Ionic Liquids in the Extraction and Recycling of Critical Metals from Urban Mines

Ionic liquids (ILs), salts with a melting temperature below the boiling point of water, are one of the most recent fashion trends in modern chemistry. Nowadays, and taking into account the extensive research found in literature, it seems hard to imagine a sustainable world in the near future without the involvement of ILs, since they have low vapor pressure, are non-flammable, and display excellent chemical/electrochemical/thermal stabilities. Accordingly, ILs are considered to be advantageous replacers of the traditional organic solvents, therefore, much more environmentally-friendly. ILs can be easily produced to tune their physicochemical properties to specific applications, and that is the case currently occurring for several separation processes. This review aims to highlight and discuss some of the most relevant key-achievements, developed at laboratory scale, focusing on the use of ILs for the hydrometallurgical extraction of critical metals from urban mines, particularly the platinum-group metals (PGMs): ruthenium, rhodium, palladium, osmium, iridium, and platinum. A few decades of investigation brought a well-recognized scientific knowledge, still with a wide space to go, but work has yet to be conducted on testing the most promising ILs for the recycling of metals from real urban mines, and at a scaled-up level. Nevertheless, based on the most significant case-studies, the upcoming of ILs to recover critical metals from end-of-life devices for further valorization is assessed.


Introduction
The inventors of modern technology employ virtually the entire periodic table of elements, a situation remarkably different from the one experienced some 25 to 40 years ago [1]. As examples, thousands of components are assembled into a single laptop computer, and several medical equipment, medical diagnostics, and other high-level technological instruments incorporate more than 70 metals [1]. Furthermore, 60 or so metals are combined into each microchip, and microchips are embedded into industrial plants, means of transportation, building equipment and appliances, consumer products, and in several other devices [1].
This state of things required that responsible people be alert, to ensure the availability of reliable supplies of all these materials. Increasing concerns about the consequences of the unbridled consumption of technology led European Union (EU) to implement the disclosure of a list of "critical raw materials" (i.e. raw materials with a high supplyrisk and a high economic importance) [2], to monitor and identify priority actions. EU undertook in charge the review and updating of the list every three years as well. In 2011, 14 critical raw materials were identified [2], followed by 20 in 2014 [3] and 27 in 2017 [4]. From the latter list, 17 are metals [4]. Platinum, palladium, rhodium, ruthenium and iridium have always been included in the lists, grouped under the designation of platinum-group metals (PGMs). Osmium does not have technological importance, although being a PGM; therefore, it is not included in those lists.
The supply risk of a material is attributed to a combination of several factors, namely, a high concentration of production in countries affected by political constraints, limited material replacement, and poor end-of-life recycling rates [5]. The major mining suppliers for PGMs are South Africa (for Pt, Rh, Ir, Ru) and Russia (Pd) [4], and EU may sometimes feel some concerns about relying on these regions from an import point of view.
The unique catalytic properties of PGMs make their utilization crucial for several applications. The main commercial utilization of Pt, Pd and Rh is in automobile catalysts (to reduce the hazardous emissions of the combustion engines) [4], but they also exhibit important expression as catalysts in the chemical, petroleum refining, and petrochemical industries [6]. Pt is also applied in medical and biomedical facilities, Pd in dental composites and in electrical joints, Rh in glass manufacture, and Ru and Ir in the chemical, electrical and electrochemical sectors. Pt and Pd are also relevant elements used in jewelry and for investment purposes [6].
The high economic value of PGMs, the concentration of their mines in a few zones of the globe, and the low index of PGMs replacement in specific applications, are the basis of the extensive demand associated with these elements [6,7]. In order to answer to all these challenges, the recovery of PGMs from secondary sources should be reinforced. The need to preserve the environment should additionally be considered.
From 2014 to now there has been an increasing effort to recycle end-of-life devices containing PGMs, figures that can generally be seen in the graphics of Fig. 1 for Pt, Pd and Rh [6]. In the graphics, the supply data represent estimates of sales by the mines of primary PGMs, the gross demand denote the sum of manufacturer demand for new metal and any changes in unrefined metal stocks, the recycling data represent estimates of the quantity of metal recovered from open-loop recycling (i.e. where the original purchaser does not retain control of the metal throughout), and net demand is equivalent to the sum of gross demand less any metal recovery from open-loop scrap [6].
Taking a look to the bars related with the total gross demands, one can observe that Pd is the most procured metal, followed by Pt; additionally, the Pt profile suffered a slight decrease from 2016 to 2018, whereas Pd slightly increased under the same period. On the other hand, the demand for Rh has been more or less constant from 2016 to 2018. By comparison of the correspondent bars in the data related to Pt, Pd and Rh of Fig. 1, one can see that the supply figures slightly surpassed the net demand for Pt and Rh in 2018, whereas for Pd the demand is still higher than the primary supply. These figures would not be so well-balanced if recycling did not significantly increase from 2014 to 2018 for all the metals.
Data regarding the total demand for Ru and Ir from 2014 to 2018 are also included in Fig. 1, but no other information is available for these latter metals in reference [6].
The recycling rate for PGMs is a reality [2][3][4], particularly from spent autocatalysts [7], but its increase is obviously advisable, taking into account the environmental sustainability of primary resources and the economic side. The main current industrial recycling companies are located in countries that do not have relevant native deposits of PGMs, 1 3 e.g., in Europe and USA [7]; accordingly, autocatalysts recycling provides a complimentary provision of PGMs from urban mining to those enterprises and, indirectly, to the countries in those geographic regions. Urban mines should therefore be regarded as a value source for PGMs, as well as for other specialty metals and materials, since they often exhibit a high grade and simpler composition when compared to the correspondent mining resources [8].
The several recycling plants around the world are primarily based on the pyrometallurgical recovery of PGMs. Hydrometallurgy seems nevertheless a better option, as it involves lower temperatures, exhibits potential for extraction of any valuable co-metals, and shows ability to be adapted to both small-and large-scale operations. Moreover, the liquid effluents produced may be easier to handle when compared to the volatile combustion emissions from the pyro-based recycling sites [7].
Although mostly supported in pyrometallurgy, the principal autocatalyst recycling facilities use hydrometallurgical refining for the final recovery of Pt, Pd and Rh [7] as well. Interested readers can find details about the commercial pyro-hydrometallurgical processes used by the recycling refiners to process urban mining, for PGMs and specialty metals recovery, in [7] and [8].
The development of an exclusive hydrometallurgical approach that could be sound, efficient and economically viable would guarantee a more environmentally friendly alternative for recovery of PGMs and specialty metals from urban mining recycling [7], and this goal has motivated intensive research in recent years.
Any hydrometallurgical approach is based on three key steps, namely [9]: 1. Leaching in which metals to be recovered are dissolved in appropriate aqueous media, usually with the help of specific agents; 2. Separation/purification to further separate the desired metals from any impurities, to separate the metals of interest from each other, or to concentrate them prior to the final step; 3. Final metal recovery either in the metallic form or as a salt.
Focusing on PGMs leaching, these are noble metals whose dissolution requires acidic, highly oxidizing environments, often in the presence of a complexing agent [7]. Chloride solutions are the most usual complexing media used to dissolve PGMs, and that is why PGMs-chloro  chemistry is generally well known and understood, for example [10]. Some recent literature focuses on the chloride complexation of PGMs from spent auto-and industrial catalysts using alternative oxidants other than chlorine, for instance [11][12][13], with the aim of turning the process more environmentally friendly and safer, and minimizing the toxic emissions of chlorine [7]. The favored option widely used for separation/purification of metals is liquid-liquid extraction or solvent extraction (SX) [14], and therefore SX has enjoyed a relatively better adoption in refining industry for PGMs recovery [7]. Accordingly, several sorts of traditional [14] and unconventional solvents [15] have been investigated in the last years, aiming to separate the metals of interest from waste recycling leaching solutions.
Ionic liquids (ILs) are within the unconventional options that have been extensively investigated for several and different purposes, under a panoply of scopes [16], and hydrometallurgy is not an exception [17]. Being salts with a melting temperature below the boiling point of water, ILs show some general properties that are independent of their specific ionic composition: they exhibit low vapor pressure, are nonflammable, and display excellent chemical/electrochemical/ thermal stabilities [18]. These latter characteristics are generally maintained, and any other features of an IL, such as being hydrophilic /hydrophobic, polar /non-polar or protic / aprotic, can be easily tuned as well [17]. This versatility justifies the testing of ILs for several application purposes, ranging from supporting media for organic, polymerization and catalysis reactions, to electrochemical recovery, liquid-liquid and solid-liquid separations, to energy storage devices, such as fuel cells and batteries [16].
This review aims to be a state-of-the-art article focusing on the description and discussion of the most significant advances that have been achieved by the application of ILs to hydrometallurgical processes, particularly those involving PGMs recovery from selected wastes. Following a chapter introducing the nature of ILs, the next sections are devoted to their use as leaching media, and as liquid-liquid extraction agents for PGMs, respectively.

