Ghana, a West Africa country, is deeply burdened by poverty, and relies on the production of gold for economic sustainability. The gold mining companies in the country have international origins and receive most of their requirements from international sources. The extraction of gold from the crushed ore requires sodium cyanide as a lixiviant, which is imported into Ghana from other countries in wooden intermediate bulk containers (IBC) for further distribution to the mines. A life cycle assessment was completed to determine the burden that this packaging, which includes the wooden container and polyethylene and polypropylene liners, places on the environment in Ghana when disposed of. It was found that the life cycle of the incinerated IBC impacted on the Ghanaian environment the most, due to the incineration and the transportation of the IBC. The International Organization for Standardization 14040 management standard was used as a methodological framework for the assessment. / Environmental Science / M.Sc. (Environmental Science)

Ghana, a West Africa country, is deeply burdened by poverty, and relies on the production of gold for economic sustainability. The gold mining companies in the country have international origins and receive most of their requirements from international sources. The extraction of gold from the crushed ore requires sodium cyanide as a lixiviant, which is imported into Ghana from other countries in wooden intermediate bulk containers (IBC) for further distribution to the mines. A life cycle assessment was completed to determine the burden that this packaging, which includes the wooden container and polyethylene and polypropylene liners, places on the environment in Ghana when disposed of. It was found that the life cycle of the incinerated IBC impacted on the Ghanaian environment the most, due to the incineration and the transportation of the IBC. The International Organization for Standardization 14040 management standard was used as a methodological framework for the assessment. / Environmental Science / M.Sc. (Environmental Science)

