A method for recovering lithium, iron and phosphorus from phosphorus-iron-lithium slag

Abstract

The application provides a method for recovering lithium, iron and phosphorus from phosphorus-iron-lithium slag, which comprises the following steps: performing electrolytic reduction treatment on a phosphorus-iron-lithium slag system to obtain a reduction system; and performing impurity removal treatment on the reduction system to obtain a divalent iron system. The method provided by the application can recover lithium, iron and phosphorus in the phosphorus-iron-lithium slag at the same time, reduce the content of impurity metal elements, and reduce the loss of phosphorus, iron and lithium.

Description

Technical Field

[0001] This application relates to the field of resource recycling technology, and in particular to a method for recovering lithium, iron and phosphorus from lithium iron phosphate slag. Background Technology

[0002] Lithium-ion batteries are widely used in consumer electronics, power batteries, and energy storage batteries due to their advantages such as high energy density, long cycle life, and no memory effect. However, since the average lifespan of lithium-ion batteries is 5-8 years, improper disposal of spent lithium-ion batteries can lead to serious environmental and safety problems. Meanwhile, due to the scarcity of energy metals and the continuous growth in market demand, the lithium, iron, and other energy metals abundant in spent lithium-ion batteries have become an important source of supplementary energy. Therefore, the recycling of spent lithium-ion batteries has become one of the research hotspots in recent years. However, because the performance of lithium-ion batteries is closely related to the quality of materials, the requirements for materials in the lithium-ion battery field are high. For example, the content of certain impurities (such as titanium, aluminum, and copper) in the cathode material must be below 100 ppm, which further increases the difficulty of metal element recycling.

[0003] Among them, lithium iron phosphate batteries occupy a significant market share in my country's lithium battery industry due to their advantages such as high safety, long cycle life, and low cost. To date, a large number of lithium iron phosphate batteries have entered the retirement stage, and the plastics, toxic electrolytes, and organic chemicals they contain pose a threat to the environment and life. On the other hand, waste LiFePO4 cathode sheets contain abundant valuable elements (Li, Fe, P). Effective recycling of these materials can reduce pollutants, alleviate resource pressure, and thus promote the healthy development of my country's lithium battery industry.

[0004] The main methods for recycling retired lithium iron phosphate (LFP) batteries are pyrometallurgical and hydrometallurgical processes. Each method has its advantages and disadvantages: pyrometallurgical processes have lower requirements for battery classification, and may even eliminate the need for pre-treatment such as dismantling, but they are energy-intensive, and the final product contains many impurities, leading to unstable performance of the resulting LFP. Hydrometallurgical processes can recover most of the lithium (Li), but leave a large amount of iron phosphate slag. Because this slag contains impurity metals such as Cu, Al, and Ti, its recovery is difficult and costly, resulting in the inability to properly dispose of the slag after lithium extraction. This not only wastes Fe and P resources but also increases the cost of waste storage and disposal. Therefore, there is an urgent need to develop a process to simultaneously recover lithium, iron, and phosphorus from waste LFP slag to achieve full resource recovery. Summary of the Invention

[0005] The purpose of this application is to provide a method for recovering lithium, iron, and phosphorus from lithium iron phosphate slag, so as to simultaneously recover lithium, iron, and phosphorus from the slag, reduce the content of impurity metal elements, and reduce the loss of phosphorus, iron, and lithium. The specific technical solution is as follows:

[0006] The first aspect of this application provides a method for recovering lithium, iron, and phosphorus from lithium iron phosphate slag, comprising the following steps:

[0007] (1) The phosphorus iron lithium slag system was electrolytically reduced to obtain a reduced system;

[0008] (2) The reduction system is subjected to impurity removal treatment to obtain a divalent iron system.

[0009] In some embodiments of this application, in the electrolytic reduction treatment step, the lithium iron phosphate slag system is used as the cathode electrolyte, and the voltage of the electrolytic reduction treatment is adjusted to 1V to 10V.

[0010] In some embodiments of this application, the impurity removal process includes at least one of a titanium ion removal step, an aluminum ion removal step, or a copper ion removal step.

[0011] In some embodiments of this application, the impurity removal process includes at least one of the following steps:

[0012] S1. Pass the reduction system through a copper removal resin to obtain a copper removal solution;

[0013] S2. Adjust the pH of the copper removal solution to 1 to 2, and adjust the reaction temperature to 40°C to 90°C to obtain the titanium removal solution;

[0014] S3. The titanium removal solution is adsorbed and purified by aluminum removal resin to obtain a divalent iron system.

[0015] In some embodiments of this application, the lithium iron phosphate slag system is a lithium iron phosphate slag leachate, which is obtained by leaching lithium iron phosphate slag with sulfuric acid solution.

[0016] In some embodiments of this application, the method further includes the step of oxidizing the divalent iron system to obtain a trivalent iron system.

[0017] In some embodiments of this application, the oxidation treatment includes at least one of oxidant oxidation treatment or electrolytic oxidation treatment.

