Method for recycling battery-grade iron phosphate and lithium salt from lithium iron phosphate waste and its application

Abstract

The present application discloses a method for recovering battery-grade iron phosphate and lithium salts from lithium iron phosphate waste materials and its application, comprising the following steps: S100, dissolving the lithium iron phosphate waste materials with a phosphoric acid solution, filtering to obtain a waste material dissolution solution; S200, adding an acid solution and an oxidant to the waste material dissolution solution to obtain a crystallization stock solution; S300, diluting the crystallization stock solution with water until its pH value is 1.0 to 1.5, and performing high-temperature crystallization to obtain hydrated iron phosphate and a lithium-containing filtrate; S400, roasting the hydrated iron phosphate to obtain battery-grade iron phosphate; S500, removing water from the lithium-containing filtrate to obtain solution a, circulating solution a to dissolve the lithium iron phosphate waste materials, and concentrating to obtain lithium salts. The present application uses a phosphoric acid solution as the leaching solvent, does not require the use of an alkali solution subsequently and can be leached cyclically, improving the economic benefits while enabling the efficient recovery of all elements of Li, Fe, and P, and also capable of obtaining a battery-grade iron phosphate product with an iron-to-phosphorus ratio meeting the requirements.

Description

Technical Field

[0001] This application relates to the technical field of battery materials, and particularly to a method for recovering battery-grade iron phosphate and lithium salts from lithium iron phosphate waste and its application. Background Art

[0002] Lithium iron phosphate batteries (LiFePO4, LFP) have become an important cathode material for lithium-ion batteries (LIBs) due to their remarkable theoretical capacity, excellent reversibility, and high-temperature stability. Lithium iron phosphate batteries currently occupy an important market share in lithium-ion batteries, especially in the emerging new energy vehicle and energy storage industries. However, the increasing penetration rate of LFP batteries in the market inevitably leads to an increase in waste LFP batteries. Improper treatment will not only cause waste of resources such as lithium, iron, and phosphorus, but also cause environmental pollution.

[0003] The prior art uses an acid dissolution-alkali precipitation process to treat lithium iron phosphate cathode waste. High-concentration strong acid and oxidant are used to dissolve the lithium iron phosphate cathode waste, and alkali solution is used to adjust the pH for precipitation to obtain iron phosphate. After filtration and washing, a lithium-containing filtrate and iron phosphate are obtained. Although this method realizes the recovery of FePO4, the process has problems such as large consumption of acid and alkali, inability to recycle leaching, large discharge of waste water and waste salts, and cannot obtain FePO4·2H2O with a complete crystal form. The iron-to-phosphorus ratio often cannot meet the requirements, and the obtained product often needs to be further processed to make the iron-to-phosphorus ratio meet the requirements, thus increasing the complexity of the process. Therefore, it is necessary to develop a lithium iron phosphate full-element recovery process with low cost, economic environmental protection, recyclable leaching, strong continuous operation, and an iron-to-phosphorus ratio that meets the requirements. Summary of the Invention

[0004] This application provides a method for recovering battery-grade iron phosphate and lithium salts from lithium iron phosphate waste and its application. This method uses a phosphoric acid system as the leaching solvent, and only phosphoric acid is added at the initial stage, and no additional addition is required in subsequent cycles, greatly reducing the consumption of acid solvent. Moreover, after diluting the phosphoric acid system in the leaching environment with water and heating, iron phosphate can be precipitated without using alkali solution. Furthermore, the phosphoric acid solution of this application can be used for recycling leaching of lithium iron phosphate waste, improving economic benefits while realizing efficient recovery of all elements of Li, Fe, and P. The method of this application can obtain an iron phosphate product with an iron-to-phosphorus ratio that meets the requirements.

[0005] In a first aspect, the present application provides a method for recovering battery-grade iron phosphate and lithium salts from lithium iron phosphate waste, comprising the following steps: S100. Obtain lithium iron phosphate waste, dissolve the lithium iron phosphate waste with a phosphoric acid solution, and filter to obtain a waste dissolution solution. S200. Add an acid solution and an oxidant to the waste dissolution solution to obtain a stock solution for crystallization. S300. Dilute the stock solution for crystallization with water until its pH value is 1.0 to 1.5, perform high-temperature crystallization, and filter to obtain hydrated iron phosphate and a lithium-containing filtrate. S400. The hydrated iron phosphate is calcined to obtain battery-grade iron phosphate. S500. After the lithium-containing filtrate is evaporated to remove water, a solution a is obtained. The solution a is returned to step S100 to continue dissolving the lithium iron phosphate waste, and then steps S200 to S300 are performed. After repeating the cycle multiple times, it is concentrated to obtain lithium salts. Among them, the order of steps S400 and S500 is not sequential. In the present application, a phosphoric acid solution is used as the leaching solvent to dissolve the lithium iron phosphate waste. Among them, phosphoric acid does not participate in the reaction. The role of phosphoric acid is to provide a stable leaching environment for the crystallization of iron phosphate. After filtration, a small amount (the amount of anions capable of coordinating with lithium ions Li + coordination) of acid and an oxidant (the amount of an oxidant capable of oxidizing Fe 2+ to Fe 3+ ) are added to the phosphoric acid system solution (i.e., the waste dissolution solution), and then diluted with water. High-temperature crystallization can precipitate iron phosphate and a lithium-containing filtrate. Moreover, after the lithium-containing filtrate is evaporated to remove water, it can continue to circulate and dissolve the lithium iron phosphate waste. After repeating this cycle until the lithium concentration in the filtrate is close to saturation, it is evaporated and concentrated to precipitate lithium salts. This process can be leached in a cycle and achieve the efficient recovery of all elements of Li, Fe, and P, and obtain an iron phosphate product with an iron-to-phosphorus ratio meeting the requirements. In the initial stage of the present application, medium-strong acid phosphoric acid is used to dissolve the lithium iron phosphate waste, and then other acids and oxidants are used to react with the lithium iron phosphate. Acids with different acidities are used in two steps, and phosphoric acid is used as the environment for the crystallization of iron phosphate, so that FePO4 meeting the battery-grade standard can be obtained.

