Some workshops practice the recovery of gold from cyanide solutions using hydrogen sulphide gas, which is passed into the solution until the liquid reddens blue litmus paper and the gas freely escapes indicating that the operation is completed, when it is allowed to rest for several hours for all the gold to precipitate. It is important to dilute the gold solution with twice its volume. For the production of the hydrogen sulphide, which is a highly poisonous gas may be used a small vessel when the volume of solution is not excessive. In order o generate hydrogen sulphide at small scale a good vessel is needed, which must be equipped with a cork pierced with two holes; a thistle funnel with a long tube is fitted in one and a gas delivery tube in the other holes. The tube of the thistle funnel must reach almost the bottom of the vessel. Some iron sulphide is placed in the vessel and the cork is replaced with the bent delivery tube and the thistle funnel; immediately ass diluted sulphuric acid in the proportion of 1 part acid and 8 parts water, down the thistle funnel until the vessel is half filled. The production of the gas begins immediately and passes through a bent tube into the gold cyanide solution. When the gold sulphide starts to precipitate to the bottom of the vessel holding the gold cyanide solution, in a very short time the gold is reduced to the condition of gold sulphide, a black precipitate readily decomposed to metallic gold on applying a dull red burning heat with exposure to air.
The ferrous sulphide must be broken into lumps about as big as 19 mm. When the gold cyanide has been successfully treated, by the removal of all the gold from the solution, the acid is poured off the iron sulphide in this gas generator and some clean water is poured in to no produce more gas. When the equipment must be used again, the water is poured away and a fresh mixture of acid and water is added into the vessel. Gold sulphide is insoluble in water. The chemical reactions proceed readily, when once started without the application of heat; the iron sulphide is decomposed by the sulphuric acid and its hydrogen combining with the sulphur of the iron sulphide forming hydrogen gas, which by means of the delivery tube provided for its conveyance, enters the vessel containing the gold solution, precipitating the gold as gold sulphide, whilst the non-decomposed or free acid in the vessel dissolves and unites with the iron, as the sulphur becomes liberated forming iron sulphate, which is retained in the solution.
Hydrogen sulphide gas is possessed of acid properties in solutions with water, and reddens blue litmus paper. Its acid properties decompose the cyanide and acts on these solutions as an aid to the complete reduction of the gold by causing it to remain insoluble in the solution. Hydrogen sulphide gas has acid properties in solution with water and reddens blue litmus paper. Its acid properties decompose the cyanide and reacts in these solutions as an aid to the complete reduction of gold by causing it to remain in the solution.