Ionic Liquids (ILs)
There are some good references about the history, nature and applications of ILs. For example, the book edited by Rogers and Seddon [16] contains the key papers presented at a symposium of a national meeting of the American Chemical Society titled "Green (or Greener) Industrial Applications of Ionic Liquids", held in April 2001, and the editors claim this symposium has been the first open international meeting on the fundamentals and applications of ILs. The preface states that the number of articles dealing with ILs in 2000 was about 114-115 [16].
A search for "ionic liquids" ranging from 2001 to 2019 in all databases of Web of Science (Clarivate Analytics), on 15 October 2019, displayed 113,280 entries, distributed yearly as shown in Fig. 2. From 2001 to 2018 the number of articles has increased six-fold, and this provides a qualitative vision of the effort carried out by researchers to investigate the nature, behavior and possible applications of ILs, and several other relevant topics as well.
Readers interested in knowing about the evolution of ILs since their beginning can access, for instance [18], but any review article focusing on properties / applications of ILs generally provides general historic information.
The research boom associated with ILs largely relies on the "green chemistry" approach. This concept represents a sort of unified effort towards environmental management of chemical facilities at large, improving a circular economy based on efficiency and cost benefits through minimizing waste and reducing hazardous streams [16]. One of the base guidelines of green chemistry is the development of processes involving cleaner solvent solutions to reduce the use of volatile organic compounds (VOCs), and ILs obviously present potential benefits as VOCs replacers [16], since they commonly have negligible vapor pressure.
ILs are however much more than just non-volatile solvents. The exclusive combination of properties, directly depending on the ILs composition, produces innovative chemistry, as well as improved selectivity for novel separation and extraction procedures [16], and these features contribute to the development of efficient and greener processes.
ILs are typically composed by an organic cation and an anion, and have low melting points, commonly below 150 °C. This large liquid range is mainly induced by packing frustration of the usual asymmetric cations [16], and also by the difference in ionic size between cations and anions. The most common attributes of ILs are as follows:  ILs are typically classified into four general types based on their cationic part [20]: The most usual cations and anions used in ILs are displayed in Fig. 3. The order of presentation of the anions follows a decreasing coordinating ability and an increasing hydrophobicity [16]. Additionally, it should be taken into account that all ILs are hygroscopic, even the most hydrophobic, hence, water may represent a significant molar fraction of an IL [15].
The most common ILs are found commercially, and usually do not involve very complicated syntheses. The general synthetic pathways adopted for the production of ILs can be found elsewhere, e.g., [17]. There are nevertheless several researchers that opt to carry out the synthesis of their own ILs, particularly to produce taskspecific ILs (TSILs), this information being important from a practical point of view. Therefore, data about the source of the ILs under investigation will be provided throughout the overall article.   Similarly to what has been adopted in reference [15], abbreviations taken on this work to designate ILs are based on their structure and not on their trade names. This procedure allows a clearer comparison of data, facilitating discussion. For instance, alkylammonium cations will be abbreviated to [NR 1 R 2 R 2 R 2 ] + , whereas phosphonium ones as [PR 1 R 2 R 2 R 2 ] + . The dialkylimidazolium and N-alkylpyridinium cations will be presented as [C n C m IM] + and [C n PYR] + , respectively. The methyl group is a frequent alkyl substituent found in dialkylimidazolium cations; these cations are widely designated in literature by the abbreviation "MIM" and, therefore, [C n MIM] + will be adopted for the alkylmethylimidazolium cations throughout this review.
As an example, the IL commercially known as Cyphos IL 101 will be designated by [PC 14 C 6 C 6 C 6 ] + [Cl] − throughout this article. Other cations or anions not listed in Fig. 3 will be presented and abbreviated case by case, but attempts to adopt similar trends will be made.

