The Chemistry of the Extraction of Gold

1S.1 General Principles 15.1.1 The chemistry of gold compounds Gold is the most noble of all the metals and this .is the key to both its eternal romance and its many practical uses in industry. It is the only metal, for example, that is not attacked in air or water by either oxygen or sulphur, and its durability under the most corrosive conditions has led to its widespread use in coinage and jewellery through the ages. Gold is the only metal that is generally found in nature in the metallic state, and the only gold compounds that occur in a natural state are the telluride and stibnite species, AuTe2 and AuSb2 • Gold reacts with all the halogens, most vigorously with bromine, which undergoes an exothermic reaction with gold powder at room temperature to produce Au2Br6 • Of greater importance to the extractive metallurgist is the chemistry of gold complexes in aqueous solutions, and this is a far broader subject. It has been traditionally accepted that gold complexes in aqueous solution can exist in one of two oxidation states, the aurous ( + 1) or the auric ( + 3), and that all the gold complexes of hydrometallurgical interest fall into these two groups. In more recent years, compounds with formal oxidation states of 1, + 2, + 4, and + 5 have been prepared and identified (Puddephatt, 1978; Schutte, 1985), but these compounds are more of an academic curiosity at present and will not be dealt with in this review. Thermodynamics predicts that neither aurous nor auric cations will be stable in aqueous solution, but will be reduced by water to metallic gold (see Section 15.1.2). In order to stabilize these ions in aqueous solutions, it is necessary to introduce a complexing ligand and to employ a suitable oxidizing agent, since no reaction occurs unless both are present in the solution. For example, gold does not dissolve in either nitric acid (an oxidizing agent) or hydrochloric acid (a complexing ligand), but dissolves quite easily in a mixture of the two, aqua regia. Whether they are solid, in solution, or in the gaseous state, gold compounds are invariably bound covalently, and by far the majority are complexes. *