Method for recycling aluminum hydroxide coarse powder

Abstract

The application relates to a recycling method of aluminum hydroxide coarse powder, and belongs to the technical field of aluminum hydroxide powder preparation. The method comprises the following steps: obtaining a sodium aluminate solution; adding aluminum hydroxide seeds into the sodium aluminate solution to perform first precipitation decomposition, so as to obtain a decomposition slurry; performing grading treatment on the decomposition slurry to obtain aluminum hydroxide coarse powder; mixing the aluminum hydroxide coarse powder with water according to a set solid content, and then performing wet grinding treatment, so that the particle size of the aluminum hydroxide coarse powder is reduced, and a wet grinding slurry is obtained; and mixing the wet grinding slurry with the decomposition slurry to perform second precipitation decomposition, so as to obtain fine powder aluminum hydroxide. The method converts the originally unusable aluminum hydroxide coarse powder into high-activity secondary seeds, realizes the closed loop circulation of "coarse powder -> wet grinding -> secondary seeds", directly uses the byproduct to replace the preparation process of the sodium aluminate solution in the traditional resolvent process, and reduces the production cost of the fine powder.

Description

Technical Field

[0001] This application relates to the field of aluminum hydroxide powder preparation technology, and in particular to a method for recycling coarse and fine aluminum hydroxide powder. Background Technology

[0002] Micronized aluminum hydroxide is a type of aluminum hydroxide with extremely small particle sizes, approximately 2 micrometers. This material undergoes a decomposition reaction at approximately 200 degrees Celsius, absorbing heat and releasing water of crystallization in the process. Due to its unique physical and chemical properties, micronized aluminum hydroxide plays an important role in various composite materials such as wires and cables, insulators, and foaming materials. It is primarily used as a filler and flame retardant, and is considered an environmentally friendly flame retardant due to its environmentally friendly nature.

[0003] The production process of micronized aluminum hydroxide primarily relies on a technique called seed precipitation. However, due to limitations in decomposition conditions, micronized aluminum hydroxide produced using this method often contains agglomerated particles with a diameter exceeding 5 micrometers. The presence of these large particles adversely affects the performance of micronized aluminum hydroxide in downstream applications; for example, they can reduce the elongation at break of cables and may cause defects in foamed materials. Therefore, methods such as sieving and classification are typically employed during the production process to remove these large particles. Large particles with a diameter between 5 and 30 micrometers are referred to as coarse micronized aluminum hydroxide powder.

[0004] In the production of micronized aluminum hydroxide, coarse aluminum hydroxide powder accounts for approximately 1.5% to 3% of the total output. Due to its non-uniform particle size, this coarse powder cannot be sold directly as a product. The usual practice is to mix this coarse powder with ordinary aluminum hydroxide and redissolve it with liquid alkali to prepare a sodium aluminate solution. Micronized aluminum hydroxide can then be obtained again through the precipitation and decomposition process of the sodium aluminate solution. However, this process still produces coarse aluminum hydroxide powder, and this recurring cycle negatively impacts the production cost of micronized aluminum hydroxide. Summary of the Invention

[0005] This application provides a method for recycling coarse and fine aluminum hydroxide powder to solve the following technical problem: how to solve the problem that coarse and fine aluminum hydroxide powder cannot be directly utilized.

[0006] This application provides a method for recycling coarse and fine aluminum hydroxide powder, the method comprising:

[0007] A sodium aluminate solution was obtained;

[0008] Aluminum hydroxide seed crystals are added to the sodium aluminate solution to carry out the first precipitation decomposition, resulting in a decomposition slurry;

[0009] The decomposed slurry is graded to obtain coarse and fine aluminum hydroxide powder.

[0010] After mixing the aluminum hydroxide coarse powder with water according to the set solid content, wet milling is performed to reduce the particle size of the aluminum hydroxide coarse powder and obtain a wet milling slurry.

[0011] The wet grinding slurry is mixed with the decomposition slurry to carry out a second precipitation decomposition, thereby obtaining micronized aluminum hydroxide.

[0012] Optionally, the volume ratio of the wet grinding slurry to the decomposed slurry is 1:(30-70).

