Industrial grade monoammonium phosphate crystals, process for their preparation and ferric phosphate crystals

Abstract

The application provides an industrial-grade monoammonium phosphate crystal and a preparation method thereof, and a ferric phosphate crystal, and belongs to the field of battery materials, wherein the industrial-grade monoammonium phosphate crystal is in a prismatic structure, the two ends of the prism are in a tetrahedral shape, the particle size is 500-700 mu m; in terms of mass percentage, the P2O5 content in the industrial-grade monoammonium phosphate crystal is greater than 61%, the N content is greater than 11.5%, and the F ion content is 50-150 mg / kg. The application uses wet-process phosphoric acid as a raw material, introduces phosphorite slurry, a defluorination agent and an ammonia-containing solution in sequence, respectively carries out desulfurization, defluorination and neutralization, and strictly controls various reaction process parameters, so that impurities are significantly reduced; the prepared product has regular particle morphology, uniform particle size distribution, and relatively high phosphorus and nitrogen content, a lithium ferric phosphate positive electrode material prepared from the precursor has good electrical properties; the preparation method has the advantages of simple operation, easy control of reaction, low cost and the like, and is suitable for large-scale industrial production.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of battery materials, in particular to an industrial-grade monoammonium phosphate crystal, a preparation method thereof, a ferric phosphate crystal, a lithium iron phosphate positive electrode material, a positive electrode sheet and a secondary battery. BACKGROUND

[0002] Industrial-grade monoammonium phosphate (MAP for short), also known as ammonium dihydrogen phosphate, is a white powdery or granular inorganic compound with a chemical formula of NH4H2PO4. After being dissolved, it dissociates into ammonium ions and phosphate ions, and has good chemical reactivity and thermal stability. It has a wide range of applications, such as being used as a fertilizer, a flame retardant, a food additive, a feed additive, a pharmaceutical buffer, a battery material, etc. Among them, in the field of new energy battery applications, ferric phosphate is an important raw material for preparing lithium iron phosphate positive electrode materials, and industrial-grade monoammonium phosphate crystal is an important raw material for preparing ferric phosphate. Therefore, the process flow design of the industrial-grade monoammonium phosphate crystal will affect the production cost and product quality of the lithium iron phosphate positive electrode material.

[0003] At present, the preparation methods of the industrial-grade monoammonium phosphate crystal mainly include the following three process routes: a hot-process ammonia neutralization process of phosphoric acid, a wet-process ammonia neutralization process of purified phosphoric acid extraction, and a wet-process direct ammonia neutralization process of phosphoric acid. Among them, the monoammonium phosphate product produced by the hot-process ammonia neutralization process of phosphoric acid has high quality, but the process has high energy consumption, high production cost and heavy pollution; the monoammonium phosphate product produced by the wet-process ammonia neutralization process of purified phosphoric acid extraction has medium quality, but the extraction and purification of the acid requires large equipment and technical investment, and has high cost; the monoammonium phosphate product produced by the wet-process direct ammonia neutralization process of phosphoric acid has lower quality than the above two, but has small investment and low production cost. It can be seen that, by using the above existing process methods of the industrial-grade monoammonium phosphate crystal, it is difficult to significantly reduce the production cost under the premise of high quality of the monoammonium phosphate product.

[0004] Therefore, it is urgent to provide a new preparation method to obtain high-quality monoammonium phosphate crystals at a low production cost. SUMMARY

[0005] In view of the technical problems in the background art, the present application provides an industrial-grade monoammonium phosphate crystal, a preparation method thereof, a ferric phosphate crystal, a lithium iron phosphate positive electrode material, a positive electrode sheet and a secondary battery, aiming to solve the problem that there is a lack of low-cost, low-pollution and high-quality monoammonium phosphate crystal preparation process in the prior art.

[0006] In a first aspect, the embodiments of the present application provide an industrial-grade monoammonium phosphate crystal, which has a prismatic structure, and the two ends of the prism are in the shape of a tetrahedron. The particle size of the industrial-grade monoammonium phosphate crystal is 500-700 μm, and the length-diameter ratio is (2-4):1. In terms of mass percentage, the content of P2O5 in the industrial-grade monoammonium phosphate crystal is greater than 61%, the content of N is greater than 11.5%, and the content of F ions is 50-150 mg / kg.

[0007] In the technical solution of the embodiments of the present application, the prepared industrial-grade monoammonium phosphate crystal has a prismatic structure, and the two ends of the prism are in the shape of a tetrahedron. The product has a regular particle morphology, a uniform particle size distribution, and a high content of P2O5 and N. The lithium iron phosphate positive electrode material prepared from the precursor has a high specific capacity and good rate performance.

[0008] In a second aspect, the embodiments of the present application provide a preparation method of an industrial-grade monoammonium phosphate crystal, which is used for preparing the industrial-grade monoammonium phosphate crystal described in the first aspect. The preparation method comprises the following steps: mixing wet-process phosphoric acid and phosphate ore slurry and performing a desulfurization reaction, then performing solid-liquid separation to obtain a first filter cake and a first filtrate; adding a defluorination agent to the first filtrate to perform a defluorination reaction, then performing solid-liquid separation to obtain a second filter cake and a second filtrate; adding an ammonia-containing solution to the second filtrate to adjust the pH to 4-5 and performing a neutralization reaction, then performing solid-liquid separation to obtain a third filter cake and a third filtrate; and performing concentration, cooling crystallization, and solid-liquid separation on the third filtrate in sequence to obtain the industrial-grade monoammonium phosphate crystal.

[0009] In the technical solution of the embodiments of the present application, the preparation steps are designed based on a wet-process phosphoric acid direct ammonia neutralization process. Wet-process phosphoric acid is used as a raw material. First, phosphate ore slurry is introduced to remove sulfur elements in the wet-process phosphoric acid. A defluorination agent is introduced into the first filtrate after sulfur removal to perform a defluorination reaction, thereby effectively removing sulfur and fluorine elements in the wet-process phosphoric acid. Then, an ammonia-containing solution is introduced to perform a neutralization reaction on the second filtrate after defluorination. The pH value and the reaction temperature of the neutralization reaction are adjusted to control the generation of monoammonium phosphate and inhibit the conversion of monoammonium phosphate to diammonium phosphate. Subsequently, the third filtrate containing monoammonium phosphate is subjected to concentration, cooling crystallization, and solid-liquid separation in sequence. In the cooling crystallization process, the cooling rate, the stirring rate, the crystallization time, and other parameters are controlled, which significantly reduces the impurity content in the industrial-grade monoammonium phosphate crystal, improves the yield of the industrial-grade monoammonium phosphate crystal, and realizes the precipitation of high-quality industrial-grade monoammonium phosphate crystal. This is conducive to the subsequent preparation of high-quality iron phosphate crystals and lithium iron phosphate positive electrode materials. The preparation method of the present application has the advantages of simple operation, easy control of reaction conditions, low cost, and is suitable for large-scale industrial production.

[0010] In some embodiments, in the step of mixing the wet-process phosphoric acid with the phosphate rock slurry and performing the desulfurization reaction, the desulfurization reaction temperature is 80-90°C, and the desulfurization reaction time is 3-4 h; the P2O5 mass percentage in the wet-process phosphoric acid is greater than 20%, and the Ca 2+ The molar ratio of SO4 2- in the wet-process phosphoric acid to Ca

[0011] In this embodiment, the phosphorite powder is added to the wet-process phosphoric acid to perform the desulfurization reaction and generate gypsum, and the phosphorite powder is used to remove the residual sulfate radical in the wet-process phosphoric acid; by adjusting the ratio of the wet-process phosphoric acid to the phosphate rock slurry (in particular, the ratio of Ca 2+ in the phosphate rock slurry to SO4 2- in the wet-process phosphoric acid) and controlling the desulfurization reaction temperature and time, the desulfurization efficiency is improved, and the energy consumption and production cost are reduced. Specifically, under the condition that the amounts of other components are the same, if the amount of the phosphate rock slurry is too large, after the residual sulfate radical in the wet-process sulfuric acid is completely reacted, more cation impurities are introduced, which reduces the subsequent phosphorus yield; if the amount of the phosphate rock slurry is insufficient, the residual sulfate radical in the wet-process sulfuric acid cannot be completely reacted, which causes insufficient desulfurization. For the control of the desulfurization reaction temperature, if the desulfurization reaction temperature is too high, the increase in the side reaction is caused, which affects the desulfurization efficiency and also possibly causes corrosion to the equipment material, thereby increasing the energy consumption; if the desulfurization reaction temperature is too low, the desulfurization efficiency is reduced. For the control of the desulfurization reaction time, if the desulfurization reaction time is too long, although the desulfurization efficiency is improved to a certain extent, the long reaction time also causes the stability of the reaction system to be reduced, thereby increasing the production cost; if the desulfurization reaction time is insufficient, the desulfurization efficiency is reduced.

[0012] In some embodiments, in the step of adding the fluoride removal agent to the first filtrate to perform the fluoride removal reaction, the fluoride removal reaction temperature is 50-80°C, and the fluoride removal reaction time is 2-4 h; the fluoride removal agent includes at least one of sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium hydroxide, potassium carbonate, and potassium bicarbonate, and the molar ratio of the cation in the fluoride removal agent to F - in the first filtrate is (1.2-1.4):3.

