Method for Recovering Lithium from Aqueous Lithium Salt Solution with Low Concentration

Abstract

To provide a method with which it is possible to produce a high-purity lithium hydroxide or lithium salt with a high yield in a short time from an aqueous lithium salt solution that has low lithium concentration.SOLUTION: A method for recovering lithium from low concentration aqueous lithium salt solution comprises: a lithium extraction step for having lithium in the low concentration aqueous lithium salt solution adsorbed onto a lithium adsorbent, and eluting lithium in the lithium adsorbent that has adsorbed lithium by means of a mineral acid to obtain a first aqueous lithium salt solution from the low concentration aqueous lithium salt solution; a purification step for purifying the first aqueous lithium salt solution to obtain a third aqueous lithium salt solution; an evaporative concentration step for evaporating and concentrating the third aqueous lithium salt solution to obtain a fourth aqueous lithium salt solution; and a membrane electrolysis step for subjecting the fourth aqueous lithium salt solution to membrane electrolysis to obtain an aqueous lithium hydroxide solution, a mineral acid, and a fifth aqueous lithium salt solution that is more dilute than the fourth aqueous lithium salt solution.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for recovering lithium from an aqueous solution of a low-concentration lithium salt.

Background Art

[0002] In recent years, lithium has attracted attention as a raw material for lithium-ion batteries such as lithium-ion secondary batteries. As its sources, in addition to those recycled from waste lithium batteries, minerals, brine, seawater, etc. are known. The brine is obtained from natural salt lakes and usually contains lithium in the form of lithium chloride. The concentration of lithium contained in the brine is an aqueous solution of a low-concentration lithium salt of about 1 g / L.

[0003] Therefore, the brine obtained from natural salt lakes is supplied to an open evaporation pond and naturally evaporated and concentrated over one year or more to remove impurities such as Mg, Ca, and B, and then reacted with a carbon source to be recovered as lithium carbonate. However, the method of concentrating the brine by natural evaporation requires a long time for concentrating the brine, is easily affected by natural conditions such as weather, and furthermore, lithium forms salts with other impurities and is lost during the concentration process. When sulfate is dissolved in the brine, lithium sulfate has a low solubility in water, and lithium in the brine precipitates as lithium sulfate, so there is a problem that lithium cannot be completely recovered from the brine. In addition, a large amount of water is required to recover lithium from brine, and in water-scarce regions, there is a problem of causing environmental pollution such as water depletion and contamination.

[0004] On the other hand, a method for producing lithium hydroxide is disclosed in Patent Document 1, which includes a neutralization step of adding sodium hydroxide to a first lithium chloride-containing liquid such as a liquid obtained by selectively adsorbing and separating lithium from brine to obtain a neutralized post-liquid, an ion exchange step of bringing the neutralized post-liquid into contact with an ion exchange resin to obtain a second lithium chloride-containing liquid, and a conversion step of subjecting the second lithium chloride-containing liquid to electrodialysis to obtain a lithium hydroxide-containing liquid.

[0005] The electrodialysis mentioned above is bipolar membrane electrodialysis. In bipolar membrane electrodialysis, hydrochloric acid and lithium hydroxide are directly generated from a purified aqueous lithium chloride solution. However, since the concentration of each of the generated hydrochloric acid and lithium hydroxide is several percent, when the hydrochloric acid generated by bipolar membrane electrodialysis is used as a lithium eluent, the lithium concentration of the lithium eluent does not become sufficiently high. Furthermore, since the concentration of the aqueous lithium hydroxide solution generated by bipolar membrane electrodialysis is also low, it is necessary to evaporate a large amount of water in the lithium hydroxide crystallization step.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] In the second lithium chloride-containing liquid to which electrodialysis is applied in the conversion step of the method for producing lithium hydroxide disclosed in Patent Document 1, sodium hydroxide is used in the neutralization step and the ion exchange step. Since the second lithium chloride-containing liquid contains sodium, there are problems that sodium hydroxide is mixed into the lithium hydroxide obtained by electrodialyzing the second lithium chloride-containing liquid, and that wasteful power is used for the movement of sodium.

