Fourth Operation

The furnace is of the same form as that employed in the second operation, excepting that there is no basin-formed recess in the bed, but a gradual slope of it, so as to discharge the whole of the fused mass readily from a hole in one of the long sides. The charge is composed of coarse metal, the product of the third operation, mixed with minerals of the fourth class, such as copper sulphide free from admixture of iron pyrites, or the carbonate or oxide of copper and copper slag from the ninth and tenth operations; together with copper scales from the rolling of copper or other copper wastes, such as are produced in the working of copper into utensils for machinery. The object of this operation is to separate the iron by combining it with silica, forming a silicate of iron, leaving the copper in combination with sulphur in such proportion as will form a sulphide of copper. With such a variety of materials to work on, a superior class of workmen is required for conducting this operation; for they are not limited by such rigid rules as in the previous operations, being allowed to operate on successive charges, as may be indicated by the results of previous workings. The materials for a charge amount to about 30 cwt.

The charge is introduced through the side door, and is spread over the bed with an iron slicer, shaped somewhat like an oar or paddle. The door is carefully closed, and the heat raised as rapidly as possible. During the first 2 hours a calcination goes on, attended with the evolution of some sulphurous acid, and an incipient fusion takes place as the heat increases. The charge flows down, until, at the end of the third hour, the fusion is complete, the scoria is very liquid, and remains quiet in the furnace. Between the fourth and fifth hours, the charge is well and rapidly stirred; the unfused portions of the charge adhering to the sides are turned into the liquid mass; and then the furnace is again closed, and the heat is urged until the whole of the interior of the furnace is raised to a dazzling whiteness. At the end of 6 hours the furnace is tapped, the matte is run into water, and thereby granulated, the very liquid scoria being conducted through another channel, after the matte has been run off into sand moulds. The scoria is sorted into 2 qualities; the first run off, having been in contact with the matte, retains 3 to 5 per cent. of copper, and is reserved for the sixth operation; the last portion run off contains much less copper, and is used as a flux in the second operation.

The matte, consisting principally of copper sulphide, still retains 4 to 8 per cent. of iron sulphide, and contains 70 to 75 per cent. copper. This matte is termed " white metal." In its purest form it is of a greyish-white colour, of a granulated porous texture, sometimes of a bluish-grey colour, with a specific gravity of 5.2 to 5.7. It is reserved for the ninth operation. The working somewhat resembles the second operation; but as the heat required is much greater, more fuel is consumed; 22 charges are worked through the furnace in a week. Fifth Operation. - The furnace employed precisely resembles that used for the fourth operation; and, indeed, the 2 operations much resemble each other. The charge on the present occasion consists of materials containing fewer metallic oxides than those of the last, but having more metallic sulphides. Excepting that some of the products of the present are brought into other operations, together with the products of previous processes, it would hardly require a separate description, but would rather be an equivalent for the fourth operation; its principal object being, by the better selection of ores containing a less variety of substances, to produce from them a " blue metal," from which blue copper may with less difficulty be elaborated.

The white metal, the product of the fourth operation, does not contain any metallic copper; but the blue metal is characterized by the presence of metallic copper, diffused throughout the mass in exceedingly minute particles.

About the same quantity of fuel is used as in the fourth operation, the reactions that take place in the furnace are similar, and the products are disposed of in much the same manner. The matte contains 70 to 75 per cent. copper.

Sixth Operation

The object of this operation is, by the combined action of the various constituents of the slags from the sixth, seventh, and eighth operations on each other, to cause the production of a matte in which the copper in these slags shall be brought together. A small quantity of copper pyrites, free from other combinations than siliceous matter in large proportions, is mixed with the slags. After the mass is fused, a little coal or other carbonaceous matter is mixed with the charge, which reacts on the oxide of copper, decomposing it, and producing a metallic copper, which alloys with any tin, nickel, cobalt, or arsenic present, and, falling to the bottom under the matte, does great service by eliminating these metals from the superincumbent matte. The same sort of furnace is used as in the last operation; but the materials of the charge, instead of being supplied to the furnace through a hopper in the roof, are introduced through the ordinary working door and another door placed more nearly in the corner of the furnace; the fused charge is drawn off from the side opposite to the working door. Each charge of 2 tons takes about 6 hours for working off.

The products are: white metal for the sixth operation; red metal, for the same operation; the bottoms, or alloy of metals, divided into tin alloy and copper alloy, for the ninth operation; scoria, for returning to the fourth operation; and slag, to be rejected to the extent of 90 per cent.; so that it will be seen that this operation is very efficient in the reduction of bulk of useless materials.

The Seventh Operation is carried on in a furnace resembling the one used in the sixth. The charge consists of about 2 tons of blue metal only, without any other flux than the sand adhering to its surface from the moulds in which it was cast. As the operation is twofold-first for calcination or oxidation, and then for fusion - the bridge is provided with an air-passage, similar to that employed in the calcining furnace for the first operation. The charge is put into the furnace in large masses, and the heat is slowly raised upon it, so as to oxidize as completely as possible the metal present. When the whole mass is fused, the iron sulphide having been converted into iron oxide and sulphurous acid, the silica reacts on the iron, and is converted into a fusible silicate; some copper oxide is also scorified, but the matte or white metal produced is much improved in quality, and rendered fit for the eighth operation. The operation requires about 12 hours for its completion; and but little alteration has taken place between the weights of the charge introduced and of the products obtained, none of the latter being finished with, but having to be redistributed to the furnaces for the second, fourth, and sixth operations.