Method for recovering lithium from low concentration aqueous lithium salt solution

Abstract

Provided is 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 a low lithium concentration. This method for recovering lithium from a 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, so as 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 so as to obtain a third aqueous lithium salt solution; an evaporative concentration step for evaporating and concentrating the third aqueous lithium salt solution so as to obtain a fourth aqueous lithium salt solution; and a membrane electrolysis step for subjecting the fourth aqueous lithium salt solution to membrane electrolysis so as 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.

Description

Method for recovering lithium from low-concentration lithium salt aqueous solution

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

[0002] In recent years, lithium has been attracting attention as a raw material for lithium-ion batteries, such as lithium-ion secondary batteries, and its supply sources 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 about 1 g / L.

[0003] Therefore, a common practice is to supply brine obtained from a natural salt lake 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. When sulfate is dissolved in the brine, lithium sulfate has low solubility in water, and lithium in the brine precipitates as lithium sulfate, resulting in a problem of 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] On the other hand, 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 lithium chloride aqueous 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 lithium hydroxide aqueous solution produced by bipolar membrane electrodialysis is also low, a large amount of water must be evaporated in the lithium hydroxide crystallization step.

[0006] Patent No. 7156322

[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 have been 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.

[0009] The present inventors have conducted extensive research in light of the above-mentioned problems and have found that a lithium salt can be produced from an aqueous solution of lithium salt with a low lithium concentration in a short time, with high purity, in high yield, at low cost, and without causing environmental pollution, by carrying out a lithium extraction step of adsorbing lithium from a low-concentration aqueous solution of lithium salt onto a lithium adsorbent, eluting the lithium from the lithium adsorbent with a mineral acid, and obtaining a first aqueous solution of lithium salt from the low-concentration aqueous solution of lithium salt, a step of purifying the first aqueous solution of lithium salt, an evaporation 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 lithium extraction step of adsorbing lithium in the low-concentration lithium salt aqueous solution onto a lithium adsorbent and eluting the lithium in 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.

[0011] The method for recovering lithium from a low-concentration lithium salt aqueous solution preferably further includes 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 include 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 from the lithium adsorbent is preferably carried out by contacting a mineral acid containing the mineral acid obtained in the membrane electrolysis step with the lithium adsorbent. 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 and obtain a second lithium salt aqueous solution. The purification step more preferably further includes 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 and obtain a third lithium salt aqueous 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 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. 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 includes the mineral acid obtained in the membrane electrolysis step. 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 dilution water used in the lithium adsorption step, catholyte in the membrane electrolysis step, and mineral acid dissolution water 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 dilution water used in the lithium adsorption step, catholyte in the membrane electrolysis step, and mineral acid dissolution water 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.

[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.

[0016] The present invention relates to a method for recovering lithium from a low-concentration lithium salt aqueous solution, and an ion-exchange membrane electrolytic cell for recovering lithium from the low-concentration lithium salt aqueous solution.

[0017] The present invention will be described in more detail. Unless otherwise specified, the "to" in a numerical range indicates a range from above to below, and both end values ​​are included. 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, identical elements are given the same reference numerals, and redundant explanations will be omitted. Also, the dimensional proportions in the drawings have been exaggerated for the sake of explanation and may differ from the actual proportions.

[0018]

[0023] The embodiment of the present invention will be described in more detail with reference to the accompanying drawings. <Lithium Extraction Step> As shown in Figure 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 contains hydrochloric acid, and even more preferably is 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 Step> 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 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 by 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)≦[amount of lithium adsorbent (kg)×lithium adsorption capacity of lithium adsorbent (g / kg)] / lithium concentration (g / kg) of low-concentration lithium salt aqueous solution 1 When formula (1) is satisfied, lithium ions in the low-concentration lithium salt aqueous solution 1 are efficiently adsorbed by 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 has adsorbed lithium from the low-concentration lithium salt aqueous solution 1 in STEP 1, and the lithium adsorbed to the adsorbent is eluted. Distilled water generated in the evaporation and concentration step and the commercialization step described below, and the mineral acid obtained in the membrane electrolysis step described below, 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 includes at least one selected from the group consisting of hydrochloric acid, sulfuric acid, and nitric acid, more preferably includes hydrochloric acid, and even more preferably is 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 step described below can be increased to 35 mass %, thereby increasing the lithium concentration in the eluent. On the other hand, the concentration of hydrochloric acid obtained by the 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 Step> 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, removing calcium, magnesium, and manganese 2 from the first lithium salt aqueous solution, and obtaining a second lithium salt aqueous solution. The content of impurities in the second lithium salt aqueous solution is 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 carbonated 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+MgCl ２ →Mg(OH) ２ ↓+2LiCl...(2) 2LiOH+MnCl ２ →Mn(OH) ２ ↓+2LiCl...(3) Li ２ CO ３ + CaCl ２ →CaCO ３ ↓+2LiCl...(4)

[0032] The purification step preferably further includes a second purification step in which the second lithium salt aqueous solution is contacted with a chelating agent to remove trace amounts of calcium, magnesium, and manganese 2 from the second lithium salt aqueous solution, thereby obtaining the third lithium salt aqueous solution. Impurities not removed in the first purification step are removed using a chelating resin. The impurity content of 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 the lithium hydroxide in the third lithium salt aqueous solution is very high. While CR11 adsorbs divalent or higher metals, it 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 aqueous solution, 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 Step> 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 dissolved water of mineral acid obtained in the membrane electrolysis step described below.