[0006] In some embodiments, in step S100, at least one of the following conditions is satisfied: Condition I. The molar concentration of the phosphoric acid solution is 1 mol / L to 10 mol / L; Condition II. The dissolution includes: a dissolution temperature of 20°C to 40°C, a solid-liquid ratio of 50 g / L to 200 g / L, and a dissolution time of 1 h to 5 h. Preferably, the molar concentration of the phosphoric acid solution is 2 mol / L to 10 mol / L, the dissolution temperature is 25°C to 35°C, the solid-liquid ratio is 80 g / L to 120 g / L, and the dissolution time is 3 h to 4 h. Among them, the solid-liquid ratio is the ratio of the mass of the lithium iron phosphate waste to the volume of the phosphoric acid solution. In the present application, by selecting an appropriate phosphoric acid concentration and solid-liquid ratio, the effective dissolution of the lithium iron phosphate waste can be achieved, and a suitable acidic environment can be provided for the subsequent crystallization of iron phosphate; by adopting an appropriate dissolution temperature, the lithium iron phosphate waste can be effectively dissolved, and it is carried out at around room temperature, which is energy-saving and environmentally friendly.

[0007] In some embodiments, in step S200, the acid solution includes hydrochloric acid solution and / or sulfuric acid solution, the molar amount of hydrogen ions in the acid solution is 1 to 1.5 times the molar amount of the leached lithium iron phosphate, the oxidant includes at least one of hydrogen peroxide, chlorine, ozone or oxygen, and the mass concentration of hydrogen peroxide is 25 wt% to 30 wt%. When the oxidant is hydrogen peroxide, its dosage is 0.5 times to 2.0 times the molar amount of iron element in the waste liquid solution, and the dosage is calculated based on the molar dosage of H2O2 in hydrogen peroxide. When the oxidant is chlorine, its dosage is that the flow rate of chlorine introduced per 10 g of lithium iron phosphate waste is 0.03 L / min to 0.05 L / min, and the introduction time is 15 min to 20 min. In the present application, an acid solution is provided, and the dosage of the acid solution only provides the function of coordinating anions with Li + coordination. The dosage of the acid solution is greatly reduced compared with the prior art, and a suitable H + concentration is provided, which is beneficial to the full reaction of the lithium iron phosphate waste; by selecting a suitable dosage of the oxidant, the Fe 2+ in the lithium iron phosphate waste can be effectively oxidized.

[0008] In some embodiments, in step S300, at least one of the following conditions is satisfied: Condition A: Calculated by volume, the amount of water added during the dilution with water is 1 to 6 times the dosage of the phosphoric acid solution in step S100. Condition B: The conditions for high-temperature crystallization are: reaction temperature: 80 °C to 110 °C, aging time: 2 h to 6 h. Dilute the stock solution to be crystallized with deionized water by 1 to 6 times (compared with the dosage of the phosphoric acid solution), adjust the pH to 1.0 to 1.5, and then utilize the physical property that the solubility of iron phosphate decreases with the increase of temperature. Heat and stir the diluted solution at a high temperature, and dihydrate iron phosphate and a lithium-containing filtrate are precipitated by crystallization. In the present application, water is used to adjust the pH, without consuming lye, reducing the consumption of lye caused by the introduction of acid, and the lithium-containing filtrate can be recycled to step S100 to continue dissolving the lithium iron phosphate waste, realizing the recycling of acid, and not introducing new cations, avoiding affecting the three-stage electrolysis of phosphoric acid, ensuring the integrity of the iron phosphate crystal form during the subsequent precipitation of crystalline iron phosphate; and selecting a suitable amount of water is also beneficial to ensuring that a product with a complete crystal form is obtained when iron phosphate is precipitated subsequently.

[0009] In some embodiments, in step S400, the conditions for roasting are: roasting temperature: 300 °C to 700 °C, roasting time: 30 min to 120 min. Dihydrate iron phosphate is roasted at a high temperature to obtain battery-grade iron phosphate. A suitable roasting temperature and roasting time are more conducive to obtaining battery-grade iron phosphate.