[0013] Optionally, the total alkali concentration NT of the sodium aluminate solution is 120 g / L to 170 g / L; the mass concentration of Al2O3 in the sodium aluminate solution is 100 g / L to 150 g / L.

[0014] Optionally, the seed coefficient of the aluminum hydroxide seed crystals is 0.5% to 3%.

[0015] Optionally, the concentration of solids is set to be 100g / L to 300g / L.

[0016] Optionally, the particle size of the aluminum hydroxide coarse powder is 5μm to 30μm.

[0017] Optionally, the aluminum hydroxide in the wet grinding slurry has a particle size of 1 μm to 3 μm.

[0018] Optionally, the decomposition time of the first precipitate is 10h to 30h.

[0019] Optionally, the temperatures for both the first and second precipitate decompositions are 60°C to 75°C.

[0020] Optionally, the decomposition time of the second precipitate is 10h to 50h.

[0021] The technical solutions provided in this application have the following advantages compared with the prior art:

[0022] This application provides a method for recycling coarse and fine aluminum hydroxide powder. The method includes: obtaining a sodium aluminate solution; adding aluminum hydroxide seed crystals to the sodium aluminate solution for a first precipitation decomposition to obtain a decomposition slurry; classifying the decomposition slurry to obtain coarse and fine aluminum hydroxide powder; mixing the coarse and fine aluminum hydroxide powder with water according to a set solid content, and then performing wet milling to reduce the particle size of the coarse and fine aluminum hydroxide powder to obtain a wet milling slurry; mixing the wet milling slurry with the decomposition slurry for a second precipitation decomposition to obtain fine aluminum hydroxide powder. The wet milling process breaks down the original coarse and fine aluminum hydroxide powder with a particle size of 5μm to 30μm to the 1μm to 3μm level, significantly increasing the specific surface area of ​​the particles and enabling them to meet the physical conditions for use as highly active secondary seed crystals. These ultrafine particles can be uniformly dispersed during subsequent decomposition, providing more nucleation sites and inhibiting the formation of large particle agglomeration. The 1μm–3μm fine aluminum hydroxide microcrystals obtained after wet milling serve as secondary seed crystals, increasing nucleation sites in the decomposition system and reducing solution supersaturation, thereby inhibiting the agglomeration tendency of microparticles during growth. The secondary seed crystals synergistically interact with the primary seed crystals (aluminum hydroxide seed crystals), their high specific surface area filling the gaps between coarse and fine powder particles through steric hindrance, reducing the probability of van der Waals-induced agglomeration and thus lowering the agglomeration rate of the final product, aluminum hydroxide microparticles. This step transforms coarse aluminum hydroxide powder that was originally unusable into highly active secondary seed crystals, achieving a closed-loop cycle of "coarse powder → wet milling → secondary seed crystals," directly utilizing byproducts to replace the traditional remelting process of sodium aluminate solution preparation, thus reducing the production cost of microparticles. Attached Figure Description

[0023] 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.

[0024] 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.

[0025] Figure 1 A schematic flowchart illustrating a method for recycling coarse and fine aluminum hydroxide powder provided in this application embodiment;

[0026] Figure 2 Electron micrograph provided for Comparative Example 1 of this application;

[0027] Figure 3 This is an electron microscope image provided for Embodiment 1 of this application;

[0028] Figure 4This is an electron microscope image provided for Embodiment 2 of this application; Detailed Implementation

[0029] 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.

[0030] 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 to 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 terms "including" and "comprise" as used herein mean "including but not limited to"; relational terms such as "first" and "second" 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 individually or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0031] Figure 1 This is a schematic flowchart illustrating a method for recycling coarse and fine aluminum hydroxide powder, provided as an embodiment of this application.

[0032] Please see Figure 1 This application provides a method for recycling coarse and fine aluminum hydroxide powder, the method comprising:

[0033] S1. A sodium aluminate solution is obtained;

[0034] A stable sodium aluminate solution is crucial to ensuring the smooth progress of the decomposition reaction.

[0035] In some embodiments, the total alkali concentration NT of the sodium aluminate solution is 120 g / L to 170 g / L; and the mass concentration of Al2O3 in the sodium aluminate solution is 100 g / L to 150 g / L.

