COPPER-SMELTING. I. The dry process.—The ores are sorted according as they contain much or little sulphur, and are mixed by the smelter to produce a mass, (1) that will contain 9 to 14 per cent of copper; (2) that after calcining will melt easily without flux; (3) that when fused will yield a mass containing about 30 per cent of copper; (4) that shall be free from impurities which would cause a low grade of copper. Three or four tons of this mixture are spread about 8 in. thick on the floor of a reverberating furnace. The fire is at first low; as the mass reddens the heat is increased, and the ore is stirred to expose all equally to the flame. The process lasts from 12 to 24 hours, depending on the quantity of silica and sulphur present. The sulphur is burned or volatilized, and the iron and sulphur pyrites are partly changed to oxides. After the calcined ore is removed from the furnace, water is added to assist oxidation, and the mass is stored for fusing. For this process, the calcined ore, with slag and broken bricks from old furnace hearths, is spread upon a reverberating hearth, the furnace is sealed with clay to exclude the air, and the heat is made intense for about 5 hours. The workman removes the slag which floats on the melted mass, and a second charge is usually added, melted, and skimmed; then the metal is run off into a pit of water, where it is granulated, or into molds, and afterwards crushed by machinery.
The processes of calcining and fusing are repeated, the first with moderate heat to consume the sulphur and oxidize the iron; the second with intense heat to remove the oxide of iron with silica in a slag, while the proportion of copper constantly increases. When the iron is mostly removed and the proportion of sulphur is much reduced, the metal is roasted in a current of air for about 24 hours, the mass being kept in a semi fluid state. The product is a sulphide of copper, Cu,S, which is at once submitted to another roasting for 24 hours, the product being nearly pure copper, which a subsequent refining fits for the market. The steps of reduction may be understood from the fol
lowing table: II. The wet process.—The ores are first burned to drive off part of the sulphur, which is utilized in the manufacture of sulphuric acid; about 30 per cent of the weight is removed. The remainder is mixed with about 15 per cent of common salt, and the whole is finely ground between heavy iron rolls. A portion of unburned, or over burned, pyrites is usually added to bring the proportion of sulphur in the mass to the proper standard. The mass is then calcined, the sulphur is oxidized, sodic sulphate is formed, while the chlorine of the salt unites with the copper to form cupric chloride; the hydrochloric acid and other gases evolved are condensed in tall chimneys as "tower water," which is saved for use in the next stage of the process. The calcined ore is washed in tight wooden tanks, with hot water, " tower water," and dilute hydrochloric acid, until all soluble copper is extracted. The solution is conducted into other tanks of wood, containing heaps of old malleable iron, by which the metallic copper is pre cipitated in a finely divided condition. When a steel blade, thrust into the liquid, is no longer reddened, the process Is ended; the tanks arc drained, and the copper is sepa rated from the iron by washing with water. This precipitate contains about 80 per cent of copper, which is further purified, as in the dry way. The liquid which contains the copper carries also the lead, silver, or gold which may have been associated with the copper in the ore, and these metals may all be profitably separated. About 14,000 tons of copper are produced annually by this process in Great Britain, out of a total annual product in the world estimated at from 126,000 to 130,000 tons.