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A promising physical method for recovery of LiCoO2 and graphite from spent lithium-ion batteries: Grinding flotation
Separation and Purification Technology 190 (2018) 45–52
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A promising physical method for recovery of LiCoO2 and graphite from spent lithium-ion batteries: Grinding flotation
MARK
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Jiadong Yua, Yaqun Heb,c, , Zhenzhou Gea, Hong Lia, Weining Xiec, Shuai Wangc a b c
School of Chemical Engineering and Technology, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China Shanghai Cooperative Center for WEEE Recycling, Shanghai Second Polytechnic University, Shanghai 201209, China Advanced Analysis & Computation Center, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Grinding flotation Spent lithium-ion battery Mixed electrode materials Dry surface modification Mechanical abrasion
Due to the limitation of secondary pollution and high equipment investment, the industrial-scale recycling technology for electrode materials from spent lithium-ion batteries (LIBs) needs urgent breakthrough. In this paper, a physical recycling method, grinding flotation, is creatively proposed for the separation and recovery of LiCoO2 and graphite from spent LIBs. According to the exploratory experiments, if the mixed electrode materials is ground in the hardgrove machine for 5 min before reverse flotation, the concentrate grade of LiCoO2 sinks and graphite floats can reach 97.13% and 73.56%, respectively. Moreover, with the help of advanced analytical technologies, the surface morphology, elemental chemical states and element distribution on the very surface of electrode particles before and after grinding were systematically analyzed to reveal the mechanism of dry surface modification. Results indicate that the mechanical grinding destroys the lamellar structure of graphite, exposing massive newborn hydrophobic surfaces. Meanwhile, the abrasion of organic film coating the LiCoO2 particles causes its original hydrophilic surface partially regained. Hence, the great wettability difference between LiCoO2 and graphite contributes to an excellent flotation separation. This grinding flotation method is a promising separation method without any toxic emissions or introducing other impurities in industrial application.
1. Introduction The attractive electrochemical characteristics (e.g. high energy density, high battery voltage, low self-discharge and no memory effect) help lithium-ion batteries (LIBs) substitute nickel-metal hydride batteries as power source in portable electronic equipment and even electric vehicle [1–4]. It is forecasted by Navigant Consulting, Inc. that in term of electric vehicle alone, the global demand for LIBs will reach $221 billion from 2015 to 2024 [5,6]. However, due to the limitation of service life (the average life is 1000 cycles), a huge quantity of discarded LIBs have been generated annually [7]. For instance in China, there will be more than 25 billion units and 500 thousand ton LIBs into waste stream in 2020 [8]. Besides, the heavy metal and hazardous organic chemicals in these end-life LIBs will pose a serious threat to the ecological system and human health [9,10]. On the other hand, spent LIBs contain a large amount of valuable metals, such as 21.60 wt% cobalt, 7.17 wt% copper, and 15.13 wt% aluminum [11]. Manufacturing the cathode materials with the cobalt and nickel from spent LIBs can save 45.3% fossil fuel, 51.3% nature ore, and 57.2% nuclear energy demand [12]. Therefore, on the basis of resource preservation
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and environmental protection, the sustainable recycling of high valueadd components from spent LIBs is imperative and significant. Furthermore, the intrinsic material value held in 1 ton of spent LIBs is $7708 and the value of each component is as follows: cathode materials ($6101), graphite ($170), copper ($654), aluminum ($103) and others ($680) [13]. This indicates that positive and negative active materials account for 81.36% of the total value of the battery. Accordingly, separation and recovery of cathode and anode active materials can promise great economic benefits. The recycling techniques of electrode materials (LiCoO2 and graphite) from waste lithium-ion batteries are mainly divided into pyrometallurgy [14,15], hydrometallurgy [16,17] and flotation [18,19]. In recent years, the pyro-metallurgical technology has developed rapidly. Li et al. [20] applied oxygen-free roasting to in situ recycle of cobalt, graphite and lithium carbonate (Li2CO3) from mixed electrode materials. The recovery rate of cobalt, lithium and graphite reach 95.72%, 98.93% and 91.05%, respectively. But the immature technology and high-risk investment make it still at the stage of theoretical research. Hydrometallurgy has a long history of development and is widely used as a necessary method to extract noble metals with high purity [21].
Corresponding author at: Advanced Analysis & Computation Center, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China. E-mail address: [email protected] (Y. He).
http://dx.doi.org/10.1016/j.seppur.2017.08.049 Received 25 June 2017; Received in revised form 16 August 2017; Accepted 20 August 2017 1383-5866/ © 2017 Elsevier B.V. All rights reserved.
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surface modification of electrode materials is revealed to establish theoretical and experimental basis for grinding flotation. Without environmental pollution or any impurity, grinding flotation technology has broad prospects for the industrialization.
