|Monday, July 18
Development of a new assay technique for the analysis of gold using a cyclotron for neutron activation analysis
* Michael Campbell, Lakehead University, Canada
Adrianna Tikka, Lakehead University, Canada
* Nathaniel Oriecuia, Lakehead University, Canada
Neutron activation (NA) is a well-established technique for the non-destructive analysis of gold containing samples. This approach traditionally requires irradiation of the sample in a nuclear reactor followed by gamma counting. Nuclear reactors are extremely efficient at neutron irradiation and have a very high thermal neutron density, unfortunately the number of available reactors is limited, and irradiation time is costly, and difficult to access. There are however an increasing number of cyclotron facilities producing medical isotopes across Canada. A by-product of the production of these isotopes is a flux of neutrons that normally goes unharnessed. This research looks to puts those neutrons to work for the non-destructive analysis of gold.
Evaluation of the Effectiveness of Cyanogenic Bacteria to Leach Gold
* Nancy Perreault, National Research Council of Canada, Canada
Sarra Sib, National Research Council of Canada, Canada
Mario Drapeau, Agnico Eagle Mines Limited, Canada
Cyanidation at an alkaline pH is the most common process used in metallurgy to extract gold from ores and concentrates. Cyanidation is carried out at a pH greater than 10.3 to maximize the concentration of cyanide (CN-) and to prevent its volatilization in the form of hydrogen cyanide (HCN). Cyanidation residues produce highly alkaline sludge rich in CN-, in stable metal cyanide complexes and cyanide transformation products. In recent years, the idea of using cyanogenic bacteria, i.e., that produce cyanide, has emerged as an alternative to the chemical leaching of gold. Most studies on bio-cyanidation have focused on understanding the natural phenomenon and, more recently, on gold leaching from electronic waste. The concept of bio-cyanidation is still at a low technology readiness level and the objective of this R&D effort was to isolate bacteria with suitable properties to carry out gold extraction under alkaline conditions. In this study, alkali-tolerant bacteria were isolated from samples collected at various mine sites. Among the 70 bacterial isolates screened, two were shown to produce cyanide (isolates AE-27 and AE-45). Both strains were phylogenetically closely related to Pseudomonas putida. AE-27 and AE-45 grew at a pH up to 9.5, at temperatures up to 37oC, and salt concentrations (NaCl) up to 6%, which was slightly better than what was observed for a known cyanogenic strain, Pseudomonas protegens Pf-5 (ATCC BAA477), which did not grow at pH 9.0 and above. Strains AE-27/AE-45 could grow in the presence of high ore concentrations with no observable toxicity at the highest concentration tested (500 g/L). Parameters that can influence cyanide production were evaluated (glycine concentration in the culture medium, pH, ore pulp density, temperature) in shake flasks. Results showed an optimal glycine concentration of 5 g/L, at a pH of 9.0, and temperature of 25oC. AE-27 could produce 16 mg free CN/L at pH 9.0. Gold extraction by AE-27 reached 14% after 48 h of incubation at 5% pulp density, pH 9.0 and at room temperature (~22oC). Gold in solution was below the ICP-MS detection limit when incubation was prolonged to 120 h in an attempt to solubilize more gold. We suspect that this was due to biodegradation of the CN-Au complex. A microbial consortium was enriched at pH 10.3. High-throughput sequencing of the consortium showed the dominance of cyanide-producing and cyanide-degrading bacteria of the Brevundimonas and Achromobacter genera. A strain of Brevundimonas isolated from the consortium was shown to produce small amounts of cyanide, but was particularly efficient towards degrade it. Overall, the results showed that bacteria isolated from our consortium successfully dissolved gold from a crushed ore sample at relatively high pH without any adaptation stage. Efforts will now focus to extend the biocyanidation period over 48 h and improve cyanide yields using bioreactors.
