Recovery of Lesser-Known Strategic Metals: The Gallium and Germanium Cases

Abstract

Being not as popular as other elements, such as cobalt, lithium, and rare earth elements, both gallium and germanium have wide use in target developments/industries, thus making them valuable and strategically critical metals. The principal sources for the recovery of both metals are secondary wastes of the bauxite (gallium) or zinc (germanium) industries; also, their recycling from waste materials is necessary. The characteristics of these materials make hydrometallurgical operations widely useful in recovering both gallium and germanium from the various sources containing them. The present work reviews the most recent applications (in 2024) of the various operations applied to the recovery of gallium or germanium from various resources.

1. Introduction

Approaching the end of the first quartile (Q1) of the XXI century, mankind urgently needs certain metals that are apparently essential for daily life. Among these metals, lithium, rare earth elements, gold, niobium, tantalum, etc., tend to receive the most attention, and there are others, like gallium and germanium, that rarely appear in the news, forums, etc., and thus attract less attention.

However, these two elements are also of the utmost importance. Gallium is a fundamental element for the fabrication of wide-band gap (WBG) semiconductors due to its properties, which enable the manufacturing of advanced military equipment by supporting voltages, temperatures, and frequencies that are higher than those of silicon-based circuits. Some gallium-bearing compounds are used in specialized developments. GaN is employed in the manufacturing of cutting-edge radars, whereas GaAs is used in the fabrication of global positioning systems (GPSs), radar, and precision weapons.

Gallium is obtained mainly from bauxite, though some gallium is also derived from the processing of sphalerite ore. Gallium is recycled from scrapping GaAs- and GaN-based devices [1]. China dominates the global primary gallium market, accounting for 94% of the global total [2].

Germanium is generally obtained as a sub-product of the zinc industry and in the recycling of electronic wastes, though there are several Ge mineral deposits scattered across the world. Similar to gallium [3], China is the main germanium producer (83%) [2].

Thus, several reviews have recently been published about the recovery/recycling of these two elements, mainly from secondary sources.

It has been shown that in the 2000–2020 period, 79% of the global gallium co-mined from bauxite ended up in red mud or entered the aluminum cycle as an impurity, indicating significant recycling potential. Different involved regions have different but complementary roles in the global gallium supply chain. As was mentioned above, China dominates the global primary gallium production, basically due to its strong alumina industry, whereas Japan and the United States are key players in high-purity gallium refining and rely on gallium to support their electronic device manufacturing [4,5].

The different technological operations and methods that are currently used or will be in the future to produce pure gallium (indium) or their oxides from waste have also been reviewed [6]. First, the process of waste pre-treatment, including disassembly and sorting in several stages, and further mechanical treatment, as well as physical, chemical, and physicochemical separations, has been reviewed. Moreover, the methods of obtaining pure metals or metal oxides and their refining processes have also been described.

Bayer liquor is an important source for recovering gallium, and thus, the next reference reviewed the recovery of this element from these liquors [7]. At present times, technologies for recovering gallium from Bayer liquor can be classified into ion exchange resins, liquid–liquid extraction, precipitation, and electrochemical processes. Comparisons between the described technologies concluded that resin ion exchange has become the primary method of gallium recovery from Bayer liquor due to its simple operation, high recovery efficiency, and low contamination. Improvements in the technology using impregnated resin methods and ion blotting are considered for further interest.

Gallium and other strategic key metals are present in the coal gangue in some mining areas of Shanxi Province and Inner Mongolia [8], and their presence has made these areas research and technological hotspots. Based on the coal gangue from the Shanxi province, a pre-enrichment method was developed. It consisted of the removal of the pyrite fraction in the gangue by high-gradient magnetic separation. Using this procedure, the gallium recovery rate was 31.88 μg/g, which is higher than the gallium content in the original ore, whereas the gallium recovery rate was 80.07%.

Waste copper–indium–gallium–diselenide (CIGS) thin-film photovoltaic (PV) modules are materials in which gallium can be recovered [9]. Methods including mechanical crushing and sorting, pyrolysis, pyrometallurgy, hydrometallurgy, and electro-deposition are described. Where selenium can be recovered by pyrometallurgical processing, hydrometallurgy appears to be the key to the recovery of gallium (and also copper and indium).

This review critically analyses different technologies for the ultra-purification of gallium required for high-performing optoelectronic devices [10]. Zone refining (zone melting) is found to be an effective technique for purifying materials in which vulnerable impurities can be reduced below parts per billion, which implies a percent purity beyond 12 N. The presence of effective resistance mechanisms to gallium and arsenic is particularly important for bioleaching Ga from GaAs and GaN [11].

Molecular recognition technology (MRT) is a methodology in which target ions achieve mutual binding under specific conditions through the synergistic action of forces such as hydrogen bonding, chelation, surface complexation, and ion exchange, which directs the design of various functionalized adsorbents for critical metal recovery nowadays. Thus, the next reference reviewed the current status of the recycling of, among others, gallium and germanium using MRT and the mechanism and application of MRT in the selective recovery of these elements [12].

Using different databases [13], the resource features of gallium and germanium (also indium) and the chemical principles, technical parameters, and metal recovery in various extraction and separation methods from monometallic and polymetallic resources were described. As was somewhat expected, the authors concluded that leaching, followed by liquid–liquid extraction or (resin) ion exchange, is the main process for the recovery of these strategic metals.

The potential of residual brines, commonly known as “bitterns”, generated during solar sea-salt extraction in traditional saltworks has been described as an alternative source of metal, including gallium and germanium, though its concentration was detected in the μg/kg concentration range [14].

The next reference [15] focused on the extraction of technologically critical elements (Ga, Ge, and In) from leach solutions of multiple sources using ionic liquids (ILs) with attention to the extraction mechanism and selectivity criteria. Generally, many ILs reviewed have low selectivity toward Ga(III), Ge(IV), and In(III) compared to Fe(III), Al(III), and Zn(II), which are typically present in high concentrations.

Since the recovery of gallium and germanium from end-of-life (EoL) products (electronic waste: e-waste) offered a potential solution to address their shortages, a review highlighted recent advances in pre-treatment and hydrometallurgical and biohydrometallurgical procedures for gallium and germanium recovery from EoL products, including spent liquid crystal displays (LCDs), light emitting diodes (LEDs), photovoltaics (PVs), and optical fibers (OFs) [16].

At the time of writing this manuscript (October 2024), the prices of these elements had risen dramatically in the last few years (

Table 1. Prices of gallium and germanium versus other strategic metals [17].

Metal	Current Price, USD/kg	Change from Last Year, %	Variation from January 2020, %
Dysprosum	403.2	−31.76	16.79
Neodymium	107.6	−4.95	65.54
Praseodymium	107.3	−4.88	47.45
Terbium	1496.8	−31.84	124.07
Gallium	911.4	20.59	205.63
Germanium	3885.6	36.85	90
Hafnium	4496.8	−3.44	187.2
Indium	687.2	22.36	118.08
Rhenium	2490	24.81	45.49
Iridium	USD 169.85/g	1.32	221.02

). Moreover, nowadays, the prices of gallium and especially germanium compare very well with others, i.e., rare earth elements, but none reached the high value of iridium.

