Sustainable Recovery of Metals from Spent Lithium-Ion Batteries: A

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Sustainable recovery of metals from spent lithium-ion batteries: A green process. Xiangping Chen, Chuanbao Luo, Jinxia Zhang, Jiangrong Kong, and tao zhou ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01000 • Publication Date (Web): 13 Nov 2015 Downloaded from http://pubs.acs.org on November 19, 2015

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ACS Sustainable Chemistry & Engineering

Sustainable recovery of metals from spent lithium-ion batteries: A green process Xiangping Chena, Chuanbao Luoa, Jinxia Zhanga, Jiangrong Konga* and Tao Zhoua* a. College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, P.R. China. *Corresponding authors: 1. Dr. Jiangrong Kong: Email: [email protected]; Tel: +86 13548773795. Department: College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, P.R. China. Address: No. 932 Lushan South Road, Changsha, Hunan Provence, P.R. China. 2. Prof. Tao Zhou: Email: [email protected]; Tel: +86 13187085986. Department: College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, P.R. China. Address: No. 932 Lushan South Road, Changsha, Hunan Provence, P.R. China.

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ACS Paragon Plus Environment


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Abstract In this study, a green process with prospective environmental and economic significances has been experimentally and theoretically established for the sustainable recovery of metals from spent lithium-ion batteries (LIBs). Three leaching systems were explored for the application of different biomass as reductants. According to leaching results, H3Cit (citric acid) & tea waste and H3Cit & H2O2 systems reveal similar leaching abilities (96% Co and 98% Li; 98% Co and 99% Li, respectively), while H3Cit & Phytolacca Americana system shows inferior leaching performance (83% Co and 96% Li) under the optimized conditions. Tentative exploration of oxidation mechanism for different biomass indicates that potential reducing substances contained in biomass can be employed as efficient reductants during leaching. Then both metal ions and waste citric acid can be simultaneously recovered by selective precipitation. About 99% Co and 93% Li could be recovered as CoC2O4·2H2O and Li3PO4, and the recycled citric acid demonstrates similar leaching capability as fresh acid according to circulatory leaching experiments. Finally, solution chemistry theory and waste stream analysis were investigated to provide theoretical foundation for the recovery process.

Key words Spent lithium-ion batteries; Green process; Metals; Leaching; Precipitation; Recovery.

Introduction Lithium-ion batteries (LIBs) had been witnessing increasing applications in mobile devices, personal computers, electric vehicles etc. Their attractive characteristics in terms of modest size and weight, superior electrochemical properties may make them an alternative superior to other batteries (e.g. nickel-cadmium, Zn/MnO2 batteries).1,2 They usually consist of valuable metals/metal oxides, organic chemicals, metallic shells and plastics with different proportions varying from manufacturers and types of batteries.3,4 Valuable materials are usually contained in these end-of-life batteries and the recycling of spent batteries may promise economic benefit.5,6 In addition, the total quantity and weight of these exhausted batteries in China will exceed 25 billion units and 500,000 tons in 2020 2


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as surveyed by Zeng et al.7 and heavy metals and noxious organic substances contained in discarded LIBs may lead to adverse impacts on environment.8,9 A boosting demand for spent LIBs recycling will be, therefore, necessary towards these exhausted batteries concerning environmental protection9 and economic benefit10. Currently, research interests were mainly focused on three aspects of pyro-, hydro- and bio-hydrometallurgical processes concerning the recovery of metals from spent LIBs.11-13 Pyro-metallurgical route may be an inadvisable method due to their obvious drawbacks in terms of intensive energy consumption, hazardous gases emission etc.11 As an emerging technology, bio-hydrometallurgical process has been attracting an increasing attention as an eco-friendly route with potential advantages in field of waste treatment.13 However, the bioleaching method may be discouraged by the inefficiency for the treatment of spent LIBs with relatively high metal content.14 Therefore, hydrometallurgical process can be taken as an alternative option for the recycling of valuable metals from spent LIBs in this work. An increasing intention has been paid to the leaching of metals from spent LIBs using innocuous organic acids as leaching reagents instead of mineral acids.15-24 Succinic-,15 malic-,16 aspartic-,16 citric-17 and ascorbic-18 acids are the commonly used organic acids and their leaching performances present in the following order: succinic acid > citric acid > ascorbic acid > malic acid > aspartic acid.15-18 In addition, citric acid is the cheapest and easily available acid with excellent leaching performance.17 During leaching, chemical reductants (e.g. NaHSO323, H2O224, HCl25) are always added to the slurry to improve leaching efficiency. Although relatively desired results could be attained by the addition of chemical reductants, their applications may be far from green,

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which may result in secondary

pollution.27 Furthermore, the secondary pollution caused by waste acid may be also a severe issue required to be conquered.28 Several studies had paid due attention to reductive leaching of manganese ore or manganese dioxide (MnO2) using different biomass (e.g. tea waste29, cornstalk30) as green reductants. It can be concluded from their discoveries that the biomass reductants demonstrate almost equivalent reductivity as chemical reductants.29,30 Despite these biomass reductants show remarkable superiorities on environmental protection and cost minimization, no relevant literature had ever reported the sustainable leaching of metals from spent LIBs using biomass as reductants. In addition, although waste acid after leaching was 3



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considered as one of the major secondary pollution source after leaching,9,28 no investigation had ever focused on the disposal of waste acid, which may cause the loss of unreacted acid and result in secondary pollution. This study was, therefore, concentrated on proposing a green strategy for the sustainable recovery of valuable metals from spent LIBs to conquer the above two bottlenecks in terms of secondary pollution and waste acid disposal. First, powders of waste cathode materials were obtained after pre-treatment operation. Then, waste cathode materials were dissolved by citric acid (C6H8O7, H3Cit) and different reductants (Tea Waste, Phytolacca Americana branches and hydrogen peroxide) as leaching reagent and reductants. Finally, Co and Li ions dissolved in the lixivium were treated with oxalic acid and phosphoric acid solutions to recover Co and Li.

