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 Art

[0002] Aluminas of different grades with different levels of purity and physical properties are used in various applications, such as advanced applications like the production of ceramics, fillers and sapphire single crystals, semiconductors, lithium ion battery applications, etc. Properties such as chemical inertness, electrical insulation and its stability at high service temperatures are used in various applications. The purity requirements are not only restricted to the total content of impurities being less than 100 ppm, but also strict in terms of achieving individual limits for each important element, for example less than 30 ppm of Na, less than 20 ppm of Si, and about 3 - 5 ppm of Fe. All other impurities need to be controlled to less than 3 ppm. The Bayer process is the main 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, heated, and aluminum hydroxide is precipitated. Most of the alumina produced worldwide is used for smelting grade applications, which is 99.7% pure. The aluminum hydroxide produced by the Bayer process also contains elemental impurities from bauxite.

[0003] The two main processes for commercially produced high-purity alumina are (i) the alkoxide method from aluminum metal and (ii) the acid pathway from alumina clay. The alkoxide method accounts for 80% of the total HPA requirements, using aluminum metal as the starting material and processing its alkoxide through multi-stage distillation to achieve the desired purity level. The advantages of these processes are their simplicity, the ease of supplying different grades of alumina, and the ability to control particle size through hydrolysis control (Shinji F. et. al, 2007). However, the main challenges include the higher cost of the raw materials and the very slow reaction rate for forming alkoxides from the metal, which are then used to produce even 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 unavoidable. Therefore, there is a series of distillation processes for purifying aluminum alkoxides, particularly from other complex alkoxides formed with silica (Bains MS, 1962 & Jeffrey H et. al, 1986).

[0004] Another process for producing high-purity alumina is an acid pathway from alumina clay, which uses special alumina clay from Australian sediments with very low iron (0.7%) and alkali content (Na2O < 0.1%). The process for preparing high-purity alumina from special alumina clay by the acid pathway is described in European Patent Application Publication No. 3530623, Australian Innovation Patent No. 2018101228, and Australian Patent Application Publication No. 2019204216. The process involves roasting the clay at 400°C followed by the use of HCl for digestion (which destroys the structure of aluminum silicate). Subsequent purification steps involve increasing the concentration of HCl using anhydrous HCl, along with the selective precipitation of AlCl3.6H2O, to remove iron and other impurities (Lewis JC, 1951). The number of purification steps depends on the concentration of impurities present in the clay. Yves Noel, 2012, discloses a similar technique for the production of high-purity alumina with greater chemical and energy consumption. U.S. Patent Application Publication No. 20180155206 describes a method for producing high-purity nanoalumina powder by dissolving common aluminum hydroxide in a sodium hydroxide solution to obtain a sodium aluminate solution, and then using a microfilter to remove most insoluble impurities other than sodium to obtain a pure sodium aluminate solution. Seeds are added thereto, and the nanoaluminum hydroxide is precipitated as a nanoslurry under optimal precipitation conditions. The nanoaluminum hydroxide slurry is filtered, dried, and broken down, and then calcined at a low temperature of less than 900°C, thereby achieving mass production of high-purity nanoalumina.

[0005] U.S. Patent No. 8124048 describes a method for producing high-purity alumina in which Si, Fe, Ca, and Na content are simultaneously removed. International Publication No. 2018040998 discloses a method for preparing ultrapure spherical alumina powder, which involves pneumatically conveying gas-phase aluminum chloride, spraying it into a heating zone at 2500°C or higher, and then thermally decomposing the ultrapure aluminum chloride to produce alumina, thereby obtaining ultrapure spherical alumina powder. The main challenges involved are the variability in impurities in the raw materials and the handling of the large amount of acidic waste generated during the multi-step refining process. Therefore, the new processes currently under development for HPA production aim to achieve the target purity with the minimum number of processing / purification steps. This is achieved through a unique combination of purification steps to control different metal impurities based on factors such as boiling point and crystallization selectivity. [Overview of the Initiative]

[0006] This invention was devised to solve the aforementioned problems. The object of the present invention is to provide a method for preparing high-purity alumina. Another special object of the present invention is to provide a method for preparing high-purity alumina using anhydrous aluminum chloride. In one embodiment, the present invention provides a method for preparing high-purity alumina, beginning with the sublimation of anhydrous aluminum chloride. The sublimation process was carried out at a specific temperature to recover the pure aluminum chloride as either a solid or a gas. The aluminum chloride recovered 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, resulting in the crystallization of the aluminum chloride and precipitation as aluminum chloride hexahydrate. The crystallized aluminum chloride hexahydrate is calcined to obtain high-purity alumina exceeding 99.99% (4N). The above 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 the description of the invention, the drawings show embodiments that are currently considered preferred and exemplary. However, it will be understood that the invention is not limited to the exact arrangements and means shown in the drawings. [Brief explanation of the drawing]

[0007] [Figure 1] This is a process diagram for producing high-purity alumina from anhydrous aluminum chloride according to an embodiment of the present invention. [Figure 2] This is a process diagram for producing high-purity aluminum hydrate from anhydrous aluminum chloride according to an embodiment of the present invention. [Modes for carrying out the invention]

[0008] In describing the present invention and in claiming it, the following technical terms are used according to their definitions set forth below. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as those generally understood by those skilled in the art to which the present invention belongs. Any methods and materials similar or equivalent to those described herein may be used when carrying out or testing the present invention, but preferred methods and materials are described herein. In this specification, each of the following terms has the meaning relating to the term in this section. The specific preferred values ​​listed below for individual process parameters, substituents, and ranges are for illustrative purposes only and do not preclude other defined values ​​or other values ​​within the defined preferred range.

