A method for sodium removal and purification of high-purity alumina precursor

Abstract

This invention belongs to the field of material purification technology, specifically relating to a sodium removal purification method for a high-purity alumina precursor. The purification method includes the following steps: (1) grinding sodium-containing aluminum hydroxide, then drying and activating it to obtain activated aluminum hydroxide powder; (2) adding the activated aluminum hydroxide powder from step (1) to a composite sodium removal agent, and stirring and washing it in a constant temperature water bath; the composite sodium removal agent is composed of hydrochloric acid, citric acid and water; (3) filtering out the aluminum hydroxide treated in step (2), rinsing the filter cake with pure water, then filtering, adding a chelation-ion exchange washing solution to the washed aluminum hydroxide, and stirring and washing; (4) rinsing the aluminum hydroxide obtained in step (3) with pure water, and then vacuum drying to obtain a high-purity alumina precursor. This invention provides a highly efficient purification method that leaves no harmful residues and can stably reduce the sodium content in aluminum hydroxide to below 5 ppm, and is more adaptable to high-sodium raw materials.

Description

Technical Field

[0001] This invention belongs to the field of material purification technology, specifically relating to a method for sodium removal and purification of high-purity alumina precursors. Background Technology

[0002] High-purity alumina (Al2O3) is widely used in high-end manufacturing fields such as LED substrates, phosphors, high-pressure sodium lamps, precision ceramics, and lithium battery separator coatings due to its excellent heat resistance, chemical stability, and electrical insulation properties. With the increasing demands for material purity from downstream applications, controlling the sodium content in alumina has become a key indicator for evaluating product grade. Sodium impurities mainly originate from adsorbed sodium, intergranular sodium, and crystalline sodium in the precursor aluminum hydroxide. During subsequent sintering, sodium forms low-activity phases such as sodium aluminate, which not only reduces the conversion rate of α-Al2O3 but also deteriorates the powder's dispersibility, density, and electrical properties. Therefore, developing efficient and low-cost methods for sodium removal and purification of aluminum hydroxide is a core step in the preparation of high-purity alumina.

[0003] Currently, the mainstream methods for industrial production of aluminum hydroxide include the Bayer process, the sintering process, and their combined methods. Among these, the modified Bayer process is widely used due to its short process and low cost; however, the sodium impurity content in the produced aluminum hydroxide fluctuates significantly, typically ranging from 100-1000 ppm (elemental sodium). The specific value depends on factors such as bauxite quality, leaching conditions, decomposition temperature, and washing efficiency. For example, when processing high-alumina-silica ores or using enhanced leaching processes, the initial sodium content of the obtained aluminum hydroxide can be as low as 100-200 ppm; while when the ore grade is low or the decomposition process is poorly controlled, the sodium content can easily rise to over 500 ppm, or even approach 1000 ppm. This sodium mainly exists in three forms: adsorbed sodium, intercrystalline sodium, and crystalline sodium. Intercrystalline sodium and crystalline sodium are difficult to remove by simple water washing because they are encapsulated by aluminum hydroxide crystals or form sodium aluminate complexes.

[0004] For aluminum hydroxide with an initial sodium content below 300 ppm, existing technologies (such as CN103204530B) can reduce sodium to around 10 ppm through two-stage acid washing or ammonium salt washing. However, when the initial sodium content exceeds 500 ppm, the effectiveness of this method drops sharply. Our experiments show that even when treating raw materials with an initial sodium content of 620 ppm under the optimal conditions of this patent, the final sodium content is still above 45 ppm, failing to meet the requirements for 4N grade high-purity alumina (sodium ≤ 10 ppm). For industrial aluminum hydroxide with even higher sodium contents (e.g., 800-1000 ppm), the treatment results are even worse, often requiring repeated washing or increasing the acid concentration. However, this exacerbates aluminum loss, increases the burden of waste acid treatment, and introduces harmful residues such as ammonium ions. Therefore, developing a highly efficient purification method that can stably reduce the sodium content to below 5 ppm without harmful residues for the wide range (100-1000 ppm) of high-sodium aluminum hydroxide produced by the modified Bayer process has become an urgent technical challenge in the field of high-purity alumina preparation.

