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Effect of biooxidation conditions on cyanide consumption and gold recovery from a refractory gold concentrate

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This study investigates the biooxidation of a refractory gold concentrate using a mixed culture of acidophilic mesophiles, moderate thermophiles and extreme thermophiles and their effect on the subsequent cyanidation and gold recovery. The experiments with high % solids using mixed mesophiles showed better oxidation potential compared to moderate thermophiles and extreme thermophiles. However, the extreme thermophiles performed better than mesophiles and moderate thermophiles during the biooxidation with

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... Bio-oxidation of refractory gold-bearing sulfide concentrates prior to cyanidation as a pretreatment step has proven to be economically competitive and environmentally friendly compared with conventional methods [7,8,11,26]. As a promising technology, bio-oxidation operations have been commissioned in a number of countries [33,35]. ...
... In addition, the toxic effect of iron in the bio-oxidation process would become obvious with the accumulation of iron ion especially under high pulp density. Ciftci and Akcil [11] have previously reported that mesophiles exhibited greater iron tolerance than moderate thermophiles and extreme thermophiles. Extreme thermophiles, with their metabolic activity at high temperatures up to 65ºC, have great potential to enhance the kinetic of bio-oxidation at a pulp density lower than 5%. ...
... The temperature dependency of pyrite oxidation kinetics has been well investigated, and the leaching kinetics could be improved via elevating the leaching temperature [4,6]. The dissolution rates (Fig. 2) of iron were 370, 641, and 811 mg/L·h at 5%, 10%, and 15%, respectively; these results were much higher than those obtained via bio-oxidation [11]. The compositions of the leaching residues are shown in Table 3. ...
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Bio-oxidation is an effective technology for treatment of refractory gold concentration. However, the unsatisfied oxidation rate and long residence time, which cause the lower cyanide leaching rate and gold recovery, is a key factor to restrict the application of traditional bio-oxidation technology. In this study, the oxidation rate of refractory gold concentration and the adaption of microorganism were analyzed to evaluate a new developed two-step pretreatment process, which includes a high temperature chemical oxidation step and a subsequent bio-oxidation step. The oxidation rates and recovery rates of gold were improved significantly after two-step process. The results showed that the highest oxidation rate of sulfide sulfur could reach to 99.01 % with extreme thermophiles microbial community when pulp density was 5 %. Accordingly the recovery rate of gold was elevated to 92.51%. Meanwhile, the results revealed that moderate thermophiles performed better than acidophilic mesophiles and extreme thermophiles whose oxidation rates declined drastically when pulp density increased to 10 % and 15 %. The oxidation rates of sulfide sulfur with moderate thermophiles were 93.94 % and 65.73 % when pulp density increased to 10 % and 15 %, respectively. All these results indicated that the two-step pretreatment increased the oxidation rate of refractory gold concentration and is a potential technology to pretreat the refractory, meanwhile, due to the sensitivity of microbial commuinty under different pulp density, the optimization of microbial community in bio-oxidation is necessary in industry.
... In the case of pyrite, a longer contact time with ferric iron in batch mode possibly caused higher oxidation after 477 h. It has been reported that arsenopyrite oxidizes faster than pyrite (Márquez et al., 2006;Ciftci andAkcil, 2010 andOssa andMárquez, 2010). The cause could be its lower oxidation potential . ...
... In the case of pyrite, a longer contact time with ferric iron in batch mode possibly caused higher oxidation after 477 h. It has been reported that arsenopyrite oxidizes faster than pyrite (Márquez et al., 2006;Ciftci andAkcil, 2010 andOssa andMárquez, 2010). The cause could be its lower oxidation potential . ...
... The highest formation of jarosite occurred after 477 h of operation in the batch mode. This could be due to the high concentration of ferric iron in solution (Fig. 5a and c) according to what was reported by Ciftci and Akcil (2010). The decrease in the output of jarosite within the continuous mode is caused by the outflow of ferric iron and some jarosite until the system reaches a steady-state with a lower average concentration of ferric iron in solution (Fig. 5a). ...
... Bioleaching of the acid-treated pyritic tailings was conducted in two 2.0-L bioreactors with a working volume of 1.0 L that were connected in series. It has been stated that even in continuous stirred tank reactor operations, the limitation of pulp density is approximately 20% due to inefficient gas transfer in the system together with microbial cell damage due to the high shear force caused by the impeller [28]. Thus, the maximal pulp density selected for the bio-oxidation was 20% (w/v). ...
... Jarosite was observed in the mesophile and thermophile BIOX ® pilot plant products from Fairview Mine, with more jarosite present (15 wt.%) in the products of the thermophile process [44]. The bioleaching of a concentrate containing 28% of pyrite in shaking flasks with a moderate thermophilic culture at a pulp density of 10% resulted in residue containing approximately 15 wt.% of jarosite [28]. The high content of jarosite (32 wt.%) was observed during a continuous stirred tank bioleaching of a flotation concentrate of the tailings from Western Tasmania containing 70.5 wt.% of pyrite with a moderate thermophilic microbial culture [45]. ...
... Carbon-in-pulp cyanidation of bio-oxidized residues provided high gold recovery (Table 3) after 12 days of bioleaching due to the high pyrite oxidation level. Previous studies have shown that the efficiency of gold recovery is strongly related to the extent of sulfide oxidation and that the dependence of gold recovery on sulfide oxidation is often linear [28,39]. Although no effects of the jarosite amount, presence of secondary residues, etc., on gold recovery from pyritic materials were usually observed, in a few studies, gold particles coated by a jarosite layer were found [50]. ...
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In this work, we proposed a method for biohydrometallurgical processing of mining (old pyritic flotation tailings) and metallurgical (slag) wastes to recover gold and other nonferrous metals. Since this processing allows the removal of toxic metals or at least decreases their content in the solids, this approach may reduce the negative environmental impacts of such waste. The proposed process was based on pyritic tailings’ bioleaching to recover metals and produce leach liquor containing a strong oxidizing agent (ferric sulfate) to dissolve nonferrous metal from slag. This approach also allows us to increase concentrations of nonferrous metals in the pregnant leach solution after pyritic waste bioleaching to allow efficient extraction. The old pyritic tailings were previously leached with 0.25% sulfuric acid for 10 min to remove soluble metal sulfates. As a result, 36% of copper and 35% of zinc were extracted. After 12 days of bioleaching with a microbial consortium containing Leptospirillum spp., Sulfobacillus spp., Ferroplasma spp., and Acidithiobacillus spp. at 35 °C, the total recovery of metals from pyritic tailings reached 68% for copper and 77% for zinc; and subsequent cyanidation allowed 92% recovery of gold. Ferric leaching of two types of slag at 70 °C with the leachate obtained during bioleaching of the tailings and containing 15 g/L of Fe3+ allowed 88.9 and 43.4% recovery of copper and zinc, respectively, from copper slag within 150 min. Meanwhile, 91.5% of copper, 84.1% of nickel, and 70.2% of cobalt were extracted from copper–nickel slag within 120 min under the same conditions.
... The biooxidation of refractory gold concentrates by mesophilic cultures with high percent solids is better comparing to the moderate and extreme-thermophilic mixed cultures (Ciftci and Akcil 2010). However, extremophiles showed better oxidation performance with 5% solids (Ciftci and Akcil 2010;Ciftci and Akcil 2013). ...
... The biooxidation of refractory gold concentrates by mesophilic cultures with high percent solids is better comparing to the moderate and extreme-thermophilic mixed cultures (Ciftci and Akcil 2010). However, extremophiles showed better oxidation performance with 5% solids (Ciftci and Akcil 2010;Ciftci and Akcil 2013). ...
... The addition of FeS 2 to arsenopyrite promotes oxidation As (III) to As (V) and increases the activity of Acidithiobacillus ferrooxidans (Tao et al. 2008). Arsenopyrite is more easily biooxidized than pyrite due to its smaller electrochemical potential (Ciftci and Akcil 2010). The addition of traces of metal catalysts such as Ag(I), Bi(III), Co (II), and Hg (II) to the bacterial culture Acidithiobacillus ferrooxidans leads to an exceptional improvement in the degree of oxidation of low-grade sulfide gold ores with arsenopyrite and pyrite (Deng et al. 2000). ...
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In nature, microorganisms developed at various places and adapted to the various weather and geological conditions. Microorganisms participate in geological transformations leading to the dissolution of some minerals and conversion to others. While some microorganisms with their metabolic activity increase the mobility of metals, others cause precipitation of metals and the formation of new minerals. These biogeochemical interactions found practical application in the recovery of metals. In the article, the proposals for improvement of existing engineering commercial processes for recovery of metals are given which can enable the formation of nanogold and nanogold compounds. Key points • Amino acids in pretreatment can increase the dissolution of the layer around the gold.• Amino acids in the complexing stage can increase gold leaching.• After the complexing stage, the bionanosynthesis of gold and its compounds is possible.Graphical abstract
... The works of Ciftci and Akcil [38] on the bacterial oxidation of refractory gold with the extreme thermophilic bacterium produced similar results, and they obtained 92% of the dissolved gold after cyanidation with a consumption of 17.5 kg per ton of sodium cyanide. However, the cost associated with the optimal growth temperature of this bacteria may decrease the efficiency of its use in a refractory gold bacterial oxidation process. ...
... Since the dissolution of refractory gold sulfide is a slow process, it is therefore necessary to find a technique to speed up it. The contribution of The works of Ciftci and Akcil [38] on the bacterial oxidation of refractory gold with the extreme thermophilic bacterium produced similar results, and they obtained 92% of the dissolved gold after cyanidation with a consumption of 17.5 kg per ton of sodium cyanide. However, the cost associated with the optimal growth temperature of this bacteria may decrease the efficiency of its use in a refractory gold bacterial oxidation process. ...
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Bacterial pre-oxidation of refractory gold concentrates generates large volumes of leachate and requires a significant supply of nutrients to support bacterial growth. Therefore, bioleachate reuse reduces both water consumption and the nutrients required for the process. However, the efficiency of this method and its benefit need to be further explored. In the present study, two tests on the reuse of bioleachate in new cycles of bacterial oxidation were carried out to evaluate the efficiency and the benefit of bioleachate reuse. Our results showed that the reuse of bioleachates could reduce nitrogen and phosphorus requirements by 40% and 36%, respectively, after a 14-day biooxidation stage in a stirred tank bioreactor. We also showed that the reuse of bioleachate had a positive effect on the recovery of gold in a subsequent 48 h treatment by cyanidation. The gold recovery rate (initial concentration of 44 mg/kg) remained unchanged at 90% after the two bioleachate recirculation loops. The reuse of bioleachate also made it possible to increase the solubilization rates of other metals from the sulfide concentrate. Thus, the solubilization yields of copper (initial concentration of 3587 mg/kg) and zinc (initial concentration of 27,315 mg/kg) increased, respectively, from 14.8% and 40.2% to 37.5% and 99.6% after the two bioleachate recirculation loops.
... The highest gold recovery results were obtained during the process at 35 °C and amounted to 85% with an 8-day biooxidation time. Sodium cyanide consumption in all cases was relatively low, with the highest consumption during experiments at 45 °C with high pH, which may be due to a higher content of sulfates (jarosite, gypsum) in the biooxidation residues (Komnitsas and Pooley 1990;Ciftci and Akcil 2010;Mubarok et al. 2017). High consumption of sodium cyanide was required during gold extraction from non-oxidized pyritic tailings, which can be caused, among other things, by the presence of water-soluble copper compounds inactivating the cyanide ion (Dai et al. 2012;Bas et al. 2015). ...
... The results demonstrate that, at linear data approximation the regression coefficient was 0.97, which indicates the applicability of this approximation for describing the dependence of gold recovery from the pyrite oxidation level in flotation tailings. The linear dependence of gold recovery on the oxidation level of sulfide sulfur or main gold-bearing sulfides was also found in many studies (Lindström et al. 1992;Miller and Hansford 1992;Muravyov and Bulaev 2013;Ciftci and Akcil 2010). This indicates that gold recovery is strongly related to the extent of sulfide biooxidation. ...