Unfortunately, its industrialization scale is restricted as the shortcomings of slow dissolution rate, expensive solvent and secondary pollution. Flotation is a physical separation method based on the wettability difference of particle surface [22]. Cathode material (LiCoO2) belongs to the ionic crystal with strong polarity and good hydrophilicity. In the flotation process, LiCoO2 is completely wetted by water and then sinks into the bottom of the flotation cell. Anode material (graphite) is a nonpolar mineral and has a good hydrophobic behavior. In flotation, graphite is attached to the bubbles and then enters the foam layer of the flotation cell. Therefore, flotation method is theoretically achievable for separating and enriching LiCoO2 and graphite. However, Li et al. [23] and Zhang et al. [24] find that both the surfaces of LiCoO2 and graphite are covered with organic composite coatings, which are consisted of polyvinylidene fluoride (PVDF) binder, carbon black conductor and CeC/CeH structure. It is difficult for conventional flotation to separate LiCoO2 and graphite because their similar surface properties lead to the same wettability. Jin et al. [25] report that the organic film on the particle surface could be removed by roasting for 2 h at 773 K, thereby effectively recovering the hydrophobicity difference and improving the flotation effect. Nevertheless, thermal treatment will make HF, P2O5, aldehydes and other toxic gases volatilized, resulting in serious environmental pollution [26]. Therefore, roasting modified flotation is hard to realize industrial-scale production. He et al. [18] propose that the flotation results could be improved after the pretreatment of Fenton modified method, which break the macromolecular material, such as PVDF, into small molecules and degrade organic impurities into H2O and CO2. Although the method of removing organic film is environmentally friendly, Fenton will introduce iron impurities [19], which decreases the economic value of LiCoO2 concentrate. Hence, in order to realize the industrial-scale separation and recovery of the electrode materials, highly effective and environmental-friendly modification flotation method remains to be further explored. In recent decades, considerable researchers have focused on grinding flotation based on mechanical attrition [27–29]. Rabieh et al. proposed that the removal of pyrite from refractory gold ores could be achieved by grinding flotation [30]. Bruckard et al. studied the effect of different grinding media on the flotation behavior of copper sulfide minerals [31]. Guern et al. presented the effects of different grinding methods on the floatability of PVC and PET plastics from waste plastic bottles [32,33]. Xia et al. showed that a large amount of newborn surface of oxidized coal with good hydrophobicity could be obtained by grinding [34]. More importantly, the impact of external oxidative surface was weakened to improve the flotation results [35]. These researches have revealed that the mechanical attrition can not only expose the primary surface of the particles, but also enhance the original wettability of the materials without introducing impurities or releasing toxic substances. As far as graphite and LiCoO2 are concerned, mechanical grinding does not have a functional damage to their structure. In the literatures, the crystallinity of graphite will not be destroyed if it is ground in a low-pressure attrition system with a short period of time [36]. The factual results are the separation of weakly bonded graphite layers and the generation of newborn hydrophobic surface. Correspondingly, cathode active materials could be renovated with some degree by grinding through mechanical energy transfer [37]. Therefore, it is feasible to modify the surface properties of electrode materials from spent LIBs by mechanical attrition. In this paper, a promising modified flotation for recovery of electrode materials from spent LIBs with high purity is reported. It is the first time that grinding flotation is utilized to separate and recover the mixed electrode powder. Through laboratory experiments, the grinding time is investigated and the optimized results of grinding flotation are obtained. Furthermore, the surface properties of the particles, including surface morphology, elemental composition and element distribution, are systematically discussed before and after grinding with the help of advanced analytical techniques. Eventually, the mechanism of dry
2. Experimental 2.1. Materials and pretreatments In order to avoid short-circuit and self-ignition, 100 pieces of spent LIBs with the same type were discharged completely in 5 wt% sodium chloride solution for 48 h and dried naturally for 24 h afterwards. All batteries were manually disassembled into metallic shell, cathode strips, anode strips and organic separators. Then the cathode strips and anode strips were placed in the impact crusher with the number ratio 1:1. After crushing and screening, the fine powder under 0.074 mm was mixed electrode materials (LiCoO2 and graphite). The content of LiCoO2 in mixed electrode powder was measured by the combination of X-ray fluorescence (XRF, Bruker S8 Tiger, Germany) and X-ray diffraction (XRD, Bruker D8 Advance, Germany). Results indicated that LiCoO2 accounted for 66.75% of the mixed electrode powder.
2.2. Grinding flotation Grinding flotation experiments were carried out in standard Hardgrove apparatus and lab-scale flotation machine. First, 40 g of mixed electrode powder together with steel balls were sealed in the grinding chamber. The grinding times were 2.5 min, 5 min, 10 min, 20 min and 30 min, respectively. Then, the grinding productions were subjected to reverse flotation with the same experimental conditions. Pulp concentration, impeller speed and aeration during flotation were 40 g/L, 1960 rpm and 0.75 L/min, respectively. The dosage of collector and frother were 200 g/t Methyl isobutyl carbinol (MIBC) and 200 g/t n-dodecane. The flotation flowsheet for obtaining high-grade LiCoO2 concentrates and graphite concentrates is shown in Fig. 1. After grinding, the hydrophobicity of graphite is improved obviously. Once the collector is added, graphite particles will quickly enter the foam layer and become the graphite concentrates. In this flotation, the dosages of flotation reagents were increased to float up most materials that could enter the foam layer. Therefore, after the frother is added, LiCoO2 concentrates and LiCoO2 middlings can be obtained from the bottom and foam layer of the flotation cell, respectively. The content of LiCoO2 in the flotation products (graphite concentrates, LiCoO2 concentrates and LiCoO2 middlings) were measured by the combination of XRF and XRD as well.
Fig. 1. The flow sheet of flotation process.
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Fig. 2. The grinding flotation results of LiCoO2 concentrates with different grinding times.
Fig. 3. The SEM images of flotation products after 5 min grinding (a, flotation feed; b, LiCoO2 middlings; c, graphite concentrates; d, LiCoO2 concentrates).
2.3. Advanced analytical methods
3. Results and discussion
The elemental composition and chemical states of carbon in the very surface of the mixed electrode powder before and after grinding were determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, USA). Microcrystalline structure of LiCoO2 and graphite were characterized by High Resolution Transmission Electron Microscopy (HRTEM, FEI Tecnai G2 F20 S, USA). The surface topography of mixed electrode powder after grinding was showed by Scanning Electron Microscopy (SEM, FEI quanta 250, USA). The element distribution on the graphite and LiCoO2 granule were revealed by the Field Emission Electron Probe Microanalyzer (FE-EPMA, Shimadzu 8050G, Japan).
3.1. The optimal results of grinding flotation experiments The concentrate grade and recovery rate are two important evaluation indexes of flotation effect in this investigation. The flotation results of LiCoO2 concentrates with different grinding times are illustrated in Fig. 2. With the increase of grinding time, the grade of flotation concentrate (LiCoO2) increases first and then decreases, and finally stabilizes at about 90%. In particularly, the optimum grade of flotation concentrates reaches 97.19% when the grinding time is 5 min. On the other hand, the recovery rate of LiCoO2 concentrate decreases 47
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Fig. 4. XPS survey scan and C1s XPS spectrum of LiCoO2 powder before and after 5 min grinding (a, survey scan of LiCoO2 powder; b, C1s XPS spectrum of LiCoO2 powder; c, survey scan of LiCoO2 grinding products; d, C1s XPS spectrum of LiCoO2 grinding products).
recovery rate is 68.23%. This indicates that the surface modification method based on mechanical grinding can significantly improve the concentrate grade of LiCoO2 but reduce the concentrate recovery rate. In order to describe the flotation separation effect more intuitively, flotation feed, concentrates and middlings of the optimum grinding flotation (grinding time was 5 min) were tested by SEM, and the results are shown in Fig. 3.
