Separations of Strategic Metals from Spent Electronic Waste Using “Green Methods”

Abstract

Next-generation recycling technologies must be urgently innovated to tackle huge volumes of spent batteries, photovoltaic panels or printed circuit boards (WPCBs). Current e-waste recycling industrial technology is dominated by traditional recycling technologies. Herein, ionic liquids (ILs), deep eutectic solvents (DESs) and promising oxidizing additives that can overcome some traditional recycling methods of metal ions from e-waste, used in our works from last year, are presented. The unique chemical environments of ILs and DESs, with the application of low-temperature extraction procedures, are important environmental aspects known as “Green Methods”. A closed-loop system for recycling zinc and manganese from the “black mass” (BM) of waste, Zn-MnO₂ batteries, is presented. The leaching process achieves a high efficiency and distribution ratio using the composition of two solvents (Cyanex 272 + diethyl phosphite (DPh)) for Zn(II) extraction. High extraction efficiency with 100% zinc and manganese recovery is also achieved using DESs (cholinum chloride/lactic acid, 1:2, DES 1, and cholinum chloride/malonic acid, 1:1, DES 2). New, greener recycling approaches to metal extraction from the BM of spent Li-ion batteries are presented with ILs ([N_8,8,8,1][Cl], (Aliquat 336), [P_6,6,6,14][Cl], [P_6,6,6,14][SCN] and [Benzet][TCM]) eight DESs, Cyanex 272 and D2EHPA. A high extraction efficiency of Li(I) (41–92 wt%) and Ni(II) (37–52 wt%) using (Cyanex 272 + DPh) is obtained. The recovery of Ni(II) and Cd(II) from the BM of spent Ni-Cd batteries is also demonstrated. The extraction efficiency of DES 1 and DES 2, contrary to ILs ([P_6,6,6,14][Cl] and [P_6,6,6,14][SCN]), is at the level of 30 wt% for Ni(II) and 100 wt% for Cd(II). In this mini-review, the option to use ILs, DESs and Cyanex 272 for the recovery of valuable metals from end-of-life WPCBs is presented. Next-generation recycling technologies, in contrast to the extraction of metals from acidic leachate preceded by thermal pre-treatment or from solid material only after thermal pre-treatment, have been developed with ILs and DESs using the ABS method, as well as Cyanex 272 (only after the thermal pre-treatment of WPCBs), with a process efficiency of 60–100 wt%. In this process, four new ILs are used: didecyldimethylammonium propionate, [N_10,10,1,1][C₂H₅COO], didecylmethylammonium hydrogen sulphate, [N_10,10,1,H][HSO₄], didecyldimethylammonium dihydrogen phosphate, [N_10,10,1,1][H₂PO₄], and tetrabutylphosphonium dihydrogen phosphate, [P_4,4,4,4][H₂PO₄]. The extraction of Cu(II), Ag(I) and other metals such as Al(III), Fe(II) and Zn(II) from solid WPCBs is demonstrated. Various additives are used during the extraction processes. The Analyst 800 atomic absorption spectrometer (FAAS) is used for the determination of metal content in the solid BM. The ICP-OES method is used for metal analysis. The obtained results describe the possible application of ILs and DESs as environmental media for upcycling spent electronic wastes.

1. Introduction

Current e-waste recycling industrial technology is dominated by traditional methods with high energy consumption in pyrometallurgical processes, using strong inorganic acids, such as H₂SO₄, HCl and HNO₃, or some organic acids, such as citric acid or tartaric acid, in hydrometallurgical processes. A short review is presented on organic solvent leaching with different oxidizing additives and surfactants at low temperatures to extract valuable metals from the waste of different batteries and printed circuit boards (WPCBs).

The following solvents were used to recover Zn and Mn metals from the “black mass” (BM) of spent zinc batteries: trihexyltetradecylphosphonium chloride, [P_6,6,6,14][Cl], (Cyphos IL 101), trihexyltetradecylphosphonium thiocyanate, [P_6,6,6,14][SCN], bis(2,4,4-trimethylpentyl)phosphinic acid, Cyanex 272, and bis-(2-ethylhexyl)phosphoric acid, D2EHPA. A flow chart of the best method of recovery for zinc and manganese has been presented [1]. A leaching efficiency of 100 wt% for Zn(II) was obtained through a combination of {Cyanex 272 + diethyl phosphite, (DPh)}, as well as 100 wt% for Zn(II) and Mn(II) using DESs (choline chloride, [N_2OH,1,1,1][Cl], ChCl:lactic acid, 1:2, DES 1, and ChCl:malonic acid, 1:1, DES 2) with the addition of the didecyldimethylammonium chloride surfactant DDACl [1]. The electrochemical recovery of zinc from acidic aqueous solutions was described. For all the procedures, the O/A ratio, temperature, time of extraction, pH and selectivity and distribution ratios were presented [1].

