Doctoral Degrees (Chemical Engineering)

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The adsorption and elution of Pt-, Pd- and Au cyanide using activated carbon
(Stellenbosch : Stellenbosch University, 2015-03) Snyders, Cornelius Albert; Bradshaw, S. M.; Akdogan, G.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: In order to exploit lower grade and complex platinum group metal resources, cheaper and more efficient alternatives to the conventional mill-float-smelt-refine route are being sought. Leaching of platinum and palladium with cyanide has been proposed a number of times as a promising precious group metals (PGM) process option, and although platinum extractions are problematic, progress into the understanding of cyanide leaching of PGM containing ore and concentrate has been made. The platinum and palladium leaching will typically take place at elevated temperatures, which can range from 55°C on heaps to 180°C in autoclaves, with a better degree of leaching occurring with higher temperatures. Although this process for Pt and Pd extraction is a promising process option, research regarding the feasibility of the subsequent upgrading and recovery of the pregnant PGM leach solution, however, has been lacking. Since the carrier-phase extraction of gold using activated carbon offers significant advantages over other processes in terms of simplicity, the high pre-concentration factor, rapid phase separation, and relatively low capital and operating costs, activated carbon was deemed the most suitable sorbent for a Pt and Pd adsorption and stripping process. Very little is published on the adsorption of PGM cyanides onto activated carbon and when the effect of impurities such as base metals and thiocyanate together with a suitable elution method, are considered, no information could be found in the open literature. This study was launched and in general it was found that the activated carbon process does seem to be a viable process consideration for the upgrading of PGMs in a cyanide leach stream. Adsorption rates for dilute PGM solutions (0.15mg/L Pt, 0.38 mg/L Pd, 0.1 mg/L Au) in a stirred vessel indicated a high rate of adsorption within the first 60 minutes (giving more than 98% recovery of precious metals). A comparison of the Pt isotherm (25°C) to Au isotherms from literature indicated a similar loading capacity, while that of Pd was found to be significantly lower. In common with most diffusion controlled processes, an increase in the adsorption rate of platinum, palladium and gold cyanide with an increase in temperature was observed, while experiments with consecutive contacts of the PGM cyanide solution onto the activated carbon revealed that with an increase in temperature, the amount of PGMs that were adsorbed, decreased with each loading. In the absence of free cyanide and base metals, it was found that after 4 consecutive contacts, 99% of the total amount of platinum and palladium adsorbed at 25°C, compared to 85% of the platinum and 83% of the palladium at 50°C. No difference could be seen between the adsorption of gold cyanide at 25 and 50°C after 4 contacts. It has also been established that the detrimental effect of free cyanide on the adsorption of PGMs will increase as the temperature increases. The detrimental effect of the presence of Cu and Ni was found to depend on the amount of these base metals adsorbed, which in turn will depend on the cyanide concentration and the solution temperature. Adsorption of Pt and Pd has been found to be significantly more affected by temperature, cyanide and base metals than the adsorption of gold and needs to be carefully taken into consideration with the design of a PGM adsorption circuit to ensure sufficient Pt and Pd recovery. It is therefore highly likely that an activated carbon recovery process for Pt and Pd cyanide will not be as robust as the gold CIS (carbon-in-solution) process, which is considered to be one of its main advantages. The feasibility of eluting platinum and palladium cyanide complexes from activated carbon was investigated. It was found that platinum and palladium elute from activated carbon almost to completion in 4 to 5 bed volumes (BV) at 80°C, while the elution of gold at this temperature is slow, with a significant amount of gold (≈ 55 %) still to be eluted after 16 bed volumes. An increase in Pt and Pd elution kinetics was demonstrated with an increase in temperature with 99% recovery achieved at 4 BVs with an elution temperature of 95°C. Cyanide pre-treatment has been found to have a large influence on PGM elution. The effect of the NaCN concentration shows an increase in the recovery of Pt, Pd and Au as the cyanide increases from 0 to 2 %, after which the recovery starts decreasing again as the NaCN concentration increases from 2 to 4%. The NaOH concentration was also found to affect the PGM recovery and at 0% NaCN, an increase in the recovery is seen, while at a higher cyanide concentration (2 and 3% NaCN) a decrease in the PGM recovery occurs when the NaOH concentration is increased from 0.22% to 1.65%. A general decrease in Pt, Pd and Au recovery was seen as the ionic strength of the elution water increased and is consistent with literature on Au elution. The effect of a hydrochloric acid pre-treatment, which forms part of the process to remove calcium build-up from the activated carbon, was investigated, and for all the cases the Pt and Pd recovery increased when an acid pre-treatment was performed, compared to no acid pre-treatment. In none of the cases did any of the Pt, Pd or Au elute with the acid or the following rinsing water. The acid pre-treatment performed at 70°C removed a significant 64% to 75% of the Ni present and an additional 9.1% to 10.5% in the following rinsing water step. In the presence of copper cyanide, the elution order has been found to be copper, palladium, platinum and gold, which is the opposite order of adsorption preference. The cyanide pre-treatment has also been found to have a major influence on the elution of Cu and can be explained by the difference in the absorbance strength between the different copper cyanide complexes. The presence of Cu did not have a negative effect on the elution of the PGMs at strong pre-treatment (2% NaCN and 0.55% NaOH) conditions, but at weak pre-treatment conditions (0% NaCN) the recovery of Pt and Pd was reduced by between 10 and 18% after 5 BVs when Cu was present. The presence of 100 mg/L KSCN salt added to the leach solution during the adsorption stage, reduces the elution recovery of the PGMs at 4 BVs from 90% for Pt and Pd, when not present, to approximately 70% when present. The addition of the additional K+ ions reduced the recovery by less that 4% at 4 BV, which indicates that the possible formation of a PGM bonding with thiocyanate ([Pt(SCN)4]2- and [Pd(SCN)4]2-), which adsorb onto carbon, but doesn’t adsorb with water, cannot be ruled out completely. A maximum amount of 0.15% for Pt, 0.28% Pd and 0.6% Au was found to report to the pre-treatment solution at 25°C. For higher pre-treatment temperatures, the amount of Pt and Pd reporting to the pre-treatment solution increased significantly to approximately 8% at 80°C, while the increase in gold was marginal to 0.8% at 80°C. For the higher loading on the activated carbon (7000 mg/kg Pt and Pd), which is expected to be a better representation of plant conditions, 0.07% Pt, 0.11% Pd and 0.12% Au reported to the pre-treatment solution. The PGMs reporting to the pre-treatment solution is attributed to the distribution of the PGMs on the carbon particle, and even though very small, a certain amount of these PGMs would readily desorb or wash off the carbon. It has thus been established here that the stripping of adsorbed platinum cyanide complexes from activated carbon consists of a two step batch process, which involves the pre-treatment of the metal-loaded activated carbon with a relatively strong sodium cyanide and sodium hydroxide solution, prior to the elution step with de-ionized water at 80°C. Through the development of a mathematical model to describe this process, it was found that the rate of release of the platinum ions is governed by the amount of platinum and sodium on the activated carbon and the concentrations of these ions in the bulk of the liquid. This is mathematically described by a modified Freundlich isotherm equation and the mass transfer diffusion equation. The dependency of the platinum elution rate on the sodium concentration on the activated carbon, as suggested by gold related literature (Van Der Merwe and Van Deventer, 1990, Stange, 1990), is confirmed. Furthermore it has been found that the rate of platinum elution interchangeably depends on the equilibrium of the Pt ions at the carbon-liquid interface and the mass transfer of these Pt ions from the interface to the bulk liquid. As both of these rate-limiting factors were found to depend on the sodium concentration, the dominant platinum elution rate limiting factor shifts as the sodium concentrations change as the elution progresses. Four main time periods are used to simplify and to describe this process. The benefits of fundamentally understanding this process can ultimately lead to improved elution, better process control, shorter elution times, smaller elution columns or assist in the development of a continuous elution process.
CFD analysis of solid-liquid-gas interactions in flotation vessels
(Stellenbosch : Stellenbosch University, 2014-04) Karimi, Mohsen; Akdogan, G.; Bradshaw, S. M.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: A Computational Fluid Dynamics (CFD) model was developed for the prediction of flotation rate constants in a stirred flotation tank and validated against experimental data. The model incorporated local, time-varying values of the turbulent flow field into an existing kinetic flotation model based on the Generalised Sutherland Equation to predict the overall flotation rate constant. Simulations were performed for the flotation of various minerals at different operational conditions and the predictions were compared with experimental data. It was found that the CFD-based model yielded improvements in the prediction of flotation rate constant for a range of hydrophobicities, agitation speeds and gas flow rates compared with existing methodologies, which use volume-averaged empirical expressions for flow variables. Moreover, comparing to the available CFD alternatives for the flotation modelling this approach eliminates the need for solving an extra partial differential equation resulting in a more computationally economic model. The model was developed in three stages. In the first, a single-phase model was used to establish the requirements for successful modelling of the velocity components and turbulent properties of water inside flotation tanks. Also, a novel use of the Grid Convergence Index for this application was carried out, which allowed determination of the maximum achievable reduction in numerical uncertainties through systematic grid refinement and adaptation. All subsequent simulations were performed at the optimal discretization level determined in this manner. It was found that the Moving Reference Frames (MRF) method was adequate for representation of the impeller movement when the rotational zone was located close to the impeller, using a time step advance of between 10◦ and 15◦ of impeller rotation. Comparison of the different turbulence models for the single-phase modelling revealed that the standard k-e and Large Eddy Simulation turbulence models both performed equally well and that the computational requirement was lower for the standard k-e model, making it the method of choice. Validation of the methodology was done by comparison with experimental data for two different stirred tanks including an unbaffled mixer and a fully baffled standard Rushton turbine tank. The validation against experimental data showed that the model was capable of predicting the flow pattern, turbulent properties and the generation of trailing vortices. The second stage of modelling used an Eulerian-Eulerian formulation for gasliquid modelling of gas-sparged fully baffled vessels (2.25 l, 10 l and 50 l) using a Rushton turbine. It was determined that the minimum model uncertainty resulting from simulation of the sparger was achieved using a disk sparger with a diameter equal to 40% of the impeller diameter. The only significant interfacial force was found to be the drag force, and this was included in the multiphase methodology. A parametric study on the available formulations for the drag coefficient was performed which showed that the effect of turbulence on the air bubbles can accurately be represented using the proposed model of Lane (Lane, 2006). Validation of the methodology was conducted by comparison of the available experimental gas holdup measurements with the numerical predictions for three different scales of Rushton turbine tanks. The results verified that the application of the designed sparger in conjunction with Lane drag coefficient can yield accurate predictions of the gas-liquid flow inside the flotation tank with the error percentage less than 6%, 13%, and 23% for laboratory, pilot and industrial scale Rushton turbine tanks, respectively. The last stage of this study broadened the Eulerian-Eulerian framework to predict the flotation rate constant. The spatially and temporally varying flow variables were incorporated into an established fundamental flotation model due to Pyke (Pyke, 2004) based on the Generalized Sutherland equation for the flotation rate constant. The computation of the efficiency of the flotation