The scoriæ are taken out by the front door, by drawing them forward with a rake. They have a great specific gravity; are brilliant with metallic lustre, very crystalline, and present, in the cavities, crystals like those of pyroxene; they break easily into very sharp-edged fragments. They contain no granulated metal in the interior; but it sometimes occurs, on account of the small thicknesses of the stratum of scoriæ, that these carry off with them, when they are withdrawn, some metallic particles.
These scoriæ, as we have already stated, under the fusion of the roasted ore, are in general melted with it. In some cases, however, a special melting is assigned to them.
The matt obtained in this second fusion is either run out into water like the first, or moulded into pigs (ingots), according to the mode of treatment which it is to undergo. This matt, called by the smelters fine metal when it is granulated, and blue metal when it is in pigs, is of a light grey colour, compact, and bluish at the surface. It is collected in the first form when it is to be calcined anew; and in the second, when it must immediately undergo the operation of roasting. Its contents in copper are 60 per cent. This operation, which is but sometimes had recourse to, lasts 5 or 6 hours. The charge is 1 ton.
(b) Particular fusion of the scoriæ of the fourth operation.—In re-melting these scoriæ, the object is to procure the copper which they contain. To effect this fusion, the scoriæ are mixed with pulverized coal, or other carbonaceous matters. The copper and several other metals are deoxidized, and furnish a white and brittle alloy. The scoriæ resulting from this melting are in part employed in the first melting, and in part thrown away. They are crystalline, and present crystals often in the cavities, which appear to belong to bisilicate of iron. They have a metallic lustre, and break into very sharp-edged fragments. The white metal is melted again, and then united to the product of the second fusion.
Fifth operation. Calcination of the second matt, or fine metal of the smelter.—This is executed in precisely the same way as that of the first matt. It lasts 24 hours; and the charge is usually 3 tons.
Sixth operation. Melting of the calcined fine metal.—This fusion is conducted like that of the first matt. The black copper, or coarse copper, which it produces, contains from 70 to 80 per cent. of pure metal; it is run into ingots, in order to undergo the operation of roasting.
The scoriæ are rich in copper; they are added to the fusion of the calcined coarse metal of the fourth operation.
In the smelting houses of Messrs. Vivian, at Hafod, near Swansea, the fifth and sixth operations have been omitted of late years. The second matt is run into pigs, under the name of blue metal, to be immediately exposed to the roasting.
The disposition of the canal a a′, [fig. 302.], which introduces a continuous current of air to the hearth of the furnace, accelerates and facilitates the calcination of the matt; an advantage which has simplified the treatment, by diminishing the number of calculations.
Seventh operation. Roasting of the coarse copper, the product of the sixth operation. The chief object of this operation is oxidizement; it is performed either in an ordinary roasting furnace, or in the one belonging to [fig. 302.], which admits a constant current of air. The pigs of metal derived from the preceding melting are exposed, on the hearth of the furnace, to the action of the air, which oxidizes the iron and other foreign metals with which the copper is still contaminated. The duration of the roasting varies from 12 to 24 hours, according to the degree of purity of the crude copper. The temperature should be graduated, in order that the oxidizement may have time to complete, and that the volatile substances which the copper still retains may escape in the gaseous form. The fusion must take place only towards the end of the operation.