A method for producing battery-grade iron phosphate from mill scale

Abstract

The application discloses a method for preparing battery-grade iron phosphate from mill scale. The method comprises the following steps: performing acid leaching on the mill scale in the presence of a metal reducing agent to obtain ferrous acid leaching solution I; when the reducing property of the metal reducing agent is stronger than that of elemental iron, adding the elemental iron into the ferrous acid leaching solution I to perform a displacement reaction, thereby obtaining ferrous acid leaching solution II and the metal reducing agent; performing neutralization and impurity removal on the ferrous acid leaching solution I or the ferrous acid leaching solution II to obtain a hydrated iron phosphate product, and performing oxidation and iron precipitation on the impurity-removed solution. The method can efficiently leach iron in the mill scale, realize deep impurity removal, and obtain high-quality battery-grade iron phosphate, and truly realizes high-value resource utilization of the mill scale. In addition, the method has the advantages of simple process, easy operation, recyclable waste acid, low cost and suitability for large-scale production.

Description

TECHNICAL FIELD

[0001] The application relates to a method for utilizing mill scale in a steel rolling plant, in particular to a method for preparing battery-grade iron phosphate from mill scale in a steel rolling plant, and belongs to the field of mill scale resource utilization. BACKGROUND

[0002] As a national basic industry, steel plants produce a large amount of steel every year. According to authoritative data, the crude steel output in 2021 was 103.279 million tons, and the crude steel output in 2022 was about 101 million tons. Among them, 3% to 5% will be lost in the form of mill scale. It can be seen that mill scale has very high recycling value. At present, mill scale is used as auxiliary raw material, to prepare reduced iron powder, to manufacture ferrosilicon alloy, and a large part of mill scale is not effectively utilized. Improving the added value of products and avoiding secondary pollution are the key directions of future research on the resource utilization of mill scale.

[0003] In addition, based on the advantages of resources, costs, cycles, safety, etc., lithium iron phosphate batteries have been widely used in many fields such as new energy vehicles, energy storage, 5G base stations, etc. Since 2014, most domestic enterprises have used iron phosphate as the raw material for the production of lithium iron phosphate, and the output reached 350,000 tons in 2021, with a year-on-year growth of 45.7%. However, there are still some problems in the production of iron phosphate that need to be solved: first, to find a suitable alternative iron source to reduce the production cost of iron phosphate and increase its competitive advantage; second, to improve the tap density of iron phosphate products to obtain higher tap density of lithium iron phosphate cathode materials.

[0004] Iron phosphate, as a high-quality precursor material for preparing lithium iron phosphate cathode materials, has broad development prospects. At present, the main iron sources for preparing iron phosphate in industry are high-purity mill scale, iron powder and ferrous sulfate. If mill scale from a steel rolling plant can be used to prepare iron phosphate precursor materials, it can not only effectively alleviate the pressure of mill scale treatment and disposal, but also is expected to become another important iron source for preparing iron phosphate. Chinese patent CN103094578B discloses a method for preparing lithium ion battery cathode material lithium iron phosphate using steel slag. The specific process is to mix the steel slag with an aqueous solution of carbonate, then heat treat and reduce, and then magnetically separate to obtain a steel slag extract; the steel slag extract is further reacted with a mixed solution of dilute sulfuric acid, hydrogen peroxide and phosphoric acid to obtain a calcium-free steel slag filter cake; the calcium-free steel slag filter cake, lithium carbonate and soluble carbon source are mixed to prepare a precursor by a wet process; and the precursor is microwave sintered to obtain a lithium iron phosphate cathode material. The preparation method involves a wet process + fire process combined process, which is complex and has high energy consumption. In addition, the composition of steel slag is not uniform, and it is difficult to ensure the stability of the product and the recovery of the main element iron by this method.

