Preparation method of high-whiteness aluminum hydroxide

Abstract

The application provides a preparation method of high-whiteness aluminum hydroxide, and belongs to the field of aluminum oxide production. The method comprises the following steps: adding a polyacrylamide composite adsorbent to a sodium aluminate dilute solution from a Bayer process dissolution process to perform adsorption decarburization treatment on the sodium aluminate dilute solution to obtain a first purified solution; adding lime milk to the first purified solution to perform co-precipitation impurity removal treatment on the first purified solution, and performing solid-liquid separation to obtain a second purified solution; adding hydrogen peroxide to the second purified solution to perform strong oxidative degradation treatment on residual organic matter in the second purified solution to obtain a third purified solution; performing seed precipitation decomposition on the third purified solution to obtain a slurry; and performing solid-liquid separation on the slurry, and washing the filter cake to obtain a high-whiteness aluminum hydroxide product. By constructing an adsorption-precipitation-oxidation three-stage synergistic purification system, the classification, segmentation and targeted removal of organic and inorganic impurities are realized, so that a Bayer process high-whiteness aluminum hydroxide preparation method capable of synergistically removing multiple impurities is developed.

Description

Technical Field

[0001] This application relates to the field of alumina production technology, and in particular to a method for preparing high-whiteness aluminum hydroxide. Background Technology

[0002] The Bayer process is the main industrial method for producing alumina. When using bauxite with a high organic content, large organic molecules such as humic acid accumulate continuously during the process, leading to a darkening of the sodium aluminate solution and an increase in total organic carbon (TOC). These organic substances, along with coexisting inorganic impurities such as iron and titanium, are encapsulated or adsorbed during the decomposition and crystallization of aluminum hydroxide, resulting in low whiteness of the final product (typically 70%–75%). This severely impacts the product's application value and market competitiveness in high-end flame-retardant fillers, water purification agents, and specialty ceramics.

[0003] Currently, various methods exist in the industry for removing organic matter from Bayer solutions, but all have limitations. For example, evaporating the mother liquor to precipitate salt and then removing organic matter through sintering is energy-intensive and limited by sintering capacity; using ion exchange resins or magnetic flotation for impurity removal is costly, has limited processing capacity, and is difficult to scale up for continuous industrial production. Therefore, there is an urgent need to develop a method for preparing high-whiteness aluminum hydroxide using the Bayer process that can synergistically remove multiple impurities. Summary of the Invention

[0004] This application provides a method for preparing high-whiteness aluminum hydroxide, in order to develop a Bayer process method for preparing high-whiteness aluminum hydroxide that can synergistically remove multiple impurities.

[0005] This application provides a method for preparing high-whiteness aluminum hydroxide, the method comprising: A polyacrylamide composite adsorbent is added to the sodium aluminate dilution solution from the Bayer process leaching to adsorb and decarbonize the sodium aluminate dilution solution, thereby obtaining a first purified solution. Lime milk is added to the first purified liquid to perform co-precipitation to remove impurities, and the second purified liquid is obtained by solid-liquid separation. Hydrogen peroxide is added to the second purification solution to strongly oxidize and degrade the residual organic matter in the second purification solution, thereby obtaining the third purification solution; The third purified liquid is subjected to seed decomposition to obtain a slurry; The slurry was subjected to solid-liquid separation, and the filter cake was washed to obtain a high-whiteness aluminum hydroxide product.

[0006] Optionally, the temperature of the sodium aluminate dilution solution is 90℃~95℃, and αk is 1.45~1.55.

[0007] Optionally, the amount of the polyacrylamide composite adsorbent added is 0.005% to 0.01% of the mass of the sodium aluminate dilution.

[0008] Optionally, the adsorption decarbonization treatment includes the following parameters: the stirring reaction temperature is 90℃~95℃, and the stirring reaction time is 45min~75min.

[0009] Optionally, the solid content of the lime slurry is 250 g / L to 290 g / L, and the effective calcium content is 170 g / L to 190 g / L.

[0010] Optionally, the mass of the added lime slurry is 5‰ to 10‰ of the total mass of the first purified liquid.

[0011] Optionally, the coprecipitation purification treatment includes the following parameters: the stirring reaction temperature is 90℃~95℃, and the stirring reaction time is 50min~70min.

[0012] Optionally, the hydrogen peroxide has a mass concentration of 25% to 30%, and the added volume of hydrogen peroxide is 1% to 5% of the volume of the second purified liquid.