Leaching by ILs
Classical leaching of primary and secondary metallic rawmaterials involves several types of reagents, mostly mineral acids, but also alkaline and complexing agents. The concentrations used are wide, depending on the phase bearing materials (oxides, sulfides, zero-valent metals, etc.), on their reactivity, and on the solubility of the reacted metal ions in aqueous media. Hydrochloric and sulfuric acids are the most commonly utilized. The use of additives, namely oxidizing or reducing agents, is also frequent, to improve the reactivity or solubility issues. For the case of noble metals like PGMs, leaching is commonly made by very concentrated HCl solutions, also requiring addition of a strong oxidant (oxygen, chorine, hydrogen peroxide, among others). Leaching, as the first and core operation of hydrometallurgy, is usually considered as a reagent-consuming step, and it is associated with technical, economic and environmental concerns.
On the other hand, besides the necessity of having favorable thermodynamic conditions for the system, the design and engineering of a leaching operation involves several other details, related with the characteristics of the solid-liquid operation, like the reactor design, mixing, promotion of accessibility of the leachant to the solid phase, efficient phase separation, washing of the residue, transport and handling of liquids and solids, and choice of the construction materials.
For rare and noble metals like PGMs, the low concentrations found in wastes give additional challenges aiming to the production of leachates with enough concentration and purity that could justify their purification and recovery through technically and economically feasible ways. Taking spent automotive catalysts as example, as they are main secondary source of PGMs: the concentrations of PGMs (Pt, Pd, Rh, with different combinations between them) can be as low as 0.01%, but higher values up to 0.3% can also be found [21]. Considering an average of 0.1% and the use of a liquid/solid ratio (L/S) of 5 L/kg (a relatively concentrated pulp density), a leachate with 0.2 g/L PGMs could be obtained. Concerning the main contaminants present in the catalyst wash-coat, aluminum and cerium, and even promoting high selective conditions, concentrations of Al and Ce in the leachates in the range of 2-12 g/L are normally found [12], and their presence causes difficulties to the subsequent purification steps. This example illustrates the selectivity problem behind processing of metal-bearing resources containing rare metals.

ILs Used in Leaching
The development of ILs and their application in metallurgy gives new opportunities to the metal extraction industry. This new field, sometimes called as iono-metallurgy, as a part of solvo-metallurgy, has been proposed, studied and developed by many authors, although being still in its infancy [7,22,23].
The most frequent studies of ILs focus on the SX operation, using them as alternative to classical extractant-diluent organic phases. However, other metallurgical operations are suitable for application of the ILs concept, such as leaching, cementation, and electrowinning [17]. Regarding the latter one, several advantages can be taken from metals electrochemical reduction in ILs, namely, by exploring the wider electrochemical potential window when compared with aqueous systems; furthermore, several side reactions, such as hydrogen evolution, are also avoided. The discussion of metals recovery be electro-reduction is nevertheless out of the scope of this review, which is focused on leaching and SX.
The use of ILs in leaching brings several advantages over classical hydrometallurgical leaching. ILs may replace the mineral acids as metals dissolution agents, providing a higher selectivity for the metals of interest against the secondary contaminants of the bearing solid matrices. This usually allows the reduction of the requirements in the subsequent separation and purification operations, involving lower reagent consumptions and lesser amounts of generated wastes. ILs can give opportunities to achieve higher selectivity patterns regarding the targeted metals through appropriate design and synthesis (taylor-made reagents). By adapting molecules to a set of metals of interest, the use of large excesses of reagents can be avoided, and the processing conditions can be adjusted to produce leach liquors with specific metals, as concentrated as possible.
The leaching operation with ILs can be designed through several alternatives, as described below: • Using the pure IL as leachant • Using the IL mixed with an additive such as an oxidant/ reducer or a complexing/ligand agent • Using an aqueous solution of the IL (or at least a mixture IL / water even with small water content) • Using an IL dissolved in an organic solvent.
There are many possibilities that can be adapted to each case, usually by choosing the appropriate combination of the cation and anion of the IL. When considering the mixture with water, the hydrophilic degree can be adjusted by the character of the anion. Alternatively, the use of a water immiscible organic phase can allow other approaches for the process, namely, a subsequent stripping operation (similarly as in SX, see more details in the next section). This arrangement provides selectivity and minimizes the number of unit operations [22].
The issue of the hydrophobic or hydrophilic character of an IL should be discussed with precaution, because several ILs are immiscible with water but they are, at the same time, very hygroscopic, absorbing substantial quantities of water. Therefore, the hydrophobic character of an IL is mostly related with immiscibility rather than with capacity of retaining and absorbing water molecules, or even to include water in its structure. Another detail that should be considered is the variation of miscibility with water of some ILs at certain conditions. As an example, literature refers [24] that some ILs such as imidazolium salts of the type [C n MIM] + [NTf 2 ] − ([NTf 2 ] − is the abbreviation for bis(trifluoromethanesulfonyl)imide anion, see Fig. 3) are generally water-insoluble, but a gap of water solubility can be observed at set temperature conditions, e.g., [C 2 MIM] + [NTf 2 ] − and [C 4 MIM] + [NTf 2 ] − . This phenomenon is usually explored to employ those ILs in the presence of water, improving mass diffusion and transfer, as will be referred to later, through some examples. The presence of water can be advantageous, since it can influence metal coordination chemistry, as water competes for metal coordination sites. The mechanisms of ILs reactions with metals are complex and are out of the scope of this review article.
A different branch of ILs frequently used are the taskspecific or functionalized ILs, in which one of the alkyl chains of the cation is replaced by another functional group, e.g., a carboxylic acid, as for example betainium cation, [C 1 C 1 C 1 NCH 3  Leaching of metals and metal compounds using ILs is still at a very early stage when compared to SX. Although there is literature reporting this topic, there are only a few studies involving some materials and wastes, namely electronic waste and rare earths, with almost no references to precious metals and PGMs. This fact is not surprising, since the leaching of PGMs involves very aggressive conditions, comprising the combination of a strongly acidic, complexing and oxidizing medium. If the first two characteristics can tentatively be attained by ILs use, the required redox potential will certainly be more difficult to achieve. Accordingly, leaching of noble metals such as PGMs is unlikely to be successfully achieved with the already known ILs. Therefore, this section focuses on some examples of application of ILs in metal leaching, namely some critical metals, serving as illustration and starting point for future developments involving noble metals.
Some examples of critical metals leaching using ILs are reviewed in the subsequent sub-sections. The starting materials are rare-earth bearing sources (primary and secondary), electric and electronic scrap, and metal oxides. Some essays on precious metals dissolution are also reported. For a better understanding of the discussion, Table 1 summarizes the overall processing conditions and the general recoveries achieved in critical metals leaching by ILs.