[0005] Currently, the following technical problems still need to be solved for taking the mill scale as the iron source for preparing the iron phosphate: (1) the iron in the mill scale mainly exists in the form of magnetite and hematite, and is difficult to be leached efficiently; (2) after the mill scale is leached by acid, the impurity metal ions such as Al, K, Na, Mn, Mg and Ca are difficult to be separated and are easy to enter the iron phosphate product; (3) the prepared iron phosphate product has unstable physicochemical properties. SUMMARY

[0006] In view of the defects in the method for preparing the iron phosphate from the mill scale, the purpose of the present application is to provide a method for preparing the battery-grade iron phosphate from the mill scale of a rolling mill, which can efficiently leach the iron in the mill scale, realize deep impurity removal, and finally obtain the high-quality battery-grade iron phosphate, and truly realize the high-value resource utilization of the mill scale of the rolling mill, and the method has the advantages of simple process, easy operation, recyclable waste acid, low cost and suitability for large-scale production.

[0007] In order to achieve the above technical purposes, the present application provides a method for preparing the battery-grade iron phosphate from the mill scale of a rolling mill, which comprises the following steps:

[0008] 1) acid leaching the mill scale in the presence of a metal reducing agent to obtain a ferrous acid leaching solution I; when the reducing property of the metal reducing agent is stronger than that of the elemental iron, adding the elemental iron into the ferrous acid leaching solution I to carry out a displacement reaction, and obtaining a ferrous acid leaching solution II and the metal reducing agent;

[0009] 2) carrying out neutralization and precipitation impurity removal on the ferrous acid leaching solution I or the ferrous acid leaching solution II to obtain a post-impurity removal solution and an impurity removal residue;

[0010] 3) adjusting the pH and the P / Fe molar ratio of the post-impurity removal solution, and then carrying out oxidation and precipitation of iron to obtain a hydrated iron phosphate product.

[0011] The specific idea of the present application for preparing the battery-grade iron phosphate from the mill scale of a rolling mill is as follows: first, the Fe 3+ in the mill scale is transferred into the solution while the Fe 3+ is reduced into Fe 2+ , so as to remove the impurities, then the impurity ions in the solution are removed by the neutralization and precipitation method, finally the battery-grade hydrated iron phosphate is prepared by oxidizing the Fe 2+ and precipitating the Fe 3+ under the condition of high acidity, and the acid iron precipitation post-solution generated in the process of precipitating the Fe 3+ can be directly recycled to the leaching process of the mill scale.

[0012] The process for preparing battery-grade iron phosphate from mill scale according to the present application mainly includes the following four steps: the first step is to reduce and strengthen the leaching of iron elements in the mill scale, first, the crystal structure of Fe3O4 and Fe2O3 minerals in the mill scale is destroyed by strong acid, so that the iron elements are transferred to the solution, at the same time, a metal reducing agent is added to reduce the iron ions to ferrous ions, which not only can promote the following chemical reaction process and improve the leaching efficiency, but also can convert the trivalent iron ions to ferrous ions, which is beneficial to the subsequent neutralization and impurity removal process. In the reduction leaching process, the following chemical reactions continuously occur in the solution system: Fe2O3+6H + =2Fe 3+ +3H2O; Fe3O4+8H + =2Fe 3+ +Fe 2+ +4H2O; 2Fe 3+ +M=2Fe 2+ +M 2+ (M is a metal reducing agent); Fe+M 2+ =Fe 2+ +M; the second step is the recovery process of the metal reducing agent, if the reducing property of the metal reducing agent is stronger than that of iron, and the ion oxidation property of the metal reducing agent is between that of iron ions and ferrous ions, then the reducing agent can be recovered by iron powder displacement, through this method, the reducing agent, as a "bridge" substance for the transformation of iron ions to ferrous ions, can be recycled; the third step is the removal process of metal impurities, which is different from the prior art, Al is removed by controlling the hydrolysis precipitation process in a weak acid environment, high-purity Cu powder is recovered by iron ball or iron sheet displacement, and other metal ions such as K, Na, Mn, Mg, Ca, etc. still exist in the solution in ionic form. The fourth step is the synthesis and crystal growth process of hydrated iron phosphate, through strictly coordinated control of the acidity and reaction temperature and reaction time of the reaction system and other conditions, not only can the co-crystallization process of K, Na, Mn, Mg, Ca and other cationic impurities be effectively prevented, and they can be controlled in the solution system, so that they do not co-precipitate with iron ions to obtain high-purity hydrated iron phosphate, but also the growth process of hydrated iron phosphate crystals can be effectively regulated at the same time, to obtain hydrated iron phosphate products meeting the battery-grade standard.

