Mineralogical characterisation of naturally floatable gangue in Merensky Reef ore flotation

During the recovery of the valuable platinum group minerals and base metal sulphides in the Merensky Reef ore by flotation, a certain amount of naturally floatable gangue (NFG) is recovered, which dilutes the concentrate. Although talc is the only silicate mineral well known for it naturally floatable properties, the proportion of NFG recovered in batch flotation tests of the Merensky ore, however, cannot be solely accounted for by the mass of pure talc present in the feed ore. Using methodology developed at the University of Cape Town to decouple the NFG from entrained gangue recovered in flotation, a series of concentrate samples have been analysed in this study using quantitative mineralogical analysis methods. Results show the major diluents of concentrate grade are orthopyroxene, talc, clinopyroxene and plagioclase in decreasing order of abundance. Orthopyroxene is however, by far the greatest diluent of concentrate grade even though the particles recovered are classified as liberated or high grade middlings. On closer examination, these orthopyroxene particles show a preferential surface association to talc and vice versa which is consistent with the low temperature alteration of anhydrous orthopyroxene to a hydrous phase, namely talc. These composite orthopyroxene particles with partial talc rims are therefore thought to be one of the main contributors to NFG in the Merensky Reef ore.