Alumina-based paint for preventing crucible pollution in high-purity aluminum preparation process and brushing method

Abstract

The invention belongs to the technical field of high-purity aluminum, and discloses an alumina-based coating for preventing crucible pollution in a high-purity aluminum preparation process and a brushing method. The brushing method comprises the following steps: cleaning the crucible, preheating, and turning off the heater after preheating; uniformly mixing an aluminum oxide-based coating, brushing the aluminum oxide-based coating on the inner wall of the crucible, drying the coating by utilizing waste heat of the crucible, then repeatedly brushing for 2-3 times to form 3-4 coatings, raising the temperature to 98-102 DEG C, and preserving heat and curing for 50-70 minutes; the preheating temperature ranges from 78 DEG C to 82 DEG C, and the heat preservation time of preheating ranges from 8 min to 12 min. The aluminum oxide-based coating comprises the following components in percentage by mass: 50-70% of high-purity aluminum oxide, 5-15% of BN powder, 1-5% of a dispersing agent and the balance of water. The method is simple, a compact film layer is formed at low temperature, energy and time are saved, and safety is high; in the preparation of high-purity aluminum, the crucible pollution prevention effect is excellent.

Description

Technical Field

[0001] This invention relates to the field of high-temperature protective coatings, specifically to an alumina-based coating and application method for preventing crucible contamination during the preparation of high-purity aluminum. Background Technology

[0002] In the smelting process of high-purity aluminum (purity ≥ 99.99%), the crucible, as the core container directly in contact with the molten aluminum, is crucial for controlling contamination of its inner wall, which is key to determining the purity of the final product. High-purity aluminum has extremely low tolerance for impurities, and traditional graphite or clay crucibles will continuously release impurities into the molten aluminum due to thermochemical corrosion at high temperatures. Simultaneously, the high activity of the molten aluminum exacerbates the interfacial reaction with the crucible substrate, leading to an increase in impurities in the molten aluminum. High-end applications such as semiconductors and sputtering targets require aluminum purity of 4N6 or higher (99.996%). Any micron-sized crucible spalling or interfacial reaction layer can severely affect product purity and significantly reduce yield. To avoid crucible contamination of the molten aluminum, applying a high-temperature protective coating to the crucible surface is an effective method; currently, alumina-based coatings are commonly used and effective.

[0003] Chinese invention patent application CN 104845413 A discloses a crucible anti-fouling coating formulation and its application method. The method involves mixing powdered alumina and aluminum dihydrogen phosphate solution at a mass ratio of 1.4:1 to form a paste. The application process is as follows: after preheating the crucible to 198-202℃, a first layer (0.4-0.8 mm) is applied, followed by holding at 500℃ for 1 hour. After the first layer is applied, the temperature is lowered to 198-202℃, and this process of application, holding, and cooling is repeated once. This method requires high preheating and holding temperatures, long holding times, high energy consumption, long downtime (>4 hours), and low efficiency. Apart from CN 104845413 A, there is a lack of publicly available data on other alumina-based coating application methods.

[0004] To address the above shortcomings, this invention provides an alumina-based coating and a coating method to prevent crucible contamination. By combining the coating formulation with a low-temperature coating process, it achieves simple operation, energy saving, time saving, and excellent protective effect. Summary of the Invention

[0005] To address the shortcomings of existing coating processes, this invention aims to provide an alumina-based coating and application method for preventing crucible contamination during the preparation of high-purity aluminum. This invention optimizes the coating formulation and application process, curing the coating at low temperatures to achieve highly efficient protection during the preparation of high-purity aluminum.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] An alumina-based coating for preventing crucible contamination during the preparation of high-purity aluminum is composed of the following components by mass percentage: 50-70% high-purity alumina (purity ≥99.9%), 5-15% BN powder, 1-5% dispersant, and the balance being water. The dispersant is BYK-110 dispersant.

[0008] The alumina-based coating is prepared by the following method: first, high-purity alumina and BN powder (boron nitride powder) are mixed evenly, then BYK-110 dispersant and water are added and stirred until a paste is formed.

[0009] The method for applying an alumina-based coating to prevent crucible contamination during the preparation of high-purity aluminum includes the following steps: cleaning and preheating the crucible, and then turning off the heater after preheating; mixing the alumina-based coating and applying it to the inner wall of the crucible; using the residual heat of the crucible to dry the coating and then applying it again 2-3 times to form 3-4 layers; then raising the temperature and maintaining the temperature for curing.

[0010] The crucible is a graphite crucible.

