Method for controlling rapid melting and homogeneity of silicon powder after granulation and remelting

Abstract

The application discloses a rapid melting and uniformity control method for silicon powder after granulation and remelting, and belongs to the technical field of silicon material preparation. The method optimizes the heat conduction condition through core-shell structure silicon particle forming, realizes gradient rapid heating by adopting double-frequency induction heating, realizes homogenization by coupling dynamic electromagnetic stirring and ultrasonic vibration, precisely controls temperature by supplementing multi-point temperature measurement, removes impurities by gradient vacuum pulse gas, and finally separates impurities by directional solidification. The innovation lies in that the core-shell structure enhances the heat conduction and fluxing effect, multi-field cooperation improves the melt uniformity, gradient vacuum and pulse ventilation strengthen the impurity removal, solves the problems of slow melting, poor uniformity and many impurities in the traditional process, significantly improves the purity and performance of silicon material, and is suitable for efficient preparation in the fields of solar-grade polycrystalline silicon, electronic-grade single crystal silicon and the like.

Description

Technical Field

[0001] This invention relates to the field of silicon material preparation technology, specifically a method for rapid melting and uniformity control of silicon powder after granulation and remelting. Background Technology

[0002] Silicon is a core material in the semiconductor and photovoltaic industries, and its quality directly affects the performance of electronic devices and solar cells. With the rapid development of the new energy and electronic information industries, the demand for high-purity silicon materials is increasing, while higher requirements are being placed on the purity and uniformity of silicon materials. Currently, silicon materials are mainly prepared using processes such as the Siemens process, the silane process, and the metallurgical process. Among these, the metallurgical process is widely used in the preparation of solar-grade polycrystalline silicon due to its advantages of low cost and low energy consumption. The preparation of polycrystalline silicon using the metallurgical method typically includes steps such as silica reduction, refining and purification, and directional solidification, with silicon powder remelting being one of the key steps.

[0003] However, traditional silicon powder remelting processes have the following technical shortcomings: First, silicon powder has a small particle size and a large specific surface area, making it prone to agglomeration and bridging during direct melting, resulting in slow melting speed, high energy consumption, and low production efficiency. The loose structure of bulk silicon powder with numerous gaps between particles leads to low overall heat transfer efficiency. The high-temperature heat generated inside the furnace cannot quickly penetrate the silicon powder mass, resulting in prolonged melting time and unnecessary consumption of electrical and thermal energy. Second, during silicon powder melting, factors such as temperature gradients and melt convection cause uneven distribution of melt composition and temperature, leading to uneven impurity distribution and affecting the performance of the final silicon material. Traditional remelting processes rely solely on a single fixed-frequency heating mode for temperature control, resulting in large temperature differences between different areas of the furnace. The lack of precise global temperature control means that the melt remains in an environment with large temperature differences for extended periods, easily causing internal flow field turbulence and further exacerbating component segregation problems.

[0004] Furthermore, in traditional remelting processes, impurity removal mainly relies on vacuum degassing and slag refining, which has limited efficiency, especially for refractory impurities such as boron and phosphorus. Traditional methods for impurity removal after silicon powder remelting are relatively simple, mostly relying on a constant high vacuum environment for basic degassing. This can only remove some volatile, light impurities, and its effectiveness in removing metallic impurities and difficult-to-remove non-metallic impurities dissolved within the silicon melt is very limited. Simultaneously, the direct melting process of silicon powder is prone to splashing and oxidation, affecting process stability and product quality consistency.

[0005] To address the aforementioned problems, some improved methods have been proposed in the prior art, such as silicon powder granulation followed by remelting, electromagnetic stirring, and vacuum refining. However, these methods still have shortcomings: The methods for granulating silicon powder are limited, resulting in low particle strength, poor thermal conductivity, and limited improvement in melting speed. Electromagnetic stirring is simple but ineffective, failing to achieve uniform homogenization of the melt. Low temperature control precision and large temperature gradients lead to uneven melt composition and temperature distribution. Impurity removal processes are also limited, making it difficult to remove multiple types of impurities simultaneously. Therefore, developing a rapid melting and homogenization control method for remelting silicon powder after granulation to address the problems in traditional processes and improve the efficiency and quality of silicon material preparation is of significant practical importance and application value. Summary of the Invention

