Method for continuous production of iron phosphate, method for production of iron phosphate and lithium iron phosphate

Abstract

The present application relates to a method for continuously preparing iron phosphate, a method for preparing iron phosphate and lithium iron phosphate. The method for continuously preparing iron phosphate comprises the following steps: mixing elemental iron, phosphorus-containing mixed acid and water to prepare a solution containing Fe 2+ by a first oxidation reaction; mixing the solution containing Fe 2+ and a first oxidizing agent to prepare a pre-oxidized solution by a second oxidation reaction; overflowing the pre-oxidized solution to a crystal growing system containing a second oxidizing agent to prepare a slurry containing Fe 3+ by a third oxidation reaction; overflowing the slurry containing Fe 3+ to an aging system for aging treatment, and after the aging is completed, performing solid-liquid separation on the slurry containing Fe 3+ to collect the filter cake; the phosphorus-containing mixed acid comprises a first acid and a second acid, the first acid is phosphoric acid, and the second acid is an inorganic strong acid other than phosphoric acid, the molar ratio of the first acid to the second acid in the phosphorus-containing mixed acid is 1:(0.1-0.4); the temperature of the second oxidation treatment is 25-60 DEG C, and the time is 20-60 min. The method is simple in process, high in production capacity, controllable in quality, and capable of preparing iron phosphate with high purity, controllable morphology and controllable and stable iron-phosphorus ratio.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, and in particular to a continuous method for preparing iron phosphate, and a method for preparing iron phosphate and lithium iron phosphate. Background Technology

[0002] Lithium iron phosphate batteries have been widely used in the new energy field due to their many advantages, such as high specific capacity, stable structure, safe performance, and long service life.

[0003] Iron phosphate (FePO4), also known as orthophosphate, is usually FePO4·2H2O with two molecules of water of crystallization. It is a nearly white, pink or pale yellow powder and is an important precursor for the preparation of lithium iron phosphate (LiFePO4). Its physicochemical properties (such as iron-to-phosphorus ratio, particle size, etc.) and crystal structure (such as morphology) determine the overall performance of LiFePO4 cathode materials to a certain extent.

[0004] The mainstream process for synthesizing iron phosphate uses Fe and P sources as raw materials, and obtains battery-grade FePO4·2H2O through oxidation / precipitation-aging crystallization. However, the traditional FePO4·2H2O preparation process generally adopts a batch single-reactor production method, which either suffers from low capacity and cumbersome operation, or has shortcomings such as large differences in morphology and iron-phosphorus ratio between different batches of products. Summary of the Invention

[0005] Based on this, the purpose of the present invention is to provide a continuous method for preparing iron phosphate, which has a simple process, high production capacity, controllable quality, and can prepare iron phosphate with high purity, controllable morphology, and controllable and stable iron-phosphorus ratio.

[0006] The technical solution is as follows:

[0007] A continuous method for preparing iron phosphate includes the following steps:

[0008] Elemental iron, phosphorus-containing mixed acid, and water are mixed to prepare an Fe-containing compound through a first oxidation reaction. 2+ The solution;

[0009] The Fe-containing 2+ The solution is mixed with the first oxidant, and a pre-oxidized feed solution is prepared through a second oxidation reaction;

[0010] The pre-oxidized feed solution is overflowed into a crystal growth system containing a second oxidant, and Fe-containing crystals are prepared through a third oxidation reaction. 3+ The slurry;

[0011] The Fe-containing 3+ The slurry overflows into the aging system for aging treatment;

[0012] After aging, the Fe-containing3+ The slurry is subjected to solid-liquid separation treatment, and the filter cake is collected;

[0013] The phosphorus-containing mixed acid includes a first acid and a second acid, wherein the first acid is phosphoric acid and the second acid is an inorganic strong acid other than phosphoric acid. In the phosphorus-containing mixed acid, the molar ratio of the first acid to the second acid is 1:(0.1~0.4).

[0014] The second oxidation treatment is carried out at a temperature of 25℃ to 60℃ for a time of 20 min to 60 min.

[0015] In one embodiment, the elemental iron is one or more of iron powder, iron bars, and iron blocks, mixed together.

[0016] In one embodiment, the second acid is one or a mixture of sulfuric acid, nitric acid, and hydrochloric acid.

[0017] In one embodiment, the molar ratio of elemental iron, phosphorus-containing mixed acid, and water is 1:(2.0-4.0):(40-100).

[0018] In one embodiment, the Fe-containing compound is prepared by a first oxidation reaction. 2+ In the solution step, the temperature of the first oxidation reaction is 40℃~90℃, and the time is 0.5h~6h.

[0019] In one embodiment, the Fe-containing 2+ Both the solution and the first oxidant are fed from the bottom of the reaction system.

[0020] In one embodiment, the first oxidant comprises one or more of air, oxygen, and ozone.

[0021] In one embodiment, the feed flow rate ratio of the first oxidant to the Fe2+-containing solution is (0.1-3):1.

[0022] In one embodiment, the air and the Fe-containing 2+ The feed flow rate ratio of the solution is (0.5~3):1.

[0023] In one embodiment, the first oxidant is oxygen, and the oxygen reacts with the Fe-containing... 2+ The feed flow rate ratio of the solution is (0.1~0.25):1.

[0024] In one embodiment, the first oxidant is ozone, and the ozone reacts with the Fe-containing... 2+ The feed flow rate ratio of the solution is (0.1~0.15):1.

[0025] In one embodiment, the second oxidant is fed from the bottom of the reaction system.

[0026] In one embodiment, the second oxidant is hydrogen peroxide, and the feed flow rate ratio of the hydrogen peroxide to the pre-oxidized feed liquid is (0.4~1):1.

[0027] In one embodiment, the second oxidant is a gaseous oxidant, which includes one or a mixture of oxygen and ozone, and the feed flow rate ratio of the gaseous oxidant to the pre-oxidized feed liquid is (0.5-5):1.

