Preparation method of lithium compound

Abstract

The present disclosure relates to a method for preparing a lithium compound, which comprises: preparing lithium phosphate; mixing the lithium phosphate and sulfuric acid to obtain a mixture; converting the lithium phosphate into lithium sulfate through a reaction in the mixture; and separating the lithium sulfate into a solid phase. In the step of converting the lithium phosphate into lithium sulfate through a reaction in the mixture, the total concentration ([P+S] mol / L) of phosphorus (P) and sulfur (S) in the liquid phase of the mixture is 5 mol / L or more.

Description

Technical Field

[0001] The present disclosure relates to a method for preparing a lithium compound that can be efficiently converted from lithium phosphate into other forms of lithium compounds. Background Art

[0002] When lithium is extracted from salt lakes or from spent batteries, the effective form of lithium compound may be lithium phosphate.

[0003] Since lithium phosphate has very low solubility, lithium phosphate can be effectively precipitated through the reaction between phosphoric acid and lithium.

[0004] However, phosphoric acid is very expensive, so if the method of extracting lithium in the form of lithium phosphate is to be economical, there must be corresponding technology that can recover and reuse high-concentration phosphoric acid from the extracted lithium phosphate.

[0005] In addition, lithium phosphate is a very stable substance and therefore cannot be easily converted into commercial lithium compounds.

[0006] Therefore, there is a need for a method to effectively convert lithium phosphate into lithium compounds with better reactivity than lithium phosphate. Summary of the Invention

[0007] Technical issues

[0008] The present disclosure provides a method for efficiently converting lithium phosphate into other forms of lithium compounds.

[0009] In addition, the present invention discloses a preparation method, which can recover phosphoric acid in lithium phosphate at a high concentration and improve the recovery rate of phosphoric acid.

[0010] The present disclosure also provides a method for preparing a high-purity lithium compound by effectively removing impurities during the preparation process.

[0011] Technical Solution

[0012] According to one embodiment of the present disclosure, a method for preparing a lithium compound includes: preparing lithium phosphate; mixing the lithium phosphate and sulfuric acid to obtain a mixture; converting the lithium phosphate into lithium sulfate through a reaction in the mixture; and separating the lithium sulfate into a solid phase. In the step of converting the lithium phosphate into lithium sulfate through a reaction in the mixture, the total concentration ([P+S] mol / L) of phosphorus (P) and sulfur (S) in the liquid phase of the mixture may be 5 mol / L or more.

[0013] The reaction of the step of converting the lithium phosphate into lithium sulfate by reacting the mixture may include the reaction of the following Reaction Formula 1.

[0014] [Reaction formula 1]

[0015] 2Li3PO4+3H2SO4+nH2O->3Li2SO4nH2O+2H3PO4

[0016] In addition, the step of preparing lithium phosphate may be a step of preparing solid-phase lithium phosphate itself or a step of preparing solid-phase lithium phosphate in a slurry state in a solvent.

[0017] The step of preparing lithium phosphate may be a step of preparing solid-phase lithium phosphate in a slurry state in a solvent, and the solvent may be water or phosphoric acid.

[0018] The step of separating the lithium sulfate into a solid phase may further include a step of recovering the high-concentration phosphoric acid filtrate remaining after separating the solid phase lithium sulfate.

[0019] At this time, the concentration of phosphoric acid in the recovered high-concentration phosphoric acid filtrate may be 40% by weight or more.

[0020] The recovered high-concentration phosphoric acid filtrate can be reused as a solvent in the step of preparing the solid-phase lithium phosphate in a slurry state in the solvent.

[0021] Alternatively, the recovered high-concentration phosphoric acid filtrate can be used to produce lithium phosphate before the step of preparing lithium phosphate.

[0022] In addition, the method for preparing the lithium compound may include, after the step of separating the lithium sulfate into a solid phase, a step of washing the separated solid phase lithium sulfate; and a step of reusing the washing water used in the washing step as a solvent in the step of preparing the solid phase lithium phosphate in a slurry form in the solvent.

[0023] The step of washing the separated solid phase lithium sulfate can utilize pure water (D1 water).

[0024] The step of washing the separated solid-phase lithium sulfate utilizes an alcohol solvent. The washing water containing the alcohol solvent can be recovered by distillation to be reused in the step of washing the solid-phase lithium sulfate.

[0025] The method for preparing the lithium compound may further include dissolving the separated lithium sulfate in pure water to prepare a lithium sulfate aqueous solution and converting the lithium sulfate aqueous solution into lithium hydroxide using an electrodialysis device having a bipolar membrane.

[0026] At this time, the method for preparing the lithium compound may further include a step of obtaining lithium carbonate by carbonizing the converted lithium hydroxide.

[0027] In addition, the step of washing the separated solid-phase lithium sulfate can utilize pure water (Dl water) and include two or more washing steps.

[0028] Specifically, the step of washing the separated solid-phase lithium sulfate may include: a step of obtaining n washing waters by washing n times; a step of reusing a portion of the washing waters in the n washing waters as a solvent for the step of preparing the solid-phase lithium phosphate in a slurry in the solvent; and a step of washing the lithium sulfate n+1 times after mixing the residual liquid in the n washing waters with added pure water.

[0029] The method for preparing the lithium compound may include: a step of using all of the n+1 washing water for n+2 washing; a step of reusing a portion of the n+2 washing water obtained through the n+2 washing as a solvent for the step of preparing a slurry of solid-phase lithium phosphate in a solvent; and a step of washing the lithium sulfate n+3 times after mixing the residual liquid in the n+2 washing water with added pure water.

[0030] In addition, in the step of converting the lithium sulfate aqueous solution into lithium hydroxide using an electrodialysis device having a bipolar membrane, the electrodialysis device may have a structure in which an anion separation membrane and a cation separation membrane are sequentially located between opposing bipolar membranes.

[0031] Regarding the step of converting the lithium sulfate aqueous solution into lithium hydroxide using an electrodialysis device having a bipolar membrane, the process condition of a pH of 3.5 or less can be met without adding additional acid.

[0032] In the step of converting the lithium sulfate aqueous solution into lithium hydroxide using an electrodialysis device having a bipolar membrane, the lithium sulfate added to the electrodialysis device can be added continuously during the process.

[0033] In the step of converting the lithium sulfate aqueous solution into lithium hydroxide using an electrodialysis device having a bipolar membrane, the lithium sulfate added to the electrodialysis device is a raw material and can also serve as a pH regulator.

[0034] In addition, the preparation method of the lithium compound further includes the step of washing the separated solid-phase lithium sulfate before the step of dissolving the separated lithium sulfate in pure water to prepare a lithium sulfate aqueous solution. The separated solid-phase lithium sulfate contains cationic impurities other than lithium. The cationic impurities can be initially purified in the step of washing the separated solid-phase lithium sulfate. The cationic impurities remaining after the initial purification can be secondary purified in the step of converting the lithium sulfate aqueous solution into lithium hydroxide using an electrodialysis device with a bipolar membrane.

[0035] The cationic impurities may be at least one selected from the group consisting of potassium (K), sodium (Na), magnesium (Mg), boron (B), nickel (Ni), and calcium (Ca).

[0036] The amount of cationic impurities purified in the step of washing the separated solid-phase lithium sulfate may be 50 wt % or more based on the total amount (100 wt %) of cationic impurities contained in the lithium phosphate added in the step of mixing the lithium phosphate and sulfuric acid to obtain a mixture.

[0037] The cationic impurities include cations of potassium (K), sodium (Na), magnesium (Mg), boron (B), nickel (Ni), or a combination thereof. Based on the total amount (100 weight %) of the cationic impurities contained in the lithium phosphate added in the step of mixing the lithium phosphate and sulfuric acid to obtain a mixture, the amount of the cations of potassium (K), sodium (Na), magnesium (Mg), boron (B), nickel (Ni), or a combination thereof purified in the step of washing the separated solid-phase lithium sulfate can be 95 weight % or more.

[0038] The cationic impurities may include calcium (Ca) cations, and the amount of calcium (Ca) cation impurities purified in the step of washing the separated solid-phase lithium sulfate may be 50 wt% or more, based on the total amount (100 wt%) of the cationic impurities included in the lithium phosphate added in the step of mixing the lithium phosphate and sulfuric acid to obtain a mixture.

[0039] According to the method for preparing the lithium compound of this embodiment, after the step of converting the lithium sulfate aqueous solution into a lithium hydroxide aqueous solution, a desalted solution and a sulfuric acid aqueous solution using the electrodialysis device having a bipolar membrane, the method may further include: a step of concentrating the lithium hydroxide aqueous solution converted in the bipolar membrane electrodialysis device to crystallize it; a step of obtaining the crystallization filtrate produced in the crystallization step; and a step of drying the crystallized lithium hydroxide to obtain powdered lithium hydroxide.

[0040] The step of separating the lithium sulfate into a solid phase may further include a step of recovering the high-concentration phosphoric acid filtrate remaining after separating the solid phase lithium sulfate. The lithium phosphate in the step of preparing lithium phosphate can be prepared using the recovered high-concentration phosphoric acid filtrate and the crystallization filtrate.

[0041] In addition, the lithium phosphate in the step of preparing lithium phosphate may also be prepared using the recovered high-concentration phosphoric acid filtrate, the crystallization filtrate, and an alkaline substance.

[0042] The alkaline substance may include at least one of sodium hydroxide, potassium hydroxide, calcium hydroxide, barium hydroxide, magnesium hydroxide, calcium oxide, lithium, potassium, and sodium.

[0043] In addition, in this embodiment, after the step of dissolving the solid-phase lithium sulfate in a solvent to obtain a lithium sulfate aqueous solution, the method may further include adding an alkaline substance to the obtained lithium sulfate aqueous solution to control the pH to be above 10.

[0044] In this case, the alkaline substance may include at least one of sodium hydroxide, potassium hydroxide, calcium hydroxide, barium hydroxide, magnesium hydroxide, calcium oxide, lithium, potassium, and sodium.

[0045] Alternatively, after the step of dissolving the solid-phase lithium sulfate in a solvent to obtain a lithium sulfate aqueous solution, the method may further include adding the lithium hydroxide aqueous solution converted by the electrodialysis device or the crystallization filtrate obtained in the step of concentrating the lithium hydroxide aqueous solution to crystallize it to the obtained lithium sulfate aqueous solution to control the pH to above 10.

[0046] In this embodiment, after the step of controlling the pH of the lithium sulfate aqueous solution to be greater than 10, the method may further include the step of removing cationic impurities from the lithium sulfate aqueous solution by solid phase separation.

[0047] The cationic impurities may be at least one selected from the group consisting of potassium (K), sodium (Na), magnesium (Mg), boron (B), and calcium (Ca).

[0048] More specifically, the cationic impurities may be divalent cationic impurities.

[0049] Next, the method may include reusing the desalted liquid converted by the electrodialysis device as a solvent in the step of dissolving the solid-phase lithium sulfate in a solvent to obtain a lithium sulfate aqueous solution.

[0050] In addition, the aqueous sulfuric acid solution converted by the electrodialysis device can be reused in the step of mixing the lithium phosphate and sulfuric acid to obtain a mixture after undergoing a concentration process.

[0051] Effects of the Invention

[0052] According to one embodiment, lithium phosphate can be efficiently converted into other forms of lithium compounds.

[0053] In addition, by recovering phosphoric acid from lithium phosphate at a high concentration and utilizing various methods such as reuse in own processes and production of lithium phosphate, economic efficiency can be ensured.

