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Microbial dissolution of Zn-Pb sulphide minerals using mesophilic iron and sulphur oxidizing acidophiles
Abstract
In this research, the bioleaching mechanism of zinc and lead from high-grade Zn–Pb ore has been investigated. It is done by using mixed culture of iron and sulfur oxidizing moderate thermophilic bacteria at 45 C. Pulp density, initial pH and ferrous concentration were studied as influential parameters in bioleaching experiments. The optimum conditions were achieved at pulp density = 50 (g/L), initial pH = 1 and FeSO4.7H2O concentration = 75 (g/L) with 98.5% zinc recovery after 25 days treatment. Generally, an
increase in ferrous concentration caused an improve zinc recovery, and an increase in initial pH and pulp density caused reduction in zinc recovery. However, in the test with optimum condition the lead dissolution was just 0.027% due to the lower Pb solubility. Furthermore, cadmium dissolution was 98% under optimum condition and results showed the cadmium dissolution was in direct proportion with zinc dissolution.
Finally, 7.82% of arsenic and 8.52% of antimony dissolved during zinc bioleaching after 25 days treatment, both under above mentioned optimum condition.
... The media pH was maintained at 1.8 with sulfuric acid. A mesophilic bacterial consortium including Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Leptospirillum ferrooxidans which was screened from Angouran Zn-Pb mine's AMD (Ghassa et al., 2015), was used for this experiment. This bacterial consortium was adapted naturally to the high concentration of zinc before being used in the tests. ...
The leaching kinetics of a sphalerite concentrate containing 38.25% zinc was studied in the presence of biological and chemical ferric reagents. To produce the biological ferric reagent (BFR), a pyrite concentrate sample was oxidized to ferric ions by iron and sulfur oxidizing bacteria, and this pregnant leach solution was then applied as oxidizing reagent in ZnS leaching. This process is commonly referred to as two-step bioleaching. This biological reagent contained 12.75 g/l ferric and its pH was 0.86. The chemical ferric reagents (CFR) were made by dissolution of Fe2(SO4)3 and FeCl3 salts in deionized water. Leaching experiments were carried out at different temperatures to study the mechanism of ZnS dissolution and its kinetics. The kinetic modeling of ZnS dissolution with BFR followed the interfacial transfer and diffusion across the product layer mechanism within the first minutes (about 60 min) while it changed to the diffusion-control mechanism after passing this initial period. On the other hand, the ZnS dissolution in presence of ferric sulfate was described by a diffusion mechanism. The surface analysis by SEM and FTIR confirmed that sulfur layer formation on the mineral surfaces could prevent the solvent diffusion to the minerals surface, and consequently it controls the dissolution reaction. The highest zinc recoveries were 70%, 99% and 83% in presence of biological ferric reagent, ferric sulfate and ferric chloride at 90 °C after 200 min, respectively. The zinc recovery for one-step bioleaching was 90% and was achieved after 20 days at 35 °C, by iron and sulfur oxidizing bacteria.
... Bioleaching of copper from ores is carried out in many countries. Copper extraction from ores containing minerals, secondary copper sulphides is carried out in heap bioleaching plants located in different countries (Rohwerder et al., 2003;Ghassa et al., 2015). ...
Biomining is defined as the technologies that utilize microbial community for the extraction of metals from its
ore or wastes and facilitate a greener recovery. Extraction of manganese by biomining is now a thing of the
present and not just a hypothesis, as it was few decades back. The severe industrial importance of manganese
has led to augmented global production of manganese in the last few years which has led to a decrease in the
amount of high grade ores. It has also resulted in pollution of both terrestrial and aquatic ecosystems due to the
generation of massive amounts of manganese containing wastes. Therefore, biomining is now being employed
to recover manganese low grade ores and solid mining wastes which serve a dual purpose of both resource
recycling and bioremediation. Manganese bio recovery can be accomplished by a wide range of bacterial and
fungal strains capable of growing under diverse environmental conditions. They solubilise manganese by direct
and indirect mechanisms thereby aiding its recovery. Bacterial solubilisation is mainly carried out by direct
mechanism which involves the direct contact of the cell with the metal. However fungal solubilisation is mostly
correlated with indirect mechanism which does not require direct contact of the cells with metal particles and
involves solubilisation by the help of bio generated metabolites that mainly includes organic acids. Many
enzymes like Muilticopper oxidase, Manganese reductase and Peptidyl-prolyl-cis-trans isomerise have been
linked to manganese solubilisation. The present scenario of commercial manganese recovery through booming
is very encouraging and this technology holds immense potential for future metal recovery and bioremediation
endeavours.
KEYWORDS: Biomining, Manganese, Bioremediation, Waste, Bacteria, Fungus
... Bioleaching of copper from ores is carried out in many countries. Copper extraction from ores containing minerals, secondary copper sulphides is carried out in heap bioleaching plants located in different countries (Rohwerder et al., 2003;Ghassa et al., 2015). ...
... Acid mine drainages (AMD) is one of the most important problems in mining industries. This metal dissolution of tailing dump can cause lots of environmental problems [39][40][41][42][43][44][45]. Removal of the heavy metals can reduce the toxicity of these drainages. ...
In this research, the bioleaching mechanism of zinc and lead from high-grade Zn–Pb ore has been investigated. It is done by using mixed culture of iron and sulfur oxidizing moderate thermophilic bacteria at 45 °C. Pulp density, initial pH and ferrous concentration were studied as influential parameters in bioleaching experiments. The optimum conditions were achieved at pulp density = 50 (g/L), initial pH = 1 and FeSO4.7H2O concentration = 75 (g/L) with 98.5% zinc recovery after 25 days treatment. Generally, an increase in ferrous concentration caused an improve zinc recovery, and an increase in initial pH and pulp density caused reduction in zinc recovery. However, in the test with optimum condition the lead dissolution was just 0.027% due to the lower Pb solubility. Furthermore, cadmium dissolution was 98% under optimum condition and results showed the cadmium dissolution was in direct proportion with zinc dissolution. Finally, 7.82% of arsenic and 8.52% of antimony dissolved during zinc bioleaching after 25 days treatment, both under above mentioned optimum condition.
In this research, the bioleaching mechanism of zinc and lead from high-grade Zn-Pb ore has been investigated. It is done by using mixed culture of iron and sulfur oxidizing moderate thermophilic bacteria at 45°C. Pulp density, initial pH and ferrous concentration were studied as influential parameters in bioleaching experiments. The optimum conditions were achieved at pulp density=50 (g/L), initial pH=1 and FeSO4.7H2O concentration= 75 (g/L) with 98.5% zinc recovery after 25 days treatment. Generally, an increase in ferrous concentration caused an improve zinc recovery, and an increase in initial pH and pulp density caused reduction in zinc recovery. However, in the test with optimum condition the lead dissolution was just 0.027% due to the lower Pb solubility. Furthermore, cadmium dissolution was 98% under optimum condition and results showed the cadmium dissolution was in direct proportion with zinc dissolution. Finally, 7.82% of arsenic and 8.52% of antimony dissolved during zinc bioleaching after 25 days treatment, both under above mentioned optimum condition.