sub-processes also incorporated the turbulent fluctuating flow characteristics. Values of the flotation rate constants were computed and volume-weight averaged for validation against available experimental data. The numerical predictions of the flotation rate constants for quartz particles for a range of particle diameters showed improvements in the predictions when compared with values determined from existing methodologies which use spatially uniform values for the important hydrodynamic variables as obtained from empirical correlations. Further validations of the developed CFD-kinetic model were carried out for the prediction of the flotation rate constants of quartz and galena floating under different hydrophobicities, agitation speeds and gas flow rates. The good agreement between the numerical predictions and experimental data (less than 12% error) confirmed that the new model can be used for the flotation modelling, design and optimization. Considering the limited number of CFD studies for flotation modelling, the main contribution of this work is that it provides a validated and optimised numerical methodology that predicts the flotation macro response (i.e., flotation rate constant) by integrating the significance of the hydrodynamic flow features into the flotation micro-processes. This approach also provides a more economical model when it is compared to the available CFD models for the flotation process. Such an approach opens the possibility of extracting maximum advantage from the computed parameters of the flow field in developing more effective flotation devices.
Characterisation and dynamic modelling of the behaviour of platinum group metals in high pressure sulphuric acid/oxygen leaching systems
(2012-12) Dorfling, Christie; Bradshaw, S. M.; Akdogan, G.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: Sulphuric acid/oxygen pressure leaching is typically employed on Base Metal Refineries (BMRs) to selectively dissolve base metals from platinum group metal (PGM) bearing nickelcopper matte. Optimal operation of this processing step requires an understanding of the system chemistry and the effects of process variables on base metal and PGM leaching behaviour. This project aimed to aid in the development of an improved understanding of the high pressure leaching system. The effects of temperature, pressure, acid concentration, and solid to liquid ratio on the leaching behaviour were determined experimentally using a two litre autoclave. For conditions comparable to that typically used at the Western Platinum Ltd. BMR, changes in the acid concentration had the largest effect on the copper leaching behaviour. Increasing the initial acid concentration from 140 g H2SO4/ℓ to 165 g H2SO4/ℓ resulted in the copper dissolution decreasing from 88.7% to 75.3% on average for the experiments performed at different temperatures (116°C, 130°C) and pressures (7 bar, 9 bar), and with different solids contents (80 g/ℓ, 130 g/ℓ). In the case of the other precious metals (OPMs), temperature was determined to be the process variable with the largest effect on the leaching kinetics. The average percentage rhodium dissolution achieved after seven hours of leaching at different conditions (pressure, acid concentration, and solids content were varied) increased from 58.3% at 116°C to 83.6% at 130°C. Similar effects were observed for ruthenium (96.2% dissolution at 130°C; 79.4% dissolution at 116°C) and iridium (81.8% dissolution at 130°C; 46.9% dissolution at 116°C). The rate of copper leaching was found to be limited by the rate of oxygen transfer from the gaseous phase to the liquid phase, while the remainder of the reactions were chemical reaction limited. The extent of OPM leaching was found to be dependent on the rate and extent of copper leaching. A set of 21 chemical reactions was proposed to describe the leaching behaviour, and the shape factors and reaction rate constants were determined by the method of least squares to minimise the error between the predicted concentrations and the experimental data. Apart from direct base metal leaching reactions, six cationic exchange reactions contribute to the leaching of copper sulphides and nickel sulphides by precipitation of OPM oxides. Three leaching reactions for each of the OPMs (one for sulphide phases, one for metallic phases, and one for oxide phases) resulted in satisfactory modelling of the system behaviour. Activation energies of -26.2 kJ/mol and -5.9 kJ/mol were calculated for the digenite acid leaching reaction and the covellite direct oxidation reaction, respectively, which confirmed that the rates of these reactions were mass transfer limited. The activation energies for the remainder of the base metal leaching reactions exceeded 30 kJ/mol. The activation energies of the reactions accounting for rhodium sulphide leaching, rhodium leaching, and rhodium oxide leaching, were calculated to be 64.2 kJ/mol, 138.5 kJ/mol, and 116.2 kJ/mol, respectively. Similar activation energies were calculated for the respective Ru and Ir leaching reactions. The rate of OPM sulphide leaching was typically an order of magnitude and three orders of magnitude larger than the rate of OPM leaching reactions and OPM oxide leaching reactions, respectively. The autoclave at the Western Platinum Ltd. BMR was modelled assuming a monosized distribution of the feed and approximating the autoclave as four ideal continuously stirred tank reactors. The steady state solution employed the sequential modular approach in MATLAB, while the dynamic simulation involved solving a set of 217 differential equations derived from mass and energy balances simultaneously in MATLAB. The model was used successfully to evaluate the effects that changes in the leaching temperature, leaching pressure, acid feed rate, and solids feed rate have on the extent of base metal and OPM leaching in the autoclave. The optimum operating conditions depend on the flow rates and compositions of the feed streams. A feed stream containing 10.3 wt% solids (825 kg solids/h) and an acid addition rate of 28.6 kg/h were considered as typical operating conditions for model-based analysis. More than 95% copper dissolution and no OPM dissolution were predicted when performing the pressure leaching at a pressure of 8 bar and a temperature of approximately 123°C. Decreasing the pressure resulted in lower copper dissolution when OPM leaching started to occur. Increasing the temperature resulted in reduced copper leaching, while decreasing the temperature resulted in a longer OPM leaching period and hence higher OPM dissolution. Model-based analysis furthermore showed that the relative amounts and relative leaching rates of digenite and covellite significantly influence the percentage copper dissolution achieved when noticeable OPM leaching start to occur.
