Browsing by Author "Bremner, Sherry"

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    Developing a methodology for reducing diamond breakage within processing plant
    (2021) Chele, Motsi John; Mainza, Aubrey; Evertsson, Carl M; Bremner, Sherry
    Diamond breakage has been a problem experienced by diamond operations. Material breakage characterisation methods has been used to determine the hardness or resistance to breakage of diamond host rock, ceramic diamond simulants and simulants embedded in the concrete blocks. This establishes a relationship between specific input energy and degree of breakage that can be used for size reduction to minimise diamond breakage. Ceramic diamond simulants have been used in the process to identify areas that are more prevalent to diamond breakage. It was found that sections of high impact such as the cone crushers and drop height in the surge bins had the highest risk of diamond breakage. Kimberlite ore and ceramic diamond simulants were subjected to compressive breakage in drop weight test. The progeny particle size distribution and degree of breakage were compared. Standard breakage characterisation models were fitted to the breakage data of tested material and relative hardness parameters determined to establish the energy threshold. The breakage tests results showed that the ceramic diamond simulants were very hard while the kimberlite ore and concrete blocks were medium to soft. The material hardness parameters were determined from fitting the breakage data to the standard impact breakage characterisation models (t10-Ecs breakage model and Size dependent breakage model). Concrete blocks and Kimberlite ore showed less resistance to compressive breakage as demonstrated by higher A values compared to the ceramic diamond simulants. Applying material hardness categories presented by Napier-Munn et al (1999), Kimberlite ore was soft, concrete blocks ranged medium to soft and ceramic diamond simulants very hard. The remedial measures implemented in the process were to rubber line the concentrate bins in the recovery to minimise the impact forces, as well the surge bins in the process plant were controlled in such a way that reduces the drop height. Finally, the cone crushers and pan feeders operating philosophy has been improved to start at high speed to achieve choke feed conditions faster and to promote interparticle crushing. The close side settings were also optimized to reduce liner to liner interaction rather enhance particle-to-particle interaction. Through the optimised process, it had been observed that the diamond breakage had dropped below 5% level of the total stones recovered at +5cts. The methodology developed proved to be working after being tested in the chosen flowsheet.
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    A granular flow model of an annular shear cell
    (2016) Bremner, Sherry; Govender, Indresan; Mainza, Aubrey Njema
    Machinery such as an IsaMillTM used in communition to produce fine particle sizes that allow minerals to be extracted are best modelled using granular flows. A single rheological description that captures all the features of granular flows has not yet been realised, although considerable progress towards a complete theory has been made. Existing models of such horizontally stirred mills are empirical, tend to be extremely dependent on boundary conditions and do not allow for confident extrapolation beyond their window of design. As a first step to understanding the dynamics inside the IsaMillTM,a constitutive stress model of a horizontal annular shear cell is developed. This shear stress model was used in an athermal energy balance to develop a description of the power dissipation, which drives the communition purpose of the IsaMillTM. The key ingredients (velocity, shear rate and volume fraction distributions) to the granular ow model are extracted from experiments using Positron Emission Particle Tracking (PEPT), as well as Discrete Element Method (DEM) simulations. 5mm glass beads were used to fill an annulus 51mm wide. In the PEPT experiments, two different surfaces of the driving wall (the inner cylinder of the shear cell) were used, over two shearing velocities. The effect of two friction coefficients over a range of shearing wall velocities were examined in the DEM simulations. The data were examined over 3 selected radial lines and utilised to calculate the shear stress distribution and the power dissipation from the developed models. It was found that even the usually simple relations describing the dynamics within a vertical shear cell are greatly modified by changing the orientation of the rotation axis.