[0010] In some embodiments, in step S500, the lithium-containing filtrate is evaporated to remove water to obtain solution a, and the molar concentration of solution a is 1 mol / L to 10 mol / L. The molar concentration of solution a is based on the molar amount of H3PO4 in solution a. In the present application, the molar concentration of solution a may be the same as or different from the molar concentration of the phosphoric acid solution in step S100. Preferably, the molar concentration of solution a is the same as the molar concentration of the phosphoric acid solution in step S100 (with an error of ±0.5 mol / L). In the present application, ensuring that the phosphoric acid concentration in solution a is the same as the phosphoric acid concentration of the phosphoric acid solution used in step S100 can make the acid environment in the system more stable, which is beneficial to the dissolution of lithium iron phosphate waste and provides a more stable environment for the precipitation and crystallization of iron phosphate. Preferably, the number of cycles is 5 to 20 times. In the present application, through multiple cycles, the lithium ion concentration in the lithium-containing filtrate can be increased, such as being saturated or nearly saturated, and then lithium extraction by concentration is carried out, and the lithium extraction efficiency will be higher. The present application can leach lithium iron phosphate waste in a cyclic manner, requiring less acid amount than traditional processes, being more environmentally friendly and having stronger operability, not consuming alkali solution, and realizing the full element recovery of Fe, Li, and P.

[0011] In a second aspect, the present application provides battery-grade iron phosphate prepared by any of the methods in the first aspect above.

[0012] In some embodiments, the iron-to-phosphorus ratio in the battery-grade iron phosphate is 0.97 to 1.0.

[0013] In a third aspect, the present application provides the application of the battery-grade iron phosphate prepared by any of the methods in the first aspect above in the preparation of a lithium iron phosphate cathode material, or the application of the battery-grade iron phosphate in the second aspect above in the preparation of a lithium iron phosphate cathode material.

[0014] The method of the present application uses a phosphoric acid solution to dissolve lithium iron phosphate waste in the initial stage to obtain a waste dissolution solution. The phosphoric acid solution can also provide a stable environment for the subsequent precipitation and crystallization of iron phosphate. Then, an acid and an oxidant are added to the obtained waste dissolution solution to obtain a stock solution for crystallization, so as to construct reaction conditions for reacting with lithium iron phosphate. After the stock solution for crystallization is diluted with water and heated for aging, battery-grade iron phosphate (with an iron-to-phosphorus ratio of 0.97 to 1.0) and a lithium-containing filtrate can be obtained. The present application further removes water from the lithium-containing filtrate and circulates it to the initial stage to dissolve lithium iron phosphate waste. After multiple reciprocating dissolution and precipitation, the Li concentration in the lithium-containing filtrate + is saturated with crystallization, and the lithium salt is precipitated by evaporation and concentration, realizing the efficient recovery of all elements of Li, Fe, and P. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] To more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the accompanying drawings required for the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only some embodiments of the present application. For those skilled in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0016] Figure 1 It is a process flow diagram of the method for recovering battery-grade iron phosphate and lithium salt from lithium iron phosphate waste in the present application;

[0017] Figure 2 It is the XRD test spectrum of the filter cake powder in Example 1 of the present application;

[0018] Figure 3 It is the XRD test spectrum of anhydrous FePO4 in Example 1 of the present application;

[0019] Figure 4 It is the XRD test spectrum of the product in Comparative Example 1 of the present application. Detailed implementation manners

[0020] In order to make the purpose, technical solutions and advantages of the present application more clear, the following further details the present application in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0021] Currently, the mature method applied in the recycling of lithium iron phosphate batteries is the acid leaching method, that is, strong acid and oxidant are used to leach Li element, Fe element, and P element in the LFP (lithium iron phosphate) cathode material into the solution, and then alkali solution is used to precipitate Fe element in the form of iron phosphate or iron hydroxide, and then sodium carbonate is added to the solution to recover Li element in the form of lithium carbonate. However, this recycling method has large consumption of acid and alkali and cannot be recycled, resulting in a large amount of waste water and waste salt emissions, and moreover, FePO4·2H2O with a complete crystal form cannot be obtained, and the iron-phosphorus ratio cannot meet the requirements. The present application proposes a method for recovering battery-grade iron phosphate and lithium salt from lithium iron phosphate waste to solve the above problems.

[0022] Method for Recycling Battery-Grade Iron Phosphate and Lithium Salt from Lithium Iron Phosphate Waste

[0023] See Figure 1 , which mainly includes the following steps:

[0024] S100. Obtain lithium iron phosphate waste, dissolve the lithium iron phosphate waste with phosphoric acid solution, and filter to obtain a waste dissolution solution;

[0025] S200. Add an acid solution and an oxidant to the waste dissolution solution to obtain a mother liquor to be crystallized;

[0026] S300. Dilute the original solution to be crystallized with water until its pH value is 1.0 to 1.5, perform high-temperature crystallization, and filter to obtain hydrated iron phosphate (iron phosphate dihydrate) and a lithium-containing filtrate;

[0027] S400. Bake the hydrated iron phosphate to obtain battery-grade iron phosphate;

[0028] S500. After evaporating the water from the lithium-containing filtrate, obtain solution a, return solution a to step S100 to continue dissolving the lithium iron phosphate waste, and then perform steps S200 to S300. After repeating multiple times, concentrate to obtain a lithium salt.

[0029] The order of step S400 and step S500 is not sequential.