ILs in the Leaching of Rare-Earth Sources
The recovery of critical rare-earth elements (RE) from primary and secondary resources is certainly the most studied topic regarding the leaching of metals by ILs. The application of [Hbet] + [NTf 2 ] − was tested in a low grade bastnasite ore (a rare-earth fluorocarbonate) [25]. Extractions of light and heavy REs, in the ranges 40-100% depending on each element, were achieved, without significant dissolution of the contaminants Fe and Si, but other metals such as Mg and Ca were also leached. The reactions seem to involve an acid-base mechanism, the carboxylic acid chain playing an important role. Addition of water (as 30-50v%) provided better yields and reduced viscosity, and adopted temperatures were in the range 50-90 °C. The IL was subsequently stripped by aqueous HCl, with pH values near 0-1, and reused. Reutilization tests (3 cycles) showed the system was stable. Regarding miscibility with water, [Hbet] + [NTf 2 ] − shows a thermomorphic character, being able to change its behavior: it is immiscible below 55 °C, and miscible above that temperature. Stripping of leached metals from the IL was also possible using an acid solution at lower temperatures (e.g., room temperature); two phases were therefore formed, one with the aqueous metal strip liquor and the other with the regenerated IL, allowing its use in a new cycle.
In another application, the same IL was also applied in the leaching of REs from a red mud bauxite residue [26]. The selectivity of RE dissolution against iron was again Pt -soluble (*) [38] successful, attaining RE yields in the range 70-85%. In this study, high temperatures such as 100-160 °C were found more advisable. Water dilution also showed to be important, increasing recoveries by improving diffusion through a substantial decrease of the viscosity of the medium. Viscosity measurements showed a decrease of more than 80% when water addition increased from 5v% to 20v%. Stripping with relatively high concentrated HCl solutions, e.g., above 4 M, were used to recover the REs to a new aqueous phase, and to regenerate the IL.
Another example of REs recovery using IL leaching, again with [Hbet] + [NTf 2 ] − , was in the treatment of NdFeB magnets (after previous oxidation by roasting at 950 °C) [27]. As found in other studies, the authors also tested the addition of water to the leaching system (until 1:1 wt ratio). The developed process took advantage of the thermomorphic property of the IL. During leaching, at 80 °C, a single phase existed, while after cooling to room temperature, two-phases coexisted: an aqueous phase rich in REs and Co (the targeted metals) and an IL phase containing mostly Fe. The REs and Co were therefore recovered by precipitation with oxalic acid, followed by separation and recovery operations, while the IL was regenerated by adding a KCl solution, which complexes iron, removing it from the organic IL phase.
Another secondary source bearing REs, the phosphor powder used in spent fluorescent lamps, was also treated with [Hbet] + [NTf 2 ] − [28]. The leaching of yttrium oxide (YOX) was selective, allowing the dissolution of yttrium and europium (the doped element in the YOX phosphor), while other RE phosphors remained insoluble. The leaching efficiency was better after addition of a small amount of water (e.g., up to 5 wt%), at a temperature of 90 °C. Yttrium was recovered from the IL by precipitation with oxalic acid, regenerating the IL.
Another investigation showed the use of [HMIM] + [HSO 4 ] − (30v% in water) in the extraction of the REs La, Ce, Nd and Y from a slag material obtained from pyrometallurgical recycling of spent NiMH battery electrodes [29]; the leaching yields attained were relatively poor (15-20% for La, Ce and Nd), at 80 °C, explained by the formation of low soluble RE sulfates.

ILs in the Leaching of Metals from Waste of Electric and Electronic Equipment
The application of ILs was also tested in metals recovery from waste of electric and electronic equipment (WEEE). Some reviews are already available on this topic, including the reference to ILs in a wider evaluation of metallurgical processes [39,40], others specifically addressing ILs [15]. Leaching of copper from printed circuit board scrap was successfully achieved using [C 4 [32], at 90 °C. In this work, a mixture of 50v% IL with ascorbic acid was found selective for indium extraction against iron, the main present metal impurity, when compared with the use of aqueous solutions of the IL. After leaching, the leachate was cooled, the displacement of the two phases being observed: the IL organic phase rich in In and the ascorbic acid aqueous solution containing most of Fe, allowing the separation of the two metals. The authors claimed that ascorbic acid allowed a substantial decrease of the Fe distribution ratio in the IL organic phase, when compared with the use of water, promoting selectivity for the process.
In another investigation, the possibility of using ILs in the leaching of precious metals (gold and silver) from WEEE has been tested. Mixtures of [C 4 MIM] + [HSO 4 ] − with thiourea (complexing agent) and ferric sulfate (oxidant) were considered suitable for that purpose [33]. The IL was utilized in aqueous solution (10-50wt%), and the temperature was 25 °C. In spite of the efforts, relatively low recoveries were found (below 25%), and the reason probably lies on the difficulty in achieving a high enough redox potential.

ILs in the Leaching of Several Metal Oxide Sources
Metal oxides are normally much more easily solubilized than zero-valent metallic forms, and this is the reason why one of the suggested applications of ILs leaching is in the treatment of oxides [41]. Furthermore, different oxides reveal diverse leaching behaviors, allowing selective leaching, as demonstrated in [34] for several metal species using [Hbet] + [NTf 2 ] − . Fifteen trivalent RE oxides were solubilized, as well as oxides of Pb, Zn, Cd, Hg, Ag, Ni, Pd and Mn, while Fe, Co, Al and Si oxides were insoluble. Mixtures of the IL with water gave better results than the pure IL, as found in other studies. In addition, the structural changes in the IL by varying temperature (thermomorphism, as previously addressed) or acidity also provided good possibilities for separation of metals and regeneration of the IL.

ILs in Precious Metals Leaching in Combination with Other Reagents
An example of ILs application to precious metals is leaching of silver and gold from a pyrite ore using [C 4 MIM] + [HSO 4 ] − combined with thiourea and ferric sulfate [35]. At set conditions (25 °C, 20 g thiourea and 0.5 g Fe 2 (SO 4 ) 3 , both per kg of ore, in pure IL), leaching recoveries over 85% for Au and 60% for Ag were attained. In another investigation, the same IL was tested for the treatment of anode slimes produced in industrial copper electrowinning [36]. Using a 60v% aqueous solution of [C 4 MIM] + [HSO 4 ] − at 50 °C, 97% Au recovery was achieved with co-dissolution of 88% Cu, while Ag dissolution was inefficient. An alternative leachant was attempted, in another study, for the treatment of the same waste, the IL [C 2 MIM] + [HSO 4 ] − , and similar conclusions were achieved, with an optimized Au recovery of 89% using 80 v% IL in water and 75 °C [37].
Concerning PGMs leaching, a screening of several ILs was carried out in a recent investigation, using several conditions, namely, the addition of stabilizing ligands and/or oxidizing agents, at different temperatures, in order to evaluate the solubility of Pd and Pt metals [38]. In this experimental screening of leachants, a very high L/S ratio was utilized , and with addition of nitric acid, tetrabutylammonium nitrate and iodine (for the third IL). In some of these combinations, other reagents (ligands/adducts) were also added (see details in [38]). Attempts to use the same imidazolium-based ILs combined with other anions such as [NTf 2 ] − or [PF 6 ] − were unsuccessful. The phosphonium chloride IL [C 14 C 6 C 6 C 6 P] + [Cl] − was also tested, at 60 °C, and it was effective for Pd leaching in combination with some ligands and oxidants (namely tetrabutylammonium nitrate). Platinum dissolution was also checked using several ILs from the imidazolium and phosphonium cation groups. Pt dissolution was clearly less effective than with Pd, the only successful combination involved [C 8 MIM] + [Cl] − , with a substantial addition of HNO 3 as oxidant, at 90 °C. A question arises from this investigation: the combination of ILs with strong mineral acids or oxidizing agents can be very questionable, since the apparent advantage of using ILs for environmental reasons is lost when using such combinations. However, providing that a reduction of the amounts of acids/ oxidants is guaranteed, this possibility can be considered for further investigation.
It is worth to mention a specific class of IL reagents, the deep eutectic solvents (DES). DES are formed by mixtures of Lewis or Brønsted acids and bases. Although the individual components of the mixture have high melting points, at a specific composition (the eutectic point) the mixture reveals a substantial decrease in the melting temperature, being liquid at low temperatures. Examples are quaternary ammonium halides (a type of IL) and a hydrogen bond donor (alcohols, amides or carboxylic acids). DES show good efficiencies in solubilizing several ferrous and non-ferrous metal oxides [23,42]. Common DES are based on choline chloride -[(CH 3 ) 3 N(CH 2 ) 2 OH] + [Cl − ] -a relatively available and inexpensive IL, combined with urea, thiourea, ethylene glycol, citric acid, malonic acid, among many others.
Like with the other ILs, any reference on the use of DES in the leaching of PGMs was not found.

