A method for recovering iron phosphate compounds from spent lithium iron phosphate batteries

Abstract

This application provides a method for recovering iron-phosphorus compounds from spent lithium iron phosphate batteries. The method includes: acid leaching, copper removal, and aluminum removal of spent lithium iron phosphate battery powder; adjusting the iron-phosphorus ratio of the obtained copper-aluminum removal solution; adjusting the pH value to 0.4-1.2 in the first round; adding an oxidant; performing solid-liquid separation; adjusting the pH value of the filtrate to 1.5-2.0 in the second round; continuously adding an oxidant; and after solid-liquid separation, obtaining iron-phosphorus slag and the final filtrate; combining the iron-phosphorus slag; washing and drying to obtain iron-phosphorus compounds. The method provided in this application can improve the purity and recovery rate of iron-phosphorus compounds by controlling the pH value of the copper-aluminum removal solution. At low pH values, the aluminum content in the precipitated iron-phosphorus compounds is extremely low. By subsequently increasing the pH value, the low phosphorus concentration also prevents the precipitation of aluminum phosphate, thus ensuring both high purity and high yield of the iron-phosphorus compounds.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of battery material preparation, and particularly relates to a method for recovering phosphorus-iron compounds from waste lithium iron phosphate batteries. BACKGROUND

[0002] The lithium iron phosphate battery has been widely applied in the new energy field due to high specific capacity, stable structure, safe performance, long service life and many other characteristics. However, the service life of the lithium ion battery is generally 3-5 years. With the passage of time, more and more lithium ion batteries will be retired and scrapped. According to relevant agencies, the subsequent retired amount will continue to increase. How to properly handle these retired and scrapped batteries has become a hot issue at present.

[0003] The mainstream treatment method for the waste lithium iron phosphate battery is to selectively leach or fully dissolve valuable elements such as lithium and copper in the battery by the hydrometallurgical method, and then recover the elements step by step. Since the lithium iron phosphate positive electrode powder in the waste lithium iron phosphate battery is attached to the aluminum foil, the lithium iron phosphate positive electrode powder or the battery powder will inevitably contain a high aluminum content during the leaching of the valuable elements, so that the leaching solution contains a high concentration of aluminum, and the aluminum concentration in the subsequently prepared liquid-phase method is too high.

[0004] In current practice, the aluminum removal process can be divided into pre-treatment or post-treatment. The pre-treatment method mainly uses an alkaline solution such as sodium hydroxide to soak the battery powder before acid leaching, and removes aluminum by taking advantage of the solubility of sodium meta-aluminate in strong alkaline solution. However, the prepared sodium meta-aluminate solution is difficult to handle and needs to be added with acid to prepare aluminum hydroxide in the subsequent process. The production process is complex and the solution volume is large. Moreover, the aluminum removal effect is not ideal due to the adsorption of aluminum in the solution by carbon in the general lithium iron phosphate powder. The post-treatment method generally removes aluminum in the leaching solution, such as resin aluminum removal or precipitation method. For example, CN113800494A discloses a method for selectively recovering aluminum from waste lithium iron phosphate battery material acid leaching solution, which uses phosphonic acid-based cation chelating resin to selectively adsorb and separate aluminum in waste lithium iron phosphate battery material acid leaching solution. Although this method can selectively extract aluminum, the amount of cation chelating resin used is large, and new impurity ions are introduced. The amount of acid and alkali used for adsorption and desorption is extremely large, which greatly increases the production cost and cannot be applied to industrial production. In addition, there are also organic complex precipitation methods for removing aluminum. For example, CN113880063A discloses a method for removing aluminum from waste lithium iron phosphate lithium extraction phosphorus iron slag, which selects pyridine carboxylic acid compounds, quinoline carboxylic acid compounds and isoquinoline-3-carboxylic acid compounds as aluminum removal agents. Although these organic aluminum removal agents effectively remove aluminum impurities, the aluminum removal agents cannot be recycled, and the generated organic complex precipitate can only be treated as waste slag, greatly reducing the economic value. There are also methods for removing aluminum by neutralization and precipitation. For example, CN116477591A discloses a method for comprehensive utilization of waste lithium iron phosphate positive material, which can efficiently separate and recover copper, iron, lithium and other useful metal elements in waste lithium iron phosphate positive material by combining two-stage leaching, copper removal by reduction, aluminum removal by neutralization and precipitation, iron precipitation by oxidation, and lithium precipitation by carbonate. The method uses a one-step addition of a neutralizing agent to adjust the pH value to remove aluminum. However, this aluminum removal method has limited effect, and the obtained aluminum-removed solution still contains a certain amount of aluminum. In the subsequent iron precipitation by oxidation step, which is usually carried out under acidic conditions, iron and aluminum co-precipitation easily occurs due to the close ion product constant of iron phosphate and aluminum phosphate (the Ksp of iron phosphate is about 1.3*10 -22 , and the Ksp of aluminum phosphate is about 9.84*10 -21 ). Therefore, in the process of preparing iron phosphate by single pH adjustment and iron precipitation by oxidation, iron and aluminum co-precipitation easily occurs, which is the main reason for the high aluminum content in the prepared iron phosphate product from waste lithium iron phosphate batteries.

[0005] Although there are various methods for removing aluminum from waste battery powder at present, all of them have various defects, and the aluminum in the general solution is difficult to clean up, which makes the aluminum content in the subsequently prepared iron phosphate not meet the market demand. Therefore, it is of great significance to seek a method that can solve the problem of excessive aluminum content in iron phosphate and stably produce iron phosphate. SUMMARY

[0006] In a first aspect, the embodiments of the present application provide a method for recovering a phosphorus-iron compound from a waste lithium iron phosphate battery, which comprises:

[0007] S1: acid leaching the waste lithium iron phosphate battery powder, and obtaining a leaching solution after solid-liquid separation;

[0008] S2: removing copper and aluminum from the leaching solution, and obtaining a copper and aluminum removal solution after solid-liquid separation;

[0009] S3: adjusting the iron-phosphorus ratio of the copper and aluminum removal solution, adjusting the pH value to 0.4-1.2 in the first round, adding an oxidizing agent, and obtaining phosphorus-iron residue and filtrate after solid-liquid separation; adjusting the pH value to 1.5-2.0 in the second round for the filtrate, continuously adding the oxidizing agent, and obtaining phosphorus-iron residue and final filtrate after solid-liquid separation; and combining the phosphorus-iron residue, and obtaining a phosphorus-iron compound after washing and drying. In this step, the adjustment of the pH value and the addition of the oxidizing agent in the first round can make most of the phosphorus and iron precipitate, so that the content of phosphorus in the solution is greatly reduced, and in the process of adjusting the pH value in the second round, the precipitation of aluminum is also greatly reduced due to the extremely low content of phosphorus, so that the content of aluminum in the obtained phosphorus-iron compound is extremely low.

[0010] In an optional embodiment of the present application, the iron-phosphorus ratio in step S3 is (0.96-1.02):1.