[0005] A patent CN103204530B has proposed a method for removing sodium during the preparation of high-purity alumina. This method uses a two-stage purification process to obtain fluffy ultra-low sodium aluminum hydroxide. The sodium content of aluminum hydroxide with an initial sodium content of 125-327 ppm can be reduced to 5-9 ppm after treatment, showing a significant apparent sodium removal effect. However, this method still has the following prominent problems: (1) The amount of sodium removal agent used is large, the residual risk is high, and it is also easy to cause aluminum loss; although the sodium content can be further reduced by using ammonium chloride or ammonium carbonate, the final product still contains ammonium ions (NH4+). + (1) High residual content may release nitrogen oxide gas during subsequent high-temperature sintering, causing secondary pollution and affecting the densification behavior of alumina. (2) Limited adaptability to high sodium raw materials: This method works well for raw materials with initial sodium content below 300 ppm, but the treatment effect of industrial grade aluminum hydroxide with sodium content > 500 ppm is significantly reduced.

[0006] Therefore, there is an urgent need to develop a new sodium removal and purification method that is highly efficient in removing sodium, leaves no residue, and is applicable to raw materials with a wide range of sodium contents. Summary of the Invention

[0007] The purpose of this invention is to address the shortcomings of existing methods for removing sodium from high-purity alumina precursors, such as large amounts of sodium removal agents that are prone to residue, poor adaptability to high-sodium raw materials, and narrow operating windows. This invention provides a highly efficient purification method that leaves no harmful residues and can stably reduce the sodium content in aluminum hydroxide to below 5 ppm, thereby meeting the requirements for the preparation of high-purity alumina.

[0008] To achieve the above objectives, the present invention provides the following technical solution: A method for sodium removal and purification of high-purity alumina precursor includes the following steps: (1) Sodium-containing aluminum hydroxide is ground, then dried and activated to obtain activated aluminum hydroxide powder; (2) Add the activated aluminum hydroxide powder from step (1) to the composite sodium removal agent and stir and wash it in a constant temperature water bath; the composite sodium removal agent is composed of hydrochloric acid, citric acid and water; (3) Filter out the aluminum hydroxide after step (2), rinse the filter cake with pure water, then filter, add chelation-ion exchange washing solution to the washed aluminum hydroxide, and stir to wash; (4) Rinse the aluminum hydroxide obtained in step (3) with pure water and then vacuum dry it to obtain a high-purity alumina precursor.

[0009] Preferably, the initial sodium content in the sodium-containing aluminum hydroxide in step (1) is 100-1000 ppm.

[0010] Preferably, in step (1), sodium-containing aluminum hydroxide is ground to a particle size ≤300μm, and then dried and activated at 100-150℃ for 30-60min to obtain activated aluminum hydroxide powder.

[0011] Preferably, in step (2), 5-10L of composite sodium removal agent is added per kilogram of aluminum hydroxide.

[0012] Preferably, the composite sodium removal agent is composed of hydrochloric acid, citric acid and water, wherein the concentration of hydrochloric acid is 10-15 g / L, the concentration of citric acid is 5-10 g / L, and the balance is water.

[0013] This invention is the first to use a composite sodium removal agent composed of hydrochloric acid and citric acid to pickle aluminum hydroxide. Citric acid acts as a complexing agent, forming a soluble complex with the sodium ions exposed on the surface to promote sodium removal. On the other hand, it can inhibit excessive aluminum dissolution and increase aluminum yield. This composite pickling system avoids the corrosion and aluminum loss problems caused by high concentration hydrochloric acid.

[0014] Preferably, the chelation-ion exchange washing solution contains 2-5 g / L EDTA-2Na, 1-3 g / L sodium polyacrylate, and the pH is adjusted to 6.0-7.0 using 1-2 mol / L dilute hydrochloric acid, with the remainder being water.

