Method for preparing lithium hydroxide monohydrate from lithium-containing solution recovered from waste lithium iron phosphate battery and application thereof

Abstract

The application discloses a method for preparing lithium hydroxide monohydrate from lithium-containing solution recovered from waste lithium iron phosphate batteries and application thereof. The method comprises the following steps: S1, taking the lithium-containing solution, adjusting the pH to 4-8, removing the precipitate after reaction, and collecting the filtrate I; S2, adjusting the pH of the filtrate I collected in step S1 to 11-13, removing the precipitate after reaction, and collecting the filtrate II; S3, mixing the filtrate II with a sodium hydroxide solution, evaporating and concentrating to obtain a causticizing solution; S4, pre-cooling and low-temperature freezing the causticizing solution, precipitating sodium sulfate decahydrate, and collecting the frozen solution; S5, evaporating and crystallizing the frozen solution, and separating the solid and the liquid to obtain crude lithium hydroxide monohydrate; S6, washing the crude lithium hydroxide monohydrate, dissolving it into a lithium hydroxide solution with a lithium concentration of 25-40 g / L, evaporating and crystallizing, and thus battery-grade lithium hydroxide monohydrate is obtained. The method has multiple advantages such as simple operation, short process, low cost and environmental friendliness.

Description

Technical Field

[0001] This invention relates to the field of waste battery material recycling technology, specifically to a method for preparing lithium hydroxide monohydrate from waste lithium iron phosphate battery lithium-containing solution and its application. Background Technology

[0002] Olivine-type lithium iron phosphate (LFP) cathode materials have attracted much attention due to their low cost, safety, structural stability, abundant raw materials, and excellent electrochemical performance. The typical lifespan of lithium-ion batteries is 6-8 years, and a large number of spent LFP batteries will inevitably be retired at the end of their service life. Furthermore, spent LFP batteries contain toxic and hazardous substances, including heavy metals, electrolytes, organic binders, and conductive agents, causing environmental pollution and ecological harm. From another perspective, spent LFP batteries are also a resource treasure trove, with lithium being the most valuable element for recycling. Typically, Fe and P in lithium iron phosphate are recovered as iron phosphate precipitation, while the most valuable lithium remains in the solution, accompanied by various impurities such as Co, Ni, Mn, P, Fe, Cu, Ca, Mg, and Al, resulting in a very complex composition. Therefore, developing an environmentally sustainable process for recovering lithium from spent LFP batteries is crucial. For lithium recovery, sodium carbonate solution precipitation is often used to precipitate lithium carbonate.

[0003] In recent years, lithium hydroxide monohydrate, one of the two most important basic lithium salts along with lithium carbonate, has received increasing attention. Compared to lithium carbonate, the high solubility and low melting point of lithium hydroxide monohydrate make it more suitable for various types of lithium-ion battery cathode materials. Especially for high-nickel ternary lithium-ion battery cathode materials, lithium hydroxide monohydrate can provide higher activity and complete cathode synthesis at lower temperatures (650 ℃~700 ℃), thereby extending battery life and enhancing safety performance. Patent CN111017964A discloses a method for producing lithium hydroxide by evaporation crystallization. This method obtains a mixture of anhydrous sodium sulfate and lithium hydroxide during the first evaporation crystallization, and the mixed salt contains only 12 wt% lithium. By redissolving the above mixed salt and then evaporating and crystallizing again, lithium hydroxide is obtained, but its dry basis purity is only about 98%, with high impurity content, and only industrial-grade lithium hydroxide can be obtained. Patent CN116581414A discloses a method for recycling acid leaching mother liquor to recover lithium, phosphorus, and iron from waste lithium iron phosphate. This method uses calcium hydroxide or calcium oxide to adjust the pH to 8-12 to purify the lithium sulfate solution, then adds barium hydroxide (0.9-1 times the lithium content of the purified solution), and separates the solid and liquid phases to obtain barium sulfate and lithium hydroxide solutions. To remove calcium and barium ions from the solution, carbon dioxide is bubbled through the solution to generate barium carbonate and calcium carbonate, which are then filtered out. Carbon dioxide is bubbled through the solution again to adjust the pH to 9-11, resulting in a lithium carbonate slurry. This slurry is heated to above 90 °C, and EDTA-2Na or EDTA-4Na is added for further impurity removal. After solid-liquid separation, lithium carbonate carbide is obtained. The lithium carbonate carbide is then washed with hot water, filtered, and dried to obtain battery-grade lithium carbonate. Although this method can recover lithium from lithium iron phosphate in the form of battery-grade lithium carbonate, the process is complex and lengthy, with high energy consumption and production costs. In particular, the use of toxic and hazardous substances such as barium hydroxide not only increases the safety risks for operators and personnel but is also environmentally unfriendly. Furthermore, the conditions for removing calcium and barium ions using carbon dioxide are difficult to control, causing lithium carbonate to enter the slag and resulting in lithium loss.

