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

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

Simulation of solid-liquid flow in stirred tanks at high solid loading

Solid liquid stirred tanks are commonly used in mineral industry for operations like concentration, leaching, adsorption, effluent treatment, etc. Hydrodynamic study is necessary to evaluate the performance of such systems. Especially, in the cases of high solid concentration, the flow field, slip velocity, turbulence and drag are significantly different from the single phase values and therefore, such studies become indispensable. In this study, the change in these parameters in the presence of solids and the effect of high solid concentration is discussed. Eulerian-eulerian multiphase modelling approach was used to simulate the solid suspension in stirred tanks. Multiple reference frame (MRF) approach was used to simulate the impeller rotation in a fully baffled tank. Simulations were conducted using commercial CFD solver ANSYS Fluent 13.0. The CFD simulations were conducted for concentration 20 wt% and the impeller speeds at the just suspension speed. The solid - liquid interaction was taken into account using modified Brucato drag model. A substantial decrease in the flow number was observed due to the presence of solids. The dampening of turbulence was evident in the impeller region where the solid concentration was the maximum. The drag was able to account the increase in drag at high turbulent intensities. The predictions in terms of the velocity profiles were found to be in reasonable agreement with the experimental data of Guida et al. (2010). The work provides an insight into the solid liquid flow in stirred tanks with high solid concentration and will be useful for applications such as carbon in leach circuit which operates at high solid loading

Simulation and Analysis of Carbon-in-Leach (CIL) Circuits

Carbon in leach (CIL) is an important process in gold processing involving simultaneous leaching and adsorption. The process holds the key to profitability in gold extraction. While the mechanism of leaching and adsorption are well known, the effect of different operating and design parameters on the CIL circuit performance is still empirical. The focus of this paper is to study the effect of parameters like cyanide concentration, oxygen concentration and mean particle diameter on the overall efficiency of CIL circuit. A dynamic model based on first principles is developed for the entire CIL circuit. Suitable kinetic models for both leaching and adsorption are adopted from the literature. Customizable simulator is written in MATLAB to simulate the model. Simulation results are first validated using previously published results. The validated model is then used to perform sensitivity studies on different parameters that affect the gold extraction process. The model can be used for controlling the operating parameters and to optimize the CIL circuit in order to achieve higher efficiency

CFD simulation of solid–liquid stirred tanks

Solid liquid stirred tanks are commonly used in the minerals industry for operations like concentration, leaching, adsorption, effluent treatment, etc. Computational Fluid Dynamics (CFD) is increasingly being used to predict the hydrodynamics and performance of these systems. Accounting for the solid–liquid interaction is critical for accurate predictions of these systems. Therefore, a careful selection of models for turbulence and drag is required. In this study, the effect of drag model was studied. The Eulerian–Eulerian multiphase model is used to simulate the solid suspension in stirred tanks. Multiple reference frame (MRF) approach is used to simulate the impeller rotation in a fully baffled tank. Simulations are conducted using commercial CFD solver ANSYS Fluent 12.1. The CFD simulations are conducted for concentration 1% and 7% v/v and the impeller speeds above the “just suspension speed”.It is observed that high turbulence can increase the drag coefficient as high as forty times when compared with a still fluid. The drag force was modified to account for the increase in drag at high turbulent intensities. The modified drag is a function of particle diameter to Kolmogorov length scale ratio, which, on a volume averaged basis, was found to be around 13 in the cases simulated. The modified drag law was found to be useful to simulate the low solids holdup in stirred tanks. The predictions in terms of velocity profiles and the solids distribution are found to be in reasonable agreement with the literature experimental data. Turbulent kinetic energy, homogeneity and cloud height in the stirred tanks are studied and discussed in the paper. The presence of solids resulted in dampening of turbulence and the maximum deviation was observed in the impeller plane. The cloud height and homogeneity were found to increase with an increase in impeller speed. The work provides an insight into the solid liquid flow in stirred tanks

Modeling of Cryogenic Liquefied Natural Gas Ambient Air Vaporizers

The ambient air vaporizer (AAV) technology is a promising option for vaporization of cryogenic fluids including liquefied natural gas (LNG). In this study, the heat transfer between ambient air and cryogenic LNG under supercritical conditions has been studied by using computational fluid dynamics (CFD). The process of regasification was first analyzed from the thermodynamics standpoint. In the absence of actual data for LNG, the empirical correlations and experimental data for supercritical flows of water and carbon dioxide were used to validate the CFD model. The model was then used to investigate the supercritical flow of LNG inside AAV. Operating conditions such as air flow velocity and operating pressure were studied. Furthermore, optimization of fin configurations including the number of fins, fin length, and fin thickness was also investigated. The methodology and results discussed in this study are of critical importance for designing AAV