Comprehensive recovery method for preparing battery-grade iron phosphate dihydrate and aluminum and lithium from lithium phosphorus aluminum ore nitric acid leaching solution

Abstract

The invention provides a comprehensive recovery method for preparing battery-grade iron phosphate dihydrate, aluminum and lithium from a lithium phosphorus aluminum ore nitric acid leaching solution, and belongs to the technical field of comprehensive recovery of lithium ores, and the method comprises the following steps: adding an iron source into the lithium phosphorus aluminum ore nitric acid leaching solution, dissolving, and filtering to obtain an iron dissolving mother solution and insoluble slag; sodium fluoride is added into the iron dissolving mother liquor for aluminum precipitation to obtain sodium hexafluoroaluminate and aluminum-precipitated liquid; heating and precipitating the aluminum-precipitated liquid to obtain iron phosphate dihydrate crystals and a lithium-containing solution; performing chemical impurity removal on the lithium-containing solution, and adding a sodium carbonate solution to obtain a battery-grade lithium carbonate product; and washing and drying the iron phosphate dihydrate crystal to obtain the battery-grade iron phosphate dihydrate. According to the method, phosphorus, aluminum and lithium are efficiently separated and extracted, battery-grade iron phosphate dihydrate, sodium hexafluoroaluminate and battery-grade lithium carbonate products are obtained, the resource utilization rate of the phosphorus-aluminum-lithium ore is greatly increased, and the economic value of the phosphorus-aluminum-lithium ore is increased. According to the method, raw materials and auxiliary materials are easy to obtain, the production cost is low, products are diversified, and industrial application is easy to achieve.

Description

Technical Field

[0001] The invention belongs to the technical field of comprehensive recovery of lithium ore, and specifically relates to a method for preparing battery-grade iron phosphate dihydrate and comprehensive recovery of aluminum and lithium by nitric acid leaching solution of lithium aluminum phosphate. Background Art

[0002] Ferrous phosphate dihydrate is an important chemical raw material. Through observation and analysis of the market of the ferrous phosphate dihydrate industry, it is found that the ferrous phosphate dihydrate industry now has huge development potential and is widely used in the manufacture of lithium iron phosphate dihydrate batteries, catalysts, ceramics, agricultural fertilizers, food additives, medicine, metal processing and other fields. With the rapid rise of the new energy industry, lithium iron phosphate dihydrate batteries are a type of battery commonly used in energy storage technology, with high overall performance, long cycle life and low cost. The market demand for consumption continues to increase, and the future development prospects of the ferrous phosphate dihydrate industry are broad. At present, the market prepares ferrous phosphate dihydrate by dissolving phosphorus source phosphate ore or phosphate, and the process has become mature and stable; while the process of extracting phosphorus from nitric acid leachate of lithium aluminum phosphate ore to prepare battery-grade ferrous phosphate dihydrate and comprehensively recycling aluminum and lithium is still relatively backward. The present invention uses technological innovation to upgrade products, diversify by-products, expand the market, improve the level of industry development and market competitiveness, and achieve sustainable development.

[0003] The invention can efficiently separate and extract phosphorus, aluminum and lithium from the nitric acid leachate of lithium-phosphite ore to obtain battery-grade ferric phosphate dihydrate, high-purity sodium hexafluoroaluminate and battery-grade lithium carbonate products, so as to diversify the products and achieve the purpose of comprehensive resource recycling. It provides a new idea for the treatment of complex nitric acid leachate containing phosphorus, aluminum and lithium, realizes the comprehensive recycling of multiple elements, reduces environmental pollution and improves economic benefits. Summary of the invention

[0004] The present invention provides a method for separating and extracting various valuable elements from nitric acid leachate of lithium aluminum phosphate ore to prepare battery-grade iron phosphate dihydrate. The method utilizes the principles of chemical precipitation and other methods to efficiently separate and extract phosphorus, aluminum and lithium from phosphorus, aluminum and lithium acid solutions to obtain battery-grade iron phosphate dihydrate, high-purity sodium hexafluoroaluminate and battery-grade lithium carbonate products. The process and equipment used are simple, the raw and auxiliary materials are cheap, the energy consumption cost is low, the product quality is high, and the resource utilization is maximized. It has the advantages of being green, low-carbon and environmentally friendly, and having high economic utilization value.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A method for preparing battery-grade iron phosphate dihydrate and comprehensive recovery of aluminum and lithium from nitric acid leaching solution of lithium aluminum phosphate ore, comprising the following steps:

