Novel silicon carbide production process

Abstract

The invention discloses a novel silicon carbide production process which comprises the following steps: S1, placing a carbon source and sodium chloride at the bottom of a synthetic reaction container, and removing part of impurities again; s2, placing the pretreated silicon source at the bottom of a purification container, gradually heating to 2250 DEG C step by step, and removing low-boiling-point impurities; s3, inert gas is introduced into the bottom of the purification container in the step S2, low-boiling-point impurities are further removed, the mixture enters a synthesis reactor, the carbon source in the synthesis reactor is gradually heated through gas-phase silicon entering the synthesis reactor, the reaction time is 10-20 h, and a silicon carbide crude product is obtained; and S4, purifying the silicon carbide crude product obtained in the step S3, wherein the purity is greater than 99.999%. The preparation method disclosed by the invention is simple and low in cost, CO2 emission is reduced, silicon carbide with higher purity can be produced, the purity of the prepared silicon carbide is greater than 99.993%, and the silicon carbide is applied to the fields of electronic products, chips and the like.

Description

Technical Field

[0001] The invention relates to the technical field of silicon carbide, and in particular to a new silicon carbide production process. Background Art

[0002] Silicon Carbide (SiC) is an inorganic non-metallic material with extremely high hardness and good thermal and chemical stability. It is a wide bandgap semiconductor material with the following main uses: 1. Abrasives and grinding materials: Due to its high hardness (Mohs hardness 9.5, second only to diamond) and strong wear resistance, silicon carbide is often used to make grinding wheels, sandpaper, grinding paste, etc. It is suitable for precision grinding and cutting of hard materials such as metal, stone and ceramics.

[0003] 2. Refractory materials and ceramics: Silicon carbide has excellent high temperature resistance and good thermal shock stability. It is an ideal material for manufacturing high temperature refractory components such as high temperature furnaces, kiln furniture, heat exchanger linings, rocket nozzles, etc.

[0004] 3. Metallurgical industry: In metallurgical processes such as ore dressing and steel manufacturing, silicon carbide is used as a wear-resistant lining material, such as pump rooms, impellers, wear-resistant pipes and cyclones, and its wear resistance far exceeds that of rubber and cast iron.

[0005] 4. Electronic and semiconductor devices: As a semiconductor material, silicon carbide is used to manufacture electronic devices with high power, high frequency, high temperature and excellent radiation resistance, such as power electronic switches, high temperature sensors, light-emitting diodes (LEDs), etc.

[0006] 5. Aerospace: Due to its light weight, high strength and high temperature resistance, silicon carbide composites are used to manufacture structural components, braking systems and heat protection systems of aircraft and spacecraft.

[0007] 6. Automotive industry: In electric vehicles, silicon carbide-based semiconductor devices are used for power conversion and control, which can effectively improve energy efficiency and reduce energy consumption.

[0008] 7. Nuclear energy: Silicon carbide has good radiation resistance and can be used as structural and protective materials for nuclear reactors.

[0009] Due to its wide range of uses, its synthesis methods have been studied extensively, mainly including the following methods: 1. Acheson method: This is the earliest and most traditional method for industrial synthesis of silicon carbide, invented by Edward G. Acheson in 1891. The method is carried out in a resistance furnace filled with silica sand (SiO2) as a silicon source, graphite as a carbon source, and a small amount of sawdust or salt (NaCl, only used when producing green silicon carbide). At high temperatures (about 2000-2500°C), silica sand is reduced to form silicon, which then reacts with carbon to form silicon carbide. This process involves complex chemical reactions, including the reduction of silica sand, the oxidation of graphite, and the formation of silicon carbide. The final product is granular silicon carbide, which needs to be screened, crushed and graded before use.

[0010] 2. Chemical Vapor Deposition (CVD): In the CVD process, silicon source and carbon source are introduced into a high-temperature reaction chamber in gaseous form, and a chemical reaction occurs on the substrate surface to directly deposit and form a silicon carbide film. This method can accurately control the thickness and composition of the film and is suitable for manufacturing high-quality silicon carbide coatings or ultra-thin layers in electronic devices.