Article
Large volumes of waste from the mining and processing industry, particularly flotation tailings of polymetallic ores, have accumulated all over the world. Mine tailings can cause severe environmental pollution and pose significant risks to both human and environmental health. However, by the contents of their metals, mine tailings can be regarded as sources for extracting copper, zinc, gold, and other metals, specifically, by using biohydrometallurgy. Stored pyritic flotation tailings of copper-zinc ores, containing 0.26% of copper, 0.22% of zinc, and 0.67 g/t of gold were studied. A continuous process of biooxidation of this waste was studied at three temperature settings of 35, 40, and 45 °C. Processing at 35 °C for 8 days caused the pyrite oxidation level to reach 73.6%, and the gold recovery by carbon-in-pulp cyanidation of the leach residue to reach 85%. The effect of pH on biooxidation of pyritic tailings at 45 °C was also studied. The pyrite oxidation level was 18% higher when the process was carried out in the pH range of 1.2–1.5, as compared to the pH range of 1.7–2.0. The results obtained are important for the development of mine waste utilization using eco-friendly biotechnology.
... These ores need a suitable pretreatment to achieve high gold and silver extractions rates during cyanidation [1]. Roasting [2], pressure oxidation [3], biooxidation [4,5] and ultra fine grinding [6] have been commercially practiced to enhance the gold/silver recoveries from refractory ores. The most common cause of refractoriness in gold and silver ores is the encapsulation of fine gold and silver particles within the mineral matrix in which these particles are not accessible to the lixiviating action of cyanide and oxygen [7]. ...
... In stirred media mills, comminution process is controlled intimately by the number (N s ) and intensity (I s ) of the stress generated within the grinding environment. Stress number is referred as the average number of stress events of each product particle and affects product fineness for a given stress intensity and expressed as [14,33] p s c s N P N N (5) where N c , P s and N p are the number of media contacts, the probability of stressing of a particle at a media contact and the number of product particles inside the mill, respectively. In the case of grinding of crystalline materials, the following proportionality is derived [14]. ...
Article
Ultra-fine grinding of the plant tailings of a refractory silver ore was studied using a laboratory type vertical stirred media mill. Preliminary tests have confirmed that ultra-fine grinding substantially improves the extraction of silver from the tailings in cyanide leaching (i.e. 36% Ag extraction from the as-received tailings (d80: 100 µm) c.f. 84% extraction after ultra-fine grinding of the tailings (d80: 1.2 µm)). In the ultra-fine grinding tests, the effects of ball diameter (2-4.5 mm), stirring speed (200-800 rpm) and ball charge ratio (50-80%) on the fineness of grind (d80, m) were investigated through a Box-Behnken design. Increasing stirrer speed and ball charge ratio decreased fineness of grind while larger balls resulted in the coarser products. These tests have also demonstrated that a fineness of grind of <5 m can be achieved under suitable conditions. Analysis of stress intensity indicated an optimum range of stress intensity of ~0.8-2*10-3 Nm for all power inputs.
... Pretreatment of ores and concentrates via biooxidation process is achieved by the means of acidophilic chemolithotrophic microorganisms, which oxidizes the gold-bearing sulfidic minerals, mainly pyrite and arsenopyrite thereby liberating gold and silver from the sulfide matrix. The liberated gold is then leached by different methods, mainly by cyanidation [11] [12] [13] [14]. Cyanide consumers include ions of iron, copper, zinc, cobalt and nickel, etc. [5]. ...
... It has also been reasoned that cyanide consumption is related to the formation of iron and arsenic precipitates during biooxidation and their subsequent carry-over to cyanidation [15]. Moreover, Komnitsas and Pooley [14] suggest that high cyanide consumption is related to the formation and precipitation of iron and silver jarosites, particularly at low pulp densities. Considering the increase of recent environmental accidents at various gold recovery processes around the world causes severe contamination of natural water resources over the use of cyanide as a lixivalent. ...
Article
The thermodynamic equilibria and kinetic aspect of gold dissolution in iodine–iodide leaching were studied with emphasis on the effects of pH value and temperature on the system. The results of thermodynamic analysis of iodine in aqueous solution were given and numerous forms of iodine exist mainly in the acid region of pH values. An increase of the potential of the system results in an increase of iodine speciation. The oxidizing potential of the system will increase by the addition of element iodine. The IO3– anions are stable in the potential range from –2.0 to –0.75 V and at pH value greater than 12.1. An increase of the temperature shifts boundaries of existence of various iodine species in the acid region of pH values. Some of them become unstable. The determined values of the diffusion coefficients and the thickness of the diffusion boundary layer, as well as the solvent concentration on the disc surface (14 mg/L) indicate that the process proceeds in the external diffusion region. Thus, while choosing the conditions of leaching from gold-containing materials of different origins of iodide solvents, it is necessary to carry out the process within the acidic region of pH values, where I−, I3− and IO4− ions are capable to form complex compounds with metals.
... Biooxidation is a well-known technology for the pretreatment of such ores that allows an efficient recovery of dispersed gold by the cyanidation method (Muravyov and Bulaev, 2013;Sun et al., 2012a, b). Mixed and pure cultures of acidophilic mesophiles and moderate and even extreme thermophiles have been used for the biooxidation of refractory gold ores; at high pulp densities (used in commercial applications) mesophiles showed better biooxidation potential compared to moderate and extreme thermophiles (Ciftci and Akcil, 2010). Even when there are some contradictory results in the literature, indigenous microbial communities usually seem to be superior to microbial consortia constructed using strains from other environments (Bryan et al., 2011;Chandraprabha et al., 2002). ...
... It means that biooxidation pre-treatment allowed a 30% increase in gold recovery with a cyanide consumption of 15% lower than that observed for the residue from abiotic control. The last result is in contradiction to that usually reported indicating that although the biooxidation pre-treatment achieves better gold recoveries, the cyanide consumption is moderately increased (Ciftci and Akcil, 2010). ...
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Two enrichment cultures (one iron oxidizing and one sulfur oxidizing) obtained from an acid mine drainage were physiological and molecular characterized; the first of them showed 99% 16S rRNA gene sequence similarity with Leptospirillum ferrooxidans, while the sulfur oxidizing species was highly related to Acidithiobacillus ferrooxidans and Acidithiobacillus ferrivorans sequences although, unlike them, the species in the enrichment culture does not seem to be able to oxidize iron. The consortium constituted by both enrichment cultures achieved a successful biooxidation of Hualilan ore allowing the increase of gold recovery up to 96.4 % in the best culture condition (low pulp density in 1K medium). At the same time, this condition showed an effective zinc bioleaching (up to 86 %) although the recovery was much higher in cultures with initially supplemented iron; kinetics studies suggested that bioleaching rate in 1K medium at low pulp densities is controlled by diffusion through a layer mainly constituted by jarosite (rather than sulfur) which was detected in X-ray diffraction diagrams. The recovery of zinc as subproduct of the pretreatment to optimize gold recovery would also allow decrease the metallic charge of the leachates from the biooxidation tanks which is an environmental advantage.
... The biooxidation pretreatment of refractory gold ores and concentrates prior to cyanidation has been widely used for the past two decades, mainly because of its environmental friendliness and applicability to low-grade ores (Harvey et al., 2002;Donati and Sand, 2007;Corkhill and Vaughan, 2009;Fantauzzi et al., 2011;Vera et al., 2013;Kaksonen et al., 2014). However, the combination of biooxidation and cyanidation results in many problems (La Brooy et al., 1994;Ciftci and Akcil, 2010;Karthikeyan et al., 2015). For example, the high toxicity of cyanide causes an increasing pressure on the environment. ...
... It implied the consumption of Fe 3 + as the oxidant in the gold extraction reactions and even a decrease in iron oxidation. The same phenomenon has been reported by other researchers, who showed that pulp density was relevant to the cell viability and oxidizing activity (Astudillo and Acevedo, 2008;Ciftci and Akcil, 2010). Insufficient oxygen mass transfer and the high shearing force of impellers caused by the high pulp density significantly inhibited microbial growth and activity (Sun et al., 2012). ...
Article
Biooxidation-thiourea (TU) leaching is considered to be a more environmentally friendly and efficient method than cyanidation to recover gold from refractory sulfide gold-bearing ores/concentrates. However, high consumption of the TU reagent and additional oxidants hinders its commercial adoption. In this work, single TU leaching and novel two-step TU leaching processes were designed and performed after the biooxidation of refractory sulfide gold-bearing concentrate from Axi, China. The novel method contained an additional six-hour bioprocess and a six-hour lixiviant leaching. During the two-step TU leaching, the microbes remaining on the residue from biooxidation regenerated Fe3 + as oxidant. Compared with the highest gold recovery rate of the single TU leaching (92.2 ± 1.54%), which had additional Fe3 + and Na2SO3 supplied after prewashing, the rate reached 95.0 ± 0.73% in the two-step TU leaching process without additional oxidant and reductant. A microbial community analysis by 16S rRNA gene amplicon sequencing indicated that the genera Acidithiobacillus and Leptospirillum greatly contributed to the regeneration of ferric ions during the two-step TU leaching process and further improved the recovery of gold. As a non-cyanide method, combining the two-step TU leaching process with biooxidation takes full advantage of the microorganisms and is worthy for industrial application.
... In the case of pyrite, the following equations describe the typical reactions that occur in a bacterially-catalyzed oxidation system (Ubaldini et al., 2000Bioleaching can take place in the form of heap leaching or stirred tank leaching. Although, the bioleaching in stirred tanks at industrial scale is limited to sulfidic concentrates, for recovery of gold and base metals, it has two main advantages:1) the economic potential for its application into industrial scale, 2) microbial growth selectivity in the bioreactors (Dominique, 2007;Ciftci and Akcil, 2010). Many Saudi gold ores are often found as very finely disseminated particles in the sulfide matrix which hampers their extraction by cyanidation. ...
... This bacterium is robust, thriving in H 2 SO 4 environments at pH less than 2.5. None of these bacteria has been found to cause plant or animal diseases (Ciftci and Akcil, 2010). 2.4. ...
... Mineral residues from biooxidation had a consumption of 6.6 kg/t of NaCN, while sterile control demanded 3.6 kg/t. Metals, sulfurs and iron complexes are considered "cyanicides"; the presence of these compounds in leaching systems reduces available CN − , so NaCN consumption increased [3,23]. The large iron precipitation that took place in cultures (detected by XRD analysis) was consistent with this result. ...
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A defined mesophilic consortium including an iron oxidizing bacterium and a sulfur oxidizing bacterium was constructed to evaluate its ability for bioleaching a flotation concentrate from Andacollo mine in Neuquén, Argentina. Experiments were performed in shake flasks with a pulp density of 10% (w/v), using a basal salt medium containing ferrous iron at pH 1.8. The leaching solutions were analyzed for pH, redox potential (using specifics electrodes), ferrous iron (by UV-Vis spectrophotometry) and metal concentrations (by atomic absorption spectroscopy). The results showed that the consortium was able to reduce the refractory behavior of the concentrate, allowing 91.6% of gold recovery; at the same time, high dissolution of copper and zinc was reached. These dissolutions followed a shrinking core kinetic model. According to this model, the copper solubilization was controlled by diffusion through a product layer (mainly jarosite), while zinc dissolution did not show a defined control step. This designed consortium, composed of bacterial strains with specific physiological abilities, could be useful not only to optimize gold recovery but also to decrease the leachates metallic charge, which would be an environmental advantage.
... Arsenopyrite (FeAsS), the most common arsenic (As)-bearing sulfide mineral in nature, is often found in auriferous sulfide ores. Due to the fact that arsenopyrite is one of the most common hosts of invisible gold (Au), which is optically undetectable Au particles because of its incorporation into the structure of host minerals, auriferous sulfide ores are typically processed by flotation to recover the bulk of gold-bearing arsenopyrite, and then flotation concentrates are processed via pretreatment (e.g., acidic pressure oxidation (Gudyanga et al., 1999), alkaline pretreatment (Mesa Espitia and Lapidus, 2015), biooxidation (Ciftci and Akcil, 2010), pyrolysis (Dunn and Chamberlain, 1997), ultra-fine grinding (Corrans and Angove, 1991)) to expose Au particles encapsulated in host minerals followed by cyanidationda conventional hydrometallurgical technique for the selective leaching of gold (Asamoah et al., 2018;Deol et al., 2012;Pokrovski et al., 2019;Tabelin et al., 2020a). Because arsenopyrite is not dissolved during cyanidation, it ends up with the leaching residues typically disposed of into tailings storage facilities (TSF). ...
