Browsing by Author "Mainza, Aubrey"

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    Open Access
    A study of the charge structure and energy utilisation in a Stirred Media Detritor using DEM-SPH
    (2022) Ndimande, Conrad B; Mainza, Aubrey
    The Stirred Media Detritor, (SMD), is a grinding device used for fine and ultra-fine grinding applications in mineral processing. The SMD has a vertically orientated shell that supports a shaft, with protruding impeller arms for agitating the charge. There is currently limited understanding of charge structure and motion in the SMD, particularly the interaction of the media and the slurry. Additionally, the number of arms and their arrangement on the shaft, are important aspects of the impeller that determine flow, energy consumption and grinding efficiency. Impeller geometry choices affect these characteristics of the process. This work focuses on studying the flow of grinding media and slurry for the industrial scale SMD 1100- E. This information is used to explore charge dynamics and energy utilisation in the SMD. To investigate the effect of impeller arm configuration on the operational behaviour of the SMD, the commercially available impeller configuration of the industrial scale SMD 335-E is used as the base case. Mill charge dynamics, transport and mixing, patterns of energy absorption on the mill surfaces are examined for the base case and compared to three different impeller arm arrangements. A two-way transient coupled Discrete Element Method (DEM) and Smoothed Particle Hydrodynamics model is used to achieve this. The ceramic grinding media is represented by the DEM component of the model, which is fully resolved, while the slurry (water and fine particles) is represented by the Smoothed Particle Hydrodynamics (SPH) model. The focus is on steady state operation therefore discharge from and feed into the mill are omitted. A nominal media size of 8 mm is used. The rotational action of the impeller forces the charge to the mill wall creating vortex centred on the mill shaft. The vortex is conical with a large diameter at the bottom, which decrease towards the bottom of the mill. Abrasion is found to be the dominant breakage mechanism in the SMD. Mixing behaviour is complex with media transfer past layers of impeller arms being influenced by the fall distance of media between impeller arm encounters.
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    Assessing the effect of cone ratio, feed solids concentration and viscosity on hydrocyclone performance
    (2014) Muzanenhamo, Pharaoh Kudzaishe; Mainza, Aubrey; Waters, Jason
    In the mineral processing industry, comminution circuits contain slurries composed of a mixture of particles of varying degrees of liberation and size. Hydrocyclones are commonly used to classify these particles. If the classification stage is not efficient, both grinding and flotation cannot be optimised or operated efficiently. Ores that are mined in industry contain metals of varying specific gravity, which makes recovery of the desired metal challenging. This study focussed on the effect of the hydrocyclone spigot to vortex finder diameter ratio, termed as the cone ratio, solids concentration and viscosity on the classification of two dual density ores. The rheological characteristic of the overflow was also evaluated. A UG2 ore (Upper Group), which consists mainly of silicates, and chromite, and an iron ore, containing mainly iron and silicates, were used as feed material. The UG2 test work was carried out on a University of Cape Town (UCT) 4 inch Multotec cyclone, while the iron ore test work was carried out on an Anglo-American 4 inch Krebs cyclone. The cyclone performance was assessed using the corrected cut size, water recovery to the underflow, sharpness of separation and feed throughput. The rheological characterisation of both the UG2 and iron ore were carried out using an AR (ARES-G2) 1000EX vane rheometer. The results obtained indicated that the cyclone cone ratio, feed solids concentration and viscosity influence the cyclone performance. For the UG2 it was observed that as the cone ratio increased the cut size decreased and levelled off at a cone ratio of 1. However, for the iron ore it was observed that the cut size increased with an increase in the cone ratio, until it reached a peak at a cone ratio of approximately 0.68, before decreasing. The water recovery to the underflow increased with cone ratio and solids concentration and for both ore types. However it was observed that the water recovery was more sensitive to the cone ratio within the range of conditions investigated. The sharpness of separation for the UG2 ore increased with cone ratio for all solids concentrations investigated and reached a peak at a cone ratio of approximately 1 then decreased. The sharpness of separation for the iron ore illustrated different trends at different feed solids concentration. Between 10 and 20 wt. % feed solids concentration the sharpness of separation for iron ore was fairly constant, while at 50 wt. % solids concentration the sharpness of separation increased with cone ratio and then levelled off at a cone ratio of 0.67. An increase in the volumetric throughput with cone ratio was observed for both ore types. Rheological characterization revealed Bingham plastic behaviour for both ore types. An increase in the feed viscosity led to an increase in the cut size, water recovery and sharpness of separation for both UG2 and iron ore. A comparison of the results with a semi mechanistic model revealed a good fit for the volumetric throughput, water recovery and viscosity. However, the sharpness of separation and cut size had more scatter. The standard error for the sharpness of separation model fit was 21% for UG2 and 23% for iron ore while the error for the cut size was 41 % for the UG2 ore and 43 % for the iron ore. It was recommended that for future work, test work should be carried at a constant pressure in order to assess purely the effect of cone ratio. Furthermore, a coarser ore should be used in order to evaluate the effect of cone ratio and feed viscosity on the individual deportment of the prevalent components in the dual density ore types investigated by carrying out assays.