[0007] S1: adding an iron source into the nitric acid leaching solution of lithium aluminum phosphate, dissolving and filtering to obtain an iron-dissolving mother liquor and an insoluble slag;

[0008] S2: adding NaF to the iron-dissolving mother liquor of step S1, filtering to obtain the aluminum-precipitated liquid and crude Na3AlF6; the crude Na3AlF6 is washed to obtain a high-purity product;

[0009] S3: heating the aluminum precipitation solution obtained in step S2 and filtering it to obtain iron phosphate dihydrate crystals and a lithium-containing solution;

[0010] S4: chemically removing impurities from the lithium-containing solution obtained in step S3, and then adding sodium carbonate solution to precipitate lithium to obtain battery-grade lithium carbonate;

[0011] S5: The FePO4·2H2O obtained in step S3 is washed and dried to obtain battery-grade iron phosphate dihydrate.

[0012] Preferably, in step S1, the chemical composition of the nitric acid leachate of lithium aluminum phosphate includes: Li: 6.5-8 g / L, Na: 0.1-0.3 g / L, Fe: 0.3-0.8 g / L, Al: 25-30 g / L, Mn: 0.05-1 g / L, Ca: 0.01-0.05 g / L, Mg: 0.1-0.15 g / L, Si: 0.01-0.05 g / L, P: 25-30 g / L, K: 0.2-0.5 g / L, H + :3~4mol / L. The nitric acid leaching solution should contain enough H + To dissolve the iron source, H + If it is insufficient, add nitric acid.

[0013] Preferably, in step S1, the iron source is an acid-soluble iron-containing substance, selected from at least one of reducing iron powder, iron oxide, and iron hydroxide, such as iron powder, iron hydroxide, etc. The amount of the iron source is determined by the phosphorus-iron molar ratio, preferably the phosphorus-iron molar ratio is (0.8-1.2): 1. The appropriate phosphorus-iron ratio can ensure the stability of the chemical structure and chemical properties of ferric phosphate dihydrate.

[0014] Preferably, in step S2, the amount of NaF used is 4 to 10 times the molar amount of aluminum ions in the iron-dissolving mother liquor, the reaction system pH=0 to 2, the reaction temperature is 20 to 100°C, the reaction time is 0.5 to 6 hours, and the crude Na3AlF6 obtained is washed to obtain high-purity Na3AlF6. Within the sodium fluoride dosage range of the present invention, the pH of the reaction system can be controlled at 0 to 2. If too much sodium fluoride is added, the pH of the system is too high, and the iron ions will be precipitated in advance in the form of ferric hydroxide, affecting the quality of sodium fluoroaluminate and reducing the efficiency of subsequent phosphorus precipitation; if the amount of sodium fluoride added is too low, the aluminum precipitation is incomplete, the product recovery efficiency is reduced, and the subsequent purity of dihydrate iron phosphate is greatly affected.

[0015] Preferably, in step S2, the crude Na3AlF6 is washed by using a dilute nitric acid solution with a pH of 1 at room temperature, controlling the liquid-solid mass ratio to be 3:1, stirring and washing twice, and then washing twice with pure water, each washing time being 0.5 h.

[0016] Preferably, in step S3, the heating temperature is 80-150°C, the reaction time is 2-6h, and the pH of the reaction system is 0.5-2. If the heating temperature is too low, dihydrate iron phosphate cannot be precipitated; if the upper limit of the heating temperature is exceeded, it will lead to excessive energy consumption, but the precipitation efficiency will not be significantly improved, so there is no need to increase the upper limit of the temperature. After aluminum precipitation, the iron ions in the liquid are +3 valent. If the pH of the solution system is greater than 2, the iron ions will be hydrolyzed and precipitated in the form of iron hydroxide, affecting the quality of iron phosphate, so the pH of the reaction system needs to be controlled. The reaction equation is: Fe 3+ +H2O⇌Fe(OH)3↓+3H + .