Details regarding the metal extraction behaviour of the BM of spent Li-ion batteries were presented using ILs ([N_8,8,8,1][Cl], [P_6,6,6,14][Cl], [P_6,6,6,14][SCN] and [Benzet][TCM]), eight DESs and two popular organophosphorous-based acids, Cyanex 272 and D2EHPA [2]. Much better results were obtained, at 90–100 wt% recovery of Co(II), Ni(II) and Li(I), using DESs than those obtained with ILs. The extraction efficiency using [P_6,6,6,14][SCN] or mixtures of two ILs showing synergistic effects at a low temperature, T = 303 K, was very low. The mixture of {[P_4,4,4,14][Cl] + NaCl + H₂O + BM} (ABS method—Aqueous Biphasic System), with the addition of H₂O₂ and DDACl, revealed a very low extraction efficiency for Co(II), at 39.6 wt%, and a successful 90.8 wt% efficiency for Li(I) at pH = 11. The results indicated promising extraction for Li(I) (41–92 wt%) and Ni(II) (37–52 wt%) using (Cyanex 272 + DPh). A complete flow chart for the selective recovery of Co(II), Li(I) and Ni(II) from the BM was presented [2]. Only DESs miscible with water, DES 2 and (ChCl:phenylacetic acid, 1:2) or (benzethonium chloride, [Benzet][Cl]: lactic acid, 1:2), showed better extraction efficiency and were proposed for the extraction of Co(II), Li(I) and Ni(II) with high extraction efficiency [2]. The use of Cyanex 272 and D2EHPA with different additives showed an extraction efficiency of Co (II), Li(I) and Ni(II) at a level of ca. 100 wt% [2]. In the following work, new DESs and bi-functional ILs were used for the leaching of valuable elements from spent Li-ion portable batteries (LiPBs) [3]. The leaching and separation process of Co(II), Ni(II), Cu(II), Li(I) and Mn(II) from the BM, obtained from LiPBs with different solvents, were presented [3]. The results of extraction with six DESs {DES 1, DES 2, ChCl + succinic acid (1:1), glutaric acid (1:1) and citric acid (1:1) as well as citric acid (2:1)} with different additives, such as DDACl, H₂O₂, trichloroisocyanuric acid (TCCA), sodium dichloroisocyanurate (NaDCC), pentapotassium bis(peroxymonosulphate) bis(sulphate) (PHM), (glycine + H₂O₂) or (glutaric acid + H₂O₂) were presented [3]. TCCA is well known as an inexpensive oxidant for the disinfection of swimming pools and is used in the processes of the extraction of metal ions.

The recovery of metals from the BM of waste Ni-Cd batteries was also under our investigations [4]. The extraction efficiency of DES 1 and DES 2 was much better than those observed for ILs [P_6,6,6,14][Cl] and [P_6,6,6,14][SCN]. The results under optimal conditions were 30 wt% for Ni(II) and 100 wt% for Cd(II) [4].

The leaching of copper and other metals from the spent printed circuit boards (WPCBs) with ILs, DESs and Cyanex 272 was presented using two different procedures: after the physical, mechanical and thermal pre-treatment at T = 1023 K for 7h and the leaching process with acids [5], or straight from the solid phase after the physical, mechanical and thermal pre-treatment at T = 1023 K for 7 h [6]. In the first method, the ILs, ABS method with ILs, Cyanex 272 and two DESs were investigated [5]. In the second method, two DESs, DES 2 and (ChCl + ethylene glycol, 1:2), and four ILs, didecyldimethylammonium propionate, [N_10,10,1,1][C₂H₅COO], didecylmethylammonium hydrogen sulphate [N_10,10,1,H][HSO₄], didecyldimethylammonium dihydrogen phosphate, [N_10,10,1,1][H₂PO₄] and tetrabutylphosphonium dihydrogen phosphate, [P_4,4,4,4][H₂PO₄], were used [6]. Various additives, such as DDACl, H₂O₂, TCCA, glycine or PHM, were added to ILs during leaching [6].

