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Parameters for Control and Optimization of Bioleaching of Sulfide Minerals

  • Süleyman Demirel University/Beijing University of Chemical Technology/Nazarbayev University

Abstract and Figures

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.
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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
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
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
Bacteria Type Culture Operating Temperature Range
Mesophile At. ferrooxidans, L. ferrooxidans, At.
20-40 °C
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
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.
20 30 40 50 60 70
Temperature (°C)
Oxygen uptake rate (mg/l/min)
Mesophilic bacteria
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 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.
1.2 1.4 1.6 1.8 2.0
Dissolution rate of zinc (mg/l/h)
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
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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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
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
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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
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 50 100 150 200
[Fe+2] (mM)
Critical Oxygen Concentration
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 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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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.
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
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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0.0 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.
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
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Modeling, Control and Optimization in Nonferrous and Ferrous Industry
1. D.W. Dew, E.N. Lawson, J.L. Broadhurst, “The BIOX® process for biooxidation of gold bearing
ores or concentrates,” Biomining: Theory, Microbes and Industrial Processes, ed. D.E. Rawlings
(Berlin: Springer-Verlag, 1997), 45-79.
2. D.E. Rawlings, D. Dew, C. du Plessis, “Biomineralization of Metal Containing Ores and
Concentrates,” Trends in Biotechnology, 21 (1) (2003), 38-44.
3. P.C. Miller et al., “Commercialisation of bioleaching for metal extraction,” Minerals and
Metallurgical Processing,16 (4) (1999), 42-50.
4. W.J. Ingledew, “Thiobacillus ferrooxidans: The bioenergetics of an acidophilic
chemolithotroph,” Biochimica et Biophysica Acta, 683 (1982), 89-117.
5. I. Suzuki, “Microbial leaching of metals from sulphide minerals,” Biotechnology Advances, 19
(2001), 119-132.
6. W. Sand et al., “(Bio)chemistry of bacterial leaching-direct vs. indirect bioleaching,”
Hydrometallurgy, 59 (2-3) (2001), 159-175.
7. P.R. Norris, N.P. Burton and N.A.M. Foulis, “Acidophiles in Bioreactor Mineral Processing,”
Extremophiles, 4 (2000), 71-76.
8. P. d’Hugues et al., “Continuous Bioleaching of Chalcopyrite Using a Novel Extremely
Thermophilic Mixed Culture,” International Journal of Mineral Processing, 66 (2002), 107-
9. K. Bosecker,. “Bioleaching: Metal Solubilization by Microorganisms,” FEMS Microbiology
Reviews, 20 (1997), 591-604.
10. A.E. Torma, “The Role of Thiobacillus ferrooxidans in Hydrometallurgical Processes,”
Advances in Biochemical Engineering, eds. T.K. Ghose, A. Fiechter and N. Blakebrough (Berlin:
Springer-Verlag, 1977), 1-37.
11. A.P. Harrison, “Genomic and Physiological Diversity Amongst Strains of Thiobacillus
ferroxidans and Genomic Comparison with Thiobacillus thiooxidans,” Arc. Microbiology, 131
(1982), 68-76.
12. D.B. Johnson et al.,Biodiversity of acidophilic moderate thermophiles isolated from two sites
in Yellowstone National Park and their roles in the dissimilatory oxido-reduction of iron,”
Biodiversity, Ecology and Evolution of Thermophiles in Yellowstone National Park, eds. A.L.
Resenbach and A. Voytek (New York NY: Plenum Press, 2000).
13. M.I. Sampson, “Influence of the Cell Properties of Acidophilic Bacteria During Attachment to
Mineral Sulphides and Consumption of Oxygen During the Oxidation of Ferrous Iron” (PhD
Thesis, Camborne School of Mines, University of Exeter, 1999).
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14. R. Guay, J. Ghosh, A.E. Torma, “Kinetics of Microbiological Production of Ferric Iron for Heap
and Dump Leaching,” Biotechnology in Mineral and Metal Processing, eds. B.J. Scheiner, F.M.
Doyle and S.K. Kawatra (Colorado: SME Inc, 1989), 95-106.