with the grinding time increasing and then increases from 49.32% to 64.85%. These mean that the grade and recovery rate of LiCoO2 concentrates show the opposite variation trend. More importantly, it can be deduced that short grinding can effectively improve the floatability difference between LiCoO2 and graphite, but long grinding will have a negative impact on them. In addition, the LiCoO2 concentrate grade of direct flotation of mixed electrode materials is 71.63%, and the 48
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3.2. Analyses on surface properties of electrode powder
Table 1 Chemical states of carbon in the surface of LiCoO2 powder before and after grinding. Peak BE/eV
Name
Raw materials At.%
Grinding products At. %
284.300 284.840 286.050 287.140 288.060 289.990 290.550
Graphite CeC/CeH e(CF2CH2)ne/CeO OeCeO C]O OeCOOR e(CF2CH2)ne
30.29 18.74 25.93 4.69 3.21 3.62 13.52
54.81 14.75 13.37 5.32 3.64 5.75 2.35
3.2.1. Composition and chemical state analysis of surface element The LiCoO2 powder before and after 5 min grinding with graphite were texted by XPS, and the results are shown in Fig. 4 and Table 1. In Fig. 4a and c, XPS survey scans show that in the case of little change in carbon content on the surface of LiCoO2 particles, the F content greatly is reduced from 13.82% to 6.7%, and the decrease rate reaches 50%. Meanwhile, the contents of Co and O are significantly improved. It indicates that the organic film, which is composed of PVDF binder containing F element, has been significantly removed from the particle surface, and the surface of LiCoO2 are partially exposed. Before and after 5 min grinding, the carbon content of the particles surface is about 60%. In order to understand the chemical states of carbon on the particle surface, the C1s XPS spectrum of the samples were analyzed, as shown in Fig. 4b and d, and the details are in Table 1. The results show that the hydrophobic functional groups on the surface of LiCoO2 powder can be classified into e(CF2CH2)ne/CeO, OeCOOR, C]O and OeCeO, while the hydrophilic functional groups are CeC/CeH and graphite carbon. After mechanical grinding, the fluorinated functional groups, such as e(CF2CH2)ne, on the particle surfaces are greatly reduced, from 39.45% to 15.72%. It further indicates that the content of organic film on the particle surface decreases after grinding. On the other hand, the graphite carbon structure on the powder surface increases significantly from 30.29% to 54.81%. It could be deduced that during the crushing process of electrode strips, the interaction between LiCoO2 and graphite causes graphite to partly adhere to the LiCoO2 particles, and grinding process will exacerbate this adhesion effect. In order to verify this conjecture, the peeling behavior of graphite and the interaction of graphite and LiCoO2 in the grinding process should be further explored under the analyses of microcrystalline structure and surface morphology from TEM and SEM.
Pictures in Fig. 3 shows the particle distribution that is magnified 1300 times in back scattering mode. In Fig. 3, the LiCoO2 particles containing cobalt with high atomic number will become bright tone, while graphite particles containing light element will become dark tone. Fig. 3a is the flotation feed which is mixed electrode powder, and the grade of LiCoO2 is 66.75%. Fig. 3b is LiCoO2 middling, the grade of LiCoO2 is 59.96% and the recovery rate is 28.45%. In Fig. 3b, the black graphite particles are enriched to some extent compared to Fig. 1, but there are still some residual particles of white LiCoO2. Fig. 3c is graphite concentrate, the grade of graphite is 82.57% and the recovery rate is 73.56%. Fig. 3d is LiCoO2 concentrate, the grade of LiCoO2 is 97.19% and the recovery rate is 49.32%. As shown in Fig. 3c and d, the separation of LiCoO2 and graphite are effectively realized, with only a small fraction of the impurities mixed into the concentrates. In other words, the grinding flotation can be a qualified physical method for the separation of LiCoO2 and graphite, and also has a high potential in industrial application. During the flotation process, the vast majority of graphite are mixed with a few LiCoO2 particles into the foam layer, so that the LiCoO2 sinking at the bottom of the flotation cell has high grade, but the recovery rate is still low. In order to explore the grinding effect on the electrode particles from microcosmic level, the physical and chemical properties of particle surface before and after mechanical grinding are characterized in detail. With the help of SEM, TEM, XPS and EPMA, the analyses of surface morphology, chemical state of elements and elements distribution are carried out systematically, and eventually the surface modification mechanism based on mechanical attrition is revealed.
3.2.2. Analyses of microcrystalline structure and surface morphology by TEM and SEM The separation of LiCoO2 and graphite in the concentrates is relatively good, but there are lots of bonding phenomenon in the LiCoO2 middling. Therefore, in order to investigate the interaction of LiCoO2 and graphite, LiCoO2 middling is the focus of research. After sample preparation, the LiCoO2 middlings were texted by the HR-TEM and the images are shown in Fig. 5. Fig. 5a presents a typical structure of flake graphite, and the lamellar structure of the graphite is in a state of staggered at different heights. It can be speculated that under the
Fig. 5. The peeling behavior of graphite and the adhesion state of graphite and LiCoO2 by HR-TEM (a, the graphite particle; b and c, the LiCoO2 particle adhered by graphite).
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Fig. 6. The peeling behavior of graphite and the adhesion state of graphite and LiCoO2 by SEM (after grinding: a, the peeling of graphite; b, small sheet structure of graphite; c, the LiCoO2 particle adhered by graphite; d, the point element analysis of the particles from c).
Fig. 7. The micro-probe element distribution in the adhesion of LiCoO2 and graphite particles (a, the graphite particle adhered by LiCoO2 particles; b, an enlarged view of the bonded portion; c, element distribution of Co; d, element distribution of F).
and the black matters are polycrystalline LiCoO2. Fig. 5b and c shows the adhesion of graphite to LiCoO2 occurs in the form of partial contact rather than in a fully encapsulated overlay. In addition, LiCoO2 particles have a prism structure from Fig. 5c.
horizontal shearing force, the graphite sheet is slipping and its lamellar structure is opened, exposing a large number of newborn graphite surface. According to the electron diffraction patterns in Fig. 5c, the materials with lighter color in Fig. 5b and c are single-crystal graphite,
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Fig. 8. Dry modification mechanism based on mechanical abrasion.
the dark point indicates that element content is low. As can be seen from Fig. 7c and d, the F elements are abundant on the surface of LiCoO2 particles, and appear an uneven distribution. Deduced from this result, the organic film on the surface of LiCoO2 particles are unevenly distributed due to the partial abrasion. Although the organic film is not completely removed, the mechanical abrasion partially exposes the original surface of LiCoO2 particles, enhancing the surface hydrophilicity. More importantly, there is little F element distribution on the surface of graphite particle in Fig. 7d. The main reason is that the lamellar structure of graphite particles are peeled off to expose a large number of newborn surfaces, and this newborn surface does not contain the boring organic film. Therefore, the hydrophobicity of the graphite particles is greatly improved.
