Abstract
Tailings are often poorly characterized. Several tailing dams contain sufficient concentrations of metals that are potentially economically feasible and can easily be reprocessed. Chromium, for example, is currently one of the most important industry elements. South Africa has a considerable resource of chromium, which makes it one of the highest producers in the world.
Current market conditions allow for the profitable recovery of chromium and platinum-group elements from flotation tailings of mines. Tailings are materials left over after the process of separating the economic fraction from the uneconomic fraction of an ore. Commonly, the volume of tailings produced by mining operations often exceeds the volume of the recovered minerals or metals. It is therefore important to run secondary inspections assuming that the tailings may contain significant amounts of minerals or metals that are of commercial interest. These minerals or metals may have been residuals of other minerals or metals that were primary targets but managed to escape the previous beneficiation process or may have been minerals or metals that were primarily never considered for extraction.
The mineralogical characterization and liberation of valuable minerals are a priority in the mineral processing industry. Norplats and Eastplats are currently investigating the potential of reprocessing chromite-rich tailings resulting from the processing of UG2 for PGE at their tailings storage facilities. The present study focuses on the characterization of these flotation tailings and their potential for the recovery of chromite and possibly PGE. To achieve this, UG2 flotation tailings were collected from the mines for the purpose of better understanding them from an academic standpoint. A geometallurgical study using the MLA and geochemical analysis was conducted for chromite and PGE minerals hosted in these flotation tailings. The geochemical analyses (which include XRF, fire assay, ICP-MS and ICP-OES analysis) were conducted at Intertek Genalysis while the mineralogical analysis was conducted on the FEI 600 field emission mineral liberation analyzer (MLA) at the University of Johannesburg. The MLA was used to obtain a full audit of
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mineralogical characteristics which included modal mineralogy, mineral grain size distribution, and mineral liberation, while geochemical analysis of the samples gave information on the elemental chemistry of these tailings.
The data resulting from the MLA and geochemical analysis showed that the bulk of chromite contained in these samples occurs as well liberated grains (> 90 %), regardless of grain sizes. Chromite particles that occur as middlings or are locked in other minerals, were typically associated with the coarsest particles of the samples. This association was commonly related to the silicate gangue minerals i.e., feldspar (mainly anorthite) and pyroxene (mainly enstatite). On the contrary, base-metal sulphides and associated PGE minerals mainly occurred as locked minerals which were commonly associated with the gangue minerals (i.e., feldspar, pumpellyite, or pyroxene) in which they were hosted.
MLA-based characterization of chromite indicated that the bulk of the chromite, which was primarily not considered for recovery, exists as well-liberated and fine grained (~85 % passing 75 μm) equant to sub-rounded minerals. Particle density distribution indicated that the bulk of particles are concentrated in the density group between 4.2 and 4.3 g/cm2 which is representative of the chromite which comprises the examined tailings. These findings suggest that chromite within these tailings may be amenable to gravity separation given that the bulk of it is well liberated and with the use of the correct density cut for recovery. In the case of BMS and PGE minerals, a substantial amount (<0.1% as typical in the UG2) of unrecovered minerals were found in the examined tailings. These minerals may have escaped the previous beneficiation process due to their very fine sizes (typically < 10 μm), making recovery difficult. Alternatively, the grind used in the attempt to recover these minerals may have been too small and thus reduced the surface area of the minerals resulting in slimes lost with silicate gangue in the tailings.
Keywords
Bushveld Complex, UG2 chromitite layer, Platinum-group metal (PGM), Platinum-group element (PGE), PGE minerals, Mineral liberation analyzer (MLA), Beneficiation, Geometallurgy, Flotation, Tailings.