A method for directly separating impurities from solid lithium phosphate

Abstract

The present invention discloses a method for directly separating impurities from solid lithium phosphate. The method of the present invention comprises the following steps: pre-treating the lithium phosphate raw material; adding the pre-treated lithium phosphate raw material to a mixed solution containing a penetrant, a complexing agent, and a buffer solution, cooling after a high temperature reaction, and continuing a low temperature reaction; filtering and separating after the reaction is completed, washing the obtained filter cake, and recycling the filtrate; drying the washed filter cake to obtain the lithium phosphate after impurities are removed. The method of directly separating impurities from solid lithium phosphate of the present invention does not need to dissolve the solid lithium phosphate, can effectively remove calcium and magnesium impurities in the solid lithium phosphate, has a high product yield, and the lithium phosphate product obtained after purification meets the requirements of the battery production industry.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium phosphate purification and refining, and relates to a method for directly separating impurities from solid lithium phosphate, and in particular to a method for directly separating calcium and magnesium impurities from solid lithium phosphate, and in particular to a method for directly separating calcium and magnesium impurities from insoluble solid lithium phosphate. Background Art

[0002] With the rise of the new energy industry, the market demand for lithium iron phosphate cathode materials continues to increase. In addition, as lithium power batteries have higher and higher requirements for the impurity content of raw materials, the content of each impurity is limited to a very low level.

[0003] Among them, since the relative mass of calcium and magnesium is higher than that of lithium, if the lithium iron phosphate positive electrode material contains high calcium and magnesium, these calcium and magnesium will exist in the form of calcium phosphate / magnesium phosphate; magnesium doping has a sintering effect, larger grains, and greater capacity attenuation. In addition, there is no active Mg 2+ Replaced Li + , resulting in a decrease in battery capacity and a decrease in gram capacity; in addition, a small amount of calcium doping has the effect of increasing the cycle capacity, but too high a calcium content will lead to a low capacity of the entire battery. Since the radius of calcium ions is larger than that of lithium ions, this not only affects its transport in the electrode, but also easily causes some irreversible damage to the structure of the electrode material, leading to poor electrochemical performance. Therefore, it is very necessary to remove calcium and magnesium impurities in the raw materials for synthesizing lithium iron phosphate.

[0004] At present, the industry usually uses precipitation conversion method to remove calcium and magnesium from lithium phosphate, the raw material of lithium iron phosphate. One method is to remove impurities and purify lithium phosphate waste residue containing calcium and magnesium by precipitation conversion method, and convert lithium phosphate waste residue into lithium hydrogen phosphate, lithium chloride, calcium chloride and magnesium chloride by adding hydrochloric acid. Lithium hydrogen phosphate reacts with calcium chloride and magnesium chloride to generate phosphate with lower solubility, and then obtains purified lithium chloride solution after filtration. Finally, lithium carbonate is produced by sodium carbonate precipitation method, and lithium carbonate meets YS / T 582-2013 standard; In addition, there is another method to decalcify lithium phosphate in the process of preparing battery-grade lithium carbonate, first hydrogenate lithium carbonate to generate lithium bicarbonate solution, then add lithium phosphate to convert calcium carbonate into calcium phosphate with lower solubility, without introducing other impurities, and obtain purified lithium bicarbonate solution after filtration and washing, and then heat the solution to decompose lithium carbonate.

[0005] However, the above methods for decalcifying magnesium all require the raw materials to be subjected to a dissolution-precipitation conversion-precipitation process, and these steps require large energy consumption and considerable consumption of reagents, which are costly and not conducive to industrial production.

[0006] CN111740102A discloses a method for preparing lithium phosphate for use as a new energy battery based on low-grade lithium phosphate, which includes the steps of preparing lithium phosphate slurry, complex reaction, ultrasonic crystallization, purification and drying. The method converts the impurities of calcium compounds, magnesium compounds and iron compounds in the low-grade lithium phosphate into water-soluble compounds by preparing slurry and complex reaction on the low-grade lithium phosphate, and avoids the impurities in the low-grade lithium phosphate from being re-incorporated into the crystal in the subsequent ultrasonic crystallization process, so that the purity of lithium phosphate crystals can be effectively improved and the particle size of lithium phosphate crystals can be reduced, thereby realizing the large-scale conversion of low-grade lithium phosphate into lithium phosphate that can be used for new energy batteries and improving the reuse efficiency of low-grade lithium phosphate and reducing resource waste and environmental pollution. However, the efficiency of removing calcium and magnesium impurities and the purity of the product of the method need to be further improved.