[0013] As a preferred solution, the temperature in the process of the acid leaching is 30-95°C, the time is 0.5-7.0h, the acid dosage is 0.8-1.4 times of the theoretical amount, the liquid-solid ratio is 3-10L:1kg, and the metal reducing agent is metered at 2.0-30.0g / L. The acid used includes at least one of sulfuric acid, hydrochloric acid and phosphoric acid. The theoretical amount of the acid is calculated according to the stoichiometric ratio of the metal substances / metal oxides in the iron scale that can chemically react with the acid. Under the preferred acid leaching conditions, the iron minerals in the steel slag in the form of Fe3O4 and Fe2O3 and other difficult-to-leach minerals can be efficiently leached. The metal reducing agent is further preferably metered at 2.0-15.0g / L. The acid dosage is further preferably 1.0-1.2 times of the theoretical amount.

[0014] As a preferred solution, the metal reducing agent includes at least one of Zn, Fe and Cu. The further preferred metal reducing agent is Fe or Cu, which has less impact on the subsequent iron recovery. The key role of the metal reducing agent is to promote the chemical reaction process of the iron ion leaching and improve the leaching efficiency, and on the other hand, to convert the ferric ion into ferrous ion, which is beneficial to the subsequent neutralization and impurity removal process.

[0015] As a preferred solution, the acid includes at least one of sulfuric acid, hydrochloric acid and phosphoric acid.

[0016] As a preferred solution, in the process of the displacement reaction, the elemental iron dosage is 1.0-4.0 times of the theoretical amount, the temperature is 20-70°C, and the time is 0.1-3.0h. The displacement reaction process mainly recovers the metal reducing agent, such as elemental copper. The theoretical amount of the elemental iron is calculated according to the chemical amount of the chemical reaction with the metal reducing agent. The elemental iron dosage is further preferably 1.5-3.0 times of the theoretical amount. The temperature is further preferably 20-50°C, and the time is further preferably 0.5-1.0h.

[0017] As a preferred solution, in the process of the neutralization and precipitation impurity removal, a pH regulator is used to adjust the pH to 3.0-6.0, at a temperature of 30-90°C for 2-7h. The pH adjustment is further preferably 4.0-5.0. The temperature is further preferably 45-70°C. The time is further preferably 2.5-4h.

[0018] As a preferred solution, the pH regulator includes at least one of elemental iron, ammonia water, sodium hydroxide, potassium hydroxide and calcium oxide.

[0019] As a preferred scheme, the pH of the post-impurity-removing solution is adjusted to 0.5-1.8, and the P / Fe molar ratio is adjusted to 1.0-1.3. The P / Fe molar ratio is further preferably 1.05-1.2. The pH is further preferably adjusted to 0.8-1.4. The phosphorus source used in adjusting the P / Fe molar ratio comprises at least one of a phosphate, a monohydrogen phosphate, a dihydrogen phosphate, and phosphoric acid.

[0020] As a preferred scheme, in the process of oxidation, the theoretical amount of 1-1.5 times of oxidizing agent is added, and the oxidizing agent comprises at least one of hydrogen peroxide, thiosulfate, peroxy acid salt, ozone, oxygen, and air. The theoretical amount of oxidizing agent is calculated based on the oxidation of ferrous ions to ferric ions. The amount of oxidizing agent added is further preferably 1.0-1.2 times of the theoretical amount.

