Method for preparing battery-grade lithium dihydrogen phosphate by secondary lithium precipitation using a hydrated phosphorus source

Abstract

This invention belongs to the field of lithium-ion battery material technology and discloses a method for preparing battery-grade lithium dihydrogen phosphate using a hydrated phosphorus source through secondary lithium precipitation. The method includes: mixing a lithium-containing purified solution with a hydrated phosphorus source for a first lithium precipitation reaction, followed by solid-liquid separation to obtain primary crude lithium phosphate and primary lithium precipitation mother liquor; mixing the primary lithium precipitation mother liquor with a hydrated phosphorus source for a second lithium precipitation reaction, followed by solid-liquid separation to obtain secondary lithium phosphate; combining the primary crude lithium phosphate and secondary lithium phosphate and dissolving them in a phosphoric acid solution to obtain a crude lithium dihydrogen phosphate solution; purifying the crude lithium dihydrogen phosphate solution by crystallization, centrifuging, and drying to obtain battery-grade lithium dihydrogen phosphate. This invention utilizes a hydrated phosphorus source through a secondary lithium precipitation and primary crystallization process to achieve efficient enrichment of lithium and deep removal of impurities, effectively improving the purity and yield of lithium dihydrogen phosphate.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery materials technology, and in particular to a method for preparing battery-grade lithium dihydrogen phosphate by secondary lithium precipitation using a hydrated phosphorus source. Background Technology

[0002] Currently, existing methods for refining high-purity lithium dihydrogen phosphate mainly focus on multi-stage precipitation and resin impurity removal. Representative methods are as follows: Multi-stage impurity removal and reconstruction method: Taking Chinese application CN118164445A as an example, this type of method adopts the path of "primary lithium precipitation - acidification and dissolution - secondary impurity removal with addition of precipitant - purification with ion exchange resin". Although this scheme can effectively reduce the metal ion content, the process is extremely cumbersome, and the resin is very prone to "poisoning" and failure when treating high-concentration lithium-rich solutions due to organic matter or metal scale, requiring frequent regeneration. The production cycle (usually exceeding 25 hours) makes it difficult to achieve industrial economics.

[0003] Regarding the preparation of battery-grade lithium dihydrogen phosphate, existing technologies have the following drawbacks: 1. Traditional battery-grade lithium dihydrogen phosphate preparation methods mainly rely on high-purity lithium carbonate or lithium hydroxide as the lithium source, resulting in high production costs and strict requirements on resource grade, which limits the source of raw materials. For example, Chinese patent applications CN118561247A and CN118164445A, etc.

[0004] 2. Traditional neutralization methods heavily rely on battery-grade lithium salts, and the concentrated liquid phosphoric acid commonly used in industrial production is highly hygroscopic, leading to concentration fluctuations. This uncertainty makes it difficult to precisely control the "phosphorus-lithium molar ratio" during the reaction, easily generating lithium phosphate or lithium hydrogen phosphate impurity phases, directly affecting the crystallization purity of the product. Examples include Chinese patents CN100482577C and CN117509579B.

[0005] 3. The purity of battery-grade lithium dihydrogen phosphate prepared from lithium-rich solutions is not ideal. Lithium-rich solutions are liquid mixtures with a high concentration of lithium ions produced during lithium salt production. These mixtures often contain high concentrations of impurity ions such as sodium, potassium, magnesium, and calcium, which severely affect electrical performance. Existing technologies struggle to efficiently and thoroughly remove these impurities, resulting in less than ideal purity of the prepared lithium dihydrogen phosphate. Examples include Chinese patent applications CN116143091A (lithium chloride brine) and CN121778682A (lithium-containing purification solution).

[0006] 4. Existing process routes often involve multiple crystallization purification and separation steps, which are complex, energy-intensive, and may generate large amounts of waste liquid or residue, putting pressure on the environment. For example, Chinese patent applications CN104445128A and CN120208170A.

[0007] 5. During the preparation process, the reaction between lithium carbonate and phosphoric acid releases a large amount of carbon dioxide gas, which can easily cause material spillage and foam entrainment in the equipment, resulting in material loss. It also poses safety hazards, as shown in Chinese application CN101767782A.

[0008] 6. Existing technologies still have shortcomings in improving lithium leaching rates and lithium dihydrogen phosphate yields, making it difficult to achieve efficient resource utilization. Examples include Chinese patent applications CN120208170A and CN121247747A.

[0009] To address the challenges of high raw material costs, difficulty in impurity removal, and complex processes in the preparation of battery-grade lithium dihydrogen phosphate, some existing literature includes: Two-step physical impurity removal method: Taking Chinese application CN101767782A as an example, this type of method generates lithium phosphate through a two-step reaction of lithium hydroxide and phosphoric acid, followed by acidification. However, the inventors of this application recognize that although this type of method utilizes the lithium phosphate precipitation process to exclude some soluble impurities, it still lacks an effective in-situ blocking method for the physical encapsulation effect of non-alkali metal impurities when processing lithium-rich solutions with complex compositions.

