Browsing by Author "Mainza, Aubrey Njema"

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    Open Access
    Contribution to the understanding of the three-product cyclone on the classification of a dual density platinum ore
    (2006) Mainza, Aubrey Njema; Powell, Malcolm
    The 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.
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    Open Access
    Developing a methodology for characterising in-situ viscosity profiles in tumbling mills
    (2012) Mangesana, Nobathembu; Mainza, Aubrey Njema; Govender, Indresan
    Milling is the most expensive operation in a mineral processing circuit and accounts for the highest amount of energy utilized. The efficiency of the milling process is affected by the rate of breakage of bigger rocks and the rate of transport of the slurry within and out of the mill. The transport of material in the mill is dependent on the rheological properties of the slurry such as viscosity. Viscosity is therefore one of the important parameters that influences the transport of the material in the mill. Slurries in tumbling mills are known to exhibit non-Newtonian behaviour. This means slurry viscosity is not constant but is a function of shear rate. To characterize slurry transport in the mill it is necessary to study the viscosity distributions in order to determine regions of high and low resistance to flow in the mill. To determine the influence of slurry viscosity on the transport of particles it is important to quantify the typical shear rate ranges that occur inside the tumbling mill. The aim of this project w as to develop a methodology for characterising viscosity distributions inside a tumbling mill using in-situ shear rate distributions obtained from PEPT at different solids concentrations and mill speeds. The P EPT technique was used to study the in-situ mot ion of slurry particles with in tumbling mill charge using a single radioactive tracer. Rheology experiments were conducted using a U-tube rheometer. The rheometer experiments were performed to characterise the rheological behaviour of the El Soldado slurry at a wide range of shear rates. The PEPT results provided information about the typical shear rate ranges that occur inside the mill. The combination of these results enabled the quantification of viscosity distribution from the slope of the rheogram at typical shear rates found in the tumbling mill. The rheology results indicated that the Bingham model is the rheological model which gives the best description of the rheology of El Sol dado slurry. It had the highest R² adjusted values at all tested solids concentrations concentration ranges. The rheological behaviour of El Soldado slurry with a particle size fraction of -75+53 microns can be described as Newtonian. This means that the viscosity is constant and does not vary with shear rate. At low solids concentrations up to 30wt%, the viscosity values attained are equivalent to that of water at room temperature at 0.001 Pa.s. At higher solids concentrations up to 60wt%, the viscosity increases to 0.007 Pa.s for the shear rate range tested. The Bingham viscosity and yield stress increases in an exponential form with increasing solids concentration. PEPT experiments were conducted to quantify and characterise in-situ shear rates in a laboratory scale tumbling mill. The highest shear rates were obtained at the lowest solids concentration for both mill speeds. It was 30s-¹ at 60% critical mill speed and 36s-¹ at 75% critical mill speed. Mill shear rates decreased with increasing slurry solids concentration. This was attributed to increased particle-particle interactions and reduced voidage. There is less volume available for particle shearing. The magnitude of the maximum shear rate is higher for the 75% critical mill speed at all slurry solids concentrations compared to the shear rates at the lower mill speed. At a higher speed the mill charge is fairly dilated by the strong centrifugal effects that oppose the natural packing structure which results in an overall decrease in bulk density. The increased voidage allows more volume for relative motion, producing an overall increase in shear rate. The narrow shear rate range and the Bingham rheological behaviour of the slurry resulted in a constant viscosity value that could be used in a viscosity model provided the solids concentration is uniform across all regions of the mill. Viscosity is more significantly impacted by solids concentration than shear rate distributions in the tumbling mill. Work should be done to study the solids concentration profiles in the tumbling mill to provide better insight on areas of active transport.
