Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/380

Rh 350 . C P P E K metal in the ladle is favourable, the charge is ready for the operation of poling. A quantity of charcoal or anthracite coal is first thrown upon the metal to prevent oxidation by the air, and then the end of a large pole of green wood, generally of birch or oak, is inserted into the melted copper, and kept pressed down to the bottom of the metal, which spurts and boils violently. This operation, it will be at once apparent, consists in the reduction of an oxide or suboxide. Since oxide of copper dissolves in metallic copper, as a salt dissolves in water, and makes it brittle, to put pieces of wood or charcoal upon the surface would not remove the oxygen ; hence the necessity of poling, in order to bring the carbonaceous matters into contact with the dissolved oxide. As the poling proceeds the refiner takes from time to time small samples called assays, which he hammers and breaks for examination. When the copper reaches the proper &quot; pitch &quot; the assay bends without break ing, and if cut and broken the fracture is fibrous, and pre sents a silky lustre. When this pitch is attained the pole is withdrawn, and a quantity of charcoal thrown upon the surface ; and, if the copper is for rolling or hammering, a little lead is added to the charges to insure toughness. In making what is termed best selected copper, the refining is performed in the manner described, but no lead is added. This quality of copper is used for the manufacture of fine alloys, such as the best brass, or Muntz s yellow metal. Copper a little over-poled is generally preferred for these purposes. When the copper is brought to the proper pitch by the refining operation it is ladled out into moulds. The follow ing are the forms in which British smelted copper is usually cast : Cake, 19 x 12^ x 1J inches, weight 1 cwt. 1 qr. Tile, 19xl2|x 1 qr. 3 tt&amp;gt;. Ingot, 11 x 3xl^ 14 to 16 Ib. During the ladling out the refiner takes an assay at short intervals, as the metal is liable to get out of pitch, or be come dry, as under-poled copper is termed, in which case poling has to be resumed. So much depends upon refining, that the best copper by a defect in this operation will be rendered unmarketable. A great variety of improvements in copper-smelting have been proposed and patented, one or two of which have been usefully applied. Several modifications of the various processes are also adopted, to suit the quality of the ores and the kind of copper to be produced. These are all suggested by the experience of the smelters in dealing with the materials at their disposal. WET PROCESSES. Several methods of extracting copper by the wet way have been more or less in practice at various periods ; but it is only of recent years that one of these has been established on a scale of great commercial extent and importance. From a very early time it has been known that the water which drained from mines containing pyritous copper ores, and which from the oxidation of the sulphide of copper contained some proportion of cupric sul phate, yielded metallic copper by precipitation in the presence of malleable or cast iron. The copper obtained in this way is known as cementation copper, and from the Spanish and Portuguese pyrites mines a considerable amount of metallic copper has long been so precipitated. The process now very extensively adopted for treating Spanish and Portuguese pyrites, and some ores of similar composition from other countries is that patented by Mr William Henderson in 1859. Mr Henderson s process is in several essential particulars the same as one patented in 184:2 by Mr William Longmaid, which, however, was chiefly designed for the production of sulphate of soda, copper being only a by-product. There can be no doubt that Mr Henderson is the practical originator of the wet process, which in Great Britain now occupies a most important position among metallurgical industries. The ores treated by the Henderson process are remarkably constant in character, and the following may be taken as representing their average composition : Sulphur 49-00 Iron 43 55 Copper 3-20 Lead 93 Arsenic 47 Zinc 0-35 Lime, with traces of silver, gold, &c 2 50 100-00 The pyrites is first employed by alkali manufacturers and other consumers of sulphuric acid as a source of that sub stance, in burning for which the ore loses about 30 per cent, of its weight. It is this burnt pyrites which forms the raw material of the process. The various stages it undergoes are briefly as under. I. Grinding. The burnt ore, as received from the acid burners, is first mixed with about 15 per cent, of common salt, and ground to a fine powder by passing it between a pair of heavy cast-iron rolls. As the amount of sulphur left in the burnt ore is apt to vary, it is necessary to ascertain its proportion in each parcel of burnt pyrites. When the sulphur falls short of the proportion necessary for effecting the decomposition which follows, a sufficient quantity of &quot; green &quot; or unburned pyrites, is added to pro duce a proper balance. If, on the other hand, the sulphur has been insufficiently extracted, &quot; dead &quot; roasted ore is added. II. Calcination. This operation is accomplished in several kinds of furnaces, that used by the Tharsis Sulphur and Copper Company being a large muffle or close furnace. By others a patent furnace with a revolving hearth and mechanical stirring arrangement has been adopted with good results ; and some use open reverberatory furnaces heated by gas from Siemens s generators. During the roasting the mixture is frequently stirred, and, in the case of hand-worked furnaces, turned with long rabbles, and the completion of the operation is ascertained by test assays. When the copper has been brought into a soluble condition, the charge is raked out of the furnace and permitted to cool under a screen at its mouth. By the calcination the sulphur in the compound is first oxidized, sulphate of sodium is formed, and at the same time the chlorine from the sodium chloride unites with the copper to form cupric chloride. A small proportion of cuprous chloride is also formed, and special precautions have to be taken to prevent the extensive formation of this compound, which is dissolved only with difficulty. The hydrochloric acid and other gaseous products evolved during the calcination are con densed as &quot; tower liquor &quot; in ordinary condensing towers, and the product is used in the subsequent process of lixiviation. III. Lixiviation. The calcined ore is conveyed to tightly- caulked wooden tanks, in which it receives repeated wash ings with hot water, tower liquor, and dilute hydrochloric acid, till all the soluble copper is thereby extracted. The product of the later washings is pumped or drawn up by a modification of Giffard s injector, to serve as a first liquor for subsequent charges of the lixiviating tanks, and no solution under a definite strength is permitted to pass on to the next stage in the process. The insoluble residue in the tanks consists of &quot;purple ore,&quot; an almost pure ferric oxide, largely used in &quot; fettling &quot; blast furnaces, and for smelting purposes; besides which it is available as jewel ler s rouge. IV. Precipitation. The precipitation of metallic copper from the solution of its chloride is accomplished in large