Lithium dihydrogen phosphate iron removal agent and preparation process thereof, and lithium dihydrogen phosphate electrolysis iron removal process

Abstract

The invention relates to the technical field of lithium dihydrogen phosphate purification, and aims to solve the problems of insufficient chelating stability and easy decomposition and failure in a high-temperature electrolysis process in an existing process for removing iron from a lithium dihydrogen phosphate battery material through a chelating agent. The lithium dihydrogen phosphate iron removal agent is prepared from the following components in percentage by mass: 50 to 60 weight percent of a main chelating agent, 15 to 20 weight percent of an auxiliary reducing agent, 5 to 10 weight percent of an auxiliary dispersing agent and the balance of a pH (Potential of Hydrogen) regulator and a solvent; the main chelating agent is selected from one or more of amino trimethylene phosphonic acid, 1-hydroxyethylidene-1, 1-diphosphonic acid and ethylenediamine tetramethylene phosphonic acid, the auxiliary reducing agent is selected from one or more of L-ascorbic acid, erythorbic acid, sodium gluconate and thiolactic acid, and the auxiliary dispersing agent is selected from one or more of polyaspartic acid, polymaleic anhydride and polyacrylic acid. The lithium dihydrogen phosphate iron removal agent is adaptive to a lithium dihydrogen phosphate electrolysis process, can realize efficient adsorption iron removal, and is low in iron removal agent consumption and low in lithium loss rate.

Description

Technical Field

[0001] This invention relates to the field of lithium dihydrogen phosphate purification technology, and more specifically, to a lithium dihydrogen phosphate iron removal agent and its preparation process, and a lithium dihydrogen phosphate electrolytic iron removal process. Background Technology

[0002] Lithium dihydrogen phosphate, as a core raw material for cathode materials and electrolyte additives in lithium-ion batteries, requires extremely high purity, especially in the removal of iron impurities (Fe). 2+ / Fe 3+ Iron impurities can severely affect battery cycle stability and rate performance, and their content must be controlled below 5 ppm. In the lithium dihydrogen phosphate electrolytic preparation process, iron impurities are easily introduced through raw material slag, corrosion of electrolytic equipment, and electrolyte circulation.

[0003] Existing iron removal technologies have significant drawbacks: Traditional neutralization precipitation methods generate Fe(OH)3 precipitate by adjusting the pH, but Fe(OH)3 exists in colloidal form, resulting in extremely poor filtration performance and easy adsorption of lithium ions, leading to lithium loss rates exceeding 5%; while single chelating agents can complex iron ions, their chelation stability is insufficient under acidic electrolysis conditions, and they are prone to decomposition and failure at high temperatures, with some chelating agents exhibiting poor biodegradability and high costs; and reduction precipitation methods require strict control of the pH range, making operation difficult and easily leading to a decrease in the purity of lithium dihydrogen phosphate crystals.

[0004] For existing technologies involving chelation for iron removal, carboxylic acid chelating agents or amino acid chelating agents are commonly used. For example, patent CN106414805A discloses a chemical cleaning method and apparatus, where the cleaning solution includes a chelating agent, a reducing agent, or a mixture of both. The chelating agent is any one of amino acid-based, hydroxy acid-based, or organophosphorus chelating agents, and the reducing agent is any one of metal ions, sulfites, oxalic acid, formic acid, ascorbic acid, pyrogallol, hydrazine, or hydrogen. However, these methods of iron removal via chelating agents suffer from insufficient chelation stability and are prone to decomposition and failure during high-temperature electrolysis.

[0005] In existing lithium dihydrogen phosphate (LiH2PO4) electrolysis processes, the iron content of the resulting LiH2PO4 product is typically 10-20 ppm, achieved by adding chelating agents and other iron removal aids. Therefore, there is an urgent need to develop an iron removal agent and process that is suitable for the acidic and high-temperature environmental characteristics of LiH2PO4 electrolysis, and that offers high iron removal efficiency, low lithium loss, environmental friendliness, and simplicity. Summary of the Invention

[0006] The purpose of this invention is to solve the problems of insufficient chelation stability and easy decomposition and failure during high-temperature electrolysis in the existing process of removing iron from lithium dihydrogen phosphate battery materials using chelating agents.

[0007] This invention is achieved through the following technical solution:

[0008] This invention provides a lithium dihydrogen phosphate iron removal agent employing a chelation-reduction-dispersion ternary composite system, comprising the following components by mass fraction: (1) 50wt%-60wt% main chelating agent: phosphonic acid chelating agent. Phosphonic acid chelating agent molecules contain stable CP bonds and multiple phosphonic acid groups (-PO3H2), with high charge density and strong CP bond stability. They are not easily hydrolyzed in an acidic environment of pH≈2-6 and at an electrolysis temperature of 60-80℃. They can coordinate with three phosphonic acid groups through the central nitrogen atom and react with Fe. 3+ / Fe 2+ It forms stable five- or six-membered ring chelates, with chelation priority higher than lithium ions, and does not affect the crystallization of lithium dihydrogen phosphate.

[0009] Specifically, the phosphonic acid chelating agent can be selected from one or more of aminotrimethylphosphonic acid (ATMP), hydroxyethylidene diphosphonic acid (HEDP), and ethylenediaminetetramethylphosphonic acid (EDTMP). More preferably, it is aminotrimethylphosphonic acid (ATMP), which has a chelation stability constant of up to 10. 30 The above methods offer fast complexation rates, moderate drug costs, environmental friendliness, and superior compatibility.

