Method for preparing battery-grade iron phosphate from wet-process crude phosphoric acid and raffinate acid

Abstract

The invention discloses a method for preparing battery-grade iron phosphate from wet-process crude phosphoric acid and raffinate acid, which comprises the following steps of: mixing and leaching an iron source and wet-process crude phosphoric acid or wet-process crude phosphoric acid and raffinate acid to obtain ferrous dihydrogen phosphate containing metal impurities, ferrous phosphate leachate I and leaching residue; carrying out pH regulation and control on the leachate I to precipitate and remove aluminum in the form of aluminum phosphate, and carrying out multi-stage precipitation to remove aluminum until the content of aluminum ions is lower than 500ppm, so as to obtain leachate II; hydrofluoric acid is added into the leachate II, the pH value is adjusted to 1.6-2.5 with refined phosphoric acid, Al ions are precipitated, and leachate III is obtained after solid-liquid separation; an oxidizing agent is added into the leachate III for a precipitation reaction, and iron phosphate dihydrate and a phosphoric acid solution are obtained; and calcining the iron phosphate dihydrate to obtain the iron phosphate dihydrate. Crude phosphoric acid and raffinate acid can be directly utilized to produce battery-grade iron phosphate, the process is simple, compared with a traditional iron-method iron phosphate production process, the raw material cost is greatly reduced, generation of waste water and waste acid in the refining process is avoided, and the method is suitable for industrial production.

Description

Technical Field

[0001] The invention relates to a method for preparing battery-grade iron phosphate, and in particular to a method for preparing battery-grade iron phosphate by using wet-process crude phosphoric acid or a mixed acid of wet-process crude phosphoric acid and raffinate acid as a phosphorus source, and belongs to the field of new energy battery materials. Background Art

[0002] In the wet phosphoric acid production process, phosphoric acid is prepared by leaching phosphate ore with sulfuric acid. This process is widely used in the industrial field and has the advantages of low cost and mature technology. However, in addition to phosphorus, the main component of phosphate ore, it often contains a large amount of impurities, including metal elements such as aluminum, magnesium, and calcium. These impurities will be dissolved into the phosphoric acid solution during the sulfuric acid leaching process, so that the obtained phosphoric acid solution contains a high level of impurities, which is called crude phosphoric acid. Due to the high impurity content, crude phosphoric acid is difficult to be directly applied to industrial occasions with high requirements for phosphoric acid purity. Therefore, in actual production, crude phosphoric acid needs to be further extracted, washed, and other steps to remove impurities to obtain refined phosphoric acid.

[0003] In the production of high-quality iron phosphate, the iron process has become the preferred process for preparing high-purity iron phosphate in recent years due to its advantages such as high product purity and high production efficiency. The iron process uses reduced iron powder to react with phosphoric acid to generate ferrous dihydrogen phosphate, which is then oxidized to prepare iron phosphate. For example, Chinese patent application CN117125688A discloses a method for preparing iron phosphate using phosphorus iron slag after lithium extraction, providing a method to avoid the problem of difficult removal of copper and aluminum in subsequent solutions, improving the purity and applicability of the resulting iron phosphate, while simplifying the preparation process and reducing the recycling cost. However, the process has high requirements for the purity of the raw materials, and high-purity reduced iron powder and refined phosphoric acid must be used, resulting in an increase in the cost of raw materials, especially the cost of refined phosphoric acid accounts for more than 50% of the raw material cost of the entire process, which puts enterprises under great economic pressure in large-scale production. In addition, the demand for the use of high-purity raw materials also places higher demands on the stability of the supply chain.

[0004] Therefore, using wet-process crude phosphoric acid as a phosphorus source and developing a suitable iron phosphate preparation process is expected to be an effective way to reduce impurities in wet-process phosphoric acid production, the cost of refined phosphoric acid, and the discharge of wastewater and waste acid. Summary of the invention

[0005] In view of the problems of high raw material cost and high impurity content in the existing iron phosphate production process, the purpose of the present invention is to provide a method for preparing battery-grade iron phosphate using wet crude phosphoric acid and raffinate acid. This method can directly use crude phosphoric acid and raffinate acid as phosphoric acid raw materials for iron-based iron phosphate production, thereby replacing expensive refined phosphoric acid in traditional processes, which can not only greatly reduce the cost of phosphorus sources in the iron-based process, but also avoid the wastewater and waste acid discharge problems caused by extraction and refining, which is green and environmentally friendly and suitable for industrial production. In addition, the iron phosphate obtained by the method of the present invention has a low impurity content and can be directly used as a battery-grade material.

