A novel biphasic leaching approach for the recovery of Cu and Zn from polymetallic bulk concentrate

Bioresource Technology 157 (2014) 310–315

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A novel biphasic leaching approach for the recovery of Cu and Zn from polymetallic bulk concentrate Bhargav C. Patel a,b, Manish Kumar Sinha c, Devayani R. Tipre b, Abhilash Pillai c, Shailesh R. Dave b,⇑ a

Institute of Forensic Science, Gujarat Forensic Sciences University, Gandhinagar 382007, Gujarat, India Department of Microbiology and Biotechnology, School of Sciences, Gujarat University, Ahmedabad 380009, Gujarat, India c CSIR-National Metallurgical Laboratory, Jamshedpur 831007, India b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 28 November 2013 Received in revised form 20 January 2014 Accepted 24 January 2014 Available online 5 February 2014 Keywords: Leptospirillum ferriphilum Bioleaching Two stage leaching Solvent extraction

1 2

3 4

950 mm

5

CO2

1200 mm

Air

6

13

7

Gas-mixing chamber

3909 mg/L/h at no metallic stress in column reactor.  It showed 51 times higher IOR compared to wild type consortium in metallic stress.  Bioregenerated ferric iron yielded 52.2 and 2.6 g/L Zn and Cu, respectively. Ò  CYANEX 301 used first time for metal recovery from PBC leachate.  First biphasic process for PBC and recovery of metals.

Support matrix (UHMWPE)

 Consortium gave highest IOR of

10 9

8

Sparger

12

14

11

ID 160 mm

a b s t r a c t In scale-up biphasic leaching process of polymetallic concentrate, the ferric bioregeneration cycles were performed in 15.0 L down flow packed bed reactor; whereas the chemical leaching cycles were done using the biogenerated ferric in an indigenously designed 10.0 L stirred tank reactor. The consortium took 25 cycles for proper biofilm formation. It showed highest iron oxidation rate (IOR) of 3908.21 mg/L/h at 25th cycle under no polymetallic stress. Even under stressed conditions, it was 2650–558 mg/L/h. Cu extractions were 86.63–46.51 and Zn extractions were 67.89–14.74% in 1st–4th cycle, respectively. The developed consortium exhibited 17–51 times higher IOR compared to original wild type consortium. Extraction isotherm for zinc with 30% CyanexÒ 301 indicated that a total of two stages are required for its complete extraction using the phase ratio of 2:1 at equilibrium pH 1.5, leaving behind Fe(II) in the raffinate. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Ferrous iron re-oxidation is essential in the bioleaching process because Fe3+ is an important electron shuttle and a chemical oxidant. Ferrous iron can be oxidized chemically in acid solutions, but microbial oxidation occurs 105–106 times faster as compared to the chemical oxidation (Bosecker, 1997). Recirculation of Fe2+ ⇑ Corresponding author. Tel.: +91 79 26303225; fax: +91 79 26302654. E-mail address: [email protected] (S.R. Dave). http://dx.doi.org/10.1016/j.biortech.2014.01.101 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.

1. Rotating shaft 2. Temperature sensor 3. Sampling port 4. Lid clamp 5. Lid (removable) 6. Inner chamber 7. Middle chamber (filled with silicon oil) 8. Outer jacket (filled with glass wool) 9. Agitator 10. Baffles 11. Heating coil 12. Drainage valve 13. Silicon oil filling port 14. Silicon oil drainage port

containing leach solutions back to the process has been practiced in two stage leaching techniques like BRISA (Biolixiviación Rápida Indirecta con Separación de Acciones: Fast Indirect Bioleaching with Actions Separation) and IBES (Indirect Bioleaching with Effects Separation) (Carranza et al., 1997). Due to the fast metal extraction rate and the reuse of the spent iron, these biphasic leaching operations are the promising and economic future technologies which make use of autotrophic as well as heterotrophic acidophilic iron oxidizers in controlled bioreactors (Ehrlich, 2001). However, it leads to the accumulation of high concentrations of both dissolved iron

