Selective Leaching of LiFePO4 by H2SO4 in the Presence of NaClO3

Abstract: Herein, problems commonly observed for the wet leaching of waste LiFePO4 cathode materials, namely extensive Fe leaching and impurity removal, are mitigated through the use of NaClO3, and the effects of leaching parameters on Fe, P, Li, and Al leaching efficiencies are probed. As a result, optimal leaching conditions are determined as temperature = 90°C, H2SO4 concentration = 1 M, liquid-to-solid ratio = 5:1, leaching time = 1 h, stirring speed = 150 rpm, and NaClO3 dosage = 25 g per 100 g raw material, with the corresponding Fe, P, Li, and Al leaching efficiencies obtained as 0.21, 0.03, 97.23 and 11.87%, respectively.


Introduction
Methods of spent LiFePO4 battery disposal include the cascade utilisation and recovery of valuable metals [1,2]. Cascade utilisation aims to ensure the quality and safety of target products but is only applied in some demonstration plants, as the performance of waste batteries is unpredictable [3].
At present, the hydrometallurgical recovery of Li from waste LiFePO4 batteries is widely employed in the lithium battery recovery industry [4][5][6]. Zhen et al. [7] leached the LiFePO4 cathode material with a mixture of H2SO4 and H2O2 and adjusted the pH of the obtained solution with alkali to precipitate Fe as Fe(OH)3, further adjusting pH to 5.0-8.0 to remove other heavy metal ions and thus obtain a solution of Li2SO4. The latter solution was treated with solid Na2CO3, concentrated, and crystallised to obtain Li2CO3. The above method consumes large amounts of alkali and produces much slag, therefore not complying with the concept of environmental protection and economy.
Yang et al. [8] probed the effects of mechanochemical activation of spent cathode powder and its potential to achieve selective Li recovery, revealing that after mechanochemical activation, ~97.67% Fe and 94.29% Li could be recovered under optimised conditions. The lixivium was evaporated and concentrated to recover FePO4, and the filtrate pH was adjusted to neutral to afford Li3PO4. The above process is lengthy, featuring the drawbacks of high energy/alkali consumption and affording a product with high impurity content. Li et al. [9] found that the use of dilute H2SO4 as a leachant and H2O2 as an oxidant allows Li to be selectively leached into solution, while Fe and P remain in the residue as FePO4, which is different from the traditional process of using excess mineral acid to leach all elements into solution.
Under optimised conditions, Li, Fe, and P leaching efficiencies of 96.85, 0.027, and 1.95%, respectively, were recorded. However, the above study is incomplete, employing an insufficient amount of experimental materials and therefore providing unconvincing results. Moreover, the chemical stability of H2O2 is low, the leaching process is prone to overflow, and the achieved oxidation efficiency is far lower than that observed for NaClO3 and NaClO.
When waste LiFePO4 cathode material is treated with H2SO4, Fe is solubilised as Fe 2+ , and its removal consumes large amounts of alkali liquor and produces much solid waste [10,11]. Importantly, the metal value of LiFePO4 is lower than that of lithium nickel cobalt manganate, which results in low market value and low recovery enthusiasm [12][13][14]. Herein, LiFePO4 was leached with H2SO4 in the  Figure 1 shows that the dominant area of FePO4 in the Li-Fe-P-H2O system is characterised by high potential and low pH. During the leaching of LiFePO4 by acid, control of pH and system potential allows Fe and Li to be present as FePO4 and Li + , respectively, thus enabling selective leaching and FePO4 isolation.
In the absence of an oxidant, the reaction of LiFePO4 with H2SO4 can be described as
A beaker filled with H2SO4 of a certain concentration and volume was charged with LiFePO4 (100 g) and put into a water bath held at a specified temperature and equipped with an overhead mixer. Subsequently, NaClO3 was added, and after a fixed retention time, the reaction mixture was filtered, and the Fe, P, Li and Al contents of the dry filter cake were determined. The filtration residue was analysed by diffraction of x-rays (XRD Ultima IV, JP) and scanning electron microscope (SEM Apreo C, US), The filtrate was analysed by inductively coupled plasma spectrometry (ICAP 7000SERIES, UK), and leaching efficiency (ηi) was calculated as  where mA is the mass of a given element in the raw material, and ma is the mass of this element in the filter residue. Figure 2 shows the effects of temperature (T) on leaching efficiency at [H2SO4] = 1 M, liquid-tosolid ratio (L/S) = 5:1, leaching time (t) = 1 h, NaClO3 dosage (D) = 25 g, and stirring speed (s) = 150 rpm.

