Biomining

Biomining refers to any process that uses living organisms to extract metals from ores and other solid materials. Typically these processes involve prokaryotes, however fungi and plants (phytoextraction also known as phytomining) may also be used. Biomining is one of several applications within biohydrometallurgy with applications in ore refinement, precious metal recovery, and bioremediation. The largest application currently being used is the treatment of mining waste containing iron, copper, zinc, and gold allowing for salvation of any discarded minerals. It may also be useful in maximizing the yields of increasingly low grade ore deposits. Biomining has been proposed as a relatively environmentally friendly alternative and/or supplementation to traditional mining. Current methods of biomining are modified leach mining processes. 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. Biomining has many applications outside of metal recovery, most notably is bioremediation which has already been used to clean up coastlines after oil spills. There are also many promising future applications, like space biomining, fungal bioleaching and biomining with hybrid biomaterials.

The possibility of using microorganisms in biomining applications was realized after the 1951 paper by Kenneth Temple and Arthur Colmer. 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. 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. The byproducts of the bacterial growth caused the media to turn very acidic, in which the microorganisms still thrived. Following this experiment, the potential to use fungi to leach metals from their environment and use microorganisms to take up radioactive elements like uranium and thorium have also been explored.

While the 1960s was when industrial biomining got its start, humans have been unknowingly using biomining practices for hundreds of years. 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. However, today we know that it is a fairly simple chemical reaction.

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.

In China, the use of biomining techniques has been documented as early as 6th-7th century BC. 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. 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.

Biological pre-treatment utilizes the natural oxidation abilities of microorganisms to remove unwanted minerals that interfere with the extraction of the target metals. This is not always necessary but is widely used in the removal of arsenopyrite and pyrite from gold (Au). Adidithiobacillus spp. release the gold by the following reaction.

Stirred tank bioreactors are used for the biooxidation of gold. While stirred tanks have been used to bioleach cobalt for copper mine tailings, these are costly systems that can reach sizes of >1300m³ meaning that they are almost exclusively used for very high value minerals like gold.