Biolixiviation of Refractary Minerals

The biolixiviation of refractary minerals which contain gold and silver is a relatively new concept. It is rapidly coming to be established as an alternative means of pretreatment. Numerous studies have been performed in laboratories and in a pilot scale using the Thiobacillus ferrooxidans, a microorganism which oxydes iron and sulphur under acid conditions.

A complete oxydation of the sulphurs may not be necessary to reach a liberation of the precious metals. For example, if the gold is encapsulated in the arsenopyrite, the conditions of the process must be adjusted to avoid the unnecessary oxidation of the accompanying pyrite. The speed of oxidation is better at low concentrations of solids (10% to 15% of solids). The time of retention can vary from 15 to 150 hours depending on the mineralization and the degree of oxydation required for the liberation of the precious metals.

For the process to occur effectively, the pH must be near 2,0; if there were consumers of acids a pretreatment with sulphuric acid would be necessary, but with the majority of concentrates a lower dosage of acid is sufficient to adjust the pH; if the pH goes down to lower values, milk of lime must be added.

To corroborate results, to perform a pilotage is necessary to escalate and confirm the speed of aeration, density of the pulp and the time of residence. Among other aspects which also should be investigated during the pilotage we can mention: the need of change of sollution for the control of acidity, effectiveness of the biomass, concentration of metals, definition of the balance of cooling and heating and the efficiency of recycling of water in a closed system.

The process using the Thiobacillus ferrooxidans was piloted with success at a scale of 7 TPD in the mine of Equity Silver B.C. in 1984-85. from then on, the technology of the bacterian lixiviation has shown to be appropriate for a large number of refractary deposits. It has been reported that small commercial plants are in operation in South Africa and Zimbabwe.

Many mining companies presently routinely include the biooxidation as one of the alternative processes of pretreatment of their refractary minerals.