[0011] In an optional embodiment of the present application, the adjustment of the pH value in step S3 is stepwise adjustment according to the order from low to high.

[0012] In an optional embodiment of the present application, the adjusting agent for the first round of pH value adjustment in step S3 is selected from at least one of sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid; and the adjusting agent for the second round of pH value adjustment is selected from at least one of sodium hydroxide, ammonia, lithium phosphate, lithium carbonate, sodium carbonate, calcium hydroxide, or calcium oxide.

[0013] In an optional embodiment of the present application, the oxidizing agent is continuously added in step S3.

[0014] In an optional embodiment of the present application, iron powder is used for copper removal in step S2.

[0015] In an optional embodiment of the application, the aluminum removal in step S2 is performed by neutralization precipitation, wherein the neutralization precipitation is performed under the condition that the pH value is adjusted to 2.5-3.5 with an excess of the reducing agent.

[0016] In an optional embodiment of the application, the reducing agent is iron powder.

[0017] In an optional embodiment of the application, the reducing agent further comprises at least one of sodium sulfide and sodium persulfate.

[0018] In an optional embodiment of the application, the iron content of the final filtrate in step S3 is less than 1 g / L.

[0019] In an optional embodiment of the application, the method for recovering phosphorus-iron compounds from waste lithium iron phosphate batteries further comprises step S4: preparing slurry by using the phosphorus-iron compounds, and adjusting the pH value of the slurry to not less than 2.3, and obtaining hydrated ferric phosphate by washing and drying the obtained precipitate.

[0020] In a second aspect, the disclosure provides a circulating oxidation precipitation system for implementing step S3 of the method of the first aspect, comprising an oxidation iron precipitation tank, a solid-liquid separation device, and a solid residue washing tank, wherein the oxidation iron precipitation tank is provided with a circulating oxidation system; the liquid inlet of the solid-liquid separation device is communicated with the lower part of the oxidation iron precipitation tank, the liquid outlet of the solid-liquid separation device is communicated with the top of the oxidation iron precipitation tank, and the residue outlet of the solid-liquid separation device is communicated with the solid residue washing tank.

[0021] In an optional embodiment of the application, the circulating oxidation system comprises a circulating pump and a Venturi tube, wherein the liquid inlet of the circulating pump is communicated with the lower part of the oxidation iron precipitation tank, the liquid outlet of the circulating pump is communicated with the injection port of the Venturi tube, and the liquid outlet of the Venturi tube is communicated with the top of the oxidation iron precipitation tank.

[0022] In a third aspect, the disclosure provides hydrated ferric phosphate obtained by the method of the first aspect.

[0023] In an optional embodiment of the application, the aluminum content in the hydrated ferric phosphate is ≤58 ppm.

[0024] In a fourth aspect, the disclosure provides the use of the method of the first aspect in treating waste batteries.

[0025] Beneficial effects:

[0026] The inventors of the present application find that when ferrous iron is oxidized at a lower pH value, ferric ions will precipitate in preference to aluminum ions, so that a phosphorus-iron compound precipitate with very low aluminum content (less than 100 ppm) can be obtained, wherein the aluminum content meets the requirements of battery-grade iron phosphate; however, at a lower pH value (especially less than 1.2), iron phosphate still has considerable solubility, so that it is difficult for iron phosphate to precipitate completely, which will directly affect the recovery rate of iron phosphate, therefore, in order to greatly reduce the aluminum content in the phosphorus-iron compound and maximize the recovery of the phosphorus-iron compound, the present application adjusts the pH in stages to carry out iron precipitation, that is, the pH value is first adjusted to a lower range (0.4-1.2) to carry out the first round of iron precipitation, at this time, iron precipitates in preference to aluminum, obtaining a high-purity phosphorus-iron compound precipitate (i.e., phosphorus-iron slag, which may contain iron phosphate, monohydrogen iron phosphate, dihydrogen iron phosphate, etc.), in this process, a large amount of phosphate ions react with iron ions and are precipitated, and the concentration of the remaining phosphate ions in the solution is greatly reduced; then the pH value is adjusted to a relatively high range (1.5-2.0) to carry out the second round of iron precipitation, at this pH value range, the solubility of iron phosphate is very small, and almost all the remaining iron ions will precipitate in the form of iron phosphate to form a new phosphorus-iron compound (mainly composed of iron phosphate), and at this stage, since the concentration of phosphate ions has been reduced to a very low level, the co-precipitation of aluminum is very weak, thereby ensuring the maximum recovery of the phosphorus-iron compound under the premise of very little aluminum precipitation.

[0027] In order to achieve better iron precipitation effect, the first round of pH adjustment and the second round of pH adjustment in the present application can be in the form of step-by-step adjustment, that is, the pH value is first adjusted to a specific pH value within the preset range, and after the iron concentration in the solution is stabilized, solid-liquid separation is carried out, the filtrate enters the next step of pH adjustment, and this cycle continues until the current round of adjustment is completed, and the specific number of steps can be determined according to the actual situation, in this way, fine classification precipitation of iron ions is realized, and each solid-liquid separation can effectively promote the precipitation reaction in the next step to be more complete, thereby maximizing the iron precipitation effect of each round of pH adjustment, which is beneficial to further improving the recovery rate of the phosphorus-iron compound, and specifically, the iron content in the final filtrate after all pH adjustments can be less than 1 g / L.

[0028] The phosphorus-iron compound obtained by the method provided by the present application contains very low aluminum impurities and has high purity, which can fully meet the requirements of aluminum content in battery-grade iron phosphate. BRIEF DESCRIPTION OF DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings needed in the embodiment description. Obviously, the drawings in the following description only show some of the embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort based on these drawings.

[0030] In order to more completely understand the present application and its beneficial effects, the following will be described in conjunction with the drawings, wherein the same reference numerals in the following description represent the same parts.

[0031] Figure 1 The structure diagram of the iron removal device in the system for preparing phosphorus iron compound provided by the embodiments of the present application is shown. DETAILED DESCRIPTION

[0032] The technical solutions in the embodiments of the present application will be described clearly and completely in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, not all. The following description of at least one exemplary embodiment is actually only illustrative, but not as any limitation on the present application and its application or use. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without any creative effort are within the protection scope of the present application.

[0033] In this document, the term "embodiment" or "implementation" means that the specific features, structures or characteristics described in conjunction with the embodiment or implementation can be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily mean the same embodiment, nor is it an independent or alternative embodiment to other embodiments. Those skilled in the art explicitly and implicitly understand that the embodiments described herein can be combined with other embodiments.

[0034] For the aluminum removal process in the treatment process of waste old lithium iron phosphate battery, the pretreatment or post-treatment method is generally used. If the pretreatment method is used, a large amount of sodium metaaluminate solution will be produced, and due to the presence of carbon powder, the aluminum removal effect is not ideal. If the post-treatment method is used, the production cost will be increased, or new impurities will be introduced. At present, although the above-mentioned shortcomings can be overcome to some extent, the present application finds that since the solubility product of iron phosphate and aluminum phosphate is close, they are easy to form coprecipitation, therefore, there is still no better solution for how to avoid introducing aluminum impurities into iron phosphate and still ensure the yield of iron phosphate.

