JP7767705B2 - Method for recovering lithium from lithium-ion secondary batteries - Google Patents

Description

The present invention relates to a method for recovering lithium from lithium-ion secondary batteries.

Lithium-ion secondary batteries are lightweight, have high capacity, and have high electromotive force compared to conventional lead-acid batteries, nickel-cadmium secondary batteries, etc., and are used as secondary batteries in personal computers, electric vehicles, portable devices, etc. Valuable materials such as cobalt and nickel are used in the positive electrode of lithium-ion secondary batteries as lithium cobalt oxide (LiCoO ₂ ), ternary positive electrode materials (LiNi _x Co _y Mn _z O ₂ (x + y + z = 1)), etc.

As the use of lithium-ion secondary batteries is expected to continue to expand, from the perspective of resource recycling, it is desirable to recover valuable materials such as lithium from lithium-ion secondary batteries that are discarded due to defective products generated during the manufacturing process, or due to the end of the lifespan of the equipment or batteries used. When recovering valuable materials such as lithium from lithium-ion secondary batteries, it is important to separate and recover the various metals and impurities used in the batteries, as this increases the value of the recovered materials. In particular, when lithium carbonate recovered from discarded lithium-ion secondary batteries is used as a battery material, high-purity lithium carbonate with few impurities is required, as the electrical properties deteriorate if the lithium carbonate contains a large amount of impurities.

One proposed technique for recovering lithium carbonate from lithium-ion secondary batteries while minimizing the introduction of impurities is to produce lithium carbonate from a solution obtained by calcining used lithium-ion secondary batteries and leaching lithium into water (see, for example, Patent Document 1). In this proposed technique, for example, used lithium-ion secondary batteries are calcined, and the resulting solution, obtained by leaching lithium into water, is stored in a crystallization tank containing an anode and a cathode. Electricity is then applied to the solution containing lithium ions and carbonate ions to precipitate lithium carbonate at and near the cathode, and lithium is recovered as lithium carbonate salt.

Patent No. 5872788

However, the conventional technology described in Patent Document 1 above has the problem that lithium is not concentrated sufficiently near the cathode, resulting in an insufficient recovery rate of lithium carbonate.

The present invention aims to solve the problems of the past and achieve the following objectives. Specifically, the present invention aims to provide a method for recovering lithium from lithium ion secondary batteries that suppresses lithium loss when concentrating the lithium leachate obtained by leaching lithium ion secondary batteries with water, and that can recover high-purity lithium at a high recovery rate.

The means for solving the above problems are as follows:
<1> A heat treatment step of heating a lithium ion secondary battery containing at least one of cobalt, nickel, and manganese at 700°C or higher;
a crushing step of crushing the heat-treated lithium ion secondary battery obtained in the heat treatment step;
a classification step of classifying the crushed material obtained in the crushing step at a classification point of 0.6 mm or more and 2.4 mm or less to obtain a coarse product and a fine product;
a lithium leaching step in which lithium contained in the fine granule product recovered in the classification step is leached into water to obtain a lithium leached solution having a lithium concentration of less than 1,500 mg/L;
a membrane separation step of separating the lithium leachate into a first lithium concentrate using a nanofiltration (NF) membrane or a reverse osmosis (RO) membrane;
The present invention relates to a method for recovering lithium from a lithium ion secondary battery, the method comprising the steps of:
<2> The method for recovering lithium from a lithium ion secondary battery according to <1>, wherein the lithium leaching solution has a fluorine concentration of 20 mg/L or more and a pH of 10.5 or more.
<3> The method for recovering lithium from a lithium ion secondary battery according to <1> or <2>, wherein the first lithium concentrated solution has a lithium concentration of 1,500 mg/L or more and less than 4,000 mg/L, and the solution temperature is 50°C or less.
<4> The method for recovering lithium from a lithium ion secondary battery according to any one of <1> to <3>, further comprising a fluorine removal step of adding calcium hydroxide to the first lithium concentrated solution and removing solidified fluorine by solid-liquid separation to obtain a fluorine-removed solution.
<5> The method for recovering lithium from a lithium ion secondary battery according to <4>, further comprising a calcium removal step of removing calcium ions in the first lithium concentrated solution with a cation exchange resin to obtain a calcium-removed solution having a calcium concentration of 10 mg/L or less.
<6> The method for recovering lithium from a lithium ion secondary battery according to <5>, further comprising an electrodialysis step of performing electrodialysis either before or after the calcium removal step, or both before and after the calcium removal step, to obtain a second lithium concentrate having a lithium concentration of 4,000 mg/L or more.
<7> The method for recovering lithium from a lithium ion secondary battery according to <5> or <6>, further comprising a lithium carbonate recovery step of heating the calcium-removed solution or the second lithium concentrated solution and adding carbon dioxide at 60°C or higher to recover lithium carbonate.
<8> The method for recovering lithium from a lithium ion secondary battery according to any one of <1> to <7>, wherein a fluorine content in a leaching residue after the lithium leaching step is 3% or less.

The present invention provides a method for recovering lithium from lithium-ion secondary batteries that suppresses lithium loss when concentrating the lithium leachate obtained by leaching lithium-ion secondary batteries with water, and enables high-purity lithium to be recovered at a high recovery rate.

FIG. 1 is a flow chart showing an example of the method of recovering lithium from a lithium ion secondary battery according to the present invention.

(Method of recovering lithium)
The method for recovering lithium from a lithium ion secondary battery of the present invention includes a heat treatment step, a crushing step, a classification step, a lithium leaching step, and a membrane separation step, and preferably includes a fluorine removal step, a calcium removal step, an electrodialysis step, and a lithium carbonate recovery step, and may further include other steps as necessary.

The prior art disclosed in Patent Document 1 (Japanese Patent No. 5872788) uses electrophoresis as a method for concentrating lithium and removing impurities such as fluorine. However, electrophoresis not only requires complicated steps, but also has the problem of high implementation costs, as lithium is concentrated near the cathode. Furthermore, when the inventors conducted follow-up tests on the technology disclosed in Patent Document 1, they found that it was difficult to achieve a lithium recovery rate of 50% or more, as shown in Comparative Example 2 below.

