Bioleaching of copper and cobalt arsenic-bearing ores: a chemical and mineralogical study

Abstract

Arsenic is a major impurity present in numerous sulfide deposits. Two types of problems are associated with the presence of this element: metallurgical and environmental problems. Both enargite (Cu3AsS4) and cobalt bearing sulfides ((Co,Fe)(As,S)2) contain arsenic as a main component. They are present in important Chilean deposits and have been the object of very scarce studies. This work is a multidisciplinary study that includes chemical, biological and mineralogical aspects. Both minerals could present important variations in their chemical composition and belong to isomorphe series. Enargite forms part of the enargite-luzonite-famatinite series and the cobalt bearing phase to a complex family of sulfides that includes arsenopyrite (FeAsS), löllingite (FeAs2) and glaucodot ((Fe, Co)AsS). Both minerals also contain variable amounts of minor elements like Sb, Bi and Ag in the case of enargite and like Ni, Pb and other elements for the cobalt bearing sulfides. The presence of these elements affects significantly the crystal structure and the defect density and consequently modifies the susceptibility of the minerals to leaching and bioleaching processes. Depending on the control stage of the leaching process, these changes in the structure and composition of the mineral can modify the global kinetics of the leaching, inducing changes in the chemical reaction, its kinetics and the extension and composition of the passivating layer or enhancing local attack on crystal disturbed zones. Furthermore, these elements are generally toxic for microorganisms and their dissolution could inhibit bacterial activity in the bioleaching process, particularly in a concentrate leaching process. A bacterial adaptation and a control of dissolved impurities would then be required. Small changes in the mineral composition and structure can then explain the great differences observed in the result of a same process applied to minerals that were expected to be similar but indeed have different origin and composition. This work demonstrates and illustrates the importance of a complete mineralogical characterization of the ores and concentrates at both macroscopic and microscopic scales.