Imperial College London

ProfessorStephenNeethling

Faculty of EngineeringDepartment of Earth Science & Engineering

Professor of Minerals Processing
 
 
 
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Contact

 

+44 (0)20 7594 9341s.neethling

 
 
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Location

 

RSM 2.35Royal School of MinesSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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105 results found

Avalos Patino J, Dargaville S, Neethling S, Piggott Met al., 2021, Impact of inhomogeneous unsteady participating media in a coupled convection-radiation system using finite element based methods, International Journal of Heat and Mass Transfer, Vol: 176, Pages: 1-16, ISSN: 0017-9310

Combined convection–radiation is a common phenomenon in many engineering problems. A differentially–heated rectangular enclosure is a widely–used benchmark for testing numerical techniques developed for solving the coupled momentum and energy equations related to combined convection–radiation. Previous studies have tended to describe the phenomenon in cases using simplified characteristics for the participating media including the assumptions of: (i) uniform distribution, (ii) homogeneous cross section, (iii) grey gas radiation and (iv) under steady state conditions. The effects of an inhomogeneous unsteady participating media, e.g. composed of a mixture of gases, are arguably understudied. In this work the effect of an inhomogeneous unsteady participating media on combined convection–radiation inside a rectangular enclosure is considered, under both grey and non-grey gas modelling approaches involving a mixture of gases. A key novelty in this work is the inclusion of the ability to handle inhomogeneous participating media which change in space, time and absorption cross section values as a result of the convection–radiation coupling, allowing us to assess different gas modelling approaches. A global gas radiation model is used and a new non–uniform discretisation method for the absorption distribution function is introduced; this method allows a better handling of those energy groups in which the Planck absorption coefficient is low, improving the performance of the spherical harmonics method and mitigating ray–effects on finite elements in angle discretisation. The momentum and energy equations are solved numerically using finite element based discretisation methods. The radiative transfer equation is solved numerically using both spherical harmonics and finite elements for the angular discretisation, with their relative performance compared. The results highlight the importance that the characteristics of the partic

Journal article

Quintanilla P, Neethling SJ, Brito-Parada PR, 2021, Modelling for froth flotation control: A review, Minerals Engineering, Vol: 162, ISSN: 0892-6875

Flotation is a conceptually simple operation; however, as a multiphase process with inherent instability, it exhibits complex dynamics. One of the most efficient ways to increase flotation performance is by implementing advanced controllers, such as Model Predictive Control (MPC). This type of controller is very dependent on the model that represents the dynamics of the process. Although model development is one of the most crucial parts in MPC, flotation models have been mainly developed for simulation purposes (i.e. analysis and design) rather than control purposes. This paper presents a critical literature review on modelling for froth flotation control. Models reviewed have been sub-classified as empirical, phenomenological and hybrid according to their characteristics. In particular, it is highlighted that models have so far primarily focused on the pulp phase, with the froth phase often neglected; when the froth phase is included, kinetics models such as those used for the pulp phase, are commonly used to represent it. Froth physics are, however, dominated by processes such as coalescence, liquid motion and solids motion, which have been previously modelled through complex, steady-state models used for simulation purposes, rather than control purposes. There remains a need to develop appropriate models for the froth phase and more complex models for the pulp phase that can be used as part of MPC strategies. The challenges associated with the development of such models are discussed, with the aim of providing a pathway towards better controlled froth flotation circuits.

Journal article

Snelling B, Neethling S, Horsburgh K, Collins G, Piggott Met al., 2020, Uncertainty quantification of landslide generated waves using Gaussian process emulation and variance-based sensitivity analysis, Water, Vol: 12, ISSN: 2073-4441