[0013] Optionally, the strong oxidative degradation treatment includes the following parameters: reaction temperature of 90℃~95℃ and reaction time of 60min~90min.

[0014] Optionally, the high-whiteness aluminum hydroxide product meets the following properties: whiteness ≥85%, sodium oxide content ≤0.20%.

[0015] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides a method for preparing high-whiteness aluminum hydroxide. By constructing a multi-stage synergistic purification process, key impurity factors affecting the whiteness of aluminum hydroxide are eliminated at the source, thereby significantly improving the whiteness of the final product. The reduction in the whiteness of aluminum hydroxide products mainly stems from two types of contamination: first, organic matter, especially humic acid macromolecules, is encapsulated or adsorbed by crystals during crystallization, resulting in a darker product color; second, inorganic coloring impurities such as iron and titanium are incorporated into the product's crystal lattice, affecting the product's appearance quality. To address these two types of contamination sources, this method designs three progressive purification stages.

[0016] First, adsorption and decarbonization are performed by adding a polyacrylamide-based composite adsorbent to the sodium aluminate dilution solution. This adsorbent utilizes its specific affinity for macromolecular organic compounds such as humic acid to preferentially capture and remove the main colored organic components in the solution, yielding the first purified solution. This step reduces the initial content of organic matter at the source, preventing most of the organic matter from entering the subsequent crystallization process.

[0017] Subsequently, lime slurry was added to the first purified liquid after adsorption treatment for co-precipitation and impurity removal, and the second purified liquid was obtained through solid-liquid separation. In this process, the calcium ions introduced by the lime slurry react chemically with the residual inorganic impurity ions such as iron and titanium in the solution to form insoluble precipitates. These inorganic coloring impurities are removed from the liquid phase by sedimentation separation, thereby achieving efficient removal of inorganic impurities.

[0018] Finally, hydrogen peroxide was added to the second purified solution after co-precipitation treatment for strong oxidative degradation, yielding the third purified solution. Hydrogen peroxide exerts a strong oxidizing effect under alkaline conditions, further oxidizing and decomposing the trace residual organic matter that could not be completely removed in the adsorption step, converting it into inorganic small molecules, thus achieving deep purification of colored organic matter.

[0019] After undergoing the three-stage treatment of adsorption, precipitation, and oxidation, the third purified liquid contains significantly reduced organic and inorganic coloring impurities to extremely low levels, achieving a highly pure solution. When this high-purity third purified liquid is subjected to fractional decomposition, aluminum hydroxide crystals precipitate and grow in a clean environment, preventing impurities from being encapsulated or adsorbed by the crystals during crystallization. This ensures that the precipitated aluminum hydroxide crystals retain their inherent white appearance. The final aluminum hydroxide product obtained through solid-liquid separation exhibits significantly improved whiteness, achieving a quality upgrade from ordinary aluminum hydroxide to high-whiteness aluminum hydroxide. Attached Figure Description

[0020] 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.

[0021] 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.

[0022] Figure 1 This is a schematic flowchart illustrating a method for preparing high-whiteness aluminum hydroxide, as provided in an embodiment of this application. Detailed Implementation

[0023] 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.

[0024] 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 "contains" 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.

[0025] Figure 1 This is a schematic flowchart illustrating a method for preparing high-whiteness aluminum hydroxide, as provided in an embodiment of this application.

[0026] like Figure 1 As shown in the embodiment of this application, a method for preparing high-whiteness aluminum hydroxide is provided, the method comprising: S1. Add a polyacrylamide composite adsorbent to the sodium aluminate dilution solution from the Bayer process leaching process to adsorb and decarbonize the sodium aluminate dilution solution to obtain the first purified solution. S2. Add lime milk to the first purified liquid to perform co-precipitation to remove impurities, and obtain the second purified liquid through solid-liquid separation. S3. Add hydrogen peroxide to the second purification solution to strongly oxidize and degrade the residual organic matter in the second purification solution to obtain the third purification solution. S4. The third purification liquid is decomposed into a slurry. S5. The slurry is subjected to solid-liquid separation, and the filter cake is washed to obtain a high-whiteness aluminum hydroxide product.