[0010] Crude lithium salt reverse reconstruction method: Taking Chinese application CN113044820A as an example, this type of method follows the path of "lithium salt → primary lithium dihydrogen phosphate solution → precipitation of refined lithium phosphate → secondary acidification". However, the inventors of this application recognize that the essence of this scheme is "reverse reconstruction", that is, first generating lithium dihydrogen phosphate and then precipitating it back into lithium phosphate. This not only adds an extra round of reagent consumption for acid-base neutralization, but also results in extremely high theoretical acid consumption during subsequent acidification to completely convert lithium phosphate into lithium dihydrogen phosphate. Summary of the Invention

[0011] According to one embodiment of the present invention, the objective is to provide a method for preparing battery-grade lithium dihydrogen phosphate by secondary lithium precipitation using a hydrated phosphorus source, so as to solve the problems of high production cost, unsatisfactory product purity, and complex process flow in the existing battery-grade lithium dihydrogen phosphate preparation technology.

[0012] The above objective can be achieved through the following technical solutions: According to one aspect of the present invention, a method for preparing battery-grade lithium dihydrogen phosphate by secondary lithium precipitation using a hydrated phosphorus source is provided, comprising: Step S1: Add a hydrated phosphorus source to the lithium-containing purification solution, adjust the pH of the reaction solution to 7-9, carry out the first lithium precipitation reaction, and separate the solid and liquid to obtain primary crude lithium phosphate and primary lithium precipitation mother liquor. Step S2: Add a hydrated phosphorus source to the primary lithium precipitation mother liquor, adjust the pH of the reaction solution to 7-9, carry out the second lithium precipitation reaction, separate the solid and liquid, and obtain secondary lithium phosphate and secondary lithium precipitation mother liquor. Step S3: Combine the secondary lithium phosphate with the primary crude lithium phosphate, and dissolve them in phosphoric acid solution to obtain a crude lithium dihydrogen phosphate solution. Step S4: The crude lithium dihydrogen phosphate solution is crystallized and purified, centrifuged and dried to obtain battery-grade lithium dihydrogen phosphate.

[0013] Preferably, the hydrated phosphorus source is a hydrated phosphate containing water of crystallization, soluble under lithium precipitation conditions, and capable of providing at least one ion among phosphate, hydrogen phosphate, and dihydrogen phosphate.

[0014] Preferably, in the lithium-containing purification solution, Li + It is 7-12 g / L, Na + 6-12 g / L, K + The concentration is 1.6-3.5 g / L; The pH of the lithium-containing purification solution is 10-12.

[0015] Preferably, the hydrated phosphorus source is at least one of ammonium phosphate trihydrate, trisodium phosphate dodecahydrate, and sodium dihydrogen phosphate dihydrate.

[0016] Preferably, the hydrated phosphorus source is mixed in as a salt solution. More preferably, the concentration of the salt solution is 20-40% by weight.

[0017] Preferably, in step S1, the temperature of the reaction solution is 60-90°C, so that the first lithium precipitation reaction lasts for 20-60 minutes; Preferably, in step S1, the purity of the crude lithium phosphate obtained is 80-92% by weight.

[0018] Preferably, step S1 further includes: washing the primary crude lithium phosphate with deionized water.

[0019] Preferably, in step S2, the temperature of the reaction solution is 60-90°C, so that the second lithium precipitation reaction lasts for 20-60 minutes.

[0020] Preferably, in step S2, the temperature of the reaction solution is higher than that in step S1.

[0021] Preferably, step S2 further includes: returning the secondary lithium precipitation mother liquor to step S1 to participate in the primary lithium precipitation reaction.

[0022] Preferably, step S2 further includes: washing the slurry with deionized water and lithium secondary phosphate.

[0023] Preferably, in step S3, after merging, the mixture is washed with deionized water to obtain a wet lithium phosphate cake with a purity of 95-99% by weight. More preferably, the mass ratio of crude lithium phosphate to deionized water after merging is (1-3):1.

[0024] Preferably, in step S3, the concentration of the phosphoric acid solution is 45-90% by weight.

[0025] Preferably, in step S3, during acid dissolution, the pH of the mixture is controlled to be 3-5.

[0026] Preferably, in step S3, the concentration of lithium dihydrogen phosphate in the crude lithium dihydrogen phosphate solution is 1800-2820 g / L.

[0027] Preferably, step S4 further includes: returning the filtrate obtained by centrifugation to step S3 for use as a phosphoric acid solution.

[0028] Preferably, in step S4, the conditions for crystallization and purification include: a temperature of 65-90°C, a pressure of 0.038-0.045 MPa, and a solid content of 18-30% after concentration.

[0029] Preferably, in step S4, the centrifugation and drying step is performed at a drying temperature of 60-70°C for 4-6 hours.

[0030] Compared with the prior art, the embodiments of the present invention provide a method for preparing battery-grade lithium dihydrogen phosphate using a hydrated phosphorus source through a secondary lithium precipitation process, which has the following beneficial effects: 1. This invention uses hydrated phosphorus sources such as ammonium phosphate trihydrate, trisodium phosphate dodecahydrate, and sodium dihydrogen phosphate dihydrate to replace traditional anhydrous phosphorus sources, and uses lithium-containing purification liquid as the lithium source, which significantly reduces production costs, expands the sources of raw materials, reduces the requirements for resource grade, and realizes the efficient utilization of lithium resources.

[0031] 2. This invention achieves efficient enrichment of lithium and deep removal of impurities through a secondary lithium deposition process, combined with the hydrated lattice slow-release effect and stepwise phase transformation of a hydrated phosphorus source. This effectively improves the purity of lithium dihydrogen phosphate and overcomes the shortcomings of existing technologies where impurities affect battery performance.