[0030] In this application, first, the lithium iron phosphate waste is dissolved in a phosphoric acid solution to obtain a phosphoric acid system mixture (i.e., the waste dissolution solution). The role of the phosphoric acid solution is, on the one hand, to dissolve the lithium iron phosphate waste and can be recycled later, and on the other hand, it can also provide a stable environment for the crystallization of iron phosphate, which is conducive to obtaining battery-grade standard FePO4. Then, a small amount of acid is added to the above phosphoric acid system mixture, and at the same time, an oxidant is added to oxidize Fe 2+ , after dilution with water, high-temperature crystallization is carried out to precipitate iron phosphate, and by high-temperature baking it, battery-grade iron phosphate can be obtained. Moreover, after evaporating the excess water from the filtrate after the precipitation of iron phosphate, it can continue to recycle and dissolve the lithium iron phosphate waste. After such repeated cycles, the lithium concentration in the solution is close to saturation, and by evaporation and concentration, lithium is precipitated. This process can be recycled for leaching and realizes the efficient recovery of all elements of Li, Fe, and P. It can be seen that the process method of this application only initially consumes a certain amount of phosphoric acid, and the phosphoric acid is recycled later. Therefore, only a small amount of hydrochloric acid / sulfuric acid needs to be provided, which greatly reduces the dosage of acid reagents. Moreover, this process method can be recycled for leaching and crystallization, which more meets the requirements of industrial continuous production from the economic and environmental protection perspectives. And the entire leaching process does not consume lye, has strong recyclability, and the method is simple. It is worth noting that in this process, medium-strong acid phosphoric acid is used to dissolve lithium iron phosphate at the initial stage, and then other acids and oxidants are used to react with lithium iron phosphate. Acids with different acidity are used in two steps, and phosphoric acid is used as the environment for crystallizing and precipitating iron phosphate, and battery-grade standard FePO4 can be obtained, that is, the Fe:P ratio is 0.97 to 1.0:1.

[0031] Optionally, the original solution to be crystallized is diluted with water until its pH value is 1.0 to 1.3, and the pH value is 1.0, 1.2, 1.3, 1.5, or a value within the range composed of any two of these values.

[0032] In some embodiments, in step S100, the molar concentration of the phosphoric acid solution is 1 mol / L to 10 mol / L, the dissolution temperature is 20°C to 40°C, the solid-liquid ratio is 50 g / L to 200 g / L, and the dissolution time is 1 h to 5 h. Preferably, the molar concentration of the phosphoric acid solution is 2 mol / L to 10 mol / L, the dissolution temperature is 25°C to 35°C, the solid-liquid ratio is 80 g / L to 120 g / L, and the dissolution time is 3 h to 4 h. By regulating the molar concentration of the phosphoric acid solution within the above range, and at the same time, controlling the dissolution temperature, solid-liquid ratio, and solution time within the above ranges, on the one hand, it is beneficial to dissolve the lithium iron phosphate waste and achieve the full leaching of elements. By adding a small amount of other acids and oxidants, the efficient recovery of all elements of Li, Fe, and P can be achieved. On the other hand, the phosphoric acid solution can be recycled in this process method and no new cations are introduced, which is beneficial to obtaining battery-grade iron phosphate with a complete crystal form. Exemplarily, the molar concentration of the phosphoric acid solution is 1 mol / L, 2 mol / L, 3 mol / L, 4 mol / L, 5 mol / L, 6 mol / L, 7 mol / L, 8 mol / L, 9 mol / L, 10 mol / L or a value within the range composed of any two of these values. Exemplarily, the dissolution temperature is 20°C, 22°C, 25°C, 28°C, 30°C, 32°C, 35°C, 38°C, 40°C or a value within the range composed of any two of these values. Exemplarily, the solid-liquid ratio is 50 g / L, 60 g / L, 80 g / L, 90 g / L, 100 g / L, 110 g / L, 130 g / L, 150 g / L, 180 g / L, 200 g / L or a value within the range composed of any two of these values. Exemplarily, the dissolution time is 1 h, 1.5 h, 2 h, 2.5 h, 3 h, 3.5 h, 4 h, 4.5 h, 5 h or a value within the range composed of any two of these values.

[0033] In some embodiments, in step S200, the acid solution includes hydrochloric acid solution and / or sulfuric acid solution, the molar amount of hydrogen ions in the acid solution is 1 to 1.5 times the molar amount of the leached lithium iron phosphate, and the oxidant includes at least one of hydrogen peroxide, chlorine gas, ozone, or oxygen. The mass concentration of hydrogen peroxide is 25 wt% to 30 wt%. The acid solution is only used to provide anions to coordinate with Li + . By regulating the dosage of the acid solution within a suitable range, it is beneficial to its full reaction with the lithium iron phosphate waste and can also significantly reduce the dosage of the acid. Exemplarily, the molar amount of hydrogen ions in the acid solution is 1 times, 1.1 times, 1.2 times, 1.3 times, 1.4 times, or 1.5 times the molar amount of the leached lithium iron phosphate or a value within the range composed of any two of these values. Exemplarily, the mass concentration of hydrogen peroxide is 25 wt%, 26 wt%, 27 wt%, 28 wt%, 29 wt%, 30 wt% or a value within the range composed of any two of these values.