[0028] In one embodiment, the temperature of the third oxidation reaction is 60°C to 80°C, and the time is 0.5h to 4h.

[0029] In one embodiment, the aging system includes a multi-stage series aging tank.

[0030] In one embodiment, the multi-stage series aging reactor is a four-stage, five-stage, or six-stage aging reactor, with each aging reactor having an independent temperature of 80℃ to 100℃ and an independent aging time of 1.0h to 3.0h.

[0031] In one embodiment, after the solid-liquid separation step, the process further includes washing the filter cake multiple times and drying the washed product.

[0032] In one embodiment, the washing process includes the following steps:

[0033] The filter cake is washed with water at temperatures ranging from 20°C to 100°C until the conductivity of the wash water is ≤300 μS / cm.

[0034] In one embodiment, the drying process is carried out at a temperature of 80°C to 200°C for 1 hour to 4 hours.

[0035] The present invention also provides an iron phosphate, which is prepared by the continuous preparation method of iron phosphate as described above.

[0036] This invention also provides a method for preparing lithium iron phosphate, comprising the following steps:

[0037] Ferric phosphate was prepared according to the continuous preparation method described above;

[0038] Lithium iron phosphate is prepared by mixing the iron phosphate with a lithium source and a carbon source and then calcining it.

[0039] The present invention has at least the following beneficial effects:

[0040] This invention prepares Fe-containing compounds by mixing phosphorus-containing mixed acids and elemental iron. 2+The solution, combined with a two-stage oxidation process, aging, and solid-liquid separation, allows for the controllable preparation of iron phosphate crystal form and properties. On one hand, the PO4 content is adjusted by using a phosphorus-containing mixed acid. 3- Concentration, while enhancing the efficient dissolution (oxidation reaction) of elemental iron, is also beneficial to improving the crystallization process; on the other hand, the second oxidation reaction (i.e., pre-oxidation) adjusts the Fe concentration. 2+ / Fe 3+ The proportion of iron phosphate crystals is adjusted to control the number of crystal nuclei and the growth rate of iron phosphate crystals, thereby achieving the goal of controlling the morphology of iron phosphate, the selection of iron phosphate crystal form, and the stability of the iron-to-phosphorus ratio of the product. Furthermore, compared to the monoclinic crystal system, the orthorhombic crystal system of iron phosphate dihydrate exhibits higher electrochemical activity. This invention enables low-cost, high-efficiency, and continuous preparation of high-purity orthorhombic crystal system battery-grade iron phosphate.

[0041] Tests have shown that in some specific examples of this invention, battery-grade iron phosphate is orthorhombic with an iron-to-phosphorus ratio (Fe / P) of 0.970–0.980, an S impurity content of less than 60 ppm, and a Cl impurity content of less than 10 ppm. This demonstrates that this invention ensures stable morphology and product performance while continuously preparing iron phosphate.

[0042] In addition, this invention has the advantages of simple and flexible operation, low equipment investment, green and environmentally friendly, controllable quality and easy industrialization, which can generate good economic and social benefits and has broad application prospects. Attached Figure Description

[0043] Figure 1 The XRD pattern of battery-grade iron phosphate prepared in Example 1;

[0044] Figure 2 The image shows the SEM pattern of the battery-grade iron phosphate prepared in Example 1. Detailed Implementation

[0045] The present invention will be further described in detail below with reference to specific embodiments and accompanying drawings. The present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of the present invention.

[0046] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0047] As used herein, the terms “and / or,” “or / and,” and “and / or” may include any one of two or more of the related listed items, as well as any and all combinations of the related listed items, including any two related listed items, any more related listed items, or a combination of all the related listed items.

[0048] When using “including,” “having,” and “contains” as described herein, the intention is to cover non-exclusive inclusion, unless an explicit qualifying term such as “only,” “consisting of,” etc., is used, in which case another component may be added.

[0049] The terms "optionally," "preferredly," "more preferably," "better," and "even better" used in this document refer to embodiments of the present invention that may provide certain beneficial effects under certain circumstances. However, other embodiments may also be preferred under the same or other circumstances. Furthermore, the description of one or more preferred embodiments does not imply that other embodiments are unavailable, nor is it intended to exclude other embodiments from the scope of the present invention. That is, in this document, "optionally," "preferredly," "more preferably," "better," and "even better" are merely descriptions of implementations or examples with better effects, but do not constitute a limitation on the scope of protection of the present invention. For example, optionally, A is one or both of a and b, indicating that A may be selected from a, b, and combinations of a and b, or other options.

[0050] In this document, terms such as "further," "even more," and "particularly" are used for descriptive purposes to indicate differences in content, but should not be construed as limiting the scope of protection of this invention.

[0051] In this document, "above" or "below" includes the stated number. For example, "below 1" includes 1.

[0052] In this document, "at least one" means one or more, such as one, two, or more. "Multiple" or "several" means at least two, such as two, three, etc., and "multi-layered" means at least two layers, such as two layers, three layers, etc., unless otherwise explicitly specified. In the description of this invention, "several" means at least one, such as one, two, etc., unless otherwise explicitly specified.

[0053] When a numerical range is disclosed herein, the range is considered continuous and includes the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0054] Unless otherwise specified, all steps of this invention may be performed sequentially or randomly. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the method may also include step (c), indicating that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0055] Unless otherwise specified, all raw materials, reagents, and other materials used in the following specific implementation methods are commercially available products.

[0056] Lithium iron phosphate (LiFePO4) batteries possess numerous advantages, including high specific capacity, stable structure, safe performance, and long lifespan, leading to their widespread application. Iron phosphate is a crucial precursor in the preparation of LiFePO4, and its physicochemical properties largely determine the overall performance of the LiFePO4 cathode material. However, traditional FePO4·2H2O preparation processes typically employ batch-type single-reactor production methods, which suffer from either low production capacity and cumbersome operation, or significant variations in morphology and iron-phosphorus ratio between different batches of products.

