The behaviors of reduction and carbon deposition during the isothermal reduction of porous limonite ore fines by CO-N-2 gas mixtures were investigated using a rotary drum reactor in the temperature range of 750-900 degrees C. As the results suggested, the CO level and reaction temperature significantly impacted the reduction degree and the rate of reduction. It was found that in pure CO atmosphere at 750 degrees C, the reduction of iron oxides occurred with the carbon deposition reaction, which formed filamentous carbon before 60 min reduction. While with the extension of reduction time, the filamentous carbon disappeared, and the porous graphite clusters were formed due to the elevating level of deposited carbon in the reduced particles. A new mechanism including eight steps was proposed to clarify this phenomenon. Yet at 900 degrees C, the reduced particles gradually showed a sponge-like structure with reduction proceeded, and the iron whiskers precipitated on the particle surface and spread out significantly in 50 vol% CO-N-2 atmosphere. As the mathematical modeling of the experimental data suggested, the overall reduction of Fe2O3 to Fe could be presented by a 3D diffusion model in the whole temperature range investigated. The value of apparent activation energy for the reduction of limonite to metallic iron was found to be nearly 104.4 kJ/mol. (C) 2019 Elsevier B.V. All rights reserved.