[0035] In the method for preparing iron phosphate compounds from waste lithium iron phosphate batteries provided in this application, only during the iron precipitation step, the pH value of the solution is adjusted to keep the phosphorus in the solution at a low pH or low concentration, which avoids iron-aluminum co-precipitation, ensures that the final iron phosphate compound can meet the requirements of battery-grade iron phosphate, and the yield of iron phosphate compound is high, avoiding increased costs.

[0036] This application provides a method for recovering ferrophosphate compounds from spent lithium iron phosphate batteries, the method comprising:

[0037] S1: The waste lithium iron phosphate battery powder is acid-leached, and the leachate is obtained after solid-liquid separation.

[0038] S2: The leachate is subjected to copper and aluminum removal treatment, and a copper-aluminum removal solution is obtained after solid-liquid separation;

[0039] S3: Adjust the iron-phosphorus ratio of the copper-aluminum removal solution. In the first round of adjustment, the pH value is adjusted to 0.4-1.2. An oxidant is added, and after solid-liquid separation, phosphorus-iron slag and filtrate are obtained. In the second round of adjustment, the pH value of the filtrate is adjusted to 1.5-2.0. An oxidant is continuously added, and after solid-liquid separation, phosphorus-iron slag and final filtrate are obtained. The phosphorus-iron slag is combined, washed, and dried to obtain phosphorus-iron compound.

[0040] The pH value of 0.4-1.2 can be 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, etc., and the pH value of 1.5-2 can be 1.6, 1.7, 1.8, 1.9, etc.

[0041] This application provides a method for preparing an iron-phosphorus compound, specifically a method for recovering and preparing an iron-phosphorus compound that meets the requirements for battery-grade iron-phosphorus phosphate from spent lithium iron phosphate batteries. In the method provided in this application, the spent lithium iron phosphate battery powder is subjected to acid leaching, copper removal, aluminum removal, and iron precipitation to obtain an iron-phosphorus compound that meets the requirements for battery-grade iron-phosphorus phosphate. In the iron precipitation step, the pH of the solution is first adjusted to a low pH environment. At a low pH, ferrous ions are oxidized and precipitated, resulting in an extremely low aluminum content in the obtained iron-phosphorus compound. Then, the pH is increased, resulting in a low phosphate concentration, which also avoids the formation of aluminum phosphate precipitation. Therefore, the method provided in this application can largely avoid aluminum-iron co-precipitation, yielding battery-grade iron-phosphorus phosphate with extremely high purity. Furthermore, the method provided in this application has a simple preparation process and a high yield of iron-phosphorus compound, showing promising application prospects.

[0042] For step S1: The waste lithium iron phosphate battery powder is acid-leached, and the leachate is obtained after solid-liquid separation.

[0043] The lithium iron phosphate battery powder described in this application can be either carbon-containing or carbon-free iron phosphate slag. Specifically, it can be the iron phosphate slag remaining after the lithium element recovery process from waste lithium iron phosphate cathode material collected from waste lithium-ion batteries. It generally contains carbon powder and some impurities such as alumina and copper oxide. Before acid leaching, the collected iron phosphate slag can be crushed into iron phosphate slag powder for better acid leaching.

[0044] In a specific embodiment of this application, acid leaching can be carried out using inorganic acid. Specifically, the step of acid leaching waste lithium iron phosphate battery powder to obtain leachate includes: mixing iron phosphate slag with inorganic acid solution at a liquid-to-solid ratio of 1:(4-10), such as 1:5, 1:6, 1:7, 1:8, 1:9, etc., and then continuously leaching at 40-90℃, such as 45℃, 50℃, 60℃, 70℃, 80℃, etc., for 6-12 hours, such as 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, etc., followed by pressure filtration to separate solid and liquid, obtaining carbon powder and leachate. The liquid-to-solid ratio is expressed in mL:g. The inorganic acid solution can be selected from at least one of hydrochloric acid solution, sulfuric acid solution, and phosphoric acid solution. Taking sulfuric acid solution as an example, a sulfuric acid solution with a concentration of 2.2-2.6 mol / L, such as 2.3 mol / L, 2.4 mol / L, or 2.5 mol / L, can be used. The solution is mixed with the iron phosphate slag according to the above liquid-to-solid ratio for acid leaching. This can effectively leach iron, copper, and aluminum from the iron phosphate slag.

[0045] Furthermore, during the above acid leaching process, stirring speeds of 100-300 rpm / min, such as 150 rpm / min, 200 rpm / min, and 250 rpm / min, can be used to assist in increasing the leaching rate. The leaching endpoint is defined as an iron leaching rate of 99.5% or higher. The acid leaching solution containing phosphorus, iron, copper, and aluminum obtained by filtration can be considered as the initial leaching solution. For carbon-containing iron phosphate slag, the solid obtained after filtration at this time is carbon-containing powder filter residue.

[0046] For step S2: the leachate is subjected to copper and aluminum removal treatment, and after solid-liquid separation, a copper-aluminum removed solution is obtained, comprising:

[0047] S21 (Copper Removal by Displacement): A measured amount of iron powder is added to the leaching solution to carry out a displacement reaction, thereby obtaining a copper removal solution;

[0048] S22 (Neutralization and Precipitation for Aluminum Removal): A reducing agent is added to the copper removal solution and the pH is adjusted to 2.5-3.5 using a pH adjuster. Precipitation and solid-liquid separation are then performed to obtain the copper-aluminum removal solution.

[0049] In a specific embodiment of this application, the step of adding a measured amount of iron powder to the leaching solution to carry out a displacement reaction to obtain a copper-removing solution includes: adding iron powder to the leaching solution according to a molar ratio of copper ions to iron powder of 1:(1.2-1.5), such as 1:1.3, 1:1.4, etc., carrying out a displacement reaction at 40-90°C, such as 45°C, 50°C, 60°C, 70°C, 80°C, etc., and filtering to obtain a copper-removing solution.

[0050] In one specific embodiment of this application, the reaction time of the displacement reaction is about 1-5 hours, such as 2 hours, 3 hours, 4 hours, etc. Specifically, the displacement reaction can be carried out under stirring conditions of 100-300 r / min, such as 150 rpm / min, 200 rpm / min, 250 rpm / min, etc. After the reaction is completed, copper can be removed by methods such as vacuum filtration.

[0051] The purpose of adding iron powder is to remove copper ions in the leachate through a displacement reaction. The copper ions will precipitate as elemental copper. Iron powder is chosen because it can prevent the introduction of other elements as a reducing agent. The addition of excessive iron powder can ensure that all copper ions react and prevent the formation of ferric ions, so that the iron ions in the solution are always in the divalent state.