[0018] In some embodiments of this application, the oxidation treatment method is selected from one of the following methods:

[0019] (a) Hydrogen peroxide is added to the ferrous iron system, wherein the molar ratio of hydrogen peroxide in the hydrogen peroxide to ferrous ions in the ferrous iron system is (1.5 to 3):1;

[0020] (b) Introducing oxygen into the ferrous iron system, wherein the molar ratio of oxygen to ferrous ions in the ferrous iron system is (5-7):1;

[0021] (c) Electrolytic oxidation treatment of the ferrous iron system: using the ferrous iron system as the anolyte, and adjusting the voltage of the electrolytic oxidation treatment to 2V to 10V.

[0022] In some embodiments of this application, the method further includes the step of preparing iron phosphate, lithium iron phosphate, or lithium manganese iron phosphate by using the trivalent iron system as an iron source and phosphorus source.

[0023] In some embodiments of this application, the method further includes the step of preparing lithium salts using the trivalent iron system as a lithium source.

[0024] The beneficial effects of this application are:

[0025] This application provides a method for recovering lithium, iron, and phosphorus from lithium iron phosphate slag, comprising the following steps: electrolytically reducing the lithium iron phosphate slag system to reduce ferric ions to ferrous ions, obtaining a reduced system; and removing impurities from the reduced system to obtain a ferrous iron system. The method provided by this application can simultaneously recover lithium, iron, and phosphorus from lithium iron phosphate slag, reduce the content of impurity metal elements, and minimize the loss of phosphorus, iron, and lithium.

[0026] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Detailed Implementation

[0027] The technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0028] For the recycling of waste lithium iron phosphate slag, most existing technologies employ wet recycling processes that selectively extract expensive lithium, leaving behind iron phosphate slag. This not only wastes phosphorus and iron resources but also increases the cost of slag storage and disposal. This application proposes a method for simultaneously recovering lithium, iron, and phosphorus from lithium iron phosphate slag, reducing the content of impurity metals and minimizing the loss of lithium, iron, and phosphorus. The specific technical solution is as follows:

[0029] This application provides a method for recovering lithium, iron, and phosphorus from lithium iron phosphate slag, which includes the following steps:

[0030] (1) Electrolytic reduction treatment of the lithium iron phosphate slag system is carried out to reduce the ferric ions to ferrous ions to obtain a reduced system;

[0031] (2) The reduction system is purified to obtain the divalent iron system.

[0032] Waste lithium iron phosphate slag contains a large amount of metallic impurities, such as Cu, Al, and Ti, which leads to high costs for recovering lithium and iron phosphate from the slag. The recovery process also results in significant losses of lithium, phosphorus, and iron, and the recovered iron phosphate still contains high levels of impurities, failing to meet the battery-grade iron phosphate chemical industry standard (HG / T 4701-2021). To address the issues of high lithium, phosphorus, and iron losses and difficulties in impurity removal during the recovery process, this application addresses the problem that ferric ions (Fe3+) hydrolyze at a low pH (around 1.5) and precipitate, while ferrous ions (Fe2+) hydrolyze at a much higher pH. Therefore, this application reduces the ferric ions in the lithium iron phosphate slag system to ferrous ions before subsequent impurity removal, thus minimizing the loss of lithium, iron, and phosphorus. Conventional chemical reduction methods, such as iron powder reduction, can significantly increase the solution pH, leading to the hydrolysis of impurity metal elements or ferric ions, resulting in the loss of phosphorus, iron, and lithium. This application employs an electrolytic reduction method to reduce ferric iron to ferrous iron, maintaining the overall solution pH at a low level and effectively reducing hydrolysis caused by pH increases. This application combines electrolytic reduction of ferric ions with impurity removal to effectively remove impurity metal elements from the solution while minimizing the loss of phosphorus, iron, and lithium.

[0033] In some embodiments of this application, the lithium iron phosphate slag system is used as the cathode electrolyte in the electrolytic reduction treatment step, and the voltage of the electrolytic reduction treatment is adjusted to 1V to 10V. For example, the voltage of the electrolytic reduction treatment can be 1V, 2V, 3V, 4V, 5V, 6V, 7V, 8V, 9V, 10V, or a range of any two of these values. This application uses the lithium iron phosphate slag system as the cathode electrolyte in the electrolytic reduction treatment, where a reduction reaction occurs on the cathode side of the electrolytic cell, reducing ferric iron to ferrous iron. The overall pH of the solution is maintained at a low level, which effectively reduces hydrolysis problems caused by increased solution pH and reduces the loss of phosphorus, iron, and lithium.

[0034] There are no particular restrictions on the type of anolyte used in the electrolytic reduction process described in this application, as long as it has reducing properties and can lose electrons to undergo an oxidation reaction. For example, the anolyte can be selected from water, solutions containing low-valence metal ions, low-carbon organic matter, etc. The oxidation reaction occurring in the anode chamber can be one or more reactions such as the oxygen evolution reaction of water electrolysis, the oxidation of low-valence metal ions to high-valence metal ions, methanol oxidation, formic acid oxidation, etc.

