Method for preparing 6H-SiC powder by treating photovoltaic silicon sludge in variable atmosphere

Abstract

The invention discloses a method for preparing 6H-SiC powder through variable atmosphere treatment of photovoltaic silicon sludge, and belongs to the technical field of high-value utilization of photovoltaic silicon sludge. A photovoltaic silicon sludge filter cake is subjected to weak acid dynamic washing and impurity removal, pulping, batching, homogenization and granulation to prepare secondary granules, a low-temperature section in a sintering furnace is controlled to be in an oxidizing atmosphere, so that organic matter in silicon sludge and an organic binder added for granulation are fully decomposed and escaped, after vacuumizing is conducted to 0.1 MPa or below, argon is introduced, the oxygen concentration in a hearth is controlled to be 0.05 vol. Vacuumizing is conducted again till the pressure is 10 <-2 > Pa or below, the temperature is further increased to 1450-1600 DEG C, a reaction is conducted for 1-8 h, finally, the temperature is increased to 2100-2500 DEG C, crystal phase transformation is completed, and finally the high-quality 6H-SiC powder is obtained through cooling. According to the method, the photovoltaic silicon sludge can be effectively utilized to prepare the 6H-SiC powder with excellent performance, and the problems of resource utilization of the photovoltaic silicon sludge and high preparation cost and low quality of the traditional 6H-SiC powder are solved.

Description

Technical Field

[0001] This invention belongs to the field of high-value utilization technology of photovoltaic silicon mud, specifically involving a method for preparing 6H-SiC powder by treating photovoltaic silicon mud with a variable atmosphere. Background Technology

[0002] With the rapid development of the photovoltaic industry, a large amount of silicon is lost during the diamond wire cutting of silicon wafers. This lost silicon exists in the form of cutting waste slurry containing silicon chips, which forms photovoltaic silicon sludge after pressure filtration. Photovoltaic silicon sludge is mainly composed of elemental silicon, water-based cutting fluid, and trace amounts of metallic and non-metallic impurities. Due to its complex composition and the difficulty in removing the various impurity elements it contains, it is currently largely unutilized. At present, the resource utilization of this silicon sludge mainly focuses on the preparation of metallurgical-grade silicon, ferrosilicon alloys, silicon carbide, and silicon-carbon anode materials.

[0003] Silicon carbide (SiC) possesses excellent properties such as high hardness, high thermal conductivity, low coefficient of thermal expansion, high temperature resistance, and corrosion resistance, making it widely used in aerospace, machinery manufacturing, and power electronics. SiC exists in various crystal forms, with α-SiC exhibiting a hexagonal or rhombohedral structure and containing over 100 polymorphs including 2H, 4H, 15R, and 6H. 6H-SiC belongs to the hexagonal crystal system with a band gap of 3.02 eV, and is crucial in high-voltage power devices and high-temperature, high-radiation environments, such as aerospace, nuclear power equipment, and thermal sensors, demonstrating excellent thermal stability and radiation resistance. Currently, the mainstream industrial method for producing α-SiC is the Acheson process. This involves mixing quartz sand, coke, a small amount of sawdust, and sodium chloride, then heating the mixture in a resistance furnace to 2600-2700 °C to react and produce SiC and CO. CO gas burns on the furnace surface to generate CO2, while a small portion of unburned CO disperses into the atmosphere, polluting the air. After the reaction, the SiC furnace charge is graded and sorted, then crushed to obtain granules. These granules are then subjected to acid-base washing, water washing, and magnetic separation to remove impurities and improve purity. Finally, the α-SiC product is dried and screened. This method has poor environmental impact, a long process, and high energy consumption. Furthermore, the yield of 6H-SiC in the α-SiC product is low, and it contains impurities such as 4H-SiC, which cannot meet the requirements of the semiconductor industry. Other methods, such as chemical vapor deposition (CVD), can produce high-purity silicon carbide, but they have high equipment and operational requirements and are costly. Laser synthesis and electron beam evaporation also have limitations, either requiring demanding equipment or having a narrow range of applications.

