Method for producing ultralow-calcium high-purity industrial silicon by 33MVA large submerged arc furnace

Abstract

The invention discloses a method for producing ultra-low-calcium high-purity industrial silicon through a 33MVA large submerged arc furnace, and belongs to the technical field of metallurgy and energy. Silica, bonded silica coal, unbonded silica coal and semi-coke are adopted as core production raw materials, meanwhile, wood chips are added to improve the air permeability of the materials, and introduction of iron impurities is reduced from the source by strictly controlling the iron content in all the raw materials; meanwhile, the submerged arc furnace operation technology is optimized, the iron melting phenomenon in the operation process is reduced, and the product iron content is further controlled; and the contents of aluminum and calcium in the product are obviously reduced by improving an external refining process. According to the method, ultra-low-calcium high-purity industrial silicon with marks such as Si3301, Si3302 and Si4202 can be stably produced, the minimum calcium content can be stably controlled to be 0.01% or below, compared with an existing production process, the raw materials used in the method are sufficient in market supply, the process controllability is high, the production path of the ultra-low-calcium high-purity industrial silicon is widened, production energy consumption and cost are reduced, and the requirement for green metallurgy development is met.

Description

Technical Field

[0001] This invention belongs to the field of metallurgy and energy technology, specifically relating to a method for producing ultra-low calcium high-purity industrial silicon using a 33MVA large-scale submerged arc furnace. Background Technology

[0002] Industrial silicon is a key basic material in metallurgy, chemical industry, photovoltaics and other fields. Among them, ultra-low calcium high-purity industrial silicon (such as Si3301, Si3302, Si4202, etc.) is in increasing demand in high-end application fields such as high-end organosilicon and electronic-grade polycrystalline silicon due to its extremely low impurity content. The purity of industrial silicon is mainly determined by the content of impurities such as iron, aluminum and calcium. In particular, controlling the calcium content is the most difficult and directly affects the application performance of the product.

[0003] Currently, industrial silicon produced by large-scale submerged arc furnaces (around 33 MVA) in China generally suffers from high impurity content, making it difficult to consistently produce ultra-low calcium high-purity products with a calcium content ≤0.01%. The main reasons include: firstly, inappropriate raw material selection, with traditional reducing agents having high impurity content, especially excessive iron content, introducing a large amount of impurities at the source; secondly, imperfect operating processes during smelting, leading to iron melting and further increasing the iron content of the product; and thirdly, outdated ladle refining processes, with traditional oxygen blowing refining having limited effectiveness in removing aluminum and calcium, and posing environmental risks, making it difficult to meet the production requirements of ultra-low calcium high-purity industrial silicon. Furthermore, while some small-scale submerged arc furnaces have attempted to produce high-purity industrial silicon, they suffer from low capacity, high energy consumption, and reliance on imported raw materials, preventing large-scale production. Therefore, developing a method for producing ultra-low calcium high-purity industrial silicon based on a 33 MVA large-scale submerged arc furnace, and solving key technical challenges such as raw material impurity control, process iron melting, and efficient aluminum and calcium removal, has become an urgent need for the industry. Summary of the Invention

[0004] This invention provides a method for producing ultra-low calcium high-purity industrial silicon using a 33MVA large-scale submerged arc furnace, solving the problem that large-scale submerged arc furnaces are difficult to use for large-scale production of ultra-low calcium high-purity industrial silicon in the prior art.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A method for producing ultra-low calcium high-purity industrial silicon using a 33MVA large-scale submerged arc furnace includes the following steps: Raw material pretreatment: The semi-coke is crushed and sieved, with the particle size controlled within the range of 5-30mm. This particle size range can increase the specific surface area of ​​the semi-coke and improve its reactivity. The sieved semi-coke is then sent to a steam pipeline for drying and preheating. The drying temperature is 120-150℃, and the drying time is 2-3 hours. The preheating temperature is 200-250℃, and the preheating time is 1-1.5 hours. This process can effectively remove moisture from the semi-coke and increase its porosity, further enhancing its reactivity. The silica is washed and sieved to remove impurities such as mud and stones. The silica particle size is controlled within the range of 30-200mm, the caking silica coal particle size is controlled within the range of 3-20mm, and the non-caking silica coal particle size is controlled within the range of 3-15mm. This ensures that the particles of each raw material are uniform and guarantees the uniformity of subsequent batching and mixing.