In order to directly observe the peeling behavior of graphite layered structure and the morphology of the adhesion between LiCoO2 and graphite, SEM analyses were performed on the LiCoO2 middling and the images are shown in Fig. 6. In the backscattering mode of SEM, the black particles are graphite and the bright white particles are LiCoO2. In Fig. 6a, the massive graphite is cracked according to its layered structure. It is indicated that the unique crushing behavior of graphite is sliding and flaking along the lamellar structure. In Fig. 6b, lots of small flaky graphite can be seen. This is the result that mechanical grinding makes a large number of graphite flakes stripped. Fig. 6c shows a LiCoO2 particle, which is adhered by graphite, and Fig. 6d is a point element analysis of the left and right parts of the particles in Fig. 6c. The results show that the black part is graphite and the bright part is LiCoO2. In Fig. 6c, there are obvious traces of abrasion on the surface of bright LiCoO2 particle. This suggests that only the angular or convex part of LiCoO2 particles can be smoothed by mechanical grinding. In addition, the graphite is easily attached to the surface, particle gap and concave convex source of LiCoO2 particles under mechanical rolling process. In order to understand the occurrence state of organic films on the surface of LiCoO2 and graphite during mechanical grinding, FE-EPMA was used to analyze the element distribution on the particle surface.
3.3. Dry modification mechanism Based on the above analyses, the dry modification mechanism is revealed and shown in Fig. 8. The surface of electrode materials (LiCoO2 and graphite) from waste LIBs are attached by a layer of organic film after crushing and screening. During the mechanical grinding, the grinding media (steel ball) produces a horizontal shear force and a vertical rolling pressure on the particle surface. Under the horizontal shear force, the internal structure of graphite is sliding and flaking along the location of graphite layer, producing a large number of new hydrophobic surfaces. Due to the high hardness of LiCoO2 particles, mechanical grinding can only wear down and smooth the angular or protrusions of the abrasive particles to partially expose the original surface of LiCoO2, which promotes the improvement of the surface hydrophilicity. Meanwhile, under the vertical rolling pressure of grinding media, some LiCoO2 and graphite particles are adhered together. This makes the hydrophobicity of those LiCoO2 particles enhanced, leading them to follow graphite into the foam layer during the flotation and ultimately lost in the tailings. However, the wettability difference between newborn surface of graphite and primary surface of LiCoO2 is significant, so presenting an excellent effect of flotation separation. More importantly, under the condition of a short period of grinding, both graphite and graphite with a small amount of LiCoO2 can float into the foam layer, so that the grade of LiCoO2 concentrate at the bottom of flotation cell reaches about 95%. However, it should be noted that due to the smaller particle size and
3.2.3. The element distribution analysis based on FE-EPMA With the help of FE-EPMA, the adhesion portion between the LiCoO2 and graphite particle was subjected to element distribution analysis, as shown in Fig. 7. Fig. 7a shows a black graphite particle and white LiCoO2 particles, whose adhesion portions are enlarged in Fig. 7b. In Fig. 7b, the edges of LiCoO2 and graphite are touched tightly, but the boundary is clear. In other words, the adhesion is the effect of mechanical force. During the grinding process, the pressure in the vertical direction from grinding medium results in the adhesion between LiCoO2 and graphite. Since the fluorine element is a characteristic element in PVDF, which is the main component in organic film, and there is no fluorine on the initial surface of electrode material, then the F element distribution can be used to characterize the occurrence of organic film. The micro-probe distribution of cobalt and fluorine elements on the particles are shown in Fig. 7c and d, respectively. In Fig. 7c and d, the counting point with light color represents a high element content and 51
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more quantity of LiCoO2 particles, graphite particles will be adsorbed by more LiCoO2 particles to its outer surface to form the agglomerates of large particles as the grinding time increases, improving the hydrophilicity of graphite. These graphite particles will no longer be able to enter the foam layer and remain in the LiCoO2 concentrate. Therefore, with the grinding time increasing, the grade of LiCoO2 concentrate decreases but the recovery rate increases. 4. Conclusion Grinding flotation is a physical sorting method proposed for the special structural characteristics of LiCoO2 and graphite. With the optimum condition of 5 min grinding, the concentrate grade of LiCoO2 and graphite are 97.19% and 82.57%, and their recovery rate are 49.32% and 73.56%, respectively. Under the horizontal shear force produced by the grinding media, the lamellar structures of graphite are sliding and flaking, exposing a large number of new hydrophobic surfaces. On the other hand, the organic film coating LiCoO2 particles are partially worn down to restore the original hydrophilic surface. With the action of vertical rolling pressure, the adhesion of LiCoO2 and graphite will occur and gradually become serious. Although the adhesion behavior in the 5 min grinding process will lead some LiCoO2 concentrate to follow graphite into the foam layer to reduce the flotation recovery rate, the great wettability difference results in an extremely good flotation concentrate grade. The advantages of reliable, high efficiency and no secondary pollution help grinding flotation to be a good choice for industrial application. Acknowledgments The financial support for this work was from the National Natural Science Foundation of China (No. 51574234) and Shanghai Cooperative Centre for WEEE Recycling (No. ZF1224). We also would like to thank the Advanced Analysis and Computation Center of China University of Mining and Technology for its technical support. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.seppur.2017.08.049. References [1] L. Li, L. Zhai, X. Zhang, J. Lu, R. Chen, F. Wu, K. Amine, Recovery of valuable metals from spent lithium-ion batteries by ultrasonic-assisted leaching process, J. Power Sources 262 (2014) 380–385. [2] L. Li, W. Qu, X. Zhang, J. Lu, R. Chen, F. Wu, K. Amine, Succinic acid-based leaching system: a sustainable process for recovery of valuable metals from spent Li-ion batteries, J. Power Sources 282 (2015) 544–551. [3] N.B. Horeh, S.M. Mousavi, S.A. Shojaosadati, Bioleaching of valuable metals from spent lithium-ion mobile phone batteries using Aspergillus niger, J. Power Sources 320 (2016) 257–266. [4] E.G. Pinna, M.C. Ruiz, M.W. Ojeda, M.H. Rodriguez, Cathodes of spent Li-ion batteries: dissolution with phosphoric acid and recovery of lithium and cobalt from leach liquors, Hydrometallurgy 167 (2017) 66–71. [5] Navigant Research, The Global Market for Lithium Ion Batteries for Vehicles is Expected to Total $221 Billion from 2015 to 2024, 2016. < http://www. navigantresearch.com > (accessed 17 February 2016). [6] J. Hu, J. Zhang, H. Li, Y. Chen, C. Wang, A promising approach for the recovery of high value-added metals from spent lithium-ion batteries, J. Power Sources 351 (2017) 192–199. [7] X. Chen, H. Ma, C. Luo, T. Zhou, Recovery of valuable metals from waste cathode materials of spent lithium-ion batteries using mild phosphoric acid, J. Hazard. Mater. 326 (2017) 77–86. [8] X. Zeng, J. Li, B. Shen, Novel approach to recover cobalt and lithium from spent lithium-ion battery using oxalic acid, J. Hazard. Mater. 295 (2015) 112–118. [9] X. Wang, G. Gaustad, C.W. Babbitt, C. Bailey, M.J. Ganter, B.J. Landi, Economic and environmental characterization of an evolving Li-ion battery waste stream, J.
