Bioleaching

Bioleaching ( literally in German : Biolaugung , more precisely: microbial ore leaching , rarely biomining ) refers to the extraction of heavy metals from your ores by chemical conversion of insoluble ore minerals to water-soluble salts by microorganisms . This allows the metals to be extracted from the ore.

The bioleaching forms a branch of biohydrometallurgy , the metal recovery through biological wet chemical processes, in which mainly copper , zinc , cobalt , nickel , gold and uranium are obtained. The most important representatives of leaching-active microorganisms are bacteria and archaea , which oxidize sulphide and elemental sulfur to sulphate and sometimes also divalent iron to trivalent iron . Among the bacteria, these are primarily the sulfur bacteria Acidithiobacillus ferrooxidans (sulfide, sulfur and iron oxidizing) and Acidithiobacillus thiooxidans (sulfide and sulfur oxidizing).

history

It is believed that copper was extracted from mine water in the Mediterranean around 3000 years ago . There is historical evidence that the Spaniards extracted copper in the 18th century on the Rio Tinto by microbial leaching of sulfidic copper ores. Since the discovery of the role of iron- and sulfur-oxidizing bacteria in ore leaching in 1940, the process of bioleaching has been investigated, clarified and technically used because of the increasingly scarce metal deposits . In Canada , uranium is bio-extracted directly from ores, in the USA and Chile copper is extracted from sulphidic pain, and in South Africa, gold has been extracted by bio-leaching since 1980.

Bio-leaching process

requirements

In order to be able to use bio-leaching effectively, various requirements must be met:

• Water must be readily available in large quantities.
• The ores must contain substances that can be oxidized by microorganisms ( sulfur , sulfides, iron (II) compounds). For rocks that are poor in iron and sulfur compounds or elemental sulfur, cheap pyrite (FeS ₂ , pebbles = iron (II) disulphide ), elemental sulfur, iron (II) sulphate (FeSO ₄ ) or iron (III) -Sulfate (Fe ₂ (SO ₄ ) ₃ ) can be added.
• Since solutions with the metals to be extracted are produced in low concentrations during bio-leaching, an inexpensive option for extraction or precipitation must be available.
• Growth substrates for the corresponding microorganisms must be available.

Importance of iron and sulfur bacteria

Iron and sulfur-oxidizing bacteria and archaea make a decisive contribution through their oxidative energy metabolism processes to converting poorly soluble sulfides such as copper iron disulfide (copper pebbles = chalcopyrite , CuFeS ₂ ) into water-soluble leachable sulfates ( copper sulfate and iron (II) sulfate).

The first and most important step in the dissolution of heavy metal sulfides that are difficult to dissolve in water is the abiotic oxidation of sulfide sulfur by iron (III) ions (Fe ³⁺ ) to elemental sulfur (S) or thiosulfate (S ₂ O ₃ ²⁻ ), which converts the heavy metals as ions become free and are dissolved in the aqueous solution. The iron (III) ions are reduced to iron (II) ions (Fe ²⁺ ). The role of iron- and sulfur-oxidizing bacteria and archaea is to (1) re-oxidize the iron (II) ions back to iron (III) ions and thus make further heavy metal sulfide available for the abiotic oxidation, and ( 2) to oxidize the resulting elemental sulfur or the resulting thiosulfate to sulfuric acid, whereby the aqueous solution is acidified and the dissolution of the heavy metal sulfides is favored. The abiotic and biotic oxidation of the sulfide releases the heavy metals from the sulfide minerals as dissolved ions. Iron and sulfur oxidizing bacteria work closely together in this way.