[0008] The problem to be solved by the present invention is to provide a method for producing a lithium salt with high purity, high yield, low cost, and no environmental pollution in a short time from an aqueous lithium salt solution with a low lithium concentration.

Means for Solving the Problems

[0009] In view of the above problems, the inventors have conducted extensive studies and found that lithium in a low-concentration lithium salt aqueous solution can be adsorbed onto a lithium adsorbent, the lithium in the lithium adsorbent adsorbed with lithium can be eluted with a mineral acid, and a first lithium salt aqueous solution can be obtained from the low-concentration lithium salt aqueous solution. By subjecting the first lithium salt aqueous solution to a purification step, an evaporation concentration step, and a membrane electrolysis step in this order, it is possible to produce a lithium salt with high purity, high yield, and low cost in a short time from a lithium salt aqueous solution with a low lithium concentration without causing environmental pollution. The present invention has been completed based on these findings.

[0010] The present invention relates to a method for recovering lithium from a low-concentration lithium salt aqueous solution, which comprises a step of adsorbing lithium in the low-concentration lithium salt aqueous solution onto a lithium adsorbent, eluting the lithium in the lithium adsorbent adsorbed with lithium with a mineral acid, and obtaining a first lithium salt aqueous solution from the low-concentration lithium salt aqueous solution. Following the step of obtaining the first lithium salt aqueous solution from the low-concentration lithium salt aqueous solution, The method for recovering lithium from the low-concentration lithium salt aqueous solution further comprises a purification step of purifying the first lithium salt aqueous solution to obtain a third lithium salt aqueous solution, an evaporation concentration step of evaporating and concentrating the third lithium salt aqueous solution to obtain a fourth lithium salt aqueous solution, and a membrane electrolysis step of subjecting the fourth lithium salt aqueous solution to membrane electrolysis to obtain an aqueous lithium hydroxide solution, a mineral acid, and a fifth lithium salt aqueous solution that is more dilute than the fourth lithium salt aqueous solution.

[0011] The method for recovering lithium from the low-concentration lithium salt aqueous solution preferably further comprises a pretreatment step of adding an amount of alkali that does not cause precipitation to the low-concentration lithium salt aqueous solution. When the low-concentration lithium salt aqueous solution is brine derived from at least one selected from the group consisting of salt lakes, seawater, and brackish water, the method may further comprise a returning step of returning the post-adsorption liquid obtained by adsorbing lithium onto the lithium adsorbent to at least one selected from the group consisting of salt lakes, seawater, and brackish water.

[0012] The elution of lithium in the lithium adsorbent is preferably carried out by contacting the lithium adsorbent with a mineral acid containing the mineral acid obtained in the membrane electrolysis step. The purification step preferably includes a first purification step of adding an aqueous lithium hydroxide solution and an aqueous lithium carbonate solution to the first lithium salt aqueous solution to remove calcium, magnesium, and manganese in the first lithium salt aqueous solution and obtain a second lithium salt aqueous solution. The purification step more preferably further includes a second purification step of bringing the second lithium salt aqueous solution into contact with a chelating agent to remove calcium, magnesium, and manganese in the second lithium salt aqueous solution and obtain a third lithium salt aqueous solution. The method for recovering lithium from the low-concentration lithium salt aqueous solution preferably includes a third purification step of treating residual chlorine and chloric acid generated by electrodialysis in the fifth lithium salt aqueous solution obtained in the electrodialysis step with a mineral acid to obtain a sixth lithium salt aqueous solution, and the sixth lithium salt aqueous solution is subjected to evaporation concentration together with the third lithium salt aqueous solution in the evaporation concentration step.

[0013] The method for recovering lithium from the low-concentration lithium salt aqueous solution preferably further includes a productization step of subjecting the aqueous lithium hydroxide solution obtained in the electrodialysis step to at least one selected from the group consisting of crystallization and carbonation. The carbon dioxide gas used in the carbonation preferably includes carbon dioxide gas generated by absorbing carbon dioxide from the atmosphere. The mineral acid preferably includes the mineral acid obtained in the electrodialysis step. The aqueous lithium hydroxide solution preferably includes the aqueous lithium hydroxide solution obtained in the electrodialysis step, and the aqueous lithium carbonate solution includes an aqueous lithium carbonate solution obtained by carbonating the aqueous lithium hydroxide solution obtained in the electrodialysis step.

