Method for recovering lithium from low concentration aqueous lithium salt solution

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 a low-concentration aqueous lithium salt solution. [Background technology]

[0002] In recent years, lithium has been attracting attention as a raw material for lithium-ion batteries, such as lithium-ion secondary batteries. Known sources of lithium include minerals, brine, seawater, and the like, as well as lithium recycled from waste lithium batteries. The brine is obtained from natural salt lakes and typically contains lithium in the form of lithium chloride. The brine is a low-concentration lithium salt solution with a lithium concentration of approximately 1 g / L.

[0003] Therefore, a common practice is to supply brine obtained from natural salt lakes to an open-air evaporation pond, concentrate it by natural evaporation over a period of one year or more, remove impurities such as Mg, Ca, and B, and then react it with a carbonate source to recover lithium carbonate. However, concentrating brine by natural evaporation requires a long time to concentrate the brine, is easily affected by natural conditions such as weather, and lithium is lost during the concentration process by forming salts with other impurities. In addition, if sulfate is dissolved in the brine, lithium sulfate has low solubility in water, so lithium in the brine precipitates as lithium sulfate, resulting in incomplete recovery of lithium from the brine. Furthermore, recovering lithium from brine requires a large amount of water, which can cause environmental pollution such as water depletion and contamination in water-scarce regions.

[0004] Meanwhile, Patent Document 1 discloses a method for producing lithium hydroxide, 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 liquid; an ion exchange step of bringing the neutralized 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 is bipolar membrane electrodialysis. In bipolar membrane electrodialysis, hydrochloric acid and lithium hydroxide are produced directly from a purified aqueous lithium chloride solution. However, since the concentrations of the produced hydrochloric acid and lithium hydroxide are each only a few percent, when the hydrochloric acid produced by bipolar membrane electrodialysis is used as a lithium eluent, the lithium concentration of the lithium eluent is not sufficiently high. Furthermore, since the concentration of the aqueous lithium hydroxide solution produced by bipolar membrane electrodialysis is also low, a large amount of water must be evaporated in the lithium hydroxide crystallization step. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 7156322 Summary of the Invention [Problem to be solved by the invention]

[0007] In the second lithium chloride-containing solution subjected to electrodialysis in the conversion step in the lithium hydroxide production method disclosed in Patent Document 1, sodium hydroxide is used in the neutralization step and the ion exchange step, and the second lithium chloride-containing solution contains sodium. Therefore, there are problems in that sodium hydroxide is mixed into the lithium hydroxide obtained by electrodialysis of the second lithium chloride-containing solution, and that electric power is wasted 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 from an aqueous lithium salt solution having a low lithium concentration in a short time, with high purity, high yield, at low cost, and without causing environmental pollution. [Means for solving the problem]

[0009] The present inventors have conducted extensive research in light of the above-mentioned problems and have found that a lithium salt with high purity, high yield, low cost, and no environmental pollution can be produced from an aqueous lithium salt solution with a low lithium concentration in a short time by adsorbing lithium from a low-concentration aqueous lithium salt solution onto a lithium adsorbent, eluting the lithium from the lithium adsorbent with a mineral acid, and obtaining a first aqueous lithium salt solution from the low-concentration aqueous lithium salt solution, and then subjecting the first aqueous lithium salt solution to a purification step, an evaporation and concentration step, and a membrane electrolysis step in that order.The present invention was completed based on these findings.