The use of tributylmethylammonium chloride, [N_4,4,4,1][Cl] with TCCA was presented in the literature earlier with an extraction efficiency of 100% for Au, Pd, Cu, and Ag at low temperature, T = 298 K [7]. Glycine was also investigated earlier with sodium cyanide for the extraction of Au, Ag, Pd, Pt from WPCBs [8], as well as Cu(II) using the S:L = 1:100 with the addition of 10% of H₂O₂ at a temperature T = 303 K for 2 h at pH = 6–6.5 [9]. Glycine, as an amino acid, is known as the complexing agent of metals. The extraction efficiency of Cu(II) was 94% [9].

Nowadays, reviews discuss many current technologies for the secondary utilization of used Li-ion batteries and recycling of valuable metals [10,11,12,13,14,15,16,17,18,19,20]. BM of Li-ion batteries contains highly precious metals, such as Co, Ni, Li and Mn, which are rare in nature. Therefore, the aims of current scientific developments are to recover valuable metals from the spent BM with preferable alternative methods using new solvents such as DESs and ILs [2,3,21]. An excellent overview of widely used ILs and the recent advances to extract metals from Li-ion batteries and their comparative assessment of performance and selectivity, minimum solute interaction and possible technological use is presented in [22] and the literature cited therein.

Mobile phones are devices operating worldwide, are expected to reach 18bn by 2025, and are a challenging waste stream. The use of [BMIM][HSO₄], [HMIM][HSO₄], [BMIM][Br], [BMIM][Cl] was presented for the extraction of Ag and Au from WPCB. High extraction efficiency was observed using 60% solutions of [BMIM][Br] or [BMIM][Cl] at a temperature of 333 K [23]. The recirculation of the valuable metallic elements from waste WPCB of mobile phones has been presented with ILs [BMIM][Cl], [EMIM][Cl], [BMIM][BF₄] and [BMIM][PF₆] with the addition of H₂O₂ [24]. The best extraction efficiency was obtained with [EMIM][Cl] and [BMIM][PF₆] [24]. The use of ILs mainly with the [NTf₂]⁻ anion and organophosphorous acids with different additives was also presented for the extraction of metal ions from e-waste [25,26,27]. The use of ammonium IL [N_1,1,8,H][HSO₄] with the addition of H₂O₂ relieved the extraction of copper from the WPCB with a 20–30% extraction efficiency after the 1 h extraction in at a temperature of 343 K [28]. Much better extraction efficiency of copper (98%) from WPCB was presented with the following ILs, [BMIM][HSO₄], [PS-MIM][HSO₄], [CM-MIM][HSO₄], [di-Ac-IM][HSO₄], [C₆(di-Ac-IM)[HSO₄], with the addition of H₂O₂, after the primary pre-treatment with [BMIM][BF₄] [29]. The extraction of Ag and Au from the WPCB after the leaching with HCl was developed with [BMIM][NTf₂], [BMIM][PF₆] and Cyphos 101 ([P_6,6,6,14][Cl]) [30]. The extraction efficiency for Cu was around 90% and 99% using [BMIM][PF₆] and Cyphos 101, respectively, at temperatures of 298 K, 323 K and 343 K with the rate of IL/solid phase = 1:1 [31].

The recycling of metals from the e-waste of batteries, printed circuit boards or photovoltaic panels is an emerging field due to its environmental and economic benefits. The goal is to analyse the different methods and process flow sheets and to find the best way to further reduce the cost and the mass and energy balances for further process improvements such as the minimization of acid consumption and to be more “green”.

