Iron Recovery from Ilmenite via Mechanochemical Treatment, Acid Leaching, and Electrodeposition for Lunar Resource Utilization

Abstract

The recent expansion of space development has increased the interest in establishing sustainable lunar bases. Because transporting materials from Earth is extremely costly, the utilization of lunar resources is essential for future lunar exploration. Lunar regolith contains abundant ilmenite (FeTiO₃), which is a potential source of iron; however, conventional terrestrial smelting methods are unsuitable for the lunar environment. In this study, a novel process for iron recovery from ilmenite-containing lunar regolith was developed by using mechanochemical treatment, acid leaching, and electrodeposition. Mechanochemical treatment with sodium hydroxide converted ilmenite into soluble species, followed by selective iron extraction in a hydrochloric acid solution and subsequent iron recovery by electrodeposition. The proposed process demonstrates a potential pathway for producing iron from lunar resources under lunar-compatible conditions. This study contributes to the development of in situ resource utilization technologies for future lunar base construction and sustainable space exploration.

Introduction

In recent years, space development has intensified worldwide, and concrete plans have been made to establish a lunar base. The Artemis program, led by NASA in collaboration with international partners, including JAXA (Japan), CSA (Canada), ASA (Australia), and ESA (European Union), aims to establish a sustained human presence on the Moon from 2030. A critical challenge for lunar infrastructure is the supply of construction material. Transporting materials from Earth remains extremely costly, with estimated costs of approximately 1.2 million USD per kg. Consequently, the development of construction materials based on in situ resource utilization (ISRU), that is, leveraging locally available lunar resources, has emerged as a research hotspot. , Accordingly, a few studies have investigated the production of construction materials from lunar regolith and its constituent minerals. −

Iron is a critical structural material in modern civil engineering because of its strength, durability, and versatility. On Earth, iron is produced from iron ores using carbonaceous reductants (coke) and fluxes, such as limestone. However, these ores are not available on the Moon, and alternative iron resources and processing routes suitable for the lunar environment must be identified. Ilmenite (FeTiO₃), which is widely distributed in lunar regolith at concentrations of approximately 10%, has attracted attention as a promising lunar resource for iron extraction. − Ilmenite is typically concentrated by magnetic or electrostatic separation, which selectively enriches iron-bearing minerals while minimizing reagent consumption and reducing the mass of material processed. − Although the separation efficiency is strongly influenced by particle size, mineral liberation, and the complex mineralogy of lunar regolith, extensive research has been conducted to address these challenges through optimization of beneficiation processes, with the ultimate goal of improving the efficiency and selectivity of ilmenite concentration.

The development of iron extraction processes applicable to ilmenite concentrates obtained through such beneficiation routes is important for future lunar ISRU. Conventional ilmenite processing methods used on Earth, such as direct reduction and sulfuric acid leaching, rely on high-temperature treatments and large quantities of chemical reagents. − These approaches are difficult to implement on the Moon owing to the constraints associated with energy supply, infrastructure, and material transport. For iron extraction from concentrated ilmenite or lunar regolith, electrochemical technologies, including molten salt electrolysis and molten oxide electrolysis, have attracted considerable attention. − Molten salt electrolysis enables simultaneous oxygen and metal production, whereas molten oxide electrolysis directly reduces metal oxides without carbon reductants. These processes are attractive because they are compatible with solar electricity and closed-loop ISRU concepts. Nevertheless, these approaches generally require operating temperatures above 900–1600 °C, corrosion-resistant reactor materials, large electrical power supplies, and sophisticated thermal management systems. Despite significant progress in electrochemical ISRU technologies, simpler pretreatment processes that operate under mild conditions and require only a limited chemical inventory remain largely unexplored. From an ISRU perspective, minimizing the number and complexity of reagents imported from Earth is essential. Therefore, pretreatment strategies based on a simple alkali–acid system are particularly attractive, as these reagents can potentially be regenerated electrochemically and reused in a closed-loop process. Such an approach could complement existing beneficiation technologies while simplifying downstream metal recovery, making it a promising route toward practical lunar resource utilization.