15. W.A. Apel, P.R. Dugan, “Hydrogen Ion Utilisation by Iron-grown Thiobacillus ferrooxidans,”
Metallurgical Applications of Bacterial Leaching and Related Microbiological Phenomena, eds.
E.L. Murr, A.E. Torma and J.A. Brierley (New York NY: Academic Press, 1978), 45-59.
16. P.C. Miller, The Design and Operating Practice of Bacterial Oxidation Plant Using Moderate
Thermophiles,” Biomining: Theory, Microbes and Industrial Processes, ed. D.E. Rawlings
(Berlin: Springer-Verlag, 1997), 81-100.
17. H. Deveci, “Bioleaching of Complex Zinc/Lead Sulphides Using Mesophilic and Thermophilic
Bacteria” (Paper presented at IXth International Mineral Processing Symposium 18-20 September
2002, Cappadocia, Turkey).
18. G. Rossi, Biohydrometallurgy (Hamburg: McGraw-Hill, 1990) 595.
19. M.P. Silverman, D.G. Lundgren, “Studies on the Chemolithotrophic Iron Bacterium
Ferrobacillus ferrooxidans: I. An Improved Medium and Harvesting Procedure for Securing
High Cell Yields,” J. Bacteriology, 77 (1959), 642-677.
20. C. Gomez, M.L. Blazquez, A. Ballester, “Bioleaching of a Spanish Complex Sulphide Ore-Bulk
Concentrate,” Minerals Engineering, 12 (1) (1999), 93-106.
21. O.H. Tuovinen, D.P. Kelly, “Studies on the Growth of Thiobacillus ferrooxidans: I. Use of
Membraine Filters and Ferrous Iron Agar to Determine Viable Numbers and Comparison with
CO2 Fixation and Iron Oxidation as Measure of Growth,” Arch. Microbiology 88 (1973), 285-
22. P.R. Norris, D.W. Barr, “Growth and Iron Oxidation by Acidophilic Thermophiles,” FEMS
Microbiology Letters, 28 (1985), 221-224.
23. W. Leathen, N.A. Kinsel, I.A. Braley, “Ferrobacillus ferrooxidans: A Chemosynthetic
Autotrophic Bacterium,” J. Bacteriology, 72 (1956), 700-704.
24. M.S. Liu, R.M.R. Branion, D.W. Duncan, “Oxygen Transfer to Thiobacillus Cultures,”
Biohydrometallurgy: Proc. of the Int. Symp., eds. P.R. Norris and D.P. Kelly (Warwick: 1987),
25. A.S. Myerson, “Oxygen Mass Transfer Requirements During the Growth of Thiobacillus
ferrooxidans on Iron Pyrite,” Biotechnology and Bioengineering, 23 (1981), 1413-1416.
26. P. d’Hugues et al., “Bioleaching of a Cobaltiferous Pyrite: A Continuous Laboratory-scale Study
at High Solids Concentration,” Minerals Engineering, 10 (5) (1997), 507-527.
27. P.R. Norris, “Factors Affecting Bacterial Mineral Oxidation: The Example of Carbon Dioxide in
the Context of Bacterial Diversity,” Biohydrometallurgy, eds. J. Salley, R.G.L. McCready and
P.L. Wichlacz, (CANMET SP89-10, 1989), 3-14.
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28. J.Y. Witne, C.V. Phillips, “Bioleaching of Ok Tedi Copper Concentrate in Oxygen- and Carbon
Dioxide-Enriched Air,” Minerals Engineering, 14 (1) (2001), 25-48.
29. M. Boon, J.J. Heijnen, “Gas-liquid Mass Transfer Phenomena in Biooxidation Experiments of
Sulphide Minerals: A Review of Literature Data,” Hydrometallurgy, 48 (1998), 187-204.
30. A.D. Bailey, G.S. Hansford, “Factors Affecting the Biooxidation of Sulphide Minerals at High
Concentrations of Solids: A Review,” Biotechnology and Bioengineering, 12(10) (1993), 1164-
31. H. Deveci, “Effect of Solids on Viability of Acidophilic Bacteria,” Minerals Engineering, 15
(2002), 1181-1190.
