A method for efficiently separating and recovering lithium from waste lithium iron phosphate

Abstract

The invention discloses a method for efficiently separating and recovering lithium elements from waste lithium iron phosphate, and belongs to the technical field of battery recovery. The invention first obtains waste lithium iron phosphate powder from waste lithium iron phosphate batteries, and then accurately calculates the optimal pH value of the reaction system at the real-time temperature according to the leaching reaction temperature and related thermodynamic data, and introduces a low-concentration phosphate solution into the leaching reaction and adjusts the pH value of the system to the optimum, thereby ensuring that most of the iron ions can be effectively precipitated in the form of iron phosphate during the reaction process, thereby achieving efficient separation of lithium and iron elements, and finally adjusting the pH value of the filtrate and adding a saturated phosphate solution to recover high-purity lithium phosphate. The invention can accurately calculate the optimal pH value corresponding to different reaction temperatures according to the reaction temperature and related thermodynamic data, thereby saving a large amount of early exploration experimental cost and time, improving the overall production efficiency, and enhancing the universality of the process.

Description

Technical Field

[0001] The invention belongs to the technical field of batteries, and in particular relates to a method for efficiently separating and recovering lithium elements from waste lithium iron phosphate. Background Art

[0002] In recent years, lithium iron phosphate batteries have been widely used due to their long service life, high safety, good thermal stability and cycle performance, and less environmental pollution. Especially in the field of power batteries and energy storage batteries, their market share and output have continued to grow; however, the service life of lithium iron phosphate batteries is usually 5 to 8 years. Although lithium iron phosphate batteries with reduced capacity can continue to play a role through cascade utilization, they still need to be recycled and regenerated in the end to achieve a closed loop of the lithium-ion battery life cycle; therefore, it is of great significance to develop technologies for efficient recycling and resource regeneration of lithium iron phosphate batteries.

[0003] At present, in the field of recycling of waste lithium iron phosphate batteries, the selective leaching method has been widely studied and applied due to its advantages of low cost, high leaching efficiency, low energy consumption, low environmental impact and high product purity. This method oxidizes the divalent iron in the lithium iron phosphate into trivalent iron through the synergistic effect of acid / oxidant, so that the iron ions and phosphate ions in the lithium iron phosphate lattice reach a charge balance, thereby prompting the lithium ions to be released from the lattice and enter the solution, thereby obtaining a lithium-containing filtrate and a filter residue containing iron and phosphorus elements. Subsequently, the filtrate and the filter residue are treated separately to achieve the recycling of all elements. However, in the process of lithium leaching, a certain degree of iron leaching will inevitably occur. A higher iron leaching rate will increase the difficulty of removing impurities from the lithium-containing filtrate, which will lead to a decrease in the purity of the recovered lithium salt.

[0004] Accurately and efficiently extracting lithium resources while strictly controlling the leaching of iron elements are key issues in the selective recovery of waste lithium iron phosphate. At present, the existing technology mainly conducts large-scale, repetitive exploratory experiments to explore the optimal leaching conditions of lithium iron phosphate positive electrode materials, striving to maximize the lithium leaching rate and minimize the iron leaching rate. However, the optimal leaching conditions determined by a large number of exploratory experiments in the existing technology often require maintaining a higher reaction temperature. [1-3] In some studies, the temperature required for the optimal leaching reaction is even as high as 90°C. [4-6] The continuous high temperature requirement will greatly increase the energy consumption cost of the production process, especially when the ambient temperature is low. In addition, the existing optimal leaching conditions are mostly set for a single fixed temperature. Once the reaction temperature needs to be changed, it is necessary to re-explore the experiment to determine the optimal conditions, which increases the experimental cost and time cost.

[0005] [1] Jin H., Zhang JL, Wang DD, et al. Facile and efficient recovery of lithium from spent LiFePO4 batteries via air oxidation-waterleaching at room temperature [J]. Green Chemistry, 2022, 24(1): 152-62.

[0006] [2] Zhang ZC, Tang JF, Su MH, et al. Design and Optimization of an Economically Viable and Highly Efficient Strategy for Li Recycling fromSpent LiFePO4 Batteries [J]. ACS Sustainable Chemistry & Engineering, 2023,11(45): 16124-32.

[0007] [3] Zhou Jikui, Liu Mudan, Liu Yong, et al. Experimental study on leaching lithium from waste lithium iron phosphate by sulfuric acid-hydrogen peroxide[J]. Mining and Metallurgical Engineering, 2020, 40(06): 79-81.

[0008] [4] Zhang Haitao, Ma Libin, Liu Yanxia, ​​et al. Method for recovering phosphorus, iron and lithium from waste lithium iron phosphate batteries[Z]. 2022

[0009] [5] Jin H., Zhang JL, Yang C., et al. Selective recovery of lithium from spent LiFePO4 battery via a self-catalytic air oxidation method[J]. Chemical Engineering Journal, 2023, 460.

[0010] [6] Duan Jinliang, Ruan Dingshan, Chen Ruokui, et al. Selective leaching kinetics of lithium from spent lithium iron phosphate battery powder[J]. Hydrometallurgy, 2022, 41(06): 518-22. Summary of the invention

[0011] The purpose of the present invention is to provide a method for efficiently separating and recovering lithium from waste lithium iron phosphate, so as to solve the problems in the above-mentioned background technology.