[0006] In order to achieve the above technical purpose, the present invention provides a method for preparing battery-grade iron phosphate by using wet-process crude phosphoric acid and raffinate acid. The method comprises the following steps: leaching an iron source and wet-process crude phosphoric acid or a mixed acid of wet-process crude phosphoric acid and raffinate acid to obtain ferrous dihydrogen phosphate and ferrous phosphate leachate I containing metal impurities and leaching residue; the pH of the leachate I is regulated to precipitate and remove aluminum in the form of aluminum phosphate, and aluminum is removed by multi-stage precipitation until the aluminum ion content is less than 500 ppm to obtain a leachate II; hydrofluoric acid is added to the leachate II, and the pH value is adjusted to 1.6-2.5 with refined phosphoric acid to precipitate Al ions, and after solid-liquid separation, a leachate III is obtained; an oxidant is added to the leachate III to carry out a precipitation reaction to obtain dihydrate iron phosphate and a phosphoric acid solution; and the dihydrate iron phosphate is calcined to obtain the obtained product.

[0007] The present invention cleverly utilizes the solubility of ferrous phosphate and the pH interval difference of phosphate precipitation of iron phosphate and impurity cations, removes impurity cations in iron phosphate through AlPO4, AlF3 selective precipitation and impurity removal and iron phosphate selective precipitation, and prepares battery-grade iron phosphate at low cost. Specifically, the impurities affecting the quality of iron phosphate in the phosphorus source wet crude phosphoric acid and raffinate used in the present invention are mainly four metal cations of Al, Mg, Ca, and Zn, among which the chemical properties of Al ions are extremely similar to those of Fe ions. When oxidative iron precipitation is performed, it is difficult to selectively remove them by pH value regulation. Therefore, the present invention utilizes the water-soluble characteristics of ferrous phosphate. In the leachate, Fe ions are controlled to be divalent, while Al ions are trivalent. Through multi-stage pH adjustment, aluminum phosphate precipitation can be formed with phosphate in the leachate, while Fe ions are not precipitated and removed by solid-liquid separation. The remaining small part of Al ions is subsequently removed by reacting with HF to generate AlF3 precipitation. As for the other three metal ions, the pH range of phosphate precipitation is quite different from that of iron phosphate precipitation. When oxidizing iron, the pH value of the leaching solution can be controlled to selectively precipitate iron phosphate dihydrate, while Mg, Ca, and Zn ions are not precipitated, thereby removing cationic impurities in iron phosphate. The resulting iron phosphate dihydrate precipitate can be directly calcined to obtain battery-grade iron phosphate, and the resulting phosphoric acid solution can be reused.

[0008] As a preferred solution, the wet-process crude phosphoric acid is obtained by leaching phosphate ore with sulfuric acid, including phosphoric acid, Al, Mg, Ca, Zn and Fe, and the mass concentration of phosphoric acid in the wet-process crude phosphoric acid is 50-55%; the raffinate acid is derived from the process of purifying the wet-process crude phosphoric acid to prepare refined phosphoric acid, including phosphoric acid, Al, Mg, Ca, Zn and Fe, and the mass concentration of phosphoric acid in the raffinate acid is 30-33%; wherein the phosphoric acid is obtained by converting P2O5. The phosphoric acid in the crude phosphoric acid and the raffinate acid is the source of phosphorus in the iron phosphate, and its content cannot be too low, and the reaction activity of the raffinate acid is insufficient when used alone; Fe can be used as a supplementary iron source, and the remaining metal impurities need to be precipitated and separated in combination with the process method of the present invention.

[0009] As a preferred solution, the wet-process crude phosphoric acid contains 10,000 to 13,000 ppm Al, 10,000 to 12,500 ppm Mg, 800 to 950 ppm Ca, 80 to 130 ppm Zn, and 5,000 to 7,000 ppm Fe.

[0010] As a preferred solution, the raffinate contains 13800-15000 ppm Al, 13000-14000 ppm Mg, 500-650 ppm Ca, 10-32 ppm Zn and 8000-10000 ppm Fe.