[0011] The preheating temperature is 78-82℃, and the preheating holding time is 8-12 minutes.

[0012] The alumina-based coating is mixed evenly and then applied to the inner wall of the crucible. The thickness of each coat of the alumina-based coating is 0.4-0.6 mm, and the drying time after each coat is 2-4 minutes.

[0013] The application was performed three times.

[0014] The temperature increase refers to raising the temperature to 98-102℃. The holding time is 50-70 minutes.

[0015] The method for applying an alumina-based coating to prevent crucible contamination during the preparation of high-purity aluminum specifically includes the following steps:

[0016] (1) Clean the crucible: Clean the inner wall of the crucible thoroughly to remove old coatings and peeling materials;

[0017] (2) Preheating: Place the graphite crucible in the heating device for preheating. The preheating temperature is 78-82℃. Keep it at this temperature for 8-12 minutes. After keeping it at this temperature, turn off the heating device.

[0018] (3) Stirring and brushing: Stir the alumina-based coating evenly, brush the first coating layer on the inner wall of the crucible, with a coating thickness of 0.4-0.6 mm, and then let it stand for 2-4 minutes until the residual temperature dries the coating; after the first coating layer is completed, repeat the brushing and standing steps 2-3 times to form 3-4 coating layers.

[0019] (4) Curing: Turn on the heating device, raise the temperature to 98-102℃, and keep it at that temperature for 50-70 minutes to allow the 3-4 layers of coating to be completely cured, forming a dense alumina cured film layer.

[0020] A crucible with an alumina-cured film layer obtained by the method is used for the smelting of high-purity aluminum.

[0021] The purity of the high-purity aluminum is 4N6 or higher.

[0022] This invention selects alumina-based coatings with specific components and combines them with a low-temperature coating process (preheating temperature 78-82℃, curing temperature 98-102℃). Through three coats and residual heat drying, a dense film layer is formed at low temperature, which effectively prevents crucible contamination.

[0023] Compared with existing technologies, the present invention has the following advantages and beneficial effects:

[0024] (1) The process of this invention is simple and the equipment requirements are not high, so small enterprises can deploy it quickly; the temperature throughout the process does not exceed 102℃, no special protection is required, and the safety is high.

[0025] (2) The present invention uses preheating at 78-82℃ and single curing at 98-102℃, with the temperature throughout the process being less than 102℃, resulting in low energy consumption; there are no multiple cooling and heating steps, shortening the operation time by more than 50%;

[0026] (3) The present invention uses 3 to 4 coats and utilizes the residual heat of the crucible for drying, which increases the number of coatings, improves the overall density, and also shortens the coating time;

[0027] (4) The present invention has excellent anti-crucible contamination effect. After 4N6 high-purity aluminum liquid is kept in the crucible for 12 hours, the contamination of a single element is ≤1ppm and the total contamination is ≤3ppm. Detailed Implementation

[0028] The present invention will be further described in detail below with reference to specific embodiments, but the embodiments of the present invention are not limited thereto. The alumina-based coating of the present invention contains high-purity alumina with a purity ≥99.9% and a particle size ≤30 micrometers, for example: 200nm~5μm; the BN powder has a purity ≥99% and a particle size ≤20μm, for example: 100nm~5μm.

[0029] Example 1

[0030] In this embodiment, the alumina-based coating is prepared by mass percentage as follows: 50% high-purity alumina (purity ≥99.9%, average particle size 1μm), 5% BN powder (average particle size 1μm), 1% BYK-110 dispersant, and the remainder is deionized water.

[0031] This embodiment describes a method for applying an alumina-based coating to prevent crucible contamination, including the following steps:

[0032] (1) Surface cleaning: Clean the inner wall of the crucible to remove old coatings and peelings;

[0033] (2) Preheating: Place the graphite crucible in the resistance furnace for preheating. Set the temperature to 78°C. After the temperature reaches the set value, keep it warm for 8 minutes. After keeping it warm, turn off the resistance furnace.

[0034] (3) Stirring and brushing: Stir the alumina-based coating evenly, use a new brush to apply the first layer of coating with a thickness of 0.4 mm, and then let it stand for 2 minutes until the residual heat dries the coating; after the first layer is applied, repeat the brushing and standing steps twice to form a 3-layer coating.

[0035] (4) Curing: Turn on the resistance furnace, heat up to 98°C, and keep it at that temperature for 50 minutes to allow the three coating layers to be completely cured and form a dense alumina cured film layer.

