Recovery of Lithium, Iron, and Phosphorus from Spent LiFePO4

Aug 10, 2017 - A selective leaching process is proposed to recover Li, Fe, and P from the cathode materials of spent lithium iron phosphate (LiFePO4) ...
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Research Article pubs.acs.org/journal/ascecg

Recovery of Lithium, Iron, and Phosphorus from Spent LiFePO4 Batteries Using Stoichiometric Sulfuric Acid Leaching System Huan Li, Shengzhou Xing, Yu Liu, Fujie Li, Hui Guo, and Ge Kuang* Institute of Chemical Engineering and Technology, Fuzhou University, Fuzhou 350108, China S Supporting Information *

ABSTRACT: A selective leaching process is proposed to recover Li, Fe, and P from the cathode materials of spent lithium iron phosphate (LiFePO4) batteries. It was found that using stoichiometric H2SO4 at a low concentration as a leachant and H2O2 as an oxidant, Li could be selectively leached into solution while Fe and P could remain in leaching residue as FePO4, which is different from the traditional process of using excess mineral acid to leach all the elements into solution. Under the optimized conditions (0.3 M H2SO4, H2O2/Li molar ratio 2.07, H2SO4/Li molar ratio 0.57, 60 °C, and 120 min), the leaching rates of 96.85% for Li, 0.027% for Fe, and 1.95% for P were recorded. The Li contained in solution was then recovered by introducing Na3PO4 as a precipitant. Around 95.56% Li was precipitated and recovered in the form of Li3PO4 under the experimental conditions. In addition, the FePO4 in the leaching residue was directly recovered by burning at 600 °C for 4 h to remove carbon slag. This study illustrates an effective process for the recycling of spent LiFePO4 batteries in a simple, efficient, and costeffective way, which give it potential to be industrially applied. KEYWORDS: Lithium iron phosphate batteries, Recovery, Stoichiometric, Sulfuric acid, Selective leaching, Lithium phosphate, Iron phosphate

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INTRODUCTION For over a decade now, lithium-ion batteries (LIBs) have been considered the best power source for a sustainable transport system that can operate at a higher voltage and achieve a higher energy density than the previously commercialized batteries and have been applied widely in electric vehicles (EVs) and hybrid electric vehicles (HEVs),1−3 with the annual production of LIBs having increased by 800% worldwide from 2000 to 2010.4 It is forecasted that there will be a huge growth for the LIB industry with an estimated global market close to $32 billion by 2020.5 The olivine structured lithium iron phosphate (LiFePO4, LFP) first reported in the 1980s6,7 has been recognized as one of the most promising cathode materials for LIB because of its advantages of having high power capability, low cost, low toxicity, excellent thermal safety, and high reversibility.8 Especially, LiFePO4 has been considered as a very safe cathode material because of its low electrochemical potential.9 As a result, the LIBs using LiFePO4 as cathode materials (abbreviated as LiFePO4 batteries) have been widely used in electronic equipment, large-scale energy storage devices, EVs, and HEVs (particularly in electric buses) in recent years.10,11 In 2014 alone, 12 500 tonnes of LiFePO4 were sold globally for use in LiFePO4 batteries.12 As the major LiFePO4 production and consumption country, China’s shipment amount of LiFePO4 reached 32 400 tonnes in 2015, accounting for 65% of the worldwide market. Driven by the © 2017 American Chemical Society

