Advanced Materials Research Vol. 825

Abstract: In the bioleaching of uranium ore, fluoride that ever accreted with the ore floats into solution, inhibiting the growth and metabolic processes of microorganism or even killing them, further reducing the efficiency of uranium leaching. In order to study the fluoride tolerance of the bioleaching microorganisms, five typical bioleaching strains including Acidithiobacillus ferrooxidans ATCC 23270, Leptospirillum ferriphilum YSK, Sulfobacillus thermosulfidooxidans ST, Acidithiobacills thiooxidans A01, Acidithiobacills caldus S1 were selected to compare their growth fluctuation and their oxidation rate of iron or sulfur in the presence of different concentration of fluoride. The results showed that At. ferrooxidans ATCC 23270 held the best fluoride tolerance, while S. thermosulfidooxidans ST took the worst. In order to explore the mechanism of microbial resistance to fluoride, whole-genome array (WGA) was used to analyze the genome-wide expression profiling of At. ferrooxidans ATCC 23270 upon 4.8 mM fluoride stress. The results showed that gene categories closely related to fluoride tolerance include cell membrane, energy metabolism, transport and binding functions of proteins, DNA metabolism, cell processing, synthesis and transportation of protein and other functional and metabolic pathways. Most of the genes associated with iron-sulfur metabolic system of At. ferrooxidans were highly expressed during 10 to 120 minutes, while in the long-term stress for 4 hours, the gene expression returned to normal level or even were down regulated. All these results provide useful information for further research on domesticating bacteria and increasing the efficiency of uranium leaching.

Abstract: The design of the industrial copper bioleaching plant at Escondida mine involves the recycling of the solutions in the system, with a consequent increase of ionic solutes in the raffinate solution and Pregnant Leach Solution (PLS). It is known that elevated concentrations of solutes in these solutions have a negative effect on the microbial population leading to decrease the dissolution of copper containing ore. To study the effect of the ionic load on the microbial oxidizing activity, a regression model was built using the systematic data obtained for the concentrations of sulfate, aluminum, chloride and manganese in solutions, during five years of operation. Then, this model was used to support a design of experiment (DOE). The DOE revealed an important effect on the microbial activity produced by sulfate and/or aluminum, decreasing the activity to minimum values, but the effect of chloride and manganese were not significant on the microbial oxidizing activity. This work highlights the importance of determining operational limits for the ionic load of biomining solutions in order to prevent future inhibitions of the microbial oxidizing-activity.

Abstract: Bacillus subtilis was used to demonstrate microbially induced selective flocculation to separate kaolinite and hematite. In neutral pH range of 7 - 8, 90 - 95% of hematite was selectively flocculated whereas 80 - 85% of kaolinite was dispersed using hematite - grown cells. Hematite- grown cells exhibited significant adsorption onto hematite than onto kaolinite, compared to unadapted cells. Kaolinite grown Bacillus subtilis secreted significant amounts of mineral specific proteins which conferred surface hydrophobicity whereas hematite-grown cells secreted more polysaccharides rendering hematite hydrophilic. Bacterial extracellular protein (EP) was isolated and the protein profiles of bacteria grown in the absence and presence of minerals were established.

Abstract: Marine hydrothermal polymetallic sulfide ores contain high amounts of valuable metals such as Cu, Pb, Zn, Au, Ag, as well as In, Ge, Bi, and Se. Samples from a site in the Indian Ocean were taken during a BGR ship cruise, crushed and sieved for bioleaching experiments to reveal the extraction of the various metals. Chalcopyrite was the main mineral, the total copper content was 38.5 %wt. Comparative bioleaching with mesophilic, moderate thermophilic and thermophilic acidophilic iron- and sulfur-oxidizing bacteria and archaea was investigated. Batch culture experiments were conducted at 2% (w/v) pulp density in shake flasks in the presence of Acidithiobacillus ferrooxidans, Acidiphilium sp. and Acidithiobacillus thiooxidans as mesophiles (30°C), a mixed culture of moderate thermopilic iron- and sulfur oxidizing bacteria (50°C) and the thermophile Acidianus brierleyi (70°C). The results after four weeks showed most effective dissolution of copper in the presence of A. brierleyi (up to 4.3 g/l), compared with moderate thermophiles and mesophiles (3.3 g/l and 2.5 g/l, respectively). Furthermore, the bioleaching performance was approved with dissolved iron concentrations. Conclusively, an increase in temperature from 30 °C to 70 °C had a major impact on bioleaching efficiency. Copper and iron extraction efficiency occurred in the order thermophiles, moderate thermophiles, mesophiles.

