Alkaline amino acids in gold leaching: chemistry and application to Witwatersrand tailings
| dc.contributor.advisor | Tadie, Margreth | en_ZA |
| dc.contributor.advisor | Akdogan, Guven | en_ZA |
| dc.contributor.author | Tapfuma, Anthony | en_ZA |
| dc.contributor.other | Stellenbosch University. Faculty of Engineering. Dept. of Chemical Engineering. | en_ZA |
| dc.date.accessioned | 2025-04-25T12:06:04Z | |
| dc.date.available | 2025-04-25T12:06:04Z | |
| dc.date.issued | 2024-12 | |
| dc.description | Thesis (PhD)--Stellenbosch University, 2024. | en_ZA |
| dc.description.abstract | In gold processing, high-grade gold reserves with readily accessible gold deposits are being exhausted thereby increasing interest in the mining and processing of low-grade gold sources. The readily available low-grade gold sources are gold tailings, which are abandoned and abundant all over the globe. The processing of abandoned gold tailings is a crucial component of gold processing which can benefit economic and environmental perspectives. Currently, the cyanidation process is used in extracting gold from tailings due to its high selectivity and the ability to form very stable gold complexes, which leads to high gold recoveries. Despite the popularity of the cyanidation process, it poses environmental hazards, leading to the search for alternative lixiviants for the leaching of gold tailings. Amino acids have been identified as environmentally benign alternatives for leaching gold. Therefore, this study investigated the use of glycine, alanine, cysteine, and histidine in the leaching of gold, focusing on understanding the novel complexation/bonding chemistry, pure gold dissolution kinetics and the application in the leaching of Witwatersrand tailings. In fulfilling the investigations, a number of objectives were achieved and these included : (i) to assess the current status of amino acid utilisation in gold leaching, (ii) investigate gold-amino acid complexation through Fourier-transform infrared spectroscopy (FTIR), (iii) utilise Density Functional Theory (DFT) calculations to investigate gold-amino acid complexes, (iv) investigate pure gold dissolution conditions, (v) analyse gold electrochemical oxidation through linear sweep voltammetry, (vi) evaluate leaching of gold from Witwatersrand tailings, and (vii) propose a leaching mechanism for the study. The literature review covered the current status of amino acid use in gold leaching, looking at potential studies conducted and possible applications in secondary sources. The FTIR analysis showed that all four amino acids interacted with gold using the anchoring bonds on their carboxylic and amine ends. In addition, cysteine showed an extra bond using the sulphur in its R group. Furthermore, the FTIR work demonstrated that all four amino acids exhibited high bond shifts on the amine bond (NH2, NH3+) and carboxylic (COO-) bond at deprotonation pH compared to isoelectric pH. DFT calculations were done to predict the different complexes that different amino acids can form with gold. The geometric optimisation done showed that bonding of amino acids and gold was possible through donor atoms such as N, O and S. The bonding/complexation energy analysis also showed that the deprotonated amino acids can easily form complexes with gold compared to zwitterion, and this supported the FTIR finding. The thermodynamic behaviour investigations showed that the deprotonated amino acid complexes with gold were more stable than their counterparts. In addition to the FTIR interactions and DFT work, dissolution showed that gold dissolution was higher for the deprotonated system than for the isoelectric system (system containing the zwitterion). Moreover, the dissolution trend for the deprotonated amino acids was found to be cysteine> alanine> glycine> histidine, which was in line with the FTIR peak shift trend and the DFT bonding energy trend for the complexes. Furthermore, tests were done to improve gold dissolution, and these investigated oxidant type, amino acid effect, copper effect, oxidant concentration and pre-oxidation effect. Results showed that potassium permanganate was the best oxidant for glycine, alanine and cysteine, whereas hydrogen peroxide was the best oxidant for histidine. Furthermore, gold dissolution was enhanced with increasing amino acid concentration, copper concentration, and oxidant concentration, as well as the introduction of the pre-oxidation stage of pre-oxidising the pure gold. In addition, the electrochemical oxidation work also qualitatively supported the findings from the pure gold dissolution, showing that gold oxidation was enhanced by increasing variables investigated in the pure gold dissolution studies. Preliminary tests done on tailings leaching yielded low extraction leading to design experiments that improved the extractions. The Design of Experiment (DOE) was limited to alanine and glycine tests due to their promising ability in the pure gold dissolution and preliminary tailings tests. Results from the DOE showed a maximum 91.4% and 94.8% gold extraction upon using alkaline glycine and alanine, respectively at low solid liquid ratio after pre-oxidation of the tailing. Finally, the work gave a relationship between FTIR, DFT, gold dissolution and electrochemical oxidation of gold followed by proposing a mechanism for this work. The mechanism showed that potassium permanganate dissolved Au to Au+ in the pre-oxidation stage which could be a promising pretreatment method. | en_ZA |
| dc.description.version | Doctoral | en_ZA |
| dc.format.extent | 311 pages | en_ZA |
| dc.identifier.uri | https://scholar.sun.ac.za/handle/10019.1/131925 | |
| dc.publisher | Stellenbosch : Stellenbosch University | en_ZA |
| dc.rights.holder | Stellenbosch University | en_ZA |
| dc.title | Alkaline amino acids in gold leaching: chemistry and application to Witwatersrand tailings | en_ZA |
| dc.type | Thesis | en_ZA |