464 



SILVER 



the ore, a regulus consisting chiefly of sulphide of 

 iron and some sulphide of copper is first formed. 

 When in the molten state this is mixed with one- 

 third of its weight of lead, which, after taking up 

 most of the silver, separates by its own weight. 

 From this silver-lend the silver, amounting to 

 6 per rent, of the alloy, is separated by cu|iellation. 

 At Wyandotte a slightly richer silver-lead is pro- 

 duced which is also cupelled. 



Combined lead and silver smelting, aa conducted 

 in Great Britain from native ore (argentiferous 

 galena), is described under LEAD. There are no 

 silver ores properly so called produced in the United 

 Kingdom, but foreign ores are smelted along with 

 lead ores or rich lead slags. 



Liquation Protest. Silver can be separated from 

 argentiferous copper by this process, which consists 

 in heating a fused mixture of the copper with a 

 large excess of lead to a temperature above the 

 melting-point of the latter, but below that of the 

 copper. In this state of matters the lead licj nates 

 or sweats out of the mass, carrying with it the 

 greater part of the silver which the copper con- 

 tained. This silver can then be extracted from the 

 lead by cupellatimi. 



CupelUition. This process consists in melting 

 lead rich in silver on the hearth of a small furnace, 

 and blowing air over it- surface, the oxygen "f 

 which, under the influence of the heat, rapidly 

 converts the lead into litharge or plumbic oxide. 

 Fused litharge has the property of dissolving the 

 oxides of some other metals, such as copper, rinc, 

 tin, antimony, &c. Silver, however, is not oxidised 

 bv this treatment. The result of the operation is 

 that the whole of the lead put in the furnace, to- 

 gether with small quantities of other oxidisable 

 metals (present as impurities), are removed as 

 oxides, and the silver is left on the bed of the fur- 

 nace. In the English cupellation-furnace the bed 

 or hearth is movable, and is formed of an oval iron 

 frame or test rammed full of powdered bone-ash. 

 The German cupellation-furnace (shown in fig. 3) 



Kig. 3. -Silver Cuj>ellation-furnaoe : 



a, hmrth of marl or clajr ; l>, bricks ; c," bed of slag ; d, movable 

 cover ; , , tu yen* connected with bellows |Jt fireplace ; a. 

 crmne for lining cover. 



is of larger capacity than the English, and has a 

 movable domed cover constructed of iron. The 

 diameter of this furnace is about 10 feet, and the 

 hearth is covered with marl. These furnaces differ 

 only in details and in the way in which they are 

 charged. In the United States what is termed a 

 steit/ water-back cupel a hollow casting through 

 which water flows is used in the first stage of 

 cupellation. 



There are three processes for teparating tUver 

 ' I practised on a large scale. One of these 

 very old method. It is called 'parting' by 



nitric acid. The alloy, which may consist of one 

 part by weight of gold to two of silver or of one of 

 the former to three of the latter, in melted and 

 granulated i.e. formed into small beads. It is 

 then Ixiiled (in platinum or earthenware vessels) 

 with nitric acid, of 1 -4 sp. gr. and an equal bulk of 

 water, for several hours, till tln> -il\i-r is almost all 

 converted into nitrate, the gold forming the residue 

 after the dissolved silver is removed. To this solu- 

 tion of nitrate of silver common salt is added to 

 precipitate the silver as chloride, from which the 

 metal is reduced by the action of granulated zinc 

 and weak sulphuric acid. The silver is then 

 washed, pressed, dried, and melted, producing bars 

 almost pure from the :illo\s MBCculy treated. 



Another method of 'parting' i by bailing the 

 alloy in concentrated sulphuric acid. Auriferous 

 silver, eit]ier granulated or in bars, is placed in 

 cast-iron pots almg with sulphuric acid, which is 

 cautiously raisedtotheboiling ]>oint. Thesilver, if it 

 does not much exceed 200 Ib. in weight, is dissolved 

 in about eight hours, but sooner if granulated. 

 The gold fans down as a sediment, and the sul- 

 phate of silver is drawn off by a platinum siphon 

 or otherwise. The solution of" sulphate of silver is 

 then diluted with water, and the metal precipitated 

 by scrap-iron. Impurities in the alloy sometimes 

 render the sulphuric acid method of parting trouble- 

 some. 



Separation of silver from argentiferous gold is 

 now effected by chlorine, a method patented by F. 

 B. Miller in 1867, and soon afterwards used at the 

 Sydney mint and at other places. Gold, to the 

 amount of 600 or 700 ounces, is melted in a clay 

 crucible, and a little melted borax thrown in. 

 Chlorine gas in then admitted through a suitable 

 pipe, the end of which almost reaches the bottom 

 of the crucible, so that the gas is forced to bubble 

 up through the molten gold. At first volatile 

 chlorides of some of the baser metals, which may 

 l>e present in very small quantity in the alloy, 

 escape through holes in the cover of the crucible, 

 but the chloride of silver does not. Chlorine is 

 supplied till it is found that no more is absorbed, 

 which shows that practically all the silver has been 

 changed into chlonde. When this is the ease the 

 crucible is allowed to cool sufficiently to allow the 

 gold to solidify, and the red-hot liquid chloride of 

 silver is then formed into slabs in moulds. This 

 chloride generally contains 2 per cent, of gold, but it 

 is eliminated by adding metallic silver, with which 

 the gold forms an alloy. From the slabs of chloride 

 of silver the metal is reduced by using them as 

 one of the elements of a galvanic battery. 



The production of tilver has increased enormously 

 since the discovery of the rich deposite in the 

 western states of North America, the development 

 of which may be said to have l>egun with the sur- 

 face-workings of the Comstock lode, Nevada, in 

 1859. But in 1861 the yield of the United States 

 was still comparatively small, amounting in value 

 to not more than 400,000. In 1871 it had risen 

 to 4,600,000 ; in 1881 the value of the production 

 was 8,600,000 ; and in 1889 it reached the yearly 

 total of 1'2,92!,000. Colorado and Montana are 

 now the great silver-producing states. The direc- 

 tor of the United States mint gives the value of the 

 silver production for I8H!t in the following coun- 

 tries: Mexico, 11,103,400; Chili, Bolivia, and 

 Peru together, 4,085,000; European countries, 

 2,136,000; and he makes out the total production 

 i >f the world in that year to be 32,583,000. 

 (The annual produce of the silver districts in the 

 Barrier Range region, New South Wales, now 

 amounts in value to nearly 2,000,000. The silver 

 district of Zeehan, on the west coast of Tasmania, 

 from which in 1891 galena ore containing 110 

 ounces of silver per ton reached England, promised 