Considering these aspects, the present study aimed to investigate a processing route that could operate under the simplified chemical constraints expected for lunar ISRU, rather than to reduce energy consumption or optimize process efficiency relative to terrestrial technologies. Mechanochemical treatment was selected because it enables chemical reactions to proceed through mechanical energy without bulk high-temperature processing. Mechanochemical reactions are driven by repeated collisions and friction between milling media and particles, generating fresh reactive surfaces, lattice defects, and structural disorder that enhance solid-state diffusion and reduce the activation energy for chemical reactions. − Consequently, milling dynamics, including collision frequency and impact energy, strongly influence the extent of phase transformation and reaction progress. Therefore, optimization of the milling conditions is essential for promoting the formation of reactive intermediate phases that can subsequently enhance the dissolution behavior of ilmenite. Sodium hydroxide was employed as a representative alkali to convert ilmenite into alkali-derived iron compounds during mechanochemical treatment, while hydrochloric acid was used to dissolve these products and produce an electrolyte suitable for iron electrodeposition. − Sodium hydroxide and hydrochloric acid were selected as representative alkali and acid reagents because they are simple, well-established chemicals that can potentially be regenerated electrochemically within a closed-loop ISRU system. These characteristics make the alkali–acid system a practical model for evaluating the feasibility of the proposed concept under simplified ISRU conditions. The proposed process differs from conventional lunar electrochemical processing technologies, such as molten salt and molten oxide electrolysis, which generally require extremely high temperatures and complex reactor systems despite their ability to simultaneously recover oxygen and metals from lunar regolith. In contrast, the present approach combines mechanochemical alkali treatment, acid dissolution, and electrochemical recovery under comparatively mild conditions. This electrically driven process has the potential to utilize solar-generated power, support closed-loop reagent recycling, and reduce reactor complexity, making it attractive for ISRU applications.

Although this study focused on the chemical feasibility of the proposed process, future work should evaluate its integration with magnetic or electrostatic beneficiation using realistic lunar regolith simulants to improve the iron recovery and reduce impurity-related effects. Future studies should therefore evaluate the proposed process using lunar regolith simulants with realistic mineral compositions and investigate its integration with beneficiation processes for practical ISRU applications.

Accordingly, this study investigated whether mechanochemical alkali treatment followed by hydrochloric acid dissolution could convert iron in ilmenite to soluble species suitable for electrodeposition. The proposed approach is intended to be an alternative iron recovery route that may complement existing lunar ISRU technologies. Representative lunar iron recovery technologies are summarized and compared in Table to place the present work in the context of current ISRU research.

1. Comparison of Representative Lunar Iron Recovery Technologies and the Proposed Mechanochemical Process for Lunar ISRU.

technology	principle	operation temperature	advantages	limitations	applicability to lunar ISRU
magnetic separation	physical beneficiation	ambient	simple, no reagents	limited enrichment	high
electrostatic separation	physical beneficiation	ambient	low reagent consumption	sensitive to particle properties	high
molten salt electrolysis	electrochemical reduction	∼900–950 °C	Fe and O2 production	high temperature, corrosion	medium–high
molten oxide electrolysis	direct oxide electrolysis	>1600 °C	direct reduction	extremely high temperature	medium
this study	mechanochemical activation + hydrometallurgy + electrodeposition	near ambient (pretreatment)	simple reagent system, mild pretreatment	reagent recycling required	potentially high

Methods

Mechanochemical Treatment

Ilmenite (0.8 g, Sigma-Aldrich, U.K.), sieved under Φ100 μm using a 150 mesh sieve, and sodium hydroxide (NaOH, 0.1–0.8 g, Wako, Japan) were placed in a zirconia container (80 mL) with zirconia grinding balls (10 mm diameter, 25 pieces) such that the ilmenite, sodium hydroxide, and grinding balls occupied one-third of the container. The mechanochemical treatment was performed at a rotational speed of 200–600 rpm and a treatment time of 0–180 min. After treatment, the grinding balls were placed in a strainer, the sample powder was sieved using a 150 mesh sieve, and the powder sample was collected from the container using a spatula.