[0054] Alternatively, lithium phosphate is slurried in low-concentration phosphoric acid and reacted with sulfuric acid to precipitate lithium sulfate, and then high-concentration phosphoric acid is extracted. The precipitated lithium sulfate is washed, and low-concentration phosphoric acid is obtained with washing water. The low-concentration phosphoric acid is then used to slurry lithium phosphate, thereby improving the recovery rate of phosphoric acid.

[0055] At the same time, the desalted liquid and crystallized filtrate produced in the bipolar membrane electrodialysis device are reused in the preparation process of the lithium compound, thereby recovering lithium from the desalted liquid and crystallized filtrate.

[0056] Furthermore, by adjusting the pH of the lithium sulfate aqueous solution to remove impurities, this process enables stable electrodialysis and the production of high-purity lithium compounds. Furthermore, the resulting desalted liquid and acid solution can be reused, reducing costs even when treated as wastewater, thereby ensuring economic efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 This is a graph showing changes in lithium solubility according to the concentrations of phosphorus (P) and sulfur (S) in the recovered filtrate.

[0058] Figure 2 and Figure 3 is a schematic diagram of an electrodialysis device according to an exemplary embodiment of the present invention.

[0059] Figure 4 These are experimental results relative to electrodialysis conditions.

[0060] Figure 5 FIG. 4 exemplarily shows a process for removing cationic impurities in a process for preparing a lithium compound according to an embodiment.

[0061] Figure 6 This is a flowchart according to Example 2.

[0062] Figure 7 It is a flow chart according to Examples 3 and 5.

[0063] Figure 8 is a flowchart showing a process according to Example 6.

[0064] Figure 9 is a flowchart showing a process according to Example 7.

[0065] Figure 10 and Figure 11 This is based on the experimental results of Example 8.

[0066] Figure 12 The process of obtaining washed solid-phase lithium sulfate from lithium phosphate is shown in FIG.

[0067] Figure 13 Schematically shows a process for reusing the desalted liquid and crystallization filtrate produced in a bipolar membrane electrodialysis device to prepare a lithium compound.

[0068] Figure 14Schematically shows the process of converting lithium phosphate into lithium sulfate and then into lithium hydroxide using a bipolar membrane electrodialysis device (BPED) in the process of Example 10.

[0069] Figure 15 exemplarily shows a process for preparing a lithium sulfate aqueous solution by converting lithium phosphate extracted using a recovered high-concentration phosphoric acid filtrate and a crystallization filtrate into lithium sulfate. DETAILED DESCRIPTION

[0070] The following describes various embodiments of the present invention in detail so that those skilled in the art can easily implement the present invention. The present invention can be implemented in various ways and is not limited to the embodiments described herein.

[0071] In addition, when the specification describes a part as “including” a certain constituent element, unless there is an obvious description to the contrary, other constituent elements may also be included and other constituent elements are not excluded.

[0072] The following describes a method for preparing a lithium compound according to one embodiment of the present disclosure.

[0073] A method for preparing a lithium compound according to an exemplary embodiment of the present disclosure may include: preparing lithium phosphate; mixing the lithium phosphate and sulfuric acid to obtain a mixture; converting the lithium phosphate into lithium sulfate through a reaction in the mixture; and separating the lithium sulfate into a solid phase.

[0074] In particular, this embodiment is characterized in that, in the step of converting the lithium phosphate into lithium sulfate through a reaction in the liquid phase of the mixture, the total concentration of phosphorus (P) and sulfur (S) in the mixture ([P+S] mol / L) is greater than 5 mol / L.

[0075] In this regard, the inventors have experimentally confirmed that lithium sulfate has a high solubility in water or low-concentration phosphoric acid, but its solubility decreases dramatically in high-concentration phosphoric acid solutions in the presence of sulfate ions. Specifically, lithium above its solubility in high-concentration phosphoric acid solutions in the presence of sulfate ions precipitates as lithium sulfate (LiSO), and a high-concentration phosphoric acid solution is generated through a direct reaction of lithium phosphate with sulfuric acid. The precipitated lithium sulfate can then be separated through solid-liquid separation, and the filtrate containing the high-concentration phosphoric acid can be directly recovered.

[0076] Figure 1 This is a graph showing changes in lithium solubility based on the concentrations of phosphorus (P) and sulfur (S) in the recovered filtrate. Figure 1 As can be seen, when the concentrations of phosphoric acid and sulfuric acid increase, the concentration of lithium in the solution drops sharply. In other words, most of the lithium precipitates as lithium sulfate.

[0077] Therefore, in the step of converting the lithium phosphate into lithium sulfate by a reaction in the liquid phase of the mixture, when the total concentration ([P+S] mol / L) of phosphorus (P) and sulfur (S) in the mixture is 5 mol / L or more, the recovery rate of lithium can be increased.

[0078] For example, the combined concentration of phosphorus (P) and sulfur (S) ([P+S] mol / L) can be 6 mol / L or higher and 15 mol / L or lower. The upper limit of the range is a theoretically calculated maximum upper limit. That is, the higher the combined concentration of phosphorus and sulfur within an economically feasible range, the higher the concentration of phosphoric acid in the recovered filtrate and the lithium recovery rate based on it.

[0079] More specifically, the total concentration of phosphorus (P) and sulfur (S) in the mixture may be 5.5 mol / L or more and 15 mol / L or less, 6 mol / L or more and 15 mol / L or less, 7 mol / L or more and 15 mol / L or less, 8 mol / L or more and 15 mol / L or less, 9 mol / L or more and 15 mol / L or less, 10 mol / L or more and 15 mol / L or less, or 12 mol / L or more and 15 mol / L or less.

[0080] When these ranges are met, the solubility of lithium sulfate decreases, allowing it to precipitate into a solid phase. In other words, the concentration of residual lithium in the solution where lithium sulfate is precipitated into a solid phase is significantly reduced. More specifically, the concentration of residual lithium in the solution where lithium sulfate is precipitated into a solid phase can be below 4 mol / L, below 3 mol / L, below 2 mol / L, below 1 mol / L, or below 0.5 mol / L. This indicates that most of the lithium has been precipitated.

[0081] More specifically, the lithium phosphate used can be in powder form. The lithium phosphate powder has a particle size of 1 to 200 μm or larger. Sulfuric acid, as the solution phase, has a concentration of 5 to 98% by weight. Higher concentrations of sulfuric acid allow for the extraction of higher concentrations of phosphoric acid.

[0082] Lithium phosphate powder can be slurried by mixing it with a 1-50 wt% phosphoric acid solution or water. While the slurrying process can be omitted, direct reaction of lithium phosphate powder with sulfuric acid reduces the uniformity of the reaction, making it difficult to precipitate the highly permeable lithium sulfate, which results in a lower phosphoric acid recovery rate.

[0083] The reaction of converting the lithium phosphate into lithium sulfate by the reaction in the mixture may include the reaction of the following Reaction Formula 1. That is, sulfuric acid is added to the lithium phosphate powder or slurry to convert the lithium phosphate into lithium sulfate according to the following reaction formula, thereby separating phosphoric acid.

[0084] [Reaction formula 1]

[0085] 2Li3PO4+3H2SO4+nH2O->3Li2SO4nH2O+2H3PO4

[0086] As the phosphoric acid concentration increases, the converted lithium sulfate precipitates. Furthermore, as the reaction proceeds, the solubility of lithium in phosphoric acid decreases as the phosphoric acid concentration increases. The solubility of lithium in phosphoric acid can be expressed as a function of the phosphorus and sulfur concentrations in the phosphoric acid, as shown below.

[0087] The following formula 1 is Figure 1 Approximate values ​​of the data.

[0088] [Formula 1]

[0089] Lithium concentration in filtrate = 0.048*(P concentration + S concentration) 2 -1.2773*(P concentration + S concentration)+9.4367(±0.6)

[0090] The unit of lithium solubility, P concentration and S concentration in Formula 1 is mol / L.

[0091] After the reaction is completed, the phosphoric acid-lithium sulfate mixture is separated by solid-liquid separation.

[0092] The separated lithium sulfate filter cake is washed with an organic solvent such as ethanol or methanol or water.

[0093] The washing liquid contains a large amount of phosphoric acid. When washing is performed with an organic solvent, the phosphoric acid is recovered by distillation, and the washing liquid or the recovered phosphoric acid can be reused in the lithium phosphate slurry preparation process.

[0094] In addition, the step of preparing lithium phosphate may be a step of preparing solid-phase lithium phosphate itself or a step of preparing solid-phase lithium phosphate in a slurry state in a solvent.

[0095] When lithium phosphate is prepared in a slurry form, the reaction rate with sulfuric acid can be further improved.

[0096] In addition, the step of preparing lithium phosphate may be a step of preparing solid-phase lithium phosphate in a slurry state in a solvent, and the solvent may be water or phosphoric acid. However, the present invention is not limited thereto.

[0097] The step of separating the lithium sulfate into a solid phase may further include a step of recovering the filtrate remaining after separating the solid phase lithium sulfate, that is, the high-concentration phosphoric acid filtrate.

[0098] In this case, the concentration of phosphoric acid in the recovered high-concentration phosphoric acid filtrate can be 40% by weight or higher. More specifically, the concentration of phosphoric acid in the recovered high-concentration phosphoric acid filtrate can be 50% by weight or higher, or 60% by weight or higher. Recycling of lithium containing high-concentration phosphoric acid at this level has not been reported in academia.

[0099] As described above, the recovered high-concentration phosphoric acid filtrate can be reused as a solvent in the step of preparing the solid-phase lithium phosphate in a slurry state in the solvent.

[0100] Alternatively, the recovered high-concentration phosphoric acid filtrate can be used to produce lithium phosphate before the step of preparing lithium phosphate.

[0101] For example, when phosphoric acid is added to a lithium-containing solution, lithium phosphate is easily precipitated.

[0102] In this embodiment, the lithium-containing solution may be in various forms such as seawater, salt water, mineral leachate, waste battery recovery solution, etc. This is well known in the art and will not be described in detail below.

[0103] After the step of separating the lithium sulfate into a solid phase, the method may further include washing the separated solid phase lithium sulfate; and reusing the washing water used in the washing step as a solvent in the step of preparing the solid phase lithium phosphate in a slurry in the solvent.

[0104] This is because the filtrate will still remain on the surface of the separated lithium sulfate, and washing can be performed to improve the purity of the lithium sulfate and the phosphoric acid in the recovered filtrate.

[0105] The separated solid-phase lithium sulfate can be washed with pure water (D1 water). In this case, due to the high solubility of lithium sulfate in water, lithium sulfate may dissolve into the washing water to some extent, resulting in lithium loss. However, the lost lithium will still be mixed into the lithium phosphate slurry, thus not affecting the lithium recovery rate of the overall reaction.

[0106] In addition, the step of washing the separated solid-phase lithium sulfate may utilize pure water (Dl water), and may include two or more washing steps.

[0107] More specifically, the step of washing the separated solid-phase lithium sulfate may include: a step of obtaining n washing waters by washing n times; a step of reusing a portion of the washing waters from the n washing waters as a solvent for the step of preparing the solid-phase lithium phosphate in a slurry in the solvent; and a step of washing the lithium sulfate n+1 times after mixing the residual liquid in the n washing waters with added pure water.

[0108] More specifically, the method may further include: using all of the n+1 washing water for the n+2 washing; reusing a portion of the n+2 washing water obtained through the n+2 washing as a solvent for the step of preparing a slurry of solid lithium phosphate in a solvent; and washing the lithium sulfate n+3 times after mixing the remaining liquid in the n+2 washing water with added pure water.