[0027] It should be noted that step S1 is the initial stage of the entire purification process, and its core function is to selectively remove the main organic coloring substances in the sodium aluminate solution. A polyacrylamide composite adsorbent is added to the sodium aluminate dilution solution from the Bayer process leaching. Utilizing its specific adsorption capacity for humic acid-based macromolecular organic compounds, the macromolecular organic compounds, mainly humic acid, are captured and separated from the liquid phase. This pretreatment not only initially reduces the color and total organic carbon content of the solution, lessening the load on subsequent processing steps, but more importantly, by removing macromolecular organic compounds that are easily encapsulated by crystals, it eliminates the main organic interference source for the subsequent aluminum hydroxide crystallization process, preventing them from being encapsulated by the product during decomposition and causing a decrease in whiteness.

[0028] Step S2 primarily targets inorganic coloring impurities in the solution, especially metal ions such as iron and titanium. By adding lime slurry to the first purification solution, calcium ions are introduced as a precipitant, reacting with iron ions to form calcium ferrite precipitate and with titanium ions to form calcium titanate precipitate. The ingenuity of this process lies in the synergistic formation of perovskite-type composite precipitates by calcium ferrite and calcium titanate, achieving efficient and simultaneous removal of multiple inorganic impurities. After the reaction, solid-liquid separation removes these inorganic impurities from the system as precipitates, resulting in a purer sodium aluminate solution. This step solves the problem of traditional methods' difficulty in simultaneously and efficiently removing multiple inorganic impurities, laying an important foundation for improving the product's whiteness.

[0029] Step S3 serves to further treat these residual macromolecular organic compounds. By adding hydrogen peroxide to the second purification solution, its strong oxidizing properties under alkaline conditions further oxidize and degrade the residual organic molecules in the solution, ultimately breaking them down into inorganic small molecules such as oxalates, carbonates, and water. This advanced oxidation process effectively complements the adsorption decarbonization step—the adsorption step primarily targets humic acid-based macromolecular organic compounds, while the oxidation step effectively treats residual macromolecular organic compounds that are difficult for adsorbents to capture. Through this dual organic removal mechanism of "adsorption + oxidation," deep purification of organic matter in the solution is achieved, ensuring that the solution entering the decomposition process has extremely high purity.

[0030] Step S4 is a crucial step in the transformation from a purified solution to a solid product. The third-stage purification solution, after three levels of deep purification, undergoes seed crystal decomposition. Utilizing the spontaneous decomposition of supersaturated sodium aluminate solution under seed crystal induction, as seen in the Bayer process, aluminum hydroxide crystallizes out in an orderly manner around the seed crystals. Because the preceding steps have significantly removed organic and inorganic coloring impurities from the solution, a clean raw material environment is provided for the crystallization process. This allows the precipitated aluminum hydroxide crystals to retain their intrinsic white color, preventing impurities from being encapsulated or adsorbed within or on the surface of the crystals. Simultaneously, the clean solution environment also promotes orderly crystal growth, positively impacting the particle size distribution and crystallinity of the product.

[0031] Step S5 is the final step in the entire preparation process, and its function is to perform solid-liquid separation on the slurry containing aluminum hydroxide crystals obtained after decomposition. Through a series of operations including sedimentation, filtration, washing, and drying, the solid aluminum hydroxide is separated from the mother liquor, and a small amount of mother liquor and soluble impurities adhering to the crystal surface are washed away. The washing operation is crucial for further reducing the sodium oxide content in the product, directly affecting the chemical purity of the final product. After this step, a high-value-added aluminum hydroxide product with significantly improved whiteness and low impurity content is finally obtained, realizing a complete closed-loop process from solution purification to high-quality product preparation.

[0032] In some embodiments, the temperature of the sodium aluminate dilution solution is 90°C to 95°C, and αk is 1.45 to 1.55.

[0033] The temperature of the sodium aluminate dilution solution is limited to 90℃–95℃ to ensure suitable fluidity and reactivity. This temperature range is beneficial for the polyacrylamide composite adsorbent to fully exert its specific adsorption capacity for humic acid macromolecular organic compounds, while avoiding the increase in solution viscosity due to excessively low temperatures, which would affect adsorption efficiency, and the energy waste or increased operational difficulty due to excessively high temperatures. The αk of the sodium aluminate dilution solution is limited to 1.45–1.55 to maintain the solution at a suitable caustic ratio. This molecular ratio range is a typical process parameter for the dilution solution after the Bayer process leaching, ensuring that the sodium aluminate solution has suitable supersaturation and stability. This provides a stable chemical environment for subsequent purification operations such as adsorption decarbonization and co-precipitation, and also creates favorable conditions for the final crystallization process in the seed decomposition step. For example, the temperature of the sodium aluminate dilution solution can be 90℃, 90.7℃, 91.4℃, 92.1℃, 92.8℃, 93.5℃, 94.2℃, 95℃, etc.; αk can be 1.45, 1.46, 1.47, 1.48, 1.49, 1.50, 1.52, 1.55, etc.

