Experimental Study of the Inactivation of Reduced Pellets at Lower Temperatures

Abstract

Synopsis:
It is generally recognized that iron oxide pellets reduced by hydrogen at lower temperature are easily re-oxidized by the atmosphere even at room temperature. However, those pellets which were slightly re-oxidized by the inert gas containing the lower oxygen potentials showed inactive behaviors. Very thin oxide layer of less than 9 Å thickness was formed on the pore surface of the pellet during re-oxidation with N₂-O₂ gas mixture of below 1% O₂ content. When O₂ content in the gas mixture was below 1%, re-oxidation degree was nearly 0.9% independent of oxygen potential. But if the O₂ content exceeded 1%, the re-oxidation degree increased rapidly with the O₂ content.
It was also shown that the rate of the re-oxidation was controlled by the diffusion of oxygen through the boundary layer and the reacted shell layer. Therefore, the exposure time necessary for inactivation could be calculated and it was dependent on the oxygen potential.
Furthermore, when the resistance of the gas transport could be negligible, the rate of the re-oxidation was represented by the logarithmic rate law.