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The experimental work described in this thesis is aimed primarily towards elucidation of the speciation of zinc-cyanide systems at elevated pH. In this study the formation and stability of H+-eN-, binary Zn2+-eN- and ternary Zn2+-CN--QH- complexes were studied by glass electrode potentiometry in aqueous solutions at 25.0°0 and in a medium of ionic strength of 0.1 mol dm-3. The solution pH was varied to cover the range 4 to 11. The study was undertaken with a view to establishing whether and under what conditions soluble binary zinc-cyanide complexes and ternary zinc-cyanide-hydroxide complexes form, and to determine formation constants for any such species that are found. This information would be useful in defining more precisely the speciation of solutions containing zinc and cyanide ions at elevated pH values. A titration method was used, in which hydrogen ion concentration was monitored by means of a glass indicating electrode. The cell was calibrated to allow measurements of hydrogen ion concentration rather than hydrogen ion activity. Owing to precipitation difficulties, the reagents were used at sub-millimolar concentration levels. The potentiometric data was interpreted with the aid of various formation function plots together with the use of various computer programs, such as HALTAFALL and ESTA. The results show that the ternary complex Zn(ON)3(OH)2- is formed in significant amounts in solutions of pH > 8.5. Some evidence was also obtained for the existence of the five coordinated species Zn(CN)3(OH)~- and Zn(CN)~- in these solutions, but existence of the latter two species cannot yet be regarded as firmly established. No polynuclear complexes were detected at the sub-millimolar concentrations used. Formation constants are reported for H+-eN- and both binary Zn2+-eN- and ternary Zn2+-eN--QH- species. / Thesis (M.Sc.)-University of Natal, Durban, 1990.

A thesis submitted to the Faculty of Engineering and The Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy in Engineering Johannesburg 12 September 2016 / Comminution and leaching unit processes play a major role in extracting valuable minerals from ore. Most of the research reported in the literature has focused on optimising individual unit operations rather than on integrating the whole process. This thesis develops an integrated approach to mineral processing systems and flow sheets and is intended to create a methodology for process synthesis that can be applied throughout the extractive metallurgical industry. This could lead to improved efficiency in the overall process by obtaining optimum recovery and, most important, a reduction in energy and material costs. In order to illustrate the methodology a particular example was chosen, namely optimizing the joint comminution and leaching of a particular gold ore. In this investigation laboratory scale grinding and leaching profiles for a gold feed sample (1700–850 μm) were measured. In a laboratory mill various combinations of grinding media, filling level and ball size were investigated, and of the three ball sizes used (10, 20 and 30mm) breakage was most pronounced for the 20 mm. Thus for instance it was also established that when using a higher filling ( =30%) and a ball size of 30 mm, more energy was consumed but less liberation occurred, thus a lower amount of gold was extracted during a 24-hour leaching period. Finally, the breakage kinetics of the gold ore was looked at. Using a standard population model the breakage and selection function parameters were successfully calculated. An investigation into the dissolution kinetics of gold ore in a solution of NaCN was also done. These were found to depend on the stirring rate, reaction temperature, particle diameter and the concentration of the leachant. The rate increased with the stirring speed, reaction temperature and leachant concentration, but decreased when the particle size was greater. The activation energy for the dissolution was estimated at about 3 kcal/mol. Furthermore, the linear relationship between the rate constant and the reciprocal of the square of the particle size is a strong indication that the gold dissolution process is diffusion-controlled. The experimental results were well-fitted to a shrinking core model. In attempting to understand the results, the researcher carried out a number of experiments that involved an investigation into the relationship between comminution and leaching in terms of energy usage and particle size, the former to establish the most efficient application of energy, and the latter to identify the degree of fineness that would ensure optimal recovery. The Attainable Region (AR) method was then used to establish ways of finding the leaching and milling times required to achieve minimum cost (maximise profit). No work on utilizing the AR technique to minimise the cost of milling and leaching on a real industrial ore has previously been published. The investigation aims to show how the AR technique can be used to develop ways of optimising an industrial process that includes milling and leaching. The experimental results were used to show how this method could be successfully applied to identifying opportunities for higher efficiency when performing these operations. The approach however is general and could in principle be used for any two or more unit operations in determining how the product from one unit should be prepared to feed to the next unit so as to optimize the overall process. / MT2017