rapid development of EVs and HEVs, the LiFePO4 battery market is expected to expand continuously with an estimated increase rate of 20% during 2016−2020.13 However, due to the huge usage and growing demand for LiFePO4 batteries, there will be plenty of spent LiFePO4 batteries obtained during production and after terminal lifespans that may soon bring serious problems.14−16 For example, the organic electrolytes containing toxic LiPF6 and the metal ions will gradually transfer to soil and groundwater when the spent LiFePO4 batteries are directly disposed of in landfill or recycled in improper ways.4 Moreover, the contents of some metals in spent LiFePO4 batteries are at fairly high levels and even greater than those in crude minerals,17 which indicates considerable profitability for recycling. Particularly, the demand for lithium, a primary ingredient of LiFePO4 batteries, has experienced a noticeable increase along with the development of EVs and HEVs. A lithium shortage may emerge in a few decades.17 The recovery of lithium from spent LiFePO4 batteries, with a current recovery rate of less than 1%,18 would be an appropriate way to relieve the shortage of lithium resources and make the supply of lithium diversified.18 Therefore, the recycling of spent LiFePO4 batteries is an Received: May 21, 2017 Revised: July 20, 2017 Published: August 10, 2017 8017

DOI: 10.1021/acssuschemeng.7b01594 ACS Sustainable Chem. Eng. 2017, 5, 8017−8024

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ACS Sustainable Chemistry & Engineering Table 1. Reported Methods for the Recycling of Spent LiFePO4 Batteries recycling method hydrometallurgical method

direct regeneration

reagents/process

products

ref

H2SO4 and H2O2 as leaching agents, NaOH as precipitant, with LiFePO4/H2SO4/NaOH molar ratio of 1:8:15 6 M HCl as leaching agent, 6.25% NH3·H2O as precipitant H3PO4 as leaching agent with LiFePO4/H3PO4 molar ratio of 1:3.16 2.5 M H2SO4 as leaching agent, NH3·H2O and Na2CO3 as precipitants heating cathode scraps at 400−600 °C for 30 min under N2 heating cathode powders at 650 °C for 1 h soaking cathode plates in DMAC solvent at 30 °C for 30 min

Fe(OH)3, FePO4·2H2O, and Li3PO4 FePO4·2H2O FePO4·2H2O and LiH2PO4 FePO4·2H2O and Li2CO3 LiFePO4 LiFePO4 LiFePO4

27 24 10 19 37 35 36

been reported for recycling LiCoO2 or LiNixCoyMn1−x−yO2 batteries in recent years. Nevertheless, the costs for those organic acids are inevitably higher than that of common mineral acids. Considering that the recycling of spent LiFePO4 batteries is not as profitable as that of spent LIBs containing valuable metals (e.g., Co, Ni), achieving a reasonable cost in the above hydrometallurgical methods may be challenging. The method of direct regeneration refers to the recovery of cathode materials by some simple steps for direct reuse in battery fabrication. In the case of recycling spent LiFePO4 batteries, LiFePO4 powders were directly separated and recovered from cathodes by heating at high temperatures or soaking in an organic solvent (Table 1). However, the recovered cathode materials easily contain impurities, and their structure is usually destroyed after numerous charging and discharging cycles, resulting in poor electrochemical performance in its direct reuse.27,35,36 Overall, if an industrially feasible process for the recycling of spent LiFePO4 batteries is to be developed, it is essential to make the process more simple, high-efficient, costeffective, and eco-friendly. Different from the traditional process of using excess and high concentration mineral acid to leach all the elements into solution and separate them subsequently, in this study, near stoichiometric H2SO4 at a low concentration associating with H2O2 was used to selectively leach Li into solution from the cathode materials of spent LiFePO4 batteries, and simultaneously, Fe and P could remain in the leaching residue, which means both leaching and separation were achieved in one step of leaching. As a result, the recycling process was largely simplified. This study uses common reagents and a simple process to recover Li, Fe, and P from spent LiFePO4 batteries, which could be promising in industrial applications.