[0036] NT (Total Na₂O) represents the total alkali concentration in the sodium aluminate solution, expressed in g / L. In the decomposition process, this parameter, along with AO (Al₂O₃ concentration), constitutes the fundamental conditions of the sodium aluminate solution system. In this embodiment, the total alkali concentration NT of the sodium aluminate solution is controlled within the range of 120 g / L to 170 g / L. This ensures the stability of the sodium aluminate solution, avoids excessive alkalinity leading to seed dissolution or side reactions, and guarantees sufficient alkalinity to support the decomposition reaction. The mass concentration of Al₂O₃ in the sodium aluminate solution is controlled within the range of 100 g / L to 150 g / L, maintaining the supersaturation of the solution at an appropriate level. This ensures sufficient driving force for the decomposition reaction while preventing excessively rapid crystal nucleation due to excessive concentration, thus contributing to the production of uniformly sized micronized aluminum hydroxide. With an Al₂O₃ mass concentration within the range of 100 g / L to 150 g / L, the decomposition reaction is more controllable, helping to reduce reaction anomalies caused by concentration fluctuations and improving production stability and reliability. By precisely controlling the composition of the sodium aluminate solution, the conditions for the decomposition reaction can be optimized, reducing unnecessary energy consumption and raw material waste. This helps to lower production costs and improve overall economic efficiency. For example, the total alkali concentration NT of the sodium aluminate solution can be 120 g / L, 130 g / L, 140 g / L, 150 g / L, 160 g / L, 170 g / L, etc.; the mass concentration of Al2O3 in the sodium aluminate solution can be 100 g / L, 110 g / L, 120 g / L, 130 g / L, 140 g / L, 150 g / L, etc.

[0037] S2. Add aluminum hydroxide seed crystals to the sodium aluminate solution to carry out the first precipitation decomposition and obtain the decomposition slurry;

[0038] The addition of aluminum hydroxide seed crystals provides initial nucleation sites for the decomposition reaction of sodium aluminate solution. These seed crystals, acting like "seeds," significantly lower the activation energy threshold of the decomposition reaction, promoting the tight bonding of aluminum ions and hydroxide ions in the sodium aluminate solution on the seed crystal surface, gradually nurturing aluminum hydroxide crystals and accelerating the decomposition reaction. The addition of aluminum hydroxide seed crystals allows for precise control of the particle size distribution of the decomposition products (i.e., the size of the aluminum hydroxide particles). The seed crystals guide the orderly growth of crystals, effectively avoiding excessive expansion or agglomeration, resulting in aluminum hydroxide particles with uniform size and excellent dispersibility. The addition of aluminum hydroxide seed crystals undoubtedly injects strong momentum into the decomposition efficiency of sodium aluminate solution. It acts as a "catalyst," significantly accelerating the decomposition process and greatly increasing the amount of decomposition products within the same reaction time. This is highly beneficial for shortening the production cycle and improving production efficiency.

[0039] In some embodiments, the seed coefficient of the aluminum hydroxide seed crystals is 0.5% to 3%.

[0040] The addition of seed crystals not only provides nucleation sites but also significantly reduces the activation energy of the decomposition reaction through a heterogeneous nucleation mechanism, thereby accelerating the decomposition process of sodium aluminate solution. The seed crystal coefficient refers to the percentage of seed crystals (i.e., the initial particles that initiate crystallization) added to the total amount of material in the reaction system during the precipitation decomposition process. Its core function is to regulate the nucleation and growth process of aluminum hydroxide particles by controlling the seed crystal ratio. In the embodiments of this application, a seed crystal coefficient <0.5% will lead to insufficient nuclei, a reduced decomposition reaction rate, and potentially a prolonged decomposition cycle; while a coefficient >3% may cause excessive particle growth due to excessive seed crystals, leading to increased agglomeration. A seed crystal coefficient range of 0.5% to 3% can balance the contradiction between reaction kinetics and product particle size distribution. For example, the seed crystal coefficient of aluminum hydroxide can be 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, etc.

[0041] In some embodiments, the first precipitate decomposes over a period of 10 to 30 hours.

