The ore and flux having been introduced through the hopper, the workman rapidly spreads the charge over the bod b of the furnace, and then introduces the scoria, evenly distributing it over the surface of the ore. The furnace door in carefully closed, and for 3 1/2 hours the fire is steadily raised, the charge being left undisturbed. Within 1/2 hour after the closing of the furnace, the fusion of the scoria commences: it flows down, and carries the best rapidly through the charge, which immediately begins to give on sulphurous and other gases, causing an ebullition of the liquid scoria; a rapid reaction is produced between the earthy and metallic constituents, the iron and silica entering into combination, and, with the assistance of lime of the fluor-spar, forming with the alumina present a very liquid scoria, through which the heavier fused copper sulphide falls to the bottom, where, from the peculiar hollow form of the bed, it is all brought together. At the end of 3 1/2 hours, the furnace door is opened, the furnace-man clears off with his rake the unfused portions remaining around the sides of the furnace, turning them into the liquid mass, which acts rapidly on them, reducing them also to the liquid condition. The door is again closed, and the fire Is urged on to greater intensity.

At the end of another { hour, the furnace is carefully tapped at the bottom of the basin, so as to allow the liquid mass to run off into an iron cylinder immersed in water. In this manner the mass is granulated, and brought into a suitable form for the next operation. While the liquid mass is still running off, the furnace door is opened, and the liquid scoria is raked off into moulds, formed by digging oblong cavities in sand; or it is sometimes cast, if sufficiently clear of copper, in cast-iron moulds, forming blocks suitable for a variety of building purposes. As soon as the charge is drawn, the furnace is re-charged, and the work is continued as before; so that in 24 hours, 6 charges are passed through the furnace. Ores containing fluor-spar are more valuable than those which contain silica alone; not only do they not require so much silica to be removed, but they also provide a flux for other ores which are siliceous. It is evident, therefore, that much judgment is requisite for the due assortment of the ores for this process.

The object of this operation is to separate the metallic from the earthy matters; the products consist of about 1/3 coarse metal, composed principally of copper and iron sulphides in nearly equal quantities, and 2/3 scoria, containing iron, silica, alumina, lime, and calcium fluoride (fluor-spar), all fused together into one homogeneous substance. The cast blocks of scoria are carefully examined when cold, to determine whether any coarse metal be left in. The peculiar colour and form of fracture of the brick give a very nice indication of the quantity of copper, down to so minute a proportion as 1/1000 to 2/1000, part. The portions containing matte or coarse metal, usually amounting to about 7 per cent., are separated, and reserved for a new charge for the same operation; the remainder is carried off to the waste heap.

Fiq. 10.

Copper Part 5 30012

The fluor-spar employed as a flux is peculiarly valuable, both as a mechanical and as a chemical agent. Under the influence of heat, it fuses on coming into contact with silica, the calcium obtaining oxygen; the silicon and fluorine set free from the calcium combine, and, as a gaseous product, in passing off, produce an agitation of the fluid mass, which is highly conducive to the perfect reaction of the various constituents on each other. The lime thus set free immediately enters into combination with another portion of silica, forming with the other earthy matters present, through the assistance of a portion of the iron, a very fluid slag; the cylinder containing the granulated matte is lifted out of the water by a crane fixed to the furnace, and is removed in wheelbarrows or waggons to the furnace employed for the next operation.

Third Operation

For this, precisely the same sort of furnace is used as for the first operation. It is charged in the same manner, from the hoppers on the roof of the furnace, with the product of the second operation, which may be regarded as copper pyrites free from gan-gue or earthy matrix, and containing about 33 per cent. copper. The heat employed in this operation is much greater than in the first) as the constituents of the charge are not nearly so fusible. The object is to drive off as much as possible of the remaining sulphur. The fire requires to be managed so as to produce an oxidizing flame, that is, flame greatly supercharged with atmospheric air, undecomposed, but highly heated by the fuel. This necessity arises from the fact that heat alone is not sufficient for the expulsion of the sulphur; but when heated oxygen is brought into contact with the materials of the charge, it partly combines with the sulphur, forming sulphurous acid, the detachment of the sulphur from the iron and the copper being facilitated by the intervention of other quantities of oxygen entering into combination with the metallic bases, forming oxides, principally of copper and iron.

The alternate employment of an oxidizing or clear flame, and of a reducing or smoky flame, is advantageous, as, under the influence of the former, sulphates are sometimes formed. The residuary sulphides, with the sulphates, are more rapidly decomposed by their reaction on each other, under the influence of the reducing flame, than they would otherwise have been, the sulphur of the sulphide combining with a portion of the oxygen of the sulphuric acid of the sulphate, its base obtaining its oxygen from the same source, the sulphurous acid passing off.

The charge begins to throw off vapours as soon as it gets red-hot throughout, or in about 2 hours after it is put into the furnace. It is then well raked over, to cause every portion in succession to be exposed to the action of the heated air passing through the furnace. The raking or rabbling is repeated every 2 hours, the heat being maintained as high as possible, short of causing the charge to fuse. At the end of 24 hours, the heat will be equal to about cherry-red; and at the end of 36 hours, at a bright red, by which time the calcination should be complete, and the charge ready to draw. The product of this operation is "coarse metal," which should be as nearly as possible pure oxides of iron and copper. It is much altered in colour, and reduced to a coarse granular condition much smaller than before. If well done, the fragments are of a deep black colour, with some indications of incipient fusion on the grains. Much more coals are consumed in this than in the first operation, but not so much as in the second.