[0021] As a preferred scheme, in the process of precipitating iron, the reaction is carried out at a temperature of 80-120℃ for 3.0-24h. By synergistically controlling the initial pH and the reaction temperature in the process of precipitating iron, the nucleation process of iron phosphate can be effectively avoided from entering the crystal lattice of iron phosphate, and the crystallinity of iron phosphate can be controlled to be high. 2+ 2+ 2+ The temperature is further preferably 95-105℃, and the time is further preferably 4.0-11.0h. After the post-impurity-removing solution is subjected to the process of precipitating iron, the mother liquor can be directly used in the process of acid leaching.

[0022] The method for preparing iron phosphate from mill scale according to the present application comprises the following specific steps:

[0023] (1) acid leaching process: an acid solution is added according to 0.8-1.4 times of the stoichiometric ratio, water is supplemented according to a liquid-solid ratio (3-10) L:1 kg, then a metal reducing agent is added after complete dissolution, the concentration of the solution system is 2.0-30.0g / L, and the stirring is carried out at a temperature of 30-95℃ for 0.5-7.0h, after the reaction endpoint is reached, liquid-solid separation is carried out, and an acid leaching solution and an acid leaching residue are obtained;

[0024] (2) recovery of reducing agent process: elemental iron is added according to 1.0-4.0 times of the stoichiometric ratio, the stirring is carried out at a temperature of 20-70℃ for 0.1-3.0h, and liquid-solid separation is carried out, and a ferrous acid leaching solution and a metal reducing agent are obtained;

[0025] (3) impurity removal process: the pH of the ferrous acid leaching solution is adjusted to 3.0-6.0, the stirring is carried out at a temperature of 30-90℃ for 2-7h, after the reaction endpoint is reached, liquid-solid separation is carried out, and a post-impurity-removing solution and an impurity removal residue are obtained;

[0026] ​​(4) Preparation of iron phosphate process: to the impurity removal after the liquid according to the P / Fe molar ratio is 1.0~1.3 supplement the required phosphorus source, stirring dissolution after adjusting the solution pH to 0.5~1.8, slowly add the stoichiometric ratio 1.0~1.5 times of oxidizing agent, after the ferrous ion is completely oxidized to iron ion, the reactor is transferred to the heater, stirring at 80~120℃ temperature for 3~24h, liquid-solid separation, filter cake washing, drying, the hydrated iron phosphate product is obtained.

[0027] Compared with the prior art, the technical scheme of the present application has the following advantages:

[0028] The present application realizes the resource utilization of the mill scale by wet method, specifically, Fe 3+ In the steel slag is transferred to the solution, and then Fe 3+ Is reduced to Fe 2+ So as to remove the impurity process, and then the impurity ions in the solution are removed by neutralization precipitation method, and finally the battery grade hydrated iron phosphate is prepared by oxidizing Fe 2+ And precipitating Fe 3+ Under high acidity conditions, and the acidic Fe 3+ Precipitation post-solution produced in the process is directly reused to the leaching process of the mill scale, and the method has the advantages of simple process, high iron leaching rate, good impurity removal effect, and battery grade phosphoric acid product, etc., and the economic and environmental benefits are significant, and it is suitable for industrial scale production, which opens up a new way for the high value utilization of the mill scale.

[0029] In the process of preparing iron phosphate from the mill scale, the metal reducing agent is used to promote the leaching process of the mill scale, so that the leaching and reduction processes of iron elements are carried out synchronously, which not only effectively improves the leaching efficiency of iron elements, but also cooperatively completes the process of reducing iron ions to ferrous ions, and the metal reducing agent has low loss and can be recycled, and the cost is low.

[0030] In the process of preparing iron phosphate from the mill scale, the iron is reduced to divalent iron, and then the impurity removal process by weak acid neutralization precipitation can reduce the loss of iron, and deeply remove the main harmful impurities such as aluminum.

