Ammonium phosphate solution and preparation method thereof, iron phosphate crystal and lithium iron phosphate positive electrode material

Abstract

The invention provides an ammonium phosphate solution and a preparation method thereof, an iron phosphate crystal and a lithium iron phosphate positive electrode material, and belongs to the field of battery materials.The preparation method comprises the following steps that a defluorinating agent and first ammonia-containing alkali liquor are added into wet-process phosphoric acid for primary neutralization reaction, then solid-liquid separation is conducted, and a first filter cake and first filtrate are obtained; the first filtrate and second ammonia-containing alkali liquor are mixed for secondary neutralization reaction, then solid-liquid separation is performed to obtain a second filter cake and second filtrate, and the second filtrate is an ammonium phosphate solution; and drying the first filter cake and the second filter cake, and recycling as a slow-release nitrogen-phosphorus fertilizer. The high-quality ammonium phosphate solution is prepared by adopting two neutralization reaction procedures, the ammonium phosphate solution can be used as a raw material to be directly applied to preparation of the iron phosphate crystal, and compared with the prior art, the preparation procedures are remarkably simplified; meanwhile, the obtained filter cake can be reused as a slow-release nitrogen-phosphorus fertilizer after being dried, so that the technological process is more environment-friendly; reaction conditions are easy to control, and the method is low in cost and suitable for large-scale industrial production.

Description

Technical Field

[0001] The present application relates to the technical field of battery materials, and in particular to an ammonium phosphate solution and a preparation method thereof, iron phosphate crystals, a lithium iron phosphate positive electrode material and a secondary battery. Background Art

[0002] Monoammonium phosphate (MAP), also known as diammonium phosphate, is a white powdery or granular inorganic compound with the chemical formula NH4H2PO4. After dissolving, it dissociates into ammonium ions and phosphate ions, and has good chemical reactivity and thermal stability. It has a wide range of applications and can be used as fertilizers, flame retardants, food additives, feed additives, pharmaceutical buffers, battery materials, etc. Among them, in the field of new energy battery applications, iron phosphate is an important raw material for the preparation of lithium iron phosphate cathode materials, and monoammonium phosphate is an important raw material for the preparation of iron phosphate, which makes the preparation process of monoammonium phosphate crystals affect the production cost and product quality of lithium iron phosphate cathode materials.

[0003] In the prior art, the steps of producing monoammonium phosphate crystals by direct ammonia neutralization of wet-process phosphoric acid mainly include: first desulfurizing and / or defluorinating the wet-process phosphoric acid, then neutralizing with ammonia to generate a monoammonium phosphate solution, and removing impurities such as aluminum, magnesium, calcium, iron, copper, cadmium, and nickel in the wet-process phosphoric acid, and then successively filtering, concentrating, crystallizing, separating, and drying to obtain monoammonium phosphate crystals (MAP crystals for short). When using MAP crystals to prepare iron phosphate, it is usually necessary to first dissolve the MAP crystals to prepare a solution containing MAP, and then mix the MAP-containing solution with an iron source solution and an oxidant and perform an oxidation reaction. During the reaction, ammonia water is added to adjust the pH value. After the reaction is completed, filtering, washing, flash drying, and calcining are performed successively to obtain iron phosphate crystals. It can be seen that the above-mentioned existing process route is relatively lengthy, and in the process of preparing MAP crystals, the three steps of desulfurization, defluorination and neutralization will affect the morphology, uniformity and yield of the MAP crystals. That is, these three steps need to be precisely controlled to prepare high-quality MAP crystals. For large-scale industrial production, the process conditions are more demanding and the cost is higher, which is not conducive to large-scale promotion and application.

[0004] Therefore, there is an urgent need to provide a new MAP preparation method to streamline the existing preparation process and reduce costs. Summary of the invention

[0005] In view of the technical problems existing in the background technology, the present application provides an ammonium phosphate solution and a preparation method thereof, iron phosphate crystals, a lithium iron phosphate positive electrode material and a secondary battery, aiming to solve the problem of the lack of a simple, low-cost, high-quality monoammonium phosphate preparation process in the prior art.

[0006] In a first aspect, an embodiment of the present application provides a method for preparing an ammonium phosphate solution, characterized in that it includes the following steps: adding a defluorinating agent and a first ammoniacal alkali solution to wet-process phosphoric acid for a primary neutralization reaction, followed by solid-liquid separation to obtain a first filter cake and a first filtrate; the first filtrate is mixed with a second ammoniacal alkali solution for a secondary neutralization reaction, followed by solid-liquid separation to obtain a second filter cake and a second filtrate, wherein the second filtrate is the ammonium phosphate solution; the first filter cake and the second filter cake are dried and recycled as a slow-release nitrogen and phosphorus fertilizer; the pH of the primary neutralization reaction is 4.6~5.2.

[0007] In the technical scheme of the embodiment of the present application, wet-process phosphoric acid is used as a raw material, and a phosphate ammonium solution containing monoammonium phosphate is prepared by two neutralization reaction steps, wherein the first neutralization reaction simultaneously removes fluorine, iron, and aluminum, and the second neutralization reaction removes impurities such as calcium and magnesium, thereby obtaining a high-quality phosphate ammonium solution with a low impurity content; the phosphate ammonium solution can be directly used as a raw material for the preparation of iron phosphate crystals. Compared with the existing wet-process phosphoric acid direct ammonia neutralization process, there is no need for separate sulfur removal, and the concentration, crystallization, and drying steps are omitted, and the monoammonium phosphate crystals are dissolved again to prepare iron phosphate crystals. At the same time, the obtained first filter cake and the second filter cake can be recycled as slow-release nitrogen and phosphorus fertilizers after drying, avoiding the generation of by-product waste, making the process more green and environmentally friendly; it can be seen that the present application scheme significantly simplifies the preparation process, the reaction conditions are easy to control, the cost is low, and it is suitable for large-scale industrial production.

[0008] In some embodiments, during a neutralization reaction, a first ammoniacal alkali solution is used to adjust the pH to 4.6-5.2 and a neutralization reaction is performed, the reaction temperature is 90-100°C, the reaction time is 1-3 h, and the mass ratio of the defluorinating agent to the wet-process phosphoric acid is (0.2-1.0):100.

[0009] In this embodiment, a defluorinating agent and a first ammoniacal alkali solution are introduced into a neutralization reaction, so that the defluorinating reaction and the neutralization reaction are carried out simultaneously, and F, Fe, and Al are removed simultaneously by adjusting the pH, reaction temperature, reaction time, and the amount of defluorinating agent used. Specifically, under the condition that the amounts of other components are the same, if the amount of defluorinating agent is too much, although the defluorinating efficiency can be improved, the excessive amount of defluorinating agent not only increases the cost of raw materials, but its excessive cationic part will combine with phosphate to form insoluble phosphate fluoride, resulting in a decrease in phosphorus yield in the subsequent preparation process; if the amount of defluorinating agent is insufficient, it cannot completely react with the fluorosilicate remaining in the first filtrate after desulfurization, resulting in insufficient defluorination. Regarding the control of the pH value of the primary neutralization reaction, if the pH value is too high (i.e., too much ammoniacal alkali solution is added), the crystallization efficiency of monoammonium phosphate will be significantly reduced, and impurities such as iron, aluminum, calcium, and magnesium will be deposited during the primary neutralization reaction. Excessive precipitation will bring out part of the phosphorus, resulting in a decrease in the subsequent phosphorus yield; if the pH value is too low (i.e., insufficient ammoniacal alkali solution is added), not only will impurities such as iron and aluminum be difficult to remove, but phosphoric acid will also be excessive and unable to fully react to form monoammonium phosphate. Regarding the control of the reaction temperature, if the reaction temperature is too high or the reaction time is too long, although the defluorination efficiency and iron and aluminum removal efficiency can be improved to a certain extent, the improvement effect of the defluorination efficiency and iron and aluminum precipitation efficiency is not obvious after exceeding a certain reaction temperature, but it will greatly increase energy consumption and production costs; if the reaction temperature is low or the reaction time is too short, the defluorination efficiency and iron and aluminum precipitation efficiency will be reduced, which is not conducive to production.

[0010] In some embodiments, the defluorinating agent includes at least one of soluble aluminum salts, aluminum hydroxide, aluminum oxide, soluble iron salts, iron hydroxide, and iron oxide; the first ammonia-containing alkaline solution includes at least one of liquid ammonia and ammonia water, and the second ammonia-containing alkaline solution includes at least one of liquid ammonia and ammonia water.

[0011] In this embodiment, soluble iron / aluminum salts, oxides or hydroxides are used as defluorinating agents, and liquid ammonia or ammonia water is used as an alkaline regulator, which can avoid the introduction of new cationic impurities during a neutralization reaction, and can simultaneously remove iron and aluminum impurities during wet-process phosphoric acid defluorination by controlling pH, reaction temperature and reaction time.

