With the exception of the native copper from Lake Superior, and smaller quantities from Peru and Bolivia, nearly all the copper of the world is obtained from sulphuretted or oxidized ores, which require for the separation of the pure metal a long and somewhat costly process. This is effected either in the dry or the wet way; that is to say, by operations in the furnace, or by processes which allow the copper to be separated in a dissolved form in watery solution, and subsequently precipitated therefrom. The term smelting applies more properly to the first process, but we shall briefly describe both the dry and wet methods. - The treatment of the native oxides and carbonates is extremely simple, requiring only their fusion with charcoal and a proper flux in a small blast furnace, by which metallic copper is at once obtained. In the case of the sulphuretted ores, which generally contain more or' less iron and other foreign metals, the treatment is much more complicated, and depends upon the fact that the copper has a greater affinity for sulphur than the associated metals; so that if a double sulphuret of copper and iron be partially oxidized by roasting in the open air, and then melted in a furnace, the iron in an oxidized condition unites with the foreign matters to form a more or less fusible cinder or slag, while the copper, still retaining its sulphur, separates in the shape of a dense brittle mass known as regulus or matt.
To facilitate this process, various ores are often mixed together in order to secure greater fusibility, and oxidized ores are sometimes mixed with highly sulphuretted ones, by which means the copper of both is got in the form of regulus. In some districts low-grade ores, of perhaps 4 or 5 per cent., are thus converted into a regulus of 30 per cent, or more, which is then shipped to some other place for further treatment. This regulus holds also any gold or silver which may have been in the ore, and hence the copper ores of Colorado, rich in precious metals, are there converted into regulus, which is sent elsewhere for further treatment. The regulus or concentrated copper ore thus obtained, still containing a portion of iron and sulphur, is again roasted and fused with some flux, whereby a richer regulus is obtained; and this operation is repeated until a nearly pure disulphide of copper results. For the extraction of the copper from this two plans are adopted. The first, still followed in Wales and in most European works, consists in partially roasting the refined regulus, by which a part of the sulphur is removed as sulphurous acid, and the combined copper is converted into oxide; and then causing the latter to react upon the yet unoxid-ized portion, the sulphur of which fixes upon the oxygen of the oxide to form sulphurous acid, while the copper of both is left in the metallic state.
The reaction may be thus represented: Cu2S+2Cu2O=Cu6 + SO2. Thesec-ond plan consists in continuing the roasting and oxidation of the purified and pulverized regulus until the whole of the sulphur is expelled. The copper, being then in the condition of oxide or suboxide, is treated directly with charcoal in the same way as the native oxides and carbonates. The latter method, known from its inventor as Napier's, is now generally pursued in Chili, and is said to offer great advantages on the score of economy of fuel and labor over the older system. The copper obtained by either of these processes is still somewhat impure, from the presence of small quantities of sulphur and foreign metals, chiefly arsenic, tin, iron, and sometimes nickel. To remove these it is fused in contact with the air, by which these are oxidized and the fixed oxides combine with the silicious matters of the hearth. In this process, however, the copper absorbs a portion of oxygen and holds in solution a quantity of suboxide, which impairs its malleability. This is removed by covering the surface of the melted metal with anthracite or charcoal, and stirring it with wooden poles, which by giving off reducing gases deoxidize the metal and give it the toughness of pure copper, after which it is cast into ingots.
The details of these processes vary considerably with different ores and in different localities. The consumption of fuel in the treatment of copper ores is very great. Thus in the old Welsh process it is said that as much as 15 tons of coal is used in extracting one ton of copper from the low-grade ores of Cornwall. In Chili, however, owing to the great improvements introduced there, a ton of ingot copper is now made from ores of 13 per cent with the aid of five tons of coal. - The extraction of copper from its ores by solution has been long known. The spontaneous oxidation of the sulphuretted ores in certain mines gives rise to solutions of sulphate of copper, which when brought in contact with metallic iron yield their copper in a metallic state, while the iron is dissolved. The copper thus obtained is in a granular, more or less crystalline form, and is known as cement copper. By the artificial oxidation, by roasting in furnaces or in heaps with burning wood, of sulphuretted ores, a large proportion of the metal is converted into sulphate, and may then be dissolved out by water.
