to three of these "slag-eye" furnaces, two of which are run together, the third being in reserve. From the flue a fan (6 feet diameter and 3 feet face), run at 290 revolutions per minute, draws the fumes and gases, first, through a pair of iron cylinders 7 feet diameter and 20 feet high, lined with firebrick, through which the 18-inch blast main, temporarily enlarged to 3 feet, passes in order to heat the blast, and, secondly, through a series of four inverted U pipes, 3 feet diameter and 20 feet high, set in brickwork and exposed to the air. These cooling arrangements are shown in plan and elevation in Figs. 28, 29, and 30. Some fume settles out in the pipes, but it is of poor colour and

SHOPPERS IN EACH

ROW

55

Fig. 31.-Lewis & Bartlett Process-Filter.

is therefore returned to the furnace. After passing through the cooling pipes the fumes enter the white bag room, or "paint house," which is 90 feet by 40 feet by 45 feet high in two storeys with a boarded floor, the arrangement of which may be seen from Fig. 31. There are three rows of hoppers, nine in each row, and above each hopper are suspended twenty bags of coarse flannel made from unscoured wool, 20 inches in diameter and 35

feet long, costing $9 apiece, and tied over nipples projecting from the top of the hopper. The bags are suspended from the rafters, the hoppers and flues are also suspended by means of iron straps from a skeleton flooring of 14-inch pipe laid 2 feet apart across a framework of 24-inch pipe, which is itself supported by eighteen columns of 31-inch pipe. Each bag has 168 square feet surface, so that the filtering surface of the whole 540 bags is 88,704 square feet.

The paint from different parts of the room is of about the same quality, and has good body and colour. It is packed in barrels holding 500 lbs. each, and sells at a slightly higher price per lb. than pig lead, 1 lb. of which makes 1.6 lbs. of paint. The daily product of each furnace is about 4250 lbs.

Analyses of the white paint as well as of the various intermediate products of the process are given in the following table:

CHAPTER VI.

THE ROASTING OF LEAD ORES.

The Roast-Reduction Process.-The roast and reduction process is adapted to the treatment of ores with any kind of metallic and non-metallic admixture, siliceous ores, and those containing arsenic and antimony being always treated in this way. comprises the separate operations of roasting and smelting.

It

In the roasting process as much as possible of the arsenic, antimony, and sulphur are driven off, and the lead is, as far as possible, converted into oxide. Some lead sulphate is unavoidably formed, but its amount is usually kept to the lowest attainable limit, since, in the ordinary reducing smelting, it would be reduced to sulphide. In many cases, towards the end of the operation, the temperature is raised in order that the silica may react upon the sulphate, forming silicate, and at the same time partially fusing the material so as better to fit it for smelting.

In the ordinary smelting process conducted in blast furnaces ("cupolas" or "stacks"), the lead is reduced from its oxide, partly by means of carbonic oxide, but chiefly by direct contact with masses of glowing, porous fuel (coke or charcoal). Such lead sulphide as may remain in the roasted ore or become reduced from the sulphate is also decomposed by the action of iron reduced from its oxide, the lead being liberated while the sulphur forms an iron matte. Other metallic constituents of the ore, except gold and silver, which are collected in the lead, pass either into the slag or the matte or become volatilised.

Ores may be roasted in lump form or as fines. All classes of ore were formerly roasted in lump form, but the method has been quite superseded as regards ores in which lead is the predominant constituent, and is now only employed for highly pyritic ores, which roast very well in large lumps.

Reactions of the Roasting Process.*- -The commonest class of lead ore is a more or less finely pulverised mixture containing besides galena, varying proportions of blende, pyrites, chalcopyrite, quartz, and other gangue materials.

The reactions which take place in roasting such mixtures are as follows:

The first action, as the temperature of the charge rises, is the distillation of about one-third of the sulphur in the pyrites, which

v. Roberts-Austen, Pres. Address, Brit. Assoc. Meeting at Cardiff, 1891; also Rose, Metallurgy of Gold, in this series, pp. 216-220.