[0017] Preferably, in step S4, the specific method of chemical impurity removal is: adding sodium hydroxide to the lithium-containing solution to adjust the system pH to 7 to remove trace iron and aluminum, adjusting the filtrate to pH = 12 after filtering to remove residual calcium and magnesium ions, and then adding a 30% by mass sodium carbonate solution thereto after filtering to prepare battery-grade lithium carbonate.

[0018] Preferably, in step S5, the obtained ferric phosphate dihydrate crystals are heated in a water bath at 60°C, and stirred and washed twice with a phosphoric acid solution having a concentration of 0.1 mol / L, each washing time being 0.5 h, and the liquid-solid mass ratio of the ferric phosphate dihydrate crystals to the phosphoric acid solution is 5:1. After filtration, the crystals are washed twice with pure water at a liquid-solid mass ratio of 5:1, each washing time being 30 min, to obtain refined FePO4·2H2O.

[0019] Preferably, in step S5, the refined FePO4·2H2O is dried at 95°C for 4 hours to obtain battery-grade FePO4·2H2O.

[0020] The above technical solution provided by the present invention at least brings the following beneficial effects:

[0021] The method of the present invention uses nitric acid leachate of lithium aluminum phosphate as raw material, adds compounds such as an iron source to an acidic solution, selectively precipitates by controlling the pH of the reaction system and the reaction temperature, separates aluminum, phosphorus, lithium and other metal impurities, and efficiently separates and extracts phosphorus, aluminum and lithium in the solution to obtain battery-grade iron phosphate dihydrate, high-purity sodium hexafluoroaluminate and battery-grade lithium carbonate products. It realizes the extraction of iron phosphate dihydrate from phosphorus, aluminum and lithium nitric acid leachate and maximizes the comprehensive recycling and utilization of valuable elements, breaking the process barriers.

[0022] In the present invention, in step S1, the iron source is dissolved in the system by the nitric acid remaining in the leaching solution;

[0023] In step S2, sodium fluoride and aluminum ions are added to the iron molten mother liquor to generate sodium hexafluoroaluminate, and aluminum is selectively separated in the form of precipitation by controlling the reaction temperature and pH to obtain a sodium hexafluoroaluminate product, and a high-purity product is obtained by washing;

[0024] In step S3, phosphorus is precipitated in the form of ferric phosphate dihydrate crystals by controlling the temperature and pH of the liquid after aluminum removal to obtain crude ferric phosphate dihydrate. In step S5, the crude ferric phosphate dihydrate is washed with dilute phosphoric acid and pure water, and dried to obtain battery-grade ferric phosphate dihydrate.

[0025] In the above process, the efficient recovery of aluminum not only produces high-purity sodium hexafluoroaluminate products and reduces resource waste, but also reduces the difficulty of the subsequent preparation of dihydrate iron phosphate, improves the purity of dihydrate iron phosphate, reduces the loss of phosphorus and lithium during aluminum precipitation, and improves the recovery rate of phosphorus and lithium. The utilization of phosphorus resources to produce battery-grade dihydrate iron phosphate products increases economic benefits, greatly reduces the difficulty of subsequent impurity removal processes, reduces the cost of treating phosphorus in wastewater, and reduces environmental pollution. The lithium-containing solution is chemically removed by controlling the pH, and a 30% by mass sodium carbonate solution is added to the liquid after impurity removal to produce a battery-grade lithium carbonate product. From then on, the efficient comprehensive recovery of phosphorus, aluminum, and lithium in the nitric acid leaching solution of lithium-phosphorus aluminum ore is achieved.