[0007] Therefore, it is necessary to develop a method that does not require dissolving solid lithium phosphate, directly separates impurities from solid lithium phosphate, and has good impurity removal effect and high product purity. Summary of the invention

[0008] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a method for directly separating impurities from solid lithium phosphate, which does not require dissolving the solid lithium phosphate and can effectively remove calcium and magnesium impurities in the solid lithium phosphate. The product yield is high. The calcium and magnesium contents in the lithium phosphate purified by this method are both lower than 100 ppm, and the purity of the lithium phosphate product reaches 99.5%. Under continuous feeding production conditions, the lithium phosphate yield reaches more than 99%, and the lithium phosphate product obtained after purification meets the requirements of the battery production industry.

[0009] The object of the present invention is to provide a method for directly separating impurities from solid lithium phosphate. To achieve this object, the present invention adopts the following technical solutions:

[0010] A method for directly separating impurities from solid lithium phosphate comprises the following steps:

[0011] Pre-treating the lithium phosphate raw material;

[0012] The pretreated lithium phosphate raw material is added to a mixed solution containing a penetrant, a complexing agent, and a buffer solution, and the temperature is lowered after a high-temperature reaction, and a low-temperature reaction is continued;

[0013] After the reaction is completed, the filter cake is washed and the filtrate is recycled;

[0014] The washed filter cake is dried to obtain the impurity-free lithium phosphate.

[0015] The method for directly separating impurities from solid lithium phosphate of the present invention can achieve the effect of separating calcium and magnesium impurities without dissolving solid lithium phosphate, and the dosage of the reagent is small, and it can be recycled, and the overall cost is low; more impurities are exposed on the surface of lithium phosphate particles through pretreatment to increase the reaction area, and the phosphate of the impurities is complexed with the complexing agent at the dissolution equilibrium to generate soluble substances, thereby separating from lithium phosphate, and the addition of the permeating agent can make the complexing agent act on the encapsulated calcium and magnesium impurities, and because the solubility of lithium phosphate in cold water is low, the dissolution of lithium phosphate can be avoided as much as possible, and the product yield can be increased, so after the reaction is completed, the temperature is lowered, and then filtered, separated, and washed to separate the calcium and magnesium impurities without dissolving the lithium phosphate. The calcium and magnesium contents in the lithium phosphate purified by the method are all lower than 100ppm, the purity of the lithium phosphate product reaches 99.5%, and the lithium phosphate yield reaches more than 99% under the condition of continuous feeding production, and the lithium phosphate product obtained after purification meets the requirements of the battery production industry.

[0016] The penetrant is a nonionic surfactant or an anionic surfactant, including one or more of isooctyl alcohol polyoxyethylene ether phosphate (AEP), fatty alcohol polyoxyethylene ether (AEO), Triton X-100, sodium dodecyl sulfate, and sodium dodecyl sulfate, preferably sodium dodecyl sulfate. The addition of the penetrant increases the surface tension of the solution, making it easier for the complexing agent to penetrate into the interior of the lithium phosphate solid.

[0017] Preferably, the mass of the penetrant accounts for 0.5-2% of the total mass of the mixed liquid, for example, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, etc.

[0018] The complexing agent is an organic chelating agent, including one or more of ATMP, EDTMPA, EDTMPAS, EGTA, EMEDTA, EDTA-2Na, EDTA-2NH4, preferably EDTA-2NH4 and / or EDTA-2Na.

[0019] The buffer solution includes an NH3-NH4Cl solution with a pH of 8-9.

[0020] Preferably, the mass of the complexing agent is more than 22% of the mass of the pretreated lithium phosphate raw material.

[0021] Preferably, the liquid-to-solid ratio of the mixed solution to the pretreated lithium phosphate raw material is (5:1)-(10:1), for example, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, etc.

[0022] The preparation of the mixed solution includes: dissolving the permeation agent and the complexing agent in the buffer solution, and obtaining the mixed solution after the complexing agent is completely dissolved.

[0023] The impurities in the lithium phosphate raw material are calcium impurities and magnesium impurities. The mass content of the calcium impurity is less than 3500 ppm, and the mass content of the magnesium impurity is less than 1000 ppm.