As described above, the present inventors have found that the conventional techniques have a problem in that the recovery rate of lithium may be low when concentrating lithium in the lithium leachate.
Therefore, the present inventors have further intensively investigated a lithium recovery method capable of recovering lithium from a lithium leachate at a high recovery rate. As a result, they have found that a method for recovering lithium from a lithium ion secondary battery, which includes: a heat treatment step of heating a lithium ion secondary battery containing at least one of cobalt, nickel, and manganese at 700°C or higher; a crushing step of crushing the heat-treated lithium ion secondary battery obtained in the heat treatment step; a classification step of classifying the crushed product obtained in the crushing step using a classification point of 0.6 mm or more and 2.4 mm or less to obtain a coarse particle product and a fine particle product; a lithium leaching step of leaching lithium contained in the fine particle product recovered in the classification step into water to obtain a lithium leachate having a lithium concentration of less than 1,500 mg/L; and a membrane separation step of converting the lithium leachate into a first lithium concentrate using a nanofiltration (NF) membrane or a reverse osmosis (RO) membrane, can suppress lithium loss during concentration and recover a high-purity lithium salt from the lithium concentrate, which led to the present invention.

<Heat treatment process>
The heat treatment step is a step of heat treating (roasting) a lithium ion secondary battery containing at least one of cobalt, nickel, and manganese at 700° C. or higher.

- Lithium-ion secondary battery -
The lithium ion secondary battery is not particularly limited and can be appropriately selected depending on the purpose. Examples include defective lithium ion secondary batteries generated during the manufacturing process of lithium ion secondary batteries, lithium ion secondary batteries discarded due to a defect in the equipment used or the end of the life of the equipment used, and used lithium ion secondary batteries discarded due to the end of the life of the equipment.

The shape, structure, size and material of the lithium ion secondary battery are not particularly limited and can be appropriately selected depending on the purpose.
The shape of the lithium ion secondary battery is not particularly limited and can be appropriately selected depending on the purpose. Examples include laminate type, cylindrical type, button type, coin type, square type, and flat type.
Examples of lithium-ion secondary batteries include those including a positive electrode, a negative electrode, a separator, an electrolytic solution containing an electrolyte and an organic solvent, and an outer container that is a battery case that houses the positive electrode, the negative electrode, the separator, and the electrolytic solution. When the secondary battery is covered with the outer container, the inside of the secondary battery is likely to be reduced by carbon derived from the negative electrode active material during heat treatment. Note that the lithium-ion secondary battery may be in a state in which the positive electrode, the negative electrode, etc. have fallen off.
The form of the lithium-ion secondary battery is not particularly limited and can be appropriately selected depending on the purpose, and examples thereof include a battery cell, a battery module, and a battery pack. Here, a battery module refers to a battery in which a plurality of battery cells, which are unit batteries, are connected and assembled into a single housing, and a battery pack refers to a battery in which a plurality of battery modules are assembled into a single housing. The battery pack may also include a controller and a cooling device.

--Positive electrode--
The positive electrode is not particularly limited as long as it has a positive electrode active material, and can be appropriately selected depending on the purpose.
The shape of the positive electrode is not particularly limited and can be appropriately selected depending on the purpose. Examples include a flat plate shape and a sheet shape.

--- Positive electrode current collector ---
The shape, structure, size, material, etc. of the positive electrode current collector are not particularly limited and can be appropriately selected depending on the purpose.
The positive electrode current collector may be in the form of, for example, a foil.
Examples of materials for the positive electrode current collector include stainless steel, nickel, aluminum, copper, titanium, tantalum, etc. Among these, aluminum is preferred.

The positive electrode material is not particularly limited and can be appropriately selected depending on the purpose. For example, it preferably contains at least a positive electrode active material containing lithium, a conductive agent, and a binder resin, and further contains other components as necessary.
Examples of positive electrode active materials include lithium manganese oxide (LiMn ₂ O ₄ ), lithium cobalt oxide (LiCoO ₂ ), lithium cobalt nickel oxide (LiCo _1/2 Ni _1/2 O ₂ ), LiNi _x Co _y Mn _z O ₂ (x + y + z = 1) which is called a ternary system or an NCM system, LiNi _x Co _y Al _z (x + y + z = 1) which is called an NCA system, lithium iron phosphate (LiFePO ₄ ), and lithium titanate (Li ₂ TiO ₃ ). Among these, lithium manganate, lithium cobaltate, lithium cobalt nickelate (LiCo _1/2 Ni _1/2 O ₂ ), LiNi _x Co _y Mn _z O ₂ (x + y + z = 1) which is called a ternary system or an NCM system, and LiNi _x Co _y Al _z (x + y + z = 1) which is called an NCA system are preferred because lithium can be easily converted into a water-soluble form by heat treatment.
The conductive agent is not particularly limited and can be appropriately selected depending on the purpose. Examples thereof include carbon black, graphite, carbon fiber, and metal carbides.
The binder resin is not particularly limited and can be appropriately selected depending on the purpose. Examples include homopolymers or copolymers of vinylidene fluoride, tetrafluoroethylene, acrylonitrile, ethylene oxide, and the like, and styrene-butadiene rubber.

--Negative electrode--
The negative electrode is not particularly limited as long as it has a negative electrode active material, and can be appropriately selected depending on the purpose.
The shape of the negative electrode is not particularly limited and can be appropriately selected depending on the purpose. Examples include a flat plate shape and a sheet shape.

--- Negative electrode current collector ---
The shape, structure, size, material, etc. of the negative electrode current collector are not particularly limited and can be appropriately selected depending on the purpose.
The negative electrode current collector may be in the form of, for example, a foil.
Examples of materials for the negative electrode current collector include stainless steel, nickel, aluminum, copper, titanium, and tantalum, with copper being preferred among these.

There are no particular restrictions on the negative electrode material and it can be selected appropriately depending on the purpose. Examples include carbon materials such as graphite and hard carbon, and titanates.

The temperature in the heat treatment (heat treatment temperature) is 700° C. or higher, preferably 750° C. or higher and 1,080° C. or lower, and more preferably 750° C. or higher and 900° C. or lower. By setting the heat treatment temperature to 700° C. or higher, it is possible to convert the lithium in Li(Ni/Co/Mn)O ₂ in the positive electrode active material and the lithium in LiPF ₆ in the electrolyte into substances in a form in which lithium is soluble in an aqueous solution, such as lithium fluoride (LiF), lithium carbonate (Li ₂ CO ₃ ), or lithium oxide (Li ₂ O), and it is possible to separate impurities other than fluorine during lithium leaching.
The heat treatment temperature refers to the temperature of the lithium ion secondary battery during the heat treatment. The heat treatment temperature can be measured by inserting a thermometer such as a couple or thermistor into the lithium ion secondary battery at the heat treatment temperature.

The time for heat treating the lithium-ion secondary battery (heat treatment time) is not particularly limited and can be appropriately selected depending on the purpose, but is preferably from 1 minute to 5 hours, more preferably from 1 minute to 2 hours. The heat treatment time may be any time that allows the lithium-containing compound to reach the desired temperature, and by slowing the rate of temperature increase, the formation of insoluble lithium oxides can be suppressed and the lithium leaching rate can be improved. Furthermore, the time for maintaining the temperature after the temperature increase may be short.
The heat treatment time is preferably within the above range, which is advantageous in terms of the cost required for the heat treatment and the productivity in producing the lithium solution.

