782 PROPERTIES AND USES OF SILVER. This metal upon the small portion of sulphuretted hydrogen which is constantly floating in the air, especially of large towns. tarnish is readily removed by means of a solution of cyanide of potassium. The best solvent for silver is nitric acid, which, if diluted with an equal bulk of water, acts upon the metal with great violence, dissolving it rapidly and evolving nitric oxide, while nitrate of silver is formed. Hydrochloric acid acts but slightly upon it. Aqua regia attacks it more rapidly. Diluted hydriodic acid attacks it with evolution of hydrogen. Boiling oil of vitriol dissolves it with evolution of sulphurous anhydride. If common salt be fused in a silver dish, or if it be moistened and left in contact with silver, it gradually corrodes it; soda being formed by the absorption of oxygen from the air, while the liberated chlorine attacks the silver. Neither the hydrated alkalies nor their nitrates exert any considerable action upon it, whether in solution or when fused by heat, and hence crucibles for the fusion of refractory minerals with caustic potash are commonly made of this metal. The value of silver as a medium of exchange has caused it to be adopted as such by all civilized nations from the earliest ages of the world. When alloyed with certain proportions of copper it is used for the current coin of the realm, and for the various articles of plate. From its superior power of reflecting light, it forms the best surface for the reflectors employed in lighthouses at sea. (932) Extraction of Silver from its Ores.-Silver is frequently met with in the native state; either pure, when it occurs in fibrous masses, or crystallized in cubes or octohedra; or sometimes combined with gold, mercury, or antimony: generally, however, it is found in combination with sulphur, mixed with sulphides of lead, antimony, copper, and iron. The mines of Peru and Mexico are the most extensive sources of silver. In Europe, those of Kongsberg in Norway, and of Schneeberg and Freyberg in Saxony, are celebrated: there are also numerous other mines from which smaller quantities are obtained. The ores of silver occur usually among the primitive rocks, frequently in calcareous veins, traversing either gneiss, or slaty and micaceous deposits. Sulphide of lead is nearly always accompanied by small quantities of sulphide of silver, and a considerable quantity of silver is extracted during the refining of lead by Pattinson's process (891), as well as by cupellation (892). At Freyberg, silver is for the most part obtained from the FREYBERG PROCESS OF AMALGAMATION. 783 The plumbiferous ores sulphide by the method of amalgamation. are in this case rejected, as they are not adapted to this method of proceeding, but are treated in the manner already described. The ores are usually sorted, so that they shall contain about 0'24 parts of silver in 100, or about 80 ounces per ton of ore, and not more than 1 per cent. of copper; the proportion of iron pyrites is not allowed to exceed, or greatly to fall short of 35 per cent. The metalliferous mass, after being reduced to a coarse powder, is mixed with a tenth of its weight of common salt, and sifted, to ensure its intimate incorporation : it is then roasted, at first at a low red heat; during this operation, care is taken to keep the mixture constantly stirred, in order as far as possible to prevent it from concreting into lumps. Meantime arsenic and antimony are expelled in dense white fumes of arsenious anhydride and oxide of antimony, and the sulphides of the other metals are partially oxidized; the silver obtains chlorine from the salt, the sodium of which unites with oxygen and sulphur, chloride of silver and sulphate of sodium being formed; the copper and the iron are changed partly into sulphates, partly into chlorides, and partly into oxides, as the equations subjoined will show : 2 Ag2S+ 2 NaCl + 2 →2 = 2 AgCl + Na2SO4 ; 2 ЄuS+4 NaCl +4 →2=Єu2Cl2+Cl2 + 2 Na2SO4 2 6 During the early stages of this operation, fumes of sulphurous anhydride are given off abundantly; and the roasting is continued until these have in great measure given place to those of chlorine and perchloride of iron. A charge of 4 cwt. requires 6 hours' roasting. The roasted mass is now raked out of the furnace, and allowed to cool: it is next sifted in order to separate the lumps, which are powdered and again submitted to the same operation. About 85 per cent. of the silver is thus converted into chloride at the first roasting. The portions which have passed through the sieve are ground to powder, and