[0010] (2) 15wt%-20wt% auxiliary reducing agent: a mild organic reducing agent, specifically one or more of L-ascorbic acid, isoascorbic acid, sodium gluconate, thiolactic acid, etc. This type of reducing agent has mild reducing properties and can reduce Fe... 3+ Efficient reduction to Fe 2+ ATMP for Fe 2+ The complexation rate compared to Fe 3+ The complexation rate is more than 30% faster, which can effectively improve the complexation efficiency of the main chelating agent and shorten the chelation time; at the same time, it does not produce irritating gases in the electrolytic acidic environment, avoids the introduction of harmful impurities such as chloride ions and sulfate ions, has a high biodegradability rate, and is also environmentally friendly.

[0011] The preferred chelating agent is L-ascorbic acid, which has a controllable reduction rate, is not easily oxidized and degraded under electrolytic conditions, has a biodegradability rate of over 90%, and can form a synergistic effect with the main chelating agent to further enhance chelation stability.

[0012] (3) 5wt%-10wt% dispersant: Water-soluble polycarboxylic acid dispersant, specifically one or more of polyaspartic acid, polymaleic anhydride, or polyacrylic acid, with the relative molecular mass of polyaspartic acid in the range of 1000-2000, and the relative molecular masses of polymaleic anhydride and polyacrylic acid in the range of 800-1500. The molecular chain of this type of dispersant contains multiple hydrophilic groups, which can be adsorbed on the surface of the chelate to form steric hindrance, preventing the chelate from agglomerating and forming large particle precipitates that adhere to the electrode surface. At the same time, it inhibits the formation of Fe(OH)3 colloids that may be generated in small amounts, thereby improving the solid-liquid separation performance. It can also help chelate some iron ions, producing a synergistic effect with ATMP, and does not affect the purity of lithium dihydrogen phosphate products.

[0013] (4) The remainder is pH adjuster and solvent: The pH adjuster is lithium hydroxide and the solvent is deionized water. The pH adjuster can be a lithium hydroxide solution with a mass concentration of about 10wt%, which is used to adjust the pH value of the iron removal agent product to 2.5-3.5 to adapt to the acidic environment of lithium dihydrogen phosphate electrolyte and avoid changing the pH of the electrolyte system after addition.

[0014] Preferably, the lithium dihydrogen phosphate iron removal agent proposed in this invention, by mass fraction, comprises 55wt% ATMP, 18wt% L-ascorbic acid, 8wt% polyaspartic acid, 2wt% lithium hydroxide (to adjust pH to 3.0), and 17wt% deionized water.

[0015] This invention also proposes a preparation process for the above-mentioned lithium dihydrogen phosphate iron removal agent, comprising the following steps: (1) Raw material pretreatment: First, place the main chelating agent, auxiliary reducing agent, and dispersant in a vacuum drying oven and dry for 2 hours at 60℃ and -0.08MPa to remove moisture and trace impurities, thus avoiding affecting the chelating activity.

[0016] (2) Mixing and dissolving: Add deionized water to the reactor equipped with a constant temperature stirring device according to the metered amount, control the water temperature at 40-50℃, and the stirring speed at 200-300r / min. Add the pretreated main chelating agent and dispersant in sequence, and continue stirring for 1-2 hours until the solid is completely dissolved and a uniform and transparent initial mixed solution is formed.

[0017] (3) Constructing a reduction system: Maintain a water temperature of 40-50℃ and a constant stirring speed, slowly add the auxiliary reducing agent, and continue stirring for 20-40 minutes to obtain a premix. While adding the agent, nitrogen gas is also introduced at a flow rate of 0.5L / min for protection to prevent the auxiliary reducing agent from being oxidized by oxygen in the air and to ensure the reducing activity.

[0018] (4) Adjusting pH and aging treatment: The pH adjuster is slowly added dropwise to the reaction vessel, and the pH value of the solution is monitored in real time. The addition is stopped when the pH value of the solution system is adjusted to 2.5-3.5. Then the temperature is raised to 50-60℃ and the mixture is stirred for 0.5-2 hours to mature the system, so that the components can work together to form a stable composite system.

[0019] (5) Filtration and storage: After maturation, the solution system is cooled to room temperature and filtered through a 0.22μm ultrafiltration membrane to remove a small amount of insoluble impurities. The resulting pale yellow transparent liquid is the iron removal agent product. It should be sealed and stored in a cool, dry place, with an effective shelf life of 6 months.

[0020] The lithium dihydrogen phosphate iron removal agent proposed in this invention utilizes chelation to remove Fe 2+ / Fe 3+ Iron impurities are removed through adsorption, primarily suitable for conventional lithium dihydrogen phosphate electrolysis processes, i.e., electrolysis environments where the electrolytic cell is made of titanium alloy, the anode uses a platinum-iridium alloy, and the cathode uses a copper sheet. Using this lithium dihydrogen phosphate iron removal agent, this invention also proposes a lithium dihydrogen phosphate electrolysis iron removal process, specifically including the following steps: (1) Pretreatment before electrolysis: Crude lithium dihydrogen phosphate was dissolved in deionized water to prepare an electrolyte with a concentration of 0.8-1.2 mol / L. The electrolyte was placed in a pretreatment tank, heated to 60°C, and stirred at a rate of 150 r / min. Simultaneously, the lithium dihydrogen phosphate iron removal agent proposed in this invention was added to the pretreatment tank at a rate of 0.3 wt%-0.5 wt% of the electrolyte mass. The reaction was continued with stirring for 2 hours to allow the iron removal agent to fully chelate with the free iron ions in the electrolyte, forming a stable phosphonic acid-iron chelate.

[0021] After the reaction is completed, a plate and frame filter press is used to filter the electrolyte with a filter membrane with a pore size of about 0.1 μm to remove a small amount of agglomerates and mechanical impurities, and the pretreated electrolyte is obtained. At this time, the iron content in the pretreated electrolyte can be reduced to below 20 ppm.

