Method for preparing high-purity ferric phosphate by low-temperature acidolysis of phosphatizing slag

Abstract

The application relates to the technical field of inorganic compound preparation, and particularly discloses a method for preparing high-purity iron phosphate from phosphating slag by low-temperature acidolysis, which comprises the following steps: washing, drying and smashing the phosphating slag to obtain phosphating slag powder; performing low-temperature acidolysis reaction on the phosphating slag powder and inorganic acid in an aqueous medium at 60-90 DEG C to obtain an acidolysis solution; adding an oxidizing agent to the acidolysis solution and adjusting the pH value of the system to 2.5-4.5 at 65-85 DEG C to remove impurities by oxidation, and then filtering to obtain an iron phosphate solution; selectively supplementing an iron source according to the molar ratio of iron to phosphorus in the solution, adjusting the pH value to 1.0-3.0, and performing a precipitation reaction at 65-75 DEG C to generate iron phosphate. The application realizes low-energy-consumption resource utilization of the phosphating slag by combining low-temperature acidolysis with oxidation impurity removal and component regulation, the purity of the product reaches the battery grade requirement, and the product is suitable for preparation of a lithium battery positive electrode material precursor.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of inorganic compound preparation, and more particularly to a method for preparing high-purity iron phosphate from phosphating residue at low temperature. BACKGROUND

[0002] Phosphating residue is the main solid waste generated in the phosphating process of metal surfaces such as steel and aluminum, and its main components are insoluble phosphate salts such as iron phosphate and zinc phosphate, and it also contains heavy metal impurities such as zinc, manganese and nickel. The traditional disposal method is mainly stacking or landfill, which not only occupies land resources, but also has the risk of environmental leakage of heavy metals. In order to realize the resource utilization of phosphating residue, researchers have proposed a method to convert it into high-value iron phosphate, which is a key precursor for preparing lithium ion battery cathode material lithium iron phosphate.

[0003] At present, there are many technologies dedicated to the preparation of iron phosphate from phosphating residue. The closest prior art discloses a treatment method: phosphating iron waste is mixed with acid, oxidizing agent and adsorbent and stirred, and then heated to 80-100℃ for leaching treatment, and after complete dissolution, solid-liquid separation is performed; then, the pH of the filtrate is adjusted by adding alkali, and iron oxide is obtained by heat treatment; then, the iron oxide is activated by high-energy ball milling, and then mixed with phosphoric acid to prepare iron phosphate. The advantage of this method is that it realizes the resource utilization of waste residue, and can obtain high-purity products.

[0004] However, the above prior art still has some disadvantages in actual use: first, the energy consumption is high and the process flow is complex: the prior art uses high temperature for acid leaching, which consumes a lot of energy to maintain the reaction temperature, increasing the production cost; the process flow is long, involving multiple phase change treatment and high-temperature heat treatment steps, the operation process is complex, the equipment investment and maintenance cost is high, which is not conducive to the efficiency and economic benefit of large-scale industrial production. Second, the stability of product purity is challenged: although this technology strives for high purity through various additives and complex processes, the high-temperature acid leaching process may exacerbate the dissolution and dispersion of impurity elements; in the complex multi-step process, any control deviation may affect the quality of the intermediate product, thus affecting the product purity, which brings technical risks to the stable production of high-specification battery-grade iron phosphate. Third, the resource comprehensive utilization rate and economy need to be improved: the process route is long, and the treatment and recycling of intermediate products and by-products in the process is not perfect, which may lead to the loss of valuable components; the cumulative energy consumption and material consumption of multi-step reaction and treatment are high, and the adaptability to raw materials may be poor, which directly affects the economy and resource recycling efficiency of the overall process. SUMMARY

[0005] In order to overcome the above-mentioned defects of the prior art, the present application provides a method for preparing high-purity ferric phosphate by low-temperature acidolysis of phosphating slag, which solves the problems raised in the above background art through the following scheme.

[0006] To achieve the above object, the present application provides the following technical scheme: a method for preparing high-purity ferric phosphate by low-temperature acidolysis of phosphating slag, comprising the following steps:

[0007] S1, pretreatment: washing, drying and crushing the phosphating slag to obtain phosphating slag powder;

[0008] S2, low-temperature acidolysis: reacting the phosphating slag powder with inorganic acid in an aqueous medium at a temperature in the range of 60-90℃ to obtain an acidolysis solution;

[0009] S3, oxidation and impurity removal: adding an oxidizing agent to the acidolysis solution and adjusting the pH value of the system to the range of 2.5-4.5 at 65-85℃ to make heavy metal impurities precipitate, and then filtering to obtain a ferric phosphate-containing solution;

[0010] S4, precipitation reaction: selectively adding an iron source according to the molar ratio of iron to phosphorus in the ferric phosphate-containing solution, adjusting the pH value of the solution to the range of 1.0-3.0, and performing a precipitation reaction at a temperature in the range of 65-75℃ to generate a ferric phosphate precipitate;

[0011] S5, post-treatment: aging, filtering, drying and calcining the ferric phosphate precipitate to obtain a high-purity ferric phosphate product.

