Method for recovering lithium using positive electrode material for lithium-ion secondary battery, and method for manufacturing lithium adsorbent

Abstract

The present invention relates to a method for recovering lithium, said method having a high lithium recovery rate and excellent stability.　That is, the present invention relates to a method for recovering lithium from a positive electrode material (A) for a lithium-ion secondary battery represented by formula (A): LiaMnbFecMxPO4, wherein, after completing a step (I) for adding an oxidizing agent and water to a positive electrode material (A) for a lithium-ion secondary battery to obtain a mixed liquid i, a step (II) for adjusting the pH of the obtained mixed liquid i to be 4 to 9 and mixing the mixed liquid i to obtain a mixed liquid ii, and a step (III-1) for filtering the obtained mixed liquid ii to obtain a filtrate iii containing lithium, the obtained filtrate iii is used to obtain, as a lithium-rich liquid, a filtrate iii' obtained by performing prescribed steps (I)' to (III-1)', and the obtained filtrate iii' is possibly used again as the filtrate iii in step (I)' to thereby repeat steps (I)' to (III-1)' multiple times to obtain, as a lithium-rich liquid, the ultimately obtained filtrate iii'.

Description

Method for recovering lithium using a positive electrode material for lithium-ion secondary batteries, and method for producing a lithium adsorbent

[0001] The present invention relates to a method for recovering lithium using a positive electrode material for a lithium ion secondary battery composed of olivine-type lithium phosphate, and a method for producing a lithium adsorbent.

[0002] Positive electrode materials made of compounds such as Li(Fe,Mn)PO4 having an olivine structure are highly useful as positive electrode materials for lithium-ion secondary batteries and are widely used. However, since the raw material lithium is expensive, there is a strong demand for the recovery and reuse of such lithium.

[0003] In this context, for example, Patent Document 1 discloses a method for recovering lithium. 1.6 Mn 1.6 Granules for adsorbing lithium obtained from lithium manganate such as O4, so-called LMO type materials, have been disclosed, and attempts have been made to adsorb Li contained in salt lake brine.

[0004] JP 2023-90401 A

[0005] However, in the case of recovering lithium using the lithium adsorption granules described in the above patent documents, there is a concern about elution of manganese, and there is also a risk that the recovery rate of lithium may decrease due to repeated recovery, and therefore there is still room for improvement.

[0006] Therefore, the present invention relates to a method for recovering lithium with a high lithium recovery rate and excellent stability.

[0007] Therefore, the present inventors have conducted extensive research to solve the above problems and have found a method for efficiently recovering lithium from a positive electrode material for a lithium ion secondary battery that is composed of a specific olivine-type lithium phosphate.

[0008] That is, the present invention provides a compound represented by the following formula (A): a Mn b Fe c M xPO4 ... (A) (in formula (A), M represents Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd; a, b, c, and x represent numbers that satisfy 0<a≦1.2, 0≦b≦1.2, 0≦c≦1.2, 0≦x≦0.3, and b+c≠0, and that satisfy a+(valence of Mn)×b+(valence of Fe)×c+(valence of M)×x=3), the method for recovering lithium from a positive electrode material for a lithium ion secondary battery (A) represented by the following steps (I) to (III-1): (I) a step of adding an oxidizing agent and water to the positive electrode material for a lithium ion secondary battery (A) to obtain a mixed solution i (II) adjusting the pH of the obtained mixed liquid i to 4 to 9 and mixing to obtain a mixed liquid ii; and (III-1) filtering the obtained mixed liquid ii, and then obtaining a lithium-containing filtrate iii. Then, using the obtained filtrate iii, the following steps (I)' to (III-1)' are carried out: (I) adding a positive electrode material for a lithium ion secondary battery (A) and an oxidizing agent to the obtained filtrate iii, and obtaining a mixed liquid i'. (II) adjusting the pH of the obtained mixed liquid i' to 4 to 9 and mixing to obtain a mixed liquid ii'. (III-1) filtering the obtained mixed liquid ii', and then obtaining a lithium-containing filtrate iii'. The obtained filtrate iii' is obtained as a high-lithium liquid, or the obtained filtrate iii' is used again as the filtrate iii in step (I)', thereby repeating the above steps (I)' to (III-1)' multiple times, and finally obtaining a high-lithium filtrate iii'.