[0012] In order to achieve the above object, the present invention provides the following technical solutions:

[0013] A method for efficiently separating and recovering lithium from waste lithium iron phosphate comprises the following steps:

[0014] 1) Discharge and disassemble the waste lithium iron phosphate batteries to obtain positive electrode sheets, and then obtain waste lithium iron phosphate powder after peeling, crushing and screening;

[0015] 2) Under stirring conditions, adding an appropriate amount of dilute phosphate solution and hydrogen peroxide to the waste lithium iron phosphate powder obtained in step 1), and adding sulfuric acid to adjust the pH of the slurry to the optimal pH value at the current reaction temperature for leaching reaction. After the reaction is completed, filtering and collecting the filtrate;

[0016] 3) adding alkaline solution to the filtrate obtained in step 2) to adjust the pH to alkaline, filtering out impurities, and then adding saturated phosphate solution to carry out precipitation reaction. After the reaction is completed, filtering out the precipitate, washing and drying to obtain lithium phosphate.

[0017] Among them, the optimal pH value at the current reaction temperature is equal to the pH value at which the critical iron ion concentration required to generate iron hydroxide and iron phosphate in the solution reaches an equal value minus the A value, and the A value is 0.1 to 0.2.

[0018] Preferably, the particle size of the waste lithium iron phosphate powder in step 1) is less than 100 μm.

[0019] Preferably, the reaction temperature in step 2) is 10-55° C., the reaction time is 10-60 min, the stirring rate is 200-1000 rpm, and the concentration of the sulfuric acid solution is 4-8 mol / L.

[0020] Preferably, in the step 2), the concentration of P element in the dilute phosphate solution is 0.01-0.1 mol / L, the phosphate is one or more of sodium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, potassium phosphate, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, ammonium phosphate, diammonium hydrogen phosphate and ammonium dihydrogen phosphate, the molar ratio of H2O2 in hydrogen peroxide to Fe element in waste lithium iron phosphate powder is 0.5-1:1, and the ratio of the total mass of the mixed solution of dilute phosphate solution and hydrogen peroxide to the mass of waste lithium iron phosphate powder is 4-16:1.

[0021] Preferably, the method for calculating the critical iron ion concentration required to generate iron hydroxide and iron phosphate in the solution to reach an equal pH value is:

[0022] a. Calculate the standard generation free energy of each level of iron ion hydrolysis reaction, each level of phosphoric acid ionization reaction, iron phosphate dissolution reaction, iron hydroxide dissolution reaction and water autoionization reaction at the current temperature according to the following formula (1): , unit is kJ / mol;

[0023] (1)

[0024] Where: is the standard free energy of formation of each reaction at 25°C, in kJ / mol; is the standard entropy change of each reaction at 25°C, in J / (K∙mol); is the standard molar heat capacity of each reaction, in J / (K∙mol);

[0025] b. Based on the following formula (2) and the standard free energy of formation of each reaction calculated in step a , calculate the equilibrium constant of each reaction at the current temperature ;

[0026] (2)

[0027] Where: R is the ideal gas constant, R=8.314 J / (K∙mol); T is the thermodynamic temperature, unit is K;

[0028] c. According to the following formulas (3) and (4), and the iron ion hydrolysis constant K calculated in step b bn (n=1, 2, 3, 4) and the ionization constants K of each phosphate level an (n=1, 2, 3), calculate the distribution coefficient of iron ions at different pH values , and the distribution coefficient of phosphate ions ;

[0029] (3)

[0030] (4)

[0031] d. According to the following formulas (5) and (6), the phosphorus concentration in the mixed solution of dilute phosphate solution and hydrogen peroxide is , the solubility product constant of ferric hydroxide calculated in step b , the solubility product constant of phosphoric acid , and the ionic product constant of water and the distribution coefficient of iron ions calculated in step c and the distribution coefficient of phosphate ions , calculate the critical iron ion concentration of ferric hydroxide at different pH values , and the critical iron concentration of iron phosphate ; (5)

[0032] (6)

[0033] e. The critical iron ion concentration of iron hydroxide at different pH values ​​calculated in step d. and critical iron concentration of ferric phosphate , draw a curve relationship between the critical iron ion concentration and the pH value, calculate the pH value corresponding to the intersection of the two curves, and this pH value is the pH value at which the critical iron ion concentration required to generate iron hydroxide and iron phosphate reaches an equal level.

[0034] Preferably, the alkali solution in step 3) is one or more of saturated sodium hydroxide solution, potassium hydroxide solution or ammonia water, and the pH value ranges from 10 to 12.

[0035] Preferably, the phosphate solution in step 3) is one or more of saturated sodium phosphate, disodium hydrogen phosphate, potassium phosphate, dipotassium hydrogen phosphate, ammonium phosphate, and diammonium hydrogen phosphate solution, and the molar ratio of the P element in the phosphate solution to the Li element in the filtrate is 1-1.2:3.

[0036] Preferably, the drying process in step 3) is carried out at a temperature of 60-100° C. and for a time of 12-24 h.

[0037] Preferably, in step 2) and step 3), a filter membrane with a pore size of 0.22 μm is used for filtration.