[0022] (2) Synergistic iron removal during electrolysis: The pretreated electrolyte is injected into the electrolytic cell for electrolysis. The electrolysis process parameters are controlled as follows: electrolysis temperature 65-75℃, flux density 200-250 A / m³. 2 The electrolytic cell voltage is 2.8-3.2V, the electrolyte circulation rate is 3-8L / h, the electrolysis time is 8-10h, and lithium dihydrogen phosphate crystals are obtained after electrolysis.

[0023] During electrolysis, an iron-removing agent is precisely added via a metering pump at a rate of 0.03-0.08% (volume fraction) of the electrolyte circulation volume per hour. This agent complexes newly generated iron ions due to equipment corrosion and raw material residue during electrolysis, preventing iron ion deposition on the electrode surface and thus avoiding impacts on electrolysis efficiency and product purity. Furthermore, samples are taken every 2 hours during electrolysis to test the iron content in the electrolyte, ensuring that the iron content remains consistently below 5 ppm.

[0024] (3) Separation after electrolysis: The lithium dihydrogen phosphate crystallization solution in the electrolytic cell is exported and first vacuum filtered, with the thickness of the filter cake controlled at 2-3 cm, to separate lithium dihydrogen phosphate crystals and mother liquor. The mother liquor contains unreacted iron removal agent and chelates, which can be returned to the pretreatment tank in step (1) for recycling. Every 2-3 cycles, a new iron removal agent needs to be added, with the amount added being 20%-50% (volume fraction) of the initial amount, thereby reducing the consumption of iron removal agent. The separated lithium dihydrogen phosphate crystals are washed 2-3 times with deionized water, with the washing water volume controlled at about 1.5 times the crystal mass. After washing, they are vacuum dried at 80℃ and -0.09MPa for 4 hours to obtain a refined lithium dihydrogen phosphate product with an iron content ≤3ppm and a purity ≥99.95%.

[0025] The technical solution of the present invention has the following beneficial effects: (1) The iron removal agent of lithium dihydrogen phosphate proposed in this invention achieves the synergistic effect of chelation, reduction and dispersion through the synergistic action of aminotrimethylphosphonic acid, L-ascorbic acid and polyaspartic acid. In the high temperature and acidic lithium dihydrogen phosphate electrolysis environment, it can efficiently adsorb and remove iron ion impurities. Adding this iron removal agent during the lithium dihydrogen phosphate electrolysis process can make the iron content in the final lithium dihydrogen phosphate product stably controlled below 3ppm, which is far lower than the iron content of lithium dihydrogen phosphate products obtained by the existing electrolytic iron removal process.

[0026] (2) The lithium dihydrogen phosphate iron removal agent is suitable for lithium dihydrogen phosphate electrolysis process. When the lithium dihydrogen phosphate iron removal agent is introduced during the electrolysis process, it not only has an outstanding adsorption and impurity removal effect on iron impurities, but also has a very low lithium loss rate. It can effectively avoid the lithium loss caused by the adsorption of lithium ions by Fe(OH)3 colloid in the traditional neutralization precipitation method, and improve the utilization rate of lithium resources to a greater extent.

[0027] (3) The present invention proposes a lithium dihydrogen phosphate iron removal agent for the lithium dihydrogen phosphate electrolysis process. It has a strong compatibility with the electrolysis process. The pH value and other characteristics of the iron removal agent are compatible with the electrolyte. It will not affect the acidity or alkalinity of the electrolysis system during the electrolysis process. In addition, the polyaspartic acid in the lithium dihydrogen phosphate iron removal agent can prevent chelates from adhering to the electrode, ensuring stable electrolysis efficiency without the need for additional equipment modification.

[0028] (4) The lithium dihydrogen phosphate iron removal agent, its preparation process and application method proposed in this invention have the advantages of being environmentally friendly and economical. All components are environmentally friendly, biodegradable, and generate no harmful waste. The mother liquor can be recycled, and the overall consumption of the iron removal agent can be reduced by about 40% or even more, making it suitable for large-scale industrial production. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, they are performed according to conventional conditions or conditions recommended by the manufacturer; where the manufacturers of the instruments, equipment, reagents, or raw materials used are not specified, they are all conventional products that can be purchased commercially.

[0030] Using the same electrolysis equipment and basic processes, the following examples and comparative examples were performed. Specifically, the electrolytic cell was made of titanium alloy, the anode was a platinum-iridium alloy, the cathode was a copper sheet, and the raw electrolyte was a 1.0 mol / L crude lithium dihydrogen phosphate electrolyte prepared by mixing crude lithium dihydrogen phosphate with deionized water, where the initial iron content was approximately 50 ppm. The electrolysis temperature was approximately 70°C, and the current density was 220 A / m³. 2 The electrolytic cell voltage is 3.0V, the electrolyte circulation rate is 5L / h, and the electrolysis time is 9h.

[0031] Example 1 (1) Preparation of lithium dihydrogen phosphate iron removal agent: 55 wt% ATMP, 18 wt% L-ascorbic acid, 8 wt% polyaspartic acid with a relative molecular mass of approximately 1500, 2 wt% lithium hydroxide, and 17 wt% deionized water were prepared as raw materials. Each of the above solid raw material components was dried under vacuum at 60℃ and -0.08 MPa for 2 hours. Separately, deionized water was heated to 45℃ and stirred at 250 rpm. ATMP and polyaspartic acid were added while stirring, and stirring continued for 1.5 hours after the addition was complete to obtain a preliminary mixed solution. Then, under a protective atmosphere of nitrogen continuously purging at 0.5 L / min, L-ascorbic acid was added to the preliminary mixed solution, and the mixture was stirred at 250 rpm for 30 minutes to obtain a premix. The pH of the premix was adjusted to approximately 3.0 by slowly adding a 10wt% lithium hydroxide solution. The mixture was then heated to 55°C, stirred for 1 hour, and filtered through a 0.22μm ultrafiltration membrane to remove solid impurities, yielding a liquid lithium dihydrogen phosphate iron removal agent.