[0004] Therefore, it is of great significance to develop a new method for recovering lithium from spent lithium iron phosphate batteries. Summary of the Invention

[0005] This invention aims to at least solve one of the technical problems existing in the prior art. To this end, this invention proposes a method for preparing lithium hydroxide monohydrate from recycled lithium iron phosphate battery lithium-containing solution. This method can efficiently, cost-effectively, and environmentally friendly produce battery-grade lithium hydroxide monohydrate using recycled lithium iron phosphate battery lithium-containing solution as raw material.

[0006] The present invention also proposes applications of the above method.

[0007] According to one aspect of the present invention, a method for preparing lithium hydroxide monohydrate from lithium-containing solution recovered from waste lithium iron phosphate batteries is provided, comprising the following steps: S1. Take a lithium-containing solution, adjust the pH to 4-8, react, remove the precipitate, and collect filtrate I; S2. Adjust the pH of filtrate I collected in step S1 to 11-13. After the reaction, remove the precipitate and collect filtrate II. S3. Mix the filtrate II with sodium hydroxide solution, evaporate and concentrate to obtain causticizing solution; S4. After pre-cooling and freezing the causticizing solution at low temperature, sodium sulfate decahydrate is precipitated, and the freezing solution is collected. S5. Evaporate and crystallize the freezing liquid, and separate the solid and liquid to obtain crude lithium hydroxide monohydrate; S6. After washing the crude lithium hydroxide monohydrate, dissolve it into a lithium hydroxide solution with a lithium concentration of 25~40g / L, evaporate and crystallize to obtain battery-grade lithium hydroxide monohydrate.

[0008] According to some embodiments of the present invention, the lithium-containing solution is the mother liquor obtained by separating the iron phosphate precipitate after acid leaching, pH adjustment, and oxidation with an oxidant from waste lithium iron phosphate black powder. This solution is the mother liquor after the first step of the two-step process for preparing iron phosphate from black powder, after separating the yellow material. Some technologies use it to prepare lithium carbonate, but only crude or industrial-grade products can be obtained, and it requires multiple processes such as carbonization and thermal decomposition, which are lengthy and complex. The present invention can cleverly convert it directly into battery-grade lithium hydroxide monohydrate, which can be directly used in the preparation of cathode materials such as ternary lithium batteries, significantly improving economic value.

[0009] According to some embodiments of the present invention, the lithium-containing solution contains Co: 0.15~1.2 g / L, Ni: 0.8~1.8 g / L, Ca: 0.02-0.12 g / L, Mg: 0.01~0.1 g / L, Fe: 1.5~3.0 g / L, Na: 15~50 g / L, Al: 0.2~0.6 g / L, P: 0.2~0.7 g / L, Li: 3~8 g / L, and SO42-. 2- :80~130g / L.

[0010] According to some embodiments of the present invention, the pH is adjusted to 6-8 in step S1.

[0011] According to some embodiments of the present invention, the reaction temperature in step S1 is 25~95℃; the reaction time is 30~90min.

[0012] According to some embodiments of the present invention, the reaction temperature in step S2 is 25~95℃; the reaction time is 30~90min.

[0013] According to some embodiments of the present invention, the reactions in steps S1 and S2 are both carried out under stirring, and the stirring speed is independently selected from 150 to 300 rpm.

[0014] According to some embodiments of the present invention, the pH is adjusted using sodium hydroxide solution in steps S1 and S2. In the recycling method of the present invention, battery-grade lithium hydroxide monohydrate can be directly obtained using only sodium hydroxide solution as the single processing reagent. No other impurity removal agents or auxiliary materials are required throughout the process, thus significantly simplifying the operation, reducing raw material and processing costs, and effectively improving the environmental friendliness of the process.