important issue for both environmental protection and resource conservation. So far, there has been a large amount of literature published focusing on the recycling of LIBs containing precious and toxic metals, such as the previously commercialized LiCoO2 batteries and the ternary-material-based LiNixCoyMn1−x−yO2 batteries, but LiFePO4 batteries have been largely neglected.19 Overall, the existing methods for the recycling of spent LIBs can be divided into pyrometallurgical,20,21 biohydrometallurgical,22,23 and hydrometallurgical10,19,24−32 methods and the method of direct regeneration. Some recycling processes combine the above methods for recovering different materials and often involve some pretreatment steps to obtain individual components of spent LIBs. For the recycling of spent LiFePO4 batteries, as concluded in Table 1, the hydrometallurgical method and direct regeneration have been reported so far. The pyrometallurgical method, with heavy investments for smelting at high temperatures and disposing of dangerous gases, has already been adopted by various industries for recovering Co, Ni, Fe, and Cu from spent LIBs, but the lithium recovery is very limited since it is always oxidized and goes into the slag.14,18 The newly reported biohydrometallurgical method has attracted much attention due to its lower operational costs, high level of safety, and environmental friendliness. However, its bottleneck is the very low leaching efficiency (ranging from several hours to days22), which almost makes it impossible to be industrially developed at the current stage. The hydrometallurgical method, as a traditional method for the recycling of spent LIBs, involves the leaching of cathode materials obtained in the pretreatment step and selectively separating the metals from the leaching solution, which is then purified to obtain different products. In the reported leaching process of recycling spent LiFePO4 batteries (Table 1), the mineral acids of H2SO4 (mostly associated with H2O2), HCl, and H3PO4 were used to leach all the elements from cathode materials into solution and followed with a complicated separation process by chemical precipitation (using NaOH or NH3·H2O). However, in order to ensure all the metals are leached into a solution for recovery, the acid additions were largely in excess and at high concentrations (ranging from 2 to 6 M), which makes the following separation process require a significant amount of alkali for neutralization.19,24,27 Cai et al.27 optimized the H2SO4 leaching and alkali separation processes with a LiFePO4/ H2SO4/NaOH molar ratio of 1:8:15, which indicates the H2SO4 addition was 16 times higher than its stoichiometric value and brings high costs of both acid and alkali. However, there seems to be no discussion in the publications of leaching with less mineral acid. In addition, to simplify the process for separating metals after leaching, some organic acids as alternatives to mineral acids, such as citric acid,25,33 ascorbic acid,25 lactic acid,34 L-tartaric acid,29 and formic acid,26 have

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EXPERIMENTAL SECTION

Materials and Reagents. The spent LiFePO4 batteries used in this study (supplied by Shenzhen Just Real Power Tech Co., Ltd., Shenzhen, China) are a kind of spent power LIBs using LiFePO4/C as cathode materials. All the chemicals, including H2SO4, H2O2, Na3PO4· 12H2O, and NaOH, were of analytical grade purchased from Fuzhou Guangda Chemical Co., Ltd., Fuzhou, China. All the solutions used in this study were prepared with ultrapure water (electrical conductivity < 0.055 μs/cm, UPH-II-5/10/20T, Ulupure Technology Co., Ltd.). To obtain cathode materials for the leaching process, the spent LiFePO4 batteries were first subjected to a pretreatment process (SI, page S2). The obtained cathode material was found to be LiFePO4 (Figure S1), and its major elemental contents (wt %) were 30.80% Fe, 3.85% Li, and 17.05% P, respectively (SI, page S2). Selective Leaching of Cathode Materials. A total of 5 g of cathode materials was mixed with a different amount of H2SO4 and H2O2 in a three-neck flask to conduct each leaching experiment. Among all the leaching experiments, the stirring speed was fixed at 200 r/min. The oxidation−reduction theory was used to leach and separate Li and Fe from the cathode materials. During the leaching process, the 8018