[0011] As a preferred solution, the effective iron component in the iron source is a combination of elemental iron or iron oxide and elemental iron, and the iron source is selected from at least one of hematite, magnetite, sulfate slag, red mud, iron oxide scale and scrap steel and reduced iron powder. When the effective component in the iron source is a combination of iron oxide and elemental iron, the molar ratio thereof needs to be controlled at 1: (1.2-3). In the leaching process of the present invention, the Fe ions must be controlled to be divalent so that water-soluble ferrous phosphate can be formed to achieve separation from aluminum ions. Therefore, the content of elemental iron in the iron source is very critical. If the proportion of elemental iron added is low, part of the iron ions in the iron-dissolving liquid will exist in a trivalent form. When the pH is low, this part of the iron ions will not precipitate in the form of dihydrated iron phosphate. However, when the pH value of the iron-dissolving liquid is increased to precipitate aluminum, part of the trivalent iron will precipitate together with aluminum phosphate in the form of dihydrated iron phosphate, resulting in an increase in the iron content in the aluminum-precipitated slag, resulting in a loss of iron recovery rate. By adding an appropriate amount of iron oxide, the cost of using elemental iron can be reduced while ensuring the separation effect.

[0012] As a preferred solution, during the leaching process, the concentration of phosphoric acid in the crude phosphoric acid or mixed acid is 3-8 mol / L, the liquid-to-solid ratio is 6-20 ml / g, the temperature is 30-100° C., and the time is 6-10 h. If the liquid-to-solid ratio is too low during the leaching process, the pH value of the leaching system is too high, and the trivalent iron ions react preferentially with the phosphoric acid to form a dihydrate ferric phosphate precipitate. There is not enough trivalent iron ions in the leachate to react with the iron element to form ferrous dihydrogen phosphate, and the iron recovery rate is reduced. If the liquid-to-solid ratio is too high, the iron ion concentration in the leachate is too low, which reduces the subsequent oxidation precipitation iron production efficiency and consumes too much phosphoric acid raw material.

[0013] As a preferred solution, the multi-stage precipitation aluminum removal is 2 to 5 stages of precipitation aluminum removal, wherein the leachate of each stage of precipitation aluminum removal is adjusted to pH 2.3 to 3.5 by adding an adjusting agent and then separating the aluminum precipitate slag. In this step, aluminum is mainly precipitated in the form of aluminum phosphate, and the precipitation reaction of aluminum phosphate is a reversible reaction. With the decrease of aluminum ion concentration in the leachate and the increase of aluminum phosphate precipitation content, the aluminum precipitation reaction gradually reaches equilibrium. If aluminum in the leachate cannot be further precipitated by only one step of aluminum precipitation, it is necessary to separate aluminum phosphate and gradually increase the solution pH to break the precipitation reaction equilibrium and promote aluminum ions in the leachate to continue to precipitate in the form of aluminum phosphate. Therefore, the deep removal of aluminum ions can be achieved through the 2 to 5 stages of precipitation aluminum removal of the present invention.

[0014] As a preferred solution, the adjusting agent is at least one of sodium hydroxide, sodium carbonate, ammonia water and ammonium carbonate. The alkaline adjusting agent selected by the present invention can achieve pH adjustment in the process of multi-stage precipitation aluminum removal.

[0015] As a preferred solution, the molar ratio of the amount of hydrofluoric acid to the Al ions in the leachate II is (3-3.5): 1. The amount of hydrofluoric acid used in the present invention is mainly determined according to the content of Al ions in the leachate II, and its purpose is to make the aluminum ions form AlF3 precipitation and remove them, so it is sufficient to ensure that the amount is 3 times the amount of Al ions and slightly excessive.

[0016] As a preferred solution, the oxidant is at least one of hydrogen peroxide, air and oxygen.

[0017] As a preferred solution, when the oxidant is hydrogen peroxide, Fe 2+ The molar ratio of ions to H2O2 in hydrogen peroxide is 1:(0.6~2.5).

[0018] As a preferred solution, when the oxidant is air or oxygen, the gas flow rate is 50 to 200 mL / min.

[0019] By controlling the amount of the added oxidant, the ferrous ions in the leachate can be oxidized to ferric ions and combined with phosphoric acid to form ferric phosphate dihydrate, while other metal ions such as Mg, Ca, Zn, etc. exist in the phosphoric acid solution in the form of phosphates.

