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Artículo V

94

Chalcopyrite (CuFeS2) bioleaching

Mejía E. R.1, a

Ospina J. D.2,b

Márquez M. A.3,c

, Morales A. L.

4,d

1, 2, 3, Materials Engineering School, Applied Mineralogy and Bio-process Group "GMAB", National University

of Colombia, Medellín AA 1027, Colombia. 4 Solid State Group, University Research Centre, Antioquia University, Medellín AA 1226, Colombia. a ermejiare@unalmed.edu.co,

b judospinaco@unalmed.edu.co,

c mmarquez@unalmed.edu.co,

d

amoral@fisica.udea.edu.co

ABSTRACT

This study aims to identify mineral phases formed during chalcopyrite bioleaching process using

Acidithiobacillus ferrooxidans-like bacteria and a bacteria mixed culture of Acidithiobacillus

ferrooxidans-like and Acidithiobacillus thiooxidans-like. Two mineral particle sizes were evaluated,

200 and 325 Tyler meshes. The strains were adapted by gradually decreasing the main energy sources

and increasing the mineral content. The experiments were performed in absence of ferrous sulphate and

elemental sulfur. The new phases formed and alterations of chalcopyrite surface were characterized

using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy with energy

dispersive X-ray spectroscopy (SEM/EDX) and X-ray diffraction (XRD). Chalcopyrite was partially

oxidized and the analysis showed ammoniumjarosite as the main phase formed. The formation and

precipitation of ammoniumjarosite was favored by the high concentration of Fe3+

, which was produced

by bacterial action after the 15th

day of process. This phase may limit the diffusion of Fe3+

ions to the

mineral surface. Moreover it was important to note that the bacteria played an important role in the

process, already in an uninoculated control copper extraction was less that 6% while in an inoculate test

copper extraction was around 40%. The results support that jarosite is the cause of passivation in

chalcopyrite bioleaching.

Keywords: Chalcopyrite, bioleaching process, jarosite, redox potential.

1. Introduction

Chalcopyrite (CuFeS2) is the most important copper ore with about 70% of copper reserves in the

world (Dutrizac 1981, Rivadeneira 2006). In metallurgical applications, it is mainly subjected to

pyrometallurgy treatment after concentration by flotation process (Córdoba et al., 2008a and b). The

interest in bio-hydrometallurgy has increased recently in order to minimize the sulphur dioxide

emissions, and also to reduce energy consumption (Marsden & House, 1992, Brierley & Luinstra, Hsu

& Roger 1995, 1993, Watling, 2006, Al-Harahssheh et al., 2006). However, the chalcopyrite copper

leaching rate is slower than other copper minerals such as chalcocite (Cu2S), covelita (CuS) and bornite

(Cu5FeS4) [Hiroyoshi et al., 2000]. Initial rapid leaching rates decline with time and bioleaching

processes release only part of the copper [Parker et al., 2003], due to so called passivation reactions at

the surface of the mineral (Yin et al., 1995, Hackl et al., 1995). After almost a century of research into

the mechanisms of chalcopyrite dissolution in ferric medium, there is consensus with respect to the

formation of a passivating film on the surface (Dutrizac 1981, Parker et al., 2003, Yin et al., 1995,

Hackl et al., 1995, Mikhlin et al., 2004, Harmer et al., 2006, Bevilaqua et al., 2002, Córdoba et al.,

2008). But despite this, the nature of this film is still unknown, although it has been postulated that it

must have low porosity and be a bad electric conductor (Córdoba et al., 2008a). Various models,

describing mass transfer diffusion and chemical reactions on the chalcopyrite surface, have been

proposed to explain the nature and composition of the passivation film which causes a slow oxidation

95

of chalcopyrite: Metal-deficient sulphides, elemental sulphur-Sº, polysulphides-XSn and jarosites-

XFe3(SO4)2(OH)6 (Dutrizac 1981, Parker et al., 2003, Yin et al., 1995, Hackl et al., 1995, Mikhlin et

al., 2004, Harmer et al., 2006, Bevilaqua et al., 2002, Córdoba et al., 2008b).

Klauber (2008) has reviewed the chemical characteristics of the surface layer of chalcopyrite

leached with ferric sulfate and suggested that metal-deficient sulphides as well elemental sulphur-Sº

were among the passivation candidates. This author suggested that metal-deficient sulfide is formed by

non-stoichiometric dissolution of sulfides, based on analytical evidence. Parker et al., (2003), were

using XPS analysis detected elemental sulphur, sulfate and disulfide phases in solid form in

chalcopyrite bioleaching experiments. Additionally, bioleaching of chalcopyrite results in the

dissolution of iron, which potentially leads to precipitation of Fe3+

hydroxysulfates such as jarosite

(Parker et al., 2003). Under these conditions, chalcopyrite leaching may involve iron-deficient

secondary mineral and intermediates (Sasaki et al., 2009).

These studies agreed in that jarosite precipitation is linked to the passivation of chalcopyrite. These

different theories require substantial further research in this area. It is, therefore, very important to

tackle the issue from different angles in an attempt to understand the nature of the recalcitrant

chalcopyrite.

Mineralogical characterization of the products in different types of beneficiation processes, called

―process mineralogy‖ has been performed as a fundamental key for the planning, optimization, and

monitoring of different minerals (Marsden a& House, 1992, Petruk, 2000). In this way, an appropriate

understanding of the mineralogy in the chalcopyrite and its transformation is essential to understand the

passivation mechanism. The main objective of this research was characterizing the mineral phases

generated in the bioleaching process of chalcopyrite and understanding the evolution or transformation

of these phases, as a support to elucidate its influence on the passivation of the process.

2. Materials and Methods

2.1. Mineral

All experiments were carried out with a natural chalcopyrite sample, from ―La Chorrera‖ Mine (El

Limón, Antioquia, Colombia). The mineral was subjected to crushing and milling processes, followed

by a gravimetric separation in a Wilfley table. Afterwards, a manual concentration in stereographic

microscopy was performed. The mineral composition of the sample, measured by countdown points,

was: 85,23% chalcopyrite (CuFeS2), 1,27% quartz (SiO2), 1,69% covellite (CuS), 2,53% sphalerite and

3,37% molibdenite (MoS2). The mineral was milled using an agate mortar. This guaranteed two

particles sizes: pass through 200 Tyler mesh (~75μm), and pass through 325 Tyler mesh, (~45μm), and

then it was sterilized in a furnace for 90 minutes at 80°C.