Effect of iron endpoint during Peirce-Smith converting on matte mineralogy and downstream processing of base and platinum-group metals
(Stellenbosch : Stellenbosch University, 2014-12) Thyse, Elton Llyle; Akdogan, G.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: The process route for the production of base and platinum-group metals from natural sulfide ores commonly requires the conversion of high-iron furnace matte into an iron-lean converter matte. This is followed by pre-treatment through cooling of the iron-lean molten matte, physical processing of the solidified matte and hydrometallurgical metal extraction. Lonmin is the third largest producer of platinum-group metals in the world and utilizes Peirce-Smith converters for blowing high-iron furnace matte with air to a final iron concentration or endpoint. The molten matte is water granulated and solidification occurs via fast-cooling. The solidified matte is ground in a closed circuit ball mill with hydrocyclone classification and subjected to first stage atmospheric leaching. The specification of an ideal or desirable converter iron endpoint requires careful consideration. Most importantly, it must ensure the crystallization of converter matte with mineralogical qualities that are within the setpoints of the downstream unit processes and techniques. An additional consideration is for the final blown converter matte to achieve an optimum bulk concentration of the base metals Ni and Cu and platinum-group metals Pt, Pd, Rh, Ru and Ir. Mattes characteristic of variable iron endpoints were regularly produced at the Lonmin converter plant section. Uncertainty by plant metallurgists in knowing the desirable iron endpoint, particularly within the context of the Lonmin base metal refinery, and poor control has had detrimental effects on the mineralogical quality of the final matte and hence on the processing characteristics of the solidified matte particles downstream. A desirable iron endpoint required investigation, selection and implementation at Lonmin. The primary focus of this study was therefore to quantify the effect of a specific iron endpoint on the mineralogy and mineral chemistry of solidified converter matte. A fundamental examination of the solidification process upon cooling was regarded as critical to an in-depth understanding of the attained mineralogy and mineral chemistry as a function of a specific iron endpoint. It became equally important to quantify the effect of the resultant mineralogy, and hence iron endpoint, on the physical property of mineral structures in relation to downstream grinding, liberation and leaching characteristics. Despite considerable industry context, limited in-depth and coherent studies on the effect of a specific iron endpoint on fast-cooled converter matte systems were found in both industrial and scholarly literature. Previous findings in literature offered a limited quantitative understanding of the effect on mineralogy and mineral chemistry. Phase and cooling equilibria of multi-component, iron endpoint specific Ni-Cu-S matte systems were also not fully available. These would have been particularly useful in understanding the complexities of converter matte solidification as a function of iron endpoint. Physical property knowledge of converter matte mineral structures was hardly available and even less so in relation to grinding, liberation and leaching processes. A comprehensive investigation was therefore required to address these extensive knowledge gaps with respect to fastcooled converter matte systems in an industrial framework. Three Peirce-Smith converter production samples, representative of the extent in variability of iron endpoints attained at the converter plant, were used in a systematic investigation coupled to a novel combination of modern analytical techniques, computational thermochemistry and metallurgical testwork. The modern analytical techniques included the application of high resolution transmission electron microscopy and focused ion beam scanning electron microscopy tomography. Computational thermochemistry was applied through the use of MTDATA phase diagram software. Metallurgical testwork involved laboratory batch grinding at various specific energies. Closely associated leach experiments were also considered relevant to this wide-ranging investigation. The Peirce-Smith converter samples investigated were indicative of mattes that attained specific endpoints of 5.17%, 0.99% and 0.15 weight% Fe. The highest combined bulk concentration of the important base and platinum-group metals was achieved in the matte which attained a specific iron endpoint of 0.99%. The mineralogy of all three converter mattes was dominated by nickel sulfide mineral structures matched to the natural mineral of heazlewoodite. Mineral structures of copper sulfide, NiCu-alloy, spinel and OsRu-alloy were also constituents of the different converter mattes. The attainment of a specific iron endpoint was found to result in measurable mineralogical differences with respect to relative mineral abundances, external morphological characteristics and mineral chemistry. The mineralogical differences were particularly distinct between mineral structures of the high (5.17%) and low (0.99% and 0.15%) iron mattes. Subtle mineralogical differences were evident between mineral structures of the low iron mattes. The 0.99% Fe matte was characteristic of a significantly higher NiCu-alloy relative abundance, compared to the 5.17% Fe matte. The NiCu-alloy structures were found to act as the primary collectors of the economically significant platinum-group metals. Mineralogical observations were used to develop an understanding of the underlying mineralization mechanism of NiCu-alloy structures. High-fidelity color and grayscale 3D reconstructions were produced of the resultant mineralized structures. It was shown theoretically that variations in iron endpoint specific starting compositions of oxygen-free liquid matte systems alter the solidification pathway towards the eutectic. Moreover, a quantitative understanding of liquid phase solidification of the high and low iron matte systems, including oxygen, was developed to within ±2.5 oC. Most of the specific energy available for grinding was expended breaking the nickel sulfide matrix, particularly of the high iron matte. The breakage rates of copper sulfide mineral structures in the 5.17% Fe matte were calculated to be higher than in the 0.15% Fe matte at 25kWh/t specific energy. The degree of copper sulfide liberation was shown to be higher for the 5.17% Fe matte than for the 0.15% Fe matte at the same specific energy of grinding. A higher degree of Ni extraction and Cu cementation could be achieved when leaching low iron matte particles. The production of converter matte attaining a specific iron endpoint of 0.99% was found to be the most suitable with respect to endpoint selection criteria. A practical iron endpoint range of 1.6% to 1.0% was recommended for the production of converter matte with a resultant mineralogical quality within the constraints of the Lonmin base metal refinery. This study offers an integrated understanding of base and platinum-group metals production as a function of a desirable iron endpoint at Lonmin. This was not previously available in metal production literature. New technology for the monitoring and consistent control of such a practical iron endpoint range can subsequently be implemented.
