Hydrometallurgical Extraction of Valuable Metals by Mixed Acid Leaching System for Used Lithium-Ion Batteries

Abstract

Lithium-ion battery recycling includes discharging and processing exhausted batteries to recover valuable metals for reuse in new battery production. The improper disposal of e-waste draws attention to the possibility of reprocessing used lithium-ion batteries to make progress in recovering valuable metals. In this study, using biodegradable mixed organic acids, valuable metals were extracted from used batteries by a hydrometallurgical process under optimal conditions such as a stirring speed of 200 rpm, mixed acid concentration of ascorbic acid/citric acid (AA/CA) of 50:50 mM, temperature of 50 °C, time of 50 min, and slurry density of 20 g/L. Kinetic studies verified that the apparent activation energies, 43.6, 70.5, 49.8, 60.6, 45, and 6 kJ/mol, and surface chemical reactions controlled the leaching process for Li, Mn, Co, Ni, and Cu from cathode powder obtained from used LIBs. XRD and FT-IR confirmed the crystalline nature of the cathode powder. UV–visible spectra showed a Co(II) complex with λ_max at 380 nm by reduction of the Co(III) complex. Lithium was recovered by LiF and as MnO₂ using ammonium persulfate. Our efforts aimed to recover it through an economical and environmentally friendly approach.

1. Introduction

In order to maintain the socio-economical–environmental balance, there has been a lot of interest in the extraction of valuable metals from used lithium-ion batteries (LIBs). As a result, LIBs that have been properly disposed of are being utilized. By the end of 2030, an estimated 11 million tons of used LIBs will have been generated [1]. Electronic waste of spent LIBs is rapidly growing due to extensive usage in transportable gadgets such as electric cars, trucks, motorbikes, laptop batteries, and mobile phone batteries [2]. However, the limited availability of metal resources needed to produce cathode material and the possibility for value generation from these metals’ rescue resources raise the necessity for recycling wasted LIBs from electronic waste [3]. The recycling of essential and rare earth elements (REEs) is a necessary goal for all governments to ensure sustainable resources [4]. Although the current metal market is steadily changing and improving, more efforts have to be made to sustain these metals for technology [5]. The recycling of spent LIBs has several financial advantages in addition to the well-known environmental ones [6,7].

Current hydrometallurgical methods are primarily based on the use of different organic acids for leaching purposes. Such methods achieve necessary high leaching and filtering efficiencies for the recovery of metals (Li, Ni, Mn, and Co) [8]. Discharging and heat treatment processes followed by mechanical crushing are the major steps for manufacturing LIBs to remove organic and halogen-containing compounds. The resulting material is a black substance and comprises valuable elements including Li, Co, Ni, Cu, and Mn and trace quantities of Cu, Al, and Fe as an impurity [9,10]. For the recovery of valuable metals from waste slags using old pyrometallurgical procedures, high temperatures were frequently needed to melt the metals and metal oxides. This process resulted in considerable emissions of toxic gasses into the environment, such as SO₂ and NO₂ [11]. Despite being weaker than inorganic acids, organic acids are nonetheless regarded as attractive leaching options due to their special role as strong chelating agents [12,13]. The literature states that many research groups have turned their attention to mixed organic acids as leaching agents due to the aggressive character of inorganic acids. Certain organic acids have been suggested, including acetic acid, malic acid, glucose, and ascorbic acid [14]. A combination of mixed acids can be used to modify the leaching process to avoid the use of H₂O₂-related toxic materials. Among them, some organic acids are used as reducing agents as well as complexing agents. Ascorbic acid is an organic chemical that occurs naturally and acts as a vinyl carboxylic acid in addition to being a moderate reducing agent. Therefore, ascorbic acid is used to assess an acid combination’s optimal conditions and abilities [15,16]. However, because of its low toxicity, citric acid has a high demand and is mostly used as an acidulant in the food and pharmaceutical industries [17].

