journ. f. pr. Ch., xii., 257.

with much antimony, lead, copper, arsenic, nickel, &c.; this is fused with lead and cupelled for its silver.

At Tarnowitz* (Hungary), experiments were made to desilverise raw lead containing 1 or 2 ozs. of silver by this method, and 1 per cent of zinc was required to effect this desilverisation; the remaining lead contained o'008 of an oz. of silver. The desilverisation of the zinc was effected by distillation in muffles; the cost for 5 tons of raw lead treated by this method amounted to about £2, and the cost of cupelling the same quantity to about £6.

B. EXTRACTION OF SILVER IN THE WET WAY.

To extract silver from ores or products by the wet way it must be converted

1. Into chloride of silver, from which the silver may be extracted by mercury (amalgamation); or the chloride of silver is dissolved in a solution of common salt and precipitated by copper (Augustin's process); or the chloride of silver is dissolved by hyposulphite of soda, and the silver thrown down as a sulphide by polysulphide of sodium (Patera's method).

2. The silver is dissolved as a sulphate and precipitated by copper (Ziervogel's process).

3. The argentiferous product is treated with dilute sulphuric acid, and the silver extracted from the argentiferous residue either by fusion with lead or in the wet way (extraction by means of sulphuric acid).

The choice between these different processes depends chiefly on the composition of the substances under treatment, whether they contain more or less lead, antimony, arsenic, and zinc.

1. Extraction of Silver by means of Mercury (Amalgamation).

This processt is based upon the property which mercury possesses of forming an alloy or amalgam with silver which decomposes when heated, the mercury being volatilised,

* KARSTEN'S Archiv., 2 R., xxv., 192. B. u, h. Ztg., 1852, p. 828.

+ KARSTEN'S Archiv., 2 R., i., 161.