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    Assessing the influence of lifter profiles on the velocity profile and the charge toe and shoulder using data from the PEPT system
    (2013) Hartmut, Brodner; Mainza, Aubrey
    The research focus of this thesis is lifter height effect on different charge characteristics. Thecharge characteristics identified were shoulder and toe angle, charge size and toe height. Anattempt was made to develop a model for the velocity profile incorporating lifter height.Therefore the objectives of the study are to: •Determine the effect of lifter height on the velocity profile using the particle tracking data. •Determine the effect the lifter height on the charge shoulder and toe. •Develop a velocity profile model including lifter height using granular flow theory and to compare the model to experimental data. The motivation for the study is that lifters are one of the most important design variables in a mill. Without lifters the mill’s energy efficiency would decrease. In Meaders & MacPherson, (1964) the effect of lifters on energy was quantified to be between 20% and 30%. The lifters control the height and angle of departure from the charge at the mill shell and therefore control the impact area and magnitude. The area and magnitude of the charge impact will affect the grind of the mill and energy utilization in the mill. The thesis was also aimed at generating data that can be used to model the velocity profile that can be incorporated in power models. Most of the power models do not account for the effect of lifters. The experiments involved collecting data from three dimensional particle tracking of selected particles in the charge. using the PEPT system. The PEPT experiments were conducted at IThemba Labs in Cape Town South Africa using a 300 mm x 285 mm experimental mill. The charge used for the experiments were glass beads with an approximate specific gravity (SG) of 2.7. The PEPT system operates by tracking the x, y and z coordinates with respect to time of an irradiated particle (tracer). The experiments were run under different conditions to evaluate the effect of the lifter height. The experiments were operated by varying mill speed (55%, 70% and 85%), mill fill (20%, 30% and 40%) and lifter heights (1.5 mm, 3 mm, 6 mm and 10 mm).
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    Assessing the Influence of mineralogy and texture on the ore breakage characteristics of drill core and crushed ore using the JKRBT
    (2020) Hill, Herbert Hill; Becker, Megan; Mainza, Aubrey
    Textural variability is a key component in addressing process challenges resulting from variability in the ore being mined. Textural variability arises from differences in the types of mineral grains present, their relative abundance and the type of interactions they have with one another. Increased textural variability is the largest contributor to mineral processing challenges in terms of mill throughput and flotation concentrate grades. Processing of ores with high textural variability often results in reduced throughput and the recovery of lower grade product, if low-grade material is not eliminated prior to arriving at the concentrator. Geometallurgy provides a powerful tool to manage ore variability better by using geological and metallurgical information during plant design and operation. The geometallurgical approach contributes towards minimising and controlling operational and technical risk of ore variability. Ore breakage characterisation is a pivotal part of geometallurgy which aims to quantify the relationship between the energy supplied for breakage and the size of the resultant progeny. The Julius Kruttschnitt Rotary Breakage Tester® (JKRBT) is an ore breakage characterisation device designed as a geometallurgical tool which can use both crushed ore and drill core samples. Drill core is especially important as it the material used for geometallurgical testing during exploration and resource definition. The JKRBT is more accurate and the test work is less time consuming than its predecessors. However, sample availability is a major concern when performing metallurgical testing as numerous tests need to be performed to get a complete view of the metallurgical response of the given ore type. This means that very little of the sample is available for ore breakage characterisation. The aim of this work is to understand the relationship between mineral texture and the ore breakage characteristics of several samples for both drill core and crushed ore which are the two sample types used at exploration and operational levels. The work is aimed at addressing the problem of sample availability in geometallurgical testing by developing an ore breakage characterisation test protocol that uses minimal sample to extract relative hardness indices. To assess the influence of mineral texture on the ore breakage characteristics, five different ore types were used. The ore was prepared by coring different size drill core and crushing using a jaw crusher. The samples were subjected to controlled single impact breakage tests using the JKRBT. A standard test consisted of 3 energy levels (low, medium and high) tested on 3-4 particle size fractions (small, medium, large and very large). The least particle protocol was developed using an ore which was considered to be fine-grained and homogenous. In developing the protocol, all the steps of the standard procedure were followed except the number of particles per test was progressively reduced from 30 particles to a threshold of 5 particles. From ore breakage characterization tests performed, it was observed that ore S was the most resistant to breakage while ore P was the least resistant to breakage. Ore A was found to be more competent than ore B and ore C but less competent that ore P. The differences in the ore breakage characteristics were attributed to the grain size distribution of the dominant constituent minerals for each sample. The findings were attributed to an increase in the surface area available for contact due to the decrease in grain size which results in less stress per unit area and thus more resistance to breakage. The hardness of an ore can be considered to be a function of the mineral hardness and its relative abundance. Using the relative mineral abundance and Mohs hardness scale, it was concluded that the more abundant the harder minerals in an ore, the more resistant to breakage the ore is. Drill core particles consistently produced a coarser progeny than crushed particles at the same conditions. The observed trend was attributed to differences in the particle shape between the crushed particles (angular) and drill core particles (cylindrical). Angular particles have a larger surface area exposed for energy absorption and therefore break more easily than drill cores. Chapter 5 showed that the proposed abridged ore breakage characterisation test that uses the minimal number of particles to extract ore breakage indices can be applied for both homogenous and heterogenous ore types. The results also show that the proposed least particles protocol can be used for ore breakage characterisation tests using both crushed ore and drill core particles. The number of particles can be reduced to as little as 10 particles per test while still obtaining the same ore breakage indices as those obtained from the standard procedure. The abridged protocol will be especially useful in situations where the amount of ore available for metallurgical testing is limited.
