A method for separating acid from waste aluminum etching solution and synthesizing battery-grade iron phosphate.

Abstract

This invention discloses a method for separating acid from waste aluminum etching solution and synthesizing battery-grade iron phosphate, comprising the following steps: S1, collecting acid solution; S2, recovering acetic acid and nitric acid; S3, preparing battery-grade iron phosphate. This invention can effectively separate nitric acid and acetic acid for full resource utilization, while avoiding the intrusion of other impurities during the synthesis process. It obtains high-purity battery-grade iron phosphate from recovered and refined phosphoric acid and inexpensive iron sources, which not only realizes the high-value utilization of etching waste liquid, but also combines waste liquid resource utilization with new energy materials. The process has no waste discharge and no environmental hazards.

Description

Technical Field

[0001] This invention relates to the fields of acidic waste liquid resource utilization and new energy material preparation technology, specifically to a method for separating acid from waste aluminum etching solution and synthesizing battery-grade iron phosphate. Background Technology

[0002] Thin-film transistor liquid crystal displays (TFT-LCDs) have advantages such as light weight and low power consumption, and have a large production market in China. A key step in the TFT-LCD manufacturing process is the wet etching process for creating aluminum conductors. This process uses phosphoric acid etching solution to remove the molybdenum-aluminum metal film on the TFT matrix glass substrate. This process generates a large amount of highly acidic industrial hazardous waste, namely aluminum-phosphoric acid etching waste liquid. The main components of this etching waste liquid are 60%-75% phosphoric acid, about 5% nitric acid, and about 10% acetic acid.

[0003] Currently, the main treatment methods for phosphorus-containing aluminum etching waste liquid include acid-base neutralization, reuse in the production of phosphate compound fertilizer, and evaporation and concentration to recover various acids. All of these methods have certain drawbacks. For example, simple acid-base neutralization is costly and generates secondary waste. Directly using aluminum etching liquid to produce phosphate compound fertilizer results in product quality being greatly affected by nitric acid, acetic acid, and impurities in the etching liquid. Evaporation and concentration to recover waste etching liquid increases the utilization rate to some extent, but the phosphoric acid produced lacks high-value utilization. At the same time, the acidic components in the waste liquid are not fully utilized, leading to resource waste and secondary pollution.

[0004] Ferric phosphate is an important chemical raw material widely used in battery production, ceramic and glass synthesis, and metal surface passivation. Due to its superior catalytic and electrochemical properties, ferric phosphate is increasingly used in catalysis and the synthesis of lithium iron phosphate batteries. Currently, the main synthesis method for ferric phosphate is precipitation, including using ferrous sulfate as the iron source and phosphoric acid as the phosphorus source supplemented with hydrogen peroxide, or using ferrous sulfate as the iron source and diammonium hydrogen phosphate as the phosphorus source mixed with a certain proportion of hydrogen peroxide. Besides ferrous sulfate, ferric nitrate and ferrous chloride can also be used as iron sources. However, all of these methods have certain drawbacks. For example, using ferrous chloride or ferrous sulfate as iron sources easily introduces sulfur and chloride ions into the ferric phosphate product, affecting battery performance, while using ferric nitrate as the iron source significantly increases recycling costs, impacting economic efficiency.

[0005] Therefore, there is an urgent need for an etching waste liquid recycling method that can effectively separate, recycle, and utilize the components of waste phosphoric acid etching solution, solve the problems of large emissions and low resource utilization, reduce costs, improve the purity of the final product, and realize the high-value application of the product. Based on this, the present invention provides a method for separating acid from waste aluminum etching solution and synthesizing battery-grade iron phosphate. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a method for separating acid from waste aluminum etching solution and synthesizing battery-grade iron phosphate.

[0007] The technical solution of this invention is: a method for separating acid from waste aluminum etching solution and combining it into battery-grade iron phosphate, comprising the following steps:

[0008] S1. Collect acid solution

[0009] The waste aluminum etching solution was subjected to vacuum fractionation to separate the phosphoric acid base solution and the acetic acid-nitric acid mixture, which were then collected separately.

[0010] S2, Recover acetic acid and nitric acid

[0011] Sodium hydroxide was added to the acetic acid-nitric acid mixture obtained from S1 at a molar mass ratio of sodium hydroxide to nitric acid of 1.1 to 1.2:1 to obtain solid sodium nitrate.

[0012] The remaining acetic acid and nitric acid mixture was refluxed, and the acetic acid condensate was collected. Calcium hydroxide was added to the acetic acid condensate at a ratio of 1 to 2:1, and calcium acetate was obtained by evaporation and crystallization using a rotary evaporator. The evaporation and crystallization parameters were: pressure of -0.09 to -0.1 MPa, temperature of 85 to 95 °C, and heating time of 15 to 30 min.

[0013] S3, Preparation of Battery-Grade Iron Phosphate

[0014] Add pure water to the phosphoric acid base solution obtained in step S1 until the mass concentration of phosphoric acid is 20-25% to obtain a diluted phosphoric acid base solution. Then add reduced iron powder to the diluted phosphoric acid base solution and stir to dissolve at 50-70℃ to obtain a mixed solution of iron and phosphoric acid. Add a pH adjuster to adjust the pH of the solution to 1.8-2.5 while stirring. Then add an oxidant to the mixture according to the molar ratio of oxidant to reduced iron powder of 0.75-0.85:1 for oxidation. Heat and stir, cool naturally, and centrifuge at 7900-8100 r / min for 3-5 min to generate iron phosphate hydrate. After treatment, obtain iron phosphate solid. Finally, weigh 0.1% citric acid according to the weight-volume ratio of iron phosphate solid to citric acid of 0.007-0.009 g: 1 mL and wash the iron phosphate solid 3-5 times. Then wash it with deionized water 1-3 times and let it air dry to obtain battery-grade iron phosphate.

