Functionalization of activated carbon for the recovery of gold thiosulfate

Amalgamation and cyanidation are common processes applied in artisanal and small-scale gold mining, despite their toxicity, causing everlasting environmental damage. Thiosulfate is a greener choice, but its employment is constrained mainly by the difficulty at recovering gold thiosulfate from the leachate by activated carbon, which in contrast, works well for gold cyanide adsorption. Up to date, the proposed alternatives to overcome this problem are impracticable or incompatible with current hydrometallurgical processes. Therefore, the use of thiosulfate remains, in the practice, limited to preg-robbing ores, where cyanide shows low performance. In this context, the main goal of this Ph.D thesis is to propose an alternative for gold thiosulfate recovery by the functionalization of activated carbon. Indeed, the presence of different types of functional groups changes the chemical properties of the material and increases its gold adsorption capacity. In this research, different functional groups have been targeted: oxygen functions that serve as anchor sites for other functionalities with known affinities for gold complexes and increase the solid’s acidity, nitrogen functional groups, specifically primary, quaternary amine sites and urea derivatives, and sulfur functional groups. Furthermore, the interaction between gold thiosulfate and the created functions has been studied employing a wide array of characterization techniques (XPS, Tof-SIMS, FTIR, AAS, PZC, Boehm Titrations, PXRD, NMR, HPLC, among others). In this way, several gold adsorption mechanisms have been proposed: aminated and urea-enriched carbons adsorb gold thiosulfate by ion-exchange interactions, whereas the formation of Au-S chains was discovered in thiolated activated carbons. Additionally, high elution efficiencies have been accomplished with all the gold-loaded solids using Na₂S₂O₃ and NH₄OH stripping solutions. Finally, it was found that all the functionalized solids showed competitive Au adsorption performances, while keeping its mechanical properties, giving this research good practical application perspectives in the gold mining industry.