Pyrolysis-ultrasonic-assisted flotation technology for recovering

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Pyrolysis-ultrasonic-assisted flotation technology for recovering graphite and LiCoO2 from spent Lithium-ion battery Guangwen Zhang, Yaqun He, Yi Feng, Haifeng Wang, and Xiangnan Zhu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02186 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018

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ACS Sustainable Chemistry & Engineering

Pyrolysis-ultrasonic-assisted flotation technology for recovering graphite and LiCoO2 from spent Lithium-ion battery Guangwen Zhang†, Yaqun He†‡*, Yi Feng†, Haifeng Wang†, Xiangnan Zhu§ †

School of Chemical Engineering and Technology, China University of Mining and Technology,

No.1 Daxue Road, Xuzhou, Jiangsu 221116, China; ‡

Advanced Analysis and Computation Center, China University of Mining and Technology, No.1

Daxue Road, Xuzhou, Jiangsu 221116, China. §

College of Chemical and Environmental Engineering, Shandong University of Science and

Technology, No. 579 Qianwangang Road, Qingdao, Shandong 266590, China. *Address correspondence to Yaqun He, School of Chemical Engineering and Technology, China University of Mining and Technology, No.1 Daxue Road, Xuzhou 221116, Jiangsu, China. Tel +86 516 83592928, Fax +86 516 83995026, E-mail: [email protected] (Y. He). First author: Guangwen Zhang, Email: [email protected] (G. Zhang).

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ABSTRACT: An environmental-friendly technology of pyrolysis-ultrasonic-assisted flotation for recovering graphite and LiCoO2 from spent Lithium-ion battery has been conducted in this paper. Surface properties and morphology of graphite and LiCoO2 derived from spent Lithium-ion battery were carefully studied and on this basis their pyrolysis characteristics were investigated by thermogravimetry gas chromatograph-mass spectroscopy. Advanced analysis techniques, scanning electron microscope, X-ray fluorescence spectrometer and X-ray photoelectron spectroscopy, were utilized to analyze the effect of pyrolysis and ultrasonic on the surface properties and morphology of graphite and LiCoO2. Flotation tests were conducted to evaluate the reinforcing effect of pyrolysis-ultrasonic on flotation behavior. Results show that organic binder and electrolyte is the main reason that graphite and LiCoO2 are hard to be separated by flotation, meanwhile, pyrolysis can effectively decompose organic binders at pyrolysis temperature of 500 °C and ultrasonic cleaning can effectively remove residual pyrolysis products. Pyrolysis-ultrasonic-assisted flotation can make the LiCoO2 grade improved from 67.25% to 93.89% with the recovery improving from 74.62% to 96.88%. This research work may provide an alternative process for the preparation of high purity LiCoO2 particles for the subsequent chemical metallurgy. KEYWORDS: Spent Lithium-ion battery; Recycling; Electrode materials; Pyrolysis-ultrasonic flotation

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INTRODUCTION Since Lithium-ion batteries (LIBs) are the important link between sustainable energy and people’s daily life, they have been extensively used in various electronics, such as cameras, mobile phones, laptop computers etc. because of their high energy density, low self-discharge, long storage life, and safe handling 1. With the rapid development of new-energy vehicle, large amounts of LIBs will play more important role in energy storage and conversion. However, the average life time of LIBs is 2-3 years, this means that the expanding demand for LIBs will produce a large quantity of spent LIBs in the next few years 2. These spent LIBs contain valuable materials including metallic shell, anode, cathode, membrane separator and other hazardous materials such as electrolyte. Cathodes are mainly composed of aluminum foil and cathode materials i.e. LiCoO2, LiMn2O4, LiFePO4, as well as other lithium metal oxides while anode contains copper foil and graphite 3. Therefore, it is a significant process for disposal of spent LIBs from the viewpoint of recourse recycling and environmental protection. Because of the high content of valuable metals, especially for Co, Li, Mn, Ni, Cu, recycling spent LIBs has attracted more and more attention. Researchers have proposed many sophisticated technologies to recycle spent LIBs. Hydrometallurgy processes play an important role in recycling process of valuable metals release from spent LIBs and it accounts for approximate 52% in the total recycling technologies 4. And the main processes of hydrometallurgy comprise chemical leaching 5-7, bio-leaching 8, ion exchange 9, chem ical deposition 10, electrochemistry 11, as well as mechanochemistry

