Hydrometallurgy 88 (2007) 3 – 18
www.elsevier.com/locate/hydromet
Silver-catalyzed bioleaching of low-grade copper ores.
Part I: Shake flasks tests
J.A. Muñoz a , D.B. Dreisinger b,⁎, W.C. Cooper b , S.K. Young c
a
Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica, Facultad de Ciencias Químicas,
Universidad Complutense, 28015 Madrid, Spain
b
Department of Metals and Materials Engineering, The University of British Columbia,
Vancouver, Canada V6T 1Z2
c
Versitech Inc., 1438 W. San Lucas Drive, Tucson, Arizona, 85704-1122, USA
Received 1 November 2006; received in revised form 31 March 2007; accepted 17 April 2007
Available online 24 April 2007
Abstract
A study of the effect of different variables (inoculation, pulp density, [Ag], nutrient medium, pH and [Fe3+]) on the silvercatalyzed bioleaching of a low-grade copper sulfide ore has been carried out in shake flasks. Chalcopyrite was the dominant copper
mineral in the ore. Preliminary tests showed that addition of other ions (Sb, Bi, Co, Mn, Ni and Sn) did not enhance the copper
dissolution rate. Conversely, an inoculation with mesophilic microorganisms and the addition of silver had a markedly catalytic effect
on the extraction of copper. The kinetics of the silver-catalyzed chalcopyritic ore bioleaching was greatly affected by pulp density and
silver concentration. Small amounts of silver (14.7 g Ag/kg Cu) dramatically accelerated the copper dissolution process while large
amounts (294.12 g Ag/kg Cu) had an inhibitory effect. The copper dissolution rate was slightly affected in the range of pH between
1.2 and 2.5 but was significantly slower at pH 3.0. The effect of [Fe3+] in the presence of silver was studied both in abiotic and biotic
conditions. High ferric iron concentrations in abiotic tests recovered similar copper amounts (∼ 95%) to those obtained without or
with low [Fe3+] in the presence of bacteria. The leaching of copper from the low-grade copper ore can be very effectively enhanced
with silver and mesophilic microorganisms. For that system, the onset of oxidizing conditions starts at an Eh value slightly higher
than 650 mV. Above that critical value of potential the copper dissolution rate slows down. This also corresponds with the completion
of the leaching process. As the potential rises past 650 mV, the copper extraction reaches a plateau.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Low-grade copper ore; Chalcopyrite; Silver catalysis; Shake flasks
1. Introduction
The spectacular rise in copper prices, at present above
US $3/lb, makes the exploitation of copper reserves
around the world very attractive. The rapid development
⁎ Corresponding author.
E-mail addresses: [email protected] (D.B. Dreisinger),
[email protected] (J.A. Muñoz).
0304-386X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.hydromet.2007.04.004
of plant operations to meet this increasing demand must
however be done in an environmentally responsible manner with due consideration to avoid air, water and landfill
pollutions to the greatest extent possible.
Currently, around 20–25% of the primary world
copper production is derived from hydrometallurgical
plants where the dominant process is copper leach–
SX–EW. In Chile, the largest copper-producing country
in the world, hydrometallurgical processing accounted
for 35% in 2002 (Edelstein, 2002).
4
J.A. Muñoz et al. / Hydrometallurgy 88 (2007) 3–18
Copper, a typical chalcophilic element, is found in
nature as sulfides orebodies. Chalcopyrite, the most important primary source of copper metal, accounts for approximately 70% of the world's known copper reserves
(Wang, 2005). Most of these deposits are porphyries, with
copper minerals finely disseminated in a host rock of
quartz. Thus, the principal obstacle for the hydrometallurgical treatment of this type of ores has been the presence of
chalcopyrite.
In response to the wide variety of copper raw materials
around the world, new leaching processes have been
developed to achieve the highest efficiency. Recently,
several authors have overviewed the various alternative
hydrometallurgical technologies developed to extract
copper from chalcopyrite ores: atmospheric chemical
(chloride or sulfate) and biological leaching and pressure
sulfate leaching (Wang, 2005; Clark et al., 2006; Dreisinger, 2006; Watling, 2006).
Moreover, copper extraction from low-grade chalcopyritic ores requires simple and economically feasible
methods. In this sense, the change in copper ore quantities
and qualities may act as the driving force for both the
development of new technologies and the improvement of
existing ones.
One of the most promising strategies developed to
optimise copper dissolution from low-grade ores has been
the utilization of acidophilic microorganisms as biological
catalysts. As early as 1922, Rudolfs and Helbronner advanced the hypothesis that bacterial leaching could be an
option to extract metals economically from low-grade
sulfide minerals (Rudolfs and Helbronner, 1922). In fact,
modern commercial application of biohydrometallurgical
processes began with bioleaching of run-of-mine copper
ores (Bryner et al., 1954).
Bioleaching is probably the most widely studied
method for the treatment of low-grade copper ores in the
mining industry. The economic feasibility of in-situ and
heap leaching of copper sulfide ores coupled with microbial activity has received much attention over time
(Murr, 1980; Sand et al., 1993; Bosecker, 1997; Rawlings,
2002; Renman et al., 2006). This has led to the implementation of different industrial practices to maximize
heap copper recovery such as: acid ore agglomeration;
high acid pre-conditioning stage; forced aeration; changes
in heap lift height and leaching cycle; or application of
chemical surfactants (Sen et al., 1993; Jenkins, 1994,
Lizama, 2001).
