Preparation method of high-purity aluminum metaphosphate

Abstract

This application relates to a method for preparing high-purity aluminum metaphosphate, belonging to the field of inorganic material preparation technology. The method includes: reacting an aqueous solution of aluminum hydroxide with phosphoric acid to obtain an aluminum dihydrogen phosphate precursor solution; sequentially evaporating and concentrating the precursor solution and drying it to obtain a dried material; subjecting the dried material to low-temperature calcination to obtain a low-temperature calcined material; mixing the low-temperature calcined material with a solvent to form a slurry, and performing solid-liquid separation to obtain a filter cake and a filtrate; then subjecting the filter cake to high-temperature calcination to obtain a high-purity aluminum metaphosphate product. This method, through the synergistic effect of solvent debinding and filtration separation after low-temperature calcination, simultaneously achieves particle size control and purification improvement without the aid of mechanical crushing. The resulting product has high purity and a concentrated particle size distribution, and the process is green and environmentally friendly, making it suitable for the large-scale preparation of high-purity aluminum metaphosphate raw materials in fields such as high-end laser glass.

Description

Technical Field

[0001] This application relates to the field of inorganic material preparation technology, and in particular to a method for preparing high-purity aluminum metaphosphate. Background Technology

[0002] High-end phosphate optical glasses, such as laser glass, are key materials for high-power laser systems (e.g., inertial confinement fusion devices), and therefore have extremely stringent requirements for raw material purity. Transition metal impurities (such as Fe, Cu, Cr, Ni, Co, Mn, etc.) contain unpaired electrons, which can easily cause strong absorption, fluorescence quenching, and phase transition scattering at laser wavelengths (e.g., around 1 μm), severely reducing laser output efficiency. According to mainstream international standards, the total metal impurities in laser glass raw materials must be ≤10 ppm, and the individual content of key coloring ions must be <1 ppm, or even lower.

[0003] Aluminum metaphosphate (Al(PO3)3) is one of the main raw materials for preparing phosphate optical glasses, and its purity directly determines the optical quality and laser output efficiency of laser glass. Currently, domestically produced aluminum metaphosphate products are mostly 3N-4N grade, with high impurity content, wide particle size distribution, and high free acid content, making it difficult to meet the stringent requirements of high-end laser glasses. High-end products mainly rely on imports. Existing aluminum metaphosphate preparation processes often employ direct high-temperature calcination of precursors, spray granulation followed by crushing and grinding, which suffers from high energy consumption, easy introduction of impurities, toxic emissions of phosphorus pentoxide, and uneven particle size.

[0004] In view of this, it is necessary to design a method for preparing high-purity aluminum metaphosphate to solve the above problems. Summary of the Invention

[0005] This application provides a method for preparing high-purity aluminum metaphosphate to solve the following technical problem: how to develop a method for preparing high-purity aluminum metaphosphate to meet the application needs of high-end laser glass and other fields. In a first aspect, embodiments of this application provide a high-purity aluminum metaphosphate product, wherein the purity of the high-purity aluminum metaphosphate product is ≥99.999%; By mass percentage, the total metal impurity content in the high-purity aluminum metaphosphate is ≤10ppm, and the content of each individual metal impurity is <1ppm; The mass fraction of free acid in the high-purity aluminum metaphosphate is ≤0.01%; The median particle size of the high-purity aluminum metaphosphate is 30μm~80μm, and the particle size distribution width index D90 / D10 of the high-purity aluminum metaphosphate is ≤3.0.

[0006] Secondly, this application provides a method for preparing the high-purity aluminum metaphosphate described in the first aspect, comprising the following steps: Provide an aqueous solution of aluminum hydroxide; Phosphoric acid is added to the aqueous solution of aluminum hydroxide, and then the first reaction is carried out to obtain an aluminum dihydrogen phosphate precursor solution. The aluminum dihydrogen phosphate precursor solution was subjected to evaporation and concentration treatment followed by drying treatment to obtain dried material; The dried material is subjected to low-temperature roasting treatment to obtain low-temperature roasted material; The low-temperature roasted material is mixed with a solvent to obtain a slurry; The slurry is subjected to solid-liquid separation treatment to obtain filter cake and filtrate; and, The filter cake was subjected to high-temperature calcination to obtain the high-purity aluminum metaphosphate.

[0007] Optionally, the calcination temperature of the low-temperature calcination treatment is 450°C to 600°C.

[0008] Optionally, the calcination time for the low-temperature calcination treatment is 1 hour to 3 hours.

[0009] Optionally, the calcination temperature of the high-temperature calcination treatment is 750°C to 850°C.

[0010] Optionally, the high-temperature roasting treatment takes place for 2 to 4 hours.

[0011] Optionally, the solvent may include deionized water.

[0012] Optionally, the mass of the solvent is 0.3 to 0.7 times the mass of the low-temperature roasted material.