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
A promising physical method for recovery of LiCoO2 and graphite from spent lithium-ion batteries: Grinding flotation
MARK
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Jiadong Yua, Yaqun Heb,c, , Zhenzhou Gea, Hong Lia, Weining Xiec, Shuai Wangc a b c
School of Chemical Engineering and Technology, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China Shanghai Cooperative Center for WEEE Recycling, Shanghai Second Polytechnic University, Shanghai 201209, China Advanced Analysis & Computation Center, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Grinding flotation Spent lithium-ion battery Mixed electrode materials Dry surface modification Mechanical abrasion
Due to the limitation of secondary pollution and high equipment investment, the industrial-scale recycling technology for electrode materials from spent lithium-ion batteries (LIBs) needs urgent breakthrough. In this paper, a physical recycling method, grinding flotation, is creatively proposed for the separation and recovery of LiCoO2 and graphite from spent LIBs. According to the exploratory experiments, if the mixed electrode materials is ground in the hardgrove machine for 5 min before reverse flotation, the concentrate grade of LiCoO2 sinks and graphite floats can reach 97.13% and 73.56%, respectively. Moreover, with the help of advanced analytical technologies, the surface morphology, elemental chemical states and element distribution on the very surface of electrode particles before and after grinding were systematically analyzed to reveal the mechanism of dry surface modification. Results indicate that the mechanical grinding destroys the lamellar structure of graphite, exposing massive newborn hydrophobic surfaces. Meanwhile, the abrasion of organic film coating the LiCoO2 particles causes its original hydrophilic surface partially regained. Hence, the great wettability difference between LiCoO2 and graphite contributes to an excellent flotation separation. This grinding flotation method is a promising separation method without any toxic emissions or introducing other impurities in industrial application.
1. Introduction The attractive electrochemical characteristics (e.g. high energy density, high battery voltage, low self-discharge and no memory effect) help lithium-ion batteries (LIBs) substitute nickel-metal hydride batteries as power source in portable electronic equipment and even electric vehicle [1–4]. It is forecasted by Navigant Consulting, Inc. that in term of electric vehicle alone, the global demand for LIBs will reach $221 billion from 2015 to 2024 [5,6]. However, due to the limitation of service life (the average life is 1000 cycles), a huge quantity of discarded LIBs have been generated annually [7]. For instance in China, there will be more than 25 billion units and 500 thousand ton LIBs into waste stream in 2020 [8]. Besides, the heavy metal and hazardous organic chemicals in these end-life LIBs will pose a serious threat to the ecological system and human health [9,10]. On the other hand, spent LIBs contain a large amount of valuable metals, such as 21.60 wt% cobalt, 7.17 wt% copper, and 15.13 wt% aluminum [11]. Manufacturing the cathode materials with the cobalt and nickel from spent LIBs can save 45.3% fossil fuel, 51.3% nature ore, and 57.2% nuclear energy demand [12]. Therefore, on the basis of resource preservation
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and environmental protection, the sustainable recycling of high valueadd components from spent LIBs is imperative and significant. Furthermore, the intrinsic material value held in 1 ton of spent LIBs is $7708 and the value of each component is as follows: cathode materials ($6101), graphite ($170), copper ($654), aluminum ($103) and others ($680) [13]. This indicates that positive and negative active materials account for 81.36% of the total value of the battery. Accordingly, separation and recovery of cathode and anode active materials can promise great economic benefits. The recycling techniques of electrode materials (LiCoO2 and graphite) from waste lithium-ion batteries are mainly divided into pyrometallurgy [14,15], hydrometallurgy [16,17] and flotation [18,19]. In recent years, the pyro-metallurgical technology has developed rapidly. Li et al. [20] applied oxygen-free roasting to in situ recycle of cobalt, graphite and lithium carbonate (Li2CO3) from mixed electrode materials. The recovery rate of cobalt, lithium and graphite reach 95.72%, 98.93% and 91.05%, respectively. But the immature technology and high-risk investment make it still at the stage of theoretical research. Hydrometallurgy has a long history of development and is widely used as a necessary method to extract noble metals with high purity [21].
Corresponding author at: Advanced Analysis & Computation Center, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China. E-mail address: [email protected] (Y. He).
http://dx.doi.org/10.1016/j.seppur.2017.08.049 Received 25 June 2017; Received in revised form 16 August 2017; Accepted 20 August 2017 1383-5866/ © 2017 Elsevier B.V. All rights reserved.
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surface modification of electrode materials is revealed to establish theoretical and experimental basis for grinding flotation. Without environmental pollution or any impurity, grinding flotation technology has broad prospects for the industrialization.
Unfortunately, its industrialization scale is restricted as the shortcomings of slow dissolution rate, expensive solvent and secondary pollution. Flotation is a physical separation method based on the wettability difference of particle surface [22]. Cathode material (LiCoO2) belongs to the ionic crystal with strong polarity and good hydrophilicity. In the flotation process, LiCoO2 is completely wetted by water and then sinks into the bottom of the flotation cell. Anode material (graphite) is a nonpolar mineral and has a good hydrophobic behavior. In flotation, graphite is attached to the bubbles and then enters the foam layer of the flotation cell. Therefore, flotation method is theoretically achievable for separating and enriching LiCoO2 and graphite. However, Li et al. [23] and Zhang et al. [24] find that both the surfaces of LiCoO2 and graphite are covered with organic composite coatings, which are consisted of polyvinylidene fluoride (PVDF) binder, carbon black conductor and CeC/CeH structure. It is difficult for conventional flotation to separate LiCoO2 and graphite because their similar surface properties lead to the same wettability. Jin et al. [25] report that the organic film on the particle surface could be removed by roasting for 2 h at 773 K, thereby effectively recovering the hydrophobicity difference and improving the flotation effect. Nevertheless, thermal treatment will make HF, P2O5, aldehydes and other toxic gases volatilized, resulting in serious environmental pollution [26]. Therefore, roasting modified flotation is hard to realize industrial-scale production. He et al. [18] propose that the flotation results could be improved after the pretreatment of Fenton modified method, which break the macromolecular material, such as PVDF, into small molecules and degrade organic impurities into H2O and CO2. Although the