Nevertheless, in the treatment of lohw-grade copper
sulfides low extractions and slow kinetics remain as the
main drawbacks of these processes as they operate at low
temperatures. The leaching of copper in dumps or heaps
from sulfidic ores is often slow and far from complete.
This is especially true to chalcopyrite oxidation where
extractions much less than 50% are observed even after
many years of leaching.
Unlike copper oxides, the leaching of sulfide ores in
sulfuric acid media is ineffective and requires the presence
of an oxidizing agent. It is well known that the copper
dissolution from chalcopyrite takes place through an
electrochemical reaction which is greatly affected by the
semiconducting properties of the surface in contact with the
leaching medium. Although different oxidizing chemicals
have been proposed for the leaching of chalcopyrite, ferric
ion is by far the most widely-used oxidant in acid medium
(Dutrizac and MacDonald, 1974).
Since the pioneering work of Sullivan (1933),
chemical and bacterial leaching of chalcopyrite with
ferric ions has been explored in detail by a significant
number of researchers (Dutrizac et al., 1969; Jones and
Peters, 1976; Majima et al., 1985; Torma and Apel, 1992).
These processes are based on the standard oxidation
potential of the Fe3+/Fe2+ redox couple and are influenced
whether by physical, chemical or biological factors.
However, ferric sulfate, unlike ferric chloride, is not a
very effective lixiviant for the dissolution of copper from
chalcopyrite. Both chemical and bacterial leaching of
chalcopyrite in acid ferric sulfate displays parabolic kinetics
due to the formation of a thickening film that prevents
further reaction between the leach solution and the chalcopyrite surface. Although complete oxidation of chalcopyrite can be represented by the following reactions:
CuFeS2 þ 2Fe2 ðSO4 Þ3 →CuSO4 þ 5FeSO4 þ 2S°
ð1Þ
CuFeS2 þ 8Fe2 ðSO4 Þ3 þ 8H2 O→CuSO4
þ 17FeSO4 þ 8H2 SO4
ð2Þ
reaction (1) prevails over reaction (2).
Additionally, the bioleaching of chalcopyrite with
ferric sulfate is a relatively slow process compared to
secondary copper sulfides (Murr, 1980; Donati et al.,
1996; Acar et al., 2005; Watling, 2006). Iron- and sulfuroxidizing bacteria are presumed to have a beneficial
effect on the copper leaching rate by maintaining a high
redox potential and by oxidizing the sulfur layer to
soluble products:
1
4
1
2
FeSO4 þ O2 þ H2 SO4
Feoxidizing bacteria 1
→
2
Fe2 ðSO4 Þ3
1
2
þ H2 O
2S-þ3O2 þ2H2 O
ð3Þ
Soxidizing bacteria
Y
2H2 SO4
ð4Þ
J.A. Muñoz et al. / Hydrometallurgy 88 (2007) 3–18
As the ferric ion concentration in the medium increases (reaction (3)), ferric sulfate undergoes hydrolysis
generating jarosite precipitates:
3Fe2 ðSO4 Þ3
þ 14H2 O →2ðH3 OÞFe3 ðSO4 Þ2 ðOHÞ6
þ 5H2 SO4
ð5Þ
However, different studies have shown that chalcopyrite dissolution at low temperature is highly dependent
on Eh, with better copper extraction rates at low than at
high redox potentials (Hiroyoshi et al., 2000; Third et al.,
2002). In addition, at high redox potentials the chalcopyrite leaching is suppressed and pyrite oxidation
may become the dominant reaction. There is a general
consensus on the refractoriness of chalcopyrite towards dissolution and on the key role that solid products, formed on the chalcopyrite surface, play during
leaching. But the nature of that film is still being debated.
The two main theories proposed assume the formation of a very thin solid product (b1 μm thickness) on
the chalcopyrite surface which consist of: S° according
to reaction (1) (Muñoz et al., 1979) or an intermediate
copper-rich sulfide formed as a result of solid state
transformations in the chalcopyrite (Ammou-Chokroum
et al., 1977). In both cases, the semiconducting properties of the new surface would be completely different
from chalcopyrite. The fact is that both theories have
received support from different studies whether using
surface analysis or electrochemical techniques (Parker
et al., 1981; Warren et al., 1982; Biegler and Horne,
1985; Almendras et al., 1988; Dutrizac, 1989; Hackl
et al., 1995; Parker et al., 2003; Mikhlin et al., 2004). In
consequence, this has brought more uncertainties into the interpretation of the mechanism of chalcopyrite
dissolution.
In addition, there is still another theory on chalcopyrite passivation that deals with the formation of a
protective layer of iron-hydroxide precipitates jarositetype (reaction (5)). In fact, this theory has been particularly important to explain chalcopyrite passivation
in bioleaching processes (Boon and Heijren, 1993, Stott
et al., 2000; Sandstrom et al., 2005; Kinnunen et al.,
2006). The formation of a protective layer of crystalline
jarosite on chalcopyrite bioleaching residues has been
detected by XPS at times as short as 20 h (De Filipo et al.,
1988). Moreover, it has been postulated that bacterial cells
can serve as nucleation sites for jarosite formation (Pesic
and Kim, 1991).