[0034] In some embodiments, the amount of polyacrylamide composite adsorbent added is 0.005% to 0.01% of the mass of sodium aluminate dilution.

[0035] The addition of polyacrylamide-based composite adsorbent is limited to 0.005%–0.01% of the mass of the sodium aluminate dilution solution, achieving economical addition while ensuring adsorption efficiency. This dosage range provides sufficient adsorption active sites, allowing the composite adsorbent to fully contact and capture humic acid macromolecular organic matter in the solution, effectively removing color-causing organic matter, while avoiding excessive adsorbent addition that would waste resources or have potential impacts on subsequent processes. For example, the addition amount of polyacrylamide-based composite adsorbent is 0.005%, 0.006%, 0.007%, 0.008%, 0.009%, or 0.010% of the mass of the sodium aluminate dilution solution.

[0036] In some embodiments, the adsorption decarbonization treatment includes the following parameters: a stirring reaction temperature of 90°C to 95°C and a stirring reaction time of 45 min to 75 min.

[0037] The stirring reaction temperature for the adsorption-decarbonation treatment is limited to 90℃~95℃, providing optimal thermodynamic conditions for the adsorption process. This temperature range matches the incoming temperature of the sodium aluminate dilution solution, eliminating the need for additional heating or cooling, which helps maintain process continuity and economy. Simultaneously, at this temperature, the molecular chains of the polyacrylamide composite adsorbent can fully extend, maximizing its adsorption activity. The stirring reaction time for the adsorption-decarbonation treatment is limited to 45min~75min, ensuring the adsorption reaction proceeds fully to equilibrium. This time range guarantees sufficient contact time between the composite adsorbent and the humic acid macromolecular organic matter to complete the adsorption process and achieve the expected adsorption efficiency, while also being compatible with the continuous operation rhythm of the industrial production line, avoiding incomplete adsorption due to excessively short reaction times or decreased production efficiency due to excessively long reaction times. For example, the stirring reaction temperature for adsorption decarbonization treatment can be 90℃, 90.7℃, 91.4℃, 92.1℃, 92.8℃, 93.5℃, 94.2℃, 95℃, etc.; and the stirring reaction time can be 45min, 48min, 51min, 54min, 57min, 60min, 66min, 75min, etc.

[0038] In some embodiments, the solid content of lime slurry is 250 g / L to 290 g / L, and the effective calcium content is 170 g / L to 190 g / L.

[0039] The lime slurry is limited to a solid content of 250 g / L to 290 g / L and an effective calcium content of 170 g / L to 190 g / L, ensuring a sufficient and stable calcium source supply for the co-precipitation reaction. Lime slurry within this concentration range exhibits suitable fluidity and reactivity, stably introducing calcium ions into the solution. This ensures the full reaction of iron ions with calcium ions to form calcium ferrite precipitate and titanium ions with calcium ions to form calcium titanate precipitate, forming a perovskite-type composite precipitate. This achieves efficient and simultaneous removal of inorganic coloring impurities from iron and titanium. For example, the solid content of lime slurry can be 250g / L, 256g / L, 262g / L, 268g / L, 274g / L, 280g / L, 286g / L, 290g / L, etc.; and the effective calcium content can be 170g / L, 173g / L, 176g / L, 179g / L, 182g / L, 185g / L, 188g / L, 190g / L, etc.

[0040] In some embodiments, the mass of lime slurry added is 5‰ to 10‰ of the total mass of the first purification liquid.

[0041] By limiting the amount of lime slurry added to 5‰ to 10‰, the introduction rate and total amount of calcium ions can be precisely controlled. This range matches the processing scale of industrial production, ensuring that calcium ions react fully with iron and titanium ions in the solution according to stoichiometric ratios. This avoids both insufficient calcium ion addition leading to incomplete impurity removal and excessive addition causing waste or introducing new impurities, while maintaining stable operating conditions within the co-precipitation reaction tank. For example, the amount of lime slurry added can be 5‰, 5.7‰, 6.4‰, 7.1‰, 7.8‰, 8.5‰, 9.2‰, or 10‰.

