Process for producing high purity alumina

Abstract

Provided herein is a process for producing high purity alumina, comprising: (a) sublimating anhydrous aluminum chloride at a predetermined temperature to recover pure aluminum chloride; (b) dissolving the aluminum chloride from step (a) in water to obtain an aluminum chloride solution; (c) introducing HCl gas into the aluminum chloride solution of step (b) to crystallize aluminum chloride hexahydrate; and (d) calcining the aluminum chloride hexahydrate of step (c) to obtain high purity alumina.

Description

[Technical field]

[0001] The present invention relates to a process for producing high purity alumina. [Background technology]

[0002] Different grades of alumina with different levels of purity and physical properties are used in various applications, e.g., for high-grade applications such as the fabrication of ceramics, fillers and sapphire single crystals, semiconductors, lithium-ion battery applications, etc. Properties such as chemical inertness, electrical insulation and its stability to high operating temperatures are used in various applications. Purity requirements are strict not only in terms of total impurity content being limited to less than 100 ppm, but also in terms of achieving individual limits for each key element, e.g., Na less than 30 ppm, Si less than 20 ppm, Fe about 3-5 ppm. All other impurities need to be controlled to less than 3 ppm. The Bayer process is the primary process by which alumina is extracted in the form of aluminum hydroxide. In the Bayer process, bauxite is dissolved in a caustic soda (NaOH) solution and heated to precipitate aluminum hydroxide. Most of the alumina produced worldwide is used for smelting grade applications, which is 99.7% pure. Aluminum hydroxide produced by the Bayer process also contains elemental impurities from the bauxite.

[0003] The two main processes for producing high purity alumina that are commercially practiced are (i) the alkoxide route from aluminum metal and (ii) the acid route from alumina clay. The alkoxide route accounts for 80% of the total HPA requirement and uses aluminum metal as the starting material, the alkoxide of which is processed in multi-stage distillation to achieve the desired purity level. The advantages of these processes are a simpler process, easy supply of different grades of alumina, as well as the ability to control particle size through controlled hydrolysis (Shinji F. et. al, 2007). However, the main challenges include the higher cost of raw materials, very slow reaction rate for forming alkoxides from metals, which are further used to make finer metal powders. Although the aluminum ingots or metal powders used are of high purity, the presence of significant impurities such as iron (500-1500 ppm) and silicon (400-800 ppm) is still inevitable. Therefore, there is a range of distillation processes to purify aluminium alkoxides from other complex alkoxides formed especially with silica (Bains MS, 1962 & Jeffrey H et. al,1986).

[0004] Another process for high purity alumina production is the acid route from alumina clay, which uses special alumina clay from Australian deposits with very low iron (0.7%) and alkali content (Na2O<0.1%). Processes for preparing high purity alumina from special alumina clay by the acid route are described in EP 3530623, AU Innovation Patent 2018101228 and AU Patent Application Publication No. 2019204216. The process uses HCl for digestion of the clay after roasting at 400°C (which destroys the structure of the aluminum silicate). Subsequent purification steps involve the removal of iron and other impurities with selective precipitation of AlCl3.6H2O by increasing the concentration of HCl using anhydrous HCl (Lewis JC, 1951). The number of purification stages depends on the concentration of impurities present in the clay. Yves Noel, 2012 discloses a similar technology for the production of high purity alumina with higher chemical and energy consumption. US Patent Application Publication No. 20180155206 describes a method for producing high purity nano alumina powder, which involves dissolving common aluminum hydroxide in sodium hydroxide solution to yield a sodium aluminate solution, and using a microfilter to remove most insoluble impurities other than sodium to yield a pure sodium aluminate solution. Seeds are added to it to precipitate nano aluminum hydroxide as a nano slurry under optimal precipitation conditions. The nano aluminum hydroxide slurry is filtered, dried, disintegrated, and then calcined at a low temperature below 900°C, thereby achieving mass production of high purity nano alumina.

