Method for preparing electronic-grade silicon carbide through coal ash dynamic impurity removal-gas phase purification method

Abstract

The invention relates to the technical field of preparation methods of silicon carbide, in particular to a method for preparing electronic-grade silicon carbide through a coal ash dynamic impurity removal-gas phase purification method. The method comprises the following steps: pretreating a fly ash raw material with a KOH solution, carrying out acid leaching in an oxalic acid / citric acid mixed solution, introducing chlorine-containing gas to carry out gas-phase impurity removal, mixing with a biomass carbon source, carrying out heating reduction in a nitrogen-hydrogen mixed gas, introducing silicon steam, and carrying out a high-temperature reaction to obtain the high-purity beta-SiC. According to the process, presintering and a pore forming agent are not needed, and the limitation of a sandwich structure is broken through. According to the dynamic aluminum-silicon ratio feedback system based on viscosity real-time regulation and control, the problem of impurity regeneration caused by the fact that the aluminum-silicon ratio is difficult to precisely regulate and control in a coal ash hydrothermal melting method is effectively solved; the Cl gas phase deep impurity removal technology decomposes mullite into volatile chlorides under high pressure, and breaks through the acid leaching impurity removal limit; according to the silicon vapor partial pressure control model, generation of alpha-SiC is inhibited by accurately regulating and controlling the concentration of gas-phase SiO, and beta-phase oriented crystallization is achieved.

Description

Technical Field

[0001] This invention relates to the technical field of silicon carbide preparation methods, specifically to a method for preparing electronic-grade silicon carbide using dynamic impurity removal and gas-phase purification of fly ash. Background Technology

[0002] In the field of preparing electronic-grade silicon carbide from fly ash, existing technologies such as CN102583468A and CN119039021A have made some progress in extracting alumina from fly ash and preparing low-expansion structural ceramics, but these technologies have failed to effectively solve the key problems in preparing high-purity electronic-grade silicon carbide.

[0003] Although CN102583468A solved the problems of equipment corrosion and shortened process chain through ammonium sulfate activation, it did not involve the preparation of silicon carbide, nor did it mention how to control the crystal phase and purity of silicon carbide.

[0004] While the low-expansion structural ceramic preparation method disclosed in CN119039021A utilizes raw materials such as coal gangue and secondary aluminum ash, its final product is not silicon carbide, and it does not provide a specific technical solution for extracting and purifying silicon carbide from fly ash.

[0005] The shortcomings of these existing technologies are that they fail to effectively control and remove impurities in fly ash, especially making it difficult to prepare high-purity, highly oriented β-SiC phases. They also fail to effectively utilize carbon sources and control silicon vapor partial pressure to promote the oriented crystallization of β-SiC.

[0006] In addition, there is room for improvement in the existing technology in terms of cost control and environmental friendliness, such as improving the utilization rate of raw materials and the recycling rate of reagents, as well as achieving a closed-loop process with zero wastewater discharge. Summary of the Invention

[0007] In order to overcome the shortcomings of the prior art, the present invention provides a method for preparing electronic-grade silicon carbide by dynamic impurity removal-gas phase purification of fly ash. The purpose is to solve the problems of metal ion toxicity, easy hydrolysis of silicon dioxide, long process and high cost in the preparation of silicon carbide from fly ash.

[0008] A method for preparing electronic-grade silicon carbide using dynamic impurity removal and gas-phase purification of fly ash includes the following steps: (1) Fly ash pretreatment - precise control of aluminum-silicon ratio Fly ash and KOH solution are mixed and added to a reaction vessel, which is then sealed. The temperature is gradually increased to 110-130℃ by heating an oil bath, and the viscosity of the reaction system is monitored in real time. The hydrothermal reaction temperature and time are automatically adjusted through a feedback system to stabilize the Al / Si ratio. (2) Graded acid leaching-gas phase synergistic impurity removal First-stage acid leaching: After the fly ash is melted by potassium hydroxide, it is poured into the graded leaching tank, and a mixed acid solution composed of oxalic acid and citric acid is added at the same time. After stirring evenly, it is heated and kept warm. Secondary enhancement: The mixture is transferred to a high-pressure reactor, where chlorine (Cl2) or hydrogen chloride (HCl) gas is used as the gas-phase transport medium. The reaction is carried out under high temperature and pressure. Chlorine / hydrogen chloride reacts directly with aluminosilicates such as mullite to generate gaseous volatile products such as aluminum chloride (AlCl3), thereby achieving deep impurity removal and ultimately achieving an Al / Si ratio of 2-3; thus obtaining impurity-removed powder. (3) In-situ β-SiC crystal phase induction The impurity-removing powder and biomass carbon source were ball-milled and mixed at a ratio of 1:2 by weight, then placed in a vacuum tube furnace and evacuated to below 10 Pa. First stage: In a mixed atmosphere of N2 and H2, slowly raise the temperature to 1300-1400℃ and hold it at that temperature; Second stage: Injection of dimethyldichlorosilane; The third step: Under a carbon-rich reducing atmosphere of CH4 and O2, the temperature is slowly increased to 1550-1650℃ and held.