All of the above is reflected by the fact that both elements are included in the fifth list of critical raw materials (CRMs) from 2023 for the European Union [2], although their inclusion in this list can be dated even earlier. Together with these two elements, others are also listed as “strategic raw materials”: cobalt, light and heavy rare earth elements, manganese, copper, nickel, etc.

Thus, the recovery and/or recycling of these two metals is of the utmost interest, given that hydrometallurgical processing is widely used to reach this goal. This manuscript reviewed the most recent advances (2024 year) in the use of the different methodologies aimed at the recovery of these valuable and strategic elements.

2. Research Methodology

References were taken from the Scopus “www.scopus.com” and Web of Science https://webofscience.clarivate.cn/wos/woscc/basic-search databases (both accessed on 1–30 September 2024) under the terms: gallium (germanium) recovery, gallium (germanium) and leaching, gallium (germanium) and liquid-liquid extraction, gallium (germanium) and ion exchange resins, and gallium (germanium) and adsorption, for the year 2024.

3. Gallium Recovery Operations

3.1. Leaching

Corundum flue dust (generated during bauxite smelting) is considered one of the potential sources of gallium and zinc. The recovery of both metals was investigated using a process that utilizes ultrasound to enhance the sulfuric leaching efficiency of both metals from this flue dust [18]. Under the various experimental conditions tested, it was found that the maximum gallium and zinc leaching efficiencies of 81.22% and 96.72% were obtained at optimum leaching conditions: temperature of 90 °C, reaction time of one hour, 3 M sulfuric acid, solid/liquid ratio of 1 g/5 mL, and ultrasound power of 750 W. However, the leaching effect on zinc was not improved by ultrasonic treatment, but the leaching efficiency on gallium was beneficial (more than 18% increase).

Fly ash was the source of gallium recovery in reference [19]. As a first step, the material was activated by Na₂CO₃ or K₂CO₃, forming the phases of kalsilite and (Na,K)AlSiO₄. Further, and under the acidic leaching (HCl medium), aluminum, lithium, and gallium were leached with respective rates of 86.17%, 89%, and 80% in the fly ash–sodium carbonate system, and efficiencies of aluminum (92.38%), lithium (95%), and gallium (83%) in the fly ash–potassium carbonate system. Based on previous work [20], it was mentioned that gallium can be recovered by adsorption methodology.

The recovery of gallium from coal fly ash (CFA) was described in reference [21]. Firstly, the CFA material was roasted at 800 °C for two hours in the presence of sodium carbonate. Then, the roasted material was leached in 1 M hydrochloric acid medium to produce a solution containing lithium (71.12% leaching efficiency), whereas gallium was leached with 18.14% efficiency. This gallium was adsorbed onto the H₂SiO₃ formed in this step. In a second acidic step (2 M HCl) of the gallium residue, the leaching efficiencies rated 83.93% and 2.63% for gallium and lithium, respectively. The use of citric acid, instead of hydrochloric acid, to leach the CFA was also compared. The results from this investigation are shown in

Table 2. Efficiencies (%) in the leaching of CFA with HCl or citric acid.

Acid	Li	Ga	REEs
HCl 0.5 M	43.9	1.5	24.8
Citric acid 0.5 M	80.1	80.2	71.5
HCl 2 M	90.4	86.8	80.3
Citric acid 2 M	79.8	80.8	78.9

Adapted from [21].

. These results indicated that though citric acid produced efficiencies similar to (or better than) HCl, the authors decided to choose the mineral acid because, in their own words (Conclusion section), “The simultaneous leaching efficiency of Li, Ga and rare earth elements in HCl environment was better than that in citric acid environment”. It was not mentioned how gallium was recovered from the acidic environment.

The leaching behavior of gallium and scandium from gibbsite bauxite was investigated [22]. Gallium was leached (73% efficiency) in a sodium hydroxide medium and temperatures above 120 °C. Scandium remained in the residue.

Using zinc powder replacement residue (ZPRR) as the source phase, one study investigated the recovery of both gallium (0.51 wt%) and germanium (0.15 wt%) contained in the residue [23]. The leaching process consisted of two stages. In the first stage, by employing 15 g/L sulfuric acid solutions, germanium was solubilized. In the second stage, the use of 150 g/L sulfuric acid allowed the dissolution of gallium. Moreover, the utilization of a strong magnetic field (10 T), ultrasonication power (90 W), and ultrasonic frequency (20 kHz) increased the rate of gallium and germanium recoveries (150 °C) to 95.53% and 97.11%, respectively.

GaAs scraps were used as a resource to recycle gallium [24]. The process included three main steps: alkaline oxidative leaching, cooling crystallization, and cyclone electrowinning. In the alkaline oxidative leaching (NaOH in the presence of H₂O₂), the gallium recovery rate reached 98.7%. The cooling crystallization yielded gallium solutions from which gallium was electrowon (99.993% purity) under a current efficiency of 34.43% and overall gallium recovery of nearly 90.5%.

An investigation was performed to obtain gallium from a carbon concentrate, where the carbon was a waste product of the aluminum industry [25]. Ashing (600 °C) of the concentrate eliminated the carbon, and from the resulting ashes, gallium was leached using acidic conditions. It was shown that the best dissolution rate (98%) was derived when 6 M hydrochloric acid solutions were used. From the leachate, gallium was uptook onto anion exchange resins (Purolite A300 and Purolite A500) and then quantitatively eluted with water. The anion exchange resin step (uptake–elution) allowed for the separation of gallium from the accompanying elements (Fe(III), Ni(II), Co(II), Zn(II), V(V), Al(III), Ca(II), Mg(II), Na(I) and K(I)). As a final step in the recovery process, NaOH (200 g/L solution) was added to the eluate, resulting in the cementation of gallium on aluminum gallana (liquid gallium–aluminum alloy).

Another work investigated the recovery of major metals from a ZnS concentrate [26], building a model to simulate the oxidative pressure leaching process. The model demonstrated that zinc, sulfur, indium, gallium, cadmium, cobalt, germanium, and copper could be recovered effectively (

Table 3. Efficiencies (%) in the recovery of the elements in the simulation.

Zn	S	In	Ga	Ge	Cu	Cd	Co
95.6	86.6	82.4	82.6	68.9	62.5	85.9	85.6

Adapted from [26].

). The main chemical inputs were sulfuric acid, oxygen, and lime, while zinc electrowinning constituted over 90% of the total electricity consumption. Among other recommendations, enhancement of iron removal processes, and exploration of alternative chemicals for Ga and Ge recovery were suggested.