[0012] More specifically, in some embodiments, the defluorination agent includes an aluminum-containing component and an iron-containing component, the aluminum-containing component includes at least one of a soluble aluminum salt, aluminum hydroxide, and aluminum oxide, the iron-containing component includes at least one of a soluble iron salt, iron hydroxide, and iron oxide, and the mass ratio of the aluminum-containing component to the iron-containing component is 100:(10~50); during a primary neutralization reaction, the aluminum-containing component and the iron-containing component are respectively added and mixed with wet-process phosphoric acid and the first ammonia-containing alkali solution for reaction.

[0013] In this embodiment, a composite method of aluminum-containing components and iron-containing components is used as a defluorinating agent, and a certain amount of iron-containing components is introduced to replace the aluminum-containing components. As the proportion of iron-containing components increases, the defluorination effect shows a trend of first increasing and then decreasing. It is difficult to obtain the best defluorination effect if the proportion of iron-containing components is too high or too low, so it is necessary to strictly limit the ratio of aluminum-containing components to iron-containing components. At the same time, when the composite method of aluminum-containing components and iron-containing components is used as a defluorinating agent, if aluminum-containing components and iron-containing components are added at the same time, the solubility is low, which makes it difficult to give full play to the defluorination effect, so it is necessary to add the aluminum-containing components and iron-containing components separately.

[0014] In some embodiments, the secondary neutralization reaction comprises the following steps: adding a second ammoniacal alkali solution to control the pH of the first filtrate to 6.5-7.5, the reaction temperature to 50-70°C, and the reaction time to 0.5-2 h.

[0015] In this embodiment, a second ammoniacal alkali solution is added to the first filtrate after the primary neutralization reaction to carry out a secondary neutralization reaction. The pH value of the secondary neutralization reaction is adjusted by the second ammoniacal alkali solution, and the reaction temperature and time are controlled at the same time. This process mainly removes calcium and magnesium impurities, and further removes impurities such as F, Fe, and Al, so that the impurity content in the obtained second filtrate is less and the phosphorus yield is higher, and it can be used as a direct raw material for preparing iron phosphate. Specifically, for the control of the pH value of the secondary neutralization reaction, as the pH value increases, the crystallization efficiency of monoammonium phosphate tends to increase first and then decrease. If the pH value is too high (i.e., too much of the second ammoniacal alkali solution is added), the crystallization efficiency of monoammonium phosphate will be significantly reduced; if the pH value is too low (i.e., the amount of the second ammoniacal alkali solution added is insufficient), not only calcium and magnesium impurities are difficult to remove, but also excess phosphoric acid cannot fully react to form monoammonium phosphate. Regarding the control of the secondary neutralization reaction temperature, if the temperature is too high, the generation of monoammonium phosphate will be accelerated. At the same time, the solubility of monoammonium phosphate increases sharply with the increase of temperature, and the amount of ammonia released increases with the temperature, thus affecting the stability of the generated monoammonium phosphate; if the temperature is too low, the generation of monoammonium phosphate is slow, and calcium and magnesium impurities are difficult to fully remove. Regarding the control of the secondary neutralization reaction time, the impurity removal efficiency gradually increases with the extension of the reaction time, and gradually slows down after reaching a certain level. Therefore, appropriately extending the reaction time is helpful to improve the impurity removal efficiency.

[0016] In another embodiment, the secondary neutralization reaction includes the following steps: adding a second ammoniacal alkali solution to control the pH of the first filtrate to 8.0-9.0, the reaction temperature is 50-70°C, the reaction time is 0.5-2 h, and solid-liquid separation is performed to obtain a filtrate; thereafter, adding phosphoric acid to the filtrate to adjust the pH to 6.5-7.5, the reaction temperature is 50-70°C, the reaction time is 0.5-2 h, and an ammonium phosphate solution is obtained; wherein the purity of the phosphoric acid is greater than 85%.

[0017] In this embodiment, a different method from the above is provided for adjusting the pH during the secondary neutralization reaction. This method first adds a second ammoniacal alkali solution to adjust the pH to a relatively high level, and then adds high-purity phosphoric acid to adjust the pH and obtain an ammonium phosphate solution. Compared with the above secondary neutralization reaction method, the method of adjusting the pH after alkali treatment adopted in this embodiment can further improve the removal effect of Fe, Al, Ca, and Mg, and further improve the quality of the ammonium phosphate solution. In this embodiment, the adjustment method of the reaction temperature and time of the neutralization reaction is the same as the adjustment method of the relevant process parameters of the above secondary neutralization reaction, which will not be repeated here.

[0018] In a second aspect, an embodiment of the present application provides a phosphate ammonium solution, which is prepared by the preparation method of the phosphate ammonium solution in the first aspect mentioned above.

[0019] In the technical solution of the embodiment of the present application, the prepared ammonium phosphate solution has a low impurity content and high quality, and can be directly used as a raw material for the preparation of iron phosphate crystals. Compared with the existing wet phosphoric acid direct ammonia neutralization process, it significantly simplifies the preparation process and reduces costs.

[0020] In the third aspect, an embodiment of the present application provides an iron phosphate crystal, which is prepared by a mixed reaction of an iron salt solution, an oxidant, and the ammonium phosphate solution in the second aspect of the present application; wherein the iron salt in the iron salt solution includes at least one of ferrous sulfate, ferrous chloride, ferric sulfate, and ferric chloride, and the oxidant includes hydrogen peroxide.

[0021] In the technical solution of the embodiment of the present application, there is no need to adopt the traditional process to dissolve the monoammonium phosphate crystals again after preparing the monoammonium phosphate crystals. Instead, the iron phosphate crystals can be directly prepared using the aforementioned ammonium phosphate solution as a raw material. The iron phosphate crystals prepared from the aforementioned ammonium phosphate solution have excellent performance, and the lithium iron phosphate positive electrode material prepared from the iron phosphate crystals has a higher specific capacity and better rate performance.

[0022] In a fourth aspect, an embodiment of the present application provides a lithium iron manganese phosphate positive electrode material, which is obtained by a mixed reaction of the iron phosphate crystals provided in the third aspect of the present application and a lithium source.

[0023] The lithium iron manganese phosphate positive electrode material of the present application is prepared from the aforementioned iron phosphate crystals, and thus has the advantages of high specific capacity and good rate performance.

[0024] In a fifth aspect, an embodiment of the present application provides a secondary battery, the positive electrode plate of the secondary battery includes the lithium iron manganese phosphate positive electrode material provided in the fourth aspect of the present application.

[0025] The secondary battery of the present application includes the aforementioned positive electrode plate, and thus has the advantages of high specific capacity and good rate performance.

[0026] Compared with the prior art, the beneficial effects of this application include: The present application uses wet-process phosphoric acid as a raw material and adopts two neutralization reaction steps to prepare an ammonium phosphate solution containing monoammonium phosphate, wherein the first neutralization reaction simultaneously removes fluorine, iron and aluminum, and the second neutralization reaction removes impurities such as calcium and magnesium, thereby obtaining a high-quality ammonium phosphate solution; the ammonium phosphate solution can be directly used as a raw material for the preparation of iron phosphate crystals. Compared with the existing wet-process phosphoric acid direct ammonia neutralization process, it omits the steps of desulfurization, concentration, crystallization, drying and re-dissolving MAP crystals, thereby significantly simplifying the preparation process; at the same time, the obtained first filter cake and the second filter cake can be recycled as slow-release nitrogen and phosphorus fertilizers after drying, thereby avoiding the generation of by-product waste and making the process more green and environmentally friendly; the reaction conditions of the present application scheme are easy to control, the cost is low, and it is suitable for large-scale industrial production.

[0027] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solution of the present application, the following is a brief introduction to the drawings used in the present application. Obviously, the drawings described below are only some embodiments of the present application, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0029] Figure 1 A process flow chart of an embodiment of the method for preparing the ammonium phosphate solution provided in the present application; Figure 2 This is a process flow chart of another embodiment of the method for preparing ammonium phosphate solution provided in this application. DETAILED DESCRIPTION

[0030] The following embodiments of the technical solution of the present application are described in detail in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application, and are therefore only used as examples, and cannot be used to limit the scope of protection of the present application.

[0031] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by technicians in the technical field to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" in the specification and claims of this application and the above-mentioned figure descriptions and any variations thereof are intended to cover non-exclusive inclusions.

[0032] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "multiple" is more than two, unless otherwise clearly and specifically defined.

