Method for recovering lithium in lithium iron phosphate battery black powder

Abstract

The application belongs to the field of lithium battery recycling, and particularly relates to a lithium recovery method from lithium iron phosphate battery black powder, comprising the following steps: pretreatment; screening the lithium iron phosphate battery black powder through a 200-mesh sieve, oxidizing the powder with oxygen for 20-40 min to obtain powder a; dry ball milling: weighing sodium phosphate acid salt, powder a and potassium superphosphate, mixing and dry milling for 2-3 h to obtain powder b; iron removal: dissolving the dry-milled powder b in water and filtering to obtain solid phase a and filtrate a; adding 0.1-0.3 wt.% of alkali to the filtrate a, filtering to obtain filtrate b; lithium leaching; adding a saturated water-soluble carbonate solution to the filtrate b obtained in S3, heating and concentrating, and filtering while hot; washing and enriching; filtering after hot water washing, collecting the solid phase, and completing lithium recovery. The application replaces lithium ions with potassium ions and sodium ions, cooperates with mechanical ball milling and the oxidation effect of persulfate, cooperates to leach lithium, has small equipment corrosion, and the lithium leaching rate can reach more than 98%.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of lithium battery recycling, and relates to a lithium battery black powder recycling method, in particular to a lithium iron phosphate battery black powder lithium recycling method. BACKGROUND

[0002] Lithium ion batteries are the most widely used secondary batteries at present, which 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 cobaltate batteries, lithium manganate batteries, ternary batteries and LiFePO4 batteries.

[0003] LiCoO2 has a high working voltage, a high specific energy, a large discharge rate, and a simple preparation process, and is still used in many fields at present, but due to the limitation of cobalt resources and cost, the future development prospect is not good, and it cannot be widely applied; LiMn2O4 has excellent charging characteristics and reversibility, but distortion occurs during discharge, resulting in a low discharge specific capacity. The energy density of ternary batteries is high, the performance is adjustable, and the low-temperature performance is good, and they are also widely used in new energy vehicle markets, but the ternary material is prone to phase change, and the safety hazard is high.

[0004] LiFePO4 batteries have high safety and good rate performance, do not contain heavy metal elements such as Ni and Co compared with ternary materials, have low cost, and have low environmental impact and resource requirements, therefore, based on the above advantages, the proportion of LiFePO4 batteries in the power battery field is gradually increasing.

[0005] During the charging process of LiFePO4, Li + is released from LiFePO4, and LiFePO4 phase is converted into FePO4 phase, and during discharge, Li + is released from the negative electrode, and FePO4 phase is converted into LiFePO4 phase. LiFePO4 and FePO4 have similar crystal structures, and the volume of the battery changes little during the entire charging and discharging process, and the internal stress is small, so that the LiFePO4 battery has good cycle performance and high safety.

[0006] However, lithium ion battery failure can cause battery life to be shortened and safety problems. The manifestations of battery failure include serious capacity attenuation, increased internal resistance, and internal short circuit. The causes of battery failure include active material structure damage, material particle breakage, and negative electrode lithium precipitation. When the performance of the battery is reduced to a certain extent due to failure, it will not meet the use requirements and be scrapped.

[0007] With the rapid development of new energy industry, the use of lithium ion power battery is increasing year by year. LiFePO4 battery contains lithium, copper, aluminum and iron, and recycling will cause resource waste. At the same time, waste batteries also have certain harm to the environment and human body. For example, the HF generated by the hydrolysis of LiPF6 in organic electrolyte has strong corrosion effect, and phosphorus element is easy to cause eutrophication of water body, causing serious pollution to the environment. Therefore, it is necessary to study the recycling of retired LiFePO4 battery.

[0008] At present, some researchers have recovered iron and lithium in lithium iron phosphate battery, and achieved positive results: method 1. Using phosphoric acid or sulfuric acid combined with hydrogen peroxide as leaching system, 30-60℃ conditions, and using alkali to remove iron, finally leaching lithium; method 2. Using hydrochloric acid and hydrogen peroxide oxidant system to selectively leach lithium, and then precipitating and separating lithium, and recovering high-purity lithium; method 3. Using formic acid and hydrogen peroxide to oxidize and leach waste lithium iron phosphate black powder, and then using sodium phosphate or sodium carbonate to recover lithium; method 4. Using methyl sulfonic acid and p-toluene sulfonic acid to leach lithium iron phosphate black powder.

