5.8 Metallurgical Methods for Metals Recovery
149
5.8.4.1 Bioleaching
As mentioned before, bioleaching is the extraction of metals from ores or concentrates
by interactions between microorganisms and minerals. It is a commercially feasible
and a low-cost solution to metals recovery and has been successfully applied to the
recovery of metals from metallic sulfides. The reaction is carried out in an acidic
environment with a pH value from 1 to 3, in which most metal ions remain in the
solution. Three major groups of microorganisms commonly used for bioleaching are
[17, 34]:
– Mesophilic bacteria: microorganisms that grow best at temperatures of 25 to 35 °C.
The most important mesophilic bacteria in bioleaching include: (1) Thiobacillus:
This is the most active bacteria in bioleaching. Most of the thiobacillus bacteria
are chemolithoautotrophic species that are autotrophic microorganisms obtaining
energy from simple substances such as carbon dioxide. Two types of thiobacillus,
thiobacillus thiooxidans and thiobacillus ferrooxidans, are acidophilic species
that are able to oxidize sulfur and sulfides in an acidic environment. Thiobacillus
thiooxidans is famous for speedy oxidation of sulfur, while thiobacillus ferrooxidans is well known for oxidizing ferrous iron; (2) Leptospirillum: Leptospirillum
ferrooxidans is also an acidophilic chemolithoautotroph. This bacterium is active
at lower pH values and a higher concentration of metals compared to thiobacillus
ferrooxidans. However, it is unable to oxidize sulfur or sulfur compounds by itself
and needs to be used in combination with thiobacillus bacteria.
– Thermophilic bacteria: microorganisms that grow at a temperature range of 50
°C. They use ferrous iron as their energy source but they need yeast extract for
their growth. At anaerobic conditions, these bacteria use elemental sulfur as an
electron acceptor and reduce it to H 2 S. Acidianus brierleyi are a genus of the
sulfolobus and an aerobic, chemolithoautotrophic species that grow on ferrous
iron, elemental sulfur and metal sulfides while oxidizing them.
– Heterotrophic bacteria: microorganisms that require organic carbon for growth,
as opposed to autotrophs like algae that use sunlight. These microorganisms are
not very useful for metals leaching. The most effective ones are the members of
the genus Bacillus from bacteria and the genera Aspergillus and Penicillium from
fungi.
At present, bacterial bioleaching of metal sulfides is of great importance. There
are two possible mechanisms for the release of metals from sulfide minerals: the
direct and indirect bacterial leaching [20, 34].
– Direct bacterial leaching
In the direct mechanism, the microorganisms directly oxidize the mineral sulfides
and solubilize metals through several enzymatic reactions according to the following
simplified reactions:
MS + H 2 SO 4 + 0.5O 2 bacteria
− −−−→
MSO 4 + S
0
+ H 2 O
(5.17)
149
5.8.4.1 Bioleaching
As mentioned before, bioleaching is the extraction of metals from ores or concentrates
by interactions between microorganisms and minerals. It is a commercially feasible
and a low-cost solution to metals recovery and has been successfully applied to the
recovery of metals from metallic sulfides. The reaction is carried out in an acidic
environment with a pH value from 1 to 3, in which most metal ions remain in the
solution. Three major groups of microorganisms commonly used for bioleaching are
[17, 34]:
– Mesophilic bacteria: microorganisms that grow best at temperatures of 25 to 35 °C.
The most important mesophilic bacteria in bioleaching include: (1) Thiobacillus:
This is the most active bacteria in bioleaching. Most of the thiobacillus bacteria
are chemolithoautotrophic species that are autotrophic microorganisms obtaining
energy from simple substances such as carbon dioxide. Two types of thiobacillus,
thiobacillus thiooxidans and thiobacillus ferrooxidans, are acidophilic species
that are able to oxidize sulfur and sulfides in an acidic environment. Thiobacillus
thiooxidans is famous for speedy oxidation of sulfur, while thiobacillus ferrooxidans is well known for oxidizing ferrous iron; (2) Leptospirillum: Leptospirillum
ferrooxidans is also an acidophilic chemolithoautotroph. This bacterium is active
at lower pH values and a higher concentration of metals compared to thiobacillus
ferrooxidans. However, it is unable to oxidize sulfur or sulfur compounds by itself
and needs to be used in combination with thiobacillus bacteria.
– Thermophilic bacteria: microorganisms that grow at a temperature range of 50
°C. They use ferrous iron as their energy source but they need yeast extract for
their growth. At anaerobic conditions, these bacteria use elemental sulfur as an
electron acceptor and reduce it to H 2 S. Acidianus brierleyi are a genus of the
sulfolobus and an aerobic, chemolithoautotrophic species that grow on ferrous
iron, elemental sulfur and metal sulfides while oxidizing them.
– Heterotrophic bacteria: microorganisms that require organic carbon for growth,
as opposed to autotrophs like algae that use sunlight. These microorganisms are
not very useful for metals leaching. The most effective ones are the members of
the genus Bacillus from bacteria and the genera Aspergillus and Penicillium from
fungi.
At present, bacterial bioleaching of metal sulfides is of great importance. There
are two possible mechanisms for the release of metals from sulfide minerals: the
direct and indirect bacterial leaching [20, 34].
– Direct bacterial leaching
In the direct mechanism, the microorganisms directly oxidize the mineral sulfides
and solubilize metals through several enzymatic reactions according to the following
simplified reactions:
MS + H 2 SO 4 + 0.5O 2 bacteria
− −−−→
MSO 4 + S
0
+ H 2 O
(5.17)