[0042] In some embodiments, the coprecipitation purification process includes the following parameters: a stirring reaction temperature of 90°C to 95°C and a stirring reaction time of 50 min to 70 min.

[0043] The stirring reaction temperature for the co-precipitation impurity removal treatment is limited to 90℃~95℃, providing a suitable reaction temperature for the formation of perovskite-type composite precipitates. This temperature range maintains consistency with the upstream adsorption decarbonization treatment, facilitating process flow integration. At this temperature, the reaction rates of calcium ferrite and calcium titanate formation are moderate, the growth and aggregation of precipitate crystals proceed in an orderly manner, and the resulting composite precipitate exhibits good settling properties, facilitating subsequent solid-liquid separation operations. The stirring reaction time for the co-precipitation impurity removal treatment is limited to 50min~70min, ensuring the complete precipitation reaction of iron and titanium impurities. This time range guarantees sufficient time for calcium ions to contact and react with iron and titanium ions to form precipitates and reach equilibrium, while also considering the continuous operation rhythm of the industrial production line, matching the reaction time with the processing capacity of upstream and downstream processes, and maintaining the stable operation of the entire production system. For example, the stirring reaction temperature for coprecipitation purification can be 90℃, 90.7℃, 91.4℃, 92.1℃, 92.8℃, 93.5℃, 94.2℃, 95℃, etc.; and the stirring reaction time can be 50min, 53min, 56min, 59min, 62min, 65min, 68min, 70min, etc.

[0044] In some embodiments, the mass concentration of hydrogen peroxide is 25% to 30%, and the volume of hydrogen peroxide added is 1% to 5% of the volume of the second purification liquid.

[0045] Limiting the hydrogen peroxide concentration to 25%–30% ensures sufficient and stable oxidizing power. Within this concentration range, hydrogen peroxide releases highly reactive oxygen species under alkaline conditions, effectively attacking the chemical bonds in residual organic molecules and gradually degrading them into inorganic small molecules such as oxalates, carbonates, and water, achieving deep purification of trace organic matter in the solution. Limiting the added volume of hydrogen peroxide to 1%–5% of the second purification liquid volume ensures economical addition while maintaining oxidation effectiveness. This dosage range provides sufficient oxidant dosage to ensure the strong oxidation reaction proceeds fully, completely degrading residual large-molecule organic matter that was not removed in the adsorption and decarbonization steps, while avoiding excessive hydrogen peroxide addition that could lead to reagent waste or potential impact on subsequent decomposition processes. For example, the mass concentration of hydrogen peroxide can be 25%, 25.7%, 26.4%, 27.1%, 27.8%, 28.5%, 29.2%, 30%, etc.; the volume of hydrogen peroxide added is 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4.5%, 5%, etc. of the volume of the second purification liquid.

[0046] In some embodiments, the strong oxidative degradation treatment includes the following parameters: reaction temperature of 90℃~95℃ and reaction time of 60min~90min.

[0047] The reaction temperature for the strong oxidative degradation treatment is limited to 90℃–95℃, providing optimal thermodynamic conditions for the oxidation reaction. This temperature range maintains consistency with the upstream purification process, ensuring continuous operation of the entire deep purification process under similar temperature conditions. At this temperature, the decomposition rate of hydrogen peroxide and the generation rate of reactive oxygen species are moderate, ensuring both sufficient oxidation reaction and preventing excessively rapid and ineffective decomposition of hydrogen peroxide due to excessively high temperatures. The reaction time for the strong oxidative degradation treatment is limited to 60–90 minutes, ensuring thorough oxidation and degradation of residual organic matter. This time range guarantees sufficient time for hydrogen peroxide to fully contact and complete the oxidation reaction with the residual macromolecular organic carbon in the solution, minimizing its content, while also matching the time rhythm of the entire deep purification process, ensuring that the purified solution can stably and continuously enter the subsequent fractionation decomposition process. For example, the reaction temperature for strong oxidative degradation treatment can be 90℃, 90.7℃, 91.4℃, 92.1℃, 92.8℃, 93.5℃, 94.2℃, 95℃, etc.; and the reaction time can be 60min, 64min, 68min, 72min, 76min, 80min, 85min, 90min, etc.

[0048] In some implementations, the high-whiteness aluminum hydroxide product meets the following performance requirement: whiteness ≥ 85%.

