Test work performed in a pilot-scale flotation column (4 in height x 0.057 in diameter) processing an industrial zinc concentrate (51% w/w Zn as sphalerite, 10.5%, Fe, 0.77% Pb, 0.62% Cu, 7.3% NSG, d(80) = 110 mu m), confirmed the findings of previous work conducted by the authors, that showed there exists a limit in the mass flow rate of solids that can be processed in the column without adversely affecting recovery and solids carrying-rate; this limit is related to the onset of an unusual accumulation of gas in the lower section of the cell due to overloading of gas bubbles. In the present work, the effect of slurry rate (J(t) = 0.3-1.7 cm/s) and slurry density (15-35% w/w solids) onto solids recovery and solids carrying-rate were studied under the following experimental conditions: J(g) = 1.45 cm/s, 15 ppm Dowfroth, pH = 9.5 and 60 g isopropyl xanthate/ton; froth depth = 0.3 m. The results showed that solids carrying-rate may be maximized by operating the column with a combination of a relatively dense slurry and a relatively small slurry rate. The above behavior is explained in terms of the solids load that air bubble transport under the different operating conditions imposed, which is reflected by the axial air-holdup profile established in the column, as a result of the accumulation of overloaded bubbles in the lower part of the collection zone. It is argued that the slurry rate plays an important role on the onset of this phenomenon since it directly affects the rising velocity of overloaded bubbles, thus being the responsible of such unusual accumulation of gas and of phenomena such as bubble coalescence and lost of bubble surface area. (C) 2007 Published by Elsevier Ltd.