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Pulp density

  1. 1. Conventional and electrochemical bioleaching of chalcopyrite concentrates by moderately thermophilic bacteria at high pulp density Ali Ahmadi a,b , Mahin Schaffie c , Jochen Petersen d , Axel Schippers e , Mohammad Ranjbar a,b, ⁎ a Department of Mining Engineering, Shahid Bahonar University of Kerman, Iran b Mineral Industries Research Centre (MIRC), Shahid Bahonar University of Kerman, Iran c Department of Chemical Engineering, Shahid Bahonar University of Kerman, Iran d Centre for Bioprocess Engineering Research, University of Cape Town, South Africa e Federal Institute for Geosciences and Natural Resources (BGR), Hannover, Germany a b s t r a c ta r t i c l e i n f o Article history: Received 22 September 2010 Received in revised form 8 December 2010 Accepted 8 December 2010 Available online 16 December 2010 Keywords: Electrochemistry Bioleaching Electro-bioreactor Chalcopyrite Moderately thermophilic bacteria Conventional and electrochemical bioleaching were investigated to extract copper from Sarcheshmeh chalcopyrite concentrate at high pulp densities. Experiments were conducted in the presence and absence of a mixed culture of moderately thermophilic iron- and sulphur oxidizing bacteria using a 2-L stirred electro- bioreactor at 20% (w/v) pulp density, an initial pH of 1.4–1.6, a temperature of 50 °C, a stirring rate of 600 rpm and Norris nutrient medium with 0.02% (w/w) yeast extract addition. The results of 10 day leaches showed that, when using electrochemical bioleaching in an ORP range of 400 to 430 mV, copper recovery reaches about 80% which is 3.9, 1.5 and 1.17 times higher than that achieved in abiotic electrochemical leaching, conventional bioleaching, and electrochemical bioleaching at 440–480 mV ORP, respectively. It appears that applying current directly to the slurry optimises both, the biological and chemical subsystems, leading to an increase in both, the dissolution rate and the final recovery of copper from the concentrate. Mineralogical analysis of the solid residues of electrochemical leaching in both, biotic and abiotic media, showed the formation of chalcocite and covellite minerals on the surface of not leached chalcopyrite. It is postulated that the reduction of refractory chalcopyrite to more soluble minerals such as chalcocite and covellite is achieved through both, electron transfer upon electrode contact and by ferrous reduction at the low ORP of the slurry. These secondary minerals are then rapidly dissolved through bioleaching, while at the same time a formation of a passive layer of jarosites is minimised. This process also appears to promote an increased bacteria–solid ratio due to favourable growth conditions. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Currently, pyrometallurgical processes are the predominant route to treat chalcopyrite flotation concentrates, which from an environ- mental perspective have some major problems, especially the emission of SO2 (Dimitrijevic et al., 2009). Over the last 30 years, many researchers in universities and industry have focused their efforts on finding ways to extract copper by bioleaching. However, for chalcopyrite due to the slow dissolution kinetics, caused primarily by passivation of the mineral surface, bioleaching has not been implemented at the full commercial scale as yet. A number of recent studies have reported that the oxidation reduction potential (ORP) is one of the main parameters governing the chemical and biological leaching rate of chalcopyrite. In this regard, several authors (Hiroyoshi et al., 1997, 2000, 2001; Pinches et al., 2001; Third et al., 2002; Sandström et al., 2005; Cordoba et al., 2008) have reported that during the chemical leaching of chalcopyrite in ferric sulfate media, both the rate and yield of copper dissolution is at a maximum in a narrow range of ORP around 400–450 mV (vs. Pt, Ag/AgCl), whereas at ORPs above this range, the surface passivation of the mineral could occur. Moreover, it has been found that applying direct current into the bacterial slurry significantly enhances both the activity and growth of microorganisms (Natarajan, 1992; Nakasono et al., 1997; Ahmadi et al., 2010a). It should be mentioned that in conventional tank bioleaching of chalcopyrite, extremely thermophilic microorganisms (temperatures as high as 70–80 °C) are required to extract copper rapidly whilst maintaining economic viability. The leading process is BioCOP™ which operates at 78 °C and 12% (w/w) pulp density (Batty and Rorke, 2006). At these high temperatures, difficulties such as low solubility of oxygen, high rate of evaporation, high corrosion of reactor construction materials, and high sensitivity of the thermophilic cells to metabolic stress caused by excess turbulence (in the contact of increased pulp density) occur. These are less of a problem at 50 °C or below (Rawlings et al., 2003; Olson et al., 2003; Okibe et al., 2003). Hydrometallurgy 106 (2011) 84–92 ⁎ Corresponding author. Department of Mining Engineering, Shahid Bahonar University of Kerman, Iran. Tel./fax: +98 341 2113663. E-mail address: m.ranjbar@web.de (M. Ranjbar). 0304-386X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2010.12.007 Contents lists available at ScienceDirect Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet
  2. 2. A previous study found some promising results to extract copper from chalcopyrite concentrates by electrochemical bioleaching at lower temperatures (≤50 °C)(Ahmadi et al., 2010a). In that research, electrochemical and conventional bioleaching experiments were carried out on a chalcopyrite concentrate using a mixed culture of mesophilic bacteria at 35 °C and a mixed culture of moderately thermophilic bacteria at 50 °C, operating at 10% (w/v) pulp density. The results showed that the control of solution ORP around 425 mV (vs. Pt, Ag/AgCl), by applying current directly to the slurry, significantly increases both, the cell concentrations and copper recovery in both cultures, especially in the presence of moderately thermophilic bacteria. One of the main limitations of bioleaching processes in stirred tanks, especially in the case of extreme thermophiles, is the negative effect of high pulp densities on both extraction rate and final recovery of metals. Oxygen and carbon dioxide availability, low bacteria–solids ratio, metabolic stress by high shear stress and abrasive conditions, inhibition of bacterial attachment, and the build-up of toxic leach products or other detrimental substances (such as some flotation reagents) have been reported as the most significant problems for a successful operation of bioleaching at high solid contents (Bailey and Hansford, 1993; Acevedo and Gentina, 2007). To overcome these problems and meet the requirements of industry, microorganisms must be adapted to high pulp densities (Mishra et al., 2005). However, to date, there is no information in the literature on the electrochem- ical control of the bioleaching process at high pulp densities of stirred reactors (at 10% ; Ahmadi et al., 2010a). For this reason, the present research work investigates the process of electrochemical bioleaching at high pulp density and compares its efficiency to that of conventional and electrochemical leaching processes in the presence and absence of moderately thermophilic bacteria. The work was executed by leaching of a Sarcheshmeh chalcopyrite flotation concentrate in a stirred electrobioreactor. 2. Experimental 2.1. Materials A mixed culture of moderately thermophilic bacteria supplied by Sarcheshmeh Copper Mine, Kerman, Iran, was used. The microorgan- isms were grown at 50 °C on Norris medium (0.4 g/L (NH4)2SO4, 0.4 g/L K2HPO4, 0.5 g/L MgSO4.7H2O) with the copper concentrate at pulp densities from 2% to 20% (w/v) replacing the energy source. The flotation concentrate was obtained from Sarcheshmeh Copper Mine, and contained 44.02% chalcopyrite (CuFeS2), 23.99% pyrite (FeS2), 6.87% covellite (CuS), 5.84% chalcocite (CuS2), 13.61% non-metallic minerals and 4.79% copper oxide minerals as shown by mineralogical analysis. The chemical analysis of the representative sample is presented in Table 1. Mineralogical investigations on the feed and solid residues were performed by optical microscopy using a Leica phase contrast microscope (DMLP). Since during the quantitative determination of phases, the iron present in the iron hydroxide precipitates is ascribed to chalcopyrite and pyrite minerals, the quantitative determination of these residue analyse are not scientifically reliable; hence in this study the results are reported only qualitatively. The transformation is visualised in order to underline our understanding of the mechanism of dissolution. The particle size distribution of the concentrate was determined by wet sieving and cyclosizer and showed that 80% is passing 76 μm (Fig. 1). 2.2. Electrobioreactor The leaching experiments were performed in a three-electrode, 2 L glass electro-bioreactor with 4 baffles, thermostated at the desired temperature by circulating water from a constant temperature bath through the double-wall jacket (Fig. 2). The reactor had a medium volume of 1.3 L with a medium height/diameter ratio equal to 1.1. The leach slurry was mechanically stirred by a pitched blade impeller (diameter=5 cm) mounted on a rotating shaft. A titanium–platinum mesh (15 cm×9 cm×0.1 cm), acting as the cathodic working electrode, was immersed into the reactor solution. A platinum foil was used as a counter electrode and was put into a separate small anodic compart- ment, separated from the cathode chamber by a glass frit. An Ag/AgCl reference electrode was in contact with the electrolyte in the main chamber through a Luggin capillary, which ended just short of the working electrode. Air was supplied through a stainless-steel ring sparger underneath the impeller. 