155 results found
Yu Y, Cilliers J, Hadler K, et al., 2024, Dry particle size separation using electrostatic traveling wave methods, Separation and Purification Technology, Vol: 336, ISSN: 0950-4214
Electrostatic traveling wave (ETW) methods have grown in popularity as a method to manipulate and separate fine particles. This is of interest to mineral processing and In-situ Space Resource Utilization. Here, we demonstrate that ETW can be used to separate particles by size. First, our numerical calculation and simulation results show the competing effect of DEP and Coulomb force on particle motion direction, which depends on the frequency, particle size and charge. Then we define the “crossover frequency” allowing prediction of operating parameters to separate particles. At the “crossover frequency” – a particle is equally likely to move forward or backward. The crossover frequency decreases as particle size increases, and the proportion of particles moving backward increases as frequency increases. Four size groups of ballotini particles (glass) were transported and collected at different wave field frequencies. Finally, we have demonstrated experimentally the separation of particles between 30–50 µm and 75–110 µm. Simulation and experiment results revealed the effects of frequency, particle size and charge on particle motions. And the model can be used to guide the design of separation systems.
Pukkella AK, Cilliers J, Hadler K, 2024, Design of parabolic conic gas cyclones for coarse particle classification: A CFD study with Response Surface Methodology, Powder Technology, Vol: 433, ISSN: 0032-5910
Gas cyclones have traditionally been used as particle separators to remove dust from gas streams, with the goal of achieving a dust-free gas flow at the exit pipe and recovering particles to the dust outlet. Typically, the particle size for removal to the dust outlet is less than 1 μm. However, gas cyclones can also be targeted for particle classification rather than particle removal. In this study, the effect of the shape of the curved conical wall, specifically the parabolic concave and convex conic designs, on particle classification in a 31 mm diameter gas cyclone was evaluated using Computational Fluid Dynamics (CFD). The CFD model predictions for standard small-scale gas cyclones were validated using experimental data available in the literature for various inlet flow rates. The curvature of the conical wall varied from convex to concave, resulting in the observation of sharper and coarser cut sizes for the parabolic conic designs. The cut size increases to 4.91 μm for the concave design at a flow rate of 30 liters per minute, compared to 3.38 μm and 2.65 μm for the convex and standard design cyclones, respectively. The axial, tangential, pressure, and turbulent kinetic energy profiles were used to provide an explanation for the observed results. The interaction effects between the parameters were explored using the Response Surface Methodology. Further, Monte Carlo simulations are performed to observe the trends in predictions by exploring the design space.
Pukkella AK, Cilliers JJ, Hadler K, 2023, A comprehensive review and recent advances in dry mineral classification, MINERALS ENGINEERING, Vol: 201, ISSN: 0892-6875
Cruise RD, Starr SO, Hadler K, et al., 2023, Triboelectric charge saturation on single and multiple insulating particles in air and vacuum, Scientific Reports, Vol: 13, ISSN: 2045-2322
Triboelectric charge transfer is complex and depends on contact properties such as material composition and contact area, as well as environmental factors including humidity, temperature, and air pressure. Saturation surface charge density on particles is inversely dependent on particle size and the number of nearby particles. Here we show that electrical breakdown of air is the primary cause of triboelectric charge saturation on single and multiple electrically insulating particles, which explains the inverse dependence of surface charge density on particle size and number of particles. We combine computational simulations with experiments under controlled humidity and pressure. The results show that the electric field contribution of multiple particles causes electrical breakdown of air, reducing saturation surface charge density for greater numbers of particles. Furthermore, these results show that particles can be discharged in a low pressure environment, yielding opportunities for improved industrial powder flows and dust mitigation from surfaces.