[0031] In the process of preparing iron phosphate from the mill scale, the initial pH and reaction temperature are used to cooperatively control the nucleation process of iron phosphate when precipitating iron, which can effectively avoid the entry of Mn 2+ , Mg 2+ , Ca 2+ And other divalent metal ions into the iron phosphate lattice, and the crystallinity of the iron phosphate can be controlled to be high, which meets the requirements of battery grade. DETAILED DESCRIPTION

[0032] The following examples are intended to further illustrate the present application and are not intended to limit the scope of the claims.

[0033] The main components of the oxide scale are as follows: Fe 61.8%, Al 0.41%, Ca 0.48%, Mg 0.21%, Mn 0.30%, Zn 0.14%, S 0.46%. The XRD results of the oxide scale show that the main components in the oxide scale are Fe203 and Fe304.

[0034] Example 1

[0035] a First, 30 g of the oxide scale powder is weighed, deionized water and concentrated sulfuric acid solution are added according to the liquid-solid ratio of 6:1 and the acid dosage of 1.0 times, respectively, then 2.0 g of elemental copper is added, the temperature is raised to 80°C, and stirring is performed for 40 min, and then liquid-solid separation is performed, thereby obtaining the acid leaching solution and the acid leaching residue. The iron element leaching rate is 98.5%.

[0036] b Then, 3.5 g of elemental iron is added to the acid leaching solution, the reaction temperature is adjusted to 25°C, constant temperature stirring is performed for 0.5 h, liquid-solid separation is performed, the filter cake is washed and dried, thereby obtaining the recovered reducing agent and the ferrous acid leaching solution. The copper ion concentration in the ferrous acid leaching solution is 0.004 g / L, and the calculated copper recovery rate is 99.96%.

[0037] c Then, the ferrous acid leaching solution is adjusted to pH=1.0 with ammonia water, 2.0 g of elemental iron is added, the temperature is raised to 50°C, stirring is performed for 5.0 h, and then liquid-solid separation is performed to obtain the impurity removal residue and the impurity removal solution. The composition of the impurity removal solution (pH=4.5) is measured, the yield and removal rate are calculated: the iron yield is 98%, the aluminum removal rate is 97.5%, and copper is not detected.

[0038] d Finally, ammonium dihydrogen phosphate solid is added to the impurity removal solution according to the P / Fe molar ratio of 1.05:1, the solution is dissolved, the pH of the reaction solution is adjusted to 0.6, then ferrous ion is slowly added dropwise until the iron ion dosage is 1.1 times the hydrogen peroxide solution, after complete oxidation, the reaction solution is transferred to a constant temperature reactor at 90°C, stirring is performed for 14 h, after the reaction is completed, liquid-solid separation is performed, the filter cake is washed and dried, thereby obtaining the hydrated ferric phosphate product. The composition and tap density of the product are shown in Table 1.

[0039] Example 2

[0040] a The elemental copper addition amount in the leaching process is increased to 4.0 g, and the rest of the operations and reagent addition are the same as in Example 1a. The iron element leaching rate is 99.2%.

[0041] b 7.5 g of elemental iron and 20 ml of deionized water are added to the acid leaching solution, and the rest of the process is the same as in Example 1b. The copper ion concentration in the ferrous acid leaching solution is 0.009 g / L, and the calculated copper recovery rate is 99.93%.

[0042] c The pH of the ferrous solution was adjusted to 2.0 with 20% sodium hydroxide solution, and 1.5 g of elemental iron was added. The rest of the process was the same as in Example 1c. The final pH of the purified solution was 4.2, the yield of iron was 96.2%, the removal rate of aluminum was 99.3%, and copper was not detected.

[0043] d Ammonium phosphate solid was added to the purified solution according to a P / Fe molar ratio of 1.1:1. After dissolution, the pH of the reaction solution was adjusted to 1.0, and then ferrous ions were slowly added dropwise until the amount of hydrogen peroxide solution was 1.2 times the amount of iron ions. After complete oxidation, the solution was transferred to a constant-temperature reactor at 95°C, and stirred for 9 h. After the reaction was completed, the liquid and solid were separated, the filter cake was washed and dried, and the hydrated ferric phosphate product was obtained. The composition and tap density of the product are shown in Table 1.