[0035] For example, when the anolyte is water, the oxidation reaction occurring on the anolyte side is as follows:

[0036] 22O-4e - =2↑+4H + (1);

[0037] The reduction reaction on the cathode side is as follows:

[0038] Fe 3+ + - = e 2+ (2);

[0039] 22I+4e - = 2H2↑ + 4OH - (3).

[0040] From the reactions occurring on the cathode and anode sides, the cathode undergoes a side reaction where water gains electrons to generate hydrogen and hydroxide ions, while the anode side undergoes a reaction where H2O loses electrons to generate oxygen and hydrogen ions. The hydrogen ions are transferred to the cathode through the membrane, and the pH of the cathode electrolyte is always maintained at a low level, reducing the occurrence of hydrolysis problems caused by an increase in solution pH.

[0041] This application does not impose any particular restrictions on the cathode and anode materials used in the above-mentioned electrolytic reduction process, as long as they can achieve the purpose of this application. For example, the anode material can be selected from titanium plates or titanium electrodes with an anode coating. The anode coating may include, but is not limited to, at least one of ruthenium-iridium alloy, ruthenium-iridium coating, iridium-tantalum alloy, ruthenium-iridium-tantalum alloy, titanium suboxide, platinum, gold, or silver. The cathode material may be selected from stainless steel electrodes, copper plates, nickel plates, or stainless steel electrodes with a cathode coating. The cathode coating may include, but is not limited to, at least one of tin-iron alloy or carbon materials.

[0042] The cathode and anode sides of the electrolytic cell used for the electrolytic reduction treatment of the lithium iron phosphate slag system described above can be separated by a diaphragm. This application does not impose any restrictions on the type of diaphragm, as long as it can achieve the purpose of this application. For example, the diaphragm can be selected from proton membranes or ion membranes. For example, the diaphragm can be selected from polybenzimidazole membranes (PBI), perfluorosulfonic acid membranes (PFSA), sulfonated polyether ether ketone membranes (SPEEK), or polyphenylene ether membranes (PPO), etc.

[0043] In this application, the specific operation of the impurity removal process is not limited, as long as it can achieve the separation of iron and phosphorus elements from impurities. The specific method depends on the differences between ferrous iron and impurity metal elements. For example, ferrous iron and other impurity metal elements can be separated by differences in valence or solubility.

[0044] In some embodiments of this application, the impurity removal process includes at least one of a titanium ion removal step, an aluminum ion removal step, or a copper ion removal step.

[0045] In one specific embodiment, since the oxidation states of iron in the ferrous iron system differ from those of some impurity metal elements (trivalent aluminum, trivalent titanium, and tetravalent titanium), this difference in oxidation states can be used to separate high-valence impurity metal ions from the reduction system. In practical applications, a metal ion adsorption resin can be used to adsorb the impurity metals into the reduction system, thereby separating the high-valence metal ions from the ferrous iron by adsorbing them into the resin.

[0046] Alternatively, the pH value of the ferrous iron system can be adjusted. For example, a pH adjuster can be added to the ferrous iron ion solution to adjust the pH to 2, causing Ti, Al, etc., to form precipitates with smaller Ksp values, thereby achieving removal. For divalent or monovalent impurity metal ions, in one embodiment, a reducing agent can be added to the ferrous iron system for reduction treatment. In the reduction treatment, the reducing agent reduces metal impurity ions with a higher oxidation potential than ferrous iron to elemental form, which is then separated by filtration. For example, taking copper ions as an example, the ferrous iron system is reduced using a reducing agent, and the copper ions are reduced to elemental copper, which is then separated from the ferrous iron system.

[0047] In some embodiments of this application, the impurity removal process includes at least one of the following steps:

[0048] S1. Pass the reduction system through a copper removal resin to obtain a copper removal solution;

[0049] S2. Adjust the pH of the copper removal solution to 1 to 2, and adjust the reaction temperature to 40℃ to 90℃ to obtain the titanium removal solution; for example, the pH of the copper removal solution can be adjusted to any two values ​​from 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, to 2; and the reaction temperature of the copper removal solution can be adjusted to any two values ​​from 40℃, 45℃, 50℃, 55℃, 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, to 90℃.

[0050] S3. The titanium removal solution is adsorbed and purified by aluminum removal resin to obtain a divalent iron system.

[0051] This application does not impose any particular limitation on the flow rate of the reduction system through the copper removal resin and the flow rate of the titanium removal solution through the aluminum removal resin, as long as the purpose of this application can be achieved. For example, the flow rate of the reduction system through the copper removal resin can be 2BV / h to 3BV / h, and the flow rate of the titanium removal solution through the aluminum removal resin can be 2BV / h to 3BV / h.

[0052] The above-mentioned impurity removal steps help remove copper, titanium and aluminum ions from the solution without introducing new impurities. The solution pH is also maintained at a low level, reducing the risk of hydrolysis and minimizing the loss of lithium, iron and phosphorus.