[0009] The above-mentioned lithium recovery process in waste lithium iron phosphate has the following problems:

[0010] 1. Method 1 and method 2 use inorganic acid combined with peroxide to leach lithium. This scheme is currently applied on a large scale, but the use of a large amount of inorganic acid will cause equipment corrosion, and harmful gas and acid mist will also be generated during the leaching process, increasing the burden of enterprise environmental protection treatment.

[0011] 2. Method 3 uses organic acid instead of traditional inorganic acid, but the amount of oxidizing agent such as hydrogen peroxide is large, and the cost of recovery treatment increases;

[0012] 3. Method 4 discards inorganic acid and peroxide oxidant, and uses sulfonic acid with methyl to leach black powder, which 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 leaching rate of more than 97% in methods 1-3, even up to 99%.

[0013] In summary, the existing lithium leaching methods have their own shortcomings, and it is necessary to propose a method for recycling lithium with less chemical reagent consumption, low equipment requirement and high leaching rate. SUMMARY

[0014] In order to optimize the technical problems of the prior art, such as the large amount of chemical reagents used for leaching lithium from lithium iron phosphate battery black powder, high requirements for equipment in 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 first removes impurities and oxidizes, then dry ball mills, uses ion replacement in crystals and the synergistic effect of sodium phosphate acid salt and potassium superphosphate to replace lithium, then adds alkali to remove iron, and concentrates and filters to complete the recovery of lithium, discarding the acid leaching method, and the amount of oxidizing agent, strong acid and strong alkali is trace amount, and the leaching rate of the acid leaching method can be reached.

[0015] The present application provides a method for recovering lithium from lithium iron phosphate battery black powder, which adopts the following technical scheme:

[0016] A method for recovering lithium from lithium iron phosphate battery black powder, comprising the following steps:

[0017] S1. Pretreatment; pass the lithium iron phosphate battery black powder through a 200 mesh sieve, oxidize the powder less than or equal to 200 mesh with oxygen, control the temperature to be 600-800℃ for 20-40min, and obtain powder a;

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

[0019] S3. Iron removal: dissolve the dry milled powder b in water and filter to obtain solid phase a and filtrate a; add 0.1-0.3wt.% of alkali to the filtrate a, filter to obtain filtrate b;

[0020] S4. Leaching of lithium; add a saturated water-soluble carbonate solution to the filtrate b obtained in S3, heat and concentrate until the liquid volume is concentrated to 30-40% of that before heating, and filter while hot;

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

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

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

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

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

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

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

[0028] The present application has at least one of the following beneficial technical effects: 1. The method of the present application discards the method of using a large amount of strong acid and strong base in the traditional process, and only a small amount of alkali is used in the step of removing iron. The ionization ability of hydrogen ions of phosphoric acid salt is weak, the corrosion to the equipment is small, and the method is easy to implement; 2. Through the synergistic effect of potassium ions and sodium ions, supplemented by mechanical ball milling, lithium is recovered by the method of ion replacement between crystals, the recovery rate is high, and the addition of chemical reagents is small. The amount of potassium ions is only one tenth of LiFePO4; 3. The present application creates a low pH environment by monohydrogen phosphate or dihydrogen phosphate, and then uses the oxidation effect of persulfate to displace the lithium of LiFePO4 that has not been replaced. Cooperate with subsequent precipitation enrichment, synergistically, under the premise of using less acid and alkali and less chemical reagents, the leaching rate of lithium is improved. 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 amount of chemical reagents, acid leaching corrosion of equipment, etc. The newly emerging process of using organic acid or sulfonic acid leaching is difficult to guarantee the leaching rate of lithium. Therefore, the recovery process of iron and lithium in lithium iron phosphate black powder is still a field worthy of attention and research and development.

[0031] Lithium iron phosphate black powder is a general term for waste lithium iron phosphate cathode materials. Generally, the waste lithium iron phosphate cathode is simply crushed, and due to the presence of carbon, it appears black, hence the name "black powder." The main components of black powder include lithium iron phosphate, loose aluminum foil and copper, carbon powder, organic adhesives, and separators. Therefore, to leach lithium from it, pretreatment is required. The pretreatment method provided in this application mainly includes two steps: sieving and oxidation. Sieving specifically refers to filtering the lithium iron phosphate black powder through a 200-mesh sieve and collecting the sieved material. Sieving removes large impurities such as copper, aluminum, and carbon powder. The subsequent oxidation step aims to remove organic adhesives, separators, and doped carbon powder. After pretreatment, the subsequent lithium leaching process can proceed.

