A general rate expression based on a proposed reaction model has been developed to describe the rate-controlling process in reduction of porous iron oxide with hydrogen. Transport fluxes in the external boundary layer and the porous iron layer and kinetic flux at the unreduced core surface are considered simultaneously in driving the rate equation in gaseous reduction of iron oxide.
Quoting the experimental data appeared on the literatures, the developed rate equation was examined to consider the rate-controlling step for the reduction of iron oxide. Such analysis showed that the interface reaction is the controlling step for the particle sizes smaller than about 1 cm in diameter, while the gaseous diffusion is the controlling step for the larger particles. It was found that mass transfer of gas to the surface on the reduction rate can be neglected.
From the mathematical analysis on the reduction rate of iron oxide, the range of particle sizes in which the effect of the diffusion rate is greater than that of the interface reaction rate was also determined.