The regrinding stage is necessary to achieve the particle size (liberation) for the final upgrade in the cleaning flotation process. At present, vertical stirred mills are increasingly employed at industrial scale. The regrinding process characterization requires mixing regime knowledge along with the mean residence time of the liquid and solid phases. In this work, residence time distribution (RTD) measurements were performed in two different industrial flotation plants. Both plants operate with Vertimills of 10001500 HP in the regrinding process with different circuit arrangements according to the plant treatment capacity. The RTD was measured using the radioactive tracer technique, which allows for non-invasive tracer detection in real time. It was found that mixing was not perfect in agitated Vertimills and consequently the RTD was better modeled by the more flexible Large and Small Tanks in Series (LSTS) model which allowed the characterization of the different mixing regimes for both liquid and solids in the regrinding mills. Effective mean residence times in the range of 1.7-12.5 mm were obtained with RTD shapes similar to those obtained in tumbling ball mills. It was found that residence times of liquid and solid particles were similar. RTD estimations are useful for better understanding the Vertimill behavior and to identify the actual breakage rates, as well as for improving Vertimill modeling and simulation. (C) 2013 Elsevier Ltd. All rights reserved.