The sulfur bacteria Acidithiobacillus ferrooxidans (also iron-oxidizing) and Acidithiobacillus thiooxidans , the iron-oxidizing bacterium Leptospirillum ferrooxidans and the sulfur- and iron-oxidizing archaea Acidianus brierleyi and Sulfolobus acidocaldarius are acidophilic (acid-loving) and even produce sulfuric acid by sulfide, sulfur and thiosulfate oxidation. Acidianus brierleyi and Sulfolobus acidocaldarius are also thermophilic (loves high temperatures). In the leaching process, Acidithiobacillus ferrooxidans , Leptospirillum ferrooxidans , Acidianus brierleyi and Sulfolobus acidocaldarius oxidize divalent iron to trivalent iron, Acidithiobacillus ferrooxidans , Acidithiobacillus thiooxidans , Acidianus brierleyi and Sulfolobus acidocaldarius sulfur to oxidize elemental sulfur.

Chemical process

Heavy metal sulfide minerals

First, the sulphide sulfur of the heavy metal sulphides is abiotic oxidized by trivalent iron ions (Fe ³⁺ ), whereby these are reduced to iron (II) ions (Fe ²⁺ ). In the case of monosulfides and chalcopyrite (CuFeS ₂ ), the oxidation product is elemental sulfur; in the case of disulfides, thiosulfate (S ₂ O ₃ ²⁻ ) is the oxidation product (see equations 1 and 5). One consequence of this oxidation is the release of heavy metals as cations, which are water-soluble and are transported with the leaching liquid. However, heavy metal mobilization would soon come to a standstill for three reasons if it were not followed by biotic oxidations: (1) There would be a deficiency of Fe ³⁺ , which is required as an oxidant for the abiotic sulfide sulfur oxidation , as soon all Fe ³⁺ ions would occur would have been reduced to Fe ²⁺ ions. This is counteracted by the microbial oxidation of Fe ²⁺ to Fe ³⁺ . (2) The elemental sulfur formed would cover the mineral surfaces and prevent the Fe ^{3+ from} attacking . This is counteracted by the microbial oxidation of the elemental sulfur to sulfuric acid. (3) The pH value of the medium would increase due to the consumption of H ⁺ ions during iron oxidation (see equation 2). Even at medium pH values, Fe ³⁺ forms poorly soluble compounds with water, such as Fe (OH) ₃ and FeOOH, and this would lead to clogging and the Fe ³⁺ concentration would drop even further. This is counteracted by the microbial oxidation of elemental sulfur and thiosulfate, in which H ⁺ ions are formed, i.e. the pH value is lowered.

In the example of sphalerite (zinc blende, ZnS), the following oxidation reactions cause the mobilization of zinc:

(1)      abiotic ${\displaystyle \mathrm {ZnS+2\ Fe^{\,3+}\longrightarrow Zn^{\,2+}+2\ Fe^{\,2+}+S^{0}} }$

(2)      microbial (iron oxidizer) ${\displaystyle \mathrm {2\ Fe^{\,2+}+0,5\ O_{2}+2\ H^{+}\longrightarrow 2\ Fe^{\,3+}+H_{2}O} }$

(3)      microbial (sulfur oxidizer) ${\displaystyle \mathrm {S^{0}+1,5\ O_{2}+H_{2}O\longrightarrow SO_{4}^{\,2-}+2\ H^{+}} }$

(4) Sum:    ${\displaystyle \mathrm {ZnS+2\ O_{2}\longrightarrow Zn^{\,2+}+SO_{4}^{\,2-}} }$

Example pyrite (pebbles, FeS ₂ ):

(5)      abiotic ${\displaystyle \mathrm {FeS_{2}+6\ Fe^{\,3+}+3\ H_{2}O\longrightarrow 7\ Fe^{\,2+}+S_{2}O_{3}^{\,2-}+6\ H^{+}} }$

(6)      microbial (iron oxidizer) ${\displaystyle \mathrm {6\ Fe^{\,2+}+1,5\ O_{2}+6\ H^{+}\longrightarrow 6\ Fe^{\,3+}+3\ H_{2}O} }$

(7)      microbial (sulfur oxidizer) ${\displaystyle \mathrm {S_{2}O_{3}^{\,2-}+2\ O_{2}+H_{2}O\longrightarrow 2\ SO_{4}^{\,2-}+2\ H^{+}} }$

