A double-stage velocity gradient conical fluidized bed flocculator (DG-FBF) was employed to investigate the issue of floc breakage that occurred in a single-stage velocity gradient conical fluidized bed flocculator (SG-FBF), using 150 mg/L Kaolin clay suspension as the influent and 60 mg/L polymer aluminum chloride as the flocculant dosage. Two kinds of particles with significant differences in effective weight (i.e. silica gel beads and resin beads) were packed into the DG-FBF; these particles separated into two grades along the axial direction due to the fluid shearing force. The back-mixing problem, which lead to floc breakage in the SG-FBF was apparently solved, as confirmed by both the observed flow patterns and the floc size distributions. The two-stage velocity gradient and voidage observed in the DG-FBF created a more suitable environment for growth of the aggregates. The flocs formed in the DG-FBF tended to be denser and presented an advanced self-similarity as evidenced by the results of fractal dimensioning. In addition, the abatement efficiencies of Kaolin suspensions in the DG-FBF increased by about 10% at most of the loading rates investigated (at superficial velocities of 6.7-17.9 mm/s) and became more stable compared with those from the SG-FBF. (C) 2012 Elsevier B.V. All rights reserved.