[0052] In a specific embodiment of this application, the steps of adding excess reducing agent to the copper removal solution and adjusting the pH to 2.5-3.5 using a pH adjuster to perform precipitation and solid-liquid separation to obtain the copper-aluminum removal solution include: adding iron powder, sodium sulfide, or sodium persulfate as a reducing agent to the copper removal solution, and simultaneously adjusting the pH of the solution to 2.5-3.5 using a pH adjuster, such as 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, or 3.4, to perform an aluminum precipitation reaction and solid-liquid separation to obtain the copper-aluminum removal solution.

[0053] In one embodiment, the amount of iron powder added is 0.1-0.5 wt% of the solution volume, such as 0.2 wt%, 0.3 wt%, 0.4 wt%, etc. The pH adjuster is selected from alkaline substances such as sodium hydroxide, ammonia, lithium phosphate, lithium carbonate, sodium carbonate, calcium hydroxide, and calcium oxide. The aluminum precipitation reaction can be carried out at 40-90℃, such as 45℃, 50℃, 60℃, 70℃, 80℃, etc., for 1-5 hours, such as 2 hours, 3 hours, 4 hours, etc. Specifically, it can be carried out under stirring conditions of 100-300 r / min, such as 150 rpm / min, 200 rpm / min, 250 rpm / min, etc., so as to better precipitate aluminum.

[0054] In specific embodiments of this application, the aluminum precipitation reaction of the copper removal solution can not only remove a large amount of aluminum, but also remove other impurity ions that may be present. For example, if titanium (Ti) ions are present, the aluminum precipitation under the above conditions can basically completely precipitate and remove the titanium ions. If Ca is also present... 2+ Mg 2+ Plasma can achieve partial precipitation, and these impurity ions can be removed during the aluminum precipitation process, which can further improve the purity of subsequent iron phosphate compound products, thereby improving product quality.

[0055] For step S3: Adjust the iron-phosphorus ratio of the copper-aluminum removal solution. First, adjust the pH to 0.4-1.2, add an oxidant, and after solid-liquid separation, obtain phosphorus-iron slag and filtrate. Second, adjust the pH of the filtrate to 1.5-2.0, continuously add an oxidant, and after solid-liquid separation, obtain phosphorus-iron slag and final filtrate. Combine the phosphorus-iron slag, wash and dry to obtain phosphorus-iron compound.

[0056] K based on iron phosphate sp =1.3×10 -22 It is known that, theoretically, copper-aluminum solutions will partially precipitate and not completely dissolve at pH values ​​between 0.4 and 1.2. Therefore, if the iron precipitation reaction is carried out within the pH range of 0.4-1.2, ferric phosphate will not precipitate completely, resulting in waste of ferric phosphate and increased production costs. Meanwhile, aluminum phosphate has a K... sp =9.84×10 -21 The solubility product constants of iron phosphate and aluminum phosphate are extremely close, which makes them very easy to co-precipitate. This is the main reason why the aluminum content in iron phosphate prepared from waste lithium iron phosphate batteries is too high.

[0057] However, this application found that although ferric phosphate cannot be completely precipitated at pH values ​​of 0.4-1.2, the aluminum content in the ferric phosphate prepared by oxidizing ferrous iron within this pH range is extremely low, reaching 100 ppm or even below 60 ppm, which is sufficient to meet the requirements for battery-grade ferric phosphate. The reason for this is speculated to be that phosphoric acid ionizes at low pH values ​​to produce a low level of phosphate ions. Furthermore, since this application has already performed an aluminum removal step, the aluminum content in the solution is at a relatively low level. This causes ferrous iron to precipitate only as ferric phosphate during oxidation, rather than as aluminum phosphate, thus effectively avoiding iron-aluminum co-precipitation.

[0058] Based on this, this application provides a method for iron precipitation. First, the pH value is adjusted to 0.4-1.2. Within the pH range of 0.4-1.2, some ferrous iron in the solution will continuously precipitate in the form of ferric phosphate slag (which may contain ferric phosphate, ferric hydrogen phosphate, ferric dihydrogen phosphate, etc.) during the oxidation process, and the aluminum impurities contained therein are extremely low. Solid-liquid separation is performed to separate the precipitated ferric phosphate slag. At this time, the total phosphorus concentration in the solution is at a low level. Then, after adjusting the pH value to 1.5-2.0 in the second round, since it cannot meet the solubility product condition for aluminum phosphate precipitation but can meet the solubility product condition for ferric phosphate, the ferric phosphate slag (mainly composed of ferric phosphate) can continue to precipitate, and the aluminum contained in the solution cannot co-precipitate with iron ions. Therefore, it can be ensured that the prepared ferric phosphate compound fully meets the requirements for aluminum content in battery-grade ferric phosphate.

[0059] In a specific embodiment of this application, the pH adjustment in step S3 is performed stepwise, from low to high pH. This adjustment method enables fine-grained precipitation of iron ions, and each round of solid-liquid separation effectively promotes a more thorough precipitation reaction in the next step, thereby maximizing the iron precipitation effect of each round of pH adjustment and further improving the recovery rate of ferrophosphate compounds.

[0060] In a specific embodiment of this application, the process of adjusting the pH of the copper-aluminum removal solution to 0.4-1.2 in the first round and causing the ferrous ions to undergo an oxidation reaction can be carried out in steps, including but not limited to one or more adjustment processes. The pH value of each subsequent adjustment only needs to be higher than the pH value of the previous adjustment. This adjustment method can ensure that when the pH value is within the range of 0.4-1.2, the precipitation of ferrous ions consumes as many phosphate ions as possible. Finally, the pH value is adjusted to 1.5-2.0. At this time, the total phosphorus content in the solution is low and insufficient to cause aluminum to form aluminum phosphate precipitate. Basically, the precipitate generated is all ferric phosphate compound. Therefore, the iron precipitation method provided by this application can ensure the purity and yield of ferric phosphate.

[0061] In a specific embodiment of this application, the process of adjusting the pH value of the copper-aluminum removal solution to 1.5-2.0 in the second round can also be carried out in steps, including but not limited to one or more adjustment processes, where the pH value of the later adjustment is higher than the pH value of the previous adjustment.

[0062] In specific embodiments of this application, the iron-phosphorus ratio in step S3 is (0.96-1.02):1, for example, 0.97:1, 0.98:1, 0.99:1, 1:1, 1.01:1, etc. The order of the pH adjustment steps and the iron-phosphorus ratio adjustment steps in this application is not strictly defined. The main point is to ensure that the pH of the copper-aluminum removal solution is 0.4-1.2 and the iron-phosphorus ratio is (0.96-1.02):1 before ferrous oxide oxidation.

[0063] Furthermore, in the process of adjusting the phosphorus-iron ratio described above, ferric salts or phosphorus sources can be used for adjustment. The ferric salts are selected from at least one of anhydrous ferric sulfate, ferric sulfate hydrate, anhydrous ferric chloride, ferric chloride hydrate, and ferric nitrate. The phosphorus source is selected from at least one of phosphoric acid, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, potassium dihydrogen phosphate, ammonium monohydrogen phosphate, sodium monohydrogen phosphate, sodium phosphate, potassium phosphate, and ammonium phosphate.