Thesis (PhD)--Stellenbosch University, 2015. / 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. / AFRIKAANSE OPSOMMING: Nuwe prosesse om komplekse laer graad waardevolle metaal ertse te onwikkel in plaas van die konventionele maal-flotasie-smelt-raffineer proses word tans ondersoek. Loging van platinum en palladium met sianied is verskeie kere al voorgestel as n belowende proses opsie en alhoewel die ekstraksie van platinum nog 'n probleem is, is vordering al gemaak om die logings proses beter te verstaan. Die loging van platinum en palladium met sianied sal tipies plaasvind by hoër temperature wat kan wissel van 55°C in n hoop logins proses tot 180°C onder druk. Beter loging vind plaas soos wat die temperatuur styg. Alhoewel hierdie proses belowend is, is daar nog geen navorsing beskikbaar t.o.v die volgende stap in die proses wat die opgradering van die logings oplossing behels. Aangesien die gebruik van geaktiveerde koolstof in die goud industrie baie voordele inhou soos die eenvoudigheid van die proses, hoë opgraderings faktor, relatiewe vinnige adsorpsie en lae kapitaal en operationele kostes, mag hierdie tegnologie ook geskik wees vir Pt en Pd. Baie min is gepubliseer oor die adsorpsie van waardevolle metale (PGMs) met geaktiveerde koolstof en wanneer die effek van onsuiwerhede soos basis metale en thiosianied saam met n afstropings metode in ag geneem word, kon geen informasie in the literatuur gvind word nie. Hierdie studie is dus geloots en in die algemeen is daar bevind dat die geaktiveerde koolstof metode wel n geskikte oplossing bied vir die opgradering van PGMs in 'n sianied logings stroom. Adsorpsie snelheid vir verdunde PGM oplossings (0.15mg/L Pt, 0.38 mg/L Pd, 0.1 mg/L Au) in bekers wat geroer is, was vinnig vir die eerste 60 minutte (98% herwinning van die PGMs) en 'n vergelyking tussen die Pt adsorpsie isoterm en gepubliseerde Au isoterms wys op n relatiewe soorgelyke ladings kapasiteit. Die isotherm van Pd was egter laer. Soortgelyk aan meeste diffusie beheerde prosesse, het 'n verhoging van temperatuur gelei tot vinniger adsorpsie snelhede vir Pt, Pd en Au. Indien oplossings egter herhaaldelik met dieselfde koolstof in kontak gebring word, is bevind dat met 'n verhoging in temperatuur, die totale hoeveelheid PGMs wat absorbeer, met elke kontak verminder. In die afwesigheid van sianied en enige basis metale, het die totale hoeveelheid Pt en Pd wat geabsobeer is na 4 opeenvolgende kontake verminder van 99% by 25°C tot 85% adsorpsie van Pt en 83% Pd by 50°C. Daar was geen veskil tussen die adsorpsie van Au by 25°C en 50°C na 4 kontakte nie. Dit is ook vasgestel dat die negatiewe effek van vrye sianied in die oplossing, groter word soos wat die temperatuur verhoog. Die negatiewe effek van die teenwoordigheid van Cu en Ni, hang af van die hoeveelheid van hierdie metale wat absorbeer word wat weer bepaal word deur die sianied konsentrasie asook die temperatuur van die oplossing. Die mate waar in Pt en Pd ge-absorbeer word deur koolstof, word aansienlik meer beïnvloed deur temperatuur, sianied konsentrasie en die teenwoordigheid van basis metale as wat die adsorpsie van Au beïnvloed word. Met die ontwerp van n PGM adsorpsie proses, sal dit sterk in aanmerking geneem moet word, om seker te maak dat so min as moontlik Pt en Pd verlore gaan. Dit wys ook dat 'n geaktiveerde koolstof proses vir Pt en Pd, nie so onvatbaar vir proses veranderings sal wees soos wat die Au proses is nie, wat as een van die groot voordele van die Au proses beskou word. Die afstropery van die Pt en Pd sianied komplekse is ondersoek en gevind dat by 80°C, amper al die Pt en Pd binne 4 to 5 bed volumes (BV) van die geaktiveerde koolstof gestroop kan word. Die afstropery van Au by hierdie temperatuur is stadig en sowat 55% van die Au bly nog oor op die koolstof na 16 BVs. Die snelheid waarteen die Pt en Pd afgestroop word verhoog indien die temperatuur verhoog word en by 95°C is 99% van die Pt en Pd herwin na 4 BVs. Voorafbehandeling van die gelaaide koolstof met sianied, het 'n groot uitwerking op die PGM afstroping. Soos wat die NaCN konsentrasie verhoog word vanaf 0 tot by 2%, verhoog die PGM herwinning maar daal ook weer indien die NaCN konsentrasie verder verhoog word vanaf 2% tot by 4%. Die NaOH konsentrasie