The origin of a rational (scientific) approach to extraction of metal values from ores with the aid of microorganisms (bioleaching) is traced. The removal by microbiological means of ore constituents that interfere with metal extraction (biobeneficiation), an outgrowth from bioleaching, is also traced. © 2004 SDU. All rights reserved.
Biomining is currently used successfully for the commercial-scale recovery of met- als such as copper, cobalt, and gold from their ores. The mechanism of metal extraction is mainly chemistry-driven and is due to the action of a combination of ferric and hydrogen ions, depending on the type of mineral. These ions are produced by the activity of chemolithotrophic microorganisms that use either iron or sulfur as their energy source and grow in highly acidic conditions. Therefore, metal extraction is a combination of chemistry and microbiology. The mixture of organisms present may vary between processes and is highly dependent on the temperature at which mineral oxidation takes place. In general, rel- atively low-efficiency dump and heap irrigation processes are used for base metal recovery, while the biooxidation of difficult-to-treat gold-bearing arsenopyrite concentrates is carried out in highly aerated stirred-tank reactors. Bioleaching reactions, the debate as to whether the reactions are direct or indirect, the role of microorganisms, and the types of processes by which metals are extracted from their ores are described. In addition, some new processes under development and the challenges that they present are discussed.
A scanning electron microscope (SEM) study was performed to provide a visual insight into the oxidation patterns of sulfide minerals during chemical and bacterial leaching of a complex ore for 3 days. The mineral grains were studied under SEM before and after bacterial and chemical leaching with or without the addition of ferrous iron to generate ferric iron in situ by bacteria or chemical oxidant (MnO2). Both mesophilic and moderately thermophilic cultures of bacteria were used in bioleaching tests. A limited oxidation of sphalerite and pyrite, similar to those in acid leaching (control), was observed to occur when no ferrous iron was added. However, the initial addition of ferrous iron into bioleaching media was shown to significantly improve the oxidation of sphalerite and pyrite. Galena was readily oxidized in the presence or absence of bacteria. Sphalerite was oxidized more extensively/selectively than chalcopyrite and pyrite, consistent with their respective nobility/electrochemical activity. Provided that chemical/biological oxidation of sphalerite was intensive, a sulfur-rich layer appeared to form on mineral surface. But, no such layer on pyrite surfaces was discernable. Supplementary bioleaching data were also provided to support SEM observations and to further elucidate the bioleaching characteristics of these sulfide phases. It can be inferred from this study that the oxidation of sulfides proceeds most discernibly via “indirect mechanism” and the generation of ferric iron by bacteria in sufficient quantity is essential for the effective oxidation of sulfide minerals.
This research aimed at finding the factors with more impact on the bioleaching of a spent hydrocracking catalyst by thermophilic acidophilic archaea called Acidianus brierleyi and determining the optimal condition in order to reach the maximum metals recovery. Initially, four more significant factors affecting the bioleaching efficiency including pH, pulp density, inoculation and elemental sulfur concentration, were screened and identified among ten key factors using Plackett-Burman factorial design. Next, a central composite design was applied to obtain the optimum conditions for achieving the maximum efficiency of the bioleaching process. pH 1.6, pulp density 0.6% (w/v), inoculation 4% (v/v) and elemental sulfur concentration 4 g/L were determined as the best conditions to reach the maximum recovery of the process. The experimental recoveries in optimal conditions were 35%, 83% and 69% for Al, Mo and Ni, respectively. To better understanding the mechanism of the leaching reaction, the rate limiting step of the process was determined under the optimum conditions and it revealed that diffusion through product layer is the rate controlling step.
The leachability tests for manufacturing scrap TV boards (STVB) have indicated the release of metals beyond the limit levels with potential problems for environmental pollution. Treatment of STVB is therefore requisite for its safe disposal in landfills. The nitric acid leaching of STVB for the removal/recovery of valuable metals (Cu and Ag) was studied by adopting the Box–Behnken design. Statistical analysis of data has revealed that the concentration of nitric acid is the most influential parameter affecting the leaching process. Effects of solids ratio and temperature on the rate and extent of the extraction of copper were also proved to be statistically significant. However, the interaction effects of these parameters were found to be insignificant. The leaching kinetics were consistent with the shrinking particle model under chemical control with an activation energy of 38.6 kJ/mol. High concentrations of nitric acid (2–5 M HNO3) were required to achieve high copper extractions (88.5–99.9%) at a pulp density of 6% w/v. The extraction of silver was enhanced from 14% to 68% with increasing the concentration of nitric acid from 1 to 5 M. These findings also demonstrate that copper may well be selectively extracted from STVB by adjusting the concentration of nitric acid.
Mixed mesophilic and extreme thermophilic bioleaching were evaluated to remove copper from the molybdenite concentrate. Bioleaching tests were carried out in shake flasks and in a 50-L bioreactor. The shake flask tests were performed with different inoculum size, solids density, pH, and temperature in order to identify optimum conditions. The highest amount of copper elimination, 75% was obtained with extreme thermophilic microorganisms (at 12% inoculation, 10% solids, 65 °C and a pH of 1.5). The highest copper elimination by mesophilic microorganisms was 55% (at 12% inoculation, 5% solids, 30 °C at pH 2). The optimum conditions in shake flask tests were applied to 7 days batch tests in a 50-L bioreactor. Extreme thermophilic experiment gave the best copper elimination of 60% (at 12% inoculation, 10% solids, 65 °C and pH 1.5). Mesophilic test removed 50% of the copper (at 12% inoculation, 10% solids, 35 °C at pH 2).
Focusing bands, as “hot spots”, severely influence the clean-up verification of contaminated soil remediation projects using electrokinetic remediation (EKR) technology. In this experiment, artificial high conductance bands (HCB) were created in the Cr(VI)-contaminated kaolin–gypsum soil to imitate the conductance heterogeneity of nature soils. Potential gradient monitoring was used to trace the behavior of HCBs. Visual observation and X-ray fluorescence analysis were used to trace the generation and moving patterns of the focusing bands caused by HCBs. Results show that the HCBs caused focusing bands in the soil by ion-induced potential gradient trapping effect (IIPGWTE), not chemical precipitation or isoelectric effect, and the heavier the HCB, the heavier the focusing phenomenon. The HCBs gradually disappeared due to the ionic diffusion increase caused by temperature increase during EKR, whereas the focusing bands remained and moved toward the anodes. In addition to causing focusing bands, some HCBs also could prolong the EKR duration resulting in higher energy consumption. In this case, the HCB’s location plays more significant role than its magnitude. Only the HCB that is caught up with by the tail of chromium (VI) plug-flow can slow down the EKR process resulting in lower energy efficiency.