Experimental section Materials and reagents Spent LIBs were kindly supplied by a local battery recycling centres and the waste cathode materials powders were obtained from spent LIBs after pre-treatment (see the following section). The biomass reductants employed in this study (powders of Phytolacca Americana branch (PA) and Tea Waste (TW)) were locally obtained (Changsha, China) after treatments of drying and milling. Then, the industrial composition, elemental analysis, main biomass constituent and heat output of biomass reductants employed in this study were proximately characterized and analyzed to determine the detailed composition of TW and PA (see Table 1). All chemical reagents employed in this study were of analytical grade and different solutions at specified concentrations were prepared or diluted using deionized water.

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Table 1 The detailed composition of Phytolacca Americana (PA) and Tea Waste (TW). Analysis

PA

TW

Analysis technique

Moisture: 5.79

Moisture: 3.51

A1

Industrial

Volatiles: 4.18

Volatiles: 10.16

A2

composition (%)

Ash: 64.31

Ash: 68.67

A3

Fixed carbon: 25.72

Fixed carbon: 17.66

A1

C: 45.15

C: 41.89

B1

Elemental analysis

H: 5.27

H: 5.74

B1

(%, dry basis)

O: 41.63

O: 42.16

B2

N: 1.18

N: 4.97

B2

Main biomass

Cellulose: 34.19

Crude protein: 21.76

C1-C4

constituent (%)

Hemicellulose: 22.07

Saccharides: 23.17

C1-C4

Lignin: 17.31

Tea polyphenol: 19.13

C1-C4

15.13

16.70

D

Heat output (MJ/kg)

A1: ASTM El 756-08 Standard Test Method, A2: ASTM E872-82 (2006) Standard Test Method, A3: ASTM El 755-01 (2007) Standard Test Method; B1: ASTM E777-08 Standard Test Method for Carbon and Hydrogen, B2: ASTM E870-82(2006) Standard Test Methods; C1: High Performance Liquid Chromatography (HPLC), C2: Ultraviolet Spectrophotometer (UV-2550, Japan), C3 : Fibber Analysis System (FOSS 2010, Sweden) and C4: Kjeltec 2300 Analyzer Unit; D: ASTM E711-87 Standard Test Method.

Pre-treatment procedure Spent LIBs were commonly pre-treated before the recycling of metals and the detailed pre-treatment steps were listed as follows: (a). Discharging: Spent batteries were immersed in an electrolyte solution (10 w/v% Na2SO4) for about 24 h to discharge the remained electricity. These exhausted batteries were then washed with deionized water and then dried at 80oC for 12 h in an oven;

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(b). Manual dismantling: Exhausted batteries were then manually dismantled and separated into following scraps: metallic shells, separators, anodes and cathodes. Metallic shells and separators were recycled directly. The anodes and cathodes were cut into small pieces (about 1×1cm) for the peeling off operation; (c). Recycling of Al/Cu foils: Then the small pieces of anodes and cathodes foils (about 1×1cm) were treated with a green solvent (N-methyl-2-pyrrolidone, NMP) at about 100 oC to dissolve PVDF binder and detach Al/Cu foils from the cathode/anode materials.15,17 Then Al/Cu foils were recycled in their metallic forms and NMP could be repeatedly used after evaporation and reclamation; (d). Thermal and mechanical treatment: The obtained cathode materials were then filtered and calcined in a muffle furnace at 700oC for 2h to remove the carbon materials.14 Finally, the waste cathode materials were ground into finer fractions with higher specific surface area which will be beneficial for the following leaching process. Fig. 1 demonstrates simplified pre-treatment flow-sheet of the above process. Metallic shells and separators were directly recycled after dismantling, and the peeled off Al/Cu foils were recovered in metallic forms. Then obtained powders of cathode materials were used as raw materials for the following leaching process and Table 2 shows composition of different metals in the obtained LiCoO2. Table 2 Contents of different metals in waste LiCoO2. Metal element

Content (wt. %)

Li

6.81

Co

58.79

Ni

0.58

Al

0.71

Others

0.76

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Al foils

Cathodes

Recycled directly

Separator

A

B Anodes

Cu foils

Metallic shells

Recycled directly

Active materials

Spent lithium-ion batteries

Dismantled scraps

For leaching

Fig.1. Simplified pre-treatment process of spent LIBs (A- Manual dismantling; B- peeling off Al/Cu foils and recycling of Al and Cu).