[0109] More specifically, the step of reusing a portion of the washing water in the n washing waters as a solvent for the step of preparing a slurry of solid-phase lithium phosphate in a solvent may be a step of reusing 30 to 70 weight % of the washing water in the n washing waters as a solvent for the step of preparing a slurry of solid-phase lithium phosphate in a solvent.

[0110] Furthermore, in the step of washing the lithium sulfate for n+1 times after mixing the residual liquid in the n-times washing water with the added pure water, the added pure water may be added according to the total amount used in the n-times washing.

[0111] In addition, the washing water used for the n+1 washing can be fully used for the n+2 washing.

[0112] That is, even if the number of washing times increases, the amount of pure water to be added is equivalent to the amount of solvent used in the step of reusing the washing water as the solvent in the step of preparing the solid-phase lithium phosphate in the form of a slurry in the solvent.

[0113] By this method, the concentration of phosphoric acid in the recovered filtrate can be higher, and the purity of the recovered lithium sulfate can also be improved.

[0114] The step of washing the separated solid-phase lithium sulfate may utilize an alcohol solvent. The washing water containing the alcohol solvent is recovered by distillation to be reused in the step of washing the solid-phase lithium sulfate.

[0115] A specific example of the alcohol solvent may be ethanol.

[0116] For example, the solubility of lithium sulfate in ethanol is very low, so the amount of lithium lost through the lithium sulfate washing step will be very small.

[0117] However, in order to reuse the wash water, the ethanol needs to be separated again, and in this case, the ethanol can be separated by methods such as distillation.

[0118] The separated ethanol can be used again for washing the solid lithium sulfate. The main component of the filtrate left after distillation is phosphoric acid, so it can be used again for lithium phosphate slurry as mentioned above.

[0119] The method may further include dissolving the separated lithium sulfate in pure water to prepare an aqueous lithium sulfate solution and converting the aqueous lithium sulfate solution into lithium hydroxide using an electrodialysis device having a bipolar membrane.

[0120] Lithium sulfate is highly soluble in water and can therefore be easily converted into an aqueous solution. This aqueous lithium sulfate solution can be converted into lithium hydroxide using an electrodialysis device equipped with a bipolar membrane and a membrane separating monovalent and divalent ions.

[0121] Figure 2 is a schematic diagram of an electrodialysis device according to an exemplary embodiment of the present invention.

[0122] In order to add the solid lithium sulfate produced in the previous process to the electrodialysis device, the solid lithium sulfate is dissolved in pure water (Dl water) to prepare a raw material solution.

[0123] The raw material solution was Figure 2 When the bipolar membrane electrodialysis device shown is used for treatment, SO4 migrates through the anion exchange membrane. 2- The bipolar membrane on the anode side encounters hydrogen from hydrolysis to obtain sulfuric acid (H2SO4), while the lithium ions that migrate to the cathode through the cation exchange membrane react with OH generated from the bipolar membrane. - The reaction proceeds to obtain LiOH. That is, the overall reaction is as follows.

[0124] Overall reaction: Li2SO4+2H2O->2LiOH+H2SO4

[0125] In an exemplary embodiment of the present invention, to ensure stable and efficient operation of the process for obtaining lithium hydroxide, an operating method for easily managing the concentration of lithium added to the lithium sulfate aqueous solution and an appropriate pH range were derived. This improves the current efficiency and conversion rate of bipolar membrane electrodialysis.

[0126] first, Figure 2 The lithium sulfate supplied may be the lithium sulfate produced according to the process of an exemplary embodiment of the present invention described above, rather than the existing commercial lithium sulfate.

[0127] Due to the characteristics of the preparation process according to an exemplary embodiment of the present invention, when lithium sulfate is dissolved, the pH is maintained below 3, which is lower than the pH range of commercial lithium sulfate aqueous solution (pH 4 or above).

[0128] This property, derived from the lithium sulfate produced in the previous process, allows the solution to maintain the pH below 3.5, the pH range required for smooth electrodialysis operation. Therefore, there is no need to add additional acid (typically sulfuric acid in the case of lithium sulfate) to adjust the pH, ensuring process stability.

[0129] When the pH of the raw solution, ie, the lithium sulfate aqueous solution, is greater than 3.5, reverse diffusion occurs through the dialysis membrane, resulting in a sharp increase in the pH of the raw solution to greater than 11, making it impossible to carry out the bipolar membrane process.

[0130] More specifically, since the lithium sulfate obtained in the process proposed above is used as the raw material, the pH of the lithium sulfate added in the electrodialysis process can be controlled to below 3 without adding additional acid, and the increase in pH that occurs during the process can be adjusted by adding a small amount of lithium sulfate aqueous solution, thereby eliminating the need to use additional acid and allowing the pH of the raw solution to be managed at 3.5 as a continuous operation condition in the process.

[0131] This process is shown in Figure 3 middle.

[0132] like Figure 3 As shown, the initial pH of the solution tank supplying the lithium sulfate aqueous solution is controlled to be below 3. When the pH increases during the process, a small amount of pre-prepared lithium sulfate aqueous solution is added, and 30 g / L is added based on the lithium concentration to control the pH to below 3.

[0133] The 30 g / L lithium sulfate added at this time is used to adjust the pH and also serves as a raw material solution, thereby having the characteristics of stably maintaining the process without introducing acid from the outside or generating by-products during the process.

[0134] In addition, in conventional bipolar membrane electrodialysis, the concentration of the initial solution, i.e., the aqueous lithium sulfate solution, is strictly controlled because the efficiency of producing the lithium hydroxide depends on the concentration of the initial solution.

[0135] That is, when the initial concentration is low, the time to obtain lithium becomes longer, and when the initial concentration is too high, the sulfur (S or SO4 2- ) migrates to the lithium hydroxide aqueous solution, thus reaching a value of several thousand ppm, which exceeds the limit of several hundred ppm. In order to improve this problem, the concentration of the lithium sulfate added initially is strictly controlled, which is a problem of process management.

[0136] As in an exemplary embodiment of the present invention, not only Figure 3 The pH is adjusted by operating in this manner. When the concentration of the initially added lithium solution changes, a 30 g / L lithium sulfate aqueous solution is supplied as a new lithium source, thereby controlling the concentration of the added solution.

[0137] At this time, the lithium concentration of the supplementary solution used is about 30 g / L, which is the saturation concentration. The concentration range can be 20 g / L to 34 g / L to reduce the amount of solution used.

[0138] The experimental results are shown in Figure 4 middle.

[0139] like Figure 4 As shown in the graph above, as lithium is consumed over time, the concentration of the raw solution decreases during electrodialysis. Consequently, the electrodialysis time and efficiency decrease.

[0140] In an exemplary embodiment of the present invention, when a decrease in conductivity due to such a decrease in lithium concentration is detected, Figure 4 A high-concentration lithium raw material solution is added to the arrow portion of the lower curve to stabilize the lithium concentration of the solution initially added.

[0141] This allows for a stable electrodialysis process to be maintained for a long period of time while continuously keeping the pH below 3 during the process.

[0142] When operating the electrodialysis process with this process layout and adjustment of the initial pH and concentration, the conversion rates of lithium and sulfur (S) are less dependent on the initial concentrations, allowing for greater freedom in process management and operation.

[0143] Furthermore, in contrast to the process of obtaining lithium in the form of lithium hydroxide, a process of obtaining sulfuric acid occurs simultaneously on the opposite electrode, thereby enabling the recovery of sulfuric acid. The initial concentration of the lithium sulfate aqueous solution used under the above conditions ranges from 4 g / L to 25 g / L.

[0144] Furthermore, the process does not require maintaining a high lithium concentration in the initial lithium sulfate aqueous solution, thereby maintaining a low sulfur concentration, which is proportional to the lithium concentration. This method is used to obtain a high-concentration lithium hydroxide aqueous solution and is the primary method for suppressing the increase in sulfur concentration that migrates to the final lithium hydroxide aqueous solution when using a high-concentration initial solution. This method can minimize the sulfur concentration in the lithium hydroxide product.

[0145] An additional advantage of the process is that the initial supply of raw liquor from which lithium is removed can be used to dissolve the initial lithium sulfate solids, thereby enabling a process layout with no waste or by-products generated during the process and no lithium loss.

[0146] The method may further comprise the step of obtaining lithium carbonate by carbonizing the converted lithium hydroxide.

[0147] The carbonization method of lithium hydroxide may include, for example, a reaction with a carbon source such as carbon dioxide, but is not limited thereto, and various methods may be used.

[0148] In addition, according to one embodiment of the present disclosure, before the step of dissolving the separated lithium sulfate in pure water to prepare the lithium sulfate aqueous solution, the step of washing the separated solid-phase lithium sulfate may be further included.

[0149] That is, the preparation method of the lithium compound according to this embodiment is characterized in that the preparation method includes: a step of preparing lithium phosphate; a step of mixing the lithium phosphate and sulfuric acid to obtain a mixture; a step of converting the lithium phosphate into lithium sulfate through a reaction in the mixture; a step of separating the lithium sulfate into a solid phase; a step of washing the separated solid phase lithium sulfate; a step of dissolving the washed solid phase lithium sulfate in pure water to prepare an aqueous lithium sulfate solution; and a step of converting the aqueous lithium sulfate solution into lithium hydroxide using an electrodialysis device having a bipolar membrane, wherein in the step of converting the lithium phosphate into lithium sulfate through a reaction in the mixture, the total concentration ([P+S] mol / L) of phosphorus (P) and sulfur (S) in the liquid phase of the mixture is 5 mol / L or more.

[0150] At this time, the separated solid-phase lithium sulfate contains cationic impurities other than lithium. The cationic impurities are initially purified in the step of washing the separated solid-phase lithium sulfate. The cationic impurities remaining after the initial purification can be secondary purified in the step of converting the lithium sulfate aqueous solution into lithium hydroxide using an electrodialysis device with a bipolar membrane.

[0151] To help understand, Figure 5 The process of removing cationic impurities according to this embodiment is exemplarily shown in FIG.

[0152] The solid-phase lithium sulfate washed in the process of washing the separated solid-phase lithium sulfate may be solid-phase lithium sulfate from which more than 50% of the cationic impurities have been removed, based on the total amount (100 weight %) of cationic impurities contained in the lithium phosphate added in the process of mixing the lithium phosphate and sulfuric acid to obtain the mixture.

[0153] Here, the cationic impurities may be at least one selected from the group consisting of potassium (K), sodium (Na), magnesium (Mg), boron (B), nickel (Ni), and calcium (Ca).

[0154] Specifically, based on the total amount (100 weight %) of cationic impurities contained in the lithium phosphate added in the process of mixing the lithium phosphate and sulfuric acid to obtain the mixture, potassium (K), sodium (Na), magnesium (Mg), boron (B) and nickel (Ni) can be removed by more than 90%, more specifically more than 95%, and calcium (Ca) can also be removed by more than 20%, more specifically more than 50%.

[0155] Therefore, when the washed solid-phase lithium sulfate is used to prepare lithium hydroxide, the impurity content of the final product will also be significantly reduced.

[0156] That is, the solid-phase lithium sulfate contains cationic impurities other than lithium. The cationic impurities are initially purified in the step of washing the solid-phase lithium sulfate, and the remaining small amount of cationic impurities are secondary purified in the bipolar membrane electrodialysis process, so that high-purity lithium hydroxide can be recovered.

[0157] Therefore, the recovered lithium hydroxide and the lithium compounds (e.g., lithium carbonate) converted from the recovered lithium hydroxide have low impurity content and can be used in lithium secondary batteries without undergoing a washing process or only requiring a minimal washing process, thereby significantly improving economic efficiency.