[0015] Explanation: Adding an alkaline neutralizing agent to a mixture of acetic acid and nitric acid, followed by evaporation and crystallization, yields nitrate and acetic acid distillate. Further treatment of the nitric acid and acetic acid with these fractions yields nitrate and acetic acid of good purity, enabling the resource utilization of acetic acid and nitric acid. Using reduced iron powder as the iron source and the separated phosphoric acid bottom liquid as the phosphorus source, the mixture undergoes dissolution, pH adjustment, oxidation, centrifugation, precipitation, drying, and calcination to obtain ferric phosphate with a satisfactory iron-to-phosphorus ratio. After pH adjustment, oxidation yields ferric phosphate hydrate with good crystallinity. This method produces ferric phosphate with high purity. Washing the solid ferric phosphate with 0.1% citric acid and deionized water further reduces the sodium and potassium content, meeting the composition requirements for battery-grade ferric phosphate.

[0016] Further, in step S1, the method of reduced pressure fractionation is as follows: the system pressure is reduced to -0.09 to -0.1 MPa by evacuation, the system temperature is increased to 80 to 100°C, and fractionation is carried out by rotary evaporator for 60 to 100 minutes.

[0017] Explanation: Acetic acid has a boiling point of 118℃ and nitric acid has a boiling point of 120.5℃. Due to their low boiling points, acetic acid and nitric acid evaporate from the tube opening, and the evaporated gas condenses, resulting in a mixed condensate of liquid acetic acid, nitric acid, and water in the collection bottle. Phosphoric acid has a boiling point of 158℃, so the unevaporated phosphoric acid bottom liquid is obtained in the rotating flask. The above fractionation method can effectively separate the phosphoric acid bottom liquid and the acetic acid-nitric acid mixture, with high fractionation efficiency and good results.

[0018] Furthermore, in step S3, the pH adjuster is sodium hydroxide or ammonia; the oxidant is hydrogen peroxide.

[0019] Note: Using sodium hydroxide or ammonia as a pH adjuster allows for quick and easy pH adjustment; hydrogen peroxide, as an oxidant, is non-toxic and harmless, has low requirements for experimental conditions, and does not pollute the environment. At the same time, hydrogen peroxide reacts quickly with iron, and the generated iron ions and oxygen are easily separated, which also facilitates subsequent mixing with phosphoric acid solution, thereby improving the preparation efficiency of ferric phosphate.

[0020] Further, in step S3, the heating and stirring parameters are: stirring for 5 to 10 minutes at a temperature of 70 to 90°C;

[0021] Note: The above parameters can effectively accelerate the reaction rate of hydrogen peroxide and iron. Temperatures above 90°C will cause a large amount of oxygen to decompose, leading to increased pressure inside the reaction vessel and thus posing a danger. Temperatures below 70°C will not promote the reaction.

[0022] Furthermore, in step S3, the heated and stirred liquid is subjected to aging treatment;

[0023] Note: Aging the heated and stirred liquid allows the aging mother liquor to be collected and added to the phosphoric acid base solution, achieving high-value utilization of phosphoric acid and further realizing the resource utilization of waste aluminum etching solution.

[0024] Furthermore, the aging process is as follows: first, the heated and stirred liquid is placed in an aging tank for aging for 0.5 to 1 hour, and then it is placed in a clarification tank with a rotating rake for sedimentation and thickening for 0.5 to 2 hours, so that impurity ions settle and accumulate at the bottom. At this time, solid-liquid separation is performed to obtain ferric phosphate dihydrate and aging mother liquor. The aging mother liquor is collected and returned to the phosphoric acid bottom liquid in step S1, and then the ferric phosphate dihydrate is washed with pure water 3 to 5 times.

[0025] Explanation: The above aging process utilizes a clarification tank with a rotating rake to separate ferric phosphate dihydrate from the aging mother liquor, thereby achieving efficient recovery of the mother liquor. Simultaneously, ferric phosphate dihydrate exhibits good crystallinity, facilitating improved preparation efficiency of solid ferric phosphate. The clarification tank with a rotating rake for sedimentation and thickening allows impurity ions and ferric phosphate dihydrate to settle simultaneously, further enhancing the purity of the aging mother liquor (phosphoric acid) and increasing its reuse value. Washing ferric phosphate dihydrate with pure water followed by centrifugation reduces impurities on its surface, thus improving the purity of the prepared solid ferric phosphate.

[0026] Furthermore, the drying method is as follows: First, the ferric phosphate hydrate is dried with hot air at a temperature of 35-40℃, with the air speed adjusted to 15-18m / s. After drying for 1-3 minutes, the ferric phosphate hydrate is placed in a microwave oven, with the microwave power adjusted to 1000-1200W, and heated to 55-60℃ for another 3-5 minutes. Then, it is taken out and placed in a heating furnace with an air vent. The initial temperature of the heating furnace is adjusted to 50-55℃ and the temperature is increased at 3-5℃ / min. At the same time, hot air is introduced from the air vent, and the air speed of the hot air is reduced at a rate of 0.5-1m / s until the air speed drops to 0.