Reductive leaching In this work, various effects were investigated and determined during the leaching process. Two leaching systems (H3Cit & PA and H3Cit & TW) were investigated and H3Cit & H2O2 system was taken as a comparison. All leaching experiments were conducted in a 250-mL three-necked and round-bottomed thermostatic reactor, which was placed in a water bath to control reaction temperature. An impeller stirrer and a vapour condenser were installed in the reactor to control the stirring rate and water evaporation respectively. A fixed amount of waste cathode materials (~2.0 g) were precisely measured and a known quantity of acid and reductant (based on the experimental conditions) were prepared as leaching reagents. They were simultaneously added to the reactor. During leaching, samples of leaching solution with fixed volume (~2.0 mL) were periodically drawn out, filtered and analysed by ICP-OES to determine concentrations of different metals. Leaching efficiencies of different metals can be calculated according to Eq. (1):

LE =

M × C 0 × V0 × 100% m × w%

(1)

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where LE is the leaching efficiency; M, C0 and V0 are the molecular mass, concentration of different metals and volume of leaching liquor, respectively; m and w% are the mass and mass fraction of metals in raw materials.

Materials recovery After leaching, the leachate (Co3(Cit)2, Co(HCit), Co(H2Cit)2, Li3Cit, Li2(HCit) and Li(H2Cit)) was successively treated with oxalic acid (H2C2O4, 0.5 mol·L-1) and phosphoric acid (H3PO4, 0.5 mol·L-1). Co and Li were respectively recovered as CoC2O4·2H2O and Li3PO4 and waste citric acid can be simultaneously displaced by stronger acids (H2C2O4 and H3PO4), indicating that both metals and waste citric acid can be recovered or reused after precipitation. Finally, the recycled citric acid was tested the possibility of reuse as leaching reagent for leaching. Based on previous literature 4,8,17, related reactions were listed as follows (Eq. (2) to Eq. (7)): Co3(Cit)2 + 3H2C2O4 = 3CoC2O4 ↓ + 2H3Cit

(2)

Co(HCit) + H2C2O4 = CoC2O4 ↓ + H3Cit

(3)

Co(H2Cit)2 + H2C2O4 = CoC2O4 ↓ + 2H3Cit

(4)

Li3Cit + H3PO4 = Li3PO4 ↓ + H3Cit

(5)

3Li2(HCit) + 2H3PO4 = 2Li3PO4 ↓ + 3H3Cit

(6)

3Li(H2Cit) + H3PO4 = Li3PO4 ↓ + H3Cit

(7)

Precipitation reactions were carried out in a 250-mL, three-necked and round-bottomed thermostatic reactor, which was placed in a water bath to control the reaction temperature. An impeller stirrer and a vapor condenser were installed in the reactor to control the agitation rate and water evaporation, respectively. The precipitation efficiencies of different metals can be calculated according to Eq. (8):

PE =

C1V1 − C 2 V2 × 100% C1V1

(8)

where PE stands for the precipitation efficiency; C1 and C2 are concentrations of metal ions in the solution before and after precipitation; V1 and V2 are volumes of the solutions before and after filtration.

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Analytical methods First, waste cathode materials were completely dissolved in a concentrated hydrochloric acid (about 10 mol·L-1 HCl) and Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES Optima 2100 DV, Perkin Elmer instruments, U.S.A.) was employed to analyse concentrations of different metals. Leaching and precipitation efficiencies of different metals were also measured and determined by ICP-OES. And XRD (Rigaku, Cu-Kα) was employed for the characterization of obtained products (CoC2O4·2H2O and Li3PO4). Ultraviolet spectrophotometer (UV752N) was used for the verification of the existence of citric acid in the precipitation solution and approximate determination of the concentration of citric acid for each cycle in the circulatory leaching (from Cycle 1 to Cycle 5). FT-IR spectrum (Thermo Scientific Nicolet iS10 FR-IR Spectrometer) was employed for the identification of relevant vibrational characteristic bands of different biomass reductants before and after leaching. For the purity analysis of the obtained precipitates, XRD was firstly used to determine the chemical compositions. Then the precipitates were completely dissolved in acid solution and ICP was used to determine the concentrations of Co2+ or Li+ ions in the solution. The purities of CoC2O4·2H2O and Li3PO4 could be calculated according to the following equation:

P=

C×V×M × 100% m0

(9)

where P is the purity of different precipitates; C and V are molar concentration of different metals and volume of the solution; M and m0 are molar mass and mass of the corresponding precipitate. Three parallel experiments were simultaneously conducted during the whole leaching and precipitation process to avoid random errors, and mean values of the analytical results would be treated as the final experimental results.

Results and discussion Reductive leaching Optimization of the leaching experiments Effect of leaching time. To obtain the reaction equilibrium, the effect of leaching time was investigated at a range from 20min to 160min with an interval of 20min under conditions 9



of reaction temperature-60oC, acid concentration-1.0M, reductant dosage-0.2g/g (mass ratio of reductant and waste cathode materials and the mass of H2O2 can be calculated according to its volume, concentration and density etc.) and slurry density-50g/L. It can be observed from the leaching results (Fig. 2) that leaching efficiencies of Co and Li experience a steady increase as the prolonging of leaching time from 20 min to 80 min and the H3Cit & H2O2 system presents the highest leaching efficiency with shorter leaching time. Besides, higher leaching efficiencies could be obtained using TW as reductant than PA. For instance, only about 52% Co and 81% Li can be leached in H3Cit & PA leaching system, while the leaching efficiencies of Co and Li are 67% and 88% in H3Cit & TW leaching system. Then leaching efficiencies almost level off after 100 min, indicating that leaching reactions have attained the reaction equilibrium within about 100 min. In this study, leaching time of 120 min would be taken as the optimal reaction time to ensure the reaction equilibrium.