[0033] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0034] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of the associated objects, indicating that there may be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0035] In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

[0036] In the prior art, the direct ammonia neutralization process of wet-process phosphoric acid is a method for preparing monoammonium phosphate crystals by ammonia neutralization reaction of wet-process phosphoric acid. The specific steps include: first desulfurizing and / or defluorinating the wet-process phosphoric acid, then neutralizing it with ammonia to generate a MAP solution, and removing impurities such as aluminum, magnesium, calcium, iron, copper, cadmium, and nickel in the wet-process phosphoric acid, and then obtaining MAP crystals after filtering, concentrating, crystallizing, separating and drying. Since the quality of MAP crystals directly affects the quality of subsequent preparation of iron phosphate crystals, and thus affects the performance of lithium iron phosphate positive electrode materials and their secondary batteries, the preparation quality of MAP crystals is extremely important. The raw material wet-process phosphoric acid contains F, SO4 2- , Fe, Al, Ca, Mg and other impurities. These impurities will exist in the form of phosphate complex salts during the ammonia neutralization process. If the above impurity elements are not fully removed during the ammonia neutralization process, the preparation quality of MAP crystals will be seriously affected; SO4 2- For example, SO4 2- When the content is high, the crystal will grow dislocated, forming X-shaped crystals, which will significantly change the microscopic morphology. Therefore, in the process of preparing high-quality MAP crystals in the prior art, F and SO4 are usually 2-, Fe, Al, Ca, Mg and other impurities need to be removed, which makes the preparation process of MAP crystals lengthy, and many process parameters need to be controlled, resulting in high production costs.

[0037] On the other hand, after obtaining MAP crystals, it is usually necessary to dissolve the MAP crystals to prepare a solution containing MAP, and then mix the MAP-containing solution with an iron source solution and an oxidant to carry out an oxidation reaction. During the reaction, ammonia water is added to adjust the pH value. After the reaction is completed, it is successively subjected to filter pressing, washing, flash drying and calcination to obtain iron phosphate crystals. It can be seen that the process route for preparing iron phosphate crystals from MAP crystals in the prior art is also relatively lengthy, which makes the technical investment of the entire preparation process large and the cost high. Under the premise of ensuring the preparation of high-quality monoammonium phosphate, how to achieve process improvement and reduce costs and increase efficiency has become the core issue of the research.

[0038] In order to solve the problem of lack of simple process, low cost and high quality monoammonium phosphate preparation process in the prior art, the present application provides a phosphate ammonium solution and its preparation method, iron phosphate crystal, lithium iron phosphate positive electrode material and secondary battery. The present application uses wet phosphoric acid as raw material, and adopts two neutralization reaction processes to prepare a phosphate ammonium solution containing monoammonium phosphate, wherein the first neutralization reaction is synchronously defluorinated, and the second neutralization reaction removes impurities such as iron, aluminum, calcium, and magnesium, so as to obtain a high-quality phosphate ammonium solution with less impurities; the phosphate ammonium solution can be directly applied to the preparation of iron phosphate crystals as a raw material, and compared with the existing wet phosphoric acid direct ammonia neutralization process, there is no need to perform desulfurization separately, and the concentration, crystallization, and drying processes are omitted, and the monoammonium phosphate crystals are dissolved again to prepare iron phosphate crystals, which significantly simplifies the preparation process and reduces costs; at the same time, the obtained first filter cake and the second filter cake can be recycled as slow-release nitrogen and phosphorus fertilizers after drying, avoiding the generation of by-product waste, making the process more green and environmentally friendly.

[0039] See also Figure 1 In a first aspect, the present application provides a method for preparing an ammonium phosphate solution, characterized in that the method comprises the following steps: S1, adding a defluorinating agent and a first ammoniacal alkali solution into wet-process phosphoric acid for a neutralization reaction, followed by solid-liquid separation to obtain a first filter cake and a first filtrate; S2, the first filtrate is mixed with the second ammoniacal alkali solution for secondary neutralization reaction, followed by solid-liquid separation to obtain a second filter cake and a second filtrate, the second filtrate being the ammonium phosphate solution; The first filter cake and the second filter cake are dried and recycled as slow-release nitrogen and phosphorus fertilizers; the pH of a neutralization reaction is 4.6-5.2.

[0040] In the technical solution of the embodiment of the present application, wet-process phosphoric acid is used as a raw material, and a two-step neutralization reaction process is used to prepare a phosphate ammonium solution containing monoammonium phosphate, wherein the first neutralization reaction simultaneously removes fluorine, iron, and aluminum, and the second neutralization reaction removes impurities such as calcium and magnesium, thereby obtaining a high-quality phosphate ammonium solution with a low impurity content. Since the phosphate ammonium solution can be directly used as a raw material for the preparation of iron phosphate crystals, there is no need to consider the impurity SO4 2- The influence of the content on the MAP crystals does not need to be considered, and there is no need to consider the regulation of the crystallization process of the MAP crystals. This enables the present application to omit a series of processes related to the preparation of MAP crystals, significantly simplifying the preparation process and reducing production costs.

[0041] Furthermore, in some embodiments, in step S1, a first ammoniacal alkali solution is used to adjust the pH to 4.6-5.2 and a neutralization reaction is carried out, the reaction temperature is 90-100°C, the reaction time is 1-3 h, and after the reaction is completed, the solid-liquid separation is carried out to obtain a first filter cake and a first filtrate; wherein the mass ratio of the defluorinating agent to the wet-process phosphoric acid is (0.2-1.0):100. Specifically, the pH value includes but is not limited to any point value among 4.6, 4.8, 5.0, and 5.2, or any range value between two of them; the reaction temperature includes but is not limited to any point value among 90°C, 93°C, 95°C, 98°C, and 100°C, or any range value between two of them; the reaction time includes but is not limited to any point value among 1 h, 1.5 h, 2 h, 2.5 h, and 3 h, or any range value between two of them; the mass ratio of the defluorinating agent to the wet-process phosphoric acid includes but is not limited to any point value among 0.2:100, 0.4:100, 0.6:100, 0.8:100, and 1.0:100, or any range value between two of them. In this embodiment, a filter press is used for solid-liquid separation. In other embodiments, the specific solid-liquid separation method or device used can be adaptively adjusted according to actual needs, and is not limited here. In this embodiment, the first filter cake can be rinsed after filtration, and after rinsing, the washing water and the first filtrate are combined to enter the subsequent secondary neutralization reaction process. The amount of washing water used and the degree of rinsing can be adaptively set according to actual needs and are not limited here.

[0042] In the technical scheme of the embodiment of the present application, a defluorinating agent and a first ammoniacal alkali solution are introduced into a neutralization reaction, so that the defluorinating reaction and the neutralization reaction are carried out simultaneously, and the simultaneous removal of F, Fe, and Al is mainly achieved by adjusting the pH, reaction temperature, reaction time, and the amount of defluorinating agent used. Specifically, under the condition that the amounts of other components are the same, if the amount of defluorinating agent is too much, although the defluorinating efficiency can be improved, too much defluorinating agent not only increases the cost of raw materials, but its excessive cationic part will combine with phosphate and form insoluble phosphate fluoride, resulting in a decrease in phosphorus yield in the subsequent preparation process; if the amount of defluorinating agent is insufficient, it cannot completely react with the fluorosilicate remaining in the first filtrate after desulfurization, resulting in insufficient defluorination. Regarding the control of the pH value of the primary neutralization reaction, if the pH value is too high (i.e., too much of the first ammoniacal alkali solution is added), the crystallization efficiency of monoammonium phosphate will be significantly reduced, and impurities such as iron, aluminum, calcium, and magnesium will be deposited during the primary neutralization reaction. Excessive precipitation will bring out a portion of phosphorus, resulting in a decrease in the subsequent phosphorus yield; if the pH value is too low (i.e., insufficient amount of the first ammoniacal alkali solution is added), not only will impurities such as iron and aluminum be difficult to remove, but phosphoric acid will also be excessive and unable to fully react to form monoammonium phosphate. Regarding the control of the reaction temperature, if the reaction temperature is too high or the reaction time is too long, although the defluorination efficiency and iron and aluminum removal efficiency can be improved to a certain extent, the improvement effect of the defluorination efficiency and iron and aluminum precipitation efficiency is not obvious after exceeding a certain reaction temperature, but it will greatly increase energy consumption and production costs; if the reaction temperature is low or the reaction time is too short, the defluorination efficiency and iron and aluminum precipitation efficiency will be reduced, which is not conducive to production. Therefore, it is necessary to control the defluorinating agent content, pH, reaction temperature and reaction time in the neutralization reaction process within the above-mentioned appropriate range to obtain the best F, Fe and Al removal effect. There is no need to carry out defluorination and iron and aluminum removal separately, which simplifies the process.

