Method for recycling and purifying silicon carbide furnace core waste graphite

Abstract

The invention provides a method for recycling and purifying silicon carbide furnace core waste graphite, and belongs to the technical field of inorganic non-metallic material recovery. The method comprises the following steps: firstly, respectively carrying out grading pretreatment on first waste graphite and second waste graphite, and carrying out winnowing purification to increase the carbon content; and then mixing the first waste graphite and the second waste graphite, and carrying out two-stage gas-solid purification, namely introducing mixed gas into a vacuum furnace, directionally removing metal impurities at 1600-2600 DEG C, continuously heating to 3100 DEG C under the protection of inert gas, deeply removing impurities such as boron, vanadium and the like, and finally obtaining the 6N-grade ultra-high-purity graphite. The gas-solid reaction replaces an acid pickling process, zero wastewater discharge is achieved, the purification efficiency can be improved, the total amount of product impurities is smaller than 1 ppm, and compared with a chemical vapor deposition method, energy consumption can be greatly reduced, and the semiconductor element supply requirement can be met.

Description

Technical Field

[0001] This invention belongs to the field of inorganic non-metallic material recycling and high purification technology, and relates to a method for the reuse and purification of waste graphite from silicon carbide furnace cores. Background Technology

[0002] With the booming development of high-tech industries, especially the rapid rise of industries such as semiconductors, photovoltaics and new energy, the demand for ultra-high purity graphite (carbon content ≥99.99%) has increased dramatically. Due to its unique physical and chemical properties, ultra-high purity graphite plays an irreplaceable role in modern high-tech fields.

[0003] Third-generation semiconductor wafer manufacturing uses silicon carbide (SiC) as the core substrate material. The high-purity silicon carbide powder required for its synthesis must be prepared by reacting ultra-high-purity carbon powder (≥99.999%) with high-purity silicon powder. The content of boron (B), a key impurity in the carbon powder, must be strictly controlled to <0.1 ppm to avoid device leakage current problems caused by lattice defects. However, existing processes have the following significant drawbacks: ① Incompatibility in impurity removal: Silicates (SiO2-Al2O3) and metal oxides (FeO) X Due to the large differences in physicochemical properties, traditional processes are difficult to remove them simultaneously and efficiently, resulting in a total impurity content >1ppm, which cannot meet the requirements for semiconductor-grade graphite (impurities <0.2ppm). ② Limitations due to the refractory nature of boron / vanadium: The BC bond energy is 536 kJ / mol, resulting in low fracture efficiency at conventional pyrolysis temperatures (<2800℃), with boron residue >0.1 ppm, directly affecting the electrical properties of silicon carbide single crystals; the traditional acid pickling method uses a mixed solution of hydrofluoric acid and nitric acid for immersion treatment, which can partially remove metallic impurities, but the boron residue is >0.5 ppm, and it generates highly corrosive wastewater (pH <1), with a treatment cost exceeding 3000 yuan per ton; ③ Poor environmental and economic efficiency: The cost of treating wastewater from pickling is >3,000 yuan / ton, and the energy consumption of chemical vapor deposition (CVD) is >15,000 kWh / ton. At the same time, CVD relies on high-temperature etching with chlorine / Freon. Although it can reduce the boron content to 0.1~0.3 ppm, the process energy consumption is as high as 15,500 kWh / ton, and there is a risk of leakage of highly toxic gases, which seriously restricts its large-scale application. ④ Resource waste: Existing technologies have a carbon phase recovery rate of less than 70% in waste graphite, resulting in landfill losses of 80,000 to 120,000 yuan per ton. Furthermore, the carbon purity after purification is only 4N grade (99.99%), which cannot be directly used for the preparation of 6N grade semiconductor toner.

[0004] Therefore, this invention aims to solve the problems of uncoordinated impurity removal, high residual refractory impurities, significant environmental pollution, high energy consumption, and resource waste by using physical pretreatment and a two-stage gas-solid high-temperature reaction purification process, thereby achieving efficient, green, and high-value recycling of waste graphite. Summary of the Invention

[0005] The purpose of this invention is to address the problems existing in the prior art by providing a method for the reuse and purification of waste graphite from silicon carbide furnace cores. This method can solve the problems of uncoordinated impurity removal, high residual refractory impurities, high energy consumption, and resource waste in existing carbon powder purification methods.

