Preparation method of iron phosphate

Abstract

The invention discloses a preparation method of iron phosphate, and relates to the technical field of lithium ion batteries. According to the method, an electrochemical double-replacement process is adopted, a ferric sulfate solution, a phosphoric acid solution or a phosphate solution is taken as a main raw material, the ferric phosphate mother liquor is prepared through an electrochemical double-replacement system, and meanwhile, sulfuric acid or sulfate is byproduct. Compared with a traditional iron phosphate synthesis system, a complex synthesis reaction kettle system can be reduced, and the influence of sodium ions or ammonia radical ions introduced by a sodium method or an ammonia method on an iron phosphate product is reduced; through an electrochemical double-replacement reaction, kinetic energy driving is provided through electric energy in the system, iron ions and phosphate ions are enriched for synthesis, an iron phosphate product is produced, economic benefits are produced, the amount of wastewater generated in the process is small, and the purpose of green production can be achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a method for preparing iron phosphate. Background Art

[0002] At present, lithium batteries have been widely used in the new energy vehicle industry. Lithium iron phosphate has become a research hotspot in recent years due to its abundant raw material sources, low price, high capacity, good safety and other advantages. FePO4 is used as a precursor to participate in the preparation of lithium iron phosphate, the positive electrode material of lithium batteries. The chemical composition, structure, morphology, particle size and performance of the precursor iron phosphate largely determine the comprehensive performance of the lithium iron phosphate positive electrode material. Therefore, the preparation process of high-quality iron phosphate is crucial.

[0003] Conventional ferric phosphate preparation is mainly divided into ammonium method and sodium method as the mainstream preparation processes. The ammonium method is generated by the reaction of ferrous sulfate, ammonium dihydrogen phosphate, ammonia water, hydrogen peroxide, etc., and the sodium method is generated by the reaction of ferrous sulfate, sodium hydroxide, phosphoric acid, hydrogen peroxide, etc.

[0004] At present, the production process of iron phosphate mostly uses ammonium dihydrogen phosphate and ferrous sulfate as raw materials. However, the above production process will produce other by-products, thereby generating a large amount of wastewater. For example, pollutants containing a large amount of nitrogen, phosphorus, sulfur, and iron are present in the wastewater, and wastewater treatment will increase the cost of the above production process, resulting in an increase in the cost of terminal electric vehicles. In addition, the presence of by-products has a great impact on the purity of iron phosphate, thereby reducing the electrochemical performance of lithium iron phosphate batteries.

[0005] Therefore, there is an urgent need to develop a short-process, low-cost, environmentally friendly, and stably controllable method for preparing iron phosphate to solve the technical problems existing in the prior art in the preparation of iron phosphate, such as high wastewater treatment costs, high costs of raw and auxiliary materials, and difficulty in disposing of by-products.

[0006] In view of this, the present invention is proposed. Summary of the invention

[0007] The object of the present invention is to provide a method for preparing ferric phosphate, aiming to provide a green production process for ferric phosphate with a short process and low wastewater generation.

[0008] The present invention is achieved in that:

[0009] In a first aspect, the present invention provides a method for preparing iron phosphate, comprising: using iron sulfate solution, phosphoric acid solution or phosphate solution as main raw materials, preparing iron phosphate mother solution through an electrochemical double displacement system, and simultaneously producing sulfuric acid or sulfate as a by-product;

[0010] Among them, the electrochemical double replacement system controls the flow direction of anions and cations through multiple anion exchange membranes and multiple cation exchange membranes arranged in an alternating manner, and uses compartments separated by multiple anion exchange membranes and multiple cation exchange membranes to input raw materials and output target products.

[0011] In an optional embodiment, the electrochemical double replacement system includes an anode plate at one end and a cathode plate at the other end, a plurality of anion exchange membranes and a plurality of cation exchange membranes are arranged between the anode plate and the cathode plate, the anion exchange membranes and the cation exchange membranes are arranged alternately to form at least one repeating unit, and each repeating unit includes a first fresh water chamber, a first concentrated water chamber, a second fresh water chamber, and a second concentrated water chamber arranged in sequence;

[0012] A ferric sulfate solution is introduced into the first fresh water chamber, and a phosphoric acid solution or a phosphate solution is introduced into the second fresh water chamber;

[0013] The first concentrated water chamber produces iron phosphate mother liquor, and the second concentrated water chamber produces sulfuric acid or sulfate.

[0014] In an optional embodiment, the ion exchange membranes adjacent to the anode plate and the cathode plate are both anion exchange membranes, and there are polar water chambers between the anode plate and the cathode plate and the corresponding anion exchange membranes, and a sulfuric acid solution with a mass fraction of 5%-15% is circulated in the polar water chambers.

[0015] In an optional embodiment, the ferric sulfate solution introduced into the first desalted water chamber is adjusted to a pH value of 0.5-1.0 by a sulfuric acid solution; preferably, the mass fraction of the ferric sulfate solution is 5%-20%, and the pH value is adjusted to 0.5-1.0 by a sulfuric acid solution with a mass fraction of 5%-10%;

[0016] Preferably, the mass fraction of the phosphoric acid solution or phosphate solution introduced into the second fresh water chamber is 5%-10%;

[0017] Preferably, a phosphoric acid aqueous solution having a pH value of 1.0-2.0 is introduced into the first concentrated water chamber;

[0018] Preferably, the set flow rates of the polar water chamber, the first fresh water chamber, the second fresh water chamber, the first concentrated water chamber and the second concentrated water chamber are all 400L / h-600L / h, the set voltage is 10V-15V, and the current is 30A-50A.

