Leaching of chalcopyrite with ferric ion. Part IV: The role of redox potential in the presence of mesophilic and thermophilic bacteria

This paper reports a study on the effect of redox potential in chalcopyrite bioleaching in the presence of iron- and sulphur-oxidizing bacteria. Bioleaching tests were carried out in stirred Erlenmeyer flasks at 180 rpm, with 0.5 g of chalcopyrite mineral, 99 ml of a sulphate solution of Fe³⁺/Fe²⁺ (with the redox potential ranging between 300 and 600 mV Ag/AgCl) at pH 1.8 and 1 ml of a mesophilic (35 °C) or thermophilic (68 °C) culture. The overoxidation of the leaching solution, due to the activity of iron-oxidizing microorganisms (Acidithiobacillus ferrooxidans, Leptospirillum ferrooxidans and Sulfolobus BC), favoured the precipitation of jarosite on chalcopyrite surfaces followed by passivation. Iron- and sulphur-oxidizing microorganisms, such as A. ferrooxidans and Sulfolobus BC adapted for 4 months to elemental sulphur as the sole energy source, recovered their iron-oxidizing ability after being in contact with Fe²⁺.     

Molecular characterization of microbial populations in a low-grade copper ore bioleaching test heap

A culture-independent approach based on PCR amplification and denaturing gradient gel electrophoresis (DGGE) and sequencing of 16S rRNA gene fragments from both Bacteria and Archaea were used to analyze the microbial community inhabiting a low-grade copper sulfide run-of-mine (ROM) test heap of a project in Chile. In this paper, we summarize results of a 1-year monitoring study. Phylogenetic analyses of 16S rRNA fragments revealed that the retrieved sequences clustered together with Acidithiobacillus ferrooxidans, Leptospirillum ferriphilum, Ferroplasma acidiphilum and environmental clones related to them. In addition, some sequences were distantly related (< 95% similarity in the 16S rRNA gene fragment analyzed) to cultured microorganisms from the Sulfurisphaera and Sulfobacillus genera. Thus, the prokaryotic assemblage might be mainly composed of sulfur- and iron-oxidizing microorganisms. The remaining sequences were related to uncultured chrenarchaeota clones or had only partial homology with known microorganisms. Attempts were made to estimate the dynamic of phylogenetic microbial groups in different stages of the leaching cycle and to correlate them with chemical and physical parameters in the heap. The temporal distribution of microbial 16S rRNA gene sequences could be divided in three periods. In the bioleaching cycle, first stage A. ferrooxidans and Sulfurisphaera-like archaea were dominant within each respective phylogenetic domain. In the second stage (from days 255 to 338), Leptospirillum and Ferroplasma groups were mainly detected, respectively. Finally (the third period from operation days 598 to 749), Sulfobacillus-like microorganisms became predominant, while Ferroplasma was the only Archaea detected. These data are now being used to obtain more detailed and quantitative information on prokaryotic community structure over time and to explore the nature of the community metabolic pathways. These results extend our knowledge on microbial dynamics in bioheaps, a key issue required to improve commercial applications.     

Rapid and specific detection of Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans by PCR

Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans are two of the most important bacteria in heap bioleaching processes of copper sulphide at common operating temperatures (18–24 °C). In this paper, both microorganisms were detected in solutions and ores coming from processes in 2 days, using specific amplification of 16S rDNA sequences by PCR. The technique was first validated using template DNA from pure cultures of the microorganisms. Then it was applied to samples of solutions and ores from bioleaching processes. Results were confirmed using tRFLP (terminal Restriction Fragment Length Polymorphism) with universal primers and by identification of isolated bacteria by means of culture. This methodology is more rapid and specific than the identification by tRFLP or by culture, which require from 1 to three weeks to positively detect the bacteria. The detection limit of this technique is 10⁵ cells per ml.     

