Reduction of iron oxides by reaction with sodium borohydride / Redução de óxidos de ferro por reação com borohidreto de sódio




xviii The chemical reduction of transition metal ions in solution by action of the borohydride group ion has been applied by several researchers attempting to synthesize amorphous Fe-B alloys. In this project iron-oxide particles suspended in water, rather than ions in solution, were used for that purpose. The reducing chemical was chosen to be sodium borohydride (NaBH4) and as such a novel route for the production of amorphous Fe-B alloys and metallic iron has eventually emerged. The iron oxides considered in this work were natural and synthetic hematite (a-Fe2O3) and synthetic goethite (a-FeOOH). The principle technique relied on to characterize both the reactants and the produced composites was Mössbauer spectroscopy, but various other complementary analytical tools, some which being state-of-art (e.g., electron diffraction and ILEEMS), were used in addition. This combination of relevant material-research approaches has made it possible to acquire a comprehensive qualitative and quantitative picture of the obtained products. The chemical reduction of an iron-ore concentrate consisting of almost pure specularite has formed the first (and chronologically initial) part of this work. For this exploratory study, only one a-Fe2O3/NaBH4 precursor mixture was considered. Subsequently, the addition of different amounts of sodium borohydride to a given amount of hematite or goethite powder was investigated. Three intimate mixtures having weight proportions for iron oxide and NaBH4 of 1:2, 1:8 and 1:16, respectively, were taken into account. From this latter part of the experimental study (i.e., concerning the effects of different concentrations of the reducer agent in the precursor material), it has become clear that the boron content in the produced amorphous Fe-B increases as the initial amount of borohydride increases. The boron content in the samples is further observed to imply a marked effect upon the crystallization and subsequent oxidation processes of the amorphous phase. Finally, ILEEMS has indicated that the reducing reactions preferentially occur at the surface of the oxide grains. Hence, the larger the surface area, the higher the extent of reduction, as observed. On the other hand, even though the synthetic goethite had particle size similar to that of the synthetic hematite, it reacted much less with NaBH4 than the hematite. In summary, the results of this research have showed that the production of amorphous Fe-B alloys, starting from Fe-oxide particles in suspension, is possible, and that for high borohydride concentrations in the precursor a-Fe2O3/NaBH4 mixtures it is possible to obtain in addition a-Fe particles. However, a total reduction of the iron oxide could not be achieved under applied conditions.


redução (química) Óxido de ferro análise espectral. mossbauer espectrometria de engenharia quimica materiais mossbauer efeito

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