[0004] Utilizing photovoltaic silicon sludge to prepare 6H-SiC can solve the problem of high-value disposal of photovoltaic silicon sludge and reduce environmental pollution. It also opens up a new raw material route for 6H-SiC preparation, reducing production costs and yielding significant environmental and economic benefits. Patent application CN107651691A, entitled "A Method for Preparing High-Quality Silicon Carbide from Crystalline Silicon Cutting Waste," describes a process where crystalline silicon cutting waste, carbon source, and purifying agent are weighed and mixed in a specific ratio, pressed into pellets, dried, and smelted at high temperature to obtain SiC crystalline blocks. These blocks are then crushed, acid-washed, and dried to obtain high-quality silicon carbide powder with a purity ≥95%. This document specifically mentions that to prevent raw material oxidation and improve silicon carbide yield, a 10-60 mm carbon layer is placed above the pellets during sintering in an induction furnace to create a reducing atmosphere. However, this method does not control the silicon carbide crystal structure and easily leads to an increase in the free carbon content in the product. Using a high-oxygen atmosphere at atmospheric pressure with excessive carbon can easily generate carbon monoxide gas. Patent application CN105293498A, entitled "A Method for Preparing Silicon Carbide Powder from Polycrystalline Silicon Cutting Waste," describes a process where polycrystalline silicon cutting waste is ground, sieved, wet-leached to remove iron, filtered, washed, and dried to obtain iron-removed polycrystalline silicon waste. Carbon powder is then added and ground to mix the waste. A binder is added, and the mixture is pressed into blocks and sintered at 1400-1600 °C in an argon atmosphere for 1-5 hours to prepare a mixture of α-SiC and β-SiC. The reaction temperature in this patent is significantly lower than the thermodynamic formation temperature of 6H-SiC, resulting in a low conversion rate for 6H-SiC, making it suitable only as a raw material for low-end silicon carbide ceramics. Patent application CN105480979A, entitled "A Method for Producing Silicon Carbide from Photovoltaic Industry Cutting Waste," describes a process where cutting waste is mixed with petroleum coke in a specific ratio, briquetted, dried, and then smelted in a resistance furnace under electric current. The furnace temperature is controlled at 2400℃ for 30 hours, followed by cooling for 24 hours, and then water cooling before removal from the furnace. The waste is then layered, graded, crushed, chemically treated, dried, sieved, and magnetically separated to obtain silicon carbide micropowder. This method involves a long reaction time, a complex production process, and high energy consumption. In summary, existing methods for preparing silicon carbide powder using photovoltaic silicon sludge suffer from long process flows, poor product quality, and low added value. Shortening the process, improving product quality, and reducing energy consumption are urgent problems that need to be addressed in existing processes. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a method for preparing high-quality 6H-SiC powder using photovoltaic silica mud with varying atmosphere treatment. This method effectively utilizes photovoltaic silica mud to prepare 6H-SiC powder with excellent properties, solving the problems of resource utilization of photovoltaic silica mud and the high cost and low quality of existing 6H-SiC powder preparation methods. The specific technical solution is as follows:

[0006] A method for preparing 6H-SiC powder by treating photovoltaic silicon mud with varying atmospheres specifically includes the following steps:

[0007] (1) Raw material pretreatment: The photovoltaic silicon mud is made weakly acidic by washing and then filtered to obtain silicon mud filter cake;

[0008] (2) Granulation and drying: Add carbon powder and binder to the silica mud filter cake, fully wet and mix evenly to obtain a blank, then granulate or wet press molding, and dry to obtain granules;

[0009] (3) Variable atmosphere gradient sintering: The dried granules are placed in a sintering furnace and heated to 250~500 ℃ in an air atmosphere and held for 1~5 h; then the temperature is raised to 600~700 ℃ and held for 0.5~2 h; after the holding is completed, the furnace is evacuated and argon gas is introduced to replace the atmosphere in the furnace. Then the furnace is evacuated and heated to 1450~1600 ℃ and held for 1~8 h; finally the temperature is raised to 2100~2500 ℃ and held for 4-12 h to obtain 6H-SiC powder.

