The percolation leaching of heavy metals from pyrite cinder in a packed bed is considered in this work. Flow rate and physicochemical parameters have been analysed in a gravity percolation leaching system with a simultaneous dissolution reaction. The structure of the unconsolidated bed of porous cinder changes during operation as a consequence of the leaching phenomenon. The variables that influence the flow rate are: bed length, particle size distribution and leaching liquor characteristics. Head loss changes during operation are mainly explained in terms of the fine particle redistribution, migration along the bed and thickening at the bottom. Experimental values of effective void fraction are compared with values from Ergun's equation modified to be applied to a layered bed.