[0013] Optionally, the aqueous solution of aluminum hydroxide is prepared by mixing high-purity aluminum hydroxide with deionized water.

[0014] Optionally, the temperature of the aluminum hydroxide aqueous solution is 75°C to 95°C.

[0015] Optionally, the stirring rate of the first reaction is 150 rpm to 400 rpm; the stirring time of the first reaction is 2 hours to 3 hours.

[0016] Optionally, the temperature of the first reaction is 75°C to 95°C.

[0017] Optionally, the first reaction time is 2 to 3 hours.

[0018] Optionally, the molar ratio of the high-purity aluminum hydroxide to the phosphoric acid is 1:3.1~4.0.

[0019] Optionally, the purity of the high-purity aluminum hydroxide is >99.99%.

[0020] Optionally, the phosphoric acid is analytical grade phosphoric acid, and the mass fraction of the analytical grade phosphoric acid is 85%.

[0021] Optionally, the filtrate may be refluxed to the step of preparing the aluminum dihydrogen phosphate precursor solution, replacing part or all of the deionized water.

[0022] The technical solutions provided in this application have the following advantages compared with the prior art: 1. This application provides a high-purity aluminum metaphosphate with a purity ≥99.999%; the total metal impurity content in the high-purity aluminum metaphosphate is ≤10ppm by mass percentage, and the content of each individual metal impurity is <1ppm; the mass fraction of free acid in the high-purity aluminum metaphosphate is ≤0.01%; which can meet the application requirements of high-end laser glass and other fields.

[0023] 2. This application provides a method for preparing high-purity aluminum metaphosphate. High-purity aluminum hydroxide is mixed with deionized water to form an aluminum hydroxide aqueous solution. Phosphoric acid is then added to the aluminum hydroxide aqueous solution and a first reaction is carried out to obtain an aluminum dihydrogen phosphate precursor solution. After evaporation, concentration, and drying, the solution is subjected to low-temperature calcination. Utilizing the incomplete conversion characteristics of the aluminum metaphosphate polycondensation reaction under these temperature conditions, the material is precisely controlled in a thermodynamically metastable intermediate state that exhibits adhesion but is not yet solidified. Subsequently, the low-temperature calcined material is mixed with a solvent. The solvent's wetting and penetration effect is used to directionally disrupt the metastable adhesive structure, allowing the material to naturally decompose without introducing any external mechanical force. The loose aggregate consists of particles with a specific particle size distribution. Simultaneously, the solvent dissolves free phosphoric acid and soluble metal impurity salts, and these impurities are removed along with the filtrate through filtration. This allows particle size control and purification to be completed synergistically in the same unit operation. The resulting filter cake is then subjected to high-temperature calcination to achieve thermodynamic equilibrium of complete polycondensation. Since there is no excess free phosphoric acid or low-melting-point phase in the filter cake that would cause sintering and adhesion, the particles only undergo internal densification without significant inter-particle sintering. This directly fixes the pre-formed particle morphology from the solvent-based debinding step into the final product's particle size distribution, ultimately yielding a high-purity aluminum metaphosphate product. This method breaks away from the traditional process of first roasting into blocks and then crushing into powder, avoiding equipment wear and impurity introduction and particle size distribution deterioration caused by mechanical processing. At the same time, it realizes the recycling of solvent and phosphoric acid through filtrate reflux, and avoids the generation of toxic phosphorus pentoxide gas during the high-temperature roasting stage. It has comprehensive advantages such as high product purity, uniform particle size, low energy consumption, and green environmental protection, and is suitable for the large-scale preparation of high-purity aluminum metaphosphate raw materials in fields such as high-end laser glass. Attached Figure Description

[0024] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0025] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 A scanning electron microscope (SEM) 295X image of high-purity aluminum metaphosphate provided for an embodiment of this application. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0028] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1~6" cover all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the content, amount added, etc., mentioned herein are based on mass; for example, an addition amount of 1% means that the additive accounts for 1% of the total mass of the system; a content of 10% means that the component has a mass fraction of 10% in the composition. Unless otherwise specified, the terms "comprising," "including," etc., used herein mean "including but not limited to"; relational terms such as "first," "second," etc., are used only to distinguish different entities or operations and do not imply an actual order or relationship; "and / or" indicates that multiple situations can exist alone or simultaneously; expressions such as "at least one," "multiple," "at least one," etc., refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned in this article, such as mass ratio and molar ratio, should be understood as the correspondence between the antecedent and consequent of a proportional equation, according to the order in which they are described. All raw materials, reagents, instruments, and equipment used in this article can be purchased commercially or prepared using existing methods.