Leaching efficiency of Co (%)

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80

A

70 60 50 40

H3Cit + PA

30

H3Cit + TW H3Cit + H2O2

20 10 0

20

40

60

80

100

120

140

Leaching time (min)

10


160

180

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100

Leaching efficiency of Li (%)

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B

90 80 70 60 50

H3Cit + PA H3Cit + TW

40

H3Cit + H2O2

30 20 0

20

40

60

80

100

120

140

160

180

Leaching time (min) Fig.2. Effect of reaction time on leaching of Co (A) and Li (B) (Reaction temperature- 60oC, acid concentration1.0M, reductant dosage- 0.2g/g and slurry density- 50g/L).

Effect of reaction temperature. As an important factor, reaction temperature presents a significant effect on the leaching of different metals under conditions of leaching time- 120 min, acid concentration- 1.0 M, reductant dosage- 0.2 g/g and slurry density- 50 g/L (see Fig. 3). The leaching efficiencies of Co and Li witness a steady increase with the increase of reaction temperature from 50oC to 70oC, and Li can be leached from waste cathode materials much more easily than Co. In H3Cit & H2O2 system, the optimal reaction temperature can be achieved at a low temperature of 70oC, with the maximum leaching efficiencies of 78% Co and 93% Li. However, the leaching efficiency of Co may be discouraged by excessively high temperature, which may be attributed to the decomposition of H2O2. In H3Cit & TW system, the leaching efficiency of Co increases continuously from 70 to 90oC and about 85% Co can be leached at 90oC, indicating that higher temperature would be beneficial for the leaching of Co. In H3Cit & PA system, the optimal reaction temperature is 80oC, under which about 70% Co and 91% Li can be leached out.

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90

80

A

70

60


50

H3Cit + H2O2 40

30 50

60

70

80

90

100

o

Reaction temperature ( C)

95


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B

90

85

80


75

H3Cit + H2O2 70

65 50

60

70

80

90

100

o

Reaction temperature ( C) Fig.3. Effect of reaction temperature on the leaching of Co (A) and Li (B) (leaching time- 120 min, acid concentration- 1.0 M, reductant dosage- 0.2 g/g and slurry density- 50 g/L).

Effect of acid concentration. Fig. 4 illustrates experimental results concerning the effect of acid concentration on the leaching of waste LiCoO2. It can be observed that the leaching efficiencies of Co and Li experience a gradual increase from 0.5 M to 1.5 M regardless of the leaching systems. In addition, lower acid concentration is required in H3Cit & PA and H3Cit & TW systems (1.5 M) than H3Cit & H2O2 system (2.0 M), indicating that the H3Cit & PA and H3Cit & TW systems can be also capable under mild leaching conditions. The maximum 12


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leaching efficiencies for Co and Li are 88% and 97% in H3Cit & TW system and 78% and 94% in H3Cit & PA system under optimized citric acid concentration of 1.5 M. The optimal concentration for H3Cit & H2O2 system is 2 M, under which about 97% Co and 98% Li can be extracted from waste LiCoO2. However, the leaching efficiencies of Co and Li witness a slight decline as the acid concentration increase from 2 to 2.5 M. This phenomenon may be ascribed to salting-out effect during leaching at the presence of Li+, Co2+, Cit3-, HCit2- and H2Cit- ions as the enhanced ionic potential due to the increase of acid concentration.31,32


100 90

A

80 70 60

H3Cit + PA 50


40 30 0.5

1.0

1.5

2.0

2.5

Acid concentration (M)

100


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95

B

90 85 80

H3Cit + PA

75

H3Cit + TW

70

H3Cit + H2O2

65 0.5

1.0

1.5

2.0

2.5

Acid concentration (M) Fig.4. Effect of citric acid concentration on the leaching of Co (A) and Li (B) (Leaching time- 120min, reaction 13



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temperature- 70oC, 80oC and 90oC for H3Cit & H2O2, H3Cit & PA and H3Cit & TW systems, reductant dosage0.2g/g and slurry density- 50g/L).

Effects of reductant dosage and slurry density. Fig. 5A shows effects of the reductant dosage and slurry density on the leaching of Co and Li under the above optimized leaching conditions. Without the addition of reductant, leaching efficiencies of Co and Li are only 36% and 61%, which is insufficient for the extraction of Co and Li from waste LiCoO2. Then a steady increase of the leaching efficiencies can be obtained with the addition of reductant dosage from 0 to 0.4 g/g. Afterwards, the leaching efficiency of Co experiences an obvious decline by adding excessive TW, which may be attributed to the adsorption of heavy metal (Co ions) in surface/inner of TW and this had been thoroughly investigated by previous studies.33-36 Besides, there is also a decline trend in H3Cit & PA system, which may be also ascribed to the adsorption effect of PA. However, the leaching efficiencies of Co and Li almost level off in the H3Cit & H2O2 system from 0.6 g/g to 0.8 g/g. Therefore, the optimal reductant dosages are 0.4, 0.4 and 0.6 g/g for H3Cit & PA, H3Cit & TW and H3Cit & H2O2 systems, respectively. The slurry density also plays an important role on leaching (as reflected in Fig. 5B). The leaching efficiencies of different metals decline with the enhancement of slurry density form 40 g/L to 100 g/L and leaching efficiencies of Co and Li almost level off from 10 to 40 g/L. The highest leaching efficiencies can be achieved in H3Cit & H2O2, under which about 98% Co and 99% Li can be leached at 50 g/L. In H3Cit & TW system, similar leaching efficiencies (96% Co and 98% Li) can be also attained at 30 g/L. And the maximum leaching efficiencies for Co and Li are about 83% and 96% at 40 g/L in H3Cit & PA system.