[0032] 3. The hydrated phosphorus source of this invention has an in-situ stripping effect. The specific hydrated phosphorus source used in this invention generates a micro-kinetic environment in the secondary lithium precipitation reaction, which plays a similar role to "in-situ stripping". It can effectively remove trace acid radicals and cationic impurities between crystals (i.e., prevent impurities from entering the crystal interior), overcome the problem of low impurity removal efficiency in conventional short-process technology, improve crystal quality, and increase the purity of the final product.

[0033] 4. This invention employs a secondary lithium precipitation process, based on a "forward refining" logic, which reduces acid and alkali consumption and lithium loss. Compared to the reverse reconstruction method of crude lithium salt, this invention shifts from "reverse reconstruction" to "forward in-situ enhancement," differing from existing technologies (such as CN113044820A) that involve "re-precipitating" dissolved lithium dihydrogen products. This invention utilizes a hydrated phosphate source salt solution for secondary lithium precipitation, with the core being the precise enhancement of the crystal phase purity of lithium phosphate in the intermediate stage. Through this "forward refining" logic, the inefficient acidification-reprecipitation cycle is skipped, significantly reducing overall acid and alkali consumption and lithium loss.

[0034] 5. This invention obtains battery-grade lithium dihydrogen phosphate by improving the purity of lithium phosphate, without using lithium dihydrogen phosphate to precipitate refined lithium phosphate in reverse, and also avoids the safety hazards caused by the release of a large amount of carbon dioxide gas from the reaction of lithium carbonate and phosphoric acid in traditional methods.

[0035] 6. This invention employs a two-stage lithium precipitation and one-stage crystallization process, which simplifies the process flow, reduces energy consumption, decreases the generation of waste liquid and waste residue, reduces the environmental burden, and meets the requirements of green environmental protection.

[0036] 7. The method of the present invention is applicable to the treatment of lithium-containing waste liquid or low-grade lithium resources, and has good practicality and economy, enabling efficient recovery and utilization of lithium resources.

[0037] 8. Closed-loop circulation of the present invention. This invention utilizes the "mother liquor after the second lithium precipitation" to directly return to the first lithium precipitation stage to participate in the reaction, achieving a closed-loop circulation of the entire system. Compared to methods relying on expensive resin systems, this approach significantly reduces the overall consumption of auxiliary reagents and has stronger industrial robustness. Detailed Implementation

[0038] The embodiments of the present invention are described in detail below. However, those skilled in the art will understand that the embodiments can be modified in various ways without departing from the spirit and scope of the present invention.

[0039] The terminology used herein is for descriptive purposes only and is not intended to limit the scope of the invention. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It should be further understood that, when used in this specification, the terms “comprising” and / or “including” specify the presence of the stated features, integers, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or combinations thereof. As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items.

[0040] Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains. Terms such as those commonly used and already found in dictionaries should be interpreted as having a meaning that matches the contextual meaning in this art. In this specification, unless explicitly defined, terms should not be ideally over-interpreted as having a formal meaning.

[0041] Existing technologies suffer from high raw material costs, difficulty in impurity removal, complex processes, and significant environmental impact in the preparation of battery-grade lithium dihydrogen phosphate. These problems manifest in several ways: First, the high cost of existing raw materials such as lithium salts and phosphates significantly increases production costs. Second, impurities generated during preparation are difficult to remove, affecting the purity and performance of the final product and posing challenges for subsequent processing. Furthermore, existing processes are cumbersome and difficult to operate, increasing production complexity. Finally, byproducts such as waste liquid and waste gas generated during production burden the environment, increasing the difficulty of environmental remediation. In addition, the inventors recognize that the two-step reaction physical impurity removal method lacks effective in-situ blocking mechanisms for the physical encapsulation effect of non-alkali metal impurities when processing complex lithium-rich solutions; and the crude lithium salt reverse reconstruction method suffers from high reagent consumption for acid-base neutralization and high theoretical acid consumption.

[0042] To address the aforementioned problems, this invention provides a method for preparing battery-grade lithium dihydrogen phosphate using a hydrated phosphorus source through a secondary lithium precipitation and primary crystallization process. By optimizing the process route and improving reaction conditions, the aim is to reduce raw material consumption, increase product purity, simplify the process flow, and reduce environmental impact, thereby achieving a more efficient and environmentally friendly production process.

[0043] Explanation of the mechanism for preparing lithium dihydrogen phosphate by lithium precipitation-acid dissolution with hydrated phosphorus source in this invention: This invention uses a hydrated phosphorus source to replace the traditional anhydrous phosphorus source, and achieves the preparation of battery-grade lithium dihydrogen phosphate through the hydrated lattice slow-release effect and stepwise phase transformation. The detailed mechanism is as follows: 1) Hydrated phosphorus sources such as ammonium phosphate trihydrate, trisodium phosphate dodecahydrate, and sodium dihydrogen phosphate dihydrate all contain stable water of crystallization molecules, forming a solvation shell. Compared with anhydrous phosphorus sources, the dissociation of hydrated phosphorus sources in aqueous solutions at 20-50℃ follows a gradual peeling mechanism of the hydrated layer at the solid-liquid interface:

[0044] The presence of water of crystallization significantly reduces the instantaneous release rate of phosphate ions, avoiding local supersaturation when anhydrous phosphorus sources are added, and allowing the lithium precipitation reaction to proceed uniformly under a mild pH gradient (7-9). This slow-release effect inhibits the colloidal encapsulation of lithium ions caused by local high pH, ​​improving the density and filtration performance of the lithium precipitation crystals (the filtration rate is typically 30-50% higher than that of anhydrous phosphorus sources).

