Microbial biotransformation of kimberlite ores.
Loading...
Date
2008
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Microbial leaching plays a significant role in the natural weathering of silicate containing
ores such as diamond-bearing kimberlite. Harnessing microbial leaching
processes to pre-treat mined kimberlite ores has been proposed as a means of
improving diamond recovery efficiencies. The biomineralization of kimberlite is
rarely studied. Therefore, this study investigated the feasibility of exploiting both
chemolithotrophic and heterotrophic leaching processes to accelerate the weathering
of kimberlite.
Preliminary investigations using mixed chemolithotrophic leaching cultures were
performed on four finely ground kimberlite samples (<100μm) sourced from different
mines in South Africa and Canada. Mixed chemolithotrophic cultures were grown in
shake flasks containing kimberlite and inorganic basal media supplemented either
with iron (Fe2+, 15g/l) or elemental sulfur (10g/l) as energy sources. Weathering due
to dissolution was monitored by Inductive Coupled Plasma (ICP) analyses of Si, Fe,
K, Mg and Ca in the leach solutions at known pH. Structural alterations of kimberlite
after specified treatment times were analyzed by X-ray Powder Diffraction (XRD).
The results of the preliminary investigation showed that weathering can be
accelerated in the presence of microbial leaching agents but the degree of
susceptibility and mineralogical transformation varied between different kimberlite
types with different mineralogical characteristics. In general, the results showed that
the kimberlite sample from Victor Mine was most prone to weathering while the
sample from Gahcho Kue was the most resistant. It was therefore deduced that
kimberlite with swelling clays as their major mineral component weathered relatively
more easily when compared to kimberlite that consisted of serpentine and phlogopite
as their major minerals. Gypsum precipitates were also distinguished indicating that a
partial alteration in the kimberlite mineralogical structure occurred. Both energy
sources positively influenced the dissolution process, with sulfur producing superior
results. This was attributed to the generation of sulfuric acid which promotes cation
dissolution and mineral weathering.
Success in the preliminary investigations led to further experimental testing
performed to determine the effect of particle size and varying energy source concentrations on the biotransformation of kimberlite. It was observed that although
weathering rates of the larger kimberlite particles (>2mm<5mm) were lower than that
of the finer particles, slight changes in their mineralogical structures represented by
the XRD analyses were seen. Optimisation studies of energy source concentration
concluded that although the highest concentration of elemental sulfur (20% w/w) and
ferrous iron (35% w/w) produced the most pronounced changes for each energy
source tested, the leaching efficiency at these concentrations were not drastically
greater than the leaching efficiency of the lower concentrations, as expected.
Following the success of batch culture shake flasks weathering tests, the effect of
continuous chemolithotrophic cultures on the biotransformation of larger kimberlite
particles (>5mm<6.7mm) was investigated. A continuous plug-flow bioleach column
was used to model the behaviour of chemolithotrophic consortia in a dump- or heap
leaching system. Two sequential columns were setup, in which the first consisted of
kimberlite mixed with sulfur and the second purely kimberlite. Inorganic growth
medium was pumped to the first column at a fixed dilution rate of 0.25h-1 and the
leachate from the first column dripped into the second. After an 8 week investigation
period, the ICP and XRD data showed that weathering did occur. However, the pH
results showed that the leaching process is governed by the amount of acid produced
by the growth-rate independent chemolithotrophic consortia. Data from pH analyses
also showed that the leaching bacteria reached ‘steady state’ conditions from day 45
onwards. The pH also remained higher in the second column than in the first column
highlighting the alkaline nature of the kimberlite ores and its ability to act as a
buffering agent and resist weathering. This important factor, as well as further
optimisation studies in process operating conditions and efficiency, needs to be
considered when establishing heap-leaching technology for these kimberlite ores.
In the preliminary heterotrophic investigation, Aspergillus niger was used to produce
organic metabolites to enhance kimberlite mineralization. The results demonstrated
that the organic acid metabolites generated caused partial solubilization of the
kimberlite minerals. However, it was deduced that for more significant changes to be
observed higher amounts of organic acids need to be produced and maintained. The
results obtained in this study also showed that the type of kimberlite presents a different susceptibility to the dissolution process and the presence of the fungal cells
may improve the leaching efficiency.
The results in this study provided an optimistic base for the use of microbial leaching
processes in accelerating the weathering of kimberlite. These findings may also serve
to supply data to formulate recommendations for further and future column microbial
leach tests as well as validation and simulation purposes.
Description
Thesis (M.Sc.) - University of KwaZulu-Natal, Pietermaritzburg, 2008.
Keywords
Microbial biotechnology., Minerals--Biotechnology., Bacterial leaching., Ores--Metallurgy., Theses--Microbiology.