The industrial application of biohydrometallurgy principally comprises the contribution of microbial activity to low-grade ore heap leaching and the use of stirred tank bioreactors for the liberation of gold from pyrite and arsenopyrite. Large scale heap bioleaching promotes recovery of target metals from copper ores (copper) and from polymetallic sulfides (currently nickel and zinc, and potentially copper, cobalt and uranium). Bioreactors for treatment of gold concentrates have been used in industry for over thirty years, with some reactors of over 1000 m3 currently in operation.
Particular acidophilic microorganisms use ferrous iron and/or sulfur as growth substrates, oxidizing them to produce ferric iron and sulfuric acid. These chemicals further attack mineral sulfides, mobilising the target metals and, in the case of refractory gold concentrates, result in the destruction of the sulfide matrix, releasing the gold for conventional downstream recovery.
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, Leptsospirillum ferriphilum)
- Sulphur oxidisers (e.g. Acidothiobacillus thiooxidans)
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, regenerating protons that are consumed by the initial leaching reactions.
Grinding Solutions’ biohydromet lab was set up in collaboration with Dr Chris Bryan and Dr Dave Dew from the Environmental and Sustainability Institute at the University of Exeter. Below is an overview of the services offered by Grinding Solutions:
Diagnostic Batch Test
Assessing the amenability of a sample to biological leaching is carried out in our batch bioreactor tests. The purpose-built bioreactors have full process control and can be run for up to 2 weeks at a time if required. These batch tests are run in duplicate and are used to determine the feasibility of using bioleaching as an enrichment method for your material. Stirred tank bioleaching is usually carried out on high grade ore or concentrates.
A more robust indicator of long term performance, or purely a means of creating a larger pregnant leach solution sample, is the continuous test. These tests can be run for a predetermined length of time and use our own design reactor vessels. Again, these vessels have complete process control and allow the determination of long term performance.
The BIOX process is technically the same as bioleaching, with bacteria assisting in the oxidation and thus dissolution of sulphide minerals. However, as the target mineral is not the sulphides but the gold locked within them, it is not referred to as bioleaching. The BIOX process is an alternative pre-treatment for gold leaching with cyanide. It is a much more environmentally friendly method in comparison to the traditional oxidative roasting of ores that release sulphur dioxide into the atmosphere – which significantly contributes to global occurrences of acid rain. The product slurry from BIOX is dewatered and the residue is then neutralised, re-pulped, and fed into the cyanide leach.
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.