[0038] The beneficial effects of the present invention are:

[0039] 1. According to the equilibrium conditions of dissolution and precipitation of iron phosphate, the present invention creatively introduces a low-concentration dilute phosphate solution in the leaching reaction process, increases the concentration of phosphate ions in the system, reduces the iron concentration required for the precipitation of iron phosphate, and effectively promotes the formation of iron phosphate precipitation, and reduces the iron content in the leaching filtrate. By adjusting the pH of the leaching reaction system to a pH close to and slightly lower than the optimal pH value (the pH value at which the critical iron ion concentration required for the formation of iron hydroxide and iron phosphate in the solution reaches the same value), it is ensured that most of the iron ions in the reaction system can be effectively precipitated in the form of iron phosphate, while the soluble iron hydroxide generated by the hydrolysis reaction is very small, thereby optimizing the entire reaction process, greatly improving the lithium leaching rate, and reducing the leaching rate of the iron element;

[0040] 2. The present invention innovatively calculates the optimal pH value corresponding to different reaction temperatures based on the real-time reaction temperature and related thermodynamic data. When the reaction temperature changes, the pH value can be accurately adjusted to keep the leaching reaction close to the optimal conditions at all times. This can not only greatly improve the lithium leaching rate, reduce the iron leaching rate, reduce the subsequent impurity removal pressure, and improve the purity of the recovered lithium salt, but also save a lot of early exploration experimental costs and time, improve overall production efficiency, enhance process universality, and meet the needs of large-scale industrial continuous production;

[0041] 3. The present invention converts lithium sulfate in the filtrate into solid lithium phosphate by adding a saturated phosphate solution to the filtrate, and a higher precipitation rate can be obtained without concentrating the lithium-containing filtrate. In addition, increasing the pH value of the filtrate and filtering impurities before precipitating the lithium phosphate can help obtain high-purity lithium phosphate and further improve its precipitation rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 The critical iron ion concentration for generating iron hydroxide and iron phosphate in the solution corresponding to different temperatures and pH values ​​is calculated;

[0043] Figure 2 The leaching rates of lithium and iron elements after the selective leaching process in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 4;

[0044] Figure 3 is the XRD pattern of lithium phosphate obtained in Example 1;

[0045] Figure 4 is the XRD pattern of lithium phosphate obtained in Example 2;

[0046] Figure 5 This is a SEM image of the lithium phosphate obtained in Example 1;

[0047] Figure 6 This is the SEM image of the lithium phosphate obtained in Example 2. DETAILED DESCRIPTION

[0048] The present invention is described in detail below by way of examples. It is necessary to point out here that the following examples are only used to further illustrate the present invention and cannot be understood as limiting the scope of protection of the present invention. The technicians in this field can make some non-essential improvements and adjustments to the present invention according to the above-mentioned content of the present invention. In the following examples, reagents and instruments not specifically described are all commercially available, and experimental operations not specifically described are implemented according to the manufacturer's instructions or conventional techniques in the field. Unless otherwise defined, all professional and scientific terms used in the text have the same meaning as those familiar to those skilled in the art. In addition, any method and material similar to or equal to the recorded content can be applied to the present invention; the endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed herein.

[0049] A method for efficiently separating and recovering lithium from waste lithium iron phosphate is provided, comprising the following steps:

[0050] 1) Discharge and disassemble the waste lithium iron phosphate batteries to obtain positive electrode sheets, and then obtain waste lithium iron phosphate powder after peeling, crushing and screening;

[0051] 2) Under stirring conditions, adding an appropriate amount of dilute phosphate solution and hydrogen peroxide to the waste lithium iron phosphate powder obtained in step 1), and adding sulfuric acid to adjust the pH of the slurry to the optimal pH value at the current reaction temperature for leaching reaction. After the reaction is completed, filtering and collecting the filtrate;

[0052] 3) adding alkaline solution to the filtrate obtained in step 2) to adjust the pH to alkaline, filtering out impurities, and then adding saturated phosphate solution to carry out precipitation reaction. After the reaction is completed, filtering out the precipitate, washing and drying to obtain lithium phosphate;

[0053] The reaction principle of the leaching process in the above step 2) is as follows:

[0054] Hydrogen peroxide can oxidize the divalent iron in the lithium iron phosphate lattice into trivalent iron. Since the trivalent iron ions and phosphate ions in the iron phosphate crystal have reached charge balance at this time, the lithium ions will be expelled from the lattice and enter the solution. This reaction process will consume the hydrogen ions in the solution, so the acid needs to be continuously replenished; in the process of selectively leaching lithium iron phosphate using hydrogen peroxide and sulfuric acid, the reactions that occur are as follows:

[0055] 2LiFePO4+H2O2+H2SO4→2FePO4+Li2SO4+2H2O

[0056] The reaction principle of the lithium phosphate precipitation process in the above step 3) is as follows:

[0057] The solubility of lithium phosphate is much lower than that of lithium sulfate. When the ion product of phosphate ions and lithium ions in the solution is greater than the solubility product of lithium phosphate, lithium phosphate will precipitate, and the lithium sulfate in the filtrate will be converted into lithium phosphate solid. The reactions that occur during the precipitation process are as follows:

[0058] 3Li2SO4+2Na3PO4→2Li3PO4↓+3Na2SO4

[0059] Among them, the optimal pH value at the current reaction temperature is equal to the pH value at which the critical iron ion concentration required to generate iron hydroxide and iron phosphate in the solution reaches an equal value minus the A value, and the A value is 0.1 to 0.2.