[0009] Preferably, in step (1), the fly ash is any one of power plant fly ash, steel plant slag, or iron plant slag.

[0010] Preferably, in step (1), the mass fraction of the KOH solution is 10-15%, and the liquid-solid ratio of the KOH solution to the fly ash is 0.8:1.

[0011] Preferably, in step (2), the oxalic acid concentration in the mixed acid solution is 10-15%, and the citric acid concentration is 5-10%; the liquid-to-solid ratio of the mixed acid solution to fly ash is 4:1.

[0012] Preferably, in step (2), the temperature is kept at 80°C for 2 hours.

[0013] Preferably, in step (2), the reaction is carried out at 240°C and 2.5 MPa for 8 hours.

[0014] Preferably, in step (3), the biomass carbon source is one or more of the following: water lily, coniferous wood, eucalyptus wood, bamboo wood, or rice husk.

[0015] Preferably, in step (3), the volume ratio of N2 to H2 in the first segment is (8-10):1.

[0016] Preferably, in step (3), the first stage involves heating to 1300-1400℃ at a rate of 3-8℃ / min and holding at that temperature for 0.5-2h.

[0017] Preferably, in step (3), the volume ratio of CH4 to O2 in the third segment is (1.5-2.5):1.

[0018] Preferably, in step (3), the third stage involves heating to 1550-1650℃ at a rate of 3-8℃ / min and holding at that temperature for 2-4 hours.

[0019] Compared with the prior art, the present invention has the following beneficial effects: The "dynamic aluminum-silicon ratio feedback system" pioneered in this invention effectively solves the problem of the difficulty in accurately controlling the aluminum-silicon ratio in the hydrothermal melting method of fly ash.

[0020] This invention is the first to propose using rice husk char as a carbon source and employing a two-stage carbothermal reduction and β-SiC crystal phase-induced synthesis process. This eliminates the need for pre-sintering and the introduction of pore-forming agents, overcoming the limitations of the "sandwich structure" proposed by Dobbs-Heiniger, reducing production costs, and expanding the scope of applications.

[0021] This invention employs a dynamic aluminum-silicon ratio feedback system (based on real-time viscosity control) to replace the traditional fixed ratio method, thus solving the problem of impurity regeneration caused by aluminum-silicon ratio imbalance.

[0022] This invention develops a deep purification technology using Cl gas phase, which decomposes mullite into volatile chlorides under high pressure, breaking through the limits of acid leaching purification.

[0023] This invention proposes a silicon vapor partial pressure control model, which suppresses the formation of α-SiC by precisely controlling the concentration of gaseous SiO, thereby achieving β-phase directional crystallization. Attached Figure Description

[0024] Figure 1 A process flow diagram for preparing electronic-grade silicon carbide using dynamic impurity removal-gas phase purification of fly ash; Figure 2 The image shows the Raman spectrum of silicon carbide obtained in Example 1.

[0025] The reference numerals in the attached figures are explained as follows: 1. Fly ash raw material storage silo; 2. KOH pretreatment stirred reactor; 3. Dynamic aluminum-silicon ratio feedback system viscosity monitoring and control instrument; 4. Pretreatment material temporary storage silo; 5. Stirred and graded leaching tank; 6. Oxalic acid / citric acid mixed acid storage tank; 7. Gas phase impurity removal high-pressure reactor; 8. Cl2 / HCl gas cylinder; 9. Filter; 10. Dryer; 11. High-purity silicon source powder storage silo; 12. Ball mill; 13. Biomass carbon source storage silo; 14. First stage carbothermic reduction zone; 15. Second stage silicon vapor partial pressure control zone; 16. Third stage directional crystallization zone; 17. Dimethyldichlorosilane gas cylinder; 18. High-purity β-SiC product collection tank. Detailed Implementation

[0026] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0027] Example 1 A method for preparing electronic-grade silicon carbide using dynamic impurity removal and gas-phase purification of fly ash, the process flow diagram of which is shown below. Figure 1 As shown, the specific steps include: (1) Pre-treatment of fly ash - precise control of aluminum-silicon ratio: Large-particle impurities are removed from the fly ash raw material storage silo (1) of the power plant, and it is mixed with a KOH solution with a mass fraction of 12% at a liquid-solid ratio of 0.8:1 in a KOH pre-treatment stirred reactor (2) to form a slurry. The viscosity change is monitored in real time by a dynamic aluminum-silicon ratio feedback system viscosity monitoring and control instrument (3) (NDJ-1B type digital display viscometer). The reaction temperature is automatically adjusted to 110℃ and the time is adjusted to 30 minutes through the feedback system to keep the viscosity of the system stable, so that the alkaline solution and fly ash react fully and the glass is converted into crystals (size 1-5μm) in large quantities, and a slurry with good fluidity and stable aluminum-silicon ratio is obtained, which is temporarily stored in the pre-treatment material storage silo (4).

