Photomicrographic investigation of the reduction of ilmenite
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In an investigation of natural weathered ilmenite, optical microscopy and x-ray spectral analysis jointly give a qualitative picture of the changes which have occurred. A microprobe can only partly be used to get detailed quantitative determinations, because it cannot be used to determine the finely intermingled weathered products.
Reduction of natural ilmenite by carbon monoxide is a topochemical process. During reduction below 1000°C, manganese or magnesium diffuses ahead of the approaching reaction front and forms a narrow enriched zone in which the Fe2+ in the ilmenite lattice is replaced by Mn2+ or Mg2+, while Ti4+ is replaced by Ti3+ to a lesser extent. In some particles the dissolved concentrations in the enriched zone reach levels high enough to prevent reduction of Fe2+, and thus isolate large regions of the particle from reduction. This “boundary effect” is less marked below 1000°C.
Oxidation converts monocrystalline particles of ilmenite to polycrystalline ones of pseudobrookite with disseminated particles of rutile. During gas reduction of preoxidized material at 1000°C, the fragments of the particles are reduced topochemically, each in its own zone of liquid enrichment, and metallic iron is concentrated at the boundaries of the fragments. The total amount of ilmenite isolated under the influence of the liquid enrichment zone is reduced, and the sintering tendency of the particles is minimized.
Gas reduction of preoxidized ilmenite at 1200°C leads to the formation of metallic iron and a solid solution of anosovite (Fe3−xTixO5 containing dissolved Mn2+).
Reduction of preoxidized ilmenite by carbon at 1200°C creates more favorable conditions for reduction than those in a gaseous mixture of CO and CO2.
KeywordsManganese Carbon Monoxide Optical Microscopy Rutile Gaseous Mixture
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