[0032] (2) Lithium dihydrogen phosphate electrolysis and iron removal treatment: The crude lithium dihydrogen phosphate electrolyte was placed in a pretreatment tank, heated to 60°C, and stirred at 150 r / min for 2 h. Simultaneously, a lithium dihydrogen phosphate iron removal agent was added at a concentration of 0.4 wt% of the electrolyte mass. The electrolyte was then filtered using a plate and frame filter press with a 0.1 μm pore size membrane to obtain the pretreated electrolyte. Inductively coupled plasma mass spectrometry (ICP-MS) analysis showed that the iron content of the pretreated electrolyte was 12.3 ppm.

[0033] The pretreated electrolyte was injected into an electrolytic cell for electrolysis to obtain lithium dihydrogen phosphate crystals. During electrolysis, lithium dihydrogen phosphate iron removal agent was added hourly at a rate of 0.05% of the electrolyte circulation volume. The lithium dihydrogen phosphate crystals were then exported from the electrolytic cell and vacuum filtered, with the filter cake thickness controlled to approximately 2.5 cm. Lithium dihydrogen phosphate crystals and mother liquor were separated. The lithium dihydrogen phosphate crystals were washed twice with 1.5 times their mass of deionized water and then dried under vacuum at 80°C and -0.09 MPa for 4 hours to obtain electrolytically refined lithium dihydrogen phosphate product.

[0034] Example 2 (1) Preparation of lithium dihydrogen phosphate iron removal agent: 58 wt% HEDP, 16 wt% isoascorbic acid, 7 wt% polymaleic anhydride with a relative molecular mass of approximately 1200, 3 wt% lithium hydroxide, and 16 wt% deionized water were prepared as raw materials. Each of the above solid raw material components was dried under vacuum at 60℃ and -0.08 MPa for 2 hours. Separately, deionized water was heated to 45℃ and stirred at 250 rpm. ATMP and polyaspartic acid were added while stirring, and stirring continued for 1.5 hours after the addition was complete to obtain a preliminary mixed solution. Then, under a protective atmosphere of nitrogen continuously purging at 0.5 L / min, L-ascorbic acid was added to the preliminary mixed solution, and the mixture was stirred at 250 rpm for 30 minutes to obtain a premix. The pH of the premix was adjusted to approximately 3.0 by slowly adding a 10wt% lithium hydroxide solution. The mixture was then heated to 52°C, stirred for 1.2 hours, and filtered through a 0.22μm ultrafiltration membrane to remove solid impurities, yielding a liquid lithium dihydrogen phosphate iron removal agent.

[0035] (2) Lithium dihydrogen phosphate electrolysis and iron removal treatment: The crude lithium dihydrogen phosphate electrolyte was placed in a pretreatment tank, heated to 60°C, and stirred at 150 r / min for 2 h. Simultaneously, a lithium dihydrogen phosphate iron removal agent was added at a concentration of 0.35 wt% of the electrolyte mass. The electrolyte was then filtered using a plate and frame filter press with a 0.1 μm pore size membrane to obtain the pretreated electrolyte. Inductively coupled plasma mass spectrometry (ICP-MS) analysis showed that the iron content of the pretreated electrolyte was 14.7 ppm.

[0036] The pretreated electrolyte was injected into an electrolytic cell for electrolysis to obtain lithium dihydrogen phosphate crystals. During electrolysis, lithium dihydrogen phosphate iron removal agent was added hourly at a rate of 0.04% of the electrolyte circulation volume. The lithium dihydrogen phosphate crystals were then exported from the electrolytic cell and vacuum filtered. The thickness of the filter cake obtained was controlled to be approximately 2.5 cm. Lithium dihydrogen phosphate crystals and mother liquor were separated. The lithium dihydrogen phosphate crystals were washed twice with 1.5 times their mass of deionized water and then dried under vacuum at 80℃ and -0.09 MPa for 4 hours to obtain electrolytically refined lithium dihydrogen phosphate product.

[0037] Example 3 (1) Preparation of lithium dihydrogen phosphate iron removal agent: 52 wt% EDTMP, 19 wt% sodium gluconate, 9 wt% polyacrylic acid with a relative molecular mass of approximately 1000, 2 wt% lithium hydroxide, and 18 wt% deionized water were prepared as raw materials. Each of these solid raw material components was dried under vacuum at 60°C and -0.08 MPa for 2 hours. Separately, deionized water was heated to 45°C and stirred at 250 rpm. ATMP and polyaspartic acid were added while stirring, and stirring continued for 1.5 hours after the addition was complete to obtain a preliminary mixed solution. L-ascorbic acid was then added to the preliminary mixed solution under a protective atmosphere of nitrogen continuously purging at 0.5 L / min, and stirred at 250 rpm for 35 minutes to obtain a premix. The pH of the premix was adjusted to approximately 3.0 by slowly adding 10 wt% lithium hydroxide solution. The mixture was then heated to 58°C and stirred for 1 hour. Solid impurities were removed using a 0.22 μm ultrafiltration membrane to obtain a liquid lithium dihydrogen phosphate iron removal agent.

[0038] (2) Lithium dihydrogen phosphate electrolysis and iron removal treatment: The crude lithium dihydrogen phosphate electrolyte was placed in a pretreatment tank, heated to 60°C, and stirred at 150 r / min for 2 h. Simultaneously, a lithium dihydrogen phosphate iron removal agent was added at a concentration of 0.45 wt% of the electrolyte mass. The electrolyte was then filtered using a plate and frame filter press with a 0.1 μm pore size membrane to obtain the pretreated electrolyte. Inductively coupled plasma mass spectrometry (ICP-MS) analysis showed that the iron content of the pretreated electrolyte was 13.5 ppm.