[0005] US Pat. No. 8,124,048 describes a method for producing high purity alumina in which the Si, Fe, Ca and Na contents are simultaneously removed. WO2018040998 discloses a method for preparing ultra-pure spherical alumina powder, comprising pneumatically pumping and spraying gas-phase aluminum chloride into a heating zone at 2500°C or higher, and then pyrolyzing the ultra-pure aluminum chloride to produce alumina, thereby obtaining ultra-pure spherical alumina powder. The main challenges involved are dealing with the variability in impurities in the raw materials as well as the large amounts of acidic waste generated during the multi-stage purification steps. Therefore, new processes for HPA production currently being developed aim to achieve the target purity with a minimum number of processing / purification steps, which is achieved through a unique combination of purification steps to control different metal impurities based on boiling point, selectivity in crystallization, etc. Summary of the Invention

[0006] The present invention has been devised to solve the above-mentioned problems. An object of the present invention is to provide a method for preparing high purity alumina. Another specific object of the present invention is to provide a process for preparing high purity alumina using anhydrous aluminum chloride. In one aspect, the present invention provides a method for preparing high purity alumina starting from the sublimation of anhydrous aluminum chloride. The sublimation process was carried out at a specific temperature to recover pure aluminum chloride as either a solid or a gas. The recovered aluminum chloride in either gaseous or solid form is further dissolved in water to obtain an aluminum chloride solution. Subsequently, anhydrous HCl gas is introduced into the aluminum chloride solution, which results in the aluminum chloride crystallizing and precipitating as aluminum chloride hexahydrate. The crystallized aluminum chloride hexahydrate is subjected to calcination to obtain high purity alumina of greater than 99.99% (4N). The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the accompanying drawings. For the purpose of assisting in explaining the invention, there are shown in the drawings embodiments which are presently considered to be preferred and exemplary. It will be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown in the drawings. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a process diagram for producing high purity alumina from anhydrous aluminum chloride according to an embodiment of the present invention. [Diagram 2] FIG. 1 is a process diagram for producing high purity aluminum hydrate from anhydrous aluminum chloride according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] In describing and claiming the present invention, the following terminology will be used in accordance with the definitions set out below. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in practicing or testing the present invention, the preferred methods and materials are described herein. As used herein, each of the following terms has the meaning associated with it in this section. The specific preferred values ​​listed below for individual process parameters, substituents, and ranges are for illustrative purposes only and do not exclude other defined values ​​or other values ​​within the defined preferred ranges.

[0009] As used herein, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. The terms "preferred" and "preferably" refer to embodiments of the invention that may offer certain advantages, under particular circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention. As used herein, the terms "comprising," "including," "having," "containing," "involving," and the like, should be understood to be open-ended, i.e., meaning including but not limited to. As used herein, "sublimation" refers to the direct conversion of a substance between solid and gas phases upon the application of heat. As used herein, "calcination" refers to heating a solid to high temperatures for the purposes of removing volatile materials (chlorides, hydroxides, carbonates, etc.) and effecting targeted phase transformations.

[0010] As used herein, the term "4N grade" refers to a sample that is 99.99% pure and has 100 ppm total impurities. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described. All publications and other references mentioned herein are incorporated by reference in their entirety. Numerical ranges are inclusive of the numbers defining the range. Conventional methods for preparing high purity alumina generally consume a lot of energy and produce waste by-products. From the foregoing, the inventors of the present invention have extensively studied a process for producing high purity alumina in which elemental metals as impurities are substantially removed. As a result, the inventors have proposed a process for preparing high purity alumina with a unique combination of purification steps selected based on the impurities contained in the feedstock. In one embodiment, the anhydrous aluminum chloride / chloride feedstock is produced from any alumina feedstock, such as bauxite, aluminum hydrate, aluminum dross or red mud, by carbo-chlorination / chlorination.

[0011] That is, the present invention provides a process for producing high purity alumina, comprising: (a) sublimating anhydrous aluminum chloride at a predetermined temperature to recover pure aluminum chloride; (b) dissolving the aluminum chloride from step (a) in water to obtain an aluminum chloride solution; (c) introducing HCl gas into the aluminum chloride solution of step (b) to obtain crystallized aluminum chloride hexahydrate; (d) calcining the crystallized aluminum chloride hexahydrate of step (c) to obtain high purity alumina; The present invention provides a process including: The heating in step (a) is carried out at a temperature in the range of 170-300° C. to promote sublimation. Sublimation of the anhydrous aluminum chloride results in pure aluminum chloride.

[0012] During sublimation, impurities are selectively separated. Low boiling impurities such as chlorides of silicon, arsenic and titanium are selectively removed by removing a vapor fraction below the boiling point of aluminum chloride, below 170°C, preferably between 50°C and 130°C. The aluminum chloride is then heated to a temperature in the range of 170°C to 300°C. This removes some other metal chlorides with higher boiling points than aluminum chloride, such as iron, zinc, nickel, lead and sodium. In another embodiment of the invention, discarding at least a portion, preferably 5% by weight, of the sublimated anhydrous aluminum chloride at the beginning and end of the sublimation process improves the production of high purity materials of aluminum chloride (AlCl3), as well as aluminum hydroxide (Al(OH)3) and aluminum oxide (Al2O3).

