Biomining
Encyclopedia
Biomining is a new approach to the extraction of desired mineral
s from ore
s being explored by the mining industry in the past few years. Microorganism
s are used to leach out the minerals, rather than the traditional methods of extreme heat or toxic chemicals, which have a deleterious effect on the environment.
, Brazil
and Australia
. Iron-and sulfur-oxidizing microorganisms are used to release occluded copper
, gold
and uranium
from mineral sulfides. Most industral plants for biooxidation of gold-bearing concentrates have been operated at 40°C with mixed cultures of mesophilic bacteria of the genera Thiobacillus or Leptospirillum ferrooxidans. In subsequent studies the dissimulatory iron-reducing archaea
Pyrococcus furiosus
and Pyrobaculum islandicum were shown to reduce gold chloride to insoluble gold.
Using Bacteria such as Acidithiobacillus ferrooxidans 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.
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. This 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.
The potential of thermophilic sulfide-oxidizing archaea in copper extraction has attracted interest due to the efficient extraction of metals from sulfide ores that are recalcitrant to dissolution
. Microbial leaching
is especially useful for copper ores because copper sulfate, formed during the oxidation of copper sulfide ores is very water-soluble. Approximately 25% of all copper mined worldwide is now obtained from leaching processes. The acidophilic
archaea Sulfolobus metallicus and Metallosphaera sedula tolerate up to 4% of copper and have been exploited for mineral biomining. Between 40 and 60% copper extraction was achieved in primary reactors and more than 90% extraction in secondary reactors with overall residence times of about 6 days.
The oxidation of the ferrous ion (Fe2+) to the ferric ion (Fe3+) is an energy producing reaction for some microorganisms. As only a small amount of energy is obtained, large amounts of (Fe2+) have to be oxidized. Furthermore, (Fe3+) forms the insoluble precipitate in H2O. Many Fe2+ oxidizing microorganisms also oxidize sulfur and are thus obligate acidophiles that further acidify the environment by the production of H2SO4. This is due in part to the fact that at neutral pH Fe2+ is rapidly oxidized chemically in contact with the air. In these conditions there is not enough Fe2+ to allow significant growth. At low pH, however, Fe2+ is much more stable. This explains why most of the Fe2+ oxidizing microorganisms are only found in acidic environments and are obligate acidophiles.
The best studied Fe2+ oxidizing bacterium is Thiobacillus ferrooxidans, an acidophililic chemolithotroph. The microbiological oxidation of Fe2+ is an important aspect of the development of acidic pH’s in mines, and constitutes a serious ecological problem. However, this process can also be usefully exploited when controlled. The sulfur containing ore pyrite (FeS2) is at the start of this process. Pyrite
is an insoluble cristalline structure that is abundant in coal- and mineral ores. It is produced by the following reaction:
S + FeS → FeS2
Normally pyrite is shielded from contact with oxygen and not accessible for microorganisms. Upon exploitation of the mine, however, pyrite is brought into contact with air (oxygen) and microorganisms and oxidation will start. This oxidation relies on a combination of chemically and microbiologically catalyzed processes. Two electron acceptors can influence this process: O2 and Fe3+ ions. The latter will only be present in significant amounts in acidic conditions (pH < 2.5).
First a slow chemical process with O2 as electron acceptor will initiate the oxidation of pyrite:
FeS2 + 7/2 O2 + H2O → Fe2+ + 2 SO42- + 2 H+
This reaction acidifies the environment and the Fe2+ will be formed is rather stable. In such an environment Thiobacillus ferrooxidans will be able to grow rapidly. Upon further acidification Ferroplasma
will also develop and further acidify. As a consequence of the microbial activity (energy producing reaction):
Fe2+ → Fe3+
This Fe3+ that remains soluble at low pH reacts spontaneously with the pyrite:
FeS2 + 14 Fe3+ + 8 H2O → 15 Fe2+ + 2 SO42- + 16 H+
The produced Fe2+ can again be used by the microorganisms and thus a cascade reaction will be initiated.
