Method for recovering lithium in lithium iron phosphate battery black powder

Abstract

The invention belongs to the field of lithium battery recycling, and particularly relates to a method for recycling lithium in lithium iron phosphate battery black powder. The method comprises the following steps: sieving the lithium iron phosphate battery black powder with a 200-mesh sieve, oxidizing the powder with oxygen for 20-40 minutes to obtain powder a; dry ball milling: weighing phosphoric acid salt of sodium, the powder a and potassium superphosphate, mixing, and carrying out dry milling for 2-3 hours to obtain powder b; iron removal: dissolving the powder b subjected to dry grinding in water, and filtering to obtain a solid phase a and filtrate a; adding 0.1-0.3 wt.% of alkali into the filtrate a, and filtering to obtain filtrate b; leaching out lithium; adding an aqueous solution of saturated soluble carbonate into the filtrate b obtained in the step S3, heating and concentrating, and filtering while hot; washing and enriching; and washing with hot water, filtering, and collecting a solid phase to complete lithium recovery. According to the method, lithium ions are replaced through potassium ions and sodium ions, mechanical ball milling and the oxidation effect of persulfate are matched, lithium is leached in a synergistic mode, corrosion to equipment is small, and meanwhile the leaching rate of lithium can reach 98% or above.

Description

Technical Field

[0001] The present application belongs to the field of lithium battery recycling, and relates to a method for recycling and treating lithium battery black powder, and specifically to a method for recycling lithium in lithium iron phosphate battery black powder. Background Art

[0002] Lithium-ion batteries are the most widely used secondary batteries at present. They are mainly composed of positive electrodes, negative electrodes, separators, electrolytes and shells. According to the composition of the positive electrode material, lithium-ion batteries can be divided into lithium cobalt oxide batteries, lithium manganese oxide batteries, ternary batteries and LiFePO4 batteries.

[0003] LiCoO2 has a high working voltage, high specific energy, high discharge rate, and simple preparation process. It is still used in many fields, but due to the limitation of cobalt resources and cost, it has poor future development prospects and cannot be widely used; LiMn2O4 has excellent charging characteristics and reversibility, but due to distortion during the discharge process, the discharge specific capacity is not high. Ternary batteries have high energy density, adjustable performance, and good low-temperature performance. They are also widely used in the new energy vehicle market, but ternary materials are prone to phase change and have high safety risks.

[0004] LiFePO4 batteries have high safety and good rate performance. Compared with ternary materials, they do not contain heavy metal elements such as Ni and Co, have lower costs, and have relatively low environmental impacts and resource requirements. Therefore, based on the above advantages, the proportion of LiFePO4 batteries in the power battery field has gradually increased.

[0005] During the charging process of LiFePO4, Li + From LiFePO4, LiFePO4 phase transforms into FePO4 phase, and Li + The FePO4 phase is transformed into the LiFePO4 phase by escaping from the negative electrode. The crystal structures of LiFePO4 and FePO4 are similar, and the battery volume does not change much during the entire charge and discharge process, and the internal stress is small, which makes the LiFePO4 battery have good cycle performance and high safety.

[0006] However, the failure of lithium-ion batteries will lead to shorter battery life and safety issues. The manifestations of battery failure include severe capacity attenuation, increased internal resistance and internal short circuit. The reasons for battery failure include damage to the active material structure, rupture of material particles and lithium deposition in the negative electrode. When the battery failure causes the performance to drop to a certain extent, it will not meet the use requirements and will be scrapped.

[0007] With the rapid development of the new energy industry, the use of lithium-ion power batteries has increased year by year. LiFePO4 batteries contain lithium, copper, aluminum and iron, etc. Failure to recycle them will cause a waste of resources. At the same time, waste batteries also pose certain hazards to the environment and human body. For example, the HF generated by the hydrolysis of LiPF6 in the organic electrolyte has a strong corrosive effect, and phosphorus easily leads to eutrophication of water bodies, causing serious pollution to the environment. Therefore, research on the recycling and reuse of retired LiFePO4 batteries is essential.

[0008] At present, some researchers have recovered the iron and lithium in lithium iron phosphate batteries and achieved positive results: Method 1. Use phosphoric acid or sulfuric acid in combination with hydrogen peroxide as the leaching system, enter and exit at 30-60°C, and use alkali to remove iron, and finally leach out lithium; Method 2. Use hydrochloric acid and hydrogen peroxide oxidation agent system to selectively enter and exit lithium, then separate the lithium precipitation and recover lithium with higher purity; Method 3. Use formic acid and hydrogen peroxide to oxidatively leach waste lithium iron phosphate black powder, and then use sodium phosphate or sodium carbonate to recover lithium; Method 4. Use methanesulfonic acid and p-toluenesulfonic acid to leach lithium iron phosphate black powder.