[0012] Preferably, the inorganic acid in step S2 is at least one of hydrochloric acid, sulfuric acid and nitric acid.

[0013] Preferably, the aqueous medium in step S2 is deionized water.

[0014] Preferably, the reaction temperature in step S2 is in the range of 65-85℃.

[0015] Preferably, the oxidizing agent in step S3 is at least one of hydrogen peroxide, sodium chlorate and oxygen, and the reaction temperature is in the range of 70-80℃.

[0016] Preferably, the pH value of the system in step S3 is adjusted to the range of 3.0-4.0.

[0017] Preferably, the pH value of the solution in step S4 is adjusted to the range of 1.5-2.5.

[0018] Preferably, the precipitation reaction temperature in step S4 is in the range of 68-72℃.

[0019] Preferably, before adding the iron source in step S4, the molar ratio of iron to phosphorus in the iron-containing phosphate solution is analyzed, and when the molar ratio of iron to phosphorus is less than 1, the iron source is supplemented to adjust the molar ratio of iron to phosphorus to 1:1-1:1.05.

[0020] Preferably, the iron source is ferrous sulfate or iron oxide powder.

[0021] Technical effects and advantages of the present application:

[0022] 1. Energy consumption is reduced and the process is simplified: The present application uses low-temperature acidolysis process, which significantly reduces the reaction energy consumption, avoids high-temperature operation, optimizes the process flow integration, saves multiple phase changes and heat treatment steps, reduces equipment investment and operating cost, and improves the industrial production efficiency;

[0023] 2. Product purity and stability are improved: Through oxidation and precise pH control, heavy metal impurities are effectively removed, low-temperature acidolysis inhibits the dissolution of impurities, combined with precise temperature control and iron-phosphorus ratio control, to ensure product consistency and meet the requirements of battery-grade materials;

[0024] 3. Resource utilization rate and economy are improved: The iron-phosphorus molar ratio analysis and control technology is used to realize accurate proportioning of raw materials, avoid waste of components, fully utilize process intermediates, improve resource utilization rate, reduce material consumption and processing cost, and enhance process economy and environmental protection. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 The process flow diagram of the present application is shown in the figure. DETAILED DESCRIPTION

[0026] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0027] The present application provides a method for preparing high-purity iron phosphate from phosphating slag by low-temperature acidolysis, as shown in the figure. Figure 1 The method comprises the following steps:

[0028] S1, pretreatment: washing, drying and crushing the phosphating slag to obtain phosphating slag powder;

[0029] S2, low-temperature acidolysis: reacting the phosphating slag powder with inorganic acid in an aqueous medium at a temperature of 60-90℃ to obtain an acidolysis solution;

[0030] S3, oxidizing impurities: an oxidizing agent is added to the acid hydrolysis solution, and the pH of the system is adjusted to 2.5-4.5 at 65-85°C, so that heavy metal impurities are precipitated, and after filtration, an iron phosphate solution is obtained;

[0031] S4, precipitation reaction: according to the molar ratio of iron to phosphorus in the iron phosphate solution, an iron source is selectively added, the pH of the solution is adjusted to 1.0-3.0, and a precipitation reaction is carried out at a temperature of 65-75°C to generate an iron phosphate precipitate;

[0032] S5, post-treatment: the iron phosphate precipitate is subjected to aging, filtration, drying and calcination treatment to obtain a high-purity iron phosphate product.

[0033] In some embodiments, in step S1, the particle size D90 of the crushed phosphating slag powder is ≤45 μm.

[0034] In some embodiments, in step S5, the calcination temperature is 550-650°C.