[0010] Furthermore,

[0011] The photovoltaic silicon mud raw material selected in step (1) contains Si≥95 wt.%, O≤4 wt.%, and the balance is impurity elements including at least one of Fe, Ni, Al, S, P, and Cl. The pH of the photovoltaic silicon mud is 4.5~6.7 after washing. The carbon powder includes one or more of petroleum coke powder, graphite powder, and recycled deposited carbon, with a carbon content ≥99.0 wt.%.

[0012] In step (1), the silica mud filter cake is slurried and ground until the average particle size is 0.5~3 μm; carbon powder is added, or a binder is added and the grinding continues until all particles pass through an 800~2000 mesh sieve.

[0013] Furthermore,

[0014] In step (2), when mixing the silica sludge filter cake and carbon powder, the Si / C molar ratio is controlled to be 1.2~1, preferably 1.05~1.02.

[0015] Step (2) involves processing the secondary granules using one of spray granulation, extrusion granulation, or wet pressing. The secondary granules after granulation have a particle size of 0.1~3 mm. The mold is designed according to the product size requirements for wet pressing. After drying, the moisture content of the secondary granules and the wet-pressed blanks is ≤1 wt.%. Spray granulation is the preferred granulation method.

[0016] Step (2) The binder includes one or more of polyvinyl alcohol, carboxymethyl cellulose, and ethyl cellulose. The amount of binder is 2-8 wt.% of the dry basis weight of the photovoltaic silicon mud and carbon powder mixture; preferably 3-5 wt.%. The binder is dissolved in a solvent, and then the silicon mud filter cake and carbon powder are added to it and stirred evenly to obtain a blank. When spray granulation is used, the solid content in the blank is 40-70 wt.%, preferably 45-55 wt.%. When extrusion granulation is used, the solid content in the blank is 75-85 wt.%. When wet pressing is used, the solid content in the blank is 80-95 wt.%.

[0017] The solvent for polyvinyl alcohol and carboxymethyl cellulose is water, and the solvent for ethyl cellulose is anhydrous ethanol.

[0018] Furthermore,

[0019] Step (3) Variable atmosphere gradient sintering: Place the granules obtained from granulation or drying of wet-pressed blanks into a sintering furnace. In an air atmosphere, control the heating rate at 3~20 ℃ / min, raise the temperature to 250~500 ℃ and hold for 1~5 h; raise the temperature to 600~700 ℃ at a rate of 5~30 ℃ / min and hold for 0.5~2 h; after the holding period, evacuate to below 0.1 MPa and then introduce argon gas to replace the atmosphere, controlling the oxygen content in the furnace to below 0.05 vol.‰, and then evacuate to 10 ℃. -2 Below Pa, the heating rate is controlled at 8~15 ℃ / min, heated to 1450~1600 ℃ and held for 1~8 h, and finally heated to 2100~2500 ℃ at a heating rate of 10~20 ℃ / min and held for 4~12 h to obtain 6H-SiC powder.

[0020] Furthermore,

[0021] Step (3) In an air atmosphere, the heating rate is preferably controlled at 5~8 ℃ / min, and the temperature is raised to 250~500 ℃ and held for 1~5 h to allow the organic binder added during the granulation process and the trace organic matter remaining in the silica mud to fully decompose and generate CO2 or small molecule gas to escape; the temperature is raised to 600℃~700℃ and held to carbonize the organic binder inside the particles; after the holding period, the vacuum is first evacuated to below 0.1MPa and then argon gas is introduced to fully remove air and carbon dioxide gas in the furnace, control the oxygen content to be below 0.05 vol.‰, and then the vacuum is evacuated again to 10 -2 Below Pa, the preferred heating rate is 10~12 ℃ / min, heating to 1450~1600 ℃, and preferably holding for 4~6 h, so that the silica mud is completely melted and fully reacts with carbon powder to generate 3C-SiC; further heating at a rate of 10~20 ℃ / min to 2300~2400 ℃ and holding for 6~8 h, at which temperature 3C-SiC is transformed into 6H-SiC powder.