passed through a bolting sieve to procure a very fine meal. The powder is next placed, with from a third to half its weight of water, in large casks, which are charged with half a ton of the ore. These casks are caused to revolve upon horizontal axes, about 20 times per minute; I cwt. of scrap wrought iron is then introduced into each cask, and after the lapse of an hour, 5 cwt. of mercury is added, after which the casks are again made to revolve for about 20 hours; during this operation a slight rise of temperature is observed. The powder 784 EXTRACTION OF SILVER BY AMALGAMATION. when placed in the casks consists principally of chloride of silver mixed with large quantities of cupric sulphate and cupreous chloride, as well as of ferric chloride, with a variable proportion of the oxides of copper and iron. The object of agitating the mixture with the iron before adding the mercury is to reduce the ferric chloride to ferrous chloride in the first instance; if this precaution were not taken, the mercury would be partially converted into calomel, which would not subsequently be decomposed, and would thus be lost: the excess of iron afterwards removes the chlorine from the chloride of silver and cupreous chloride, and the sulphuric acid from the copper. = Fe+ FeCl 3 FeCl2 ; 2 AgCl + Fe=FeCl2+ Ag2. The presence of the mercury favours this reaction, by establishing a voltaic current, and the silver and copper thus set at liberty unite immediately with the mercury, forming a liquid amalgam. At the expiration of 18 or 20 hours the casks are filled up with water, and are again set in motion for a couple of hours to allow the amalgam to be washed out of the spent materials; after which the fluid amalgam is drawn off into sacks of ticking; these sacks form a kind of rude filter, through which the greater part of the mercury runs into a stone trough, leaving behind it a soft solid containing from 15 to 17 per cent. of silver. The mud in the casks is again submitted to washing; the residual amalgam subsides, owing to its greater density, and the lighter portions are rejected. The filtered part of the mercury, which retains a small quantity of silver, is used again for the amalgamation of a fresh portion of ore. The silver in the solid amalgam has now to be FIG. 357. separated from the remaining mercury; for this purpose it is placed in trays, supported on a tripod, c, fig. 357, under a large distillatory iron bell, B, round the upper part of which a fire, a, is lighted; the bell and its contents are thus brought to a red heat, by which means the mercury is driven off; its vapour descends, and is condensed in the water contained in the vessel, D. The operation is generally performed on 5 cwt. of amalgam at a time, and occupies 8 hours. The resi dual spongy mass of silver and copper is then fused and cast into ingots, which in the Saxon mines AMERICAN PROCESS OF AMALGAMATION. 785 contain usually about 70 per cent. of pure silver and 28 of copper.* (933) American Process of Amalgamation.-In the mining districts of Mexico and Chili, where fuel is expensive, and where ores are often worked of a much poorer description than in Europe, the process of amalgamation is different. A good deal of the silver occurs in the native state, so that it unites directly with the mercury. The mineral is stamped and ground to a fine powder in mills, then moistened with water, and mingled with from 1 to 5 per cent. of salt; the mixing is effected by the trampling of horses during 6 or 8 hours. The ore thus blended with the salt is allowed to remain undisturbed for some days, after which an addition of or T of its weight of what is technically termed magistral is made. This substance consists of roasted copper pyrites, and contains about 10 per cent. of sulphate of copper, the remainder being sulphate of iron and other impurities; mercury, to the extent of twice the quantity of silver that the ore contains, is then added, the mixture being effected, as before, by An improvement upon this process has been introduced by Augustin, who dispenses with the use of mercury altogether. After the ore has been roasted first by itself, and again a second time with chloride of sodium, it is digested in a concentrated solution of common salt; such a solution dissolves chloride of silver readily: a dilute solution of chloride of sodium exerts little or no solvent action; and the concentrated liquid when diluted deposits the chloride of silver which it had previously dissolved. In practice it is found better, instead of diluting the liquid, to digest it upon metallic