32. H. Deveci, I. Alp, T. Uslu, “Effect of Surface Area, Growth Media and Inert Solids on
Bioleaching of Complex Zinc/Lead Sulphides,” Proc. of the 18th Int. Mining Congress and
Exhibition of Turkey, IMCET 2003, ed. G. Ozbayoglu (Ankara: The Chamber of Mining
Engineers of Turkey, 2003), 415-423.
33. C. Komnitsas, F.D. Pooley, “Optimisation of the Bacterial Oxidation of an Arsenical Gold
Sulphide Concentrate from Olympias, Greece,” Minerals Engineering, 4(12) (1991), 1297-1303.
34. O.H. Tuovinen, S.I. Niemela, H.G. Gyuenberg, “Tolerance of Thiobacillus ferrooxidans to Some
Metals,” Antonie van Leeuwenhoek, 37 (1971), 489-496.
35. K.A. Natarajan, “Electrochemical Aspects of Bioleaching of Base Metal Sulphides,” Microbial
Mineral Recovery, eds. H.L. Ehrlich and C.L. Brierley (McGraw-Hill, 1990), 79-106.
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... Due to process economics, it is preferred to operate the process at high solid concentration. Increasing pulp densities involves some practical limitations (Deveci et al., 2003). ...
... Chemical composition of different culture media for bioleaching studies(Deveci et al., 2003) ...
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After the computer and mobile revolution, electric and electronic waste had become a serious threat to urban and rural communities equally. Prevention of the hazardous exposure and proper management are challenging in developing nations. One way to turn the crisis to opportunity is to extract metals from this Waste Electronic and Electric Equipment (WEEE) is making waste into a source of metal ores. The involvement of microbes in this technology could increase the boons by being an eco-friendly technique for reducing the hazardous nature. This article reviews the mechanisms involved in the process of bioleaching, the microorganisms employed, methods used and various developments as well as limitations along with recent advances and future prospects of the process of bioleaching of metals from WEEE.
... The optimum pH range in a specific bioleaching process depends on both the operating system and the used microorganisms (Patel et al., 2015). Acidophiles highly require an acidic environment so that the optimum range of pH value of 2e2.5 is needed for the growth and bioleaching functionality of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans, and the bioleaching yield is reduced considerably under the conditions with pH values higher than 2.5 (Gu et al., 2018;Deveci et al., 2003). To study the influence of pH on the growth of acidephiles, Fig. 5f shows the growth of a mixed culture of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans at various pH values, in which the optimum pH range is between 1.8 and 2.5 and the greatest concentration of the bacterial cells is obtained at pH of 2 (Wang et al., 2014a). ...
... Nitrogen, phosphorus, magnesium is some of the needed elements in a sufficient medium. One of the most utilized mediums in the bioleaching processes is 9 K medium which consists of ammonium, potassium, and phosphate (Deveci et al., 2003). S 0 , as an electron donor, serves as the energy source of the bioleaching microorganisms and plays a vital role in sulfur metabolism. ...
The rapidly growing demand for lithium has resulted in a sharp increase in its price. This is due to the ubiquitous use of lithium-ion batteries (LIBs) in large-scale energy and transportation sectors as well as portable devices. Recycling of the LIBs for being the supply of critical metals hence becomes environmentally and economically viable. The presently used approaches for the recovery of spent LIBs like pyrometallurgical process can effectively recover nickel, cobalt, and copper, while lithium is usually lost in slag. Bioleaching process as an alternative method of extraction and recovery of valuable metals from the primary and secondary resources has been attracting a large pool of attraction. This method can provide with higher recovery yield even for low concentration of metals which makes it viable among conventional methods. The bioleaching process can work with lower operating cost and consumed water and energy along with a simple condition, which produces less hazardous by-products ultimately. Here, we comprehensively review the biology and chemistry mechanisms of the bioleaching process with a conclusive discussion to help how to extend the use of bioleaching for lithium extraction and recovery from the spent LIBs with a focus on recovery yields improvement. We elaborate on the three main types of the reported bioleaching with considering effective parameters including temperature, initial pH, pulp density, aeration, and medium and cell nutrients to sustain microorganism activity. Finally, practical challenges and future opportunities of lithium are discussed to inspire future research trends and pilot studies to realize the full potential of lithium recovery using sustainable bioleaching processes to extend a clean energy future.