[0022] The final 6H-SiC content obtained by this invention is ≥98.5 wt.%, the free carbon content is ≤0.2 wt.%, and the powder particle size is D10 of 1~3 μm, D50 of 5~10 μm, and D90 of 12~18 μm.

[0023] Beneficial effects of the invention

[0024] (1) This invention effectively utilizes the silicon mud waste generated by the photovoltaic industry, realizes the recycling and reuse of resources, reduces environmental pollution, and has significant environmental and economic benefits.

[0025] (2) In the process of raw material processing and batching, the present invention precisely controls the particle size of silica mud particles and carbon powder, and adopts a two-stage particle mixing granulation method to make the silica mud particles and carbon powder in the most compact packing state, thereby shortening the mass transfer distance in the subsequent reaction process.

[0026] (3) In the high-temperature treatment process, the present invention adopts a variable atmosphere gradient sintering process. In the low-temperature section, an oxidizing atmosphere is used and sufficient heat preservation is carried out to fully decompose and release the trace organic matter remaining in the binder and silica mud and carbonize it. After vacuuming, argon gas is introduced to remove oxygen. Finally, vacuuming is carried out again and the temperature is raised to 1450~1650 ℃ and kept at the temperature to allow silicon and carbon to fully react to generate 3C-SiC, reducing the residual carbon content in the silicon carbide product. Finally, the temperature is raised to the preferred temperature of 2300~2400 ℃ and kept at the temperature for 6~8 h to convert 3C-SiC into 6H-SiC powder. The obtained product has a 6H-SiC content ≥98.5 wt.% and an FC (free carbon) content ≤0.2 wt.%, and high-quality powder can be obtained directly. This avoids the low purity of SiC products prepared by silica mud in the existing disclosed methods, which require crushing and sieving to obtain SiC powder. This further shortens the process and reduces production costs.

[0027] (4) The present invention uses photovoltaic silicon mud as raw material, which greatly reduces the cost of raw materials compared with traditional silicon carbide raw materials. At the same time, the optimized preparation process has a lower sintering temperature, which can effectively reduce energy consumption and other production costs, and improve the market competitiveness of the product. Attached Figure Description

[0028] Figure 1 XRD patterns of the products in the embodiments of this invention.

[0029] Figure 2 XRD patterns of the products in the comparative examples of this invention. Detailed Implementation

[0030] The following examples are intended to further illustrate the present invention, but not to limit it.

[0031] The 6H-SiC content and FC (free carbon) of this invention were determined using the chemical analysis method for silicon carbide in common abrasives, GB / T 3045-2024.