[0019] In an optional embodiment, the ferric sulfate solution is obtained by oxidation reaction of ferrous sulfate solution.

[0020] In an optional embodiment, the preparation process of the ferric sulfate solution includes: mixing the ferrous sulfate solution and the oxidant to carry out an oxidation reaction, the reaction temperature is 30° C.-60° C., and the reaction time is 45 min-60 min;

[0021] Preferably, the oxidant is a hydrogen peroxide solution, and the molar ratio of hydrogen peroxide to ferrous ions is controlled to be (0.5-0.6):1. After the reaction, the pH value of the hydrogen peroxide solution is adjusted to 0.5-1.0 by adding sulfuric acid solution.

[0022] In an optional embodiment, the method further includes: crystallizing and aging the iron phosphate mother liquor produced by the electrochemical double replacement system to obtain solid iron phosphate; and calcining the solid iron phosphate to obtain anhydrous iron phosphate.

[0023] In an optional embodiment, the process of crystallizing and aging the iron phosphate mother liquor comprises: adjusting the pH value of the iron phosphate mother liquor to 1.0-2.0, stirring for 30 min-120 min at a temperature of 20° C.-100° C. to generate iron phosphate, and obtaining iron phosphate solid after solid-liquid separation;

[0024] Preferably, solid-liquid separation is carried out by filter pressing and washing.

[0025] In an optional embodiment, the process of preparing anhydrous ferric phosphate from ferric phosphate solid includes: flashing the ferric phosphate solid, and then calcining;

[0026] Preferably, the flash temperature is controlled to be 200°C-250°C;

[0027] Preferably, the calcination temperature is controlled to be 450° C.-650° C., and the calcination time is 4 h-6 h.

[0028] In an optional embodiment, the by-produced sulfuric acid is removed from impurities to obtain a sulfuric acid product;

[0029] Preferably, the by-product sulfate is removed from the water and then evaporated and crystallized to obtain the sulfate product.

[0030] The present invention has the following beneficial effects: the present invention adopts an electrochemical double replacement process, uses ferric sulfate solution, phosphoric acid solution or phosphate solution as the main raw material, and prepares ferric phosphate mother liquor through an electrochemical double replacement system, while producing sulfuric acid or sulfate as a byproduct. Compared with the traditional ferric phosphate synthesis system, it can reduce the complex synthesis reactor system and reduce the influence of sodium ions or ammonia ions introduced by the sodium method or ammonia method on the ferric phosphate product; through the electrochemical double replacement reaction, the system is driven by electric energy to provide kinetic energy, enrich iron ions and phosphate ions for synthesis, and produce ferric phosphate products to generate economic benefits, and the amount of wastewater generated in the process is relatively small, which can achieve the purpose of green production. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0032] Figure 1 A process flow chart of the method for preparing ferric phosphate provided by the present invention;

[0033] Figure 2 It is the working schematic diagram of the electrochemical double displacement system;

[0034] Figure 3 This is a diagram of the electrochemical double displacement system device;

[0035] Figure 4 The process flow chart of preparing ferric phosphate from sodium phosphate in Example 2 is shown;

[0036] Figure 5 The equipment operation trend chart analysis of the test process of Example 2;

[0037] Figure 6 Example 2 is an XRD diagram of the precipitate obtained after the electrochemical double replacement system and subsequent series of treatments;

[0038] Figure 7 This is a process flow chart for preparing ferric phosphate using the sodium method in Comparative Example 1. DETAILED DESCRIPTION

[0039] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0040] In view of the problems existing in the traditional ammonium method and sodium method for preparing ferric phosphate, the embodiment of the present invention has developed a new process for preparing ferric phosphate, which uses ferric sulfate solution, phosphoric acid solution or phosphate solution as the main raw materials, prepares ferric phosphate mother liquor through an electrochemical double displacement system, and simultaneously produces sulfuric acid or sulfate as a by-product, which can solve the technical problems such as high wastewater treatment cost, high cost of raw and auxiliary materials, and difficult disposal of by-products in the preparation process of ferric phosphate prepared by the existing method.

[0041] Please refer to Figure 1 The present invention provides a method for preparing iron phosphate, the steps of which are as follows:

[0042] (1) Preparation

[0043] Prepare the raw and auxiliary materials, ferrous sulfate solution S1, hydrogen peroxide solution S2, phosphoric acid or phosphate S5, and pure water S6, test the solution whose average value meets the requirements and send it to the next process.

[0044] The ferrous sulfate solution S1 and the hydrogen peroxide solution S2 are used in step (2) to oxidize iron to prepare a ferric sulfate solution, the phosphoric acid or phosphate S5 is used in step (3), and the pure water S6 is used to adjust the pH value after adding acid.

[0045] (2) Iron oxidation reaction

[0046] The ferrous sulfate solution S1 and the hydrogen peroxide solution S2 are sent to an oxidation reaction tank S3, and after chemical heating, stirring and oxidation, a ferrous sulfate solution S4 is obtained.

