BIOLOGY-BIOMINING
BIOMINING -
Biohydrometallurgy
can be defined as the control of natural (biochemical) processes of
interactions between microbes and minerals to recover valuable metals. Advances
in
biotechnology have permitted the extraction of metals from low-grade ores, improved recovery rates at operations, and reduced operating costs.
biotechnology have permitted the extraction of metals from low-grade ores, improved recovery rates at operations, and reduced operating costs.
Biomining
is defined as extracting mineral ores or enhancing the mineral recovery from
mines using microorganisms instead of traditional mining methods.
mines using microorganisms instead of traditional mining methods.
Copper was the first metal extracted using microorganisms
in the ancient past in the Mediterranean region. The efficiency of bio- mining
can be increased either by finding suitable strains of microorganisms or by genetically
modifying existing microorganisms, made possible due to rapid advances in the
field of biotechnology and microbiology.
Biomining allows environmentally friendly ways of
extracting metals from low-grade ores (ores that have small amounts of valuable
metals scattered throughout).
Biomining
includes two different chemical processes called
(i) bioleaching and
(ii) biooxidation..
(ii) biooxidation..
Bioleaching refers to the use of bacteria,
principally Thiobacillus ferrooxidans, Leptospirillum ferrooxidans and
thermophilic species of Sulfobacillus, Acidianus and Sulfolobus, to leach a metal
of value such as copper, zinc, uranium, nickel and cobalt from a sulphide
mineral. Bioleaching places the metal values of interest in the solution phase
during oxidation. These solutions are handled for maximum metal recovery and
the solid residue is discarded. Mineral biooxidation refers to a pretreatment
process that uses the same bacteria as bioleaching to catalyze the degradation
of mineral sulphides, usually pyrite or arsenopyrite, which host or occlude
gold, silver or both. Biooxidation leaves the metal values in the solid phase
and the solution is discarded.
B. MICROORGANISMS IN BIOMINING:
There are different types of bacteria
present in nature that oxidize metal sulfides and
solubilize minerals, thus, helping in their
extraction from the ores.
Characteristics of the bacteria used in
bio-mining:
- Mineral extraction involves the production of high temperatures
so the bacteria should be able to survive the heat, hence, they should be
thermophilic.
- b. Bio-mining involves using strong acids and alkalis, hence,
bacteria should be
chemophilic.
- c. Bacteria should produce energy from inorganic compounds,
hence, should also be autotrophic (characteristic of an organism capable
of making nutritive organic molecules from inorganic sources via
photosynthesis i.e., involving light energy or chemosynthesis i.e.,
involving chemical energy).
- d. The bacteria should be able to adhere to the solid surfaces
or have the ability to form biofilms.
Examples of Bacteria
Useful for Biomining Operations
Thiobacillus ferrooxidans is a chemophilic, moderately thermophilic
bacteria which can produce energy from oxidation of inorganic compounds like sulfur
and iron. It is the most commonly used bacteria in biomining.
bacteria which can produce energy from oxidation of inorganic compounds like sulfur
and iron. It is the most commonly used bacteria in biomining.
Several other bacteria such as T.thioxidans, Thermothrix thiopara,
Sulfolobus acidocaldarius and
S. brierleyi are also widely used to extract various
minerals.
Thermothrix thiopara
is an extremely thermophilic bacteria that can survive very high
temperatures between 60-75C and is used in extraction of sulfur.
temperatures between 60-75C and is used in extraction of sulfur.
T.ferrooxidans and T.thiooxidans. The bacteria are applied in the form of dots on a
nitrocellulose film. Antigen-antibody reaction is carried out on the film and then treated
with a secondary antibody to make the reaction visible by producing a color. The sample
can be approximated by comparison of the test sample with that of a known sample.
nitrocellulose film. Antigen-antibody reaction is carried out on the film and then treated
with a secondary antibody to make the reaction visible by producing a color. The sample
can be approximated by comparison of the test sample with that of a known sample.
C. BIOMINING RECOVERY:
Minerals are recovered
from ores by the microorganisms mainly by two mechanisms: (a)
Oxidation and (b)
Reduction.
(a) Oxidation
The microorganisms like T.ferroxidans and
T.thioxidans are used to release iron and
sulfur respectively. T.ferroxidans oxidize
ferrous ion to ferric ion.
4Fe++ + O2 + 4H+→ Fe++++ 2H2O
The bacteria attach to the surface of the ore and oxidize by a direct and indirect method.