Simulations of landslide generated waves (LGWs) are prone to high levels of uncertainty. Here we present a probabilistic sensitivity analysis of an LGW model. The LGW model was realised through a smooth particle hydrodynamics (SPH) simulator, which is capable of modelling fluids with complex rheologies and includes flexible boundary conditions. This LGW model has parameters defining the landslide, including its rheology, that contribute to uncertainty in the simulated wave characteristics. Given the computational expense of this simulator, we made use of the extensive uncertainty quantification functionality of the Dakota toolkit to train a Gaussian process emulator (GPE) using a dataset derived from SPH simulations. Using the emulator we conducted a variance-based decomposition to quantify how much each input parameter to the SPH simulation contributed to the uncertainty in the simulated wave characteristics. Our results indicate that the landslide’s volume and initial submergence depth contribute the most to uncertainty in the wave characteristics, while the landslide rheological parameters have a much smaller influence. When estimated run-up is used as the indicator for LGW hazard, the slope angle of the shore being inundated is shown to be an additional influential parameter. This study facilitates probabilistic hazard analysis of LGWs, because it reveals which source characteristics contribute most to uncertainty in terms of how hazardous a wave will be, thereby allowing computational resources to be focused on better understanding that uncertainty.

Journal article

Snelling BE, Collins GS, Piggott MD, Neethling SJet al., 2020, Improvements to a smooth particle hydrodynamics simulator for investigating submarine landslide generated waves, International Journal for Numerical Methods in Fluids, Vol: 92, Pages: 744-764, ISSN: 0271-2091

Submarine landslides can exhibit complex rheologies, including a finite yield stress and shear thinning, yet are often simulated numerically using a Newtonian fluid rheology and simplistic boundary conditions. Here we present improvements made to a Smoothed Particle Hydrodynamics simulator to allow the accurate simulation of submarine landslide generated waves. The improvements include the addition of Bingham and Herschel‐Bulkley rheologies, which better simulate the behavior of submarine mudflows. The interaction between the base of the slide and the slope is represented more accurately through the use of a viscous stress boundary condition. This condition treats the interface between the seafloor and the slide as a fluid boundary layer with a user‐defined viscosity and length scale. Modifications to the pressure and density calculations are described that improve their stability for landslide generated wave scenarios. An option for pressure decomposition is introduced to prevent particle locking under high pressure. This facilitates the application of this simulator to landslide scenarios beneath significant water depths. Additional modifications to the reaveraging and renormalization routines improve the stability of the free surface and fluid density. We present the mathematical formulations of these improvements alongside commentary on their performance and applicability to landslide generated wave modeling. The modifications are verified against analytical fluid flow solutions and a wave generation experiment.

Journal article

Lombardo AG, Simon BA, Taiwo O, Neethling SJ, Brandon NPet al., 2019, A pore network model of porous electrodes in electrochemical devices, JOURNAL OF ENERGY STORAGE, Vol: 24, ISSN: 2352-152X

Journal article

Wang P, Cilliers JJ, Neethling SJ, Brito-Parada PRet 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.

Journal article

Wang P, Cilliers JJ, Neethling SJ, Brito-Parada PRet 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.

Journal article

Neethling SJ, Brito Parada P, Hadler K, Cilliers Jet 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

Journal article

Reyes F, Cilliers JJ, Neethling SJ, 2019, Quantifying mineral liberation by grade and surface exposure using X-ray micro-tomography for flotation processes, Pages: 3985-3994

Liberation is a key driver in all mineral separation processes as it limits the maximum possible grade for a given recovery. In flotation, this is further complicated by the fact that it is surface exposure of the floatable minerals that determines the ultimate performance. Liberation, grade and surface exposure are commonly quantified using Scanning Electron Microscopy coupled to Energy Dispersive X-ray spectroscopy (SEM/EDX) analysis of polished sections. The intrinsically 2D nature of this technique can result in significant sampling errors and stereological effects that can affect the quantification of the ore's textural characteristics. X-ray micro-Tomography (XMT) is an imaging method that can non-invasively and non-destructively delineate ore fragments in 3D, thus providing an alternative method that eliminates the need for stereological corrections and readily provides surface exposure. A methodology and automated algorithm were designed for extracting this information from images of closely packed particles, thus allowing samples containing a large number of particles to be assessed. The main drawback of XMT is that, unlike SEM/EDX, it cannot directly measure the mineralogy of the sample, instead producing a 3D X-ray attenuation map. We therefore also present an algorithm for calibrating the thresholding of the XMT images based on SEM/EDX images of sections through the same sample, thus allowing the liberation analysis to be carried out on a 3D mineral map in which the uncertainty in the mineral assignment is small and statistically quantified. The methodology was tested on low grade porphyry copper ore as this is both an industrially relevant and traditionally difficult system to quantify using XMT due to the similarity in the X-ray attenuation of the sulphide species, especially the pyrite and chalcopyrite. As each 3D volume imaged contain 100-1000s of grains, large data sets can be readily produced. By dividing these particles into classes based on both