[0009] The above-mentioned process for recovering lithium from waste lithium iron phosphate has the following problems:

[0010] 1. Method 1 and Method 2 use inorganic acid in combination with peroxide to leach lithium. This solution is currently used on a large scale, but the large-scale use of inorganic acid will cause equipment corrosion. At the same time, harmful gases and acid mist will also be accompanied by the leaching process, increasing the burden of environmental protection treatment for enterprises.

[0011] Second, method 3 uses organic acid instead of traditional inorganic acid, but the amount of oxidants such as hydrogen peroxide is large, and the cost of recycling and treatment increases;

[0012] 3. Method 4 abandons oxidants such as inorganic acids and peroxides and uses sulfonic acid with methyl to leach black powder. It has low cost and less secondary pollution. However, the leaching rate of lithium leached by this method is only about 95%, which is lower than the 97% or even 99% leaching rate in methods 1 to 3.

[0013] In summary, the methods for leaching lithium in the prior art each have their own shortcomings. It is necessary to propose a method for recovering lithium that uses less chemical reagents, has low requirements on equipment, and has a considerable leaching rate. Summary of the invention

[0014] In order to optimize the technical problems in the prior art of leaching lithium from lithium iron phosphate battery black powder, such as the large amount of chemical reagents used, high equipment requirements for acid treatment, and difficulty in balancing green environmental protection and leaching rate, the present application provides a method for recovering lithium from lithium iron phosphate battery black powder, which firstly performs pretreatment to remove impurities and oxidize, then performs dry ball milling, and utilizes ion replacement in the crystal and the synergistic effect of sodium phosphate acid salt and potassium superphosphate to replace the lithium, then adds alkali to remove iron, and concentrates and filters to complete the recovery of lithium, abandons the acid leaching method, uses trace amounts of oxidants and strong acids and alkalis, and can achieve the leaching rate of the acid leaching method.

[0015] The present application provides a method for recovering lithium from black powder of lithium iron phosphate batteries using the following technical solution:

[0016] A method for recovering lithium from black powder of lithium iron phosphate batteries comprises the following steps:

[0017] S1. Pretreatment: The lithium iron phosphate battery black powder is passed through a 200-mesh sieve, and the powder less than or equal to 200 mesh is oxidized by oxygen, and the temperature is controlled to 600-800°C for 20-40 minutes to obtain powder a;

[0018] S2. Dry ball milling: Weigh 10 to 20 parts by weight of sodium phosphate, 10 to 20 parts by weight of powder a, 0.3 to 0.6 parts by weight of potassium superphosphate and mix, control the ball-to-material ratio to 5 to 10:1, and dry mill at a ball milling rate of 200 to 300 r / min for 2 to 3 hours to obtain powder b;

[0019] S3. Iron removal: The dry-milled powder b was dissolved in water and filtered to obtain a solid phase a and a filtrate a; 0.1 to 0.3 wt.% of alkali was added to the filtrate a and filtered to obtain a filtrate b;

[0020] S4. Lithium leaching; adding a saturated aqueous solution of soluble carbonate to the filtrate b obtained in S3, heating and concentrating until the liquid volume is concentrated to 30% to 40% of the filtrate b before heating, and filtering while hot;

[0021] S5. Washing and enrichment: After washing with hot water, filter and collect the solid phase to complete the recovery of lithium.

[0022] In a specific implementation scheme of the present application, in step S1, after the battery black powder is passed through a 200-mesh sieve, gas a is used to oxidize the powder with a mesh size of less than or equal to 200; gas a includes at least 55% N2 by volume and 40-42% O2 by volume, and in gas a, the concentrations of CO2 and CO are both less than 50 ppm.

[0023] In a specific implementation manner of the present application, in step S2, the sodium phosphate acid salt includes any one of sodium monohydrogen phosphate and sodium dihydrogen phosphate or a combination thereof in any proportion.

[0024] In a specific implementation scheme of the present application, in step S3, powder b is dissolved in water at a solid-liquid ratio of 50 to 200 g / L.

[0025] In a specific implementation scheme of the present application, in step S3, the solid phase a is dehydrated to a constant weight under an oxygen atmosphere at a temperature of 200-300° C. to complete the recovery of iron.