Direct Method -
In this
method the ore is oxidized by the microorganisms due to the direct contact with
the compound.
2FeS2 + 7O2 + 2H2O → 2FeSO4 + 2H2SO4
the compound.
2FeS2 + 7O2 + 2H2O → 2FeSO4 + 2H2SO4
Indirect Method -
In this
method the mineral is indirectly oxidized by an agent that is produced by
direct
oxidation. For example, the ferric ion produced by the above reaction is a powerful
oxidizing agent and can release sulfur from the metal sulfides. Thus production of ferric ion indirectly causes oxidation of metal sulfide resulting in the breaking of the crystal
lattice of the heavy metal sulfide and separating the heavy metal and sulfur.
CuS + Fe+++→ Cu+ + S + Fe++
oxidation. For example, the ferric ion produced by the above reaction is a powerful
oxidizing agent and can release sulfur from the metal sulfides. Thus production of ferric ion indirectly causes oxidation of metal sulfide resulting in the breaking of the crystal
lattice of the heavy metal sulfide and separating the heavy metal and sulfur.
CuS + Fe+++→ Cu+ + S + Fe++
(b) Reduction -
Bacteria
like Desulfovibro desulfuricans play an active role in reduction of sulfates
which
results in the formation of hydrogen sulfides.
4H2 + H2SO4 → H2S + 4H2O
results in the formation of hydrogen sulfides.
4H2 + H2SO4 → H2S + 4H2O
D. TYPES OF BIOMINING:
1. Stirred Tank
Biomining - This method is used for leaching from substrates
with high
mineral
concentration. Since the method is expensive and time consuming, substrates
with lower concentration are not used for leaching. Copper and refractory gold ores are
well suited for this type of method. Special types of stirred tank bioreactors lined with
rubber or corrosion resistant steel and insulated with cooling pipes or cooling jackets are
used for this purpose.
with lower concentration are not used for leaching. Copper and refractory gold ores are
well suited for this type of method. Special types of stirred tank bioreactors lined with
rubber or corrosion resistant steel and insulated with cooling pipes or cooling jackets are
used for this purpose.
Thiobacillus
is the commonly used bacteria. Since it is aerobic the bioreactor is provided
with an abundant supply of oxygen throughout the process provided by aerators, pumps
and blowers. This is a multi-step process consisting of large numbers of bioreactors
connected to each other. The substrate moves from one reactor to another and in the final
stage it is washed with water and treated with a variety of chemicals to recover the
mineral.
with an abundant supply of oxygen throughout the process provided by aerators, pumps
and blowers. This is a multi-step process consisting of large numbers of bioreactors
connected to each other. The substrate moves from one reactor to another and in the final
stage it is washed with water and treated with a variety of chemicals to recover the
mineral.
The name
is fairly self-explanatory, as the process requires constructing large aerated
tanks that are generally arranged in a series, so that runoff from one tank serves as raw
material for the next. In this way, the reactor can operate in continuous flow mode, with
fresh ore being added to the first tank while the runoff from the final tank is removed and
treated. The ore to be processed is generally crushed to a very small particle size, to
ensure that the solids remain suspended in the liquid medium. Mineral nutrients in the
form of (NH4)2SO4 and KH2PO4 are also added to the tanks to ensure maximal microbial density is maintained.
tanks that are generally arranged in a series, so that runoff from one tank serves as raw
material for the next. In this way, the reactor can operate in continuous flow mode, with
fresh ore being added to the first tank while the runoff from the final tank is removed and
treated. The ore to be processed is generally crushed to a very small particle size, to
ensure that the solids remain suspended in the liquid medium. Mineral nutrients in the
form of (NH4)2SO4 and KH2PO4 are also added to the tanks to ensure maximal microbial density is maintained.
Due to the
extremely high cost of stirred tank reactors, they are only used for highly
valuable materials. For gold extraction for example, this technique is usually used when
the ore body contains high concentrations of arsenopyrite (AsFeS).
valuable materials. For gold extraction for example, this technique is usually used when
the ore body contains high concentrations of arsenopyrite (AsFeS).
2. Bioheaps - Bioheaps are large amounts of low grade ore and effluents from
extraction
processes
that contain trace amounts of minerals. Such effluents are usually stacked in
large open space heaps and treated with microorganisms to extract the minerals. Bioheaps
are also called biopiles, biomounds and biocells.
large open space heaps and treated with microorganisms to extract the minerals. Bioheaps
are also called biopiles, biomounds and biocells.