[0006] The purpose of this invention is to provide a method for rapid melting and uniformity control of silicon powder after granulation and remelting. Through steps such as silicon powder pretreatment granulation, rapid melting by gradient induction heating, homogenization by dynamic electromagnetic stirring combined with ultrasonic vibration, precise temperature field control, gradient vacuum pulse degassing and directional solidification, this method solves the problems of low melting efficiency, poor melt uniformity and uneven impurity distribution in traditional silicon powder remelting, thereby improving the preparation efficiency and quality of silicon materials.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A method for rapid melting and uniformity control of silicon powder after granulation and remelting, characterized by comprising the following steps performed sequentially: S1. Grading and pretreatment of silicon powder raw materials and preparation of core-shell structured silicon particles: The raw silicon powder is graded and screened to remove impurities and then mixed with a binder. Solid silicon particles with a core-shell structure are prepared by extrusion granulation. S2. Gradient induction heating for rapid melting: Core-shell silicon particles are placed in an induction furnace and heated gradually at a set rate to above the melting point of silicon under an inert atmosphere to form a silicon melt. S3. Dynamic electromagnetic stirring combined with ultrasonic vibration homogenization: Dynamic electromagnetic stirring is applied to the melt, and ultrasonic vibration is introduced at the same time. The stirring speed and vibration frequency are controlled to promote the uniform distribution of melt components. S4. Precise temperature field control: The melt temperature is monitored in real time through a multi-point temperature measurement system, and the heating power is adjusted to maintain a uniform and stable melt temperature. S5. Gradient vacuum pulse degassing: Gradient vacuum method combined with pulsed introduction of reaction gas is used to remove volatile impurities from the melt; S6. Directional solidification and post-processing: The homogenized melt is directionally solidified, and after solidification, the impurity-rich areas are cut away to obtain high-purity silicon material.

[0008] Preferably, step S1 specifically includes the following steps: S101. Using air classifier and mechanical screening equipment in tandem, the raw silicon powder is screened by particle size, and fine powder with a particle size of less than 50μm and coarse particles with a particle size of more than 800μm are removed, leaving qualified silicon powder raw material with a particle size of 100-500μm. S102. Mix the screened qualified silicon powder with the binder at a ratio of 100:1 to 100:5, control the mixing temperature at 50-80℃, and mix for 30-60 minutes to ensure uniform mixing. S103. Core-shell structured silicon particles are prepared using a double-layer extrusion granulator. The core is a mixture of silicon powder and binder with a diameter controlled at 5-10 mm. The outer shell is a mixture of silicon powder and flux with a thickness controlled at 1-3 mm. The flux is selected from one of boric anhydride, boric acid, or sodium carbonate. S104. Place the formed core-shell structured silicon particles in a drying device and dry them at 100-150℃ for 2-4 hours to remove moisture and improve particle strength. Alternatively, microwave drying can be used with a microwave frequency of 2450MHz, a drying power of 500-1000W, and a drying time of 30-60 minutes.

[0009] Preferably, step S2 specifically includes the following operation steps: S201. Inert gas with a purity of not less than 99.999% is introduced into the induction furnace, and the gas flow rate is controlled at 5-20L / min to maintain an oxygen-free environment inside the furnace. S202 employs a combined high-frequency and medium-frequency heating method, with a high-frequency frequency of 100-300kHz and a medium-frequency frequency of 10-50kHz. The heating process is divided into three stages: The first stage involves heating from room temperature to 800℃ at a rate of 10-20℃ / min. The second stage involves heating from 800℃ to 1200℃ at a rate of 5-10℃ / min. The third stage involves heating from 1200℃ to 1450-1500℃ at a rate of 2-5℃ / min. S203, after complete melting, hold at temperature for 10-30 minutes to form a uniform silicon melt; during the dual-frequency induction heating process, the power ratio of high frequency to medium frequency is 1:1-1:3. In the early stage of heating, the high frequency power accounts for 70-80% and the medium frequency power accounts for 20-30%; in the later stage of heating, the high frequency power accounts for 30-50% and the medium frequency power accounts for 50-70%; after complete melting, the high frequency power accounts for 20-30% and the medium frequency power accounts for 70-80%.

[0010] Preferably, step S3 specifically includes the following operation steps: S301: A variable frequency electromagnetic stirring device is adopted, with a stirring frequency of 5-50Hz. The stirring process is divided into three stages: The initial stirring speed is 10-30 rpm, and it is maintained for 5-10 minutes. The stirring speed during the intermediate stage is 50-80 rpm, and it lasts for 10-20 minutes. In the later stage, the stirring speed is 30-50 rpm for 5-10 minutes. S302. Uses an ultrasonic generator with a power of 500-2000W, a vibration frequency of 20-40kHz, and an ultrasonic vibration time of 5-15min. S303. Ultrasonic vibration and electromagnetic stirring are performed alternately, with each ultrasonic vibration lasting 1-3 minutes and the electromagnetic stirring lasting 2-5 minutes. The alternation cycle is 3-5 times to promote the uniform distribution of melt components.

[0011] Preferably, step S4 specifically includes the following operation steps: S401. Arrange 3-5 thermocouples in the upper, middle and lower parts of the melt respectively, with a temperature measurement accuracy of ±0.5℃, and build a multi-point temperature measurement system; S402. A PID control system is adopted to adjust the induction heating power in real time according to the temperature measurement data, so as to maintain the melt temperature fluctuation range within ±2℃. S403. A 5-10mm thick insulation layer is provided on the surface of the melt, made of graphite felt or alumina fiber, to reduce heat loss and improve temperature uniformity.

