A method of stripping a silicon carbide coating from a graphite surface and recovering the graphite substrate

Abstract

This invention discloses a method for peeling off the silicon carbide coating on the surface of graphite and recovering the graphite matrix. The method involves covering the graphite part containing the failed silicon carbide coating with alkali metal compound powder, then heating under positive pressure to melt the alkali metal compound powder into a molten alkali metal compound to soak the failed silicon carbide coating. The pressure is then reduced to below 100 mbar within 10 minutes to allow the molten alkali metal compound to completely volatilize. After complete volatilization, the pressure is maintained below 100 mbar. After this holding period, cooling gas is introduced to lower the furnace temperature below 300°C within 15 minutes. The furnace door is then opened, and the material is further cooled to room temperature in the atmosphere. The method provided by this invention is simple, has good peeling effect, and causes almost no damage to the graphite matrix.

Description

Technical Field

[0001] This invention belongs to the field of graphite substrate recycling technology for ceramic coatings, specifically relating to a method for peeling off the silicon carbide coating on the graphite surface and recycling the graphite substrate. Background Technology

[0002] Graphite materials are widely used in metallurgy, chemical industry, photovoltaic and semiconductor industries due to their excellent high temperature resistance, electrical and thermal conductivity and chemical stability. In order to further improve the wear resistance, oxidation resistance and corrosion resistance of graphite parts, silicon carbide coatings are often prepared on their surface by methods such as chemical vapor deposition.

[0003] After use, the silicon carbide coating on these coated parts may fail due to localized damage caused by mechanical impact or thermal shock, or due to process failures in the coating preparation process, or due to excessive adhering impurities (such as metals or slag) on ​​the surface during the process. Currently, the conventional method for handling these failed parts is to discard them entirely. This not only wastes the expensive graphite substrate but also increases the cost of solid waste disposal.

[0004] The core challenge in recycling these components lies in effectively removing the hard silicon carbide coating while protecting the internal graphite matrix from damage to the greatest extent possible. Mechanical peeling methods can easily scratch or damage the graphite matrix; while strong acid and alkali chemical corrosion methods can dissolve the silicon carbide, they will also severely corrode the graphite matrix, leading to a decrease in its strength and a porous surface that cannot meet the requirements for reuse. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the first objective of this invention is to provide a method for peeling off the silicon carbide coating from the surface of graphite and recovering the graphite matrix. The method provided by this invention is simple, provides excellent peeling results, and causes virtually no damage to the graphite matrix.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] This invention discloses a method for peeling off the silicon carbide coating on the surface of graphite and recovering the graphite matrix. The method involves covering a graphite part containing a failed silicon carbide coating with alkali metal compound powder, then heating under positive pressure to melt the alkali metal compound powder into a molten alkali metal compound, which is then used to soak the failed silicon carbide coating for 0.5-3 hours. The pressure is then reduced to below 100 mbar within 10 minutes to allow the molten alkali metal compound to completely volatilize. After complete volatilization, the pressure is maintained below 100 mbar. After this holding period, cooling gas is introduced to lower the furnace temperature below 300°C within 15 minutes. Finally, the furnace door is opened, and the material is allowed to cool further to room temperature in the atmosphere.

[0008] The method of this invention first involves immersing a failed silicon carbide coating in a molten alkali metal compound to initially dissolve, corrode, and swell the coating. Then, the pressure inside the container is reduced to below 100 mbar, allowing the molten alkali metal compound to completely evaporate. During this evaporation process, the coating further swells. The temperature is then maintained, and the thermal stress generated by the different coefficients of thermal expansion between the coating and the graphite substrate causes the swollen and embrittled coating to separate from the graphite substrate. Finally, heating is stopped, and the material is rapidly cooled to room temperature. During cooling, the difference in shrinkage rates between the coating and the substrate generates further thermal stress, causing the coating to peel off further from the substrate and be removed, resulting in a reusable graphite substrate.

[0009] In a preferred embodiment, the alkali metal compound powder is selected from at least one of potassium hydroxide powder, sodium sulfate powder, sodium carbonate powder, and sodium hydroxide powder.

[0010] Further preferably, the alkali metal compound powder is of industrial grade. In this invention, using industrial-grade alkali metal compound powder is sufficient, which can significantly reduce processing costs.

[0011] In a preferred embodiment, a graphite part containing a failed silicon carbide coating is covered with alkali metal compound powder in a graphite crucible, placed in a high-temperature furnace, and then heated to 200-300°C at a rate of 5-10°C / min under vacuum conditions, held for 10-30min to expel air from the furnace, and then a protective gas is introduced to 101.5-105 kPa, and the pressure is maintained while the temperature is increased to 800-1100°C at a rate of 5-10°C / min to melt the alkali metal compound powder into a molten alkali metal compound.

