Method for producing high-purity aluminum monohydrate and α-alumina

Abstract

The present invention relates to a method for producing aluminum monohydrate, comprising the steps of: i) mixing an alumina raw material with ethylenediaminetetraacetic acid to obtain a raw material mixture; and ii) subjecting the raw material mixture to a hydrothermal treatment, wherein the pH of the raw material mixture is at least 8. The obtained aluminum monohydrate exhibits excellent purity despite starting from low-purity raw materials and can be calcined to obtain high-purity α-alumina.

Description

Technical Field

[0001] Description The present invention relates to a method for producing high-purity aluminum monohydrate (e.g., boehmite and / or diaspore) and high-purity α-aluminum oxide from low-purity alumina raw materials using hydrothermal treatment. Further, the use of complexing agents is described.

[0002] The production of boehmite using hydrothermal treatment is known in the art. European Patent No. 0304721 discloses a method for producing a microcrystalline boehmite product, which includes heating a precursor compound convertible to boehmite under autogenous pressure in an aqueous medium having a pH of 5 or less or a pH of 8 or more in the presence of boehmite seed particles finer than 0.02 μm. Aluminum oxide, aluminum hydroxide, or aluminum alkoxide can be used as the precursor. After hydrothermal treatment, microcrystalline boehmite is obtained. Chinese Patent Application Publication No. 110357135 teaches a method for producing alumina for a lithium battery separator using aluminum hydroxide and seed crystals of ultrafine aluminum hydroxide and mixing them with ammonia. However, both methods are technically complex and require a large amount of expensive seed particles.

[0003] In view of the prior art, the underlying problem of the present invention is to provide a more inexpensive and easily controllable method for producing high-purity aluminum monohydrate using low-purity (impure), and thus less expensive raw materials and a minimal amount of technical equipment. A further problem of the present invention is to provide a specific use of complexing agents.

[0004] The solution to the above-mentioned problems is described in the independent claims. The dependent claims include advantageous embodiments of the present invention.

[0005] Accordingly, the problem of the above heading is solved by a method for producing aluminum monohydrate such as boehmite and / or diaspore, the method comprising a step of mixing an alumina raw material with a complexing agent selected from a nitrogen-containing complexing agent, acetic acid, and citric acid to obtain a raw material mixture, and a step of subjecting the raw material mixture to hydrothermal treatment. Surprisingly, it has been found that due to the addition of a specific complexing agent to the alumina raw material before hydrothermal treatment, the purity of the produced aluminum monohydrate can be significantly improved.

[0006] Accordingly, an important first aspect of the present invention is the following steps: - a step of mixing an alumina raw material with ethylenediaminetetraacetic acid to obtain a raw material mixture, and - a step of subjecting the raw material mixture to hydrothermal treatment including, wherein the pH of the raw material mixture is at least 8, a method for producing aluminum monohydrate.

[0007] In the prior art, it has not been recognized that in a simple and efficient method, a high-purity aluminum monohydrate (or α-alumina upon calcination of the aluminum monohydrate) can be produced using (low-purity) raw materials, wherein ethylenediaminetetraacetic acid is used in the raw material mixture subjected to hydrothermal treatment and the pH of the raw material mixture is at least 8.

[0008] In particular, when using alumina raw materials with a purity of 99.8% or less, especially 99.6% or less, the purity of the aluminum monohydrate after implementing the method of the present invention is at least 99.90%, preferably up to 99.99%. Representative impurities contained in the alumina raw material and removable by the method of the present invention include Na2O, CaO, Fe2O3, ZnO, Li2O, MnO, CuO, Ga2O3, and BeO. Such impurities are often occluded in the particles or aggregates of the alumina raw material. In the present invention, alumina raw materials with a purity of 99.8% or less, especially 99.6% or less, are referred to as "low-purity" alumina raw materials. In a preferred embodiment of the present invention, all purity values are reported on an Al2O3 basis in order to be able to compare the purity of the raw material before treatment (e.g., smelter-grade alumina or aluminum monohydrate) with the aluminum monohydrate product obtained by the method described herein.

[0009] Thus, through the step of "mixing the alumina raw material with ethylenediaminetetraacetic acid to obtain a raw material mixture", as is clear from the disclosure herein, a raw material mixture is prepared that contains not only the alumina raw material but also ethylenediaminetetraacetic acid and optionally other components. The pH of the raw material mixture (i.e., before being subjected to hydrothermal treatment) is at least 8.