[0064] Further, the pH adjuster used in the first round of pH adjustment in step S3 is selected from at least one of sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid; the pH adjuster used in the second round of pH adjustment is selected from at least one of sodium hydroxide, ammonia, lithium phosphate, lithium carbonate, sodium carbonate, calcium hydroxide, or calcium oxide.

[0065] Furthermore, the oxidant in step S3 is added continuously.

[0066] In a specific embodiment of this application, the oxidant used for the oxidation reaction of ferrous ions can be air or oxygen. In the well-adjusted copper and aluminum removal solution, air or oxygen is introduced, and the solution is brought into full contact with air or oxygen using an aeration device, so that ferrous ions are fully oxidized to ferric ions. During this process, the copper and aluminum removal solution is circulated through a circulating pump, a Venturi tube, and an iron oxide precipitation tank, so that air or oxygen is fully mixed with the solution and aeration is performed. This causes the ferrous ions in the solution to be oxidized to ferric ions due to the presence of oxygen. After iron precipitation under certain conditions, high-purity ferric phosphate dihydrate can be obtained. Since air or oxygen is used as the oxidant, there is no need to add an additional oxidant in the reaction vessel, so high-purity ferric phosphate dihydrate can be obtained at low cost.

[0067] Specifically, the process of adjusting the pH value to 1.5-2.0 to form a solid precipitate can be carried out at 40-90℃, such as 45℃, 50℃, 60℃, 70℃, 80℃, etc., for 3-5 hours, such as 3.5 hours, 4.0 hours, 4.5 hours, etc., and can be carried out under stirring conditions of 100-300 r / min, such as 150 rpm / min, 200 rpm / min, 250 rpm / min, etc., for 3-5 hours. Finally, the ferric phosphate dihydrate is obtained by filtration and separation.

[0068] Furthermore, the iron precipitation reaction in step S3 can be carried out in a circulating oxidation precipitation system, such as... Figure 1 As shown, the circulating oxidation precipitation system includes an iron oxide settling tank, a solid-liquid separation device, and a solid slag washing tank. The circulating oxidation system is installed on the iron oxide settling tank. The liquid inlet of the solid-liquid separation device is connected to the lower part of the iron oxide settling tank, the liquid outlet of the solid-liquid separation device is connected to the top of the iron oxide settling tank, and the slag outlet of the solid-liquid separation device is connected to the solid slag washing tank.

[0069] In a specific embodiment of this application, the circulating oxidation precipitation system further includes a circulating pump and a venturi tube, wherein the inlet of the circulating pump is connected to the lower part of the iron oxide precipitation tank, the outlet of the circulating pump is connected to the spray port of the venturi tube, and the outlet of the venturi tube is connected to the top of the iron oxide precipitation tank.

[0070] In the circulating oxidation precipitation system, the copper-aluminum solution circulates internally via a circulating pump, a Venturi tube, and an iron oxide precipitation tank. Air (oxidant) is introduced into the Venturi suction pipe through at least one opening, and the liquid phase is introduced into the Venturi jet pipe through at least one opening. Because the Venturi tube includes both a Venturi jet pipe and a Venturi suction pipe, the solution mixes with the air drawn in through the Venturi suction pipe as it flows through the Venturi tube, and then is ejected from the Venturi jet pipe. This thorough mixing of gas and liquid results in aeration, causing the ferrous ions in the solution to completely convert to ferric ions due to the presence of oxygen in the air.

[0071] In a specific embodiment of this application, after the iron precipitation reaction, the iron content of the final filtrate is less than 1 g / L.

[0072] Furthermore, a specific embodiment of this application also includes step S4: preparing a slurry using the ferric phosphate compound, adjusting the pH value of the slurry to not less than 2.3, and obtaining hydrated ferric phosphate after washing and drying the precipitate.

[0073] In a specific embodiment of this application, after obtaining the ferric phosphorus compound in step S3, the method further includes a step of post-processing the ferric phosphorus compound to obtain hydrated ferric phosphate. Specifically: a slurry is prepared using the ferric phosphorus compound, and the pH value of the slurry is adjusted to be not less than 2.3, for example, 2.5, 3, 3.5, 4, 5, 6, 7, etc., and then washed. Hydrated ferric phosphate is obtained by heat treatment at 300-800℃, for example, calcination at 300-800℃, such as 400℃, 500℃, 600℃, 700℃, etc., for 2-12 hours, such as 3 hours, 4 hours, 5 hours, 6 hours, 8 hours, 10 hours, 11 hours, etc. After calcination and combination, post-processing (such as pulverization) is performed to obtain battery-grade hydrated ferric phosphate.

[0074] Furthermore, the hydrated iron phosphate prepared by the preparation method of this application embodiment can be further processed into anhydrous iron phosphate, and the method also includes a step of mixing anhydrous phosphoric acid with a lithium source and a carbon source and then performing carbothermic reduction to obtain lithium iron phosphate. This allows the prepared hydrated iron phosphate to be converted into lithium iron phosphate, which can be used as the positive electrode active material for lithium-ion batteries. The lithium source can be lithium carbonate, lithium nitrate, etc., and the carbon source can be organic matter such as glucose. Since lithium iron phosphate is prepared by the method of this application embodiment, it has low cost and high purity, thus reducing the cost of the positive electrode active material lithium iron phosphate.

[0075] Secondly, this disclosure provides a cyclic oxidation precipitation system for implementing step S3 of the first aspect, such as... Figure 1 As shown, the system includes an iron oxide settling tank, a solid-liquid separation device, a solid slag washing tank, a circulating pump, and a venturi tube. The iron oxide settling tank is equipped with a circulating oxidation system. The inlet of the solid-liquid separation device is connected to the lower part of the iron oxide settling tank, the outlet of the solid-liquid separation device is connected to the top of the iron oxide settling tank, the slag outlet of the solid-liquid separation device is connected to the solid slag washing tank, the inlet of the circulating pump is connected to the lower part of the iron oxide settling tank, the outlet of the circulating pump is connected to the spray port of the venturi tube, and the outlet of the venturi tube is connected to the top of the iron oxide settling tank.

[0076] Thirdly, embodiments of this application provide a method for obtaining hydrated iron phosphate as described in the first aspect.

[0077] In one optional embodiment of this application, the aluminum content in the hydrated ferric phosphate is ≤58ppm, such as 50ppm, 45ppm, 40ppm, etc.

[0078] By controlling the phosphorus concentration or pH value in the iron precipitation reaction to keep it low, the co-precipitation of iron and aluminum can be avoided to the greatest extent possible. Therefore, the aluminum impurity content in the hydrated iron phosphate prepared in the end is extremely low, which is sufficient to meet the application requirements of battery-grade iron phosphate.

[0079] Fourthly, embodiments of this application provide an application of the method described in the first aspect in the treatment of waste batteries.

[0080] The following description is based on specific embodiments.