[0158] A variation of the method for preparing the lithium compound according to this embodiment may include: preparing lithium phosphate; mixing the lithium phosphate and sulfuric acid to obtain a mixture; converting the lithium phosphate into lithium sulfate through a reaction in the mixture; separating the lithium sulfate into a solid phase; dissolving the solid phase lithium sulfate in a solvent to obtain a lithium sulfate aqueous solution; and converting the lithium sulfate aqueous solution into a lithium hydroxide aqueous solution, a desalted solution, and a sulfuric acid aqueous solution using an electrodialysis device having a bipolar membrane.

[0159] Figure 13 Schematically shows a process for reusing the desalted liquid and crystallization filtrate produced in a bipolar membrane electrodialysis device to prepare a lithium compound.

[0160] See also Figure 13 First, lithium phosphate is prepared and converted into lithium sulfate.

[0161] The step of converting lithium phosphate into lithium sulfate is performed by mixing the lithium phosphate and sulfuric acid to obtain a mixture and then reacting the mixture.

[0162] In the step of converting the lithium phosphate into lithium sulfate through a reaction in the mixture, a total concentration ([P+S] mol / L) of phosphorus (P) and sulfur (S) in the liquid phase of the mixture may be 5 mol / L or more.

[0163] The total concentration of phosphorus and sulfur in the liquid phase of the mixture and the reaction formula are described in detail. Figure 1 , Reaction Formula 1 and the aforementioned contents explained on the basis of Formula 1 are the same, so they are not repeated here.

[0164] After the reaction is completed, the phosphoric acid and lithium sulfate mixture is separated by solid-liquid separation, thereby separating solid-phase lithium sulfate.

[0165] Next, the solid lithium sulfate is dissolved in a solvent to obtain a lithium sulfate aqueous solution, which is then added to an electrodialysis device with a bipolar membrane to convert it into a lithium hydroxide aqueous solution. Pure water can be used as the solvent.

[0166] In the process of converting the aqueous lithium hydroxide solution using the electrodialysis device having the bipolar membrane, desalted liquid and aqueous sulfuric acid solution are simultaneously generated.

[0167] Lithium sulfate is highly soluble in water and can therefore be easily converted into an aqueous solution. This aqueous lithium sulfate solution can be converted into lithium hydroxide using an electrodialysis device equipped with a bipolar membrane and a membrane separating monovalent and divalent ions.

[0168] See also Figure 2 The electrodialysis device with a bipolar membrane used in the conversion process of the lithium hydroxide aqueous solution of this embodiment can be a structure in which an anode chamber (anode cell) with an anode (+), a first bipolar membrane BP, an anion selective dialysis membrane A, a cation selective dialysis membrane C, a second bipolar membrane BP, and a cathode chamber (cathode cell) with a cathode (-) are sequentially arranged.

[0169] In order to add the solid phase lithium sulfate separated in the previous step to the electrodialysis device, the surrounding phase lithium sulfate is dissolved in pure water (Dl water) to prepare a lithium sulfate aqueous solution.

[0170] Using the aforementioned electrodialysis device, the lithium sulfate aqueous solution prepared as above is added between the anion selective dialysis membrane A and the cation selective dialysis membrane C, and water is added between the first bipolar membrane BP and the anion selective dialysis membrane A and between the second bipolar membrane BP and the cation selective dialysis membrane C, respectively, to prepare for bipolar membrane electrodialysis.

[0171] When power is applied to the bipolar membrane electrodialysis device to which the lithium sulfate aqueous solution and the water are added, hydrolysis of the concentrate water occurs at each bipolar membrane, and due to the electrophoretic effect, the cations and anions in the lithium sulfate aqueous solution migrate to the cathode and the anode sides, respectively.

[0172] That is, SO4 2- The first bipolar membrane on the anode side encounters hydrogen from hydrolysis, thereby obtaining sulfuric acid (H2SO4). In addition, lithium ions that migrate to the cathode through the cation selective dialysis membrane react with OH generated from the second bipolar membrane. - The reaction proceeds to obtain lithium hydroxide (LiOH). That is, the overall reaction is as follows.

[0173] Overall reaction: Li2SO4+2H2O->2LiOH+H2SO4

[0174] In summary, in the conversion process using an electrodialysis device with a bipolar membrane, lithium hydroxide aqueous solution (LiOH) and sulfuric acid aqueous solution (H2SO4-) as well as desalted liquid (Li2SO4) can be obtained.

[0175] The lithium hydroxide aqueous solution obtained by the method described above can be recovered into powder through the following steps: a step of crystallization by concentration; a step of obtaining the crystallization filtrate produced in the crystallization step; and a step of drying the crystallized lithium hydroxide to obtain powdered lithium hydroxide.

[0176] In addition, the step of separating the lithium sulfate into a solid phase may further include the step of recovering the high-concentration phosphoric acid filtrate remaining after separating the solid phase lithium sulfate and preparing lithium phosphate using the recovered phosphoric acid filtrate.

[0177] As described above, lithium phosphate can be prepared by using the recovered high-concentration phosphoric acid filtrate and the crystallization filtrate obtained in the lithium hydroxide crystallization process.

[0178] The lithium phosphate prepared in this manner can be used as the lithium phosphate in the step of preparing lithium phosphate in the lithium compound preparation process of this embodiment.

[0179] In this case, as described above, the lithium phosphate in the lithium phosphate preparation step can also be prepared using the recovered high-concentration phosphoric acid filtrate and the crystallization filtrate. Alternatively, the lithium phosphate can be prepared by controlling the pH of the recovered high-concentration phosphoric acid filtrate and the crystallization filtrate to 9.5 or higher using an alkaline substance.

[0180] When the high-concentration phosphoric acid filtrate and the crystallization filtrate are mixed after adjusting the chemical equivalent of lithium phosphate, the pH is lower than the pH suitable for lithium phosphate extraction, thereby reducing the lithium phosphate extraction rate. In other words, because the pH of the crystallization filtrate is high, the pH can be adjusted to above 9 by increasing the content of the crystallization filtrate in the recovered high-concentration phosphoric acid filtrate during mixing, thereby recovering lithium.

[0181] However, when preparing lithium phosphate, if the recovered high-concentration phosphoric acid filtrate and the crystallization filtrate are mixed in an appropriate proportion according to chemical equivalents and an additional alkaline substance is added to adjust the pH to above 9.5, the lithium recovery rate can be further improved compared to the case where only the crystallization filtrate is mixed into the recovered high-concentration phosphoric acid filtrate to adjust the pH to above 9.

[0182] The alkaline substance is used to adjust the pH of the mixture of the recovered high-concentration phosphoric acid filtrate and the crystallization filtrate to above 9.5, and may include, for example, at least one of sodium hydroxide, potassium hydroxide, calcium hydroxide, barium hydroxide, magnesium hydroxide, calcium oxide, lithium, potassium, and sodium.

[0183] Next, in the step of dissolving the solid lithium sulfate in a solvent to obtain a lithium sulfate aqueous solution, a lithium hydroxide aqueous solution or the crystallization filtrate obtained in the step of concentrating the lithium hydroxide aqueous solution to crystallize it can be added to the obtained lithium sulfate aqueous solution. In this manner, the pH of the lithium sulfate aqueous solution can be adjusted to 10 or above by adding the lithium hydroxide aqueous solution or the crystallization filtrate, thereby preparing a purified lithium sulfate aqueous solution. In this case, the pH of the purified lithium sulfate aqueous solution can also be 11 or above.

[0184] In this embodiment, after controlling the pH of the lithium sulfate aqueous solution to 10 or above, the process may further include removing cationic impurities from the lithium sulfate aqueous solution by solid phase separation. That is, when the pH of the lithium sulfate aqueous solution is adjusted as described above, the cationic impurities contained in the initial lithium sulfate aqueous solution can be effectively reduced.

[0185] For example, the cationic impurities may be at least one selected from the group consisting of potassium (K), sodium (Na), magnesium (Mg), boron (B), and calcium (Ca). More specifically, the cationic impurities may be divalent cationic impurities.

[0186] When cationic impurities are present in the lithium sulfate aqueous solution, precipitates may form in the solution of the bipolar membrane electrodialysis device, depending on the operating conditions of the bipolar membrane electrodialysis device. In this case, the lifespan of the ion exchange membrane of the bipolar membrane electrodialysis device may be adversely affected. Therefore, by controlling the pH of the lithium sulfate aqueous solution added to the bipolar membrane electrodialysis device, a lithium sulfate aqueous solution with a relatively low cationic impurity content can be added, thereby operating the conversion process using the bipolar membrane electrodialysis device more stably.

[0187] Next, the purified lithium sulfate aqueous solution from which impurities have been removed by controlling the pH is again adjusted to pH, and then added to the bipolar membrane electrodialysis device. At this time, in order to dilute the lithium sulfate from which impurities have been removed, as Figure 13 As shown, the pH can be adjusted, for example, by adding desalted water.

[0188] That is, in an exemplary embodiment of the present invention, the pH of the lithium sulfate aqueous solution added to the bipolar membrane electrodialysis device to remove impurities is adjusted to a pH control range for smooth operation of the bipolar membrane electrodialysis device, that is, the pH is adjusted to below 3.5.

[0189] When the pH of the lithium sulfate solution fed into an electrodialysis unit with a bipolar membrane exceeds 3.5, back diffusion through the membrane will occur. This can cause the pH of the lithium sulfate solution fed into the bipolar membrane electrodialysis unit to increase dramatically, resulting in a pH exceeding 11, making bipolar membrane processing impossible.

[0190] Therefore, the solution tank connected to the bipolar membrane electrodialysis device and supplied with the lithium sulfate aqueous solution has an initial pH controlled to be below 3. When the pH of the solution tank increases during the process, a small amount of pre-prepared lithium sulfate aqueous solution is added, with 30 g / L added based on the lithium concentration, to control the pH to below 3.

[0191] The 30 g / L lithium sulfate added at this time is used to adjust the pH and also serves as a raw material solution, thereby having the characteristics of stably maintaining the process without introducing acid from the outside or generating by-products during the process.

[0192] More details and references on the pH range suitable for operation of bipolar membrane electrodialysis units Figure 3 and Figure 4 The above contents of the description are the same, so they will not be repeated here.

[0193] Furthermore, the desalted liquid converted by the electrodialysis device can be reused as a solvent in the step of dissolving the solid-phase lithium sulfate in a solvent to obtain a lithium sulfate aqueous solution. In this case, no additional solvent is required, which is economical. Furthermore, when preparing the lithium sulfate aqueous solution, lithium remaining in the desalted liquid can be recovered, thereby improving the lithium recovery rate in the preparation process.

[0194] In addition, the aqueous sulfuric acid solution converted by the electrodialysis device can be reused in the step of mixing the lithium phosphate and sulfuric acid to obtain a mixture after undergoing a concentration process.

[0195] As mentioned above, the sulfuric acid aqueous solution and the desalted liquid generated in the bipolar membrane electrodialysis device can be reused in the next process, thereby improving the productivity of the lithium compound preparation process according to this embodiment.

[0196] In addition, in the preparation process, when the pH of the lithium sulfate aqueous solution is adjusted to above 10 after solid lithium sulfate is dissolved in a solvent, the impurity content of the sulfuric acid aqueous solution and the desalted liquid generated by the bipolar membrane electrodialysis process is also greatly reduced. Therefore, when the wastewater is discharged through wastewater treatment, the wastewater treatment cost can also be reduced, thereby ensuring excellent economic efficiency.

