Modelling and simulation of carbon-in-leach circuits

A CIL circuit is a process of continuous leaching of gold from ore to liquid using a counter-current adsorption of gold from liquid to carbon particles in a series of tanks. It concentrates gold from 2.5-3.5 g/t in ore to 10000 to 15000 g/t on carbon, thus playing an important role on the economics of a gold refinery.In this study, a dynamic model of CIL circuits has been developed to study the transient nature of the system. The effect of various operating parameters on the performance of the system has also been assessed. For example, the particle size and cyanide concentration were predicted to play a critical role on the gold leaching. A decrease in the particle size increased the efficiency of the process, whereas an opposite effect was observed on increasing the cyanide concentration. The recovery also increased on increasing the carbon transfer interval. On the other hand, oxygen concentration did not show a significant effect on the efficiency.The hydrodynamics of CIL tanks is also a complex phenomenon, and it affects both leaching and adsorption kinetics. Current models account for the effect of hydrodynamics in lumped manner. One needs to incorporate the hydrodynamic parameters explicitly in order to make the model applicable over a wider range of operating conditions. Therefore, rigorous CFD simulations of CIL tanks have also been carried out in this study. However, current multiphase CFD simulations require validation especially for interphase closures (such as drag). Therefore, simulations have been conducted using a number of drag models. The modified Brucato drag model was found to be the most appropriate for the CIL tanks, and hence was used in conducting the majority of the simulations in this study. Subsequently, the simulations were conducted to study the effect of various parameters, such as solid loading, and impeller speed and type, on the hydrodynamics of CIL tanks.At low solid loadings, the effect of it on the liquid hydrodynamics was minimal, however, at high solid concentrations, substantial impact on the hydrodynamics was predicted. For example, ‘false bottom effect’ was predicted at very high solid concentration indicates the presence of dead zones. Similarly, at higher solid loadings, higher slip velocities were observed below the impeller, near the wall and near the impeller rod. Finally, the higher solid loadings also caused the dampening of turbulence due to the presence of particles, thus resulting in significant power consumption to counteract this dampening.Other than ore particles, CIL tanks also contain the larger carbon particles. The flow of carbon particles is affected by the flow of ore-liquid slurry. No model is currently available for calculating the drag force on the carbon particles. For obtaining the drag force, a novel macroscopic particle model (MPM) based on RDPM approach was used after validation. The predictions from the MPM model were compared with the available experimental data, and a new drag model has been proposed for the carbon particles in the CIL slurry.The research develops a phenomenological model, validates the drag model for ore particles and proposes a drag model for carbon particles. These models along with the methodology presented in the thesis can be applied on the industrial scale CIL tanks for any ore type provided the rate terms and kinetic constants are known

CFD modelling of flow and solids distribution in carbon-in-leach tanks

The Carbon-in-Leach (CIL) circuit plays an important role in the economics of a gold refinery. The circuit uses multiphase stirred tanks in series, in which problems such as dead zones, short-circuiting, and presence of unsuspended solids are detrimental to its efficiency. Therefore, the hydrodynamics of such a system is critical for improving the performance. The hydrodynamics of stirred tanks can be resolved using computational fluid dynamics (CFD). While the flow generated by the impellers in the CIL tanks is complex and modelling it in the presence of high solid concentration is challenging, advances in CFD models, such as turbulence and particle-fluid interactions, have made modelling of such flows feasible. In the present study, the hydrodynamics of CIL tanks was investigated by modelling it using CFD. The models used in the simulations were validated using experimental data at high solid loading of 40 wt. % in a lab scale tank. The models were further used for examining the flow generated by pitched blade turbine and HA-715 Mixtec impellers in lab scale CIL tanks with 50 wt. % solids. The effect of design and operating parameters such as off-bottom clearance, impeller separation, impeller speed, scale-up, and multiple-impeller configuration on flow field and solid concentrations profiles was examined. For a given impeller speed, better solids suspension is observed with dual impeller and triple impeller configurations. The results presented in the paper are useful for understanding the hydrodynamics and influence of design and operating parameters on industrial CIL tanks