[0010] During hydrothermal treatment, which is typically carried out in an autoclave under pressure and increasing temperature, the alumina raw material recrystallizes to form a more thermodynamically stable phase. When using the complexing agent of the present invention, impurity removal becomes possible by complex formation and optionally dissolution during hydrothermal treatment, and these impurities can be easily separated by filtering, washing, and drying the product obtained after the hydrothermal treatment is completed and the reaction mixture is cooled, thereby obtaining high-purity aluminum monohydrate having a purity of at least 99.90%. The high-purity aluminum monohydrate can ultimately be dried.

[0011] The method of the present invention does not require specific additional method steps and only requires adding a complexing agent selected from a nitrogen-containing complexing agent, acetic acid, and citric acid to the alumina raw material and performing hydrothermal treatment to prepare the raw material mixture. At this time, the complexing agent may be used alone or in combination of two or more complexing agents. The above-mentioned complexing agents, particularly nitrogen-containing complexing agents, are well-known in the art and are available at low cost. Since nitrogen-containing complexing agents are generally non-toxic, their use is easy and does not require specific preventive measures. The method of the present invention can be easily controlled and can be implemented with a short treatment time, so the cost of the method of the present invention is low.

[0012] The pH of the raw material mixture is preferably set to at least 8, more preferably at least 9, and even more preferably at least 10, whereby surprisingly, the purity of the obtained aluminum monohydrate can be increased. From the viewpoint of obtaining an extremely high purity of 99.99%, the pH of the raw material mixture is particularly set in the range of 10 to 12, and most preferably in the range of 10 to 11. As used herein, the "pH of the raw material mixture" refers to the pH of the raw materials before hydrothermal treatment, that is, before the raw material mixture is subjected to hydrothermal treatment.

[0013] In order to improve the solubility of impurities and promote the recrystallization of the alumina raw material during hydrothermal treatment, the raw material mixture preferably contains ammonia. At this time, the mass ratio of the alumina raw material to ammonia is preferably 100:1 to 10:1, particularly 50:1 to 10:1. Ammonia can be used as an aqueous solution. As an example, a 25% by mass aqueous ammonia solution can be prepared and added to the raw material mixture. Ammonia can preferably be used in an amount of 22 g of pure ammonia per 100 g of the alumina raw material.

[0014] Surprisingly, it has been further found that it is preferable that the raw material mixture contains ammonia and the content of ammonia relative to the content of the alumina raw material is in the range of 1.25% to 3.70% by mass.

[0015] Since alumina raw materials are readily available at low cost, they are preferably selected from smelting-grade alumina, chemical-grade alumina, transition alumina (i.e., λ-alumina, δ-alumina, η-alumina, θ-alumina, κ-alumina, ρ-alumina, and χ-alumina), and aluminum trihydrate. Among these, smelting-grade alumina, chemical-grade alumina, transition alumina (i.e., λ-alumina, δ-alumina, η-alumina, θ-alumina, κ-alumina, ρ-alumina, and χ-alumina), and aluminum trihydrate, particularly smelting-grade alumina or aluminum trihydrate, are particularly preferred. Using the pulverized product of the above-mentioned raw materials is particularly preferred from the viewpoint of shortening the processing time. Also, using these pulverized products can contribute to improving the purity. Therefore, the D50 particle size of the pulverized product of the above-mentioned raw materials after pulverization is preferably in the range of 0.1 μm to 100 μm, more preferably in the range of 1 μm to 45 μm. The term "alumina raw material" is used herein to include the above-mentioned raw materials, including aluminum trihydrate. Therefore, the term "alumina raw material" is used herein as a synonym for "raw material" or "raw material for the preparation of aluminum monohydrate" to define any raw material suitable for the preparation of aluminum monohydrate (and α-alumina during the calcination of aluminum monohydrate).

[0016] In the present invention, in the method for preparing aluminum monohydrate described herein, it has been found that the use of a specific complexing agent from a group of at least nitrogen-containing complexing agents preferably containing at least one electron-donating nitrogen is very advantageous.