In the industrial microbial leaching process, low grade ore is dumped in a large pile (the leach dump) and a dilute sulfuric acid solution (pH 2) is percolated down through the pile. The liquid coming out at the bottom of he pile, rich in the mineral is collected and transported to a precipitation plant where the metal is reprecipitated and purified. The liquid is then pumped back to the top of the pile and the cycle is repeated.
Thiobacillus ferrooxidans is able to oxidize the Cu+ in chalcocite
(Cu2S) to Cu2+, thus removing some of the cupper in the soluble form, Cu2+, and forming the mineral covellite
(CuS). This oxidation of Cu+ to Cu2+ is an energy yielding reaction (such as the oxidation of Fe2+ to Fe3+). Covellite can then be oxidized, releasing sulfate and soluble Cu2+ as products.
A second mechanism, and probably the most important in most mining operations, involves chemical oxidation of the copper ore with ferric (Fe3+) ions formed by the microbial oxidation of ferrous ions (derived from the oxidation of pyrite).
Three possible reactions for the oxidation of copper ore are:
Cu2S + 1/2 O2 + 2 H+ → CuS + Cu2+ + H2O
CuS + 2 O2 → Cu2+ + SO42-
CuS + 8 Fe3+ + 4 H2O → Cu2+ + 8 Fe2+ + SO42- + 8 H+
The copper metal is the recovered by using Fe0 from steel cans:
Fe0 + Cu2+ → Cu0 + Fe2+
The temperature inside the leach dump often rises spontaneously as a result of microbial activities. Thus, thermophilic iron-oxidizing chemolithotrophs such as thermophilic Thiobacillus species and Leptospirillum and at even higher temperatures the thermoacidophilic archaeon Sulfolobus (Metallosphaera sedula) may become important in the leaching process above 40°C. Similarly to copper, Thiobacillus ferrooxidans can oxidize U4+ to U6+ with O2 as electron acceptor. However, it is likely that the uranium leaching process depends more on the chemical oxidation of uranium by Fe3+, with T. ferrooxidans contributing mainly through the reoxidation of Fe2+ to Fe3+ as described above.
UO2 + Fe(SO4)3 → UO2SO4 + 2 FeSO4
Gold
is frequently found in nature associated with minerals containing arsenic
and pyrite. In the microbial leaching process T. ferrooxidans and relatives are able to attack and solubilize the arsenopyrite minerals, and in the process, releasing the trapped gold (Au):
2 FeAsS[Au] + 7 O2 + 2 H2O + H2SO4 → Fe(SO4)3 + 2 H3AsO4 + [Au]
The above information can be verified in "NBIAP News Report." U.S. Department of Agriculture (June 1994).
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s from ore
Ore
An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....
s being explored by the mining industry in the past few years. Microorganism
Microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...
s are used to leach out the minerals, rather than the traditional methods of extreme heat or toxic chemicals, which have a deleterious effect on the environment.
Overview
The development of industrial mineral processing has been established now in several countries including South AfricaSouth Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
, Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
and Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
. Iron-and sulfur-oxidizing microorganisms are used to release occluded copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
and uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
from mineral sulfides. Most industral plants for biooxidation of gold-bearing concentrates have been operated at 40°C with mixed cultures of mesophilic bacteria of the genera Thiobacillus or Leptospirillum ferrooxidans. In subsequent studies the dissimulatory iron-reducing archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
Pyrococcus furiosus
Pyrococcus furiosus
Pyrococcus furiosus is an extremophilic species of Archaea. It can be classified as a hyperthermophile because it thrives best under extremely high temperatures—higher than those preferred of a thermophile...
and Pyrobaculum islandicum were shown to reduce gold chloride to insoluble gold.
Using Bacteria such as Acidithiobacillus ferrooxidans to leach copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
from mine tailings
Tailings
Tailings, also called mine dumps, slimes, tails, leach residue, or slickens, are the materials left over after the process of separating the valuable fraction from the uneconomic fraction of an ore...
