Due to continually declining ore grades, increasing mineralogical complexity, and increasing metal demand, models for the design and optimisation of minerals processing operations are of critical importance. These models do not currently incorporate particle shape, which, although rarely quantified, is known to affect numerous unit operations. Automated Scanning Electron Microscopy (Auto-SEM-EDS) is a widely used tool for mineralogical analysis. It also provides an opportunity for simple, quantitative and mineral-specific shape characterisation. Existing mineralogical databases could therefore become useful resources to facilitate the incorporation of shape effects in minerals processing models. A robust Auto-SEM-EDS shape characterisation methodology is required to ensure that the particle shape information in these databases is interpreted appropriately. For this work, a novel methodology for Auto-SEM-EDS shape characterisation was developed that is suitable for the analysis of fine particles (<75 μm). This involved testing the response of various shape descriptors to image resolution, and measurement with different devices and image processing routines. The most widely used shape descriptor in minerals processing, circularity, was found to be highly dependent on both image resolution and image processing settings, making it a poor choice for shape characterisation of fine particles. Roundness and aspect ratio were found to be more robust descriptors. However, in the interest of being able to compare particulate shape measurements across different studies, the precise definition of aspect ratio is important as variation in ‘length’ and ‘width’ definitions can significantly impact aspect ratio measurements. The possibility that preferential orientation of particles would introduce bias to the 2-D cross-sectional measurements was also addressed through comparison of roundness distributions measured from orthogonal cross-sections of a particulate sample mounted within a block of resin. The excellent repeatability of these measurements indicated that the particles were randomly orientated, and thus it can be inferred that 2-D measurements of a sufficient number of particles will be directly related to the particulate sample’s 3-D properties. Roundness and aspect ratio were then used in conjunction to produce surface frequency distributions that allow for distinction between non-rounded particles that were smooth and elongated and non-rounded particles that were neither elongated nor smooth. Three applications of the shape characterisation methodology developed were then demonstrated, which highlighted some of the potential contributions that this methodology can make towards minerals processing. The applications were all based on a case study of the Upper Group 2 (UG2) Chromitite, a platinum group mineral (PGM) ore of key economic significance to South Africa. The first application of the shape characterisation methodology constituted a novel approach to quantify phase boundary fracture, a mode of breakage in which cracks tend to propagate along the boundaries between mineral grains. The mineral specific shape characterisation that is possible with Auto-SEM-EDS was used to assess the conservation of grain shape during breakage, quantifying the extent to which phase boundary fracture led to liberation by detachment of the chromite grains. The critical importance of phase boundary fracture to the liberation of platinum group minerals and thus the economic processing of UG2 ore was demonstrated. Both the data presented, and the novel approach, can provide valuable evidence to support the development of liberation models. The second application of the methodology involved fine grinding of UG2 ore in ball mills and stirred mills, with particle shape analysis used to provide insight into breakage mechanisms. The particle shape characteristics of feed and product samples from both laboratory and plantscale ball mills and stirred mills were compared. None of the milling conditions led to product particles being more rounded than feed particles. This suggested that the common perception that fine grinding involves a higher degree of abrasion than primary grinding is inappropriate if the definition of abrasion used is that which is commonly linked to rounding of particles. Particle shape was clearly dependent on ore texture and phase boundary fracture at coarse sizes, but below the chromite grain size distribution (less than 75 μm) there were minimal differences in shape between the major mineral components. The particle roundness distributions were similar for the laboratory stirred mill sample and the ball mill and IsaMill samples taken from the plant, suggesting that ore characteristics are the dominant factor controlling particle shape. The similarity in shape characteristics of samples from the plantscale ball mill and IsaMill led to the deduction that differences in the performance of a UG2 ore flotation circuit with an IsaMill on- or off-line are unlikely to be attributable to particle shape effects. Finally, entrainment is an important issue in most flotation circuits as it reduces concentrate grade. It is particularly important for UG2 ore due to the negative impact of chromite gangue on smelter performance. Little appears to exist in the published literature correlating particle shape with entrainment, and therefore, the third application for the shape characterisation methodology was focused on this topic. This required an adaptation of the methodology in order to ensure that particle size was not confounding the results. The findings based on samples taken from a UG2 ore concentrator indicated that the chromite particles recovered to the concentrate streams were significantly more rounded than chromite particles of the same size in the feed and tails streams. This suggests that shape could have a significant effect on entrainment, although in this study it was not possible to identify the mechanisms responsible, and more research would be required to determine whether the observations are generalizable. The approach to decouple the effects of shape from size could be applied to other areas of minerals processing research, and could prove particularly useful for the incorporation of effects of shape in classification models for devices such as screens, hydrocyclones, thickeners, reflux classifiers and air classifiers.

show more