separating the two sections of the vat is so made that only the electric current may pass through it, as in the previous plant. Fig. 144.-Borchers' modified electrolytic cell The cathode is placed in a wide compartment, and consists of a hollow cylinder of sheet-metal bent over a wooden roller (Fig. 143) that can be rotated from without; or, the core being omitted, it may be rotated by two narrow rollers placed above the liquid as shown in Fig. 144. Connection is made with the negative conductor by the brushes placed above, these being pressed against the surface by a spring. The electrolyte is introduced into the cathode compartment, where the metal is deposited on the surface of the revolving cathode; it then passes through a series of openings, o, into the anode compartment, and, flowing first downwards, and circulating among the granules of the alloy, it finally rises again into the free space to the right. This flow of solution carries down with it all the insoluble residue washed off the surface of the anode granulations. A current of liquid is caused to enter the contracted well, g, at the bottom of the vat through a long and narrow slot, and this liquid mingles with the electrolyte, and so increases the rate at which it travels upwards through the right-hand portion of the vat. By regulating this rate of flow it is possible to effect a separation of the light from the heavy particles of the residue in suspension. The heavier portion of the slime escapes through the slot and the tube, r, which is connected with it as in the apparatus previously described. The solution that is admitted mechanically to assist the separation enters the slot by the tube, z. The electrolyte, after passing through the vat, overflows through a channel above, exactly as shown in Figs. 141 and 142. After filtering the saturated solution, and, if necessary, separating some of the dissolved constituents, it is returned to the cathode cells where the metal that is most easily separated electrolytically from the remainder will be deposited. The author has treated ores and smelting products containing the precious metals, with apparatus of the type just described, but put together in the most primitive manner from scrap and lumber; and yet the results were quite satisfactory. The experiments were made in an American factory, and extended over some ten years. A properly constructed apparatus was then put together, but was not used on account of the author's return to Germany. The Dietzel Apparatus.—An apparatus patented by Dietzel* for the same purpose is based to some extent on the same idea. A vat with a bottom, sloping either towards the middle or to one side, is lined with carbon plates, on which the granulated metal to be treated is gradually moved downwards with the aid of suitable mechanism. The cathodes, which are suspended in the copper nitrate electrolyte, consist of straight copper plates or of sheet copper, bent over wooden cores into cylinders, and are enclosed within open wooden vats, of which the bottoms are formed of a dense filtering medium, such as linen. These vats must be of sufficient size to allow ample room for the introduction and movement of the grains of alloy. The latter is con*German Patent 68,990. nected, as the anode in the circuit, by contact with plates of conducting, but not soluble, material at the bottom of the bath. The circulation of the electrolyte and the progress of the operation is as follows: (1) Introduction into the cathode compartment, with separation of copper. (2) Penetration through the filter at the bottom of the cathode cell into the space around the anode, where copper and silver are dissolved, and gold is left as a residue. (3) Transfer to the filtering and precipitating plant, where any gold previously held in suspension is caught, and silver is precipitated by means of copper. (4) Re-introduction of the copper nitrate solution into the cathode compartment. Since the plates to be employed in making connection between the positive lead and the anode must be conductant but insoluble," it will be remarked that carbon is not here available. It is well known that carbon plates have but a short existence under the influence of such high current densities as are necessary [for economic reasons] in the treatment of valuable raw materials. In this case the ordinary disintegration of the plates would be greatly hastened by the mechanical action of the grains of alloy as they slowly move over the surface of the carbon. No doubt the mere consumption of carbon would not be a a very heavy charge, taking into account the value of the metal under treatment, for even the best carbon plates in these days are not costly. But the carbon powder resulting from the disintegration of the plates would mix with the pulverulent gold residue from the anodes in such quantities, that the separation of the two materials would prove to be a more difficult problem even than the separation of the original alloy components. In a newer modification of this apparatus, described in a private communication written by