Structural and morphological changes during reduction of hematite to magnetite and wustite in hydrogen rich reduction gases under fluidised bed conditions

Abstract

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.

Reduction of electric arc furnace dust pellets by simulated reformed natural gas

“…7 and 8 show no sintered structure at 700°C while EAFD pellets reduced up to 900°C features a sintered structure. This fact has influence on the reduction mechanisms since as soon as sintering occurs, the porosity decreases, which hampers the reducing gas diffusion [30,43].…”

Reduction Kinetics of Fine Hematite Ore Particles with a High Temperature Drop Tube Furnace

“…[14][15][16][17][18] The polished sections of the partially reduced particles were observed with optical microscope as shown in Figs. 7-11.…”

Isothermal Reduction Kinetics of Powdered Hematite and Calcium Ferrite with CO–N₂ Gas Mixtures

“…Maeda et al 7) studied the final stage of reduction of quaternary calcium ferrite with CO-CO 2 gas mixtures. Weiss et al 8) investigated the reduction process of hematite to magnetite and found that the system gradually forms a micro-porous layer of magnetite while the porosity initially increases and then decreases. Turkdogan et al [9][10][11][12] explored the gaseous reduction of iron oxides in H 2 or CO-CO 2 mixtures and discussed the rate-controlling step of reduction process.…”