The mineral industry has been using cyanidation of aerated slurries to recover gold from ores for more than a century. However, the leaching plant is usually designed as a series of agitated tanks of the same size, without any attempt to find an optimal plant design for improving the circuit efficiency, either by decreasing the cyanide consumption, or increasing the gold recovery, or decreasing the total plant volume. The objective of the study is to test, by simulation, if it would be profitable to use plant designs differing from the usual ones. The focus is put on the selection of the volumes of the tanks in the cascade of leaching reactors. The methodology involves the use of gold dissolution and cyanide consumption kinetic models incorporated into a simulator, and the definition of a performance criterion for the plant optimization. The performance is characterized by a cost function containing a term representing the value of the unleached gold and a term accounting for the cyanide consumption costs. It is shown that, for the same total volume, using a sequence of increasing size reactors improves the performance of the plant. The results are produced for different size of the ore particles and different numbers of tanks in the leaching circuit. (c) 2005 Elsevier Ltd. All rights reserved.