Reduction kinetics of mechanically activated hematite concentrate with hydrogen gas using nonisothermal methods

摘要

The reduction kinetics of both non-activated and mechanically activated hematite concentrate in a vibratory mill for different grinding periods have been studied using themogravimetry (TG). Changes in the structure of hematite were studied using X-ray diffraction analysis. The isoconversional method of Kissinger-Akahira-Sunose (KAS) was used to determine the activation energy of the different reactions. The Vyazovkin model-free kinetic method was also used for prediction of kinetic behavior of the samples for a given temperature. Fe2O3 was found to reduce to Fe in a two-step via Fe3O4. Intensive grinding resulted in improved resolution of overlapping reduction events. It was also established that the mechanical activation had a positive effect on the first step of reduction. With increasing the grinding time, the activation energy at lower extent of conversion (alpha <= 0.11) decreased from 166 to 106 kJ mol(-1) range in the initial sample to about 102-70 kJ mol(-1) in the sample ground for 9 h. The complexity of the reduction of hematite to magnetite and magnetite to iron was illustrated by the dependence of E on the extent of conversion, alpha(0.02 <= alpha <= 0.95). The values of E decreased sharply with alpha for 0.02 <= alpha <= 0.11 range in the initial sample and mechanically activated samples, followed by a slight decrease in the values of E during further reduction by alpha <= 0.85 in the ground samples up to 3 h. A slight increasing dependence of E on alpha for mechanically activated sample within 9 h in the second step of reduction was observed due to the finely agglomerated particles during intensive milling and subsequently the formation of a dense layer during the reduction processes. In addition, the dependence of ln A(alpha) on alpha was detected and it was found that the ln A(alpha) shows the same dependence on alpha as the apparent activation energies. (c) 2007 Elsevier B.V. All rights reserved.