The flow behavior of the liquid phase of an industrial gold leaching tank is studied using lithium chloride tracer. The residence time distribution is interpreted using an arrangement of simple ideal reactors, the parameters of which are calibrated by a standard least squares method. The measured net volume of the reactor is 62% of its nominal volume, due to the space occupied by the agitation assembly, the gas phase and the accumulation of solid material on the tank walls. The results show that the tank mixing behavior is dominated by a perfectly mixed volume, corrupted by the presence of bypass and stagnant zones. The mixing pattern is modeled as two parallel flows, one which contains the dominant perfect mixer and a series of perfect mixer reactors with cross-flow to match the stagnant zone, and the other one which account for the bypass represented by a series of perfectly mixed reactors. © 2004 Elsevier Ltd. All rights reserved.