[0006] Therefore, the present invention adopts the following technical solution: A method for the reuse and purification of waste graphite from silicon carbide furnace cores, comprising the following steps: Step 1, Graded Pretreatment: Obtain the first and second waste graphite generated from the smelting of abrasive-grade silicon carbide; Specifically, the first type of waste graphite is furnace core waste graphite generated during the smelting of abrasive-grade green silicon carbide, with a carbon content ≥90%, and the second type of waste graphite is furnace core waste graphite generated during the smelting of abrasive-grade black silicon carbide, with a carbon content ≥85%. The first and second waste graphite were subjected to primary processing to obtain primary first graphite powder and primary second graphite powder with a particle size of 23~45μm. Specifically, the primary processing includes coarse screening, crushing, and fine screening performed sequentially; coarse screening refers to removing particles with a size > 5 mm; crushing refers to crushing the material after coarse screening to a particle size ≤ 2 mm; and fine screening refers to separating the crushed graphite powder with a particle size of 23~45 μm.

[0007] The primary graphite powder and the secondary graphite powder were subjected to two-stage processing to obtain secondary primary graphite powder with a carbon content ≥99.2% and secondary secondary graphite powder with a carbon content ≥99%. Then, the secondary primary graphite powder and the secondary secondary graphite powder were mixed at a mass ratio of 1~2:1 to obtain graphite to be purified with a carbon content ≥99.2% and a particle size of 23~45μm.

[0008] Specifically, the secondary treatment involves using a cyclone classifier to remove impurities with a density >2.2 g / cm³ from the primary graphite powder; the operating parameters of the cyclone classifier are set as follows: wind speed 8~12 m / s, classifier wheel speed 250~400 rpm.

[0009] Step 2: Two-stage gas-solid purification: First stage of gas-solid purification: Place the graphite to be purified in a vacuum furnace and evacuate to an initial vacuum level ≤10. -2Pa, a mixture of CHClF2 and helium with a molar ratio of 1:1~5 is introduced at a flow rate of 8~15 m³ / h, and the mixture is kept at 1600~2600℃ for 30~180 minutes to obtain primary purified graphite with a carbon content ≥99.99%. The second stage of gas-solid purification involves introducing a protective gas, which is at least one of argon or helium with a purity ≥99.999% into the vacuum furnace; heating to 3100℃ at a rate of 10~20℃ / min while maintaining a vacuum degree ≤10. -3 Pa is then kept at a constant temperature for 20-40 minutes; then, the temperature is rapidly reduced to 2900-3000℃ at a rate greater than 15℃ / min, and kept at 2900-3000℃ for 60-180 minutes while maintaining an inert gas atmosphere, to obtain secondary purified graphite with a carbon content ≥99.9999%.

[0010] In the first stage of gas-solid purification, the molar ratio of CHClF2 to helium is adjusted according to the type of impurities in the raw material. Specifically, a molar ratio of 1:1 for CHClF2 to helium is suitable for raw materials with high silicate content, while a molar ratio of 1:5 for raw materials with metal oxides as the main component is suitable.

[0011] In the second stage of gas-solid purification, rapid cooling of the furnace temperature at a rate greater than 15℃ / min is a key element of the purification process. This significantly improves the removal efficiency of refractory impurities such as boron and vanadium by inhibiting the re-adsorption of impurities at the graphite grain boundaries and enhancing diffusion kinetics through temperature gradient. If the cooling rate is lower than 15℃ / min, the impurity removal effect will decrease significantly.

[0012] The criticality of the quenching rate is verified through gradient cooling experiments. The data are shown in the table below (under the same raw materials and process conditions): Table 1 As shown in Table 1, when the cooling rate is ≤14℃ / min, due to insufficient diffusion kinetics and intensified impurity re-adsorption, the boron residue is ≥0.3ppm, which cannot meet the 6N grade standard (boron ≤0.01ppm). When the cooling rate is >15℃ / min, the diffusion-dominated deboring pathway is activated, the boron content is ≤0.015ppm and the total impurities are <1ppm. Therefore, a quenching rate strictly >15℃ / min is a necessary condition for achieving deep deboring (≤0.015ppm).