[0081] Example 1

[0082] This embodiment provides a method for recovering phosphorus iron compounds from spent lithium iron phosphate batteries, including:

[0083] S1 (Acid Immersion Step):

[0084] 1000g of waste lithium iron phosphate battery powder and 2.5mol / L sulfuric acid were mixed at a solid-liquid ratio of 4:1, and then stirred at 300rpm / min at 80℃ for 12h. The leaching endpoint was set at a lithium ion leaching rate of over 99%. After acid leaching, the leachate and carbon powder were obtained by filtration.

[0085] S21 (Copper Removal Step):

[0086] Take 1000 mL of leachate (copper ion concentration 4.8 g / L), add 5.1 g of reduced iron powder, stir at room temperature (25-30℃) for 2 h at a stirring rate of 150 rpm / min. After the reaction is complete, copper ions are recovered as elemental copper products. Excess iron powder is dissolved in acid leaching solution to obtain copper-free solution, which contains: ferrous ion concentration 34.2 g / L, aluminum ion concentration 4.3 g / L, and phosphorus ion concentration 18.6 g / L.

[0087] S22 (Aluminum Removal Step):

[0088] Take 800 mL of the copper-removed solution and add 6 g of reduced iron powder. Stir at room temperature at a stirring speed of 200 rpm / min. Then add sodium hydroxide solution to adjust the pH to 2.5. Stir for a total of 120 min. After the reaction is complete, filter to obtain aluminum-containing slag and copper-removed aluminum solution.

[0089] S3 (Iron sinking step):

[0090] Take 600 mL of the aluminum-removed solution, add 12.7 g of 85% phosphoric acid to adjust the iron-to-phosphorus ratio to 1:1.02, and simultaneously add 2.5 mol / L sulfuric acid to adjust the pH to 1.0 in the first round. Continuously purge with oxygen and stir at 70°C for 30 min at a stirring rate of 300 rpm. When the total iron concentration in the solution no longer changes, filter for solid-liquid separation. Collect the filter cake, wash it, and return the filtrate to the container. Add liquid alkali to adjust the pH in the second round, first adjusting it to 1.5, and continue purging with oxygen to allow precipitation. When the total iron concentration in the solution no longer changes, separate the solid and liquid, wash the filter cake, and return the filtrate. Repeat the pH adjustment to 2.0, and when the total iron concentration in the solution no longer changes, separate the solid and liquid to obtain the filtrate and filter cake. Combine the three filter cakes, wash, and dry them to obtain a low-aluminum iron-phosphorus compound.

[0091] S4 (Conversion Steps):

[0092] A slurry was prepared using the ferric phosphate compound, and the pH of the slurry was adjusted to 2.5. The resulting precipitate was washed and dried to obtain ferric phosphate dihydrate.

[0093] The S3 (iron precipitation step) is carried out in a circulating oxidation precipitation system, which includes an iron precipitation tank, a solid-liquid separation device, and a solid slag washing tank. The iron precipitation tank is equipped with a circulating oxidation system, which includes a circulating pump and a Venturi tube. The inlet of the circulating pump is connected to the lower part of the iron precipitation tank, the outlet of the circulating pump is connected to the spray port of the Venturi tube, and the outlet of the Venturi tube is connected to the top of the iron precipitation tank. The inlet of the solid-liquid separation device is connected to the lower part of the iron precipitation tank, the outlet of the solid-liquid separation device is connected to the top of the iron precipitation tank, and the slag discharge port of the solid-liquid separation device is connected to the solid slag washing tank. This circulating oxidation precipitation system is also used in other embodiments of the present invention.

[0094] Example 2

[0095] This embodiment provides a method for recovering phosphorus iron compounds from spent lithium iron phosphate batteries, including:

[0096] S1 (Acid Immersion Step):

[0097] 1000g of waste lithium iron phosphate battery powder and 2.5mol / L sulfuric acid were mixed at a solid-liquid ratio of 4:1, and then stirred at 300rpm / min at 80℃ for 12h. The leaching endpoint was set at a lithium ion leaching rate of over 99%. After acid leaching, the leachate and carbon powder were obtained by filtration.

[0098] S21 (Copper Removal Step):

[0099] Take 1000 mL of leachate (copper ion concentration 4.8 g / L), add 5.1 g of reduced iron powder, stir at room temperature (25-30℃) for 2 h at a stirring rate of 150 rpm / min. After the reaction is complete, copper ions are recovered as elemental copper products. Excess iron powder is dissolved in acid leaching solution to obtain copper-free solution, which contains: ferrous ion concentration 34.2 g / L, aluminum ion concentration 4.3 g / L, and phosphorus ion concentration 18.6 g / L.

[0100] S22 (Aluminum Removal Step):

[0101] Take 800 mL of the copper-removed solution and add 6 g of reduced iron powder. Stir at room temperature at a stirring speed of 200 rpm / min. Then add sodium hydroxide solution to adjust the pH to 2.5. Stir for a total of 120 min. After the reaction is complete, filter to obtain aluminum-containing slag and copper-removed aluminum solution.

[0102] S3 (Iron sinking step):

[0103] Take 600 mL of the aluminum-removed solution, add 12.7 g of 85% phosphoric acid to adjust the iron-to-phosphorus ratio to 1:1.02, then add 2.5 mol / L sulfuric acid for the first round of pH adjustment, initially adjusting the pH to 1.2. Oxygen is continuously introduced, and the mixture is stirred at 70°C for 30 min at a stirring rate of 300 rpm / min. When the total iron concentration in the solution no longer changes, filter to separate the solid and liquid. Collect the filter cake, wash it, and return the filtrate to the container. Then, add liquid alkali to the filtrate for the second round of pH adjustment to 2.0. The specific operation method can refer to the second round pH adjustment method described in Example 1 to obtain the final filtrate and filter cake. Combine the two filter cakes, wash and dry them to obtain a low-aluminum-content iron-phosphorus compound.

[0104] S4 (Conversion Steps):

[0105] A slurry was prepared using the ferric phosphate compound, and the pH of the slurry was adjusted to 2.3. The resulting precipitate was washed and dried to obtain ferric phosphate dihydrate.

[0106] Example 3

[0107] This embodiment provides a method for recovering phosphorus iron compounds from spent lithium iron phosphate batteries, including:

[0108] S1 (Acid Immersion Step):

[0109] 1000g of waste lithium iron phosphate battery powder and 2.5mol / L sulfuric acid were mixed at a solid-liquid ratio of 4:1, and then stirred at 300rpm / min at 80℃ for 12h. The leaching endpoint was set at a lithium ion leaching rate of over 99%. After acid leaching, the leachate and carbon powder were obtained by filtration.

[0110] S21 (Copper Removal Step):

[0111] Take 1000 mL of leachate (copper ion concentration 4.8 g / L), add 5.1 g of reduced iron powder, stir at room temperature (25-30℃) for 2 h at a stirring rate of 150 rpm / min. After the reaction is complete, copper ions are recovered as elemental copper products. Excess iron powder is dissolved in acid leaching solution to obtain copper-free solution, which contains: ferrous ion concentration 34.2 g / L, aluminum ion concentration 4.3 g / L, and phosphorus ion concentration 18.6 g / L.

