Method for synthesizing lithium phosphate serving as raw material of low-cost battery-grade lithium dihydrogen phosphate

Abstract

The invention relates to the technical field of preparation of lithium battery materials, and discloses a synthesis method of a low-cost battery-grade lithium dihydrogen phosphate raw material lithium phosphate. The synthesis method comprises the following steps: dissolving crude lithium sulfate in alkaline lithium phosphate lithium precipitation mother liquor; carrying out chemical precipitation on the filtered lithium sulfate solution to preliminarily remove calcium, and subsequently, deeply removing calcium and COD (Chemical Oxygen Demand) by sequentially using activated carbon and calcium removal resin to obtain a lithium sulfate finished solution; mixing the lithium sulfate completion liquid and the alkali liquid, freezing to-10 DEG C to-3 DEG C, and performing centrifugal separation to obtain a lithium sulfate freezing liquid; and reacting the refrigerating fluid with a dilute phosphoric acid solution to precipitate lithium phosphate, stirring, washing and centrifuging the filtered crude product lithium phosphate, and drying to obtain the battery-grade lithium dihydrogen phosphate raw material lithium phosphate. The lithium phosphate prepared at low cost is used for replacing battery-grade lithium carbonate, lithium hydroxide and the like as a lithium source for producing the lithium dihydrogen phosphate, so that the production cost of the battery-grade lithium dihydrogen phosphate can be effectively reduced, the lithium phosphate mother liquor is recycled, and the problem of treating a large amount of wastewater caused by other methods is solved.

Description

Technical Field

[0001] The present invention relates to the field of lithium battery material preparation, and in particular to a method for synthesizing lithium phosphate, a low-cost battery-grade lithium dihydrogen phosphate raw material. Background Art

[0002] As an important raw material for lithium iron phosphate, lithium dihydrogen phosphate can provide both phosphorus and lithium sources, which not only simplifies the production process of lithium iron phosphate synthesis, saves procurement and storage costs, but also avoids the problem of large amounts of ammonia gas polluting the environment caused by the use of ammonium dihydrogen phosphate in other synthesis methods. The application of lithium dihydrogen phosphate is becoming one of the main choices for lithium iron phosphate battery manufacturing. The synthesis methods of lithium dihydrogen phosphate are mainly divided into extraction method and neutralization method. The extractant is mostly one or a mixture of methanol, ethanol, and propanol. It is expensive and a hazardous chemical, and it is difficult to recycle. Therefore, the extraction method is difficult to use in large-scale industrial production. In the neutralization method, the traditional domestic production of battery-grade lithium dihydrogen phosphate generally uses battery-grade lithium carbonate and lithium hydroxide as lithium sources, so the manufacturing cost is very high.

[0003] Publication No. CN113979413A proposes a method for synthesizing lithium dihydrogen phosphate by freezing and impurity separation, which uses lithium sulfate as a lithium source, mixes sodium dihydrogen phosphate and then freezes it, and separates mirabilite to obtain a lithium dihydrogen phosphate solution. However, this method still has many defects. On the one hand, the lithium oxide concentration of the obtained lithium dihydrogen phosphate solution is low, and a large amount of steam and time are required for subsequent evaporation and concentration. On the other hand, the content of impurities such as Ca is high and further treatment is required to meet battery-grade requirements. Publication No. CN118515250A uses lithium sulfate and monoammonium phosphate to precipitate lithium phosphate under alkaline conditions, and lithium phosphate and phosphoric acid react to obtain a lithium dihydrogen phosphate solution. Although this method can obtain battery-grade lithium dihydrogen phosphate, a large amount of ammonium sulfate wastewater and barium sulfate salt will be generated during the production process, which brings great challenges to wastewater treatment and environmental protection. In addition, these patents do not mention the conventional impurity removal process of Ca, Mg and other impurities. It can be seen that there are certain technical requirements for the lithium sulfate raw materials used, which brings challenges to cost reduction. In the application with publication number CN118561247A, crude lithium phosphate is obtained by lithium precipitation reaction between lithium-containing mother liquor and phosphate. Before obtaining the raw material of lithium dihydrogen phosphate for refined lithium phosphate, the crude lithium phosphate needs to consume a large amount of phosphoric acid and sodium hydroxide for recrystallization and impurity removal. The recrystallization process not only increases the production cost, but also produces a large amount of phosphorus-containing wastewater.