[0013] In this embodiment, the fluoride removal agent is added to the first filtrate after the desulfurization to perform the fluoride removal reaction, the fluoride removal agent can be selected from soluble sodium salts or potassium salts and does not contain sulfur elements, so as to avoid the introduction of sulfur elements again after the desulfurization, and sodium fluorosilicate or potassium fluorosilicate that is slightly soluble in water is generated during the fluoride removal reaction, and the fluorine elements are removed after the solid-liquid separation; by adjusting the ratio of the fluoride removal agent to the first filtrate (in particular, the ratio of Na + and / or K + in the fluoride removal agent to F -and production cost. Specifically, under the condition that the amounts of other components are the same, if the amount of the defluorination agent is too large, although the defluorination efficiency can be improved, the excessive defluorination agent not only increases the raw material cost, but also combines with the phosphate to form insoluble fluorophosphate, which reduces the phosphorus yield in the subsequent preparation process; if the amount of the defluorination agent is insufficient, it cannot completely react with the residual fluorosilicate in the first filtrate after desulfurization, resulting in insufficient defluorination. For the control of the defluorination reaction temperature, although the defluorination efficiency can be improved to a certain extent when the defluorination reaction temperature is too high, the defluorination rate does not improve significantly when the reaction temperature exceeds a certain value, and the energy consumption is greatly increased; if the defluorination reaction temperature is too low, the defluorination efficiency is reduced. For the control of the defluorination reaction time, although the defluorination efficiency can be improved to a certain extent when the defluorination reaction time is too long, the defluorination rate does not improve significantly when the reaction time exceeds a certain value, and the production cost is greatly increased; if the defluorination reaction time is insufficient, the defluorination efficiency is reduced.

[0014] In some embodiments, in the step of neutralization reaction, an ammonia-containing solution is added to adjust the pH to 4-5 and perform the neutralization reaction, the neutralization reaction temperature is 70-90°C, and the neutralization reaction time is 1.5-3 h; wherein the ammonia-containing solution comprises at least one of liquid ammonia and aqueous ammonia.

[0015] In this embodiment, an alkaline ammonia-containing solution is added to the second filtrate after desulfurization and defluorination to perform a neutralization reaction, the pH value of the neutralization reaction is adjusted by the ammonia-containing solution, and the neutralization reaction temperature and time are controlled to make the phosphoric acid fully react with the ammonium ion to generate monoammonium phosphate, and at the same time remove impurities such as iron, aluminum, calcium, and magnesium. Specifically, for the control of the neutralization reaction pH value, as the pH value increases, the crystallization efficiency of monoammonium phosphate first increases and then decreases, if the pH value is too high (i.e. the amount of the alkaline ammonia-containing solution added is too much), the crystallization efficiency of monoammonium phosphate will be significantly reduced; if the pH value is too low (i.e. the amount of the alkaline ammonia-containing solution added is insufficient), not only the impurities such as iron, aluminum, calcium, and magnesium are difficult to remove, but also the phosphoric acid is excessive and cannot fully react to generate monoammonium phosphate. For the control of the neutralization reaction temperature, the temperature will affect the ionization equilibrium of NH4OH and HPO4 2- , if the temperature is too high, it will accelerate the generation of monoammonium phosphate, at the same time the solubility of monoammonium phosphate increases sharply with the increase of temperature, and the ammonia escape amount increases with the temperature being too high, thereby affecting the stability of the generated monoammonium phosphate, which is not conducive to the subsequent crystallization; if the temperature is too low, the generation of monoammonium phosphate is slow, and the impurities such as iron, aluminum, calcium, and magnesium are difficult to be fully removed. For the control of the neutralization reaction time, the impurity removal efficiency gradually increases with the extension of the reaction time, and then gradually slows down after reaching a certain degree, so that appropriately prolonging the reaction time helps to improve the impurity removal efficiency.

[0016] In some embodiments, the specific conditions of the cooling crystallization include: a cooling rate of 0.2-0.6°C / min, a stirring rate of 150-300 rpm, a crystallization time of 4-7 h, and a solid-liquid ratio of 30-60% after cooling.

[0017] In this embodiment, the third filtrate obtained after the neutralization reaction is sequentially concentrated, cooled and crystallized, and solid-liquid separated. In the cooling crystallization process, the cooling rate, the stirring rate, the crystallization time, and the solid-liquid ratio after cooling are key factors affecting the growth of ammonium phosphate crystals and the quality of the product. For the control of the cooling rate, the stirring rate, and the crystallization time, if the cooling rate is too slow, or the stirring rate is too low, or the crystallization time is too long, although it is conducive to the growth of complete crystals, the efficiency is low, which is not conducive to industrial production; if the cooling rate is too fast, or the stirring rate is too fast, or the crystallization time is too short, the crystal growth is incomplete and the crystal size is small, and the quality is low. For the control of the solid-liquid ratio after cooling, if the solid-liquid ratio is too high, the crystal growth rate is slow and the crystal size increases; if the solid-liquid ratio is too low, the crystal growth rate is fast and the crystal size decreases, so the solid-liquid ratio after cooling needs to be controlled in an appropriate range to make the obtained crystals not too large or too small.

[0018] In some embodiments, after cooling crystallization and solid-liquid separation, the obtained solid phase is an industrial-grade ammonium phosphate crystal, and the liquid phase is returned to the wet-process phosphoric acid and reacts with the phosphate rock slurry again to perform desulfurization.

[0019] In this embodiment, after cooling crystallization and solid-liquid separation, the obtained liquid phase is used as a mother liquor and is reused for desulfurization reaction with the wet-process phosphoric acid and the phosphate rock slurry, so that the remaining phosphorus is fully recovered, thereby improving the utilization rate of phosphorus.

[0020] In some embodiments, the first filter cake is reused in the extraction process section of the wet-process phosphoric acid preparation process to prepare wet-process phosphoric acid; the second filter cake is dried and reused as a fluosilicate product; and the third filter cake is dried and reused as a slow-release nitrogen and phosphorus fertilizer.

[0021] In this embodiment, the first filter cake obtained after the desulfurization reaction mainly contains phosphate, sulfate, etc., is reused in the extraction process section to prepare wet-process phosphoric acid again, which can effectively improve the utilization rate of phosphorus; the second filter cake obtained after the defluorination reaction mainly contains sodium or potassium fluosilicate, etc., which can be recycled in the form of a fluosilicate product; and the third filter cake obtained after the neutralization reaction mainly contains a double salt containing ammonia and phosphate, which can be reused as a slow-release nitrogen and phosphorus fertilizer after drying. Thus, the first filter cake, the second filter cake, and the third filter cake can be reused respectively, effectively avoiding the generation of waste materials in the preparation process, and making the process more green and environmentally friendly.

[0022] In a third aspect, the embodiments of the present application provide a ferric phosphate crystal, which is prepared by mixing and reacting an iron salt solution, an oxidizing agent and the industrial-grade monoammonium phosphate crystal in the first aspect.

[0023] In the technical solution of the embodiments of the present application, the ferric phosphate crystal product prepared from the industrial-grade monoammonium phosphate crystal has regular particle morphology, uniform particle size distribution and excellent performance, and the lithium iron phosphate positive electrode material prepared from the ferric phosphate crystal has high specific capacity and good rate performance.

[0024] In a fourth aspect, the embodiments of the present application provide a lithium iron manganese phosphate positive electrode material, which is prepared by mixing and reacting the ferric phosphate crystal provided in the third aspect of the present application and a lithium source.

[0025] The lithium iron manganese phosphate positive electrode material of the present application is prepared from the aforementioned ferric phosphate crystal, and thus has the advantages of high specific capacity and good rate performance.

[0026] In a fifth aspect, the embodiments of the present application provide a positive electrode sheet, which comprises the lithium iron manganese phosphate positive electrode material provided in the fourth aspect of the present application.

[0027] The positive electrode sheet of the present application comprises the aforementioned lithium iron manganese phosphate positive electrode material, and thus has the advantages of high specific capacity and good rate performance.

[0028] In a sixth aspect, the embodiments of the present application provide a secondary battery, which comprises the positive electrode sheet provided in the fifth aspect of the present application.

[0029] The secondary battery of the present application comprises the aforementioned positive electrode sheet, and thus has the advantages of high specific capacity and good rate performance.

[0030] Compared with the prior art, the present application has the following advantages:

[0031] In the present application, wet-process phosphoric acid is used as a raw material, and phosphorite slurry, a defluorination agent and an ammonia-containing solution are introduced in sequence to perform desulfurization reaction, defluorination reaction and neutralization reaction, respectively, and the process parameters of the desulfurization reaction, the defluorination reaction and the neutralization reaction are strictly controlled, so that the impurities such as sulfur, fluorine, iron, aluminum, calcium and magnesium are significantly reduced. The prepared industrial-grade monoammonium phosphate crystal has a prismatic structure, the product particle morphology is regular, the particle size distribution is uniform, and the content of phosphorus and nitrogen is high. The lithium iron phosphate positive electrode material prepared from the precursor has high specific capacity and good rate performance. The preparation method of the present application has the advantages of simple operation, easy control of reaction conditions, low cost and the like, and is suitable for large-scale industrial production.

[0032] The above description is only a summary of the technical solutions of the present application. In order to make the technical means of the present application more clearly understood and implemented according to the content of the description, and in order to make the above and other purposes, characteristics and advantages of the present application more obvious and easy to understand, the following specific embodiments of the present application are described. BRIEF DESCRIPTION OF DRAWINGS

[0033] In order to more clearly illustrate the technical solutions of the present application, the drawings used in the present application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of these drawings.

[0034] Figure 1 Process flow chart of an embodiment of the preparation method of industrial-grade monoammonium phosphate crystals provided by the present application;

[0035] Figure 2 SEM images of the industrial-grade monoammonium phosphate crystals obtained in Examples 1-5 of the present application: a-e are the industrial-grade monoammonium phosphate crystals prepared in Examples 1-5, respectively. DETAILED DESCRIPTION

[0036] The embodiments of the technical solutions of the present application will be described in detail below with reference to the drawings. The following examples are only used to more clearly illustrate the technical solutions of the present application, and therefore only serve as examples, and cannot limit the protection scope of the present application.

[0037] 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 the present application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the present application; the terms "include" and "have" and any variations thereof in the specification and claims of the present application and the above description of drawings are intended to cover non-exclusive inclusion.

[0038] In the description of the embodiments of the present application, the technical terms "first", "second" and the like 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 technical features indicated. In the description of the embodiments of the present application, the meaning of "a plurality of" is two or more, unless otherwise explicitly specified.

