Recovery of metals from spent lithium-ion batteries using organic acids

Journal Pre-proof Recovery of metals from spent lithium-ion batteries using organic acids

Jessica de Oliveira Demarco, Jéssica Stefanello Cadore, Franciele da Silveira de Oliveira, Eduardo Hiromitsu Tanabe, Daniel Assumpção Bertuol PII:

S0304-386X(18)30964-2

DOI:

https://doi.org/10.1016/j.hydromet.2019.105169

Reference:

HYDROM 105169

To appear in:

Hydrometallurgy

Received date:

20 December 2018

Revised date:

9 May 2019

Accepted date:

14 June 2019

Please cite this article as: J. de Oliveira Demarco, J.S. Cadore, F. da Silveira de Oliveira, et al., Recovery of metals from spent lithium-ion batteries using organic acids, Hydrometallurgy(2018), https://doi.org/10.1016/j.hydromet.2019.105169

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© 2018 Published by Elsevier.

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RECOVERY OF METALS FROM SPENT LITHIUM-ION BATTERIES USING ORGANIC ACIDS Jessica de Oliveira Demarco; Jéssica Stefanello Cadore; Franciele da Silveira de Oliveira; Eduardo Hiromitsu Tanabe; Daniel Assumpção Bertuol* Environmental Processes Laboratory (LAPAM), Chemical Engineering Department, Federal University of Santa Maria (UFSM), Av. Roraima 1000, Santa Maria, RS, 97105-900, Brazil ABSTRACT

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An effective and environmentally friendly process was developed for recovering metals present in lithium-ion batteries (LIBs), using a mechanical process followed by heat treatment and leaching using malic, citric, and formic acids at concentrations of 2 M. The techniques employed to characterize the process were TGA, DSC, XRD, SEM, FT-IR, and EDXRF. The leaching was carried out using different experimental conditions of temperature, volume of hydrogen peroxide (H2O2), S/L ratio, and extraction time. The leachate solutions were analyzed by FAAS. The characterization results showed that heat treatment at 700 °C for 2 h was effective for degradation of the graphite and PVDF present in the LIBs. Over 90% of the Co, Li, and Mn present could be extracted using the following conditions: 2 M DL-malic acid, 6% (v/v) H2O2, S/L of 1:20 (m/v), 95 ºC, and extraction time of 60 min. The process for recovering metals from spent LIBs using DL-malic acid could be considered economically and environmentally correct, avoiding negative impacts in the environment and recovering metals with high added value that could be used in the manufacturing of new products.

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1 INTRODUCTION

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Keywords: LIB; Recycling; Leaching; Valuable metal; Organic acid.

In the last decades, the requirements for batteries as mobile power sources have steadily increased, largely due to the implementation of new technologies in the electronics industry, attractive designs for the consumer, and the wide range of equipment used on a daily basis. Portable devices, such as mobile phones and microcomputers, make a major contribution to the increased demand. Currently, lithium-ion batteries (LIBs) are the type most widely employed, with their use expected to increase further as a result of their application in the automotive sector (Freitas et al., 2010; Meshram et al., 2015). The widespread and growing use of LIBs generates large quantities of spent batteries that must be recycled by means of environmentally friendly and economically viable processes. Ideally, closed-loop recycling should provide materials for the production of new batteries. LIBs contain high amounts of valuable metals, such as aluminum, copper, lithium, ⁎ Corresponding author at: Chemical Engineering Department, Federal University of Santa Maria – UFSM, Avenida Roraima 1000, 97105-900 Santa Maria, RS, Brazil. E-mail address: [email protected] (Daniel Assumpção Bertuol).

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cobalt, nickel, and manganese, of which the most valuable is cobalt (Co) (Georgi-Maschler et al., 2012). Based on the assumption that the metal content can be completely recovered in the metallic form, the monetary value of the Co present in 1 ton of spent batteries is approximately US$ 7200 (Georgi-Maschler et al., 2012). The value of Li is significantly lower, on average US$ 4530 per ton, but has increased since 2009 (U.S. Geological Survey, 2011). Therefore, the recovery of these metals has a strong economic influence on the development of a battery recycling process (Georgi-Maschler et al., 2012). Furthermore, the

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irresponsible disposal of spent LIBs, in addition to failing to meet the requirement for sustainable use of valuable materials, can lead to environmental contamination (Chen et al.,

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2015).

Currently, pyrometallurgical and hydrometallurgical processes are the two routes

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commonly used to recover valuable metals from spent LIBs (Al-Thyabat et al., 2013; Chen et

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al., 2015; Georgi-Maschler et al., 2012). However, pyrometallurgical processes have disadvantages, such as the release of hazardous gases and high energy consumption.

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Furthermore, a hydrometallurgical process is usually needed in order to refine the waste and obtain purer forms, such as salts, hydroxides, and metals (Chen et al., 2015). Consequently,

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several studies have shown that use of a hydrometallurgical process can be advantageous for recovering valuable metals from spent LIBs (Lee and Rhee, 2003; Nayaka et al., 2016; Swain

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et al., 2007). However, it is necessary to use processes that do not cause new environmental threats. One option is to use organic acids, instead of the traditional leaching agents, since these acids are less corrosive and can be considered ecologically correct (Argenta et al., 2017).

The recovery of valuable metals from spent LIBs generally employs acid leaching in the presence of a reducing agent, which converts the metals into a more soluble oxidation state (Bertuol et al., 2016). When a strongly acidic solution is used for the leaching of Co and Li, more than 99% (by mass) of these metals can be recovered. However, Cl2, SO3, and NOx are released during this process, while the acid residue obtained after leaching also presents a risk to the environment (Li et al., 2010b). Because of the growing interest in the sustainable management of natural resources and in reducing environmental pollution, the recovery of metals from spent LIBs is becoming increasingly important, because it can help to alleviate potential environmental pressures and resolve the crisis of scarcity of Co and Li (Wang et al., 2016).