[0026] The present invention realizes efficient separation and recovery of phosphorus, aluminum and lithium through simple processes and equipment, reduces environmental pollution and improves economic benefits. The materials used are easy to purchase and cheap; the process is short, the operation is simple and easy to control, the cost is low, the raw material source is wide, the energy consumption is low, and it is green and environmentally friendly, and it is easy to realize industrialization. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 The present invention is a schematic flow chart of a method for preparing battery-grade iron phosphate dihydrate and a comprehensive recovery method of aluminum and lithium from a nitric acid leaching solution of lithium-aluminum phosphate ore. DETAILED DESCRIPTION

[0028] In order to make the objectives, technical solutions and advantages of the present invention more clear, the embodiments of the present invention will be described in further detail below.

[0029] The analysis results of the components of the nitric acid leaching solution used in the following examples and comparative examples are shown in Table 1, the battery-grade lithium carbonate standards in the industry are shown in Table 2, and the battery-grade iron phosphate dihydrate standards are shown in Table 3. According to Table 2, except for the quality of lithium carbonate, the content of other impurities is less than or equal to the standards shown; according to Table 3, except for the tap density, the content of other impurities is less than or equal to the standards shown.

[0030] Table 1 Analysis results of components of nitric acid leaching solution of lithium aluminum phosphate

[0031]

[0032] Table 2 Battery grade lithium carbonate standard (wt%) (YS / T582-2013)

[0033]

[0034] Table 3 Battery grade iron phosphate dihydrate standard (w%) (HG / T 4701-2021)

[0035]

[0036] Example 1

[0037] like Figure 1 As shown, a method for preparing battery iron phosphate dihydrate and comprehensive recovery of aluminum and lithium from nitric acid leaching solution of lithium aluminum phosphate ore specifically comprises the following steps:

[0038] S1: Add iron powder into nitric acid leaching solution at a phosphorus to iron molar ratio of 0.9:1, and filter to obtain iron-dissolving mother liquor and insoluble slag.

[0039] S2: Control the temperature of the mother liquor obtained in step S1 at 70°C, slowly add saturated NaF solution to the mother liquor, the amount of sodium fluoride used is 7 times the molar amount of aluminum in the mother liquor, and control the system pH=1, stir the reaction for 6 hours, and filter to obtain the aluminum precipitation liquid and crude Na3AlF6 product.

[0040] S3: The crude Na3AlF6 product obtained in step S2 is washed with a dilute nitric acid solution with a pH value of 1, the liquid-to-solid mass ratio is controlled to be 3:1, and the product is washed twice at room temperature for 30 minutes each time, and then washed twice with pure water, and filtered to obtain a high-purity Na3AlF6 product.

[0041] S4: The pH of the aluminum precipitation solution obtained in step S2 is adjusted to 1 with dilute nitric acid, and the mixture is stirred at 150° C. for 5 hours at a stirring speed of 100 rpm, and the dihydrate iron phosphate crystals and the lithium-containing solution are obtained by filtration.

[0042] S5: The ferric phosphate dihydrate crystals obtained in step S4 are stirred and washed twice with 0.1 mol / L dilute phosphoric acid at 60° C. in a water bath, each time for 30 min, and then washed twice with pure water, each time for 30 min, and filtered to obtain high-purity ferric phosphate dihydrate.

[0043] S6: drying the iron phosphate dihydrate after washing in step S5 at 95° C. for 4 hours to obtain battery-grade iron phosphate dihydrate.

[0044] S7: adding sodium hydroxide to the lithium-containing solution obtained in step S4 to adjust the system pH to 7 to remove trace iron and aluminum, filtering and adjusting the filtrate to pH = 12 to remove residual calcium and magnesium ions, filtering and adding 30% by mass sodium carbonate solution to the impurity-free solution to precipitate lithium to obtain battery-grade lithium carbonate.

[0045] The aluminum recovery rate obtained in this embodiment is 92.5%, the phosphorus recovery rate is 97.8%, and the lithium recovery rate is 98.2%. Product quality: Sodium hexafluoroaluminate purity: 99.55%, iron phosphate dihydrate: iron-phosphorus ratio 0.98, lithium carbonate purity: 99.74%. The specific content of each element in the battery-grade product is shown in Table 4 and Table 5.