[0034] In some embodiments, when the oxidant is hydrogen peroxide, its dosage is 0.5 times to 2.0 times the molar amount of iron element in the waste dissolution solution, and the dosage is based on the molar dosage of H2O2 in hydrogen peroxide. When the oxidant is chlorine, its dosage is that the flow rate of chlorine introduced per 10 g of lithium iron phosphate waste is 0.03 L / min to 0.05 L / min, and the introduction time is 15 min to 20 min. The role of the oxidant is to oxidize Fe 2+ to Fe 3+ , and by regulating the addition amounts of different types of oxidants within the above ranges, it is beneficial for their full reaction with the lithium iron phosphate waste. Exemplarily, the dosage of hydrogen peroxide is 0.5 times, 0.8 times, 1.0 times, 1.2 times, 1.5 times, 1.8 times, 2.0 times the molar amount of iron element in the waste dissolution solution or values within the range composed of any two of these values. Exemplarily, the flow rate of chlorine is 0.03 L / min, 0.032 L / min, 0.035 L / min, 0.038 L / min, 0.04 L / min, 0.045 L / min, 0.048 L / min, 0.05 L / min or values within the range composed of any two of these values. Exemplarily, the introduction time is 15 min, 16 min, 17 min, 18 min, 19 min, 20 min or values within the range composed of any two of these values.

[0035] In some embodiments, in step S300, calculated by volume, the amount of water added during the dilution process is 1 time to 6 times the amount of phosphoric acid solution used in step S100. Preferably, the amount of water added during the dilution process is 2 times to 5 times the amount of phosphoric acid solution used in step S100. Exemplarily, the amount of water added is 1 time, 2 times, 3 times, 4 times, 5 times, 6 times the amount of phosphoric acid solution or values within the range composed of any two of these values.

[0036] In some embodiments, in step S300, the conditions for high-temperature crystallization are: reaction temperature: 80 °C to 110 °C, aging time: 2 h to 6 h. By regulating the temperature and aging time of high-temperature crystallization within the above ranges, it is beneficial to ensure that when iron phosphate precipitates subsequently, a product with a complete crystal form is obtained. Exemplarily, the reaction temperature for high-temperature crystallization is 80 °C, 85 °C, 90 °C, 95 °C, 100 °C, 105 °C, 110 °C or values within the range composed of any two of these values. The aging time is 2 h, 3 h, 4 h, 5 h, 6 h or values within the range composed of any two of these values, where the aging time includes the crystallization time required for the precipitation process and the time to maintain the temperature for the crystal grains to form.

[0037] In some embodiments, in step S400, the calcination conditions are as follows: calcination temperature: 300 °C to 700 °C, calcination time: 30 min to 120 min. Exemplarily, the calcination temperature is 300 °C, 350 °C, 400 °C, 450 °C, 500 °C, 550 °C, 600 °C, 650 °C, 700 °C or a value within the range formed by any two of these values. Exemplarily, the calcination time is 30 min, 40 min, 50 min, 60 min, 80 min, 100 min, 120 min or a value within the range formed by any two of these values.

[0038] In some embodiments, in step S500, the lithium-containing filtrate is evaporated to remove water to obtain solution a, and the molar concentration of solution a is 1 mol / L to 10 mol / L. The molar concentration of solution a is based on the molar amount of H3PO4 in solution a. Exemplarily, the molar concentration of solution a is 1 mol / L, 2 mol / L, 3 mol / L, 4 mol / L, 5 mol / L, 6 mol / L, 7 mol / L, 8 mol / L, 9 mol / L, 10 mol / L or a value within the range formed by any two of these values.

[0039] In some embodiments, in step S500, the number of cycles is 5 to 20 times. Exemplarily, the number of cycles is 5 times, 6 times, 8 times, 10 times, 12 times, 13 times, 15 times, 18 times, 20 times or a value within the range formed by any two of these values.

[0040] The following introduces possible implementation manners

[0041] Reaction equation:

[0042] 2LiFePO4 + 2HCl + H2O2 + H3PO4 → 2FePO4 + 2H2O + 2LiCl + H3PO4

[0043] 2Fe2+ + H2O2 + 2H+ = 2Fe3+ + 2H2O

[0044] This patent uses phosphoric acid to dissolve lithium iron phosphate waste. The concentration of phosphoric acid is 1 - 10 mol / L, the temperature for dissolving lithium iron phosphate is 20 - 40 °C, the solid-liquid ratio is 50 - 200 g / L, the leaching time is 1 - 5 h. After acid leaching is completed, filtration is carried out, and a small amount of hydrochloric acid or sulfuric acid is added. The amount of acid added in this process only provides the coordination effect of anions with Li. Then an oxidant is added to the solution. The oxidant is one of hydrogen peroxide, chlorine, ozone, oxygen, etc., and the purpose is to oxidize Fe 2+ to Fe 3+ , and at this time the solution contains Fe 3+ , PO4 3- , Li + , Cl- / SO4 2- 。The obtained leaching solution is diluted 1 - 6 times (dosage of phosphoric acid) with deionized water, the pH is adjusted to 1.0 - 1.5, and then, taking advantage of the physical property that the solubility of iron phosphate decreases with increasing temperature, the obtained diluted leaching solution is heated and stirred at a high temperature of 80 - 110°C for an aging time of 2 - 6 h. Crystalline iron phosphate precipitates out, and the obtained iron phosphate dihydrate is filtered; the iron phosphate dihydrate is calcined at a high temperature to obtain battery-grade iron phosphate, with a calcination temperature of 300 - 700°C and a calcination time of 30 - 120 min. The lithium-containing acid solution is evaporated to remove water to reach the initial phosphoric acid concentration, and then recycled to leach lithium iron phosphate (no acid addition is required in this process, and the initial phosphoric acid is used to dissolve lithium iron phosphate). The above cycle is repeated 5 - 20 times, and the lithium concentration in the solution gradually crystallizes. Lithium chloride is recovered by high-temperature crystallization. Compared with the traditional recovery of lithium in the form of lithium carbonate, the introduction of sodium carbonate material is saved, and it is more energy-saving and environmentally friendly.