[0024] The pretreatment is to grind the lithium phosphate to control the particle size within 1 μm. Preferably, the grinding method is ball milling and sand milling. The particle size D100 of the treated lithium phosphate is ≤ 1 μm. Reducing the particle size can increase the contact area between the impurity particles and the solution and improve the purification effect.

[0025] The molar ratio of the complexing agent to the impurity calcium is (2:1)-(10:1), for example, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, etc. The theoretical dosage of the complexing agent is: the molar ratio of EDTA:Ca is 1:1. If the amount of the complexing agent added is the theoretical amount, the complexing reaction of calcium and magnesium impurities will not be sufficient, and the degree of purification will be reduced; increasing the dosage of the complexing agent can improve the complexing effect and increase the degree of purification.

[0026] The temperature of the high temperature reaction is 50-90°C, for example, 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, etc., and the time of the high temperature reaction is 1-2h, for example, 1h, 1.1h, 1.2h, 1.3h, 1.4h, 1.5h, 1.6h, 1.7h, 1.8h, 1.9h, 2h, etc.

[0027] The temperature of the low-temperature reaction is 1-5°C, for example, 1°C, 2°C, 3°C, 4°C, 5°C, etc.; the time of the low-temperature reaction is 12-24h, for example, 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, 20h, 21h, 22h, 23h, 24h, etc.

[0028] Increasing the reaction temperature can increase the dissolution degree of lithium phosphate on the particle surface and increase the contact area between the impurity particles and the solution. The role of low-temperature reaction is to recrystallize the lithium phosphate that enters the liquid phase during high-temperature reaction, thereby increasing the product yield.

[0029] The mass contents of calcium impurities and magnesium impurities in the lithium phosphate after impurities removal are both below 100 ppm.

[0030] The filter cake is washed with ethanol. Lithium phosphate has a certain solubility in water. Flushing with ethanol can avoid the dissolution of lithium phosphate during the washing process, which causes a decrease in yield.

[0031] The drying temperature is 70-80°C, for example, 70°C, 71°C, 72°C, 73°C, 74°C, 75°C, 76°C, 77°C, 78°C, 79°C, 80°C, etc.

[0032] As a preferred embodiment of the present invention, the method comprises the following steps:

[0033] The lithium phosphate raw material is ball-milled and sand-milled to control the particle size within 1 μm, and the mass content of impurity calcium in the lithium phosphate raw material is less than 3500 ppm, and the mass content of impurity magnesium is less than 1000 ppm;

[0034] The pretreated lithium phosphate raw material is added to a mixed solution containing a penetrant, a chelating agent, and a buffer solution, wherein the penetrant is a nonionic surfactant or an anionic surfactant, the mass of the penetrant accounts for 0.5-2% of the total mass of the mixed solution, the chelating agent is an organic chelating agent, the buffer solution is an NH3-NH4Cl solution with a pH of 8-9, and the liquid-to-solid ratio of the mixed solution to the pretreated lithium phosphate raw material is (5:1)-(10:1), reacting at a high temperature of 50-90°C, cooling for 1-2h, and continuing the low-temperature reaction at 1-5°C for 12-24h;

[0035] After the reaction is completed, the filter cake is washed with ethanol and the filtrate is recycled;

[0036] The washed filter cake is dried to obtain lithium phosphate with calcium and magnesium contents less than 100 ppm after impurities are removed.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] The method for directly separating impurities from solid lithium phosphate of the present invention does not need to dissolve the solid lithium phosphate, can effectively remove calcium and magnesium impurities in the solid lithium phosphate, and has a high product yield. The calcium and magnesium contents in the lithium phosphate purified by the method are both lower than 100 ppm, and the purity of the lithium phosphate product reaches 99.5%. Under the condition of continuous feeding production, the lithium phosphate yield reaches more than 99%, and the lithium phosphate product obtained after purification meets the requirements of the battery production industry.

[0039] The method for directly separating impurities from solid lithium phosphate of the present invention can achieve the effect of separating calcium and magnesium impurities without dissolving the solid lithium phosphate in the purification process, and the dosage of the reagent is small, and the product can be recycled, and the overall cost is low. During the purification process, the filtrate can be recycled after being treated by another process, and no solid waste is generated, and there is no environmental pollution problem. The recovery process is simple, the process is easy to control, and it is easy to realize industrialization. DETAILED DESCRIPTION

[0040] The technical solution of the present invention is further illustrated below through specific implementation methods.

