The effect of the fluidization air flow rate and the transport air velocity on the solids mass flow rate and the pressure drop along the transport pipe was investigated in a vertical air-lift. The use of two different materials, glass (150 mu m) and zirconium oxide (260 mu m) particles (Geldart's B class), enabled to estimate the effect of particle properties on the air-lift performance. Different levels of the solids mass flow rate were obtained by varying the fluidization air flow rate while keeping the transport air velocity constant. The solids loading ratio varied between 2 and 16 for both types of particles. The pressure drop was non-linearly related to the solids mass flow rate at most of the transport air velocities tested. State diagrams presenting the relationship between the transport air velocity, the solids mass flow rate, and the pipeline pressure drop were constructed to characterize the flow pattern and to compare the behavior of the glass and zirconium oxide particles. Owing to the larger size and higher density, the zirconium oxide particles displayed higher pressure drop values than the glass beads and the minimum pressure drop shifted towards higher transport air velocities. The velocity at the transition between dilute and dense phase conveying was approximately in the range of 7 to 10 m/s for glass beads and 10 to 12 m/s for zirconium oxide, at the solids mass flow rates tested. The flow patterns could be also effectively characterized from the analysis of the pressure drop fluctuations at different transport air velocities. (C) 2010 Elsevier B.V. All rights reserved.