In this study the performance of Fe(III) formed in situ, generated by oxidizing Fe(II) with H2O2 or KMnO4 (denoted as H2O2–Fe(II) or KMnO4–Fe(II) process), for As(V) removal was compared with FeCl3 coagulation and the mechanisms were explored. The optimum oxidant/Fe(II) molar ratios for the H2O2–Fe(II) and KMnO4–Fe(II) processes were 1:2 and 1:3, respectively. The advantage of Fe(III) formed in situ over pre-formed Fe(III) for As(V) removal was strongly dependent on the Fe/As ratio and the advantage became less significant at high Fe/As ratio. The hydrolysis of Fe(III) formed in situ generated much more Feb species than the pre-formed Fe(III) did. Moreover, Fe(III) formed in situ hydrolyzed more slowly and generated flocs with smaller size compared to the pre-formed Fe(III) did over the pH range of 6.0–8.0. An introduction of high shear force for 1 min in the flocculation stage to break the aggregates into small particles enhanced the uptake of As(V) by pre-formed Fe(III) to a larger extent than that by Fe(III) formed in situ. The good performance of Fe(III) formed in situ for As(V) removal should be ascribed primarily to slow floc aggregation rate and small floc size and secondly to the formation of large amount of polymeric hydrolysis species.
The leaching kinetics of zinc residue, having total Zn content of 12.31%, along with other metallic components such as Fe and Pb, is leached using sulfuric acid as solvent, augmented with ultrasound is presented. The effects of variables such as the leaching temperature, sulfuric acid concentration, particle size, liquid/solid ratio and the ultrasound power have been assessed. The results show the maximum recovery of zinc to be 80% at an ultrasound power of 160 W, leaching temperature of 65 °C, sulfuric acid concentration of 1.4 mol/L, particle size range of 74–89 μm and liquid/solid ratio of 4. The kinetics of leaching is modeled using shrinking core model and the rate controlling step is identified to be the diffusion through the product layer. The raw and the leached residue are characterized using XRD and SEM/EDX analysis. The activation energy is estimated to be 6.57 KJ/mol, while the order of reaction with respect to sulfuric acid concentration is 0.94 and particle size is 0.12 respectively.
A study was carried out to examine the possibility for Aspergillus niger strain KBS4 to bioleach metals from sulphide ore with low concentration of arsenic and to optimize the parameters that affect this process by orthogonal array optimization. Fungal sample was collected, purified and sequenced. The bioleaching process was optimized with L25 Taguchi orthogonal experimental array design. Five factors were investigated and 25 batch bioleaching tests were run at five levels for each factor. The parameters were initial pH, particle size, pulp density, initial inoculums and residence time for bioleaching. The experimental results showed that under optimized leaching conditions: pH 5.5, particle size 180 μm, initial inoculums size 3×107 spores per ml, pulp density 15% and residence time of 20 days, the bioleach ability of metals were 63% Fe, 68% Zn, 60% As, 79% Cu and 54% Al. The biosorption of metal ions by fungal biomass might occur during the bioleaching process but it did not hinder the removal of metal ions by bioleaching.
The application of the response surface methodology and the central composite design (CCD) technique for modeling and optimization of the influence of some operating variables on copper, molybdenum and rhenium recoveries in a bioleaching process was investigated. Three main bioleaching parameters, namely pH, solid concentration and inoculum percent, were changed during the bioleaching tests based on CCD. The ranges of the bioleaching process variables used in the design were as follows: pH 1.46–2.14, solid concentration 0.95%–11.05%, and inoculum percent 1.59%–18.41%. A total of 20 bioleaching tests were carried out by the CCD method according to software-based designed matrix. Empirical model equations were developed according to the copper, molybdenum and rhenium recoveries obtained with these three parameters. Model equations of responses at the base of parameters were achieved by using statistical software. The model equations were then individually optimized by using quadratic programming to maximize copper, molybdenum and rhenium recoveries individually within the experimental range. The optimum conditions for copper recovery were pH 1.68, solid concentration 0.95% and the inoculum 18.41% (v/v), while molybdenum and rhenium recoveries were 2.18% and 24.41%, respectively. The predicted values for copper, molybdenum and rhenium recoveries were found to be in good agreement with the experimental values. Also jarosite formation during bioleaching tests was also investigated.
A study of the effect of different variables (inoculation, pulp density, [Ag], nutrient medium, pH and [Fe3+]) on the silver-catalyzed bioleaching of a low-grade copper sulfide ore has been carried out in shake flasks. Chalcopyrite was the dominant copper mineral in the ore. Preliminary tests showed that addition of other ions (Sb, Bi, Co, Mn, Ni and Sn) did not enhance the copper dissolution rate. Conversely, an inoculation with mesophilic microorganisms and the addition of silver had a markedly catalytic effect on the extraction of copper. The kinetics of the silver-catalyzed chalcopyritic ore bioleaching was greatly affected by pulp density and silver concentration. Small amounts of silver (14.7 g Ag/kg Cu) dramatically accelerated the copper dissolution process while large amounts (294.12 g Ag/kg Cu) had an inhibitory effect. The copper dissolution rate was slightly affected in the range of pH between 1.2 and 2.5 but was significantly slower at pH 3.0. The effect of [Fe3+] in the presence of silver was studied both in abiotic and biotic conditions. High ferric iron concentrations in abiotic tests recovered similar copper amounts (∼ 95%) to those obtained without or with low [Fe3+] in the presence of bacteria. The leaching of copper from the low-grade copper ore can be very effectively enhanced with silver and mesophilic microorganisms. For that system, the onset of oxidizing conditions starts at an Eh value slightly higher than 650 mV. Above that critical value of potential the copper dissolution rate slows down. This also corresponds with the completion of the leaching process. As the potential rises past 650 mV, the copper extraction reaches a plateau.
In this study, we have investigated the bioleaching potential of a native strain of Acidithiobacillus ferrooxidans isolated from zinc and lead sulfide mines under varied ambient conditions of growth and substrate consumption. The effects of intermittent irrigation, type of agglomeration, feed formulation in terms of acid, enriched salt solution, initial number of microorganism, and their interactions on the bioleaching of low-grade zinc sulfide ores (containing 5.78% zinc) on growth and biooxidation efficiency of the bacteria have been evaluated. Bioleaching capacities of the isolate were optimized by the utilization the Taguchi method (e-qualitic-4) for design of experiments. As a result, eight column bioreactors with 5.7kg ore (100%
Both Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans attached to pyrite rapidly, reaching the adsorption equilibrium in 20–25min. Furthermore, the adsorption content of substrate-grown bacteria was larger than that of solution-grown ones. Zeta potential studies revealed that isoelectric points (IEP) of pyrite shifted to lower pH after interacting with bacteria.Iron solubilisation by the mixed culture was faster than that for the A. ferrooxidans. There was no close relation between iron recovery and fineness. Circular, square and elongated etch pits were visible by scanning electron microscopy (SEM). Moreover, jarosite in the leaching of 3%(w/v) pulp density was detected by Fourier transform infrared spectroscopy(FTIR) and X-ray diffraction(XRD), though the pH of the solutions declined to below 1.00. S was not detected in the leaching residues. Furthermore, the contact angles showed there was no S produced. These results point to a thiosulfate mechanism in pyrite bioleaching, where the thiosulfate leads to sulphate without S formation.