[0041] Unless otherwise specified, various raw materials of the present invention can be purchased from the market or prepared according to conventional methods in the art.

[0042] Example 1

[0043] A method for directly separating calcium and magnesium impurities from solid lithium phosphate in this embodiment includes the following steps:

[0044] (1) After pretreatment of lithium phosphate containing a small amount of calcium and magnesium impurities by ball milling and sand milling, 100 g (D100 ≤ 1 μm) was weighed;

[0045] (2) preparing 500 g of a mixed solution comprising 7.33 g (twice the theoretical amount of the complexing agent) of EDTA-2NH4, a buffer solution of NH3-NH4Cl with a pH of 8 to 9, and 5 g of an osmotic agent, sodium dodecyl sulfate, and maintaining the temperature at 50° C.;

[0046] (3) adding the pre-treated lithium phosphate to the mixed solution, stirring for 1 hour, lowering the temperature to 2° C., continuing stirring for 24 hours, filtering and separating, washing the filter cake with ethanol, and recycling the filtrate after another process treatment;

[0047] (4) After the filter cake is dried, a solid lithium phosphate product is obtained after purification of calcium and magnesium. The composition analysis of the lithium phosphate raw material and the purified lithium phosphate product is shown in Table 1.

[0048] The yield of the purified lithium phosphate obtained in this example is 99.1%.

[0049] Table 1 Lithium phosphate product composition

[0050]

[0051] Example 2

[0052] A method for directly separating calcium and magnesium impurities from solid lithium phosphate in this embodiment includes the following steps:

[0053] (1) After pretreatment of lithium phosphate containing a small amount of calcium and magnesium impurities by ball milling and sand milling, 100 g (D100 ≤ 1 μm) was weighed;

[0054] (2) preparing 600 g of a mixed solution comprising 7.33 g (twice the theoretical amount of the complexing agent) of EDTA-2NH4, a buffer solution of NH3-NH4Cl with a pH of 8 to 9, and 6 g of an osmotic agent, sodium dodecyl sulfate, and maintaining the temperature at 50° C.;

[0055] (3) adding the pre-treated lithium phosphate to the mixed solution, stirring for 1 hour, lowering the temperature to 2° C., continuing stirring for 24 hours, filtering and separating, washing the filter cake with ethanol, and recycling the filtrate after another process treatment;

[0056] (4) After the filter cake is dried, a solid lithium phosphate product is obtained after purification of calcium and magnesium. The composition analysis of the lithium phosphate raw material and the purified lithium phosphate product is shown in Table 2.

[0057] The yield of the purified lithium phosphate obtained in this example is 99.2%.

[0058] Table 2 Lithium phosphate product composition

[0059]

[0060] From the data comparison in Table 2, it can be seen that compared with Example 1, Example 2 has better removal effect of calcium and magnesium impurities. Compared with Example 1, since the reaction liquid-solid ratio of Example 2 is increased from 5:1 in Example 1 to 6:1, the fluidity of the slurry is improved, the contact between lithium phosphate and the solution is more sufficient, the reaction liquid-solid ratio is increased, the degree of calcium and magnesium purification is increased, and the improvement effect of changing the reaction conditions on purification is in line with expectations.

[0061] Example 3

[0062] A method for directly separating calcium and magnesium impurities from solid lithium phosphate in this embodiment includes the following steps:

[0063] (1) After pretreatment of lithium phosphate containing a small amount of calcium and magnesium impurities by ball milling and sand milling, 100 g (D100 ≤ 1 μm) was weighed;

[0064] (2) preparing 600 g of a mixed solution comprising 22.0 g (6 times the theoretical amount of the complexing agent) of EDTA-2NH4, a buffer solution of NH3-NH4Cl with a pH of 8 to 9, and 6 g of an osmotic agent, sodium dodecyl sulfate, and maintaining the temperature at 50° C.;

[0065] (3) adding the pre-treated lithium phosphate to the mixed solution, stirring for 1 hour, lowering the temperature to 2° C., continuing stirring for 24 hours, filtering and separating, washing the filter cake with ethanol, and recycling the filtrate after another process treatment;

[0066] (4) After the filter cake is dried, a solid lithium phosphate product is obtained after purification of calcium and magnesium. The composition analysis of the lithium phosphate raw material and the purified lithium phosphate product is shown in Table 3.