[0012] Preferably, step S5 specifically includes the following operation steps: S501, gradient vacuum is divided into three stages: The first stage has a vacuum of 10-50 Pa and a holding time of 10-20 min to remove gaseous impurities such as hydrogen and nitrogen. The second stage involves a vacuum of 1-10 Pa and a holding time of 20-30 min to remove volatile metallic impurities. The third stage involves a vacuum of 0.1-1 Pa and a holding time of 10-20 min to remove residual volatile impurities. S502. The reaction gas is pulsed in, with a purity of not less than 99.99%, a pulse frequency of 1-5 times / min, a duration of 5-15 seconds per pulse, and a pressure of 0.01-0.1 MPa. The reaction gas is selected from chlorine, hydrogen chloride, or hydrogen fluoride. S503. During pulse ventilation, 0.1-0.5 vol% oxygen can be introduced to promote the oxidation of carbon impurities into carbon monoxide gas. At the same time, low-frequency electromagnetic stirring is applied with a stirring frequency of 1-5 Hz and a stirring speed of 5-10 rpm to promote the uniform distribution of reaction gases.

[0013] Preferably, step S6 specifically includes the following operation steps: S601. A crucible-moving directional solidification furnace is used, with a solidification rate of 0.5-5 mm / min and a temperature gradient of 5-20℃ / cm. During solidification, the melt temperature is kept 5-20℃ higher than the melting point of silicon. S602. After solidification, the impurity-rich areas at the bottom and top are removed by wire cutting, with a removal thickness of 5-15mm. S603. The cut silicon material is subjected to pickling treatment. The pickling solution is a mixture of hydrofluoric acid and nitric acid with a volume ratio of 1:3-1:5. The pickling temperature is 20-40℃ and the pickling time is 10-30min to remove residual impurities on the surface.

[0014] Preferably, 0.1-0.5 wt% of a surfactant is added to the outer shell of the core-shell structured silicon particles prepared in S103. The surfactant is one of sodium dodecylbenzenesulfonate, Tween 80 or Span 80, which improves the bonding strength between the outer shell and the core and reduces the breakage rate during the granulation process. The binder is one of silica sol, polyvinyl alcohol or methyl cellulose, which improves the particle formability.

[0015] Preferably, argon or helium is used as the inert gas in S201. Argon has a lower cost and is suitable for large-scale production, while helium has good thermal conductivity and is suitable for rapid heating scenarios.

[0016] Preferably, in S502, the pulsed reaction gas can be selected according to the type of impurity. Chlorine is effective in removing metal impurities, hydrogen chloride is effective in removing boron impurities, and hydrogen fluoride is effective in removing phosphorus impurities. In S503, low-frequency electromagnetic stirring can promote the diffusion of reaction gas in the melt, enhance the reaction effect, and improve the impurity removal efficiency.

[0017] This invention provides a method for rapid melting and uniformity control of silicon powder after granulation and remelting, which has the following beneficial effects: Firstly, this invention differs from traditional solid silicon granules and bulk silicon powder. The outer coating layer of the granules contains fluxing components, which can reduce the temperature threshold required for silicon raw material to melt and accelerate the melting process of the surface material. At the same time, the dense granule molding structure can reduce the material accumulation gaps and significantly improve the overall heat conduction rate. This completely solves the problems of slow heat conduction and asynchronous melting of bulk silicon powder, effectively reduces the overall operation time from room temperature to complete melting, reduces the energy consumption of the high-temperature melting process, and meets the needs of large-scale mass production and cost reduction.

[0018] Secondly, the heating mode adopts a three-stage gradient heating combined with dual-frequency dynamic power adjustment. It can flexibly adjust the heat source output form according to the physical state changes of silicon particles in three different stages: solid softening, surface melting, and overall full melting. In the early stage of heating, high-frequency heat source is used to quickly accumulate heat and shorten the preheating time. In the melting and heat preservation stage, medium-frequency heat source is used to achieve uniform heating throughout the entire area, avoiding local overheating and burn-out or local insufficient heating. Compared with the single-frequency constant temperature heating mode, the heat field distribution is more reasonable and orderly.

[0019] Furthermore, it abandons the single stirring and homogenization mode and adopts a multi-field synergistic homogenization method that combines segmented electromagnetic stirring with intermittent ultrasonic vibration. Low-speed stirring completes the overall mixing of the melt over a large area, while high-frequency ultrasonic vibration deeply breaks down the fine-scale component-rich clumps inside the melt. The two effects work alternately to complement each other's shortcomings, which can not only achieve uniform flow field at the macro level of the melt, but also optimize the component distribution at the micro level. This eliminates the long-standing component segregation defects in silicon melt from two dimensions, ensuring a high degree of material consistency inside the silicon material after subsequent solidification.