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Due to the environmental concerns, it is necessary to separate and recycle all constituents of LIBs, in order to recover the valuable metals (Natarajan et al., 2018). The present work focuses on the recovery of Co and Li. The main methods found in the literature for recovering these metals are presented in Table 1.

Table 1 Hydrometallurgical methods used for recovering cobalt and lithium from spent LIBs. Leaching agent

H2O2 (v/v)

T (ºC)

Li et al., 2010a

Citric acid (1.25 M)

1%

90

Golmohammadzadeh et al., 2017

Citric acid (2 M)

1.25%

Li et al., 2012

Ascorbic acid (1.25 M)

-

Sun and Qiu, 2011

Sulfuric acid (2 M)

Chen and Zou, 2014

Li et al., 2010b

Pinna et al., 2017 Gao et al., 2018b

S/L ratio (m/v)

Leaching efficiency

1:20

91% - Co 99% - Li

300

1:30

96.46% - Co 99.8% - Li

70

20

1:25

94.8% - Co 98.5% - Li

5%

80

60

1:50

99% - Co 99% - Li

Citric acid (2 M)

2%

80

90

1:30

95% - Co 99% - Li

Oxalic acid (1 M)

-

95

150

1:15

97% - Co 98% - Li

DL-malic acid (1.5 M)

2%

90

40

1:20

93% - Co 94% - Li

Phosphoric acid (2%)

2%

90

60

1:8

99% - Co 88% - Li

Acetic acid (3.5 M)

4%

60

60

1:40

93.62% - Co 99.93% - Li

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Zeng et al., 2015

Time (min)

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Reference

As shown in Table 1, Co and Li can be readily leached from LIB residues using dilute acid solutions. However, reducing agents such as hydrogen peroxide (H2O2) may be required in order to achieve dissolutions comparable to those obtained using higher concentration acid solutions (Ferreira et al., 2009). Furthermore, the studies shown in Table 1 generally used processes different to those employed in the present work, such as manual separation of battery components and ultrasonic shaking, which may be impracticable on a large scale.

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The aim of this study was to develop an effective and environmentally friendly technique for recovering metals present in LIBs, using a mechanical process followed by heat treatment and leaching using malic, citric, and formic acids. Different characterization techniques were used to confirm the presence of the target metals in the LIBs, as well as to evaluate the efficiency of the heat treatment for removal of graphite and PVDF. In order to obtain the best process parameters, evaluation was made of the effects of the solid/liquid ratio, volume of hydrogen peroxide, temperature, and extraction time.

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2 EXPERIMENTAL

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2.1 MATERIALS AND REAGENTS

A total of 33 batteries of the same brand and model were collected from a mobile

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phone maintenance company. The leaching procedures employed hydrochloric acid (HCl,

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Synth), nitric acid (HNO3, Synth), anhydrous sodium sulfate (Na2SO4, Synth), anhydrous DLmalic acid (C4H6O5, Synth), anhydrous citric acid (C6H8O7, Proquimios), and anhydrous formic acid (CH2O2, Êxodo Científica). All these reagents were analytical grade. Hydrogen

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peroxide (H2O2, 30%, Synth) was used as a reducing agent. The solutions were prepared at

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different concentrations by dilution using deionized water.

2.2 DISMANTLING AND CHARACTERIZATION OF THE LIBS

Mass characterization was performed by manually disassembling three LIBs of the same brand and model, with separation of the different components including the external covering, aluminum case, cathode, anode, polymeric material, aluminum foil (support for the LiCoO2 cathode), and copper foil (support for the LiCγ anode). The mass of each component was determined. The LiCoO2 powder was separated by scraping, followed by weighing. The LiCoO2 powder used in the leaching stages was obtained from 30 batteries of the same brand and model. The charge level of each battery was checked by measuring the voltage, in order to avoid the possibility of short circuit and self-ignition. The batteries that had any charge were discharged in 10% Na2SO4 solution for 2 h, followed by washing with deionized water and drying in an oven at 80 ºC for 12 h. The batteries were then comminuted in a hammer mill (Model A4, Tiger) with a grid opening size of 5 mm, for release of the

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metals from the electrodes, with the aim of reducing the cost of the separation steps (Silveira et al., 2017). The particles that passed through the grid were dried for 24 h in an oven at 80 ºC, for evaporation and elimination of the organic solvents present in the electrolyte of the batteries. The material was then subjected to granulometric separation using vibrating sieves (Tyler 65, with opening of 212 µm) to separate the fine fraction (LiCoO2 and graphite) from the metal and polymer fractions. The particle size separation increased the selectivity towards the active materials, hence decreasing the cost of subsequent separation steps (Lee and Rhee, 2002).

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Thermogravimetric analysis (TGA) was performed using a Shimadzu TGA-50 instrument, obtaining the change in mass as a function of the thermal degradation of the

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sample. The sample was analyzed in an atmosphere of synthetic air at a flow rate of 50 mL.min-1, with heating from ambient temperature to 1000 ºC, at a rate of 10 ºC.min-1.

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Differential scanning calorimetry (DSC) was used to observe changes during heating of the

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sample from room temperature to 600 ºC, at a rate of 10 ºC.min-1, under a flow of nitrogen gas supplied at 50 mL.min-1.

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Samples of the fraction with granulometry <212 µm were subjected to pretreatment in a muffle furnace at 600, 650, and 700 ºC, during 2 h for each temperature, in order to

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eliminate graphite and organic compounds.