Conference paper

Reyes F, Cilliers JJ, Neethling SJ, 2019, Quantifying mineral liberation by grade and surface exposure using X-ray micro-tomography for flotation processes, Pages: 1570-1579

Liberation is a key driver in all mineral separation processes as it limits the maximum possible grade for a given recovery. In flotation, this is further complicated by the fact that it is surface exposure of the floatable minerals that determines the ultimate performance. Liberation, grade and surface exposure are commonly quantified using Scanning Electron Microscopy coupled to Energy Dispersive X-ray spectroscopy (SEM/EDX) analysis of polished sections. The intrinsically 2D nature of this technique can result in significant sampling errors and stereological effects that can affect the quantification of the ore's textural characteristics. X-ray micro-Tomography (XMT) is an imaging method that can non-invasively and non-destructively delineate ore fragments in 3D, thus providing an alternative method that eliminates the need for stereological corrections and readily provides surface exposure. A methodology and automated algorithm were designed for extracting this information from images of closely packed particles, thus allowing samples containing a large number of particles to be assessed. The main drawback of XMT is that, unlike SEM/EDX, it cannot directly measure the mineralogy of the sample, instead producing a 3D X-ray attenuation map. We therefore also present an algorithm for calibrating the thresholding of the XMT images based on SEM/EDX images of sections through the same sample, thus allowing the liberation analysis to be carried out on a 3D mineral map in which the uncertainty in the mineral assignment is small and statistically quantified. The methodology was tested on low grade porphyry copper ore as this is both an industrially relevant and traditionally difficult system to quantify using XMT due to the similarity in the X-ray attenuation of the sulphide species, especially the pyrite and chalcopyrite. As each 3D volume imaged contain 100-1000s of grains, large data sets can be readily produced. By dividing these particles into classes based on both

Conference paper

Ilankoon S, Neethling S, 2019, Inter-particle liquid spread pertaining to heap leaching using UV fluorescence based image analysis, Hydrometallurgy, Vol: 183, Pages: 175-185, ISSN: 0304-386X

The visualisation of individual flow paths in a porous media and how they relate to the overall hydrodynamics is crucial to improving our understanding of a number of systems. Unsaturated flow through larger particles (order of millimetres and a few centimetres) in heap leaching systems where both gravity and capillarity play an important role in the structure of the flow paths is complex and still not fully understood. A variety of laboratory methods have been employed for this purpose, with UV light being used in several studies. Flow studies have typically involved millimetre and sub-millimetre scale particles and the flow is typically capillary-dominated. This paper instead concentrates on gravity-dominated flows pertaining to heap leaching and is particularly focused on developing methods for using the UV fluorescence to obtain quantitative measurements of liquid spreading, such as average lateral liquid spread for a point source liquid addition. This study uses a pseudo2-D system with larger ore particles than in previous UV flow studies. The UV fluorescence results were analysed using image analysis to determine the overall liquid spreading and the locations of the flow paths. Both narrowly sized and realistic ore systems were investigated to understand the effect of length scale on flow paths and dynamics of liquid spreading through the gravity-dominated ore beds. It was shown that both ore systems experience distinct flow channelling compared to the more uniform flow profiles observed in capillary-dominated systems. Lateral liquid spreading coefficients were calculated, with the narrowly sized ore system showing higher values, probably due to the larger effective length scale of the inter-particle spaces.