[0197] The following describes examples of the present invention and comparative examples. However, the following examples are merely examples of the present invention, and the present invention is not limited to the following examples.

[0198] Example 1

[0199] Under the conditions shown in Table 1 below, lithium phosphate was reacted with sulfuric acid, and then an experiment was conducted to recover a high-concentration phosphoric acid filtrate by solid-liquid separation. The results are shown in Table 2.

[0200] Lithium phosphate extracted from used batteries was used. After drying at 105°C for 24 hours, the weight change was measured, revealing a moisture content of approximately 35%. The dried lithium phosphate was analyzed by ICP (inductively coupled plasma) analysis, and the amount of sulfuric acid equivalent to one equivalent according to Reaction Formula 1 was calculated. The purity of the sulfuric acid used was 95%.

[0201] Phosphoric acid used for slurrying lithium phosphate was a 30-50 wt% phosphoric acid solution prepared by mixing a phosphoric acid solution (Oi Kakin Co., Ltd.) and ultrapure water at a ratio of 85:15 (phosphoric acid solution: ultrapure water).

[0202] The phosphoric acid solution and lithium phosphate were mixed in a reactor to form a slurry, and then sulfuric acid was added while stirring at a speed of 200 rpm. At this time, the sulfuric acid was added at a rate of about 10 g / min.

[0203] After the sulfuric acid addition was complete, the reaction slurry was stirred for approximately 40 minutes, followed by solid-liquid separation by conventional vacuum filtration at a pressure of approximately 50 mbar. The recovered liquid phase was sampled and analyzed by ICP analysis, the results of which are shown in Table 2.

[0204] The experimental results show that the recovered liquid phase is a high-concentration phosphoric acid filtrate with a phosphoric acid concentration of 51-62% by weight, containing 6.5-11 g / L of lithium. The lithium content is inversely proportional to the sum of the phosphorus (P) and sulfur (S) contents in the phosphoric acid component.

[0205]

Table 1

[0206]

[0207]

Table 2

[0208]

[0209] Example 2

[0210] Figure 6 This is a flow chart according to Example 2, which specifically includes an ethanol washing step.

[0211] Following the reaction of lithium phosphate with sulfuric acid under the conditions shown in Table 3, an experiment was conducted to recover the high-concentration phosphoric acid filtrate by solid-liquid separation. Furthermore, the composition of the washing water used to recover the lithium sulfate was analyzed, and the results are shown in Tables 4, 5, and 6.

[0212] Lithium phosphate was extracted from used batteries. To reduce experimental error due to differences in moisture content, the material was dried at 105°C for 24 hours to remove any moisture. The dried lithium phosphate was analyzed by ICP (inductively coupled plasma) analysis, and the amount of sulfuric acid corresponding to the equivalent ratio according to Reaction Formula 1 was calculated. The sulfuric acid used had a purity of 95%.

[0213] Phosphoric acid used for slurrying lithium phosphate was a phosphoric acid solution having a concentration of 21 to 30 wt %. The phosphoric acid solution was prepared by mixing a phosphoric acid solution (Oi Kakin Co., Ltd.) and ultrapure water at a ratio of 85:15 (phosphoric acid solution: ultrapure water).

[0214] The phosphoric acid solution and lithium phosphate were mixed in a reactor to form a slurry, and then sulfuric acid was added while stirring at a speed of 200 rpm. At this time, the sulfuric acid was added at a rate of about 10 g / min.

[0215] After the addition of sulfuric acid was completed, the reactant slurry was further stirred for about 40 minutes, and then solid-liquid separation was performed by conventional vacuum filtration at a pressure of about 50 mbar.

[0216] After solid-liquid separation, the solid filter cake was washed with ethanol by conventional vacuum filtration and evenly spraying a fixed amount of ethanol onto the filter cake. For Sample 4, 400 g of ethanol was sprayed followed by 500 g of ethanol, while Samples 5 and 6 were sprayed with 500 g of ethanol.

[0217] The recovered phosphoric acid filtrate, washing water (ethanol), and solid matter were subjected to component analysis using ICP.

[0218]

Table 3

[0219]

[0220]

Table 4

[0221]

[0222]

Table 5

[0223]

[0224]

Table 6

[0225]

[0226] The experimental results confirmed that the recovered liquid phase was a high-concentration phosphoric acid filtrate with a phosphoric acid concentration of 45-54% by weight and contained 7.5-12.1 g / L of lithium. Furthermore, the lithium content was inversely proportional to the sum of the phosphorus (P) and sulfur (S) contents in the phosphoric acid component. The wash water (ethanol) contained 45-59 g / L of P and 3.7-12.2 g / L of S, with the content of other elements (including lithium) being less than 1 g / L. The solids were primarily composed of lithium and sulfur, with a P content of 0.76-1.25% by weight. This indicates that more than 90% of the P was recovered through the high-concentration phosphoric acid filtrate and wash water (ethanol).

[0227] Example 3

[0228] Figure 7 This is a flow chart according to Example 3. Specifically, it includes a pure water (D1 water) washing step.

[0229] An experiment was conducted to recover high-concentration phosphoric acid by solid-liquid separation after reacting lithium phosphate with sulfuric acid under the conditions shown in Table 7. Furthermore, the composition of the washing water used to recover the lithium sulfate was analyzed, and the results are shown in Tables 8, 9, and 10.

[0230] Lithium phosphate was extracted from used batteries. To reduce experimental error due to differences in moisture content, the material was dried at 105°C for 24 hours to remove any moisture. The dried lithium phosphate was analyzed by ICP (inductively coupled plasma) analysis, and the amount of sulfuric acid corresponding to the equivalent ratio according to Reaction Formula 1 was calculated. The sulfuric acid used had a purity of 95%.

[0231] Phosphoric acid used for slurrying lithium phosphate was a phosphoric acid solution with a concentration of 21 to 30 wt %. The phosphoric acid solution was prepared by mixing a phosphoric acid solution (Oi Kakin Co., Ltd.) and ultrapure water at a ratio of 85:15 (phosphoric acid solution: ultrapure water).

[0232] The phosphoric acid solution and lithium phosphate were mixed in a reactor to form a slurry, and then sulfuric acid was added while stirring at a speed of 200 rpm. At this time, the sulfuric acid was added at a rate of about 10 g / min.

[0233] After the addition of sulfuric acid was completed, the reactant slurry was further stirred for about 40 minutes, and then solid-liquid separation was performed by conventional vacuum filtration at a pressure of about 50 mbar.

[0234] After solid-liquid separation, the solid filter cake is washed with water (ultrapure water) by performing conventional vacuum filtration and uniformly spraying a fixed amount of water onto the filter cake.

[0235] The recovered phosphoric acid and washing water were analyzed for components by ICP, and the solid matter was dried at 105° C. for 24 hours and then analyzed for components by ICP.

[0236]

Table 7

[0237]

[0238]

Table 8

[0239]

[0240]

Table 9

[0241]

[0242]

Table 10

[0243]

[0244] Experimental results show that the recovered liquid phase is a high-concentration phosphoric acid filtrate with a concentration of 49-59% by weight and contains 8.7-9.8 g / L of lithium. The lithium content is inversely proportional to the sum of the phosphorus (P) and sulfur (S) contents in the phosphoric acid component. The wash water contains 116-147 g / L of P and 64-70 g / L of S. The lithium content is 21.2-23.5 g / L, which is inversely proportional to the sum of the phosphorus (P) and sulfur (S) contents in the wash water.

[0245] The main components of the solid matter were lithium and sulfur, and the P content was 0.52 to 1.09% by weight. This indicated that 90% or more of the P was recovered by the high-concentration phosphoric acid filtrate and the washing water.

[0246] Example 4

[0247] A process cycle experiment was carried out using the washing water recovered from the sample 7.

[0248] The washing water obtained in Sample 7 was used again for slurrying of lithium phosphate, and this cycle was repeated twice.

[0249] The results are shown in Table 11 below.

[0250]

Table 11

[0251]

[0252] From the results, it can be confirmed that when the process is continuously carried out, the recovery rate of P is 94-96%, and the recovery rate of Li is 96-98%.

[0253] Example 5

[0254] Example 5 also adopts Figure 7Although Example 5 is not shown separately in the accompanying drawings, Example 5 includes a continuous process in which the washing liquid from the previous time is reused.

[0255] An experiment was conducted to recover high-concentration phosphoric acid by solid-liquid separation after reacting lithium phosphate with sulfuric acid under the conditions shown in Table 12. Furthermore, the composition of the washing water used to recover the lithium sulfate was analyzed, and the results are shown in Tables 13, 14, and 15.

[0256] Lithium phosphate was extracted from used batteries. To reduce experimental error due to differences in moisture content, the material was dried at 105°C for 24 hours to remove any moisture. The dried lithium phosphate was analyzed by ICP (inductively coupled plasma) analysis, and the amount of sulfuric acid corresponding to the equivalent ratio according to Reaction Formula 1 was calculated. The sulfuric acid used had a purity of 95%.

[0257] Phosphoric acid used to slurry the lithium phosphate was the washing liquid recovered in the previous experiment, and its concentration was 26 to 26.5% by weight.

[0258] The phosphoric acid solution and lithium phosphate were mixed in a reactor to form a slurry, and then sulfuric acid was added while stirring at a speed of 200 rpm. At this time, the sulfuric acid was added at a rate of about 10 g / min.

[0259] After the addition of sulfuric acid was completed, the reactant slurry was further stirred for about 40 minutes, and then solid-liquid separation was performed by conventional vacuum filtration at a pressure of about 50 mbar.

[0260] After solid-liquid separation, the solid filter cake is washed with water (ultrapure water) by performing conventional vacuum filtration and uniformly spraying a fixed amount of water onto the filter cake.

[0261] The recovered phosphoric acid and washing water were analyzed for components by ICP, and the solid matter was dried at 105° C. for 24 hours and then analyzed for components by ICP.

[0262]

Table 12

[0263]

[0264]

Table 13

[0265]

[0266]

Table 14

[0267]

[0268]

[0269]

Table 15

[0270]

[0271] Experimental results show that the liquid phase recovered from a continuous process that reuses the previous wash solution is a high-concentration phosphoric acid filtrate with a concentration of 63-65.7% by weight and 6.9-6.6 g / L of lithium. The lithium content is inversely proportional to the sum of the phosphorus (P) and sulfur (S) contents in the phosphoric acid component.

[0272] The wash water contained 110-113 g / L of phosphorus (P), 72 g / L of sulfur (S), and 25-25.8 g / L of lithium, which is inversely proportional to the sum of the phosphorus (P) and sulfur (S) contents in the wash water. The solids were primarily lithium and sulfur, with 1.03-1.19% P by weight. This indicates that over 90% of the P was recovered by the phosphoric acid solution and wash water.

[0273] This result shows that it is possible to stably Figure 3 The continuous process shown in FIG2 reuses the previous washing liquid.

[0274] Example 6

[0275] Figure 8 : is a flow chart showing the process according to Example 6. Specifically, it shows a continuous process in which the washing step is repeated two or more times.

[0276] An experiment was conducted to recover high-concentration phosphoric acid by solid-liquid separation after reacting lithium phosphate with sulfuric acid under the conditions shown in Table 16. Furthermore, the composition of the washing water used to recover the lithium sulfate was analyzed, and the results are shown in Tables 17, 18, and 19.

