Bioleaching

Bioleaching ( German literally: bioleaching, more precisely, microbial ore leaching, rarely Biomining ) refers to the recovery of heavy metals from their ores by converting insoluble ore minerals to water-soluble salts by microorganisms. The bioleaching is a branch of biohydrometallurgy, metal extraction by organic - wet-chemical processes, obtained under mainly copper, zinc, cobalt, nickel, gold and uranium. The most important representatives laugungsaktiver microorganisms are bacteria and archaea that oxidize sulfide and elemental sulfur to sulfate and partially divalent to trivalent iron. Among the bacteria that are especially the sulfur bacteria Acidithiobacillus ferrooxidans ( sulfide, sulfur - and iron - oxidizing) and Acidithiobacillus thiooxidans ( sulphide and sulfur oxidising).

• 4.1 copper recovery by bioleaching
• 4.2 Uranium recovery by bioleaching

History

Presumably, copper was recovered from mine water already about 3000 years ago in the Mediterranean. Historically documented is the copper recovery of the Spaniards in the 18th century on the river Rio Tinto by microbial leaching of sulphidic copper ores. Since the discovery of the role of iron - and sulfur -oxidizing bacteria in ore leaching and 1940 was studied in more detail because of increasingly scarce metal deposits in the process of bioleaching, enlightened and used industrially. In Canada, uranium is bioextrahiert directly from ores, in the United States and Chile Copper is extracted from low-grade ores and sulfidic in South Africa since 1980 gold by bioleaching.

The end of the bioleaching

Requirements

To use a bioleaching sense, various conditions must be met:

• Water must be readily available in large quantities.
• The ores must oxidizable substances of microorganisms (sulfur, sulfides, iron ( II) compounds ). In rocks that are low in iron and sulfur compounds or elemental sulfur, can cheap pyrite ( FeS2, pyrite = iron (II ) disulfide ), elemental sulfur, iron (II) sulfate ( FeSO4 ) or iron (III ) sulfate ( Fe2 (SO4 ) 3) are added.
• Since in the bioleaching solutions with the metals to be recovered, in small concentrations, must ensure a cost effective way for the extraction or precipitation exist.
• Growth substrates for the microorganisms must be present.

Importance of iron and sulfur bacteria

Iron - and sulfur - oxidizing bacteria and archaea, through their oxidative energy metabolism processes instrumental in sparingly soluble sulfides such as Kupfereisendisulfid ( chalcopyrite = chalcopyrite, CuFeS2 ) in water soluble leachable sulfate (copper sulfate and iron ( II ) sulfate ) to implement.

The first and most important step to the dissolution of poorly water heavy metal sulfides is the abiotic oxidation of sulfide sulfur by iron (III ) ions (Fe3 ) to elemental sulfur ( S) or thiosulfate ( S2O32 ), so that the heavy metals are released as ions and aqueous in the solution have been solved. Iron (III ) ions to iron ( II) ions reduced thereby ( Fe2 ). The role of iron - and sulfur - oxidizing bacteria and archaea is ( 1) the iron (II ) ions to reoxidize to iron ( III ) ions and thus to provide for the abiotic oxidation of other heavy metal sulfide available, and ( 2) oxidizing the resultant elemental sulfur and the resulting sulfuric acid to thiosulphate, whereby the aqueous solution is acidified and the dissolution of the heavy metal sulfide is promoted. Due to the abiotic and biotic oxidation of the sulfide so the heavy metals from the sulphide minerals as dissolved ions are released. Iron - and sulfur -oxidizing bacteria work together in this way tight.

The sulfur bacteria Acidithiobacillus ferrooxidans (including 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), the sulfur - oxidizer even create self- sulfuric acid by sulfide, sulfur and thiosulfate oxidation. Acidianus and Sulfolobus acidocaldarius brierleyi are also thermophilic ( high temperature loving). In the leaching process Acidithiobacillus ferrooxidans, Leptospirillum ferrooxidans, Acidianus and Sulfolobus acidocaldarius brierleyi oxidize divalent to trivalent iron, Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans, Sulfolobus acidocaldarius and Acidianus brierleyi oxidize elemental sulfur to sulfuric acid.

Chemical drain

Heavy metal sulfide minerals

First, the sulfide sulfur in the heavy metal sulphides is oxidized abiotic by trivalent iron ions ( Fe3 ) to said iron ( II) ions ( Fe2 ) to be reduced. The oxidation product is at monosulfides and chalcopyrite ( CuFeS2 ), elemental sulfur is at disulfides thiosulfate ( S2O32 - ), the oxidation product (see equations 1 and 5). A consequence of this oxidation, the release of heavy metals as cations are thus water-soluble and are transported to the leach liquor. The heavy metal mobilization would, however, for three reasons soon to a grinding halt if not biotic oxidations would join in: (1) There would be a lack of Fe3 , which is required as the oxidant for the abiotic sulphide oxidation, because soon all Fe3 ions to Fe2 ions would have been reduced. This is counteracted by the microbial oxidation of Fe2 to Fe3 . ( 2) The elemental sulfur formed would cover the mineral surfaces and hinder the attack of Fe3 . This is counteracted by the microbial oxidation of elemental sulfur to sulfuric acid. (3) The pH of the medium was (see equation 2) to increase the consumption of H ions in the iron oxidation. Even at intermediate pH values ​​forms Fe3 with poorly water- soluble compounds such as Fe (OH ) 3 and FeOOH and there would be a clogging and the Fe3 concentration would fall further. Which is counteracted by the microbial oxidation of elemental sulfur and thiosulfate, where H ions are formed, the pH is thus reduced.