[0004] Considering the production cost, economic benefit and environmental protection, the existing technology still has much room for improvement.

[0005] Therefore, a new technical solution is needed to solve the above technical problems. Summary of the invention

[0006] The purpose of the present invention is to provide a method for synthesizing lithium phosphate, which is a raw material of battery-grade lithium dihydrogen phosphate at a low cost. The method is simple to operate, has low requirements on raw materials and equipment, has low production cost, and production water can be recycled for multiple times. The method is green and environmentally friendly and suitable for large-scale production. The obtained lithium phosphate and the lithium dihydrogen phosphate prepared using the lithium phosphate as raw materials are of good quality and can meet the quality requirements of battery-grade lithium dihydrogen phosphate.

[0007] The technical solution adopted by the present invention is:

[0008] A method for synthesizing lithium phosphate, a low-cost battery-grade lithium dihydrogen phosphate raw material, comprises the following steps:

[0009] Step 1: The crude lithium sulfate monohydrate is stirred, washed, dissolved and filtered in sequence to obtain a lithium sulfate solution that has been preliminarily freed of impurities;

[0010] Step 2: The preliminarily impurity-removed lithium sulfate solution is preliminarily decalcified with carbonate, and the decalcified solution is passed through an ion exchange resin column and an activated carbon adsorption tower to obtain a lithium sulfate solution after deep decalcification and COD removal;

[0011] Step 3: Add liquid caustic soda to the lithium sulfate solution to adjust the Na + / SO4 2- After the concentration ratio is reached, the temperature is lowered and frozen to precipitate sodium sulfate decahydrate, and the frozen slurry is centrifuged to obtain lithium sulfate frozen liquid and mirabilite;

[0012] Step 4: premix the diluted phosphoric acid solution and part of the lithium sulfate frozen liquid, control the pH of the premixed liquid to be weakly acidic, then add the remaining lithium sulfate frozen liquid to the premixed liquid, carry out lithium phosphate precipitation reaction at room temperature, continue the reaction for 15 to 20 minutes after the addition is completed, filter to obtain a crude lithium phosphate filter cake and a lithium phosphate precipitation mother liquor, rinse the filter cake with a small amount of water, and the lithium phosphate mother liquor can be returned to the previous process for dissolving the crude lithium sulfate monohydrate;

[0013] Step 5: The crude lithium phosphate is stirred and washed with stirring water, and the filter cake is rinsed with a small amount of water and dried after filtering to obtain lithium phosphate for battery-grade lithium dihydrogen phosphate raw material. The stirring and washing mother liquor is recycled and returned to the previous process to dissolve the crude lithium sulfate after the sulfate ion concentration is enriched;

[0014] Step 6: Add water and phosphoric acid to dissolve the prepared battery-grade lithium dihydrogen phosphate raw material lithium phosphate to obtain a lithium dihydrogen phosphate solution, control the pH of the solution at 2-3, then evaporate and concentrate, cool and crystallize, centrifuge, rinse the filter cake with a saturated lithium dihydrogen phosphate solution and dry, and obtain a battery-grade lithium dihydrogen phosphate product.

[0015] By adopting the above process, the operation is simplified, the requirements for raw materials and equipment are reduced, the production cost is reduced, and the production water can be recycled for many times, which is green and environmentally friendly and suitable for large-scale production. The obtained lithium phosphate and the lithium dihydrogen phosphate prepared with it as raw materials are of good quality and can meet the quality requirements of battery-grade lithium dihydrogen phosphate.

[0016] Preferably, in step 1, the crude lithium sulfate monohydrate is dissolved in an alkaline lithium phosphate lithium precipitation mother solution, and the main ion concentrations of the lithium precipitation mother solution include: Li2O: 2-4 g / L, P: ≤0.05 g / L, OH - :3~5g / L, add 1.5g / L diatomaceous earth to assist filtration during filtration to obtain preliminary removal of impurities Mg 2+ ≤0.001g / L, Li2O 40~50g / L lithium sulfate solution.