[0011] 3. Physical vapor transport (PVT): Similar to the Acheson method, but more focused on the preparation of high-purity and large single-crystal silicon carbide. In this method, in a sealed chamber, the silicon carbide raw material is sublimated into a gaseous state at high temperature (about 2000°C or above), and then under the action of the temperature gradient, the gaseous material is transported and deposited on the seed crystal in the low-temperature area, gradually growing into a single-crystal silicon carbide.

[0012] 4. Solution growth method: Utilizing the solubility characteristics of silicon carbide, the silicon carbide raw material is dissolved in a specific solvent, and then the silicon carbide crystallization is promoted by slowly cooling or adding seed crystals to form single crystals or polycrystalline materials of a specific form.

[0013] The existing production process is to use SiO 2 The raw materials (purity 99.3%) are added with carbon materials and salt, and silicon carbide is generated at high temperature. In the process of generating silicon carbide, carbon is needed to replace the oxygen in silicon dioxide, which increases the use of carbon materials and also increases the emission of carbon dioxide.

[0014] CN 113788480 B discloses a method for preparing high-purity silicon carbide, comprising the following steps: S1, placing high-purity carbon powder in a graphite crucible, and purifying the high-purity carbon powder using a high-temperature vacuum method; S2, placing block silicon in a graphite crucible, heating to melt the block silicon, rotating the graphite crucible to evenly mix the high-purity carbon powder and the block silicon, and reacting to synthesize high-purity silicon carbide. The present application uses block silicon instead of silicon powder, and the purity of block silicon is much higher than that of silicon powder; after the block silicon is melted into liquid silicon, the graphite crucible is rotated at high speed to form convection inside the silicon solution, and the high-purity carbon powder can be evenly distributed in the silicon solution, and high-purity, high-synthesis degree high-purity silicon carbide can be formed after cooling. The purity of high-purity silicon carbide is generally higher than 99.999%, but the preparation cost is too high. Summary of the invention

[0015] In view of the deficiencies of the prior art, the present invention provides a new silicon carbide production process, comprising the following steps: S1: placing the carbon source and sodium chloride at the bottom of the synthesis reaction vessel, introducing inert gas, preheating to 700-750°C, and removing some impurities again. The temperature in step S1 must be controlled below 800°C to prevent the sodium chloride from melting and blocking the active sites of the carbon source; S2: Place the pretreated silicon source at the bottom of the purification container and gradually heat it to 2250°C in steps to remove low-boiling impurities. The temperature in step S2 must be controlled below 2300°C to prevent silicon vapor from volatilizing during the impurity removal process and waste raw materials; S3: In step S2, an inert gas is introduced into the bottom of the purification container, and the temperature is raised to 2350-2400°C. The inert gas brings the gaseous silicon from the purification container into a buffer tank, and the temperature of the buffer tank is 2250-2350°C. Low-boiling impurities are further removed, and then the gaseous silicon enters the synthesis reactor. The gaseous silicon entering the synthesis reactor gradually heats the temperature of the carbon source in the synthesis reactor to 1900-2200°C. At the same time, the synthesis reactor starts to heat up, and the reaction temperature is maintained at 1900-2200°C. The reaction time is 10-20h to obtain a crude silicon carbide product. In step S3, after further purification in the buffer tank, the purity of the silicon source is further improved, so that high-purity silicon carbide can be synthesized; S4: The crude silicon carbide obtained in step S3 is cooled to 700-750°C, ultrasonically shattered, and oxygen is introduced again for purification. After purification, the crude silicon carbide is cooled, washed with water, and dried to obtain a pure silicon carbide product with a purity of >99.99%. Ultrasonic shattering in step S4 allows the excess carbon source and sodium chloride to be removed in the subsequent purification step.

[0016] Furthermore, in step S2, the silicon source is photovoltaic silicon mud waste of about 2 microns, with a silicon content greater than 80%. The pretreatment method of the silicon source is that the silicon mud waste is heated to 2200°C by a medium frequency furnace, and then slowly heated to 2350°C for 3 hours to melt into blocks, remove impurities in the silicon mud, and increase the content to more than 99.9%. Then, the block silicon material with a content of more than 99.9% is crushed into particles of 0~20mm. Only by slow heating can the impurities in the silicon source be completely removed and the purity of the silicon source be improved.