[0015] This invention employs a chelation-ion exchange washing solution composed of EDTA-2Na and sodium polyacrylate, which deeply removes intergranular and adsorbed sodium through a dual mechanism of chelation capture and ion exchange. Experiments have shown that the optimal ratio of EDTA-2Na to sodium polyacrylate results in better sodium removal efficiency. This invention completely avoids the use of ammonium salts, leaves no harmful residues, and meets the requirements for high-purity alumina preparation.

[0016] Preferably, in step (2), the activated aluminum hydroxide powder from step (1) is added to the composite sodium removal agent and stirred and washed 1-2 times in a constant temperature water bath at 40-60℃, with each washing time being 0.5-1.0h.

[0017] Preferably, in step (3), a chelating-ion exchange washing solution is added to the cleaned aluminum hydroxide, and the mixture is stirred and washed 1-2 times at 30-50°C, each time for 0.5-1.0 h.

[0018] Preferably, the aluminum hydroxide obtained in step (3) is rinsed with pure water until the conductivity of the rinsing solution is ≤5μS / cm.

[0019] Preferably, the sodium content in the high-purity alumina precursor is ≤5ppm.

[0020] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows: 1. This invention abandons the traditional ammonium salt and high-concentration acid washing process, and adopts a technical route combining citric acid-hydrochloric acid composite acid washing with chelation-ion exchange for deep sodium removal. The entire process avoids the use of nitrogen-containing reagents such as ammonium chloride and ammonium carbonate, thus preventing the residue of ammonium ions in the product and eliminating the release of harmful nitrogen oxide gases during subsequent high-temperature sintering. Simultaneously, the citric acid in the composite sodium removal agent inhibits aluminum dissolution, and the washing wastewater can be treated by conventional neutralization and precipitation, making it environmentally friendly. This method achieves the green and pollution-free preparation of high-purity alumina precursors.

[0021] 2. This invention utilizes a synergistic process of activation, composite acid washing, chelation-ion exchange, and low-temperature vacuum drying to achieve deep sodium removal from aluminum hydroxide with varying initial sodium contents. The sodium content in the treated precursor can be stably controlled below 5 ppm. This performance is significantly superior to existing technologies and fully meets the stringent market requirements for sodium impurities in high-purity alumina.

[0022] 3. This invention uses a chelation-ion exchange washing solution composed of EDTA-2Na and sodium polyacrylate to deeply remove intergranular and adsorbed sodium through a dual mechanism of chelation capture and ion exchange. Experiments have shown that the optimal ratio of EDTA-2Na to sodium polyacrylate results in better sodium removal efficiency. This invention completely avoids the use of ammonium salts, leaves no harmful residues, and meets the requirements for high-purity alumina preparation.

[0023] 4. This invention uses a composite sodium removal agent composed of hydrochloric acid and citric acid to pickle aluminum hydroxide. Citric acid acts as a complexing agent, forming a soluble complex with the sodium ions exposed on the surface to promote sodium removal. On the other hand, it can inhibit excessive aluminum dissolution and increase aluminum yield. This composite pickling system avoids the corrosion and aluminum loss problems caused by high concentration hydrochloric acid. Detailed Implementation

[0024] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] All raw materials used in the following embodiments of the present invention are commercially available products.

[0026] Example 1 This embodiment provides a method for sodium removal and purification of high-purity alumina precursor, including the following steps: (1) Grind sodium-containing aluminum hydroxide to a particle size ≤300μm, and then dry and activate it at 120℃ for 50min to obtain activated aluminum hydroxide powder; (2) Add the activated aluminum hydroxide powder from step (1) to the composite sodium removal agent. Add 7L of composite sodium removal agent to every kilogram of aluminum hydroxide. Stir and wash twice in a constant temperature water bath at 50°C, with each washing time being 0.8h. The composite sodium removal agent is composed of hydrochloric acid, citric acid and water, wherein the concentration of hydrochloric acid is 12g / L, the concentration of citric acid is 6g / L, and the remainder is water.