[0043] Specifically, in some embodiments, the defluorinating agent includes at least one of soluble aluminum salts, aluminum hydroxide, aluminum oxide, soluble iron salts, iron hydroxide, and iron oxide; soluble aluminum salts include but are not limited to aluminum sulfate, aluminum nitrate nonahydrate, and aluminum chloride, and soluble iron salts include but are not limited to iron sulfate, iron nitrate nonahydrate, and iron chloride; the first ammonia-containing alkali solution includes at least one of liquid ammonia and ammonia water, and the second ammonia-containing alkali solution includes at least one of liquid ammonia and ammonia water.

[0044] In the technical solution of the embodiment of the present application, soluble iron / aluminum salts, oxides or hydroxides are used as defluorinating agents, and liquid ammonia or ammonia water is used as an alkaline regulator, which can avoid the introduction of new cationic impurities during a neutralization reaction. By controlling the pH, reaction temperature and reaction time during a neutralization reaction, fluorine impurities in wet-process phosphoric acid can be removed in the form of fluorosilicates, and iron and aluminum impurities can be removed in the form of hydroxides.

[0045] More specifically, in some embodiments, the defluorinating agent includes an aluminum component and an iron component, the aluminum component includes at least one of a soluble aluminum salt, aluminum hydroxide, and aluminum oxide, and the iron component includes at least one of a soluble iron salt, iron hydroxide, and iron oxide; the mass ratio of the aluminum component to the iron component includes but is not limited to any point value of 100:10, 100:20, 100:25, 100:30, 100:40, and 100:50 or a range value between any two; further preferably, the defluorinating agent includes a soluble aluminum salt and a soluble iron salt, and the mass ratio of the soluble aluminum salt to the soluble iron salt is 100:25. During the primary neutralization reaction, the aluminum component and the iron component are added separately and mixed with the wet-process phosphoric acid and the first ammonia-containing alkali solution for reaction.

[0046] In the technical scheme of the embodiment of the present application, the aluminum-containing component and the iron-containing component are compounded as a defluorinating agent, which has a better defluorinating effect than a single aluminum-containing component or a single iron-containing component as a defluorinating agent; the compounding method can be regarded as introducing a certain amount of iron-containing component to replace the aluminum-containing component. As the proportion of the iron-containing component increases, the defluorination effect shows a trend of first increasing and then decreasing. It is difficult to obtain the best defluorination effect if the proportion of the iron-containing component is too high or too low, so it is necessary to strictly limit the ratio of the aluminum-containing component to the iron-containing component. At the same time, when the aluminum-containing component and the iron-containing component are compounded as a defluorinating agent, in the actual implementation process, if the aluminum-containing component and the iron-containing component are added at the same time, for the mixed solution system, the two components exist at the same time than only containing a single component, and the solubility is lower, which makes it difficult for the compound component to give full play to the defluorination effect, so it is necessary to add the aluminum-containing component and the iron-containing component separately to make it play a better defluorination effect; among them, the order of applying wet-process phosphoric acid, aluminum-containing component, iron-containing component, and the first ammonia-containing alkali solution has a variety of ways, which can be set according to actual needs, and is not limited here.

[0047] Further, in some embodiments, the secondary neutralization reaction in step S2 includes the following steps: adding a second ammoniacal alkali solution to control the pH of the first filtrate to 6.5-7.5, the reaction temperature is 50-70°C, the reaction time is 0.5-2 h, and after the reaction is completed, solid-liquid separation is performed to obtain a second filter cake and a second filtrate. Specifically, the pH value includes but is not limited to any point value of 6.5, 6.7, 7.0, 7.2, and 7.5 or a range value between any two, the reaction temperature includes but is not limited to any point value of 50°C, 55°C, 60°C, 65°C, and 70°C or a range value between any two, and the reaction time includes but is not limited to any point value of 0.5 h, 1 h, 1.5 h, and 2h or a range value between any two. In this embodiment, a filter press is used for solid-liquid separation. In other embodiments, the specific solid-liquid separation method or device used can be adaptively adjusted according to actual needs, and is not limited here.

[0048] In the technical scheme of the embodiment of the present application, a second ammoniacal alkali solution is added to the first filtrate after the primary neutralization reaction for a secondary neutralization reaction, and the pH value of the secondary neutralization reaction is adjusted by the second ammoniacal alkali solution, while the reaction temperature and time are controlled. Since the pH value of the secondary neutralization reaction is higher than that of the primary neutralization reaction, the process is mainly used to remove calcium and magnesium impurities, and further remove impurities such as F, Fe, and Al, so that the impurity content in the obtained second filtrate is further reduced and the phosphorus yield is high, and it can be used as a direct raw material for preparing iron phosphate. Specifically, for the control of the pH value of the secondary neutralization reaction, as the pH value increases, the crystallization efficiency of monoammonium phosphate tends to increase first and then decrease. If the pH value is too high (i.e., too much of the second ammoniacal alkali solution is added), the crystallization efficiency of monoammonium phosphate will be significantly reduced; if the pH value is too low (i.e., the amount of the second ammoniacal alkali solution added is insufficient), not only calcium and magnesium impurities are difficult to remove, but also excess phosphoric acid cannot be fully reacted to form monoammonium phosphate. Regarding the control of the secondary neutralization reaction temperature, if the temperature is too high, the generation of monoammonium phosphate will be accelerated. At the same time, the solubility of monoammonium phosphate increases sharply with the increase of temperature, and the amount of ammonia released increases with the temperature, thus affecting the stability of the generated monoammonium phosphate; if the temperature is too low, the generation of monoammonium phosphate is slow, and calcium and magnesium impurities are difficult to fully remove. Regarding the control of the secondary neutralization reaction time, the impurity removal efficiency gradually increases with the extension of the reaction time, and gradually slows down after reaching a certain level. Therefore, appropriately extending the reaction time is helpful to improve the impurity removal efficiency.

[0049] For further information, see Figure 2 In another embodiment, the secondary neutralization reaction process includes the following steps: adding a second ammoniacal alkali solution to control the pH of the first filtrate to 8.0-9.0, the reaction temperature is 50-70°C, the reaction time is 0.5-2 h, and solid-liquid separation is performed to obtain a filtrate; thereafter, phosphoric acid is added to the filtrate to adjust the pH to 6.5-7.5, the reaction temperature is 50-70°C, the reaction time is 0.5-2 h, and an ammonium phosphate solution is obtained; wherein the purity of the phosphoric acid is greater than 85%.

[0050] Specifically, during the secondary neutralization reaction, the regulation of the pH value includes two stages. The first stage is to control the pH within the range of 8.0-9.0 by adding a second ammoniacal alkali solution, and obtain a filtrate by solid-liquid separation, wherein the pH value includes but is not limited to any point value among 8.2, 8.3, 8.5, 8.7, and 9.0 or a range value between any two of them, the reaction temperature includes but is not limited to any point value among 50°C, 55°C, 60°C, 65°C, and 70°C or a range value between any two of them, and the reaction time includes but is not limited to any point value among 0.5 h, 1 h, 1.5 h, and 2 h or a range value between any two of them; the second stage is to control the pH of the filtrate within the range of 6.5-7.5 by adding phosphoric acid, wherein the pH value includes but is not limited to any point value among 6.5, 6.7, 7.0, 7.2, and 7.5 or a range value between any two of them, the reaction temperature includes but is not limited to any point value among 50°C, 55°C, 60°C, 65°C, and 70°C or a range value between any two of them, and the reaction time includes but is not limited to 0.5 h, 1h, 1.5h, 2h, or any point value or range value between any two of them. The phosphoric acid is high-purity phosphoric acid, and the high-purity phosphoric acid can be selected from chemically pure, analytically pure, premium pure, super pure and above phosphoric acid preparations as an acidic pH regulator, which is not limited here.

[0051] In the technical scheme of the embodiment of the present application, a different method from the above is provided for adjusting the pH during the secondary neutralization reaction. Since the method first adds a second ammoniacal alkali solution to adjust the pH to neutral or alkaline, and too high a pH will inhibit the formation of monoammonium phosphate, it is necessary to add high-purity phosphoric acid for neutralization reaction again to adjust the pH to neutral or acidic to obtain a phosphate ammonium solution with a high P2O5 content. Compared with the above-mentioned secondary neutralization reaction method, the method of adjusting the pH after over-alkalinity adopted in this embodiment can further improve the removal effect of Fe, Al, Ca, and Mg, and further reduce the impurity content to improve the quality of the obtained phosphate ammonium solution. As for the adjustment method of the reaction temperature and time of the neutralization reaction in this embodiment, the adjustment method of the relevant process parameters of the above-mentioned secondary neutralization reaction is the same as that of the above-mentioned secondary neutralization reaction, which will not be repeated here.