[0049] The whiteness of aluminum hydroxide products is mainly affected by organic coloring impurities (such as humic acid macromolecules) and inorganic coloring impurities (such as iron and titanium compounds). This application eliminates the adverse effects of these two types of impurities on product whiteness at the source through a three-step deep purification process.

[0050] First, adsorption decolorization treatment (S1) targets and removes large-molecule organic colorants such as humic acid. A polyacrylamide composite adsorbent is added to the sodium aluminate dilution solution. Utilizing its specific adsorption capacity for large-molecule organic humic acid, the most abundant organic colorant in the solution is preferentially captured and separated. This step reduces the absorbance of the solution by 5%–10%, effectively cutting off the pathway for most organic matter to be encapsulated or adsorbed by subsequent aluminum hydroxide crystals, thus laying the foundation for improved product whiteness.

[0051] Secondly, co-precipitation purification (S2) simultaneously removes inorganic coloring impurities. Lime slurry is added to the first purified solution after adsorption treatment, introducing calcium ions to react with iron ions in the solution to form calcium ferrite precipitate, and with titanium ions to form calcium titanate precipitate. These two react synergistically to form a perovskite-type composite precipitate. Through this chemical precipitation process, inorganic coloring ions such as iron and titanium are efficiently and simultaneously removed, completely eliminating the possibility of them doping into the product lattice during aluminum hydroxide crystallization, resulting in an extremely high inorganic purity solution.

[0052] Finally, a strong oxidative degradation treatment (S3) deeply degrades the residual macromolecular organic matter. Hydrogen peroxide is added to the second purified solution after co-precipitation treatment, and an advanced oxidation reaction is carried out under alkaline conditions. This process can completely oxidize and degrade the trace residual macromolecular organic matter that could not be removed in the adsorption step into oxalate, carbonate, and water, further reducing the absorbance of the solution by 20%–25%. Through the dual organic removal mechanism of "adsorption + oxidation," the content of discoloring macromolecular organic carbon in the solution is reduced to a minimum, providing a near-pure liquid phase environment for the subsequent crystallization process.

[0053] Under the action of the above three-stage purification system, the coloring impurities in the third purification liquid entering the seed decomposition process are purified. In the subsequent seed decomposition process (S4), aluminum hydroxide crystals precipitate and grow in an orderly manner under the induction of crystal seeds. Due to the high purity of the mother liquor environment, the precipitated crystals can maintain their intrinsic white color, avoiding impurities from being wrapped or adsorbed inside or on the surface of the crystals. The whiteness of the final product can be stably achieved to over 85%.

[0054] 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 standards / industry standards / the disclosure herein; if there are no corresponding national standards / industry standards / the disclosure herein, they are performed according to generally accepted international standards, conventional conditions, or conditions recommended by the manufacturer.

[0055] Example 1 This embodiment provides a method for preparing high-whiteness aluminum hydroxide, specifically including the following steps: S1. Add 0.006% of the mass of the sodium aluminate dilution solution (temperature 95℃, αk 1.50) to the sodium aluminate dilution solution after the Bayer process. Stir and react for 60 minutes to carry out adsorption and decarbonization treatment to obtain the first purified solution. At this time, the absorbance can be reduced by about 10%.

[0056] S2. Add lime milk (solid content 270g / L, effective calcium 180g / L) to the first purified liquid that enters the co-precipitation reaction tank. The added mass is 10‰ of the total mass of the first purified liquid. Stir and react at 95℃ for 60min to carry out co-precipitation to remove impurities. After sedimentation and separation, the second purified liquid is obtained. At this time, the absorbance can be reduced by about 5%.

[0057] S3. 5% hydrogen peroxide (27.5%) is introduced into the second purification liquid entering the decomposition tank. Under alkaline conditions, a gas-liquid oxidation reaction is carried out at a reaction temperature of 95°C for 60 minutes to perform strong oxidative degradation treatment, resulting in the third purification liquid, which is used to further degrade the residual organic matter. At this time, the absorbance can be reduced by about 30%.

[0058] S4. The third purified liquid is subjected to seed decomposition and cyclone classification to obtain slurry; S5. Filter the obtained slurry, wash the filter cake with hot water, and dry it to obtain a high-whiteness aluminum hydroxide product.