[0015] According to some embodiments of the present invention, the mass concentration of sodium hydroxide in the sodium hydroxide solution added in step S3 and the sodium hydroxide solution used for pH adjustment are independently selected from 30-40%, such as 32%.

[0016] According to some embodiments of the present invention, in step S3, the sodium hydroxide solution has a molar ratio of Li:OH - Add in a ratio of 1:1 to 1.35.

[0017] According to some embodiments of the present invention, in step S3, the sodium hydroxide solution has a molar ratio of Li:OH - Add at a ratio of 1:1 to 1.2.

[0018] According to some embodiments of the present invention, the lithium concentration in the causticizing solution is 10~35 g / L.

[0019] According to some embodiments of the present invention, the lithium concentration in the causticizing solution is 15~25 g / L.

[0020] According to some embodiments of the present invention, the precooling temperature is 10~35°C.

[0021] According to some embodiments of the present invention, the precooling temperature is 10~25°C.

[0022] According to some embodiments of the present invention, the freezing temperature is 0 to -15°C.

[0023] According to some embodiments of the present invention, the freezing temperature is -6 to -12°C.

[0024] According to some embodiments of the present invention, step S4 further includes the step of adding seed crystals before evaporation and crystallization.

[0025] According to some embodiments of the present invention, the seed crystal is lithium hydroxide monohydrate or anhydrous lithium hydroxide.

[0026] According to some embodiments of the present invention, the amount of seed crystals added is 0~35 g / L, such as 5 g / L, 10 g / L, 15 g / L, 20 g / L, 25 g / L, 30 g / L, etc.

[0027] According to some embodiments of the present invention, the amount of seed crystals added is 25~35 g / L.

[0028] According to some embodiments of the present invention, the temperature of the evaporation crystallization is 60~100°C.

[0029] According to some embodiments of the present invention, in step S5, the concentration of the crude lithium hydroxide monohydrate evaporation slurry is controlled at 20% to 50%. After the slurry concentration reaches 20% to 50%, evaporation is stopped and the mixture is filtered while still hot.

[0030] According to some embodiments of the present invention, the washing includes agitation, and the agitation is performed 1 to 4 times.

[0031] According to some embodiments of the present invention, the solid-to-liquid mass ratio of the washing liquid is 1:0.9~1.1, the washing temperature is 30℃~90℃, and the washing time for a single wash is 5 min~50 min.

[0032] According to some embodiments of the present invention, the agitation temperature is 60 ℃ to 90 ℃, and the agitation time for a single agitation is 5 min to 30 min.

[0033] According to some embodiments of the present invention, the agitation speed is 150~300 rpm.

[0034] According to some embodiments of the present invention, the concentration of the evaporated crystal slurry in step S6 is 30% to 50%.

[0035] According to some embodiments of the present invention, the concentration of the evaporated crystal slurry in step S6 is 30%~40%.

[0036] According to some embodiments of the present invention, step S6 further includes a step of vacuum drying after evaporation and crystallization, wherein the temperature of vacuum drying is 50~90°C.

[0037] According to some embodiments of the present invention, the temperature of the vacuum drying is 60~80°C.

[0038] According to another aspect of the present invention, the application of the above method in the preparation of lithium battery cathode materials is provided.

[0039] Compared with the prior art, the present invention has at least the following beneficial effects: 1. Based on the high Al and P content in lithium-containing solutions, this invention proposes a two-step precipitation method for phosphorus and impurity removal. The first step adjusts the pH to 4-8, causing Al and P to precipitate as aluminum hydroxide and monohydrogen phosphate, respectively. The second step adjusts the pH to 11-13, allowing metal ion impurities such as Co, Ni, Mn, Mg, and Ca to be removed as hydroxide precipitates. This avoids the use of resins, EDTA, and phosphorus removal agents, and other chemical reagents for impurity removal. Furthermore, the entire impurity removal process uses only sodium hydroxide, avoiding the risk of introducing impurities through additional auxiliary materials. The resulting lithium sulfate solution fully meets the requirements for producing high-purity battery-grade lithium hydroxide monohydrate.

[0040] 2. The present invention avoids the use of EDTA, phosphorus removal agents, and especially barium salts, which greatly reduces the safety risks of personnel operation, alleviates environmental pressure, and shortens the production process and reduces production costs.

