(Altenau, Freiberg, Stolberg); oxidation smelting in air reverberatory furnaces (England, Stolberg, Rouet); oxidation smelting in cupola furnaces (Freiberg, Antonshütte); smelting with oxidising and purifying fluxes (methods of Baker, Pontifex, and Glassford).

DIVISION I.

SMELTING OF SULPHURETTED LEAD ores.

The production of lead is effected in hearth, cupola, and reverberatory furnaces. Galena free from earthy substances and foreign sulphides, or containing only small quantities of lime, heavy spar, and blende, is advantageously worked on hearths and in reverberatory furnaces; but those ores which contain a large amount of earthy and siliceous substances, as well as other metallic sulphides, are best treated in cupola furnaces, though local circumstances sometimes render the employment of reverberatory furnaces advisable also for these ores. The choice of a reverberatory furnace or a hearth furnace, for pure galena, depends chiefly on the quality and price of the available fuel, and for this reason hearth furnaces are principally used in those countries where good coal is expensive, and wood and turf cheap.

THE REVERBERATORY PROCESS is simple, very economical, and occasions the least loss of metal; it admits of the use of raw fuel, and of the process being watched throughout. It occasions but a small residue, and requires neither blast machines nor expensive fluxes. On the other hand, this process is only adapted for very pure ores containing little or no silica; small amounts of lime, heavy spar, and zinc blende, are not so injurious. As objections to this process, compared with the cupola furnace process, we may mention the loss of copper in poor copper schlich, a deterioration of the resulting lead by its alloying with copper, and, lastly, greater difficulties in making the slags. profitable. The different modifications of the reverberatory process are dependent on the presence of foreign substances, and also on the greater or less amount of silver contained in lead Although the smallest

quantity of silica contained in galena occasions a loss of

metal, fuel, and time, yet if the ore is worked in reverberatory furnaces, it is still possible that local circumstances may recommend this process likewise for ores which are rich in silica.

The different reverberatory processes may be thus classified, according to their chemical reactions, and according to the quality of the ores:

A. THE ROASTING REDUCTION PROCESS.

a. Carinthian Process, for very pure galena, poor in silver, and for ores containing a small amount of lime.

b. English Process, for galena containing more lime, and sufficient silver to make it worth extracting, and also containing some zinc blende and heavy spar.

c. French Process, for ores containing a large amount of pyrites, blende, and earthy matters, but not more than 5 per cent silica, and more or less silver.

B. DECOMPOSITION OF GALENA BY MEANS OF IRON.

French Precipitation or Vienne Process, for ores containing much quartz and aluminous substances, with more or less silver.

*

A. DECOMPOSITION OF GALENA BY ROASTING PRO-
CESSES IN REVERBERATORY FURNACES.*
In all these processes the three following periods are to
be observed:

Period 1. (Roasting.) Galena is roasted at a more or less. increased temperature for a longer or shorter time, and is thereby converted more or less into oxide of lead and sulphate of lead; part of the galena, however, remains undecomposed. If the temperature is raised rapidly, less sulphate of lead will be produced than when the temperature is raised slowly, or continued for a longer time. The most suitable temperature, and the proper time required for the roasting, depend on the quality of the ore; and it is essential * Theorie des Flammöfen processes. KARSTEN, Arch. 1, R. vi., 110. die Entschwefelung des Bleiglanzes. Ibid, vi., p. 328. Ann. des Min., 1833, tom. ii., p. 3. LAMPADIAS, Fortschritte, 1839, p. 55. PUVIS, Theorie der Zugutemachung des Bleiglanzes in Flammöfen. KARSTEN, Arch. i., R. vi., 236. KERL, Ueber die Zugutemachung der Bleierze in Flammöfen, B. u. h. Ztg., 1854, p. 165. PHILLIPS, in B. u. H. Ztg., 1859, p. 368. RIVOT, Métallurgie du Plomb et de l'Argent, 1860, p. 32.

UEBER

to know whether the ore contains substances which may cause the operation to proceed more rapidly, as in this case there is danger of a slagging together of the mass, thereby injuring the entire operation.

Pure galena requires a lower temperature for roasting, and hence there is less loss of lead by volatilisation (Carinthian process) than when calcareous galena is employed (English process); whilst galena containing pyrites must be roasted at a temperature rising gradually but not getting too high (French process).

Amongst foreign substances which may be present, Quartz, Clay, and Silicates generally, exert an injurious influence even when not more than or per cent of silica is present, by forming at the commencement easily liquefied slags, which cover the roasting mass like a varnish, and render perfect oxidation impossible. If the ore contains from 5 to 6 per cent of silica, the reactions are seriously interrupted, and the more so the more the silica is disseminated through the galena.* It appears, also, that a finer grain of the pounded and dressed ores assists the formation of slags.