DOI: 10.1021/acssuschemeng.7b01594 ACS Sustainable Chem. Eng. 2017, 5, 8017−8024

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Figure 1. Effects of various variables on the leaching rates of Li and Fe. (a) H2SO4 concentration. (b) H2O2/Li molar ratio. (c) Temperature. (d) H2SO4/Li molar ratio. (e) Time. Fe2+ in LiFePO4 would be oxidized to Fe3+ by H2O2 and then would react with PO43− to form FePO4 precipitation. The Li was expected to leach out from the cathode materials and appear in solution. Therefore, Li and Fe would be separated during the selective leaching process. After vacuum filtration and drying at 120 °C for 4 h, the obtained residue was weighted by analytical balance (BS 224S, Sartotius). A certain amount of leaching residue (dissolved in aqua regia solution, HNO3/HCl = 1:3, v/v) and leaching solution were sampled from each experiment to measure the Li and Fe contents by inductively coupled plasma-optical emission spectrometry (ICP-OES,

ICPE-9000, Shimadzu Co., Ltd.). The reactions during the leaching process can be represented as 2LiFePO4 + H 2SO4 + H 2O2 → Li 2SO4 + 2FePO4 ↓ + 2H 2O (1) In order to establish the technical feasibility of the leaching of cathode materials and separation of Li and Fe during the leaching process, the effects of different variables on the leaching rates of Li and Fe were evaluated. The pH of the filtrate of leaching solution, as an important parameter for FePO4 precipitation, was also measured by pH meter (PHS-3C, Shanghai INESA Scientific Instrument Co., Ltd.). 8019

DOI: 10.1021/acssuschemeng.7b01594 ACS Sustainable Chem. Eng. 2017, 5, 8017−8024

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ACS Sustainable Chemistry & Engineering Recovery of Li from Leaching Solution. After selective leaching, the Li was expected to be separated with Fe and present in solution for further recovery. Before recovering Li, the leaching solution was first treated by adding NaOH to remove minor impurities, resulting in a small number of residues generated. To avoid possible Li loss by coprecipitation, the residues can be recycled to the next cycle of the leaching process. Afterward, the leaching solution was heated to concentrate, and stoichiometric sodium phosphate dodecahydrate (Na3PO4·12H2O) was then introduced for the precipitation and recovery of Li3PO4.17 The precipitation reaction can be represented as 3Li 2SO4 + 2Na3PO4 → 3Na 2SO4 + 2Li3PO4 ↓

3.66, while the Fe always remained at a low leaching rate (less than 0.012%). However, when the H2SO4 concentration continuously rose from 0.3 to 1.0 M resulting in the H2SO4/ Li molar ratio rising from 0.55 to 1.82, the Fe was leached significantly with the leaching rate increasing from 0.0027 to 78.39%, and the leaching rate of Li increased slightly (from 95.74 to 99.88%). This means that the precipitated Fe was gradually becoming soluble with decreasing pH. In order to achieve the maximum separation of Li and Fe and, simultaneously, make more Li leach into solution, 0.3 M H2SO4 was chosen in further experiments. Under this concentration, the H2SO4/Li molar ratio was 0.55. Effect of H2O2/Li Molar Ratio. H2O2, used as oxidant or reductant, has been shown to enhance the dissolution of Li, Co, Mn, Ni, and Cu from spent LiCoO2 and LiNixCoyMn1−x−yO2 batteries.30,38−41 In order to investigate the effect of H2O2 addition on the extraction rates of Li and Fe, some experiments with different H2O2/Li molar ratios were performed. The leaching experiments were conducted for 120 min at 60 °C with a H2SO4 concentration of 0.3 M, an L/S ratio of 10 mL/g (H2SO4/Li molar ratio 0.55), and a stirring speed of 200 r/min. The results (Figure 1b) indicate that when associating with H2O2, the leaching of Li was enhanced considerably and that of Fe was inhibited accordingly. The reasonable explanation is that H2O2 served as an oxidant transferring Fe from a bivalent to a trivalent state, which favored the formation of a FePO4 precipitate and hence made the structure of LiFePO4 easily decompose in H2SO4 solution (eq 1). When the H2O2/Li molar ratio increased from 0 (no H2O2 added) to 2.07, the Li leaching rate rose by around 45% (from 49.25 to 96.08%) and the leaching rate of Fe decreased from 36.75 to 0.0024%. Meanwhile, the pH of the leaching solution experienced an upward trend, which is because the oxidation−reduction reaction between H2O2 and Fe2+ consumes H+ (eq 3). However, when the H2O2/Li molar ratio was greater than 2.07, both the leaching rates of Li and Fe remained relatively stable, indicating that the leaching reaction nearly reached equilibrium. Considering that greater addition of H2O2 makes the reaction more intense and easily produces a large amount of foam overflowing the reactor, a H2O2/Li molar ratio of 2.07 was recommended for the following experiments. At this ratio, the volume fraction of H2O2 added was 2.02 vol %, and the pH of the filtrate was 3.74. As can be seen from eq 1, the theoretical H2O2/Li molar ratio is 0.5, which means the proper addition amount of H2O2 was about 4 times as high as its theoretical value.