2.2 Bioleaching Experiments

For the bioleaching experiments Acidithiobacillus ferrooxidans-like and Acidithiobacillus

thiooxidans-like strains were employed. The strains, from ―El Vampiro II‖ coal mine (Morales, Cauca,

Colombia), were isolated by Cardona (2008). The microorganisms were previously grown in T&K

medium by successively replacing the ferrous sulfate by chalcopyrite. The medium was acidified to pH

1.8 with H2SO4. The flasks were sterilized by autoclaving for 20 min, 120°C at 18 psi. The experiment

was inoculated with Acidithiobacillus ferrooxidans 10% (v/v), for the single culture, and 5%(v/v)

Acidithiobacillus ferrooxidans and 5%(v/v) Acidithiobacillus thiooxidans, for the mixed culture. The

experiments were carried out for 45 days, in 500ml shake flasks, containing 300ml medium with 10%

(w/v) chalcopyrite, at 180rpm and 30ºC. All conditions were duplicated and the respective abiotic

control was included.

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2.3 Chemical analysis

Measurements of pH (HACH HQ40d multi PHC30103) and redox potential (Shot Handylab 1 Pt

6880) in situ (reference electrode Ag0/AgCl) were performed every day. Samples were aseptically

withdrawn from the flasks after 24 hours and then every five days for the SEM/EDS, FTIR and XRD

analisis. The samples were separated in a DIAMOND IEC DIVISION centrifuge, for 15min at

3000rpm. Iron and sulphate concentration were measured with an UV-visible spectrophotometer

GENESYS™ 10. The methods employed were 3500-FeD (O-phenantroline) for ferrous and total iron

according to the Standard Methods for water analysis (Leonore et al., 1999).

2.4 Mineralogical analysis

Combinations of analytical techniques were used to do the mineralogical characterization of the

samples. The FTIR spectra, for the solid samples, were recorded in a FTIR Spectrophotometer

Shimadzu Advantage 8400 with KBr pellets (transmission mode). A sample of KBr mixture at 1:200

ratio was used. The total number of scans was 20 with a spectral resolution of 4 cm−1

, a range of 400–

4000 cm-1

and Happ-Henzel correction were used.

The biooxidation samples were mounted in epoxy resin and were polished with sequential finer SiC

grit paper and a final polish with 0.05-µm sized alumina powder. The polished sections analysis were

performed with JEOL JSM 5910 LV scanning electron microscopy (SEM) in backscattering electron

mode and energy dispersive X-ray (EDX) detector (Oxford instrument), using a beam voltage of 18kV.

XRD analyses of the samples were conducted in a Bruker D8ADVANCE diffractometer with Cu λ=

1.5406 Å radiation, generated at 35 kV and 30 mA. X-ray diffraction data were obtained using

computer controlled Xray Diffractometer Panalytical X'Pert Pro MPD. The initial characterizations of

the mineral polished sections were performed in a plane polarizared optical microscopy (reflected light)

(PPOM-RL).

3. Results

3.1 Chalcopyrite leaching experiment

Chalcopyrite oxidation in both cultures, with different grain sizes, presented a relatively low redox

potential at the beginning of the test, followed by an increment and finally a small decrease with time.

The pH values were always around 2,1 (data not shown) for both cultures. Redox potential, pH values

and concentration of Fe2+

and Fe3+

in abiotic controls showed little change throughout the time (Fig.

1A, Fig. 1B and 1C). The dissolution of Fe2+

presented a maximum value around 10th

day (arround

1200 ppm), then it decreases sharply (until 580 ppm), and becomes stationary around the 16th

day up to

the 41th day (Fig. 1B). The Fe3+

dissolution shows a very small concentration up to the 11th

day

(around 100 ppm), a sharp increase up to the 15th

day (until 9200 ppm), and a small oscillatory

increasing trend up to the 41th

day (Fig. 1C). The concentration of SO42-

in liquid phase present a

maximum value arround 15th

day (around 55600 ppm), then decreases (until 19000 ppm), Fig 1E. The

concentration of SO4-2

in solid phase show a sharp increase up to the 15th

day and become stationary

arround the 30th

day, Fig 1F. Copper extraction was around 50% for grain size 200 Tyler mesh and

40% for grain size 325 Tyler mesh. Less than 6% Cu was solubilized in the chemical controls (Fig.

1D). Fig.2. Shows the amount copper extracted as function of the redox potential. According to this

figure chalcopyrite bioleaching showed the critical potential, ~ 430 mV, where enhanced copper

extraction is obtained. In the higher potential region, the amounts of extracted copper are smaller than

in the lower potential region.

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3.2 Mineralogical analysis

3.2.1 FTIR measurements

Results obtained by FTIR (Fig. 3) showed that the predominant mineral phase present was jarosite,

confirmed by the presence of the bands ν3 (anti-symmetric stretching triply degenerate vibration) at

1190cm-1

, 1085cm-1

and 1008cm-1

, ν4 (deformation vibration) at 629cm-1

and ν2 (deformation vibration

doubly degenerate) at 513cm -1

and 470cm-1

(Chernyshova 2003, Marquez et al., 2006, Sasaki et al.,

2009, Gunneriusson et al., 2009). Also there were absorptions at 740cm−1

, 870cm−1

and 1414cm−1

that

may correspond to ν3 of NH4+ in ammoniumjarosite (Sasaki et al., 2009). In addition, typical bands of

quartz at 798cm-1

, 779cm-1

and 694cm-1

could be observed (Marquez 1999) as well as hydroxyl groups

at 3400cm-1

, and water at 1640cm-1

of jarosite mineral (Xuguang 2005, Gunneriusson et al., 2009).

Bands around 2935 cm-1

related to the total carbon present on the cell surface showed a permanent

increase (Naumann & Helm 1995, Sharma & Hanumarha 2005Xia et al., 2008,). During the first five

days the samples did not show a significant difference, but between the 5th

and the 10th

day the width

and intensity of the jarosite bands significantly increased, afterwards the increasing of the bands in the

samples was very slow. It is important to note that both test showed the same bands. FTIR of

uninoculated samples for all tests (Fig. 3) showed a spectrum with little variation compared with un-

leached sample (Fig.3).

3.2.2 SEM/EDX analysis

SEM images of un-leached and leached chalcopyrite surface are shown in Figs. 4-7. SEM images of

uninoculated samples, for all tests (Fig. 4), showed a surface with a few alterations such as isolated

cracks. These alterations were interpreted as mineral genetic defects. The grains showed well defined

edges. EDX analysis of the grains showed the chalcopyrite stoichiometric composition (Fig. 5).

Morphology of grains exposed to bacteria is illustrated in Figs. 6 and 7. All the samples show typical

corrosion features such as pits, groove and gulfs on the surface of chalcopyrite grains (Figs. 6a and 7b

arrow indicated pits and gulfs), which increased from de edge of the grain towards the core. These

characteristics were observable since the first days of the process and became more evident with time.

Also it was characterized by EDX an aggregate containing S, O, and Fe, where the atomic ratio Fe:S

was 2,92:0,96, suggesting that this aggregate is mainly jarosite containing small grains of chalcopyrite

(Fig. 5, arrow indicated the small chalcopyrite grains). Average pit size and pitting density on surface

increased with reaction time. After 15 days surface pitting was extensive, resulting in discrete euhedral

and elongate pits and grooves (Fig. 7b). The formation of jarosite aggregates showed an increase with

time and presents a sharp increase up to the end of the process. Moreover some grains showed a partial

jarosite film covering chalcopyrite grains since the first day of process (Fig. 6a, b and 7b, arrow

indicated the chalcopyrite layer).

