### Browsing by Author "Powell, Malcolm"

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- ItemOpen AccessApplication of CFD to hydrocyclone flow prediction(2004) Dlamini, Mduduzi Felix; Powell, Malcolm; Meyer, ChrisThe mechanical simplicity of the hydrocylone deceptively suggests non-intricate and well-understood device hydrodynamics. On the contrary, improved understanding on the hydrodynamic behaviour and operation of the hydrocyclone are still being sought in order that, its full classification potential may be realised. Current models describing such behaviour are predominantly highly empirical hence the need for continued complementary research via computational techniques such as Computational Fluid Dynamics (CFD). This dissertation presents the outcome of an investigation into the flow characteristics of a hydrocyclone. Assessment of its hydrodynamic behaviour was conducted via interpretation of numerically predicted velocity and pressure field profiles arising from single phase (water-only) and multiphase (water/air) flow fields.
- ItemOpen AccessContribution to the understanding of the three-product cyclone on the classification of a dual density platinum ore(2006) Mainza, Aubrey Njema; Powell, MalcolmThe detrimental effects exhibited by the conventional hydrocyclone in classifying ores that contain different density components motivated the AMIRA P9 project to look into modifications to the conventional hydrocyclone that can improve separation by component density. In this project a cyclone termed the three-product cyclone has been developed and tested. The three-product cyclone is a modification of the conventional hydrocyclone with an additional vortex finder termed the inner vortex finder inserted concentric to the existing one, termed the outer vortex finder, resulting in three products from the same hydrocyclone. The three-product cyclone produces a finer overflow stream, an intermediate overflow stream comprising fine high density particles and medium sized light particles, and a coarse underflow stream. The work presented in this thesis involved designing an overflow arrangement which allowed interchangeable inner vortex finders to be tested, and a special rig for industrial scale experiments using a 600mm diameter hydrocyclone.
- ItemOpen AccessThe effect of circuit configuration on regrind circuit performance(2004) Mathye, Musa W; Powell, Malcolm; Sweet, CraigIt was identified that potential improvements in recoveries were to be found in the better use of the regrind stages at a number of Anglo-Platinum concentrators. Historically, these concentrators used to operate in an open circuit configuration, over a relatively short period, the circuits were changed to closed circuit configurations because of the reported benefits of operating in this configuration. The effect of closing the circuit had, however not been quantified in these concentrators. This dissertation looks at quantifying the effects of each configuration with the purpose of drawing meaningful comparisons between them. The circuit configurations studied are open circuit, I-stage and 2-stage closed circuit configurations. The primary difference between these configurations is the network of hydrocyclones. This study pays special attention to the effect of cyclone performance on the regrind circuit performance. Comparisons were made between the open and the 2-stage closed circuits in three concentrators, and surprisingly the closed circuit did not show any improvement over the open circuit configuration from a combination efficiency perspective. The cyclones performance was analysed, and it was observed that the cyclones were operating inefficiently, most likely because the cyclones employed were not appropriate for regrind circuits. A further comparison was made between the I-stage and 2-stage circuit configurations and the I-stage closed circuit configuration proved to be more efficient than the 2-stage closed circuit configuration. Simulations were conducted to predict the performance of the circuit configurations by using more appropriate cyclones which are smaller in size. These cyclones are currently being used in a similar application in another concentrator. The simulations indicated that significant improvements in comminution performance could be realised with the utilisation of more appropriate cyclones, to achieve the closed circuit configuration.
- ItemOpen AccessMeasurement of particle interaction properties for the incorporation into Discrete Element Methods(2005) Chandramohan, Rajiv; Powell, MalcolmThe principle aim of this research project is to measure parameters which are pertinent for numerical simulations in discontinuous media. One such numerical tool, the Discrete Element Method (DEM), is a promising technique for predicting the dynamics of charge motion with in a mill. Particle interactions in DEM are calculated by contact force and force displacement laws at each particle contact. These contact events are characterized by parameters that are often fitted or estimated due to the lack of accurate experimental measurements. The aim of this project is to experimentally measure the necessary interaction properties required for the DEM analysis and to test the DEM models against the measured experimental results. An in-ﬂight binary collisions drop tester is constructed to measure the material interaction properties of two spheres. The collision event is captured photographically and pre- and post- relative velocities are measured. The binary collisions of the particles are carefully controlled by relay timing circuits and they are captured on digitized images using a SLR digital camera. The particles are illuminated using digital strobes controlled by a signal generator. The heights of the colliding particles are adjusted to vary the drop velocities prior to collision. The measured relative velocities arc applied in rigid body theory of binary impact to extract the required material interaction properties. The parameters measured from the binary collision include coefficients of tangential and normal restitution and friction. The analysis presented here draws on the work of Maw et al and Foerster et al, which is an extension of the Hertz theory of impact to the oblique impact of the elastic bodies with circular contacts. Initial numerical simulations using the viscous damping model is performed in Particle Flow Code (PFC) and a comparison between experimental and numerical results presented.