In seeking solutions to overcome chalcopyrite
passivation in acidic ferric sulfate solutions, several
processes have been considered for the removal of the
5
passivating layer: a) the increase of temperature, b) the
use of Fe- and S-oxidizing microorganisms and/or c) the
addition of different catalytic ions. Apparently, the
increase of copper dissolution rate in these processes is
due to a change in the morphology of the solid products
formed on the chalcopyrite surface.
On the one hand, the joint action of the two first
options have found a potential application in the bioleaching process which combined high temperatures
with the use of acidophilic thermophilic archaebacteria
(mainly Sulfolobus and Acidianus) to decompose the
passive layer. These bacterial leaching processes have
received a major consideration in the scientific literature
(Brierley, 1990; Clark and Norris, 1996; Gericke and
Pinches, 1999) and their industrial implementation is
now demonstrated in the BioCOP™ process (Batty and
Rorke, 2006).
On the other hand, the joint action of options b and c,
given above, has found potential application in the
bioleaching process which combined the addition of
different metal ions (Ag(I), Hg(II), Co(II), Bi(III), As
(V), Mn(II), Ru(III)) with the use of acidophilic
mesophilic bacteria (mainly Acidithiobacillus ferrooxidans, At. thiooxidans, Leptospirillum ferrooxidans).
The role of these catalysts in the chalcopyrite bioleaching has been attributed to the formation of a metal
sulfide on the chalcopyrite surface which dissolves the
original sulfide material by galvanic action or by substitution in the crystal lattice (Barriga et al., 1987;
Ahonen and Tuovinen, 1990; Ballester et al., 1990;
Escudero et al., 1993). That galvanic corrosion effect
was already known to play an important role during the
leaching of natural chalcopyrite samples in which the
presence of impurities in the chalcopyrite lattice or the
presence of some mineral sulfides, like pyrite and
molybdenite, accelerates the dissolution of chalcopyrite
(Dutrizac and MacDonald, 1973; Berry et al., 1978;
Mehta and Murr, 1983; Romano et al., 2001). However,
the importance of galvanic couples in leaching processes lies in maintaining a good permanent electrical contact between the different mineral sulfides in the system
and this can be further complicated by the formation of
poorly conducting films on the chalcopyrite mineral
grains.
In the present investigation, the leaching of low-grade
copper ores using chemical (silver ions) and biological
(mesophilic microorganisms) catalysts has been explored in three different systems: shake flasks, stirred
tanks and column reactors. Further insights on the silvercatalyzed chalcopyrite process will be given in part II
and part III, with especial emphasis on literature works
on stirred tanks and column reactors.
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J.A. Muñoz et al. / Hydrometallurgy 88 (2007) 3–18
2. Materials and methods
2.1. Low-grade copper ore
A low-grade copper ore from San Manuel mine
(Tucson, Arizona) was used. The ore was supplied by
Magma Copper Company (taken over by BHP in the
late 1990's) and corresponded to the porphyry Lower
Kalamazoo orebody, named lower K-ore, with an
average copper composition of 0.62% concentrated
almost entirely into the sulfide fraction. The following
chemical analysis of the as-received ore was obtained:
0.68% Cu, 0.012% acid-soluble Cu, 3.50% Fe, 3.11% S
and 84.50% of acid insoluble. Acid-soluble copper was
determined in quadruplicate following the AP-101G
analytical procedure from Magma Copper Co. X-ray
diffraction analysis of the ore showed the presence of
quartz and silicates as the main components and chalcopyrite, pyrite and galena as minor phases.
The as-received ore was initially screened through
−0.6 mm and the remaining larger fraction crushed to
−0.6 mm prior to the beginning of the shake flask experiments. The particle size distribution of the ground ore
tested is shown in Fig. 1.
2.2. Bacterial cultures
All bioleaching tests were inoculated with active
maintenance cultures of thiobacilli and leptospirilli
constantly renewed. Bacterial cultures were maintained
routinely in an orbital shaker at 35 °C and grown on
75 mL of the Fe-free 9K, named 0K, nutrient medium
(3.0 g (NH4)2SO4, 0.5 g KH2PO4, 0.5 g MgSO4·7H2O,
0.1 g KCl and 0.01 g Ca(NO3)2·4H2O) adjusted to pH
2.0, with 4 g of low-grade copper ore as the sole source
of energy and 5 mL aliquots of exponential-phase bacterial culture.
Two different mesophilic cultures were subcultivated:
in the absence and in the presence of silver as Ag2SO4.
Silver resistance was enhanced by subsequent adaptation
of the bacterial culture to increasing silver concentrations. Periodically, pH, Eh and bacterial population of
the cell suspensions were measured and bacteria transferred to a fresh nutrient medium after reaching the
exponential growth phase.
In order to reproduce the data, the same bacterial culture was used as inoculum in the study of each variable.
2.3. Bioleaching tests
Bioleaching experiments were carried out in 250 mL
bottom-baffled Erlenmeyer flasks incubated at a constant
temperature of 35 °C in an orbital shaker agitating at
215 rpm. Each flask contained 75 mL of 1/10 0K medium,
8 g of low-grade copper ore (10% pulp density, unless
otherwise stated) and the corresponding additives (metal
ions, halides and ferric sulfate) according to each case.