3.2.3 XRD measurements

According to the XRD initial qualitative analysis, it was found that the main mineral phase present

in the samples was chalcopyrite with small quantities of quartz (SiO2), molibdenite (MoS2) and

covellite (CuS), chlorite, wollastonite (Fig. 8). X-ray diffraction spectra of bioleached samples are

shown in Fig. 10 and 11. Mineralogical evolution of the mineral phases consists of a gradual reduction

of molibdenite and chalcopyrite peaks and an appearance of ammoniumjarosite

((NH4)2Fe6(SO4)4(OH)12. Wollastonite was dissolved from the beginning of the process. XRD for

uninoculated controls did not show any apparent change after 30 days (Fig. 9).

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Figure 1. A. Changes of Eh. B. changes in the concentration of Fe2+. C. changes in the concentration of Fe3+. D. copper dissolution. E.

SO42- in liquid phase and F. SO4

2- in solid phase for pure and mixed culture, where 200F: test with Acidithiobacillus ferrooxidans and 200

Tyler mesh, 325F: test with Acidithiobacillus ferrooxidans and 325 Tyler mesh, 200FT : test with mixed culture and 200 Tyler mesh,

325FT: test with mixed culture and 325 Tyler mesh, 200C: test with abiotic and 200 Tyler mesh, 325C: test with abiotic and 325 Tyler

mesh.

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Figure 2. Relationship between the (%) copper dissolution and redox potential (mV) where 200F: test with Acidithiobacillus ferrooxidans

and 200 Tyler mesh, 325F: test with Acidithiobacillus ferrooxidans and 325 Tyler mesh, 200FT: test with mixed culture and 200 Tyler

mesh, 325FT: test with mixed culture and 325 Tyler mesh, 200C: test with abiotic and 200 Tyler mesh, 325C: test with abiotic and 325

Tyler mesh.

Figure 3. FT-IR spectra of solid residues after bioleaching of chalcopyrite. a) pure culture -200 Tyler mesh, b) pure culture -325 Tyler

mesh, c) Mixed culture -200 Tyler mesh and d) Mixed culture -325 Tyler mesh.

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Figure 4. SEM micrographs of uninoculated residues after bioleaching of chalcopyrite. 200 and 325 Tyler mesh. Where Cpy:

Chalcopyrite and Qz: Quartz.

Figure 5. SEM/EDX micrograph and analysis of the residues after bioleaching of chalcopyrite whit 200 Tyler mesh. Where Cpy:

Chalcopyrite, J: Jarosite.

Cpy

b) c)

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Figure 6. SEM micrographs of the residues after bioleaching of chalcopyrite. 200 Tyler mesh. Where Cpy: Chalcopyrite, J: Jarosite and

Qz: Quartz.

Figure 7. SEM micrographs of the residues after bioleaching of chalcopyrite. 325 Tyler mesh. Where Cpy: Chalcopyrite, J: Jarosite, and

Qz: Quartz.

Figure 8. XRD spectra for chalcopyrite of abiotic controls before the bioleaching process. a). particle size 200 mesh. b). particle size 325

mesh CPy: chalcopyrite, Qz: quartz, Mo: molibdenite, W: wollastonite, Cl: chlorite, CuS: covellite

J Film

J Film

a)

a) b) c)

Groove

Gulf

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Figure 9. X-ray diffractograms of uninoculated samples after 30 days of the of the bioleaching process. a). particle size 200 mesh. b).

particle size 325 mesh CPy: chalcopyrite, Qz: quartz, Mo: molibdenite, Cl: chlorite, CuS: covellite

Figure 10. XRD spectra for chalcopyrite before bioleaching process. A. particle size 200 Tyler mesh for pure culture. B.

particle size 325 Tyler mesh for pure culture. C. particle size 200 Tyler mesh for mixed culture. D. particle size 325 Tyler mesh

for mixed culture. CPy: chalcopyrite, Qz: quartz, Mo: molibdenite, W: wollastonite, Cl: clorite, J: jarosita, Cv: covellite.

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Figure 11. Relative abundance images of peaks in XRD spectra for chalcopyrite during bioleaching process. A. particle size 200 Tyler

mesh for pure culture. B. particle size 325 Tyler mesh for pure culture. C. particle size 200 Tyler mesh for mixed culture. D. particle size

325 Tyler mesh for mixed culture. CPy: chalcopyrite, Mo: molibdenite, J: jarosita, Cv: covellite.

4. Discusion

4.1 Chalcopyrite Leaching Experiment

From the results previously stated, it is possible to conclude that the bioleaching process of

chalcopyrite was passivated. Chemical analysis showed that around the 15th

day, the system had a low

ferrous ion concentration, high ferric ion concentration, an increased redox potential and a decrease in

copper dissolution. The results also suggest that a low concentration of Fe3+

is favorable for Cu

lixiviation, since at high Fe3+

concentration the copper released start to be slower. Hiroyoshi et al.

(2001) found that when the concentration of ferrous and cupric ions is low in the system, the overall

reaction of chalcopyrite leaching is controlled by ferric ions and the copper extraction rate is slower

than high concentrations of ferrous and cupric ions. This could explain why after 15th

day the release of

copper occurs more slowly. Furthermore, ferrous ions promoted chalcopyrite bioleaching below the

―critical‖ potential, in this case around of 430 mV, causing enhanced copper extraction between days 1

- 15. In the higher potential region (day 16- 45), the released of copper was smaller than in the lower

potential region, because possibly there was not enough ferrous ion to promote chalcopyrite

bioleaching. The present findings agree with previous works (Hiroyoshi et al., 2000, 2001, 2008,

Sandstrom et al., 2005, Córdoba et al., 2008 a and b, Mejía et al., 2009). The hypotheses in these works

were that the chalcopyrite dissolution was catalyzed by the ferrous ion according to the following

reactions:

CuFeS2 + 4H+ + O2 → Cu

2+ + 2S

0 + Fe

2+ + H2O (4)

104

CuFeS2 + 3Cu2+

+ 3Fe2+

→ 2Cu2S + 3Fe3+

(5)

2Cu2S + 8Fe3+

→ 4Cu2+

+ 8Fe2+

+ 2S0

(6)

These researchers found that the inhibition of chalcopyrite bioleaching by mesophilic

microorganisms is due to the consumption of ferrous ion by the bacteria. In these reactions the role of

microorganisms as Acidithiobacillus ferrooxidans was not evident and the copper dissolution was a

purely chemical process. However, in the present study it was observed that Acidithiobacillus

ferrooxidans play a fundamental role in the copper dissolution, as can be seen in the chemical controls,

where dissolution practically does not appear, whereas in the inoculated medium a leaching around

50% was reached (Fig. 1).