[0045] 2) Selective separation mechanism of different hydrated phosphorus sources: Ammonium phosphate trihydrate reacts with lithium-containing purification solution to form lithium phosphate precipitate, with ammonium ions entering the mother liquor. Due to the significant difference between the radius of ammonium ions (143 pm) and lithium ions (76 pm), isomorphous substitution into the lithium phosphate lattice is difficult.

[0046] The endothermic dissolution effect of the dodecahydrate in trisodium phosphate dodecahydrate stabilizes the reaction temperature, preventing impurity entrapment caused by localized overheating. The 12 water molecules of crystallization form a hydration barrier on the surface of the lithium phosphate crystal, hindering the lattice penetration of impurity ions (sodium and potassium). The lithium phosphate solid formed after lithium precipitation with sodium dodecahydrate has a more porous mesoporous structure (specific surface area of ​​approximately 15-25 m²). 2 / g, anhydrous phosphorus source lithium precipitation product approximately 5-8m 2 The trace amount of hydrated phosphorus source introduced by / g makes it easier for sodium ions physically adsorbed in the washing step to be removed by deionized water (washing efficiency of over 40%).

[0047] Sodium dihydrogen phosphate dihydrate, as an acidic salt, undergoes a lithium precipitation reaction essentially based on a competition between acid-base neutralization and precipitation-dissolution equilibrium. This reaction spontaneously generates phosphoric acid, automatically maintaining the system's pH in the weakly acidic range of 4.5-5.5. At this pH, the solubility of lithium phosphate is sufficient for quantitative precipitation, while sodium ions remain in the liquid phase due to the high solubility of sodium dihydrogen phosphate. The key role of the dihydrate structure in this process is to bind the hydrated ionic radius of sodium ions through the hydrogen bond network of the water of crystallization, reducing their diffusion rate into the lithium phosphate solid phase. Trace amounts of ammonium and sodium ions introduced by the hydrated phosphorus source, due to differences in ionic radius or the common ion effect with lithium ions, accumulate in the mother liquor during crystallization, thus achieving impurity removal.

[0048] In some embodiments of the present invention, a method for preparing battery-grade lithium dihydrogen phosphate using secondary lithium precipitation from a hydrated phosphorus source includes the following steps: (1) Add a hydrated phosphorus source to the lithium-containing purification solution and adjust the pH of the reaction solution to 7-9. After the first lithium precipitation reaction, the solid and liquid are separated to obtain crude lithium phosphate and lithium precipitation mother liquor. (2) Add a hydrated phosphorus source to the primary lithium precipitation mother liquor, adjust the pH of the reaction solution to 7-9, and after the second lithium precipitation reaction, separate the solid and liquid to obtain secondary lithium phosphate and secondary lithium precipitation mother liquor; return the secondary lithium precipitation mother liquor to step (1) to participate in the reaction again. (3) The primary crude lithium phosphate and the secondary lithium phosphate are combined and washed and / or washed and combined to obtain a lithium phosphate wet cake. The cake is then acid-dissolved with a phosphoric acid solution until the pH of the resulting mixture is 3-5 to obtain a crude lithium dihydrogen phosphate solution. (4) The crude lithium dihydrogen phosphate solution is crystallized and purified, centrifuged, and dried to obtain battery-grade lithium dihydrogen phosphate. In addition, the filtrate obtained by centrifugation is returned to the acid-soluble reaction solution in step (3) and used as mother liquor water.

[0049] By utilizing a hydrated phosphorus source and employing a two-stage lithium precipitation process, efficient lithium enrichment and deep impurity removal are achieved, effectively improving the purity of lithium dihydrogen phosphate and reducing production costs. The mother liquor from the second lithium precipitation stage is returned to the first stage for reuse in the reaction, realizing a closed-loop cycle. This two-stage lithium precipitation and one-stage crystallization process simplifies the battery-grade lithium dihydrogen phosphate process, reduces energy consumption, and minimizes waste liquid and residue generation. This method is suitable for treating lithium-containing waste liquid or low-grade lithium resources, enabling efficient recovery and utilization of lithium resources to obtain high-purity, high-yield battery-grade lithium dihydrogen phosphate products.

[0050] "Lithium-containing purification solution" is a lithium-containing solution that contains lithium ions but also contains a certain amount of impurities, obtained after lithium-containing raw materials have undergone purification processes such as leaching and impurity removal.

[0051] Further, in step (1), the lithium-containing purification solution contains Li + It is 7-12 g / L, Na + 6-12 g / L, K + The concentration of SO4 is 1.6-3.5 g / L. 2- The concentration was 70.32 g / L. The initial pH of the lithium-containing purification solution was 10-12.

[0052] A hydrated phosphorus source refers to a hydrated phosphate containing stable water of crystallization, soluble under lithium precipitation reaction conditions, and capable of providing phosphate, hydrogen phosphate, and / or dihydrogen phosphate ions. The water of crystallization in a hydrated phosphorus source can act as a hydration buffer and regulate the crystal interface during dissolution and lithium precipitation, which helps reduce local supersaturation and minimizes the embedding or entrainment of impurity ions in the lithium phosphate precipitate. Preferably, the hydrated phosphorus source is at least one of ammonium phosphate trihydrate, trisodium phosphate dodecahydrate, and sodium dihydrogen phosphate dihydrate. Further, the hydrated phosphorus source is mixed in as a salt solution, preferably with a salt solution concentration of 20-40% by weight. "Weight %" and "mass percentage" are the same concept.

