Biomining

Biomining is a specialized field of biohydrometallurgy that uses living organisms to extract metals from ores and other solid materials. This process allows humans to recover valuable elements from complex sources that are otherwise difficult to process. Currently, the largest use for biomining is treating mining waste that contains iron, copper, zinc, and gold. It also offers a way to maximize yields from low-grade ore deposits, where the concentration of metal is very low. Because it can serve as a supplement to traditional mining, it is often viewed as a relatively environmentally friendly alternative.

The primary mechanism used in biomining is a process called bioleaching. This method often involves the use of ferric ions (Fe3+) to perform the oxidation of sulfide minerals. During this reaction, microorganisms promote chemical changes that release metals into a liquid solution. For example, when processing chalcopyrite, the bacteria help convert the mineral into copper ions, ferrous iron, and sulfur. The organisms used in these processes are highly specialized. They must be able to tolerate very high metal concentrations and a low pH, which means they thrive in highly acidic environments.

There are several distinct ways to perform bioleaching in industrial settings. One method is dump bioleaching, where waste rock is piled into massive mounds taller than 100 meters. These mounds are saturated with sulfuric acid to encourage mineral oxidation by the bacteria already present in the rock. A newer version is heap bioleaching. In this method, rocks are ground into a fine grain and stacked in much smaller piles, only 2 to 10 meters high. These heaps are well-irrigated to ensure oxygen and carbon dioxide reach the bacteria, and they are often inoculated with specific microbes. The liquid that flows out of the bottom is called leachate, which is rich in processed minerals.

Another advanced method is in situ biomining. This involves flooding and inoculating fractured ore bodies while they are still underground. Once the bacteria are introduced, they begin leaching precious metals directly from the ore deposit. The metals are then extracted using a recovery well. This method is a viable replacement for traditional mining because it avoids the need to haul or smelt ore. By leaving the ore in the ground, it eliminates the need for waste rocks or mineral tailings on the surface. However, this method does face environmental challenges, such as the potential for groundwater contamination.

Humans have been using biomining for centuries, even before they understood the science. In China, documented techniques were used as early as the 6th or 7th century BC. During the Tang and Song dynasties, copper production reached 1,000 tons per year using hydrometallurgical techniques. In the Middle Ages, miners in Portugal, Spain, and Wales discovered that flooding mine shafts with leftover iron helped them obtain copper. They did not know bacteria were responsible, so they viewed the process as alchemy. Modern science only began to explain this in 1951. Kenneth Temple and Arthur Colmer published a paper proving that the bacterium Acidithiobacillus ferrooxidans is an iron oxidizer.

Biomining has specific, vital applications for different metals. For gold, biological pre-treatment uses microbes to dissolve minerals like arsenopyrite and pyrite. This exposes the gold that was trapped inside. In copper mining, Acidithiobacillus ferrooxidans is used to solubilize copper from sulfidic ores. Some specialized archaea, such as Sulfolobus metallicus, can even tolerate copper concentrations as high as 4%. Biomining was also used in Canada during the 1970s to extract additional uranium from mines that had already been exploited. In these cases, the bacteria help oxidize the uranium, making it easier to collect.

Looking toward the future, the possibilities for biomining are expanding into new frontiers. Scientists are investigating the use of fungi, such as Aspergillus and Penicillium, to recover gold from electronic waste. There is also the exciting concept of space biomining. Space agencies believe this could allow astronauts to extract metals, minerals, water, and oxygen from extraterrestrial regolith, which is space soil. This could be part of a biological life support system (BLSS). Such a system would use microorganisms to break down waste and regolith to create a cyclical, regenerative environment for humans living in space.