Towards a Novel Bioleaching Mechanism

Abstract

Research on the chemistry of pyrite bioleaching demonstrated-contradictory to the text book theory on direct or indirect leaching mechanisms-that only the indirect one is functioning. Cells of Thiobacillus ferrooxidans and Leptospirillum ferrooxidans primarily attach to the surface of pyrite particles by electrostatic interactions caused by iron(III) ion containing extracellular polymeric compounds. If sufficient free iron(III) ions are available (0.2 g/l) pyrite degradation starts. The first degradation products are thiosulfate and iron(II) ions. Thiosulfate will be rapidly oxidized by iron(lll) ions to tetrathionate. Tetrathionate adsorbs to the pyrite surface and is hydrolized to sulfane-monosulfonic acid and sulfate plus one proton. From the sulfane-monosulfonic acid several polythionales and elemental sulfur may arise. Consequently, the function of the leaching bacteria is “only” the maintenance of a high redox potential by keeping the iron(III) ions mainly in the oxidized state to optimize the indirect attack on the metal sulfide.     

Kinetics of leaching chalcopyrite-bearing waste rock with thermophilic and mesophilic bacteria

The leaching characteristics and kinetics of a thermophilic, chemolithotrophic microorganism were studied and compared with those of mesophilic T. ferrooxidans. Chalcopyrite waste rock was used for laboratory scale shake flask leaching systems. The optimum (leaching) temperatures were determined to be between 30 and 40°C for Thiobacillus ferrooxidans and between 50 and 60°C for the thermophiles. The thermophilic microorganisms were observed to function as a kind of take-over organism with increasing temperature. Viable cell counts (most probable number measurements) for both microorganisms were observed to be highly correlated with total copper in solution after 6 weeks of leaching, and over a range of temperatures (20–70°C). Observations by scanning electron microscopy showed the bulk of precipitates formed during leaching were comprised of gypsum crystals, and that thermophilic organisms were attached in profusion to some mineral surfaces seen by X-ray energy dispersive analysis to be somewhat uniformly coated with a thin sulfur layer. The propensity for attachment of the thermophiles as compared with T. ferrooxidans was consistent with observations of other thermophilic microbes. The reaction order for T. ferrooxidans was 0.92, compared with 1.29 for the thermophilic microorganisms. Correspondingly, the activation energy for the thermophiles was 26.77 kcal mole^? compared with 22.22 kcal mole^?1 and 7.11 kcal mole^?1 for the T. ferrooxidans and sterile control reactions, respectively. The rate of reaction increased by 1.93 for each 10°C temperature increase for the thermophiles, compared with 1.83 for T. ferrooxidans.     

Effect of suspension potential on the oxidation rate of copper concentrate in a sulfuric acid solution

A chalcopyrite concentrate containing 17 pct pyrite was oxidized in 1 mol/dm³ sulfuric acid solution at 90 °C (363.2 K). The suspension potential (Ptvs SCE, in the presence of Fe³⁺/Fe²⁺) was maintained constant in the range 0.30 to 0.65 V by controlled additions of KMnO₄ solution. The oxidation appeared to be under surface reaction control. The rate constant was nearly independent of total Fe concentration (0.01 to 0.5 mol/dm³), but increased rapidly with a rise in suspension potential until it reached a maximum at 0.40 to 0.43 V, after which there was marked decrease at around 0.45 V. Chalcopyrite in the concentrate was oxidized to form elemental sulfur over the whole of the suspension potential range, whereas the oxidation of pyrite took place only above 0.45 V and yielded sulfate ion. At 0.40 V the apparent activation energy was 47 kJ/mol. An analogy between the potential dependence of the rate and the Tafel correlation for an electrode process is discussed.     

Column Bioleaching of a Low-Grade Silicate Ore of Uranium

The bioleaching of a low-grade Indian uraninite ore (triuranium octoxide, U₃O₈: 0.024%), containing ferro-silicate and magnetite as the major phases, and hematite and pyrite in minor amounts, has been reported. Experiments were carried out in laboratory scale column reactors inoculated with enriched culture of Acidithiobacillus ferrooxidans isolated from the source mine water. The pH effect on uranium recovery was examined with the same amounts of ores in different columns. With the presence of 10.64% Fe in the ore as ferro-silicate, the higher uranium biorecovery of 58.9% was observed with increase in cell count from 6.4 × 10⁷ to 9.7 × 10⁸ cells/mL at pH 1.7 in 40 days as compared to the uranium recovery of 56.8% at pH 1.9 with a corresponding value of 9.4 × 10⁸ cells/mL for 2.5-kg ore in the column. The dissolution of uranium under chemical leaching conditions, however, recorded a lower value of 47.9% in 40 days at room temperature. Recoveries were similar with 6-kg ore when column leaching was carried out at pH 1.7. The bioleaching of uranium from the low-grade ore of Turamdih may be correlated with the iron(II) and iron(III) concentrations, and redox potential values.     