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100

90

A1



100

80 70 60

H3Cit + AB 50

H3Cit + TW

40

H3Cit + H2O2

95

A2

90 85 80

H3Cit + AB

75

H3Cit + TW

70

H3Cit + H2O2

65 60

30 0.0

0.2

0.4

0.6

0.8

0.0

1.0

0.2

Reductant dosage (g/g)

0.4

0.6

0.8

1.0

Reductant dosage (g/g)

100

B1

90

H3Cit + AB

80

H3Cit + TW

70

H3Cit + H2O2

B2

100



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60 50 40 30 20

90

80

H3Cit + AB 70


60

50

20

40

60

80

100

20

Slurry density (g/L)

40

60

80

100

Slurry density (g/L)

Fig.5. Effect of reductant dosage and slurry density on leaching of Co (A1, B1) and Li (A2, B2) (Leaching time120min, reaction temperature- 70oC, 80oC and 90oC, citric acid concentration- 2.0 M, 1.5 M and 1.5 M for H3Cit & H2O2, H3Cit & PA and H3Cit & TW systems, respectively).

Tentative exploration of the oxidation mechanism As a typical reductive leaching reaction, leaching efficiencies of different metals can be greatly facilitated by the addition of reductant. As investigated by Li et al.,17 H2O2 can be decomposed and reduced into H2O and O2 in H3Cit & H2O2 system. Besides, PA can be also treated as reductant for its high content of cellulose/hemicellulose (see Table 1), which can be degraded into D-glucose, fructose etc. in acidic condition and reductive groups contained in these degradation products can be taken as potential reductant source.37 The reducing substances (e.g. D-glucose) will be eventually oxidized into other eco-friendly organics, which had been thoroughly investigated by Pagnanelli et al.38 In addition, relatively high contents of reducing substances (e.g. epigallocatechin gallate or EGCG, tea polyphenol) contained in TW can also improve the leaching efficiency, which has been experimentally confirmed by the leaching experiments. 15



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According to analytical results of infrared spectra of PA and TW (see Fig. 6), significant changes of stretching vibration and bending vibration could be detected for both PA and TW after leaching. For PA, the characteristic peaks before and after leaching of 1741.5/1743.4, 1637.3/1647.0, 1512.0/1512.0, 1039.5/1053.3 (see Fig. 6A) indicate the changes of C=O stretch (esters, saturated aliphatic), -C=C- stretch (alkenes), N-O asymmetric stretch (nitro compounds), C-N stretch (aliphatic amines), respectively. It can be found that obvious violet shifts (increased wave numbers) occurred during leaching, indicating that the oxidation of PA accompanied with the leaching reaction. Besides, the reducible substances degraded such as D-glucose may also promote the leaching efficiency.39 In H3Cit & TW system (see Fig. 6B), characteristic peaks of different functional groups also experience significant changes after leaching. For instance, the absorbance of -OH stretch (at 3411.5/3336.4) witnesses an obvious decline after leaching, indicating that this polyhydric groups were oxidized to carboxylic or aldehyde groups (e.g. wave numbers at 2364.4, 1722.2). Furthermore, there are remarkable changes of other characteristic peaks in oxidation reaction zone (e.g. 1633.5: -C=C- stretch; 1569.8 and 1486.9: C-C stretch in-ring; 1043.4: C-N stretch after leaching), which can further demonstrate the reducibility of TW. The possible oxidation pathways for TW were proposed based on the above analytical results (see Fig. 7), in which the main reducing substances (epigallocatechin gallate (EGCG), epigallocatechin (EGC) and theaflavin-3,3’digallate) may be oxidized into different oxidation products in the presence of Co(Ⅲ). And the above leaching reactions in H3Cit & PA and H3Cit & TW systems can be briefly expressed as Eq. (10) and Eq. (11). LiCoO2 + H3Cit + TW → Co3(Cit)2 + Co(HCit) + Co(H2Cit)2 + Li3Cit + Li2(HCit) + Li(H2Cit) + OD1

(10)

LiCoO2 + H3Cit + PA → Co3(Cit)2 + Co(HCit) + Co(H2Cit)2 + Li3Cit + Li2(HCit) + Li(H2Cit) + OD2

(11)

where OD1 and OD2 stand for the corresponding oxidized derivatives.

16


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Page 17 of 28

A

T (%)

1743.4 1647.0 1512.0 1053.0

PA PA residues

Oxidation reaction zone

1741.5 1512.0 1637.3

4000

3500

3000

2500

2000

1500

1039.5

1000

500

-1

Wavelength (cm )

B 1629.6

2925.5


T (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3411.5

848.6 1448.3

2364.4

1519.7 2925.6

1722.2

3336.4

667.3

1043.4

1633.5

TW TW residues 4000

3500

3000

2500

2000

1500

1000

500

-1

Wave number (cm ) Fig.6. Infrared spectra of PA (A) and TW (B) before and after leaching (the red dotted box represents the main oxidation reaction zone).