[0052] In a second aspect, an embodiment of the present application provides a phosphate ammonium solution, which is prepared by the preparation method of the phosphate ammonium solution in the first aspect mentioned above.

[0053] In the technical solution of the embodiment of the present application, the prepared ammonium phosphate solution has a low impurity content and high quality, and can be directly used as a raw material for the preparation of iron phosphate crystals. Compared with the existing wet phosphoric acid direct ammonia neutralization process, it significantly simplifies the preparation process and reduces costs.

[0054] In the third aspect, an embodiment of the present application provides an iron phosphate crystal, which is prepared by a mixed reaction of an iron salt solution, an oxidant, and the ammonium phosphate solution in the second aspect of the present application; wherein the iron salt in the iron salt solution includes at least one of ferrous sulfate, ferrous chloride, ferric sulfate, and ferric chloride, and the oxidant includes hydrogen peroxide.

[0055] In the technical solution of the embodiment of the present application, there is no need to dissolve the monoammonium phosphate crystals again, and iron phosphate crystals are directly prepared using the aforementioned ammonium phosphate solution as raw material. The iron phosphate crystals prepared from the aforementioned ammonium phosphate solution have excellent performance, and the lithium iron phosphate positive electrode material prepared from the iron phosphate crystals has a higher specific capacity and better rate performance.

[0056] In a fourth aspect, an embodiment of the present application provides a lithium iron manganese phosphate positive electrode material, which is obtained by a mixed reaction of the iron phosphate crystals provided in the third aspect of the present application and a lithium source.

[0057] In the technical solution of the embodiment of the present application, the lithium iron phosphate manganese positive electrode material is prepared by uniformly mixing the industrial-grade iron phosphate crystals provided in the third aspect of the present application with a lithium source and then adopting a high-temperature solid-phase method; or by uniformly mixing the aforementioned industrial-grade iron phosphate crystals, a lithium source, and a carbon source and then adopting a high-temperature solid-phase method. Among them, the molar ratio of iron phosphate crystals to lithium source is 1: (1~1.03), the molar ratio of iron phosphate crystals to carbon source is 1: (0.01~0.2), the lithium source includes at least one of lithium carbonate and lithium hydroxide, and the carbon source includes at least one of glucose, sucrose, fructose, cellulose, starch, citric acid, polyacrylic acid, polyethylene glycol and dopamine. The high-temperature solid phase method is high-temperature gradient calcination. During the high-temperature gradient calcination, the calcination temperature of the first stage is 350~450℃, and the insulation time of the first stage is 2~6h; the calcination temperature of the second stage is 760~790℃, and the insulation time of the second stage is 2~6h; the calcination temperature of the third stage is 600~700℃, and the insulation time of the third stage is 2~6h. The iron manganese phosphate lithium positive electrode material of the present application is prepared from the above-mentioned iron phosphate crystals, so it has the advantages of high specific capacity and good rate performance. In other embodiments, other preparation methods or other lithium sources and carbon sources can be selected as raw materials according to actual needs, which are not limited here.

[0058] In a fifth aspect, an embodiment of the present application provides a secondary battery, the positive electrode plate of the secondary battery includes the lithium iron manganese phosphate positive electrode material provided in the fourth aspect of the present application.

[0059] The secondary battery of the present application includes the aforementioned positive electrode plate, and thus has the advantages of high specific capacity and good rate performance.

[0060] Some specific examples are listed below. It should be noted that the examples described below are exemplary and are only used to explain the present application, and cannot be understood as limiting the present application. If no specific technology or conditions are specified in the examples, the technology or conditions described in the literature in the field or the product instructions are used. The reagents or instruments used without indicating the manufacturer are all conventional products that can be purchased commercially. Among them, the composition analysis of the wet-process phosphoric acid used in the following examples and comparative examples is shown in Table 1: Table 1 Composition analysis of wet-process phosphoric acid

[0061] 1. Preparation method Example 1 The specific steps of preparing the ammonium phosphate solution in this embodiment are as follows: (1) Aluminum nitrate nonahydrate and liquid alkali are added to wet-process phosphoric acid for a neutralization reaction. The aluminum nitrate nonahydrate accounts for 0.5% of the mass of the wet-process phosphoric acid. The slurry pH is controlled at 5.0. The reaction temperature is 95°C and the reaction time is 2 h. Subsequently, a first filter cake and a first filtrate are obtained by filter pressing.

[0062] (2) The first filter cake is washed with water, the washing water and the first filtrate are pooled, and liquid alkali is added to carry out a secondary neutralization reaction. The pH of the slurry is controlled at 7.0, the reaction temperature is 60°C, and the reaction time is 1 h. Then, the second filter cake and the second filtrate are obtained by filter pressing. The second filtrate is the ammonium phosphate solution.

[0063] (3) The first filter cake and the second filter cake are dried and recycled as slow-release nitrogen and phosphorus fertilizers.

[0064] Example 2 The specific steps of preparing the ammonium phosphate solution in this embodiment are as follows: (1) Ferric nitrate nonahydrate and liquid alkali are added to wet-process phosphoric acid for a neutralization reaction. Ferric nitrate nonahydrate accounts for 0.5% of the mass of the wet-process phosphoric acid. The pH of the slurry is controlled at 5.0. The reaction temperature is 95°C and the reaction time is 2 h. Subsequently, a first filter cake and a first filtrate are obtained by filter pressing.

[0065] (2) The first filter cake is washed with water, the washing water and the first filtrate are pooled, and liquid alkali is added to carry out a secondary neutralization reaction. The pH of the slurry is controlled at 7.0, the reaction temperature is 60°C, and the reaction time is 1 h. Then, the second filter cake and the second filtrate are obtained by filter pressing. The second filtrate is the ammonium phosphate solution.

[0066] (3) The first filter cake and the second filter cake are dried and recycled as slow-release nitrogen and phosphorus fertilizers.

[0067] Example 3 The specific steps of preparing the ammonium phosphate solution in this embodiment are as follows: (1) Ferric nitrate nonahydrate, liquid alkali, and aluminum nitrate nonahydrate are sequentially added to wet-process phosphoric acid for a neutralization reaction, wherein the total amount of ferric nitrate nonahydrate and aluminum nitrate nonahydrate accounts for 0.5% of the mass of the wet-process phosphoric acid, and the mass ratio of ferric nitrate nonahydrate to aluminum nitrate nonahydrate is 25:100. The pH of the slurry is controlled at 5.0, the reaction temperature is 95°C, the reaction time is 2 h, and then filter pressing is performed to obtain a first filter cake and a first filtrate.

[0068] (2) The first filter cake is washed with water, the washing water and the first filtrate are pooled, and liquid alkali is added to carry out a secondary neutralization reaction. The pH of the slurry is controlled at 7.0, the reaction temperature is 60°C, and the reaction time is 1 h. Then, the second filter cake and the second filtrate are obtained by filter pressing. The second filtrate is the ammonium phosphate solution.

[0069] (3) The first filter cake and the second filter cake are dried and recycled as slow-release nitrogen and phosphorus fertilizers.

[0070] Example 4 Compared with Example 3, the only difference is that in step (1), the mass ratio of ferric nitrate nonahydrate to aluminum nitrate nonahydrate is 10:100.

[0071] Example 5 Compared with Example 3, the only difference is that in step (1), the mass ratio of ferric nitrate nonahydrate to aluminum nitrate nonahydrate is 50:100.

[0072] Example 6 Compared with Example 3, the only difference is that in step (1), the total amount of ferric nitrate nonahydrate and aluminum nitrate nonahydrate accounts for 0.2% of the mass of wet-process phosphoric acid.

[0073] Example 7 Compared with Example 3, the only difference is that in step (1), the total amount of ferric nitrate nonahydrate and aluminum nitrate nonahydrate accounts for 1.0% of the mass of wet-process phosphoric acid.

[0074] Example 8 Compared with Example 3, the only difference is that in step (1), the pH of the slurry is controlled at 4.6 during the neutralization reaction.

[0075] Example 9 Compared with Example 3, the only difference is that in step (1), the pH of the slurry is controlled at 5.2 during the neutralization reaction.

[0076] Example 10 Compared with Example 3, the only difference is that in step (1), the reaction temperature during the neutralization reaction is 90°C.

[0077] Embodiment 11 Compared with Example 3, the only difference is that in step (1), the reaction temperature during the neutralization reaction is 100°C.

[0078] Example 12 Compared with Example 3, the only difference is that in step (2), the pH of the slurry is controlled at 6.5 during the secondary neutralization reaction.

[0079] Embodiment 13 Compared with Example 3, the only difference is that in step (2), the pH of the slurry is controlled at 7.5 during the secondary neutralization reaction.

[0080] Embodiment 14 Compared with Example 3, the only difference is that in step (2), the reaction temperature during the secondary neutralization reaction is 50°C.