Solvent Extraction (SX) of Platinum-Group Metals (PGMs) by ILs
As previously mentioned, SX is one of the most preferred options for separation/purification of PGMs in the industrial recycling facilities adopting hydrometallurgical methods [7]. SX traditionally includes an extraction and a stripping stage, e.g., [14,43]. In extraction, the aqueous feed is contacted with an immiscible organic solvent, which should simultaneously be selective for the desired metal species and promote its efficient transfer to the organic medium; in stripping, the loaded solvent is equilibrated with a stripping aqueous solution, allowing the transfer of the metal species to the new aqueous medium. Prior to the stripping stage, and if some undesired contaminants accumulate in the solvent, an additional step involving the use of a scrubbing solution may be included. Successful SX schemes should effectively separate the metal of interest from the impurities, left in the aqueous raffinate, and also enable the reutilization of the solvent in successive extraction-stripping cycles, an indication that the solvent is robust and did not suffer degradation. A recovery stage should follow the SX process to retrieve the metal ion from the stripping solution; the metal itself, or a salt containing the metal, are then finally produced.
The efficiency of a SX process is commonly evaluated through the calculation of the extraction percentage (%E)-Eq. (1)-and/or the distribution ratio (D)-Eq. (2)-of the metal of interest.

3
In Eqs. (1) and (2), [M]org., [M]aq.in., and [M]aq. stand for the metal ion concentrations in the organic phase after equilibration, in the initial aqueous phase, and in the aqueous phase in equilibrium, respectively, whereas Vorg. and Vaq. indicate the volumes of organic and aqueous phases, respectively. The expression relating the %E and D is also very common, and can be depicted as in Eq. (3).
In several SX processes the volume of the aqueous and organic phases is the same, hence, volume ratio (A/O) is unitary, leading to simpler forms for Eqs. (1) and (3). Furthermore, similar expressions to calculate the stripping percentage (%S) and/or the correspondent distribution ratio (D) of a given metal ion after stripping can be derived.
To determine the selectivity of a SX system for the metal A over the metal B, for instance, the separation factor (SF or β) of A towards B is defined as the ratio of the D values for metal A over metal B, as shown in Eq. (4). The higher the SF value, the more selective the solvent will be for metal A.
One of the determinant requisites for a robust SX process is that the solvent should not be significantly lost to the aqueous phase; the contamination of the aqueous streams would compromise the correct functioning of the process, and the economic side cannot be neglected as well. Accordingly, when considering the use of ILs as extracting media for SX of metals, their choice should, in principle, rely on the ones with a higher hydrophobicity. The necessary hydrophobicity for the cations can be assured by the inclusion of long alkyl chains, nevertheless, a compromise may have to be established with the associated anions. In Fig. 3, the sequence of appearance of the anions follows a growing hydrophobicity; therefore, the use of ILs bearing [BF 4 ] − , [PF 6 ] − and [NTf 2 ] − may be expected.
The solvents used in traditional SX usually involve an extractant (the active species of the solvent for the extraction of the desired solute or solutes) dissolved or diluted in an organic solvent, called diluent, which should act as an inert carrier for the extractant (often mixtures of hydrocarbons). Sometimes a modifier is also added, to improve the physicochemical properties of the overall system [14]. Two different approaches are perceptible in literature when ILs are used in the SX of metals: some authors opt to use ILs in a pure form, e.g., ILs are simultaneously the extractant and diluent of the SX system, others use ILs as the extractants, namely, they place ILs in a diluent. The first option is obviously more environmentally-friendly, although the latter one helps in Vorg.
. the improvement of the physicochemical properties of the SX systems. In fact, ILs usually exhibit a certain degree of viscosity, and this drawback has to be overcome when ILs are used in pure form. This section reviews some of the most significant data on the SX of PGMs from HCl solutions involving ILs, and mainly focuses on information obtained in the last ten years. The classification of the ILs based on the cationic part is the adopted option.

