Research at the Parker Centre which has determined the mechanism of chalcopyrite leaching could lead to successful industrial bioleaching of chalcopyrite.

Most of the world's copper comes from the processing of ores dominated by sulphides, of which chalcopyrite is the most abundant example. Pyrometallurgical processes are still economically favoured for copper extraction from chalcopyrite, as chalcopyrite is resistant to hydrometallurgical processes due to passivation.

Consequently, the mechanisms of microbial chalcopyrite leaching and passivation are of considerable interest to the minerals industry. If improvements can be made, microbial leaching may become a viable alternative to pyrometallurgy.

Passivation - the formation of a passive surface layer that hinders leaching - has been variously attributed to four principal surface species. Some researchers report that a layer of sulphur is responsible, some have proposed polysulphides whilst others suggest jarosites (insoluble iron hydroxy sulphates) are the culprits.

Another possibility is that the passivating layer is not an adsorbed species but an alteration of the sample chemistry at the surface, due to migration of the copper near the surface through the crystal lattice, leaving a metal deficient sulphide.

X-ray photoelectron spectroscopy (XPS) has been used to identify the surface species present before and after leaching. A number of previous studies on chalcopyrite leaching used XPS with somewhat conflicting conclusions. This is most likely due to possible flaws in sample preparation and/or data analysis.

Hence, in this PhD study, particular attention was paid to preserving the integrity of samples and surfaces and to XPS spectral decomposition. This enabled the identification of a range of elemental sulphur, jarosite and sulphide phases on the surface. Other proposed species such as polysulphides and metal deficient sulphides were not observed.

As a consequence of determining the leaching mechanism and identifying the passivating pre-cursors and species, the conditions which reduce or eliminate their formation have been established. This has lead to efforts to develop sulphide oxidising microbes capable of surviving the conditions that prevent/delay passivation. It is intended that these microbes will be able to be applied industrially to enhance current heap leaching practice.

For further information, contact
Dr Craig Klauber (Craig.Klauber@minerals.csiro.au)
or Andrew Parker (Andrew.Parker@minerals.csiro.au).

First published in Australia's Paydirt.