[0020] As a preferred solution, the precipitation reaction process is: stirring the reaction at a temperature of 50 to 80° C. for 1 to 3 hours.

[0021] As a preferred solution, the calcination conditions are: temperature of 500-700° C. and time of 60-180 min. The calcination temperature and time of the present invention can remove the crystal water in the dihydrated iron phosphate, thereby obtaining battery-grade iron phosphate.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1) The present invention uses wet crude phosphoric acid or a mixture of crude phosphoric acid and raffinate acid as a phosphorus source in the iron-based ferric phosphate production process to prepare a high-value-added battery-grade ferric phosphate product. Compared with the traditional ferric phosphate iron-based process, the raw material cost is greatly reduced. More importantly, the discharge of wastewater and waste acid in the phosphoric acid extraction and refining process is avoided, which is green and environmentally friendly.

[0024] 2) The present invention cleverly utilizes the solubility of ferrous phosphate and the difference in pH ranges of phosphate precipitation of ferric phosphate and impurity cations, and uses a two-step impurity removal process of selective precipitation of AlPO4 and AlF3 and selective precipitation of ferric phosphate to directionally remove impurity cations in ferric phosphate and prepare battery-grade ferric phosphate at low cost.

[0025] 3) The present invention uses multi-stage precipitation to remove aluminum and multi-stage pH value adjustment to form aluminum phosphate precipitation with Al ions and phosphate in the leaching solution, thereby achieving deep removal of Al ions. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The present invention is a process flow chart of a method for preparing battery-grade iron phosphate from wet crude phosphoric acid and raffinate acid, wherein the number of multi-stage precipitation aluminum removal stages is 3.

[0027] Figure 2 This is the SEM microscopic morphology of the iron phosphate dihydrate prepared in Example 1.

[0028] Figure 3 This is the XRD pattern of iron phosphate dihydrate prepared in Example 1. DETAILED DESCRIPTION

[0029] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention and the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] Example 1

[0031] In this embodiment, reduced iron powder is used as the iron source and wet-process crude phosphoric acid is used as the phosphorus source. The main components of the two raw materials are shown in Table 1 and Table 2, respectively.

[0032] Table 1 Main chemical composition of reduced iron powder / %

[0033]

[0034] Table 2 Main chemical components of crude phosphoric acid / ppm

[0035]

[0036] Battery-grade iron phosphate was produced using the above two raw materials, with the liquid-solid ratio of phosphoric acid to reduced iron powder in crude phosphoric acid controlled at 20 ml / g, the phosphoric acid concentration at 3 mol / L, and the leaching temperature at 70°C. After 6 hours of leaching, the reaction between reduced iron powder and phosphoric acid was basically completed, and the leached residue was filtered to obtain a ferrous dihydrogen phosphate solution containing metal impurities, with an initial pH value of 1.6.

[0037] Using 1 mol / L NaOH solution as pH adjuster, in the first stage of aluminum precipitation, the pH value of the leachate was adjusted to 2.4 to obtain aluminum precipitate slag I. After solid-liquid separation, NaOH solution was continued to be added to the leachate to adjust the pH value of the leachate to 2.5 to obtain aluminum precipitate slag II. After solid-liquid separation, NaOH solution was continued to be added to the leachate to adjust the pH value of the leachate to 2.6 to obtain aluminum precipitate slag III. After solid-liquid separation, NaOH solution was continued to be added to the leachate to adjust the pH value of the leachate to 2.8 to obtain aluminum precipitate slag IV. The chemical composition of the comprehensive aluminum precipitate slag is shown in Table 3. At this time, the Al ion content in the leachate is 358ppm. In order to further reduce the Al ion content in the leachate, HF acid with a molar content of 3 times that of aluminum ions is added to continue aluminum precipitation. Subsequently, refined phosphoric acid is used to adjust the pH value of the leachate to 2.2. After solid-liquid separation, the Al ion concentration in the leachate drops to 56ppm. Using H2O2 as an oxidant, Fe 2+ The molar ratio of ions to H2O2 was controlled at 1:0.6, the temperature of the hydrogen peroxide oxidation precipitation reaction was controlled at 65°C, and the mixture was stirred for 2 hours. After solid-liquid separation, the dihydrated iron phosphate product was obtained, which was calcined at 500°C for 3 hours to finally obtain the iron phosphate product, the chemical composition of which is shown in Table 4.