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

HO HO

HO

OH OH

O

O

OH

HO

OH

O

HOOC

OH OH

O OH

Co3+

OH OH

O O

OH

OH OH OH

O

HOOC

O

OH

A HO HO

OH

HO OH

O OH

EGC

OH O

O

OH

OH

Co3+

O

O OH

OH

O

OH OH OH

OH

O

OH

O

+

O

O

Theaflavin-3, 3’ digallate

O

OH HO

HOOC

OH

OH

HOOC

OH HOOC

OH

O

HOOC

O

OH OH OH

O

OH

OH

O

O

OH

OH O

OH

O

OH

B

OH OH

OH

O

HOOC

OH

O

OH

Co3+

COOH

HO

OH

O

OH

EGCG

HO

HOOC

+

HOOC

Page 18 of 28

O O OH

OH OH OH

O OH

C

Fig.7. Possible oxidation pathways of the main reductants contained in TW (A- oxidation pathway of EGCG; Boxidation pathway of EGC and C- oxidation pathway of theaflavin-3,3’digallate).

Material recovery procedure Recovery of metals After leaching, the lixivium (H3Cit & H2O2 system will be taken as an example for the following metal and waste acid recovery process) was consecutively treated with H2C2O4 and H3PO4 solutions, and Co and Li were recovered as CoC2O4·2H2O and Li3PO4. Eq. (12) to (14) list the chemical equations and the corresponding solubility product constants40 (pKsp) of relevant precipitates (CoC2O4, Li2C2O4 and Li3PO4). CoC2O4 ↓ = Co2+ + C2O42-

pKsp = 7.2

(12)

Li2C2O4 ↓ = 2Li+ + C2O42-

pKsp = 1.9

(13)

Li3PO4 ↓ = 3Li+ + PO43-

pKsp = 3.4

(14)

18


Page 19 of 28

It can be clearly concluded from above equations that the pKsp of CoC2O4 (7.2) is much greater than that of Li2C2O4 (1.9), indicating Co2+ will be precipitated before the precipitation of Li+ by addition H2C2O4 to leaching liquor. Then the lithium ions were precipitated using H3PO4 solution. As reflected in Fig. 8A, almost all Co (about 99%) and about 93% Li can be recovered as CoC2O4·2H2O (as reported in our previous study13) and Li3PO4 8 (99.3% and 98.5% in purities, respectively) at conditions of 60oC, 30 min, 300 rpm and n(H2C2O4):n(Co2+) =1.05 or n(H3PO4):n(Li+)=0.4 (molar ratio). Fig. 8B shows XRD patterns and real products of Li3PO4 and CoC2O4·2H2O. It can be discovered that obtained products are relatively pure compounds.

100

A

98.9% 99.3%

B

98.5% 92.6%

CoC2O4·2H2O 80

60

Intensity / a.u.

Recovery rate or purity / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Recovery rate Purity

40

Li3PO4

20

0 Co

Li

0

10

20

30

40

50

60

70

80

2 Theta / degree

Fig.8. Recovery of Co and Li from leaching liquor (A: recovery rate and purity of CoC2 O4·2H2O and Li3PO4; B: XRD patterns and the real products recovered).

Recycling of waste citric acid In this study, a closed-loop process was proposed to recycle waste acid and the recycled citric acid will be reused as leaching reagent. The leaching reaction (H3Cit & H2O2 system) could be expressed as follows (Eq. (15)): H3Cit + LiCoO2 + H2O2 → Co3(Cit)2 + Co(HCit) + Co(H2Cit)2 + Li3Cit + Li2(HCit) + Li(H2Cit) + H2O + O2

(15)

If we superimpose Eq. (2), (3), (4), (5), (6), (7) and (15), a total reaction equation can be then obtained as follows (Eq. (16)) 4,8,17: LiCoO2 + H2O2 + H2C2O4 + H3PO4 → CoC2O4 + Li3PO4 + O2 + H2O

19


(16)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

It can be concluded from Eq. (16) that citric acid seems to be eliminated during the total recovery process and products (CoC2O4 and Li3PO4) can be directly obtained from the waste LiCoO2 after reductive leaching and selective precipitation. Residues can be then returned to circulatory leaching process and H2O and O2 are the only by-products, indicating that metals can be recovered in precipitate forms and citric acid can be repeatedly used as leaching reagent after precipitation without adverse impact on environment (produced only H2O and O2 theoretically). Tentatively theoretical exploration (solution chemistry theory analysis in terms of acidity coefficient (Ka or pKa) analysis) and characterization of recycled citric acid were carried out to verify the existence of the recycled citric acid in precipitation solution. Table 3 shows these constants of different acids in aqueous phase at 25oC.40 It can be found that Ka1 and pKa1 (determining the acidity) present in the following order: H2C2O4 > H3PO4 > H3Cit (Ka1: 5.4×10-2, 7.5×10-3 and 7.4×10-4 and pKa1: 1.27, 2.12 and 3.13 for H2C2O4, H3PO4 and H3Cit, respectively). Therefore, it is theoretically feasible by the addition of stronger acids (H2C2O4 and H3PO4) to replace weaker acid (H3Cit) from leaching solution by selective precipitation. Then circulatory leaching experiments were conducted to verify the practical feasibility and Fig. 9 presents the results of circulatory leaching experiments at optimized leaching conditions. It can be concluded that the leaching efficiency of Co experiences a mild decline from 96% to 92% and the leaching efficiency of Li keeps almost stable after 5 cycles (about 98%, 96%, 98%, 95% and 97% from Cycle 1 to 5). Then ultraviolet spectrophotometer was employed for the approximate determination of the acid concentration, and the concentrations of citric acid are 1.97, 1.91, 1.85, 1.87 and 1.83 from Cycle 1 to 5. Based on the above analysis results, it can be concluded that circulatory leaching may be not discouraged by the un-conspicuous decrease of Co leaching efficiency and relatively stable concentration of the citric acid, which also indicate that the recycled citric acid can be repeatedly used as leaching reagent. In addition, it can be also discovered from Fig. 10 (ultraviolet spectra analysis) that the fresh citric acid (Cycle 1) and the recycled citric acid (Cycle 2) demonstrate the maximum absorbance at similar wavelengths (235nm and 225nm) which are within the characteristic absorption wavelength range of citric acid (210-240nm). The minor shift in wavelength may 20


