Bubble size was measured in two lab scale cells using the HUT bubble size analyzer. The influence of operating conditions on bubble size was investigated. The Sauter mean bubble size was found to vary over the range of 1.2-2.9 mm. The experimental bubble size distributions were satisfactorily represented by the upper-limit distributions. The frothers were found to have a profound impact on bubble size. It seems that frothers not only prevent bubble coalescence, but they also, affect the bubble break-up process. The air flow rate was also found to have a strong influence on bubble size. The Sauter mean bubble diameter increased as the air flow rate increased. The Sauter mean bubble diameter was found to decrease with increasing impeller speed. The addition of quartz on the other hand was found to increase the size of bubbles, this effect was more evident at solid concentration exceeding 20% (kg/kg). The size of bubbles generated with the multi-mix and free-flow mechanisms were different at similar operating conditions, in a non-coalescing environment. The free-flow design generated broader bubble size distributions than the multi-mix mechanism. (c) 2005 Elsevier Ltd. All rights reserved.