In this research, a mixed acid (AA-CA) process was used to extract valuable metals from used LIBs, following sustainable hydrometallurgical principles, and preserve the leaching agent’s ability to recover valuable metals compared to traditional single-acid leaching. Ascorbic acid (AA) and citric acid provide a new perspective on the application of organic acids (CA-AA) as a novel, eco-friendly reagent for the recycling of valuable metals from various e-wastes. The total concentration of critical metals was determined by atomic absorption spectroscopy (AAS) and inductively coupled plasma optical emission spectroscopy (ICP-OES). The morphology of cathode powder-containing metals was studied by XRD and SEM.

2. Experimental Procedure

2.1. Materials

The discarded lithium-ion batteries (LIBs) used in our research were collected from the local market. The leaching agents were citric acid, hydrogen peroxide (H₂O₂), and ascorbic acid. Analytical-grade reagents were used as received from the supplier. The complete extraction process was performed according to the given flowsheet diagram (Figure 1).

2.2. Collection of Samples

Used LIBs of laptops, mobiles, and e-mobility (LiNiMnCoO₂) with a cathode made of Li, Mn, Co, and Ni were collected from a local market. The circuit was checked individually using a voltmeter to avoid spontaneous combustion and short circuits. The circuit board attached to the diffusion assembly was then removed by solder cutting. Taking all precautions into account, the entire procedure was carried out in a local battery workshop.

2.3. Sample Pre-Treatment

2.3.1. Discharging

Lithium-ion batteries were discharged according to the methods in the literature with slight modifications [18]. LIBs were immersed in a brine solution (10%) for about 24 h and then removed from the discharge solution and air-dried.

2.3.2. Dismantling

The spent lithium-ion batteries were disassembled once exhausted into anode and cathode, while steel casing, caps, and plastic covers were removed manually [19,20].

2.3.3. Brine Dissolution Procedure

Due to the amphoteric properties of aluminum foil, a brine solution approach was used to separate the cathode powder from the aluminum foil. Dismantled batteries were dissolved in a brine solution (10%). After 5 h at room temperature, more than 90% of the aluminum was dissolved in the brine solution and the cathode powder was separated by simple filtration. This was an effective and convenient method to obtain pure cathode powder, which was used for further studies [21,22].

2.4. Leaching Experiment

For comparison, the following series of experiments were performed with single and mixed organic acids to recover valuable/critical metals from exhausted LIBs.

2.4.1. Leaching of Cathode Powder

The metals (Li, Ni, Co, Mn, and Cu) from the cathode powder were extracted using single organic acids (ascorbic acid and citric acid) with different concentrations (1.5 to 2.5 M) to form the lixiviant solution. An appropriate amount of cathode powder was mixed with the organic acids separately. The mixture was stirred at a speed of 300–600 rpm. The organic acids reduced the metals in the cathode material by producing a di-hydro acid complex (DHAC). By taking an aliquot of the material at regular intervals and filtering it with Whatman filter paper, the leaching efficiency of the process was checked. The amount of metal (Li, Ni, Co, Mn, and Cu) present in the filtrate was determined by AAS and ICP-OES [23,24].

2.4.2. Leaching of Cathode Powder with Mixed Organic Acid

The cathode material was mixed with a mixture of ascorbic and citric acids (AA-CA and CA-AA) in an aqueous medium in two cycles. In the first cycle, AA-CA with varying ratios like 90:10, 80:20, 70:30, 60:40, and 50:50 mM were used, while, in the second cycle, CA-AA with different ratios like 90:10, 80:20, 70:30, 60:40, and 50:50 mM were used. The interaction of metals Li, Ni, Co, Mn, and Cu with these acids is expressed in Figure 2. Typically, a cathode sample (2.0 g) was added to the distilled water with the addition of mixed organic acids and stirred for 1 h. The pH of the reaction mixture was changed regularly and optimized. The sample was regularly collected from the reaction flask, filtered, and quantified for various metal ions by AAS and the residue was subjected to AAS and ICP-OES. Undissolved dark residue was discarded.