Article
Mineral processing, pyro- and hydrometallurgical processes of auriferous sulfide ores and porphyry copper deposits (PCDs) generate arsenopyrite-rich wastes. These wastes are disposed of into the tailings storage facilities (TSF) in which toxic arsenic (As) is leached out and acid mine drainage (AMD) is generated due to the oxidation of arsenopyrite (FeAsS). To suppress arsenopyrite oxidation, this study investigated the passivation of arsenopyrite by forming ferric phosphate (FePO 4) coating on its surface using ferric-catecholate complexes and phosphate simultaneously. Ferric iron (Fe 3+) and catechol form three types of complexes (mono-, bis-, and triscatecholate complexes) depending on the pH, but mono-catecholate complex (i.e.,[Fe(cat)] +) became unstable in the presence of phosphate because the chemical affinity of Fe 3+ —PO 4 3– is most probably stronger than that of Fe 3+ —catechol in [Fe(cat)] +. When two or more catechol molecules were coordinated with Fe 3+ (i.e., [Fe(cat) 2 ] – and [Fe(cat) 3 ] 3–), however, these complexes were stable irrespective of the presence of phosphate. The treatment of arsenopyrite with [Fe(cat)2 ] – and phosphate could suppress its oxidation due to the formation of FePO 4 coating, evidenced by SEM-EDX and XPS analyses. The mechanism of FePO 4 coating formation by [Fe(cat) 2 ] – and phosphate was confirmed by linear sweep voltammetry (LSV): (1) [Fe(cat) 2 ] – was oxidatively decomposed and (2) the resultant product (i.e., [Fe(cat)] +) reacts with phosphate, resulting in the formation of FePO 4.
... While emphasis has been placed largely on the bio-oxidation of sulfidic-refractory gold concentrates in stirred-tank reactors development is underway on heap leaching of low-grade chalcopyrite ores (Gericke et al., 2009). Ciftci and Akcil (2010) studied the bio-oxidation of a refractory gold concentrate using a mixed culture of acidophilic mesophiles, moderate thermophiles and extreme thermophiles and their effect on the subsequent cyanidation and gold recovery. The experiments with high % solids using mixed mesophiles showed better oxidation potential compared to moderate thermophiles and extreme thermophiles. ...
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This paper presents an overview of the various methodologies used in the recovery of gold from secondary sources. Gold recovery is interesting due to its vast industrial applications, high market prices and extensively used precious metal, the sanctuary value attributed to gold during international political and economical crises, and the limited resource of this metal may explain the recent increasing gold share value. The state of art in recovery of gold from spent sources by pyrometallurgy; hydrometallurgy; bio-hydrometallurgy techniques is highlighted in this paper. This article also provides an overview of past achievements and present scenario of recovery studies carried out on the use of some promising methods which could serve as an economical means for recovering gold. The present review also highlights the used varieties of leaching, cementing, reducing agents, peeling, coagulants, adsorbents, agglomeration solvents, ion exchange resins and bio-sorbents in real situations and hopes to provide insights into recovery of gold from spent sources. Evaluation of lucrative and environmentally friendly technologies to recover gold from primary and secondary spent sources was made in this study.
... This concentration level represents a considerable percentage (39%) of the cyanide being consumed for non-leaching purposes and this may likely be the cause of the suspected cyanide depletion in the column tests after 14 days. Ciftci and Akcil (2010) report a similar effect in their test work which involved the cyanide leaching of bioleached residue concentrate. The first stage of bioleaching was intended to liberate refractory Au minerals for cyanide leaching. ...
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Samples of low-grade flotation concentrate derived from Platreef ore were treated with a two-stage heap leach process to determine the potential to run this process parallel to the convential concentrate–smelt–refine process for extracting platinum group metals, thereby adding value to the convential process via economic treatment of low-grade materials. Using bench-scale columns, a first stage bioleach achieved extractions of 91.1% Cu, 98.5% Ni and 83.5% Co in a space of 88 days at a temperature of 65 °C. Samples of residual concentrate from this process were similarly treated via columns in a second stage leach using cyanide solution, achieving extractions of up to 96.5% Pd, 97.5% Au and 35% Pt in a space of 45 days at 50 °C. Mineral liberation analysis of a sample of concentrate from the cyanide leach experiment revealed that the bulk of the remaining Pt was in the form of sperrylite which appeared to leach very slowly in cyanide solution. Further to this, analysis of the cyanide leachate solution showed high levels of thiocyanate.
... Elemental compositions of DRGO were determined by X-ray fluorescence (XRF) spectroscopy (Rigaku, ZSX Primus II, Akishima, Japan) and CHN analysis (Yanaco CHN MT-5, Kyoto, Japan). The XRF analysis showed that the major elemental compositions of the DRGO are 6.1 wt% S, 15.7 wt% Si, 2.0 wt% K, and 13.4 wt% Fe, 1.0 wt% Na, 1.7 wt% Mg, 3.1 wt% Ca, 0.6 wt% Ti, and 56.5 wt%, whereby K is the potential for the formation of jarosite which may hinder gold recovery through cyanide consumption [24] and gold grains re-encapsulation [25]. CHN analysis revealed carbon content of 5.27 wt%, which may include both organic and inorganic forms. ...
Article
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Environmentally friendly pretreatment of double refractory gold ores (DRGO) to improve gold recovery without emitting pollutant gas is challenging. Sequential biotreatment, including iron-oxidizing microorganisms to decompose sulfides, followed by the enzymatic decomposition of carbonaceous matter, was recently developed. The effect of acid washing by 1 M HCl for 24 h between two bioprocesses was evaluated using a real double refractory gold ore from the Syama mines, Mali, which includes 24 g/t of Au and 5.27 wt % of carbon with a relatively higher graphitic degree. The addition of the acid washing process significantly improved gold recovery by cyanidation to yield to 84.9 ± 0.7% from 64.4 ± 9.2% (n = 2). The positive effects of acid washing can be explained by chemical alteration of carbonaceous matter to facilitate the accessibility for lignin peroxidase (LiP) and manganese peroxidase (MnP) in cell-free spent medium (CFSM), although the agglomeration was enhanced by an acid attack to structural Fe(III) in clay minerals. Sequential treatment of DRGO basically consists of the oxidative dissolution of sulfides and the degradation of carbonaceous matter prior to the extraction of gold; however, the details should be modified depending on the elemental and mineralogical compositions and the graphitic degree of carbonaceous matter.
... These results may attribute to that bacteria uses acid for its growth and reproduction at the beginning process (1) [24]. And then, the gold ore bioleaching involves in sulfur and sulfide minerals oxidation (2) to (4) [25,26], which leads to a decrease in pH value [27]. These results were similar to the study of Zhou et al. [28]. ...
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In order to improve the bioleaching efficiency of arsenic-rich gold concentrates, a mixed bacterial flora had been developed, and the mutation breeding method was adopted to conduct the research. The original mixed bacterial flora had been enrichedin acid mine drainage of Dexing copper mine, Jiangxi Province, China. It was induced by UV (ultraviolet), ultrasonic, and microwave, and their combination mutation. The most efficient bacterial flora after mutation was collected for further bioleaching of arsenic-rich gold concentrates. Results indicated that the bacterial flora after mutation by UV 60 s combined with ultrasonic 10 min had the best oxidation rate of ferrous, the biggest density of cells, and the most activity of total protein. During bioleaching of arsenic-rich gold concentrates, the density of the mutant bacterial cells reached to 1.13 × 10(8) cells/mL at 15 days, more than 10 times compared with that of the original culture. The extraction of iron reached to 95.7% after 15 days, increased by 9.9% compared with that of the original culture. The extraction of arsenic reached to 92.6% after 12 days, which was increased by 46.1%. These results suggested that optimum combined mutation could improve leaching ability of the bacterial flora more significantly.
... In this process either metabolic activities or products of microorganisms are involved. This process is based on the ability of micro-organisms (bacteria and fungi) to transform solid compounds into soluble and extractable elements, which can be recovered (Akcil and Deveci, 2010;Ciftci and Akcil, 2010;Ehrlich, 2004;Anjum et al., 2012). Halide systems such as bromine-bromide, iodine-iodide and bromide-chloride are capable of dissolving gold at very fast rates. ...
Article
The aim of this paper consists in defining optimal conditions of controlled and selective gold leaching contained in copper - gold slag, by wet chlorination. Studies were conducted on three types of copper - gold slag, resulting as by-products from pyrometallurgical processing of non-ferrous raw materials. Due to high levels of copper in slag, an oxidative pre-treatment is necessary to separate gold from accompanying elements and also for copper recovery. After nitric acid pre-treatment, nearly 87% Cu were readily extracted, and lead and silver content in the all three types of solid samples has been reduced by 91.34% and 96.29% respectively. Selective leaching of the solid phase resulting after HNO3 pre-treatment was done by wet chlorination, using nascent chlorine (Cl2) as leaching agent, in presence of hydrochloric acid (HCl). The chlorine was obtained «in situ» by the reaction of sodium hypochlorite (NaOCl, 5% Cl2) and HCl. In this study, the effect of nitric acid pre-treatment, redox potential (Eh) given by sodium hypochlorite (NaOCl) consumption, hydrochloric acid concentration, temperature and leaching time have been studied. The potential required for a rapid leaching rate and for avoiding the reduction of gold is 950 mV (vs AgCl/Ag(KCl sat)).The 5% NaOCl consumption required for gold leaching from pre-treated samples was lower than NaOCl consumption necessary for original samples leaching. The highest gold extraction yield (98%) was obtained after 6 hours at ambient temperature, solid/liquid ratio (w/v) 1:2 and 4M HCl. In order to explain experimental results a mathematical model of the simultaneous gold, copper, lead and silver dissolving has been developed. It considers at particle level, the competition between surface reaction and the diffusion of the active species by a gel coating that covers the dissolving particle.
... Refractory gold ores which do not respond to direct cyanidation require pretreatment to liberate the contained gold and silver prior to cyanidation so that gold and silver extractions can be improved (La Brooy et al., 1994). Roasting (Dunn and Chamberlain, 1997), pressure oxidation (Gunyanga et al., 1999), biooxidation (Ciftci and Akcil, 2010;Iglesias and Carranza, 1994) and ultrafine grinding (Corrans and Angove, 1991) have been commercially practised to enhance gold recoveries from refractory ores. Another technique is CANMET's Enhanced Leach Process (CELP) developed to treat silver-rich gold ores (Rajala et al., 2011). ...
... In this process either metabolic activities or products of microorganisms are involved. This process is based on the ability of micro-organisms (bacteria and fungi) to transform solid compounds into soluble and extractable elements, which can be recovered (Akcil and Deveci, 2010;Ciftci and Akcil, 2010;Ehrlich, 2004;Anjum et al., 2012). Halide systems such as bromine-bromide, iodine-iodide and bromide-chloride are capable of dissolving gold at very fast rates. ...
Article
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In this study, cyanidation experiments on massive sulfide sample was investigated. The grade of precious metals (Au and Ag) in the ore are 4.2 g/t and 311.9 g/t. Also, the average grade of other elements are As-1751.6, Sb-619.7, Zn-781.4, Cu-454.6, Pb-101.8, Hg-324.3, and Bi-0.6 (all assays are in g/t). The first objective of test work focused on the determination of conditions for extracting gold from the ore ground to −75 μm. Different influential parameters such as cyanide concentration, particle size fraction, pH and leaching time have been comprehensively investigated. The optimum parameters were -25μm for size fraction, 2000 mg/L for cyanide concentration, pH=11 and 24hrs cyanidation time. The results showed that highest gold and silver leaching efficiencies were 76.88% and 59.63% in optimum condition. In addition, roasting as a pretreatment in temperature of 700oC in 2 hrs was evaluated. Cyanidation test on the roasted ore in optimized conditions decreased the gold and silver leaching efficiencies reached from 76.88% and 59.63% to 85.97% and 67.57%, respectively. In addition, the antimony extraction increased from 23.73% to 43.10%.
... The literature appears to focus essentially on refractory ores of gold rather than silver. Several pre−treatment processes prior to cyanidation such as roasting (Dunn and Chamberlain 1997), pressure oxidation (Gunyanga et al. 1999) and biooxidation (Iglesias and Carranza 1994;Ciftci and Akcil 2010) have been commercially applied to enhance gold leaching from refractory ores. ...