[0017] Therefore, the nitrogen-containing complexing agent is preferably ethylenediaminetetraacetic acid, which surprisingly is particularly advantageous in the process for preparing aluminum monohydrate (or α-alumina) having a very high level of purity as described herein, even when starting from low-purity starting materials (alumina raw materials), where the amounts of impurities such as Na (calculated as Na2O), Fe (calculated as Fe2O3) and / or Ca (as CaO) are very small. Surprisingly, ethylenediaminetetraacetic acid appears to balance the acidity and alkalinity of the raw material mixture and enables the obtaining of high-purity aluminum monohydrate by a simple and direct method starting from low-purity alumina raw materials as described herein. The disodium salt of ethylenediaminetetraacetic acid is one of the most commonly used complexing agents, but pure ethylenediaminetetraacetic acid has been found to give better results, especially in the range of at least pH 8, preferably at least pH 9, more preferably pH 10 - 12, and most preferably pH 10 - 11, without increasing the total Na2O concentration.

[0018] Furthermore, it has been found that the impurities can be most effectively separated from the raw material mixture when the mass ratio of the alumina raw material to the complexing agent (especially ethylenediaminetetraacetic acid) is in the range of 50:1 to 20:1. Regarding the improvement of the impurity level of aluminum monohydrate, it was not the case that the greater the amount of the complexing agent, the significantly better the results were obtained. However, the lower the amount of the complexing agent, the higher the impurity level. It has been found that it is particularly preferable to use 20 - 30 g of the complexing agent per 1000 g of the alumina raw material. As described above, according to a preferred embodiment of the present invention, the complexing agent is ethylenediaminetetraacetic acid. According to one embodiment of the present invention, the mass ratio of the alumina raw material to the complexing agent (especially ethylenediaminetetraacetic acid) is calculated based on the dry mass of the alumina raw material used in the raw material mixture (to be subjected to hydrothermal treatment) and the dry mass of the complexing agent (especially ethylenediaminetetraacetic acid) present in the raw material mixture (i.e., preferably without taking into account the residual moisture of materials generally less than 1% by mass, for example, about 0.5% by mass or less).

[0019] According to a further preferred embodiment, the raw material mixture contains ammonium hydroxide. Without being bound by theory, ammonium hydroxide seems to function as a buffer, especially when combined with ammonia, and by adding ammonium hydroxide, the desired pH value can be easily maintained. This increases the solubility of impurities and makes the recrystallization of alumina proceed more rapidly and easily.

[0020] The raw material mixture may further contain an acid, if necessary, to balance / regulate the pH value of the raw material mixture (to above pH 8 or above pH 9, especially within the range of pH 10 - 11), where the acid is especially selected from the group consisting of hydrochloric acid, phosphoric acid, sulfuric acid, nitric acid, boric acid, hydrofluoric acid, and carboxylic acids, and the carboxylic acid is preferably selected from carbonic acid, formic acid, and propionic acid. Surprisingly, it has been found that the addition of one of these acids or a mixture thereof contributes to a more efficient reduction of Na2O and CaO.

[0021] More preferably, the raw material mixture contains hydrogen peroxide, sodium hydroxide, or potassium hydroxide. Thereby, the pH value can be adjusted more easily.

[0022] From the perspective of improving the purity of the prepared aluminum monohydrate, it is more advantageous if the mass ratio of the solid compound to the liquid compound in the raw material mixture is in the range of 1:1 to 1:2.

[0023] From the perspective of the high purity of the produced aluminum monohydrate, it is more preferable to set the temperature during the hydrothermal treatment within the range of 150°C to 260°C. Although higher temperatures also have an effect, the higher the temperature is set, the more costly the method becomes. The lower the temperature is set, the slower the recrystallization becomes. When the temperature is set within the range of 150°C to 260°C, a balance of all advantageous effects is achieved, and good results are obtained from the perspective of the purity of the product.

[0024] The pressure during the hydrothermal treatment is not particularly limited, and the maximum value will depend on the stability of the container used. However, when the hydrothermal pressure during the hydrothermal treatment is in the range of 1 bar to 300 bar, particularly 1 bar to 50 bar, a good reaction rate and cost efficiency can be achieved.

[0025] More preferably, the hydrothermal treatment is carried out for 1 to 100 hours.

[0026] The reaction during the hydrothermal treatment can preferably be accelerated by stirring the raw material mixture during the hydrothermal treatment.

[0027] As a further aspect of the present invention, there is provided a method for producing α-alumina by calcining the aluminum monohydrate obtained according to the above method. The calcination can be carried out at a temperature above 700 °C, more specifically above 800 °C, and most specifically between 900 and 1500 °C. In addition, the calcination can be carried out in an electric, gas or other combustion furnace / kiln (continuous or periodic), and the residence / holding time is flexible. The calcination parameters are preferably set to adjust the specific crystal size and surface area of α-alumina. Depending on the desired product to be produced, the primary crystal size is preferably 0.1 to 6 μm, and the specific surface area is 0.1 to 50 m 2 / g, where the particle size distribution is measured according to DIN EN 725 / T5 and the surface area is measured according to ISO 9227. By using high-purity aluminum monohydrate, α-alumina also exhibits high purity.

