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  • Journal article
    Cole K, Brito-Parada PR, Hadler K, Mesa D, Neethling SJ, Norori-McCormac AM, Cilliers JJet 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.

  • Journal article
    Wang P, Brito-Parada PR, 2022,

    Dynamics of a particle-laden bubble colliding with an air-liquid interface

    , Chemical Engineering Journal, Vol: 429, Pages: 1-10, ISSN: 1385-8947

    The collision, bouncing and potential bursting of air bubbles with air-liquid interfaces are key processes involved in the initial stage of foam formation. While fundamental studies of these processes, especially for gas-liquid-solid froth systems, are very valuable for a better understanding of various chemical engineering separation systems, these are scarce. This paper investigates the dynamics of rising bubbles, without particles attached and with various particle coverages, as they collide with an air-liquid interface. For uncoated bubbles, an increase in distance from the bubble releasing point to the air-liquid interface resulted in higher bubble approach velocities, although with minor changes in the velocity fluctuation frequency. This increase in approach velocity was not observed for bubbles with relatively high particle coverage. For particle-laden bubbles, the collision with the interface is associated with movement of the particles over the surface of the decelerating bubble. This particle motion on the bubble surface, combined with bubble shape pulsation, contributes to the kinetic energy dissipation of the approaching bubble. A damped oscillation model was derived to represent the velocity of the bubble interacting with the interface, which shows that the amplitude of the velocity decreases gradually with the increase in particle coverage. The damping coefficient in the model, introduced to quantify the influence of attached particles, is shown to increase with particle coverage, confirming the key role that particles play in bubble collision dynamics at an air-liquid interface and allowing, for the first time, the prediction of their behavior.

  • Journal article
    Marmiroli B, Rigamonti L, Brito-Parada PR, 2021,

    Life Cycle Assessment in mineral processing – a review of the role of flotation

    , The International Journal of Life Cycle Assessment, Vol: 27, Pages: 62-81, ISSN: 0948-3349

    PurposeThe aim of this literature review is to investigate the role of the beneficiation stage in the Life Cycle Assessment (LCA) of metals and minerals with a focus on the flotation process.MethodsThe systematic literature search included LCA studies comprising the beneficiation stage in their system boundaries and resulted in 29 studies that met the criteria requirements and were analysed. First, the system boundaries are investigated, along with the level of detail in the description of the sub-processes (e.g. flotation) and data granularity. Then, the life cycle inventories are scrutinised: data transparency and the relation between system granularity and data availability is commented. Of particular relevance, the way in which the functional unit is dealt with is examined. Finally, studies impact assessments are compared and discussed, and key parameters are highlighted.Results and discussionFor system boundaries, beneficiation is generally embedded into the mining stage. Even when described on its own, important sub-processes (e.g. flotation) are not considered, except for eight cases analysed. Functional unit definition is hindered by the output of the system being an intermediate product. Indeed, most studies use a declared functional unit but fail to provide its relevant characteristics, which is essential for a correct interpretation of results and for comparisons. Most studies rely on secondary data, not always presented transparently, to describe beneficiation. Results on the role of beneficiation in the metal value chain environmental impacts are conflicting, partly because of its site dependency. Site-dependent parameters found to be determining are ore grade, energy mix, mining technique, concentrate grade and ore mineralogy.ConclusionsThe flotation process, and more generally the beneficiation stage, is typically overlooked in LCA studies despite its growing relevance. Beneficiation not being assessed as a standalone stage, detailed in its subprocess

  • Journal article
    Mesa D, Cole K, van Heerden MR, Brito-Parada PRet al., 2021,

    Hydrodynamic characterisation of flotation impeller designs using Positron Emission Particle Tracking (PEPT)