Planning and realization of a plant for the recovery of gold from thin coatings by hydro-biotechnological methods
* Jens Markowski, Brandenburg University of Technology, Germany
Anja Lohse, m&k gmbh, Germany
Javier Garcia, Rhode+Wagner GmbH, Germany
Production waste from the manufacture of printed circuit boards often has gold-containing contact strips and dots whose gold content is very low. Recycling of these gold coatings by means of conventional melting processes is technically hardly possible, since the carrier materials often contain mechanical reinforcements and flame retardants in addition to thermosetting plastics. Furthermore, there would be significant gold losses in the smelting process. Together with two industrial partners, at BTU in the last years a technology was developed, which can be used to decoat gold-bearing waste from PCB-production and contact stripes using biotechnological methods. As a result, a complete and separate recovery of the gold tinsel and the carrier material copper with high purity is possible. The bioleaching process is realized with iron and sulfur oxidizing bacteria, especially Leptospirilum ferrooxidans and Acidithiobacillus ferrooxidans. The first pilot plant with a planned throughput of nearly 2000 kg per year is currently in realization. The biotechnological apparatus consists of a closed leaching reactor (with a capacity of approx. 300 liters of fluid) as the core element and the peripheral equipment (fermenter, cementation reactor, filter systems etc.). The plant is designed as a compact unit. All vessels are made of plastic and are closed. Pipes, fittings, filters, etc. are also made of plastic and have a modular design. The fluid-carrying vessels are moderately ventilated and are therefore equipped with an exhaust system. Compared to processes using inorganic acids, shorter leaching times can be achieved, partial regeneration and multiple use of the bioleaching solution is possible (over 12 times/charges with the same leaching solution after a short recreation period). After optimizing the conditions, a process time of only 60 hours per leaching-charge is possible. Over a period of 2-3 days, the microorganisms dissolve the copper layers present as gold carriers on the polymer. The dissolved gold flakes are filtered off and the leaching liquid containing copper is fed to the next process stage. There, during approx. 3-4 days, the dissolved copper is recovered by feeding metallic iron (with a sacrificial anode). The microorganisms present in the solution accelerate this process. All process steps take place at moderate temperatures (< 40°C) and in a slightly acidic environment. The separated gold tinsel as main product with a content of over 900 mg/g Au is a suitable input for the precious metal smelter. The copper precipitates in metallic form and can be recovered in high purity (> 90%) as a by-product. Due to the biological leaching and the resulting concentration of the gold components, only < 1 wt.% of the waste containing precious metals has to be thermally treated. This can significantly reduce the CO2-emissions from the gold recycling process compared with thermal processes and with conventional gold mining.
Recovery of Gold from Synthetic Thiosulfate-based Leach Solutions, by Dielectric-barrier Discharge (DBD) Plasma Treatment
* Jean-François Sauvageau, Université Laval, Canada
Natalia Milaniak, Université Laval, Canada
Marc-André Fortin, Université Laval, Canada
Cyanide is used as a leaching agent for gold extraction by the mining industry. Despite its high efficiency, cyanide is a toxic chemical associated with risks for workers and the environment. Usually, gold is complexed by free cyanide in solution and selectively recovered by adsorption on activated carbon. However, activated carbon must be periodically regenerated in rotary kilns at high temperature, which comes with significant greenhouse gases emissions. These drawbacks motivate the development of cyanide-free leaching systems, such as thiosulfate leaching. However, gold-thiosulfate complexes must be recovered by resins, which are expensive consumables and subjected to strict operating conditions. Recovery techniques for gold-thiosulfate complexes must be developed in order for thiosulfate to become an economically viable alternative to cyanide. In this study, an atmospheric plasma process was developed to recover gold-thiosulfate complexes from pregnant leach solutions (PLS). Plasma technologies have been demonstrated efficient to selectively recover noble metals from PLS.