Method for recovering lithium

Abstract

Provided is a means capable of improving lithium recovery efficiency. Provided is a lithium recovery method for recovering lithium from a lithium ion-containing aqueous solution, the method comprising: obtaining, from the lithium ion-containing aqueous solution and a gas, a mixture of a lithium ion-concentrated aqueous solution and a gas hydrate; separating the gas hydrate from the mixture to obtain the lithium ion-concentrated aqueous solution; and disposing the lithium ion-concentrated aqueous solution on one main surface side of a lithium ion conductor, disposing recovery water on the other main surface side, and transmitting lithium ions contained in the lithium ion-concentrated aqueous solution from the one main surface side to the other main surface side to obtain a lithium ion recovery aqueous solution.

Description

How to recover lithium

[0001] The present invention relates to a method for recovering lithium, and more particularly to a method for recovering lithium from an aqueous solution containing lithium ions.

[0002] Lithium (Li) is widely used in batteries such as lithium-ion secondary batteries, in the ceramics industry as an additive for pottery and glass, as a moisture absorbent for air conditioning equipment, and as a flux and grease material in continuous casting. In recent years, lithium consumption has been on the rise due to the growing demand for automotive batteries and other applications.

[0003] Lithium exists on Earth in minerals (e.g., spongemen, petalite, etc.) and brines (e.g., seawater, salt lake water, geothermal brine, hot spring water, etc.). From these, various lithium compounds or metallic lithium are produced, mainly via lithium carbonate. Known methods for recovering lithium from brine include ion exchange, coprecipitation, solvent extraction, solar crystallization, and electrochemical processes.

[0004] As a technology relating to a recovery method using an electrochemical process, for example, Japanese Patent No. 6,818,334 (corresponding to the specification of U.S. Patent Application Publication No. 2019 / 0032230) discloses a lithium permselective membrane having a permselective membrane body mainly composed of a lithium ion conductor and a lithium adsorption layer formed on one main surface of the permselective membrane body. This document also discloses a lithium recovery method in which a raw solution (aqueous solution containing lithium) and a recovery solution (water) are separated by a lithium permselective membrane so that the ion adsorption layer contacts the raw solution, lithium ions are transferred from the raw solution to the recovery solution, and then lithium is recovered from the recovery solution. According to this document, by using a lithium permselective membrane having the above configuration, lithium ions in the raw solution can be recovered into the recovery solution at a high recovery rate.

[0005] However, even with the techniques described in the above documents, the lithium recovery efficiency is still insufficient, and further improvements have been desired.

[0006] Therefore, an object of the present invention is to provide a means for improving the efficiency of lithium recovery.

[0007] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result have found that the above-mentioned problems can be solved by separating water from an aqueous solution containing lithium ions in advance as a gas hydrate and then subjecting the separated water to an electrochemical process, thereby completing the present invention.

[0008] That is, one aspect of the present invention is a method for recovering lithium from a lithium ion-containing aqueous solution, the method comprising: obtaining a mixture of gas hydrate and a lithium ion-enriched aqueous solution from a gas and the lithium ion-containing aqueous solution; separating the gas hydrate from the mixture to obtain the lithium ion-enriched aqueous solution; arranging the lithium ion-enriched aqueous solution on one main surface side of a lithium ion conductor and arranging recovery water on the other main surface side; and allowing lithium ions contained in the lithium ion-enriched aqueous solution to permeate from the one main surface side to the other main surface side to obtain a lithium ion-recovery aqueous solution.

[0009] One aspect of the present invention is a method for recovering lithium from a lithium ion-containing aqueous solution, the method comprising the steps of: obtaining a mixture of gas hydrate and a lithium ion-enriched aqueous solution from a gas and the lithium ion-containing aqueous solution (hereinafter also referred to as a “gas hydrate generation step (A)” or “step (A)”); separating the gas hydrate from the mixture to obtain the lithium ion-enriched aqueous solution (hereinafter also referred to as a “gas hydrate separation step (B)” or “step (B)”); and disposing the lithium ion-enriched aqueous solution on one main surface of a lithium ion conductor and disposing recovery water on the other main surface, and allowing lithium ions contained in the lithium ion-enriched aqueous solution to permeate from the one main surface to the other main surface to obtain a lithium ion-recovery aqueous solution (hereinafter also referred to as a “lithium ion recovery step (C)” or “step (C)”). This aspect can improve lithium recovery efficiency.