[0042] Within a 10-30 hour timeframe, Al₂O₃ in the sodium aluminate solution begins to precipitate and decompose, gradually forming a stable crystal nucleus structure. This forms the basis for subsequent crystal growth and provides a uniform crystal growth interface for the second precipitation and decomposition stage. By controlling the first precipitation and decomposition time to 10-30 hours, the relationship between the crystal nucleation rate and the growth rate can be balanced. A time shorter than 10 hours may lead to insufficient crystal nucleus formation, while a time longer than 30 hours may cause excessively rapid crystal growth, resulting in large particles. Therefore, a time range of 10-30 hours helps to obtain micronized aluminum hydroxide with a relatively uniform particle size distribution. After the first precipitation and decomposition is completed, micronized aluminum hydroxide particles with a certain basic particle size distribution have formed in the decomposition slurry. At this time, adding secondary seed crystals (1μm-3μm) after wet milling can more effectively disperse the subsequently generated microcrystalline particles and reduce the agglomeration effect caused by Brownian motion. Therefore, a first precipitation and decomposition time of 10-30 hours provides a suitable opportunity for the addition of secondary seed crystals. Compared to a longer timeframe, controlling the first precipitation decomposition time to 10-30 hours can shorten the production cycle and improve production efficiency. Furthermore, since crystal nucleation and crystal growth have reached a certain equilibrium at this stage, an excessively long decomposition time is sufficient to meet subsequent production needs. Precisely controlling the first precipitation decomposition time avoids unnecessary energy waste and increased production costs. A time exceeding 30 hours not only increases energy consumption but may also lead to excessive equipment occupancy, affecting the overall production schedule. A time range of 10-30 hours, however, allows for effective cost and energy control while ensuring product quality. For example, the first precipitation decomposition time can be 10 hours, 14 hours, 18 hours, 22 hours, 26 hours, or 30 hours.

[0043] S3. The decomposed slurry is graded to obtain coarse aluminum hydroxide powder.

[0044] Grading effectively removes impurities from the decomposition slurry, improving the purity of the coarse and fine aluminum hydroxide powder. This plays a crucial role in subsequent wet milling, secondary precipitation decomposition, and the improvement of the final product quality. Grading effectively separates aluminum hydroxide particles in the decomposition slurry according to their particle size, resulting in coarse and fine aluminum hydroxide powder with a particle size within a certain range. This particle size control provides ideal raw materials for subsequent wet milling and secondary precipitation decomposition, facilitating gradient control of particle size distribution. Grading can precisely separate coarse and fine powder with a particle size of 5μm to 30μm. Particles in this size range possess the physical characteristics suitable for wet milling (easily broken down to 1μm to 3μm). Secondary seed crystals (1μm to 3μm) prepared through wet milling are reintroduced into the decomposition system, forming a closed-loop cycle of "coarse and fine powder → wet milling → secondary seed crystals," achieving gradient control of particle size. By converting the coarse and fine aluminum hydroxide powder obtained through graded processing into valuable secondary seed crystals and reusing them in the production process, efficient resource utilization is achieved. This circular economy model not only reduces waste emissions but also improves the comprehensive utilization rate of resources.

[0045] In some embodiments, the particle size of the aluminum hydroxide coarse powder is 5 μm to 30 μm.

[0046] This application utilizes the coarse and fine aluminum hydroxide powder obtained after classification to prepare secondary seed crystals through wet milling, which synergize with the primary seed crystals. The wet-milled aluminum hydroxide microcrystals (particle size reduced to 1μm–3μm) serve as secondary seed crystals, providing more nucleation sites during subsequent precipitation and decomposition processes, inhibiting the formation of large particle agglomerations. This optimized secondary seed crystal performance contributes to obtaining a fine aluminum hydroxide product with a more uniform particle size distribution and lower agglomeration rate. Coarse and fine aluminum hydroxide powder with a particle size range of 5μm–30μm possesses suitable physical properties and is easily refined through wet milling. Particles in this size range can be more effectively broken down to 1μm–3μm during wet milling, thereby preparing high-quality secondary seed crystals. By controlling the particle size of the coarse and fine aluminum hydroxide powder within the range of 5μm–30μm and performing wet milling and secondary precipitation and decomposition in subsequent processes, the yield of the final product, fine aluminum hydroxide, can be significantly improved. By converting coarse aluminum hydroxide powder with a particle size of 5μm to 30μm into valuable secondary seed crystals and recycling them into the production process, efficient resource utilization is achieved. This circular economy model not only reduces waste emissions but also improves the comprehensive utilization rate of resources. For example, the particle size of the coarse aluminum hydroxide powder can be 5μm, 10μm, 15μm, 20μm, 25μm, 30μm, etc.