[0046] Table 4 Content of each element in the lithium carbonate product of Example 1 (wt%)

[0047]

[0048] Table 5 Content of each element in the ferric phosphate dihydrate product of Example 1 (wt%)

[0049]

[0050] Example 2

[0051] A method for preparing battery iron phosphate dihydrate and comprehensive recovery of aluminum and lithium from nitric acid leaching solution of lithium aluminum phosphate ore is implemented by the method of Example 1, except that:

[0052] In step S1, the iron source is ferric hydroxide, and the amount used is at a phosphorus to iron molar ratio of 0.9:1;

[0053] In step S2, the amount of sodium fluoride used is 6 times the molar amount of aluminum in the iron-dissolving mother solution, the reaction temperature is controlled at 65°C, the reaction is kept warm for 5 hours, and the system pH is controlled at 0.5;

[0054] In step S4, the pH of the aluminum-removed liquid is controlled to 0.5 with dilute nitric acid and reacted at 140° C. for 6 hours;

[0055] The aluminum recovery rate obtained in this example is 92.4%, the phosphorus recovery rate is 96.7%, and the lithium recovery rate is 98.4%. Product quality: Sodium hexafluoroaluminate purity: 99.67%, iron phosphate dihydrate: iron-phosphorus ratio 0.97, lithium carbonate purity: 99.76%.

[0056] Example 3

[0057] A method for preparing battery iron phosphate dihydrate and comprehensive recovery of aluminum and lithium from nitric acid leaching solution of lithium aluminum phosphate ore is implemented by the method of Example 1, except that:

[0058] In step S1, the iron source is iron powder, and the amount used is in a phosphorus to iron molar ratio of 1:1;

[0059] In step S2, the amount of sodium fluoride used is 5 times the molar amount of aluminum in the iron-dissolving mother solution, the reaction temperature is controlled at 80°C, the reaction is kept warm for 4 hours, and the system pH is controlled at 1;

[0060] In step S4, the pH of the aluminum-removed solution is adjusted to 1 with dilute nitric acid and reacted at 120° C. for 5 hours;

[0061] The aluminum recovery rate obtained in this example is 93.7%, the phosphorus recovery rate is 97.5%, and the lithium recovery rate is 97.3%. Product quality: Sodium hexafluoroaluminate purity: 99.64%, iron phosphate dihydrate: iron-phosphorus ratio 0.97, lithium carbonate purity: 99.66%.

[0062] Example 4

[0063] A method for preparing battery iron phosphate dihydrate and comprehensive recovery of aluminum and lithium from nitric acid leaching solution of lithium aluminum phosphate ore is implemented by the method of Example 1, except that:

[0064] In step S1, the iron source is iron powder, and the amount used is at a phosphorus to iron molar ratio of 0.8:1;

[0065] In step S2, the amount of sodium fluoride used is 4 times the molar amount of aluminum in the iron-dissolving mother solution, the temperature is controlled at 100°C during the reaction, the reaction is kept warm for 3 hours, and the pH of the system is controlled at 0.5;

[0066] In step S4, the pH of the solution after aluminum removal is adjusted to 0.5 with dilute nitric acid and reacted at 130° C. for 5 hours;

[0067] The aluminum recovery rate obtained in this example is 91.9%, the phosphorus recovery rate is 96.4%, and the lithium recovery rate is 95.3%. Product quality: Sodium hexafluoroaluminate purity: 99.52%, iron phosphate dihydrate: iron-phosphorus ratio 0.98, lithium carbonate purity: 99.62%.

[0068] Example 5

[0069] A method for preparing battery iron phosphate dihydrate and comprehensive recovery of aluminum and lithium from nitric acid leaching solution of lithium aluminum phosphate ore is implemented by the method of Example 1, except that:

[0070] In step S1, the iron source is iron powder, and the amount used is at a phosphorus to iron molar ratio of 1.2:1;

[0071] In step S2, the amount of sodium fluoride used is 8 times the molar amount of aluminum in the iron-dissolving mother solution, the reaction temperature is controlled at 20°C, the reaction is kept warm for 6 hours, and the system pH is controlled at 2;

[0072] In step S4, the pH of the solution after aluminum removal is adjusted to 2 with dilute nitric acid and reacted at 80° C. for 6 hours;

[0073] The aluminum recovery rate obtained in this example is 94.1%, the phosphorus recovery rate is 95.8%, and the lithium recovery rate is 94.9%. Product quality: Sodium hexafluoroaluminate purity: 99.52%, iron phosphate dihydrate: iron-phosphorus ratio 0.97, lithium carbonate purity: 99.58%.