[0059] Example 2 This embodiment provides a method for preparing high-whiteness aluminum hydroxide, specifically including the following steps: S1. Add 0.005% of the mass of the sodium aluminate dilution solution (temperature 90℃, αk 1.50) to the sodium aluminate dilution solution after the Bayer process, stir and react for 60 min, and perform adsorption and decarbonization treatment to obtain the first purified solution. At this time, the absorbance can be reduced by about 5%.

[0060] S2. Add lime milk (solid content 270g / L, effective calcium 180g / L) to the first purified liquid entering the co-precipitation reaction tank. The added mass is 5‰ of the total mass of the first purified liquid. Stir and react at 95℃ for 60min to carry out co-precipitation to remove impurities. After sedimentation and separation, the second purified liquid is obtained. At this time, the absorbance can be reduced by about 3%.

[0061] S3. 3% hydrogen peroxide (27.5%) is introduced into the second purification liquid entering the decomposition tank. Under alkaline conditions, a gas-liquid oxidation reaction is carried out at a reaction temperature of 90°C for 60 minutes to perform strong oxidative degradation treatment, resulting in the third purification liquid, which is used to further degrade the residual organic matter. At this time, the absorbance can be reduced by about 20%.

[0062] S4. The third purified liquid is subjected to seed decomposition and cyclone classification to obtain slurry.

[0063] S5. Filter the obtained slurry, wash the filter cake with hot water, and dry it to obtain a high-whiteness aluminum hydroxide product.

[0064] Comparative Example 1 Using traditional processes: the diluted solution and the original decomposition solution after dissolution are not subjected to adsorption, co-precipitation, or strong oxidation for deep purification. Aluminum hydroxide is prepared only through conventional seed decomposition.

[0065] Product performance testing The aluminum hydroxide products obtained in Examples 1, 2 and Comparative Example 1 were tested for whiteness using a WSB-2 digital whiteness meter from Shanghai Xinrui Instruments Co., Ltd. The results are shown in Table 1.

[0066] Table 1. Performance of aluminum hydroxide products from the examples and comparative examples.

[0067] As shown in Table 1, the aluminum hydroxide product prepared using the method of this embodiment has a significantly improved whiteness of over 85%, and its sodium oxide content and particle size are basically equivalent to or better than those of traditional products. This significant improvement in product performance enables it to meet the application requirements of industries such as high-end titanium dioxide and water purification agents (e.g., polyaluminum chloride). In contrast, the aluminum hydroxide product of Comparative Example 1 has a significantly lower whiteness, failing to meet the application requirements of high-end aluminum hydroxide.

[0068] Furthermore, one or more technical solutions in the embodiments of this application have at least the following technical effects or advantages: (1) Significantly improved product quality to meet high-end application requirements: The high-whiteness aluminum hydroxide product prepared in the embodiments of this application has a stable whiteness of over 85%, which is 10-15 percentage points higher than that of aluminum hydroxide produced by the traditional Bayer process (whiteness 70%-75%). The product is pure white and has excellent appearance. At the same time, the sodium oxide content in the product can be controlled below 0.20%. This product characteristic of high whiteness and low sodium content enables it to meet the stringent quality requirements of high-end flame retardant fillers, special ceramics, and high-end water purification agents (such as polyaluminum chloride), significantly expanding the application scope and market competitiveness of the product.

[0069] (2) High impurity removal efficiency and significant purification effect: The embodiments of this application achieve efficient graded removal of organic and inorganic coloring impurities through the orderly coupling of three-stage purification. Among them, the adsorption decarbonization step utilizes the specific adsorption capacity of the composite adsorbent to reduce the absorbance of the solution by 5% to 10%, effectively removing most of the humic acid macromolecular organic matter; the strong oxidation degradation step, through the deep oxidation effect of hydrogen peroxide, can further reduce the absorbance of the solution by 20% to 30%, realizing the conversion of residual macromolecular organic matter into small molecule inorganic matter; the coprecipitation impurity removal step, through the chemical precipitation effect of calcium ions, simultaneously removes inorganic coloring impurities such as iron and titanium. After the three-stage purification treatment, the total organic carbon content and the iron and titanium impurity content in the solution are reduced to extremely low levels, providing a highly pure raw material solution for subsequent high-quality crystallization, fundamentally ensuring the whiteness quality of the final product.

