Bioleaching is leaching where the extraction of metal from solid minerals into a solution is facilitated by the metabolism of certain microbes - bioleaching microbes. Bioleaching is a process described as "the use of microorganisms to transform elements so that the elements can be extracted from a material when water is filtered trough it".
Bacterial oxidation such as bioleaching or biooxidation on a commercial scale has been done on sulfide metal bearing materials such as arsenopyrite, pyrite, pyrrhotite, covellite and chalcocite ores and concentrates, the one exception to this processing being the oxidation of chalcopyrite ores and concentrates.
Gold and copper are the dominating valuable metals that are commercially extracted:
- Copper from low-grade secondary copper sulfide ores by heap bioleaching.
- Gold from refractory gold concentrates by stirred tank leaching as a pretreatment step. The biooxidation residue is further treated by cyanide leaching to recover gold.
Co is also commercially extracted. Ni and Zn may become so in the future.
Heap leaching is the most common method for bioleaching and is mainly used for secondary copper ores. Stirred tank leaching is used for refractory gold concentrates where gold is locked into the pyrite/arsenopyrite matrix.
As the microbes do not necessarily need to contact the valuable metal-bearing material that is bioleached, they can be physically separated from it:
- Direct bioleaching. The microbes are kept together with the valuable metal-bearing material
- Indirect bioleaching. The microbes are kept in a pond external to the valuable metal-bearing material and provide the leaching chemicals at a distance.
Bioleaching involves abiotic and biotic reactions, often with different physicochemical requirements. Indirect bioleaching is a way of satisfying the requirements independently by separating the biotic and abiotic reactions. In direct bioleaching the challenge is to select microbes whose living conditions are as close to the optimal conditions of the abiotic leaching reactions as possible.
- Ores and concentrates of lower metal concentration can be treated economically. Therefore the concentrating process can stop earlier, before the concentrate is sent for leaching. This means that loss of metal value during concentration is avoided.
- "Difficult" - refractory - concentrates can be processed.
- Concentrates with contaminants like arsenic, bismuth and magnesia are often expensive to treat in conventional metal-production. Mining companies often have to pay penalties for these difficult-to-treat contaminants when they sell concentrate to a smelter.
- The arsenic in the concentrates can be removed in an environmentally stable form.
- Possible to make use of existing SX/EW plant capacaties once the oxidic ore cap has been mined out.
- Economic exploitation of smaller deposits, in remote locations, becomes viable because of reduced infrastructural costs.
- Rapid start-up. Easy and reliable process when it comes to maintenance.
- The process takes place at atmospheric pressure and low temperatures.
- Water-based process means less dust.
- No emissions of sulfur dioxide. Therefore, purification of smoke gas and sulfuric acid plants are superfluous.
Despite the advantages with bioleaching it is not always easy to choose among the different methods of metal extraction in order to explore a potential mine. The Techno-Economic factors of a resource need to be evaluated from case to case.
Tools and materials
- Sulfide ore or concentrate → binding or hosting the valuable metal; energy for bioleaching microbes
- Air (supplied actively or passively)
- O2 → bioleaching microbes are aerobes and crave oxygen to extract energy from sulfide minerals.
- CO2 → bioleaching microbes need the macro-nutrient carbon to build cell mass
- N, P, K, Mg → nutrients for bioleaching microbes
- pH-regulators (to keep pH 1-2)
- Bioleaching microbes like T. ferrooxidans, T. thiooxidans & L. ferrooxidans
- Control of temperature (affected by climate)
- Air regulation
- Cooling tower
- Do a bleed in order to neutralize and precipitate metals (mostly iron)
- Distribution system, stirring (in tanks), sprinklers, airflow, tubes - "blood-circulation" of the heap.
- Reaction catalysts
- Bioleaching Technology in Minerals Processing by courtesy of John Neale
- Considerations regarding bioleaching galena concentrate by courtesy of Teodor Velea and Liliana Gerghe
- Amenability test
- Mixtures of bioleaching microbes for innoculation
- History of biohydrometallurgy which includes history of bioleaching.