Hydrogen reduction mechanisms of ilmenite between 823 and 1353 K
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In situ gravimetric measurements and microscopic examinations were used to determine the mechanisms of oxygen removal from synthetic ilmenite disks between 823 and 1353 K. Under a hydrogen atmosphere, iron was observed to form a layer of low porosity on the surface of samples early in the reduction. This created diffusion limitations for hydrogen to the reaction front and for the escape of water vapor. A shrinking core reduction model, modified to include the growth of this iron film, was capable of predicting the conversion-time relationships of ilmenite samples. An activation energy of 43.2 ± 2.6 kcal/gmole was determined to be representative of reaction control over the temperature range 823-1023 K.
I. INTRODUCTION
The National Aeronautics and Space Administration (NASA), in response to a directive put forth by the President of the United States and the National Space Council, recently proposed the construction of a lunar base during the first decade of the next century.1 A possible activity at the future base could be the production of oxygen from local sources of ilmenite (FeTiO3), a mineral found in abundance (up to 20%) in the lunar maria. A method currently under examination for producing oxygen from ilmenite reduces the mineral in the solid state with hydrogen. The general reactions involved in producing oxygen from ilmenite in the proposed process are shown in Eqs. (l)-(3): FeTiO3 + H 2 -* Fe + TiO2 + H2O H2O -» H 2
(electrolysis)
(1) (2)
FeTiO3 Fe + TiO2 + \O2 (3) Understanding the hydrogen reduction mechanisms of ilmenite [Eq. (1)] is important for the design of a process utilizing this mineral for lunar oxygen production. II. PREVIOUS STUDIES ON IRON-TITANIUM OXIDE REDUCTION
Ilmenite falls within a group of minerals referred to as iron-titanium oxides. Iron-titanium oxides have been processed by the iron industry for many years. Naturally occurring iron-titanium oxides are mixtures of a variety of solid phases including ilmenite, pseudobrookite (Fe2TiO5), rutile (TiO2), magnetite (Fe3O4), and hematite (Fe2O3).2 Studies on the rates and mechanisms of reduction of these materials have been carried out using solid carbon,3"6 carbon monoxide,7"13 and hydrogen.514"22 The reduction mechanisms appear to be influenced by a number of factors including the quantity 574 http://journals.cambridge.org
J. Mater. Res., Vol. 6, No. 3, Mar 1991 Downloaded: 13 Mar 2015
of ferric iron,22 24 the presence of foreign cations,13'25 and the Fe:Ti ratio present in the particular ore under examination.911 Table I lists the ferric iron content in experimental samples from some previous investigations. Also listed in Table I are impurities present in these samples and the experimental techniques employed to contact the gas and the solid phases. From previous work, an investigation of stoichiometric ilmenite with no foreign cations present is necessary to determine the reduction mechanisms of ilmenite. It is important to limit or eliminate ferric iron from samples, as the oxidation state of iron has been shown to alter the
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