Method for recycling sodium iron pyrophosphate positive electrode material and application thereof

Abstract

The application discloses a recycling method and application of sodium iron pyrophosphate positive electrode material, relates to the technical field of sodium ion batteries, and adds a strong alkali solution to the retired sodium iron pyrophosphate material to obtain a mixed solution; the mixture is heated in a contact environment with air; a double decomposition reaction occurs between hydroxyl ions in the alkali solution and the sodium iron pyrophosphate to form a solid-liquid mixture containing iron precipitate; the mixed system is continuously heated; sodium pyrophosphate is hydrolyzed into soluble sodium phosphate in a high-alkalinity and heating environment; the system is cooled and solid-liquid separation is performed to obtain a filtrate, iron is separated from phosphorus and sodium, the filtrate is evaporated and crystallized to obtain a sodium phosphate product. The reaction is performed through a gradually increasing temperature alkali solution system, so that the sodium iron pyrophosphate is subjected to an alkaliolysis reaction, the phosphorus recovery rate in the reaction system is greater than or equal to 92.5%, the iron leaching rate is less than 1%, and then the iron is maximally separated from the phosphorus and sodium, the conversion rate of phosphorus is significantly improved, and the product purity is improved.

Description

Technical Field

[0001] This application relates to the technical field of sodium-ion batteries, specifically to a method for recycling sodium iron pyrophosphate cathode material and its application. Background Technology

[0002] Sodium-ion batteries are secondary batteries that use sodium ions as charge carriers. They are charged and discharged by the migration of sodium ions between the positive and negative electrodes.

[0003] In some related technologies, methods for recovering phosphorus from phosphorus-containing battery cathode materials often employ strong acids, such as sulfuric acid and hydrochloric acid, to leach phosphorus into soluble phosphoric acid or phosphates, which are then recovered through precipitation, crystallization, or other methods. However, for structurally stable compounds like sodium iron pyrophosphate, direct acid leaching is often inefficient and simultaneously leaches iron, making subsequent separation of iron, phosphorus, and sodium difficult. Summary of the Invention

[0004] In order to solve at least one of the problems mentioned in the background art, this application provides a method and application for the recovery of sodium iron pyrophosphate cathode material. By heating an alkaline solution of a certain concentration to carry out a metathesis reaction with sodium iron pyrophosphate, iron combines with hydroxide ions to form iron precipitate, while phosphorus and sodium enter the liquid phase as soluble phosphates, thereby achieving efficient separation of iron and sodium phosphate.

[0005] The specific technical solutions provided in this application are as follows: In a first aspect, a method for recovering sodium iron pyrophosphate cathode material is provided, the method comprising: A strong alkaline solution is added to decommissioned sodium iron pyrophosphate material to obtain a mixture. Under conditions of contact with air, the mixture is heated to a first-level temperature to carry out an alkaline hydrolysis reaction to form a solid-liquid mixture containing a first iron-containing precipitate and dissolved sodium pyrophosphate. The solid-liquid mixture is heated to a secondary temperature, which is higher than the primary temperature, to continue the reaction until the pyrophosphate ions in the solid-liquid mixture are converted into phosphate ions to form a phosphate system solution. The phosphate system solution is cooled, the solid phase filtrate in the cooled solution is separated, and the filtrate is evaporated to obtain sodium phosphate product.

[0006] In one specific embodiment, the concentration of the strong alkali solution is 3 mol / L to 18 mol / L; Preferably, the concentration of the strong alkali solution is 5 mol / L to 12 mol / L.

[0007] In one specific embodiment, the strong alkaline solution is one or more of sodium hydroxide solution, potassium hydroxide solution, and lithium hydroxide solution; preferably, the strong alkaline solution is sodium hydroxide solution.

[0008] In one specific embodiment, the ratio of the molar amount of the strong alkaline solution to the molar amount of the decommissioned sodium iron pyrophosphate material is greater than or equal to 8.5.