[0047] In some embodiments, the preparation process of ferrous sulfate solution S4 includes: mixing ferrous sulfate solution and oxidant for oxidation reaction, the reaction temperature is 30°C-60°C, the reaction time is 45min-60min, and the reaction is fully carried out by regulating the reaction temperature and reaction time. Specifically, the reaction temperature can be 30°C, 40°C, 50°C, 60°C, etc.; the reaction time can be 45min, 50min, 55min, 60min, etc.

[0048] In some embodiments, the oxidant may be a hydrogen peroxide solution, and the molar ratio of hydrogen peroxide to ferrous ions is controlled to be (0.5-0.6):1, such as 0.50:1, 0.53:1, 0.55:1, 0.58:1, 0.60:1, etc. Before the reaction, the pH value of the hydrogen peroxide solution is adjusted to 0.5-1.0 (such as 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, etc.) by adding sulfuric acid solution, so that the reaction is carried out under acidic conditions to enhance the oxidizability of hydrogen peroxide.

[0049] (3) Electrochemical double replacement

[0050] The working principle of electrochemical double replacement is: under the action of a direct current electric field, the selective permeability of the ion exchange membrane to ions in the solution is utilized to allow the anions and cations in the solution to pass through the anion and cation exchange membranes respectively, thereby achieving the purpose of desalination, concentration, and resynthesis.

[0051] By applying an electric field during the electrochemical double replacement process, the charged ions in the solution can migrate under the action of the direct current electric field under the driving force of the electric field. The working principle of the ion exchange membrane used is based on its special structure and selective permeability. This membrane is a polymer membrane containing ionic groups, which is composed of a polymer skeleton, a fixed group and a movable ion on the group. It has selective permeability to ions in the solution and is mainly divided into a cation exchange membrane and an anion exchange membrane. When an electric field is applied to both sides of the membrane or there is a concentration difference, the ions will be driven to move, the cations will move to the cathode, and the anions will move to the anode. Due to the selective permeability of the ion exchange membrane, the cations can only pass through the positive membrane and the anions can only pass through the negative membrane, thereby achieving the separation of anions and cations. Specifically, the anion exchange membrane can be a commercially available brand LANCYTOM, model ATG-10; the cation exchange membrane can be a commercially available brand LANCYTOM, model CT-4. The electrochemical double replacement system used in the embodiment of the present invention can be improved on the basis of a traditional electrodialysis device.

[0052] Please combine Figure 1 , Figure 2 and Figure 3 The electrochemical double replacement system controls the flow direction of anions and cations through multiple anion exchange membranes and multiple cation exchange membranes that are staggered. The compartments formed by multiple anion exchange membranes and multiple cation exchange membranes are used to input raw materials (ferric sulfate solution, phosphoric acid solution or phosphate solution) and output the target product (ferric phosphate mother liquor, and simultaneously produce sulfuric acid or sulfate as a by-product).

[0053] In some embodiments, the electrochemical double replacement system includes an anode plate at one end and a cathode plate at the other end, and multiple anion exchange membranes and multiple cation exchange membranes are arranged between the anode plate and the cathode plate. The anion exchange membranes and the cation exchange membranes are arranged alternately to form at least one repeating unit, and each repeating unit includes a first fresh water chamber, a first concentrated water chamber, a second fresh water chamber, and a second concentrated water chamber arranged in sequence, and the number of repeating units is adjusted by regulating the logarithm of the ion exchange membrane. During the preparation process, a ferric sulfate solution is introduced into the first fresh water chamber, and a phosphoric acid solution or a phosphate solution is introduced into the second fresh water chamber; the first concentrated water chamber produces a ferric phosphate mother liquor, and the second concentrated water chamber produces sulfuric acid or sulfate.

[0054] Specifically, the electrochemical double replacement device includes an anode plate, a cathode plate, a cation exchange membrane and an anion exchange membrane, and the arrangement is anode plate-polar water chamber-anion exchange membrane-first fresh water chamber-cation exchange membrane-first concentrated water chamber-anion exchange membrane-second fresh water chamber-cation exchange membrane-second concentrated water chamber-anion exchange membrane-first fresh water chamber-cation exchange membrane-first concentrated water chamber-anion exchange membrane-first concentrated water chamber-cation exchange membrane-first fresh water chamber-anion exchange membrane-first concentrated water chamber-anion exchange membrane-second fresh water chamber-cation exchange membrane-second concentrated water chamber (repeated according to the rules)-anion exchange membrane-polar water chamber-cathode plate. The anode plate and the cathode plate can both be iridium-coated titanium electrode plates, but are not limited thereto. The polar water chamber adopts dilute sulfuric acid circulation, such as a sulfuric acid solution with a mass fraction of 5%-15% (such as 5%, 10%, 15%, etc.) can be passed. The first fresh water chamber and the second fresh water chamber serve as feed channels (for example, the first fresh water chamber is fed with ferric sulfate solution, and the second fresh water chamber is fed with phosphoric acid), and the first concentrated water chamber and the second concentrated water chamber are used as production channels (for example, iron phosphate mother liquor is synthesized in the first concentrated water chamber, and the first concentrated water chamber adopts a wide flow barrier. When ions migrate over, they are gradually enriched by being flushed to the outside of the container through a large flow rate, and the concentrations of iron ions and phosphate ions in the iron phosphate storage tank increase, forming iron phosphate solids and starting to settle to the bottom; at the same time, dilute sulfuric acid is produced in the second concentrated water chamber.