[0010] The present invention relates to a method for recovering lithium from a low-concentration lithium salt aqueous solution, comprising: a step of adsorbing lithium in the low-concentration lithium salt aqueous solution onto a lithium adsorbent, eluting the lithium in the lithium adsorbent with a mineral acid, and obtaining a 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 and 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 a lithium hydroxide aqueous 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 a low-concentration lithium salt aqueous solution preferably further comprises a pretreatment step of adding an alkali to the low-concentration lithium salt aqueous solution in an amount that does not cause precipitation. When the low-concentration lithium salt aqueous solution is brine derived from at least one selected from the group consisting of a salt lake, seawater, and brackish water, the method may further comprise a return step of returning a post-adsorption solution obtained by adsorbing lithium onto the lithium adsorbent to at least one selected from the group consisting of a salt lake, 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, including 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 aqueous lithium salt solution to remove calcium, magnesium, and manganese from the first aqueous lithium salt solution to obtain a second aqueous lithium salt solution. More preferably, the purification step further includes a second purification step of contacting the second aqueous lithium salt solution with a chelating agent to remove calcium, magnesium, and manganese from the second aqueous lithium salt solution, thereby obtaining the third aqueous lithium salt solution. The method for recovering lithium from a low-concentration lithium salt aqueous solution preferably includes a third purification step of treating residual chlorine and chloric acid produced 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 the sixth lithium salt aqueous solution is evaporated and concentrated together with the third lithium salt aqueous solution in the evaporation and concentration step.

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

[0014] The distilled water generated in the evaporation and concentration step is preferably used as at least one selected from the group consisting of an eluent used in the lithium adsorption step, a catholyte in the membrane electrolysis step, and water in which a mineral acid is dissolved obtained in the membrane electrolysis step. The distilled water generated in the commercialization step is preferably used as at least one selected from the group consisting of an eluent used in the lithium adsorption step, a catholyte in the membrane electrolysis step, and water in which a mineral acid is dissolved obtained in the membrane electrolysis step. The mineral acid used in the lithium extraction step preferably includes at least one selected from the group consisting of hydrochloric acid, sulfuric acid, and nitric acid, and more preferably includes hydrochloric acid. The electricity used in the membrane electrolysis step preferably includes electricity obtained from renewable energy, and more preferably includes electricity obtained from at least one selected from the group consisting of solar power generation, wind power generation, hydroelectric power generation, and biomass power generation. [Effects of the Invention]

[0015] The method of recovering lithium from a low-concentration lithium salt aqueous solution of the present invention provides a method by which a high-purity lithium salt can be produced from the lithium salt aqueous solution having a low lithium concentration in a short time, in a high yield, at low cost, and without causing environmental pollution. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is an explanatory diagram showing one embodiment of the method for recovering lithium from a low-concentration aqueous lithium salt solution of the present invention. [Figure 2] FIG. 1 is an explanatory cross-sectional view showing one embodiment of the structure of an ion-exchange membrane electrolytic cell used in the method of recovering lithium from a low-concentration lithium salt aqueous solution of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention will now be described in further detail. Unless otherwise specified, the symbol "to" in a numerical range indicates a range from above to below, and both ends of the range are included. Furthermore, when a numerical range is indicated, the upper and lower limits can be combined as appropriate, and the resulting numerical range is also considered to be disclosed. Furthermore, in the description of the drawings, the same elements are denoted by the same reference numerals, and duplicate explanations will be omitted. Also, the dimensional proportions of the drawings are exaggerated for the convenience of explanation and may differ from the actual proportions.

[0018] The embodiments of the present invention will be described in more detail with reference to the accompanying drawings. <Lithium extraction process> As shown in Fig. 1, the method for recovering lithium from a low-concentration lithium salt aqueous solution of the embodiment (hereinafter, sometimes referred to as a lithium recovery method) starts with a low-concentration lithium salt aqueous solution 1. 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 source selected from the group consisting of a salt lake, 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 lithium-containing solid such as an active material powder.