Nevertheless, some researchers have found that release of copper is favored at high concentration of

Fe3+

, where this ion contributes to the overall efficiency of the leach of chalcopyrite, in which ferric

ions acts as oxidizer producing elemental sulphur, according to the following reactions (Schippers &

Sand 1999, Bevilaqua et al., 2002, Parker et al., 2003, Walting 2006, Klauber 2008, Akcil et al., 2007):

CuFeS2 + 4Fe3+

→ 5Fe2+

+ Cu2+

+ 2S° (7)

It is important to emphasize that the behavior of the kinetic parameters analysis were similar for

both types of cultures. This may be due to the pH value around 2,1, throughout the process, which

inhibited Acidithiobacillus thiooxidans. This suggested that this type of microorganisms was unable to

obtain the energy sources from chalcopyrite becoming necessary the elementary sulfur addition as an

additional energy source (Bevilaqua et al., 2002).

4.2 Mineralogical analysis

Mineralogical studies indicated that the predominant mineral product was jarosite, which could be

considered as the ―unfavorable phase‖, because its presence apparently would pasivate the release of

copper. (Schippers & Sand 1999, Hiroyoshi et al., 2000, Bevilaqua et al., 2002, Parker et al., 2003,

Sandstrom et al., 2005, Walting 2006, Akcil et al., 2007, Córdoba et al., 2008b, Mejía et al., 2009).

The jarosite formation was more marked from 15th

day, when the system showed a lower concentration

of ferrous ion; high concentration of ferric ion, increase in redox potential and higth concentration of

SO42-

in the solution. The jarosite formation was favored when redox potentials increased above of the

―critical‖ value (430mV), around 15th

day, favoring the hydrolysis of ferric ion, promoting jarosite

precipitation and, apparently, generating a chalcopyrite passivation (Cordoba et al., 2008b).

The formation of jarosite was confirmed by FTIR spectra, showing a permanent increase in the

typical bands. The sharp increase of jarosite bands in 15th

day and its further slow increase, was

consistent with the observed in the chemical data. On the other hand, the increase of the band at 2935

cm-1

(OM) was possibly due to an increase in bacterial population, indicating bacterial activity (Xia et

al., 2008). Furthermore, the SEM analysis indicated that chalcopyrite dissolution, in the presence of the

microorganisms, took place on the surface due to the presence of roughening on the grains and the

formation of dissolution pits, which increase with time. Also, the pH reigning in the test and the high

concentration of Fe3+

and SO42-

, could generate instability in the system, favoring the precipitation and

nucleation of jarosite, agglomerating on small chalcopyrite particles, increasing after the 10th

day and

forming a non-uniform film on the biggest chalcopyrite grains. The phenomenon of jarosite

precipitation was interpreted due to the breakage in the solubility limit of iron and sulfates in the

solution and it could be mitigated by a reduction of the concentration of sulfates in the medium

(Dutrizac 1981, Hiroyoshi et al., 2000, Sandstrom et al., 2005, Xuguang 2005, Córdoba et al., 2008b).

105

On the other hand, XRD analysis (Fig. 9 and 10) indicated that jarosite was formed at the expense of

chalcopyrite dissolution. In contrast, the chalcopyrite in the control reaction system was visually

unaltered. Nevertheless the dissolution of a minority phases as molybdenite, wollastonite and chlorite

can be observed. This mineralogical analysis showing, the predominance galvanic effect, where the

molybdnite with a lowers potential dissolves in contact with chalcopyrite (Da silva et al., 2003, Urbano

et al., 2007). Wereover, chlorite and wollastonite are soluble in acid environmental (Marsend and

House 1992) and small quantities of the Cl- ion are not toxic to bacterial population (Gahan et al.;

2009). The increase abundance relative in the process (Fig. 11) could indicate the reduction of

chalcopyrite to CuS and therefore confirm the hypotheses raised by Hiroyoshi et al. (1997, 2000, 2001,

2002, 2008), where the ferrous ions promoted chalcopyrite leaching, favored the reduction of

chalcopyrite to Cu2S and the simultaneously oxidation of this mineral. However, in this case the ferrous

ions apparently favored the reduction of chalcopyrite to CuS and the simultaneously oxidation. The

hypothesis of this work was that the chalcopyrite dissolution was catalyzed initially by the ferrous ion

according to the following reactions:

CuFeS2 + 4H+ + O2 → Cu

2+ + 2S

0 + Fe

2+ + H2O (8)

CuFeS2 + Cu2+

+ Fe2+

→ 2CuS + Fe3+

(9)

CuS + Fe3+

→ Cu2+

+ Fe2+

+ S0 (10)

Finally, the particle size used generated differences in the tests. The smaller particle size (-325 Tyler

mesh) has a larger surface area and therefore it was much copper in solution in the beginning of the

process. Hence the medium may be more toxic to bacteria, which made hinder its activity. Moreover,

according to peaks intensity in the spectrums of XRD, it was seen that there was a higher precipitation

of jarosite, as well as a higher chalcopyrite oxidation ratio, for the higher particle size samples (-200

Tyler mesh). It‘s possible to conclude that the leaching process of chalcopyrite is moderately dependant

on the mineral particle size, in agreement with other studies (Shrihari et al., 1991 and 1995, Harvey &

Crundwell, 1996, Breed et al., 1996, Makita et al., 2004, Schippers, 2007, Jiang et al., 2008), and they

suggested that this was due to a greater efficiency of bacteria attachment to the biggest and not the

small particles.

5. Conclusions

It was shown that the chalcopyrite bioleaching is not a typical bioprocess, because this result in

faster and greater extractions when leaching was done at low redox potential values, compared to high

values when the chalcopyrite was inhibited. In this case, the ferrous ions apparently favored the

reduction of chalcopyrite to CuS and the simultaneously oxidation of this new phase formed, increasing

the release of copper. High concentration of Fe3+

produces a chemical instability in the process,

favoring the precipitation of jarosite principal phase formed in the processes. This phenomenon could

be responsible for inhibiting the chalcopyrite bioleaching. The results suggest that the rate of

dissolution of the mineral was affected by the formation of jarosite. This mineral may limit the

diffusion of ions through the chalcopyrite surface and the access of the leaching solution. The results

suggest that the bacteria play an important role in the chalcopyrite bioleaching process and the process

is moderately dependant on the mineral particle size.

106

Acknowledgements

The authors would like to thank the Program of Biotechnology of Colciencias, Colombia, the

laboratory of Biomineralogy of National University of Colombia, the laboratory of Mineralurgy,

University of Antioquia, and laboratory of Coal, National University of Colombia,. ALM thanks CODI,

Programa de Sostenibilidad, University of Antioquia, for partial support

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109

Artículo VI

110

Galena (PbS) bioleaching

Mejía E. R.1,a

, Ospina J. D. 2,b

, Márquez M. A.3,c

, Morales A. L.