B.C. Patel et al. / Bioresource Technology 157 (2014) 310–315

and heavy metal in the leach liquor (Nurmi et al., 2009). This creates adverse condition for the activity of iron oxidizers. The biphasic leaching operation (BLP) is a brilliant unconventional option for the treatment of base metal concentrates, which have been extensively evaluated over the years. In the biphasic process, the bacterial oxidation of ferrous iron to ferric iron is performed in a separate vessel/reactor (first phase), which is physically separate from the leach reactor (second phase). (Carranza et al., 1997, 2004). The sulphidic feed material in the leach reactor is contacted with Fe3+ iron solution generated by bacteria. From the reactor product, the liquid and solid phases are separated, with the liquid phase proceeding to metal recovery by, for example, solvent extraction and electrowinning (SX–EW) and returning to the first phase, which completes the liquor circulation loop between the leach reactor and the biooxidation vessel. (Fomchenko and Biryukov, 2009; Palencia et al., 2002; Romero et al., 1998, 2003). The process is based on bioleaching by the indirect contact mechanism (Sand et al., 2001). According to this mechanism, metallic sulphides are chemically oxidized by ferric sulphate leading to elemental sulphur and copper in solution Eqs. (1) and (2). The resulting ferrous iron Eqs. (1) and (2) is again converted to ferric iron by iron oxidising microorganisms Eqs. (3):

CuFeS2 ðchalcopyriteÞ þ 2Fe2 ðSO4 Þ3 ! CuSO4 þ 5FeSO4 þ 2S0

ð1Þ

ZnSðsphaleriteÞ þ 2Fe3þ ! Zn2þ þ 0:125S8 þ 2Fe2þ

ð2Þ

Iron oxidising acidophiles

2Fe2þ þ 0:5O2 þ 2Hþ ƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒ! 2Fe3þ þ H2 O

ð3Þ

Two major reasons given by the authors (Fomchenko and Biryukov, 2009; Palencia et al., 2002) to separate the chemical from the biological stage are: (1) the possibility to perform the chemical leaching at high temperature in order to increase the kinetics as suffered in heap and stirred tank bioleaching reactors; and (2) the inhibition of the bacterial growth by heterogeneous ecosystem (heap leaching) and the harmful physico-chemical effects that exert on the bacteria when using a single stirred tank bioleaching reactor (Ballester et al., 2007). From known diverse iron oxidising microorganisms, Acidithiobacillus ferrooxidans, Leptospirillum ferrooxidans and Leptospirillum ferriphilum plays a pivotal role in the oxidation of Fe2+ iron (Rawlings, 2005). For many years A. ferrooxidans was considered the dominant iron-oxidizing microorganism. However, based on kinetics reasoning (Boon et al., 1999a,b) and molecular ecology studies (Rawlings et al., 1999), it was reasoned that L. ferrooxidans might be the most important microorganism for ferrous iron oxidation. The optimal pH for the growth of A. ferrooxidans is reported to be within the range of 1.8–2.5 (Rawlings et al., 1999). L. ferrooxidans is more acid-resistant than A. ferrooxidans and can grow at lower pH values (<1.0). With regard to the temperature, A. ferrooxidans is considered to be more tolerant to low temperatures and less tolerant to high temperatures as compared to L. ferrooxidans (Rawlings et al., 1999). Moreover, as reported by Galleguillos et al. (2009), the metal resistance ability of the L. ferriphilum is far greater than A. ferrooxidans and L. ferrooxidans, which make it the candidate of choice for bioreoxidation of Fe2+ iron from leachate containing high concentration of different metals. However due to the development of above mentioned adverse situation for biooxidation activity process need to be optimised and the concerned microorganisms require adaptation. In attempt to develop an efficient BLP for a polymetallic bulk concentrate (PBC), the development of a laboratory scale down flow packed bed column reactor (first phase) involving an efficient L. ferriphilum dominant iron oxidising consortium has already been attempted with the optimisation of the chemical leaching process