[0047] S4. After mixing the aluminum hydroxide coarse powder with water according to the set solid content, perform wet milling to reduce the particle size of the aluminum hydroxide coarse powder and obtain a wet milling slurry.

[0048] Wet milling significantly reduces the particle size of coarse aluminum hydroxide powder, refining it from the original 5μm–30μm range to 1μm–3μm. This particle size reduction increases the specific surface area of ​​the particles, providing more nucleation sites for subsequent precipitation and decomposition reactions, thus contributing to obtaining more uniformly sized and better-dispersed micronized aluminum hydroxide powder. The refined aluminum hydroxide particles (i.e., microcrystals in the wet milling slurry) exhibit higher reactivity. During precipitation and decomposition, these microcrystals can more quickly adsorb aluminum and hydroxide ions in the solution, forming new aluminum hydroxide crystals and accelerating the decomposition reaction. The wet-milled aluminum hydroxide microcrystals, added as secondary seed crystals to the decomposition system, can be uniformly dispersed in the slurry. By providing additional nucleation sites, they inhibit the agglomeration of large particles. This inhibition helps reduce the proportion of agglomerated particles in the final product, improving the product's dispersibility and flowability. The wet grinding slurry obtained by wet grinding can be used as a secondary seed crystal to prepare micronized aluminum hydroxide products with excellent performance. These products have more uniform particle size distribution, higher specific surface area and better dispersibility, thus showing better application performance in fields such as wires and cables, insulators, and foaming materials.

[0049] Wet milling allows for the direct utilization of coarse and fine aluminum hydroxide powder as a byproduct. After particle size reduction, it can be reused as a secondary seed crystal in the production process, avoiding the cost of preparing sodium aluminate solution in traditional resolution processes. This recycling method helps reduce overall production costs and improve resource utilization efficiency. The addition of wet milling slurry allows for better control of supersaturation in the decomposition system, reducing abnormal crystal growth or agglomeration caused by excessive local supersaturation. This contributes to improving the stability of the entire production process and ensuring the uniformity of product quality.

[0050] In some embodiments, the set solid content is 100g / L to 300g / L.

[0051] A solids content range of 100 g / L to 300 g / L balances the slurry viscosity and the contact area of ​​the grinding media. This avoids excessively high concentrations that could overload the wet mill or exacerbate particle agglomeration, while also preventing excessively low concentrations that could reduce grinding efficiency. Ultimately, this achieves the goal of effectively reducing the particle size of coarse and fine powders from 5 μm to 30 μm to 1-3 μm. The 100 g / L to 300 g / L solids content design maintains the uniformity of slurry dispersion, preventing particle settling or excessively high local concentrations during wet milling, thus providing the physical basis for subsequent addition as secondary seed crystals to the decomposition tank. The 100 g / L to 300 g / L concentration range strikes a balance between equipment processing capacity (such as wet mill capacity) and material circulation efficiency, reducing water and energy waste while avoiding operational difficulties caused by insufficient slurry fluidity, ultimately lowering overall production costs. By precisely controlling the solid content, the surface activity of the wet-milled slurry particles is more easily matched with the decomposition system, thereby effectively suppressing the agglomeration of micronized aluminum hydrogen during the secondary seed return process and improving the final product qualification rate.

[0052] In some embodiments, the aluminum hydroxide in the wet grinding slurry has a particle size of 1 μm to 3 μm.