[0029] In a first aspect, embodiments of this application provide a high-purity aluminum metaphosphate product, wherein the purity of the high-purity aluminum metaphosphate product is ≥99.999%; By mass percentage, the total metal impurity content in the high-purity aluminum metaphosphate is ≤10ppm, and the content of each individual metal impurity is <1ppm; The mass fraction of free acid in the high-purity aluminum metaphosphate is ≤0.01%; The median particle size of the high-purity aluminum metaphosphate is 30μm~80μm, and the particle size distribution width index D90 / D10 of the high-purity aluminum metaphosphate is ≤3.0.

[0030] A purity of 99.999% or higher means that the mass fraction of aluminum metaphosphate, the main component, in high-purity aluminum metaphosphate products is not less than 99.999%, i.e., the total mass fraction of all impurities does not exceed one part per hundred thousand. A total metal impurity content of 10 ppm means that the sum of the contents of all metal element impurities in the product does not exceed ten parts per hundred thousand of the total mass; total metal impurities are the sum of the contents of each individual metal impurity. A total metal impurity content of less than 1 ppm means that the content of any single metal element impurity in the product does not exceed one part per million of the total mass. A free acid mass fraction of 0.01% means that the content of free phosphoric acid or acidic phosphate species in the product, calculated as phosphoric acid, does not exceed one part per ten thousand of the total mass.

[0031] Median particle size refers to the particle size value corresponding to a cumulative particle size distribution percentage of 50% of the product particles, measured in micrometers (μm). D90 / D10 is the ratio of the particle size corresponding to a cumulative particle size distribution percentage of 90% to the particle size corresponding to a cumulative particle size distribution percentage of 10%. The smaller the ratio, the more concentrated the particle size distribution.

[0032] Secondly, this application provides a method for preparing the high-purity aluminum metaphosphate described in the first aspect, comprising the following steps: Provide an aqueous solution of aluminum hydroxide; Phosphoric acid is added to the aqueous solution of aluminum hydroxide, and then the first reaction is carried out to obtain an aluminum dihydrogen phosphate precursor solution. The aluminum dihydrogen phosphate precursor solution was subjected to evaporation and concentration treatment followed by drying treatment to obtain dried material; The dried material is subjected to low-temperature roasting treatment to obtain low-temperature roasted material; The low-temperature roasted material is mixed with a solvent to obtain a slurry; The slurry is subjected to solid-liquid separation treatment to obtain filter cake and filtrate; and, The filter cake was subjected to high-temperature calcination to obtain the high-purity aluminum metaphosphate.

[0033] The first step involves reacting an aqueous solution of aluminum hydroxide with phosphoric acid to obtain an aluminum dihydrogen phosphate precursor solution. This step serves to neutralize the solid, high-purity aluminum hydroxide with the liquid phosphoric acid, generating soluble aluminum dihydrogen phosphate, which then dissolves in deionized water to form a homogeneous solution. Aluminum dihydrogen phosphate has good solubility in water, and this step converts the solid aluminum source into a solution state. The reaction equation for this step is: Al(OH)3 + 3H3PO4 → Al(H2PO4)3 + 3H2O. The advantage of using a solution-state precursor is that it allows for uniform mixing of aluminum and phosphate at the molecular scale, ensuring a homogeneous final product.

[0034] The aluminum dihydrogen phosphate precursor solution was sequentially subjected to evaporation and concentration treatment followed by drying treatment to obtain dried material. The dried material was then subjected to low-temperature calcination treatment to obtain low-temperature calcined material. The purpose of evaporation and concentration treatment and drying treatment is to remove most of the deionized water from the aluminum dihydrogen phosphate precursor solution, causing the dissolved aluminum dihydrogen phosphate to precipitate as a solid, facilitating subsequent transfer and calcination treatment. The purpose of low-temperature calcination is to induce preliminary thermal decomposition and condensation reaction of aluminum dihydrogen phosphate. Solvent is added to the low-temperature roasted material to break down its agglomerates and form a slurry. The slurry is then filtered to obtain a filter cake and a filtrate. The purpose of adding solvent to the low-temperature roasted material is to utilize its wetting and penetrating properties to disrupt the agglomerate structure formed during the low-temperature roasting process. Upon contact with the solvent, the internal agglomerates of the low-temperature roasted material are broken down and loosened, forming a slurry composed of particles of varying sizes. This step, without the aid of any mechanical crushing or grinding equipment, allows for the natural loosening of agglomerated aggregates into a particle aggregate with a specific size distribution simply by adding solvent, directly achieving control over the product particle size.

[0035] The purpose of filtration in separating the slurry is to separate the loose solid particles from the liquid containing dissolved impurities. During the addition of solvent to the low-temperature roasted material, unreacted phosphoric acid remaining in the material and soluble metal impurity salts generated during the reaction are dissolved by the solvent and enter the liquid phase. After filtration, these impurities are removed with the filtrate, while the solid particles are retained as a filter cake. This step achieves two objectives simultaneously: firstly, the physical separation of solid particles from the impurity solution, and secondly, the chemical separation of soluble impurities from the target product. The synergistic effect of these two aspects significantly improves the purity of the filter cake compared to the low-temperature roasted material.