[0017] Furthermore, in the step S1, the carbon source is at least one of petroleum coke, anthracite, the bottom material in the distillation kettle after the distillation of crude 2-ethylanthraquinone in the process of producing 2-ethylanthraquinone, and succinilide. When the carbon source is succinilide, the purity of the prepared product is the highest, because silicon atoms can react deeply inside succinilide, the reaction is thorough, and the purity of the prepared product is also higher.

[0018] Furthermore, in the step S1, the mass ratio of the carbon source to sodium chloride is 6-9:1, and in the synthesis reaction in the step S3, the molar ratio of carbon to silicon is 1-1.05:1.

[0019] Further, when the carbon source in step S1 is petroleum coke, anthracite, or the bottom material in the distillation kettle after distilling the crude 2-ethylanthraquinone in the process of producing 2-ethylanthraquinone, the carbon source needs to be pretreated, and the pretreatment method includes the following steps: S21: crushing the carbon source, and then washing with ethanol and water to obtain a washed carbon source; S22: The washed carbon source obtained in step S21 is immersed in a 3 wt % sodium hydroxide solution for activation for 2 to 3 hours, and then filtered and washed to obtain an alkali-washed carbon source; S23: soaking the carbon source after alkali washing in step S22 in an acid solution at a temperature between 20 and 60° C. for 3 hours, and then filtering, washing, neutralizing and drying to obtain a pretreated carbon source.

[0020] Furthermore, in the step S23, the contents of hydrofluoric acid and hydrochloric acid in the acid solution are 0.5wt% and 1wt% respectively.

[0021] Furthermore, the inert gas is one of argon and helium. When the inert gas is helium, the effect is better because helium molecules are smaller and can provide an inert environment at a deep level.

[0022] Furthermore, the heating process in step S2 is to first heat to 750°C, maintain for 2 hours, then heat to 1600°C, maintain for 1 hour, and finally heat to 2250°C, maintain for 3 hours. By distributed heating, impurities can be separated step by step to improve the purity of the silicon source.

[0023] The preparation method of the present invention is simple, low-cost, and reduces CO 2 Emissions can be reduced to produce higher purity silicon carbide. The purity of the prepared silicon carbide is greater than 99.993%, which is used in electronic products, chips and other fields. DETAILED DESCRIPTION

[0024] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present invention.

[0025] It should be understood that the specific embodiments described herein are only used to explain the present invention, and are not used to limit the present invention.

[0026] Embodiment 1: A new silicon carbide production process comprises the following steps: S1: 120g of petroleum coke and 18g of sodium chloride are placed at the bottom of a synthesis reaction vessel, argon is introduced, and the mixture is preheated to 700-750°C, and some impurities are removed again. The petroleum coke pretreatment method comprises the following steps: S21: crushing the petroleum coke, and then washing with ethanol and water to obtain washed petroleum coke; S22: soaking and activating the washed petroleum coke obtained in step S21 in a 3 wt % sodium hydroxide solution for 2 to 3 hours, and then filtering and washing to obtain alkali-washed petroleum coke; S23: soaking the petroleum coke after alkali washing in step S22 in an acid solution at a temperature between 20 and 30° C. for 3 hours, and then filtering, washing, neutralizing and drying to obtain pretreated petroleum coke, wherein the contents of hydrofluoric acid and hydrochloric acid in the acid solution are 0.5 wt % and 1 wt % respectively; S2: Place 275g of pretreated silicon source at the bottom of the purification container and gradually heat it to 2250°C in steps. The specific heating process is to first heat it to 750°C, keep it for 2h, then heat it to 1600°C, keep it for 1h, and finally heat it to 2250°C, keep it for 3h to remove low-boiling impurities. The silicon source before pretreatment is silicon mud waste of about 2 microns produced by photovoltaics, and the silicon content is greater than 80%. The pretreatment method of the silicon source is to first heat the silicon mud waste to 2200°C through a medium frequency furnace, and then slowly heat it to 2350°C for 3h, melt it into blocks, remove impurities in the silicon mud, and increase the content to more than 99.9%, and then crush the block silicon material with a content of more than 99.9% into particles of 0~20mm; S3: In step S2, argon is introduced into the bottom of the purification container, and the temperature is raised to 2350-2400°C. The argon brings the gaseous silicon from the purification container into the buffer tank, and the temperature of the buffer tank is 2250-2350°C, and further removes low-boiling impurities, and then enters the synthesis reactor. The gaseous silicon entering the synthesis reactor gradually heats the temperature of the petroleum coke in the synthesis reactor to 1900-2000°C. At the same time, the synthesis reactor starts to heat up, and the reaction temperature is maintained at 1900-2000°C. The reaction time is 15 hours, and a crude silicon carbide product is obtained; S4: Cooling the crude silicon carbide obtained in step S3 to 700-750°C, ultrasonically shattering it, introducing oxygen again for purification, cooling, washing with water, and drying after purification to obtain pure silicon carbide with a purity of 99.996%.