[0010] Each step of the recovery method of this embodiment will be described in detail below.

[0011] The present embodiment relates to a method for recovering lithium from a lithium-ion-containing aqueous solution. The lithium-ion-containing aqueous solution to be recovered can be any water containing dissolved lithium ions, such as brine (e.g., seawater, salt lake water, geothermal brine, hot spring water, etc.), a lithium-ion-containing aqueous solution obtained during the recycling process of lithium-containing products (e.g., lithium-ion batteries (LiBs)), concentrated seawater that is a by-product of seawater desalination, or concentrated seawater that is a by-product of soda salt production. The following is an example of a method for obtaining a lithium-ion-containing aqueous solution from LiBs. First, the LiBs are deactivated and crushed, and the casing, resin, and current collector metal are removed. The remaining black mass (concentrated slag containing nickel, manganese, cobalt, lithium, etc.) is brought into contact with water to dissolve the lithium in the water, thereby obtaining a lithium-ion-containing aqueous solution. Other elements than lithium, such as nickel, manganese, and cobalt, are insoluble in water, so they are recovered separately by hydrometallurgy, in which the black mass obtained after dissolving lithium in water is dissolved in an acid solution to extract the metal ions.

[0012] The concentration of lithium ions contained in the lithium ion-containing aqueous solution is not particularly limited, but is preferably 10 －５ mol / kg or more 10 －１ mol / kg or less, more preferably 10 －５ mol / kg or more 10 －２ mol / kg or less, and more preferably 10 －５ mol / kg or more 10 －３ mol / kg or less.

[0013] The form of lithium obtained by the recovery method of this embodiment may be any of lithium ions, lithium compounds, and metallic lithium, and is not particularly limited. In the lithium ion recovery step described below, lithium is recovered as a lithium ion recovery aqueous solution (in the form of an aqueous solution in which lithium ions are dissolved in water). Alternatively, lithium may be recovered after being converted from the lithium ion recovery aqueous solution to various lithium compounds (e.g., lithium carbonate or lithium hydroxide). That is, according to one embodiment of the present invention, there is provided a method for producing a lithium ion-containing aqueous solution, which includes the lithium recovery method of this embodiment. Furthermore, according to another embodiment of the present invention, there is provided a method for producing a lithium compound (e.g., lithium carbonate or lithium hydroxide), which includes the lithium recovery method of this embodiment.

[0014] <Gas Hydrate Formation Step (A)> In step (A), a mixture of gas hydrate and a lithium ion-enriched aqueous solution is obtained from a gas and the lithium ion-containing aqueous solution. Gas hydrate is an ice-like solid substance in which gas molecules are trapped inside a cage-like structure of water molecules. Gas hydrate can be formed by applying low-temperature and high-pressure conditions in the presence of water and gas.

[0015] The gas used in step (A) is not particularly limited, but may be, for example, a hydrocarbon, carbon dioxide (CO ２ ), oxygen (O ２ ), nitrogen (N ２ ), argon (Ar), krypton (Cr), xenon (Xe), hydrogen sulfide (H ２S). Specific examples of hydrocarbons include methane, ethane, propane, n-butane, iso-butane, n-pentane, ethylene, 1-butene, 2-butene, isobutene, cyclopentane, methylcyclopentane, chlorocyclopentane, bromocyclopentane, cyclohexane, methylcyclohexane, tetrahydrofuran, 1,2-dioxolane, 1,3-dioxolane, and the like. These gases may be used alone or in combination of two or more. Among these, from the viewpoint of reducing the cost required for lithium recovery, it is preferable that the gas contains carbon dioxide. Furthermore, since this enables the production of gas hydrate under higher temperature and lower pressure conditions (milder conditions), it is more preferable that the gas contains carbon dioxide and a hydrocarbon, and even more preferable that the gas contains carbon dioxide and cyclopentane, and carbon dioxide (CO ２ ) and cyclopentane are particularly preferred.

