The Eighth Operation is conducted in a furnace precisely similar in every respect to that employed for the seventh. The materials, constituting the charge are white metal, produced from the sixth and seventh operations, together with the red metal from the sixth, without any other flux than the sand adhering to the blocks from the moulds in which they were cast. The operation lasts about 4* hours, the weight of the charge being about 1 1/2 ton. At first, as in the last operation, the effect is that of oxidation; but as the fusion proceeds, the copper oxide which has been produced, coming into contact with copper sulphide, a reaction takes place, sulphurous acid is evolved, and metallic copper is precipitated, or a copper sub-sulphide is produced. The products are 3, all of which have to be re-worked. They are-1st, a regalus, consisting of 81 copper, .2 iron, nearly 2 per cent. sulphur; 2nd, a slag, consisting almost entirely of silica, and iron and copper oxides, with about 10 per cent. of copper mechanically mixed; 3rd, copper bottoms, or alloys of copper with other metals.
These are reserved for the ninth operation; and some of the slag is returned to the furnace for the fourth operation.
The same sort of furnace is used as in the last. The charge, amounting to, 2 1/2 to 3f tons, consists of white metal from the fourth operation, regulus from the eighth operation, copper bottoms from the sixth and eighth, and a small proportion of rich oxide or carbonate ores, associated with quartzose matrix or gangue. No other flux is added than the silica of the rich ore. The heat of the furnace is at first regulated for the calcining of the charge, and is gradually raised until the whole mass is perfectly fused. The object of the calcination is to promote the oxidation of the metals and of the sulphur: the sulphur, being converted into sulphurous acid, passes off, together with arsenic if it be present. By fusion of the oxidized matters, copper oxide is brought into contact with copper sulphide, oxygen leaving the copper and combining with the sulphur; the 2 in combination) assuming the gaseous form, produce a most desirable agitation of the constituents of the charge, whereby in succession matters having chemical reactions on each other are brought into contact, and the desired changes are effected.
As only a limited supply of oxygen is contained within the charge, and this is soon exhausted by the reactions that take place within the fluid mass, it becomes necessary to obtain a fresh supply. In the fused state, the charge presents but a very small amount of surface for the reaction of the heated air passing through the furnace. The production of the necessary surface for the further oxidation is effected in a simple but ingenious manner. The furnace is allowed to cool, by opening the doors, down to a dull-red heat; in so doing, the crust formed over the surface of the fluid mass is broken up by the sulphurous acid passing off. As the charge throughout becomes more and more pasty, it is rendered more porous by the gas evolved, through the agency of the oxygen of the heated air coming into contact with it, which at the same time that it is expelling the sulphur, is also being stored up within the mass by the combining action of the copper, iron, and other metals present. At the end of about 12 hours, the charge is so far cooled that the disengagement of gas ceases.
The doors are again closed, the heat is gradually raised during the next 6 hours, fusion slowly takes place, calcination at first going on; and after the mass has again become fluid, the mutual reactions of the oxides and sulphides, as before indicated, again take place. The heat is then urged on to the utmost; the silica and iron, entering into combination, produce a slag, together with the remaining small proportions of antimony, arsenic, etc, through which the fused metallic copper finds its way to the sole of the furnace.
At the end of 24 hours the charge is drawn, the slag being skimmed off, and the metal cast into blocks about 3 it. long and 18 in. wide. This product is known as "blistered" copper, and amounts to about 60 per cent. of the weight of the charge put into the furnace. The residuary slag, which not unfrequently contains 15 to 20 per cent. of copper, is sorted and returned to the furnaces for the fourth and sixth operations. The blistered copper is so called because the surface of the ingots is covered with blisters; the interior is full of cavities, the fractured surface, when fresh, being of a deep-red colour.
Refining the coarse metal produced in the last process is conducted in a similar furnace, having a larger fire-place and the roof somewhat higher above the sole, so as to allow of the piling of the charge of ingots, 7 to 10 tons weight, on and over each other. These require to be so arranged as to allow of a free draught through the furnace, but with a regular equable distribution of heat. During the first 18 hours, the workman has only to maintain the fire so as steadily to raise the heat of the furnace.
Calcination and oxidation at first go on during the slow fusion, as in the last operation, more sulphur being evolved; and when the fusion is complete, the silica adhering to the ingots fuses, together with some of the copper oxide, at the same time laying hold of most, if not all, of the other metallic oxides remaining in combination. At the end of 22 hours, the scoria thus produced is raked off, leaving the surface of the metal as clean as possible. A few\ shovelfuls of powdered wood-charcoal, or finely-pulverized anthracite of the best quality, are thrown on the surface of the charge, and rapidly spread over it. A short time after this, a pole of green wood is plunged into the fused metal; a violent ebullition takes place, causing a more intimate mixture of the copper and the charcoal; and probably the steam and gaseous products evolved assist in the elimination of any traces of sulphur left within the metal. This action with the pole, termed " poling,' is maintained for about 20 minutes. The superintendent then takes out a small sample of the metal in a ladle; when cooled, he examines it by cutting the ingot half-way through with a steel chisel, then bending it in a vice back to a double, by which means the fibre of the metal is developed, the colour and flexibility are ascertained, and it is determined whether the refining is complete.