[0041] 3. The method for preparing battery-grade lithium hydroxide monohydrate from lithium-containing solutions recycled from waste lithium iron phosphate batteries proposed in this invention has the characteristics of simple operation, short process, low cost and environmental friendliness, high economic added value, and is conducive to large-scale production.

[0042] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0043] Figure 1 The image shows the XRD pattern of the battery-grade lithium hydroxide monohydrate prepared in Example 1.

[0044] Figure 2 The forms in which Al exists under different pH conditions.

[0045] Figure 3 The forms in which H3PO4 exists under different pH conditions.

[0046] Figure 4 The values ​​represent the P and Al content of the phosphorus removal solution obtained by the two-step precipitation method in Example 1.

[0047] Figure 5 The content of metal ion impurities in the purification solution obtained by the two-step precipitation method in Example 1. Detailed Implementation

[0048] The following will clearly and completely describe the concept and technical effects of the present invention in conjunction with embodiments, so as to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the scope of protection of the present invention. Unless otherwise specified, the experimental methods used in the embodiments are conventional methods; the materials and reagents used, unless otherwise specified, are commercially available. Unless otherwise specified, the same parameter value is the same in all embodiments. The embodiments described below are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0049] In the description of this invention, references to terms such as "some embodiments" indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0050] In the description of this invention, the use of I, II, etc., is only for the purpose of distinguishing technical features and should not be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated or the order of the technical features indicated.

[0051] Unless otherwise specified, "room temperature" in this invention means 25℃±5℃, and 25℃ in the examples and comparative examples.

[0052] Unless otherwise specified, "%" in this invention represents the mass fraction.

[0053] In the description of this invention, "lithium concentration" refers to the concentration based on elemental lithium.

[0054] This invention provides a method for preparing battery-grade lithium hydroxide monohydrate from lithium-containing solutions of spent lithium iron phosphate batteries. Addressing the complex multi-ion system of this lithium-containing solution, this invention first employs a two-step pH method to selectively remove aluminum, phosphorus, iron, and other metal ion impurities, obtaining filtrate II with low impurity content. Subsequently, through causticization and freezing operations, battery-grade lithium hydroxide monohydrate product is obtained. Specifically, the lithium-containing solution obtained from the recycling of spent lithium iron phosphate batteries undergoes phosphorus and impurity removal treatment. The resulting purified solution is mixed with a sodium hydroxide solution and then concentrated by evaporation to increase the lithium-ion concentration. The concentrated solution is then frozen and crystallized at low temperature to precipitate sodium sulfate decahydrate, achieving the transformation of the lithium sulfate solution into a lithium hydroxide solution. The resulting crude lithium hydroxide solution is evaporated and crystallized to obtain crude lithium hydroxide monohydrate crystals. These crystals are then subjected to refining steps such as stirring and recrystallization to finally obtain a lithium hydroxide monohydrate product that meets battery-grade standards. The method of this invention achieves efficient recovery of lithium from waste lithium iron phosphate batteries, with significant advantages such as high lithium recovery rate, low energy consumption, short process, and environmental friendliness. While effectively reducing the environmental pollution pressure of solid waste, it realizes the high added value utilization of lithium resources and has good economic benefits and industrial application prospects.

[0055] The lithium-containing solutions used in the following examples and comparative studies are the filtrate from the first step of the two-step process for preparing iron phosphate from waste lithium iron phosphate battery black powder after acid leaching, following the separation of the yellow material. Its composition information is shown in Table 1 below.

[0056] Table 1 Main components of lithium-containing solutions

[0057] If described otherwise, the stirring speed during stirring and washing in the following examples and comparative examples is 200 rpm.

[0058] Example 1 This example provides a method for preparing lithium hydroxide monohydrate from lithium iron phosphate battery-containing solutions after recycling waste lithium iron phosphate batteries. The specific steps are as follows: S1. Take 4 L of lithium-containing solution from waste lithium iron phosphate batteries, adjust the pH to 7 with 32 wt% sodium hydroxide solution at room temperature, react and stir for 30 min, and filter to obtain the filtrate after the reaction is complete.

[0059] S2. The filtrate obtained in step S1 is heated to 40 °C, and 32 wt% sodium hydroxide solution is added to adjust the pH of the solution to 12. The reaction is carried out and stirred for 35 min. After the reaction is completed, the filtrate is filtered.