[0074] Note: The main content and impurity content of battery-grade iron phosphate dihydrate and battery-grade lithium carbonate obtained in Examples 2 to 5 all meet the standards.

[0075] Comparative Example 1

[0076] A method for preparing battery iron phosphate dihydrate and comprehensive recovery of aluminum and lithium from nitric acid leaching solution of lithium aluminum phosphate ore is implemented according to the method described in Example 1, except that:

[0077] In step S1, the iron source is iron powder, which is added at a phosphorus to iron molar ratio of 1.5:1.

[0078] The recovery rate of aluminum obtained in this comparative example is 92.4%, the recovery rate of phosphorus is 85.9%, and the recovery rate of lithium is 89.7%. Product quality: purity of sodium hexafluoroaluminate: 99.62%, iron phosphate dihydrate: iron-phosphorus ratio 0.93, purity of lithium carbonate: 99.53%.

[0079] The phosphorus-iron molar ratio of this comparative example is higher than that of Example 1, the iron-phosphorus ratio of ferric phosphate dihydrate is reduced, which does not meet the battery-grade product standards, and the phosphorus yield is reduced, and phosphorus precipitation is incomplete, resulting in increased lithium loss, reduced recovery rate, and reduced economic benefits during subsequent impurity removal and lithium precipitation.

[0080] Comparative Example 2

[0081] A method for preparing battery iron phosphate dihydrate and comprehensive recovery of aluminum and lithium from nitric acid leaching solution of lithium aluminum phosphate ore is implemented according to the method described in Example 1, except that:

[0082] In step S2, the amount of sodium fluoride used is twice the molar amount of aluminum in the iron-dissolving mother solution, the reaction temperature is controlled at 60°C, the reaction is kept warm for 2 hours, and the system pH is controlled at 3;

[0083] The aluminum recovery rate obtained in this comparative example is 79.6%, the phosphorus recovery rate is 91.6%, and the lithium recovery rate is 91.6%. Product quality: Sodium hexafluoroaluminate purity: 75.3%, iron phosphate dihydrate: iron-phosphorus ratio 0.94, lithium carbonate purity: 99.62%.

[0084] In this comparative example, the amount of sodium fluoride used is lower than that in Example 1, and the pH of aluminum precipitation is higher than that in Example 1, which results in iron hydroxide and aluminum hydroxide precipitates being wrapped in the sodium hexafluoroaluminate precipitate, resulting in a reduced product yield and an impure sodium hexafluoroaluminate product; the aluminum content in the liquid after aluminum precipitation is high, resulting in an excessive aluminum impurity content in the dihydrate ferric phosphate product and reduced product purity.

[0085] Comparative Example 3

[0086] A method for preparing battery iron phosphate dihydrate and comprehensive recovery of aluminum and lithium from nitric acid leaching solution of lithium aluminum phosphate ore is implemented according to the method described in Example 1, except that:

[0087] In step S4, the pH of the liquid after aluminum removal is controlled at 3, the temperature for preparing ferric phosphate dihydrate is controlled at 70° C., and the reaction is performed for 5 hours;

[0088] The recovery rate of aluminum obtained in this comparative example is 92.8%, the recovery rate of phosphorus is 84.2%, and the recovery rate of lithium is 89.4%. Product quality: Sodium hexafluoroaluminate purity: 99.62%, iron phosphate dihydrate: iron-phosphorus ratio 0.92, lithium carbonate purity: 99.62%.

[0089] In this comparative example, the temperature for preparing ferric phosphate dihydrate is lower than that in Example 1, and the pH of the system is higher than that in Example 1. The higher the pH, the easier it is for iron ions in the system to produce iron hydroxide precipitation, which reduces the yield of ferric phosphate dihydrate and the iron-phosphorus ratio does not meet the product standard. At this pH, iron hydroxide easily forms a colloid, wraps the lithium ions, and entrains the lithium ions during filtration, resulting in the loss of lithium ions and a reduced lithium recovery rate.