The crystalline structures of the original material and the samples obtained after the

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different thermal treatments were characterized by X-ray diffraction (XRD), using a Rigaku Miniflex 300 instrument operated in the 2θ range 5-98º, with Cu Kα radiation (γ = 1.5418 А), voltage of 30 kV, and current of 10 mA. Analysis by scanning electron microscopy (SEM) with energy dispersive spectroscopy (Vega-3G, Tescan) was performed in order to obtain the morphological characteristics and compositions of the samples before and after the heat treatment. For this analysis, the samples were metalized by sputter-coating with gold (using a current of 20 mA for 90 s) and were immobilized using Ni/Cu tape. Analyses by Fourier transform infrared spectrometry (FTIR), employing a Shimadzu Prestige 21 instrument, were used to confirm the effectiveness of the heat treatment for removal of PVDF. Semi-quantitative characterization of the elements present in the samples was performed by energy dispersive X-ray fluorescence (EDXRF), using a Bruker S2 Puma instrument.

2.3 RECOVERY OF METALS

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The system used for the leaching experiments consisted of a heating plate with a magnetic stirrer operated at 300 rpm, which was submerged in a water bath and connected to a recirculation cooling system composed of a reflux condenser and ultrathermostatic bath at 5 ºC, in order to avoid losses by evaporation. All the leaching stages were performed with the material that had been submitted to heat treatment. The sequence of these steps is shown in

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Fig. 1.

Fig. 1. Processes carried out to recover the metals from the spent LIBs.

Aqua regia (HCl:HNO3, 3:1 ratio) was used for complete digestion of the LIB powder prior to quantification of the metal contents, using 1:50 S/L ratio, temperature of 90 ºC, duration of 120 min, and agitation at approximately 300 rpm (Li et al., 2012; Wang et al., 2016; Gao et al., 2017; Golmohammadzadeh et al., 2017). The leaching processes using DL-malic acid, formic acid, and citric acid were performed with the acids at concentrations of 2 M. The experiments were conducted using different conditions of temperature, volume of hydrogen peroxide (H2O2), S/L ratio, and

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extraction time, which were selected based on studies reported in the literature (Li et al., 2010b; Chen and Zou, 2014; Golmohammadzadeh et al., 2017). The leachate solutions were separated from the solid material by filtration and were analyzed by flame atomic absorption spectrometry (FAAS), using an Agilent 240 FS instrument. The first step was to determine the best S/L ratio for the process, using ratios of 1:10, 1:20, 1:30, and 1:50 (m/v), while the other variables remained fixed. The best ratio was then used in the second stage, evaluating the influence of H2O2 at concentrations of 0, 1, 2, 4, and 6%. In the third step, the effect of temperature on extraction of the desired metals was

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evaluated using temperatures of 55, 65, 75, 85, and 95 ºC. Finally, the effect of the leaching time was investigated using times of 30, 60, 120, 180, and 240 min, while keeping the other

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3 RESULTS AND DISCUSSION

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variables at their optimum values.

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3.1 CHARACTERIZATION OF THE LITHIUM-ION BATTERIES

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3.1.1 Mass balance

Fig. 2 shows the different constituents present in a LIB (a) after manual separation:

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polymeric casing (b); metallic casing (c); connectors making the electrical contacts of the battery (d); anode composed of a copper electrode covered with layers of graphite (e); polymer separator between the cathode and the anode (f); cathode composed of an aluminum electrode covered by LiCoO2 (g); LiCoO2 powder after scraping (h).

Fig. 2. Different components of a dismantled spent lithium-ion battery: a) lithium-ion battery; b) polymeric casing; c) metallic casing; d) connectors; e) anode; f) polymeric separator; g) cathode; h) LiCoO2 powder.

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Table 2 shows the masses of the components of three batteries. The total masses of the batteries before dismantling were from 20.12 to 21.47 g. The cathode mass was between 8.24 and 9.6 g, containing from 3.05 to 5.94 g of active LiCoO2 material. As pointed out by Georgi-Maschler et al. (2012), battery producers produce their own specific types of LIBs, so it is difficult to provide accurate general values for the masses of the components, because the composition varies according to the manufacturing process (Bertuol et al., 2016). As can be observed here, even batteries of the same brand and model can differ in terms of their mass

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composition.

Mass (g)

Fraction (%)

0.39

1.94

2.11

Connectors

Fraction (%)

Battery 3 Mass (g)

Fraction (%)

1.68

0.22

1.03

10.49

2.37

11.08

3.03

14.11

1.49

7.41

1.54

7.20

1.16

5.40

Anode

4.08

20.28

4.12

19.25

4.63

21.56

Cathode

8.78

43.64

8.24

38.50

9.60

44.72

casing

Polymeric

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Metallic

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casing

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Polymeric

Mass (g)

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Table 2 Characterization of the masses of the LIB components. Battery 1 Battery 2

0.59

2.93

0.71

3.32

0.81

3.77

5.73

28.48

3.42

15.98

3.66

17.05

3.05

15.16

4.82

22.52

5.94

27.67

Copper foil

1.72

8.55

1.47

6.87

2.06

9.59

Graphite

2.36

11.73

2.65

12.38

2.57

11.97

Electrolyte

2.68

13.31

4.06

18.97

2.02

9.41

Ʃ

20.12

100

21.40

100

21.47

100

separator Aluminum foil LiCoO2

The prior segregation of battery components would be very profitable for recycling, considering that the plastic case, the polymer foil, the solvent, the steel case, the electrical contacts, and the aluminum and copper foils are all directly recyclable after separation (Paulino et al., 2008). On average, these components constitute 65% (by weight) of the spent

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battery. However, the manual separation of the different components would be impracticable on a large scale. Therefore, the mechanical processes were carried out using whole batteries and the segregation of components was performed by particle size separation.