Electrochemical noise analysis of bioleaching of bornite (Cu5FeS4) by Acidithiobacillus ferrooxidans

Electrochemical noise (EN) is a generic term describing the phenomenon of spontaneous fluctuations of potential or current noise of electrochemical systems. Since this technique provides a non-destructive condition for investigating corrosion processes, it can be useful to study the electrochemical oxidation of mineral sulfides by microorganisms, a process known as bacterial leaching of metals. This technique was utilized to investigate the dissolution of a bornite electrode in the absence (first 79 h) and after the addition of Acidithiobacillus ferrooxidans (next 113 h) in salts mineral medium at pH 1.8, without addition of the energy source (Fe²⁺ ions) for this chemolithotrophic bacterium. Potential and current noise data have been determined simultaneously with two identical working bornite electrodes which were linked by a zero resistance ammeter (ZRA). The mean potential, E_coup, coupling current, I_coup, standard deviations of potential and current noise fluctuations and noise resistance, R_n, have been obtained for coupled bornite electrodes. Noise measurements were recorded twice a day in an unstirred solution at 30 °C. Significant changes in these parameters were observed when the A. ferrooxidans suspension was added, related with bacterial activity on reduced species present in the sulfide moisture (Fe²⁺, S²⁻). ENA was a suitable tool for monitoring the changes of the corrosion behavior of bornite due to the presence of bacterium.     

Bacterial leaching of nickel laterites using chemolithotrophic microorganisms: Process optimisation using response surface methodology and central composite rotatable design

The depletion of easily processed nickel sulphides and demand for the nickel metal pose a challenge of finding new effective methods for nickel recovery from low grade ores. Solving these problems successfully requires optimisation of the processes. In this study, a statistically-based optimisation strategy has been used in the optimisation of pH, pulp density and particle sizes during the bacterial leaching of nickel laterites using a mixed culture of chemolithotrophic bacteria (Acidithiobacilllus ferrooxidans, Acidithiobacillus caldus and Leptospirillum ferroxidans). The central composite rotatable design (CCRD) was used to collect the data for fitting the second order response. A mathematical model equation was then derived by computer simulation programming applying least squares method using MATLAB R2006a. This second order model representing the nickel recovery from nickel laterite ore is expressed as a function of the three variables (pH, particle size, and pulp density). A statistical analysis (ANOVA) was carried out to study the effects of the individual variables as well as their combined interactive effects on the recovery of nickel. The results showed that the effects of the individual variables and their quadratic terms were statistically significant whilst the interactions among the variables were statistically insignificant. Response surface plots drawn for spatial representation of the model showed that the nickel recovery depends more on particle size than on pH and pulp density. Using the model, optimised values of 2.6% pulp density, initial pH of 2.0 and 63 μm particle size resulted in a nickel recovery of 79.8%. Confirmatory test at these optimum conditions resulted in a nickel recovery of 74.1%; thus verifying that the model is valid and plausibly fits the experimental data with a marginal error of 7.7%. The significance of this study is that it has opened up an opportunity for the potential application of chemolithotrophic microorganisms for the commercial processing of the difficult-to-process low grade nickel laterite ores. Some of the examples where this process may be applied include silicate ores, oxidic converter furnace slags and refractory oxides.     