[0028] (2) Graded acid leaching-gas phase synergistic impurity removal: Primary acid leaching: The slurry from the temporary storage tank (4) is pumped into the stirring and grading leaching tank (5). At the same time, a mixed acid solution (liquid-solid ratio 4:1) consisting of 12% oxalic acid and 8% citric acid by mass is added from the oxalic acid / citric acid mixed acid storage tank (6). The temperature is controlled at 80°C, and the mixture is stirred for 2 hours to convert the free metal ions in the fly ash into a soluble state.

[0029] Secondary enhancement: The material after primary acid leaching is transferred to a high-pressure gas-phase impurity removal reactor (7). A Cl2 / HCl gas cylinder (8) is connected, and chlorine gas is introduced into the reactor as a gas-phase transport and reaction medium. The reactor is treated at 240℃ and 2.5MPa for 8 hours to remove impurities such as Al by forming volatile AlCl3, and the mullite decomposition rate is greater than 99.5%. After the reaction, the material is washed with deionized water, filtered by a filter (9), dried by a dryer (10), and then ground to obtain high-purity silicon source powder, which is stored in a high-purity silicon source powder storage silo (11).

[0030] (3) In-situ β-SiC crystal phase induction: Weigh 20 grams of high-purity silicon source powder from storage silo (11) and mix it with 40 grams of rice husk char from biomass carbon source storage silo (13) in a ball mill (12) at a weight ratio of 1:2. Place the mixture into an alumina crucible and put it into a vacuum tube furnace with programmable temperature control, and evacuate it to below 10 Pa.

[0031] First stage (carbothermic reduction): Under an atmosphere of N2 / H2 = 9:1 (volume ratio), the temperature is increased to 1350℃ (corresponding to...) at a rate of 5℃ / min. Figure 1 The first section of the central region, carbothermal reduction zone 14, is kept at a temperature of 1 hour for carbothermal reduction.

[0032] Second stage (silicon vapor partial pressure control): Maintain temperature, inject dimethyldichlorosilane gas (corresponding to) from dimethyldichlorosilane gas cylinder (17) through a precision flow system. Figure 1 The second section of the middle region (silicon vapor partial pressure control zone 15) is used to precisely regulate the silicon vapor partial pressure.

[0033] The third stage (directional crystallization): switch the atmosphere to CH4 / O2=2 (volume ratio), and increase the temperature to 1600℃ (corresponding to...) at a rate of 5℃ / min. Figure 1 The third directional crystallization zone (16) in the central region is kept at a constant temperature for 3 hours. Residual impurities are encapsulated through epitaxial growth to form a dense layered structure. After the reaction is completed, the product is cooled under a protective atmosphere, and the resulting high-purity β-SiC product enters the collection tank (18).

[0034] Performance Testing and Analysis To verify the effectiveness of the product prepared in this embodiment, the following tests were performed: (1) X-ray diffraction (XRD); (2) Raman spectroscopy: 532nm laser, 2cm -1 (2) Resolution, 5mW power; (3) Field emission scanning electron microscope (FESEM); (4) X-ray photoelectron spectroscopy (XPS); (5) High temperature nitrogen adsorption-desorption spectrometer (BET); (6) Hall effect spectrometer (THS). The test data of several batches of products prepared in this embodiment are shown in Table 1.

[0035] Figure 2 For the Raman spectrum of the product, at approximately 796 cm⁻¹ -1 and 972cm -1 The characteristic peaks appearing at this location belong to β-SiC, indicating that the product is mainly composed of the β-SiC phase.

[0036] Table 1 Comparative experimental data

[0037] *Note: In Comparative Example 2, due to the presence of a large amount of mullite and other impurity phases, the β-SiC peak intensity in the XRD pattern is relatively weak, and it is accompanied by obvious impurity peaks.* Table 2 Gas phase purification process parameters and effects

[0038] Note: Comparative Example 2 is an example of extending the acid leaching time.