[0028] A further aspect of the present invention relates to the use of ethylenediaminetetraacetic acid (EDTA) in the preparation of aluminum monohydrate, particularly in the hydrothermal treatment. A further aspect of the present invention is that the aluminum monohydrate is obtained from the raw material mixture to be subjected to the hydrothermal treatment, and the pH of the raw material mixture before the hydrothermal treatment is at least 8, preferably at least 9, more preferably in the range of 10 to 12, and most preferably in the range of 10 to 11, and the mass ratio of the raw material to ethylenediaminetetraacetic acid is 50:1 to 20:1, optionally 40:1 to 20:1, regarding the aforementioned use.

[0029] As a further aspect of the present invention, the use of a combination of a complexing agent selected from nitrogen-containing complexing agents, acetic acid and citric acid and ammonia in the production of aluminum monohydrates such as boehmite and / or diaspore, particularly in hydrothermal treatment, is disclosed. Using a specific complexing agent in combination with ammonia improves the purity of the produced aluminum monohydrate.

[0030] The following embodiments of the present invention are also disclosed herein: 1. A method for producing an aluminum monohydrate, comprising the following steps: - mixing an alumina raw material with a complexing agent selected from nitrogen-containing complexing agents, acetic acid and citric acid to obtain a raw material mixture, and - subjecting the raw material mixture to hydrothermal treatment The method comprising. 2. The method according to embodiment 1, wherein the pH of the raw material mixture is at least 5, preferably at least 8, more preferably at least 9, even more preferably at least 10, and in this case, the pH of the raw material mixture is most preferably in the range of 10 to 11. 3. The method according to embodiment 1 or 2, wherein the raw material mixture contains ammonia, and in this case, the mass ratio of the alumina raw material to ammonia is 100:1 to 10:1. 4. The method according to any one of embodiments 1 to 3, wherein the raw material mixture contains ammonia, and in this case, the content of ammonia relative to the content of the alumina raw material is in the range of 1.25% by mass to 3.70% by mass. 5. The method according to any one of embodiments 1 to 4, wherein the alumina raw material is selected from smelting grade alumina, pretreated (leached) smelting grade alumina, chemical grade alumina, pretreated (leached) chemical grade alumina, transition alumina, and aluminum trihydrate. 6. The nitrogen-containing complexing agent contains at least one electron-donating nitrogen, and in this case, the nitrogen-containing complexing agent is particularly nitrilotriacetic acid, ethylenediaminetetraacetic acid, disodium ethylenediaminetetraacetate, 1,2-cyclohexylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, bis(aminoethyl)-glycol ether-N,N,N'N'-tetraacetic acid, N-(2-hydroxyethyl)-ethylenediamine-N,N,N'-triacetic acid trisodium salt, triethylenetetraminehexaacetic acid, N,N-bis(carboxymethyl)-L-glutamic acid tetrasodium salt, L-glutamic acid, N-N-diacetic acid tetrasodium salt GLDA, ethylenediamine-N,N'-bis(2-hydroxyphenylacetic acid), ethylenediamine, 2-(2-aminoethylamino)ethanol, diethylenetriamine, diethylenetetramine, iminodiacetate, triethylenetetramine, triaminotriethylamine, bis(salicylidene)ethylenediamine, ethylenediaminetriacetate, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate, dimethylglyoxime, 8-hydroxyquinoline, 2,2'-bipyridine and 1,10-phenanthroline, and is particularly selected from any of the methods according to any one of Embodiments 1 to 5. 7. The nitrogen-containing complexing agent is ethylenediaminetetraacetic acid, and the method according to any one of Embodiments 1 to 6. 8. The mass ratio of the alumina raw material to the nitrogen-containing complexing agent is 50:1 to 20:1, and the method according to any one of Embodiments 1 to 7. 9. The raw material mixture further contains ammonium hydroxide, and the method according to any one of Embodiments 1 to 8. 10. The raw material mixture further contains an acid, and in this case, the acid is particularly selected from the group consisting of hydrochloric acid, phosphoric acid, sulfuric acid, nitric acid, boric acid, hydrofluoric acid and carboxylic acid, and in this case, the carboxylic acid is preferably selected from carbonic acid, formic acid and propionic acid, and the method according to any one of Embodiments 1 to 9. 11. The raw material mixture further contains hydrogen peroxide, sodium hydroxide or potassium hydroxide, and the method according to any one of Embodiments 1 to 10. 12. The method according to any one of Embodiments 1 to 11, wherein the mass ratio of the solid compound of the raw material mixture to the liquid compound of the raw material mixture is in the range of 1:1 to 1:2, and / or the temperature during the hydrothermal treatment is in the range of 150°C to 260°C, and / or the hydrothermal pressure during the hydrothermal treatment is in the range of 1 bar to 300 bar, particularly 1 bar to 50 bar, and / or the hydrothermal treatment is carried out for 1 to 100 hours, and / or the raw material mixture is stirred during the hydrothermal treatment. 13. A method for producing α-alumina, comprising a step of calcining aluminum monohydrate obtained according to the method described in any one of Embodiments 1 to 12. 14. The method according to Embodiment 13, wherein the purity level of α-alumina is 99.8 mass% to 99.99 mass%. 15. Use of a complexing agent selected from nitrogen-containing complexing agents, acetic acid, and citric acid in the production of aluminum monohydrate, particularly in hydrothermal treatment.

