Advanced Materials Research Vols. 20-21

Abstract: The exploitation of mineral deposits generates large amounts of wastes such as fine size particles, known as tailings. Mine wastes are characterized by the presence of pyrite, pyrrotite, galene, sphalerite, chalcopyrite and arsenopyrite. One of the most important environmental problems derived from the treatment and disposition of mining tailings is acid mine drainage (AMD) which results from the oxidation of the sulfurous minerals contained in these tailings. The acid generated is produced by a series of complex chemical reactions; the production rate is controlled by leaching microorganisms, which are present mining processes. In this work the bioleaching kinetics of heavy metals such as Cu, Fe, Zn, Cd, Pb, Ni and As, from a Tizapa tailing sample in presence of mesophilic and moderately thermophilic bacteria were studied and compared. The microorganisms used in this work were previously adapted to the tailing sample by the conventional method of successive steps. The percentage of dissolution of heavy metals at 45°C was smaller than at 35°C and the value of parameters such as redox potential and bacterial growth increased with temperature. Bioleaching studies were compared with the corresponding control systems.

Abstract: Several studies and different successful commercial applications had demonstrated that bioleaching can be an innovative approach that is capable to provide mining industry opportunities for essential growth in the medium term[2-18]. To identify the commercial application potential of bioleaching in Iranian copper industry, a research program was initiated. The objective of the main part of this program was to evaluate bacterial leaching processes for copper recovery from (i) high grade ores and flotation concentrates and (ii) low grade ores and flotation tailings. The latest results of these studies indicates the general operability of the bioleaching in both cases. At optimum conditions, the copper extraction from low grade materials was more than 80% and that from high grade ores and flotation concentrates about 95%, which should be high enough to justify the process economically.

Abstract: The Bioshale project, involving 13 partners throughout Europe, is co-funded by the European Commission under the FP6 program. The main objective of this project (which started in October 2004) is to identify and develop innovative biotechnological processes for ‘’eco-efficient’’ exploitation of metal-rich, black shale ores. Three extensive deposits have been selected for R&D actions. These are: (i) a site (in Talvivaara, Finland) that, at the outset of the project, had not been exploited; (ii) a deposit (in Lubin, Poland) that is currently being actively mined, and (iii) a third site (in Mansfeld, Germany) where the ore had been actively mined in the past, but which is no longer exploited. The black shale ores contain base (e.g. copper and nickel), precious (principally silver) and PGM metals, but also high contents of organic matter that potentially handicap metal recovery by conventional techniques. The main technical aspects of the work plan can be summarized as: (i) evaluation of the geological resources and selection of metal-bearing components; (ii) selection of biological consortia to be tested; (iii) assessment of bioprocessing routes, including hydrometallurgical processing; (iv) techno-economic evaluation of new processes from mining to metal recovery including social, and (v) assessing the environmental impacts of biotechnological compared to conventional processing of the ores. An overview of the main results obtained to date are presented, with special emphasis on the development of bioleaching technologies for metal recovery that can be applied to multielement concentrates and black shale ores.

Abstract: Oil shales are one of the alternative sources of hydrocarbon fuels (“synthetic petroleum”), characterized by the increased sulfur and nitrogen content which represent even greater ecological problem in use, compared to classical fuels. Acidithiobacillus ferrooxidans is capable of oxidizing pyrite to iron (III)-ion, providing a strong oxidation agent at low pH. We have used this oxidizing agent for oxidation of sulfur present in DBT as a substrate model to demonstrate its potential to oxidize organically bound sulfur in oil shales. An HCl-concentrate was used as the hydrocarbon matrix. Acidithiobacillus ferrooxidans is already recognized to oxidize the pyritic sulfur component, thereby potentially providing a complete sulfur removal system. By applying GC-MS we established that DBT transformation occurred by oxidation or elimination of sulfur. The products obtained are more soluble in water than parent compounds and this reduces concentration of organic sulfur.

Abstract: Cyanogenic Chromobacterium violaceum, Pseudomonas fluorescens, and P. plecoglossicida were able to mobilize silver, gold and platinum when grown in the presence of various metal-containing solids such as powdered platinum, platinum-containing automobile catalytic converters, powdered silver, or gold-containing electronic scrap. Five percent of silver was mobilized from powdered jewelry scrap as dicyanoargentate after one day, although 96% was mobilized when non-biological cyanide leaching was applied. Dicyanoargentate proved to inhibit growth at concentrations >20 mg/L. Gold was microbially solubilized from electronic scrap (shredded printed circuit boards). Maximum dicyanoaurate concentration corresponded to a 68.5% dissolution of the total gold added. Additionally, cyanide-complexed copper was detected during treatment of electronic scrap due to its high copper content of approximately 100 g/kg scrap. Small amounts of platinum were mobilized from pure platinum powder after 10 days. The process proved to be very slow. In summary, all findings demonstrate the potential of microbial mobilization of metals as cyanide complex from solid materials and represent a novel type of microbial metal mobilization which might find industrial application.