4,d

1, 2, 3, Materials Engineering School, Applied Mineralogy and Bio-process Group "GMAB", National University

of Colombia, Medellín AA 1027, Colombia.

4 Solid State Group, University Research Centre, Antioquia University, Medellín AA 1226, Colombia. a ermejiare@unalmed.edu.co,

b judospinaco@unalmed.edu.co,

c mmarquez@unalmed.edu.co,

d

amoral@fisica.udea.edu.co

ABSTRACT

This study aims to identify mineral phases formed during galena bioleaching process using Acidithiobacillus ferrooxidans-like bacteria and a bacteria mixed culture of Acidithiobacillus ferrooxidans-like and Acidithiobacillus thiooxidans-like. Two mineral particle sizes were evaluated, 200 and 325 Tyler meshes. The strains were adapted by gradually decreasing the main energy sources and increasing the mineral content. The experiments were performed in absence of ferrous sulphate and elemental sulfur. The new phase formed and alterations of galena surface were characterized using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX) and X-ray diffraction (XRD). Galena was partially oxidized and the analysis showed anglesite as the main phase formed. This phase may limit the diffusion of leaching solution to the mineral surface. Keywords: Acidithiobacillus ferrooxidans, anglesite, biooxidation, bioleaching of lead, acid mine

drainage.

1. Introduction

The use of microbial leaching of metals from sulfide minerals has been shown strongly (Marsden &

House, 1992, Brierley e Luinstra, Hsu et al., 1993, Partha & Nataraja2006, Watling, 2006, Al-

Harahssheh et al., 2006). However, little attention has been paid to the bacterial oxidation of galena,

mainly due to the fact that, in a sulphate system, galena is oxidized to insoluble lead sulphate (Santhiya

et al., 2000; Da Silva 2004a and b). Formation of lead sulphate prevents the recovery of lead from the

traditional solvent extraction via electrowinning routes (Da Silva, 2004b).

Galena (PbS) is a mineral of vast industrial importance, not only for being the world‘s main source

of lead, but also for being a semiconducting material with a band gap around of 0,4 eV (Muscat et al.,

2003). Furthermore, sulphide materials are of interest from an environmental perspective, being the

major cause of the acidification of water systems in mining operations.

In contrast to galena, great attention has been shown in the bioleaching of sphalerite (Muscat & Gale

2003). This interest has arisen due to the increasing need of processing lower grade ores of mixed

mineralogy (Da Silva 2004b; Muscat & Gale 2003). One particular problem is the common association

of sphalerite with galena, especially at fine particle sizes which can particularly complicate the

differential flotation of the two minerals (Da Silva 2004, Liao & Deng 2004, Bolorunduro et al., 2003).

Nowadays, kinetics and mechanism of sphalerite bioleaching is well known (Da Silva 2004a, Boon

et al., 1998, Paar et al., 1984, Rodrigez et al., 2003, Zapata et al., 2007), but the kinetics and

mechanism of galena bioleaching is not complete understood (Da Silva, 2004a and b).

It is well known that two different minerals can be selectively bioleached, due to galvanic

interactions (Da Silva, 2004b). Galvanic interaction causes the mineral of lower rest potential to be

sacrificed, whereas the mineral of higher potential is passivated (Das et al., 1999; Suzuki, 2001, Da

Silva, 2004, Abraitis et al., 2004, Cruz et al., 2005, Urbano et al., 2007).

111

The mechanism of galena oxidation is also important in flotation processes, where the mineral

oxidation, through the grinding/flotation circuit, can affect its hydrophobicity and therefore the

interaction with surfactants (Da Silva 2004; Jañezuk et al., 1993; Nowak et al., 2000; Peng et al.,

2002).

For other hand, pretreatment of refractory ores to recover metals from sulphide lower grade ores or

refractory minerals is not usual in Colombia (Muñoz et al., 2003). For this reason, economic losses

related to mining processes are common, especially on subsistence mining. Their implementations in

mining and metallurgical industries are very attractive (Flower et al., 1999; Rohwerder et al., 2003;

Olson et al., 2003).

In this work, a biological oxidation of galena, using Acidithiobacillus ferrooxidans-like bacteria and

mixed culture was carried out. Characterization techniques such as SEM, XRD and FT-IR were used to

follow morphologic and chemical changes occurring during the process.

2. Materials and Methods

2.1 Mineral

All experiments were carried out with a galena sample from El Silencio miner, property of Frontino

Gold Mines Company (Segovia, Antioquia, Colombia). The mineral was subjected to crushing and

milling processes, followed by a gravimetric separation in a Wilfley table. Afterwards, a manual

concentration using stereographic microscope was performed. Mineralogical composition of the

concentrate, measured by countdown points was: 93,3% galena (PbS), 6,2% sphalerite (ZnS) and 0,5%

chalcopyrite (CuFeS2) for -200 Tyler and 90% galena (PbS), 7,5% sphalerite (ZnS), 0.7%

chalcopyrite(CuFeS2) and 1.8% gangue (SiO2), for -325 Tyler. An agate mortar was used to obtain two

particle sizes: pass through 200 Tyler mesh (~75μm), and pass through 325 Tyler mesh (~45μm). X-

ray diffraction results confirmed that the principal mineral phase was the galena in both sizes (Fig. 1).

The mineral was sterilized in a furnace at 80°C for 90 minutes.

A. Galena 200

Lin

(Co

un

ts)

0

1000

2000

3000

4000

5000

2-Theta - Scale

16 20 30 40

Gn

Gn

Gn

Sp

y

Sp

y

Arg

Qz

Qz

Arg

50

B. Galena 325Gn

Gn

Lin

(Co

un

ts)

0

1000

2000

3000

4000

5000

6000

7000

8000

2-Theta - Scale

15 20 30 40 50

Gn

Sp

y Sp

y

Qz

Figure 1. X- ray spectra for the concentrates. A) Mineral pass through 200 and B) mineral pass through 325.

2.2 Bioleaching Experiments

For the bioleaching experiments Acidithiobacillus ferrooxidans-like and Acidithiobacillus

thiooxidans-like strains were employed. The strains were isolated by Cardona (2008). The

112

microorganisms were previously grown in T&K medium by successively replacing the ferrous sulfate

by galena. The medium was acidified to pH 1.8 with H2SO4. The flasks were sterilized by autoclaving

for 20 min, 120°C at 18 psi. The experiments was inoculated with Acidithiobacillus ferrooxidans 10%

(v/v), for the single culture, and 5%(v/v) Acidithiobacillus ferrooxidans and 5%(v/v) Acidithiobacillus

thiooxidans, for the mixed culture. The experiments were carried out for 30 days, in 500ml shake flasks

containing 300ml medium, with 10% (w/v) galena, at 180rpm and 30ºC. All conditions were duplicated

and the respective abiotic control was included.

