For a long time, metallurgists have done series of important researches upon the recovery and extraction of gold and silver from old cyanide solutions. According to the results, if hydrochloric acid is added to a solution of silver in cyanide until the liquid exhibits an acid reaction, it is possible to obtain a white precipitate of silver chloride, which, when submitted to heat, melts into a yellow mass. If this was silver cyanide, the application of a red heat would have left a regulus of silver. The addition of the hydrochloric acid precipitates all the silver present in the liquid in the form of chloride.
If the solution is evaporated to dryness, and heat the residue to redness, until the mass is in a state of quiet fusion, and has assumed a brown color, there remains, when mass is washed with water, metallic and porous silver. The wash waters, when filtered, still contain a little silver in solution, because, if hydrochloric acid is added to them, it produces a precipitate of silver chloride. In evaporating and calcining a solution of gold in cyanide, the result is similar; it is possible to obtain metallic gold. The wash waters, acidulated with hydrochloric acid, give, when treated with hydrogen sulphide, a brown precipitate of gold sulphide; and with the salt of tin a violet precipitate (purple of Cassius), a proof that these liquids still contain a little gold in solution.
Extraction of Silver by the Wet method involves the addition of hydrochloric acid until the liquid exhibits a strongly acid reaction. The precipitate of silver chloride which is thus obtained, will be a reddish-white color, because of the copper cyanide is precipitated with it when the solution has been used a long time for silvering objects containing copper. In this precipitation by hydrochloric acid, there is hydrocyanic acid gas set free, therefore the operation should only be performed in the open air, or in a place where there is good ventilation; if the precipitate is very red, it must be treated with hot hydrochloric acid, which will dissolve the copper cyanide. The silver chloride having been washed with water, must be dried and then fused with sodium carbonate in a crucible coated with borax, in the ordinary manner for obtaining metallic silver.
This method is very simple in its application, and very economical, considering that by the aid of the hydrochloric acid all the silver contained in the solution of cyanide is precipitated, and there remains no trace of it in the liquid. But the quantity of hydrocyanic gas which is disengaged is a circumstance which must be taken into serious consideration when operating on large quantities of silver solution, the vapor of which is most deleterious, and nothing but the most perfect ventilation, combined with arrangements for the escape of the poisonous gases, will admit of the process being carried on without danger to the workmen; when, however, it is important to take precautions. The liquid should be poured into very capacious vessels, because the addition of the acid produces a large amount of froth.
If we consider the extraction of silver by the Dry Method, the solution of silver cyanide may be evaporated to dryness, the residue fused at a red heat and the resulting- mass, when cold, is washed with water. The remainder is the silver in a porous metallic condition. There still remains in the wash waters a little silver, which may be precipitated by the addition of hydrochloric acid.