[0067] The yield of the purified lithium phosphate obtained in this example is 99.2%.

[0068] Table 3 Lithium phosphate product composition

[0069]

[0070] From the data comparison in Table 3, it can be seen that, compared with Example 2, due to the increase in the amount of complexing agent used in Example 3, the complexing effect is improved, the degree of calcium and magnesium purification is increased, and the improvement effect of changing the reaction conditions on purification is in line with expectations.

[0071] Example 4

[0072] A method for directly separating calcium and magnesium impurities from solid lithium phosphate in this embodiment includes the following steps:

[0073] (1) After pretreatment of lithium phosphate containing a small amount of calcium and magnesium impurities by ball milling and sand milling, 100 g (D100 ≤ 1 μm) was weighed;

[0074] (2) preparing 600 g of a mixed solution comprising 7.33 g (twice the theoretical amount of the complexing agent) of EDTA-2NH4, a buffer solution of NH3-NH4Cl with a pH of 8 to 9, and 6 g of an osmotic agent, sodium dodecyl sulfate, and maintaining the temperature at 90° C.;

[0075] (3) adding the pre-treated lithium phosphate to the mixed solution, stirring for 1 hour, lowering the temperature to 2° C., continuing stirring for 24 hours, filtering and separating, washing the filter cake with ethanol, and recycling the filtrate after another process treatment;

[0076] (4) After the filter cake is dried, a solid lithium phosphate product is obtained after purification of calcium and magnesium. The composition analysis of the lithium phosphate raw material and the purified lithium phosphate product is shown in Table 4.

[0077] The yield of the purified lithium phosphate obtained in this example is 99.2%.

[0078] Table 4 Lithium phosphate product composition

[0079]

[0080] From the comparison of the data in Table 4, it can be seen that, relative to Example 2, since the reaction temperature of Example 4 is increased from 50°C to 90°C, the solubility of lithium phosphate on the solid surface is increased, the reaction contact area between the impurities and the solution is increased, the degree of calcium and magnesium purification is increased, and the improvement effect of changing the reaction conditions on purification is in line with expectations.

[0081] Comparative Example 1

[0082] A method for directly separating calcium and magnesium impurities from solid lithium phosphate in this comparative example comprises the following steps:

[0083] (1) After pretreatment of lithium phosphate containing a small amount of calcium and magnesium impurities by ball milling and sand milling, 100 g (D100 ≤ 1 μm) was weighed;

[0084] (2) preparing 600 g of a mixed solution comprising 3.67 g (the theoretical amount of the complexing agent) of EDTA-2NH4, a buffer solution of NH3-NH4Cl with a pH of 8 to 9, and 6 g of an osmotic agent, sodium dodecyl sulfate, and maintaining the temperature at 90° C.;

[0085] (3) adding the pre-treated lithium phosphate to the hot solution, stirring for 1 hour, lowering the temperature to 2° C., continuing to stir for 24 hours, filtering and separating, washing the filter cake with ethanol, and recycling the filtrate after another process treatment;

[0086] (4) After the filter cake is dried, a solid lithium phosphate product is obtained after purification of calcium and magnesium. The composition analysis of the lithium phosphate raw material and the purified lithium phosphate product is shown in Table 5.

[0087] The yield of the purified lithium phosphate obtained in this comparative example is 99.2%.

[0088] Table 5 Lithium phosphate product composition

[0089]

[0090] It can be seen from the data in Table 5 that, relative to Example 4, after maintaining the reaction liquid-solid ratio and reaction temperature unchanged and reducing the complexing agent to the theoretical amount, the complexing reaction of calcium and magnesium impurities is not fully carried out, the degree of calcium and magnesium purification is reduced, and the residual calcium and magnesium content does not meet the production requirements of the positive electrode material.