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    Circulation rate modelling of tumbling mill charge using positron emission particle tracking (PEPT)
    (2013) Von Kallon, Daramy Vandi; Govender, Indresan; Mainza, Aubrey
    This research is focused on developing theoretical understandings of charge circulation trends as observed in tumbling mills at different operating conditions. Of particular interest is the underlying assumptions being made by many mill models that a particle imparts energy for potential breakage only once per revolution of the mill to the charge body – that is, that the circulation rate of mill charge can be assumed to be constant irrespective of the speed at which the mill is run. The trajectory data used in this thesis is derived from positron emission particle tracking (PEPT) experiments conducted at the University of Birmingham positron imaging centre and further experiments were conducted at the iThemba LABS in Cape Town. The experimental approach is highly suited to allow the effective examination of the assumption that the grinding charge in these mills circulates at a constant rate of unity.
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    Developing a ball mill model that incorporates grate and overflow discharge configurations
    (2023) Mwale, Adolph; Mainza, Aubrey
    The purpose of this thesis was to investigate performance differences between the grate and overflow discharge type ball mills of the same size under similar operating conditions using platreef ore which is classified as rheologically complex. The results from this investigation were used to develop a ball mill model that incorporates discharge mechanism and variables that influence mill power draw and product size distribution. Discharge configuration effects and power draw being one of the most important factors in ball mill performance, this thesis was set out to include these in performance prediction and analysis. The pilot ball mill operated by Magotteaux installed at the Anglo Platinum plant in Rustenburg was used to collect performance and operational data for use in the modelling. Two discharge configuration types were tested. The grate at two set open areas of 9.5% and 3.4% were used as well as the overflow type. The 1.25 m diameter and 2.20 m length mill was operated at two ball-load set points and a feed rate range of 1.50 tph to 2.25 tph for each configuration. Results showed that the grate configured ball mill produced much finer product compared to the overflow when operated under similar conditions. Results further showed that the grate ball mill had relatively higher breakage and discharge rates when operated at same conditions as the overflow. It was also shown that the grate discharge ball mill will draw an average of 18% more power than the overflow ball mill of the same size under similar operating conditions. Using these results, a new ball mill model was developed. The model is based on the principle that breakage distribution of the ore is unique to its competence and specific energy. The model incorporates mill operating conditions and retains the use of the appearance function. The model further incorporates influences of design and operating variable on mill power draw to predict product size distribution. It was found that the model agrees well with the pilot ball mill performance data. The model was later sub-developed into two models each specific to the discharge configuration type, the grate and overflow. For each configuration type, the model was found to give good predictions in agreement with real data. Thus, a model that considers the unique properties of the ore and discharge configuration incorporating mill power draw was developed.
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    Developing a mechanistic model for flow through a perforated plate with application to screening of particulate materials
    (2016) Ogunmodimu, Olumide; Mainza, Aubrey; Govender, Indresan; Franzidis, Jean-Paul
    Screening in mineral processing is the practice of separating granulated ore materials into multiple particle size fractions, and is employed in most mineral processing plants. Models of screening performance have been developed previously with the aim of improving process efficiency. Different methods have been used, such as physical modelling, empirical modelling, and mathematical modelling including the discrete element method (DEM). These methods have major limitations in practice, and experimental data to validate the models have been difficult to obtain. Currently, the design and scale-up of screens still relies on rules of thumb and empirical factor methods rather than a fundamentally based understanding of the behaviour of the granular system. To go beyond the current state-of-the-art in screen modelling requires a clear understanding of the particle motion along a dynamic (vibrating) inclined plane. Central to this understanding is the notion that granular systems exhibit a unique rheology that is not observed in fluids; i.e. neither Newtonian nor non-Newtonian. It is thus imperative to fully quantify the granular rheology, which is determined by the depth of the particle bed along the screen, the solids concentration, and the average velocity of the granular avalanche on the screen. The concept of granular rheology is important. Existing empirical models of vibrating screens tend to be extremely dependent on boundary conditions of a particular machine design. The concept of granular rheology is important because, akin to fluid flow, granular flow exhibits different flow regimes depending on the extent of energy input in the system. This work employed DEM to quantify the granular rheology of particles moving along a vibrating inclined screen in order to begin the development of a phenomenological model of screening. The model extends the visco-plastic rheology formation of Pouliquen et al. (2006) to capture the kinetic and turbulent stresses obtained in granular flow on an inclined vibrating screen. In general, DEM was employed to develop a mechanistic model of screening which includes a description of the rheology of granular flow on a vibrating screen. Microscopic properties of granular flow were used in DEM to simulate screening of particulate materials. Granular mixtures of two particle constituents (3 mm and 5 mm) were simulated on an inclined vibrating screen of 3.5 mm apertures. For the base case, frequency and amplitude are 4 Hz and 1 mm, respectively. While microscopic properties were employed for the simulation, the properties extracted from the simulations are macroscopic fields which are consistent with the continuum equations of mass, momentum and energy balance. From the continuum equations, a micro-macro transition method called the coarse-graining approach was employed to obtain the volume fraction and the tangential velocity as a function of the depth of flow along the inclined surface. This approach is suitable for this work because the produced fields satisfy the equations of continuum mechanics; even near the base of the flow. The continuum analysis of the flowing layer reveals a coexistence of flow regimes- (i) quasi-static, (ii) dense (liquid-like), and (iii) inertial. The regimes are consistent with the measured solids concentrations spanning these regimes on inclined vibrating screens. The quasi-static regime is dominated by frictional stress and corresponds to low inertial number (I). Beyond the quasi-static regime, the frictional stress chains break and the collisional-kinetic and turbulent stress begin to dominate. The variation of the effective frictional coefficient with the inertial number (I) characterises the flow. Finally, an effective frictional coefficient model that is based on frictional, collisionalkinetic and turbulent stress was developed. Data analyses for this model were done at a steady flow in the base case where a coexistence of three flow regimes were observed. It was observed that each regime of flow is dominated by corresponding shear stresses. While the quasi-static regime is dominated by frictional stress, the kinetic and the inertial regimes are dominated by kinetic and turbulent shear stresses, respectively. This model was tested by varying the intensity of vibration in the base case and it was observed that at higher frequencies and amplitudes, the quasi-static regime gradually disappeared. Furthermore, the inertial number at which transition occurs to different regimes varies in response to the intensity of vibration. This is an important step in developing a phenomenological model of screening. The model presents a fundamental understanding of the mechanisms governing transport of granular matter on an inclined vibrating screen.