3.1.2 Evaluation of calcination processes and chemical composition

The techniques described below were used to characterize both the LIBs powder and

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the efficiency of the heat treatment for removal of graphite and PVDF.

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3.1.2.1 Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC)

The results of the TGA and DSC analyses of the LIB powder are presented in Fig. 3.

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The thermogravimetric analysis curves showed that there were significant mass losses when

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the temperature was ramped from 50 to 800 °C.

Fig. 3. (a) Thermogravimetric analysis (TGA) and (b) differential scanning calorimetry (DSC) analysis of the LIB powder.

The first mass decrease, below 500 ºC, could be attributed to the formation of H2O and CO2 in the gas phase (Li et al., 2012). A significant mass loss of 22.8% occurred from 580 to 845 ºC, probably associated with degradation of the graphite and redox reactions between the cathodic conductor and the active materials. During these reactions, the metals in the active cathode materials were reduced from a high charge to a low charge state. The reduction reactions of the transition metals make their leaching easier and more efficient (Li et al., 2012;

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Yang et al., 2016). A maximum mass loss peak was present at around 700 ºC. According to Kurajica et al. (2011), the reduction of Co3+ to Co2+ is thermodynamically favored at temperatures higher than 700 ºC. The heat treatment eventually causes breakdown of the metal oxide cathode material, releasing oxygen. The positively charged active materials can disproportionate at elevated temperatures, as shown in Equations 1 and 2 (Wang et al., 2012; Diaz et al., 2019).

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(

)

(1) (2)

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A weight loss of 1.7% wt. was observed from room temperature to 275 ºC, with a

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corresponding endothermic peak, due to the loss of water bound in the material (Li et al., 2012). A small endothermic peak between 197 and 215 ºC could be attributed to the melting

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of LiPF6 (Yang et al., 2006). The decomposition of LiPF6 is represented by Equation 3 (Ravdel et al., 2003), where the second product is a strong Lewis acid that can react with

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water to form HF and POF3 (Genieser et al., 2018).

(3)

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A mass loss of 10.8% from 275 to 580 ºC was associated with several exothermic peaks. The mass loss above 500 ºC could be explained by the thermal degradation of PVDF, which decomposes at around 500 ºC (Yang et al., 2016). As reported by Wang et al. (2012), the reactions do not proceed in a precise order, since they can influence each other, hence inducing erratic behavior. For example, the PVDF– LixC6 reactions are strongly affected by the degree of lithiation of the graphite, only occurring when the carbon electrode is lithiated. PVDF is dehydrofluorinated according to Equation 4. A possible reaction between the binder and the LixC6 electrode is represented by Equation 5 (Wang et al., 2012).

(4) (5)

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The gas generated in the pretreatment of LIBs can be removed using activated carbon. Adsorption is considered one of the most viable options that can be applied in industrial flue gas treatment processes (Shafeeyan et al., 2011). Among several adsorbents, carbonaceous materials show excellent potential because of their large surface area, plentiful pore structure, high stability, and relatively low cost. Furthermore, most of the material adsorbed onto the carbon materials can be recycled by means of desorption processes performed under various conditions (Zhang et al., 2017).

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3.1.2.2 Characterization by X-ray diffraction

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Fig. 4 shows X-ray diffractograms of the cathodic and anodic active materials (powders) of the batteries without heat treatment and after heat treatment. For the sample

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without calcination, all the XRD peaks could be indexed to LiCoO2 and graphite (Silveira et

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al., 2017; Bertuol et al., 2016; Zhang et al., 2014; Ferreira et al., 2009). The graphite in the sample was from the anode and the LiCoO2 was from the cathode.

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After two hours of calcination, when the temperature reached about 700 ºC, the absence of the graphite peaks indicated that the graphite had combusted. Hence, 700 ºC was

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considered the ideal temperature for the calcination prior to the leaching step. This was in agreement with the TGA data, corroborating the results of Zheng et al. (2018) and enabling a

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shorter heat treatment time, compared to several earlier studies (Li et al., 2010b; Li et al., 2012; Golmohammdadzadeh et al., 2017).

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Fig. 4. X-ray diffractograms of the LIB powder before and after heat treatment at different

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temperatures.

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3.1.2.3 Energy dispersive X-ray fluorescence (EDXRF)

The residues of LIBs contain many valuable metals, such as Cu, Al, Fe, Li, Co, Mn, and Ni (Huang et al., 2016; Xiao et al., 2017). Table 3 shows the chemical composition of the LIB powder with granulometry <212 µm, after the heat treatment at 700 ºC. The results showed that after comminution in a hammer mill and particle size separation, Mn was the main element present in the LIB powder (51.87%), followed by Co (34.15%), Li (3.97%), Ni (3.35%), Fe (2.83%), and Al (1.39%). The Li concentration was obtained by FAAS, due to the low molecular weight of the element, and was added to the EDXRF results. The Mn was probably derived from the LiMn2O4 complex, which is used in some cathodes in order to reduce the production costs of batteries, since Co is expensive (Deboer and Lammertsma, 2013; Swart et al., 2014; Huang et al., 2016; Xin et al., 2016; Xiao et al., 2017). Therefore, considering the high Mn content in the sample, the extraction efficiency was evaluated for this metal, as well as for Co and Li.

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Increasing attention has been given to lithium manganese oxides, due to their high energy density, low cost, and environmentally friendly characteristics. Fisher et al. (2013) developed film cathodes for lithium-ion batteries using a Li–Mn–O system with Mn mass contribution of 53.2%. Here, the presence of phosphorus suggested that the electrolyte was lithium hexafluorophosphate (LiPF6) (Silva et al., 2018). The results shown are averages of three replicates.

