Abstract
Arsenopyrite (FeAsS) is often associated with gold, but pre-treatment is necessary prior to gold leaching, mainly due to the gold encapsulation in the matrix of FeAsS. Bio-oxidation is attractive and promising, largely due to its simplicity, low cost and environmental friendliness. A critical problem that still impedes the large-scale applications of this green technology is its slow leaching kinetics. Some metal ions such as Ag+ have previously been found to expedite the bioleaching process. In this paper, the role of Ag+ in the arsenopyrite bioleaching by Acidithiobacillus ferrooxidans was investigated in detail by bioleaching experiments and a series of analyses including thermodynamics, X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Experimental results suggested that addition of 5 mg/L Ag+ to the leaching system could significantly improve the final As leaching efficiency from 30.4% to 47.8% and shorten the bioleaching period from 19 days to 15 days. Thermodynamic analysis indicates that Ag+ destabilises As2S2, As2S3 and S0 via forming Ag2S, which is confirmed by the XRD analysis on the phase transformation during bioleaching. SEM and XPS analyses further showed that Ag+ removed the passivating film consisting mainly of As2S2, As2S3 and S0 because Ag2S formed on the arsenopyrite surface from the start bioleaching of 36 h. In the presence of Fe3+, Ag2S could easily be dissolved to Ag+ again, likely leading to the establishment of the Ag+/Ag2S cycle. The bacteria utilised the two synergistic cycles of Fe3+/Fe2+ and Ag+/Ag2S to catalyse the bioleaching of arsenopyrite.
Highlights
In nature, significant amounts of gold are often found in arsenopyrite (FeAsS) [1,2,3]
Based on these research findings, this paper reports results from a detailed investigation into the role of Ag+ in the bioleaching of pure arsenopyrite in 9K culture medium containing Acidithiobacillus ferrooxidans (A. ferrooxidans)
Experimental results from the arsenopyrite bioleaching by A. ferrooxidans in 9K culture medium suggested that addition of 5 mg/L Ag+ to the solution had no significant influence on the pH, but resulted in a lower level of the solution potential
Summary
Significant amounts of gold are often found in arsenopyrite (FeAsS) [1,2,3]. Most gold in the arsenopyrite is difficult to leach, mainly because the gold tends to be encapsulated in the mineral matrix [4,5]. It is, necessary to pre-treat arsenopyrite gold ores before leaching to improve gold extraction. Bio-oxidation is a simple, low-cost and eco-friendly technology that is appropriate to extract valuable metals from a series of ores/concentrates and waste materials with substantially reduced environmental pollution. This promising green technology has drawn significant attention, and its applications in bio-hydrometallurgy have been on the increase [8,9,10]
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