During the reduction of iron ore fines structural changes in particles have a significant influence on the rate of reduction. Investigations regarding porosity, specific surface area and mean pore diameters in the reduction of hematite with hydrogen rich reducing gases were performed by mercury porosimetry. Morphological changes were examined by metallographic analyses of polished sections in reflected light. In the magnetite equilibrium phase, significant influence of temperature on structural parameters and sintering effects were found. For wustite phases, the influence of temperature was less pronounced. For the reduction of hematite to magnetite and magnetite to wustite topochemical phase growth and microporous product layers were observed. In the single step, reduction of hematite to wustite progressive conversion and significant increase in the mean pore diameter were found. The results presented in this work are of high importance for understanding the reaction kinetics of iron ore fines and essential for modelling heterogeneous reactions.List of symbols a 1 constant for increase in specific surface area, g 22 min 21 a 2 constant for decrease in specific surface area, g 22 min 21/2 A Sp specific surface area, m 2 g 21 A Sp,0 initial specific surface area, m 2 g 21 A Sp,de term for decrease in specific surface area, m 2 g 21 A Sp,in term for increase in specific surface area, m 2 g 21 p i partial pressure p tot total pressure, bar t time, min T temperature, K c parameter for decrease in specific surface area 14 h temperature, uC Structural changes during heterogeneous reactionsDirect reduction of iron ore fines receives increasing attention in the area of alternative iron making. For the design and optimisation of direct reduction processes, detailed knowledge of kinetics of iron ore fines reduction is necessary, to be able to model heterogeneous reactions. Numerous models were developed by several authors, 1-12 including structural parameters such as porosity, specific surface area and mean pore diameter. Investigations regarding the changes of particle structure including also sintering effects during reaction were done. [11][12][13][14][15][16][17][18][19] During heterogeneous reactions, chemical and physical effects are competing for the changes of porosity, specific surface area and pore size distribution. Chemical reaction can result in an increase and decrease in porosity and specific surface area. Decisive therefore are the molar volume of solid reaction products, 9,20-22 the stoichiometry of the reaction and the initial porosity of the reactant.Furthermore, decrease in porosity and specific surface area during heterogeneous reactions is caused by sintering effects, recrystallisation and reordering of solid species. Sintering effects during iron ore fines reduction were investigated by several authors. [22][23][24][25][26][27] Decrease in specific surface area observed by Heizmann et al. 25 was interpreted as reorganisation effects of solid reaction products.

For optimization of iron ore fines reduction processes, fundamental knowledge of the reduction kinetics under industrial operating conditions is required. Reduction tests with coarse hematite iron ore were performed in H2‐rich reduction gas atmospheres of H2, H2O, CO, CO2 and CH4 considering elevated pressures similar to industrial process conditions. The reaction kinetics of the reactions hematite to magnetite, magnetite to wüstite and hematite to wüstite were investigated in a laboratory‐scale pressurized fluidized bed reactor. To facilitate kinetic measurements, the laboratory‐scale fluidized bed reactor is equipped with a sampling system, which allows sampling of the bed material at operating conditions. Process conditions were chosen close to industrial plants in a temperature range from 400 to 700 °C. Significant influence of temperature on the rate of reduction was found in all tests. Variation of reduction gas composition showed no influence on the rate of reduction in the range of concentrations investigated. Results presented herein proved the feasibility of depicting industrial process conditions in laboratory scale. For the reduction of hematite to magnetite and magnetite to wüstite topochemical phase growth were observed. In the single step reduction of hematite to wüstite, magnetite appeared as intermediate product formed by topochemical reaction while progressive conversion of magnetite to wüstite was found.

For the investigation and modelling of direct reduction processes of iron ore fines detailed knowledge of stability areas of Fe, Fe 32d O 4 , Fe 12y O and further of kinetic and structural parameters as functions of temperature and gas composition is necessary. Kinetic and structural parameters are supplied by detailed experimental work. For planning experimental test programmes, information about stability areas of iron and iron oxides is essential. As the calculation of equilibrium plots of quartary gas mixtures containing H 2 , H 2 O, CO and CO 2 is rather complex, a simplified empirical method for the determination of stability areas of iron and iron oxides is useful. To make experimental data acquisition easier, a detailed description on the calculation of empirical reduction diagrams including the non-stoichiometries of magnetite and wustite is given in this work. Stability fields of iron, wustite and magnetite are given as functions of temperature and gas composition of H 2 , H 2 O, CO and CO 2 . The influence of non-stoichiometries concerning the achievable degree of reduction was developed for wustite and magnetite. In the case of wustite a significant effect was found.

In metallurgical processes, fluidized-bed technology is gaining more importance because of its advantages. Processes with H 2 -rich and CO-rich reducing gases were developed for the reduction of iron ore fines (e.g. FINEX ® ). For improvement of these new technologies, greater knowledge about the chemical kinetics of iron ore reduction in fluidized beds is necessary. The scope of this work is to evaluate the limiting regime of the iron ore fines reduction. Therefore, experimental results of reduction tests were compared with theoretically investigated reduction rates. These reduction rates were based on a limitation either of mass transfer through the external gas film to the particle surface, diffusion in a porous product layer (pore diffusion and Knudsen diffusion), diffusion in a dense product layer (solid diffusion) or the phase boundary reaction. The phase boundary reaction was found to be the most likely limiting reaction regime.

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