Journal article

Nunez Rattia JM, Percival J, Neethling S, Piggott MDet al., 2018, Modelling local scour near structures with combined mesh movement and mesh optimisation, Journal of Computational Physics, Vol: 375, Pages: 1220-1237, ISSN: 0021-9991

This paper develops a new implementation coupling optimisation-based anisotropic mesh adaptivity algorithms to a moving mesh numerical scour model, considering both turbulent suspended and bedload sediment transport. The significant flexibility over mesh structure and resolution, in space and time, that the coupling of these approaches provides makes this framework highly suitable for resolving individual marine structure scales with larger scale ocean dynamics. The use of mesh optimisation addresses the issue of poor mesh quality and/or inappropriate resolution that have compromised existing modelling approaches that apply mesh movement strategies alone, especially in the case of extreme scour. Discontinuous Galerkin finite element-based discretisation methods and a Reynolds Averaged Navier–Stokes-based turbulent modelling approach are used for the hydrodynamic fluid flow. In this work the model is verified in two dimensions for current-dominated scour near a horizontal pipeline. Combined adaptive mesh movement and anisotropic mesh optimisation is found to maintain both the quality and validity of the mesh in response to morphological bed evolution changes, even in the case where it is severely constrained by nearby structures.

Journal article

Reyes F, Lin Q, Cilliers JJ, Neethling SJet al., 2018, Quantifying mineral liberation by particle grade and surface exposure using X-ray microCT, Minerals Engineering, Vol: 125, Pages: 75-82, ISSN: 0892-6875

Liberation is a key driver in all mineral separation processes as it limits the maximum possible grade for a given recovery. In flotation, this is further complicated by the fact that it is surface exposure of the floatable minerals that determines the ultimate performance. Liberation, grade and surface exposure are commonly quantified using Scanning Electron Microscopy coupled to Energy Dispersive X-ray spectroscopy (SEM/EDX) analysis of polished sections. The intrinsically 2D nature of this technique can result in significant sampling errors and stereological effects that can affect the quantification of the ore's textural characteristics. X-ray microCT (XMT) is an imaging method that can non-invasively and non-destructively delineate ore fragments in 3D, thus providing an alternative method that eliminates the need for stereological corrections and readily provides surface exposure. A methodology and automated algorithm were developed for extracting this information from images of closely packed particles. By dividing these particles into classes based on both their surface exposure and grade, the extent to which there is preferential breakage of the particles can be assessed—an important consideration if sufficient surface liberation for good flotation performance is to be achieved at coarser particle sizes. Using low energy scanning simple 3D mineral maps can be obtained via XMT, allowing for the assessment of liberation and surface exposure for each mineral species. The methodology was tested on low grade porphyry copper ore as this is representative of the most commonly treated ore types for copper production.

Journal article

Shean B, Hadler K, Neethling S, Cilliers JJet al., 2018, A dynamic model for level prediction in aerated tanks, Minerals Engineering, Vol: 125, Pages: 140-149, ISSN: 0892-6875

Stirred aerated tanks are a key unit operation in many industries, including froth flotation. Reliable and robust level control is of great importance in maintaining steady operation for successful implementation of higher level optimising control strategies, particularly when such tanks are arranged in series. When changes are made to the rate of aeration, there is a corresponding change in the pulp bubble size and gas holdup (the volume fraction of air in the tank), and consequently the pulp height. Stable operation of flotation tanks must, therefore, include the effect of air rate on pulp height in level control systems, especially if air rate is being actively controlled. In this paper, a model is developed from first principles to link the change in gas holdup with variation in air rate under dynamic conditions, accounting for the variability in gas holdup with height that results from differences in gas compressibility. This is validated experimentally. In order to test the model, experiments were carried out using a 70 L laboratory tank comprising water and reagent systems. For both simple and complex changes in air rate, the model showed good agreement with the experimental results when predicting the change in pulp height at steady state. Under dynamic conditions, the experimental system exhibited a slightly slower response than is predicted by the model; this is likely to be due to the well mixed assumption not being adequately met. This model provides a method to improve the operating stability of aerated tanks through better modelling of the dynamic pulp height changes that result from changes in air flowrate. In flotation tanks, this will enable greater control over froth height, which has been found to affect significantly mass pull, froth stability and flotation performance.