[0060] S3. Add 32 wt% sodium hydroxide solution to the filtrate obtained in step S2, controlling the molar ratio Li:OH. - The solution was prepared by mixing the lithium in a 1:1 ratio and then evaporating and concentrating it in a rotary evaporator to obtain a causticized solution with a lithium concentration of 15 g / L.

[0061] S4. Cool the causticizing liquid obtained in step S3 to 10 ℃, place it in a high and low temperature integrated constant temperature bath and freeze it to -6 ℃, and then filter and separate it to obtain the cryogenic liquid.

[0062] S5. Add 25 g / L of lithium hydroxide monohydrate seed crystals to the cryosol obtained in step S4, and place it in a rotary evaporator at 70 °C for evaporation and crystallization. Control the crystal slurry concentration to 20%, and obtain crude lithium hydroxide monohydrate after filtration.

[0063] S6. The crude lithium hydroxide monohydrate obtained in step S5 is washed twice with a liquid-to-solid mass ratio of 1:1 at a temperature of 70 °C for 5 min each time. The solution is then redissolved to obtain a lithium hydroxide solution with a lithium concentration of 25 g / L. This solution is then evaporated and crystallized in a rotary evaporator at 70 °C, maintaining a crystal slurry concentration of 30%. After filtration, the solution is dried in a vacuum drying oven at 60 °C to obtain the final product.

[0064] Example 2 This example provides a method for preparing lithium hydroxide monohydrate from lithium iron phosphate battery-containing solutions after recycling waste lithium iron phosphate batteries. The specific steps are as follows: S1: Take 4 L of lithium-containing solution from waste lithium iron phosphate batteries, heat it to 40 ℃, add 32 wt% sodium hydroxide solution to adjust the pH to 6, react and stir for 45 min, and filter to obtain the filtrate after the reaction is complete.

[0065] S2: The filtrate obtained in step S1 is heated to 50 ℃, and 32 wt% sodium hydroxide solution is added to adjust the pH of the solution to 11. The reaction is carried out and stirred for 35 min. After the reaction is completed, the filtrate is obtained by filtration.

[0066] S3: Add 32 wt% sodium hydroxide solution to the filtrate obtained in step S2, controlling the molar ratio Li:OH - The ratio was 1:1.05, and then the mixture was placed in a rotary evaporator for evaporation and concentration to obtain a causticized solution with a lithium concentration of 20 g / L.

[0067] S4: Cool the causticizing liquid obtained in step S3 to 15 ℃, place it in a high and low temperature constant temperature bath and freeze it to -8 ℃, and then filter and separate it to obtain the cryosol.

[0068] S5: Add 30 g / L of lithium hydroxide monohydrate seed crystals to the cryosol obtained in step S4, and place it in a rotary evaporator at 80 °C for evaporation and crystallization. Control the crystal slurry concentration to 25%, and obtain crude lithium hydroxide monohydrate after filtration.

[0069] S6: The crude lithium hydroxide monohydrate obtained in step S5 is washed twice with a liquid-to-solid mass ratio of 1:1 at 70°C for 10 minutes each time. It is then redissolved to form a lithium hydroxide solution with a lithium concentration of 30 g / L. This solution is then evaporated and crystallized in a rotary evaporator at 80°C, controlling the crystal slurry concentration to 35%. After filtration, it is dried in a vacuum drying oven at 65°C to obtain the final product.

[0070] Example 3 This example provides a method for preparing lithium hydroxide monohydrate from lithium iron phosphate battery-containing solutions after recycling waste lithium iron phosphate batteries. The specific steps are as follows: S1: Take 4 L of lithium-containing solution from waste lithium iron phosphate batteries, heat to 50 ℃, add 32 wt% sodium hydroxide solution to adjust the pH to 6.5, react and stir for 60 min, and filter to obtain the filtrate after the reaction is complete.

[0071] S2: The filtrate obtained in step S1 is heated to 60 ℃, and 32wt% sodium hydroxide solution is added to adjust the pH to 11.5. The mixture is reacted and stirred for 50 min. After the reaction is completed, the filtrate is filtered.