[0009] The present invention also provides a compound represented by the following formula (A): a Mn b Fe c M xPO4...(A) (in formula (A), M represents Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd; a, b, c, and x represent numbers that satisfy 0<a≦1.2, 0≦b≦1.2, 0≦c≦1.2, 0≦x≦0.3, and b+c≠0, and that satisfy a+(valence of Mn)×b+(valence of Fe)×c+(valence of M)×x=3), the method for producing a lithium adsorbent from a positive electrode material for a lithium ion secondary battery (A) represented by the following steps (I) to (III-2): (I) a step of adding an oxidizing agent and water to the positive electrode material for a lithium ion secondary battery (A) to obtain a mixed solution i (II) adjusting the pH of the obtained mixed solution i to 4 to 9 and mixing the mixture to obtain a mixed solution ii; and (III-2) filtering the obtained mixed solution ii to obtain filter cake iii.

[0010] The lithium recovery method of the present invention targets the positive electrode material (A) for a lithium ion secondary battery, and therefore can effectively improve the recovery rate while ensuring excellent stability, thereby obtaining a highly versatile high-lithium solution. Furthermore, since a lithium adsorbent having excellent lithium adsorption properties can also be obtained, the method is also highly useful as a method for producing a lithium adsorbent.

[0011] The present invention will be described in detail below. The method for recovering lithium of the present invention comprises the steps of: a Mn b Fe c M xPO4 ... (A) (in formula (A), M represents Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd; a, b, c, and x represent numbers that satisfy 0<a≦1.2, 0≦b≦1.2, 0≦c≦1.2, 0≦x≦0.3, and b+c≠0, and that satisfy a+(valence of Mn)×b+(valence of Fe)×c+(valence of M)×x=3). Generally, a positive electrode material that is an olivine-type lithium phosphate as represented by the above formula (A) has a crystal structure that is very stable compared to so-called LTO-type positive electrode materials or LMO-type positive electrode materials such as lithium titanate or lithium manganate, and therefore it has been difficult to extract lithium directly from such materials, making it difficult to recover a sufficient amount. However, the method of the present invention makes it possible to effectively recover lithium even from such a positive electrode material (A).

[0012] In the above formula (A), a is preferably 0.6≦a≦1.2, more preferably 0.65≦a≦1.15, and even more preferably 0.7≦a≦1.1. b is preferably 0.4≦b≦0.8. c is preferably 0.2≦c≦0.6. x may be 0≦x≦0.2, or may be 0≦x≦0.15, or may be 0≦x≦0.1. M may also be Mg, Al, Ti, Zn, Nb, Co, Zr, or Gd.

[0013] Specifically, for example, LiMnPO4, LiFePO4, LiMn 0.3 Fe 0.7 PO4, LiMn 0.4 Fe 0.6 PO4, LiMn 0.45 Fe 0.55 PO4, LiMn 0.7 Fe 0.3 PO4, LiMn 0.9 Fe 0.1 PO4, LiMn 0.8 Fe 0.2 PO4, LiMn 0.75 Fe 0.15 Mg 0.1 PO4, LiMn0.75 Fe 0.19 Zr 0.03 PO4, LiMn 0.6 Fe 0.4 PO4, LiMn 0.5 Fe 0.5 PO4, Li 1.2 Mn 0.63 Fe 0.27 PO4, Li 0.6 Mn 0.84 Fe 0.36 PO4, etc. Among them, LiMn 0.8 Fe 0.2 PO4, LiMn 0.4 Fe 0.6 PO4, LiMn 0.45 Fe 0.55 PO4, LiMn 0.7 Fe 0.3 PO4, LiMn 0.6 Fe 0.4 PO4, Li 1.2 Mn 0.63 Fe 0.27 PO4 is preferred.