[0039] Example 2 A method for preparing electronic-grade silicon carbide using dynamic impurity removal and gas-phase purification of fly ash specifically includes the following steps: (1) Precise control of fly ash pretreatment and aluminum-silicon ratio Using slag from the steelmaking process as raw material, the slag is first crushed and sieved through a 200-mesh sieve to remove large-particle impurities. 100 grams of the treated slag powder is weighed and mixed with 80 ml of a 10% KOH solution (liquid-to-solid ratio of 0.8:1) in a polytetrafluoroethylene liner, stirring to form a homogeneous slurry. The sealed reactor is placed in a programmable temperature-controlled oil bath, and the dynamic aluminum-silicon ratio feedback system is activated. This system automatically adjusts the reaction parameters by monitoring the viscosity changes of the reaction system in real time (using an NDJ-1B digital viscometer). When the viscosity of the system changes due to silicate dissolution and crystal transformation, the feedback system stabilizes the hydrothermal reaction temperature at 120°C and controls the reaction time to 40 minutes.

[0040] (2) Graded acid leaching-gas phase synergistic impurity removal First-stage acid leaching (chemical leaching): Pour the slurry pretreated in step (1) into a graded leaching tank. At the same time, add 400 ml of a mixed acid solution consisting of 10% oxalic acid and 10% citric acid (i.e., a liquid-to-solid ratio of 4:1). Mechanically stir to ensure that the material is in full contact with the acid solution, and keep the reaction in a constant temperature water bath at 80°C for 2 hours.

[0041] Secondary intensification (deep vapor-phase impurity removal): After primary acid leaching, the material, along with the acid solution, is transferred to a high-pressure reactor equipped with a stirrer. Hydrogen chloride (HCl) gas is introduced into the reactor as a vapor-phase transport and impurity removal medium. The reactor is sealed, and the reaction is carried out for 7 hours under intensified conditions of 230℃ and 2.2MPa. After this step, the Al / Si ratio of the material is precisely reduced to approximately 2.5. After the reaction, the product is filtered, repeatedly washed with deionized water until neutral, then dried in an oven at 110℃ for 12 hours, and finally ground to obtain thoroughly impurity-removed silicon source powder for later use.

[0042] (3) In-situ β-SiC crystal phase induction Weigh 20 grams of the purified powder obtained in step (2) and place it together with 40 grams of bamboo charcoal (made from bamboo carbonization, used as a biomass carbon source, with a weight ratio of 1:2) in a planetary ball mill. Ball mill the mixture at 300 rpm for 2 hours to ensure uniform mixing of the carbon and silicon phases. Load the uniformly mixed powder into an alumina ceramic boat and place it in the quartz tube constant temperature zone of a vacuum tube furnace. Evacuate the furnace to below 10 Pa.

[0043] First stage (carbothermic reduction and pre-reaction): A N2 / H2 mixture with a volume ratio of 8:1 is introduced into the furnace tube. The temperature is programmed to rise to 1400℃ at a heating rate of 3℃ / min and held at that temperature for 0.5 hours.

[0044] The second stage (precise control of silicon vapor partial pressure): Under the condition of maintaining a temperature of 1400℃, gaseous dimethyl dichlorosilane ((CH3)2SiCl2) is injected into the furnace tube through a precision flow control system.

[0045] The third stage (directional crystallization and growth of β-SiC): The atmosphere inside the tube furnace was switched to a CH4 / O2 mixture with a volume ratio of 1.5:1. Subsequently, the temperature was further increased to 1650℃ at a heating rate of 8℃ / min, and held at this high temperature for 2 hours.

[0046] After the reaction was complete, heating was stopped, and the tube furnace was allowed to cool naturally to room temperature under a protective N2 atmosphere. The reaction product was then removed, yielding a grayish-green final product.

[0047] Performance Characterization and Effects The product obtained in Example 2 was tested: X-ray diffraction (XRD) analysis: The spectrum shows strong β-SiC characteristic diffraction peaks, and no obvious characteristic peaks of α-SiC or other impurity crystal phases were observed.

[0048] Chemical analysis: Inductively coupled plasma optical emission spectrometry (ICP-OES) was used to determine that the purity of silicon carbide in the product reached 99.97%, and the content of major metal impurities was extremely low.

[0049] Raman spectroscopy: at approximately 796 cm⁻¹ -1 and 972cm -1 The observation of characteristic scattering peaks of β-SiC further confirmed the crystal phase of the product.