TABLE XII.-ANALYSES OF COMPLEX LEAD SULPHIDE ORES.

NOTES.-1, 4, 5 are mixtures of ore and concentrates; 6 and 7 are natural ores which have to be concentrated before roasting.

References.-1. E. and M. J., July 16, 1892. 2. Hofman, Metallurgy of Lead, p. 167. 3. Schnabel, Handbuch der Metallhüttenkunde, vol. 1., p. 389. 4. E. and M. J., Oct. 21, 1893. 5. Schnabel, op. cit., p. 393. 6. Taylor and Brunton, E. and M. J., May 8, 1886. 7 and 8. Author.

burns with a blue flame to SO, part of which again in contact with oxygen and with the hot porous charge and furnace brickwork is converted into SO. The next action is the successive oxidation of the metallic sulphides by the current of hot air, approximately in the following order, Fe,S,, CuFeS, Ag S, Cu,S, PbS, ZnS. Of these, silver sulphide is soon reduced to metal, and would remain so if it occurred alone, but the presence of much base metal sulphides and excess of free SO, enables it to be converted into sulphate; iron and copper sulphides are converted, in the first instance, into sulphates, lead and zinc sulphides chiefly into oxides, free SO, in the vapours reconverting part of the oxides into sulphates.

*This is not silica, but a mixture of silicates, including rhodonite, garnet, felspar, &c., as well as quartz.

This process of oxidation is not continuous, except in a strong draught, for as soon as the sulphurous acid reaches a certain tension chemical equilibrium is established and the oxidation is arrested, even in presence of an excess of oxygen, unless the temperature be raised considerably.

Besides a temperature which should be easily controlled, efficient oxidising roasting, therefore, requires abundant access of air to the hot ore and rapid removal of the products of combustion. If the temperature be kept moderately high, and the draught and supply of air low, most of the lead and zinc will be converted into sulphates, partly by direct action of SO, and O, partly by the SO, evolved from decomposing iron sulphates The temperature must, in any case, be well under control, for, if raised too suddenly, portions of the ore will undergo incipient fusion, and particles of unaltered sulphides be protected by an envelope impervious to the further action of the gases.

As the temperature rises, and before the oxidation of the sulphides is complete, the sulphates formed begin to dissociate in the following order-Fe,(SO4)3 FeSO4, CuSO4, AgSO4, ZnSO. Iron and copper sulphates give off, at first, SO, and, afterwards, SO, and O; silver sulphate remains undecomposed until all the iron and nearly all the copper sulphates have been decomposed, when it in turn breaks up into metallic silver, oxygen, and SO; zinc sulphate is nearly unchanged in simple roasting, giving off only part of its SO, at a full red heat, the basic sulphate remaining not being decomposable under a bright yellow heat. Lead sulphate is unaltered.

Among gangue substances quartz and barytes remain unaltered, chalybite is converted into magnetic oxide, and calcite into calcium sulphate.

3

3

As regards removal of the deleterious substances, arsenic and antimony, it may be remarked that mispickel (FeSAs) is pretty thoroughly decomposed, the greater part of its arsenic being volatilised direct as As,S,, which is oxidised into As, and SO, by the current of hot gases, the remainder being oxidised direct to AsO at a low temperature in presence of a good draught. Most sulph-antimonides and sulph-arsenides (e.g., fahlerz, bournonite, &c.), like mispickel, give up the greater part of their arsenic (antimony) as As,S(Sb,S), which are at once oxidised to As,O,(Sb,O) and SO.. Another portion of the arsenic (or antimony) is converted into arseniates (or antimoniates), which resist the effects of simple heating. These compounds are, however, reduced in contact with metallic sulphides, especially with free pyrites, and, therefore, on rabbling the arseniate (or antimoniate) formed in the oxidised surface crust gets turned over into the bottom layer of the ore, where there is more or less free sulphur, and is re-volatilised to the surface as