- ItemOpen AccessSegregation and material flow along low aspect SAG mills(2006) Mwansa, Sonny; Powell, MalcolmThe significance of this study is primarily intended to provide better understanding of axial segregation and flow of particles along SAG mills. Additionally, the current SAG mill models developed using "short" Australian and North American style mills exposes inaccuracies when applied to predict the performance of "long" mills. Thus the results of this study are expected to be useful for improving modelling of long SAG mills. Several studies of radial and axial segregation in tumbling horizontal devices at laboratory-scale and a few on pilot-scale have been conducted in the past by various researchers. Recently, industrial-scale studies of axial segregation in kilns and dry milling, particularly in the cement industry have also been conducted. However, there seems to be no work reported and cited in the literature on axial segregation studies along SAG mills. Thus the lack of previous segregation research along SAG mills has exposed a knowledge gap in the subject.
- ItemOpen AccessUsing discrete element modelling (DEM) and breakage experiments to model the comminution action in a tumbling mill(2008) Kulya, Chisenga; Powell, Malcolm; Mainza, Aubrey; Govender, Indresan; Sarracino, RobertThe Discrete Element Method (DEM) is a powerful modelling tool that characterises the system at the individual particle level. This makes it particularly well suited for simulating tumbling mills whose charge is principally individual particles (steel balls, rocks and fines). The use of DEM to simulate tumbling mills has proliferated since the early 1990s and been successfully employed to predict important milling parameters such as charge motion, power draw, liner wear and impact energy distribution. The ultimate aim of any model of the tumbling mill is to predict the product of the milling process. Current DEM simulations of the tumbling mill however do not simulate the breakage of the particles and as such can not directly predict the product. In order to predict the performance of industrial-scale tumbling mills, laboratory-scale mills are used to experimentally obtain data, which is then scaled up using black box mathematical models. In this thesis a tumbling mill model that utilises the power of DEM to provide the mechanical environment and the energies available for breakage is proposed. The incorporation of DEM eliminates the need to scale up because DEM is able to simulate the actual industrial-scale device. Data from breakage experiments on the ore being treated is also incorporated into the model to determine the breakage functions. Population balance techniques are applied in the mathematical framework of the model to predict the product of the comminution process. In order to test the proposed tumbling mill model, DEM simulations of a 1.695m diameter pilot SAG mill using charge based on actual operation data were performed and analysed. Results from the DEM simulation and Drop Weight Tester breakage experiments were then used in the proposed tumbling mill model to predict the evolution of the product size distribution.
- ItemOpen AccessX-ray motion analysis of charge particles in a laboratory mill(2005) Govender, Indresan; Powell, MalcolmThe work reported herein is of an automated X-ray vision system used to track the 3D trajectories of a typical bulk charge particle within an experimental Perspex mill, the intention of which is to simulate the grinding motion of rock found in typical industrial mills The experimental rig is constructed through an optimisation scheme that determines the maximum allowed dimensions of the mill that can be imaged by the X-ray system, however, the optimisation principle is not limited to the current study and can be employed to maximise any volume being investigated with the X-ray system. The raw data for each position of the tracked particle comprises of two X-ray images of the tumbling mill generated orthogonal to each other, with a phase lag between them. The correction of the phase lag between the biplanar images is a new addition to the usual usage of the system resulting in an effective sampling rate of 100 frames per second, thereby ensuring that the resolution is sufficient to conduct detailed kinematic studies. The processing of the raw images are achieved with a dynamic template matching algorithm followed by a modiﬁed, and improved, implementation of the Canny edge detector, while the centering of the edge images are based on an adapted conic ﬁtting routine, resulting in an overall subpixel centering accuracy. The processed images are then mapped to object space using the direct linear transformation (DLT), equipped with a physically valid variance model that is shown to improve the standard implementation even when robust solvers are employed. The ﬁnal reconstruction accuracy of the tracked particle was shown to be 0.15 mm and is achieved without iteration due to the appropriateness of the variance model. The high accuracy data. was initially used to benchmark the discrete element method (DEM), providing the ﬁrst numerical comparisons that surpassed the usual end-window snapshots employed by other investigators for the purpose of validation. The analysis of the data was not restricted to DEM veriﬁcation, and in some sense, surpassed the initial objective by yielding trends useful to communication practices. Amongst these analyses was the ﬁnding that the circulation rate of the charge is not once per mill revolution but greater, depicting a linear trend with mill speed. The slip between charge layers was shown to follow a linear pattern, with the degree of slip increasing linearly with mill speed. The phenomenon of charge surging was quantiﬁed, resulting in a trend for the variation of the surge amplitude with mill speed. A mechanism for the surging phenomenon was also proposed in this study. A particularly useful outcome of the data analyses was the formulation of a power model through heuristic trends of the center of mass (CoM) and center of circulation (CoC) of the charge. The methodology outlined by the model was shown to be robust, providing a correct approach to obtaining a truly fundamental power model based on generally applicable principles.