[0050] Example 3 A method for preparing electronic-grade silicon carbide using dynamic impurity removal and gas-phase purification of fly ash specifically includes the following steps: (1) Precise control of fly ash pretreatment and aluminum-silicon ratio Iron ore slag was used as raw material. First, the raw material was crushed, ground, and passed through a 200-mesh sieve to obtain a uniform powder. 100 grams of this slag powder was weighed and thoroughly mixed with 80 ml of a 15% KOH solution (strictly maintaining a liquid-to-solid ratio of 0.8:1) in a polytetrafluoroethylene liner within a reactor to form a slurry. The sealed reactor was placed in a programmable temperature-controlled oil bath, and the dynamic aluminum-silicon ratio feedback system was activated. This system automatically stabilized the hydrothermal reaction temperature at 130°C and controlled the reaction time to 25 minutes by monitoring the viscosity changes of the reaction system in real time.

[0051] (2) Graded acid leaching-gas phase synergistic impurity removal First-stage acid leaching (chemical leaching): Transfer the pretreated slurry to a staged leaching tank. Add 400 ml of a mixed acid solution containing 15% oxalic acid and 5% citric acid (liquid-to-solid ratio maintained at 4:1). React under constant temperature of 80°C with mechanical stirring for 2 hours.

[0052] Secondary Enhancement (Deep Gas-Phase Purification): After primary acid leaching, all materials are transferred to a high-pressure reactor. Chlorine gas (Cl2) is introduced into the reactor as a gas-phase purification medium. The reactor is sealed, and the reaction is carried out for 9 hours under more intense conditions of 250℃ and 2.8MPa. After this deep treatment, the Al / Si ratio of the material is successfully reduced to 2.2. The reaction product is filtered, thoroughly washed with water until neutral, dried at 110℃, and then ground to obtain high-purity, high-activity silicon source powder.

[0053] (3) In-situ β-SiC crystal phase induction Weigh 20 grams of the above-mentioned purified powder and mix it with 40 grams of charcoal obtained from the carbonization of coniferous wood (weight ratio 1:2) by ball milling for 2 hours. Place the mixed powder in an alumina crucible and put it in the constant temperature zone of a vacuum tube furnace. Evacuate to below 10 Pa.

[0054] First stage (carbothermic reduction and pre-reaction): A N2 / H2 mixture with a volume ratio of 10:1 is introduced. The temperature is programmed to rise to 1300℃ at a rate of 8℃ / min and held at this temperature for 2 hours.

[0055] The second stage (precise control of silicon vapor partial pressure): Under the condition of maintaining 1300℃, dimethyldichlorosilane ((CH3)2SiCl2) is injected into the furnace tube through a precision delivery system.

[0056] The third stage (directional crystallization and growth of β-SiC): The atmosphere was switched to a CH4 / O2 mixture with a volume ratio of 2.5:1. Subsequently, the temperature was raised to 1550℃ at a relatively slow heating rate of 3℃ / min and held for 4 hours. After the reaction was completed, the furnace was cooled to room temperature under an inert atmosphere, and the final product was removed.

[0057] Performance Characterization and Effects The product obtained in Example 3 was characterized as follows: X-ray diffraction (XRD) analysis: The diffraction pattern shows that the product is well-crystallized β-SiC, and no diffraction peaks of α-SiC or other impurity phases were detected.

[0058] Chemical analysis: Inductively coupled plasma optical emission spectrometry (ICP-OES) analysis showed that the purity of the silicon carbide product obtained in this example was as high as 99.99%.

[0059] Raman spectroscopy: at approximately 796 cm⁻¹ -1and 972cm -1 Sharp characteristic Raman peaks of β-SiC were observed at the location.

[0060] Comparative Example 1 (without dynamic aluminum-silicon ratio feedback) This comparative example aims to verify the criticality and irreplaceable role of the "dynamic aluminum-silicon ratio feedback system" used in step (1) of this invention in obtaining high-purity, β-phase silicon carbide products. By eliminating this feedback system and using fixed process parameters for pretreatment, the negative impact on subsequent impurity removal and final product performance is compared.

[0061] A method for preparing silicon carbide from fly ash, the steps of which are basically the same as those in Example 1, the only difference being the pretreatment process in step (1): Step (1) of this comparative example: Fixed parameter preprocessing 100 grams of fly ash from the same batch as in Example 1 were mixed with 80 ml of a 12% KOH solution (liquid-to-solid ratio 0.8:1) to form a slurry. A sealed reactor was placed in an oil bath and directly set to a fixed temperature of 110°C for a fixed reaction time of 30 minutes. During this process, the dynamic aluminum-silicon ratio feedback system was not activated, the system viscosity was not monitored in real time, and the reaction temperature and time were not automatically adjusted based on viscosity changes. After the reaction was completed, subsequent steps were performed.

[0062] All subsequent steps, including the graded acid leaching-gas phase synergistic impurity removal in step (2) (primary acid leaching and secondary Cl2 strengthening) and the in-situ β-SiC crystal phase induction in step (3) (three-stage heating and injection of dimethyl dichlorosilane), have all process parameters, reagent dosages and operating conditions that are completely consistent with those in Example 1.