... The reason of effective chalcopyrite leaching with the extremely thermophilic microorganisms can be that passivating sulfur layers which occur during the leaching are less stable at higher temperature (Stott et al., 2000, Rodríguez et al., 2003Rawlings et al., 2003;Kinnunen, 2004). Also, processes using thermophilic microorganism can run up to 85°C (Deveci et al., 2003;Rawlings et al., 2003). Torma (1977) mentioned that bioleaching of most of the sulfide minerals runs at the optimum pH level of 1.5-2.3. ...
... The solution contained 4500 ml 0 K medium (Table 2) with the adapted microbial culture as inoculum (500 ml). The effect of solid concentration (w/v) (5%, 7.5%, 10% and 12.5%) on metal extraction was investigated at 32°C and 300 rpm Table 1 Working temperature of acidophilic microorganisms in bioleaching processes (modified from Deveci et al., 2003;Holanda et al., 2016 Mineral salts and elements in 0 K medium (modified 9 K medium without ferrous iron, Silverman and Lundgren, 1959). ...
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This study focuses on investigating the extraction of gold, copper, iron, nickel, cobalt, and zinc present in the flotation tailings. The studied sample contained iron (3.56%), copper (0.09%), and gold (0.2 ppm) as major target elements, whereas cobalt (0.04%), nickel (0.03%) and zinc (0.04%) were trace elements of interest. Primarily, bioleaching with mixed acidophilic culture was applied as a pretreatment process for the recovery of nickel, cobalt, and zinc, as well as for iron removal. The effect of solid concentration (5–12.5%) in bioleaching was investigated at pH 1.8 and the temperature was kept at 32 °C. The highest extractions of nickel, cobalt, zinc, and iron at 5% and 7.5% solid concentrations in the bioleaching experiments were 90%, 60%, 86% and 67%, respectively. Dissolution of gold and copper was not observed. The residues from bioleaching pretreatment were applied for chemical chloride leaching to extract gold and copper into the solution. In chloride leaching, the highest extractions of copper and gold were 98% and 63%, respectively. In addition, residual nickel, cobalt, and zinc were dissolved into the solution with the extraction of 99%, 80%, and 90%, respectively. In all chloride leaching experiments, the highest extractions of iron, copper, gold, nickel, cobalt, and zinc were observed with biologically pretreated feed. Alternatively, residues from bioleaching were also subjected to conventional cyanide leaching. Dissolutions of copper, nickel, cobalt and zinc were shown to be higher in chloride solution, however, 7%-unit more of gold could be extracted by cyanidation. With these findings, it appears that the combination of biological pretreatment and chloride leaching can provide a non-toxic process for improved valuable metals extraction from low-grade tailings.
... In general, multiple factors affect the bioleaching processes, which include the presence of metal ions, contacting method, pulp density (PD), nutrients, initial pH, temperature, inoculation ratio and aeration [193][194][195]. Excess metal ions in the medium can be detrimental to the microorganisms and an environment without excessive toxins is also required for the growth of the microorganisms. ...
Lithium-ion batteries (LIBs) are widely used as a critical energy storage system for internet of things (IoT), electric vehicles (EV) and various renewable energy sources. However, the widespread use of LIBs has resulted in the significant accumulation of batteries with different chemistries in landfills. Existing methods for LIB recycling are unsustainable, non-environmentally friendly and ineffective at recycling spent LIBs with mixed chemistry. These methods are inadequate to achieve recovery and repurposing of the valuable and sometimes toxic components. This mandates an inherent need to improve the existing processes or develop a novel, sustainable, environmentally friendly and effective alternative process. In this paper, we present a comprehensive review of the current recovery technologies; demonstrating the gaps in understanding, the challenges and opportunities available in the recycling processes. This review will also examine and discuss different fundamental scientific principles and methods that can be employed to develop sustainable and effective recycling processes with an aim to facilitate the LIB recycling industry to shift towards a circular economy.
