Industrial silicon directional solidification purification method and purification system thereof

Abstract

The invention discloses a directional solidification purification method and system for industrial silicon. The method comprises the following steps: soaking silicon powder with mixed acid to remove a surface oxide layer and metal impurities; mixing and melting the soaked silicon powder and a slag former according to the ratio of 10: 1, and continuously stirring for 30 minutes; and after standing for 10 minutes, removing furnace slag on the surface of the melt, and enabling the furnace slag to flow into a slag tank to obtain the silicon melt, so that the removal rate of Fe impurities is greater than or equal to 98%. After a flux is added into the silicon melt, stirring is performed, so that impurities such as Al, Ca and B can be effectively removed; a reactive gas is blown into the bottom of the silicon melt, and the silicon melt is partially vacuumized, so that Al, Ca, Mg and P can be efficiently removed, and B impurities can be partially removed; by means of crucible high-temperature control and induction coil arrangement, Lorentz force distribution can be optimized for Al / Ca impurities, impurity segregation is more thorough, crystal defects are reduced, and the requirement for solar grade silicon purity is met. And finally, a directional solidification method is adopted, so that most of metal impurities can be gathered at the tail end of the silicon ingot, and the product purity reaches 99.5% or above.

Description

Technical Field

[0001] This invention relates to the field of industrial silicon purification technology, and in particular to a method and system for directional solidification purification of industrial silicon. Background Technology

[0002] Silicon plays a vital role in national production. Silicon materials of different purities can be applied to various fields. For example, industrial silicon with a purity of 99% (2N) is mainly used as an industrial additive, while electronic-grade polycrystalline silicon with a purity of 99.9999999% (9N) or higher is mainly used for semiconductor chips. Solar cells require polycrystalline silicon with a purity of 99.9999% (6N) or higher; therefore, polycrystalline silicon with a purity of (6N) or higher is also called solar-grade polycrystalline silicon. In the photovoltaic industry, the purity requirements for industrial silicon are high, typically reaching 6N solar-grade silicon.

[0003] Current traditional methods for preparing silicon materials mainly rely on ore smelting and physicochemical reactions. While these processes are mature, they suffer from drawbacks such as high investment, high cost, high energy consumption, severe pollution, and difficulty in controlling impurities, hindering the development and application of silicon materials in the photovoltaic industry. Therefore, researching a low-investment, low-cost directional solidification purification process is of paramount importance. Summary of the Invention

[0004] The purpose of this application is to provide a method and system for directional solidification purification of industrial silicon, in order to study an industrial silicon purification process with low investment and low cost, and to improve the purity of industrial silicon products.

[0005] To address the aforementioned technical problems, this application provides a method for directional solidification purification of industrial silicon, comprising: Silicon blocks are crushed and sieved to obtain silicon powder, which is then soaked in a mixed acid. The soaked silicon powder and slagging agent were mixed at a ratio of 10:1, and the mixture was added to a refining furnace and heated to 1400°C under a vacuum of ≤6.67×10⁻³Pa to melt, and stirred continuously for 30 minutes. After standing for 10 minutes, remove the slag from the surface of the melt and let the slag flow into the slag pot to obtain the silicon melt; After adding flux to the silicon melt and stirring, the slag on the surface of the silicon melt is removed after standing and flows into the slag pot. Reactive gas is blown into the bottom of the silicon melt, and a vacuum is drawn into part of the silicon melt. The silicon melt is transferred to a crucible coated with Si3N4, heated to 1650°C and held for 30 minutes with continuous stirring, and an induction coil is installed on the outer periphery of the crucible to enrich impurities at the rear end of the silicon ingot. After solidification, the crucible is cooled to 800°C and annealed for 5 hours. After annealing, the impurity accumulation area at the rear end of the silicon ingot is removed to obtain the purified silicon ingot.

[0006] As a preferred embodiment, an industrial silicon directional solidification purification method, during the process of heating to 1400°C under a vacuum degree ≤6.67×10⁻³Pa, further includes using an electron beam to bombard and melt the silicon to promote the volatilization of phosphorus impurities.

[0007] The plan also requires detailed explanation of a method for directional solidification and purification of industrial silicon, which, after obtaining the silicon melt, further includes: A plasma torch is used to scan the surface of the silicon melt so that boron impurities can be removed by oxidation under the action of active plasma.