[0090] It can be seen from the above embodiments and comparative examples that the embodiment scheme of the present invention can more efficiently separate phosphorus, aluminum and lithium, obtain better yields of phosphorus, aluminum and lithium, and the three products meet national standards. The comparative examples that are not within the scope of the present invention cannot achieve the significantly improved effect of the present invention.

[0091] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A method for preparing battery-grade iron phosphate dihydrate and comprehensive recovery of aluminum and lithium from nitric acid leaching solution of lithium aluminum phosphate, characterized in that: The following steps are involved: S1: adding an iron source into the nitric acid leaching solution of lithium aluminum phosphate, dissolving and filtering to obtain an iron-dissolving mother liquor and an insoluble slag; S2: adding NaF to the iron-dissolving mother liquor of step S1, filtering to obtain the aluminum-precipitated liquid and crude Na3AlF6; the crude Na3AlF6 is washed to obtain a high-purity product; S3: heating the aluminum precipitation solution obtained in step S2 and filtering it to obtain iron phosphate dihydrate crystals and a lithium-containing solution; S4: chemically removing impurities from the lithium-containing solution obtained in step S3, and then adding sodium carbonate solution to precipitate lithium to obtain battery-grade lithium carbonate; S5: The FePO4·2H2O obtained in step S3 is washed and dried to obtain battery-grade iron phosphate dihydrate.

2. The method according to claim 1, characterized in that In step S1, the chemical composition of the nitric acid leachate of lithium aluminum phosphate includes: Li: 6.5-8 g / L, Na: 0.1-0.3 g / L, Fe: 0.3-0.8 g / L, Al: 25-30 g / L, Mn: 0.05-1 g / L, Ca: 0.01-0.05 g / L, Mg: 0.1-0.15 g / L, Si: 0.01-0.05 g / L, P: 25-30 g / L, K: 0.2-0.5 g / L, H + :3~4mol / L.

3. The method according to claim 1, characterized in that: In step S1, the iron source is selected from at least one of reduced iron powder, iron oxide, and iron hydroxide, and the molar ratio of phosphorus to iron is (0.8~1.2):

1.

4. The method according to claim 1, characterized in that In step S2, the amount of NaF used is 4 to 10 times the molar amount of aluminum ions in the iron-dissolving mother liquor, the pH of the reaction system is 0 to 2, the reaction temperature is 20 to 100° C., and the reaction time is 0.5 to 6 h.

5. The method according to claim 1, characterized in that In step S2, the crude Na3AlF6 is washed by using a dilute nitric acid solution with a pH of 1 at room temperature, controlling the liquid-solid mass ratio to 3:1, stirring and washing twice, and then washing twice with pure water, each washing time being 0.5 h.

6. The method according to claim 1, characterized in that In step S3, the heating temperature is 80-150° C., the reaction time is 2-6 hours, and the pH of the reaction system is 0.5-2.

7. The method according to claim 1, characterized in that In step S4, the specific method of chemical impurity removal is: adding sodium hydroxide to the lithium-containing solution to adjust the system pH to 7 to remove trace iron and aluminum, adjusting the filtrate to pH = 12 after filtering to remove residual calcium and magnesium ions, and then adding a 30% by mass sodium carbonate solution thereto after filtering to prepare battery-grade lithium carbonate.

8. The method according to claim 1, characterized in that: In step S5, the obtained ferric phosphate dihydrate crystals are heated in a water bath at 60° C., and are stirred and washed twice with a 0.1 mol / L phosphoric acid solution, each washing time being 0.5 h, and the liquid-solid mass ratio of the ferric phosphate dihydrate crystals to the phosphoric acid solution being 5:1; after filtration, the crystals are washed twice with pure water at a liquid-solid mass ratio of 5:1, and each washing time is 30 min to obtain refined FePO4·2H2O.

9. The method according to claim 1, characterized in that: In step S5, the refined FePO4·2H2O is dried at 95°C for 4 hours to obtain battery-grade FePO4·2H2O.

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