ILs with Dialkylimidazolium Cations
Research involving ILs with dialkylimidazolium cations for the SX of PGMs has been carried out with ILs in pure form, e.g., in the absence of a diluent. Table 2 summarizes some of the most relevant information available in literature on the use of pure dialkylimidazolium cations in ILs for SX of PGMs. Data about the structure of the ILs and the targeted metals to be extracted / separated, the adopted general experimental conditions, the main results obtained, and some pertinent observations are included.
It  [48,52]. Attempts to strip Pt(IV) from the loaded ILs are not extensive; in [46], the degradation of [C 4 MIM] + [PF 6 ] − upon Pt(IV) stripping by HCl or HNO 3 was noticed, whereas the unfeasibility to recycle [C 8 MIM] + [NTf 2 ] − for Pt(IV) extraction has also been reported [49]. Some authors have nevertheless the idea of avoiding the stripping step by exploring the possibility of the direct electrolysis of the ILs for the metals recovery, e.g., [17,49], but results related with those efforts have not yet been described in literature.
Yang and coworkers developed mixed hydrophilic − hydrophobic IL systems for Pt(IV) SX [50], finding that the metal ion extractability can be highly enhanced when compared with the performance shown by single hydrophobic ILs. The best results were attained for [C 16 [53] such as Al(III) and Fe(III) were extracted at about 4%. Pd(II) was also 85% extracted. Pt(IV) in the IL system has been efficiently stripped by hydrazine hydrate, leading to the formation of Pt powder and allowing the regeneration of [C 16 MIM] + [Cl] − . Authors reached a certain degree of success when testing the reutilization of the IL system for Pt(IV) extraction, but the overall performance showed a declining tendency. It should be stressed that those latter essays were carried out with a 0.8 M HCl solution containing Pt(IV) only [50]. Following a similar trend, Yang and collaborators proposed a scheme for extraction and separation of Pt(IV), Pd(II), Ru(III), and Rh(III) [53]. Accordingly, Pt(IV) was firstly separated by [C 6  , and 80% Rh(III). Some reutilization experiments were also carried out [51]. The same group of authors has previously published a work reporting an efficient and selective Pd(II) extraction over Pt(IV) by synthesized ILs with tetrahydropyran-2H-yl-protected thiol moieties [54].
Generally, the authors of the articles cited in Table 2 made efforts to characterize the types of PGMs extraction reactions involving ILs. In several ones the main identified mechanism is the anion exchange type [45][46][47][48][49][50][52][53][54][55], namely, the anion of the IL exchanges with the PGMs anions, leading to the extraction of the metal ions. The species [PtCl 6 ] 2− is known to be predominant even for very low HCl concentrations, e.g., [10] [10,53,55]. Some authors suggest that the preference of the several ILs containing dialkylimidazolium cations for extracting Pt(IV) over Pd(II) is due to the lower charge density and larger diameter of the Pt(IV) chlorocomplex species, the Pd(II) ones having a bigger hydration shell, therefore, disfavoring the formation of ion-pairs with the IL cation [48,49].
According to the anionic exchange mechanism, the transfer of IL anions to aqueous phases is, of course, a drawback, since raffinate aqueous phases containing [NTf 2 ] − or [PF 6 ] − are not advisable, and this is probably also the reason why stripping and recycling of these ILs are not straightforward; in fact, after the first cycle, the ILs do not have the same composition anymore. The situation may improve when the so-called hydrophilic ILs are used, e.g., [50,53], particularly because the anion is easier to recycle. However, the overall selectivity results achieved up to now, both for PGMs extraction over other metals, or within PGMs, cannot be considered outstanding.
In addition to the hydrophobic [NTf 2 ] − and [PF 6 ] − anions, imidazolium cations with long alkyl chains such as [C 8 MIM] + or [C 16 MIM] + , used to achieve hydrophobicity of the IL systems for SX, are known to be toxic [56], therefore, contradicting the environmentally friendly properties usually associated with ILs.

Pure ILs
Some of the most relevant works dealing with SX of PGMs employing solely ILs with phosphonium cations are displayed in Table 3. When [PC 14 C 6 C 6 C 6 ] + is involved, the best results for the extraction of Pt(IV) and Pd(II) are accomplished by taking [Br] − as the counter anion, but when [Cl] − is used instead of [Br] − the overall results are also very good [49]. Papaiconomou and coworkers [57] [52]. These authors claimed HNO 3 to be a good stripping agent for both Pt(IV) and Pd(II) loaded from 1 M HCl, and they managed to accomplish five reutilization stages 1 3 with the ILs mixture without a significant loss of efficacy. Although the phosphonium IL should be transformed in [PC 14 C 6 C 6 C 6 ] + [NO 3 ] − during stripping, authors suppose [52] that the IL should be regenerated to the initial form during the extraction stage. The extraction reactions of PGMs by these ILs have generally been evaluated [49,52,57], and with this aim the system [PC 14 C 6 C 6 C 6 ] + [Cl] − and Rh(III) has been recently investigated [60]. All the collected data support the formation of PGMs ion-pairs through anion-exchange extraction reactions, as stated in the previous section for ILs with dialkylimidazolium cations.
The IL [PC 12 C 8 C 8 C 8 ] + [Cl] − has been synthesized and tested for Pt(IV), Pd(II), and Rh(III) extraction from 0.1 M to 5 M HCl solutions [58]. In comparison to [PC 14 C 6 C 6 C 6 ] + [Cl] − , this novel IL is more hydrophobic and less viscous than the available commercially one. Both Pt(IV) and Pd(II) were quantitatively extracted within the studied HCl range, whereas Rh(III) extraction significantly decreased for higher HCl concentrations. When loaded from a 0.5 M HCl solution, the best stripping agents for Pt(IV), Pd(II) and Rh(III) were HNO 3 , thiourea, and HCl, respectively [58]. Authors performed three reutilization cycles for this IL system, and the overall results were again better when compared with those by [PC 14 C 6 C 6 C 6 ] + [Cl] − under the same experimental conditions.
Following the general idea of using temperature to promote the homogeneous liquid − liquid extraction of metal ions, firstly introduced by Binnemans and coworkers [61], Papaiconomou and collaborators proposed the development of acidic aqueous biphasic systems to recover metal ions by playing with the thermomorphic behavior those systems can exhibit [59]. Accordingly, they explored the ability to separate highly acidic solutions into two phases in presence of an IL by changing temperature, studying different compositions for mixtures of [PC 14 C 4 C 4 C 4 ] + [Cl] − -HCl-H 2 O at various temperatures. High %E results were achieved for Pt(IV), Fe(III) and Co(II) [59]. Changes in temperature allow the possibility of transforming a biphasic system in a monophasic one, and vice-versa, also improving the overall kinetic phenomena in comparison with the traditional two-immiscible liquid systems. Additionally, stripping was not necessary, since the overall system is aqueous. The developed concept seems promising, but extensive studies should be carried out to evaluate the viability of this novel approach when applied in hydrometallurgical solutions produced directly from spent devices.