[0277] Lithium phosphate was extracted from used batteries. To reduce experimental error due to differences in moisture content, the material was dried at 105°C for 24 hours to remove any moisture. The dried lithium phosphate was analyzed by ICP (inductively coupled plasma) analysis, and the amount of sulfuric acid corresponding to the equivalent ratio according to Reaction Formula 1 was calculated. The purity of the sulfuric acid used was 70%.

[0278] As the phosphoric acid used to slurry the lithium phosphate, Samples 12, 13, and 14 used a 40% by weight phosphoric acid solution prepared by mixing an 85% phosphoric acid solution (Oi Kakin Co., Ltd.) with ultrapure water. Sample 15 used the recovered washing liquid obtained in Sample 8 as the phosphoric acid used to slurry the lithium phosphate. The specific composition of the solution is shown in Table 9.

[0279] The phosphoric acid solution and lithium phosphate were mixed in a reactor to form a slurry, and then sulfuric acid was added while stirring at a speed of 200 rpm. At this time, the sulfuric acid was added at a rate of about 10 g / min.

[0280] After the addition of sulfuric acid was completed, the reactant slurry was further stirred for about 40 minutes, and then solid-liquid separation was performed by conventional vacuum filtration at a pressure of about 50 mbar.

[0281] After solid-liquid separation, the process of washing the solid filter cake is carried out in two steps, namely washing 1 / 2 and washing 2 / 2. Ultrapure water is used as the washing water added in the washing 2 / 2 step, and the washing liquid recovered from the previous washing 2 / 2 step is used as the washing water added in the washing 1 / 2 step.

[0282] However, sample 9 used ultrapure water in both wash 1 / 2 and wash 2 / 2 because there was no recovered washing liquid in the first cycle of the continuous process.

[0283] The washing method is to implement conventional vacuum filtration and evenly spray a certain amount of water onto the filter cake.

[0284] The recovered phosphoric acid and washing water were analyzed for components by ICP, and the solid matter was dried at 105° C. for 24 hours and then analyzed for components by ICP.

[0285] Table 16

[0286]

[0287]

Table 17

[0288]

[0289]

Table 18

[0290]

[0291]

[0292]

Table 19

[0293]

[0294] The experimental results show that the washing is carried out in two steps, namely washing 1 / 2 and washing 2 / 2. In the washing 1 / 2 step, the washing liquid recovered in the previous washing 2 / 2 step is used. Figure 4 In the process, the washing liquid recovered in the washing 1 / 2 step with a phosphoric acid concentration of 30 to 40 wt % and the washing liquid recovered in the washing 2 / 2 step with a phosphoric acid concentration of 15 to 7 wt % can be obtained.

[0295] The main components of the solid matter are lithium and sulfur, and the P content is 0.80 to 1.74% by weight. It is known that 90% or more of the P is recovered by the phosphoric acid solution and the washing water.

[0296] From the results of Examples 1, 2, 3 and 5, it can be seen that the higher the concentration of phosphoric acid used to slurry the lithium phosphate, the higher the concentration of the recovered phosphoric acid. The continuous process in Example 6 based on two steps of reusing the previous washing liquid as phosphoric acid for slurrying the lithium phosphate can obtain high-concentration recovered phosphoric acid.

[0297] The relationship between the phosphorus, sulfur, and lithium components of the recovered phosphoric acid and the washing water in Examples 1, 2, 3, 5, and 6 satisfies the following formula: lithium concentration = 0.048*(P concentration + S concentration) 2 -1.2773*(P concentration+S concentration)+9.4367(±0.6), where the units of lithium solubility, P concentration, and S concentration are mol / L.

[0298] Example 7

[0299] Figure 9 This is a flow chart illustrating the process according to Example 7. Specifically, it illustrates a continuous process in which the washing step is repeated two or more times. After solid-liquid separation, the solid cake is washed in two steps: washing 1 / 2 and washing 2 / 2. Ultrapure water is used as the washing water added in the washing 2 / 2 step, and the washing liquid recovered from the previous washing 2 / 2 step is used as the washing water added in the washing 1 / 2 step.

[0300] The washing method is to implement conventional vacuum filtration and evenly spray a certain amount of water onto the filter cake.

[0301] The amount of lithium phosphate added in each step or the concentration and amount of sulfuric acid, the amount of recovered phosphoric acid and the amount of phosphoric acid contained in the washing solution are as follows: Figure 9 shown.

[0302] Example 8

[0303] Example 8 is an example of the preparation of a lithium hydroxide aqueous solution, using the raw material of the lithium sulfate aqueous solution obtained by the bipolar membrane electrodialysis proposed above.

[0304] The lithium concentration of the initial solution used ranges from 7.1 g / L to 17.3 g / L, at which time the obtained lithium conversion rate (for preparing lithium hydroxide) is 94.5 to 96.1%, and the S conversion rate (for preparing sulfuric acid) is 96.8 to 97.8%. The influence of the concentration change of the added raw material solution is very low, which means that the lithium and sulfur conversion rates can be maintained regardless of the initial concentration, thus having great advantages in process operation.

[0305] Table 20

[0306]

[0307] The experimental results are shown in Figure 10 and Figure 11 middle.

[0308] first, Figure 10 shows the concentration change of lithium hydroxide obtained in the bipolar membrane electrodialysis process, Figure 11 The obtained changes in the concentration of sulfuric acid are shown.

[0309] At this time, since the lithium concentration cannot be quantified in real time during the process, the lithium concentration in the obtained lithium hydroxide aqueous solution is quantified by utilizing the characteristic that the conductivity increases with the increase of lithium ions, so as to control the process in real time. Therefore, the figure shows the change in conductivity relative to time, rather than the change in concentration relative to time.

[0310] That is, in the graph, the electrical conductivity indicates the change in lithium concentration of lithium hydroxide and the change in sulfuric acid solution concentration of the sulfuric acid aqueous solution.

[0311] In the Figure 10 In the figure, the circular points indicate that the lithium concentration of the initial lithium sulfate aqueous solution added was 7 g / L, the square points indicate that the lithium concentration was 12 g / L, and the diamond points indicate that the lithium concentration was 17.25 g / L. In general, when the amount of solution added is the same, the final lithium concentration depends on the difference in the initial lithium concentrations. The final concentration when the initial concentration is 17 g / L is higher than the final concentration when the initial concentration is 7 g / L, and the final concentration increases proportionally.

[0312] However, as an embodiment of the present invention Figure 10 As shown, the speed of the initial migrating ions is the same regardless of the concentration. As a result, the trends (slope and general shape) of the actual lithium concentration changes relative to the three concentration changes are almost the same.

[0313] The results show that lithium hydroxide can be obtained regardless of the initial concentration. At this point, at the circular point, that is, when the lithium concentration is low, when no more lithium ions migrate, the reaction is interrupted and ends after about 90 minutes.

[0314] As proposed in the present invention, when a high concentration (30 g / L) stock solution is added to continuously supply ions and lower the pH, the reaction can continue. Figure 10 The results of the embodiment of the present invention were confirmed without supplying additional initial solution. The results show that maintaining the lithium concentration of the initial solution is not a problem during the process of using electrical energy to migrate lithium ions in the solution.

[0315] exist Figure 11 In the case of obtaining lithium hydroxide and sulfuric acid at the same time, sulfur (S or SO4 2- ) ion migration is also the same as the lithium ion migration described above and is independent of the change in initial concentration.

[0316] From the results, the Figure 10 and Figure 11 The examples show that stable operation of the process can be achieved if the concentration variation of the initially added lithium sulfate aqueous solution is kept within a wide range and a small amount of high-concentration lithium sulfate solution (30 g / L) is added each time the lithium concentration of the initial solution decreases during the process.

[0317] Comparative Example 1

[0318] The composition and density of commercial phosphoric acid solutions according to their concentration are as follows: Generally, the content of elements such as Li in phosphoric acid is very low, and there is no technology in academia that controls the Li content by adjusting the P+S content.

[0319] Table 21 below shows changes in components based on the concentration of commercial phosphoric acid (Oi Kakin Co., Ltd.).

[0320] Table 21

[0321]

[0322] Experimental Example 1

[0323] The relationship between the phosphorus, sulfur, and lithium components of the recovered phosphoric acid and the washing water in Examples 1, 2, 3, 5, and 6 satisfies the following formula: lithium concentration = 0.048*(P concentration + S concentration) 2 -1.2773*(P concentration+S concentration)+9.4367(±0.6), where the units of lithium solubility, P concentration, and S concentration are mol / L.

[0324] By using the above formula, when lithium phosphate is separated into lithium sulfate and phosphoric acid for recovery, the ratio of the solid phase to the liquid phase of the recovered Li is shown in Table 22. In the calculation, it is assumed that lithium phosphate has a theoretical chemical formula (Li3PO4), except for Li + and PO4 3- In addition, there were no impurities, and no phosphoric acid and water were further added for slurrying. The purity of the added sulfuric acid was 100%, and the S concentration of the recovered phosphoric acid was 50 g / L.

[0325] Table 22

[0326]

[0327] As shown in the above results, it can be theoretically calculated that a P+S concentration of at least 5 mol / L is required to allow solid-phase lithium sulfate to exist after lithium phosphate is separated into lithium sulfate and phosphoric acid.

[0328] Preparation Examples 1 to 5

[0329] Figure 12 The process of obtaining washed solid-phase lithium sulfate from lithium phosphate is shown in FIG.

[0330] In such Figure 12 Lithium phosphate was reacted with sulfuric acid under the conditions shown, followed by solid-liquid separation to produce lithium sulfate. The lithium phosphate used was extracted from brine, and its composition was analyzed by ICP (Inductively Coupled Plasma) and is shown in Table 23 below.

[0331] Table 23

[0332] Li K Ca Na B Mg P Ni Other elements 17.35 0.0009 0.031 0.90 0.090 0.022 25.05 0.0006 ＜0.0005

[0333] The unit omitted in Table 23 is g / L.

[0334] As a solvent for preparing the lithium phosphate into a slurry, a portion of the first washing water obtained in the previous process was used. The solvent and solid lithium phosphate were mixed in a reactor to form a slurry, and then stirred at a speed of 200 rpm and sulfuric acid was added.

[0335] At this time, the rate of addition of sulfuric acid was about 1 kg / min.

[0336] After the addition of sulfuric acid was complete, the reactant slurry was further stirred for about 40 minutes, and then solid-liquid separation was performed by conventional vacuum filtration at a pressure of about 50 mbar.

[0337] After solid-liquid separation, the solid cake washing process is carried out in two steps: primary washing and secondary washing. Ultrapure water is used as the washing water added in the secondary washing process, while the secondary washing liquid recovered from the previous secondary washing process is used as the washing water in the primary washing process.

[0338] The recovered solid lithium sulfate was dried at 105° C. for 24 hours, and then subjected to component analysis by ICP. The results are shown in Table 24 below.

[0339] Table 24

[0340]

[0341] It can be confirmed from Table 24 that the lithium sulfate prepared in Preparation Examples 1 to 5 has removed more than 95% of impurities such as K, Na, Mg, B and Ni present in lithium phosphate, and Ca has been reduced by more than 50%.

[0342] Example 9

[0343] Example 9 is an example of the preparation of a lithium hydroxide aqueous solution, using a raw material of a lithium sulfate aqueous solution obtained by bipolar membrane electrodialysis.

[0344] Specifically, solid-phase lithium sulfate was prepared using the lithium phosphate of Table 23 by the same method as Preparation Examples 1 to 5.

[0345] The prepared solid-phase lithium sulfate solid was dissolved in ultrapure water to prepare a lithium sulfate aqueous solution with a lithium (Li) concentration of 20 g / L, and the components were analyzed using ICP. The results are shown in Table 25.