[0055] It should be noted that under the action of the electric field, the iron ions in the first fresh water chamber pass through the cation exchange membrane to reach the first concentrated water chamber, and the phosphate ions in the second fresh water chamber pass through the anion exchange membrane to reach the first concentrated water chamber. There are a large number of iron ions and phosphate ions in the first concentrated water chamber. Their concentrations gradually increase during ion migration and combine to form iron phosphate mother liquor; the hydrogen ions (or metal ions, such as Na + ) passes through the cation exchange membrane to reach the second concentrated water chamber, and the sulfate ions in the first fresh water chamber pass through the anion exchange membrane to reach the second concentrated water chamber. There are a large number of hydrogen ions (or metal ions) and sulfate ions in the second concentrated water chamber. Their concentration gradually increases during ion migration and combines to form sulfuric acid (sulfate). The double replacement chemical formula of the reaction in this process is:

[0056] Fe2(SO4)3+2H3PO4→2FePO4+3H2SO4.

[0057] In some embodiments, the ferric sulfate solution introduced into the first fresh water chamber is adjusted to a pH value of 0.5-1.0 (such as 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, etc.) by a sulfuric acid solution, and the synthesis of ferric phosphate can be promoted within this pH range. Preferably, the mass fraction of the ferric sulfate solution is 5%-20% (such as 5%, 10%, 15%, 20%, etc.), and the pH value is adjusted to 0.5-1.0 by a sulfuric acid solution with a mass fraction of 5%-10% (such as 5%, 8%, 10%, etc.).

[0058] In some embodiments, the mass fraction of the phosphoric acid solution or phosphate solution introduced into the second desalted water chamber is 5%-10%, such as 5%, 8%, 10%, etc. The phosphoric acid aqueous solution with a pH value of 1.0-2.0 introduced into the first concentrated water chamber can be prepared by mixing pure water and phosphoric acid solution, and the pH value of the phosphoric acid aqueous solution can be 1.0, 1.5, 2.0, etc.

[0059] It should be noted that after the ferrous sulfate solution S4 and the phosphoric acid / phosphate solution S5 enter the electrochemical double replacement system S7, the pure water S6 is added with phosphoric acid to adjust the pH and then enters the electrochemical double replacement system S7 as a circulating stock solution. After adjusting the pH value, applying an electric field and reacting the ion exchange process, the main product ferrous phosphate mother liquor is obtained and enters the ferrous phosphate storage tank S12, and the by-product is the sulfuric acid / sulfate solution S9. During the reaction, the solution ion concentrations in S4 and S5 will gradually decrease due to ion migration, and the dilute solutions S4 and S5 can be concentrated and reused separately through their corresponding reverse osmosis systems S8.

[0060] Furthermore, the set flow rates of the polar water chamber, the first fresh water chamber, the second fresh water chamber, the first concentrated water chamber, and the second concentrated water chamber are all 400L / h-600L / h, the set voltage is 10V-15V, and the current is 30A-50A. By adjusting the parameters of the electrochemical double replacement, the reaction of the synthetic iron phosphate and the by-products can be carried out quickly and fully. Specifically, the set flow rates of the polar water chamber, the first fresh water chamber, the second fresh water chamber, the first concentrated water chamber, and the second concentrated water chamber can be the same or different, and can be independently 400L / h, 450L / h, 500L / h, 550L / h, 600L / h, etc.; the set voltage can be 10V, 11V, 12V, 13V, 14V, 15V, etc., and the current can be 30A, 35A, 40A, 45A, 50A, etc.

[0061] (4) Sulfuric acid or sulfate purification

[0062] like Figure 1 As shown, when the by-product in step (3) is a sulfuric acid solution, the by-product sulfuric acid solution S9 is sent to the acid impurity removal system S10 to obtain relatively pure sulfuric acid S11, and the sulfuric acid S11 is sold as a by-product or recycled.

[0063] like Figure 4 As shown, when the by-product in step (3) is a sulfate solution, the by-product sulfate (such as sodium sulfate solution S9) is sent to the impurity removal / evaporation system S10, and then the sulfate product (such as sodium sulfate S11) is obtained by evaporation and crystallization.

[0064] (5) Iron phosphate crystallization and aging

[0065] Under acidic conditions, the iron phosphate mother liquor transported to the iron phosphate storage tank by the electrochemical double replacement system is crystallized / aged S12, and iron phosphate is generated after pH adjustment, temperature control, time control, stirring control and other conditions are controlled. The generated iron phosphate is separated from the solid by liquid separation (such as filter press washing S13) to obtain clean iron phosphate solid.

[0066] In some embodiments, the process of crystallizing and aging the iron phosphate mother liquor includes: adjusting the pH value of the iron phosphate mother liquor to 1.0-2.0, stirring for 30min-120min at a temperature of 20°C-100°C to generate iron phosphate, and obtaining iron phosphate solid after solid-liquid separation; the solid-liquid separation method can be pressure filtration washing to fully remove residual ions on the surface.