[0039] The pretreated electrolyte was injected into an electrolytic cell for electrolysis to obtain lithium dihydrogen phosphate crystals. During electrolysis, lithium dihydrogen phosphate iron removal agent was added hourly at a rate of 0.06% of the electrolyte circulation volume. The lithium dihydrogen phosphate crystals were then exported from the electrolytic cell and vacuum filtered, with the filter cake thickness controlled to approximately 2.5 cm. Lithium dihydrogen phosphate crystals and mother liquor were separated. The lithium dihydrogen phosphate crystals were washed twice with 1.5 times their mass of deionized water and then dried under vacuum at 80°C and -0.09 MPa for 4 hours to obtain electrolytically refined lithium dihydrogen phosphate product.

[0040] Example 4 (1) Preparation of lithium dihydrogen phosphate iron removal agent: 56 wt% ATMP, 17 wt% thiolactic acid, 6 wt% polyaspartic acid with a relative molecular mass of approximately 1300, 2 wt% lithium hydroxide, and 19 wt% deionized water were prepared as raw materials. Each of the above solid raw material components was dried under vacuum at 60℃ and -0.08 MPa for 2 hours. Separately, deionized water was heated to 45℃ and stirred at 250 rpm. ATMP and polyaspartic acid were added while stirring, and stirring continued for 1.5 hours after the addition was complete to obtain a preliminary mixed solution. L-ascorbic acid was then added to the preliminary mixed solution under a protective atmosphere of nitrogen continuously purging at 0.5 L / min, and stirred at 250 rpm for 30 minutes to obtain a premix. The pH of the premix was adjusted to approximately 3.0 by slowly adding 10 wt% lithium hydroxide solution. The mixture was then heated to 55℃ and stirred for 1 hour. Solid impurities were removed using a 0.22 μm ultrafiltration membrane to obtain a liquid lithium dihydrogen phosphate iron removal agent.

[0041] (2) Lithium dihydrogen phosphate electrolysis and iron removal treatment: The crude lithium dihydrogen phosphate electrolyte was placed in a pretreatment tank, heated to 60°C, and stirred at 150 r / min for 2 h. Simultaneously, a lithium dihydrogen phosphate iron removal agent was added at a concentration of 0.4 wt% of the electrolyte mass. The resulting mother liquor was recycled three times, with 0.12% (by weight of the electrolyte mass) of fresh lithium dihydrogen phosphate iron removal agent added during the third cycle. The electrolyte was then filtered using a plate and frame filter press with a 0.1 μm pore size membrane to obtain the pretreated electrolyte. Inductively coupled plasma mass spectrometry (ICP-MS) analysis showed that the iron content of the pretreated electrolyte, after three cycles of mother liquor pretreatment, was 15.1 ppm.

[0042] The pretreated electrolyte was injected into an electrolytic cell for electrolysis to obtain lithium dihydrogen phosphate crystals. During electrolysis, lithium dihydrogen phosphate iron removal agent was added hourly at a rate of 0.05% of the electrolyte circulation volume. The lithium dihydrogen phosphate crystals were then exported from the electrolytic cell and vacuum filtered, with the filter cake thickness controlled to approximately 2.5 cm. Lithium dihydrogen phosphate crystals and mother liquor were separated. The lithium dihydrogen phosphate crystals were washed twice with 1.5 times their mass of deionized water and then dried under vacuum at 80°C and -0.09 MPa for 4 hours to obtain electrolytically refined lithium dihydrogen phosphate product.

[0043] Example 5 (1) Preparation of lithium dihydrogen phosphate iron removal agent: 50 wt% ATMP, 20-18 wt% L-ascorbic acid, 10 wt% polyaspartic acid with a relative molecular mass of approximately 2000, 2 wt% lithium hydroxide, and 18 wt% deionized water were prepared as raw materials. Each of the above solid raw material components was dried under vacuum at 60℃ and -0.08 MPa for 2 hours. Separately, deionized water was heated to 45℃ and stirred at 250 rpm. ATMP and polyaspartic acid were added while stirring, and stirring continued for 2 hours after the addition was complete to obtain a preliminary mixed solution. Then, under a protective atmosphere of nitrogen continuously purging at 0.5 L / min, L-ascorbic acid was added to the preliminary mixed solution, and the mixture was stirred at 250 rpm for 30 minutes to obtain a premix. The pH of the premix was adjusted to approximately 3.0 by slowly adding a 10wt% lithium hydroxide solution. The mixture was then heated to 55°C, stirred for 2 hours, and filtered through a 0.22μm ultrafiltration membrane to remove solid impurities, yielding a liquid lithium dihydrogen phosphate iron removal agent.

[0044] (2) Lithium dihydrogen phosphate electrolysis and iron removal treatment: The crude lithium dihydrogen phosphate electrolyte was placed in a pretreatment tank, heated to 60°C, and stirred at 150 r / min for 2 h. Simultaneously, a lithium dihydrogen phosphate iron removal agent was added at a concentration of 0.3 wt% of the electrolyte mass. The electrolyte was then filtered using a plate and frame filter press with a 0.1 μm pore size membrane to obtain the pretreated electrolyte. Inductively coupled plasma mass spectrometry (ICP-MS) analysis showed that the iron content of the pretreated electrolyte was 16.8 ppm.

[0045] The pretreated electrolyte was injected into an electrolytic cell for electrolysis to obtain lithium dihydrogen phosphate crystals. During electrolysis, lithium dihydrogen phosphate iron removal agent was added hourly at a rate of 0.03% of the electrolyte circulation volume. The lithium dihydrogen phosphate crystals were then exported from the electrolytic cell and vacuum filtered, with the filter cake thickness controlled to approximately 2.5 cm. Lithium dihydrogen phosphate crystals and mother liquor were separated. The lithium dihydrogen phosphate crystals were washed twice with 1.5 times their mass of deionized water and then dried under vacuum at 80°C and -0.09 MPa for 4 hours to obtain electrolytically refined lithium dihydrogen phosphate product.