[0112] S22 (Aluminum Removal Step):

[0113] Take 800 mL of the copper-removed solution and add 6 g of reduced iron powder. Stir at room temperature at a stirring speed of 200 rpm / min. Then add sodium hydroxide solution to adjust the pH to 2.5. Stir for a total of 120 min. After the reaction is complete, filter to obtain aluminum-containing slag and copper-removed aluminum solution.

[0114] S3 (Iron sinking step):

[0115] Take 600 mL of the aluminum-removed solution, add 12.7 g of 85% phosphoric acid to adjust the iron-to-phosphorus ratio to 1.02:1, then add 2.5 mol / L sulfuric acid for the first round of pH adjustment to 0.8. Afterwards, continuously purge with oxygen and stir at 70°C for 30 min at a stirring rate of 300 rpm / min. When the total iron concentration in the solution no longer changes, filter for solid-liquid separation. Collect the filter cake, wash it, and return the filtrate to the container. Then add liquid alkali for the second round of pH adjustment, first adjusting the pH to 1.5, continuing oxygen purging to allow precipitation. When the total iron concentration in the solution no longer changes, separate the solid and liquid, wash the filter cake, return the filtrate, and repeat the pH adjustment to 2.0. When the total iron concentration in the solution no longer changes, separate the solid and liquid to obtain the final filtrate and filter cake. Combine the three filter cakes, wash, and dry to obtain a low-aluminum-content iron-phosphorus compound.

[0116] S4 (Conversion Steps):

[0117] A slurry was prepared using the ferric phosphate compound, and the pH of the slurry was adjusted to 2.4. The resulting precipitate was washed and dried to obtain ferric phosphate dihydrate.

[0118] Comparative Example 1

[0119] This comparative example provides a method for recovering phosphorus compounds from spent lithium iron phosphate batteries.

[0120] The difference from Example 1 is that step S3 is:

[0121] Take 600 mL of the aluminum-removed solution, add 12.7 g of 85% phosphoric acid to adjust the iron-to-phosphorus ratio to 1:1.02, then add 2.5 mol / L sulfuric acid to adjust the pH to 1.0. Continuously purge with oxygen and stir at 70°C for 30 min at a stirring rate of 300 rpm. Once the total iron content in the solution no longer changes, separate the solid and liquid, wash the filter cake, and return the filtrate. Add another 6.2 g of 85% phosphoric acid to adjust the iron-to-phosphorus ratio to 1.02, and repeatedly adjust the pH to 2.0 with liquid alkali. Once the total iron content in the solution no longer changes, separate the solid and liquid, obtaining the filtrate and filter cake. Combine the three filter cakes, wash, and dry to obtain a low-aluminum-content iron-phosphorus compound.

[0122] Although this comparative example used a first and second round of pH adjustment, phosphoric acid was added before the second round of pH adjustment to restore the phosphorus content in the solution. Then, the second round of pH adjustment was performed to precipitate iron. The purpose was to compare the effect of the phosphorus content in the solution on the precipitation process of aluminum and iron.

[0123] Comparative Example 2

[0124] This comparative example provides a method for recovering phosphorus compounds from spent lithium iron phosphate batteries.

[0125] The difference from Example 1 is that step S3 is:

[0126] Take 600 mL of the aluminum-removed solution, add 12.7 g of 85% phosphoric acid to adjust the iron-phosphorus ratio to 1:1.02, then add 2.5 mol / L sulfuric acid to adjust the pH to 1.0, continuously purge oxygen, stir at 70℃ for 30 min at a stirring rate of 300 rpm / min, and after all the ferrous iron in the solution is oxidized to ferric iron, add liquid alkali to adjust the pH to 2.0, filter to separate solid and liquid, and obtain filtrate and filter cake. Dry the washed filter cake to obtain the iron-phosphorus compound.

[0127] In this comparative example, solid-liquid separation was not performed during the first round of pH adjustment, and the second round of pH adjustment was carried out directly. That is, after the first round of pH adjustment reaction was completed, the phosphorus and iron slag was not separated, and the second round of pH adjustment was carried out directly to precipitate iron. The purpose was to compare the effect of solid-liquid separation operation on the precipitation process of aluminum and iron.

[0128] Comparative Example 3

[0129] This comparative example provides a method for recovering phosphorus compounds from spent lithium iron phosphate batteries.

[0130] The difference from Example 1 is that step S3 is:

[0131] Take 600 mL of the aluminum-removed solution, add 12.7 g of 85% phosphoric acid to adjust the iron-phosphorus ratio to 1:1.02, add 2.5 mol / L sulfuric acid to adjust the pH to 2.0, continuously purge oxygen, stir at 70℃ for 30 min at a stirring rate of 300 rpm / min. When the total iron concentration in the solution no longer changes, filter to separate the solid and liquid, collect the filter cake, wash it, return the filtrate to the container, adjust the pH to 2.0, continue to purge oxygen to allow the precipitate to form, wait until the total iron concentration in the solution no longer changes, separate the solid and liquid, wash the filter cake, dry it after washing, and obtain the iron-phosphorus compound.

[0132] In this comparative example, the pH value was adjusted to a relatively high 2.0 during the first round of pH adjustment, and then the precipitation of iron was carried out in the first round. The pH value was then adjusted again for another round of iron precipitation. That is, the precipitation of iron was carried out directly at a higher pH value. The purpose was to compare the effect of solution pH value on the co-precipitation process of aluminum and iron.

[0133] Comparative Example 4

[0134] This comparative example provides a method for preparing iron phosphate from spent lithium iron phosphate batteries, referring to CN116477591A, as follows:

[0135] (1) Acid leaching: In the first leaching process, 290 mL of concentrated sulfuric acid (98.3% by mass) was poured into 3700 mL of deionized water to prepare the leaching agent. Then, 1000 g of waste lithium iron phosphate cathode powder was slowly added to the leaching agent. The mixture was stirred at 30°C for 3.0 h. Solid-liquid separation was performed to obtain a first-stage leaching solution (subsequent experimental raw material) and a first-stage leaching residue (for the second-stage leaching).

[0136] (2) Copper and aluminum removal: Take 200 mL of the first leaching mixture, add 1.2 g of elemental iron, stir at 50°C for 30 min and then filter. The filtrate obtained is the copper-removed liquid.

[0137] Add 2.1g of sodium hydroxide and 3.0g of elemental iron to the copper-removed solution, seal and stir immediately. After the sodium hydroxide is completely dissolved and stirred evenly, heat the solution in a water bath to 70℃ and stir for 4.0h. Then separate the solid and liquid. The resulting solution is the aluminum-removed solution.

