The dissolution of gold in chloride media under ambient conditions-a re-assessment of mechanisms

Abstract

This study has shown that the use of methods that measure the initial rates of dissolution of gold using iron(III) as the oxidant under ambient conditions can be deceptive in that, in the absence of efficient methods to re-oxidize iron(II) to iron(III), the rates rapidly decrease with time resulting in predictable poor recoveries. Nevertheless, the results obtained have confirmed that gold(I) is the main product of dissolution with iron(III) as the oxidant and that the initial rate increases parabolically with increasing chloride concentration, is proportional to the square root of the iron(III) concentration and is independent of the acidity. These observations are consistent with the effects of chloride and acid concentrations on the anodic dissolution of gold. The mixed potential model of oxidative dissolution was shown to be operative in this system.The rate of homogeneous re-oxidation of iron(II) to iron(III) by dissolved oxygen under ambient conditions is too slow to be of value in sustaining the high potentials required. On the other hand, the use of low concentrations of chlorate have been shown to be effective in sustaining the reaction as a result of the generation of small quantities of chlorine by oxidation of chloride.