Journal article

Brito Parada P, Neethling S, 2018, Predicting flotation behaviour – the interaction between froth stability and performance, Minerals Engineering, Vol: 120, Pages: 60-65, ISSN: 0892-6875

Froth behaviour has a major impact on the overall performance of flotation cells, with the froth controlling the water recovery and entrainment, as well as having a significant impact on the recovery. Froth stability, including bubble coalescence and the bursting of the bubbles at the froth surface, are the key drivers of froth performance. Even though the froth stability is hard to directly control, it is important to understand how this stability impacts froth performance parameters such as the water recovery. In this paper it is shown how a theoretical understanding of froth behaviour based on foam physics can be used to link stability to performance. The extent to which these simplified theoretical relationships can describe the complex behaviour seen in real flotation systems at both the laboratory and industrial scale are explored. The paper shows how bursting flux depends upon gas flux and how this influences the relationship between the gas flux and the water recovery.

Journal article

Reyes F, Lin Q, Udoudo O, Dodds C, Lee PD, Neethling Set al., 2017, Calibrated X-ray micro-tomography for mineral ore quantification, Minerals Engineering, Vol: 110, Pages: 122-130, ISSN: 0892-6875

Scanning Electron Microscopy (SEM) based assessments are the most widely used and trusted imaging technique for mineral ore quantification. X-ray micro tomography (XMT) is a more recent addition to the mineralogy toolbox, but with the potential to extend the measurement capabilities into the three dimensional (3D) assessment of properties such as mineral liberation, grain size and textural characteristics. In addition, unlike SEM based assessments which require the samples to be sectioned, XMT is non-invasive and non-destructive. The disadvantage of XMT, is that the mineralogy must be inferred from the X-ray attenuation measurements, which can make it hard to distinguish from one another, whereas SEM when coupled with Energy-Dispersive X-ray Spectroscopy (EDX) provides elemental compositions and thus a more direct method for distinguishing different minerals. A new methodology that combines both methods at the mineral grain level is presented. The rock particles used to test the method were initially imaged in 3D using XMT followed by sectioning and the 2D imaging of the slices using SEM-EDX. An algorithm was developed that allowed the mineral grains in the 2D slice to be matched with their 3D equivalents in the XMT based images. As the mineralogy of the grains from the SEM images can be matched to a range of X-ray attenuations, this allows minerals which have similar attenuations to one another to be distinguished, with the level of uncertainty in the classification quantified. In addition, the methodology allowed for the estimation of the level of uncertainty in the quantification of grain size by XMT, the assessment of stereological effects in SEM 2D images and ultimately obtaining a simplified 3D mineral map from low energy XMT images. Copper sulphide ore fragments, with chalcopyrite and pyrite as the main sulphide minerals, were used to demonstrate the effectiveness of this procedure.

Journal article

McBride D, Ilankoon IMSK, Neethling SJ, Gebhardt JE, Cross Met al., 2017, Preferential flow behaviour in unsaturated packed beds and heaps: Incorporating into a CFD model, HYDROMETALLURGY, Vol: 171, Pages: 402-411, ISSN: 0304-386X

Journal article

Ilankoon IMSK, Neethling SJ, Huang Z, Cheng Zet al., 2017, Improved inter-particle flow models for predicting heap leaching hydrodynamics, Minerals Engineering, Vol: 111, Pages: 108-115, ISSN: 0892-6875

Heap leaching is one of the most important hydrometallurgical techniques for the extraction of valuable metals from low grade ores due to the relatively low capital costs and limited comminution requirements. However, it is typically affected by low recoveries, compared to conventional mineral extraction techniques such as froth flotation followed by smelting. These low recoveries indicate that there is still significant scope for the improvement of the process performance. This overall process performance is governed by chemical kinetics, mass transport and hydrodynamic effects, with hydrodynamics being particularly not completely understood at the heap scale. The authors have previously presented a model for the hydrodynamics which accounts for both liquid holdup hysteresis and the influence of particle porosity on the fluid flow. In this previous work the model form was only validated for narrowly sized particles. This shortcoming is addressed in this paper, with the validation of the models being extended to cover a range of more complex size distributions representative of those encountered in industrial heap leaching. In this work load cell based gravimetric measurements of liquid holdup values were complimented with electrical capacitance tomography (ECT) measurements, which gave not only the average holdup, but also the liquid distribution within the columns. This study demonstrates that these new hydrodynamic models remain applicable for more realistic particle size distributions, with the need to distinguish between the behaviour of the liquid held within the particle porosity compared to that flowing around the particles being critical to accurate prediction of the hydrodynamics.