[0014] The distilled water generated in the evaporation concentration step is preferably used as at least one selected from the group consisting of the eluent used in the Obtaining the first lithium salt aqueous solution from the low-concentration lithium salt aqueous solution step, the catholyte of the electrodialysis step, and the water for dissolving the mineral acid obtained in the electrodialysis step. The distilled water generated in the productization step is preferably the Obtaining the first lithium salt aqueous solution from the low-concentration lithium salt aqueous solutionIt is used as at least one selected from the group consisting of an eluent used in the process, the catholyte in the membrane electrolysis process, and the dissolved water of the mineral acid obtained in the membrane electrolysis process. The mineral acid used in the lithium extraction process preferably contains at least one selected from the group consisting of hydrochloric acid, sulfuric acid, and nitric acid, and more preferably contains hydrochloric acid. The electric power used in the membrane electrolysis process preferably includes electric power obtained from renewable energy, and more preferably includes electric power obtained by at least one selected from the group consisting of solar power generation, wind power generation, hydropower generation, and biomass power generation.

Advantages of the Invention

[0015] The method for recovering lithium from the low-concentration lithium salt aqueous solution of the present invention provides a method capable of producing a high-purity lithium salt in a short time, with a high yield and low cost, and without causing environmental pollution, from the lithium salt aqueous solution having a low lithium concentration.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0017] The present invention will be described in more detail. In addition, "~" in the numerical range represents "from... to..." and includes both end values unless otherwise specified. When a numerical range is indicated, the upper limit value and the lower limit value can be appropriately combined, and the numerical range thus obtained is also disclosed. In the following description of the drawings, the same elements are denoted by the same reference numerals, and redundant descriptions are omitted. Also, the dimensional ratios in the drawings are exaggerated for convenience of explanation and may be different from the actual ratios.

[0018] The embodiments of the present invention will be described in more detail with reference to the accompanying drawings. <Lithium extraction step> As shown in FIG. 1, the method for recovering lithium from the low-concentration lithium salt aqueous solution of the embodiment (hereinafter sometimes referred to as the lithium recovery method) uses the low-concentration lithium salt aqueous solution 1 as a starting material. The lithium content in the low-concentration lithium salt aqueous solution 1 is 2 mol / kg or less. The low-concentration lithium salt aqueous solution 1 is preferably brine derived from at least one selected from the group consisting of salt lakes, seawater, and brackish water, more preferably brine derived from a salt lake.

[0019] The low-concentration lithium salt aqueous solution 1 may be an aqueous solution obtained from a solid containing lithium such as active material powder.

[0020] The active material powder will be described. When the used lithium-ion battery is a used lithium-ion battery whose battery life as a battery product has expired, or a lithium-ion battery discarded as a defective product or the like in the manufacturing process, first, a discharge treatment is performed. Various highly safe methods such as resistive discharge can be adopted for the discharge treatment. All the remaining charges are discharged by the discharge, and then, after forming an opening in the casing of the waste lithium-ion battery, for example, after heat treatment (roasting) at a temperature in the range of 100 to 800 ° C, or without heat treatment, it is pulverized with a pulverizer such as a hammer mill or a jaw crusher, and the active material powder can be obtained by removing (classifying) the casing, current collector, etc. constituting the waste lithium-ion battery by sieving. Alternatively, the waste lithium-ion battery after the discharge treatment may be pulverized with the pulverizer, the casing, current collector, etc. may be removed by sieving, and then heat treatment may be performed at the temperature in the above range to obtain the active material powder.

[0021] When the waste lithium-ion battery is the remaining positive electrode material or the like used for commercialization in the manufacturing process, the discharge treatment and the formation of the opening are not performed, and after heat treatment at the temperature within the above range, or without heat treatment, it may be pulverized by the pulverizer, and the current collector and the like are removed by sieving to obtain the active material powder. Further, the waste lithium-ion battery may be pulverized by the pulverizer, the current collector and the like are removed by sieving, and then heat treatment is performed at the temperature within the above range, or the active material powder may be obtained without heat treatment.