[0032] Example 1

[0033] The silica mud from a photovoltaic cell processing company contained 99.2 wt.% Si, 0.3 wt.% O, 0.04 wt.% Fe, 0.025 wt.% Ni and 0.016 wt.% S. After washing with dilute oxalic acid solution until the pH of the washing water is 6.5±0.1, the filter cake is obtained by pressure filtration. The filter cake is then ground with water to form a silica mud slurry with an average particle size of 0.5 μm. Simultaneously, graphite powder with a carbon content of 99.9 wt.% is selected as the carbon source, with the amount of graphite powder used based on a Si / C molar ratio of 1.05. The binders are polyvinyl alcohol and sodium carboxymethyl cellulose, with amounts of 5 wt.% and 1.5 wt.% of the dry basis mass of the silica mud and graphite mixed powder, respectively. Polyvinyl alcohol and sodium carboxymethyl cellulose are first dissolved in water, and then added to the silica mud slurry along with the graphite powder and thoroughly mixed to form a slurry with a solid content of 55 wt.%. The slurry is further ground until all particles pass through a 2000-mesh sieve, and then spray granulation is used to obtain secondary granules with an average particle size of 0.3 mm. The granules are then dried to a moisture content of 1 wt.%. The granules were then placed in a sintering furnace and heated from room temperature to 300°C at a rate of 5°C / min under air atmosphere. The temperature was held for 3 hours to allow the binder added during granulation and any residual organic matter in the silica sludge to fully decompose and release CO2. The temperature was then further increased to 600°C at a rate of 5°C / min and held for 2 hours. After holding, a vacuum of 0.1 MPa was applied, and argon gas was introduced to completely purge air and carbon dioxide from the furnace, resulting in an oxygen content of 0.05 vol.‰. The furnace was then evacuated to 10... -2 The temperature was increased to 1450 °C at a rate of 10 °C / min and held for 5 h to completely melt the silica sludge and allow it to fully react with the carbon powder to form 3C-SiC. Finally, the temperature was increased to 2300 °C at a rate of 15 °C / min and held for 6 h. At this temperature, 3C-SiC was converted into 6H-SiC powder. After the holding period, the powder was cooled in the furnace to obtain silicon carbide powder. XRD analysis showed that the product was 6H-SiC. Chemical analysis showed that the 6H-SiC content was 98.9 wt.%, the FC (free carbon) content was 0.1 wt.%, and particle size analysis showed that the powder particle size was D10 2.4 μm, D50 8.6 μm, and D90 13.5 μm.

[0034] Example 2

[0035] The silica mud from a photovoltaic cell processing company contained 99.2 wt.% Si, 0.3 wt.% O, 0.04 wt.% Fe, 0.025 wt.% Ni and 0.016 wt.% S. After washing with dilute acetic acid until the pH of the washing water is 5.0±0.1, the filter cake is obtained by pressure filtration. The filter cake is then ground with water to form a silica mud slurry with an average particle size of 1 μm. Simultaneously, graphite powder with a carbon content of 99.9 wt.% is selected as the carbon source, with the amount of graphite powder used based on a Si / C molar ratio of 1.05. The binders are polyvinyl alcohol and sodium carboxymethyl cellulose, with amounts of 5 wt.% and 1.5 wt.% of the dry basis mass of the silica mud and graphite mixed powder, respectively. Polyvinyl alcohol and sodium carboxymethyl cellulose are first dissolved in water, and then added to the silica mud slurry along with the graphite powder and thoroughly mixed to form a slurry with a solid content of 55 wt.%. The slurry is further ground until all particles pass through a 1600 mesh, and secondary agglomerates with an average particle size of 0.3 mm are obtained by spray granulation and dried to a moisture content of 1 wt.%. The granules were then placed in a sintering furnace and heated from room temperature to 300°C at a rate of 5°C / min under air atmosphere. The temperature was held for 3 hours to allow the organic binder added during granulation and any residual organic matter in the silica sludge to fully decompose and release CO2. The temperature was then further increased to 650°C at a rate of 5°C / min and held for 1.5 hours. After this holding period, a vacuum of 0.1 MPa was applied, and argon gas was introduced to completely purge air and carbon dioxide from the furnace. The oxygen content in the furnace was then reduced to 0.04 vol.‰. Finally, a vacuum of 10... -2 The temperature was increased to 1450 °C at a rate of 10 °C / min and held for 5 h to completely melt the silica sludge and allow it to fully react with the carbon powder to form 3C-SiC. Finally, the temperature was increased to 2350 °C at a rate of 15 °C / min and held for 6 h under vacuum. At this temperature, 3C-SiC was converted into 6H-SiC powder. After the holding period, the powder was cooled in the furnace to obtain silicon carbide powder. XRD analysis showed that the product was 6H-SiC. Chemical analysis showed that the 6H-SiC content was 99.2 wt.%, the FC (free carbon) content was 0.1 wt.%, and particle size analysis showed that the powder particle size was D10 2.9 μm, D50 9.4 μm, and D90 14.9 μm.