[0008] The solution requires further detailed explanation of an industrial silicon directional solidification purification method, in which, after scanning the surface of the silicon melt with a plasma torch, the process returns to the step of transferring the silicon melt into a crucible.

[0009] As a preferred embodiment, a method for directional solidification and purification of industrial silicon, wherein heating to 1650°C and holding at that temperature for 30 minutes specifically includes the following three stages: Resistance heating is used at the top of the crucible to maintain the temperature of the crucible top at 1500°C - 1650°C; Induction heating is used in the middle of the crucible to maintain the temperature in the middle of the crucible at 1450°C - 1520°C; A water-cooling assembly is installed at the bottom of the crucible to maintain the temperature at the bottom of the crucible below 1450°C.

[0010] In a preferred embodiment, an industrial silicon directional solidification purification method, after obtaining the silicon ingot, further includes: The silicon ingot is sliced ​​using a multi-wire dicing machine to obtain silicon wafers; The surface roughness Ra of the silicon wafer is reduced to <0.1 μm by removing the cutting damage layer using chemical mechanical polishing or hydrofluoric acid cleaning. The quality of the silicon wafer was detected by electron probe microscopy or inductively coupled plasma mass spectrometry.

[0011] To address the aforementioned technical problems, this application also provides an industrial silicon directional solidification purification system, based on any one of the above-described industrial silicon directional solidification purification methods, comprising: The refining furnace, a crucible connected to the outlet of the refining furnace, a crusher, a screening device connected to the crusher, and an soaking tank, wherein the outer side of the crucible is coated with a Si3N4 coating. An electromagnetic stirrer is installed in the upper part of the molten pool of the refining furnace. A slag pot is installed at the slag outlet of the refining furnace through a slag ditch. A high-temperature resistant jet pipe is connected to the bottom of the molten pool of the refining furnace. A high-temperature resistant vacuum pipe is connected to the upper part of the molten pool of the refining furnace. The vacuum pipe is connected to an air compressor. The crucible is provided with a graphite heating element, an induction coil, and a water cooling pipe arranged sequentially from top to bottom on its outer periphery.

[0012] Compared with the prior art, the present invention provides an industrial silicon directional solidification purification method, which includes crushing and screening silicon blocks to obtain silicon powder, soaking the silicon powder in a mixed acid to remove the surface oxide layer and metal impurities; mixing the soaked silicon powder with a slag-forming agent at a ratio of 10:1, adding the mixture to a refining furnace, heating it to 1400℃ under a vacuum degree ≤6.67×10⁻³Pa to melt it, and continuously stirring for 30 minutes; after standing for 10 minutes, removing the slag from the surface of the melt and allowing the slag to flow into a slag pot to obtain silicon melt, which can achieve an Fe impurity removal rate ≥98%. Flux is added to the molten silicon and stirred. After settling, slag is skimmed off the surface of the molten silicon and flows into a slag pot, effectively removing Al, Ca, and B impurities. Reactive gas is blown into the bottom of the molten silicon, and a partial vacuum is drawn to efficiently remove Al, Ca, Mg, and P, and partially remove B impurities. The molten silicon is then transferred to a crucible coated with Si3N4, heated to 1650℃ and held for 30 minutes with continuous stirring. An induction coil is installed around the outer perimeter of the crucible to concentrate impurities at the rear end of the silicon ingot. This step allows for more thorough impurity segregation and reduces crystal defects. Compared to traditional low-speed solidification, this increases production capacity and meets the purity requirements for solar-grade silicon. After solidification, the crucible is cooled to 800℃ and annealed for 5 hours. After annealing, the impurity accumulation area at the rear end of the silicon ingot is removed to obtain the purified silicon ingot. Finally, directional solidification is used, resulting in very high purity at the front end of the silicon ingot after solidification, while the rear end accumulates most of the Fe, Al, Ca, Ti, and other metallic impurities. High-purity silicon ingots can be obtained by removing the "impurity accumulation zone" at the end. Using this method for industrial silicon purification, the product purity can reach over 99.5%. The industrial silicon directional solidification purification system provided in this application achieves the above-mentioned results. Attached Figure Description

[0013] To more clearly illustrate the technical solution of this application, the drawings used in the embodiments will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without any creative effort.