2.3. Leaching experiments Batch experiments were carried out in the electro-bioreactor at 20% (w/v) pulp density, an initial pH of around 1.5, a temperature of 50 °C, a stirring rate of 600 rpm, an aeration rate of 1 vvm (volume of air/volume of slurry/min) and Norris nutrient medium with 0.02% (w/w) yeast extract addition. The intense agitation is needed both for maintaining a homogeneous suspension and increasing the rate of mass transfer (especially of oxygen and carbon dioxide from the gas phase). During the leaching experiments, the pH of the suspensions was monitored periodically and adjusted to around 1.5 by addition of H2SO4 (6 M). The variations of ORP were recorded daily throughout the leaching period. The pH and ORP values were measured with a Jenway 3540 pH meter and a Pt electrode in reference to an Ag/AgCl electrode (+207 mV vs. SHE at 25 °C), respectively. Samples were periodically taken from the slurry and filtered through Whatman No.41 filter paper. After that the filtrate was used for copper and iron analysis by the atomic adsorption method. The remaining solids were returned to the reactor. The evaporated liquid was periodically replaced by adding acidified distilled water (pH=1.5). The biotic experiments were inoculated with 20% (v/v) of a culture previously adapted to 20% pulp density. To maintain the ORP in the desired range (400–430 or 440–480 mV) during electrochemical bioleaching, the potential of the working electrode was controlled with respect to the reference electrode with a Solartron Sl 1287 potentiostat/ galvanostat. To keep the ORP in the desired range, the applied potential was manually set slightly lower than the set point value during the experiments, however it was always higher than 250 mV. The initial solutions of the conventional bioleaching, electrochem- ical bioleaching at 400–430 mV and electrochemical bioleaching at 440–480 mV experiments contained 2.46, 3.15 and 2.35 g/L iron, respectively, which originated from their inoculum solutions. Table 1 Chemical analysis of the copper sulphide concentrate. Elements Cu Fe S Si Al Zn Mg K (wt.%) 27.50 23.03 14.82 3.87 1.45 0.99 0.40 0.24 Cumulativepassing(%) Particle size (micrometer) 100 80 60 40 20 0 1 10 100 Fig. 1. Particle size distribution of copper concentrate. 85A. Ahmadi et al. / Hydrometallurgy 106 (2011) 84–92
  3. 3. To evaluate the contribution of acid solution to copper recovery, an abiotic test of chemical leaching was carried out under the same conditions as the bioleaching test, except the initial composition of their solutions (in the abiotic tests there was no iron in the initial solution). In addition, an abiotic electro-leaching test was conducted by applying a fixed 100 mA current (no ORP control), to investigate the effect of applied DC current to the slurry on the recovery of copper and iron, while other conditions were kept similar to the abiotic chemical leaching test. It should be mentioned that this electro-leaching test was different from that performed in a previous study (Ahmadi et al., 2010a), in which the working electrode was set as anode causing different electrochemical reactions expected here are different from those that occurred in the previous test. In the abiotic tests of this new study, the medium was sterilized with 2% (v/v) bactericide (2% (w/w) thymol in ethanol) added to prevent microbial growth. In order to sterilize the reactor before each test, it was operated for 2 h at 85 °C and 800 rpm in a solution of 10% (v/v) bactericide and 5% (v/v) HCl. Microbial growth was periodically verified by observation under a Nikon optical microscope (ECLIPSE, TE 2000-U). Free cells in solution were counted by direct counting using a Thoma chamber of 0.1 mm depth and 0.0025 mm2 area with the optical microscope (magnification=1500×). 3. Results and discussion In order to examine the influence of ORP and presence of bacteria on the dissolution of chalcopyrite concentrate, various processes, namely chemical leaching (control test), bioleaching, electro-leaching and electrochemical bioleaching were carried out in an electro-reactor at 20% (w/v) pulp density. During evaluating the results and comparing them with those obtained from the previous research at the lower pulp density (Ahmadi et al., 2010a), it should be considered that the experimental conditions of this study are different from those in the previous work. One of the main differences relates to their different stirring rate. In that research, because of fear of bacterial adaptation, the stirring rate was set at 300 rpm in the first 4 days with the result that during this period, a portion of the concentrate had settled at the bottom of the reactor and didn't take part in the leachingreactions. This problem was solved in the high pulp density by consecutive adaptation tests (4 times) at stirring rate of 600 rpm which was constant at the rate during the main experiments. 3.1. Chemical leaching Figs. 3 and 4 show the results of copper recovery (Fig. 3) and iron dissolution (Fig. 4) from the concentrate by the various methods over Fig. 2. Schematic illustration of thermostated electrobioreactor. Fig. 3. Copper recovery as a function of leaching time at 50 °C and 20% pulp density for different experimental conditions: chemical leaching (CL), electrochemical leaching (ECL), bioleaching (BL) and electrochemical bioleaching (EBL). 86 A. Ahmadi et al. / Hydrometallurgy 106 (2011) 84–92