[0017] By adopting the above process, the excess OH in the lithium phosphate precipitation mother liquor is utilized. - It reacts with impurities such as Mg and Al in crude lithium sulfate to produce Mg(OH)2 and Al(OH)3 precipitates to remove impurities. At the same time, it recovers excess Li2O in the mother liquor to increase the lithium yield, dissolve lithium sulfate, and reduce wastewater discharge.

[0018] Preferably, after the liquid alkali in step 3 is added to the lithium sulfate solution, the Na 2+ / SO4 2- The molar ratio is 2.0 to 2.4:1, the freezing temperature is -10 to -3°C, and the SO4 content in lithium sulfate freezing liquid is 2- ≤40g / L.

[0019] By adopting the above process, Na in the mixed solution 2+ / SO4 2- The molar ratio is 2.0-2.4:1. If the actual ratio is lower than this value, the sulfate ion in the filtrate after freezing will be too high. If the ratio is higher than this value, on the one hand, too much liquid alkali will be consumed, increasing production costs, and on the other hand, the impurity Na ion in the filtrate will also increase.

[0020] Preferably, in step 4, the Li / P molar ratio of the lithium sulfate frozen solution and the phosphoric acid is 3:0.95, the volume ratio of the phosphoric acid solution and the lithium sulfate frozen solution is 1:5, the pH of the premixed solution is 4.5-6.5, and the remaining lithium sulfate frozen solution is added for 1-2 hours.

[0021] By adopting the above process, the amount of phosphoric acid is controlled to be lower than the theoretical value, P in the lithium phosphate precipitation mother liquor is ≤0.05g / L, and lithium phosphate precipitation will not be generated when the mother liquor returns to the previous process to dissolve the crude lithium sulfate, thereby avoiding the slag carrying too high a lithium amount. Too low a phosphoric acid amount will cause the lithium yield to be too low during lithium precipitation. The pH value of the mixed solution after premixing the dilute phosphoric acid solution with part of the lithium sulfate frozen liquid is required to be 4.5-6.5, presenting a weakly acidic environment, and ensuring that no lithium phosphate crystals are generated. Premixing can speed up the reaction time, improve production efficiency, and control the dripping time of the remaining frozen liquid to avoid dripping too quickly resulting in a high lithium phosphate impurity level and dripping too slowly affecting production efficiency.

[0022] Preferably, in step 5, the volume mass ratio of the stirring and washing water to the crude lithium phosphate, i.e., ml:g, is 5 to 7:1, the stirring and washing temperature is 40 to 60° C., and the stirring and washing time is 10 to 20 min.

[0023] By adopting the above process, the temperature is pre-raised to 40-60°C to increase the agitation and cleaning effect.

[0024] Preferably, the impurity Na≤0.1% and SO4 2- ≤0.35%.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] 1. The lithium source used in the process of the present invention is crude lithium sulfate, and there is no obvious requirement for purity. Therefore, compared with synthesizing battery-grade lithium dihydrogen phosphate using battery-grade lithium carbonate or lithium hydroxide as the lithium source, the production cost will be greatly reduced.

[0027] 2. The process flow of the present invention is simple. It uses lithium sulfate frozen liquid to synthesize lithium phosphate, a raw material of battery-grade lithium dihydrogen phosphate, in one step. There is no need for the lithium phosphate recrystallization and impurity removal step. Compared with using refined lithium phosphate after recrystallization of crude lithium phosphate as the raw material of battery-grade lithium dihydrogen phosphate, it saves a large amount of phosphoric acid, sodium hydroxide, water and the resulting production wastewater treatment problems.