[0057] For example, a method for preparing morphology-controllable iron phosphate and lithium iron phosphate has been reported: ferrous salt, phosphoric acid, and hydrogen peroxide are mixed in a single step, and the pH of the system is adjusted by adding alkali to prepare monoclinic iron phosphate dihydrate. This method allows for flexible control of the size and morphology of the iron phosphate, but it uses an intermittent operation, resulting in high production costs and significant batch-to-batch variations in performance such as the iron-to-phosphorus ratio. Alternatively, a low-cost method and equipment for preparing stable iron phosphate has been proposed: phosphorus and iron source solutions are continuously fed and sequentially passed through three reactors in series. By adjusting the amounts of phosphorus and iron sources, the mixing, reaction, and iron-to-phosphorus ratio of the materials are controlled, ultimately yielding a monoclinic iron phosphate dihydrate product. This method uses a continuous operation and allows for directional control of the iron-to-phosphorus ratio, but it suffers from uncontrollable primary particle size and morphology.

[0058] In addition, some studies have shown that, compared with the monoclinic crystal system, the orthorhombic crystal system of iron phosphate dihydrate has higher electrochemical activity. However, the traditional methods for preparing orthorhombic iron phosphate are basically electrolytic methods, which have disadvantages such as high energy consumption and cumbersome operation, and urgently need to be improved.

[0059] The purpose of this invention is to provide a continuous method for preparing iron phosphate, which has a simple process, high production capacity, controllable quality, and can prepare battery-grade iron phosphate with high purity, controllable morphology, and controllable and stable iron-phosphorus ratio.

[0060] The technical solution is as follows:

[0061] A continuous method for preparing iron phosphate includes the following steps:

[0062] Elemental iron, phosphorus-containing mixed acid, and water are mixed to prepare an Fe-containing compound through a first oxidation reaction. 2+ The solution;

[0063] The Fe-containing 2+ The solution is mixed with the first oxidant, and a pre-oxidized feed solution is prepared through a second oxidation reaction;

[0064] The pre-oxidized feed solution is overflowed into a crystal growth system containing a second oxidant, and Fe-containing crystals are prepared through a third oxidation reaction. 3+ The slurry;

[0065] The Fe-containing 3+ The slurry overflows into the aging system for aging treatment;

[0066] After aging, the Fe-containing 3+ The slurry is subjected to solid-liquid separation treatment, and the filter cake is collected;

[0067] The phosphorus-containing mixed acid includes a first acid and a second acid, wherein the first acid is phosphoric acid and the second acid is an inorganic strong acid other than phosphoric acid. In the phosphorus-containing mixed acid, the molar ratio of the first acid to the second acid is 1:(0.1~0.4).

[0068] The second oxidation treatment is carried out at a temperature of 25℃ to 60℃ for a time of 20 min to 60 min.

[0069] This invention prepares Fe-containing compounds by mixing mixed acid and elemental iron. 2+ The solution, combined with a two-stage oxidation process, as well as aging and solid-liquid separation treatment, allows for the controllable preparation of iron phosphate crystal form and properties. On one hand, the PO4 content is adjusted by mixing acids. 3- Concentration, while enhancing the efficient dissolution of elemental iron, is beneficial for improving the crystallization process; on the other hand, the second oxidation reaction (pre-oxidation) adjusts the Fe concentration. 2+ / Fe 3+The proportion of iron phosphate is adjusted to control the number of crystal nuclei and the growth rate, thereby achieving the goal of controlling the morphology of iron phosphate, the selection of iron phosphate crystal form, and the stability of the iron-to-phosphorus ratio of the product. Furthermore, compared to the monoclinic crystal system, the orthorhombic crystal system of iron phosphate dihydrate exhibits higher electrochemical activity. This invention enables low-cost, high-efficiency, and continuous preparation of high-purity orthorhombic iron phosphate.

[0070] In one embodiment, the elemental iron is one or more of iron powder, iron bars, and iron blocks, mixed together.

[0071] Elemental iron has poor solubility in a single acid. This invention uses a phosphorus-containing mixed acid, which can promote the forward reaction of dissolution (or oxidation reaction of elemental iron) and allows for more flexible adjustment of the concentration of phosphate ions.

[0072] In one embodiment, the second acid is one or more of sulfuric acid, nitric acid, and hydrochloric acid, and in the phosphorus-containing mixed acid, the molar ratio of the first acid to the second acid is 1:(0.1-0.4). Controlling the molar ratio of different acids in the phosphorus-containing mixed acid can better adjust the concentration of phosphate ions, which is more conducive to crystallization, promotes the formation of orthorhombic iron phosphate, and achieves the purpose of regulating the morphology of iron phosphate, the selection of iron phosphate crystal form, and the stability of the iron-phosphorus ratio of the product.

[0073] In one embodiment, the phosphorus-containing mixed acid is a mixture of phosphoric acid and sulfuric acid, wherein the molar ratio of phosphoric acid to sulfuric acid is 1:(0.1 to 0.4), including but not limited to 1:0.1, 1:0.2, 1:0.3 or 1:0.4.

[0074] In one embodiment, the phosphorus-containing mixed acid is a mixture of phosphoric acid and nitric acid, wherein the molar ratio of phosphoric acid to nitric acid is 1:(0.1 to 0.4), including but not limited to 1:0.1, 1:0.2, 1:0.3 or 1:0.4.

[0075] In one embodiment, the phosphorus-containing mixed acid is a mixture of phosphoric acid and hydrochloric acid, wherein the molar ratio of phosphoric acid to hydrochloric acid is 1:(0.1 to 0.4), including but not limited to 1:0.1, 1:0.2, 1:0.3 or 1:0.4.

