Effect of Titanium Dioxide on Aggregation of Reduced Metallic Iron in Molten Slag

Abstract

The reduction process of lunar regolith simulant was designed to obtain metallic iron (Fe) and utilize lunar resources, focusing mainly on the effect of titanium dioxide (TiO 2 ) on the recovery of metallic Fe from molten slag. Thermodynamic calculations were conducted to predict the equilibrium phases existing at high temperature upon heating the regolith simulant to 1600°C under an oxygen partial pressure of 10 ¹15 atm, in which the metallic Fe coexisted with the molten slag. Based on these predictions, two steps were applied in the experimental procedure: a reduction step followed by a melting step. The results showed that metallic Fe droplets were obtained by aggregating fine metallic Fe particles in the melting step. The viscosity of molten slag was found to decrease when TiO 2 was added to the regolith simulant, but the recovery ratio® defined as the weight ratio of the metallic Fe obtained from the aggregation of fine metallic Fe particles to the total amount of Fe in the regolith simulant®also decreased. The wettability between liquid Fe and molten slag improved upon increasing the TiO 2 content in the latter. These results indicate that wettability exerts a substantially more dominant effect than the viscosity of molten slag does in the recovery of metallic Fe from regolith simulant.

Evaluation of Wettability at Interface between Molten Slag and Liquid Fe on Recovery of Metallic Fe from Lunar Regolith

“…TiO 2 is known to lower the viscosity of molten silicate-based slag 16) , which is generally advantageous for metal recovery. However, our previous study 15) showed that recovery of the Fe alloy decreased with addition of TiO 2 in the slag, even though the viscosity of the molten slag decreased. We concluded that when the molten slag wetted the liquid Fe, a repulsive capillary force acted on the contact points between Fe particles in the slag 15,17) , making it difficult for the fine liquid Fe particles to coalesce.…”

“…The lunar regolith simulant was the same as that used in our previous study 15) . The regolith simulant was crushed to less than 125 µm, dried at 473 K for 24 h to remove moisture, and then uniformly mixed with various oxides: ilmenite, albite, and Na 2 O•xSiO 2 compounds.…”

“…The regolith simulant was crushed to less than 125 µm, dried at 473 K for 24 h to remove moisture, and then uniformly mixed with various oxides: ilmenite, albite, and Na 2 O•xSiO 2 compounds. Figure 1 shows the experimental process designed from thermodynamic prediction by FactSage in our previous study 15) . The experimental process was divided into two steps, as shown in Fig.…”

“…We applied a pyrometallurgical process to utilize lunar resources based on the similarities of slag, volcanic rock, and lunar regolith. In our previous study 15) , we successfully extracted metallic iron (Fe) from regolith simulant at low oxygen partial pressure using H 2 . We observed that liquid Fe particles reduced from the regolith simulant were dispersed in the molten slag, and a bulk Fe alloy was recovered after coalescence of these particles.…”

“…In this follow-up study, we investigated the effect of the wettability between molten slag and liquid Fe on the coalescence of Fe particles by addition of various oxides when recovering metallic Fe from lunar regolith simulant. In particular, as a part of the realization of lunar metallurgy, we focused on the change in Fe recovery on addition of ilmenite, an abundant mineral of the mare region, and albite (NaAlSi 3 O 8 ), a common mineral of the highland region on the Moon 1,15) .…”