A Closed-Loop Process for Selective Metal ... - ACS Publications

Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9972-9980

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A Closed-Loop Process for Selective Metal Recovery from Spent Lithium Iron Phosphate Batteries through Mechanochemical Activation Yongxia Yang,†,‡ Xiaohong Zheng,‡,§ Hongbin Cao,‡ Chunlong Zhao,‡,∥ Xiao Lin,‡ Pengge Ning,‡ Yi Zhang,†,‡ Wei Jin,‡ and Zhi Sun*,‡ †

School of Materials Science and Engineering, Northeastern University, Shenyang, Liaoning 110819, China Beijing Engineering Research Center of Process Pollution Control, Division of Environment Technology and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China § University of Chinese Academy of Sciences, Beijing 100190, China ∥ State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China ‡

S Supporting Information *

ABSTRACT: With the increasing consumption of lithium ion batteries (LIBs) in electric and electronic products, the recycling of spent LIBs has drawn significant attention due to their high potential of environmental impacts and waste of valuable resources. Among different types of spent LIBs, the difficulties for recycling spent LiFePO4 batteries rest on their relatively low extraction efficiency and recycling selectivity in which secondary waste is frequently generated. In this research, mechanochemical activation was developed to selectively recycle Fe and Li from cathode scrap of spent LiFePO4 batteries. By mechanochemical activation pretreatment and the diluted H3PO4 leaching solution, the leaching efficiency of Fe and Li can be significantly improved to be 97.67% and 94.29%, respectively. To understand the Fe and Li extraction process and the mechanochemical activation mechanisms, the effects of various parameters during Fe and Li recovery were comprehensively investigated, including activation time, cathode powder to additive mass ratio, acid concentration, the liquid-to-solid ratio, and leaching time. Subsequently, the metal ions after leaching can be recovered by selective precipitation. In the whole process, about 93.05% Fe and 82.55% Li could be recovered as FePO4·2H2O and Li3PO4, achieving selective recycling of metals for efficient use of resources from spent lithium ion batteries. KEYWORDS: Spent LiFePO4, Mechanochemical activation, Lithium recovery, Leaching

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INTRODUCTION With the rapid development of electric vehicles (EVs), it has been forecasted that the global consumption of lithium-ion batteries for vehicles is expected to total $221 billion from 2015 to 2024.1,2 Since it was first reported in 1997, lithium iron phosphate (LiFePO4) for lithium-ion batteries has been recognized as one of the excellent cathode materials for applications in large vehicles or facilities because of its superior thermal safety, relatively high theoretical capacity, theoretical energy density (580 Wh kg−1), acceptable operating voltage (3.45 vs Li+/Li), low material cost, nontoxicity, and high reversibility.3 Commercial LiFePO4 batteries have been used in electric vehicles by improving poor intrinsic electronic conductivity and the low diffusion coefficient of Li+ with carbon coating and reduction to the particle size.4 The more reliance on lithium ion batteries (LIBs) in electronic equipment and electric vehicles, the more spent LIBs will be generated due to their limited life spans and the wastes from the production process.5,6 Although LiFePO4 material is considered to be © 2017 American Chemical Society

relatively environmental friendly, the corresponding spent LIBs may still cause environmental problems (from the electrolyte) and waste of valuable resources such as lithium for the increasing accumulation of quantity and improper disposal with a discarding manner. As a lithium-containing secondary resource, the necessity to develop an efficient and cost-effective route to recycle spent LiFePO4 cathode materials is significant. The treatment technology of spent LiFePO4 batteries mainly includes two categories: direct regeneration of cathode materials and recycling as individual compounds. Chen et al.7 reported a small-scale model line to direct regenerate cathode materials from spent LiFePO4 batteries. The recycled cathode powder exhibited almost the same discharge capacities and specific energy densities as the fresh cathode material at high discharge current densities by heat-treatment for 650 °C under Received: June 13, 2017 Revised: September 20, 2017 Published: September 29, 2017 9972

