Advanced Materials Research Vols. 71-73

Abstract: Chalcopyrite bioleaching process using Acidithiobacillus ferrooxidans and a mixed culture of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans like bacterium was carried out. Two mineral particle sizes were evaluated, 200 and 325 Tyler mesh. The strains were adapted by gradually decreasing of the main energy sources and increasing in the mineral content. The experiments were performed in absence of ferrous sulphate and elemental sulfur. When the mixed culture was used, pH values were always over 2,1, indicating a probable passivation of Acidithiobacillus thiooxidans in the consortium. For both cultures, the Cu2+ dissolution occurred at relatively low redox potential values, around 400mV–450mV, while at high redox potential values, 550mV, chalcopyrite dissolution was inhibited. Copper lixiviation was around 40% for both tests. The Fourier Transform-Infrared spectra showed that the main oxidation phase is jarosite. The results showed that chalcopyrite oxidation is more dependent on the redox potential than particle size or type of culture used.

Abstract: In the present work the occurrence of surface transformations triggered during chemical and biological leaching of chalcopyrite at 70 °C in basal medium were investigated with the aid of the Rietveld technique. Leaching experiments were conducted in 250 ml shake flasks at 150 rpm, contacting 1 g of a -80 # + 120# chalcopyrite concentrate with 100 ml of iron-free basal medium at pH 1.5 at 70°C. Three different conditions were used: experiment (a) in aerated conditions with basal medium inoculated with Sulfolobus metallicus (4x108 cells/ml); experiment (b) in aerated abiotic conditions; experiment (c) in abiotic conditions under N2 atmosphere. Copper and iron dissolved in solution was analyzed with atomic absorption. Samples of the initial chalcopyrite and the treated samples obtained after 16 days under the various experimental conditions were analyzed by Grazing-Incident X-ray diffraction (GID). The obtained x-ray spectra were then analyzed with the Rietveld refinement technique using the Topas software. Results showed that the initial concentrate consisted mainly of tetragonal chalcopyrite (JCPDS : 37-0471). Chalcopyrite leached under abiotic-anaerobic conditions was crystallographically unaffected. In chalcopyrite leached under aerated abiotic conditions there was evidence of surface texturization showed as preferential orientation to the crystallographical planes ( 3 1 2 ) and (1 1 6). Finally, surface texturization was also observed in chalcopyrite leached under inoculated conditions, but in this case showed as preferential orientation to the crystallographical planes ( 0 2 4 ) and ( 2 2 0). These results give a first evidence of surface texturization phenomena during dissolution of chalcopyrite at 70 °C. They suggest that chalcopyrite dissolution occurs via selective leaching of specific crystallographic planes, and this selectivity is influenced by both the chemical action of oxygen and the microorganism actvity.

Abstract: Bioleaching of secondary sulfides is a technical and economical well established bioprocess. However in the case of bioleaching of primary sulfides, like chalcopyrite and enargite, still is necessary to improve the bioprocess kinetic. One alternative that has been proposed to overcome this situation is to use hyperthermophilic bioleaching microorganisms. Nevertheless, higher process temperatures modifies in a different extent operation parameters like CO2 and O2 transfer rates, both gases being essential to the bioprocess. The aim of this work was to establish quantitatively, using a continuous bioreactor operating at steady state conditions, which gas limits the growth and activity of bioleaching microorganisms when air is used as source of both of them. The experiments were run in a 3.3 L agitated continuous bioreactor operated at 70°C, pH 1.8 and 150 g/L of copper concentrate rich in chalcopyrite. Operating at steady state condition with an aeration rate of 1.2 vvm, a copper solubilization volumetric productivity of 1.7 g(Cu2+)(L•day)-1 and a copper extraction of 55% were obtained. Enriching the inflow air with CO2 up to 5% (v/v) showed the maximum values of these indexes, increasing 34% the volumetric productivity of copper solubilization and reaching 74% of copper extraction. On the opposite way, enriching air with oxygen or increasing the aeration rate did not bring about any change either of the volumetric productivity or the percentage of copper extraction. The results allow concluding that the CO2 is the limiting nutrient for cell growth in the continuous bioleaching of a chalcopyrite concentrate using hyperthermophilic microorganisms.