[0009] In one specific embodiment, heating the mixture to a primary temperature for an alkaline hydrolysis reaction involves a solid-liquid mixture containing a first iron-containing precipitate and dissolved sodium pyrophosphate, specifically including: The mixture is heated to 85°C~95°C for 2~4 hours for alkaline hydrolysis. The OH- in the strong alkali solution... - Fe in the decommissioned sodium ferric pyrophosphate material 3+ The first iron-containing precipitate is formed by the combination; Furthermore, the alkaline hydrolysis reaction causes phosphorus and sodium to form soluble sodium pyrophosphate, thereby forming a solid-liquid mixture containing the first iron-containing precipitate and dissolved sodium pyrophosphate.

[0010] In one specific embodiment, the solid-liquid mixture is further heated to a secondary temperature to carry out the reaction, specifically including: Continue heating the solid-liquid mixture to 95°C~100°C and stir the reaction for 2h~4h.

[0011] In one specific embodiment, the solid-liquid mixture is further heated to a boiling state to carry out the reaction.

[0012] In one specific embodiment, the phosphate system solution is cooled to 10°C to 20°C.

[0013] In one specific embodiment, the solid phase includes the first iron-containing precipitate and the second iron-containing precipitate formed by decomposing the tetrahydroxyferric hydroxide in the phosphate system solution by cooling to 10°C~20°C. And / or, the solid phase is ferric hydroxide precipitate.

[0014] In one specific embodiment, the phosphorus recovery rate in the reaction system is greater than or equal to 92.5%, and the iron leaching rate is less than 1%.

[0015] Secondly, an application is provided in which the sodium phosphate product obtained by the recovery method of sodium iron pyrophosphate cathode material as described above is used in the industrial or food fields.

[0016] The embodiments of this application have the following beneficial effects: By using an alkaline solution of a certain concentration to disrupt the complex structure in decomposing sodium ferric pyrophosphate, hydroxide ions combine with oxygen and water to undergo a metathesis reaction with sodium ferric pyrophosphate. By gradually increasing the temperature in the reaction system, the reaction boundary is effectively controlled. This ensures that iron can be effectively and completely precipitated at the alkaline temperature, and that pyrophosphate is converted to phosphate at the secondary temperature system. This maximizes the separation of iron from sodium phosphate, significantly improving the conversion rate of phosphorus and the purity of the product. Attached Figure Description

[0017] To more clearly illustrate the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a schematic diagram of a method for recovering sodium iron pyrophosphate cathode material provided in an embodiment of this application. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0020] The "range" disclosed herein is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also expected. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0021] Unless otherwise specified in this application, all embodiments and preferred embodiments mentioned herein can be combined to form new technical solutions.

[0022] Unless otherwise specified, all technical features and preferred features mentioned herein can be combined to form new technical solutions.

[0023] In this application, unless otherwise specified, all steps mentioned herein may be performed sequentially or randomly, but are preferably performed sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0024] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0025] This application provides a method for recovering sodium iron pyrophosphate cathode material, such as... Figure 1 As shown, the method includes the following steps: S1. A strong alkaline solution is added to the decommissioned sodium iron pyrophosphate material to obtain a mixture. Under conditions of contact with air, the mixture is heated to a first-level temperature to carry out an alkaline hydrolysis reaction to form a solid-liquid mixture containing a first iron-containing precipitate and dissolved sodium pyrophosphate.