Thesis(MTech (Chem.Eng.Technology))--//Cape Technikon, 1999 / The gold mining industry has mainly relied upon the use of a highly polluting chemicals, such as mercury and cyanide, to recover gold from its ores. As environmental legislation has become more stringent in all countries and environmental protection has become the focus of world-wide research, development of environmental sound processes has been favoured. The Coal Gold Agglomeration (CGA) process is such a process which was developed some years ago and has the advantage in that gold is recovered by a procedure which has little or no effect on the environment. The CGA process is based on the hydrophobic characteristics of coal, gold and oil. Gold particles which are substantially free become attached to the coal-oil agglomerates during collision, and eventually penetrate into the agglomerates. The resulting agglomerates are recycled to increase the gold loading, separated from the slurry, burnt, ashed and smelted to produce gold bullion. Laboratory scale batch tests were performed on an artificial/synthetic gold ore, containing fine gold powder. The slurry was contacted with a mixture of coal and oil. i.e. coal-oil agglomerates, after which both the agglomerates and ore were analysed for gold. Operating parameters, such as the mode of contact between the coal-oil phase and the gold containing slurry, contact time of the slurry and the coal-oil phase, means of separating the coal-oil gold agglomerates from the slurry, coal to ore, coal to oil and water to ore ratios, type of oil, effect of collectors and the mineralogy of the ore on the gold recovery were investigated. Results have shown that stirring the coal-oil phase and the slug yielded higher gold loadings than shaking and the traditional rolling bottle technique. BI increasing the time of contact between the coal-oil phase and the gold slurry. the final gold loading in the agglomerates increases, until an equilibrium value is reached. An increase in the amount of coal, together with a decrease in the amount of water used in the slurry, has shown to increase gold recoveries. Furthermore, by varying the concentration and volume of a collector. such as potassium amyl xanthate (PAX) enhanced the settling rate and enabled the effectiveness of separation. Moreover, it was found that the gold loading on the coal-oil phase increased after recycling it. Further tests were performed on a real ore sample and after X-ray Diffraction (XRD) analysis, it was found that certain minerals other than gold was transferred to the coal-oil phase. The theoretical foundation of the CGA process is based on the difference in free energy and was expressed as a function of the interfacial tensions and three-phase contact angles between gold, oil and water, together with the ratio of coal-oil agglomerate to gold particle radii, as the free energy is a measure of the thermodynamic stability and hence, partly a measure of gold recoveries, meaningful predictions as to gold recoveries were made by performing a sensitivity analysis on the variables connected to the free energy, It was, however, found that some operating parameters, which were linked to other factors, such as the maximum gold transfer into coal-oil phase and the separation efficiency of the agglomerates. were vital to be taken into account when predictions as to gold recoveries were made. Therefore, the gold recoveries were found to be a function of the thermodynamic stability as well as the maximum gold transfer into the coal-oil phase and the separation efficiency of the agglomerates, The meaningful information gained by performing the theoretical investigations were applied and linked to gold recoveries, thereby providing useful explanations as to the typical gold recoveries obtained during experimentation. A comparative study on mercury amalgamation was done to evaluate the performance of the CGA process. It was found that the CGA process yielded better gold recoveries than amalgamation, which makes it the better process both in terms of recoveries as well as environmental safety, A further application of the theoretical knowledge was, however, very useful to explain the tendency of the CGA process yielding the better results.

Thesis (MTech (Chemical Engineering))--Cape Technikon, Cape Town, 2001. / Carbon-in-pulp technology is used extensively in the mining industry to recover metal cyanides from solution. Also this technology has found increasing application in the gold mining sector, replacing the less efficient zinc precipitation procedure. The extensive use of carbon in such processes have prompted many researchers to investigate the mechanism of metal cyanide adsorption. Not only has this provided many viable theories in the understanding of the mechanism, but has also led to an improved understanding of the effects of the various operating conditions on the ClP circuit. Also the modelling of this process has resulted in proposed rate equations of which the famous "kn" model is the most widely used in design. This is a single rate equation that could result in significant errors and hence a dual resistance model was developed. However this model is mathematically complex. Recently in an attempt to overcome the shortcomings of previous models, empirical calculations to accurately describe adsorption kinetics were developed at the Cape Technikon. These correlations were derived using batch experimental data. In this study the focus was on modeling the adsorption process on a continuous scale using a laboratory scale cascade system. This study utilized the fact that solution concentration is the main driving force for aurocyanide adsorption onto activated carbon and that carbon loading has an indirect effect on adsorption kinetics. The metal was ultimately tested against actual plant data and provided very accurate results.

Thesis (MTech(Chemical engineering))--Cape Technikon, Cape Town, 2001 / Over the years the carbon-in-pulp technology has been refined to become the highly efficient process that is used in our present-day system of recovering dissolved gold from cyanide leached pulps. The efficiency of a CIP circuit mainly depends on the effectiveness ofthe adsorption section as it not only determines the amount of soluble gold lost in the residues, but also indirectly affects the function of the other processes in the plant. Research in this area has declined over the past few years as a result of a decrease in the gold price. It is now more than ever important to investigate the operating conditions ofthe adsorption process to ensure that a highly effective system is maintained.The adsorption of gold cyanide onto activated carbon is to a large extent dependent on maintaining operating conditions well above those of equilibrium. The Freundlich and the Langmuir isotherms have been used by many researchers to describe the equilibrium conditions of the adsorption process. The general practice in the carbonin- pulp technology is to use an isotherm for the prediction of a circuit's performance. As confidence has increased in the reliability of these predictions, it has become important to acquire knowledge of the equilibrium condition that is driving the process. Previous research findings have indicated that the equilibrium isotherm of gold cyanide adsorption onto activated carbon is influenced by changes in the adsorption conditions down the adsorption train. This equilibrium or isotherm shift may lead to errors in the prediction of gold adsorption rates, which results in the filct that the simulations of the performance of the CIP circuits are not reliable. In this study the aim was to investigate the combined influence of various operating conditions on the adsorption equilibrium

