A novel fluidized bed reactor, which is an improvement of the loop seal, was applied successfully to iron ore reduction roasting. For this U-type reduction chamber, the fluid dynamic behaviors of different-sized particles were investigated via a cold experimental apparatus using the pressure measurement method in this study. The results showed that the measured U-mf of 71, 101 and 147 mu m-sized particles were 0.033, 0.040 and 0.059 m/s, while the U-c were 0.167, 0.190 and 0.223 m/s, respectively, demonstrating that the decrease of particle size caused a rapid transition from both the fixed to bubbling regime, and the bubbling to turbulent flow regime. Under stable operation, the pressure drop, average solids holdup and axial nonuniformity index across the FC increased when particle size increased, along with the solids height and pressure drop gradient in the SC, though the differential pressure fluctuation decreased. These consequences indicated that larger particle size promotes the opportunity for particle mixing and contacting, accordingly increasing the preferential formation of the particle aggregation and deteriorating the reduction performance. These can provide guidelines for the regulation and control of industrial iron ore fluidized bed roasting. (C) 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.