for the admission of air to the hearth; H, H are the hoppers for charging the furnace, and t, a platform over which the barrows of ore are conveyed to the hoppers. The fuel used in roasting the ore is chiefly anthracite, a coal which, under ordinary management, yields no flame. Flame, however, is absolutely necessary to the proper roasting of the copper ore experience has taught the copper smelter to obtain this desideratum by limiting the supply of air to the fuel in the fire-grate, thus causing the carbonic anhydride which is formed at the lower part of the fire to be converted into carbonic oxide. By a nice adjustment of the supply of air through g, the other apertures of the furnace being closed, the carbonic oxide is gradually burned as it plays over the ore upon the hearth, c c; the maximum of heat is thus obtained at the minimum cost of fuel, the carbonic oxide being completely burned before it reaches the flue. An 718 MELTING FOR COARSE AND FINE METAL. admirable analysis of this operation is given by Le Play in his elaborate memoir on the Welsh method of copper smelting (Ann. des Mines, IV. xiii. 128).* (870) 2. Melting for Coarse Metal.-The roasted ore is now subjected to fusion in the ore furnace with certain proportions of slag, the produce of a subsequent operation, of siliceous ore free from sulphur, and of fluor-spar if necessary: by this means the charge is converted into a fusible slag, consisting chiefly of silicate of iron, and into sulphides of copper and of iron, which sink through the slag, and form what is termed a matt. This fusion occupies about 5 hours, each charge containing about 1 ton of roasted ore. The matt thus procured contains about 33 per cent. of copper it is run off while liquid into water, by which it is granulated. The product goes by the name of coarse metal. The slag which floats above the matt is raked out of the furnace at a separate aperture. It ought to contain no appreciable quantity of copper. 3. Calcination of the Coarse Metal.-The granulated metal is again roasted for 24 hours, during which operation a large proportion of the sulphide of iron is converted into oxide. 4. Melting for Fine Metal.—A second fusion is performed upon this calcined matt with the addition of a portion of copper ore known to be rich in oxide of copper and in silica, and to contain but little iron pyrites. By this means the oxide of iron is removed in the form of a fresh slag of silicate of iron, and the oxygen contained in the freshly added oxide of copper completes the oxidation of any portion of sulphide of iron still remaining; the oxide of copper and the whole of the sulphide of this metal being reduced to the state of subsulphide of copper (Eu,S) or fine metal. The slags from this process, and all the subsequent ones, are preserved. This matt contains about 80 per cent. of copper. It is cast into pigs. If a very pure metal be required, the roasting is carried a little further; a portion of the metal is thus reduced: this portion contains the greater part of the foreign metals, which give up their sulphur more readily than the copper; the reduced metal, from *The heat emitted during the combustion of anthracite is very intense, so that it causes a rapid oxidation of the fire-bars of the furnace. This fuel has also the inconvenience of splitting into small fragments, which choke the air-ways between the bars, if the heat be suddenly applied. The copper smelter overcomes these difficulties by employing a grate consisting only of a few bars, which do not come into contact with the fuel itself, but only serve as a support for the clinker produced during the combustion of the coal. A bed of clinkers, 12 or 16 inches thick, rests upon the fire-bars, and above this the fuel is burned: from time to time the fireman removes portions of the clinker as it accumulates. COPPER SMELTING-BLISTERED COPPER-REFINING. 719 its greater density, sinks to the bottom; the upper parts of the pigs are subsequently detached from the lower portions, and the metal extracted from the upper portions of the ingots is known in the market as best selected copper. 5. Roasting for Blistered Copper.-The fine metal or subsulphide is now to be freed from the sulphur, which has hitherto been useful by forming a fusible compound with the copper, thus facilitating its separation from the impurities by which it was accompanied. With this view the pigs of fine metal are next subjected, for several hours, upon the bed of a reverberatory, to a heat just short of that required to fuse them; the metal by this means becomes oxidized at the surface, and a part of the sulphur which it still retains is also oxidized; at last it is fused: a remarkable reaction then begins to take place. When oxide of copper and subsulphide of copper are heated together, they decompose each other; sulphurous anhydride and metallic copper are liberated; Єu2S+2 €u✪=SO2+4 €u. It is not desirable that the temperature should be too strongly raised, as the oxide of copper would then combine with the silica still present in the mass, and would cease to exert its oxidizing influence. After the charge has become liquid, the temperature of the furnace is allowed to fall; the melted mass solidifies upon its surface, and an appearance of violent ebullition is produced from the formation of sulphurous anhydride, and its efforts to escape from the tenacious mixture: when this ceases, the desulphuration is complete. The heat is now rendered very intense, the copper melts and sinks to the bottom, and separates completely from the slag, which consists chiefly of silicate of copper; the reduced metal is then run off into sand moulds. The ingots thus obtained, being full of bubbles, are termed pimple or blistered copper. (871) 6. Refining or Toughening.