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Table 3 Semi-quantitative analysis of the LIB powder by EDXRF. Element Weight (%)

1.39 (± 0.18 )

Mn

51.87 (± 0.18)

Fe

2.83 (± 0.03)

Ni

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Cu

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Li

34.15 (± 0.32) 3.97 (± 0.29)

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Co

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Al

3.35 (± 0.10) 0.45 (± 0.04) 1.8 (± 0.17)

Others

0.19 (± 0.11)

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Dutta et al. (2018) analyzed the composition of the cathodic material present in different brands and models of LIBs and obtained mass concentrations of 20% for Co, 2.4% for Li, 2.25% for Mn, 1.2% for Al, 0.3% for Fe, and 0.8% for Ni, after comminution of the batteries and centrifugation to separate the polymer, metal, and LiCoO2 powder fractions. However, the analysis employed a sample with 90% of the particles smaller than 1040 μm, while the present work employed particles smaller than 212 μm. Natarajan et al. (2018) studied the metals content of the cathodic material and noted that the composition of LIBs can differ according to the manufacturer, obtaining values of 5.1% Li, 26.3% Co, and 28.2% Mn. Differently, Ku et al. (2016) reported a composition of 15.3% Ni, 14.3% Mn, 6.0% Co, 2.3% Al, 0.3% Cu, and 61.8% other elements.

3.1.2.4 Fourier transform infrared spectrometry (FT-IR)

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Fig. 5 shows the FT-IR spectra of the LIB powder before and after heat treatment at 700 ºC. A broad absorption peak between 3600 and 3000 cm-1 could be attributed to O–H stretching vibration, while a peak at 2933 cm-1 was associated with stretching vibrations of methyl and methylene groups. A peak at 1644 cm-1 corresponded to vibration of C=C in H2C=CF–R groups. Peaks at 1394 cm-1 and between 1194 and 866 cm-1 indicated the presence of the fluorocarbon group from the PVDF binder (Sun and Qiu, 2011; Pant and Dolker, 2017). After the heat treatment, these absorption peaks attributed to the PVDF binder disappeared, confirming the effectiveness of the heat treatment at 700 ºC for the removal of

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organic fluorocarbon compounds. The results of Fourier transform infrared spectroscopy analysis reported by Hanisch et

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al. (2015) indicated the production of unwanted compounds such as HCN, HF, CH4, HCHO, COF2, HNCO, higher hydrocarbons, nitrogen oxides, CO, and CO2, among others, when

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electrodes were incinerated in air. These byproducts must not be released to the environment,

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since they may be hazardous to humans, the environment, and equipment. Adsorptive and absorptive methods such as the use of activated carbon and gas scrubbers can prevent the

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release of these compounds into the atmosphere.

Fig. 5. FTIR spectra obtained before (a) and after (b) heat treatment of the LIB powder at 700 ºC. 3.1.2.5 Characterization by scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS)

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Morphological analysis using scanning electron microscopy was used to observe the microstructural changes caused by the heat treatment of the different samples (Fig. 6). Prior to the heat treatment, the particles formed agglomerates (Fig. 6a), while after the heat treatment, these agglomerates were broken up, resulting in dispersed particles (Fig. 6b). This provided further evidence of the effectiveness of the calcination at 700 ºC during 2 h, facilitating contact with the leaching agent.

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Fig. 6. SEM micrographs obtained before (a) and after (b) heat treatment at 700 ºC (magnification of 2500x).

3.2 RECOVERIES OF THE METALS

The recoveries of the metals were evaluated varying the S/L ratio, H2O2 volume, temperature, and extraction time. The recovery achieved using extraction with aqua regia (3 HCl: 1 HNO3) was assumed to be 100% for all the metals.

3.2.1 Effect of the solid/liquid ratio

The effect of the solid/liquid ratio was studied in the range from 1:10 to 1:50 (m/v), while keeping the other parameters constant at 2 M acid, 1% (v/v) H2O2, temperature of 65 ºC, and time of 60 min. The leaching efficiencies obtained for Co, Li, and Mn are shown in Fig. 7.

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Fig. 7. Effect of the S/L ratio on the leaching efficiencies for (a) Co, (b) Li, and (c) Mn, using 2 M acid, 1% (v/v) H2O2, and 65 ºC, for 60 min. The results (Fig. 7) indicated that S/L of 1:20 was most suitable for recovering the metals from the LiCoO2 powder, in agreement with the results reported by Sun and Qiu (2011), Chen et al. (2017), and Gao et al. (2017). It can be seen that the leaching efficiencies for Co, Li, and Mn did not increase significantly when lower S/L ratios were used. At a low S/L ratio, the leaching solution included abundant water molecules, so the organic acids became increasingly weak. At a high S/L ratio, there was weaker ionization capacity and higher acid concentration. These characteristics provided an explanation for the fact that the leaching efficiencies of Co and Li initially increased and then decreased, as the S/L ratio was reduced (Zheng et al., 2018). The use of S/L of 1:20 resulted in a higher concentration gradient of H+ between the solid-liquid interface and the solution, which promoted ion transfer in the solution, leading to higher leaching efficiency. With further addition of the acids, the effects of diffusion velocity

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and viscosity became weaker, with the leaching efficiency decreasing for Li and not changing significantly for Co and Mn (Guo et al., 2016). Therefore, S/L of 1:20 was considered ideal from both economic and extraction efficiency perspectives, providing Co extraction of 30.63% for DL-malic acid, 20.78% for formic acid, and 43.39% for citric acid. For Li, the extraction efficiencies were 95.77, 63.25, and 82.18%, respectively, while for Mn the efficiencies were 98.90, 18.81, and 19.09% for DL-malic acid, formic acid, and citric acid, respectively. As can be seen from the results, Li was leached more easily than Co. This could be explained by the lamellar structure of LiCoO2

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and the different distribution patterns of Co and Li in the cathodic materials (Li et al., 2017;

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3.2.2 Effect of the hydrogen peroxide volume

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Chen et al., 2018a).