[0053] Both the copper removal resin and the aluminum removal resin described in this application can be purchased. For example, the copper removal resin can be selected from KF340 (Jiangsu Haipu), and the aluminum removal resin can be selected from Tulsimer T-62MP (Dusheng aluminum removal resin) or HP4020 (Jiangsu Haipu).

[0054] In some embodiments of this application, the lithium iron phosphate slag system is a lithium iron phosphate slag leachate, which is obtained by leaching lithium iron phosphate slag with sulfuric acid solution.

[0055] This application does not particularly limit the process of obtaining lithium iron phosphate slag leachate by acid leaching with sulfuric acid solution, as long as the purpose of this application can be achieved. In some embodiments of this application, the molar concentration of sulfuric acid solution is 1 mol / L to 10 mol / L, and the mass ratio of sulfuric acid to lithium iron phosphate slag in the sulfuric acid solution is (0.2 to 0.8):1. For example, the molar concentration of the sulfuric acid solution can be 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L, 3.5 mol / L, 4 mol / L, 4.5 mol / L, 5 mol / L, 5.5 mol / L, 6 mol / L, 6.5 mol / L, 7 mol / L, 7.5 mol / L, 8 mol / L, 8.5 mol / L, 9 mol / L, 9.5 mol / L, 10 mol / L, or a range of any two of these values; the mass ratio of sulfuric acid to lithium iron phosphate slag in the sulfuric acid solution can be 0.2:1, 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, or a range of any two of these values. In some embodiments of this application, the sulfuric acid solution is a mixture of hydrogen peroxide and sulfuric acid, wherein the concentration of hydrogen peroxide is 10 vol% to 30 vol%, and the molar concentration of the sulfuric acid solution is 1 mol / L to 10 mol / L. For example, the concentration of hydrogen peroxide can be 10 vol%, 15 vol%, 20 vol%, 25 vol%, 30 vol%, or any combination of two of these values, and the molar concentration of the sulfuric acid solution can be 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L, 3.5 mol / L, 4 mol / L, 4.5 mol / L, 5 mol / L, 5.5 mol / L, 6 mol / L, 6.5 mol / L, 7 mol / L, 7.5 mol / L, 8 mol / L, 8.5 mol / L, 9 mol / L, 9.5 mol / L, 10 mol / L, or any combination of two of these values. Adding a mixed solution of hydrogen peroxide and sulfuric acid during the acid leaching process can improve the leaching rate of lithium while efficiently leaching phosphorus and iron. In some embodiments of this application, the acid leaching temperature is 40°C to 90°C, and the acid leaching time is 2 hours to 6 hours. For example, the pickling temperature can be 40℃, 50℃, 60℃, 70℃, 80℃, 90℃, or any two of these values; the pickling time can be 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, 6h, or any two of these values.

[0056] In some embodiments of this application, the method further includes an oxidation treatment of the divalent iron system to obtain a trivalent iron system. The oxidation treatment includes at least one of oxidant oxidation treatment or electrolytic oxidation treatment.

[0057] In some embodiments of this application, the oxidation treatment method involves adding hydrogen peroxide to a ferrous iron (Fe2+) system, wherein the molar ratio of hydrogen peroxide in the hydrogen peroxide to ferrous ions in the ferrous iron system is (1.5–3):1. For example, the molar ratio of hydrogen peroxide in the hydrogen peroxide to ferrous ions in the ferrous iron system can be 1.5:1, 1.8:1, 2:1, 2.2:1, 2.5:1, 2.8:1, 3:1, or any range of two of these values. By adjusting the molar ratio of hydrogen peroxide in the hydrogen peroxide to ferrous ions in the ferrous iron system within the range specified in this application, ferrous ions in the solution can be oxidized to ferric ions.

[0058] In some embodiments of this application, the oxidation treatment method is as follows: oxygen is introduced into a ferrous iron (Fe2+) system, and the molar ratio of the introduced oxygen to the ferrous ions in the ferrous iron system is (5-7):1. For example, the molar ratio of the introduced oxygen to the ferrous ions in the ferrous iron system can be 5:1, 5.2:1, 5.5:1, 5.8:1, 6:1, 6.2:1, 6.5:1, 6.8:1, 7:1, or any range of two of these values. By adjusting the molar ratio of the introduced oxygen to the ferrous ions in the ferrous iron system within the range of this application, the ferrous ions in the solution can be oxidized to ferric ions.

[0059] In some embodiments of this application, the oxidation treatment method is as follows: electrolytic oxidation treatment of the ferrous iron system: using the ferrous iron system as the anolyte, and adjusting the voltage of the electrolytic oxidation treatment to 2V to 10V. For example, the voltage of the electrolytic oxidation treatment can be 2V, 3V, 4V, 5V, 6V, 7V, 8V, 9V, 10V, or a range of any two of these values. This method can oxidize ferrous ions in the solution to ferric ions, resulting in high conversion efficiency and low cost.