Page 20 of 28

Page 21 of 28

be attributed to the skewing of carbonyl absorption peaks in different solutions,41 indicating that waste citric acid can be replaced by stronger acids during precipitation reactions. Table 3 Ionization constants for different acids in aqueous phase (25oC). Acid H2C2O4 H3PO4

H3Cit

Ionization constant

Ionization equation -

Ka +

pKa

-2

H2C2O4 = HC2O4 + H HC2O4- = C2O42- + H+ H3PO4 = H2PO4- + H+ H2PO4- = HPO42- + H+ HPO42- = PO43- + H+ H3Cit = H2Cit- + H+ H2Cit- = HCit2- + H+ HCit2- = Cit3- + H+

5.4×10 (K1) 5.4×10-5 (K2) 7.5×10-3 (K1) 6.3×10-8 (K2) 4.4×10-13 (K3) 7.4×10-4 (K1) 1.7×10-5 (K2) 4.0×10-7 (K3)

1.27 (pKa1) 4.27 (pKa2) 2.12 (pKa1) 7.20 (pKa2) 12.36 (pKa3) 3.13 (pKa1) 4.76 (pKa2) 6.40 (pKa3)

120

Co Li

Circulatory leaching under optimized conditions 100

Leaching efficiency / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

60

40

20

0 1

2

3

4

5

Cycle(s)

Fig. 9. Circulatory leaching experiments under the optimized conditions (80 min, 70 oC, 2.0 M, reductant dosage- 0.6 g·g-1 and slurry density- 50 g·L-1).

21



3.5 235nm

3.0

225nm

2.5

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 28

2.0

Fresh citric acid Recycled citric aicd

1.5 1.0 0.5 0.0 -0.5 200

300

400

500

600

700

800

Wavelength (nm) Fig. 10. Analysis results of ultraviolet spectra for the fresh citric acid (Cycle 1) and the recycled citric acid (Cycle 2).

Waste stream analysis In the whole recovery process, metals in the waste cathode materials after pre-treatment, reductive leaching and selective precipitation can be recovered in their metallic or precipitate forms (Al/Cu foils, CoC2O4·2H2O and Li3PO4). Other lower-valued scraps (e.g. separators, metallic shells) can be also recovered as their original forms. It can be concluded from above results that citric acid can be reused with excellent leaching performance. Theoretically, H2O and O2 are the only by-products in the whole recovery process. Therefore, this green process may promise a closed-loop route for the sustainable recovery of metals from spent LIBs.

Conclusions Exhausted LIBs without proper disposal or recycling may present potential threats to the environment and human health. On the other hand, the sustainable recovery of metals from spent LIBs may promise economic and environmental benefits. This work is exactly focused on a green strategy for the sustainable recovery of metals from spent LIBs. Based on the experimental and theoretical results, following conclusions could be obtained. (1). Reductive leaching

22


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Three leaching systems (H3Cit & H2O2, H3Cit & PA and H3Cit & TW systems) were explored for the innovative application of different biomass as reductants. In H3Cit & H2O2 system, about 98% Co and 99% Li could be leached under the optimized conditions of 80 min, 70oC, 2.0M, reductant dosage- 0.6g·g-1 and slurry density- 50g·L-1. Similar leaching efficiencies (96% Co and 98% Li) could be attained in H3Cit & TW system at the optimal conditions: 120min, 90oC, 1.5M, reductant dosage- 0.4g·g-1 and slurry density- 30g·L-1. For H3Cit & PA system, inferior leaching results (83% Co and 96% Li) can be obtained under the optimized conditions: 120min, 80oC, 1.5M, reductant dosage- 0.4g·g-1 and slurry density- 40g·L-1. According to the oxidation mechanism of PA and TW, degraded products or reducible substances contained will facilitate the extraction of Co and Li from waste LiCoO2 during the leaching reactions. (2). Materials recovery Both metals and waste citric acid could be simultaneously recycled in the selective precipitation process. About 99% Co and 93 % Li could be recovered as CoC2O4·2H2O and Li3PO4 and the recycled citric acid also demonstrates similar leaching performance with fresh acid after 5 cycles under the same conditions. Then solution chemistry theory, ultraviolet spectrum analysis and waste stream analysis confirmed that it is theoretically feasible for the recovery of metals and waste acid in a single precipitation process.

Acknowledgements This study was financially supported by the Fundamental Research Funds for the Central Universities of Central South University (No. 72150050350) and National Natural Science Foundation of China (contract No. 21176266).