—The blistered copper now undergoes the concluding operation of refining. From 7 to 8 tons of the metal are placed in a reverberatory furnace and kept in a melted state for upwards of 20 hours, in order to oxidate the last traces of foreign metals: during this process a large quantity of oxide of copper is formed; part of this oxide is absorbed by the melted metal, and the copper, if examined at this stage, is found to be of a dull red colour, coarse grained and brittle. To reduce this oxide, the slags are skimmed off, and the surface is covered with a few shovelfuls of anthracite or of charcoal; the metal is then subjected to the process of poling, in which the trunk of a young tree is thrust into the molten metal. The inflammable gases disengaged from the green wood as it chars, produce a 720 DEFECTIVE AND COMMERCIAL COPPER. powerful agitation of the whole mass, and in about 20 minutes the poling is finished. The reducing influence of the combustible gases has in the mean time been brought to bear upon every portion of the melted metal. In this way the oxide diffused through the mass is deprived of oxygen. If the poling be carried too far, the copper again becomes brittle, and is said to be overpoled. This defect may be remedied by exposing the surface of the melted metal to a current of air. If too little poling be used, the metal is still brittle, and it is then said to be underpoled. The progress of the poling, therefore, requires careful watching: the refiner tests the metal from time to time by dipping a small test-ladle into the melted mass; a sample of copper is thus removed, and cooled suddenly by immersion in water: the grain of the copper is judged of by cutting the hammered button partially through with a chisel or shears, and then bending it by placing it in a vice. If properly refined, the broken surface will display a fibrous structure with a beautiful silky lustre. If underpoled, the fracture will be granular, with a number of red points. If overpoled, the fibres become coarser, and the fracture has a strong metallic lustre, but the silky appearance is wanting. When, upon testing, the copper appears to be fine grained, fibrous, and silky, of good colour, and malleable, it is either ladled out and cast into ingots, or it is cooled suddenly at the surface, by allowing water to run upon it; in the latter case rose copper is produced, and successive films are made and removed till all the metal is consumed. There appears to be no doubt that the brittleness of underpoled copper is due to the presence of red oxide of copper in the metal, and Mr. Vivian has suggested that overpoled copper may be defective from the presence of a little carbon. Percy, however, was unsuccessful in the attempt to discover carbon in overpoled specimens. An interesting paper by Abel on the non-metallic impurities of refined copper will be found in the Journ. Chem. Soc. 1864, p. 164. The slags from the various operations are carefully remelted, and the copper which is extracted from them is termed black copper; it is run into pigs, which are subsequently refined. The presence of a small quantity of tin in the refined copper is considered to be advantageous, as the toughness and tenacity of the metal are thereby increased. Antimony is singularly injurious; so small a quantity as IO Ounces in the ton renders copper unfit for making brass that is required for rolling; and minute traces of nickel and of bismuth are also said greatly to injure the tenacity of the metal. (872) Kernel roasting.-When cupriferous iron pyrites, con KERNEL ROASTING- 721 taining from 2 to 3 per cent. of copper, is broken into lumps of about the size of the fist, and subjected to a very gradual roasting, a large portion of the copper becomes concentrated in the middle of the lump, and a nucleus of sulphide of copper and iron is formed. This nucleus, or kernel, is surrounded by a more or less porous shell, composed mainly of peroxide of iron, which may be detached from the nucleus by a blow. Upon these observations a method of roasting copper ore has been founded, to which the name of kernel roasting has been given. This roasting is conducted in the Venetian Alps, at Agordo, in kilns, and at Mülbach in enormous heaps in the open air. These heaps are in the form of a truncated square pyramid, the base of which is about 30 feet square (406). The roasting is a very slow operation, requiring from 5 to 6 months for its completion. Spring and autumn are the most favourable seasons in which to commence it. Sulphur distils off to the extent of o2 per cent. of the ore; the kernels constitute from 13 to 14 per cent. of the roasted mass, and they contain about 5 per cent. of metallic copper. The cause of this concentration of copper in the interior of the mass is entirely unexplained. The shells, which retain a small proportion of sulphate of copper, are washed to extract this as far as practicable, and the roasted ore is then subjected to processes not essentially differing from those already described. (Percy, Metallurgy, i. 439.) In many copper mines the water which is pumped up is impregnated with sulphate of copper derived from the oxidation of the sulphide by exposure to the air: the copper is easily separated in the metallic form, by collecting the water in tanks containing scrap-iron the iron unites with the oxygen and the acid, whilst the copper is set at liberty: CuSO4+Fe=FeSO4 +¤u. : When the ore consists of the oxides and carbonates of copper, it is easily reduced to the metallic state by simple fusion with coke or charcoal, oxide of iron and lime being added in quantity sufficient to form a fusible slag with the silica which usually accom panies these ores; the copper is rendered tough by a process analogous to that of poling. The copper of commerce is often very nearly pure. It contains minute quantities of arsenic, of iron, of lead, and sometimes of tin and silver. Abel has detected traces of selenium in some specimens, and of sulphur in others. Copper may be readily obtained in a state of perfect purity, by decomposing a solution of pure sulphate of copper by means of the voltaic battery: it is then deposited in coherent plates upon the negative electrode. (873) Properties.-Copper is one of the metals which has been II. 3 A |