A novel method for extracting metals from asteroids using non-aqueous deep eutectic solvents

Extra-terrestrial mining and metal processing are vital for access to strategic metals for space exploration. This study demonstrates for the first time the catalytic dissolution of metals from meteorite proxies of metal-rich asteroids using a deep eutectic solvent (DES). DESs are of particular interest for extra-terrestrial mining as they can be designed to have relatively low vapour pressures and could potentially be made from organic waste products created in extra-terrestrial settlements. Three types of meteorites were investigated: two chondrites (H3, H5) and one iron (IAB-MG) meteorite. Chondrite samples were composed of silicates (olivine, pyroxene) with metal-rich phases occurring as native metal alloys, sulphides and oxides. Metallic Fe–Ni and troilite (FeS) are the most abundant metal-bearing phases in all three samples, particularly in the iron-rich meteorite. The samples were subjected to chemical micro-etching experiments with iodine and iron(III) chloride as oxidising agents in a DES formed from the mixture of choline chloride and ethylene glycol. Micro-etching experiments demonstrated that Fe–Ni rich phases are effectively leached out in this system, while other mineral phases remain unreactive.

Overall, chondrite meteorites are characterised by their diverse mineralogy.Olivine, pyroxene, plagioclase and kamacite tend to be the most prominent minerals, with composition depending on the degree of metamorphism to which they were subjected 52,53 .In this research, the mineral constituents of the meteoritic samples were determined by automated SEM-EDX mapping and correlated to high resolution reflected light images.Bulk analysis of meteorites can be found in Tables S1, S2 and S3 as Supplementary Material.
Figure 4 shows the mineral distribution of iron and nickel in the studied meteoritic samples.In both the chondrite samples, iron is distributed across a range of silicate and Fe-Ni rich mineral phases.In sample NWA 13876 the bulk of the contained iron is hosted in Fe-Ni rich phases such as kamacite (43%), troilite (10%), taenite (3%) and Ni-wüstite (2%) though a significant amount also occurs in silicate phases including olivine (23%) and pyroxene (16%).Nickel is hosted exclusively in the Fe-Ni rich minerals with kamacite (58%), taenite (36%) and Ni-wüstite (5%) being the major mineral hosts.Sample NWA 7160 contains a higher proportion of silicate minerals, and this is reflected in the distribution of target metals.Iron is hosted mainly in silicate phases including olivine (43%), pyroxene (14%) and augite (3%), while Fe-Ni minerals contain a lower proportion with kamacite (13%), Ni-wüstite (13%) and troilite (11%) being most notable.The Campo del Cielo sample, as an iron-nickel meteorite, is composed entirely of Fe-Ni rich mineral phases, with kamacite hosting the majority of both iron (99%) and nickel (95%), with the remaining nickel being hosted in taenite (4%).The elemental distribution in the different mineral phases can be found in Fig. S2 in the Supplementary Material.

Redox catalysts in DESs-a brief summary
The catalytic oxidation of metals has already been demonstrated for several solids, such as superalloys and minerals 58,59 , as well as in the recycling of spent electronic devices 41,60 .The advantage of electrocatalysis is that no direct contact is required between the anode and the material to be oxidised, and the chemical oxidising agent can be re-generated in situ by an electrochemical reaction (if it has a reversible electrochemical behaviour) 61 .While electrocatalysis can be carried out in a variety of media, non-aqueous eutectic mixtures can decrease passivation of some metals, while careful selection of the redox catalyst can enable selective solubilisation of metals from complex mixtures 37,61 .For instance, iodine has been reported as a strong oxidising agent for metal and mineral digestion, particularly for solubilising gold in a wide variety of solvents, both aqueous and nonaqueous systems 58,59,[61][62][63][64][65][66][67][68] .Iron(III) chloride (FeCl 3 ) and copper(II) chloride (CuCl 2 ) have also been reported as redox catalysts in different ionic fluids and brines, for the dissolution of metals from solar photovoltaic cells and printed circuit boards.Both of them tend to have high solubility and fast electron transfer 41,60 .The ChCl: 2EG system was considered as a solvent for processing the meteorites as has a relatively low viscosity.However, there would be other similar, non-aqueous, ionic fluids which would be more suitable for extra-terrestrial applications 69 .A general overview of the physical properties of this particular eutectic solvent, as well as its constituents, is shown in Table 1.
Chemical etching was investigated for all three samples using iodine and FeCl 3 , separately, as redox catalyst at a concentration of 0.1 mol dm -3 .Iodine was selected due to its strong oxidative capacity, whereas FeCl 3 was chosen due to the high concentration of iron in the samples.Notice that both redox catalysts have high solubility and display a reversible redox behaviour 61,73 , which can be explained by the following redox processes:

