The discharge of fine powders from circulating fluidized beds (CFB) decreases their loading in the bed which is believed to be an important key parameter affecting the whole performance and fluidization quality of the fluidizing system. The discharge rates of micron-size powders from a semi-batch CFB were investigated using a binary mixture of fine aluminium hydroxide powders (3.0 mu m mean size) and coarse FCC particles (66 mu m mean size). The discharge rates of fine powders, measured at the top of the cyclone, are believed to be affected not only by gas humidity but also by the water content of the bed particles. The effects of the equilibrium water content of FCC particles on the discharge rates of fine powders were investigated under different gas velocities and starting loadings of fine powders of 3 and 5 wt.%. The discharge rates increased greatly using FCC particles of higher moisture contents as 0.054-0.067 wt(H2O)/wt(dry FCC), where large discharge rates of fine powders were measured at higher loadings of fine powders. Dry FCC particles and FCC particles of lower moisture contents as 0.038 wt(H2O)/wt(dry FCC), were found not to be appropriate for CFB as extremely low discharge rates of fine powders were obtained at higher loadings of fine powders. High gas relative humidity as 85% could not increase the discharge rates of fines in the presence of totally dry bed particles. (C) 2013 Elsevier B.V. All rights reserved.