A simple and efficient recovery technique for gold ions from ammonium thiosulfate medium by galvanic interactions of zero-valent aluminum and activated carbon: A parametric and mechanistic study of cementation

Abstract

Cyanide- and halide-based extraction methods are the most common ways of leaching gold (Au) from Au-bearing materials, but because of the toxicity and/or highly corrosive nature of lixiviants employed in these techniques, alternative strategies are being explored. Thiosulfate is a promising lixiviant with a non-toxic, more selective, and less corrosive alternative, but its application on an industrial scale is still limited since efficient methods for Au(I) ion recovery from thiosulfate media remain elusive. This study describes a simple and highly efficient technique for Au(I) ion recovery from thiosulfate medium based on galvanic interactions between aluminum (Al) and activated carbon (AC) under various industrially relevant conditions. The results showed that when only AC or Al are used, Au recoveries were negligible under the following conditions: 0.15 g of AC or Al with 10 ml thiosulfate solution containing 100 mg/l of Au ions at 25 °C for 24 h with 120 rpm. With Al (0.15 g)-AC (0.15 g) mixture, however, Au recovery significantly increased reaching over 99%. The effects on Au recovery of various parameters, including recovery time, dissolved oxygen, mixing ratio of Al and AC, solid-to-liquid ratio, and temperature, were also evaluated and the highest Au recovery was obtained under the following conditions: 1 h recovery time without oxygen, 1:1 of Al and AC, and 0.3 g/10 ml at 25 °C. Using scanning electron microscopy with energy dispersive spectroscopy coupled with electrochemical experiments, the critical role of AC on Au recovery was identified as a reduction mediator between Al and Au(I)-thiosulfate complex in the solution. Finally, the following sequence of processes are proposed to explain Au recovery by Al-AC in thiosulfate medium: (i) AC is lodged into Al oxide micro-cracks or “attached” on Al oxide film, (ii) electrons moved from Al to AC, (iii) Au(I) ions receive electrons from AC, and (iv) metallic Au is cemented on AC.