[0027] Embodiment 2: A new silicon carbide production process comprises the following steps: S1: 120g of the bottom material in the distillation kettle after the distillation of crude 2-ethylanthraquinone in the 2-ethylanthraquinone process (hereinafter referred to as the distillation bottom material) and 18g of sodium chloride are placed at the bottom of the synthesis reaction vessel, argon is introduced, and preheated to 700-750°C to remove some impurities again. The distillation bottom material pretreatment method comprises the following steps: S21: crushing the distillation bottom material, and then washing with ethanol and water to obtain a washed distillation bottom material; S22: The washed distillation bottom material obtained in step S21 is immersed in a 3wt% sodium hydroxide solution for activation for 2-3 hours, and then filtered and washed to obtain an alkali-washed distillation bottom material; S23: soaking the distillation bottom material after alkali washing in step S22 in an acid solution at a temperature between 40 and 50° C. for 3 hours, and then filtering, washing, neutralizing and drying to obtain a pretreated distillation bottom material, wherein the contents of hydrofluoric acid and hydrochloric acid in the acid solution are 0.5wt% and 1wt% respectively; S2: Place 275g of pretreated silicon source at the bottom of the purification container and gradually heat it to 2250°C in steps. The specific heating process is to first heat it to 750°C, keep it for 2h, then heat it to 1600°C, keep it for 1h, and finally heat it to 2250°C, keep it for 3h to remove low-boiling impurities. The silicon source before pretreatment is silicon mud waste of about 2 microns produced by photovoltaics, and the silicon content is greater than 80%. The pretreatment method of the silicon source is to first heat the silicon mud waste to 2200°C through a medium frequency furnace, and then slowly heat it to 2350°C for 3h, melt it into blocks, remove impurities in the silicon mud, and increase the content to more than 99.9%, and then crush the block silicon material with a content of more than 99.9% into particles of 0~20mm; S3: Helium is introduced into the bottom of the purification container in step S2, and the temperature is raised to 2350-2400°C. The helium brings the gaseous silicon from the purification container into the buffer tank. The temperature of the buffer tank is 2250-2350°C, and the low-boiling impurities are further removed. Then the gaseous silicon enters the synthesis reactor. The gaseous silicon entering the synthesis reactor gradually heats the temperature of the distillation bottom material in the synthesis reactor to 2100-2200°C. At the same time, the synthesis reactor starts to heat up, and the reaction temperature is maintained at 2100-2200°C. The reaction time is 18 hours to obtain crude silicon carbide; S4: Cooling the crude silicon carbide obtained in step S3 to 700-750°C, ultrasonically shattering it, introducing oxygen again for purification, cooling, washing with water, and drying it after purification to obtain pure silicon carbide with a purity of 99.999%.