[0081] Embodiment 15 Compared with Example 3, the only difference is that in step (2), the reaction temperature during the secondary neutralization reaction is 70°C.

[0082] Example 16 Compared with Example 3, the only difference is that in step (2), the first filter cake is washed with water, the washing water and the first filtrate are pooled, and liquid alkali is added for neutralization reaction, the slurry pH is controlled at 8.0, the reaction temperature is 60°C, and the reaction time is 1 h. Subsequently, filter press is performed to obtain a second filter cake and a second filtrate, and high-purity phosphoric acid (purity>85%) is added to the second filtrate and neutralization reaction is carried out again. During this neutralization reaction, the slurry pH is controlled at 7.0, the reaction temperature is 60°C, and the reaction time is 1 h. The solution obtained after the reaction is an ammonium phosphate solution.

[0083] Embodiment 17 Compared with Example 3, the only difference is that in step (2), the first filter cake is washed with water, the washing water and the first filtrate are pooled, and liquid alkali is added for neutralization reaction, the slurry pH is controlled at 8.5, the reaction temperature is 60°C, and the reaction time is 1 h. Subsequently, the second filter cake and the second filtrate are obtained by filter pressing, and high-purity phosphoric acid (purity>85%) is added to the second filtrate and neutralization reaction is carried out again. During this neutralization reaction, the slurry pH is controlled at 7.0, the reaction temperature is 60°C, and the reaction time is 1 h. The solution obtained after the reaction is the ammonium phosphate solution.

[0084] Embodiment 18 Compared with Example 3, the only difference is that in step (2), the first filter cake is washed with water, the washing water and the first filtrate are pooled, and liquid alkali is added for neutralization reaction, the slurry pH is controlled at 9.0, the reaction temperature is 60°C, and the reaction time is 1 h. Subsequently, filter press is performed to obtain a second filter cake and a second filtrate, and high-purity phosphoric acid (purity>85%) is added to the second filtrate and neutralization reaction is carried out again. During this neutralization reaction, the slurry pH is controlled at 7.0, the reaction temperature is 60°C, and the reaction time is 1 h. The solution obtained after the reaction is an ammonium phosphate solution.

[0085] Embodiment 19 Compared with Example 3, the only difference is that in step (2), the first filter cake is washed with water, the washing water and the first filtrate are pooled, and liquid alkali is added for neutralization reaction, the slurry pH is controlled at 8.5, the reaction temperature is 60°C, and the reaction time is 1 h. Subsequently, filter press is performed to obtain a second filter cake and a second filtrate, and high-purity phosphoric acid (purity>85%) is added to the second filtrate and neutralization reaction is carried out again. During this neutralization reaction, the slurry pH is controlled at 6.5, the reaction temperature is 60°C, and the reaction time is 1 h. The solution obtained after the reaction is an ammonium phosphate solution.

[0086] Embodiment 20 Compared with Example 3, the only difference is that in step (2), the first filter cake is washed with water, the washing water and the first filtrate are pooled, and liquid alkali is added for neutralization reaction, the slurry pH is controlled at 8.5, the reaction temperature is 60°C, and the reaction time is 1 h. Subsequently, filter press is performed to obtain a second filter cake and a second filtrate, and high-purity phosphoric acid (purity>85%) is added to the second filtrate and neutralization reaction is carried out again. During this neutralization reaction, the slurry pH is controlled at 7.5, the reaction temperature is 60°C, and the reaction time is 1 h. The solution obtained after the reaction is an ammonium phosphate solution.

[0087] Comparative Example 1 Compared with Example 3, the only difference is that in step (1), the mass ratio of ferric sulfate to aluminum sulfate is 5:100.

[0088] Comparative Example 2 Compared with Example 3, the only difference is that in step (1), the mass ratio of ferric sulfate to aluminum sulfate is 70:100.

[0089] Comparative Example 3 Compared with Example 3, the only difference is that in step (1), the total amount of iron sulfate and aluminum sulfate accounts for 0.05% of the mass of wet-process phosphoric acid.

[0090] Comparative Example 4 Compared with Example 3, the only difference is that in step (1), the total amount of iron sulfate and aluminum sulfate accounts for 2.0% of the mass of wet-process phosphoric acid.

[0091] Comparative Example 5 Compared with Example 3, the only difference is that in step (1), the pH of the slurry is controlled at 4.0 during the neutralization reaction.

[0092] Comparative Example 6 Compared with Example 3, the only difference is that in step (1), the pH of the slurry is controlled at 6.0 during the neutralization reaction.

[0093] Comparative Example 7 Compared with Example 3, the only difference is that in step (1), the reaction temperature during the neutralization reaction is 70°C.

[0094] Comparative Example 8 Compared with Example 3, the only difference is that in step (1), the reaction temperature during the neutralization reaction is 120°C.

[0095] Comparative Example 9 Compared with Example 3, the only difference is that in step (2), the pH of the slurry is controlled at 5.5 during the secondary neutralization reaction.

[0096] Comparative Example 10 Compared with Example 3, the only difference is that in step (2), the pH of the slurry is controlled at 8.5 during the secondary neutralization reaction.

[0097] Comparative Example 11 Compared with Example 3, the only difference is that in step (2), the reaction temperature during the secondary neutralization reaction is 40°C.

[0098] Comparative Example 12 Compared with Example 3, the only difference is that in step (2), the reaction temperature during the secondary neutralization reaction is 80°C.

[0099] Comparative Example 13 Compared with Example 3, the only difference is that in step (2), the first filter cake is washed with water, the washing water and the first filtrate are pooled, and liquid alkali is added for neutralization reaction, the slurry pH is controlled at 7.5, the reaction temperature is 60°C, and the reaction time is 1 h. Subsequently, filter press is performed to obtain a second filter cake and a second filtrate, and high-purity phosphoric acid (purity>85%) is added to the second filtrate and neutralization reaction is carried out again. During this neutralization reaction, the slurry pH is controlled at 7.0, the reaction temperature is 60°C, and the reaction time is 1 h. The solution obtained after the reaction is an ammonium phosphate solution.

[0100] Comparative Example 14 Compared with Example 3, the only difference is that in step (2), the first filter cake is washed with water, the washing water and the first filtrate are pooled, and liquid alkali is added for neutralization reaction, the slurry pH is controlled at 9.5, the reaction temperature is 60°C, and the reaction time is 1 h. Then, the second filter cake and the second filtrate are obtained by filter pressing, and high-purity phosphoric acid (purity>85%) is added to the second filtrate and neutralization reaction is carried out again. During this neutralization reaction, the slurry pH is controlled at 7.0, the reaction temperature is 60°C, and the reaction time is 1 h. The solution obtained after the reaction is the ammonium phosphate solution.

[0101] Comparative Example 15 Compared with Example 3, the only difference is that in step (2), the first filter cake is washed with water, the washing water and the first filtrate are pooled, and liquid alkali is added for neutralization reaction, the slurry pH is controlled at 8.5, the reaction temperature is 60°C, and the reaction time is 1 h. Subsequently, filter press is performed to obtain a second filter cake and a second filtrate, and high-purity phosphoric acid (purity>85%) is added to the second filtrate and neutralization reaction is carried out again. During this neutralization reaction, the slurry pH is controlled at 6.0, the reaction temperature is 60°C, and the reaction time is 1 h. The solution obtained after the reaction is an ammonium phosphate solution.

[0102] Comparative Example 16 Compared with Example 3, the only difference is that in step (2), the first filter cake is washed with water, the washing water and the first filtrate are pooled, and liquid alkali is added for neutralization reaction, the slurry pH is controlled at 8.5, the reaction temperature is 60°C, and the reaction time is 1 h. Subsequently, filter press is performed to obtain a second filter cake and a second filtrate, and high-purity phosphoric acid (purity>85%) is added to the second filtrate and neutralization reaction is carried out again. During this neutralization reaction, the slurry pH is controlled at 8.0, the reaction temperature is 60°C, and the reaction time is 1 h. The solution obtained after the reaction is an ammonium phosphate solution.

[0103] Comparative Example 17 Compared with Comparative Example 16, the only difference is that in step (2), the first filter cake is washed with water, the washing water and the first filtrate are pooled, and liquid alkali is added for neutralization reaction, the slurry pH is controlled at 8.5, the reaction temperature is 60°C, the reaction time is 1 h, and then the second filter cake and the second filtrate are obtained by filter pressing, and the neutralization reaction is not performed again to adjust the pH. The obtained second filtrate is the ammonium phosphate solution.

