Parameters for Control and Optimization of Bioleaching of Sulfide Minerals

Abstract

Bioleaching/biooxidation is essentially a dissolution process with the involvement of acidophilic bacteria acting as the "catalyst" to accelerate the dissolution of metals from sulfide minerals. The contribution of bacteria to the metal dissolution is closely controlled by the growth of bacteria, which is itself affected by the physico-chemical conditions within the bioleaching environment. There are a number of operating parameters controlling bioleaching processes, which are required to be maintained within a certain range in the leaching environment whereby the activity of bacteria with the resultant oxidation of sulfide minerals can be optimized. In this regard temperature, acidity, oxidizing conditions, availability of nutrients, oxygen and carbon dioxide, surface area and presence of toxic ions are of prime importance for control and optimization of bioleaching of sulfide ores/concentrates. Bioleaching processes are temperature and pH dependent with optimum metal dissolution occurring in a particular range where the bacterial strain is most active e.g. mesophiles at 35-40°C and pH 1.6-2.0. Provision of nutrient salts is required to maintain the optimum growth and hence metal dissolution with the quantity of nutrients apparently being dependent on the availability of substrate i.e. head grade/pulp density of an ore/concentrate. Oxygen transfer is one of the most critical factors since the oxygen levels below 1-2 mg/l may adversely affect the oxidizing activity of bacteria. Bioleaching rate tends to improve with increasing the surface area at low pulp densities but, in practice, the pulp density is limited to ~20% w/v. Increasing concentrations of ions such as Cl -may also adversely affect the oxidative activity of bacteria.

Full text

Parameters for Control and Optimization of Bioleaching of Sulfide Minerals
H. Deveci1, A. Akcil2 and I. Alp1
1Karadeniz Technical University, Mining Eng. Dept., TR61080 Trabzon, Turkey
2Suleyman Demirel University, Mining Eng. Dept., TR32260 Isparta, Turkey
Keywords: Bioleaching, Biooxidation, Acidophilic bacteria
Abstract
Bioleaching/biooxidation is essentially a dissolution process with the involvement of acidophilic
bacteria acting as the “catalyst” to accelerate the dissolution of metals from sulfide minerals. The
contribution of bacteria to the metal dissolution is closely controlled by the growth of bacteria, which is
itself affected by the physico-chemical conditions within the bioleaching environment.
There are a number of operating parameters controlling bioleaching processes, which are required to be
maintained within a certain range in the leaching environment whereby the activity of bacteria with the
resultant oxidation of sulfide minerals can be optimized. In this regard temperature, acidity, oxidizing
conditions, availability of nutrients, oxygen and carbon dioxide, surface area and presence of toxic ions
are of prime importance for control and optimization of bioleaching of sulfide ores/concentrates.
Bioleaching processes are temperature and pH dependent with optimum metal dissolution occurring in a
particular range where the bacterial strain is most active e.g. mesophiles at 35-40°C and pH 1.6-2.0.
Provision of nutrient salts is required to maintain the optimum growth and hence metal dissolution with
the quantity of nutrients apparently being dependent on the availability of substrate i.e. head grade/pulp
density of an ore/concentrate. Oxygen transfer is one of the most critical factors since the oxygen levels
below 1-2 mg/l may adversely affect the oxidizing activity of bacteria. Bioleaching rate tends to improve
with increasing the surface area at low pulp densities but, in practice, the pulp density is limited to ~20%
w/v. Increasing concentrations of ions such as Cl- may also adversely affect the oxidative activity of
bacteria.
Introduction
Biooxidation of refractory, gold bearing arsenopyrite/pyrite concentrates as a pretreatment step prior to
cyanidation have already proved an economically viable and competitive process with reduced
environmental impact and low capital costs involved [1,2]. This has stimulated the extension of the
technology to the treatment of low grade and/or difficult-to-treat ores/concentrates in particular, for the
recovery of copper, nickel, cobalt and zinc [2,3].