[0027] (3) Filter out the aluminum hydroxide after step (2), rinse the filter cake once with pure water at 40℃ with a liquid-to-solid ratio of 2:1 for 10 min, then filter, add chelating-ion exchange washing solution to the washed aluminum hydroxide at a liquid-to-solid ratio of 7:1 (L / kg), stir and wash twice at 40℃ for 0.8 h each time; the concentration of EDTA-2Na in the chelating-ion exchange washing solution is 3 g / L, the concentration of sodium polyacrylate is 2 g / L, the pH is adjusted to 6.5 with 1 mol / L dilute hydrochloric acid, and the remainder is water.

[0028] (4) The aluminum hydroxide obtained in step (3) is rinsed twice with pure water at 40°C for 0.4 h each time until the conductivity of the rinsing solution is 5 μS / cm; then it is vacuum dried at 120°C for 4 h to obtain a high-purity alumina precursor.

[0029] Example 2 This embodiment provides a method for sodium removal and purification of high-purity alumina precursor, including the following steps: (1) Grind sodium-containing aluminum hydroxide to a particle size ≤300μm, and then dry and activate it at 120℃ for 50min to obtain activated aluminum hydroxide powder; (2) Add the activated aluminum hydroxide powder from step (1) to the composite sodium removal agent. Add 7L of composite sodium removal agent to every kilogram of aluminum hydroxide. Stir and wash twice in a constant temperature water bath at 50°C, with each washing time being 1.0h. The composite sodium removal agent is composed of hydrochloric acid, citric acid and water, wherein the concentration of hydrochloric acid is 14g / L, the concentration of citric acid is 5g / L, and the remainder is water.

[0030] (3) Filter out the aluminum hydroxide after step (2), rinse the filter cake once with pure water at 40℃ with a liquid-to-solid ratio of 2:1 for 10 min, then filter, add chelating-ion exchange washing solution to the washed aluminum hydroxide at a liquid-to-solid ratio of 7:1 (L / kg), stir and wash twice at 40℃ for 0.8 h each time; the concentration of EDTA-2Na in the chelating-ion exchange washing solution is 5 g / L, the concentration of sodium polyacrylate is 1 g / L, the pH is adjusted to 6.5 with 1 mol / L dilute hydrochloric acid, and the remainder is water.

[0031] (4) The aluminum hydroxide obtained in step (3) is rinsed twice with pure water at 40°C for 0.4 h each time until the conductivity of the rinsing solution is 5 μS / cm; then it is vacuum dried at 120°C for 4 h to obtain a high-purity alumina precursor.

[0032] Example 3 This embodiment provides a method for sodium removal and purification of high-purity alumina precursor, including the following steps: (1) Grind sodium-containing aluminum hydroxide to a particle size ≤300μm, and then dry and activate it at 120℃ for 50min to obtain activated aluminum hydroxide powder; (2) Add the activated aluminum hydroxide powder from step (1) to the composite sodium removal agent. Add 7L of composite sodium removal agent to every kilogram of aluminum hydroxide. Stir and wash twice in a constant temperature water bath at 50°C, with each washing time being 0.8h. The composite sodium removal agent is composed of hydrochloric acid, citric acid and water, wherein the concentration of hydrochloric acid is 10.5g / L, the concentration of citric acid is 8.8g / L, and the remainder is water.

[0033] (3) Filter out the aluminum hydroxide after step (2), rinse the filter cake once with pure water at 40℃ with a liquid-to-solid ratio of 2:1 for 10 min, then filter, add chelating-ion exchange washing solution to the washed aluminum hydroxide at a liquid-to-solid ratio of 7:1 (L / kg), stir and wash twice at 40℃ for 0.8 h each time; the concentration of EDTA-2Na in the chelating-ion exchange washing solution is 2.8 g / L, the concentration of sodium polyacrylate is 2.6 g / L, the pH is adjusted to 6.5 with 1 mol / L dilute hydrochloric acid, and the remainder is water.

[0034] (4) The aluminum hydroxide obtained in step (3) is rinsed twice with pure water at 40°C for 0.4 h each time until the conductivity of the rinsing solution is 5 μS / cm; then it is vacuum dried at 120°C for 4 h to obtain a high-purity alumina precursor.