Abstract: An overview is presented of a multi-year research effort on developing high-rate fluidized-bed bioprocesses for ferric sulfate production to be used as a unit process in various hydrometallurgical applications including indirect tank leaching of ore concentrates, regeneration of heap leach liquors and control of iron containing acidic mine wastewater. Iron oxidation rates of over 26 kg m-3 h-1 were achieved at hydraulic retention times of less than 1 h at 37 °C. Oxygen supply became the rate-limiting factor even with 99.5% dioxygen aeration. Fe2+ oxidation proceeded at pH below 1 even in the presence of 60 g Fe3+ L-1 allowing the regeneration of concentrated ferric sulphate solutions required in indirect tank leaching of sulfidic ore concentrate applications. Of several tested FBR carrier materials activated carbon was the most suitable based on its availability, long-term durability and the achieved high iron oxidation rates. Jarosite precipitates accumulating to the top of the inert carrier materials played an important role in the FBR biomass retainment. For regeneration of synthetic and actual sulfidic ore heap leaching liquors, a gravity settler was installed in the recycle line of the FBR. The system produced iron precipitates with good settling characteristics and settling tank effluent with low turbidity and suspended solids concentrations. These results revealed the potential of FBR process in both heap leach liquor regeneration and controlling the iron containing waste streams. The PCR-DGGE-partial seguencing of the 16S rRNA gene protocol revealed that the FBR culture at 25-37 °C remained dominated by Leptospirillum ferriphilum over a range of operational conditions studied over the years. A modeling approach for managing Fe3+ production by FBR in combination with heap leaching was based on an artificial neural network-back propagation algorithm (ANN-HEAP) and resulted in excellent match between the measured and the predicted concentrations. High-rate fluidized-bed iron oxidation is amenable to regeneration of tank and heap leaching solutions as well as controlling iron containing waste streams.

Abstract: The focus of this paper is on the development of a bioleaching process for the treatment of complex polymetallic concentrates, containing chalcopyrite, sphalerite, galena and silver, using a moderately thermophilic culture operating around 45°C. Initial development R&D work and subsequent research for process improvements carried out in bench-scale piloting facilities will be discussed.

Abstract: In the frame of a European project (BioMinE - FP6), a continuous bioleaching operation was carried out in a laboratory-scale unit using a cobaltiferous pyrite. The objective of the work was to use this system to investigate mechanisms of microbial activity and mineral oxidation in continuous stirred bioreactors (1x50L - 3x20L). A combination of scientific and technical approaches (molecular ecology, biochemistry and microscopy) was used and various key operating parameters were tested (temperature, nitrogen source, CO2 availability, designed consortia). An increase of temperature of 10°C (35°C to 45°C) had no major influence on the bioleaching efficiency. When the ammonium source was limiting, there was a negative influence on both bacterial growth and bioleaching efficiency. This result was related to a combination of factors such as less bacterial attachment to the pyrite surface and less precipitate formation. CO2 limitation had a very significant negative effect on the bacterial productivity and consequently on the bioleaching efficiency. Nevertheless, the population composition remained unchanged. An important decrease of EPS (sugar) production was also observed. The bacterial strains, that dominate the culture, originated from the deposit in Uganda. It seems that their bioleaching ability was improved over the time when cultured in continuous mode. The culture composition was very stable. The iron-oxidizer L. ferriphilum was the dominant organism in standard (not limiting) conditions, and was always very well represented during the first 3-4 days of residence time. Sulfobacillus sp. BRGM2 also played an important role in the process. This study gives new insights for the application of this technology, and more specifically on the influence of key operating parameters on bioleaching performances, population dynamics and attachment of bacteria to the solid surfaces.

Abstract: Identifying influential factors in the bacterial leaching of nickel laterites using a mixed culture of chemolithotrophic micro-organisms was explored using the approach of statistical design of experiments. In a series of experiments, pH, particle size, pulp density, type of substrate and inoculum size were statistically combined using a quarter fractional factorial designs 2 5−2 III and tested for their influence on nickel recovery using chemolithotrophic microorganisms. The results indicated that inoculum size was not statistically significant while the rest of the factors were statistically significant. Under the ranges studied the interaction between the variables was found to be weak. The results also showed that recovery was maximized at low pH and low pulp density. In the range studied, particles of less than 38μm had a negative influence on nickel recovery. Sulphur substrate also showed better effects than pyrite.

Abstract: A model was prepared to study the performance of a thermophilic bioleaching heap that employs mixed mesophilic and thermophilic microbes for copper extraction from CuFeS2. Mesophiles’ preference for and ease of dissolving additional FeS2 provided to the heap enables the transition from a mesophilic to a thermophilic bioleaching state without the necessity of additional energy supply. In this sense, the mathematical description of the bioleaching process is done taking into consideration the dependency of both microbes’ biological states on physicochemical factors such as the temperature and O2 availability. With regard to the flow rates of the liquid and air phases, simulation results have shown that these flow rates govern not just the heat transfer and variation of cell distribution, but also the leaching rate regardless of the fraction of CuFeS2 per FeS2 leached (FCP) which is the other variable influencing to the heat accumulation in the heap.