[0072] S3: Add 32 wt% sodium hydroxide solution to the filtrate obtained in step S2, controlling the molar ratio Li:OH - The ratio was 1:1.10, and then the mixture was evaporated and concentrated in a rotary evaporator to obtain a causticized solution with a lithium concentration of 25 g / L.

[0073] S4: Cool the causticizing liquid obtained in step S3 to 20 ℃, place it in a high and low temperature constant temperature bath and freeze it to -10 ℃, and then filter and separate it to obtain the cryosol.

[0074] S5: Add 28 g / L of anhydrous lithium hydroxide seed crystals to the cryosol obtained in step S4, and place it in a rotary evaporator at 85 °C for evaporation and crystallization. Control the crystal slurry concentration to 30%, and obtain crude lithium hydroxide monohydrate after filtration.

[0075] S6: The crude lithium hydroxide monohydrate obtained in step S5 is washed twice with a liquid-to-solid mass ratio of 1:1 at 80℃ for 20 min each time. It is then redissolved to form a lithium hydroxide solution with a lithium concentration of 35 g / L. This solution is then evaporated and crystallized in a rotary evaporator at 85℃, controlling the crystal slurry concentration to 40%. After filtration, it is dried in a vacuum drying oven at 70℃ to obtain the finished product.

[0076] Example 4 This example provides a method for preparing lithium hydroxide monohydrate from lithium iron phosphate battery-containing solutions after recycling waste lithium iron phosphate batteries. The specific steps are as follows: S1: Measure 4 L of lithium-containing solution from waste lithium iron phosphate batteries, heat to 70 ℃, add 32 wt% sodium hydroxide solution to adjust pH to 8, react and stir for 55 min, and filter to obtain filtrate after the reaction is complete.

[0077] S2: The filtrate obtained in step S1 is heated to 85 ℃, and 32wt% sodium hydroxide solution is added to adjust the pH to 13. The mixture is reacted and stirred for 25 min. After the reaction is completed, the filtrate is filtered.

[0078] S3: Add 32 wt% sodium hydroxide solution to the filtrate obtained in step S2, controlling the molar ratio Li:OH - The ratio was 1:1.20, and then the mixture was evaporated and concentrated in a rotary evaporator to obtain a causticized solution with a lithium concentration of 23 g / L.

[0079] S4: Cool the causticizing liquid obtained in step S3 to 25 ℃, place it in a high and low temperature constant temperature bath and freeze it to -12 ℃, and then filter and separate it to obtain the cryosol.

[0080] S5: Add 35 g / L of anhydrous lithium hydroxide seed crystals to the cryosol obtained in step S4, and place it in a rotary evaporator at 75 °C for evaporation and crystallization. Control the crystal slurry concentration to 35%, and obtain crude lithium hydroxide monohydrate after filtration.

[0081] S6: The crude lithium hydroxide monohydrate obtained in step S5 is washed twice with a liquid-to-solid mass ratio of 1:1 at 85°C for 30 min each time. It is then redissolved to form a lithium hydroxide solution with a lithium concentration of 38 g / L. This solution is then evaporated and crystallized in a rotary evaporator at 75°C, controlling the crystal slurry concentration to 32%. After filtration, it is dried in a vacuum drying oven at 75°C to obtain the final product.

[0082] The compositional analysis of the finished products prepared in Examples 1, 2, 3 and 4 is shown in Table 2.

[0083] Table 2 shows the compositional analysis of the battery-grade lithium hydroxide monohydrate prepared in the examples.

[0084] In the table, " / " indicates a value below the detection limit of ICP (inductively coupled plasma).

[0085] As can be seen from Table 1, the lithium hydroxide monohydrate prepared in different embodiments has high purity, few impurities and good quality, and fully meets the requirements of the national standard (GB / T 26008-2020) for battery-grade lithium hydroxide monohydrate.

[0086] Figure 1 This is the XRD powder diffraction pattern of the battery-grade lithium hydroxide monohydrate prepared in Example 1. Figure 1It can be seen that the diffraction peaks of the sample are completely consistent with the standard diffraction card of lithium hydroxide monohydrate, and no diffraction peaks of other impurities were observed, which indicates that the prepared lithium hydroxide monohydrate is a high-purity phase.