[0020] The temperature control mode, featuring multi-point temperature measurement and intelligent power regulation across the entire area, can capture real-time temperature changes of the melt at different locations within the melting furnace. It automatically adjusts the heating power, and combined with a surface insulation structure to reduce heat loss, it effectively minimizes overall melt temperature fluctuations. This stable temperature environment prevents internal convection disturbances caused by temperature differences within the melt, further solidifying the already formed uniform melt distribution and providing a stable operating environment for subsequent impurity removal processes. The temperature control employs a PID control system, adjusting the induction heating power in real-time based on temperature measurement data to maintain melt temperature fluctuations within ±2℃, thus improving temperature uniformity.

[0021] The graded vacuum combined with pulsed functional gas introduction method breaks the limitations of constant vacuum impurity removal. It utilizes different vacuum levels to adapt to impurities with different boiling points and different forms of existence. Then, by taking advantage of the directional chemical reaction between the reactive gas and the impurities, the solid dopants that were originally difficult to remove are converted into gaseous substances and escape smoothly. This greatly expands the range of impurities that can be removed and improves the overall purity of the remelted silicon material. At the same time, the pulsed gas introduction mode can precisely control the amount of gas used and avoid excessive gas introduction that could cause molten environment disturbance.

[0022] The entire process is tightly and rationally connected. The pre-granulation process optimizes the basic melting conditions, the mid-stage multi-field control stabilizes the melt quality, and the post-stage directional solidification precisely separates impurity areas. The entire process is interconnected to form a complete closed loop. It can not only be adapted to the remelting of virgin high-quality silicon powder, but also to the upgrading and remelting of various recycled waste silicon powder and low-purity industrial silicon powder. The process has a wide range of applicability, the overall production process is easy to operate, and the process is highly stable. It can produce finished silicon materials with uniform quality and compliant physicochemical properties in a long-term and stable manner, and has extremely high practical industrial application value. Attached Figure Description

[0023] Figure 1 This is a flowchart of the process of the invention. Detailed Implementation

[0024] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0025] Example 1 S1. Photovoltaic-cut silicon powder is screened using a combination of airflow classification and sieving to remove fine powder with a particle size less than 50 μm and coarse particles with a particle size greater than 800 μm, resulting in silicon powder with a particle size of 100-500 μm. The silicon powder is mixed with silica sol binder at a ratio of 100:3, with the mixing temperature controlled at 60℃ and the mixing time at 45 min. Core-shell structured silicon granules are prepared using a double-layer extrusion granulator. The core is a mixture of silicon powder and silica sol with a diameter of 8 mm, and the outer shell is a mixture of silicon powder and boron anhydride flux with a thickness of 2 mm. After granulation, the granules are dried at 120℃ for 3 h to remove moisture.

[0026] S2. Place the silicon granules in an induction furnace and introduce 99.999% pure argon gas at a flow rate of 10 L / min. Use a dual-frequency induction heating method with a high frequency of 200 kHz and a medium frequency of 30 kHz. The heating process is divided into three stages: the first stage heats from room temperature to 800℃ at a rate of 15℃ / min; the second stage heats from 800℃ to 1200℃ at a rate of 8℃ / min; and the third stage heats from 1200℃ to 1480℃ at a rate of 3℃ / min. After complete melting, hold at this temperature for 20 minutes to form a uniform silicon melt. During the dual-frequency induction heating process, the power ratio of the high frequency to the medium frequency is 1:2. In the initial stage of heating, the high frequency power accounts for 75% and the medium frequency power accounts for 25%; in the later stage of heating, the high frequency power accounts for 40% and the medium frequency power accounts for 60%; and after complete melting, the high frequency power accounts for 25% and the medium frequency power accounts for 75%.

[0027] S3. Dynamic electromagnetic stirring combined with ultrasonic vibration homogenization, using a variable frequency electromagnetic stirring device with a stirring frequency of 25Hz; the stirring process is divided into three stages: The initial stirring speed is 20 rpm for 8 minutes; The stirring speed during the intermediate stage is 65 rpm for 15 minutes. The stirring speed was 40 rpm for 7 minutes during the later stage. At the same time, an ultrasonic generator with a power of 1000W and a vibration frequency of 30kHz is used. Ultrasonic vibration and electromagnetic stirring are performed alternately. Each ultrasonic vibration lasts for 2 minutes and the electromagnetic stirring lasts for 3 minutes. The alternation cycle is 4 times.

[0028] S4. Precise temperature field control uses four thermocouples, which are respectively arranged in the upper, middle and lower parts of the melt, with a temperature measurement accuracy of ±0.5℃. Temperature control adopts a PID control system, which adjusts the induction heating power in real time according to the temperature measurement data to maintain the melt temperature fluctuation range within ±1.5℃. An 8mm thick graphite felt insulation layer is set on the surface of the melt to reduce heat loss.

[0029] S5, gradient vacuum pulse degassing, the gradient vacuum is divided into three stages: The first stage vacuum degree is 30Pa, and the holding time is 15min; The second stage has a vacuum degree of 5 Pa and a holding time of 25 min; The third stage has a vacuum of 0.5 Pa and a holding time of 15 min. Chlorine gas with a purity of 99.99% is pulsed in at a frequency of 3 times / min, with each pulse lasting 10 seconds and a pressure of 0.05 MPa.