[0039] Reference to“an embodiment” herein means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the application. The appearances of the phrase“in

[0040] In the description of the embodiments of the application, the term“and / or” only means an association relationship of the associated objects, which means that there can be three relationships, for example, A and / or B, which can mean that A exists alone, A and B exist together, and B exists alone. In addition, the character“ / ” herein generally means that the front and rear associated objects are in an“or” relationship.

[0041] In the description of the embodiments of the application, the term“a plurality of” means more than two (including two), and similarly, “a plurality of groups” means more than two groups (including two groups), and “a plurality of pieces” means more than two pieces (including two pieces).

[0042] The wet-process phosphoric acid direct ammonia neutralization process is a method for preparing monoammonium phosphate by ammonia neutralization reaction of wet-process phosphoric acid. In the prior art, one-time ammonia neutralization or multiple ammonia neutralization is generally used, followed by concentration, crystallization and drying, thereby preparing monoammonium phosphate crystals. Although this process has small investment and low production cost, the prepared monoammonium phosphate crystals generally have low quality, and it is difficult to obtain high-quality crystals with regular micro-morphology and uniform particle size distribution. The reason is that the raw material wet-process phosphoric acid contains F, SO4 2- , Fe, Al, Ca, Mg and other impurities, which exist in the form of phosphoric acid complex in the ammonia neutralization process, and the existing process is difficult to fully remove the above impurity elements in the ammonia neutralization process, resulting in the influence of impurity residues on the preparation of monoammonium phosphate crystals. For example, in the existing ammonia neutralization process, impurity F - still remains in a large amount after ammonia neutralization, which reduces the phosphorus yield in the preparation of monoammonium phosphate, and the presence of F - may accelerate the aging and corrosion of production equipment, and the emission of F - may pollute the environment; and the presence of impurity SO4 2- will reduce the width of the metastable zone, not only causing the morphology change of the prepared monoammonium phosphate crystals due to misplacement growth, but also affecting the particle size and uniformity in the crystallization process.

[0043] The other two existing processes for preparing monoammonium phosphate crystals also have shortcomings. The monoammonium phosphate produced by the thermal ammonia phosphate neutralization process has high quality, but this process has high energy consumption, high production cost, and heavy pollution. The monoammonium phosphate produced by the wet phosphoric acid extraction purification ammonia neutralization process has medium quality, but the extraction, refining and purification acid process requires large investment in equipment and technology, resulting in high cost.

[0044] To address the technical problem of lacking a low-cost, low-pollution, and high-quality monoammonium phosphate (MAP) crystal preparation process in existing technologies, this application provides an industrial-grade MAP crystal and its preparation method, as well as iron phosphate crystals, lithium iron phosphate cathode materials, cathode sheets, and secondary batteries. This application uses wet-process phosphoric acid as raw material, sequentially introducing phosphate rock slurry, a defluorinating agent, and an ammonia-containing solution to carry out desulfurization, defluorination, and neutralization reactions, respectively. The process parameters for these reactions are strictly controlled to significantly reduce impurities such as sulfur, fluorine, iron, aluminum, calcium, and magnesium. The prepared industrial-grade MAP crystals exhibit a prismatic structure, with regular particle morphology and uniform particle size distribution, and high phosphorus and nitrogen content. The lithium iron phosphate cathode material prepared from this precursor possesses high specific capacity and good rate performance.

[0045] In a first aspect, embodiments of this application provide an industrial-grade monoammonium phosphate crystal. The industrial-grade monoammonium phosphate crystal has a prismatic structure with four-sided pyramidal ends. Its particle size is 500~700 μm and its aspect ratio is (2~4):1. By mass percentage, the industrial-grade monoammonium phosphate crystal has a P2O5 content greater than 61%, a N content greater than 11.5%, and an F ion content of 50~150 mg / kg.

[0046] In the technical solution of this application embodiment, the prepared industrial-grade monoammonium phosphate crystals have a prismatic structure with tetrahedral pyramidal ends, exhibiting a specific microstructure and aspect ratio range. During the preparation process, the impurity content affects the microstructure and aspect ratio; therefore, strict control of the impurity content is necessary to obtain the aforementioned industrial-grade monoammonium phosphate crystals. For example, SO4... 2- When the content is too high, it will cause the crystal to grow in a dislocation manner, forming an X-shaped crystal, which will significantly change the microstructure. The presence of impurities such as iron and aluminum ions will inhibit the growth of monoammonium phosphate crystals. At the same time, too many impurities will cause the prismatic structure of the crystal to become longer, increasing the aspect ratio, which will make the prepared monoammonium phosphate crystals difficult to filter.

[0047] Please see Figure 1 Secondly, embodiments of this application provide a method for preparing industrial-grade monoammonium phosphate crystals, comprising the following steps:

[0048] S1. Wet-process phosphoric acid is mixed with phosphate rock slurry and subjected to desulfurization reaction, followed by solid-liquid separation to obtain the first filter cake and the first filtrate;

[0049] S2, a defluorination agent is added into the first filtrate to perform a defluorination reaction, and then solid-liquid separation is performed to obtain a second filter cake and a second filtrate;

[0050] S3, an ammonia-containing solution is added into the second filtrate to adjust the pH value to 4-5 and perform a neutralization reaction, and then solid-liquid separation is performed to obtain a third filter cake and a third filtrate;

[0051] S4, the third filtrate is subjected to concentration, cooling crystallization and solid-liquid separation in sequence to obtain industrial-grade monoammonium phosphate crystals.

[0052] In the technical scheme of the embodiment, the preparation steps are designed based on the wet-process phosphoric acid direct ammonia neutralization process, and the wet-process phosphoric acid is used as the raw material to perform desulfurization reaction, defluorination reaction and neutralization reaction in sequence. First, the phosphorite slurry is introduced to remove the sulfur elements in the wet-process phosphoric acid, and then a defluorination agent is introduced into the first filtrate after the sulfur removal to perform a defluorination reaction, so that the sulfur and fluorine elements in the wet-process phosphoric acid can be effectively removed. Then, an ammonia-containing solution is introduced to perform a neutralization reaction on the second filtrate after the defluorination, the pH value and the reaction temperature of the neutralization reaction are adjusted, and the generation of monoammonium phosphate is controlled and the conversion of monoammonium phosphate to diammonium phosphate is inhibited. Subsequently, the third filtrate containing monoammonium phosphate is subjected to concentration, cooling crystallization and solid-liquid separation in sequence. In the cooling crystallization process, the cooling rate, the stirring rate and the crystallization time are controlled, which significantly reduces the impurity content in the industrial-grade monoammonium phosphate crystals, improves the yield of the industrial-grade monoammonium phosphate crystals, and realizes the precipitation of high-quality industrial-grade monoammonium phosphate crystals, thereby facilitating the subsequent preparation of high-quality iron phosphate crystals and lithium iron phosphate positive electrode materials.

[0053] Further, in some embodiments, in step S1, the desulfurization reaction temperature is 80-90℃, and the desulfurization reaction time is 3-4 h. Specifically, the desulfurization reaction temperature includes but is not limited to any one of 80℃, 83℃, 85℃, 88℃, 90℃ or a range value between any two of them, and the desulfurization reaction time includes but is not limited to any one of 3 h, 3.5 h, 4 h or a range value between any two of them. The mass percentage of P2O5 in the wet-process phosphoric acid is greater than 20%, and the mass percentage of Ca 2+ The molar ratio of Ca 2- to SO4 2+ in the wet-process phosphoric acid is (0.8-1.0):1. Specifically, the molar ratio of Ca 2- to SO4 2- in the wet-process phosphoric acid includes but is not limited to any one of 0.8:1, 0.9:1, 1:1 or a range value between any two of them. In this embodiment, the pressure filtration method is used for solid-liquid separation, and in other embodiments, the specific solid-liquid separation method or device can be adaptively adjusted according to actual needs, which is not limited herein.

[0054] In the technical scheme of the embodiments of the present application, phosphorite powder is added to the wet-process phosphoric acid to carry out desulfurization reaction and generate gypsum, and the phosphorite powder is used to remove the residual sulfate radical in the wet-process phosphoric acid. After the sulfate radical is sufficiently removed, it can significantly weaken the influence on the micro-morphology of the crystal, so that the crystal can be crystallized in a prismatic structure. Specifically, by adjusting the ratio of the wet-process phosphoric acid to the phosphorite slurry (especially the ratio of Ca 2+ to SO4 2- in the wet-process phosphoric acid), and controlling the desulfurization reaction temperature and time, the desulfurization efficiency is improved, while the energy consumption and production cost are reduced. Specifically, under the condition that the amounts of other components are the same, if the amount of the phosphorite slurry is too large, after the residual sulfate radical in the wet-process sulfuric acid is completely reacted, more cation impurities will be introduced, which will reduce the subsequent phosphorus yield. At the same time, it is found through experiments that when the content of sulfate radical is too low, after the temperature is reduced for crystallization, the proportion of fine MAP crystals (the average particle size is much lower than 500 μm) increases, thereby reducing the proportion of the MAP crystals with regular and uniform particle size (the particle size is 500-700 μm) obtained in a single preparation process, that is, it is difficult to obtain an ideal MAP crystal particle size distribution state. Although the fine MAP crystals can be used as a mother liquor and recycled to the preparation process of the next cycle, the recycling of too many fine MAP crystals will undoubtedly reduce the preparation amount of the MAP crystals with regular and uniform particle size in a single preparation process. If the amount of the phosphorite slurry is insufficient, it cannot completely react with the residual sulfate radical in the wet-process sulfuric acid, so that the desulfurization is insufficient. Therefore, it is necessary to strictly control the sulfur content of the prepared MAP crystals by controlling the amount of the phosphorite powder, so as to achieve a high phosphorus yield and reduce the content of fine grains in the preparation process of the MAP crystals, and obtain MAP crystals with ideal particle size distribution and low impurity content. For the control of the desulfurization reaction temperature, if the desulfurization reaction temperature is too high, the side reaction will increase, which will affect the desulfurization efficiency, and may also cause corrosion to the equipment materials, thereby increasing the energy consumption. If the desulfurization reaction temperature is too low, the desulfurization efficiency will be reduced. For the control of the desulfurization reaction time, if the desulfurization reaction time is too long, although the desulfurization efficiency can be improved to some extent, the stability of the reaction system will be reduced due to the long reaction time, thereby increasing the production cost. If the desulfurization reaction time is insufficient, the desulfurization efficiency will be reduced. Therefore, the desulfurization reaction temperature, the desulfurization reaction time, and the molar ratio of Ca 2+ to SO4 2- in the phosphorite slurry should be controlled within the above suitable range, so as to obtain the best desulfurization effect.