[0014] The BET specific surface area of ​​the positive electrode material (A) is preferably 5 m from the viewpoint of increasing the lithium recovery efficiency. 2 / g to 30m 2 / g, more preferably 10m 2 / g~25m 2 / g. From the viewpoint of ensuring such a suitable BET specific surface area, the positive electrode material (A) is preferably a granule. The BET specific surface area of ​​the positive electrode material (A) means a value determined from an adsorption isotherm by a nitrogen adsorption method, and can be measured, for example, using a flow-type automatic specific surface area measuring device (FlowSorb III 2305, manufactured by Shimadzu Corporation) under conditions of a nitrogen / helium mixed gas containing 30% nitrogen.

[0015] The average particle size of the positive electrode material (A) is preferably 1 μm to 40 μm, more preferably 5 μm to 35 μm, from the viewpoint of increasing the lithium recovery efficiency. As the positive electrode material (A) having such an average particle size, for example, a positive electrode material obtained by hydrothermal synthesis is preferably used. The average particle size of the positive electrode material (A) is determined by the D of the volume-based particle size distribution based on the laser diffraction / scattering method.50 The particle diameter at 50% of the cumulative particle size (median diameter) is used as the particle diameter.

[0016] There are no particular limitations on whether or not the positive electrode material (A) is supported by carbon. However, from the viewpoint of enhancing the versatility of the high-lithium content liquid obtained by the present invention, it is preferable that the positive electrode material (A) is supported by carbon.

[0017] In the method for recovering lithium of the present invention, step (I) is a step of adding an oxidizing agent and water to a positive electrode material (A) for a lithium ion secondary battery (positive electrode material (A)) to obtain a mixed solution I. Following this step, by going through the subsequent step (II), lithium can be effectively extracted from the positive electrode material (A).

[0018] Specific examples of oxidizing agents that can be used include one or more selected from peroxoacid salts, permanganates, and hydrogen peroxide salts. More specific examples of peroxoacid salts include sodium peroxodisulfate, potassium peroxodisulfate, ammonium peroxodisulfate, peroxochromate, and peroxotitanate. More specific examples of permanganates include hydrogen permanganate, potassium permanganate, and sodium permanganate. More specific examples of hydrogen peroxide salts include sodium hydrogen peroxide, potassium hydrogen peroxide, and magnesium hydrogen peroxide. Among these, from the viewpoint of effectively increasing the recovery efficiency of lithium, peroxoacid salts are preferred, and sodium peroxodisulfate (NaSO) is more preferred.

[0019] The amount of the oxidizing agent added may vary depending on the composition of the positive electrode material (A), but is preferably 0.1 to 0.9 parts by mass, more preferably 0.15 to 0.85 parts by mass, and even more preferably 0.2 to 0.8 parts by mass, relative to 1 part by mass of the positive electrode material (A).

[0020] From the viewpoint of efficiently recovering lithium, the solids concentration of the mixed solution i obtained in step (I) is preferably 0.1% by mass to 40% by mass, more preferably 0.5% by mass to 35% by mass, and even more preferably 1% by mass to 30% by mass, based on 100% by mass of the mixed solution i. The amount of water added may be adjusted as appropriate depending on the amount of the positive electrode material (A) and the amount of the oxidizing agent added, and so as to provide an appropriate solids concentration in the resulting mixed solution i.

[0021] In step (II), the pH of the mixed solution i obtained in step (I) is adjusted to 4 to 9 and mixed to obtain mixed solution ii. This allows the reaction related to the extraction of lithium from the positive electrode material (A) to proceed efficiently. The pH of mixed solution i is 4 to 9, preferably 4.5 to 8.5, and more preferably 5 to 8. A pH adjuster such as sodium hydroxide or potassium hydroxide may be used to adjust the pH. The pH is measured at 25°C. The temperature of mixed solution i is preferably adjusted to 5°C to 60°C, and may be 15°C to 45°C. If the pH of mixed solution i obtained in step (I) is already within the above range, further pH adjustment is not necessary. Furthermore, the temperature of mixed solution ii obtained in step (II) is preferably 5°C to 60°C, and more preferably 15°C to 45°C.