2. Methods Used in the Leaching Processes with ILs and DESs

The BM sample of all batteries used was washed and analysed using A. Analyst 800 atomic absorption spectrometer, PerkinElmer, Inc., Shelton, Connecticut 06484-4794, U.S.A (FAAS, FAES techniques, pre-analysed for elemental composition by SEM/EDS), as was described in our published works [1,2,3,4,5,6] (see Figure 1 and Figure 2). The concentration was determined using ICP-OES method. All the chemicals were used as delivered without further purification [1,2,3,4,5,6]. The synthesis of ILs and DESs, as well as extraction and stripping procedures used for the different BMs, obtained from different batteries, was described in published works [1,2,3,4,5,6]. For example, for the leaching of metals from 1.5 g of BM, a mixture of 7.1 cm³ IL ([P_6,6,6,14][SCN] or [P_6,6,6,14][Cl]) or mixture of these two ILs showing the synergistic effect of the extraction and 4.5 cm³ H₂O₂ (30 wt% aqueous solution) and 8.0 cm³ of DDACl, (50% aqueous solution) was used [1]. The following mixtures {[P_4,4,4,14][Cl], or [P_8,8,8,8][Br], or [NBz,1,1,R][Cl] + NaCl + H₂O + BM} were used in ABS method [1]. The selectivity of extraction was calculated from the concentration of metal ions in both phases [1]. The leaching of Li-ions batteries BM was proposed with 8 DESs with different additives (1.5 g of BM + 15 cm³ of DES + 15 cm³ of water + 2 cm³ of DDACl + 4.5 cm³ of H₂O₂) [2]. Extraction was conducted for 2h at T = 303 K, pH = 2.5–7 and O:A ratio 1:1 [2]. The extraction with ILs was similar: 1.5 g of BM + 11 g of IL ([P_6,6,6,14][SCN]) or 15.5 g of trihexyl tetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate, [P_6,6,6,14][BTMPP] + 5 g of toluene + 4 cm³ of water + 8 cm³ of DDACl + 4.5 cm³ of H₂O₂) for 2–6 h at T = 313 K [2]. The leaching of LiPBs was provided with 5 DESs and 3 bi-functional ILs. A mixture of 10 g of BM + 15 g of DES + 10 cm³ of water + 8 cm³ of DDACl + 5 cm³ of H₂O₂ (or 8 g of TCCA in 22 cm³ of acetone, or 15 g of glycine) was stirred under reflux for 2 h, 3000 rpm at T = 333 K at pH = 3, and then the liquid phase was analysed for the metal ion content [3]. For the extraction procedure for the Ni-Cd batteries with ILs ([P_6,6,6,14][SCN] and ([P_6,6,6,14][Cl]), equal volumes of 15 cm³ of aqueous phase and 15 cm³ of organic phase with 1.5 g of the BM as for Li-ion batteries were used [4]. The mixtures were stirred for 30 min, 5000 rpm at T = 318 K [4]. Also, two DESs were used for the extraction of Ni, Cd-DES 1 and DES 2 [4]. The leaching procedure was similar to ILs—a mixture of 15 cm³ of DES, 1.5 g of the BM and 15 cm³ of water was used for 30 min, at T = 318 K at pH = 3 (ratio O:A = 1:1) [4].

Two methods of acidic leaching were used: I (4M H₂SO₄ + 100 g/dm³ (NH₂)₂CS + 13 g/dm³ Fe₂(SO₄)₃), and after two steps of leaching, II {5M HNO₃ and (4M H₂SO₄ + 100 g/dm³ (NH₂)₂CS + 13 g/dm³ Fe₂(SO₄)₃)} was used for the WPCB material after the thermal pre-treatment at a temperature T = 1023 K for 7 h [5]. The extraction of metal ions was performed using the acidic leachate [5] or from BM of WPCBs only after the thermal pre-treatment [6]. In the first work, the extraction was performed by using 4 cm of the liquid leachate phase of pH = 3–7, 5 cm³ of IL and 1 cm³ of H₂O₂ [5]. The [P_6,6,6,14][Cyanex272] (from acidic leachate only), or [N_10,10,1,1][D2EHPA], or [N_10,10,1,1][Cyanex272],or [N_10,10,1,1][SAL] was used in toluene (6.4 g of IL + 2.07 g of toluene). Mixture was stirred for 30 min at a temperature T = 303 K with A/O = 1:2 [5]. For the ABS method, 10 cm³ of the liquid leachate phase of pH = 3 + 2.2 g of NaCl + 8 g of IL ([P_8,8,8,8][Br], or [P_4,4,4,14][Cl]) + 1.5 cm³ H₂O₂ was used [5]. The mixture was stirred for 2 h at a temperature T = 303 K at pH = 7. The extraction was carried out for 2 h at a temperature T = 318 K [5]. Two DESs were used, DES 1 and DES 2, at temperature T = 333 K for 2 h at pH = 3–5. To 1.5 g of the BM after the thermal pre-treatment; 15 cm³ of DES + 8 cm³ of DDACl + 4 cm³ of H₂O₂ + 3 cm³ of water were mixed. The ratio is O:A = 1:1 v/v [5]. In the second work, 1.5 g of the BM of WPCBs only after the thermal pre-treatment was used with mixture of 15 cm³ of DES 1 or DES 2, 8 cm³ of DDACl, 4 cm³ of H₂O₂ and 3 cm³ of water as an extracting solution for 2 h, 3000 rpm at T = 333 K at pH = 5, O/A = 1:1 [6]. The following ILs were used with the addition of the oxidizing agent TCCA: [P_4,4,4,4][Cl], [P_4,4,4,14][Cl], [P_8,8,8,8][[Br], [P_6,6,6,14][Cl], [N_10,10,1,1][C₂H₅COO], [N_10,10,1,H][HSO₄], [N_10,10,1,1][H₂PO₄], or [P_4,4,4,4][H₂PO₄]. The mixture of 16 g of the IL + 1.5 g of the BM of WPCBs + 10 cm³ of H₂O + 8 cm³ of DDACl + 4-12 g of TCCA (in 11–33 cm³ of acetone) for 2 h at 3000 rpm at T = 318 K, at pH = 2 was used for the extraction of metal ions. The O/A ratio = 2:1 for one-stage or two-stage extractions. The addition of glycine (4–12 g) and H₂O₂ (4 cm³) was also performed using the following ILs: [P_4,4,4,4][Cl], [P_8,8,8,8][Br], or [N_10,10,1,1][C₂H₅COO]. The extraction process was carried out for 2 h at 3000 rpm at T = 333 K, at pH = 6. Extraction with the addition of PHM (4–16 g) was also performed [6].