[0053] The original coarse-fine powder with a particle size of 5μm to 30μm is reduced to a particle size of 1μm to 3μm after wet milling. This particle size characteristic endows it with a higher specific surface area, thus meeting the physical requirements for use as a secondary seed crystal. This provides more nucleation sites for subsequent precipitation and decomposition reactions. These nucleation sites can accelerate the decomposition reaction of sodium aluminate solution, promoting the combination of more aluminum ions and hydroxide ions to form aluminum hydroxide crystals. The refined aluminum hydroxide particles, introduced into the decomposition system as secondary seed crystals, can be uniformly dispersed in the slurry. By providing additional nucleation sites, they inhibit the agglomeration of large particles. This inhibition helps reduce the proportion of agglomerated particles in the final product, improving the product's dispersibility and flowability. Finally, aluminum hydroxide microcrystals with a particle size of 1μm to 3μm have higher reactivity and can more quickly adsorb aluminum ions and hydroxide ions in the solution to form new aluminum hydroxide crystals. This helps to shorten the decomposition reaction time and improve production efficiency. The coarse and fine aluminum hydroxide powder obtained from classification is treated as a byproduct in this application and converted into a secondary seed slurry of 1μm to 3μm in size through wet milling. This slurry is then reintroduced into the decomposition process. This targeted recycling avoids the repeated recycling and consumption of traditional mixing and resolution processes, directly reducing the preparation cost of sodium aluminate solution. Because the aluminum hydroxide particles in the wet milling slurry are small and uniform, the prepared micronized aluminum hydroxide product has a more uniform particle size distribution, higher specific surface area, and better dispersibility. These optimized product properties enable micronized aluminum hydroxide to exhibit better application performance in fields such as wires and cables, insulators, and foaming materials. For example, the particle size of aluminum hydroxide in the wet milling slurry can be 1μm, 1.4μm, 1.8μm, 2.2μm, 2.6μm, 3μm, etc.

[0054] S5. The wet grinding slurry is mixed with the decomposition slurry to carry out a second precipitation decomposition to obtain micronized aluminum hydroxide.

[0055] As a secondary seed crystal, the wet grinding slurry, with its ultrafine particles (1μm–3μm in diameter), significantly increases nucleation sites when mixed into the decomposition slurry. These nucleation sites provide more starting points for the decomposition reaction of sodium aluminate solution, thereby accelerating the decomposition rate and shortening the production cycle. The addition of the secondary seed crystal helps suppress the agglomeration of aluminum hydroxide particles during decomposition. Its high specific surface area and uniform dispersion characteristics can fill the interparticle gaps through physical steric hindrance, reducing the probability of agglomeration caused by van der Waals forces. This results in a lower agglomeration rate and more uniform particle size distribution in the final micronized aluminum hydroxide powder. The micronized aluminum hydroxide powder obtained through the second precipitation decomposition exhibits optimized dispersibility, flowability, and specific surface area due to the suppression of particle agglomeration. Converting the coarse aluminum hydroxide powder byproduct into a valuable secondary seed crystal and reusing it in the production process achieves efficient resource utilization. This circular economy model not only reduces waste emissions but also improves the comprehensive utilization rate of resources.

[0056] In some embodiments, the volume ratio of the wet grinding slurry to the decomposed slurry is 1:(30-70).

[0057] The volume ratio of wet grinding slurry to decomposition slurry is 1:(30-70), ensuring uniform dispersion of the wet grinding slurry (as secondary seed crystals) in the decomposition slurry. A wet grinding slurry ratio >1:30 leads to excessively high seed crystal concentration, increasing the probability of particle collision and exacerbating agglomeration; while a ratio <1:70 fails to provide sufficient nucleation sites, affecting decomposition efficiency. The 1:(30-70) ratio balances seed crystal activity and reaction system stability, thereby reducing the agglomeration of micronized aluminum hydroxide particles. Secondly, the volume ratio of wet grinding slurry to decomposition slurry of 1:(30-70) maintains the supersaturation of the sodium aluminate solution system in a dynamic equilibrium state. The 1μm-3μm microcrystals (secondary seed crystals) carried by the wet grinding slurry can quickly consume the supersaturated Al2O3 in the solution, ensuring the driving force of the decomposition reaction and avoiding abnormal crystal growth caused by a sudden drop in supersaturation. Finally, the 1:(30-70) ratio works synergistically with the high specific surface area of ​​the wet grinding slurry microcrystals to suppress particle agglomeration through physical steric hindrance. The ultrafine particles (1μm-3μm) of the secondary seed crystals can fill the gaps between the coarse powder particles (5μm-30μm), reducing the van der Waals forces between particles and thus lowering the agglomeration rate of the final product, micronized aluminum hydroxide. For example, the volume ratio of the wet grinding slurry to the decomposition slurry can be 1:30, 1:35, 1:40, 1:45, 1:50, 1:55, 1:60, 1:65, 1:70, etc.

