Modelling and simulation of carbon-in-leach circuits

Abstract

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.