Abstract
Booysendal South UG2 (BSU) mine owned by Northam Platinum Limited, located in the Eastern Limb of the Bushveld Complex, is recognised for mining and the mineral beneficiation of the Upper Group 2 (UG2) Reef. This reef is widely characterised by two chromitite layers, comprising an upper chromitite leader seam (UL) and the lower main chromitite seam (UM), with a combined thickness of 1.4 metres. Where the two seams are superposed, the UG2 is referred to as the UG2 normal reef-type. Locally, an internal pyroxenite layer between these two seams, as well as multiple bifurcations of pyroxenite within the chromitite layers, ranging in thickness from 10 – 200 cm, and in some cases up to 10 m thick, give rise to a variation of the UG2 Reef known as the UG2 split reef-type. This disruption in the stratigraphy of the ‘normal’ UG2 Reef has resulted in a range of theoretical mining cuts. This presented an opportunity to study the UG2 split reef facies and classify it according to textural attributes, lithological proportions of the chromitite layers and performance behaviour determined through bench-top upstream processing.
The present study seeks to offer insights into the significance of key properties in the process behaviour of the UG2 split reef. Drill core samples representing the ‘normal’ UG2 Reef and the UG2 split reef were subjected to detailed petrographic studies, chemical assays, and benchtop milling and flotation test work. It is through an examination of the mineralogical characteristics that affect metallurgical performance, such as bulk mineralogy, sulphide and PGM grain size distribution, liberation and associations (including mode of occurrence) that the ‘normal’ UG2 Reef has better PGM recovery than the UG2 split reef. This is expected due to the more lithological variable nature of the latter. This increased complexity arises primarily from the presence of internal pyroxenite layers, the pronounced disruption, and the reduced proportion of the lower main chromitite layer (UM), which also affect the bulk mineralogy and distribution of the PGE mineralisation.
The variability in the lithological nature of the reef led to the classification of eight mining resource-cuts, namely: N_full cut and N_ULT210CUT that represent the ‘normal’ UG2 Reef and SR_UMC1, SR_UMC2, SR_UMC3, SR_LMC1, SR_LMC2 and SR_full cut that represent the UG2 split reef. Despite being classified as a UG2 split reef, theoretical mining resource-cuts SR_UMC1 depict good flotation performance (based on grade-recovery and enrichment ratio-recovery curves) compared to its counterparts. However, the associated prolonged milling times (over 40 minutes) are problematic and negatively affect energy consumption and costs...