Prep. We will not attempt to give a minute description of the various complex processes by which the reduction of copper from its ores is effected, but will merely give an outline of the common or Welsh process. This process includes six distinct operations, as follows:—1. The ore (copper and iron pyrites), containing from 8 to 10% of copper, is roasted in a reverberatory furnace, called a ‘calciner,’ by which much of the sulphide of iron is converted into oxide. 2. The calcined ore is melted with ‘metal slag’ (a product of a subsequent operation—No. 3), in a melting furnace called the ‘ore furnace.’ The products are a regulus, termed ‘coarse metal,’ containing about 35% of copper, and ‘ore-furnace slag,’ which is thrown away. Much of the iron, and the whole of the so-called earthy matter of the ore, are thus separated as slag. 3. The coarse metal, having been granulated by causing it to flow from the furnace into water, is calcined with free access of air in a calciner, and a considerable amount of sulphur is expelled. 4. The calcined granulated, coarse metal is melted with the addition of matters rich in oxides of copper, namely, ‘roaster’ and ‘refinery slags’ (from the two remaining operations, Nos. 5 and 6, respectively), and native carbonates of copper, or ores containing oxide of copper. The products are a regulus, termed ‘metal,’ which contains about 75% of copper, and metal slag (see No. 2). The metal should be in the state of ‘white metal,’ compact and brittle, with a feeble metallic lustre and a dark, bluish-grey colour. It is tapped off into sand moulds. 5. The pigs of regulus obtained by the last operation are roasted in a furnace through which air passes. The temperature is so regulated that the regulus may be melted in from 6 to 8 hours. The slag is skimmed off, and after a time the heat is lowered, to allow the regulus to solidify. It is again melted and tapped into sand moulds, the product being called ‘blister copper.’ 6. This, the last operation, is termed ‘refining.’ From 6 to 8 tons of blister copper, in pigs, are melted in a furnace, and kept exposed for about 15 hours to the oxidising influence of the air. The slag is skimmed off through the end opening. When the oxidation has been sufficiently prolonged, anthracite or free-burning coal, as pure as possible, is thrown upon the surface of the metal, and after a short time the thick end of a long birch or oak pole is plunged into the molten mass. This part of the operation is termed ‘poling.’ The wood in contact with the copper is rapidly decomposed; much gas is evolved, which causes the metal to be splashed about, and every part of it to be exposed to the reducing action of the coal. When the refiner finds the metal to be at the state of ‘tough pitch,’ the pole is taken out, and the coal pushed back from the end opening, through which the copper is then ladled out as quickly as possible, and cast into suitable moulds. For full details of this and other processes, the reader is referred to Dr Percy’s work on ‘Metallurgy,’ and Ure’s ‘Dictionary of Arts, Manufactures, and Mines.’
In the laboratory copper is commonly employed under the following forms:
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1. Bean-shot copper. Produced by simply lading the melted copper from the refining furnace into hot water. In small lumps like peas and beans; hence its name. Used to make alloys, solutions, &c.
2. Electrotype copper. A very pure form, obtained by decomposing sulphate of copper in an electrotype apparatus. It does not contain lead, whereas most varieties of commercial copper do contain that metal.
3. Feather-shot copper, granulated C. Produced by lading the refined copper from the furnace into cold water. In small pieces, with a feathered edge. Used to make calamine, brass, solution of copper, &c.
4. Copper in plates or foil. Those of commerce (best, annealed) are generally employed.
5. Copper in powder.—a. A solution of sulphate of copper is heated to the boiling-point, and precipitated with distilled zinc; the precipitated copper is then separated from the adherent zinc by dilute sulphuric acid, washed with water, and dried by exposure to a moderate temperature.
6. Copper prepared by the hydrometallurgical method.—One of the oldest processes of this kind, is that known as the ‘cementation’ method, and consists in precipitating copper from a solution of the sulphate of the metal, by means of metallic iron. In some mines solutions of the sulphate are met with occurring naturally, in others they are prepared artificially by treating poor ores containing oxide of copper, with sulphurous acid or diluted sulphuric acid, and sometimes by roasting copper pyrites and afterwards washing them with water to extract the resulting sulphate. The copper obtained by any of the above processes is called ‘cementation copper.’ In the Isle of Anglesea the cementation liquid containing the dissolved sulphate of copper, is first run into large vessels where the suspended matters are allowed to subside; from these it is conveyed to tanks containing old scrap-iron, which serves as the precipitating agent. The scrap-iron is occasionally stirred up so as to renew the metallic surface presented to the solution. The muddy liquor which contains metallic copper as a spongy mass, besides impurities, is run into vessels where it deposits the copper, which after the removal of the supernatant fluid, is removed and dried in a furnace.
7. Wet Process. (Henderson’s process.) The ores (Spanish and Portuguese pyrites) treated by this method vary very slightly in composition, rarely containing much more than 3 per cent. of copper, nearly 50 per cent. of sulphur, from 43 to 44 per cent. of iron, with small quantities of lead, arsenic, zinc, lime, &c. The ores are first employed by the vitriol manufacturers, as a source of sulphuric acid. In the process of burning they lose about 30 per cent. of their sulphur. The copper is extracted from the residue by subjecting this latter to the following processes, which are thus described in the ‘Encyclopædia Brittanica.’