Abstract: It is believed that the dissolution of chalcopyrite (CuFeS2) in acid medium can be accelerated by the addition of Cl- ions, which modify the electrochemical reactions in the leaching system. Electrochemical noise analysis (ENA) was utilized to evaluate the effect of the Cl- ions and Acidithiobacillus ferrooxidans on the oxidative dissolution of a CPE-chalcopyrite (carbon paste electrode modified with chalcopyrite) in acid medium. The emphasis was on the analysis of the admittance plots (Ac) calculated by ENA. In general, a stable passive behavior was observed, mainly during the initial stages of CPE-chalcopyrite immersion, characterized by a low passive current and a low dispersion of the Ac plots, mainly after bacteria addition. This can be explained by the adhesion of bacterial cells on the CPE-chalcopyrite surface acting as a physical barrier. The greater dispersions in the Ac plots occurred immediately after the Cl- ions addition, in the absence of bacteria characterizing an active-state. In the presence of bacteria the addition of Cl- ions only produced some effect after some time due to the barrier effect caused by bacteria adhesion.

Abstract: Based on the bioleaching mechanism and electrochemical studies of metal sulfides, the dissolution rate of secondary copper sulfides and pyrite are controlled by redox potentials. Experiment on the bioleaching of the secondary copper sulfides under constant potential by sparging with nitrogen gas demonstrated, by analyzing Cu and Fe content of the bioleaching solution and residues, the pyrite and secondary cooper sulfides dissolution rates have large difference in various redox potential. The pyrite and secondary cooper sulfides have good selection when the redox potential controlled between 700mV and 760mV, we can realize the secondary copper sulfides bioleaching process be controlled, then supply theoretical guide for the iron-acid balance during copper bioleaching process.

Abstract: During large-scale column tests at BHP Billiton’s Johannesburg Technology Centre (JTC) during 2005/6 on a low-grade copper ore, the concentrations of both oxygen and CO2 were continuously monitored in feed and exit gas as well as at various intermediate positions over the height of the column. This paper describes results from a test run at 40 °C fed with an air stream enriched to between 1000 and 2000 ppm CO2. Oxygen consumption very closely tracks iron and copper leaching. CO2 is consumed rapidly from the bottom up, resulting in significant depletion midway through the column, even though an enriched feed was used. Oxidation rates decline in CO2 depleted zones, but were not observed to cease completely. This rate of decline is postulated to be linked to a slowly decaying population unable to regenerate itself. Comparison between O2 and CO2 consumption rates shows a linear correlation beyond a minimum oxidation rate. This minimum rate corresponds to a non-growth maintenance energy requirement, and the slope of the linear correlation to the growth yield. Both are functions of available CO2 in the range 50 to 1000 ppm, with maintenance declining and yield increasing. The findings of this study imply that CO2 supplementation in bioheaps will stimulate microbial growth and CO2 consumption, but not necessarily increase the rate of oxygen uptake and hence leaching. Absence of CO2 is expected to result in gradual population decline, but a degree of oxidation continues on the basis of maintenance. In tall heaps CO2 depletion with height is likely and may therefore result in impaired leaching in the upper zones.

Abstract: The effects of several variables on the column bioleaching of copper sulphide ore have been investigated. The copper ore contained chalcopyrite as the main sulfide minerals and bornite and chalcocite as the minor minerals. The experiment was carried out using bench-scale column leach reactors designed in Key Lab of Biometallurgy of Ministry of Education, which were inoculated with the pure mesophile bacteria (Acidithiobacillus ferrooxidans) and thermophile bacteria (Sulfobacillus), respectively, and the mixed bacteria which contain both iron- and sulfur-oxidizing bacteria. The results show that the mixed cultures were more efficient than the pure cultures alone and the maximum copper recovery 53.64% was achieved using the mixed cultures after 85 days. The leaching rate of chalcopyrite tended to increase with the increased dissolved ferric iron concentration. The effect of particle size on the rate of the copper leaching was also investigated, and it was shown that the copper bioleaching rate decreases as the amount of fines increase, which limits the permeability, thus decreases leaching rate. Jarosite and elemental sulphur formed in the column were characterized by the X-ray and EDS.