Biooxidation/bioleaching is essentially a dissolution process with the aid of acidophilic bacteria to
enhance the dissolution of metals from minerals. The exploitation of acidophilic chemolitotrophic
bacteria in mineral leaching is inherently based on the ability of these microorganisms to derive the
energy required for their growth and other metabolic functions from the oxidation of ferrous iron (Eq.1)
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and/or elemental sulfur (Eq.2) or reduced sulfur compounds [4,5]. The oxidation products, ferric iron
and/or sulfuric acid, act as lixiviant for the oxidative dissolution of sulfide minerals (MeS) (Eq.3) in
acidic environments [2,6]. However, sulfide minerals may exhibit different dissolution characteristics in
bioleaching environments i.e. during the oxidation of pyrite (FeS2) thiosulfate is the primary sulfur
intermediate while the oxidation of sphalerite (ZnS) proceeds via the formation of polysulfides [6].
2Fe2+ + 1/2O2 + 2H+bacteria
→ 2Fe3+ + H2O(1)
S0 + 3/2O2 + H2O bacteria
→ H2SO4 (2)
MeS + 2Fe3+ → Me2+ + 2Fe2+ + S0(3)
Although a variety of iron- and sulfur-oxidizing microorganisms may be involved in the oxidation of
sulphide minerals (Table I), mesophilic bacteria e.g. Acidithiobacillus ferrooxidans (previously
Thiobacillus ferrooxidans), Leptospirillum ferrooxidans and At. thiooxidans (previously T. thiooxidans
operating at ≤40°C are the most commonly used microorganisms for the bioleaching of sulphide
minerals within ore dumps/heaps or commercial bioreactors [2]. Thermophilic bacteria with their ability
to operate at high temperatures up to 85°C have great potential for use in bioleaching processes probably
due to the improvement expected in the kinetics of metal dissolution particularly from the recalcitrant
minerals such as chalcopyrite [2,7,8].
Table I Classification of acidophilic bacteria used in bioleaching processes according to operating
temperature
Bacteria Type Culture Operating Temperature Range
Mesophile At. ferrooxidans, L. ferrooxidans, At.
thiooxidans
20-40 °C
Moderate
Thermophile
S. termosulfidooxidans, S. acidophilus, At.
caldus, A. ferooxidans
40-55 °C
Extreme Thermophile Sulfolobus-like archaea, S. metallicus,
A. brierleyi
55-85 °C
Bioleaching of sulfide minerals is naturally a complex process since chemical and microbiological
reactions occur concomitantly within the system. The strains of bacteria used as the mediator of
oxidative reactions (Eq.1-2) themselves establish optimum conditions under which they optimally grow.
The optimum growth conditions could be adjusted to maximize the rate and extent of metal dissolution
from sulfide ores/concentrates [9]. This assumes the primary consideration is the overall chemical and
microbiological aspects of the leaching process.
There are a number of factors controlling the activity of bacteria with the resultant oxidation of substrate
(i.e. sulfide minerals), which have to be identified and maintained within a certain range in the leaching
environment in order to optimize bioleaching performance. In this paper, the parameters including
temperature, acidity, oxidizing conditions, availability of nutrients, oxygen and carbon dioxide, surface
area and presence of toxic ions that are deemed of prime importance for the control and optimization of
a bioleaching process was examined and their effect on the dissolution process was discussed.
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Parameters for Control and Optimization of Bioleaching of Sulfide Minerals
Temperature
Optimum activity of each type of bacteria takes place in a relatively well-defined range of temperatures
(Table I) at which these microorganisms operate most efficiently. This indicates the temperature
dependent character of bioleaching processes.
Figure 1 illustrates the temperature dependence of oxidizing activity of mixed cultures suggesting an
optimum temperature of 35°C and 50-55°C for mesophilic MES1 (dominated by At. ferrooxidans-like
bacteria) and moderately thermophilic MOT6 respectively. The decrease in the oxidative activity of the
bacteria at temperatures beyond the optimum may be attributed to the likely denaturation of the proteins
involved in the oxidizing system of the bacteria [10]. The data presented in Figure 1 are consistent with
the optimum temperatures reported for a variety of mesophilic and moderately thermophilic bacteria
[11,12]. The optimum temperature may differ for the growth of a particular strain of bacteria. Sampson
[13] and Guay et al. [14] determined the optimum temperatures of 37 °C and 35 °C respectively for the
oxidation of ferrous iron by At. ferrooxidans. Both L. ferrooxidans and At. thiooxidans are known to be
more tolerant to temperature than At. ferrooxidans with their ability to operate efficiently at 40°C at
which biooxidation of gold concentrates is commercially practiced [1].