[0055] In some embodiments, in the step of adding a defluorination agent to the first filtrate for defluorination reaction, the defluorination reaction temperature is 50-80℃, and the defluorination reaction time is 2-4 h; specifically, the defluorination reaction temperature includes but is not limited to any one of 50℃, 55℃, 60℃, 65℃, 70℃, 75℃, 80℃ or a range value between any two of them, and the defluorination reaction time includes but is not limited to any one of 2 h, 2.5 h, 3 h, 3.5 h, 4 h or a range value between any two of them. The defluorination agent includes at least one of sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium hydroxide, potassium carbonate, and potassium bicarbonate, and the molar ratio of the cation in the defluorination agent to F - in the first filtrate is (1.2-1.4):3; the molar ratio of the cation in the defluorination agent to F - in the first filtrate includes but is not limited to any one of 1.2:3, 1.3:3, 1.4:3 or a range value between any two of them. In this embodiment, pressure filtration is used for solid-liquid separation, and in other embodiments, the specific solid-liquid separation method or device can be adjusted according to actual needs, which is not limited here.

[0056] In the technical scheme of the embodiments of the present application, a defluorination agent is added to the first filtrate after desulfurization for defluorination reaction, the defluorination agent can be a soluble sodium salt or potassium salt and does not contain sulfur element, which avoids the introduction of sulfur element again after desulfurization, and generates slightly soluble sodium fluorosilicate or potassium fluorosilicate during defluorination reaction, which is removed after solid-liquid separation. After the fluorine element is removed, the defluorination pressure in the subsequent neutralization reaction process can be significantly reduced, the fluorine ion residue is reduced, and the phosphorus yield in the subsequent preparation process is improved. Specifically, by adjusting the ratio of the defluorination agent to the first filtrate (especially the Na + and / or K + in the defluorination agent to F -and the production cost. Specifically, under the condition that the amounts of other components are the same, if the amount of the defluorination agent is too large, although the defluorination efficiency can be improved, the excessive defluorination agent not only increases the raw material cost, but also forms insoluble fluorophosphates by combining with the phosphate, which reduces the phosphorus yield in the subsequent preparation process. In addition, it is found through experiments that a small amount of fluoride ions can promote the crystallization of MAP. When excessive defluorination agent is introduced to sharply reduce the fluoride ions, the small amount of fluoride ions cannot promote the crystallization of MAP, resulting in small crystal grains. If the amount of the defluorination agent is insufficient, the defluorination agent cannot completely react with the residual fluorosilicate in the first filtrate after desulfurization, resulting in insufficient defluorination. Therefore, the amount of the defluorination agent needs to be controlled to strictly regulate the fluorine content of the prepared MAP crystal, so as to achieve good phosphorus yield and crystal size at the same time. For the control of the defluorination reaction temperature, if the defluorination reaction temperature is too high, the defluorination efficiency can be improved to a certain extent, but the improvement effect of the defluorination rate is not obvious when the reaction temperature exceeds a certain value, and the energy consumption is greatly increased. If the defluorination reaction temperature is too low, the defluorination efficiency is reduced. For the control of the defluorination reaction time, if the defluorination reaction time is too long, the defluorination efficiency can be improved to a certain extent, but the improvement effect of the defluorination rate is not obvious when the reaction time exceeds a certain value, and the production cost is greatly increased. If the defluorination reaction time is insufficient, the defluorination efficiency is reduced. Therefore, the defluorination reaction temperature, the defluorination reaction time and the molar ratio of the cations in the defluorination agent to the F - must be controlled within the above suitable range to obtain the best defluorination effect.

[0057] In some embodiments, in the step of neutralization reaction, the pH is adjusted to 4-5 by adding an ammonia-containing solution, and the neutralization reaction is carried out at a temperature of 70-90°C for 1.5-3 hours. Specifically, the pH value includes but is not limited to any one of 4.0, 4.2, 4.4, 4.6, 4.8, 5.0 or a range value between any two of them, the neutralization reaction temperature includes but is not limited to any one of 70°C, 75°C, 80°C, 85°C, 90°C or a range value between any two of them, and the neutralization reaction time includes but is not limited to any one of 1.5 hours, 2 hours, 2.5 hours, 3 hours or a range value between any two of them. The ammonia-containing solution includes at least one of liquid ammonia and ammonia water. In this embodiment, pressure filtration is used for solid-liquid separation, and in other embodiments, the specific solid-liquid separation method or device can be adjusted as needed, which is not limited here.

[0058] In the technical scheme of the embodiments of the present application, the alkaline ammonia-containing solution is added to the second filtrate after desulfurization and defluorination to perform a neutralization reaction, the pH value of the neutralization reaction is adjusted by the ammonia-containing solution, and the neutralization reaction temperature and time are controlled to make the phosphoric acid fully react with the ammonium ion to generate monoammonium phosphate crystals, and at the same time, the impurities such as iron, aluminum, calcium and magnesium are removed. Specifically, for the control of the pH value of the neutralization reaction, with the increase of the pH value, the crystallization efficiency of monoammonium phosphate shows a trend of first increasing and then decreasing, if the pH value is too high (i.e. the amount of alkaline ammonia-containing solution added is too much), a sticky precipitate will be formed, which will make it difficult for monoammonium phosphate to be shaped, and its crystallization efficiency will be significantly reduced; if the pH value is too low (i.e. the amount of alkaline ammonia-containing solution added is insufficient), not only the impurities such as iron, aluminum, calcium and magnesium are difficult to remove, but also the phosphoric acid is excessive and cannot fully react to generate monoammonium phosphate. For the control of the neutralization reaction temperature, the temperature will affect the ionization equilibrium of NH4OH and HPO4 2-

[0059] In some embodiments, the specific conditions of the cooling crystallization include that the cooling rate is 0.2-0.6 ℃ / min, the stirring rate is 150-300 rpm, the crystallization time is 4-7 h, and the solid-liquid ratio after cooling is 30-60%; specifically, the cooling rate includes but is not limited to any one value or a range value between any two values of 0.2 ℃ / min, 0.4 ℃ / min and 0.6 ℃ / min, the stirring rate includes but is not limited to any one value or a range value between any two values of 150 rpm, 200 rpm, 250 rpm and 300 rpm, the crystallization time includes but is not limited to any one value or a range value between any two values of 4 h, 5 h, 6 h and 7 h, and the solid-liquid ratio after cooling includes but is not limited to any one value or a range value between any two values of 30%, 40%, 50% and 60%.

[0060] ​In the technical scheme of the embodiments of the present application, the third filtrate obtained after the neutralization reaction is sequentially concentrated, cooled and crystallized, and solid-liquid separated. In the cooling and crystallization process, the cooling rate, stirring rate, crystallization time, and solid-liquid ratio after cooling are key factors affecting the growth of monoammonium phosphate crystals and the quality of the product. For the control of the cooling rate, stirring rate and crystallization time, if the cooling rate is too slow or the crystallization time is too long, although it is beneficial to the growth of complete crystals, the efficiency is low and is not conducive to industrial production; if the cooling rate is too fast or the crystallization time is too short, the crystal growth is incomplete and the crystal grains are small, and the quality is low. For the control of the solid-liquid ratio after cooling, if the solid-liquid ratio is too high, the crystal growth rate is slow and the crystal grain size increases; if the solid-liquid ratio is too low, the crystal growth rate is fast and the crystal grain size decreases, so the solid-liquid ratio after cooling needs to be controlled within an appropriate range to make the obtained crystals not too large or too small. If the stirring speed is too slow, the solid-phase mass transfer rate and solute growth rate are low, so the average particle size of the formed crystals is small; when a certain rotating speed of 180 rpm is reached, the solid-phase mass transfer rate and solute growth rate rapidly increase, and the average particle size of the crystals rapidly increases; if the stirring speed is too fast, the impact strength between the crystals and the stirring paddle will further increase, at this time, the secondary nucleation phenomenon between the crystals and the crystals and the probability of crystal rupture will greatly increase, and the violent collision between the crystals and the stirring paddle will also greatly increase, finally resulting in a decrease in the average particle size of the product. Therefore, the cooling rate, stirring rate, crystallization time and solid-liquid ratio after cooling need to be controlled within the above appropriate ranges to obtain the best crystallization effect.

[0061] In some embodiments, after cooling and crystallization and solid-liquid separation, the obtained solid phase is an industrial-grade monoammonium phosphate crystal, and the liquid phase is returned to the wet-process phosphoric acid and is subjected to a desulfurization reaction with the phosphate rock slurry again. In the present embodiment, a filter pressing method is used for solid-liquid separation, and in other embodiments, the specific solid-liquid separation method or device used can be adaptively adjusted according to actual needs, which is not limited herein.

[0062] In the technical scheme of the embodiments of the present application, after cooling and crystallization and solid-liquid separation, the obtained liquid phase is used as a mother liquor and is subjected to a desulfurization reaction with the wet-process phosphoric acid and the phosphate rock slurry again, so that the remaining phosphorus is fully recovered, thereby improving the utilization rate of phosphorus; at the same time, since the impurities after the neutralization reaction are mainly separated out by the third filter cake, the impurity content in the third filtrate is extremely low, so that when the liquid phase after cooling and crystallization and solid-liquid separation is used as a mother liquor, there will be no many impurities enriched with the return flow, so that the liquid phase used as a mother liquor will not affect the crystallization of monoammonium phosphate after multiple cycles of reaction, and continuous industrial production of high-quality industrial-grade monoammonium phosphate crystals is realized.