Chemical etching of chondrite meteorites
The reflected light images of meteorites NWA 13876 and NWA 7160 before and after chemical etching in the presence of iodine and FeCl 3 can be seen as Supplementary in Figures S3 and S4, respectively.A large portion of the surface exposed to the catalytic solutions, consisting of silicate materials (e.g., olivine), did not show signs of chemical oxidation in either sample.Figure 5 shows that chemical etching was observed for the kamacite and wüstite, with weak etching also in the taenite.The troilite, however, developed a yellow tarnish colour possibly due to the generation of sulphur or a passivating species able to change the optical properties of the underlying mineral.Such behaviour has already been observed when processing sulphide minerals with the same eutectic solvent [74][75][76] .The effect of iodine and FeCl 3 as redox catalysts seems to have a similar effect, as both of them preferentially etched the Fe-Ni/Fe-O mineral phases over other mineral compounds.
Figure 6 shows the etching depth in meteorites NWA 13876 and NWA 7160 after chemical contact with iodine in the ChCl: 2EG eutectic solvent.Both 3D images confirmed that Fe-Ni minerals were preferentially etched.The average depth obtained in these particles, after 1 h of experiment, was 33 ± 3.3 and 13 ± 1.3 μm for NWA 13876 and NWA 7160, respectively.These minerals were etched at an average rate of ca.0.4 μm min -1 , which was comparable to the rate obtained with FeCl 3 .The difference in etching rates was due to the distribution of iron within the difference mineral phases.In sample NWA 7160, iron was mostly concentrated in unreactive silicate minerals, such as olivine, pyroxene and augite, whereas in sample NWA 13876 iron was mostly concentrated in more soluble Fe-Ni rich phases, such as kamacite, troilite, taenite and Ni-wüstite (Fig. 4).The dissolution of nickel could also have contributed to the difference in etching rates, as it is known that it can form complexes either with the anion or with an oxygen donor (usually in the form of OH -ions) of the DESs depending on its concentration and temperature 44 .For instance, nickel can form complexes with EG molecules, i.e. [Ni(EG) 3 ] 2+ , whereas trichloro complexes with two or three hydrate water molecules can also be formed in the presence of water molecules (water content > 5 wt.%) 77,78 .Such complexes are likely to develop passivating layers on the surface of the particles 79 . (1)

Chemical etching of "Campo del Cielo" meteorite
Figure 7 shows the reflected light images of the Campo del Cielo meteorite before and after chemical etching with iodine and FeCl 3 as oxidising agents.A dissimilar etching effect was observed on the sample.It is known that the behaviour of iron species in this DES is particularly interesting since only the Fe III/II couple is stable 58 .
As kamacite, a mineral composed principally of iron, is the major constituent in this sample, in the presence  of FeCl 3 , the contained iron can be dissolved into the solution as Fe II species (Eq.5).In theory, part of the Fe II species will be re-oxidised over time into Fe III as long as there is sufficient oxygen present in the surrounding medium (Eq.6).However, the development of both chemical reactions will be restricted by the presence of Fe III in the solution.Eventually, Fe II could have been subjected to secondary chemical reactions by forming a stable oxide/hydroxide complex, leading to the formation of a passivating film 79,80 .This could explain the dullness of the sample after etching.The formation of such a complex could hinder both the re-generation of Fe III and the dissolution of metallic iron.
In the case of iodine, it has been reported that both [I 2 Cl] − and [I 3 ] − are stable in ChCl: 2EG eutectic mixture (distribution between complexes tending to change as function of water addition) 81,82 , suggesting that both species should be able to oxidise iron according to Eqs. (7, 8).However, both the Fe III/II and iodine redox couples are at very similar potentials in ChCl: 2EG 58 , suggesting that Fe II can also be oxidised to Fe III by iodine.This can explain the differences between the surface morphologies obtained after etching, as in the presence of iodine, iron does not seem to passivate.The etching rate was evaluated by measuring the height profile across the sample, as shown in Fig. 8.The etch depth was measured using the level of the substrate (insoluble phase) as a reference height.As can be seen in Fig. 9, iodine allows a more efficient dissolution of metals than FeCl 3 .The average dissolution rate of the kamacite in the presence of iodine (ca.0.22 μm min −1 ) was almost twice the dissolution rate obtained with FeCl 3 (0.11 μm min −1 ).This confirmed that iodine is capable of providing stronger oxidising conditions than FeCl 3 .This would be expected given the difference in redox potential between these two oxidising agents.This clearly demonstrates that metal rich phases can be selectively dissolved from meteorites.In fact, the process could be further enhanced by considering electrochemical oxidation as an alternative extracting methodology.Pateli et al. have already demonstrated that the dissolution rate of iron oxide compounds increases through electrooxidation in a similar DES, while metals can be recovered on a counter electrode 36 .A similar methodology has been demonstrated for the anodic dissolution of copper sulfide minerals, and the simultaneous recovery of copper at the cathode during electrowinning 75 .
Given that metal recovery from these types of liquids has been extensively demonstrated 77,83,84 , it indicates that if a DES can be synthesised from extra-terrestrial sources or bio-produced waste, sustainable metal recovery using just solar power could be possible under extreme conditions.