[0104] 2. Test Method 1. Phosphorus pentoxide test The general method for determining the content of phosphorus pentoxide in inorganic chemical products, namely the quinoline molybdate weight method, was used to determine the P2O5 in the test solution. The principle of this method is that in an acidic medium, the phosphorus pentoxide in the test solution and the added precipitant quinoline molybdate form a precipitate, and the P2O5 content is calculated by filtering, drying and weighing.

[0105] Test reagents: hydrochloric acid or nitric acid, quinoline molybdate solution.

[0106] Test equipment: glass sand crucible (filter plate pore size is 5 pm~15 pm), electric constant temperature drying oven (temperature controlled at 180 ℃±5℃ or 250 ℃±10℃).

[0107] Test steps: 1) Preparation of test solution: Weigh an appropriate amount of sample (containing about 600 mg of phosphorus pentoxide), place in a 100 mL beaker, add 20 mL of water and 5 mL of hydrochloric acid or nitric acid, cover with a watch glass, and boil for 10 min. After cooling, transfer to a 500 mL volumetric flask, add 10 mL of hydrochloric acid or nitric acid, dilute to the mark with water, and shake well.

[0108] 2) Preparation of blank test solution: Except for not adding sample, the amount of other reagents added is exactly the same as that of the test solution, and they are processed in the same way as the sample.

[0109] 3) Determination and calculation: Use a pipette to transfer 10 mL of the test solution and blank test solution to a 250 mL beaker, and add water to a total volume of about 100 mL. Cover with a watch glass, heat in a water bath until the temperature of the contents reaches 75℃±5℃, maintain for 30 seconds, and add 35 mL of quinoline molybdate solution. Cool to room temperature, and stir 3 to 4 times during the cooling process. Use a glass crucible that has been dried to a constant mass at 180℃±5℃ or 250℃±10℃ to filter the supernatant liquid, and wash the precipitate 5 to 6 times by decantation, using about 20 mL of water each time. Transfer the precipitate to a glass crucible and continue to wash with water 3 to 4 times. Place the glass crucible in an electric constant temperature drying oven at 180℃±5℃ for 45 minutes or at 250℃±10℃ for 15 minutes, take it out, place it in a desiccator to cool to room temperature, weigh it, and calculate and count the mass percentage of phosphorus pentoxide.

[0110] 3. Fluoride ion test The sample to be tested is decomposed with hydrochloric acid, and the pH of the solution is controlled to maintain 5.5-6.0 with citric acid-sodium citrate buffer solution, while eliminating the interference of aluminum, iron and other ions. A potentiometer is used, with a saturated calomel electrode as the reference electrode and a fluoride ion selective electrode as the indicator electrode, to measure the electrode potential, and the fluoride content is calculated using the working curve method.

[0111] Test reagents: hydrochloric acid, nitric acid, sodium hydroxide (200 g / L), citric acid-sodium citrate buffer solution (weigh 24 g citric acid, 270 g trisodium citrate, dissolve in water, and dilute to 1000 mL, mix well, pH 5.5-6.0), fluorine standard solution (1.0 mg / mL), bromocresol green indicator solution (1 g / L).

[0112] Test equipment: fluoride ion selective electrode, saturated calomel electrode, potentiometer, magnetic stirrer.

[0113] Test steps: 1) The sample is passed through a 125 μm test sieve, dried at 105-110°C for more than 2 h, and placed in a desiccator to cool to room temperature.

[0114] 2) Weigh 0.1-0.2 g of the sample, accurate to 0.0001 g, place in a 50 mL beaker, moisten with a small amount of water, add 10 mL of hydrochloric acid solution, stir for a while, let stand for 30 min, and stir every 10 min. Transfer to a 100 mL volumetric flask, dilute to the mark with water, and shake well. Let stand to clarify or dry filter, and take 10.0 mL of the clear solution and place in a 50 mL volumetric flask.

[0115] 3) Add five drops of citric acid-sodium citrate buffer solution and two drops of bromocresol green indicator solution, neutralize with sodium hydroxide solution until the solution turns blue, and then adjust the solution to yellow with nitric acid solution. Add 20 mL of citric acid-sodium citrate buffer solution, dilute to the mark with water, shake well, and pour into a dry 50 mL beaker.

[0116] 4) Insert a fluoride ion selective electrode and a saturated calomel electrode, measure the potential value at equilibrium under constant stirring with a magnetic stirrer, and find the corresponding fluoride amount on the working curve.

[0117] 5) Take 1.0, 2.0, 3.0, 5.0, and 10.0 mL of fluorine standard solution and place them in a set of 50 mL volumetric flasks, respectively, and add 1.0 mL of hydrochloric acid solution. Measure the potential value at equilibrium according to 7.2 to 7.3. Draw a working curve on semi-logarithmic coordinate paper with the potential value (mV) as the ordinate and the corresponding fluorine amount (mg) as the abscissa, and calculate the corresponding fluoride ion content.

[0118] 3. Product component testing The components of the solutions prepared in the examples and comparative examples were tested using an inductively coupled plasma spectrometer (ICP).

[0119] III. Analysis of test results of various embodiments and comparative examples I. Test and analyze the effects of the process conditions of the single neutralization reaction of the present application on the impurity removal effect and phosphorus yield. Specifically referring to Examples 1 to 11 and Comparative Examples 1 to 8, the parameters of the selection of defluorinating agent components, the compounding ratio of the defluorinating agent, the amount of defluorinating agent added, the pH of the single neutralization reaction, and the temperature of the single neutralization reaction were adjusted, and the impurity content (ppm) and the P2O5 yield (%) in the first filtrate were tested, wherein the amount of the defluorinating agent added was expressed as the mass percentage of the defluorinating agent relative to the wet-process phosphoric acid, and the impurities measured included Al, Fe, Mg and F, as shown in Table 2.

[0120] Table 2

[0121] It can be seen from Table 2 that: A) As for the selection of defluorinating agent components, it can be seen from Examples 1 to 5 that single aluminum nitrate nonahydrate exhibits a high P2O5 yield but a high impurity content, and single ferric nitrate nonahydrate exhibits a low impurity content but a low P2O5 yield. Compared with the same mass of single aluminum nitrate nonahydrate or single ferric nitrate nonahydrate, the compounding method of aluminum nitrate nonahydrate + ferric nitrate nonahydrate can achieve a more balanced performance in terms of impurity removal effect and P2O5 yield, thereby proving that the use of a compound of aluminum nitrate nonahydrate + ferric nitrate nonahydrate as a defluorinating agent can achieve a high P2O5 yield and a low impurity content.

[0122] B) As for the compounding ratio of the defluorinating agent, it can be seen from Examples 3 to 5 and Comparative Examples 1 to 2 that by using a compound defluorinating agent of aluminum nitrate nonahydrate + ferric nitrate nonahydrate, the ratio of the two is adjusted under the condition that the total mass of the compound defluorinating agent remains unchanged. As the ratio of the iron-containing component increases, the defluorination effect first increases and then decreases. It is difficult to obtain the best defluorination effect when the iron-containing component ratio is too high or too low. Therefore, it is necessary to limit the ratio of the aluminum-containing component to the iron-containing component in order to enable the compound defluorinating agent to obtain a better defluorination effect.

[0123] C) As for the amount of defluorinating agent added, it can be seen from Examples 3, 6-7 and Comparative Examples 3-4 that a composite defluorinating agent of aluminum nitrate nonahydrate + ferric nitrate nonahydrate is used and the relative proportions of aluminum nitrate nonahydrate and ferric nitrate nonahydrate remain unchanged. As the total mass proportion of the composite defluorinating agent gradually increases, the impurity content gradually decreases, but the P2O5 yield also gradually decreases, indicating that excessive addition of the defluorinating agent will be detrimental to the improvement of the P2O5 yield. It is more appropriate to control the mass ratio of the defluorinating agent to the wet-process phosphoric acid at (0.2-1.0):100.

[0124] D) For the pH of the primary neutralization reaction, it can be seen from Examples 3, 8-9 and Comparative Examples 5-6 that when a composite defluorinating agent of aluminum nitrate nonahydrate + ferric nitrate nonahydrate is used and the relative proportions of aluminum nitrate nonahydrate and ferric nitrate nonahydrate remain unchanged, when the pH of the primary neutralization reaction is too low, the P2O5 yield is high but the impurity content is also high; when the pH of the primary neutralization reaction is too high, the impurity content is low but the P2O5 yield is also low, and it is more appropriate to control the pH of the primary neutralization reaction at 4.6-5.2.

[0125] E) For the primary neutralization reaction temperature, in combination with Examples 3, 10-11 and Comparative Examples 7-8, it can be seen that when a composite defluorinating agent of aluminum nitrate nonahydrate + ferric nitrate nonahydrate is used and the relative proportions of aluminum nitrate nonahydrate and ferric nitrate nonahydrate remain unchanged, when the primary neutralization reaction temperature is too low, the P2O5 yield is high, but impurities are difficult to fully remove; when the primary neutralization reaction temperature is too high, although the impurities can be fully removed, the P2O5 yield decreases again, and it is more appropriate to control the primary neutralization reaction temperature at 90-100°C.