[0076] In one embodiment, the molar ratio of elemental iron, the phosphorus-containing mixed acid, and water is 1:(2.0–4.0):(40–100), including but not limited to 1:2.0:40, 1:2.0:50, 1:2.0:60, 1:2.0:70, 1:2.0:80, 1:2.0:90, 1:2.0:100, 1:2.5:40, 1:2.5:50, 1:2.5:60, 1:2.5:70, 1:2.5:80, 1:2.5:90, 1:2.5:100, and 1:3. 0:40, 1:3.0:50, 1:3.0:60, 1:3.0:70, 1:3.0:80, 1:3.0:90, 1:3.0:100, 1:3.5:40, 1:3.5:50, 1:3.5:60, 1:3.5:70, 1:3.5:80, 1:3.5:90, 1:3.5:100, 1:4.0:40, 1:4.0:50, 1:4.0:60, 1:4.0:70, 1:4.0:80, 1:4.0:90 or 1:4.0:100.

[0077] In one embodiment, during the preparation of Fe-containing... 2+ In the solution step, the temperature of the first oxidation reaction is 40℃ to 90℃, including but not limited to 40℃, 45℃, 50℃, 55℃, 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, or 90℃. Under this temperature range, it is more advantageous for the present invention to ensure high purity and yield of ferric phosphate while achieving deep dissolution of elemental iron, and the ferric phosphate has a high Fe / P ratio.

[0078] In one embodiment, during the preparation of Fe-containing... 2+ In the solution step, the first oxidation reaction takes 0.5 h to 6 h. Under these time conditions, it is more advantageous for the present invention to ensure high purity and yield of ferric phosphate while achieving deep dissolution of elemental iron, and the ferric phosphate has a high Fe / P ratio.

[0079] In one embodiment, the first oxidant comprises one or more of air, oxygen, and ozone.

[0080] In one embodiment, the first oxidant reacts with the Fe-containing... 2+ The feed flow rate ratio of the solution is (0.1–3):1, including but not limited to 0.1:1, 0.15:1, 0.2:1, 0.25:1, 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, 1.2:1, 1.5:1, 2:1, 2.5:1, 2.8:1, or 3:1. Controlling such a feed ratio is beneficial for sufficient contact of the materials, thereby achieving the purpose of regulating the number of crystal nuclei and the growth rate of iron phosphate crystals.

[0081] In one embodiment, the first oxidant is air. Further, the air is combined with the Fe-containing... 2+ The feed flow rate ratio of the solution is (0.5~3):1.

[0082] In one embodiment, the first oxidant is oxygen. Further, the oxygen reacts with the Fe-containing... 2+ The feed flow rate ratio of the solution is (0.1~0.25):1.

[0083] In one embodiment, the first oxidant is ozone. Further, the ozone reacts with the Fe-containing... 2+ The feed flow rate ratio of the solution is (0.1~0.15):1.

[0084] In one embodiment, the Fe-containing 2+ Both the solution and the first oxidant are fed from the bottom of the reaction system, which is conducive to full contact of the materials and achieves the purpose of controlling the number of crystal nuclei and growth rate of iron phosphate crystals.

[0085] In one embodiment, the temperature of the second oxidation reaction (pre-oxidation treatment) is 25°C to 60°C, including but not limited to 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, and 60°C. Low-temperature pre-oxidation is beneficial for adjusting Fe. 2+ / Fe 3+ The ratio is adjusted to control the number of crystal nuclei and growth rate of iron phosphate crystals, thereby achieving the goal of stable crystal form selection and product iron-phosphorus ratio.

[0086] In one embodiment, the second oxidation reaction (pre-oxidation treatment) takes place for 20 to 60 minutes, including but not limited to 20, 30, 40, 50, or 60 minutes. Controlling the pre-oxidation time to 20 to 60 minutes is more conducive to adjusting Fe... 2+ / Fe 3+ The ratio is adjusted to control the number of crystal nuclei and growth rate of iron phosphate crystals, thereby achieving the goal of stable crystal form selection and product iron-phosphorus ratio.

[0087] In one embodiment, the second oxidant is fed from the bottom of the reaction system to ensure sufficient contact of the materials, further regulate the number of crystal nuclei and growth rate of iron phosphate crystals, and achieve crystal form selection.

[0088] In one embodiment, the second oxidant is hydrogen peroxide, and the feed flow rate ratio of the hydrogen peroxide to the pre-oxidized feed solution is (0.4 to 1):1, including but not limited to 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, or 1:1. Controlling such a feed ratio ensures sufficient contact of the materials, further regulating the number of crystal nuclei and growth rate of iron phosphate crystals, and achieving crystal form selection.

[0089] In one embodiment, the second oxidant is a gaseous oxidant, comprising one or a mixture of oxygen and ozone, and the feed flow rate ratio of the gaseous oxidant to the pre-oxidized feed liquid is (0.5-5):1. Controlling this feed ratio ensures sufficient contact of the materials, further regulating the number of crystal nuclei and growth rate of iron phosphate crystals, thereby achieving crystal form selection.

[0090] In one embodiment, the temperature of the third oxidation reaction is 60°C to 80°C, and the time is 0.5h to 4h.

[0091] In one embodiment, the aging system includes multi-stage aging tanks connected in series, which can improve the crystallinity, iron-phosphorus ratio and refine the product particles.

[0092] In one embodiment, the multi-stage aging reactor is a four-, five-, or six-stage aging reactor. Further, the temperature of each aging reactor is independently set to 80°C to 100°C, which can improve the crystallinity, iron-phosphorus ratio, and refine the product particle size. It is understood that the temperature of each aging reactor includes, but is not limited to, 80°C, 85°C, 90°C, 95°C, or 100°C. Even further, the aging time of each aging reactor is independently set to 1.0h to 3.0h, which can improve the crystallinity, iron-phosphorus ratio, and refine the product particle size. It is understood that the aging time of each aging reactor includes, but is not limited to, 1.0h, 1.5h, 2.0h, 2.5h, or 3.0h.