[0060] The electrolytic cell for electrolytic oxidation treatment of the divalent iron system described above also includes electrode materials and a cathode electrolyte. This application does not impose any particular limitation on the electrode materials, as long as they achieve the purpose of this application. For example, the electrode materials can be selected from the cathode and anode materials described in the electrochemical reduction treatment above. This application also does not impose any particular limitation on the cathode electrolyte that matches the divalent iron system as the anode electrolyte, as long as it achieves the purpose of this application. For example, the cathode electrolyte can be a sulfuric acid solution, ferric sulfate solution, copper sulfate solution, or ferrous sulfate solution, etc.

[0061] The cathode and anode sides of the electrolytic cell used for the electrolytic oxidation treatment of the divalent iron system described above can be separated by a diaphragm. This application does not impose any restrictions on the type of diaphragm, as long as it achieves the purpose of this application. For example, the diaphragm can be selected from proton exchange membranes or ion exchange membranes, such as polybenzimidazole membranes (PBI), perfluorosulfonic acid membranes (PFSA), sulfonated polyether ether ketone membranes (SPEEK), or polyphenylene ether membranes (PPO). In some embodiments of this application, a diaphragm may not be provided between the cathode and anode chambers of the electrolytic cell. In this case, a compensation method is needed to suppress unnecessary material migration between the anode and cathode chambers. This application does not impose any particular restrictions on the compensation method, as long as it achieves the purpose of this application. For example, compensation methods include, but are not limited to, optimizing electrode arrangement, selecting electrode materials, adjusting electrode spacing, controlling flow rate and stirring speed, setting up in-tank flow guides, and adjusting electrolysis parameters through online analysis.

[0062] In some embodiments of this application, the method further includes the step of using the trivalent iron system as an iron and phosphorus source to prepare iron phosphate, lithium iron phosphate, or lithium manganese iron phosphate. Specifically, the trivalent iron system described in this application is used as an iron and phosphorus source to generate iron phosphate, which can then be used as a precursor material to prepare lithium iron phosphate or lithium manganese iron phosphate.

[0063] This application does not specifically limit the steps for preparing ferric phosphate using the trivalent iron system as both an iron and phosphorus source, as long as the objective of this application is achieved. For example, the steps for generating ferric phosphate using the trivalent iron system as both an iron and phosphorus source are as follows: after adding a phosphorus source to the trivalent iron system, an alkali source is added to adjust the pH of the solution, followed by separation to obtain a precipitate and a lithium-containing solution. The precipitate is then washed, dried, and heat-treated to obtain ferric phosphate. The phosphorus source is selected from at least one of phosphoric acid, ammonium hydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate. The mass ratio of phosphorus to iron in the solution is adjusted to (1.2–1.5):1 by adding the phosphorus source to the trivalent iron system. The alkali source is selected from at least one of sodium hydroxide and ammonia water. The pH of the solution is adjusted to 2–6 by adding the alkali source. The drying temperature is 60°C to 120°C, the heat treatment temperature is 400°C to 1000°C, and the heat treatment time is 1 hour to 10 hours.

[0064] In some embodiments of this application, a step of preparing lithium salts using the trivalent iron system as a lithium source is included. This application does not limit the method for preparing lithium salts, as long as it meets the objectives of this application. In some embodiments of this application, after adding a phosphorus source to the trivalent iron system, an alkali source is added to adjust the pH of the solution, followed by separation to obtain a precipitate and a lithium-containing solution. The lithium-containing solution is boiled and concentrated, and then sodium carbonate is added to precipitate lithium ions as lithium carbonate. After filtration, washing, and drying, a lithium carbonate product is obtained.

[0065] Test methods and equipment:

[0066] Phosphorus recovery rate calculation

[0067] Inductively coupled plasma (ICP) spectroscopy was performed on samples of the lithium iron phosphate (PFP) slag system. The mass percentage of phosphorus in the PFP slag system was obtained as W1, and the mass of the PFP slag system was M1. The mass of phosphorus in the PFP slag system was calculated as W1 × M1. ICP testing was also performed on the purified ferrous iron (Fe2) system. The mass percentage of phosphorus in the Fe2 system was obtained as W2, and the mass of the Fe2 system was M2. The mass of phosphorus in the Fe2 system was calculated as W2 × M2. Therefore, the following calculations were performed...

[0068] Iron recovery rate calculation

[0069] ICP testing was performed on samples of the lithium iron phosphate slag system. The mass percentage of iron in the lithium iron phosphate slag system was obtained as X1, and the mass of the lithium iron phosphate slag system was M1. The mass of iron in the lithium iron phosphate slag system was calculated as X1 × M1. ICP testing was also performed on the divalent iron system after impurity removal treatment. The mass percentage of iron in the divalent iron system was obtained as X2, and the mass of the divalent iron system was M2. The mass of iron in the divalent iron system was calculated as X2 × M2. Therefore, the following calculations were performed...