Attachment of acidophilic bacteria to solid surfaces: The significance of species and strain variations

Sixteen strains of acidophilic bacteria were screened for their abilities to adhere to pyrite ore, glass beads and ferric hydroxysulfates. These were three culture collection and two isolated strains of the iron- and sulfur-oxidizer, Acidithiobacillus ferrooxidans, two each of the sulfur-oxidizer Acidithiobacillus thiooxidans and the iron-oxidizer Leptospirillum ferrooxidans (the type strain and a mine isolate in either case), five heterotrophic acidophiles (four Acidiphilium and one Acidocella sp.) and two moderately thermophilic iron/sulfur-oxidizers (Sulfobacillus thermosulfidooxidans and Sulfobacillus acidophilus). Considerable variations were found between different species of acidophiles, and also between different strains of the same species, in how they attached to the three solid materials tested. Attachment to the solid substrata generally increased with time (over 100 min) though > 99% of one At. ferrooxidans isolate (strain OP14) were attached to pyrite after just 10 min exposure. Most acidophiles attached more readily to pyrite than to glass beads, and attachment to ferric hydroxysulfates was highly variable, though one At. ferrooxidans isolate (strain SJ2) and one heterotrophic acidophile (Acidocella sp. het-4) both attached strongly to ferric iron precipitates (jarosites and schwertmannite) that formed in cultures of At. ferrooxidans grown at pH > 2. The results of these experiments showed that even closely related strains of acidophilic bacteria can display very different propensities to attach to solid materials, an observation that may explain the somewhat disparate results reported on occasions by research groups that have examined single, or limited numbers of strains, of acidophiles (mostly At. ferrooxidans). The significance of differential attachment of mineral-oxidizing and other acidophiles to pyrite and other solids is discussed in the context of biohydrometallurgy.     

Bioleaching of apatite rich low grade Indian uranium ore

Abstract

Uranium ore from Narwapahar Mines, UCIL contains 0·047% U₃O₈ with some refractory minerals and high apatite (5%) results in a maximum 78% recovery through conventional processing at UCIL, with a fairly high consumption of sulphuric acid and pyrolusite, and loss of uranium as uranium phosphate. To avoid usage of non-ecofriendly oxidants, obviate the influence of phosphate and improve the overall process output of uranium, an alternate extraction technology using microbial isolate(s) is elucidated in this study. A. ferrooxidans isolated from Narwapahar mine water was used in bioleaching of uranium from this apatite rich low grade uraninite ore. Optimum uranium biorecovery of 96% is achieved at 10% pulp density (w/v), pH 1·7 and 35°C in 40 days with the fine particles of <45 μm size. Under the optimum condition at pH 1·7, rise in redox potential is recorded to be 594–708 mV in 40 days. Bioleaching of uranium seems to follow the indirect mechanism of leaching with the involvement of Fe(III) biogenically generated by Acidithiobacillus ferrooxidans (A. ferrooxidans). Uranium recovery was also examined using another mesophilic isolate of Leptospirillum ferrooxidans (L. ferrooxidans) which showed 98% uranium leaching at 40°C, which shows the possibility of improving the kinetics of the process. The high R² values in the temperature range (298–308 K) indicated uranium dissolution by the chemical reaction occurring at the ore surface with Fe(III) generated biogenically, with E_a value of 28·3 kJ mol^?1. The mechanism of uranium bioleaching is also elucidated with X-ray diffraction phase identification of the leach residues with time, followed by observing the surface morphology through SEM at varying temperatures.