[0087] Figure 2 and Figure 3 These represent the forms in which Al and H3PO4 exist under different pH conditions. Figure 2 and Figure 3 It can be seen that within the pH range of 6-8, Al mainly exists stably as Al(OH)3 precipitate, while P mainly exists as HPO4. 2- HPO4 2- The metal salts are precipitates. It is worth noting that as the pH increases, Al will also precipitate as AlO2. - Al and P can be redissolved into the solution in the form of hydroxides. Therefore, a two-step precipitation method can be used: first, Al and P are removed within the pH range of 6-8 to avoid the redissolution of Al at high pH values ​​and the use of phosphorus removal agents; then, the pH is adjusted to 11-13 to further precipitate the metal ions as hydroxides.

[0088] Figure 4 The values ​​represent the P and Al content of the phosphorus removal solution obtained by the two-step precipitation method in Example 1. Figure 4 It can be seen that under pH=7 conditions, the P and Al in the phosphorus removal solution decreased from 400 ppm and 44 ppm to less than 5 ppm. Figure 5 The content of metal ion impurities in the purification solution obtained by the two-step precipitation method in Example 1. Figure 5 It can be seen that by further adjusting the pH to 12, the metal ions in the impurity removal solution can be significantly reduced to below 3 ppm, which is perfectly suitable for the subsequent preparation of high-purity battery-grade lithium hydroxide monohydrate.

[0089] Examples 5-8 This example provides a method for preparing lithium hydroxide monohydrate from lithium iron phosphate battery recycled lithium-containing solution. The other steps are the same as in Example 1, except that the pH value in step S1 is changed to 4, 5, 6, and 8. The effect on the removal of P and Al in the first step of the two-step precipitation method is shown in Table 3.

[0090] Comparative Examples 1-2 This example provides a method for preparing lithium hydroxide monohydrate from lithium iron phosphate battery recycled lithium-containing solution. The other steps are the same as in Example 1, except that the pH value in step S1 is changed to 3 and 9. The effect on the removal of P and Al in the first step of the two-step precipitation method is shown in Table 3.

[0091] Table 3. Effects of pH value on phosphorus and al removal.

[0092] Data analysis from the above examples and comparative examples (Table 3) shows that the concentrations of P and Al in the solution decrease significantly with increasing pH. When the pH is between 4 and 8, the concentrations remain stable below 20 ppm; when the pH is between 6 and 8, the concentrations remain stable below 5 ppm. Further increases or decreases in pH lead to redissolution of P and Al, resulting in increased concentrations. Therefore, the pH value for the first step of the two-step precipitation method to remove P and Al should be controlled between 4 and 8, preferably between 6 and 8.

[0093] Examples 9-10 This example provides a method for preparing lithium hydroxide monohydrate from lithium-containing solutions recycled from waste lithium iron phosphate batteries. The other steps are the same as in Example 1, except that the pH value in step S2 is changed to 11 or 13. The effect of this change on the removal of metal ions in the second step of the two-step precipitation method is shown in Table 4.

[0094] Comparative Examples 3-4 This example provides a method for preparing lithium hydroxide monohydrate from lithium iron phosphate battery recycled lithium-containing solution. The other steps are the same as in Example 1, except that the pH value in step S2 is changed to 9 or 10. The effect of this change on the removal of metal ions in the second step of the two-step precipitation method is shown in Table 4.

[0095] Table 4. Effect of pH value on the removal of metal ions

[0096] Data analysis from the above embodiments (Table 4) shows that the total amount of metal impurity ions in the solution of the second step of the two-step precipitation method decreases significantly with increasing pH value, reaching less than 8 ppm within the pH range of 11-13, which fully meets the requirements for producing high-purity battery-grade lithium hydroxide monohydrate. Therefore, the pH value for removing metal impurity ions in the second step of the two-step precipitation method should be controlled between 11 and 13.

[0097] Examples 11-17 This example provides a method for preparing lithium hydroxide monohydrate from recycled lithium iron phosphate battery lithium-containing solution. The other steps are the same as in Example 1, except that the amount of lithium hydroxide monohydrate seed crystals added in step S5 is changed to 0 g / L, 5 g / L, 10 g / L, 15 g / L, 20 g / L, 30 g / L, and 35 g / L, to control the Na and SO4 content in the crude lithium hydroxide monohydrate. 2- The effect of content is shown in Table 5.