... However, it should be mentioned that direct evidence for poor bulk mixing in bioreactors comes mostly from ferric iron measurements during the propagation of microorganisms on minerals in order to recover valuable metals. Some authors (Acevedo et al., 1998;Deveci et al., 2003) have reported poorly circulating regions in which dissolved oxygen concentrations approached zero while high concentrations were maintained in the impeller zones in such a way that it would be interesting to study the use of air-sparged concentrated mineral pulp suspensions to correlate terminal blending time to an ion ferric production rate in terms of impeller geometry and silver and gold recovery. ...
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Microbiological leaching of low-grade ores for the recovery of various metals in solution has been practiced for quite some time. More recently a variety of research projects has been directed toward the study of leaching of high-grade concentrates under controlled conditions. The rationale for such studies is that this kind of leaching could be competitive with smelting. Microbiological leaching lends itself to smaller plants, would be less energy intensive, and would result in less air pollution problems. The ability of some microorganisms to assist in the recovery or remotion of metals from ores or tailings is now well known and their contribution to the solubilization of metal sulphides, while the mechanism is not completely clear, is accepted and well documented. They are the only microbes to be used for metal recovery or removal on a commercial scale.
... These results suggested that the gold dissolution depends on the activities of a-1 strain in this study although the dissolution of elements from ore is generally promoted with the increase in temperature [26]. It is therefore important to operate the gold bioleaching with IOB and iodide under the optimal temperature for the activities of IOB. ...
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Iodide-oxidizing bacteria (IOB) oxidize iodide into iodine and triiodide which can be utilized for gold dissolution. IOB can be therefore useful for gold leaching. This study examined the impact of incubation conditions such as concentration of the nutrient and iodide, initial bacterial cell number, incubation temperature, and shaking condition on the performance of the gold dissolution through the experiments incubating IOB in the culture medium containing the marine broth, potassium iodide and gold ore. The minimum necessary concentration of marine broth and potassium iodide for the complete gold dissolution were determined to be 18.7 g/L and 10.9 g/L respectively. The initial bacterial cell number had no effect on gold dissolution when it was 1 × 104 cells/mL or higher. Gold leaching with IOB should be operated under a temperature range of 30–35 °C, which was the optimal temperature range for IOB. The bacterial growth rate under shaking conditions was three times faster than that under static conditions. Shaking incubation effectively shortened the contact time compared to the static incubation. According to the pH and redox potential of the culture solution, the stable gold complex in the culture solution of this study could be designated as gold (I) diiodide.
Nowadays, sulfide ores are a huge source of precious metals. One of the main problems for working with sulfide ores is their low solubility in acids (Ksp<10−20) and the production of toxic and harmful by-products. In the present study, the use of aluminum permanganate [Al(MnO4)3] oxidizer for sulfide ore dissolution and metal extraction has partially solved this problem. Taguchi experimental design based on Aspen Hysys modeling assembled has been applied to dissolution study of sulfide ores with Al(MnO4)3 oxidizer as a novel plan from experimental to industrial scale. The optimum results have been utilized as the primary data for the simulation and sensitivity analysis of the process by Aspen Hysys software. The effects of operating parameters including pH, retention temperature, agitation rate, retention time, amount of Al(MnO4)3 consumed, leaching density, grain size and oxygen pressure have been investigated on the extraction efficiency of metals from sulfide ores. Under optimized conditions, Zn, Cu, and Pb metal extraction efficiency was obtained above 77%, 73%, and 70%, respectively.