[0020] The active material powder will now be described. When the used lithium-ion batteries are used lithium-ion batteries whose battery life as a battery product has expired or lithium-ion batteries discarded as defective products during the manufacturing process, they are first subjected to a discharge treatment. Various highly safe methods, such as resistance discharge, can be used for the discharge treatment. After discharging all remaining charge, openings are formed in the casings of the used lithium-ion batteries. The batteries are then heat-treated (roasted) at a temperature ranging from 100 to 800°C, or the batteries are pulverized in a pulverizer such as a hammer mill or jaw crusher without being heat-treated. The casings, current collectors, and other components of the used lithium-ion batteries are removed by sieving (classification), thereby obtaining the active material powder. Alternatively, the used lithium-ion batteries after the discharge treatment may be pulverized in the pulverizer, the casings, current collectors, and other components are removed by sieving, and the active material powder may be obtained by heat-treating the batteries at a temperature within the range.

[0021] When the used lithium ion batteries are residual positive electrode materials or the like used in commercialization in a manufacturing process, the active material powder may be obtained by pulverizing the used lithium ion batteries in the pulverizer after heat treatment at a temperature in the above range or without heat treatment without performing the discharge treatment and forming openings, and removing current collectors and the like by sieving.Furthermore, the used lithium ion batteries may be pulverized in the pulverizer, removing current collectors and the like by sieving, and then heat treatment at a temperature in the above range or without heat treatment to obtain the active material powder.

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

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

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

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

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

[0027] The amount (kg) of the low-concentration lithium salt aqueous solution 1 stored 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 above formula (1) is satisfied, 1 lithium ion in the low-concentration lithium salt aqueous solution is efficiently adsorbed onto the lithium adsorbent.

[0028] The lithium adsorbent may be a known spinel-type lithium manganese oxide, which may be produced by the method described in Japanese Patent No. 3937865, for example.

[0029] <Elution step> In STEP 2, a mineral acid is added to the adsorbent that adsorbed lithium from the low-concentration lithium salt aqueous solution 1 in STEP 1, and the lithium adsorbed on the adsorbent is eluted. Distilled water generated in the evaporation and concentration process and the commercialization process, described later, and the mineral acid obtained in the membrane electrolysis process, described later, are added to an eluent tank and sent to an adsorption tower filled with the adsorbent with adsorbed lithium. In the adsorption tower, lithium and hydrogen ions are exchanged, and the resulting solution becomes weakly acidic and returns to the eluent tank. The above operations are repeated, and 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 hydrochloric acid, and even more preferably hydrochloric acid. When the low-concentration lithium salt aqueous solution 1 is a lithium chloride aqueous solution, the hydrochloric acid concentration obtained in the membrane electrolysis process, described later, can be increased to 35% by mass, thereby increasing the lithium concentration in the eluent. 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 a salt lake, seawater, and brackish water, the lithium recovery method of the embodiment may include a returning step of returning an adsorbed liquid obtained by adsorbing lithium onto the lithium adsorbent to at least one selected from the group consisting of a salt lake, seawater, and brackish water.

[0031] <Purification process> The lithium recovery method of the embodiment includes a purification step in STEP 3 of purifying the first lithium salt aqueous solution to obtain a third lithium salt aqueous solution. The purification step preferably includes a first purification step of adding a lithium hydroxide aqueous solution and a lithium carbonate aqueous solution to the first lithium salt aqueous solution to remove calcium, magnesium, and manganese from the first lithium salt aqueous solution to obtain a second lithium salt aqueous solution. The second lithium salt aqueous solution has an impurity content of several ppm. At least a portion of the lithium hydroxide aqueous solution used in the first purification step may be the lithium hydroxide aqueous solution obtained in the membrane electrolysis step described below. Furthermore, at least a portion of the lithium carbonate aqueous solution used in the first purification step may be the lithium carbonate aqueous solution remaining after carbonating the lithium hydroxide aqueous solution obtained in the membrane electrolysis step described below. When the first lithium salt aqueous solution is a lithium chloride aqueous solution, reactions represented by the following formulas (2) to (4) occur in the first purification step. 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 contacting the second aqueous lithium salt solution with a chelating agent to remove trace amounts of calcium, magnesium, and manganese 2 from the second aqueous lithium salt solution, thereby obtaining the third aqueous lithium salt solution. Impurities not removed in the first purification step are removed by a chelating resin. The impurity content in the third lithium salt aqueous solution is less than 100 ppb. An example of the chelating resin is 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 is very high. Note that the CR11 adsorbs divalent or higher metals but does not easily capture monovalent metals. Furthermore, 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 is not mixed into the third lithium salt, the purity of the product obtained in the production step described below is increased, and the efficiency of the membrane electrolysis step described below is also improved.