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The effect of the H2O2 volume on the extraction of the metals was evaluated in the range 0-6% (Fig. 8). A volume of 8% was also used for the extraction of Co and Li using DL-

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malic acid, because the leaching efficiency increased as the H2O2 volume was increased in the range 0-6%. The other parameters were kept constant at 2 M acid concentration, temperature

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of 65 ºC, time of 60 min, and S/L ratio of 1:20.

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b)

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a)

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-p

ro

c)

Fig. 8. Effect of H2O2 volume on the leaching efficiencies for (a) Co, (b) Li, and (c) Mn,

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using 2 M acid, S/L ratio of 1:20, and 65 ºC, during 60 min.

As shown in Fig. 8(a), the leaching efficiency for Co using DL-malic acid increased when the H2O2 volume was increased from 0 to 6%. The chemical bond between Co and oxygen is strong, making acid leaching of LiCoO2 difficult. When hydrogen peroxide was added, the oxygen produced from the decomposition of H2O2 converted Co3+ to Co2+, which assisted the dissolution (Saeki et al., 2004; Li et al., 2010a). While hydrogen peroxide assisted the dissolution of Co, the dissolution of Li was also promoted, because the two metals were contained in the same oxide compound (Shin et al., 2005). The use of 1% H2O2 with DL-malic acid resulted in Mn extraction of 98.90% (Fig. 8(c)). However, while the Co extraction increased progressively using H2O2 volumes of 2, 4, and 6%, the Mn extraction decreased from 98.90 to 85.01%. It could be concluded from these results that the use of 1% H2O2 resulted in easier dissolution of the LiMn2O4 complex, compared to the LiCoO2 complex (Zhang et al., 2018).

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When formic acid was used together with 1% (v/v) H2O2, the Co leaching efficiency increased from 1.19 to 20.78 %, while the use of 1% (v/v) H2O2 with citric acid increased the efficiency from 21.62 to 43.39%. When the H2O2 volume was increased above 1%, the efficiency decreased. Therefore, the use of H2O2 at a concentration of 1% was most suitable for both acids. According to Zheng et al. (2018), Co3+ is reduced to Co2+ with H2O2. However, when the dosage of the reducing agent is increased (Fig. 8), the molecules of H2O2 break down, due to the excess of H2O2, so the agent changes from being a reducer to being an oxidant

)

(

)

( )

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(

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(Equation 6).

(6)

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The use of 1% (v/v) H2O2 with citric acid resulted in extraction efficiencies of 82.18%

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for Li and 19.09% for Mn. The same volume of H2O2 in formic acid provided leaching efficiencies of 63.28% for Li and 18.81% for Mn.

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The use of 6% (v/v) H2O2 with DL-malic acid resulted in leaching efficiencies of 67.21% for Co, 99.30% for Li, and 85.01% for Mn, so this volume of H2O2 was defined as

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being most suitable for use with DL-malic acid. The leaching efficiencies of the metals observed in the absence of H2O2 could be

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explained by the fact that Li+ and Co2+ ions can be readily leached by DL-malic acid, while Co3+ and Mn4+ require the presence of a reducing agent (Yao et al., 2015; Zhang et al., 2015; Sun et al., 2017).

3.2.3 Effect of temperature

The effect of temperature was studied while maintaining the other parameters constant at S/L of 1:20, H2O2 volumes of 6% with DL-malic acid and 1% with citric and formic acids, and extraction time of 60 min. The leaching efficiencies obtained are shown in Fig. 9.

Journal Pre-proof 20

a)

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b)

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re

-p

ro

c)

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Fig. 9. Effect of temperature on the leaching efficiencies for (a) Co, (b) Li, and (c) Mn, using 2 M acid, S/L ratio of 1:20, and H2O2 volumes of 6% with DL-malic acid and 1% with formic and citric acids, during 60 min. When the temperature was increased to 95 ºC, 90.57% extraction of Co was achieved using DL-malic acid, indicating that under these conditions, the higher temperature accelerated the leaching, according to a mechanism of homogeneous corrosion of the particles (Joulié et al., 2014). The ability of cobalt to form chelates with organic acids is based on the potential required for Co3+ to Co2+ conversion. The chelate formation reaction of cobalt with this organic acid is favored by the dissociation of DL-malic acid at high temperatures (Li et al., 2014; Golmohammadzadeh et al., 2017). This does not occur for formic and citric acids, so the Co leaching efficiency did not increase, with maximum efficiencies of 20.78 and 43.39%, respectively, obtained at 65 ºC. The kinetic energies of molecules containing Li and Co generally increase with increasing temperature, leading to the ability to break down hydrogen bonds. Hence, the hydrogen bonds between the organic acids and the metals were weakened at temperatures above 65 ºC (Yu et al., 2018).