12,13

. In addition, pyrometallurgy is another effective technology for recycling

valuable metals from spent LIBs

14,15

. However, previous hydrometallurgy and pyrometallurgy

technologies always focus on high-economic value metals recovery from cathode materials 3



because their high cost and potential secondary pollution. How to obtain cathode materials with high purity is the core process that determines the hydrometallurgy efficiency. Now, the spent LIBs are firstly dismantled and then cathodes are collected to be as objective in current hydrometallurgy process. The manual dismantling process make the large-scale industrialize difficult because of economic and healthy perspective. It is urgent to develop pretreating technology of spent LIBs to obtain high purity cathode materials for hydrometallurgy process. Recently, physical processes are proposed to pretreat spent LIBs to obtain high purity cathode materials, which can provide raw materials for the hydrometallurgy technologies. Physical processes consist of discharging, crushing, sieving, and separation 16-18. The spent LIBs were fully discharged and the impact crusher was utilized to break up the spent LIBs and realize the liberation among components. The crushing products were sieved to obtain fine electrode mixture. The high-efficient flotation technology has been proved to realize the separation between cathode and anode materials because of their obvious hydrophilic/hydrophobic difference caused by different crystal texture

19,20

. By this flowchart, the high purity cathode materials were obtained.

However, electrode particles from the mechanical crushing process are enveloped by organic binder and residual organic electrolyte, which results in their original surfaces are not exposed and flotation process cannot realize the efficient separation of cathode and anode materials because of the same hydrophilia and hydrophobicity. Removing of organic binders and electrolyte from cathode and anode particles will determine the flotation efficiency. Removal of organic binders and electrolyte using Fenton solution has been reported by He et al. 21. However, the addition of Fe2+ in Fenton solution will remain in electrode particles, which make the subsequent chemical purification complex

22

. Roasting technology has been used to remove organic binders and 4


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electrolyte, but secondary pollution will be produced in the combustion process

23,24

. High

flotation efficiency has been obtained, but it should not be recommended. Pyrolysis is a useful method to recover organic materials, and it has been used in recycling of waste printed circuit boards and waste plastics

25,26

. Pyrolysis also has been studied to make electrode materials

liberated from copper/aluminum foils by Sun and Qiu

27,28

. They found that pyrolysis is an

efficiency method to remove organic binders from electrode materials. Pyrolysis is used to remove organic binders and electrolyte, it cannot only remove the organic binder but also can recycle organics. Ultrasonic technology has been utilized to clean particle’s surface to improve their flotation behavior

29,30

. Therefore, pyrolysis-ultrasonic pretreatment may remove organic binder

and electrolyte on the surface of electrode particles and enhance their flotation process. In this paper, a novel method of pyrolysis-ultrasonic-assisted flotation was proposed to recycle graphite and LiCoO2 from spent LIBs. Pyrolysis characteristics of electrode materials were analyzed by thermogravimetry gas chromatograph-mass spectroscopy (TG-GC/MS). Advanced analysis technologies of scanning electron microscope (SEM), X-ray fluorescence spectrometer (XRF) and X-ray photoelectron spectroscopy (XPS) were used to obtain the morphology, chemical composition, and chemical states of surface elements of electrode material particles before and after pyrolysis. Based on these analysis, the non-floatability mechanism of electrode materials was analyzed. Effect of pyrolysis on the surface chemical properties and floatability of electrode materials was investigated. Pyrolysis-ultrasonic-assisted flotation experiments were conducted to evaluate the separating behavior of electrode materials. EXPERIMENTAL Sample preparation. Spent LIBs from waste mobile phones were collected and pretreated 5