2.4.3. Reductive Leaching

Citric acid is considered a weak organic acid that releases three H⁺ with pK_a1 = 3.14, pK_a2 = 4.79, and pK_a3 = 6.39 at 50 °C. Ascorbic acid is quite acidic at room temperature with pK_a1 = 4 and pK_a2 = 11.7. All reductive leaching experiments were performed in a 250 mL round bottom flask fitted with a condenser, which was heated in a heating mantle with continuous stirring at high temperature. A 2.0 g sample of black cathode powder was mixed with a 200 mL solution of the desired concentration of ascorbic acid. Ascorbic acid was used as a reducing agent and oxidized to dihydro ascorbic acid [25]. By increasing the contact surface, ascorbic acid accelerated the adsorption of organic acids on the cathode powder [17,26]. The following parameters were studied to investigate the leaching efficiency. Kinetics analysis indicated that the leaching process of spent lithium-ion battery material was controlled by a combination of surface chemical reaction and diffusion, while the Avrami equation explained the kinetics of crystallization and phase transition using the Avrami equation model (Equation (1)):

In(−In(1 − x)) = Ink₄ + nInt

(1)

2.5. Recovery of Valuable Metals

A stoichiometric amount of oxalic acid was added to the previously described leaching solution containing trace amounts of Co, Li, and Mn. The reaction mixture was stirred for 5 min at room temperature and allowed to stand overnight to settle the precipitate. After filtration, precipitates were dried at 105 °C for 3 h and characterized by XRD and SEM, while the filtrate was analyzed by AAS and ICP-OES. Cobalt precipitated as cobalt oxalate, lithium precipitated as LiF, and the remaining filtrate was further mixed with 100 mM NH₄F/CoO₃. The amount of cobalt and lithium in the filtrate was determined by AAS analysis. During iron removal, manganese was selectively precipitated and recovered as MnO₂ using ammonium persulfate. Copper was deposited on the cathode surface due to the dissolution of the copper electrode. The subsequent acidic nickel(II)–lithium(I) compound at various pH values was mixed with sodium hydroxide or another cleaning agent to produce virtually all nickel metal in the form of nickel(II) hydroxide, which was reused in the manufacturing of the cathode material.

3. Analytical Methods

The total amount of valuable metals in the cathode material was determined by a couple of techniques such as inductively coupled plasma optical emission spectroscopy (ICP-OES-Avio^® Max brand, Shelton, CT, USA and atomic absorption spectrophotometer (AAS, Trace AI 1200, Aurora, ON, Canada). For phase identification, powder X-ray diffraction, brand XRD-Raku, with Cu Kα radiation was used. Scanning electron microscopy (brand SEM-Tescan GmbH, Dortmund, Germany) was used to investigate the morphology. FT-IR spectroscopy (Bruker, Billerica, MA, USA) was used to confirm the binder removal and change in ligand environment during the experiment and spectra were recorded at 4000–400 cm⁻¹. The complexation of the metal ions with organic acids was further confirmed by a spectrophotometer (UH5300-HITECH, Tokyo, Japan).

4. Results and Discussion

4.1. Optimization of Leaching Conditions

4.1.1. Effect of Temperature

The optimum temperature had a significant influence on the leaching process (Figure 3a). At a 50 mM concentration (ascorbic acid/citric acid), time of 50 min, 200 rpm stirring speed, and 20 g/L slurry density, the temperature was optimized. The leaching efficiency of Li, Co, Ni, Cu, and Mn increased to 98%, 97%, 97%, 94%, and 95%, respectively, when the temperature changed from 35 to 50 °C. This means that an endothermic reaction occurred throughout the leaching process. As the temperature increased, the leaching reaction rate increased [27]. However, at temperatures above 50 °C, the leaching performance of Li, Cu, Ni, Co, and Mn decreased. This happened when the heat generated by the cavitation and reaction of the acid mixture was superimposed. However, 50 °C was considered an ideal temperature because the ascorbic acid/citric acid leaching system became very unstable at high temperatures.

