Mineralogical characterisation of chromite in the UG2 Reef from Waterval Mine, Western bushveld : implications for minerals processing
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 iii 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.