One important feature of hydrometallurgical operations is the temperature dependency of dissolution
process such that the rate and extent of dissolution of sulfides increases with temperature. However, this
can be partially applied to bioleaching processes. In effect, these processes establish a certain
temperature range beyond which the rise in the rate of dissolution with temperature is not commensurate
with the decrease in the oxidizing activity of bacteria. This may be ascribed to the likely denaturation of
the proteins caused by the increase in the rate of thermal death of the microorganisms [10]. In this
respect, the optimum temperature for the bioleaching operations may well be defined as the temperature
at which the rate of biooxidation of desired minerals is maximised.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
20 30 40 50 60 70
Temperature (°C)
Oxygen uptake rate (mg/l/min)
Mesophilic bacteria
Moderately
thermophilic bacteria
Figure 1: Effect of temperature on the rate of oxygen uptake during the oxidation of ferrous iron (100
mM, pH 1.7) by the mixed cultures, mesophile MES1 and moderately thermophile MOT6.
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The oxidation of sulfide minerals is an exothermic reaction and in large bioreactors this results in a
gradual rise of temperature or in heap or dump leaching operations in the development of “hot spots”.
Since mesophilic strains of bacteria operate in an optimum temperature range of 20-40°C, over-heating
due to exothermic reaction would cause a sharp decrease in the dissolution rate. The process may
eventually cease at 45-50°C because these temperatures are beyond the upper limit of these
microorganisms. Accordingly, bioleaching systems require intimate control of operating temperature to
maintain optimum range for the activity of bacteria i.e. 35-40°C for mesophiles.
Acidity
Acidity of the environment controls the bacterial activity within a system. The H+ ion is in fact vital for
acidophilic microorganisms since it is utilized by bacteria as a proton source for the reduction of O2
[15]. It is therefore one of the principal components in the mechanism of derivation of energy by
bacteria from the oxidation of substrate.
0
4
8
12
16
20
1.2 1.4 1.6 1.8 2.0
pH
Dissolution rate of zinc (mg/l/h)
Mesophilic
Bacteria
(30°C)
Moderately
Thermophilic
Bacteria
Figure 2: Effect of pH on the leaching activity of mesophilic (WJM mixed culture) and moderately
thermophilic (S thermosulfidooxidans) bacteria during bioleaching of a complex sulphide ore (16.2%
Zn, 7.95% Fe, 5.6% Pb, 15.2% S).
Figure 2 shows the effect of pH on the dissolution rate of zinc from a complex sulfide ore by mesophilic
and moderately thermophilic bacteria. The bioleaching efficiency by mesophiles and moderate
thermophiles tended to increase with decreasing acidity (pH 1.0-2.0) although a slight decrease above
pH 1.6 for moderately thermophilic bacteria was apparent. Statistical analysis of data indicated that the
differences in the performance of both mesophiles were insignificant in the pH range of 1.6-2.0. This
was consistent with the optimum pH 1.5 to 2.3 for bacterial leaching/oxidation of most sulphide
minerals/ferrous iron reported [10]. However, in commercial applications the operating pH is often
lower than the optimum values for bacteria e.g. pH 1.2-1.8 for the BIOX® process [1] and pH 1.3-1.5 for
the BacTech® process [16]. The optimum pH range may be identified as that the optimum growth of
bacteria and the most efficient oxidation of minerals are attained.
In a given bioleaching environment, acidity would probably be controlled by the oxidation of iron,
sulfur and metal sulfides as well as by the dissolution of carbonate minerals and by the formation of
ferric precipitates. The latter is in particular undesirable since it may adversely affect the progress of
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dissolution process by forming a protective layer on the mineral surface [17]. Therefore, the acidic
environment should be maintained to minimize or preclude the ferric iron precipitation. This may also
determine the upper pH limit for optimum operation of a bioleaching process and the highest level of
acidity that bacteria can tolerate can establish the lower pH limit. In heap and dump leaching practice,
acidity of the leaching environment is extremely difficult to control while in tank leaching operations the
optimum range of acidity can be readily maintained.