Bioleaching processes for extraction of zinc from sphalerite are more environmentally friendly and consume less energy than conventional technologies but are as yet less economic. One necessary step towards arriving at a cost-effective sphalerite bioleaching process is the use of appropriate methodology for the optimization of pertinent factors in such processes. Previous studies on Zn bioleaching systems have reported a fairly wide range of values as the optimum level of relevant physicochemical parameters for Zn bioleaching processes. This is partly due to the different strains and Zn source type employed but another reason could be that important parameters in this process interact with each other. In order to shed more light on this matter, in the present work Response Surface Methodology was employed for the study and optimization of important factors in a sphalerite bioleaching process by Acidithiobacillus ferrooxidans using shaking bioreactors. The effect of change in the levels of temperature, pH, initial Fe(II) concentration and pulp density – in the range 30–36°C, 1.4–2.0, 3–11gL−1 and 4–6% wt/vol respectively – on the rate Zn bioleaching was studied using a Central Composite Design. The results showed a statistically significant effect of pH and pulp density – and to a lesser extent temperature and initial Fe(II) concentration – on the rate of bioleaching of Zn. A statistically significant interaction was found between pH and temperature, which means that the optimum values of these two parameters can only be correctly obtained through the use of factorial design of experiments. Additionally, the optimum level of temperature and pH was found to depend on the level of pulp density. This means that when employing shaking bioreactors for optimization of these parameters the level of pulp density should be carefully chosen. However, there was no statistically significant interaction between initial Fe(II) concentration and the other three factors studied.
Acid mine drainage (AMD) represents a serious environmental problem related to sulfide minerals and coal mining. High content of toxic metals and high acidity in AMD adversely affects surface water, groundwater and soil. The abandoned mine of the Smolník deposit in Slovakia is a typical example in this respect. The quality of AMD needs to be monitored and suitable treatment methods need to be developed.The aim of this paper was to demonstrate the technical feasibility of heavy metals removal from AMD using physical–chemical and biological–chemical methods. The base of the physical–chemical method was electrowinning. The principle of the biological–chemical method was the selective sequential precipitation (SSP) of metals with the application of hydrogen sulfide produced by sulfate-reducing bacteria and sodium hydroxide solution. Both the electrowinning and SSP processes decrease the content of heavy metals in AMD. The pre-treatment of AMD by chemical iron–aluminum precipitation (in the case of electrowinning tests) and chemical iron precipitation (in the case of SSP tests) improved the selectivity of the processes. A further aim of the work was the improvement of the SSP.During the electrochemical experiments, 99% Zn removal – under metallic form – and 94% Mn removal – under MnO2 form – both with a high degree of purity, were achieved. The SSP process reached the selective precipitation of chosen metals with 99% efficiency – Fe, Al and Mn in the form of metal hydroxides, Cu and Zn as metal sulfides. The results achieved may be used for designing a process appropriate for the selective recovery of metals from the AMD discharged from the Smolník deposit.
Previous studies identifying Fe3+ as the main oxidizing agent in CuFeS2 bioleaching suggests that if the precipitation of additional Fe3+ could be prevented, the Cu extraction yields should be enhanced. In this respect, the acid-generating nature of the biologically mediated oxidation of additional S0 to H2SO4 should theoretically serve as a pH regulator as well as biomass generator. This should prevent the precipitation of Fe3+ and attenuate the biomass reduction caused by the inhibitory effect of high Fe3+ concentrations. To prove the former hypothesis, three thermophile strains were employed for bioleaching of chalcopyrite under various additional S0 and Fe3+ concentrations. The hypotheses about additional S0 application were fully confirmed; the addition of S0 alone enhanced the leaching rates with A. brierleyi and M. sedula in media at initial pH 2. Although higher initial leaching rates were obtained with additional Fe3+, its role as the main leaching factor is questioned; leaching with thermophiles appears to depend on the availability of protons and ORP rather than on the prevention of Fe3+ precipitation. Additional S0 in media with high Fe3+ concentrations has shown the best improvements in the case of bioleaching with A. brierleyi, whereas improvements in bioleaching with M. sedula and S. metallicus were less notable.
Bioleaching, also referred to as minerals biooxidation, and bioremediation have been widely employed commercially for heap and dump bioleaching of secondary copper sulfide ores, sulfidic-refractory gold concentrates and treatment of acid rock drainage. Technical and commercial challenges, identified in this paper, remain for bioleaching of primary sulfides and complex ores. New frontiers for the technology exist in processing massive sulfides, silicate-locked minerals and in the more distant future in-situ leaching. Decommissioning of cyanide heap leach operations and stabilizing mine wastes using biotechnology are opportunities requiring intensive and focused research, development and engineering efforts.
The purpose of this study was to test the feasibility of using iron and sulphur oxidizing bacteria for the acid leaching of a high grade Pb–Zn ore material. Three strains (ATCC 13661, NCIMB 13537 and C2-TF) of Acidithiobacillus ferrooxidans and two strains (MT-TH1 and MT-13) of Sulfobacillus thermosulfidooxidans were tested in this study. The bioleaching was monitored by measuring the dissolved metals and by X-ray diffraction analysis of leach residues. The bioleaching efficiency varied between 0.014 and 0.35. The maximum dissolution of lead was achieved with the mesophilic At. ferrooxidans (NCIMB 13537) at 30 °C. The maximum recovery of zinc was achieved with moderately thermopilic S. thermosulfidooxidans (MT-TH1) at 45 °C.
The electrochemical response of massive chalcopyrite electrodes bioleached with mesophile microorganisms in both presence and absence of Ag(I) ions was studied using linear and cyclic voltammetry. The electrolyte employed for the electrochemical tests was the medium used to bioleaching the electrodes (3.0 g/L (NH4)2·SO4, 0.5 g/L K2HPO4, 0.5 g/L MgSO4·7H2O and 0.1 g/L Ca(NO3)2).The results showed small differences in voltagramms carried out in presence and absence of Ag(I): In bioleached electrodes tested in presence of Ag(I) ions, the anodic and cathodic current signals were bigger than when Ag(I) ions were not present. Moreover, the passivation of chalcopyrite occurred at two different times: First, the passivation was observed in the bioleached electrodes in absence of Ag(I) and after in the bioleached electrodes in presence of Ag(I). The pre-wave observed in the anodic dissolution of the chalcopyrite was not seen in the bioleached electrodes in absence of Ag(I) ions, but in the electrode treated with bacteria in presence of Ag(I) ions an increase in current signals was observed.