311

parameters on bench scale level (Patel et al., 2012a). The development of the efficient consortium required for BLP and its efficiency under very high multi-metal stress has also been optimally studied (Patel et al., 2012b). In this paper, an attempt has been put in place to describe the scale-up in biphasic leaching process of polymetallic bulk concentrate (PBC) and the purification of metals by solvent extraction (SX) using CyanexÒ 301. 2. Methods 2.1. Polymetallic bulk concentrate The polymetallic bulk concentrate (PBC) was supplied by the Gujarat Mineral Development Corporation (GMDC), Ambamata Multimetal Mine Project, Gujarat, India. The PBC majorly contained sphelarite along with chalcopyrite, galena, pyrite and sulphur (Patel et al., 2012a,b). 2.2. Iron oxidizing consortium Recently developed multistress resistant, L. ferriphilum dominated iron oxidising consortium (Patel et al., 2012b) was used for the generation of ferric iron at pH 1.8in 90 mL SDB1 medium consisting (g/L): (NH4)2SO4 3.0, MgSO45H2O 0.5, K2HPO4 0.5 supplemented with FeSO47H2O, 20.0 g/L. 2.3. A cyclic biphasic leaching operation 2.3.1. First phase: ferric regeneration in 15.0 L down flow packed bed column reactor The larger PVC (polyvinyl chloride) bioreactor (Fig. 1) used in the study was of 1200 mm height and 160 mm inner diameter with the basic construction features as described by Patel et al. (2012a). The column was packed with 3.0 kg prewashed shredded threads of Ultra High Molecular Weight Polyethylene (UHMWPE) as supporting matrix. As shown in schematic diagram (Fig. 1), the air and CO2 were first mixed in a gas-mixing chamber filled with distilled water and then supplied through a sparger located at the bottom of the column. The air was fed at a rate of 3.0 L/min using a mini oil free air compressor and CO2 was supplied at a concentration of 0.3% (v/v) of the total aeration. The active biofilm was developed as described by Patel et al. (2012a) with some modifications in the steps. Briefly, the prepared column was filled with 14.5 L of culture medium having pH 1.8 and 420 mV, which contained g/L: (NH4)2SO4, 3.0; K2HPO4, 0.5; MgSO47H2O 0.5 and FeSO47H2O 20.0. A 500 ml of actively growing SR-BH-L consortium having 4  108 cells per mL were inoculated. Throughout the study, whole column reactor was set up in a specially designed incubator chamber with temperature set at 32 ± 2 °C. Once 95% oxidation of Fe2+ iron was achieved, 50% volume of the medium from the column was withdrawn and 50% new medium was fed to the column. The cycle was repeated for 10 times to develop required biofilm. The development of the biofilm was checked by running five consecutive cycles of ferrous iron biooxidation with complete removal of the spent medium from the column and 100% new medium was added in the column. Biofilm formation was also confirmed by the microscopic observation. Single cycle was considered when >90% Fe2+ iron was oxidised. With a fully developed biofilm, the ferrous biooxidation was continued till the Fe3+ iron concentration reached to 1.5% (w/v) in the solution. The produced biogenic Fe3+ iron was used for the chemical leaching of metals in second phase as described in following section. This was considered as one cycle of biphasic leaching operation. The next cycle was started by filling the column reactor

B.C. Patel et al. / Bioresource Technology 157 (2014) 310–315

CO2

Gas-mixing chamber

ber. The middle chamber was filled with industrial grade silicon oil, and a microprocessor controlled high efficiency heating coil at the bottom. The outer chamber was packed with glass wool as an insulator. A drainage valve was provided at the bottom of the main tank. A sampling and temperature sensor ports were provided through the removable lid. The rotating shaft, having open turbine of variable pitch type agitator at the bottom, was fitted on the motor vertically. The operating temperature range of the reactor was from ambient to 150 °C temperature. Biogenerated ferric iron from the first phase column bioreactor was used for the extraction of Cu and Zn under the optimised conditions (Patel et al. 2012a). The tank was filled with 7.0 L leachate having 1.5% w/v Fe3+ iron concentration and system was heated to 90 ± 2 °C. Then 10% w/v PBC was added and agitated. After agitating about 1.0 min at 650 rpm, 10.0 mL sample was withdrawn through the sampling port for 0 h reading. The system was run for total 240 min of contact time. During the process at regular interval of time, samples were withdrawn for different physico-chemical analysis. Then the leachate was allowed to cool. When the PBC particles settled down, the supernatant (i.e., leachate) was withdrawn and used again for the bioregeneration of the Fe3+ iron using 15.0 L column reactor. The spent PBC was collected in a separate vessel for further analysis. This leaching process was repeated for four cycles using fresh PBC at each new cycle. To compare the scale up process, metal extractions by the same biogenic ferric iron was performed in agitating 200 and 500 mL system on hot plate as well as 1000 mL capacity stirred tank reactor under the similar conditions for one cycle.

1200 mm

Air

950 mm

Support matrix (UHMWPE)

312

Sparger

ID 160 mm Fig. 1. Schematic diagram of 15.0 L down flow packed bed column reactor.