method of removing organic film is environmentally friendly, Fenton will introduce iron impurities [19], which decreases the economic value of LiCoO2 concentrate. Hence, in order to realize the industrial-scale separation and recovery of the electrode materials, highly effective and environmental-friendly modification flotation method remains to be further explored. In recent decades, considerable researchers have focused on grinding flotation based on mechanical attrition [27–29]. Rabieh et al. proposed that the removal of pyrite from refractory gold ores could be achieved by grinding flotation [30]. Bruckard et al. studied the effect of different grinding media on the flotation behavior of copper sulfide minerals [31]. Guern et al. presented the effects of different grinding methods on the floatability of PVC and PET plastics from waste plastic bottles [32,33]. Xia et al. showed that a large amount of newborn surface of oxidized coal with good hydrophobicity could be obtained by grinding [34]. More importantly, the impact of external oxidative surface was weakened to improve the flotation results [35]. These researches have revealed that the mechanical attrition can not only expose the primary surface of the particles, but also enhance the original wettability of the materials without introducing impurities or releasing toxic substances. As far as graphite and LiCoO2 are concerned, mechanical grinding does not have a functional damage to their structure. In the literatures, the crystallinity of graphite will not be destroyed if it is ground in a low-pressure attrition system with a short period of time [36]. The factual results are the separation of weakly bonded graphite layers and the generation of newborn hydrophobic surface. Correspondingly, cathode active materials could be renovated with some degree by grinding through mechanical energy transfer [37]. Therefore, it is feasible to modify the surface properties of electrode materials from spent LIBs by mechanical attrition. In this paper, a promising modified flotation for recovery of electrode materials from spent LIBs with high purity is reported. It is the first time that grinding flotation is utilized to separate and recover the mixed electrode powder. Through laboratory experiments, the grinding time is investigated and the optimized results of grinding flotation are obtained. Furthermore, the surface properties of the particles, including surface morphology, elemental composition and element distribution, are systematically discussed before and after grinding with the help of advanced analytical techniques. Eventually, the mechanism of dry
2. Experimental 2.1. Materials and pretreatments In order to avoid short-circuit and self-ignition, 100 pieces of spent LIBs with the same type were discharged completely in 5 wt% sodium chloride solution for 48 h and dried naturally for 24 h afterwards. All batteries were manually disassembled into metallic shell, cathode strips, anode strips and organic separators. Then the cathode strips and anode strips were placed in the impact crusher with the number ratio 1:1. After crushing and screening, the fine powder under 0.074 mm was mixed electrode materials (LiCoO2 and graphite). The content of LiCoO2 in mixed electrode powder was measured by the combination of X-ray fluorescence (XRF, Bruker S8 Tiger, Germany) and X-ray diffraction (XRD, Bruker D8 Advance, Germany). Results indicated that LiCoO2 accounted for 66.75% of the mixed electrode powder.
2.2. Grinding flotation Grinding flotation experiments were carried out in standard Hardgrove apparatus and lab-scale flotation machine. First, 40 g of mixed electrode powder together with steel balls were sealed in the grinding chamber. The grinding times were 2.5 min, 5 min, 10 min, 20 min and 30 min, respectively. Then, the grinding productions were subjected to reverse flotation with the same experimental conditions. Pulp concentration, impeller speed and aeration during flotation were 40 g/L, 1960 rpm and 0.75 L/min, respectively. The dosage of collector and frother were 200 g/t Methyl isobutyl carbinol (MIBC) and 200 g/t n-dodecane. The flotation flowsheet for obtaining high-grade LiCoO2 concentrates and graphite concentrates is shown in Fig. 1. After grinding, the hydrophobicity of graphite is improved obviously. Once the collector is added, graphite particles will quickly enter the foam layer and become the graphite concentrates. In this flotation, the dosages of flotation reagents were increased to float up most materials that could enter the foam layer. Therefore, after the frother is added, LiCoO2 concentrates and LiCoO2 middlings can be obtained from the bottom and foam layer of the flotation cell, respectively. The content of LiCoO2 in the flotation products (graphite concentrates, LiCoO2 concentrates and LiCoO2 middlings) were measured by the combination of XRF and XRD as well.
Fig. 1. The flow sheet of flotation process.
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Fig. 2. The grinding flotation results of LiCoO2 concentrates with different grinding times.
Fig. 3. The SEM images of flotation products after 5 min grinding (a, flotation feed; b, LiCoO2 middlings; c, graphite concentrates; d, LiCoO2 concentrates).
2.3. Advanced analytical methods
3. Results and discussion
The elemental composition and chemical states of carbon in the very surface of the mixed electrode powder before and after grinding were determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, USA). Microcrystalline structure of LiCoO2 and graphite were characterized by High Resolution Transmission Electron Microscopy (HRTEM, FEI Tecnai G2 F20 S, USA). The surface topography of mixed electrode powder after grinding was showed by Scanning Electron Microscopy (SEM, FEI quanta 250, USA). The element distribution on the graphite and LiCoO2 granule were revealed by the Field Emission Electron Probe Microanalyzer (FE-EPMA, Shimadzu 8050G, Japan).
3.1. The optimal results of grinding flotation experiments The concentrate grade and recovery rate are two important evaluation indexes of flotation effect in this investigation. The flotation results of LiCoO2 concentrates with different grinding times are illustrated in Fig. 2. With the increase of grinding time, the grade of flotation concentrate (LiCoO2) increases first and then decreases, and finally stabilizes at about 90%. In particularly, the optimum grade of flotation concentrates reaches 97.19% when the grinding time is 5 min. On the other hand, the recovery rate of LiCoO2 concentrate decreases 47
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Fig. 4. XPS survey scan and C1s XPS spectrum of LiCoO2 powder before and after 5 min grinding (a, survey scan of LiCoO2 powder; b, C1s XPS spectrum of LiCoO2 powder; c, survey scan of LiCoO2 grinding products; d, C1s XPS spectrum of LiCoO2 grinding products).
recovery rate is 68.23%. This indicates that the surface modification method based on mechanical grinding can significantly improve the concentrate grade of LiCoO2 but reduce the concentrate recovery rate. In order to describe the flotation separation effect more intuitively, flotation feed, concentrates and middlings of the optimum grinding flotation (grinding time was 5 min) were tested by SEM, and the results are shown in Fig. 3.