[0038] Table 3 Comprehensive analysis of chemical composition of aluminum slag / %

[0039]

[0040] Table 4 Chemical composition analysis of iron phosphate / ppm

[0041]

[0042] Example 2

[0043] In this embodiment, high-grade hematite and reduced iron powder are used as iron sources, and wet-process crude phosphoric acid is used as phosphorus source. The main components of hematite are shown in Table 5, and the crude phosphoric acid is still the crude phosphoric acid used in Example 1.

[0044] Table 5 Main chemical components of hematite / %

[0045]

[0046] The above three raw materials are used to produce battery-grade iron phosphate, the molar ratio of Fe2O3 and reduced iron powder in hematite is controlled to be 1:1.4, the liquid-solid ratio of phosphoric acid to solid iron source is 15ml / g, the phosphoric acid concentration is 3.5mol / L, the leaching temperature is controlled at 60℃, and the leaching time is 7h. After the leaching reaction is completed, the leached residue is filtered to obtain a ferrous dihydrogen phosphate solution containing metal impurities, and the initial pH value of the leaching solution is 1.7.

[0047] Using 1 mol / L NaOH solution as pH adjuster, the first stage of aluminum precipitation, the pH value of the leachate is adjusted to 2.5, and aluminum precipitation slag I is obtained. After solid-liquid separation, NaOH solution is continued to be added to the leachate to adjust the pH value of the leachate to 2.6, and aluminum precipitation slag II is obtained. After solid-liquid separation, NaOH solution is continued to be added to the leachate to adjust the pH value of the leachate to 2.65, and aluminum precipitation slag III is obtained. After solid-liquid separation, NaOH solution is continued to be added to the leachate to adjust the pH value of the leachate to 2.7, and aluminum precipitation slag IV is obtained. After solid-liquid separation, NaOH solution is continued to be added to the leachate to adjust the pH value of the leachate to 2.85, and aluminum precipitation slag V is obtained. The chemical composition of the comprehensive aluminum precipitation slag is shown in Table 6. At this time, the Al ion content in the leachate was 326ppm. In order to further reduce the Al ion content in the leachate, HF acid with a molar content of 3 times that of aluminum ions was added to continue aluminum precipitation. Subsequently, the pH value of the leachate was adjusted to 2.3 using refined phosphoric acid. After solid-liquid separation, the Al ion concentration in the leachate dropped to 48ppm. H2O2 was used as an oxidant, the molar ratio of Fe ions to H2O2 was controlled at 1:0.6, the temperature of the hydrogen peroxide oxidation precipitation reaction was controlled at 50℃, and stirred for 1h. After solid-liquid separation, a dihydrate iron phosphate product was obtained, which was calcined at 500℃ for 3h to finally obtain an iron phosphate product, the chemical composition of which is shown in Table 7.

[0048] Table 6 Comprehensive analysis of chemical composition of aluminum slag / %

[0049]

[0050] Table 7 Chemical composition analysis of iron phosphate / ppm

[0051]

[0052] Example 3

[0053] In this embodiment, reduced iron powder is used as the iron source, and a mixed acid of wet-process crude phosphoric acid and raffinate acid is used as the phosphorus source. Reduced iron powder is the raw material in Example 1, and the composition is shown in Table 1. Crude phosphoric acid is the raw material in Example 1, and the composition is shown in Table 2. Raffinate acid is the waste acid produced by the refining of wet-process phosphoric acid, and the chemical composition is shown in Table 8.

[0054] Table 8 Main chemical components of raffinate / ppm

[0055]

[0056] Battery-grade iron phosphate was produced using the above three raw materials, wherein the volume percentages of mixed acid crude phosphoric acid and raffinate acid were 80% and 20%, respectively, the liquid-solid ratio of phosphoric acid to reduced iron powder was controlled to be 25 ml / g, the phosphoric acid concentration was 3 mol / L, and the leaching temperature was controlled at 80° C. After 7 hours of leaching, the reduced iron powder and the mixed acid basically reacted, and the impure ferrous dihydrogen phosphate solution was obtained after filtering the leached residue, and the initial pH value of the leaching solution was 1.6.