[0045] The principle of dissolving lithium iron phosphate and crystalline iron phosphate using a phosphoric acid system in this patent is as follows:

[0046] Phosphoric acid is a moderately strong acid, which can more easily regulate the pH. At the same time, the characteristics of the three-stage ionization of phosphoric acid are beneficial to the crystallization of iron phosphate.

[0047] The three-stage ionization equation of phosphoric acid:

[0048]

[0049]

[0050]

[0051] Phosphoric acid can ionize H2PO4 - 、HPO4 2- 、PO4 3- ,while the dissolved iron ions exist in the form of Fe 3+ after oxidation. Fe 3+ can form complexes with phosphoric acid H3PO4 and phosphate ions H2PO4 - 、HPO4 2- 、PO4 3- to form Fe(H2PO4)3. The equations are shown in (4)(5)(6). Utilizing the instability of Fe(H2PO4)3, increasing the pH of the solution and then heating it at a high temperature can convert it into FePO4 and phosphoric acid. The equation is shown in (7). Therefore, by using phosphoric acid as a solvent during the precipitation and crystallization process of iron phosphate, FePO4 with a Fe:P ratio meeting the requirements (Fe:P ratio is 0.97 - 1.0:1) can be obtained, and the obtained FePO4 has a good phase structure and high purity. Using water to regulate the increase in pH not only promotes the ionization of phosphoric acid, generating more H2PO4 - 、HPO42- , which helps in the formation of iron phosphate. More notably, H2O does not introduce new cationic impurities (such as ammonia water, which would introduce NH4 + ), avoiding the possible reaction of cationic impurities with phosphate ions H2PO4 - 、HPO4 2- 、PO4 3- . This is conducive to the formation of the complex Fe(H2PO4)3, resulting in a product that meets the iron-phosphorus ratio requirements. Using the phosphoric acid system to crystallize out iron phosphate, phosphoric acid is only used as an intermediate and does not consume phosphoric acid, realizing the stable crystallization and recovery of iron phosphate in the phosphoric acid system.

[0052] Fe 3+ +2H3PO4+H2PO4 - →Fe(H2PO4)3+2H + (4)

[0053] Fe 3+ +2H3PO4+HPO4 2- →Fe(H2PO4)3+H + (5)

[0054] Fe 3+ +2H3PO4+PO4 3- →Fe(H2PO4)3 (6)

[0055]

[0056] The difference between the process method of this application and the traditional process lies in: First, this process uses phosphoric acid as the leaching solvent, only adding phosphoric acid in the initial stage, and no additional supplementation is required in subsequent cycles, greatly reducing the consumption of acid solvent. At the same time, using phosphoric acid as the solvent can provide a stable environment for the precipitation and crystallization of iron phosphate. Second, diluting with water in the phosphoric acid system, heating to crystallize iron phosphate does not require the use of lye and can be recycled for leaching, improving economic benefits. In addition, it is also conducive to obtaining iron phosphate products with an iron-phosphorus ratio meeting the requirements and a complete crystal form.

[0057] Hereinafter, examples and comparative examples are given to illustrate the implementation mode of this application more specifically. Unless otherwise stated, the parts, percentages, and ratios listed are based on mass.

[0058] The lithium iron phosphate waste materials in the following examples and comparative examples include unqualified lithium iron phosphate and discarded lithium iron phosphate batteries after being phased out. The lithium iron phosphate obtained by disassembling unqualified lithium iron phosphate and / or discarded lithium iron phosphate batteries is simply calcined in an inert atmosphere to obtain lithium iron phosphate waste materials free of carbon and organic substances.

[0059] Example 1

[0060] 10 g of lithium iron phosphate waste was placed in 100 mL of phosphoric acid solution with a concentration of 6 mol / L, and continuously stirred and dissolved at 25 °C for 4 h. The remaining residue was removed by filtration, and the leaching rate of lithium iron phosphate reached 86.2%, obtaining a solution containing Fe 2+ 、PO4 3- solution. 6.4 mL of 10 mol / L hydrochloric acid was added to the filtrate, and at the same time, 6.8 mL of 30% hydrogen peroxide was added. Slowly stirred for 10 min to oxidize Fe 2+ to Fe 3 + . Then, 400 mL of water was added to the filtrate to adjust the pH to 1.0, and it was placed in a water bath at 90 °C for high-temperature crystallization. After aging for 4 h, it was filtered to obtain a FePO4·2H2O filter cake and a filtrate containing LiCl. The filtrate was evaporated to remove excess water and recycled to the acid leaching section to continue leaching lithium iron phosphate. The subsequent recycling process and parameters were the same as the first time. After 20 times of cyclic leaching, the leaching rate of lithium iron phosphate still reached 85.6%. The filtrate containing LiCl was subjected to evaporation crystallization. Specifically, the water was heated and evaporated at 100 °C. Since LiCl was close to saturation, lithium chloride crystallized out as the water evaporated, and LiCl solid could be obtained by filtration. The purity of LiCl obtained by evaporation crystallization was 99.0%. The filter cake powder was tested by XRD to determine its composition. XRD is shown in Figure 2 . The obtained white filter cake was placed in a high-temperature furnace and heated at 400 °C for 120 min to remove the crystal water, obtaining anhydrous FePO4 with a complete crystal form. XRD is shown in Figure 3 , and the element content of anhydrous FePO4 was tested to determine the main element and impurity content, as shown in Table 1. It was calculated that Fe:P = 0.99 and the impurity content was less than 50 ppm, meeting the battery-grade FePO4 standard (the battery-grade standard Fe:P ratio is 0.97 - 1.0:1).