4.1.2. Effect of Retention Time

Time had a significant impact on the efficiency of metal leaching. Greater leaching efficiency was observed at 50 and 90 min [27]. The effect of time was studied by using a mixed acid (CA/AA) with a concentration of 50:50 mM, temperature of 50 °C, stirring speed of 200 rpm, and slurry density of 20 g/L [28]. The leaching efficiency of Li, Co, Ni, Cu, and Mn improved rapidly when the leaching time was increased from 20 to 50 min, as shown in Figure 3b. After 50 min, the leaching efficiency of Li, Ni, Co, Mn, and Cu remained constant, indicating that the process had reached equilibrium. Consequently, 50 min was the ideal leaching time. The leaching efficiency increased steadily after 50 min to 90% for Cu, Ni, and Mn and to 98% for Li. Consequently, it was decided that 50 min was the appropriate leaching time. Conversely, as the reaction time increased, the growth rate decreased.

4.1.3. Effect of Concentration

The effect of acid concentration was investigated by using two combinations of mixed acids, ascorbic–citric acid (A-C) and citric acid–ascorbic acid (C-A), with molar concentrations of 90:10, 80:20, 70:30, 60:40, and 50:50 mM; other parameters remained unchanged, including a temperature of 50 °C, agitation speed of 200 rpm, S/L of 20 g/L, and reaction time of 50 min [29]. The results are shown in Figure 3c,d. The data show that the leaching efficiency of the valuable metal was directly influenced by the concentration of the acid. The leaching efficiency for Li, Co, Ni, Cu, and Mn increased to 85% at equal concentrations of citric–ascorbic acids and ascorbic acid–citric acid (CA 50:50). Such changes in metal leaching behavior suggest a modification to the rate mechanism. It makes sense that at low acid concentrations, the rate of diffusion became slow. So higher concentrations of mixed acids increased the rate of reaction and also increased the frequency of metal interaction with specific groups of acids. At higher concentrations of mixed acids, the leaching efficiency of metals decreased.

4.1.4. Effect of Stirring Speed

The rate of stirring is important in heterogeneous processes to promote a noticeable mass transfer in the solid–liquid phase and increase the leaching process in relevant lixiviants. In order to ascertain how the agitation speed affected the leaching of metals, the cathode material was stirred at 200–600 rpm. Other parameters, such as acid concentration (50:50), pulp density (20 g/L), time, and temperature (50 °C), remained constant [30]. The results are shown in Figure 3d, which shows that the leaching efficiency of all metals increased as the stirring speed increased. It was observed that at a stirring speed of 200 rpm, the leaching efficiency increased to 95%.

4.1.5. Effect of Solid–Liquid Ratio

The influence of the solid–liquid ratio on leaching performance is shown in Figure 3f. By keeping all other factors constant, such as a 50:50 mM ascorbic acid and citric acid concentration, temperature of 50 °C, 200 rpm agitation speed, 8 vol/vol H₂O₂ dose, and reaction time of 50 minutes, the effect of variation in the solid–liquid ratio (g/L) was investigated. Leaching efficiency increased from 10 g/L to 20 g/L and remained constant at 20 g/L, which corresponded to an optimum solid–liquid ratio. So, a 20 g/L solid–liquid ratio was preferred for efficient leaching because when the ratio was increased to above 20 g/L, the leaching efficiency decreased. This shows that an appropriately high leaching reagent is required because a low leaching reagent did not lead to high leaching efficiency.

4.1.6. Leaching Kinetics

Metal ions were extracted from the cathode powder by the leaching process, which was formed from oxides of solid metals and then changed to a liquid state. The reaction mechanism of leaching can be understood by using the Avrami equation. The Avrami equation explains the kinetics of crystallization and phase transition. It was applied to calculate the activation energy for the leaching reaction minerals [30,31].