Article
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Ultra-fine grinding (UFG) was proposed as a pre-treatment method prior to cyanide leaching of old plant tailings of a refractory silver ore. Direct cyanidation of the as-received tailings (d80: 100 µm) led to a low silver extraction of only 43% over 24 h. A size-based diagnostic study has demonstrated that the refractoriness of the tailings is essentially physical in character. Therefore, effects of particle size/UFG (d99: 5-75 µm), concentrations of NaCN (0.75-2.25 g/L) and Pb(NO3)2 (0-500 g/t) on the extraction of silver were investigated using a Box-Behnken design. The statistical analysis of the experimental data (%Ag extraction at 1 h) revealed that particle size/UFG was the most significant parameter for the extraction of silver, which was substantially improved after UFG. A positive effect of increasing the concentration of NaCN was also observed particularly at finer particle sizes while the effect of concentration of Pb(NO3)2 was insignificant. Almost complete extraction for silver (i.e. 99%) was demonstrated to be possible at even higher concentrations of cyanide (>2.25 g/L NaCN) when using the finest material (d99: 5 µm) over only 1 h of leaching. Albeit, the consumption of cyanide tended to increase with UFG or increasing the initial concentration of cyanide. The findings showed that ultra-fine grinding can be used as a suitable and environmentally sound pre-treatment method to improve the extraction of silver from the refractory silver ore tailings. The diagnostic approach adopted in the current study has proved to be a useful analytical tool to determine the amenability of the ore to ultrafine-grinding as a pre-treatment process.
... The literature appears to focus essentially on refractory ores of gold rather than silver. Several pre−treatment processes prior to cyanidation such as roasting (Dunn and Chamberlain 1997), pressure oxidation (Gunyanga et al. 1999) and biooxidation (Iglesias and Carranza 1994;Ciftci and Akcil 2010) have been commercially applied to enhance gold leaching from refractory ores. ...
Article
Full-text available
Ultra−fine grinding (UFG) was proposed as a pre−treatment method prior to cyanide leaching of old plant tailings of a refractory silver ore. Direct cyanidation of the as− received tailings (d 80 : 100 µm) led to a low silver extraction of only ≤43% over 24 h. A size-based diagnostic study has demonstrated that the refractoriness of the tailings is essentially physical in character. Therefore, effects of particle size/UFG (d 99 : 5−75 µm), concentrations of NaCN (0.75−2.25 g/L) and Pb(NO 3) 2 (0−500 g/t) on the extraction of silver were investigated using a Box-Behnken design. The statistical analysis of the experimental data (%Ag extraction at 1 h) revealed that particle size/UFG was the most significant parameter for the extraction of silver, which was substantially improved after UFG. A positive effect of increasing the concentration of NaCN was also observed particularly at finer particle sizes while the effect of concentration of Pb(NO 3) 2 was insignificant. Almost complete extraction for silver (i.e. ≥99%) was demonstrated to be possible at even higher concentrations of cyanide (>2.25 g/L NaCN) when using the finest material (d 99 : 5 µm) over only 1 h of leaching. Albeit, the consumption of cyanide tended to increase with UFG or increasing the initial concentration of cyanide. The ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 2 findings showed that ultra−fine grinding can be used as a suitable and environmentally sound pre−treatment method to improve the extraction of silver from the refractory silver ore tailings. The diagnostic approach adopted in the current study has proved to be a useful analytical tool to determine the amenability of the ore to ultrafine−grinding as a pre−treatment process.
... A mixed consortium of acidophilic bacteria is generally considered to be more effective for bioleaching than a pure culture (Akcil et al., 2007;Ciftci and Akcil, 2010;Deveci et al., 2004;Fu et al., 2008). Therefore, a laboratory stock culture of mixed meso-acidophilic chemolithotrophic microbial consortium comprising predominantly A. ferrooxidans, L. ferrooxidans and A. thiooxidans strains was used as the inoculum in the present bioleaching studies. ...
Article
Dumping of low-grade chalcopyrite encompasses several environmental problems. Despite slow dissolution rate, meso-acidophilic bioleaching is preferred for the extraction of copper from such ores. In the present study, meso-acidophilic bioleaching of a low-grade chalcopyrite in presence of an acid-processed waste newspaper (PWp) is discussed for the first time. The study illustrated a strong catalytic response of PWp with enhanced bio-recovery of copper from acid-conditioned chalcopyrite. A maximum of 99.13% copper recovery (0.36% Cu dissolution/day) was obtained in 6 days of bioleaching in presence of 2 gL- 1 PWp in contrast to only 5.7% copper in its absence. FTIR analysis of bioleached residues revealed similar spectral patterns to the original acid-conditioned ore in presence of PWp, thus indicating less development of passivation layer which was also confirmed through a complementary raman characterization of the bioleached residues. Further, a reaction mechanism (chemistry) was proposed suggesting the possible role of PWp as the electron donor under oxygen limiting conditions which facilitated microbial reduction of Fe (III). The resulting biochemical changes provided an energy source for the bacteria, thus allowing free flow of electrons through the ore surface, thus contributing towards enhanced bioleaching of copper.
... A laboratory stock of mixed meso-acidophillic chemolithotrophic microbial consortium was used for bioleaching studies (discussed in Section 2.4.2). Since, mixed consortium is considered to be more effective than a pure culture (Ciftci and Akcil, 2010;Akcil et al., 2007), a consortium predominantly comprising At. ferrooxidans, Leptospirillum ferrooxidans and Acidithiobacillus thiooxidans strains were used in the present study. The standard 9 K + media containing (NH 4 ...
Article
Dumping of poor but metal containing industrial waste is associated with several environmental issues. Exposure of these wastes to the natural environment offers serious concerns for the mineral processing industries to utilize them for metal recovery and check environmental pollution. In the present study, a novel sequential bioreduction-bioleaching and bioreduction-chemical leaching route as a hybrid process is compared and discussed for the enhanced recovery of copper from an industrial concentrator plant ball milling unit rejected sample. A mixed consortium of metal reducing bacteria (DMRB) initially adapted to high Fe(III) concentrations was found to cause mineralogical/matrix alteration (possibly silicate weathering) including Fe(III) bioreduction in the sample and dissolute 29.73% copper during the first 35 days under facultative anaerobic conditions. Sequential leaching of the bioreduced waste sample (generated from the first step) using a mixed meso-acidophilic bacterial consortium predominantly Acidithiobacillus ferrooxidans showed additional 28.72% copper dissolution within 2 days using 1 gL- 1 Fe(II). On the other hand, a comparative chemical leaching of the same bioreduced sample using 0.5 M H2SO4 yielded additional 32.17% copper within 4 days of leaching and indicated better performance than the bioleaching tests.
... [8,9] . Ciftci 等 [10] ...
Article
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In order to enhance the efficiency of bio-oxidation of Axi gold concentrate, the effects of three mixed bacteria cultures and two-step treatment approach on the intensification of bio-oxidation were studied. The results revealed that the optimum mixed culture was moderate thermophiles, the oxidation rates declined drastically with increasing of pulp density. The extraction rates of iron and sulfur were 98.35% and 91.90% at 2% pulp density, when the pulp density was elevated to 15%, the extraction rates of iron and sulfur declined to 50.25% and 49.51%, respectively. The two-step treatment could further improve the efficiency of pretreatment with moderate thermophiles, the extraction rates of iron and sulfur boosted to 64.93% and 65.73% at 15% pulp density after the two-step treatment, which was equivalent to the result obtained at 10% pulp density with normal bio-oxidation process.
... Indigenous bacterial strains are highly resistant against the recalcitrant nature of heavy metals dissolved in the leaching solution (Latorre et al., 2016). Normally, consortia of acidophilic bacteria are much more efficient in metals recovery compared to pure inoculum (Ciftci and Akcil, 2010;Yang et al., 2013;Panda et al., 2015;Utimura et al., 2017;Deng et al., 2017). The reason could be due to the high resistance of consortia towards copper, zinc, chloride, and arsenic present in the leaching solution. ...
Article
Disposing of low-grade ores involves numerous environmental issues. Bioleaching with acidophilic bacteria is the preferred solution to process these ores for metals recovery. In this study, indigenous iron-oxidizing bacteria Acidithiobacillus ferrooxidans, Leptospirillum ferriphilum, and Leptospirillum ferrooxidans were used in consortia supplemented with acid-treated bamboo sawdust (BSD) for copper and zinc recovery. Findings showed the extreme catalytic response of BSD with the best recovery of metals. Maximum of 92.2 ± 4.0% copper (0.35%) and 90.0 ± 5.4% zinc (0.33%) were recovered after 8 days of processing in the presence of 2 g/L BSD. Significant variations were reported in physicochemical parameters during bioleaching in the presence of a different concentration of BSD. Fourier Transform Infrared spectroscopy results of bioleached residues showed significant variations in spectral pattern and maximum variations were reported in 2.0 g/L BSD, which indicates maximum metals dissolutions. The impact of bacterial consortia and BSD on iron speciation of bioleached ores was analyzed by using Mössbauer spectroscopy and clear variations in iron speciation were reported. Furthermore, the bacterial community structure dynamics revealed significant variations in the individual bacterial proportion in each experiment. This finding shows that the dosage concentration of BSD influenced the microenvironment, which effect the bacterial abundance and these variations in the bacterial structural communities were not associated with the initial proportion of bacterial cells inoculated in the bioleaching process. Moreover, the mechanism of chemical reactions was proposed by explaining the possible role of BSD as a reductant under micro-aerophilic conditions that facilitates the bacterial reduction of ferric iron. This type of bioleaching process with indigenous iron-oxidizing bacteria and BSD has significant potential to further upscale the bioleaching process for recalcitrant ore bodies in an environment friendly and cost-effective way.
... However, in the two-step process, these values increased to 69.88%, 70.03% and 78.67%. It had been stated that insufficient gas transfer together with microbial cell damage due to high shear force at high pulp density influenced the bio-oxidation efficiency [20,21]. After chemical leaching, the pulp density declined to 8.5% in the biological leaching step, while in the one-step process, the pup density was 10%. ...
Article
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In order to improve the bio-oxidation efficiency of Axi refractory gold concentrate, a two-step process including a high temperature chemical oxidation and a subsequent bio-oxidation, combined with pH control during the bio-oxidation step was used. The results revealed that the optimum mode was to maintain solution pH at 1.0-1.2 during the biological oxidation stage. Under this condition, the activity of mixed culture could be sustained and the formation of jarosite could be diminished, thus the oxidation efficiency was improved. The oxidation levels of iron and sulfur were improved by 12.50% and 15.49%, and the gold recovery was increased by 21.02%. Therefore, the two-step process combined with pH control is an effective method for oxidizing the biohydrometallurgical process of Axi gold concentrate, and it will have a broad prospect of application in dealing with complex refractory gold concentrate.
... In the case of pyrite, the following equations describe the typical reac- tions that occur in a bacterially-catalyzed oxidation system ( Ubaldini et al., 2000) Bioleaching can take place in the form of heap leaching or stirred tank leaching. Although, the bioleaching in stirred tanks at industrial scale is limited to sulfidic concentrates, for recovery of gold and base metals, it has two main ad- vantages:1) the economic potential for its application into industrial scale, 2) microbial growth selectivity in the biore- actors (Dominique, 2007;Ciftci and Akcil, 2010). Many Saudi gold ores are often found as very finely dissem- inated particles in the sulfide matrix which hampers their extraction by cyanidation. ...
Article
Sometimes gold recovery from its ores represents a challenge. This is due to fine dissemination and interlocking of the gold within the associated sulfide minerals. Many approaches were tried to solve this problem, they included roasting, oxidation in addition to bioprocessing. In the last approach, application of bacteria enhances sulfides bio-oxidation and consequently facilitates their leaching. Therefore, this paper aims at investigating gold biorecovery from Alhura area gold ore, located at Kingdom of Saudi Arabia. Investigated parameters included Feed Size, mm; Dose of bacteria, ml; Retention time, day; Steering speed, rpm; Bacteria nutrient addition rate, K 2SO 4, kg/t; Bacteria nutrient addition rate, (NH 4) 3PO 4, kg/t. Statistical screening of these parameters showed that the most significant ones are: ore feed size, dose of bacteria and K 2SO 4 nutrition in addition to retention time. However at optimum conditions, (10 ml bacterial dose, 6 days retention time, and 6.5 Kg/t K 2SO 4 as bacteria nutrient) a gold concentrate containing up to 107 g/t gold from an ore containing 1.14 g/t gold was obtained.