[0036] Example 3

[0037] The silica mud from a photovoltaic cell processing company contained 99.4 wt.% Si, 0.2 wt.% O, 0.021 wt.% Fe, 0.023 wt.% Ni, 0.012 wt.% P and 0.016 wt.% S. The filter cake was obtained by washing with dilute acetic acid until the pH of the washing water was 4.6±0.1. The filter cake was then mixed with water and ground into a silica mud slurry with an average particle size of 2 μm. Simultaneously, graphite powder with a carbon content of 99.5 wt.% was selected as the carbon source, with the amount of graphite powder used based on a Si / C molar ratio of 1.02. The binders were ethyl cellulose and sodium carboxymethyl cellulose, used at 3 wt.% and 1 wt.% of the dry basis mass of the silica mud and graphite mixed powder, respectively. Ethyl cellulose was first dissolved in anhydrous ethanol, and sodium carboxymethyl cellulose was dissolved in water. These were then mixed and added to the silica mud slurry along with the graphite powder to prepare a slurry with a solid content of 52 wt.%. This slurry was further ground until all particles passed through an 800-mesh sieve, and then filtered to produce a billet with a solid content of 80 wt.%. Secondary granules with an average particle size of 3 mm were obtained by extrusion granulation and further dried to a moisture content of 0.8 wt.%. The granules were then placed in a sintering furnace and heated from room temperature to 450 °C at a rate of 3 °C / min under air atmosphere, and held for 1 hour to allow the organic binder added during the granulation process and the trace organic matter remaining in the silica sludge to fully decompose and generate CO2. The temperature was then increased to 700 °C at a rate of 3 °C / min and held for 1 hour. After the holding period, a vacuum of 0.1 MPa was applied, and argon gas was introduced to completely purge air and carbon dioxide from the furnace, resulting in an oxygen content of 0.03 vol.‰ in the furnace. Finally, a vacuum of 8 × 10⁻⁶ MPa was applied. -3 The temperature was increased to 1600 °C at a rate of 15 °C / min and held for 3 h to completely melt the silica mud and allow it to fully react with the carbon powder to form 3C-SiC. Finally, the temperature was increased to 2400 °C at a rate of 12 °C / min and held for 4 h. At this temperature, 3C-SiC was converted into 6H-SiC powder. After the holding period, the powder was cooled in the furnace to obtain silicon carbide powder. XRD analysis showed that the product was 6H-SiC. Chemical analysis showed that the 6H-SiC content was 99.3 wt.%, the FC (free carbon) content was 0.08 wt.%, and the particle size analysis showed that the powder particle size was D10 2.6 μm, D50 9.4 μm, and D90 15.6 μm.

