Suspension magnetization roasting technology is a new energy-saving technology in iron ore separation, and good results have been achieved in the treatment of complex and refractory iron ore resources. To solve the problem of uneven quality of the reduced product and poor discharge of the material, a U-type reduction chamber, with dual functions of reduction chamber and sealing valve, is designed in this work. Within the self-designed reduction chamber structure, the effects of the initial bed height and aeration velocity on the minimum fluidization velocity were studied using different sizes of alumina powder particles. Based on the experimental data measured using the Delta P - U-f and 6p - U-f methods, it is found that particle size and aeration velocity are the main factors affecting the minimum fluidization velocity, whereas the initial bed height has little effect. Both methods show good consistency, which illustrates their reliability for measurement of the minimum fluidization velocity. A power spectrum analysis revealed that the formation, coalescence, and fragmentation of bubbles were the fundamental reasons for pressure fluctuation. In addition, with decreasing particle size, there were higher frequencies of bubble coalescence and fragmentation, and the size distribution of the bubbles in the reduction chamber was more dispersed. (C) 2018 Elsevier B.V. All rights reserved.