[0030] S6. Directional solidification and post-treatment are carried out using a crucible-moving directional solidification furnace with a solidification rate of 2 mm / min and a temperature gradient of 10℃ / cm. During solidification, the melt temperature is kept 10℃ higher than the melting point of silicon. After solidification, the impurity-rich areas at the bottom and top are removed by wire cutting, with a removal thickness of 10 mm. The cut silicon material is then acid-washed using a mixture of hydrofluoric acid and nitric acid in a volume ratio of 1:4. The acid washing temperature is 30℃, and the acid washing time is 20 min.

[0031] Example 2 S1. Pretreatment of silicon powder raw materials and core-shell structure granulation: Industrial virgin silicon powder is screened by a combination of air classification and sieving to remove fine powder with a particle size of less than 50 μm and coarse particles with a particle size of more than 800 μm, so as to obtain silicon powder with a particle size of 100-500 μm; Silicon powder and polyvinyl alcohol binder are mixed at a ratio of 100:5, the mixing temperature is controlled at 80℃, and the mixing time is 60 min. Core-shell structured silicon granules were prepared using a double-layer extrusion granulator. The core was a mixture of silicon powder and polyvinyl alcohol with a diameter of 10 mm, and the outer shell was a mixture of silicon powder and boric acid flux with a thickness of 3 mm. 0.3 wt% of Tween 80 surfactant was added to the outer shell. After granulation, the granules were dried by microwave at a frequency of 2450 MHz, a drying power of 800 W, and a drying time of 45 min to remove moisture.

[0032] S2. Gradient induction heating for rapid melting: Silicon granules are placed in an induction furnace, and helium gas with a purity of 99.999% is introduced, with the gas flow rate controlled at 20L / min; a dual-frequency induction heating method is adopted, with a high frequency of 300kHz and a medium frequency of 50kHz. The heating process is divided into three stages: The first stage involves heating from room temperature to 800℃ at a rate of 20℃ / min. The second stage involves heating from 800℃ to 1200℃ at a rate of 10℃ / min. The third stage involves heating from 1200℃ to 1500℃ at a rate of 5℃ / min. After complete melting, the mixture is kept at a constant temperature for 30 minutes to form a uniform silicon melt. During the dual-frequency induction heating process, the power ratio of high frequency to medium frequency is 1:3. In the early stage of heating, the high frequency power accounts for 80% and the medium frequency power accounts for 20%. In the later stage of heating, the high frequency power accounts for 30% and the medium frequency power accounts for 70%. After complete melting, the high frequency power accounts for 20% and the medium frequency power accounts for 80%.

[0033] S3. Dynamic electromagnetic stirring combined with ultrasonic vibration homogenization, using a frequency conversion electromagnetic stirring device with a stirring frequency of 50Hz. The stirring process is divided into three stages: The initial stirring speed is 30 rpm for 10 minutes; The stirring speed during the intermediate stage is 80 rpm for 20 minutes. The stirring speed in the later stage was 50 rpm for 10 minutes. At the same time, an ultrasonic generator with a power of 2000W and a vibration frequency of 40kHz is used. Ultrasonic vibration and electromagnetic stirring are performed alternately. Each ultrasonic vibration lasts for 3 minutes, and the electromagnetic stirring lasts for 5 minutes. The alternation cycle is 5 times.

[0034] S4. Precise temperature field control employs five thermocouples, respectively positioned at the top, middle, and bottom of the melt, with a temperature measurement accuracy of ±0.5℃. Temperature control utilizes a PID control system, which adjusts the induction heating power in real time based on the measured temperature data to maintain the melt temperature fluctuation within ±1.2℃. A 10mm thick alumina fiber insulation layer is installed on the surface of the melt to reduce heat loss.

[0035] S5, gradient vacuum pulse degassing, the gradient vacuum is divided into three stages: The first stage vacuum degree is 50Pa, and the holding time is 20min; The second stage has a vacuum level of 10 Pa and a holding time of 30 min; The vacuum level in the third stage is 1 Pa, and the holding time is 20 min. Hydrogen chloride with a purity of 99.99% was pulsed through the system at a frequency of 5 pulses / min, with each pulse lasting 15 seconds and a pressure of 0.1 MPa. Simultaneously, 0.3 vol% oxygen was introduced to promote the oxidation of carbon impurities. During the pulsed ventilation process, a low-frequency electromagnetic stirrer was applied at a frequency of 3 Hz and a speed of 8 rpm.

[0036] S6. Directional solidification and post-treatment are carried out using a crucible-moving directional solidification furnace with a solidification rate of 5 mm / min and a temperature gradient of 20℃ / cm. During solidification, the melt temperature is kept 20℃ higher than the melting point of silicon. After solidification, the impurity-rich areas at the bottom and top are removed by wire cutting, with a removal thickness of 15 mm. The cut silicon material is then acid-washed using a mixture of hydrofluoric acid and nitric acid in a volume ratio of 1:5. The acid washing temperature is 40℃ and the washing time is 30 min.

