Iron removal from kaolin. Comparison between “in situ” and “two-stage” bioleaching processes

The filamentous fungus Aspergillus niger can produce large amounts of organic acids (especially oxalic acid) which can be used for the removal of the iron contained as impurity in kaolin. This allows an increase of kaolin whiteness index, a very important factor for its industrial application. Iron leaching may be carried out “in situ” by growing A. niger on kaolin, the efficiency being largely dependent on the strain used, the medium pH and the ratio: culture medium/mass of kaolin. In addition, the presence of kaolin in growing cultures affects negatively both fungus development and active metabolite secretion. In order to overcome the drawbacks, an alternative two-stage bioleaching process was examined. Firstly, fungus was cultured in optimum conditions for active metabolite production. Then, the spent medium was used as leaching agent. Large concentrations of oxalic acid are produced in the culture stage and a kaolin 80% whiteness index is obtained after the leaching process.