some experiments made by Cohen,* who observed that when cuprous chloride was electrolysed under a low current-density, the cupric chloride formed at the anode sank to the bottom of the containing vessel in the form of a solution, of which the specific gravity was higher than that of the surrounding medium, and, collecting at the bottom, it formed a layer of gradually increasing thickness. It the cathode were so long that it dipped into this layer, copper became dissolved from the former within the immersed area; and the deposition of copper could be effected most satisfactorily without a diaphragm with the aid of the apparatus shown in Fig. 132. A carbon anode, A, is used of such length that it dips into a collecting-trough formed at the bottom of the bath, and it is suspended opposite a cathode of sheet copper, K, of only half its length. The cuprous chloride liquor is admitted into the upper part of the deep electrolysing Cu, Cl2 Solution. Permanent Level of- Fig. 132.-Cohen's single-compartment electrolyte cell. vat, whilst the cupric chloride solution, which streams downward from the anode, is withdrawn through a siphon from the deepest part of the trough in which it collects. With a current density of 20 amperes per sq. metre of cathode area [1.86 amperes per sq. ft.] the deposit of copper answers all requirements in respect of both quality and quantity. The E.M.F. required under these conditions amounts scarcely to volt. It is evident that the Hoepfner process, and possibly that of Siemens' also, has been advanced a stage by this very ingenious and simple device. But this is the last step towards the direct treatment of sulphide ores and metallurgical products of which any description need be given in this place. Of new suggestions and patents there is no lack; but they will remain for the most part as they are at present. And it may be said that the quest * Zeitschrift für Elektrochemie, 1895, vol. ii., p. 25. for a practical method for the direct electrolytic treatment of copper ores is still open. Applications of Copper. -The uses of copper are very numerous on account of the valuable properties of the metal. Copper serves for the production of a large number of implements, apparatus, parts of machinery, and the like, both for household and for factory use; in electrical work it finds a special application in the form of wire. In the service of art it is used both in coppersmith work and in electrotyping. In the mixed-metal trades copper forms the basis of very many important alloys, such as bronze (including copper-tin, coppertin zinc, copper - manganese, copper - aluminium, and copper silicon), brass (copper-zinc), German silver (copper-nickel zinc), &c. For the production of such copper compounds as, for example, copper sulphate, cupric oxide, phosphor copper, so far as they do not occur as bye-products in metallurgical works, metallic copper, and the scrap from the rolling mill and the coppersmith's works are commonly used. = CHAPTER II. SILVER. Properties of the Metal.-Silver (Ag; atomic weight = 108; specific gravity 105) is a white, tough, malleable metal with a brilliant lustre, and a crystalline structure (in the regular system), and with a hardness intermediate between those of copper and gold. It has the highest electrical conductivity of all the metals. Its fusing point approximates 1000° C., and it is volatile at high temperatures, so that it may be distilled in the oxyhydrogen flame. A special characteristic of silver that is of great importance in connection with the refining and working of the metal is its power of dissolving oxygen when in the liquid state. At the moment of solidification, and after a thin crust of solid metal has formed over the surface, the oxygen which is given out from the still fluid metal within, forces its way through the outer crust, and often gives rise to a considerable loss of metal by the projection of small fragments to a distance; the phenomenon is known as the spitting, sprouting, or vegetation of silver. Among the metals which dissolve, or are dissolved by, silver, lead, mercury, copper, and zinc may be especially noted. Silver belongs to those metals which cannot be oxidised directly either at high or at low temperature, either in moist or in dry air. Of the metalloids, the halogens, and especially chlorine, are most liable to combine with silver; but it may be very readily united also with sulphur by direct fusion. Among the compounds of sulphur, hydrogen sulphide is able to attack this metal energetically. The best chemical solvents are nitric and concentrated sulphuric acids. Many metallic chlorides (CuCl2, HgCl2, FeCl) are capable of converting silver into its chloride, and so of making it soluble in other salts; cyanides also can form double salts, which are soluble in water by direct action upon either metallic silver, its haloid salts, or its sulphide. Occurrence of Silver in Nature.—Silver is found native; alloyed with gold, copper, and mercury; as chloride in horn silver, AgCl, as bromide and iodide in bromargyrite (AgBr), and iodargyrite (AgI) respectively; as sulphide in silver glance, Ag2S, and in combination with other sulphides, as in the case of red silver ores and fahlerz which may be given as examples of thioantimonites and thioarsenites. It is also present as sulphide in more or less considerable quantity in nearly all the sulphide ores of other metals. The extraction of silver is effected according to one (or to a combination) of the following principles : 1. Solution of the silver in another metal, with subsequent concentration and separation. 2. Separation of the silver by processes of chemical solution, sometimes with subsequent chemical precipitation. 3. Electrolysis. 1. SOLUTION OF THE SILVER IN ANOTHER METAL. Treatment with Lead.