Residence time distribution functions were determined and modelled for a pilot plant mixer-settler and an electrowinning cell. The mixer RTD is modelled by a small plug flow reactor, a small perfect mixer and a large size one. The settler RTD is modelled by Cholette-Cloutier units in parallel with a series of small perfect mixers, although frequently the settler is assumed to behave as a plug flow. Finally the electrowinning cell RTD is very close to being a perfect mixer.