[0028] Embodiment 3: A new silicon carbide production process comprises the following steps: S1: 120g of octylene and 18g of sodium chloride are placed at the bottom of a synthesis reaction vessel, argon is introduced, and the mixture is preheated to 700-750°C, and some impurities are removed again. The ethylene pretreatment method comprises the following steps: S21: crushing the sucrose olefin, and then washing it with ethanol and water to obtain washed sucrose olefin; S22: The washed sucrose obtained in step S21 is immersed in a 3wt% sodium hydroxide solution for activation for 2-3 hours, and then filtered and washed to obtain alkali-washed sucrose; S23: soaking the alkali-washed ethylene in step S22 in an acid solution at a temperature between 20 and 30° C. for 3 hours, and then filtering, washing, neutralizing and drying to obtain pretreated ethylene, wherein the contents of hydrofluoric acid and hydrochloric acid in the acid solution are 0.5 wt % and 1 wt % respectively; S2: Place 275g of pretreated silicon source at the bottom of the purification container and gradually heat it to 2250°C in steps. The specific heating process is to first heat it to 750°C, keep it for 2h, then heat it to 1600°C, keep it for 1h, and finally heat it to 2250°C, keep it for 3h to remove low-boiling impurities. The silicon source before pretreatment is silicon mud waste of about 2 microns produced by photovoltaics, and the silicon content is greater than 80%. The pretreatment method of the silicon source is to first heat the silicon mud waste to 2200°C through a medium frequency furnace, and then slowly heat it to 2350°C for 3h, melt it into blocks, remove impurities in the silicon mud, and increase the content to more than 99.9%, and then crush the block silicon material with a content of more than 99.9% into particles of 0~20mm; S3: In step S2, argon is introduced into the bottom of the purification container, and the temperature is raised to 2350-2400°C. The argon brings the gaseous silicon from the purification container into the buffer tank, and the temperature of the buffer tank is 2250-2350°C, and further removes low-boiling impurities, and then enters the synthesis reactor. The gaseous silicon entering the synthesis reactor gradually heats the temperature of the ethylene in the synthesis reactor to 1900-2000°C. At the same time, the synthesis reactor starts to heat up, and the reaction temperature is maintained at 1900-2000°C. The reaction time is 15h, and a crude silicon carbide product is obtained; S4: Cooling the crude silicon carbide obtained in step S3 to 700-750° C., ultrasonically shattering it, introducing oxygen again for purification, cooling, washing with water, and drying it after purification to obtain pure silicon carbide with a purity of 99.9996%.

[0029] Embodiment 4: A new silicon carbide production process comprises the following steps: S1: 120g of anthracite and 18g of sodium chloride are placed at the bottom of a synthesis reaction vessel, argon is introduced, and the mixture is preheated to 700-750°C, and some impurities are removed again. The anthracite pretreatment method comprises the following steps: S21: crushing the anthracite, and then washing with ethanol and water to obtain washed anthracite; S22: The washed anthracite obtained in step S21 is immersed in a 3wt% sodium hydroxide solution for activation for 2-3 hours, and then filtered and washed to obtain alkali-washed anthracite; S23: soaking the anthracite after alkali washing in step S22 in an acid solution at a temperature between 20 and 30° C. for 3 hours, and then filtering, washing, neutralizing and drying to obtain pretreated anthracite, wherein the contents of hydrofluoric acid and hydrochloric acid in the acid solution are 0.5wt% and 1wt% respectively; S2: Place 275g of pretreated silicon source at the bottom of the purification container and gradually heat it to 2250°C in steps. The specific heating process is to first heat it to 750°C, keep it for 2h, then heat it to 1600°C, keep it for 1h, and finally heat it to 2250°C, keep it for 3h to remove low-boiling impurities. The silicon source before pretreatment is silicon mud waste of about 2 microns produced by photovoltaics, and the silicon content is greater than 80%. The pretreatment method of the silicon source is to first heat the silicon mud waste to 2200°C through a medium frequency furnace, and then slowly heat it to 2350°C for 3h, melt it into blocks, remove impurities in the silicon mud, and increase the content to more than 99.9%, and then crush the block silicon material with a content of more than 99.9% into particles of 0~20mm; S3: In step S2, argon is introduced into the bottom of the purification container, and the temperature is raised to 2350-2400°C. The argon brings the gaseous silicon from the purification container into the buffer tank. The temperature of the buffer tank is 2250-2350°C, and the low-boiling impurities are further removed. Then, the gaseous silicon enters the synthesis reactor. The gaseous silicon entering the synthesis reactor gradually heats the temperature of the anthracite in the synthesis reactor to 1900-2000°C. At the same time, the synthesis reactor starts to heat up, and the reaction temperature is maintained at 1900-2000°C. The reaction time is 15 hours, and a crude silicon carbide product is obtained; S4: The crude silicon carbide obtained in step S3 is cooled to 700-750°C, ultrasonically pulverized, and oxygen is introduced again for purification. After purification, the crude silicon carbide is cooled, washed with water, and dried to obtain pure silicon carbide with a purity of 99.994%.