[0036] After the coating is applied, 4N6 aluminum ingots with known composition are placed in a crucible, melted, and kept at that temperature for 12 hours. Samples are then taken to analyze the degree of contamination. The specific steps are as follows:

[0037] (1) Preheating: Set the temperature of the resistance furnace to 700℃ and keep it warm for 1 hour;

[0038] (2) Melting: Put 15kg of 4N6 aluminum ingot into the crucible at one time, and check the melting of the aluminum ingot every 30 minutes until the aluminum ingot is completely melted into molten aluminum;

[0039] (3) Heat preservation: The aluminum liquid is kept at 700℃ for 12 hours;

[0040] (4) Sampling and analysis: After heat preservation, a small amount of aluminum liquid is taken with a clean graphite sampling spoon, cast into the specified shape, and its composition is detected by an analyzer. The composition is compared with that of 4N6 aluminum ingot to analyze the degree of aluminum liquid contamination.

[0041] Example 2

[0042] In this embodiment, the alumina-based coating is formulated by mass percentage as follows: 60% high-purity alumina, 10% BN powder, 3% BYK-110 dispersant, and the remainder is deionized water.

[0043] This embodiment describes a method for applying an alumina-based coating to prevent crucible contamination, including the following steps:

[0044] (1) Surface cleaning: Clean the inner wall of the crucible to remove old coatings and peelings;

[0045] (2) Preheating: Place the graphite crucible in a resistance furnace to preheat at 80°C for 10 minutes, and then turn off the resistance furnace.

[0046] (3) Stirring and brushing: Stir the alumina-based coating evenly, use a new brush to apply the first layer of coating with a thickness of 0.5 mm, and then let it stand for 3 minutes until the residual heat dries the coating; after the first layer is applied, repeat the brushing and standing steps twice to form a 3-layer coating.

[0047] (4) Curing: Turn on the resistance furnace, set the temperature to 100℃, and keep it at that temperature for 60 minutes to allow the three coating layers to be completely cured and form a dense alumina cured film layer.

[0048] After the coating is applied, the degree of contamination of the crucible with the molten aluminum is tested. The specific operating conditions are the same as in Example 1.

[0049] Example 3

[0050] In this embodiment, the alumina-based coating is prepared by mass percentage as follows: 70% high-purity alumina, 15% BN powder, 5% BYK-110 dispersant, and the remainder is deionized water.

[0051] This embodiment describes a method for applying an alumina-based coating to prevent crucible contamination, including the following steps:

[0052] (1) Surface cleaning: Clean the inner wall of the crucible to remove old coatings and peelings;

[0053] (2) Preheating: Place the graphite crucible in the resistance furnace for preheating, raise the temperature to 82°C, keep it at that temperature for 12 minutes, and then turn off the resistance furnace.

[0054] (3) Stirring and brushing: Stir the alumina-based coating evenly, use a new brush to apply the first layer of coating with a thickness of 0.6 mm, and then let it stand for 4 minutes until the residual heat dries the coating; after the first layer is applied, repeat the brushing and standing steps twice to form a 3-layer coating.

[0055] (4) Curing: Turn on the resistance furnace, heat up to 102°C, and keep it at that temperature for 70 minutes to allow the three coating layers to be completely cured and form a dense alumina cured film layer.

[0056] After the coating is applied, the degree of contamination of the crucible with the molten aluminum is tested. The specific operating conditions are the same as in Example 1.

[0057] Comparative Example 1

[0058] In this comparative example, the crucible surface was cleaned without coating, and the degree of contamination of the crucible with the molten aluminum was directly tested. The testing method was the same as in Example 1.

[0059] Comparative Example 2

[0060] The specific steps for this comparative example are as follows:

[0061] (1) Surface cleaning: Clean the inner wall of the crucible to remove old coatings and peelings;

[0062] (2) Preheating: Place the graphite crucible in a resistance furnace to preheat at 78°C for 8 minutes, and then turn off the resistance furnace.

[0063] (3) Stirring and brushing: Stir the alumina-based coating evenly (the composition is the same as in Example 1), use a new brush to apply one layer of coating with a thickness of 0.4 mm, and then let it stand for 2 minutes until the residual heat dries the coating.

[0064] (4) Curing: Turn on the resistance furnace, heat up to 98°C, and keep it at that temperature for 50 minutes to allow the coating to be completely cured and form a dense alumina cured film.

[0065] After the coating is applied, the degree of contamination of the crucible with the molten aluminum is tested. The specific operating conditions are the same as in Example 1.