Nutrient Requirements and Culture Media
A culture medium for isolation and growth of bacteria is essentially a mixture of necessary chemical
compounds to provide all the elements required for cell mass production and sufficient energy for
biosynthesis and maintenance [18]. A typical nutrient solution is mainly composed of nitrogen
introduced as an ammonium salt, phosphorus as a potassium salt of phosphoric acid, magnesium as
magnesium sulfate and other salts such as calcium nitrate or calcium chloride are sometimes added. A
number of growth media (Table II) essentially as derivatives of above mentioned compounds in varying
amounts have been proposed for microbial leaching studies among which the often-quoted “9K”
medium [19] may be the most extensively utilized liquid medium. One of the major detractions to “9K”
liquid medium is the possible precipitation of phosphate, potassium and ammonium as jarosites due to
their high concentrations in the medium.
A minimum concentration of salts in the liquid medium is essential to maintain the desired level of
bacterial activity as illustrated in Figure 3. The occurrence of comparable growth as indicated by the
dissolution rate of zinc in both ES (enriched salt solution) and 0.1ES media suggested that quantity of
nutrient salts in 0.1ES media (despite being 10 times diluted) was nearly adequate to support the build-
up of biomass and to achieve significant dissolution rate of metal (Figure 3). The limitation of the
bacterial growth in DDW (double distilled water) and TW (tap water) appeared to be due to the limited
availability of the nutrient components of enriched salt solution (ES). The better bioleaching activity of
microorganism in TW than in DDW was most likely due to the presence of anions and cations at
relatively high concentrations in TW i.e. Mg2+ (2.93 ppm c.f. <0.001 ppm), PO43- (0.31 c.f. 0.15 ppm)
and NO3- (5.4 c.f. 0.1 ppm).
Gomez et al. [20] investigated the influence of growth media using five different media formulations,
with varying the concentrations of salts, on the bioleaching of complex bulk concentrate by a mixed
culture. They observed significantly higher extractions of metals (Zn, Cu and Fe) within 9K medium
than those within the Norris medium (Table II). The concentration of salts in the other three media used
by Gomez et al. [20] was significantly lower than that in the Norris medium.
Table II Different formulations of nutrient media used in bioleaching/biooxidation studies
Nutrient Media (NH4)2SO4
(g/l)
MgSO4.7H2O
(g/l)
KH2PO4
(g/l)
KCl
(g/l)
Ca(NO3)2.H2O
(g/l)
9K 19] 3 0.5 0.5 0.1 0.01
T&K [21] 0.4 0.4 0.4 - -
ES [22] 0.2 0.4 0.1 0.1 -
Leathen [23] 0.15 0.5 0.01 0.05 0.05
Norris [20] 0.2 0.2 0.2 - -
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0
4
8
12
16
ES 0.1ES DDW TW
Dissolution rate of zinc (mg/l/h)
At. ferrooxidans (pH 1.7 & 30°C)
Figure 3: Effect of growth media on the activity of At. ferrooxidans during bioleaching of a complex
sulphide ore (“ES”: Enriched salt solution (Table II), “0.1ES”: 10 times diluted ES, “DDW”: Double
distilled water, “TW”: Tap water).
The data in Figure 3 suggest that the limitation of the extraction of metals in the Norris medium
observed by Gomez et al. [20] was probably due to the high metal content of the bulk concentrate
(17.1% Zn, 25.0% Fe and14.0% Cu) and operating pulp density (5% w/v) compared with the ore sample
(1% w/v) used in the current study (16.2% Zn, 7.95% Fe, 5.6% Pb, 15.2% S). It can be also inferred
from these data that the requirement of growth media i.e. the concentrations of salts to be added would
be determined by the quantity of the substrate available (i.e. head grade and/or pulp density) for bacterial
oxidation.