[0014] Figure 1 A flowchart of an industrial silicon directional solidification purification method provided in this application embodiment; Figure 2 This is a schematic diagram of an industrial silicon directional solidification purification system provided in an embodiment of this application; Figure 3 This is a schematic diagram illustrating a process comparison and summary provided in the embodiments of this application; In the diagram: 1. Crusher; 2. Screening equipment; 3. Soaking tank; 4. Refining furnace; 5. Crucible; 6. Electromagnetic stirrer; 7. Slag pot; 8. High-temperature resistant jet pipe; 9. High-temperature resistant vacuum tube; 10. Air compressor. Detailed Implementation

[0015] To enable those skilled in the art to better understand the technical solutions in this application, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings.

[0016] The core of this application is to provide a method and system for directional solidification purification of industrial silicon, in order to study an industrial silicon purification process with low investment and low cost, and improve the purity of industrial silicon products.

[0017] Figure 1 This is a flowchart of a method for directional solidification purification of industrial silicon provided in an embodiment of this application. Figure 2 This is a schematic diagram of an industrial silicon directional solidification purification system provided in an embodiment of this application. Figure 3 This is a schematic diagram illustrating a process comparison and summary provided in an embodiment of this application. See also... Figures 1 to 3 As shown.

[0018] Example 1 A method for directional solidification purification of industrial silicon includes the following steps: S1: The silicon block is crushed and sieved to obtain silicon powder, and the silicon powder is soaked in a mixed acid.

[0019] The particles are crushed to 150-500μm and sieved to expose the grain boundary metal impurity phases (Fe-Si, Al-Si alloys). The mixed acid can be a mixture of HCl-HF-HNO3, which can dissolve and wash away the metal impurity phases enriched at the grain boundaries.

[0020] S2: Mix the soaked silicon powder with the slagging agent at a ratio of 10:1, add the mixture to the refining furnace, heat it to 1400℃ under a vacuum of ≤6.67×10⁻³Pa to melt it, and continue stirring for 30 minutes.

[0021] S3: After standing for 10 minutes, remove the slag from the surface of the melt and let the slag flow into the slag pot to obtain the silicon melt.

[0022] Silicon melt refers to high-temperature liquid silicon that is fluid and formed after solid silicon completely transforms into a liquid state. The slag-forming agent can be CaO, SiO2, and MgO in a mass ratio of 3:2:1. Electromagnetic stirring can be used, with a stirring frequency of 50-100Hz, a stirring intensity of 0.2-0.4T, and a stirring time of 30 minutes. This allows Fe impurities to react with the slag-forming agent to form FeO·SiO2 slag. After standing for 10 minutes, the slag on the surface of the melt is removed, allowing the slag to flow into a slag pot to obtain the silicon melt, thus achieving the separation of Fe impurities (removal rate ≥98%) from the silicon melt.

[0023] S4: After adding flux to the silicon melt and stirring, let it stand and then remove the slag from the surface of the silicon melt and let it flow into the slag pot.

[0024] Specifically, SiO2, CaO, and CaF2 fluxes are added to the molten silicon to form slag. Impurity elements, such as Al, Ca, and B, have different distribution coefficients between the molten silicon and the slag, and will tend to enter the slag. This can effectively remove Al, Ca, and B. For example, alkaline oxide slag is effective in removing B. Electromagnetic stirring is also used to promote the reaction.

[0025] S5: Blow reactive gas into the bottom of the silicon melt and evacuate part of the silicon melt.

[0026] The reactive gas can be a mixture of oxygen, chlorine, and argon. Oxygen can oxidize impurities such as Al and Ca into oxides that enter the slag, and it can also oxidize B into B2O that volatilizes. Chlorine can form chlorides with impurities such as Al, Ca, and Fe, and due to its low boiling point, it volatilizes to remove AlCl3 and FeCl3. Argon, as an inert gas, acts as a stirrer and carries away the volatiles.

[0027] S6: Transfer the silicon melt to a crucible coated with Si3N4, heat it to 1650℃ and hold it for 30 minutes while stirring continuously, and install an induction coil on the outer periphery of the crucible to enrich impurities at the rear end of the silicon ingot.

[0028] S7: After solidification, cool the crucible to 800℃, hold for 5 hours for annealing, and then remove the impurity accumulation area at the rear end of the silicon ingot to obtain the purified silicon ingot.