[0022] Furthermore, the solid containing lithium may be spodumene, which is a kind of silicate mineral containing lithium and aluminum.

[0023] The solid containing lithium is dissolved in a mineral acid to obtain an acid-dissolved solution of the solid containing lithium. The solid containing lithium may contain valuable metals such as iron, aluminum, manganese, cobalt, and nickel in addition to lithium. The mineral acid used for dissolving the solid containing lithium preferably contains at least one selected from the group consisting of hydrochloric acid, sulfuric acid, and nitric acid, more preferably contains hydrochloric acid, and still more preferably is hydrochloric acid.

[0024] Next, the acid-dissolved solution is neutralized by adding lithium hydroxide (LiOH). The neutralized acid-dissolved solution is then subjected to solvent extraction. In the solvent extraction, among the valuable metals, manganese, cobalt, and nickel excluding lithium are separately solvent-extracted, or iron and aluminum are separated and removed as aqueous solutions of their respective metal sulfates, and the low-concentration lithium salt aqueous solution 1 is obtained.

[0025] The low-concentration lithium salt aqueous solution 1 may be brine derived from at least one selected from the group consisting of salt lakes, seawater, and brackish water.

[0026] <Lithium adsorption step> The low-concentration lithium salt aqueous solution 1 is preferably stored in a storage tank, an amount of alkali that does not cause precipitation is added to the low-concentration lithium salt aqueous solution 1, and the solution is sent to an adsorption tower filled with a lithium adsorbent. The adsorption tower is preferably of a column type. In the adsorption tower, lithium and hydrogen ions are exchanged, and the resulting solution exhibits acidity and is returned to the storage tank. The above operations are repeated, and lithium in the low-concentration lithium salt aqueous solution 1 is adsorbed by the lithium adsorbent. Examples of the alkali added to the low-concentration lithium salt aqueous solution 1 include sodium hydroxide, potassium hydroxide, etc. From the perspective of cost, the preferred alkali is sodium hydroxide.

[0027] The storage amount (kg) of the low-concentration lithium salt aqueous solution 1 in the storage tank is adjusted to satisfy the following formula (1). Storage amount (kg) ≦ [Lithium adsorbent amount (kg) × Lithium adsorption capacity of lithium adsorbent (g / kg)] / Lithium concentration of low-concentration lithium salt aqueous solution 1 (g / kg) ··· (1) When the formula (1) is satisfied, lithium ions in the low-concentration lithium salt aqueous solution 1 are efficiently adsorbed by the lithium adsorbent.

[0028] As the lithium adsorbent, known lithium manganate spinel may be used. The lithium adsorbent is produced, for example, by the production method described in Japanese Patent No. 3937865.

[0029] <Elution step> In STEP1, the adsorbent that has adsorbed lithium in the low-concentration lithium salt aqueous solution 1 has a mineral acid added thereto in STEP2, and the lithium adsorbed on the adsorbent is eluted. The distilled water generated in the evaporation concentration step and the productization step described later, and the mineral acid obtained in the membrane electrolysis step described later are added to the eluate tank, and are liquid-fed to an adsorption tower filled with the adsorbent on which the lithium is adsorbed. In the adsorption tower, lithium and hydrogen ions are exchanged, and the resulting solution exhibits weak acidity and returns to the eluate tank. The above operations are repeated, and the lithium in the low-concentration lithium salt aqueous solution 1 is eluted from the lithium adsorbent. The mineral acid preferably contains at least one selected from the group consisting of hydrochloric acid, sulfuric acid, and nitric acid, more preferably contains hydrochloric acid, and still more preferably is hydrochloric acid. When the low-concentration lithium salt aqueous solution 1 is an aqueous lithium chloride solution, the hydrochloric acid concentration obtained in the membrane electrolysis step described later can be increased to 35% by mass, so the lithium concentration in the eluate can also be increased. On the other hand, the hydrochloric acid concentration obtained by electrodialysis described in Patent Document 1 is only a few percent.

[0030] When the low-concentration lithium salt aqueous solution 1 is brine derived from at least one selected from the group consisting of salt lakes, seawater, and brackish water, the lithium recovery method of the embodiment may include a returning step of returning the post-adsorption liquid obtained by adsorbing lithium on the lithium adsorbent to at least one selected from the group consisting of salt lakes, seawater, and brackish water.