  4. 4. 10 days. As can be seen, the rate of chalcopyrite concentrate dissolution in chemical leaching process (control test) is significantly lower than those obtained in the other processes in which final values of copper recovery and iron dissolution did not exceed 13% and 5%, respectively. The main portion of this copper extraction is attributed to the acid leaching of copper oxides and partial leaching of chalcocite (Eqs. (1) and (4)) and covellite (Eqs. (2) and (5)) minerals. It is presumed that chemical dissolution of chalcopyrite (Eqs. (3) and (6)) is negligible due to its high lattice energy (Habashi, 1978). Cu2S þ 2H þ →Cu 2þ þ Cu o þ H2S ð1Þ CuS þ 2H þ →Cu 2þ þ H2S ð2Þ CuFeS2 þ 4H þ →Cu 2þ þ Fe 2þ þ 2H2S ð3Þ Cu2S + 1 =2O2 + 2H þ →Cu 2+ + CuS + H2O ð4Þ CuS + 1 =2O2 + 2H þ →Cu 2+ + S B + H2O ð5Þ CuFeS2 þ O2 þ 4H þ →Cu 2þ þ Fe 2þ þ 2S∘ þ 2H2O ð6Þ The smell of rotten eggs during the process could be related to the production of H2S through reactions (1) and (2). Fig. 5 shows the variation of pH during various processes. The pH value rises during the chemical leaching of the concentrate, which is due to acid consumption by the reactions of copper oxide and sulphide minerals (Eqs. (1)–(6)) as well as gangue minerals. It was controlled at around pH 1.5 by the addition of H2SO4 (during the first 6 days). Fig. 6 shows that the ORP values remained low in the range of 300–330 mV. The low extraction ratio of Fe:Cu is probably due to the dissolution of iron-free minerals i.e. copper oxides, covellite and chalcocite and the insolubility of refractory iron bearing minerals i.e. pyrite and chalcopyrite under the conditions studied. Comparison of the mineralogical analysis of the solid residue (after 10 days) (Fig. 7b) with that of the feed concentrate (Fig. 7a) clearly reveals that secondary copper bearing minerals i.e. covellite and chalcocite were not dissolved. 3.2. Conventional bioleaching To investigate the efficiency of the mixed culture of moderate thermophiles at high pulp density, a conventional bioleaching experiment was carried out in the stirred bioreactor. The optimal conditions (temperature, 50 °C; initial pH, 1.5; nutrient medium, Norris; and yeast extract, 0.02% (w/w)) obtained from a previous study (Ahmadi et al., 2010b) were employed. During preliminary bioleaching experiments (data not shown), the bacteria were adapted to a high solid content and high degree of slurry agitation by increasing the pulp density from 2 to 20% (w/v) and the stirring rate from 300 to 600 rpm and then maintaining these conditions. The adapted culture (the 4th generation of a culture maintained at 20% pulp density and 600 rpm) was used to inoculate the bioleaching experiment discussed here (inoculation volume=20% (v/v)). Looking at Fig. 3, the copper recovery profile can be divided into three distinct phases. Initially, approximately 21% of copper is rapidly leached within the first day; this is mainly associated with the dissolution of copper oxides by H2SO4 and the first stage leaching of chalcocite (Eq. (7)). Thehigher initial copper extraction in this biotic test compared to the chemical leaching test (21.4% vs. 8.1%) is mainly related to their different initial solution compositions. In the bioleaching test, the iron concentration in the initial solution is 2.46 g/L (mostly as ferric iron), originating from the inoculation solution, which acts as a leaching agent for copper sulphides (Eqs. (7) to (9)) and pyrite (Eq. (10)). Cu2S þ 2Fe 3þ →Cu 2þ þ 2Fe 2þ þ CuS ð7Þ CuS þ 2Fe 3þ →Cu 2þ þ 2Fe 2þ þ S∘ ð8Þ CuFeS2 þ 4Fe 3þ →5Fe 2þ þ Cu 2þ þ 2S∘ ð9Þ FeS2 þ 8H2O þ 14Fe 3þ →15Fe 2þ þ 2SO 2− 4 þ 16H þ ð10Þ Fig. 5. Variation of pH as a function of leaching time at 50 °C and 20% pulp density for different experimental conditions: chemical leaching (CL), electrochemical leaching (ECL), bioleaching (BL) and electrochemical bioleaching (EBL). Fig. 6. ORP variation as a function of leaching time at 50 °C and 20% pulp density for different experimental conditions: chemical leaching (CL), electrochemical leaching (ECL), bioleaching (BL) and electrochemical bioleaching (EBL). Fig. 4. Total iron recovery as a function of leaching time at 50 °C and 20% pulp density for different experimental conditions: chemical leaching (CL), electrochemical leaching (ECL), bioleaching (BL) and electrochemical bioleaching (EBL). 87A. Ahmadi et al. / Hydrometallurgy 106 (2011) 84–92