Yu Y, Luo Y, Cilliers J, et al., 2023, Numerical solution of the electric field and dielectrophoresis force of electrostatic traveling wave system, Micromachines, Vol: 14, Pages: 1347-1347, ISSN: 2072-666X
Electrostatic traveling wave (ETW) methods have shown promising performance in dust mitigation of solar panels, particle transport and separation in in situ space resource utilization, cell manipulation, and separation in biology. The ETW field distribution is required to analyze the forces applied to particles and to evaluate ETW design parameters. This study presents the numerical results of the ETW field distribution generated by a parallel electrode array using both the charge simulation method (CSM) and the boundary element method (BEM). A low accumulated error of the CSM is achieved by properly arranging the positions and numbers of contour points and fictitious charges. The BEM can avoid the inconvenience of the charge position required in the CSM. The numerical results show extremely close agreement between the CSM and BEM. For simplification, the method of images is introduced in the implementation of the CSM and BEM. Moreover, analytical formulas are obtained for the integral of Green’s function along boundary elements. For further validation, the results are cross-checked using the finite element method (FEM). It is found that discrepancies occur at the ends of the electrode array. Finally, analyses are provided of the electric field and dielectrophoretic (DEP) components. Emphasis is given to the regions close to the electrode surfaces. These results provide guidance for the fabrication of ETW systems for various applications.
Malone L, Cardin M-A, Cilliers JJ, et al., 2023, Exploring Novel Architectures in Lunar In-Situ Resource Utilisation, Brisbane, Australia, 26th World Mining Congress
Yu Y, Cilliers J, Hadler K, et al., 2023, The motion of small particles in electrostatic travelling waves for transport and separation, Powder Technology, Vol: 425, Pages: 1-16, ISSN: 0032-5910
Electrostatic travelling waves can be used to transport small particles across a surface. This technology has received particular attention for dust mitigation on solar panels and for manipulating small particles on the Moon as part of in-situ resource utilization processes. Electrostatic travelling wave systems have no moving parts and are particularly well suited to low humidity environments. Here, we analyse the motion of small particles with the aim of exploiting differences in motion and velocity to separate particles by size. We investigate the effects of voltage, frequency, particle size and charge, wavelength and initial conditions on the properties of particle motion, such as particle velocity, levitation height and motion mode. We calculate the electrostatic fields using accurate boundary conditions based on the Fourier expansion method, which shows more detail near the surface of electrodes. We solve the equations of motion using the implicit Runge-Kutta method, and measure the particle charge with a free fall system. We show that the numerical results have a good agreement with the analytical results of a particle moving in a certain mode. We have observed three modes of motion with a high-speed camera, and these results provide guidance for the development of electrostatic travelling wave systems for various applications.
Rasera JN, Cilliers JJ, Lamamy J-A, et al., 2023, Experimental investigation of an optimised tribocharger design for space resource utilisation, Planetary and Space Science, Vol: 228, Pages: 1-11, ISSN: 0032-0633
Triboelectric charging and free-fall separation are attractive technologies for lunar mineral beneficiation. Here, an optimised tribocharger design was built and evaluated under terrestrial conditions. The charging behaviour of pure silica and ilmenite were tested using the optimised design, as were mixtures of silica and ilmenite, and samples of lunar regolith simulant JSC-1. Pure silica and ilmenite acquired negative and positive charge, respectively, through contact with the tribocharger. The tribocharger affected significantly the movement of the pure minerals in the electrostatic field. Results from the binary mixtures indicate that ilmenite recovery is independent of initial ilmenite concentration, and can be predicted from the mass distribution of pure ilmenite samples. For JSC-1, the tribocharger was found to increase the density of the material in certain collectors, indicating an upgrading of denser constituents. The optimised tribocharger design has a significant effect on the charging and separation performance.
Cilliers J, Hadler K, Rasera J, 2023, Toward the utilisation of resources in space: knowledge gaps, open questions, and priorities, npj Microgravity, Vol: 9, Pages: 1-5, ISSN: 2373-8065
There are many open science questions in space resource utilisation due to the novelty and relative immaturity of the field. While many potential technologies have been proposed to produce usable resources in space, high confidence, large-scale design is limited by gaps in the knowledge of the local environmental conditions, geology, mineralogy, and regolith characteristics, as well as specific science questions intrinsic to each process. Further, the engineering constraints (e.g. energy, throughput, efficiency etc.) must be incorporated into the design. This work aims to summarise briefly recent activities in the field of space resource utilisation, as well as to identify key knowledge gaps, and to present open science questions. Finally, future exploration priorities to enable the use of space resources are highlighted.