[0066] Comparative Example 3

[0067] The specific steps for this comparative example are as follows:

[0068] (1) Surface cleaning: Clean the inner wall of the crucible to remove old coatings and peelings;

[0069] (2) Preheating: Place the graphite crucible in a resistance furnace for preheating at 82°C for 12 minutes. After preheating, turn off the resistance furnace.

[0070] (3) Stirring and brushing: Stir the alumina-based coating evenly (the composition is the same as in Example 1), use a new brush to brush the first layer of coating, the coating thickness is 0.6 mm, and then let it stand for 4 minutes until the residual heat dries the coating; after the first layer is brushed, repeat the brushing and standing steps once to form two layers of coating.

[0071] (4) Curing: Turn on the resistance furnace, set the temperature to 102℃, and keep it warm for 70 minutes to allow the two coating layers to be completely cured and form a dense alumina cured film layer.

[0072] After the coating is applied, the degree of contamination of the crucible with the molten aluminum is tested. The specific operating conditions are the same as in Example 1.

[0073] Comparative Example 4

[0074] This comparative example uses protective coatings for other metal smelting processes (commercially available phosphate-based coatings, mainly composed of aluminum dihydrogen phosphate and aluminum oxide). The specific steps are as follows:

[0075] (1) Surface cleaning: Clean the inner wall of the crucible to remove old coatings and peelings;

[0076] (2) Preheating: Place the graphite crucible in a resistance furnace to preheat at 78°C for 8 minutes, and then turn off the resistance furnace.

[0077] (3) Stirring and brushing: Stir the phosphate-based coating evenly, and use a new brush to brush it onto the inner wall of the crucible in 3 layers. The thickness of each layer is 0.5 mm. After each layer is brushed, let it stand for 3 minutes to dry using residual heat.

[0078] (4) Curing: Turn on the resistance furnace, heat to 98°C, and keep warm for 50 minutes to cure the coating.

[0079] After the coating is applied, the degree of contamination of the crucible with the molten aluminum is tested. The specific operating conditions are the same as in Example 1.

[0080] Comparative Example 5

[0081] The comparative example uses a high coating temperature; the specific steps are as follows:

[0082] (1) Surface cleaning: Clean the inner wall of the crucible to remove old coatings and peelings;

[0083] (2) Preheating: Place the graphite crucible in the resistance furnace for preheating, raise the temperature to 150°C, keep it at that temperature for 10 minutes, and then turn off the resistance furnace.

[0084] (3) Stirring and brushing: Stir the alumina-based coating (with the same composition as in Example 1) evenly, and use a new brush to brush it onto the inner wall of the crucible in 3 layers with a single layer thickness of 0.5 mm. After each layer is brushed, let it stand for 3 minutes to dry using residual heat.

[0085] (4) Curing: Turn on the resistance furnace, heat to 150°C, and keep warm for 60 minutes to cure the coating.

[0086] After the coating is applied, the degree of contamination of the crucible with the molten aluminum is tested. The specific operating conditions are the same as in Example 1.

[0087] Comparative Example 6

[0088] The comparative example uses a low coating temperature; the specific steps are as follows:

[0089] (1) Surface cleaning: Clean the inner wall of the crucible to remove old coatings and peelings;

[0090] (2) Preheating: Place the graphite crucible in the resistance furnace for preheating, raise the temperature to 60°C, keep it at that temperature for 10 minutes, and then turn off the resistance furnace.

[0091] (3) Stirring and brushing: Stir the alumina-based coating (with the same composition as in Example 1) evenly, and use a new brush to brush it onto the inner wall of the crucible in 3 layers with a single layer thickness of 0.5 mm. After each layer is brushed, let it stand for 3 minutes to dry using residual heat.

[0092] (4) Curing: Turn on the resistance furnace, heat to 80°C, and keep warm for 60 minutes to cure the coating.

[0093] After the coating is applied, the degree of contamination of the crucible with the molten aluminum is tested. The specific operating conditions are the same as in Example 1.

[0094] The degree of contamination of the crucibles on the 4N6 aluminum melt in Examples 1-3 and Comparative Examples 1-6 is summarized in Tables 1 and 2, with the unit being ppm.

