| Summary: | The application of thermophilic bacteria to the leaching of a chalcopyrite (CuFeS2)
concentrate from the Gibraltar Mine has been investigated in both shake flask and stirred
tank reactor experiments.
The conventional hydrometallurgical treatment of chalcopyrite is difficult due to a
surface passivation phenomenon of the leaching particles. This passivation can be
overcome through biological leaching at temperatures exceeding 50°C. The thermophilic
leaching system is characterized by low temperatures (with respect to autoclave
processes), high acidity and most importantly, a low redox potential. Due to the
thermodynamic, chemical and biological properties inherent to high temperature
bioleaching, the redox potential of the leaching slurry is held within the narrow potential
range of 500 - 600 mV SHE. It is thought that in this range, chalcopyrite can be leached
successfully.
Of the various thermophilic species selected, a mixed thermophilic culture (growing at
70°C) outperformed all other thermophiles as well as the standard mesophile (growing at
35°C). It was observed that the thermophilic system is very sensitive to changes in
temperature and consequently, leaching at the highest possible temperature (80-85°C) is
advantageous. Inhibitions due to copper revealed that bacterial activity and therefore the
leaching rate begins to be inhibited at copper concentrations above 10 g/L. Stirred tank
reactor (STR) experiments resulted in copper extractions in excess of 90 % in 15 days.
Approximately 75 to 99 % of the sulfide oxidized reported as elemental sulfur. It was
found that the relative rates of leaching and the extent of sulfide oxidation (copper
extraction) decreased with increasing pulp density over the range 2 to 10 %.
Thermophilic bioleaching of the Gibraltar concentrate is thought to proceed under a
biologically assisted indirect chemical leaching mechanism with the avoidance of
chalcopyrite passivation as the result of low solution ORP.
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