It has been noted the recovery and precipitation of gold may be performed by adding minerals acids. Since the process involves the liberation of hydrocyanic acid (prussic acid) the operations must always be conducted with the utmost caution, and should always be carried on in the open air. It is well to remark here that when acid is added to a cyanide solution, not only hydrocyanic acid but also carbonic acid is liberated; and since this heavy gas cannot escape through the flue of an ordinary chimney, owing to its gravity, but flows over the vessel in a dense white vapor, the operator should be careful not to disturb these fumes, so as to cause them to rise upward, but to allow them to flow over the sides of the vessel and escape into the open air, where they will be dispersed by the wind.
The reduction of the gold by the dry way is, however, less hazardous, not so offensive, and fully as economical. To precipitate gold from cyanide solutions, hydrochloric acid is to be gradually added, until no further precipitation takes place; the solution may be heated and the vessel set aside to cool. The precipitate, which is of a yellowish, color, must then be separated from the supernatant liquor by decantation, and then filtered. Since a small portion of gold, however, still remains in the liquor, it must not be thrown away, but should be heated, and zinc filings added, which, will throw down the remainder of the gold; the clear liquor is now to be poured off, and the residue treated with dilute hydrochloric acid, to remove excess of zinc, and after washing, the deposit is to be added to the other portions.
Ignite and fuse the mixture in a platinum or ordinary crucible, with an equal weight of potassium sulphate. Dissolve the saline residue in sulphuric acid, then wash it with water, when perfectly pure gold will remain. Some metallurgists recommend the following method at small for recovering gold from old solutions: The solution should be evaporated to dryness, the residue then finely powdered, and intimately mixed with an equal weight of litharge and fused at a strong heat; the lead is extracted from the resulting button of gold and lead alloy by warm nitric acid, when the gold will remain as a loose brown spongy mass. If hydrochloric acid is poured into a pure solution of gold in cyanide, there is slowly formed at ordinary temperatures, and immediately on the application of heat, a yellow precipitate; the filtered liquid which has given this precipitate still contains a little gold in solution.
On evaporating the liquid to dryness, fusing, dissolving, and filtering afresh, there remains upon the filter the remainder of the gold. Crystallized 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 heat. When it is strongly ignited, mixed with an equal weight of sodium carbonate, a button of metallic gold is obtained. Cyanide gilding solutions, when mixed with sulphuric, nitric, or hydrochloric acid, slowly deposits gold; and when boiled with hydrochloric acid, it is completely resolved into cyanide of gold and potassium chloride. The same result is obtained with nitric and sulphuric acid, and even with oxalic, tartaric, and acetic acids. When heated with sulphuric acid, it gives off hydrocyanic acid gas, and after ignition, leaves a mixture of gold and sodium.
Iodine sets free cyanogen gas, forms potassium iodide of potassium, and throws down the cyanide of gold. Gold may be precipitated from washing waters containing traces of the precious metal by adding a solution of iron sulphate of iron, when a brown deposit of pure gold is obtained; the subsidence of the metal may be hastened by heating the liquid.