with the grinding time increasing and then increases from 49.32% to 64.85%. These mean that the grade and recovery rate of LiCoO2 concentrates show the opposite variation trend. More importantly, it can be deduced that short grinding can effectively improve the floatability difference between LiCoO2 and graphite, but long grinding will have a negative impact on them. In addition, the LiCoO2 concentrate grade of direct flotation of mixed electrode materials is 71.63%, and the 48
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3.2. Analyses on surface properties of electrode powder
Table 1 Chemical states of carbon in the surface of LiCoO2 powder before and after grinding. Peak BE/eV
Name
Raw materials At.%
Grinding products At. %
284.300 284.840 286.050 287.140 288.060 289.990 290.550
Graphite CeC/CeH e(CF2CH2)ne/CeO OeCeO C]O OeCOOR e(CF2CH2)ne
30.29 18.74 25.93 4.69 3.21 3.62 13.52
54.81 14.75 13.37 5.32 3.64 5.75 2.35
3.2.1. Composition and chemical state analysis of surface element The LiCoO2 powder before and after 5 min grinding with graphite were texted by XPS, and the results are shown in Fig. 4 and Table 1. In Fig. 4a and c, XPS survey scans show that in the case of little change in carbon content on the surface of LiCoO2 particles, the F content greatly is reduced from 13.82% to 6.7%, and the decrease rate reaches 50%. Meanwhile, the contents of Co and O are significantly improved. It indicates that the organic film, which is composed of PVDF binder containing F element, has been significantly removed from the particle surface, and the surface of LiCoO2 are partially exposed. Before and after 5 min grinding, the carbon content of the particles surface is about 60%. In order to understand the chemical states of carbon on the particle surface, the C1s XPS spectrum of the samples were analyzed, as shown in Fig. 4b and d, and the details are in Table 1. The results show that the hydrophobic functional groups on the surface of LiCoO2 powder can be classified into e(CF2CH2)ne/CeO, OeCOOR, C]O and OeCeO, while the hydrophilic functional groups are CeC/CeH and graphite carbon. After mechanical grinding, the fluorinated functional groups, such as e(CF2CH2)ne, on the particle surfaces are greatly reduced, from 39.45% to 15.72%. It further indicates that the content of organic film on the particle surface decreases after grinding. On the other hand, the graphite carbon structure on the powder surface increases significantly from 30.29% to 54.81%. It could be deduced that during the crushing process of electrode strips, the interaction between LiCoO2 and graphite causes graphite to partly adhere to the LiCoO2 particles, and grinding process will exacerbate this adhesion effect. In order to verify this conjecture, the peeling behavior of graphite and the interaction of graphite and LiCoO2 in the grinding process should be further explored under the analyses of microcrystalline structure and surface morphology from TEM and SEM.
Pictures in Fig. 3 shows the particle distribution that is magnified 1300 times in back scattering mode. In Fig. 3, the LiCoO2 particles containing cobalt with high atomic number will become bright tone, while graphite particles containing light element will become dark tone. Fig. 3a is the flotation feed which is mixed electrode powder, and the grade of LiCoO2 is 66.75%. Fig. 3b is LiCoO2 middling, the grade of LiCoO2 is 59.96% and the recovery rate is 28.45%. In Fig. 3b, the black graphite particles are enriched to some extent compared to Fig. 1, but there are still some residual particles of white LiCoO2. Fig. 3c is graphite concentrate, the grade of graphite is 82.57% and the recovery rate is 73.56%. Fig. 3d is LiCoO2 concentrate, the grade of LiCoO2 is 97.19% and the recovery rate is 49.32%. As shown in Fig. 3c and d, the separation of LiCoO2 and graphite are effectively realized, with only a small fraction of the impurities mixed into the concentrates. In other words, the grinding flotation can be a qualified physical method for the separation of LiCoO2 and graphite, and also has a high potential in industrial application. During the flotation process, the vast majority of graphite are mixed with a few LiCoO2 particles into the foam layer, so that the LiCoO2 sinking at the bottom of the flotation cell has high grade, but the recovery rate is still low. In order to explore the grinding effect on the electrode particles from microcosmic level, the physical and chemical properties of particle surface before and after mechanical grinding are characterized in detail. With the help of SEM, TEM, XPS and EPMA, the analyses of surface morphology, chemical state of elements and elements distribution are carried out systematically, and eventually the surface modification mechanism based on mechanical attrition is revealed.
3.2.2. Analyses of microcrystalline structure and surface morphology by TEM and SEM The separation of LiCoO2 and graphite in the concentrates is relatively good, but there are lots of bonding phenomenon in the LiCoO2 middling. Therefore, in order to investigate the interaction of LiCoO2 and graphite, LiCoO2 middling is the focus of research. After sample preparation, the LiCoO2 middlings were texted by the HR-TEM and the images are shown in Fig. 5. Fig. 5a presents a typical structure of flake graphite, and the lamellar structure of the graphite is in a state of staggered at different heights. It can be speculated that under the
Fig. 5. The peeling behavior of graphite and the adhesion state of graphite and LiCoO2 by HR-TEM (a, the graphite particle; b and c, the LiCoO2 particle adhered by graphite).
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Fig. 6. The peeling behavior of graphite and the adhesion state of graphite and LiCoO2 by SEM (after grinding: a, the peeling of graphite; b, small sheet structure of graphite; c, the LiCoO2 particle adhered by graphite; d, the point element analysis of the particles from c).
Fig. 7. The micro-probe element distribution in the adhesion of LiCoO2 and graphite particles (a, the graphite particle adhered by LiCoO2 particles; b, an enlarged view of the bonded portion; c, element distribution of Co; d, element distribution of F).
and the black matters are polycrystalline LiCoO2. Fig. 5b and c shows the adhesion of graphite to LiCoO2 occurs in the form of partial contact rather than in a fully encapsulated overlay. In addition, LiCoO2 particles have a prism structure from Fig. 5c.
horizontal shearing force, the graphite sheet is slipping and its lamellar structure is opened, exposing a large number of newborn graphite surface. According to the electron diffraction patterns in Fig. 5c, the materials with lighter color in Fig. 5b and c are single-crystal graphite,
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Fig. 8. Dry modification mechanism based on mechanical abrasion.
the dark point indicates that element content is low. As can be seen from Fig. 7c and d, the F elements are abundant on the surface of LiCoO2 particles, and appear an uneven distribution. Deduced from this result, the organic film on the surface of LiCoO2 particles are unevenly distributed due to the partial abrasion. Although the organic film is not completely removed, the mechanical abrasion partially exposes the original surface of LiCoO2 particles, enhancing the surface hydrophilicity. More importantly, there is little F element distribution on the surface of graphite particle in Fig. 7d. The main reason is that the lamellar structure of graphite particles are peeled off to expose a large number of newborn surfaces, and this newborn surface does not contain the boring organic film. Therefore, the hydrophobicity of the graphite particles is greatly improved.