... Oxidation slow is the most disadvantage of biooxidation process [2]. The amount of pyrite is common far higher than arsenopyrite in the gold-bearing sulfide ores , and the oxidation of the former is far slower than the latter [3]. ...
... These data show that the efficient recovery of gold from residues had a proportional dependence on the level of oxidation of sulfides, while gold recovery from untreated concentrate was 49%. These results correspond with previous published data regarding biooxidation of sulfidic concentrates (Muravyov and Bulaev 2013;Ciftci and Akcil 2010;Muravyov et al. 2015). ...
Article
A practical problem in the biohydrometallurgical processing of sulfide raw materials is the need to increase the efficiency and stability. The heating of pulp in bioreactors causes exothermic lysis of microorganisms and cessation of the production process. To find solutions to these problems, the present study examines the biooxidation of a gold-bearing sulfidic concentrate in a two-step (ferric leaching + biooxidation) process with different temperatures in the bioreactors at the biological step: 50 °C in the first bioreactor and 39 °C in the second. The main minerals of the flotation concentrate were pyrrhotite, pyrite, arsenopyrite, and stibnite. The preliminary ferric leaching of the concentrate for 5 h at 80 °C and pH 1.15–1.35 allowed oxidization of 26.8% of arsenopyrite and 54.4% of sulfur in the sulfides. At the biological step, a moderately thermophilic culture of acidophilic microorganisms dominated by representatives of the genus Sulfobacillus was used. The residence time of the pulp in each bioreactor was 2 days with a total residence time of 4 days. This study indicated that the efficiency of two-step process with the use of a two-temperature mode is comparable with the constant temperature mode and much higher than that of the one-step process. The final oxidation levels of arsenopyrite and sulfidic sulfur for the two-step process with elevated temperature in the primary bioreactor were 73.1 and 65.9%, respectively, with 82% gold recovery. Graphical abstract Open image in new window
... Free-milling gold ores, which are amenable to direct cyanidation (gold extraction ≥ 85-90%) without any need of chemical pre-treatment, have been treated and consumed too much in the past (Zhou and Fleming, 2007). On the other hand, refractory types of gold ores (gold extraction ≤~85-90%) often require some chemical and/or physical pre-treatment methods such as roasting (Dunne et al., 2013), bio-oxidation (BiOx) (Ciftci and Akcil, 2010), pressure-oxidation (POX) (Baron et al., 2016), ultra-fine grinding (UFG) (Celep et al., 2015) prior to cyanidation (Fig. 1), and also alkaline sulphide leach process (Anderson et al., 2005;Celep et al., 2011). Refractoriness is a term generally used to express the reason(s) for low or poor gold extractions (La Brooy et al., 1994). ...
Article
With the rapid depletion of free-milling types of gold ores, sulphidic gold ores are often oxidized prior to cyanidation due to the refractoriness. This results in the increase in the processing of oxidized gold ores. Thus, evaluation of the electrochemical dissolution of more complex and oxidized gold ores become ever important for the mining industry and for R&D. In this study, past achievements and recent developments in terms of electrochemical methods used in gold dissolution and passivation studies of pure gold, sulphidic, and oxidized gold ores are presented.
... Several studies have confirmed the presence of elemental sulphur in the oxidation process of pyrite (Sasaki et al, 1995;Xu and Schoonen, 1995;Schippers et al, 1996). Iron and arsenic precipitates formed in the BIOX ® process also contribute to the increased cyanide consumption (Hackl and Jones, 1997;Miller and Brown, 2005;Ciftci and Akcil, 2010). The residual iron, arsenic and other dissolved metals in solution after the CCD wash will also contribute to high cyanide consumption (Mudder et al, 2001). ...
Conference Paper
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In the application of the BIOX® technology, high tailings grade (2-16 g/t) may be generated with a decrease in overall gold recovery, which sometimes make it economically unattractive. This paper investigated the causes of the high tailings grade by using samples from the Bogoso BIOX plant. Partial chemical analysis, diagnostic tests and cyanidation optimisation studies were conducted to invoke understanding into the high tailings grades. Diagnostic study on the tailings showed 48.34% of gold in carbonaceous matter, 17.12% in quartz, 13.40% as liberated gold, 11.51% imbedded in sulphides and the remaining 9.65% in carbonates. Optimum cyanidation recovery of 90.4% was established for the BIOX concentrate at 7 kg/t NaCN, 40-50 g/L activated carbon and residence time of 40 hours. The recovery was sensitive to carbon and cyanide concentrations but additional cyanide was not economical. High preg-robbing indices of 72.7% and 64.4% were recorded for the BIOX product and the flotation concentrate respectively. This confirms the inability of the BIOX process to deactivate carbonaceous matter, which remains a serious precursor for low overall gold recoveries and high CIL tailings.
... Roasting is commercially used for oxidation of sulphide minerals and combustion of carbonaceous material (using O 2 ) (Baron et al., 2016;Dunn and Chamberlain, 1997). Pressure oxidation and biooxidation are also available technologies for oxidation of gold-bearing sulphides (Ciftci and Akcil, 2010;Gudyanga et al., 1999;Iglesias and Carranza, 1994). Oxidative pretreatment of refractory gold ores/concentrates (and passivation of carbonaceous material) using nitric acid based processes (i.e., Nitrox and Arseno (Redox)) were also tested in the literature (La Brooy et al., 1994;Miller et al., 2016). ...
Conference Paper
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In this study, the effects of ultra-fine grinding (from 27 µm (d 80 , as-received) to 7.5 µm), cyanide concentration (2-5 g/L NaCN) and pulp density (30-50% w/w) on the rate and extent of extraction of gold and silver were studied. The tailings (d 80 =27 µm) used in the leaching tests contained 17.8 g/t Au, 99.5 g/t Ag, 6.4% Zn, 9.1% Fe and 560 g/t Cu. Dissolution of zinc, iron and copper was also monitored during the leaching tests. The findings demonstrated that, the tailings sample is highly refractory with low gold recoveries (i.e. 40.2% Au over 24 h.) in cyanide leaching (2 g/L NaCN). Reducing the particle size from 27 to 7.5 µm (d 80) by ultra-fine grinding adversely affected the leaching performance in that the extraction of Au and Ag markedly decreased from 40.2% to 10.4% and from 67.3% to 54.4%, respectively, over 24 h. This could be attributed to the fact that fine grinding can increase pulp viscosity, surface area of sulphides and their possible activation. Copper dissolution was observed to be high (i.e. ≤62.2%). This could have also contributed to low gold/silver extractions due to the excessive consumption of cyanide through complexation with copper. The extraction of gold was shown to improve up to 73.0% by increasing the cyanide concentration or decreasing the pulp density.
... The utilization of carbon adsorbent based on anthracite enabled an Ag purification better than 90% and 87-88%, for thiosulphate and thiocyanate solutions, respectively [21,8]. After cyanidation, Bio-oxidation of Au concentrates using extremophiles rendered a maximum Au recovery of 92% [22,2]. The utilization of activated carbon enabled purification rates ≥ 98,15% up to nearly 100% using optimal conditions [11,2]. ...
Article
Due to the global rising demand there is an enormous interest to recover noble metals most efficiently from aqueous solutions, in particular industrial wastewater. Herein we demonstrate a new concept to recover Ag and Au from sulfate, chlorine and nitrate containing waters e.g. from catalysis production. Therefore two different treatment procedures, called 2-step ferritization and Lt-delafossite process, are presented which are based on a modified ferrite process. The phase assemblage can be controlled by the adjustment of the reaction parameters. During the 2-step ferritization process, Ag0 and Au0 are precipitated within one separate step. The precipitated phases obtained by the Lt-delafossite process are pure delafossite AgFeO2 and composite particles of Ag0, AgFeO2 and Fe3O4. By using these low energy, fast (<20 min) and easy to handle precipitation processes, Ag and Au can be recovered to ≥99.99% from wastewater. In consequence, these procedures help to avoid additional waste streams.
... It has been proposed that the oxidation rate of sulfide by ferric ion increases as temperature rises (Li et al., 2009). Though the optimal temperature of extremely thermophiles is higher than the mesophiles and moderate thermophiles, the cell walls of extremely thermophiles are not strong enough as compared to the mesophiles and moderate thermophiles if the pulp density is higher than 5%, which results in the disruption of microbial cells due to shear and stress caused by stirring (Ciftci and Akcil, 2010;Ciftci and Akcil, 2013). The regeneration of ferric ion as oxidizing agent in the process of bio-oxidation would be hindered. ...
Article
A two-stage chemical-biological oxidation approach, which applied a high temperature chemical oxidation stage prior to the bio-oxidation stage, was employed to accelerate the bio-oxidation rate of Axi gold concentrate with pyrite as the main sulfide phase. The extraction levels of Fe and S were 67.0% and 72.9% in the traditional one-stage bio-oxidation approach, while the values were elevated respectively to 78.0% and 79.6% with the two-stage approach. Accordingly, the extraction rate of gold was increased from 83.6% to 90.3% when subjected to the two-stage approach. The intensification mechanism of the two-stage approach was carried out by using Scanning Electron Microscope-Energy Dispersive X-ray Spectroscopy (SEM- EDX), X-ray photoelectron spectroscopy (XPS) and particle size analysis. The results indicated that pyrite on the surface of the concentrate was partly oxidized and particle size of the concentrate diminished after the first stage (chemical oxidation), which would facilitate the subsequent bio-oxidation process.
... Indigenous microorganisms are more resistant towards the recalcitrant behavior of heavy metals present in leaching solutions after dissolution described by Latorre et al. (2016). Generally, mixed consortia of acidophiles are considered more effective for metals' dissolution compared to pure culture (Ciftci and Akcil 2010;Yang et al. 2013;Panda et al. 2015;Utimura et al. 2017;Deng et al. 2017) because consortium has higher capability to resist copper, zinc, arsenic and chloride ions compared to pure culture. In this study, consortia of bacterial strains were found very effective in copper and zinc dissolutions under optimized conditions. ...
Article
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Indigenous iron-oxidizing bacteria were isolated on modified selective 9KFe2+ medium from Baiyin copper mine stope, China. Three distinct acidophilic bacteria were isolated and identified by analyzing the sequences of 16S rRNA gene. Based on published sequences of 16S rRNA gene in the GenBank, a phylogenetic tree was constructed. The sequence of isolate WG101 showed 99% homology with Acidithiobacillus ferrooxidans strain AS2. Isolate WG102 exhibited 98% similarity with Leptospirillum ferriphilum strain YSK. Similarly, isolate WG103 showed 98% similarity with Leptospirillum ferrooxidans strain L15. Furthermore, the biotechnological potential of these isolates in consortia form was evaluated to recover copper and zinc from their ore. Under optimized conditions, 77.68 ± 3.55% of copper and 70.58 ± 3.77% of zinc were dissolved. During the bioleaching process, analytical study of pH and oxidation-reduction potential fluctuations were monitored that reflected efficient activity of the bacterial consortia. The FTIR analysis confirmed the variation in bands after treatment with consortia. The impact of consortia on iron speciation within bioleached ore was analyzed using Mössbauer spectroscopy and clear changes in iron speciation was reported. The use of indigenous bacterial consortia is more efficient compared to pure inoculum. This study provided the basic essential conditions for further upscaling bioleaching application for metal extraction.