(2)

The precipitation experiment was conducted at 65 °C for 2 h with a stirring speed of 250 r/min and was repeated five times. Since the solubility of Li3PO4 decreases with increasing temperature, a higher temperature can make more Li precipitate. The precipitated Li3PO4 was then collected through filtrating and washed with 80 °C deionized water to eliminate the excess Na3PO4. The Li contents before and after precipitation were measured by ICP-OES for the calculation of Li3PO4 precipitation %. The crystal structure of the Li3PO4 product was measured by X-ray diffraction (XRD, MiniFlex II, Rigaku Co., Ltd.). Its surface characteristic and particle size were also characterized by scanning electron micrometer (SEM, Nova NanoSEM 230, FEI Technologies Inc.) and laser particle size analyzer (Mastersizer 2000, Malvern Instruments Co., Ltd.), respectively. Analysis of Leaching Residue. During the leaching step, the Fe2+ in the cathode materials could be easily oxidized to Fe3+ by H2O2 and was then expected to be precipitated as FePO4 at a proper pH range. After filtration, the FePO4 would accordingly be contained in the leaching residue. Also, the carbon slag from the coating layer on LiFePO4 would be present in the residue. In order to confirm the form of the precipitation of Fe, the leaching residue was treated by burning at 600 °C for 4 h to remove carbon slag and was then tested by XRD.

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RESULTS AND DISCUSSION Effects of Various Variables on the Leaching Rates of Li and Fe. Effect of H2SO4 Concentration. To study the effect Table 2. Experimental Results with Different Leaching Timea leaching solution obtained after filtration

leaching rate (%)

time (min)

Li (g/L)

Fe (mg/L)

P (g/L)

pH

Li

Fe

P

80 100 120 mean

3.497 3.504 3.546 3.516

1.228 1.216 1.717 1.387

0.324 0.323 0.312 0.319

3.72 3.87 3.66 3.75

95.09 95.30 96.85 95.75

0.013 0.011 0.027 0.017

1.98 1.97 1.95 1.97

[H2SO4] = 0.3 M, H2O2/Li molar ratio = 2.07, T = 60 °C, and H2SO4/Li molar ratio = 0.57.

a

2Fe2 + + H 2O2 + 2H+ → 2Fe3 + + 2H 2O

(3)

Effect of Reaction Temperature. The effect of reaction temperature on the Li and Fe leaching rates was investigated in the range of 25 to 80 °C. The other conditions were as follows: H2SO4 concentration, 0.3 M; H2O2/Li molar ratio, 2.07; L/S ratio, 10 mL/g (H2SO4/Li molar ratio 0.55); and leaching time, 120 min. The results (Figure 1c) show that the reaction temperature had no obvious influence on the leaching rates of Li and Fe. Even when the reaction temperature was at 25 °C, the leaching rate of Li could reach 92.60% and that of Fe could be as low as 0.00013%. In the studied range of temperature, the pH of leaching solution always remained between 3.63 and 3.82, which made Fe precipitate and separate with Li almost completely. Since the leaching rate of Li peaked at 96.08% at a temperature of 60 °C, this temperature was chosen for further experiments. However, as the temperature has a slight influence