DOI: 10.1021/acssuschemeng.7b01914 ACS Sustainable Chem. Eng. 2017, 5, 9972−9980

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ACS Sustainable Chemistry & Engineering an Ar/H2 flow. Song et al.8 developed a process by adding new LiFePO4 approach to regenerate spent cathode materials. In the course of this process, the cathode scrap was soaked in DMAC solvent to separate the cathode materials and Al foil at optimal conditions of 30 min at 30 °C and a solid to liquid ratio of 50 g/L, and the separate cathode materials were directly regenerated with addition of new LiFePO4 by solid-phase sintering method, in which battery capacities can reach 144 mAh/g at 0.1C when adding at a ratio of 3:7 for new and spent materials at 700 °C. On the other hand, Fe and Li can be recovered as individual compounds. Bian et al.9 introduced phosphoric acid (H3PO4) as the leaching reagent to treat the cathode material, which was already separated from Al foil by using NaOH aqueous solution with ultrasound-assistance. The Fe and Li are recovered as FePO4·2H2O by aging and LiH2PO4 from the filtrate by ethanol solvent extraction. Zheng et al.10 utilized sulfuric acid to leach spent cathode materials after its separation from the current collector at 600 °C annealing. The Fe can be recovered as FePO4 by adjusting the pH value of the solution to 2, and the Li was recovered as Li2CO3 by adding saturated sodium carbonate. Huang et al.11 designed a leaching−flotation−precipitation process to separate and recover Li/Fe/Mn from the mixed types of cathode materials (hybrid waste of LiFePO4 and LiMn2O4). The purity for obtained Li3PO4, FeCl3, and MnO2/MnO3 is 99.32 ± 0.07%, 97.91 ± 0.05%, and 98.73 ± 0.05%, respectively. For the spent LiFePO4 batteries, direct regeneration is not suitable for cathode scrap with a high amount of impurities. Olivine structured lithium iron phosphate is fairly stable that effective extraction of lithium and iron has to rely on strong acid/ alkaline or a much larger amount of leachate than the stoichiometric requirement. Pretreatment with high temperature roasting is often used to improve the leaching recovery. In these cases, secondary pollution is inevitable, and the excess acid/alkaline needs to be treated, which can increase the process cost significantly.12 In recent years, the possibility of metal extraction selectively from a highly complex industrial information and communication technology (ICT) waste by using a hydrometallurgical method has been presented.13 Mechanochemical activation begins with prehistoric times, when reactions could be initiated during grinding and rubbing accidentally.14,15 With the aid of high-energy ball-milling of materials, the mechanically induced changes subsequently influence their physical and chemical properties.16 Takacs reported that mechanically activated self-sustaining hightemperature reactions (MSRs) represented a class of selfpropagating high-temperature synthesis in the preparation of nanocrystalline materials, amorphous alloys, and metastable crystalline alloys.17−19 Sepelak reported on the single-step synthesis nanocrystalline Ca2SnO4 and Fe2SiO4 by mechanochemical activation.20 It demonstrates that mechanically induced reactions provide novel opportunities for the nonthermal manipulation of materials and the tailoring of their properties. Balaz introduced the mechanochemical route to synthesize well-crystallized ZnS, CdS, and PbS nanoparticles and discussed the suitability of mechanochemistry application in chalcogenide synthesis.21 Although mechanochemical activation achieves good results in material preparation, it can also be used for the recovery of wastes. In recent years, mechanochemical activation method has been increasingly applied in metal recycling from wastes, for example, recycling indium, lead, cobalt, lithium, tungsten, gold, and the REEs.22