2.3.3. Analysis In all the experiments at regular time interval, concentration of copper, zinc and total iron in the leach liquor samples were analysed by atomic absorption spectrophotometer (Elico Ltd, model BL-194, India). For the estimation, as per requirement, 0.1 N HCl was used to dilute the leachate samples. The soluble Fe2+ iron was estimated by titrating against 0.1 N K2Cr2O7 using diphenylamine as an indicator in highly acidic condition (Vogel, 1962). The pH and redox potential was measured using micro processor-pH meter (Systronics, India; model 361). Fe3+ iron concentration was calculated from the difference of total iron and Fe2+ iron estimated by AAS and titrimetric method, respectively. Total dissolved solids (TDS) were measured by portable TDS meter (Eutech, Singapore, model TDSTestr 11+).

with the leachate generated in the second phase (i.e., chemical leaching) for Fe3+ iron regeneration. This operation was continued up to four cycles. 2.3.2. Second phase: 10.0 L stirred tank reactor for chemical leaching under optimised conditions A 10.0 L stirred tank reactor having 7.0 L working volume was indigenously designed for the biphasic leaching operation. The schematic diagram of the reactor is illustrated in Fig. 2. The reactor was made up of SS-316 consisting of a 10.0 L baffled inner tank surrounded by two jackets making middle chamber and outer cham1 2

3 4

5 6

13

7 10 9

8

12

14

1. Rotating shaft 2. Temperature sensor 3. Sampling port 4. Lid clamp 5. Lid (removable) 6. Inner chamber 7. Middle chamber (filled with silicon oil) 8. Outer jacket (filled with glass wool) 9. Agitator 10. Baffles 11. Heating coil 12. Drainage valve 13. Silicon oil filling port 14. Silicon oil drainage port

11

Fig. 2. Schematic diagram of the chemical leaching reactor (10.0 L).

B.C. Patel et al. / Bioresource Technology 157 (2014) 310–315

2.4. Solvent extraction of copper and zinc The obtained leach liquor was subjected to solvent extraction for the recovery of Cu and Zn. Various parameter were studied such as extractant concentration, phase ratio variation etc. All solvent extraction experiments were carried out by mixing equal volumes (except for the construction of McCabe Thiele diagram (McCabe and Thiele, 1925)) of leach liquor and desired extractant (CyanexÒ 301) of known concentration in a multi-point magnetic stirrer (Spectralab, Model – Whirlmatic Mega, India) at room temperature for 15 min. Fifteen minute shaking time was found to be sufficient to reach equilibrium. The pH of the aqueous solution was adjusted to the desired value by adding dilute H2SO4 or NaOH solutions. After phase disengagement, aqueous and organic phases were separated. Metal ion concentration in the aqueous phase was analysed by Atomic Absorption Spectrometer (Elico, SL-194, India). Metal contents of the organic phases were determined by mass balance. Stripping of metal ions from the loaded organic phase was carried out with dilute sulphuric acid. 3. Results and discussion 3.1. IOR profile of the consortium in down flow packed bed column reactor Ferrous iron oxidation in the column is shown in Table 1. The obtained results indicate that proper biofilm formation of the employed consortium required 25 cycles. During biofilm formation process in the column IOR was increased from 36.57 mg/L/h to as high as 3909 mg/L/h from first to 25th cycles. When the organisms were inoculated in the first cycle, the IOR was as low as 36.57 mg/ L/h which gradually increased almost 21.5 times in the next eight cycles. The increase was obviously due to the increase in the cell density in each cycle. From the 11th to 16th cycles, at each cycle the column was washed with the acidified distilled water followed by replacement of the spent medium with the fresh medium to remove the planktonic cell. Due to the washing in 11th and 12th cycle, the IOR decreased to as low as 232.7, which could be due to removal of the planktonic cells and till the biofilm may not have developed fully. However, in the next four cycles the IOR reached up to 930.8 mg/L/h, and it continued to increase up

Table 1 IOR profile of the consortium SR-BH-L during biofilm development in down flow packed bed column reactor (First Phase). Cycle no.

IOR (mg/L/h)

Standard deviation

1a 3b 5b 7b 9b 10b 11c 12c 13c 14c 15c 16c 25d 27d 29d

36.57 209.44 418.88 628.31 744.67 785.63 313.55 232.71 465.42 558.50 698.13 930.83 3909.50 3907.24 3908.21

±0.92 ±1.07 ±1.04 ±1.00 ±0.60 ±0.91 ±0.98 ±1.36 ±0.94 ±0.96 ±1.07 ±0.87 ±0.59 ±0.49 ±0.63