Abstract: A descriptive mathematical model is a valuable tool that can help understand the relationship between the heap leaching process at the Escondida mine in Chile, the microbial community that participates in the process, and the physical characteristics of the heap, such as the arrangement and the mineral composition of the individual leaching strips. However, the bioleaching process at Escondida is a system, which presents many challenges to modelling. The main challenges relate to heap's design and mineral characteristics, the complex interactions between biological and physicochemical parameters, and the unexpected changes in the heap's operational conditions. The heap is sampled periodically and more than 20 variables, including 16S rRNA gene copy number for 16 different microorganisms, are recorded. The data exhibit complex behaviour, including variable dynamics between strips, systematic differences between lifts of the heap, and spatial and temporal correlations. In this work, we develop a non-linear descriptive model for the microbial population trajectory along the leaching cycle and across the different strips. The parameterisation of the model considers the different dynamics between lifts, and strip specific parameters characterise the behaviour of data from individual strips. The parameterisation also allows for spatial correlation by incorporating the effect of adjacent strips on the microbial population trajectory. The model is found to provide a good fit to the data and captures its behaviour across strips. Residuals showed no systematic patterns of departure between the observed and modelled response. The R² values ranged from 0.53 to 0.71, indicating a reasonable level of predictive power.

Abstract: A novel apparatus was developed to test the ferrous iron oxidation kinetics of Leptospirillum ferriphilum in predominantly sessile culture, by combining a CSTR under wash-out conditions with a packed bed of inert ceramic saddles. Results indicate that a dense culture of sessile bacteria is established rapidly, which achieves high oxidation rates in all experiments with a yield in terms of CO₂ uptake rates comparable or higher to what has been measured in planktonic culture. However, the yield in terms of cell counts changes dramatically, indicating a substantial shift in carbon utilisation in the sessile culture. The apparatus is feasible as a method to study iron oxidation kinetics of sessile cultures, providing it is operated at sufficiently high recycle ratios.

Abstract: Continuous high-rate iron oxidation and removal of jarosite precipitates from solution at low pH and ambient temperature and pressure was successfully demonstrated. The bio-catalysed iron oxidation and jarosite precipitation is promising as a unit process for a variety of hydrometallurgical process flow sheets, where it allows for iron removal from ferrous solutions without the requirement for chemical addition and with negligible base metal co-precipitation losses. The process demonstrated performance that could be used in a large scale industry unit. A two-stage airlift bioreactor (ALBR) system comprised of two ALBRs, each with its own settler, was operated for iron oxidation and precipitation at room temperature with a mixed culture of mesophilic iron oxidisers. The two-stage reactor design allowed for optimization of overall reactor kinetics by facilitating the growth of low (430 mV vs Ag/AgCl) and high (517 mV) redox potential iron oxidizers in the respective reactors. The influent (pH 1.5) contained (g L^-1) 15 Fe²⁺, 1.5 Cu, 1.5 Ni, nutrients and trace elements. The hydraulic retention time (HRT) was decreased stepwise to evaluate process performance. With the lowest HRTs (8 h in ALBR1 and 10 h in ALBR2), the overall iron oxidation and precipitation rates of the two-stage system were 0.75 ± 0.02 g L^-1 h^-1 and 0.15 ± 0.01 g L^-1 h^-1, respectively and overall iron oxidation and precipitation efficiencies of 94 ± 3% and 18 ± 1 %, respectively. The percent of influent Fe, S, Cu and Ni removed as precipitates from settlers were 30.9%, 16.7%, 1.1% and 0.2%, respectively. The precipitates were predominately comprised of (>95%) jarosite with potassium jarosite being the dominant form, followed by hydronium, ammonium and sodium jarosites. In conclusion, the two-stage ALBR system allowed efficient iron oxidation and precipitation of the oxidised iron as well settling jarosite with only minor loss of Cu and Ni via co-precipitation.

