The recovery of platinum-group elements from the UG-2 chromitite, Bushveld Complex - a mineralogical perspective

Abstract

¶The behaviour of platinum-group elements (PGE) during milling and flotation of the UG-2 chromitite layer is still poorly understood. The PGE in the UG-2 chromitite occur primarily in discrete platinum-group mineral phases, varying from sulphide assemblages (predominantly cooperite, braggite, malanite and laurite) to assemblages comprising a significant component of alloys (such as Pt-Fe alloy) or various telluride, bismuthinide, bismuthotelluride, arsenide and sulpharsenide phases. The mode of occurrence of platinum-group minerals ranges from being predominantly associated with base-metal sulphides (either locked within, or at the grain boundaries of, sulphide minerals) to being predominantly associated with gangue minerals (locked in chromite or silicate, or at grain boundaries of chromite and silicate grains). During milling, more than 50% of the platinum-group mineral grains in the samples investigated were liberated from the ore matrix. During flotation tests, most of the platinum-group mineral grains that remained associated with gangue reported to the flotation tailings, and those associated with liberated base-metal sulphides to the flotation concentrates. Because of their small grain size (generally less than ∼10 μm), the response of the liberated platinum-group mineral grains was more difficult to predict. It was found that most of these grains did report to the flotation concentrates, albeit at a slower rate than the base-metal sulphides. The flotation behaviour of these grains was determined both by grain size and type of mineral, with coarser braggite, cooperite, and to a lesser extent malanite being the fastest floating species.