[0063] The final product was analyzed, and the results showed a stark contrast with those of Example 1: Chemical analysis: Inductively coupled plasma optical emission spectrometry (ICP-OES) was used to determine that the purity of silicon carbide in the product was only 99.2%. Analysis of major impurities showed that the content of Al2O3 was significantly higher than that in Example 1.

[0064] X-ray diffraction (XRD) analysis: In the diffraction pattern, in addition to the characteristic peaks of β-SiC, distinct characteristic diffraction peaks of α-SiC (6H or 4H type) appeared. Furthermore, trace amounts of unreacted SiO2 or aluminosilicate impurity peaks may also have been detected.

[0065] Raman spectroscopy: The spectrum shows the characteristic peak of β-SiC (~796 cm⁻¹). -1 ~972cm -1 In addition, at approximately 766cm -1 788cm -1Characteristic peaks belonging to α-SiC appeared at positions such as [position missing], confirming that the product is a mixed phase of β-SiC and α-SiC.

[0066] The results of Comparative Example 1 clearly reveal that: Incomplete impurity removal: Due to the use of fixed parameters in the pretreatment stage, the conditions were not optimized in real time according to the slight fluctuations of the raw materials or the reaction process, resulting in inaccurate and unstable initial control of the aluminum-silicon ratio. This "imbalanced" precursor makes it impossible for subsequent acid leaching and gas-phase chlorination to achieve the best results in impurity removal. Some aluminum impurities are "locked" in the phase and are difficult to remove completely, ultimately leading to a high content of impurities such as Al2O3 in the product.

[0067] Phase control failure: Residual aluminum and other impurities act as catalysts or heterogeneous nucleation sites during the high-temperature carbothermic reduction stage, promoting the formation of more thermodynamically stable α-SiC. This disrupts the induction effect of the "silicon vapor partial pressure control model" on the directional crystallization of β-SiC, resulting in a large number of α-SiC impurity phases in the product.

[0068] System synergy is disrupted: The three core innovations of this invention form an organic whole. Dynamic aluminum-silicon ratio control is the foundation for deep impurity removal and crystal phase control. Comparative Example 1 demonstrates that once this foundation is undermined, even with subsequent advanced vapor-phase impurity removal and silicon vapor partial pressure control, the defects in the preceding steps cannot be compensated, ultimately failing to obtain a high-purity β-SiC product.

[0069] In summary, Comparative Example 1 strongly demonstrates that the "dynamic aluminum-silicon ratio feedback system" is a key component of this invention, and is indispensable for achieving electronic-grade high purity and β-phase directional crystallization.

[0070] Comparative Example 2 (Deep impurity removal in the gas phase without Cl2 / HCl) This comparative example aims to verify the key role of the "Cl2 / HCl gas-phase deep impurity removal technology" in step (2) of this invention. By omitting this step and using only the conventional acid leaching process for comparison, its negative impact on the removal of stubborn impurities such as mullite and the purity of the final product is revealed.

[0071] Experimental methods: A method for preparing silicon carbide from fly ash, the steps of which are basically the same as those in Example 1, the only difference being the impurity removal process in step (2): First-stage acid leaching: The procedure is exactly the same as in Example 1. The pretreated slurry is subjected to acid leaching. The mixed acid used is a solution of 12% oxalic acid and 8% citric acid by mass in a volume ratio of 1:1 (liquid-solid ratio 4:1). The mixture is stirred at 80°C for 2 hours.

[0072] Secondary enhancement was cancelled: the gas-phase impurity removal step of transferring to a high-pressure reactor and introducing Cl2 gas was omitted. In an attempt to enhance impurity removal, after completing the first-stage acid leaching, the leaching time was extended by 4 hours in the same acid solution environment at atmospheric pressure and 80°C (total acid leaching time 6 hours).

[0073] Subsequent processing: The acid-leached material was washed, dried, and ground in the same manner as in Example 1 to obtain impurity-free powder.

[0074] The in-situ β-SiC crystal phase induction in the subsequent step (3) is strictly consistent with the process parameters and conditions of Example 1.

[0075] Results and Analysis: Analysis of the final product showed that its performance was significantly inferior to that of Example 1: Chemical purity: ICP-OES analysis showed that the purity of the SiC product was only 98.5%, which did not meet the requirements for electronic grade. Elemental analysis showed that the Al / Si ratio was as high as 15, indicating that the removal of aluminum impurities was extremely poor.