After 1 day of chemical conditioning, the pH of the
leach suspensions was adjusted to 2.0 and 5 mL of an
active microbial culture, previously adapted to the conditions used, was added. Abiotic control tests received
5 mL of 2 g/L thymol in methanol instead of the inoculum.
The redox potential was measured using a Pt combination redox electrode with a Ag/AgCl reference electrode.
pH was monitored using a gel-filled combination pH probe
with a Ag/AgCl reference electrode.
Water lost by evaporation was compensated by adding
deionized water. Periodically, pH was readjusted to 2.0
with 6M H2SO4, the redox potential measured and 0.5 mL
of sample removed from the leach solutions and analyzed
for copper, iron and silver by atomic absorption spectrophotometry. The sample removed was replaced with
1/10 0K nutrient medium. Bacterial population was determined weekly using a counting chamber and an optical
microscope. At the end of the experiments, the solids were
filtered and a chemical analysis of the residues was accomplished to complete the mass balance for the calculation of final copper and iron extractions.
3. Results and discussion
3.1. Preliminary experiments
Fig. 1. Particle size distribution of the reground low-grade copper ore
used in shake flask tests.
A series of preliminary experiments were carried out
previously to test the silver-catalyzed bioleaching process.
These tests were focused on the use of different catalytic
J.A. Muñoz et al. / Hydrometallurgy 88 (2007) 3–18
ions and the use of halides as possible complexing agents
for them.
3.1.1. Effect of different catalytic ions
Different authors have proposed the addition of foreign
ions (Ag, Bi, Sn, Co, etc.) to speed up the dissolution of
7
copper from chalcopyrite-containing ores (Barriga et al.,
1987; Ballester et al., 1990; Escudero et al., 1993).
The catalysis of the low-grade copper ore was tested
with six different metal ions (Ag, Sb, Bi, Co, Mn, Ni and
Sn). Ions were added as soluble solid compounds
(Ag2SO4, Sb2(SO4)3, CoSO4·7H2O, NiSO4·6H2O,
Fig. 2. Effect of different catalytic ions and halides on the bioleaching of the low-grade chalcopyritic ore over 27 days: copper (a and d) and iron
extracted (b and e) and redox potential (c and d) versus time respectively.
8
J.A. Muñoz et al. / Hydrometallurgy 88 (2007) 3–18
MnCl2·4H2O, SnCl2·2H2O) except for bismuth which
was added as liquid previous dissolution of Bi° in diluted sulfuric acid at pH 1.8. All the experiments were
performed at 5% pulp density and with the addition of
1 g cation/kg ore or 2 g Ag/kg ore in the case of silver.
Except for silver, the presence of other metal cations
apparently did not modify the chemistry of the chalcopyrite dissolution process since the enhancement of copper
extraction was negligible compared with the biotic control
test (Fig. 2a). In fact, only the addition of silver effectively
accelerated the copper dissolution kinetics after an
induction period of 10 days. In addition, the amount of
iron dissolved was practically unaffected by the presence
of catalytic ions in the leaching solution (Fig. 2b).
Nevertheless, iron was extracted in higher amounts than
copper in all the experiments except for silver.
The differences observed in copper extraction seem
to be related to the different evolution of the redox
potential when silver is present (Fig. 2c). The control
that silver ions exerts on Eh is the key to understand the
high copper recoveries in this system as will be
discussed later. In all cases, high bacterial populations
(N109 cells/mL) were counted in agreement with the
high potential values recorded.
3.1.2. Effect of halide concentration in the absence of
catalyst
The effect of chloride ions on the chalcopyrite dissolution in acidic solutions has been studied in chemical (Lu
et al., 2000) and in bacterial leaching (Leong et al., 1993;
Kinnunen and Puhakka, 2004). But high copper leaching
rates and extractions were restricted to the use of high
temperatures (N67 °C). A main concern in the microbiological process might be bacterial inhibition by chloride
ions, though concentrations as high as 5 (Kinnunen and
Puhakka, 2004) or 7.5 g/L of Cl− (Leong et al., 1993) have
been reported not to affect the iron oxidation rate by
mesophilic bacteria. Therefore, since halides are toxic
substances to microorganisms, few studies have considered the effect of halides in the bioleaching process of
chalcopyrite. However, in the silver-catalyzed process
they could be used as complexing agents of silver.
A comparative study of the effect of the addition of
halides (chloride and bromide) in the uncatalyzed
chalcopyrite bioleaching was carried out. NaCl and
KBr salts were used to supply chloride and bromide ions
to the diluted nutrient medium.
Copper extractions with halides were low (b 15%) and
always below the values obtained in the biotic control test
to which no halides were added (Fig. 2d). Higher chloride
concentrations, but especially the presence of bromide
ions, had a more detrimental effect on the copper
dissolution process. A similar trend of iron extraction
was observed for all tests with halides (Fig. 2e).
The redox potential of the solution was clearly affected
by the presence of halides (Fig. 2f). This parameter decreased with increasing halide concentration and with
the addition of bromide ions. This suggests that halides,
particularly bromide ions, inhibited the bacterial growth
strongly.
Final metal extractions and Fe/Cu and S(H2SO4) consumed/Cu extracted molar ratios based on the chemical
analysis of the residues are shown in Table 1. The lower
the Fe/Cu molar ratio the higher is the selectivity of the
process to copper in terms of metals in solution. The other
molar ratio gives some indication about the amount of
acid consumed by the ore in terms of copper dissolution
efficiency. For the present tests few differences of these
parameters were found.