Brief Description of the Drawings

[0031]

Figure 1

Figure 2

Figure 3

[0032] Example Example 1 In an autoclave, 50 g of aluminum hydroxide (Al(OH)3) (see Table 1 for details) having an impurity (Na2O, Fe2O3, SiO2, MgO, TiO2, CaO, Li2O, and ZnO) concentration of about 0.25 mass% was mixed with 0.5 g of EDTA (ethylenediaminetetraacetic acid), 1.5 ml of pure NH3, and 50 g of H2O. The hydrothermal treatment was carried out at a steam pressure of 190°C and 12 bar for 48 hours while stirring. After filtering the reaction mixture, washing, and drying the filtrate, high-purity boehmite was obtained in an amount of 41.75 g.

[0033] The results regarding the reduction of impurities are shown in Table 1 below. In this case, the second row shows the initial amounts of the respective impurities. Unless otherwise defined, the numerical values shown in Table 1 are indicated in mass%. To enable comparison of the purity of the raw material before treatment (e.g., smelting-grade alumina or aluminum hydrate) with the aluminum monohydrate product, all purity values are reported on an Al2O3 basis.

Table 1

[0034] It can be seen that a significant reduction in the impurity levels was achieved for the impurities Na2O, Fe2O3, MgO, CaO, and Li2O.

[0035] Examples 2 - 10 Examples 2 - 10 were prepared in the same manner as Example 1, except that the amounts of EDTA and NH3 were adjusted as shown in Table 1 above. It can be seen that a significant reduction in the impurity levels was achieved for the impurities Na2O, Fe2O3, MgO, CaO, Li2O, and ZnO.

[0036] From Table 1 above, it can also be seen that even when the ammonia content was increased from 6 ml to 10 ml (see Examples 9 and 10), no further improvement, i.e., no further reduction in impurities, was achieved. Instead, a slight increase in some impurities (Na2O, CaO, Li2O) was observed.

[0037] In Figure 1, the test results regarding the impurity levels of Examples 1, 2, 3 and 5 are compared, and an overview of the influence of the amount of EDTA is shown. Example 1 has the least amount of EDTA (0.5 g of EDTA per 50 g of alumina raw material - Al(OH)3: alumina raw material: EDTA mass ratio = 100:1). Example 2 contains 1 g of EDTA per 50 g of alumina raw material, which corresponds to an alumina raw material: EDTA mass ratio of 50:1. Example 3 contains 1.2 g of EDTA per 50 g of alumina raw material, which corresponds to an alumina raw material: EDTA mass ratio of 42.7:1. Example 5 contains 1.5 g of EDTA per 50 g of alumina raw material, which corresponds to an alumina raw material: EDTA mass ratio of 33.3:1. It can be seen that the best test results can be obtained when the mass ratio of the alumina raw material to the nitrogen-containing complexing agent is in the range of 50:1 to 20:1. When the amount of EDTA increases, the cost increases significantly, but the impurity level does not decrease significantly further.