[0053] First lithium precipitation reaction conditions: After adding a hydrated phosphorus source, adjust the pH to 7-9; the reaction temperature is 60-90℃, and the reaction time is 20-60 minutes. Second lithium precipitation reaction conditions: After adding a hydrated phosphorus source, adjust the pH to 7-9; the reaction temperature is 60-90℃, and the reaction time is 20-60 minutes.

[0054] The purposes and targets of the two lithium precipitation processes differ. The first precipitation primarily aims to precipitate and recover most of the lithium ions in the lithium-containing purification solution. The mother liquor after the first precipitation usually still contains a certain amount of lithium ions, as well as impurities such as sodium ions, potassium ions, and sulfate ions. The second precipitation involves adding a hydrated phosphorus source to the mother liquor from the first precipitation stage, resulting in a solution composition and ion concentration that differ from the first precipitation stage. Adjustments can be made within these ranges depending on specific circumstances, such as the target being treated and the system after adding the hydrated phosphorus source; the conditions for the two precipitation processes do not necessarily need to be exactly the same. Furthermore, an appropriate amount of deionized water can be added to the mother liquor from the first precipitation stage for the second precipitation.

[0055] In some preferred embodiments, the reaction temperature during the second lithium precipitation can be appropriately increased. For example, when performing a second lithium precipitation on the mother liquor from the first lithium precipitation, the reaction temperature can be 75-90°C. Taking the following limits as an example, the first lithium precipitation of the lithium-containing purified liquid is performed at 60°C, and the second lithium precipitation of the mother liquor from the first lithium precipitation is performed at 75°C. Appropriately increasing the reaction temperature for the second lithium precipitation compared to the first lithium precipitation can promote the dissolution of the added hydrated phosphorus source and the ion mass transfer within the system, allowing the residual lithium ions in the mother liquor from the first lithium precipitation to further form lithium phosphate precipitate, making the second lithium precipitation more complete, reducing the residual lithium content in the liquid after the second lithium precipitation, and improving the overall lithium recovery rate; it also facilitates the maturation of the secondary lithium phosphate precipitate and subsequent solid-liquid separation.

[0056] The solid-phase crude lithium phosphate obtained from the two lithium precipitation reactions and solid-liquid separation can be washed with deionized water at room temperature before being combined; alternatively, they can be combined first and then washed with deionized water at room temperature. Preferably, during washing, deionized water is added at a ratio of 1-3:1 to the solid-phase crude lithium phosphate (primary crude lithium phosphate and / or secondary crude lithium phosphate) for washing / slurrying.

[0057] In step (1), after the first lithium precipitation reaction, the purity of the crude lithium phosphate obtained is 80-92% by weight. In step (3), the lithium phosphate wet cake obtained by combining and washing the products after the two lithium precipitation reactions has a purity of 95-99% by weight. Compared with the crude lithium phosphate obtained after the first lithium precipitation reaction, the purity of the combined lithium phosphate wet cake is further improved. This invention improves the selective precipitation efficiency of lithium by using a two-stage lithium precipitation process, thereby increasing the purity and overall recovery rate of lithium phosphate.

[0058] In step (3), the high-purity lithium phosphate obtained from the two lithium precipitation processes is dissolved in phosphoric acid to obtain a lithium dihydrogen phosphate solution. To further improve the acid dissolution effect, preferably, the concentration of the phosphoric acid solution is 45-90% by weight.

[0059] In addition, the concentration of lithium dihydrogen phosphate in the crude lithium dihydrogen phosphate "solution" obtained after acid dissolution is 1800-2820 g / L, which is more conducive to subsequent concentration, crystallization and drying to obtain battery-grade lithium dihydrogen phosphate products.

[0060] In step (4), a single crystallization process is used, followed by centrifugation and drying to obtain battery-grade lithium dihydrogen phosphate. The process is simple. Further, a single concentration (evaporation) crystallization purification method is employed. Preferably, the crystallization purification conditions include: a temperature of 65-90℃, a pressure of 0.038-0.045 MPa, and a solid content of 18-30% after concentration.

[0061] Solid-liquid separation was achieved by centrifugation. After centrifugation, the crystals were dried at 60-70℃ for 4-6 hours to obtain high-purity, high-yield battery-grade lithium dihydrogen phosphate product.

[0062] In some embodiments, after secondary lithium precipitation and primary crystallization, the obtained battery-grade lithium dihydrogen phosphate product has a purity of ≥99.5%, and the impurity content meets the standard requirements for battery-grade lithium dihydrogen phosphate; the lithium yield is ≥94.8%. This product has lower impurity content and higher product purity, while maintaining a high yield.

[0063] The technical solution and effects of the present invention will be described below with reference to specific embodiments: Example 1 The preparation of battery-grade lithium dihydrogen phosphate using a hydrous phosphorus source through secondary lithium precipitation includes the following steps: Step (1): Take 600 mL of the purification solution, the composition of which is Li + =7.2g / L, Na + =6.5g / L, K + =1.8g / L, pH=10.

[0064] The system was heated to 60°C under stirring, and a 20% (w / w) ammonium phosphate trihydrate solution was slowly added. The pH of the reaction solution was adjusted to 7, and the reaction was continued for 20 minutes before solid-liquid separation was performed to obtain primary crude lithium phosphate and primary lithium precipitation mother liquor.

[0065] Test: After washing the crude lithium phosphate with deionized water, the purity of the crude lithium phosphate was 82%.