The presence of gallium in NdFeB wastes not only serves as a resource for the recovery of rare earth elements (REEs) but also as a source for gallium recovery. Thus, a process including leaching, reduction, liquid–liquid extraction, precipitation, and calcination was developed [27]. Firstly, Pr, Nd, and Ga were leached in 5 M HCl medium and a liquid–solid ratio (mL/g) of 5:1 at 90 °C for 150 min, resulting in leaching rates of 98%, 99%, and 98% for Pr, Nd, and Ga respectively. As a second step in the process, Fe(III) was reduced in the leachate to Fe(II) by the use of iron powder. In the liquid–liquid extraction step, tri-butylphosphate (TBP) was used to extract gallium from the Pr, Nd, and Ga feed solution; in order to eliminate the formation of an undesirable third phase after extraction, 10% v/v iso-decanol was added to the organic phase as a modifier. Gallium was completely stripped from the Ga-loaded organic phase by the use of water as a strippant. From this stripped solution, gallium was finally obtained as Ga₂O₃ with a purity of 98.6%. In the whole process, the gallium recovery rate was around 93%.

The presence of gallium in brown corundum fly ash (BCFA) wastes resulted in an investigation to recover this strategic metal from these materials [28]. In the process, gallium was recovered by hydrothermal leaching and alkali regeneration while producing silica–potassium compound fertilizer (SPCF) and zeolite F. In the leaching step and under the use of 210 g/L KOH solutions, a liquid/solid ratio of 25 mL/g, and 160 °C during one hour, the extraction efficiencies of Ga, K, and Si were 95.91%, 51.78%, and 69.57%, respectively. The residue from this operation was further used as SPCF for agricultural production.

Spent surface-mounted devices with light-emitting diodes (SMDs LEDs) are a source for gallium (and indium) recovery. The thermal pre-treatment of such powders presented a gallium concentration of near 290 mg/kg, and it was subjected to acidic leaching (3 M HCl + 30% v/v H₂O₂) [29]. This operation allowed for the dissolution of gallium (97%), but also the presence of several impurities in the leachate (copper (29 g/L), iron (0.2 g/L), tin (3.2 g/L), etc.). Thus, an alternative by the use of desferrioxamin E (DFOE) as a leachate agent was investigated. By the use of this siderophore, gallium can be recovered (about 14%) at pH 4 and 14 days. Further, and employing advanced “GaLIophore technology” based on preparative columns, Ga/In-DFOE complexes were separated from other accompanying metals [30].

Since the leaching of red mud with sulfuric acid led to the adsorption of dissolved gallium onto polysilicic acid, a replacement of the above acid by phosphoric acid was investigated [31]. Under the conditions: 100 °C, six hours, a liquid/solid ratio of 9/1, and 10 M phosphoric acid, it was found that gallium was recovered into the solution with 95.9% efficiency. The residue of the leaching operation can be used for preparing mineral fertilizers.

3.2. Adsorption

Using catechol/polyethyleneimine as shell and KIT-6 as core, a series of KIT-6@x-catechol/polyethyleneimine composites (x = 18, 27, 36, 54, 72 mg catechol) was fabricated and used to remove gallium from synthetic solutions [32]. By the use of composites with x equal to 36, the percentage of gallium removed from the solution of pH 3 exceeded 80% (

Table 4. Adsorption efficiencies of KIT-6@x-CA/PEI.

Adsorbent, x =	a [Ga]e, mg/g
18	75.01
27	64.85
36	88.65
54	60.88
72	55.58

Feed solution: 50 mg/L Ga(III) at pH 3. Adsorbent: 5 mg. Time: one day. Temperature: 30 °C. ^a based on the Langmuir equation. Adapted from [32].

). The extraction mechanism responded to cation exchange between Ga³⁺, Ga(OH)²⁺, and Ga(OH)₂⁺ species in the solution and protons from the phenolic group in the composite. Moreover, chelation also occurred due to the coordination of the oxygen atom on the phenolic hydroxyl group of catechol. The best gallium elution efficiencies (around 90%) were reached using 0.1–0.5 M HCl solutions.

The residue of the acidic leaching operation on coal fly ash was subjected to a hydrothermal treatment (120 °C for one day) with the objective of synthesizing mesoporous silica materials [33]. The removal of gallium from the solution responded to ion exchange and chelation, with maximum gallium removal occurring at pH 8 (presence of Ga(OH)4⁻ species in the solution). Gallium was conveniently removed (91.71%) from the loaded adsorbent by the use of 0.1 M HCl solution.

Tetradentate amidoxime-functionalized MIL-53(Al) nanofiber membranes (PAN/MIL-53(Al)-AO) were prepared to investigate their performance in the removal of gallium (and other elements) from a solution [34]. In this case, maximum gallium removal was accomplished at pH 4, whereas desorption rate (90%) was reached using 0.1 M HCl. The adsorption operation responded to the next equation:

$$4\mathrm{AO}+{\mathrm{Ga}}^{3+}=\mathrm{Ga}{\left(\mathrm{AO}\right)}_4^{-}+4{\mathrm{H}}^{+}$$

(1)

Column adsorption experiments carried out for 12 h showed a water flux of PAN/MIL-53(Al)-AO over 95.49 L/m²⋅h, with a gallium removal rate of 96.66%.

Saltworks bitterns (the product after sodium chloride crystallization of saltworks) were used to investigate their performance in the recovery of gallium using commercial N-methylglucamine adsorbents (S108, CRB03, CRB05) under dynamic conditions [35]. The three adsorbents were useful in removing 90% of the element from the solution. Elution experiments, also performed on packed columns, indicated that gallium could be eluted by the use of 1 M HCl solutions.

The removal of gallium from synthetic solutions was investigated by impregnation of Amberlite XAD7-type polymeric resin with the amino acid DL-valine (C₅H₁₁NO₂) [36]. The best conditions for maximum gallium removal were established at pH > 9, reaction time: 1.5 h, and temperature: 45 °C. No elution data were given in the work.

Impregnation of P507 extractant (2-ethylhexyl 2-2ethylhexyl phosphate) (C₁₆H₃₅O₃P) onto coconut shell-derived mesoporous activated carbon produced P507@MAC adsorbent, which was used for gallium (and indium) recovery [37]. The maximum gallium removal (near 95%) from the aqueous feed occurred at pH 6. Moreover, experiments were performed on a leach solution derived from the leaching of CIGS photovoltaic modules. The leaching of the material with nitric acid resulted in a solution containing Al(III) (0.6 mg/L), Cd(II) (1.3 mg/L), Cu(II) (10.7 mg/L), Ga(III) (5.2 mg/L), In(III) (12.1 mg/L), Mg(II) (0.1 mg/L), and Zn(II) (36.5 mg/L). Separation of indium first occurred by adsorption onto the adsorbent at pH 1, followed by a second step in which gallium was removed, using fresh adsorbent, from the leachate at pH 2. The best gallium desorption efficiency (93%) resulted from the use of 0.5 M nitric acid solutions as eluant. In column experiments performed on synthetic solutions, the maximum adsorption capacity was 45.63 mg/g; these results fit with the Yoon–Nelson model.