copper; the chloride of silver is decomposed, chloride of copper is formed and dissolved, whilst metallic silver is precipitated. The presence of chloride of copper in the solution of the chloride of sodium does not prevent the liquid from being employed again for the extraction of chloride of silver from fresh portions of the roasted ore. Another important improvement in the operation was made by Ziervogel. He avoids the preparation of chloride of silver entirely, and merely roasts the sulphurous ores in such a manner that the sulphates of iron and copper are completely decomposed, whilst the sulphate of silver, which withstands a much higher temperature, remains undecomposed in the mass. In this operation the powdered ore is roasted till it gives off no odour of sulphurous anhydride and yields no sensible amount of sulphate of copper when thrown red hot into water: boiling water then dissolves out the sulphate of silver, but the oxides of copper and iron remain undissolved. The silver is precipitated from the liquid by means of metallic copper as before. A small quantity of silver is still retained in the undissolved residue, from which it may be advantageously extracted by the method of Augustin. Both these processes have been patented and practised on a large scale in England. Percy has suggested the use of hyposulphite of sodium as a solvent for the chloride of silver. The mineral after roasting with chloride of sodium is washed first with hot and then with cold water, and afterwards is digested in a dilute solution of the alkaline hyposulphite, which dissolves the chloride of silver; from this solution the silver is precipitated as sulphide, by means of sulphide of sodium, whilst hyposulphite of sodium is reproduced as before. II. 3 E 786 AMERICAN PROCESS OF AMALGAMATION. the trampling of horses. It is again allowed to rest for 16 or 20 days during this period a considerable portion of the silver becomes united with the mercury, forming a hard, brilliant amalgam, and at the same time a large quantity of calomel is formed. Another equal quantity of mercury is added, and a still longer interval of rest is allowed; then a third dose of mercury to the same extent follows; by this last addition a fluid amalgam is obtained, which is separated by washing, filtered, and the mercury is expelled from the silver by distillation. The quantity of mercury consumed in this process varies from 130 to 150 parts for each 100 parts of silver extracted, great waste being incurred owing to the formation of calomel, which is not recovered. It is calculated that up to the close of the last century, 6 million cwt. of mercury had thus been lost by the processes adopted in the American mines in the course of 2c0 years.* The theory of this operation is rather obscure. The sulphate of copper of the magistral, and the chloride of sodium decompose each other, cupric chloride and sulphate of sodium being formed. Cupric chloride, in the presence of metallic silver, is converted into cupreous chloride, whilst chloride of silver is produced; 2 Єu"Cl2+Ag2=Єu ́¿Cl2+2 AgCl. When cupreous chloride, with excess of common salt and water, is brought into contact with sulphide of silver, the cupreous chloride is dissolved by the solution of chloride of sodium; this solution of cupreous chloride decomposes the sulphide of silver, and is converted into cupreous sulphide, whilst chloride of silver is formed; Eu',Cl2+ Ag2S=Єu'2S +2 AgCl. The excess of salt dissolves the chloride of silver, and the addition of mercury decomposes this dissolved chloride; calomel is formed, and an amalgam of silver is procured; 2 AgCl + 2 Hg=2 HgCl+Ag2. If too much magistral be added, an excess of cupric chloride (EuCl) is produced; this state of the mixture is easily perceived, for in such a case the globules of mercury in the mixture appear to be too minutely divided; the addition of lime then becomes necessary in order to decompose the excess of the cupric chloride, otherwise this salt would reconvert the silver into chloride, and the mercury into calomel. (934) Separation of Silver from Copper by Liquation.-It occa *Dumas proposes to recover this mercury by treating the washed residue with a quantity of chloride of lime or nitrate of sodium, proportioned to the mercury they contain, then adding hydrochloric acid in slight excess; the calomel would thus be converted into corrosive sublimate. This is to be removed by methodical washing, and the mercury precipitated by copper. The solution of copper thus obtained would furnish the magistral required for a new operation upon fresh ore. |