[0070] Lithium recovery rate calculation

[0071] ICP testing was performed on samples of the lithium iron phosphate slag system. The mass percentage of lithium in the lithium iron phosphate slag system was obtained as Y1, and the mass of the lithium iron phosphate slag system was M1. The mass of lithium in the lithium iron phosphate slag system was calculated as Y1 × M1. ICP testing was also performed on the ferrous iron system after impurity removal treatment. The mass percentage of lithium in the ferrous iron system was obtained as Y2, and the mass of the ferrous iron system was M2. The mass of iron in the ferrous iron system was calculated as Y2 × M2. Therefore, the following calculations were performed...

[0072] Testing of aluminum, titanium, and copper content in ferric phosphate products

[0073] The testing was conducted according to the industry standard HG / T4701-2021, "Method for Testing Iron Phosphate for Batteries." Specifically, inductively coupled plasma atomic emission spectrometry (ICP-AES) was used to test the aluminum, titanium, and copper content in the iron phosphate products.

[0074] Example

[0075] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0076] Example 1

[0077] The lithium iron phosphate slag used in this application is lithium iron phosphate slag obtained from battery recycling companies. The lithium iron phosphate slag was sampled and analyzed by inductively coupled plasma (ICP) technology. The content of the main elements in the lithium iron phosphate slag is shown in Table 1.

[0078] (1) Add a mixed solution of hydrogen peroxide and sulfuric acid to 100g of the above-mentioned lithium iron phosphate slag. The volume concentration of hydrogen peroxide is 20 vol%, the concentration of sulfuric acid is 4 mol / L, the mass ratio of sulfuric acid to lithium iron phosphate slag in the sulfuric acid solution is 0.6:1, the acid leaching temperature is 70℃, the acid leaching time is 3h, and after the acid leaching is completed, filter to obtain the lithium iron phosphate slag system and filter residue. The mass of the lithium iron phosphate slag system is 800g.

[0079] ICP analysis was performed on samples of the lithium iron phosphate slag system. The contents of the main elements in the lithium iron phosphate slag system are shown in Table 2. Therefore, it can be calculated that the leaching rate of iron during the acid leaching process of the lithium iron phosphate slag is 91%, the leaching rate of phosphorus is 91%, and the leaching rate of lithium is 95%.

[0080] (2) The above-mentioned lithium iron phosphate slag system was subjected to electrolytic reduction treatment to reduce the ferric ions in the solution to ferrous ions. An electrolytic cell was assembled, using the lithium iron phosphate slag system as the cathode electrolyte, a 1 mol / L sulfuric acid solution as the anode electrolyte, an iridium-tantalum-titanium electrode as the anode material, a stainless steel electrode as the cathode material, and a perfluorosulfonic acid proton exchange membrane as the diaphragm. The voltage for the electrolytic reduction treatment was adjusted to 4V, the electrolysis time to 6h, and the electrode area to be 10×10cm². 2 Take a drop of cathode electrolyte onto filter paper, then add a 0.1 mol / L potassium thiocyanate acidic solution to the filter paper. Stop electrolysis when the filter paper does not change color, and a reduced system is obtained.

[0081] (3) Pass the reduction system through copper removal resin (model: KF340 Jiangsu Haipu) at a flow rate of 3 BV / h to obtain copper removal solution;

[0082] Add ammonia to the copper removal solution to adjust the pH to 1.5, set the reaction temperature to 80℃, and the reaction time to 3 hours. Filter to obtain the titanium removal solution.

[0083] The above titanium removal solution was passed through aluminum removal resin (model: HP-4020 Jiangsu Haipu) at a flow rate of 3 BV / h to obtain an aluminum removal solution, namely a divalent iron system.

[0084] (4) Add hydrogen peroxide to the ferrous iron system, wherein the molar ratio of hydrogen peroxide in the hydrogen peroxide to ferrous ions in the ferrous iron system is 2:1, to oxidize the ferrous iron in the solution to ferric iron, thereby obtaining the ferric iron system.

[0085] (5) Add phosphoric acid to the above trivalent iron system to adjust the mass ratio of phosphorus to iron in the solution to 1.3:1. Then add ammonia to the solution to adjust the pH to 5. The reaction time is 2h. Filter to obtain iron phosphate precipitate and lithium-containing solution. Wash the iron phosphate precipitate and dry it at 80℃. Then heat treat it at 600℃ for 6h to obtain iron phosphate product.

[0086] (6) Boil the above lithium-containing solution, add sodium carbonate, the molar ratio of sodium carbonate to lithium ions in the solution is 0.6:1, filter to obtain precipitate, wash the precipitate and dry it at 80°C to obtain lithium carbonate.

[0087] Example 2

[0088] Except for adjusting the voltage of the electrolytic reduction process to 1V and the electrolysis time to 30h in step (2), the rest is the same as in Example 1.

[0089] Example 3

[0090] Except for adjusting the voltage of the electrolytic reduction process to 10V and the electrolysis time to 2h in step (2), the process is the same as in Example 1.