following the procedure shown in Fig.1. They were firstly discharged with 5wt% NaCl solution for 48 h and then naturally air dried. To obtain pure cathode and anode materials, manual dismantling process instead of mechanical crushing was conducted, and then the cathode and anode were obtained from this process, respectively. Electrode materials were obtained by combining crushing and sieving process. In order to prevent the over-crushing of aluminum and copper foils, the crushing time is controlled in 25 s. Previous studies have demonstrated that electrode materials are mainly concentrated in fine size fraction. Therefore, the 0.075 mm size screen hole was used to obtain electrode materials with fine size in this study. Three kinds of samples were prepared including cathode materials (LiCoO2), anode materials (Graphite), and electrode material mixture (LiCoO2 and graphite). As a result, the cathode material and anode material are obtained to respectively analyze their physico-chemical properties and pyrolysis characteristics. Electrode material mixtures will be processed by pyrolysis or pyrolysis-ultrasonic and then used for flotation.

Fig.1 Pretreating flowchart of spent LIBs Sample pyrolysis procedures. Firstly, pyrolysis characteristics of cathode and anode materials

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were respectively analyzed by TG-GC/MS that included Thermogravimetric Analyzer (TGA) (Labsys Evo1600, France), Auto Injector System (Automation Autoinjector Corp., Italy) and GC/MS (TRACE GC Ultra/ISQ Single Quadrupole MS, USA). TG tests were first conducted to analyze the decomposition process of organics in raw LiCoO2 and graphite particles. And then pyrolysis products at weight loss temperature were collected and injected into GC-MS system by auto injector system. Pyrolysis experiments of electrode material mixtures were conducted in a controlled atmosphere tube furnace (MXG1200-80, Shanghai Micro-X furnace Co., Ltd.). In each experiment, the 5 g sample was placed into the corundum tube and were sealed by flanges at both ends. Vacuum pump was firstly applied to provide the vacuum environment, and then the corundum tube was filled with high purity nitrogen with the gas-flow of 200 ml/min. Gas pyrolysis products could be taken out of tube by nitrogen flow and then macromolecular organics could be collected by condensation while small molecule organic gases were treated by gas-treating system. Pyrolysis temperature increased from an initial temperature of 30 °C to terminal temperature of 500 °C at a heating rate of 10 °C/min. Samples would be processed at isothermal temperature of 500 °C for 30 min. Afterwards, the samples were naturally cooled to 30 °C at nitrogen atmosphere. After pyrolysis, a part of pyrolytic electrode materials were treated by ultrasound at solid/liquid ratio of 1:10 g/ml with an ultrasonic frequency of 25 kHz, electric power of 800 W for 2 h. Afterwards, electrode materials were separated by filtration method and dried at 80 °C. By these processes, three samples were prepared for flotation: raw electrode material mixture (REM), pyrolytic electrode material mixture (PEM), and pyrolysis-ultrasonic pretreated electrode material mixture (PUEM). Flotation procedures. Three kinds of flotation feeds (REM, PEM, PUEM) were utilized in the 7



flotation process. LiCoO2 is hydrophilic due to the polarity ionic crystal whereas anode material graphite is hydrophobic due to its nonpolarity. Thus, graphite will attach to the bubble’s surface and then is recovered from the top foam layer, while LiCoO2 is collected in the tailings in the flotation process. Flotation experiments were conducted using XFD-63 flotation machine at pulp density of 40 g/L, impeller speed of 1800 rpm, aeration quantity of 2.0 L/min, the flow sheet of flotation is shown in Fig.2. n-Dodecane and methyl isobutyl carbinol (MIBC) were used as collector and frother, and their dosage were 300 g/t and 150 g/t, respectively. LiCoO2 concentrate and graphite concentrate were obtained by filtration and drying, and then their compositions are obtained from XRF analysis. The recovery rate of LiCoO2 defined by formula (1) is adopted to evaluate the separation efficiency. Recovery rate

(1)

Where Gt is the grade of LiCoO2 in LiCoO2 concentrate (%), Yt is the yield of LiCoO2 concentrate (%) and GT is the LiCoO2 grade in electrode mixture (%).