  5. 5. The initial copper extraction rate in this research is significantly higher than that obtained in the previous study (Ahmadi et al., 2010a), which could be attributed to the higher stirring rate employed here (which prevents the setting of the solids. The first phase is followed by another linear increase to about 51%, between days 1 and 7. This increase is probably associated with the leaching of chalcopyrite and covellite (natural mineral and the product of chalcocite leaching in Eq. (7)) according to Eqs. (9) and (8), respectively. This phase is followed by ceasing the copper dissolution in the remaining days to the end of experiment. Comparing the various processes in Figs. 3 and 4, it can be found that the final values of copper recovery (51.6%) and iron dissolution (15.9%) are, respectively, around 3.8 and 3.2 times higher than those of the chemical leaching process. Mineralogical examinations showed that secondary copper bearing minerals such as chalcocite and covellite were not found in the bioleaching residue (Fig. 7c), although they were present in the feed concentrate (Fig. 7a) and in the residue of the chemical leaching experiment as well (Fig. 7b). In the bioleaching process, bacteria oxidize the insoluble metal sulphides, such as chalcopyrite, by indirect and/or contact mechanisms (Sand et al., 2001). Iron- and sulfur-oxidizing bacteria catalytically generate the leaching agents of [Fe3+ ] and [H+ ] according to Eqs. (11) and (12), respectively, and then these agents, especially ferric iron, dissolve the sulphide minerals (Eqs. (1)–(10)). In this regard, sulphur Fig. 7. Mineralogical images of feed (a); solid residues of (b) chemical leaching, (c) bioleaching, (d ) electrochemical leaching, (e and f) electrochemical bioleaching, (Cv=covellite; Cc=chalcocite; Ccp=chalcopyrite; Py=pyrite). 88 A. Ahmadi et al. / Hydrometallurgy 106 (2011) 84–92
  6. 6. oxidizing bacteria remove elemental sulphur, as a passivation layer, from the surface of sulphide minerals during the acid production process (Eq. (12)). 4Fe 2þ þ O2 þ 4H þ þ Iron oxidizing acidophiles →4Fe 3þ þ 2H2O ð11Þ S B + H2O + 3 =2O2þ Sulphur oxidizing acidophiles →H2SO4 ð12Þ Schippers and Sand (1999) proposed that the indirect mechanism occurs via the thiosulphate route (primarily pyrite) or via the polysulphides and sulphur route (most other sulphide minerals such as chalcopyrite). However, in the contact mechanism bacteria attach to the mineral surface and prepare the medium and then facilitate the mineral attack through an electrochemical dissolution involving ferric ions contained in the microbe's extracellular poly- meric substances (EPS) (Sand et al., 2001). Tributsch (2001) concluded that in practice suspended bacteria feed on chemical species released by attached bacteria (cooperative mechanism). The rapid increase of ORP to relatively high levels (Fig. 6) and the pH decrease (Fig. 5) indicate that both, activity and growth of the bacteria are very favourable under the conditions studied. It can be seen that ORP increases from 390 to 540 mV (after day 7) with the lag phase of bacterial growth being less than 1 day, whereas it was significantly longer in the bioleaching experiment of the previous study (Ahmadi et al., 2010a). Furthermore, Fig. 5 shows the variation of pH during the bioleaching experiment. During the first day of the experiment, pH increases and sulfuric acid needs to be added to keep the reactor pH around 1.5. After that the pH decreased gradually to a final value of 1.3, which indicates that the amount of acid-production is more than that of acid consumption during the transition phase. It should be noticed that the initial upward trend of pH is due to the acid consuming reactions such as the dissolution of copper oxides, chalcocite (Eqs. (1) and (4)), covellite (Eq. (2) and (5)), chalcopyrite (Eq. (3) and (6)) and gangue minerals as well as the bacterial oxidation of Fe(II) to Fe(III) (Eq. (11)), while the subsequent decrease of pH is due to the activity of bacteria to produce acid (Eq. (12)), the dissolution of pyrite (Eq. (10)) and the hydrolysis of ferric iron to form jarosite (Eq. (13)). Jarosite is formed under the conditions of high pH and high ferric iron concentration, as might be expected from Eq. (13) (Stott et al., 2000). 