To meet the requirements of Ga(III) selective adsorption from acid leaching of fly ash, a hydrophilic ion imprinted polymer based on graphene oxide (IIP-GO/PAA) was synthesized by using surface imprinting technology with Ga(III) as the template ion, and acrylic acid (AA) as the hydrophilic functional monomer [38]. Once the synthesis procedure finished, the material was washed with 1 M HCl (in order to eliminate the Ga(III) template ions), and after washing and drying, the resulting material was ready to use to remove Ga(III) from the solution. Maximum gallium removal was attained at pH 3, showing the imprinted adsorbent had good selectivity towards Ga(III) in the presence of Al(III), Fe(III), Mg(II), and Ca(II) (experiments were carried out on a synthetic solution simulating the acidic leachate of the ash, though no data about the pH of the solution were given in the published manuscript). Gallium(III) was loaded onto the imprinted polymer via a cation exchange reaction with the protons of the –COOH groups of acrylic acid. As somewhat expected, a 1 M HCl medium was used to remove Ga(III) from the metal-loaded adsorbent.

Aiming to recover thorium from aqueous industrial effluents, the synthesis of an adsorbent was investigated by (i) synthesizing an adsorbent bearing phosphonate groups and carbonyl/amide groups and (ii) integrating an ion-imprinting step in the synthesis [39]. However, this adsorbent also removed gallium(III) from a multielemental solution, where the removal was pH-dependent with a maximum for gallium at an equilibrium pH of 5.36. At this pH, the adsorption efficiency followed the order: Th > U > Ga > Si > Mn > Fe > Al > Na. Though no data were given in the manuscript about gallium elution, presumably, and based on the thorium results, it could be performed by acidic solutions, i.e., 0.3 M nitric acid.

Vanadium slag processing residue (VSPR) is another source for gallium recovery; thus, the recovery of this element from the residue was investigated by 4-amino-3-hydrazino-1,2,4-triazol-5-thiol (AHTZT)-modified graphene oxide adsorbent (GO-AHTZT) [40]. However, and despite the first sentence of this paragraph, it should be noted here that all the experimentation was performed on synthetic solutions. Gallium (III) removal was extremely sensitive to the pH of the solution (

Table 5. Influence of pH on Ga(III) removal.

pH	[Ga]e, mg/g
2	2.9
2.5	4.5
3	23.9
3.5	1.3

Feed phase: 50 mg/L Ga(III). Adsorbent dosage: 5 mg. Temperature: 25 °C. Adapted from [40].

), rating maximum at pH 3. In addition, 1 M NaOH solutions were selected to remove Ga(III) from the metal-loaded adsorbent. The performance of the adsorbent from Ga(III), Sc(III), and In(III) mixed solutions (resembling real VSPR) was also investigated, and the experimentation showed that the removal order was found to be Ga > Sc >> In (zero removal), but the as-obtained gallium adsorption capacity was much lower than that obtained from the single gallium-bearing solution. Thus, the coexistence of other metal ions will impede the migration of ions; this is attributable to an increase in the activation energy of ion migration and resulting decrease in the removal capacity of the adsorbent [41].

3.3. Ion Exchange Resins

Anion exchange AN-31 and D-403 resins (OH form) were used in batch and column experiments to investigate their performance on the removal of gallium(III) from alumina–alkaline solutions [42]. Batch experiments in solutions of pH 14 containing both gallium and aluminum showed that the removal of gallium from the solution increased with the increase in aluminum concentration in the alkaline feed phase. These results also showed that D-403 resin performed better than AN-31 resin, which was attributable to the higher basicity of the former resin. Gallium (III) uptake onto the resins responded to the following equilibrium:

$$\mathrm{R}-\mathrm{OH}+\mathrm{Ga}{\left(\mathrm{OH}\right)}_4^{-}\leftrightarrow \mathrm{R}-\mathrm{Ga}{(\mathrm{OH})}_4+{\mathrm{OH}}^{-}$$

(2)

In column experiments, the separation factors Ga/Al values were 150 and 100 for D-403 and AN-31 resins, demonstrating again the best performance of DN-403 with respect AN-31. Elution experiments showed that 2 M NaOH solutions conveniently removed the metal from gallium-loaded resin. In this case, the elution was represented by the following reactions:

$$\mathrm{R}-\mathrm{Ga}{\left(\mathrm{OH}\right)}_4+2{\mathrm{OH}}^{-}\to \mathrm{R}-\mathrm{OH}+\mathrm{Ga}{\left(\mathrm{OH}\right)}_5^{2-}$$

(3)

$$\mathrm{R}-\mathrm{Ga}{\left(\mathrm{OH}\right)}_4+3{\mathrm{OH}}^{-}\to \mathrm{R}-\mathrm{OH}+\mathrm{Ga}{\left(\mathrm{OH}\right)}_6^{3-}$$

(4)

where these gallium polycharged anions not retained by the resin. Due to the gallium concentration presented by the eluates (about 4.5 g/dm³), they were suitable for the electrolytic production of the metal.

Taking as a basis the recovery of gallium from a pressure leaching solution in a zinc hydrometallurgical plant, a hydroxamic acid chelating resin (HA-PDR) was synthesized by introducing hydroxamic acid groups into the pre-made polyacrylate resin (PDR) with a methyl ester group through the hydroxylamine reaction [43]. Batch adsorption experiments, carried out on synthetic solutions, showed that the removal of gallium from the solution increased with the increase in the pH from zero to 2.5. The metal loading onto the resin resulted from a combination of cation exchange of metal ions (Ga³⁺) with protons of the N-OH group and chelation with oxygen atoms of the C=O group of hydroxamic acid. Gallium could be eluted with 0.9–1.2 M sulfuric acid solutions.

In reference [44], the removal of gallium(III) by a strong acidic styrene cation exchange resin (JK resin) from a simulated Bayer solution (200 mg/L Ga, 120 mg/L V, 30 g/L Al and 3 M NaOH) was investigated by batch experiments. There is something strange with this investigation, because it is hard to understand why a strong acidic cation exchange resin (used to remove cations) was used to extract anionic metal species (Ga(OH)₄⁻) from alkaline solutions.

Again, to reach the goal of gallium removal from simulated Bayer solutions, an amidoxime-grafted polyacrylonitrile-styrene–divinylbenzene (A-PSD) resin was prepared by suspension acrylonitrile-styrene–divinylbenzene resins and subsequent electrophilic addition of hydroxylamine hydrochloride with amidoxime [45]. Gallium uptake responded to the reaction of Ga(OH)₄^- species with the protons of the C=N-OH group and formation of C=N-OGa(OH)₃ species in the resin. The problem then was that the oxygen was electrostatically unbalanced. Desorption was carried out with 1 M HCl.