[0346] Table 25

[0347]

[0348] The lithium sulfate aqueous solution thus prepared was converted into a lithium hydroxide aqueous solution by bipolar membrane electrodialysis. The components of the converted lithium hydroxide aqueous solution, the recovered sulfuric acid solution, and the desalted liquid were analyzed by ICP. The results are shown in Table 26.

[0349] Table 26

[0350] Ingredients (g / L) LiOH solution <![CDATA[H2SO4 solution]]> Desalted liquid Li 23.44 1.17 0.101 K 0.082 0.003 ＜0.003 Ca 0.008 0.014 ＜0.003 B 0.003 0.003 0.007 S 1.2 51.88 0.192 Na 0.183 0.047 ＜0.003 P ＜0.003 1.46 1.1 Other elements ＜0.003 ＜0.003 ＜0.003

[0351] A lithium hydroxide aqueous solution prepared by bipolar membrane electrodialysis was concentrated under vacuum to crystallize, thereby preparing a LiOHH2O filter cake. This filter cake was then washed with ultrapure water equal in weight to the filter cake and then dried at low temperature to prepare a LiOHH2O powder. The components of the prepared LiOHH2O powder were analyzed by ICP, and the results are shown in Table 27.

[0352] Table 27

[0353] Ingredients (wt.%) Li Ca S Na Mg Other elements <![CDATA[LiOHH2O]]> 26.59 ＜0.0005 0.0023 ＜0.0005 ＜0.0005 ＜0.0005

[0354] It can be confirmed from Tables 23 and 25 that more than 95% of impurities such as K, Na, Mg, B and Ni present in lithium phosphate are removed from the solid-phase lithium sulfate prepared by the process of converting lithium phosphate into lithium sulfate as in one embodiment of the present invention, and Ca is reduced by more than 50%.

[0355] In addition, it can be confirmed from Table 26 that some impurities such as P and Ca present in lithium sulfate are removed, and it can be confirmed from Table 27 that the impurities of the LiOHH2O powder washed with a very small amount of ultrapure water with the same weight as the prepared filter cake are controlled sufficiently to be used as a lithium compound for lithium secondary batteries.

[0356] In summary, the process of washing the solid lithium sulfate separated from the lithium phosphate after conversion provides a primary purification of cationic impurities other than lithium. Furthermore, the small amount of cationic impurities remaining after the primary purification undergoes a secondary purification in a bipolar membrane electrodialysis process, thereby recovering high-purity lithium hydroxide.

[0357] The recovered lithium hydroxide and the lithium compound (e.g., lithium carbonate) converted from the recovered lithium hydroxide have low impurity content and therefore can be used in lithium secondary batteries without undergoing a washing process or only requiring a minimal washing process, thereby significantly improving economic efficiency.

[0358] Example 10

[0359] (1) Lithium sulfate conversion and lithium hydroxide conversion

[0360] Figure 14 Schematically shows the process of converting lithium phosphate into lithium sulfate and then into lithium hydroxide using a bipolar membrane electrodialysis device (BPED) in the process of Example 10.

[0361] Specifically, lithium phosphate and sulfuric acid are mixed, and the lithium phosphate is converted into lithium sulfate through a reaction in the mixture, and then the solid lithium sulfate is separated. The solid lithium sulfate is then dissolved using the desalted liquid from the bipolar membrane electrodialysis device in the previous process, and then converted into lithium hydroxide using bipolar membrane electrodialysis.

[0362] For example Figure 14 Experiments were conducted using desalted water to dissolve lithium sulfate and prepare lithium sulfate aqueous solutions under the conditions shown. The composition of the desalted water used for dissolution is shown in Table 28, and the compositions of samples 1 to 5 of the prepared lithium sulfate aqueous solutions (LS solutions) are shown in Table 29. Each component was analyzed by ICP (Inductively Coupled Plasma).

[0363] Table 28

[0364]

[0365]

[0366] The unit omitted in Table 28 is g / L.

[0367] Table 29

[0368]

[0369] The unit omitted in Table 29 is g / L.

[0370] The desalted solution contained 1.62 g / L of lithium, 4.36 g / L of sulfur (S), 0.314 g / L of phosphorus (P), and a small amount of sodium (Na). The lithium sulfate aqueous solution prepared from the desalted solution contained approximately 34 g / L of lithium.

[0371] Therefore, when the desalted liquid is used to dissolve the surrounding lithium sulfate to prepare the lithium sulfate aqueous solution, the lithium remaining in the desalted liquid is recovered by the sulfuric acid solution, thereby improving the lithium recovery rate in the process.

[0372] The lithium sulfate aqueous solution thus prepared was diluted with the same desalted solution and ultrapure water and then converted into lithium hydroxide using a bipolar membrane electrodialysis device. The composition of the lithium sulfate aqueous solution (LS stock solution) added to the bipolar membrane electrodialysis device, the prepared lithium hydroxide aqueous solution (BASE), sulfuric acid (acid), and the desalted solution (salt) were analyzed by ICP (Inductively Coupled Plasma) and are shown in Table 30.

[0373] Table 30

[0374] Classification Li S P Ca Mg Na K other LS stock solution 12.790 30.730 0.629 ＜0.003 0.004 0.083 0.007 ＜0.003 <![CDATA[H2SO4(Acid)]]> 0.318 43.623 0.417 ＜0.003 ＜0.003 0.005 ＜0.003 ＜0.003 LiOH(Base) 24.176 0.518 ＜0.003 ＜0.003 ＜0.003 0.156 0.008 ＜0.003 Desalted liquid (Salt) 2.135 5.798 0.478 ＜0.003 0.010 0.006 ＜0.003 ＜0.003

[0375] The unit omitted in Table 30 is g / L.

[0376] From Table 30, it can be confirmed that when the desalted liquid produced from the bipolar membrane electrodialysis device in the previous process is used to prepare the lithium sulfate aqueous solution and then converted into lithium hydroxide, the lithium content contained in the lithium hydroxide aqueous solution is 24.176 g / L. Therefore, the following can be successfully completed. Figure 14 In the process shown, the lithium in the desalted solution is recovered again as a component of the converted lithium hydroxide aqueous solution.

[0377] (2) Removal of impurities from lithium sulfate aqueous solution

[0378] For passing Figure 14 The lithium sulfate aqueous solution prepared by the process shown was analyzed by ICP (Inductively Coupled Plasma) for components based on pH changes, as shown in Table 31 below. The pH of the lithium sulfate aqueous solution was adjusted by adding the lithium hydroxide aqueous solution obtained from the bipolar membrane electrodialysis device in the previous process.

[0379] Table 31

[0380]

[0381] The pH of the prepared lithium sulfate aqueous solution is about 2.5, and it contains a considerable amount of impurities such as phosphorus (P) and magnesium (Mg). The amount of these impurities can vary depending on the lithium sulfate preparation process and the purity of the added lithium phosphate.

[0382] As shown in Table 31, when the pH of the lithium sulfate aqueous solution is adjusted to 10 or higher, divalent or higher cation impurities such as phosphorus (P) and magnesium (Mg) are removed, and their content is reduced to 0.003 g / L or lower. Therefore, by adjusting the pH of the lithium sulfate aqueous solution to 10 or higher, a high-purity lithium sulfate aqueous solution with impurities removed can be prepared.

[0383] Furthermore, the pH of the lithium sulfate aqueous solution can be adjusted by adding an alkaline lithium aqueous solution such as lithium hydroxide. Therefore, in this exemplary embodiment, the pH of the lithium sulfate aqueous solution can be easily adjusted to 10 or above by using the lithium hydroxide aqueous solution obtained in a bipolar membrane electrodialysis device or the crystallization filtrate obtained in a crystallization process of the lithium hydroxide aqueous solution as the alkaline lithium aqueous solution, without adding an additional alkaline solution.

[0384] Next, the purified lithium sulfate aqueous solution, adjusted to a pH of 10 or above, was diluted with ultrapure water and then converted into lithium hydroxide using a bipolar membrane electrodialysis device. The components of the lithium sulfate aqueous solution (LS stock solution) added to the bipolar membrane electrodialysis device, the prepared lithium hydroxide aqueous solution (BASE), sulfuric acid (acid), and the desalted solution (salt) were analyzed using ICP (Inductively Coupled Plasma) and are shown in Table 32.

[0385] Table 32

[0386]

[0387] The unit omitted in Table 32 is g / L.

[0388] It can be confirmed from Table 32 that when a lithium sulfate aqueous solution purified by adjusting the pH is converted into a lithium hydroxide solution using a bipolar membrane electrodialysis device, the concentrations of phosphorus (P) and divalent or higher cationic impurities in the sulfuric acid and desalted liquid produced at the same time are below 0.003 g / L.

[0389] In addition, when there are divalent or higher cationic impurities in the lithium sulfate aqueous solution, the precipitate generated according to the operating conditions of the bipolar membrane electrodialysis device may be generated in the solution of the bipolar membrane electrodialysis device. In this case, the life of the ion exchange membrane of the bipolar membrane electrodialysis device may be adversely affected. Therefore, by removing the divalent cationic impurities in the lithium sulfate aqueous solution added to the bipolar membrane electrodialysis device, the conversion process utilizing the bipolar membrane electrodialysis device can be operated more stably.

[0390] Furthermore, residual phosphorus (P) in the sulfuric acid aqueous solution and desalted liquid produced along with the lithium hydroxide solution in the bipolar membrane electrodialysis unit is removed, allowing the sulfuric acid aqueous solution and desalted liquid to be reused in the next process. This significantly improves the productivity of the lithium compound production process.

[0391] Furthermore, when the aqueous sulfuric acid solution and desalted liquid generated in the bipolar membrane electrodialysis device are discharged through wastewater treatment, the wastewater treatment cost can be significantly reduced, thereby improving economic efficiency.

[0392] (3) Preparation of lithium phosphate using crystallization filtrate and recovered phosphoric acid

[0393] The crystallized filtrate produced in the bipolar membrane electrodialysis device can also be used in the preparation process of lithium phosphate.

[0394] After lithium phosphate is converted into lithium sulfate, the filtrate remaining after separation, i.e., high-concentration phosphoric acid, can be recovered in the process of separating the solid-phase lithium sulfate.

[0395] See also Figure 14 , using the high-concentration phosphoric acid and the crystallization filtrate, the raw material for the lithium compound preparation process according to this embodiment, namely lithium phosphate, can be prepared.

[0396] At this time, the crystallization filtrate generated in the bipolar membrane electrodialysis device includes the filtrate generated in the process of preparing lithium hydroxide (LiOH·H 2 O) using a lithium hydroxide aqueous solution and washing water, and generally contains 20 g / L or more of lithium.

[0397] The filtrate composition and lithium recovery rate based on pH in the process of preparing lithium phosphate using the crystallization filtrate and the recovered high-concentration phosphoric acid filtrate are shown in Table 33 below.

[0398]

Table 33

[0399]

[0400] When lithium phosphate was extracted using the recovered high-concentration phosphoric acid filtrate and the crystallization filtrate, the lithium recovery rate was approximately 87.7% under conditions of a pH of 9 or higher.

[0401] In addition, when the pH of the mixture of the recovered high-concentration phosphoric acid filtrate and the crystallization filtrate was adjusted to 9.5 by adding sodium hydroxide (NaOH), the recovery rate of lithium could be improved to about 98%.

[0402] The lithium recovery rate is the amount of lithium recovered as lithium phosphate relative to the total amount of lithium contained in the recovered high-concentration phosphoric acid and the crystallization filtrate. This lithium recovery rate can vary depending on the lithium concentration of the recovered high-concentration phosphoric acid and the crystallization filtrate.

