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roasted, only one side of the double stall is filled and lit, the other being reserved for the re-roasting. The length of each half stall is 8 to 10 feet, and the width 10 to 12 feet across the four fireplaces.

The only advantage of stalls of this kind is that any kind of cheap inferior fuel can be used in them; they are, however, awkward and expensive to fill and empty, especially when wood

of any kind is very scarce; they are only in use for lead ores at a few out-of-the-way places. The form of stall here figured, however, is of very wide application, one of its special advantages being the air passage, a, in each of the division walls communicating with the open air, by which means the ore in each stall is virtually exposed to the air (through the openings, b, b) on both sides as well as on the front, and not on the front only as in the old pattern of stall.

Kiln Roasting.-Kiln roasting is only employed when the roast gases produced from pyritous lead ores can be usefully made into sulphuric acid, or when they must be rendered innocuous to the surrounding country. The ores treated must

be poor in lead, or it is impossible to prevent them from sintering.

Kiln roasting requires no fuel, as, once started, the necessary heat is supplied by combustion of the sulphur; skilled labour is, however, indispensable, and the roasting with leady ores is so imperfect that it must be followed by a re-roasting of the lump ore in heaps, and of the smalls in reverberatory furnaces. Ordinary pyrites burners are unsuitable for ores containing much galena, as the column of ore is too low to give the requisite temperature.

Lead ores containing much pyrites are successfully roasted in

kilns at Oker (Harz) and at Freiberg. The construction of those at the latter place is shown in Figs. 34 and 35. S is the shaft of the kiln, 94 feet high, 71⁄2 feet long, and 4 feet 3 inches wide. It is filled with ore through an opening in the arch by means of a hopper closed by a sliding door, or, better, by means of a cone and bell. The ore falls upon the pierced saddle grate, G, and is ignited through the ashpit doors, A, through which also the supply of air is regulated. The roast-gases are led off through a flue, F, in the back wall into a main flue, which is common to the whole system of kilns, and communicates with the sulphuric acid works. The roasted ore is raked out through the discharging doors, D, and the descent of the column is regulated by means of the working doors, W, through which bars can be driven into the charge to loosen it and break up halffused masses. Each kiln turns out 1 tons of roasted ore per twenty-four hours.

At Oker similar kilns, arranged in groups of five, communicating with each other under the arch, are used for roasting mixed copper and lead ores. The kilns are somewhat higher than those at Freiberg, being 13 feet high by 7 feet 6 inches long and 4 feet wide, and the charge per division varies from 1.4 up to 2 tons per twenty-four hours. Clean pyritous copper ores are so well roasted in these kilns as to leave barely enough sulphur to form matte with the copper; but the leady or mixed ores averaging blende 25 per cent., pyrites 25 per cent., chalcopyrites 15 per cent., galena 14 per cent., heavy spar 11 per cent., and other gangue 7 per cent., and of which 18 to 2 tons are charged per twenty-four hours, cannot be sufficiently desulphurised at one roasting. They are, therefore, re-roasted twice in heaps to fit them for smelting.

Comparison between Different Systems of Roasting Lump Ore. The following table shows at a glance the advantages and disadvantages possessed by heaps, stalls, and kilns respectively:

It is, of course, understood that only the coarser parts of ores which are rich in pyrites can be at all advantageously roasted in lump form. Such ores, however, which are usually roasted in heaps, can frequently be handled to greater advantage in stalls or kilns. In many cases the value of the sulphuric acid

produced by condensation of the roast-gases added to the amount of compensation for damage to crops, &c., saved by preventing the escape of the fumes will more than pay interest and wear and tear on a complete plant of kilns and acid chambers; again, in other cases, the arsenic contained in the ore will pay a good interest on the cost of kilns and settling flues, even if the whole of the sulphurous and sulphuric acids in the roast gases be allowed to escape, while in cases where none of these byeproducts can be advantageously utilised or rendered marketable, and when the surrounding country is barren, simple heap-roasting is often the cheapest and best system, even when it has to be repeated on part of the ore.

