Carbothermal reduction of ilmenite and fayalite

Abstract

In order to eliminate thermodynamic limitations during H₂ and CO reduction processes, a novel carbothermal reduction process is proposed to generate lunar oxygen for propellant and life support on the lunar surface. The kinetics and mechanism of the carbothermal reduction of synthetic ilmenite and fayalite (simulants for lunar ilmenite and fayalite) were investigated in the present study. Carbothermal reduction of ilmenite with charcoal powder was studied between 975°C and 1100°C. It was found that the reduction process is controlled by the carbon gasification reaction instead of by the rate of ilmenite reduction with carbon monoxide, which has been claimed to be the rate limiting step by several prior researchers. The activation energy obtained using a simplified carbon gasification model for this reduction is 27.2 kcal/mole. The reduction products were studied by SEM and XRD and it was found that the major products are α-Fe and TiO₂ at temperatures below 1000°C; at 1050°C, α-Fe and Ti₉O₁₇ were observed; at 1100°C, α-Fe and Ti₄O₇ were observed. Iron is completely segregated from the titanium oxides in the product. Carbothermal reduction of ilmenite with deposited carbon was investigated between 775°C and 1000°C. An extremely fast reduction rate (more than ten times faster than charcoal powder reduction) was observed. The reduction rate-limiting step is believed to be the ilmenite reduction with carbon monoxide. The activation energy calculated by a simplified model is 50 kcal/mole between 775°C and 900°C, and 17.6 kcal/mole above 900°C. It was also found that TiO₂ can be reduced to much lower oxygen content titanium oxides than during powdered charcoal reduction. The temperature and particle size effects during carbothermal reduction of synthetic fayalite were investigated. The product morphology of this reduction showed that α-Fe and α-cristobalite are the main products at temperatures above 1100°C, at lower temperatures, α-Fe, α-quartz and amorphous silica are the main products. The iron produced by reduction is segregated from the SiO₂ phases and agglomerates in large particles, which is different from the product morphology observed during hydrogen reduction of fayalite. In order to better understand the mechanism and kinetics of the carbothermal reduction process, a mathematical model was developed to simulated the CO₂/CO ratio, CO and CO₂ partial pressure distributions, conversion, etc. during the reduction process. Using the model to treat the reduction of ilmenite with charcoal powder reproduces experimental results very well.