Diluted ILs
Some of the most representative research carried out for SX of PGMs with ILs containing phosphonium cations dissolved in a diluent are depicted in Table 4, the majority of them from the Wisniewski group. The general results point out to a very good extraction performance of [PC 14 C 6 C 6 C 6 ] + [Cl] − in toluene towards Pd(II) recovery from dilute HCl, typically below 1 M HCl [62][63][64]70]. When chloride anion is replaced by bromide, the overall data concerning Pd(II) extraction improves [65,70]. Stripping of Pd(II) can be accomplished by aqueous ammonia from both ILs [63,65] or by acidic thiourea [70].
Both [PC 14 C 6 C 6 C 6 ] + [Cl] − [64,70] and [PC 14 C 6 C 6 C 6 ] + [Br] − [66,70] in toluene extracted Pt(IV) similarly to Pd(II), namely, Pd(II) was better extracted than Pt(IV) from 0.1 M HCl by [PC 14 C 6 C 6 C 6 ] + [Cl] − , but Pt(IV) was more effectively extracted than Pd(II) from 3 M HCl [64]. Similar overall results were obtained by Lee and collaborators [67], who tested [PC 14 C 6 C 6 C 6 ] + [Cl] − in xylene for Pd(II) and Pt(IV) extraction. The SX system was able to efficiently extract both PGMs from 0.1 M HCl, leaving Rh(III) in the raffinate, and then a sequential stripping scheme with sodium thiocyanate to recover Pt(IV), and thiourea in HCl to strip Pd(II), showed promising results. Another IL with the same cation but with the bis(2,4,4-trimethylpentyl)phosphinate anion has also been tested for Pd(II) and Pt(IV) extraction [64,70], generally exhibiting a similar or a bit worse performance when compared with that of [PC 14 C 6 C 6 C 6 ] + [Cl] − . Rh(III) and Ru(III) extraction by ILs with phosphonium cations, in toluene, also called the attention of the Wisniewski group [64,66,[68][69][70]. The extraction figures obtained for these two PGMs are generally worse than those attained for Pd(II) and Pt(IV) by the ILs mentioned above, but potential useful data has been collected regarding the effects of the ageing of the solutions, since ageing plays a determinant role on the extraction of Rh(III) and Ru(III) [66,69]. Recently a sequential extraction scheme has been investigated and developed with the aim of separating Pd(II), Pt(IV), Rh(III) and Ru(III) by the three commercial ILs, dissolved in toluene, containing the same phosphonium cation [70]. The best SX system to achieve the desired aim was [PC 14 C 6 C 6 C 6 ] + [Cl] − ; hence, authors found that a first extraction stage allowed the separation of Pd(II) and Pt(IV), whereas a second extraction stage with the raffinate promoted Ru(III) uptake. Pd(II) was stripped from the first loaded IL by thiourea in HCl, followed by HNO 3 to strip Pt(IV), and the second loaded Ru(III) IL was subject to stripping by thiourea in HCl, and finally HCl, to recover Ru(III). Rh(III) remained in the final raffinate [70].
A MSc thesis developed under the supervision of the authors of the current review focused on the recovery of PGMs, namely Pt(IV), Pd(II) and Rh(III), from leaching solutions of two spent automobile catalysts [71]. The best conditions found for the leaching stage have been established and sequentially adopted to produce solutions in order to allow the application of SX to separate the PGMs. Traditional extractants and one IL, [PC 14 C 6 C 6 C 6 ] + [Cl] − in  Extractions with individual or binary metal mixtures; IL recycling possible but with contaminations [64] [PC 14 [70] toluene, were therefore tested. Under the adopted experimental conditions, the IL quantitatively extracted Pt(IV) and Pd(II) (corresponding to about 245 mg/L and 70 mg/L in the organic phase, respectively), and about 30-50% of Rh(III) from 3 M HCl. From the metals analyzed, it could be observed that Fe(III), La(III) and Nd(III) were 89-99%, 97% and 89% extracted, respectively, while the most abundant elements, Al(III) and Ce(III), also showed %E around 20%, with overall concentrations of about 150 and 220 mg/L in the organic solution, respectively. This is only a preliminary study indicating that the IL, though being unselective, did not lose its efficacy towards Pd(II) and Pt(IV) extraction. These results are also important to highlight that real hydrometallurgical solutions are always much more complex and more concentrated in contaminating elements than any simulated ones. In fact, even a highly promising SX system for ideal conditions may be useless when applied in practical situations.

ILs with Ammonium Cations
This class of cations in ILs has not been extensively explored for PGMs extraction as imidazolium and phosphonium ones have. Some of the most relevant results obtained with ILs containing ammonium cations are displayed in Table 5, and all those works are discussed in more detail in sequence.
[NC 1 C 8 C 8 C 8 ] + [Cl] − [73], and [NC 6 C 6 C 6 C 6 ] + and [NC 8 C 8 C 8 C 8 ] + bromides [75] are available commercially, all the other ammonium IL derivatives have been synthesized. The former IL is very well known for decades in hydrometallurgical solvent extraction, named as Aliquat ®336, and its properties towards PGMs recovery from hydrometallurgical solutions have been previously explored, e. g. [76,77]. Katsuta [72], finding that the best results were achieved with a mixture of the two ILs. Pd(II) and Pt(IV) were extracted almost completely from 0.1 M HCl; under similar conditions, Na(I), Mg(II), K(I), Ca(II), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II), Ru(III), Rh(III), and Cd(II) were practically not extracted. Both PGMs can be stripped from the IL mixture by HNO 3 , allowing the recycling of the ILs for further SX cycles.
[NC 1 C 8 C 8 C 8 ] + [Cl] − in limonene proved its efficiency to extract Pd(II) from a real leach liquor produced by hydrometallurgical treatment of ceramic microcircuits from printed circuit boards (PCBs). According to the authors, limonene was chosen as diluent due to its low-toxicity, simple availability and easy renewability. Together with 83-92 mg/L of Pd(II), some traces of Ag(I) and almost 3 g/L of Ba(II) were removed [73]. The final recovery of Pd has been accomplished by direct reduction with a sodium borohydride solution, the Pd powder exhibiting a slight contamination by Ag.
Protic and aprotic ILs containing ammonium cations have been tested for extraction of Pt(IV) and Pd(II) from 0.1 M to 2 M HCl solutions [74]. Encouraging results for Pd(II) and Pt(IV) extraction have been reached, with good selectivity profiles against other metals from 0.1 M HCl, but with worse figures when more concentrated HCl solutions were involved. Both metal ions could be stripped by thiourea, or thiourea in HCl and / or in HNO 3 [74].
Finally, tetra-hexyl and tetra-octyl ammonium IL derivatives, together with several anions, have been investigated for the extraction of Pt(II) and Pt(IV) from water, modeling aqueous effluents [75]. The ILs combining both cations with bromide, thiocyanate and dicyanamide (DCA) anions showed good extraction values for both Pt(II) and Pt(IV), but the cations combined with [NTf 2 ] − did not extract Pt [75]. Pt(IV) could be stripped from [NC 6