[0027] Explanation: The above drying method first uses hot air to preliminarily dry the surface of the iron phosphate hydrate, and then uses microwave drying to further dry the iron phosphate hydrate. Microwave drying has the characteristic of drying from the inside out, drying the inner layer of the material first and then gradually drying it from the outside, avoiding the problem of poor drying effect caused by only drying the outside, which prevents the moisture inside the iron phosphate hydrate from moving out. Then, slowly increasing the microwave temperature and decreasing the hot air velocity can promote the drying of iron phosphate hydrate from the inside out, further improving the drying efficiency.

[0028] Further, in step S3, the processing method is as follows: the ferric phosphate hydrate obtained after centrifugation is dried and calcined; wherein, the calcination temperature is 500-700℃ and the calcination time is 3-5h;

[0029] Note: Drying and calcining ferric phosphate hydrate yields anhydrous ferric phosphate, a precursor for battery synthesis. Optimal conditions are achieved at the calcination temperature described above. Temperatures above 700℃ cause excessive thermal decomposition of ferric phosphate hydrate, leading to decreased yield and purity, as well as structural changes that reduce its electrochemical performance. Conversely, calcination temperatures below 500℃ result in incomplete dehydration, leading to excessive moisture content, which affects purity and chemical reactivity. Incomplete thermal decomposition also negatively impacts the electrochemical performance of ferric phosphate.

[0030] Furthermore, the centrifuged ferric phosphate hydrate is subjected to pressure filtration before drying. The pressure filtration method is as follows: the ferric phosphate hydrate is fed into a filter press and filtered once at 8-10 MPa for 10-15 min to obtain filtrate and filter cake. The filtrate is returned to the phosphoric acid bottom solution in step S1. The filter cake is then washed with an iron-containing solution with a mass concentration of 95-98 mg / L, and the washing pressure is used again for pressure filtration at 8-10 MPa for 10-15 min. After 1-3 pressure filtrations, each filter cake is dried and calcined at 120-140℃ for 20-30 min. The drying parameters are: drying temperature: 40-50℃, drying time: 15-20 min.

[0031] Note: The above-mentioned pressure filtration method enables further utilization of phosphoric acid, while improving the purity of ferric phosphate hydrate and further increasing the preparation efficiency of ferric phosphate.

[0032] Further, in step S3, the molar ratio of P in the diluted phosphoric acid solution to Fe in the reduced iron powder is P:Fe = 3:1 to 1.1;

[0033] Note: The above ratio can effectively meet the production requirements of ferric phosphate. At the same time, a slight excess of reduced iron powder can reduce the formation of hydrates, improve the stability of ferric phosphate, and enhance its electrochemical performance, such as increasing energy density and discharge voltage. In addition, an excess of reduced iron powder can ensure the quantity of iron ions, further improving the preparation efficiency of ferric phosphate.

[0034] Furthermore, in step S1, the waste aluminum etching solution is wet etching waste liquid from the semiconductor industry;

[0035] Note: The experimental data shows that this method can effectively utilize the components of wet etching waste liquid in the semiconductor industry, reducing waste liquid pollution while improving the resource utilization effect of etching waste liquid.

[0036] The beneficial effects of this invention are:

[0037] (1) The method of the present invention can effectively separate nitric acid, acetic acid and phosphoric acid and then make full use of resources. At the same time, it avoids the addition of other impurities during the synthesis process. It obtains high-purity iron phosphate from the recovered and refined phosphoric acid and the cheap iron source. It not only realizes the high-value utilization of etching waste liquid, but also combines waste liquid resource utilization with new energy materials. The process has no waste discharge and no environmental hazards.

[0038] (2) The present invention uses reduced iron powder as an iron source, which further reduces the preparation cost. At the same time, the present invention adopts aging treatment and pressure filtration to further treat iron phosphate hydrate, which can effectively improve the utilization rate of phosphoric acid and further improve the purity of battery-grade iron phosphate, thereby improving the performance of the battery. Attached Figure Description

[0039] Figure 1 This is a flowchart of the method in Embodiment 1 of the present invention;

[0040] Figure 2 This is the mass spectrum of sodium nitrate, a byproduct prepared by the method in Example 1 of this invention;

[0041] Figure 3 This is a mass spectrum of the battery-grade iron phosphate synthesized in this invention using waste liquid and pure phosphoric acid as raw materials respectively;

[0042] Figure 4 This is the mass spectrum of ferric phosphate dihydrate synthesized according to different ferric phosphorus addition ratios in this invention;

[0043] Figure 5 This is a mass spectrum of anhydrous ferric phosphate synthesized by the methods of Examples 1, 8 and 9 of this invention after calcination at 700°C.

[0044] Figure 6 This is a mass spectrum of the battery-grade iron phosphate products synthesized in Example 1 and Comparative Example 3 of the present invention;

[0045] Figure 7 This is a morphology diagram of the battery-grade iron phosphate product prepared in Example 1 of this invention;

[0046] Figure 8 This is a morphology diagram of the battery-grade iron phosphate product prepared in Comparative Example 8 of this invention;

[0047] Figure 9 This is a morphology diagram of the iron phosphate dihydrate product synthesized in Comparative Example 8 of this invention;

[0048] Figure 10 This is a morphological diagram of the iron phosphate dihydrate product synthesized in Comparative Example 9 of this invention. Detailed Implementation

[0049] The present invention will now be described in more detail with reference to specific embodiments, so as to better demonstrate the advantages of the present invention.