Rasera JN, Cilliers JJ, Lamamy J-A, et al., 2023, A methodology for tribocharger design optimisation using the Discrete Element Method (DEM), Powder Technology, Vol: 413, Pages: 1-14, ISSN: 0032-5910
Tribocharger design optimisations presented in the literature are based typically on experimental investigations. While this approach is useful and necessary to evaluate the performance of a design, experimental investigations are limited to studying a finite matrix of parameters. Computational approaches, such as the discrete element method (DEM), offer greater flexibility, however they have not been used previously for tribocharger design optimisation. This work presents a novel approach using the DEM to study the effect of different tribocharger designs on the charging process using particle–wall and particle–particle contact areas as proxies for charge transfer. The bulk sample charge output from the model are compared with bulk charges measured experimentally, showing good agreement. Furthermore, a method to predict approximately the charging behaviour of complex mixtures from linear combinations of the simulation outputs of single species, single size particle samples is presented, demonstrating good agreement.
Malone L, Cardin M-A, Cilliers J, et al., 2022, Development of a Comprehensive Lunar Mining Simulator to Study Design and Decision-Making under Uncertainty, Paris, France, International Astronautical Congress
Yu Y, Cilliers J, Hadler K, et al., 2022, A review of particle transport and separation by electrostatic traveling wave methods, Journal of Electrostatics, Vol: 119, Pages: 1-16, ISSN: 0304-3886
The controlled movement of dry particles using non-mechanical means is desirable in a number of different applications, including solar panel dust mitigation, toner particle motion and in the handling and beneficiation of regolith for In-Situ Resource Utilization (ISRU). The electric curtain, the electrostatic traveling wave (ETW) and the electro-dynamic screen (EDS) are examples of techniques that can transport and separate particles with no moving parts nor fluid medium. This review paper brings together the research carried out on these techniques.We provide a comprehensive review on the particle movement mechanisms and the development and application of ETW methods, featuring a diverse range of hardware and circuitry, particulate material and process objectives. We focus on the evaluation of experimental development in the area of dust mitigation, particle transport and ISRU processes. We also detail the current knowledge about theory and modelling methods. Moreover, we provide a guide for possible improvement of the effectiveness of ETW devices, by outlining the limitations in application, theoretical understanding and potential research aspects.
Many discrete element method (DEM) tribocharging models presented in the literature rely on ill-defined or poorly quantified charging parameters. This work presents a straightforward experimental method to quantify key parameters, namely the charge transfer limit, Γ, and the charging efficiency, κc. These parameters are then used in both 2D and 3D DEM simulations to evaluate the applicability of faster 2D models to tribocharge modelling. Both the 2D and 3D models are found to perform well against the experimental data for single-contact and single-particle, multi-contact systems. However, the 2D model fails to produce good agreement with experimental data for multi-contact, multi-particle systems. This approach for determining experimentally the parameters for the DEM tribocharging model is found to be effective and produces good agreement between simulated and experimental data. This method will improve and simplify the DEM modelling of triboelectric charging in dry material handling processes.
Cole K, Brito-Parada PR, Hadler K, et al., 2022, Characterisation of solid hydrodynamics in a three-phase stirred tank reactor with positron emission particle tracking (PEPT), Chemical Engineering Journal, Vol: 433, Pages: 1-13, ISSN: 1385-8947
It is challenging to measure the hydrodynamics of stirred tank reactors when they contain multiphase flows comprising liquid, gas bubbles and particles. Radioactive particle tracking techniques such as positron emission particle tracking (PEPT) are the only established techniques to determine internal flow behaviour due to the inherent opacity and density of fluid and the vessel walls. The profiles of solids flow are an important tool for robust reactor design and optimisation and offer insight into underlying transport processes and particle–fluid–bubble interactions for applications such as froth flotation. In this work, measurements with PEPT were performed with two tracer particles differing in surface hydrophobicity to characterise the solids hydrodynamics in a baffled vessel agitated with a Rushton turbine. The location data from PEPT were averaged with time to estimate the probability density function (PDF) of particle velocity in individual voxels. The peaks of these voxel distributions were used to produce profiles of solids flow in different azimuthal and horizontal slices. Bimodal vertical velocity distributions were observed in the impeller radial jet which suggest the particles experienced trajectory crossing effects due to inertia. Statistical tests were performed to compare the velocity distributions of the hydrophilic and hydrophobic tracer particles, which indicated similar average flow behaviour in the liquid or pulp phase of the vessel and differences near the air inlet, in the impeller discharge stream and pulp–froth interface. With tracers designed to represent gangue and valuable mineral species, the differences in velocity reveal interactions such as bubble–particle attachment and entrainment.
