Knowledge Modelling and Comparison of Cyanide-free Gold Leaching Processes -Research Summary

Abstract. There is increasing pressure from authorities in many countries to find cyanide-free methods for gold processing due to environmental and safety risks. Therefore, it is now under investigation whether the knowledge extraction and formalization procedure can be applied for comparing cyanide-free leaching methods for a certain type of raw material. Research is needed to determine and code the similarity models related to cyanide-free processes. The task is challenging, since most of the processes are still in the development phase and there is not much information available on the new-coming processes

Recovering the lost gold of the developing world : bibliographic database

This report contains a library of 181 references, including abstracts, prepared for Project R 7120 "Recovering the lost gold of the developing world" funded by the UK' s Department for International Development (DFID) under the Knowledge and Research (KAR) programme. As part of an initial desk study, a literature review of gold processing methods used by small-scale miners was carried out using the following sources; the lSI Science Citation Index accessed via Bath Information and Data Services (BIDS), a licensed GEOREF CD-ROM database held at the BGS's Library in Keyworth and IMMage a CD-ROM database produced by the Institution of Mining and Metallurgy held by the Minerals group ofBGS. Information on the search terms used is available from the author

An electrochemical model for the leaching kinetics of gold ore in cyanide solutions

This project consisted of a literature and a laboratory study to investigate the applicability of an electrochemical model, to the leaching kinetics of gold ore. The procedure was to get an approximate measurement of the gold surface potential and then, to use only this potential and an area term in a two parameter rate expression. The potential measurements in a pulp, with a gold and gold-silver electrode, were attempted but, the electrodes became passive. The results showed that the rate controlling mechanism was a combination of the rate of both oxygen and cyanide diffusion and it was concluded that the applicability of an electrochemical model to the leaching kinetics of gold ore, could not be ruled out. It was found that the leaching kinetics could be modelled very accurately by using the solution redox potential, however, no theoretical basis could be found to explain this observation

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

Application of HPGR and X-Ray CT to investigate the potential of Witwatersrand gold ore for heap leaching : a process mineralogy approach

Includes bibliographical references.Auriferous conglomerates of the Archaean Witwatersrand Basin in South Africa host one of the largest known gold resources and rate as the world’s most outstanding example of a fossil megaplacer deposit. For the past 40 years, Witwatersrand gold production in South Africa has been progressively declining due to problems related to high energy costs, decreasing grade, accessibility to greater depths, health and safety issues, labour union unrest and economic uncertainties: thus the overall viability of current gold production is questionable. Ultimately, the future of Witwatersrand gold mining relies on devising smarter strategies across the entire industry, but in particular critical areas such as comminution and extraction. With the continuous increase in mining depth, dominance of low-grade gold ores and strict safety regulations, metallurgical processing options have become limited. Heap leaching is a well-established technology which continues to grow in use and provides several benefits to solve some of these problems. High pressure grinding rolls (HPGR) is another technology with significant potential, especially for its application in coarse particle heap leaching due to its ability to induce micro-cracks as well as its high grinding efficiency and low energy requirements. This study explores the use of these two technologies in a process mineralogical framework using novel 3D X-ray computed tomography mineralogical analysis in order to assess a potential of the Witwatersrand gold ore for heap leaching

The Development of Sustainable Hydrometallurgical Processes for the Recovery of Precious Metal