[0006] Ingredient mixing: The pretreated silica, caking coal, uncaking coal, semi-coke, and wood chips are mixed in batches of 200kg:20kg:100kg:10kg:40kg on a mixing belt to ensure that all raw materials are fully and evenly mixed to obtain a mixture. In this batching ratio, the proportion of silica is fixed, and the proportions of other raw materials can be adjusted appropriately according to the actual fixed carbon content of each raw material. This can ensure that the reduction reaction proceeds fully while minimizing the introduction of impurities.

[0007] Submerged arc furnace smelting: The mixed material is fed into a 33MVA large submerged arc furnace via a conveyor belt. The tamping depth is controlled to be no less than 500mm from the material surface to ensure the stability of the material layer structure in the furnace, from the raw material zone to the clinker zone, the SiC formation zone, the SiC decomposition zone, and the crucible zone, thus preventing local reaction imbalances caused by material layer collapse. Current is introduced into the furnace through three-phase electrodes. The arc heat generated by the arcing at the electrode tip and the resistance heat generated by the current passing through the furnace charge provide the required temperature for the reaction. The material surface temperature is controlled to not exceed 480℃, and the reaction temperature in the crucible zone is controlled to be above 1600℃. At the same time, the electrode insertion depth is optimized to 2000-2800mm to ensure stable furnace operation and reduce process power consumption. The iron impurity content is further controlled during the tamping, tapping, and casting processes.

[0008] Ladle refining: The molten silicon produced by the submerged arc furnace is introduced into the refining furnace. Before introduction, the scouring of the empty ladle walls must be checked. If brick joints are found, the ladle is returned to the auxiliary workshop. At the same time, the turnover of ladles is increased, and the auxiliary workshop increases the frequency of ladle inspections and pays attention to maintenance quality and scouring. The oxygen-enriched bottom-blown oxidation refining process is adopted. The core of the process is to achieve precise control of refining time and gas ratio through segmented oxygen-air regulation. Oxygen with a purity of ≥98% is introduced. During silicon molten refining, the ratio and flow rate of oxygen and air need to be controlled: when the silicon molten temperature is high, reducing the oxygen ratio and increasing the air ratio can effectively reduce the silicon molten temperature. Conversely, when the silicon molten temperature is low, increasing the oxygen ratio and decreasing the air ratio can effectively increase the silicon molten temperature by utilizing the exothermic oxidation reaction. The specific control parameters are as follows: Before the empty ladle enters, the oxygen adjustment value is 10-15 Nm³, and the air opening is 20-30% (adjustment deviation ±2%, air should be adjusted so that the molten silica can be agitated); after 15 minutes of refining, the oxygen adjustment value is 20-35 Nm³, and the air opening is 20-30%; after 1 hour of refining until the ladle is unloaded, the oxygen adjustment value is 40-50 Nm³, and the air opening is 10-20%; 30 minutes after the ladle is pulled (when the molten silica temperature is high), the oxygen adjustment value is 30±5 Nm³, and the air opening is 20-40%. The blowing temperature is controlled at 1550-1650℃, and the water pouring judgment standard is 70 minutes after the ladle is pulled or the ladle is hung at 50mm, at which time the air opening is maintained at 10-20%. The reasons for the differences in control standards at different stages are as follows: During the first 15 minutes of refining, air is mainly used to agitate the molten silicon and prevent pore blockage, with a small amount of oxygen playing a refining role; from 15 to 60 minutes of refining, oxygen is increased to accelerate refining, and the air opening is just enough to keep the molten silicon tumbling; from 60 minutes of refining until the end of the furnace discharge, the oxygen flow rate is increased to accelerate the refining rate, while the air flow rate is reduced to ensure that the refining temperature meets the standard; 30 minutes after the package is unloaded, when the temperature of the molten silicon is high, the oxygen flow rate needs to be reduced and the air flow rate increased to lower the temperature of the molten silicon.

[0009] Based on the order of oxygen affinity (calcium > aluminum > silicon > iron), oxygen preferentially reacts with calcium and aluminum impurities in the silicon melt to form low-melting-point oxides. If the silicon ladle fails to turn over, the oxygen flow rate should be gradually increased or decreased until the ladle turns over. When changing the gas supply pipe, the air and oxygen supply should be gradually shut off, and after replacing it with another completely closed gas pipe, the air and oxygen supply should be gradually reopened. During casting, oxygen should be turned on first, and after the core leaks out, the air supply should be increased to blacken the core. If conditions permit, the oxygen blowing time should be extended, and water should not be poured in for casting immediately. After blowing is completed, a refining agent (a mixture of fluorite and soda ash, mass ratio 3:1) should be added at a rate of 0.8-1.2% of the silicon melt mass. The refining agent can lower the slag melting point, improve slag fluidity, and allow the calcium and aluminum oxides generated by oxidation to fully enter the slag. After standing for 15-20 minutes, the slag should be removed using a slag skimmer to separate the silicon melt from the impurities.