[0091] Comparative Example 2

[0092] A method for directly separating calcium and magnesium impurities from solid lithium phosphate in this comparative example comprises the following steps:

[0093] (1) After pretreatment of lithium phosphate containing a small amount of calcium and magnesium impurities by ball milling and sand milling, 100 g (D100 ≤ 1 μm) was weighed;

[0094] (2) preparing 600 g of a mixed solution comprising 66 g (18 times the theoretical amount of the complexing agent) of EDTA-2NH4, a buffer solution of NH3-NH4Cl with a pH of 8 to 9, and 6 g of an osmotic agent, sodium dodecyl sulfate, and maintaining the temperature at 90° C.;

[0095] (3) adding the pre-treated lithium phosphate to the mixed solution, stirring for 1 hour, lowering the temperature to 2° C., continuing stirring for 24 hours, filtering and separating, washing the filter cake with ethanol, and recycling the filtrate after another process treatment;

[0096] (4) After the filter cake is dried, a solid lithium phosphate product is obtained after purification of calcium and magnesium. The composition analysis of the lithium phosphate raw material and the purified lithium phosphate product is shown in Table 6.

[0097] The yield of the purified lithium phosphate obtained in this comparative example is 99.1%.

[0098] Table 6 Lithium phosphate product composition

[0099]

[0100] It can be seen from the data in Table 6 that, relative to Example 4, this comparative example maintains the reaction liquid-solid ratio and reaction temperature unchanged, and after increasing the amount of complexing agent to 66 g, the calcium-magnesium purification effect is slightly increased, but there is not much room for improvement. At this time, the slight reduction in the calcium-magnesium impurity content in lithium phosphate has little effect on the production of positive electrode materials, and continuing to increase the amount of complexing agent is not cost-effective.

[0101] Comparative Example 3

[0102] A method for directly separating calcium and magnesium impurities from solid lithium phosphate in this comparative example comprises the following steps:

[0103] (1) After pretreatment of lithium phosphate containing a small amount of calcium and magnesium impurities by ball milling and sand milling, 100 g (D100 ≤ 1 μm) was weighed;

[0104] (2) preparing 600 g of a mixed solution comprising 7.33 g (twice the theoretical amount of the complexing agent) of EDTA-2NH4, a buffer solution of NH3-NH4Cl with a pH of 8 to 9, and 6 g of an osmotic agent, sodium dodecyl sulfate, and maintaining the temperature at 40° C.;

[0105] (3) adding the pre-treated lithium phosphate to the mixed solution, stirring for 1 hour, lowering the temperature to 2° C., continuing stirring for 24 hours, filtering and separating, washing the filter cake with ethanol, and recycling the filtrate after another process treatment;

[0106] (4) After the filter cake is dried, a solid lithium phosphate product is obtained after purification of calcium and magnesium. The composition analysis of the lithium phosphate raw material and the purified lithium phosphate product is shown in Table 7.

[0107] The yield of the purified lithium phosphate obtained in this comparative example is 99.1%.

[0108] Table 7 Lithium phosphate product composition

[0109]

[0110] It can be seen from the data in Table 7 that, relative to Example 4, this comparative example maintains the reaction liquid-solid ratio and the amount of complexing agent unchanged. After the reaction temperature is lowered to 40°C, the calcium and magnesium impurity particles are insufficiently in contact with the solution, some calcium and magnesium impurities cannot react with the complexing agent, the degree of calcium and magnesium purification is reduced, and the residual calcium and magnesium content does not meet the production requirements of the positive electrode material.

[0111] Comparative Example 4

[0112] A method for directly separating calcium and magnesium impurities from solid lithium phosphate in this comparative example comprises the following steps:

[0113] (1) After pretreatment of lithium phosphate containing a small amount of calcium and magnesium impurities by ball milling and sand milling, 100 g (D100 ≤ 1 μm) was weighed;

[0114] (2) preparing 600 g of a mixed solution, which includes 7.33 g (twice the theoretical amount of the complexing agent) of EDTA-2NH4, a buffer solution of NH3-NH4Cl with a pH of 8 to 9, without adding sodium dodecyl sulfate, an osmotic agent, and maintaining the temperature at 90° C.;

[0115] (3) adding the pre-treated lithium phosphate to the mixed solution, stirring for 1 hour, lowering the temperature to 2° C., continuing stirring for 24 hours, filtering and separating, washing the filter cake with ethanol, and recycling the filtrate after another process treatment;

[0116] (4) After the filter cake is dried, a solid lithium phosphate product is obtained after purification of calcium and magnesium. The composition analysis of the lithium phosphate raw material and the purified lithium phosphate product is shown in Table 8.