[0050] Example 1

[0051] A method for separating acid from waste aluminum etching solution and combining it into battery-grade iron phosphate includes the following steps:

[0052] S1. Collect acid solution

[0053] The waste aluminum etching solution was subjected to vacuum fractionation to separate the phosphoric acid base solution and the acetic acid-nitric acid mixture by utilizing the difference in boiling points between the components, and the two solutions were collected separately. The waste aluminum etching solution is a wet etching waste liquid from the semiconductor industry.

[0054] The method of vacuum fractionation is as follows: evacuate to -0.095 MPa, raise the temperature to 90°C, and fractionate by rotary evaporator for 80 min;

[0055] S2, Recover acetic acid and nitric acid

[0056] Sodium hydroxide was added to the acetic acid-nitric acid mixture obtained from S1 at a molar mass ratio of sodium hydroxide to nitric acid of 1.15:1 to obtain solid sodium nitrate.

[0057] The remaining acetic acid and nitric acid mixture was refluxed, and the acetic acid condensate was collected. Calcium hydroxide was added to the acetic acid condensate at a molar ratio of 3:2 to calcium hydroxide, and calcium acetate was obtained by evaporation and crystallization using a rotary evaporator. The evaporation and crystallization parameters were: pressure of -0.095 MPa, temperature of 90 °C, and heating time of 22 min.

[0058] S3, Preparation of Battery-Grade Iron Phosphate

[0059] Add pure water to the phosphoric acid base solution obtained in step S1 until the mass concentration of phosphoric acid is 23% to obtain a diluted phosphoric acid base solution. Then, add reduced iron powder to the diluted phosphoric acid base solution and stir to dissolve it at 60°C to obtain a mixed solution of iron and phosphoric acid. Add sodium hydroxide to adjust the pH of the solution to 2.0 while stirring. Then, add hydrogen peroxide to the mixed solution at a molar ratio of hydrogen peroxide to reduced iron powder of 0.8:1 for oxidation. Heat and stir at 80°C for 7 minutes. Aging treatment is performed on the heated and stirred liquid. After natural cooling, centrifugation is performed at 8000 r / min for 4 minutes to generate iron phosphate hydrate. After treatment, iron phosphate solid is obtained. Finally, 0.1% citric acid is weighed according to the weight-volume ratio of iron phosphate solid to citric acid of 0.008 g: 1 mL to wash the iron phosphate solid 4 times, then wash it twice with deionized water, and air dry to obtain battery-grade iron phosphate. The molar ratio of P in the diluted phosphoric acid base solution to Fe in the reduced iron powder is P:Fe = 3:1.05.

[0060] The aging process is as follows: First, the heated and stirred liquid is placed into an aging tank for aging for 0.8 hours, and then it is placed into a clarification tank with a rotating rake for sedimentation and thickening for 1.2 hours, so that impurity ions settle to the bottom and accumulate. At this time, solid-liquid separation is performed to obtain ferric phosphate dihydrate and aging mother liquor. The aging mother liquor is collected and returned to the phosphoric acid bottom liquid in step S1, and then the ferric phosphate dihydrate is washed 4 times with pure water.

[0061] The treatment method is as follows: the ferric phosphate hydrate obtained after centrifugation is dried and calcined; the calcination temperature is 600℃ and the calcination time is 4h.

[0062] The drying method is as follows: First, the iron phosphate hydrate is dried with hot air at a temperature of 37℃ and the air speed is adjusted to 17m / s. After drying for 2 minutes, the iron phosphate hydrate is placed in a microwave oven, the microwave power is adjusted to 1100W, and the temperature is heated to 58℃ for another 4 minutes. Then, it is taken out and placed in a heating furnace with an air vent. The initial temperature of the heating furnace is adjusted to 53℃ and the temperature is increased at 4℃ / min. At the same time, hot air is introduced from the air vent, and the air speed of the hot air is reduced at a rate of 0.8m / s until the air speed drops to 0.

[0063] Example 2

[0064] Unlike Example 1, in step S1, the method of reduced pressure fractionation is as follows: the system pressure is reduced to -0.09 MPa by evacuation, the system temperature is increased to 80°C, and fractionation is carried out by rotary evaporator for 60 min.

[0065] Example 3

[0066] Unlike Example 1, in step S1, the method of reduced pressure fractionation is as follows: the system pressure is reduced to -0.1 MPa by evacuation, the system temperature is increased to 90°C, and fractionation is carried out by rotary evaporator for 100 min.

[0067] Example 4

[0068] Unlike Example 1, in step S2, sodium hydroxide is added to the acetic acid-nitric acid mixture obtained in S1 at a molar mass ratio of sodium hydroxide to nitric acid of 1.1:1 to obtain solid sodium nitrate.

[0069] The remaining acetic acid and nitric acid mixture was refluxed to collect the acetic acid condensate. Calcium hydroxide was added to the acetic acid condensate at a molar ratio of 1:1 to calcium hydroxide and acetic acid condensate, and calcium acetate was obtained by evaporation and crystallization using a rotary evaporator. The evaporation and crystallization parameters were: pressure of -0.09 MPa, temperature of 85 °C, and heating time of 30 min.

[0070] Example 5

[0071] Unlike Example 1, in step S2, sodium hydroxide is added to the acetic acid-nitric acid mixture obtained in S1 at a molar mass ratio of sodium hydroxide to nitric acid of 1.2:1 to obtain solid sodium nitrate.

[0072] The remaining acetic acid and nitric acid mixture was refluxed to collect the acetic acid condensate. Calcium hydroxide was added to the acetic acid condensate at a molar ratio of 2:1 to calcium hydroxide to the acetic acid condensate, and calcium acetate was obtained by evaporation and crystallization using a rotary evaporator. The evaporation and crystallization parameters were: pressure of -0.1 MPa, temperature of 95 °C, and heating time of 15 min.

