Mining and

UNIT 16 MINING AND BIOLEACHING Bioleaching

Structure
16.1 Introduction
16.2 Beginning of bioleaching process
16.3 Microorganisms in bioleaching
16.4 Operating factors that affect the process of bioleaching
16.5 Recovery of metals
16.6 Methods in mineral recovery
16.7 Commercial processes of Bioleaching
16.8 Recovery of Copper by dump leaching:
16.9 Uranium bioleaching
16.10 Microbial sorption in metal recovery
16.11 Oil recovery
16.12 Petroleum prospecting
16.13 Let us Sum Up
16.14 Suggested further reading/ References
16.15 Answers to check your Progress Exercise

16.1 INTRODUCTION
With the advent of industrialization as well as urbanization, there is an
exponential increase in the demand of industrially important minerals along
the exponential increase in world population. As the technology advances so
does the need for minerals of industrial importance. The high-grade deposits
of ores were easily available earlier as there were huge reserves however;
many of them are either depleted or soon be depleted with their present rate
of extraction and consumption. Hence, it becomes increasingly important to
find innovative and economical methods to recover such metals from lower-
grade deposits, which for technical or economic reasons have not been
extracted for usage. Various physicochemical and biological methods are
available and can play an important role in recovering of valuable minerals
from low-grade ores. Among biological methods, microbes are used in
recovering low-grade ores acting as biocatalyst in bio mining processes
meeting some of the needs of industrialized society. Such recovery processes
employ microbial metabolic activities to gain access to, rather than to
produce, desired products; in the form of soluble minerals or metals, this
process is called bioleaching. During bioleaching for the recovery of metals,
methods modify the physicochemical properties of metallic ore, so that
metals can be extracted. Bioleaching is unaffected by low concentrations of
the metals in the solution. Currently, biomining is at the forefront of the
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Ecofriendly accessible applied mining sciences. The techniques of biomining are
Bioprocesses
inexpensive, nontoxic and efficient. Moreover, the techniques are
environment friendly as bioleaching results in less air pollution and little
disruption to geological formations, and the microbes used are naturally
present. In short mining with microbe is both ecofriendly and economical.

Bioleaching (or biomining) is a biohydrometallurgy process that extracts

valuable metals from a low-grade ore with the help of microorganisms. In
other words, it is also defined as metal dissolution from their mineral
resources by certain naturally occurring microorganisms or their utilization to
change elements, so that when water is sifted through it, elemental extraction
from a material is possible. Bioleaching generally refers to the transformation
of metals via microorganisms into their water-soluble form. For instance,
copper sulfide (CuS2) is oxidized microbially to copper sulphate (CuSO4) in
case of copper (Cu), and metals are available in the fluid stage and remaining
solids are disposed off. Conversely, bio-oxidation” (a type of bioleaching),
defines the microbial oxidation of minerals, containing metal compounds of
interest. Subsequently, metals stay in solid deposits in concentrated form.
Additional terms such as bio-extraction, biomining, and bio-recovery are also
used to describe metal mobilization from solid materials interceded by
microorganisms or parasites or planktonic build-ups. Biomining, primarily
concerns the wide-ranging implementation of microbial courses for economic
metal regeneration in the mining sector.

Fig.16.1: Electron microscopic view of bacteria embedded with mineral ore during
bioleaching

16.2 BEGINNING OF BIOLEACHING PROCESS

Evidences shows that ancient people figured out, how to naturally recover the
bioleached Copper. For example, one model was found at the hour of the
Roman Empire, Spain, harking back to the eighteenth century, where
recouping of copper from acidic water has been proved.However, this
biotechnology development began in around 1950, after the isolation and
characterization of bacteria capable of leaching Copper. This fundamental
knowledge enabled the inception of understanding relationship between this
unique microbial action and Cu disintegration, and its capability as an
elective innovation for Copper recovery. Nowadays, this procedure is very
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popular in various countries with several thousand tons of extraction in case Bioleaching
of Copper and other commercial metals. The extraction of metal sulfides was
first demonstrated with the mobilization of zinc from zinc sulfide (ZnS). It
was found that the transformation of ZnS to ZnSO4 was microbially
mediated. Later in 1947, Thiobacillus ferrooxidans, one the significant
organism in bioleaching methods was found in an acid mine drainage. A first
patent in this field of metal extraction was granted in 1958. Further
exploration in this field continues which led to the popularization of this
technique. Now biomining is used to recover lead (Pb), arsenic (As),
antimony (Sb), nickel (Ni), molybdenum (Mo), gold (Au), silver (Ag) and
cobalt (Co) in various countries esp. in India, China, Chile, South Africa,
Australia, Iran, Mexico, and the United States.

