Biomining
Technique of extracting metals from ores using prokaryotes or fungi
From Wikipedia, the free encyclopedia
Biomining refers to processes that use organisms to extract metals from ores and other solid materials. Biomining is a subset of biohydrometallurgy with applications in ore refinement, precious metal recovery, and, possibly, bioremediation.[1] The largest application currently being used is the treatment of mining waste containing iron, copper, zinc, and gold. It may also be useful in maximizing the yields of increasingly low grade ore deposits.[2] Biomining has been proposed as a relatively environmentally friendly alternative and/or supplementation to traditional mining.[1] Current methods of biomining are modified leach mining processes.[3] These aptly named bioleaching processes most commonly includes the inoculation of extracted rock with bacteria and acidic solution, with the leachate salvaged and processed for the metals of value.[3] Biomining has applications outside of metal recovery, most notably is bioremediation, which has been deployed in an attempt to clean up oil spills.[4] Aspirational applications include space biomining, fungal bioleaching and biomining with hybrid biomaterials.[5][6]
History
The possibility of using microorganisms in biomining applications was realized after the 1951 paper by Kenneth Temple and Arthur Colmer.[7] In the paper the authors presented evidence that the bacteria Acidithiobacillus ferrooxidans (basonym Thiobacillus ferrooxidans) is an iron oxidizer that thrive in iron, copper and magnesium-rich environments.[7] In the experiment, A. ferrooxidans was inoculated into media containing between 2,000 and 26,000 ppm ferrous iron, finding that the bacteria grew faster and were more motile in the high iron concentrations.[7] The byproducts of the bacterial growth caused the media to turn very acidic, in which the microorganisms still thrived.[8] Following this experiment, the potential to use fungi to leach metals from their environment[9] and use microorganisms to take up radioactive elements like uranium and thorium[10] have also been explored.[9]
While the 1960s was when industrial biomining got its start, humans have been unknowingly using biomining practices for hundreds of years.[11] In western Europe the practice of extracting copper from metallic iron by placing it into drainage streams, used to be considered an act of alchemy.[11] However, today we know that it is a fairly simple chemical reaction.[11]
Cu2+ + Fe0 → Cu0 + Fe2+
In the Middle Ages in Portugal, Spain and Wales, miners unknowingly used this reaction to their advantage when they discovered that when flooding deep mine shafts for a period with some leftover iron they were able to obtain copper.[12]
In China, the use of biomining techniques has been documented as early as 6th-7th century BC.[13] The relationship between water and ore to produce copper was well documented, and during the Tang dynasty and Song dynasty copper was produced using hydrometallurgical techniques.[13] Though the mechanism of oxidation via bacteria was not understood, the unintended use of biomining allowed copper production in China to reach 1000 Tons per year.[13]
Mechanism
The processes often involve the use ferric ions (Fe3+) for oxidation of sulfide minerals.[14] The organisms that promote these reactions tolerate high metal concentrations and low pH:
- CuFeS2+4H++O2 → Cu2++Fe2++2S0+2H2O
- 4Fe2++4H++O2 4Fe3++2H2O,
- 2S0+3O2+2H2O→2SO2−4+4H+,
- CuFeS2+4Fe3+→Cu2++2S0+5Fe2+
Bioleaching technologies
Heap or dump leaching
Bioleaching was one of the first widely used applications of biomining.[15] It is practiced in two broad venues:
- rock is treated with an extractant (lixiviant), which percolates through the solid and the metals are recovered from the leachate.[16]
- Dump bioleaching, waste rock is piled into mounds (>100m tall) and saturated with sulfuric acid to encourage mineral oxidation from native bacteria.[17] Inoculation of the rock with bacteria is often not performed in dump bioleaching which instead relies on the bacteria already present in the rock.[17]
- Heap bioleaching is a newer take on dump leaching.[17] The process includes more processing in which the rocks are ground into a finer grain size.[17] This finer grain is then stacked only 2 – 10 m high and is well irrigated allowing for plenty of oxygen and carbon dioxide to reach the bacteria.[17] The mounds are also often inoculated with bacteria.[17] The liquid coming out at the bottom of the pile, called leachate, is rich in the processed mineral. The heaps reside on large non-porous platforms which are used to catch the leachate for processing.[17] Once collected the leachate is transported to a precipitation plant where the metal is reprecipitated and purified. The waste liquid, now void of the valuable minerals, can be pumped back to the top of the pile and the cycle is repeated.[17]
The temperature inside the leach dump often rises spontaneously as a result of microbial activities.[17] Thus, thermophilic iron-oxidizing chemolithotrophs such as thermophilic Acidithiobacillus species and Leptospirillum and at even higher temperatures the thermoacidophilic archaeon Sulfolobus (Metallosphaera sedula) may become important in the leaching process above 40 °C.[17]