[0026] Specifically, decommissioned sodium ferric pyrophosphate powder is placed in a reactor equipped with a stirrer and reflux device, and a strong alkaline solution is added. The mixture is heated to a certain temperature and then stirred at a constant temperature to carry out the reaction. In this embodiment, the strong alkaline solution is preferably a sodium hydroxide solution. The mixture is heated to 85℃~95℃ for an alkaline hydrolysis reaction for 2h~4h, so that the OH- in the strong alkaline solution... - Fe in the decommissioned sodium ferric pyrophosphate material 3+ The reaction combines to form a first iron-containing precipitate; and through the alkaline hydrolysis reaction, phosphorus and sodium elements form soluble sodium pyrophosphate, thus forming a solid-liquid mixture containing the first iron-containing precipitate and dissolved sodium pyrophosphate. The reaction mechanism during this process is as follows: 3Na2FeP2O7+6NaOH+0.75O2+1.5H2O→3Fe(OH)3↓+3Na4P2O7 By mixing materials containing sodium ferric pyrophosphate with concentrated sodium hydroxide solution and reacting under heating conditions, the concentrated NaOH provides a high concentration of OH- during this stage. - It undergoes a metathesis reaction with Na₂FeP₂O₇, Fe 3+ With OH - The combination forms a water-insoluble reddish-brown Fe(OH)3 precipitate, while phosphorus and sodium enter the liquid phase as soluble sodium pyrophosphate (Na4P2O7). The alkaline environment directly achieves the separation of iron, phosphorus, and sodium in both solid and liquid forms, avoiding the problem of iron and phosphorus co-dissolution in the acid leaching method.

[0027] In a specific embodiment, the strong alkaline solution used in this embodiment is selected from one or more of sodium hydroxide solution, potassium hydroxide solution, and lithium hydroxide solution. Preferably, the strong alkaline solution is sodium hydroxide solution. Sodium hydroxide solution can achieve the alkaline hydrolysis reaction with sodium ferric pyrophosphate without introducing new impurities, ensuring the purity of the subsequently recovered material. The ratio of the molar amount of the strong alkaline solution to the molar amount of the decommissioned sodium ferric pyrophosphate material is greater than or equal to 8.5. By setting the amount of strong alkaline solution to far exceed the molar amount of decommissioned sodium ferric pyrophosphate, multiple cyclic leaching of sodium ferric pyrophosphate is achieved, ensuring that sodium ions and phosphorus in the decommissioned material are released into the solution as much as possible in the form of soluble sodium ferric pyrophosphate.

[0028] In this embodiment, the concentration of the strong alkali solution is 3 mol / L to 18 mol / L. Specifically, the concentration of the strong alkali solution can be set to 3 mol / L, 4 mol / L, 5 mol / L, 6 mol / L, 7 mol / L, 8 mol / L, 9 mol / L, 10 mol / L, 11 mol / L, 12 mol / L, 13 mol / L, 14 mol / L, 15 mol / L, 16 mol / L, 17 mol / L, 18 mol / L, or any range of two of the above values. The alkali solution of the above concentration provides a high concentration of OH-. - If the concentration of the alkaline solution is insufficient, the concentration of hydroxide ions in the above reaction system will not meet the Fe... 3+ The requirement for complete precipitation leads to incomplete precipitation of ferric hydroxide, with some iron remaining in the liquid phase as a soluble iron-pyrophosphate complex, reducing the iron removal rate. Simultaneously, excessively low alkaline solution concentrations reduce the reaction rate, resulting in fine precipitate particles that easily form colloids, making filtration difficult and affecting the purity of subsequent phosphorus recovery. Experiments show that under low alkaline conditions, a gray-green intermediate precipitate, a mixture of Fe(OH)₂ and Fe(OH)₃, may appear in the reaction system. In this case, the oxidation process in the reaction system is hindered, the efficiency of O₂ participation is reduced, and the reaction is incomplete. Excessively high alkaline solution concentrations increase the viscosity of the solvent system, improving operational safety and convenience, but also drastically increasing the ionic strength of the solution. This causes Fe(OH)₃ precipitate particles to agglomerate and become denser, which, while facilitating sedimentation, can trap unreacted phosphorus components, reducing phosphorus recovery. Localized excessive alkalinity may also accelerate O₂ escape, weakening the oxidation driving force and affecting reaction equilibrium. Therefore, it is necessary to control the concentration of the strong alkaline solution in this example to ensure the stable operation of the reaction system.

[0029] S2. Continue heating the solid-liquid mixture to a secondary temperature, which is higher than the primary temperature, to carry out the reaction. The pyrophosphate ions in the solid-liquid mixture are converted into phosphate ions to form a phosphate system solution.

[0030] Specifically, in this embodiment, the solid-liquid mixture is heated to 95°C to 100°C and stirred for 2 to 4 hours. Preferably, in this embodiment, the solid-liquid mixture is heated to a slight boiling state to carry out the reaction.