313

to the highest observed IOR of 3909 mg/L/h at 25th cycle. The obtained IOR (3909 mg/L/h) remained constant in next five cycles. The resulted very high IOR was due the formation of the biofilm on the support medium. The electron microscopic observation of the support medium of 30th cycle also indicated the presence of a dense biofilm of spiral shaped bacteria (Microphotograph not included). 3.2. Biphasic leaching operation The results of Cu and Zn extractions from the PBC for four cycles using biogenerated ferric iron under the optimized conditions in fed batch process are given in Fig. 3(A) and (B), respectively. Cu and Zn extraction gradually decreased with each succeeding fedbatch chemical leaching cycle and it resulted in 86.63%, 81.98%, 75.58% and 46.51% copper and 67.89%, 62.46%, 47.37% and 14.74% zinc leaching in 1st, 2nd, 3rd and 4th cycle, respectively. Decrease in the Zn extraction was 2.47 times higher as compared to copper extraction during the process. When the metal loaded leachate generated in each cycle was added in 15.0 L down flow packed bed column reactor having biofilm of developed consortium SR-BH-L, the obtained maximum IOR in 1st, 2nd, 3rd and 4th cycle was 2650, 1390, 650 and 558 mg/L/h, respectively. The observed gradual decrease in the IOR was obviously due to increase in concentration of various dissolved metals in the leachate and increase in osmotic pressure of the medium due to increased ionic strength. However the developed consortium exhibited nearly 17–51 times higher IOR compared to original wild type consortium under similar stress conditions (Table 2). In case of metal extraction the observed gradual decrease in Zn and Cu extraction with increasing number of fed-batch cycles could be due to accumulation of respective metals and other impurities in the leachate (Table 3). As can be seen from the data, the dissolved percent metal concentration in the leachate during four cycles of fed batch chemical leaching was 19.0, 36.3, 49.2 and 52.2 g/L for Zn and 1.5%, 2.1%, 2.6% and 2.2% for Cu in 1st, 2nd, 3rd and 4th cycle, respectively. Comparison of IOR of wild type and developed consortium indicates the toxic effect of the stress developed due to the presence of increasing metals concentrations and increased osmotic pressure in the system. In case of Cu extraction, galvanic interactions play an important role, which was responsible for decrease in soluble Cu concentration after particular period of contact time. Specifically in 3rd cycle, at 30 min of contact time the Cu concentration in the leachate was 0.26% and in 4th cycle it was 0.22%. But in the 3rd cycle Cu concentration in pre-leachate at the end of the 180 min of contact time decreased 0.13%. This amount increased to 0.22% in the 4th cycle at 30 min of contact time and there after it decreased gradually and reached to 0.14% at the end of experiment. Thus, in case of polymetallic concentrate, it could be advisable to separate extracted Cu at particular period of time. In present experiment, the optimum time for Cu stripping from the leachate was 30 min of contact time. Overall results indicated that use of bioregenerated ferric iron from the leachate yielded as high as 52.2 and 2.6 g/L Zn and Cu, respectively in the solution, which can be further subjected to metal recovery by solvent extraction. 3.3. Solvent extraction study for the recovery of copper and zinc from leachate

a

First inoculation. In these cycles 50% of the spent medium was replaced in each cycle with new medium supplemented with 4% (w/v) ferrous sulphate. c In these cycles, the spent medium was removed and the column was washed with acidified distilled water and the fresh medium was filled in each cycle with new medium supplemented with 4% (w/v) ferrous sulphate. d After the fully developed biofilm in the column. b

The effect of CyanexÒ 301 concentration on the extraction of copper, zinc and iron at equilibrium pH 1.5 was investigated. The concentration of CyanexÒ 301 was varied in the range 5–50% (v/v) in kerosene. It was observed that the extraction of zinc increased with the increase in extractant concentration. Whereas,

314

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(A)

(B)

Fig. 3. (A) Copper and (B) Zinc leaching profile during biphasic leaching operation.

Table 2 Comparison of IOR of the wild type and developed consortium under the stressed conditions.

100

IOR (mg/L/h)

1st 2nd 3rd 4th

80

Wild type consortium

Developed consortium

116 60 23 8.5

1960 1210 545 430

% Extraction

Leaching cycle

60 Zn Cu Fe

40 20

Table 3 Characteristics of leachate after each leaching cycle. Cycle no.

pH

1 2 3 4

1.67 1.61 1.63 1.68

0 0

Redox potential (mV)

Cu concentration (g/L)

Zn concentration (g/L)

Total dissolved solids (g/L)

461 468 479 488

1.5 2.1 2.6 2.2

19.0 36.3 49.2 52.2

143.21 263.16 356.23 398.51

10

20

30

% CYANEX

40

50

60

301 Conc. (v/v)

Fig. 4. Solvent extraction of Cu, Zn and Fe2+ from polymetallic leachate by different concentrations of CyanexÒ 301.