Cole K, Barker DJ, Brito-Parada PR, et al., 2022, Standard method for performing positron emission particle tracking (PEPT) measurements of froth flotation at PEPT Cape Town, MethodsX, Vol: 9, Pages: 101680-101680, ISSN: 2215-0161
Positron emission particle tracking (PEPT) is a technique for measuring the motion of tracer particles in systems of flow such as mineral froth flotation. An advantage of PEPT is that tracer particles with different physical properties can be tracked in the same experimental system, which allows detailed studies of the relative behaviour of different particle classes in flotation. This work describes the standard operating protocol developed for PEPT experiments in a flotation vessel at PEPT Cape Town in South Africa. A continuously overflowing vessel with constant air recovery enables several hours of data acquisition at steady state flow and consistent flotation conditions. Tracer particles are fabricated with different coatings to mimic mineral surface hydrophobicity and size, and a data treatment derived from a rotating disk study is utilized to produce high frequency (1 kHz) location data relative to the tracer activity. Time averaging methods are used to represent the Eulerian flow field and occupancy of the tracer behaviour based on voxel schemes in different co-ordinate systems. The average velocity of the flow in each voxel is calculated as the peak of the probability density function to represent the peak of asymmetrical or multimodal distributions.•A continuously overflowing flotation vessel was developed for extended data acquisition at steady state flow.•The data treatment enabled the direct comparison of different particle classes in the flotation vessel.•The solids flow fields was described by the probability density function of tracer particle velocity measured in different voxel schemes.
Cruise RD, Hadler K, Starr SO, et al., 2022, The effect of particle size and relative humidity on triboelectric charge saturation, Journal of Physics D: Applied Physics, Vol: 55, Pages: 1-14, ISSN: 0022-3727
Triboelectric charging is present in numerous technologies and everyday processes, providing both problems and opportunities. Despite this, there is no generalised model for the amount of charge that will build up on surfaces in contact. Here, we develop a new model for the saturation charge on triboelectrically charged spherical insulators, accounting for both equalisation of surface potentials and electrical breakdown of the surrounding medium. Experiments are conducted under controlled temperature and humidity using two independent methods, measuring the saturation charge on polymer spheres contacting grounded stainless steel. The results verify our equalisation of surface potentials model which describes how saturation charge density increases for smaller particle sizes. Key triboelectric properties are calculated: The estimated saturation charge on a flat surface and the equalisation potential between different materials, which can be used to predict charge saturation and quantify a triboelectric series. The transition radius below which electrical breakdown will cause saturation of charge is also calculated theoretically. Limitations to the model are demonstrated experimentally. As particle size reduces, a point is reached at which the electrostatic adhesion of particles to the grounded charging surface prevents further charge build-up. Furthermore, it is found that the saturation charge for smaller particles in humid conditions is greatly reduced. These calculations, and the demonstrated procedure, can serve as a tool for the design of technologies and processes influenced by triboelectric charge build-up, including triboelectric nanogenerators and electrostatic mineral separators.
Vega-Garcia D, Cilliers JJ, Brito-Parada PR, 2020, CFD modelling of particle classification in mini-hydrocyclones, Separation and Purification Technology, Vol: 251, Pages: 1-9, ISSN: 1383-5866
This work presents validated Computational Fluid Dynamics (CFD) predictions of the effect that changes in vortex finder and spigot diameters have on the classification performance of mini-hydrocyclones. Mini-hydrocyclones (e.g. 10 mm in diameter) have been applied successfully to the separation of micron-sized particles since their bypass fraction is larger than the water recovery, which results in a high particle recovery to the underflow, as well as low water recovery. However, a larger bypass fraction can be a disadvantage when the purpose of the hydrocyclone is particle classification, because of the large amount of fine particles that are misplaced in the underflow. Although it is well known that changes in the outlets of the hydrocyclone affect its performance, there is limited research on the effect of these design parameters in mini-hydrocyclones, in particular with regard to particle classification. The aim of this study is to computationally explore the influence of spigot and vortex finder on the classification process. To this end, CFD simulations were carried out and the predictions experimentally validated in a 3D printed mini-hydrocyclone using glass beads (below 20μm) as the particulate system. The numerical results showed very good agreement with the experimental data for recovery of solids, concentration ratio, pressure drop and particle size distribution. A trade-off was observed between the solids recovery and concentration ratio, while the solids recovery was found to be inversely proportional to the pressure drop when vortex finder diameters were kept constant. It was found that the design that yielded the lowest recovery among those tested also resulted in a particle size distribution furthest away from that of the feed. We show how the model can be used to assess changes in design parameters in order to inform the selection of designs that exhibit lower energy requirements without compromising separation performance.
