No abstract

A process mineralogical study based on three texturally and mineralogically different chromite-bearing ore types at the Nkomati nickel mine was undertaken, with focus on chromite. Chromite is a by-product of the Ni-Cu-Co-PGE ore at Nkomati Nickel mine. These being the PCMZ_MG (medium-grade Ni-Cu sulphide silicate ore with disseminated chromite), PCMZ_HG (high-grade Ni-Cu sulphide silicate ore containing disseminated chromite) and MCHR (massive chromite unit) ore types. These were processed using benchtop flotation followed by gravity concentration using a shaking table at different grind sizes. Quantitative mineralogical data was obtained using a 600F Mineral Liberation Analyser for the unprocessed and processed ores at three selected target grinds. The Mineral Liberation Analyser data indicated that increased milling does not relate to increased chromite grades and recoveries, particularly for the disseminated PCMZ type ores based on laboratory-scale gravity concentration. The recovery is controlled largely by the chromite chemistry. The results also showed that the MCHR samples that underwent a pre-flotation stage before gravity separation had better Cr2O3 grades (45% to 47%) and recoveries (52% to 61%) than MCHR ore that did not undergo a pre-flotation stage, which recorded grades ranging from 44% to 46% and recoveries ranging from 43% to 60%. This holds promise for the blending of MCHR ores with the PCMZ ores. The PCMZ ores also displayed better Cr2O3 grades and recoveries at coarser grinds. The optimal target grind to process all three ore types is a P80 of 75 μm, which is the current grind size employed at Nkomati Nickel mine. Due to the low nickel price and grade the Nkomati Nickel mine is currently under care and maintenance.

The mineralogy and texture of Ni-sulfide ores at the Nkomati nickel mine are highly variable, and this results in often erratic nickel recovery at the mine. The variability of the ore presents an opportunity to study the influence of grind size on the flotation-based recovery of Ni in highly heterogeneous sulfide ores, which would be applicable to this ore type at many other mines worldwide. In view of this, a process mineralogy investigation was conducted on thirteen mineralogically and texturally different nickel-sulfide ores from the Nkomati Nickel Mine, with a view on the influence of grind size on the flotation performance of pentlandite. Ore types presented include medium- and high-grade variants of the bleb, disseminated, massive, semi-massive, and net-textured sulfide ores of the Main Mineralized Zone (MMZ), as well as disseminated chromite-rich nickel sulfide ore and massive chromitite ore of the Peridotitic Chromitite Mineralized Zone (PCMZ). Laboratory scale metallurgical test work, comprising of sequential grinding and bench-top flotation testing of the ores, was conducted in combination with quantitative mineralogical investigation of the flotation feed and associated flotation products, using a FEI 600F Mineral Liberation Analyzer. The ore types under consideration require a variety of grind sizes (i.e., milling times) in order to attain optimal recovery of nickel through flotation. This is predominantly controlled by ore texture, and also partly by the abundance of the major constituent minerals in the ore, being pyroxenes, base metal sulfides, and chromite. Liberation of pentlandite is directly correlated with grind size (milling time), which is also positively correlated with the level of nickel recovery through flotation. A grind size of P80 at 75 µm results in the highest concentrate nickel grades of 7.5–8.1% in the PCMZ ores’ types which is the current grind for the PCMZ ores at Nkomati. A grind size of P77 at 75 µm yields the best overall pentlandite liberation, Ni recoveries of 84–88% and grades of 5.3–5.6% in the MMZ ores. This holds the potential to produce the best overall pentlandite liberation, nickel grades, recoveries from blending the MMZ and PCMZ ore types, and milling the composite ore at a target grind of P80 at 75 µm.