Oxygen and Carbon Dioxide Transfer
The bacteria involved actively in the biodegradation of sulphide minerals are, in general, autotrophic
aerobes and hence the oxidizing activity of bacteria depends largely on the availability of oxygen and
carbon dioxide. Oxygen and carbon dioxide are required for these microorganisms to complete the
cycle of respiration. During the respiration process oxygen functions as the terminal electron acceptor
while carbon derived from the fixation of the carbon dioxide is utilized in the synthesis of biomolecules
[18]. In this respect, oxygen and carbon dioxide mass transfer to a given system to support the bacterial
activity is one of the most important factors in the bioleaching processes.
The solubility of oxygen (~7.8 mg/l) in water at 30°C (Figure 4) would not be sufficient even to oxidize
(stoichiometrically) 0.1 g ferrous iron. Additionally, the role of bacteria in these processes appertains to
the growth of the bacteria and carbon is the major component in cellular material. Therefore, the gas
phase (oxygen and carbon dioxide) is continuously transferred from an external environment i.e. air to
liquid phase so as to fulfill the requirement of the system for both gases. The rate of gas transfer from
atmosphere to a liquid phase (assuming that no gas consuming chemical reaction takes place in the
medium) as a function of mass transfer coefficient and concentration driving force can be expressed as
follows:
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0
2
4
6
8
10
12
14
16
0 20406080100
Temperature (°C)
Oxygen solubility (mg/l)
Figure 4: Solubility of oxygen in water at different temperatures (1 atm air).
OTR = kLa (C*-C) (4)
where OTR is the rate of gas transfer; kL is the mass transfer coefficient; a is the gas-liquid interfacial
area per unit liquid volume; C* is the saturated dissolved gas concentration and C is the actual dissolved
gas concentration of the liquid phase. The saturated dissolved gas concentration (C*) depends on a
number of parameters. These include the gas concentration in the gas phase (i.e. enriched or normal
air), operating temperature (Figure 4), presence of solids and dissolved ionic and/or non-ionic species in
the liquid phase.
Bioleaching systems assert a lowest limit, known as the critical dissolved oxygen concentration below
which the activity of bacteria is limited due to the inadequate concentration of dissolved oxygen in the
medium. Liu et al. [24] observed that the growth of At. ferrooxidans on ferrous iron was limited at a
dissolved oxygen concentration of 0.7 mg/l and the oxidation process completely ceased at the
concentrations below 0.2 mg/l. Myerson [25] reported that dissolved oxygen concentration was not
limiting in bioleaching of pyrite insofar as the concentration was maintained at above 0.5 mg/l. The
highest value of dissolved oxygen concentration with 1.2 mg/l below which bioleaching of a
cobaltiferous pyrite became limited (at 20% pulp density) was reported by d’Hugues et al. [26]. Figure
5 illustrates that the critical dissolved oxygen concentration depends on the type of bacteria used and the
concentration of substrate available in the bioleaching medium. Therefore, the demand for oxygen
would be dependent on the sulfide content of the feed and accordingly on the solids concentration and
determination of the minimum concentration is required for a particular feed in a given operation.
Otherwise, based on the above citations and the data presented in Figure 5, a minimum level of
dissolved oxygen of >1-2 mg/l is to be maintained for an optimum operation in a given bioleaching
system.
In addition to oxygen, the adequate supply of carbon dioxide is a prerequisite for cell growth. Norris
[27] revealed that the limitation of CO2 to the activity of the thermophilic strains of bacteria was almost
completely ameliorated with the introduction of 0.1% (v/v) CO2 in air. Witne and Phillips [28] observed
significant improvements (for thermophiles in particular) in the bioleaching of pyrite and copper
concentrates by the enrichment of air with oxygen and carbon dioxide.
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0.0
1.0
2.0
3.0
4.0
5.0
0 50 100 150 200
[Fe+2] (mM)
Critical Oxygen Concentration
(mg/l)
Moderately thermophilic bacteria (50°C)
Mesophilic bacteria (30°C)
Figure 5: Critical dissolved oxygen concentration for mixed cultures of mesophilic (MES1) and
moderately thermophilic (MOT6) bacteria at different concentrations of ferrous iron.