1 − (1 − x)^1/3 = k₁t

(2)

1 − 2/3x − (1 − x)^2/3 = k₂t

(3)

(−In(1 − x))² = k₃t

(4)

In(−In(1 − x)) = Ink₄ + nInt

(5)

Equations (2)–(5), representing the surface chemical reaction, diffusion control, logarithmic rate control, and Avrami equation, respectively, are the typical models for controlling the reaction rate. The energy of activation (E_a (kJ/mol)) was calculated by applying the Arrhenius equation (Equation (6)). Different variables of these equations are

k₁, k₂, k₃, k₄ = Constant of control model

x = Leaching efficiency of metal

t = Leaching time (min)

n = Fitting result

A = Frequency factor

T = Absolute temperature (K)

k = A_exp^−Ea/RT

(6)

where k is the reaction rate constant (min⁻¹)

E_a = Activation energy

R = Universal constant of gas (8.3145 J/K/mol)

The logarithmic form of Equation (5) is as follows:

$$Ink=InA {\displaystyle \frac{-E_a}{RT}}\times {\displaystyle \frac{1}{T}}$$

(7)

Plots of lnk vs. 1000/T in Figure 4f show a greater fitting degree. Leaching kinetics at different temperatures and time periods were studied. Four equation models showed correlations with the leaching data, as shown in Figure 4. Plots of lnK vs. 1000/T and activation energy (E_a) for the leaching of different metals from spent lithium-ion battery material can be obtained by following

Table 1. Kinetics parameters of leaching by different models.

Time °C	Model 1		Model 2		Model 3		Model 4		Fitting Result
	InK	R2	InK	R2	InK	R2	InK	R2	n
50 °C	−3.1806	0.97	−3.1806	0.99	−3.1806	0.99	−3.1806	0.99	0.5
45 °C	−3.3287	0.91	−3.1806	0.97	−3.1806	0.99	−3.1806	0.98	0.7
40 °C	−3.4527	0.89	−3.4527	0.91	−3.4527	0.99	−3.4527	0.81	0
35 °C	−3.5575	0.85	−3.5575	0.86	−3.5575	0.98	−3.5575	0.86	0
30 °C	−3.3287	0.86	−3.1806	0.97	−3.1806	0.99	−3.1806	0.86	0.7

. The leaching data were correlated with four well-known leaching models, Equations (2)–(7).

The fitting degree is shown by the value of the regression value, R². Compared to other models, the Avrami equation model (Equation (5)) showed the best-fitting correlation, suggesting that the primary cause of the kinetic behavior was the reverse crystallization process [32]. Consequently, for some heterogeneous solid–liquid systems, the leaching of many metals could be satisfactorily explained by the Avrami equation. Depending on the activation energy, the chemical processes at the surface level regulated the reaction rate during leaching. Furthermore, the fitting levels of the other three models were good at low temperatures but deteriorated as the temperature increased. This could have been due to the increase in temperature leading to significant changes in the mass fraction of metals in the solid state. However, this was not taken into account in the other three models [33].

In(−In(1 − x)) versus time at different temperatures for all the metals is shown in Figure 4. The activation energies of each metal, which were governed by a completely regulated process, were also found using the scattered value of the slope line. The highest E_a value of cobalt showed imperative behavior, like the presence of different valance states of cobalt (Co II and Co III), which needed additional energy to cross the barrier of dissolution. The values of energy of activation obtained for Li, Mn, Ni, and Cu in the organic leaching system were lower than that of the cobalt leaching system, indicating that these elements may readily react with mixed organic acids.

The lowest E_a value of lithium showed that it can be more relatively easily dissolved than cobalt, so the lithium fraction could be located between the oxygen atom and cobalt octahedral molecule. The apparent activation energies (43.6 kJ/mol for Li, 70.5 kJ/mol for Co, 49.8 kJ/mol for Ni, 60.6 kJ/mol for Cu, and 45.6 kJ/mol for Mn) proved that the leaching process is a chemical reaction-controlled process and temperature-dependent [33].