[0016] The specific method for producing gas hydrate from the gas and the water in the lithium ion-containing aqueous solution is not particularly limited, and any known method for producing gas hydrate can be appropriately adopted. For example, ２ The following method can be used to generate hydrate: First, a lithium ion-containing aqueous solution is mixed with carbon dioxide to prepare a mixed solution. Next, the mixed solution is subjected to low-temperature and high-pressure conditions to form a CO hydrate. ２ In this case, the mixture may be subjected to low-temperature, high-pressure conditions while continuing to supply carbon dioxide to the mixture. ２ In the phase equilibrium diagram of a hydrate system, there are no particular limitations as long as the temperature and pressure are lower than the phase equilibrium conditions. ２ , gaseous CO ２ , CO ２ The quadruple point at which the four phases of hydrate can coexist is a temperature of 10.5°C and a pressure of 4.46 MPa (44.0 atm).

[0017] In step (A), water in the lithium ion-containing aqueous solution is taken in as gas hydrate, and a lithium ion-enriched aqueous solution in which lithium ions are concentrated is produced. Thus, step (A) produces a mixture of gas hydrate and a lithium ion-enriched aqueous solution.

[0018] <Gas Hydrate Separation Step (B)> In step (B), the gas hydrate is separated from the mixture to obtain a lithium ion-enriched aqueous solution. The specific separation method is not particularly limited, and any known method for solid-liquid separation can be appropriately adopted. If the density of the gas hydrate is lower than the density of the lithium ion-enriched aqueous solution, the sherbet-like gas hydrate will float in the mixture. In this case, they can be easily separated by extracting the gas hydrate from the top of the mixture or extracting the lithium ion-enriched aqueous solution from the bottom of the mixture.

[0019] The concentration of lithium ions contained in the lithium ion concentrated aqueous solution is not particularly limited, but is preferably 10 －４ mol / kg or more, more preferably 10 －４ mol / kg or more and 10 mol / kg or less, and more preferably 10 －３ mol / kg or more and 10 mol / kg or less, and particularly preferably 10 －２ mol / kg or more and 10 mol / kg or less, and most preferably 10 －１ mol / kg or more and 10 mol / kg or less.

[0020] <Lithium ion recovery step (C)> In step (C), the concentrated lithium ion aqueous solution is placed on one main surface side of a lithium ion conductor, recovery water is placed on the other main surface side, and lithium ions contained in the concentrated lithium ion aqueous solution are allowed to permeate from the one main surface side to the other main surface side, thereby obtaining a lithium ion recovery aqueous solution.

[0021] The lithium ion conductor is not particularly limited as long as it has the function of selectively transmitting lithium ions. Examples of such lithium ion conductors include those described in Japanese Patent No. 6818334 (corresponding to the specification of U.S. Patent Application Publication No. 2019 / 0032230) as represented by Formula 1: (Li ｘ , La ｙ ) TiO ｚ (where x=3a-2b, y=2 / 3-a, z=3-b, 0<a≦1 / 6, 0≦b≦0.06, x>0) or lithium lanthanum titanate (LLTO) having a composition of formula 2: Li １＋ｘ＋ｙ Al ｘ (Ti, Ge) ２－ｘ Si ｙ P ３－ｙ O １２ (where 0≦x≦0.6, 0≦y≦0.6) Li-substituted NASICON (Na Super Ionic Conductor) crystals having the composition are known. ０．２９ La ０．５７ TiO ３ LLTO having the composition (in the above formula 1, a≈0.1, b≈0) is preferred.

[0022] In step (C), the lithium ion concentrated aqueous solution is placed on one main surface of the lithium ion conductor, and recovery water is placed on the other main surface. More specifically, for example, in a dialysis tank consisting of two compartments separated by a lithium ion conductor, the lithium ion concentrated aqueous solution is placed in one compartment (hereinafter also referred to as the "first compartment"), and recovery water is placed in the other compartment (hereinafter also referred to as the "second compartment"). Here, from the viewpoint of recovering high-purity lithium, the recovery water is preferably water that contains no or almost no impurities (pure water (resistivity of about 1 to 10 MΩ cm) or ultrapure water (resistivity of more than about 10 MΩ cm)). Pure water and ultrapure water may be RO water, deionized water, or distilled water. The recovery water may also be water obtained by the gas hydrate decomposition step described below, or water obtained by purifying the water to remove impurities (particularly divalent metal ions such as magnesium ions and calcium ions). The use of water derived from gas hydrate is preferable because it eliminates the need to separately procure water for recovery, thereby reducing the cost required for lithium recovery.