[0013] The beneficial effects of this invention are as follows: This invention achieves a carbon phase purity of ≥99.9999% (6N grade) in the purified waste graphite through raw material pretreatment and a two-stage high-temperature gas-solid reaction purification process, especially through dynamic temperature field control, namely ultra-high temperature activation, heat preservation diffusion, and rapid cooling at a rate greater than 15℃ / min. Simultaneously, it eliminates process wastewater discharge and significantly reduces energy consumption compared to traditional vapor deposition methods. Specifically: 1. Deep removal of impurities: This invention can achieve boron content ≤0.01ppm and total impurity content <1ppm, while simultaneously and efficiently removing silicates and metal oxides; 2. High-temperature kinetic optimization: This invention uses ultra-high temperature combined with a dynamic temperature field, namely rapid heating, high-temperature sustained effect, rapid cooling activation, and medium-temperature stability, which effectively promotes the diffusion and volatilization of refractory impurities such as boron and vanadium, and significantly shortens the purification time. 3. Green and low-carbon: The purification process of this invention is completely acid-free, with zero wastewater discharge. The exhaust gas is treated efficiently to meet emission standards (fluoride ≤1mg / m³, chloride ≤5mg / m³), and the energy consumption is ≤9800kWh / ton, which is >36% lower than the traditional chemical vapor deposition method (>15000kWh / ton). 4. Highly efficient resource utilization: The carbon recovery rate in this invention is ≥98.5%, significantly higher than that of traditional processes (<70%), maximizing resource value; 5. Semiconductor compatibility: The product obtained by this invention has a purity of 6N grade and a boron content of ≤0.01ppm, which can meet the stringent requirements of semiconductor-grade silicon carbide synthesis for toner. 7. Key process innovation: The rapid cooling step of the present invention at a rate greater than 15℃ / min is a necessary condition for achieving deep removal of refractory impurities such as boron and vanadium. By generating micro-defects to maintain the volatilization channel, the problem of low boron removal efficiency in traditional high-temperature methods is solved. Attached Figure Description

[0014] Figure 1 This is a screenshot of the test report for Example 1; Figure 2 This is a screenshot of the test report for Example 2; Figure 3 This is a screenshot of the test report for Example 3; Figure 4 This is a screenshot of the test report for Example 4. Detailed Implementation

[0015] The technical solution of the present invention will be described in detail below with reference to Examples 1 to 4.

[0016] Examples 1 and 2 both adopted the purification process of the present invention, Example 3 served as the slow cooling control group, and Example 4 served as the low-speed cooling group.

[0017] The raw materials for Examples 1-4 all originated from waste graphite from the furnace core of an Atchison resistance furnace used for smelting abrasive-grade silicon carbide. For ease of description, the waste graphite obtained from the preparation of green silicon carbide will be referred to as first waste graphite, and the waste graphite obtained from the preparation of black silicon carbide will be referred to as second silicon waste graphite. Specifically, the first waste graphite has an initial carbon content ≥90%, and its main impurities are composed of SiO2-Al2O3, with the impurities exhibiting silicate inclusions and a dense structure. The second waste graphite has an initial carbon content ≥85%, and its main impurities are Al2O3-FeO. x The composition and impurity morphology are characterized by dispersed metal oxides with high surface activity.

[0018] The purification equipment used in Examples 1-4 is a high-temperature vacuum induction furnace with a maximum temperature of 3200℃, an operating temperature of ≤3100℃, a furnace pressure of 0.1MPa, and is equipped with a high-purity inert gas circulation system, with an ultimate vacuum degree of ≤5×10⁻⁶. -4 Pa.

[0019] Example 1 Step 1, Graded Pretreatment: First waste graphite with a carbon content ≥92.3% and second waste graphite with a carbon content ≥87.5% were subjected to primary and secondary treatments, respectively. Secondary first graphite powder and secondary second graphite powder in a mass ratio of 1:1 were fed into an LDH-3000 ribbon mixer with a speed of 10-60 r / min. The mixture was mixed for 120 minutes in a three-stage mixing mode of premixing-main mixing-homogenization to obtain graphite to be purified with a carbon content of 99.3% and a particle size of 23~45μm.

[0020] Step 2: Two-stage gas-solid purification: The first stage of gas-solid purification includes: Place the graphite to be purified in a vacuum of 1×10⁻⁶. -3 In a vacuum furnace of Pa, CHClF2 and helium with a molar ratio of 1:5 are introduced as a mixed gas with a flow rate of 10 m³ / h. The temperature is increased to 1600℃ at 5℃ / min and held for 30 min, then increased to 2600℃ at 8℃ / min and held for 90 min to obtain primary purified graphite with a carbon content ≥99.9903%.

[0021] The second stage of gas-solid purification includes: Argon gas with a flow rate of 5 L / min and a purity of ≥99.999% was continuously introduced into the ultra-high temperature vacuum induction furnace as a protective gas; the temperature was increased from 2600℃ to 3100℃ at a rate of 15℃ / min and held for 30 min, and then rapidly cooled to 2950℃ at a rate of 18℃ / min and held for 120 min to obtain secondary purified graphite.

[0022] The properties of the graphite product obtained from the secondary purification in this embodiment are shown in Table 2, and were measured by GDMS glow discharge mass spectrometry. The testing unit was Suzhou Bofike Analytical Technology Service Co., Ltd., and the test report is as follows. Figure 1 As shown.