ILs with Miscellaneous Cations
As previously mentioned, the main classes of ILs developed for PGMs extraction contain imidazolium and phosphonium cations. Nevertheless, some examples of other cations less explored can also be picked up from literature, although less interesting from a practical point of view. For instance, Prausnitz and collaborators [78] found that functionalized pyridinium with methyl and butyronitrile groups, and similar piperidinium cations, when combined with [NTf 2 ] − , were able to extract Pd(II) from water at pH 7. These results were corroborated by Lee [79], who additionally reported that other functionalized piperidinium, pyrrolidinium and pyridinium cations, some of them possessing disulfide, nitrile or alkene groups, combined with [NTf 2 ] − , showed D values ranging from 50 to 10,000 for Pd(II) extraction, also from water.
Yang and coworkers [80] used hexadecylpyridinium chloride, [C 16 PYR] + [Cl] − , in chloroform, for studying Pd(II) extraction from hydrochloric acid media. Adopting a contact time of 10 min and a ratio A/IL of 5, the authors found a complete Pd(II) extraction by the IL at 0.1 M HCl, but the metal ion extraction sharply decreased to 70% at 1 M HCl. Selectivity for Pd(II) extraction from 0.1 M HCl, against several other metals, was also evaluated, denoting %E below 10% for Al(III), Co(II), Cu(II), Fe(III), Ni(II) and Sn(IV). Metallic Pd could be obtained from the IL loaded phase by refluxing with an oxalic acid solution at 65 °C for 4 h [80]. • There are comprehensive efforts to understand how ILs work in the extraction of PGMs, and these fundamental data is of course very useful. The collected results suggest the occurrence of anion-exchange extraction reactions for all PGMs. The increase of HCl concentration in the aqueous phases above 2-3 M reduces PGMs extraction by ILs in a higher or lesser extent; this effect is explained by the competitive extraction of chloride anions and even HCl, this latter leading to the formation of ion pairs in the organic phase involving IL cations and [HCl 2 ] − [67]. The presence of a more hydrophilic anion in the IL, such as [Cl] − or [Br] − , helps the transfer of Pt(IV) and especially Pd(II) to the IL organic phase, and a phosphorus site reveals a higher affinity for Pd(II) species than the nitrogen atoms, particularly those of the dialkylimidazolium cations.
• Some information about selectivity for the PGMs extraction by ILs has also been collected; however, this is commonly accomplished in general terms, e.g., without a focus on specific compositions of real solutions coming from the hydrometallurgical treatment of end-of-life devices, such as spent industrial or automobile catalysts. Additionally, the majority of the studies have been carried out with very dilute HCl aqueous media, and the real leaching solutions are generally much more HCl-concentrated. A good option to adopt by the research groups could be the production of real leaching solutions by themselves, directly from the hydrometallurgical treatment of the devices mentioned above.
• Some attempts to check recycling capabilities of ILs have been reported, but those are often carried out with solutions containing only the metal under consideration. Efficient stripping agents to recover the PGMs from the loaded ILs have not been easy to find; accordingly, alternatives to skip the stripping step of SX should be pursued. There is almost no information about the loading capacities of ILs, another very important feature from a practical point of view as well.
• TSILs have also been investigated for PGMs extraction. These dedicated efforts may be valuable from a fundamental perspective, but the results obtained up to now do not seem to justify an investment on their more expensive production. The development of homogeneous / heterogeneous SX through temperature change may be an option deserving a deeper consideration in the near future, as the information appearing in literature is still preliminary.
• The fundamental information available on the SX of PGMs by ILs can be a good starting point for the people directly involved in the establishment of new and / or managing already existing industrial facilities devoted to the hydrometallurgical treatment of secondary resources containing those metals. The best compromise should be the combination of efforts by research institutions (where the novel knowledge is mainly located) and the enterprises possessing the raw materials (as well as the industrial experience). This has probably been the spirit behind broad projects funded by the European Commission (EC). Regarding the problematics of PGMs recovery from spent automobile catalysts by ILs, the project Platirus [81] should be mentioned. This European-funded project claims the development of an innovative process for recovering PGMs from spent autocatalysts by means of a dual extraction and separation process (leaching and SX) with ILs. The first step involves the use of a lowcost hydrophilic choline-based IL, and the second a hydrophobic IL. More information about the achievements of this project is expected.

Challenges and Further Developments
The optimization of ILs towards their application in metallurgical processes still requires a long path of research and development, in the following fields: (a) cost reduction; (b) knowledge on thermodynamics and equilibrium behavior; (c) kinetics and reaction mechanisms; (d) metals recovery from IL (stripping, direct precipitation/electrowinning), regeneration and reuse; (e) process integration and equipment design; (f) interaction with materials and corrosion assessment; (g) effluents and waste management as well as environmental impact; (h) more precise health effects associated with long term exposure to ILs.
Although the versatility of ILs may enable a wide range of formulae combination and operating possibilities (e.g., used in pure form or diluted, in aqueous or organic media), most research works choose to use them in dilute forms. In SX, the use of pure ILs as organic phase raises problems due to their high levels of viscosity, with negative effects in mixing, diffusion of species, reaction rate inhibition, and phase separation. In leaching, acidic and hydrophilic ILs (instead of aqueous insoluble ones) are prevalent, and water mixtures are common, allowing to improve efficiency; the option of mixing water can have relation with viscosity control by improving mass transfer, but a precise explanation about the behavior of IL mixtures and their causes was not really accomplished, and it is essential to be done. A more accurate knowledge about reaction mechanisms and mass transfer phenomena will allow developing adequate formulas, to design new ILs or to improve those already existing, to adapt their properties to each requirement, and to decrease synthesis costs.
The progressive advances in ILs development also need to be related with engineering and application. In the leaching operation it corresponds to a new kind of solid-liquid extraction, eventually in a non-aqueous media or in an aqueous solution containing a dissolved IL, which significantly changes not only the leaching operation itself but all the process. For instance, the solid-liquid separation (e.g., flocculation/settling), and the recovery and reuse of the IL after the metal recovery, among many other issues, are details needing a thorough consideration. In SX, the possibility of using pure ILs as organic phase also involves challenges regarding the equipment design and the mixing.
After leaching, SX and subsequent metal recovery operations, the reuse of ILs is a critical issue. ILs are expensive and their reutilization will be mandatory. The knowledge of reaction mechanisms is again essential: the reaction products containing the metal species should not be substantially different from the initial IL composition, in order to allow its reuse after metal recovery to a new stream. If the mechanism involves loss of part of IL structure when extracting the metal, a very precise evaluation of the viability of IL reuse should be made; if not seeming feasible, such process may probably have to be abandoned. Auxiliary operations for IL regeneration, prior to reuse, would be eventually needed, if they can be fulfilled in a relatively simple way.
In hydrometallurgical processing, several water effluents are generated, such as washing of the insoluble residue after leaching operation, in washing of solvent phases, etc. The wash waters can eventually be reused in the process, at least partially, and the remaining is treated in well-established wastewater treatment plants. How these procedures will be changed if using an IL in leaching or in SX is an unsolved question. What will be the washing media? Water, as usual? That depends on the use, or not, of aqueous solutions in the process, and on the solubility of the organic IL in water. And in what level the wastewater treatment will be affected, and what will be the effect of contamination of the residue regarding its safe disposal? The fact that ILs are normally considered as "green" reagents (essentially because they are non-volatile), it does not mean that there are no concerns regarding the correct management of effluents and residues containing traces of ILs. These are crucial issues related to handling of new types of solutions in a process. However, experience of the organic chemical industry can be very useful, and principles and procedures can be transferred to the metallurgical industry dealing with ILs.
Research will continue and many of the previous questions will be answered in due time. The success of industrial utilization of ILs in extractive metallurgy is still unknown, but its immense potential cannot be disregarded.