3Fe 3þ þ X þ þ 2HSO − 4 þ 6H2O→XFe3ðSO4Þ2ðOHÞ6 þ 8H þ ð13Þ where X+ =K+ ,Na+ ,NH4 + and H3O+ . Despite using a dilute nutrient medium (Norris), due to the high solid content, the concentration of alkali ions increased in the solution, which is a favorable factor for precipitation of jarosite at the high solution ORPs facilitated by iron oxidizing bacteria. Moreover, the precipitation is not reversible in chalcopyrite systems (Leahy and Schwarz, 2009); hence, once formed, the later increase of acid concentration doesn't dissolve the precipitate. Jarosite formation causes the removal of Fe3+ and essential bacterial nutrients, suchas K+ orNH4 + , from solution, potentially resulting in a slowed-down process or even a complete stop. It restricts the flow of bacteria, nutrients, oxidants and reaction products to and away from the mineral surface (Hackl et al., 1995). The stoppage of copper dissolution after day 7 is likely related to the passivation of chalcopyrite by jarosite precipitates. In our previous study (Ahmadi et al., 2010a), a significant amount of jarosite was found on the bioleaching residue, which was confirmed by SEM/EDS analyses (Fig. 8). The low leaching rate in the chemical leach could also be caused by the passivation of chalcopyrite surface by polysulphide compounds (Biegler and Horn, 1985). It is likely that both jarosite and polysulphides passivate jointly the mineral surface. The increase of the rate of iron dissolution and the concomitant decrease of the rate of copper dissolution in the final days of the experiment (Fig. 4) are attributed to the leaching of pyrite and could be explained by the mechanism described by Petersen and Dixon (2006), in which the dissolution of chalcopyrite is preferentially accelerated at low ORPs, while at higher ORPs, pyrite and covellite are leached much faster than chalcopyrite. During the analysis of iron dissolution profiles, it should be borne in mind that these values do not take into account the portion of iron leached from the concentrate which was subsequently precipitated as iron hydroxides especially jarosite. This problem occurs to a a significant extent during the conventional bioleaching test, where the ORP values surpass 500 mV and jarosite precipitation would occur. As noted above, this phenomenon was encountered in our previous study (Ahmadi et al., 2010a). It should be noticed that iron dissolution reported here represents the minimum iron recovery from the concentrate. 3.3. Electro-leaching It can be assumed that the electrical charging of semiconducting metallic sulphide minerals such as chalcopyrite could be done as a result of periodic electrical contact between mineral particles and the working electrode. These contact interactions may occur in the electrochemical bioleaching experiment when current is applied to control the solution ORP. Hence, to investigate the occurrence of this phenomenon and its influence on the copper recovery and iron dissolution from the concentrate, an electro-leaching experiment was carried out at a current density of 1 mA cm-2 (total direct curren- t=100 mA) and 20% (w/v) pulp density. Such a low current density was chosen to minimize the evolution of hydrogen (Eq. (14)) and was Fig. 8. SEM image and EDS analysis of the solid residue of conventional bioleaching. 89A. Ahmadi et al. / Hydrometallurgy 106 (2011) 84–92
  7. 7. in the range of the current passing in the electrochemical bioleaching tests discussed in the next subsection 2H þ þ 2e − →H2 ð14Þ The values of copper recovery and iron dissolution from the concentrate by the electro-leaching method are also presented in Figs. 3 and 4. Fig. 3 shows that the final copper recovery by electro- leaching (~20%) is significantly higher than that in the chemical leaching process, but substantially lower than those in both the conventional bioleach and the electrochemical bioleaching processes discussed below. However, iron dissolution is initially very low, whereas, the final dissolution is significantly higher than that obtained in the chemical leachingtest (Fig. 4). As Fig. 6 depicts, duringthe electro- leaching process, the ORP profile is slightly below that measured in the chemical leaching process. Because of low ORP, the dissolution of pyrite as the main iron bearing phase is very slow. Moreover, similar to what was measured in the other experiments, the solution pH rises initially due to the initial acid consuming reactions (Fig. 5). Using the electro-leaching method, some reactions would be expected to occur as a result of passing direct current across the slurry. As noted by Warren et al. (1982), electrochemical reactions of a mineral are a direct result of the thermodynamic properties of the mineral, properties of the electrolyte, and their interaction at the mineral-electrolyte interface. The data in Figs. 3 and 4 suggests that during the electro-leaching process, the extraction ratio of Fe:Cu rises from 0.28 in day 1 to 0.45 at the end of the test. This increase could be related to the electro-reduction of chalcopyritetochalcocite(Eqs.(15)and (16))andremovingironfromthe chalcopyrite lattice as described by Biegler and Swift (1976). 2CuFeS2 þ 6H þ þ 2e − →Cu2S þ 2Fe 2 þ 3H2S ð15Þ CuFeS2 þ 3Cu 2þ þ 4e − →2Cu2s þ Fe 2þ ð16Þ This result was confirmed by mineralogical analysis (Fig. 7d), in which chalcocite was found around chalcopyrite particles in the solid residue (blue-gray region). Chalcocite could also be produced by electro- reduction of covellite according to the following equation (Elsherief et al., 1995). 