The reduction in particle size, the increase in specific surface area, point defects and dislocations in crystalline structures, polymorphic transformations, bond breakage, or even chemical reaction were identified as the main processes occurring during the mechanochemical activation.23 These physical and chemical changes indicate a decrease in activation energy and an increase in reaction activity after mechanochemical activation, which is expected to help in relieving the aforementioned problems during the recycling of spent LiFePO4 batteries.22,24 For lithium-based systems, James and co-workers studied the electrochemical insertion of Li in mechanochemically prepared Zn2SnO4.25 Sepelak prepared nanosized LiFe5O8 by the mechanochemical method.26 All of the previous findings show that it is possible to in situ enhance/manipulate ion diffusion through mechanical activation. In this research, this technology was investigated to recover metals (Fe and Li) from an industrially provided cathode scrap. The aims are to reduce the acid consumption with less strong acidic leachate, improve the recovery selectivity of lithium, and decrease the emission of secondary pollution, which finally develop a closed-loop recycling process for spent LiFePO4 batteries. To fulfill the objectives, this work focuses on identifying the effect of mechanochemical activation behavior of spent cathode powder and the potential to recover lithium selectively. The examined parameters including activation time, cathode powder to additive mass ratio, acidic concentration, the liquid-to-solid ratio, and leaching time were systematically discussed.

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EXPERIMENTAL SECTION

Materials. The spent LiFePO4 batteries were supplied by Brunp Recycling Co. Ltd. in China and were dismantled using a manual procedure after discharging. The plastic cases of the batteries were first removed, which was followed by removing the steel case mechanically. Anode and cathode uncurled manually, and copper and aluminum foils were collected for recycling separately. The cathode materials in the form of powder were separated from Al foil by ultrasonic-assisted and mechanical enhancement technique in water after 1 h. The obtained powder was dried and stored for further investigation. Table 1 presents

Table 1. Main Element Composition of the Cathode Scrap Powder elements composition (wt %)

Fe 31.25

Li 4.08

P 18.94

Al 0.16

the composition of the powder separated from cathode scrap. The Xray diffraction of the cathode powder (Figure 1) shows the presence of LiFePO4 as the only phase in the cathode powder. All chemical

Figure 1. XRD pattern of the cathode powder. 9973


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ACS Sustainable Chemistry & Engineering reagents used in this work were of analytical grade, and all solutions were prepared with ultrapure water (Milli-Q, Millipore). Mechanochemical Activation of Cathode Powder. The separated cathode powder and additive were mixed proportionally (cathode powder/EDTA-2Na mass ratio from 6:1 to 1:1) and with mechanochemical activation using a planetary ball mill (QMQX2, Nanjing University Instrument Plant, China) with four symmetrical pots of 250 mL. The rotation speed of the pot is twice that of the disk. Approximately 5 g of cathode powder and one of the additives were mixed with 120 g Φ5 mm zirconia beads. The planetary ball mill was operated at 550 rpm for 0.5−6 h under ambient atmosphere. During activation, an interval of 15 min was set after each milling run of 15 min, to avoid the accumulation of heat. Activated samples were further leached with diluted phosphoric acid. Acidic Leaching. The leaching experiments were performed in a series of 100 mL conical flasks conducted at room temperature by magnetic stirring. For each run, a fixed amount of the cathode powder (∼2.0 g) was precisely measured, and a known concentration of phosphoric acid was prepared as leaching reagent. They were simultaneously added to the flask under continuous stirring. After a certain time, the flask was taken down and filtered immediately by vacuum filtration using a cellulose acetate membrane (pore size 0.45 μm). The residues were washed several times with ultrapure water and dried at 60 °C for 24 h in air. The concentrations of metals in the leachate were determined by induction coupled plasma-optical emission spectrometry (ICP-OES, iCAP 6300 Radial, Thermo Scientific). The leaching efficiency of metals from the cathode powder was calculated by

LEM =

Figure 2. Leaching efficiency of Fe and Li from different samples (mechanochemical activation sample, activation time = 5 h, mass ratio of cathode powder to EDTA-2Na = 3:1; leaching parameters, H3PO4 = 0.5 M, S/L ratio = 40 g/L, leaching time = 60 min).