[0012] In a further preferred embodiment, the protective gas is nitrogen or argon.

[0013] The preferred approach is to then reduce the pressure to below 100 mbar within 3-10 minutes.

[0014] In the preferred embodiment, after the molten alkali metal compound has completely volatilized, the temperature is maintained at below 100 mbar for 0.5-2 hours.

[0015] In the preferred embodiment, after the heat preservation is completed, heating is stopped and cooling gas is introduced. Under the combined action of cooling gas and cooling water, the furnace temperature is kept below 300℃ within 5-15 minutes.

[0016] In a further preferred embodiment, the cooling gas is compressed nitrogen or compressed argon, and the amount of cooling gas introduced is such that the gas flow rate per unit cubic meter volume of the furnace cavity is 30-70 L / min.

[0017] In a further preferred embodiment, the flow rate of the cooling water is the maximum limit value of the high-temperature furnace.

[0018] Principles and advantages

[0019] The process of this invention involves four steps: "molten alkali metal compound corrosion - volatilization and swelling - thermal stress separation - rapid cooling". Each step is designed to solve a specific problem and creates conditions for the next step, resulting in a synergistic effect of "1+1>2".

[0020] Step 1: Corrosion of molten alkali metal compounds – chemical weakening and structural loosening:

[0021] Mechanism of Action: At high temperatures of 800-1100℃, the selected alkali metal compound powders (KOH / Na2SO4 / Na2CO3 / NaOH) are in a molten state and possess high chemical activity. The molten alkali metal compounds react chemically with silicon carbide (SiC). Taking Na2SO4 as an example, the reaction is: SiC + 2Na2SO4 + 2O2 → SiO2 + Na2SiO3 + Na2CO3 + 2SO2↑. This reaction process starts from the coating surface and gradually erodes inwards. It not only directly dissolves some SiC, but more importantly, the silicates and carbonates produced in the reaction have different molecular volumes and crystal structures than the original SiC, generating internal stress within the coating and leading to microcracks and pores. The graphite matrix itself has certain micropores. The molten alkali metal compounds penetrate the coating through these cracks and pores into the interface between the coating and graphite, as well as into the micropores of the graphite matrix itself. The cracks and pores within the coating itself are also filled by the molten alkali metal compounds. This makes the originally dense and hard coating loose and porous, laying the foundation for subsequent steps. Although the matrix graphite (C) can also react with alkali at high temperatures, its reaction rate is much lower than that of SiC, thus achieving selective corrosion, preferentially attacking the target coating while protecting the matrix.

[0022] Step 2, Volatilization and Swelling - Stress Amplification:

[0023] Mechanism of action: Rapidly reducing the pressure to below 100 mbar within 3-10 minutes causes the molten alkali metal compounds to volatilize rapidly. During this volatilization, salts at the coating-substrate interface, within coating cracks and pores, and in graphite micropores rapidly vaporize, generating steam from the inside out. This outward pressure expands existing cracks, amplifying the destructive effect of the first step and further causing the coating and substrate to peel apart. This internally generated physical destructive force is more effective than simply applying external force. It can disrupt the integrity of the coating-substrate interface structure and the coating's own structure, but because the force originates from within the coating or at the coating-graphite interface, it avoids direct damage to the graphite matrix.

[0024] Step 3, Thermal Stress Separation - Further peeling is achieved by utilizing the difference in intrinsic physical properties:

[0025] Mechanism of Action: After the first two steps, the coating contains numerous defects such as pores, cracks, pits, and bubbles. At this point, during the continuous high-temperature insulation process, although silicon carbide and graphite are both high-temperature resistant materials, their coefficients of thermal expansion differ. At high temperatures, this difference leads to different expansion rates, generating shear stress at the interface where the bonding force has been weakened. Under the continuous action of this thermal stress, the bonding interface between the coating and the substrate is damaged, causing the already defective coating to detach from the substrate.

[0026] Step 4, rapid cooling - to further peel off the coating:

[0027] Mechanism of action: During rapid cooling and contraction, due to the difference in thermal expansion coefficients, the coating and substrate shrink at different rates, generating new stresses in opposite directions at the interface. This causes the coating to crack and peel off during the cold-heat, expansion-contraction cycle. Rapid cooling increases the temperature difference, amplifying this thermal stress effect and ensuring sufficient separation.