Stirred tank
The major alterntive to heap or dump leaching is continuously stirred tank reactor (STR).[16] Alternatives include the airlift reactor (ALR) or pneumatic reactor (PR) of the Pachuca type to extract the low concentration mineral resources efficiently.[2]
In situ biomining
In situ biomining involves the flooding and inoculation of fractured ore bodies that have yet to be removed from the ground.[17] Once the bacteria are introduced to the ore deposits, they begin leaching the precious metals, which can then be extracted as leachate with a recovery well.[18] In-situ mining also shows promise for applications in cost-effective deep subsurface extraction of metals.[19]
In situ biomining, is the one current method utilizing bioleaching that serves as an effective and viable replacement for traditional mining.[20] Because in-situ biomining, negates the need for the extraction of the ore bodies, this method stops the need for hauling or smelting of the ore.[19] This would mean there would be no waste rocks or mineral tailings that contaminate the surface.[19] In-situ biomining poses environmental challenges, such as the contamination of ground water.[19][20]
Applications
Gold
Biological pre-treatment utilizes the natural oxidation abilities of microorganisms to solubilize minerals that interfere with the extraction of gold.[17] Plants for biooxidation of gold-bearing concentrates have been operated at 40 °C with mixed cultures of mesophilic bacteria of the genera Acidithiobacillus or Leptospirillum ferrooxidans.[21] Gold is frequently found in nature associated with arsenopyrite and pyrite. In the microbial leaching process Acidithiobacillus ferrooxidans, etc. dissolve these minerals, exposing trapped gold (Au).[22] The following reaction summarizes the process:[22]
- 2 FeAsS[Au] + 7 O2 + 2 H2O + H2SO4 → Fe(SO4)3 + 2 H3AsO4 + [Au]
Copper
One of the largest applications of these leaching methods is in the mining of copper. Acidithiobacillus ferrooxidans has the ability to solubilize copper from its sulfidic ores.[23] The acidophilic archaea Sulfolobus metallicus and Metallosphaera sedula can tolerate up to 4% of copper. The main application is for extraction from low grade ores using Thiobacillus thiooxidans.[24] – an important consideration in the face of the depletion of high grade ores.[2]
The copper can then be recovered from the solution by plating it out on scrap iron or electrowinning.
- Fe0 + Cu2+ → Cu0 + Fe2+
Uranium
Biomining was used in Canada in the 1970s to extract additional uranium out of exploited mines.[25] Similarly to copper, Acidithiobacillus 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 At. ferrooxidans contributing mainly through the reoxidation of Fe2+ to Fe3+.
- UO2 + Fe2(SO4)3 → UO2SO4 + 2 FeSO4
Economic feasibility and potential drawbacks
Bioleaching can be economical mainly as a complement to traditional mining. It allows for economic extraction of low-grade ore and allows exploitation of abandoned mine tailings.[26]
Future prospects
Additional capabilities, of current bioleaching technologies include the bioleaching of metals from sulfide ores, phosphate ores, and concentrating of metals from solution.[3]
Beyond gold and copper
Stirred tanks have been investigated for bioleaching cobalt mine tailings,[27]
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Coal desulfurization
Biological methods have demonstrated some promise for the removal of sulfur from coal, giving a cleaner-burning fuel. This concept has not progressed beyond demonstration phase, however.[28]
Biomining in space
The concept of space biomining is creating a new field in the world of space exploration.[5] The main space agencies believe that space biomining may provided an approach to the extraction of metals, minerals, nutrients, water, oxygen and volatiles from extraterrestrial regolith.[29][30][5] Bioleaching in space also shows promise for application in building biological life support systems (BLSS).[5] BLSS do not usually contain biological component, however, the use of microorganisms to breakdown waste and regolith, while being able to capture their byproducts like nitrates and methane would theoretically allow for a cyclical system of regenerative life support.[5]
Fungi in biomining
Fungi and plants (phytoextraction also known as phytomining) may also be used.[31] Species of filamentous fungi, specifically those in the genera of Aspergillus and Penicillium have been shown as effective bioleaching agents.[6] Fungi have the ability to solubilize metals through acidolysis, redoxolysis and chelation reactions.[6] Like bacteria, fungi have been studied for their ability to extract rare earth elements and to process low grade ore. But their most promising and studied usage is in the breakdown of E-waste and the recovery of valuable metals from it, like gold.[6][32] Despite the promise of fungal bioleaching, there has been no industrial applications of it as it does not out compete its bacterial counterparts.[6]