4.2. SEM Analysis

The SEM analysis of the spent cathode powder before and after leaching and mixed acid leaching is shown in Figure 5a–c, respectively. Figure 5a shows that the pure cathode compound particles were irregular and agglomerated and had a diameter of 1–5 μm. There was a large particle size distribution with particles that were bound together and completely closed, with small areas; the penetration of the leaching agent into the cathode powder was reduced and the particles of the cathode powder were covered with a binder [34]. After single-acid leaching, the residue had open surfaces, indicating the non-uniform surface morphology, with less deposition and less particle shrinkage, as shown in Figure 5b. The porous surfaces of the mixed-acid residues became smaller with decreasing particle size, as shown in Figure 5c. They showed an amorphous structure that increased the area of the sample at optimal conditions, like 50:50 mM organic acids (CA-AA), 50 °C, 50 min, 200 rpm, and 20 g/L pulp density. After leaching with ascorbic and citric acids, particles with visibly clean primary surfaces were dispersed, indicating that the thermal treatment completely decomposed the binder. Leaching reduced the size of the cathode powder particles. In hydrometallurgical leaching, the organic acid mixture interacts with the surface of the cathode particles and simultaneously converts the high-valent metal ion into a low-valent ion by forming chelates. The edges of the particles in the residue were rougher than in pure cathode powder. Before leaching, particles were irregular and agglomerated; leaching with mixed organic acids created microstructural changes, with some cracks on the surface of the cathode film. SEM analysis further confirmed that traces of residual carbon were also present, along with leached cathode material due to organic burn-off, which might have been from the acetylene black that was used to confirm the supply of electric conductivity in the cathodes.

4.3. XRD Analysis

Figure 6 shows the XRD pattern of organic acid (AA-CA) and the leaching residue of ascorbic and citric acid at different time intervals. Weak peaks at 2θ = 38°, 47°, and 61° and intense peaks at 2θ = 28°, 29°, and 27° indicated the presence of the crystalline phase of the cathode powder. This cobalt oxalate could be used as a precursor for the preparation of LiCoO₂ cathode material. As the amount of acid increased, certain residue properties decreased. These values are similar to those in previous reports. After confirming the results of previous studies, the crystal-like orthorhombic cathodic powder phase was identified by XRD analysis [35].

When the concentration of both acids, ascorbic acid and citric acid, reached 50 mM, the residue consisted of graphite and small amounts of cathode material. Before leaching, a 5–10% weight of residual carbon was present in the cathode material due to organic combustion. Figure 6 represents the active components of the cathode material, LiCoO₂ and Co₃O₄. Peaks at 2θ = 17° and 42° belonged to Co₃O₄ of the cathode powder. The peak at 2θ = 26° and some enlarged peaks at 2θ = 17–20° showed the coexistence of M₃O₄ and M₂O₄, indicating a phase transition from an initial hexagonal layer arrangement to a spinel arrangement taking place after leaching. The peak at 2θ = 28° was due to the presence of undissolved LiCoO₂, which disappeared as the concentration of acids (CA-AA) increased. The peak at 2θ = 75 was attributed to graphite. Further peaks of other crystalline materials were not observed, confirming the solubility of the cathode material during the leaching process.

4.4. UV–Visible Analysis

The complexation of the metal ions with organic acids was further confirmed by UV–Vis spectra (Figure 7). The maximum absorption (λ_max) at 380 and 512 nm was due to Co(II) and Co(III) complexes, respectively. Co(III) was dominant in the cathode powder (LiCoO₂). An hour later, citric acid reduced Co(III) to Co(II), as confirmed by an increase in the concentration of the Co(II) complex. Consequently, the intense peak of the Co(II) complex grew faster than that of the Co(III) complex. However, the hump at 500 nm remained because of the production of Co(II) complexes. In other words, the interaction of metals with acids at the higher absorption, CA at λ_max = 380 nm, was greater than the reduction at 500 nm. Hence, UV–Vis spectra proved that the metals formed the following types of complexes: Co–citrate, Co–ascorbate, Ni–citrate, Ni–ascorbate, M–chelates, etc., with citric and ascorbic acids.

Spectra at different acid concentrations, as shown in Figure 7, showed that with increasing intensity, Co(II) was released from the cathode powder through the complex formation behavior of ascorbic acid and citric acid. However, due to the complex reaction of the mixed organic acids, the acid content decreased, which further increased the acid content, which could result in cobalt complexes being precipitated. At a high mixed acidic ratio, the cobalt peak disappeared, indicating that cobalt had precipitated. Positively charged cobalt, created by the leaching process, could combine with six water molecules to form hydrated cobalt ions. Then, hydrogen bonds in the hydrated cobalt formed a complex with negatively charged citrate ions. In summary, the cathodic powder exhibited hydrating and complexing behavior in the leaching system.