[0035] The method described in the present application is mainly aimed at the waste slag generated in the treatment process of phosphating wastewater from the metal surface treatment industry of steel, galvanized parts, aluminum materials, etc. The typical components include insoluble phosphates such as iron phosphate, ferrous phosphate, zinc phosphate, manganese phosphate, etc., and calcium carbonate, and contain various heavy metal impurity elements such as zinc, manganese, nickel, magnesium, etc. Among them, iron elements may exist in mixed valence states of Fe 2+ and Fe 3+ . The raw material usually also contains physical impurities such as oil stains and suspended solids. The present application aims to provide a method for recovering and preparing battery-grade high-purity iron phosphate from phosphating slag. The battery-grade standard requires that the purity of high-purity iron phosphate is usually not less than 99.5%. The purity of the iron phosphate product prepared by the method can be as high as 99.5% or more (see Examples 1 and 4), and the content of key heavy metal impurities meets the battery-grade standard, i.e. the content of each of zinc, manganese, nickel, chromium, copper, sodium, etc. is less than 50 ppm. In the following examples and comparative examples, unless otherwise specified, step S2 preferably uses sulfuric acid with a concentration of 98% as inorganic acid; step S3 preferably uses hydrogen peroxide with a concentration of 30% as oxidizing agent to ensure that Fe 2+ is completely oxidized to Fe 3+ ; when adjusting the pH in step S4, concentrated ammonia water with a concentration of 10% is preferably used to avoid sodium ion contamination of the product.

[0036] The present application is further described in detail below in combination with examples, comparative examples and performance test experiments, and these examples should not be understood as limiting the scope of the present application. Examples

[0037] Take 1000g of zinc-based phosphating slag (moisture content about 20%) from a steel plant, wash with 80℃ hot water for 3 times, each time with 2000mL water, until the filtrate is neutral. Dry the washed phosphating slag at 105℃ for 12 hours, then pass through a Raymond mill air jet and sieve through a 325 mesh screen to obtain about 800g of phosphating slag powder with a particle size D90≤45μm.

[0038] Take 500g of the above phosphating slag powder and place it in a 2000mL three-necked flask with stirring and temperature control, add 1000mL of deionized water, and stir to form a slurry. Slowly add 200g of concentrated sulfuric acid with a concentration of 98% under continuous stirring, control the reaction temperature at 75±2℃, and react for 2 hours until no bubbles are generated, to obtain a dark green acidolysis solution.

[0039] Slowly add 30% hydrogen peroxide to the acidolysis solution, and test with potassium ferricyanide test paper until no blue color is detected, a total of about 50mL of hydrogen peroxide is consumed. Then slowly adjust the pH of the system to 3.5 with 10% sodium hydroxide solution, and incubate at 75℃ for 1 hour with stirring, a large amount of light-colored precipitate is generated. Perform hot filtration to obtain about 1200mL of red-brown iron-containing phosphate solution.

[0040] Perform component analysis on the iron-containing phosphate solution by ICP-OES, and measure the iron ion concentration to be 0.092g / mL, the phosphate ion concentration to be 0.050g / mL (calculated as phosphorus), and the iron-phosphorus molar ratio to be 1.02:1. No phosphorus source or iron source needs to be supplemented. Adjust the pH of the solution to 2.0 with 10% concentrated ammonia water, control the temperature at 70±2℃, and stir for 2 hours to generate a large amount of white iron phosphate precipitate. After aging the iron phosphate precipitate slurry for 4 hours, perform suction filtration, and repeatedly wash with deionized water until the filtrate is free of sulfate ions as detected by barium chloride. Place the filter cake in an oven at 110℃ and dry for 12 hours to obtain iron phosphate dihydrate precursor. Finally, place the iron phosphate dihydrate precursor into a muffle furnace and calcine at 600℃ for 3 hours, and after cooling, obtain 300g of anhydrous iron phosphate.

[0041] XRF analysis shows that the product iron phosphate is a white powder with a purity of 99.5%; ICP-MS detection shows that the main impurity contents are: zinc 18ppm, manganese 15ppm, nickel 8ppm, chromium 5ppm, and sodium 22ppm, all of which meet the requirements of battery-grade materials. According to the iron content (24%) in the phosphating slag powder and the product mass, the iron element yield is 92%. In this invention, the iron element yield is calculated based on the phosphating slag powder dried at 105℃, and the calculation formula is: iron element yield (%) = [(product iron phosphate mass × theoretical mass fraction of iron in iron phosphate) / (phosphating slag powder mass participating in the reaction × phosphating slag powder iron content)] × 100%.

[0042] In the present application, the iron content of the phosphating residue powder, the iron / phosphorus ion concentration in the iron-containing phosphate solution, the heavy metal impurity content in the final product, and the product iron content required for calculating the yield are determined by inductively coupled plasma optical emission spectrometry (ICP-OES).

[0043] In the present application, the purity of the product iron phosphate is determined by X-ray fluorescence spectrometry (XRF) and is calculated based on FePO4. The crystal structure of the product iron phosphate is characterized by X-ray diffraction (XRD), and the morphology is observed by scanning electron microscopy (SEM).