[0073] Example 6

[0074] Unlike Example 1, in step S3, pure water is added to the phosphoric acid base solution obtained in step S1 until the mass concentration of phosphoric acid is 20%, resulting in a diluted phosphoric acid base solution.

[0075] Example 7

[0076] Unlike Example 1, in step S3, pure water is added to the phosphoric acid base solution obtained in step S1 until the mass concentration of phosphoric acid is 25%, resulting in a diluted phosphoric acid base solution.

[0077] Example 8

[0078] Unlike Example 1, in step S3, the molar ratio of P in the diluted phosphoric acid solution to Fe in the reduced iron powder is P:Fe = 3:1.

[0079] Example 9

[0080] Unlike Example 1, in step S3, the molar ratio of P in the diluted phosphoric acid solution to Fe in the reduced iron powder is P:Fe = 3:1.1.

[0081] Example 10

[0082] Unlike Example 1, in step S3, sodium hydroxide was added while stirring to adjust the pH of the solution to 1.8.

[0083] Example 11

[0084] Unlike Example 1, in step S3, sodium hydroxide was added while stirring to adjust the pH of the solution to 2.5.

[0085] Example 12

[0086] Unlike Example 1, in step S3, hydrogen peroxide was added to the mixed solution at a molar ratio of hydrogen peroxide to reduced iron powder of 0.75:1 for oxidation. The solution was heated and stirred at 70°C for 5 minutes. The heated and stirred liquid was then aged and naturally cooled. After centrifugation at 7900 r / min for 3 minutes, ferric phosphate hydrate was generated. The ferric phosphate hydrate obtained after centrifugation was dried and calcined to obtain solid ferric phosphate.

[0087] Example 13

[0088] Unlike Example 1, in step S3, hydrogen peroxide was added to the mixed solution at a molar ratio of hydrogen peroxide to reduced iron powder of 0.85:1 for oxidation. The mixture was heated and stirred at 90°C for 10 minutes. The heated and stirred liquid was then aged, naturally cooled, and centrifuged at 8100 r / min for 5 minutes to generate iron phosphate hydrate. The iron phosphate hydrate obtained after centrifugation was dried and calcined to obtain solid iron phosphate.

[0089] Example 14

[0090] Unlike Example 1, in step S3, the aging process is as follows: the heated and stirred liquid is first placed into an aging tank for aging for 0.5 hours, and then it is placed into a clarification tank with a rotating rake for 0.5 hours for sedimentation and thickening, so that impurity ions settle and accumulate at the bottom. At this time, solid-liquid separation is performed to obtain ferric phosphate dihydrate and aging mother liquor. The aging mother liquor is collected and returned to the phosphoric acid bottom liquid in step S1.

[0091] Example 15

[0092] Unlike Example 1, in step S3, the aging process is as follows: the heated and stirred liquid is first placed into an aging tank for 1 hour of aging, and then it is placed into a clarification tank with a rotating rake for 2 hours of sedimentation and thickening, so that impurity ions settle and accumulate at the bottom. At this time, solid-liquid separation is performed to obtain ferric phosphate dihydrate and aging mother liquor. The aging mother liquor is collected and returned to the phosphoric acid bottom liquid in step S1.

[0093] Example 16

[0094] Unlike Example 1, in step S3, 0.1% citric acid was weighed out and rinsed three times with ferric phosphate solid and 1 mL of citric acid at a weight-volume ratio of 0.007 g: 1 mL. The ferric phosphate solid was then rinsed once with deionized water and air-dried to obtain battery-grade ferric phosphate.

[0095] Example 17

[0096] Unlike Example 1, in step S3, citric acid with a mass concentration of 0.1% was weighed out at a weight-volume ratio of 0.009g:1mL to rinse the ferric phosphate solid 5 times, then rinsed 3 times with deionized water, and air-dried to obtain battery-grade ferric phosphate.

[0097] Example 18

[0098] Unlike Example 1, in step S3, the calcination temperature is 500°C and the calcination time is 5 hours.

[0099] Example 19

[0100] Unlike Example 1, in step S3, the calcination temperature is 700°C and the calcination time is 3 hours.

[0101] Example 20

[0102] Unlike Example 1, in step S3, the drying method is as follows: First, the ferric phosphate hydrate is dried with hot air at a temperature of 35°C with a wind speed of 15 m / s. After drying for 1 minute, the ferric phosphate hydrate is placed in a microwave oven with a microwave power of 1000W and heated to 55°C for another 3 minutes. Then, it is taken out and placed in a heating furnace with an air vent. The initial temperature of the heating furnace is adjusted to 50°C and increased at a rate of 3°C / min. At the same time, hot air is introduced from the air vent, and the wind speed of the hot air is reduced at a rate of 0.5 m / s until the wind speed drops to 0.

[0103] Example 21

[0104] Unlike Example 1, in step S3, the drying method is as follows: First, the ferric phosphate hydrate is dried with hot air at a temperature of 40°C with a wind speed of 18 m / s. After drying for 3 minutes, the ferric phosphate hydrate is placed in a microwave oven with a microwave power of 1200W and heated to 60°C for another 5 minutes. Then, it is taken out and placed in a heating furnace with an air vent. The initial temperature of the heating furnace is adjusted to 55°C and increased at a rate of 5°C / min. At the same time, hot air is introduced from the air vent, and the wind speed of the hot air is reduced at a rate of 1 m / s until the wind speed drops to 0.