Figure 2. Effect of temperature on leaching efficiency
With increasing T, the leaching efficiency of Li increased, that of Fe declined, and that of P did not significantly change, and that of Al increased first and then decreased. At 90°C, the leaching efficiencies of Fe, P, Li, and Al equalled 0.07, 0.017, 98.12, and 16.72%, respectively. In order to achieve the results of high lithium leaching rate and low iron leaching rate, the above temperature (90 °C) was therefore selected as optimal.   Figure 4 shows    With increasing t, the leaching efficiency of Li increased and then saturated, while those of Fe and P did not significantly change. After 1-h leaching, the leaching efficiencies of Fe, P, Li, and Al reached 0.025, 0.001, 98.099, and 4.54%, respectively. Thus, t = 1 h was chosen as optimal.

Effect of leaching time on leaching efficiency
The leaching efficiency of P and Fe significantly increased under conventional leaching. In the presence of NaClO3, all Fe 2+ ions were oxidised to Fe 3+ and precipitated together with P as FePO4. Figure 6 shows the effects of D on leaching efficiency (T = 90 °C, [H2SO4] = 1 M, L/S = 5:1, t = 1 h, s = 150 rpm). With increasing D, the leaching efficiency of Li increased, while those of Fe and P decreased. At D = 25 g per 100 g raw material, Fe, P, Li, and Al leaching efficiencies reached 0.13, 0.004, 96.88, and 12.71%, respectively, and the above dosage was therefore chosen as optimal.  When s increased from 50 to 100 rpm, the leaching efficiencies of all elements increased. Upon a further increase to 150 rpm, P, Fe, and Al leaching efficiencies did not change significantly, while Li leaching efficiency increased to 96.26%, remaining stable at higher s. This behaviour was ascribed to the promotional effect of high stirring speed on substance diffusion.

Verification test
On the basis of single-factor experiments, optimum leaching conditions were determined as T = 90 °C, [H2SO4] = 1 M, L/S = 5:1, t = 1 h, D = 25 g per 100 g raw material. Under these conditions, the leaching efficiencies of Fe, P, Li, and Al equalled 0.21, 0.03, 97.23, and 11.87%, respectively.  The morphological features of leaching residue particles obtained after leaching at optimum leaching conditions are displayed in Figure 8(a,b), It can be observed that the particles of leached residue are of different sizes (Figure 8a), and the surface of leached slag is of flocculent structure (Figure 8b). In order to identify the crystalline structure of leaching residue particles, XRD spectra were recorded and presented in Figure 8c. In order to identify the crystalline structure of leaching residue particles, XRD spectra were recorded and presented in Figure 8c. As can be seen from the above figures, LiFePO4 was leached with H2SO4 in the presence of NaClO3 at controlled pH and system potential，FePO4 is the main component of the leaching residue. This indicates in the technology，we can removal of Fe in the form of insoluble FePO4 to afford a solution of Li2SO4 and thus achieve selective Li leaching.

Conclusions
Herein, we realised selective leaching of Li from spent LiFePO4 cathodes, showing that in the presence of NaClO3, all Fe 2+ ions in solution were oxidised to Fe 3+ and precipitated together with P as FePO4 to afford a P-and Fe-free solution containing Li. 1) Optimum conditions for the selective leaching of LiFePO4 were determined as leaching temperature = 90°C, [H2SO4] = 1 M, L/S = 5:1, leaching time = 1 h, NaClO3 dosage = 25 g per 100 g raw material.
2) FePO4 can be obtained by selective leaching. After purification and modification, pure FePO4 was obtained, which was used to prepare LiFePO4 and realize resource recycling.
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