Reference [46] investigated the removal of gallium (and vanadium) from Bayer mother liquor. The removal of both metals from the solution was investigated by the use of LSC-600S resin (OH form). Since both elements were uploaded onto the resin, the strategy was to elute both in an acidic medium (no data were given in the manuscript) followed by the selective precipitation of vanadium and the electrolytic recovery of gallium to yield Ga₂O₃. The loading of gallium (and vanadium) onto the resin was attributed to the formation of species that had the same unbalancing situation as that in the previous reference.

Though it is difficult to establish comparisons between the different adsorption/resins used to remove gallium from the solutions, mainly due to the very different experimental conditions used in the investigations,

Table 6. Different Ga(III) maximum loadings using adsorbent and ion exchange resins.

Adsorbent/Resin	a [Ga(III)]m, mg/g	Ref.
KIT-6@36-CA/PEI	88.65	[32]
Mesoporous silica	386.84	[33]
PAN/MIL-53(Al)-AO	231.08	[34]
Amberlite XAD-7/DL-valine	28.86	[36]
II-GO/PAA	236.2	[38]
GO-AHTZT	41.4	[40]
HA-PDR	27.7	[43]
A-PSD	14.67	[45]
LSC-6005 (OH form)	38.46	[46]

^a Based on the Langmuir isotherm model.

summarizes some of the results derived from the use of the various adsorbents or ion exchange resin systems reviewed in this work.

Table 5 shows the wide range of concentrations reached within the various adsorbents/resins used to uptake gallium(III) from the different aqueous media.

3.4. Liquid–Liquid Extraction

Reference [47] is not of practical use since the authors dissolved TBP (tri-n-butyl phosphate) extractant in dichloromethane (CH₂Cl₂). Dichloromethane is toxic and a recognized carcinogenic [48,49].

Ionic liquids are chemicals that, due to their properties: tuneable nature to achieve higher selectivity, superior physicochemical properties, diverse structural properties, etc., are considered green solvents and used in a number of applications including the separation of metals. Among the various types of ionic liquids, phosphonium ionic liquids (basically, quaternary phosphonium salts) have been used in the recovery of gallium [50]. Several phosphonium-based ionic liquids have been used either to leach gallium from GaAs semiconductors or as extractants mainly from HCl solutions. One study described the use of [P₄₄₁₀⁺][Br⁻] (tributyldecylphosphonium tribromide) as a leaching reagent from Ga(III) in a bromide medium; this metal completely solubilizes as [P₄₄₁₀⁺][GaBr₄⁻] species after five hours, and at the very same time, arsenic(III) and indium(III) also are completely co-leached with gallium [51]. Two other quaternary phosphonium salts were used as extractants for the recovery of gallium(III) from Zener diodes and LCD screens (Cyphos IL104, trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate) or Cyphos IL101 (trihexyl(tetradecyl)phosphonium chloride) to investigate the extraction of gallium(III) from 2 M HCl medium. It was described that using both extractants, the species formed in the organic phase responded to [R₃R′P⁺][GaCl₄⁻] stoichiometry. Cyphos IL 104 extracted the metal with a 99.8% efficiency at 3 M HCl [52], whereas in the case of Cyphos IL101, the extraction percentage also exceeded 99% [53].

One type of extractant is named solvation reagents, and in this class, ketones and ethers are found. The next investigation used these two types of extractants in the recovery of gallium from concentrated HCl solutions [54]. Metal extraction was greatly dependent on the HCl concentration in the feed solution (

Table 7. Percentages of gallium extraction using different solvation extractants.

Extractant	HCl 3 M	HCl 5 M
1,2-dimethoxybenzene	30	99
Methyl isobutylketone	90	99
2-nonanone	30	99
Dibutyl carbitol	25	90
Valerophenone	nil	90
Propiophenone	55	85

Feed phase: 0.07 g/L Ga(III). Organic phase. Undiluted extractants. O/A ratio: 0.2. Adapted from [54].

), increasing with the increase in the acid concentration from nil to 5 M. As expected from these results, and using 2-nonanone as an extractant, gallium stripping was well performed with low HCl concentration (0.01–1 M) solutions and with water (89.8% efficiency).

This investigation not only used phosphonium ionic liquids but also ammonium (quaternary ammonium salts) ionic liquids to model (UNIQUAC model with the extended Pitzer–Debye–Huckel (PDH*) framework) parameters in the liquid–liquid extraction of gallium from HCl media [55]. The extractants used in the investigation were Cyphos IL101, ALi-CY, ALi-D2, and ALi-PC (Ali: Aliquat 336 quaternary ammonium salt; CY: Cyanex 272, organophosphinic acid; D2: PC-88A, organophosohonate; PC: D2EHPA, organophosphoric acid). Extraction of Ga(III) from single metal solutions exhibited deviation between 0.5 and 0.8%, and using binary metal mixtures (Ga/Cu, Ga/Zn, Ga/Fe(III) and Ga/In), the values ranged from 0.85 to 1.5%.

3.5. Miscellaneous Operations

Ion flotation technology has been applied using rhamnolipid biosurfactant as an ion collector to recover gallium [56]. Experiments showed that the sole use of the biosurfactant produced foams of high stability; thus, 1,2-decanediol was added to induce foam instability and improve concentrate collection. Near 80% gallium was removed at pH 7 using the mixture rhamnolipid/1,2-decanediol.

A process involving microwave pyrolysis, thermal oxidation, and thermal chlorination technologies was used to recover copper, indium, gallium, and selenide from CIGS solar cells [57]. Microwave pyrolysis produced nearly 90% weight loss of the cells, where the temperature requirements for this operation were lower than in conventional pyrolisis. Selenium was completely recovered by thermal oxidation at 800 °C and 0.5 h. Gallium was recovered (56% effectivity) by thermal chlorination at 260 °C, whereas the metal was completely recovered by increasing the temperature to 380 °C and after 2.23 h. The complete recovery of indium needed 580 °C and 1.79 h.

The next work investigated the recovery of the metals on a gallium–indium eutectic used as a surface coating to induce aluminum reactivity in water [58]. It was experimentally found that the addition of imidazole (0.02 M) to seawater led to rapid reactions being completed in under 10 min, allowing for the recycling of nearly 90% of the gallium–indium eutectic and producing 99% of the anticipated hydrogen output based on the mass of aluminum. Furthermore, the usage of elevated temperatures favored the swift and complete reaction of aluminum in saltwater.

