The KCCL plant reprocesses a pyrite concentrate produced and disposed during the operation of the Kilembe copper mine (Uganda). This disposal was causing an environmental damage for the Queen Elizabeth National Park since the 1970s. The bioleaching process aims at extracting the valuable cobalt trapped in sulphide minerals by destroying the mineral matrix, thus avoiding further pollution from these sulphides. Both for economic and environmental reasons, the process has to oxidise a maximum of sulphides. A gravity circuit was to be introduced to recycle the sulphides remaining after bioleaching, and a simulation study was undertaken to check the effects of this recycling: increased recovery, accumulation of "refractory" minerals in the closed loop. For the simulation of the bioleaching/gravity circuit of the KCCL plant, the objective of bioleaching modelling was to establish a link between a particle size and its capability to be bioleached, to study the impact of the gravity section position inside the bioleaching circuit. Tests were carried out at lab-scale, which proved a relation between the dissolution kinetic and the particle size. That validated the possibility to use a shrinking-core approach for the modelling of the dissolution. The model theory is presented and simulation of different plant configurations, aiming at reducing the sulphide and heavy metals content in the treatment wastes, are described in the paper. (C) 2003 Elsevier Ltd. All rights reserved.