(8) Sum:    ${\displaystyle \mathrm {FeS_{2}+3,5\ O_{2}+H_{2}O\longrightarrow Fe^{\,2+}+2\ SO_{4}^{\,2-}+2\ H^{+}} }$

Example chalcopyrite (copper pyrites, CuFeS2):

(9)      abiotic ${\displaystyle \mathrm {CuFeS_{2}+4\ Fe^{\,3+}\longrightarrow Cu^{\,2+}+5\ Fe^{\,2+}+2\ S_{0}} }$

(10)      microbial (iron oxidizer) ${\displaystyle \mathrm {4\ Fe^{\,2+}+O_{2}+4\ H^{+}\longrightarrow 4\ Fe^{\,3+}+2\ H_{2}O} }$

(11)      microbial (sulfur oxidizer) ${\displaystyle \mathrm {2\ S^{0}+3\ O_{2}+2\ H_{2}O\longrightarrow 2\ SO_{4}^{\,2-}+4\ H^{+}} }$

(12) Sum:    ${\displaystyle \mathrm {CuFeS_{2}+4\ O_{2}\longrightarrow Cu^{\,2+}+Fe^{\,2+}+2\ SO_{4}^{\,2-}} }$

The decisive primary attack on the practically water-insoluble heavy metal sulfides is the abiotic oxidation with Fe ³⁺ as oxidant. The heavy metals are released as water-soluble ions. The greater the ratio of Fe (III) to Fe (II) ions, the more effective this abiotic oxidation is. The implementation would soon come to a standstill if the microbial iron and sulfur oxidation did not follow.

The oxidation of elemental sulfur, thiosulphate and Fe ²⁺ ions serve as an energy source for the microorganisms.

Uraninite

Uranium occurs in nature mainly as the poorly water-soluble uraninite (pitchblende, UO ₂ ). Through abiotic oxidation with Fe ³⁺ , the tetravalent uranium in this is oxidized to hexavalent uranium, which forms water-soluble uranyl ions (UO ₂ ) ²⁺ . The Fe ³⁺ , which is reduced to Fe ²⁺ , is regenerated by iron oxidizers.

(13)      abiotic ${\displaystyle \mathrm {UO_{2}+2\ Fe^{\,3+}\longrightarrow (UO_{2})^{\,2+}+2\ Fe^{\,2+}} }$

(14)      microbial (iron oxidizer) ${\displaystyle \mathrm {2\ Fe^{\,2+}+0,5\ O_{2}+2\ H^{+}\longrightarrow 2\ Fe^{\,3+}+H_{2}O} }$

(15) Sum:    ${\displaystyle \mathrm {UO_{2}+0,5\ O_{2}+2\ H^{+}\longrightarrow (UO_{2})^{\,2+}+H_{2}O} }$

Since H ⁺ ions are consumed here, the pH value rises. As a result, Fe ³⁺ ions are converted into Fe (III) compounds that are difficult to dissolve in water and are no longer available for the oxidation of uraninite. It is therefore necessary that the pH value is kept low by adding acid or - as is common in practice - by the natural presence or addition of pyrite. The oxidation of pyrite produces sulfuric acid (see equation 8).

Leaching with heterotrophic microorganisms

Leaching with carbon -heterotrophic microorganisms uses their ability to form rock -dissolving metabolites, especially organic acids such as fatty acids and citric acid , to produce metabolite-chelate complexes . The disadvantage of these leaching processes is the need to provide organic substances as a carbon and energy source.

Technical process

For microbial leaching, large quantities of crushed ore are piled up in heaps and sprayed with water from above. As the water seeps through, the iron- and sulfur-oxidizing bacteria and archaea multiply within the moist rock. They adhere to the surfaces of the minerals and are mostly not carried away by the leaching liquid. The metal-containing liquid seeps out at the foot of the heap and is collected in collecting basins. The dump should therefore be built on a water-impermeable subsoil (e.g. a layer of clay). The leaked leaching liquid is returned to the surface of the heap. If it has been sufficiently enriched with the desired heavy metals in a constant cycle, these can be extracted or precipitated. The low-metal leaching liquid is redistributed on the heap.