[0060] In some preferred embodiments of the present invention, the method for calculating the critical iron ion concentration required to generate iron hydroxide and iron phosphate in the solution to reach an equal pH value is:

[0061] a. Calculate the standard free energy of formation of the iron ion hydrolysis reaction, phosphate ionization reaction, iron phosphate dissolution reaction, and iron hydroxide dissolution reaction at the current temperature according to the following formula (1): , in kJ / mol:

[0062] (1)

[0063] Where: is the standard formation free energy of each reactant at the current temperature, in kJ / mol; is the standard entropy change of each reactant at the current temperature, in J / (K∙mol); is the standard molar heat capacity of each reactant, in J / (K∙mol); the standard entropy change, standard molar heat capacity and standard generation freedom of each reactant can be found in the Lang's Handbook of Chemistry, industrial software HSC and Factsage;

[0064] b. According to the following formula (2) and the standard free energy of formation of each reaction calculated in step a , calculate the equilibrium constant of each reaction at the current temperature ;

[0065] (2)

[0066] Where: R is the ideal gas constant, R=8.314 J / (K∙mol); T is the thermodynamic temperature, unit is K;

[0067] c. According to the following formulas (3) and (4), and the iron ion hydrolysis constant K calculated in step bbn (n=1, 2, 3, 4) and the ionization constants K of each phosphate level an (n=1, 2, 3), calculate the distribution coefficient of iron ions at different pH values

[0068] , and the distribution coefficient of phosphate ions ;

[0069] (3)

[0070] (4)

[0071] d. According to the following formulas (5) and (6), the phosphorus concentration in the mixed solution of dilute phosphate solution and hydrogen peroxide is , the solubility product constant of ferric hydroxide calculated in step b , the solubility product constant of phosphoric acid , and the ionic product constant of water and the distribution coefficient of iron ions calculated in step c and the distribution coefficient of phosphate ions , calculate the critical iron ion concentration of ferric hydroxide at different pH values , and the critical iron concentration of iron phosphate ; (5) (6)

[0072] e. The critical iron ion concentration of iron hydroxide at different pH values ​​calculated in step d. and critical iron concentration of ferric phosphate , draw a curve relationship between the critical iron ion concentration and the pH value, calculate the pH value corresponding to the intersection of the two curves, and this pH value is the pH value at which the critical iron ion concentration required to generate iron hydroxide and iron phosphate reaches an equal level.

[0073] It should be noted that the reaction formulas of the hydrolysis reactions of iron ions at various levels in the above step a are the following formulas ①, ②, ③ and ④, the reaction formulas of the ionization reactions of phosphoric acid at various levels are the following formulas ⑤, ⑥ and ⑦, the reaction formula of the dissolution reaction of iron phosphate is the following formula ⑧, the reaction formula of the dissolution reaction of iron hydroxide is the following formula ⑨, and the reaction formula of the water autoionization reaction is the following formula ⑩;

[0074]

[0075]

[0076]

[0077] This embodiment calculates the standard formation free energy of reaction equations ①-⑩ when the reaction temperature is 25°C, 35°C, 45°C and 55°C according to formula (1): ; Then, under the condition that the phosphorus concentration [P]0 = 0.01 mol / L in the mixed solution of dilute phosphate solution and hydrogen peroxide, the critical iron ion concentrations for the formation of iron hydroxide and iron phosphate at different pH values ​​at 25°C, 35°C, 45°C, and 55°C were calculated according to formulas (2)-(6), and a relationship diagram between the critical iron ion concentration and the pH value was drawn, as shown in the figure. Figure 1 shown by Figure 1 It can be seen that at 25°C, when pH is less than 3.92, as the pH value increases, the amount of iron ions dissolved and entering the solution decreases, and the critical iron ion concentration for the formation of iron phosphate in the solution also decreases, indicating that a larger proportion of iron ions will precipitate in the form of iron phosphate, so the iron content in the solution decreases; when pH is greater than 3.92, as the pH value increases, at the same iron ion concentration, iron hydroxide is more likely to be generated in the solution, but the newly generated iron hydroxide will exist in the form of colloidal particles. Before the colloidal particles agglomerate and grow to a large enough size, they cannot be separated from the liquid phase by filtration, which instead leads to the iron content of the solution. The content rises back, so the formation of iron hydroxide should be avoided as much as possible, so the pH of the slurry in the leaching process at this temperature should not exceed 3.92; at this time, the iron ions dissolved and oxidized are relatively small, and most of the iron ions will precipitate in the form of iron phosphate, and at the same time, hydrolysis reaction occurs to produce very little soluble iron hydroxide; similarly, the pH of the slurry in the leaching process at 35°C, 45°C, and 55°C should not exceed 3.80, 3.67, and 3.52, respectively. In actual operation, the use of sulfuric acid solution to control the slurry pH value below this value by 0.1-0.2 can effectively reduce the iron content in the solution and achieve better iron-lithium separation effect.

[0078] In some preferred embodiments of the present invention, the particle size of the waste lithium iron phosphate powder in step 1) is less than 100 μm, which can ensure that the powder is fully contacted and reacted with hydrogen peroxide during the selective leaching process.