[0022] In addition, when mixing the mixed solution i, it is preferable to stir the mixed solution i from the viewpoint of efficiently proceeding with the reaction for extracting lithium from the positive electrode material (A). The stirring time is preferably 1 hour to 72 hours, more preferably 2 hours to 48 hours, and even more preferably 3 hours to 24 hours.

[0023] Step (III-1) is a step of filtering the mixed solution (ii) obtained in step (II) and then obtaining a lithium-containing filtrate (iii). This allows for the production of a filtrate (iii) containing ions of lithium extracted from the positive electrode material (A). When filtering the mixed solution (ii), it is preferable to use a method involving centrifugation in order to efficiently increase the lithium recovery efficiency. When performing filtration using such a method involving centrifugation, a conventional centrifuge may be used, and the rotor speed is preferably 1,000 rpm to 5,000 rpm, more preferably 1,500 rpm to 4,500 rpm.

[0024] In the method for recovering lithium of the present invention, after the above steps (I) to (III-1), the filtrate iii obtained in step (III-1) is used to undergo the following steps (I)' to (III-1)'. Note that the filtrate iii obtained only through the above steps (I) to (III-1) can also be used as a lithium-rich liquid.

[0025] Step (I)' is a step of adding the positive electrode material for a lithium ion secondary battery (A) and an oxidizing agent to the filtrate iii obtained in step (III-1) and mixing them to obtain a mixed solution i'. The oxidizing agent that can be used and the amount thereof added are the same as the oxidizing agent that can be used in step (I) and the amount thereof added. The solids concentration of the mixed solution i' obtained in step (I)' is the same as the solids concentration of the mixed solution i obtained in step (I), and may be adjusted to such a solids concentration by adding water as appropriate.

[0026] Step (II)' is a step of adjusting the pH of the mixed solution i' obtained in step (I)' to 4 to 9 to obtain a mixed solution ii'. The pH, temperature, and stirring conditions during mixing of the mixed solution i' and the resulting mixed solution ii' in step (II)' are the same as those of the mixed solution i and the resulting mixed solution ii in step (I).

[0027] Step (III-1)' is a step of filtering the mixed solution ii' obtained in step (II)' and then obtaining a lithium-containing filtrate iii'. The filtration in step (III-1)' is similar to the filtration in step (III-1).

[0028] In the method for recovering lithium of the present invention, the filtrate iii' obtained in step (III-1)' through the above steps (I)' to (III-1)' is obtained as a lithium-rich liquid, or the obtained filtrate iii' is reused as the filtrate iii in step (I)', thereby repeating the above steps (I)' to (III-1)' multiple times, and finally obtaining the filtrate iii' obtained in step (III-1)' as a lithium-rich liquid.

[0029] That is, one aspect of the present invention is a method in which, after having undergone the above steps (I) to (III-1), the filtrate iii obtained thereafter is used to perform the above steps (I)′ to (III-1)′ once to obtain a filtrate iii′ obtained in step (III-1)′ as a high-lithium liquid. Another aspect of the present invention is a method in which, after having undergone the above steps (I) to (III-1), the filtrate iii obtained thereafter is used to perform the above steps (I)′ to (III-1)′ multiple times to obtain a filtrate iii′ obtained finally in step (III-1)′ as a high-lithium liquid.

[0030] Thus, the method for recovering lithium of the present invention is a method in which lithium can be effectively and efficiently recovered from the positive electrode material (A) by performing the above steps (I) to (III-1) and then repeating the above steps (I)' to (III-1)' once or multiple times, thereby obtaining a high-lithium solution containing the recovered lithium as ions at an extremely high concentration. Therefore, from the viewpoint of obtaining a high-lithium solution containing lithium at a higher concentration, it is preferable to repeat the above steps (I)' to (III-1)' multiple times rather than performing the above steps (I)' to (III-1)' once.

