Comparative studies on isothermal and non-isothermal reduction of haematite in carbon monoxide atmosphere

Abstract

Iron ore fines was isothermally and non-isothermally reduced with CO and the total mass loss was continuously recorded. The different phases developed during the reduction process were identified by X-ray diffraction analysis. The structural changes accompanying the reduction reactions were microscopically examined. During the isothermal reduction tests, temperature has a significant effect on the reduction reaction. At a given temperature, the highest rate was obtained at initial stages whereas the minimum rate was observed at the later stages due to the formation of dense iron layer. The activation energy values (Ea) at the early stages was 39.23kJ mol -1 revealed that the reduction is most likely controlled by a combined effect of gaseous diffusion and interfacial chemical reaction mechanism. At later stages, the Ea values were 54.19 kJ mol-1 indicated that the interfacial chemical reaction is the rate controlling mechanism. Testing of the mathematical formulations derived from the gas-solid reaction model confirmed these controlling mechanisms. The non-isothermal reduction experiments were carried out using different heating rates which showed a considerable effect on the degree of reduction. The reduction conversion continuously increased with rise in temperature. The reduction mechanism was predicted from model free and model fitting. The activation energy values were ranging from 135-40 kJ.mol-1 indicating that gas diffusion is the rate controlling step.

Isothermal and Non-Isothermal Reduction Behaviors of Iron Ore Compacts in Pure Hydrogen Atmosphere and Kinetic Analysis

“…At the initial stages, the Ea value indicates that the reduction process seems to be controlled by a mixed mechanism of gaseous diffusion and interfacial chemical reaction. During the final stages, the Ea value reveals that the reaction is controlled by interfacial chemical reaction and this matched with other studies [17,26,32].…”

“…Increasing heating rates leads to shorter time for the sample at any particular temperature. At the same heating rate, the degree of reduction is higher in H 2 than in CO atmosphere [26].…”

“…For any given reduction temperature, the rate of reduction is highest in the early stages and gradually decreases until the end of the reduction reaction. At low temperatures, the reduction takes longer and the reduction time decreases as reduction In the case of reduction using CO at temperatures 80--900°C , samples did not complete reduction and stopped at different extents of reduction, depending on the applied temperature [26]. Moreover, during experiments, an increase in the weight of the compacts was observed.…”

“…4 Reduction behaviors of iron ore compacts at 800-1100°C by pure H 2 core and the diameter of the unreacted core becomes smaller and smaller until zero. The reduction mechanisms of iron ore during both the initial and final stages can be deduced from the correlations between the apparent activation energy values, the gas-solid chemical reaction, and the microstructure of the reduced samples in the reduction procedure [26,[28][29][30][31]. The kinetics of iron ore reduction can be described using a shrinking unreacted core model to elucidate the reduction reaction.…”

“…where K and X express the rate of reaction and the fractional reduction degree, respectively. In order to confirm the reduction mechanism prediction from the activation energy values, the above chemical reaction [26] formulations were tested against experimental results as presented in Figs. 7 and 8.…”