[0091] Example 4

[0092] The lithium iron phosphate slag raw material used in this application is lithium iron phosphate slag obtained by battery recycling companies. In Example 4, another batch of lithium iron phosphate slag was sampled, analyzed and recovered. The content of the main elements in the lithium iron phosphate slag of Example 4 is shown in Table 1.

[0093] (1) Add a mixed solution of hydrogen peroxide and sulfuric acid to 100g of the above-mentioned lithium iron phosphate slag. The volume concentration of hydrogen peroxide is 20 vol%, the concentration of sulfuric acid is 4 mol / L, the mass ratio of sulfuric acid to lithium iron phosphate slag in the sulfuric acid solution is 0.6:1, the acid leaching temperature is 70℃, the acid leaching time is 3h, and after the acid leaching is completed, filter to obtain the lithium iron phosphate slag system and filter residue. The mass of the lithium iron phosphate slag system is 780g.

[0094] ICP analysis was performed on samples of the lithium iron phosphate slag system. The contents of the main elements in the lithium iron phosphate slag system are shown in Table 2. Therefore, it can be calculated that during the acid leaching process of the lithium iron phosphate slag, the leaching rate of iron is 89%, the leaching rate of phosphorus is 90%, and the leaching rate of lithium is 94%.

[0095] (2) The above-mentioned lithium iron phosphate slag system was subjected to electrolytic reduction treatment to convert the ferric ions in the solution into ferrous ions. An electrolytic cell was assembled, using the lithium iron phosphate slag system as the cathode electrolyte, a 1 mol / L sulfuric acid solution as the anode electrolyte, an iridium-tantalum-titanium electrode as the anode material, a stainless steel electrode as the cathode material, and a perfluorosulfonic acid proton exchange membrane as the diaphragm. The applied power supply voltage was adjusted to 4V, the electrolysis time to 6h, and the electrode area to be 10×10cm². 2 Take a drop of cathode electrolyte onto filter paper, then add a 0.1 mol / L potassium thiocyanate acidic solution to the filter paper. Stop electrolysis when the filter paper does not change color, and a reduced system is obtained.

[0096] (3) The reduction system was passed through a copper removal resin (model: KF340 Jiangsu Haipu) at a flow rate of 2 BV / h to obtain a copper removal solution; ammonia was added to the copper removal solution to adjust the pH to 1.5, the reaction temperature was 60℃, the reaction time was 4h, and the solution was filtered to obtain a titanium removal solution; the titanium removal solution was passed through an aluminum removal resin (model: HP-4020 Jiangsu Haipu) at a flow rate of 2 BV / h to obtain an aluminum removal solution, i.e., a divalent iron system. Subsequent processing steps were the same as in Example 1.

[0097] Example 5

[0098] Except for step (4), where electrolytic oxidation is used to oxidize ferrous ions in the ferrous system to ferric ions, the process is the same as in Example 1. Specifically, the electrolytic oxidation process involves placing the ferrous system in the anode chamber of the electrolytic cell as the anolyte, with an iridium-tantalum-titanium electrode as the anode material; and using a 1 mol / L sulfuric acid solution as the cathode electrolyte, with a stainless steel electrode as the cathode material and a perfluorosulfonic acid proton exchange membrane as the diaphragm. The electrolytic oxidation process uses a voltage of 6V, an electrolysis time of 7 hours, and an electrode area of ​​10 × 10 cm². 2 Take a drop of anolyte onto filter paper, then add a 0.1 mol / L o-phenanthroline solution to the filter paper. Stop electrolysis when the filter paper does not change color, and you will get a trivalent iron system.

[0099] Example 6

[0100] Except for step (4), where the voltage of the electrolytic oxidation treatment is adjusted to 2V and the electrolysis time is 24h, the rest is the same as in Example 5.

[0101] Example 7

[0102] Except for step (4), where the voltage of the electrolytic oxidation treatment is adjusted to 10V and the electrolysis time is 5h, the rest is the same as in Example 5.

[0103] Example 8

[0104] Except for step (4), where oxygen is introduced into the ferrous iron system to oxidize the ferrous ions to ferric ions, the process is the same as in Example 1. The molar ratio of the introduced oxygen to the ferrous ions in the ferrous iron system is 6:1, and the flow rate is 10 m / s. 3 / h, oxidizing ferrous ions in the solution to ferric ions, to obtain a ferric system.

[0105] Comparative Example 1

[0106] Except for step (2), which did not employ electrolytic reduction, the following method was used to reduce the ferric ions in the lithium iron phosphate slag system to ferrous ions: iron powder was added to the lithium iron phosphate slag system, wherein the molar ratio of the iron powder to the ferric ions in the solution was 1.5:1, and the reaction time was 4 hours, resulting in a reduced system. The remaining processing steps were the same as in Example 1.

[0107] Table 1 shows the content of major elements in the lithium iron phosphate slag raw materials in Examples 1 and 4.