From the solutions thus obtained the copper is either thrown down by iron in the metallic state, or in some cases by lime as an oxide, or by sulphuretted hydrogen gas as a sulphide, to be subjected to further-treatment. Other methods of rendering the copper soluble have been devised. Thus, in some cases, the naturally or artificially oxidized ores are directly treated with sulphuric or hydrochloric acid, by which the copper is dissolved as sulphate or chloride, and then precipitated by metallic iron. Another plan consists in calcining the ores with a portion of common salt, by which, with the aid of the oxidized sulphur, sulphate of soda and chloride of copper are formed. This is known as Longmaid's process, and has been extensively applied by Henderson in England to the treatment of the pyritous ores from Spain, of which several hundred thousand tons are annually imported into England and calcined in proper kilns for the extraction of the large amount of sulphur which they contain, which is used for the manufacture of sulphuric acid. The residues, still containing a little sulphur and from one to four per cent of copper, are calcined with salt as above described, and give up to water, with the help of a little hydrochloric acid, the whole of their copper.
The consumption of iron by these processes is considerable, amounting to from one and a quarter to two tons for each ton of copper produced. When the copper is present as sulphate, the resulting sulphate of iron is a valuable by-product. The protochloride of iron obtained when the copper is precipitated from its chloride has been converted by exposure to air or by chlorine gas into perchloride, and used to dissolve oxide of copper from oxidized ores. The occurrence of large quantities of low-grade copper ores in localities remote from cheap supplies of fuel has caused increased attention to be given to wet processes for the extraction of copper from its ores, and two newly devised methods have been brought forward and patented. The one, known as Monnier's, differs from that of Longmaid in substituting sulphate of soda for chloride of sodium in the calcining furnace. By this means it is possible to convert the whole of the copper in a sulphuretted ore into sulphate, which is then decomposed by metallic iron, or, as has been lately proposed, is partially separated by a process of concentration from the sulphate of soda, and then decomposed by calcination with charcoal.
The other, known as the Hunt and Douglas process, is based on a new principle, and applied either to native oxides and carbonates or to the artificially prepared oxides resulting from the roasting of sulphuretted ores. Protoxide of copper reacts with protochloride of iron to form peroxide of iron and a mixture of protochloride and di-chloride of copper: 3CuO + 2FeCl=Fe2O3 + Cu2Cl + CuCl. The dichloride is insoluble in water, but readily soluble in hot and concentrated brine; and from the solution holding the two chlorides of copper, the metal is readily thrown down by metallic iron, regenerating the protochloride of iron, with a consumption of about two thirds of a ton of iron for a ton of metallic copper. In practice the ores (oxidized if necessary by roasting) are digested with a hot brine mixed with protochloride of iron; and the resulting solution, being brought in contact with metallic iron, gives up its copper, and is then ready for the treatment of a fresh portion of ore. The cement copper thus obtained is very pure, and is readily melted down into ingot copper.
The small quantities of silver very often present in copper ores are dissolved in the form of chlorides in the Hunt and Douglas and Longmaid processes, and may be economically separated therefrom by the ingenious process of Claudet, which consists in precipitating the silver in combination with iodine. - Swansea in Wales, which is in a coal district, has been for a long time one of the great copper-smelting centres, and receives the ores and regulus not only of Cornwall, but from many other parts of the globe. In former years much of the copper ore from Chili was carried to England for smelting, but it, is now in great part smelted at home, although in 1870 two thirds of the copper produce of that country was sent to Swansea in the shape of regulus. Large quantities of ores from Australia, South Africa, Cuba, and British North America are shipped to England for treatment. The smelting works of Great Britain are the most extensive on the globe. In 1871 there were 24 copper mills, employing 3,525 hands; of which 13 were in Glamorganshire, 3 in Carmarthenshire, 3 in Surrey, 2 in Staffordshire, 2 in Worcestershire, and 1 in Anglesey. According to the census of 1870, the United States contained 27 establishments for milling and smelting copper, with 43 steam engines of 3,244 horse power, 1 water wheel of 39 horse power, and 1,082 hands; capital, $3,158,500; wages paid, $577,129; value of materials, $10,715,400; value of product, $11,-684,123. They were thus distributed:
Value of products.
The other 3 (1 in Arizona and 2 in California) were unimportant. The Maryland works are in Baltimore co., the Massachusetts works in Bristol co., the Rhode Island works in Newport co., and the Tennessee works in Polk co. Of the Michigan works 12, employing 511 hands, and producing $6,752,228, were in Houghton co.; 1, with 62 hands, producing $2,003,760, was in Wayne co.; and 6 (quartz mills), with 63 hands, producing $504,988, were in Keweenaw county.