Boon and Heijnen [29] examined the kinetic data in the literature with respect to the oxygen and carbon
dioxide limitation and concluded that, in most cases, the observed decrease in the biooxidation rates at
high solids densities was, to a large extent, probably as a consequence of exhaustion of carbon dioxide
in the liquid phase. These findings imply the positive effect of using carbon dioxide enriched air in
bioleaching processes particularly using thermophilic bacteria.
Particle Size and Pulp Density
Bacterial oxidation of sulphide minerals occurs through surface chemical reactions via the attachment of
bacteria and/or the leaching by bacterially generated ferric iron and/or acid. Therefore, the increased
surface area through particle size reduction would lead to a higher rate and extent of extraction as shown
in Figure 6. However, the extent of size reduction required is of practical importance. The complete
liberation of valuable minerals may not be necessary considering the expensive nature of comminution
process, particularly fine grinding. In practice biooxidation of refractory gold concentrates is carried out
at a particle size of -75 µm [1]. The optimum particle size requires a compromise between the size
reduction costs and the improved kinetics and recoveries.
In addition to particle size, the pulp density of the concentrate or ore determines the available surface
area for the bioleaching process. It is of particular interest to operate bioleaching processes at high
solids concentrations due to process economics. However, there are certain practical limitations to
increasing pulp densities [30,31] and operating pulp density is often limited to a threshold level of 20%
solids by weight in industrial stirred tank biooxidation practice [1].
Figure 7 demonstrates that the residence time required to achieve the desired level of metal extraction
tends to increase with increasing pulp density. The adverse effect of increasing pulp density can be
attributed mainly to the decrease in bacteria-to-solid ratio, mechanical damage to bacterial cells by solid
particles, the inhibitory effect of increasing concentrations of metal ions in solution and the limited
availability (i.e. transfer) of oxygen and carbon dioxide [29,30-33].
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0
20
40
60
80
100
0 50 100 150 200 250
Time in hours
Zinc extraction (%)
-20 µm -45+20 µm
-63+45 µm -90+63 µm
-125+90 µm -250+125 µm
Figure 6: Effect of particle size on the extraction of zinc from a complex sulphide ore (1% w/v) (16.2%
Zn, 7.95% Fe, 5.6% Pb, 15.2% S) using At. feroooxidans at 30°C and pH 1.7.
Komnitsas and Pooley [33] argued that at high pulp densities with an increase in the relative surface area
the population of bacteria in solution phase could become too low to maintain a high ferric/ferrous ratio.
This has the implication of the accumulation of ferrous iron and arsenite (As3+) in solution. The latter
would probably lead to the product inhibition given the relatively high toxic character of As3+ in
comparison with As5+ (ferric iron aids the oxidation of As3+ to As5+). The accumulation of ferrous iron
would gradually deteriorate the oxidizing conditions i.e. low redox potential producing the unfavourable
conditions for the oxidation of pyrite in particular. Deveci et al. [32] also observed that the capability of
bacteria to maintain oxidising conditions required to efficiently drive the extraction of zinc (>400 mV vs
Ag/AgCl) deteriorated as the pulp density increased. Increasing the bacteria-to-solid ratio via using a
strong inoculum the authors noted a significant improvement in the dissolution rate and extent of zinc at
high pulp densities. Boon and Heijnen [29] concluded that the limited availability of oxygen and carbon
dioxide was the main cause for the limitation in the kinetics of metal dissolution at high pulp densities.
Deveci [31] showed that the attrition of bacterial cells by solid particles occur in stirred tank reactors
resulting in the loss of viability of bacterial cells and the first order deactivation rate of bacterial
population increases exponentially with increasing the concentration of solids and becomes significant at
≥20% w/w.
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0
20
40
60
80
100
3% 5% 8% 10% Control
Pulp density (% w/v)
Extraction of zinc (at t=216 h) (%)
Figure 7: Effect of pulp density on the extraction of zinc from a complex sulfide ore (16.2% Zn, 7.95%
Fe, 5.6% Pb, 15.2% S) using mixed mesophilic bacteria (MES1) at 30°C and pH 1.7.