Le minerai d’uranium des Mines de Narwapahar, d’UCIL, contient 0·047% d’U₃O₈ avec quelques minéraux réfractaires et une teneur élevée en apatite (5%). On note une récupération maximale de 78% par traitement conventionnel à UCIL, avec une consommation relativement élevée d’acide sulfurique et de pyrolusite, ainsi qu’une perte d’uranium sous forme de phosphate d’uranium. Afin d’éviter l’utilisation d’agents oxydants non écologiques, de prévenir l’influence du phosphate, et d’améliorer la production globale d’uranium du procédé, dans cette étude on examine une autre technologie d’extraction utilisant un (des) isolat(s) microbien(s). On a utilisé A. ferrooxidans, isolée de l’eau de mine de Narwapahar, pour la biolixiviation de l’uranium de ce minerai pauvre en uraninite et riche en apatite. La bio récupération optimale de l’uranium de 96% est obtenue à 10% PD (poids/volume) au pH de 1·7 et à 35°C en 40 jours avec les particules fines d’une taille <45 μm. Sous la condition optimale d’un pH de 1·7, on a enregistré l’augmentation du potentiel rédox à 594–708 mV en 40 jours. La biolixiviation de l’uranium semble suivre le mécanisme indirect de lixiviation avec l’implication de Fe(III) engendré bio génétiquement par A. ferrooxidans. On a également examiné la récupération de l’uranium en utilisant un autre isolat mésophile de L. ferrooxidans, qui a montré une lixiviation de 98% de l’uranium à 40°C, ce qui montre la possibilité d’améliorer la cinétique du procédé. Les valeurs élevées de R² dans la gamme de température (298–308 K) indiquaient que la dissolution de l’uranium par réaction chimique se produisait à la surface du minerai avec Fe(III) engendré bio génétiquement, à une valeur de Ea de 28·3 kJ mol^?1. Le mécanisme de biolixiviation de l’uranium est également examiné avec l’identification de phase par XRD des résidus de lixiviat en fonction du temps, suivi par l’observation de la morphologie de la surface au moyen du SEM, à des températures variées.     

Dissolution of uranium from silicate-apatite ore by Acidithiobacillus ferrooxidans

Bioleaching of a low-grade Indian silicate-apatite uranium ore containing 0.024% U₃O₈ and 10.6% iron with minor amounts of base metals has been reported. The studies involved extraction of uranium using enriched culture containing Acidithiobacillus ferrooxidans (A. ferrooxidans) derived from the source mine water employing bio-chemically generated ferric ion as an oxidant. Parameters such as particle size of the ore, pulp density, and pH of lixiviant media were optimised. Maximum uranium bio-dissolution of 98% was achieved using ore of mixed particles of < 76 μm size. Uranium bio-recovery was found to be 96% at the pulp density (PD) of 10% (w/v) and 20% (w/v) with the particles of < 76 μm size in 40 days at 2.0 pH and 35 °C temperature. At 1.7 pH and 20% (w/v) PD, 98% uranium bio-recovery was achieved with a rise in redox potential from 595 mV to 715 mV in 40 days. Uranium bio-dissolution may be correlated with the generation of ferric ions through the bio-chemical action on the ore. The work illustrated the efficacy of leaching of uranium by the involvement of bacteria by indirect mechanism.     

Direct oxidation of copper sulfide by a biofilm of Acidithiobacillus ferrooxidans

The direct oxidation of synthetic copper sulfide by Acidithiobacillus ferrooxidans was studied under iron-free conditions in two different types of biofilm reactors. It was shown that copper was oxidized biologically in the absence of intermediate oxidants such as ferric ions, but at rates much lower than these due to the indirect mechanism within the iron reduction–oxidation cycle. The effect of temperature on the rate of copper sulfide biooxidation was studied and the results show that the optimal temperature for the direct copper sulfide oxidation by A. ferrooxidans was close to 35 °C. The variation of acidity of the liquid medium between pH 1.5 and 3.5 did not affect the oxidation rate significantly.     

Visualisation of pyrite leaching by selected thermophilic archaea: Nature of microorganism–ore interactions during bioleaching

In this study, pyrite (FeS₂) was leached by Acidianus brierleyi, Metallosphaera sedula and Sulfolobus metallicus during a 60 day experimental period. Leaching occurred over a redox potential range of 800 to 860 mV (S.H.E.) and in the presence of increasing Fe³⁺ levels. A modified ferrozine assay was developed to detect the increase of iron in solution as bioleaching of the ore progressed. For the first time, the interactions of these extreme thermophiles with the metal sulfide ore particles were extensively documented using SEM and TEM. As the pyrite degraded, there appeared to be a progression of deposited structures forming, ranging from sub-micron precipitates and disc-shaped structures on the ore's surface, which ultimately were similar for all leaching cultures. Furthermore, the residues resulting from the leaching of pyrite by M. sedula, the most active thermophile, were characterised using SEM/EDX, and appeared to be dominated by iron sulfate precipitates. The nature of the deposits formed, together with our other results, indicate that A. brierleyi, M. sedula and S. metallicus acted through the ‘contact’ and ‘non-contact’ sub-mechanisms of the indirect bioleaching mechanism for the dissolution of pyrite. The role of the bioleaching microorganisms is thus to maintain sufficient levels of Fe³⁺ and acid during pyrite leaching, for maximal mineral dissolution.     