[0037] Example 3 S1. Pretreatment of silicon powder raw materials and core-shell granulation: Ultrafine waste silicon powder is screened by a combination of air classification and sieving to remove fine powder with a particle size of less than 50 μm and coarse particles with a particle size of more than 800 μm, so as to obtain silicon powder with a particle size of 100-500 μm; Silicon powder and methylcellulose binder are mixed at a ratio of 100:1, the mixing temperature is controlled at 50℃, and the mixing time is 30 min; Core-shell structured silicon granules were prepared using a double-layer extrusion granulator. The core was a mixture of silicon powder and methylcellulose with a diameter of 5 mm, and the outer shell was a mixture of silicon powder and sodium carbonate flux with a thickness of 1 mm. 0.1 wt% sodium dodecylbenzenesulfonate surfactant was added to the outer shell. After granulation, the granules were dried at 100 °C for 2 h to remove moisture.

[0038] S2. Gradient induction heating for rapid melting: Silicon granules are placed in an induction furnace and 99.999% pure argon gas is introduced at a flow rate of 5L / min. Dual-frequency induction heating is used, with a high frequency of 100kHz and a medium frequency of 10kHz. The heating process is divided into three stages: The first stage involves heating from room temperature to 800℃ at a rate of 10℃ / min. The second stage involves heating from 800℃ to 1200℃ at a rate of 5℃ / min. The third stage involves heating from 1200℃ to 1450℃ at a rate of 2℃ / min. After complete melting, the mixture is kept at a constant temperature for 10 minutes to form a uniform silicon melt. During the dual-frequency induction heating process, the power ratio of high frequency to medium frequency is 1:1. In the early stage of heating, the high frequency power accounts for 70% and the medium frequency power accounts for 30%. In the later stage of heating, the high frequency power accounts for 50% and the medium frequency power accounts for 50%. After complete melting, the high frequency power accounts for 30% and the medium frequency power accounts for 70%.

[0039] S3. Dynamic electromagnetic stirring combined with ultrasonic vibration homogenization adopts a frequency conversion electromagnetic stirring device with a stirring frequency of 5Hz. The stirring process is divided into three stages: The initial stirring speed is 10 rpm for 5 minutes; The stirring speed during the intermediate stage is 50 rpm for 10 minutes. The stirring speed in the later stage was 30 rpm for 5 minutes. At the same time, a 500W ultrasonic generator with a vibration frequency of 20kHz is used. Ultrasonic vibration and electromagnetic stirring are alternated. Each ultrasonic vibration lasts for 1 minute and the electromagnetic stirring lasts for 2 minutes. The alternation cycle is 3 times.

[0040] S4. Precise temperature field control uses three thermocouples, which are respectively arranged in the upper, middle and lower parts of the melt, with a temperature measurement accuracy of ±0.5℃; Temperature control adopts a PID control system, which adjusts the induction heating power in real time based on temperature measurement data to maintain the melt temperature fluctuation range within ±1.8℃; a 5mm thick graphite felt insulation layer is set on the melt surface to reduce heat loss.

[0041] S5, gradient vacuum pulse degassing, the gradient vacuum is divided into three stages: The first stage vacuum degree is 10Pa, and the holding time is 10min; The second stage has a vacuum level of 1 Pa and a holding time of 20 min; The vacuum level in the third stage is 0.1 Pa, and the holding time is 10 min. Hydrogen fluoride with a purity of 99.99% was pulsed through the system at a frequency of 1 pulse / min, with each pulse lasting 5 seconds and a pressure of 0.01 MPa. Simultaneously, 0.1 vol% oxygen was introduced to promote the oxidation of carbon impurities. During the pulsed ventilation process, a low-frequency electromagnetic stirrer was applied at a frequency of 1 Hz and a speed of 5 rpm.

[0042] S6. Directional solidification and post-treatment are carried out using a crucible-moving directional solidification furnace with a solidification rate of 0.5 mm / min and a temperature gradient of 5℃ / cm. During solidification, the melt temperature is kept 5℃ higher than the melting point of silicon. After solidification, the impurity-rich areas at the bottom and top are removed by wire cutting with a removal thickness of 5 mm. The cut silicon material is subjected to pickling treatment. The pickling solution is a mixture of hydrofluoric acid and nitric acid with a volume ratio of 1:3. The pickling temperature is 20℃ and the pickling time is 10min.