[0023] Table 2 This embodiment verifies the purification effect of mixing first and second waste graphite at a 1:1 mass ratio. In this embodiment, wastewater discharge adopts a completely acid-free washing process, and waste gas is treated by a two-stage catalytic oxidation process. Fluoride emission concentration is ≤1 mg / m³, chloride ≤5 mg / m³, and emissions comply with GB16297-1996 standards. The overall power consumption is 9200 kWh / ton, lower than the traditional process's 15500 kWh / ton, and the carbon recovery rate is ≥98.5%, higher than the traditional process's ≤70%. Specifically, the two-stage catalytic oxidation process refers to using a CuO-MnO2 catalyst to treat the waste gas at 300℃.

[0024] Example 2 Step 1, Graded Pretreatment: First waste graphite with a carbon content ≥92% and second waste graphite with a carbon content ≥86.5% were subjected to primary and secondary treatments, respectively. Secondary first waste graphite and secondary second waste graphite in a mass ratio of 2:1 were fed into an LDH-3000 ribbon mixer with a speed of 10-60 r / min and mixed for 160 minutes according to a three-stage mixing mode of premixing-main mixing-homogenization to obtain graphite to be purified with a carbon content of 99.3% and a particle size of 23~45μm.

[0025] Step 2: Two-stage gas-solid purification: The first stage of gas-solid purification includes: Place the graphite to be purified in a vacuum of 1×10⁻⁶. -3 In a vacuum furnace of Pa, CHClF2 and helium in a molar ratio of 1:5 were introduced as a mixed gas at a flow rate of 15 m³ / h. Then, the temperature was increased to 2000℃ at a rate of 5℃ / min and held for 60 min. Next, the temperature was increased to 2600℃ at a rate of 8℃ / min and held for 180 min to obtain primary purified graphite with a carbon content ≥99.9959%, ​​a boron residue of 0.8 ppm, and a femite residue of 0.5 ppm.

[0026] The second stage of gas-solid purification includes: Argon gas with a flow rate of 5 L / min and a purity of ≥99.999% was continuously introduced into the ultra-high temperature graphitization furnace as a protective gas; the temperature was increased from 2600℃ to 3100℃ at a rate of 15℃ / min and held for 30 min; then it was rapidly cooled to 2950℃ at a rate of 20℃ / min and held for 120 min to obtain secondary purified graphite.

[0027] The properties of the graphite product obtained from the secondary purification in this embodiment are shown in Table 3, and were measured by GDMS glow discharge mass spectrometry. The testing unit was Eurotherm Evans Materials Technology (Shanghai) Co., Ltd., and the test report is as follows. Figure 2 As shown.

[0028] Table 3 This embodiment verifies the purification effect of mixing first and second waste graphite at a mass ratio of 2:1. In this embodiment, wastewater discharge is zero, and waste gas treatment employs catalytic oxidation and molecular sieve adsorption, achieving a fluoride capture rate >99.99%, an emission concentration ≤0.5mg / m³, and compliance with GB16297 emission standards. The overall power consumption is 9800kWh / ton, lower than the traditional process's 15500kWh / ton, and the carbon recovery rate is ≥99%, higher than the traditional process's ≤70%.

[0029] Example 3 Except for the second gas-solid purification step, the remaining steps in this embodiment are completely consistent with those in Embodiment 2.

[0030] The second stage of gas-solid purification in this embodiment includes: Argon gas with a flow rate of 5 L / min and a purity of ≥99.999% was continuously introduced into the ultra-high temperature graphitization furnace as a protective gas; the temperature was increased from 2600℃ to 3100℃ at a rate of 15℃ / min and held for 30 min; then it was slowly cooled to 2950℃ at a rate of 5℃ / min (rate <15℃ / min) and held for 120 min to obtain secondary purified graphite.

[0031] The performance test results of the graphite product after secondary purification in this embodiment are shown in Table 4. These results were obtained by Eurotherm Evans Materials Technology (Shanghai) Co., Ltd. using glow discharge mass spectrometry (GDMS). The test report is as follows: Figure 3 As shown. Table 4 As shown in Table 4, the slow cooling method used in the second gas-solid purification step of this embodiment resulted in the total carbon content (99.99972%) of the secondary purified graphite being slightly lower than the 6N grade standard, the boron content (<0.02ppm) exceeding the standard, and the total impurity content (2.71ppm) being significantly higher than that of Example 2 (0.77ppm) and the standard requirement (≤1ppm).

[0032] Example 4 Except for the second gas-solid purification step, the remaining steps in this embodiment are completely consistent with those in Embodiment 2.

