Biohydrometallurgy focusses on the bacterially-assisted oxidation of metallic sulphide minerals. It utilises bacterial cultures that have adapted overtime to metabolize energy from this oxidative breakdown of sulphide minerals. Biological oxidation (bioleaching) can be used to process a number of different minerals, including not just sulphides but also oxides such as pitchblende. It can also be utilised as a pre-treatment for refractory gold (BIOX).
The bacteria do not actually cause the dissolution of sulphides, but merely catalyse the oxidation of the metal sulphides to metal sulphates. The main role of the leaching bacteria is the regeneration of ferric ions and the conversion of intermediate sulphur compounds to sulphate.
This is carried out through 2 main types of bacteria:
- Iron oxidisers (e.g. Acidothiobacillus ferrooxidans – Af, Leptsospirillum ferriphilum – Lf)
- Sulphur oxidisers (e.g. Acidothiobacillus thiooxidans – At)
The regeneration of ferric ions allows for further oxidation of the sulphide mineral. The oxidation of the sulphur compounds to sulphuric acid is also beneficial, as it regenerates protons that are consumed by the initial leaching reactions. Elemental sulphur may also occur as a product of the initial leach reactions (for example with polysulphides such as chalcopyrite). Elemental sulphur can form as a film coating the mineral surfaces. With elemental sulphur being so inert, this film cannot be broken abiotically in acidic environments, however sulphur oxidisers such as At are still capable of oxidising. Breaking the sulphur coating, or preventing it from forming in the first place, increases the leach kinetics.
Our biohydromet lab has been set up with input from Professor Chris Bryan and Dr Dave Dew from the Environmental and Sustainability Institute, Exeter University. For more information on how biohydrometallurgy may be applicable for your project, or simply to find out more about biohydrometallurgy, please do not hesitate to contact us.