[0032] The process of leaching lithium mainly includes four steps: ball milling, iron removal, lithium leaching, and washing and enrichment. In the ball milling process, this application uses dry ball milling, supplemented by sodium ions, monohydrogen phosphate ions (or dihydrogen phosphate ions), potassium ions and persulfate ions, and controls the ball-to-material ratio and ball milling rate to carry out dry grinding, aiming to achieve ion exchange in the crystal and replace lithium ions with sodium ions and potassium ions.

[0033] Regarding the role of sodium salts: LiFePO4 has a typical olivine-type structure, in which Li + They occupy specific lattice positions. These positions, due to the crystal field, have certain requirements regarding the size and charge of the ions. Na₂HPO₄ or NaH₂PO₄ and K₂S₂O₈ are all ionic compounds, and in their crystal structures, Na₂HPO₄ or NaH₂PO₄ and K₂S₂O₈ are ionic compounds. + Sodium is tightly bound to anions through ionic bonds; when sodium phosphate salts are mechanically ball-milled with LiFePO4, the crystal structures of both are disrupted due to grinding and collisions. Mechanical ball milling also provides energy, allowing the ions in the system to overcome energy barriers, thus making the ions more active. + Due to its small ionic radius and its similarity to Li + Similar charges enter the LiFePO4 crystal and destroy the Li + Location, will Li + Displaced and combined with the original Na + The bound anions combine to form an electrically neutral substance.

[0034] Regarding the role of potassium persulfate: ① The role of potassium: In the process of breaking down LiFePO4 crystals, Na... + Although the ionic radius is similar to that of Li + Similar, but K + The ionic radius is larger. In the synergistic effect, K + It may first enter the larger interstitial spaces of the LiFePO4 crystal or disrupt the crystal structure, becoming Na. + The entry of K creates more opportunities, namely K +First, the crystal structure of LiFePO4 is destroyed, and the "large block" crystal structure is divided into "small blocks". Then, Na + enters the "small block" LiFePO4 crystal and completes 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 K + may lower the reaction energy barrier, making it easier for Na

[0035] ② The role of persulfate: complexing with monohydrogen phosphate or dihydrogen phosphate, it plays an oxidation role under acidic conditions, specifically oxidizing LiFePO4 into FePO4, thereby removing most of the iron, and precipitating the un-replaced lithium as Li2SO4. The specific chemical equation is as follows: 2LiFePO4 + K2S2O8 = 2FePO4 + K2SO4 + Li2SO4. In combination with the subsequent saturated carbonate, lithium sulfate is precipitated as lithium carbonate, which complements the leaching of lithium from another direction and increases the leaching rate of lithium.

[0036] Based on the above, the contributions of the present application at least include the following: 1. Abandoning the method of using a large amount of strong acid and strong base in the traditional process, the present application only uses a small amount of alkali in the iron removal step, the ionization ability of hydrogen ions of phosphoric acid salt is weak, the corrosion to equipment is small, and it is easy to implement; 2. The present application recovers lithium through the synergistic effect of potassium ions and sodium ions, supplemented by mechanical ball milling, through the method of ion replacement between crystals, the recovery rate is high, and the amount of chemical reagents added is small. The amount of potassium ions playing the role of "crushing" is only one tenth of LiFePO4; 3. The present application creates a low pH environment by using monohydrogen phosphate or dihydrogen phosphate, and then uses the oxidation effect of persulfate to replace the lithium in the un-replaced LiFePO4, combined with subsequent precipitation enrichment, synergistically, under the premise of using less acid and alkali and less chemical reagents, the leaching rate of lithium is improved.

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

[0038] Example 1

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

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

[0041] S2. Dry ball milling: 10 g of sodium phosphate monohydrate, 10 g of powder a, and 0.3 g of superphosphate are weighed and mixed, the ball-to-material ratio is controlled at 5:1, the ball milling speed is 300 r / min, and dry milling is performed for 2 h to obtain powder b.

[0042] S3. Iron removal: the powder b is dissolved in water according to a solid-liquid ratio of 50 g / L, and then filtered to obtain a solid phase a and a filtrate a; 0.1 wt.% of base is added to the filtrate a, and then filtered to obtain a filtrate b; the solid phase a is dehydrated to a constant weight under an oxygen atmosphere at a temperature of 200°C to complete the recovery of iron.