The heat treatment method is not particularly limited and can be appropriately selected depending on the purpose, and examples thereof include a method using a roasting furnace.
The roasting furnace is not particularly limited and can be appropriately selected depending on the purpose. Examples include batch furnaces such as rotary kilns, fluidized bed furnaces, tunnel furnaces and muffle furnaces, cupola furnaces and stoker furnaces.

<Crushing process>
The crushing step is not particularly limited as long as it is a step of crushing the lithium ion secondary battery (roasted product) that has been heat-treated in the heat treatment step to obtain crushed products, and can be appropriately selected depending on the purpose. The crushing step is preferably, for example, a step of crushing the roasted product by impact to obtain crushed products. If the outer container of the lithium ion secondary battery does not melt during the heat treatment, it is more preferable to perform pre-crushing by cutting the roasted product with a cutter before applying an impact to the roasted product.

Examples of methods for crushing by impact include a method in which the roasted material is thrown by a rotating striking plate and struck against a collision plate to apply an impact, and a method in which the roasted material is hit by a rotating striker (beater). Specifically, this can be done by using a hammer crusher or the like.
An example of a method for crushing by impact is a method in which the roasted material is hit with ceramic or other balls, which can be performed using a ball mill, etc. Crushing by impact can also be performed using, for example, a biaxial crusher with a short blade width and length for crushing by compression.

There are no particular restrictions on the crushing time in the crushing step and it can be selected appropriately depending on the purpose, but the crushing time per kg of lithium-ion secondary batteries is preferably from 1 second to 30 minutes, more preferably from 2 seconds to 10 minutes, and particularly preferably from 3 seconds to 5 minutes.

<Classification process>
The classification step is not particularly limited as long as it is a step of classifying the crushed material into a coarse product and a fine product and obtaining recovered products from each of them, and can be appropriately selected depending on the purpose.

The classification method is not particularly limited and can be appropriately selected depending on the purpose, and can be carried out using, for example, a vibrating sieve, a multistage vibrating sieve, a cyclone, a standard sieve according to JIS Z8801, etc. By classification, copper, iron, aluminum, etc. can be separated into a coarse-grained product, and lithium can be concentrated in a fine-grained product.
The classification point in the classification is preferably 0.6 mm or more and 2.4 mm or less, and more preferably 0.85 mm or more and 1.7 mm or less. When the classification point in the classification is 2.4 mm or less, the incorporation of impurities such as copper, iron, and aluminum into the fine-grained product can be suppressed, and the amount of lithium recovered per unit weight when the fine-grained product is leached can be improved. Furthermore, when the classification point in the classification is 0.6 mm or more, the recovery of lithium in the coarse-grained product can be suppressed, and the amount of lithium recovered when the fine-grained product is leached can be improved.
The classification into coarse and fine products may be repeated several times, and this reclassification can further reduce the impurity content of each product.

<Lithium leaching process>
The lithium leaching step is a step in which lithium contained in the fine granule product recovered in the classification step is leached into water to obtain a lithium leachate having a lithium concentration of less than 1,500 mg/L.
Here, the lithium concentration of the obtained lithium leachate is less than 1,500 mg/L, although it depends on the heat treatment conditions of the lithium ion secondary battery, the solid-liquid ratio during leaching, etc. The inventors have found that the lithium in the fine granule product is present mainly as lithium carbonate, lithium fluoride, or lithium oxide. They have also found that the leaching rate of lithium salts into water can be improved by setting the lithium concentration of the lithium leachate to less than 1,500 mg/L. The lithium in the lithium leachate is dissolved mainly as lithium carbonate and lithium fluoride. In other words, CO ₃ ^2- ions and F 2 ^- ions are dissolved in amounts approximately equal to the lithium equivalent. On the other hand, if the lithium leachate is concentrated by some method to increase the lithium concentration to 1,500 mg/L or more and then heated to 60°C or higher (reducing the solubility of lithium carbonate), lithium carbonate will precipitate.
Here, the lithium concentration can be measured by ICP-AES analysis using, for example, ICAP 6300DuO (manufactured by Thermo Fisher Scientific Co., Ltd.).

By leaching lithium into water in the lithium leaching process, it is possible to recover almost all (100%) of impurities such as nickel (Ni), cobalt (Co), and manganese (Mn) as residue. Therefore, when leaching lithium into water, it is preferable to perform solid-liquid separation of the slurry water from which the lithium has been leached using filter paper or a solid-liquid separator in order to recover residues such as nickel, cobalt, and manganese. In other words, in the lithium leaching process of the lithium recovery method, it is preferable to remove residues such as nickel, cobalt, and manganese by solid-liquid separation.

The water used in the lithium leaching process is not particularly limited and can be selected appropriately depending on the purpose. Examples include industrial water, tap water, ion-exchanged water, ultrafiltered water, reverse osmosis water, distilled water, and other pure and ultrapure water.

The pH (hydrogen ion exponent) of the lithium leaching solution is preferably 10.5 or more, more preferably 10.5 or more and 12.5 or less, and particularly preferably 10.5 or more and 12.0 or less.
By adjusting the pH of the lithium leaching solution to 10.5 or higher, the amount of calcium hydroxide or the like added to increase the pH can be reduced. Furthermore, the amount of dissolved calcium ions supplied by the added calcium hydroxide or the like can be reduced. Furthermore, when the calcium removal step is performed in a later stage, the amount of remover (e.g., cation exchange resin or carbonate) used can be reduced. On the other hand, when the pH of the lithium leaching solution is 12.5 or lower, the added components are more easily dissolved, and the fluorine removal efficiency can be improved.

The leaching residue contains valuable materials such as cobalt, nickel, and carbon. Fluorine can be removed from the leaching residue by the lithium leaching step. It is preferable that the fluorine content in the leaching residue after the lithium leaching step is 3% or less.
Since the amount of fluorine removed is inversely proportional to the solid-liquid ratio during leaching, leaching at a low solid-liquid ratio is preferable in order to remove fluorine from the leaching residue, but the smaller the solid-liquid ratio, the lower the lithium/fluorine ratio that can be obtained as a low-concentration lithium leachate. By applying reverse osmosis membrane separation to this low-concentration lithium leachate, a solution with an increased lithium concentration (first lithium concentrated solution) can be recovered, enabling more efficient lithium purification (e.g., recovery of lithium carbonate).