[0044] Examples 2-4 each provide a method for preparing high-purity iron phosphate from phosphating residue at low temperature.

[0045] The above examples differ from Example 1 in that the process parameters, raw material characteristics, and intermediate product characteristics are different, as shown below. Example

[0046] The acidolysis reaction temperature is controlled at 85±2℃; 10% sodium hypochlorite is used as the oxidizing agent in the oxidation and impurity removal step; the system pH is adjusted to 4.0; the oxidation and impurity removal reaction temperature is 85℃; the solution pH is adjusted to 2.5 in the precipitation reaction step; the precipitation reaction temperature is controlled at 75±2℃; about 1180 mL of iron-containing phosphate solution is obtained; the iron ion concentration is measured to be 0.089 g / mL, the phosphate ion concentration is measured to be 0.049 g / mL (calculated based on phosphorus), and the iron / phosphorus molar ratio is 1.00:1; finally, 285 g of anhydrous iron phosphate is obtained; the product iron phosphate purity is 99.2%; the impurity content is: zinc 21 ppm, manganese 18 ppm, nickel 10 ppm, chromium 6 ppm, and sodium 28 ppm; and the iron element yield is 87.5%. Example

[0047] The acidolysis reaction temperature is controlled at 65±2℃; the acidolysis reaction time is 2.5 hours; about 40 mL of 30% hydrogen peroxide is consumed in the oxidation and impurity removal step; the system pH is adjusted to 3.0; the oxidation and impurity removal reaction temperature is 65℃; the oxidation and impurity removal is stirred for 1.5 hours; the solution pH is adjusted to 1.5 in the precipitation reaction step; the precipitation reaction temperature is controlled at 65±2℃; the precipitation is stirred for 2.5 hours; about 1230 mL of iron-containing phosphate solution is obtained; the iron ion concentration is measured to be 0.088 g / mL, the phosphate ion concentration is measured to be 0.049 g / mL (calculated based on phosphorus), and the iron / phosphorus molar ratio is 1.00:1; finally, 290 g of anhydrous iron phosphate is obtained; the product iron phosphate purity is 99.1%; the impurity content is: zinc 23 ppm, manganese 17 ppm, nickel 9 ppm, chromium 7 ppm, and sodium 22 ppm; and the iron element yield is 89%. Example

[0048] The phosphating slag powder raw material with an iron content of 24%, a phosphorus content of 14.8% and an iron-phosphorus molar ratio of 0.89 is analyzed by ICP; about 1200 mL of iron-phosphate-containing phosphate solution is obtained after acidolysis; the iron ion concentration is 0.080 g / mL, the phosphate radical concentration is 0.045 g / mL (calculated as phosphorus), and the iron-phosphorus molar ratio is 0.99:1; according to the detection results, 10 g of ferrous sulfate is supplemented to adjust the iron-phosphorus molar ratio to 1.02; finally, 295 g of anhydrous ferric phosphate is obtained; the product has a purity of 99.5% of ferric phosphate; the impurity content is: zinc 19 ppm, manganese 16 ppm, nickel 8 ppm, chromium 5 ppm, and sodium 23 ppm; and the iron element yield is 90.1%.

[0049] The other process parameters in the above examples are the same as those in example 1.

[0050] Comparative examples 1-12 respectively provide methods for preparing high-purity ferric phosphate by low-temperature acidolysis of phosphating slag.

[0051] Comparative example 1 differs from example 1 in that the acidolysis reaction temperature is controlled at 50±2℃.

[0052] The other process parameters in the above comparative examples are the same as those in example 1.

[0053] Result: After 4 hours of acidolysis reaction, there are still a large amount of undissolved solids in the slurry, and the acidolysis reaction is still incomplete even if the acidolysis reaction time is extended to 7 hours. The product has a purity of 94.2% of ferric phosphate, and the iron element yield is 78.5%.

[0054] In the present application, the "acidolysis reaction to precipitation reaction time" specifically refers to the total time consumption of the three core chemical reaction stages of acidolysis, oxidation and impurity removal, and heat preservation and precipitation stirring, and does not include the auxiliary operation time such as filtration, aging and washing.

[0055] Comparative example 2 differs from example 1 in that the pH value of the system is adjusted to 5.0 in the oxidation and impurity removal step.

[0056] The other process parameters in the above comparative examples are the same as those in example 1.