The technical viability of the BRISA process (Biolixiviación Rápida Indirecta con Separación de Acciones: Fast Indirect Bioleaching with Actions Separation) for the copper recovery from chalcopyrite concentrates has been proved. Two copper concentrates (with a copper content of 8.9 and 9.9 wt.%) with chalcopyrite as the dominant copper mineral have been leached with ferric sulphate at 12 g/L of ferric iron and pH 1.25 in agitated reactors using silver as a catalyst. Effects of temperature, amount of catalyst and catalyst addition time have been investigated. Small amounts of catalyst (from 0.5 to 2 mg Ag/g concentrate) were required to achieve high copper extractions (>95%) from concentrates at 70 °C and 8–10 h leaching. Liquors generated in the chemical leaching were biooxidized for ferrous iron oxidation and ferric regeneration with a mixed culture of ferrooxidant bacteria. No inhibition effect inherent in the liquor composition was detected. The silver added as a catalyst remained in the solid residue, and it was never detected in solution. The recovery of silver may be achieved by leaching the leach residue in an acid-brine medium with 200 g/L of NaCl and either hydrochloric or sulphuric acid, provided that elemental sulphur has been previously removed by steam hot filtration. The effect of variables such as temperature, NaCl concentration, type of acid and acidity–pulp density relationship on the silver extraction from an elemental sulphur-free residue has been examined. It is possible to obtain total recovery of the silver added as a catalyst plus 75% of the silver originally present in concentrate B (44 mg/kg) by leaching a leach residue with a 200 g/L NaCl–0.5 M H2SO4 medium at 90 °C and 10 wt.% of pulp density in two stages of 2 h each. The incorporation of silver catalysis to the BRISA process allows a technology based on bioleaching capable of processing chalcopyrite concentrates with rapid kinetics.
Affecting factors, such as ore size, leaching temperature, holding time, alkaline concentration and liquid: solid ratio (volume/weight) (L/S), were studied to leach refractory hemimorphite [Zn4(Si2O7)(OH)·H2O] zinc oxide ores with NaOH solution in this paper. The impact of leaching recovery of Zn and its concomitant metals was checked through experiments. Results showed that when the ores of 65–76 µm size were leached for 2 h at 358 K in the presence of 5 mol/L sodium hydroxide and liquid:solid ratio of 10:1, the leaching rate of Zn, Al, Pb and Cd were about 73%, 45%, 11% and 5%, respectively, but that of Fe was less than 1‰. Moreover, the leaching experiments were repetitive and reliable. And the kinetic study indicates that the calculated activation energy is 45.7 kJ/mol, which illuminates that alkaline leaching the refractory hemimorphite [Zn4(Si2O7)(OH)⁎H2O] zinc oxide ores is controlled by the chemical process of the reaction of leach liquor and ores in the whole leach process.
The development of molecular tools for the detection and quantification of both species as well as functional traits, aids in a better understanding and control of microbial processes. Presently, these methods can also be used to assess the activity of these organisms or functions, even in complex ecosystems and difficult matrices such as ores and low pH samples. In this paper we present the versatility of one of these tools, Q-PCR, to allow accurate and fast insight in changes in two types of microbial processes representing two ways in which microbes can interact with metals, bioleaching and bioprecipitation. Using the Q-PCR technique it was possible to identify and quantify the thermoacidophilic archaeon Acidianus sp. to be the main microbial strain responsible for biooxidation of arsenite in a low pH reactor. The method was also used to study the dynamics between the iron oxidizing and sulfur oxidizing acidophiles during bioleaching of a zinc concentrate in a batch reactor system and showed that the iron oxidizer Leptospirillum ferriphilum that dominated the starting culture disappeared upon addition of the concentrate. Gradually, bacterial activity was regained starting with growth of sulfur oxidizers and at later stage iron oxidizers started to grow. Molecular analysis can be used to direct research to the relevant organisms involved and concentrate on improving their application (in the arsenite case Acidianus sp.) or in understanding appearances and disappearances of microorganisms (during leaching of zinc concentrate the disappearance of Leptospirillum after high inoculation levels) in order to allow optimization of leaching efficiencies at the lowest (oxygen) costs.
The extraction of molybdenum and cobalt from spent petroleum catalyst (Co–Mo/Al2O3) was investigated using sulphuric and nitric acid solution. Direct leaching of spent catalyst with sulphuric acid was not effective, whereas the combination of sulphuric and nitric acid was significant for the recovery of both molybdenum and cobalt. The effect of experimental factors such as reaction time, acid concentration, temperature, solid–liquid ratio and particle size were studied to determine the best conditions for the solubilisation of metal values. The parameters, temperature and nitric acid concentration are found to be critical factors especially on leaching of molybdenum from the spent catalyst. Under optimum conditions (pulp density 10% (w/v), [H2SO4] 0.5 M, [HNO3] 4.0 M, particle size 51–70 μm, temperature 50 °C and time 5 h), about 99.7% of molybdenum and 99.6% of cobalt could be extracted with 14.9% extraction of aluminium.
Statistically based experimental designs were applied to screen and optimize the bioleaching of spent hydrocracking catalyst by Penicillium simplicissimum. Eleven factors were examined for their significance on bioleaching using a Plackett-Burman factorial design. Four significant variables (pulp density, sucrose, NaNO(3), and yeast extract concentrations) were selected for the optimization studies. The combined effect of these variables on metal bioleaching was studied using a central composite design (CCD). Second-order polynomials were established to identify the relationship between the recovery percent of the metals and the four significant variables. The optimal values of the variables for maximum metals bioleaching were as follows: pulp density (4.0%, w/v), sucrose (90 g/L), NaNO(3) (2 g/L) and yeast extract (0.36 g/L). The maximum metals recovery percentages from the predicted models were 97.6% Mo, 45.7% Ni, and 14.3% Al. These values were in perfect agreement with the actual experimental values, which were (98.8 +/- 0.9)% Mo, (46.5 +/- 0.6)% Ni, and (13.7 +/- 0.4)% Al. The growth kinetics of the fungus in the presence of the spent catalyst at various pulp densities (2-11%) and optimal condition was modeled using the modified Gompertz model. The kinetic parameters in the system were estimated using MATLAB R2008a. Results showed that the modified Gompertz model fit the experimental data well. The relationship between the specific growth rate and pulp density was found by modifying the Luong inhibition model which gave maximum specific growth rate of 0.034 day(-1), optimal pulp density of 3.95% w/v and critical inhibitor concentration of 10.9% w/v.
In this research, the role of zinc ferrite in the hydrometallurgical recovery of zinc in both primary zinc production and zinc-containing waste treatment processes is discussed. In the present work, experiments were performed on the caustic leaching of pure zinc ferrite. The effects of leaching time, caustic concentration and temperature were investigated in the deceleratory period. Atomic Absorption Spectroscopy (AAS) and X-Ray Diffraction (XRD) analyses were employed to analyze both the leach solutions and the residues. It was found that the percentage of decomposed zinc ferrite increased linearly with increasing leaching time. Although the decomposition rate increased with the caustic concentration, it was limited by the high viscosity of the concentrated leach solutions. The maximum percentage of decomposed zinc ferrite was only about 9% under the experimental conditions. Based on the experimental results, it was postulated that the dissolution rate of zinc ferrite in the deceleratory period, was controlled by the diffusion of zinc ions in the imperfect zinc ferrite lattice. The dissolution reaction could be represented as follows:
Zn(II) and Pb(II) from Nigerian sphalerite and galena ores were bioleached by a mixed culture of acidophilic bacteria. The influences of pH and ferric ion on the bioleaching rates of sphalerite and galena were examined. The result shows that pH 2.1 and 2.7 are favourable for the leaching of Zn(II) and Pb(II) from sphalerite and galena, respectively. It was observed that the use of agarose-simulated media caused cells to excrete exopolymers containing ferric ions which enhanced oxidation. The oxidation equilibrium for sphalerite and galena took 3 and 4 d, respectively. About 38.3% sphalerite and 34.2% galena were leached within 1 d and approximately 92.0% Zn(II) and 89.0% Pb(II) were recovered in 5 d, respectively. The unleached residual products were examined by X-ray diffraction for sphalerite, revealing the presence of elemental sulphur(S), zinc sulphate (ZnSO4) and few traces of calcium aluminate (Ca3Al2O6). The XRD pattern also indicates the presence of elemental sulphur (S), lead sulphate (PbSO4) and few traces of itoite [Pb(S,Ge)(O,OH)4] and cobalt lead silicate [Pb8Co(Si2O7)3] in the unleached galena ore.
