Though these improvements were important, more economical processes in the wet way have of late been introduced. Since the year 1859, the rich lead ores have been submitted to the precipitation process in Vogl's furnace. Ores containing 71 per cent of lead, cause a loss of only o'9 per cent of lead when smelted in that furnace. Silver ores containing no lead, or not more than 15 per cent, are treated according to Patrera's process (vide Silver).

At the Marienhütte, in Kinzigthal (Baden), mixtures of the following composition are worked in cupola furnaces :14 cwts. of galena, containing 48 to 55 lbs. of lead and I to 1 ounces of silver, 23 cwts. of mixed ores (galena, copper pyrites, malachite), containing 7 lbs. of lead and ounce of silver, and 4 cwts. of silver ore with 2 ounces of silver, 15 cwts. of refinery cinders, and 18 cwts. of copper slags.

At the smelting works in Münsterthal (Baden), galena schlich, containing 50 lbs. of lead and 2 ounces of silver per cwt., is worked in cupola furnaces 6 feet high. In the matt of those works, Sandberger found Ni, Cu, Sb,; the same alloy was found by Hausmann in furnaces at the Upper Hartz. These works produce annually 470 lbs. of silver and 65 to 70 tons of litharge.

At the smelting works at Ems (Nassau), the precipitation process was formerly adopted, but it is now discarded.

In Sala (Sweden) argentiferous galena, which occurs in granular limestone, is worked, associated with zinc blende, a little iron pyrites, some gypsum, quartz, and silicates of magnesia. It is smelted in cupola furnaces 19 feet high.

At Ruszkberg (Banat), a mixture of 24 cwts. of argentiferous galena, 24 cwts. of brown lead ore, 29 cwts. of argillaceous iron ore, 12 cwts. of roasted lead matt, I cwt. of iron pyrites, and 10 cwts. of limestone is profitably worked in cupola furnaces 20 feet high.

TREATMENT OF ROASTED ORES WITH IRON, or SOME FERRUGINOUS FLUX.

Roasting Reduction Precipitation Process.

This process is used for ores which are either so rich in foreign sulphides, antimonides, and arsenides, as to be unfit

I

for the common precipitation process, or so rich in earthy siliceous matter, that they cannot be worked in reverberatory furnaces. It would be possible to work these last ores by the common precipitation process; but in some cases local circumstances, chiefly the price of materials, make a previous roasting desirable (Commern, Ems). Ores poor in lead and rich in silver, associated with different gangues, and which, on account of the presence of this amount of silver, cannot be sufficiently dressed, are best adapted for this process. The disadvantages have been spoken of on page 83. The process is carried on in the different works with modifications dependent on the nature and the aggregation of the ore, and on local circumstances. The reaction process is intended to transform the sulphides as much as possible into oxides, and to volatilise antimony, arsenic, zinc, &c.; the oxides are then submitted to a reducing and purifying smelting. Matt is thus formed in small quantity, but containing the greater part of the copper present; the degree of roasting must be regulated according to the amount of copper contained in the ore.

The roasting of the ores is conducted,

1. In Heaps, either with or without shelter; used for ores containing a very large quantity of foreign sulphides, and which cannot be completely roasted on account of the copper contained in them (Hartz). Those ores contain sufficient sulphur to burn by themselves. Pot-like depressions are stamped in the porous covering of the roasting pile, and the sulphur being sublimed from below collects in them, and is dipped out from time to time in a liquid state; the quantity is about sufficient to pay the cost of roasting. When the ores contain less sulphur, they must be provided with intermediate layers of fuel (Müsen, Rothenbach.)

2. In Mounds; they permit the fuel and temperature to be better regulated, and are used for the same ores as those before described; but the roasting process cannot be carried on completely and uniformly with these arrangements.

3. In Cupola Furnaces: chiefly used when the sulphur

is employed for the production of sulphuric acid (Oker in the Hartz).

4. In Reverberatory Furnaces, with one or two hearths, either one above the other, or side by side; they give the best result in roasting, and are mostly employed. Furnaces with two hearths are more difficult to work, but they effect a saving of fuel. According to Vogl, furnaces with one. hearth and a long grate are more economical to work than English furnaces with short grates. If the sulphurous acid, formed by the roasting, is to be used for the production of sulphuric acid, these furnaces must be combined with muffle furnaces (Przibram, Freiberg), and the roasted ore must have been previously more or less ground.