Following mechanochemical treatment with sodium hydroxide, 0.2 g of the treated ilmenite sample was added to a 1.0 mol/L hydrochloric acid solution (20 mL) in a 50 mL centrifuge tube, and the tube was shaken for 2 h. The suspension was centrifuged, and the supernatant was filtered. The pH of the supernatant was measured using a pH meter (LAQUA F-72, HORIBA, Japan), and the concentrations of Fe and Ti were determined using atomic absorption spectrometry (Analyst 200, PerkinElmer). The dissolved amounts were calculated using the following eq

$$E=\frac{C·V}{M·m}$$ 1

where E is the amount of metal dissolved (mmol/g), C is the metal concentration (mg/L), V is the solution volume (L), M is the atomic weight (mg/mmol), and m is the sample weight (g). The mineral phases of the prepared samples and residues were determined using powder X-ray diffraction (XRD; D8 ADVANCE, Bruker AXS).

For comparison, two additional samples were prepared: a mechanochemically treated sample of ilmenite (0.8 g) alone and a sample of ilmenite (0.8 g) and sodium hydroxide (0.2 g) thoroughly mixed with a mortar.

Acid Leaching and Electrodeposition

The dissolution of metals from mechanochemically treated ilmenite in hydrochloric acid was examined during the acid leaching stage. For the treated samples, a mixture of ilmenite (0.8 g) and sodium hydroxide (0.2 g) was processed in a planetary ball mill at 600 rpm for 120 min.

Samples (0.1–1.0 g) were added to a 0–1.0 mol/L hydrochloric acid solution (10 mL) in a 50 mL centrifuge tube and shaken for 2 h. The suspensions were centrifuged, and the supernatant was filtered using a 0.2 μm pore membrane filter. The pH of the filtrate was measured, and the concentrations of Fe and Ti in the filtrate were determined using an atomic absorption spectrometer. The dissolved amounts of Fe and Ti were calculated using eq . The mineral phases of the prepared samples and the residues after acid leaching were determined using XRD.

For electrodeposition, the leachate obtained by acid leaching of mechanochemically treated ilmenite (0.8 g) with sodium hydroxide (0.2 g) in a 1 mol/L hydrochloric acid solution (10 mL) for 0.5 h was used. The leachate had a pH of 0.3 and Fe and Ti concentrations of 136 and 140 mg/L, respectively. The leachate (2 mL) with or without the addition of oxalic acid dihydrate ((COOH)₂·2H₂O) (0.2 g) was poured into an electrochemical cell (10 mm × 20 mm × 20 mm). Two carbon electrodes (Φ2 mm × 25 mm) were positioned at intervals of 10 mm and immersed to a depth of 10 mm. A direct current of 5 V was applied to the carbon electrode, and the electrode was observed using a stereomicroscope (SZM223T, AS ONE, Japan) and scanning electron microscopy (SEM; JSM-6510A, JEOL, Japan) equipped with an energy-dispersive X-ray spectroscopy detector (EDS; JED-2300, JEOL).

Results and Discussion

Table summarizes the results of control experiments conducted to clarify the individual effects of mechanochemical treatment and sodium hydroxide addition on the dissolution behavior of ilmenite. The dissolution amounts of Fe and Ti were evaluated for raw ilmenite, a simple mixture of ilmenite and sodium hydroxide, and mechanochemically treated ilmenite without the addition of sodium hydroxide. The rotation speed was 200 rpm, and the processing time was 60 min. The dissolution amounts of raw ilmenite were 0.080 mmol/g for iron and 0.081 mmol/g for Ti. For the mixture, the values were 0.081 mmol/g for Fe and 0.084 mmol/g for Ti. The corresponding values for the mechanochemically treated sample without sodium hydroxide addition were 0.089 and 0.088 mmol/g for iron and titanium, respectively. These results indicate that neither mechanochemical treatment alone nor simple physical mixing with sodium hydroxide significantly enhances the dissolution of ilmenite. Therefore, the enhanced dissolution observed in this study originates from the mechanochemical reaction between ilmenite and sodium hydroxide rather than from particle size reduction or simple mixing.

2. Dissolution Amounts of Fe and Ti from Raw Ilmenite, a Mixture of Ilmenite and Sodium Hydroxide, and a Mechanochemically Treated Sample Without Sodium Hydroxide Addition.

solubility (mmol/g)	raw ilmenite	mixture	mechanochemically treated ilmenite
Fe	0.080	0.081	0.089
Ti	0.081	0.084	0.088

To clarify the mechanism responsible for the enhanced dissolution of ilmenite, the effects of NaOH addition, milling intensity (rotation speed), and processing time were systematically investigated. The dissolved amounts of Fe and Ti obtained by atomic absorption spectroscopy were interpreted together with the phase evolution identified by XRD to determine how mechanochemical reactions promoted the formation of acid-soluble phases.