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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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    Developing a methodology that incorporates ceramic media properties to model power draw in a M4 IsaMill™
    (2012) Durant, Abraham Cristiaan; Mainza, Aubrey
    Sustainability in the mineral processing industry is guaranteed by the efficient operation of unit processes to maximize profitability within the prevailing economic conditions at the time. Efficient operation can be measured based on cost, metallurgical and energy efficiency of the operation or as a combination of the three, with energy efficiency becoming increasingly more important in the world and current South African context even more so. With ore bodies becoming more complex and finely disseminated the need for ultra fine grinding to maximise mineral recoveries in PGM concentrating operations by means of mainstream grinding has become more important. Conventional grinding technology was found to be inefficient to address this challenge. Introduction of new stirred milling technology for mainstream grinding using ceramic media successfully addressed the challenge posed by finely disseminated and complex PGM ore bodies. Introduction of new technology also brings about optimisation opportunities on various levels to ensure that the new technology is performing optimally in order to maximise profitability. As relatively new technology, several potential optimisation areas for the horizontal stirred mill has been identified for study. Operating costs for IsaMills™ horizontal mills can be classified into 3 groups i.e. media, maintenance and energy cost. Ceramic media type used can potentially play a role in determining the cost contribution of each of these groups to the overall mill operating costs. An area found very important to study, taking into consideration the enormous strain on the South African electricity supply system, was the impact of ceramic media properties on the energy efficiency of the horizontal stirred mill.
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    The development and demonstration of a practical methodology for fine particle shape characterisation in minerals processing
    (2016) Little, Lucy; Becker, Megan; Mainza, Aubrey; Wiese, Jenny
    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.
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    Effect of HPGR on platinum bearing ores and the flotation response as compared to the conventional ball mill
    (2011) Solomon, Nomonde; Mainza, Aubrey; Becker, Megan; Petersen, Jochen; Franzidis, Jean-Paul
    This thesis focuses on the application of the high pressure grinding rolls (HPGR) on platinum bearing ores Merensky, UG2 and Platreef. Conventional tumbling mills such as the ball mill that are typically applied on these ore types are highly energy intensive with a small percentage of the input energy being used for actual breakage. Rapidly increasing energy costs have contributed to the rising interest of the HPGR in the platinum industry, particularly in plants processing UG2 and Platreef ores. Therefore, this thesis seeks to determine if the HPGR can be used as an alternative to the ball mill. Key aspects of interest are throughput, energy efficiency, PGE grade and recovery and PGM liberation.