[0028] 3. In the process of the present invention, the lithium phosphate precipitation mother liquor is returned to the previous step to dissolve the crude lithium sulfate. On the one hand, the recycling of the mother liquor greatly reduces the generation of production wastewater, making the manufacturing process economical and environmentally friendly. On the other hand, the OH in the mother liquor is - Li + Comprehensive utilization not only effectively removes common impurities such as Mg and Al in crude lithium sulfate, but also improves the overall lithium yield, making the product more competitive. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The figure is a process flow chart of the synthesis method of the present invention. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below. It should be understood that the specific implementation cases described herein are only used to explain the present invention and make the disclosure of the present invention more thorough and comprehensive, and are not used to limit the present invention.

[0031] Example 1

[0032] like Figure 1 As shown in the flow chart, 600g of crude lithium sulfate was weighed and added to 3L of lithium phosphate lithium precipitation mother liquor, and stirred and dissolved continuously. After adding 4.5g of diatomaceous earth as a filter aid for 15 minutes, the initial impurity-free Li2O was filtered out with a concentration of 42g / L and Mg 2+ 0.0002 g / L, Ca 2+ The 0.22g / L lithium sulfate solution and Mg(OH)2 residue were added to 3L lithium sulfate solution to remove impurities chemically to obtain Ca 2+ The lithium sulfate solution after primary decalcification is 0.02g / L. The primary decalcified lithium sulfate solution is passed through an activated carbon adsorption tower and an ion exchange resin to obtain a COD of 150mg / L and a Ca 2+ 0.001 g / L, Na + 24.6g / L, SO4 2- The lithium sulfate solution is 160.5g / L. 379.0g of 48% liquid caustic soda is added to 2L of lithium sulfate solution. The mixed solution is frozen to below -5°C and kept cold for 15 minutes. SO4 is obtained by centrifugation. 2- The lithium sulfate freezing liquid is 36.8g / L, and 200ml of dilute phosphoric acid solution is prepared according to Li:P=3:0.95. 300ml of the freezing liquid is pre-mixed with the dilute phosphoric acid solution, and the pH is 5. Then 700ml of the remaining freezing liquid is added to the pre-mixed liquid within 1.5h to precipitate lithium phosphate. After the dropwise addition is completed, stirring and reacting is continued for 15min. Crude lithium phosphate and lithium phosphate precipitation mother liquor are obtained by suction filtration. The filter cake is rinsed with 200ml of high-temperature water, and then the crude lithium phosphate is put into the stirring and washing water at a mass volume ratio of 1:6 and stirred for 10min. After suction filtration, the filter cake is dried at 110°C for 2h to obtain lithium phosphate for battery-grade lithium dihydrogen phosphate raw material.

[0033] The chemical composition of lithium phosphate and lithium dihydrogen phosphate products prepared therefrom were tested, and the results are shown in Table 1.

[0034] Example 2

[0035] like Figure 1 As shown in the flow chart, 600g of crude lithium sulfate was weighed and added to 3L of lithium phosphate lithium precipitation mother liquor, and stirred and dissolved continuously. After adding 4.5g of diatomaceous earth as a filter aid for 15 minutes, the initial impurity-free Li2O was filtered out with a concentration of 42g / L and Mg2+ 0.0002 g / L, Ca 2+ The 0.22g / L lithium sulfate solution and Mg(OH)2 residue were added to 3L lithium sulfate solution to remove impurities chemically to obtain Ca 2+ The lithium sulfate solution after primary decalcification is 0.02g / L. The primary decalcified lithium sulfate solution is passed through an activated carbon adsorption tower and an ion exchange resin to obtain a COD of 150mg / L and a Ca 2+ 0.001 g / L, Na + 24.6g / L, SO4 2- The final solution of lithium sulfate is 160.5g / L. 48% liquid caustic soda is added to 2L of the final solution of lithium sulfate. The mixed solution is frozen to below -8°C and kept cold for 15 minutes. SO4 is obtained by centrifugation. 2- The lithium sulfate freezing liquid is 24.8g / L, and 200ml of dilute phosphoric acid solution is prepared according to Li:P=3:0.95. 300ml of the freezing liquid is pre-mixed with the dilute phosphoric acid solution, and the pH is 6. Then 700ml of the remaining freezing liquid is added to the pre-mixed liquid within 1.5h to precipitate lithium phosphate. After the dropwise addition is completed, stirring and reacting is continued for 15min. The crude lithium phosphate and lithium phosphate precipitation mother liquor are obtained by suction filtration. The filter cake is rinsed with 200ml of high-temperature water, and then the crude lithium phosphate is put into the stirring and washing water at a mass volume ratio of 1:6 and stirred for 10min. After suction filtration, the filter cake is dried at 110°C for 2h to obtain lithium phosphate for battery-grade lithium dihydrogen phosphate raw material.

