Method for preparing battery-grade lithium phosphate based on step-by-step precipitation and double washing and application thereof

Abstract

The application discloses a method for preparing battery-grade lithium phosphate based on step-by-step precipitation and double washing and application thereof. The method takes crude lithium sulfate as raw material, and through an integrated process of step-by-step precipitation and double washing, the problem of the wrapping of impurity sodium ions and sulfate in the lithium precipitation process of the crude lithium sulfate is solved, and the production cost of the lithium phosphate is reduced, so that the method is suitable for the industrialized production of lithium ion battery positive material precursors. The application mainly controls the impurity content from two aspects. Through step-by-step precipitation and crystal type induced generation, the wrapping of impurities in the product crystal grains can be reduced, and the double washing scheme can remove the residual impurities in the crystal grain gaps. The purpose of the crystal type induced generation is to guide the generation of large crystals and reduce the specific surface area, so that the Na adsorption amount is reduced.

Description

Technical Field

[0001] This invention relates to the field of lithium salt material preparation technology, specifically to a method and application for preparing battery-grade lithium phosphate based on stepwise precipitation and dual washing. Background Technology

[0002] As lithium salt prices continue to decline, profit margins for companies are shrinking. To address this issue, technological improvements to existing production processes are needed to reduce costs, increase efficiency, and enhance corporate competitiveness.

[0003] Battery-grade lithium phosphate (Li3PO4) must meet the following requirements: purity ≥ 99.5%, Na... + ≤100ppm, SO4 2- Requirements include ≤200ppm. Traditional production processes primarily involve reacting high-purity lithium salts (such as lithium carbonate, lithium sulfate, and lithium hydroxide) with phosphoric acid / phosphate. However, this approach is costly, and in the highly competitive lithium salt market, profit margins are very limited. Driven by this trend, companies have implemented technological improvements to their production processes, aiming to prepare high-purity battery-grade lithium phosphate from lower-cost lithium salt crude products to reduce production costs. However, these new processes have also brought about new problems:

[0004] 1. Severe impurity encapsulation: In lithium precipitation reactions (such as the preparation of lithium phosphate from crude lithium sulfate: Li₂SO₄ + Na₄PO₄ → Li₃PO₄↓ + Na₂SO₄), Na₂SO₄... + With SO4 2- It is easily encapsulated by lithium phosphate crystals, which are difficult to remove with conventional water washing, resulting in excessive residue.

[0005] 2. Uncontrollable crystal form: Amorphous or microcrystalline lithium phosphate has a large specific surface area and enhanced ability to adsorb impurities.

[0006] 3. Low washing efficiency: Traditional static washing requires multiple centrifugations (5-8 times), which consumes a lot of water and produces high-salt wastewater.

[0007] Therefore, how to develop a method for preparing battery-grade lithium phosphate from crude lithium salts (taking crude lithium sulfate as an example) with acceptable product impurity residues, lower process costs, and controllable crystal morphology is a problem that urgently needs to be solved by technicians in this field. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method and application for preparing battery-grade lithium phosphate based on stepwise precipitation and dual washing. This method uses crude lithium sulfate (containing Li₂SO₄, Na ...) as the raw material. + K +Using impurities such as sodium ions (Na+) as raw materials, and through an integrated process of stepwise precipitation and double washing, the production cost of lithium phosphate is reduced while the problem of sodium ions (Na+) being impurities during the precipitation of crude lithium sulfate is addressed. + ) and sulfate (SO4) 2- This addresses the issue of encapsulation and is applicable to the industrial production of lithium-ion battery cathode material precursors.

[0009] To achieve the above objectives, the technical solution designed by the present invention is as follows:

[0010] This invention provides a method for preparing battery-grade lithium phosphate based on stepwise precipitation and dual washing, comprising the following steps:

[0011] 1) Dissolve crude lithium sulfate in deionized water, filter, add diammonium hydrogen phosphate to the filtrate to adjust the pH to 8.0-9.0, and heat from room temperature to 40-50℃ to carry out the reaction to obtain an intermediate reaction solution;

[0012] 2) Continue to add diammonium hydrogen phosphate to adjust the pH to 10.5-11.5, and when the temperature is raised to 80-90℃, add nano alumina sol to react and obtain a reaction solution. Filter to obtain a solid product (primary lithium phosphate containing impurities).