[0035] <Membrane Electrolysis Step> The lithium recovery method of the embodiment includes a membrane electrolysis step 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 in STEP 5. 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 has 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 including the anode plate 12 and a cathode chamber 18 including the cathode plate 13.

[0037] In the membrane electrolysis 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 are converted into residual chlorine (Cl ２ ), while the lithium ions migrate through the ion exchange membrane 16 to the cathode chamber 18.

[0038] In the cathode chamber 18, the catholyte (H ２ O) is hydroxide ion (OH － ) and hydrogen ions (H ＋ ) and hydrogen ions are ionized into hydrogen gas (H ２ ), while hydroxide ions combine with lithium to produce 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 produced 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 above embodiment, the hydrogen gas (H ２ ) and residual chlorine (Cl ２) to obtain hydrochloric acid 4, which can be used to dissolve solid 1 in STEP 1. Although not shown in FIG. 2 , when the fourth lithium salt aqueous solution contains sulfate ions, sulfuric acid can be obtained in anode chamber 17. When the fourth lithium salt aqueous solution contains nitrate ions, nitric acid can be obtained in anode chamber 17. That is, mineral acid 4 can be obtained in the membrane electrolysis step, and 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] <Production Step> The lithium hydroxide aqueous solution 3 obtained in the membrane electrolysis step is crystallized in STEP 6 to form lithium hydroxide monohydrate (LiOH.H ２ 0)5, and by carbonating it in STEP 7, lithium carbonate (Li ２ CO ３ ) 6, which can be recovered as lithium oxide monohydrate (LiOH·H ２ O)5 and lithium carbonate (Li ２ CO ３ During the crystallization, distilled water 8 is produced. The carbonation can be carried out by, for example, absorbing carbon dioxide from the atmosphere and then rinsing the lithium hydroxide aqueous solution 3 with carbon dioxide gas (CO ２ This may be carried out by reacting the compound with

[0042] The lithium hydroxide component obtained in the membrane electrolysis step is a high-purity lithium hydroxide aqueous solution free of 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% by 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 eluting lithium in the lithium adsorbent with mineral acid and diluting the 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 is 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 eluting lithium and the third purification step, and the lithium carbonate obtained in the production 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 it circulates the chemical solution and does not discharge salt.

[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 above 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), and the residual chlorine concentration in the catholyte is reduced to a very low level. 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)). LiClO ３ +6HCl→LiCl+3Cl ２ +3H ２ O... (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 %.

[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: a lithium extraction 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 performing membrane electrolysis on the fourth lithium salt aqueous solution 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 as described in claim 1, further comprising a pretreatment step of adding an alkali to the low-concentration lithium salt aqueous solution in an amount that does not cause precipitation.

3. A method for recovering lithium from a low-concentration lithium salt aqueous solution as described in 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 as described in claim 1, wherein the purification step includes a first purification step of adding an aqueous solution of lithium hydroxide and an aqueous solution of lithium carbonate to the first aqueous solution of lithium salt, removing calcium, magnesium, and manganese from the first aqueous solution of lithium salt, and obtaining a second aqueous solution of lithium salt.

5. A method for recovering lithium from a low-concentration aqueous lithium salt solution as described in claim 4, wherein the purification step further comprises 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 to obtain the third aqueous lithium salt solution.

6. A method for recovering lithium from a low-concentration lithium salt aqueous solution as described in claim 1, comprising a third purification step of treating with a mineral acid the residual chlorine and chloric acid produced by membrane electrolysis in the fifth lithium salt aqueous solution obtained in the membrane electrolysis step 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. A method for recovering lithium from a low-concentration lithium salt aqueous solution as described in 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. A method for recovering lithium from a low-concentration lithium salt aqueous solution as described in claim 7, wherein the carbon dioxide gas used in the carbonation includes carbon dioxide gas produced by absorbing carbon dioxide from the atmosphere.

9. A method for recovering lithium from a low-concentration lithium salt aqueous solution as recited in claim 6, wherein said mineral acid includes the mineral acid obtained in said membrane electrolysis step.

10. A method for recovering lithium from a low-concentration lithium salt aqueous solution as recited in claim 4, wherein the lithium hydroxide aqueous solution includes the lithium hydroxide aqueous solution obtained in the membrane electrolysis process, and the lithium carbonate aqueous solution includes a lithium carbonate aqueous solution obtained by carbonated the lithium hydroxide aqueous solution obtained in the membrane electrolysis process.

11. A method for recovering lithium from a low-concentration lithium salt aqueous solution as described in claim 1, wherein the distilled water generated in the evaporation and concentration process is utilized as at least one selected from the group consisting of diluted water of the mineral acid used in the lithium adsorption process, catholyte in the membrane electrolysis process, and diluted water of the mineral acid obtained in the membrane electrolysis process.

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

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

14. A method for recovering lithium from a low-concentration aqueous lithium salt solution as recited in claim 1, wherein said mineral acid comprises hydrochloric acid.

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

16. A method for recovering lithium from a low-concentration lithium salt aqueous solution as described in 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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