    , Separation and Purification Technology, Vol: 276, Pages: 1-19, ISSN: 0950-4214

    Impellers play a key role in flotation cells, as the turbulence generated through agitation aids particle suspension, air dispersion and particle–bubble collision. Therefore, it is important to understand the effect that different impeller designs have on flotation hydrodynamics, as small variations could enhance flotation performance. The study of flotation hydrodynamics is, however, a complex task due to the nature of flotation systems, which are opaque, multiphase, and polydisperse. In this paper, the impact of impeller design modifications on the hydrodynamics of a flotation cell was quantified for the first time in a three-phase system. Two different impeller designs, with and without a stator, were assessed using positron emission particle tracking (PEPT), a technique that allows the position and velocity of radioactive particle tracers within an opaque vessel to be determined. A novel PEPT data analysis strategy, as well as a statistical analysis on the basis of the Jensen–Shannon distance, were used. This statistical analysis, applied for the first time to PEPT data, facilitated the comparison of the different designs, by generating a robust quantification of their hydrodynamic differences. The experimental results showed that the stator significantly modified the hydrodynamics within the flotation cell, distorting the lower mixing loop that is characteristic of radial impellers. The use of a stator also resulted in the reduction of particle velocity and swirling outside of the impeller–stator region, both at the level of the impeller and, notably, at the pulp–froth interface. These findings have important implications for impeller–stator design, evidencing that the impeller has a direct effect on the hydrodynamics of the pulp and froth.

  • Journal article
    Quintanilla P, Neethling SJ, Navia D, Brito-Parada PRet al., 2021,

    A dynamic flotation model for predictive control incorporating froth physics. Part I: Model development

    , Minerals Engineering, Vol: 173, Pages: 1-23, ISSN: 0892-6875

    It is widely accepted that the implementation of model-based predictive controllers (MPC) ensures optimal operation if an accurate model of the process is available. In the case of froth flotation, modelling for control purposes is a challenging task due to inherent process instabilities. Most models for control have only focused on the pulp phase rather than the froth phase, which is usually oversimplified or even neglected. Despite the fact that froth stability can significantly affect the overall performance of flotation cells, there is still a gap in literature regarding flotation models for control purposes that properly include froth physics.In this paper we describe the development of a dynamic flotation model suitable for model predictive control, incorporating equations that describe the physics of flotation froths. Unlike other flotation models for control in the literature, the model proposed here includes important variables related to froth stability, such as bursting rate and air recovery, as well as simplified equations to calculate froth recovery and entrainment. These model equations allow estimating the amount of valuable material reporting to the concentrate, which can be used as a proxy to estimate grade and recovery. Additionally, pulp-froth interface physics was also included in our model, which enables a more accurate prediction of relevant flotation variables.A sensitivity analysis of the parameters showed that two out of seven parameters were highly sensitive. The highly sensitive parameters are the exponential factor n of the equation for the overflowing bubble size, and the constant value a of the equation for the bursting rate. Although the other parameters showed a reasonably lower sensitivity than n and a, the results also revealed that there is a significant difference in the prediction accuracy if the parameters are poorly estimated. Further simulations of important variables for control exhibited a good adaptability to changes in typ

  • Journal article
    Quintanilla P, Neethling SJ, Mesa D, Navia D, Brito-Parada PRet al., 2021,

    A dynamic flotation model for predictive control incorporating froth physics. Part II: Model calibration and validation

    , Minerals Engineering, Vol: 173, Pages: 1-15, ISSN: 0892-6875

    Modelling for flotation control purposes is the key stage of the implementation of model-based predicted controllers. In Part I of this paper, we introduced a dynamic model of the flotation process, suitable for control purposes, along with sensitivity analysis of the fitting parameters and simulations of important control variables. Our proposed model is the first of its kind as it includes key froth physics aspects. The importance of including froth physics is that it improves the estimation of the amount of material (valuables and entrained gangue) in the concentrate, which can be used in control strategies as a proxy to estimate grade and recovery.In Part II of this series, experimental data were used to estimate the fitting parameters and validate the model. The model calibration was performed to estimate a set of model parameters that provide a good description of the process behaviour. The model calibration was conducted by comparing model predictions with actual measurements of variables of interest. Model validation was then performed to ensure that the calibrated model properly evaluates all the variables and conditions that can affect model results. The validation also allowed further assessing the model’s predictive capabilities.For model calibration and validation purposes, experiments were carried out in an 87-litre laboratory scale flotation tank. The experiments were designed as a randomised full factorial design, manipulating the superficial gas velocity and tailings valve position. All experiments were conducted in a 3-phase system (solid-liquid–gas) to ensure that the results obtained, as well as the behaviour of the flotation operation, are as similar as possible to those found in industrial flotation cells.In total, six fitting parameters from the model were calibrated: two terms from the equation for overflowing bubble size; three parameters from the bursting rate equation; and the number of pulp bubble size classes. After the mode