[0031] Following the alkaline hydrolysis reaction in step S1, the mixed system is maintained at high temperature or boiling conditions. Under these conditions of high alkalinity and heating, the sodium pyrophosphate in the reaction system undergoes hydrolysis, transforming into more stable and versatile soluble sodium phosphate (Na3PO4). The reaction mechanism is: Na4P2O7 + 2NaOH → 2Na3PO4 + H2O. This step completely converts phosphorus from pyrophosphate to phosphate, which is beneficial for the subsequent recycling of phosphorus products.

[0032] In this embodiment, the reaction system uses a gradually increasing temperature system, with the secondary temperature being higher than the primary temperature. The alkaline hydrolysis stage can be carried out efficiently at a moderate temperature, i.e., 85℃~95℃. - With Fe in the structure of sodium iron pyrophosphate 3+ Combined to form Fe(OH)3 precipitate, Na + Phosphorus is released into the solution in the form of soluble sodium pyrophosphate, achieving the first in-situ separation of iron from phosphorus and sodium. The setting of the primary temperature range avoids the exacerbation of the partial back dissolution of Fe(OH)3 precipitate in strong alkali to form Fe(OH) when the boiling temperature is used in the initial stage. 4- This process, in turn, contaminates the liquid phase. The temperature is then raised to a boiling point of approximately 100°C. At this point, the reaction system enters the hydrolysis stage. The high energy input causes the POP-bridged oxygen bonds in sodium pyrophosphate to break, and under the combined action of water molecules and excess NaOH, it is completely converted into sodium orthophosphate. The boiling state not only provides sufficient thermodynamic driving force for the hydrolysis reaction but also acts as a stirrer and stripper, which helps shift the reaction equilibrium to the right, significantly improving the phosphorus conversion rate and product purity. By gradually increasing the temperature, the boundaries of the two-step reaction are effectively controlled: the alkaline hydrolysis temperature ensures the selectivity and completeness of iron precipitation, and the boiling hydrolysis temperature ensures the complete conversion of phosphorus forms. Simultaneously, the final cooling and solid-liquid separation re-precipitates and fixes any trace amounts of iron that may have dissolved at high temperatures, preventing iron contamination of the phosphorus product. The overall process ensures high recovery rates of phosphorus and sodium while achieving maximum separation of iron from phosphorus and sodium.

[0033] S3. Cool the phosphate system solution, separate the solid phase in the solution after cooling to obtain filtrate, and evaporate the filtrate to obtain sodium phosphate product.

[0034] Specifically, the solid phase includes the first iron-containing precipitate and the second iron-containing precipitate formed by decomposing the tetrahydroxyferric hydroxide in the phosphate system solution by cooling to 10℃~20℃. Specifically, the solid phase is ferric hydroxide precipitate. In this embodiment, the phosphate system solution is cooled to 10℃~20℃ to decompose the tetrahydroxyferric hydroxide in the phosphate system solution and form ferric hydroxide precipitate. The ferric hydroxide precipitate, after washing and drying, can be used as an iron source. The filtrate is an alkaline solution rich in Na3PO4, which can be used to obtain sodium phosphate product through evaporation and crystallization. The reaction system is cooled because ferric hydroxide can partially dissolve in a strong alkaline solution under high temperature conditions to generate tetrahydroxyferric hydroxide [Fe(OH)4]. - The complex is formed, and cooling the reaction system can terminate the complexation process.

[0035] The alkaline hydrolysis method in this embodiment enables in-situ separation of iron as Fe(OH)3 precipitation and phosphorus as a soluble salt in the liquid phase in one step. Phosphorus recovery is high (>92%), while iron leaching is extremely low (<1%), fundamentally solving the problem of iron-phosphorus co-dissolution and difficult separation in acid leaching. Subsequent heating and hydrolysis converts phosphorus from sodium pyrophosphate to sodium phosphate, which has a wider range of applications. Furthermore, by selecting a strong alkaline solution, the reaction can be carried out under mild conditions, without producing acidic iron-containing wastewater. Post-treatment is simple and environmentally friendly. Simultaneously, the Fe(OH)3 precipitate has high purity and is easily utilized as a resource.