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.
The potential applications of biotechnology to mining and processing are countless. Some examples of past projects in biotechnology include a biologically assisted in situ
In situ
In situ is a Latin phrase which translated literally as 'In position'. It is used in many different contexts.-Aerospace:In the aerospace industry, equipment on board aircraft must be tested in situ, or in place, to confirm everything functions properly as a system. Individually, each piece may...
mining program, biodegradation methods, passive bioremediation of acid rock drainage, and bioleaching of ores and concentrates. This 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.
The potential of thermophilic sulfide-oxidizing archaea in copper extraction has attracted interest due to the efficient extraction of metals from sulfide ores that are recalcitrant to dissolution
Dissolution (chemistry)
Dissolution is the process by which a solid, liquid or gas forms a solution in a solvent. In solids this can be explained as the breakdown of the crystal lattice into individual ions, atoms or molecules and their transport into the solvent. For liquids and gases, the molecules must be compatible...
. Microbial leaching
Bioleaching
Bioleaching is the extraction of specific metals from their ores through the use of living organisms. This is much cleaner than the traditional heap leaching using cyanide...
is especially useful for copper ores because copper sulfate, formed during the oxidation of copper sulfide ores is very water-soluble. Approximately 25% of all copper mined worldwide is now obtained from leaching processes. The acidophilic
Acidophile (organisms)
Acidophilic organisms are those that thrive under highly acidic conditions . These organisms can be found in different branches of the tree of life, including Archaea, Bacteria, and Eukaryotes...
archaea Sulfolobus metallicus and Metallosphaera sedula tolerate up to 4% of copper and have been exploited for mineral biomining. Between 40 and 60% copper extraction was achieved in primary reactors and more than 90% extraction in secondary reactors with overall residence times of about 6 days.
The oxidation of the ferrous ion (Fe2+) to the ferric ion (Fe3+) is an energy producing reaction for some microorganisms. As only a small amount of energy is obtained, large amounts of (Fe2+) have to be oxidized. Furthermore, (Fe3+) forms the insoluble precipitate in H2O. Many Fe2+ oxidizing microorganisms also oxidize sulfur and are thus obligate acidophiles that further acidify the environment by the production of H2SO4. This is due in part to the fact that at neutral pH Fe2+ is rapidly oxidized chemically in contact with the air. In these conditions there is not enough Fe2+ to allow significant growth. At low pH, however, Fe2+ is much more stable. This explains why most of the Fe2+ oxidizing microorganisms are only found in acidic environments and are obligate acidophiles.
The best studied Fe2+ oxidizing bacterium is Thiobacillus ferrooxidans, an acidophililic chemolithotroph. The microbiological oxidation of Fe2+ is an important aspect of the development of acidic pH’s in mines, and constitutes a serious ecological problem. However, this process can also be usefully exploited when controlled. The sulfur containing ore pyrite (FeS2) is at the start of this process. Pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
is an insoluble cristalline structure that is abundant in coal- and mineral ores. It is produced by the following reaction:
S + FeS → FeS2
Normally pyrite is shielded from contact with oxygen and not accessible for microorganisms. Upon exploitation of the mine, however, pyrite is brought into contact with air (oxygen) and microorganisms and oxidation will start. This oxidation relies on a combination of chemically and microbiologically catalyzed processes. Two electron acceptors can influence this process: O2 and Fe3+ ions. The latter will only be present in significant amounts in acidic conditions (pH < 2.5).