40% of iron could be leached out with direct acid leaching. However, 94.29% of Fe and 92.04% of Li were recovered when mechanochemical activation was applied with EDTA-2Na being cogrinded with LiFePO4 cathode powder, indicating that mechanochemical activation significantly accelerated the process of acid leaching for the extraction of metal. This behavior could be explained by the fact that mechanical forces such as shearing, impact, and squeezing exerted by ball milling will transmit energy to powder, diminish powder particles, and destroy the crystal structures.22 Figure S1 demonstrates the effect of acidic type on the leaching of spent LiFePO4 powder. It is clear that strong acids are possibly leaching both lithium and iron, while mild phosphate acid shows less effectiveness on the leaching. These leaching priority behaviors can be explained on the basis of hard−soft-acid−base (HSAB) interaction theory.29 By introducing mechanical activation, it facilitates effective metal recovery from spent LiFePO4 when mild acidic solution (H3PO4) is used. Effect of Activation Time. The effect of activation time on Fe and Li extraction from LiFePO4 cathode powder was carried out at the solution concentration of 0.5 M H3PO4, solid/liquid ratio (g/L) of 40. There was a significant increase in the leaching efficiency of Fe and Li with a prolonged activation time especially in the first 2 h. As given in Figure 3, the leaching efficiency of Fe and Li reached from 37.81% to 95.96% and 61.73−91.84%, respectively. Considering the energy consump-

C M, tV C M,f V + Wf

(1)

where LEM is the metal leaching efficiency; and CM,t, CM,f, V, and Wf refer to the concentration of metal ions in the leachate at time t (g/L), metal contents in the final solution (g/L), leachate volume (L), and metal contents in the final residue after leaching, respectively. Materials Recovery. The precipitation experiments were conducted to determine the optimum recovery of lithium and possibly iron. The leachate was refluxed for 9 h at 90 °C in a 250 mL threenecked flask with a vapor condenser. The precipitate was collected by filtering, washing at least three times with ultrapure water, and drying at 60 °C for 24 h in air. The precipitation efficiency PM can be calculated by

⎛ CV ⎞ PM = ⎜1 − 1 1 ⎟ × 100% C 2V2 ⎠ ⎝

(2)

where PM is the metal precipitation efficiency; C1 and C2 are the concentrations of metals in the solution before and after precipitation (g/L), respectively; and V1 and V2 refer to the volumes of the liquor before and after precipitation (L), respectively. Characterization. The separated cathode powder, mechanochemical activation powder, and precipitated products were analyzed by Xray diffraction spectrometer (X’pert PRO, PANalytical) with Cu Kα radiation. The data were collected by step scanning with a scanning speed of 10°/min and a scanning angle (2θ) of 5−90°. The morphology of the solid samples was characterized with a scanning electron microscope (SEM, JEOL JSM-7610F), and the samples were deposited by a gold film to enhance electric conductivity. Liquid samples were diluted and prepared for analyzing by ICP-OES.

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RESULTS AND DISCUSSION Metal Recovery from Cathode Powder through Mechanochemical Activation. Effect of Mechanochemical Activation. The EDTA-2Na is considered to be an excellent metal chelating reagent, which has been widely used for the removal of metals from industrial waste, such as alkali metals, rare-earth elements, or metals of spent LiCoO2 batteries.27,28 As can be seen in Figure 2, around 60% of lithium and less than

Figure 3. Effect of activation time on Fe and Li leaching efficiency (mass ratio of cathode powder to EDTA-2Na = 3:1; leaching parameters: H3PO4 = 0.5 M, S/L ratio = 40 g/L, leaching time = 60 min). 9974