According to Vogl,* an adddition of coal is advantageous in the roasting of lead ores. When the roasting is well conducted, the sulphides will be nearly all transformed into oxides. In contrast to the common roasting, the temperature in the last period is sometimes raised so high that the roasting mass either cakes together, or completely fuses. This is done either with the object of doing away with the pounded form of the ore, which is injurious in cupola furnaces; or in order to decompose the sulphate of lead by silica so as to counteract the formation of matt, and to save ferruginous fluxes. As the components of the roasting mass (oxide of lead, sulphate of lead, and some undecomposed galena) are brought into nearer contact by the caking, a reaction of the oxide upon the sulphuretted substances will be produced during the process in the cupola furnace; and, as in the reverberatory furnace, metallic lead and sulphurous acid will be formed. When the ore does not contain sufficient silica, the deficiency must be supplied by an addition of quartz sand. The silicate of lead which has been formed together with other silicates must be decomposed by ferruginous substances. The sulphuration will be most complete when the galena is treated at a gradually rising temperature, as in the French roasting

process, when it mostly changes into sulphate of lead, which again is decomposed by silica at a high temperature; but the drawback to this operation is the greater consumption of fuel. If a great amount of sulphates of lime and baryta has been formed in the roasting mass, they must be decomposed by silica at a continued high temperature. A complete scorification is advisable if a sufficient quantity of ferruginous substances can be added in the subsequent smelting; also when a saving in fuel and labour is desired in working ores poor in lead, and when the scorification is facilitated by the presence of white lead ore (Commern, Ems, Stolberg).

Foreign substances associated with the ore are sometimes advantageous and sometimes injurious to the roasting process,

Iron Pyrites requires a lower temperature: containing a arge amount of sulphur, it requires more time for roasting, and forms sulphates in large quantity, thus necessitating a large addition of silica and a high temperature for decomposition; but these disadvantages are more than counterbalanced by the favourable influence of the oxide of iron formed. Undecomposed sulphide of iron increases the yield of matt; the presence of pyrites acts advantageously in preventing the volatilisation of lead and silver, by transforming them into sulphates.

Zinc Blende has an invariably injurious action in this process. The best treatment for ores containing a great amount of blende is to roast them first by themselves, and afterwards with coal several times.

Sulphide of Antimony in ores is highly deleterious, and a roasting of such ores with coal is advisable.

Arsenical Pyrites likewise causes a loss of metal.

Copper Pyrites does not interfere with the roasting process, but part of its copper always enters into the lead and decreases its value (Lower Hartz, Müsen, Rothenbach.)

Grey Copper Ore acts similarly, and, on account of the antimony and arsenic it contains, also increases the loss of metal.

Sulphide of Silver is easily roasted.

Sparry Iron Ore is converted by the roasting process into peroxide of iron and counteracts caking of the ore; it acts favourably in the smelting process; if there is a large amount associated with the galena, siliceous fluxes must be added, and the smelting process conducted in low furnaces and with a low pressure of blast.

Heavy Spar has no influence upon the roasting process, and acts favourably upon the fusibility of the slags in the smelting process.

Lime hastens the roasting by absorption of sulphuric acid, and acts, like baryta, as a flux for silica.

Smelting the Roasted Ores.

The result depends chiefly on the roasting, the choice of suitable fluxes, and on the furnaces. The roasting mass chiefly contains metallic oxides, reducible in different degrees; and this process is intended to reduce the oxide of lead as quickly as possible, and to scorify those oxides which are difficult to reduce.

Experiments have been tried so to compose the mixture* as to form slags of mono-silicates, or of a combination of mono- and sub-silicates with a metallic base, chiefly protoxide of iron (basic silicate of iron and lime), which may be formed at a sufficiently low temperature, and which, not having a high specific gravity, allow a separation of the smelting products, except when zinc blende is present in large quantities. If the slags are too basic, and contain excess of iron, their easy chilling and reduction of too much iron cause an impure result and shorter operations; and if their specific gravity is too high they do not separate sufficiently from the matt, thus causing loss of metal. If they are too acid, contain too little iron, and are not rendered more easily fusible by oxide of lead, they require a higher temperature for their smelting, causing a considerable volatilisation of lead and greater reduction of foreign oxides,

* Oesterr. Zeitschr., 1857, No. 1. B. u. h. Ztg., 1857, p. 51, 91.