The typical nickel-copper-PGE/PGM ore type has an igneous origin, and hosts the nickel and copper generally as pentlandite (Fe,Ni)9S8 and chalcopyrite CuFeS₂, often accompanied by varying amounts of pyrrhotite Fe1-xS (where 0 < x <0.2). The pyrrhotite could occur in any of three forms: monoclinic (common); hexagonal, or troilite (rare). In the case of monoclinic pyrrhotite, it is common to see both nickel in solid solution with the pyrrhotite, and as micro-sized blebs intruding the pyrrhotite crystal matrix. In pentlandite, it is also common to see cobalt in solid solution (generally in the range 0.1-0.5%) as well as palladium (in the range 10-50 ppm). The processing implications from this information are:
- In the Bushveld Complex of South Africa, the monoclinic pyrrhotite is enriched with platinum and palladium in solid solution, necessitating its successful flotation for higher PGE recovery, often with finer grinding and an optimised mixed collector system (Kinloch, 1982; Lotter, 1995; Lotter and Bradshaw, 2010; Wiese et al., 2005).
- Additionally for the Bushveld Merensky ore, some of the Platinum Group Elements (PGE) occur as discrete Platinum Group Minerals (PGM), which are recoverable by a gravity extraction process. Unrecovered residual PGM report to the final flotation concentrate, making the correct sampling and assaying thereof challenging.
- In Canada, the amount of pyrrhotite is sufficient to class it as a sulphide gangue that has to be rejected to tailings for environmental reasons. In the latter case, this causes a certain amount of nickel recovery loss due to the high degree of nickel in solid solution (Kelebek et al., 1996).
- The sampling and assaying of these ores for PGE thus becomes challenging, since due to the textural associations of the PGE and PGM, the frequency distributions of grades in metal accounting streams are multi-modal and skewed, and need to be treated in order to obtain unbiassed assay grades (Lotter et al., 2000).
The silicate mineralogy is ultramafic. The ferro-magnesian silicates are abundant in the whole rock assembly, and are prone to serpentinisation, a post-formation alteration caused by water interacting with the hot rock assembly, converting pyroxene to serpentine, and producing talc and magnetite as by- products. The processing implications from this geological history are:
- This hardens the work index of the ore as well as reducing the sulphide grain size distribution, bringing about finer and more costly grinding requirements.
- Another processing implication from serpentinisation is the control of talc in the flotation process, since it is naturally-floating and interferes with the sulphide flotation, reducing nickel recovery.
- The serpentinisation process is that it draws part of the nickel into solid solution in the serpentine, rendering that part of the nickel unrecoverable by flotation. Depending on the degree of serpentinisation, this loss has been seen in the range 5-40% (Peyerl, 1983).
- Where orthopyroxene has formed, this mineral is easily activated for flotation by the use of copper sulphate in the flotation circuit to activate the pentlandite. Such activation of orthopyroxene causes the mineral to respond well to collectors such as xanthate, and to float successfully at the expense of nickel recovery, as well as increasing the content of magnesium in the flotation concentrate. The latter causes elevated liquidus problems in slag formation at the smelter, however changes to the flotation reagent scheme may partly compensate for this by orthopyroxene depression (Kerr et al., 2003; Lotter et al., 2008).
In short, the expertise needed to develop a sustainable flowsheet for this type of ore is specialist. Please contact Flowsheets for any assistance in this matter. We have extensive experience and expertise in the flowsheet development of this range of ores.
References
Kelebek, S., Wells, P.F., Fekete, S.O., 1996. Differential flotation of chalcopyrite, pentlandite and pyrrhotite in Ni-Cu sulphide ores. Can. Metall. Q. https://doi.org/10.1179/000844396109608615
Kerr, A., Bouchard, A.P., Truskoski, J., Barrett, J., Labonté, G., 2003. The “Mill Redesign Project” at Inco’s Clarabelle mill. CIM Bull.
Kinloch, E.D., 1982. Regional trends in the platinum-group mineralogy of the critical zone of the Bushveld complex, South Africa. Econ. Geol. https://doi.org/10.2113/gsecongeo.77.6.1328
Lotter, N.O., 1995. A Quality Control Model for the Development of High Confidence Flotation Test Data, M.Sc. Thesis. University of Cape Town.
Lotter, N.O., Bradshaw, D.J., 2010. The formulation and use of mixed collectors in sulphide flotation, in: Minerals Engineering. https://doi.org/10.1016/j.mineng.2010.03.011
Lotter, N.O., Bradshaw, D.J., Becker, M., Parolis, L.A.S., Kormos, L.J., 2008. A discussion of the occurrence and undesirable flotation behaviour of orthopyroxene and talc in the processing of mafic deposits. Miner. Eng. https://doi.org/10.1016/j.mineng.2008.02.023
Lotter, N.O., O’Connor, C.T., Clark, I., 2000. The Relative Bias Errors of Gravimetric Fire-Assaying Practice for Platinum-Group Elements in Bushveld Merensky Ore at Rustenburg, in: SME Annual Meeting and Exhibit. Salt Lake City.
Peyerl, W., 1983. Metallurgical Implications of the Mode of Occurrence of Platinum-Group Metals in the Merensky Reef and UG-2 Chromitite of the Bushveld Complex, in: Special Publication – Geological Society of South Africa.
Wiese, J., Harris, P., Bradshaw, D., 2005. The influence of the reagent suite on the flotation of ores from the Merensky reef, in: Minerals Engineering. https://doi.org/10.1016/j.mineng.2004.09.013