[0057] Result: Fe 3+ A large amount of iron ions are hydrolyzed and precipitated, and co-precipitated with impurities. The iron element yield is less than 60%, and the product has a purity of 92.1% of ferric phosphate.

[0058] Comparative example 3 differs from example 1 in that the pH value of the solution is adjusted to 3.5 in the precipitation reaction step.

[0059] The other process parameters in the above comparative examples are the same as those in example 1.

[0060] Result: Colloidal precipitate was generated, which was very difficult to filter. The yield of iron element was reduced to 83%, and the purity of the product iron phosphate was reduced to 95.5%.

[0061] Comparative Example 4 differs from Example 1 in that the precipitation reaction temperature was controlled at 60±2℃.

[0062] The other process parameters in the above comparative example are the same as those in Example 1.

[0063] Result: The tap density of the product iron phosphate is 0.65 g / cm3, which does not meet the requirement of the physical property of the battery-grade material that the tap density should be greater than 0.8 g / cm3. The physical property is not up to standard, and the purity of the product iron phosphate is 98.8%.

[0064] Comparative Example 5 differs from Example 1 in that 10% sodium hypochlorite is used as the oxidizing agent in the oxidation and impurity removal step.

[0065] The other process parameters in the above comparative example are the same as those in Example 1.

[0066] Result: The oxidation and impurity removal uses 10% sodium hypochlorite, and the total time from acidolysis reaction to precipitation reaction is 5 hours. The sodium impurity content in the product iron phosphate is 185 ppm, which exceeds the requirement of the battery-grade material.

[0067] Comparative Example 6 differs from Example 1 in that 37% concentrated hydrochloric acid is used as the inorganic acid in the acidolysis reaction.

[0068] The other process parameters in the above comparative example are the same as those in Example 1.

[0069] Result: The acidolysis reaction uses 37% concentrated hydrochloric acid, and the total time from acidolysis reaction to precipitation reaction is 5 hours. The chlorine impurity content in the product iron phosphate is 255 ppm, which does not meet the requirement of the battery-grade material that the chlorine impurity content should be less than 100 ppm. The product iron phosphate is unqualified, and the purity and sodium impurity content of the product iron phosphate do not need to be detected.

[0070] Comparative Example 7 differs from Example 1 in that the same phosphorus slag powder raw material as in Example 4 is used, with a molar ratio of iron to phosphorus of 0.89, but no ICP-OES component analysis and iron source supplement are performed.

[0071] The other process parameters in the above comparative example are the same as those in Example 1.

[0072] Result: The purity of the product iron phosphate is only 94.0%, and high-purity iron phosphate cannot be obtained.

[0073] Comparative Example 8 uses the method disclosed in the prior art.

[0074] The specific steps are as follows: 500 g of phosphating slag powder from the same source and after pretreatment is mixed with 200 g of 98% concentrated sulfuric acid, 50 mL of 30% hydrogen peroxide and 20 g of activated carbon adsorbent, and stirred at 95°C for 2 hours. The subsequent process includes: after solid-liquid separation, sodium hydroxide is added to the filtrate to adjust the pH to neutral, and the obtained precipitate is heat treated at 600°C for 2 hours to obtain iron oxide; the iron oxide is activated by high-energy ball milling, and then mixed with phosphoric acid at a molar ratio of 1:1.05, and reacted at 80°C for 3 hours to prepare iron phosphate.

[0075] Results: The purity of the product iron phosphate is 98.2%, the zinc impurity content is 85 ppm, the total process time from raw material treatment to obtaining the final product is about 12 hours, and the iron element yield is 82%. Compared with Example 1, the purity of the product iron phosphate is lower, the impurity content is significantly higher, and the process flow contains multiple high-temperature and high-energy consumption treatments, resulting in a total consumption time much longer than the present application and higher energy consumption.

[0076] The difference between Comparative Example 9 and Example 1 is that the reaction temperature in the oxidation impurity removal step is controlled at 60±2°C.

[0077] The other process parameters in the above comparative example are the same as those in Example 1.

[0078] Results: The impurity removal reaction rate is slow, and the heavy metal impurities are not completely precipitated. The purity of the product iron phosphate is 96.8%, the zinc impurity content is 52 ppm, the manganese impurity content is 38 ppm, and the iron element yield is 85.2%.

[0079] The difference between Comparative Example 10 and Example 1 is that the reaction temperature in the oxidation impurity removal step is controlled at 90±2°C.

[0080] The other process parameters in the above comparative example are the same as those in Example 1.

[0081] Results: High temperature causes some precipitated impurities to redissolve and a small amount of colloidal material to be generated. The purity of the product iron phosphate is 97.5%, but the zinc impurity content increases to 45 ppm, the product iron phosphate color is slightly yellow, and the iron element yield is 86.8%.