[0063] In some embodiments, the first filter cake is recycled to the extraction process section in the wet-process phosphoric acid preparation process and is used for preparing the wet-process phosphoric acid; the second filter cake is dried and recycled as a fluosilicate product; and the third filter cake is dried and recycled as a slow-release nitrogen and phosphorus fertilizer.

[0064] In the technical scheme of the embodiment of the present application, the first filter cake obtained after the desulfurization reaction mainly contains phosphate, sulfate and the like, is reused to the extraction process section and is used to prepare wet-process phosphoric acid again, and the prepared wet-process phosphoric acid can be used as raw material to perform the above preparation steps again, thereby effectively improving the utilization rate of phosphorus. In other embodiments, the first filter cake can also be reused to other process sections for preparing wet-process phosphoric acid to achieve similar reuse effects as described above, which are not limited herein. The second filter cake obtained after the defluorination reaction mainly contains sodium or potassium fluorosilicate, and can be recovered in the form of fluorosilicate product. The recovered fluorosilicate can be used as building materials, metal plating materials, preservatives, pesticides, plastic fillers and the like, and the specific application mode can be adaptively selected according to actual needs, which are not limited herein. The third filter cake obtained after the neutralization reaction mainly contains a double salt containing ammonia and phosphate, and can be recovered and utilized as slow-release nitrogen and phosphorus fertilizer after drying. The third filter cake can also be adaptively adjusted according to actual needs to obtain slow-release nitrogen and phosphorus fertilizer meeting the requirements, which are not limited herein. In this way, the first filter cake, the second filter cake and the third filter cake can be reused respectively, so that all the products obtained in the foregoing process steps are utilized, effectively avoiding the generation of waste materials in the preparation process, and making the process flow more green and environmentally friendly.

[0065] In a third aspect, the embodiment of the present application provides a phosphoric acid iron crystal, which is prepared by mixing and reacting an iron salt solution, an oxidizing agent and the industrial-grade monoammonium phosphate crystal in the first aspect. The iron salt in the iron salt solution includes at least one of ferrous sulfate, ferrous chloride, ferric sulfate and ferric chloride, and the oxidizing agent includes hydrogen peroxide.

[0066] In the technical scheme of the embodiment of the present application, the phosphoric acid iron crystal product prepared from the industrial-grade monoammonium phosphate crystal has regular particle morphology, uniform particle size distribution and excellent performance, and the lithium iron phosphate positive electrode material prepared from the phosphoric acid iron crystal has high specific capacity and good rate performance. In other embodiments, other conventional raw materials can be selected as the iron salt or the oxidizing agent for preparing the phosphoric acid iron crystal according to actual needs, which are not limited herein.

[0067] In a fourth aspect, the embodiment of the present application provides a lithium iron manganese phosphate positive electrode material, which is prepared by mixing and reacting the phosphoric acid iron crystal provided in the third aspect of the present application with a lithium source.

[0068] In the technical scheme of the embodiment of the present application, the lithium iron manganese phosphate positive electrode material is prepared by mixing the industrial-grade iron phosphate crystal and a lithium source uniformly and then using a high-temperature solid-phase method, or by mixing the aforementioned industrial-grade iron phosphate crystal, a lithium source and a carbon source uniformly and then using a high-temperature solid-phase method. The molar ratio of the iron phosphate crystal to the lithium source is 1: (1-1.03), and the molar ratio of the iron phosphate crystal to the 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 a high-temperature gradient calcination. In the process of the high-temperature gradient calcination, the calcination temperature of the first stage is 350-450 DEG C, the holding time of the first stage is 2-6 h, the calcination temperature of the second stage is 760-790 DEG C, the holding time of the second stage is 2-6 h, and the calcination temperature of the third stage is 600-700 DEG C. The holding time of the third stage is 2-6 h. The lithium iron manganese phosphate positive electrode material of the present application is prepared from the aforementioned iron phosphate crystal, and thus 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 herein.

[0069] In a fifth aspect, the embodiment of the present application provides a positive electrode sheet, which comprises the lithium iron manganese phosphate positive electrode material provided in the fourth aspect of the present application.

[0070] The positive electrode sheet of the present application comprises the aforementioned lithium iron manganese phosphate positive electrode material, and thus has the advantages of high specific capacity and good rate performance.

[0071] In a sixth aspect, the embodiment of the present application provides a secondary battery, which comprises the positive electrode sheet provided in the fifth aspect of the present application.

[0072] The secondary battery of the present application comprises the aforementioned positive electrode sheet, and thus has the advantages of high specific capacity and good rate performance.

[0073] Some specific embodiments are listed below. It should be noted that the embodiments described below are exemplary and are used to explain the present application, and cannot be understood as a limitation on the present application. If a specific technology or condition is not specified in the embodiments, the technology or condition is performed according to the technology or condition described in the literature in the art or according to the product manual. If the reagent or instrument is not specified by the manufacturer, it is a conventional product that can be obtained from the market. In the following examples and comparative examples, the composition analysis of the wet-process phosphoric acid and the phosphate ore slurry is shown in Table 1:

[0074] Table 1 Composition analysis of wet-process phosphoric acid and phosphate ore slurry

[0075]

[0076] I. Preparation Method

[0077] Example 1

[0078] The specific steps for preparing industrial-grade monoammonium phosphate crystals in this embodiment are as follows:

[0079] (1) Wet-process phosphoric acid is mixed with phosphate rock slurry and subjected to desulfurization reaction. The Ca in the phosphate rock slurry 2+ SO4 in wet-process phosphoric acid 2- The molar ratio was 1:1, the desulfurization reaction temperature was 80℃, and the desulfurization reaction time was 4 h. Then, the first filter cake and the first filtrate were obtained by pressure filtration. The first filter cake was recycled to the extraction process section and wet phosphoric acid was prepared again.

[0080] (2) A defluorinating agent, sodium bicarbonate, is added to the first filtrate to carry out a defluorination reaction. The cations in the defluorinating agent react with the F in the first filtrate. - The molar ratio was 1.2:3, the defluorination reaction temperature was 60℃, and the defluorination reaction time was 2 h. Subsequently, the second filter cake and the second filtrate were obtained by pressure filtration. The second filter cake was dried and recovered as a fluorosilicate byproduct.

[0081] (3) Liquid ammonia was added to the second filtrate to adjust the pH to 4.5 and a neutralization reaction was carried out. The neutralization reaction temperature was 80℃ and the neutralization reaction time was 1.5 h. Then, the third filter cake and the third filtrate were obtained by pressure filtration. The third filter cake was dried and recovered as a slow-release nitrogen and phosphorus fertilizer by-product.

[0082] (4) After the third filtrate is concentrated, it is cooled and crystallized. The temperature is reduced from 90℃ to 25℃ room temperature at a cooling rate of 0.3℃ / min, a stirring rate of 180 rpm, and a crystallization time of 5 h. After cooling, the solid-liquid ratio is 60%. Then, it is filtered by pressure. After filtration, the liquid phase is refluxed into wet phosphoric acid and reacted with phosphate rock slurry again for desulfurization. The solid phase is dried to obtain industrial grade monoammonium phosphate crystals.

[0083] Example 2

[0084] The specific steps for preparing industrial-grade monoammonium phosphate crystals in this embodiment are as follows:

[0085] (1) Wet-process phosphoric acid is mixed with phosphate rock slurry and subjected to desulfurization reaction. The Ca in the phosphate rock slurry 2+ SO4 in wet-process phosphoric acid 2- The molar ratio was 0.9:1, the desulfurization reaction temperature was 80℃, and the desulfurization reaction time was 4 h. Then, the first filter cake and the first filtrate were obtained by pressure filtration. The first filter cake was recycled to the extraction process section and wet phosphoric acid was prepared again.

[0086] (2) A defluorinating agent, sodium carbonate, is added to the first filtrate to carry out a defluorination reaction. The cations in the defluorinating agent react with the F in the first filtrate. -The molar ratio was 1.2:3, the defluorination reaction temperature was 60℃, and the defluorination reaction time was 2 h. Subsequently, the second filter cake and the second filtrate were obtained by pressure filtration. The second filter cake was dried and recovered as a fluorosilicate byproduct.

[0087] (3) Liquid ammonia was added to the second filtrate to adjust the pH to 4.5 and a neutralization reaction was carried out. The neutralization reaction temperature was 80℃ and the neutralization reaction time was 1.5 h. Then, the third filter cake and the third filtrate were obtained by pressure filtration. The third filter cake was dried and recovered as a slow-release nitrogen and phosphorus fertilizer by-product.

[0088] (4) After the third filtrate is concentrated, it is cooled and crystallized. The temperature is reduced from 90℃ to 25℃ room temperature at a cooling rate of 0.3℃ / min, a stirring rate of 180 rpm, and a crystallization time of 5 h. After cooling, the solid-liquid ratio is 60%. Then, it is filtered by pressure. After filtration, the liquid phase is refluxed into wet phosphoric acid and reacted with phosphate rock slurry again for desulfurization. The solid phase is dried to obtain industrial grade monoammonium phosphate crystals.

[0089] Example 3

[0090] The specific steps for preparing industrial-grade monoammonium phosphate crystals in this embodiment are as follows:

[0091] (1) Wet-process phosphoric acid is mixed with phosphate rock slurry and subjected to desulfurization reaction. The Ca in the phosphate rock slurry 2+ SO4 in wet-process phosphoric acid 2- The molar ratio was 0.8:1, the desulfurization reaction temperature was 80℃, and the desulfurization reaction time was 4 h. Then, the first filter cake and the first filtrate were obtained by pressure filtration. The first filter cake was recycled to the extraction process section and wet phosphoric acid was prepared again.