The fluorine concentration of the lithium leaching solution is preferably 20 mg/L or more, because when a fluorine removal step described later is carried out, fluorine can be solidified and separated, and the fluorine concentration can be reduced to less than 20 mg/L.
The fluorine concentration can be measured, for example, using an apparatus in which an F-2021 fluoride ion electrode (manufactured by DKK-TOA Corporation) is connected to an IM-32P glass electrode type hydrogen ion concentration indicator (manufactured by DKK-TOA Corporation).

The lithium leaching method in the lithium leaching step is not particularly limited and can be selected appropriately depending on the purpose. For example, the fine granular product of the lithium ion secondary battery recovered in the classification step can be introduced into a liquid (water) and stirred to leach lithium into the liquid. The stirring speed of the liquid in the lithium leaching step is not particularly limited and can be selected appropriately depending on the purpose, and can be, for example, 200 rpm.
The leaching time in the lithium leaching step is not particularly limited and can be appropriately selected depending on the purpose, and can be, for example, 1 hour.

<Membrane separation process>
The membrane separation step is a step of concentrating the lithium leached solution obtained in the lithium leaching step using a nanofiltration (NF) membrane or a reverse osmosis (RO) membrane to obtain a first lithium concentrate.
The concentration treatment in the membrane separation step is preferably carried out at a temperature of the lithium leaching solution of 50° C. or less. By carrying out the membrane separation step at a temperature of the lithium leaching solution of 50° C. or less, it is possible to prevent lithium carbonate from crystallizing and blocking the membrane surface, and lithium and carbonate ions can be concentrated in the obtained first lithium concentrated solution at low cost. Note that fluorine is also concentrated in the first lithium concentrated solution, but this fluorine is efficiently removed in the fluorine removal step described below.
The lithium concentration of the first lithium concentrated solution can be 1,500 mg/L or more and less than 4,000 mg/L. By setting the lithium concentration of the first lithium concentrated solution to less than 4,000 mg/L, it is possible to prevent lithium carbonate from crystallizing and blocking the membrane surface during membrane separation, thereby enabling continuous, stable membrane separation over a long period of time (with reduced increase in operating pressure). When an NF membrane is used, fluorine can be removed from the permeate side.
In the present invention, a lithium leachate having a lithium concentration of less than 1,500 mg/L is subjected to membrane separation (membrane concentration), and therefore the lithium concentration can be adjusted to 1,500 mg/L or more and less than 4,000 mg/L more cheaply than other concentration methods (e.g., evaporation concentration or electrodialysis).

In this way, in the present invention, lithium ion secondary batteries can be treated by performing a heat treatment process, a crushing process, a classification process, a lithium leaching process, and a membrane separation process, and a first lithium concentrate solution having a lithium concentration of 1,500 mg/L or more can be produced.

<Fluorine removal process>
The fluorine removal step is a step of adding calcium hydroxide to the first lithium concentrated solution and removing the solidified fluorine by solid-liquid separation to obtain a fluorine-removed solution. In other words, while both lithium and fluorine can be concentrated in the first lithium concentrated solution by the membrane separation step, the solidified fluorine can be removed by adding calcium hydroxide, which solidifies the fluorine contained in the first lithium concentrated solution, to the first lithium concentrated solution.
The fluorine removal step can reduce the fluorine concentration in the post-fluorine-removal solution to about 5 mg/L to 20 mg/L. Even if the fluorine removal step is performed on the lithium leaching solution without performing the concentration treatment using the membrane separation step, the fluorine concentration in the resulting post-fluorine-removal solution is at a similar level of 5 mg/L to 20 mg/L. Therefore, performing fluorine removal on the first lithium concentrated solution can reduce the ratio of the fluorine concentration to the lithium concentration compared to the case where the membrane separation step is not performed, and can reduce the fluorine content of the finally obtained recovered lithium product.

The calcium hydroxide may be added in the form of a solid calcium hydroxide to the first lithium concentrate, or in the form of a solution or slurry in which calcium hydroxide is dissolved.
Here, an example of a preferred form for adding calcium hydroxide will be described.
As described above, in the lithium leaching step, the pH of the lithium leaching solution can be adjusted to 10.5 or higher by adjusting the lithium concentration when leaching lithium into water. This is thought to be because the lithium leaching solution becomes a lithium hydroxide solution. By increasing the pH during leaching, the amount of alkali added to raise the pH to 12 or higher after the addition of calcium hydroxide can be reduced. Furthermore, if the pH of the first lithium concentrated solution after the addition of calcium hydroxide is less than 12, it is preferable to add additional alkali to adjust the pH to 12 or higher.
Under these conditions, in view of the solubility product of calcium, the amount of dissolved calcium ions (Ca ²⁺ ) in the post-fluoride removal solution after the fluoride removal step can be suppressed to 1,000 mg/L or less, thereby reducing the amount of cation exchange resin used to remove calcium ions (Ca ²⁺ ) in the calcium removal step.
By using calcium hydroxide for fluoride solidification, the addition of calcium and the adjustment of pH can be carried out simultaneously by simply adding calcium hydroxide (slaked lime).
On the other hand, fluorine can be sufficiently removed even under conditions where the dissolved amount of calcium ions (Ca ²⁺ ) is small. For example, when the fluorine concentration in the first lithium concentrated solution before fluorine removal is 2,000 mg/L, the fluorine concentration in the lithium leaching solution can be reduced to about 20 mg/L by performing the fluorine removal step.

Here, solidifying fluorine in the fluorine removal step means solidifying fluorine so that it can be removed from the lithium leachate. Therefore, for example, when removing fluorine from the lithium leachate by solid-liquid separation, the fluorine can be solidified so that it can be separated into solid and liquid. In this case, for example, the solidified fluorine may be in the form of fine particles, or may be in the form of a precipitate (sediment) having a certain size.

The method for removing solidified fluorine in the fluorine removal process is not particularly limited and can be selected appropriately depending on the purpose. Examples include a method using solid-liquid separation and a method for removing a precipitate (sediment) formed from solidified fluorine. Among these, a method using solid-liquid separation is preferred. In other words, it is preferable to remove solidified fluorine by solid-liquid separation in the fluorine removal process. By removing solidified fluorine by solid-liquid separation, fluorine can be removed from the lithium leachate with greater precision.

The solidified fluorine can be removed by solid-liquid separation, for example, by filtering the lithium leachate (which may be in the form of a slurry) using filter paper or a solid-liquid separator.
The filter paper used for removing solidified fluorine by solid-liquid separation is not particularly limited and can be appropriately selected depending on the purpose. However, it is preferable to use quantitative filter paper, and it is more preferable to use filter paper classified as Type 5C in JIS P3801.
The solid-liquid separator used for removing solidified fluorine by solid-liquid separation is not particularly limited and can be appropriately selected depending on the purpose, and examples thereof include a screw press, a roller press, a belt screen, a vibrating sieve, a multi-plate wave filter, a vacuum dehydrator, a pressure dehydrator (filter press), a belt press, a screw press, a centrifugal concentration dehydrator (screw decanter), and a multi-disk dehydrator.