[0126] II. Test and analyze the influence of the secondary neutralization reaction process conditions on the impurity removal effect and phosphorus yield when the present application adopts the secondary neutralization reaction mode without alkali. Specifically, the reaction process of the secondary neutralization reaction process is subdivided and recorded as the first stage and the second stage in sequence. The aforementioned secondary neutralization reaction process without alkali only contains the first stage, which represents the neutralization reaction without alkali; and the aforementioned secondary neutralization reaction process with pH adjusted after alkali includes the first stage and the second stage, wherein the first stage represents the neutralization reaction after adding liquid alkali, and the second stage represents the neutralization reaction after adding phosphoric acid. Specifically refer to Examples 3, 12-15 and Comparative Examples 9-12, adjust the parameters of the secondary neutralization reaction pH and the secondary neutralization reaction temperature, and test the impurity content (ppm) and P2O5 yield (%) in the second filtrate, wherein the measured impurities include Mg and F, as shown in Table 3.

[0127] Table 3

[0128] It can be seen from Table 3 that: A) For the secondary neutralization reaction pH, it can be seen from Examples 3, 12-13 and Comparative Examples 9-10 that, under the premise that the secondary neutralization pH is not too alkaline, as the pH value increases, the impurity content gradually decreases, while the P2O5 yield of the second filtrate shows a trend of first increasing and then decreasing, which proves that it is difficult to obtain a good P2O5 yield if the secondary neutralization pH is too high or too low, and it is more appropriate to control the secondary neutralization reaction pH at 6.5-7.5.

[0129] B) As for the secondary neutralization reaction temperature, it can be seen from Examples 3, 14-15 and Comparative Examples 11-12 that, under the premise that the secondary neutralization pH is not too alkaline, as the temperature increases, the impurity content gradually decreases, while the P2O5 yield of the second filtrate shows a decreasing trend, which proves that too high a secondary neutralization temperature will affect the formation of monoammonium phosphate, and it is more appropriate to control the secondary neutralization reaction temperature at 50-70°C.

[0130] III. Test and analyze the effects of the conditions of the first and second stages of the secondary neutralization process on the impurity removal effect and phosphorus yield when the present application adopts a secondary neutralization reaction mode of adjusting the pH after alkali (i.e., the pH of the secondary neutralization reaction is adjusted after the pH is adjusted in the first stage and in the second stage). Specifically referring to Examples 3, 16-20 and Comparative Examples 13-17, the parameters of the first stage pH, the second stage pH, and whether the second stage is performed after the first stage alkali are adjusted are adjusted, and the impurity content (ppm) and P2O5 yield (%) in the second filtrate are tested, wherein the impurities measured include Mg and F, as shown in Table 4.

[0131] Table 4

[0132] It can be seen from Table 4 that: A) For the pH of the first stage, it can be seen from Examples 16 to 18 and Comparative Examples 13 to 14 that, under the premise that the pH of the first stage is too alkaline, high-purity phosphoric acid needs to be added to adjust the pH of the second stage neutralization reaction. At this time, the pH of the second stage remains unchanged. In the first stage, as the pH value increases, the impurity content gradually decreases, and the P2O5 yield of the ammonium phosphate solution also shows a decreasing trend. At this time, it is more appropriate to control the pH of the first stage of the secondary neutralization reaction at 8.0 to 9.0.

[0133] B) As for the pH of the second stage, it can be seen from Examples 17, 19-20 and Comparative Examples 15-16 that, under the premise that the pH of the first stage is too alkaline, high-purity phosphoric acid needs to be added to adjust the pH of the second stage neutralization reaction. At this time, the pH of the first stage remains unchanged, and the impurity content gradually decreases with the increase of pH in the second stage, while the yield of P2O5 of the ammonium phosphate solution shows a trend of first increasing and then decreasing. At this time, it is more appropriate to control the pH of the second stage of the secondary neutralization reaction at 6.5-7.5.

[0134] C) As to whether the second stage of the secondary neutralization reaction is performed after the alkali treatment, it can be seen from the comparison between Example 3 and Comparative Example 10 that if the pH adjustment of the neutralization reaction in the second stage is not performed, when the neutralization reaction in the first stage is too alkaline, the P2O5 yield will be reduced, which is not conducive to the preparation of the ammonium phosphate solution. At the same time, it can be seen from the comparison between Example 3 and Example 17 that the pH adjustment after the secondary neutralization reaction is too alkaline, compared with the method of not performing the secondary neutralization reaction with alkali treatment, the P2O5 yield of the ammonium phosphate solution is slightly reduced, but the impurity removal effect is improved to a certain extent. Therefore, it is easier to obtain an ammonium phosphate solution with fewer impurities and higher quality by performing the pH adjustment after the secondary neutralization reaction is too alkaline.

[0135] In summary, the ammonium phosphate solution prepared in the present application has low impurity content and excellent quality, and the LMFP positive electrode material prepared from the ammonium phosphate solution has good electrochemical properties.

[0136] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A method for preparing an ammonium phosphate solution, characterized in that: The following steps are involved: Adding a defluorinating agent and a first ammoniacal alkali solution into wet-process phosphoric acid for a neutralization reaction, followed by solid-liquid separation to obtain a first filter cake and a first filtrate; The first filtrate is mixed with the second ammoniacal alkali solution to undergo a secondary neutralization reaction, followed by solid-liquid separation to obtain a second filter cake and a second filtrate, wherein the second filtrate is an ammonium phosphate solution; The first filter cake and the second filter cake are dried and recycled as slow-release nitrogen and phosphorus fertilizers; The pH of the primary neutralization reaction is 4.6-5.

2.

2. The method for preparing the ammonium phosphate solution according to claim 1, characterized in that: During the primary neutralization reaction, the reaction temperature is 90-100° C., the reaction time is 1-3 h, and the mass ratio of the defluorinating agent to the wet-process phosphoric acid is (0.2-1.0):

100.

3. The method for preparing the ammonium phosphate solution according to claim 2, characterized in that: The defluorinating agent includes at least one of soluble aluminum salt, aluminum hydroxide, aluminum oxide, soluble iron salt, iron hydroxide, and iron oxide; and / or The first ammonia-containing alkaline solution includes at least one of liquid ammonia and aqueous ammonia, and the second ammonia-containing alkaline solution includes at least one of liquid ammonia and aqueous ammonia.

4. The method for preparing the ammonium phosphate solution according to claim 3, characterized in that: The defluorination agent comprises The invention relates to a novel nanostructured carbon nanotube containing a ... During the primary neutralization reaction, the aluminum-containing component and the iron-containing component are added separately and mixed with the wet-process phosphoric acid and the first ammonia-containing alkali solution for reaction.

5. The method for preparing the ammonium phosphate solution according to claim 3, characterized in that: The secondary neutralization reaction comprises the following steps: The second ammoniacal alkali solution is added to control the pH of the first filtrate to be 6.5-7.5, the reaction temperature is 50-70° C., and the reaction time is 0.5-2 h.

6. The method for preparing the ammonium phosphate solution according to claim 3, characterized in that: The secondary neutralization reaction comprises the following steps: The second ammoniacal alkali solution is added to control the pH of the first filtrate to 8.0-9.0, the reaction temperature is 50-70°C, the reaction time is 0.5-2 h, and a filtrate is obtained by solid-liquid separation; thereafter, phosphoric acid is added to the filtrate to adjust the pH to 6.5-7.5, the reaction temperature is 50-70°C, the reaction time is 0.5-2 h, and an ammonium phosphate solution is obtained; Wherein, the purity of the phosphoric acid is greater than 85%.

7. An ammonium phosphate solution, characterized in that: The ammonium phosphate solution is prepared by the method for preparing the ammonium phosphate solution according to any one of claims 1 to 6.

8. An iron phosphate crystal, characterized in that: The iron phosphate crystals are prepared by a mixed reaction of an iron salt solution, an oxidant and the ammonium phosphate solution according to claim 7; Wherein, the iron salt in the iron salt solution includes at least one of ferrous sulfate, ferrous chloride, ferric sulfate and ferric chloride, and the oxidant includes hydrogen peroxide.

9. A lithium iron phosphate positive electrode material, characterized in that: The lithium iron phosphate positive electrode material is prepared by a mixed reaction of the iron phosphate crystals described in claim 8 and a lithium source.

10. A secondary battery, characterized in that: The positive electrode plate of the secondary battery comprises the lithium iron manganese phosphate positive electrode material as claimed in claim 9.

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