[0105] Example 22

[0106] Unlike Example 1, in step S3, the centrifuged ferric phosphate hydrate is subjected to pressure filtration before drying. The pressure filtration method is as follows: pressure filtration is performed once at 8 MPa for 15 min to obtain filtrate and filter cake. The filtrate is returned to the phosphoric acid bottom solution in step S1. Then, the filter cake is washed with an iron-containing solution with a mass concentration of 95 mg / L. The washing pressure is then pressure filtration is performed again at 8 MPa for 15 min. After one pressure filtration, each filter cake is dried and calcined at 120°C for 30 min. The drying parameters are: drying temperature: 40°C, drying time: 20 min.

[0107] Example 23

[0108] Unlike Example 1, in step S3, the centrifuged ferric phosphate hydrate is subjected to pressure filtration before drying. The pressure filtration method is as follows: pressure filtration is performed once at 9 MPa for 12 min to obtain filtrate and filter cake. The filtrate is returned to the phosphoric acid bottom solution in step S1. Then, the filter cake is washed with an iron-containing solution with a mass concentration of 96 mg / L. The washing pressure is then pressure filtration is performed again at 9 MPa for 12 min. After pressure filtration twice, each filter cake is dried and calcined at 130°C for 25 min. The drying parameters are: drying temperature: 45°C, drying time: 17 min.

[0109] Example 24

[0110] Unlike Example 1, in step S3, the centrifuged ferric phosphate hydrate is subjected to pressure filtration before drying. The pressure filtration method is as follows: pressure filtration is performed once at 10 MPa for 15 min to obtain filtrate and filter cake. The filtrate is returned to the phosphoric acid base solution in step S1. Then, the filter cake is washed with an iron-containing solution with a mass concentration of 98 mg / L. The washing pressure is then pressure filtration is performed again at 10 MPa for 10 min. After pressure filtration is performed 3 times, each filter cake is dried and calcined at 140°C for 20 min. The drying parameters are: drying temperature: 50°C, drying time: 15 min.

[0111] Comparative Example

[0112] Comparative Example 1: Unlike Example 1, ferrous sulfate was added to the diluted phosphoric acid solution at a molar ratio of P:Fe = 3:1.05.

[0113] Comparative Example 2: Unlike Example 1, reduced iron powder was added to the diluted phosphoric acid liquid at a molar ratio of P:Fe = 3:0.8.

[0114] Comparative Example 3: Unlike Example 11, reduced iron powder was added to the diluted phosphoric acid liquid at a molar ratio of P:Fe = 3.6:1.1.

[0115] Comparative Example 4: Unlike Example 1, a pH adjuster was added while stirring to adjust the pH of the solution to 3.

[0116] Comparative Example 5: Unlike Example 1, the mixture was heated and stirred at 50°C for 7 minutes.

[0117] Comparative Example 6: Unlike Example 1, the drying method was to directly dry the ferric phosphate hydrate with hot air.

[0118] Comparative Example 7: Unlike Example 1, ammonia was used as the pH adjuster.

[0119] Comparative Example 8: Unlike Comparative Example 7, ammonia was added while stirring to adjust the pH of the solution to 1.5.

[0120] Experimental Example

[0121] The ferric phosphate prepared in Example 1 was subjected to ion type and content detection. The detection results are shown in Table 1.

[0122] Table 1. Detection results of various ions and their contents in ferric phosphate in Example 1.

[0123]

[0124]

[0125] The iron phosphates prepared in Examples 1-24 and Comparative Examples 1-8 were used as sample groups. Three to five samples were taken from each group, and the average value of the test results was taken. The sulfur content, chlorine content and iron-phosphorus ratio in the samples were determined.

[0126] 1. Investigate the effects of iron source and the molar ratio of phosphorus to iron on the properties of iron phosphate.

[0127] Table 1. Performance determination of ferric phosphate prepared in Examples 1, 8-9, and Comparative Examples 1-3.

[0128]

[0129] Conclusion: Compared with Example 1, the phosphorus-iron molar ratio in Comparative Example 2 was adjusted to 3:0.8, which, compared with the phosphorus-iron molar ratio of 3:1 in Example 8, shows that the phosphorus-iron ratio added during the synthesis process has a certain impact on the iron-phosphorus ratio in the final iron phosphate product. Reducing the amount of iron added can save reagents, but may cause a decrease in the iron-phosphorus ratio of the product. Compared with Example 1, the phosphorus-iron molar ratio in Comparative Example 4 was adjusted to 3.6:1.1. Increasing the amount of phosphoric acid added during the synthesis process helps dissolve iron powder to some extent, but has a significant impact on the purity of the final product and is prone to generating other iron-phosphorus byproducts. In Comparative Example 1, ferrous salt was directly added to the etching waste liquid, resulting in the synthesis product being doped with Cl or S elements, which affected the purity of the battery. At the same time, the nitric acid and acetic acid in the product were not utilized, resulting in resource waste. Furthermore, the acetic acid odor during the synthesis process was quite pungent, leading to a poor production environment. Meanwhile, the cost of Example 1 of this invention is much lower than that of Comparative Example 1. The main reason for the huge cost difference is the different iron sources added. The iron source used in Example 1 of this invention is reduced iron powder, which is inexpensive and readily available. The iron salt used in Comparative Example 1 has a higher unit mass cost and is added in large quantities, which significantly increases the cost.