2CuS þ 2H þ þ 2e − →Cu2S þ H2S ð17Þ This reduction increases the overall dissolution rate of covellite or stage (II) of chalcocite leaching (Eq. (8)) as the rate of stage (I) chalcocite leaching (Eq. (7) is more rapid. Biegler and Swift (1976) reported that at high current densities (N10mAcm−2 ) metallic-copper could be deposited on the cathode electrode. This deposition may be as a result of a cathodic reaction described by Eq. (18) or due to the direct electroplating of Cu2+ in solution (Eq. (19)). Cu2S þ 2H þ þ 2e − →2Cu∘ þ H2S ð18Þ Cu 2þ þ 2e − →Cu ∘ ð19Þ It should be noted that to check the formation of copper on the cathode during the process, a portion of the slurry was occasionally taken out and returned instantly to the reactor. No copper deposit was observed at any stage during the test. However, if copper is deposited, it can be a reductive agent for chalcopyrite leaching according to Eq. (20) as reported by Hiskey and Wadsworth (1981). Therefore the deposit could not have been formed. 2CuFeS2 þ Cu o þ 2H þ →Cu2S þ Fe 2 þ H2S ð20Þ In terms of galvanic interactions between different metallic sulphide minerals, Mehta and Murr (1983) observed that when pyrite, chalcopyrite, chalcocite and covellite are in contact with each other, due to their different rest potentials the rate of covellite dissolution would be the fastest of all (anodic corrosion), followed by chalcocite and chalcopyrite, with the rate of pyrite dissolution the lowest of all (cathodic protection). Furthermore, it has been reported (Warren et al., 1982; Lu et al., 2000) that chalcopyrite may be electrochemically converted to other copper sulphide phases such as talnakhite (Cu9Fe8S16) (Eq. (21)) or bornite (Cu5FeS4) (Eqs. (22) and (23)), which would be dissolved faster than chalcopyrite. 9CuFeS2 þ 4H þ þ 2e − →Cu9Fe8S16 þ Fe 2þ þ 2H2S ð21Þ 5CuFeS2 þ 12H þ þ 4e − →Cu5FeS4 þ 4Fe 2þ þ 6H2S ð22Þ 2CuFeS2 þ 3Cu 2þ þ 4e − →Cu5FeS4 þ Fe 2þ ð23Þ H2S produced during the process could be oxidized in the presence of Fe(III) (Eq. (24)) and/or oxygen (Eq. (25)) or undesirably led to the precipitation of covellite in the presence of Cu(II) (Eq. (26)). H2S þ 2Fe 3þ →S∘ þ 2Fe 2þ þ 2H þ ð24Þ H2S + 1 =2O2→SB + H2O ð25Þ Cu 2þ þ H2S→CuS þ 2H þ ð26Þ 3.4. Electrochemical bioleaching Two electrochemical bioleaching experiments were carried out, one at an ORP in the range of 400–430 mV and one at an ORP in the range of 440–480 mV over 10 days. The values of copper recovery and iron dissolution from the concentrate by electrochemical bioleaching are shown in Figs. 3 and 4, respectively. As can be seen in Fig. 3, the highest copper recovery among the various processes was obtained in the experiment conducted at the ORP range of 400–430 mV, in which copper recovery after 10 days reached 77%. When the ORP was kept higher, in the range of 440–480 mV, Cu dissolution leveled off after the 3rd day despite showing the highest initial rate of copper extraction in all tests. The final recovery (66%) was significantly lower than that obtained during the experiment conducted in the range of 400– 430 mV, however it was still higher than that obtained by conven- tional bioleaching (~51%), electro-leaching (~20%) and chemical leaching (~13%). The higher initial copper extraction rate in the first 3 days of electrochemical bioleaching at 440–480 mV could be related to the higher ORP values in this period which is more favorable for the leaching of chalcocite and covellite minerals. In these biotic tests, the initial iron concentrations, originating from the inoculated solution, were 3.15 g/L (ferric iron concentration=2.21 g/L) and 2.35 (mostly as ferric iron) g/L in tests conducted at 400–430 mV and 440–480 mV, respectively. Ferric iron acts as a leaching agent for copper sulphides and would lead to a high initial rate of copper dissolution from the concentrate. Fig. 4 illustrates that the dissolution of iron during electrochemical bioleaching in the range of 400–430 mV increases linearly up to around 30%, as compared to 23%, 16%, 9% and 5% during electrochemical bioleaching at the range of 440–480 mV, conventional bioleaching, electro-leaching and chemical leaching processes, respec- tively. It should be noted that in electrochemical bioleaching the dissolution of iron is significantly lower than that of copper (29% against 77%), similar to what was observed in the other processes 90 A. Ahmadi et al. / Hydrometallurgy 106 (2011) 84–92
