A Process Mineralogical Evaluation of Chromite at the Nkomati Nickel Mine, Uitkomst Complex, South Africa

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.

Using Process Mineralogy as a Tool to Investigate Blending Potential of the Pentlandite-Bearing Ores at the Nkomati Ni Mine in South Africa

“…The geology of the Uitkomst Complex and mineralization at Nkomati Nickel Mine have been well documented by [11,[13][14][15][16][17][18][19]. This study focuses primarily on the MMZ and the PCMZ orebodies that, at the time of undertaking this study, were being exploited at the Nkomati mine (the mine is currently under care and maintenance).…”

“…The mine feed cut-off grade is 0.3% Ni for the MMZ ores and the target grind employed at the mine is 67 wt% passing 75 µm, while the mine cut-off grade of the PCMZ ores is 0.25% Ni with a target grind of 80 wt% passing 75 µm. Metal recovery is accomplished through froth flotation [18,19].…”

“…The samples were hand-sorted according to sulfide texture types. Following the approach documented by [19], ten samples, about 12 kg each, were collected from the MMZ ore stockpiles (high-and medium-grade ore stockpiles) and three samples were collected from the PCMZ ore stockpiles, respectively. The ten samples from the MMZ comprised high-and medium-grade variants of the bleb (denoted by Bleb_HG for the high-grade variant and Bleb_MG for the medium-grade variant), disseminated (Dis_HG and Dis_MG), massive (MS_HG and MS_MG), semi-massive (SMS_HG and SMS_MG), and net-textured (Net_HG and Net_MG) sulfide ores.…”

“…Before starting flotation, 65 mL (or about 60 g dry weight) of the swirling pulp in the flotation cell was collected for chemical analysis ('assay') of the flotation feed. Details of the reagents used, additional sequence and conditioning times are as per [19]. The reagents added included primary collector sodium isobutyl xanthate (SIBX), secondary collector Senkol 700, frother Senfroth 200, and depressant Sendep 300.…”

“…Nkomati is the only primary nickel producing mine in South Africa, with pentlandite being the main nickel-bearing mineral. Two ore horizons are being mined for nickel, these being the Main Mineralized Zone (MMZ) as well as the Peridotitic Mineralized Zone (PCMZ), with each of these characterized by a variety of ore textures and mineralogy [11,18,19]. The ore types of the MMZ ores are comprised of: high-and medium-grade bleb (Bleb_HG and Bleb_MG); disseminated (Dis_HG and Dis_MG); massive (MS_HG and MS_MG); semi-massive (SMS_HG and SMS_MG); and net-textured (Net_HG and Net_MG) varieties.…”