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    Open Access
    Development of a wet fine screen model integrating the effect of operating and design variables on screening performance
    (2016) Mabote, Seipati; Mainza, Aubrey Njema; Bepswa, Aaron Paul
    Mineral processing is the process that involves liberation and beneficiation of valuable constituents of an ore. Several physical beneficiation processes exist and one such process is classification. Screens are classification devices sometimes used in the classification stage of closed grinding circuits to separate mill product into different size classes. Poor classification of particles results in reduced throughput, high power consumption and over - grinding. Most of the research on screening has been done in scalping applications or classification at relatively large cut sizes. There is limited work done on screening at feed sizes of minus 150 μm and there are no robust models for wet fine screening application for use in circuit simulation studies. The effect of feed flow rate, solids concentration and aperture size on wet fine screening performance was evaluated in this study. The range of values of the factors investigated were the feed rate (9, 13, 19, 25, 30 and 35 t/h), screen aperture size (45, 75, 106 and 150 μm) and solids content (30, 40, 50 and 60%). A pilot Derrick screen plant at Mintek in Johannesburg was used for the experiments on a UG2 and chromite ore blend. Screen undersize and oversize samples were collected for particle size distribution analysis and mass balance calculations. The samples collected were filtered, de - lumped and split down to masses ranging between 200 and 300 grams for wet screening using the Malvern MasterSizer particle analyser. The results were used to analyse the effect of the investigated factors on the wet fine screening performance. These results were used to develop a wet fine screening model. Results indicate that increased feed flow rate and solids concentration lead to finer cut sizes, reduced sharpness of separation and higher water recoveries to the oversize. An increase in aperture size increased the sharpness of separation and decreased the water recoveries to the oversize. The solids concentration appeared to have a higher effect on cut size than the feed flow rate. The highest cut size and sharpness of separation and lowest water recovery to oversize were attained at the lowest feed rate. The lowest solids concentration produced the best performance with regards to all partition curve properties. The cut size approached the aperture size at the lowest throughput and solids concentration for all aperture sizes. All the efficiency curves exhibited fish hooks at fine particle sizes with the fish hooks becoming more pronounced at higher feed flow rates and solids concentration and smaller aperture sizes. A wet fine screen model that includes multi component ores as well as changes in operating conditions was developed using the 2 - parameter Whiten screen model as a basis. The dimensional analysis approach was applied in developing the sub - models that relate the operating and design parameters to the Whiten model parameters. The dimensional analysis approach was further applied to develop the model that describes the fish hook effect for subsequent incorporation into the overall modified fine screen model. Generally, the modified model is capable of predicting the performance of the wet fine screen reasonably well with minor errors and accommodates for the data that exhibits the fish hook. The model also reduces the fitting process required in the original Whiten model.
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    Open Access
    Effect of operating variables on IsaMill™ performance using platinum bearing ores
    (2011) Chaponda, Brian; Mainza, Aubrey Njema
    Comminution involves crushing and grinding operations. The grinding operations use the traditional tumbling mills and stirred mills to reduce the ore to the required fineness. This thesis intends to investigate the influence of design and operating variables on the IsaMillTM specific energy and product size, when grinding UG2 platinum-bearing ore. The main objectives of this work were to study the effects of operating variables on specific energy consumption and product fineness, and to investigate IsaMillTM scale-up protocol. The experimental studies were conducted using the M4 IsaMillTM on a laboratory scale and the M10 000 IsaMillTM on an industrial scale.
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    Open Access
    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.
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    Open Access
    Investigation and modelling of the progression of zinc leaching from large sphalerite ore particles
    (Elsevier, 2013-01) Ghorbani, Yousef; Petersen, Jochen; Becker, Megan; Mainza, Aubrey Njema; Franzidis,Jean-Paul
    X-ray Computed Tomography (CT) was used to follow the progression of Zn leaching in a number of individual sphalerite ore particles, which were subjected to a long-term simulated heap bioleaching environment. The ore was prepared by two different modes of comminution – HPGR at 90 bar and cone crusher – and individual particles were selected from three different size fractions. Investigation of the reacted fraction of Zn vs distance from the centre of each particle indicated that leaching from large particles leads to near complete conversion near the surface, but only partial conversion in the zones that are closer to the centre of particles. The cores of the cone-crushed particles show hardly any conversion at all, especially in the larger particle sizes. Mathematical analysis shows that leaching from the large particle does not follow the shrinking core model. It is shown that the progression can rather be described by a combined reaction-diffusion process progressing through the network of cracks and pores closer to the particle surface. Extent and depth of this network are a function of particle size and comminution method. A simplified rate model is proposed that describes the extent of leaching as a function of time in terms of a set of parameters that can all be related to just particle size and crushing mode.
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    Investigation of particles with high crack density produced by HPGR and its effect on the redistribution of the particle size fraction in heaps
    (Elsevier, 2013) Ghorbani, Yousef; Petersen, J; Becker, Megan; Mainza, Aubrey Njema; Franzidis, Jean-Paul; Kalala, JT
    The application of comminution technology such as the High-pressure grinding rolls (HPGR), which is able to generate a high density of cracks in the ore particles, is favourable for leaching processes. Extraction of metallic values by the heap leach process, can take place on the particles with partial exposure of mineral grains, if it can provide sufficient surface front for chemical attack by leaching solution. The aim of this study was to assess the benefits of high crack density in the ore particles produced using the HPGR and how it could diminish due to inadequate percolation of the leaching agent. A zinc ore was comminuted using HPGR at three different pressure settings and with a cone crusher for the control experiment. Subsamples from the (+23/-25, +14/-16, +5.25/-6.75 mm) size fractions were characterized and packed into leach reactors. The reactors were stopped from time to time to investigate the progress of crack and micro-crack growth and its effect on metal extraction using the X-ray computed tomography (CT). The results are validated with those obtained using traditional techniques such as SEM and QEMSCAN. Investigation of the leach reactors residue indicated significant changes in the particle size distribution (PSD) of initial feed toward the fine size fraction. The residues from the reactors leaching the material prepared using the HPGR product contained more fine particles than the reactors, which were fed by cone crusher product. These differences were up to 10.3%.