Article
Microbial enhanced sulfide oxidation is one of the alternatives to roasting or pressure oxidation (POX). High grade whole ores or upgraded sulfide concentrates can be processed using POX or roasting processes but the high capital and operating costs often prohibit to process lower grade materials using these process methods. Sand Farming process is a novel alternative to POX or roasting for medium to low grade materials. The key advantage of the process is to use a relatively coarser material compared to POX or roasting. The POX and roasting processes utilize fine ground materials with P80 75 µm or smaller to achieve higher gold recovery. The Sand Farming process uses agglomerated sample of P80 0.60 mm and the comminution cost saving is significant. Ore samples were collected from a gold mine in Mexico and two metallurgical composites were prepared for the study. Comp #1 contains 2.7 g/t of gold and 15.4 g/t of silver with 0.33% copper and 0.11% arsenic. The total sulfur was 5.7% and the sulfide sulfur was 5.6%. Gold and silver grade of Comp #2 was 1.2 g/t and 8.8 g/t, respectively. Copper and arsenic concentrations were lower than 0.07% and 0.03%. Total sulfur was 5.0% with 4.9% sulfide sulfur. The gold recovery from the Sand Farming was comparable to POX and roasting. Roasting calcine yielded the gold extraction of 70% while gold extraction from POX residue showed about 90%. After 44 days of Sand Farming biooxidation, 80% and 90% gold extractions were observed from Comp #1 and Comp #2, respectively. The gold extractions were further increased to 92% and 96% after 83 days of biooxidation.
Article
Arsenic and antimony are environmental pollutants associated with several diseases, including skin cancer. One of the main sources of both elements is the oxidation of sulphide minerals, particularly arsenopyrite and therefore any of both elements is a concern in gold mining operations because arsenic- and antimony-bearing sulphides may contain significant gold contents. Bioleaching is considered more environmentally-friendly than chemical processes to recovery gold, particularly from low-grade ores and residues bearing such sulphides. Thus, the current work has investigated the bio-oxidation of both a gold-bearing sulphide concentrate and a flotation tailing by Acidithiobacillus ferrooxidans, at 32 °C, focusing on the behaviour of both arsenic and antimony. Fe2+ supplementations were required in order to increase the oxidation efficiency and the optimum pH value was 1.75. The cyanidation tests confirmed the refractory nature of the samples under study (21%). The solid products of the bio-oxidation stage showed gold extractions four times as large (85%) as that of the initial sample. The dissolution of antimony was low with no evidence of the formation antimony precipitates. Unlike arsenic, the latter reported mostly to the solid product of the bioleaching step as inert sulphide particles.
Article
Drawing on recent experimental and commercial developments, this review reappraises potential substitute leach reagents for cyanide in the gold mining sector. In addition to extraction, the application of these reagents in the gold processing, pre-treatment and gold purification stages is explored. The main objective here is to discuss how the gold extraction process can be more sustainable, with a view to developing more greener leaching reagents.
Article
Pyrite, usually associated with gold deposit, is disadvantageous to gold leaching. In this study, an environmentally friendly oxidizing agent, chlorine dioxide (ClO2), was used for the first time to efficiently oxidize pyrite under mild experimental conditions, so as to remove the adverse effects of pyrite on gold leaching. Firstly, with established content determination method, the prepared pure ClO2 solutions with different concentrations were investigated on their impacts on pyrite oxidation efficiency. Secondly, solid NaClO2 was used to oxidize pyrite under acidic condition, providing a new oxidation approach for practical application. In fact, the key of the approach is the use of solid NaClO2 in acidic environment to generate ClO2 and then oxidize pyrite. For the pyrite pulp with 400 mL volume with an initial concentration of 0.025 M, the oxidation efficiencies of Fe and S in pyrite reached 99.8% and 98% at a NaClO2 concentration of 0.125 M and C4H6O6 concentration of 0.0025 M with magnetic stirring for 1 h at 30 °C in a constant temperature water bath. The addition of C4H6O6 can increase the acidity of the system and form a complex with iron ions, thereby preventing the precipitation of ferrihydrite from acidity changes caused by overdose NaClO2. After redox reaction, the acidity and oxidation–reduction potential (ORP, Eh) of the system were noticeably increased. The results of chemical composition analysis and X ray diffraction (XRD) analysis of pyrite oxidation residue confirmed that pyrite was oxidized to ions by ClO2, and no solid by-products such as Fe2O3 or Fe(OH)3 were formed. The scanning electron microscope (SEM) analysis of residual pyrite indicated that small pyrite particles were oxidized first, and then oxidants eroded the defects of large particles, gradually oxidizing all pyrites. This method is expected to be used as a pretreatment approach for pyrite oxidation before gold leaching process.
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Finding a cleaner, environmentally friendly and cost-effective way of metal and mineral extraction has a great importance in today’s world. Using microorganisms in bio-leaching and bio-oxidation process is of great value. From Archaea to bacteria and fungi, microorganisms can play an important role in extraction of metals from mine drainage and un-accessible sources, both in aquatic and terrestrial environments. Optimization of environmental factors such as the temperature, pH and substrate concentration is crucially important to access the optimum extraction of selected metals from an ore or mine drainage. The present paper will review the bio-leaching and bio-oxidation process of minerals with emphasis on the most well-known species of bacterial communities of such ability, through the literature.
Article
We examined the ferric leaching and biooxidation steps in a two-step biooxidation process of a gold-bearing sulfide concentrate containing pyrrhotite, arsenopyrite and pyrite. Ferric leaching of the concentrate (pulp density at 200 g/L) for 2 hours at 70°C at pH 1.4 by ferric sulfate solution (initial concentration at 35.6 g/L), which was obtained by microbial oxidation of ferrous sulfate allowed to oxidize 20.4% of arsenopyrite and 52.1% of sulfur. The most effective biooxidation of ferric leached concentrate was observed at 45°C in the presence of yeast extract. Oxidation of the sulfide concentrate in a two-step process proceeded more efficiently than in one-step. In a two-step mode, gold extraction from the residue was 10% higher and the content of elemental sulfur was two times lower than in a one-step process.
Article
The ongoing depletion of gold and silver-containing ore deposits demands a shift towards the processing of more complex sources. Leaching of gold or silver from these sources, which are named refractory and are typically low-grade, can be complicated due to one or more of the following effects: (1) kinetic or thermodynamic impediments on cyanide diffusion or on the dissolution of minerals, (2) readsorption of leached gold or silver (preg-robbing), and (3) excessive cyanide consumption due to the presence of cyanicidal agents. This article reviews the occurrence of these effects as a function of the elemental composition and compiles strategies to overcome them. The first type of refractory behavior was found to be related to arsenic or antimony sulfides and sulfosalts, manganese oxides, permanganates, iron sulfides, oxides, and hydroxides, tellurides, as well as different sulfur-containing minerals. In general, most kinetic or thermodynamic impediments are addressed by performing intensive grinding and through oxidation processes for sulfides, such as roasting, pressure oxidation, chemical oxidation or biological oxidation. On the other hand, preg-robbing activity was described as common in ores that contain carbonaceous matter, silicates, and sulfides. Strategies to deal with preg-robbing ores often include the destruction or deactivation of the preg-robbing surface, or the addition of another adsorbent to compete with the preg-robber (carbon-in-leach). Finally, cyanicidal agents include sulfides and compounds containing base metals, mainly copper, iron, zinc, and nickel. In the case of sulfides, performing oxidation processes is common, while base metals often require one or more pre-leaching stages in which they can be partially removed from the ore prior to cyanidation. Even though this review intends to serve as a compendium of strategies directed at improving cyanidation processes depending on the nature of the elements present in a feed, a final discussion stressing the importance of developing ore-specific strategies is given.
Article
The effect of bio-oxidation and neutral atmospheric oxidation pre-treatment of refractory gold concentrates on gold and silver extraction during cyanidation was investigated. A series of pre-treatment experiments was conducted on three high sulfur (> 9%) refractory concentrates with 0.92–1.25 g/t gold and 64–90 g/t silver. Cyanidation of the pre-treated concentrates using both methods was compared with respect to gold and silver leaching efficiency and reagent consumption. Bio-oxidation experiments were carried out using a mixed culture of Acidithiobacillus ferroxidans, Acidithiobacillus thiooxidans, and Leptospirillum ferrooxidans. The experimental results showed that both neutral atmospheric oxidation and bio-oxidation processes led to improvements in gold and silver extractions. The best result was achieved using neutral atmospheric oxidation at 20% pulp density, increasing gold and silver extraction to 78 and 73%, respectively. This corresponds to increases of 47 and 37% over direct cyanidation of the untreated concentrate. For bio-oxidation pre-treatment, increasing the duration had a negative effect on silver leaching efficiency due to jarosite formation. Cyanide consumption was reduced after neutral atmospheric oxidation and increased after bio-oxidation pre-treatment. Lime consumption increased while thiocyanate production was decreased after both pre-treatment methods. In terms of economic efficiency, neutral atmospheric oxidation pre-treatment was superior to bio-oxidation pre-treatment.Graphical AbstractPotential value in $USD per ton of three refractory concentrates with and without pre-treatment with neutral atmospheric oxidation and bio-oxidation.
Chapter
Microbe–mineral interactions are very pervasive in nature. Since coal is a chief source of nonrenewable energy and finds its application in a wide variety of sectors, the importance of microbe–mineral interaction is indispensible for developing a sustainable microbial coal biotechnology. The underlying necessity of microbe–mineral interaction is also linked with acid mine drainage that is a universal environmental problem in iron- and sulfur-rich environments. In the view of the fact that microbes act as a storehouse of several novel biomolecules or enzymes, they can be used for bioprocessing on an industrial scale incorporating innovative ideas and advanced technologies. The coal mines comprise of several synergistic interactions occurring between microbes and minerals which vary according to pH, temperature, mineralogy, and metal concentration, ultimately forming a viable microbial community.
Article
Growing demand of gold makes it essential to recover gold from refractory gold-bearing sulfide concentrates. Bio-oxidation method of refractory gold-bearing concentrates is a promising technology, but the widely utilization of bio-oxidation in the gold pre-treatment industry is limited by the low reaction rate and long oxidation cycle. Therefore, the intensifying approaches become the key factor to improve this technology. To achieve the intensification goal, three strategies were summarized in this work. Firstly, optimization of the microbial community was analyzed with the purpose of constructing more promising mixed cultures. Secondly, the chemical conditions were controlled to enhance the metabolism and function of the microorganisms. Thirdly, new processing approaches were improved to create more favorable conditions for biological and chemical reactions. When the bio-oxidation process is subjected to these strengthening strategies, the bio-oxidation efficiency will be elevated significantly. ©, 2015, Central South University of Technology. All right reserved.
Article
BACKGROUND Biomining is one of the few cost‐effective technologies that can be applied to low‐grade ores and also mining tailings. In the current work, the bio‐oxidation of a gold‐sulphide flotation tailing at high pulp densities was investigated and information for the high cyanide consumption commonly reported in bio‐oxidative processes are discussed in order to promote less use of this chemical reagent during cyanidation. RESULTS Different reactors were compared alongside the performance of strains of At. ferrooxidans and S. thermosulfidooxidans . Shaking flasks tests performed using 5% solids (w/v) revealed that the mesophiles presented a faster adaptation to the solids and resulted in better tailings oxidation as compared to the moderate thermophile. Rolling bottles (pilot scale rotating‐drum) experiments, carried out afterwards, enabled the adaptation of the At. ferrooxidans strain to 20% solids (w/v). This adapted strain was thus utilized in a 10 litre‐stirred tank reactor (pilot scale stirred tank) experiment at the same pulp density. The concentration of the ferrous iron used in the bio‐oxidation step had a detrimental effect on the cyanide consumption during cyanidation of the bio‐oxidised material. The cyanide consumption varied from 6.9kg/t to 13.7kg/t as the initial Fe²⁺ concentration increased from 0.0g/L to 10.0g/L Fe²⁺, respectively in the shaking flasks. CONCLUSION Higher pulp densities (20% solids (w/v)) resulted in a lower cyanide consumption, which was around 7.0kg/t in the experiments carried out in both rolling bottle and stirred tank reactor when 5.0g/L Fe²⁺ was used during bio‐oxidation. This cyanide consumption was associated to a gold extraction around 95%. This article is protected by copyright. All rights reserved.
Article
Separation of NaCN from the mixture of NaCN and Na2CO3 (which is mainly produced by urea and sodium carbonate method) was studied. The soluble possibility of NaCN and Na2CO3 in different mixed solvents was theoretically investigated. The experimental solubility of NaCN and Na2CO3 in water, ethanol and mixed solvent (water + ethanol) at the temperature range of 285.15K-323.15K was determined by a static method. e-NRTL model was used to estimate the experimental solubility of NaCN and Na2CO3. It is found out that optimal separation of NaCN ranges at the temperature of 318.15K below and at the ethanol mass composition of 0.7–0.8.