[0130] As can be seen from the above analysis and comparison, the method of the present invention can fully utilize nitric acid and acetic acid, while avoiding the doping of other impurities during the synthesis process. It obtains high-purity iron phosphate from recycled phosphoric acid and inexpensive iron sources, which not only realizes the high-value utilization of etching waste liquid, but also combines waste liquid resource utilization with new energy materials. The process has no waste discharge and no environmental hazards.

[0131] 2. Investigate the effects of pH value and pH adjusters on the properties of ferric phosphate.

[0132] Table 2. Performance determination of ferric phosphate prepared in Examples 1, 10-11, Comparative Examples 4, and Comparative Example 8.

[0133]

[0134] Conclusion: Compared with Example 1, in Comparative Example 4, the pH was adjusted from 2.0 to 3.0, which is higher than the preferred range of 1.8 to 2.5. The iron-phosphorus ratio of the product is closely related to the synthesis pH. When the pH is higher than the preferred value, the iron-phosphorus ratio of the synthesized product is lower than the standard value. Other impurities are easily generated during the synthesis process, resulting in impurity of the product. Furthermore, in Comparative Example 7, the use of ammonia as a pH adjuster generated other complexation intermediates, which caused impurity of the product and thus led to a decrease in the iron-phosphorus ratio.

[0135] 3. Investigate the effect of stirring temperature on the properties of ferric phosphate.

[0136] Table 3. Performance determination of ferric phosphate prepared in Examples 1, 12-13, and Comparative Example 5.

[0137] Group Example 1 Example 12 Example 13 Comparative Example 5 Sulfur content % wt ＜0.0001％ ＜0.0001％ ＜0.0001％ ＜0.0001％ Chlorine content %wt —— —— —— —— Iron-to-phosphorus ratio 0.99 0.98 0.98 0.90

[0138] Conclusion: Compared with Example 1, Comparative Example 5 adjusted the synthesis temperature to 50°C, which is lower than the preferred conditions of 70-90°C. The synthesis temperature has a significant impact on the product. Lowering the temperature can save energy, but it will cause a significant decrease in the iron-phosphorus ratio, resulting in the product failing to meet the standards.

[0139] 4. Investigate the effect of drying method on the properties of ferric phosphate products.

[0140] Table 4. Performance determination of ferric phosphate prepared in Examples 1, 20-21, and Comparative Example 6.

[0141] Group Example 1 Example 20 Example 21 Comparative Example 6 Sulfur content % wt ＜0.0001％ ＜0.0001％ ＜0.0001％ ＜0.0001％ Chlorine content %wt —— —— —— —— Iron-to-phosphorus ratio 0.99 0.99 0.98 0.94

[0142] Conclusion: As shown in Table 4, the iron-to-phosphorus ratio of the iron phosphate prepared in Comparative Example 6 was significantly reduced. This is because the hot air drying process on the outer surface of the iron phosphate hydrate resulted in the formation of a hard shell, which prevented the moisture inside the iron phosphate hydrate from being discharged, thus affecting the drying effect of the iron phosphate and reducing the iron-to-phosphorus ratio.

[0143] 5. Investigate the effect of pressure filtration on the properties of ferric phosphate products.

[0144] Table 5. Performance determination of ferric phosphate prepared in Examples 1, 22-24

[0145]

[0146]

[0147] Conclusion: As shown in Table 5 and the data from Example 1, pressure filtration can further improve the purity of ferric phosphate hydrate. At the same time, pressure filtration can make fuller use of phosphoric acid liquid, so that the resource utilization rate of the entire preparation process is higher.

[0148] Depend on Figure 2 It can be seen that the synthesis byproduct in the process is sodium nitrate. The method of the present invention can utilize the etching waste liquid, and the iron phosphate of the synthesized product is the same as that synthesized with pure phosphoric acid.

[0149] in, Figure 4 , Figure 5 , Figure 6 The linear graphs in each image correspond from top to bottom to the phosphorus-iron molar ratio in the chart labels from top to bottom; from Figure 4It can be seen that when the iron-phosphorus ratio is 3:1, amorphous iron phosphate dihydrate is synthesized, while the iron-phosphorus ratios of 3:1.05 and 3:1.1 result in iron phosphate dihydrate with better crystallinity. After calcination, all three form battery-grade iron phosphate. Among them, the iron phosphate dihydrate synthesized at the 3:1.1 ratio and the anhydrous iron phosphate after calcination have the best crystallinity, obvious characteristic peaks, and low impurity content.

[0150] Depend on Figure 7 , Figure 8 It can be seen that the ferric phosphate dihydrate obtained by using sodium hydroxide as a pH adjuster is more uniform and has fewer impurities. However, the ferric phosphate dihydrate obtained by using ammonia as a pH adjuster has an altered morphology due to the formation of other complex intermediates, which affects its charge-discharge performance.

[0151] The effect of different pH values ​​on the morphology of battery-grade iron phosphate was investigated using ammonia as a pH adjuster. Figure 9 , Figure 10 It can be seen that when the pH of the mixture is adjusted to 2.0, the synthesized iron phosphate dihydrate is a fine and uniform sphere, while when the pH is 1.5, the morphology of iron phosphate dihydrate changes, a rod-like structure appears, and the particles become larger, which will affect the charge and discharge performance. It is evident that the discharge performance of battery-grade iron phosphate can only reach its optimal state when the pH is within the range defined by this invention.