For all systems, the amount of solvents and additivities, temperature of the process pH and liquid/solid, as well as O/A ratios and the extraction efficiency and distribution ratios were described [1,2,3,4,5,6]. The utilization of new ILs/DESs with new additives may serve as a potential alternative to the acidic leaching methods.

2.1. Effects of the Extraction from Different Sources

To verify the effectiveness of the leaching, the extraction efficiency for the metal ion content was usually described as follows:

$$E={\displaystyle \frac{gE,Aq+gE,O}{g0}}\times 100$$

(1)

where g₀/(g) is the initial metal content in BM, g_E,Aq/(g) is the metal ion content in the aqueous phase, and g_E,O/(g) is the metal ion content in the organic/DES phase after the extraction. The distribution ratio is as follows: [D = (content of metal ions after extraction/initial metal content in BM) × V_Aq (H₂O + H₂O₂ + DDACl)/V_IL or _DES], where V_Aq or V_IL or _DES are the volumes of the measured probe [1]. The extraction efficiencies of metals from the BM or WPCB material after the thermal pre-treatment using ILs, mixtures of two ILs (synergistic effect) or ABS method were not as successful (E = 13–38%) as the extraction of metal ions from the aqueous solutions with the same methods (E = 60–99%) measured earlier [10].

The well-known organophosphorous-based acids were used as extraction solvents for different metals with good extractability and high extraction efficiency in many works [32]. In works presented here, a mixture of Cyanex 272 and DPh at low temperature T = 318 K, pH = 2.5, O/A = 1/1 was used with the addition of DDACl surfactant [1]. The extraction efficiency was satisfactory for zinc, E_Zn = 85.5–100 wt%, D_Zn = 0.9–1 at O/A = 1/1 at pH = 2.5 [1]. Extraction with DES 1 and DES 2 was also very successful for Zn(II), (E_Zn = 70–92 wt%) [1]. See the Patent Application in [33]. Zinc deposits of 95–100% purity were obtained during the potential controlled electrolyses [1]. The selective leaching performances of DESs miscible with water for the extraction of Co(II) (88.5 wt%), Li(I) (86.7 wt%) and Ni(II) (84.5 wt%) from the Li-ion batteries were shown [2]. DES at pH = 2.5 and DES (ChCl:phenylacetic acid, 1:2) for Co(II) (98.7 wt%) and for Li (I) (100 wt%) as well as DES (benzethonium chloride, [Benzet][Cl]:lactic acid, 1:2} for cobalt(II) (89.2 wt%) were presented [2]. The new proposed DES (ChCl/phenylacetic acid, 1:2) was very successful, with a leaching efficiency of 100 wt% of Li(I), DES:A = 1:1 [2]. The addition of Na₂SO₄ to Cyanex 272 revealed a Li(I) extraction efficiency of 92.0 wt%. The best results were obtained with D2EHPA (+ DDACl, H₂O₂ and Na₂SO₄) for all metals {Co(II), Li(I) and Ni(II)} at the level ca. 100 wt% [2].