het ook n invloed gehad. By 0% NaCN, het die PGM herwinning verhoog soos wat die NaOH konsentrasie verhoog is maar by hoër sianied konsentrasies van 2 en 3%, is n daling in die herwinning waargeneem soos wat die NaOH verhoog is vanaf 0.22% tot by 1.65%. N algemene daling in die Pt, Pd and Au herwinning is waargeneem soos wat die ioniese sterkte van die stropings water toegeneem het. Dit stem ooreen met gepubliseerde navorsing oor die afstroping van Au. Om kalsium van die geaktiveerde koolstof te verwyder, word die koolstof vooraf met soutsuur gewas. Hierdie stap is vir Pt en Pd ondersoek en daar is bevind dat vir al die gevalle, die herwinning hoër was wanneer die gelaaide koolstof met soutsuur behandel is teenoor geen behandeling nie. In geen van die gevalle, is Pt, Pd of Au saam met die soutsuur of die daaropvolgende was water afgestroop nie. Voorafbehandeling met soutsuur teen 70°C lei wel daartoe dat tussen 64 en 75% Ni afgestroop word saam met die soutsuur en n verdere 9.1 to 10.5% saam met die daaropvolgende was water. In die teenwoordigheid van koper sianied, word Cu eerste afgestroop met Pd, Pt en dan Au wat daarop volg. Dit is presies die teenoorgestelde orde waarin die metale geabsorbeer word. Die voorafbehandeling met sianied, het ook 'n beduidende effek op die stroping van Cu. Dit kan verduidelik kan word aan die hand van die verskillende koper sianied komplekse wat vorm wat elkeen 'n verskillende affiniteit het vir adsorpsie. Wanneer 2% NaCN en 0.6% NaOH in die vooraf behandeling stap gebruik word, het die teenwoordigheid van koper geen negatiewe invloed op die afstroping van die PGMs gehad nie maar die herwinning is wel met 10% en 18% verlaag by 5 BVs wanneer geen sianied in die voorafbehandeling stap gebruik is nie. Wanneer 100 mg/L KSCN sout by die adsorpsie stap gevoeg word, daal die herwinning van die PGMs in die stropings stap van 90% tot 70% by 4 BVs. Die addisionel K+ katione verminder die herwinning met slegs 4% by 4 BVs wat beteken dat die vorming van adisionele komplekse soos [Pt(SCN)4]2- en [Pd(SCN)4]2-, wat nie op die normale metode afgestroop kan word, 'n moontlikheid mag wees. N maksimum van 0.15% vir Pt, 0.28% Pd en 0.6% Au word in die vooraf behandelings stap af gestroop. Indien die temperatuur van hierdie stap verhoog word na 80°C, verhoog die hoeveelheid Pt en Pd wat na hierdie stroom raporteer na 'n beduidende 8% tewyl Au basies onveranderd bly by 0.8%. Vir geaktiveerde koolstof wat hoër gelaai is (7000 mg/Kg Pt en Pd) en dus 'n beter verteenwoordiging van aanleg kondisies is, het 0.07% Pt, 0.11% Pd en 0.12% Au raporteer na die vooraf behandelings stap. Dit word toegeskryf aan die verspreiding van die PGMs op die koolstof wat hoofsaaklik op die oppervlakte voorkom en alhoewel die hoeveelheid klein is, word 'n sekere hoeveelheid slegs afgewas. Dit is dus vasgestel hier, dat die afstroping van Pt 'n twee stap proses is. Die eerste stap is die voorabehandeling van die koolstof met 'n sianied oplossing en daarna volg die afstroping van die Pt met suiwer water teen ongeveer 80°C. Met die ontwikkeling van n wiskundige model, is bevind dat die snelheid waarteen die Pt afgestroop word, beïnvloed word deur die hoeveelheid Pt en Na wat op die koolstof oppervlakte is, asook die konsentrasies van die metale in die vloeistof. Dit word wiskundig beskryf deur n gemodifiseerde Freundlich isotherm vergelyking asook deur die massa diffusie vergelyking. Die Pt afstropings snelheid wat afhanklik is van die Na konsentrasie kan vergelyk word met die afstroping snelheid van Au wat ook afhanklik is van die Na konsentrasie (Van Der Merwe and Van Deventer, 1990, Stange, 1990). Verder is bevind dat die snelheid van Pt stroping afhanklik is van beide die ewewig van Pt ione by die koolstof-vloeistof grens asook die massa beweging van Pt ione van die koolstof-vloeistof grens na die vloeistof. Beide hierdie snelheids bepalende faktore word bepaal deur die Na konsentrasie en skuif soos wat die Na konsentrasie verander soos wat die afstroping plaasvind. Vier hoof tyd periodes word gebruik om hierdie verskynsel te verduidelik. Deur die proses fundamenteel te verstaan, kan uiteindelik lei tot 'n beter proses, beter beheer, korter afstropings tye, kleiner toerusting of die ontwikkeling van 'n kontinue proses.