[0057] Using 1mol / L NaOH solution as pH adjuster, the first stage of aluminum precipitation, the pH value of the leachate is adjusted to 2.3, and aluminum precipitation slag I is obtained. After solid-liquid separation, NaOH solution is continued to be added to the leachate to adjust the pH value of the leachate to 2.5, and aluminum precipitation slag II is obtained. After solid-liquid separation, NaOH solution is continued to be added to the leachate to adjust the pH value of the leachate to 2.7, and aluminum precipitation slag III is obtained. After solid-liquid separation, NaOH solution is continued to be added to the leachate to adjust the pH value of the leachate to 2.8, and aluminum precipitation slag IV is obtained. The chemical composition of the comprehensive aluminum precipitation slag is shown in Table 9. At this time, the Al ion content in the leachate is 459ppm. In order to further reduce the Al ion content in the leachate, HF acid with a molar content of 3.5 times that of aluminum ions is added to continue aluminum precipitation, and then the pH value of the leachate is adjusted to 2.1 using refined phosphoric acid. After solid-liquid separation, the Al ion concentration in the leachate drops to 112ppm. H2O2 was used as the oxidant, the molar ratio of Fe ions to H2O2 was controlled at 1:0.6, the temperature of the hydrogen peroxide oxidation precipitation reaction was controlled at 65°C, and the mixture was stirred for 2 hours. After solid-liquid separation, the dihydrated iron phosphate product was obtained, which was calcined at 500°C for 3 hours to finally obtain the iron phosphate product, the chemical composition of which is shown in Table 10.

[0058] Table 9 Comprehensive analysis of chemical composition of aluminum slag / %

[0059]

[0060] Table 10 Chemical composition analysis of iron phosphate / ppm

[0061]

[0062] Comparative Example 1

[0063] This comparative example uses hematite, reduced iron powder and crude phosphoric acid in Example 2 as raw materials. The molar ratio of Fe2O3 and reduced iron powder in hematite is controlled to be 1:1.4, the liquid-solid ratio of phosphoric acid to solid iron source is 15ml / g, the phosphoric acid concentration is 3.5mol / L, the leaching temperature is controlled at 60°C, and the leaching time is 7h. After the leaching reaction is completed, the leached residue is filtered to obtain an impure ferrous dihydrogen phosphate solution, and the initial pH value of the leaching solution is 1.7.

[0064] Only one-stage aluminum precipitation was used with 1 mol / L NaOH solution as pH adjuster. In order to completely precipitate aluminum, the pH value of the leachate was adjusted to 3.5 to obtain aluminum precipitation slag. The chemical composition of the slag is shown in Table 11. At this time, the Al ion content in the leachate was 865 ppm. In order to further reduce the Al ion content in the leachate, HF acid with a molar amount of 3 times the Al ion was added to continue aluminum precipitation. Subsequently, the pH value of the leachate was adjusted to 2.1 with refined phosphoric acid. After solid-liquid separation, the Al ion concentration in the leachate dropped to 321 ppm. H2O2 was used as an oxidant, the molar ratio of Fe ions to H2O2 was controlled at 1:0.6, the hydrogen peroxide oxidation precipitation reaction temperature was controlled at 50°C, and stirred for 1h. After solid-liquid separation, a dihydrate iron phosphate product was obtained, which was calcined at 500°C for 3h to finally obtain an iron phosphate product. Its chemical composition is shown in Table 12.

[0065] Table 11 Comprehensive analysis of chemical composition of aluminum slag / %

[0066]

[0067] Table 12 Chemical composition analysis of iron phosphate / ppm

[0068]

[0069] It can be seen from Table 11 that compared with the embodiment, the iron content in the aluminum precipitation slag in the comparative example is higher, resulting in the loss of iron in the aluminum removal process. The main reason is that if only one-step aluminum precipitation is used, in order to maximize the aluminum removal rate in the leachate, a higher pH is required for aluminum precipitation. At this time, ferrous dihydrogen phosphate will also precipitate in the form of precipitation, resulting in a loss of iron recovery rate. Therefore, this process must adopt the principle of multi-stage precipitation and slowly increase the pH value of the leachate to improve the aluminum removal rate and ensure the iron recovery rate.

[0070] Comparative Example 2

[0071] This comparative example uses hematite, reduced iron powder and crude phosphoric acid in Example 2 as raw materials. The molar ratio of Fe2O3 and reduced iron powder in hematite is controlled to be 1:1.4, the liquid-solid ratio of phosphoric acid to solid iron source is 15ml / g, the phosphoric acid concentration is 3.5mol / L, the leaching temperature is controlled at 60°C, and the leaching time is 7h. After the leaching reaction is completed, the leached residue is filtered to obtain an impure ferrous dihydrogen phosphate solution, and the initial pH value of the leaching solution is 1.7.