In the present study, the effect of some important parameters including particle size, pulp density and temperature on the rate of Zn dissolution from sphalerite concentrate by Acidithiobacillus ferrooxidans was investigated. The highest rate of sphalerite bioleaching was obtained at particle size, pulp density and temperature of 38–150 μm, 4% wt/vol and 33 °C respectively. The formation of a product layer over sphalerite concentrate particles was confirmed by SEM images whereas XRD, EDS and BET analysis showed that this layer is composed of elemental sulfur and is non-porous. Based on these results, it was decided that a kinetic model in which the rate of Zn dissolution is limited by diffusion of ions through a non-porous product layer is appropriate to describe the sphalerite bioleaching process. Determinations of ferrous iron ion concentration during bioleaching showed that the concentration of this ion varies significantly during bioleaching and hence the kinetic model was revised to take account of this fact. The predictions of this model had a good compatibility with the experimental data and the value of activation energy was determined as 39 kJ/mol.
The main concern of the present study is to find a more feasible and economical method to extract metal ions from low grade discarded ores like black shale by Aspergillus niger. A.niger exhibited a good potential in generating varieties of organic acids effective for metal ions solubilization. The effectiveness of organic acids was enhanced when sulphuric acid was added to the medium. Different agricultural wastes as substrates were evaluated. Maximum solubilization of copper (68.5%), zinc (49.0%) and cobalt (60.4%) was achieved in the media containing mango peel, rice bran and glucose as substrates. The extraction of low concentrations of metal ions from this ore indicated that this low grade discarded ore may be a potential source for metal ions in the future.
The acidified ferric chloride leaching of an artificial matte was carried out to investigate the extraction behaviour of copper, nickel, and cobalt. The composition of the synthetically prepared Cu–Ni–Co–Fe matte was: 24.95% Cu, 35.05% Ni, 4.05% Co, 11.45% Fe, and 24.5% S. The major mineral phases of the matte were: CuFeS2, CuS2, (FeNi)9S8, (FeNi)S2, Ni9S8, Ni3S2, (CoFeNi)9S8 and Co metal. The effects of time (0–7 h), temperature (30–90 °C), FeCl3 (0.5–2.0 M) and HCl (0.1–0.5 M) concentrations were studied with and without the presence of an organic solvent (CCl4). The Cu extraction was fast compared to Ni and Co and reached equilibrium within 3 h in FeCl3 medium. The extraction of all the three metals increased with increase of temperature. The effect was found to be the maximum for Co, followed by Ni and Cu. Under optimum conditions (1.5 M FeCl3, 0.3 M HCl, 90 °C and 7 h), 99.5% Cu, 93.2% Ni and 85.2% Co was extracted to the solution. When carbon tetrachloride was added to the reaction medium, the extraction of metals increased due to the solubilisation of elemental sulphur coated around the sulphide mineral particles. From the XRD study the undissolved Ni phase was found to be (FeNi)9S8 and undissolved Co phase was cobalt metal.
This study investigates the bioleaching of the complex Pb/Zn ore/concentrate using mesophilic (at 30 jC), moderate (at 50 jC), and extreme thermophilic (at 70 jC) strains of acidophilic bacteria. The effects of bacterial strain, pH, iron precipitation, and external addition of Fe 2 + on the extraction of zinc were evaluated. The results have shown that the ore is readily amenable to the selective extraction of zinc and lead using the acidophilic strains of bacteria [i.e., majority of lead (>98%) reports to the residue]. Moderate thermophiles displayed superior kinetics of dissolution of zinc compared with the other two groups of bacteria. The pH was found to exert a profound effect on the leaching process controlling the bacterial activity and precipitation of ferric iron mainly as K-jarosite. The K + released presumably from the alteration of the silicate phases such as K-feldspar present in the ore appeared to promote the formation K-jarosite in moderately thermophilic leaching systems. The external addition of iron was shown to be required for the bacteria to efficiently drive the extraction of zinc from the bulk concentrate. These findings place the emphasis on the prime importance of ferric iron for the dissolution of zinc and of mineralogical properties (i.e., iron and silicate content) of an ore/concentrate to be treated via bioleaching processes. D 2004 Elsevier B.V. All rights reserved.
The Angouran Zn-(Pb–Ag) deposit, Zanjan Province, NW Iran, is located within the central Sanandaj-Sirjan Zone of the Zagros orogenic belt. The deposit has proven and estimated resources of 4.7Mt of sulfide ore at 27.7% Zn, 2.4% Pb, and 110g/t Ag, and 14.6Mt of oxidized carbonate ores at 22% Zn and 4.6% Pb. It is hosted by a metamorphic core complex that is unconformably overlain by a Neogene volcanic and evaporite-bearing marine to continental sedimentary sequence. The sulfide orebody, precursor to the significant nonsulfide ores, is located at the crest of an open anticline at the contact between Neoproterozoic to Cambrian footwall micaschists and hanging wall marbles. 40Ar–39Ar data on muscovite from mineralized and unaltered footwall micaschists suggest a rapid Mid-Miocene exhumation of the metamorphic basement (∼20Ma) and yield an upper age constraint for mineralization. The fine-grained sulfide ore is massive, replacive, often brecciated, clearly postmetamorphic and dominated by Fe-poor sphalerite, with minor galena, pyrite, anhydrite, quartz, muscovite, dolomite, and rare calcite. Sphalerite contains Na–Ca–Cl brine inclusions (23–25mass% total dissolved solids) with homogenization temperatures of 180–70C. Fluid inclusion chemistry (Na–K–Li–Ca–Mg–Cl–Br), ore geochemistry, S, and Pb isotope data suggest that the Angouran sulfide ore formed by the interaction of modified, strongly evaporated Miocene seawater and the lithotypes of an exhumed metamorphic core complex. Minor contributions of metals from Miocene igneous rocks cannot be excluded. Mineralization occurred in a collisional intra-arc setting with high heat flow, probably during the transition from an extensional to a compressional regime. The Angouran deposit may represent a new type of low-temperature carbonate-hosted Zn–Pb ore that is distinct from Mississippi Valley type and sedimentary-exhalative deposits.