[0036] The chemical composition of lithium phosphate and lithium dihydrogen phosphate products prepared therefrom were tested, and the results are shown in Table 1.

[0037] Example 3

[0038] like Figure 1 As shown in the flow chart, 600g of crude lithium sulfate was weighed and added to 3L of lithium phosphate lithium precipitation mother liquor, and stirred and dissolved continuously. After adding 4.5g of diatomaceous earth as a filter aid for 15 minutes, the initial impurity-free Li2O was filtered out with a concentration of 42g / L and Mg 2+ 0.0002 g / L, Ca 2+ The 0.22g / L lithium sulfate solution and Mg(OH)2 residue were added to 3L lithium sulfate solution to remove impurities chemically to obtain Ca 2+ The lithium sulfate solution after primary decalcification is 0.02g / L. The primary decalcified lithium sulfate solution is passed through an activated carbon adsorption tower and an ion exchange resin to obtain a COD of 150mg / L and a Ca 2+ 0.001 g / L, Na + 24.6g / L, SO4 2-The lithium sulfate solution is 160.5g / L. 48% liquid caustic soda is added to 2L of lithium sulfate solution. The mixed solution is frozen to below -8°C and kept cold for 15 minutes. SO4 is obtained by centrifugation. 2- The lithium sulfate freezing liquid is 24.8g / L, and 200ml of dilute phosphoric acid solution is prepared according to Li:P=3:0.95. 300ml of the freezing liquid is pre-mixed with the dilute phosphoric acid solution, and the pH is 6. Then 700ml of the remaining freezing liquid is added to the pre-mixed liquid within 1.0h to precipitate lithium phosphate. After the dropwise addition is completed, stirring and reacting is continued for 15min. The crude lithium phosphate and lithium phosphate precipitation mother liquor are obtained by suction filtration. The filter cake is rinsed with 200ml of high-temperature water, and then the crude lithium phosphate is put into the stirring and washing water at a mass volume ratio of 1:5 and stirred for 10min. After suction filtration, the filter cake is dried at 110°C for 2h to obtain lithium phosphate for battery-grade lithium dihydrogen phosphate raw material.

[0039] The chemical composition of lithium phosphate and lithium dihydrogen phosphate products prepared therefrom were tested, and the results are shown in Table 1.

[0040] Table 1 Chemical composition of lithium phosphate and lithium dihydrogen phosphate products prepared therefrom

[0041]

[0042] In Table 1, LiH2PO4-1 and LiH2PO4-2 are the product indicators of two brands of battery-grade lithium dihydrogen phosphate in "Battery-grade lithium dihydrogen phosphate" YS / T967-2014.

[0043] The battery-grade lithium dihydrogen phosphate raw material lithium phosphate prepared by the methods of Examples 1 to 3 of the present invention has a low impurity content, and the process of the present invention is simpler to operate, reduces the requirements for raw materials and equipment, reduces production costs, and the production water can be recycled multiple times, which is green and environmentally friendly and suitable for large-scale production. The obtained lithium phosphate and the lithium dihydrogen phosphate prepared using the lithium phosphate as raw materials are of good quality and can meet the quality requirements of battery-grade lithium dihydrogen phosphate.

[0044] The embodiments described above are merely descriptions of preferred implementations of the present invention and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by those skilled in the art should be included in the protection scope determined by the claims of the present invention.