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    Effect of VRM on a polymetallic sulfide ore and the flotation response as compared to conventional wet and dry rod milling
    (2019) Nyakunuhwa, Hebert Simbarashe; Mainza, Aubrey; Corin, Kirsten
    Comminution is an energy intensive, size reduction and mineral dressing process which consumes up to 50% of concentrator energy consumption. Conventional methods use mainly a combination of crushers and tumbling mills in comminution circuits. Energy consumption in these circuits has been found to be relatively high. To reduce the energy requirements, compression grinding equipment, Vertical Roller Mills (VRMs) and High-Pressure Grinding Rolls (HPGRs) have been identified as potential solutions, and they have been adopted in the cement industry. Reports from plants where these technologies have been installed in circuits indicate they are more energy efficient than the conventional comminution circuits. Studies have also suggested that the use of VRMs results in comminution products with relatively higher mineral liberation degrees. Unlike in the cement industry, comminution equipment in mineral processing circuits are also required to produce particles that can be separated and recovered in downstream processes. Froth flotation is a selective separation process that utilises differences in surface properties to separate value minerals from unwanted gangue. The success of flotation is dependent on chemistry, operational and equipment factors. The chemistry factors consider the interaction between flotation reagents and solids particles surface. The operational factors consider the effect of particle size distribution, mineralogy, feed rate, pulp density, pulp potential (Eh), bubble size, temperature and circuit design on flotation. The use of different comminution procedures may result in flotation feeds of different particle size distributions (PSDs), mineral liberation characteristics and pulp potential. Due to these differences, the resultant flotation response may differ. The present study was aimed at assessing the particle size distribution, mineral liberation profiles and the flotation response from material comminuted using the VRM floated under batch flotation conditions in a 3 litre Barker flotation cell. A complex polymetallic sulfide ore containing chalcopyrite (1.3 %), galena (2.4 %) and sphalerite (1.8 %) as the main value minerals and magnetite (68.0 %) and quartz (15.7 %) as dominant gangue minerals was used for the study. The ore was milled to target grinds of 55 %, 60 %, 65 %, 70 % and 75 % passing 75 µm respectively, at a grinding pressure of 600 kPa, air temperature of 300 K. For the benchmarking grind of 65 % passing 75 µm, the ore was also milled using heated air of temperature of 373 K and at elevated grinding pressures of 800 kPa and 1000 kPa. Further work was performed to evaluate if the VRM results are comparable to conventional dry and wet rod milling products floated under the same batch flotation conditions. An increase in grinding pressure was observed to result in an increase in throughput and a general decrease in specific energy consumption without a change in product particle size distribution nor the recovery of chalcopyrite, galena and sphalerite. Using heated air (373 K) resulted in the production of slightly less fines in the comminution products. The recovery of chalcopyrite, galena and sphalerite were not affected by the change in operating temperature. However, concentrate grade (selectivity) was compromised at elevated temperatures of comminution probably due to surface oxidation. The results indicated that the grind range to achieve the best flotation performance when using the VRM as a comminution device is between 60 % and 70 % passing 75 µm. The results also indicated that at the benchmarking grind of 65 % passing 75 µm, the specific energy consumption for comminution using the VRM was 54.3 % lower than that of the conventional tumbling mill circuit. The grind of 55 % passing 75 µm resulted in lower flotation efficiencies as the minerals were unlikely liberated enough whereas the grind of 75 % passing 75 µm resulted in poor performances due to low water recovery. Comparing VRM with wet and rod milling, the different comminution procedures resulted in flotation feed of similar PSDs for all grinds compared. The wet and dry rod milling products of grinds 55 % and 75 % passing 75 µm achieved better recoveries of chalcopyrite, galena and sphalerite as compared to the VRM performance mainly due to high water recoveries achieved. While mineral recoveries were above 90 % for the grinds of 60 % and 70 % passing 75 µm, the rod milling products had statistically better flotation recoveries at 95 % confidence compared to the VRM products. The mineral recoveries after dry rod milling were marginally better than after wet rod milling due to the minimisation of galvanic interactions during dry rod milling. For the benchmarking grind of 65 % passing 75 µm, VRM grinding resulted in 84 %, 84 % and 90 % liberated chalcopyrite, galena and sphalerite respectively. The liberation of chalcopyrite, galena and sphalerite after wet and dry rod milling were 80 %, 78 % and 90 % respectively. Chalcopyrite recovery was 96.7 %, 96.3 % and 96.7 % for the VRM, dry rod mill (RD) and wet rod mill (RW) products respectively. Galena recovery was 94.3 %, 94.3 % and 92.9 % for the VRM, RD and RW products respectively. Sphalerite recovery was 96.6 %, 97.4 % and 97.4 % for the VRM, RD and RW products respectively. The differences in recovery were statistically insignificant at 95 % confidence. Liberation differences did not translate to differences in recoveries as the ore was coarse grained. The recovery kinetics were very fast and independent of comminution procedure. Reference to the benchmarking grind therefore, the VRM can be retrofitted into existing plant installations as it is more energy efficient and the flotation performance was similar when using the flotation procedure tailored for tumbling mill-flotation systems.
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    Evaluating the effect of operating variables on energy consumption in stirred mills Mussa Lisso.
    (2013) Lisso, Mussa; Mainza, Aubrey
    High grade ores have largely been depleted and those currently being treated are low grade, complex and sometimes finely disseminated, requiring fine grinding to liberate valuable minerals. For fine grinding applications, conventional tumbling mills are energy intensive. More energy efficient technologies such as stirred mills have been developed and widely used for fine and ultra-fine grinding. In this study, the effects of residence time, solids concentration, impeller speed, impeller type, media size and media density on energy consumption in a batch vertical stirred mill were investigated. The effect of energy on mill performance was assessed using the perfect mixing mill model. In addition, the effect of media stress intensity on grind and energy efficiency at constant residence time was also investigated. It was found that irrespective of the method of altering the energy input, the fineness of grind improved with increase in the specific energy input. This suggests that energy is the key driver for size reduction. The perfect mixing model can be used to assess mill performance and the breakage rates generally increased with increase in the specific energy input, impeller speed and solids concentration. The media stress intensity approach is useful in assessing mill performance in stirred mills at constant residence time. The fineness of grind improved when the media stress intensity was varied from 4.41×10-3 to 27.41×10-3Nm. In addition, the specific energy required to produce material below 25μm and 38μm decreased with an increase in the media stress intensity. When slurry density effects were considered, an optimum stress intensity was observed with respect to specific energy required to produce material below 25μm and 38μm. It was recommended that additional test work be carried out to investigate the effect of media size in the range -6.7mm + 2mm on energy efficiency. It was also recommended that tests be carried out at impeller speed between 600rpm and 1500rpm to assess how mill performance increases even at relatively high impeller speeds. In addition, a model predicting the specific energy using the impeller speed and solids concentration can also be developed.