[0072] In the aluminum removal step, this comparative example does not adjust the pH value of the leachate, but directly uses HF to precipitate the aluminum ions in the leachate. After reduction leaching, the aluminum ion concentration in the leachate is 5881ppm, and the aluminum ions in the original acid basically all enter the leachate. HF acid with a molar content of 3.5 times the aluminum ion is added to the leachate. After solid-liquid separation, the Al ion concentration in the leachate drops to 1893ppm. Subsequently, a 1mol / L NaOH solution is used to adjust the pH value of the leachate to 2.1, H2O2 is used as an oxidant, the molar ratio of Fe ions to H2O2 is controlled at 1:0.6, and the reaction temperature of the hydrogen peroxide oxidation step is controlled at 50°C and stirred for 1h. After solid-liquid separation, a dihydrate iron phosphate product is obtained, which is calcined at 500°C for 3h to finally obtain an iron phosphate product, and its chemical composition is shown in Table 13.

[0073] Table 13 Chemical composition analysis of iron phosphate / ppm

[0074]

[0075] It can be seen from Table 13 that the aluminum removal rate is only about 68% when HF aluminum precipitation is used alone, and there is still 1893ppm of aluminum ions remaining in the leachate, which is far below the limit required for battery-grade iron phosphate production. When oxidative iron precipitation is carried out, aluminum phosphate will precipitate in the process of increasing pH. After adding hydrogen peroxide, iron phosphate will precipitate out together with aluminum phosphate and cannot be separated, resulting in excessive aluminum content in the iron phosphate product, which cannot be used.

[0076] Comparative Example 3

[0077] This comparative example uses hematite, reduced iron powder and crude phosphoric acid in Example 2 as raw materials. The molar ratio of Fe2O3 and reduced iron powder in hematite is controlled to be 1:1.1, the liquid-solid ratio of phosphoric acid to solid iron source is 15ml / g, the phosphoric acid concentration is 3.5mol / L, the leaching temperature is controlled at 60°C, and the leaching time is 7h. After the leaching reaction is completed, the leached residue is filtered to obtain an impure ferrous dihydrogen phosphate solution, and the initial pH value of the leaching solution is 1.7.

[0078] 1 mol / L NaOH solution was used as pH adjuster, and aluminum precipitation was performed in the first stage. The pH value of the leachate was adjusted to 2.3 to obtain aluminum precipitation slag I. After solid-liquid separation, NaOH solution was continued to be added to the leachate to adjust the pH value of the leachate to 2.5 to obtain aluminum precipitation slag II. After solid-liquid separation, NaOH solution was continued to be added to the leachate to adjust the pH value of the leachate to 2.7 to obtain aluminum precipitation slag III. The chemical composition of the comprehensive aluminum precipitation slag is shown in Table 14. At this time, the Al ion content in the leachate was 429 ppm. In order to further reduce the Al ion content in the leachate, HF acid with a molar content of 3 times that of aluminum ions was added to continue aluminum precipitation. Subsequently, refined phosphoric acid was used to adjust the pH value of the leachate to 2.1. After solid-liquid separation, the Al ion concentration in the leachate dropped to 105 ppm. H2O2 was used as an oxidant, and the molar ratio of Fe ions to H2O2 was controlled at 1:0.6. The reaction temperature of the hydrogen peroxide oxidation step was controlled at 50°C and stirred for 1h. After solid-liquid separation, dihydrate iron phosphate product was obtained, which was calcined at 500°C for 3h to finally obtain iron phosphate product, whose chemical composition is shown in Table 15.

[0079] Table 14 Comprehensive analysis of chemical composition of aluminum slag / %

[0080]

[0081] Table 15 Chemical composition analysis of iron phosphate / ppm

[0082]

[0083]

[0084] As can be seen from Table 14, compared with the embodiment, the iron content in the aluminum precipitate slag in the comparative example is higher, resulting in the loss of iron in the aluminum removal process. The main reason is that in the first step of leaching, the reduced iron powder is added at a low ratio, resulting in part of the iron ions in the iron-dissolving solution being in a trivalent form. When the pH is low, this part of the iron ions will not be precipitated in the form of dihydrated iron phosphate. However, when the pH value of the iron-dissolving solution is increased to precipitate aluminum, part of the trivalent iron will be precipitated together with aluminum phosphate in the form of dihydrated iron phosphate, resulting in an increase in the iron content in the aluminum precipitate slag and a loss of iron recovery rate. Therefore, in the first step of the iron-dissolving process, attention should be paid to the amount of reduced iron powder added to ensure that the iron ions in the iron-dissolving solution are in a divalent form.