The process heat is dissipated with a delay, depending on the thermal conductivity of the heap material. If the microbial oxidation takes place quickly under favorable conditions, the heap material heats up considerably, sometimes up to around 60 ° C. The composition of the microorganism society changes when heated so that thermophilic iron- and sulfur-oxidizing bacteria and archaea predominate or are exclusively present, and the leaching process is further accelerated.

Importance of the process

Today, microbes deliberately supply pure metal from large quantities of low-quality poor ores. In the United States , Canada , Chile , Australia and South Africa produce a quarter of the total copper by bioleaching ( bioleaching ) worldwide. More than 10% of the gold and 3% of the cobalt and nickel are obtained biotechnologically.

The bio-leaching processes are more environmentally friendly than other smelting methods. In contrast to conventional smelting processes, bio-leaching does not release any harmful substances if the operation is carried out correctly, but large quantities of process water containing sulfuric acid are produced, which must be neutralized and freed from the heavy metals contained.

Examples

Copper extraction through bioleaching

Copper is mainly leached from ores containing chalcopyrite , which also contain pyrite. This produces sulfuric acid and the easily soluble, blue-colored copper sulfate. The copper is extracted from the solution by so-called cementation : The divalent copper ions (Cu ²⁺ ) present in the solution are reduced with elemental iron (scrap) to elemental copper, which precipitates, iron is in the form of divalent ions (Fe ²⁺ ) in solution. The increased demand and the simultaneously decreasing stock of copper have led in recent years to the fact that mining had to be pushed into ever deeper zones. Energy and development costs have increased, so the cheaper bioleaching is used.

Uranium extraction through bioleaching

When uranium is leached from its minerals with tetravalent uranium, especially uraninite (UO ₂ ), bacteria and archaea from pyrite (pyrites, FeS ₂ ) or dissolved divalent iron (Fe ²⁺ ) act as an oxidant ("aggressive") ) produces dissolved trivalent iron (Fe ³⁺ ). This oxidizes uranium to hexavalent uranium, which is present in uranyl ions ((UO ₂ ) ²⁺ ), which are readily soluble in dilute sulfuric acid. This is how uranium is extracted in Canada (Agnew Lake Mine and Denison Mines, Ontario).

Others

Researchers are investigating whether biomining could possibly be used in asteroid mining in the future .

literature

• Douglas E. Rawlings, Barrie D. Johnson (Eds.): Biomining . Springer-Verlag, Berlin, Heidelberg, New York 2007, ISBN 978-3-540-34909-9 .
• Giovanni Rossi: Biohydrometallurgy . McGraw-Hill, Hamburg a. a. O. 1990, ISBN 3-89028-781-6 .
• Henry L. Ehrlich, Corale L. Brierley (Eds.): Microbial Mineral Recovery . McGraw-Hill, New York et al. a. O. 1990, ISBN 0-07-007781-9 .

Web links

• NZZ: Bacteria as miners

Individual evidence

1. ^ John Neale: Bioleaching technology in minerals processing (PDF; 573 kB), Mintek Biotechnology Division, South Africa , September 2006
2. ↑ H. Brandl: Microbial leaching of metals . In: HJ Rehm (Ed.) Biotechnology , Vol. 10. Wiley-VCH, Weinheim u. a. O. 2001, pp. 191-224, ISBN 3-527-28328-5 .
3. ↑ Axel Schippers: Investigations on the sulfur chemistry of the biological leaching of metal sulfides . ( Reports from metallurgy ). Shaker Verlag, Aachen 1998, ISBN 3-8265-4076-X .
4. ↑ Biomining for In-Situ Resource Utilization pdf, niac.usra.edu; 'Biomining' Microbes Could Extract Minerals From Asteroids nbcnews.com, accessed February 3, 2015