[0013] 3) Dissolve sodium polyacrylate in water to obtain a sodium polyacrylate dispersion, and use the sodium polyacrylate dispersion to perform ultrasonic countercurrent washing on the solid product obtained in step 2).

[0014] 4) Then, under supercritical CO2 and carrying ethanol conditions, micro-interface scouring is performed, followed by spray drying to obtain battery-grade lithium phosphate.

[0015] Further, in step 1), the lithium ions (Li) in the crude lithium sulfate + ) and diammonium hydrogen phosphate phosphate (PO4) 3- The molar ratio of ) is 1:1.0-1.10.

[0016] Furthermore, the lithium ions (Li) in the crude lithium sulfate + ) and diammonium hydrogen phosphate phosphate (PO4) 3- The molar ratio of ) is 1:1.05.

[0017] Furthermore, in step 1), the heating rate is 3-5℃ / min, and the reaction time is 30-60min.

[0018] Furthermore, in step 2), the amount of nano-alumina sol used is 0.1-0.5% of the mass of crude lithium sulfate;

[0019] The heating rate is 3-5℃ / min, and the reaction time is 30-60min.

[0020] Furthermore, in step 3), the sodium polyacrylate dispersion is prepared by dissolving sodium polyacrylate in deionized water at a temperature of 80°C, and the mass fraction of the sodium polyacrylate dispersion is 0.1%.

[0021] The ultrasonic power is 150-250W, and the countercurrent velocity is 1-3m / s.

[0022] Furthermore, in step 3), the ultrasonic power is 200W and the countercurrent flow velocity is 2m / s.

[0023] Furthermore, in step 4), the supercritical CO2 pressure is 10 MPa and the ethanol concentration is 5 vol%.

[0024] The spray dryer has an inlet temperature of 180℃ and an outlet temperature of 80℃.

[0025] Furthermore, in step 4), the D of battery-grade lithium phosphate 50 =20-50μm, specific surface area <5m² 2 / g.

[0026] The present invention also provides an application of the battery-grade lithium phosphate prepared by the above method in the preparation of precursors for lithium-ion battery cathode materials.

[0027] The principle of this invention:

[0028] 1. Stepwise precipitation mechanism: SO4 is precipitated through the formation and transformation of the intermediate Li(NH4)2PO4·H2O. 2- Pre-removal (removal rate > 95%); details are as follows:

[0029] ① First stage: pH = 8.0-9.0, temperature 40-50℃, reaction time 30-60 min, generating intermediate Li(NH4)2PO4·H2O (preferably precipitates, avoiding SO4). 2- Co-crystallization); The heating rate can be 3-5℃ / min, and controlling the heating rate is more conducive to controlling the crystal form and impurity encapsulation;

[0030] ② Second stage: Adjust the pH to 10.5-11.5, raise the temperature to 80-90℃, add nano-alumina sol to convert it into well-defined Li3PO4, and simultaneously release NH3 to promote SO42-. 2- It dissolves in the form of (NH4)2SO4.

[0031] 2. Crystal formation: The surface hydroxyl groups of the nano-alumina sol selectively combine with the Li3PO4 crystal planes, inducing the formation of a solid with a solid size D. 50 =20-50μm, reducing specific surface area by <5m² 2 / g, reducing Na + Adsorption (Na)+ (Adsorption capacity reduced by 60%).

