EXHAUSTED SOLUTIONS. ] UXDULATORY FORCES. ELECTRO-METALLURGY. 



237 



cyanide liquid, the silver may bo recovered in the me- 

 tallic state thus : Evaporate the solution nearly to dry- 

 ness ; reduce the resulting salt to powder ; mix it with 

 its own weight of a mixture of one part of nitrate of 

 potash and two parts of common salt; and roast the 

 whole in an iron pan to dryness. Fuse the dried mix- 

 ture at a bright red heat in an earthen crucible, until the 

 silver collects at the bottom of the vessel in a melted 

 state ; then pour it slowly into a large quantity of water. 

 The resulting granules of silver should not be used in 

 making a new plating liquid, because they generally con- 

 tain copper derived from the articles suspended in the 

 plating solution (see 151), but should be exchanged at a 

 silver refiner's for pure silver. 



224. " The crystallised double cyanide of gold and 

 potassium fuses and effervesces by heat, and is resolved 

 into cyanogen gas, ammonia, and cyanide of potassium, 

 if air be present ; its complete decomposition requires a 

 strong red heat. When it is strongly ignited, and mixed 

 with an equal weight of carbonate of potash, a button of 

 metallic gold is obtained. When heated with sulphuric 

 acid, it gives off hydrocyanic acid gas, and, after igni- 

 tion, leaves a mixture of gold and sulphate of potash. 

 Iodine sets free cyanogen gas, forms iodide of potassium, 

 ami throws down the cyanide of gold. The aqueous 

 solution of cyanide of gold and potassium gilding liquid, 

 when mixed with sulphuric, hydrochloric, or nitric acid, 

 K! >wly deposits cyanide of gold ; and, when boiled with 

 hydrochloric acid, it is completely resolved into cyanide 

 of gold and chloride of potassium. Similar effects are 

 "I by sulphuric or nitric acid, and even by oxalic, 

 ric, and acetic acid." 



1 1 .'-.traction of Gold and Silver from Exhausted 

 Solutions. " To obtain the remaining gold from gilding 

 solutions which have become inactive, they should be 

 evaporated to dryness, the residue finely powdered, and 

 intimately mixed with an equal weight of litharge, fused 

 at a strong red heat, and the lead extracted from the 

 alloy button of gold and lead by warm nitric acid ; the 

 gold will then remain as a loose yellowish- brown spongy 

 mum." 



L'-'J. The following extracts are from the works of 

 Bcettger, J. Pr. Chein., 36, 1G9; Eisner, Redtel, Hessen- 

 berg, J. Pr. Chem., and other foreign writers : " I have 

 undertaken a series of researches upon this subject, and 

 hasten to communicate the results to the public; but, 

 before proceeding to the communication, I think it 

 necessary to mention the results of the experiments upon 

 which are based the methods given further on for ex- 

 tracting both the silver and the gold of old cyanide of 

 potassium liquids. 



227. "1st. If wo add hydrochloric acid to a solution 

 of silver iu cyanide of potassium, until the liquid exlu- 

 bits an acid reaction, we obtain a white precipitate of 

 chloride of silver, which, when submitted to heat, melts 

 into a yellow mass. If this were cyanide of silver, the 

 application of a red heat would have loft a regulus of 

 silver. The addition of the hydrochloric acid precipi- 

 tates all the silver present in the liquid in the form of 

 chloride of silver. 



228. "2nd. If wo evaporate a solution of silver in 

 of potassium to dryness, and heat the residue to 



redness until the mass is in a state of quiot fusion, and 

 has assumed a brown colour, there remains, when we 

 wash the mass with water, metallic and porous silver. 

 The wash-waters, when filtered, still contain a little silver 

 in solution ; becatiso, if hydrochloric acid be added to 

 them, it will produce a precipitate of chloride of silver. In 

 evaporating aiid calcining a solution of gold in cyanide 

 of potassium, the result is the same we obtain metallic 

 gold. The wash-waters, acidulated with hydrochloric 

 acid, give, when treated with sulphuretted hydrogen, a 

 brown precipitate of sulphide of gold ; and, with the salt 

 of tin, a violet precipitate purple of Cassius a proof 

 that these liquids still contain a little gold in solution. 



li'J!!. "3rd. If we pour upon finely-divided silver for 

 instance, silver-leaf, or silver precipitated in the porous 

 state by zinc from a solution of silver a concentrated 

 solution of cyanide of potassium, at the ordinary tempe- 



rature, and shake it frequently, the liqxiid, at the end of 

 a certain time, exhibits silver in solution ; and by adding 

 hydrochloric acid to it, we produce an abundant precipi- 

 tate of chloride of silver. This experiment explains why, 

 in the wash-waters of the various combinations of gold or 

 silver with cyanide of potassium, we can still demon- 

 strate the presence of gold and silver, after the most 

 minute separation. 



