This paper presents the results of measurements and modelling the residence time distribution (RTD) in industrial flotation columns from several plants, using radioactive and salt tracers. Columns under study had different geometry (Le. square, rectangular and circular transverse sections) and size (from ca. 2.5 m(3) to 180 m3). Data were obtained for liquid (both radioactive and salt tracers), floatable and non-floatable solids by size classes (radioactive tracers). Model structures including the axial dispersion, perfect mixer (PM), large and small tanks in series (LSTS) and two parallel perfect mixers were evaluated and compared. Based on the results, it is concluded that the mixing regime in flotation columns cannot be described by a unique model structure nor can they be related to the cross-sectional area or size of the columns. All industrial datasets presented in this paper showed poor fitting with the perfect mixer model. On the other hand, the LSTS as well as the two parallel perfect mixers models were outstandingly flexible compared to the other structures. The tests allowed the identification of the flow regime and the effective residence times of the different phases, which ranged from 9 to 41 min. The RTD characterization has been useful to detect process failure such as bypass flow identification as well as unbalanced flow distributions in parallel columns at industrial scale.