Modelling gas-liquid flow and local mass transfer in stirred tanks

This doctorial thesis offers a guideline for modelling gas-liquid flow in stirred tanks with computational fluid dynamics (CFD). Particularly the effect of varying physical properties and industrial operating conditions is highlighted. The most important thing in modelling mass transfer in stirred vessels is the accurate prediction of local bubble size. Population balances for bubbles are needed for accurate description of the local mass transfer rate. There are many pitfalls in gas-liquid modelling at the transitional turbulence regime, and they need to be recognised and dealt with at a reasonable computational cost. Details of the work are presented in the included publications, this thesis sums up the findings. Backbone of this thesis is the experimental work done on 14 and 200 dm³ vessels. Experimental techniques were compared in making bubble size distribution (BSD) measurements. A variety of experiments were made to investigate: physical properties, vapour-liquid equilibrium, gas hold-up, gas-liquid mass transfer, bubble size distributions, local mixing times, flow fields and bubble swarm interactions. Parameters for a number of phenomenological models were fitted with a computationally less demanding multiblock model and were then used to simulate stirred reactors with CFD. The early systems were lean dispersions of low viscosity; at the end of this work opaque shear thinning G-L dispersions were modelled. The effect of impeller geometry on G-L mass transfer was studied by simulating three impeller geometries. There were no differences in the volumetric mass transfer rate between the impellers, although the flow patters and gas hold-up showed clear differences between the impellers. Heterogeneous behaviour like gas slug creation and reactor dead-spaces were successfully modelled. The simulated dispersions were highly heterogeneous: 50% of mass transfer took place in less than 10% of the reactor volume. A xanthan fermentation batch lasting for days was modelled; the reaction speed was bottlenecked by both mixing and mass transfer. These findings strongly support the use of spatially detailed models over ideal mixing assumption

CFD simulation of solid-liquid stirred tanks for low to dense solid loading systems

The hydrodynamics of suspension of solids in liquids are critical to the design and performance of stirred tanks as mixing systems. Modelling a multiphase stirred tank at a high solids concentration is complex owing to particle-particle and particle-wall interactions which are generally neglected at low concentrations. Most models do not consider such interactions and deviate significantly from experimental data. Furthermore, drag force, turbulence and turbulent dispersion play a crucial role and need to be precisely known in predicting local hydrodynamics. Therefore, critical factors such as the modelling approach, drag, dispersion, coefficient of restitution and turbulence are examined and discussed exhaustively in this paper. The Euler-Euler approach with kinetic theory of granular flow, Syamlal-O'Brien drag model and Reynolds stress turbulence model provide realistic predictions for such systems. The contribution of the turbulent dispersion force in improving the prediction is marginal but cannot be neglected at low solids volume fractions. Inferences drawn from the study and the finalised models will be instrumental in accurately simulating the solids suspension in stirred tanks for a wide range of conditions. These models can be used in simulations to obtain precise results needed for an in-depth understanding of hydrodynamics in stirred tanks

Solid Suspension and Gas Dispersion in Mechanically Agitated Vessels

RÉSUMÉ La conception et l’opération réussies de cuves agitées mécaniquement liquide-solide (LS) ou gazliquide- solide (GLS) requièrent la détermination précise du niveau adéquat de suspension solide qui est essentiel pour le procédé. Les ingénieurs et les scientifiques doivent définir des conditions géométriques et opératoires pour un milieu spécifique (propriétés physiques spécifiques) afin de fournir le niveau optimal de suspension solide. Ceci nécessite une connaissance approfondie de comment l’état de la suspension solide peut être influencé par des variations des paramètres physiques, opératoires et géométriques. De même, des corrélations empiriques ou des concepts théoriques précis sont nécessaires pour atteindre cet objectif. Le fait de ne pas concevoir la cuve agitée pour atteindre les conditions optimales et pour maintenir le système dans ces conditions durant l’opération peut amener des inconvénients significatifs concernant la qualité du produit (sélectivité et rendement) et le coût. Cette étude implique un travail expérimental et théorique extensif sur la suspension et la dispersion du solide dans un système de mélange liquide-solide. Le système étudié a été une cuve agitée mécaniquement. En utilisant différentes techniques de mesure, comme la densitométrie aux rayons gamma et les fibres optiques, il a été possible d’obtenir des résultats très intéressants qui permettront d’améliorer la conception et la montée en échelle de systèmes de mélange liquidesolide. Une revue de la littérature approfondie à propos de la suspension de solide en cuves agitées et des discussions détaillées avec des partenaires industriels nous ont menés à nous concentrer sur trois objectifs principaux pour améliorer la connaissance des systèmes de mélange liquide-solide denses : 1. Introduire une méthode prometteuse pour la caractérisation fine de la vitesse de suspension (Njs) dans un système de mélange liquide-solide à haute concentration.----------ABSTRACT The successful design and operation of liquid-solid (LS) and gas-liquid-solid (GLS) mechanically agitated vessels require the accurate determination of the proper level of solid suspension that is essential for the process at hand. Engineers and scientists must define geometrical and operating conditions for a specific medium (specified physical properties) in such a way that provides the optimum level of solid suspension. This requires comprehensive knowledge about how the state of solid suspension may be affected by changing physical, operational, and geometrical parameters. Also, accurate empirical correlations or theoretical concepts are necessary to fulfill that objective. Failure to design the agitated vessel to achieve optimum conditions and maintain the system at these conditions during operation may cause significant drawbacks concerning product quality (selectivity and yield) and cost. This research involves extensive experimental and theoretical work on solid suspension and dispersion in a liquid-solid mixing system. The system under study was a mechanically agitated vessel. By using different measurement methods, i.e., Gamma Ray Densitometry, and Optical Fibre, attention-grabbing results have been obtained, which will help to improve the design and scale-up of liquid-solid mixing systems. A thorough literature survey on solid suspension in agitated tanks and comprehensive discussions with industrial partners led us to focus on three major objectives to improve our knowledge of dense liquid-solid mixing systems: 1. To introduce a promising method for accurate characterizing just off-bottom suspension speed (Njs) in high concentration liquid-solid mixing system