[0082] The difference between Comparative Example 11 and Example 1 is that the precipitation reaction temperature is controlled at 55±2°C.

[0083] The other process parameters in the above comparative example are the same as those in Example 1.

[0084] Results: The precipitation and crystallization rate is too slow, fine crystals are generated, and filtration is difficult. The tap density of the product iron phosphate is only 0.58 g / cm³, the purity of the product iron phosphate is 97.2%, and the iron element yield is 82.5%.

[0085] Comparative Example 12 differs from Example 1 in that the precipitation reaction temperature is controlled at 80±2℃.

[0086] The other process parameters in the above comparative examples are the same as those in Example 1.

[0087] Result: The precipitation rate is too fast, the crystal growth is uneven, and part of the amorphous precipitate is produced. A small amount of ammonium iron phosphate impurities is detected in the product iron phosphate, the purity of the product iron phosphate is 96.5%, and the yield of iron element is 84.3%.

[0088] Performance detection test: Record the yield of iron element and the purity of product iron phosphate in the preparation of high-purity iron phosphate from phosphating slag by low-temperature acidolysis in the examples and comparative examples.

[0089] The test results are shown in Table 1.

[0090] Table 1 Performance detection results of high-purity iron phosphate prepared from phosphating slag by low-temperature acidolysis

[0091] Note: NA means that the data is not detected or cannot be effectively obtained.

[0092] In combination with Table 1, by comparing the test results of Examples 1-4 and Comparative Examples 1-12, it can be seen that:

[0093] In Example 1, the acidolysis temperature is controlled at 75±2℃, the pH for oxidation and impurity removal is adjusted to 3.5, the pH for precipitation reaction is adjusted to 2.0, the precipitation temperature is controlled at 70±2℃, and 30% hydrogen peroxide is used as the oxidizing agent and 98% sulfuric acid is used as the acidolysis agent. The purity of the product iron phosphate is as high as 99.5%, the impurity content is extremely low, and the yield of iron element reaches 92%. This proves that within the process parameter range required by the present application, high-performance battery-grade iron phosphate can be efficiently prepared.

[0094] In Example 2, the acidolysis temperature is controlled at 85±2℃, the pH for oxidation and impurity removal is adjusted to 4.0, the pH for precipitation reaction is adjusted to 2.5, and the precipitation temperature is controlled at 75±2℃. Although the use of 10% sodium hypochlorite results in a slightly higher sodium impurity than Example 1, the purity of the product iron phosphate still reaches 99.2%, and the yield of iron element is 87.5%. This indicates that within the parameter range of the present application, even if the oxidizing agent is not the optimal choice, the main performance indicators are still good, proving the effectiveness and fault tolerance of the process parameter range.

[0095] In Example 3, relatively milder process conditions are adopted: acidolysis temperature 65±2℃, pH for oxidation and impurity removal 3.0 and temperature 65℃, pH for precipitation 1.5 and temperature 65±2℃. The purity of the product iron phosphate still reaches 99.1%, and the yield of iron element is 89%. This further indicates that as long as the key parameters are controlled within the range required by the present application, even if non-optimal conditions at the lower end of the range are used, the results are still significantly better than the comparative examples and the prior art, and the process is stable.

[0096] The raw material used in Example 4 has an insufficient iron-phosphorus molar ratio (0.89), but after adjusting the iron-phosphorus molar ratio to 1.02 by ICP-OES component analysis and supplementing the iron source, the product iron phosphate purity is as high as 99.5%, and the iron element yield reaches 90.1%. This proves the necessity of the "component monitoring and precise feeding" step in the process of the present application, as well as the good adaptability to raw materials of different sources, ensuring high purity and high yield of the product iron phosphate.

[0097] In summary, Examples 1-4 of the present application consistently achieve excellent results of short process, high yield, high purity and low impurity content by controlling the process parameters of key steps such as acid hydrolysis, oxidation impurity removal, precipitation, etc. within the optimized range determined by the present application, fully verifying the effectiveness and superiority of the technical scheme of the present application.

[0098] In Comparative Example 1, the acid hydrolysis temperature is controlled at 50°C, which is lower than the range of 60-90°C required by the present application, resulting in a 5-hour extension of the acid hydrolysis reaction compared to Example 1, an acid hydrolysis reaction time of 7 hours, incomplete acid hydrolysis, an iron element yield of 78.5%, and a product iron phosphate purity of only 94.2%. In contrast, Example 1 of the present application performs acid hydrolysis at 75°C, with short reaction time, high yield and high purity.