16.3 MICROORGANISMS IN BIOLEACHING

Bioleaching includes various ferrous iron and sulfur oxidizing bacteria,
including one of the most commonly applied Acidithiobacillus ferrooxidans
and Acidithiobacillus (formerly called Thiobacillus). It is a bacterium that is a
non-spore forming, motile, rod-shaped and gram-negative. It is a chemo-
lithotrophic bacterium which derives growth energy from sulfur or iron
oxidation. It oxidizes ferrous iron (Fe+2) into ferric form (Fe+3), and convert
soluble or insoluble sulfides, thiosulfate to sulfate (SO2-4).

T.ferrooxidans and T. thiooxidans, are synergistic bacteria and improves the

efficiency of metal extraction from the ores when put together. Mixture of
Leptospirillum ferrooxidans and Thiobacillus organoparpus can efficiently
degrade pyrite (FeS2) and chalcopyrite (CuFeS2) to extract minerals of
interest. Besides this, the other microbes used in bioleaching process are
Sulfolobus acidocaldarius and S. brierlevi. These are thermophilic and
acidophilic bacteria which grow in acidic hot springs and often used to
extract Mo and Cu respectively from molybdenite (MoS2) and CuFeS2.
Pseudomonas aeruginosais also used in mining low grade Uranium (0.02%)
ores. Among fungal strains Aspergillus niger can extract Cu and Ni while
Aspergillus oryzae is employed for the extraction of Au. Rhizopus arrhizus is
known to extract Uranium (U) from waste water. Bioleaching, in nutshell is
the oxidative sulfide mineral solubilization interceded by microbial activity.
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Ecofriendly Chemolithotrophs, a group of microbes specifically use inorganic reduced
Bioprocesses
compounds as a source of energy. These organisms are regularly utilized as
acidophilic bioleaching microorganisms. They utilize Fe+2 or reduced sulfur
composites as vitality source and commonly found chemical elements in ores,
especially in Cu ores. The generation of ferric ion by the activity of
microbes, is a strong Cu2S oxidant, leading to the deliverance of metal into
the solution. Reduced sulfur compounds are oxidized to sulfuric acid (H2SO4)
via microbes during ferric attacks, upholding low pH, which is vital for
acidophiles and the solubility of ferric iron. Microorganisms re-oxidize
reduced iron, produced during mineral attacks at the end of the cycle.

16.4 OPERATING FACTORS THAT AFFECT

THE PROCESS OF BIOLEACHING
Microorganisms are undoubtedly, the primary performers in bioleaching.
Since pile bioleaching is not conducted under aseptic settings, the process is
taken care of by the mine site's in-situ mixed microbial consortia. A
microbial consortium is a group of two or more organisms living together. A
wide variety of microbes generally bacteria and archaea are available during
heap bioleaching process. .Acidithiobacillus ferrooxidans, Leptospirillum
ferrooxidans, and Acidithiobacillus thiooxidans are some of the most
important and first recognized bioleaching bacteria. These microorganisms
need nutrients for growth and to bioleach, which they must discover in their
environment. A significant number of nutrients become accessible from
similar ores and leaching solution, however, some of them like phosphorus
and nitrogen may turn out to be rare and influence the bioleaching procedure.
If economically practicable, these nutrients need to be supplemented to the
leaching solution.

Bioleaching is profoundly influenced by biological, physicochemical, and

environmental parameters, which in turn influence the yield of metal
extraction. Two gaseous components are needed by the bioleaching
microorganisms: carbon dioxide as an energy/carbon source, and oxygen as
an electron acceptor. They must be shifted from gaseous phase to leaching
solution, to become accessible to microorganisms. This implies that their
accessibility will rely on the bioleaching framework's mass exchange
characteristics.