[0032] 3. Washing Technology: Ultrasonic disruption of the coating layer + supercritical micro-interface rinsing reduces the number of washes from 8 to 2, resulting in a 70% reduction in wastewater discharge. An ultrasonic countercurrent-supercritical washing system (coupled) is employed: the combined action of ultrasonic countercurrent and supercritical washing (coupled) leverages the high solubility of CO2 in the supercritical state, resulting in effective washing of many impurities, as detailed below:

[0033] ① First-stage washing: Use a sodium polyacrylate dispersion with a mass fraction of 0.1%, ultrasonic power of 200W, and countercurrent flow rate of 2m / s;

[0034] ② Secondary washing: Supercritical CO2 (10 MPa, 50℃) carrying ethanol (5 vol%) is used for micro-interface rinsing to remove residual Na at the grain boundaries. + With SO4 2- .

[0035] The beneficial effects of this invention are:

[0036] This invention primarily controls impurity content through two aspects: stepwise precipitation and crystal form induction to reduce impurity encapsulation within the product crystals, and a dual washing process to remove impurities remaining between crystal grains. The purpose of crystal form induction is mainly to guide the formation of large crystals, reducing their specific surface area and thus decreasing Na adsorption. In the comparative example, amorphous small crystals with a larger specific surface area are formed, exhibiting stronger Na adsorption, resulting in a relatively higher impurity content compared to the example. Specific effects are as follows:

[0037] 1. Deep removal of impurities: Na + ≤50ppm, SO4 2- ≤150ppm, better than battery-grade standards;

[0038] 2. Good crystal size: The tap density of plate-like lithium phosphate is ≥1.5 g / cm³. 3 Improve the processing performance of cathode materials;

[0039] 3. Green and efficient: Water consumption is reduced by 77%. The only organic liquid used in the process is a small amount of ethanol used in the secondary washing, which can be recycled multiple times and can also be recovered later. Therefore, there is basically no organic waste liquid produced.

[0040] 4. Cost advantage: The cost is reduced by 40% compared to the complexing agent method, making it suitable for production lines with a capacity of tens of thousands of tons. Detailed Implementation

[0041] The present invention will now be described in further detail with reference to specific embodiments, so that those skilled in the art can understand it.

[0042] Example 1

[0043] A method for preparing battery-grade lithium phosphate 1 based on stepwise precipitation and double washing includes the following steps:

[0044] 1) Crude lithium sulfate (composition: Li₂SO₄ 92.5%, Na₂SO₄ 92.5%) + =1.2%, SO4 2- =4.8%) was dissolved in deionized water to obtain a crude lithium sulfate solution with a concentration of 1.8 mol / L. After filtration, diammonium hydrogen phosphate (Li₂) was added to the filtrate. + ) and diammonium hydrogen phosphate phosphate (PO4) 3- The molar ratio of the two components is 1:1.05. The pH is adjusted to 8.5, and the temperature is increased from room temperature to 45℃ at a rate of 3-5℃ / min for 30 min to obtain an intermediate reaction solution.

[0045] 2) Continue to add diammonium hydrogen phosphate to adjust the pH to 11.0, and add nano alumina sol when the temperature is increased to 85℃ at a rate of 3-5℃ / min. React for 1 hour to obtain a reaction solution, and filter to obtain a solid product; wherein, the amount of nano alumina sol is 0.3% of the mass of crude lithium sulfate.

[0046] 3) Dissolve sodium polyacrylate in deionized water at 80°C to obtain a sodium polyacrylate dispersion with a mass fraction of 0.1%. Use the sodium polyacrylate dispersion to perform ultrasonic countercurrent washing on the solid product obtained in step 2).

[0047] 4) Then, under supercritical CO2 conditions (supercritical CO2 (10 MPa, 50 °C) carrying ethanol (5 vol%)), micro-interface rinsing was performed for 20 min, followed by spray drying at an inlet temperature of 180 °C and an outlet temperature of 80 °C to obtain battery-grade lithium phosphate 1.

