The mixing characteristics of large-size industrial flotation equipment, consisting of banks of mechanical cells and pneumatic columns, has been observed using non-invasive experimental techniques, such as a radioactive tracer to measure the residence time distribution of the liquid and solid. The classical tanks-in-series model showed a good fitting to describe the operation of industrial banks with a different number of flotation cells in series. On the other hand, results for large-size pneumatic flotation columns were also close to well-mixed and can be well represented using a simple model of one large mixer in series with two small mixers plus a dead time, which can be related to the physical phenomena occurring inside a flotation column. The column mixing model is shown to compare well with tracer data from a variety of column designs from different industrial operations. A comparison with the axial dispersion model considering a closed-closed boundary conditions is presented. (c) 2004 Elsevier Ltd. All rights reserved.