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For Li, the highest extraction efficiencies obtained using formic and citric acids were 63.25 and 82.18%, respectively, at 65 ºC. In the case of DL-malic acid, the highest efficiency was 99.33% at 65 ºC, while lower extraction of 93.22% was obtained at 95 ºC. Increase of the temperature from 55 to 65 ºC (Fig. 9) resulted in the recovery of Li increasing, while the leaching efficiency decreased, due to decomposition of the Li complex at higher temperatures (Golmohammadzadeh et al., 2018). In the case of Mn, the highest leaching efficiency was 99.53%, using DL-malic acid at 95 ºC, while values of 19.09 and 39.72% were obtained for citric acid at 65 ºC and formic

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acid at 75 ºC, respectively. However, since Co is the most valuable metal present in LIBs, the recovery efficiency for this metal has a strong economic impact on the development of a

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suitable battery recycling process (Georgi-Maschler et al., 2012). Therefore, the temperatures selected for recovering the metals from LIBs were 95 ºC for malic acid and 65 ºC for formic

re

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and citric acids.

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3.2.4 Effect of leaching time

Evaluation of the effect of the leaching process time was performed in experiments in

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which the other parameters were kept constant at the best values identified previously: S/L ratio of 1:20, H2O2 volumes of 6% with DL-malic acid and 1% with formic and citric acids,

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and temperatures of 95 ºC with malic acid and 65 ºC with formic and citric acids. The results are shown in Fig. 10.

Journal Pre-proof 22

a)

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b)

na

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-p

ro

c)

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Fig. 10. Effect of leaching time on the extraction efficiencies of (a) Co, (b) Li, and (c) Mn, using 2 M acid, S/L ratio of 1:20, 6% (v/v) H2O2 and 95 ºC with DL-malic acid, and 1% (v/v) H2O2 and 65 ºC with formic and citric acids. As can be seen in Fig. 10, the Co, Li, and Mn leaching efficiencies tended to increase and then decrease, as the duration of the process was extended. Leaching for 60 min resulted in extraction efficiencies for Co of 90.57, 20.67, and 43.39%, using DL-malic, formic, and citric acids, respectively. The values for Li were 93.22, 63.25, and 82.18%, respectively, and for Mn were 99.53, 18.82, and 19.09%, respectively. Extending the leaching time for longer than 60 min did not increase the extraction of Co and Mn, which could be attributed to the instability of H2O2 and its decomposition to H2O and O2 (Chen and Zou, 2014). It is also important to mention the possibility that Mn could react with the O2 released by the decomposition of H2O2, leading to the precipitation of Mn, which is feasible according to the diagram shown in Fig. 11. There have been few studies reporting the behavior of Mn with these acids. In most cases, the extraction times used were longer than 1 h, which highlights

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the relevance of the present study. In the work of Chen et al. (2018b), it was found that an +] M [ n 3could = result 10 .00 M  in lower extraction of Li, Co, and Mn. extended leaching time TO T

0 H

+

OH



-4 M n 3+

M n 2O 3 (c r)

-6

-8 4

6

8

pH

10

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2

of

L og C on c .

-2

12

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Fig. 11. Speciation diagram for Mn. Source: HYDRA-MEDUSA speciation diagrams (available from: https://www.kth.se/che/medusa/downloads-1.386254).

The Pourbaix diagrams for Co, Li, and Mn are provided in Fig. 12, showing the

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na

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possible equilibrium phases of the aqueous electrochemical systems.

Journal Pre-proof 24 M [ n 3+ ]TO T= [C o 3+ ]TO T=

10 .00 M  10 .00 M 

[L

10 .00 M 

M [ n 3+ ]TO T= + = i ]TO T[C = o 31+0]T.0O0TM 

10 .

10 .00 M 

[L i+ ]TO T=

10 .00 M 

10 .

C oO 2 (c r ) CoO ( H )3 (s )

05 .

05 .

00 .

L i+

00 .

E

SH E

CoO ( H )2 (s )

E

SH E

/V

/V

C o 2+

-0 5 .

-0 5 . C o (c r)

-1 0 . 2

4

6

8

10

of

-1 0 .

2

12

pH M [ n 3+ ]TO T= 10 .00 M 

t= 25 C 

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[C o 3+ ]TO T=

4

10 .00 M 

-p

8

10

12

pH

10 .00 M 

[L i+ ]TO T=

10 .

6

M nO 4 

M nO 2 (c r )

M n 2O 3 (c r ) M n 3O 4 (s )

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00 .

M n 2+

E

SH E

/V

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05 .

M nO ( H )2

na

-0 5 .

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-1 0 .

2

4

6 pH

8

10

12 t= 25 C 

Fig. 12. Pourbaix diagrams for (a) Co, (b) Li, and (c) Mn. Source: HYDRA-MEDUSA speciation diagrams (available from: https://www.kth.se/che/medusa/downloads-1.386254). The pH values fluctuated in the ranges 2.1 to 2.98 for DL-malic acid, 0.14 to 3.28 for formic acid, and 1.81 to 2.59 for citric acid. The potential values varied from 0.4579 to 0.4909, from 0.4299 to 0.6149, and from 0.4689 to 0.5119 for DL-malic, formic, and citric acids, respectively. Hence, considering the Pourbaix diagrams presented in Fig. 12, all the leaching stages were in the corrosion area, enabling the recovery of metals by dissolution. However, the variations in the physicochemical properties of different acid solutions could lead to difficulties in establishing the best parameters for the recycling of metals from LIBs. The characteristics of the interactions between the materials and the acid solutions, the variation in the physicochemical properties of the solutions, and the relationship between the

t= 25 C 

Journal Pre-proof 25

leaching parameters are critical aspects of a comprehensive evaluation of the effectiveness of a leaching process (Gao et al., 2018a). The possible reactions that may have occurred during the leaching processes are described below. Citric acid presented extraction efficiency below 50%, showing that it was not an ideal leaching agent for this process. The leaching reaction of the LiCoO2 residue with a solution of citric acid can be represented by Equations 7, 8, and 9 (Li et al., 2010b): (

)

( )

(

)

(

)

(

)

(

( ) )

( )

(

(7)

of

(

)

(

)

( )

(

)

ro

( ) (

)

)

(

)

(

(

)

(

) (8)

)

(

)

-p

( )

re

(9)

Equations 7, 8, and 9 suggest that the leaching efficiencies of Li and Co depend on the

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concentration of H2O2. Moreover, Li dissolves more readily in the presence of citric acid, compared to Co. In the leaching reaction, the Co and Li were leached as Co(C6H7O7)2,

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Li(C6H7O7), Co2+, and Li+. The residues were Co3O4 and C, which could not be leached because Co3O4 does not dissolve completely in citric acid (Li et al., 2010b).