[0048] Example 2

[0049] A method for preparing battery-grade lithium phosphate 2 based on stepwise precipitation and double washing includes the following steps:

[0050] 1) Crude lithium sulfate (composition: Li₂SO₄ 92.5%, Na₂SO₄ 92.5%) + =1.2%, SO4 2- =4.8%) was dissolved in deionized water to obtain a crude lithium sulfate solution with a concentration of 1.5 mol / L. After filtration, diammonium hydrogen phosphate (Li₂) was added to the filtrate. + ) and diammonium hydrogen phosphate phosphate (PO4) 3- The molar ratio of the two components is 1:1.05. The pH is adjusted to 8.0, and the temperature is increased from room temperature to 50℃ at a rate of 3-5℃ / min for 30 min to obtain an intermediate reaction solution.

[0051] 2) Continue to add diammonium hydrogen phosphate to adjust the pH to 11.5, and when the temperature is increased to 90℃ at a rate of 3-5℃ / min, add nano-alumina sol and react for 1 hour to obtain a reaction solution. Filter to obtain a solid product; wherein, the amount of nano-alumina sol is 0.3% of the mass of crude lithium sulfate.

[0052] 3) Dissolve sodium polyacrylate in deionized water at 80°C to obtain a sodium polyacrylate dispersion with a mass fraction of 0.1%. Use the sodium polyacrylate dispersion to perform ultrasonic countercurrent washing on the solid product obtained in step 2).

[0053] 4) Then, under supercritical CO2 conditions (supercritical CO2 (10 MPa, 50 °C) carrying ethanol (5 vol%)), micro-interface scouring was performed for 20 min, followed by spray drying at an inlet temperature of 180 °C and an outlet temperature of 80 °C to obtain battery-grade lithium phosphate 2.

[0054] Example 3

[0055] A method for preparing battery-grade lithium phosphate 3 based on stepwise precipitation and double washing includes the following steps:

[0056] 1) Crude lithium sulfate (composition: Li₂SO₄ 92.5%, Na₂SO₄ 92.5%) + =1.2%, SO4 2- =4.8%) was dissolved in deionized water to obtain a crude lithium sulfate solution with a concentration of 2.0 mol / L. After filtration, diammonium hydrogen phosphate (Li₂) was added to the filtrate. + ) and diammonium hydrogen phosphate phosphate (PO4) 3- The molar ratio of the two components is 1:1.05. The pH is adjusted to 9.0, and the temperature is increased from room temperature to 50℃ at a rate of 3-5℃ / min for 30 min to obtain an intermediate reaction solution.

[0057] 2) Continue to add diammonium hydrogen phosphate to adjust the pH to 10.5, and add nano alumina sol when the temperature is increased to 80℃ at a rate of 3-5℃ / min. React for 1 hour to obtain a reaction solution, and filter to obtain a solid product; wherein, the amount of nano alumina sol is 0.3% of the mass of crude lithium sulfate.

[0058] 3) Dissolve sodium polyacrylate in deionized water at 80°C to obtain a sodium polyacrylate dispersion with a mass fraction of 0.1%. Use the sodium polyacrylate dispersion to perform ultrasonic countercurrent washing on the solid product obtained in step 2).

[0059] 4) Then, under supercritical CO2 conditions (supercritical CO2 (10 MPa, 50 °C) carrying ethanol (5 vol%)), micro-interface scouring was performed for 20 min, followed by spray drying at an inlet temperature of 180 °C and an outlet temperature of 80 °C to obtain battery-grade lithium phosphate 3.

[0060] Comparative Example 1

[0061] The direct precipitation method (traditional process) has the following steps:

[0062] Lithium precipitation stage: Dissolve crude lithium sulfate to 1.8 mol / L, filter to remove mechanical impurities, and then add (NH4)2HPO4 (lithium ions (Li)) to the filtrate. + ) and phosphate (PO4) 3- (Molar ratio = 1:3.05), directly adjust the pH of the reaction solution to 11, react at 85℃ for 1 hour, and then filter and separate while hot;

[0063] Washing stage: Wash at 85℃ for 30 minutes with pure water mass: filter cake (liquid-solid ratio) = 3:1, filter, and repeat the above operation 6 times;

[0064] Drying stage: Drying in an oven at 110℃ or under vacuum at 80-90℃.

