Bioleaching

Bioleaching is a biohydrometallurgical technique that uses living organisms to extract or mobilise metals from their ores. It provides an alternative to conventional metallurgical processes, particularly for ores containing low concentrations of valuable metals. Bioleaching is employed for the recovery of copper, zinc, nickel, cobalt, gold, silver, arsenic, antimony, molybdenum and other metals, using organisms whose metabolic activities promote oxidation, dissolution or mobilisation of minerals. This approach offers an environmentally and economically attractive complement to traditional smelting and chemical leaching.

Categories of Bioleaching

Bioleaching practices can be broadly grouped into two main categories:

• Oxidation of refractory minerals to release precious metals, notably gold and silver. These ores commonly contain pyrite or arsenopyrite, whose oxidation enables the liberation of enclosed or chemically bound metals.
• Leaching of sulphide minerals to extract associated metals such as copper or nickel. Examples include the dissolution of pentlandite for nickel recovery or the leaching of chalcocite, covellite, or chalcopyrite to obtain copper.

These processes rely on microbial activity to accelerate oxidative reactions, improve solubilisation and regenerate oxidising agents essential for continuous metal release.

Microorganisms Involved

A variety of ferrous-iron and sulphur-oxidising bacteria play central roles in bioleaching systems. Frequently used species include Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans. Their metabolic pathways enable them to oxidise ferrous iron to ferric iron and convert reduced sulphur compounds to sulphate. Microbial activity is essential for regenerating ferric ions, which are powerful oxidants of many metal-bearing minerals.
Two principal mechanisms describe their involvement:

• Indirect leaching, in which ferric ions oxidise the ore minerals independently of microbes, while bacteria regenerate ferric iron from ferrous iron.
• Direct microbial attack, in which the organisms oxidise mineral surfaces directly, facilitating metal release.

Both pathways contribute to the dissolution of target metals, depending on mineralogy and environmental conditions.

Pyrite Bioleaching

Pyrite (FeS₂) bioleaching is a well-studied system illustrating the interplay of chemical and microbial reactions. In the initial step, ferric ions oxidise disulphide units to yield thiosulphate and ferrous ions. The ferrous ions are subsequently re-oxidised by iron-oxidising bacteria using oxygen as the terminal electron acceptor. Thiosulphate is then converted to sulphate by sulphur-oxidising microorganisms. The overall outcome produces soluble ferrous sulphate and sulphuric acid.
These reactions occur partly at the bacterial cell membrane, where electrons generated from iron and sulphur oxidation support microbial energy metabolism. The microbial regeneration of ferric iron is fundamental to maintaining reaction cycles involved in continuous mineral breakdown.

Copper Minerals and Chalcopyrite Leaching

Bioleaching methods for copper exhibit varying efficiencies depending on mineral type. Supergene minerals such as chalcocite (Cu₂S) and covellite (CuS) leach readily under microbial conditions. By contrast, chalcopyrite (CuFeS₂), despite its abundance, is less responsive due to kinetic barriers and passivating surface layers. As a result, conventional copper production continues to rely heavily on flotation followed by smelting.
Chalcopyrite dissolution involves an initial ferric-driven oxidation step that releases copper ions into solution, followed by microbial regeneration of ferric iron and oxidation of elemental sulphur to sulphate. Although technically feasible, the process proceeds slowly, limiting industrial adoption for high-grade ores.

Bioleaching of Non-sulphidic Ores

Non-sulphide minerals can also be treated through bioleaching when ferric iron acts as the oxidant, as in the case of pitchblende. Here, microbial involvement is limited to regenerating ferric ions from ferrous ions. Supplementing the system with sulfidic iron minerals enhances the supply of iron for oxidation. Layering non-sulphidic materials with sulfides and elemental sulphur colonised by Acidithiobacillus species provides an effective approach for mobilising metals from ores lacking inherent sulphide content.

Solvent Extraction and Electrowinning

Once metals such as copper enter the aqueous solution, further processing is required to obtain purified products. A widely used technique is ligand exchange solvent extraction, in which copper ions form neutral complexes with specially designed organic ligands. These complexes dissolve into organic solvents such as kerosene, separating copper from other ions.
Adjusting the pH releases copper ions back into an aqueous phase, after which electrowinning produces high-purity metal. Copper ions migrate to negatively charged cathodes under an applied electric current, depositing as metallic copper. Alternatively, displacement reactions using scrap iron can precipitate copper from solution, with iron serving as the reducing agent.
For precious metals such as gold, cyanide leaching may follow bio-oxidation, with dissolved gold subsequently recovered on charcoal through adsorption.

Fungal Bioleaching

In addition to bacterial systems, several fungi have demonstrated potential in metal mobilisation. Species such as Aspergillus niger and Penicillium simplicissimum can leach metals from substrates including electronic waste, catalytic converters and fly ash. These fungi produce organic acids, such as citric acid, that dissolve metals directly rather than through oxidative pathways. Studies show high mobilisation rates for metals including aluminium, nickel, lead, and zinc, underscoring the versatility of fungal systems.

Economic Feasibility

Bioleaching offers economic benefits under suitable conditions. It is generally simpler and less labour-intensive than conventional metallurgical operations. Microorganisms efficiently target metal-bearing minerals even in low-grade ores, achieving extraction yields exceeding 90% in some systems. The reduced need for intensive crushing and grinding lowers energy and capital costs.
However, bioleaching proceeds more slowly than smelting, resulting in delayed metal recovery and cash flow. Consequently, smelting remains the preferred method for high-grade copper ores. Nonetheless, for low-grade deposits and wastes, bioleaching often provides a cost-effective alternative. At major mining operations such as Escondida in Chile, bioleaching has played an important role in supplementing conventional extraction technologies.