Fig.2 The flotation flowchart of electrode materials 8


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Measurement methods. The TG-GC/MS was used to analyze the pyrolysis products. An XPS (ESCALAB 250Xi, America) at room temperature in an ultra-high vacuum (UHV) system was used to analyze the chemical states of surface elements. By XPS analysis, element content on the surface of the electrode particles could be obtained to present the non-floatability of electrode materials. The morphology of the electrode material particles before and after pyrolysis was determined using an SEM (FEI quanta 250, United States). Chemical components of electrode materials, LiCoO2 concentrate, and graphite concentrate were analyzed by XRF (Bruker S8 Tiger, Germany). RESULTS AND DISCUSSION Pyrolysis characteristics of electrode materials. Pyrolysis characteristics of raw LiCoO2 and graphite derived from spent LIBs are analyzed by TG-GC-MS. The TG results are shown in Fig.3 and pyrolysis products at pyrolysis temperature of 120 °C and 500 °C that corresponding to the weight loss peaks in DTG curve are present in Fig.4. The first weight loss stage appears when pyrolysis temperature ranges from 30 °C to 150 °C, this is mainly caused by the volatilization of residual electrolyte. The decomposition of organic binder causes the second weight loss during the temperature of 450 °C to 550 °C. Similar TG and DTG curves of LiCoO2 and graphite demonstrate that the LiCoO2 and graphite used in this study have the same organic binder of PVDF. The DTG analysis shows the maximum weight loss appears at 500 °C, and it indicates that pyrolysis can make organic binders fully decomposed at 500 °C. Therefore, the pyrolysis experiments are conducted at temperature of 500 °C.

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Fig.3 Thermogravimetric analysis of LiCoO2 and graphite Based on the TG analysis, the pyrolysis products of electrode materials at temperature of 120 °C and 500 °C was sampled via the auto injector system that was connected with the GC and TGA system and gas analysis were conducted in GC/MS system. Fig.4 demonstrates that pyrolysis products at pyrolysis temperature of 120 °C come from ester electrolyte, such as Dimethyl

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Carbonate, Ethylene Carbonate, and Propylene Carbonate. Organic pyrolysis products at pyrolysis temperature of 500 °C are mostly fluorides that are mainly come from the decomposition of PVDF, such

as

Vinylidene

fluoride,

1,3,5-Trifluorobenzene,

1,4-difluorobenzene,

1,1,1,3,3,3-hexafluoro-Propane, and 1,2,4-Trifluorobenzene, which demonstrates that PVDF has been decomposed at pyrolysis temperature of 500 °C. In the pyrolysis process, a part of polymer structure of -(CH2-CF2)-n decompose to single molecule of CH2=CF2 while others separate out HF and their carbon chain fracture to form small molecules. Ethylene Carbonate and Propylene Carbonate are from residual electrolyte. These results demonstrate that the main organics in electrode materials are organic binder and electrolyte and pyrolysis can effectively remove organic binder and electrolyte.

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Fig.4 Pyrolysis products of electrode materials at different pyrolysis temperature: (a) Pyrolysis temperature of 120 °C; (b) Pyrolysis temperature of 500 °C. Effects of pyrolysis on the physico-chemical properties of electrode materials. After crushing and sieving processes, phase compositions of the raw electrode materials and pyrolytic electrode materials with size of -0.075 mm were tested by XRD, and their results were shown in Fig.5. No new material peaks are detected by XRD in LiCoO2 or graphite, which indicates that crystal structures of LiCoO2 or graphite are not changed at pyrolysis temperature of 500 °C. At pyrolysis

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temperature of 500 °C, only decomposition of organic binders happens on the surface of electrode material particles and it does not influence the physico-chemical properties of electrode materials, this is advantage for the subsequent separating process. XRD results demonstrate that 500 °C can not only remove organic binders but also not change the properties of electrode materials.