Furthermore, by using an aluminum compound such as aluminum sulfate, calcium aluminate, or aluminum chloride as a second additive component in addition to calcium hydroxide, the fluoride concentration can be reduced even further than when a calcium compound is used alone; for example, the fluoride concentration in the post-fluoride removal solution can be reduced to 10 mg/L or less. Furthermore, the aluminum leached during lithium leaching can be coprecipitated during fluoride solidification, allowing the aluminum to be used as a fluoride remover along with the aluminum removal.

The amount of the additive component added relative to the total amount of the first lithium concentrate is not particularly limited and can be appropriately selected depending on the purpose. When the first lithium concentrate contains carbonate ions, the additive component reacts with the carbonate ions and is consumed, so it is preferable to increase the amount of the additive component added. From the viewpoint of stably solidifying and separating fluorine, the amount of the additive component added is preferably an amount that allows the additive component to react with both the fluorine and the carbonate ions contained in the first lithium concentrate when the total amounts of fluorine and carbonate ions contained in the first lithium concentrate are X moles (mol) of fluorine and Y moles (mol) of carbonate ions.
The amount of the additive component added relative to the moles (mol) of fluorine (X) and moles (mol) of carbonate ions (Y) in the first lithium concentrated solution may be, for example, 0.5 × (X + 2Y) mol or more and 10 × (X + 2Y) mol or less when the valence of the additive component ions is divalent. More preferably, the amount of the additive component added relative to the moles (mol) of fluorine (X) and moles (mol) of carbonate ions (Y) in the first lithium concentrated solution may be, for example, 0.75 (X + 2Y) mol or more and 5 (X + 2Y) mol or less.
For example, when calcium hydroxide (where the valence of calcium ions is divalent) is used as the compound containing the additive component, the amount of calcium hydroxide added relative to X moles (mol) of fluorine and Y moles (mol) of carbonate ions in the first lithium concentrated solution is preferably 0.5 × (X + 2Y) mol or more and 10 × (X + 2Y) mol or less, and more preferably 0.75 × (X + 2Y) mol or more and 5 × (X + 2Y) mol or less.

In the fluorine removal step, it is preferable to stir the first lithium concentrated solution after adding calcium hydroxide.
The stirring speed of the first lithium concentrate is not particularly limited and can be appropriately selected depending on the purpose. For example, the stirring speed is preferably 20 rpm or more and 2,000 rpm or less, and more preferably 50 rpm or more and 1,000 rpm or less.
The stirring time for the first lithium concentrated solution is not particularly limited and can be appropriately selected depending on the purpose, but is, for example, preferably from 5 minutes to 240 minutes, more preferably from 15 minutes to 120 minutes. By setting the stirring time (reaction time) to 240 minutes or less, re-dissolution of solidified fluorine can be prevented.

Methods for confirming the fluoride concentration in the solution after fluoride removal include measuring the fluoride concentration using, for example, an ion electrode method or ion chromatography.

<Calcium removal process>
The calcium removal step is a step of removing calcium ions from the first lithium concentrated solution using a cation exchange resin to obtain a calcium-removed solution having a calcium concentration of 10 mg/L or less. For the adsorption and removal of calcium ions using a cation exchange resin, multiple types of ion adsorption resins or ion adsorption resin towers can be used.
By contacting the cation exchange resin with the first lithium concentrated solution, calcium ions can be adsorbed onto the cation exchange resin and removed while leaving lithium dissolved. This allows the calcium content in lithium carbonate to be further reduced. Furthermore, since cation exchange resin adsorption removes Ca ²⁺ ions (obtaining a calcium-removed solution) without supplying CO ₃ ^2- ions, CO ₃ ^2- ions do not dissolve in the calcium-removed solution. As a result, it is possible to concentrate the calcium-removed solution without precipitating lithium carbonate. Note that lithium ions (cations) do not adsorb onto the cation exchange resin.
It is preferable to carry out electrodialysis either before or after the calcium removal step, or both, to concentrate the solution after calcium removal.

In the fluoride removal process, calcium hydroxide is used to remove fluoride, and the calcium concentration in the post-fluoride removal solution can be suppressed to 1,000 mg/L or less. This means that the amount of cation exchange resin used per unit amount of post-fluoride removal solution is small, making calcium removal using a cation exchange resin low-cost (economical). While the addition of carbonate and the resulting calcium carbonate are also conceivable as methods for removing calcium ions, the calcium removal process using a cation exchange resin is preferred because it is more accurate and can produce Li carbonate with a lower Ca content. Furthermore, the calcium removal process using a cation exchange resin does not supply CO ₃ ²⁻ during calcium removal, so it can selectively remove calcium ions without precipitating or losing lithium carbonate.

<Electrodialysis process>
The electrodialysis step is a step of performing electrodialysis either before or after the calcium removal step, or both, to obtain a second lithium concentrated solution having a lithium concentration of 4,000 mg/L or more. Note that the electrodialysis step may be performed by multi-stage treatment to concentrate the lithium to a predetermined concentration.
Since most of the CO ₃ ^2- ions in the calcium-removed solution are removed as calcium carbonate by the fluoride removal step and the calcium removal step, the calcium-removed solution contains almost no CO ₃ ^2- ions. Therefore, even if lithium ions are concentrated to 4,000 mg/L or more, crystallization of lithium carbonate does not occur in the lithium carbonate recovery step described below. In addition, since the ion exchange membrane used in electrodialysis has higher resistance to alkali than the nanofiltration (NF) membrane or reverse osmosis (RO) membrane used in membrane concentration, it is preferable to concentrate the fluoride-removed solution (pH 12 or higher) after fluoride removal by electrodialysis.
The concentration ratio when concentrating the post-fluoride removal solution is not particularly limited and can be appropriately selected depending on the purpose, but is preferably 1.5 to 100 times, more preferably 3 to 50 times, and particularly preferably 5 to 10 times. If the concentration ratio exceeds 100 times, lithium fluoride may precipitate inside the device during electrodialysis. Furthermore, if the concentration ratio is 10 times or less, concentration may be completed in only one stage of electrodialysis rather than multistage treatment, which is particularly preferred from the perspective of equipment installation costs. Note that concentration by electrodialysis can increase the lithium carbonate production rate by 5 times or more compared to evaporation concentration with equipment of the same scale.

