Biooxidation of Minerals & Refractory Concentrates of Gold & Silver

The use of the biooxidation for the treatment of minerals and refractory concentrates of gold and silver is being developed with great potential of immediate application. An ample range of materials, from residues of low degree to concentrates contain submicroscopically disseminated gold in their matrix, which can not be recuperated by the conventional process of cyanidation, unless such materials are pretreated, for example by fine grinding, toasting or lixiviation under pressure: processes which result to be very costly.

The present work includes an evaluation of the bacterian lixiviation as an alternative means of pretreatment. There are unfinished states, among others, aspects related to the biomass bacterial production, methodology of work in laboratories and results of tests for the recuperation of silver of pyritic materials. In the same manner, a scheme of the mechanism which takes into account the solubility of the metallic sulphurs in acid solution is proposed; the product of solubility seems to be the more decisive parameter for the mechanism of the bacterian lixiviation which uses microorganisms of Thiobacillus ferrooxidans type.

Many minerals of gold and silver are refractary to technologies of conventional processing. The refractary nature of these materials can be attributed to components such as iron-sulphured minerals, silicose minerals, telurures, organic compounds and sulphosalts.

The precious metals enclosed within the sulphured minerals of iron represent a large number of refractory minerals of gold. These minerals consist mainly of pyrite and arsenopyrite; these must be oxidated to liberate the precious metals encapsulated in its matrix, permitting in this manner a contact with the lixiviant agent.

There are various methods for the oxidation of the sulphured minerals of iron (toasting, oxidation by pressure, and chemical oxidation), those who make that these refractary materials be docile to the processes of conventional recuperation. Nevertheless, the use of such methods sometimes results non attractive due to the high costs of capital and operation. One alternative to such methods is the biological pretreatment with which the sulphured minerals of iron are degraded and the precious metals liberated can be recuperated by the conventional technologies.