[0034] <Evaporation and concentration process> The lithium recovery method of the embodiment includes, in STEP 4, an evaporation and concentration step 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 and concentration step together with residual chlorine generated in the membrane electrolysis step described below and a sixth lithium salt aqueous solution obtained by treating chloric acid with a mineral acid. An example of an evaporation and concentration device used in the evaporation and concentration step is a multi-effect evaporator. Furthermore, distilled water 7 generated in the evaporation and concentration step may be used as at least one selected from the group consisting of an eluent used in the lithium adsorption step, a catholyte in the membrane electrolysis step described below, and a mineral acid dissolution solution obtained in the membrane electrolysis step described below.

[0035] <Membrane electrolysis process> The lithium recovery method of the embodiment includes a membrane electrolysis step in STEP 5 of performing membrane electrolysis on the fourth lithium salt aqueous solution to obtain a lithium hydroxide aqueous solution, a mineral acid 4, and a fifth lithium salt aqueous solution that is more dilute than the fourth lithium salt aqueous solution. The membrane electrolysis step in STEP 5 can be performed using, for example, a membrane electrolytic cell 11 shown in FIG.

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

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

[0038] In the cathode chamber 18, the catholyte (HO) converts hydroxide ions (OH - ) and hydrogen ions (H + ), and the hydrogen ions generate hydrogen gas (H2) on the cathode plate 13, while the hydroxide ions combine with lithium to generate a lithium hydroxide aqueous solution 3. A part of at least one selected from the group consisting of the distilled water 7 and distilled water 8 generated by crystallization, which will be described later, may be used as the catholyte.

[0039] The electricity used in the membrane electrolysis step preferably includes electricity obtained from renewable energy, and more preferably includes electricity obtained from 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 embodiment, hydrochloric acid 4 can be obtained by reacting hydrogen gas (H2) generated in the membrane electrolysis step with residual chlorine (Cl2), and hydrochloric acid 4 can be used to dissolve the solid 1 in STEP 1. Although not shown in FIG. 2 , if the fourth lithium salt aqueous solution contains sulfate ions, sulfuric acid can be obtained in the anode chamber 17. If 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 can be used in at least one step selected from the group consisting of dissolving 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 be recovered as lithium hydroxide monohydrate (LiOH HO) 5 by crystallization in STEP 6, or as lithium carbonate (LiCO) 6 by carbonation in STEP 7. The crystallization and carbonation can also recover lithium oxide monohydrate (LiOH HO) 5 and lithium carbonate (LiCO) 6. Distilled water 8 is produced during the crystallization. The carbonation can be carried out, for example, by reacting the lithium hydroxide aqueous solution 3 with carbon dioxide (CO) produced 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, the lithium hydroxide monohydrate obtained by crystallization and the lithium carbonate obtained by adding carbon dioxide gas are high-purity lithium salts. Lithium carbonate is slightly soluble in water (the solubility of lithium carbonate in water at 20°C is 1.31 mass %, or 2.46 g / kg Li). The water generated in the production step is reused in at least one step selected from the group consisting of elution of lithium in the lithium adsorbent with mineral acid and dilution of mineral acid produced in the membrane electrolysis step, and water is circulated in the method for recovering lithium from a low-concentration lithium salt aqueous solution of the present invention. Therefore, the addition of water from outside the system can be minimized, making the method for recovering lithium from a low-concentration lithium salt aqueous solution of the present invention environmentally friendly. Furthermore, the mineral acid obtained in the membrane electrolysis step is reused in at least one step selected from the group consisting of the lithium elution step and the third purification step, and the lithium carbonate obtained in the commercialization step is reused in the first purification step. Therefore, the method of recovering lithium from a low-concentration lithium salt aqueous solution of the present invention can reduce chemical costs and is environmentally friendly because the chemical solution is recycled and salt is not discharged.