[0044] Example 3

[0045] a In the leaching process, 4.0 g of elemental iron was used instead of elemental copper, and 20 ml of deionized water was added. The rest of the process was the same as in Example 2a. The leaching rate of iron was 98.8%.

[0046] b The pH of the acid leaching solution was adjusted to 4.5 with 20% sodium hydroxide solution, and the reaction temperature was adjusted to 60°C. The solution was stirred at constant temperature for 4.0 h, and then the purified residue and the purified solution were obtained by liquid-solid separation. The composition, yield and removal rate of the purified solution were measured: the yield of iron was 95.2%, the removal rate of aluminum was 99.8%, and copper was not detected.

[0047] c Ammonium monohydrogen phosphate solid was added to the purified solution according to a P / Fe molar ratio of 1.2:1. After dissolution, the pH of the reaction solution was adjusted to 1.5, and then ferrous ions were slowly added dropwise until the amount of 30% ammonium persulfate solution was 1.2 times the amount of iron ions. After complete oxidation, the solution was transferred to a constant-temperature reactor at 100°C, and stirred for 5 h. After the reaction was completed, the liquid and solid were separated, the filter cake was washed and dried, and the hydrated ferric phosphate product was obtained. The composition and tap density of the product are shown in Table 1.

[0048] Example 4

[0049] a In the leaching process, 3.0 g of elemental zinc was used instead of elemental copper, and the rest of the process was the same as in Example 2a. The leaching rate of iron was 97.3%.

[0050] b The pH of the solution was adjusted to 3.0 with calcium oxide solid, and the insoluble white precipitate was filtered. Then, 1.5 g of elemental iron was added, and the temperature was adjusted to 80°C. The solution was stirred at constant temperature for 1.5 h, and then the purified residue and the purified solution were obtained by liquid-solid separation. The composition, yield and removal rate of the purified solution (pH = 3.9) were measured: the yield of iron was 93.2%, the removal rate of aluminum was 97.1%, and copper was not detected.

[0051] cThe calcium dihydrogen phosphate solid was added to the impurity removal solution according to a P / Fe molar ratio of 1.2:1, stirred for 15 min, and then filtered. The filtrate was adjusted to a reaction liquid pH of 1.2, and then slowly added with ferrous ion until the amount of hydrogen peroxide solution was 1.2 times the amount of iron ion. After complete oxidation, the reaction liquid was transferred to a constant temperature reactor at 105°C, and stirred for 4 h. After the reaction was completed, the liquid and solid were separated, the filter cake was washed and dried, and the hydrated ferric phosphate product was obtained. The composition and tap density of the product are shown in Table 1.

[0052] Comparative Example 1

[0053] aFirst, 30 g of iron oxide scale powder was weighed, and deionized water and concentrated sulfuric acid solution were added according to a liquid-solid ratio of 6:1 and an acid dosage of 1.0 times, respectively. The temperature was raised to 80°C, and the reaction was stirred for 1.0 h. The liquid and solid were separated, and the acid leaching liquid and acid leaching residue were obtained. The iron element leaching rate was 75.6%. The results show that under the same conditions, without adding a reducing agent, the iron element leaching rate of the iron oxide scale is significantly lower.

[0054] bThe pH was adjusted to 2.0 using 20% sodium hydroxide solution, and then 5.0 g of elemental iron was added. The rest of the process was the same as in Example 1c. After impurity removal, the final pH of the solution was 3.3, the iron yield was 69.6%, and the aluminum removal rate was 10.7%. The results show that without a reduction-leaching process, the subsequent impurity removal cannot achieve the desired effect through precipitation impurity removal, resulting in large iron loss and low aluminum removal rate. The reason is that when the concentration of trivalent iron ions in the system is high, elemental iron preferentially reacts with trivalent iron, without consuming hydrogen ions in the system, and thus cannot achieve the purpose of increasing pH and promoting hydrolysis.