Investigation of the gas dispersion and mixing characteristics in column flotation using Computational Fluid Dynamics (CFD)
(Stellenbosch : Stellenbosch University, 2016-03) Mwandawande, Ikukumbuta; Akdogan, G.; Bradshaw, S. M.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: In this thesis, Computational Fluid Dynamics (CFD) was applied to study gas dispersion and mixing characteristics of industrial and pilot scale flotation columns. An Eulerian-Eulerian multiphase modelling approach with appropriate interphase momentum exchange terms was applied to simulate the multiphase flow inside the column while turbulence in the continuous phase was modelled using the k-ϵ realizable turbulence model. The CFD simulations in this research were performed using the Ansys Fluent 14.5 CFD solver. In the first part of the research, CFD was used to predict the average gas holdup and the axial gas holdup variation in the collection zone of a 0.91 m diameter pilot flotation column operated in batch mode. The axial gas holdup profile was achieved in the simulations using the Ideal Gas law to impose compressibility effects on the air bubbles. With mean absolute relative error (MARE) ranging from 6.2 to 10.8%, the predicted average gas holdup values were in good agreement with experimental data. The axial gas holdup prediction was generally good for the middle and top parts of the column where the mean absolute relative error values were less than 10% while the gas holdup was over-predicted for the bottom part of the column (MARE exceeding 20%), especially at lower superficial gas velocities. The axial velocity of the air bubbles decreased with height along the column. The axial decrease in the bubble velocity may be due to the increase in the drag force resulting from the upward increase in gas holdup in the column. Simulations were also conducted to compare the gas holdup predicted with three different drag models, the Universal drag coefficient, the Schiller-Naumann, and the Morsi-Alexander drag models. The gas holdup predictions for the three drag models were not significantly different. Flotation columns are known for their improved metallurgical performance compared to conventional flotation cells. However, increased mixing in the column can adversely affect its grade/recovery performance. In the second part of this research, the mixing characteristics of the collection zone of industrial flotation columns were investigated using CFD. Liquid and particle residence time distribution (RTD) data were computed from CFD simulations and subsequently used to determine the mixing parameters (the mean residence time and the vessel dispersion number). Liquid RTD was modelled using the Species Model available in Ansys Fluent while the particle RTD was modelled using a user defined scalar (UDS) transport equation that computes the age of the particles in the column. The mean residence time of particles in the column was well predicted with a mean absolute relative error equal to 7.8%. The results obtained showed that particle residence time decreases with increasing particle size. The residence time of the coarser particles (125 μm) was found to be about 50% of the liquid residence time while the finer particles (44 μm) had residence time similar to the liquid one. These findings are in agreement with experimental data available in the literature. The relationship between the liquid and solids axial dispersion coefficients was also investigated by comparing the water and the solids flow patterns. The flow patterns between the phases revealed that their dispersion coefficients were similar. In addition, the effects of the bubble size and particle size of the solids on the liquid dispersion were investigated. It was found that increasing particle size of the solids resulted in a decrease in the liquid vessel dispersion number. On the other hand, a decrease in the bubble size caused a significant increase in the liquid vessel dispersion number. Flotation columns are normally operated at optimal superficial gas velocities to maintain bubbly flow conditions. However, with increasing superficial gas velocity, loss of bubbly flow may occur with adverse effects on column performance. It is therefore important to identify the maximum superficial gas velocity above which loss of bubbly flow occurs. The maximum superficial gas velocity is usually obtained from a gas holdup versus superficial gas velocity plot in which the linear portion of the graph represents bubbly flow while deviation from the linear relationship indicates a change from the bubbly flow to the churn-turbulent regime. However, this method is difficult to use when the transition from bubbly flow to churn-turbulent flow is gradual as happens in the presence of frothers. Two alternative methods are presented in the final part of the present research in which the flow regime prevailing in the column is related to radial gas holdup profiles and gas holdup versus time plots obtained from CFD simulations. The results showed that radial gas holdup profiles can be used to distinguish bubbly flow (saddle shaped gas holdup profiles) from churn turbulent flow (steep parabolic gas holdup profiles). However, the transitional regime between these two extremes was difficult to characterize due to its gradual nature. Another important finding of this research was that different radial gas holdup profiles could result in opposite liquid flow patterns. For example, a liquid circulation pattern with upward flow in the centre and downward flow near the column walls was always present when the radial gas holdup profile is parabolic. On the other hand, an inverse flow pattern was observed in which the liquid rises near the column wall but descends in the centre and adjacent to the wall. This profile was accompanied by corresponding saddle shaped radial gas holdup profiles.
