Crystallographic control in ilmenite reduction

Scanning electron microscopy and transmission electron microscopy techniques were used to characterize the products from hydrogen reduction of polygranular synthetic ilmenite discs at temperatures in the range 823 to 1173 K and pressures in the range 1 to 13 atm. Reduction commences at grain boundaries and cracks and advances progressively to grain interiors. Within individual grains, the morphology of the reduction products was found to be crystallographically controlled. Near parallel bands of metallic iron (Fe _m ) form within each grain, aligned with the basal plane of ilmenite (il) (0001) _il . The separation between bands is of the order of 1 μm and is relatively constant with change of pressure and temperature. In the interband region, conversion of ilmenite to rutile occurs preferentially parallel to {112̄0}_il ilmenite planes, generating platelets of rutile that grow normal to the Fe _m bands. The intergrain duplex morphology of the reduction products closely resembles cellular precipitation in alloys. At reduction temperatures above ∼1000 K, the interband region comprises dense, nonporous oriented intergrowths of rutile platelets and residual ilmenite, whereas below ∼900 K, the interband region contains a fine, filamentary network of pores. In the intermediate temperature regime, a change from dense to porous interband region occurs with increasing pressure. The observations have been interpreted in terms of the relative rates of interfacial chemical reaction and solid-state diffusion, with the latter having a controlling influence at lower temperatures or higher pressures.