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    Evaluating the influence of lifter face angle on the trajectory of particles in a tumbling mill using PEPT
    (2014) Takalimane, Motena; Mainza, Aubrey; Govender, Indresan
    The work performed in this thesis was aimed at evaluating the influence of lifter face angle on the charge kinematics for a laboratory scale mill. The study involved tracking a single particle representing the ensemble using the Positron Emission Particle Tracking (PEPT) to obtain the location of the particle with time. The particle was radiated with a radionuclide; ⁶⁸8Ga, which has a half-life of 68 minutes. The objectives of the study involved tests with different lifter face angles at different mill speeds and volumetric mill filling. After performing the tests the data was analysed to obtain probability density distributions for each test conditions from key charge descriptors. Charge descriptors such as the Centre of Circulation (CoC), shoulder angle, toe angle, the free surface and also kinematic information such as the velocity profile along a carefully chosen radial line from the centre of the mill that passes through the CoC were obtained. The time averaged velocity data was used when assessing the influence of the lifter face angle on the velocity profile. The results showed notable effects of lifter face angle on charge characteristics. No real definitive trend was observed for the CoC as the lifter face angle was altered at all mill speed and filling conditions. However, the CoC showed an outward shift toward the mill shell with an increase in mill speed but an inward shift toward the mill centre with increase in charge filling degrees. Mill speed is expected to cause a load expansion as the charge approaches centrifugation.
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    Fine-grinding Characterisation of PGM Ore Using HIGmill
    (2022) Mabiza, Audrey; Mainza, Aubrey; Bepswa, Paul
    Historically, mineral processing industries use conventional tumbling mills to carry out milling operations However, the ore bodies that are currently being explored and mined are low-grade complex ores. These ore bodies often require fine grinding to liberate their valuable fine grained minerals. Conventional mills have proven to be energy inefficient when they are used for fine grinding. Energy efficient technologies, such as stirred mills, are therefore being introduced to the minerals market for use in fine-grinding applications. The HIGmill is an example of a stirred mill that is used for fine grinding mineral ores. This thesis reports the results obtained from the investigating of the influence of operating parameters on the HIGmill product size and specific energy when grinding UG2 platinum ore. The milling experiments were conducted using the HIG 5, which is a laboratory scale HIGmill. The operating variables tested were mill tip speed, solids concentration and grinding media size. Tip speeds tested in this study were between 300 rpm and 1050 rpm. The solids concentration tested was between 40% and 60%. Grinding media sizes used were 2 mm, 3.5 mm and 5 mm. Using these parameters, a three-factorial design was used for the experiments. From the milling results, particle size distributions, stress intensity, specific energy and size specific energy were analysed for each variable tested. Signature plots were also produced for the milling results. Flotation tests were then carried out on the HIG 5 products to assess the downstream response of the HIGmill. The milling results showed that the optimal tip speed was 600 rpm. The optimal solids concentration for the UG2 ore was 60%. For the feed top size of −1000 µm, using 5 mm grinding media was found to be energy intensive and it is recommended that 2 mm and 3.5 mm grinding media are considered when for the grinding the UG2 ore. The flotation results highlighted that grinding finer increases the recovery potential of the ore from 73.2% to an average of 81%. It was also noted that grinding finer increases the chromite recoveries. Flotation kinetic models showed that there is no significant difference in the flotation kinetics of the HIGmill products.
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    Hybridization of electrical resistance tomography to population balance model for accurate bubble column reactor hydrodynamic parameter predictions
    (2016) Adetunji, Olubode Caleb; Rawatlal, Randhir; Adler, Andy; Mainza, Aubrey
    A novel approach of obtaining bubble size and spatial distribution is developed by hybridising techniques of Electrical Resistance Tomography and the Gas Disengagement Technique using a Population Balance as a framework. As a result, detailed hydrodynamic predictions suitable for Bubble Column Reactor [BER] optimisation results with minimal computing effort. Electrical Resistance Tomography [ERT] is a technique for creating 3D images of objects occurring in space. The images are obtained through current stimulations through a body surface electrodes and measurements of resulting voltage signals due to interior spatial conductivity field distribution. The use of ERT imaging method for hydrodynamic parameter predictions in a BCR has a benefit of yielding high temporal resolution but low spatial resolution. The low spatial resolution in electrical imaging accounts for underestimated or overestimated hydrodynamic parameter predictions similar to results obtained from the use of alternative techniques. The population balance model [PBM] is a mathematical framework with which the spatial transport of properties of bubble population can be described. The PBM also allows for the description of the time-variant bubble population properties by a division of bubble population into size classes. Moreover, the PBM allows for the inclusion of models of bubble coalescence and breakage phenomena, which affect the distribution of bubble population properties during bubble swarming. The included source terms enable accurate modelling of the bubble evolution either in a steady or unsteady state fluid flow regime. The objective of the present study is to develop an ERT interpretation technique yielding a high accuracy reconstruction of bubble population distribution through coupling ERT measurements to a PBM. It is hypothesized that a higher accuracy interpretation of ERT measurements will result from coupling ERT measurements to a PBM. The ERT technique has the capacity to image the steady and time-dependent gas void fractions in column sections as bubbles swarm and during dynamic gas disengagement [DGD]. This ERT potential is explored in hybridizing ERT and a PBM in the present work.
