Abstract
A. ferrooxidans and their metabolic products have previously been explored as a viable alternative depressant of pyrite for froth flotation; however, the mechanism by which separation is achieved is not completely understood. Scanning electron microscopy (SEM), photoemission electron microscopy (PEEM), time-of-flight secondary ion mass spectrometry (ToF-SIMS) and captive bubble contact angle measurements have been used to examine the surface physicochemical properties of pyrite upon exposure to A. ferrooxidans grown in HH medium at pH 1.8. C K-edge near edge X-ray absorption fine structure (NEXAFS) spectra collected from PEEM images indicate hydrophilic lipids, fatty acids and biopolymers are formed at the mineral surface during early exposure. After 168 h, the spectra indicate a shift towards protein and DNA, corresponding to an increase in cell population and biofilm formation on the surface, as observed by SEM. The Fe L-edge NEXAFS show gradual oxidation of the mineral surface from Fe(II) sulfide to Fe(III) oxyhydroxides. The oxidation of the iron species at the pyrite surface is accelerated in the presence of A. ferrooxidans and extracellular polymeric substances (EPS) as compared to HH medium controls. The surface chemical changes induced by the interaction with A. ferrooxidans show a significant decrease in surface hydrophobicity within the first 2 h of exposure. The implications of these findings are the potential use of EPS produced during early attachment of A. ferrooxidans, as a depressant for bioflotation.
Highlights
Gold- and copper-containing ore grades are declining, suggesting the necessity for new and innovative processing procedures [1,2,3,4]
Over the first 24 h of exposure to A. ferrooxidans, mineral debris was observed on the surface of the pyrite and the pitting character was similar between the sample exposed to culture (Figure 1A,B) and HH medium (Figure 1E,F)
A difference in measured contact angle as little as 3◦ can alter mineral recovery by up to 18%, depending on the size fraction of the particles [4]. These results suggest that an exposure of as little as 2 h could promote the depression of pyrite by A. ferrooxidans, while longer periods of exposure showed no significant separation of contact angle compared to the sterile control sample
Summary
Gold- and copper-containing ore grades are declining, suggesting the necessity for new and innovative processing procedures [1,2,3,4]. Copper sulfide minerals are abundant in nature and contain valuable base metals [1,4,5,6,7]. These copper sulfides are commonly associated with gangue materials of similar surface physicochemical properties [1,4,5,6,7]. The traditional method for separating pyrite from the valuable ore is froth flotation This method is heavily dependent on the surface properties of the minerals being separated, requiring the materials of interest to be hydrophobic and for the gangue material to be hydrophilic, or vice versa [5,6,7,8]. In order to alter the surface properties of the sulfides, a variety of depressants, collectors, frothers and activators are used, many of which are undesirable to Minerals 2018, 8, 132; doi:10.3390/min8040132 www.mdpi.com/journal/minerals
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