[0035] Comparative Example 1 The difference between this comparative example and Example 1 is that citric acid is not added to the composite sodium removal agent in step (2), and only a hydrochloric acid aqueous solution with a concentration of 12 g / L is used, while the rest remains unchanged.

[0036] Comparative Example 2 The difference between this comparative example and Example 1 is that: in step (3), chelation-ion exchange washing solution is not used, but ammonium chloride solution from patent CN103204530B is used: the concentration is 10g / L, the pH is adjusted to 6.5 with dilute hydrochloric acid, the liquid-solid ratio is the same 7:1, and the solution is stirred and washed twice at 40°C for 0.8h each time.

[0037] Comparative Example 3 The difference between this comparative example and Example 1 is that the composite sodium removal agent is composed of hydrochloric acid, citric acid and water, wherein the concentration of hydrochloric acid is 20 g / L, the concentration of citric acid is 1 g / L, and the remainder is water.

[0038] Comparative Example 4 The difference between this comparative example and Example 1 is that the concentration of EDTA-2Na in the chelation-ion exchange washing solution is 0.5 g / L, the concentration of sodium polyacrylate is 5 g / L, the pH is adjusted to 6.5 using 1 mol / L dilute hydrochloric acid, and the remainder is water.

[0039] Comparative Example 5 The difference between this comparative example and Example 1 is that the concentration of EDTA-2Na in the chelation-ion exchange washing solution is 10 g / L, the concentration of sodium polyacrylate is 0.5 g / L, the pH is adjusted to 6.5 using 1 mol / L dilute hydrochloric acid, and the remainder is water.

[0040] Comparative Example 6 The difference between this comparative example and Example 1 is as follows: the method of Example 1 (CN103204530B) is adopted: aluminum hydroxide powder is ground to ≤500μm for later use; the finely ground aluminum hydroxide powder is washed three times with 45g / L hydrochloric acid in a constant temperature water bath at 50℃ to obtain pure aluminum hydroxide; the purified pure aluminum hydroxide is washed with water until pH 7.0 and dried at 300℃; the washed and dried aluminum hydroxide is added to 10g / L ammonium chloride and washed three times in a constant temperature water bath at 50℃ for secondary purification; the secondary purified aluminum hydroxide is washed with pure water at 50℃ and then dried at 300℃.

[0041] Performance testing High-sodium aluminum hydroxide powders No. 1, No. 2, and No. 3 were used as raw materials. All high-sodium aluminum hydroxide powders were prepared using a modified Bayer process, and their elemental composition (ICP-OES determination, unit ppm) is shown in Table 1. Following the purification methods described in Examples 1-3 and Comparative Examples 1-6, each powder was subjected to sodium removal treatment sequentially. Each experiment was repeated three times, and the average value was taken. The high-purity alumina precursors obtained after treatment were digested, and the sodium content (elemental Na, unit ppm) was determined by ICP-OES. The results are shown in Table 2.

[0042] Table 1. Elemental composition (ppm) of high-sodium aluminum hydroxide powder Table 2. Sodium content test results (ppm) in high-purity alumina precursors. As shown in Table 2, the purification methods in Examples 1-3 can stably reduce the sodium content of No. 1, No. 2 and No. 3 high-sodium aluminum hydroxide powders to below 5 ppm.

[0043] The sodium removal efficiency of Comparative Examples 1 and 6 decreased significantly, especially the sodium content of high-sodium raw materials No. 2 and No. 3 increased significantly after treatment.

[0044] Comparative Example 2 used ammonium chloride to replace the chelation-ion exchange washing solution, which had some effect, but ammonium residue was detected in the product, and the sodium content still exceeded 15 ppm under high sodium raw material conditions.

[0045] In Comparative Example 3, the concentration of hydrochloric acid in the composite sodium removal agent was too high and the concentration of citric acid was too low, resulting in a poor sodium removal effect. Furthermore, the high concentration of hydrochloric acid led to increased aluminum dissolution, while the insufficient citric acid reduced the complexing and protective effect.