[0031] <Purification process> The lithium recovery method of the above embodiment includes a purification step of purifying the first lithium salt aqueous solution to obtain a third lithium salt aqueous solution in STEP3. The purification step preferably includes adding an aqueous lithium hydroxide solution and an aqueous lithium carbonate solution to the first lithium salt aqueous solution to remove calcium, magnesium, and manganese 2 in the first lithium salt aqueous solution, and includes a first purification step of obtaining a second lithium salt aqueous solution. The content of impurities in the second lithium salt aqueous solution is several ppm. At least a part of the aqueous lithium hydroxide solution used in the first purification step may be the aqueous lithium hydroxide solution obtained in the membrane electrolysis step described later. Further, at least a part of the aqueous lithium carbonate solution used in the first purification step may be the aqueous lithium carbonate solution of the residual liquid obtained by carbonating the aqueous lithium hydroxide solution obtained in the membrane electrolysis step described later. When the first lithium salt aqueous solution is an aqueous lithium chloride solution, in the first purification step, the reactions represented by the following formulas (2) to (4) occur. 2LiOH + MgCl2 → Mg(OH)2↓ + 2LiCl ··· (2) 2LiOH + MnCl2 → Mn(OH)2↓ + 2LiCl ··· (3) Li2CO3 + CaCl2 → CaCO3↓ + 2LiCl ····· (4)

[0032] The purification step preferably further includes a second purification step of bringing the second lithium salt aqueous solution into contact with a chelating agent to remove trace amounts of calcium, magnesium, and manganese 2 in the second lithium salt aqueous solution to obtain the third lithium salt aqueous solution. Impurities that were not removed in the first purification step are removed by a chelating resin. The content of impurities in the third lithium salt aqueous solution is less than 100 ppb. Examples of the chelating resin include Diaion CR11 manufactured by Mitsubishi Chemical Corporation. Since other heavy metals are also removed by the chelating resin, the third lithium salt aqueous solution does not contain heavy metals, and the purity of lithium hydroxide in the third lithium salt aqueous solution becomes very high. Note that the CR11 adsorbs metals with a valence of 2 or more and is less likely to capture monovalent metals. Also, since calcium, magnesium, and manganese are removed in the first purification step, the load on the expensive chelating resin used in the second purification step is reduced.

[0033] Since lithium hydroxide is used in the first purification step, sodium does not mix into the third lithium salt, the purity of the product obtained in the productization step described later is increased, and the efficiency of the membrane electrolysis step described later is also improved.

[0034] <Evaporation concentration step> The lithium recovery method of the above embodiment includes an evaporation concentration step in STEP4 of evaporating and concentrating the third lithium salt aqueous solution to obtain a fourth lithium salt aqueous solution. The third lithium salt aqueous solution is preferably subjected to the evaporation concentration step together with the residual chlorine generated in the membrane electrolysis step described later and the sixth lithium salt aqueous solution obtained by treating chloric acid with a mineral acid. Examples of the evaporation concentration device used in the evaporation concentration step include a multiple-effect can evaporation device. Further, the distilled water 7 generated in the evaporation concentration step may be used as at least one selected from the group consisting of the eluent used in the lithium adsorption step, the cathode liquid of the membrane electrolysis step described later, and the dissolution water of the mineral acid obtained in the membrane electrolysis step described later.

[0035] <Membrane electrolysis step> In the lithium recovery method of the above embodiment, in STEP5, the fourth lithium salt aqueous solution is subjected to membrane electrolysis to obtain an aqueous lithium hydroxide solution, a mineral acid 4, and a fifth lithium salt aqueous solution that is more dilute than the fourth lithium salt aqueous solution, including a membrane electrolysis step. The membrane electrolysis step of STEP5 can be performed using, for example, the membrane electrolysis cell 11 shown in FIG. 2.