[0038] Example 4

[0039] The silica mud from a photovoltaic cell processing company contained 99.4 wt.% Si, 0.2 wt.% O, 0.021 wt.% Fe, 0.023 wt.% Ni, 0.012 wt.% P and 0.016 wt.% S. After dynamic washing with citric acid solution until the pH of the washing water is 6.0±0.1, the filter cake is obtained by pressure filtration. Water is added and the mixture is ground to a silica mud slurry with an average particle size of 2 μm. Simultaneously, graphite powder with a carbon content of 99.5 wt.% is selected as the carbon source, with the amount of graphite powder used based on a Si / C molar ratio of 1.12. The binders are ethyl cellulose and sodium carboxymethyl cellulose, used at 3 wt.% and 1 wt.% of the dry basis mass of the silica mud and graphite mixed powder, respectively. Ethyl cellulose is first dissolved in anhydrous ethanol, and sodium carboxymethyl cellulose is dissolved in water, then mixed. This mixture, along with the graphite powder, is added to the silica mud slurry and thoroughly mixed to prepare a mixed slurry with a solid content of 56 wt.%. This slurry is further ground until all particles pass through an 1100-mesh sieve and then pressure filtered to obtain a billet with a solid content of 80 wt.%. Secondary granules with an average particle size of 3 mm are obtained by extrusion granulation and further dried to a moisture content of 0.8 wt.%. The granules were then placed in a sintering furnace and heated from room temperature to 450 °C at a rate of 3 °C / min under an air atmosphere. The temperature was held for 1 hour to allow the organic binder added during granulation and any residual organic matter in the silica sludge to fully decompose and release CO2. The temperature was then further increased to 700 °C at a rate of 3 °C / min and held for 0.5 hours. After this holding period, a vacuum of 0.1 MPa was applied, and argon gas was introduced to completely purge air and carbon dioxide from the furnace. The oxygen content in the furnace was then reduced to 0.03 vol.‰. Finally, a vacuum of 8 × 10⁻⁶ MPa was applied. -3 The temperature was increased to 1550 °C at a rate of 15 °C / min and held for 3.5 h to completely melt the silica sludge and allow it to fully react with the carbon powder to form 3C-SiC. Finally, the temperature was increased to 2200 °C at a rate of 18 °C / min and held for 10 h. At this temperature, 3C-SiC was converted into 6H-SiC powder. After the holding period, the powder was cooled in the furnace to obtain silicon carbide powder. XRD analysis showed that the product was 6H-SiC. Chemical analysis showed that the 6H-SiC content was 98.8 wt.%, the FC (free carbon) content was 0.01 wt.%, and the particle size analysis showed that the powder particle size was D10 1.9 μm, D50 8.8 μm, and D90 14.9 μm.

[0040] Comparative Example 1

[0041] Using the same silica mud and graphite powder as in Example 1, the batching, mixing, and granulation process control requirements were the same as in Example 1. However, the variable atmosphere gradient sintering process was not used during sintering. The secondary granules obtained from granulation were placed in a sintering furnace and heated from room temperature to 2300 ℃ at a heating rate of 10 ℃ / min under an argon atmosphere. After holding at this temperature for 6 h, the furnace was cooled to obtain silicon carbide powder. XRD analysis showed that the product was a mixture containing 6H-SiC (88 wt.% of total SiC), 4H-SiC (8 wt.% of total SiC), and 3C-SiC (4 wt.% of total SiC). Chemical analysis showed that the SiC content in the product was 97.5 wt.%, the FC (free carbon) content was 0.3 wt.%, and particle size analysis showed that the powder particle size D10 was 2.8 μm, D50 was 8.7 μm, and D90 was 14.1 μm.

[0042] Comparative Example 2

[0043] Using the same silica mud and graphite powder as in Example 1, the batching, mixing, and granulation processes were controlled in the same way as in Example 1. However, instead of a variable atmosphere gradient sintering process, sintering was carried out under vacuum conditions. The secondary granules obtained from granulation were placed in a sintering furnace and sintered under a vacuum of 8 × 10⁻⁶. -3 Silicon carbide powder was obtained by heating from room temperature to 2350 ℃ at a heating rate of 10 ℃ / min under Pa conditions and holding for 6 h, followed by furnace cooling. XRD analysis showed that the product was a mixture containing 6H-SiC (59.5 wt.% of total SiC), 4H-SiC (37 wt.% of total SiC), and 3C-SiC (3.5 wt.% of total SiC). Chemical analysis showed that the SiC content in the product was 97.9 wt.% and the FC (free carbon) content was 0.5 wt.%. Particle size analysis showed that the powder particle size D10 was 3.5 μm, D50 was 11.4 μm, and D90 was 18.9 μm.