[0046] In Comparative Example 4, the concentration of EDTA-2Na in the chelation-ion exchange washing solution was too low and the concentration of sodium polyacrylate was too high. Due to the lack of sufficient EDTA chelation, the deep sodium removal capacity was significantly insufficient, and the sodium content was high under high sodium raw materials.

[0047] In Comparative Example 5, the concentration of EDTA-2Na was too high and the concentration of sodium polyacrylate was too low. The sodium removal effect was close to that of Example 1, but doubling the amount of EDTA led to a significant increase in cost. In addition, high concentration of EDTA may introduce more organic residues. At the same time, the low concentration of sodium polyacrylate affected the dispersibility of the slurry and easily caused particle agglomeration.

[0048] The above results indicate that the composition of the composite sodium removal agent, the concentration range of the chelation-ion exchange washing solution, and the process steps specified in this invention have a synergistic effect, enabling deep sodium removal from high-sodium raw materials without harmful residues.

[0049] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for sodium removal and purification of a high-purity alumina precursor, characterized in that, Includes the following steps: (1) Sodium-containing aluminum hydroxide is ground, then dried and activated to obtain activated aluminum hydroxide powder; (2) Add the activated aluminum hydroxide powder from step (1) to the composite sodium removal agent and stir and wash it in a constant temperature water bath; the composite sodium removal agent is composed of hydrochloric acid, citric acid and water; (3) Filter out the aluminum hydroxide after step (2), rinse the filter cake with pure water, then filter, add chelation-ion exchange washing solution to the washed aluminum hydroxide, and stir to wash; (4) Rinse the aluminum hydroxide obtained in step (3) with pure water and then vacuum dry to obtain a high-purity alumina precursor.

2. The sodium removal and purification method for the high-purity alumina precursor according to claim 1, characterized in that, In step (1), the initial sodium content in the sodium-containing aluminum hydroxide is 100-1000 ppm.

3. The sodium removal and purification method for the high-purity alumina precursor according to claim 1, characterized in that, In step (1), sodium-containing aluminum hydroxide is ground to a particle size ≤300μm, and then dried and activated at 100-150℃ for 30-60min to obtain activated aluminum hydroxide powder.

4. The sodium removal and purification method for the high-purity alumina precursor according to claim 1, characterized in that, In step (2), 5-10L of composite sodium removal agent is added per kilogram of aluminum hydroxide.

5. The sodium removal and purification method for the high-purity alumina precursor according to claim 1, characterized in that, The composite sodium removal agent is composed of hydrochloric acid, citric acid and water, wherein the concentration of hydrochloric acid is 10-15 g / L, the concentration of citric acid is 5-10 g / L, and the balance is water.

6. The sodium removal and purification method for the high-purity alumina precursor according to claim 1, characterized in that, The chelation-ion exchange washing solution contains 2-5 g / L EDTA-2Na, 1-3 g / L sodium polyacrylate, and the pH is adjusted to 6.0-7.0 using 1-2 mol / L dilute hydrochloric acid, with the remainder being water.

7. The sodium removal and purification method for the high-purity alumina precursor according to claim 1, characterized in that, In step (2), the activated aluminum hydroxide powder from step (1) is added to the composite sodium removal agent and stirred and washed 1-2 times in a constant temperature water bath at 40-60℃, with each washing time being 0.5-1.0h.

8. The sodium removal and purification method for the high-purity alumina precursor according to claim 1, characterized in that, In step (3), a chelating-ion exchange washing solution is added to the cleaned aluminum hydroxide, and the mixture is stirred and washed 1-2 times at 30-50℃, each time for 0.5-1.0h.

9. The sodium removal and purification method for the high-purity alumina precursor according to claim 1, characterized in that, The aluminum hydroxide obtained in step (3) is rinsed with pure water until the conductivity of the rinsing solution is ≤5μS / cm.

10. The sodium removal and purification method for the high-purity alumina precursor according to claim 1, characterized in that, The sodium content in the high-purity alumina precursor is ≤5ppm.

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