This experimental work on sphalerite flotation investigated the effect on flotation performance of three particle size fractions, namely, coarse (d(80) = 100 mu m), medium (d(80) = 39 mu m) and fine (d(80) = 15 mu m), bubble size distribution, superficial air velocity, and collector dosage. Bubble size distributions were characterized with the image analysis technique. The two-phase (liquid-gas) centrifugal pump and frother addition (MIBC, 5-30 ppm) allowed generating bubble diameters between 150 and 1050 mu m, and air holdup ranging from 0.2% and 1.3%. Main results showed that each particle-size distribution required an optimal bubble-size profile, and that sphalerite recovery proceeded from mechanisms involving true flotation (when J(g) = 0.04 cm/s and 1.9 x 10(-4) M SIPX). However, cluster-flotation occurs at high collector dosage (when J(g)= 0.04 cm/s and d(32) between 285 and 1030 mu m), and requiring further investigation. (C) 2014 Elsevier Ltd. All rights reserved.