[0023] Using the dialysis tank described above, lithium ions contained in the lithium ion concentrated aqueous solution are permeated from the one main surface side to the other main surface side. From the viewpoint of improving the lithium recovery rate, the permeation is preferably carried out in an electrodialysis mode. That is, according to a preferred embodiment, in the step (C), an anode is disposed on the one main surface side (first chamber), a cathode is disposed on the other main surface side (second chamber), and a voltage is applied between the two electrodes. In this case, it is preferable to dispose the anode so as to be in contact with the one main surface, and the cathode so as to be in contact with the other main surface. This makes it possible to significantly improve the migration rate of lithium ions from the one main surface side (first chamber) to the other main surface side (second chamber). Note that, according to electrodialysis, oxygen (O ２ ) and hydrogen (H ２ ) can be produced as a by-product.

[0024] It is also possible to perform dialysis without applying voltage by utilizing the energy of the difference in lithium ion concentration between the one main surface side (first chamber) and the other main surface side (second chamber) (reverse electrodialysis mode). In this case, the lithium ion migration speed is slower than in electrodialysis, but electrical energy can be extracted because a potential difference occurs between the one main surface side (first chamber) and the other main surface side (second chamber).

[0025] The recovery method of this embodiment is characterized in that the lithium ion concentrated aqueous solution obtained by the steps (A) and (B) is used in the step (C). The inventors' investigations have revealed that this configuration accelerates the migration of lithium ions in the lithium ion conductor in the step (C), thereby improving the lithium recovery efficiency. In the past, in lithium recovery methods using lithium ion conductors (such as the aforementioned LLTO and Li-substituted NASICON-type crystals), techniques have been studied in which the applied voltage is increased in electrodialysis mode in order to improve the lithium ion migration rate. However, with these conventional techniques, increasing the voltage sometimes fails to sufficiently improve the lithium ion migration rate. This is thought to be due to the fact that increasing the voltage causes electronic conduction in the lithium ion conductor, resulting in a decrease in current efficiency and an increase in power consumption for water electrolysis. On the other hand, according to the recovery method of this embodiment, by using the lithium ion concentrated aqueous solution in the step (C), the lithium ion migration rate can be significantly improved without increasing the applied voltage. The lithium ion-containing aqueous solution can be concentrated by sun drying or other methods. However, while the sun drying method requires one to two years for concentration, the separation of gas hydrate in steps (A) and (B) allows concentration to be completed in a few days. This makes it possible to recover a larger amount of lithium in a shorter period of time.

[0026] <Other Steps> [Gas Hydrate Decomposition Step] In the recovery method of this embodiment, it is preferable to use water (decomposition water) obtained by decomposing the gas hydrate separated in the step (B) as the recovery water in the step (C). As described above, the recovery water in the step (C) needs to be water that contains no or almost no impurities (pure water, ultrapure water). By using water derived from the gas hydrate generated in the step (A) as the recovery water, it is possible to reduce the procurement cost of the recovery water, and therefore it is possible to further improve the efficiency of lithium recovery.

[0027] The decomposition water can be used as recovery water without being purified, but from the viewpoint of recovering high-purity lithium, it is preferable to use the decomposition water as recovery water after purifying it.

[0028] That is, a preferred embodiment of the method for recovering lithium further includes decomposing the gas hydrate separated in the step (B) to obtain decomposed water, and arranging the decomposed water, with or without purification, on the other main surface side as at least a part of recovery water in the step (C).

[0029] The means for obtaining decomposed water from gas hydrate is not particularly limited, and decomposed water can be easily obtained by subjecting the gas hydrate to conditions of higher temperature and lower pressure than the phase equilibrium conditions of the gas hydrate. Note that, when the gas hydrate is decomposed, gas that was enclosed by water molecules in the gas hydrate is also generated. This gas can be recovered as needed and reused in the above step (A).

