Selective adsorption of gold over copper cyanocomplexes on activated carbon

Abstract

Copper cyanocomplexes have been recognized as competitors for the adsorption sites of activated carbon during gold adsorption. In this paper, the effects of practical parameters (excess free cyanide, calcium ions and aeration conditions) on the selective adsorption of Au over Cu cyanocomplexes on activated carbon samples from different sources are discussed. In addition to the adsorption experiments, the aqueous and solid phases were characterized by Raman spectroscopy in an attempt to elucidate the observed trends. Four solid samples with different structures as regards the relative amount of graphitic planes and density of surface functional groups were selected. The results indicated the highest adsorption of Au and the lowest adsorption of Cu on the sample with the lowest density of surface groups and relative higher crystalline character. In contrast, the highest adsorption of Cu and the lowest adsorption of Au were obtained using the sample with the highest density of surface groups and relative lower crystalline character. These findings support the industrial application of activated carbons with low density of surface functional groups for gold recovery, such as the product obtained from coconut shell, and are consistent with the hypothesis of different preferential sites for the adsorption of Au and Cu. The results also indicate that if there is an excess of free cyanide in the solution, a higher Au/Cu selectivity is observed, but the Au loading on the activated carbon samples declines. In the presence of Ca2+ ions in the cyanide solution, positive effects on Au adsorption and negative effects on Cu adsorption were demonstrated for a CN/Cu molar ratio above 4 (i.e., Cu(CN)43− as the predominant species), and these behaviors resulted in a higher Au/Cu selectivity. A previous adsorption of Ca2+ ions on the activated carbon surface creates new sites for Au(CN)2− adsorption, in addition to the graphene layers. From aerated solutions, the adsorption experiments revealed a positive effect on Cu adsorption, and negative impact on Au adsorption, thus affecting the Au/Cu selectivity. The aeration of the solution results in a decrease of the cyanide concentration, which reduces the CN/Cu molar ratio. Therefore, the addition of Ca2+ ions in the process solution and an optimal concentration of free cyanide, which should take into account the loss of cyanide by oxidation and volatilization, may help to improve Au/Cu selectivity.