A process performance monitoring methodology for mineral processing plants
(Stellenbosch : Stellenbosch University, 2014-04) Groenewald, Jacobus Willem De Villiers; Aldrich, C.; Bradshaw, S. M.; Akdogan, G.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: Key to remaining competitive within the mineral industry is ensuring that all processes are always being operated optimally. Process performance monitoring is an ideal initiative with which to accomplish this. Not only can it be used to ensure fault free process operation, but it can also be applied for plant performance improvement through a better understanding of the contributors to the success or failure of the process operation. Critical to the success of any proposed monitoring approach would be its ability to cater for the fact that these mineral processes are typically highly complex, dynamic and non-linear. The purpose of this study was to propose and evaluate a methodical approach to plant-wide process performance monitoring for mineral processing plants. Crucial to this approach is the concept of integrating process causality maps with data-based systems for event detection and diagnosis. To this end, process causality maps were developed to provide a means of structuring process data through the use of fundamental process knowledge. Statistical data-based fault detection techniques, being especially powerful with regards to data compression and dimensionality reduction, were employed to allow huge data sets to be analysed more easily. Change point detection techniques allowed for the identification of stationary segments of data in otherwise non-stationary data sets. Variable importance analysis was used to identify and interpret the variable(s) responsible for the event conditions. Using simulated data sets, different techniques were evaluated in order to acquire an appreciation for their effectiveness and reliability. While it was found that no single technique significantly outperformed any other, it was confirmed that for data having different structures and characteristics, none of the techniques were effective in analysing all potential event conditions. It was suggested that all available techniques be run in parallel, with expert interpretation of the results, ensuring a more comprehensive analysis to be performed. Furthermore, given that only process measurements being monitored could be used to detect events and be analysed for importance, the consequences of monitoring too few process measurements were highlighted. A generic analytical methodology for multivariate process performance monitoring was defined, ensuring the use of appropriate techniques and interpretations. The methodology was subsequently successfully applied to a mineral processing concentrator case study. The application of process causality maps was found to significantly simplify the challenge of monitoring the process, not only improving the ability of the techniques applied through a better focussed application, but also the interpretability of the results due to the reduction in complexity. Extreme learning machine, a robust and computationally inexpensive algorithm, was identified as a potential core algorithm for the data analysis techniques forming part of a process performance monitoring solution. With different drivers, at different times, having different effects on the process, visual representation of the data through canonical variate analysis biplots, combined with a sound understanding of the process under investigation, contributed significantly to a better understanding of the important variables for each event condition. From an implementation perspective, adoption of the methodology remains the biggest barrier to success, requiring the most attention in the immediate future.
Pyrometallurgical refining of high grade PGM leach residues prior to precious metals separation
(Stellenbosch : Stellenbosch University, 2014-12) Bezuidenhout, Gert Adrian; Bradshaw, S. M.; Akdogan, G.; Eksteen, Jacques J.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: Primary platinum mining companies use a complex multistage recovery and refining process. The removal of base metals in the base metal refinery (BMR) leaves a residue that requires a number of hazardous and expensive unit operations to prepare the platinum group metal (PGM) concentrate for final separation and refining. In the PGM industry, hydrometallurgy is used almost exclusively to refine material with a PGM content of more than a couple of per cent. This thesis proposes and investigates an alternative pyrometallurgical refining method to remove contaminants from a high grade PGM residue (e.g. residue from a BMR after the bulk of the base metals has been removed), whilst minimising aqueous wastes and potential PGM losses. The hypothesis behind the use of a pyrometallurgical process was the effective separation of a number of elements from the PGMs through the use of only a few processing steps. The noble nature of PGMs, i.e. their resistance to oxidation and low vapour pressure at high temperature, allows for effective separation. The theoretical feasibility of the concept was explored using thermochemical modelling. Modelling suggested that the use of a controlled atmosphere roasting step, followed by smelting and atomisation, could lead to a technically feasible process to separate PGMs from other metals. Throughout this thesis, thermochemical modelling was shown to be a useful tool to interpret and understand elemental behaviour across the roasting and smelting steps. It was experimentally illustrated that roasting of PGM residues in an oxidising environment at low temperatures (700 °C - 900 °C range) will selectively vaporise the volatile oxides of S, Se, Te and As (to varying degrees), that would otherwise be quite stable in the alloy or matte phase during a melt at low oxygen partial pressure. Arsenic volatilisation proceeded only partially (30 wt% to 60 wt%), probably due to the formation of a temperature stable species of arsenate. Osmium volatilisation was not properly described by modelling and proceeded only partially (30 wt% to 70 wt%), possibly due to the presence of Osmium in a solid solution phase that depresses the activity and correspondingly the fugacity. Although a number of the PGMs oxidise in the roasting