3.2. Silver catalysis experiments
Based on the preliminary tests, a new series of tests
were conducted in order to understand the chemistry of
the silver-catalyzed bioleaching of the low-grade copper
ore under study. In all cases silver was added as
Ag2SO4. In this work, silver concentration is sometimes
expressed in g Ag/kg Cu instead of mg Ag/kg ore since
the application of silver in this process is linked to
the copper extraction from chalcopyrite. Further, the
economic efficiency of use of silver for copper extraction is more easily assessed by reporting the specific
addition of silver.
3.2.1. Influence of inoculation
This variable was studied both in the uncatalyzed and
catalyzed-silver bioleaching process. Fig. 3 shows the
evolution of metals extracted (copper and iron) and
Table 1
Final metal extractions and molar ratios based on material balances after 27 days of bioleaching
Test
No halides
2.5 g/L Cl−
5 g/L Cl−
10 g/L Cl−
5 g/L Cl− + 5 g/L Br−
5 g/L Br−
%Cu
%Fe
Fe/Cu
S(H2SO4)/Cu
17.8
16.4
5.5
14.1
17.1
7.3
2.5
14.0
22.8
12.4
3.1
9.1
20.4
14.3
4.2
12.4
15.8
18.3
6.6
17.3
15.8
15.4
5.5
19.4
J.A. Muñoz et al. / Hydrometallurgy 88 (2007) 3–18
9
Fig. 3. Effect of inoculation on the non-catalyzed and catalyzed bioleaching of the low-grade chalcopyritic ore over 15 days: copper (solid lines) and
iron (dash lines) extracted (a and c) and redox potential (b and d) versus time respectively.
redox potential over time in the absence of silver and in
the presence of 14.7 g Ag/kg Cu.
In the uncatalyzed processes poor copper and iron
recoveries were obtained with a slight effect of inoculation (Fig. 3a). However, the redox potential was
clearly influenced by the presence of bacteria in the
leach solution (Fig. 3b). Thus, these results give a good
indication of the refractoriness of the material tested.
In the catalyzed processes the effect of silver on
the copper extraction was markedly affected by inoculation (Fig. 3c) and show that the addition of
silver is not enough to catalyze the copper dissolu-
tion without bacteria. Again clear differences in the
evolution of the redox potential are observed between the inoculated and uninoculated tests (Fig. 3d).
The slightly higher copper dissolution in uninoculated tests than in sterilized tests with thymol can be
attributed to the difficulty of getting complete sterilization of the medium and the detrimental effect
caused by thymol. Brickett et al. (1995) compared
the effect of different bacterial inhibitors and concluded that the use of thymol modifies the ore surface properties overestimating the role of bacteria in
bioleaching.
Table 2
Final metal extractions and molar ratios based on material balances after 15 days of bioleaching
Test
Uninoculated
Uninoculated (thymol)
Inoculated
Uninoculated (Ag)
Uninoculated (thymol + Ag)
Inoculated (Ag)
%Cu
%Fe
Fe/Cu
S(H2SO4)/Cu
18.9
8.4
2.7
13.2
14.1
12.4
5.3
17.6
17.8
16.4
5.5
14.1
29.2
11.2
2.3
9.7
17.6
8.5
2.9
16.1
93.8
19.1
1.2
2.6
10
J.A. Muñoz et al. / Hydrometallurgy 88 (2007) 3–18
As shown in Table 2, the silver-catalyzed
bioleaching of the low-grade copper ore extracted
more copper and enabled more efficient consumption
of acid.
3.2.2. Influence of pulp density
This variable was studied in the range between 2.5
and 15% using 294.1 g Ag/ kg Cu.
The availability of solid substrate can have an important
effect on the bioleaching process. As shown in Fig. 4a, the
percentage of copper extracted during the silver-catalyzed
bioleaching experiments was greatly affected by a high
pulp density whereas minor differences were found for pulp
densities between 5 and 10%. For a pulp density of 15%,
mass transfer processes appear to play an important role. At
lower pulp densities, the copper extraction from the lowgrade ore displays parabolic kinetics and proceeds
according to a two-stage process with a plateau after
reaching the maximum copper dissolution rate. After that,
the system is practically unable to recover more copper.
Since the copper extraction was not complete, the last
unleached fraction must be subjected either to the action of
a protective film or to the diffusion of reactants through the
chalcopyrite surface. Majima et al. (1985) and Dutrizac
(1989) have pointed out that ∼10% of the Cu which
remains in chalcopyrite is lost because the chalcopyrite is
coated with a layer of reaction products that hinder
transport phenomena.
The long lag phase observed in these experiments,
approximately 10 days, would be related to the toxic
effect exerted by the relative high amount of silver used.
In fact, the analyses of silver in solution (results not
shown) indicated that the removal of silver from the
solution was related to the disappearance of the toxic
effect of this element during the lag phase. Of course, this
induction phase can be shortened by culture adaptation
or by using a lower amount of catalysts as will be shown
in later tests.
Fig. 4. Effect of pulp density on the silver-catalyzed bioleaching of the low-grade chalcopyritic ore: copper (a) and iron extracted (b) and redox
potential (c) versus time; and final Cu and Fe extractions (d) after 44 days.