[0036] The membrane electrolysis cell 11 is provided with an anode plate 12 on one inner surface, a cathode plate 13 on the inner surface facing the anode plate 12. The anode plate 12 is connected to the anode 14 of the power supply, and the cathode plate 13 is connected to the cathode 15 of the power supply. Further, the membrane electrolysis cell 11 is partitioned by an ion exchange membrane 16 into an anode chamber 17 provided with the anode plate 12 and a cathode chamber 18 provided with the cathode plate 13.

[0037] In the membrane electrolysis cell 11, when, for example, lithium chloride is supplied as the fourth lithium salt aqueous solution to the anode chamber 17 for electrolysis, chloride ions generate residual chlorine (Cl2) on the anode plate 12, while lithium ions move to the cathode chamber 18 through the ion exchange membrane 16.

[0038] In the cathode chamber 18, the cathode liquid (H2O) ionizes into hydroxide ions (OH - ) and hydrogen ions (H + ). Hydrogen ions generate hydrogen gas (H2) on the cathode plate 13, while hydroxide ions combine with lithium to generate an aqueous lithium hydroxide solution 3. At least a part selected from the group consisting of the distilled water 7 and the distilled water 8 generated by crystallization described later may be used as the cathode liquid.

[0039] The electric power used in the membrane electrolysis step preferably includes electric power obtained from renewable energy, and more preferably includes electric power obtained by at least one selected from the group consisting of solar power generation, wind power generation, hydroelectric power generation, and biomass power generation.

[0040] In the above-described embodiment, hydrochloric acid 4 can be obtained by reacting the hydrogen gas (H2) generated in the membrane electrolysis step with residual chlorine (Cl2), and the hydrochloric acid 4 can be used for dissolving the solid 1 in STEP1. Although not shown in FIG. 2, when the fourth lithium salt aqueous solution contains sulfate ions, sulfuric acid can be obtained in the anode chamber 17. When the fourth lithium salt aqueous solution contains nitrate ions, nitric acid can be obtained in the anode chamber 17. That is, mineral acid 4 can be obtained in the electrolysis step, and the mineral acid 4 may be used for at least one selected from the group consisting of dissolution of the active material powder and the third purification step.

[0041] <Productization process> The lithium hydroxide aqueous solution 3 obtained in the membrane electrolysis step can also be recovered as lithium hydroxide monohydrate (LiOH·H2O) 5 by crystallization in STEP6, and can also be recovered as lithium carbonate (Li2CO3) 6 by carbonation in STEP7. Lithium hydroxide monohydrate (LiOH·H2O) 5 and lithium carbonate (Li2CO3) 6 can also be recovered by subjecting the crystallization and carbonation thereto. At the time of the crystallization, distilled water 8 is generated. The carbonation may be carried out, for example, by reacting the lithium hydroxide aqueous solution 3 with carbon dioxide gas (CO2) generated by absorbing carbon dioxide from the atmosphere.

[0042] The lithium hydroxide component obtained in the membrane electrolysis step is a high-purity lithium hydroxide aqueous solution containing no impurities. Therefore, lithium hydroxide monohydrate obtained by crystallization and lithium carbonate obtained by adding carbon dioxide gas become high-purity lithium salts. Lithium carbonate is slightly soluble in water (the solubility of lithium carbonate in water at 20°C is 1.31% by mass, 2.46 g / kg in terms of Li amount). The water generated in the productization process is reused in at least one selected from the group consisting of elution of lithium in the lithium adsorbent with mineral acid and dilution of the mineral acid generated in the membrane electrolysis step, and water circulates in the method for recovering lithium from the low-concentration lithium salt aqueous solution of the present invention. Therefore, the addition of water from outside the system can be minimized, and the method for recovering lithium from the low-concentration lithium salt aqueous solution of the present invention is environmentally friendly. Furthermore, the mineral acid obtained in the membrane electrolysis step is reused in at least one selected from the group consisting of the elution of lithium and the third purification step, and the lithium carbonate obtained in the productization step is reused in the first purification step. Therefore, the method for recovering lithium from the low-concentration lithium salt aqueous solution of the present invention can reduce chemical costs, and by circulating the chemical solution, salts are not discharged, which is environmentally friendly.