Claims

1. A method for separating acid from waste aluminum etching solution and synthesizing battery-grade iron phosphate, characterized in that, Includes the following steps: S1. Collect acid solution The waste aluminum etching solution was subjected to vacuum fractionation to separate the phosphoric acid base solution and the acetic acid-nitric acid mixture by utilizing the difference in boiling points between the components, and the solutions were collected separately. S2, recovery of acetic acid and nitric acid Sodium hydroxide was added to the acetic acid-nitric acid mixture obtained from S1 at a molar ratio of sodium hydroxide to nitric acid of 1.1 to 1.2:1 to obtain solid sodium nitrate. The remaining acetic acid and nitric acid mixture was refluxed, and the acetic acid condensate was collected. Calcium hydroxide was added to the acetic acid condensate at a molar ratio of 1 to 2:1, and calcium acetate was obtained by evaporation and crystallization using a rotary evaporator. The evaporation and crystallization parameters were: pressure of -0.09 to -0.1 MPa, temperature of 85 to 95 °C, and heating time of 15 to 30 min. S3, Preparation of Battery-Grade Iron Phosphate Add pure water to the phosphoric acid base solution obtained in step S1 until the mass concentration of phosphoric acid is 20-25% to obtain a diluted phosphoric acid base solution. Then add reduced iron powder to the diluted phosphoric acid base solution and stir to dissolve at 50-70℃ to obtain a mixture of iron and phosphoric acid. Add a pH adjuster to adjust the pH of the mixture to 1.8-2.5 while stirring. Then add an oxidant to the mixture according to the molar ratio of oxidant to reduced iron powder of 0.75-0.85:

1. Heat and stir, cool naturally, and centrifuge at 7900-8100 r / min for 3-5 min to generate iron phosphate hydrate. After drying and calcination, obtain iron phosphate solid. Finally, weigh 0.1% citric acid according to the weight-volume ratio of iron phosphate solid to citric acid of 7-9 g:1 L to wash the iron phosphate solid 3-5 times, then wash with deionized water 1-3 times, and air dry to obtain battery-grade iron phosphate. In step S3, the heated and stirred liquid is aged. The aging process is as follows: the heated and stirred liquid is first placed in an aging tank for 0.5 to 1 hour of aging, and then placed in a clarification tank with a rotating rake for 0.5 to 2 hours of sedimentation and thickening to allow impurity ions to settle and accumulate at the bottom. Then, solid-liquid separation is performed to obtain ferric phosphate dihydrate and aging mother liquor. The aging mother liquor is collected and returned to the phosphoric acid bottom liquid in step S1. Then, the ferric phosphate dihydrate is washed with pure water 3 to 5 times. In step S3, the centrifuged ferric phosphate hydrate is subjected to pressure filtration before drying. The pressure filtration method is as follows: the ferric phosphate hydrate is sent to a filter press and filtered once at 8-10 MPa for 10-15 min to obtain filtrate and filter cake. The filtrate is returned to the phosphoric acid bottom solution in step S1. The filter cake is then washed with an iron-containing solution with a mass concentration of 95-98 mg / L. The washing liquid is then pressure-filtered again at 8-10 MPa for 10-15 min. After filtration 1-3 times, each filter cake is dried and calcined at 120-140℃ for 20-30 min. The drying parameters are: drying temperature: 40-50℃, drying time: 15-20 min.

2. The method for separating acid from waste aluminum etching solution and synthesizing battery-grade iron phosphate according to claim 1, characterized in that, In step S1, the method of reduced pressure fractionation is as follows: evacuate to -0.09~-0.1MPa, raise the temperature to 80~100℃, and fractionate by rotary evaporator for 60~100min.

3. The method for separating acid from waste aluminum etching solution and synthesizing battery-grade iron phosphate according to claim 1, characterized in that, In step S3, the pH adjuster is sodium hydroxide or ammonia; the oxidant is hydrogen peroxide.

4. The method for separating acid from waste aluminum etching solution and synthesizing battery-grade iron phosphate according to claim 1, characterized in that, In step S3, the heating and stirring parameters are: stirring for 5 to 10 minutes at a temperature of 70 to 90°C.

5. The method for separating acid from waste aluminum etching solution and synthesizing battery-grade iron phosphate according to claim 1, characterized in that, The drying method is as follows: First, the iron phosphate hydrate is dried with hot air at a temperature of 35~40℃, with the air speed adjusted to 15~18m / s. After drying for 1~3 minutes, the iron phosphate hydrate is placed in a microwave oven, and the microwave power is adjusted to 1000~1200W. It is heated to 55~60℃ and dried for another 3~5 minutes. Then, it is taken out and placed in a heating furnace with an air vent. The initial temperature of the heating furnace is adjusted to 50~55℃ and the temperature is increased at 3~5℃ / min. At the same time, hot air is introduced from the air vent. The air speed of the hot air is reduced at a rate of 0.5~1m / s until the air speed drops to 0.

6. The method for separating acid from waste aluminum etching solution and synthesizing battery-grade iron phosphate according to claim 1, characterized in that, In step S3, the calcination temperature is 500~700℃ and the calcination time is 3~5h.

7. The method for separating acid from waste aluminum etching solution and synthesizing battery-grade iron phosphate according to claim 1, characterized in that, In step S3, the molar ratio of P in the diluted phosphoric acid solution to Fe in the reduced iron powder is P:Fe = 3:1~1.

1.

8. The method for separating acid from waste aluminum etching solution and synthesizing battery-grade iron phosphate according to claim 1, characterized in that, In step S1, the waste aluminum etching solution is a wet etching waste solution from the semiconductor industry.

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