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ACS Sustainable Chemistry & Engineering tion and recovery efficiencies, it is recommended that 2 h is the optimum milling time. Generally, small particles tend to dissolve more rapidly as compared to large particles because of large specific surface areas.30 Amorphous materials have a larger fraction of highly energetic sites such as dislocations and defects. Therefore, if milling the sample for a longer time, the particle size will be smaller and the crystal structure may be destroyed continuously, so the leaching efficiency will be improved. This is well consistent with Fe and Li leaching efficiency after milling for different times. Meantime, small particles will be reunited with the increase in the activation time, and thus the leaching platform appeared in the leaching curves of activated samples. Effect of Mass Ratio of Cathode Powder to Activation Additive. The mass ratio is another key parameter in the mechanochemical activation, because the chelating efficiency of EDTA-2Na with ions in the cathode powder can be influenced. As shown in Figure 4, the leaching efficiency increases sharply

Figure 5. Effect of acid concentration on Fe and Li leaching efficiency (S/L ratio = 40 g/L, leaching time = 60 min; activation parameters: activation time = 2 h, mass ratio of cathode powder to EDTA-2Na = 3:1).

The reason is most likely related to the thermodynamic interaction between phosphate ionization and leaching reaction of LiFePO4. In an aqueous solution, ionization of phosphate is under thermodynamic equilibrium at a certain temperature (K1 and K2 are constants) as shown by eqs 3, 4, and 5.31 It can be concluded that further increase of the phosphoric acid concentration may not be able to further increase the activation of H+. Therefore, all further experiments were carried out using 0.6 M H3PO4. H3PO4 = H2 PO4 − + H+ H2 PO4 − = HPO4 2 − + H+

Figure 4. Effect of cathode powder to additive mass ratio on Fe and Li leaching efficiency (activation time = 2 h; leaching parameters: H3PO4 = 0.5 M, S/L ratio = 40 g/L, leaching time = 60 min).

K1 =

[H 2PO4 −][H+] [H3PO4 ]

K2 =

[HPO4 2 −][H+] [H2 PO4 −]

(3)

(4)

HPO4 2 − = PO4 3 − + H+ K3 =

with the decrease of mass ratio of cathode powder to EDTA2Na that 52.46% for Fe and 72.14% for Li can be leached out at a mass ratio of 6:1, while 95.96% of Fe and 91.84% of Li can be at a mass ratio of 3:1. Leaching efficiency was almost unchanged when the mass ratio is further decreased. The above results indicate that 3:1 can be the appropriate mass ratio for mechanochemical activation process. This is consistent with a previous report that EDTA generally coordinates with metal ions at the molar ratio of 1:1.28 In this research, it indicates the mass ratio of LiFePO4 to EDTA-2Na shall be theoretically 2.4:1. This is calculated by considering a perfect LiFePO4 crystal structure, and the spent cathode material needs less EDTA-2Na for full chelation. However, taking economic feasibility into account, we used a small amount of EDTA2Na to recover spent LiFePO4 in which the mass ratio of cathode powder to EDTA-2Na is 3:1 (as given in Figure S5, the recovery efficiencies show minor difference). Effect of Acid Concentration. After mechanochemical activation, leaching of Fe and Li was investigated at varying H3PO4 concentrations in the range of 0.2−0.7 M while maintaining all other parameters unchanged. As shown in Figure 5, the leaching efficiency was increased from 51.59% to 99.75% for Fe and from 64.33% to 94.75% for Li, respectively, with increasing H3PO4 concentration from 0.2 to 0.7 M. It was observed that the dissolution efficiency of Fe and Li remained constant when the H3PO4 concentration was larger than 0.6 M.

[PO4 3 −][H+] [HPO4 2 −]

=

[PO4 3 −][H+]3 [H3PO4 ]K1K 2

(5)

4LiFePO4 + 4H+ + O2 = 4Fe3 + + 4Li+ + 4PO4 3 − + 2H 2O

(6)