[0038] In Figure 2, the test results regarding the impurity levels of Examples 4 to 10 are compared, and an overview of the influence of the amount of NH3 is shown. In Example 4, the ammonia content is the least (2 ml of ammonia corresponding to 1.5 g of ammonia: alumina raw material: ammonia mass ratio = 32.4:1). In Example 5, 3 ml of ammonia corresponding to 2.3 g of ammonia is contained: alumina raw material: ammonia mass ratio = 21.6:1. In Example 6, 3.5 ml of ammonia corresponding to 2.7 g of ammonia is contained: alumina raw material: ammonia mass ratio = 18.5:1. In Example 7, 4 ml of ammonia corresponding to 3.1 g of ammonia is contained: alumina raw material: ammonia mass ratio = 16.2:1. In Example 8, 4.5 ml of ammonia corresponding to 3.5 g of ammonia is contained: alumina raw material: ammonia mass ratio = 14.4:1. In Example 9, 6 ml of ammonia corresponding to 4.6 g of ammonia is contained: alumina raw material: ammonia mass ratio = 10.8:1. In Example 10, 6 ml of ammonia corresponding to 7.7 g of ammonia is contained: alumina raw material: ammonia mass ratio = 6.5:1.

[0039] If the mass ratio of the alumina raw material to ammonia is in the range of 100:1 to 10:1, it can be seen that an excellent impurity reduction effect can be obtained. Even if the amount of ammonia increases, it does not further significantly affect the impurity level.

[0040] Example 11 Using aluminum trihydrate as the starting material having a D50 of 8 μm, Example 11 was prepared in the same manner as Example 1. Aluminum trihydrate was mixed with 1.1 g of EDTA and 4.5 ml of NH3. The hydrothermal treatment was carried out for 24 hours at 190 °C and a steam pressure of 12 bar while stirring. High-purity boehmite was obtained, which was filtered, washed, and dried.

[0041] The results regarding the reduction of impurities are shown in Table 2 below. In this case, the second row shows the initial amounts of the respective impurities, and the last row shows the results of the impurity measurements after the hydrothermal treatment. Unless otherwise defined, the numerical values shown in Table 2 are expressed in mass%. To enable comparison of the purity of the raw material before treatment (e.g., smelting-grade alumina or aluminum hydrate) with the aluminum monohydrate product, all purity values are reported on an Al2O3 basis. [Table 2]

[0042] Regarding the impurities Na2O, Fe2O3, MgO, CaO, Li2O, ZnO, CuO, Ga2O, and BeO, it can be seen that a significant reduction in the impurity level was achieved.

[0043] Example 12 Using smelting-grade alumina (SGA) raw material having a particle size of 84 μm and a particle size of 5 μm as the starting material, Example 12 was prepared in the same manner as Example 1. The smelting-grade alumina was mixed with 1.5 g of EDTA and 6 ml of NH3. The hydrothermal treatment was carried out for 60 hours at 190 °C and a steam pressure of 12 bar while stirring. High-purity boehmite was obtained, which was filtered, washed, and dried.

[0044] The results regarding the reduction of impurities are shown in Table 3 below. Unless otherwise defined, the numerical values shown in Table 3 are expressed in mass %. To enable comparison of the purity of the raw material before treatment (e.g., smelting grade alumina or aluminum hydrate) with the aluminum monohydrate product, all purity values are reported on an Al2O3 basis. [Table 3]

[0045] It can be seen that a significant reduction in impurity levels was achieved for the impurities Na2O, Fe2O3, MgO, and CaO.

[0046] Example 13 Using a smelting grade alumina (SGA) raw material, Example 13 was prepared in the same manner as Example 12. Sample A was not leached. Sample B was leached with an HCl / water mixture. EDTA was added in an amount of 1.5 g, and NH3 was added in an amount of 6 ml to the unleached smelting grade alumina sample A and the leached smelting grade alumina sample B. The hydrothermal treatment was carried out at 190 °C and a steam pressure of 12 bar for 48 hours with stirring. The resulting boehmite was filtered, washed, and dried.

[0047] The results regarding the reduction of impurities are shown in Table 4 below. Unless otherwise defined, the numerical values shown in Table 4 are expressed in mass %. To enable comparison of the purity of the raw material before treatment (e.g., smelting grade alumina or aluminum hydrate) with the aluminum monohydrate product, all purity values are reported on an Al2O3 basis.

[0048] Example 14 Using the same smelting grade alumina (SGA) raw material, Example 14 was prepared in the same manner as Example 13. Sample C was not leached. Sample D was leached with an HCl / water mixture. Although EDTA was not added, NH3 was added to the non-leached smelting grade alumina sample C in an amount of 6 ml to leach the smelting grade alumina sample D. The hydrothermal treatment was carried out at 190 °C and a steam pressure of 12 bar for 48 hours with stirring. The resulting boehmite was filtered, washed, and dried.