<Lithium carbonate recovery process>
The lithium carbonate recovery step is a step of heating the calcium-removed solution or the second lithium-concentrated solution and adding carbon dioxide at 60° C. or higher to recover lithium carbonate. That is, lithium carbonate can be selectively crystallized by utilizing the difference in solubility between lithium carbonate and other lithium salts (e.g., lithium sulfate, lithium fluoride), and lithium carbonate can be recovered at a high concentration.
The heating temperature of the second lithium concentrate is preferably 60° C. or higher, more preferably 80° C. or higher, and particularly preferably 95° C. or higher. The higher the heating temperature, the more the solubility of lithium carbonate can be reduced, and the more the amount of lithium carbonate crystallized can be increased. Examples of the heating method include heating with an electric heater or a pipe made of copper, stainless steel, Teflon (registered trademark), or the like through which heated steam passes.

The method for adding carbon dioxide to the calcium-removed solution or the second lithium-enriched solution is not particularly limited and can be appropriately selected depending on the purpose, but is preferably carried out by blowing in a gas containing carbon dioxide or adding a carbonate. Examples of the carbon dioxide-containing gas include air and carbon dioxide gas. Examples of the carbonate include sodium carbonate and potassium carbonate.
The carbonate ion concentration of the lithium-containing solution after the supply of carbon dioxide is preferably 2 or more relative to the lithium concentration of 1, more preferably 3 or more relative to the lithium concentration of 1, and particularly preferably 4 to 32.3 relative to the lithium concentration of 1. If the carbonate ion concentration is less than 2 relative to the lithium concentration of 1, the amount of lithium sulfate precipitated in addition to lithium carbonate increases, and industrial grade lithium carbonate (lithium carbonate purity of 99.0% or more) may not be crystallized. On the other hand, if the carbonate ion concentration exceeds 32.3 relative to the lithium concentration of 1, lithium carbonate may become hydrated in a solution containing dissolved metals derived from carbonates added to dissolve carbonate ions, and the metal purity derived from carbonates in the lithium carbonate may become excessive.

It is preferable to continue stirring the liquid during the recovery of lithium carbonate. Stirring can make the concentrations of carbonate ions and impurity ions (e.g., fluorine ions) in the liquid uniform, and also make the particle size of the lithium carbonate precipitate uniform, thereby reducing the generation of impurity crystals or the entrainment of the liquid into the lithium carbonate precipitate, and therefore reducing the quality of impurities in the recovered lithium carbonate.
When recovering lithium carbonate, crystallization may be started or continued in a state in which lithium carbonate crystals have been added beforehand or during crystallization. The addition of these crystals (seed crystals) can improve and homogenize the particle size of the lithium carbonate recovered by crystallization, thereby reducing the water content of the crystals and further reducing the quality of impurities derived from water-containing components.
In the lithium carbonate recovery step, lithium carbonate can also be crystallized by heating the liquid to a temperature equal to or higher than the temperature at which the liquid evaporates during crystallization and evaporating and concentrating the liquid.

The precipitated lithium (lithium carbonate) can be recovered using known tools such as a spoon, rake, or scraper. Because the slurry containing the precipitated lithium carbonate has low viscosity, it does not clog pipes and can be recovered by pumping the slurry while stirring it and feeding it to a solid-liquid separator (e.g., a filter press).

As described above, lithium leachate obtained by treating lithium-ion secondary batteries is subjected to a membrane separation process to concentrate lithium and fluorine, and lithium is then recovered as lithium carbonate from the concentrated lithium solution, making it possible to easily obtain lithium carbonate with a low fluorine content and high purity (grade).

<Other processes>
The other steps are not particularly limited and can be appropriately selected depending on the purpose.

Here, with reference to the drawings, an example of an embodiment of the method for recovering lithium from a lithium-ion secondary battery of the present invention will be described.

First Embodiment
FIG. 1 is a flow chart showing an example of a method for recovering lithium from a lithium ion secondary battery according to the first embodiment.
In the first embodiment of Fig. 1, a lithium ion secondary battery (LIB) is first subjected to a heat treatment (heat treatment step) to obtain a heat-treated LIB. As a result, for example, lithium in Li(Ni/Co/Mn) _O2 in the positive electrode active material of the LIB and _LiPF6 in the electrolyte can be converted into substances in which lithium is soluble in aqueous solutions, such as lithium fluoride (LiF), _lithium carbonate ( _Li2CO3 ), or lithium oxide ( _Li2O ), making it possible to separate nickel, cobalt, manganese, etc., and also melting and separating aluminum (Al) in the LIB.
Next, in the first embodiment, the LIB heat-treated product is crushed and classified (crushing step and classification step) to obtain a coarse product and a fine product. Here, impurities such as copper (Cu), iron (Fe), and aluminum (Al) can be separated and removed from the coarse product.

In the first embodiment, lithium is then leached from the fine product into water, and a residue containing impurities such as nickel (Ni), cobalt (Co), and manganese (Mn) is formed in the lithium leaching solution.
Then, residues containing nickel (Ni), cobalt (Co), and manganese (Mn) are removed from the solution into which lithium has been leached by solid-liquid separation to produce a lithium leachate.
The obtained lithium leachate is subjected to membrane separation by passing it through a nanofiltration (NF) membrane or a reverse osmosis (RO) membrane to obtain a first lithium concentrate.

Next, calcium hydroxide, which solidifies the fluorine contained in the first lithium concentrate, is added to the lithium solution to remove the solidified fluorine, yielding a fluorine-removed liquid (fluorine removal process). The fluorine-removed liquid from which the solidified fluorine has been removed is passed through a cation exchange resin to adsorb and remove calcium ions to a concentration of 10 mg/L or less (calcium removal process). The resulting calcium-removed liquid is subjected to electrodialysis (electrodialysis process) to obtain a second lithium concentrate with a lithium concentration of 4,000 mg/L or more. The resulting second lithium concentrate is heated to 60°C or higher, and carbon dioxide (gas) is added to crystallize and recover lithium carbonate (lithium carbonate recovery process).

Examples of the present invention are described below, but the present invention is not limited to these examples in any way.

Example 1
<Preparation of lithium solution>
A lithium ion secondary battery (approximately 10 kg) containing cobalt, nickel, and manganese as a positive electrode active material was subjected to heat treatment using a muffle furnace (KBF66812-S, manufactured by Koyo Thermo Systems Co., Ltd.) as a heat treatment device under the conditions of a heat treatment temperature of 800°C (heated over 1 hour and then maintained for 2 hours) and an air supply rate of 5 L/min.
Next, the heat-treated lithium ion secondary battery (heat-treated lithium ion secondary battery) was crushed using a hammer crusher (Makino-type swing hammer crusher HC-20-3.7, manufactured by Makino Sangyo Co., Ltd.) as a crushing device under conditions of 50 Hz (hammer peripheral speed 38 m/s) and a hole diameter of the punched metal at the outlet portion of 10 mm, to obtain crushed lithium ion secondary battery material.