[0030] Comparative Example 1: The petroleum coke in step S1 of Example 1 is used directly without pretreatment, and the rest is the same as Example 1. The purity of the finally prepared silicon carbide is only 99.1%, which will not be repeated here. This shows that the pretreatment of the carbon source has an obvious technical effect.

[0031] Comparative Example 2: Step S2 of Example 1 is omitted, and the rest is the same as Example 1. The purity of the silicon carbide finally prepared is only 99.93%, which will not be repeated here, indicating that S2 is effective in improving the purity of the product.

[0032] Comparative Example 3: The ultrasonic crushing in step S4 of Example 1 was omitted, and the rest was the same as Example 1. The purity of the silicon carbide finally prepared was only 99.98%, which will not be repeated here. This shows that the ultrasonic crushing in step S4 is effective in improving the purity of the product.

Claims

1. A new silicon carbide production process, characterized in that: The following steps are involved: S1: placing the carbon source and sodium chloride at the bottom of the synthesis reaction vessel, introducing inert gas, preheating to 700-750°C, and removing some impurities again; S2: placing the pretreated silicon source at the bottom of the purification container and gradually heating it to 2250°C in steps to remove low boiling point impurities; S3: In the bottom of the purification container in step S2, an inert gas is introduced, and the temperature is raised to 2350-2400°C. The inert gas brings the gaseous silicon from the purification container into a buffer tank, and the temperature of the buffer tank is 2250-2350°C, and further removes low-boiling impurities, and then enters the synthesis reactor. The gaseous silicon entering the synthesis reactor gradually heats the temperature of the carbon source in the synthesis reactor to 1900-2200°C. At the same time, the synthesis reactor starts to heat up, and the reaction temperature is maintained at 1900-2200°C. The reaction time is 10-20h, and a crude silicon carbide product is obtained; S4: Cooling the crude silicon carbide obtained in step S3 to 700-750°C, ultrasonically shattering it, introducing oxygen again for purification, and after purification, cooling, washing, and drying to obtain pure silicon carbide with a purity of >99.99%.

2. The silicon carbide production process according to claim 1, characterized in that: In step S2, the silicon source is photovoltaic silicon mud waste of about 2 microns, with a silicon content greater than 80%. The pretreatment method of the silicon source is that the silicon mud waste is first heated to 2200°C by a medium frequency furnace, and then slowly heated to 2350°C for 3 hours to melt into blocks, remove impurities in the silicon mud, increase the content to more than 99.9%, and then crush the block silicon material with a content of more than 99.9% into particles of 0-20 mm.

3. The silicon carbide production process according to claim 1, characterized in that: The carbon source in step S1 is at least one of petroleum coke, anthracite, bottom material in the distillation kettle after distilling crude 2-ethylanthraquinone in the process of producing 2-ethylanthraquinone, and octylene.

4. The silicon carbide production process according to claim 1, characterized in that: The mass ratio of the carbon source to sodium chloride in step S1 is 6-9:1, and in the synthesis reaction in step S3, the molar ratio of carbon to silicon is 1-1.05:

1.

5. The silicon carbide production process according to claim 1, characterized in that: In the step S1, when the carbon source is petroleum coke, anthracite, or the bottom material in the distillation kettle after distilling the crude 2-ethylanthraquinone in the process of producing 2-ethylanthraquinone, the carbon source needs to be pretreated. The pretreatment method The method comprises the following steps: S21: crushing the carbon source, and then washing with ethanol and water to obtain a washed carbon source; S22: The washed carbon source obtained in step S21 is immersed in a 3wt% sodium hydroxide solution for activation for 2 to 3 hours, and then filtered and washed to obtain an alkali-washed carbon source; S23: The alkali-washed carbon source in step S22 is immersed in an acid solution at a temperature between 20 and 60°C for 3 hours, and then filtered, washed, neutralized and dried to obtain a pretreated carbon source.

6. The silicon carbide production process according to claim 5, characterized in that: In the step S23, the contents of hydrofluoric acid and hydrochloric acid in the acid solution are 0.5wt% and 1wt% respectively.

7. The silicon carbide production process according to claim 1, characterized in that: The inert gas is one of argon and helium.

8. The silicon carbide production process according to claim 1, characterized in that: The heating process in step S2 is to first heat to 750° C., maintain for 2 hours, then heat to 1600° C., maintain for 1 hour, and finally heat to 2250° C., maintain for 3 hours.