Journal article

Dobson KJ, Harrison STL, Lin Q, Bhreasail AN, Fagan-Endres MA, Neethling SJ, Lee PD, Cilliers JJet al., 2017, Insights into ferric leaching of low grade metal sulfide-containing ores in an unsaturated ore bed using x-ray computed tomography, Minerals, Vol: 7, ISSN: 2075-163X

The distribution of the metal-bearing mineral grains within a particulate ore prepared for leaching, and the impact of this spatial heterogeneity on overall extraction efficiency is of key importance to a mining industry that must continuously target ever-reducing grades and more complex ore bodies. If accessibility and recovery of the target minerals is to be improved, a more detailed understanding of the behaviour of the system must be developed. We present an in situ analysis using X-ray computed tomography to quantify the rates of volume reduction of sulfide mineral grains in low grade agglomerated copper bearing ores during a miniature laboratory scale column leaching experiment. The data shows the scale of the heterogeneity in the leaching behaviour, with an overall reduction of sulphide mineral grains of 50%, but that this value masks significant mm3 to cm3 scale variability in reduction. On the scale of individual ore fragments, leaching efficiency ranged from 22% to 99%. We use novel quantitative methods to determine the volume fraction of the sulfide that is accessible to the leachate solution.

Journal article

Tong M, Cole K, Brito-Parada PR, Neethling S, Cilliers JJet al., 2017, Geometry and Topology of Two-Dimensional Dry Foams: Computer Simulation and Experimental Characterization, LANGMUIR, Vol: 33, Pages: 3839-3846, ISSN: 0743-7463

Pseudo-two-dimensional (2D) foams are commonly used in foam studies as it is experimentally easier to measure the bubble size distribution and other geometric and topological properties of these foams than it is for a 3D foam. Despite the widespread use of 2D foams in both simulation and experimental studies, many important geometric and topological relationships are still not well understood. Film size, for example, is a key parameter in the stability of bubbles and the overall structure of foams. The relationship between the size distribution of the films in a foam and that of the bubbles themselves is thus a key relationship in the modeling and simulation of unstable foams. This work uses structural simulation from Surface Evolver to statistically analyze this relationship and to ultimately formulate a relationship for the film size in 2D foams that is shown to be valid across a wide range of different bubble polydispersities. These results and other topological features are then validated using digital image analysis of experimental pseudo-2D foams produced in a vertical Hele–Shaw cell, which contains a monolayer of bubbles between two plates. From both the experimental and computational results, it is shown that there is a distribution of sizes that a film can adopt and that this distribution is very strongly dependent on the sizes of the two bubbles to which the film is attached, especially the smaller one, but that it is virtually independent of the underlying polydispersity of the foam.

Journal article

Ferrier RJ, Cai L, Lin Q, Gorman GJ, Neethling SJet al., 2016, Models for apparent reaction kinetics in heap leaching: A new semi-empirical approach and its comparison to shrinking core and other particle-scale models, Hydrometallurgy, Vol: 166, Pages: 22-33, ISSN: 0304-386X

Particle-scale effects are critically important to the performance of heap leaching operations. In a heap-scale simulation, the transport of fluid phases and reactive species external to the ore particles might be modelled with thousands of grid elements by the finite volume or finite element method. The inter- and intra-particle diffusions and reactions are usually parametrised by a deterministic model, such as the shrinking core model (SCM), that translates the external conditions into an effective product extraction rate. However, the rate equation takes the form of an implicit or partial differential equation for all but the simplest models and kinetic regimes, becoming expensive to solve on large grids. We instead propose an economical, easily calibrated semi-empirical approach in which the dependencies on external conditions and the current state of the ore are considered to be mathematically separable. We show that the standard SCM does not suffer greatly from this approximation even when there is a mixed control regime with nonlinear kinetics. The dependency on the state of the ore is derived empirically, inherently capturing heterogeneous features and cluster-scale effects. We demonstrate that this method scales correctly when fitted to data from physical column leaching experiments.

