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Synopsis : A mathematical model is presented for describing the hydrogen reduction of porous basic hematitepellets which show extremely low reduction-rate in the final stage at relatively high temperatures. From the fact that the reduction rate in the final stage depends on the amount and diameter of wastite particles encircled with dense iron, such slow rate is considered to be controlled by the rate of solid-state diffusion of oxygen through the recrystallizing iron layer. To take into account the solid-state diffusion , the multi-stage zone-reaction model presented previously is partly modified , i.e., cross section of a pellet is devided into the following four reaction zones: hematite-magnetite , magnetite-wiistite, wiAstite-iron layer in which wastite particles are dotted with iron nuclei, and the other wastite-iron layer composed of wilstite particles encircled with dense iron. In the former three layers , chemical reaction and gaseous diffusion proceed simultaneously. In the last layer , the reduction rate of wastite particles is controlled by the rate of solid-state diffusion mentioned above. The calculated results agree well with experimental data up to the final stage of reduction . The values of oxygen permeability through iron layer show good agreement with those reported previously.

Multi stage Zone-reaction Model with Solid-state Diffusion for the Hydrogen Reduction of Porous Hematite Pellets