[0030] [Decomposition Water Purification Step] The decomposition water obtained in the above step is purified as necessary to recover high-purity lithium, and then reused in step (C). In particular, when brine (e.g., seawater, salt lake water, geothermal brine, hot spring water, etc.) is used as the lithium-containing aqueous solution, divalent alkaline earth metal ions (e.g., magnesium ions, calcium ions, etc.) contained in the brine may adhere to the surface of the gas hydrate, resulting in the alkaline earth metal ions being mixed into the decomposition water. If the decomposition water is used as recovery water as is in step (C), the alkaline earth metal ions may be mixed into the lithium ion recovery aqueous solution, which may cause a decrease in the purity of lithium. In terms of preventing this decrease in purity, when brine is used as the lithium-containing aqueous solution, it is preferable to purify the decomposition water before using it as recovery water.

[0031] The specific method for removing alkaline earth metal ions from decomposition water is not particularly limited, but alkaline earth metal ions can be easily removed from decomposition water by bubbling carbon dioxide gas through the decomposition water to precipitate carbonates of alkaline earth metal ions and then separating the carbonates. Bubbling carbon dioxide gas through decomposition water precipitates alkaline earth metal carbonates, which have low solubility in water. Purified decomposition water can be obtained by separating the precipitated alkaline earth metal carbonates using a known solid-liquid separation method. According to the above purification method, the concentration of alkaline earth metal ions contained in the decomposition water can be reduced to about 1 / 100,000.

[0032] That is, in a preferred embodiment of the lithium recovery method, when the decomposed water obtained in the above step is purified and placed on the other main surface side as at least a part of the recovery water in the step (C), the purification includes bubbling carbon dioxide gas through the decomposed water to precipitate an alkaline earth metal carbonate, and separating the alkaline earth metal carbonate.

[0033] The alkaline earth metal carbonate precipitated by the bubbling of carbon dioxide gas floats in the decomposition water in the form of a fine powder. While this fine powder can be separated by known solid-liquid separation techniques (e.g., filtration), the following technique allows for easier separation. The carbon dioxide gas bubbled in to precipitate the alkaline earth metal carbonate is dissolved in the decomposition water. When the decomposition water is decompressed, the dissolved carbon dioxide gas foams. The carbon dioxide gas bubbles generated by this foaming adhere to the surface of the alkaline earth metal carbonate fine powder, causing the fine powder to float to the surface of the decomposition water. The alkaline earth metal carbonate can be easily separated by removing this floated fine powder from the top of the decomposition water. In this case, the floated fine powder can be removed by a simple separation technique using a dam or weir. The degree of decompression is not particularly limited as long as it is under conditions that allow carbon dioxide gas bubbles to be generated. Therefore, carbon dioxide bubbles may be generated by returning a pressure that was pressurized during the bubbling of carbon dioxide gas to normal pressure, or by reducing the pressure that was normal during the bubbling of carbon dioxide gas. Such a solid-liquid separation technique using carbon dioxide bubbles enables solid-liquid separation in a shorter time than when fine powder is separated by precipitation.

[0034] That is, in a preferred embodiment of the lithium recovery method, the purification includes bubbling carbon dioxide gas into the decomposed water to precipitate alkaline earth metal carbonate, reducing the pressure of the decomposed water to foam the carbon dioxide gas in the decomposed water and generate carbon dioxide bubbles, and separating the alkaline earth metal carbonate that has floated up due to the carbon dioxide bubbles.

[0035] [Lithium Carbonate Precipitation Step] The lithium contained in the lithium ion recovery aqueous solution obtained in the step (C) may be recovered after being further converted into the form of lithium carbonate. The conversion to lithium carbonate can be carried out by bubbling carbon dioxide gas into the lithium ion recovery aqueous solution.