[0067] Specifically, the pH value of the iron phosphate mother liquor after adjusting the pH value can be 1.0, 1.3, 1.5, 1.8, 2.0, etc., the crystallization temperature can be 20°C, 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, etc., and the stirring time can be 30min, 50min, 80min, 100min, 120min, etc.

[0068] (6) Drying / Calcination

[0069] The iron phosphate solid is calcined to obtain anhydrous iron phosphate. In actual operation, the process of preparing anhydrous iron phosphate from the iron phosphate solid includes: flashing the iron phosphate solid S14 to obtain dihydrate iron phosphate S15, and then calcining S16 to obtain anhydrous iron phosphate S17. The anhydrous iron phosphate S17 can be mixed / screened S18 and then packaged S19 to obtain the product iron phosphate S20.

[0070] Furthermore, the flash temperature is controlled to be 200°C-250°C, such as 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, etc.; the calcination temperature is controlled to be 450°C-650°C, such as 450°C, 480°C, 500°C, 530°C, 550°C, 580°C, 600°C, 630°C, 650°C, etc.; the calcination time is 4h-6h, such as 4h, 4.5h, 5h, 5.5h, 6h, etc.

[0071] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0072] It should be noted that the following concentrations not specified are all mass fractions.

[0073] Example 1

[0074] Please refer to Figure 1 This embodiment provides a method for preparing iron phosphate, and the specific steps are as follows:

[0075] (1) Raw material storage tank: prepare the raw material ferrous sulfate solution S1 (85% by mass ferrous sulfate heptahydrate), hydrogen peroxide solution S2 (27.5% by mass hydrogen peroxide), phosphoric acid S5 (85% by mass phosphoric acid), and pure water S6 (conductivity ≤ 10us / cm). The solution with the required average value will be sent to the next process.

[0076] (2) Iron oxidation reaction: The ferrous sulfate solution S1 and the hydrogen peroxide solution S2 are sent to the oxidation reaction tank S3. During this process, the pH value is adjusted to 0.5-1.0 by adding dilute sulfuric acid (mass fraction is 8%, the same below). Under acidic conditions, hydrogen peroxide has strong oxidizing properties. Hydrogen peroxide, as an oxidant, can remove Fe in the ferrous sulfate. 2+ All oxidized to Fe 3+ Iron ions; the molar ratio of hydrogen peroxide and ferrous ions is controlled to be 0.55, the oxidation reaction temperature is controlled to be 40°C, stirring is performed during the reaction, and the ferric sulfate solution S4 is obtained after the oxidation reaction for 60 minutes.

[0077] The chemical formula of the reaction of this process is: 2FeSO4+H2O2+H2SO4→Fe2(SO4)3+2H2O.

[0078] (3) Electrochemical double replacement system: Sulfuric acid H2SO4 (8% sulfuric acid) is continuously added to the ferric sulfate solution S4 (8% by mass of ferric sulfate) to adjust the pH value to 0.5-1, and the acidic ferric sulfate solution is passed into the first fresh water chamber; phosphoric acid S5 (8% by mass of phosphoric acid solution) is passed into the second fresh water chamber; pure water S6 and 8% by mass of phosphoric acid solution are mixed to obtain a phosphoric acid aqueous solution with a pH value of 1.0-2.0 as a circulating stock solution into the first concentrated water chamber. After entering the electrochemical double replacement system S7, after the replacement reaction through the electric field and ion exchange process (the set flow rate of the 5 compartments is about 500L / h, the set voltage: 12V, the current: 40A, the polar solution: 4% dilute sulfuric acid, the number of membrane pairs is 40 pairs, and the area of ​​a single membrane is 0.2*0.4m 2 ) to obtain the main product, ferric phosphate mother liquor, which enters the ferric phosphate storage tank S12 (pH < 2.0). During the process, the dilute solutions S4 / S5 are concentrated and reused separately through the reverse osmosis system S8 (acidic membrane concentration system), and the electrochemical double replacement system S7 obtains the sulfuric acid solution S9 (5% to 15%) as a by-product.

[0079] The double displacement chemical formula of the reaction of this process is:

[0080] Fe2(SO4)3+2H3PO4→2FePO4+3H2SO4.

[0081] (4) Acid purification: The sulfuric acid solution S9 (5% to 15% sulfuric acid) is sent to the acid impurity removal system S10 to obtain relatively pure sulfuric acid S11, which is sold as a by-product or reused. The impurity removal principle of the acid impurity removal system S10 is as follows: The membrane used in the system is a nanofiltration membrane, which is a special semi-permeable membrane with a pore size range of several nanometers to tens of nanometers. It can intercept organic matter with a molecular weight between hundreds and thousands, while allowing small molecules to pass through. Nanofiltration membrane technology uses this characteristic to separate ions, molecules and colloids in the solution under pressure to achieve solution separation and concentration. In this membrane concentration system, the nanofiltration membrane can effectively intercept impurities in the acid solution and concentrate the acid solution that needs to be recovered to achieve resource recycling.

[0082] (5) Crystallization and aging of iron phosphate: Under acidic conditions (pH < 2.0), the solution in the iron phosphate storage tank S12 is mixed with a phosphoric acid solution having a mass fraction of 8% to adjust the pH to 1.0-2.0, the temperature is controlled to 50° C., and the mixture is stirred at a speed of 300 rpm for 90 min to generate iron phosphate. The generated iron phosphate is washed by filter press S13 to obtain clean iron phosphate solid.