[0058] In some embodiments, the second precipitate decomposition time is 10h to 50h.

[0059] After wet milling the coarse and fine powder, it is reintroduced into the decomposition process and decomposed for another 10 to 50 hours before being discharged. This time range ensures that Al2O3 in the sodium aluminate solution is fully converted to form aluminum hydroxide crystals. Simultaneously, the aluminum hydroxide particles gradually grow to the ideal micron size range, ensuring uniform particle size distribution in the product. By controlling the decomposition time of the second precipitation to 10 to 50 hours, the agglomeration of micronized aluminum hydroxide particles can be effectively suppressed. This is because within this time range, particle growth and dispersion reach a relatively balanced state, avoiding agglomeration problems caused by excessively long or short times. During the second precipitation decomposition process, the secondary seed crystals (1μm to 3μm) after wet milling require sufficient time to perform their nucleation and dispersion functions. A decomposition time of 10 to 50 hours ensures that the secondary seed crystals have enough time to distribute uniformly in the system and effectively inhibits the formation of large particles. A decomposition time of less than 10 hours may result in incomplete release of solution supersaturation, while a time exceeding 50 hours will increase energy consumption and may induce abnormal crystal growth. The secondary seed crystals (1μm–3μm) after wet milling provide abundant highly active surface sites during the decomposition stage. Setting a lower limit of 10 hours for the decomposition time ensures sufficient growth period for the crystal nuclei, promoting the gradual growth of particles to the target micro-powder size range. The upper limit of 50 hours effectively controls the contact time between particles, reducing agglomeration caused by Brownian motion. Compared with the traditional single-stage decomposition process, the agglomeration rate is significantly reduced. The 1μm–3μm microcrystals treated by wet milling serve as secondary seed crystals, working synergistically with the primary seed crystals (seed coefficient of 0.5%–3%). Leveraging their high specific surface area, they fill the voids between coarse and fine powders using steric hindrance, effectively weakening the agglomeration tendency caused by van der Waals forces, thereby reducing the agglomeration rate of the final product, aluminum hydroxide powder. For example, the decomposition time for the second precipitation can be 10h, 15h, 20h, 25h, 30h, 35h, 40h, 45h, 50h, etc.

[0060] In some embodiments, the temperatures for both the first and second precipitate decompositions are 60°C to 75°C.

[0061] The decomposition rate of sodium aluminate solution is significantly accelerated when the temperature is controlled within the range of 60℃ to 75℃ during the first precipitation decomposition (i.e., initial decomposition). This is because appropriately increasing the temperature increases the molecular motion speed, leading to a higher collision frequency between reactant molecules and thus promoting the decomposition reaction. Simultaneously, controlling the temperature within this range effectively prevents abnormal growth of aluminum hydroxide particles. Temperatures above 75℃ may cause excessively rapid particle growth, resulting in irregularly large particles, while temperatures below 60℃ may slow the reaction rate too much, affecting production efficiency. The 60℃–75℃ temperature range ensures both a stable reaction rate and maintains particle growth in a relatively stable state. In the second precipitation decomposition (i.e., decomposition after the addition of secondary seed crystals), the wet-milled secondary seed crystals (particle size 1μm–3μm) work better under the 60℃–75℃ temperature conditions. This temperature range helps prevent the formation of new crystal nuclei, resulting in well-crystallized aluminum hydroxide with high strength. Temperatures of 60℃ to 75℃ allow secondary seed crystals to disperse more uniformly in the decomposition slurry, providing more nucleation sites, thereby reducing the supersaturation of the solution and further inhibiting the agglomeration of micronized particles. For example, the temperatures for the first and second precipitation decompositions can be 60℃, 63℃, 66℃, 69℃, 72℃, 75℃, etc.

[0062] 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.

[0063] Comparative Example 1

[0064] Sodium aluminate solution NT 140 g / L, AO 125 g / L, decomposition temperature 70℃, seed coefficient 1.5%, decomposition for 24 h, followed by washing and drying to obtain micronized aluminum hydroxide powder with d10 1.319 μm, d50 2.523 μm, and d90 5.429 μm, as shown in electron microscopy. Figure 2 Large, aggregated particles were clearly visible in both 50μm and 100μm fields of view.