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Formic acid also showed low extraction efficiency. In addition to being a weak acid, formic acid is a reducer of ions with high oxidative potential, due to its aldehyde. Unlike other acids, formic acid can selectively leach aluminum to obtain high purity aluminum, while nickel, cobalt, and manganese can precipitate as hydroxides. The chemical reactions that could occur during the leaching process with formic acid are described by Equations 10 and 11 (Gao et al., 2017): ( )

(

)

(

( ) (

)

( (

)

)

)

( ) (

( )

(10)

) (

( )

) (11)

The leaching of Co may have decreased over time due to the formation and precipitation of hydroxide. Furthermore, the solubility of Co in formic acid is relatively low, compared to the solubility of Li. Hence, formic acid is not indicated as a leaching agent for

Journal Pre-proof 26

Co when entire LIBs are comminuted in hammer mills, because a large number of other constituents are present, such as aluminum, which may have a significant influence on the leaching of Co and Li. However, this acid can be used for the selective extraction of aluminum, consequently facilitating the extraction of Co and Li (Gao et al., 2017). The leaching reaction between LiCoO2 and malic acid is a multiphase process determined by the chemical reaction and by the transfer of ions in the solution. The presence of a reducing agent during the leaching of LiCoO2 with malic acid can facilitate the direct reaction, since the Co3+ is reduced to Co2+. The chemical reaction can be represented by

(

) ()

(

)

( )

)

(

) (

)

()

(

)

( )

) (12)

(

) (13)

re

(

(

( )

-p

( )

ro

( )

of

Equations 12 and 13 (Li et al., 2010a):

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The rates of both the chemical reaction and the ion transfer are significantly affected by the temperature (Li et al., 2010a). At lower temperatures, the leaching is determined by the

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chemical reaction, for which the rate increases with increase of the temperature. Hence, the progress of the leaching is determined by the ionic transfer. At 95 ºC, leaching using DL-

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malic acid for 60 min resulted in over 90% extraction of Co, Li, and Mn, showing the suitability of this acid for the recovery of metals from spent LIBs. DL-malic acid degrades more easily under aerobic and anaerobic conditions, compared to HCl, HNO3, and H2SO4. Furthermore, this acid can be recycled and reused for subsequent leaching (Li et al., 2010a; Bahaloo-Horeh and Mousavi, 2017). Therefore, the process using DL-malic acid to recover metals from spent LIBs can be considered economically and ecologically advantageous, avoiding adverse impacts in the environment and recovering metals with high added value that can be used in the manufacturing of new products. If not treated appropriately, spent LIBs are classified as hazardous waste that can cause harm to the environment, animals, and human health (Ordoñez et al., 2016).

6 CONCLUSIONS

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The characterization results showed that heat treatment at 700 ºC for 2 h was effective for degradation of the graphite and polyvinylidene fluoride components of the LIBs, making leaching of the metals easier and more efficient. Since manual separation of the different components would be impracticable on a large scale, mechanical processes using whole batteries and segregation of the components by granulometric separation were adopted. Particle size separation increased the selectivity towards the active materials, while also reducing the costs associated with different separation stages.

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Evaluation was made of the effects of the parameters S/L ratio, H2O2 volume, temperature, and leaching time. S/L of 1:20 was most suitable for recovering Co, Li, and Mn

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from the LiCoO2 powder using DL-malic, citric, and formic acids. The best H2O2 volumes were 1% with citric and formic acids and 6% with DL-malic acid. The best leaching

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temperatures were 65 ºC, using citric and formic acids, and 95 ºC, using DL-malic acid.

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Under these conditions, leaching for 60 min resulted in Co extraction of 90.57, 20.67, and 43.39%, Li extraction of 93.22, 63.25, and 82.18%, and Mn extraction of 99.53, 18.82, and

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19.09%, using DL-malic, formic, and citric acids, respectively. Based on these results and considering the need to minimize the consumption of energy and reagents, while achieving

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satisfactory leaching efficiency, the best conditions were determined for the leaching of Co, Li, and Mn from spent LIBs. It was found that over 90% of these metals could be leached

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using a solution of 2 M DL-malic acid, 6% (v/v) H2O2, S/L of 1:20 (m/v), temperature of 95 ºC, and leaching time of 60 min.

The process for the recovery of metals from spent LIBs using DL-malic acid could be considered economically and ecologically correct, avoiding negative impacts in the environment and recovering metals with high added value that could be used in the manufacturing of new products.

ACKNOWLEDGMENTS

Financial support for this work was provided by the Brazilian agencies CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior), CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico), FAPERGS (Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul), and SDECT (Secretaria de Desenvolvimento Econômico, Ciência e Tecnologia do Rio Grande do Sul).

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HIGHLIGHTS

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- Mechanical processes of whole batteries and granulometric separation of components - Investigating the possibility of leaching of LIBs by using three organic acids - Efficiency of the heat treatment in the degradation of the graphite and PVDF - Recovery of more than 90% of Co, Li, and Mn in DL-malic acid and H2O2