Toxicity
Resistance to the toxic effect of metal ions is peculiar to the microorganisms utilised in the bioleaching
processes where metals are solubilised and released from ore/concentrate into leaching environment.
The concentrations of some elements in solution may exert a toxic effect on the bacteria. The intrinsic
toxicity of a cation or anion will probably manifest itself as the reduction in the ability of bacteria to
complete the oxidation of a substrate. Different strains of bacteria exhibit varying sensitivity to
toxicants.
Tuovinen et al. [34] showed that At. ferrooxidans was able to oxidize ferrous iron in the presence of high
concentrations (10 g/l) of Zn, Ni, Cu, Co, Mn and Al whilst Ag and anions of Te, As and Se were
proved to have an inhibitory effect on the iron oxidizing activity of the bacteria at concentrations of only
50-100 mg/l. The adaptation of bacterial species to a particular environment is therefore essential to
mitigate for the inhibitory effects of toxic metal ions or of increasing concentrations of metals. This
would enhance the rate and extent of oxidation in bacterial leaching systems. Natarajan [35] reported the
development of specially adapted strains of At. ferrooxidans with tolerance for metals; 50 g/l Cu and 72
g/l Ni.
Furthermore, the quality (i.e. salinity) of process water available may be of practical importance for the
application of a bioleaching process using mesophilic bacteria since the chloride ions (≥1% Cl-) suppress
the bioleaching activity of these microorganisms (Figure 8). However, the extreme thermophiles can
operate successfully under extremely saline conditions with no discernible effect on their activity
(Figure 8). It is however of importance to note that the formation of jarosite-type precipitates could
present problems particularly in the extremely thermophilic systems (70°C) due to the availability of
counter ions e.g. Na+ in saline environments. Such precipitates could be detrimental to the metal
dissolution due to the encrustation of the precipitates on the unreacted mineral surface hindering the
progress of the dissolution process. Therefore the suitability of local water for the process should be
tested during the development of a bioleaching process.
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0
4
8
12
16
0.0 1.0 2.0 4.0
Concentration of chloride added (% Cl-)
Dissolution rate of zinc (mg/l/h)
Mesophilic Bacteria (30°C)
Extremely Thermophilic Bacteria (70°C)
Figure 8: Effect of added chloride on the extraction rate of zinc from a complex sulfide ore (16.2% Zn,
7.95% Fe, 5.6% Pb, 15.2% S) using mesophilic (WJM mixed culture at pH 1.4) and extremely
thermophilic (S. metallicus at pH 1.2) bacteria.
Conclusions
In bioleaching of sulfide minerals, the rate and extent of metal dissolution are closely governed by the
activity/growth of bacteria and the chemistry of the dissolution process. Temperature, pH, growth
media, availability of oxygen and carbon dioxide, surface area (i.e. particle size and pulp density) and
toxic effects of metal ions in solution may be identified as the main parameters of fundamental
importance for optimization of biooxidation/bioleaching of sulfide minerals. These parameters would
control the activity of bacteria and hence the dissolution of sulfide minerals. Following conclusions can
be drawn from the current study:
i) The optimum metal dissolution would occur in a well-defined range of temperature and pH where
the bacteria used thrives most effectively i.e. 35-40°C and pH 1.6-2.0 for mesophiles and 50-55°C
and pH 1.6-2.0 for moderate thermophiles.
ii) Addition of nutrient salts is essential to maintain the optimum bioleaching activity with the quantity
of nutrient salts to be added being dependent apparently on the head grade and/or pulp density.
iii) A dissolved oxygen concentration above a critical level (>1-2 mg/l) that depends on the type of
bacteria and the availability of substrate is required to be maintained within the bioleaching
environment for an optimum metal dissolution.
iv) Increase in the surface area via size reduction enhances the bioleaching efficiency at low pulp
densities i.e. at 1% w/v, but, increase in pulp density may adversely affect the dissolution process
with ~20% w/v being regarded as the threshold level for an industrial operation.
v) Quality of process water is of practical importance since salinity (≥1% Cl-) adversely affects the
activity of mesophiles albeit the extreme thermophiles can operate efficiently under saline
conditions.
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