[0036] Meanwhile, by controlling the concentration of the strong alkali in this embodiment to preferably be a concentrated sodium hydroxide solution of 5 mol / L to 12 mol / L, a sufficiently high OH concentration can be provided. - The concentration is chosen to efficiently destroy stable complexes, ensuring rapid reaction while avoiding reagent waste, increased viscosity, and operational safety issues caused by excessive concentration. The alkaline hydrolysis temperature of 85℃~95℃ effectively accelerates the reaction process and improves efficiency. The hydrolysis stage uses boiling or near-boiling temperatures to drive the hydrolysis reaction completely to the right, ensuring complete conversion of pyrophosphate to phosphate. This achieves a phosphorus recovery rate of greater than or equal to 92.5% and an iron leaching rate of less than 1% in this reaction system, thereby maximizing the separation of iron from sodium phosphate and significantly improving phosphorus conversion rate and product purity.

[0037] Corresponding to the above embodiments, this application provides an application of sodium phosphate products obtained by the recovery method of sodium iron pyrophosphate cathode material as described above in the industrial or food fields.

[0038] Example 1 Step 1: Weigh 10g of sodium iron pyrophosphate powder and place it in a reactor equipped with a stirring and reflux device.

[0039] Step 2: Add 100 mL of 8 mol / L sodium hydroxide solution (NaOH solution in large excess).

[0040] Step 3 (Alkaline hydrolysis): Heat the mixture to 85°C and react it at a constant temperature with stirring for 3 hours in an open system with full contact with air. A large amount of reddish-brown precipitate can be observed to form.

[0041] Step 4 (Hydrolysis): Heat the reaction system to a gentle boiling state and continue reflux for 3 hours to allow the sodium pyrophosphate to be fully hydrolyzed.

[0042] Step 5: After the reaction is complete, cool to 20℃, filter, and wash the precipitate 2-3 times with a small amount of pure water. Combine the filtrates, add some phosphoric acid to obtain a pure trisodium phosphate solution, evaporate to crystallize, and dry to obtain pure trisodium phosphate powder. Dry the filter residue at 105℃ and weigh it.

[0043] Example 2 Step 1: Weigh 10g of sodium ferric pyrophosphate powder.

[0044] Step 2: Add 120 mL of 10 mol / L sodium hydroxide solution.

[0045] Step 3 (Alkaline hydrolysis): Heat the mixture to 85°C and stir at a constant temperature for 2.5 hours.

[0046] Step 4 (Hydrolysis): Heat the reaction system to boiling and reflux for 4 hours.

[0047] Step 5: After the reaction is complete, cool to 10℃, filter, and wash the precipitate 2-3 times with a small amount of pure water. Combine the filtrates, add some phosphoric acid to obtain a pure trisodium phosphate solution, evaporate to crystallize, and dry to obtain pure trisodium phosphate powder. Dry the filter residue at 105℃ and weigh it.

[0048] Example 3 Step 1: Weigh 10g of sodium ferric pyrophosphate powder.

[0049] Step 2: Add 80 mL of a 12 mol / L sodium hydroxide solution.

[0050] Step 3 (Alkaline hydrolysis): Heat the mixture to 85°C and stir for 2 hours at a constant temperature.

[0051] Step 4 (Hydrolysis): Heat the reaction system to boiling and reflux for 5 hours.

[0052] Step 5: After the reaction is complete, cool to 15℃ and filter. Wash the precipitate 2-3 times with a small amount of pure water. Combine the filtrates, add some phosphoric acid to obtain a pure trisodium phosphate solution, evaporate to crystallize, and dry to obtain pure trisodium phosphate powder. Dry the filter residue at 105℃ and weigh it.

[0053] Example 4 Step 1: Weigh 10g of sodium ferric pyrophosphate powder.