2.3Chemical analysis

Measurements of pH (HACH HQ40d multi PHC30103) and redox potential (Shot Handylab 1 Pt

6880) in situ (reference electrode Ag0/AgCl) were performed every day. Samples were aseptically

withdrawn from the flasks after 24 hours and then every five days. The samples were separated in a

DIAMOND IEC DIVISION centrifuge, for 15min at 3000rpm. Iron and sulphate concentration were

measured with an UV-visible spectrophotometer GENESYS™ 10. The methods employed were 3500-

FeD (O-phenantroline) for ferrous and total iron according to the Standard Methods for water analysis.

2.4 Mineralogical analysis

Combinations of analytical techniques were used to do the mineralogical characterization of the

samples. The FTIR spectra, for the solid samples, were recorded in a FTIR Spectrophotometer

Shimadzu Advantage 8400 with KBr pellets (transmission mode). A sample of KBr mixture at 1:200

ratio was used. The total number of scans was 20 with a spectral resolution of 4 cm−1

, a range of 400–

4000 cm-1

and Happ-Henzel correction were used.

The biooxidation samples were mounted in epoxy resin and were polished with sequential finer SiC

grit paper and a final polish with 0.05-µm sized alumina powder. The polished sections analysis were

performed with JEOL JSM 5910 LV scanning electron microscopy (SEM) in backscattering electron

mode and energy dispersive X-ray (EDX) detector, Oxford instrument, using a beam voltage of 18kV.

XRD analyses of the samples were conducted on a Bruker D8ADVANCE diffractometer with Cu λ=

1.5406 Å radiation generated at 35 kV and 30 mA. X-ray diffraction data were obtained using

computer controlled Xray Diffractometer Panalytical X'Pert Pro MPD. The initial characterizations of

the mineral polished sections were performed by optical microscopy of reflected light (OMRL).

3. Results

3.1 Galena leaching experiments with Acidithiobacillus ferrooxidans

In Fig. 2, variation on pH and redox potential (Eh) values for the inoculated systems and the abiotic

controls are presented. In order to prevent the inhibition of the bacteria, H2SO4 was added to maintain

the pH values around 2,0 until the day 15. The pH values first increase and then decrease with time to

levels around to 1,1. The pH values in the abiotic controls stabilized around 2,0 after day 15.

The variation on redox potential in both cultures, with different grain sizes, presented a relatively

low redox potential at the beginning of the test until the day 11, followed by an increment and finally

presented a stationary phase whit small decrease in the time. Eh values in the abiotic controls were

around 284 mV through all the process. The SO42-

concentration in solution as well as in the solid

phase, showed a gradually increased whit the time. However, the increase of sulphate concentration in

the solid phase was stronger than in solution (Fig. 3). The dissolution of Fe2+

increased from day 6 to

15 where presented the maximum value. Then, it decreased sharply, become more or less stationary

from with small decrease the 25th

day (Fig. 4). In the abiotic experiments, ferrous iron increased

between 5th

and 10th

remaining stationary until the end of the process. The Fe3+

increased from day 10th

to 15th

, then becoming stationary until the day 25 and finally decreased sharply even about 1ppm,

113

staining like that until the end of the experiment. Lead extraction was around 57% for all texts. On the

other hand, less than 5% Pb was solubilized in chemical controls (Fig. 5).

3.2 Fourier Transform infrared Spectroscopy (FT-IR)

Results obtained by FT-IR for the bioleached galena samples, show typical bands of the anglesite, the

main mineral product of the process, with absorption bands at 950-1000, 1165-1765, 1115-1125, 1050-

1060 and 592-620 cm-1

(Chernyshova 2003). Scotlandite (PbSO3) was also identified by bands at 920,

870, 970 and 600-620 cm-1

(Paar et al., 1984, Chernyshova 2003). The bands around 2935 cm-1

related

to the total carbon present on the cell surface showed a permanent increase (Naumann & Helm., 1995,

Sharma & Hynumantha 2005, Xia et al., 2008).The FTIR spectra showed in the beginning of the

process, for all test, an increased in the bands of the anglesite and scontraldite.

For the tests conducted to Acidithiobacillus ferrooxidans, with different grain sizes, presented a

continuous increment in the bands of the anglesite and scontlandite until 10th

day, followed the strong

increased in this bands. This was possibly due to strong anglesite and scontlandite precipitate. Then it

increased continuously until 20th

day. Finally, the typical bands of anglesite and scontrandite showed a

strong increased (Fig. 6 a and b). However, the FTIR spectra for the test conduced to mixed culture,

whit different grain size, presented continues increment in the bands of anglesite and scontralndite

through the process (Fig. 6 c and d).

Fig 2. Changes in pH and redox potential (Eh) during the bacterial oxidation process. Where 200F is test with Acidithiobacillus

ferrooxidans and 200 Tyler mesh, 325F is test with Acidithiobacillus ferrooxidans and 325 Tyler mesh, 200FT is test with consortium and

200 Tyler mesh, 325FT is test with consortium and 325 Tyler mesh, 200C is a inoculate test with 200 Tyler mesh and 325C is a inoculate

test with 325 Tyler mesh.

114

Fig3. Changes in SO4

2- during the bacterial oxidation process. Where 200F is test with Acidithiobacillus ferrooxidans and 200 Tyler

mesh, 325F is test with Acidithiobacillus ferrooxidans and 325 Tyler mesh, 200FT is test with consortium and 200 Tyler mesh, 325FT is

test with consortium and 325 Tyler mesh, 200C is a inoculate test with 200 Tyler mesh and 325C is a inoculate test with 325 Tyler mesh.

Fig4. Changes in Fe2- and Fe3+ during the bacterial oxidation process. Where 200F is test with Acidithiobacillus ferrooxidans and 200

Tyler mesh, 325F is test with Acidithiobacillus ferrooxidans and 325 Tyler mesh, 200FT is test with consortium and 200 Tyler mesh,

325FT is test with consortium and 325 Tyler mesh, 200C is a inoculate test with 200 Tyler mesh and 325C is a inoculate test with 325

Tyler mesh.

115

Fig 5. Galena oxidation during the bacterial oxidation process. Where 200F is test with Acidithiobacillus ferrooxidans and 200 Tyler

mesh, 325F is test with Acidithiobacillus ferrooxidans and 325 Tyler mesh, 200FT is test with consortium and 200 Tyler mesh, 325FT is

test with consortium and 325 Tyler mesh, 200C is a inoculate test with 200 Tyler mesh and 325C is a inoculate test with 325 Tyler mesh.

Fig 6. FT-IR spectra of solid residues after bioleaching of galena. Where OM: Organic matter and SO4

2-: anglesite.

116

3.3 Scanning electron microscope (SEM/EDX)

SEM images of leached galena are shown in the Figs. 7, 8 and 9. All the samples corrosion features,

such as pits and gulfs on the surface of galena grains (Fig. 7 H and I). Moreover, was observed a coarse

particle size porous films coating grains (Fig. 7 C, F and L), and aciculate precipitates of anglesite (Fig.