[0093] In one embodiment, the slurry from the aging reactor is overflowed into a centrifuge for solid-liquid separation to obtain mother liquor and wet ferric phosphate.

[0094] In one embodiment, after the solid-liquid separation step, the process further includes washing the filter cake multiple times and drying the washed product.

[0095] In one embodiment, the washing process includes the following steps:

[0096] The filter cake is washed with water at temperatures ranging from 20°C to 100°C until the conductivity of the wash water is ≤300 μS / cm.

[0097] Understandably, the washing temperatures include, but are not limited to, 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C, 70°C, 80°C, 85°C, 90°C, 95°C, or 100°C. Furthermore, the washing process is repeated multiple times, with each washing process occurring at an independent temperature, which may be the same or different.

[0098] In one embodiment, after the washing step, a drying step is also included.

[0099] In one embodiment, the drying temperature is 80°C to 200°C, including but not limited to 80°C, 85°C, 90°C, 95°C, 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, 190°C or 200°C.

[0100] In one embodiment, the drying time is 1 hour to 4 hours, including but not limited to 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, or 4 hours.

[0101] In one embodiment, a method for continuous preparation of iron phosphate includes the following steps:

[0102] S1. Dissolve elemental iron in a mixture of acid and water, then remove the residue to obtain Fe. 2+ liquid;

[0103] S2, Fe is transferred via a flow pump / flow meter 2+ Liquid and air are continuously injected into the pre-oxidation reactor;

[0104] S3. The pre-oxidation reactor liquid overflows naturally from the overflow port at the top of the reactor to the crystal growth reactor, and hydrogen peroxide is continuously injected.

[0105] S4. The slurry from the crystal growth reactor overflows naturally from the overflow port at the top of the reactor to the multi-stage series aging reactor;

[0106] S5. The slurry from the aging kettle overflows naturally from the overflow port at the top of the kettle to the centrifuge, where solid and liquid are separated to obtain mother liquor and wet ferric phosphate.

[0107] S6. The wet product is washed and dried to obtain battery-grade iron phosphate.

[0108] The present invention also provides an iron phosphate, which is prepared by the continuous preparation method of iron phosphate as described above.

[0109] In one embodiment, the ferric phosphate is in the form of block particles, with D10, D50, D90 and D99 being 15μm to 25μm, 40μm to 45μm, 100μm to 130μm and 180μm to 220μm, respectively.

[0110] In one embodiment, the ferric phosphate is in the form of block particles with D10, D50, D90 and D99 of 19.219 μm, 43.519 μm, 104.96 μm and 191.366 μm, respectively.

[0111] This invention also provides a method for preparing lithium iron phosphate, comprising the following steps:

[0112] Ferric phosphate was prepared according to the continuous preparation method described above;

[0113] Lithium iron phosphate is prepared by mixing the iron phosphate with a lithium source and a carbon source and then calcining it.

[0114] The continuous preparation method of iron phosphate of this invention has the advantages of simple operation, low equipment investment, green and environmentally friendly, controllable quality, and easy industrialization, which can generate good economic and social benefits. The iron phosphate prepared according to the method of this invention has the advantages of high purity, controllable morphology, and controllable and stable iron-phosphorus ratio. It can also produce orthorhombic iron phosphate with high purity, controllable morphology, and controllable and stable iron-phosphorus ratio. The iron phosphate prepared by this invention can be further used as a precursor to prepare lithium iron phosphate. Battery-grade lithium iron phosphate with excellent comprehensive performance such as high purity, controllable morphology, and controllable and stable iron-phosphorus ratio can be prepared under simple and low-cost conditions, and then used in lithium iron phosphate batteries, which has broad application prospects.

[0115] The following description, in conjunction with specific embodiments and comparative examples, provides further details.

[0116] Example 1

[0117] 500 kg of iron powder, 1500 kg of phosphoric acid, 500 kg of sulfuric acid, and 7000 kg of pure water were mixed and reacted at 90 °C for 0.5 h. After the reaction was completed, the solid and liquid phases were separated to obtain Fe. 2+ Liquid. Fe is transferred via a flow pump / flow meter. 2+ Liquid and air are continuously injected from the bottom into the second oxidation reactor (hereinafter referred to as the pre-oxidation reactor) at a flow rate ratio of 100 L / h: 400 L / h, with the pre-oxidation time controlled at 20 min and the reactor temperature at 60 °C. The pre-oxidation slurry continuously flows naturally from the overflow port of the pre-oxidation reactor into the crystal growth reactor. Hydrogen peroxide is continuously injected from the bottom into the crystal growth reactor at a flow rate of 8 L / h and the reactor temperature at 80 °C. The slurry from the crystal growth reactor continuously flows naturally from the overflow port into the fourth-stage aging reactor, with the temperature of each aging reactor controlled at 100 °C and the aging time per reactor at 1.0 h. The slurry from the aging reactor continuously overflows naturally into a centrifuge for solid-liquid separation, yielding mother liquor and wet iron phosphate. The wet product is washed with pure water until the conductivity of the iron phosphate washing water is ≤300 μs / cm, and then dried at 200 °C for 1 h to obtain battery-grade iron phosphate.

[0118] The iron phosphate prepared above was subjected to XRD analysis to obtain... Figure 1 The diffraction pattern shown will Figure 1 The diffraction pattern was compared with the corresponding product standard card, confirming that the product obtained in this embodiment is indeed orthorhombic iron phosphate.

[0119] The iron phosphate prepared above was subjected to SEM testing to obtain... Figure 2 As shown in the SEM image, the product particles are large, regularly shaped crystals.

[0120] The prepared ferric phosphate was analyzed by atomic emission spectrometry (ICP), and the contents of impurities sodium, magnesium, silicon, potassium, calcium, chromium, cobalt, nickel, manganese, zinc, cadmium, and lead were all less than 10 ppm. Infrared spectroscopy analysis showed that the impurity SO4 was present. 2- The content of [unspecified substance] was 50 ppm; the content of impurity Cl was 15 ppm by turbidimetric analysis; and the Fe / P ratio was 0.975 by chemical titration analysis.