  • Journal article
    Moore KR, Moradi S, Doyle K, Sydd O, Amaral V, Bodin J, Brito-Parada PR, Dudley F, Fitzpatrick R, Foster P, Goettmann F, Roberts D, Roethe R, Sairinen R, Sambrook T, Segura-Salazar J, Thomas Get al., 2021,

    Sustainability of switch on-switch off (SOSO) mining: Human resource development tailored to technological solutions

    , Resources Policy, Vol: 73, Pages: 1-13, ISSN: 0301-4207

    Adaptable, mobile, modularised technical solutions were piloted for switch on-switch off (SOSO) mining at test sites in the west Balkans. Pre-training occurred at the site of module construction and on the mine site in order to transfer knowledge relating to the rapid deployment, commissioning and operation of mining and processing units, in a mature health and safety culture. Translation of extensive documentation, describing operation of the equipment, into local languages and visual aids supported communication. Consideration of the activities required to deploy and operate prototype solutions revealed how characteristics of a SOSO workforce differed from other types of mining. Deployment of modularised plant employed fewer workers than traditional stick-build of a processing plant, but selective mining and processing of complex and variable deposits limited the potential for automation and required operator control. A workforce with mixed levels of experience was most amenable to development of a mature health and safety culture. The total number of employees was small at an individual site and might remain small, even in a multi-deposit, regional business model. However, employment is higher per unit of production than for conventional large-scale mining. The duration of employment is shorter than for large-scale mining but employment can nevertheless be important where there are few alternative opportunities and where it can increase the skills-base to support a more diversified local economy. SOSO mining constitutes a new relationship between society and the mining industry, which needs further consideration for greater resilience in the local community and increased social sustainability.

  • Journal article
    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
    Beylot A, Muller S, Segura-Salazar J, Brito-Parada P, Paneri A, Yan X, Lai F, Roethe R, Thomas G, Goettmann F, Braun M, Moradi S, Fitzpatrick R, Moore K, Bodin Jet al., 2021,

    Switch on-switch off small-scale mining: Environmental performance in a life cycle perspective

    , Journal of Cleaner Production, Vol: 312, ISSN: 0959-6526

    A switch on-switch off (SOSO) approach to mining relies on an integrated modular and mobile plant. It is appropriate for rapid start-up and cessation of production from ore deposits that have economic viability dependent on fluctuating commodity prices. This study aims at assessing the environmental performance of the SOSO approach in a life cycle perspective, using evidence collected during its pilot-scale implementation on a high-grade lead deposit located in Bosnia-Herzegovina. The deployed modules include: i) a selective mining tool, ii) a comminution module with crusher and screen, and iii) a gravity separation module with screen, spirals and a shaking table. The Life Cycle Inventory is based on a comprehensive and transparent set of data, drawn from the on-site pilot tests, completed with additional calculations (including process simulation with USIM-PAC®), scenarios (regarding energy), and some generic data and assumptions. The potential environmental impacts are calculated considering 13 impact categories of the EF 2.0 impact assessment method. The production of one tonne of Pb concentrate, with a Pb-content of 58.7%, induces in particular the potential generation of 897 kg CO2-eq. Moreover, the contribution analysis enables identification of the environmental hotspots, including the consumption of electricity supplied by on-site diesel generators that contributes to more than 90% of the total impacts in seven impact categories. Finally, this article discusses i) how advantageous the SOSO approach is compared to large-scale lead mining regarding some of its associated key environmental hotspots, ii) the potential for reduction of the impacts associated with electricity consumption, in particular through the implementation of a renewable electricity supply mix (solar photovoltaics and biomass-based), iii) the contributions of equipment and tailings to the total impacts, and iv) the contribution of the SOSO approach to resource accessibility and depletion

  • Journal article
    Wang H, Brito-Parada PR, 2021,

    Shape deformation and oscillation of particle-laden bubbles after pinch-off from a nozzle

    , Chemical Engineering Journal, Vol: 412, Pages: 1-10, ISSN: 1385-8947

    The rise of bubbles in liquid is a common phenomenon in chemical engineering applications. Bubble dynamics, however, are not fully understood, particularly at the early stages after bubbles are released from submerged nozzles, or when particles coat the bubble surface. In this work, a detailed investigation of microparticle-laden bubbles rising in water after being released from a nozzle was carried out to determine the influence of bubble surface coverage on the interface dynamics after pinch-off. The use of high-speed photography, at up to 25170 frames per second, allowed two regimes to be systematically investigated for the first time, i.e. an initial bubble shape deformation and shape oscillations. Surface pressure analysis shows that microparticles reduce the apparent surface tension of the interface by generating surface pressure during the initial bubble deformation. In contrast, during shape oscillations, little effect was observed on the period of the dominant harmonic, indicating that surface tension does not change during the oscillations. Harmonic analysis also showed that microparticles at bubble surfaces significantly increase the damping rate of the dominant harmonic, with a dependency on the bubble surface coverage. By quantifying the effect of particles on bubble dynamics, this work contributes to a better understanding of gas–liquid–solid reactors in which particle attachment plays a key role.