[0076] Phase analysis: In the XRD diffraction pattern, in addition to the silicon carbide diffraction peak, strong diffraction peaks of aluminum-containing impurity phases such as mullite (3Al2O3·2SiO2) can be clearly observed.

[0077] Morphological observation: SEM showed that the product morphology was uneven and contained unreacted inclusions or impurity phases.

[0078] in conclusion: This comparative example clearly reveals: Acid leaching for impurity removal has its limitations: mixed acids are effective against free metallic impurities, but they are insufficient to destroy stable crystal structures such as mullite. Simply extending the acid leaching time cannot solve this fundamental problem.

[0079] The irreplaceable gas-phase impurity removal technology: The Cl2 / HCl gas-phase impurity removal technology used in this invention directly reacts the gas with mullite under high temperature and pressure to generate volatile AlCl3, achieving "gas-phase transport" and deep removal of impurities. This process effectively overcomes the mass transfer resistance of solid-liquid reaction and is the core of breaking through the acid leaching limit and reducing the Al / Si ratio to the ideal range (2-3).

[0080] Decisive impact on the product: Residual aluminum impurities not only directly reduce the chemical purity of the product, but their high melting point also interferes with the uniform growth of SiC crystals during subsequent high-temperature processes, affecting the final product performance.

[0081] In summary, Comparative Example 2 strongly demonstrates that the "Cl2 / HCl gas-phase deep impurity removal technology" is a key innovative step in the preparation of electronic-grade β-SiC from fly ash, and its effect is far superior to that of conventional acid leaching processes.

[0082] Comparative Example 3 (Changing Silicon Vapor Conditions) This comparative example aims to verify the key role of the "silicon vapor partial pressure control model" in step (3) of the present invention in achieving directional crystallization of β-SiC. By changing the gaseous silicon source (dimethyldichlorosilane) to a conventional solid-phase silicon powder mixing method, its negative impact on the crystal phase of the product was examined.

[0083] Experimental methods: A method for preparing silicon carbide from fly ash, the steps of which are basically the same as those in Example 1, the core difference being the silicon source introduction method in step (3): Change the silicon source and mixing method: In the ball milling mixing stage, in addition to weighing 20 grams of impurity-removing powder and 40 grams of rice husk charcoal, add micron-sized silicon powder with a molar amount of silicon provided by dimethyldichlorosilane. Ball mill the silicon powder, carbon source, and impurity-removing powder together to ensure uniform mixing.

[0084] Cancel the gas phase injection step: During subsequent heating, cancel the "Second stage: injection of dimethyldichlorosilane" operation.

[0085] Heating process: First step: Under an atmosphere of N2 / H2=9:1, the temperature is increased to 1350℃ at a rate of 5℃ / min and held for 1 hour (same as Example 1).

[0086] (The second paragraph is omitted) Third stage: Directly in a CH4 / O2=2 atmosphere, the temperature is increased to 1600℃ at 5℃ / min and reacted for 3 hours (same as Example 1).

[0087] Except for the changes mentioned above, all parameters and conditions remain the same as in Example 1.

[0088] Results and Analysis: The performance of the final product differs significantly from that of Example 1: Phase composition: XRD patterns show a large number of strong diffraction peaks of α-SiC (6H / 4H type). Spectrum fitting indicates that the β-SiC content is less than 60%, and the α phase becomes the main crystalline phase.

[0089] Chemical purity: The product purity decreased to 99.0%. The decrease in purity may be related to the introduction of impurities and disordered crystal structure by the solid-phase silicon powder.

[0090] Product morphology: The product agglomerates severely after the reaction and requires strong crushing. SEM observation shows that there are a large number of non-cubic crystal morphologies such as plates and rods, which are consistent with the characteristics of α-SiC.

[0091] Raman spectrum: at approximately 766 cm⁻¹ -1 788cm -1 966cm -1α-SiC characteristic peaks were detected in various locations, while β-SiC characteristic peaks were weaker, confirming that the hexagonal polymorph (α-SiC) is dominant.

[0092] in conclusion: This comparative example clearly reveals: The decisive factor in controlling the partial pressure of silicon vapor: Solid-phase silicon doping methods struggle to achieve uniform and precise control of the concentration of gaseous silicon species (such as SiO) within the reaction zone, easily leading to localized silicon concentration imbalances and creating conditions conducive to the thermodynamically more stable nucleation and growth of α-SiC. This invention, through the instantaneous decomposition of dimethyldichlorosilane, creates a uniform and controllable silicon vapor environment, thereby strongly promoting the uniform nucleation and directional growth of metastable β-SiC.

[0093] Advantages of gas-phase reactions: Gas-phase transport enables reactants to mix thoroughly at the molecular / atomic level, resulting in a uniform reaction. Solid-phase mixing, on the other hand, introduces microscopic inhomogeneities, which can easily lead to local deviations in the stoichiometry, causing the formation of impurity phases and product agglomeration.