[0026] In a specific embodiment of the present application, in step S4, the saturated soluble carbonate includes saturated sodium carbonate.

[0027] In a specific implementation scheme of the present application, in step S5, the filter residue of S4 is washed at least twice with water having a temperature not lower than 90° C., and the solid phase is collected to complete the recovery of lithium.

[0028] The present application includes at least one of the following beneficial technical effects: 1. It abandons the method of using a large amount of strong acids and strong bases in traditional processes, and only uses a small amount of base in the iron removal step. The ability of phosphoric acid salts to ionize hydrogen ions is also weak, and it has little corrosion to equipment and is easy to implement; 2. Through the synergistic effect of potassium ions and sodium ions, assisted by mechanical ball milling, lithium is recovered by the method of ion replacement between crystals, with a high recovery rate and less chemical reagents added. The amount of potassium ions used is even one-tenth of LiFePO4; 3. The present application creates a low pH environment through monohydrogen phosphate or dihydrogen phosphate, and then uses the oxidation effect of persulfate to replace the lithium of LiFePO4 that has not been replaced, and cooperates with subsequent precipitation enrichment to synergistically improve the lithium leaching rate under the premise of using less acid and alkali and less chemical reagents. DETAILED DESCRIPTION

[0029] The present application discloses a method for recovering lithium from lithium iron phosphate black powder.

[0030] In the traditional process of leaching lithium from lithium iron phosphate black powder, there are technical problems such as the use of a large number of chemical reagents and acid leaching corrosion equipment. The emerging process of leaching using organic acid or sulfonic acid is difficult to guarantee the lithium leaching rate. Therefore, the recovery process of lithium iron in lithium iron phosphate black powder is still an area worthy of attention and research and development.

[0031] Lithium iron phosphate black powder is a general term for waste lithium iron phosphate positive electrode waste. Generally, the waste lithium iron phosphate positive electrode is directly broken into pieces. Due to the presence of carbon, it is black in appearance, so it is called black powder. The components of black powder mainly include lithium iron phosphate, scattered aluminum foil and copper, carbon powder, organic glue and diaphragm, etc. Therefore, if you want to leach the lithium therein, you need to pre-treat it. The pre-treatment method provided in this application mainly includes two steps: screening and oxidation. Screening specifically refers to filtering the lithium iron phosphate black powder through a 200-mesh sieve, collecting the sieved material, and screening can remove large particles of impurities such as copper, aluminum and carbon powder. The subsequent oxidation step is intended to remove organic glue, diaphragm, and doped carbon powder. After pre-treatment, the subsequent lithium leaching process can be carried out.

[0032] The steps of leaching lithium mainly include four steps: ball milling, iron removal, leaching lithium and washing enrichment; during the ball milling process, the present application utilizes dry ball milling, assisted by sodium ions, monohydrogen phosphate ions (or dihydrogen phosphate), potassium ions and persulfate ions, controls the ball-to-material ratio and the ball milling rate for dry grinding, aiming to achieve ion exchange in the crystals, replacing lithium ions with sodium ions and potassium ions.

[0033] About the role of sodium salt: LiFePO4 has a typical olivine structure, in which Li + Occupies specific lattice positions. These positions have certain requirements on the size and charge of the ions due to the effect of the crystal field. Na2HPO4 or NaH2PO4 and K2S2O8 are all ionic compounds. + The anions are tightly bound by ionic bonds. When sodium phosphate and LiFePO4 are mechanically milled, the crystal structures of the two are destroyed due to grinding and collision. Mechanical milling also provides energy to enable the ions in the system to break through the energy barrier, thus making the ions active. + ions due to their smaller ionic radius and Li + Similar charges enter the destroyed LiFePO4 crystal + Location, Li + Replaced with the original Na + The bound anions combine to form electrical neutrality.

[0034] About the role of potassium persulfate: ① The role of potassium: In the destruction process of LiFePO4 crystals, Na + Although the ionic radius is similar to that of Li + Similar, but K + The ionic radius of K is larger. + It may first enter the larger gap in the LiFePO4 crystal or destroy the crystal structure, forming Na + The entry of K +First, the LiFePO4 crystal structure is destroyed, and the "large block" crystal structure is divided into "small blocks", and then Na + Enter the "small piece" of LiFePO4 crystal to complete the replacement. Therefore, the synergistic effect of potassium and sodium may improve the replacement efficiency by optimizing the reaction path and rate. + The addition of may reduce the reaction energy barrier, making Na + It is easier to enter the crystal structure. At the same time, the combined effect of the two ions may make the ion exchange more thorough and improve the lithium recovery rate.