[1-2] A new prototype of dielectric-barrier discharge plasma reactor was developed. Synthetic gold-thiosulfate PLS (5.9 ppm Au, 321 ppm Cu, 52 g/L thiosulfate) were plasma-treated (95%Ar+5%H2, 14.5 kVpp, 25 kHz). The plasma reduces Au and Cu ions into gold-containing nanoparticles (NPs), a value-added product. The recovery of gold and copper was evaluated by microwave plasma-assisted emission spectroscopy (MP-AES), while the hydrodynamic diameter of NPs was measured by dynamic light scattering (DLS). To determine the effect of plasma treatment on thiosulfate oxidation to polythionates, thiosulfate content was measured by ionic chromatography before and after treatment. Fundamental parameters of the plasma (electron density, electron temperature, power) were calculated from optical emission spectroscopy (OES) spectra and current-voltage waveforms. MP-AES results revealed a rapid and strong reduction of gold (81.4 ± 2.3%) and copper (90.8 ± 0.5%) ions to NPs. Despite an initial Cu/Au ratio of 55/1 in the leach solutuions, Au recovery was comparable to copper recovery in the final NP sediment (1:1) which suggests a selectivity for Au. DLS results demonstrated that NPs form agglomerates (100 nm). Thiosulfate concentration decrease was minimal (0.5%), which indicate that metastable species generated by plasma react with noble metals rather than thiosulfate. OES spectra of discharges allowed for detection of electronic transitions related to argon, OH radicals and sulfur species. Calculations were performed from characteristic argon peaks to obtain an electron temperature of 1.234 eV and electron density of 8.94x10-15cm-3. The power of discharges was stable during experiments (39.9 ± 0.2 W), proving the robustness of the process in a pilot-plant industrial setting. The results demonstrate that Au and Cu-containing thiosulfate PLS can be plasma-treated without degrading thiosulfate. Metal NPs (Cu-Au) are generated, which can be collected for further refining. The addition of a thickener in the process flowsheet process could improve gold recovery, as unreacted ions could be recirculated under plasma discharges. Additional experiments with leach solutions provided by the industry are being performed to fully prove the economic potential of this technology. References:  Sauvageau, J.-F. and M.-A. Fortin, Hydrometallurgy, 2020. 197: p.105483.  Sauvageau, J.F., et al., Particle & Particle Systems Characterization, 2018. 35(4).
Multi-level Modelling of Gold Nanoparticles in Various Environments
* Rika Tandiana, Université Paris Saclay, France
Van Oanh Nguyen-Thi, Université Paris Saclay, France
Cécile Sicard-Roselli, Université Paris Saclay, France
Carine Clavaguéra, Université Paris Saclay, France
Gold Nanoparticles (GNPs) have been demonstrated to be promising materials across various research fields, thanks to their tunable chemical, physical, and optical properties. The complexity of GNPs systems can be partly attributed to the dynamic interactions with surface ligands and solvent. Therefore, with the precise understanding of these dynamic interactions, a rational design of materials with desired properties would be achievable. While various spectroscopic techniques have been widely employed for this effort, computational approaches have been proven to be a powerful method to provide important atomistic insights. Therefore, it is our objective to employ multi-level computational approaches in an attempt to understand the interaction of GNPs with various environments. The interfacial interaction of water on a series of increasing size GNP has been systematically probed with classical molecular dynamics and semi-empirical DFTB methodologies. The structural properties of the first solvation shell have been investigated with orientational relaxation lifetime, radial distribution functions, and the distribution of orientations of water molecules. The re-arrangement of water network to form an extended 2D hydrogen bond network has been observed as the size of GNP increases. The DFTB method was then employed to optimize the geometry and calculate the vibrational density of states (VDOS) of the first solvation shell of the GNP series. A blue shift in the VDOS corresponding to OH stretch has been observed for GNP water shell as compared to the water droplet, which further supported the observation of re-arrangement of water network. In addition to water molecules, the dynamics of interaction between GNP and surface ligand is important. Therefore, the adsorption of organic molecules on GNP has been systematically investigated at DFT level. Firstly, the geometry of a series of substituted aromatic compounds adsorbed on Au32 has been optimized. Topological analyses (QTAIM and NCI) have subsequently been performed on each of the complexes to identify the formation of multiple non-covalent interactions with varied strength. The energy decomposition analyses were further performed to find that the electrostatic and dispersive interactions were the main contributors to these non-covalent interactions. Vibrational analyses were lastly performed to investigate how the adsorption on GNP affects the IR spectroscopy of the organic compounds. Interestingly, the information on the orientation of the aromatic ring can be deduced from the variation in the intensity of vibrational mode corresponding to the CH stretch of the phenyl ring. To conclude, this work has highlighted that the interfacial water arrangement changes with respect to the size of GNP, and that the adsorption of organic ligands is driven by electrostatics and dispersion. The insight from the adsorption study at the quantum level can then be applied to refine classical force field parameters describing the interaction of GNP and aromatic ligands. Finally, it would be interesting to include these ligands in molecular dynamics simulations to probe the dynamic of the interactions and to mimic the presence of biomolecules. The influence of the solvent in the adsorption geometry of ligands on GNP could be assessed with vibrational analysis, and directly compared to experimental observation.