[0094] The technology is irreplaceable: simply providing an equal amount of silicon cannot achieve the goal of crystal phase control. Only by precisely controlling the silicon partial pressure through a gas-phase approach can the limitation of the traditional carbothermal reduction method, which easily generates the α phase, be overcome, and the directional crystallization of β-SiC be achieved.

Claims

1. A method for preparing electronic-grade silicon carbide using dynamic impurity removal and gas-phase purification of fly ash, characterized in that, Includes the following steps: (1) Fly ash pretreatment - precise control of aluminum-silicon ratio Fly ash and KOH solution are mixed and added to a reaction vessel, which is then sealed. The temperature is gradually increased to 110-130℃ by heating an oil bath, and the viscosity of the reaction system is monitored in real time. The hydrothermal reaction temperature and time are automatically adjusted through a feedback system to stabilize the Al / Si ratio. (2) Graded acid leaching-gas phase synergistic impurity removal First-stage acid leaching: After the fly ash is melted by potassium hydroxide, it is poured into the graded leaching tank, and a mixed acid solution composed of oxalic acid and citric acid is added at the same time. After stirring evenly, it is heated and kept warm. Secondary intensification: The mixture is transferred to a high-pressure reactor, where chlorine or hydrogen chloride gas is used as the gas-phase transport medium. The reaction is carried out under high temperature and pressure, causing Al to form volatile AlCl3. The oxygen partial pressure in the system is continuously removed to decompose the mullite, and finally the Al / Si ratio reaches 2-3, resulting in purified powder. (3) In-situ β-SiC crystal phase induction The impurity-removing powder and biomass carbon source were ball-milled and mixed at a ratio of 1:2 by weight, then placed in a vacuum tube furnace and evacuated to below 10 Pa. First stage: In a mixed atmosphere of N2 and H2, slowly raise the temperature to 1300-1400℃ and hold it at that temperature; Second stage: Injection of dimethyldichlorosilane; The third step: Under a carbon-rich reducing atmosphere of CH4 and O2, the temperature is slowly increased to 1550-1650℃ and held.

2. The method for preparing electronic-grade silicon carbide by dynamic impurity removal-gas phase purification of fly ash according to claim 1, characterized in that, In step (1), the fly ash is any one of power plant fly ash, steel plant slag or iron plant slag; preferably, in step (1), the mass fraction of KOH solution is 10-15%, and the liquid-solid ratio of KOH solution to fly ash is 0.8 mL: 1 g.

3. The method for preparing electronic-grade silicon carbide by dynamic impurity removal-gas phase purification of fly ash according to claim 1, characterized in that, In step (2), the oxalic acid concentration in the mixed acid solution is 10-15%, and the citric acid concentration is 5-10%; the liquid-solid ratio of the mixed acid solution to fly ash is 4:

1.

4. The method for preparing electronic-grade silicon carbide by dynamic impurity removal-gas phase purification of fly ash according to claim 1, characterized in that, In step (2), the temperature is kept at 80℃ for 2 hours.

5. The method for preparing electronic-grade silicon carbide by dynamic impurity removal-gas phase purification of fly ash according to claim 1, characterized in that, In step (2), the reaction is carried out at 240℃ and 2.5MPa for 8 hours.

6. The method for preparing electronic-grade silicon carbide by dynamic impurity removal-gas phase purification of fly ash according to claim 1, characterized in that, In step (3), the biomass carbon source is one or more of the following: water lily, coniferous wood, eucalyptus wood, bamboo wood, or rice husk.

7. The method for preparing electronic-grade silicon carbide by dynamic impurity removal-gas phase purification of fly ash according to claim 1, characterized in that, In step (3), the volume ratio of N2 to H2 in the first segment is (8-10):

1.

8. The method for preparing electronic-grade silicon carbide by dynamic impurity removal-gas phase purification of fly ash according to claim 1, characterized in that, In step (3), the first stage involves heating to 1300-1400℃ at a rate of 3-8℃ / min and holding at that temperature for 0.5-2h.

9. The method for preparing electronic-grade silicon carbide by dynamic impurity removal-gas phase purification of fly ash according to claim 1, characterized in that, In step (3), the volume ratio of CH4 to O2 in the third segment is (1.5-2.5):

1.

10. The method for preparing electronic-grade silicon carbide by dynamic impurity removal-gas phase purification of fly ash according to claim 1, characterized in that, In step (3), the third stage involves heating the temperature to 1550-1650℃ at a rate of 3-8℃ / min and holding it at that temperature for 2-4 hours.

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