[0029] This step is directional solidification, which utilizes the significant difference in solubility of impurities in solid and liquid silicon. Most impurities are far more soluble in liquid silicon than in solid silicon. As the silicon melt slowly solidifies from one end, the impurities are continuously pushed towards the last part to solidify. After solidification, the front end of the silicon ingot (the first part to solidify) has very high purity, while the end (the last part to solidify) accumulates most of the metallic impurities such as Fe, Al, Ca, and Ti. By removing the "impurity accumulation zone" at the end, a high-purity silicon ingot can be obtained. The solidification rate can be controlled between 0.5-2 mm / min.

[0030] Example 2 Based on Example 1, an industrial silicon directional solidification purification method, in the process of heating to 1400℃ under a vacuum degree ≤6.67×10⁻³Pa, also includes using an electron beam to bombard and melt silicon to promote the volatilization of phosphorus impurities.

[0031] In a high vacuum environment, silicon is bombarded and melted with an electron beam. The extremely high vacuum level can promote the volatilization of phosphorus. Because P has an extremely high vapor pressure, the removal rate can reach more than 90% under the high temperature and vacuum of the electron beam.

[0032] Based on Example 2, an industrial silicon directional solidification purification method further includes, after obtaining the silicon melt, the following: A plasma torch is used to scan the surface of the silicon melt to oxidize and remove boron impurities under the action of active plasma. In this embodiment, an industrial silicon directional solidification purification method involves scanning the surface of the silicon melt with a plasma torch and then transferring the silicon melt back into a crucible.

[0033] Under high temperature and active plasma (often containing oxidizing atmospheres such as O2 and H2O), boron is oxidized to BO or B2O3 and volatilizes. Using Example 2, B and P can be strongly removed. Finally, the process returns to the directional solidification stage, utilizing the segregation effect to push the remaining trace metal impurities to the end of the silicon ingot. After removal, the final high-purity silicon ingot is obtained.

[0034] The embodiments of this application employ a physical metallurgical method (rather than a traditional chemical method), which can purify industrial silicon (99%~99.5%) to solar-grade (99.9999% or higher). Compared with the traditional chemical method, the process of the embodiments of this application has the following performance indicators: Figure 3 As shown.

[0035] Based on Example 1, a method for directional solidification purification of industrial silicon, which involves heating to 1650°C and holding for 30 minutes, specifically includes the following three stages: For initial solidification, resistance heating is used at the top of the crucible to maintain the temperature at 1500°C - 1650°C; the purpose is to establish an absolutely flat and stable solidification interface at the bottom (starting end) of the crucible.

[0036] Solidification of the main body: Induction heating is used in the middle of the crucible to maintain the temperature in the middle of the crucible at 1450°C - 1520°C; the purpose is to efficiently "sweep" impurities such as Al and Ca into the liquid phase while maintaining a flat interface, and to pursue a balance between purification efficiency and production speed.

[0037] Final solidification: A water-cooling system is installed at the bottom of the crucible to maintain the temperature below 1450°C. The purpose is to suppress interfacial instability and impurity backmixing caused by extremely high impurity concentrations during the final 10-20% of melt solidification. This reduces the temperature difference in the liquidus region, thereby suppressing natural convection and bringing the melt closer to a "static" state.

[0038] Based on Example 1, an industrial silicon directional solidification purification method, after obtaining silicon ingots, further includes: Silicon wafers are obtained by slicing silicon ingots using a multi-wire cutting machine; this cutting method causes less damage and less material loss.

[0039] Chemical mechanical polishing (CMP) or hydrofluoric acid cleaning removes the dicing damage layer from silicon wafers, achieving a surface roughness Ra < 0.1 μm. Mechanical dicing leaves a micrometer-thick damage layer on the silicon wafer surface. This layer disrupts the crystal structure, contains numerous microcracks and stresses, and may be contaminated by the dicing medium, severely affecting the accuracy of some performance tests. CMP achieves global planarization and damage layer removal, yielding an atomically smooth, damage-free surface (Ra far less than 0.1 μm), representing the most advanced surface treatment method. Hydrofluoric acid cleaning is a more economical method, typically using an HF-HNO3 mixed acid system. HNO3 oxidizes and etches the silicon, while HF dissolves the generated SiO2. By adjusting the ratio and temperature, the etching depth can be precisely controlled, effectively removing the damage layer and cleaning surface metallic impurities.