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    The influence of slurry viscosity on hydrocyclone performance
    (2012) Waters, Jason G; Mainza, Aubrey; Govender, Indresan
    Many of the empirical hydrocyclone cut size models available consider the feed solids content to be an important variable in determining the efficiency and resultant classification. However, feed viscosity as a whole is admittedly a more accurate variable to consider as it can be affected by many factors, including solids content, particle size distribution, particle size, mineralogy, pulp chemistry, particle shape and carrier fluid temperature. Earlier theoretical models did try to incorporate slurry viscosity but were developed for very low solids content systems due to the difficulty in measuring slurry viscosity at higher concentrations. Current hydrocyclone models being applied in industry have difficulty in predicting rheological fluctuations at constant solids content, for example with an increase in feed clay content. Investigations, focussed on expanding the studies of previous researchers in this area, were conducted. Experiments involved the design and operation of two test rigs incorporating three hydrocyclone sizes (75,100, 165 mm), with two different ore types (platreef ore, copper ore) from a secondary stage operation used as the feed material. A change in relative viscosity was investigated by altering the viscosity of the carrier fluid (water) by the addition of sucrose, and modification of the slurry temperature. Hydrocyclone feed flow rate and solids content were also modified. A custom made on-line tube rheometer allowed viscosities of feed concentrations of up to 43% (by vol.) to be measured over a range of shear rates (200/s to 1500/s). The slurries under investigation were found to be settling in nature, and therefore a decision was made to exclude data below the critical settling velocity of the tube. Rheological characterisations revealed both ore types exhibited Bingham plastic behaviour. A concentration versus viscosity relationship was determined from the rheological data and the resultant viscosity values were then linked to hydrocyclone efficiency. The significant findings of this work included the following: * Increased pulp viscosity achieved by 1) sucrose addition and 2) decreased slurry temperatures resulted in a drop in hydrocyclone performance attributed to the combined effect on the partition curve parameters namely - an increased cut size (d50c), decreased water split to O/F (C) and reduced value of alpha. *Increased pulp viscosity achieved by 3) feed solids content had dissimilar effects on the partition curve parameters. An optimum viscosity point was reached for water split to O/F and alpha parameters, however cut size increased with increased pulp viscosity. * Rheological effects on the cut size parameter appeared more significant for the largest of the three cyclone body diameters used in the study. This can be attributed to the decreased tangential velocities inside the larger cyclone radius. The combined rheology and hydrocyclone data from this thesis will provide useful validation data for the new hydrocyclone models currently being developed as part of the P9 project. The models are to be incorporated in the JKSimMet simulation package and consider the effects of viscosity in their equations for cut-size and water split.
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    Investigating multi-directional inhomogeneous granular suspensions
    (2019) De Klerk, David; Govender Indresan; Mainza, Aubrey
    Granular flows in rotating drums find many applications in industry, even though the dynamics of their granular media is not fully understood. Several models of granular flow and granular suspensions (where a viscous fluid is present in the voids between the granular particles) have been proposed in the last decade and a half. These models are unified in the way that dimensional analysis is employed to describe bulk properties of the flow in terms of a number of dimensionless parameters. However, applicability to rotating drums has not been demonstrated for many of these models. Furthermore, most studies rely on numerical simulations or experiments of slowly rotating drums that are not easily identified with industrial applications that operate in higher Froude regimes. This thesis presents a series of Positron Emission Particle Tracking (PEPT) experiments and Discrete Element Method (DEM) simulations of rotating drums with a viscous fluid. The three aims of the thesis are, to investigate the use of the Ergodic hypothesis when analysing PEPT data, to test the use of the lubrication approximation in the DEM simulations and to compare results from rotating drums to the latest models of granular rheology and granular suspensions. Two sets of PEPT experiments were carried out with a drum (radius R = 230 mm and length L = 200 mm) which was forced to rotate around its axis. The first series of experiments, used to investigate the use of the ergodic hypothesis, used a fixed rotation rate (ω = 0.6ωc = 0.6 p g/R) and three different particle sizes (5 mm, 8 mm and 10 mm). A radio labelled tracer particle’s location was recorded for 10 h for each of the three particle sizes. The second series of experiments, intended to test rheology models of dense suspensions and the use of the lubrication approximation in DEM, used 10 mm diameter glass spheres and glycerol/water mixtures in a drum. The second configuration was simulated with DEM using the Hertz-Mindlen contact model for particle-particle interactions. The effect of a viscous force between particles in close proximity to each other was captured by a lubrication approximation. Particle level data from experiments and simulations are transformed to smooth fields by a coarsegraining method which is described in detail. The ergodic assumption (which states that time averages of the PEPT tracer is equivalent to the ensemble average and central to analysing PEPT data) is evaluated using the first series of experiments. It was found that the average velocity can be established after 15 min tracking time, however the solids fraction still shows under sampled regions after tracking for 3 h. Several techniques were used to investigate this, including as Poincaré maps and the global mixing index. A variation on the averaging technique is shown to account for under sampled regions in the solids fraction.
