This case study briefly highlights the potential to derive, in an approximate sense, likely processing behaviour from geological reconstructions. In turn, this information can feed into geometallurgical planning allowing the early stage recognition of potential processing flowpaths that can be used to optimise recovery. The case study is pulled together from analysis undertaken on the Merensky reef at Northam Platinum Ltd in South Africa.
The Merensky Reef at Northam Platinum shows a complex range of reef developments with several distinct reef types that are processed through the run-of-mine. These differences can be related to the paragenetic history of the deposit with the differing mineralogy related to the changing footwall mineralogy at the time of the hanging wall deposition. This case study looks at three of those reef types (the Normal Reef, the transitional Pothole reef and the full Pothole reef) which contain distinct differences in their mineralogical deportment.
The differing mineralogy of the footwall at the time of hanging wall deposition resulted in differences in modal mineralogy, the amount of floatable gangue and the sulphide textural development. These differences in turn led to predictable differences in milling times, mineral liberation and sulphide flotation performance. As the Merensky reef is a platinum-group element (PGE) ore with the majority of the platinum-group minerals contained within sulphides, these differences are crucial.
The Merensky reef, which was discovered in 1924 by Dr Hans Merensky, formed for many years the worlds’ premium resource of platinum. It consists of the Merensky Cyclic Unit (MCU) deposited unconformably as a drape over a stratified and mineralogically variable footwall (Figure 1; Viring & Cowell, 1999; Roberts et al., 2007). Mineralisation within the Merensky reef is present at both the base of the MCU and within the upper portion of the footwall. The variability in the footwall mineralogy at Northam Platinum mine during MCU depodition has significant processing implications which will be explored in this article. For further reference, similar research at the Bafokeng Rasimone Platinum mine also shows the effect of geological variations on flotation performance (Smith et al., 2013). All results presented in this article are derived from a laboratory scale set of experiments. The results are therefore presented as a comparison across standardised conditions but are not necessarily completely indicative of the behaviour on a processing plant scale.
At the Northam Platinum mine variation in the footwall mineralogy has led to development of at least three distinct reef types; Normal, NP2 and P2 reef. The Normal and P2 reefs are broadly mineralogically similar in that the footwall is melanocratic in each case and with the immediate lithology below the MCU chromitite consisting of pegmatitic pyroxenite underlain by another basal chromititic stringer. For the Normal reef this basal chromite stringer is underlain by anorthosite, whilst for the P2 reef the basal stringer is underlain by a more dunitic harzburgite. The NP2 reef is the most mineralogically distinctive of the three reef types consisting of the single MCU chromite overlying a predominantly anorthositic footwall. Within the anorthosite there is the development of a troctolite band which marks the downward extent of fluid infiltration from the MCU (Figure 2; Viring & Cowell, 1999).
Critically, the difference in footwall mineralogy, particularly of the NP2 reef effects the modal mineralogy (Table 1), the degree of floatable gangue, the sulphide textural development (Figure 3), expected milling times (Table 2), as well as sulphide grades and recoveries during flotation (Figure 4). The focus on factors affecting the sulphide development is key because platinum group minerals (PGM) show an intimate association with base-metal sulphides in the Merensky Reef (Becker et al., 2008; Brough et al., 2010), though this does not necessarily hold for the other reef types present within the Bushveld Complex (i.e. the UG2 and Platreef).
QEMSCAN analysis of the milled feeds of the three reef types show distinct differences in their modal mineralogy (Table 1). In comparison to the NP2 reef which is plagioclase rich (62.6 wt%), the Normal and P2 reef are plagioclase poor (16.7 wt% and 7.2 wt%, respectively). With relatively little plagioclase the principal gangue mineral in the Normal and P2 reefs is orthopyroxene (51.2 wt% and 68.6 wt%, respectively), which constitutes just 25.4 wt% in the NP2 reef. There are also differences in the total proportions of alteration minerals (defined as the sum of the minerals amphibole, serpentine, talc, chlorite as well as magnetite). The Normal reef contains the highest amount of alteration minerals (2.3 wt%), followed by the P2 reef (1.8 wt%) and lastly the NP2 reef (1.0 wt%). Biotite which may be primary or secondary is also greatest within the Normal reef (1.8 wt%) compared to the P2 (0.9 wt%) and NP2 reefs (1.1 wt%). The relative proportions of low-temperature alteration minerals such as talc are important since they are naturally floatable and may induce inadvertent gangue flotation if they are associated with other common gangue minerals (Becker et al., 2009; Jasieniak and Smart, 2009).