[0092] (2) A defluorinating agent, sodium hydroxide, is added to the first filtrate to carry out a defluorination reaction. The cations in the defluorinating agent react with the F in the first filtrate. - The molar ratio was 1.2:3, the defluorination reaction temperature was 60℃, and the defluorination reaction time was 2 h. Subsequently, the second filter cake and the second filtrate were obtained by pressure filtration. The second filter cake was dried and recovered as a fluorosilicate byproduct.

[0093] (3) Liquid ammonia was added to the second filtrate to adjust the pH to 4.5 and a neutralization reaction was carried out. The neutralization reaction temperature was 80℃ and the neutralization reaction time was 1.5 h. Then, the third filter cake and the third filtrate were obtained by pressure filtration. The third filter cake was dried and recovered as a slow-release nitrogen and phosphorus fertilizer by-product.

[0094] (4) After the third filtrate is concentrated, it is cooled and crystallized. The temperature is reduced from 90℃ to 25℃ room temperature at a cooling rate of 0.3℃ / min, a stirring rate of 180 rpm, and a crystallization time of 5 h. After cooling, the solid-liquid ratio is 60%. Then, it is filtered by pressure. After filtration, the liquid phase is refluxed into wet phosphoric acid and reacted with phosphate rock slurry again for desulfurization. The solid phase is dried to obtain industrial grade monoammonium phosphate crystals.

[0095] Example 4

[0096] Compared with Example 3, the only difference is that in step (1), the desulfurization reaction temperature is 85°C.

[0097] Example 5

[0098] Compared with Example 3, the only difference is that in step (1), the desulfurization reaction temperature is 90°C.

[0099] Example 6

[0100] Compared with Example 3, the only difference is that in step (1), the desulfurization reaction time is 3 h.

[0101] Example 7

[0102] Compared with Example 3, the only difference is that in step (1), the desulfurization reaction time is 3.5 h.

[0103] Example 8

[0104] Compared with Example 3, the only difference is that in step (2), the molar ratio of the cation in the defluorination agent to F in the first filtrate is 1.4:3. -

[0105] Example 9

[0106] Compared with Example 3, the only difference is that in step (2), the molar ratio of the cation in the defluorination agent to F in the first filtrate is 1.3:3. -

[0107] Example 10

[0108] Compared with Example 3, the only difference is that in step (2), the defluorination reaction temperature is 65°C.

[0109] Example 11

[0110] Compared with Example 3, the only difference is that in step (2), the defluorination reaction temperature is 70°C.

[0111] Example 12

[0112] Compared with Example 3, the only difference is that in step (2), the defluorination reaction time is 3 h.

[0113] Example 13

[0114] Compared with Example 3, the only difference is that in step (2), the defluorination reaction time is 2.5 h.

[0115] Example 14

[0116] ​​The difference compared with Example 3 is only that in step (3), liquid ammonia is added into the second filtrate to adjust pH to 4.0 and perform the neutralization reaction.

[0117] Example 15

[0118] The difference compared with Example 3 is only that in step (3), liquid ammonia is added into the second filtrate to adjust pH to 5.0 and perform the neutralization reaction.

[0119] Example 16

[0120] The difference compared with Example 3 is only that in step (3), the neutralization reaction time is 1 h.

[0121] Example 17

[0122] The difference compared with Example 3 is only that in step (3), the neutralization reaction time is 2 h.

[0123] Example 18

[0124] The difference compared with Example 3 is only that in step (3), the neutralization reaction temperature is 70℃.

[0125] Example 19

[0126] The difference compared with Example 3 is only that in step (3), the neutralization reaction temperature is 90℃.

[0127] Example 20

[0128] The difference compared with Example 3 is only that in step (4), the cooling rate is 0.2 ℃ / min.

[0129] Example 21

[0130] The difference compared with Example 3 is only that in step (4), the cooling rate is 0.4 ℃ / min.

[0131] Example 22

[0132] The difference compared with Example 3 is only that in step (4), the stirring rate is 150 rpm.

[0133] Example 23

[0134] The difference compared with Example 3 is only that in step (4), the stirring rate is 200 rpm.

[0135] Example 24

[0136] The difference compared with Example 3 is only that in step (4), the crystallization time is 3 h.

[0137] Example 25

[0138] The difference compared with Example 3 is only that in step (4), the crystallization time is 4 h.

[0139] Example 26

[0140] The difference compared with Example 3 is only that in step (4), the solid-liquid ratio after cooling is 50%.

[0141] Example 27

[0142] The difference compared with Example 3 is only that in step (4), the solid-liquid ratio after cooling is 70%.

[0143] Comparative Example 1

[0144] The difference compared with Example 3 is only that in step (1), the Ca 2+ The molar ratio of SO4 2- in the wet-process phosphoric acid is 0.7:1.

[0145] Comparative Example 2

[0146] The difference compared with Example 3 is only that in step (1), the Ca 2+ The molar ratio of SO4 2- in the wet-process phosphoric acid is 1.1:1.

[0147] Comparative Example 3

[0148] The difference compared with Example 3 is only that in step (1), the desulfurization reaction temperature is 70°C.

[0149] Comparative Example 4

[0150] The difference compared with Example 3 is only that in step (1), the desulfurization reaction temperature is 100°C.

[0151] Comparative Example 5

[0152] The difference compared with Example 3 is only that in step (1), the desulfurization reaction time is 2 h.

[0153] Comparative Example 6

[0154] The difference compared with Example 3 is only that in step (1), the desulfurization reaction time is 5 h.

[0155] Comparative Example 7

[0156] The difference compared with Example 3 is only that in step (2), the molar ratio of cation in the defluorination agent to F - in the first filtrate is 1.0:3.

[0157] Comparative Example 8

[0158] The difference compared with Example 3 is only that in step (2), the molar ratio of the cation in the defluorination agent to F in the first filtrate is 1.5:3. -

[0159] Comparative Example 9

[0160] The difference compared with Example 3 is only that in step (2), the defluorination reaction temperature is 50°C.

[0161] Comparative Example 10

[0162] The difference compared with Example 3 is only that in step (2), the defluorination reaction temperature is 80°C.

[0163] Comparative Example 11

[0164] The difference compared with Example 3 is only that in step (2), the defluorination reaction time is 1.5 h.

[0165] Comparative Example 12

[0166] The difference compared with Example 3 is only that in step (2), the defluorination reaction time is 1 h.

[0167] Comparative Example 13

[0168] The difference compared with Example 3 is only that in step (3), liquid ammonia is added to the second filtrate to adjust the pH to 6.0 and a neutralization reaction is performed.

[0169] Comparative Example 14

[0170] The difference compared with Example 3 is only that in step (3), liquid ammonia is added to the second filtrate to adjust the pH to 3.5 and a neutralization reaction is performed.

[0171] Comparative Example 15

[0172] The difference compared with Example 3 is only that in step (3), the neutralization reaction time is 3 h.

[0173] Comparative Example 16

[0174] The difference compared with Example 3 is only that in step (3), the neutralization reaction time is 0.5 h.

[0175] Comparative Example 17

[0176] The difference compared with Example 3 is only that in step (3), the neutralization reaction temperature is 60°C.

[0177] Comparative Example 18

[0178] The difference compared with Example 3 is only that in step (3), the neutralization reaction temperature is 100°C. ​

[0179] Comparative Example 19

[0180] The difference compared with Example 3 is only that in step (4), the cooling rate is 0.5 ℃ / min.

[0181] Comparative Example 20

[0182] The difference compared with Example 3 is only that in step (4), the cooling rate is 0.8 ℃ / min.

[0183] Comparative Example 21

[0184] The difference compared with Example 3 is only that in step (4), the stirring rate is 250 rpm.

[0185] Comparative Example 22

[0186] The difference compared with Example 3 is only that in step (4), the stirring rate is 300 rpm.

[0187] Comparative Example 23

[0188] The difference compared with Example 3 is only that in step (4), the crystallization time is 7 h.

[0189] Comparative Example 24

[0190] The difference compared with Example 3 is only that in step (4), the crystallization time is 8 h.

[0191] Comparative Example 25

[0192] The difference compared with Example 3 is only that in step (4), the solid-liquid ratio after cooling is 30%.

[0193] Comparative Example 26

[0194] The difference compared with Example 3 is only that in step (4), the solid-liquid ratio after cooling is 80%.

[0195] Comparative Example 27

[0196] The difference compared with Example 3 is only that the desulfurization reaction process is not performed.

[0197] Comparative Example 28

[0198] The difference compared with Example 3 is only that the defluorination reaction process is not performed.

[0199] Comparative Example 29

[0200] The difference compared with Example 3 is only that the desulfurization reaction process and the defluorination reaction process are not performed.

[0201] II. Test Method

[0202] 1. SEM test

[0203] The MAP crystals prepared in Examples 1-5 were subjected to SEM test by using MERLIN Compact field emission scanning electron microscope (Model: Quanta200FEG) produced by Zeiss Company, and the test results are shown in Figure 2 .

[0204] 2. Phosphorus pentoxide test

[0205] The general method for determining the content of phosphorus pentoxide in inorganic chemical products, i.e., quinoline molybdenum ketone gravimetric method, was used to determine P2O5 in the test solution. The principle of this method is that in an acidic medium, phosphorus pentoxide in the test solution reacts with the added precipitant quinoline molybdenum ketone to form a precipitate, which is filtered, dried, weighed, and the content of P2O5 is calculated.

[0206] Test reagents: hydrochloric acid or nitric acid, quinoline molybdenum ketone solution.

[0207] Test equipment: glass sand crucible (filter plate aperture is 5 pm-15 pm), electric heating constant temperature drying oven (temperature control at 180 ℃±5 ℃ or 250 ℃±10 ℃).

[0208] Test steps:

[0209] 1) Preparation of test solution: weigh an appropriate amount of sample (containing about 600 mg of phosphorus pentoxide), place it 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 it into a 500 mL volumetric flask, add 10 mL of hydrochloric acid or nitric acid, dilute to the mark with water, and shake well.

[0210] 2) Preparation of blank test solution: except that no sample is added, the amount of other reagents added is exactly the same as that for the preparation of test solution, and they are treated in the same way as the sample.

