Solid State Phenomena Vol. 262

Abstract: Bioleaching processes are usually open systems where introduced and native microorganisms survive to changes in pH, temperature, salt and metal concentration, among others. Spatial and temporal description of the microbial community could be relevant for better comprehension of copper extraction process and help in the development of operative procedures to improve the metal extraction. We performed metagenomics and high-throughput sequencing of 16S rRNA genes analyses on samples from Escondida mine bioleaching heap and laboratory columns tests. Archaeal community structure in samples was assessed using three pairs of Archaea-specific primers, and results were highly depending on the primers pairs used. Similarly, three pairs of Bacteria-specific primers were used to assess the bacterial community. Moreover, according to the metagenomics analysis, At. thiooxidans, F. acidarmanus, Leptospirillum spp., Acidiphilium sp. JA12-A1, Acidiphilium spp., At. ferrivorans, and Leptospirillum ferriphilum were the most representative microorganisms. The repercussion of the different methodologies and outputs in the characterization of the bioleaching microbial community is discussed. A better understanding of the microbial community in bioleaching processes could improve the analysis regarding environmental changes in the heap process, its metallurgical performance and, can be used to assist in the decision-making process.

Abstract: In-situ realtime method that can monitor the target bacteria should be used to determine the real situation of the bacteria in deep parts of heaps in heap bioleaching plants. This study suggest to apply flow cytometry technology to in-situ realtime monitoring of target bacteria. Flow cytometry is a method that can rapidly quantify the bacterial cells in bacterial suspension based on the detection of lights that are emitted from bacterial cells. In this study, we estimated the possibility of the application of flow cytometry to the selective detection of target bacteria. The bacterial culture solution that had been diluted by water including other bacteria was provided for fluorescence spectral analysis and scattered light analysis that were functions of flow cytometry. Our target bacteria could be selectively detected by those analyses in this study, therefore, it was shown that the flow cytometry could be useful for detecting target bacteria selectively. Because the measurement principle of flow cytometry is quite simple, it can be expected to be installed into deep heaps through the monitoring wells and determine the dominance of target bacteria in-situ and realtime in the future.

Abstract: Microbial association with and colonisation of mineral surfaces plays a key role in enhancing the extraction of metals from ores during heap bioleaching processes. On the other hand, if uncontrolled, the same association can also lead to the generation of acid rock drainage (ARD) effluents from mine waste. This study aims to measure microbial metabolic activity of a mixed mesophilic culture on the surfaces of pyrite-bearing waste rocks of different grades over time. The waste rocks are milled, size fractionated and coated onto glass beads, to provide a defined surface area. The metabolic activity on the mineral surface is measured with isothermal microcalorimetry (IMC) complemented with scanning electron microscopy (SEM) and analysis of solution chemistry to measure leach agents and metal release into the pregnant leach solution (PLS). The waste rock samples showed a similar degree of leaching when the solution chemistry was analysed, despite having different sulphide content. However, when metabolic activity of the micro-organisms on the mineral surface was measured, greater activity was seen with higher sulphide content. This data informs an ongoing study to establish a flow-through configuration of the biokinetic test for ARD prediction accounting for both leach solution and microbial-mineral interaction as well as differing kinetics of acid-neutralising and generating reactions to enable the refinement of the current batch method.

Abstract: In this study, a dynamically-controlled column was used to evaluate two ores known to cause heap overheating. This enabled the simulation of heap self-heating under controlled conditions. The lixiviant was inoculated with a consortia of mesophilic and moderately thermophilic microorgaisms, and the impact of rapid temperature increases on biological activity and cell numbers was evaluated. During the leaching of ore sample A, the temperature lagged for 29 days before increasing rapidly from 26 to 88 °C. Cell numbers and solution potential increased concomitantly, before both were reduced as the temperature increased past maximum microbial tolerances. Cell numbers began increasing again within 10 days of reaching temperatures that would facilitate mesophilic growth being restored. During the leaching of ore B, the temperature lagged for 4 days before exhibiting a rapid increase in temperature, increasing from 30 to 76 °C over a six-day period. Cell numbers were reduced with the sudden temperature increase, and did not recover over the remainder of the experiment.

