Abstract
M.Sc.
The leaching of chalcopyrite (CuFeS2) concentrate in a ferrous iron promoted
aerobic/anaerobic controlled low potential sulphate system was investigated
by using the duel metabolic (aerobic ferrous iron oxidation and anaerobic
ferric iron reduction) capabilities of Ferroplasma JTC 3. The experimental
work conducted in this study was divided in three sections.
The first section focussed on the identification and phylogenetic
classification of Ferroplasma JTC 3, first identified amongst a mixed microbial
population in a 55 oC pyrite concentrate-fed bioreactor operated at
Johannesburg Technology Centre (BHP Billiton, JTC). Based on the 16S
rDNA sequence and the phylogenetic analysis, Ferroplasma JTC 3 represents
a new species member under the genus of Ferroplasma. The optimal growth
temperature of Ferroplasma JTC 3 was determined at approximately 53 oC
(moderate thermophile).
The second section of this study focussed on the isolation, basic metabolism
and growth conditions of Ferroplasma JTC 3, specifically directed towards the
chalcopyrite leaching related experimental work. An important aspect of this
study was to compare low potential chalcopyrite leaching (potential below 400
mV vs. Ag/AgCl) against high potential chalcopyrite bioleaching (potential
above 600 mV vs. Ag/AgCl) in terms of the rate of copper extraction. Microbial
growth and the rate of ferrous iron oxidation are essential in order to maintain
a high potential during an extended leach period, which was typically the case
in the high potential chalcopyrite leaching experiments performed during this
study. Ferroplasma JTC 3 required yeast extract as sole carbon source
(chemo-heterotrophic) for growth via aerobic ferrous iron oxidation. Taking
into account no carbon dioxide enrichment via aeration, chemo-autotrophic
growth by means of ferrous iron oxidation was poor with carbon dioxide as
sole carbon source. The anaerobic metabolism of Ferroplasma JTC 3 was
utilized in assisting with solution potential control during the low potential
chalcopyrite leaching work. The anaerobic metabolism enabled the reduction
of ferric iron (decrease redox potential) in the presence of elemental sulphur
and yeast extract. Elemental sulphur was shown to be a requirement for
Ferroplasma JTC 3 assisted ferric iron reduction, which was not influenced by
different ferrous/ferric iron based redox potentials.
The third section covers the main focus of this study, which was the low
potential leaching of chalcopyrite in combination with the metabolic
capabilities of Ferroplasma JTC 3. The major challenge of low potential
chalcopyrite leaching in an acidic environment is maintaining the solution
potential below the critical upper limit (400 mV vs. Ag/AgCl) of the low
potential window for prolonged periods of time. The reason is the slow
chemical oxidation of ferrous iron in the presence of oxygen, which increases
the leach solution potential above the critical upper limit before complete
copper dissolution is obtained. The aim of this study was to maintain a low
solution potential environment in a bioreactor via a programmable electronic
gas control system, capable of creating an aerobic environment until the
solution potential would reach the upper low potential limit (400 mV vs.
Ag/AgCl) due to ferrous iron oxidation (chemically or via Ferroplasma JTC 3)
and then switch to an anaerobic environment. During the anaerobic
environment, the aim was to decrease the solution potential to a lower
potential set point via chalcopyrite oxidation by ferric iron (ferric iron reduction)
and by employing the ferric iron reduction metabolism of Ferroplasma JTC 3.
With the particular aerobic/anaerobic solution potential control system, in
conjunction with the metabolic capabilities of Ferroplasma JTC 3, the solution
potential could be controlled within the critical low potential region, but no
chalcopyrite leaching could be obtained during the anaerobic phase. The lack
of chalcopyrite leaching during the anaerobic phase was due to inability of
ferric iron to act as oxidant of chalcopyrite after the mineral was pre-leached
in the preceding aerobic phase. The “oxidative acid leach” mechanism was
identified as the dominant leach reaction that prevailed during the aerobic low
potential stage in each of the aerobic/anaerobic control experiments
conducted, in which oxygen acts as oxidant of chalcopyrite (electron acceptor)
in the presence of protons (H+) (acidic environment), instead of ferric iron in
an acid environment. The “boundary potential”, which is the maximum solution
where no electron flow occurred to the ferrous/ferric couple from “pre-leached”
chalcopyrite, was identified in the region of 450 mV (Ag/AgCl). Under the
experimental conditions within this study, the leaching of chalcopyrite within
the aerobic phase of the aerobic/anaerobic control experiments was superior
to the Ferroplasma JTC 3 mediated high potential leaching, but complete
copper dissolution could not be obtained with the combined aerobic and
anaerobic system.
Ferric iron precipitation as a function of pH was proposed as a tool for solution
potential control, instead of controlling the potential by limiting oxygen to the
leach system. In controlling the solution potential via pH, almost complete
copper dissolution from chalcopyrite was obtained, while maintaining the
solution potential below the critical upper limit of the low potential region.