[0066] Step (2): Slowly add 20% ammonium phosphate trihydrate solution to the primary lithium precipitation mother liquor at 75°C, adjust the pH of the reaction solution to 7.0, and react for 30 min to carry out the second lithium precipitation; after the second lithium precipitation, solid and liquid separation is performed to obtain secondary lithium phosphate and secondary lithium precipitation mother liquor.

[0067] Step (3): Combine the primary crude lithium phosphate from step (1) and the secondary lithium phosphate from step (2), and wash with room temperature deionized water to obtain a wet lithium phosphate cake with a purity of about 98%.

[0068] The combined and washed lithium phosphate wet cake was acid-dissolved in a 45% phosphoric acid solution, and the pH of the mixture was controlled to 5 to obtain a crude lithium dihydrogen phosphate solution. The concentration of lithium dihydrogen phosphate in the solution was approximately 1816 g / L.

[0069] Step (4): The crude lithium dihydrogen phosphate solution is crystallized and purified under the conditions of 65°C and 0.038 MPa, and concentrated to a solid content of 18%. Then, it is centrifuged and dried at 60°C for 4 hours to obtain battery-grade lithium dihydrogen phosphate product.

[0070] Example 2

[0071] The preparation of battery-grade lithium dihydrogen phosphate using a hydrous phosphorus source through secondary lithium precipitation includes the following steps: Step (1): Take 700 mL of the purification solution, the composition of which is Li + =7.5g / L, Na + =8.8g / L, K + =2.6g / L, pH=10.

[0072] The system was heated to 75°C under stirring, and a mixed solution of 30% by mass of trisodium dodecahydrate and sodium dihydrogen phosphate dihydrate was slowly added. The pH of the reaction solution was adjusted to 8, and the reaction was continued for 40 minutes before solid-liquid separation was performed to obtain primary crude lithium phosphate and primary lithium precipitation mother liquor.

[0073] Test: After washing the crude lithium phosphate with deionized water, the purity of the crude lithium phosphate was 88%.

[0074] Step (2): Slowly add a mixed solution of 30% by mass of trisodium dodecahydrate and sodium dihydrogen phosphate dihydrate to the primary lithium precipitation mother liquor at 85°C, adjust the pH of the reaction solution to 8, and react for 20 minutes to carry out the second lithium precipitation; after the second lithium precipitation, solid-liquid separation is performed to obtain secondary lithium phosphate and secondary lithium precipitation mother liquor.

[0075] Step (3): Combine the primary crude lithium phosphate from step (1) and the secondary lithium phosphate from step (2), and wash with room temperature deionized water to obtain a wet lithium phosphate cake with a purity of about 97%.

[0076] The lithium phosphate wet cake was added to a 60% phosphoric acid solution for acid dissolution. The pH of the mixture was controlled to 4 to obtain a crude lithium dihydrogen phosphate solution. The concentration of lithium dihydrogen phosphate in the solution was approximately 2400 g / L.

[0077] Step (4): The crude lithium dihydrogen phosphate solution is crystallized and purified under the conditions of 80°C and 0.042 MPa, and concentrated to a solid content of 22%. Then, it is centrifuged and dried at 65°C for 5 hours to obtain battery-grade lithium dihydrogen phosphate product.

[0078] Example 3

[0079] The preparation of battery-grade lithium dihydrogen phosphate using a hydrous phosphorus source through secondary lithium precipitation includes the following steps: Step (1): Take 1000 mL of the purification solution, the composition of which is Li + =12g / L, Na + =11.5g / L, K + =3.4g / L, pH=12.

[0080] The system was heated to 80°C under stirring, and a mixed solution of 40% by mass of trisodium phosphate dodecahydrate and sodium dihydrogen phosphate dihydrate was slowly added. The pH of the reaction solution was adjusted to 9, and the reaction was continued for 60 minutes before solid-liquid separation was performed to obtain primary crude lithium phosphate and primary lithium precipitation mother liquor.

[0081] Test: After washing the crude lithium phosphate with deionized water, the purity of the crude lithium phosphate was 91%.

[0082] Step (2) The obtained primary lithium precipitation mother liquor is slowly added at 90°C to a mixed solution of 40% by mass of trisodium dodecahydrate and sodium dihydrogen phosphate dihydrate, and the pH of the reaction solution is adjusted to 9. The reaction is carried out for 30 minutes. After the second lithium precipitation, solid-liquid separation is performed to obtain secondary lithium phosphate and secondary lithium precipitation mother liquor.

[0083] Step (3): Combine the primary crude lithium phosphate from step (1) and the secondary lithium phosphate from step (2), and wash them with room temperature deionized water to obtain a wet lithium phosphate cake with a purity of 98%.

[0084] The lithium phosphate wet cake was added to a phosphoric acid solution with a concentration of 85% for acid dissolution. The pH of the mixture was controlled to 3 to obtain a crude lithium dihydrogen phosphate solution. The concentration of lithium dihydrogen phosphate in the solution was approximately 2820 g / L.

[0085] Step (4): The crude lithium dihydrogen phosphate solution is crystallized and purified under the conditions of 90°C and 0.045 MPa, and concentrated to a solid content of 30%. Then, it is centrifuged and dried at 70°C for 6 hours to obtain battery-grade lithium dihydrogen phosphate product.

[0086] Comparative Example 1

[0087] Unlike Example 1, step (2) of second lithium deposition is not performed; instead, battery-grade lithium dihydrogen phosphate is prepared by first-stage lithium deposition.