Subsequently, the crushed lithium ion secondary battery material was sieved using a sieve with 1.2 mm mesh size (diameter 200 mm, manufactured by Tokyo Screen Co., Ltd.) After sieving, the sieved material (coarse product) and the sieved material (fine product) that passed through the 1.2 mm sieve were collected.
The resulting fine product (fluorine content: 5.5%) was used to leach lithium into water under the following conditions: 100 L of leachate volume, 10% solid-liquid ratio, 200 rpm stirring speed, and 1 hour of leaching time. The lithium-leached water (slurry) was then subjected to solid-liquid separation using Class 5C filter paper (manufactured by Toyo Roshi Kaisha, Ltd.), and a residue containing nickel (Ni), cobalt (Co), and manganese (Mn) was removed to produce a lithium leachate. The fluorine content of the residue was 2.0%, and fluorine was successfully removed from the valuables in the residue into the lithium leachate.

The lithium concentration of the prepared lithium leachate was measured by high-frequency inductively coupled plasma atomic emission spectrometry (ICP-AES analysis) using an ICAP 6300DuO (manufactured by Thermo Fisher Scientific K.K.), and the lithium concentration was found to be 772 mg/L.
The fluoride ion concentration was measured using an apparatus in which an F-2021 fluoride ion electrode (manufactured by DKK-TOA Corporation) was connected to an IM-32P glass electrode type hydrogen ion concentration indicator (manufactured by DKK-TOA Corporation), and the fluoride concentration was found to be 372 mg/L.
The pH of the prepared lithium leaching solution was measured using a pH meter (HM-25R, manufactured by DKK-TOA Corporation) and found to be 11.1.

Next, a reverse osmosis (RO) membrane separation test was performed using a flat membrane testing apparatus with a reverse osmosis membrane (product name: ESPA2-LD, manufactured by Nitto Denko Corporation) and an operating pressure of 1.5 MPa on 13,290 mL of the lithium leachate (liquid temperature: 20°C), to obtain 4,500 mL of a first lithium concentrated solution and 8,790 mL of a permeate (membrane separation step).
Next, 4,500 mL of the first lithium concentrate was added to a 10-L glass beaker and stirred at 150 rpm using a stirrer with a rotor blade diameter of 150 mm. 21 g/L of slaked lime powder was added from the top of the beaker, and stirring was continued for 1 hour, after which the stirring was stopped and the mixture was subjected to suction filtration using 5C filter paper for solid-liquid separation into a filtrate (liquid after fluoride removal) and a residue ( _CaF2, etc.), thereby carrying out a fluoride removal step.
A calcium removal step was carried out by passing 4,380 mL of the fluoride-removed solution through a cation exchange resin tower (product name: Duolite C20SC, manufactured by Sumika Chemtex Co., Ltd.) at a rate of SV (space velocity) = 1 to obtain a calcium-adsorbed and removed solution (4,370 mL). The entire amount of this solution was subjected to electrodialysis using an electrodialysis apparatus (Acilyzer EX-3B, manufactured by Astom Corporation) (electrodialysis step) and concentrated four-fold to obtain 1,100 mL of a second lithium concentrate.
While 1,000 mL of the obtained second lithium concentrated solution was heated and stirred with a hot magnetic stirrer to a liquid temperature of 90°C, _CO2 gas was diffused into the solution until the pH of the solution decreased to 11.0. After the gas diffusion, the slurry in which lithium carbonate had crystallized was subjected to suction filtration with 5C filter paper, and lithium carbonate was recovered (lithium carbonate recovery step).

<Evaluation of grade, recovery rate, and burning loss>
The mass of the lithium carbonate was measured using an electromagnetic balance (product name: GX-8K, manufactured by A&D Co., Ltd.) after drying at 100°C for 1 hour using a dryer (product name: DRM620DD, manufactured by Advantec Toyo Kaisha, Ltd.).
The quality of impurities in the lithium carbonate was analyzed by heating and dissolving the recovered lithium carbonate in aqua regia (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.), and analyzing the solution using a high-frequency inductively coupled plasma optical emission spectrometer (iCaP6300, manufactured by Thermo Fisher Scientific K.K.) to determine the content ratios (quality) of various impurities other than fluorine and ignition loss.
Regarding the fluorine content and chlorine content, lithium carbonate was burned at 1,000°C using an automatic sample combustion apparatus (product name: AQF-100/ICS-1500, manufactured by Dia Instruments Co., Ltd.) to volatilize fluorine and chlorine, which were then absorbed into an absorption liquid, and the absorption liquid was analyzed by ion chromatography.
The sulfate radicals were analyzed by the gravimetric method (lithium carbonate was dissolved in hydrochloric acid, filtered, and barium hydroxide was added to the filtrate to make barium sulfate, which was then incinerated in a muffle furnace and measured as barium sulfate weight, and the sulfuric acid content was calculated from this weight).
Regarding the loss on ignition, the lithium carbonate after drying was dried at 550° C. for 3 hours, and then the loss in weight was measured.

Next, the results of the reverse osmosis membrane test are shown in Table 1, the composition of the lithium solution after each step and the amount of lithium (%) in each solution when the amount of lithium in the lithium leaching solution is taken as 100% are shown in Table 2, and the quality of the recovered lithium carbonate is shown in Table 3.

Example 2
The same operation as in Example 1 was carried out, except that an NF membrane (product name: NANO-SW, manufactured by Nitto Denko Corporation) was used instead of the RO membrane in Example 1, and reverse osmosis membrane separation was performed on 14,500 mL of a lithium leachate (liquid temperature: 20°C) using a flat membrane testing apparatus at an operating pressure of 1.0 MPa, to obtain 4,500 mL of a first lithium concentrate and 10,000 mL of a permeate. The results of the reverse osmosis membrane test are shown in Table 1, and the quality of the recovered lithium carbonate is shown in Table 3.

Example 3
Lithium carbonate was recovered in the same manner as in Example 1, except that the fluorine removal step was not performed. The quality of the recovered lithium carbonate is shown in Table 3.

Example 4
Lithium carbonate was recovered in the same manner as in Example 1, except that the calcium removal step was not performed. The quality of the recovered lithium carbonate is shown in Table 3.