[0043] Comparative Example 1 (Traditional Direct Remelting of Silicon Powder) S1. Pretreatment of silicon powder raw materials: The silicon powder is sieved to obtain silicon powder with a particle size of 100-500μm; S2. Direct induction heating melting: Silicon powder is placed directly into the induction furnace, and argon gas with a purity of 99.999% is introduced. The gas flow rate is controlled at 10L / min. Single-frequency induction heating is used with a frequency of 100kHz and a heating rate of 5℃ / min. The temperature is raised to 1480℃ and held for 30min to form a silicon melt. S3. Static heat preservation: the melt is kept at a constant temperature for 30 minutes without electromagnetic stirring or ultrasonic vibration. S4. Vacuum degassing, vacuum degree is 1Pa, heat preservation time is 30min; S5. Directional solidification and post-treatment are carried out using a crucible-moving directional solidification furnace with a solidification rate of 2 mm / min and a temperature gradient of 10℃ / cm. After solidification, the impurity-rich area is cut off and acid-washed.

[0044] Comparative Example 2 (Common granulation and remelting method for silicon powder) S1. Pretreatment and ordinary granulation of silicon powder raw materials: The silicon powder is sieved to obtain silicon powder with a particle size of 100-500μm; the silicon powder and silica sol binder are mixed at a ratio of 100:3 to prepare ordinary spherical silicon particles with a diameter of 8mm; after granulation, the particles are dried at 120℃ for 3h. S2. Induction heating melting: Place silicon granules in an induction furnace and introduce argon gas with a purity of 99.999% at a flow rate of 10 L / min. Use single-frequency induction heating with a frequency of 100 kHz and a heating rate of 10 °C / min to heat to 1480 °C and hold for 20 min to form a silicon melt. S3. The electromagnetic stirring adopts a fixed frequency electromagnetic stirring with a stirring frequency of 25Hz, a stirring speed of 50rpm, and a stirring time of 30min. S4. Vacuum degassing, vacuum degree is 1Pa, heat preservation time is 30min; S5. Directional solidification and post-treatment are carried out using a crucible-moving directional solidification furnace with a solidification rate of 2 mm / min and a temperature gradient of 10℃ / cm. After solidification, the impurity-rich area is cut off and acid-washed.

[0045] Comparison of effects between the examples and the comparative examples ; Conclusion: The comparison results show that the melting time of Examples 1-3 of this invention is significantly shorter than that of Comparative Examples 1 and 2, the melt temperature uniformity is significantly better than that of the comparative examples, the impurity content is lower, the resistivity uniformity of the silicon material is better, and the process stability is higher. This indicates that the rapid melting and uniformity control method for remelting silicon powder after granulation of this invention, through innovative technologies such as core-shell structure granulation, dual-frequency induction heating, dynamic electromagnetic stirring combined with ultrasonic vibration, gradient vacuum pulse degassing, and directional solidification, effectively solves the problems existing in traditional silicon powder remelting, improves the preparation efficiency and quality of silicon materials, and has significant technical advantages and application value.

[0046] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for rapid melting and uniformity control of silicon powder after granulation and remelting, characterized in that, The steps are as follows, performed sequentially: S1. Grading and pretreatment of silicon powder raw materials and preparation of core-shell structured silicon particles: The raw silicon powder is graded and screened to remove impurities and then mixed with a binder. Solid silicon particles with a core-shell structure are prepared by extrusion granulation. S2. Gradient induction heating for rapid melting: Core-shell silicon particles are placed in an induction furnace and heated gradually at a set rate to above the melting point of silicon under an inert atmosphere to form a silicon melt. S3. Dynamic electromagnetic stirring combined with ultrasonic vibration homogenization: Dynamic electromagnetic stirring is applied to the melt, and ultrasonic vibration is introduced at the same time. The stirring speed and vibration frequency are controlled to promote the uniform distribution of melt components. S4. Precise temperature field control: The melt temperature is monitored in real time through a multi-point temperature measurement system, and the heating power is adjusted to maintain a uniform and stable melt temperature. S5. Gradient vacuum pulse degassing: This method uses a gradient vacuum approach combined with pulsed introduction of reaction gas to remove volatile impurities from the melt. S6. Directional solidification and post-processing: The homogenized melt is directionally solidified, and after solidification, the impurity-rich areas are cut away to obtain high-purity silicon material.

2. The method for rapid melting and uniformity control of silicon powder after granulation and remelting according to claim 1, characterized in that, Step S1 specifically includes the following operations: S101. Using air classifier and mechanical screening equipment in tandem, the raw silicon powder is screened by particle size, and fine powder with a particle size of less than 50μm and coarse particles with a particle size of more than 800μm are removed, leaving qualified silicon powder raw material with a particle size of 100-500μm. S102. Mix the screened qualified silicon powder with the binder at a ratio of 100:1-100:5, control the mixing temperature at 50-80℃, and mix for 30-60 minutes. S103. Core-shell structured silicon granules are prepared using a double-layer extrusion granulator. The core is a mixture of silicon powder and binder, and the outer shell is a mixture of silicon powder and flux. The flux is selected from boric anhydride, boric acid, or sodium carbonate. S104. Place the formed core-shell structured silicon particles in a drying device and dry them at 100-150℃ for 2-4 hours to remove moisture.

