BIOMINING
INTRODUCTION
Biomining is
defined as extracting mineral ores or enhancing the mineral recovery from mines
using microorganisms instead of traditional mining methods. Copper was the
first metal extracted using microorganisms in the ancient past in the
Mediterranean region. Biomining is becoming popular because it is cheap,
reliable, efficient, safe, and environmentally friendly, unlike traditional
mining methods. The efficiency of biomining 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 is an application of biotechnology
and is also known as microbial leaching or alternately, bio-oxidation.
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. It is
very important to select suitable microorganisms to ensure the success of biomining,
a process which requires knowledge of properties of microorganisms, both
physiological and biochemical. Bacteria are found to be the most suitable
microorganisms that can be used in biomining.
Characteristics of the bacteria used
in biomining.
1. Mineral extraction involves the production
of high temperatures so the bacteria should be able to survive the heat, hence,
they should be thermophilic.
2. Biomining
involves using strong acids and alkalis, hence, bacteria should be chemophilic.
3. Bacteria
should produce energy from inorganic compounds, hence, should also be
autotrophic.
4. The
bacteria should be able to adhere to the solid surfaces or have the ability to
form biofilms.
Identification of Bacteria Useful for
Biomining Operations
There are
wide varieties of bacteria with varying capabilities existing on earth;
therefore, it is essential to identify precisely the types that can perform
biooxidation/bioleaching effectively.
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. 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.
Techniques
like genetic engineering and conjugation are used to produce bacteria with
desired characteristics to increase the rate of biooxidation thus increasing
the mineral yield through biomining. It
is also important to identify biomining bacteria present in colonies of other
bacteria. Techniques developed for this purpose include: immune fluorescence,
dot immunoassay, and dot-blot hybridization. Immuno fluorescence, this
technique is generally used to identify specific antibodies or antigens present
in biological fluids. Fluorescent antibodies are used to identify biomining
bacteria. Dot Immunoassay This technique is used to identify ore-adhering
bacteria like 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. Dot-blot Hybridization, This is a
DNA based technique to identify biomining bacteria such as T.ferrooxidans. The
bacteria are isolated from samples of ores and soil treated with sodium dodecyl
sulfate (SDS). The cells are disrupted to extract DNA and the extracted DNA is
then purified. The DNA obtained from ore sample is fixed on nitrocellulose
membrane using southern blotting technique. Genetic probes are used to identify
and distinguish various biomining bacteria used in this procedure. The DNA
fragments on the membrane are treated with standard probes.
BIOMINING RECOVERY
Minerals are
recovered from ores by the microorganisms mainly by two mechanisms: oxidation
and reduction.
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
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 sulphides.
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++
Reduction
Bacteria
like Desulfovibro desulfuricans play an active role in reduction of sulfates
which results in the formation of hydrogen sulphides.
4H2 +
H2SO4 -->
H2S + 4H2O
TYPES OF
BIOMINING
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. 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.
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. They are also used
for biodegradation of petroleum and chemical wastes. 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:
·
Cost
effective
·
of
simple design and easy to implement
·
and
very effective in extracting from low concentration ores
Disadvantages of using bioheaps are that they:
·
Are
time consuming (takes about 6-24 months),
·
have
a very low yield of mineral,
·
require
a large open area for treatment,
·
Have
no process control,
·
are
at high risk of contamination,
·
Have
inconsistent yields because bacteria may not grow uniformly in the heap.
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 biomining 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.
Factors Effecting Biomining
Biomining
success depends on various factors some of which are discussed below.
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.
Crystal Lattice Energy
This
determines the mechanical stability and degree of 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.
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.
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.
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.
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.
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.
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.
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.
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.
Procedure
Low grade
copper ore is brought to the dump leaching site. The dump surface is wetted
uniformly with water and sulfuric acid using sprayers to maintain acidity which
helps the growth of acidophilic bacteria and bacterial inoculum. Air is
supplied to the dump through channels constructed for this purpose while
building the dump. Biooxidation takes place over the course of time and copper
is leached into the solution which is collected at the bottom of the heap. The
leach solution rich in copper is treated chemically using electrowinning and
solvent extraction techniques to extract pure copper.
Electrowinning
In this
technique the leach solution containing copper leached from the dump is
circulated through an electrowining cell and electricity is passed. Pure copper
is obtained from the cell in the form of electro-won cathode. An Electrowining
cell is basically a simple electro-voltaic cell with a lead or graphite anode
and aluminum cathode with the leach solution being the electrolyte. When the
electricity is passed through the cell, the copper ions present in the
electrolyte are reduced to metallic copper and become deposited over the
cathode.
Solvent Extraction
Copper
solvent extraction systems consist of three loops. In the first loop the leach
solution containing copper obtained from dump leaching is passed through the
extraction chamber. Here the leach solution comes in contact with organic extractant
which extracts copper from it. Leach solution and organic extractant are passed
through the leaching chamber for further leaching. The copper-rich organic
extractant then enters the second loop and passes through stripping chamber.
The stripping chamber consists of highly acidic electrolyte which strips copper
from the organic extractant. The organic extractant is directed back to the
extraction chamber in the first loop. The copper-rich acidic electrolyte enters
the third loop and is subjected to electrowinning to extract pure copper. The
spent electrolyte is directed back to the stripping chamber in the second loop.
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