Role of ferric ions in bioleaching of uranium from low tenor Indian ore

Abstract

The role of biogenic ferric ions in leaching of uranium by Acidithiobacillus ferrooxidans from a low grade ore of Turamdih mines, India, has been investigated. Using the enriched culture of bacterial isolate containing mainly A. ferrooxidans from the source mine water, biorecovery of 98% uranium at 20% (w/v) pulp density, pH 1·7 and 35°C temperature using <76 μm particles in 40 days was obtained. The effect of temperature on bioleaching of uranium showed higher recovery at 35°C. The uranium dissolution was facilitated by iron(III) available in the leach liquor because of bacterial oxidation of pyrite and chemical dissolution of magnetite present in the ore under acidic conditions. The biogenically generated Fe(III) ions enhanced uranium dissolution from the uraninite ore. The bioleaching of uranium appeared to follow a chemical control kinetic model with the reaction of lixiviant, Fe(III) and acid on the surface of the solid in the temperature range 25–35°C. Phase identification by XRD and the study of surface morphology of the ore and the residue by SEM study corroborated the above mechanism of uranium leaching.

On a étudié le rôle des ions ferriques biogènes dans la lixiviation de l’uranium par A. ferrooxidans d’un minerai pauvre des mines de Turamdih, en Inde. En utilisant la culture enrichie d’isolat bactérien contenant principalement Acidithiobacillus ferrooxidans de l’eau de source de la mine, on a obtenu une biorécupération de 98% d’uranium à 20% de densité de la pulpe (poids/vol), avec un pH de 1·7 et une température de 35°C, en utilisant des particules de <76 μm, en 40 jours. L’effet de la température sur la biolixiviation de l’uranium montrait une récupération plus élevée à 35°C. La dissolution de l’uranium était facilitée par le fer(III), disponible dans la liqueur de lixiviat grâce à l’oxydation bactérienne de la pyrite et à la dissolution chimique de la magnétite présentes dans le minerai en conditions acides. Les ions Fe(III) engendrés biogéniquement augmentaient la dissolution de l’uranium du minerai d’uraninite. La biolixiviation de l’uranium semblait suivre un modèle cinétique à contrôle chimique avec la réaction du lixiviant – Fe(III) et de l’acide à la surface du solide dans la gamme de température de 25 à 35°C. L’identification de phase par XRD et l’étude de la morphologie de la surface du minerai et du résidu par étude au SEM corroboraient le mécanisme ci-dessus de lixiviation de l’uranium.     

Catalytic-Oxidative Leaching of Low-Grade Complex Zinc Ore by Cu (II) Ions Produced from Copper Ore in Ammonia-Ammonium Sulfate Solution

The catalytic-oxidative leaching of a mixed ore, which consists of low-grade oxide copper ore and oxide zinc ore containing ZnS, was investigated in ammonia-ammonium sulfate solution. The effect of the main parameters, such as mass ratio of copper ore to zinc ore, liquid-to-solid ratio, concentration of lixivant, leaching time, and temperature, was studied. The optimal leaching conditions with a maximum extraction of Cu 92.6?pct and Zn 85.5?pct were determined as follows: the mass ratio of copper ore to zinc ore 4/10?g/g, temperature 323.15?K (50?°C), leaching time 6?hours, stirring speed 500?r/min, liquid-to-solid ratio 3.6/1?cm³/g, concentration of lixivant including ammonia 2.0?mol/dm³, ammonium sulfate 1.0?mol/dm³, and ammonium persulfate 0.3?mol/dm³. It was found that ZnS in the oxide zinc ore could be extracted with Cu(II) ion, which was produced from copper ore and was used as the catalyst in the presence of ammonium persulfate.     