Abstract: Methods for improving the treatment efficiency of a refractory gold-bearing sulphidic concentrate are proposed. These methods consist of the oxidation of the concentrate through a two-step process: a high-temperature ferric leaching step and a subsequent biooxidation step, which further involves the use of organic nutrients. The concentrate contained 34.7% pyrite and 7.9% arsenopyrite. The biooxidation of the concentrate (in a one-step process) was conducted at 45°C in bioreactors under continuous conditions. The pyrite and arsenopyrite oxidation levels after 240 hours were 60.2 and 92.0%, and the gold recovery level by cyanidation was 65.7%. The two-step process involved the leaching of the concentrate by a Fe³⁺-containing solution and the subsequent biooxidation of the leach residue. The pyrite andarsenopyrite oxidation levels after 240 hours of biooxidation were 65.7 and 94.1%, and the gold recovery level was 71.7%. The effect of an organic nutrient (yeast extract) on biooxidation during the two-step process was studied. The pyrite and arsenopyrite oxidation levels after 240 hours of biooxidation under mixotrophic conditions were 73.5 and 95.1%, and the gold recovery level was 77.9%. The effect of the organic nutrient on the microbial population was determined. Sulfobacillus thermosulfidooxidans and Acidithiobacillus caldus were the predominant microorganisms studied under both autotrophic and mixotrophic conditions. Archaeon Acidiplasma sp. MBA-1 was a minor component of the microbial community under autotrophic conditions but was one of the predominant microorganisms studied under mixotrophic conditions. The strain MBA-1 could oxidise both ferrous iron and elemental sulphur in the presence of yeast extract. These results suggest that the organic nutrient changed the composition and increased the activity of the microbial population. Thus, a two-step process with organic nutrients added during biooxidation may be considered an effective strategy for treating refractory pyrite-arsenopyrite concentrates.

Abstract: Stirred tank reactor (STR) leaching of non-ferrous metals from copper smelting slag was conducted using biogenic Fe³⁺ solution containing H₂SO₄. The solution was initially obtained through biooxidation of FeS₂+S° by using a moderately thermophilic microbial culture such as Sulfobacillus thermosulfidooxidans at 45°C and was applied as lixiviant in STR leaching experiments. The effect of pH, pulp density, temperature and initial Fe³⁺ concentration on leaching behavior of Cu, Fe, Zn and Ni from the slag were investigated. The presence of high concentration of Fe³⁺ ions was found to promote the leaching of copper and nickel while inhibiting the leaching of zinc and iron. Process pH was considered as the most significant parameter and the optimum leaching results were obtained at pH 1.8 with low fayalite dissolution and jarosite precipitation. During the leaching at this pH (1.8), 25% (w/v) pulp density and 65°C temperature in presence of 20 g/L initial Fe³⁺ concentration, maximum extraction of Cu (90%) and Ni (85%) was observed in 4 hours whereas Fe extraction was minimum (19%). A schematic flowsheet of multistage leaching process was proposed. The result obtained from multistage batch leaching experiments will be step forward for continuous stirred tank reactor leaching for industrial scale implementation.

Abstract: Moderately thermophilic bioleaching of a pyritic chalcopyrite concentrate was mimicked in an electrochemical vessel. The bioleaching was carried out for 28 days at 45°C with 2.5% (wt/vol) solid content at pH 1.5. Data from the redox potential development was used to program a redox potential controller in an electrochemical vessel to reproduce the same leaching conditions in the absence of microorganisms. Despite precipitation of iron as jarosite and formation of elemental sulphur in the electrochemical experiment, the copper recoveries were almost the same in both experiments.