published_or_final_version / Environmental Management / Master / Master of Science in Environmental Management

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.

Pseudomonas fluorescens NCIMB 11764 (Pf11764) is uniquely able to grow on the poison cyanide as its sole nitrogen source. It does so by converting cyanide oxidatively to carbon dioxide and ammonia, the latter being assimilated into cellular molecules. This requires a complex enzymatic machinery that includes nitrilase and oxygenase enzymes the nature of which are not well understood. In the course of a proteomics analysis aimed at achieving a better understanding of the proteins that may be required for cyanide degradation by Pf11764, an unknown protein of 17.8 kDa was detected in cells exposed to cyanide. Analysis of this protein by ESI-coupled mass spectrometry and bioinformatics searches gave evidence of strong homology with a protein (Hyp1) of unknown function (hypothetical) present in the bacterium Photorhabdus luminescens subsp. laumondii TTO1 (locus plu_1232). A search of available microbial genomes revealed a number of Hyp1 orthologs the genes of which are found in a conserved gene cluster known as Nit1C. Independent studies revealed that in addition to Hyp1, Pf11764 possesses a gene (nit) specifying a nitrilase enzyme whose closest homologue is a nitrilase found in Nit1C gene clusters (77% amino acid identity). DNA sequence analysis has further revealed that indeed, hyp1Pf11764 and nitPf11764 are contained in a cluster that includes also a gene specifying an oxygenase. Given the possible connection of Nit1C-endoded nitrilase and oxygenase enzymes to enzymatic cyanide degradation, there is strong reason for thinking that the genes specifying these enzymes contribute to bacterial growth on cyanide in those bacteria containing the Nit1C cluster. Because the biological function of the Hyp1 protein is currently unknown, it was cloned and the protein expressed in E. coli so that its properties could further be explored. Unfortunately, the expression of the protein in an insoluble form complicated these analyses. However, at least two lines of evidence suggest a possible role as a regulator of gene expression. First, over-expression of the protein was accompanied by the parallel elevation of the putative vector-encoded b-lactamase, implying that Hyp1Pf11764 can affect the expression of other genes. Second, a comparison of the amino acid sequence of select peptide fragments of Hyp1Pf11764, by conducting searches for homology with proteins in the existing nonredundant protein database, consistently revealed motifs in common with those present in bacterial response regulators that are part of two-component signal transduction systems widely distributed in bacteria.

M.Sc. / Atmospheric emissions of dust and chemicals from gold mine tailings are ongoing environmental management issues on the Witwatersrand, where residential areas have developed in proximity to dumps. Residual amounts of cyanide, used as a chemical in the gold extraction process, are deposited with the mineral tailings. This study deals with the evaluation of population exposure to cyanide vapour emitted from gold mine tailings dams, an issue of environmental impact and public health concern. The first part of this study deals with the determination of the emission factors of cyanide vapour from three selected gold mine tailings dams: one under reclamation; one no longer operational; and one fully operational as a slimes deposition site. These three sites are located in the Boksburg area of the Ekurhuleni Metropolitan Municipality. Twelve samples were collected in all: six from the Cason (2628AAL91) mine dump (under reclamation in 2006); four from the ERPM tailings complex [comprising the dormant deposits 4/L/47, 4/L/48, 4/L/49 and 4/L/50, which are north of the N17 national highway; and the 2628ACL1 deposit, which is south of the N17]; and two from the Rooikraal (active) deposition site. All samples were collected in the year 2006. In the second part of the study, the obtained emission factors were used in an Industrial Source Complex dispersion model to measure the degree of population exposure to cyanide vapour for communities that are in proximity to the operational gold mine tailings dams in the Johannesburg Metropolitan Municipality. In the third part, a health risk assessment was carried out for communities in proximity to the selected gold mine tailings dams. The study revealed the average flux (± std. dev.) of Cason to be (4.7 ± 0.8) x 10-7 g m-2 s-1, the average flux of ERPM (3.9 ± 0.6) x 10-7 g m-2 s-1, the average flux of Rooikraal (1.7 ± 0.2) x 10-5 g m-2 s-1. The concentrations of cyanide vapour emitted from the older tailings deposits (sand) that are under reclamation and the dormant slimes dams are very low (and pose little health risk to occupants of adjacent land). However, HCN emission factors from operational tailings dams, which are receiving continuous depositions, are 3 to 80 times higher than the emission factors from dormant mine tailings. Ambient cyanide concentrations near the active tailings deposits may sometimes exceed selected international health standards. The average hourly, daily and annual dispersion model calculations for the Nasrec tailings deposition sites in Johannesburg revealed that 117 938, 18 722 and 8 130 people respectively were exposed to hydrogen cyanide concentrations that were above international (Ontario, Canada) standards. This raises environmental concerns that require institution of cyanide monitoring and setting of emission limits applicable to South African legal and environmental circumstances. These conclusions are based on a small number of emission factor determinations and hence these findings should be regarded as provisional. Further testing and verification are required for emission factors from a larger number of dormant, reworked and active tailings dams.