Effect of S/L Ratio. The leaching of Fe and Li in the function of S/L ratio in the range of 40−100 (g/L) was studied, in which other parameters were kept constant. Results given in Figure 6 clearly show that the leaching efficiency decreases from 99.86% to 76.17% for Fe and 95.4−72.29% for Li, respectively, with increasing S/L ratio of 50−100 g/L, which indicated that the lower solid to liquid ratio could enlarge the contact areas of activation powder and phosphate solution to accelerate the leaching reaction. Generally speaking, both the increasing acid concentration and the decreasing S/L ratio facilitate the leaching efficiencies of metals because of the increase of reagent in the reaction system.32,33 However, a low solid to liquid ratio would lead to the increase of leach solution volume, which is not favorable for following metal separation and recovery.34,35 Hence, 50 g/L can be taken as the optimum value for the rest of the experiments. Effect of Leaching Time. The effect of leaching time (0−60 min) on the leaching efficiency of metals was examined using 0.6 M H3PO4 and solid to liquid ratio of 50 g/L at room temperature after mechanochemical activation. It is apparent 9975


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Figure 6. Effect of S/L ratio on Fe and Li leaching efficiency (H3PO4 = 0.6 M, leaching time = 60 min; activation parameters: activation time = 2 h, mass ratio of cathode powder to additive = 3:1).

that the dissolution of Fe and Li progressed well (Figure 7) with increasing leaching time. No further increased leaching

Figure 8. Cathode powder for mechanochemical activation at different milling time: (a) XRD patterns, and (b) FTIR patterns.

Figure 7. Effect of leaching time on Fe and Li leaching efficiency (H3PO4 = 0.6 M, S/L ratio = 50 g/L; activation parameters: activation time = 2 h, mass ratio of cathode powder to EDTA-2Na = 3:1).

efficiencies were observed after 97.67% Fe and 94.29% Li being recovered at reaction duration over 30 min. This can be attributed to that the residue hinders the diffusion of H+ from the solution to the interface of residual materials. Mechanochemical Activation Mechanisms. To elucidate the mechanochemical activation mechanisms in details, physicochemical changes were also investigated. According to X-ray diffraction (XRD) patterns presented in Figure 8a, the intensity of characteristic diffraction peaks (the position can be found in Figure 1) decreased with an increase in milling time; specifically, the X-ray diffraction intensities of (101), (111), (211), and (311) lattice planes decrease as the milling time increases (Figure 9). However, the mechanochemical activation powder (311) crystal plane intensity decreased faster than did (101), (111), and (211). The FTIR patterns indicate that P− O/PO4 bond/tetrahedra is probably the structure destroyed during mechanochemical activation. Meanwhile, as shown in Figure 10, the full width at half-maximum (FWHM) of peaks also broadened with a simultaneous increase of milling time, and the FWHM of the cathode powder (311) crystal plane is wider than those of other lattice planes. These results indicate that the (311) plane of cathode powder can be easily destroyed in accordance with mechanochemical activation. During mechanochemical activation, it is easier to destroy the atomic bonds with longer lengths, which is the diagonal of an

Figure 9. X-ray diffraction intensity of lattice faces with milling time.

Figure 10. The FWHM of lattice faces with milling time (all data were calculated by Jade6.5 software).

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ACS Sustainable Chemistry & Engineering orthorhombic crystal cell. This is reflected by (311) or (211) planes in the XRD patterns because the crystal constant of the a-axis is the longest for the LiFePO4 crystal. The SEM micrographs of samples were presented in Figure S4. Although the particle size is not significantly changed, agglomeration of particles is observed. It may come from reactions between the agglomeration additive materials and the cathode powder. With the mechanochemical activation proceeding, lithium and iron were chelated, and subsequent leaching using a diluted leachate solution can be facilitated. Recovery of Iron. After mechanochemical activation leaching, a leachate with pH of 1.5 was obtained for materials recovery. As can be seen in the E−pH diagrams in Figure S2, the FePO4 phase is thermodynamically stable under acidic conditions. All diagrams were made using HSC 6.0 chemistry software. Accordingly, crystalline iron phosphate precipitation can be obtained by refluxing air for 9 h at 90 °C in a 250 mL three-necked flask with a vapor condenser. Iron(II) can be oxidized into iron(III), which can be further precipitated out from the solution. The main reaction is shown as Fe3 + + PO4 3 − + 2H 2O = FePO4 ·2H 2O↓

Figure 12. SEM images of the recycled FePO4·2H2O. Inset: Microstructure of FePO4·2H2O.