[0036] The filter cake is calcined at high temperature to obtain high-purity aluminum metaphosphate. The purpose of high-temperature calcination is to further condense the aluminum metaphosphate oligomers in the filter cake into fully polymerized aluminum metaphosphate as the final product.

[0037] The method described in this application does not require any spray granulation or mechanical crushing and grinding treatment on the high-temperature roasted product, and can directly obtain a high-purity aluminum metaphosphate product with uniform particle size.

[0038] In some embodiments, the calcination temperature of the low-temperature calcination treatment is 450°C to 600°C.

[0039] In some embodiments, the calcination time for the low-temperature calcination treatment is 1 to 3 hours.

[0040] Within a temperature range of 450°C to 600°C, aluminum dihydrogen phosphate molecules undergo dehydration condensation, gradually transforming into oligomeric or amorphous aluminum metaphosphate. The low-temperature roasting process simultaneously forms adhesive agglomerates with a certain mechanical strength. If the low-temperature roasting temperature is below 450°C, the decomposition and condensation reaction of aluminum dihydrogen phosphate is insufficient, and the material remains primarily powdery, failing to form controllable adhesive agglomerates. Adding deionized water will not yield slurry particles with a suitable particle size. If the low-temperature roasting temperature is above 600°C, the material will be over-sintered and form hard lumps. Subsequent addition of deionized water will be insufficient to effectively break down its adhesiveness, failing to achieve particle size control, and significantly increasing energy consumption.

[0041] The low-temperature roasting time determines the extent of thermal decomposition and condensation reactions of the dried material at temperatures between 450°C and 600°C. During heating, aluminum dihydrogen phosphate in the dried material first removes residual adsorbed water and water of crystallization. Subsequently, condensation reactions occur between aluminum dihydrogen phosphate molecules, removing water molecules and forming an aluminum-oxygen-phosphorus bridging structure. The degree of this condensation reaction gradually increases with the extension of the holding time. The chemical composition of the low-temperature roasted material gradually transitions from aluminum dihydrogen phosphate to oligomeric aluminum metaphosphate, and the physical form of the material gradually changes from loose powder to agglomerates with a certain degree of cohesion.

[0042] The low-temperature roasting time also controls the content of residual free phosphoric acid in the low-temperature roasted material. Within the temperature range of 450°C to 600°C, some of the free phosphoric acid produced by the decomposition of aluminum dihydrogen phosphate escapes in gaseous form, while some remains in an adsorbed state inside the low-temperature roasted material. The longer the holding time, the more fully the free phosphoric acid escapes, and the less residual free acid remains in the low-temperature roasted material. However, this also increases the material's adhesiveness.

[0043] In this embodiment, the low-temperature calcination time, ranging from 1 to 3 hours, and the low-temperature calcination temperature constitute a synergistic control parameter. Within the low-temperature calcination temperature range of 450°C to 600°C, the low-temperature calcination time and temperature are complementary: when the low-temperature calcination temperature is close to 450°C, the polycondensation reaction rate is slow, requiring a low-temperature calcination time of nearly 3 hours to achieve the desired degree of conversion; when the low-temperature calcination temperature is close to 600°C, the reaction rate accelerates, and a low-temperature calcination time of nearly 1 hour can achieve the same level of adhesion.

[0044] In some embodiments, the calcination temperature of the high-temperature calcination treatment is 750°C to 850°C.

[0045] In some embodiments, the high-temperature roasting treatment takes 2 to 4 hours.

[0046] Within a temperature range of 750°C to 850°C, the polymerization reaction of aluminum metaphosphate reaches completion, and the product structure tends to be stable. Simultaneously, within this temperature range, the filter cake particles do not undergo significant melting or deformation, and the particle morphology is maintained intact. Therefore, the particle size distribution formed by adding water to break up adhesion is directly inherited as the particle size distribution of the final product. If the high-temperature calcination temperature is below 750°C, the polycondensation reaction of aluminum metaphosphate is incomplete, and the product contains residual aluminum dihydrogen phosphate or oligomers. The chemical composition of the product deviates from the stoichiometry of aluminum metaphosphate, and the free acid content is high. If the high-temperature calcination temperature is above 850°C, local melting or sintering adhesion will occur on the particle surface, destroying the already formed particle morphology and particle size distribution. At the same time, energy consumption increases significantly, and the requirements for calcination equipment are correspondingly increased.