230. "4th. When hydrochloric acid, or ordinary sul- 

 phuric acid, is added to a solution of cyanide of copper 

 and cyanide of potassium, until the liquid exhibits an 

 acid reaction, there results a reddish-white precipitate, 

 which is a cyanide of copper in the anhydrous state. If 

 the precipitate be well washed and boiled in potash lye, 

 protoxide of copper will be separated, of a beautiful red 

 colour ; and if to the filtered alkaline liquid we add a 

 solution of green copperas (sulphate of iron), a dirty-blue 

 precpitate will be obtained. A solution of carbonate of 

 soda furnishes the same results, and yields, with the cop- 

 peras, the same dirty-blue precipitate. If the reddish- 

 white precipitate be dissolved in pure nitric acid, and a 

 solution of nitrate of silver added to it, an abundant 

 white precipitate will be produced, which, when washed, 

 dried, and calcined, yields silver in the metallic state a 

 proof that the precipitate is cyanide of silver. 



231. "The reddish- white precipitate is soluble in an 

 excess of hydrochloric acid, in nitric acid, and in aqua 

 regia ; it is also soluble in aqueous ammonia, and in a 

 solution of cyanide of potassium. 



232. "5th. If we potir hydrochloric acid into a very 

 pure solution of gold in cyanide of potassium, there will be 

 slowly formed, at ordinary temperatures, and immedi- 

 ately on the application of heat, a yellow precipitate, 

 which is cyanide of gold ; the filtered liquid which has 

 given this precipitate, still contains a little gold in solu- 

 tion. In evaporating to dryness, fusing, dissolving, and 

 filtering afresli, there is found upon the filter the remain- 

 der of the gold. 



233. " When a solution of silver prepared for silvering 

 articles of bronze or of brass, has been employed a certain 

 time for that purpose, the precipitate produced in it by 

 the addition of hydrochloric acid, is not pure white, but 

 reddish, in consequence of the reddish-white cyanide of 

 copper which is precipitated with it ; for we know that 

 those silvering liquids which have been used for some 

 time, contain copper in solution. The same thing occurs 

 with the solutions for gilding, in which articles of silver, 

 copper, bronze, and brass, have been gilded for a long 

 time ; the liquid contains, after a certain time of service, 

 not only gold, but also silver and copper. This case 

 presents itself especially when gilded articles of silver, 

 containing copper or other alloys of silver, are in the 

 solution of gold ; then the precipitate of cyanide of gold, 

 produced by the addition of hydrochloric acid, does not 

 possess its proper pure yellow colour. It has happened 

 to mo to observe a precipitate of this kind, which, instead 

 of being yellow, was green ; and, in fact, articles of iron 

 have been gilded in a solution, and the precipitate con- 

 tained, besides cyanide of gold, Prussian blue, so as to 

 be demonstrated in an examination, which consisted in 

 boiling the green precipitate in aqua regia, filtering to 

 separate the dirty-green residue, evaporating the filtered 

 liquid to dryness, and dissolving the dry salt in water 

 acidulated with hydrochloric acid ; the addition of sul- 

 phate of iron to this new liquid gave a brown precipitate, 

 and the salts of tin a reddish-brown precipitate. In 

 treating by aqua regia, the cyanide of gold was then de- 

 composed, and converted into chloride of gold. 



234. " Based upon the preceding facts, we may found 

 several methods for recovering all the silver and gold of 

 old cyanide of potassium solutions. The extraction of 

 these precious metals may be effected either by the wet 

 or by the dry process. 



235. "Extraction of Silver by the Wet Method. Add 

 hydrochloric acid until the liquid exhibits a strongly acid 

 reaction (230). The precipitate of chloride of silver 

 which is thus obtained, will bo, as we liave already said, 

 of a reddish-white colour, because of the cyanide of cop- 

 per which is precipitated with it when the solution has 