Coal gangue and fly ash are coal-based solid wastes, and both are considered aluminum and gallium resources. Simulating leaching solutions (nitric acid medium) from these materials [59], an investigation was developed in order to separate gallium and iron from aluminum by the use of cupferron solutions in water to precipitate both gallium and iron. The efficiency in the precipitation from a solution containing 34 g/L Al, 0.5 g/L Fe(III), and 0.07 g/L Ga pH: 0.5–0.9 was found to be 99.1%, 99.9%, and 4.7% for gallium, iron, and aluminum, respectively. Cupferron can be regenerated (nearly 82%) by the use of 8 M ammonium hydroxide solutions; from this regeneration operation, gallium and iron(III) were separated.

Another work investigated the pre-enrichment of the metals contained in fly ash from the Togtoh Power Plant in Inner Mongolia [60]. While gallium was primarily found in the glass phase and in corundum, with concentrations from 43.9 to 81.9 μg/g, the second-row fly ash presented the highest contents of gallium and other accompanying metals, being in this case not necessary for the pre-processing to achieve the extraction of these elements. Physical separation could further concentrate Li, Ga, Nb, lanthanides, and yttrium in fly ash. Particularly, particle size separation concentrated these elements in the <20 μm size range, and magnetic separation was favorable to the recovery of gallium together with lithium, niobium, and all the lanthanides but cerium.

4. Germanium Recovery Operations

4.1. Leaching

One investigation presented results on the leaching of liquation-feeding furnace dross using sulfuric and oxalic acid solutions [61]. The dross contained mostly zinc (68.0% wt.) but also an elevated germanium concentration (0.68% wt.). The influence of temperature, time, initial acid concentration, and the liquid/solid phase ratio (L/S) was examined. It was found that germanium availability via leaching was limited (

Table 8. Germanium recovery efficiencies from liquation-feeding furnace dross.

Acid	T, °C	Time, h	[Acid], % wt.	L/S	% Recovery
Sulfuric	80	2	15	25/1	60
Oxalic	80	3	12.5	10/1	57

Adapted from [61].

), with recoveries not exceeding 60%. It was stated that the preprocessing of dross before leaching was necessary.

In reference [62], a complete ash valorization process for integrated gasification with combined cycle fly ash was proposed. Germanium was first extracted from fly ash using water, with an extraction yield of 85% (after 24 h), and subsequently, the leached fly ash was used in the manufacture of fire-resistant boards containing 60% ash, thereby avoiding its disposal in a landfill. After this treatment, the final effluent was considered to achieve a zero-discharge objective.

In the leaching process of secondary zinc oxide, there is a problem of germanium loss caused by the colloidal adsorption of germanium by iron hydroxide (Fe(OH)₃) formed by Fe³⁺ hydrolysis. To avoid this issue, the next reference investigated the hydrolysis conditions of Fe³⁺ and the adsorption mechanism of germanium onto colloidal Fe(OH)₃ [63]. The coexistence of Fe³⁺ and H₂GeO₃ requires high acidity conditions (pH < 1.53 at 25 °C). During leaching, the adsorption of germanium by colloidal Fe(OH)₃ could be inhibited by increasing the reaction temperature, controlling the pH value of the solution system, and suppressing the formation of Fe³⁺ in the feed solution by adding a reducing agent.

Another work seemed to be a continuation of the previous reference using ultrasonic enhanced reduction leaching of zinc suboxide to optimize leaching conditions [64]. The optimized conditions for the ultrasonic enhanced reduction leaching process were found to be 85 °C, FeS of 0.6% zinc suboxide mass, 150 g/L sulfuric acid, and 300 W of ultrasonic power. The leaching efficiency of germanium can reach 91.34% under these conditions, exhibiting an improvement of 8.51% compared with conventional conditions. Moreover, the Fe³⁺ concentration in the leaching solution was maintained at 15 mg/L, satisfying the requisite criteria for germanium precipitation. Both the conventional and ultrasonic leaching processes are governed by internal diffusion and fit the Drozdov kinetic model:

$${\displaystyle \frac{1}{\mathrm{t}}}\ln \left({\displaystyle \frac{1}{1-\mathrm{x}}}\right)-\left({\displaystyle \frac{\mathrm{x}}{\mathrm{t}}}\right)\mathsf{\gamma }=\mathrm{k}$$

(5)

In the above Equation (5), x represented the leaching efficiency, γ the self-resistance coefficient, k the reaction time constant, and t the leaching time.

The next reference investigated the leaching behavior of germanium in a zinc oxide dust from a Pb–Zn smelter located in Sichuan Province, China [65]. Mineralogical analysis of the Ge-containing zinc oxide dust, combined with sulfuric acid leaching tests, revealed negative correlation between iron and silicon content in the dust and the leaching rate of germanium. According with these results, the leaching behavior of germanium and its occurrence state in the leach residue was further studied using mixed zinc oxide dust (1179.4 g/t germanium). It was difficult to dissolve Ge-containing insoluble phases in the sulfuric acid system, and germanium primarily existed in silicate and insoluble solids (mainly zinc ferrite) with the leach residue, accounting for 69.74% and 18.42% of its distribution, respectively. Fe and Si insoluble phases from the residue were decomposed through a three-stage enhanced leaching process: (i) leaching with 160 g/L sulfuric acid of the mixed zinc oxide dust, (ii) leaching of the residue of step (i) with 160 g/L hydrochloric acid, and (iii) leaching of the residue of step (ii) with 40 g/L hydrofluoric acid. The results indicated that the leaching rate of germanium increased to 95.98% by dissolving insoluble solids such as zinc ferrite and disrupting the lattice of silicate minerals. No data were provided about the treatment of the solution containing germanium in the last step (iii), which also contained appreciable concentrations of zinc and iron.

A product, germanium-bearing iron cake (GBIC), produced through the germanium removal process in a zinc smelter was used as starting material for the recovery of zinc and germanium [66]. The investigation introduced a roasting–leaching process in which willemite (Zn₂(SiO₄)) transformed into ammonia-soluble hemimorphite (Zn₄Si₂O₇(OH)₂·H₂O), favoring zinc leaching. The optimal roasting conditions included a particle size range of 49–74 mµ, a temperature of 490 °C, and a reaction time of four hours. Under optimized leaching conditions: [NH₄⁺+NH₃] (7 M), NH₄⁺/NH₃ molar ratio (2.5), temperature (50 °C), liquid/solid ratio (20 mL/g), and time (3 h), zinc could be separated from germanium. The source for ammonium ions influenced the separation factor Zn/Ge (

Table 9. Approximate separation factors (β_zn/Ge) at various sources of ammonium ions.

Chemical	ΒZn/Ge
NH3	470
NH3+NH4Cl	883
NH3+(NH4)2SO4	993
NH3+(NH4)2CO3	948
NH3+NH4HCO3	678

Adapted from [66].

), and though ammonium sulfate had a slightly greater separation factor value than ammonium carbonate, the latter was preferred because, with its use, the presence of impurities in the leachate was lower than with the former salt. Zinc and germanium had leaching rates of 84.92% and 0.59%, respectively. After three cycles of leaching, the Zn concentration in leachate increased to 20 g/L, facilitating the subsequent recovery of zinc. The germanium concentration in the germanium-bearing leaching residue increased by 1.5 times compared with the initial GBIC, though no data were given in the publication about how this germanium could be recovered from this residue.