Fig.5 XRD results of cathode and anode materials before and after pyrolysis treatment Changes in surface properties of electrode particles by pyrolysis. Surface chemical compositions of raw LiCoO2 and pyrolytic LiCoO2 were analyzed by XPS. XPS wide energy

13



spectrums of LiCoO2 before and after pyrolysis are shown in Fig.6. Fig.6 demonstrates that the main element compositions detected from surface of LiCoO2 are C, O, Co, and F. The high content of F in raw LiCoO2 particles indicates LiCoO2 particles are cladded by PVDF. By pyrolysis, the content of F decreases from 26.69% to 6.54% while the content of Co increases from 3.28% to 10.66%, which demonstrates that the cathode binder (PVDF) has been decomposed effectively and the surface of LiCoO2 has been exposed.

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Fig.6 XPS wide energy spectrums of LiCoO2 before and after pyrolysis Chemical states of surface elements of raw LiCoO2 and pyrolytic LiCoO2 were carefully analyzed using XPS and the C1s spectrum of raw LiCoO2 and pyrolytic LiCoO2 particles were shown in Fig.7. and Table 1. The C1s spectrum of raw LiCoO2 particles presents a peak at 284.22 eV which is caused by the additive of carbon black and its atomic percentage is 36.73%. Two

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main peaks of -(CH2CF2)-n and -(CH2CF2)-n that are attributed to PVDF were observed at 286.05 eV and 290.30 eV respectively, and their atomic percentage is 19.03% and 5.18% in raw LiCoO2 particles, which indicates the organic binder still remain on the surfaces of electrode powders. Peaks presented at 287.14 eV, 288.06 eV, and 289.00 eV are related to O-C-O, C=O, and O-C=O which are from ester electrolyte, such as Ethylene Carbonate and Propylene Carbonate. After pyrolysis, peaks of -(CH2CF2)-n and -(CH2CF2)-n derived from PVDF and C=O and O-C=O derived from electrolyte have been removed. But at the same time, four new peaks are found on the surface of pyrolytic LiCoO2 particles, they are pyrolytic carbon, CF, C-CF in trifluorobenzene and C-CF in trifluorobenzene, which indicates that a part of pyrolysis products and residue remain on the surface of LiCoO2 particles. The main existing form of C on the surface of LiCoO2 particles is pyrolytic carbon and its content is up to 63.50%. The decrease of oxygen-containing functional groups on the surfaces of electrode particles is advantages for the flotation of LiCoO2 and graphite. But the residue of pyrolytic carbon may be disadvantage for the flotation, therefore, ultrasonic cleaning process is utilized to remove residual pyrolytic products on the surface of pyrolytic electrode particles.

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Fig.7 C1s XPS spectrum of raw LiCoO2 and pyrolytic LiCoO2 particles

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Table 1 Chemical states of carbon in the surface of raw LiCoO2 and pyrolytic LiCoO2 Raw LiCoO2 Components

Pyrolytic LiCoO2

BE

FWHM

Atomic

(eV)

(eV)

(%)

Carbon black

284.22

0.85

36.73

C-C/C-H

284.80

1.01

11.90

C-COOR

285.40

1.00

8.89

-(CH2CF2)-n

286.05

1.25

19.03

O-C-O

287.14

0.96

4.43

C=O

288.06

1.00

3.10

O-C=O

289.00

1.05

4.83

O-COOR

289.99

0.75

5.92

-(CH2CF2)-n

290.30

1.00

5.18

Components

BE

FWHM

Atomic

(eV)

(eV)

(%)

Pyrolytic carbon

283.97

0.97

63.50

C-C/C-H

284.80

1.01

17.52

C-CF in trifluorobenzene

286

1.27

7.04

O-C-O

287.14

1.75

3.45

C-CF in trifluorobenzene

288.10

1.00

0.78

CF

289.4

1.57

7.71

Surface morphology analysis of LiCoO2 and graphite before and after pyrolysis. Surface morphology analysis of raw LiCoO2 and graphite with -0.075 mm size derived from spent LIBs were conducted by SEM, and the SEM images are shown in Fig.8. Because the organic binders have not been removed in the crushing process of electrode materials, the agglomeration still exists in raw LiCoO2 and graphite particles. In addition, the individual particles that have been liberated each other also comprise some organic binders on their surfaces. The SEM images present electrode particles have rough surfaces because of the residual organic binders. Surface morphology analysis indicates two main reason that raw LiCoO2 and graphite are hard to be 18


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separated by flotation: (1) Electrode particles are cladded by organic binders to form agglomeration that decrease the hydrophilic differences between LiCoO2 and graphite; (2) The individual particles also comprise some organic binder and their surfaces are rough, this increase the hydrophilia of graphite. SEM analysis demonstrate that removing organic binders is an essential process in recycling process of spent LIBs.