[0079] The present invention can accurately calculate the optimal pH value corresponding to different reaction temperatures, improve the lithium leaching rate and reduce the iron leaching rate. In order to reduce the energy consumption cost of the production process, the leaching reaction temperature in step 2) of the present invention can be preferably 10-55°C, and can also be 15°C, 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, 50°C.

[0080] In some preferred embodiments of the present invention, in the step 2), the reaction time is 10-60 min, the stirring rate is 200-1000 rpm, and the concentration of the sulfuric acid solution is 4-8 mol / L; the reaction time can also be selected from 20min, 30min, 40min, 50min; the stirring rate can also be selected from 300rpm, 400rpm, 500rpm, 600rpm, 700rpm, 800rpm, 900rpm; the molar ratio of H2O2 in hydrogen peroxide to Fe element in waste lithium iron phosphate powder can also be selected from 0.6:1, 0.7:1, 0.8:1, 0.9:1; the concentration of the sulfuric acid solution can also be selected from 5 mol / L, 6 mol / L, 7 mol / L.

[0081] In some preferred embodiments of the present invention, the concentration of P element in the dilute phosphate solution in step 2) is 0.01-0.1 mol / L, the phosphate is one or more of sodium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, potassium phosphate, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, ammonium phosphate, diammonium hydrogen phosphate and diammonium dihydrogen phosphate, the molar ratio of H2O2 in hydrogen peroxide to Fe element in waste lithium iron phosphate powder is 0.5-1:1, and the ratio of the total mass of the mixed solution of dilute phosphate solution and hydrogen peroxide to the mass of waste lithium iron phosphate powder is 4-16:1; the molar ratio of H2O2 in hydrogen peroxide to Fe element in waste lithium iron phosphate powder can also be selected from 0.6:1, 0.7:1, 0.8:1, 0.9:1, and the concentration of phosphorus element in the dilute phosphate solution can also be selected from 0.01-0.1 Any value in mol / L; the ratio of the total mass of the mixed solution of dilute phosphate solution and hydrogen peroxide to the mass of the waste lithium iron phosphate powder can also be selected as 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, 11:1, 12:1, 13:1, 14:1, 15:1;

[0082] Among them, according to the dissolution and precipitation equilibrium conditions of iron phosphate, under the same temperature and pH conditions, the higher the phosphate ion concentration, the lower the iron ion concentration required for precipitation. Therefore, the critical iron ion concentration in the leaching system with a certain phosphate is lower than that in the leaching system without phosphate. The addition of dilute phosphate solution can reduce the iron ion concentration and thus reduce the iron content of the leaching filtrate.

[0083] In some preferred embodiments of the present invention, the alkali solution in step 3) is one or more of saturated sodium hydroxide, potassium hydroxide solution or ammonia water, and the alkali solution is used to adjust the pH value of the filtrate to a range of 10-12. The pH value of the filtrate can also be adjusted to 10.5, 11, or 11.5.

[0084] In some preferred embodiments of the present invention, the phosphate solution in step 3) is one or more of saturated sodium phosphate, disodium hydrogen phosphate, potassium phosphate, dipotassium hydrogen phosphate, ammonium phosphate, and diammonium hydrogen phosphate, and the molar ratio of the P element in the phosphate solution to the Li element in the filtrate is 1-1.2:3. The molar ratio of the P element in the phosphate solution to the Li element in the filtrate can also be selected as 1.05:3, 1.1:3, and 1.15:3.

[0085] In some preferred embodiments of the present invention, the drying process temperature in step 3) is 60-100°C and the drying time is 12-24 h. The drying process temperature can also be 65°C, 70°C, or 75°C; the drying time can also be 13 h, 14 h, 15 h, 16 h, 17 h, 18 h, 19 h, 20 h, 21 h, 22 h, or 23 h.

[0086] In some preferred embodiments of the present invention, the preferred pore size of the filter membrane used in the filtration process in steps 2)-3) is 0.22 μm, which can ensure that solid impurities are fully removed and lithium phosphate precipitates are retained.

[0087] It should be noted that the calculation formulas for the leaching rates of lithium and iron in the embodiment of the present invention are as follows:

[0088] (7)

[0089] In the formula, ε M is the leaching rate of lithium or iron, dimensionless ω L and ω S are the lithium or iron content in the leaching filtrate and the waste lithium iron phosphate powder, dimensionless, m L and m S are the masses of leaching filtrate and waste lithium iron phosphate powder, respectively, in g.

[0090] The following are specific embodiments:

[0091] Example 1

[0092] 1) First, the waste lithium iron phosphate battery is discharged and disassembled to obtain the waste lithium iron phosphate positive electrode sheet, and then the aluminum foil is peeled off and removed, and the waste lithium iron phosphate powder (iron mass fraction is 31.01%) with a particle size less than 100 μm is obtained;

[0093] 2) Under the conditions of a reaction temperature of 45°C and a stirring rate of 600 rpm, 200 g of the waste lithium iron phosphate powder obtained in step 1), 137.28 g of hydrogen peroxide with a mass fraction of 27.5%, and a 0.01 mol / L dilute phosphate solution prepared by mixing 0.76 g of sodium phosphate and 1862 g of water were added to the reactor, and the pH value of the slurry was adjusted to 3.5 using 8 mol / L sulfuric acid solution. After continuous stirring for 10 min, the slurry was filtered to obtain a filtrate;

[0094] 3) Determine the lithium and iron content in the filtrate obtained in step 2) and calculate the leaching rate. The calculation result is as follows: Figure 2 As shown, under this condition, the leaching rates of lithium and iron elements were 97.67% and 0.051%, respectively;

[0095] 4) Weigh 1000 g of the filtrate obtained in step 2), add saturated sodium hydroxide solution to adjust the pH value of the filtrate to 10, filter out impurities, and then add saturated sodium phosphate solution to the filtrate at a molar ratio of P:Li=1.1:3, filter out the precipitate, and dry the precipitate in a 60°C forced air drying oven for 24 h to obtain the target product, lithium phosphate.