Claims

1. A method for preparing 6H-SiC powder from a photovoltaic silicon slurry by changing atmosphere treatment, characterized in that, Specifically comprising the following steps: (1) Raw material pretreatment: the photovoltaic silicon sludge is made weakly acidic by washing, and a silicon sludge filter cake is obtained after filtration; (2) Granulation and drying: the silicon sludge filter cake is fully wetted and mixed with carbon powder and a binder to obtain a blank, then granulation or wet pressing is performed, and drying is performed to obtain granules; (3) Variable atmosphere gradient sintering: the dried granules are placed in a sintering furnace, heated to 250-500 ℃ in an air atmosphere for 1-5 h; then heated to 600-700 ℃, and kept for 0.5-2 h; after the end of the holding, vacuum is drawn and argon is introduced to replace the atmosphere in the furnace, then vacuum is drawn again and heated to 1450-1600 ℃ for 1-8 h; finally, heated to 2100-2500 ℃, and kept for 4-12 h to obtain 6H-SiC powder.

2. The method of claim 1, wherein In step (1), the Si content of the selected photovoltaic silicon sludge raw material is ≥95 wt.%, the O content is ≤4 wt.%, and the balance is impurity elements including at least one of Fe, Ni, Al, S, P, and Cl, and the photovoltaic silicon sludge is made to have a pH of 4.5-6.7 by washing; the carbon powder includes one or more of petroleum coke powder, graphite powder, and recycled deposited carbon, and the carbon content is ≥99.0 wt.%.

3. The method of claim 1 or 2, wherein In step (1), the silicon sludge filter cake is ground to an average particle size of 0.5-3 μm; the carbon powder is added, or a binder is further added and ground to 800-2000 mesh screen full pass.

4. The method of claim 1, wherein In step (2), the Si / C molar ratio is controlled to be 1.2-1 when the silicon sludge filter cake is mixed with the carbon powder.

5. The method of claim 1, wherein In step (2), one of spray granulation, extrusion granulation, or wet pressing is used for processing, the secondary granule particle size after granulation is 0.1-3 mm, wet pressing is performed according to product size requirements, and the water content of the secondary granule and the wet pressing blank after drying is ≤1 wt.%.

6. The method according to claim 1 or 5, characterized in that, In step (2), the binder includes one or more of polyvinyl alcohol, carboxymethyl cellulose, and ethyl cellulose, and the binder amount is 2-8 wt.% of the dry basis mass of the mixed powder of the photovoltaic silicon sludge and the carbon powder; the binder is dissolved in a solvent, then the silicon sludge filter cake and the carbon powder are added and stirred to obtain a blank, the solid content in the blank is 40-70 wt.% when spray granulation is used, the solid content in the blank is 75-85 wt.% when extrusion granulation is used, and the solid content in the blank is 80-95 wt.% when wet pressing is used.

7. The method of claim 1, wherein Step (3) atmosphere changing gradient sintering: the granules obtained by drying the granulated or wet-pressed green compact are placed in a sintering furnace, the temperature increasing rate is controlled at 3-20 ℃ / min in an air atmosphere, the temperature is increased to 250-500 ℃ and kept for 1-5 h; the temperature is increased to 600-700 ℃ at a rate of 5-30 ℃ / min and kept for 0.5-2 h; after the keeping, vacuum is drawn to below 0.1 MPa, argon is introduced to replace the atmosphere, the oxygen content in the furnace is controlled below 0.05 vol.‰, then vacuum is drawn again to below 10 -2 Pa, the temperature increasing rate is controlled at 8-15 ℃ / min, the temperature is increased to 1450-1600 ℃ and kept for 1-8 h, finally the temperature is increased to 2100-2500 ℃ at a rate of 10-20 ℃ / min and kept for 4-12 h to obtain 6H-SiC powder.

8. The method of claim 1, wherein The obtained 6H-SiC has a content of ≥98.5 wt.%, a free carbon content of ≤0.2 wt.%, a powder particle size D10 of 1-3 μm, a powder particle size D50 of 5-10 μm, and a powder particle size D90 of 12-18 μm.

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