J.A. Muñoz et al. / Hydrometallurgy 88 (2007) 3–18
Table 3
Final metal extractions and molar ratios based on material balances
after 44 days of bioleaching
Pulp density
2.5%
5%
7.5%
10%
15%
%Cu
%Fe
Fe/Cu
S(H2SO4)/Cu
97.1
49.6
3.0
4.6
92.3
28.2
1.8
4.1
86.0
21.8
1.5
4.7
84.2
16.9
1.2
4.5
72.2
9.4
0.8
4.8
The iron was dissolved to a lesser extent in tests with
pulp densities higher than 5% (Fig. 4b). Whether this is
because of iron precipitation (not detected in a SEMEDXR study carried out on several residues) or of a
different dissolution rate than copper, the fact is that the
silver-catalyzed bioleaching process becomes more
selective, dissolving a smaller amount of iron, as the
pulp density increases. At a pulp density of 10% only
10% of Fe was dissolved versus 80% of Cu after
44 days.
11
As in the former experiment, with different pulp
densities there was an effect of silver on the onset of
oxidizing conditions. The redox potential of the solution
was controlled by pulp density until maximum copper
dissolution rates were reached, indicating that a similar
mechanism must be taking place in all tests (Fig. 4c). All
the S-shaped Eh curves are characterized by an initial
drop until a preset redox potential is reached. After that,
the redox potential rapidly rises up to a high oxidizing
potential (∼ 850 mV) and then stabilizes. Interestingly,
high oxidizing conditions correspond to a marked
decrease in copper dissolution.
Fig. 4d shows the final metal recoveries as a function
of pulp density. An increase of the pulp density
decreases the Fe/Cu molar ratio in the leach solution
and, therefore, the process becomes more selective to
copper (Table 3). The S(H2SO4) consumed/Cu extracted
molar ratio practically remains constant (∼ 4.5) in the
range of pulp density studied.
Fig. 5. Effect of silver concentration on the silver-catalyzed bioleaching of the low-grade chalcopyritic ore: copper (a) and iron extracted (b) and redox
potential (c) versus time; and final Cu and Fe extractions (d) after 27 days.
12
J.A. Muñoz et al. / Hydrometallurgy 88 (2007) 3–18
Table 4
Final metal extractions and molar ratios based on material balances
after 27 days of bioleaching
[Ag] (g/kg Cu)
0
14.7
36.8
58.1
147.1
220.6
294.1
%Cu
%Fe
Fe/Cu
S(H2SO4)/Cu
17.8
16.4
5.5
14.1
96.7
31.6
1.9
3.6
97.2
29.1
1.8
3.6
97.2
28.2
1.7
3.6
97.4
28.3
1.7
4.0
89
23.7
1.6
4.9
n.a.
n.a.
n.a.
n.a.
3.2.3. Influence of silver concentration
The preliminary study on the catalytic effect of
different ions demonstrated that the copper extraction
from the low-grade copper ore tested was only accelerated in the case of silver. In the previous experiment, a
silver concentration of 294.1 g/kg Cu was used. However, for economic reasons, the minimum silver
concentration for effective catalysis should be used.
The catalytic effect of silver was tested in the range
between 14.7 and 294.1 g Ag/kg Cu.
The results show that smaller additions of silver
dissolve higher amounts of copper and with faster
kinetics (Fig. 5a). This is a direct consequence of the
disappearance of the induction period observed in previous experiments as in the test with the highest silver
concentration. In a similar way, iron dissolution was also
more favourable with minor amounts of silver (Fig. 5b).
As in the previous tests, similar S-shaped Eh curves were
obtained in the presence of different silver concentrations with the differences being on the onset of oxidizing
conditions. Apparently, the adverse toxic effect of the
highest silver concentration to microorganisms delays
the appearance of oxidizing conditions (Fig. 5c).
The effect of silver concentration on the final metal
extractions is shown in Fig. 5d. Material balances of the
residues indicated that 97% of Cu was extracted in the
tests with silver versus 18% in the absence of silver
(Table 4). The Fe/Cu and S(SO4H2)/Cu ratios were close
to 2.0 and 4.0 respectively in the presence of silver and
Fig. 6. Effect of pH on the silver-catalyzed bioleaching of the low-grade chalcopyritic ore: copper (a) and iron extracted (b) and redox potential (c)
versus time; and final Cu and Fe extractions (d) after 14 days.
J.A. Muñoz et al. / Hydrometallurgy 88 (2007) 3–18
Table 5
Final metal extractions and molar ratios based on material balances
after 14 days of bioleaching
Test
pH 1.2 pH 1.5 pH 1.8 pH 2.0 pH 2.5 pH 3.0
%Cu
85.5
%Fe
38.3
Fe/Cu
2.6
S(H2SO4)/Cu 14.9
91.4
38.9
2.5
7.7
92.1
21.8
1.4
4.6
89.0
9.2
0.5
2.8
82.0
2.9
0.2
1.3
25.9
0
0
1.3
5.5 and 14.0 in its absence. Therefore, the silvercatalyzed process dissolves less iron and consumes less
acid per each mol of copper produced.