temperature range, their vapour pressures do not allow measurable losses to the gas phase (apart from Osmium that is not recovered in most precious metal refineries). Almost all the PGMs reported to two separate solid solution phases during roasting. The smelting step allows the PGMs to dissociate from oxygen, alloy and melt, without the addition of a collector. The liquidus temperature was shown to be strongly influenced by the concentrations of impurities (S, Se, Te, As, Fe, Ni and Cu) in the feed to the melt and ranged between 1 235 °C and 1 510 °C. The most important variable necessary to avoid non-PGM elements (especially As, Pb, Fe and Ni) from joining the alloy phase is control of the partial oxygen pressure in the melt. Au losses due to volatilisation of species such as AuS, AuSe and AuTe were shown possible at higher temperatures, with 21% recovery loss measured at 1 700 °C in the presence of 6% Se, Te and S. Ru losses (ranging from 9% to 39%) across the smelting step were shown to be sensitive to the combined Fe, Ni and Cu content of the feed, the melting temperature and the partial oxygen pressure across the melt. With Cu addition (to achieve Cu content in the alloy phase >60% by weight), melting temperatures as low as 1 200 °C is sufficient to collect PGMs and separate them from the slag phase. However, Ru is insoluble in the Cu-rich alloy and varying Ru recoveries were measured, pointing to a possible loss mechanism. After casting and solidification of the melt, the alloy phase can be separated from the slag phase by mechanical means. Atomisation is necessary to break the alloy phase into fine particulate that allows improved leaching kinetics. From experimental test work and literature studies, it appears that fine particulate sizing (D50 of 20 μm) can be achieved at high pressure requirements (in excess of 400 bar). The necessity of re-melting the alloy phase before atomisation allows the opportunity for a high temperature treatment step. It was experimentally shown that near complete volatilisation of Pb and Bi can be achieved if the alloy is kept at 1 700 °C for 30 minutes in an inert environment. Significant Ag losses to the vapour phase were also measured across this high temperature treatment step. This study fundamentally and practically illustrated that it is possible to use pyrometallurgy to separate PGMs from a large number of metals/oxides/amphoterics present in leach residues. By effectively using a combination of roasting and smelting, it is possible to upgrade the PGM content of a pressure leach residue from low forty per cent to high eighty per cent, with low PGM losses. The proposed roasting, smelting and atomisation processes were able to remove the bulk of S, Se, Te, Bi, Pb, oxides (such as SiO2 and Cr2O3) and partially remove Os, As, Fe, Ni and Cu. It was experimentally shown that the PGM containing alloy is amenable to leaching in a chlorine environment as used in a precious metal refinery, but more work is necessary to achieve near complete dissolution. This investigation spanned a number of processing steps and measured the associated impact of each on multiple elements, and in that regard it does not follow a classical or typical doctoral thesis approach. However, a field of research has been studied that enjoys very little publication, apart from specialist alloys or specialist characteristics. This is in part due to the proprietary nature of PGM refining. This study contributes to the existing scientific knowledge base, but also to the process engineering knowledge base of the behaviour of high grade PGM residues at high temperatures.
The recovery of platinum group metals from low grade concentrates to an iron alloy using silicon carbide as reductant
(Stellenbosch : Stellenbosch University, 2014-12) Malan, Willem du Toit; Bradshaw, S. M.; Akdogan, G.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: In this study, SiC reduction of Rowland and Easterns LG (Low Grade) concentrates was investigated. The purpose of the study was to investigate the feasibility of SiC as reductant with respect to metal fall, PGM grade in the alloy, slag composition, Cr solubility and overall PGM recovery. The integration of such process in the current matte-based collection process was also investigated. Currently, the matted-based collection process is most widely used for PGM recovery, but because PGM containing concentrates are becoming more enriched with UG2 (Upper Group 2) LG concentrates, it is expected to be integrated or replaced with an alloy collection process. This kind of process offers greater flexibility to the different types of ore that could be used. The process is chromium tolerant and environmentally friendly. For this purpose Rowland and Easterns UG2 LG Concentrate samples from Lonmin Western Platinum Limited were analysed with XRD, XRF and ICP-MS and it was found that SiO2 and MgO are the most abundant oxides and Pd is the most abundant element from the PGMs. Sulphide bearing minerals such as chalcopyrite were detected in low concentrations (below 1 %) and Cr2O3 concentrations are between 2 – 4 %. The FeO/SiO2 ratio was lower in Rowland LG concentrate. SiC reduction of Rowland and Easterns concentrate was done at 1600℃. Reductant to concentrate ratios for laboratory scale experiments were ranged from 2.5 to 3.5 kg SiC / 100 kg concentrate. SiC reduction of Rowland concentrate had different reduction times. The duration of reduction experiments ranged from 30 - 180 min. PGM recoveries from SiC reduction of Rowland concentrate were very poor (below 10 %) and Fe recoveries were lower than 50 %. A slag viscosity at the end of the melt of more than 4 poise was responsible for poor phase separation. SEM images revealed metal prills entrained in the slag phase instead of settling and combining to the alloy globule at the bottom of the crucible. However, PGM recoveries from SiC reduction of Easterns concentrate was significantly better. More than 85 % of Ir and Pd and almost 60 % of Pt were recovered in a test with a reductant to concentrate ratio of 3.5 kg SiC / 100 kg Easterns concentrate. Fe recovery was also the highest at 66%. Cr and Si concentrations were below 5 % in total. The slag viscosity at the end of melt was calculated to be less than 4 poise and a SEM image of a slag sample revealed