  • 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
    Segura-Salazar J, Brito-Parada PR, 2021,

    Stibnite froth flotation: A critical review

    , Minerals Engineering, Vol: 163, Pages: 1-23, ISSN: 0892-6875

    Antimony has a long and diverse history of applications, and concerns about its future supply have emerged in recent years. Stibnite-bearing ores continue to be the main source of this critical element, although its sourcing from secondary resources (e.g. mine tailings) is becoming increasingly relevant yet technically challenging. In this sense, froth flotation plays a key role in the recovery of stibnite from both sources. In view of this, a comprehensive review of technical and environmental aspects related to the flotation of stibnite across scales (microflotation, bench-scale and industrial-scale) has been carried out. One of the major topics reviewed is stibnite flotation reagents; while the overall practice has been mostly conservative, relying on the use of xanthate collectors and lead nitrate as an activator, less common (bio)reagents for the flotation of stibnite ores of varying complexity have been shown to be a promising alternative. Other challenges identified from the literature are discussed, highlighting areas of opportunities for further research to enhance stibnite flotation, minimise antimony losses throughout the mine life cycle and thus make antimony more available for future generations.

  • Journal article
    Wang H, Brito-Parada PR, 2021,

    Deformation dynamics of particle-laden bubbles: The effect of surfactant concentration and particle contact angle

    , Minerals Engineering, Vol: 160, Pages: 1-6, ISSN: 0892-6875

    The pinch-off dynamics of bubbles coated with silica particles in deionised water and in dodecylamine (DDA) solution, at 0.2 μM and 20.0 μM, was studied using high-speed photography. Surface pressure generated at the deforming particle-laden interface during bubble pinch-off was obtained based on the fitting to a pinch-off model. It was observed that the pinch-off dynamics of these particle-laden bubbles remained almost unchanged at the low DDA concentration of 0.2 μM, while the dynamics slowed down significantly at the DDA concentration of 20.0 μM. Notably, both the 0.2 μM and 20.0 μM DDA concentrations have a negligible effect on the surface tension and pinch-off dynamics of uncoated bubbles. The difference in DDA concentration, however, is known to change the contact angle of silica particles from approximately 27° to 45°. It can be concluded that it is the change in particle contact angle that affects the pinch-off dynamics of particles-laden bubbles. Indeed, at a concentration of 0.2 μM DDA there is no significant change in contact angle of the silica particles with respect to that in DI water only, resulting in similar dynamics. It is suggested that the increase in the particle contact angle changes particle interactions, leading to a change in the surface pressure and apparent surface tension of particle-laden bubbles, which in turn slows down the pinch-off process. The findings in this work are relevant to our understanding of fundamental aspects of deforming particle-laden interfaces, such as those in the coalescence of flotation froths.

  • Journal article
    Wang H, Brito-Parada PR, 2020,

    The role of microparticles on the shape and surface tension of static bubbles

    , Journal of Colloid and Interface Science, Vol: 587, Pages: 14-23, ISSN: 0021-9797

    HYPOTHESIS: Surface tension is a critical parameter in bubbles and foams, yet it is difficult to assess when microparticles are attached at the interface. By considering the interaction force between an air-liquid interface and microparticles, modified equations for sessile bubble tensiometry can be derived to determine the surface tension and shape of static microparticle-laden bubbles. EXPERIMENTS: A modified sessile bubble method, in which the forces between microparticles and the air-liquid interface are considered, was developed and used to analyse the surface tension of bubbles fully coated by a monolayer of silica microparticles of different sizes. The results are compared to those obtained using classical sessile bubble tensiometry. The new method is also used to investigate the contours of particle-laden bubbles of varying particle radius and contact angle. FINDINGS: While the classical sessile bubble method overestimates the surface tension, results obtained using the modified sessile bubble method show that the surface tension of static microparticle-laden bubbles remains the same as that of uncoated bubbles, with no dependency on the particle size. The discrepancy is due to the fact that microparticles attached to the air-liquid interface deform a bubble in a similar way that changes in surface tension do for uncoated bubbles.