The study investigates the utilization of cedar wood bark as bioadsorbent for the adsorption and simultaneous precipitation of gold as flakes. This is with a view to establishing the electrochemical study of the adsorption and evaluate pre-treated cedar wood bark as possible adsorbent for gold in various solutions. The research plan for this project is divided into two parts. Part one focuses on understanding the adsorption of gold using the cedar wood bark as adsorbent. The second part focuses on the electrochemical study of the redox reaction during adsorption process using cyclic voltammetry technique. Synthetic solution of gold is prepared with dissolution of gold (III) chloride in hydrochloric acid, sodium thiosulfate and sodium thiourea lixiviants. Cedar wood bark is pre-treated with dilute and concentrated sulfuric acid under various experimental conditions to obtain three bioadsorbents, dilute-air dried (D-AD), concentrated washed-air dried (CW-AD) and concentrated not washed-oven dried (CNW-OD). The gold solutions are electrochemically tested for redox reaction using cyclic voltammetry (CV) techniques. One-point adsorption test is carried out on the various gold solutions to determine the suitable samples for the research. The outcome of the CV experiment indicates that redox reaction of gold in hydrochloric acid medium is easily measured through the anodic and cathodic peak formation. The one-point adsorption test favors the use of D-AD as adsorbent in acidic gold solution with percentage adsorption of 99.999%. Hence, the research is narrowed down to the use of D-AD adsorbent and acidic gold solution. Solid/liquid ratio and hydrochloric acid concentration tests indicate that 1.5 and 0.5 M, respectively, are the best suitable for the research. For the kinetic study of the adsorption process at temperatures of 298, 303 and 313 K in 96 hours, pseudo-second order model has determination coefficients of 0.988, 0.996 and 0.998, respectively, while the pseudo-first order model has determination coefficients of 0.91, 0.77 and 0.62 at those three different temperatures. Hence, the adsorption process follows the pseudo-second order model. The activation energy from the pseudo-second order rate constant indicates that the process is chemisorption with a value of 59.86 kJ/mol. The adsorption isotherm is found to follow Freundlich isotherm model, which might have favored the formation of gold flakes on the adsorbent. The CV experiment shows the disappearance of anodic peaks as the adsorption of gold progresses, which is an indication of reduction reaction synonymous to adsorption process. X-ray diffractometer (XRD) and Fourier transform infrared (FTIR) instruments were used to determine the presence of gold precipitates and the spectra obtained from the two experiments confirm the presence of gold. In conclusion, the study established cedar wood bark as a potential source of biomass for adsorption of gold (III) ions from acidic chloride solution, and that cyclic voltammetry (CV) technique was successfully used to examine the adsorption process

Adsorption of Gold from Thiosulfate Leaching Solutions using Polyethylenimine Functionalised Magnetic Nanoparticles

In this research polyethylenimine coated iron oxide magnetic nanoparticles (PEI-MNPs), as a novel adsorbent, was evaluated for the adsorption of gold from thiosulfate leaching solutions that contain gold, copper and calcium thiosulfate. This novel adsorbent can simply be separated from the solution using an external magnetic field, followed by gold adsorption. Gold elution from the adsorbent was simple and rapid

Effect of scale of operation on heap leaching performance

The extent to which differences in bulk density and in irrigation geometry contribute towards the differences observed in leaching performance between laboratory-scale columns versus commercial-scale heaps was investigated. Furthermore, the contributions of segregation and stratification, which are absent from columns but unavoidable in heaps, were also investigated. Four case studies provided data whereby the column and heap leaching of the respective sample materials could be directly compared. The first involved gold-bearing ore, the second gold-bearing sand, a third was conducted on oxide-copper ore and the fourth case study was performed on sulphidic copper ore. The first case study was published by others, while the other three were conducted by the author himself. Furthermore, custom-designed boxes were constructed whereby the segregation and stratification of ore could be simulated. Tests were performed to observe the leaching performance of segregated and stratified ore samples, compared to unsegregated and unstratified ore samples. The leaching characteristics of each case was quantified in terms of the parameters of a mathematical model, based on dual-porosity hydrology and diffusion with chemical reaction, which provided optimised fits to the experimental data. Furthermore, fundamental relations were derived for the impact that changes in bulk density and changes in drip-irrigation spacing are theoretically expected to have on the same model parameters. These relations were tested against the actual trends observed in the model parameters that best fitted the data of the four case studies and of the segregation/stratification experiments. It was found that the diffusional distance, which places a mass-transfer limitation on the over-all rate of leaching, can be governed by either the ore bulk density, or by the irrigation dripper spacing. The relations derived under this study can be used to estimate the maximum dripper spacing that can be permitted to prevent dripper spacing from becoming the rate-limiting parameter. Estimates of the impact of dripper spacing on the ultimate extent of extraction are also provided. However, neither segregation nor stratification exhibited significant or reproducible effects on leaching kinetics. These findings can place the drafting of heap leaching design parameters on a more fundamental footing