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

Over the last 100 Years, the cyanidation process has been the most popular method for recovering gold from its ores. Despite this, there are still efforts to improve the efficiency of the process, particularly as ores become more difficult to treat. Many investigators have studied the cyanidation process, although a large proportion of these studies have obtained contradictory results. This thesis presents a kinetic and electrochemical study of the leaching of gold in cyanide solutions, and emphasis is placed on rationalising the conflicting results which have been published in the past.The leaching rate of gold was measured using a rotating electrochemical quartz crystal microbalance, an instrument which allows the simultaneous measurement of electrochemical data and mass changes at the solid-solution interface in real time. A proportion of this project was devoted to the on-going design of this instrument, and a number of modifications are discussed in detail. Initially, the leaching of gold in cyanide solutions was investigated under conditions of high purity. Under these conditions, it was found that the gold surface is blocked by a passive film, presumably AuCN. The presence of such a film results in the reaction being chemically controlled, and under typical cyanidation conditions (4 mM cyanide, pH 10.0), the rate of dissolution is very low. These kinetic results were supported by complimentary electrochemical studies, which showed that gold is passive in the potential region where cyanidation occurs.The second part of this thesis presents a study of the effect of system purity on the leaching of gold in cyanide solutions. Solution phase purity was investigated by adding controlled amounts of lead or silver to the leach solutions. It was found that in the presence of low concentrations of lead, the dissolution of gold in 20 mM cyanide solutions was oxygen ++ / diffusion controlled (as compared to chemical control for gold in the absence of lead). However, high concentrations of lead were found to be detrimental to the leaching process. It is believed that the role of lead is to modify the surface by cementation, hence reducing the effect of the passive film. Silver was also found to be effective at reducing passivation, and the role of silver believed to be similar to that of lead. It was found that unlike lead, high concentrations of silver are not detrimental to the dissolution of gold in cyanide solutions.Solid phase purity was also found to be important in the leaching of gold, and it was found that the leaching of a gold sample which contains 1 % silver is diffusion controlled. This finding is important from an industrial viewpoint, as most native gold contains some silver. Consequently, attempts were made to rationalise the leaching of gold/silver with current plant practice. Discussion on the effect of cyanide and oxygen concentrations, temperature and lead addition is presented.

Owing to the favored Coulombic attraction between the ammonium group and anion which stabilizes the B-F/B-CN bond against heterolysis, cationic borane [25]+ has great affinity toward anions than its neutral analog, and is capable of capturing fluoride or cyanide from water under bi-phasic conditions. By placing the fluorophilic silyl group adjacent to an electrophilic carbocation, a novel fluoride sensor [45]+ was obtained. Sensing occurs via a fluoride induced methyl migration from the silicon to adjacent electrophilic methylium center which is unprecedented. As a result of its strong fluoride affinity, [45]+ is able to react with KF in aqueous media at pH 7.0. The electrochemistry study of these cationic Lewis boranes reveals that the cationic character of these boranes serves to decrease their reduction potential and increase the stability of the resulting radicals. In this part of the research, we have prepared a cationic borane [27]+, which features two reversible reduction waves at -0.86 and -1.56 (vs. Fc/Fc+) corresponding to the formation of stable neutral and anionic derivatives. The one-electron reduction of [27]+ leads to the formation of a boron containing neutral radical featuring an unusual boron-carbon one-electron π bond. Further reduction of 27• results in the formation of the borataalkene derivative [27]-, which features a formal B=C double bond. The structural changes accompanying the stepwise population of the B-C π-bond are also determined, and this sequential population of B-C π-bonding orbital is also supported by theoretical computations. In order to understand the impact of the cationic nature of these boranes on their oxidative power, three novel cationic boranes ([34]+, [35]2+, and [36]3+) have been synthesized and their oxidative power were examined via cyclic voltammetry. The CV data of these compounds shows that the reduction potential of these triarylboranes is linearly proportional to the number of the pendant cationic substituents. Substitution of a mesityl group by an ArN+ group leads to an increase of the reduction potential by 260 mV.