[0028] Compared with the prior art, the present invention has at least the following advantages:

[0029] 1. Existing technologies rely on forceful destruction, whether mechanical crushing or strong acid corrosion. Essentially, these methods use forces far exceeding the material's tolerance limits to destroy the coating. However, this inevitably damages the substrate, making it even more difficult to recycle large, complex structural components. This invention, however, employs precise induced separation. It utilizes physicochemical methods to precisely weaken the interfacial bonding between the coating and the substrate, inducing internal stress to cause the coating to actively detach. In this invention, the force is concentrated on the coating itself and at its interface with the substrate. The selective corrosion mechanism ensures that the graphite substrate is minimized from chemical attack, avoiding any form of mechanical scratching, impact, or abrasion. The substrate experiences uniform, predictable stress, rather than localized impact forces. Therefore, the graphite substrate does not develop cracks, scratches, or structural damage, making it particularly advantageous for recycling complex-shaped structural components.

[0030] 2. Since the graphite matrix is ​​not damaged in this invention and can be completely recycled, it can significantly save costs. Isostatically pressed graphite parts are very expensive, with large or complex-shaped graphite parts costing thousands or even tens of thousands of RMB per unit. By recycling the matrix, more than 90% of the cost of purchasing new materials can be saved. Furthermore, the treatment cost of solid waste containing SiC is high, and treating it as ordinary solid waste requires paying treatment fees. This method converts it into recyclable resources, directly saving this cost. Attached Figure Description

[0031] Figure 1A photograph of a graphite part with a failed silicon carbide coating before treatment in Example 1.

[0032] Figure 2 A photograph of the actual product in Example 1 after being treated with molten alkali metal compounds and then exiting the furnace.

[0033] Figure 3 A photograph of the graphite matrix after final cleaning in Example 1.

[0034] Figure 4 XRD pattern of the coated sample before treatment with molten alkali metal compound in Example 1.

[0035] Figure 5 XRD pattern of the graphite matrix after final cleaning in Example 1.

[0036] Figure 6 A photograph of the actual product after being treated with molten alkali metal compounds in Example 2, taken out of the furnace.

[0037] Figure 7 A photograph of the graphite matrix after final cleaning in Example 2.

[0038] Figure 8 A photograph of the actual product after being treated with molten alkali metal compounds in Example 3, taken out of the furnace.

[0039] Figure 9 A photograph of the graphite matrix after final cleaning in Example 3.

[0040] Figure 10 The graphite part with a failed silicon carbide coating before treatment in Comparative Example 1.

[0041] Figure 11 A physical image of the graphite part after processing in Comparative Example 1.

[0042] Figure 12 A photograph of the graphite part after processing in Comparative Example 2. Detailed Implementation

[0043] Example 1

[0044] Take a used silicon carbide-coated graphite block and place it in a brand new corundum crucible. Add sufficient industrial-grade potassium hydroxide powder to completely cover the silicon carbide-coated graphite block.

[0045] The corundum crucible was placed in a tube furnace, and the furnace door was closed. Under atmospheric pressure, the temperature was increased to 200°C at a rate of 10°C / min, held for 30 min, and then nitrogen gas was introduced to a slightly positive pressure of 103 kPa. This pressure was maintained, and the temperature was increased to 1000°C at a rate of 10°C / min, and held at this temperature for 1 h. Subsequently, the vacuum system was activated, and the furnace pressure was reduced to 50 mbar within 3 min and maintained at this state for 1 h to ensure complete volatilization of the molten potassium hydroxide. Afterward, the pressure was maintained below 50 mbar, and the temperature was held at 1000°C for another 1 h. Finally, the heating was turned off, nitrogen gas was introduced at a rate of 40 L / min, and the cooling water flow rate was increased. The furnace was cooled to 300°C in 15 min, and then the furnace door was opened to allow it to cool to room temperature in the atmosphere.

[0046] Upon removing the processed sample, it was observed that the silicon carbide coating on the surface had largely peeled off and could be easily removed with a gentle sweep. Observation and measurement of the graphite substrate after the coating was removed revealed a smooth surface with no obvious corrosion pits, and its appearance was similar to that of the original graphite. Figure 1 The image shows a graphite part with a SiC coating before processing. Figure 2 The image shows the state of the coating after it has been treated with molten alkali metal compounds and is now removed from the graphite substrate. Figure 3 The image shows the graphite substrate after the coating has been removed. As can be seen, the original processing texture and edges of the graphite surface remain after the coating is removed, indicating that the method does not cause significant damage to the graphite substrate. Figure 4 This is the XRD pattern of the coated sample before processing. Figure 5 The XRD pattern of the final stripped coating is shown in the image. Comparing the two images, it can be seen that the process has completely removed the coating from the graphite surface.