Based on these results, it is clear that silver creates the
necessary conditions for a rapid dissolution of chalcopyrite. Considering a random distribution of chalcopyrite in the lower K-ore, the addition of silver must be very
effective as to cover all the chalcopyrite surfaces.
13
3.2.4. Influence of pH
The pH plays an important role in bioleaching
processes in two ways: affecting the chemistry of reactions and establishing the ranges of predominance of
different microorganisms. A range of pH between 3.0
and 1.2 was studied. All experiments were carried out
with a silver concentration of 14.7 g Ag/kg Cu and
inoculated with a bacterial inoculum adapted to the same
amount of silver ion.
The copper dissolution rate was markedly affected at
pH 3.0, with little influence in the range between 1.2 and
2.5 and an optimum pH between 1.5 and 2.0 (Fig. 6a). A
reasonable explanation is that in that range of pH bacterial consortia related to the catalyzed process are more
active than at higher or lower pH. Besides, pH established differences in the initial amount of copper dissolved during the chemical stage (t = 0). Assuming that
the amount of soluble copper present in the ore is similar
Fig. 7. Effect of nutrient medium composition on the silver-catalyzed bioleaching of a low-grade chalcopyritic ore: copper (a) and iron extracted
(b) and redox potential (c) versus time; and final Cu and Fe extractions (d) after 14 days.
14
J.A. Muñoz et al. / Hydrometallurgy 88 (2007) 3–18
Table 6
Final metal extractions and molar ratios based on material balances
after 14 days of bioleaching
Test
Deionized
water
1/10 0K
medium
0K
medium
Cl-free
0K medium
%Cu
%Fe
Fe/Cu
S(H2SO4)/Cu
67.3
2.9
0.3
3.6
93.8
19.1
1.2
2.6
92.5
38.0
2.4
2.7
88.1
38.4
2.6
2.8
in all cases, there must be certain protonic attack responsible for those little differences observed.
Iron, however, was dissolved in higher amounts at
lower pH (Fig. 6b). This suggests that ferric precipitation
occurred during tests carried out at higher pH. After 8 days,
tests at pH 3.0, 2.5 and 2.0 were acid-producing while the
others pH tested were acid-consuming until the end of the
experiments.
Silver in solution was only detected in the experiment at
pH 3.0 (results not shown). For the other pH, silver
remained precipitated practically during all the experiment. That fact could be responsible for the low redox
potentials recorded during the test at high pH unlike the
high values recorded in the rest of tests (Fig. 6c).
Therefore, the bacterial activity in the test at pH 3.0 was
insufficient to create adequate oxidizing conditions for
copper dissolution (Fig. 6c). For the other tests, similar Sshaped Eh curves were obtained but the onset of oxidizing
conditions were established at different times in each case.
These Eh time-dependent differences are probably the
result of changes in the bacterial consortium which, in turn,
was affected by the pH value.
X-ray diffraction analyses (results not shown)
carried out on the residues did not show appreciable
differences in chemical composition, all peaks corresponding to the unreacted material (mainly silicoaluminates and quartz).
The effect of pH on the final metal extractions is
shown in Fig. 6d. The Fe/Cu molar ratio decreases with
increasing pH, indicating a clear dependency of iron
Fig. 8. Effect of initial ferric ion concentration on the silver-catalyzed chemical leaching of a low-grade chalcopyritic ore: copper (a) and iron
extracted (b) and redox potential (c) versus time; and final Cu and Fe extractions (d) after 14 days.
J.A. Muñoz et al. / Hydrometallurgy 88 (2007) 3–18
dissolution with the acid conditions of the medium
(Table 5). At lower initial pH more acid was consumed
but iron extraction practically remained the same. The S
(H2SO4) consumed/Cu extracted molar ratio was an
exponential function of the initial pH.
3.2.5. Influence of the nutrient medium composition
The standard nutrient solution reported in the literature to grow acidophilic Fe- and S-oxidizing bacterial
cultures, the so-called 9K medium, contains different
mineral salts necessary for the metabolic process and
Fe2+ as energy source to sustain bacterial growth. However, iron is frequently omitted in sulfide mineral bioleaching because the amount of iron sulfides is sufficient
per se. In our case, the Fe-free medium used, named 0K
medium, contains the amounts of nutrient salts given by
Silverman and Lundgren (1959).
In practical situations, some of these salts are not
required because some essential elements are present in
the mineral matrix. Therefore, the examination of the
15
chemical composition of the solution in bioleaching
processes may be important to ensure the presence of the
necessary nutrients. Four Fe2+-free nutrient media were
tested: deionized water, 1/10 0K, 0K and Cl-free 0K
(without KCl) at pH 2.0 and with 14.7 g Ag/kg Cu.
All experiments except that with deionized water had a
similar behaviour for copper extraction (Fig. 7a). This
suggests that the low-grade copper ore used was unable to
provide enough salts to the leaching medium to maintain
adequate bacterial activity. However, based on the similar
Eh curves (Fig. 7c) another possible explanation is that
silver is interacting with some anion of the nutrient medium
absent in the ore. In fact the elimination of chloride in the
nutrient medium slightly decreased the copper extraction,
although in this case the bacterial culture used was
different. The concentrated nutrient medium (0K) had a
larger influence on iron than on copper extraction (Fig. 7b).