Leaching of marine manganese nodules by acidophilic bacteria growing on elemental sulfur

This article describes the bioleaching of manganese nodules by thermophilic and mesophilic sulfuroxidizing bacteria, in which oxidized sulfur compounds are biologically produced from elemental sulfur added to liquid medium and are simultaneously used to leach nodules. The thermophile Acidianus brierleyi solubilized the manganese nodules faster at 65 °C than did the mesophiles Thiobacillus ferrooxidans and Thiobacillus thiooxidans at 30 °C. Leaching experiments with A. brierleyi growing on elemental sulfur were used to optimize various process parameters, such as medium pH, initial sulfur-liquid loading ratio, and initial cell concentration. The observed dependencies of the leaching rates at a pH optimum on the initial amounts of elemental sulfur and A. brierleyi cells were qualitatively consistent with model simulations for microbial sulfur oxidation. Under the conditions determined as optimum, the leaching of nodule particles (−330+500 mesh) by A. brierleyi yielded 100 pct extraction of both copper and zinc within 4 days and high extractions of nickel (85 pct), cobalt (70 pct), and manganese (55 pct) for 10 days. However, the iron leaching was practically negligible.     

Model for ferric sulfate leaching of copper ores containing a variety of sulfide minerals: Part II. Process modeling of in situ operations

The Lin-Sohn Leach simulation (DLS) computer model was used to simulate copper extraction from run-of-mine fragment size distributions expected for underground modifiedin situ solution mining. Ventilation was found to be a key variable in obtaining good copper recoveries, and the rate of ventilation was found to vary with ore type and through time. Improving recoveries and recovery rates with improved fragmentation was found to be more difficult than expected. Limitations on surface kinetics and bacterial ferric ion generation often erode benefits expected from reduction in diffusion limitation. Control of fines formation is difficult with the current state of the art in blasting. Fines may also improve the initial leaching rate at the expense of increased ventilation cost and more limited ultimate recovery. Ore grade was found to be less important to solution mining than to conventional copper recovery. Mineralogy was found to be crucial to solution mining economics. Mineralogy affects the grade of the ore and the importance of that grade. Pyrite was found to be helpful in increasing the ultimate copper recovery from ore but, at the same time, was found to reduce the rate of that recovery. Recovery rates were found to be very sensitive to mineral grain size, and calibration of DLS to a given ore may be especially important in this variable. Tortuosity and porosity of ore fragments appear relatively unimportant over the ranges typical of sulfide copper porphyry ore. Modifiedin situ solution mining appears to be a competitive option when a favorable ore type is available. The DLS computer code may be a valuable aid in screening options and selecting the most favorable orebodies.     

Research and Development Activities for the Recovery of Gold From Alkaline Cyanide Solutions

The use of autotrophic and heterotrophic microorganisms in the solublisation of gold from refractory sulphides and oxide ores is discussed. Basic microbiological characteristics of the leaching bacteria are outlined. Reaction mechanisms in the biooxidation of gold-bearing arscnopyrite and pyrite an: illustrated with special reference of Thiobacillus ferrooxidans. Biological and engineering factors influencing biooxidation for the liberation of gold are critically examined. Available reactor designs as welt as pilot-plant operations are brought out. The utility of heterotrophs in the direct leaching of gold is also evaluated. The role of biosorption in the recovery of precious metals from solutions and in environmental control is also examined. The potential of biomineral technology for processing gold-bearing ores and concentrates in the Indian context is highlighted.     

Electrochemical study of enargite bioleaching by mesophilic and thermophilic microorganisms

The bioleaching effects of two microorganisms on two different enargite (Cu₃AsS₄) minerals have been followed by solution titration and rest potential measurements and studied by cyclic voltammetry. The investigation focused on the superficial modifications produced in the minerals by the activity of the mesophilic bacterium Acidithiobacillus ferrooxidans at 35 °C and the thermophilic archaeon Sulfolobus sp. at 68 °C after 3 and 12 days of attack, comparing the modified surfaces with that of untreated samples. The electrochemical characterization of all the electrodes was done by means of potentiometric assays performed under the same conditions: acidulated deionized water as electrolyte (pH 2.0), and temperature of 26 ± 2 °C. Additionally, a comparative SEM and EDX study of untreated and biotreated samples was carried out.