[0047] The high-temperature calcination time determines the depth of the condensation reaction in the filter cake at temperatures between 750°C and 850°C. After low-temperature calcination and water-based debinding treatment, the chemical composition of the filter cake's solid matrix consists of low-polymerization-degree aluminum metaphosphate species, whose degree of polymerization has not yet reached the theoretical state of fully polymerized aluminum metaphosphate. During the high-temperature calcination holding stage, dehydration condensation reactions continue to occur between oligomers, and the crosslinking density of the aluminum-oxygen-phosphorus bridging structure gradually increases with the extension of the holding time, with the chemical composition approaching the stoichiometric ratio of Al(PO3)3.

[0048] The high-temperature roasting time also determines the final degree of removal of residual free phosphoric acid in the filter cake. Even after the water filtration step, the filter cake still contains trace amounts of adsorbed phosphoric acid or phosphate species. These residues further volatilize or decompose into phosphorus oxide gases and escape from the system during the high-temperature roasting holding stage. The longer the holding time, the more thoroughly the residual free acid is removed, and the lower the free acid content of the final product.

[0049] The high-temperature calcination time also affects the final morphology and structural stability of the product particles. During the holding process at 750°C to 850°C, the particles formed by the water-addition de-adhesion step are further densified at high temperatures. The micropores and defects inside the particles gradually heal under the drive of surface tension, and the mechanical strength and chemical stability of the particles are correspondingly improved. However, the holding time must be controlled within a reasonable range to avoid irreversible sintering and adhesion between particles. In the embodiments of this application, the high-temperature calcination time is 2 to 4 hours, which constitutes a synergistic control parameter with the high-temperature calcination temperature of 750°C to 850°C. When the high-temperature calcination temperature is close to 750°C, the polycondensation reaction rate is relatively slow, requiring a high-temperature calcination time of nearly 4 hours to ensure complete reaction and sufficient removal of free acid; when the high-temperature calcination temperature is close to 850°C, the reaction rate accelerates, and a high-temperature calcination time of nearly 2 hours can achieve the same degree of complete polycondensation.

[0050] In some embodiments, the solvent includes deionized water.

[0051] In some embodiments, the mass of the solvent is 0.3 to 0.7 times the mass of the low-temperature roasted material.

[0052] When the added solvent is 0.3 to 0.7 times the mass of the low-temperature roasted material, the total amount of solvent is just enough to completely wet all the low-temperature roasted material and effectively break down its adhesiveness, making the material uniformly loose into slurry particles with a suitable particle size distribution. Within this solvent range, the solid content of the slurry is at an appropriate level, which ensures both the efficiency of the filtration and separation operation and the product yield, while also ensuring that soluble impurities are fully dissolved and removed with the filtrate.

[0053] In some embodiments, the aqueous solution of aluminum hydroxide is prepared by mixing high-purity aluminum hydroxide with deionized water.

[0054] In some embodiments, the temperature of the aqueous aluminum hydroxide solution is 75°C to 95°C.

[0055] In some embodiments, the stirring rate of the first reaction is 150 rpm to 400 rpm; the stirring time of the first reaction is 2 hours to 3 hours.

[0056] In some embodiments, the temperature of the first reaction is 75°C to 95°C.

[0057] Although the neutralization reaction between high-purity aluminum hydroxide and phosphoric acid exhibits a thermodynamically spontaneous tendency, the solid-liquid reaction between solid high-purity aluminum hydroxide and liquid phosphoric acid requires sufficient energy input to accelerate the reaction process and ensure its completeness. The combined conditions of a first-reaction temperature of 75°C to 95°C and a stirring time of 2 to 3 hours ensured the complete and thorough reaction of high-purity aluminum hydroxide and phosphoric acid in deionized water, producing a clear and transparent homogeneous aluminum dihydrogen phosphate precursor solution. The first-reaction temperature of 75°C to 95°C can be achieved using a water bath heating method. Water bath heating uses hot water as a medium to uniformly transfer heat to the reaction vessel, providing a uniform and stable thermal environment for the reaction and promoting its full progress.

[0058] In some implementations, the first reaction takes 2 to 3 hours.

[0059] In some embodiments, the molar ratio of the high-purity aluminum hydroxide to the phosphoric acid is 1:3.1 to 4.0.

[0060] The theoretical molar ratio of phosphoric acid to aluminum hydroxide is 3:1. In this embodiment, phosphoric acid is used in appropriate excess relative to the theoretical stoichiometric ratio to ensure that the high-purity aluminum hydroxide is completely converted into aluminum dihydrogen phosphate, avoiding incomplete reaction and residual unreacted aluminum hydroxide due to insufficient phosphoric acid. However, the excess of phosphoric acid should not be too much (the molar ratio of phosphoric acid to high-purity aluminum hydroxide should not be higher than 4.0), otherwise it will increase the content of free acid in subsequent processes.

[0061] In some embodiments, the purity of the high-purity aluminum hydroxide is >99.99%.