[0108] Table 1

[0109] Li (%) Fe (%) P(％) Al (mg / kg) Ti (mg / kg) Cu (mg / kg) Example 1 2.46 31.2 17.5 5738 4526 598 Example 4 2.13 30.1 15.8 4387 4066 624

[0110] Table 2 shows the content of the main elements in the lithium iron phosphate slag systems of Examples 1 and 4.

[0111] Table 2

[0112] Li (mg / kg) Fe (mg / kg) P (mg / kg) Al (mg / kg) Ti (mg / kg) Cu (mg / kg) Example 1 2921 35627 19893 429 515 92 Example 4 2578 34489 18398 389 503 64

[0113] Table 3 shows the results of phosphorus recovery rate, iron recovery rate, lithium recovery rate, and the mass percentage of aluminum, titanium, and copper in the iron phosphate product for each example and comparative example.

[0114] Table 3

[0115]

[0116] This application employs electrolytic reduction to reduce ferric ions to ferrous ions in the lithium iron phosphate slag system, combined with the impurity removal method of this application, to effectively remove impurity metal elements from the solution while reducing the loss of phosphorus, iron, and lithium. As can be seen from Examples 1 to 8 and Comparative Example 1 in Table 3, the examples of this application show higher recovery rates of phosphorus, iron, and lithium with less loss, and the final iron phosphate product has lower aluminum, titanium, and copper contents. Comparative Example 1, however, did not use electrolytic reduction to reduce ferric ions. The recovery rates of phosphorus, iron, and lithium in Comparative Example 1 were lower, with greater loss, and the final iron phosphate product had higher aluminum and titanium contents.

[0117] As can be seen from Examples 1 and 4, different batches of lithium iron phosphate slag can be recycled using the method provided in this application. Furthermore, after acid leaching, the leaching rates of iron, phosphorus, and lithium in the lithium iron phosphate slag raw materials of Examples 1 and 4 are relatively high, indicating that the acid leaching method of this application can efficiently leach iron, phosphorus, and lithium from the lithium iron phosphate slag, which is beneficial to improving the recovery rate of iron, phosphorus, and lithium.

[0118] As can be seen from Examples 1 to 3, by using electrolytic reduction to reduce ferric iron to ferrous iron and controlling the voltage of electrolytic reduction within the range of this application, the recovery rates of phosphorus, iron and lithium in Examples 1 to 3 are high and the losses are small. At the same time, the aluminum content, titanium content and copper content in the final iron phosphate product are all low.

[0119] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A method for recovering lithium, iron, and phosphorus from lithium iron phosphate slag, comprising the following steps: (1) Electrolytic reduction treatment of the lithium iron phosphate slag system was carried out to obtain a reduced system; (2) The reduction system is subjected to impurity removal treatment to obtain a divalent iron system.

2. The method according to claim 1, wherein, In the electrolytic reduction process, the lithium iron phosphate slag system is used as the cathode electrolyte, and the voltage of the electrolytic reduction process is adjusted from 1V to 10V.

3. The method according to claim 1, wherein, The impurity removal process includes at least one of the following steps: titanium ion removal, aluminum ion removal, or copper ion removal.

4. The method according to claim 1, wherein, The impurity removal process includes at least one of the following steps: S1. Pass the reduction system through a copper removal resin to obtain a copper removal solution; S2. Adjust the pH of the copper removal solution to 1 to 2, and adjust the reaction temperature to 40°C to 90°C to obtain the titanium removal solution; S3. The titanium removal solution is adsorbed and purified by aluminum removal resin to obtain a divalent iron system.

5. The method according to any one of claims 1 to 4, wherein, The lithium iron phosphate slag system is a lithium iron phosphate slag leachate, which is obtained by leaching lithium iron phosphate slag with sulfuric acid solution.

6. The method according to any one of claims 1 to 4, wherein, It also includes the step of oxidizing the divalent iron system to obtain a trivalent iron system.

7. The method according to claim 6, wherein, The oxidation treatment includes at least one of oxidant oxidation treatment or electrolytic oxidation treatment.

8. The method according to claim 6, wherein, The oxidation treatment method is selected from one of the following methods: (a) Hydrogen peroxide is added to the ferrous iron system, wherein the molar ratio of hydrogen peroxide in the hydrogen peroxide to ferrous ions in the ferrous iron system is (1.5 to 3):1; (b) Introducing oxygen into the ferrous iron system, wherein the molar ratio of oxygen to ferrous ions in the ferrous iron system is (5-7):1; (c) Electrolytic oxidation treatment of the ferrous iron system: using the ferrous iron system as the anolyte, and adjusting the voltage of the electrolytic oxidation treatment to 2V to 10V.

9. The method according to any one of claims 6 to 8, wherein, It also includes the step of using the trivalent iron system as an iron source and phosphorus source to prepare iron phosphate, lithium iron phosphate or lithium manganese iron phosphate.

10. The method according to any one of claims 6 to 8, wherein, It also includes the step of preparing lithium salts by using the trivalent iron system as a lithium source.

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