Rasera J, Cilliers J, Lamamy J-A, et al., 2020, The beneficiation of lunar regolith for space resource utilisation: A review, Planetary and Space Science, Vol: 186, ISSN: 0032-0633
Space Resource Utilisation (SRU) technology will enable further exploration and habitation of space by humankind. The production of oxygen on the Moon is one of the first objectives for SRU; this can be achieved through the thermo-chemical reduction of the lunar regolith. Several techniques, such as hydrogen reduction and molten salt electrolysis, have been proposed. All reduction techniques require a consistent feedstock from the regolith to reliably and consistently produce oxygen. The preparation of this feedstock, known as beneficiation, is a critical intermediate stage of the SRU flowsheet, however it has received little consideration relative to the preceding excavation, and the subsequent oxygen production stage. This review describes the physics of the main beneficiation methods suitable for SRU. Further, we collate and review all of the previous studies on the beneficiation of lunar regolith.
Wang P, Reyes F, Cilliers J, et al., 2020, Evaluation of collector performance at the bubble-particle scale, Minerals Engineering, Vol: 147, Pages: 1-3, ISSN: 0892-6875
Particle attachment and detachment in froth flotation are complex processes and their measurement presents many challenges. Of particular interest is the effect of collectors at the bubble-particle scale, in order to assess the strength or collecting ability of these reagents. However, studies of the effect of collectors on particle attachment at the bubble-particle scale are scarce. In this work, we propose a methodology to characterise collector strength by measuring the attachment rate of particles to a capillary-pinned bubble. An image processing technique was developed to quantify bubble surface coverage over time, which was then used to determine particle attachment kinetics. The image analysis strategy is based on the sessile drop method and uses curve fitting to determine accurately the particle coverage. The methodology was used to assess the collecting ability of three chalcopyrite collectors. Interestingly, although very similar contact angle measurements were found for two of the collectors, they showed distinctly different particle attachment kinetics. It is proposed that this particle-bubble attachment method can be used to gain additional information not currently available from either contact angle measurements or bulk collector performance tests.
Hadler K, Martin DJP, Carpenter J, et al., 2020, A universal framework for Space Resource Utilisation (SRU), Planetary and Space Science, Vol: 182, Pages: 1-5, ISSN: 0032-0633
Space Resource Utilisation (SRU) or In Situ Resource Utilisation (ISRU) is the use of natural resources from the Moon, Mars and other bodies for use in situ or elsewhere in the Solar System. The implementation of SRU technologies will provide the breakthrough for humankind to explore further into space. A range of extraction processes to produce useable resources have been proposed, such as oxygen production from lunar regolith, extraction of lunar ice and construction of habitation by 3D printing. Practical and successful implementation of SRU requires that all the stages of the process flowsheet (excavation, beneficiation and extraction) are considered. This requires a complete ‘mine-to-market’ type approach, analogous to that of terrestrial mineral extraction.One of the key challenges is the unique cross-disciplinary nature of SRU; it integrates space systems, robotics, materials handling and beneficiation, and chemical process engineering. This is underpinned by knowledge of the lunar or planetary geology, including mineralogy, physical characteristics, and the variability in local materials. Combining such diverse fields in a coordinated way requires the use of a universal framework. The framework will enable integration of operations and comparison of technologies, and will define a global terminology to be used across all fields. In this paper, a universal SRU flowsheet and terminology are described, and a matrix approach to describing regolith characteristics specifically for SRU is proposed. This is the first time that such an approach has been taken to unify this rapidly-developing sector.