[0095] Table 1. Components and content of 4N6 raw material and molten aluminum (unit: ppm)

[0096]

[0097] Table 2. Components and content of 4N6 raw material and molten aluminum (unit: ppm)

[0098]

[0099] As shown in Table 1, in Examples 1-3, the 4N6 aluminum melt contained 16 elements, including Fe, Si, and Cu, without significant contamination. The most severe contamination was only 0.8 ppm lower than that of the raw material. The total contamination of the 16 elements in the three examples was 1.1 ppm, 2.6 ppm, and 1.5 ppm, respectively, all less than 3 ppm. In contrast, the aluminum melt was significantly contaminated in the comparative examples. In Comparative Example 1, which was uncoated, the Fe contamination was as high as 126 ppm, and the B contamination was also relatively high at 9.8 ppm. In Comparative Example 2, when one coating was applied, the contamination of Fe, Si, Cu, and B all exceeded 1 ppm, reaching a maximum of 5.8 ppm, with a total elemental contamination of 16 ppm. In Comparative Example 3, when two coatings were applied, the Si and B contamination exceeded 1 ppm, at 7 ppm and 1.9 ppm, respectively, with a total elemental contamination of 11 ppm. Comparative Example 4, using other coatings, had a total contamination level of 8.5 ppm; Comparative Example 5, with a coating temperature exceeding the upper limit, had a total contamination level of 10.4 ppm; and Comparative Example 6, with a coating temperature below the lower limit, had a total contamination level of 9.2 ppm. This demonstrates that only within the coating formulation and coating temperature range of this invention (preheating 78-82°C, curing 98-102°C) can the contamination of the molten aluminum by the crucible be effectively prevented. Temperatures that are too high or too low will result in insufficient coating curing and a decreased protective effect.

Claims

1. An alumina-based coating for preventing crucible contamination during the preparation of high-purity aluminum, characterized in that: It is composed of the following components by mass percentage: 50-70% high-purity alumina, 5-15% BN powder, 1-5% BYK-110 dispersant, and the balance is water; High-purity alumina has a purity of ≥99.9%.

2. A method for applying an alumina-based coating to prevent crucible contamination during the preparation of high-purity aluminum, characterized in that: The process includes the following steps: cleaning and preheating the crucible, then turning off the heater; mixing the alumina-based coating and applying it to the inner wall of the crucible; using the residual heat of the crucible to dry the coating, then repeating the application 2-3 times to form 3-4 layers; subsequently raising the temperature and maintaining it for curing; the preheating temperature is 78-82℃, and the preheating time is 8-12 minutes; raising the temperature refers to raising the temperature to 98-102℃, and the curing time is 50-70 minutes; The alumina-based coating is as defined in claim 1.

3. The method for applying an alumina-based coating to prevent crucible contamination during the preparation of high-purity aluminum according to claim 2, characterized in that: The alumina-based coating is prepared by the following method: first, high-purity alumina and BN powder are mixed evenly, then BYK-110 dispersant and water are added and stirred until a paste is formed; The crucible is a graphite crucible; The alumina-based coating is mixed evenly and then applied to the inner wall of the crucible. The thickness of each coat of the alumina-based coating is 0.4-0.6 mm, and the drying time after each coat is 2-4 minutes.

4. The method for applying an alumina-based coating to prevent crucible contamination during the preparation of high-purity aluminum according to claim 2, characterized in that: The application was performed three times.

5. The method for applying an alumina-based coating to prevent crucible contamination during the preparation of high-purity aluminum according to claim 2, characterized in that: The alumina-based coating has a high-purity alumina particle size of 200 nm to 5 μm and a BN powder particle size of 100 nm to 5 μm.

6. The method for applying an alumina-based coating to prevent crucible contamination according to claim 2, characterized in that: Specifically, the following steps are included: (1) Clean the crucible: Clean the inner wall of the crucible thoroughly to remove old coatings and peeling materials; (2) Preheating: Place the crucible in the heating device for preheating. The preheating temperature is 78-82℃. Keep it at this temperature for 8-12 minutes. After keeping it at this temperature, turn off the heating device. (3) Stirring and brushing: Stir the alumina-based coating evenly, brush the first coating layer on the inner wall of the crucible, with a coating thickness of 0.4-0.6 mm, and then let it stand for 2-4 minutes until the residual temperature dries the coating; after the first coating layer is completed, repeat the brushing and standing steps 2-3 times to form 3-4 coating layers. (4) Curing: Turn on the heating device, raise the temperature to 98-102℃, and keep it at that temperature for 50-70 minutes to allow the 3-4 layers of coating to be completely cured, forming a dense alumina cured film layer.

7. A crucible with an alumina-cured film layer obtained by the method of any one of claims 2 to 6 for smelting high-purity aluminum.

8. The application according to claim 7, characterized in that: The purity of the high-purity aluminum is 4N6 or higher.

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