Claims

1. A method for synthesizing lithium phosphate, a low-cost battery-grade lithium dihydrogen phosphate raw material, characterized in that: The following steps are involved: Step 1: The crude lithium sulfate monohydrate is stirred, washed, dissolved and filtered in sequence to obtain a lithium sulfate solution that has been preliminarily freed of impurities; Step 2: The preliminarily impurity-removed lithium sulfate solution is preliminarily decalcified with carbonate, and the decalcified solution is passed through an ion exchange resin column and an activated carbon adsorption tower to obtain a lithium sulfate solution after deep decalcification and COD removal; Step 3: Add liquid caustic soda to the lithium sulfate solution to adjust the Na + / SO4 2- After the concentration ratio is reached, the temperature is lowered and frozen to precipitate sodium sulfate decahydrate, and the frozen slurry is centrifuged to obtain lithium sulfate frozen liquid and mirabilite; Step 4: premix the diluted phosphoric acid solution and part of the lithium sulfate frozen liquid, control the pH of the premixed liquid to be weakly acidic, then add the remaining lithium sulfate frozen liquid to the premixed liquid, carry out lithium phosphate precipitation reaction at room temperature, continue the reaction for 15 to 20 minutes after the addition is completed, filter to obtain a crude lithium phosphate filter cake and a lithium phosphate precipitation mother liquor, rinse the filter cake with a small amount of water, and the lithium phosphate mother liquor can be returned to the previous process for dissolving the crude lithium sulfate monohydrate; Step 5: The crude lithium phosphate is stirred and washed with stirring water, and the filter cake is rinsed with a small amount of water and dried after filtering to obtain lithium phosphate for battery-grade lithium dihydrogen phosphate raw material. The stirring and washing mother liquor is recycled and returned to the previous process to dissolve the crude lithium sulfate after the sulfate ion concentration is enriched; Step 6: Add water and phosphoric acid to dissolve the prepared battery-grade lithium dihydrogen phosphate raw material lithium phosphate to obtain a lithium dihydrogen phosphate solution, control the pH of the solution at 2-3, then evaporate and concentrate, cool and crystallize, centrifuge, rinse the filter cake with a saturated lithium dihydrogen phosphate solution and dry, and obtain a battery-grade lithium dihydrogen phosphate product.

2. The method for synthesizing a low-cost battery-grade lithium dihydrogen phosphate raw material lithium phosphate according to claim 1, characterized in that: In step 1, the crude lithium sulfate monohydrate is dissolved in an alkaline lithium phosphate lithium precipitation mother solution, and the main ion concentrations of the lithium precipitation mother solution include: Li2O: 2-4 g / L, P: ≤0.05 g / L, OH - :3~5g / L, add 1.5g / L diatomaceous earth to assist filtration during filtration to obtain preliminary removal of Mg 2+ Lithium sulfate solution with pH ≤0.001g / L and Li2O of 40~50g / L.

3. The method for synthesizing a low-cost battery-grade lithium dihydrogen phosphate raw material lithium phosphate according to claim 1, characterized in that: After the liquid alkali in step 3 is added to the lithium sulfate solution, the Na 2+ / SO4 2- The molar ratio is 2.0~2.4:1, the freezing temperature is -10~-3℃, and the SO4 in lithium sulfate freezing liquid 2- ≤40g / L.

4. The method for synthesizing a low-cost battery-grade lithium dihydrogen phosphate raw material lithium phosphate according to claim 1, characterized in that: In step 4, the Li / P molar ratio of the lithium sulfate frozen solution and the phosphoric acid is 3:0.95, the volume ratio of the phosphoric acid solution and the lithium sulfate frozen solution is 1:5, the pH of the premixed solution is 4.5-6.5, and the remaining lithium sulfate frozen solution is added for 1-2 hours.

5. The method for synthesizing a low-cost battery-grade lithium dihydrogen phosphate raw material lithium phosphate according to claim 1, characterized in that: The volume mass ratio of the washing water and the crude lithium phosphate, i.e. ml:g, is 5-7:1, the washing temperature is 40-60°C, and the washing time is 10-20min.

6. The method for synthesizing a low-cost battery-grade lithium dihydrogen phosphate raw material lithium phosphate according to claim 1, characterized in that: The impurities Na≤0.1% and SO4 2- ≤0.35%.

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