[0043] When the mineral acid is hydrochloric acid, in the membrane electrolysis step, chlorate is generated and accumulated by hydroxide ions that back-diffuse from the cathode chamber 18. The accumulated chlorate causes deterioration of the quality of lithium in the membrane electrolysis of the lithium hydroxide aqueous solution, deterioration of the equipment material, and deterioration of the ion exchange resin. Therefore, the lithium recovery method of the embodiment preferably includes, as STEP8, a third purification step of treating residual chlorine and chloric acid generated by membrane electrolysis in the fifth lithium salt aqueous solution obtained in the membrane electrolysis step with a mineral acid to obtain a sixth lithium salt aqueous solution. The sixth lithium salt aqueous solution may be evaporated and concentrated together with the third lithium salt aqueous solution in the evaporation and concentration step. This operation contributes to an improvement in the lithium recovery rate. Furthermore, at least a part of the mineral acid used in the third purification step may be the mineral acid obtained in the membrane electrolysis step.

[0044] In the third purification step, the cathode liquid in the cathode chamber 17 that has been subjected to membrane electrolysis in the membrane electrolysis step is brought into contact with air (aerated) to remove gas, and the residual chlorine concentration in the cathode liquid is made extremely small. Furthermore, a part of the cathode liquid is treated with the mineral acid to obtain the sixth lithium salt aqueous solution (see the following formula (2)). LiClO3 + 6HCl → LiCl + 3Cl2 + 3H2O ··· (2) When the method for recovering lithium from the low-concentration lithium salt aqueous solution of the present invention includes the third purification step and the lithium salt aqueous solution is a lithium chloride aqueous solution, the hydrochloric acid concentration obtained in the membrane electrolysis step can be 35% by mass, and the lithium hydroxide concentration in the lithium hydroxide aqueous solution can be 5 to 11% by mass.

Explanation of Symbols

[0045] 1···Low-concentration lithium salt aqueous solution, 2···Calcium, magnesium and manganese 3···Lithium hydroxide aqueous solution, 4···Mineral acid or hydrochloric acid 5···Lithium hydroxide monohydrate, 6···Lithium carbonate, 11…Electrolytic cell 12…Anode plate, 13…Cathode plate, 14…Anode, 15…Cathode, 16…Ion exchange membrane 17…Anode chamber, 18…Cathode chamber

Claims

1. A method for recovering lithium from a low-concentration lithium salt aqueous solution, comprising: adsorbing lithium in the low-concentration lithium salt aqueous solution onto a lithium adsorbent, eluting lithium in the lithium adsorbent adsorbed with lithium with a mineral acid, and obtaining a first lithium salt aqueous solution from the low-concentration lithium salt aqueous solution; subsequent to the step of obtaining the first lithium salt aqueous solution from the low-concentration lithium salt aqueous solution, a purification step of purifying the first lithium salt aqueous solution to obtain a third lithium salt aqueous solution; an evaporation concentration step of evaporating and concentrating the third lithium salt aqueous solution to obtain a fourth lithium salt aqueous solution; and a membrane electrolysis step of subjecting the fourth lithium salt aqueous solution to membrane electrolysis to obtain an aqueous lithium hydroxide solution, a mineral acid, and a fifth lithium salt aqueous solution that is more dilute than the fourth lithium salt aqueous solution, the method for recovering lithium from a low-concentration lithium salt aqueous solution.

2. A method for recovering lithium from a low-concentration lithium salt aqueous solution according to Claim 1, further comprising a pretreatment step of adding an amount of alkali that does not cause precipitation to the low-concentration lithium salt aqueous solution.

3. A method for recovering lithium from a low-concentration lithium salt aqueous solution according to Claim 1, wherein the elution of lithium in the lithium adsorbent is carried out by contacting the lithium adsorbent with a mineral acid containing the mineral acid obtained in the membrane electrolysis step.

4. A method for recovering lithium from a low-concentration lithium salt aqueous solution according to Claim 1, wherein the purification step includes a first purification step of adding an aqueous lithium hydroxide solution and an aqueous lithium carbonate solution to the first lithium salt aqueous solution to remove calcium, magnesium, and manganese in the first lithium salt aqueous solution and obtain a second lithium salt aqueous solution.

