Bioleaching of zinc and nickel from silicates using Aspergillus niger cultures

In this work, we investigated the role of bacteria from the genera Bacillus and Pseudomonas and fungi from the genera Aspergillus and Penicillium in the leaching process of two different silicates (calamine and garnierite). Since the results obtained with A. niger were better than those with different bacteria, a more detailed investigation of the leaching process with this microorganism was conducted. Moreover, although it is clear that the citric acid generated by fungi could be an important leaching agent acting in the solubilization of the used silicates, other products of metabolism could be involved. Related to this, the results obtained with chemical leaching using low concentrations of citric acid (lower than 10 mM) showed, for both calamine and garnierite, that the respective dissolution of zinc and nickel was much lower when compared to those processes in which cultures or supernatant liquor of A. niger cultures were used and in which the maximum concentration of citric acid was 8 mM, The results obtained also suggest that the type of mineral (and/or the metal present in it) presents a different susceptibility to the bioleaching process and also demonstrate that depending of the situation, the presence of the fungi cells seem to improve the leaching process. From a practical point of view, the high yield rate of extracting metals from silicates obtained by using for example, supernatant liquors of A. niger cultures, is noteworthy. This bioleaching process present two advantages as compared to conventional chemical leaching processes: (a) the very low concentrations of organic compounds present in such a situation represent a lower ecological risk; and (b) even with a lower final yield, the economical cost of a such process. Both characteristics could facilitate its industrial application. (C) 2000 Elsevier Science B.V. All rights reserved.