Nonisothermal kinetics of iron-ore reduction

Abstract

Non-isothermal kinetic studies on reduction of iron ore have been carried out under increasing temperature conditions. These studies use a commercial thermal analyser (STA-780) and the moving bed technique (MBT). The heating rate is linear in the former and non-linear in the latter. The theory of non-isothermal kinetics and methods of kinetic parameters evaluation employed in this paper are described briefly. Recently the MBT has been used for generating non-isothermal kinetic data with and without external gas flow. The MBT results presented in the paper show that transport processes significantly govern reduction kinetics. The method proposed can be extended to analysing dynamic non-isothermal type reactors. Simultaneous thermal analysis (STA) plots show stepwise reduction of iron ore (coupled with oxidation of coal) and a change of reaction mechanism at about 600-degrees-C. The moving bed experiments employing larger and bulk samples with external gas flow are presented and their data have been compared with STA data and with MBT data obtained for identical systems without external gas flow. The reducing gas mixture of carbon monoxide and nitrogen in the fixed composition of 30:70 was used in this investigation. The MBT results with external gas flow show stepwise reduction of iron ore, but in the absence of an external gas flow there is considerable overlap of reactions. The variation of apparent activation energy values (12-80 kJ mol-1) indicates the change of kinetic mechanism. Apparently the gas phase mass transport and the initial rate determining processes govern the reduction process under the temperature conditions imposed during the course of the reaction. It is shown that the presence of an external gas affects the kinetics of the reacting system significantly. The M BT can be effectively used to support and substantiate the kinetic data obtained by standard commercial equipment and to carry out non-isothermal kinetic studies employing larger and bulk samples, under conditions close to those of actual systems.