5. A method for recovering lithium from a low-concentration lithium salt aqueous solution according to Claim 4, wherein the purification step further includes a second purification step of contacting the second lithium salt aqueous solution with a chelating agent to remove calcium, magnesium, and manganese in the second lithium salt aqueous solution and obtain the third lithium salt aqueous solution.

6. ​ ​ A method for recovering lithium from a low-concentration lithium salt aqueous solution according to claim 1, comprising a third purification step of treating residual chlorine and chloric acid generated by membrane electrolysis in the fifth lithium salt aqueous solution obtained in the membrane electrolysis step with a mineral acid to obtain a sixth lithium salt aqueous solution, and evaporating and concentrating the sixth lithium salt aqueous solution together with the third lithium salt aqueous solution in the evaporation and concentration step. A method for recovering lithium from a low-concentration lithium salt aqueous solution.

7. A method for recovering lithium from a low-concentration lithium salt aqueous solution according to claim 1, further comprising a productization step of subjecting the lithium hydroxide aqueous solution obtained in the membrane electrolysis step to at least one selected from the group consisting of crystallization and carbonation. A method for recovering lithium from a low-concentration lithium salt aqueous solution.

8. A method for recovering lithium from a low-concentration lithium salt aqueous solution according to claim 7, wherein the carbon dioxide gas used in the carbonation contains carbon dioxide gas generated by absorbing carbon dioxide from the atmosphere. A method for recovering lithium from a low-concentration lithium salt aqueous solution.

9. In the method for recovering lithium from a low-concentration lithium salt aqueous solution according to claim 6, the mineral acid includes the mineral acid obtained in the membrane electrolysis step. A method for recovering lithium from a low-concentration lithium salt aqueous solution

10. In the method for recovering lithium from a low-concentration lithium salt aqueous solution according to claim 4, the lithium hydroxide aqueous solution includes the lithium hydroxide aqueous solution obtained in the membrane electrolysis step, and the lithium carbonate aqueous solution includes a lithium carbonate aqueous solution obtained by carbonating the lithium hydroxide aqueous solution obtained in the membrane electrolysis step. A method for recovering lithium from a low-concentration lithium salt aqueous solution.

11. A method for recovering lithium from a low-concentration lithium salt aqueous solution according to claim 1, wherein the distilled water generated in the evaporation and concentration step is used as at least one selected from the group consisting of dilution water of the mineral acid used in the step of obtaining the first lithium salt aqueous solution from the low-concentration lithium salt aqueous solution, the cathode liquid of the membrane electrolysis step, and dilution water of the mineral acid obtained in the membrane electrolysis step. A method for recovering lithium from a low-concentration lithium salt aqueous solution.

12. A method for recovering lithium from a low-concentration lithium salt aqueous solution according to claim 7, wherein the distilled water generated in the commercialization step is at least one selected from the dilution water of the mineral acid used in the step of obtaining a first lithium salt aqueous solution from the low-concentration lithium salt aqueous solution, the catholyte of the electrodialysis step, and the dilution water of the mineral acid obtained in the electrodialysis step, and is used as such, a method for recovering lithium from a low-concentration lithium salt aqueous solution.

13. A method for recovering lithium from a low-concentration lithium salt aqueous solution according to claim 1, wherein the mineral acid contains at least one selected from the group consisting of hydrochloric acid, sulfuric acid, and nitric acid, a method for recovering lithium from a low-concentration lithium salt aqueous solution.

14. A method for recovering lithium from a low-concentration lithium salt aqueous solution according to claim 1, wherein the mineral acid contains hydrochloric acid, a method for recovering lithium from a low-concentration lithium salt aqueous solution.

15. A method for recovering lithium from a low-concentration lithium salt aqueous solution according to any one of claims 1 to 14, wherein the electric power used in the electrodialysis step includes electric power obtained from renewable energy, a method for recovering lithium from a low-concentration lithium salt aqueous solution.

16. A method for recovering lithium from a low-concentration lithium salt aqueous solution according to claim 15, wherein the electric power obtained from renewable energy includes electric power obtained by at least one selected from the group consisting of solar power generation, wind power generation, hydropower generation, and biomass power generation, a method for recovering lithium from a low-concentration lithium salt aqueous solution.

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