[0013] The impurity-free aluminum chloride can be collected as a gas and subsequently cooled to a temperature below 170°C to obtain solid aluminum chloride as a powder or granules, or it may be dissolved in gaseous form without cooling in water and further processed. The residue resulting from the sublimation process contains graphitic impurities with higher boiling points as well as inorganic impurities such as iron, zinc, lead, sodium and other chlorides. In certain embodiments, sublimation is carried out in the presence or absence of a carrier gas, which is preferably an inert gas such as nitrogen or argon. In another embodiment, the pure aluminum chloride obtained from sublimation can be collected as a solid product or in gaseous form. The gaseous or solid pure aluminum chloride obtained from sublimation is dissolved in water to obtain an aluminum chloride solution, the temperature of the solution being kept below 50°C to avoid loss of aluminum chloride.

[0014] In one embodiment, the gas produced during sublimation may be passed directly through a water spray column / absorption unit to produce an aluminum chloride solution having a concentration in the range of 100-400 gm of aluminum chloride per liter, or in other words a 10-40% aluminum chloride solution. In another embodiment, the solids produced during sublimation may be dissolved directly in water to produce an aluminum chloride solution having a concentration in the range of 100-400 gm of aluminum chloride per liter, or in other words a solution of 10-40% aluminum chloride. The aluminum chloride solutions are subjected to further purification based on their solubility in highly supersaturated HCl acidic solution (+36%). The aluminum chloride solutions are made supersaturated by condensing the HCl using anhydrous HCl gas while maintaining a temperature in the range of 25-65° C. until the saturation of HCl reaches 30-40%. In some embodiments, anhydrous Cl2, AlCl3 or any chlorine-containing gas can be used. Once saturation with HCl in the range of 30-40% is reached, aluminum chloride hexahydrate (AlCl3.6H2O) crystallizes. Impurities such as arsenic, silicon, zinc and iron, which have high solubility in acidic solutions, remain in solution and are separated in the process.

[0015] In another embodiment, at least a portion of the aluminum chloride hexahydrate is further dissolved in water and reacted with ammonia or ammonium hydroxide to precipitate ultra-high purity aluminum hydroxide for special applications, as shown in Figure 2. The aluminum hydroxide is further calcined to obtain high purity alumina. In one embodiment, impurities can be minimized by controlling the flow rate of HCl gas, the initial concentration of the aluminum chloride solution, and the temperature to control the crystallization rate of aluminum chloride hexahydrate (AlCl3.6H2O). In another embodiment, aluminum chloride hexahydrate seed crystals may be added for the crystallization step. In some embodiments, the sublimation and crystallization steps can be repeated two or more times to produce high purity alumina material, such as 4N, 5N, or 6N (+99.99). In some embodiments, the crystallization step is repeated multiple times to prepare different grades of high purity alumina, such as 4N, 5N, or 6N. The aluminum chloride hexahydrate precipitate is calcined at a temperature in the range of 1000-1100°C to obtain high purity alumina (HPA). In another embodiment of the invention, the sublimation and / or crystallization steps can be combined with any other purification methods known in the art, such as solvent extraction / organic precipitation, to prepare high purity aluminum hydrate or alumina with a purity of >99.99%.

[0016] The final HPA product obtained by this process has a purity of 99.99%, ensuring the stringent quality standards prescribed for the 4N grade. In one embodiment, calcination is carried out at temperature stages, particularly at 90°C, 360°C and 1000-1100°C, to recover the HCl and chlorine released during the process and subsequently recycle them. The process of the present application successfully minimizes the impurities present in the alumina to levels of less than 20 ppm Na, less than 10 ppm Si, less than 1 ppm Fe, and less than 3 ppm all other impurities. The resulting material is suitable for high purity alumina applications such as sapphire single crystal fabrication, coating applications in lithium ion batteries, LEDs, etc. Conventional methods for producing high purity alumina require multiple purification steps to achieve the desired purity requirements. However, the process claimed in this application takes into account the significant impurities present in the feedstock and devise a method to separate the impurities. Thus, high purity alumina or aluminum hydrate is produced in a minimum number of steps. Moreover, the process of the present application generates low side streams, thereby ensuring the sustainability of the process with maximum recycling.Furthermore, the process of the present application is not very chemical / energy intensive. EXAMPLES