The basic gangue minerals are the same for each reef type (orthopyroxene, plagioclase, olivine, clinopyroxene, chromite and alteration phases) but relative to the NP2 reef, the Normal and P2 reef contain a far higher abundance of primary ferromagnesian minerals and their alteration phases. One important alteration phase is talc which is present in slightly greater abundance in both the Normal and P2 reefs. Talc is a highly floatable gangue mineral, capable of not only entering the concentrate through true flotation but also of carrying other gangue minerals such as orthopyroxene through association (Becker et al., 2009). Furthermore, talc has a froth stabilising effect promoting increased water recovery and therefore mass recovery by entrainment.
This increased quantity of floatable gangue has two main effects. The first is to lower base-metal sulphide (BMS) grades and this is seen for the Normal and P2 reefs, where initial grades are lower than the NP2 reef (Figure 4). The second is to slow the rate of BMS recovery and thereby reduce total BMS recovery (Figure 4). It is worth noting that this latter effect is only key in batch floats which are froth limiting and that the plant response may not be identical.
Across the three reef types there is one main sulphide texture and three subsidiary textures; the main sulphide texture is fine to medium grained (0.5-5mm), composites (Figure 3a), predominantly of pyrrhotite, pentlandite and chalcopyrite, but also with minor amounts of bornite, cubanite and possibly mackinawite. The three subsidiary textures are: fine grained (<0.2mm), largely monomineralic inclusions in the major silicate phases (Figure 3b) and chromite; sulphides concentrated within microfractures that occurred during brittle deformation (Figure 3c); and very fine unidentifiable sulphides located within secondary silicate minerals such as paragonite and serpentine (Figure 3d).
The Normal and P2 reefs contain all four sulphide textures, whereas the NP2 reef contains only composite sulphides (Figure 3a) and fine-grained sulphides (Figure 3b). The lack of very fine sulphide development and sulphide veining suggests that sulphides within the NP2 reef will be the easiest to liberate as is the case for chalcopyrite, pyrrhotite and the composite BMS (Brough et al., 2010).
The reason for this difference between the NP2 reef and the Normal and P2 reefs is the pegmatitic nature of the footwall, which is related to the initial ‘pre-MCU event’ footwall. Since the footwall to the Normal and P2 reef was melanocratic, the subsequent deposition of a new hot magma pulse (the MCU), led to reconstitution and grain coarsening. This encouraged the development of coarse composites as well as complex secondary remobilisation of sulphides, generating three subsidiary textures. In contrast the leucocratic NP2 footwall responded very differently to the deposition of the MCU. Rather than grain coarsening, the magma infiltrated between the plagioclase grains occasionally reacting with rare orthopyroxene grains. The result was largely undisturbed anorthositic or leucocratic layers within which a thin troctolized layer represents the downward extent of magma infiltration.
Milling times for the three reefs suggest that the NP2 reef is the softest, relative to the Normal and P2 reefs, with the P2 reef being the hardest (Table 2). This difference in hardness can be attributed to mineralogy, texture and degree of alteration. It implies that the plagioclase rich rocks (i.e. the NP2 reef) are softer that the orthopyroxene rich rocks (i.e. the Normal and P2 reefs). Furthermore, since the mineralogy of the Normal and P2 reefs are similar the role of alteration and texture are also critical. The Normal reef is slightly more altered that the P2 reef, and contains a much thicker inter-chromitite pegmatite (~150 cm, compared with ~40 cm). This suggests that the increased alteration and pegmatitic character of the Normal reef decreases its milling time relative to the P2 reef. The major implication of this is the expected ore throughput, with the NP2 reef capable of being processed quicker than either the Normal of P2 reefs (Brough et al., 2010).
The variations in sulphide development described above can be linked in with observed variations in liberation and grade recovery. Firstly, the dominance within all reef types of medium grained composite sulphides results in excellent liberation (>80%) for all BMS at both grind sizes (Brough et al., 2008). Since PGM are invariably associated with sulphides, any such PGM associated with the major composite sulphides can also be expected to be recovered.
Secondly, the presence of fine-grained BMS locked within primary orthomagmatic (e.g. orthopyroxene) minerals explains why sulphide recovery is not optimised for each of the three reefs. This is because after grinding the fine-grained sulphides will be locked or only partially liberated, and being trapped within hydrophilic minerals will be retained in the pulp. The lower sulphide recoveries observed within the Normal and P2 reefs values will partly be a function of the greater quantity of fine grained sulphides minerals present (Figure 4).
From this particular study the NP2 reef is the easiest of the three reefs to process, producing optimum sulphide recoveries and the highest sulphide grades and that these differences in performance can be traced back to the conditions at the time of formation. The different footwall conditions during MCU deposition resulted in a different modal and textural mineralogy. These mineralogical differences ultimately controlled the processing performance, having a direct effect on milling times, floatable gangue and sulphide liberation. Essentially, the paragenesis controlled the processing requirements. This case study shows the potential that accurate geological reconstructions have to impact on geometallurgical considerations.