[0083] (6) Drying / calcination: The iron phosphate after filter pressing and washing is flashed S14 (230° C.) to obtain dihydrate iron phosphate S15, and then calcined (550° C. / 5 h) S16 to obtain anhydrous iron phosphate S17.

[0084] (7) Mixing / screening / packaging: The anhydrous ferric phosphate S17 is mixed / screened S18 and then packaged S19 to obtain the product ferric phosphate S20.

[0085] Example 2

[0086] Please refer to Figure 4 This embodiment provides a method for preparing iron phosphate, and the specific steps are as follows:

[0087] (1) Raw material storage tank: prepare the raw material ferrous sulfate solution S1 (85% ferrous sulfate heptahydrate), hydrogen peroxide solution S2 (27.5% hydrogen peroxide), sodium phosphate S5 (8% sodium phosphate), and pure water S6 (conductivity ≤ 10us / cm). The solution with the required average value will be sent to the next process.

[0088] (2) Iron oxidation reaction: The ferrous sulfate solution S1 and the hydrogen peroxide solution S2 are sent to the oxidation reaction tank S3. During this process, the pH value is adjusted to 0.5-1.0 by adding dilute sulfuric acid. Under acidic conditions, hydrogen peroxide has a strong oxidizing property. Hydrogen peroxide, as an oxidant, can remove Fe in the ferrous sulfate. 2+ All oxidized to Fe 3+ iron ions; the molar ratio of hydrogen peroxide and ferrous ions is controlled to be 0.55, the oxidation reaction temperature is controlled to be 40°C, stirring is performed during the reaction, and the ferric sulfate solution S4 is obtained after the oxidation reaction for 60 minutes. The chemical formula of the reaction of this process is: 2FeSO4+H2O2+H2SO4→Fe2(SO4)3+2H2O.

[0089] (3) Electrochemical double replacement system: The ferric sulfate solution S4 (iron sulfate mass fraction of 5%) is continuously added with sulfuric acid H2SO4 (8% sulfuric acid) to adjust the pH value to 0.5-1, and the acidic ferric sulfate solution is passed into the first fresh water chamber; sodium phosphate S5 (sodium phosphate aqueous solution with a mass fraction of 8%) is passed into the second fresh water chamber; pure water S6 and 8% phosphoric acid solution are mixed to obtain a phosphoric acid aqueous solution with a pH value of 1.0-2.0 as a circulating stock solution and enter the first concentrated water chamber. After entering the electrochemical double replacement system S7, after the replacement reaction through the electric field and ion exchange process (the set flow rate of the 5 compartments is about 500L / h, the set voltage: 12V, the current: 40A, the polar solution: 4% dilute sulfuric acid, the number of membrane pairs is 40 pairs, and the area of ​​a single membrane is 0.2*0.4m 2 ) to obtain the main product, ferric phosphate mother liquor, which enters the ferric phosphate storage tank S12 (pH < 2.0). During the process, the dilute solution is concentrated and reused by the reverse osmosis system S8 (acidic membrane concentration system), and the electrochemical double replacement system S7 by-product obtains sodium sulfate solution S9 (5% to 15% sodium sulfate).

[0090] The double displacement chemical formula of the reaction of this process is:

[0091] Fe2(SO4)3+2H3PO4→2FePO4+3H2SO4.

[0092] (4) Purification: The sodium sulfate solution S9 (5% to 15% sodium sulfate) is sent to the impurity removal system S10, and then the sodium sulfate product is obtained by evaporation, crystallization and drying to be sold as a by-product or reused.

[0093] (5) Crystallization and aging of iron phosphate: Under acidic conditions (pH < 2.0), the solution in the iron phosphate storage tank S12 is mixed with a phosphoric acid solution having a mass fraction of 8% to adjust the pH to 1.0-2.0, the temperature is controlled to 50° C., and the mixture is stirred at a speed of 300 rpm for 90 min to generate iron phosphate. The generated iron phosphate is washed by filter press S13 to obtain clean iron phosphate solid.

[0094] (6) Drying / calcination: The iron phosphate after filter pressing and washing is flash evaporated S14 (230° C.) to obtain dihydrate iron phosphate S15, and then calcined (550° C., 5 h) S16 to obtain anhydrous iron phosphate S17.

[0095] (7) Mixing / screening / packaging: The anhydrous ferric phosphate S17 is mixed / screened S18 and then packaged S19 to obtain the product ferric phosphate S20.

[0096] The element analysis of the solution before the experiment is shown in Table 1 and Table 2:

[0097] Table 1 Element analysis of solution before experiment

[0098]

[0099] Table 2 Element analysis of each compartment after the experiment

[0100]

[0101] It can be seen from Table 1 and Table 2 that the sodium ions in the second fresh water chamber migrated to the second concentrated water chamber, the iron ions in the first fresh water chamber migrated to the first concentrated water chamber, the phosphate ions in the second fresh water chamber migrated to the first concentrated water chamber, and the sulfate ions in the first fresh water chamber migrated to the second concentrated water chamber. The entire system completed the process of double replacement electrochemical migration and resynthesis of compounds.