  • Journal article
    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.

  • Journal article
    Mesa Pena D, Morrison AJ, Brito Parada PR, 2020,

    The effect of impeller-stator design on bubble size: implications for froth stability and flotation performance

    , Minerals Engineering, Vol: 157, ISSN: 0892-6875

    The impeller in a mechanical flotation tank plays a key role in keeping particles in suspension, dispersing and breaking-up air bubbles, and promoting particle-bubble collision. However, the turbulent regime generated by the impeller can also affect the pulp-froth interface, destabilising the lower regions of the froth and affecting the overall flotation performance. The effects that pulp zone design modifications have on the froth are, however, poorly understood and have not been well-studied.In this work, we study the impact of impeller-stator design on the performance of a large laboratory-scale flotation tank. Two different impeller designs, with and without a stator, were assessed under a range of air flow rates to determine changes in pulp bubble size, froth stability, and metallurgical recovery. The results allow us to quantify, for the first time, the reduction in bubble size in a three-phase flotation system and the improvement in froth stability due to the use of a stator, and thus the enhancement in flotation performance. An inverse relationship is found between the pulp bubble size and froth stability. It is shown that the impeller designs that exhibited smaller bubble sizes resulted in higher froth stability values and also higher flotation recoveries. These findings provide insights into the links between pulp and froth zone phenomena, paving the way for improvements in flotation tank design that lead towards flotation optimisation.

  • Journal article
    Wang H, Brito-Parada PR, 2020,

    The pinch-off dynamics of bubbles coated by microparticles

    , Journal of Colloid and Interface Science, Vol: 577, Pages: 337-344, ISSN: 0021-9797

    HypothesisWhile the pinch-off dynamics of bubbles is known to be influenced by changes in surface tension, previous studies have only assessed changes due to liquid properties or surfactant effects at the air-liquid interface but not due to the presence of particles. The current study proposes that particles at the air-liquid interface play an important role in changing the surface tension and thus the pinch-off dynamics of particle-laden bubbles.ExperimentsHigh-speed photography was used to study the pinch-off dynamics of air bubbles coated by a monolayer of silica microparticles. The influence of bubble surface coverage and particle size classes on the bubble pinch-off dynamics were explored.FindingsWe identify that although the scaling exponent of the power law that governs the pinch-off of coated and uncoated bubbles is the same, the pinch-off dynamics is distinctly different when particles are present at the air-liquid interface due to a decrease in surface tension with time in the neck region. We suggest that the surface pressure generated by particle interaction reduces the pinch-off speed by reducing the apparent surface tension. We observe that the apparent surface tension is dependent on particle size but not on the percentage of bubble surface coated by particles.

  • Journal article
    Sommer A-E, Ortmann K, Van Heerden M, Richter T, Leadbeater T, Cole K, Heitkam S, Brito-Parada PR, Eckert Ket al., 2020,

    Application of Positron Emission Particle Tracking (PEPT) to measure the bubble-particle interaction in a turbulent and dense flow

    , Minerals Engineering, Vol: 156, Pages: 1-10, ISSN: 0892-6875

    In a flotation cell, turbulence influences the motion of solid particles relative to the bubble surface, and, thus,affects the recovery rate. But, the impact of turbulence on the probability of a bubble-particle aggregationis still difficult to measure, especially in a dense flow. Therefore, the focus of this work was to applyPositron Emission Particle Tracking (PEPT) as a method to investigate the effect of turbulence on theparticle movement and bubble-particle interaction in an opaque flow. Single air bubbles (db = 2.5 mm) weregenerated on a needle in a water flow channel. Upstream, a grid produced an isotropic turbulent flow with5 % to 15 % turbulence intensity and a Kolmogorov microscale of 20 µm. Depending on the distance to thegrid, the flow near the captive bubble (Reb ≈ 450) was characterized by eddies of different length scalesand magnitude with tomographic Particle Image Velocimetry (PIV). The solid suspension contained up to0.3 % polymethylmethacrylate (PMMA) particles (dp = 200 µm–400 µm) and up to six radiolabelled particles(dp = 300 µm–400 µm) coated with PMMA. The trajectories of the labelled particles were used to determinethe average particle distribution in the turbulent field and describe the bubble-particle interactions. Theseresults provide valuable information on the applicability of PEPT in turbulent and dense flow fields as wellas on particle trajectories close to bubbles, enhancing our understanding of key flotation phenomena.