[0043] When the mineral acid is hydrochloric acid, chlorate is generated and accumulated in the membrane electrolysis step by hydroxide ions migrating backward from the cathode chamber 18. The accumulated chlorate causes deterioration of lithium quality, deterioration of equipment materials, and deterioration of ion exchange resins during membrane electrolysis of a lithium hydroxide aqueous solution. Therefore, the lithium recovery method of the embodiment preferably includes, as STEP 8, a third purification step in which residual chlorine and chloric acid generated by membrane electrolysis in the fifth lithium salt aqueous solution obtained in the membrane electrolysis step are treated 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 improving the lithium recovery rate. Furthermore, at least a portion 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 catholyte in the cathode chamber 17, which has been subjected to membrane electrolysis in the membrane electrolysis step, is degassed by contacting it with air (aeration), thereby significantly reducing the residual chlorine concentration in the catholyte. Further, a portion of the catholyte 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 a 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 concentration of hydrochloric acid obtained in the membrane electrolysis step can be 35 mass %, and the concentration of lithium hydroxide in the lithium hydroxide aqueous solution can be 5 to 11 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. 1. A method for recovering lithium from a low-concentration aqueous lithium salt solution, comprising the steps of: a step of adsorbing lithium in the low-concentration lithium salt aqueous solution onto a lithium adsorbent, and eluting lithium from the lithium adsorbent with a mineral acid to obtain a 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 and 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 a lithium hydroxide aqueous solution, a mineral acid, and a fifth lithium salt aqueous solution that is more dilute than the fourth lithium salt aqueous solution.

2. A method for recovering lithium from a low-concentration lithium salt aqueous solution according to claim 1, comprising the steps of: The method for recovering lithium from a low-concentration aqueous lithium salt solution further comprises a pretreatment step of adding an alkali to the low-concentration aqueous lithium salt solution in an amount such that precipitation does not occur.

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

4. 2. The 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 a lithium hydroxide aqueous solution and a lithium carbonate aqueous solution to the first lithium salt aqueous solution to remove calcium, magnesium, and manganese from the first lithium salt aqueous solution to obtain a second lithium salt aqueous solution.

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

6. 2. The 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 produced 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.

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

8. 8. The 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 includes carbon dioxide gas produced by absorbing carbon dioxide from the atmosphere.

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

10. 5. The method for recovering lithium from a low-concentration lithium salt aqueous solution according to claim 4, wherein the lithium hydroxide aqueous solution contains the lithium hydroxide aqueous solution obtained in the membrane electrolysis step, and the lithium carbonate aqueous solution contains a lithium carbonate aqueous solution obtained by carbonate the lithium hydroxide aqueous solution obtained in the membrane electrolysis step.

11. 2. The 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 utilized as at least one selected from the group consisting of dilution water of the mineral acid used in the lithium adsorption step, a catholyte in the membrane electrolysis step, and dilution water of the mineral acid obtained in the membrane electrolysis step.

12. 8. The method for recovering lithium from a low-concentration lithium salt aqueous solution according to claim 7, wherein the distilled water generated in the production step is utilized as at least one selected from the group consisting of dilution water of the mineral acid used in the lithium adsorption step, a catholyte in the membrane electrolysis step, and dilution water of the mineral acid obtained in the membrane electrolysis step.

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

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

15. The method for recovering lithium from a low-concentration lithium salt aqueous solution according to any one of claims 1 to 14, wherein the electricity used in the membrane electrolysis step includes electricity obtained by renewable energy.

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

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