furnaces, and in smelting the residues of reverberatory furnaces.

The cost of one cwt. of lead, when working residues and poor ores containing only from 9 to 18 per cent of lead, amounts to from 3s. 6d. to 4s. 9d.; when working argentiferous ores with a medium percentage of metal, to from 4s. 9d. to 5s. IId.; and when working ores with 35 to 40 per cent down to 20 per cent, to about 5s. 6d. In the reverberatory process the expense is about 3s.

The loss by volatilisation is seldom less than 5 or 6 per cent, but it is considerably lessened by condensation chambers; the yield from the smoke, containing from 30 to 40 per cent of argentiferous lead, is estimated to be 7 or 8 per cent of the worked materials; the vitreous slags containing 1 or 1 per cent, seldom as much as 2 per cent of lead, are thrown aside.

REDUCTION OF SULPHATE OF LEAD.

Sulphate of Lead seldom occurs in nature in large quantity, and is usually worked together with sulphuretted ores, without modifying the smelting process; sometimes the artificial sulphate of lead produced in calico works by decomposing sulphate of alumina by acetate of lead, has to be treated by a smelting process. This operation is performed either in reverberatory furnaces or in cupola furnaces.

Only rich substances, such as the artificial sulphate of lead, can be treated in reverberatory furnaces, which always cause a great loss of metal when the sulphate is not mixed with galena (Poullaouen), but is worked by itself.

The following method is recommended by Rivot and Phillips :

Artificial sulphate of lead, containing a little excess of sulphuric acid, is heated in an English roasting reverberatory furnace till no more sulphuric acid is evolved; it is then mixed with 20 per cent of quartz and 1 per cent of pulverised coal, and heated in an English copper matt furnace, at a gradually rising temperature, till the whole mass fuses, and till no more sulphurous and sulphuric acids are expelled by the quartz. This takes about three hours when working a charge of a ton.

The coal assists the decomposition of sulphuric acid, and extracts some metallic lead.

The product chiefly consists of silicate of lead, and may be decomposed for the extraction of the contained lead.

a. By Metallic Iron, in the form of scrap iron, &c. About 15 per cent of this is uniformly distributed over the surface of the smelting mass, and repeatedly stirred up with it. This will cause the formation of metallic lead, and of slags containing silicate of iron, which are tapped off together with the lead, and are worked again in a high cupola furnace if they contain as much as 18 or 20 per cent of lead. This process of decomposition is finished in about five hours. If it is continued longer, and if more iron is added, it will be possible to produce poorer slags, but in this case the volatilisation of lead, already very great, increases, and the consumption of materials is greater. A ferruginous lead may also be formed, which would have to be treated by a refining process. The resulting scum which contains iron and lead is mixed with the charge in the next operation. From a dried sulphate containing from 65 to 68 per cent of lead, only 50 per cent is produced, 6 per cent remaining in the slags, and 10 per cent being volatilised; therefore the fume must be collected and added to the sulphate of the next charge. When calculating the yield from slags and fume, from 58 to 59 per cent of lead is produced from the dried sulphate, giving a loss of 9 or 10 per cent. As the consumption of fuel, iron, and iron tools is very considerable in the different operations, this process is applicable only to rich substances.

b. By Charcoal, Coke or Coal, and Lime.-The pasty mass is mixed as uniformly as possible with 10 per cent of lime in small pieces, and 8 per cent of fuel, heated till completely liquid, and then tapped off. Purer lead is obtained than when using iron, but at the same time richer slags are produced; these have to be re-smelted, and the volatilisation of lead is considerable.

c. By Iron Pyrites.-If from 20 to 22 per cent of this substance is mixed with the silicate of lead, metallic lead,

silicate of iron, and sulphurous acid will be produced, the excess of sulphur being volatilised:

3PbO, SiO, + FeS = 3Pb+ FeO, SiO, + SO2.

If iron pyrites is added in excess, poorer slags will be obtained, but a matt will also be produced which will be expensive to work up. To avoid this, the pyrites must be mixed well and uniformly with the silicate, so that the former is in no place in excess; the slags are re-smelted in cupola furnaces. Though the reaction takes place in a very short time, yet the temperature required must be high, and therefore the volatilisation of lead is considerable.