[0046] Comparative Example 1 (1) Preparation of iron removal agent: Take 60wt% ATMP and 40wt% deionized water by mass fraction. First, place the ATMP in a vacuum dryer at 60℃ and -0.08MPa for 2h. Take the deionized water, heat it to 45℃ and stir at 250r / min. Add the ATMP while stirring. After the addition is completed, continue stirring for 1.5h to obtain the iron removal agent.

[0047] (2) Lithium dihydrogen phosphate electrolysis and iron removal treatment: The crude lithium dihydrogen phosphate electrolyte was placed in a pretreatment tank, heated to 60°C, and stirred at 150 r / min for 2 h. An iron-removing agent was added simultaneously, at a concentration of 0.8 wt% of the electrolyte mass. The electrolyte was then filtered using a plate and frame filter press with a 0.1 μm pore size membrane to obtain the pretreated electrolyte. Inductively coupled plasma mass spectrometry (ICP-MS) analysis showed that the iron content of the pretreated electrolyte was 35.2 ppm.

[0048] The pretreated electrolyte was injected into an electrolytic cell for electrolysis to obtain lithium dihydrogen phosphate crystals. The lithium dihydrogen phosphate crystals were then exported from the electrolytic cell and vacuum filtered. The thickness of the filter cake obtained by vacuum filtration was controlled to be about 2.5 cm. Lithium dihydrogen phosphate crystals and mother liquor were separated. The lithium dihydrogen phosphate crystals were washed twice with 1.5 times their mass of deionized water and then dried under vacuum at 80℃ and -0.09 MPa for 4 hours to obtain electrolytically refined lithium dihydrogen phosphate product.

[0049] Comparative Example 2 (1) Preparation of iron removal agent: 65 wt% ATMP, 10 wt% polyaspartic acid with a relative molecular mass of approximately 1500, and 25 wt% deionized water were prepared as raw materials. Each of these solid raw material components was dried under vacuum at 60℃ and -0.08 MPa for 2 hours. Separately, deionized water was heated to 45℃ and stirred at 250 rpm. ATMP and polyaspartic acid were added while stirring, and stirring continued for 1.5 hours after the addition was complete to obtain a preliminary mixed solution. A trace amount of 10 wt% lithium hydroxide solution was then slowly added dropwise to adjust the pH of the premix to approximately 3.0. The mixture was then heated to 55℃ and stirred for 1 hour. Solid impurities were removed using a 0.22 μm ultrafiltration membrane to obtain a liquid iron removal agent.

[0050] (2) Lithium dihydrogen phosphate electrolysis and iron removal treatment: The crude lithium dihydrogen phosphate electrolyte was placed in a pretreatment tank, heated to 60°C, and stirred at 150 r / min for 2 h. An iron-removing agent was added simultaneously, at a concentration of 0.4 wt% of the electrolyte mass. The electrolyte was then filtered using a plate and frame filter press with a 0.1 μm pore size membrane to obtain the pretreated electrolyte. Inductively coupled plasma mass spectrometry (ICP-MS) analysis showed that the iron content of the pretreated electrolyte was 23.6 ppm.

[0051] The pretreated electrolyte was injected into an electrolytic cell for electrolysis to obtain lithium dihydrogen phosphate crystals. During electrolysis, an iron-removing agent was added hourly at a rate of 0.05% of the electrolyte circulation volume. The lithium dihydrogen phosphate crystals were then exported from the electrolytic cell and vacuum filtered. The thickness of the filter cake obtained was controlled to be approximately 2.5 cm. Lithium dihydrogen phosphate crystals and mother liquor were separated. The lithium dihydrogen phosphate crystals were washed twice with 1.5 times their mass of deionized water and then dried under vacuum at 80℃ and -0.09 MPa for 4 hours to obtain electrolytically refined lithium dihydrogen phosphate product.

[0052] Comparative Example 3 (1) Preparation of iron removal agent: 60 wt% ATMP, 20 wt% L-ascorbic acid, and 20 wt% deionized water were prepared as raw materials. Each solid raw material component was dried under vacuum at 60℃ and -0.08 MPa for 2 hours. Deionized water was heated to 45℃ and stirred at 250 rpm. ATMP was added while stirring, and stirring continued for 1.5 hours after the addition was complete to obtain a preliminary mixed solution. L-ascorbic acid was then added to the preliminary mixed solution under a protective atmosphere of nitrogen continuously purging at 0.5 L / min, and stirred at 250 rpm for 30 minutes to obtain a premix. A trace amount of 10 wt% lithium hydroxide solution was then slowly added dropwise to adjust the pH of the premix to approximately 3.0. The mixture was then heated to 55℃ and stirred for 1 hour. Solid impurities were removed using a 0.22 μm ultrafiltration membrane to obtain a liquid iron removal agent.

[0053] (2) Lithium dihydrogen phosphate electrolysis and iron removal treatment: The crude lithium dihydrogen phosphate electrolyte was placed in a pretreatment tank, heated to 60°C, and stirred at 150 r / min for 2 h. An iron-removing agent was added simultaneously, at a concentration of 0.4 wt% of the electrolyte mass. The electrolyte was then filtered using a plate and frame filter press with a 0.1 μm pore size membrane to obtain the pretreated electrolyte. Inductively coupled plasma mass spectrometry (ICP-MS) analysis showed that the iron content of the pretreated electrolyte was 18.7 ppm.