ROASTING OF FINE ORES.

The greater part of the lead ore received by any smelting establishment is in a more or less fine condition, whether in the form of natural smalls from the mines or, as is more common, of concentrates from dressing works, which may vary from bean size jig concentrates down to the finest dust from revolving tables and other slime machines. The variety of appliances which have been used for roasting fine ores is very great; they may be described as reverberatory, cylinder, muffle, and shaft furnaces, examples of all of which will be described in the volume of this series on the Metallurgy of Copper.

The roasting of lead ores presents, however, much more difficulty than that of copper ores, since they are so much more fusible, while it is often complicated by the presence of large amounts of blende, one of the most difficult of all metallic sulphides to thoroughly oxidise. Some of the roasting furnaces which have been most successful on pyritic copper ores have proved quite unsuitable to the roasting of ordinary galena ores carrying blende, while other forms of furnace are so obviously unsuitable that they have not been even tried.

The furnace which has been most uniformly successful in roasting lead ores is the old-fashioned long hearth reverberatory, which, though not economical of fuel or of labour, is adapted to ores carrying any proportions of galena or of pyrites.

In all cases the reverberatory hearth is connected with a system of flues and condensing chambers in order to catch the particles of ore mechanically carried over by the draught, together with some lead volatilised in the metallic condition through the reaction of PbO and PbSO, on PbS in the hottest part of the furnace, and a still larger amount of PbS* there volatilised.

4

Lead sulphide being far more volatile than the metal itself, its oxide or sulphate.

REVERBERATORY FURNACES.

The old style of reverberatory furnace with a single hearth, on which the charge was spread evenly for roasting and from which it was raked at the end of the operation, is now almost extinct (except, perhaps, in Wales), as it is not only of small capacity, but is extremely wasteful of fuel.

Double Hearth Reverberatories having the hearths superposed ("double bedded calciners") are still to be found at Mechernich and one or two other works in Germany and in England, but have been generally abandoned elsewhere on account of the heavy labour required to work the charge on the upper hearth by means of movable platforms on which the men stand, and of the difficulty of getting at the lower hearth and arch for effecting repairs when necessary. The saving in fuel effected by the superposition of hearths, though real enough, is, in most cases, insufficient to compensate for the above disadvantages. Beyond this and a slight saving in first cost, the chief advantage of the "double bedded calciner" is the economy of space in old metallurgical works where there is not room to erect new furnaces of the long type.

The superposed hearths of the Mechernich furnaces are very long, being each 50 feet long by 13 feet wide. In twenty-four hours, each furnace puts through 9 tons of lead ore dressed up to 50 to 60 per cent. lead, with a consumption of 27 cwts. of coal (i.e., 15 per cent. by weight of the ore), the roasted ore, which is sintered together before being drawn, averaging only 0.6 to 0.7 per cent. of sulphur. No such perfect roasting of lead ore is conducted in America, but it takes a good deal of labour, and would be, in most cases, far too expensive under American conditions.

Other double hearth reverberatories are in use for roasting silver ores, and will be referred to in Part II.

Single Long Hearth Reverberatories.-The commonest kind of roasting furnace is the German "Fortschaufelungsofen or long hearth reverberatory, containing at the same time several charges in different stages of the process, which are successively raked and pushed forward as the completed charges are drawn.

When the subsequent treatment of the roasted ore requires it to be in a fused or agglomerated condition, the roasting hearth at the firebox end terminates, either in a "sump" or depression in which the semi-fused mass can be collected, or, preferably, in a vertical channel or flue through which the roasted ore is raked into a separate smelting hearth or fuse box, and through which the flames passing over the latter ascend to the roasting hearth. When a separate fusebox is used, the ore is almost always made so fluid as to be drawn off through a spout into slag pots, and only if very infusible is it raked out in agglomerated cakes and