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Cyanide is the reagent of choice for gold and silver extraction, but also a toxic chemical that may cause severe environmental pollution problems. Vascular plants possess an enzyme system that detoxifies cyanide by converting it to the amino acid asparagine. The phytotoxicity of cyanide is indirectly connected to the efficiency of this enzyme system: Plants only survive cyanide exposure up to a dosage they can metabolize. Cyanide phyto-toxicity was measured for the subtropical grass Sorghum bicolor. Potassium cyanide was not toxic when added to the irrigation water at up to 125mg KCN/l (50mg CN/l). In a degradation test, cyanide was efficiently degraded by sorghum roots and leaves. Cyanide elimination using plants seems to be a feasible option for gold and silver mine waste and wastewater. Theoretical estimates indicate that a large area of land is needed. But the process is cost effective, sustainable, and has less critical emissions than any competing technology. Until now, phytotreatment of gold mining wastewater has only been tested on a lab scale. With the current knowledge, a pilot-scale demonstration could be implemented immediately
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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.
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This short communication highlights important biological features of a pyritic refractory gold ore heap-leaching environment. Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans were to be two of only three bacterial species detected on the ore surfaces at ambient temperature. Sulfobacillus was not originally introduced in the inoculum, but was later detected in the column. Only organisms of the Leptospirillum genus were present in the ferric sulfate leaching solution. Ignoring that the ore hosted approximately 500 times more microorganisms than the solution can greatly underestimate the oxidizing potential. One or more of these microorganisms oxidized ferrous ions, leading to a rise in the solution potential. They also oxidized elemental sulfur, resulting in a higher sulfate yield than measured in sterile medium.
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In higher plants, the alternative oxidase (AOX) is the terminal oxidase in a mitochondrial electron transport chain thought to allow carbon flow through glycolysis and the citric acid cycle when cellular energy levels are high. In phytoplankton, information is lacking on the taxonomic distribution and metabolic properties of the AOX. We used cyanide-resistant respiration to indicate the presence of the AOX, and the AOX inhibitors, salicylhydroxamic acid (SHAM) and propyl gallate (PG), to estimate the relative activity and capacity of the AOX in axenic cultures of the marine phytoplankton, Chlorella sp. (Chlorophyceae), Closterium sp. (Zygnematophyceae), Thalassiosira sp. (Bacillariophyceae), Cryptomonas sp. (Cryptophyceae), Ochromonas sp. (Chrysophyceae), and Amphidinium carterae (Dinophyceae), and the freshwater green alga Chlamydomonas reinhardtii. AOX inhibitor effects were higher in stationary phase (nutrient-limited) cultures compared to linearly growing cultures. With the exception of Closterium, in which respiration was almost completely inhibited by cyanide, estimates of AOX capacity, determined as the effect of AOX inhibitors on cyanide-resistant respiration, were nearly identical using the 2 AOX inhibitors, and ranged from 46 to 113 % of the respiration rates of cultures not exposed to inhibitors. The presence of the AOX in 5 of the 6 marine phytoplankton species tested suggests that the AOX is widespread among phytoplankton. Furthermore, the pronounced inhibitory effect of SHAM and PG added alone to stationary phase cultures of Chlorella, Thalassiosira, Cryptomonas, and A. carterae (21 to 63 % of uninhibited respiration rates) implies that the AOX contributes substantially to oxygen and carbon cycling in many species of phytoplankton during nutrient deficiency.
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The Microbial hydrometallurgy and microbial mineral processing of metal sulphides is currently a well established technology. Over past years there has been a huge amount of developments with regards to the understanding of its both engineering perspective as well as fundamental approach with regards to the microorganisms. The huge diversity of the microorganisms, which has come into picture over the years of research and development have made the engineers to go beyond several limitations of working temperature to salt tolerance of the microorganisms in harsh conditions to deliver better technologies for the future operative plants. Today scientists have been able to deliver the various mechanisms involved in bioleaching but still there are facets to be really understood and more importantly on the front how lab scale research can be turned out into full scale operation by scaling up the research and optimizing the engineering aspects of the research. Most of the bioleaching operation has shown their productivity in commercial application of refractory gold concentrates using mesophilic microorganisms followed by the cyanide leaching to recover optimum amount of gold with an environment friendly method compared to the conventional method of roasting. Research in the area of chalcopyrite bioleaching is still continuing o solve the mysteries of jarosite precipitation and formation of passivation layer, which inhibits the copper recovery in a heap leaching of chalcopyrite by biological methods. Use of extreme thermophiles in chalcopyrite bioleaching is making a revolutionary movement to solve the mystery behind the scaling up the process, which could be possible to be solved in future. Bioleaching with other sulphide minerals together with Acid Mine Drainage (AMD) mitigation, which is a serious concern today, is taking is taking shape today in order to cater the needs of the mankind. However the biohydrometallurgy research seems to contribute to a greater extent in framing environmental friendly process with regards to hydrometallurgical operations in future and establish a developed technology to benefit human beings needs by its upcoming research and development. Keywords: Biomining, refractory gold, copper, chalcopyrite, bioleaching, nickel sulphide, biooxidation, acid mine drainage
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Applications of biotechnology are in use or have been proposed for almost all sectors of the mining and minerals industries for metal extraction, metal recovery, and environmental control. A recently completed study in Canada reviewed the status of biotechnological process development in different sectors of the industry and by commodity. This paper provides an overview of the findings of the study including a discussion of the sectors of the industry in which biotechnology enjoys commercial success and those for which future applications are indicated. Special emphasis is given to the commercial metal extraction processes and to applications for environmental control for which future technical and economic advantages are likely as environmental regulations become more stringent.
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Modern commercial application of biohydrometallurgy for processing ores became reality in the 1950s with the advent of copper bioleaching at the Kennecott Copper Bingham Mine. Early application entailed dump leaching of low-grade, low-value, run-of-mine material. Dump bioleaching has evolved into a commercially accepted option for bioheap copper leaching of higher-grade, higher value ores. This commercial practice is exemplified by at least 11 mining operations. Paradoxically, application of biohydrometallurgy in the pretreatment of refractory gold ores began with processing high value concentrates, using biooxidation-tank processes and was followed by extension to processing low-grade, lower value ores in heaps. Now, bioleaching has been extended to the commercial extraction and recovery of cobalt. Even with the current success of biohydrometallurgical applications in the mining industry, the real potential of biotechnology in mining remains to be realized. As confidence in commercial bioprocessing grows and experience extends the application's knowledge base, innovations and new commercial practices will emerge. Near-term future commercial applications will likely remain focused on recoveries of copper, gold and possibly nickel. Recent technical advances show that very refractory chalcopyrite can be successfully bioleached. Processes for copper recovery from this mineral will include both heap and stirred-tank reactors. Next generation technologies for pretreatment of refractory gold ores will be based on use of thermophilic bacteria for sulfide oxidation. For biohydrometallurgy to commercially advance, the microbiologist must work cooperatively with the practitioners of the technology for mutual understanding of operational limitations and practical constraints affecting the microbiological component. q 2001 Elsevier Science B.V. All rights reserved.
Article
The Bacterial Oxidation (BIOX) process for the pre-treatment of refractory ores for gold recovery developed by Genmin, the mining division of Gencor, has been operating successfully at the Fairview Mine since October 1986. The continuous operation of a pilot plant at Genmin Process Research has resulted in further process developments to improve plant operating efficiencies. These new developments, which include a higher solids content of the pulp feed to BIOX, lower retention times, higher operating temperatures, lower power utilization for oxygen dispersion and reduced nutrient addition, have all been successfully implemented on the Fairview plant. It has furthermore been confirmed that the arsenic precipitate formed in the neutralization of effluent solutions complies to the United States of America EPA standards. Continuous cyanidation pilot plant work conducted on BIOX product has indicated that cyanide consumption can be drastically decreased if a continuous flowsheet is implemented in preference to a batch leaching system.
Article
Jarosite [KFe3(SO4)2(OH)6] is a mineral that is common in acidic, sulphate-rich environments, such as acid sulphate soils derived from pyrite-bearing sediments, weathering zones of sulphide ore deposits and acid mine or acid rock drainage (ARD/AMD) sites. The structure of jarosite is based on linear tetrahedral–octahedral–tetrahedral (T–O–T) sheets, made up from slightly distorted FeO6 octahedra and SO4 tetrahedra. Batch dissolution experiments carried out on synthetic jarosite at pH 2, to mimic environments affected by ARD/AMD, and at pH 8, to simulate ARD/AMD environments recently remediated with slaked lime (Ca(OH)2), suggest first order dissolution kinetics. Both dissolution reactions are incongruent, as revealed by non-ideal dissolution of the parent solids and, in the case of the pH 8 dissolution, because a secondary goethite precipitate forms on the surface of the dissolving jarosite grains. The pH 2 dissolution yields only aqueous K, Fe, and SO4. Aqueous, residual solid, and computational modelling of the jarosite structure and surfaces using the GULP and MARVIN codes, respectively, show for the first time that there is selective dissolution of the A- and T-sites, which contain K and SO4, respectively, relative to Fe, which is located deep within the T–O–T jarosite structure. These results have implications for the chemistry of ARD/AMD waters, and for understanding reaction pathways of ARD/AMD mineral dissolution.
Article
My perceptions of the biohydrometallurgical field span four decades and stem from being a professional microbiologist conducting academic research and research for process development and applications. My experiences have given me an appreciation for knowledge gained through fundamental research and the transfer of this knowledge to development of commercial scale applications of microbial processes. The symposia series for international activities in biohydrometallurgy has been a major factor in advancing knowledge and applications for microbial bioleach systems. The first international biohydrometallurgy meeting was held in Braunschweig, Germany in 1977. This was the predecessor for the International Biohydrometallurgy Symposia. As evident from the Symposia, advances in development and applications of biohydrometallurgy technologies follow an evolutionary, rather than revolutionary progression from demonstration of knowledge at the laboratory scale to engineering commercial plants.
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Bio leaching is becoming increasingly used for the extraction of gold from refractory gold ore. It is established that in gold bio leaching two kinds of mechanism are involved namely indirect bio leaching and direct bio leaching. The reaction schemes involved in the different mechanisms and kinetic rate expressions for different reactions have been reported in the literature.In the present paper an attempt has been made to develop an approach for simulation of a gold bio reactor leach circuit. For this purpose a mathematical model for bio leaching has been developed for process design of a gold bio reactor circuit. The salient features of the model are parameter estimation from experimental data and then scaling up the process for pilot size and industrial size bio reactor operation in continuous mode. Bio reactors can be simulated for various combinations of series-parallel modes of operation. Experimental data of the laboratory scale bio reactor collected for different pulp densities and residence times have been simulated for parameter estimation. Experimental data of pilot size bio reactors, collected from the literature, have been used for vetting of the parameters estimated earlier. Analysis of the simulation results indicates the extent of conversion taking place due to indirect and direct mechanisms. The performance of the simulation model developed based on literature information has been confirmed by some experimental data collected, as well as by literature data.
Article
The application of bacterial oxidation as a pretreatment step for the extraction of gold from arsenical gold sulphide concentrates offers potentially significant economic advantages over oxidative pretreatment alternatives. In this study the bacterial oxidation of an extremely refractory gold sulphide concentrate, from Olympias, Greece, is examined.Leaching tests were conducted in air-stirred pachuca reactors in order to determine the effect of pulp density on the degree of pyrite and arsenopyrite oxidation.Cyanidation tests were conducted in order to determine the degree of gold and silver liberation in relation to the degree of each sulphide phase oxidised and to the cyanide consumed.Mathematical analysis of the leaching data allowed the gold distribution in each sulphide mineral phase to be estimated and the gold recoveries according to the degree of each mineral oxidised predicted.
Article
Refractory gold ores and concentrates are characterised by low gold recoveries and high cyanide consumptions when subjected to direct cyanide leaching. Therefore an oxidation pretreatment step is required before cyanidation that will break up the sulphide lattice and render gold particles accessible to cyanide ions. The main three options for the treatment of refractory ores and concentrates include the traditional oxidation roasting, the modern pressure oxidation and the bacterial oxidation which is still at an advanced experimental stage. The present work focuses on the mineralogical factors influencing the refractoriness of gold, the main characteristics and developments of each process and provides economic comparative data from various operations worldwide. Environmental considerations are also briefly discussed.
