Bioleaching

Bioleaching, also known as biomining, is the process of extracting metals from ores or waste materials through the metabolic activity of microorganisms. It is commonly applied to sulfide minerals to release metal ions into solution, making the metals easier to recover by conventional hydrometallurgical methods. Bioleaching can be used for copper, nickel, cobalt, zinc, uranium, and other metals, and is sometimes employed to pretreat refractory gold ores.

The process relies on chemolithotrophic microorganisms, especially acidophilic bacteria and archaea such as Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans, and Leptospirillum species. These organisms oxidize ferrous iron or reduced sulfur compounds, generating ferric iron and sulfuric acid. The ferric iron and acidity chemically dissolve mineral sulfides, releasing metal ions into solution. In some systems, microbial action also alters mineral surfaces to enhance leaching rates.

Industrial implementation includes heap bioleaching, where ore is piled and irrigated with leach solution, and stirred-tank bioreactors for greater process control. Bioleaching is used for low-grade copper oxides and sulfides, as well as for the pretreatment of refractory gold ores to free encapsulated gold for subsequent cyanidation. It can also be applied to mining wastes and tailings.

Advantages of bioleaching include lower energy consumption, reduced capital costs, and the ability to process low-grade or otherwise uneconomical ores. Challenges include slower processing rates, sensitivity to environmental conditions, need for careful pH and temperature management, and potential generation of acidic effluents or heavy-metal releases requiring proper containment and treatment.