Journal article

Lin Q, Neethling SJ, Courtois L, Dobson KJ, Lee PDet al., 2016, Multi-scale quantification of leaching performance using X-ray tomography, Hydrometallurgy, Vol: 164, Pages: 265-277, ISSN: 0304-386X

The performance of heap leaching is dictated by a large number of processes acting at a wide range of length scales. One important scale is that of the individual particles, where the interaction between the rate kinetics at the surfaces of the individual mineral grains and the mass transport through the particle combine to give the overall apparent particle scale kinetics. It has been recognised for a long time that variability in the mineralogy, size and spatial distribution of the mineral grains within the particle are likely to have a large effect on the leach performance and its variability and thus, ultimately, the performance of the heap. In this paper a new method for quantifying this behaviour and its variability at scales from the particle through to the grain and down to the surface kinetics is presented. This method is based on the use of a series of XMT (also called micro-CT) images of a column taken at regular intervals over 168 days of leaching. The key development in the analysis of this data is an algorithm that has allowed every single one of the hundreds of thousands of mineral grains within the column to be individually tracked across all the time points as they undergo dissolution. This has allowed the dependency of the mineral grain leach rate on its size and position in the particle to be decoupled from one another. It also meant that the variability in the surface kinetics of the grains could be assessed, with mineralogical variability being the key source of this variability. We demonstrate that understanding and quantifying this underlying kinetic variability is important as it has a major impact on the time evolution of the average kinetics of the leaching.

Journal article

Nunez Rattia JM, Percival JR, Yeager B, Neethling S, Piggott MDet al., 2016, Numerical simulation of scour below pipelines using flexible mesh methods, The 8th International Conference on Scour and Erosion, Pages: 101-108

Evaluating bed morphological structure and evolution (specifically the scoured bed level) accurately using numerical models is critical for analyses of the stability of many marine structures. This paper discusses the performance of an implementation within Fluidity, an open source, general purpose, Computational Fluid Dynamics (CFD) code, capable of handling arbitrary multi-scale unstructured tetrahedral meshes and including algorithms to perform dynamic anisotropic mesh adaptivity. The flexibility over mesh structure and resolution that these capabilities provide makes it potentially highly suitable for coupling the structural scale with larger scale ocean dynamics. In this very preliminary study the solver approach is demonstrated for an idealised scenario. Discontinuous Galerkin finite-element (DG-FEM) based discretisation methods have been used for the hydrodynamics and morphological calculations, and automatic mesh deformation has been utilised to account for bed evolution changes while preserving the validity and quality of the mesh. In future work, the solver will be used in three-dimensional impinging jet and other industrial and environmental scour studies.

Conference paper

Lin Q, Barker DJ, Dobson KJ, Lee PD, Neethling SJet al., 2016, Modelling particle scale leach kinetics based on X-ray computed micro-tomography images, Hydrometallurgy, Vol: 162, Pages: 25-36, ISSN: 1879-1158

The apparent leach kinetics for an ore particle within a heap leaching system depend on the chemical conditions in the fluids around the particle, the mass transport within the particle and the reaction kinetics at the surface of each mineral grain. The apparent rate kinetics thus depend upon the distribution of the mineral grains, in terms of both size and position, within the individual ore particles, as well as the evolution of this distribution. Traditionally this behaviour has been modelled using simplified relationships such as the shrinking core model. In this paper a method for simulating this evolution and the resultant kinetics based directly on 3D XMT images of the internal structure of the particles is presented. The model includes mass transport through the gangue matrix, surface reaction kinetics and the dissolution and subsequent evolution of the individual mineral grains within the ore particle. Different minerals and mineral associations will result in different surface reaction kinetics. One of the key inputs into this model is thus the distribution of the surface rate kinetics. A method for experimentally determining this distribution is presented. The simulation results are compared to the evolution of real particles as they undergo leaching as measured using a time sequence of 3D XMT images of a leaching column. It was found that these simulations are able to accurately predict both the overall leaching trends, as well as the leaching behaviour of mineral grains in classes based on their size and distance to the particle surface. The leaching behaviour did not follow that of a simple shrinking core approximation, with the actual spatial and size distribution of the grains, as well as the distribution of their surface rate kinetics, all impacting the apparent leach kinetics. For the copper ore particles used in this work the best fit to the experiments was achieved at an intermediate value of the dimensionless group that characterises the relativ