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    Investigating the fine milling of pigment in a colourant formulation on the Bühler PML2 Superflow mill
    (2021) Lewis, Emily; van der Westhuizen, Andre; Mainza, Aubrey
    Stirred media mills remain the most suitable option in the coatings industry for the ultrafine grinding of colourants. The fineness of grind and colourant tint strength are typically used as indicators for the end of a production run during colourant manufacturing. These typical parameters from the coatings industry make it difficult to recognise whether the end point of a production run to maximise the product yield has actually been reached, without excessive time or energy. Including particle size analysis, to enable the analysis of energy efficiency and production performance, could assist to understand the impact of the various milling parameters. This study compares the various experimental approaches that can be taken to understand milling performances during colourant production. A laboratory scale, Bühler Superflow, vertical, stirred media mill is used, with a yellow lead (II) chromate colourant. The milling parameters reviewed included rotor tip speed, the media filling level, the media size and solid content of the colourant suspension. A three-level factorial design of experiments was used for each parameter. The results are analysed in terms of the Kwade stress model and the one factor at a time approach, as well as a response surface study. The significant interaction effects (ie, p-value < 0.5) identified using the response surface study included the tip speed / media size (AC) and media filling level / media size (BC) interaction terms for both the energy efficiency and rate models. For the power model, the tip speed / media filling level (AB) and tip speed / solids content of the colourant suspension (AD) interaction terms were deemed significant. This confirmation disqualified the one factor at a time method as a viable means to determine optimal milling performance. The results did, however, confirm the validity of the stress energy model as a minimum stress energy of 0.067x10-3Nm was identifiable across the different specific energy input levels. This methodology would be most useful for future colourant production optimisation studies. As part of the model validation, the higher stirrer tip speed of 7.3m/s, higher solids content of 55wt% and lower media size of 0.8mm paired with a mid-level media filling level of 87% was identified as the optimal combination of parameters to maximise the production rate and energy efficiency.
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    Investigating the potential of using hydrocyclone-fine screen hybrid systems to improve the performance of classification circuits
    (2017) Muketekelwa, Saliya L; Mainza, Aubrey; Bepswa, Paul Aaron
    Classification is an integral part of comminution operations that controls the performance of the circuit. Hydrocyclones are normally used to perform the classification function. They offer numerous advantages that include, the ability to handle high throughputs, low floor space occupation and relatively low capital and running costs. Despite these advantages, hydrocyclones are inherently inefficient classifiers as they are predominantly dependent on hydrodynamics to effect separation. This effect is more prominent in operations handling complex ores such as a dual-density ore, where the heavy fine particles are misplaced to the underflow and the lighter middling particles report to the overflow. Several attempts have been made to improve the separation efficiency of cyclones either by modification of the cyclone or use of multi-stage cycloning. Most of the results obtained from experimental and simulation studies have shown considerable improvements. Even though some have not yet found wide application in the minerals industry due to practical limitations related to control and unstable operations. More recently, fine screening has gained recognition in the classification role. This development has allowed the use of fine screens in closed-circuit grinding operations resulting in significant metallurgical and economic benefits. Screens provide a sharper cut at the desired size and reduce the fraction of fines bypassing classification compared to hydrocyclones but have capacity limitations at smaller apertures. In an effort to mitigate the classification challenges of both the hydrocyclone and fine screen, this study investigated the potential of combining the high throughput performance of the hydrocyclone operation and the high precision classification characteristics of fine screening to result in a hybrid classification circuit Plant scale tests were conducted using five different classification circuit configurations at an operational Base Metal Concentrator treating a polymetallic ore. The classification circuit configurations considered included (i) a two-stage hydrocyclone with primary underflow reclassification (ii) an inclined hydrocyclone, (iii) a fine screen and (iv) selected permutations of hybrid circuit designs that included a hydrocyclone-fine screen (2 stage) and two hydrocyclones-fine screen (3 stage) variants of the hybridised configurations. The efficiency curves and their respective key performance indicators were used to assess the performance of the circuit configurations tested. The results showed that classification circuits that included fine screens exhibited higher sharpness of separation compared to circuit configurations comprised of hydrocyclones. The fine screen configuration showed the sharpest separation while the hydrocyclone-fine screen hybrid configurations gave relatively higher separation efficiencies than the configurations with hydrocyclones only. The overall sharpness of separation values obtained for the two stage and three-stage hybrid circuits were 3.0 and 2.4, respectively. The two-stage hydrocyclone and inclined hydrocyclone circuits had sharpness of separation values of 1.7 and 0.5, respectively. The inclined hydrocyclone circuit configuration performed the poorest. Furthermore, the two-stage hybrid circuit showed a higher degree of separation compared to the three-stage hybrid configuration. However, it was observed that a finer corrected cut size was realised for the three-stage hybrid circuit design. The fishhook effect was seen at particle sizes less than 38μm for the configurations incorporating a fine screen and an inclined hydrocyclone. Notably, the effect appeared to be more pronounced in configurations involving a fine screen stage. The results have shown that application of hybrid classification configurations can improve the performance of classification circuits. In addition, reclassification of hydrocyclone underflow on fine screens will results in a sharper classification while reclassifying the overflow stream on fine screens will provide a clean circuit final product. An evaluation of the capital and operating costs associated with fine screens should be done to determine the economic feasibility of incorporating the units in conventional milling circuits.