[0065] Example 1

[0066] The graded coarse and fine aluminum hydroxide powder was mixed with water at a solid content of 200 g / L and wet-milled using a wet mill to reduce the particle size of the coarse and fine aluminum hydroxide powder from 17.9 μm to 2.26 μm. The coarse and fine aluminum hydroxide powder slurry was then mixed with 60 parts of sodium aluminate solution decomposition slurry. The slurry was returned to the sodium aluminate solution from Comparative Example 1 after decomposition for 24 hours. The mixture was then further decomposed in the decomposition tank for another 24 hours before being discharged, washed, and dried to obtain fine aluminum hydroxide powder with d10 of 1.335 μm, d50 of 2.624 μm, and d90 of 5.583 μm. Electron microscopy showed a significant reduction in large agglomerates at 3, 50 μm, and 100 μm fields of view.

[0067] Example 2

[0068] The graded coarse and fine aluminum hydroxide powder was mixed with water at a solid content of 200 g / L and wet-milled using a wet mill to reduce the particle size of the coarse and fine aluminum hydroxide powder from 17.9 μm to 2.26 μm. The coarse and fine aluminum hydroxide slurry was then mixed with 60 parts of sodium aluminate solution decomposition slurry, and the slurry was returned to the sodium aluminate solution from Comparative Example 1 after 24 hours of decomposition. The mixture was further decomposed in the decomposition tank for 48 hours before being discharged, washed, and dried to obtain fine aluminum hydroxide powder with d10 1.278 μm, d50 2.459 μm, and d90 5.059 μm. Electron microscopy results are shown below. Figure 4 Large particles clustered in 50μm and 100μm fields of view were significantly reduced.

[0069] One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0070] Highly efficient resource utilization: By using wet milling, coarse and micro aluminum hydroxide powder, which was originally unusable, is transformed into highly active secondary seed crystals, achieving a closed-loop cycle of "coarse and micro powder → wet milling → secondary seed crystals". This method directly utilizes byproducts to replace the traditional sodium aluminate solution preparation process of remelting, significantly reducing the production cost of micro powder and improving resource utilization efficiency.

[0071] Balancing environmental protection and economic benefits: This invention reduces waste emissions and meets environmental protection requirements by recycling coarse and fine aluminum hydroxide powder. Simultaneously, it brings significant economic benefits to enterprises by lowering the production cost of the fine powder and improving resource utilization efficiency.

[0072] 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 method for recycling coarse and fine aluminum hydroxide powder, the method comprising: A sodium aluminate solution was obtained; Aluminum hydroxide seed crystals are added to the sodium aluminate solution to carry out the first precipitation decomposition and obtain a decomposition slurry. The first precipitation decomposition time is 10h to 30h. The decomposed slurry is graded to obtain coarse and fine aluminum hydroxide powder. After mixing the coarse aluminum hydroxide powder with water according to a set solid content, wet milling is performed to reduce the particle size of the coarse aluminum hydroxide powder and obtain a wet milling slurry. In the wet milling slurry, the particle size of aluminum hydroxide is 1μm to 3μm. The wet grinding slurry is mixed with the decomposition slurry to carry out a second precipitation decomposition, thereby obtaining micronized aluminum hydroxide.

2. The method according to claim 1, characterized in that, The volume ratio of the wet grinding slurry to the decomposed slurry is 1:(30-70).

3. The method according to claim 1, characterized in that, The total alkali concentration NT of the sodium aluminate solution is 120 g / L to 170 g / L; the mass concentration of Al2O3 in the sodium aluminate solution is 100 g / L to 150 g / L.

4. The method according to claim 1, characterized in that, The seed coefficient of the aluminum hydroxide seed crystals is 0.5% to 3%.

5. The method according to claim 1, characterized in that, The set solid content is 100g / L to 300g / L.

6. The method according to claim 1, characterized in that, The particle size of the aluminum hydroxide coarse powder is 5μm to 30μm.

7. The method according to claim 1, characterized in that, The temperatures for both the first and second precipitate decompositions are 60℃~75℃.

8. The method according to claim 1, characterized in that, The second precipitate decomposes in 10 to 50 hours.

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