The effect of nutrient medium composition on the
final metal extractions is shown in Fig. 7d. The Fe/Cu
molar ratio decreased with a decrease of the amount
Fig. 9. Effect of initial ferric ion concentration on the silver-catalyzed biological leaching of a low-grade chalcopyritic ore: copper (a) and iron
extractions (b) and redox potential (c) versus time; and final Cu and Fe extractions (d) after 14 days.
16
J.A. Muñoz et al. / Hydrometallurgy 88 (2007) 3–18
Table 7
Final metal extractions and molar ratios based on material balances after 14 days of bioleaching
Test
0
0.1 g/L
0.25 g/L
0.5 g/L
1 g/L
3 g/L
5 g/L Fe3+
Uninoculated
%Cu
%Fe
Fe/Cu
S(H2SO4)/Cu
29.2
11.2
2.3
9.7
62.6
8.4
0.8
5.0
69.3
18.3
1.6
4.1
68.8
22.2
1.9
4.1
70.6
10.9
0.9
3.0
86.2
4.7
0.3
0.8
96.5
2.5
0.2
0
Inoculated
%Cu
%Fe
Fe/Cu
S(H2SO4)/Cu
93.8
19.1
1.2
2.6
95.0
22.4
1.4
3.4
96.1
28.6
1.7
3.3
96.0
23.5
1.4
2.6
84.9
15.1
1.0
1.2
80.6
6.6
0.4
0.8
95.2
0
0
0
of salts in the nutrient medium, indicating a clear dependency of iron dissolution with this variable (Table 6).
The S(H2SO4) consumed/Cu extracted molar ratio was
slightly higher when deionized water was used as a
leaching medium.
3.2.6. Influence of [Fe 3+ ]
This study was carried out in a range of [Fe3+]
between 0.1 and 5 g/L with a fixed amount of silver
(1.5 g Ag/kg Cu) and in the absence and in the presence
of bacteria. Increasing ferric concentration has an
immediate effect on the pH and, therefore, on the acid
consumption. At high ferric concentration, ferric sulfate
undergoes hydrolysis precipitating iron and releasing
acid to the leaching solution (reaction (5)).
The amount of copper dissolved chemically in the
presence of silver ions depended on [Fe 3+ ]Initial
(Fig. 8a). For the pulp density tested (10%), the effect
was more pronounced at [Fe3+] N 3 g/L.
In the presence of bacteria, the results at high [Fe3+]
were similar after the chemical stage (Fig. 9a). However,
the final copper extractions reached similar values for all
experiments. Since the acid-soluble copper in the ore is
very low, copper extraction largely depends on the effectiveness of chalcopyrite dissolution.
Iron was extracted more readily from chalcopyrite at
low ferric concentrations both in uninoculated and
inoculated experiments (Figs. 8b and 9b, respectively).
At [ Fe3+]Initial N 1 g/L, iron precipitation on chalcopyrite
surfaces probably prevents further reaction. The redox
potential was practically not affected in the absence of
bacteria, being dramatically affected in its presence
(Figs. 8c and 9c, respectively). In abiotic experiments
the potential is controlled by chemical reactions between the solution and the ore in such a way that all tests
with ferric ion reach a common value of potential near
650 mV. In biotic experiments biochemical reactions
play an important role in variations of Eh and all tests,
including the one without initial ferric ions, tend to a
same value of potential higher than 850 mV. Bacterial
population counts (results not shown) indicated that
bacteria were able to tolerate a ferric concentration as
high as 5 g/L.
The effect of ferric concentration on the final metal
extractions in abiotic and biotic experiments is shown in
Figs. 8d and 9d, respectively. Material balances showed
that Fe/Cu and S( H2SO4)/Cu ratios increased with
decreasing [Fe3+] from 5 to 0 g/L (Table 7). From these
results it is clear that ferric ions together with silver ions
make a good lixiviant for chalcopyrite. But ferric ion has
to be maintained above certain critical value (N650 mV)
for the process to be effective.
4. Conclusions
Several conclusions can be drawn from this study:
The preliminary studies showed that, of the different
metal cations tested, silver was the only effective catalyst on the copper bioleaching of a low-grade chalcopyritic ore. Furthermore, in the absence of silver coper
dissolution was not enhanced by the addition of halides
(Cl− and Br−).
Silver concentration is a key factor in the copper
dissolution from low-grade ores. There is an optimum
silver concentration at which copper is preferentially
extracted from chalcopyrite. Small amounts of silver
catalyze the process whereas high silver concentrations
diminish the copper extraction.
pH and nutrient medium composition had a marked
influence on the copper bioleaching. At pH ~ 3.0 the
catalytic effect of silver disappears. The bioleaching of
the low-grade copper ore requires a minimum amount of
nutrient salts to obtain the maximum efficiency.
An increase of ferric ion concentration negatively
affects the silver-catalyzed bioleaching but the effect is
markedly positive on the silver-catalyzed chemical
J.A. Muñoz et al. / Hydrometallurgy 88 (2007) 3–18
leaching of the low-grade ore tested. The experiments have
demonstrated that for the tests with silver the onset of
oxidizing conditions occurs at an Eh value slightly higher
than 650 mV. When that critical value of potential is surpassed the copper dissolution rate slows down.
The Fe/Cu molar ratio decreases and, therefore, the
selectivity of the silver-catalyzed process increases with:
inoculation and low silver concentrations.
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