[0403] Figure 15Schematically shows a process for preparing an aqueous lithium sulfate solution by converting lithium phosphate extracted using recovered high-concentration phosphoric acid and crystallization filtrate into lithium sulfate as described above.

[0404] The present invention can be implemented in various ways and is not limited to the above-described embodiments. A person skilled in the art will appreciate that the present invention can be implemented in other specific ways without changing the technical concept or essential features of the present invention. Therefore, it should be understood that the above-described embodiments are illustrative and not intended to limit the present invention.

Claims

1. A method for preparing a lithium compound, comprising: Steps for preparing lithium phosphate; The step of mixing the lithium phosphate and sulfuric acid to obtain a mixture; A step of converting the lithium phosphate into lithium sulfate by a reaction in the mixture; and The step of separating the lithium sulfate into a solid phase, In the step of converting the lithium phosphate into lithium sulfate by a reaction in the mixture, the total concentration of phosphorus (P) and sulfur (S) ([P+S] mol / L) in the liquid phase of the mixture is 5.5 mol / L or more and 15 mol / L or less, wherein in the step of mixing the lithium phosphate and the sulfuric acid to obtain the mixture and in the step of converting the lithium phosphate into lithium sulfate by a reaction in the mixture, a high-concentration phosphoric acid solution is produced by a direct reaction of the lithium phosphate with the sulfuric acid, wherein sulfate ions are present in the high-concentration phosphoric acid solution, and lithium present at a concentration higher than its solubility in the high-concentration phosphoric acid solution in which sulfate ions are present is precipitated as lithium sulfate (Li2SO4), The step of preparing lithium phosphate is a step of preparing solid-phase lithium phosphate in a slurry form in a solvent. The step of separating the lithium sulfate into a solid phase further includes the step of recovering the high-concentration phosphoric acid filtrate remaining after separating the solid phase lithium sulfate, and wherein the concentration of phosphoric acid in the recovered high-concentration phosphoric acid filtrate is 40% by weight or more, in, The recovered high-concentration phosphoric acid filtrate is reused as a solvent in the step of preparing a solid-phase lithium phosphate in a slurry state in a solvent.

2. The method for preparing a lithium compound according to claim 1, wherein: The reaction of the step of converting lithium phosphate into lithium sulfate by the reaction in the mixture comprises the following reaction formula 1 [Reaction formula 1] 2Li3PO4+3H2SO4+nH2O->3Li2SO4nH2O+2H3PO4.

3. The method for preparing a lithium compound according to claim 1, wherein: The step of preparing lithium phosphate is a step of preparing solid-phase lithium phosphate in a slurry state in a solvent, and the solvent is phosphoric acid.

4. The method for preparing a lithium compound according to claim 1, wherein: The recovered high-concentration phosphoric acid filtrate is used to produce lithium phosphate before the step of preparing lithium phosphate.

5. The method for preparing a lithium compound according to claim 2, wherein: After the step of separating the lithium sulfate into a solid phase, the method comprises: The step of washing the separated solid phase lithium sulfate; A step of reusing the washing water used in the washing step as a solvent in the step of preparing the solid-phase lithium phosphate in a slurry state in the solvent.

6. The method for preparing a lithium compound according to claim 5, wherein: The step of washing the separated solid lithium sulfate utilizes pure water.

7. The method for preparing a lithium compound according to claim 6, wherein: The step of washing the separated solid phase lithium sulfate utilizes an alcohol solvent, The washing water containing the alcohol solvent is recovered by distillation to be reused in the step of washing the solid-phase lithium sulfate.

8. The method for preparing a lithium compound according to claim 1, further comprising: dissolving the separated solid-phase lithium sulfate in a solvent to prepare an aqueous lithium sulfate solution; and The step of converting the lithium sulfate aqueous solution into lithium hydroxide aqueous solution, desalted solution and sulfuric acid aqueous solution using an electrodialysis device with a bipolar membrane.

9. The method for preparing the lithium compound according to claim 8, further comprising: A step of obtaining lithium carbonate by carbonizing the converted lithium hydroxide.

10. The method for preparing a lithium compound according to claim 5, wherein: The step of washing the separated solid-phase lithium sulfate utilizes pure water and includes two or more washing steps.

11. The method for preparing a lithium compound according to claim 10, wherein: The step of washing the separated solid phase lithium sulfate comprises: a step of obtaining n wash waters by performing n washes; a step of reusing a portion of the n-times washing water as a solvent in the step of preparing the solid-phase lithium phosphate in a slurry form in the solvent; as well as The residual liquid in the washing water obtained by the n times is mixed with the added pure water, and then the lithium sulfate is washed for n+1 times.

12. The method for preparing a lithium compound according to claim 11, further comprising: The step of using all the water from the n+1 washes for the n+2 washes; a step of reusing a portion of the n+2 wash water obtained by the n+2 washes as a solvent in the step of preparing the solid-phase lithium phosphate in a slurry state in the solvent; and The residual liquid in the n+2 washing water is mixed with the added pure water, and then the lithium sulfate is washed for n+3 times.

13. The method for preparing a lithium compound according to claim 8, wherein: In the step of converting the lithium sulfate aqueous solution into lithium hydroxide using an electrodialysis device having a bipolar membrane, The electrodialysis device is a structure in which an anion separation membrane and a cation separation membrane are sequentially located between opposing bipolar membranes.

14. The method for preparing a lithium compound according to claim 13, wherein: Regarding the step of converting the lithium sulfate aqueous solution into lithium hydroxide using an electrodialysis device having a bipolar membrane, Without adding additional acid, the process condition of pH below 3.5 is met.

15. The method for preparing a lithium compound according to claim 14, wherein: In the step of converting the lithium sulfate aqueous solution into lithium hydroxide using an electrodialysis device having a bipolar membrane, The lithium sulfate added to the electrodialysis device is added continuously during the process.

16. The method for preparing a lithium compound according to claim 14, wherein: In the step of converting the lithium sulfate aqueous solution into lithium hydroxide using an electrodialysis device having a bipolar membrane, The lithium sulfate added to the electrodialysis device is a raw material and also acts as a pH regulator.

17. The method for preparing a lithium compound according to claim 8, wherein: Before the step of dissolving the separated solid phase lithium sulfate in pure water to prepare a lithium sulfate aqueous solution, the method further comprises the step of washing the separated solid phase lithium sulfate. The separated solid phase lithium sulfate contains cationic impurities other than lithium, The cationic impurities are initially purified in the step of washing the separated solid phase lithium sulfate, The cationic impurities remaining after the primary purification are subjected to secondary purification in the step of converting the lithium sulfate aqueous solution into lithium hydroxide using an electrodialysis device having a bipolar membrane.

18. The method for preparing a lithium compound according to claim 17, wherein: The cationic impurities are at least one selected from the group consisting of potassium (K), sodium (Na), magnesium (Mg), boron (B), nickel (Ni), and calcium (Ca).

19. The method for preparing a lithium compound according to claim 17, wherein: Based on the total amount (100 wt %) of cationic impurities contained in the lithium phosphate added in the step of mixing the lithium phosphate and sulfuric acid to obtain a mixture, the amount of the cationic impurities purified in the step of washing the separated solid-phase lithium sulfate is 50 wt % or more.

20. The method for preparing a lithium compound according to claim 19, wherein: The cationic impurities include cations of potassium (K), sodium (Na), magnesium (Mg), boron (B), nickel (Ni), or a combination thereof, Based on the total amount (100 wt %) of cationic impurities contained in the lithium phosphate added in the step of mixing the lithium phosphate and sulfuric acid to obtain a mixture, the amount of the cationic impurities purified in the step of washing the separated solid-phase lithium sulfate is 95 wt % or more.

21. The method for preparing a lithium compound according to claim 19, wherein: The cationic impurities include calcium (Ca) cations, The amount of calcium (Ca) cation impurities purified in the step of washing the separated solid-phase lithium sulfate is 50 wt % or more, based on the total amount (100 wt %) of cationic impurities contained in the lithium phosphate added in the step of mixing the lithium phosphate and sulfuric acid to obtain a mixture.

22. The method for preparing a lithium compound according to claim 8, wherein: After the step of converting the lithium sulfate aqueous solution into a lithium hydroxide aqueous solution, a desalted solution, and a sulfuric acid aqueous solution using the electrodialysis device having a bipolar membrane, the method further comprises: a step of concentrating and crystallizing the lithium hydroxide aqueous solution converted in the bipolar membrane electrodialysis device; a step of obtaining a crystallization filtrate produced in the crystallization step; and The step of drying the crystallized lithium hydroxide to obtain powdered lithium hydroxide.

23. The method for preparing a lithium compound according to claim 22, wherein: The step of separating the lithium sulfate into a solid phase further includes the step of recovering the high-concentration phosphoric acid filtrate remaining after separating the solid phase lithium sulfate. The lithium phosphate in the step of preparing lithium phosphate is prepared by using the recovered high-concentration phosphoric acid filtrate and the crystallization filtrate.

24. The method for preparing a lithium compound according to claim 23, wherein: The lithium phosphate in the step of preparing lithium phosphate is prepared by using the recovered high-concentration phosphoric acid filtrate, the crystallization filtrate and an alkaline substance.

25. The method for preparing a lithium compound according to claim 24, wherein: The alkaline substance includes at least one of sodium hydroxide, potassium hydroxide, calcium hydroxide, barium hydroxide, magnesium hydroxide, calcium oxide, lithium, potassium and sodium.

26. The method for preparing a lithium compound according to claim 8, wherein: After the step of dissolving the solid-phase lithium sulfate in a solvent to obtain a lithium sulfate aqueous solution, the method further comprises the step of adding an alkaline substance to the obtained lithium sulfate aqueous solution to control the pH to be above 10.

27. The method for preparing a lithium compound according to claim 26, wherein: The alkaline substance includes at least one of sodium hydroxide, potassium hydroxide, calcium hydroxide, barium hydroxide, magnesium hydroxide, calcium oxide, lithium, potassium and sodium.

28. The method for preparing a lithium compound according to claim 22, wherein: After the step of dissolving the solid-phase lithium sulfate in a solvent to obtain a lithium sulfate aqueous solution, the method further includes adding the lithium hydroxide aqueous solution converted by the electrodialysis device or the crystallization filtrate obtained in the step of concentrating the lithium hydroxide aqueous solution to crystallize it to the obtained lithium sulfate aqueous solution to control the pH to above 10.

29. The method for preparing a lithium compound according to claim 26 or 28, wherein: After the step of controlling the pH of the lithium sulfate aqueous solution to be above 10, the method further comprises the step of removing cationic impurities from the lithium sulfate aqueous solution by solid phase separation.

30. The method for preparing a lithium compound according to claim 29, wherein: The cationic impurities are at least one selected from the group consisting of potassium (K), sodium (Na), magnesium (Mg), boron (B), and calcium (Ca).

31. The method for preparing a lithium compound according to claim 30, wherein: The cationic impurities are divalent cationic impurities.

32. The method for preparing a lithium compound according to claim 8, comprising: The step of reusing the desalted liquid converted by the electrodialysis device as a solvent in the step of dissolving the solid-phase lithium sulfate in a solvent to obtain a lithium sulfate aqueous solution.

33. The method for preparing a lithium compound according to claim 8, wherein: The aqueous sulfuric acid solution converted by the electrodialysis device is subjected to a concentration process and then reused in the step of mixing the lithium phosphate and sulfuric acid to obtain a mixture.

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