  8. 8. studied. The main reason for the low Fe:Cu extraction ratio in electrochemical bioleaching could be the low solubility of pyrite as the main iron bearing phase in the concentrate at the prevailing potential. Mineralogical analysis of the solid residue from the electrochemical bioleaching at 400–430 mV (Fig. 7e) shows that the chalcopyrite was corroded much more, while the pyrite surface remained almost unaffected. The amount of current passing through the reactor varied from 100 to 450 mA. As has been shown, both, applying current directly into the slurry and controlling the ORP, have a positive effect on both the chemical and biological sub-systems. In this regard, the increased copper extraction rate is postulated to be due to the three following reasons: firstly, by electrochemical reduction of ferric iron, the concentration of ferrous iron which is a source of energy for bacteria, increases, so their growth and activity would be enhanced. Enumer- ation of bacterial populations in the final solutions showed that the cell density increased from about 4×108 cells/ml for conventional bioleaching to about 9×108 cells/ml for electrochemical bioleaching at 400–430 mV. Previous work done by the authors (Ahmadi et al., 2010a) also showed that after 5 days the cell concentrations of both, mesophilic and moderately thermophilic cultures, were about 3–4 fold higher in electrobioleaching slurries than those counted in the related conventional bioleaching processes. Natarajan (1992) and Nakasono et al. (1997) also found that applying a current into a solution containing Acidithiobacillus ferrooxidans increases the cell concentrations substantially. Industrially, this result is very important, because low bacteria–solids ratios have been cited as one of the main problems of the bioleaching processes at high pulp densities (Bailey and Hansford, 1993). The second reason is that the highest dissolution rate of chalcopyrite occurs at low solution ORPs around 400 mV which has been explained by Hiroyoshi and co-workers (Hiroyoshi et al., 1997, 2000, 2001). This effect has also been confirmed by other authors (Pinches et al., 2001; Third et al., 2002; Sandström et al., 2005; Cordoba et al., 2008; Gericke et al., 2010). Moreover, SEM/EDAX examinations of our previous research (Ahmadi et al., 2010a) showed that electrochemical bioleaching of chalcopyrite concentrate at around 425 mV, significantly reduces the amount of jarosites on the chalcopyrite surface. Conner (2005) also demonstrated that applying DC current into the bioleaching solutions significantly reduces the formation of jarosite in the solid residues. The third reason is that, as observed in the electro-leaching experiment, periodic electrical contact between chalcopyrite and the working electrode electro- chemically reduces chalcopyrite to more soluble minerals such as chalcocite and covellite. Mineralogical observations of the residue of electrochemical bioleaching at 400–430 mV clearly visualized the coverage of chalcocite and covellite minerals on the surface of not leached chalcopyrite which is attributed to the reduction of chalcopyrite to these secondary copper sulphides through both direct electron transfer to the mineral and indirect reduction by Fe(II) at the low solution ORP. This reason was verified by electrochemical analyses (Ahmadi, 2010c) and was also reported previously by Biegler and Swift (1976). 4. Conclusions Conventional and electrochemical (bio)-leaching were explored to extract copper from Sarcheshmeh chalcopyrite flotation concentrate at high pulp density using a mixed culture of moderately thermophilic bacteria adapted to 20% (w/v) pulp density. Leaching of copper was found to be most efficient during bioleaching with the ORP electrochemically controlled between 400 and 430 mV. The final copper recovery of electrochemical bioleaching at 400–430 mV (77%) was higher by a factor of 5.9, 3.9, 1.5 and 1.17 relative to that of chemical leaching (control test), conventional bioleaching, electro- leaching and electrochemical bioleaching at 440–480 mV, respectively. Mineralogical observations of the sold residues of electrochemical processes showed that chalcopyrite is converted to chalcocite and covellite minerals which is attributed to both direct electron transfer to the mineral and indirect reduction by Fe(II) at the low solution ORP. Prevention of formation of passive layers such as jarosites as a result of low slurry ORP, increase of the bacterial concentration and the electro-reduction of chalcopyrite to less refractory minerals such as chalcocite and covellite are considered to be the main reasons for enhancing both, the dissolution rate and final copper recovery of the concentrate in the electrochemical bioleaching process. From the results of this research work, it can be concluded that the electrochemical bioleaching could be considered as one of the most promising alternatives to extract copper from high grade chalcopyrite concentrates. In this regard the development of the process by establishing a continuous system and the assessment of economical parameters are needed to justify conducting the process in larger scales. 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