Fig.8 SEM image of raw LiCoO2 and graphite derived from spent LIBs Surface morphology analysis of pyrolytic LiCoO2 and graphite with -0.075 mm size derived from spent LIBs also conducted by SEM, and the SEM images are shown in Fig.9. From the SEM images, we can find that the agglomeration of electrode materials has disappeared and have adequately liberated each other. Organic binders have been removed and smooth surface has been present after pyrolysis. However, there are some residual pyrolysis products on the surface of

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pyrolytic electrode particles and their main contents are pyrolytic carbon with strong hydrophobicity. Therefore, some pyrolytic LiCoO2 particles may be easy to attach on bubble and collected in froth product, which result in a low recovery of LiCoO2. To improve the recovery rate, the residual pyrolysis products should be removed.

Fig.9 SEM image of pyrolytic LiCoO2 and graphite particles derived from spent LIBs Effects of ultrasonic cleaning on surface characteristics of pyrolytic LiCoO2 and graphite. The XPS and SEM analysis demonstrate that some pyrolysis residues remain on the surface of electrode particles. To remove the residual pyrolysis products, ultrasonic cleaning process of the pyrolytic LiCoO2 and graphite particles are carried out in a laboratory ultrasonic machine. SEM images of LiCoO2 and graphite particles treated by pyrolysis-ultrasonic are shown in Fig.10. As we can see from these images, the residual pyrolysis carbon particles with larger size have been

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removed and the surfaces of electrode materials are smooth. After the removal of pyrolysis residues, the LiCoO2 particles will be hydrophilic and remain in the pulp while graphite will attach on the bubble and be collected in froth product. In addition, ultrasonic process continues to eliminate the particle agglomeration, and it strengthen the flotation behavior of LiCoO2 and graphite particles.

Fig.10 SEM image of pyrolysis-ultrasonic LiCoO2 and graphite particles derived from spent LIBs Flotation test of LiCoO2 and graphite with different pretreatment. Flotation experiments of electrode materials (REM, PEM, PUEM) with -0.075 mm size were conducted to separate graphite and LiCoO2. The flotation results are given in Table 2 and Fig.11. The LiCoO2 grade in REM LiCoO2 concentrate is only 67.25% with the recovery of 74.62%. The LiCoO2 grade in PEM LiCoO2 concentrate is up to 94.00% with the recovery of 84.61% whereas it is up to 93.89% in PUEM LiCoO2 concentrate with the high recovery of 96.88%. The high recovery rate of LiCoO2 21



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in REM is caused by the high yield of REM LiCoO2 concentrate with large amounts of graphite remain. After pyrolysis pretreatment, the organic binder and electrolyte have been removed and the LiCoO2 grade in PEM LiCoO2 concentrate increases to 94.00%. However, the residual pyrolysis products comprising hydrophobic properties remain on the surface of pyrolytic LiCoO2 particles and result in some LiCoO2 particles also attach on bubbles and mix in froth products, it is the main reason that a low recovery of 84.61% is obtained. The flotation result of PUEM demonstrates that ultrasonic cleaning can assist flotation behavior because the pyrolysis residues have been removed. This research demonstrated that flotation is a useful process to recover LiCoO2 from electrode materials mixture, and the pyrolysis-ultrasonic pretreatment can improve the recovery efficiency of LiCoO2. Table 2 Flotation results of electrode materials with different pretreatment REM (%)

PEM (%)

PUEM (%)