The high extraction efficiency was obtained with DESs with different oxidizing additives and glycine in the solid–liquid extraction from the LiPBs BM [3]. DES 2 with the addition of (glycine + H₂O₂) or TCCA has shown good results. The extraction efficiency with DES 2 + glycine + H₂O₂ over Co(II), Ni(II), Li(I), Mn(II) was 60–65 wt% and for Cu(II) 75 wt%. DES 2, used as a new solvent with the addition of TCCA, induced an excellent extraction efficiency as large as 50–70 wt% for Co(II), Ni(II), Li(I), Mn(II) as well as 86 wt% for Cu(II) [3]. The results after the two-stage extraction and double stripping with (DES 2 + 15 g of glycine + H₂O₂) show the best 100 wt% extraction of all metals, except Cu(II) 75 wt% [3]. The extraction efficiency (E) of metal ions with DES 2 at T = 333 K, 2 h, at pH = 3 with different additives is presented in

Table 1. The extraction efficiency (E) of metal ions extraction with DES 2 at T = 333 K, 2 h at pH = 3 [3].

Leaching Mixture	Ion	E (wt%)
DES 2 + H2O2	Co(II)	14
	Ni(II)	12
	Li(I)	76
	Cu(II)	41
	Mn(II)	53
DES 2 + TCCA	Co(II)	59
	Ni(II)	50
	Li(I)	65
	Cu(II)	86
	Mn(II)	69
DES 2 + NaDCC × 2H2O	Co(II)	57
	Ni(II)	56
	Li(I)	66
	Cu(II)	37
	Mn(II)	62
DES 2 + PHM	Co(II)	20
	Ni(II)	2
	Li(I)	82
	Cu(II)	78
	Mn(II)	61
DES 2 + (15 g glycine + H2O2)	Co(II)	100
	Ni(II)	100
	Li(I)	100
	Cu(II)	75
	Mn(II)	100
DES 2 + (glutaric acid + H2O2)	Co(II)	18
	Ni(II)	27
	Li(I)	72
	Cu(II)	21
	Mn(II)	45

and in Figure 3 and Figure 4. The results obtained with new bi-functional ILs, [N_10,10,1,1][Cyanex272], [N_10,10,1,1][D2EHPA] and [P_6,6,6,14][Cyanex272]/toluene, were not so successful [3]. The results of the single-stage extraction for all metals were only at the level of 20–50 wt% [3].

Furthermore, various leaching methods were systematically investigated for the extraction of metal ions from the BM of waste Ni-Cd batteries using ILs, the ABS method, two DESs and two organophosphorous acids [4]. The leaching efficiency reached 100 wt% over cadmium at a temperature of 318 K over 20 min using DES 1 and DES 2 [4]. The IL, [P_6,6,6,14][Cl] was proposed for the extraction of Ni(II) from the aqueous phase after leaching BM with DES 1. The effective Ni/Cd separation was proved. The leaching of the solid BM with lL ([P_6,6,6,14][Cl]) or with a mixture of two ILs ([P_6,6,6,14][Cl], [P_6,6,6,14][SCN]), or the ABS method showed the extraction efficiency of metals below 10–20 wt% [4]. Also, the extraction of metal ions from the BM with organophosphorous acids was only at the level of 20–30 wt% [4].

The relationship between the structure of bi-functional ILs, the ILs in the ABS method as well as the DESs and selective extraction efficiencies was investigated for the extraction of valuable metal ions from the BM of WPCBs after the thermal pre-treatment (T = 1023 K, 7 h) and two methods of acidic leaching [5]. The well-known ILs were selected for the extraction [P_6,6,6,14][SCN], Aliquat 336, [P_6,6,6,14][Cyanex272], [P_4,4,4,14][Cl] (ABS method) and [P_8,8,8,8][Br] (ABS method) [5]. The ABS method revealed the extraction efficiency of all metal ions from the liquid acidic leachate phase of 80–100 wt% at a temperature T = 303 K, after 30 min at pH = 3 [5]. The best extraction efficiency of Cu(II), Ag(I), Al(III), Zn(II) was obtained with [P_6,6,6,14][Cyanex272]/toluene and was >60 wt% at a temperature T = 303 K after 30 min at pH = 7. The extraction efficiency of Ag(I) was E_Ag = 100 wt%, with distribution ratio D = 1 at a temperature T = 303 K for 30 min at pH = 3 using (Cyanex 272 + DPh) [5]. The results obtained with DES 2 showed lower extraction efficiencies from the solid material without acidic leaching: for Al(III), E_Al = 67.3 wt%, E_Cu = 9.6 wt% and E_Ag = 14.2 wt% at pH = 5 [5].