[0085] Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing battery-grade iron phosphate from wet crude phosphoric acid and raffinate acid, characterized in that: The iron source is mixed with wet crude phosphoric acid or wet crude phosphoric acid and raffinate acid for leaching to obtain ferrous dihydrogen phosphate and ferrous phosphate leachate I containing metal impurities and leaching residue; the pH value of the leachate I is adjusted to precipitate and remove aluminum in the form of aluminum phosphate, and aluminum is removed by multi-stage precipitation until the aluminum ion content is less than 500ppm to obtain leachate II; hydrofluoric acid is added to the leachate II, and the pH value is adjusted to 1.6-2.5 with refined phosphoric acid to precipitate Al ions, and after solid-liquid separation, leachate III is obtained; an oxidant is added to the leachate III for precipitation reaction to obtain dihydrate iron phosphate and phosphoric acid solution; the dihydrate iron phosphate is calcined to obtain.

2. The method for preparing battery-grade iron phosphate using wet-process crude phosphoric acid and raffinate according to claim 1, characterized in that: The wet-process crude phosphoric acid is obtained by leaching phosphate ore with sulfuric acid, and includes phosphoric acid, Al, Mg, Ca, Zn and Fe, and the mass concentration of phosphoric acid in the wet-process crude phosphoric acid is 50-55%; the raffinate acid comes from the process of purifying the wet-process crude phosphoric acid to prepare refined phosphoric acid, and includes phosphoric acid, Al, Mg, Ca, Zn and Fe, and the mass concentration of phosphoric acid in the raffinate acid is 30-33%; wherein the phosphoric acid is obtained by converting P2O5.

3. The method for preparing battery-grade iron phosphate using wet-process crude phosphoric acid and raffinate according to claim 2, characterized in that: The effective iron component in the iron source is elemental iron or a combination of iron oxide and elemental iron, and the iron source is selected from at least one of hematite, magnetite, sulfate slag, red mud, iron oxide scale and scrap steel and reduced iron powder.

4. A method for preparing battery-grade iron phosphate using wet crude phosphoric acid and raffinate according to any one of claims 1 to 3, characterized in that: During the leaching process, the concentration of phosphoric acid in the crude phosphoric acid or mixed acid is 3-8 mol / L, the liquid-solid ratio is 6-20 ml / g, the temperature is 30-100° C., and the time is 6-10 hours.

5. The method for preparing battery-grade iron phosphate using wet-process crude phosphoric acid and raffinate according to claim 1, characterized in that: The multi-stage precipitation aluminum removal includes 2 to 5 stages of precipitation aluminum removal, wherein an adjusting agent is added to the leaching solution of each stage of precipitation aluminum removal to adjust the pH to 2.3 to 3.5 and then the aluminum precipitate slag is separated.

6. The method for preparing battery-grade iron phosphate using wet-process crude phosphoric acid and raffinate according to claim 5, characterized in that: The adjusting agent is at least one of sodium hydroxide, sodium carbonate, ammonia water and ammonium carbonate.

7. The method for preparing battery-grade iron phosphate using wet-process crude phosphoric acid and raffinate according to claim 1, characterized in that: The molar ratio of the amount of hydrofluoric acid used to the Al ions in the leaching solution II is (3-3.5):

1.

8. The method for preparing battery-grade iron phosphate using wet-process crude phosphoric acid and raffinate according to claim 7, characterized in that: The oxidant is at least one of hydrogen peroxide, air and oxygen.

9. A method for preparing battery-grade iron phosphate using wet-process crude phosphoric acid and raffinate according to claim 1 or 5, characterized in that: The precipitation reaction process is: stirring the reaction at a temperature of 50 to 80° C. for 1 to 3 hours.

10. The method for preparing battery-grade iron phosphate using wet-process crude phosphoric acid and raffinate acid according to claim 9, characterized in that: The calcination conditions are: temperature of 500-700° C. and time of 60-180 min.

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