Biomining

The human being has taken profit from microorganisms since ancient times. For instance, all ancient cultures have produced one or more fermented beverages like beer or wine. These processes are catalyzed by microorganisms. Like many industrial processes, mining can also be conducted by microorganisms. The use of microorganisms in mineral processing technology is called biomining. Bioleaching, biooxidation, bioflotation and bioremediation are examples of technologies applied in biomining. Acidithibacillus ferroxidans was the first bacterium found to be involved in mineral leaching processes. At first, it was isolated from the Tinto River in Huelva, Spain. This river has red and acidic waters with high concentration of heavy metals. These characteristics are mainly generated by the microbial oxidation of sulphide minerals present upstream. It is currently known that bioleaching processes can be catalyzed by many species of bacteria, archaea and fungi. Among them, other Acidithobacillus species like A. thiooxidans or A. caldus and microorganisms from the genus Leptospirillum, Sulfobacillus, Sulfolobus, Acidianus, Ferroplasma, etc., can be mentioned. Most microorganisms involved in this process are acidophilic, autotrophic and lithotrophic. This means that they can live at pH 3 or below, are able to grow reducing carbon dioxide to make organic compounds and can obtain energy from the oxidation of inorganic compounds like ferrous ions or sulphides. Most bioleaching microorganisms are also heavy metal resistant, besides archaea are usually thermopiles. These microorganisms are classified as “extremophile” because they are able to grow in conditions considered banned for life.

The important role that these microorganisms have in biomining depends on all of these physiological characteristics. The biological oxidation of sulphide minerals is the fundamental mechanism for bioleaching and biooxidation. In the first case, the metal of interest is bound to the sulphide compound that is biologically oxidized. As a result, the metal of interest is directly solubilized by the bioleaching. This technology is successfully used in industrial processes for treating low grade ores (metal concentration 0.5%p/p or less). It is widely used in the USA, Mexico and Chile. The last one is the largest producer and exporter of copper in the world. More than 30% of the Chilean copper production is recovered by bioleaching. Other metals mined by means of this technology are nickel, cobalt and zinc. There are three different procedures to do this: dump, heap and in situ irrigation. Processes in stirred tanks can also be used for more expensive metals like gold. This is common in biooxidation. In the biooxidation process, the metal of interest is not forming sulphides but is occluded within a matrix formed by sulphides of other metals, often iron. Solubilization of iron makes the metal of interest more available to the next step in the process, which is usually done by using chemicals that form soluble complexes with the metal of interest. For example, cyanide is the complexing agent used for gold. There are many industrial plants using this technology in Australia, Brazil, South Africa and Peru, but the Sansu plant located in Ghana is the largest in the world.

Processes like bioleaching and biooxidation are less contaminating than conventional mining processes because they produced fewer toxic compounds and demand less energy.