Lime, in not too considerable quantity (10 to 12 per cent), acts advantageously both in the roasting process, by rendering the mass more difficult of fusion, and also in the reactions of the 3rd period.

Iron Pyrites assists in the more rapid conversion of galena into oxide and sulphate of lead.

Sulphide of Antimony is injurious even when in small. quantities (2 to 3 per cent); it facilitates a caking together of the mass, occasions loss of silver and lead on account of its volatility, and gives rise to the formation of an antimonial lead. Arsenical Pyrites is less injurious, as it only occasions a loss of metal. Grey Copper Ore, on account of the quantity of antimony, arsenic, and sulphur it contains, acts like sulphide of antimony; it also occasions the formation of cupriferous lead.

Versuche zu Poullaouen und auf dem Oberharze über den Einfluss des Quartzes, Thonschiefers und Kalks. B. u. h. Ztg, 1854, p. 193, 228. BERTHIER'S Metall. Analyt. Chemie, ii. 643.

Copper Pyrites does not interfere with the roasting process, but is objectionable on account of the copper present.

Sparry Iron Ore may be injurious, if silica is present at the same time, by forming easily fusible silicates of iron; but it is serviceable in the subsequent reactions.

(The action of the above-named substances has been proved by trials in reverberatory furnaces executed at the Clausthal Lead Works.*)

Period 2. (Heating the mass for the production of metallic lead, by the reaction of the oxidised upon the sulphuretted constituents.)

If the roasting mass, by frequent stirring and by a diminished oxidising flame, is quickly brought to a cherry-red heat, the oxide and sulphate of lead react upon sulphide of lead in such a way that metallic lead is produced. This may be proved by experiments on a small scale. † When the oxidised components have given their oxygen to the sulphur of the excess of galena, and no more lead is produced, the roasting process may be commenced again at a lower temperature, and the oxide and sulphate of lead formed can be made to react upon the remaining galena, by raising the temperature and stirring the mass till the lead. again ceases to run off. This roasting and reaction is repeated until no more lead is produced, and the mass commences to liquefy. The first or chief roasting depends on the nature of the ore, on the temperature of the roasting, and on the time it is carried on; the quantity of the resulting lead depends partly on the purity of the galena and partly on the care bestowed on the process. is of importance not to allow the temperature to rise above a cherry-red heat during the lead extraction; if a higher temperature is employed, a loss of lead by volatilisation will take place, and also a greater amount of lead will be kept back in the slags. Oxide and sulphate of lead are transformed, with galena, into metallic lead and sulphuric acid

It

KERL, Oberharzer Hüttenprocesse, 2 Aufl., 1860, p. 579. B. u. h. Ztg., 1854, p. 201. +PLATTNER, in B. u. h. Ztg., 1854, p. 22.

only at a somewhat low temperature; if a higher temperature is employed, oxysulphide will be formed, which does not decompose galena. Towards the end of the second period the temperature must be considerably raised, for the purpose of extracting the lead as completely as possible from the roasting mass, which now contains the foreign substances in a more concentrated state, and has become much more difficult to fuse. The formation of oxysulphide of lead cannot therefore be avoided, and it will be retained in the residue together with undecomposed galena, oxide of lead, and sulphate of lead. The quantity of oxysulphide of lead left in the residue depends upon the nature of the foreign substances contained in the lead ore.

The longer the reactions can be continued without rendering the mass liquid, the less oxysulphide will be formed (if blende, iron pyrites, sparry iron ore, &c., are associated with the ore). The further treatment of the residues for the extraction of the remaining lead depends on this.

In the second period, foreign substances occurring in galena may act either advantageously or injuriously. The latter is always the case if they are present in the galena in large quantities, as they then mechanically prevent the oxidised and sulphuretted substances having that complete contact which is necessary for the reaction.

Carbonate of Lime acts favourably if present in small quantity, as it decomposes galena, as well as sulphate of lead, and by preventing the too easy liquefaction of the mass facilitates the reaction; a proportion above 12 per cent is disadvantageous, owing to its mechanical action.

Fluor Spar acts in a similar manner. Its fusibility will assist the liquefaction of the mass, but it eliminates a small amount of silica by forming gaseous fluoride of silicium. Combinations of fluor spar with heavy spar and sulphate of lime are easily fusible.

Heavy Spar, in small quantity, has no influence. But 12 per cent or more interferes mechanically, and increases the bulk of the residue.

Quartz, Clay, and other Silicates render the mass easily

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