[0035] ②The role of persulfate: In combination with monohydrogen phosphate or dihydrogen phosphate, it plays an oxidizing role under acidic conditions, specifically oxidizing LiFePO4 to FePO4, thereby achieving the removal of most of the iron, and precipitating the unreplaced lithium in the form of Li2SO4. The specific chemical equation of the reaction is as follows: 2LiFePO4+K2S2O8=2FePO4+K2SO4+Li2SO4. In combination with subsequent saturated carbonate, lithium sulfate is precipitated in the form of lithium carbonate, which complements the lithium leaching method from another direction and increases the lithium leaching rate.

[0036] Based on the above, the contributions of the present application include at least the following: 1. Abandoning the method of using a large amount of strong acids and strong bases in traditional processes, the present application only uses a small amount of alkali in the iron removal step, and the ability of phosphoric acid salts to ionize hydrogen ions is also weak, with little corrosion to equipment and easy implementation; 2. The present application recovers lithium through the synergistic effect of potassium ions and sodium ions, assisted by mechanical ball milling, through the method of inter-crystal ion replacement, with a high recovery rate and less chemical reagents added, and the amount of potassium ions that play a "crushing" role is even one-tenth of LiFePO4; 3. The present application creates a low pH environment through monohydrogen phosphate or dihydrogen phosphate, and then uses the oxidation effect of persulfate to replace the lithium of LiFePO4 that has not been replaced, and cooperates with subsequent precipitation enrichment to improve the lithium leaching rate under the premise of using less acid and alkali and less chemical reagents.

[0037] The present application will be further described in detail below in conjunction with specific embodiments.

[0038] Example 1

[0039] The present application embodiment discloses a method for recovering lithium from black powder of lithium iron phosphate batteries, which specifically comprises the following steps:

[0040] S1. Pretreatment; after the battery black powder is passed through a 200-mesh sieve, the powder with a mesh size of ≤200 is oxidized using gas a; gas a is a mixed gas obtained by mixing a nitrogen cylinder and an oxygen cylinder; according to GC analysis, gas a includes 56.4% by volume of N2 and 40% by volume of O2, and in gas a, the CO2 concentration is 32ppm, the CO concentration is 14ppm, and the temperature is controlled at 600°C for 40 minutes to obtain powder a.

[0041] S2. Dry ball milling: weigh 10 g of sodium monohydrogen phosphate, 10 g of powder a, and 0.3 g of potassium superphosphate and mix them. Control the ball-to-powder ratio to 5:1. Dry mill at a ball milling rate of 300 r / min for 2 h to obtain powder b.

[0042] S3. Iron removal: dissolve powder b in water at a solid-liquid ratio of 50 g / L and filter to obtain solid phase a and filtrate a; add 0.1 wt.% of alkali from filtrate a to filtrate a, and filter to obtain filtrate b; dehydrate solid phase a to constant weight at a controlled temperature of 200°C under an oxygen atmosphere to complete iron recovery.

[0043] S4. Lithium leaching; add saturated sodium carbonate aqueous solution to the filtrate b obtained in S3 at a volume ratio of 1:1.2, heat and concentrate until the liquid volume is concentrated to 30% of the filtrate b before heating, and filter while hot.

[0044] S5. Washing and enrichment: Wash the filter residue of S4 twice with 90°C water, collect the solid phase, and complete the recovery of lithium. The calculated lithium yield is 97.4%.

[0045] Example 2

[0046] The present application embodiment discloses a method for recovering lithium from black powder of lithium iron phosphate batteries, which specifically comprises the following steps:

[0047] S1. Pretreatment; after the battery black powder is passed through a 200-mesh sieve, the powder with a mesh size of ≤200 is oxidized using gas a; gas a is a mixed gas obtained by mixing a nitrogen cylinder and an oxygen cylinder; GC analysis shows that gas a includes 58.1% by volume of N2 and 41.3% by volume of O2, and in gas a, the CO2 concentration is 34 ppm, and no CO is detected. The temperature is controlled at 700°C for 30 minutes to obtain powder a.

[0048] S2. Dry ball milling: weigh 15 g of a mixture of sodium monohydrogen phosphate and sodium dihydrogen phosphate in a weight ratio of 1:1, take 14 g of powder a, and 0.45 g of potassium superphosphate and mix them. Control the ball-to-material ratio to 8:1, and dry mill at a ball milling rate of 240 r / min for 2.5 hours to obtain powder b.

