Crystallographic Control in Ilmenite Reduction
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UCTION
REDUCTION of ilmenite with hydrogen is of current interest, both as the basis of environmentally benign metallurgical processes for producing high-titania feedstocks that can be used in titania pigment production and as a possible means of supplying water or oxygen at future lunar bases.[1] A recent patent[2] describes an ilmenite upgrading route involving hydrogen reduction of ilmenite under pressure to give titanium dioxide and metallic iron, according to Reaction [1], followed by leaching steps to remove the iron and other impurities. FeTiO3ðsolidÞ þ H2ðgasÞ ! Feðsolid metalÞ þ TiO2ðsolidÞ þ H2 OðvaporÞ
½1
The absence of published information on the influence of pressure on the hydrogen reduction of ilmenite provided the impetus for our recently published study[3] on the kinetics of hydrogen reduction of sintered polygranular synthetic ilmenite discs as a function of both temperature and pressure. The reduction kinetics were monitored by measuring the mass loss of the discs as a function of time in a specially constructed pressurized thermogravimetric microbalance. By fitting the
M.L. de VRIES, Project Scientist, and I.E. GREY, Research Scientist, are with the CSIRO Minerals, Clayton South VIC 3169, Australia. Contact e-mail: [email protected] J.D. FITZ GERALD, Technical Specialist, is with the Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia. Manuscript submitted May 29, 2006. Article published online April 25, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
kinetic curves at different temperatures and pressures using a shrinking core reaction model, a broad delineation of the relative contributions of gas mass transport and interfacial chemical reaction mechanisms was achieved.[3] However, as noted by previous researchers,[4,5,6] the experimental measurement of only one macroscopic parameter (the mass loss in this case) severely limits the interpretation of kinetic modeling data when a number of different transport resistances are contributing. In particular, we were not able to extract information on the possible contribution of solid-state diffusive mechanisms from the kinetic study. In order to help overcome these limitations, we applied electron microscopy techniques to the analysis of the morphologies of the reduced products at different stages of reduction. There have been few previously published microscopy studies of ilmenite reduction. Jones[7,8] combined optical microscopy and electron microprobe analyses of reduced natural grains of ilmenite to show how Mn and Mg impurities in the natural grains influenced the morphology of the reduced samples. The investigation by Briggs and Sacco[9] of hydrogen reduction of synthetic ilmenite discs is particularly relevant to our study, because similar sample synthesis procedures were used. Their scanning electron microscope (SEM) images of partially reduced discs showed that, although macroscopically, the reduction proceeded in a topochemical or shrinking-core manner, the metallic iron formed in distincti
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