[0088] Specifically, the following steps are included: Step (1): Take 600 mL of the purification solution, the composition of which is Li + =7.2g / L, Na + =6.5g / L, K + =1.8g / L, pH=10. The system was heated to 60℃ under stirring, and a 20% (w / w) ammonium phosphate trihydrate solution was slowly added. The pH of the reaction solution was adjusted to 7, and the reaction was continued for 20 min before solid-liquid separation to obtain crude lithium phosphate.

[0089] Test results: After washing with deionized water, the purity of crude lithium phosphate was 82%.

[0090] Step (2): Without performing a second lithium precipitation treatment, proceed directly to the subsequent step (3).

[0091] Step (3): The crude lithium phosphate obtained in step (1) is directly added to the phosphoric acid solution for acid dissolution. The pH of the mixture is controlled to be 5 to obtain a crude lithium dihydrogen phosphate solution with a purity of about 79%.

[0092] Step (4): The crude lithium dihydrogen phosphate solution is crystallized, centrifuged and dried to obtain lithium dihydrogen phosphate product. However, the impurity content of the product is relatively high and the purity is significantly lower than that of Examples 1 to 3, making it difficult to stably meet the battery-grade requirements, as shown in Table 1 of the subsequent test examples.

[0093] Comparative Example 2

[0094] Unlike Example 1, anhydrous ammonium phosphate was used instead of ammonium phosphate trihydrate as the phosphorus source in the lithium precipitation process. Except for the type of phosphorus source and the corresponding amount added, the composition of the raw material solution, reaction temperature, reaction time, pH adjustment conditions, phosphorus-lithium molar ratio, washing, acid dissolution, crystallization, centrifugation, and drying conditions were all the same as in Example 1. The amount of anhydrous ammonium phosphate added was calculated based on the same effective phosphorus molar amount as that of ammonium phosphate trihydrate in Example 1.

[0095] Specifically, the following steps are included: Step (1): Take 600 mL of the purification solution, the composition of which is Li + =7.2g / L, Na + =6.5g / L, K + =1.8g / L, pH=10.

[0096] The system was heated to 60°C under stirring, and an anhydrous ammonium phosphate solution with a mass fraction of 40% was slowly added. The pH of the reaction solution was adjusted to 7, and the reaction was continued for 20 minutes before solid-liquid separation was performed to obtain primary crude lithium phosphate and primary lithium precipitation mother liquor.

[0097] After being washed with deionized water, the purity of the crude lithium phosphate was 76%.

[0098] Compared with Example 1 of the present invention, which uses ammonium phosphate trihydrate, Comparative Example 2 shows that anhydrous ammonium phosphate dissolves and releases phosphate ions faster during the addition process, which easily causes the phosphate ion concentration near the feeding point to rise instantaneously, resulting in local supersaturation. This leads to rapid nucleation of lithium phosphate precipitate and agglomeration.

[0099] Step (2): The primary lithium precipitation mother liquor obtained in step (1) is subjected to a second lithium precipitation treatment according to step (2) of Example 1, and solid-liquid separation is performed to obtain secondary lithium phosphate; Step (3): Combine the secondary lithium phosphate with the primary crude lithium phosphate obtained in step (1) and wash with deionized water to obtain a lithium phosphate wet cake with a purity of about 81%.

[0100] The wet lithium phosphate cake was dissolved in a 45% phosphoric acid solution, and the pH of the mixture was controlled to 5 to obtain a crude lithium dihydrogen phosphate solution. The concentration of lithium dihydrogen phosphate in the solution was approximately 1263 g / L.

[0101] Step (4): The crude lithium dihydrogen phosphate solution was crystallized and purified under the conditions of 65°C and 0.038 MPa, and concentrated to a solid content of 18%. Then it was centrifuged and dried at 60°C for 4 hours to obtain a small amount of agglomerated and unevenly sized lithium dihydrogen phosphate product.

[0102] Test case

[0103] The composition and content of the products prepared in Examples 1-3 and Comparative Example 1 were determined, and the yield of lithium in Examples 1-3 and Comparative Example 1 was calculated. The results are shown in Table 1.

[0104] The main content (lithium dihydrogen phosphate content) was determined by the quinomolybdate gravimetric method, and the contents of Li, Na, K, Ca, Fe, Pb, and SO4 were also measured. 2- The content of lithium was determined by ICP, chlorine by spectrophotometry, and water-insoluble matter by gravimetric method. The yield was calculated as: lithium content in the lithium dihydrogen phosphate product / lithium content in the lithium precipitation mother liquor. 100%, and the lithium content passed the ICP test.

[0105] Table 1

[0106] As shown in Table 1, the lithium dihydrogen phosphate prepared by the method of the present invention has a content of ≥99.5%, and the impurity content meets the requirements of the YS-T967-2014 battery-grade lithium dihydrogen phosphate standard, with a lithium yield of ≥94.8%.

[0107] Compared with the primary lithium precipitation in Comparative Example 1, the secondary lithium precipitation product of the present invention has a lower impurity content and higher product purity, while the yield remains at a high level, indicating that the method of the present invention has good impurity removal capability and industrial applicability.

[0108] Compared with anhydrous phosphate in Comparative Example 2, this invention uses a hydrated phosphorus source for secondary lithium precipitation and primary crystallization. Under the same effective phosphorus molar dosage, this method helps to slow down the local release rate of phosphate ions, suppress local supersaturation and impurity entrainment during the lithium precipitation process, thereby improving the purity of the lithium phosphate intermediate and the purity and yield of the final lithium dihydrogen phosphate product.