[0049] S3. Iron removal: Powder b is dissolved in water at a solid-liquid ratio of 160 g / L and filtered to obtain solid phase a and filtrate a; 0.2 wt.% of alkali is added to filtrate a, and filtrate b is obtained by filtering; solid phase a is dehydrated to constant weight under an oxygen atmosphere at a controlled temperature of 250°C to complete iron recovery.

[0050] S4. Lithium leaching; add saturated sodium carbonate aqueous solution to the filtrate b obtained in S3 at a volume ratio of 1:1.2, heat and concentrate until the liquid volume is concentrated to 35% of the filtrate b before heating, and filter while hot.

[0051] S5. Washing and enrichment: Wash the filter residue of S4 twice with 90°C water, collect the solid phase, and complete the recovery of lithium. The calculated lithium yield is 98.1%.

[0052] Example 3

[0053] The present application embodiment discloses a method for recovering lithium from black powder of lithium iron phosphate batteries, which specifically comprises the following steps:

[0054] S1. Pretreatment; after the battery black powder is passed through a 200-mesh sieve, the powder with a mesh size of ≤200 is oxidized using gas a; gas a is a mixed gas obtained by mixing a nitrogen cylinder and an oxygen cylinder; according to GC analysis, gas a includes 57.2% by volume of N2 and 42% by volume of O2, and in gas a, the CO2 concentration is 51 ppm, and no C0 is detected. The temperature is controlled at 800°C for 20 minutes to obtain powder a.

[0055] S2. Dry ball milling: weigh 20 g of sodium dihydrogen phosphate, 20 g of powder a, and 0.6 g of potassium superphosphate and mix them. The ball-to-powder ratio is controlled to be 10:1. The ball milling speed is 200 r / min and dry milling is performed for 3 hours to obtain powder b.

[0056] S3. Iron removal: dissolve powder b in water at a solid-liquid ratio of 200 g / L and filter to obtain solid phase a and filtrate a; add 0.3 wt.% of alkali to filtrate a and filter to obtain filtrate b; dehydrate solid phase a to constant weight at a controlled temperature of 300°C under an oxygen atmosphere to complete iron recovery.

[0057] S4. Lithium leaching; add saturated sodium carbonate aqueous solution to the filtrate b obtained in S3 at a volume ratio of 1:1.2, heat and concentrate until the liquid volume is concentrated to 40% of the filtrate b before heating, and filter while hot.

[0058] S5. Washing and enrichment: Wash the filter residue of S4 twice with 90°C water, collect the solid phase, and complete the recovery of lithium. The calculated lithium yield is 96.8%.

[0059] Comparative Example 1

[0060] This comparative example is compared with Example 1, except that the potassium persulfate in step S2 of Example 1 is replaced by ammonium persulfate, and the calculated lithium yield is 92.5%.

[0061] Comparative Example 2

[0062] This comparative example is compared with Example 1, except that potassium persulfate in step S2 of Example 1 is replaced by potassium chloride, and the calculated lithium yield is 94.2%.

[0063] Comparative Example 3

[0064] This comparative example is compared with Example 1, except that the sodium monohydrogen phosphate in step S2 of Example 1 is replaced by sodium chloride, and the calculated lithium yield is 94.0%.

[0065] Comparative Example 4

[0066] This comparative example is compared with Example 1, the difference is that potassium persulfate is missing in step S2, and the calculated lithium yield is 90.2%.

[0067] Results analysis 1-4:

[0068] 1. Combining Comparative Example 1 and Example 1, it can be seen that after the potassium persulfate in Example 1 is replaced with ammonium persulfate, the lithium leaching rate in Comparative Example 1 is significantly reduced. This is because the role of potassium ions is to cooperate with sodium ions to promote the replacement of lithium ions by sodium ions, while ammonium ions cannot play the above role. Therefore, under the process steps of Example 1, the crushing effect of potassium ions is lacking. If only the replacement effect of sodium ions is to be relied on, the ball milling should be longer, or a more appropriate ball-to-material ratio should be used. Therefore, the sodium in Comparative Document 1 is not fully replaced, and Comparative Example 1 does not change the other process parameters and steps of Example 1. Therefore, even if there is an oxidizing effect of persulfate to supplement the lithium leaching rate, the lithium leaching rate is still lower than that of Example 1 as a whole.