[0009] In this specification, the singular forms "a," "an," and "the" include multiple subjects unless otherwise clearly specified by the context. The terms “preferred” and “preferred” refer to embodiments of the present invention that may offer certain advantages under specific circumstances. However, other embodiments may also be preferred under the same or other circumstances. Furthermore, the enumeration of one or more preferred embodiments does not imply that other embodiments are unhelpful, nor is it intended to exclude other embodiments from the scope of the present invention. In this specification, terms such as "comprising," "including," "possessing," "containing," and "associated with" should be understood as non-restrictive; that is, they mean including but not limiting. In this specification, "sublimation" refers to the direct conversion of a substance from its solid phase to its gaseous phase when heated. In this specification, "calcination" refers to heating a solid to a high temperature for the purpose of removing volatile substances (such as chlorides, hydroxides, and carbonates) and inducing a target phase transition.

[0010] In this specification, the term "4N grade" refers to a sample with a purity of 99.99% and a total impurity level of 100 ppm. When carrying out or testing the present invention, any methods and materials similar or equivalent to those described herein may be used, but preferred methods and materials will be described. All publications and other references mentioned herein are incorporated in their entirety by reference. Numerical ranges include the numbers defining the range. Conventional methods for preparing high-purity alumina generally consume a lot of energy and produce waste by-products. Therefore, the inventors of the present invention have conducted extensive research on processes for producing high-purity alumina in which elemental metals as impurities are substantially removed. As a result, the inventors propose a process for preparing high-purity alumina that involves selecting a unique combination of purification steps based on the impurities present in the raw material. In one embodiment, the anhydrous aluminum chloride / chloride feedstock is produced by carbon-chlorination / chlorination from any alumina feedstock, such as bauxite, aluminum hydrate, aluminum dross, or red clay.

[0011] In other words, the present invention is a process for producing high-purity alumina, (a) A step of sublimating anhydrous aluminum chloride at a predetermined temperature to recover pure aluminum chloride, (b) The step of dissolving the aluminum chloride from step (a) in water to obtain an aluminum chloride solution, (c) The step of introducing HCl gas into the aluminum chloride solution of step (b) to obtain crystallized aluminum chloride hexahydrate, (d) A step of calcining the crystallized aluminum chloride hexahydrate from step (c) to obtain high-purity alumina, It provides a process that includes this. The heating in step (a) is carried out at a temperature in the range of 170-300°C to promote sublimation. As a result of the sublimation of anhydrous aluminum chloride, pure aluminum chloride is obtained.

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

[0013] Aluminum chloride free of impurities can be collected as a gas and subsequently cooled to a temperature below 170°C to obtain solid aluminum chloride in the form of powder or granules, or it may be dissolved in water in its gaseous form without cooling and further processed. The residue produced in the sublimation process contains graphite impurities with higher boiling points, as well as inorganic impurities such as iron, zinc, lead, sodium, and other chlorides. In some embodiments, sublimation is carried out in the presence or absence of a carrier gas. The carrier gas 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 pure aluminum chloride, obtained as gas or solid from sublimation, is dissolved in water to obtain an aluminum chloride solution. The temperature of the solution is maintained below 50°C to avoid loss of aluminum chloride.

[0014] In one embodiment, the gas produced during sublimation can be passed directly through a water spray column / absorption unit to produce an aluminum chloride solution having an aluminum chloride concentration in the range of 100 to 400 gm / liter, or in other words, a 10 to 40% aluminum chloride solution. In another embodiment, the solid formed during sublimation is directly dissolved in water to produce an aluminum chloride solution having a concentration of aluminum chloride in the range of 100 - 400 gm per liter, or equivalently a solution of 10 - 40% aluminum chloride. The aluminum chloride solution is further subjected to purification based on their solubility in a highly supersaturated HCl acidic solution (+36%). The aluminum chloride solution is supersaturated by concentrating 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 certain embodiments, anhydrous Cl2, AlCl3 or any chlorine-containing gas can be used. When saturation of HCl in the range of 30 - 40% is achieved, aluminum chloride hexahydrate (AlCl3·6H2O) crystallizes. Impurities such as arsenic, silicon, zinc and iron which have high solubility in the acidic solution remain in the solution and are thus separated in this process.