[0040] Electron probe microscopy or inductively coupled plasma mass spectrometry is used to test the quality of silicon wafers to ensure that the purity and performance of silicon ingots meet the requirements.

[0041] Electron probe microscopy (EPM) specifically involves in-situ, micro-area compositional analysis on a longitudinal section of a silicon ingot. By bombarding the ingot with an electron beam, characteristic X-rays are excited, yielding "line scan" or "area distribution" maps. This allows for a direct visualization of the concentration gradients of impurities such as Al, Ca, and Fe from the high-purity region at the front to the enriched region at the back, offering high spatial resolution and excellent visualization. Inductively coupled plasma mass spectrometry (ICP-MS) involves sampling from different locations on the silicon ingot for trace element analysis. It boasts extremely high sensitivity and can detect light elements such as B and P, which are difficult to accurately measure using EPM. Combining the data from both methods allows for the accurate determination of the critical point where impurity concentration begins to rise sharply.

[0042] The present application provides a method for directional solidification purification of industrial silicon, which includes crushing and sieving silicon blocks to obtain silicon powder, soaking the silicon powder in a mixed acid to remove the surface oxide layer and metal impurities, mixing the soaked silicon powder with a slag-forming agent at a ratio of 10:1, adding the mixture to a refining furnace, heating it to 1400°C under a vacuum of ≤6.67×10⁻³Pa to melt it, and continuously stirring for 30 minutes; after standing for 10 minutes, removing the slag from the surface of the melt and allowing the slag to flow into a slag pot to obtain silicon melt, which can achieve a Fe impurity removal rate of ≥98%. Flux is added to the molten silicon and stirred. After settling, slag is skimmed off the surface of the molten silicon and flows into a slag pot, effectively removing Al, Ca, and B impurities. Reactive gas is blown into the bottom of the molten silicon, and a partial vacuum is drawn to efficiently remove Al, Ca, Mg, and P, and partially remove B impurities. The molten silicon is then transferred to a crucible coated with Si3N4, heated to 1650℃ and held for 30 minutes with continuous stirring. An induction coil is installed around the outer perimeter of the crucible to concentrate impurities at the rear end of the silicon ingot. This step allows for more thorough impurity segregation and reduces crystal defects. Compared to traditional low-speed solidification, this increases production capacity and meets the purity requirements for solar-grade silicon. After solidification, the crucible is cooled to 800℃ and annealed for 5 hours. After annealing, the impurity accumulation area at the rear end of the silicon ingot is removed to obtain the purified silicon ingot. Finally, directional solidification is used, resulting in very high purity at the front end of the silicon ingot after solidification, while the rear end accumulates most of the Fe, Al, Ca, Ti, and other metallic impurities. High-purity silicon ingots can be obtained by removing the "impurity accumulation zone" at the end. Using this method for industrial silicon purification, the product purity can reach over 99.5%.

[0043] Example 3 The foregoing has described in detail an embodiment of an industrial silicon directional solidification purification method. Based on the industrial silicon directional solidification purification method described in the above embodiment, this invention also provides an industrial silicon directional solidification purification system corresponding to the method. Since the system embodiment corresponds to the method embodiment, please refer to the method embodiment description for the system embodiment, and it will not be repeated here.

[0044] An industrial silicon directional solidification purification system, based on the industrial silicon directional solidification purification method provided in any of the above embodiments, includes: The crusher 1, the screening equipment 2 connected to the crusher 1, the soaking tank 3, the refining furnace 4, and the crucible 5 connected to the discharge port of the refining furnace 4, the outer side of the crucible 5 is coated with Si3N4. An electromagnetic stirrer 6 is installed in the upper part of the molten pool of the refining furnace 4. A slag pot 7 is installed at the slag outlet of the refining furnace 4 through the slag ditch. A high-temperature resistant jet pipe 8 is connected to the bottom of the molten pool of the refining furnace 4. A high-temperature resistant vacuum pipe 9 is connected to the upper part of the molten pool of the refining furnace 4. The high-temperature resistant vacuum pipe 9 is connected to the air compressor 10. The outer periphery of the crucible 5 is provided with a graphite heating element, an induction coil, and a water cooling pipe arranged sequentially from top to bottom.

[0045] The industrial silicon directional solidification purification system provided in this application embodiment can realize the directional solidification purification process of industrial silicon, and ultimately make the product purity meet the requirements.