[0055] cFinally, ammonium dihydrogen phosphate solid was added to the impurity removal solution according to a P / Fe molar ratio of 1.05:1. After dissolution, the reaction liquid pH was adjusted to 0.6, and then slowly added with ferrous ion until the amount of hydrogen peroxide solution was 1.1 times the amount of iron ion. After complete oxidation, the reaction liquid was transferred to a constant temperature reactor at 90°C, and stirred for 14 h. After the reaction was completed, the liquid and solid were separated, the filter cake was washed and dried, and the hydrated ferric phosphate product was obtained. The composition and tap density of the product are shown in Table 1. The results show that even at a higher acidity, the purity of the ferric phosphate product cannot meet the battery grade requirements, with an aluminum content of 0.09%, which is much higher than the standard of <0.03%.

[0056] Comparative Example 2

[0057] aFirst, 30 g of iron oxide scale powder was weighed, and deionized water and concentrated sulfuric acid solution were added according to a liquid-solid ratio of 8:1 and an acid dosage of 1.2 times, respectively. The temperature was raised to 90°C, and the reaction was stirred for 2.0 h. The liquid and solid were separated, and the acid leaching liquid and acid leaching residue were obtained. The iron element leaching rate was 85.8%. The results show that without adding a reducing agent during the leaching process, even if the acid dosage is increased, the leaching temperature is increased, and the leaching time is prolonged, the iron element in the iron oxide scale powder cannot be efficiently leached.

[0058] b Adjust the pH to 4.0 with ammonia water without adding iron powder, and the rest of the process is the same as example 1c. The yield of iron is 17.5%, and the removal rate of aluminum is 83.2%. The results show that without the acidification-leaching process, the removal rate of aluminum can be greatly improved by increasing the pH of the leaching solution, but the loss of iron is serious.

[0059] Comparative Example 3

[0060] According to the steps of example 1, the impurity-removed solution is obtained, and the process of synthesizing iron phosphate is carried out:

[0061] According to the P / Fe molar ratio of 1.1:1, ammonium dihydrogen phosphate solid is added to the impurity-removed solution, and after dissolution, the pH of the reaction solution is adjusted to 2.0. Then, ferrous ion is slowly added dropwise to 1.1 times the amount of hydrogen peroxide solution of iron ion. After complete oxidation, it is moved into a constant temperature reactor at 100°C, and stirred for 6h. After the reaction is completed, liquid-solid separation is carried out, and the filter cake is washed and dried to obtain the hydrated iron phosphate product. The composition and tap density are shown in Table 1. The results show that the content of impurities such as Mn and Mg in the iron phosphate product exceeds the standard requirement, the particles are small, the filtration is poor, the water content is high, and the caking is serious.

[0062] Comparative Example 4

[0063] According to the steps of example 1, the impurity-removed solution is obtained, and the process of synthesizing iron phosphate is carried out:

[0064] According to the P / Fe molar ratio of 1.2:1, ammonium dihydrogen phosphate solid is added to the impurity-removed solution, and after dissolution, the pH of the reaction solution is adjusted to 1.0. Then, ferrous ion is slowly added dropwise to 1.1 times the amount of hydrogen peroxide solution of iron ion. After complete oxidation, it is moved into a constant temperature reactor at 70°C, and stirred for 20h. After the reaction is completed, liquid-solid separation is carried out, and the filter cake is washed and dried to obtain the hydrated iron phosphate product. The results show that the iron concentration in the iron precipitation solution is 36.7g / L, the iron precipitation rate is less than 50%, and the product is yellow with poor crystallinity.

