The rougher circuit at Miduk Copper Concentrator, Iran, includes five Microcel (TM) flotation columns in parallel, each 12 m in height and 4 m in diameter. The main objective of this communication was to describe the mixing conditions of rougher columns using residence time distribution (RTD) data. A liquid radioactive tracer was employed to trace the material reported to tailing and concentrate. External scintillation detectors were used to record the tracer intensity without disturbing the flow pattern. Data analysis showed that large and small tank-in-series and N perfect mixers in series were the best models to present the flow pattern of liquid transferred to tailing and concentrate, respectively. Solids mean residence time from a theoretical model for counter-current columns was estimated. A 1.4:1 ratio between the residence time of liquids and solids was achieved. In order to complete the diagnosis, solid content and grade profiles along the collection and froth zone of one of the columns were also measured. The results of the present study showed that despite the presence of vertical baffles, the mixing conditions of industrial rougher columns were close to well mixed ones. The possible causes for mixing in industrial columns were described. As expected by increasing gas flow rate, the mean residence times of liquid transferred to both tailing and concentrate streams reduced and the mixing degree increased. (c) 2007 Elsevier Ltd. All rights reserved.