Thermal synthesis of nanocrystalline fcc Fe–Ni alloy by gaseous reduction of coprecipitated NiFe 2 O 4 from secondary resources

Abstract

Nanocrystalline fcc Fe–Ni alloy has been fabricated by gaseous reduction of coprecipitated NiFe₂O₄ from fly ash. Pure crystalline nickel ferrite is obtained by thermal treatment of coprecipitated hydrated sulphate mixture of iron and nickel produced from treatment of fly ash. The produced powders of NiFe₂O₄ were isothermally reduced in pure hydrogen at 800–1100°C to synthesise fcc Fe–Ni alloy. The reduction behaviour of nanosized NiFe₂O₄ was investigated. The formed phases after reduction processes were identified by X-ray phase analysis. The microstructures of partially and completely reduced samples were examined by a scanning electron microscope and an optical microscope. The kinetics data obtained from reduction process were used to elucidate the reduction mechanism under isothermal condition. It was found that nanocrystalline fcc Fe–Ni alloy (Fe_0·64Ni_0·36) can be obtained from NiFe₂O₄ powders. The reduction rate increased with increasing reduction temperature in both the initial and final reduction stages. At the initial stages of reduction, the reaction rate was controlled by gaseous diffusion mechanism while the combination of gaseous diffusion and chemical reaction mechanism controlled the final stages. Grain growth and coalescence of the formed Fe_0·64Ni_0·36 grains took place by increasing the reduction temperature.