Conference Paper
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Magnesium compounds which are caustic magnesia, sinter or dead-burnt magnesia, magnesium chloride, magnesium hydroxide, magnesium carbonate (magnesite) are produced from seawater, lake brines and minerals deposits. Magnesite ore (MgCO3) is a natural mineral mainly composed of magnesium carbonate and it is the primary source for production of magnesium and its compounds. Magnesite ore is composed of serpentine, quartz-based silica, opal and limestone, and SiO2, Fe2O3, CaO, Al2O3 content is important to determining the quality of magnesite ore and economic evaluation is done according to these values. Beneficiation of magnesite ore is performed using physical and chemical methods to produce a high-grade product to be used in the manufacture of magnesium compounds. Magnesite ore is separated from impurity silica and iron by crushing, grinding, screening and beneficiations methods. In this study is aimed to determine that beneficiation methods which used for magnesite ore is how successful. According to experiment results, the best result has been obtained by chemical beneficiation method so it is the best suitable method for beneficiation magnesite ore.
Biomachining will not be considered as a full-scale manufacturing technology until a stable, controlled and continuous metal removal rate (MRR) is achieved. In this research work, a novel strategy that could promote its industrial implementation, namely simultaneous bacterial growth and machining of copper contained in oxygen-free copper (OFC) workpieces, was investigated. This proposal has the major advantage of being a single-stage process, thereby reducing total operating times and becoming more economical in comparison with conventional biomachining (downtime due to bacterial growth would disappear). The study was carried out using mesophilic (Acidithiobacillus ferrooxidans) and thermophilic (Sulfobacillus thermosulfidooxidans) extremophile bacteria in order to prevent the progressive decrease in the amount of metal removed per unit time. A constant MRR of 43 mg h–1 was achieved with A. ferrooxidans in the simultaneous process. Despite the accomplishment of a constant MRR, this value is lower than the maximum MRR obtained in conventional biomachining (109 mg h–1), probably due to the inability of ferric ions to come into contact with the metallic surface. With regard to the culture period in MAC medium, S. thermosulfidooxidans showed a slower growth rate (0.11 h–1) and lower ferrous ion oxidation level (0.12 g Fe2+ L–1 h–1) than A. ferrooxidans (0.17 h–1 and 0.22 g Fe2+ L–1 h–1, respectively) under optimal pH (1.5) and Fe2+ concentration (6 g L–1) conditions.
Conference Paper
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The effects of surface area (as a function of particle size and pulp density), growth media and inert solids on the extraction of zinc from the complex sulphide ore/concentrate were evaluated using the mesophilic and moderately thermophilic bacteria. The results have shown that an increase in the available surface area via size reduction improves the dissolution of zinc from the ore at low pulp densities (1-2% w/v). However, excessive increase in the surface area with increasing pulp density was found to adversely influence the dissolution process due to the inability of the bacteria to maintain the oxidising conditions in favour of the mineral sought after. Addition of nutrient salts was found essential to sustain the optimum bioleaching activity and the concentration of nutrient salts to be provided appears to depend on the availability of substrate (i.e. head grade and/or pulp density) for bacterial oxidation. The addition of inert solids (quartz) was shown to have a limited effect on the bioleaching of the concentrate.
The initial pH of aerobic suspensions of iron-grown T. ferrooxidans was shown to increase with the addition of FeSO 4 . The pH increase was proportional to the Fe +2 concentration in the 2 mM − 35 mM range and appears to be due to uptake by the cells. H + removal also varied directly with initial pH of the suspension in the range of 2.0 to 3.3. Initial pH increase was followed in time by a net pH decrease; leading to the postulation that the cells require H + in proportion to the amount of electrons produced from the oxidation of Fe +2 and that the net acid production resulted from a slower abiotic hydrolysis of Fe +2 (Fe +3 + 3H 2 O → Fe (OH) 3 + 3H + ). Neither controls without cells nor control suspensions of cells without substrate showed any significant pH change. Suspensions of cells at pH 2.1 in the presence of DNP (1 mM − 10 mM) or oxalacetate (80 μM − 120 μM) showed some H + uptake in the absence of added Fe +2 leading to the postulation that these compounds effected the H + permeability of the cytoplasmic membrane.