  • Journal article
    Mesa Pena D, Brito Parada P, 2020,

    Bubble size distribution in aerated stirred tanks: Quantifying the effect of impeller-stator design

    , Chemical Engineering Research and Design, Vol: 160, Pages: 356-369, ISSN: 0263-8762

    Bubble size is an important variable in aerated stirred tanks as it determines the surface area available for reactions. In the bubble break-up process, impellers play a key role. Despite this importance, research into the effect of impeller design on bubble size is scarce. In this work we study the effect of two impellers, with and without a stator, as well as the effect of airflow rate, impeller speed and surfactant concentration on bubble size.Results show that there is a critical impeller speed above which bubble size is not further decreased, regardless of the airflow. Operating at this critical speed results in the smallest bubble size possible without additional turbulence. The reduction in bubble size caused by a stator was quantified for the first time and, interestingly, it was found that the stator also reduced the critical coalescence concentration. The implications of these findings for the design, evaluation and optimisation of impellers are discussed.

  • Journal article
    Sitorus F, Brito-Parada PR, 2020,

    A multiple criteria decision making method to weight the sustainability criteria of renewable energy technologies under uncertainty

    , Renewable and Sustainable Energy Reviews, Vol: 127, Pages: 1-11, ISSN: 1364-0321

    Selecting the most suitable renewable energy technology among feasible alternatives considering conflicting criteria is a Multiple Criteria Decision Making (MCDM) problem. One of the essential stages in the methods used to solve such problems is determining the appropriate weight of each criterion to be considered. The Shannon Entropy method is a frequently used MCDM method to calculate the criteria weights, however it is not suitable to solve problems for which uncertainty in the input data exists. This paper presents a new extended Shannon Entropy method: the Integrated Constrained Fuzzy Shannon Entropy (IC-FSE) method, by which criteria weights are obtained from uncertain input data. To show the applicability of IC-FSE, an illustrative example for the selection of a renewable energy technology in the mining industry is presented, in which three alternative renewable energy technologies, onshore wind, solar photovoltaic and concentrated solar power, were evaluated with respect to technical, social, economic and environmental categories. The results show that IC-FSE can effectively provide appropriate fuzzy solutions for weighting the sustainability criteria for renewable energy technologies. The superiority of this method is showcased by demonstrating that IC-FSE results are more robust than those obtained using other existing methods. The methodology presented can be applied broadly in the renewable energy sector to ensure better informed decision making processes.

  • Journal article
    Wang H, Brito-Parada PR, 2020,

    Coalescence dynamics of particle-laden bubbles.

    , Langmuir: the ACS journal of surfaces and colloids, Vol: 36, Pages: 5394-5399, ISSN: 0743-7463

    Understanding the coalescence of particle-laden bubbles is crucial to our understanding of the role of particles in stabilizing liquid foams. In this work, the coalescence of microparticle-laden bubbles is studied experimentally using high-speed photography. In particular, the interparticle forces in the neck region during the early stage of bubble coalescence are calculated. The results indicate that a monolayer of silica particles coating the bubble surfaces hinders the growth dynamics of the air neck formed between the coalescing bubbles. We postulate that the decrease in the growth dynamics is due to the surface pressure caused by the particle interaction after the initiation of bubble coalescence. We identify that the apparent surface tension in the neck region increases with time for particle-laden bubbles and is lower for larger particle sizes. These findings enhance our understanding of the role of particles on the dynamics of fast deforming interfaces.

  • Journal article
    Sitorus F, Brito-Parada PR, 2020,

    Equipment selection in mineral processing - A sensitivity analysis approach for a fuzzy multiple criteria decision making model