The low
grade ores like refractory sulfide gold ore and chalocite ore (copper) are
crushed first to reduce the size then treated with acid to promote growth and
multiplication of chemophilic bacteria. The crushed and acid-treated ore is
then agglomerated so that the finer particles get attached to the coarser ones,
and then treated with water or other effluent liquid. This is done to optimize
moisture content in the ore bacteria that is inoculated along with the liquid.
The ore is then stacked in large heaps of 2-10 feet high with aerating tubes to
provide air supply to the bacteria thus promoting biooxidation.
Advantages of using
bioheaps are that they are: (a) cost effective, (b) of simple design
and easy to implement,
and (c) very effective in extracting from low concentration ores.
Disadvantages
of using bioheaps are that they: (a) are time consuming (takes about 6-24
months), (b) have a very low yield of mineral, require a large open area for treatment,
have no process control, (c) are at high risk of contamination, and (d) have inconsistent
yields because bacteria may not grow uniformly in the heap.
months), (b) have a very low yield of mineral, require a large open area for treatment,
have no process control, (c) are at high risk of contamination, and (d) have inconsistent
yields because bacteria may not grow uniformly in the heap.
3. In-situ Bioleaching
- In this method the mineral is extracted directly from the
mine
instead of
collecting the ore and transferring to an extracting facility away from the
site
of the mine. In-situ bio-mining is usually done to extract trace amounts of minerals
present in the ores after a conventional extraction process is completed. The mine is
blasted to reduce the ore size and to increase permeability and is then treated with water
and acid solution with bacterial inoculum. Air supply is provided using pipes or shafts.
Biooxidation takes place in-situ due to growing bacteria and results in the extraction of
mineral from the ore.
of the mine. In-situ bio-mining is usually done to extract trace amounts of minerals
present in the ores after a conventional extraction process is completed. The mine is
blasted to reduce the ore size and to increase permeability and is then treated with water
and acid solution with bacterial inoculum. Air supply is provided using pipes or shafts.
Biooxidation takes place in-situ due to growing bacteria and results in the extraction of
mineral from the ore.
E. FACTORS EFFECTING BIOMINING:
Success of biomining
and efficiency in recovery of minerals depends on various factors
some of which are
discussed below.
(a) Choice of Bacteria
- This is the most important factor that determines the success
of
bioleaching. Suitable bacteria that can survive at high
temperatures, acid concentrations, high concentrations of heavy metals, remaining active under such
circumstances, are to be selected to ensure successful
bioleaching.
(b) Crystal
solubility of the sulfides. The sulfide ores with lower
crystal lattice energy have higher
solubility, hence, are easily extracted into solution by
the action of bacteria.
(c) Surface Area - Rate of oxidation by the
bacteria depends on the particle size of the
ore. The rate increases with reduction in size of the ore
and vice-versa.
(d) Ore Composition - Composition of ore
such as concentration of sulfides, amount of
mineral present, and the extent of contamination, has
direct effect on the rate of bio-
oxidation.
(e) Acidity - Biooxidation requires a pH of
2.5-3 for maximum results. The rate of
biooxidation decreases significantly if the Ph is not in
this range since the activity of
acidophilic bacteria is reduced.
(f) Temperature - The bacteria used in
biomining are either mesophilic or thermophilic.
Optimum temperature is required for biooxidation to proceed
at a fast rate. Optimum
temperature range for a given bacteria is between 25-35° C depending on the type of ore
being selected. The rate of biooxidation is reduced significantly if the temperature is
above or below the optimum temperature.
temperature range for a given bacteria is between 25-35° C depending on the type of ore
being selected. The rate of biooxidation is reduced significantly if the temperature is
above or below the optimum temperature.
(g) Aeration - The bacteria used in
biomining are aerobic thus require an abundant supply
of oxygen for survival and growth. Oxygen can be provided
by aerators and pipes. Mechanical
agitation is also an effective method to provide continuous air supply uniformly and also to mix the contents.
(h) Solid-liquid Ratio - The ratio of
ore/sulfide to the leach solution (water + acid
solution + bacteria inoculum) should be maintained at
optimum level to ensure that
biooxidation proceeds at maximum speed. The leach solution containing leached
minerals should be removed periodically and replaced with new solution.
biooxidation proceeds at maximum speed. The leach solution containing leached
minerals should be removed periodically and replaced with new solution.