[0010] 5. Finished product molding: The refined silicon melt is poured into the mold through a flow channel, and after naturally cooling to room temperature, it is crushed to obtain ultra-low calcium high-purity industrial silicon products.

[0011] The specific requirements for the raw materials used in the above steps are as follows: Silica: The procurement standards are SiO2 ≥ 99.4%, Fe2O3 ≤ 0.035%, Al2O3 ≤ 0.35%, CaO ≤ 0.1%, and P2O5 ≤ 60ppm; Semi-coke: The procurement standards are: air-dried volatile matter ≤6%, fixed carbon ≥80%, Fe2O3 ≤0.10%, Al2O3 ≤0.5%, CaO ≤2%, TiO2 ≤200ppm, B2O3 ≤500ppm; Caking silica coal (caking coal): The procurement standard is as follows: air-dried volatile matter ≥38%, fixed carbon ≥58%, Fe2O3≤0.15%, Al2O3≤0.4%, CaO≤0.30%, P2O5≤100ppm, TiO2≤300ppm, B2O3≤60ppm, caking index>85; Uncaking silica coal (uncaking coal): The procurement standard is air-dried volatile matter ≥38%, fixed carbon >57%, Fe2O3 ≤0.15%, Al2O3 ≤0.3%, CaO ≤0.2%, P2O5 ≤40ppm, TiO2 ≤100ppm, B2O3 ≤60ppm.

[0012] Beneficial effects: This invention provides a method for producing ultra-low calcium high-purity industrial silicon using a 33MVA large-scale submerged arc furnace, which has the following significant advantages: 1. Precise Impurity Control: By strictly limiting the iron, aluminum, and calcium impurity content of each raw material, the introduction of impurities is reduced from the source; at the same time, the operation process of the submerged arc furnace is optimized to reduce the phenomenon of in-process ironing. Combined with the efficient ladle refining process, the entire process of iron, aluminum, and calcium impurities can be controlled, and the products of grades such as Si3301, Si3302, and Si4202 can be stably produced. The calcium content can be stably controlled to below 0.01%, and the product purity meets the requirements of high-end applications.

[0013] 2. Green and efficient process: The oxygen-enriched bottom-blown oxidation refining process is used to replace the traditional chlorination refining process, which avoids the emission of toxic and harmful gases and has better environmental performance. At the same time, the proportion of raw materials and smelting parameters are optimized, and the silicon recovery rate can reach more than 85%. The average daily output of a single furnace is more than 65 tons, and the production efficiency is higher than that of the same furnace type in the same industry.

[0014] 3. Controllable raw material costs: The silica, fused silica, unfused silica, and semi-coke used are all common raw materials in the domestic market with sufficient supply. Compared with the production process that relies on imported high-quality raw materials or charcoal, the raw material costs and supply risks are significantly reduced, and the production path of ultra-low calcium high-purity industrial silicon is broadened. Attached Figure Description

[0015] Figure 1 This is a production process diagram in an embodiment of the present invention. Detailed Implementation

[0016] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments: Example 1

[0017] like Figure 1 As shown, a method for producing ultra-low calcium high-purity industrial silicon using a 33MVA large-scale submerged arc furnace includes the following steps: 1. Raw material pretreatment: The semi-coke is crushed and screened to control the particle size to 5-30mm; the screened semi-coke is sent into a steam pipeline and dried at 130℃ for 2.5h, and then preheated at 220℃ for 1.2h to remove moisture and increase porosity. Silica, caking silica and non-caking silica are screened to remove impurities, and the particle size of silica is controlled to be 30-200mm, the particle size of caking silica to be 3-20mm, and the particle size of non-caking silica to be 3-15mm.

[0018] 2. Batching and Mixing: Weigh out the pretreated raw materials according to the following batch ratio: 200 kg silica, 22 kg caking coal, 90 kg non-caking coal, 15 kg semi-coke, and 35 kg wood chips. Add these materials to the mixing conveyor belt to obtain a uniform mixture. The raw material specifications are as follows: silica (SiO2: 99.65%, Fe2O3: 0.03%, Al2O3: 0.18%, CaO: 0.08%); semi-coke (fixed carbon: 84.5%, Fe2O3: 0.085%, Al2O3: 0.16%, CaO: 1.5%); caking coal (Fe2O3: 0.12%, Al2O3: 0.25%, CaO: 0.29%, fixed carbon: 59%); non-caking coal (Fe2O3: 0.10%, Al2O3: 0.22%, CaO: 0.17%, fixed carbon: 58%); wood chips (fixed carbon: 12%, moisture ≤37%).