Cilliers J, Hadler K, Rasera J, 2020, Estimating the scale of Space Resource Utilisation (SRU) operations to satisfy lunar oxygen demand, Planetary and Space Science, Vol: 180, Pages: 1-8, ISSN: 0032-0633
The production of oxygen from lunar regolith is analogous to metal production from ore in a terrestrial mine. The process flowsheets both include excavation, haulage and beneficiation of the regolith or ore to provide the feedstock for the chemical extraction of oxygen or metal. The production rate of oxygen depends on the mass rate of regolith treated and the efficiency of converting the regolith to oxygen. To date, the development of Space Resource Utilisation (SRU) has been concerned with the technological development of the process, particularly the excavation and oxygen extraction. However, the required operating mass rates of the mine operation and the oxygen extraction stage have not been considered in any great detail.Previous estimates of mining scale for lunar oxygen production are reviewed, and converted to a comparable regolith mining rate of kg/h. Beneficiation of the regolith before oxygen extraction is considered, and the effects of pre-sizing and removal of a specific component, agglutinates, are considered. The oxygen yield and operational availability are also included. It is estimated that the minimum mining rate to produce 1000 kg of oxygen per annum is at least five times higher than previous estimates, 30 kg/h, for equivalent efficiency assumptions.Monte-Carlo simulations were performed for statistical confidence in the estimates of the mining mass rate and the required oxygen extraction feedstock rate. To be 95% confident that the 1000 kg/y O2 will be met, the designed mining rate should be at least 65 kg/h, and the beneficiated feedstock rate 16 kg/h.This study has revised and increased the estimate of the lunar regolith mining scale required for the production of a given amount of oxygen. It has also estimated the mass rate of feedstock required for oxygen extraction, if the regolith is first beneficiated.The findings have a significant impact on the practical implementation of lunar mining and oxygen extraction, particularly the process des
Harrison STL, Kotsiopoulos A, Stevenson R, et al., 2020, Mixing indices allow scale-up of stirred tank slurry reactor conditions for equivalent homogeneity, CHEMICAL ENGINEERING RESEARCH & DESIGN, Vol: 153, Pages: 865-874, ISSN: 0263-8762
Fernando F, Cilliers J, Brito Parada PR, 2019, An integrated constrained fuzzy stochastic analytic hierarchy process method with application to the choice problem, Expert Systems with Applications, Vol: 138, Pages: 1-25, ISSN: 0957-4174
The ability of the analytical hierarchy process (AHP) when applied to the choice problem in the context of group decision making under uncertainty has been often criticised. AHP is not able to fully capture the various opinions and the uncertainty associated with the lack of information. This work develops an integrated constrained fuzzy stochastic analytic hierarchy process (IC-FSAHP) method in order to deal with the aforementioned drawbacks. IC-FSAHP combines two existing fuzzy AHP (FAHP) methods and further extends its applicability by implementing stochastic simulations. A case study has been conducted in order to assess the ability of IC-FSAHP; the results showed that IC-FSAHP is able to capture the uncertainty and multiple DMs' opinions. This paper also discusses the effect that the number of DMs has in enhancing rank discrimination. Besides, the possibility of the occurrence of rank reversal because of the use of IC-FSAHP has been analysed. The results showed that the ranking of alternatives was preserved throughout the changes in the number of alternatives, however, rank reversal occurred in the case of changes in judgements scales. By comparing the U-uncertainty in fuzzy global priorities obtained using IC-FSAHP to that obtained using an existing FSAHP method, we show that our method is capable of minimising the risk of losing important knowledge during the computations. We also discuss how IC-FSAHP can decrease the uncertainty and increase the reliability of the decisions by means of robust computations.
Rasera JN, Cilliers JJ, Lamamy JA, et al., 2019, The beneficiation of lunar regolith using electrostatic separation for space resource utilisation, 70th International Astronautical Congress, ISSN: 0074-1795
Copyright © 2019 by the International Astronautical Federation (IAF). All rights reserved. Differences in the electrostatic properties of materials can be exploited for both the sizing and enrichment of minerals. In this study, the motion of silica particles falling through an electrostatic field was investigated to characterise a custom free-fall electrostatic separator. The motion was affected by varying the magnitude of the electrostatic field and the spacing of the electrodes. SiLibeads (spherical silica) were sized and tribocharged in a borosilicate glass beaker and fed into the separator. Fourteen electrostatic field strengths each generated at three different electrode spacings (75 mm, 150 mm, and 225 mm) were studied. The percentage of particles reporting to each electrode was measured. Analyses of the results indicate that the expected linear increase in the field strength does not increase proportionally the amount of material reporting to each electrode, indicating that additional underlying parameters must be characterised. Further, an analysis of the variance between the measurements indicates that there are almost no significant effects on the separator's operation due to changing either the field strength or electrode spacing. However, two statistically unique operating conditions were identified. The measurements collected at a field strength of 0.04 kV/mm with a 75-mm spacing were unique relative to other field strengths at that spacing and may indicate an optimal operating condition. Further, the data collected at each electrode spacing with a constant electric field strength of 0.06 kV/mm were also found to be unique. This implies that there may be a performance dependence on electrode spacing in addition to the field strength. Further analysis and experimentation are required to draw more detailed conclusions.