[0121] Example 2

[0122] 700 kg of iron bar, 3000 kg of phosphoric acid, 1000 kg of hydrochloric acid, and 18000 kg of pure water were mixed and reacted at 40 °C for 6 hours. After the reaction was completed, the solid and liquid phases were separated to obtain Fe. 2+ Liquid. Fe is transferred via a flow pump / flow meter. 2+ Liquid and air were continuously injected into the pre-oxidation reactor from the bottom at a flow rate ratio of 200 L / h: 500 L / h, with the pre-oxidation time controlled at 40 min and the reactor temperature at 50 °C. The pre-oxidation slurry continuously flowed naturally from the overflow port of the pre-oxidation reactor into the crystal growth reactor. Hydrogen peroxide was continuously injected into the crystal growth reactor from the bottom at a flow rate of 40 L / h and the reactor temperature at 50 °C. The slurry from the crystal growth reactor continuously flowed naturally from the overflow port into the five-stage aging reactor, with the reactor temperature controlled at 90 °C and the aging time per reactor at 2.0 h. The slurry from the aging reactor continuously overflowed naturally into a centrifuge for solid-liquid separation, yielding mother liquor and wet ferric phosphate. The wet product was washed with pure water until the conductivity of the ferric phosphate wash water was ≤300 μs / cm, and then dried at 150 °C for 2 h to obtain battery-grade ferric phosphate.

[0123] The prepared ferric phosphate was analyzed by atomic emission spectrometry (ICP), and the contents of impurities sodium, magnesium, silicon, potassium, calcium, chromium, cobalt, nickel, manganese, zinc, cadmium, and lead were all less than 10 ppm. Infrared spectroscopy analysis showed that the impurity SO4 was present. 2- The content of [unspecified substance] was 30 ppm; turbidimetric analysis showed that the content of impurity Cl was 5 ppm; chemical titration analysis showed that the Fe / P ratio was 0.977. XRD analysis confirmed that it is orthorhombic iron phosphate.

[0124] Example 3

[0125] 900 kg of iron block, 6500 kg of phosphoric acid, 400 kg of nitric acid, and 12000 kg of pure water were mixed and reacted at 80 °C for 5 h. After the reaction was completed, the solid and liquid phases were separated to obtain Fe. 2+ Liquid. Fe is transferred via a flow pump / flow meter. 2+ Liquid and air were continuously injected into the pre-oxidation reactor from the bottom at a flow rate ratio of 150 L / h: 400 L / h, with the pre-oxidation time controlled at 60 min and the reactor temperature at 25 °C. The pre-oxidation slurry continuously flowed naturally from the overflow port of the pre-oxidation reactor into the crystal growth reactor. Hydrogen peroxide was continuously injected into the crystal growth reactor from the bottom at a flow rate of 30 L / h and the reactor temperature at 75 °C. The slurry from the crystal growth reactor continuously flowed naturally from the overflow port into the fourth-stage aging reactor, with the reactor temperature controlled at 95 °C and the aging time per reactor at 3.0 h. The slurry from the aging reactor continuously overflowed naturally into a centrifuge for solid-liquid separation, yielding mother liquor and wet ferric phosphate. The wet product was washed with pure water until the conductivity of the ferric phosphate wash water was ≤300 μs / cm, and then dried at 120 °C for 2 h to obtain battery-grade ferric phosphate.

[0126] The prepared ferric phosphate was analyzed by atomic emission spectrometry (ICP), and the contents of impurities sodium, magnesium, silicon, potassium, calcium, chromium, cobalt, nickel, manganese, zinc, cadmium, and lead were all less than 10 ppm. Infrared spectroscopy analysis showed that the impurity SO4 was present. 2- The content of [unspecified substance] was 30 ppm; turbidimetric analysis showed that the content of impurity Cl was 10 ppm; chemical titration analysis showed that the Fe / P ratio was 0.974. XRD analysis confirmed that it is orthorhombic iron phosphate.

[0127] Example 4

[0128] 900 kg of iron powder, 5500 kg of phosphoric acid, 450 kg of hydrochloric acid, and 20000 kg of pure water were mixed and reacted at 50 °C for 1.5 h. After the reaction was completed, the solid and liquid phases were separated to obtain Fe. 2+ Liquid. Fe is transferred via a flow pump / flow meter. 2+ Liquid and air were continuously injected into the pre-oxidation reactor from the bottom at a flow rate ratio of 200 L / h: 500 L / h, with the pre-oxidation time controlled at 30 min and the reactor temperature at 45 °C. The pre-oxidation slurry continuously flowed naturally from the overflow port of the pre-oxidation reactor into the crystal growth reactor. Hydrogen peroxide was continuously injected into the crystal growth reactor from the bottom at a flow rate of 25 L / h and the reactor temperature at 70 °C. The slurry from the crystal growth reactor continuously flowed naturally from the overflow port into the sixth-stage aging reactor, with the reactor temperature controlled at 90 °C and the aging time per reactor at 2.0 h. The slurry from the aging reactor continuously overflowed naturally into a centrifuge for solid-liquid separation, yielding mother liquor and wet ferric phosphate. The wet product was washed with pure water until the conductivity of the ferric phosphate wash water was ≤300 μs / cm, and then dried at 80 °C for 3 h to obtain battery-grade ferric phosphate.

[0129] The prepared ferric phosphate was analyzed by atomic emission spectrometry (ICP), and the contents of impurities sodium, magnesium, silicon, potassium, calcium, chromium, cobalt, nickel, manganese, zinc, cadmium, and lead were all less than 10 ppm. Infrared spectroscopy analysis showed that the impurity SO4 was present. 2- The content of [unspecified substance] was 50 ppm; turbidimetric analysis showed that the content of impurity Cl was 5 ppm; chemical titration analysis showed that the Fe / P ratio was 0.976. XRD analysis confirmed that it is orthorhombic iron phosphate.