Abstract: Heap bioleaching operations are often faced with extended and unpredictable lag periods after inoculation, prior to the establishment of a stable oxidising environment, during which the heap is fully colonised or the inoculum overcomes the sub-optimal conditions resulting from acid agglomeration. Supplementation of laboratory scale (4kg ore) leach columns with soluble nitrogen, particularly as yeast extract, significantly reduced the lag time and promoted bacterial growth, resulting in a 50-95% increase in copper recovery post-inoculation. The effect of yeast extract addition to Acidithiobacillus ferrooxidans in controlled oxidation tests was investigated. Initial exposure of a stock culture to yeast extract resulted in a transient, dose dependent inhibition at concentrations of 0.5 g.l-1 and below. At 1.25 g.l-1 inhibition was complete over the time scale of the experiment. The inhibition phase was characterised by observable changes in cell morphology and ultrastructure. Despite the initial inhibition, the biomass yield at the end of the experiments was equivalent, or higher, in the presence of yeast extract. Cultures were adapted to growth on yeast extract as the sole nitrogen source and adapted cultures showed the highest rates of iron oxidation and cell growth, in the presence of 0.5 and 1 g.l-1 of yeast extract.

Abstract: In this work, the monitoring of bacterial and chemical dissolution of massive bornite was performed using cyclic voltammetry (CV); the mineral surface was examined using scanning electron microscopy (SEM) and the elements were indentified by Energy Dispersive X-ray Analysis (EDXA). The electrolyte employed for the electrochemical tests was the medium used for bioleaching the electrodes (3.0 g•dm-3 (NH4)2•SO4, 0.5 g•dm-3K2HPO4, 0.5 g•dm-3 MgSO4•7H2O and 0.1 g•dm-3 Ca(NO3)2).The results showed differences in voltagramms carried out by Acidithiobacillus ferrooxidans and Acidithiobacillus caldus: In bioleached electrodes tested in the presence of Acidithiobacillus ferrooxidans, the anodic and cathodic current signals were larger than with Acidithiobacillus caldus The analysis of CV results allowed attributing the different peaks observed in both the direct and reverse potential scanning to the oxidation of bornite to a secondary copper mineral(chalcocite and covellite) and its reduction via different non stoichiometric copper sulphides intermediaries. Surface evidences were produced by SEM, and chemical elements evidences were also investigated by EDAX.

Abstract: The effect of two flotation reagents: isopropyl- and ethyl-xanthates and two solvent extraction reagents (SX): Lix 984 and Lix 860IC on the bioleaching of a copper concentrate with Sulfolobus metallicus was analyzed. The experiments were performed in 250 ml shake flasks with 100 ml basal medium with an initial pH of 1.6, inoculated with a pure culture of S. metallicus and 0.5 % copper concentrate (38% of Cu) at 70oC. In the bioleaching flasks 0, 50 or 500 μg/mL of flotation reagent and 0, 10 or 100μg/mL of SX reagents was added. All organic reagents caused a strong inhibition of ferrous iron oxidation and growth of planktonic cells of S. metallicus. However, bioleaching in presence of 50 μg/mL of isopropyl-xanthate or 10 μg/mL of Lix 984 reached copper dissolutions of 70% and 72% respectively after 200 hours, compared with 98% in the process without organic reagents. These results suggest that isopropyl-xanthate and Lix 984 at the lowest concentrations used in this study would not affect the mechanism of bioleaching by contact of S. metallicus on the sulphide, however it strongly affected the indirect mechanism. Ethyl-xanthates and Lix 860IC, beginning with 50μg/mL and 10μg/mL respectively, generated a total inhibitory effect on the oxidation activity of S. metallicus on the sulphides.