Co(III) citrate showed peaks in the range of λ_max 500 nm to 515 nm. In the presence of ascorbic acid, the maximum wavelength of Co(II) shifted from 300 to 380 nm due to the oxidizing power of water to accommodate the Co(II) in the leaching residue to stabilize the black cathode powder. With longer dissolution times, the absorption of the solution increased and a color change from colorless to light pink was observed. The increase in color intensity indicated the release of Co(III) from the oxide through complexation with the ligand. Although there was not a complete conversion of Co(III) to Co(II), there was a slight reduction in peaks compared to citric acid. Both organic acids were necessary in the dissolved mixture to detect Co(II) [36,37].

4.5. FT-IR Analysis

Figure 8a–e show the functional group changes by individual acids in the leaching system, varying mixed acid concentrations, and in organic aqua regia (OAR) in the leaching solution of relevant acid systems. All leaching systems had a common residence time of 50 min at 50 °C.

The data presented in Figure 8a–e show a broad peak at 3400 cm⁻¹ to 3500 cm⁻¹ and around 1600 cm⁻¹, indicating absorption peaks of -OH-adsorbed water. The peaks around 1100 cm⁻¹ and 1400 cm⁻¹ indicated the stretching peaks of the symmetric C-O and -COO vibrations, respectively. Before leaching, compared to the organic acid solution, Figure 8d shows a peak at 1400 cm⁻¹, while the peak at 1450 cm⁻¹ and the CH₂-lactone-5 member showed a stretching vibration in the range of 1745–1720 cm⁻¹ (Figure 8c,d). This phenomenon shows that both citric acid and ascorbic acid can be chelated with different metal ions and form chelate complexes. Both the 140 cm⁻¹ and 1450 cm⁻¹ peaks were observed, shown in Figure 8e of organic aqua regia (OAR), with a 1:3 ratio citric–ascorbic acid mixture, which combined to form broader peaks than when using different concentrations of single acids. Here, it was confirmed that organic aqua regia has the ability to chelate and dissolve valuable metal ions. For a comparison of inorganic aqua regia (HCl:HNO₃ in a 3:1 v/v ratio) with mixed organic aqua regia, inorganic aqua regia was used to leach the black cathode powder, and the composition was found to be lithium 1.8%, cobalt 13.5%, nickel 26.3%, copper 14%, and manganese 2%, studied through ICP-OES [38].

During leaching, salts of acids and metals such as Li, Co, Ni, Cu, etc. were reduced to a highly stable divalent form because some of the alcohol and esters groups could present and easily release H⁺, thereby forming lone pairs of electrons to form conjugate systems of complexes with metal ions. The peak height at 1400 cm⁻¹ in samples c and e was larger than the peak at 1450 cm⁻¹ in sample d, indicating that most metal ions formed chelate complexes after 50 min of leaching with organic acids (CA-AA) instead of a single acid [39,40].

5. Conclusions

Instead of mineral acids, a mixed organic acid system was used to recover valuable metals from exhausted Li-ion batteries. In a characteristic case, cathode material from old batteries removed from used Li-ion batteries was dissolved at 50 °C in a solution containing citric acid as a chelating agent and ascorbic acid as a reducing agent in the leaching system. All valuable metals were extracted from the dissolved solution to recover Li, Co, Ni, Cu, and Mn, providing a useful starting point for battery manufacturing. In this study, we identified the best optimal conditions for metal leaching from cathode powder. In addition, the XRD pattern of the cathode and SEM analysis showed that the entire process was carried out over 50 min at a temperature of 50 °C, mixed acid concentration of 50 mM, slurry density of 20 S/L, and stirring speed of 200 rpm. Based on the findings, this work provides a new perspective on the application of organic acids (CA-AA) as a novel, environmentally friendly reagent for recycling valuable metals from various waste sources, such as electronic scrap, making recycling more economically attractive.