Activated carbon functionalized with amine sites as an efficient alternative for gold thiosulfate recovery

Thiosulfate is a non-toxic and efficient leaching agent that could be employed instead of cyanide for extracting gold from ores. Nevertheless, it is not commonly used in the gold mining industry due to the challenge that represents the recovery of the formed gold thiosulfate complex from the leachate. Several alternatives such as precipitation methods and adsorption onto amine-rich resins have been attempted for solving this problem. However, neither of them have been proven fully effective or compatible with current hydrometallurgical processes. In this research, the functionalization of granular active carbon with covalently grafted primary and quaternary amine sites is proposed, for enhancing its ability at trapping Au thiosulfate. The functionalized carbons accomplish up to 100 % Au recovery (pristine carbon recovers < 18 % Au) in 2 h (with quaternary aminated carbon), while also being able to operate under acid (pH 4) and basic (pH 9) adsorption conditions. Moreover, ASTM hardness tests estimate that these carbons conserve the mechanical properties needed for being employed in Carbon-in-pulp processes (H > 86%). In-depth solid characterization techniques (XPS, TOF-SIMS, FTIR, PXRD) have revealed that the adsorption mechanism of Au thiosulfate onto amine groups depends on the pH of the solution: at acid pH, an ion exchange mechanism seems to be the dominant process. Under basic conditions, the formation of Au-degrees species is evidenced, in addition to ionexchange phenomena. Elution of the gold loaded samples has been achieved with a yield up to 76 % with Na2S2O3 and NaCl solutions.