[0211] 3) Determination and calculation: 10 mL of the test solution and the blank test solution are pipetted into 250 mL beakers, respectively, and water is added to a total volume of about 100 mL. A watch glass is placed on top, and the temperature of the contents of the beakers is brought to 75°C ± 5°C in a water bath for 30 s, and 35 mL of the quinoline molybdate citrate solution is added. After cooling to room temperature, the solution is stirred 3 to 4 times during the cooling process. The supernatant is suction filtered using a glass sand crucible which has been previously dried to constant mass at 180°C ± 5°C or 250°C ± 10°C, and the precipitate is washed 5 to 6 times by decantation using about 20 mL of water each time. The precipitate is transferred to the glass sand crucible and washed 3 to 4 more times with water. The glass sand crucible is placed in an electrically heated constant temperature drying oven at 180°C ± 5°C for 45 min or at 250°C ± 10°C for 15 min, removed, and allowed to cool to room temperature in a desiccator. The mass is determined and the content is calculated as phosphorus pentoxide in mass percent.

[0212] 3. Fluoride ion test

[0213] The sample to be tested is decomposed with hydrochloric acid, and the pH of the solution is controlled to be maintained at 5.5 to 6.0 using a citric acid-sodium citrate buffer solution, while eliminating the interference of aluminum, iron and other ions. A potentiometric meter is used, with a saturated calomel electrode as the reference electrode and a fluoride ion selective electrode as the indicating electrode, to measure the electrode potential, and the content of fluorine is determined using a working curve method.

[0214] Test reagents: hydrochloric acid, nitric acid, sodium hydroxide (200 g / L), citric acid-sodium citrate buffer solution (24 g of citric acid and 270 g of trisodium citrate are weighed, dissolved in water, and diluted to 1000 mL, mixed well, and the pH is 5.5 to 6.0), fluoride standard solution (1.0 mg / mL), bromocresol green indicator solution (1 g / L).

[0215] Test equipment: fluoride ion selective electrode, saturated calomel electrode, potentiometric meter, magnetic stirrer.

[0216] Test steps:

[0217] 1) The sample is passed through a 125 μm test sieve and dried at 105 to 110°C for 2 h or more, and then cooled to room temperature in a desiccator.

[0218] 2) 0.1 to 0.2 g of the sample is weighed to an accuracy of 0.0001 g, placed in a 50 mL beaker, moistened with a small amount of water, 10 mL of hydrochloric acid solution is added, stirred for a few seconds, and left to stand for 30 min, stirring every 10 min. Diluted to the mark with water in a 100 mL volumetric flask, and shaken well. After standing, the clear solution is filtered or dried, and 10.0 mL of the filtrate is taken and placed in a 50 mL volumetric flask.

[0219] 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 is blue, then adjust the solution to be just 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.

[0220] 4) Insert the fluoride ion selective electrode and saturated calomel electrode, and measure the equilibrium potential value under constant stirring of the magnetic stirrer. Find the corresponding amount of fluorine on the working curve.

[0221] 5) Measure the equilibrium potential value according to 7.2-7.3. Draw a working curve on semi-logarithmic coordinate paper with the potential value (mV) as the ordinate and the corresponding amount of fluorine (mg) as the abscissa, and calculate the corresponding amount of fluoride ion. The data units ppm in the following examples and comparative examples can be converted to mg / kg.

[0222] 4. Component test of crystal product

[0223] The components of the monoammonium phosphate (MAP) crystals prepared in Examples 1-3 and Comparative Examples 27-29 were tested by inductively coupled plasma spectrometer (ICP), and the test results are shown in Table 6.

[0224] III. Analysis of test results of each example and comparative example

[0225] I. Test and analyze the influence of the desulfurization process conditions of the present application on the desulfurization effect and phosphorus yield. See Examples 1-7 and Comparative Examples 1-6 for details. Adjust the molar ratio of SO4 2+ to SO4 2- in wet-process phosphoric acid, the desulfurization reaction temperature, and the desulfurization reaction time, and test the residual mass fraction (ppm) of SO4 2- in the first filtrate and the P2O5 yield (%), where the P2O5 yield represents the ratio of the mass of P2O5 in the first filtrate to the mass of P2O5 in the wet-process phosphoric acid. Since phosphorus ore slurry is introduced for desulfurization during the desulfurization reaction, and the phosphorus ore slurry also contains a certain amount of P2O5, the calculated P2O5 yield may be greater than 100%. See Table 2 for details.

[0226] Table 2: Desulfurization process parameters and characterization parameter statistics of Examples 1-7 and Comparative Examples 1-6

[0227]

[0228] From Table 2, we can see that:

[0229] a) for the mole ratio of Ca 2+ to SO4 2- in the phosphorite slurry in the wet-process phosphoric acid, in combination with Examples 1-3 and Comparative Examples 1-2, it can be seen that as the amount of phosphorite slurry added increases, the mole ratio of Ca 2+ to SO4 2- increases gradually, although the sulfur removal effect is improved, the P2O5 yield of the first filtrate gradually decreases, proving that too much phosphorite slurry added will affect the phosphorus yield, and therefore the mole ratio of Ca 2+ to SO4 2- in the phosphorite slurry in the wet-process phosphoric acid needs to be controlled within an appropriate range to obtain better desulfurization effect and phosphorus yield.

[0230] b) for the desulfurization reaction temperature, in combination with Examples 3-5 and Comparative Examples 3-4, it can be seen that as the desulfurization reaction temperature increases, the desulfurization effect first increases and then decreases, the desulfurization effect is poor when the temperature is too low, and when the temperature exceeds 90°C, side reactions occur and the desulfurization effect gradually decreases, and therefore it is more appropriate to control the desulfurization reaction temperature at 80-90°C.

[0231] c) for the desulfurization reaction time, in combination with Examples 3, 6-7 and Comparative Examples 5-6, it can be seen that when the desulfurization time is too short, the desulfurization effect is significantly poorer, and when the time exceeds 4 h, the desulfurization effect improves very little, and therefore it is more appropriate to control the desulfurization reaction time at 3-4 h.

[0232] II. Test and analyze the influence of the defluorination process conditions of the present application on the defluorination effect and phosphorus yield. For details, see Examples 8-13 and Comparative Examples 7-12, adjust the mole ratio of the cation in the defluorination agent to F - in the second filtrate, the defluorination reaction temperature, and the defluorination reaction time, and test the residual mass fraction (ppm) of F - in the second filtrate and the P2O5 yield (%), wherein the P2O5 yield represents the ratio of the mass of P2O5 in the second filtrate to the mass of P2O5 in the wet-process phosphoric acid, as shown in Table 2.

[0233] Table 3 Defluorination process parameters and characterization parameter statistics of Examples 8-13 and Comparative Examples 7-12

[0234]

[0235] From Table 3, it can be seen that:

[0236] a) for the mole ratio of the cation in the defluorination agent to F -Based on the molar ratios of Examples 3, 8-9, and Comparative Examples 7-8, it can be seen that the defluorination effect is significantly poor when the amount of defluorinating agent added is too small. As the amount of defluorinating agent added gradually increases, the fluoride ion content gradually decreases, and the phosphorus yield also gradually decreases. When the molar ratio exceeds 1.4:3, the improvement in defluorination effect is minimal, thus proving that excessive defluorinating agent reduces the phosphorus yield. Simultaneously, the average particle size of the MAP crystals subsequently prepared in Examples 3, 8-9, and Comparative Examples 7-8 was tested. The comparison revealed that when the amount of defluorinating agent added was excessive, the average particle size of the subsequently prepared MAP crystals decreased to 300-400 μm, indicating that insufficient fluoride ions are insufficient to promote MAP crystallization. Therefore, the cations in the defluorinating agent and the F in the second filtrate... - The molar ratio of (1.2~1.4):3 is more suitable.

[0237] b) Regarding the defluorination reaction temperature, based on Examples 3, 10-11 and Comparative Examples 9-10, it can be seen that the defluorination effect is poor when the defluorination reaction temperature is too low, and the defluorination effect is only slightly improved when the defluorination reaction temperature exceeds 80℃. Therefore, it is more appropriate to control the defluorination reaction temperature at 50-80℃.

[0238] c) Regarding the defluorination reaction time, based on Examples 3, 12-13 and Comparative Examples 11-12, it can be seen that the defluorination effect is significantly worse when the defluorination reaction time is too short. Therefore, it is more appropriate to control the defluorination reaction time to 2-4 hours.

[0239] III. The effects of the neutralization process conditions of this application on the defluorination efficiency and phosphorus yield were tested and analyzed. Specifically, Examples 14-19 and Comparative Examples 13-18 were consulted. The parameters of pH, neutralization reaction temperature, and neutralization reaction time in the neutralization process were adjusted, and the F content in the third filtrate was tested. - The remaining mass fraction (ppm) and water-soluble phosphorus yield (%) are shown in Table 4, where the water-soluble phosphorus yield represents the ratio of the mass of P2O5 in the third filtrate to the mass of P2O5 in wet-process phosphoric acid.

[0240] Table 4. Statistical table of neutralization process parameters and characterization parameters for Examples 14-19 and Comparative Examples 13-18

[0241]

[0242] As can be seen from Table 4:

[0243] a) Regarding pH control during the neutralization reaction, based on Examples 3, 14-15 and Comparative Examples 13-14, it can be seen that as the pH increases, the F in the third filtrate... -The residual mass fraction of MAP decreases first and then increases, and the water-soluble phosphorus yield decreases. When the pH value is too high, a viscous precipitate is formed, which makes it difficult to form MAP, thereby significantly reducing the water-soluble phosphorus yield. Therefore, it is more appropriate to control the pH value to be 4-5.

[0244] b) For the neutralization reaction time, it can be known from Examples 3, 16-17 and Comparative Examples 15-16 that when the neutralization reaction time is too short, the F - The residual mass fraction of MAP decreases first and then increases, and the water-soluble phosphorus yield decreases. When the pH value is too high, a viscous precipitate is formed, which makes it difficult to form MAP, thereby significantly reducing the water-soluble phosphorus yield. Therefore, it is more appropriate to control the pH value to be 4-5.