[0031] The lithium contained in the high-lithium solution, i.e., the lithium contained in the filtrate iii' as the high-lithium solution finally obtained in step (III-1)', is the positive electrode material (A) used in step (I), i.e., the lithium constituting the positive electrode material (A) initially used, is preferably 0.7 mol to 1 mol, more preferably 0.8 mol to 1 mol, and even more preferably 0.9 mol to 1 mol. The lithium concentration in the high-lithium solution may be controlled by appropriately adjusting the water content of the filtrate iii', for example, 0.1 mol / L to 3000 mol / L, 1 mol / L to 3000 mol / L, 10 mol / L to 3000 mol / L, 100 mol / L to 3000 mol / L, 1000 mol / L to 3000 mol / L, or 1500 mol / L to 2500 mol / L.

[0032] Thus, according to the method for recovering lithium of the present invention, lithium can be efficiently recovered from the positive electrode material (A) composed of olivine-type lithium phosphate, and a highly versatile high-lithium solution can be obtained. For example, if the high-lithium solution is applied to olivine-type lithium phosphate that has deteriorated due to use as a positive electrode material, and lithium has been released from it, it can be effectively and efficiently restored to its pre-deteriorated state by applying the high-lithium solution obtained by the method of the present invention.

[0033] The present invention is also highly useful as a method for producing a lithium adsorbent. That is, the method for producing a lithium adsorbent of the present invention is a method for producing a lithium adsorbent from a positive electrode material (A) for a lithium ion secondary battery represented by the above formula (A), and includes the following steps (I) to (III-2): (I) adding an oxidizing agent and water to the positive electrode material (A) for a lithium ion secondary battery to obtain a mixed solution (i); (II) adjusting the pH of the obtained mixed solution (i) to 4 to 9 and mixing the materials to obtain a mixed solution (ii); and (III-2) filtering the obtained mixed solution (ii) to obtain a filter cake (iii). Thus, the method for producing a lithium adsorbent of the present invention includes steps (I) to (II) of the method for recovering lithium of the present invention, and includes step (III-2) instead of step (III-1) of the method for recovering lithium of the present invention. Therefore, steps (I) to (II) of the method for producing a lithium adsorbent of the present invention are the same as steps (I) to (II) of the method for recovering lithium of the present invention. The lithium adsorbent obtained by the present invention has an extremely high lithium adsorption capacity, and even if a low-quality lithium-containing resource is used, the material represented by the above formula (A) can be obtained.

[0034] Step (III-2) in the method for producing a lithium adsorbent of the present invention is a step of filtering the mixed solution (ii) obtained in step (II) and then obtaining filter cake (iii). After filtering the mixed solution (ii) obtained in step (II), both filtrate (iii) and filter cake (iii) are obtained. While the method for recovering lithium of the present invention uses one of the filtrates (iii), the method for producing a lithium adsorbent of the present invention uses the other filter cake (iii). As described above, the filtrate (iii) obtained in the method for recovering lithium of the present invention is a liquid containing lithium at a high concentration, while the amount of lithium in the filter cake (iii) is effectively reduced. Therefore, the filter cake (iii) can be used as a lithium adsorbent with excellent lithium adsorption properties.

[0035] The filtration of the mixed solution ii in the step (III-2) is performed in the same manner as the filtration of the mixed solution ii in the above step (III-1). The obtained filter cake iii is preferably dried.

[0036] The amount of lithium contained in the filter cake iii is preferably 0 mol to 0.3 mol, more preferably 0 mol to 0.2 mol, and even more preferably 0 mol to 0.1 mol, relative to 1 mol of lithium constituting the positive electrode material for lithium ion secondary batteries (A) used in the step (I).

