| Summary: | Thesis (MSc (Earth Sciences))--University of Stellenbosch, 2010. === ENGLISH ABSTRACT: The Bushveld Complex of South Africa contains three of the most important platinum
deposits in the world namely the Merensky Reef, the Upper Group Two (UG2) chromitite
reef and the Platreef. These three ore bodies are principally beneficiated by froth flotation.
During the beneficiation of chromite hosted PGE’s by froth flotation, chromite represents
the principal gangue mineral. This is particularly true for the UG2 main seam. An excess of
more than 3% in mass of chromite in the PGM concentrate is known to result in significant
problems in the downstream processing and extraction of PGEs. The variability in texture
and composition of chromite due to its primary crystallization and subsequent modification
by the development of potholes or through IRUP intrusions are thought to influence the
flotation behaviour of the UG2 main seam chromitite ore.
This study conducted at Waterval Mine investigated the role of mineralogical characteristics
of chromites on the flotation performance of three different environments for the UG2 main
seam: (1) “normal” UG2 main seam; (2) UG2 main seam affected by pothole formation; and
(3) UG2 main seam affected by IRUP intrusion. This was achieved through an extensive
petrographic investigation of the chromites from each environment, to individually
characterise their primary textures. This was followed by compositional characterisation of
the chromite from each environment. Finally the flotation performance of the ore from each
environment was investigated, using small scale batch flotation experiments, to establish
any linkage between the textures, the composition and the flotation performance of the
chromite from different environments.
In this study it was found that the UG2 normal reef and the UG2 reef affected by pothole
formation are both principally characterised by primary mineralogical features comprising
mainly fine‐grained chromite as the cumulate phase and orthopyroxene and plagioclase as
intercumulate phases. These two reef types were also found to be identical in the
composition of the chromites present. In addition, in both of these almost unaltered reef
types it was found that chromite showed small recoveries by flotation. On the other hand, it
was found that the UG2 affected by IRUP intrusion was affected by post‐magmatic alteration
that had overprinted primary textures and compositional features. This resulted in the
replacement of primary minerals by secondary alteration assemblages. Orthopyroxene was
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replaced by serpentine, chlorite, amphibole and talc, while plagioclase is replaced by sericitic
alteration. Furthermore, this alteration also resulted in modification of the chromite
compositions. The compositional change in the chromites from the IRUP reef type resulted
in Fe and Ti enrichment of chromite with increasing magnetic properties, and Cr, Al and Mg
depletion. The alteration also resulted in the coarsening of chromite in the IRUP affected
main seam reef particularly at the bottom and the top of the main seam.
These compositional and textural modifications, principally the post‐magmatic alteration of
intercumulate orthopyroxene, resulted in a greater recovery of chromite by flotation in the
concentrate from the IRUP affected ore compared to the two other two ore types where
there was small amount of chromite recovered. The characterisation of the recovered
chromite revealed that the principal reason for chromite flotation was caused by the mineral
association of chromite with hydrophobic Si, Mg, Fe rich phases, principally altered
orthopyroxene and associated serpentine, chlorite, amphibole and talc.
This investigation showed that the difference in mineralogical and flotation performances of
chromite from the different UG2 main seam reef types was caused by the postcrystallisation
alteration of cumulate and intercumulate phases due to the emplacement of
IRUPs. Although IRUP affected UG2 main seam ore is not currently processed, it could be
processed much more rapidly than the other two types of UG2 main seam ores because of
its softer character resulting in shorter milling times. This is most likely a function of the
presence of alteration phases and the presence of coarser chromite grains, as well as
already brecciated chromite grains. Savings associated with the shorter milling time of this
ore type are perhaps offset by the cost of the higher dosages of depressant required to
suppress the floatable chromite in this ore type. However, given the energy cost of longer
milling times, the cost of the depressant is likely to be insignificant. Moreover, the
processing of the UG2 main seam ore affected by IRUP intrusion would also require a
different approach to extraction of the ore to keep it separate from the normal reef ore.
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