Graphite

LiCoO2

Graphite

LiCoO2

Graphite

LiCoO2

concentrate

concentrate

concentrate

concentrate

concentrate

concentrate

LiCoO2 grade

48.89

67.25

25.07

94.00

6.44

93.89

Yield

31.87

68.13

40.56

59.44

31.96

68.04

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Fig.11 Flotation results of LiCoO2 and graphite with different pretreatment Mechanism of flotation enhancement of LiCoO2 and graphite by pyrolysis-ultrasonic. Surface properties and morphology of raw LiCoO2 and graphite indicate that the existence of residual electrolyte and organic binder result in the hard flotation process of LiCoO2 and graphite. Schematic diagram of LiCoO2 and graphite particles with different pretreatment is shown in Fig.12. LiCoO2 and graphite particles are cladded by organic binder with strong hydrophobicity and they cannot attach on the bubbles, this result in the low LiCoO2 grade in LiCoO2 concentrate. Pyrolysis can make organic binder and electrolyte fully decomposed, but some pyrolytic carbon remains on the surface of pyrolytic LiCoO2 and graphite particles. Both LiCoO2 and graphite particles will attach on the bubbles, this result in some LiCoO2 particles mix into froth products, it is the main reason that a low recovery of LiCoO2 in PEM is obtained. Ultrasonic cleaning is used to remove the residual pyrolysis products, the original surfaces of LiCoO2 and graphite are exposed so LiCoO2 particles is hydrophilic while graphite is hydrophobic. The graphite particles will attach on the bubble’s surface while LiCoO2 remain in the pulp. Therefore, 23



pyrolysis-ultrasonic flotation can be used to achieve high flotation separation of LiCoO2 and graphite.

Fig.12 Schematic diagram of LiCoO2 and graphite particles with different pretreatment CONCLUSION Pyrolysis-ultrasonic-assisted flotation for recovering LiCoO2 and graphite from spent Lithium-ion battery has been fully studied. Mineralogical characteristics indicate that organic binder, residual electrolyte, and rough surfaces result in the hard flotation process of LiCoO2 and graphite. Basic pyrolysis characteristics of LiCoO2 and graphite indicate that organic binders can be decomposed thoroughly under pyrolysis temperature of 500 °C and the main pyrolysis products are fluorine-containing benzene and ester electrolyte. SEM and XPS analysis indicate that some

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residual pyrolysis products remain on the surface of electrode material particles and ultrasonic cleaning can remove them and strengthen flotation behavior effectively. The LiCoO2 grade improved from 67.25% to 93.89% while their recovery improved from 74.62% to 96.88% by pyrolysis-ultrasonic-assisted flotation. Parameter optimization experiments should be conducted in the further research works to recover much purer electrode materials. This research work may provide an alternative environmental-friendly flowchart to recycle LiCoO2 and graphite from spent LIBs. ACKNOWLEDGEMENT This work is supported by the Fundamental Research Funds for the Central Universities (2018BSCXA08), the Priority Academic Program Development of Jiangsu Higher Education Institutions (KYCX18_1927). The authors would like to thank Advanced Analysis and Computation Center of China University of Mining and Technology for their technical support. REFERENCES [1] Bertuol, D.A.; Toniasso, C.; Jimenez, B.M.; Meili, L.; Dotto, G.L.; Tanabe, E.H.; Aguiar, M.L., 2015. Application of spouted bed elutriation in the recycling of lithium ion batteries. J. Power Sources 2015, 275, 627-632, DOI 10.1016/j.jpowsour.2014.11.036. [2] Wang, D.; Wen, H.; Chen, H.; Yang, Y.; Liang, H., Chemical evolution of LiCoO2 and NaHSO4·H2O mixtures with different mixing ratios during roasting process. Chem. Res. Chinese U. 2016, 32, 674-677, DOI 10.1007/s40242-016-5490-2. [3] Xiao, J.; Li, J.; Xu, Z., Recycling metals from lithium ion battery by mechanical separation and

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! Green sustainable technology of pyrolysis-ultrasonic-assisted flotation for recycling LiCoO2 and Graphite from spent lithium-ion batteries Graphical Abstract


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Pyrolysis-ultrasonic-assisted flotation technology for recovering

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