Aided by self-agglomeration, a two-stage reduction process conducted at a higher temperature ( 600 degrees C) than the single-stage process resulted in an enhancement of the k constant to more than twice that of the single-stage process. A force balance model coupled with the reduction kinetics of Fe2O3 is first proposed to explain the self-agglomeration mechanism of iron nanoparticles (NPs) during the reduction. This force balance model successfully elucidates the reason for the prevention of defluidization via a two-stage fluidized bed method. At lower temperatures, there is a long stationary phase, which is of great importance in overcoming the sintering of agglomerates and in promoting the reduction reaction. At lower temperatures, the initial NPs first self-agglomerate into particles that are tens of microns in size; they then gradually agglomerate into larger particles (> 100 mu m) at higher temperatures. By contrast, the sudden growth of iron NP agglomerates causes sintering and defluidization in the single-stage fluidized bed method. (C) 2017 Elsevier Ltd. All rights reserved.