(7)

Recovery of Lithium. The excessive amount of H3PO4 and Li+ should also be recycled or reused for the sake of comprehensive recovery of metals and waste minimization. After iron was recovered, the residual solution consisting mainly Li and phosphate ions was used for recovery lithium. To study the extraction behavior of Li in complicated Li+ solution, the thermodynamic aspects, particularly the stability regions of different phases of lithium in the aqueous solutions, were calculated by HSC 6.0 software. The E−pH diagram of Li−P− H2O system is shown in Figure S3. It can be seen that the stable domain of Li3PO4 phase is within the stability region of water in neutral and alkaline region. Hence, the Li+ solution can be easily precipitated into lithium phosphate by adjusting pH with 5 M sodium hydroxide solution. The XRD pattern of the obtained lithium phosphate is shown in Figure 13, and it agrees well with the standard pattern peaks.

The XRD pattern of the obtained iron phosphate precipitation is shown in Figure 11, and it agrees well with

Figure 11. XRD pattern of the recycled iron phosphate sample.

the standard pattern peaks. To accurately calculate the quality of iron phosphate, the precipitated iron phosphate was dissolved by 0.1 M hydrochloric acids, and its mass fraction of components was tested by ICP-OES (Table 2). From Table Table 2. Quality Analysis of the Recovered Iron Phosphate content composition

Fe Li P (wt %) Al (wt %) (wt %) (wt %) 30.31 0.04 16.97 0.13

P/Fe (molar ratio) 1.01

2, the P/Fe molar ratio of the precipitate is 1.01; even a small amount of Al remaining in iron phosphate, which doped in the cathode materials, was found to increase the rate performance and cycle stability of LiFePO4/C, indicating the high quality of FePO4·2H2O was recovered.36 The SEM images of the precipitated FePO4·2H2O are illustrated in Figure 12. It can be noticed that the precipitated FePO4·2H2O is a multistage structure of the spherical, the surface assembled by nanosheets of iron phosphate, which can be used for the synthesis of catalyst and LiFePO4 cathode material.37

Figure 13. XRD pattern of the recycled lithium phosphate.

As shown in Table 3, the purity of the obtained product (Li3PO4) is 96.51% based on the analytical results of ICP-OES analysis by dissolving in 3 M nitric acid and diluting into a suitable concentration. Because of the presence of EDTA-2Na complexing reagent in the solution, impurity iron and aluminum, especially at low concentrations, have potential to form complexes and become 9977


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more than 94%. FePO4·2H2O precipitates are recycled without regulating the pH value of the leaching solution. Lithium is recovered in the form of Li3PO4 by adjusting the filtrate pH to 8.0, which is obtained after collecting the FePO4·2H2O precipitates. After recovery of Li3PO4, the solution consists mainly of sodium ions, phosphate ions, and EDTA-2Na (Table S1). EDTA-2Na is possibly recovered by recrystallization, while the remaining solution with sodium phosphate can be used for softener and detergent, boiler scale inhibitor, dyeing, and finishing agent. Detailed research on how to recover the activation additive will be provided in our subsequent research. As shown in Table 4, the global recovery rates of Fe and Li are found to be 93.05% and 82.55%, respectively. In the course

Table 3. Mass Fraction of Metals in the Precipitated Lithium Phosphate content composition (wt %)

Al2O3 0.05

Fe2O3 0.64

Na2O 0.87

Li3PO4 96.51

others 1.93

difficult to be completely removed (lg KFeY = 25.1 > lg KAlY = 16.3 > lg KLiY = 2.79, Y is EDTA).38,39 Even so, a residual solution containing mainly sodium phosphate and EDTA-2Na can be obtained. The SEM image of the precipitated Li3PO4 is illustrated in Figure 14. It can be noticed that the precipitated