(i) Surfactants - Adding small amounts of
surfactants like Tween 20 to the leaching
process increases the rate of biooxidation of minerals from
sulfide ores. The surfactants decrease the surface tension of the leach solution, thus, wetting the
ore and resulting in increased bacterial contact which
ultimately increases the rate of biooxidation.
F. EXAMPLES OF BIOMINING:
(a) Biomining of Copper
- Copper was the first metal extracted by bioleaching. It is
the
metal most
commonly extracted from oxide ores by this method. In the United States ,
alone, about 11% of copper is produced from low grade ores by bioleaching technique
every year. Copper is available in mines across the world in more than 350 types of ores,
but it is mainly present along with sulfur. Copper from low-grade ores like copper sulfide
minerals is most commonly extracted by biooxidation since it is not economically viable
to use conventional metallurgical techniques
alone, about 11% of copper is produced from low grade ores by bioleaching technique
every year. Copper is available in mines across the world in more than 350 types of ores,
but it is mainly present along with sulfur. Copper from low-grade ores like copper sulfide
minerals is most commonly extracted by biooxidation since it is not economically viable
to use conventional metallurgical techniques
(b) Biomining of Gold - Biooxidation of refractory gold ores to extract gold is carried
out by a
commercial procedure called BIOX developed by GENCOR S.A Ltd
Johannesburg SouthAfrica in an effort to
replace existing procedures which posed severe
pollution problems. The BIOX process had several advantages over existing procedures
including lower cost.
Johannesburg South
pollution problems. The BIOX process had several advantages over existing procedures
including lower cost.
(c) Microbially
Enhanced Oil Recovery (MEOR) - Recent technological
developments
have helped to make
possible the recovery of oil. Using microorganisms is one such
technique to improve
the recovery process hence called “microbially enhanced oil
recovery” (MEOR). It was discovered in 1926 that microorganisms can be used in the
petroleum industry to
enhance oil recovery, but the concept became popular only after the
1950s. Microbes can
enhance the recovery of petroleum products directly or indirectly.
G. FUTURE TREND OF
BIOMINING FOR PROCESSING OF MINERALS :
Although
mining is one of humankind's oldest activities, the techniques used to extract
minerals haven't changed substantially for centuries. Ores are dug from the earth,
crushed, then minerals such as copper and gold are extracted by extreme heat or toxic
chemicals. The environmental and health effects of traditional mining technologies have
been deleterious.
minerals haven't changed substantially for centuries. Ores are dug from the earth,
crushed, then minerals such as copper and gold are extracted by extreme heat or toxic
chemicals. The environmental and health effects of traditional mining technologies have
been deleterious.
In the past few years,
the mining industry has been turning to a more efficient and
environmentally
salubrious method for extracting minerals from ores: microorganisms
that leach
them out. Using a bacterium such as Thiobacillus ferooxidans to leach copper
from mine tailings has improved recovery rates and reduced operating costs. Moreover, it
permits extraction from low grade ores - an important consideration in the face of the
depletion of high grade ores.
from mine tailings has improved recovery rates and reduced operating costs. Moreover, it
permits extraction from low grade ores - an important consideration in the face of the
depletion of high grade ores.
Thiobacillus
ferooxidans, which is naturally present in certain sulfur-containing
materials, gets energy by oxidizing inorganic materials, such as copper sulfide minerals.
This process releases acid and an oxidizing solution of ferric ions, which can wash out
metals from crude ore. Poor quality copper ore, which is bound up in a sulfide matrix, is
dumped outside a mine and treated with sulfuric acid to encourage the growth of T.
ferooxidans. As the bacteria chew up the ore, copper is released and collected in solution.
The sulfuric acid is recycled.
materials, gets energy by oxidizing inorganic materials, such as copper sulfide minerals.
This process releases acid and an oxidizing solution of ferric ions, which can wash out
metals from crude ore. Poor quality copper ore, which is bound up in a sulfide matrix, is
dumped outside a mine and treated with sulfuric acid to encourage the growth of T.
ferooxidans. As the bacteria chew up the ore, copper is released and collected in solution.
The sulfuric acid is recycled.