Article
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.
Article
Attachment of four strains of Acidithiobacillus ferrooxidans to pyrite, chalcopyrite, galena, sphalerite or quartz was found to be mineral-selective. The bacterial extracellular polymeric substances (EPS) are responsible for mediating this process. Attachment of cells of A. ferrooxidans as well as of Acidithiobacillus thiooxidans was diminished, when depleted of their EPS. After 5 days of cultivation cells of A. ferrooxidans cover mineral surfaces with a dense biofilm, as visualised by fluorescence microscopy and AFM. Primary attachment was restricted to surface sites with visible defects.Chemical analyses of EPS of A. ferrooxidans, A. thiooxidans and Leptospirillum ferrooxidans indicated neutral sugars, fatty acids and uronic acids. The composition differed with the strain and the growth substrate. IronIII ions were only detectable in EPS of ironII ion- and pyrite-grown cells, but not in EPS of sulfur grown cells. Pyrite oxidation rates correlated with the amount of EPS-complexed ironIII ions in the case of A. ferrooxidans and L. ferrooxidans. Furthermore, pyrite oxidation rates of L. ferrooxidans were correlated with the genetic affiliation of the strains. The data for A. ferrooxidans seem to indicate a similar correlation, however, the results were not as clear-cut as those obtained for L. ferrooxidans. Sulfur oxidation rates of A. thiooxidans did not require EPS complexed ironIII ions.
Article
In this paper, the results of a study of the leaching of pyrite in the presence and absence of bacteria at the same solution conditions are summarised. These results indicate that the leaching of pyrite occurs at a higher rate in the presence of bacteria than in the absence of bacteria. Analysis of the rate of reaction as a function of concentration indicates that the order of reaction with respect to ferric ions is the same in the presence and the absence of bacteria. However, the order of reaction with respect to H+ is −0.5 in the absence of bacteria, and −0.39 in the presence of bacteria. The results of a study of the mixed potential of pyrite are also presented. These results indicate that the mixed potential of pyrite decreases with time in the presence of bacteria, while it is constant in the absence of bacteria. A detailed mechanism of the leaching of pyrite in ferric sulphate solutions is presented, and this theory is used to interpret the results. It is argued that the effect of the bacteria on the rate of leaching and on the mixed potential is not consistent with the direct contact mechanism of bacteria leaching. Instead, it is shown that the results can be explained by an increase in the pH at the mineral surface as a result of bacterial activity. This means that the increase in the leaching rate is a result of the indirect contact mechanism.
Article
In the present work, the applicability of pre-biooxidation using mixed cultures containing Thiobacillus ferrooxidans and Leptospirillum ferrooxidans from a refractory flotation sulfide concentrate was studied. The flotation concentrate consisted mainly of arsenopyrite and pyrite. Parameters for adapting the bacteria to arsenic(III) and catalyzing oxidation of sulfides with silver ion were also studied. The optimum conditions for efficient biooxidation were 10% culture inoculation, 5% pulp density, and a 10 mg/L silver ion catalyst in a pH=2 iron-free 9K suspension at 30 °C for 6 days, using the tamed mixed cultures. Under optimal bioleaching conditions, the results demonstrated that the oxidation times were shortened by almost 9 days (from 15 to 6 days at a 95% arsenic and iron oxidation level) compared to the sterilized control. After biooxidation, the gold extraction from the above biooxidation residue was studied. When sodium sulfite powder of 2.52 g/kg concentrate was added into the pH=2 sulfuric acidic–thiourea system, the consumption of thiourea significantly decreased from 12.0 to 3.0 g/kg concentrate and the gold extraction time was remarkably shortened from 6 to 1 h.
Article
Chalcopyrite oxidation was evaluated with two acidophilic thiobacilli that are important in bioleaching processes. Acidithiobacillus thiooxidans in pure culture did not oxidize CuFeS2 but oxidized externally added S0 in the presence of CuFeS2. Acidithiobacillus ferrooxidans released Cu2+ and soluble Fe from chalcopyrite, and the time course lead to a gradual passivation of chalcopyrite whereby Cu2+ dissolution leveled off. Fe3+ acted as a chemical oxidant in CuFeS2 leaching and was reduced to Fe2+. Parallel bacterial re-oxidation of Fe2+ contributed to a high Fe3+/Fe2+ ratio and an increase in redox potential. Chemical oxidation of chalcopyrite was slow compared with A. ferrooxidans-initiated solubilization. X-ray analysis revealed new solid phases: (i) jarosite, found in solids from A. ferrooxidans cultures and in chemical controls that initially received Fe2+ or Fe3+, and (ii) S0, found mostly in iron-amended A. ferrooxidans culture and the corresponding chemical controls.
Article
Bioleaching of pyrite (FeS2) by the thermophilic bacterium Acidianus brierleyi was studied experimentally and theoretically in a stirred-tank reactor operated continuously with respect to both mineral particles and liquid medium. Continuous bioleaching experiments were performed at 65°C and pH 1.5 over the range of dilution rates (0.4–5 d−1) and feed pyrite-liquid loading ratios (5 and 20 kg/m3). A mathematical model was developed to describe quantitatively the bioleaching process in the continuous-flow mode of operation. The reactor model assumed that the growth rate of adsorbed bacteria on the mineral surface is proportional to the product of the adsorbed-cell concentration and the fraction of adsorption sites by unoccupied by cells, and took into account a distribution of residence time for the mineral particles leaving the reactor. The model developed here and the parameter values determined previously in separate batch tests gave good predictions of the mean leaching fraction and the free cell concentration in the reactor. The present model was also consistent with previous rate data for the bioleaching of zinc sulfide by the mesophilic bacterium Thiobacillus ferrooxidans in a continuous-flow reactor.
Article
This chapter illustrates commercial bacterial-oxidation practice, and considers some of the design and operational characteristics of bacterial-oxidation plants for the treatment of refractory gold concentrates. New process advancements that improve the cost effectiveness of bacterial oxidation for the treatment of concentrates by improving the rate of oxidation and reducing cyanide consumption in downstream gold recovery are discussed in the chapter. These advancements have arisen as a result of the laboratory investigations combined with the observations made on existing plants. Refinements to the bacterial-oxidation process, which alter the sulfur chemistry of the final residue, have been found beneficial in reducing cyanide consumption.
Article
Pyrite and arsenopyrite concentrates were oxidized during growth of a variety of acidophilic microorganisms over a wide temperature range. A mesophilic culture, comprising Thiobacillus ferrooxidans and Leptospirillum ferrooxidans as the principal iron-oxidizing bacteria, was used at 30C; Sulfobacillus thermosulfidooxidans was active in pure and mixed cultures at 48°C; and Sulfolobus-like, thermoacidophilic archaea were grown at up to 84°C At low mineral concentrations, the rate of pyrite/arsenopyrite dissolution was proportional to temperature. However, the use of elevated temperatures to increase rates of mineral processing over those obtainable with mesophiles appeared to be most practicable with moderately thermophilic bacteria because growth of Sulfolobus strain BC was inhibited at higher mineral concentrations. Two aspects of higher temperature bioleaching were emphasized: the unique capacity of Sulfolobus-like archaea forextensive chalcopyrite oxidation; and the requirement for progress in leaching at high mineral concentrations before the potential of these organisms can be realised in process development.
Article
Cyanide leaching is the traditional process for the extraction of gold from primary raw materials. When the gold host rock contains high concentrations of sulphides, an oxidative thermal or a chemical pretreatment is often needed to access precious metal particles. Biooxidative leaching may be an interesting alternative, if compared with the environmental effects and costs of the conventional pretreatment processes, to liberate gold from the sulphide matrix and then to make it amenable to cyanidation. In this work, bioleaching with Thiobacillus ferrooxidans and Thiobacillus thiooxidans has been investigated at a bench scale on a refractory gold-bearing arsenopyrite (2 g/t Au) ore coming from the Gölcück mine (Turkey). The factors influencing the biooxidative pretreatment, in order to enhance the gold recovery in a conventional cyanidation process, were tested using a factorial plan of experiments. Direct cyanide leaching of the arsenopyrite ground to −74 μm showed no gold dissolution at all, but with fine grinding to −30 μm, gold recovery reached about 55.3% after 48 h of cyanidation. On the contrary, cyanidation accomplished on a 72 h bioleached arsenopyrite has allowed 84.3% of gold to be solubilized in 2 h, using the following bioleaching conditions: 20% pulp density, pH 2, stirring conditions 200 rpm, temperature 30°C, time 3 days. Under these conditions, the preliminary tests in a semi-continuous lab-scale microfermentor have been performed to evaluate the scale-up of the biooxidative process. It was possible to solubilise 95.2% Au after 48 h cyanidation for the samples bioleached during 3 days, and 96.8% Au when the time of bioleaching was 7 days.
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Biological treatment of a synthetic leachate containing cyanide was accomplished in a sequencing batch biofilm reactor (SBBR). A mixed culture of organisms growing on silicone tubing were provided with cyanide as a sole carbon and nitrogen source. Organisms consumed cyanide (20 mg/liter CN−WAD) and produced ammonia in an approximate 1:1 molar yield. The SBBR was operated on a 24-h cycle. Over the course of each cycle, 20 mg/liter of cyanide was degraded to below 0.5 mg/liter. Results from four track studies are presented. It was demonstrated that, when supplied with glucose, the organisms would readily consume excess ammonia. For each mole of glucose added, 10 moles of NH3-N were removed from solution. The SBBR can be used as a mobile system for treatment of leachate from gold-mining operations. Large volumes of low concentration wastewater can be treated in the SBBR since it is not necessary to maintain a consortium of settling organisms. © 1998 Elsevier Science Ltd. All rights reserved
Article
A new species of ferrous-iron-oxidizing, moderately thermophilic, acidophilic bacteria, Acidimicrobium ferrooxidans, has been described. Two isolates of the species differed only in the tendency of one, previously known as strain TH3, to grow in filaments. The chromosomal DNA base composition is between 67 and 69 mol0/o G +C. The capacity of this species to fix CO, from air was greater than that of iron-oxidizing thermoacidophiles of the genus Sulfobacillus, which required an enhanced CO, concentration for optimum autotrophic growth. Under air, ferrous iron oxidation in mixed cultures of A. ferrooxidans with either Sulfobacillus thermosulfidooxidans or Sulfobacillus acidophilus was more extensive than in pure cultures of these three strains. The greater part of ferrous iron oxidation in mixed cultures probably resulted from activity of the Sulfobacillus species, which possess a greater tolerance of ferric iron, and which presumably grew mixotrophically utilizing organic compounds from A. ferrooxidans .
Article
The chemical oxidation of the arsenopyrite and pyrite gold-bearing concentrates with the microbially-produced Fe3+ solution was investigated. During the chemical stage, the most easily oxidized sulfidic elements were shown to be removed from the concentrates. After chemical leaching of the arsenopyrite concentrate, the oxidation levels for sulfidic iron and arsenic were 64.3% and 31.1%, respectively; for the pyrite concentrate, the oxidation levels of sulfidic iron and sulfur were 21.2 and 25.8%, respectively. Biooxidation of the arsenopyrite and pyrite gold-bearing concentrates has been carried out in the onestage (control) and two-stage (experimental) variants in reactors in feed batch mode. An activity of biooxidation of sulphidic concentrates without preliminary oxidation by Fe3+ solution (one-stage process) during the control experiments and after preliminary oxidation of Fe3+ solution (two-stage process) in experimental variants was studied. At 4days residence time, the oxidation level for sulfidic arsenic from the arsenopyrite concentrate was 38.4% in the control process and 92.8% in the experimental one; gold recovery by cyanidation was 67.76% and 92.95%, respectively. At 8days residence time, the results of the control process were lower, than of experimental process after 4days of oxidation: the oxidation level of sulfidic arsenic was 59.7%; gold recovery was 82.38%. In the control variant, the time of biooxidation of the pyrite concentrate was 36days; in the experimental variant, it was 5days. The oxidation levels for sulfidic iron and sulfur were 40.3 and 55.7%, respectively; gold recovery by cyanidation was 76.9%. The results obtained demonstrate that two-stage bacterial–chemical processes are promising for intensification of the biohydrometallurgical processing of refractory sulfidic concentrates containing not only nonferrous metals, but noble metals as well.