Numerical Study and Geometric Investigation of the Influence of Rectangular Baffles over the Mixture of Turbulent Flows into Stirred Tanks

The present work aims to define strategies for numerical simulation of the mixture of turbulent flows in a stirred tank with a low computational effort, and to investigate the influence of the geometry of four rectangular baffles on the problem of performance. Two computational models based on momentum source and sliding mesh are validated by comparison with experimental results from the literature. For both models, the time‐averaged conservation equations of mass, momentum and transport of the mixture are solved using the finite volume method (FVM) (FLUENT® v.14.5). The standard k–ε model is used for closure of turbulence. Concerning the geometrical investigation, constructal design is employed to define the search space, degrees of freedom and performance indicators of the problem. More precisely, seven configurations with different width/length (L/B) ratios for the rectangular baffles are studied and compared with an unbaffled case. The momentum source model leads to valid results and significantly reduces the computational effort in comparison with the sliding mesh model. Concerning the design, the results indicate that the case without baffles creates the highest magnitude of turbulence kinetic energy, but poorly distributes it along the domain. The best configuration, (L/B)o = 1.0, leads to a mixture performance nearly two times superior than the case without baffles

Doctor of Philosophy

dissertationFroth flotation is a highly complex, multiphase, and multiscale process that is usually performed in large tanks called mechanical flotation cells. The aim of this research is to investigate the single and multiphase flow hydrodynamics in lab scale flotation cells by decoupling the hydrodynamics from physicochemical effects. Both experimental and numerical approaches are used to study the behavior of flows in lab and pilot scale flotation cells. Nonintrusive experimental techniques such as particle image velocity (PIV) and electrical resistance tomography (ERT) techniques are used to measure flow velocities, solids holdup, mixing efficiency, and to interpret flow pattern. Eulerian-Eulerian computational fluid dynamics (CFD) models are developed and tested for solid-liquid (slurry) and gas-liquid flows in stirred tanks and flotation cells. Using single phase CFD simulations, the effect of flotation specific impeller blade shape and impeller size on mean flow and pumping behavior is tested in lab scale flotation cells for the first time. In the absence of a stator, the mean flow is found to transition from radial to axial type flow when the off-bottom clearance is below the critical value. This prediction is experimentally verified using time averaged PIV data. Based on the analysis of pumping and power number data, the rectangular shaped blade design is found to be the most efficient. The impeller blade shape is found to critically affect the flow in the vicinity of the impeller and a design with the largest surface area is needed to create an intense turbulence zone, needed for mixing and dispersion of incoming air. Eulerian-Eulerian CFD model is used to study the solid phase suspension and mixing characteristics for monosized silica particles. Experimental comparison with the results from the literature for stirred tanks and in-house ERT measurements suggest that the model performs reasonably well. Population balance equation model (PBM) is coupled with CFD to study gas dispersion, mixing, and local bubble size distribution in the stirred tank and flotation cell using quadrature method of moments (QMOM) approach in ANSYS Fluent solver. The default QMOM model in Fluent is found to be inaccurate due to independent solution of moment transport equations and therefore is supplied with a moment correction algorithm from the literature to successfully identify and correct the invalid moment sequence during the CFD simulation. The new model is found to be superior to the current models in its ability to satisfactorily predict the overall gas holdup and local bubble size distribution for stirred tanks under moderate aeration and agitation rates. This model is extended to study the development of flow regimes based on the gas dispersion pattern in a generic flotation cell. Though highly useful, the coupled CFD-PBM approach is computationally intensive and requires considerable effort to achieve an accurate solution. This motivated us to develop a PBM based on the high-order moment conserving method of classes (HMMC) approach for a pilot scale XCELL flotation cell for frother concentration over critical coalescence concentration, thus, only considering breakage of bubbles. Nonlinear optimization solvers in Matlab are used to calculate the point estimates of adjustable parameters in breakage models. The 95% bootstrap calculated using empirical bootstrap indicates very high confidence in estimated parameters. The HMMC model provides an accurate description of steady state bubble size distribution and the mean number diameters only using overall gas holdup and specific energy as inputs

