Biomining

Valuable metals can be bound up in solid minerals; there are microbes that can oxidize those metals, which then allows them to dissolve in water. This is the basic process behind biomining and can be used to recover metals when dissolved from the solid rock. Another technique for biomining uses microbes to break down the surrounding minerals to recover the metal from the rock. Biomining operations have been used to extract gold, silver, uranium, nickel, copper, cobalt, and zinc.

Biomining has been shown to be less expensive and better for the environment than traditional mining, especially as it has lower carbon dioxide emissions and carbon and water footprints when compared to traditional mining technology. And the chemicals used in biomining are naturally occurring at mining sites and are not pathogenic, unlike chemicals that have been used in traditional mining.

One of the earliest known examples of biomining came from Romans working the Rio Tinto copper mine in Spain, 2,000 years ago. There, miners noticed a blue fluid running off the mine tailings, which indicated the fluid contained copper salts. The miners were, in turn, able to use the fluid to recover the metal.

There are two over-arching methods of biomining: bioleaching and biooxidation. Both processes involve microbial reactions, which happen anywhere microbes, rocks, and the necessary nutrients for reaction, such as oxygen, occur together. In the process of bioleaching, the metal of interest is directly dissolved. In the process of biooxidation, the surrounding material around the metal can be dissolved or the metal of interest is enriched.

Biomining, in all of its methods, uses naturally occurring microorganisms that eat away at minerals surrounding the metals of interest. The process of these minerals is then sped up using acid in a solution with the bacteria and metal. The resulting solution then dismantles the metals of interest from the minerals in a liquid form, which can be reconstituted with an electrochemical process.

There are various approaches to biomining, including direct and indirect bioleaching. In direct bioleaching, minerals are used that are receptive to oxidation to create a direct enzymatic strike using the microorganisms to separate the metal and the ore. In indirect bioleaching, microorganisms are not in direct contact with the minerals in the process, but leaching agents are created by the microbes and used to oxidize the ore.

Many of the processes used for biomining still require rocks to be removed and crushed in order to be mined, or else use traditional mining waste to recover more metals of interest rather than leaving them behind. However, biomining also offers a different method of extraction from the ground, which would not require breaking rocks to dig for metal, but would pump microbes underground and extract the leachate that would move to the surface. This has been suggested to be less costly and energy-intensive than traditional methods with limited environmental disruption in a process similar to fracking for oil.

Many of the processes of biomining take place in large, close, stirred-tank reactors called bioreactors, which contain water, microorganisms, ore material, and a source of energy. The source of energy used depends on the microbe used, such as microorganisms used to leach gold and copper that derive energy from inorganic sources. Which means, for most biomining, the temperature, sugars, rate at which the tank is stirred, acidity, carbon dioxide, and oxygen levels are monitored and can be tuned to provide optimal biomining reactions. The use of bioreactors have been shown to work faster than heap methods, while using less energy than high-temperature smelting processes.

There have been experiments into using biomining procedures to recover precious metals from other waste streams where valuable metals may be lost, or where heavy metals pose a risk to health or the environments. This has included ashes from incinerators, dust from steel production, and sewage sludge containing recoverable amounts of precious and heavy metals. And in the case of consumer electronics, which often have short lifespans and poor recyclability, while using metals of interest, there have been experiments into using biomining to recover those minerals, in a process called "urban" biomining.

Biomining has also been considered to be a mining procedure to recover minerals and rare earth minerals from lunar rocks, Martian rocks, and asteroids. There were experiments into the effect of microgravity, which can alter basic physical processes like convection and liquids mixing. This includes an experiment in 2019 by BioRock, on the International Space Station. These experiments found that biomining was feasible in microgravity, and the processes worked similarly to an Earth gravity environment.

In 2020, the European Space Agency launched the BioAsteroid experiment aboard the 21st SpaceX cargo resupply mission to continue to investigate biomining aboard the International Space Station. This was to help understand what applications biomining would be capable of and useful for, especially in the case of mining asteroids and other space entities with the purpose of establishing a self-sustaining presence in space.