In the smelting process, which is preceded by roasting the ores to remove some of the sulphur, large lump ores not too complicated with gangue and other metals, are smelted in the blast furnace, while the powdered ores and those of complex composition are smelted in the reverberatory furnace.

The process of producing copper is not direct and simple, as is the case with iron, but consists of a number of treatments under separate heats, or in separate furnaces and receptacles. The object of the several steps of the process is to simplify the copper compounds into a nearly pure sulphide by removal of other parts of the ore, principally iron and an excess of sulphur, and then to break up the copper sulphide and remove its sulphur by oxidation, leaving a somewhat impure metallic copper which is then refined.

Roasting the ore removes some of the sulphur and incidentally oxidizes some of the free copper and iron. The roasted ore is then melted in a smelting furnace to convert the earthy matter into slag by use of flux, as in iron smelting, and the remaining product is a nearly pure sulphide of copper in which some iron sulphide remains. This product, known as "matte" or "coarse metal," is run in a molten state from the smelting furnace into a large refractory lined vessel called a converter, and cold air is blown through the molten mass to oxidize the sulphur and the iron.

Fig. 11. - Copper Converter.

Fig. 11 shows a cross-section of a converter. It consists of a shell k of steel plates with a lining dd of silica. The trunnion band aa supports the converter and allows it to be tilted about an axis b perpendicular to the plane of the page. The wind box / extends part way around the bottom. It receives air through one of the trunnions, which is hollow and connected to a blower. The air is forced through several openings, as at g, into the molten charge. It combines with the sulphur, forming sulphur dioxide which escapes at h into the open air. The iron in the charge is also oxidized, this oxide forming a slag with the silica lining of the converter. The oxidation of sulphur and iron supplies heat which keeps the charge molten, and the blow is continued until the flame from the converter mouth shows that these are about burned out and that copper is beginning to burn. The converter is then tilted in the direction of the arrow to pour the charge into a ladle. The product thus obtained is copper about 98 or 99% pure, known as "blister copper," because of the blisters on its surface due to expelling sulphur dioxide as the metal cools.

The last stage is the refining of blister copper either by the poling process or by electrolysis. The object is the same in both cases, viz., to remove the remaining sulphur, iron, copper oxide, antimony, arsenic and other less frequent impurities. The purpose for which the copper is to be used and the kind of impurity it contains determine the degree of refining necessary, though not over .5% of combined impurities should remain in any grade of refined copper.