This paper describes some of the important microbiological and engineering challenges in scaling up biobeneficiation of bauxite. A soil bacterium Paenibacillus polymyxa was recently shown to selectively remove calcium and iron impurities from low grade bauxite (< 50% Al), for abrasive and refractory applications. respectively. An industrial scale formulation of Bromfield medium (called ISF-2), based on cane sugar and tap/mine water, is developed to successfully culture P. polymyxa under septic conditions. The culture is found to be efficient in removing calcium from bauxite ore, carried out as cascade operations in total recycle slurry reactors. In cascade leaching experiments with pre-grown culture, calcium removal was observed to occur solely by indirect mechanism in an initial rapid phase lasting few minutes, followed by a gradual phase comprising of direct attack as well as indirect mechanisms. An alternative mechanism of indirect leaching is proposed based on solubilisation of accessible calcium in the culture metabolite, up to a saturation solubility limit. The saturation solubility theory gives an explanation for the need to perform cascade experiments, and also successfully predicts the possibility of pulse leaching experiments. Some of these recent advances are likely to enable successful commercialisation of bauxite biobeneficiation. <(c)> 2001 Elsevier Science B.V. All rights reserved.