[0049] The results regarding impurity reduction are shown in Table 4 below. Unless otherwise defined, the values shown in Table 4 are expressed in mass %. To enable comparison of the purity of the raw material before treatment (e.g., smelting grade alumina or aluminum hydrate) with the aluminum monohydrate product, all purity values are reported on an Al2O3 basis.

[0050] Example 15 Using the same smelting grade alumina (SGA) raw material, Example 15 was prepared in the same manner as Example 13. Sample E was not leached. Sample F was leached with an HCl / water mixture. EDTA was added in an amount of 1.5 g, but NH3 was not added. The hydrothermal treatment was carried out at 190 °C and a steam pressure of 12 bar for 48 hours with stirring. The resulting boehmite was filtered, washed, and dried.

[0051] The results regarding impurity reduction are shown in Table 4 below. Unless otherwise defined, the values shown in Table 4 are expressed in mass %. To enable comparison of the purity of the raw material before treatment (e.g., smelting grade alumina or aluminum hydrate) with the aluminum monohydrate product, all purity values are reported on an Al2O3 basis.

Table 4

[0052] As can be seen from the test results in Table 4, by adding EDTA and NH3 (Samples A and B), the best test results can be obtained from the perspective of impurity reduction. However, even with the addition of only EDTA (Samples E and F), the reduction of the impurities shown in Table 4 is good. The impurity level of Fe2O3 in Sample F has only increased slightly. However, the impurity concentrations of Na2O and CaO have been significantly reduced. By adding NH3 (see Samples C and D), Na2O is reduced, but the impurity levels of Fe2O3 and CaO are high.

[0053] Example 16 Example 16 shows the test results of the soda impurity levels of Samples G to V of a smelting grade alumina raw material to which 1.5 g of EDTA and 6 ml of NH3 were added. The hydrothermal treatment was carried out at a steam pressure of 190 °C and 12 bar without stirring. The obtained boehmite was filtered, washed, and dried. Samples G to V were thus treated for different times at the same temperature. In addition, the D50 of the smelting grade alumina raw material Samples G to V changed as outlined in Table 5 below.

Table 5

[0054] From Table 5, it can be seen that the addition of EDTA and NH3 had an advantageous effect on reducing the soda impurity level. It can also be seen that the higher the D50, the lower the effect of reducing the soda impurity level. A special improvement in soda reduction may be obtained for D50 in the range of 2 to 15 μm. Also, from the test results in Table 5, it can be seen that excellent reduction effects of the soda impurity level were obtained by hydrothermal treatment for 18 to 48 hours, and no further advantageous effect was obtained at a treatment time of 60 hours.

[0055] Example 17 As the starting material, a particle size of 4.5 μm (D 50Using a smelting grade alumina (SGA) raw material having ), Example 17 was prepared in the same manner as Example 1. 50 g of smelting grade alumina was mixed with 100 ml of water and different amounts of EDTA and NH3 as shown in Table 6. The hydrothermal treatment was carried out at 190 °C and a steam pressure of 12 bar for 48 hours with stirring. High-purity boehmite was obtained, which was filtered, washed, and dried.

[0056] The results regarding impurity reduction are shown in Table 6 below. Unless otherwise defined, the numerical values shown in Table 6 are expressed in mass %. To enable comparison of the purity of the raw material before treatment (e.g., smelting grade alumina or aluminum hydrate) with the aluminum monohydrate product, all purity values are reported on an Al2O3 basis.

Table 6

[0057] Example 18 Using an aluminum hydrate raw material having a particle size of 8 μm as the starting material, Example 18 was prepared in the same manner as Example 1. 50 g of aluminum hydrate was mixed with 100 ml of water and different amounts of EDTA and NH3 as shown in Table 7. The hydrothermal treatment was carried out at 190 °C and a steam pressure of 12 bar for 24 hours with stirring. High-purity boehmite was obtained, which was filtered, washed, and dried.

[0058] The results regarding impurity reduction are shown in Table 7 below. Unless otherwise defined, the numerical values shown in Table 7 are expressed in mass %. To enable comparison of the purity of the raw material before treatment (e.g., smelting grade alumina or aluminum hydrate) with the aluminum monohydrate product, all purity values are reported on an Al2O3 basis.