The first objective was to determine the optimum NaOH addition for promoting the mechanochemical conversion of ilmenite into acid-soluble compounds. The metal dissolution amounts of the samples treated mechanochemically with different amounts of NaOH are shown in Figure . The rotation speed was 200 rpm, and the processing time was 60 min. As shown in Figure , the dissolved amounts of Fe and Ti increased markedly with NaOH addition up to 0.2 g but decreased when excess NaOH was added. This behavior indicates that an appropriate NaOH content is required to maximize the mechanochemical reaction rather than simply increasing the alkali dose. The maximum dissolved amounts of Fe and Ti were 1.56 mmol/g (23.7%) and 1.63 mmol/g (24.8%), respectively.

1.

Metal dissolution amounts of samples mechanochemically treated with different amounts of sodium hydroxide.

Figure shows the XRD patterns of the mechanochemically treated samples with different amounts of sodium hydroxide and the residues after dissolution in hydrochloric acid. Notably, the sample with 0.8 g of sodium hydroxide could not be analyzed by XRD because of the large amount of unreacted NaOH. Moreover, the sample exhibited deliquescence, which inhibited mechanochemical processing. A diffraction peak related to ilmenite was observed in all samples after mechanochemical treatment. A peak ascribed to the sodium salt of ilmenite was observed in the sample with 0.2 g of sodium hydroxide.

2.

XRD patterns of (a) mechanochemically treated samples with different amounts of sodium hydroxide and (b) residue after dissolution in hydrochloric acid.

The following reaction is considered to have occurred during the mechanochemical treatment

$${\mathrm{F}\mathrm{e}\mathrm{T}\mathrm{i}\mathrm{O}}_3+2\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}\to {\mathrm{N}\mathrm{a}}_2{\mathrm{F}\mathrm{e}\mathrm{T}\mathrm{i}\mathrm{O}}_4+{\mathrm{H}}_2\mathrm{O}$$ 2

After dissolution in hydrochloric acid, only the ilmenite peak was observed, indicating that Na₂FeTiO₄ was completely dissolved in hydrochloric acid as

$${\mathrm{N}\mathrm{a}}_2{\mathrm{F}\mathrm{e}\mathrm{T}\mathrm{i}\mathrm{O}}_4+6\mathrm{H}\mathrm{C}\mathrm{l}\to {\mathrm{F}\mathrm{e}\mathrm{C}\mathrm{l}}_2+{\mathrm{T}\mathrm{i}\mathrm{O}\mathrm{C}\mathrm{l}}_2+2\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}+{3\mathrm{H}}_2\mathrm{O}$$ 3

The optimum NaOH addition (0.2 g) appears to reflect a balance between sufficient NaOH availability for sodium salt formation and effective mechanical mixing. At higher NaOH additions, excessive NaOH adhered to the zirconia ball and powder surfaces, reducing collision efficiency and suppressing mechanochemical reactions. Therefore, the optimum condition was governed not only by the stoichiometric molar ratio but also by milling dynamics and mixing behavior, depending on the condition of the equipment. The XRD results explain the dissolution behavior observed in Figure . Formation of Na₂FeTiO₄ was detected only under appropriate NaOH addition, whereas excess NaOH remained unreacted because of an insufficient milling efficiency. After acid leaching, the disappearance of Na₂FeTiO₄ indicates that this mechanochemically generated phase was selectively dissolved in hydrochloric acid. Therefore, the increased Fe and Ti dissolution shown in Figure can be attributed to the formation of acid-soluble sodium titanoferrite during mechanochemical treatment.

Previous studies on alkali roasting of ilmenite generally require temperatures above 700–900 °C to produce sodium titanates. − In contrast, the present study demonstrates that similar sodium-containing reaction products can be generated solely by mechanochemical activation without external heating, indicating the potential of mechanochemistry as an alternative pretreatment route.

After the optimum NaOH addition was established, the influence of milling energy was investigated by varying the rotational speed. The amount of metal dissolved in the mechanochemically treated samples at different rotational speeds is shown in Figure . The amount of sodium hydroxide added was 0.2 g, and the processing time was 60 min. A higher rotation speed corresponds to an increased amount of dissolved metal. Specifically, at 600 rpm, the dissolved amounts of iron and titanium were 2.09 and 2.06 mmol/g, respectively.