[0043] S4. Lithium leaching: saturated sodium carbonate aqueous solution is added to the filtrate b obtained in S3 according to a volume ratio of 1:1.2, heated and concentrated until the liquid volume is concentrated to 30% of that before heating, and then filtered while hot.

[0044] S5. Washing and enrichment: the filter residue of S4 is washed twice with water at 90°C, and the solid phase is collected to complete the recovery of lithium, and the lithium yield is calculated to be 97.4%.

[0045] Example 2

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

[0047] S1. Pretreatment; after the battery black powder is sieved through a 200 mesh sieve, the sieved powder ≤200 mesh is oxidized using gas a; gas a is a mixed gas obtained by mixing a nitrogen gas cylinder and an oxygen gas cylinder; GC analysis shows that gas a includes 56.4% N2 by volume, 40% O2 by volume, and in gas a, the CO2 concentration is 32 ppm, the CO concentration is 14 ppm, the temperature is controlled at 600°C for 40 min, and powder a is obtained.

[0048] S2. Dry ball milling: 10 g of sodium phosphate monohydrate, 10 g of powder a, and 0.3 g of superphosphate are weighed and mixed, the ball-to-material ratio is controlled at 5:1, the ball milling speed is 300 r / min, and dry milling is performed for 2 h to obtain powder b.

[0049] S3. Iron removal: the powder b is dissolved in water according to the solid-liquid ratio of 160 g / L, and then filtered to obtain a solid phase a and a filtrate a; 0.2 wt.% of alkali of the filtrate a is added into the filtrate a, and then filtered to obtain a filtrate b; the solid phase a is dehydrated to constant weight under the oxygen atmosphere and at the temperature of 250 ℃, and the iron recovery is completed.

[0050] S4. Lithium leaching: the saturated sodium carbonate aqueous solution is added into the filtrate b obtained in S3 according to the volume ratio of 1:1.2, and then heated and concentrated until the liquid volume is concentrated to 35% of that before heating, and then filtered while hot.

[0051] S5. Washing and enrichment: the filter residue of S4 is washed twice with water at 90 ℃, and then the solid phase is collected to complete the lithium recovery, and the lithium yield is calculated to be 98.1%.

[0052] Example 3

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

[0054] S1. Pretreatment: after the battery black powder is sieved through a 200-mesh sieve, the sieved powder ≤200 mesh is oxidized by using gas a; the gas a is a mixed gas obtained by mixing a nitrogen gas cylinder and an oxygen gas cylinder; according to GC analysis, the gas a comprises 57.2% of N2 by volume, 42% of O2 by volume, and the CO2 concentration in the gas a is 51 ppm, and no CO is detected; the temperature is controlled at 800 ℃ for 20 min to obtain powder a.

[0055] S2. Dry ball milling: 20 g of sodium dihydrogen phosphate, 20 g of powder a and 0.6 g of superphosphate are weighed and mixed, the ball-to-material ratio is controlled to be 10:1, the ball milling speed is 200 r / min, and dry milling is performed for 3 h to obtain powder b.

[0056] S3. Iron removal: the powder b is dissolved in water according to the solid-liquid ratio of 200 g / L, and then filtered to obtain a solid phase a and a filtrate a; 0.3 wt.% of alkali of the filtrate a is added into the filtrate a, and then filtered to obtain a filtrate b; the solid phase a is dehydrated to constant weight under the oxygen atmosphere and at the temperature of 300 ℃, and the iron recovery is completed.

[0057] S4. Lithium leaching: the saturated sodium carbonate aqueous solution is added into the filtrate b obtained in S3 according to the volume ratio of 1:1.2, and then heated and concentrated until the liquid volume is concentrated to 40% of that before heating, and then filtered while hot.

[0058] S5. Washing and enrichment: the filter residue of S4 is washed twice with water at 90 ℃, and then the solid phase is collected to complete the lithium recovery, and the lithium yield is calculated to be 96.8%.