[0070] (3) Strong process compatibility and ease of industrial implementation: The operating conditions of each unit in the embodiments of this application are mild, with the temperature controlled in the range of 90℃~95℃, which is highly matched with the incoming temperature of the diluent after the Bayer process leaching process. No additional heating or cooling is required, which is conducive to saving energy costs. The operating parameters of each step (such as reaction time, reagent dosage, etc.) are all adapted to the processing capacity and operating rhythm of the existing Bayer process production line, and can be directly embedded into the existing process flow for modification and upgrading without the need for large-scale replacement of the main equipment. This high compatibility feature gives the technology of this application a good foundation for industrial promotion and can quickly realize the product upgrade from ordinary aluminum hydroxide to high whiteness aluminum hydroxide.

[0071] (4) Good process stability and controllable product quality: The embodiments of this application achieve visualized and quantifiable management of the entire process through clear process step division and precise parameter control. The steps of adsorption decarbonization, coprecipitation impurity removal, and strong oxidative degradation are interconnected yet relatively independent, which facilitates the monitoring and control of the operating status of each link and timely detection and resolution of process fluctuation problems. The three-level purification system forms a multi-protection mechanism. Even if a slight fluctuation occurs in a certain step at the front end, the subsequent steps can still play a purification role, ensuring that the quality of the solution that finally enters the decomposition process is stable and reliable. This process design enables key quality indicators such as product whiteness and sodium oxide content to have good batch stability, providing a strong guarantee for quality control in large-scale industrial production.

[0072] (5) Good economic efficiency and controllable overall cost: The polyacrylamide composite adsorbent, lime milk, hydrogen peroxide and other reagents used in the embodiments of this application are all commonly used industrial chemicals, widely available and moderately priced, and the dosage is controlled within the optimized range (0.005% to 0.01% adsorbent, 1% to 5% hydrogen peroxide), so the reagent cost is controllable. At the same time, each unit operation is carried out under normal pressure and medium temperature conditions, and the equipment investment and operating energy consumption are low. The high value-added aluminum hydroxide product prepared by the technology of this application has a significantly higher market price than ordinary aluminum hydroxide, and the added value of the product is far higher than the cost of process modification and the increase in operating costs, which has a good input-output ratio and economic feasibility.

[0073] 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 herein.

Claims

1. A method for preparing high-whiteness aluminum hydroxide, characterized in that, The method includes: A polyacrylamide composite adsorbent is added to the sodium aluminate dilution solution from the Bayer process leaching to adsorb and decarbonize the sodium aluminate dilution solution, thereby obtaining a first purified solution. Lime milk is added to the first purified liquid to perform co-precipitation to remove impurities, and the second purified liquid is obtained by solid-liquid separation. Hydrogen peroxide is added to the second purification solution to strongly oxidize and degrade the residual organic matter in the second purification solution, thereby obtaining the third purification solution; The third purified liquid is subjected to seed decomposition to obtain a slurry; The slurry was subjected to solid-liquid separation, and the filter cake was washed to obtain a high-whiteness aluminum hydroxide product.

2. The method according to claim 1, characterized in that, The temperature of the sodium aluminate dilution solution is 90℃~95℃, and αk is 1.45~1.

55.

3. The method according to claim 1, characterized in that, The amount of the polyacrylamide composite adsorbent added is 0.005% to 0.01% of the mass of the sodium aluminate dilution.

4. The method according to claim 1, characterized in that, The adsorption decarbonization treatment includes the following parameters: the stirring reaction temperature is 90℃~95℃, and the stirring reaction time is 45min~75min.

5. The method according to claim 1, characterized in that, The lime slurry has a solid content of 250 g / L to 290 g / L and an effective calcium content of 170 g / L to 190 g / L.

6. The method according to claim 5, characterized in that, The amount of lime slurry added is 5‰ to 10‰ of the total mass of the first purified liquid.

7. The method according to claim 1, characterized in that, The coprecipitation purification process includes the following parameters: the stirring reaction temperature is 90℃~95℃, and the stirring reaction time is 50min~70min.

8. The method according to claim 1, characterized in that, The hydrogen peroxide has a mass concentration of 25% to 30%, and the volume of hydrogen peroxide added is 1% to 5% of the volume of the second purified liquid.

9. The method according to claim 1, characterized in that, The strong oxidative degradation treatment includes the following parameters: reaction temperature of 90℃~95℃, and reaction time of 60min~90min.

10. The method according to claim 1, characterized in that, The high-whiteness aluminum hydroxide product meets the following performance requirements: whiteness ≥ 85%, sodium oxide content ≤ 0.20%.

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