of acid concentration on the Li and Fe leaching rates, experiments with different H2SO 4 concentrations were conducted at a H2O2/Li molar ratio (molar ratio between the H2O2 added and Li contained in the experimented cathode materials) of 2.96, a leaching temperature of 60 °C, a liquor to solid (L/S) ratio of 10 mL/g, and a leaching time of 120 min. The results are shown in Figure 1a. In order to clearly reveal the change of H2SO4/Li molar ratio (molar ratio between the H2SO4 added and Li contained in the experimental cathode materials) with the increase of H2SO4 concentration, an additional x axis of H2SO4/Li molar ratio is also added in Figure 1a. With the H2SO4 concentration increasing from 0.1 to 0.3 M, the leaching rate of Li increased significantly from 35.19 to 95.74% and the pH of the filtrate decreased from 7.15 to 8020

DOI: 10.1021/acssuschemeng.7b01594 ACS Sustainable Chem. Eng. 2017, 5, 8017−8024

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Figure 2. XRD pattern (a), SEM image (b), and particle size distribution (c) of the precipitated Li3PO4.

ratio means that the more H2SO4 that was added, the more the total volume of the leaching solution. As shown in Figure 1d, the H2SO4/Li molar ratio could obviously affect the leaching of Li. With the molar ratio increasing in the whole selected range, the Li leaching rate rose from 67.84 to 98.40%. However, when the molar ratio was in the range from 0.33 to 0.57, there was no obvious change in the leaching rate of Fe. However, with the range continuously extending, more Fe was leached to be soluble in solution. When the molar ratio was 0.65, the leaching rate of Fe reached 0.22%. To make the volume of the leaching solution as low as possible to reduce the following evaporation costs and decrease the addition amount of H2SO4, a H2SO4/Li molar ratio of 0.57 (equivalent to an L/S ratio of 10.5 mL/g) was selected for further experiments. At this ratio, the pH of the filtrate was at 3.24. From eq 1, we can know that the appropriate addition amount of H2SO4 was very close to its theoretical value (experimental ratio of 0.57 vs theoretical ratio of 0.50), which indicates that a near stoichiometric amount of H2SO4 could already achieve the expected effect for the leaching process. As a result, the addition amount of alkali in the following step of impurities removal can be reduced. Effect of Leaching Time. The effect of leaching time was investigated in the range from 60 to 150 min. The other conditions were as follows: H2SO4 concentration of 0.3 M, H2O2/Li molar ratio of 2.07, leaching temperature of 60 °C, and H2SO4/Li molar ratio of 0.57. As can be seen from Figure 1e, when the leaching time rose from 60 to 80 min, the leaching rate of Li increased from 76.48 to 95.20% and that of Fe dropped marginally from 3.95 to 0.0084%. With the leaching

Figure 3. XRD pattern of leaching residue.

on the leaching of Li and Fe, a temperature lower than 60 °C is also feasible, particularly when the costs for heating energy are first considered in the industrial application. Effect of H2SO4/Li Molar Ratio. In order to clearly reveal the relationship between the amounts of H2SO4 and LiFePO4, experiments with different H2SO4/Li molar ratios instead of L/ S ratios were conducted under the following conditions: H2SO4 concentration of 0.3 M, H2O2/Li molar ratio of 2.07, leaching temperature of 60 °C, and leaching time of 120 min. Since both the mass of LiFePO4 used for each experiment and the concentration of H2SO4 were fixed, the higher H2SO4/Li molar 8021

DOI: 10.1021/acssuschemeng.7b01594 ACS Sustainable Chem. Eng. 2017, 5, 8017−8024

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Figure 4. Flowchart for the leaching and recovery process.