few entrained metal prills. After the above findings on the importance of viscosity, it was decided to increase the FeO content in the initial concentrate charge in order to decrease slag viscosity, increase metal fall (PGM collecting phase) and further increase PGM recovery. Peirce-Smith converter slag was used for this purpose. A test was conducted with the addition of 10 kg converter slag / 100 kg Easterns concentrate. The reductant to concentrate ratio was kept at 3.5 kg SiC / 100 kg Easterns concentrate. The results revealed that Ir and Pd recoveries were more than 95%, while Pt recovery was almost 70%. Fe recovery increased to 76 %. On the basis of the results from the test, an optimum feed ratio between Easterns LG concentrate, Rowland concentrate and Peirce-Smith converter slag was calculated. Thermodynamic phase equilibrium calculations predicted that the concentrate charge should consist of 60 - 80% Easterns concentrate with a slag addition of 30 – 40 kg converter slag / 100 kg LG concentrate. SiC reduction of this optimum LG concentrate charge is expected to recover more than 90% of all PGMs. Cr and Si concentrations in the alloy will be below 1 % in total. The amount of converter slag as an addition will be however limited by final PGM grade in the alloy, furnace slag quantities recycled and slag resistivity required in the alloy furnace. The effectiveness of SiC as reductant was also compared to C reduction. C reduction of an optimum concentrate charge had a marginally higher metal fall at the same reductant to concentrate ratio than SiC reduction of an optimum concentrate charge. However, gas emissions are on average 3 times higher for C reduction of a concentrate charge and C reduction requires at least 300 MJ more to smelt 1 ton of LG concentrate than SiC reduction. This is mostly due to C reacting endothermically with FeO to produce Fe(l) and CO(g) in contrast to SiC reacting exothermically with FeO to produce Fe(l), SiO2(l) and CO(g). Integrating SiC reduction of LG concentrates into the existing smelting route at Lonmin was also proposed through a process flow diagram. From an economic point of view, it was found that SiC reduction of 1 ton of LG concentrate charge with a converter slag addition requires almost 700 MJ more than the smelting of a UG2 blended concentrate to produce a matte phase. However it must be taken into account that the sulphide rich layers in the Bushveld complex are being depleted rapidly and alternative processes such as SiC reduction and alloy collection process will be utilized faster than expected. Moreover, gas emissions from reductive smelting is considerably lower, hence it is a more environmentally friendly process. Finally, from the findings of this study, it could be said that base metals and PGMs could be recovered in an iron alloy from SiC reduction of LG concentrate with converter slag additions. Therefore integrating such a process into the matte-based collection process could be considered as a future alternative to smelting UG2 LG concentrates.
Simulating the medium temperature chalcopyrite oxidation system in batch and continuous autoclaves
(Stellenbosch : Stellenbosch University, 2012-12) Steyl, Johann Du T.; Bradshaw, S. M.; Akdogan, G.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
ENGLISH ABSTRACT: A detailed mathematical description of the pressure oxidation of chalcopyrite concentrates in acidic sulfate solutions is presented in this dissertation, including descriptions of the most important accompanying reactions under typical medium operating temperatures (140-155ºC) in batch and steady-state continuous stirred tank reactors. The mathematical framework consists of various modules, each addressing particular thermodynamic and kinetic aspects of the primary processes occurring in a pressure oxidation reactor. Comprehensive literature investigations are presented which constituted the departure point of this study, supplemented by phenomenological data obtained from controlled batch and continuous experimentation. The different modules are each covered in a chapter of this dissertation, and include the following: i) the solution thermodynamic framework, ii) the interfacial oxygen mass transfer rate, iii) the iron(II) oxidation rate, iv) the iron(III) precipitation rate, (v) the intrinsic oxidation kinetics of the sulfide minerals, and vi) the particulate batch and continuous reactor frameworks. These modules are imbedded within a continuous mass and energy balance platform of a primary leaching – solvent extraction circuit, while the batch reactor analogy describes the conservation of mass, and its movement between phases, in a stand-alone configuration. Literature information proved particularly important in deriving the generic functional forms of the most important reaction rate expressions, while the phenomenological expressions were refined, and the associated constants obtained, from low pulp density batch experimentation. Higher pulp density batch reactor and continuous (pilot plant) data were then used to confirm the successful integration of these expressions in the overall simulation. This is the first truly comprehensive study of chalcopyrite concentrate pressure oxidation, the kinetics of the accompanying reactions and each of the underlying thermodynamic aspects. Consistent theoretical arguments and insights are presented, while conceptual proposals are tabled in cases where the fundamental information is lacking. This dissertation presents an internally consistent simulation, with each of the modules contributing to an overall mathematical description of the pressure oxidation of chalcopyrite concentrates. It provides a powerful research and process engineering tool, and may be used to simulate the performance of a primary leaching circuit on a full-scale plant. As an example, the simulation was used to study the impact of selected intrinsic properties and control parameters on the autoclave and primary leach circuit performance. Various recommendations on improving the simulation are highlighted, as well as the aspects to explore in follow-up studies.

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Browsing Doctoral Degrees (Chemical Engineering) by browse.metadata.advisor "Akdogan, G."

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