First a slow chemical process with O2 as electron acceptor will initiate the oxidation of pyrite:
FeS2 + 7/2 O2 + H2O → Fe2+ + 2 SO42- + 2 H+
This reaction acidifies the environment and the Fe2+ will be formed is rather stable. In such an environment Thiobacillus ferrooxidans will be able to grow rapidly. Upon further acidification Ferroplasma
Ferroplasma
In taxonomy, Ferroplasma is a genus of the Ferroplasmaceae.The genus Ferroplasma consists solely of F. acidophilum, an acidophilic iron-oxidizing member of the Euryarchaeota. Unlike other members of the Thermoplasmata F.acidophilum is a mesophile with a temperature optimum of approximately 35°C, at...
will also develop and further acidify. As a consequence of the microbial activity (energy producing reaction):
Fe2+ → Fe3+
This Fe3+ that remains soluble at low pH reacts spontaneously with the pyrite:
FeS2 + 14 Fe3+ + 8 H2O → 15 Fe2+ + 2 SO42- + 16 H+
The produced Fe2+ can again be used by the microorganisms and thus a cascade reaction will be initiated.
In the industrial microbial leaching process, low grade ore is dumped in a large pile (the leach dump) and a dilute sulfuric acid solution (pH 2) is percolated down through the pile. The liquid coming out at the bottom of he pile, rich in the mineral is collected and transported to a precipitation plant where the metal is reprecipitated and purified. The liquid is then pumped back to the top of the pile and the cycle is repeated.
Thiobacillus ferrooxidans is able to oxidize the Cu+ in chalcocite
Chalcocite
Chalcocite, copper sulfide , is an important copper ore mineral. It is opaque, being colored dark-gray to black with a metallic luster. It has a hardness of 2½ - 3. It is a sulfide with an orthorhombic crystal system....
(Cu2S) to Cu2+, thus removing some of the cupper in the soluble form, Cu2+, and forming the mineral covellite
Covellite
Covellite is a rare copper sulfide mineral with the formula CuS. This indigo blue mineral is ubiquitous in copper ores, it is found in limited abundance and is not an important ore of copper itself, although it is well known to mineral collectors.The mineral is associated with chalcocite in zones...
(CuS). This oxidation of Cu+ to Cu2+ is an energy yielding reaction (such as the oxidation of Fe2+ to Fe3+). Covellite can then be oxidized, releasing sulfate and soluble Cu2+ as products.
A second mechanism, and probably the most important in most mining operations, involves chemical oxidation of the copper ore with ferric (Fe3+) ions formed by the microbial oxidation of ferrous ions (derived from the oxidation of pyrite).
Three possible reactions for the oxidation of copper ore are:
Cu2S + 1/2 O2 + 2 H+ → CuS + Cu2+ + H2O
CuS + 2 O2 → Cu2+ + SO42-
CuS + 8 Fe3+ + 4 H2O → Cu2+ + 8 Fe2+ + SO42- + 8 H+
The copper metal is the recovered by using Fe0 from steel cans:
Fe0 + Cu2+ → Cu0 + Fe2+
The temperature inside the leach dump often rises spontaneously as a result of microbial activities. Thus, thermophilic iron-oxidizing chemolithotrophs such as thermophilic Thiobacillus species and Leptospirillum and at even higher temperatures the thermoacidophilic archaeon Sulfolobus (Metallosphaera sedula) may become important in the leaching process above 40°C. Similarly to copper, Thiobacillus ferrooxidans can oxidize U4+ to U6+ with O2 as electron acceptor. However, it is likely that the uranium leaching process depends more on the chemical oxidation of uranium by Fe3+, with T. ferrooxidans contributing mainly through the reoxidation of Fe2+ to Fe3+ as described above.
UO2 + Fe(SO4)3 → UO2SO4 + 2 FeSO4
Gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
is frequently found in nature associated with minerals containing arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
and pyrite. In the microbial leaching process T. ferrooxidans and relatives are able to attack and solubilize the arsenopyrite minerals, and in the process, releasing the trapped gold (Au):
2 FeAsS[Au] + 7 O2 + 2 H2O + H2SO4 → Fe(SO4)3 + 2 H3AsO4 + [Au]
The above information can be verified in "NBIAP News Report." U.S. Department of Agriculture (June 1994).