[0096] Example 2

[0097] 1) First, the waste lithium iron phosphate battery is discharged and disassembled to obtain the waste lithium iron phosphate positive electrode sheet, and then the aluminum foil is peeled off and removed, and the waste lithium iron phosphate powder (iron mass fraction is 31.01%) with a particle size less than 100 μm is obtained;

[0098] 2) Under the conditions of a reaction temperature of 45°C and a stirring rate of 600 rpm, 200 g of the waste lithium iron phosphate powder obtained in step 1), 137.28 g of hydrogen peroxide with a mass fraction of 27.5%, and a 0.01 mol / L dilute phosphate solution prepared by mixing 0.76 g of sodium phosphate and 1862 g of water were added to the reactor, and the pH value of the slurry was adjusted to 3.5 using 8 mol / L sulfuric acid solution. After continuous stirring for 10 min, the slurry was filtered to obtain a filtrate;

[0099] 3) Weigh 1000 g of the filtrate obtained in step 2), add ammonia water to adjust the pH value of the filtrate to 10, filter out impurities, and then add saturated ammonium phosphate solution to the filtrate at a molar ratio of P:Li=1.1:3, filter out the precipitate; dry the precipitate in a 60°C forced air drying oven for 24 h to obtain the target product, lithium phosphate.

[0100] Comparative Example 1

[0101] 1) First, the waste lithium iron phosphate battery is discharged and disassembled to obtain the waste lithium iron phosphate positive electrode sheet, and then the aluminum foil is peeled off and removed, and the waste lithium iron phosphate powder (iron mass fraction is 31.01%) with a particle size less than 100 μm is obtained;

[0102] 2) Under the conditions of a reaction temperature of 45°C and a stirring rate of 600 rpm, 200 g of the waste lithium iron phosphate powder obtained in step 1), 137.28 g of hydrogen peroxide with a mass fraction of 27.5%, and a 0.01 mol / L dilute phosphate solution prepared by mixing 0.76 g of sodium phosphate and 1862 g of water were added to the reactor, and the pH value of the slurry was adjusted to 2 using an 8 mol / L sulfuric acid solution. After continuous stirring for 10 min, the slurry was filtered to obtain a filtrate;

[0103] 3) Determine the lithium and iron content in the filtrate obtained in step 1) and calculate the leaching rate. The calculation result is as follows: Figure 2 As shown in the figure, under this condition, the leaching rates of lithium and iron elements are 98.94% and 0.30%, respectively.

[0104] Comparative Example 2

[0105] 1) First, the waste lithium iron phosphate battery is discharged and disassembled to obtain the waste lithium iron phosphate positive electrode sheet, and then the aluminum foil is peeled off and removed, and the waste lithium iron phosphate powder (iron mass fraction is 31.01%) with a particle size less than 100 μm is obtained;

[0106] 2) Under the conditions of a reaction temperature of 45°C and a stirring rate of 600 rpm, 200 g of the waste lithium iron phosphate powder obtained in step 1), 137.28 g of hydrogen peroxide with a mass fraction of 27.5%, and a 0.01 mol / L dilute phosphate solution prepared by mixing 0.76 g of sodium phosphate and 1862 g of water were added to the reactor, and the pH value of the slurry was adjusted to 3 using an 8 mol / L sulfuric acid solution. After continuous stirring for 10 min, the slurry was filtered to obtain a filtrate;

[0107] 3) Determine the lithium and iron content in the filtrate obtained in step 1) and calculate the leaching rate. The calculation result is as follows: Figure 2 As shown in the figure, under this condition, the leaching rates of lithium and iron elements are 98.51% and 0.13%, respectively.

[0108] Comparative Example 3

[0109] 1) First, the waste lithium iron phosphate battery is discharged and disassembled to obtain the waste lithium iron phosphate positive electrode sheet, and then the aluminum foil is peeled off and removed, and the waste lithium iron phosphate powder (iron mass fraction is 31.01%) with a particle size less than 100 μm is obtained;

[0110] 2) Under the conditions of a reaction temperature of 45°C and a stirring rate of 600 rpm, 200 g of the waste lithium iron phosphate powder obtained in step 1), 137.28 g of hydrogen peroxide with a mass fraction of 27.5%, and a 0.01 mol / L dilute phosphate solution prepared by mixing 0.76 g of sodium phosphate and 1862 g of water were added to the reactor, and the pH value of the slurry was adjusted to 4 using an 8 mol / L sulfuric acid solution. After continuous stirring for 10 min, the slurry was filtered to obtain a filtrate;

[0111] 3) Determine the lithium and iron content in the filtrate obtained in step 1) and calculate the leaching rate. The calculation result is as follows: Figure 2 As shown in the figure, under this condition, the leaching rates of lithium and iron elements are 95.62% and 0.33%, respectively.

