Investigation of the breakage mechanism in high speed disk-type impeller stirred mills and its influence on the mineral liberation

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The goal of comminution is not only size reduction but most importantly is the liberation of the minerals of interest so that they can be effectively separated from the gangue in the downstream separation process. It has been known that comminution has a significant influence on the way minerals are liberated and in recognition of this, it is important to have a better understanding of how the liberation properties of the minerals are affected by the method of size reduction. This thesis investigates the type of breakage mechanism in stirred mills, particularly at the regrind stage, and how it influences the liberation properties of the minerals of the rougher concentrates. Three rougher concentrates of the Tampakan ore were selected for this study: Primary (contains chalcopyrite as the main copper-bearing mineral); Medium Sulphur (contains bornite as the main copper-bearing mineral, with a significant amount of chalcocite; and High Arsenic (contains bornite as the main copper-bearing mineral with the presence of chalcocite and a copper-arsenic mineral in the form of enargite). These concentrates were comminuted in the M1.5 V Netzsch stirred mill at different durations to produce different product size distributions. Progeny particles were then characterised using the JKMRC’S mineral liberation analyser (MLA) on a size-by-size basis. It was found that the dominant breakage in stirred mills is attrition based on the observations on the particle size distributions of the feed and the products as well as the evolution of the shape of the particles as grinding progressed. The mode of breakage in stirred mill was assessed based on the observations of the patterns in the particle size distributions and the evolution of the particle shape in the products ground for different durations. A shifting of the peak towards finer size fractions with increasing grinding time which is characteristic of an impact breakage mechanism was not observed in the products of the stirred mill. The peaks observed in the feed were still present after grinding, although the size of the peaks at the relatively coarser size fractions significantly reduced while the peaks found in the fine size fractions increased with increasing degree of comminution. This trend in the particle size distributions between the feed and the products is an evidence of an attrition mode of breakage. Analysis of the particle shape distributions of the feed and the products showed that attrition produced two populations of particles - the core particles which inherit the shape of the parent particles and either remain in the current size fraction or goes to the next smaller size fraction with shapes that are generally elongated and smooth; and the attrited particles that are chipped off from the surface of the parent particles and report to the +10 µm and -10 µm size fractions. Furthermore, it was also observed that while breakage via attrition makes the particles smoother, it does not always result in the production of rounder particles as the result of this study showed that the particle shape distributions of the feed and the products did not change with increasing degree of comminution. The degree of comminution was not found to have any effect on the size-by-size liberation of the minerals. However, the overall liberation of the valuable copper-bearing minerals have shown an improvement with respect to an increase in the degree of comminution. This is due to the increase in the amount of fine particles generated as a result of increasing the degree of comminution. The simulation of the attrition breakage using image processing was found to predict accurately the size-by-size liberation of bornite in Medium Sulphur. However, for the bornite in High Arsenic and chalcopyrite in Primary, a deviation from the actual liberation was observed. Reasons for the deviations were discussed in terms of the possibility of the presence of differential breakage rates among the gangue minerals, dissemination of the valuable minerals in the surface of the particles and the grain size of the valuable minerals. In spite of the deviations observed in the simulated liberation from the actual, there are potential ways to refine the simulation methodology to be able to achieve better predictions. The results of this thesis clearly show that attrition is the dominant breakage mechanism in stirred mills and the degree of attrition does not change the size-by-size liberation characteristics of the minerals. This result will aid in the development of fundamental models in mineral liberation resulting from comminution using stirred mills.