7 D and L). These characteristics were observable since five days of the process (Fig. 7 A, B,G and H)

and became more evident with time. After 15 day galena grains were coated with anglesite film with

porous texture (Fig. 7 C and K). The formations of anglesite aciculate were more evident at the end of

the process (Fig. 7 L). Is important to note that, the pasivante effect that has galena on the sphalerite

and pyrite where the pyrite and sphalerite grain not showed oxidation evidences (Fig. 7C, D, J, F and

L). However, in some cases, was observed sphalerite and pyrite grains with grooves of oxidation at the

end of the process (Figs. 8 A and B). Moreover, was observed remaining nucleus of galena and

anglesite with porous texture (Fig. 7K and 8B).

On other hand, some galena grains showed oxidation along cleavage planes (Figs. 7 E and G). The

SEM images of uninoculated samples, for all tests (Fig. 8 a and b), showed a surface with a few

alterations such as small oxidation in the cleavage planes.

EDX analysis of the grains showed the galena, anglesite, pyrite and sphalerite stoichiometric

composition (Table 1).

3.4 X-ray diffraction analysis

According to the XRD initial analysis, it was found that the main mineral phases present in the

original samples was galena, with small quantities of quartz (SiO2), sphalerite (ZnS) and chalcopyrite

(CuFeS2), and aragonite, (Fig. 1). X-ray diffraction spectra of bioleached samples are shown in Fig. 11.

Mineralogical evolution of the mineral phases consists of a gradual reduction of galena peaks and an

appearance of anglesite (PbSO4). Anglesite peaks appear were formed from 5th

day, for all sample.

However, peak intensity were higher in concentrate pass through -325 Tyler mesh. The sphalerite peaks

showed unchanged throughout the process. XRD for un-inoculate controls showed a little formation of

anglesite around 30 day (Fig. 10).

117

Fig 7. SEM micrograph of the residues after bioleaching of galena pass through 200 (A, B,C, D, E and F) and 325(G, H, I, J, K and L)

Tyler mesh. Where Gn is galena, Ang is anglesite and Py is pyrite. A) Galena grain and aciculate anglesite precipitates, arrows indicates

corrosion gulfs (day 5). B) Galena grain in insipient oxidation state (day 5). C) Galena grain covered and coated with anglesite porous

film and pyrite grain without apparent oxidation (Day 15). D) Anglesite grains aciculate and anhedrals, quartz grains without apparent

oxidation and grain of pyrite in insipient oxidation state (Day 15). E) Galena grain oxidant along cleavage plane (arrows indicates

cleavage plane), and anglesite grain (Day 30). F) Galena grain covered and coated with anglesite porous film and pyrite grain without

apparent oxidation (Day 30). G) Galena grain corroded in gulfs ang cleavage plane (Day 5). H) Galena grain in insipient oxidation state

showed gulfs of corrosion (Day 5). I) Galena grain corroded with anglesite cavity formation (Day 15). J) Galena grain covered and

coated with anglesite porous film and pyrite grain without apparent oxidation (Day 15). K) Anglesite porous with remaining galena

nucleus. L) Anglesite grains aciculate and anhedrals and galena grains coated with anglesite porous film and sphalerite grain without

apparent oxidation.

118

Table 1. EDS analysis of the residues after bioleaching of galena.

Fig 8. SEM micrograph of the residues after bioleaching of galena. Where Gn is galena, Ang is anglesite, Py is pyrite and Spy is

sphalerite. A) Sphalerite and pyrite grains showed a typical corrosion groove and pits and anglesite porus.B) Pyrite grain with groove of

corrosion, remaining nucleus of galena and anglesite porous.

Figure 9. SEM micrographs of uninoculated residues after bioleaching of galena A) Galena grain with insipient oxidation state along

cleavage plane (pass through 200 Tyler mesh) and anglesite aciculate on galena surface. B) Galena grain with insipient oxidation state

along cleavage plane (pass through 325 Tyler mesh), and pyrite without oxidation state. Where Cpy: Chalcopyrite and Qz: Quartz.

119

Fig 10. X-ray diffractograms of inoculated samples after 30 days of the biooxidation process. A) sample -200 Tyler mesh B) sample -325

Tyler mesh. Where Gn is galena, sph is sphalerite and Ang is anglesite.

Fig 11. XRD spectrums for galena before bioleaching process. A. particle size 200 Tyler mesh for pure culture. B. particle size 325 Tyler

mesh for pure culture. C. particle size 200 Tyler mesh for mixed culture. D. particle size 325 Tyler mesh for mixed culture. Where Gn is

galena, sph is sphalerite, Qz is quartz and Ang is anglesite

A B

120

4. Discussion and conclusions

4.1. Galena Leaching Experiment

Acidithiobacillus ferrooxidans-like bacteria showed a good adaptation on galena with a high

oxidative capacity (SEM, FTIR, XRD and chemical data), as the microorganism was grew in mineral

concentrate that the mineral was the only source of energy for the growth. Lei Jiang et al (2008) stated

that the bacteria may directly oxidize galena taking energy from the mineral. However, several authors

suggest that Acidithiobacillus ferrooxidans does not have a direct effect in the oxidation of galena, but

only indirectly through oxidizing hydrogen sulfide (H2S) and sulfur (Da silva 2004; Muscat & Gale

2003; Garcia et al., 1995). Thus, Acidithiobacillus ferrooxidans utilizes hydrogen, sulfide dissolved in

the solution, as energy source (Dutrizac & Chen 1995; Mizoguchi & Habashi 1981), according to the

following equations:

PbS + H2SO4→ PbSO4 +H2S (1)

2H2S + O2 bacteria

→ 2S + 2H2O (2)

2S + 2H2O + 3O2 bacteria

→ 2SO42-

+ 4H+ (3)

Furthermore, this study suggests that galena biooxidation also produces anglesite (PbSO4) by

reacting with sulfuric acid according to equation 1.

This is in agreement whit the results obtained in this work, where Pb2+

was released from galena and

precipitated as lead sulphate. Nevertheless, in accordance with the results obtained by FTIR, where

there was evidence of the presence of scontlandite (PbSO3) it is possible to suggest that the anglesite

was not the only sulphate mineral phase, being able to generate other lead sulphate mineral phases as

accessories. On other hand, the solubility for Pb2+

is very low, around 45 ppm (Mousavi et al., 2006),

the SO42-

increased gradually in solid and liquid phases (Fig. 3), being higher in solids (Fig.4). It

indicates that Pb2+

and SO42-

or SO32-

ions react to form anglesite and scontlandite according equation 6

and 7. Moreover, anglesite (PbSO4) was detected in the residual solid and increased in time according

to FTIR, XRD and SEM analysis. This can be represented by the following equations:

PbS + H2SO4 +0.5O2 → PbSO4 + H2O + S0 (4)

2PbS + H2SO4 + 3/2O2 → 2PbSO3 + H2O + Sº (5)

Pb2+

+ SO42-

→ PbSO4 (6)

Pb2+

+ SO32-

→ PbSO3 (7)

However, elemental sulfur was not detected by the SEM, XRD and FTIR. Moreover, the pH (Fig.