[0069] 2. Combining Comparative Example 2 and Example 1, it can be seen that after the potassium persulfate in Example 1 is replaced with potassium chloride in Comparative Example 2, the lithium leaching rate is significantly reduced. This is because the persulfate ions can oxidize lithium iron phosphate, thereby supplementing the lithium leaching rate. The lack of persulfate ions cannot oxidize and precipitate the unreplaced lithium, resulting in a waste of lithium.

[0070] 3. Combining Comparative Example 3 and Example 1, it can be seen that in Comparative Example 3, the sodium monohydrogen phosphate in Example 1 is replaced with sodium chloride. Even if the presence of sodium ions is retained, the oxidizability of persulfate ions decreases due to the slightly higher pH, thereby causing a decrease in the leaching rate of lithium.

[0071] Combining the results of analysis 1 and 2, as well as comparative example 4 and embodiment 1, it can be seen that after the lack of potassium persulfate, the lithium leaching rate of comparative example 4 is the worst among the four comparative examples, because comparative example 4 lacks both the crystal-destroying effect of potassium ions and the oxidation and collection effect of persulfate ions.

[0072] In summary, the present invention replaces lithium ions with potassium ions and sodium ions, and cooperates with mechanical ball milling and persulfate oxidation to leach lithium. It uses less chemical reagents, less acid and alkali, less corrosion to equipment, and the lithium leaching rate can reach more than 98%.

[0073] The above description is only a preferred specific implementation manner of the present invention; however, the protection scope of the present invention is not limited thereto; any technician familiar with the technical field within the technical scope disclosed by the present invention; any equivalent replacement or change based on the technical solution and improved concept of the present invention shall be covered by the protection scope of the present invention.

Claims

1. A method for recovering lithium from black powder of lithium iron phosphate batteries, characterized in that: The steps include: S1. Pretreatment: The lithium iron phosphate battery black powder is passed through a 200-mesh sieve, and the powder less than or equal to 200 mesh is oxidized by oxygen, and the temperature is controlled to 600-800°C for 20-40 minutes to obtain powder a; S2. Dry ball milling: Weigh 10 to 20 parts by weight of sodium phosphate, 10 to 20 parts by weight of powder a, 0.3 to 0.6 parts by weight of potassium superphosphate and mix, control the ball-to-material ratio to 5 to 10:1, and dry mill at a ball milling rate of 200 to 300 r / min for 2 to 3 hours to obtain powder b; S3. Iron removal: The dry-milled powder b was dissolved in water and filtered to obtain a solid phase a and a filtrate a; 0.1 to 0.3 wt.% of alkali was added to the filtrate a and filtered to obtain a filtrate b; S4. Lithium leaching; adding a saturated aqueous solution of soluble carbonate to the filtrate b obtained in S3, heating and concentrating until the liquid volume is concentrated to 30% to 40% of the filtrate b before heating, and filtering while hot; S5. Washing and enrichment; After washing with hot water, the solid phase is collected by filtration to complete the recovery of lithium.

2. The method for recovering lithium from black powder of lithium iron phosphate battery according to claim 1, characterized in that: In step S1, after the battery black powder is passed through a 200-mesh sieve, gas a is used to oxidize the powder with a mesh size of less than or equal to 200; gas a includes N2 with a volume percentage of at least 55% and O2 with a volume percentage of 40-42%, and in gas a, the concentrations of CO2 and CO are both less than 50 ppm.

3. The method for recovering lithium from black powder of lithium iron phosphate battery according to claim 1, characterized in that: In step S2, the sodium phosphate acid salt includes any one of sodium monohydrogen phosphate and sodium dihydrogen phosphate or a combination thereof in any proportion.

4. The method for recovering lithium from black powder of lithium iron phosphate battery according to claim 1, characterized in that: In step S3, powder b is dissolved in water at a solid-liquid ratio of 50 to 200 g / L.

5. The method for recovering lithium from black powder of lithium iron phosphate battery according to claim 1, characterized in that: In step S3, the solid phase a is dehydrated to a constant weight under an oxygen atmosphere at a controlled temperature of 200-300° C. to complete the recovery of iron.

6. The method for recovering lithium from black powder of lithium iron phosphate battery according to claim 1, characterized in that: In step S4, the saturated soluble carbonate includes saturated sodium carbonate.

7. The method for recovering lithium from black powder of lithium iron phosphate battery according to claim 1, characterized in that: In step S5, the filter residue of S4 is washed at least twice with water having a temperature not lower than 90° C., and the solid phase is collected to complete the recovery of lithium.