[0102] Analysis of equipment operation trend chart during the test process Figure 5 As shown, it can be seen that: the increase in the conductivity of concentrated water 2 (the output material of the second concentrated water chamber) during the process indicates that the ion concentration of the material is slowly increasing; turbidity is produced in concentrated water 1 (the output material of the first concentrated water chamber) during the experiment, indicating that solid matter is forming.

[0103] Example 2 The XRD pattern of the precipitate obtained after the electrochemical double replacement system and subsequent series of treatments is as follows Figure 6 As shown, it can be seen that the precipitate obtained after the first concentrated water chamber and the subsequent series of treatments is iron phosphate.

[0104] Comparative Example 1

[0105] A method for preparing ferric phosphate by sodium method, please refer to Figure 7 , including the following specific steps:

[0106] (1) Raw material storage tank: prepare the raw material sulfuric acid solution S1 (H2SO4>98%), iron powder S2 (Fe>97%), phosphoric acid S4 (85% phosphoric acid), sodium hydroxide S5 (NaOH≥99%). The solution with the average value reaching the requirement will be sent to the next process.

[0107] (2) Iron oxidation reaction: The sulfuric acid solution S1 and the iron powder S2 are sent to the iron salt reaction tank S3, and a ferrous sulfate solution is obtained in the process.

[0108] The chemical formula of the reaction of this process is: Fe+H2SO4→FeSO4+H2↑.

[0109] (3) Synthesis reactor system: The ferrous sulfate solution, phosphoric acid S4, sodium hydroxide S5, hydrogen peroxide solution S8, and sodium hydroxide S9 obtained in the iron salt reaction tank S3 are introduced into the synthesis reactor system S7. After the synthesis reaction, the precipitated ferric phosphate enters the filter press washing S10, the mother liquor wastewater is adjusted and neutralized, and enters the impurity removal and concentration S11, and then the by-product sodium sulfate S22 is obtained through evaporation and crystallization S21.

[0110] The chemical formula of the reaction in this process is:

[0111] FeSO4+H3PO4+H2O2+NaOH→[Fe(PO4)]x[Fe(OH)3] 2-x 2H2O+Na2SO4;

[0112] Conversion reaction:

[0113] [Fe(PO4)]x[Fe(OH)3] 2-x ·2H2O+H3PO4→FePO4·2H2O+mH2O.

[0114] (4) Crystallization and aging of ferric phosphate: Under acidic conditions (pH < 2.0), the ferric phosphate solution after filter pressing and washing is adjusted / aged S12, the pH is adjusted to 1.0-2.0, the temperature is controlled to 80°C-100°C, and stirred at a speed of 50-500 rpm for 30-120 min to generate ferric phosphate. The generated ferric phosphate is filtered and washed S13 to obtain clean ferric phosphate solid.

[0115] (5) Drying / calcination: The iron phosphate after filter pressing and washing is flash evaporated S14 (200°C-250°C) to obtain dihydrate iron phosphate S15, and then calcined (500°C-650°C, 4h-6h) S16 to obtain anhydrous iron phosphate S17.

[0116] (6) Mixing / screening / packaging: The anhydrous ferric phosphate S17 is mixed / screened S18 and then packaged S19 to obtain the product ferric phosphate S20.

[0117] Test Example 1

[0118] Table 3 is a comparison of the processes for preparing ferric phosphate in Example 1, Example 2, and Comparative Example 1:

[0119] Table 3 Comparison of process routes

[0120] Process route comparison Example 1 Example 2 Comparative Example 1 Output products Iron phosphate Iron phosphate Iron phosphate Output by-product sulfuric acid Sodium sulfate Sodium sulfate Difference in auxiliary materials Ferrous Sulfate / Phosphoric Acid Ferrous Sulfate / Sodium Phosphate Ferrous Sulfate / Phosphoric Acid / Sodium Hydroxide Is a synthesis reactor required? no no yes Do by-products require an evaporator? no yes yes Produces wastewater, waste residue and waste gas none Evaporation of mother liquor Evaporation of mother liquor System operation security risks Low Low Low Core energy consumption type Electrically driven ion migration Electrically driven ion migration Steam heating synthesis Raw material cost Low middle high

[0121] It can be seen from Table 3 that the process used in Example 1 has simple raw materials, a single source, low cost, and basically no three wastes are generated; the process used in Comparative Example 1 is a traditional ferric phosphate synthesis method (sodium method), which produces more wastewater, waste residue and product impurities.

[0122] In summary, the present invention provides a method for preparing iron phosphate, which combines acidic membrane concentration system, electrochemical double displacement system, anion and cation exchange membrane, precipitation aging, filter press washing and other technical means to obtain iron phosphate products, sulfuric acid and sulfate byproducts, and provides a new method for preparing iron phosphate. Compared with the traditional iron phosphate synthesis system, it has the following advantages:

[0123] (1) The electrochemical double replacement system process provides a new direction and method for the preparation of iron phosphate;

[0124] (2) The ammonium method / sodium method was not used, which reduced the Na + or NH4 + The introduction of improves the purity of iron phosphate products;

[0125] (3) The sulfuric acid produced by the double displacement electrochemical system can be used to adjust the value of the raw materials, which reduces the additional pH agent addition process and reduces acid and alkali consumption;

[0126] (4) The electrochemical double displacement process equipment has a simple structure;

[0127] (5) Compared with the traditional synthesis reaction, there is no sulfuric acid in the iron phosphate compartment during the preparation process of the present invention, so that the iron phosphate will not be re-dissolved by sulfuric acid, and only the pH value needs to be controlled to ensure the smooth progress of the reaction;

[0128] (6) The element structure of the entire process system is simple, with only Fe, S, P, H, O and other elements, which can effectively control the introduction of impurities, and the by-products can be reused or sold;

[0129] (7) The process can be used for continuous feeding production;

[0130] (8) The back-end iron phosphate drying / calcination / packaging processes are basically the same as the traditional processes, which can further ensure the consistency and stability of the iron phosphate products.