Thesis (MTech (Chemical engineering)--Cape Technikon, Cape Town, 2001 / Over the years the carbon-in-pulp technology has been refined to become the highly efficient process that is used in our present-day system of recovering dissolved gold from cyanide leached pulps. The efficiency of a CIP circuit mainly depends on the effectiveness ofthe adsorption section as it not only determines the amount of soluble gold lost in the residues, but also indirectly affects the function of the other processes in the plant. Research in this area has declined over the past few years as a result of a decrease in the gold price. It is now more than ever important to investigate the operating conditions ofthe adsorption process to ensure that a highly effective system is maintained. The adsorption of gold cyanide onto activated carbon is to a large extent dependent on maintaining operating conditions well above those of equilibrium. The Freundlich and the Langmuir isotherms have been used by many researchers to describe the equilibrium conditions of the adsorption process. The general practice in the carbonin- pulp technology is to use an isotherm for the prediction of a circuit's performance. As confidence has increased in the reliability of these predictions, it has become important to acquire knowledge of the equilibrium condition that is driving the process. Previous research findings have indicated that the equilibrium isotherm of gold cyanide adsorption onto activated carbon is influenced by changes in the adsorption conditions down the adsorption train. This equilibrium or isotherm shift may lead to errors in the prediction of gold adsorption rates, which results in the filct that the simulations of the performance of the CIP circuits are not reliable.

In recent years, emissions of hydrogen cyanide from metallurgical operations have received renewed attention by legislative bodies, leading to the need for a reliable quantification method for HCN volatilisation. Subsequently, the purpose of this project, launched by Anglogold Ashanti Ltd. and in collaboration with MINTEK and the University of Pretoria, was to develop a prediction model for cyanide volatilisation from plant operations and tailings storage facilities in South Africa. The study was done in four stages, the first being a laboratory study of the equilibrium behaviour of hydrogen cyanide. Henry’s Law constant (kH) was determined at different solution cyanide concentrations, salinities and temperatures. A value for kH was established at 0.082 atm.L/mol, which was found to be independent on the solution cyanide concentration between 10 and 200 ppm cyanide. In addition, the effect of temperature on kH was found to be negligible at solution temperatures between 20 and 35ºC. It was also concluded that high salinities increase kH and promote volatilisation, but this effect was negligible at the typical salinity levels found in South African process water. The second stage entailed a detailed study of the mass transfer coefficient, KOL, for hydrogen cyanide from cyanide solutions and pulp mixtures, both in the laboratory and on-site. It followed from this investigation that the most important parameters affecting KOL are the HCN (aq) concentration in the liquid, the wind velocity across the solution or pulp surface, expressed in terms of a Roughness Reynolds number, Re*, and the moisture content, or solid to liquid ratio, of the pulp. Furthermore, it was concluded that KOL is highly sensitive to HCN (aq) concentrations at low concentrations, while it becomes rather insensitive to HCN (aq) at concentrations above 20 ppm HCN (aq) . The data generated by the laboratory and on-site test work was incorporated into the development of an empirical prediction model, based on the Roughness Reynolds number (Re*), moisture content (M), and aqueous cyanide concentration (HCN(aq) ) which may be described by the following equation: KOL= <font face="symbol">a</font> Re*b Mc HCN(aq)d + e The model coefficients were subsequently determined for application of the model to leach tanks, adsorption tanks, tailing storage facility surfaces and return water dams. The calculated model predictions for KOL were in excellent agreement with the measured test work data. Finally, the prediction model was validated at the leach and adsorption sections of a selected gold plant and a selected tailings storage facility. The model predicted that 9% of the cyanide lost in the leach and adsorption section could be attributed to HCN volatilisation. As for the tailings storage facility, the model assigned 63% of the cyanide lost from the tailings storage facility to HCN volatilisation, of which 95% occurred from the area on the tailings dam surface covered in a thin liquid film. It is recommended that the current methods available for the determination of HCN (aq) be further improved, due to the sensitivity of the model to the input value of the HCN (aq) concentration, in order to ensure that reliable predictions are made. It is also suggested that additional validation work be done in order to establish the generic applicability of the model to different sites. / Dissertation (MEng(Metallurgical))--University of Pretoria, 2007. / Materials Science and Metallurgical Engineering / unrestricted

The thesis takes the form of a collection of publications and patents concerning gold extraction which were presented over a period of 18 years while employed at the Anglo American Research Laboratories.

Please read the abstract in the front section of this document / Dissertation (M Eng (Metallurgical Engineering))--University of Pretoria, 2006. / Materials Science and Metallurgical Engineering / unrestricted