Bioleaching of Chalcopyrite
Biomining is one of the most important technologies globally for the recovery of copper, because of its low energy cost compared to traditional pyrometallurgy, and its comparative environmental benefit. However, the leaching rate of biomining has always been its limiting factor, and so to enhance this, considerable effort has been made to research the dissolution mechanism of chalcopyrite (CuFeS2) in bioleaching. To this end, one component of a larger body of work conducted at CSIRO into the bioleaching of chalcopyrite and pyrite was to investigate whether or not crystal orientation effected the ability of Acidithiobacillus ferrooxidans bacteria to adhere to the surface of chalcopyrite. Previous studies suggested that this was the case with Pyrite, and that sulphur terminated planes were more easily attacked than iron terminated planes.
To investigate this possibility on chalcopyrite it was necessary to obtain a mounted sample with a random assortment of crystal faces polished for EBSD analysis. Therefore, a sample of Chalcopyrite was crushed to a relatively fine, sand-like consistency using a mortar and pestle, then mounted and polished for EBSD analysis. This process randomised the orientation of the individual crystallites and produced the EBSD dataset and optical images shown in the figure.
Part (a) of the figure, the inverse pole figure orientation map shows that the randomisation of chalcopyrite crystals was successful. Figure (b) shows optical images of the areas indicated by the white rectangles in (a). The crystals selected in these areas have a broad range of orientations and were considered to be good examples of crystals that would indicate whether any particular orientation was favoured by the Acidithiobacillus ferrooxidans bacteria for colonisation.
(a): Inverse Pole Figure orientation map of randomised chalcopyrite grains. (b): Optical images taken before and after formation of biofilm showing attachment of Acidithiobacillus ferrooxidans bacteria to the chalcopyrite crystals indicated by the white rectangles in (a). (c): Pattern quality EBSD map showing sub-grain boundaries in chalcopyrite crystals where the misorientation across the grain boundary is less than the 5 deg used to define a grain boundary for the grain map shown in (d). (d): Grain map of the area displayed in (a) and (c) showing the different grains in the chalcopyrite crystals produced during randomisation.
This figure shows the same crystals before and after formation of the biofilm. During collection of the EBSD map, carbon contamination is burned onto the sample by the electron beam (a well known and long standing problem in electron microscopy) and must be removed prior to exposure to the bacteria since it could impede formation of the biofilm. Accordingly, the sample was repolished after the EBSD map, and so the crystals in the before and after images (b) look slightly different but are clearly the same crystals. The figure shows that the bacteria have formed evenly over the surface of all of the crystals, and that no particular orientation has been favoured. This suggests, that for chalcopyrite, it does not seem to matter if the plane exposed to the bacteria is sulphur terminated or iron terminated, the bacteria adhere to the all orientations evenly.
Part (c) shows a pattern quality image of the EBSD map. In this map, the areas with lighter coloured pixels produced a stronger diffraction pattern. This map is useful since different crystal orientations can be seen in the map, and sub-grain boundaries (i.e., boundaries whose misorientation across the boundary is less than the minimum specified to define a grain during post processing of the EBSD dataset, in this case, 5°) are visible as well. A crystal showing sub-grain boundaries is shown in the top of this image. Comparison of this crystal with the same crystal in the grains map (d) indicates the grain boundaries that are visible in the pattern quality map (c) and are not visible in the grains map (d). (c and d) also show both grain boundaries and sub-grain boundaries in the crystals within the white rectangles. Comparison of these images with Figure 9 (b) shows that the bacteria have not preferentially attacked either the grain boundaries or sub-grain boundaries.
These results suggest that for chalcopyrite, crystal orientation by itself does not greatly affect initial attachment of Acidithiobacillus ferrooxidans bacteria during formation of the biofilm. However, it is still considered possible that crystal orientation may play a part in the process since it is known that surface roughness effects initial attachment of bacteria. Different orientations corrode at different rates (this is how etching works), and during attack by leaching agents or even atmospheric attack, a surface roughness will form on multi grain chalcopyrite crystals due to their variation of crystal orientation. This work may be considered in the future.
References
Y. Yang, S. N. Tan, A. M. Glenn, S. Harmer, S. Bhargava and M. Chen; “A direct observation of bacterial coverage and biofilm formation by Acidithiobacillus ferrooxidans on chalcopytire and pyrite surfaces”; Biofouling: The Journal of Bioadhesion and Biofilm Research, 31 (7), pp 575-586, 2015; DOI http://dx.doi.org/10.1080/08927014.2015.1073720