Example 5
In Example 1, the electrodialysis step was not performed, and the entire amount (4,000 mL) of the post-crystallization solution after the first lithium carbonate recovery was mixed with a fresh lithium leaching solution (9,290 mL), and the mixture was concentrated again by RO membrane concentration until the lithium concentration reached 2,255 mg/L. This concentrate was then subjected again to the steps from the fluorine removal step onwards, excluding the electrodialysis step, to recover lithium carbonate (second time) (i.e., the post-crystallization solution was repeatedly subjected to the membrane concentration step, thereby performing two-stage treatment). Except for this, lithium carbonate was recovered in the same manner as in Example 1. The quality of the recovered lithium carbonate is shown in Table 3.

(Comparative Example 1)
Lithium carbonate was recovered by performing the same operation as in Example 1, except that a reverse osmosis membrane test using an RO membrane was performed on a lithium leachate (lithium concentration: 2,559 mg/L) obtained by setting the solid-liquid ratio when leaching lithium into water to 40% in Example 1.
In Comparative Example 1, the lithium concentration of the first lithium concentrated solution was 7,000 mg/L or more, but the test was interrupted because lithium carbonate was precipitated on the RO membrane surface, causing an increase in the operating pressure. In addition, in Comparative Example 1, the recovery rate of lithium in the first lithium concentrated solution at the time the test was interrupted was 33%.

(Comparative Example 2)
Lithium carbonate was recovered from the aqueous leachate of the lithium ion secondary battery obtained in Example 3 by the method described in Example 1 of Patent Document 1 (Japanese Patent No. 5872788).
In Comparative Example 2, the recovery rate of lithium from the lithium solution after electrophoresis was about 50%, and the remaining 50% was distributed to the impurity concentrated solution side containing a high concentration of fluorine. However, recovering lithium from this solution by electrophoresis again would be more costly, making it difficult to recover lithium, and it was therefore difficult in Comparative Example 2 to improve the recovery rate of lithium to 50% or more.

(Comparative Example 3)
Lithium carbonate was recovered in the same manner as in Example 1, except that the membrane separation step (membrane concentration) was not performed in Example 1. The fluorine content in the recovered lithium carbonate was 600 ppm (the fluorine content was high because the solution was purified without concentration by membrane separation).
In Comparative Example 3, concentration by electrodialysis alone was insufficient, and the recovery rate of lithium in the recovered lithium carbonate was only 45% when the lithium in the lithium leaching solution was taken as 100%.

From the results of Tables 1 to 3, it is clear that in Example 1, 99% of lithium was recovered in the first lithium concentrated solution. The lithium concentration was concentrated to 2,255 mg/L. In Example 1, no lithium crystallization occurred.
In addition, in Example 1, when the lithium in the lithium leaching solution was taken as 100%, 70% of the lithium was recovered in the recovered lithium carbonate.
In addition, in Example 1, high-purity lithium carbonate having a calcium content of 51 ppm and a fluorine content of 200 ppm could be purified.

In Example 2, 88% of lithium was recovered in the first lithium concentrated solution, and the lithium concentration was concentrated to 2,208 mg/L. No lithium crystallization occurred in Example 2. Furthermore, 29% of the fluorine was removed into the permeate side in Example 2.
In Example 2, when the lithium in the lithium leaching solution was taken as 100%, 60% of lithium was recovered in the recovered lithium carbonate.
In addition, in Example 2, high-purity lithium carbonate having a calcium content of 61 ppm and a fluorine content of 100 ppm could be purified.

In Example 3, lithium carbonate was obtained with a fluorine content of 113,300 ppm and a lithium carbonate content of 88.2%. Furthermore, in Example 3, when the lithium in the lithium leachate was taken as 100%, 70% of the lithium was recovered from the recovered lithium carbonate.

In Example 4, lithium carbonate was obtained with a calcium content of 24,460 ppm and a lithium carbonate content of 95% or more. Furthermore, in Example 4, when the lithium in the lithium leachate was taken as 100%, 70% of the lithium was recovered from the recovered lithium carbonate.

In Example 5, high-purity lithium carbonate with a calcium content of 64 ppm and a fluorine content of 100 ppm was obtained. Furthermore, in Example 5, when the lithium content in the lithium leaching solution was taken as 100%, 51% of the lithium was recovered from the recovered lithium carbonate. Furthermore, in Example 5, by repeatedly performing the membrane concentration process on the post-crystallization solution, it was possible to further improve the lithium recovery rate.

Claims (8)

a heat treatment step of heating a lithium ion secondary battery containing at least one of cobalt, nickel, and manganese at 700°C or higher;
a crushing step of crushing the heat-treated lithium ion secondary battery obtained in the heat treatment step;
a classification step of classifying the crushed material obtained in the crushing step at a classification point of 0.6 mm or more and 2.4 mm or less to obtain a coarse product and a fine product;
a lithium leaching step in which lithium contained in the fine granule product recovered in the classification step is leached into water to obtain a lithium leached solution having a lithium concentration of less than 1,500 mg/L;
a membrane separation step of separating the lithium leachate into a first lithium concentrate using a nanofiltration (NF) membrane or a reverse osmosis (RO) membrane;
A method for recovering lithium from a lithium ion secondary battery, comprising: The method for recovering lithium from a lithium-ion secondary battery according to claim 1, wherein the lithium leaching solution has a fluorine concentration of 20 mg/L or more and a pH of 10.5 or more. The method for recovering lithium from a lithium-ion secondary battery according to claim 1 or 2, wherein the first lithium concentrated solution has a lithium concentration of 1,500 mg/L or more and less than 4,000 mg/L, and the solution temperature is 50°C or less. The method for recovering lithium from a lithium-ion secondary battery described in any one of claims 1 to 3, further comprising a fluorine removal step of adding calcium hydroxide to the first lithium concentrated solution and removing solidified fluorine by solid-liquid separation to obtain a fluorine-removed solution. The method for recovering lithium from a lithium-ion secondary battery described in claim 4, further comprising a calcium removal step of removing calcium ions from the first lithium concentrated solution using a cation exchange resin to obtain a calcium-removed solution having a calcium concentration of 10 mg/L or less. The method for recovering lithium from a lithium-ion secondary battery described in claim 5, further comprising an electrodialysis step in which electrodialysis is performed either before or after the calcium removal step, or both, to obtain a second lithium concentrate having a lithium concentration of 4,000 mg/L or more. The method for recovering lithium from a lithium-ion secondary battery described in claim 5 or 6, comprising a lithium carbonate recovery step of heating the calcium-removed solution or the second lithium-concentrated solution and adding carbon dioxide at 60°C or higher to recover lithium carbonate. 8. The method for recovering lithium from a lithium ion secondary battery according to claim 1, wherein the fluorine content in the leaching residue after the lithium leaching step is 3% or less.