[0109] The foregoing description is provided for illustrative purposes only and is not intended to be exhaustive or to limit the invention to the forms disclosed. Many modifications and other embodiments will be apparent to those skilled in the art. The embodiments were chosen and described to better illustrate the principles and practical application of the invention and to enable those skilled in the art to understand the invention and to design various embodiments with various modifications suitable for particular purposes, and are not intended for limiting purposes.

Claims

1. A method for preparing battery-grade lithium dihydrogen phosphate by secondary lithium precipitation using a hydrated phosphorus source, characterized in that, include: Step S1: Add a hydrated phosphorus source to the lithium-containing purification solution, adjust the pH of the reaction solution to 7-9, carry out the first lithium precipitation reaction, and separate the solid and liquid to obtain primary crude lithium phosphate and primary lithium precipitation mother liquor; wherein, the hydrated phosphorus source is a hydrated phosphate containing water of crystallization that is soluble under lithium precipitation conditions and can provide at least one ion among phosphate, hydrogen phosphate and dihydrogen phosphate. Step S2: Add a hydrated phosphorus source to the primary lithium precipitation mother liquor, adjust the pH of the reaction solution to 7-9, carry out the second lithium precipitation reaction, separate the solid and liquid, and obtain secondary lithium phosphate and secondary lithium precipitation mother liquor. Step S3: Combine the secondary lithium phosphate with the primary crude lithium phosphate, and dissolve them in phosphoric acid solution to obtain a crude lithium dihydrogen phosphate solution. Step S4: The crude lithium dihydrogen phosphate solution is crystallized and purified, centrifuged and dried to obtain battery-grade lithium dihydrogen phosphate.

2. The method for preparing battery-grade lithium dihydrogen phosphate by secondary lithium precipitation using a hydrated phosphorus source according to claim 1, characterized in that, In the lithium-containing purification solution, Li + It is 7-12 g / L, Na + 6-12 g / L, K + The concentration is 1.6-3.5 g / L; The pH of the lithium-containing purification solution is 10-12.

3. The method for preparing battery-grade lithium dihydrogen phosphate using secondary lithium precipitation from a hydrated phosphorus source according to claim 1, characterized in that, The hydrated phosphorus source is at least one of ammonium phosphate trihydrate, trisodium phosphate dodecahydrate, and sodium dihydrogen phosphate dihydrate. And / or, step S2 further includes: returning the secondary lithium precipitation mother liquor to step S1 to participate in the primary lithium precipitation reaction.

4. The method for preparing battery-grade lithium dihydrogen phosphate by secondary lithium precipitation using a hydrated phosphorus source according to claim 1, characterized in that, The hydrated phosphorus source is mixed in as a salt solution; wherein the concentration of the salt solution is 20-40% by weight.

5. The method for preparing battery-grade lithium dihydrogen phosphate by secondary lithium precipitation using a hydrated phosphorus source according to claim 1, characterized in that, In step S1, the temperature of the reaction solution is 60-90℃, so that the first lithium precipitation reaction lasts for 20-60 minutes; And / or, in step S1, the crude lithium phosphate obtained has a purity of 80-92% by weight.

6. The method for preparing battery-grade lithium dihydrogen phosphate by secondary lithium precipitation using a hydrated phosphorus source according to claim 1, characterized in that, In step S2, the reaction solution temperature is 60-90℃, allowing the second lithium precipitation reaction to last for 20-60 minutes; Alternatively, in step S2, the temperature of the reaction solution is higher than that in step S1.

7. The method for preparing battery-grade lithium dihydrogen phosphate by secondary lithium precipitation using a hydrated phosphorus source according to claim 1, characterized in that, In step S3, after merging, the mixture is washed with deionized water to obtain a wet lithium phosphate cake with a purity of 95-99% by weight. The mass ratio of crude lithium phosphate to deionized water after merging is (1-3):

1.

8. The method for preparing battery-grade lithium dihydrogen phosphate by secondary lithium precipitation using a hydrated phosphorus source according to claim 1, characterized in that, In step S3, the concentration of the phosphoric acid solution is 45-90% by weight; the pH of the mixture is controlled to be 3-5. And / or, in step S3, the concentration of lithium dihydrogen phosphate in the crude lithium dihydrogen phosphate solution is 1800-2820 g / L.

9. The method for preparing battery-grade lithium dihydrogen phosphate by secondary lithium precipitation using a hydrated phosphorus source according to claim 1, characterized in that, Step S4 also includes returning the filtrate obtained by centrifugation to step S3 for use as a phosphoric acid solution.

10. The method for preparing battery-grade lithium dihydrogen phosphate by secondary lithium precipitation using a hydrated phosphorus source according to claim 1, characterized in that, In step S4, The crystallization and purification conditions include: a temperature of 65-90℃, a pressure of 0.038-0.045 MPa, and a solid content of 18-30% after concentration. In the centrifugation and drying step, the drying temperature is 60-70℃ and the drying time is 4-6 hours.

11. The method for preparing battery-grade lithium dihydrogen phosphate by secondary lithium precipitation using a hydrated phosphorus source according to claim 1, characterized in that, Step S1 also includes: washing the primary crude lithium phosphate with deionized water; And / or, step S2 further includes: washing with deionized water and lithium secondary phosphate.

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