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

Example, pyrite ( pyrite, FeS2 ):

Example chalcopyrite ( chalcopyrite, CuFeS2 ):

The crucial primary attack on the practically water-insoluble heavy metal sulfides is the abiotic oxidation with Fe3 as the oxidant. The heavy metals are released as water-soluble ions. These abiotic oxidation is more effective, the higher the ratio of Fe ( III) - and Fe (II ) ions. The reaction came soon to a grinding halt if not the microbial iron and sulfur oxidation would connect.

The oxidation of elemental sulfur, thiosulphate and Fe2 ions are used the microorganisms as an energy source.

Uraninite

Uranium occurs in nature mainly present as the poorly water-soluble uraninite ( pitchblende, UO2). By abiotic oxidation with Fe3 herein is oxidized tetravalent uranium to the hexavalent uranium, the water-soluble uranyl ion ( UO2) 2 forms. The thereby reduced Fe2 Fe3 is regenerated by Eisenoxidierer again.

Since in this case H ions are consumed, the pH increases. This has the consequence that Fe3 ions are too poorly water-soluble Fe implemented (III ) compounds and are no longer available for the oxidation of uraninite available. It is therefore necessary that the pH value is kept low, by adding acid or -. Common practice - by natural presence or addition of pyrite During the oxidation of the pyrite is produced sulfuric acid (see Equation 8).

Leaching with heterotrophic microorganisms

Leaching with kohlenstoffheterotrophen microorganisms uses their ability to form rock -resolution metabolites, mainly organic acids, such as fatty acids and citric acid, the production of metabolite -chelate complexes. A disadvantage of this leaching process is the need to provide organic substance calculated as carbon and energy source.

Technical Procedure

For the microbial leaching large amounts of crushed ore are piled in heaps and sprayed with water from above. As the water percolates, the iron - and sulfur - oxidizing bacteria and archaea reproduce within the wet rock. They adhere to the surfaces of minerals and are mostly not discharged by the leaching fluid. At the foot of the stockpile the metal-containing liquid oozes out and is collected in reservoirs. Therefore, the pile should be constructed on impermeable surface ( a layer of clay, for example). The leaked leaching fluid is reproduced on the surface of the heap. Has it enriched enough with the desired heavy metals in constant circulation, they can be extracted and precipitated. The metal-poor leaching fluid is redistributed on the heap.

The process heat is derived delayed depending on the thermal conductivity of the stockpile material. If the microbial oxidation proceeds rapidly under favorable conditions, thus the dump material heats up, partially up to about 60 ° C. The composition of the microorganisms society changes when heated so that thermophilic iron - and sulfur - oxidizing bacteria and archaea predominate, or are exclusively available, and the leaching process is further accelerated.

Importance of the process

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

The bio-leaching are more environmentally friendly than other Verhüttungsmethoden. In contrast to conventional smelting no harmful substances are released during the bioleaching, with proper management, however, large amounts of sulfuric acid-containing process water are produced due to the process, which must be neutralized and freed of heavy metals contained.

Examples

Copper recovery by bioleaching

Copper is mainly of chalcopyrite - containing ores that also contain pyrite leached. This produces sulfuric acid and slightly soluble blue colored copper sulphate. The copper is recovered from the solution by so-called cementation: the present in the solution divalent copper ions ( Cu2 ), can be reduced with elemental iron (scrap) to elemental copper, which precipitates, iron going for it in the form of divalent ions ( Fe2 ) in solution. The increased demand and simultaneously decreasing stock of copper led in recent years to the fact that the reduction had to be pushed into deeper and deeper zones. Energy and development costs rose, so that the cost-effective bioleaching is applied.

Uranium recovery by bioleaching

In the leaching of uranium from its minerals with tetravalent uranium, mainly uraninite ( UO2), is acting also by bacteria and archaea from pyrite ( pyrite, FeS2 ) or dissolved ferrous iron ( Fe2 ) as an oxidant ( " aggressive " ) dissolved ferric iron generated ( Fe3 ). This oxidized uranium to hexavalent uranium in uranyl ion ( (UO2 ) 2 ) is present, which are readily soluble in dilute sulfuric acid. In this way, uranium is recovered in Canada ( Agnew Lake Mine and Denison Mines, Ontario ).