[0036] When gas hydrate-derived water (decomposition water) is used as the recovery water in step (C), alkali metal ions other than lithium ions (e.g., sodium ions, potassium ions, etc.) contained in brine (e.g., seawater, salt lake water, geothermal brine, hot spring water, etc.) may be mixed into the recovery water. From the viewpoint of obtaining high-purity lithium carbonate, when carbon dioxide gas is bubbled into the lithium ion recovery aqueous solution, it is preferable to control the concentration of carbonate ions in the lithium ion recovery aqueous solution so that lithium carbonate precipitates and carbonates of alkali metal ions other than lithium do not precipitate. That is, a preferred lithium recovery method further includes bubbling carbon dioxide gas into the lithium ion recovery aqueous solution obtained in step (C) to precipitate lithium carbonate, and controlling the concentration of carbonate ions in the lithium ion recovery aqueous solution so that lithium carbonate precipitates and carbonates of alkali metal ions other than lithium do not precipitate. The control of the carbonate ion concentration in this case is not particularly limited, and can be performed by the following method. That is, carbonate ions are bubbled while monitoring the concentration of carbonate ions in the lithium ion recovery aqueous solution so that the concentration is equal to or greater than the solubility product of lithium carbonate and less than the solubility product of carbonates of alkali metal ions other than lithium ions (e.g., sodium carbonate, potassium carbonate, etc.). The concentration of alkali metal ions in the lithium ion recovery aqueous solution before bubbling with carbonate ions can be confirmed by high-frequency inductively coupled plasma (ICP) atomic emission spectroscopy. The concentrations of carbonate ions and alkali metal ions during carbonate ion bubbling can be monitored using a water quality measuring device based on an ion electrode method (e.g., a portable pH meter HM-40P manufactured by DKK-TOA Corporation). By bubbling carbon dioxide gas while controlling the carbonate ion concentration in this manner, high-purity lithium carbonate can be obtained.

[0037] The following embodiments are also within the scope of the present invention: the method according to claim 1 with the features of claim 2; the method according to claim 2 with the features of claim 3; the method according to any one of claims 1 to 3 with the features of claim 4; the method according to claim 3 with the features of claim 5; and the method according to claim 5 with the features of claim 6.

Claims

1. A method for recovering lithium from a lithium ion-containing aqueous solution, comprising: obtaining a mixture of gas hydrate and a lithium ion-enriched aqueous solution from a gas and the lithium ion-containing aqueous solution; separating the gas hydrate from the mixture to obtain the lithium ion-enriched aqueous solution; arranging the lithium ion-enriched aqueous solution on one main surface side of a lithium ion conductor and arranging recovery water on the other main surface side, and allowing lithium ions contained in the lithium ion-enriched aqueous solution to permeate from the one main surface side to the other main surface side to obtain a lithium ion-recovery aqueous solution.

2. The method for recovering lithium according to claim 1, further comprising: decomposing the separated gas hydrate to obtain decomposed water; and disposing the decomposed water, with or without purification, on the other main surface side as at least a part of the recovery water.

3. The method for recovering lithium according to claim 2, wherein, in a case where the decomposition water is purified and placed on the other main surface side as at least a part of the recovery water, the purification comprises: bubbling carbon dioxide gas into the decomposition water to precipitate an alkaline earth metal carbonate; and separating the alkaline earth metal carbonate.

4. The method for recovering lithium according to claim 2 or 3, further comprising bubbling carbon dioxide gas into the lithium ion recovery aqueous solution to precipitate lithium carbonate, and controlling the concentration of carbonate ions in the lithium ion recovery aqueous solution so that lithium carbonate is precipitated and carbonates of alkali metal ions other than lithium are not precipitated.

5. The method for recovering lithium as described in claim 3, wherein the purification step includes the steps of: bubbling carbon dioxide gas into the decomposed water to precipitate alkaline earth metal carbonates, reducing the pressure of the decomposed water to cause the carbon dioxide gas in the decomposed water to foam and generate carbon dioxide bubbles; and separating the alkaline earth metal carbonates that have risen to the surface due to the carbon dioxide bubbles.

6. The method for recovering lithium according to claim 5, further comprising bubbling carbon dioxide gas into the lithium ion recovery aqueous solution to precipitate lithium carbonate, and controlling the concentration of carbonate ions in the lithium ion recovery aqueous solution so that lithium carbonate precipitates and carbonates of alkali metal ions other than lithium do not precipitate.