90 80 70 60

[ Z n] or g.

copper quantitatively extracted into the organic phase with all the concentration of CyanexÒ 301 studied. Zinc extraction increased from 30–99.9% at the equilibrium pH of 1.5 with no co-extraction of Fe(II) till CyanexÒ 301 concentrations was raised to 30% (v/v) and beyond this concentration Fe(II) extraction increased up to 5.6% and at the same time Zn extraction increased up to 90% (Fig. 4). Hence, for the further experiments 30% (v/v) CyanexÒ 301 chosen as optimum extractant concentration in order to keep all the iron content in the leachate for further leaching. When extraction of Cu, Zn and Fe(II) with 30% (v/v) CyanexÒ 301 at different phase ratio (O/A = 1:5 to 5:1) was carried out at the equilibrium pH of 1.5, it was found that extraction of Cu and Zn increased with increase in phase ratio for both the extractants. It was observed that the copper completely extracted into the organic phase in the entire phase ratio studied, whereas Zn extraction increased from 28% to 99% with increase in phase ratio from 1:5 to 5:1. McCabe Thiele diagram (McCabe and Thiele, 1925) was made by using the data obtained from phase ratio variation to determine the number of theoretical counter-current stages required for complete extraction of copper and zinc (Fig. 5). Extraction isotherm for zinc with 30% CyanexÒ 301 indicates that a total of two stages are required for its complete extraction using the phase ratio of 2:1 at equilibrium pH 1.5, leaving behind Fe(II) in the raffinate.

50 40 30 20 10 0

O/A = 2 0

20

40 [Zn]aq.

Fig. 5. Extraction McCabe–Thiele isotherm for zinc with 30% CyanexÒ 301.

The iron free loaded organic thus obtained after counter-current extraction containing 26.0 and 0.6 g/L Zn and Cu, respectively was then subjected to the stripping study using different concentration of sulphuric acid (2.0–18.0 M) using phase ratio of 1. In case of Cu, the complete stripping was achieved at all the concentration of H2SO4. Whereas, stripping rate of Zn increased to 28% (data not shown) with the increase in the concentration of H2SO4 up to 7.0 M but thereafter it decreased because of formation of viscous phase. Hence, 7.0 M H2SO4 was selected for Zn stripping study at

B.C. Patel et al. / Bioresource Technology 157 (2014) 310–315

different phase ratio. It was observed that zinc almost completely back extracted into the aqueous phase in three stages using A/O ratio of 5.0. 4. Conclusions The developed biofilm showed IOR 3909 mg/L/h under no stress even in 15.0 L reactor. Moreover, under poly-stressed, it was 2650– 558 mg/L/h. The developed consortium exhibited 17–51 times higher IOR compared to original. In chemical leaching, Cu and Zn extractions were 86.63–46.51 and 67.89–14.74% in 1st–4th cycle. Extraction isotherm for zinc with CyanexÒ 301 indicated requirement of two stages for its complete extraction using the phase ratio of 2:1 at equilibrium pH 1.5, leaving behind Fe(II) in the raffinate. This scaled up process can be exploited for the polymetallic concentrate. Acknowledgements The authors are thankful to Gujarat Mineral Development Corporation (GMDC), Gujarat, India, for the project grant (No. GSRC/ 4789/2006-07) and research scholarship to B.C. Patel. References Ballester, A., Blázquez, M.L., González, F., Muñoz, J.A., 2007. Catalytic role of silver and other ions on the mechanism of chemical and biological leaching. In: Donati, E., Sand, W. (Eds.), Microbial Processes for Metal Sulphides. Springer, Dordrecht, The Netherlands, pp. 77–102. Boon, M., Brasser, H.J., Hansford, G.S., Heijnen, J.J., 1999a. Comparison of the oxidation kinetics of different pyrites in the presence of Thiobacillus ferrooxidans or Leptospirillum ferrooxidans. Hydrometallurgy 53, 57–72. Boon, M., Meeder, T.A., Thone, C., Ras, C., Heijnen, J.J., 1999b. The ferrous iron oxidation kinetics of Thiobacillus ferrooxidans in continuous cultures. Appl. Microbiol. Biotechnol. 51, 820–826. Bosecker, K., 1997. Bioleaching: metal solubilization by microorganisms. FEMS Microbiol. Rev. 20, 591–604.

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