Application of 63Ni to the reduction mechanism of Fe-Ni oxides

The reduction behavior of powdered Fe-Ni oxides in a hydrogen atmosphere at 380oC was studied by means of radioactive tracer of 63Ni. Change in the radioactivity of the surface gives information on mutual diffusion of Fe and Ni between surface and bulk during the reduction process. The radioactivity of 63Ni in the surface layer of the alloy oxides increased with the progress of reduction in the initial stages and then drastically decreased, suggesting that Ni is covered with metallic Fe at the final stage of reduction. x-Ray diffraction showed that unreduced Fe-Ni oxides were composed of a mixture of Fe2O3 (hematite) and NiO, and the reduced alloys were composed of homogeneous Fe-Ni and Fe or Ni. Fe2O3 was changed to Fe3O4 (magnetite) before metallic Ni appeared, and metallic Ni appeared earlier than Fe-Ni. Metallic Fe appeared after Fe-Ni, and no FeO was detected throughout the reduction. The following reactions were proposed to be involved in the reduction mechanism; FeO+Fe2O3→Fe3O4 and NiO+Fe+Fe2O3→Ni+Fe3O4. It was also concluded that an Fe-rich surface is formed by the belated diffusion of Fe to metallic Ni, which is produced earlier than metallic Fe. (author)