The chemical properties of gold play an important role in the performance of the recovery process to be employed, especially when an hydrometallurgical process is selected. Gold oxidizes neither at ordinary temperatures, nor at a red heat. It forms two oxides (Aurous, Au2O, and Auric, Au.2O3). It is not soluble in sulphuric, nitric nor hydrochloric acid, but dissolves in solutions containing chlorine or bromine, in solutions of sodium, calcium and cuprous-sodium hyposulphite, in solutions of cyanides, especially of sodium cyanide, and in small quantities at 100° C in ferric and cupric salts in the presence of alkaline carbonates and excess of carbon dioxide.
Also, dry chlorine gas does not attack molten gold, but attacks leaf gold at 300° C and partially dust gold at ordinary temperatures. Gold dissolves freely, however, in solutions containing chlorine, like chlorine water, or that generate chlorine, like, e.g., aqua regia, .mixtures of hydrochloric acid with chromic acid, antimonic acid, nitrates, or mixtures of nitric acid with salt ammoniac. Auric chloride, AuCl3, is thereby formed. This salt, which forms on evaporation a dark ruby-red or reddish-brown mass, is decomposed on heating to 180° C into aurous chloride, AuCl, and chlorine. When heated to over 200° C, aurous chloride is decomposed into chlorine and gold.
It is important to indicate that bromine acts like chlorine, only more slowly. Aqueous solutions of iodine attack gold, even in the sunlight ; the effect is much greater when the temperature is raised and the pressure increased. Hyposulphites of potassium, sodium, calcium and magnesium dissolve metallic gold with the formation of double salts; sodium hyposulphite, for instance, produces the salt 3Na2S2O3, Au2S2O3 + 4H2O. It has been noted that 1000 mL of sodium hyposulphite solution of the most varying degrees of concentration will dissolve 0.002 gram of gold. The dissolving power of the hyposulphites of calcium and magnesium is similar, but of potassium is less; that of the double salt of sodium and copper is the same as of sodium hyposulphite.
Sodium cyanide dissolves gold as a double salt, sodium cyanide, for instance, as aurous-sodium cyanide. Gold is but weakly combined in its solutions, and is accordingly precipitated by a whole series of substances, such as phosphorus, sulphur dioxide, nitrogen, nitrous acid and nitrites, arsenious acid, antimonious chloride, mercury, silver, palladium, platinum, the common metals, carbon, ferrous salts, stannous chloride, and many organic compounds, such as oxalic acid. Sulphides minerals such as pyrite, galena, chalcopyrite, cinnabar, stibnite precipitate metallic gold from a solution of the chloride. From cyanide solutions gold is thrown down as metal by the metals of the alkalis, by aluminum and by zinc. In gold extraction, ferrous salts (sulphate and chloride), charcoal, hydrogen sulphide and sulphides are used to precipitate gold from its solutions, zinc being used in the case of dilute aurocyanide solutions. Gold does not combine with free sulphur. Hydrogen sulphide tends to produce a brownish-black precipitate consisting of sulphide of gold, of metallic gold and of sulphur; in the cold it consists mainly of sulphide of gold, but decomposes on heating, all the gold separating in the metallic state.