[0047] Example 2

[0048] Take a used silicon carbide coated graphite block and place it in a brand new graphite crucible. Add sufficient industrial-grade sodium sulfate powder to completely cover the silicon carbide coated graphite block.

[0049] The graphite crucible was placed in a high-temperature furnace, the furnace door was closed, and the temperature was raised to 200°C at a rate of 10°C / min under vacuum. After holding at this temperature for 20 min, nitrogen gas was introduced to a slightly positive pressure of 104 kPa, and this pressure was maintained while the temperature was raised to 1000°C at a rate of 10°C / min, and held at this temperature for 1 h. Subsequently, the vacuum system was activated, and the pressure inside the furnace was reduced to below 50 mbar within 3 min, and this state was maintained for 1 h to ensure complete volatilization of the liquid molten sodium sulfate. Afterward, the pressure was maintained below 50 mbar, and the temperature was held at 1000°C for another 1 h. Finally, the heating was turned off, nitrogen gas was introduced at a rate of 50 L / min, and the cooling water flow rate was increased simultaneously. The furnace was rapidly cooled to 300°C in 10 min, and then the furnace door was opened to allow it to cool to room temperature in the atmosphere.

[0050] Upon removing the processed sample, it was observed that the silicon carbide coating on the surface had peeled off over a large area, which could be easily removed with a gentle sweep. Observation and measurement of the stripped graphite matrix revealed a smooth surface without obvious corrosion pits, and its appearance was similar to that of the original graphite. Figure 6 The image shows the state of the coating after it has been treated with molten alkali metal compounds and is now removed from the graphite substrate. Figure 7 To clean the graphite substrate after the SiC coating has been peeled off, it can be seen that the graphite substrate still has sharp edges after the coating is removed. This indicates that the method effectively removes the coating while having little impact on the graphite substrate and does not affect its reuse.

[0051] Example 3

[0052] Take a used silicon carbide-coated graphite block and place it in a brand new graphite crucible. Add sufficient industrial-grade sodium carbonate powder to completely cover the silicon carbide-coated graphite block.

[0053] The graphite crucible was placed in a high-temperature furnace, the furnace door was closed, and the temperature was raised to 200°C at a rate of 5°C / min under vacuum. After holding at this temperature for 30 minutes, nitrogen gas was introduced to a slightly positive pressure of 103 kPa. This pressure was maintained, and the temperature was raised to 900°C at a rate of 8°C / min, and held at this temperature for 1.5 hours. Subsequently, the vacuum system was activated, and the pressure inside the furnace was reduced to below 20 mbar within 5 minutes and maintained at this state for 1 hour to ensure complete volatilization of the liquid molten sodium carbonate. Afterward, the pressure was maintained below 20 mbar, and the temperature was held at 900°C for another 1.5 hours. Finally, the heating was turned off, nitrogen gas was introduced at a rate of 50 L / min, and the cooling water flow rate was increased to rapidly cool the furnace to 300°C within 15 minutes. Then, the furnace door was opened, and the furnace was allowed to cool to room temperature in the atmosphere.

[0054] Upon removing the processed sample, it was observed that a large area of ​​the silicon carbide coating on the surface had peeled off, which could be easily removed with a light sweep. Observation and measurement of the graphite matrix after peeling off the coating revealed a smooth surface without obvious corrosion pits, and its appearance was close to that of the original graphite. Figure 8 The image shows the state of the coating after it has been treated with molten alkali metal compounds and exited the furnace. At this point, the coating has swollen and bubbled over a large area and has separated from the graphite substrate. Figure 9 The graphite substrate after removing the swollen and bubbling SiC coating is shown in the figure. As can be seen, the graphite substrate is intact after the coating is removed, and the processing marks on the original graphite surface are clearly visible.

[0055] Comparative Example 1

[0056] Take a small piece of used silicon carbide-coated graphite. Place it in a brand new corundum crucible and add sufficient industrial-grade potassium hydroxide powder to completely cover the silicon carbide-coated graphite piece.

[0057] Place the corundum crucible into a tube furnace, close the furnace door, and heat it to 200°C at a rate of 10°C / min under normal pressure. Hold it at this temperature for 30 min, then introduce nitrogen gas to a slightly positive pressure of 103 kPa. Maintain this pressure and heat it to 1000°C at a rate of 10°C / min. Hold it at this temperature for 1 h, then turn off the heating. Introduce nitrogen gas at a rate of 40 L / min, while simultaneously increasing the cooling water flow rate. The furnace will rapidly cool down to 300°C in 15 min. Then open the furnace door and allow it to cool to room temperature in the atmosphere. Remove the sample and clean off any residual salt.