[0138] (3) Iron precipitation: Take 1000 mL of the aluminum-removed liquid mixture. Based on the molar ratio of total iron to total phosphorus in the aluminum-removed liquid of 1:1.02, add ammonium phosphate to the aluminum-removed liquid and stir until the ammonium phosphate solid is completely dissolved. Adjust the pH to 0.8 with 1:1 dilute sulfuric acid. Slowly add 1.2 times the theoretical amount of hydrogen peroxide solution. After complete oxidation, raise the temperature to 70℃ and stir the reaction at a constant temperature for 24 h to obtain a suspension of hydrated ferric phosphate. Separate the liquid and solid to obtain precipitated Fe. 3+ The filter cake of hydrated ferric phosphate is then dried to obtain ferric phosphate.

[0139] In this comparative example, only sulfuric acid was used for a single pH adjustment, that is, iron precipitation was carried out only in an environment with a low pH value, in order to compare the effect of segmented pH adjustment on the yield of phosphorus and iron.

[0140] Performance testing

[0141] Elemental analysis and yield calculations were performed on the hydrated iron phosphate obtained in the examples and comparative examples. The results are shown in Table 1.

[0142] Table 1

[0143]

[0144] As can be seen from the examples and performance tests, the ferrophosphate compound prepared by the method provided in this application has extremely high purity and particularly low aluminum content, which can meet the requirements of battery-grade ferrophosphate. Furthermore, the yield of ferrophosphate is high, avoiding waste of raw materials and increased costs.

[0145] A comparison of the examples and Comparative Example 1 shows that the phosphorus content in the solution has a significant impact on the co-precipitation of aluminum and iron during the iron precipitation step. Only in an environment with low phosphorus content can the pH value be increased to avoid the generation of a large amount of aluminum precipitate, thereby enabling selective iron precipitation and ensuring improved yield and purity of the ferrophosphate compound. A comparison of the examples and Comparative Example 2 shows that this application, by employing iron precipitation at a suitable pH value and timely solid-liquid separation, can avoid the influence of the obtained ferrophosphate slag on subsequent steps, ensuring optimal results for each iron precipitation reaction. This is because during the first round of pH adjustment, the obtained ferrophosphate slag may contain acidic precipitates such as ferric phosphate monohydrogen phosphate and ferric phosphate dihydrogen phosphate. If not separated in time, these precipitates will redissolve during subsequent rounds of pH adjustment, preventing an effective reduction in the phosphorus content in the solution and leading to iron-aluminum co-precipitation. The resulting ferrophosphate slag will have a high aluminum content, making it difficult to obtain ferrophosphate that meets battery-grade requirements. The comparison between Examples and Comparative Example 3 shows that pH value has a significant impact on the co-precipitation process of aluminum and iron. If iron precipitation is carried out at a consistently high pH value (e.g., 2.0), co-precipitation is likely to occur due to the similarity of the product constants of iron and aluminum ions, resulting in a high aluminum content in the obtained ferrophosphorus compound product. Even timely solid-liquid separation during the iron precipitation process cannot prevent this co-precipitation. Furthermore, the comparison between Examples and Comparative Example 4 shows that although iron precipitation at a lower pH value can effectively prevent aluminum co-precipitation, if iron precipitation is only carried out at a lower pH value, some iron precipitation is inevitable. If further precipitation and recovery are not carried out, the recovery rate of ferrophosphorus will be severely affected.

[0146] Therefore, the method provided in this application can effectively avoid the co-precipitation of aluminum and maximize the recovery of iron phosphate, thus achieving high-quality and high-efficiency recycling of waste lithium iron phosphate batteries.

[0147] The method for recovering phosphorus iron compounds from spent lithium iron phosphate batteries provided in this application has been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of this application. The description of the above embodiments is only for the purpose of helping to understand the method and core ideas of this application. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this application. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A method for recovering phosphorus-iron compounds from spent lithium iron phosphate batteries, characterized by, The method comprises: S1: acid leaching waste lithium iron phosphate battery powder, and obtaining leaching liquor after solid-liquid separation; S2: removing copper and aluminum from the leaching liquor, and obtaining copper and aluminum removal solution after solid-liquid separation; S3: adjusting the iron-phosphorus ratio of the copper and aluminum removal solution, adjusting the pH value to 0.4-1.2 in the first round, adding an oxidizing agent, and obtaining phosphorus-iron slag and filtrate after solid-liquid separation, adjusting the pH value to 1.5-2.0 in the second round for the filtrate, continuously adding the oxidizing agent, and obtaining phosphorus-iron slag and final filtrate after solid-liquid separation, and obtaining phosphorus-iron compound after washing and drying of the phosphorus-iron slag; Further comprising step S4: preparing slurry by using the phosphorus-iron compound, and adjusting the pH value of the slurry to not less than 2.3, and obtaining hydrated ferric phosphate after washing and drying of the obtained precipitate; In the hydrated ferric phosphate, the aluminum content is ≤58 ppm.

2. The method of claim 1, wherein, In step S3, the iron-phosphorus ratio is (0.96-1.02):

1.

3. The method of claim 1, wherein, In step S3, the pH value in the second round is adjusted in steps from low to high.

4. The method of claim 1, wherein, In step S3, the adjusting agent for the pH value in the first round is selected from at least one of sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid; and the adjusting agent for the pH value in the second round is selected from at least one of sodium hydroxide, ammonia, lithium phosphate, lithium carbonate, sodium carbonate, calcium hydroxide, or calcium oxide.

5. The method of claim 1, wherein, In step S3, the oxidizing agent is continuously added.

6. The method of claim 1, wherein, In step S2, copper is removed by using iron powder to replace copper. In step S2, aluminum is removed by neutralization and precipitation, wherein the conditions for neutralization and precipitation to remove aluminum are that the pH value is adjusted to 2.5-3.5 under the premise of an excess of a reducing agent; The reducing agent is iron powder; or the reducing agent comprises iron powder, and the reducing agent further comprises at least one of sodium sulfide and sodium persulfate.

7. The method according to any one of claims 1 to 6, characterized in that, In step S3, the iron content of the final filtrate is less than 1 g / L.

8. A circulating oxidation precipitation system for carrying out the S3 step according to any one of claims 1 to 7, characterized in that, It comprises an oxidation and iron precipitation tank, a solid-liquid separation device, and a solid residue washing tank, wherein a circulating oxidation system is arranged on the oxidation and iron precipitation tank; the liquid inlet of the solid-liquid separation device is communicated with the lower part of the oxidation and iron precipitation tank, the liquid outlet of the solid-liquid separation device is communicated with the top of the oxidation and iron precipitation tank, and the residue outlet of the solid-liquid separation device is communicated with the solid residue washing tank.

9. The circulating oxidic precipitation system according to claim 8, characterized in that The circulating oxidation system comprises a circulating pump and a Venturi tube, wherein the liquid inlet of the circulating pump is communicated with the lower part of the oxidation and iron precipitation tank, the liquid outlet of the circulating pump is communicated with the injection port of the Venturi tube, and the liquid outlet of the Venturi tube is communicated with the top of the oxidation and iron precipitation tank.

10. Iron phosphate hydrate obtained by the process according to any one of claims 1 to 7, characterized in that, In the hydrated ferric phosphate, the aluminum content is ≤58 ppm.

11. Use of the method according to any one of claims 1-7 in treating waste batteries.

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