2), showed a decrease after six days around to 1,3, for all test, indicating an increase in H+

concentration produced by bacterial activity. This behavior was due to the galena dissolution in an acid

environment occurring as a result of the protonation of the mineral surface. Where, the only

protonation mechanism that has been proven to be energetically favorable in aqueous solution consists

in the attachment of three H+ onto three surface S atoms surrounding a central Pb atom, which is then

replaced by fourth H+

(equation 1). In this work was detected a little changes in chemical controls in

galena oxidation and SO42-

in solid and liquid concentration at the beginning of the process indicating

that the dissolution of galena was favoring in acid medium, by means of purely chemical mechanism.

This is in agree with Gerson & O‘Deo (2003) and Acero et al., (2007).

Then, bacterial oxidize hydrogen sulfide (H2S) produced by generating elemental sulfur and water

(equation 1), and elemental sulfur is also oxidized by bacteria producing sulfates and H +

(equation 2).

121

The H+ attack back to the mineral and thus generate a cycle, this generate a great dissolution in

inoculate tests (around 57%) compared with the un-inoculate test.

On the other hand, at the 10th

day, when the concentration of Fe3+

was high, probably due to the

biooxidation of the minor quantities of sphalerite and pyrite (Fig. 4), the galena oxidation was favored,

indicating possibly that this ion contributes to the overall efficiency in the process. Lei Jiang et al.,

(2007) found that ferric can oxidize the galena with the generation of elemental sulphur, according to

the following reaction:

PbS + 2Fe3+

→ Pb2+

+ 2 Fe2+

+ S0 (8)

It is important to emphasize that the behavior of the kinetic parameters analysis were similar for

both types of cultures. Probably due to the fact than the pH value was highly unstable at the beginning

of the process, the pH was going up around 4,0, inhibiting Acidithiobacillus thiooxidans. This

suggested that this type of microorganisms was unable to obtain the energy sources from galena,

becoming necessary an elemental sulfur addition, as an additional energy source. On the other hand, the

particle size used not generated differences in the tests.

4.2. Mineralogical analysis

Mineralogical studies indicated that the predominant mineral phase, product of the galena

biooxidation was anglesite. The presence of anglesite did not show in clear form, evidences of

pasivation of galena biooxidation process, this was showed in curve of galena oxidation, which was

linear and increasing throughout the process (Fig. 5) and the SEM images where on 30th

day were

observed anglesite grains with remaining nucleus of galena (Figs. 7K and 8B). The formation of this

mineral phase was confirmed by FTIR spectra which, showed in the beginning of the process, for all

test, an increased in the bands of the anglesite and scontraldite, probably due of anglesite and

scontlandite present precipitate pulse (Fig. 6). This sharp increased was observed for 10th

and 20th

for

the tests with Acidithiobacillus ferrooxidans, and its further slow increase in the other day of the

process was consistent with the sharp increase in the chemical sulphate data (Fig. 3). While, the FTIR

spectra, for the test conduced to mixed culture presented the continue increment in the bands of

anglesite and scontralndite through the process (Fig. 6 C and D).

On the other hand, the increase of the band at 2935 and 2847 cm-1

(OM) was possibly due to an

increase in bacterial population, indicating bacterial activity (Naumann et al., 1995, Sharma et al.,

2005, Xia et al., 2008). Furthermore, the SEM analysis indicated that galena dissolution, in the

presence of the microorganisms, took place on the surface due to the presence of roughening on the

grains, the formation of dissolution gulf (Fig 7 A and H) and preferential dissolution in cleavage

planes(Fig 7 E), which increase with time. The phenomenon of anglesite precipitation occurs because

the solubility limit of lead and sulfates in the medium is exceeded and it could be mitigated by a

reduction of the concentration of sulfates in the medium. Moreover, was observed the precipitation of

anglesite film on galena grain from de day 15th

evidenced by SEM analysis (Fig. 7 C).

The preferential oxidation on galena cleavage planes was due probably this region was more

favorable potentially or chemically more reactive, because this zone have a higher surface energy and

therefore was easily oxidized, in agreement with Bennett and Tributsch (1978).

On the other hand, XRD analysis (Fig.10) indicated that anglesite was formed at the expense of

galena dissolution. In contrast, the galena in the control reaction system showed a few alterations.

Nevertheless the dissolution of a minority phases as sphalerite can be observed.

Finally, mineralogical data showed the passivating effect of galena on pyrite and sphalerite (Fig. 7 F

and L respectively), when the last one, with a higher rest potential, is provided at the expense of the

122

galena oxidation, which acts as sacrificial anode, in agreement with previous (Das et al., 1999; Suzuki,

2001, Da Silva 2004b, Abraitis et al., 2004, Cruz et al., 2005, Urbano et al., 2007).

However, in some cases, dissolution was present in pyrite and sphalerite grains (Fig. 8 A and B).

This data confirmed that de Fe2+

lixiviation comes from probably the minor quantities of sphalerite and

pyrite, present in the sample concentrate. However, the rest potential of sphalerite is less than pyrite,

possibly indicates that iron leached come to sphalerite, but high concentration of iron in solution (Fig.

4), indicate apparently that the pyrite generated an important contribution. Moreover, the iron

percentage in sphalerite (around 8,6% weight) was smaller than in pyrite(around 50,48% weight)

(Table 1).

5. Conclusions

The examinations on bioleaching of natural galena concentrate in T&K medium by

Acidithiobacillus ferrooxidans-like bacteria and mixed culture allowed to draw some conclusions:

The bacteria have an impact on higher yield in the course of reaction of oxidizing PbS into PbSO4

as compared to the control data. The level of lixiviation after 30 days of bioleaching amounted up

to 57 % whereas in the control examinations it was only 6 %.

The Fe3+

favored the biolixiviation of galena, because when its concentration increase, the galena

dissolution was favored.

The predominant new mineral phase was anglesite, its formation of porous film on galena, but this

film not limit the access of the leaching agent and microorganisms inside the grain.

In the presence of bacteria, the XRD peaks corresponding to galena decreased and, at the same

time, new peaks appeared, anglesite, during the bioleaching process. This signal became more

intense in the time.

In both culture, microorganisms gradually modified the original galena surface, increasing the rest

potential and SO42-

in solid and liquid.

The particle size apparently was not a determining factor in the process.

The galena was initially dissolved by acid medium.

Acknowledgements

The authors would like to thank to program of biotechnology of Colciencias, Colombia, laboratory

of biomineralogical of National University of Colombia, Medellínd, Professor Diego Hernan Giraldo of

University of Antioquia and laboratory and the group of molecular studies of University of Antioquia.

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