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26. Zeng, X., Li, J., Innovative Application of Ionic Liquid to Separate Al and Cathode Materials from Spent High-power Lithium-ion Batteries. J. Hazard. Mater. 2014, 271, 50-56. 27. Li, J., Zeng, X., Chen, M., Ogunseitan, O.A., Stevels, A., “Control-Alt-Delete”: Rebooting Solutions for the E-waste Problem. Environ. Sci. Technol. 2015, 49, 7095-7108. 28. Yoo, K.; Shin, S.-M.; Yang, D.-H.; Sohn, J.-S., Biological treatment of wastewater produced during recycling of spent lithium primary battery. Miner. Eng. 2010, 23, (3), 219-224. 29. Tang, Q.; Zhong, H.; Wang, S.; Li, J.-z.; Liu, G.-y., Reductive leaching of manganese oxide ores using waste tea as reductant in sulfuric acid solution. T. Nonferr. Metal. Soc. 2014, 24, (3), 861-867. 30. Cheng, Z.; Zhu, G.; Zhao, Y., Study in reduction-roast leaching manganese from low-grade manganese dioxide ores using cornstalk as reductant. Hydrometallurgy 2009, 96, (1-2), 176-179. 31. Chung, N. H.; Tabata, M., Salting-out phase separation of the mixture of 2-propanol and water for selective extraction of cobalt(II) in the presence of manganese(II), nickel(II), and copper(II). Hydrometallurgy 2004, 73, (1-2), 81-89. 32. Ma, L.; Nie, Z.; Xi, X.; Han, X. g., Cobalt recovery from cobalt-bearing waste in sulphuric and citric acid systems. Hydrometallurgy 2013, 136, 1-7. 33. Weng, C.-H.; Lin, Y.-T.; Hong, D.-Y.; Sharma, Y. C.; Chen, S.-C.; Tripathi, K., Effective removal of copper ions from aqueous solution using base treated black tea waste. Ecol. Eng. 2014, 67, 127-133. 34. Amarasinghe, B. M. W. P. K.; Williams, R. A., Tea waste as a low cost adsorbent for the removal of Cu and Pb from wastewater. Chem. Eng. J. 2007, 132, (1–3), 299-309. 35. Ng, I. S.; Wu, X.; Yang, X.; Xie, Y.; Lu, Y.; Chen, C., Synergistic effect of Trichoderma reesei cellulases on agricultural tea waste for adsorption of heavy metal Cr(VI). Bioresource Technol. 2013, 145, 297-301. 36. Malkoc, E.; Nuhoglu, Y., Removal of Ni(II) ions from aqueous solutions using waste of tea factory: Adsorption on a fixed-bed column. J. Hazard. Mater. 2006, 135, (1–3), 328-336. 37. Hariprasad, D.; Dash, B.; Ghosh, M. K.; Anand, S., Leaching of manganese ores using sawdust as a reductant. Miner. Eng. 2007, 20, (14), 1293-1295.

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38. Pagnanelli, F.; Moscardini, E.; Granata, G.; Cerbelli, S.; Agosta, L.; Fieramosca, A.; Toro, L., Acid reducing leaching of cathodic powder from spent lithium ion batteries: Glucose oxidative pathways and particle area evolution. J. Ind. and Eng. Chem. 2014, 20, (5), 3201-3207. 39. Granata, G.; Moscardini, E.; Pagnanelli, F.; Trabucco, F.; Toro, L., Product recovery from Li-ion battery wastes coming from an industrial pre-treatment plant: Lab scale tests and process simulations. J. Power Sources 2012, 206, 393-401. 40. Speight, J.G., Lange’s Handbook of Chemistry, McGraw-Hill, New York, 2005. 41. Wang, L.Q.; Li, X.Z.; Jiang, X.X.; Chen, W.S.; Hu, L.S.; Walle, M.D.; Deng, L.; Yang, M.H.; Liu, Y.N.; and Kirin, S.I., When protein-based biomineralization meets hydrothermal synthesis: the nanostructures of the as-prepared materials are independent of the protein types. Chem. Commun. 2015, DOI: 10.1039/C5CC06846K.

27



For Table of Contents Use Only

Sustainable recovery of metals from spent lithium-ion batteries: A green process Xiangping Chena, Chuanbao Luoa, Jinxia Zhanga, Jiangrong Konga* and Tao Zhoua*

A

C

T/%

1512.0 1053.0


1741.5 1512.0 1637.3

PA PA residues

1039.5

4000 3500 3000 2500 2000 1500 1000

Li3PO4

500

0

10

Wave number / cm-1

B 1629.6

2925.5

Oxidation reaction zone 3411.5

848.6 1448.3

2364.4

1519.7 2925.6 3336.4

TW TW residues

1722.2

667.3

1043.4

1633.5

4000 3500 3000 2500 2000 1500 1000

500

30

40

50

60

70

80

D

Co Li

Circulatory leaching

100

Wave number / cm-1

Recycling of Al/Cu foils

20

2 Theta / degree

120

Leaching efficiency / %

Pre-treatment

CoC2O4·2H2O

Intensity / a.u.

1743.4 1647.0

T/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 28

Reductive leaching

80 60 40 20 0

1

2

3

4

5

Cycle (s)

Materials Recovery

(A, B- infrared spectra of PA and TW before and after leaching; C-metals recovery, D-recycling of citric acid)

Synopsis: Metals in spent lithium-ion batteries were revovered by a green process combined with reductive leaching and selective precipitation.

28


Sustainable Recovery of Metals from Spent Lithium-Ion Batteries: A

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