Wang P, Cilliers JJ, Neethling SJ, et al., 2019, The behavior of rising bubbles covered by particles, Chemical Engineering Journal, Vol: 365, Pages: 111-120, ISSN: 1385-8947
A systematic investigation of the influence of particle coverage on the dynamics of rising bubbles was carried out using high-speed photography and image analysis techniques to study bubble behavior in terms of changes in velocity and aspect ratio. The buoyancy force and drag force exerted on the bubbles and the effect of particles were calculated to further understand their behavior. Results show that particles attached on the bubbles strongly dampen the oscillations observed in bubble aspect ratio and decrease its velocity and acceleration. The particles also render the bubbles more spherical and slow their velocity. It was found that the overall velocity of a bubble is directly correlated to its aspect ratio and inversely correlated to its particle coverage, while the acceleration and the aspect ratio and its change are inversely correlated. Interestingly, the trend observed in the oscillation and the oscillation period of particle-laden bubbles is similar for different levels of particle coating. A drag modification factor ηp, which quantifies the drag influence of particles on bubble velocity, was identified from force analysis. A modified drag coefficient for uncoated and particle-laden bubbles was introduced, which allows, for the first time, to predict the behavior of rising bubbles in gas-liquid-particle systems.
Sitorous F, Cilliers JJ, Brito Parada PR, 2019, Multi-criteria decision making for the choice problem in mining and mineral processing: applications and trends, Expert Systems with Applications, Vol: 121, Pages: 393-417, ISSN: 0957-4174
Despite the fact that the potential of multi-criteria decision making (MCDM) to overcome a variety of problems in mining and mineral processing has been widely recognised, no literature review in these fields has been conducted. This manuscript addresses this issue by providing a comprehensive overview of the applications and trends of MCDM methods for the choice problem (i.e., determining the best option from a set) in mining and mineral processing. 90 articles published between 1999 and 2017 were selected following a searching methodology and eligibility criteria detailed in this manuscript. In addition, for the purpose of the survey, different types of selection problems were identified. The results show that there are two phases of growth in the application of MCDM techniques to the choice problem in mining and mineral processing. The first phase, from 1999 to 2007, shows a very low number of publications with only a moderate increase by the end, whereas the second phase, from 2007 to 2017, shows a significant growth in the number of published articles. The review also shows that the most addressed problem has been the selection of mining methods, while the Analytical Hierarchy Process (AHP) has been the most used MCDM method. The rise in the application of hybrid MCDM methods is also discussed. This review paper provides insight into the current state of applications of MCDM in mining and mineral processing and discusses pathways for future research directions in the development of MCDM methods that would benefit these fields.
Wang P, Cilliers JJ, Neethling SJ, et al., 2019, Effect of particle size on the rising behavior of particle-laden bubbles, Langmuir, Vol: 35, Pages: 3680-3687, ISSN: 0743-7463
The rising behavior of bubbles, initially half and fully coated with glass beads of various sizes, was investigated. The bubble velocity, aspect ratio, and oscillation periods were determined using high-speed photography and image analysis. In addition, the acting forces, drag modification factor, and modified drag coefficient were calculated and interpreted. Results show that the aspect ratio oscillation of the rising bubbles is similar, irrespective of the attached particle size. As the particle size is increased, the rising bubbles have a lower velocity and aspect ratio amplitude, with the time from release to each aspect ratio peak increasing. Higher particle coverage is shown to decrease the bubble velocity and dampen the oscillations, reducing the number of aspect ratio peaks observed. The highest rise velocities correspond to the lowest aspect ratios and vice versa, whereas a constant aspect ratio yields a constant rise velocity, independent of the particle size. Force analysis shows that the particle drag modification factor increases with the increased particle size and is greatest for fully laden bubbles. The modified drag coefficient of particle-laden bubbles increases with the increased particle size, although it decreases with the increased Reynolds number independent of the particle size. The drag force exerted by the particles plays a more dominant role in decreasing bubble velocities as the particle size increases. The results and interpretation produced a quantitative description of the behavior of rising particle-laden bubbles and the development of correlations will enhance the modeling of industrial applications.