[0099] In Comparative Example 2, the oxidation impurity removal pH is adjusted to 5.0, which is higher than the range of 2.5-4.5 required by the present application, resulting in a Fe 3+ Ion hydrolysis precipitation, co-precipitation with impurities, iron element yield less than 60%, product iron phosphate purity decreased to 92.1%. Example 1 of the present application controls the oxidation impurity removal pH at 3.5, effectively removing impurities, with high yield and purity.

[0100] In Comparative Example 3, the precipitation reaction pH is adjusted to 3.5, which is higher than the range of 1.0-3.0 required by the present application, generating a colloidal precipitate, making filtration difficult, with an iron element yield of 83% and a product iron phosphate purity of 95.5%. Example 1 of the present application controls the precipitation pH at 2.0, generating a precipitate that is easy to filter, with high yield and purity.

[0101] In Comparative Example 4, the precipitation reaction temperature is controlled at 60°C, which is lower than the range of 65-75°C required by the present application, resulting in a product iron phosphate tap density that is low and physical properties that do not meet standards, although the product iron phosphate purity is 98.8%, the iron element yield is not optimal. Example 1 of the present application controls the precipitation temperature at 70°C, resulting in a product iron phosphate with good physical properties.

[0102] In Comparative Example 5, 10% sodium hypochlorite was used as the oxidizing agent, and the sodium ions dissociated from the sodium hypochlorite could not be removed by washing, resulting in a sodium impurity content of up to 185 ppm in the product iron phosphate, which exceeded the requirements for battery-grade materials. In Example 1 of the present application, hydrogen peroxide was used as the oxidizing agent, and the sodium impurity content was 22 ppm, which was qualified.

[0103] In Comparative Example 6, 37% concentrated hydrochloric acid was used as the inorganic acid, resulting in a chlorine impurity content of up to 255 ppm in the product iron phosphate. The introduction of chloride ions by the concentrated hydrochloric acid resulted in the product iron phosphate having no use value, so there was no need to detect the purity and sodium impurity content of the product iron phosphate. In Example 1 of the present application, sulfuric acid was used, and no chlorine impurities were introduced.

[0104] In Comparative Example 7, phosphorus slag powder raw materials with an iron-phosphorus molar ratio of 0.89 were used, but no composition analysis or iron source supplement was performed, and the purity of the product iron phosphate was only 94.0%. In Example 4 of the present application, the iron source was supplemented and analyzed by ICP-OES, and the iron-phosphorus molar ratio was adjusted to 1.02, and the purity of the product iron phosphate was as high as 99.5%.

[0105] In Comparative Example 8, the existing technology was used, and the total process time was about 12 hours, the purity of the product iron phosphate was low, the zinc impurity content was high, and the iron element yield was low. In Example 1 of the present application, the process flow was short, the purity of the product iron phosphate was high, the impurity content was low, and the iron element yield was high. It is worth noting that even in Example 3 of the present application, which is relatively non-optimal in terms of process conditions: acidolysis temperature 65±2℃, precipitation pH 1.5, and precipitation temperature 65±2℃, the purity of the product iron phosphate was 99.1%, which was still significantly higher than the purity of 98.2% in Comparative Example 8, the zinc impurity content was 23 ppm, which was much lower than the zinc impurity content of 85 ppm in Comparative Example 8, the process flow time was shortened by nearly half, and the iron element yield was increased by 7 percentage points. This fully shows that the technical scheme of the present application has made overall and significant progress compared with the existing technology, and this advantage is reflected in the entire range of process parameters.

[0106] In Comparative Example 9, the oxidation and impurity removal temperature was controlled at 60℃, which was lower than the range of 65-85℃ required by the present application, the impurity removal reaction rate was slow, the heavy metal impurities were not completely precipitated, the purity of the product iron phosphate decreased to 96.8%, the zinc impurity content was 52 ppm, the manganese impurity content was 38 ppm, and the iron element yield was 85.2%. In Example 1 of the present application, the oxidation and impurity removal temperature was controlled at 75℃, and the impurities were completely removed.

[0107] The impurity removal temperature in the comparative example 10 is controlled at 90 DEG C, which is higher than the range of 65-85 DEG C required by the present application, the high temperature causes part of the precipitated impurities to redissolve, the purity of the product iron phosphate is 97.5%, the zinc impurity content increases to 45 ppm, the product iron phosphate is slightly yellow, and the iron element yield is 86.8%. The impurity removal temperature in the present application is controlled at 75 DEG C, the product iron phosphate is white, and the impurity content is low.