Table 7

[0059] The importance of the pH of the raw material mixture (i.e., measured before hydrothermal treatment) in the purification of the material can be seen from Tables 6 and 7 for SGA and aluminum hydrate, respectively. Mixtures with a low pH of 8 or less are purified to some extent, but a pH of at least 8, preferably at least 9, is required to effectively reduce significant impurities, especially Na2O, Fe2O3, and CaO. Furthermore, Table 6 shows the effect of the presence of different amounts of complexing agent. Optimal reduction to the preferred levels of all impurities (i.e., <0.010% Na2O, <0.005% Fe2O3, and <0.0015% CaO in this example) is only observed when adding 1.5 g of EDTA within the range of 50:1 to 20:1, optionally 40:1 to 20:1, for the mass ratio of the (alumina) raw material to EDTA.

[0060] The dependence of the impurity concentration as a function of pH is graphically shown in Figure 3 for Samples 18 / 1 to 18 / 4 of Table 7 above. For the Fe2O3 and CaO impurities, different scales are used because the overall concentrations are low (right side of the figure). The highly beneficial effect of the pH of the raw material mixture containing EDTA being above 8 can be easily seen from Figure 3.

[0061] The effect of different amounts of EDTA was further tested in additional experiments / samples for the effect on the Na2O impurity level, as shown in Table 8 below. Thus, the amount of EDTA was only 0.5 g in Sample 18 / 5 and only 1 g in Sample 18 / 6. As a result, the reduction of the Na2O impurity level was inferior compared to Sample 18 / 4 with 1.5 g of EDTA, as can be seen from the comparison.

Table 8

Claims

1. The following steps: - Mixing an alumina raw material selected from smelting grade alumina, pretreated smelting grade alumina, chemical grade alumina, pretreated chemical grade alumina, transition alumina, and aluminum trihydrate with ethylenediaminetetraacetic acid to obtain a raw material mixture, and - Subjecting the raw material mixture to hydrothermal treatment A method for producing aluminum monohydrate, wherein the pH of the raw material mixture is at least 8.

2. The method according to claim 1, wherein the pH of the raw material mixture is in the range of at least 9, more preferably at least 10, particularly in the range of 10 to 12, and most preferably in the range of 10 to 11.

3. The method according to claim 1 or 2, wherein the raw material mixture contains ammonia, and the mass ratio of the alumina raw material to ammonia is 100:1 to 10:1, particularly 50:1 to 10:

1.

4. The method according to any one of claims 1 to 3, wherein the raw material mixture contains ammonia, and the content of ammonia relative to the content of the alumina raw material is in the range of 1.25% by mass to 3.70% by mass.

5. The method according to any one of claims 1 to 4, wherein the alumina raw material is selected from leached smelting grade alumina and leached chemical grade alumina.

6. The method according to any one of claims 1 to 5, wherein the mass ratio of the alumina raw material to the ethylenediaminetetraacetic acid is 50:1 to 20:1, optionally 40:1 to 20:

1.

7. The method according to any one of claims 1 to 6, wherein the raw material mixture further contains ammonium hydroxide.

8. The method according to any one of claims 1 to 7, wherein the raw material mixture further contains hydrogen peroxide, sodium hydroxide or potassium hydroxide.

9. The mass ratio of the solid compound to the liquid compound of the raw material mixture is in the range of 1:1 to 1:2, and / or the temperature during the hydrothermal treatment is in the range of 150°C to 260°C, and / or the hydrothermal pressure during the hydrothermal treatment is in the range of 1 bar to 300 bar, particularly 1 bar to 50 bar, and / or the hydrothermal treatment is carried out for 1 to 100 hours, and / or the raw material mixture is stirred during the hydrothermal treatment. The method according to any one of claims 1 to 8.

10. A method for producing α-alumina, comprising preparing aluminum monohydrate according to the method according to any one of claims 1 to 9, and then calcining the aluminum monohydrate to obtain α-alumina.

11. The method according to any one of claims 1 to 10, wherein the purity level of the aluminum monohydrate is 99.9% by mass to 99.99% by mass.

12. Use of ethylenediaminetetraacetic acid in combination with ammonia in the production of aluminum monohydrate, said production comprising the following steps: - Mixing an alumina raw material selected from smelting grade alumina, pretreated smelting grade alumina, chemical grade alumina, pretreated chemical grade alumina, transition alumina, and aluminum trihydrate with ethylenediaminetetraacetic acid to obtain a raw material mixture having a pH of at least 8, and - Subjecting the raw material mixture to hydrothermal treatment including said use.

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