3.

Amounts of dissolved metals in the mechanochemically treated samples at different rotation speeds.

The XRD patterns of the mechanochemically treated samples at different rotation speeds and the corresponding residues after dissolution in a hydrochloric acid solution are shown in Figure . The peak intensity of the sodium salt in the mechanochemically treated sample increased with an increase in rotation speed. After dissolution in hydrochloric acid, only the ilmenite peak was observed in the XRD pattern. These results indicate that increasing the rotation speed accelerated the reaction between ilmenite and sodium hydroxide, thereby increasing the amount of the soluble sodium salt. These XRD results explain why the dissolved amounts shown in Figure increased with rotational speed; stronger milling promoted the mechanochemical conversion of ilmenite into acid-soluble sodium phases.

4.

XRD patterns of (a) mechanochemically treated samples at different rotation speeds and (b) residues after dissolution in hydrochloric acid solution.

The influence of the milling time was subsequently examined to determine when the mechanochemical reaction reached completion. The amounts of metals dissolved from the samples subjected to mechanochemical treatment for different durations are shown in Figure . The amount of sodium hydroxide added was 0.2 g, and the rotation speed was set at 600 rpm. The amount of dissolved metals increased with increasing processing time and stabilized after 120 min. After 120 min of processing, the dissolution amounts were 2.34 mmol/g for iron and 2.30 mmol/g for Ti.

5.

Metal dissolution amounts of samples subjected to mechanochemical treatment for different durations.

Figure shows the XRD patterns of the mechanochemically treated samples and the residues after dissolution. The sodium salt of ilmenite was formed after 30 min of treatment, while sodium salts of iron and titanium were formed at 90 min, potentially by the following reactions

$${\mathrm{N}\mathrm{a}}_2{\mathrm{F}\mathrm{e}\mathrm{T}\mathrm{i}\mathrm{O}}_4+4\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}\to {\mathrm{N}\mathrm{a}}_4{\mathrm{F}\mathrm{e}\mathrm{O}}_3+{\mathrm{N}\mathrm{a}}_2{\mathrm{T}\mathrm{i}\mathrm{O}}_3+{2\mathrm{H}}_2\mathrm{O}$$ 4

6.

XRD patterns of (a) mechanochemically treated samples at different times and (b) residues after dissolution.

After dissolution, only the peaks of ilmenite were observed because the sodium salts mostly dissolved into hydrochloric acid. The plateau in dissolution after 120 min (Figure ) corresponds well to the saturation of sodium phase formation observed by XRD (Figure ), indicating that the mechanochemical reaction had nearly reached completion. The observed saturation after 120 min suggests that the mechanochemical activation approaches a reaction limit under the present milling conditions. A similar saturation behavior has been reported for other mechanochemical oxide activation processes.

After identifying the optimum mechanochemical treatment conditions, the dissolution step was investigated to determine the conditions for extracting iron while maintaining solution conditions suitable for electrodeposition. The amounts of metals dissolved when the samples were added to a 1 mol/L hydrochloric acid solution are shown in Figure . The pH of the leachate remained strongly acidic (0.4–0.6 pH units). Iron and titanium exhibited maximum dissolution at an addition rate of 20 g/L, with dissolved amounts of 1.21 and 1.23 mmol/g, respectively. In other words, 19.1% of iron and 19.4% of titanium present in ilmenite were dissolved in hydrochloric acid solution. The dissolved amounts decreased at addition rates above 50 g/L.

7.

Metal dissolution amounts as a function of the sample amount added to 1 mol/L hydrochloric acid solution.

Figure amounts of dissolved metals and the pH values of the leachates at various hydrochloric acid concentrations for a pulp density of 20 g/L. As the HCl concentration increased beyond 0.1 mol/L, the amount of dissolved metals increased. The pH decreased sharply between 0 and 0.1 mol/L HCl, indicating that the enhanced dissolution at higher acid concentrations can be attributed, at least partly, to the lower solution pH, which promotes the dissolution of the alkali-activated iron-bearing phases.

8.

Amounts of dissolved metals and pH as functions of hydrochloric acid concentrations.