Abstract: The thermophilic archaean, Acidianus brierleyi, was examined for its feasibility to bioleach copper from a low-grade chalcopyrite ore (1.15 % copper, 20.4 % iron and 2.63 wt% sulfur) at 65°C and pH 1.8-2.5. The chalcopyrite leaching was markedly accelerated in the presence of A. brierleyi, and an extremely high 80% leaching of copper in the low-grade ore (25-38 μm particles) was achieved in 14 days in a batch stirred reactor. By comparison, the leaching of iron was very slow and only a slight 5 % iron was leached in 14 days in the presence or absence of A. brierleyi. In other words, A. brierleyi selectively leached chalcopyrite while magnetite leaching by A. brierleyi was negligible. Moreover, bioleaching of the low-grade ore (53-75 μm particles) yielded 55% copper recovery after 20 days of operation in a column reactor. The good results for the copper bioleaching in the column reactor are very similar to those in the stirred reactor. These results lead to the conclusion that the thermophile bioleaching with A. brierleyi is attractive as an economical and environmentally friendly process for good copper extraction from low-grade chalcopyrite ore.

Abstract: In bioleaching processes using autotrophic bacteria, CO₂ is the carbon source for the growth of the microorganisms and its availability is dependent on gas mass transfer. The objective of this study was to investigate the demand in CO₂ in complex copper concentrate bioleaching operations and to optimize CO₂ supply. Batch tests in 2L-stirred reactors at 10%_w/v solid load were performed to study the need for CO₂-supplementation and to determine the adequate CO₂ partial pressure in the gas inlet. The results show that Fe oxidation (and thus microbial activity) is delayed when air is injected without CO₂-supplementation. CO₂-supplementation improves leaching kinetics since Cu dissolution rate increases from 84 mg/L/h with air solely to 120 mg/L/h when CO₂ is added to air. The study proposes also a methodology to determine G/L transfer components and to asses CO₂ limitations in the system. It shows that the microorganisms are not only sensitive to the transfer rate of CO₂ from the gas to the liquid phase, but also to the availability of CO₂ in solution.

Abstract: Heap and dump leaching of sulphide minerals have become well-established techniques for the processing of low grade ores, especially of copper, over the past 30 years. The oxidative dissolution of sulphides in heaps can be significantly enhanced by microbial colonies, but the complexities of the heap leach process overall often counteract the potential advantages, or prevent microbial colonisation and bioleaching in the first place. This overview discusses the multiple layers of complexities that govern percolation leaching processes, such as the interactions between mineral grains, particle pores and leach solution, microbial responses to solution chemistry typical of heaps, solution and solute transport in heterogeneous unsaturated ore beds, as well as heap aeration and microbial response to CO₂ supply. It becomes clear that economically successful heap bioleaching hinges on careful engineering and operation of the heap process as a whole to create an environment in which microbial colonies can thrive and the value metal is released sufficiently rapidly into solution.

Abstract: Molecular approaches to studying the microbiology of copper bioleach heaps have greatly increased our knowledge of the microbial diversity in these environments. Nevertheless, technical and operational challenges exist that prevent obtaining samples from commercial-scale heaps necessary to improve our understanding of the temporal changes that occur during heap maturation and the progression of copper leaching.

Abstract: The purpose of this work was to investigate the phenomenon of microbial iron reduction in industrial minerals and materials. These materials are generally not pure, often associated with impurities usually in the form of Fe³⁺. In all cases, the presence of iron affects the colour and the physical properties of the mineral and therefore lowers their industrial value and limits their application. In this study bentonite, kaolin and quartz sand sample were used for the experiments and compared in effectiveness of iron dissolution. The experimental results showed that after 30 days of bioleaching process, bacteria are able to remove 9.29% of Fe occurring in the kaolin sample (K-I) in amorphous form of oxyhydroxides and approximately 12% of Fe from the bentonite sample (B-JP) also in amorphous form of oxyhydroxides. In the quartz sand sample C3-15D, the concentration of Fe decreased by 15% after 15 days of bioleaching process and in the sample C3-30D after 30 days of bioleaching process by 24.7%.

Abstract: To improve the efficiency of sustainable recovery of valuable resources (such as copper, gold, uranium), advanced technologies of electrochemistry, surface chemistry and materials characterization, such as Synchrotron X ray-absorption spectroscopy (XAS), X-Ray photoelectron spectroscopy (XPS), and in situ X-ray Diffraction (XRD) have been employed to investigate the interactions between microbes and sulfide minerals in situ to gain a detailed understanding of the chemical reaction mechanisms operating in bioleaching. It has been demonstrated that alternating current Scanning Electrochemical Microscopy (ac-SECM) allows for straightforward characterization of sulfide mineral samples in aqueous solution. With the technology localized electrochemical properties of the mineral surface including the surface current mapping can be achieved. Robust chemical sensors have been developed for real time, in-situ monitoring of key leaching process parameters.