New information on the chalcopyrite bioleaching mechanism at low and high temperature

The chalcopyrite bioleaching mechanism was studied to determine its direct or indirect nature. At the same time, a study of possible causes that diminish the dissolution rate and inhibit the attack of this sulphide mineral was carried out. An electrochemical study of the mineral was also performed, which included the elimination or dissolution of possible diffusion barriers formed on the mineral surface. Results of these studies showed that the oxidation state of the dissolved iron (Fe3+) was fundamental to chalcopyrite bioleaching because Fe3+ controlled the relative rate of the oxidation reactions. In addition, the attack of chalcopyrite was controlled by elemental sulphur and intermediate, nonstoichiometric, copper sulphides forming on the chalcopyrite surface, which are less reactive than the original sulphide. Intermediate sulphides caused an important barrier effect at low temperature (35 degreesC). At higher temperature (68 degreesC), these intermediate sulphides do not constitute a diffusion barrier due to their dissolution. Microbial attachment to the pyritic phase of the copper ore was related to the dissolution rate of the mineral due to the liberation of Fe2- after the attack of the pyrite by contact bioleaching. It was concluded that the bioleaching of chalcopyrite concentrate is a cooperative effort involving the simultaneous contact bioleaching of the pyritic phase of the mineral, possibly by an indirect mechanism via thiosulphate, and the indirect bioleaching of chalcopyrite, probably by a mechanism by way of polysulphide and elemental sulphur. (C) 2003 Elsevier B.V. All rights reserved.