[0054] Step 2: Add 120 mL of sodium hydroxide solution with a concentration of 4 mol / L (close to the lower limit of the preferred range, appropriately increase the volume to ensure the amount of alkali).

[0055] Step 3 (alkali hydrolysis): The reaction is carried out at a constant temperature of 85°C with stirring for 3.5 hours.

[0056] Step 4 (Hydrolysis): Heat to boiling and reflux for 4 hours.

[0057] Step 5: Cool to 15℃, filter, wash, and separate. Combine the filtrates for phosphorus recovery crystallization. Evaporate the crystals, dry, and obtain pure trisodium phosphate powder. Dry the filter residue at 105℃ and weigh.

[0058] Example 5 Step 1: Weigh 10g of sodium ferric pyrophosphate powder.

[0059] Step 2: Add 100 mL of sodium hydroxide solution with a concentration of 13 mol / L (exceeding the preferred upper limit of 12 mol / L, the viscosity of the system increases significantly).

[0060] Step 3 (alkali hydrolysis): Stir the reaction at 85°C for 3 hours.

[0061] Step 4 (Hydrolysis): Heat to boiling and reflux for 3 hours.

[0062] Step 5: Cool to 10℃ and filter. A significant decrease in slurry filterability was observed, requiring a longer filtration time. Combine the filtrates to recover phosphorus, and dry and weigh the filter residue. Results showed a slight decrease in phosphorus recovery rate, trace amounts of dissolved iron were detectable in the filtrate, and washing became more difficult.

[0063] Comparative Example 1 Corresponding to the above embodiments, the difference between this comparative example and Example 1 is that a dilute sodium hydroxide solution with a concentration of 1 mol / L is used.

[0064] Reaction result: Insufficient alkali concentration, Fe(P2O7) 2- Incomplete ion destruction leads to a slow reaction rate and a significant decrease in phosphorus recovery.

[0065] Comparative Example 2 The difference between this comparative example and Example 1 is that the alkaline hydrolysis reaction temperature is only 25°C, corresponding to the above embodiments.

[0066] Reaction results: When the temperature is too low, the reaction kinetics are slow, and even with extended time, the iron-phosphorus separation efficiency is not high.

[0067] Comparative Example 3 Corresponding to the above embodiments, the difference between this comparative example and Example 1 is that the heating hydrolysis in step 4 is not performed, and the separation is performed directly after alkaline hydrolysis.

[0068] Reaction results: Phosphorus in the filtrate mainly exists in the form of sodium pyrophosphate, which is less stable and has a wider range of applications than sodium phosphate, resulting in lower value for phosphorus products.

[0069] Comparative Example 4 Corresponding to the above embodiments, this comparative example uses 2 mol / L sulfuric acid to leach sodium ferric pyrophosphate at 85°C for 2 hours.

[0070] Reaction result: Iron and phosphorus enter the solution together, resulting in an iron-phosphorus mixture. Additional steps (such as pH adjustment to precipitate iron) are required for separation, making the process complex and producing acidic wastewater containing iron and phosphorus.

[0071] Comparative Example 5 Step 1: Weigh 10g of sodium iron pyrophosphate powder and place it in a reactor equipped with a stirring and reflux device.

[0072] Step 2: Add 25 mL of 8 mol / L sodium hydroxide solution.

[0073] Step 3 (Alkaline hydrolysis): Heat the mixture to 85°C and react with stirring at a constant temperature for 3 hours in an open system with full contact with air. A reddish-brown precipitate will be observed to form.

[0074] Step 4 (Hydrolysis): Heat the reaction system to a gentle boiling state and continue reflux for 3 hours to allow the sodium pyrophosphate to be fully hydrolyzed.

[0075] Step 5: After the reaction is complete, cool to 20℃, filter, and wash the precipitate 2-3 times with a small amount of pure water. Combine the filtrates, add some phosphoric acid to obtain a pure trisodium phosphate solution, evaporate to crystallize, and dry to obtain pure trisodium phosphate powder. Dry the filter residue at 105℃ and weigh it.