[0065] Table 1 ICP detection results of dihydrate iron phosphate, Fe / P and tap density

[0066] Sample Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 3 Fe % 29.49 29.38 29.33 29.91 29.18 30.02 P％ 16.51 16.37 16.4 16.51 16.49 16.45 Fe / P 0.991 0.996 0.993 1.01 0.982 1.013 Tap density (g / cm3) 3 ​ 1.18 1.12 0.995 0.936 1.09 0.997 Ca ppm not detected 4 11 14 17 26 Mg ppm 2 9 25 27 49 122 Zn ppm 2 5 9 11 9 36 Cu ppm not detected not detected 1 not detected 3 9 Mn ppm 10 37 45 51 73 188 Al ppm 8 16 21 20 900 269 S ppm 11 27 45 49 1200 530

[0067] Through the comprehensive comparison of examples and comparative examples, it can be seen that acidification-reduction leaching can significantly accelerate the leaching rate and improve the leaching rate of iron element, which is crucial for the precipitation and impurity removal stage. Not only is it beneficial to the removal of impurities such as aluminum, but it also reduces the loss of iron. In addition, in the process of synthesizing iron phosphate, the pH and reaction temperature of the reaction solution need to be comprehensively controlled, otherwise it will affect the purity of iron phosphate or the yield of iron, and also affect the physicochemical properties of iron phosphate product.

Claims

1. A method for preparing battery-grade iron phosphate from iron oxide scale in steel rolling mills, characterized in that: Includes the following steps: 1) Iron oxide scale is acidically leached in the presence of a metal reducing agent to obtain ferrous acid leaching solution I; when the reducing power of the metal reducing agent is stronger than that of elemental iron, elemental iron is added to ferrous acid leaching solution I to carry out a displacement reaction to obtain ferrous acid leaching solution II and a metal reducing agent; during the acid leaching process, the temperature is 30~95℃, the time is 0.5~7.0h, the amount of acid used is 0.8~1.4 times the theoretical amount, the liquid-solid ratio is 3~10L:1kg, and the metal reducing agent is measured at 2.0~30.0g / L; the metal reducing agent includes at least one of Zn, Fe, and Cu; 2) Neutralize and precipitate ferrous acid leaching solution I or ferrous acid leaching solution II to remove impurities, and obtain the purified solution and the purified residue; 3) After adjusting the pH and P / Fe molar ratio of the purified solution, oxidize and precipitate iron to obtain hydrated iron phosphate product.

2. The method for preparing battery-grade iron phosphate from iron oxide scale in a steel rolling mill according to claim 1, characterized in that: The acid includes at least one of sulfuric acid, hydrochloric acid, and phosphoric acid.

3. The method for preparing battery-grade iron phosphate from iron oxide scale in a steel rolling mill according to claim 1, characterized in that: During the displacement reaction, the amount of elemental iron used is 1.0 to 4.0 times the theoretical amount, the temperature is 20 to 70°C, and the time is 0.1 to 3.0 h.

4. The method for preparing battery-grade iron phosphate from iron oxide scale in a steel rolling mill according to claim 1, characterized in that: During the neutralization precipitation and impurity removal process, the pH is adjusted to 3.0~6.0 using a pH adjuster, and the reaction is carried out at a temperature of 30~90℃ for 2~7 hours.

5. The method for preparing battery-grade iron phosphate from iron oxide scale in a steel rolling mill according to claim 4, characterized in that: The pH adjuster includes at least one of elemental iron, ammonia, sodium hydroxide, potassium hydroxide, and calcium oxide.

6. The method for preparing battery-grade iron phosphate from iron oxide scale in a steel rolling mill according to claim 1, characterized in that: The pH of the purified solution is adjusted to 0.5-1.8, and the P / Fe molar ratio is adjusted to 1.0-1.

3.

7. A method for preparing battery-grade iron phosphate from iron oxide scale in a steel rolling mill according to claim 1 or 6, characterized in that: During the oxidation process, an oxidant of 1 to 1.5 times the theoretical amount is added. The oxidant includes at least one of hydrogen peroxide, thiosulfate, peroxy acid salt, ozone, oxygen, and air.

8. The method for preparing battery-grade iron phosphate from iron oxide scale in a steel rolling mill according to claim 1, characterized in that: During the precipitation of iron, the reaction is carried out at a temperature of 80~120℃ for 3.0~24h.