[0061] Table 1

[0062] Element Fe P Al Ca Cu Mg Na Content / ppm 370827.7 208057.1 10.7 45.3 43.3 39.7 36.0

[0063] Example 2

[0064] 10 g of lithium iron phosphate waste was placed in 100 mL of phosphoric acid solution with a concentration of 10 mol / L, and continuously stirred and dissolved at 25 °C for 4 h. The remaining residue was removed by filtration, and the leaching rate of lithium iron phosphate reached 88.1%, obtaining a solution containing Fe 2+ 、PO4 3- solution. 12.8 mL of 5 mol / L hydrochloric acid was added to the filtrate, and chlorine gas was passed into the filtrate. The chlorine gas flow rate was 0.05 L / min, and the time was 20 min to oxidize Fe 2+ to Fe 3+; Then, 500 mL of water was added to the filtrate to adjust the pH to 1.2, and it was placed in a water bath at 90 °C for high-temperature crystallization. After aging for 4 h, it was filtered to obtain a FePO4·2H2O filter cake and a filtrate containing LiCl. The filtrate was evaporated to remove excess water and recycled to the acid leaching section to continue leaching lithium iron phosphate. The subsequent recycling process and parameters were the same as those in the first time. After twenty times of cyclic leaching, the leaching rate of lithium iron phosphate still reached 86.7%, and LiCl with a purity of 98.9% was obtained by evaporation crystallization.

[0065] Example 3

[0066] 10 g of lithium iron phosphate waste was placed in 100 mL of phosphoric acid solution with a concentration of 6 mol / L, and continuously stirred and dissolved at 25 °C for 4 h. The remaining residue was filtered off, and the leaching rate of lithium iron phosphate reached 86.2%, obtaining a solution containing Fe 2+ , PO4 3- solution. 3.2 mL of 10 mol / L sulfuric acid and 6.8 mL of 30% hydrogen peroxide were added to the filtrate, and slowly stirred for 10 min to oxidize Fe 2+ to Fe 3 + ; Then, 400 mL of water was added to the filtrate to adjust the pH to 1.0, and it was placed in a water bath at 90 °C for high-temperature crystallization. After aging for 4 h, it was filtered to obtain a FePO4·2H2O filter cake and a filtrate containing Li2SO4. The filtrate was evaporated to remove excess water and recycled to the acid leaching section to continue leaching lithium iron phosphate. The subsequent recycling process and parameters were the same as those in the first time. After twenty times of cyclic leaching, the leaching rate of lithium iron phosphate still reached 85.2%, and Li2SO4 with a purity of 99.1% was obtained by evaporation crystallization.

[0067] Examples 4 to 5

[0068] Except for adjusting the parameters according to Table 2, the rest was the same as in Example 1.

[0069] Comparative Example 1

[0070] <Sulfuric acid + a large amount of water>

[0071] 10 g of lithium iron phosphate waste was placed in 100 mL of sulfuric acid solution with a concentration of 6 mol / L, and continuously stirred and dissolved at 25 °C for 4 h. The remaining residue was filtered off, and the leaching rate of lithium iron phosphate reached 85.3%, obtaining a solution containing Fe 2+ , PO4 3- solution. 6.8 mL of 30% hydrogen peroxide was added to the filtrate, and slowly stirred for 10 min to oxidize Fe 2+ to Fe 3+。Then, 900 mL of water was added to the filtrate to adjust the pH to 1.0, and it was placed in a water bath at 90 °C for high-temperature crystallization. After aging for 4 h, it was filtered to obtain a white product without crystal form. XRD is shown in Figure 4 。

[0072] Comparing Example 1 with Comparative Example 1, it can be seen that when using the sulfuric acid system, a large amount of water needs to be added subsequently to adjust the pH, which affects the crystal form integrity of FePO4·2H2O. Using the sulfuric acid system, FePO4·2H2O with complete crystal form cannot be obtained, and efficient cyclic leaching cannot be achieved.

[0073] Comparative Example 2

[0074] <Phosphoric acid + alkali>

[0075] 10 g of lithium iron phosphate waste was placed in 100 mL of phosphoric acid solution with a concentration of 6 mol / L and continuously stirred and dissolved at 25 °C for 4 h. The remaining residue was filtered off, and the leaching rate of lithium iron phosphate reached 86.2%. A solution containing Fe 2+ , PO4 3- was obtained. 3.2 mL of sulfuric acid with a concentration of 10 mol / L and 6.8 mL of 30% hydrogen peroxide were added to the filtrate, and it was slowly stirred for 10 min to oxidize Fe 2+ to Fe 3 + . Then, ammonia water was added to the filtrate to adjust the pH to 1.0, and it was placed in a water bath at 90 °C for high-temperature crystallization. After aging for 4 h, it was filtered to obtain a white filter cake. The obtained white filter cake was placed in a high-temperature furnace and heated at 400 °C for 120 min to remove the crystal water, and anhydrous FePO4 was obtained. It was calculated that Fe:P = 1.12:1.