Environmental factors like oxidation-reduction potential (ORP), pH, and

temperature are significant operating conditions for microorganisms and
sulfide oxidation done by ferric iron. Low pH (1-2) favors both the processes.
While as, microbial tolerance fixes an upper limit in the event of temperature
which obviously doesn't favor the sulfide ferric oxidation. High temperature
loving organisms have appeared to get greater percentage of extraction than
medium and low temperature organisms. While in ORP case, microbial
action tend to increase the leaching solution as bioleaching continues. The
322 speed of bioleaching process is more dependent on the kind of metal sulfide.
 Mining and
Generally primary Cu minerals (CuFeS2, Enargite) are considerably harder to Bioleaching
solubilize than the secondary ones (Chalcocite, Covellite, Bornite). Particle
size influences the efficiency and degree of leaching in any extraction
operation. The rate and stretch out of sulfide oxidation will rely upon the
surface region presented to the leaching fluid. In turn, due to intensive
smashing of the ore, elevated interface area is acquired. Diffusion phenomena
control the solubilization with an enormous particle size. Initial bioleaching
efforts were carried out in dump bioleaching mode, described by an
exceptionally dissimilar solid bed, established by rocks as large as 1 m in
distance across. In this context, the principle working factors are not
controlled, resulting in low productivity. Heap is an improved and better
framework, where homogeneous arrangement significantly increases the
chances of bioleaching and the efficiency of the whole process. These are the
contentions, why on industrial scale, it is utmost important. An outrageous
circumstance is the utilization of unsettled reactors, where it is conceivable to
apply tight compliance over temperature, accessibility of gaseous component,
pH, and ORP. The effectiveness of bioleaching relies mainly on the expertise
of microorganisms and the structure of mineralogical and chemical ores.
Maximum metal extraction can be acquired only when leaching conditions
are an optimal range of bacterial development conditions. Bioleaching factors
and their related impacted have been listed in Table. 1
Table 16.1. Bioleaching factors and their impacts (adapted from Wasim and
group, 2019 )

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Ecofriendly
Bioprocesses 16.5 RECOVERY OF METALS
Bioleaching process is used to recover the metals from the ore’s that are
unsuitable for direct smelting because of their low-grade content. Under ideal
laboratory conditions, nearly 97% recovery of Cu from the ores takes place
by bioleaching, which is seldom attained in actual mining methods. Even 50
to 70% recovery of copper by bioleaching from an ore that would otherwise
be completely unproductive would be an important achievement.

If the ore establishment is enough porous and over layers a water-

impermeable stratum, the mineral can be leached in situ without first mining
it. An appropriate boreholes pattern is created, with some holes used for
leaching liquor injection and others used for leachate recovery. More often,
though, this bioleaching method is accomplished after mining, splitting, and
piling of the material in heaps on a water-resistant surface or on a specially
constructed apron. Water is then pumped to the top of the heap and runs
down through the ore of the apron. The leaching water and ore usually
supplies enough dissolved mineral nutrients to satisfy the needs of T.
ferroxidans, but in some cases, minerals such as ammonia and phosphate
must be added. In most of these bioleaching operations, the leached metal is
then extracted with an organic solvent and subsequently removed by
stripping from the solvent. The leaching liquor and the solvent are recycled.

The characteristics of the ore have an important effect on its susceptibility to

bioleaching. The rate of leaching is determined in large part by the size of the
mineral particles. Increasing the surface area, accomplished by crushing
and/or grinding, generally increases bioleaching however, must also be
conductive for efficient leaching to occur. Optimal conditions for bioleaching
use T. ferroxidans area temperature of 30 to 50 °C, a pH of 2.3 to 2.4, and an
iron concentration of 2 to 4 g/L of leach liquor. Available oxygen and
nutrients such as ammonium, nitrogen, phosphorus, sulfate and magnesium
are essential for the growth of T. ferroxidans.