[0062] In some embodiments, the phosphoric acid is analytical grade phosphoric acid, and the mass fraction of the analytical grade phosphoric acid is 85%.

[0063] Using high-purity aluminum hydroxide and analytical-grade phosphoric acid as raw materials allows for the control of metal impurities at the source, ensuring the purity of the final product. In some embodiments, the filtrate is refluxed to the step of preparing the aluminum dihydrogen phosphate precursor solution, replacing part or all of the deionized water.

[0064] The filtrate contains unreacted phosphoric acid and a small amount of dissolved metal impurities. By recycling the filtrate back to the step of reacting high-purity aluminum hydroxide with phosphoric acid in deionized water, the resources of phosphoric acid and deionized water can be utilized, avoiding the waste of phosphoric acid and the discharge of waste liquid.

[0065] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national / industry standards; if there is no corresponding national / industry standard, they are performed according to general international standards, conventional conditions, or conditions recommended by the manufacturer.

[0066] The high-purity aluminum hydroxide used in the examples had a purity >99.99%, and the phosphoric acid used was analytical grade phosphoric acid (mass fraction of 85%). In the examples, the particle size of the product was measured using a laser particle size analyzer, the purity and metal impurity content were measured using ICP-MS, and the free acid content was measured using ion chromatography.

[0067] Example 1 Take 1000 mL of deionized water and place it in a beaker. Heat the beaker in a water bath to 75°C. Add 130 g of high-purity aluminum hydroxide (purity > 99.99%) to the deionized water to obtain an aluminum hydroxide aqueous solution.

[0068] 600g of analytical grade phosphoric acid (mass fraction 85%) was slowly added dropwise to an aqueous solution of aluminum hydroxide. The water bath temperature was maintained at 75°C, and the mixture was stirred at a stirring rate of 300 rpm for 3 hours to obtain a clear and transparent aluminum dihydrogen phosphate precursor solution.

[0069] The aluminum dihydrogen phosphate precursor solution was heated, evaporated, concentrated, and then dried to obtain the dried material.

[0070] The dried material was placed in a muffle furnace and roasted at 450°C for 2 hours to obtain a low-temperature roasted material.

[0071] After the low-temperature roasted material is naturally cooled, deionized water is added as a solvent. The mass of the deionized water is 0.3 times the mass of the low-temperature roasted material, which breaks down the adhesiveness of the low-temperature roasted material and forms a slurry.

[0072] The slurry is poured into a Buchner funnel for filtration and separation to obtain a filter cake and a filtrate. The filtrate is then returned to the step of preparing the aluminum dihydrogen phosphate precursor solution, replacing part of the deionized water, thus achieving the resource recycling of phosphoric acid and water.

[0073] The filter cake was transferred to a clean crucible, which was then placed in a muffle furnace and calcined at 750°C for 4 hours. After calcination, the mixture was allowed to cool naturally to room temperature to obtain a high-purity aluminum metaphosphate product.

[0074] The high-purity aluminum metaphosphate product obtained in Example 1 was subjected to various performance tests. The test results showed that the median particle size of the high-purity aluminum metaphosphate product was 68.4 μm, D90 / D10 = 3.0; the purity of the high-purity aluminum metaphosphate product was 99.999% (total metal impurity content was 9.8 ppm, including 0.9 ppm Fe, 0.06 ppm Cu, 0.15 ppm Cr, 0.25 ppm Ni, 0.5 ppm Ti, 0.9 ppm Na, 0.3 ppm Ca, and 0.2 ppm Mg, with other individual metal impurities all less than 0.1 ppm); and the free acid mass fraction of the high-purity aluminum metaphosphate product was 0.01%.

[0075] Example 2 The operation steps are the same as in Example 1, except that: the water bath temperature is 85°C, the water bath stirring time is 2.5 hours; the amount of phosphoric acid is 634g; the low-temperature roasting temperature is 480°C, the low-temperature roasting time is 2 hours, and the low-temperature roasted material is obtained; after the low-temperature roasted material is naturally cooled, the mass of deionized water added is 0.5 times the mass of the low-temperature roasted material; the high-temperature roasting temperature is 800°C, and the high-temperature roasting time is 3.5 hours.

[0076] The high-purity aluminum metaphosphate product obtained in Example 2 was subjected to performance testing. The results were as follows: median particle size 55.7 μm, D90 / D10 = 2.8; total metal impurity content 8.9 ppm (sodium content 0.9 ppm, other individual metal impurity contents all less than 0.9 ppm); free acid mass fraction 0.009%.

[0077] Example 3 The operation steps are the same as in Example 1, except that: the water bath temperature is 90°C, the water bath stirring time is 2.5 hours; the amount of phosphoric acid is 670g; the low-temperature roasting temperature is 550°C, the low-temperature roasting time is 1.5 hours, and the low-temperature roasted material is obtained; after the low-temperature roasted material is naturally cooled, the mass of deionized water added is 0.6 times the mass of the low-temperature roasted material; the high-temperature roasting temperature is 850°C, and the high-temperature roasting time is 3 hours.