[0059] Comparative Example 1

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

[0061] Comparative Example 2

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

[0063] Comparative Example 3

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

[0065] Comparative Example 4

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

[0067] Results Analysis I-IV:

[0068] I. According to Comparative Example 1 and Example 1, the leaching rate of lithium in Comparative Example 1 is significantly lower after replacing the potassium persulfate in Example 1 with ammonium persulfate. This is because potassium ions work together with sodium ions to promote the displacement of lithium ions, while ammonium ions cannot perform the above function. Therefore, under the process steps of Example 1, the crushing effect of potassium ions is missing, and if only the displacement effect of sodium ions is relied on, longer ball milling time or more suitable ball-to-material ratio should be used. Therefore, the sodium in Comparative Example 1 is not fully displaced, and the process parameters and steps of Example 1 are not changed in Comparative Example 1. Therefore, even if the oxidation effect of persulfate is supplemented to increase the leaching rate of lithium, the overall leaching rate of lithium is still lower than that of Example 1.

[0069] II. According to Comparative Example 2 and Example 1, the leaching rate of lithium in Comparative Example 2 is significantly lower after replacing the potassium persulfate in Example 1 with potassium chloride. This is because persulfate ions can oxidize lithium iron phosphate, thereby supplementing the leaching rate of lithium. The absence of persulfate ions cannot oxidize and precipitate the lithium that has not been displaced, resulting in waste of lithium.

[0070] III. According to Comparative Example 3 and Example 1, the sodium phosphate monohydrate in Comparative Example 3 is replaced by sodium chloride, even though sodium ions are still present, the oxidation of persulfate is reduced due to the slightly higher pH, resulting in a decrease in the leaching rate of lithium.

[0071] From the results of analysis of the results, one, two, and comparative example 4 and example 1, it can be seen that the lithium leaching rate of comparative example 4 is the worst among the four comparative examples, because comparative example 4 lacks the effects of potassium ions on destroying the crystal and the effect of persulfate ions on supplementing the collection of lithium by oxidation.

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

[0073] The above merely provides the preferred but non-limiting embodiments of the present application; the protection scope of the present application is not limited to this; any person skilled in the art, within the technical scope disclosed by the present application, according to the technical solutions and the improved concept of the present application, makes equivalent replacement or changes, should be covered in the protection scope of the present application.

Claims

1. A method for recovering lithium from lithium iron phosphate battery soot, characterized by, Comprise the following steps: 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 using oxygen, the temperature is controlled at 600-800℃ for 20-40min, and powder a is obtained; S2. Dry ball milling: 10-20 parts by weight of sodium phosphate acid salt, 10-20 parts by weight of powder a, and 0.3-0.6 parts by weight of superphosphate are weighed and mixed, the ball-to-material ratio is controlled at 5-10:1, the ball milling speed is 200-300r / min, and dry milling is carried out for 2-3h to obtain powder b; S3. Iron removal: after dry milling, the powder b is dissolved in water and filtered to obtain solid phase a and filtrate a; 0.1-0.3wt.% of base is added to the filtrate a, and the filtrate b is obtained by filtration; S4. Lithium leaching: a saturated soluble carbonate aqueous solution is added to the filtrate b obtained in S3, heated and concentrated until the liquid volume is concentrated to 30-40% of that before heating, and filtered while hot; S5. Washing and enrichment: after hot water washing, filtration is carried out, and the solid phase is collected to complete the recovery of lithium.

2. The method according to claim 1, wherein the method is characterized by: In step S1, after the battery black powder is passed through a 200 mesh sieve, the powder less than or equal to 200 mesh is oxidized using gas a; gas a includes N2 with a volume fraction of at least 55%, O2 with a volume fraction of 40-42%, and the concentrations of CO2 and CO in gas a are both less than 50ppm.

3. The method according to claim 1, wherein the method is characterized by: In step S2, the sodium phosphate acid salt includes any one or a combination of sodium monohydrogen phosphate and sodium dihydrogen phosphate in any proportion.

4. The method of claim 1, wherein the lithium recovery method is characterized by: In step S3, the powder b is dissolved in water according to a solid-liquid ratio of 50-200g / L.

5. The method of claim 1, wherein the lithium recovery method is characterized by: In step S3, the solid phase a is dehydrated to a constant weight under an oxygen atmosphere at a temperature of 200-300℃ to complete the recovery of iron.

6. The method of claim 1, wherein the lithium recovery method is characterized by: In step S4, the saturated soluble carbonate includes saturated sodium carbonate.

7. The method of claim 1, wherein the lithium recovery method is characterized by: In step S5, the filter residue of S4 is washed at least twice with water at a temperature not lower than 90℃, and the solid phase is collected to complete the recovery of lithium.