[0131] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing ferric phosphate, characterized in that: include: Using ferric sulfate solution, phosphoric acid solution or phosphate solution as main raw materials, an electrochemical double replacement system is used to prepare ferric phosphate mother liquor, and sulfuric acid or sulfate is produced as a by-product; The electrochemical double replacement system controls the flow direction of anions and cations by staggered multiple anion exchange membranes and multiple cation exchange membranes, and uses compartments separated by multiple anion exchange membranes and multiple cation exchange membranes to input raw materials and output target products.

2. The preparation method according to claim 1, characterized in that: The electrochemical double replacement system comprises an anode plate at one end and a cathode plate at the other end, a plurality of anion exchange membranes and a plurality of cation exchange membranes are arranged between the anode plate and the cathode plate, the anion exchange membranes and the cation exchange membranes are arranged alternately to form at least one repeating unit, and each of the repeating units comprises a first fresh water chamber, a first concentrated water chamber, a second fresh water chamber and a second concentrated water chamber which are arranged in sequence; The ferric sulfate solution is introduced into the first fresh water chamber, and the phosphoric acid solution or phosphate solution is introduced into the second fresh water chamber; The first concentrated water chamber produces iron phosphate mother liquor, and the second concentrated water chamber produces sulfuric acid or sulfate.

3. The preparation method according to claim 2, characterized in that: The ion exchange membranes adjacent to the anode plate and the cathode plate are both anion exchange membranes, and the anode plate, the cathode plate and the corresponding anion exchange membranes are both polar water chambers, in which a sulfuric acid solution with a mass fraction of 5%-15% is circulated.

4. The preparation method according to claim 3, characterized in that: The ferrous sulfate solution introduced into the first fresh water chamber is adjusted to a pH value of 0.5-1.0 by a sulfuric acid solution; preferably, the mass fraction of the ferrous sulfate solution is 5%-20%, and the pH value is adjusted to 0.5-1.0 by a sulfuric acid solution with a mass fraction of 5%-10%; Preferably, the mass fraction of the phosphoric acid solution or phosphate solution introduced into the second fresh water chamber is 5%-10%; Preferably, a phosphoric acid aqueous solution with a pH value of 1.0-2.0 is introduced into the first concentrated water chamber; Preferably, the set flow rates of the polar water chamber, the first fresh water chamber, the second fresh water chamber, the first concentrated water chamber and the second concentrated water chamber are all 400L / h-600L / h, the set voltage is 10V-15V, and the current is 30A-50A.

5. The preparation method according to any one of claims 1 to 4, characterized in that: The ferric sulfate solution is obtained by oxidation reaction of ferrous sulfate solution.

6. The preparation method according to claim 5, characterized in that: The preparation process of the ferrous sulfate solution comprises: mixing the ferrous sulfate solution and the oxidant to carry out an oxidation reaction, the reaction temperature is 30° C.-60° C., and the reaction time is 45 min-60 min; Preferably, the oxidant is a hydrogen peroxide solution, the molar ratio of hydrogen peroxide to ferrous ions is controlled to be (0.5-0.6):1, and the pH value of the hydrogen peroxide solution is adjusted to 0.5-1.0 by adding sulfuric acid solution before the reaction.

7. The preparation method according to any one of claims 1 to 4, characterized in that: Also includes: Crystallizing and aging the iron phosphate mother liquor produced by the electrochemical double replacement system to obtain iron phosphate solid; The ferric phosphate solid is calcined to obtain anhydrous ferric phosphate.

8. The preparation method according to claim 7, characterized in that: The process of crystallizing and aging the iron phosphate mother liquor comprises: adjusting the pH value of the iron phosphate mother liquor to 1.0-2.0, stirring for 30 min-120 min at a temperature of 20° C.-100° C. to generate iron phosphate, and obtaining the iron phosphate solid after solid-liquid separation; Preferably, the solid-liquid separation is carried out by filter pressing and washing.

9. The preparation method according to claim 7, characterized in that: The process of preparing the anhydrous ferric phosphate from the ferric phosphate solid comprises: flashing the ferric phosphate solid and then calcining it; Preferably, the flash temperature is controlled to be 200°C-250°C; Preferably, the calcination temperature is controlled to be 450° C.-650° C., and the calcination time is 4 h-6 h.

10. The preparation method according to any one of claims 1 to 4, characterized in that: The by-product sulfuric acid is removed for impurities to obtain a sulfuric acid product; Preferably, the by-product sulfate is removed from the water and then evaporated and crystallized to obtain the sulfate product.

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