In the western limb of the Bushveld Igneous Complex, platinum is mined predominantly from two reefs – Merensky and UG2. Most of the platinum group minerals (PGM’s) in Merensky ore are associated with base metal sulphides (BMS), and thus Merensky concentrators will usually resemble simple BMS circuits. However, the mineralogy of UG2 ore is more complicated, and thus UG2 circuits are also more complex. The UG2 reef is a chromitite layer in the critical zone of the Bushveld Igneous Complex, which results in high chromite content. Chromite causes significant complications in the downstream smelter process, and therefore chromite constraints are imposed on UG2 concentrators. A further aspect complicating the treatment of UG2 ore is that PGM’s are not only associated with BMS, but ultra-fine PGM’s are also locked in gangue minerals. This affects the milling and flotation characteristics of the circuits, as it is not possible to efficiently target the liberation and recovery of relatively large BMS and ultra-fine PGM’s in the same circuit. As a result UG2 circuits have evolved to deal with these issues in a number of ways. This thesis focuses on the design of milling and flotation circuits to optimise the recovery of coarse BMS (with associated PGM’s) and ultra-fine PGM’s contained in associated siliceous gangue minerals. In order to achieve this, UG2 circuits usually feature more than one milling and flotation stage. In the primary stage, a relatively coarse grind targets the liberation and recovery of BMS, while much finer grinds in subsequent milling stages target the liberation and recovery of PGM’s locked in siliceous minerals. In recent years stirred mills have been incorporated into UG2 circuits for the liberation of fine PGM’s locked in gangue. These mills grind exclusively via attrition type breakage, as opposed to the predominant impact breakage in a ball mill. The development of these multi-stage circuits to treat UG2 ore was based to a large extent on a qualitative evaluation of the ore and mineralogy, as well as by trial and error over many years of operation, rather than being built on a scientific foundation. Therefore, the aim of this project was to gain an understanding of the effect of breakage mechanism and circuit configuration on the floatability profile of a UG2 ore. This information would then be used to establish a formal design framework for the selection of mill type, and the number of milling and flotation stages to meet specified performance criteria. In order to achieve these goals, it was necessary to design a set of experiments that would investigate multiple millfloat circuits and various milling devices. The experiments were conducted on a pilot scale with a ball mill, IsaMill and Stirred Media Detritor (SMD), together with Lonmin’s flotation pilot plant. The ball mill represented a milling device in which impact breakage would predominate, using steel balls as a grinding media, whilst the IsaMill and SMD represented devices that grind exclusively via attrition, using inert grinding media. The mills and flotation cells were arranged into different millfloat configurations to test the effect of multiple milling and flotation stages on the PGM recovery of UG2 ore. The data was fitted to a kinetic model adapted from the literature to determine the floatability of the ore as a function of its treatment in the different circuits tested. Some new modelling methodologies were developed to incorporate disparate data sets into the same modelling framework. An analysis of the performance of the different circuits showed that PGM recovery increased with an increase in the number of mill-float stages, while a single mill-float stage produced higher initial grades. On a size-by-size basis additional mill-float stages resulted in an increase in PGM recovery in all size classes, while no minimum size was detected for optimum flotation (the highest recoveries for all three circuits were achieved in the finest size fraction). A detailed analysis of the data and floatability profiles revealed the following: (i) Multi-stage circuits minimise over-grinding, and this manifests as an improved recovery in the finest size fraction that was measured (-10m). (ii) Sub 10m liberated PGM’s displayed higher floatabilities than locked or partially liberated PGM’s in coarser size fractions, resulting in the higher recoveries observed in this size fraction for all circuits. (iii) Preferential liberation of PGM’s was observed for this ore. This meant that stagewise removal also favoured recovery in coarser sizes, since partially liberated PGM’s were recovered before being liberated to a finer size fraction by additional milling. (iv) Attrition breakage is more efficient than impact breakage at liberating valuables in the finest size fraction, but also more prone to over-grinding of liberated PGMbearing BMS particles. Therefore circuits with attritioning devices are more likely to benefit from multiple mill-float stages. Impact breakage is more efficient at liberating particles from coarse size fractions – the host will usually be shattered and the valuable mineral liberated to a smaller size fraction. (vi) In contrast, attrition breakage gradually chips away at the host particle, often achieving partial liberation of the valuable particle in a relatively coarse size fraction. Therefore, attrition breakage favours recovery in coarser sizes, as valuables are liberated in a coarser size fraction than with impact breakage. This does not indicate that attrition breakage is more efficient at liberating valuables from coarse sizes, as impact breakage will usually liberate more particles from coarse sizes, but these liberated particles are recovered in finer size fractions. (vii) For UG2 ore, it was found that significantly higher PGE concentrate grades could be achieved with stirred mills. Results indicate that this was driven by the liberation of PGM’s from BMS, since the recovery of fine, liberated PGM’s would result in higher PGE grades than the recovery of PGM’s associated with BMS. From these observations and findings it was possible to construct a framework for the milling and rougher design of UG2 circuits. The information required for the design is a target size distribution, size and associations of the valuable minerals and the hardness and breakage characteristics of the ore. From this information the number and type of milling devices, as well as the number of flotation stages can be selected. The target grind for each stage is driven to a large extent by economic return-on-investment considerations, although the recovery-grind relationship of the ore can assist in making that decision. Although the work and design framework was established for UG2 ore, it can be used to design the main stream milling and rougher circuits for most ore types.

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