[0130] Comparative Example 1

[0131] Comparative Example 1 prepared iron phosphate continuously according to the same steps and processes as in Example 1, the only difference being the preparation of Fe. 2+ The solution step used 750 kg of phosphoric acid and 900 kg of sulfuric acid, and the other operations were the same as in Example 1.

[0132] The prepared ferric phosphate was analyzed by atomic emission spectrometry (ICP), and the contents of impurities sodium, magnesium, silicon, potassium, calcium, chromium, cobalt, nickel, manganese, zinc, cadmium, and lead were all less than 10 ppm. Infrared spectroscopy analysis showed that the impurity SO4 was present. 2- The content of [unspecified substance] was 900 ppm; turbidimetric analysis showed that the content of impurity Cl was 15 ppm; chemical titration analysis showed that the Fe / P ratio was 0.992. XRD analysis confirmed that it is a monoclinic ferric hydroxyphosphate.

[0133] Comparative Example 2

[0134] Comparative Example 2 prepared iron phosphate continuously using the same steps and processes as in Example 1, the only difference being the preparation of Fe. 2+ The steps involved using 1500 kg of phosphoric acid and 900 kg of sulfuric acid, with other operations being the same as in Example 1.

[0135] The prepared ferric phosphate was analyzed by atomic emission spectrometry (ICP), and the contents of impurities sodium, magnesium, silicon, potassium, calcium, chromium, cobalt, nickel, manganese, zinc, cadmium, and lead were all less than 10 ppm. Infrared spectroscopy analysis showed that the impurity SO4 was present. 2- The content was 1200 ppm; according to infrared spectroscopy analysis, the impurity SO4 2- The content of [unspecified substance] was 250 ppm; turbidimetric analysis showed that the content of impurity Cl was 18 ppm; chemical titration analysis showed that the Fe / P ratio was 0.963. XRD analysis showed that it was a mixture of monoclinic and orthorhombic iron phosphate, with the monoclinic crystals accounting for a larger proportion, approximately 40% by mass.

[0136] Comparative Example 3

[0137] Comparative Example 3 prepared ferric phosphate continuously according to the same steps and processes as in Example 1, except that air was not used in the pre-oxidation step and the second oxidation reaction was omitted, i.e., the pre-oxidation treatment was omitted. Other operations were the same as in Example 1.

[0138] The prepared ferric phosphate was analyzed by atomic emission spectrometry (ICP), and the contents of impurities sodium, magnesium, silicon, potassium, calcium, chromium, cobalt, nickel, manganese, zinc, cadmium, and lead were all less than 10 ppm. Infrared spectroscopy analysis showed that the impurity SO4 was present. 2- The content of [unspecified substance] was 900 ppm; turbidimetric analysis showed that the content of impurity Cl was 15 ppm; chemical titration analysis showed that the Fe / P ratio was 0.959. XRD analysis showed that it was a mixture of monoclinic and orthorhombic iron phosphate, with the monoclinic crystals accounting for more, approximately 80% by mass.

[0139] Comparative Example 4

[0140] Comparative Example 4 prepared iron phosphate continuously according to the same steps and processes as in Example 1, except that the pre-oxidation time was 10 min in the pre-oxidation step, and the other operations were the same as in Example 1.

[0141] The prepared ferric phosphate was analyzed by atomic emission spectrometry (ICP), and the contents of impurities sodium, magnesium, silicon, potassium, calcium, chromium, cobalt, nickel, manganese, zinc, cadmium, and lead were all less than 10 ppm. Infrared spectroscopy analysis showed that the impurity SO4 was present. 2- The content of [unspecified substance] was 800 ppm; turbidimetric analysis showed that the content of impurity Cl was 15 ppm; chemical titration analysis showed that the Fe / P ratio was 0.958. XRD analysis showed that it was a mixture of monoclinic and orthorhombic iron phosphate, with the monoclinic crystals accounting for more, approximately 70% by mass.

[0142] Comparative Example 5

[0143] Comparative Example 5 prepared ferric phosphate continuously according to the same steps and processes as in Example 1, except that the reaction temperature was 90°C in the pre-oxidation step, and the other operations were the same as in Example 1.

[0144] The prepared ferric phosphate was analyzed by atomic emission spectrometry (ICP), and the contents of impurities sodium, magnesium, silicon, potassium, calcium, chromium, cobalt, nickel, manganese, zinc, cadmium, and lead were all less than 10 ppm. Infrared spectroscopy analysis showed that the impurity SO4 was present. 2- The content of [unspecified substance] was 1200 ppm; turbidimetric analysis showed that the content of impurity Cl was 20 ppm; chemical titration analysis showed that the Fe / P ratio was 0.958. XRD analysis showed that it was a mixture of monoclinic and orthorhombic iron phosphate, with the monoclinic crystals accounting for more, approximately 90% by mass.

[0145] Results analysis:

[0146] In Examples 1 to 4 of this invention, the battery-grade iron phosphate is orthorhombic with an iron-to-phosphorus ratio (Fe / P) of 0.974 to 0.977, an S impurity content of ≤50ppm, and impurities of sodium, magnesium, silicon, potassium, calcium, chromium, cobalt, nickel, manganese, zinc, cadmium, and lead of less than 10ppm, and impurity of chlorine of less than 15ppm.

[0147] The results of Example 1 and Comparative Example 1 show that in Comparative Example 1, 750 kg of phosphoric acid and 900 kg of sulfuric acid were used in the steps. The product was monoclinic hydroxyferric phosphate with a high S impurity content and a high iron-to-phosphorus ratio, which did not achieve the purpose of the present invention to ensure stable morphology and performance.

