The present study investigates the discharge rates of ultrafine and superfine powders from a semi-batch circulating fluidized bed (CFB) using a binary mixture of fine Al(OH)(3) powders (mean size 0.5 and 8.0 mu m, Geldart-C) and coarse fluidized catalytic cracker (FCC) particles (mean size 6 mu m, Geldart-A). Discharge rates of fine powders were measured at the top of the cyclone. Effects of the equilibrium water content of coarse FCC particles on the performance of the CFB, in terms of discharge rates of fine powders, were investigated at a starting loading of fine powders of 5 wt%. The discharge rates of the cohesive ultrafine powders were much smaller than the discharge rates of the less cohesive superfine powders, irrespective of the water content of FCC particles. In the presence of 8.0 mu m superfine powders, the discharge rates decreased with decreasing the loading of superfines in the bed. At a certain loading of superfines, specifically at high equilibrium water contents in the range of 0.061 and 0.067 kg-water/kg-dry FCC, discharge rates decreased with decreasing the equilibrium water content of FCC particles. For FCC particles of low equilibrium water content as 0.038 kg-water/kg-dry FCC, discharge rates were low at the beginning then increased to maximum, within a few minutes, then decreased again with decreasing the loading of superfines in the bed. In the presence of 0.5 mu m ultrafine powders, at high water equilibrium content of FCC particles as 0.067 kg-water/kg-dry FCC, the discharge rates decreased with decreasing the loading of ultrafines in the bed. However, at the two relatively low equilibrium water contents of FCC particles of 0.038 and 0.061 kg-water/kg-dry FCC, the discharge rates were almost constant. The cohesive property of ultrafine powders prevailed. Copyright (c) 2013 John Wiley & Sons, Ltd.