Conclusions
In this study, both iron(III) chloride and iodine were used as oxidising agents for solubilising metals from three meteorite proxies of NEAs in ChCl: 2EG deep eutectic solvent.Automated SEM-EDX analysis confirmed the chemical association of nickel with iron-rich metal phases in the original samples (before chemical etching), whose average concentration was above 10 wt.%.The mineralogical matrix in the chondrite samples was mostly composed of silicate materials, whereas the Campo del Cielo sample was constituted mostly of an iron-nickel solid solution.Some other metals of interest for space technologies such as ruthenium and rhodium were also identified as traces.Optical profilometry was used to study the etching depth and dissolution rates in the different samples.In all samples iron-rich alloys and oxides containing nickel within their lattice structure were preferentially etched by the DES with either oxidising agent.Other metal-bearing minerals, such as troilite, did not dissolve and may have rapidly passivated.In all cases, the silicate groundmass showed no indication of etching.Etching with iodine was faster and is considered a more effective oxidising agent than iron(III) chloride for extracting target phases from meteoritic material.

Future perspectives
Current process technologies for metal recovery and separation rely heavily on water and high-energy consumption for operation.Although the presence of water ice at both the north and south lunar poles has already been confirmed 85 , it will likely not be available for direct use as it will be used mostly as a source of oxygen and hydrogen, and so will be an expensive commodity.On the other hand, the deployment of energy-efficient methods will be crucial for the processing of materials in low-energy and ambient temperature conditions 86,87 .Thus, chemical oxidation using DESs has a great potential for metal recovery and separation from NEAs.DESs such as the eutectic system ChCl: 2EG meets some of the criteria for its potential use in space, being a largely anhydrous solvent and having a low vapour pressure.In addition, due to the relatively high chloride concentration (ca. 5 mol dm −3 ) and the absence of oxygen in space, the formation of less reactive oxide/hydroxide is circumvented.
Eventually, metals could be extracted in situ from their solid matrix(es) injecting leach solutions through injector pipes and suction from another battery of harvesting pipes, geometrically arranged to stimulate the passage of solutions through the interior of the asteroid, forcing its dissolution.Thus, pregnant leach solutions (PLS) containing the valuable metal may be retrieved at the surface.In an electrochemical cell the oxidising agent can be regenerated and simultaneously metals can be recovered from the PLS by electrowinning.It has already been demonstrated that DESs can allow the development of closed-loop flow sheets by, for instance, integrating the metallurgical unit operations of leaching and electrowinning 61,75 .Thus, dissolved metals can be selectively deposited at the cathode by adjusting the electrode potential.Although the methodology may require an artificial pressurised atmosphere to operate, energy requirements will be relatively low as the process operates at temperatures lower than 100 °C.In terms of analytical methods for mineral exploration, we have used X-ray spectra induced by electron bombardment to identify mineral phases, but other techniques may also be applicable.Laser Induced Breakdown Spectroscopy (LIBS) represents an interesting analytical tool as enables qualitative, semi-quantitative, and quantitative analysis of all elements in the periodic table.It can be performed  in the laboratory or outside in the ambient environment for on-site analysis in situ, as it requires minimum sample preparation and enables a rapid online analysis 88,89 .While in the long-term the utilisation of asteroids as mineral and metal resources is a key step in human space exploration, further investigation is required to make this a viable economic activity.The technology hereby proposed, though in a nascent stage, is very promising with DESs possessing many favourable physical and chemical characteristics and, as was demonstrated here, can selectively extract target value phases from proxyasteroid material.Moving forward, this will require an integrated understanding of the internal structure and micro-texture of asteroids, which minerals host potentially useful metal-rich phases and their concentrations, and what electrochemical methodologies can be implemented for metal recovery.