The oxidative activities of Thiobacilli can produce high temperature in some

mineral deposits and may increase the tolerance limits of the species being
used. Obviously, this would lead to decreased bioleaching activity and
mineral production. Because of these high temperatures, thermophilic sulfur-
oxidizing microorganisms may be useful for some bioleaching processes.
Members of the genus Sulfolobus are obligate thermophiles that oxidizes Fe+2
and sulfur in a way, like that of the members of the genus Thiobacillus. These
acid-tolerant thermophilic bacteria can oxidize inorganic substrates and are
used in the bioleaching of metallic sulfides. Sulfolobus has been used for
MoS2 (Molybdenum sulfide) bioleaching, whereas Thiobacillus is not
tolerant to high Mo, mercury, and Ag concentrations. Two methods of sulfide
mineral oxidation have been proposed and relay on its composition. The
suggested pathway for thiosulfate oxidizes species like FeS2, MoS2, and
tungstenite (WS2), while sphalerite ((Zn, Fe) S), CuFeS2, arsenopyrite
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(FeAsS), and galena (PbS) are oxidized by means of polysulfide pathway. Bioleaching

16.6 METHODS IN MINERAL RECOVERY

Methods play a key role in the leaching of metals from mineral bearing rocks
but their activities were discovered only recently. Thiobacillus ferroxidans
was isolated from coalmine drainage in 1947 and was subsequently
associated with most of the natural and artificial leaching sites. Methods are
used for recovery of minerals from low-grade ores and they do metal
leaching without polluting the atmosphere. Besides T. ferroxidans, other
acidophilic chemolithotrophic bacteria that are believed to be important to the
leaching processes are T. thioxidans, Leptospirillum, ferroxidans and species
belonging to genus sulphobus.T. ferroxidans is a small gram-positive straight
rod-shaped bacterium. It grows best in acidic solution at pH range 1.5-2.5
with an optimal temperature range of 10-30 °C and upper limit of 37 °C. It
obtains energy by the oxidation of ferrous to ferric form and the reduced form
of sulfur to H2SO4 using oxygen as a terminal electron acceptor. It uses CO2
as a carbon source. Other thiobacillus sp. that also plays a role in leaching
include T. thioxidans, T. acidophilus and T. organoborous. Many commercial
minerals of utilities are MS (metal sulphite) that are extremely insoluble.
Microbial activity causes solubilization of metals from their ores either by
direct leaching or by indirect leaching. These methods can be employed
together for efficient recovery of the metals (Fig.2).

Direct leaching:

In this process, the microbe act directly on the ore to extract metal. T.
ferroxidans become attached to mineral particles. Enzymes associated with
the cell walls catalyze oxidative attack on crystal lattice of the MS and
oxidation of mineral occurs in two steps.

Cu S + 0.5 O2 + 2H+ → Cu+2 + So + H2O

So + 1.5 O2 + H2O → H2SO4

T. thioxidans and T. ferroxidans cooperate in leaching of sulphite mineral.

When sulfide minerals are oxidized to Cu+2, the So (elemental sulfur) is
formed as a byproduct and coats the (covering Cu) remaining mineral
particles and limit the further access of Cupric to mineral sulfide.T.
thioxidans attacks this “passivating” sulfur layer and enhances the leaching
process by exposing the mineral surface and by generating H2SO4.
Leptospirillum ferroxidans is somewhat more acidophilic than T. ferroxidans
and grows at pH 1.2 on FeS2 and at temperature up to 40 °C. Archaebacteria
of genus sulphobolus may also contribute to the leaching process; the
organisms grow autotrophically at pH 1-3 and at temperature ranging 50-90
°C.

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Ecofriendly Indirect leaching:
Bioprocesses
In this process the microbe produce certain substances or oxidizing agents
such as ferric iron or sulphuric acid which solubilize the metal for extraction.
For indirect leaching acidic environment is important to extract metals. It
depends on the ability of various species of acidophilic sp. like Thiobacillus
ferroxidans to generate metabolic energy by oxidizing ferrous or sulfide
leading to ferric sulphate production according to following equation:

FeS2 + 3.5 O2 + H2O → FeSO4 + H2SO4

2FeSO4 + 0.5 O2 + H2SO4 → Fe2 (SO4)3 + H2O

Ferric sulphate is a potent oxidizing agent capable of dissolving many

essential minerals of copper sulfide.
CuFeS2 + 2Fe2 (SO4)3 → CuSO4 + 5FeSO4 + 2So
(Chalcopyrite)
Cu2S + 2Fe2 (SO4)3 → 2CuSO4 + 4 FeSO4 + So
(Chalcocite)
Cu5FeS4 + 6Fe2 (SO4)3 → 5CuSO4 + 13FeSO4 + 4So
(Bornite)