The presence of germanides makes its recovery from zinc oxide dust difficult (0.20 wt% Ge). Thus, a study investigated [67] an ultrasonic-enhanced oxidation leaching procedure in which potassium permanganate and ultrasonication were introduced to favor the dissolution of sulfide species. Conditions for this operation were an initial sulfuric acidity of 160 g/L, a KMnO₄ concentration of 32.60 g/L, a liquid/solid ratio of 6 mL/g, a temperature of 90 °C, a reaction time of 30min, and an ultrasonic power of 480 W. Next, and in the roasting operation on the residue of the previous operation, the addition of Na₂CO₃ and Mg(NO₃)₂ promoted the formation of insoluble tetrahedral GeO₂ and complex germanium-containing compounds. At the same time, SnO₂ and zinc ferrite reacted with the sulfur produced in the ultrasonic-enhanced oxidation leaching and releasing germanium into its lattice. The conditions used in this operation were summarized as a roasting temperature of 800 °C, a roasting time of 30 min, and an amount of Na₂CO₃ and Mg(NO₃)₂ of 0.060 g/g and 0.050 g/g, respectively. In the final step, the roasting slag was leached using the conditions to dissolved germanium, which were a liquid/solid ratio of 3 mL/g, a reaction time of 20 min, an initial acidity of 60 g/L, and a temperature of 90 °C. The final recovery rates of Zn and Ge were 89.93% and 95.46%, respectively, though no data were given on how these elements were recovered or separated.

In order to avoid high carbon emissions and the loss of humic acid in the conventional process of germanium recovery from coal fly ash produced by the combustion of germanium-rich lignite in an industrial process, the next work utilized, on germanium-rich lignite, the co-extraction and utilization of germanium and nitrogen-rich humic acid (NRHA) [68]. These two compounds were co-extracted by ammonoxidation of lignite; the operation was performed at 80 °C for 3 h, with dissolution rates of 26.74 wt% (Ge) and 38.20 wt% (NRHA). Repeating the operation three times yielded 42.05 wt% Ge and 54.85 wt% NRHA recoveries. The acid was identified as a potential material to be used as biofertilizer or nitrogen doped carbon materials. No data about the further recovery of germanium were provided.

4.2. Adsorption

As it is mentioned in [35], the usage of commercial N-methylglucamine adsorbents (S108, CRB03 and CRB05) was investigated in the removal of critical raw materials presented in solar saltworks brinnes or bitterns. Under experimentation on synthetic bitterns, germanium was removed (batch experiments) from the solution within one hour, except in the case of S108, which needed two hours. Column experiments showed that the presence of impurities reduced the treatment capacity, whereas desorption was performed using 1 M HCl solutions, reaching a germanium concentration factor in the eluate of 35.

Due to their complexation abilities with metals, siderophores are a series of ligands for application in recycling and bioremediation technologies. The important number of these ligands (not less of 250) and their selection against the recovery of targeted metals, i.e., germanium, makes the density functional theory of assistance in choosing the best ligand. In the work [69], results of the calculations were compared to the reference siderophore desferrioxamine B. The experimental validation of the procedure was tested with a couple of these ligands: fimsbactin A and agrobacting, showing at least sevenfold better germanium binding than the reference siderophore.

Fixed-bed column experimentation was used to investigate the performance of functionalized polystyrenic beads with catechol (A-Cat), nitro-catechol (A-Cat-N), and pyrogallol (A-Py) in germanium recovery [70]. All three compounds showed germanium loadings greater than 25 mg/g, while the element was conveniently desorbed with 2 M HCl solutions from the germanium-loaded adsorbents. The reactions describing germanium upload onto the adsorbents were

$$\mathrm{Ge}{(\mathrm{OH})}_4+{\mathrm{H}}_2\mathrm{L}=\mathrm{Ge}{\left(\mathrm{OH}\right)}_2\mathrm{L}+{\mathrm{H}}_2\mathrm{O}$$

(6)

$$\mathrm{Ge}{(\mathrm{OH})}_4+{\mathrm{H}}_2\mathrm{L}=\mathrm{Ge}\left(\mathrm{OH}\right){\mathrm{L}}_2^{-}+2{\mathrm{H}}^{+}+2{\mathrm{H}}_2\mathrm{O}$$

(7)

$$\mathrm{Ge}{(\mathrm{OH})}_4+3{\mathrm{H}}_2\mathrm{L}={\mathrm{GeL}}_3^{2-}+2{\mathrm{H}}^{+}+4{\mathrm{H}}_2\mathrm{O}$$

(8)

with catechol forming the 1:1, 1:2, and 1:3 complexes and nitrocatechol and pyrogallol favoring the formation of both 1:1 and 1:3 complexes.

An adsorbent (DG-UiO-66) for the recovery of germanium was prepared by grafting D-anhydrous glucose onto UiO-66-NH₂ [71]. For both adsorbents, maximum germanium adsorption was found at pH values greater than 10, with better performance in the case of the DG-UiO-66 adsorbent. Metal adsorption included ion exchange and chelation. To desorb the metal, mixed solutions of 10% thiourea and 1% sulfuric acid were used.

Based on mucic acid, a metal–organic framework adsorbent (Ma-Zr-MOF) was synthesized [72]. The maximum adsorption capacity (82.06 mg/g) was reached at 25 °C and pH 6.0, increasing to 95 mg/g at 45 °C. Based on the experimental analysis and density functional theory calculations, it was concluded that the ortho-hydroxyl group played a key role in the adsorption of germanium and that the adsorption mechanism included chelation and ion exchange. Desorption was investigated using nitric acid or sodium hydroxide solutions (though the concentrations used in the experiments were not given in the work). With both desorbents, germanium was recovered in the solution, but the adsorbent lost efficiency under continuous use, which was more significant in the case of the NaOH medium (

Table 10. Adsorption–desorption cycles using Ma-Zr-MOF adsorbent.

Cycle	HNO3	NaOH
1	88.5–85.6	88.5 – <10
2	84.6–74.6	40 – <10
3	76.7–72.9	20 – <5
4	72.2–65.4	-
5	65.7–61.6	-

Adapted from [72].

).

A trihydroxyl functionalized titanium dioxide (TiO₂-NH₂-3OH) was synthesized by the Schiff base reaction method and applied to germanium recovery from coal acid leaching solutions (though the experiments were carried out on synthetic solutions) [73]. The best adsorption results were obtained at pH 3 with an optimal adsorbent dosage of 8 g/L. Germanium was effectively separated from Zn(II), Co(II), Mn(II), Ni(II), and Al(III)). Germanium loading onto the adsorbent was due to the ion exchange interaction between OH and Ge(OH)₄ on the adsorbent surface. Desorption was performed with 0.5 M HCl solutions.

