Dense medium separation (DMS) is a method often used to upgrade base metal sulfide (BMS) ores before their main processing stage, with varying results achieved for different ore types. The process makes use of the density differences between the BMS minerals and the lower density silicate/carbonate gangue minerals, using a separating medium of density between the two ore components. The separation is accelerated using a dense medium cyclone (DMC) to form two products: overflow (tailings) and underflow (concentrate). The purpose of DMS is to reject large quantities of gangue upfront, resulting in reduced time, energy and costs associated with processes such as milling and flotation. Preconcentration of ores using physical methods such as DMS is becoming an important consideration as lower grade ores are mined, to increase the feasibility of mining such ores. Two nickel sulfide deposits were chosen as case studies in order to understand differences in DMS efficiency for different ores. The first is the Main Mineralised Zone (MMZ) of the Nkomati Nickel deposit in Mpumalanga, South Africa, which is part of the Uitkomst Complex. The Phoenix deposit is also considered, and forms part of the Tati greenstone belt in eastern Botswana. Both deposits are magmatic Cu-Ni-PGE (platinum group element) deposits with similar sulfide mineralogy and pentlandite as the main nickel host. A process mineralogy approach was used to evaluate samples of both ores, describing the differences in the mineralogical properties within the overflow and underflow of each ore in order to understand the extent to which individual properties affect the separation. A bulk sample of each ore type was subjected to DMS using a pilot plant setup, and the overflow and underflow products further classified into a series of density classes using sinkfloat analysis. These density classes were mineralogically characterised by petrography, quantitative X-ray diffraction, QEMSCAN and electron probe microanalysis, to provide information on differences in bulk mineralogy, mineral textures, mineral chemistry and particle properties between the samples. The nickel contents of both ores were upgraded using DMS and the Nkomati ore experienced a more efficient separation than the Phoenix ore, which is contrary to previous tests on MMZ ore of similar grade. Both the Nkomati and Phoenix ores consisted of primary magmatic minerals such as pyroxene and plagioclase, as well as a variety of secondary silicates formed by alteration of the original mineral assemblages, e.g. amphibole, chlorite and talc. Three sulfide textures were observed: disseminated / bleb-textured, net-textured and massive. Both ores show more than one texture, with the Nkomati ore displaying all three textures and the Phoenix ore mostly consisting of disseminated sulfides with minor massive sulfides. Pentlandite in the disseminated zones dominantly occurs as fine exsolution lamellae in pyrrhotite, with granular pentlandite mostly located within massive sulfide regions. Apart from overall particle density, sulfide texture is the main controlling factor affecting the individual particle recovery by DMS, with massive and net-textured sulfides having larger grain sizes and therefore higher liberation than disseminated sulfides. In addition to the DMS concentration of sulfide minerals, primary and secondary silicate minerals are separated by their density differences, which can affect the recovery of finely disseminated sulfides associated with them. Silicatehosted nickel is another factor that accounts for higher nickel losses to the overflow, observed particularly in the Phoenix ore. Particle size is also an important control on DMS, where particles near the cut-point have a more-or-less equal chance of sinking or floating, and tend to separate on size rather than density. Small particles of less than ~2 mm are also more likely to float, causing even dense, sulfide-rich particles to be lost to the overflow. An evaluation of particle shapes shows that shape separation plays a minor role for the ores studied, and shape differences are most pronounced nearer to the DMS cut-point, where a higher proportion of irregular-shaped and elongated particles have been concentrated to the underflow. The ultimate aim of the characterisation of the DMC products would be to use the information gathered to be able to predict the behaviour of an ore being subjected to DMS, based on its mineralogy.

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