Leaching by ferric sulphate is called as indirect because it is independent of

presence of oxygen or microbial activity; an acidic pH is required for dump
leaching. In its absence, the oxidation of ferrous is very slow and hence
mineral leaching would be very slow. Thus, the indirect leaching means that
the microbial activity supply necessary conditions and efficiency to the
process. T. ferroxidans also derive energy by oxidizing sulfur generated in
the process and give H2SO4.
2So + 3O2 + 2H2O → 2H2SO4
H2SO4 maintains low pH that is optimal for acidophilic T. ferroxidans and
suppresses the loss of ferric sulphate by hydrolysis.
Fe2 (SO4)3 + 2H2O → 2Fe (OH)SO4 + H2SO4
H2SO4also leaches several copper oxide minerals for eg:
Cu3 (OH)2(CO3)2 + 3H2SO4 → 2CuSO4 + 2CO2 + 4H2O
(Azurite)

Fig. 16.2: The Indirect and direct mechanism of bioleaching

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16.7 COMMERCIAL PROCESSES OF Bioleaching
BIOLEACHING
The natural process of bioleaching is very slow. In order to increase the
efficiency of the process, various methods are used commercially for
maximum extraction of the minerals. The type of resource is the main factor
that decides about the process involved in bioleaching. Generally three main
methods are used according to the ore to be processed (Fig. 3).

a) Slope/ Dump Leaching:

It is one of the most commonest and cheap method in which the finely
powdered ore are made into large piles along the slopes of a range, and
water containing microbes (Thiobacillus) is continuously sprinkled over
the slope. The collected water at the bottom is used to extract the metals.
After extraction, the microbial population is regenerated in an oxidation
pond.

b) Heap Leaching:

This method is used to extract low grade minerals from ore. The
powdered ore is arranged in a big heap on an impervious natural surface,
and then the same process of metal leaching is followed as in case of
slope leaching.

c) In-Situ Leaching:

This process takes place at the point of generation of ore, hence called as
in-situ process. The ore is exposed through sub surface blasting and
passages for acidic water are drilled through this ore. The acidic water is
pumped along with the microbe (Thiobacillus sp.) through these
passages. A pit is made at the bottom of the ore surface to collect this
percolating water. This water rich in minerals is pumped out for
extraction of desired minerals. After extraction the water is reused for
generation of microbial species.

Fig. 16.3: Commercial processes of bioleaching (a) Slope (b) Heap (c) In-situ

Case studies:
Copper bioleaching
Copper, an element with high thermal conductivity and ductility has always
been in elevated demand for electricity, construction, transportation and other
industries. With its increased demand there has always been a felt a limited
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Ecofriendly supply of this resource hence, bioleaching has been commonly applied to
Bioprocesses
extract this mineral from low grade ores in many countries like the United
States, Australia, Canada, Mexico, South Africa and Japan. The United States
alone produces 10% of total copper through bioleaching. Types of copper
ores used in bioleaching processes are Covellite, chalcocite and Chalcopyrite.
These ores also contain fractions of other elements such as Chalcopyrite
contains 26% copper, 25.9% iron, 20.5% zinc and 33% sulphur. Thiobacillus
ferrooxidans oxidizes insoluble CuFeS2 and transforms it into soluble copper
sulfate (CuSO4). Sulphuric acid, a byproduct produced in this reaction,
maintains an acidic environment (low pH) necessary for microbial growth.
During the oxidation of Chalcopyrite the following reaction occurs:
2CuFeS2 + 8 ½ O2 + H2SO4 → 2 CuSO4 + Fe2 (SO4)3 + H2O
Similarly covellite is oxidized to copper sulphate:
CuS + 2O2 → CuSO4

In copper leaching processes, the action of Thiobacillus includes both direct

and indirect oxidation of CuS via generation of ferric ions from ferrous
sulfide (FeS). FeS is present in most of the important copper ores, like
CuFeS2. Copper is recovered by solvent extraction or by using scrap iron. In
the latter case, Cu replaces Fe according to the equation:

CuSO4 + Fe0 → Cu0 + FeSO4

16.8 RECOVERY OF COPPER BY DUMP

LEACHING:
Copper ore containing more than 0.5 % of Cu is prone to smelting whereas
Cu in lower grade ore is recovered by dump or heap leaching. A method in
which broken rocks are piled 100 or more feet high on an impermeable
surface and water. The same water is repeatedly circulated and recirculated
through pile of the rock. Same time FeS2 oxidize causes ore to become
strongly acidic and rich in ferric sulphate. This water slowly percolates down
through the pile and creates the applicable conditions for the growth of T.
ferroxidans within the pile.