Völckel's method for the reduction of the artificial sulphate of lead is as follows:-The substance is heated with coal in a Carinthian reverberatory furnace with an inclined hearth, in order to convert part of it into sulphide of lead, which is made to react upon the undecomposed sulphate, producing metallic lead. The yield of lead is about 60 per cent, and the resulting slags are re-smelted with an addition of refinery cinders.

At Poullaouen, the artificial sulphate of lead is added to the mixture when smelting argentiferous galena by the French method; in this case the roasting process is either shortened or altogether omitted.

Smelting in cupola furnaces is more suitable for poor sulphates, as it produces poorer slags, causes less loss of lead by volatilisation, and the consumption of material is small. Such a process is used in some smelting works in Southern France, where sulphate of lead, with a gangue of lime and clay, is worked; the ore is heated in a reverberatory furnace with two hearths, till it cakes, and, if required, enough quartz is added to form silicates containing at least 30 per cent of silica.

The caked, porous mass is smelted, together with lead products and impure slags, in furnaces from 15 to 2 metres high, and with an addition of enough ferruginous substance (refinery cinders, or roasted iron ore) to form an easily fusible slag containing from 44 to 45 per cent of silica. When using higher furnaces, the porous mass becomes already

reduced in the upper part of the furnace, and causes a volatilisation of lead. In operations of from 17 to 18 days, about 25 or 30 parts of coke are used for every 100 parts of ore smelted, and for caking 10 parts of ore, about 25 or 30 of coal are consumed. The yield of lead is about 80 per cent of the quantity contained in the ore, whilst the yield of poor ores in reverberatory furnaces amounts to only 45 per cent.

If a more complete desulphurisation is required, the mass must be completely scorified, instead of only caked; but sometimes the other minerals present in the ore make such a scorification unavoidable. In that case it is preferable to smelt the ore in furnaces from three to four metres high, as then a reduction of lead cannot easily take place in the upper part of the furnace; higher furnaces also allow a saving of fuel.

TREATMENT OF OXIDISED SMELTING PRODUCTS.

These are chiefly Litharge, Dross, and Hearth Bottoms, besides some lead slags. They are worked either with lead and silver ores, or by themselves in reverberatory, cupola, or hearth furnaces. Upon adding a percentage of iron pyrites, the greater part of the copper they contain may be, by these smelting processes, concentrated in a matt.

Reduction of Litharge.

As litharge is nearly pure oxide of lead, with only small amounts of copper, silver, antimony, arsenic, iron, &c., its reduction is effected by any of the three above-named furnaces, the choice depending chiefly on the quality and the price of the fuel, and on the smelting process which has been used in working the lead ores.

The production of lead in a certain time, as well as the ultimate yield of lead, is larger in cupola furnaces than in reverberatory and hearth furnaces, and consequently the price of labour is lower; on the other hand, the lead produced from impure litharge in cupola furnaces at a high temperature, is of inferior quality, containing more copper and antimony than that produced in reverberatory and hearth furnaces, which allow the use of raw fuel, whilst the former require coked fuel. The loss of lead in cupola furnaces

usually amounts to from 1 to 24 per cent; in reverberatory furnaces from 3 to 5 per cent; and is still greater in hearth furnaces. When litharge in the form of powder is reduced in cupola furnaces, the loss of lead exceeds that in reverberatory furnaces, which is usually somewhat larger on account of the greater volatilisation and scorification by the coal ash.

Reduction of Litharge in Reverberatory Furnaces. The reduction furnaces are usually of smaller dimensions than the furnaces for smelting ore. Those used are

a. Reverberatory furnaces with an inclined hearth, from which the lead continually flows out, either from the short side of the furnace situated opposite to the grate (Holzappel), or from one of the long sides of the furnace (England).

b. Reverberatory furnaces furnished with a sump on the short side of the furnace beneath the flue, from which the lead is ladled.

The products are

a. Lead, either marketable, or treated according to its purity and amount of silver;

b. Slags, mostly treated with the remnants of the ore smelting in slag hearths, or worked again in reverberatory furnaces mixed with coal and fluxes.

Illustrations of the Litharge Reduction Process.

In England the reduction of litharge is carried on at different places with little modification in the construction of the furnaces, as is shown by the following examples :