[0245] c) For the neutralization reaction temperature, it can be known from Examples 3, 18-19 and Comparative Examples 17-18 that when the neutralization reaction temperature is too high, the water-soluble phosphorus yield is significantly reduced. The reason is that the solubility of MAP increases sharply with the increase of temperature, and the ammonia escape amount increases when the temperature is too high, which affects the stability of the generated MAP. Therefore, it is more appropriate to control the neutralization reaction temperature to be 70-90°C.

[0246] IV. The effects of the cooling crystallization process conditions of the present application on the MAP crystal yield and the average particle size of the crystals were tested and analyzed. For details, refer to Examples 20-27 and Comparative Examples 19-26. The cooling rate, stirring rate, crystallization time, and solid-liquid ratio after cooling in the cooling crystallization process were adjusted, and the MAP crystal yield (%) and average particle size (μm) were tested. The water-soluble phosphorus yield represents the ratio of the mass of P2O5 in the third filtrate to the mass of P2O5 in the wet-process phosphoric acid, and is shown in Table 5.

[0247] Table 5 Cooling crystallization process parameters and characterization parameter statistics table of Examples 20-27 and Comparative Examples 19-26

[0248]

[0249] It can be seen from Table 5 and the above analysis that: Figure 2

[0250] a) Figure 2 The MAP crystals prepared in Examples 1-5 have a significant prismatic structure, with a four-sided conical shape at both ends of the prism, an appropriate particle size, and uniform morphology.

[0251] b) For the cooling rate in the cooling crystallization process, it can be known from Examples 3, 20-21 and Comparative Examples 19-20 that the adjustment of the cooling rate has no significant effect on the MAP crystal yield. When the cooling rate is too fast, the crystal particle size is small, and when the cooling rate is too slow, the crystal particle size is large and the morphology difference is large. Therefore, it is more appropriate to control the cooling rate to be 0.2-0.6°C / min, so as to obtain high-quality MAP crystals with appropriate particle size and uniform morphology.

[0252] ​c) For the stirring rate in the cooling crystallization process, it can be seen from Examples 3, 22-23 and Comparative Examples 21-22 that the adjustment of the stirring rate has no significant effect on the yield of MAP crystals. When the stirring speed is too slow, the solid-phase mass transfer rate and solute growth rate are low, and small crystal particles are formed. When the stirring speed reaches 180 rpm, the solid-phase mass transfer rate and solute growth rate rapidly increase, and the average particle size of the crystals rapidly increases. When the stirring speed is too fast, the impact strength between the crystals and the stirring paddle will further increase. At this time, the secondary nucleation phenomenon between the crystals and the crystals, the probability of crystal rupture, and the violent collision between the crystals and the stirring paddle will also greatly increase, ultimately resulting in a decrease in the average particle size of the product. Therefore, as the stirring speed gradually increases, the average particle size of the crystals first increases and then decreases, and it is more appropriate to control the stirring rate to 150-300 rpm.

[0253] d) For the crystallization time in the cooling crystallization process, it can be seen from Examples 3, 24-25 and Comparative Examples 23-24 that the adjustment of the crystallization time has no significant effect on the yield of MAP crystals. When the crystallization time is too short, the crystal particle size is small, and when the crystallization time is too long, the crystal particle size is large and the morphology difference is large. Therefore, it is more appropriate to control the crystallization time to 4-7 h.

[0254] e) For the solid-liquid ratio after cooling in the cooling crystallization process, it can be seen from Examples 3, 26-27 and Comparative Examples 25-26 that the adjustment of the solid-liquid ratio after cooling has a significant effect on the yield of MAP crystals. Although increasing the solid-liquid ratio after cooling is beneficial to improving the yield of MAP crystals, a too high solid-liquid ratio will result in a slow crystal growth rate, a significant increase in crystal particle size, and a significant difference in morphology, making it difficult to obtain high-quality MAP crystals with uniform morphology and appropriate particle size. Therefore, it is more appropriate to control the solid-liquid ratio after cooling to 30-60%.

[0255] V. Test and analyze whether the use of the desulfurization reaction process and the defluorination reaction process described in the present application affects the quality of the prepared MAP crystals. Specifically, refer to Examples 1-3 and Comparative Examples 27-29. The components of the final prepared MAP crystals are tested, as shown in Table 6.

[0256] Table 6

[0257]

[0258] As can be seen from Table 6, the average particle size of the MAP crystals is mainly affected by the sulfate ion. A too high concentration of sulfate ions will result in a significant increase in particle size. The principle is that SO4 2- has a strong hydrogen bonding ability similar to PO4 3- , which makes SO4 2-Cylinders and conical surfaces that can enter the MAP can affect the growth rate of the solution, resulting in increased crystal particle size, so the foregoing desulfurization reaction process is crucial for obtaining high-quality MAP crystals with uniform morphology and appropriate particle size.

[0259] When no desulfurization reaction process or defluorination reaction process is performed, the sulfur and fluorine impurities of the obtained MAP crystals significantly increase, while the sulfur and fluorine impurities of the MAP crystals obtained in Examples 1-3, which have the desulfurization reaction process and the defluorination reaction process, can be significantly reduced, thereby improving the quality of the MAP crystals.

[0260] In summary, the precursor prepared by the coprecipitation method has a regular and uniform morphology, and the LMFP positive electrode material prepared from the precursor has good electrochemical performance.

[0261] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and embodiments having the same technical idea and substantially the same function and effect within the scope of the technical solutions of the present application are included in the technical scope of the present application. In addition, within the scope of the main idea of the present application, various modifications that can be thought of by those skilled in the art, other ways constructed by combining part of the components of the embodiments are also included in the scope of the present application.

Claims

1. An industrial-grade monoammonium phosphate crystal, characterized in that, The industrial-grade monoammonium phosphate crystals have a prismatic structure with four-sided pyramidal ends. The particle size is 500~700 μm and the aspect ratio is (2~4):

1. By mass percentage, the industrial-grade monoammonium phosphate crystals contain more than 61% P2O5, more than 11.5% N, and 50-150 mg / kg F ions. The preparation method of the industrial-grade monoammonium phosphate crystals includes the following steps: Wet-process phosphoric acid is mixed with phosphate rock slurry and subjected to a desulfurization reaction, followed by solid-liquid separation to obtain a first filter cake and a first filtrate; wherein the wet-process phosphoric acid contains more than 20% P2O5 by mass, and the phosphate rock slurry contains more than 20% Ca. 2+ With the SO4 in the wet-process phosphoric acid 2- The molar ratio is (0.8~1.0):1, the desulfurization reaction temperature is 80~90℃, and the desulfurization reaction time is 3~4 h; A defluorinating agent is added to the first filtrate to carry out a defluorination reaction, followed by solid-liquid separation to obtain a second filter cake and a second filtrate; wherein, the cations in the defluorinating agent react with the F in the first filtrate. - The molar ratio is (1.2~1.4):3, the defluorination reaction temperature is 50~80℃, and the defluorination reaction time is 2~4 h; The second filtrate is mixed with an ammonia solution to adjust the pH to 4-5 and then neutralized. The solid and liquid are then separated to obtain a third filter cake and a third filtrate. The neutralization reaction temperature is 70-90℃ and the neutralization reaction time is 1.5-3 h. The third filtrate is successively concentrated, cooled and crystallized, and then separated into solid and liquid components to obtain industrial-grade monoammonium phosphate crystals. The specific conditions for the cooled crystallization include: a cooling rate of 0.2~0.6℃ / min, a stirring rate of 150~300 rpm, a crystallization time of 5~7 h, and a solid-liquid ratio of 60% or 70% after cooling.

2. A method for preparing industrial-grade monoammonium phosphate crystals as described in claim 1, characterized in that, Includes the following steps: Wet-process phosphoric acid is mixed with phosphate rock slurry and subjected to desulfurization reaction, followed by solid-liquid separation to obtain the first filter cake and the first filtrate; A defluorinating agent is added to the first filtrate to carry out a defluorination reaction, followed by solid-liquid separation to obtain a second filter cake and a second filtrate. The second filtrate is mixed with an ammonia solution to adjust the pH to 4-5 and neutralize it. Then the solid and liquid are separated to obtain the third filter cake and the third filtrate. The third filtrate was successively concentrated, cooled and crystallized, and then separated into solid and liquid components to obtain industrial-grade monoammonium phosphate crystals.

3. The method for preparing industrial-grade monoammonium phosphate crystals according to claim 2, characterized in that, The cations in the defluorinating agent react with the F in the first filtrate. - The molar ratio is (1.2~1.4):

3.

4. The method for preparing industrial-grade monoammonium phosphate crystals according to claim 2, characterized in that, The ammonia-containing solution includes at least one of liquid ammonia and ammonia water.

5. The method for preparing industrial-grade monoammonium phosphate crystals according to claim 2, characterized in that, After cooling crystallization and solid-liquid separation, the obtained solid phase is the industrial-grade monoammonium phosphate crystal, and the liquid phase is refluxed into the wet-process phosphoric acid and undergoes a desulfurization reaction with the phosphate rock slurry again. The first filter cake is recycled to the extraction process section of the wet-process phosphoric acid preparation process to prepare the wet-process phosphoric acid. The second filter cake is dried and recycled as a fluorosilicate product. The third filter cake is dried and then recycled as a slow-release nitrogen and phosphorus fertilizer.

6. A type of iron phosphate crystal, characterized in that, The iron phosphate crystals are prepared by mixing and reacting an iron salt solution, an oxidant, and the industrial-grade monoammonium phosphate crystals as described in claim 1. 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.

7. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material is prepared by mixing and reacting the iron phosphate crystals described in claim 6 with a lithium source.

8. A positive electrode sheet, characterized in that, Includes the lithium iron phosphate cathode material as described in claim 7.

9. A secondary battery, characterized in that, Includes the positive electrode sheet as described in claim 8.

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