722 GOLD AND SILVER REFINING 







and allow to stand for half an hour, during which time tho gold is precipitated. The 

 object in adding this amount of sulphuric acid is to form a clear solution, that tho 

 gold maybe enabled to settle to the bottom; water could not bo added, because it, 

 would probably cause an explosion by the heat evolved in its combination, and because 

 sulphate of silver is not very soluble in water, while it is soluble to a very large 

 extent in hot sulphuric acid. At the end of half an hour the clear liquor, containing 

 in solution the silver and copper as sulphates, is decanted and mixed with so much 

 water as shall reduce it to a sp. gr. of from 1-2080 to 1-2605, and well stirred. Copper 

 plates are then introduced, while the solution is kept hot or boiling by a jet of steam. 



The silver salt is decomposed by the copper plates, and the copper passes into 

 solution as sulphate of copper, so that at the end of the precipitation the solution con- 

 tains the copper of the original alloy, as well as tho copper which has been used to 

 precipitate the silver. The silver precipitates or falls to the bottom in a finely divided 

 or spongy form, and it is commonly thought that the whole of the silver is thrown 

 down when a portion of the solution is not rendered turbid by a solution of chloride 

 of sodium ; but in the presence of a strongly acid solution this test is not to be relied 

 on for minute quantities ; therefore, in some refineries, the solution is allowed to rest 

 for days together in leaden cisterns in which copper plates are placed, so that by these 

 means the last traces of silver are obtained. 



If the amount of gold be very minute, the original solution is well stirred and then 

 allowed to settle for some time ; when finely-divided gold, mechanically mixed with 

 crystals of sulphate of silver and crystals of sulphate of copper, is found at the bottom. 

 This deposit is boiled with water, and is then transferred to a vessel in which it is 

 kept hot, and is brought into contact with suspended copper plates, by which the silver 

 is rendered metallic, and falling to the bottom of the vessel, mixes with the gold. Tho 

 mixed precipitate of silver and gold is then dried, melted, and granulated, and treated 

 with sulphuric acid, as in the process already described. By this extra process the 

 gold becomes concentrated by the removal of the silver, and is then thrown down in 

 larger and more easily collected particles. "When the gold is finely divided and 

 precipitates slowly, the following plan is sometimes adopted : The whole precipitate, 

 containing finely-divided gold mixed with sulphate of silver, is washed well with 

 warm water, and left to rest. The sulphate of silver is dissolved, whilst the gold settles 

 to the bottom of the vessel, but it is still mixed with a minute quantity of sulphate of 

 silver. It is drained and placed in the retort or boiler of cast iron, and boiled with 

 sulphuric acid ; this boiling is twice repeated, and at last a very diluted solution of 

 sulphate of silver is obtained ; but by the boiling the gold has assumed a form which 

 enables it to precipitate rapidly ; in fact, the flocculent sponge becomes a mass of dense 

 particles, which fall readily to the bottom, are collected and well washed, to free them 

 from silver, and are then dried ready for melting. 



The solution of sulphate of silver is evaporated in leaden vessels by the agency of 

 steam, until it becomes saturated, and is then allowed to stand for an hour that all 

 the gold may separate, and is afterwards drawn off either by a tap placed about half an 

 inch from the bottom of the vessel, or by a siphon, and is then treated with copper 

 plates, as already detailed. 



In all cases the precipitated spongy silver is carefully washed to free it from sul- 

 phate of copper, and dried by heat or by hydraulic pressure ; but if dried by pressure 

 the masses obtained are found to contain from 8 to 10 percent, of water, and are 

 therefore dried by a gentle heat to avoid the breaking-up of the masses, from the 

 sudden formation of steam, as well as to save the chance of destroying the pot of 

 Picardy clay in which the silver is melted when it has been dried. 



After melting, the silver is found to retain traces of gold, which are so minute us 

 to be overlooked, since the cost of recovery would exceed the value of the gold to be 

 recovered ; but the silver is found to be alloyed with from 5 to 6 thousandths of its 

 weight of copper, which appears to be left in tho form of sulphate, notwithstanding 

 the washings to which tho silver has been subjected. It is practically impossible to 

 wash away the last traces of sulphate of copper. This small amount of copper is of 

 little importance, since it amounts to but f> parts of copper alloyed with 995 parts 

 of silver, yet this may be removed by fusion and treatment with nitrate of potassa. 



During the whole process, even if copper be not present in the original mass of 

 metal to be refined, it is to be observed that copper plates are used for precipitating 

 the silver; therefore sulphate of copper is formed in considerable quantities, and as this 

 salt has a high commercial value as giving the l>aso for many colours used in painting 

 and paper-hangings, as well as for agricultural purposes.it becomes desirable to ol.t.-iin 

 this salt in a saleable form. Tho solution is thm-tore evaporated to a sp. gr. of 

 1'3804, and allowed to rool, when crystals deposit ; hut since sulphate i.f copper de- 

 posited from strongly acid solutions is mixed with the anhydrous salt, tho whole mass 

 of crystals is re-dissolved in warm -water, and allowed to stand in leaden vessels about 



