Kinetics and mechanism of the lunar ilmenite reduction.

Abstract

The high temperature reduction of ilmenite by various reducing agents is investigated. The experiments were performed in a microgravimetric reactor system. The starting materials and products were characterized using X-ray diffraction (XRD) with a Cu Kα source, high-resolution scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) and wavelength-dispersive X-ray (WDX) analyses, mercury porosimetry and Mossbauer spectrometry. The fundamentals of ilmenite reduction by hydrogen and carbon monoxide are compared and analyzed both experimentally and through theoretical modeling. Both processes involve the diffusion of Fe product away from the reaction front through the TiO₂ phase, followed by the nucleation and growth of a separate Fe phase. The main differences between CO and H₂ reduction of ilmenite are that the rate of H₂ reduction of ilmenite is much faster than that by CO and TiO₂ can be further reduced to lower oxides of titanium at temperature higher than 876°C. A technique for the synthesis of lunar ilmenite simulants has been developed. The simulant sample have been successfully prepared with desired physical and chemical properties. The significance of this work is that it is possible to generate the kinetics data even without having access to real lunar ilmenite which is precious and difficult to obtain at the present time. The results of ilmenite reduction by a mixture of H₂ and CO indicate that the mechanism is similar to the reduction of ilmenite by H₂ or CO alone. However, the overall process is more complex because of the reaction between H₂ and CO₂ generated from the CO reduction of ilmenite. MgTiO₃, an important impurity in lunar ilmenite, has some effect on the reduction kinetics and mechanism. This depends on the reduction temperature and conversion. In samples containing MgTiO₃ impurity, in addition to the expected Fe and TiO₂ product phases, an unreduced core enriched in MgTiO₃ is observed. The movement of MgTiO₃ towards the core is a significant factor affecting the reduction rate.