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

[0016] The final HPA product obtained by this process has a purity of 99.99%, ensuring strict quality standards defined for the 4N grade. In certain embodiments, calcination is carried out at temperature stages, particularly at 90 °C, 360 °C, and 1000 - 1100 °C, to recover HCl and chlorine released during the process and then recycle it. The process of this application has successfully minimized impurities present in alumina to levels of less than 20 ppm of Na, less than 10 ppm of Si, less than 1 ppm of Fe, and less than 3 ppm of all other impurities. The resulting material is suitable for high-purity alumina applications, such as single-crystal sapphire production, 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 important impurities present in the feedstock and devises a technique for separating the impurities. Thus, high-purity alumina or aluminum hydrate is produced in a minimum number of steps. Moreover, the process of this application generates a small amount of side stream, thereby ensuring the sustainability of the process through maximum recycling. Furthermore, the process of this application does not consume a large amount of chemicals / energy.

Examples

[0017] The following specific examples are illustrative and explanatory of the present invention but should not be construed as limiting the scope of the present invention. Anhydrous aluminum chloride, produced by the chlorination of aluminum metal, was used as the raw material. The aluminum metal ingots contained impurities of Na: 10-100 ppm, Si: 400-1000 ppm, Fe: 580-1500 ppm, V: 50-150 ppm, and Ti: 50 ppm. The chlorination process 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 to a temperature range of 170°C to 300°C to induce 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 a temperature of 25 to 65°C to enable crystallization of aluminum chloride hexahydrate. The precipitated crystalline 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 an acidic medium was carried out in a single step. As a result, the purity of alumina was increased by controlling 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 the two-step crystallization of aluminum chloride hexahydrate, resulting in the production of even higher purity alumina by controlling the main metal impurities to less than 0.5 ppm. Table 1 shows the purity of the alumina produced by these processes in this application.

[0019] [Table 1]

[0020] (Example 4) In the process of Example 1, instead of calcining crystalline aluminum chloride hexahydrate to obtain alumina, aluminum hydroxide is prepared using crystalline aluminum chloride hexahydrate. The crystalline aluminum chloride hexahydrate is redissolved in water, and ammonium hydroxide is added until the pH becomes 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 it is 99.99%.

[0021] [Table 2]

[0022] The foregoing description of the present invention is illustrative and descriptive. Various modifications will become apparent to those skilled in the art in light of this disclosure. All such modifications that fall within the scope and spirit of the appended claims are intended to be encompassed thereby. Another aspect of the present invention may be as follows: [1] A process for producing high-purity alumina, (a) A step of sublimating anhydrous aluminum chloride at a predetermined temperature to recover pure aluminum chloride, (b) The step of dissolving the aluminum chloride from step (a) in water to obtain an aluminum chloride solution, (c) A step of introducing HCl gas into the aluminum chloride solution from step (b) to crystallize aluminum chloride hexahydrate, (d) A step of calcining the aluminum chloride hexahydrate from step (c) to obtain high-purity alumina, A process that includes this. [2] The process according to [1], wherein the sublimation in step (a) is a two-step process, in which anhydrous aluminum chloride is heated at different temperatures. [3] The process according to [2], wherein the two-stage sublimation is carried out at temperatures of 50°C to 170°C and 170°C to 300°C. [4] The process according to [1], wherein the sublimation is carried out until 95-97% of the aluminum chloride is recovered from the anhydrous aluminum chloride. [5] The process according to [1], wherein the sublimation of step (a) may be repeated at least four times. [6] The process according to [1], wherein the sublimation may be carried out in the presence of a carrier gas. [7] The process according to [1], wherein the aluminum chloride obtained in step (a) is either a gas or a solid. [8] The process according to [1], wherein the dissolution of aluminum chloride in step (b) is carried out at a temperature of 30 to 50°C. [9] The process according to [1], wherein step (b) is repeated at least four times.

[10] The process according to [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] The process according to [1], wherein aluminum chloride hexahydrate seed crystals may be added during step (c).

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

[13] The process described in [1] above, wherein calcination is carried out until the loss on ignition is less than 0.5%.

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

[15] The process according to

[14] , further comprising the step of calcining the high-purity aluminum hydroxide to obtain high-purity alumina.

Claims

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

2. The process according to claim 1, wherein the sublimation in step (a) is a two-step process, wherein anhydrous aluminum chloride is heated at different temperatures.

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

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

5. The process according to claim 1, wherein the sublimation of step (a) is repeated at least four times.

6. The process according to claim 1, wherein the sublimation is carried out in the presence of a carrier gas.

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

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

9. The process according to 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.

10. The process according to claim 1, wherein an aluminum chloride hexahydrate seed crystal is added during step (c).

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

12. The process according to claim 1, wherein calcination is carried out until the ignition loss is less than 0.5%.

13. The process according to claim 1, wherein the crystallized aluminum chloride hexahydrate from step (c) is dissolved in water and reacted with ammonia or ammonium hydroxide to obtain high-purity aluminum hydroxide.

14. The process according to claim 13, further comprising the step of calcining the high-purity aluminum hydroxide to obtain high-purity alumina.

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