    , Minerals Engineering, Vol: 150, Pages: 1-8, ISSN: 0892-6875

    Selecting the most suitable mineral processing equipment among feasible alternatives with respect to multiple conflicting criteria is considered a Multiple Criteria Decision Making (MCDM) problem. For example, a type of crusher that might allow a very high throughput is less likely to be used in a mobile plant, so trade-offs between these type of criteria need to be clearly defined in the decision making process. One of the most frequently used MCDM methods is the Analytical Hierarchy Process (AHP) method, which relies on judgements from decision makers that allow for comparisons to be made between alternatives (e.g. the type of equipment) or criteria (e.g. the characteristics of the equipment that are of interest). However, AHP is not able to capture the uncertainty associated with the various decision makers’ judgements and the lack of precise information. An integrated constrained fuzzy stochastic analytic hierarchy process (IC-FSAHP) is a new hybrid MCDM method that can be used to overcome the aforementioned limitations of AHP. In order to understand the robustness of AHP based methods, a sensitivity analysis of the decision making results is required. However, sensitivity analyses are not often carried out for fuzzy AHP methods, arguably because of the complexity of some of the procedures involved, the computation time required and the limited resources available to do so. The main objective of this paper is therefore to propose a new sensitivity analysis approach by applying an additional fuzzification factor and disagreement level of decision makers in order to model uncertainty. For this purpose, a case study for the selection of primary crushers was considered. Five types of primary crushers were evaluated with respect to six criteria to showcase the applicability of the proposed approach to assess IC-FSAHP. The results obtained showcase that the proposed sensitivity analysis approach is capable of providing extensive and useful “what-if”

  • Journal article
    Bhutani G, Brito Parada PR, 2020,

    A framework for polydisperse pulp phase modelling in flotation

    , Separation and Purification Technology, Vol: 236, ISSN: 0950-4214

    Froth flotation is one of the most widely-used mineral processing operations. The pulp zone in flotation tanks is polydisperse ingeneral and serves as a medium for the interaction between the solid particles and the gas bubbles in a liquid continuum, leadingto particle–bubble attachment/detachment and bubble coalescence/breakage phenomena. To better predict the hydrodynamics andinform the design of e cient flotation equipment, it is therefore important to accurately model and simulate the evolution of the sizedistribution of the dispersed phases. This has created an urgent need for a framework that can model the pulp phase in an e cientmanner, which is not currently available in the literature. The available software products are not e cient enough to allow for atractable modelling of industrial-scale flotation cells and in some cases they cannot model the polydispersity of the dispersed phaseat all. This work presents an e cient numerical framework for the macroscale simulation of the polydisperse pulp phase in frothflotation in an open-source finite element computational fluid dynamics (CFD) code that provides an e cient solution method usingmesh adaptivity and code parallelisation. A (hybrid finite element–control volume) finite element framework for modelling the pulpphase has been presented for the first time in this work. An Eulerian–Eulerian turbulent flow model was implemented in this workincluding a transport equation for attached and free solid particles. Special care was taken to model the settling velocity of the freesolids and the modification of the liquid viscosity due to the presence of these particles. Bubble polydispersity was modelled usingthe population balance equation (PBE), which was solved using the direct quadrature method of moments (DQMOM). Appropriatefunctions for bubble coalescence and breakage were chosen in the PBE. Mesh adaptivity was applied to the current problem toproduce fully-unstructured anisotropic meshes, whi

  • Journal article
    Wang P, Reyes F, Cilliers J, Brito Parada PRet 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.

  • Journal article
    Mackay I, Videla AR, Brito Parada PR, 2020,

    The link between particle size and froth stability - Implications for reprocessing of flotation tailings

    , Journal of Cleaner Production, Vol: 242, ISSN: 0959-6526

    Historic tailings dams can often be considered as valuable mineral reserves due to the declining head grades of primary deposits. The reprocessing of such material is of great interest to the minerals processing industry, not only from an economic point of view, but also from an environmental one. However, tailings material is generally comprised of fine particles, which poses a challenge for its reprocessing using froth flotation due to reduced recoveries of these particle sizes. In addition, there is some debate as to the effect that these fine particles have on the froth stability, which in turn is linked to mineral recovery.In this work, air recovery was used as a measure of froth stability to determine the flotation response of a copper tailings ore to changes in particle size distribution and superficial gas velocity. The system exhibited a maximum in air recovery, which correlates well with the local peak in dynamic froth stability presented in previous work. This maximum in froth stability is also shown to correspond to an improvement in flotation performance, thus highlighting the importance of considering the link between particle size, air rate and froth stability when determining the flotation strategy for tailings reprocessing. The results are discussed in terms of the implications for the reprocessing plant from where the ore samples were obtained and, more generally, for the efficient flotation reprocessing of tailings.

  • Journal article
    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.

  • 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
    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.

  • 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

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