[0148] The results of Example 1 and Comparative Example 2 show that in Comparative Example 2, 1500 kg of phosphoric acid and 900 kg of sulfuric acid were used, and the product was a mixture of monoclinic and orthorhombic crystal systems. The content of sulfur impurities was relatively high, and the iron-to-phosphorus ratio was relatively low, which did not achieve the purpose of this invention to ensure stable morphology and performance.

[0149] The results of Example 1 and Comparative Example 3 show that air was not used in the pre-oxidation step of Comparative Example 3, the product was a mixture of monoclinic and orthorhombic crystal systems, the S impurity content was high, and the iron-phosphorus ratio was low, which did not achieve the purpose of the present invention to ensure stable morphology and performance.

[0150] As can be seen from the results of Example 1 and Comparative Example 4, the pre-oxidation time in the pre-oxidation step of Comparative Example 4 was 10 min, the product was a mixture of monoclinic and orthorhombic crystal systems, the S impurity content was relatively high, and the iron-phosphorus ratio was low, which did not achieve the purpose of the present invention to ensure stable morphology and performance.

[0151] The results of Example 1 and Comparative Example 5 show that in Comparative Example 5, the reaction temperature in the oxidation step was 90°C, the product was a mixture of monoclinic and orthorhombic crystal systems, the S impurity content was high, and the iron-phosphorus ratio was low, which did not achieve the purpose of the present invention to ensure stable morphology and performance.

[0152] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0153] The embodiments described above are merely illustrative of several implementations of the present invention, designed to facilitate a detailed understanding of the technical solutions of the present invention, but should not be construed as limiting the scope of protection of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention. It should be understood that technical solutions obtained by those skilled in the art based on the technical solutions provided by the present invention through logical analysis, reasoning, or limited experimentation are all within the scope of protection of the appended claims. Therefore, the scope of protection of this invention patent should be determined by the content of the appended claims, and the specification and drawings can be used to interpret the content of the claims.

Claims

1. A method for the continuous production of iron phosphate characterized in that, The method comprises the following steps: Elemental iron, phosphorus-containing mixed acid and water are mixed to prepare a solution containing Fe 2+ through a first oxidation reaction; mixing the solution containing Fe 2+ and the first oxidizing agent to prepare a pre-oxidized solution by a second oxidation reaction; overflowing the pre-oxidized feed solution to a crystallization system containing a second oxidizing agent to produce a slurry containing Fe 3+ through a third oxidation reaction; overflowing the slurry containing Fe 3+ to an aging system for aging treatment; After aging is completed, the slurry containing Fe 3+ is subjected to solid-liquid separation treatment, and the filter cake is collected. The phosphorus-containing mixed acid comprises a first acid and a second acid, the first acid is phosphoric acid, and the second acid is an inorganic strong acid other than phosphoric acid, and the molar ratio of the first acid to the second acid in the phosphorus-containing mixed acid is 1:(0.1-0.4); The temperature of the second oxidation treatment is 25-60 DEG C, and the time is 20-60 min.

2. The method of claim 1, wherein, The step of preparing the solution containing Fe 2+ by the first oxidation reaction satisfies one or more of the following (1) to (4): (1) the elemental iron is one or more of a mixture of iron powder, iron bar and iron block; (2) the second acid is one or more of a mixture of sulfuric acid, nitric acid and hydrochloric acid; (3) the molar ratio of the elemental iron, the phosphorus-containing mixed acid and water is 1:(2.0-4.0):(40-100); (4) the temperature of the first oxidation reaction is 40-90 DEG C, and the time is 0.5-6 h.

3. The method of claim 1, wherein, The step of preparing the pre-oxidized feed liquid through the second oxidation reaction satisfies one or more of the following (1)-(3): (1) the solution containing Fe 2+ Both the solution containing Fe and the first oxidizing agent are fed from the bottom of the reaction system. (2) the first oxidizing agent and the solution containing Fe 2+ at a feed flow rate ratio of (0.1-3): 1; (3) the first oxidizing agent comprises one or more of a mixture of air, oxygen and ozone.

4. The method of claim 1, wherein, The step of preparing the slurry containing Fe 3+ by a third oxidation reaction satisfies one or more of the following (1) to (3): (1) the feeding mode of the second oxidizing agent is from the bottom of the reaction system; (2) the second oxidizing agent is hydrogen peroxide, and the feeding flow rate ratio of the hydrogen peroxide to the pre-oxidized feed liquid is (0.4-1):1; or the second oxidizing agent is a gaseous oxidizing agent, the gaseous oxidizing agent comprises one or both of a mixture of oxygen and ozone, and the feeding flow rate ratio of the gaseous oxidizing agent to the pre-oxidized feed liquid is (0.5-5):1; (3) the temperature of the third oxidation reaction is 60-80 DEG C, and the time is 0.5-4 h.

5. The method according to any one of claims 1 to 4, characterized in that, The aging system comprises a plurality of serially connected aging tanks.

6. The method of claim 5, wherein, The plurality of serially connected aging tanks are four, five or six aging tanks, and the temperature of each aging tank is independently 80-100 DEG C, and the aging time of each aging tank is independently 1.0-3.0 h.

7. The method according to any one of claims 1 to 4, characterized in that, After the solid-liquid separation step, the method further comprises the steps of washing the filter cake multiple times and drying the washed product.

8. The method of claim 7, wherein, One or more of the following (1)-(2) are satisfied: (1) the washing step comprises the following steps: washing the filter cake with water at 20-100 DEG C until the conductivity of the washing water is less than or equal to 300 us / cm; (2) the temperature of the drying step is 80-200 DEG C, and the time is 1-4 h.

9. A method of producing lithium iron phosphate, characterized by, The method comprises the following steps: The method for continuously preparing iron phosphate according to any one of claims 1-8 is used to prepare iron phosphate; The iron phosphate is mixed with a lithium source and a carbon source, and iron phosphate lithium is prepared through calcination treatment.

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