4.3. Ion Exchange Resins

The next reference used D201×7 in the removal of germanium from a sulfuric solution using tartaric acid as a complexing agent [74]. This resin is a strongly basic styrene-based anion exchange resin with quaternary ammonium ((CH₃)₃Cl-N⁺) groups and Cl⁻ acting as a counter anion, and thus was the anion to be exchanged with the anionic metal complex. The use of tartaric acid allowed for the formation of (GeO₂(OH)₂C₄H₄O₄)²⁻ species in the solution, thus allowing its uptake onto the resin. This uptake was pH dependent, where the maximum germanium loading was reached in the 2–3 pH value range. Different solutions were investigated to elute germanium from the metal-loaded resin; from all of them, 1 M NaOH solutions presented the best elution data (in excess of 80%). Column experiments showed that the eluted solution presented germanium concentrations as high as 36 g/L. However, these column results were confusing since the author used 5 M NaOH solutions to elute germanium, and the authors stated that at pH 2 (uptake experiments), iron was present in the solutions as Fe(II), when at pH 0, they wrote that iron was present as Fe(III).

Similarly to the case of gallium(III) (Table 6),

Table 11. Various Ge(IV) maximum loadings using adsorbents or an ion exchange resin.

Adsorbent/Resin	a [Ge(IV)]m, mg/g	Ref.
OG-UiO-66	250.04	[71]
UiO-66-NH2	216.21	[71]
Ma-Zr-MOF	82.06	[72]
TiO2-NH2-3OH	72.94	[73]
D201×7 resin	212	[74]

^a Based on the Langmuir isotherm model.

summarizes some results relative to the Ge maximum loadings onto different adsorbents and resins.

From the results shown in Table 11, the variety of results found in the investigations can be seen, which probably are in close resemblance with the different experimental conditions used in each work.

4.4. Liquid–Liquid Extraction

Among various systems [15], the extraction of germanium from a zinc-refinery residue leachate was described using Aliquat 336 in bisulfate cycle ionic liquid. Germanium was stripped from the metal-loaded organic phase using 2 M NaOH as the strippant. After this stripping operation, the ionic liquid needed to be regenerated to the original bisulfate form.

Another manuscript investigated the extraction of germanium(IV) by the tertiary amine N235/TBP system but under the formation of different metal-coordinated complexes in the aqueous feed solution [75]. The coordination abilities followed the next sequence: tartaric acid > oxalic acid > citric acid > maleic acid > salicylic acid, and thus the metal extractability into the organic phase.

An organic phase composed of 3% C7-9 hydroxamic acid (YW100), 15% di(2-ethylhexyl) phosphoric acid (D2EHPA), 35% tri(octyl-decyl) amine (N235), and 47% kerosene was used to extract germanium from a germanium slag sulfuric acid leaching solution [76]. The metal-loaded organic phase was stripped by 1 M NH₄F solution. The germanium extraction and stripping rates were 99.4% and 96.2%, respectively, after three extraction and stripping stages.

4.5. Miscellaneous Operations

In the recovery of germanium from coal, the formation of GeO₂-SiO₂ solid solution makes the recovery of this metal difficult; thus, a procedure involving the enrichment of germanium in lignite by gravity separation and low-temperature sintering was investigated [77]. In this procedure, gravity separation produced a metal concentration factor in lignite of near 2, whereas the treatment of the sinter at 300–500 °C produced a residual ash containing 1.8 g Ge/kg, with a concentration factor in the order of 10.6. As a consequence of the above procedure, more than 90% of germanium in lignite could be recovered by chlorinated distillation.

In the combustion of lignite to produce electricity, coal ash wastes were generated. Due to the presence of arsenic, germanium, and tungsten in these wastes, sequential recovery of these elements using vacuum distillation was investigated [78]. Arsenic was first volatilized at temperatures under 550 °C; then, germanium and tungsten were volatilized as sulfides by the addition of sodium sulfite. The best conditions for this removal were summarized as 1050 °C, a mass ratio of 0.6, a pressure of 1 Pa, and two hours of reaction time. The condensed products were As₂S₃ and GeS both for coal fly and coal bottom ashes, whereas WO_x (x < 3)/WS₂ and WO₃/WS₂ species were present in the above coal fly and coal bottom ashes, respectively.

With the objective to produce ultrapure germanium (7N), vacuum distillation purification of zone-fused germanium tailings (99.98%) to separate low-boiling-point trace impurities such as Zn, In, and Mg was used [79]. The results of the investigation showed that after the operation, the impurity content was below 0.05 ppm (removal rate 96.1%) for all elements except arsenic (0.87 ppm), resulting in germanium with 99.9995% purity.

5. Conclusions

As in the case of many strategic metals, gallium and germanium commercial recycling systems are not under consideration due to the low metal concentrations in major applications, and, apparently, resources in end-of-life final products tend to be wasted. Since the world demand for these two strategic metals will continue to increase in the next years, it is of the utmost importance to improve collaborations between industries, academia, and scientists so that gallium and germanium recycling can be increased. These collaborations are the key to reaching a sustainable metals supply and improving the world’s transition towards low carbon emissions.

Since the leaching operation used very different experimental conditions, it is not easy to perform a direct comparison between the different results reviewed in this work. In the case of separation technologies (including adsorption, resin ion exchangers, and liquid–liquid extraction), one can generalize some special features of each one of these operational units (

Table 12. Differential points of the reviewed separation technologies.

Operation Unit	Operational Time	Metal Concentration	Selectivity	Feed Solution	pH Solution	Problematic Phases Separation
Adsorption	min/h	10−3–103 mg/L	average–good	clarified/non-clarified	0–14	no
Resin ion exchange	min/h	10−3–103 mg/L	average–good	clarified/non-clarified	0–14	no
Liquid–liquid extraction	min	101–105 mg/L	good	clarified	0–14	yes

).

Some focal points to be addressed are listed below (without any special order):

Ion impregnation processing presented a clear feature due to its possibilities to adsorb and separate target ions from multi-metallic systems. Improvements in their fabrication processes are one of the research hotspots in the few next years.

Due to the possibilities of electrochemical processes to yield pure metals, the scaling up of them is also of interest.

Based on the applications of molecular recognition technology, developments needed to be made for adsorbent design with selective recovery of these (and others) strategic metals.

Though a number of attempts have been made to separate targeted metals from multi-elemental leaching solutions, highly selective extractants (including ionic liquids, see below) and ion exchange resins are still one research priority.

The rational development of ionic liquids and task-specific ionic liquids, their recycling, process scaling-up, and detailed economic viability investigations need to be developed to accelerate the potential commercialization of these chemicals in the selective extraction of these critical elements.

Improvements in the optimization and implementation of the e-waste recovery protocols (mostly investigations of separation technologies on real leaching solutions) will favor circularity and thus the recycling of these strategic and valuable metals.