[0112] Comparative Example 4

[0113] 1) First, the waste lithium iron phosphate battery is discharged and disassembled to obtain the waste lithium iron phosphate positive electrode sheet, and then the aluminum foil is peeled off and removed, and the waste lithium iron phosphate powder (iron mass fraction is 31.01%) with a particle size less than 100 μm is obtained;

[0114] 2) Under the conditions of a reaction temperature of 45°C and a stirring rate of 600 rpm, 200 g of the waste lithium iron phosphate powder obtained in step 1), 137.28 g of hydrogen peroxide with a mass fraction of 27.5%, and a 0.01 mol / L dilute phosphate solution prepared by mixing 0.76 g of sodium phosphate and 1862 g of water were added to the reactor, and the pH value of the slurry was adjusted to 5 using an 8 mol / L sulfuric acid solution. After continuous stirring for 10 min, the slurry was filtered to obtain a filtrate;

[0115] 3) Determine the lithium and iron content in the filtrate obtained in step 1) and calculate the leaching rate. The calculation result is as follows: Figure 2 As shown in the figure, under this condition, the leaching rates of lithium and iron elements are 89.12% and 0.49% respectively.

[0116] Experimental Example 1

[0117] The lithium phosphate powders obtained in Example 1 and Example 2 were subjected to X-ray diffraction (XRD) and the results were as follows: Figure 3 and Figure 4 As shown;

[0118] Depend on Figure 3 It can be seen that the XRD spectrum of the precipitated product of Example 1 corresponds well to Li3PO4 (PDF#01-071-1528), and there is no obvious diffraction peak of the impurity Na3PO4 (PDF#97-001-5440) in the image; Figure 4It can be seen that the XRD spectrum of the precipitation product of Example 2 corresponds well to Li3PO4 (PDF#01-071-1528), and there is no obvious diffraction peak of the impurity (NH4)3PO4·3H2O (PDF#97-001-4289) in the image; this indicates that according to the method of the present invention, adding a saturated sodium phosphate solution or an ammonium phosphate solution during the precipitation process can obtain lithium phosphate with a high degree of crystallization.

[0119] Experimental Example 2

[0120] The lithium phosphate powders obtained in Example 1 and Example 2 were observed by scanning electron microscope (SEM). Figure 5 and Figure 6 As shown;

[0121] Depend on Figure 5 It can be seen that the primary particles of lithium phosphate obtained in Example 1 are uniform in size, with a particle size of 60-80 nm, and the primary particles agglomerate to form tightly arranged secondary particles, and the gaps between the primary particles are small; Figure 6 It can be seen that the primary particles of lithium phosphate obtained in Example 2 are uniform in size, with a particle size of 100-120 nm. Compared with the lithium phosphate obtained in Experimental Example 1, the gaps between the primary particles of lithium phosphate obtained in Experimental Example 2 are larger, and the degree of agglomeration of the formed secondary particles is smaller.

[0122] The particle sizes of the primary particles of lithium phosphate obtained in Experimental Example 1 and Experimental Example 2 are different, which can be attributed to the differences in nucleation and growth rates during the crystal formation process; in Experimental Example 1, the crystal nucleation rate is faster during the process of adding saturated sodium phosphate solution to precipitate lithium phosphate, and a large number of fine particles are formed in a short period of time. In Experimental Example 2, the crystal growth rate is faster during the process of adding saturated ammonium phosphate solution to precipitate lithium phosphate, and the crystal nuclei are quickly produced into particles with larger particle sizes; this shows that according to the method of the present invention, by adding saturated sodium phosphate or ammonium phosphate solution during the precipitation process, lithium phosphate of uniform size can be obtained, and the size of lithium phosphate particles can be regulated within a certain range by adding different saturated phosphate solutions.

[0123] In summary, in the process of recycling waste lithium iron phosphate, the use of low-concentration phosphate solution and precise control of the pH value of the slurry in the leaching process can achieve efficient separation of lithium and iron elements. At 45°C, when the pH value of the leaching process is controlled to 3.5, the leaching rates of lithium and iron elements are 97.67% and 0.051%, respectively; compared with other pH conditions, the leaching rate of iron element is the lowest at this time, which is conducive to obtaining a lithium-containing filtrate with a low content of impurity elements, reducing the difficulty of the subsequent impurity removal process; in addition, in the process of precipitating lithium phosphate, lithium phosphate powder with a high degree of crystallization and uniform size can be prepared by adding a saturated phosphate solution, and the size of lithium phosphate particles can be regulated within a certain range by adding different saturated phosphate solutions.