Since the results in Figure suggested that metal dissolution was strongly influenced by solution pH, the dissolution behavior was further examined as a function of pH. Figure summarizes the dissolved amounts of iron and titanium from the treated ilmenite as a function of the leachate pH, while Figure presents the XRD patterns of the residues after dissolution in hydrochloric acid concentrations of 0, 0.2, 0.5, and 1.0 mol/L (corresponding to pH values of 12.24, 1.06, 0.61, and 0.24, respectively) to clarify the associated phase transformations during dissolution. The amount of dissolved metal increased with decreasing pH values. The sodium salts of ilmenite, iron, and titanium did not dissolve under basic and neutral conditions but began to dissolve at pH values less than 2. The sodium salt dissolved in the hydrochloric acid solution at a lower pH, which likely led to an increase in the dissolution amount. The XRD results provide direct evidence that the increase in dissolved Fe and Ti shown in Figures – originates from the dissolution of sodium-containing reaction products under strongly acidic conditions. Unlike conventional hydrochloric acid leaching of ilmenite, which often requires thermal pretreatment or reducing agents, , the present process produced acid-soluble phases through mechanochemical activation alone. This simplified reaction pathway may be advantageous for future ISRU applications.

9.

Dissolved amounts of Fe and Ti from treated ilmenite as a function of leachate pH.

10.

XRD patterns of the residue after sample dissolution at various hydrochloric acid concentrations.

Finally, the feasibility of recovering dissolved iron by electrodeposition was evaluated. Figure shows photographs of the (a) carbon electrode before electrodeposition, (b) cathode, and (c) anode after electrodeposition in leachate without oxalic acid, and (d) cathode and (e) anode after electrodeposition in leachate with oxalic acid. Bubbles formed at both electrodes during electrodeposition. Without oxalic acid, the original electrodes remained largely unchanged (Figure a–c,e), whereas precipitation was observed on the cathode surface when oxalic acid was added (Figure d). Additionally, a yellow iron hydroxide-like precipitate tentatively presumed to contain Fe­(OH)₃ species was observed in the absence of oxalic acid. Based on these observations, the leachate likely contained iron predominantly as Fe³⁺ following mechanochemical treatment with NaOH and acid leaching with HCl. In the absence of oxalic acid, Fe³⁺ reduction is challenging because of its low reduction potential, which causes it to precipitate as Fe­(OH)₃ via hydrolysis (without electrolysis). In contrast, the addition of oxalic acid promotes the reduction of Fe³⁺ to Fe²⁺ through the following reaction, enabling its electrodeposition

$${2\mathrm{F}\mathrm{e}}^{3+}+{{{\mathrm{C}}_2\mathrm{O}}_4}^{2-}\to {2\mathrm{F}\mathrm{e}}^{2+}+{2\mathrm{C}\mathrm{O}}_2$$ 5

11.

Photographs of (a) carbon electrode before electrodeposition; (b) cathode and (c) anode after electrodeposition in leachate without oxalic acid; and (d) cathode and (e) anode after electrodeposition in leachate with oxalic acid.

Figure shows SEM images and EDS spectra of the (a) cathode and (b) anode surfaces after electrodeposition in the leachate with oxalic acid, respectively. The cathode surface was covered with precipitates (Figure a), whereas the anode retained a smooth carbon surface without deposition (Figure b). Representative EDS spectra of the cathode and anode after electrodeposition are also shown in Figure . Because iron enrichment was not readily distinguishable by visual comparison of the spectra, the Fe/C atomic ratios were calculated for a quantitative evaluation. The Fe/C ratio of the cathode (0.0126) was substantially higher than that of the anode (0.0005), indicating the preferential enrichment of Fe-containing species at the cathode. Although EDS analysis does not allow for definitive identification of the chemical state of iron, these results support the deposition of Fe-containing species on the cathode during electrodeposition. These results suggest that iron can be recovered from ilmenite via a mechanochemical treatment with NaOH, acid leaching, and electrochemical deposition. Together, these observations demonstrate that mechanochemical activation, acid dissolution, and electrodeposition can be integrated into a continuous iron recovery process suitable for ISRU-oriented resource utilization.

12.

SEM images and representative EDS spectra obtained from the (a) cathode and (b) anode surfaces after electrodeposition in leachate containing oxalic acid. The corresponding Fe/C ratios calculated from the EDS results were 0.0126 and 0.0005, respectively.