[0054] The pretreated electrolyte was injected into an electrolytic cell for electrolysis to obtain lithium dihydrogen phosphate crystals. During electrolysis, an iron-removing agent was added hourly at a rate of 0.05% of the electrolyte circulation volume. The lithium dihydrogen phosphate crystals were then exported from the electrolytic cell and vacuum filtered. The thickness of the filter cake obtained was controlled to be approximately 2.5 cm. Lithium dihydrogen phosphate crystals and mother liquor were separated. The lithium dihydrogen phosphate crystals were washed twice with 1.5 times their mass of deionized water and then dried under vacuum at 80℃ and -0.09 MPa for 4 hours to obtain electrolytically refined lithium dihydrogen phosphate product.

[0055] Comparative Example 4 This comparative example uses the traditional neutralization and precipitation method for lithium dihydrogen phosphate electrolysis and iron removal: First, ammonia was added dropwise to the crude lithium dihydrogen phosphate electrolyte to adjust the pH to approximately 5.0. The solution was then heated to 60°C and stirred at 150 rpm for 2 hours. The Fe(OH)3 precipitate was removed by filtration, yielding the pretreated electrolyte. Inductively coupled plasma mass spectrometry (ICP-MS) analysis showed that the iron content of the pretreated electrolyte was 10.5 ppm.

[0056] The pretreated electrolyte was injected into an electrolytic cell for electrolysis, and the electrolysis voltage was adjusted to 3.5V to obtain lithium dihydrogen phosphate crystals. During the electrolysis process, Fe(OH)3 colloids were redissolved, resulting in a decrease in the iron removal rate. The lithium dihydrogen phosphate crystals were then exported from the electrolytic cell and vacuum filtered. The thickness of the filter cake obtained by vacuum filtration was controlled to be about 2.5cm, and lithium dihydrogen phosphate crystals were separated. The lithium dihydrogen phosphate crystals were washed twice with 1.5 times their mass of deionized water and then placed in a vacuum dryer at 80℃ and -0.09MPa for 4h to obtain electrolytically refined lithium dihydrogen phosphate product.

[0057] Comparative Example 5 (1) Preparation of iron removal agent: 55 wt% EDTA, 18 wt% L-ascorbic acid, 8 wt% polyaspartic acid with a relative molecular mass of approximately 1500, 2 wt% lithium hydroxide, and 17 wt% deionized water were prepared as raw materials. Each of these solid raw material components was dried under vacuum at 60°C and -0.08 MPa for 2 hours. Separately, deionized water was heated to 45°C and stirred at 250 rpm. ATMP and polyaspartic acid were added while stirring, and stirring continued for 1.5 hours after the addition was complete to obtain a preliminary mixed solution. L-ascorbic acid was then added to the preliminary mixed solution under a protective atmosphere of nitrogen continuously purging at 0.5 L / min, and stirred at 250 rpm for 30 minutes to obtain a premix. The pH of the premix was adjusted to approximately 3.0 by slowly adding 10 wt% lithium hydroxide solution. The mixture was then heated to 55°C and stirred for 1 hour. Solid impurities were removed using a 0.22 μm ultrafiltration membrane to obtain a liquid iron removal agent.

[0058] (2) Lithium dihydrogen phosphate electrolysis and iron removal treatment: The crude lithium dihydrogen phosphate electrolyte was placed in a pretreatment tank, heated to 60°C, and stirred at 150 r / min for 2 h. Simultaneously, an iron-removing agent was added at a concentration of 0.4 wt% of the electrolyte mass. The electrolyte was then filtered using a plate and frame filter press with a 0.1 μm pore size membrane to obtain the pretreated electrolyte. Inductively coupled plasma mass spectrometry (ICP-MS) analysis showed that the iron content of the pretreated electrolyte was 28.4 ppm. This analysis indicates that under this electrolytic environment, the main component of the iron-removing agent, EDTA, had decomposed and become ineffective, resulting in a weak iron removal effect.

[0059] The pretreated electrolyte was injected into an electrolytic cell for electrolysis to obtain lithium dihydrogen phosphate crystals. During electrolysis, an iron-removing agent was added hourly at a rate of 0.05% of the electrolyte circulation volume. The lithium dihydrogen phosphate crystals were then exported from the electrolytic cell and vacuum filtered. The thickness of the filter cake obtained was controlled to be approximately 2.5 cm. Lithium dihydrogen phosphate crystals and mother liquor were separated. The lithium dihydrogen phosphate crystals were washed twice with 1.5 times their mass of deionized water and then dried under vacuum at 80℃ and -0.09 MPa for 4 hours to obtain electrolytically refined lithium dihydrogen phosphate product.

[0060] Test case Lithium dihydrogen phosphate products obtained from electrolysis in Example 1 and Comparative Example 1 were used as different samples. The following performance tests were conducted on each sample: the iron content was detected using inductively coupled plasma mass spectrometry (ICP-MS); the lithium concentration in the electrolyte before and after electrolysis was detected using atomic absorption spectrophotometry, and the lithium loss rate was calculated based on the lithium concentration difference; the iron removal rate was calculated based on the initial iron content and the product iron content: iron removal rate = (initial iron content - product iron content) / initial iron content × 100%; the electrolysis efficiency was comprehensively evaluated based on the fluctuation range of the cell voltage during electrolysis (≤±0.1V was considered stable) and the product purity (detected by high performance liquid chromatography). The results of the above tests are summarized in Table 1 below: Table 1 Performance test results of different samples

[0061] As can be seen from the above experiments and the test results in Table 1: (1) Under the same raw material conditions, the iron content of the lithium dihydrogen phosphate products in Examples 1-5 is ≤2.9ppm and the iron removal rate is ≥94.2%, which means that the iron removal effect is significantly better than that of conventional chelating agents or traditional iron removal methods used in Comparative Examples 1-5. This shows that the lithium dihydrogen phosphate iron removal agent proposed in this invention has a more significant iron removal effect.