The Analysis of Leaching Residue. After treatment by burning, the residue was measured by XRD. As shown in Figure 3, the XRD pattern was in good agreement with the standard pattern of FePO4, which proves that the Fe was precipitated as FePO4 and appeared in the leaching residue. Therefore, the Fe was recovered in the form of FePO4. This result agrees well with the previous publication,27 in which it was indicated that Fe3+ would be precipitated as FePO4 rather than Fe(OH)3 at a pH of around 3.40 in the presence of PO43− in a sulfate solution. Development of a Novel Recycling Process for Spent LiFePO4 Batteries. On the basis of the above results and analyses, a novel recycling process for spent LiFePO4 batteries under leaching with H2SO4 and H2O2 is presented in Figure 4. The spent LiFePO4 batteries were first subjected to a pretreatment step to obtain the cathode materials for the leaching process, which has been well studied by previous researchers. After pretreatment, the cathode materials obtained from the spent LiFePO4 batteries were then leached with stoichiometric H2SO4. The H2O2 was introduced for enhancing the leaching effect and oxidizing Fe2+ in LiFePO4 to Fe3+, which will favor the subsequent precipitation of FePO4. The optimized leaching conditions are as follows: 0.3 M H2SO4; H2O2/Li molar ratio, 2.07; H2SO4/Li molar ratio, 0.57; 60 °C; and 120 min. After filtration, the solution containing Li and the leaching residue containing FePO4 and C were obtained. The

time further going up, the Li leaching rate remained at a stable level and peaked at 96.85% at 120 min. In the meantime, there was no obvious change of Fe leaching rate, and the pH of the filtrate always remained from 3.62 to 3.87. The detailed results of selected experiments (t = 80, 100, and 120 min) with relatively stable data have been listed in Table 2. From the table, we can know that around 99.98% Fe together with 98.03% P were precipitated and 95.75% Li was leached during the selective leaching process of the selected experiments. The concentrations of Fe and P in the leaching solution were around 1.387 mg/L and 0.319 g/L, respectively. In order to achieve a maximum leaching rate of Li, 120 min (with filtrate pH of 3.66) was considered as optimized leaching time. Recovery of Li from the Leaching Solution. The results obtained by repeating the experimental procedures five times are listed in Table S1. They reveal that the Li3PO4 precipitation reached around 95.56% in the first cycle of the recycling process under the experimental conditions. The obtained Li3PO4 can also be further purified to meet the requirement of battery-grade materials. The XRD pattern (Figure 2a) of the precipitated Li3PO4 indicates that its characteristic peaks agree well with the standard Li3PO4 pattern. The SEM image (Figure 2b) illustrates that the obtained Li3PO4 was composed of agglomerated globules. The particle size distribution (Figure 2c) of the Li3PO4 is measured to be 18.964 μm. 8022

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ACS Sustainable Chemistry & Engineering residue was then treated by calcination at 600 °C for 4 h to remove carbon slag and recover FePO4. Also, the leaching solution was purified by adding alkali (NaOH) solution and concentrated by evaporation. After that, Na 3 PO 4 was introduced to the leaching solution for the precipitation and recovery of Li as Li3PO4 product. Finally, the mother solution was recycled to commence the next cycle of the alkali leaching of cathode plates.

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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.7b01594. Details about the pretreatment process, XRD analysis of cathode materials, and detailed data of the Li precipitation (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Address: Room 218, Building No. 6, Xueyuan Road No. 2, Shangjie Town, Fuzhou City, Fujian Province, China 350108. Tel.: 86-13950286251. E-mail: [email protected]. ORCID

Ge Kuang: 0000-0003-4197-5069 Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acssuschemeng.7b01594 ACS Sustainable Chem. Eng. 2017, 5, 8017−8024

Research Article

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DOI: 10.1021/acssuschemeng.7b01594 ACS Sustainable Chem. Eng. 2017, 5, 8017−8024