Dr. Dietzel to the author, the platinum wires, P (Fig. 145), are substituted for carbon plates. In order to accomplish the introduction and movement of the alloy, S, beneath the diaphragm, D, more conveniently (especially in certain cases, such as the treatment of large quantities of base alloys), the vat, which is constructed in the shape of an inverted roof, or with an otherwise sloping bottom, is made as elongated as possible, and the support for the alloy, S, is provided with rollers, r, which run on the rail, s. In this way it is possible to convey the residual slime from the treatment of the alloy to openings at the ends of the long sides of the vat, or to introduce fresh material in the same way. The composition of the raw material may average 55 per cent. of copper, 36 per cent. of silver, and 5 per cent. of gold, together with some zinc, lead, tin, nickel, cobalt, iron, manganese, aluminium, platinum, &c. The alloy is cast into plates, S, about 3 to 4 mm. [in.] thick, which are then laid upon the platinum wire conductors. The electrolyte consists of a slightly acid solution of copper nitrate, with 3 to 5 per cent. of copper and small quantities of free acid. On the admission of the liquors into the cathode compartment, the proportion of free acid should be at least 0.1 per cent; but should not exceed 0.2 to 0.4 per cent. at most. This proportion may be regulated by the addition, as required, of a strongly acid solution of silver and copper, which is obtained from the treatment of the gold slimes with boiling nitric acid. The form of the diaphragm, D, remains practically the same as that originally adopted. As already indicated, the diaphragm itself consists of close linen cloth stretched obliquely across the bottom of the cathode compartment. The slanting position is Fig. 145.-Dietzel's alloy-separating vat. adopted, because after the bulk of the silver and copper have been dissolved from the anode material, a considerable quantity of oxygen is necessarily generated in contact with the insoluble residue, owing to the high current density used; and this oxygen must of course be conducted freely away from the cathode cell. The cathodes consist of rotating copper cylinders, K, the surface of which is coated with a thin layer of graphite, so that the electrolytic copper may be readily stripped from it, as soon as it has been deposited to a sufficient thickness. The methods used in conducting the operations remain unaltered. Many details, no doubt, require further development, as in the case of all new processes. But over a ton of metal has already been separated in this manner. A number of other methods, which have been brought forward for the extraction of the precious metals, will be discussed in the next chapter, at least, so far as they are founded upon practicable ideas. Applications of Silver.-Silver is not very extensively used in the pure condition on account of its inferior strength. Excepting, therefore, for the production of pure compounds of silver, which are very largely employed, especially in photography, and for scientific instruments, chemical apparatus, and silver-plated articles, it is almost always alloyed with copper. These alloys, which contain up to 90 per cent. of silver, are very generally used for coinage purposes, and for the production of household implements and objects of art, or silversmith's work. * == Properties of the Metal.-Gold (Au; atomic weight 197; specific gravity=19.3) is a yellow metal with a brilliant lustre ; it is strong, and is the most malleable of all metals. On account of its softness and its other mechanical properties a fractured surface shows no appreciable crystalline structure, but appears hackly. Gold melts at a temperature of 1035° C., and at about 2000° C. it begins to vaporise. Its conductance in respect of heat and electricity is very high, and may be represented by the numbers 60 and 70 respectively, if the conductance of silver be regarded as 100. The tendency of gold to absorb gases when in the molten condition is very small as compared with that of silver. On the other hand, the solid metal, when in the finely divided condition, is capable of condensing about 0.7 per cent. of electro-positive gases (H, CO, &c.). In its power of dissolving or being dissolved by other metals it is very similar to silver. The alloys of these metals, particularly those with lead, mercury, copper, and zinc, and the alloy of gold with silver, have a special interest for metallurgists. The presence of a very small proportion of foreign matter (0.05 per cent. of lead, bismuth, or tin; or 0.00003 per cent. of antimony) suffices to destroy the malleability of gold and to make the metal weak and brittle. In its chemical aspect, gold is one of the most stable of metals. Its oxides and sulphides can only be produced indirectly, and are very easily decomposed. Halogens, especially chlorine and * [The foreign silver currency contains 90 per cent. of silver. In England the standard alloy for coinage and plate contains 924 per cent. of silver and 7 per cent. of copper.-TRANSLATOR.] |