Acid mine drainage (AMD) from the Iberian Pyrite Belt has high acidity and metal concentrations. Earlier pilot experiments, based on limestone sand dispersed in wood shavings (dispersed alkaline substrate; DAS) have been shown to be an efficient treatment option. However, complete metal removal was not achieved, principally due to the high ferrous iron concentration in the inflow AMD. In order to oxidize and remove iron, a natural Fe-oxidizing lagoon (NFOL) was added prior to treatment with limestone-DAS. The NFOL comprises several pre-existing Fe-stromatolite terraces and cascades, and a lagoon with a volume of 100 m(3) built near the mine shaft. Downstream of the NFOL, the limestone-DAS treatment consists of two reactive tanks of 3 m(3) each filled with limestone-DAS reactive substrate, connected in series with two decantation ponds of 6 m(3) each and several oxidation cascades. The AMD emerging from the mine shaft displayed a pH near 3, a net acidity of 1800 mg/L as CaCO(3) equivalents, and mean concentrations of 440 mg/L Zn; 275 mg/L Fe (99% Fe(II)); 3600 mg/L SO(4); 250 mg/L Ca; 100 mg/L Al; 15 mg/L Mn; 5 mg/L Cu; and 0.1-1 mg/L As, Pb, Cr, Cd, Co, and Ni. The oxidation induced in the NFOL enhanced ferric iron concentration, showing an average of 65% oxidation and 38% retention during the monitoring period. The whole system removed a mean of 1350 mg/L net acidity as CaCO(3) equivalents (71% of inflow); corresponding to 100% of Fe, Al, Cu, Pb and As, and 6% of Zn.
The dissolution of metal sulfides is controlled by their solubility product and thus, the [H+] concentration of the solution, and further enhanced by several chemical mechanisms which lead to a disruption of sulfide chemical bonds. They include extraction of electrons and bond breaking by [Fe3+], extraction of sulfur by polysulfide and iron complexes forming reactants [Y+] and electrochemical dissolution by polarization of the sulfide [high Fe3+ concentration]. All these mechanisms have been exploited by sulfide and iron-oxidizing bacteria. Basically, the bacterial action is a catalytic one during which [H+], [Fe3+] and [Y+] are breaking chemical bonds and are recycled by the bacterial metabolism. While the cyclic bacterial oxidative action via [H+] and [Fe3+] can be called indirect, bacteria had difficulties harvesting chemical energy from an abundant sulfide such as FeS2, the electron exchange properties of which are governed by coordination chemical mechanisms (extraction of electrons does not lead to a disruption of chemical bonds but to an increase of the oxidation state of interfacial iron). Here, bacteria have evolved alternative strategies which require an extracellular polymeric layer for appropriately conditioned contact with the sulfide. Thiobacillus ferrooxidans cycles [Y+] across such a layer to disrupt FeS2 and Leptospirillum ferrooxidans accumulates [Fe3+] in it to depolarize FeS2 to a potential where electrochemical oxidation to sulfate occurs. Corrosion pits and high resolution electron microscopy leave no doubt that these mechanisms are strictly localized and depend on specific conditions which bacteria create. Nevertheless, they cannot be called ‘direct’ because the definition would require an enzymatic interaction between the bacterial membrane and the cell. Therefore, the term ‘contact’ leaching is proposed for this situation. In practice, multiple patterns of bacterial leaching coexist, including indirect leaching, contact leaching and a recently discovered cooperative (symbiotic) leaching where ‘contact’ leaching bacteria are feeding so wastefully that soluble and particulate sulfide species are supplied to bacteria in the surrounding electrolyte.
The kinetics and rate limiting sub-processes in the bacterial oxidation of zinc sulphide with Thiobacillus ferrooxidans were examined. The oxidation rate of synthetic ZnS in the presence and absence of bacteria at equal ferrous and ferric iron concentration and pH, was measured in parallel batch experiments. Bacteria oxidize Fe2+ produced in the chemical oxidation of ZnS with Fe3+. In the fermenter without bacteria hydrogen peroxide was added to regenerate ferric iron and maintain the Fe2+ concentration equal to that in the fermenter with bacteria. No significant differences in the oxidation rates of zinc sulphide were found between the slurry with and without bacteria at equal Fe2+ and Fe3+ concentrations. Consequently, an indirect mechanism (i.e. chemical oxidation of ZnS with Fe3+ to Zn2+, S0 and Fe2+, and bacterial oxidation of S0 to SO42− and Fe2+ to Fe3+) determines the oxidation rate of ZnS in the bacterial oxidation process. From the experiments it was shown that T. ferrooxidans prefers S0 as a substrate and the regeneration of Fe3+ can even become terminated. From the experimental data several suggestions for improved process design and experimental research are given.
The catalytic effect of activated carbon on the bioleaching of low-grade primary copper sulflde ores using mixture of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans was investigated. The results show that the addition of activated carbon can greatly accelerate the rate and efficiency of copper dissolution from low-grade primary copper sulflde ores. The solution with the concentration of 3.0 g/L activated carbon is most beneficial to the dissolution of copper. The resting time of the mixture of activated carbon and ores has an impact on the bioleaching of low-grade primary copper sulflde ores. The 2 d resting time is most favorable to the dissolution of copper. The enhanced dissolution rate and efficiency of copper can be attributed to the galvanic interaction between activated carbon and chalcopyrite. The addition of activated carbon obviously depresses the dissolution of iron and the bacterial oxidation of ferrous ions in solution. The lower redox potentials are more favorable to the copper dissolution than the higher potentials for low-grade primary copper sulfide ores in the presence of activated carbon.
The biological method has been considered as an efficient and cost-effective alternative to physicochemical treatment technologies for soil remediation. A bioleaching process was investigated for removal of heavy metals from metal-contaminated soil in this study. The optimal bioleaching operating parameters i.e., soil solid content and sulfur (substrate) dosage for this bioleaching process were studied using central composite design (CCD) and response surface methodology (RSM). Results showed that the rate of pH reduction was found decreasing with increasing of soil solid content because high soil solid content had high buffering capacity. Furthermore, the rate of pH reduction increased with an increase of sulfur dosage in the bioleaching. The maximum metal solubilization was obtained at 1% (w/v) soil solid content and 0.1% (w/v) sulfur dosage and the efficiency was higher than 80%, except Pb. In considering the application of bioleaching, the operating conditions were recommended to be selected in the range of 1–7% (w/v) solid content and 0.1–0.3% (w/v) sulfur dosage for high efficiency (>60%) of metal solubilization. After the bioleaching process, in the treated soil was stable and residual heavy metals were no longer harmful to the environment.
Biological leaching rates of copper from chalcopyrite in column reactors were accelerated upon addition of 30 to 45 mg Ag/liter leach solution (2.5 to 37.5 mg Ag/kg ore material). Silver addition produced mixed effects on the rates of sphalerite and pentlandite leaching, being slightly positively or negatively catalytic depending on the ore sample tested. The positive effect on copper leaching rates was transient and the rates eventually declined to levels similar to those determined before the Ag addition. Following the addition, silver was detected only at trace levels in leach solution. Analysis of mass balance indicated a considerable incorporation of the added silver into jarosite fraction, yielding a 140-fold silver enrichment factor relative to silver concentration in solution.