[0037] The resulting filter cake iii is an adsorbent derived from the positive electrode material (A) and can be used as a lithium adsorbent. Therefore, it has very high lithium adsorption properties. Therefore, even if a low-quality resource with a low lithium concentration (specifically, a resource with a lithium concentration of about 0.01 ppm to 1000 ppm), such as wastewater discharged in the manufacturing process of a positive electrode material or seawater, can be used, lithium can be efficiently and effectively adsorbed.

[0038] In this case, for example, the lithium adsorbent obtained by the present invention is first added to the resource to be treated for lithium adsorption, and the mixture is stirred at 1°C to 99°C, preferably 20°C to 60°C, for 1 hour to 72 hours, preferably 1 hour to 48 hours. Next, the mixture is filtered, and the resulting filter cake is dried and then recovered. The recovered material can be obtained as a material represented by the above formula (A) in the same way as the positive electrode material (A), and can be fully utilized as a positive electrode material. It can also be effectively utilized as the positive electrode material (A) in step (I)' of the above method for recovering lithium.

[0039] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. Measurements and preparations were carried out according to the following methods.

[0040] <Component Analysis of Positive Electrode Material (A), Filtrate, and Filter Cake> Component analysis of the positive electrode material (A), filtrate, and filter cake was performed by using a calibration curve method in inductively coupled plasma optical emission spectroscopy (ICP-OES), and the lithium content was confirmed.

[0041] <<Measurement of pH>> Measurement was carried out using a pH meter (F-52, manufactured by HORIBA).

[0042] <<Preparation of Positive Electrode Material (A)>> LiMn 0.7Fe 0.3 PO4 (BET specific surface area: 20m 2 / g, average particle size: 10 μm) was prepared and used as a positive electrode material (A).

[0043] <<Preparation of Simulated Solution to be Treated for Lithium Adsorption>> 1 g of a single-element standard solution for lithium ICP atomic emission spectrometry was dissolved in 1 L of water to prepare an aqueous solution with a lithium concentration of 1000 ppm. This aqueous solution was used as a simulated solution to be treated for lithium adsorption.

[0044] [Example 1] The following treatments 1) to 4) were carried out to carry out a method of producing a lithium adsorbent while carrying out a method of recovering lithium. That is, in treatment 1), the method of recovering lithium (the above steps (I) to (III-1)) was carried out, and simultaneously a lithium adsorbent was produced. Then, in treatment 2), the method of recovering lithium (the above steps (I)' to (III-1)') was carried out multiple times using the lithium adsorbent obtained in treatment 1).

[0045] 1) 100 g of positive electrode material (A) was mixed with 30 g of NaSO and 1 L of water and stirred to obtain mixed solution i. The resulting mixed solution i had a pH of 6 and a temperature of 25°C, confirming that it could be used as is without further pH adjustment. Mixed solution i was then stirred at 25°C for 1 hour to obtain mixed solution ii. The resulting mixed solution ii was filtered using a centrifuge (Tabletop Centrifuge 2010, manufactured by Kubota Shoji Co., Ltd., rotor speed 3000 rpm) to recover filtrate iii. At the same time, filter cake iii was collected separately and dried before recovery. The recovered filter cake iii contained 0.1 moles of lithium per mole of lithium constituting the positive electrode material (A), confirming that a useful lithium adsorbent was obtained.

[0046] 2) Next, the collected filter cake iii was added to 1 L of the simulated liquid and stirred at 25°C for 1 hour. This was subjected to the same filtration as in 1) above, and separated into 100 g of filter cake iii-A and wastewater, and only filter cake iii-A was collected. At this time, the collected filter cake iii-A contained LiMn 0.7 Fe 0.3It was confirmed that the material was represented by PO4 (the same as the positive electrode material (A) used), and it was found that the material could also be used as the positive electrode material (A) in step (I)' of the method for recovering lithium.

[0047] 3) 100 g of the filter cake iii-A obtained in the treatment 2) above was used as the positive electrode material (A). 30 g of NaSO and the filtrate iii obtained in the treatment 1) above were mixed with this and stirred at 25°C for 1 hour, and then filtered using a centrifuge (2010, manufactured by Kubota Shoji Co., Ltd., rotor speed 3000 rpm) to recover filtrate iii', which was simultaneously dried.