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    Investigation of the flotation behaviour of ball mill and IsaMill products
    (2012) Khonthu, Tsepang; O'Connor, Cyril; Wiese, Jenny; Mainza, Aubrey Njema
    Valuable minerals that are used in various aspects of everyday life are buried in fossilised storages below the ground in the earth's crust. These minerals are mined as rocks which then have to be crushed in order to liberate these minerals of value. The liberated valuable minerals have to be winnowed from the rock powder. Flotation is the main process which is used in mineral processing to recover valuable minerals. This process uses the differences in surface properties of particles to separate hydrophobic particles from hydrophilic ones. The strong relationship between the method of crushing the rocks and the amount of valuable minerals which can be reclaimed from the crushed rock has long been realized in mineral processing. The type of mineral, the size to which the rock must be reduced to and the amount of energy needed for this size reduction are among the most important factors which guide the decision on the type of device to be used in pulverising mineral rocks. Physical properties, with the exception of particle size, of mill products have not been investigated as thoroughly as the chemical properties. Physical properties include surface roughness and particle shape. The differences in shape, between particles produced by different mills, with respect to these properties, have been attributed to differences in the breakage mechanisms in the mills used to grind the particles. There is contradicting literature on the breakage mechanisms that dominate in various mills. This confusion is exacerbated by the fact that different breakage mechanisms occur simultaneously in any one mill. The definition and determination of particle shape are also difficult. Subsequently, the effect of particle shape on flotation is a subject that is rife with contradictions which add to the complexity of the subject.
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    A mathematical model for predicting classification performance in wet fine screens
    (2015) Mwale, Adolph Ntaja; Mainza, Aubrey Njema; Kallon, Daramy; Bepswa, Aaron Paul
    Screening is a well-known classification process in the minerals processing industry. The process involves separation of fine particles from coarse particles based on size and is applicable to both dry and fine screening. Fine screening is normally carried out wet. Until recently, fine wet screening had been limited to relatively low throughput applications. Developments in the recent past have seen the evolution of fine screening to high capacity applications. It has found application in operations such as closed circuits with a mill in place of hydrocyclones. However, even though developments are increasing, there has been a process model developmental lag. A fine wet screen model that can be used for unit simulation purposes to predict screen performance outcomes or integration into other models to simulate and predict process performance is necessary. Most existing screen models are for dry and coarse screening applications. This thesis is aimed at developing a fine wet screen process model for predicting wet screening performance in the 45 - 150 μm range. Pilot plant testwork was conducted using a UG2-Chrome ore blend as feed.
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    Using positron emission particle tracking (PEPT) to investigate the motion of granular media in a laboratory-scale tumbling mill
    (2012) Morrison, Angus James; Govender, Indresan; Mainza, Aubrey Njema
    Positron emission particle tracking is a Lagrangian, single particle tracking technique in which the trajectory of a representative tracer particle is triangulated from the decay products of the positron-emitting radioisotope with which it is labelled. Although the trajectories of a tracer particle moving in a bulk of similar particles can be of interest, it is often more informative to employ the ergodic assumption and to thus convert trajectory data in the Lagrangian reference frame of the tracer particle into a fixed Eulerian reference frame. This has, in the past, been done by dividing 3D space into voxels and assigning a location probability density to each voxel based on the number of times that triangulated tracer particle locations fall into it- a process called simple binning. A major outcome of my work has been to develop an alternative probability density based on the cumulative time spent by the tracer particle in a given voxel. This method is called residence time binning, and the resultant probability distribution- which I argue is proportional to, among other things, the mass and solidicity distributions of the tracer particle - the residence time distribution (RTD). In this work I propose, implement and test the residence time binning method, and show that it significantly outperforms the simple binning method in all situations. A second thrust of my work has been to develop a suite of general analysis routines for positron emission particle tracking (PEPT) data, based on the RTD. This suite contains routines for the triangulation, optimisation and pre-processing of PEPT data, as well as for obtaining residence time probability and time-averaged kinematic distributions in 3D space, and for aggregating and visualising the results. I have also extended this general set of routines for the special case of cylindrical symmetry through the addition of routines for the further pre-processing of RTDs, as well as for the calculation of angular measures about an arbitrary axis in space. Finally, I further extended this set of routines for application to tumbling mills. My tumbling mill analysis includes the identification of charge features and regions, and the isolation of charge in each region so-defined for further analysis. These features, particularly the shape of the bulk free and equilibrium surfaces, the angular position of the centre of circulation (CoC) of the charge, and the position of its impact toe allow me to characterise the behaviour of the charge under a range of conditions. This characterisation, together with the shear rate distributions and power draughts that I also calculate, allow me to speak meaningfully about the evolution of grinding regions in tumbling mills- information that could be used to construct charge motion and grinding models to inform the use of tumbling mills in industry. In this work, I apply these analysis routines to a small subset of the experiments performed by the UCT Centre for Minerals Research (CMR) on laboratory-scale tumbling mills, and in so-doing elucidate the behaviour of charge in its different regions- and the evolution of such behaviour with mill operating parameters- and discuss the implications of these to grinding efficacy in tumbling mills.