[0076] The phosphorus recovery rate and iron leaching rate in the above reaction process were tested using the ICP method. XRD analysis of the product after evaporation of the filtrate was used to determine whether hydrolysis had occurred.

[0077] The test results of the above embodiments and comparative examples are summarized in Table 1.

[0078] Table 1 Test Results of the Example

[0079] Based on the data in Table 1, the method described in this embodiment, which involves adding a strong alkaline solution to decommissioned sodium ferric pyrophosphate material to obtain a mixed solution, heating the mixture in an air-contaminated environment, and allowing the hydroxide ions in the alkaline solution to undergo a metathesis reaction with the sodium ferric pyrophosphate to achieve solid-liquid separation of iron, phosphorus, and sodium, and continuing to heat the mixed system, causing the sodium pyrophosphate to hydrolyze in the high alkalinity and heated environment, converting it into soluble sodium phosphate, and then cooling the system to perform solid-liquid separation, evaporating and crystallizing the filtrate to obtain the sodium phosphate product, can achieve a phosphorus recovery rate of over 92.5% and an iron leaching rate of <1% in decommissioned sodium ferric pyrophosphate. When the strong alkaline solution sodium hydroxide was treated at different concentrations, it was found that within the range of strong alkalinity in this embodiment, hydrolysis could be achieved in the reaction process as the concentration of the strong alkaline solution increased, and the recovery rate of phosphorus increased with the increase of the concentration of the strong alkaline solution. The leaching rate of iron remained <1%. When the concentration of the strong alkaline solution was 4 mol / L, hydrolysis could still occur, but the recovery rate of phosphorus was significantly reduced. When the concentration of the strong alkaline solution was 13 mol / L, although phosphorus and sodium could be recovered, the recovery rate of phosphorus was significantly lower than that in Example 1, and the iron dissolution increased, making filtration more difficult and affecting the reaction balance in this embodiment.

[0080] Combining Example 1 and Comparative Example 2, it can be found that if the temperature of the alkaline hydrolysis process is too low, the reaction kinetics are slow, and even with extended time, the iron-phosphorus separation efficiency is not high, with a phosphorus recovery rate of only 45.2%. Combining Example 1 and Comparative Example 4, it can be found that traditional acid treatment of decommissioned sodium iron pyrophosphate materials allows iron and phosphorus to enter the solution together, resulting in an iron-phosphorus mixture. Additional steps (such as pH adjustment to precipitate iron) are required for separation, making the process complex and generating acidic wastewater containing iron and phosphorus.

[0081] Combining Example 1 and Comparative Example 3, it can be seen that if the phosphorus is directly separated after alkaline hydrolysis in the reaction system, although the concentration and temperature of the strong alkaline solution are optimized, the phosphorus in the filtrate of the reaction result mainly exists in the form of sodium pyrophosphate, which has lower stability and application range than sodium phosphate, resulting in a lower value for the phosphorus product.

[0082] Combining Example 1 and Comparative Example 5, it can be found that if the volume of sodium hydroxide is reduced, although the concentration of the sodium hydroxide solution is within the preferred range, the amount of hydroxide ions provided by the sodium hydroxide is too small, the reaction is incomplete, and sodium pyrophosphate cannot be completely converted into sodium phosphate. The resulting product is a mixture of the two, which also leads to a decrease in phosphorus recovery rate and purity.

[0083] The data in Table 1 further confirms that by controlling the concentration of the strong alkali in this embodiment to preferably be a concentrated sodium hydroxide solution of 5 mol / L to 12 mol / L, a sufficiently high OH- concentration can be provided. - The concentration is chosen to efficiently destroy stable complexes, ensuring rapid reaction while avoiding reagent waste, increased viscosity, and operational safety issues caused by excessive concentration. The alkaline hydrolysis temperature range of 85℃ to 95℃ effectively accelerates the reaction process and improves efficiency. The hydrolysis stage utilizes boiling or near-boiling temperatures to drive the hydrolysis reaction completely to the right, ensuring complete conversion of pyrophosphate to phosphate. This achieves a phosphorus recovery rate of greater than or equal to 92.5% and an iron leaching rate of less than 1% in this reaction system, thereby maximizing the separation of iron from sodium phosphate and significantly improving phosphorus conversion and product purity.