Currently
25% of all copper worldwide, worth more than $1 billion annually, is produced
through bioprocessing. This ranks it as one of the most important applications of
biotechnology today. Bioprocessing is also being used to economically extract gold from
very low grade, sulfidic gold ores, once thought to be worthless.
through bioprocessing. This ranks it as one of the most important applications of
biotechnology today. Bioprocessing is also being used to economically extract gold from
very low grade, sulfidic gold ores, once thought to be worthless.
H. SUMMARY AND
CONCLUSION:
Biomining
is the sustainable, biotechnological process utilizing microorganisms to
remove metals from sulfide mineral ores and concentrates. The development of
biomining has progressed from poorly designed dumps to highly engineered heaps and
stirred tank reactors in an industrially important biotechnological process. The release of
metals from sulfide minerals is catalyzed by iron oxidizing acidophilic (optimum pH for
growth <3) microorganisms that act in consortia with heterotrophic and sulfur oxidizing
acidophiles in a mixed culture. The microorganisms catalyze metal release by
regenerating ferric iron that oxidizes the mineral sulfide bond to produce metals and
reduced inorganic sulfur compounds. The other microorganisms in the mixed culture may
oxidize the reduced inorganic sulfur compounds to sulfuric acid that is the source of the
acid in these environments.
remove metals from sulfide mineral ores and concentrates. The development of
biomining has progressed from poorly designed dumps to highly engineered heaps and
stirred tank reactors in an industrially important biotechnological process. The release of
metals from sulfide minerals is catalyzed by iron oxidizing acidophilic (optimum pH for
growth <3) microorganisms that act in consortia with heterotrophic and sulfur oxidizing
acidophiles in a mixed culture. The microorganisms catalyze metal release by
regenerating ferric iron that oxidizes the mineral sulfide bond to produce metals and
reduced inorganic sulfur compounds. The other microorganisms in the mixed culture may
oxidize the reduced inorganic sulfur compounds to sulfuric acid that is the source of the
acid in these environments.
Now,
tremendous improvements in biomining are expected with continued research in
identifying bacterial strains better suited for individual applications and large-scale
operations as well as in the genetic engineering of bacterial strains that can stand up to
high temperature processes and heavy metals such as arsenic, mercury, or cadmium.
Biomining has become one of the premier mining technologies, and the future appears
bright. The potential applications of biotechnology to mining and processing are
countless. Some examples of past projects in biotechnology include a biologically
assisted in situ mining program, biodegradation methods, passive bioremediation of acid
rock drainage, and bioleaching of ores and concentrates. Research often results in
technology implementation for greater efficiency and productivity or novel solutions to
complex problems. Additional capabilities include the bioleaching of metals from sulfide
materials, phosphate ore bioprocessing, and the bioconcentration of metals from
solutions. One project recently under investigation is the use of biological methods for
the reduction of sulfur in coal-cleaning applications. From in situ-mining to mineral
processing and treatment technology, biotechnology provides innovative and cost-
effective industry solutions
identifying bacterial strains better suited for individual applications and large-scale
operations as well as in the genetic engineering of bacterial strains that can stand up to
high temperature processes and heavy metals such as arsenic, mercury, or cadmium.
Biomining has become one of the premier mining technologies, and the future appears
bright. The potential applications of biotechnology to mining and processing are
countless. Some examples of past projects in biotechnology include a biologically
assisted in situ mining program, biodegradation methods, passive bioremediation of acid
rock drainage, and bioleaching of ores and concentrates. Research often results in
technology implementation for greater efficiency and productivity or novel solutions to
complex problems. Additional capabilities include the bioleaching of metals from sulfide
materials, phosphate ore bioprocessing, and the bioconcentration of metals from
solutions. One project recently under investigation is the use of biological methods for
the reduction of sulfur in coal-cleaning applications. From in situ-mining to mineral
processing and treatment technology, biotechnology provides innovative and cost-
effective industry solutions
References:
1. http://knol.google.com/k/partha-das-sharma/biomining/oml631csgjs7/8
1. http://knol.google.com/k/partha-das-sharma/biomining/oml631csgjs7/8
2. www.ebookee.com.cn/Biomining_142912.html
3. Davis
2000.
Rio Tinto estuary (Spain
1107-1116.
4. Brierley, C.L. and J.A. Brierley. 1997.
Microbiology for the Metal Mining Industry. in Manual of Environmental
Microbiology. (Ed.) C.J. Hurst. ASM Press, Washington D.C.
Biotechnology.
7.
Rawlings, D.E. 2002. Heavy metal mining using microbes. Annual Review of
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