[0117] The yield of the purified lithium phosphate obtained in this comparative example is 99.2%.

[0118] Table 8 Lithium phosphate product composition

[0119]

[0120] It can be seen from the data in Table 8 that, relative to Example 4, this comparative example maintains the reaction liquid-to-solid ratio, the amount of complexing agent, and the reaction temperature unchanged, and does not add the penetrant C. The calcium and magnesium impurity particles inside the particles cannot contact the solution, some calcium and magnesium impurities cannot react with the complexing agent, the degree of calcium and magnesium purification is reduced, and the residual calcium and magnesium content does not meet the production requirements of the positive electrode material.

[0121] Comparative Example 5

[0122] A method for directly separating calcium and magnesium impurities from solid lithium phosphate in this comparative example comprises the following steps:

[0123] (1) After pretreatment of lithium phosphate containing a small amount of calcium and magnesium impurities by ball milling and sand milling, 100 g (D100 ≤ 1 μm) was weighed;

[0124] (2) preparing 600 g of a mixed solution comprising 7.33 g (twice the theoretical amount of the complexing agent) of EDTA-2NH4, a buffer solution of NH3-NH4Cl with a pH of 8 to 9, and 6 g of an osmotic agent, sodium dodecyl sulfate, and maintaining the temperature at 90° C.;

[0125] (3) adding the pre-treated lithium phosphate to the mixed solution, stirring for 25 hours, filtering and separating, washing the filter cake with ethanol, and recycling the filtrate after another process treatment;

[0126] (4) After the filter cake is dried, a solid lithium phosphate product is obtained after purification of calcium and magnesium. The composition analysis of the lithium phosphate raw material and the purified lithium phosphate product is shown in Table 9.

[0127] The yield of the purified lithium phosphate obtained in this comparative example is 86.5%.

[0128] Table 9 Lithium phosphate product composition

[0129]

[0130] It can be seen from the data in Table 9 that, relative to Example 4, this comparative example maintains the reaction liquid-to-solid ratio, the dosage of the complexing agent and the penetrant unchanged, and does not undergo a low-temperature reaction. This will increase the solubility of lithium phosphate under higher temperature conditions, and lithium phosphate will not be recrystallized and precipitated, resulting in a lower recovery rate.

[0131] Comparative Example 6

[0132] A method for directly separating calcium and magnesium impurities from solid lithium phosphate in this comparative example comprises the following steps:

[0133] (1) After pre-grinding, lithium phosphate containing a small amount of calcium and magnesium impurities is weighed to obtain 100 g (D100 ≤ 1 μm);

[0134] (2) preparing 600 g of a mixed solution comprising 7.33 g (twice the theoretical amount of the complexing agent) of EDTA-2NH4, a buffer solution of NH3-NH4Cl with a pH of 8 to 9, and 6 g of an osmotic agent, sodium dodecyl sulfate, and maintaining the temperature at 90° C.;

[0135] (3) adding the pre-treated lithium phosphate to the mixed solution, stirring for 1 hour, lowering the temperature to 25° C., continuing stirring for 24 hours, filtering and separating, washing the filter cake with ethanol, and recycling the filtrate after another process treatment;

[0136] (4) After the filter cake is dried, a solid lithium phosphate product is obtained after purification of calcium and magnesium. The composition analysis of the lithium phosphate raw material and the purified lithium phosphate product is shown in Table 10.

[0137] The yield of the purified lithium phosphate obtained in this comparative example is 89.4%.

[0138] Table 10 Lithium phosphate product composition

[0139]

[0140] It can be seen from the data in Table 10 that, relative to Example 4, this comparative example keeps the reaction liquid-to-solid ratio, the amount of complexing agent and penetrant unchanged, and cools to room temperature instead of low temperature, which will cause insufficient crystallization of lithium phosphate and reduce its recovery rate.

[0141] The present invention illustrates the detailed process equipment and process flow of the present invention through the above-mentioned embodiments, but the present invention is not limited to the above-mentioned detailed process equipment and process flow, that is, it does not mean that the present invention must rely on the above-mentioned detailed process equipment and process flow to be implemented. Those skilled in the art should understand that any improvement of the present invention, equivalent replacement of various raw materials of the product of the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

[0142] The preferred embodiments of the present invention are described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, a variety of simple modifications can be made to the technical solution of the present invention, and these simple modifications all belong to the protection scope of the present invention.

