Abstract: Multi-velocity gradients along the up-flow direction was develped in a fluidized bed reactor based on the fact that different heavy solid particles could achieve a hydraulic classification under fluidized condition. Based on the results of the flocculation kinetic characteristics of fluidized bed flocculator, the characteristics of a multi-velocity gradient fluidized bed flocculator (MGF) filled with 800 μm and 1 200 μm resin particles was studied in the gradient distribution characteristics, flocculation efficiency and morphological characteristics of the flocs. The results obtained in the MGF were compared with a single-velocity gradient fluidized bed flocculator (SGF) filled with 1 200 μm resin particles directly. It was found that the environment for collision of colloids or fine particles was more suitable for the MGF, and thus the continuous growing of the clusters could be created. As a result, bigger and denser flocs could be formatted in MGF, and then the sedimentation ability of the flocs improved obviously which also confirmed by the improved flocculation efficiencies. The flocculation efficiency of kaolin suspensions and dispersed red increased 2%-4% and 7%-20% respectively in the MGF compared with the SGF.

Key words: fluidized beds, velocity gradient, flocculation, fractal dimension