The effluent becomes progressively enriched in metals such as Cu. Finally

the metal rich effluent is pumped in to a basin called as launder and iron
scraps are added to precipitate the copper by following eq.:

Cu+2 + Fe0 → Cu0 + Fe+2

This Fe+2 rich solution is transferred to shallow oxidation ponds when T.
ferroxidans rapidly oxidizes ferrous to ferric and forms some additional
H2SO4 by oxidation of sulfur compounds. Much of Fe+3 formed in oxidation
ponds precipitate as Fe(OH)3. The acidic supernatant Fe2(SO4)3 solution is
then pumped back to top of dump. A dump can be view as a continuous flow
reactor in which solubilization of metal is performed by bacteria attached to
ore particles.
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The bacteria thus play roles at two places: Bioleaching

(i) in the leached dump

(ii) in oxidation pond

At present, large-scale Cu bioleaching is mostly conducted by heaps

percolation. Minerals are squashed to a particle size of about 1 cm or more in
few smashing stages, healed with diluted H2SO4 and agglomerated before
stacking into tiny mechanically resistant spheres. Using drip irrigation or
sprinkling, an acid solution is applied over heaps and rehashed many times as
needed to get the ideal extraction and concentration of Cu. To facilitate
percolation of leaching solution, and flow of upcoming gas inoculated at
bottom, it is critical to construct a homogenous heap with a high void portion
to provide necessary oxygen and CO2. Loaded liquor at that point enters the
recovery segment, comprising of solvent extraction unit, which purifies and
concentrates fluid in Cu and Cu recuperation by means of electrowinning.
The duration of leaching may last two months or more. The schematic
diagram of a typical Cu heap bioleaching operation is shown in Fig.16. 4.

Fig. 16.4: The schematic diagram of a copper heap bioleaching operation (adapted
from Gentina, 2013)

16.9 URANIUM BIOLEACHING

Uranium (U) bioleaching is commonly used in Canada, the United States,
India and many other nations. This process helps to recover U from low
grade ores (0.01 to 0.5% U) and low-grade nuclear wastes. The recovery of
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Ecofriendly Uranium, a fuel required by the nuclear power generation industry, can easily
Bioprocesses
be enhanced by microbial activities. The microbial recovery of Uranium from
otherwise useless low-grade ores is helpful in overcoming the international
energy shortage. Nuclear safety and waste disposal problems, as well as the
limited supply of Uranium, render current nuclear fission generators
controversial; for all of these reasons, it may very well be only a stopgap
solution to the international energy problem. Although bioleaching cannot
influence safety considerations, this process can have an immediate and
direct bearing on the economics of nuclear power production by providing a
mechanism for commercial use of low-grade nuclear waste. Recovery of
Uranium from radioactive wastes is extremely important because it
overcomes the problem of waste disposal, a major shortcoming of using
nuclear power generators.

Bacterial leaching of Uranium is most feasible in geological strata where the

ore is in the tetravalent state. Insoluble tetravalent Uranium oxide (UO2)
occurs in low-grade ores. Although there is no evidence for the direct
oxidation of UO2 by T. ferroxidans, UO2 can be converted to the leachable
hexavalent form (UO2SO4) indirectly by the action of this microorganism.

Uranium ore occurs not as sulfide but as oxide UO2 and is frequently
associated with FeS2 minerals. The Uranium is leached from ore by indirect
mechanism. T. ferroxidans oxidizes Fe+2 in FeS2 (which often accompanies
the U ores) to ferric iron. The oxidized iron acts as an oxidant, converting
UO2 chemically to UO2SO4, which is then recovered through leaching. The
optimal conditions for extraction of Uranium are: 45-50 °C temperature, 1.5-
3.5 pH, and around 0.2% of incoming CO2 air. The soluble form of U from
the leach liquor is extracted into organic solvents (tributyl phosphate) which
is then precipitated and recovered through ion exchange chromatography.
Uranium recovery through this leaching process ranges from 30-90%.