In order to directly observe the peeling behavior of graphite layered structure and the morphology of the adhesion between LiCoO2 and graphite, SEM analyses were performed on the LiCoO2 middling and the images are shown in Fig. 6. In the backscattering mode of SEM, the black particles are graphite and the bright white particles are LiCoO2. In Fig. 6a, the massive graphite is cracked according to its layered structure. It is indicated that the unique crushing behavior of graphite is sliding and flaking along the lamellar structure. In Fig. 6b, lots of small flaky graphite can be seen. This is the result that mechanical grinding makes a large number of graphite flakes stripped. Fig. 6c shows a LiCoO2 particle, which is adhered by graphite, and Fig. 6d is a point element analysis of the left and right parts of the particles in Fig. 6c. The results show that the black part is graphite and the bright part is LiCoO2. In Fig. 6c, there are obvious traces of abrasion on the surface of bright LiCoO2 particle. This suggests that only the angular or convex part of LiCoO2 particles can be smoothed by mechanical grinding. In addition, the graphite is easily attached to the surface, particle gap and concave convex source of LiCoO2 particles under mechanical rolling process. In order to understand the occurrence state of organic films on the surface of LiCoO2 and graphite during mechanical grinding, FE-EPMA was used to analyze the element distribution on the particle surface.
3.3. Dry modification mechanism Based on the above analyses, the dry modification mechanism is revealed and shown in Fig. 8. The surface of electrode materials (LiCoO2 and graphite) from waste LIBs are attached by a layer of organic film after crushing and screening. During the mechanical grinding, the grinding media (steel ball) produces a horizontal shear force and a vertical rolling pressure on the particle surface. Under the horizontal shear force, the internal structure of graphite is sliding and flaking along the location of graphite layer, producing a large number of new hydrophobic surfaces. Due to the high hardness of LiCoO2 particles, mechanical grinding can only wear down and smooth the angular or protrusions of the abrasive particles to partially expose the original surface of LiCoO2, which promotes the improvement of the surface hydrophilicity. Meanwhile, under the vertical rolling pressure of grinding media, some LiCoO2 and graphite particles are adhered together. This makes the hydrophobicity of those LiCoO2 particles enhanced, leading them to follow graphite into the foam layer during the flotation and ultimately lost in the tailings. However, the wettability difference between newborn surface of graphite and primary surface of LiCoO2 is significant, so presenting an excellent effect of flotation separation. More importantly, under the condition of a short period of grinding, both graphite and graphite with a small amount of LiCoO2 can float into the foam layer, so that the grade of LiCoO2 concentrate at the bottom of flotation cell reaches about 95%. However, it should be noted that due to the smaller particle size and
3.2.3. The element distribution analysis based on FE-EPMA With the help of FE-EPMA, the adhesion portion between the LiCoO2 and graphite particle was subjected to element distribution analysis, as shown in Fig. 7. Fig. 7a shows a black graphite particle and white LiCoO2 particles, whose adhesion portions are enlarged in Fig. 7b. In Fig. 7b, the edges of LiCoO2 and graphite are touched tightly, but the boundary is clear. In other words, the adhesion is the effect of mechanical force. During the grinding process, the pressure in the vertical direction from grinding medium results in the adhesion between LiCoO2 and graphite. Since the fluorine element is a characteristic element in PVDF, which is the main component in organic film, and there is no fluorine on the initial surface of electrode material, then the F element distribution can be used to characterize the occurrence of organic film. The micro-probe distribution of cobalt and fluorine elements on the particles are shown in Fig. 7c and d, respectively. In Fig. 7c and d, the counting point with light color represents a high element content and 51
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more quantity of LiCoO2 particles, graphite particles will be adsorbed by more LiCoO2 particles to its outer surface to form the agglomerates of large particles as the grinding time increases, improving the hydrophilicity of graphite. These graphite particles will no longer be able to enter the foam layer and remain in the LiCoO2 concentrate. Therefore, with the grinding time increasing, the grade of LiCoO2 concentrate decreases but the recovery rate increases. 4. Conclusion Grinding flotation is a physical sorting method proposed for the special structural characteristics of LiCoO2 and graphite. With the optimum condition of 5 min grinding, the concentrate grade of LiCoO2 and graphite are 97.19% and 82.57%, and their recovery rate are 49.32% and 73.56%, respectively. Under the horizontal shear force produced by the grinding media, the lamellar structures of graphite are sliding and flaking, exposing a large number of new hydrophobic surfaces. On the other hand, the organic film coating LiCoO2 particles are partially worn down to restore the original hydrophilic surface. With the action of vertical rolling pressure, the adhesion of LiCoO2 and graphite will occur and gradually become serious. Although the adhesion behavior in the 5 min grinding process will lead some LiCoO2 concentrate to follow graphite into the foam layer to reduce the flotation recovery rate, the great wettability difference results in an extremely good flotation concentrate grade. The advantages of reliable, high efficiency and no secondary pollution help grinding flotation to be a good choice for industrial application. Acknowledgments The financial support for this work was from the National Natural Science Foundation of China (No. 51574234) and Shanghai Cooperative Centre for WEEE Recycling (No. ZF1224). We also would like to thank the Advanced Analysis and Computation Center of China University of Mining and Technology for its technical support. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.seppur.2017.08.049. References [1] L. Li, L. Zhai, X. Zhang, J. Lu, R. Chen, F. Wu, K. Amine, Recovery of valuable metals from spent lithium-ion batteries by ultrasonic-assisted leaching process, J. Power Sources 262 (2014) 380–385. [2] L. Li, W. Qu, X. Zhang, J. Lu, R. Chen, F. Wu, K. Amine, Succinic acid-based leaching system: a sustainable process for recovery of valuable metals from spent Li-ion batteries, J. Power Sources 282 (2015) 544–551. [3] N.B. Horeh, S.M. Mousavi, S.A. Shojaosadati, Bioleaching of valuable metals from spent lithium-ion mobile phone batteries using Aspergillus niger, J. Power Sources 320 (2016) 257–266. [4] E.G. Pinna, M.C. Ruiz, M.W. Ojeda, M.H. Rodriguez, Cathodes of spent Li-ion batteries: dissolution with phosphoric acid and recovery of lithium and cobalt from leach liquors, Hydrometallurgy 167 (2017) 66–71. [5] Navigant Research, The Global Market for Lithium Ion Batteries for Vehicles is Expected to Total $221 Billion from 2015 to 2024, 2016. < http://www. navigantresearch.com > (accessed 17 February 2016). [6] J. Hu, J. Zhang, H. Li, Y. Chen, C. Wang, A promising approach for the recovery of high value-added metals from spent lithium-ion batteries, J. Power Sources 351 (2017) 192–199. [7] X. Chen, H. Ma, C. Luo, T. Zhou, Recovery of valuable metals from waste cathode materials of spent lithium-ion batteries using mild phosphoric acid, J. Hazard. Mater. 326 (2017) 77–86. [8] X. Zeng, J. Li, B. Shen, Novel approach to recover cobalt and lithium from spent lithium-ion battery using oxalic acid, J. Hazard. Mater. 295 (2015) 112–118. [9] X. Wang, G. Gaustad, C.W. Babbitt, C. Bailey, M.J. Ganter, B.J. Landi, Economic and environmental characterization of an evolving Li-ion battery waste stream, J.
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