An oxidation process caused by microbes where the valuable metal remains (but becomes enriched) in the solid phase. Biooxidation leaves the metal values in the solid phase and the solution may be discarded. This contrasts with bioleaching where the valuable metal is solubilized. (Both bioleaching and biooxidation are oxidation processes).
If the valuable metal in the ore is hosted by, but not bound to, a surrounding undesired mineral matrix which is blocking the valuable metal from further chemical treatment the matrix can be degraded by selective dissolution of the undesired mineral matrix. Here the microbes causing the selective dissolution may enrich the valuable metal and facilitate metal extraction by selective dissolution of the undesired mineral matrix.
It can be used as a pretreatment process to degrade mineral sulfides such as pyrite or arsenopyrite.
The most common commercial example is pre-treatment of arsenopyrite refractory gold concentrates. The sulphide matrix (arsenopyrite), hosting inclusions of elemental gold particles, is biooxidated. The matrix surrounding the elemental gold will hence loosen up and partly disintegrate, thereby exposing the gold particles so that they are more accessible to subsequent treatment (usually cyanidation).
Researchers about biooxidation
"In refractory ores, small particles of gold are covered by insoluble sulfides impeding the contact between cyanide and gold. In this case, a pretreatment stage must be considered, such as pressure oxidation, chemical oxidation, roasting or bio-oxidation, the latter currently being the alternative of choice. In the biooxidation process, bacteria partially oxidize the sulfide coating covering the gold microparticles in ores and concentrates. In this way, gold recovery from refractory minerals can increase from 15-30% to 85-95%. In the last 15 years at least ten large-scale commercial gold processing units have been established in South Africa, Brazil, Australia, Ghana, Peru and USA, and eight of them use bioreactors."