[0048] 4) The procedure of subjecting the solution to the treatment in 3) and then subjecting it to the treatment in 2) was repeated 300 times to finally obtain a filtrate x as a high-lithium solution. The obtained filtrate x had a lithium concentration of 1700 mol / L, and it was confirmed that a high-lithium solution was obtained.

Claims

1. The following formula (A): Li a Mn b Fe c M x PO4 ... (A) (in formula (A), M represents Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd; a, b, c, and x represent numbers that satisfy 0<a≦1.2, 0≦b≦1.2, 0≦c≦1.2, 0≦x≦0.3, and b+c≠0, and that satisfy a+(valence of Mn)×b+(valence of Fe)×c+(valence of M)×x=3), the method for recovering lithium from a positive electrode material for a lithium ion secondary battery (A) represented by the following steps (I) to (III-1): (I) a step of adding an oxidizing agent and water to the positive electrode material for a lithium ion secondary battery (A) to obtain a mixed solution i (II) a step of adjusting the pH of the obtained mixed liquid i to 4 to 9 and mixing to obtain a mixed liquid ii; (III-1) a step of filtering the obtained mixed liquid ii, and then obtaining a lithium-containing filtrate iii; (I)' a step of adding a positive electrode material for a lithium ion secondary battery (A) and an oxidizing agent to the obtained filtrate iii, and mixing them to obtain a mixed liquid i'; (II)' a step of adjusting the pH of the obtained mixed liquid i' to 4 to 9 to obtain a mixed liquid ii'; and (III-1)' a step of filtering the obtained mixed liquid ii', and then obtaining a lithium-containing filtrate iii', and obtaining the resulting filtrate iii' as a lithium-rich liquid, or by using the resulting filtrate iii' again as the filtrate iii in step (I)', the above (I)' to (III-1)' are repeated multiple times to obtain the finally obtained filtrate iii' as a lithium-rich liquid, a method for recovering lithium.

2. The method for recovering lithium according to claim 1, wherein the oxidizing agent used in step (I) and step (I)' is one or more selected from the group consisting of peroxoacid salts, permanganates, and hydrogen peroxide salts.

3. The method for recovering lithium according to claim 1 or 2, wherein the temperature of the mixed solution ii or mixed solution ii' obtained in step (II) or step (II)' is 5°C to 60°C.

4. The method for recovering lithium according to any one of claims 1 to 3, wherein the oxidizing agent added in step (I) is 0.1 to 0.9 parts by mass per part by mass of the positive electrode material for lithium ion secondary batteries (A).

5. The method for recovering lithium according to any one of claims 1 to 4, wherein the amount of lithium contained in the filtrate iii' as the lithium-rich solution finally obtained in step (III-1)' is 0.7 mol to 1 mol per 1 mol of lithium constituting the positive electrode material (A) for lithium ion secondary batteries used in step (I).

6. The following formula (A): Li a Mn b Fe c M x PO4...(A) (in formula (A), M represents Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd; a, b, c, and x represent numbers that satisfy 0<a≦1.2, 0≦b≦1.2, 0≦c≦1.2, 0≦x≦0.3, and b+c≠0, and that satisfy a+(valence of Mn)×b+(valence of Fe)×c+(valence of M)×x=3), the method for producing a lithium adsorbent from a positive electrode material for a lithium ion secondary battery (A) represented by the following steps (I) to (III-2): (I) a step of adding an oxidizing agent and water to the positive electrode material for a lithium ion secondary battery (A) to obtain a mixed solution i (II) adjusting the pH of the obtained mixed solution i to 4 to 9 and mixing the mixture to obtain a mixed solution ii; and (III-2) filtering the obtained mixed solution ii to obtain a filter cake iii.

7. The method for producing a lithium adsorbent according to claim 6, wherein the oxidizing agent used in step (I) is one or more selected from the group consisting of peroxo acid salts, permanganates, and hydrogen peroxide salts.