-Excepting in the case in which a material containing much zinc or copper has to be treated (such a material being subjected directly to the third process), the silver is obtained as an alloy with lead from most silver ores or metallurgical products, either by smelting with lead ores or by contact with a bath of metallic lead. For smelting with lead ores, only a poor material is used, and in no case would the proportion of silver exceed 10 per cent. The smelting processes employed will be very briefly described in the Chapter on Lead. In order, as far as possible, to avoid loss of silver, an alloy (work-lead) with less than 1 per cent. of silver should be produced; this is then submitted to a process which results in the silver being concentrated in a small portion of the lead, whilst the remainder of the latter is worked up into a soft or market lead, which should be almost free from silver. The concentration process may consist in melting the worklead and submitting it to a series of systematic crystallisations, the lead crystals being ladled out from the bath as fast as they form, or else the still fluid alloy of lead and silver (which now contains a larger proportion of the latter metal) is tapped off from the purer lead crystals at a later stage, and nearer to the time when the whole mass would solidify. By this method, which is known as the Pattinson process, an alloy containing somewhat less than 2 per cent. of silver is formed, and this is afterwards treated for the separation of the two metals. Another concentration process [the Parkes process] is based on the solubility of silver in zinc when the fused lead alloy is mixed with a small proportion of melted zinc, and the separation of a ternary alloy of silver, zinc, and lead, which is less fusible than lead itself. The crust of zinc alloy which forms on the surface of the lead on cooling the mixture a little, is removed and heated by itself so that the excess of lead may liquate out, and the residue is then distilled, or it is treated, when in the molten condition, with a current of water vapour. In the latter case zinc oxide is obtained mixed with grains of a rich silver-lead, and is separated from the remaining melted silver-lead alloy, washed, dried, and sold for use as a pigment. The residue from the washing operation is purified with sulphuric acid, and is submitted with the bulk of the silver-lead to the separation process. This process is distinguished from that of Pattinson by a more thorough removal of the silver from the work lead by the production of a concentration product that is richer in silver (containing as much as 12 per cent. of silver), and by a higher yield of silver. The third process, due to Rössler and Edelmann, consists in adding a small quantity of aluminium to the zinc, which is used as in the Parkes process. A very rich silver-zinc alloy (with 25 to 35 per cent. of silver is thus formed, and is treated by electrolysis for the extraction of the silver). (See Section 3.) The richest sorts of work lead, and the enriched products of the Pattinson and Parkes processes are melted in a reverberatory furnace, and are there exposed freely to the oxidising action of the air, in the process known as cupellation, until a crude metallic silver is obtained, lead and most of the impurities having been removed in combination with oxygen as oxides. For further purification the metal is refined either by a second oxidation in a smaller reverberatory furnace, or by melting with a little silver sulphate in a plumbago crucible. If the resulting silver contain gold, it must be further treated by either the second or third systems. Experiments have also been made in the direction of separating argentiferous work-lead directly by electrolysis. (See Section 3.) Amalgamation.- Mercury dissolves silver in considerable quantities, and can also decompose most of its salts [but not the chloride] and the sulphide with the separation of metallic silver, which dissolves in the excess of mercury used. The silver may therefore be extracted from ores which contain it either as metal, certain salts, or pure sulphide, by treatment with mercury, that is to say, by amalgamation. The mercury is separated from the resulting amalgam by distillation, and is thus recovered by condensation, whilst the silver remains behind in the metallic state, but generally alloyed with other metals, such as gold or copper. If it be required to treat other silver ores by this process, the silver must be first converted into chloride by chlorination, or by a chloridising roast [and a reducing agent, such as iron, should be used in conjunction with the mercury]. A large number of amalgamators have been devised for bringing the ore into the most intimate mixture with the mercury, and the more important of them are described in the newer textbooks of metallurgy. The use of electricity in hastening amalgamation has been frequently proposed, but has not come permanently into use. Some of the more important of the patented processes will be referred to in the Chapter on Gold, for the extraction of which metal they were originally devised. Solution of the Silver in Copper. When ores containing the precious metals and copper are treated by smelting processes, a part of the silver and most of the gold are liable to pass into the bye-product that contains the copper, and, therefore, under certain circumstances, into the copper itself. Ores containing the precious metals are not thus smelted with copper ores, as they might be with those of lead, in order to obtain the gold and silver alloyed with the copper. Copper products containing the precious metals are treated according to systems (2) and (3). 2. SEPARATION OF THE SILVER BY PROCESSES OF CHEMICAL SOLUTION. The Ziervogel Process.-The matte smelted from the argentiferous copper ores is so roasted that in the first stage, among other products, copper sulphate is formed, whilst in the second stage (after previous crushing of the roasted matte) a double decomposition takes place between copper sulphate and silver sulphide with the formation of silver sulphate. The latter salt is then extracted from the roasted charge by means of hot water and acid solutions of copper sulphate, and the silver is finally precipitated from the resulting liquors by metallic copper. The Augustin, Patera, and Kiss Processes.-The ores are submitted to a chloridising roast, and the silver chloride is extracted in the Augustin process by concentrated brine, in the Patera method by sodium thiosulphate solution,* and in the Kiss modification by calcium thiosulphate solution. In the first * [This salt is still commonly known as hyposulphite of soda, but as this term belongs more properly to a different compound altogether, the name which is required by the modern system of nomenclature is adopted in the text.-TRANSLATOR.] |