Bacteria that bring about dissimilatory transformations of iron are important from both biogeochemical and industrial perspectives (Ehrlich and Brierley, 1990; Johnson, 1995). The oxido-reduction of iron in extremely acidic (pH > 3) environments is particularly interesting because of the greater solubility of ionic (particularly ferric) iron and the relative stability of soluble ferrous iron under these conditions. Acidophilic iron-oxidizing bacteria are generally considered the most significant microorganisms in the biological processing of sulfide ores (“biomining”) in which the accelerated oxidative dissolution of sulfidic minerals (e.g., pyrite, arsenopyrite, and chalcopyrite) solubilizes (e.g., copper) or releases (refractory gold) metals, thereby facilitating their recovery (Rawlings and Silver, 1995). Most research into bacterial iron transformations at low pH has focused on mesophilic chemolithotrophs, particularly Thiobacillus ferrooxidans, though a number of physiologically and phenotypically diverse mesophilic acidophiles, it is now known, are involved in the dissimilatory oxido-reduction of iron (Johnson, 1995; Norris and Johnson, 1997; Pronk and Johnson, 1992).
This paper is a progress report on the commercialization of using bioleaching for base-metal concentrates. The paper focuses on bioleach processes for recovering copper from chalcopyrite and nickel/cobalt from pentlandite/pyrrhotite. Data is discussed from pilot-plant trials in which an overall recovery >95% was obtained for the bioleaching of copper from chalcopyrite. The pilot plant was operated in closed circuit with solvent extraction and electrowinning circuits for final metal recovery. For the bioleaching of nickel and cobalt from pentlandite/pyrrhotite, an overall recovery of 97% was achieved. Precipitation routines were used to produce a final nickel/cobalt product. The pilot plants were capable of treating a few kilograms of concentrate per day. Prominent features in the design of a 1-t/day (1.1-stpd) copper-bioleach demonstration plant for the treatment of chalcopyrite concentrate are discussed. The plant, which is now under design, will be constructed and operated based on the results obtained from the laboratory pilot-plant campaigns. Issues of scale-up for the demonstration plant, together with integration into upstream and downstream processing, are also addressed. Independently derived capital and operating costs are presented for a possible commercial plant. These cost studies indicate the principle economic issues in considering the application of bioleaching to the extraction of base metals. The benefits of bioleaching complex concentrates that are not amenable to physical beneficiation and economic treatment by conventional smelting are highlighted. Issues, such as the environmental aspects, that illustrate the benefits of bioleach technology are given.
It has been most satisfying to see the emergence of bacterial leaching technology from laboratory experimentation through to commercial acceptance. The use of moderate thermophiles for commercial bacterial leaching processes is an evolving area which offers a number of possible advantages over the use of more traditional mesophilic (lower temperature) organisms.
GENCOR S.A. Ltd. has pioneered the commercialization of biooxidation of refractory gold ores. Development of the BIOX® process started in the late 197os at GENCOR Process Research, in Johannesburg, South Africa. The early work was championed by Eric Livesey-Goldblatt, the manager of GENCOR Process Research who directed pioneering and innovative research into bacterial oxidation of refractory gold ores prior to cyanidation. This work was driven by the need to replace Fairview’s outmoded Edward’s roasters, which at the time were seriously contributing to pollution in the Barberton area.
The application of bacterial oxidation as a pretreatment step for the extraction of gold from arsenical gold sulphide concentrates offers potentially significant economic advantages over oxidative pretreatment alternatives. In this study the bacterial oxidation of an extremely refractory gold sulphide concentrate, from Olympias, Greece, is examined.Leaching tests were conducted in air-stirred pachuca reactors in order to determine the effect of pulp density on the degree of pyrite and arsenopyrite oxidation.Cyanidation tests were conducted in order to determine the degree of gold and silver liberation in relation to the degree of each sulphide phase oxidised and to the cyanide consumed.Mathematical analysis of the leaching data allowed the gold distribution in each sulphide mineral phase to be estimated and the gold recoveries according to the degree of each mineral oxidised predicted.
Most of the moderately thermophilic, acidophilic iron-oxidizing bacteria which have been isolated required a source of reduced sulphur for growth on iron. One isolate (strain ALV) utilized sulphate as the sole source of sulphur. All of the isolates were capable of chemolitho-heterotrophin growth on iron in the presence of yeast extract. Autotrophic growth has been confirmed in all strains except one previously described, but now re-isolated, moderate thermophile (TH3).