[0033] The second stage of gas-solid purification in this embodiment includes: Argon gas with a flow rate of 5 L / min and a purity of ≥99.999% was introduced into the ultra-high temperature graphitization furnace for protection; the temperature was increased from 2600℃ to 3100℃ at a rate of 15℃ / min and held for 30 min; then cooled to 2950℃ at a rate of 14℃ / min and held for 120 min to obtain secondary purified graphite.

[0034] The performance test results of the graphite product after secondary purification in this embodiment are shown in Table 5. These results were obtained by Eurotherm Evans Materials Technology (Shanghai) Co., Ltd. using glow discharge mass spectrometry (GDMS). The test report is as follows: Figure 4 As shown. Table 5 As shown in Table 5, when the cooling rate is 14℃ / min in this embodiment, the total carbon content in the product drops to 99.99959%, ​​which does not meet the 6N grade standard. The boron content is 0.3ppm, which exceeds the standard by 30 times. The total impurities are 4.03ppm, which is 4 times the standard value. Therefore, when the cooling rate is lower than 15℃ / min, it is impossible to achieve deep removal of difficult-to-remove impurities such as boron and vanadium, and the total carbon purity decreases significantly.

[0035] In summary, regarding the boron content and total impurity content of graphite after secondary purification, the examples without quenching processes showed higher results than those with quenching processes. Specifically, a quenching rate greater than 15°C / min in the second-stage gas-solid purification is a crucial condition for achieving deep boron removal (≤0.01ppm) and meeting the total impurity standard (<1ppm). When the cooling rate is lower than 15°C / min (such as 5°C / min in Example 3 and 14°C / min in Example 4), both the boron content and total impurity content significantly exceed the standard, and the product purity cannot meet the requirements for semiconductor applications. This conclusion also applies to other high-boron waste graphite purification processes.

Claims

1. A method for the reuse and purification of waste graphite from silicon carbide furnace cores, characterized in that, Includes the following steps: Step 1, Graded Pretreatment: The first and second waste graphite generated by the electric resistance furnace are obtained and subjected to primary processing to obtain primary first graphite powder and primary second graphite powder with a particle size of ≤45μm. The first waste graphite has a carbon content of ≥90%, and the second waste graphite has a carbon content of ≥85%. The first graphite powder and the second graphite powder are subjected to two-stage processing to obtain the second first graphite powder and the second second graphite powder. Mix the first secondary graphite powder and the second secondary graphite powder at a mass ratio of 1~2:1 to obtain graphite to be purified with a carbon content ≥99.2% and a particle size of 23~45μm; Step 2: Two-stage gas-solid purification: The first stage of gas-solid purification includes: placing the graphite to be purified in a vacuum furnace, with an initial vacuum degree ≤10. -2 A mixture of CHClF2 and helium with a molar ratio of 1:1~5 is introduced at a flow rate of 8~15 m³ / h, and the mixture is kept at 1600~2600℃ for 30~180 minutes to obtain primary purified graphite; the primary purified graphite has a carbon content ≥99.99%; The second stage of gas-solid purification includes: introducing a protective gas into the vacuum furnace at a flow rate of 5-10 L / min, heating to 3100℃ at a rate of 10-20℃ / min, and maintaining a vacuum level of ≤10. -3 Pa, keep at a constant temperature for 20-40 minutes, then rapidly cool to 2900-3000℃ at a rate of >15℃ / min, and keep at 2900-3000℃ for 60-180 minutes while maintaining an inert gas atmosphere to obtain secondary purified graphite with a carbon content ≥99.9999%.

2. The method according to claim 1, characterized in that, The primary processing in step 1 includes: sequentially performing coarse screening, crushing, and... Fine sieving is used to obtain primary graphite powder with a particle size ≤45μm; wherein: The coarse screen is used to remove first and second waste graphite particles with a particle size >5mm. Crushing is used to crush the coarse screening product to a particle size ≤2mm. The coarse screening step also includes magnetic separation to remove iron. Fine sieves are used to separate the first and second waste graphite particles with a size of 23~45μm from the crushed products.

3. The method for recycling and purifying waste graphite from silicon carbide furnace cores according to claim 1, characterized in that, The secondary processing in step 1 includes: using a cyclone classifier to remove impurities with a density > 2.2 g / cm³ from the first and second graphite powders, respectively.

4. The method for recycling and purifying waste graphite from silicon carbide furnace cores according to claim 1, characterized in that, The protective gas is at least one of argon or helium, and the purity of the protective gas is ≥99.999%.

5. The method for recycling and purifying waste graphite from silicon carbide furnace cores according to claim 1, characterized in that, The boron content of the secondary purified graphite is ≤0.01ppm.

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