Journal article

Ilankoon IMSK, Neethling SJ, 2015, Liquid spread mechanisms in packed beds and heaps. The separation of length and time scales due to particle porosity, Minerals Engineering, Vol: 86, Pages: 130-139, ISSN: 1872-9444

The distribution of liquid within a heap is a key factor in the system performance as it has a strong effect on the transport of both reagents and leached species and thus the leaching rate. How liquid spreads from drippers and the subsequent development of flow paths and any associated channelling is thus important. In this paper a pseudo 2-D column was used to investigate the horizontal spread of liquid in the vicinity of dripper in columns packed with both narrowly sized particles and more realistic particle size distributions. Both systems had distinct separation of the time scales at which different saturation features developed. There was an initial rapid formation of flow paths in the inter-particle spaces with only local wetting of the intra-particle spaces, though this was associated with little spread. Over a much longer time period there was extensive horizontal spread of the liquid within the ore particles, though this was associated with virtually no vertical flow. The externally held liquid (liquid content between the particles) showed strong channelling behaviour, especially in the realistically sized particles, despite the care that was taken to ensure uniform packing. This effect can be reduced by changing initial bed conditions and employing dense drip emitter locations, but it cannot be completely eliminated as particle level heterogeneities in heap leaching systems affect external flow paths creation. Hysteresis in the amount of liquid spread was also demonstrated, with the total spread depending not only on the current flow rate, but also on the flow history.

Journal article

Neethling SJ, Barker DJ, 2015, Using Smooth Particle Hydrodynamics (SPH) to model multiphase mineral processing systems, Minerals Engineering, Vol: 90, Pages: 17-28, ISSN: 1872-9444

A characteristic of most minerals processing systems is that they are multi-phase. The Lagrangian nature of SPH means that it is well suited to modelling these systems as it can naturally track interfaces and free surfaces. In this paper we describe a massively parallel SPH simulator that we have developed and highlight some of the features that have been implemented. These features include surface tension and contact angles, two way coupled solid fluid-interactions and animated geometries. The capabilities of this simulator are illustrated by means of examples showing its ability to simulate a wide range of different minerals processing related systems including particle scale heap flow, multi-phase interactions in a sheared slurry and multi-phase mixing.

Journal article

Mostaghimi P, Percival JR, Pavlidis D, Ferrier RJ, Gomes JLMA, Gorman GJ, Jackson MD, Neethling SJ, Pain CCet al., 2015, Anisotropic Mesh Adaptivity and Control Volume Finite Element Methods for Numerical Simulation of Multiphase Flow in Porous Media, MATHEMATICAL GEOSCIENCES, Vol: 47, Pages: 417-440, ISSN: 1874-8961

Journal article

Lin Q, Neethling SJ, Dobson KJ, Courtois L, Lee PDet al., 2015, Quantifying and minimising systematic and random errors in X-ray micro-tomography based volume measurements, COMPUTERS & GEOSCIENCES, Vol: 77, Pages: 1-7, ISSN: 0098-3004

Journal article

Ilankoon IMSK, Neethling SJ, 2014, Transient liquid holdup and drainage variations in gravity dominated non-porous and porous packed beds, CHEMICAL ENGINEERING SCIENCE, Vol: 116, Pages: 398-405, ISSN: 0009-2509

Journal article

Barker DJ, Brito-Parada P, Neethling SJ, 2014, Application of B-splines and curved geometries to boundaries in SPH, International Journal for Numerical Methods in Fluids, Vol: 76, Pages: 51-68, ISSN: 1097-0363

Smoothed particle hydrodynamics (SPH) has been increasing in popularity rapidly in recent years and is being used for an ever wider range of applications. Central to almost all of these application is the inclusion of accurate wall boundaries. We present here a discussion of boundaries in SPH, in particular, focusing on reflected ghost-particle boundaries. We show how one can include curved shapes as geometrical objects and more generally as parametric non-uniform rational B-spline (NURBS) curves. By properly considering the reflection operation, we derive a correction factor that demonstrably improves the accuracy of the SPH solution and present examples to confirm this. NURBS are standard for representing both 2D curves and 3D surfaces. We detail how they can be practically included in an SPH implementation, including how to calculate various required quantities and reflect particles in the NURBS object.

Journal article

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