[0065] The battery-grade lithium phosphate 1-3 prepared in Examples 1-3 and the one prepared in Comparative Example 1 were tested for relevant indicators, as shown in Table 1:

[0066] Table 1

[0067]

[0068] As shown in Table 1, analysis of the lithium phosphate quality in Examples 1-3 and Comparative Example 1 reveals that the lithium phosphate product prepared using the above method has higher purity than that prepared using the traditional method. Furthermore, the lithium phosphate prepared by this invention, through stepwise precipitation and crystal induction, achieves a specific surface area of ​​<5m² for battery-grade lithium phosphate. 2 / g, to reduce the amount of Na adsorption, and at the same time, to reduce the amount of impurities encapsulated inside the product crystals. Finally, a double washing is performed to further remove Na from the product. + SO4 2- The residual impurity ions are significantly lower than in traditional methods. Furthermore, the number of water washes during production is reduced, resulting in far less wastewater. Therefore, the above preparation method is superior to traditional methods in terms of product quality, production cost, and environmental friendliness.

[0069] All other parts not described in detail are existing technologies. Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, not all embodiments. People can obtain other embodiments based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for preparing battery-grade lithium phosphate based on stepwise precipitation and dual washing, characterized in that: Includes the following steps: 1) Dissolve crude lithium sulfate in deionized water, filter, add diammonium hydrogen phosphate to the filtrate to adjust the pH to 8.0-9.0, and heat from room temperature to 40-50℃ to carry out the reaction to obtain an intermediate reaction solution; 2) Continue to add diammonium hydrogen phosphate to adjust the pH to 10.5-11.5, and when the temperature is raised to 80-90℃, add nano alumina sol to react and obtain a reaction solution. Filter to obtain a solid product. 3) Dissolve sodium polyacrylate in water to obtain a sodium polyacrylate dispersion, and use the sodium polyacrylate dispersion to perform ultrasonic countercurrent washing on the solid product obtained in step 2). 4) Then, under supercritical CO2 and carrying ethanol conditions, micro-interface scouring is performed, followed by spray drying to obtain battery-grade lithium phosphate.

2. The method according to claim 1, characterized in that: In step 1), the molar ratio of lithium ions in crude lithium sulfate to phosphate ions in diammonium hydrogen phosphate is 1:1.0-1.

10.

3. The method according to claim 1 or 2, characterized in that: The molar ratio of lithium ions in the crude lithium sulfate to phosphate ions in diammonium hydrogen phosphate is 1:1.

05.

4. The method according to claim 1 or 2, characterized in that: In step 1), the heating rate is 3-5℃ / min and the reaction time is 30-60min.

5. The method according to claim 1, characterized in that: In step 2), the amount of nano-alumina sol used is 0.1-0.5% of the mass of crude lithium sulfate; The heating rate is 3-5℃ / min, and the reaction time is 30-60min.

6. The method according to claim 1, characterized in that: In step 3), the sodium polyacrylate dispersion is prepared by dissolving sodium polyacrylate in deionized water at a temperature of 80°C, and the mass fraction of the sodium polyacrylate dispersion is 0.1%. The ultrasonic power is 150-250W, and the countercurrent velocity is 1-3m / s.

7. The method according to claim 6, characterized in that: In step 3), the ultrasonic power is 200W and the countercurrent flow velocity is 2m / s.

8. The method according to claim 1, characterized in that: In step 4), The supercritical CO2 pressure is 10 MPa, and the ethanol concentration is 5 vol%. The spray dryer has an inlet temperature of 180℃ and an outlet temperature of 80℃.

9. The method according to claim 1 or 8, characterized in that: In step 4), the D of battery-grade lithium phosphate 50 =20-50μm, specific surface area <5m² 2 / g.

10. The application of the battery-grade lithium phosphate prepared by the method of claim 1 in the preparation of a precursor for a lithium-ion battery cathode material.

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