UO2 + Fe2(SO4)3 + 2H2SO4 → [UO2 (SO4)3]-4 + 2FeSO4 + 4H+

The technical and economic feasibility of employing Thiobacillus for the

recovery of Uranium and copper minerals depends on various factors. The
geological formation in which the minerals occur is also important in
determining the suitability of the bioleaching process. In situ bioleaching is
ideal when there is a natural drainage system, as through a fault with an
impermeable basin, that will permit economic recovery of the minerals.
However, recovery of Uranium is much higher in heap leaching method.

Check Your Progress 1

Notes: a) Use the Space given for your answer.
b) Check your answer with the one given at the end of this unit.
1. Explain methods of mineral recovery?
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…………………………………………………………………………… Bioleaching
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1. Discribe Uranium biobleaching.
……………………………………………………………………………
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16.10 BIOLEACHING OF OTHER METALS

Bioleaching technique is also used for extraction of other metals such as gold,
silver, nickel, molybdenum, cobalt and antimony. This method has been
considered as most promising in case of precious metal ores like Au and Ag.
Removal of iron is again significant here, prior to the actual process of
leaching. This is done by using the organism Thiobacillus ferrooxidans
which precipitate iron under aerobic conditions. Au is acquired through
bioleaching of arsenopyrite/FeS2 ore and its cyanidation process. Ag is more
easily solubilized than Au during microbial leaching of iron sulfide.
Bioleaching is also helpful in removing certain impurities from the metal rich
ores. The microorganisms such as Rhizobium sp. and Brady rhizobium sp.
can remove silica from bauxite (aluminium ore) in metal purification process

Advantages of Bioleaching process

• Bioleaching is simple and effective technique in recovering metals from
low grade ores.
• It is an ecofriendly and cost effective technique as compared to smelting
process.
• It can used to concentrate metals from dilute mixtures and wastes.
• Compared to other processes this process does not produce any toxic
emissions and other health risk to miners
• Bioleaching also offer different ways to extract valuable metals from
low-grade ores that have already been processed.
However, the process has a major limitation or disadvantage due to its very
slow speed in recovering metals through biological processes. Though
researches are going on to make the process faster and efficient.

16.11 MICROBIAL SORPTION IN METAL

RECOVERY
In the field of biotransformation and biogeochemical cycling, metal and
mineral transformations by microbes are significant. Different microbial
properties can cause changes in metal speciation, toxicity and mobility, along 331
Ecofriendly with formation, dissolution or deterioration of minerals. Among these
Bioprocesses
properties, biosorption is most important in case of microbial transformation
of metals. Biosorption is commonly referred to as the mechanisms involved
in extracting metals from solution through microorganisms and related
materials. A wide range of microorganisms (bacteria, algae, yeasts, molds)
are used for biosorption of metals.

The cell membranes of microorganisms possess negatively charged ions due

to presence of hydroxyl (OH–), phosphoryl (PO3-4), carboxyl (COO–) and
sulfhydryl (HS–) groups. Metals being positively charged ions are easily
adsorbed on microbial cell surfaces. Several potential microbial metal
biosorbents are members from Bacillus, Pseudomonas, Streptomyces,
Aspergillus, Trichoderma, Yarrowialipolytica, Rhizopus, and Penicillium sp.
Rhodospirullum sp. can bioaccumulate Cd, Hg and Pb. Bacillus circulansis
used to bioadsorb metals such as Cd, Co, Cu and Zn. Among fungal species,
immobilized fungal biomass has been widely used in metal biosorption due
to mechanical strength, increased density, and resistance to chemical
environment. Aspergillus niger, A. oryzae, Mucor haemalis, Penicillium
chrysogenum helps in selective adsorption of several metal ions like
Uranium, Thorium. Pencillium lapidorum, P. spimuiosum are useful for the
bio sorption of metals such as Hg, Zn, Pb, Cu. Several species of fresh water
or marine algae are also known to bio accumulate metals. For example,
Chlorella vulgaris and C. regularis can accumulate metals like Pb, Hg, Cu,
Mo and U. All these processes are confirmed by use of electron microscopy,
which record the deposition of metals on the microbial cell walls which
proves the composition of cell wall plays a key role in the metal adsorption.