The combination of the mechanical and thermal processes of cutting the WPCBs into small pieces, crushing in a hydraulic press and, finally, using the thermal pre-treatment at a temperature T = 1023 K for 7 h and leaching with DES 2 and DES {ChCl choline chloride + ethylene glycol, 1:2} as well as with four new ILs, such as didecyldimethylammonium propionate, [N_10,10,1,1][C₂H₅COO], didecylmethylammonium hydrogen sulphate [N_10,10,1,H][HSO₄], didecyldimethylammonium dihydrogen phosphate, [N_10,10,1,1][H₂PO₄] and tetrabutylphosphonium dihydrogen phosphate, [P_4,4,4,4][H₂PO₄], showed promise in enhancing the leaching efficiency of metals [6]. In the leaching process, the additives such as H₂O₂, TCCA, PHM or glycine in a single or two extraction stages were used [6]. Finally, the extraction efficiency of Ag(I) ions from the BM of the WPCB material was 100 wt% using the following ILs, [P_4,4,4,4][Cl], [P_4,4,4,14][Cl], [P_8,8,8,8][Br], [N_10,10,1,1][C₂H₅COO] and [N_10,10,1,1][H₂PO₄], with the addition of H₂O₂ or TCCA, or (glycine + H₂O₂) in a single or two extraction stages [6]. The best extraction efficiency of Cu(II), E_Cu = 68.9 wt% was with [P_4,4,4,4][Cl] + TCCA (8 g, two extraction stages) and E_Cu = 50.1 wt% with [N_10,10,1,1][C₂H₅COO] + TCCA (8 g, two extraction stages) [6]. The best extraction efficiency of Zn(II) was E_Zn = 67.2–71.9 wt% with [N_10,10,1,1][C₂H₅COO] in a single or two extraction stages. The best extraction efficiency of Al(III), E_Al = 81.2 wt% was with [P_4,4,4,4][Cl] + TCCA (8 g, two extraction stages) and E_Al = 52.5 wt% with [N_10,10,1,1][C₂H₅COO] + TCCA (8 g, two extraction stages) [6].

2.2. Reaction Mechanism

The reaction mechanism between metal ions such as Li or Co extracted from the BM of LiBs batteries was discussed in many works [34,35,36]. The leaching process typically involves the formation of a soluble in the water anion [CoCl₄]²⁻ [36]. The mechanism of the interactions of metal ions in the solution is explained as an “ion exchange” and/or “ion pairing” interactions of metal ions transforming from the aqueous (Aq) to the organic phase (O) [37]:

2 Me⁺_(Aq) + [Cl]⁻_(Aq) + [COO]⁻_(O) = MeCl_(Aq) + Me[COO]_(O)

(2)

This reaction is possible after the process of the extraction of Me from the BM.

Generally, the driving forces for recycling and separations of metal technologies are described in many studies [14,15,16,17,18,19,20]. Unfortunately, usually, there is not a complete list of metals discussed in the published papers, such as Fe, Al, Cu, Mn, Li, Co, Ni. The most important problem is to separate Co and Ni discussed in many works from 20 years for Li-ion batteries [38]. An interesting analysis of the hydrometallurgical Li-ion battery recycling was presented to understand the possible connections between different flow sheets, process parameters and the environmental impacts of the processes [39]. However, the first leaching of the BM of Li-ion batteries was usually proposed with H₂SO₄ [39]. Based on our investigations on the different kinds of batteries or WPCB materials, the selective leaching/extraction systems for valuable metal recycling may be considered for future technologies.

3. Conclusions

To conclude, all these results from leaching and extraction studies may provide rapid optimization parameters, such as the type of DES, or IL, or the oxidizing additives, volume ratio, extraction time, pH of the process, etc., to maximize the extraction efficiency for different metal ions from different materials. The processes presented in our works may be used for the recovery of metal ions from e-waste in future industrial applications.