[0084] Although preferred embodiments have been described in this application, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of the embodiments of this application.

[0085] Obviously, those skilled in the art can make various modifications and variations to this application without departing from the spirit and scope of this application. Therefore, if such modifications and variations fall within the scope of the claims of this application and their equivalents, this application also intends to include such modifications and variations.

Claims

1. A method for recovering sodium iron pyrophosphate cathode material, characterized in that, The method includes: A strong alkaline solution is added to decommissioned sodium iron pyrophosphate material to obtain a mixture. The mixture is heated to a first-level temperature under air contact conditions to carry out an alkaline hydrolysis reaction to form a solid-liquid mixture containing a first iron-containing precipitate and dissolved sodium pyrophosphate. The solid-liquid mixture is heated to a secondary temperature, which is higher than the primary temperature, to continue the reaction until the pyrophosphate ions in the solid-liquid mixture are converted into phosphate ions to form a phosphate system solution. The phosphate system solution is cooled, and the solid phase in the solution after cooling is separated to obtain a filtrate. The filtrate is then evaporated to obtain the sodium phosphate product.

2. The method for recovering sodium iron pyrophosphate cathode material according to claim 1, characterized in that, The concentration of the strong alkali solution is 3 mol / L to 18 mol / L; Preferably, the concentration of the strong alkali solution is 5 mol / L to 12 mol / L; And / or, the strong alkaline solution is one or more of sodium hydroxide solution, potassium hydroxide solution, and lithium hydroxide solution.

3. The method for recovering sodium iron pyrophosphate cathode material according to claim 1 or 2, characterized in that, The ratio of the molar amount of the strong alkaline solution to the molar amount of the decommissioned sodium iron pyrophosphate material is greater than or equal to 8.

5.

4. The method for recovering sodium iron pyrophosphate cathode material according to claim 1 or 2, characterized in that, Heating the mixture to a primary temperature to initiate an alkaline hydrolysis reaction forms a solid-liquid mixture containing a first iron-containing precipitate and dissolved sodium pyrophosphate, specifically comprising: The mixture is heated to 85°C~95°C for 2~4 hours for alkaline hydrolysis to reduce the OH- ions in the strong alkali solution. - Fe in the decommissioned sodium ferric pyrophosphate material 3+ The first iron-containing precipitate is formed by the combination; Furthermore, the alkaline hydrolysis reaction causes phosphorus and sodium to form soluble sodium pyrophosphate, thereby forming a solid-liquid mixture containing the first iron-containing precipitate and dissolved sodium pyrophosphate.

5. The method for recovering sodium iron pyrophosphate cathode material according to claim 1 or 2, characterized in that, Continue heating the solid-liquid mixture to a secondary temperature to carry out the reaction, specifically including: Continue heating the solid-liquid mixture to 95°C~100°C and stir the reaction for 2h~4h.

6. The method for recovering sodium iron pyrophosphate cathode material according to claim 5, characterized in that, Continue heating the solid-liquid mixture until it boils to allow the reaction to proceed.

7. The method for recovering sodium iron pyrophosphate cathode material according to claim 4, characterized in that, The phosphate system solution was cooled to 10℃~20℃.

8. The method for recovering sodium iron pyrophosphate cathode material according to claim 7, characterized in that, The solid phase includes the first iron-containing precipitate and the second iron-containing precipitate formed by decomposing the tetrahydroxyferric oxide in the phosphate system solution by cooling to 10°C~20°C. And / or, the solid phase is ferric hydroxide precipitate.

9. The method for recovering sodium iron pyrophosphate cathode material according to claim 1 or 2, characterized in that, The phosphorus recovery rate in the reaction system is greater than or equal to 92.5%, and the iron leaching rate is less than 1%.

10. An application characterized in that, The sodium phosphate product obtained by the recovery method of sodium iron pyrophosphate cathode material according to any one of claims 1 to 9 can be used in the industrial or food fields.

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