Integrated modeling of grinding, liberation and leaching of gold ores

Avec l'augmentation rapide du prix de l'or depuis 10 ans, les producteurs d'or se tournent vers les gisements à plus faible teneur et contenant des minerais plus difficiles à traiter donnant des rendements plus faibles que les minerais conventionnels. Les producteurs doivent par conséquent s'assurer que des baisses importantes du prix de l'or affectent le moins possible la rentabilité de leur opération. Ces producteurs veuillent par conséquent à maintenir des coûts de production faibles et aussi à maximiser les rendements des opérations. Ce projet aborde des approches permettant d'étudier par simulation le choix de la réduction granulométrique d'un minerai pour maximiser la récupération des espèces de la valeur. La première étape du traitement d'un minerai d'or est la réduction granulométrique de la roche pour en libérer les grains d'or ou encore exposer leur surface pour les subséquentes étapes de récupérations. La récupération de l'or dans le minerai broyé s'effectue habituellement par des méthodes gravimétriques suivies d'une dissolution de l'or avec le cyanure. Les récupérations gravimétriques et chimiques qui suivent la réduction granulométrique sont fortement liées à la façon dont les grains d'or sont libérés de la particule de gangue. Le projet vise par conséquent à développer une méthode pour tenter de prédire les propriétés de libération et d'exposition des grains d'or à partir d'informations sur le minerai et d'utiliser ces propriétés de libération pour prédire le comportement de l'or dans un circuit de cyanuration. Le premier défi relevé dans ce doctorat a été celui de prédire la distribution des grains de l'or en fonction de la taille des particules de minerai. Des essais de broyage d'un minerai d'or ont en effet montré que l'or n'est pas distribué aléatoirement dans les classes granulométriques. Puisque la vitesse de cyanuration varie avec la taille des particules, il est important d'identifier où se trouvent les grains d'or dans le minerai. À cet effet, on a développé un modèle de broyage qui utilise des fonctions de broyage et de sélection pour la gangue non-minéralisée du minerai et pour les particules porteuses d'or. Pour arriver à prédire de la distribution de l'or dans les classe granulométriques on a aussi utilisée des mesures de la distribution des grains d'or selon leur taille, ce qui n'avait jamais été rapporté dans la littérature. Il a cependant aussi été nécessaire de poser des hypothèses comme la 11 nonbroyabilité de l'or et l'impossibilité d'avoir plus d'un grain d'or par particule de minerai. Une fois que la distribution de l'or dans les classes granulométrique est connue, il faut s'attarder à déterminer de quelle façon les grains d'or pourraient être exposés à l'environnement chimique de lixiviation. Pour obtenir cette information, on a utilisé une méthode numérique qui génère des particules de différentes formes autour d'un grain d'or de forme quelconque. La génération des particules est faite selon une tessellation de Voronoi qui n'a probablement jamais utilisé pour une application de ce type. Une fois les particules générées, on isole la particule qui contient la plus grande portion du grain d'or et on analyse les propriétés d'exposition de ce grain. On peut alors le classer comme libéré, fortement ou peu exposé à l'environnement, ou encore encapsulé dans la particule, mais accessible à traverse des pores ou des fractures dans le minerai. L'information sur le type d'exposition des particules et la distribution granulométrique des grains d'or alimentent un simulateur pour la cyanuration qui tient compte de ces données pour prédire le degré de dissolution de l'or. On dispose donc d'un premier prototype de simulateur intégrant broyage et lixiviation. Ce prototype permet d'étudier l'effet de la réduction granulométrique sur le rendement or d'un procédé Cette approche est probablement une première dans les travaux de simulation des usines de traitement des minerais d'or. Le simulateur de broyage libération et dissolution a été calibré avec des données expérimentales provenant d'essais de broyage et de cyanuration conduits avec le minerai de la mine de Géant Dormant au Québec. Les résultats obtenus sont encourageants et devraient motiver la poursuite des travaux sur la modélisation intégrée du broyage et la récupération des espèces de valeur dans les minerais