[0124] In summary, it should be noted that the above is only a preferred embodiment of the present invention and does not overly limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, for those skilled in the art, they can still make some simple deductions, replacements, or equivalent replacements for some of the technical features of the technical solutions recorded in the aforementioned embodiments without departing from the concept of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for efficiently separating and recovering lithium from waste lithium iron phosphate, characterized in that: The following steps are involved: 1) Discharge and disassemble the waste lithium iron phosphate batteries to obtain positive electrode sheets, and then obtain waste lithium iron phosphate powder after peeling, crushing and screening; 2) Under stirring conditions, adding an appropriate amount of dilute phosphate solution and hydrogen peroxide to the waste lithium iron phosphate powder obtained in step 1), and adding sulfuric acid to adjust the pH of the slurry to the optimal pH value at the current reaction temperature for leaching reaction. After the reaction is completed, filtering and collecting the filtrate; 3) adding alkaline solution to the filtrate obtained in step 2) to adjust the pH to alkaline, filtering out impurities, and then adding saturated phosphate solution to carry out precipitation reaction. After the reaction is completed, filtering out the precipitate, washing and drying to obtain lithium phosphate; The optimal pH value at the current reaction temperature is equal to the pH value at which the critical iron ion concentration required to generate iron hydroxide and iron phosphate in the solution reaches an equal value minus the A value, and the A value is 0.1 to 0.2; The method for calculating the critical iron ion concentration required to generate iron hydroxide and iron phosphate in the solution to reach an equal pH value is: a. Calculate the standard generation free energy of each level of iron ion hydrolysis reaction, each level of phosphoric acid ionization reaction, iron phosphate dissolution reaction, iron hydroxide dissolution reaction and water autoionization reaction at the current temperature according to the following formula (1): , unit is kJ / mol; (1) Where: is the standard free energy of formation of each reaction at 25°C, in kJ / mol; is the standard entropy change of each reaction at 25°C, in J / (K∙mol); is the standard molar heat capacity of each reaction, in J / (K∙mol); b. Based on the following formula (2) and the standard free energy of formation of each reaction calculated in step a , calculate the equilibrium constant of each reaction at the current temperature ; (2) Where: R is the ideal gas constant, R=8.314 J / (K∙mol); T is the thermodynamic temperature, unit is K; c. According to the following formulas (3) and (4), and the iron ion hydrolysis constant K calculated in step b bn (n=1, 2, 3, 4) and the ionization constants K of each phosphate level an (n=1, 2, 3), calculate the distribution coefficient of iron ions at different pH values , and the distribution coefficient of phosphate ions ; (3) (4) d. According to the following formulas (5) and (6), the phosphorus concentration in the mixed solution of dilute phosphate solution and hydrogen peroxide is , the solubility product constant of ferric hydroxide calculated in step b , the solubility product constant of phosphoric acid , and the ionic product constant of water and the distribution coefficient of iron ions calculated in step c and the distribution coefficient of phosphate ions , calculate the critical iron ion concentration of ferric hydroxide at different pH values , and the critical iron concentration of iron phosphate ; (5) (6) e. The critical iron ion concentration of iron hydroxide at different pH values ​​calculated in step d. and critical iron concentration of ferric phosphate , draw a curve relationship between the critical iron ion concentration and the pH value, calculate the pH value corresponding to the intersection of the two curves, and this pH value is the pH value at which the critical iron ion concentration required to generate iron hydroxide and iron phosphate reaches an equal level.

2. A method for efficiently separating and recovering lithium from waste lithium iron phosphate according to claim 1, characterized in that: The particle size of the waste lithium iron phosphate powder in step 1) is less than 100 μm.

3. A method for efficiently separating and recovering lithium from waste lithium iron phosphate according to claim 1, characterized in that: The reaction temperature in step 2) is 10-55° C., the reaction time is 10-60 min, the stirring rate is 200-1000 rpm, and the concentration of the sulfuric acid solution is 4-8 mol / L.

4. The method for efficiently separating and recovering lithium from waste lithium iron phosphate according to claim 1, characterized in that: In the step 2), the concentration of P element in the dilute phosphate solution is 0.01-0.1 mol / L, the phosphate is one or more of sodium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, potassium phosphate, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, ammonium phosphate, diammonium hydrogen phosphate and ammonium dihydrogen phosphate, the molar ratio of H2O2 in hydrogen peroxide to Fe element in waste lithium iron phosphate powder is 0.5-1:1, and the ratio of the total mass of the mixed solution of dilute phosphate solution and hydrogen peroxide to the mass of waste lithium iron phosphate powder is 4-16:

1.

5. The method for efficiently separating and recovering lithium from waste lithium iron phosphate according to claim 1, characterized in that: In step 3), the alkali solution is one or more of saturated sodium hydroxide solution, potassium hydroxide solution or ammonia water, and the pH value ranges from 10 to 12.

6. The method for efficiently separating and recovering lithium from waste lithium iron phosphate according to claim 1, characterized in that: In the step 3), the phosphate solution is one or more of saturated sodium phosphate, disodium hydrogen phosphate, potassium phosphate, dipotassium hydrogen phosphate, ammonium phosphate, and diammonium hydrogen phosphate solution, and the molar ratio of the P element in the phosphate solution to the Li element in the filtrate is 1-1.2:

3.

7. The method for efficiently separating and recovering lithium from waste lithium iron phosphate according to claim 1, characterized in that: The drying process in step 3) is carried out at a temperature of 60-100° C. and for a time of 12-24 h.

8. The method for efficiently separating and recovering lithium from waste lithium iron phosphate according to claim 1, characterized in that: In the step 2) and step 3), filtering is performed using a filter membrane with a pore size of 0.22 μm.

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