[0078] The high-purity aluminum metaphosphate product obtained in Example 3 was subjected to performance testing. The results were as follows: median particle size 42.1 μm, D90 / D10 = 2.6; total metal impurity content 8.4 ppm (sodium content 0.8 ppm, other individual metal impurity contents less than 0.7 ppm); free acid mass fraction 0.006%.

[0079] Example 4 The operation steps are the same as in Example 1, except that: the water bath temperature is 95°C, the water bath stirring time is 2 hours; the amount of phosphoric acid is 730g; the low-temperature roasting temperature is 600°C, the low-temperature roasting time is 1 hour, and the low-temperature roasted material is obtained; after the low-temperature roasted material is naturally cooled, the mass of deionized water added is 0.7 times the mass of the low-temperature roasted material; the high-temperature roasting temperature is 850°C, and the high-temperature roasting time is 2.5 hours.

[0080] The high-purity aluminum metaphosphate product obtained in Example 4 was subjected to performance testing. The results were as follows: median particle size 35.8 μm, D90 / D10 = 2.5; total metal impurity content 8.1 ppm (sodium content 0.7 ppm, other individual metal impurity contents less than 0.5 ppm); free acid mass fraction 0.004%.

[0081] Comparative Example 1 Comparative Example 1 provides a method for preparing aluminum metaphosphate, which differs from Example 1 in that it does not undergo low-temperature calcination; instead, the dried precursor material is directly calcined at high temperature. To reduce adhesion, a layer of aluminum metaphosphate powder is pre-laid at the bottom of the crucible. The remaining steps and parameters are the same as in Example 1. Testing revealed that the product obtained in Comparative Example 1 still exhibited significant adhesion to the crucible after sintering, requiring crushing and grinding to obtain granules after removal. The product contained a high sodium content of 356 ppm and an iron content of 8.9 ppm. These results indicate that direct high-temperature calcination without low-temperature treatment leads to severe material adhesion, making complete removal difficult, and impurities (especially sodium) cannot be effectively removed. Consequently, the product purity falls far short of the requirements for laser glass raw materials.

[0082] Comparative Example 2 Comparative Example 2 provides a method for preparing aluminum metaphosphate. The difference from Example 1 is that high-temperature calcination is not performed. Instead, the filter cake is dried and used directly as the final product. The remaining steps and parameters are the same as in Example 1. Analysis of the XRD pattern of the product obtained in Comparative Example 2 revealed that its main phase was aluminum metaphosphate oligomer, rather than fully polymerized Al(PO3)3; the free acid mass fraction was 0.15%, and the total metal impurity content was 11.2 ppm. These results indicate that the condensation reaction of the filter cake without high-temperature calcination was incomplete, the chemical composition deviated from the stoichiometry of aluminum metaphosphate, and the free acid content of the product significantly exceeded the standard, failing to meet the application requirements of high-end laser glass.

[0083] Comparative Example 3 Comparative Example 3 provides a method for preparing aluminum metaphosphate. The difference from Example 1 is that the low-temperature roasted material is not mixed and filtered with deionized water. Instead, the low-temperature roasted material is directly subjected to high-temperature roasting. The remaining steps and parameters are the same as in Example 1. Although the product obtained in Comparative Example 3 showed some improvement in agglomeration compared to Comparative Example 1, it was still a hard, agglomerated mass, requiring mechanical crushing to obtain particles. Testing revealed that the median particle size of the crushed product was 120.5 μm, with a D90 / D10 ratio of 5.2, indicating a wide particle size distribution. The crushing process introduced additional metallic impurities, increasing the iron content to 5.3 ppm. Furthermore, due to the lack of water addition for pulping and filtration, the sodium content remained as high as 342 ppm. These results indicate that without water addition for de-agglomeration and filtration, the sintered body formed during low-temperature calcination further densifies into a hard sintered mass after high-temperature calcination, necessitating mechanical crushing. This not only disrupts particle size uniformity but also introduces secondary pollution due to equipment wear, while simultaneously failing to effectively remove impurities such as sodium.

[0084] Appendix Figure 1 Detailed explanation: Figure 1 A scanning electron microscope (SEM) 295X image of high-purity aluminum metaphosphate provided for an embodiment of this application; as shown in the figure, the obtained product particles have regular morphology and uniform size, are spherical or nearly spherical, have good dispersibility, and no obvious agglomeration.