[0143] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0144] In addition, various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. A method for directly separating impurities from solid lithium phosphate, characterized in that: The steps include: Pre-treating the lithium phosphate raw material; Adding the pretreated lithium phosphate raw material into a mixed solution containing a penetrant, a complexing agent, and a buffer solution, cooling after a high-temperature reaction, and continuing a low-temperature reaction; After the reaction is completed, the filter cake is washed and the filtrate is recycled; The washed filter cake is dried to obtain lithium phosphate after impurities are removed; The penetrant is a nonionic surfactant or an anionic surfactant, including one or more of isooctyl alcohol polyoxyethylene ether phosphate, fatty alcohol polyoxyethylene ether, Triton X-100, sodium dodecyl sulfonate, and sodium dodecyl sulfate; The complexing agent is an organic chelating agent, including one or more of ATMP, EDTMPA, EDTMPAS, EGTA, EMEDTA, EDTA-2Na, and EDTA-2NH4; The impurities in the lithium phosphate raw material include calcium impurities and magnesium impurities; The molar ratio of the complexing agent to the impurity calcium is (2:1)-(10:1); The temperature of the high temperature reaction is 50-90°C, and the temperature of the low temperature reaction is 1-5°C.

2. The method according to claim 1, characterized in that The penetrant is sodium lauryl sulfate.

3. The method according to claim 1, characterized in that: The mass of the penetrant accounts for 0.5-2% of the total mass of the mixed solution.

4. The method according to claim 1, characterized in that The complexing agent is EDTA-2NH4 and / or EDTA-2Na.

5. The method according to claim 1, characterized in that The buffer solution includes an NH3-NH4Cl solution with a pH of 8-9.

6. The method according to claim 1, characterized in that The mass of the complexing agent is more than 22% of the mass of the pretreated lithium phosphate raw material.

7. The method according to claim 1, characterized in that The liquid-to-solid ratio of the mixed solution to the pretreated lithium phosphate raw material is (5:1)-(10:1).

8. The method according to claim 1, characterized in that: The preparation of the mixed solution includes: dissolving the osmotic agent and the complexing agent in the buffer solution, and obtaining the mixed solution after the complexing agent is completely dissolved.

9. The method according to claim 1, characterized in that: The mass content of the impurity calcium is less than 3500 ppm, and the mass content of the impurity magnesium is less than 1000 ppm.

10. The method according to claim 1, characterized in that The pretreatment is to grind the lithium phosphate to control the particle size within 1 μm.

11. The method according to claim 10, characterized in that The grinding methods are ball milling and sand milling.

12. The method according to claim 1, characterized in that The high temperature reaction time is 1-2h.

13. The method according to claim 1, characterized in that The low temperature reaction time is 12-24h.

14. The method according to claim 1, characterized in that The mass contents of calcium impurities and magnesium impurities in the lithium phosphate after impurities removal are both below 100 ppm.

15. The method according to claim 1, characterized in that The filter cake is washed with ethanol.

16. The method according to claim 1, characterized in that The drying temperature is 70-80°C.

17. The method according to claim 1, characterized in that The method comprises the following steps: The lithium phosphate raw material is ball-milled and sand-milled to control the particle size within 1 μm, and the mass content of impurity calcium in the lithium phosphate raw material is less than 3500 ppm, and the mass content of impurity magnesium is less than 1000 ppm; The pretreated lithium phosphate raw material is added to a mixed solution containing a penetrant, a chelating agent, and a buffer solution, wherein the penetrant is a nonionic surfactant or an anionic surfactant, the mass of the penetrant accounts for 0.5-2% of the total mass of the mixed solution, the chelating agent is an organic chelating agent, the buffer solution is an NH3-NH4Cl solution with a pH of 8-9, and the liquid-to-solid ratio of the mixed solution to the pretreated lithium phosphate raw material is (5:1)-(10:1), reacting at a high temperature of 50-90°C, cooling for 1-2h, and continuing the low-temperature reaction at 1-5°C for 12-24h; After the reaction is completed, the filter cake is washed with ethanol and the filtrate is recycled; The washed filter cake is dried to obtain lithium phosphate with calcium and magnesium contents less than 100 ppm after impurities are removed.

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