CFD modeling of the catalyst oil slurry hydrodynamics in a high pressure and temperature as potential for biomass liquefaction

The paper presents the simulation of a catalyst-para n oil slurry hydrodynamics under high pressure and temperature in a convex bottom reactor with a Rushton turbine which was conducted with an application of computational fluid dynamics (CFD) modeling. An analysis to obtain a uniform distribution of solid catalyst particles suspended in para n oil was carried out as a potential for biomass liquefaction. The e ects of the particle diameter, bed density, liquid viscosity, and the initial solid loading on slurry hydrodynamics in high pressure and temperature behavior were investigated using the Eulerian–Eulerian two-fluid model and the standard k-" turbulence model. The main objective was to assess the performance in agitating highly concentrated slurries to obtain slurry velocity, concentration, the degree of homogeneity, and to examine their e ect on the mixing quality. The results of the analysis are applied to predicting the impact of the most e cient conditions on slurry suspension qualities as potential for biomass liquefaction

Hydrodynamic investigation of the discharge of complex fluids from dispensing bottles using experimental and computational approaches

The discharge of non-Newtonian, complex fluids through orifices of industrial tanks, pipes, dispensers, or packaging containers is a ubiquitous but often problematic process because of the complex rheology of such fluids and the geometry of the containers. This, in turn, reduces the discharge rate and results in residual fluid left in the container, often referred to as heel. Heel formation is undesired in general, since it causes loss of valuable material, container fouling, and cross-contamination between batches. Heel may be of significant concern not only in industrial vessels but also in consumer packaging. Despite its relevance, the research in this area is significantly limited. Previous research conducted in simpler systems, such as orifices of pipes and vessels, has already shown that the discharge of fluids through orifices is significantly affected by the geometric parameters and the fluid rheology. More specifically, the geometric properties of the orifice such as the diameter ratio, aspect ratio, and orifice shape, and the rheological properties of the fluid played a critical role on the discharge of complex fluids through orifices of vessels and pipes. However, how these parameters affect the discharge of complex fluids flow from more complicated systems such as consumer dispensing bottles operating with a hand pump has remained uninvestigated. Therefore, the overall objectives of this work are to quantify the discharge hydrodynamics in dispensing bottles and the resulting heel for a wide range of geometries, operational parameters, and fluid rheology through the use of experimental and computational approaches. Particle Image Velocimetry (PIV) is the main experimental tool used in this work. A novel experimental methodology is also developed and utilized to optimize the transparency of the highly complex fluids such as pastes, for their optical hydrodynamic investigations using PIV. In addition, Computational Fluid Dynamics (CFD) is also utilized to predict the hydrodynamics and the residual heel volume. The simulation predictions are validated against the experimental data. It is found that the heel volume and profile after the discharge is strongly related to the flow during the discharge, and both static and dynamic aspects of the discharge process can be determined using PIV, and predicted using CFD. Finally, correlations to predict the heel volume based on the rheological and geometric parameters are presented. It is expected that this work will be of significant academic and industrial interest, especially for product developers and packaging engineers to optimize the shape of dispensing bottles so that the discharge process from such containers is facilitated, and the heel volume is minimized

Process Simulation of Technical Precipitation Processes - The Influence of Mixing

This work develops and shows up methods to tackle multi-scale challenges in particle formation during precipitation crystallization. Firstly, molecular, micro- and meso-scale interactions in confined impinging jet mixers are investigated and simulatively predicted. Secondly, to build up on developed methods, macroscale as present for instance in stirred tank reactors is added to the considerations