[0062] (2) In Examples 1-5, the lithium loss rate during the electrolytic treatment of lithium dihydrogen phosphate was ≤0.27%, while the lithium loss rate in Comparative Example 4 was as high as 6.82%, and the lithium loss rate in Comparative Example 1 was also 1.23%. This is because: the chelating agent of the present invention has a higher chelating priority for iron ions than for lithium ions, and there is no Fe(OH)3 colloid adsorption of lithium, and the dispersant further reduces the entrainment loss of lithium.

[0063] (3) The electrolytic cell voltage fluctuation range in Examples 1-5 is ≤ ±0.09V and the product purity is ≥99.95%, while the cell voltage fluctuation in Comparative Examples 1-5 is significantly larger and the purity is significantly lower. This indicates that the lithium dihydrogen phosphate iron removal agent proposed in this invention does not change the pH of the electrolytic system, and the dispersant can prevent the chelate from adhering to the electrode, thereby ensuring the stability of the electrolysis process and the purity of the product.

[0064] (4) In Example 4, the separated mother liquor was recycled and the iron removal effect was still maintained. The overall consumption of the iron removal agent was reduced by more than 40%. However, in Comparative Example 1, the chelating agent was doubled, but the iron removal effect was still poor. In fact, the iron removal efficiency in Comparative Example 5 dropped sharply. This proves that the lithium dihydrogen phosphate iron removal agent proposed in this invention has both long-term stability and industrial economics.

[0065] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A lithium dihydrogen phosphate iron removal agent, characterized in that, By mass fraction, it includes 50-60 wt% main chelating agent, 15-20 wt% auxiliary reducing agent, 5-10% dispersant, and the balance is pH adjuster and solvent; The main chelating agent is selected from one or more of aminotrimethylphosphonic acid, hydroxyethylidene diphosphonic acid, and ethylenediaminetetramethylphosphonic acid; the auxiliary reducing agent is selected from one or more of L-ascorbic acid, isoascorbic acid, sodium gluconate, and thiolactic acid; and the dispersant is selected from one or more of polyaspartic acid, polymaleic anhydride, or polyacrylic acid.

2. The lithium dihydrogen phosphate iron removal agent according to claim 1, characterized in that, By mass fraction, it includes 55 wt% aminotrimethylphosphonic acid, 18 wt% L-ascorbic acid, 8 wt% polyaspartic acid, 2 wt% lithium hydroxide and 17 wt% deionized water.

3. A preparation process for the lithium dihydrogen phosphate iron removal agent according to claim 1 or 2, characterized in that, Includes the following steps: A1. Place the main chelating agent, auxiliary reducing agent, and dispersant separately in a vacuum drying oven for drying and use. A2 is first taken as a solvent, heated and stirred, then the main chelating agent and the dispersing agent are added at once, and the stirring is continued until dissolved to obtain a preliminary mixed solution; A3 is kept under heating and stirring. Under nitrogen protection, an auxiliary reducing agent is added to the initial mixed solution, and stirring is continued to obtain the premix. A4 is added dropwise to the premix to adjust the pH value of the premix to acidic, then heated and stirred for aging treatment, and filtered to remove insoluble impurities, to obtain liquid lithium dihydrogen phosphate iron removal agent.

4. The preparation process of the lithium dihydrogen phosphate iron removal agent according to claim 3, characterized in that, In step A2, heat to 40-50℃, stir at a rate of 200-300 r / min, and continue stirring for 1-2 hours after adding the material; in step A3, add the auxiliary reducing agent and continue stirring for 20-40 minutes.

5. The preparation process of the lithium dihydrogen phosphate iron removal agent according to claim 3, characterized in that, In step A4, adjust the pH of the premix to 2.5-3.5, then heat to 50-60℃ and stir for 0.5-2 hours.

6. A lithium dihydrogen phosphate electrolytic iron removal process, characterized in that, The lithium dihydrogen phosphate iron removal agent prepared by means of the lithium dihydrogen phosphate iron removal agent according to claim 1 or 2, or by means of the preparation process according to any one of claims 3 to 5.

7. The lithium dihydrogen phosphate electrolytic iron removal process according to claim 6, characterized in that, Includes the following steps: B1 Pretreatment before electrolysis: Crude lithium dihydrogen phosphate was dissolved in deionized water to prepare an electrolyte; the electrolyte was heated and stirred, and the lithium dihydrogen phosphate iron removal agent was added for pretreatment, followed by filtration to obtain the pretreated electrolyte; B2 electrolysis and synergistic iron removal: The pretreated electrolyte is injected into the electrolytic cell for electrolysis, and the lithium dihydrogen phosphate iron removal agent is added periodically. After electrolysis, lithium dihydrogen phosphate crystals are obtained. B3 is separated after electrolysis: The lithium dihydrogen phosphate crystallization solution is exported, filtered, and lithium dihydrogen phosphate crystals and mother liquor are separated. The lithium dihydrogen phosphate crystals are washed and dried to obtain the lithium dihydrogen phosphate product. The separated mother liquor is returned to step B1 for recycling.

8. The lithium dihydrogen phosphate electrolytic iron removal process according to claim 6, characterized in that, In step B1, the electrolyte concentration is controlled at 0.8-1.2 mol / L, and the amount of lithium dihydrogen phosphate iron removal agent added is 0.3-0.5 wt% of the electrolyte mass.

9. The lithium dihydrogen phosphate electrolytic iron removal process according to claim 6, characterized in that, In step B2, the amount of lithium dihydrogen phosphate iron removal agent added is 0.03-0.08% of the electrolyte circulation volume per hour, based on volume fraction.

10. The lithium dihydrogen phosphate electrolytic iron removal process according to claim 6, characterized in that, In step B3, the mother liquor needs to be replenished with a new lithium dihydrogen phosphate iron removal agent every 2-3 cycles, and the replenishment amount is 20-50% of the initial amount by volume fraction.

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