Neethling SJ, Brito Parada P, Hadler K, et al., 2019, The transition from first to zero order flotation kinetics and its implications for the efficiency of large flotation cells, Minerals Engineering, Vol: 132, Pages: 149-161, ISSN: 0892-6875
Flotation cells have traditionally been modelled using first order kinetics, often distributed over multiple floatable species. This description is valid as long as the kinetics are not restricted by the available bubble surface area. If this carrying capacity limit is approached, the behaviour will transition toward zero order kinetics with respect to the concentration of floatable species in the pulp, with this transition being associated with a significant degradation in performance. In this paper we develop a model which describes the transition from first to zero order kinetics. A dimensionless group is introduced, which is the ratio of the flotation rate under first order kinetics to the rate at maximum bubble carrying capacity. At values of this dimensionless group much less than 1 the kinetic equation reduces to the familiar k-Sb relationship, but with a progressive deviation away from first order kinetics as the value increases through 1, with zero order kinetics obtained for values of the dimensionless group much greater than one. This dimensionless group is a function of the cell size, being proportional to the ratio of the cell volume to its cross-sectional area.Since mechanical flotation cells continue to get larger, mainly due to the capital and operating cost benefits that they provide for a given residence time, the potential for deleterious zero order effects is likely to increase. This is also why zero order behaviour is virtually never encountered at the laboratory scale. The propensity for zero order kinetics also increases with both the floatability and concentration of floatable material in the pulp, as well as with the fineness of the grind. This means that cleaner cells are likely to be very susceptible to exhibiting zero order kinetics, while scavenger cells are likely to continue to exhibit first order kinetics for any foreseeable flotation cell size. The cell size at which zero order kinetics effects will degrade the performance of rougher
Hadler K, Cilliers J, 2019, The effect of particles on surface tension and flotation froth stability, Mining, Metallurgy & Exploration, Vol: 36, Pages: 63-69, ISSN: 2524-3470
It is widely accepted that particles stabilise flotation froths and that stable froths result in improved flotation performance. Predicting the effect of particle addition on froth stability is, however, challenging. Dynamic surface tension measurement using maximum bubble pressure presents an attractive technique to investigate the effect of surfactant and particles at the air-water interface. The range of bubble lifetimes that can be studied (typically 0.1 to 60 s) is analogous to variations in air rate in flotation cells, and the corresponding changes in surface tension give an indication to the diffusion and adsorption rates of particles at the interface. In this paper, we use dynamic surface tension measurements to investigate the effect of particles on bubble surfaces at the microscale and link this to bulk froth stability measurements carried out using a froth column. Using the maximum bubble pressure method, the results show that the addition of particles results in lower surface tension, both at the dynamic (i.e. short) bubble lifetimes and towards equilibrium (i.e. 60 s bubble lifetime). This corresponds with the bulk froth stability measurements, where the three-phase system yielded more stable froths than the surfactant only system. Furthermore, increased particle loading at the air-water interface, whether through higher surfactant concentrations or lower air rates (longer bubble lifetimes), gave lower surface tension and higher froth stability. This demonstrates the link between bubble loading and froth stability. It is proposed that the maximum bubble pressure technique can be used to predict froth stability for two- and three-phase systems, enabling the effect of particle loading to be accounted for and quantified. Moreover, the technique has the potential to allow rapid determination of particle and surfactant diffusion at the air-water interface and prediction of the corresponding effect on bulk froth behaviour.
Cilliers JJ, Harrison STL, 2019, Yeast flocculation aids the performance of yeast dewatering using mini-hydrocyclones, SEPARATION AND PURIFICATION TECHNOLOGY, Vol: 209, Pages: 159-163, ISSN: 1383-5866
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