oa South African Journal of Chemical Engineering - The benefit of separate milling of silicate and chromite particles for chromite-rich UG-2 ores



Extraction of platinum group elements (PGE) is a major source of revenue in South Africa and the reserves represent about 75 per cent of world reserves. Most of the remaining PGE reserves are located in the UG-2 chromitite layer of the Bushveld Igneous Complex. Platinum concentrators experience significant losses of valuable PGE in their secondary milling circuits due to insufficient liberation of platinum-bearing particles. The chromium oxide (CrO) content in UG-2 concentrates is typically 3%, which results in severe operational problems in the downstream smelting process. This paper follows up from the work done in 2010 which indicated that spiral concentrators could be incorporated into the secondary milling circuit design for UG-2 platinum concentrators for separate grinding of platinum-bearing silicate particles and chromite-rich particles.

A detailed mineralogical analysis was conducted on a plant audit sample from the secondary rougher tailings (SRT) stream for comparison with the laboratory test work to assess the degree of potential losses of PGEs experienced with the conventional milling circuit.The effect of the size-by-assay analyses of the laboratory milled products and flotation tailings for a low grade UG-2 ore sample from the hydrocyclone (densifier) underflow were compared to the standard process. The effects on the recovery of PGEs, and the entrainment of CrO were measured in combined batch rougher flotation tests. Mineralogical evaluations of the SRT plant sample showed losses of about 37% of the PGEs in that stream to the coarse fractions (+75 µm). The chromite entrainment was significantly reduced (>30%) with a 3.6% increase in the PGE recovery. This paper confirms the benefit of separate ball milling in the secondary milling circuit for a chromite-rich (>50%) UG-2 platinum ore.


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