16.12 OIL RECOVERY

Bioleaching of oil shales (oil containing rocks) likewise can possibly enhance
hydrocarbons recovery. Many oil shales contains huge amount of carbonates
and FeS2, and evacuation of these minerals builds the porosity of the shale,
indirectly enhancing oil recovery. Acid dissolves the carbonates and can be
produced by Thiobacillus species growing on the sulfur and iron in FeS2.
Such microbial leaching appears to have the potential for making recovery of
hydrocarbons from oil shales economically feasible.

Recovering oil involves two to three stages. Primary recovery is a stage

where 12% to 15% of the oil in the well is recovered without need of any
external agent. Secondary recovery involves the use of water and other
substances to extract 15-20 % of more oil from well. The tertiary oil recovery
is the utilization of biological and chemical agents to improve oil recovery
from oil shales. Tertiary recovery of oil employs solvents, surfactant, and
polymers to dislodge oil from the geological formations (Fig.5). The tertiary
recovery has the potential for recovering 60 to 120 billion barrels of oil in the
United States reserves alone that otherwise could not be recovered. Xanthan
gum produced by bacteria such as Xanthomonas campestries, is a promising
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 Mining and
compound for the tertiary recovery of oil. Such polymers have higher Bioleaching
viscosity and flow characteristics that enables them to move in the rock
layers containing oil deposits through small pores. When introduced during
operations of aqueous flooding, that is, injected into petroleum reservoirs to
force out the oil, Xanthan gums help push the oil toward the production
wells. These polymers are formed through conventional fermentation
processes in which X. campestries is grown and the xanthan gums are
retrieved.

Fig. 16.5. Tertiary oil recovery process using surfactant injection method

16.13 PETROLEUM PROSPECTING

Petroleum prospecting is one of the most interesting way in which these
methods aid to the petroleum industry. Associated with liquids and solid part
of petroleum, a gaseous fraction also occurs. It consists of methane, ethane
and propane. In petroleum producing region, these gases may seep to the
surface and provide nutrient for the growth of specific hydrocarbons utilizing
bacteria. When one find bacteria capable of oxidizing these gases there is a
strong suggestion that a petroleum deposit is nearby. However, CH4 utilizing
bacteria are not the indicator of petroleum because CH4 is produced
biologically in many systems that are not related to petroleum (by cattle and
rice field). However, C2H6 is not produced biologically in significant
amounts and it is almost always associated only in with petroleum. Detection
of C2H6 utilizing methods can be used as an indication of petroleum
resources. Since geological methods of locating petroleum deposit are
adequate, microbiological prospecting at present is not in much use.
However, it may become more important in future as petroleum is getting
depleted.

16.14 LET US SUM UP

In this unit we have discussed about bioleaching process, microorganisms in
bioleachingand various operating factors that affect the process of
bioleaching. The unit also covers recovery of metal and prospects of
commercial processes of bioleaching.
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Ecofriendly
Bioprocesses 16.15 KEY WORDS
Biobleaching: Bioleaching process is used to recover the metals from the
ore’s that are unsuitable for direct smelting because of their low-grade
content.

16.16 SUGGESTED FURTHER READING/

REFERENCES
• Douglas E. Rawlings (Ed.) Biomining Theory, Microbes and Industrial
Processes. Springer Science & Business Media. 2013
• Edgardo R. Donati, Wolfgang Sand. Microbial Processing of Metal
Sulfides. Springer Science & Business Media. 2007
• Pradipta Kumar Mohapatra. Textbook of environmental biotechnology, I
K International. 2006
• K A Natarajan. Biotechnology of Metals Principles, Recovery methods
and Environmental Concerns. Elsevier. 2018

16.17 ANSWERS TO CHECK YOUR PROGRESS

EXERCISE
Check Your Progress 1

1. Refer Section Number 16.6

2. Refer Section Number 16.9

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