[0076] Comparing Example 3 with Comparative Example 2, it can be seen that when using the phosphoric acid system + alkali (ammonia water), the Fe:P of the obtained FePO4 does not meet the requirements, and the addition of the alkali solution (ammonia water) causes the phosphoric acid system to be unable to be recycled for leaching.

[0077] Table 2

[0078]

[0079]

[0080] Combined with Table 2 and Figures 2 to 4 , it can be seen that the present application can recycle and leach lithium iron phosphate waste, and recover high-cost-performance products (battery-grade iron phosphate, the standard Fe:P ratio of battery-grade FePO4 is 0.97 - 1.0:1 and high-purity lithium chloride / lithium sulfate). The amount of acid required is less than that of the traditional process, it is more environmentally friendly and has stronger operability, and it does not consume alkali solution, and realizes the full-element recovery of Fe, Li, and P.

[0081] The above are only the preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present application shall be included within the protection scope of the present application.

Claims

1. A method for recovering battery-grade iron phosphate and lithium salts from lithium iron phosphate waste, characterized in that, It includes the following steps: S100. Obtain lithium iron phosphate waste, dissolve the lithium iron phosphate waste with a phosphoric acid solution, and filter to obtain a waste dissolution solution; S200. Add an acid solution and an oxidant to the waste dissolution solution to obtain a mother liquor for crystallization, wherein the acid solution includes a hydrochloric acid solution and / or a sulfuric acid solution; S300. Dilute the mother liquor for crystallization with water until its pH value is 1.0 to 1.5, perform high-temperature crystallization, and filter to obtain hydrated iron phosphate and a lithium-containing filtrate, wherein the reaction temperature of the high-temperature crystallization is 80°C to 110°C; S400. The hydrated iron phosphate is calcined to obtain battery-grade iron phosphate; S500. After the lithium-containing filtrate is evaporated to remove water, a solution a is obtained. The solution a is returned to step S100 to continue dissolving lithium iron phosphate waste, and then steps S200 to S300 are performed. After multiple cycles, it is concentrated to obtain a lithium salt; The order of step S400 and step S500 is not sequential.

2. The method according to claim 1, wherein In step S100, at least one of the following conditions is satisfied: Condition I. The molar concentration of the phosphoric acid solution is 1 mol / L to 10 mol / L; Condition II. The dissolution includes: dissolution temperature of 20°C to 40°C, solid-liquid ratio of 50 g / L to 200 g / L, and dissolution time of 1 h to 5 h.

3. The method according to claim 1, wherein In step S100, at least one of the following conditions is satisfied: Condition I. The molar concentration of the phosphoric acid solution is 2 mol / L to 10 mol / L; Condition II. Dissolution temperature of 25°C to 35°C, solid-liquid ratio of 80 g / L to 120 g / L, and dissolution time of 3 h to 4 h.

4. The method according to any one of claims 1 to 3, characterized in that, In step S200, the molar amount of hydrogen ions in the acid solution is 1 to 1.5 times the molar amount of lithium iron phosphate leached by dissolution; The oxidant includes at least one of hydrogen peroxide, chlorine, ozone or oxygen, and the mass concentration of the hydrogen peroxide is 25 wt% to 30 wt%.

5. The method according to claim 4, wherein When the oxidant is hydrogen peroxide, its dosage is 0.5 times to 2.0 times the molar amount of iron element in the waste dissolution solution, and the dosage is based on the molar dosage of H2O2 in hydrogen peroxide; When the oxidant is chlorine, its dosage is that the flow rate of chlorine introduced per 10 g of lithium iron phosphate waste is 0.03 L / min to 0.05 L / min, and the introduction time is 15 min to 20 min.

6. The method according to any one of claims 1 to 3, characterized in that In step S300, at least one of the following conditions is satisfied: Condition A. Calculated by volume, the amount of water added during the dilution with water is 1 time to 6 times the dosage of the phosphoric acid solution in step S100; Condition B. The conditions for the high-temperature crystallization are: Aging time: 2 h to 6 h.

7. The method according to any one of claims 1 to 3, characterized in that, In step S400, the conditions for the calcination are: Calcination temperature: 300°C to 700°C; Calcination time: 30 min to 120 min.

8. The method according to any one of claims 1 to 3, characterized in that, In step S500, the molar concentration of the solution a is 1 mol / L to 10 mol / L, and the molar concentration of the solution a is based on the molar amount of H3PO4 in the solution a.

9. The method according to any one of claims 1 to 3, characterized in that, The molar concentration of the solution a is the same as the molar concentration of the phosphoric acid solution in step S100.

10. The method according to any one of claims 1 to 3, characterized in that, The number of times of the cycle is 5 to 20 times.

11. Lithium iron phosphate for battery use prepared by the method according to any one of claims 1 to 10.

12. The lithium iron phosphate for battery according to claim 11, wherein The iron-to-phosphorus ratio in the lithium iron phosphate for battery use is 0.97 to 1.

0.

13. Use of the lithium iron phosphate for battery use prepared by the method according to any one of claims 1 to 10 in the preparation of a lithium iron phosphate cathode material; or, Use of the lithium iron phosphate for battery use according to claim 12 in the preparation of a lithium iron phosphate cathode material.

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