The cyanidation of gold. 1. Mixed potential and dissolution studies

Despite the fact that the cyanidation process for the recovery of gold was patented almost 140 years ago, understanding of the dissolution of gold has proved to be particularly elusive. While it is widely accepted to be a typical electrochemically-based corrosion process, details of the published kinetics of the anodic and cathodic reactions have been found to be very variable with little agreement between many studies, predominantly of the anodic reaction. In this series of three papers, an attempt has been made to shed some light on the electrochemistry of gold in cyanide solutions. Unlike many previous studies, the use of gold‑silver alloys and an electrode fabricated using gold from an actual plant have been used in addition to pure gold. Solutions from two gold plants have also been compared to pure solutions. The first of these papers summarizes and compares the literature on the anodic characteristics of gold dissolution in cyanide solutions. The use of the mixed-potential model to describe the kinetics of gold dissolution and the important measurement of mixed potentials under typical cyanidation conditions are described in detail with an emphasis on the possible passivation of gold and the effect of the metal and solution composition on the passivation process. A number of small scale dissolution experiments using various gold materials and solutions has been undertaken in conjunction with mixed potential and anodic voltammetric measurements. This study has shown that electrochemical studies using pure gold electrodes in pure solutions cannot be used to predict the reactivity of gold under actual plant conditions. • Variable results in published studies of cyanidation. • Gold-silver alloys and plant gold samples studied. • Detailed mixed potential measurements highlight passivation. • Gold dissolution studies in conjunction with voltammetry. • Pure gold in pure solutions not relevant to plant operation.