3. The method for rapid melting and uniformity control of silicon powder after granulation and remelting according to claim 1, characterized in that, Step S2 specifically includes the following operation steps: S201. Inert gas with a purity of not less than 99.999% is introduced into the induction furnace; S202 employs a combined high-frequency and medium-frequency heating method, with a high-frequency frequency of 100-300kHz and a medium-frequency frequency of 10-50kHz. The heating process is divided into three stages: The first stage involves heating from room temperature to 800℃ at a rate of 10-20℃ / min. The second stage involves heating from 800℃ to 1200℃ at a rate of 5-10℃ / min. The third stage involves heating from 1200℃ to 1450-1500℃ at a rate of 2-5℃ / min. S203, after complete melting, hold at temperature for 10-30 minutes to form a uniform silicon melt; during the dual-frequency induction heating process, the power ratio of high frequency to medium frequency is 1:1-1:

3. In the early stage of heating, the high frequency power accounts for 70-80% and the medium frequency power accounts for 20-30%; in the later stage of heating, the high frequency power accounts for 30-50% and the medium frequency power accounts for 50-70%; after complete melting, the high frequency power accounts for 20-30% and the medium frequency power accounts for 70-80%.

4. The method for rapid melting and uniformity control of silicon powder after granulation and remelting according to claim 1, characterized in that, Step S3 specifically includes the following operational steps: S301: A variable frequency electromagnetic stirring device is adopted, with a stirring frequency of 5-50Hz. The stirring process is divided into three stages: The initial stirring speed is 10-30 rpm, and it is maintained for 5-10 minutes. The stirring speed during the intermediate stage is 50-80 rpm, and it lasts for 10-20 minutes. In the later stage, the stirring speed is 30-50 rpm for 5-10 minutes. S302. Uses an ultrasonic generator with a power of 500-2000W, a vibration frequency of 20-40kHz, and an ultrasonic vibration time of 5-15min. S303. Ultrasonic vibration and electromagnetic stirring are performed alternately. Each ultrasonic vibration lasts for 1-3 minutes, and the electromagnetic stirring lasts for 2-5 minutes. The alternation cycle is 3-5 times.

5. The method for rapid melting and uniformity control of silicon powder after granulation and remelting according to claim 1, characterized in that, S4 specifically includes the following operation steps: S401. Arrange 3-5 thermocouples in the upper, middle and lower parts of the melt respectively, with a temperature measurement accuracy of ±0.5℃, and build a multi-point temperature measurement system; S402. A PID control system is adopted to adjust the induction heating power in real time according to the temperature measurement data, so as to maintain the melt temperature fluctuation range within ±2℃. S403. A heat insulation layer with a thickness of 5-10mm is provided on the surface of the melt, and the material is graphite felt or alumina fiber.

6. The method for rapid melting and uniformity control of silicon powder after granulation and remelting according to claim 1, characterized in that, Step S5 specifically includes the following operational steps: S501, gradient vacuum is divided into three stages: The first stage has a vacuum of 10-50 Pa and a holding time of 10-20 min to remove hydrogen and nitrogen gas impurities. The second stage involves a vacuum of 1-10 Pa and a holding time of 20-30 min to remove volatile metallic impurities. The third stage involves a vacuum of 0.1-1 Pa and a holding time of 10-20 min to remove residual volatile impurities. S502. The reaction gas is pulsed in, with a purity of not less than 99.99%, a pulse frequency of 1-5 times / min, a duration of 5-15 seconds per pulse, and a pressure of 0.01-0.1 MPa. The reaction gas is selected from chlorine, hydrogen chloride, or hydrogen fluoride. S503. During pulse ventilation, 0.1-0.5 vol% oxygen can be introduced to promote the oxidation of carbon impurities into carbon monoxide gas, while low-frequency electromagnetic stirring is applied.

7. The method for rapid melting and uniformity control of silicon powder after granulation and remelting according to claim 1, characterized in that, Step S6 specifically includes the following operational steps: S601. A crucible-moving directional solidification furnace is used, with a solidification rate of 0.5-5 mm / min and a temperature gradient of 5-20℃ / cm. During solidification, the melt temperature is kept 5-20℃ higher than the melting point of silicon. S602. After solidification, the impurity-rich areas at the bottom and top are removed by wire cutting, with a removal thickness of 5-15mm. S603. The cut silicon material is subjected to pickling treatment. The pickling solution is a mixture of hydrofluoric acid and nitric acid with a volume ratio of 1:3-1:

5. The pickling temperature is 20-40℃ and the pickling time is 10-30min to remove residual impurities on the surface.

8. The method for rapid melting and uniformity control of silicon powder after granulation and remelting according to claim 2, characterized in that, In the preparation of core-shell structured silicon particles using S103, 0.1-0.5 wt% of a surfactant is added to the outer shell. The surfactant is one of sodium dodecylbenzenesulfonate, Tween 80, or Span 80. The binder is one of silica sol, polyvinyl alcohol, or methylcellulose.

9. The method for rapid melting and uniformity control of silicon powder after granulation and remelting according to claim 3, characterized in that, In S201, argon or helium is selected as the inert gas.

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