[0017] The following specific examples are illustrative and illustrative of the present invention, but should not be construed as limiting the scope of the invention. Anhydrous aluminum chloride produced by chlorination of aluminum metal was used as the feedstock. The aluminum metal ingot contained the following impurities: Na: 10-100ppm, Si: 400-1000ppm, Fe: 580-1500ppm, V: 50-150ppm, Ti: 50ppm. The process of chlorination of molten aluminum metal was carried out at a very high temperature of 600-700°C, so metal impurities that form chlorides are reported along with the aluminum chloride. Example 1 Anhydrous aluminum chloride was heated at temperatures ranging from 170°C to 300°C to effect sublimation. The sublimated aluminum chloride was dissolved in water to obtain an aluminum chloride solution. HCl gas was purged into the aluminum chloride solution at temperatures ranging from 25°C to 65°C to allow crystallization of aluminum chloride hexahydrate. The precipitated crystallized aluminum chloride hexahydrate was calcined at a temperature of 1100°C to obtain high purity alumina.

[0018] Example 2 In the process of Example 1, sublimation was repeated twice and the crystallization of aluminum chloride hexahydrate in acid medium was carried out in one step, which resulted in the increase of the purity of alumina by controlling the important impurities such as Na, Si, K, and Zn. Example 3 In the process of Example 1, sublimation was carried out in a single step in conjunction with a two-step crystallization of aluminum chloride hexahydrate, resulting in higher purity alumina by controlling the major metal impurities to less than 0.5 ppm. The purity of the alumina produced by these processes of the present application is shown in Table 1.

[0019] [Table 1]

[0020] Example 4 In the process of Example 1, instead of calcining the crystallized aluminum chloride hexahydrate to obtain alumina, the crystallized aluminum chloride hexahydrate is used to prepare aluminum hydroxide. The crystallized aluminum chloride hexahydrate is redissolved in water and ammonium hydroxide is added until the pH is 6. Thus, the aluminum hydroxide precipitates, which is filtered and dried at 105° C. to obtain a water-free product. Furthermore, the precipitated aluminum hydroxide was calcined at a temperature of 1100°C to obtain high-purity alumina. The purity of the obtained aluminum hydroxide is shown in Table 2 and is 99.99%.

[0021] [Table 2]

[0022] The foregoing description of the invention is illustrative and explanatory thereof. Various modifications will become apparent to those skilled in the art upon consideration of the present disclosure. It is intended to embrace all such modifications that fall within the scope and spirit of the appended claims.

Claims

1. 1. A process for producing high purity alumina, comprising: (a) sublimating anhydrous aluminum chloride at a predetermined temperature to recover pure aluminum chloride; (b) dissolving the aluminum chloride from step (a) in water to obtain an aluminum chloride solution; (c) introducing HCl gas into the aluminum chloride solution of step (b) to crystallize aluminum chloride hexahydrate; (d) calcining the aluminum chloride hexahydrate of step (c) to obtain high purity alumina; The process includes:

2. 2. The process of claim 1, wherein the sublimation of step (a) is a two-stage process, in which the anhydrous aluminum chloride is subjected to heating at different temperatures.

3. 3. The process of claim 2, wherein the two-stage sublimation is carried out at temperatures of 50°C to 170°C and 170°C to 300°C.

4. 2. The process of claim 1, wherein the sublimation is carried out until 95-97% of the aluminum chloride is recovered from the anhydrous aluminum chloride.

5. 10. The process of claim 1, wherein the sublimation of step (a) may be repeated at least four times.

6. The process of claim 1 , wherein the sublimation may be carried out in the presence of a carrier gas.

7. 2. The process of claim 1, wherein the aluminum chloride obtained in step (a) is either a gas or a solid.

8. 2. The process of claim 1, wherein the dissolving of aluminum chloride in step (b) is carried out at a temperature of 30 to 50°C.

9. 2. The process of claim 1, wherein step (b) is repeated at least four times.

10. 2. The process of claim 1, wherein the introduction of HCl gas in step (c) is carried out at a temperature in the range of 25 to 65° C.

11. 2. The process of claim 1, wherein aluminum chloride hexahydrate seed crystals may be added during step (c).

12. The process of claim 1, wherein the calcination is carried out at a temperature in the range of 1000°C to 1200°C.

13. 2. The process of claim 1, wherein the calcination is carried out until the loss on ignition is less than 0.5%.

14. 2. The process of claim 1, wherein the crystallized aluminum chloride hexahydrate of step (c) may be dissolved in water and reacted with ammonia or ammonium hydroxide to obtain high purity aluminum hydroxide.

15. 15. The process of claim 14, comprising calcining the high purity aluminum hydroxide to obtain high purity alumina.

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