Chris Brough1, Megan Becker2, David Reid3 and Dee Bradshaw4
Christopher Brough is a consultant mineralogist at Petrolab, Cornwall. He specialises in the application of mineralogy to a range of mining problems including the characterisation of acid-generating and acid-neutralising minerals with implications for acid rock drainage and metal leaching (ARDML) together with the characterisation of ore and gangue mineralogy with their implications for potential processing problems. He has an MEarthSc from Oxford University, an MRes from the University of Cape Town and obtained his PhD in PGE and chromite geochemistry from Cardiff University in 2011.
Megan Becker is a Senior Research Officer leading the process mineralogy initiative in the Centre for Minerals Research in the Department of Chemical Engineering at the University of Cape Town. She has a background in geology (BSc Hons, MSc – University of Cape Town) and a PhD in process mineralogy from the University of Pretoria (2009). Her research interests focus on understanding both the effect mineralogy has on the beneficiation process and the effect of the beneficiation process on the mineralogy, and how one can best use mineralogy information. She has a keen interest in teaching mineralogy and in mineralogical analysis techniques, and currently manages the QEMSCAN facility in the CMR. She has supervised close to 20 postgraduate students and has over 50 publications in international journals and conference proceedings in the area of minerals beneficiation (including comminution, flotation, hydrometallurgy and even environmental aspects such as acid rock drainage).
Dee Bradshaw is a Professorial Research Fellow and Research Leader at the Julius Kruttschnitt Mineral Research Centre (JKMRC) part of the Sustainable Minerals Institute. She joined the University of Queensland in 2008 after working for the Centre for Minerals Research at the University of Cape Town for over 25 years. Her interest in Process Mineralogy began in 2003 after a background in flotation chemistry. She is particularly interested in the processing of complex ores and in the development of a mineralogical framework for mineral processing in general. She has also been responsible for initiating and participating in post graduate courses in Flotation Chemistry, Process Mineralogy and Geometallurgy at UCT, JKMRC and the University of Tasmania. She has participated in many professional development courses for industry in Australia, South Africa and Canada. She has supervised over 40 post graduate research students and co-authored over 120 journal and conference papers.
David Reid received his Bachelor of Science degree (BSc.), Honours and MSc in Geology and Geochemistry from Victoria University of Wellington, New Zealand in 1970-73. He obtained his PhD in Geochemistry in 1977 from the University of Cape Town, South Africa where he is currently a Professor in the Department of Geological Sciences. He was a recipient of the Distinguished Teacher Award from UCT in 2000. He is a Fellow and Life Member of the Geological Society of South Africa since 1973 and has three times been awarded their Jubilee Medal in recognition of published research in South African geology and geochemistry. He is a Fellow of the Society of Economic Geologists since 1998. David Reid has published, lectured and consulted widely on topics related to economic geology and geochemistry, with particular emphasis on mineralization in Namaqualand, Bushmanland, Namibia and the Bushveld Complex. He also holds Directorships in mineral exploration companies listed on the JSE.
Becker, M., Harris, P., Wiese, J. and Bradshaw, D. (2009): Mineralogical characterisation of naturally floatable gangue in Merensky Reef ore flotation. International Journal of Mineral Processing, 93, pp 246-255
Becker, M., Brough, C., Reid, D., Smith, D., Bradshaw, D. (2008): Geometallurgical characterisation of the Merensky Reef at Northam Platinum Mine: Comparison of normal, pothole and transitional reef types, 9th International Congress for Applied Mineralogy, pp 391 – 399
Brough, C., Bradshaw, D. J., Becker, M. (2010): A comparison of the flotation behaviour and the effect of copper activation on three reef types from the Merensky reef at Northam, Minerals Engineering, 23 (11), pp 846 – 854
Jasieniak, M., Smart, R.S.C., 2009. Collectorless flotation of pyroxene in Merensky ore: residual layer identification using statistical ToF-SIMS analysis. International Journal of Mineral Processing. 92, pp 169–176.
Roberts, M.D., Reid, D.L., Miller, J.A., Basson, I.J., Roberts, M. & Smith, D. (2007): The Merensky Cyclic Unit and its impact on footwall cumulates below Normal and Pothole reef types in the Western Bushveld Complex. Mineralium Deposita, 42, pp 271- 292
Smith, A.J.B., Viljoen, K.S., Schouwstra, R., Roberts, J., Schalkwyk, & Gutzmer, J. (2013): Geological variations in the Merensky Reef at Bafokeng Rasimone Platinum Mine and its influence on flotation performance. Minerals Engineering, 52, pp 155-168
Smith, D.S., Basson, I.J. & Reid, D.L. (2004): Normal Reef Subfacies of the Merensky Reef at Northam Platinum Mine, Zwartklip Facies, Western Bushveld Complex, South Africa. The Canadian Mineralogist. 42, pp 243-260.