Towards better understanding of the gold-pyrite contacts using DFT simulations
* Dariush Azizi, Université Laval, Canada
Faïçal Larachi, Université Laval, Canada
Gold dissolution is an important hydrometallurgical route to the recovery of gold embedded in host mineral matrices. However, this process involves many complex reactions which make gold dissolution a very challenging process. Among them, the gold/hosted minerals (especially pyrite) galvanic interactions are very critical phenomena which pose perennial challenges to understanding gold dissolution. In this study, Density Functional Theory (DFT) simulations have been performed to unveil some important electronic features of gold/pyrite associations on gold dissolution. For this purpose, three typical gold/pyrite occurrences have been considered by which natural occurrence of gold in complex mineral matrices can be presented. These gold/pyrite occurrences consist of (Figure 1), 1) gold slab/pyrite slab association to emulate contactless free gold and pyrite with up to a few Å separations (model_1); 2) gold-substituted pyrite solid-solution slab to emulate atomically dispersed gold in a sulfidic matrix (model_2); and 3) pyrite slab with gold nanocluster tethered to it, in addition to isolated gold nanocluster and to pure pyrite slab (model_3). The description of these close contacts between gold and pyrite, can help us to understand better the complex behavior for gold dissolution processes [1-2]. Gold/pyrite associations for all three studied models have been shown to induce electron transfer from gold to both pyrite’s sulfur and iron atoms. Model_1 revealed that galvanic interactions between free gold and pyrite can occur up to 10 Å separation distance between the metal and the sulfide slabs. However, the efficacy of galvanic interactions decreased with increasing the inter-slab separation distances. Model_2 divulged that S-substitutive incorporation of Au in the pyrite structure resulted in expanded crystal dimensions as well as changes in electron properties of gold, iron and sulfur atoms in the solid-solution structure. Occurrence of gold nanocluster on pyrite surface (model_3) led to the formation of metal-metal Au-Fe bond and covalent Au-S bond. Sorption interactions of ligands, such as cyanide, with gold and pyrite surfaces in presence of dioxygen were investigated with the three model variants. The DFT simulations showed that CN- preferentially adsorbed on gold instead of pyrite sites provided gold/pyrite galvanic interactions deprived gold from electron density at the expense of pyrite. The cyanide adsorption energies were found to follow the sequence: model_1 with substituted Au+ > model_2 > model_3 with substituted Au+ > model_1 (short distance) > model_3 > pure pyrite (100) & model_1 (long distance contact) > isolated Au9 nanocluster (see Table 1). As Au electron deficiency via galvanic interactions or gold oxidation was enhanced, gold-cyanide interaction energy increased portending the promotion of gold dissolution. This study can lead us to better understand the complex behavior of gold-metal sulfide composites and the dissolution of the noble metal, thus helping a refined description of gold recovery from a multitude of gold-bearing mineral environments.