[0098] Table 5. Effects of Seed Crystal Addition Amount on Na and SO4 in Crude Lithium Hydroxide Monohydrate 2- Effect of content

[0099] Data analysis from the above embodiments (Table 5) shows that the addition of seed crystals can significantly reduce Na and SO4 in crude lithium hydroxide monohydrate. 2-The content of Na and SO42- in the crude lithium hydroxide monohydrate is beneficial for the subsequent preparation of battery-grade lithium hydroxide monohydrate. After the seed crystal addition reaches 25 g / L-35 g / L, the Na and SO42- content in the crude lithium hydroxide monohydrate is reduced. 2- The rate of decrease in content is reduced. Therefore, the optimal amount of seed crystals added during the evaporation and crystallization of crude lithium hydroxide monohydrate is 25 g / L-35 g / L.

[0100] The embodiments of the present invention have been described in detail above, but the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A method for preparing lithium hydroxide monohydrate from lithium-containing solution recovered from waste lithium iron phosphate batteries, characterized in that: Includes the following steps: S1. Take a lithium-containing solution, adjust the pH to 4-8, react, remove the precipitate, and collect filtrate I; S2. Adjust the pH of filtrate I collected in step S1 to 11-13. After the reaction, remove the precipitate and collect filtrate II. S3. Mix the filtrate II with sodium hydroxide solution, evaporate and concentrate to obtain causticizing solution; S4. After pre-cooling and freezing the causticizing solution at low temperature, sodium sulfate decahydrate is precipitated, and the freezing solution is collected. S5. Evaporate and crystallize the freezing liquid, and separate the solid and liquid to obtain crude lithium hydroxide monohydrate; S6. After washing the crude lithium hydroxide monohydrate, dissolve it into a lithium hydroxide solution with a lithium concentration of 25~40g / L, evaporate and crystallize to obtain battery-grade lithium hydroxide monohydrate.

2. The method for preparing lithium hydroxide monohydrate from recycled lithium iron phosphate battery solution according to claim 1, characterized in that: The lithium-containing solution is the mother liquor obtained by separating the iron phosphate precipitate after acid leaching, pH adjustment, and oxidation with an oxidant from waste lithium iron phosphate black powder; and / or, the lithium-containing solution contains Co: 0.15~1.2 g / L, Ni: 0.8~1.8 g / L, Ca: 0.02-0.12 g / L, Mg: 0.01~0.1 g / L, Fe: 1.5~3.0 g / L, Na 15~50 g / L, Al: 0.2~0.6 g / L, P: 0.2~0.7 g / L, Li: 3~8 g / L, SO42- 2- :80~130g / L.

3. The method for preparing lithium hydroxide monohydrate from recycled lithium iron phosphate battery solution according to claim 1, characterized in that: In step S1, the pH is adjusted to 6-8.

4. The method for preparing lithium hydroxide monohydrate from recycled lithium iron phosphate battery solution according to claim 1, characterized in that: In step S3, the sodium hydroxide solution has a molar ratio of Li:OH - Add in a ratio of 1:1 to 1.

35.

5. The method for preparing lithium hydroxide monohydrate from recycled lithium iron phosphate battery solution according to claim 1, characterized in that: Step S4 further includes adding seed crystals before evaporation and crystallization. The seed crystals are lithium hydroxide monohydrate or anhydrous lithium hydroxide, and the amount of seed crystals added is 0~35g / L.

6. The method for preparing lithium hydroxide monohydrate from recycled lithium iron phosphate battery solution according to claim 5, characterized in that: The amount of seed crystals added is 25~35g / L.

7. The method for preparing lithium hydroxide monohydrate from recycled lithium iron phosphate battery solution according to claim 1, characterized in that: The washing process includes agitation, which is performed 1 to 4 times.

8. The method for preparing lithium hydroxide monohydrate from recycled lithium iron phosphate battery solution according to claim 7, characterized in that: The solid-to-liquid mass ratio of the washing solution is 1:0.9~1.1, the washing temperature is 30 ℃~90 ℃, and the washing time for a single wash is 5 min~50 min.

9. The method for preparing lithium hydroxide monohydrate from recycled lithium iron phosphate battery solution according to claim 1, characterized in that: In step S6, the concentration of the evaporated crystal slurry is 30%~50%.

10. The application of the method according to any one of claims 1 to 9 in the preparation of lithium battery cathode materials.

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