[0085] In summary, the high-purity aluminum metaphosphate preparation method provided in this application controls the material to a metastable intermediate state through low-temperature calcination, and then utilizes the synergistic effect of solvent debinding and filtration separation to simultaneously achieve particle size control and purification improvement without the aid of mechanical crushing. The resulting product has a purity ≥99.999%, total metal impurities ≤10ppm, individual metal impurities <1ppm, free acid ≤0.01%, and a median particle size of 30μm~80μm with D90 / D10 ≤3.0. It has comprehensive advantages such as high product purity, concentrated particle size distribution, resource recycling, low energy consumption, and simple process, and is suitable for the large-scale preparation of high-purity aluminum metaphosphate raw materials in fields such as high-end laser glass.

[0086] Furthermore, one or more technical solutions in the embodiments of this application have at least the following technical effects or advantages: The embodiments of this application eliminate the need for spray granulation and mechanical crushing and grinding, thus removing the energy-intensive spray drying and crushing and grinding processes in traditional methods and significantly reducing production energy consumption. At the same time, the process route is simple, with fewer operating steps and a shorter production cycle, which is conducive to large-scale industrial production.

[0087] In this embodiment, the filtrate obtained from filtration and separation is returned to the step of reacting high-purity aluminum hydroxide and phosphoric acid in deionized water, replacing part or all of the deionized water for preparing aluminum dihydrogen phosphate precursor solution. This achieves the recycling of phosphoric acid and water, avoids waste of phosphoric acid and discharge of waste liquid, and reduces raw material consumption and production costs.

[0088] This application embodiment removes excess phosphoric acid by adding deionized water and filtering after low-temperature roasting, ensuring the filter cake contains almost no volatile phosphorus compounds before high-temperature roasting. Therefore, no toxic phosphorus pentoxide gas is generated during subsequent high-temperature roasting, avoiding the phosphorus pentoxide pollution problem caused by high-temperature roasting in traditional processes and achieving green and environmentally friendly production.

[0089] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed in this application.

Claims

1. A high-purity aluminum metaphosphate, characterized in that, The purity of the high-purity aluminum metaphosphate product is ≥99.999%; By mass percentage, the total metal impurity content in the high-purity aluminum metaphosphate is ≤10ppm, and the content of each individual metal impurity is <1ppm; The mass fraction of free acid in the high-purity aluminum metaphosphate is ≤0.01%; The median particle size of the high-purity aluminum metaphosphate is 30μm~80μm, and the particle size distribution width index D90 / D10 of the high-purity aluminum metaphosphate is ≤3.

0.

2. A method for preparing high-purity aluminum metaphosphate as described in claim 1, characterized in that, Includes the following steps: Provide an aqueous solution of aluminum hydroxide; Phosphoric acid is added to the aqueous solution of aluminum hydroxide, and then the first reaction is carried out to obtain an aluminum dihydrogen phosphate precursor solution. The aluminum dihydrogen phosphate precursor solution was subjected to evaporation and concentration treatment followed by drying treatment to obtain dried material; The dried material is subjected to low-temperature roasting treatment to obtain low-temperature roasted material; The low-temperature roasted material is mixed with a solvent to obtain a slurry; The slurry is subjected to solid-liquid separation treatment to obtain filter cake and filtrate; and, The filter cake was subjected to high-temperature calcination to obtain the high-purity aluminum metaphosphate.

3. The method according to claim 2, characterized in that, The roasting temperature for the low-temperature roasting treatment is 450°C to 600°C; and / or, The roasting time for the low-temperature roasting treatment is 1 to 3 hours.

4. The method according to claim 2, characterized in that, The high-temperature roasting treatment is performed at a roasting temperature of 750°C to 850°C; and / or, The high-temperature roasting treatment takes 2 to 4 hours.

5. The method according to claim 2, characterized in that, The solvent includes deionized water; and / or, The mass of the solvent is 0.3 to 0.7 times the mass of the low-temperature roasted material.

6. The method according to claim 2, characterized in that, The aluminum hydroxide aqueous solution is composed of a mixture of high-purity aluminum hydroxide and deionized water; and / or, The temperature of the aluminum hydroxide aqueous solution is 75°C to 95°C; and / or, The stirring rate of the first reaction is 150 rpm to 400 rpm; the stirring time of the first reaction is 2 hours to 3 hours; and / or, The temperature of the first reaction is 75°C to 95°C; and / or, The first reaction takes 2 to 3 hours.

7. The method according to claim 6, characterized in that, The molar ratio of the high-purity aluminum hydroxide to the phosphoric acid is 1:3.1~4.

0.

8. The method according to claim 7, characterized in that, The purity of the high-purity aluminum hydroxide is >99.99%; and / or, The phosphoric acid is analytical grade phosphoric acid, and the mass fraction of the analytical grade phosphoric acid is 85%.

9. The method according to any one of claims 2 to 8, characterized in that, The filtrate is refluxed to the step of preparing the aluminum dihydrogen phosphate precursor solution, replacing part or all of the deionized water.

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