[0046] Other embodiments of this application will readily occur to those skilled in the art upon consideration of the specification and practice of the application disclosed herein. This application is intended to cover any variations, uses, or adaptations of this application that follow the general principles of this application and incorporate common knowledge or customary techniques in the art disclosed herein. The specification and examples are to be considered exemplary only, and the true scope of this application is indicated by the claims.

[0047] It should be understood that this application is not limited to the precise structure described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The embodiments of this application described above do not constitute a limitation on the scope of protection of this application.

Claims

1. A method for directional solidification purification of industrial silicon, characterized in that, include: Silicon blocks are crushed and sieved to obtain silicon powder, which is then soaked in a mixed acid. The soaked silicon powder and slagging agent were mixed at a ratio of 10:1, and the mixture was added to a refining furnace and heated to 1400°C under a vacuum of ≤6.67×10⁻³Pa to melt, and stirred continuously for 30 minutes. After standing for 10 minutes, remove the slag from the surface of the melt and let the slag flow into the slag pot to obtain the silicon melt; After adding flux to the silicon melt and stirring, the slag on the surface of the silicon melt is removed after standing and flows into the slag pot. Reactive gas is blown into the bottom of the silicon melt, and a vacuum is drawn into part of the silicon melt. The silicon melt is transferred to a crucible coated with Si3N4, heated to 1650°C and held for 30 minutes with continuous stirring, and an induction coil is installed on the outer periphery of the crucible to enrich impurities at the rear end of the silicon ingot. After solidification, the crucible is cooled to 800°C and annealed for 5 hours. After annealing, the impurity accumulation area at the rear end of the silicon ingot is removed to obtain the purified silicon ingot.

2. The method for directional solidification purification of industrial silicon according to claim 1, characterized in that, The process of heating to 1400°C under a vacuum of ≤6.67×10⁻³Pa also includes bombarding and melting silicon with an electron beam to promote the volatilization of phosphorus impurities.

3. The method for directional solidification purification of industrial silicon according to claim 2, characterized in that, After obtaining the silicon melt, the process also includes: A plasma torch is used to scan the surface of the silicon melt so that boron impurities can be removed by oxidation under the action of active plasma.

4. The method for directional solidification purification of industrial silicon according to claim 3, characterized in that, After scanning the surface of the silicon melt with a plasma torch, the process returns to the step of transferring the silicon melt into the crucible.

5. The method for directional solidification purification of industrial silicon according to claim 1, characterized in that, The heating to 1650℃ and holding for 30 minutes specifically includes the following three stages: Resistance heating is used at the top of the crucible to maintain the temperature of the crucible top at 1500°C - 1650°C; Induction heating is used in the middle of the crucible to maintain the temperature in the middle of the crucible at 1450°C - 1520°C; A water-cooling assembly is installed at the bottom of the crucible to maintain the temperature at the bottom of the crucible below 1450°C.

6. The method for directional solidification purification of industrial silicon according to claim 1, characterized in that, After obtaining the silicon ingot, the process further includes: The silicon ingot is sliced ​​using a multi-wire dicing machine to obtain silicon wafers; The surface roughness Ra of the silicon wafer is reduced to <0.1 μm by removing the cutting damage layer using chemical mechanical polishing or hydrofluoric acid cleaning. The quality of the silicon wafer was detected by electron probe microscopy or inductively coupled plasma mass spectrometry.

7. An industrial silicon directional solidification purification system, based on the industrial silicon directional solidification purification method according to any one of claims 1 to 6, characterized in that, include: The crusher (1), the screening equipment (2) connected to the crusher (1), the soaking tank (3), the refining furnace (4) and the crucible (5) connected to the discharge port of the refining furnace (4), the outer side of the crucible (5) is coated with Si3N4 coating. An electromagnetic stirrer (6) is provided in the upper part of the molten pool of the refining furnace (4). A slag pot (7) is provided at the slag outlet of the refining furnace (4) through the slag ditch. A high-temperature resistant jet pipe (8) is connected to the bottom of the molten pool of the refining furnace (4). A high-temperature resistant vacuum pipe (9) is connected to the upper part of the molten pool of the refining furnace (4). The high-temperature resistant vacuum pipe (9) is connected to an air compressor (10). The crucible (5) is provided with a graphite heating element, an induction coil and a water cooling pipe from top to bottom on its outer periphery.

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