[0058] Appearance before processing: Figure 10 As shown. After the sample was removed, the coating did not peel off, and salt residue remained on the surface, with little change in appearance. The results after treatment are as follows. Figure 11 As shown.

[0059] Comparative Example 2

[0060] Take a used silicon carbide-coated graphite block and place it in a brand new graphite crucible. Add sufficient industrial-grade sodium carbonate powder to completely cover the silicon carbide-coated graphite block.

[0061] A graphite crucible was placed in a high-temperature furnace, the furnace door was closed, and the temperature was raised to 200°C at a rate of 5°C / min under vacuum. After holding at this temperature for 30 minutes, nitrogen gas was introduced to a slightly positive pressure of 104 kPa. This pressure was maintained, and the temperature was raised to 1000°C at a rate of 10°C / min without further holding. The vacuum system was then activated, and the furnace pressure was reduced to below 50 mbar within 3 minutes and maintained at this state for 1 hour to ensure complete volatilization of the molten sodium carbonate. Afterward, the pressure was maintained below 50 mbar, and the temperature was held at 1000°C for another hour. Finally, the heating was turned off, and nitrogen gas was introduced at a rate of 50 L / min while simultaneously increasing the cooling water flow rate. The furnace was rapidly cooled to 300°C within 10 minutes, and then the furnace door was opened to allow it to cool to room temperature in the atmosphere. After removing the sample, the coating was clearly visible and had not peeled off. The results after treatment are as follows. Figure 12 As shown.

Claims

1. A method for peeling off the silicon carbide coating from the surface of graphite and recovering the graphite matrix, characterized in that: The graphite part containing the failed silicon carbide coating is covered with alkali metal compound powder. Then, the alkali metal compound powder is heated under positive pressure to melt into a molten alkali metal compound and immersed in the failed silicon carbide coating for 0.5-3 hours. Then, the pressure is reduced to below 100 mbar within 10 minutes to allow the molten alkali metal compound to completely volatilize. After the molten alkali metal compound has completely volatilized, the temperature is maintained below 100 mbar. After the temperature maintenance is completed, cooling gas is introduced and the furnace temperature is reduced to below 300°C within 15 minutes. Then, the furnace door is opened and the part is allowed to continue cooling to room temperature in the atmosphere.

2. The method for peeling off the silicon carbide coating on the graphite surface and recovering the graphite matrix according to claim 1, characterized in that: The alkali metal compound powder is selected from at least one of potassium hydroxide powder, sodium sulfate powder, sodium carbonate powder, and sodium hydroxide powder. The alkali metal compound powder is of industrial grade.

3. The method for peeling off the silicon carbide coating on the graphite surface and recovering the graphite matrix according to claim 1, characterized in that: In a graphite crucible, a graphite part with a failed silicon carbide coating is covered with alkali metal compound powder and placed in a high-temperature furnace. Then, under vacuum conditions, the temperature is raised to 200-300°C at a rate of 5-10°C / min and held for 10-30 min to purge the air from the furnace. Then, a protective gas is introduced to 101.5-105 kPa and maintained at this pressure. The temperature is raised to 800-1100°C at a rate of 5-10°C / min to melt the alkali metal compound powder into a molten alkali metal compound.

4. The method for peeling off the silicon carbide coating on the graphite surface and recovering the graphite matrix according to claim 1, characterized in that: Then the pressure is reduced to below 100 mbar within 3-10 minutes.

5. The method for peeling off the silicon carbide coating on the graphite surface and recovering the graphite matrix according to claim 1, characterized in that: After the molten alkali metal compound has completely volatilized, it should be kept at a temperature below 100 mbar for 0.5-2 hours.

6. The method for peeling off the silicon carbide coating on the graphite surface and recovering the graphite matrix according to claim 1, characterized in that: After the heat preservation is completed, heating is stopped and cooling gas is introduced. Under the combined action of cooling gas and cooling water, the furnace temperature is kept below 300℃ within 5-15 minutes.

7. The method for peeling off the silicon carbide coating on the graphite surface and recovering the graphite matrix according to claim 6, characterized in that: The cooling gas is compressed nitrogen or compressed argon, and the amount of cooling gas introduced is such that the gas flow rate per unit cubic meter of furnace cavity is 30-70 L / min.

8. The method for peeling off the silicon carbide coating on the graphite surface and recovering the graphite matrix according to claim 6, characterized in that: The flow rate of the cooling water is the maximum limit value of the high-temperature furnace.

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