[0108] The precipitation reaction temperature in the comparative example 11 is controlled at 55 DEG C, which is lower than the range of 65-75 DEG C required by the present application, the precipitation crystallization rate is too slow, fine crystals are generated, filtration is difficult, the product iron phosphate has low tap density, the purity of the product iron phosphate is 97.2%, and the iron element yield is 82.5%. The precipitation temperature in the present application 1 is controlled at 70 DEG C, and the crystal growth is good.

[0109] The precipitation reaction temperature in the comparative example 12 is controlled at 80 DEG C, which is higher than the range of 65-75 DEG C required by the present application, the precipitation rate is too fast, the crystal growth is uneven, part of the amorphous precipitate and ammonium phosphate impurity phase are generated, the purity of the product iron phosphate is 96.5%, and the iron element yield is 84.3%. The precipitation temperature in the present application 1 is controlled at 70 DEG C, and the precipitation is uniform without impurity phase.

[0110] The purity of the products in the examples 1 and 4 reaches the high standard of 99.5% required by the battery grade. Although the purity of the products in the examples 2 and 3 is slightly low due to the use of sodium hypochlorite or milder reaction conditions, the impurity content is far lower than that in the comparative examples and the prior art, and completely meets the requirements of the battery grade material on impurities. This fully shows that the process of the present application has the significant advantage and innovation that high-performance products can be produced in a wide range of process parameters.

[0111] In summary, by controlling the parameters such as acidolysis temperature, impurity removal temperature and pH, precipitation reaction temperature and pH, the present application realizes the preparation of iron phosphate with high yield and high purity, the product iron phosphate meets the requirements of the battery grade material, and the process flow is short and the energy consumption is low.

[0112] Secondly: the present application discloses the structure involved in the present application, other structures can refer to the general design, in the case of no conflict, the same embodiment and different embodiments of the present application can be combined with each other;

[0113] Finally: the above only describes the preferred embodiments of the present application, and is not used to limit the present application, any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing high-purity ferric phosphate from phosphatizing slags by low-temperature acidolysis, characterized in that, The method comprises the following steps: S1, pretreatment: washing, drying and crushing the phosphating residue to obtain phosphating residue powder; S2, low-temperature acidolysis: reacting the phosphating residue powder with inorganic acid in an aqueous medium at a temperature in the range of 60-90℃ to obtain an acidolysis solution; S3, oxidation and impurity removal: adding an oxidizing agent to the acidolysis solution and adjusting the pH value of the system to the range of 2.5-4.5 at 65-85℃ to make heavy metal impurities precipitate in the form of a precipitate, and then obtaining an iron-containing phosphate solution after filtration; S4, precipitation reaction: determining the iron-phosphorus molar ratio in the iron-containing phosphate solution by component analysis, supplementing an iron source when the iron-phosphorus molar ratio is less than 1, adjusting the iron-phosphorus molar ratio to 1:1-1:1.05, then adjusting the pH value of the solution to the range of 1.0-3.0, and performing a precipitation reaction at a temperature in the range of 65-75℃ to generate an iron phosphate precipitate; S5, post-treatment: aging, filtering, drying and calcining the iron phosphate precipitate to obtain a high-purity iron phosphate product.

2. The method of claim 1, wherein the phosphorus slag is prepared by a process comprising: The inorganic acid in step S2 is at least one of hydrochloric acid, sulfuric acid and nitric acid. ​ 3. The method of claim 1, wherein the phosphorus slag is prepared by a process comprising: The aqueous medium in step S2 is deionized water. ​ 4. The method for preparing high-purity ferric phosphate by low-temperature acid hydrolysis of phosphating slag according to claim 1, characterized in that, The reaction temperature in step S2 is 65-85℃.

5. The method for preparing high-purity ferric phosphate by low-temperature acid hydrolysis of phosphating slag according to claim 1, characterized in that, The oxidizing agent in step S3 is at least one of hydrogen peroxide, sodium chlorate and oxygen, and the reaction temperature is 70-80℃.

6. The process for the preparation of high purity ferric phosphate from phosphorus slag by low temperature acid digestion as claimed in claim 1 wherein, The pH value of the system is adjusted to 3.0-4.0 in step S3. 7.The method according to claim 1, characterized in that, The pH value of the solution is adjusted to 1.5-2.5 in step S4. 8.The method according to claim 1, characterized in that, The precipitation reaction temperature in step S4 is 68-72℃. 9.The method according to claim 1, characterized in that, The iron source is ferrous sulfate or iron oxide powder.

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