A broader comparison between the proposed process and representative lunar iron recovery technologies is presented in the introduction (Table ); therefore, the following discussion focuses on the mechanochemical reaction mechanism and its implications for iron recovery.

The practical applicability of the proposed process under lunar conditions remains an important issue. Because mechanochemical reactions are governed by collisions and friction between milling balls and particles, the reduced lunar gravity (1/6 g) may alter ball trajectories, collision frequency, and impact energy, thereby affecting the particle activation and reaction efficiency. Consequently, optimization of mill geometry and operating conditions, together with numerical simulations and reduced-gravity experiments, will be necessary for a practical reactor design. Nevertheless, mechanochemical processing remains attractive for lunar ISRU because it avoids large high-temperature furnaces and may enable compact, electrically driven processing systems powered by solar energy.

Another important consideration is the complexity of the actual lunar regolith. While commercially available ilmenite was used as a simplified model material in this study, lunar regolith contains various silicate minerals, oxides, glassy phases, and trace elements that may consume acid, introduce impurity ions, and reduce the efficiency of iron dissolution and electrodeposition. Therefore, future studies should evaluate the proposed process by using realistic lunar regolith simulants.

Although reagent recycling and electrolyte purification were beyond the scope of this study, they will be essential for the practical ISRU implementation. In particular, the potential regeneration of NaOH and HCl within a closed-loop electrochemical system could substantially reduce the amount of consumable reagents transported from Earth.

This study was designed as a proof-of-concept investigation to demonstrate the feasibility of the proposed process. Although iron deposition was confirmed by SEM-EDS analyses, the recovered iron was not quantified because only a small amount was deposited on the electrode. As a result, neither the overall iron recovery efficiency nor a complete mass balance could be determined. Furthermore, the deposited iron could not be characterized by XRD because the amount of the deposit was insufficient for a reliable phase identification. These issues, together with the optimization of the electrodeposition process, will be addressed in future studies.

Overall, unlike conventional high-temperature reduction processes, the proposed approach employs mechanochemical activation as a pretreatment to enhance the acid dissolution of iron from ilmenite. These results demonstrate the feasibility of a simpler and potentially more compact iron recovery route that may complement future lunar ISRU technology.

Conclusions

Iron recovery from ilmenite, a mineral abundantly present in lunar regolith, was investigated by using a novel process integrating mechanochemical treatment with sodium hydroxide, acid leaching with hydrochloric acid, and electrodeposition. Mechanochemical treatment with sodium hydroxide at 600 rpm for 120 min converted ilmenite to soluble salts. After acid dissolution, ferric ions were successfully extracted into a strongly acidic hydrogen chloride solution, and iron was subsequently recovered by electrodeposition in the presence of oxalic acid.

The proposed process demonstrates the feasibility of recovering iron through a combination of mechanical activation, alkali treatment, and electrochemical recovery without reliance on conventional high-temperature smelting methods or complex reagent systems. Because iron is an essential structural material for infrastructure, machinery, and construction, the proposed approach may contribute to reducing the dependence on material transportation from Earth and support future ISRU concepts for sustainable lunar base construction.

However, the present study was conducted using commercially available ilmenite as a simplified model material rather than actual lunar regolith or lunar regolith simulants. Actual lunar regolith contains more complex mineral compositions and physical characteristics that may influence the mechanochemical reactions, dissolution behavior, and electrodeposition efficiency. Therefore, the current results should be regarded as an initial feasibility study of the proposed concept.

Further investigations will be necessary to evaluate the applicability of the process to lunar regolith simulants and ultimately actual lunar materials. In addition, future work should address reagent recycling, energy consumption, scale-up feasibility, and the influence of lunar environmental factors, such as low gravity and vacuum conditions, to assess the practicality of the proposed process for lunar applications.

Glossary

Abbreviations

NASA

National Aeronautics and Space Administration

JAXA

Japan Aerospace Exploration Agency

CSA

Canadian Space Agency

ASA

Australian Space Agency

ESA

European Space Agency

USD

United States Dollar

XRD

X-ray diffraction

SEM

scanning electron microscopy

T.W. conceived the study, designed the experiments, performed the experiments, analyzed the data, and drafted the manuscript. The authors have read and approved the final manuscript.

This study was supported by the Japan Mining Promotive Foundation Survey Test Research Project.

The author declares no competing financial interest.