Table 4. Global Recovery Rates of Different Metals from the Cathode Powder in This Research element

leaching efficiency (%)

precipitation efficiency (%)

total recovery rate (%)

main products

Fe Li

97.67% 94.29%

96.95% 87.89%

93.05% 82.55%

FePO4·2H2O Li3PO4

of recovering Fe as FePO4·2H2O, without adjusting pH, it was found that about 4.03% Fe was lost in leaching residue and 2.89% Fe into the Li solution. During the lithium recovery process, the lithium loss of the leaching process takes 5.72%, recovery Fe 0.35%, precipitation Li3PO4 11.36%. During the precipitation process, the loss of lithium mainly resulted from the complexation with EDTA-2Na and water rinsing to get high-purity products. It is obvious that the value loss of lithium tops all others in the precipitation process due to the chelation of EDTA to lithium ions. For this process, the iron and lithium can be effectively recovered without secondary waste, which contributes significantly to the recycling of metals from waste lithium-ion batteries.

Figure 14. SEM image of the recycled lithium phosphate.

Li3PO4 was presented as massive agglomerates of numerous rhombic sheets. The recovered Li3PO4 can be used for the synthesis of dental material, catalyst, and LiFePO4 cathode material. Development of a New Process for Spent LiFePO4 Battery Recovery. On the basis of the above theoretical and experimental results, a process to recover metals from spent LiFePO4 batteries can be proposed. A schematic design of the process is plotted in Figure 15. The spent LiFePO4 cathode powder with EDTA-2Na is treated by mechanochemical activation. The leaching efficiencies of Fe and Li can reach

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CONCLUSIONS The effect of the mechanochemical activation process on metal recovery from industrial waste LiFePO 4 batteries was investigated in phosphoric acid solution. About 93.05% Fe and 82.55% Li could be recovered as FePO4·2H2O and Li3PO4. With a systematic understanding of this novel process, the metal recovery can be optimized, and the following conclusions can be drawn: (1) Mechanochemical activation: An innovative process for Fe and Li recovery from spent LiFePO4 batteries through mechanochemical activation is proposed. After mechanochemical activation, about 97.67% Fe and 94.29% Li are recovered under the optimized conditions of activation time (2 h), mass ratio of cathode powder to EDTA-2Na = 3:1, 0.6 M H3PO4, S/ L ratio (50 g/L), and leaching time (20 min). According to the mechanochemical activation mechanism, the (311) face of cathode powder is more readily destroyed and transformed to disordered states, leading to the significant increase of leaching efficiency. (2) Material recovery: The thermodynamic diagrams, particularly the stable regions of lithium and iron components in the aqueous solutions, were calculated. As a result, about 93.05% Fe and 82.55% Li could be recycled in the selective precipitation process via the proton activity modulation. On the basis of the results, the advantages of the process are less dilute acid, a high selectivity recovery rate, high purity of

Figure 15. Development of a mechanochemical activation process for spent LiFePO4 batteries recycling. 9978


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product, and no secondary waste emission, which reaches a closed-loop process. This research provides a technology for selective and effective recycling of valuable metals from spent lithium iron phosphate batteries.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01914. Profiles of leaching efficiencies with different acidic solutions; the corresponding E−pH diagram of Li−Fe− P−O−H; SEM images of the cathode materials with different mechanochemical activation times; and the composition of residual solution after precipitation of lithium (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 10 82544844. Fax: +86 10 82544845. E-mail: [email protected]. ORCID

Hongbin Cao: 0000-0001-5968-9357 Zhi Sun: 0000-0001-7183-0587 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support of this research from the CAS Pioneer Hundred Talents Program (Z.S.), the National Natural Science Foundation of China under Grant nos. 51425405 and L1624051, and the National Science-Technology Support Plan Projects (2015BAB02B05).

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REFERENCES

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