The microbiological leaching of copper ores was evaluated with samples that contained chalcopyrite (CuFeS2), pyrite (FeS2), pyrrhotite (Fe1−xS), and sphalerite (ZnS) in varying proportions as the main sulfide minerals. The solubilization of copper from chalcopyrite was slow in contrast to the rapid solubilization of zinc. The addition of up to 30 mg/l silver, either as a sulfate or nitrate salt, enhanced the bacterial leaching of copper from chalcopyrite. The positive catalytic effect was related to the concentration of the silver added. The enhancement of the copper leaching due to the silver addition was negligible in the absence of bacteria. Only trace concentrations of silver were detected in leach solution samples. The leaching of iron and zinc was inhibited by silver addition and a transient period of depressed Eh was typically associated with this phase. The addition of silver also enhanced the bacterial leaching of a purified chalcopyrite sample. Increased solubilization of copper and iron was obtained concurrently with the formation of sulfate. Mass balance studies and stoichiometric calculations indicated elemental sulfur formation and the precipitation of iron and sulfate during the leaching. Potassium jarosite [KFe3(SO4)2(OH)6] was identified by X-ray diffraction in Fe(III)-containing precipitates.
The mechanism of bacterial leaching of chalcopyrite catalyzed by silver ions is studied in this paper. The copper and iron ions in the chalcopyrite crystal lattice are replaced by the silver ions, the silver ions then combining with the sulfur to form silver sulfide. The ferrous ions are oxidized to ferric ions by the thiobacillus ferrooxidans. The ferric ions can convert the silver sulfide into silver ions. Using this mechanism, a new kinetic model of bacterial leaching of chalcopyrite in the presence of silver ions was established. The model accurately predicts the copper recovery rate in the bacterial leaching of chalcopyrite in the presence of silver ions.
Modelling chalcopyrite leaching involves accounting for the precipitation of jarosite and other iron hydroxide minerals; the difficulty in modelling these processes arises from uncertainty in the precipitation rate, and its dependence on the relevant variables (such as pH, Fe3+ concentration). Furthermore, an added complexity is accounting for the clogging of the macro and micro pore space, and its effects on leaching rate and liquid flow. There has been a lack of modelling of such processes in the literature, and in this work we focus on the inclusion of a basic model of jarosite precipitation with the associated removal of ferric ions from solution. The solubility criterion is formulated from experimental data of the log of ferric concentration versus pH: a linear slope in log Fe versus pH is apparent. This rule forms the basis for the solubility of ferric, and if the rule is met a first order dependence of the overall precipitation rate on the ferric is used. A rate constant for the precipitation rate is needed and a value chosen to provide reasonable model behaviour i.e. a reasonable amount of ferric is removed from solution: comparison with experimental data is needed to ensure appropriate model parameters are chosen. Four cases are considered: a base case, a case without the effects of jarosite precipitation, a higher inlet pH, and a reduced gangue acid consumption case. The base model results show jarosite precipitates after a distance of around 0.8 m below the inlet, at a position where the pH has risen sufficiently from the (lower) inlet value. In the higher inlet pH case, the spatial position where the onset of jarosite precipitation begins is further up in height in the column: the position is higher up because the jarosite criteria is met sooner, due to the pH being higher. Consequently, copper extraction is worse and perhaps non-intuitively, only a small percentage (1%) more overall jarosite precipitates than in the base case. In the reduced gangue acid consumption case, the pH is lower due to reduced acid consumption, and spatial position where the onset of jarosite precipitates begins is lower in the column; overall extraction is far better (89%) and jarosite precipitation is far less (67% less) than the base case.
This paper presents a study for leaching kinetics of sphalerite concentrate in FeCl3–HCl solution. The shrinking core model was applied to the results of experiments investigating the effects of stirrer speed of 200–600 rpm, ferric ion concentration in range of 0–1 M, solid/liquid ratio in range of 1/100–1/5, leaching temperature range of 40–80 °C and particle size on zinc dissolution rate. The activation energy for the leaching process was found to be 45.30 kJ/mol and the Arrhenius constant was calculated to be 5.454 s−1. The order of reaction for ferric ion concentration, solid/liquid ratio and particle size were also obtained. The rate of the reaction based on reaction-controlled process can be expressed as,The dissolution of sphalerite with acidic ferric chloride solution was found to be controlled by reaction-controlled process.
A study of the effect of different variables (inoculation, aeration, silver complexants, [Ag], [Fe3+], temperature and chemical activation stage) on the silver-catalyzed bioleaching of two different low-grade copper ores has been carried out in stirred tanks. The catalyzed bioleaching process was greatly affected by bacterial activity. Aeration and the use of different complexing agents (thiosulfate and thiosulfate plus cupric ions) did not enhance but also did not inhibit the copper kinetics in the silver-catalyzed process. On the contrary, the presence of 5 g/L Cl− inhibited the catalytic effect of silver. The effect of silver concentration was tested on two different low-grade copper ores in the range between 10 and 500 mg Ag/kg for the lower K-ore and between 1.4 and 35.7 g Ag/kg Cu for the PVD ore, the former with a higher content of copper. Silver catalysis was effective for both ores but the PVD ore was basically unaffected by silver concentration in the range studied. Maximum copper extractions and copper dissolution rates were obtained with a very small amount of silver (3.6 g Ag/kg Cu). In all cases, the copper recovery was at least twice that in the absence of silver (∼ 30%). High ferric concentrations have been tested in the absence and in the presence of silver. The presence of silver was essential to improve the copper extraction from chalcopyrite in acidic ferric sulfate solutions. However, bioleaching experiments conducted with silver and 1 g/L Fe3+ produced lower copper extractions (20%) compared to experiments where ferric iron was absent (55%). The copper dissolution in the silver-catalyzed lower K-ore bioleaching is temperature dependent, with an optimum temperature around 35 °C. The activation energies of the copper dissolution process were 109.7 and 20 kJ/mol in the ranges of temperature between 15 and 28 °C and 28 and 45 °C respectively. The chemical activation stage establishes optimum conditions that promote higher copper extractions in the presence of silver.
The history of sulfidic ore leaching and the relatively recent discovery of microbial involvement in the process and its commercial exploitation are summarized. A possible future developmental direction is indicated.
The influence of silver and iron concentration and the presence or absence of oxygen was investigated in the silver-catalyzed chalcopyrite leaching. The leaching tests were performed at two different temperatures (35 and 68 °C) in stirred flasks (180 rpm) containing 0.5 g of mineral and 100 mL of Fe3+/Fe2+ sulphate solutions at pH 1.8 and at an initial redox potential of 500 mV vs. Ag/AgCl. The addition of a great excess of silver favoured the transformation of chalcopyrite into copper-rich sulphides, such as: covellite, CuS, and geerite, Cu8S5. These sulphides prevented the formation of CuFeS2/Ag2S galvanic couple and, thus, the regeneration of silver. In addition, oxygen in solution plays a key role in the regeneration of silver ions acting as the main oxidizing agent for Ag2S.