Material and methods
The DES was prepared from 1 mol eq of ChCl (Acros Organics, 99%) and 2 mol eq of ethylene glycol (EG, Fisher, 98%).The solvent was prepared by stirring the components at 50 °C until a colourless homogeneous liquid was formed.FeCl 3 (Sigma Aldrich, 97%) and iodine (Fisher Scientific, > 95%) were used as oxidising agents.
Meteoritic materials used in this study were obtained from Msg-Meteorites and consist of two stony meteorites and one iron meteorite (see Fig. S1 in the Supplementary material).Stony meteorites consist of a 0.5 cm thick, approximately 6 cm square section of meteorite NWA 13876 and 5 cm long piece of meteorite NWA 7160.Both NWA meteorites belong to the high-iron (H) chemical group of ordinary chondrites.These are distinguished by their high siderophile element content, relatively small chondrules (~ 0.3 mm), and oxygen isotope compositions that are closer to the terrestrial fractionation line than those of other chondrites.The NWA 13876 is classified as type 5, as these types of meteorites have been metamorphosed at conditions that were sufficient enough to homogenise olivine and pyroxene, convert all low-calcium pyroxene to orthopyroxene, cause the growth of various secondary minerals, and blur chondrule outlines 90 .The NWA 7160 meteorite is classified as type 3, which is typically characterised by abundant chondrules, low degrees of aqueous alteration, and the presence of non-stoichiometric mineral phases 91 .The iron meteorite consists of a piece of irregularly shaped material from the Campo del Cielo meteorite.Campo del Cielo is classified as an IAB-MG type, consisting of Fe-Ni alloy that crystallised from a melt 56,92 .
All samples were documented and photographed before visually selecting potential target areas for further study.In each case an approximately 2.5 cm 2 section was cut from the interior of the main sample, so as not to sample altered material from the fusion crust, embedded in epoxy resin and polished.Samples were imaged using a Sensofar 3D Surface Profiler to generate reflected light montages for the identification of metallic phases within the sample both prior to and post chemical etching.
Mineralogical constituents and proportions were determined by automated Scanning Electron Microscopy and Energy-dispersive X-ray spectroscopy (SEM-EDX) mineralogical analysis using a Zeiss SIGMA 200 scanning electron microscope equipped with two Bruker XFlash 6|60 energy dispersive spectroscopy detectors controlled by Zeiss Mineralogic software.Samples were imaged to provide high-resolution Back-scattered Electron (BSE) montages and analysed by EDX (accelerating voltage: 20 kV; detector aperture: 120; step size: 10 µm in NWA 13786 and NWA 7160 and 50 µm in Campo del Cielo; dwell time: 0.05 s).
In-situ etching was carried out by submerging the surface of the resin block in 80 mL DES at 50 °C with a nominal concentration of 0.1 mol dm −3 of oxidising agent 59 .Constant and gentle agitation was applied to maintain a homogeneous and continuous flow inside the reactor.The block was held stable with a clamp so that the stirrer bar did not contact the etching surface.The temperature was controlled using a hotplate stirrer with a glass-coated thermocouple placed directly in the etching solution.
Etching was performed step-wise for an effective period of 1 h at 50 °C.After each step, the sample was removed from the solvent, rinsed with acetone to remove any residue, and dried in air.Prior to leaching and then after each step, 2D and 3D optical images of the sample were captured by using a Zeta Instruments Zeta 2000 optical profiler using the inbuilt Zeta 3D software version 1.8.5.Images were constructed by determining the features of an image that are in focus at different heights.These were then analysed to produce a reconstructed 3D topography of the surface of each sample.After the corresponding images were obtained, the sample was returned to the solvent.

Figure 4 .
Figure 4. Mineral distribution of iron and nickel as determined by automated SEM-EDX analysis in (a) NWA 13876, (b) NWA 7160 and (c) Campo del Cielo.

Figure 5 .
Figure 5. Reflected light images of meteorite NWA 13876.(A) Sample before chemical etching and, (B) same sample after 60 min of chemical etching with 0.1 mol dm −3 iodine as oxidising agent in ChCl: 2EG eutectic solvent.The corresponding distribution of minerals as determined by SEM-EDX is shown in (C).

2 Figure 6 .
Figure 6.Etch depth in sample references (a) NWA 13876 and (b) NWA 7160, after 60 min of etching with 0.1 mol dm −3 iodine as oxidising agent.In the left-hand side, the 3D reflected light images (with line profiles) are described, whereas the topographic line profiles are shown in the right-hand side of the corresponding 3D image.

Figure 7 .
Figure 7. Reflected light images of the Campo del Cielo meteorite before and after 60 min of chemical etching with 0.1 mol dm −3 of iodine or FeCl 3 as oxidising agents in ChCl: 2EG eutectic solvent.

Figure 8 .Figure 9 .
Figure 8. Etch depth in the Campo del Cielo sample after 60 min of etching with 0.1 mol dm −3 (a) iodine and (b) FeCl 3 as oxidising agents.In the left-hand side, the 3D reflected light images (with line profiles) are described, whereas the topographic line profiles are shown in the right-hand side of the corresponding 3D image.