[0019] 3. Smelting in an electric arc furnace: The mixture is fed into a 33MVA large electric arc furnace via a conveyor belt. The furnace depth is controlled at 550mm, the electrode insertion depth at 2500mm, the material surface temperature at 450℃, and the crucible zone reaction temperature at 1800℃ for smelting production.

[0020] 4. Ladle Refining: The molten silicon produced by the submerged arc furnace is introduced into the refining furnace. After checking that the scouring of the empty ladle wall is normal, an oxygen-enriched bottom-blowing oxidation refining process is adopted, introducing oxygen with a purity of 99%. The process is operated according to segmented parameter control: before the empty ladle enters, the oxygen is adjusted to 20 Nm³ and the air opening is 20%; after 15 minutes of refining, the oxygen is adjusted to 35 Nm³ and the air opening is 20%; after 1 hour of refining, the oxygen is adjusted to 65 Nm³ and the air opening is 30%; 30 minutes after the ladle is pulled out... When the temperature of the silicon melt is high, the oxygen is adjusted to 32 Nm³ and the air opening is 20%. After the ladle is pulled, the water pouring judgment standard is met 50 minutes later, and the air opening is maintained at 20%. The blowing temperature is controlled at 1600℃. During the blowing process, the silicon ladle is kept stable and turned. After the blowing is completed, a mixed refining agent of fluorite and soda ash (mass ratio 3:1) is added, and the amount added is 1.0% of the mass of the silicon melt. After standing for 18 minutes, the slag is removed. When casting, oxygen is turned on first. After the core leaks out, the air is turned on to blow the core black.

[0021] 5. Finished product molding: The refined silicon melt is poured into a mold, cooled to room temperature, and then crushed to obtain ultra-low calcium high-purity industrial silicon products.

[0022] During the implementation of the method in this embodiment, the average daily output of a single furnace was 62.5 tons, and the silicon recovery rate was 84%. The product test results were: Fe 0.27%, Al 0.20%, Ca 0.006%, which met the requirements of Si3301 grade.

[0023] Example 2

[0024] A method for producing ultra-low calcium high-purity industrial silicon using a 33MVA large-scale submerged arc furnace includes the following steps: 1. Raw material pretreatment: The semi-coke is crushed and sieved to control the particle size to 5-20mm. The sieved semi-coke is then fed into a steam pipeline and dried at 140℃ for 2 hours, followed by preheating at 240℃ for 1 hour to remove moisture and increase porosity. Silica, caking silica, and uncaking silica are sieved to remove impurities, controlling the silica particle size to 50-150mm, the caking silica particle size to 5-12mm, and the uncaking silica particle size to 5-15mm.

[0025] 2. Raw Material Mixing: Weigh out the pretreated raw materials according to the following batch ratio: 200 kg silica, 20 kg caking coal, 102 kg non-caking coal, 10 kg semi-coke, and 38 kg wood chips. Add these materials to the mixing conveyor belt to obtain a uniform mixture. The raw material specifications are as follows: silica (SiO2: 99.4%, Fe2O3: 0.032%, Al2O3: 0.15%, CaO: 0.06%); semi-coke (fixed carbon: 82%, Fe2O3: 0.06%, Al2O3: 0.35%, CaO: 0.13%); caking coal (Fe2O3: 0.12%, Al2O3: 0.22%, CaO: 0.18%, fixed carbon: 58%); non-caking coal (Fe2O3: 0.13%, Al2O3: 0.20%, CaO: 0.06%, fixed carbon: 60%); wood chips (fixed carbon: 12%, moisture ≤ 44%).

[0026] 3. Smelting in an electric arc furnace: The mixture is fed into a 33MVA large electric arc furnace via a conveyor belt. The furnace depth is controlled at 320mm, the electrode insertion depth at 2300mm, the material surface temperature at 460℃, and the crucible zone reaction temperature at 1700℃ for smelting production.

[0027] 4. Ladle Refining: The molten silicon produced by the submerged arc furnace is introduced into the refining furnace. After checking that the scouring of the empty ladle wall is normal, the oxygen-enriched bottom-blowing oxidation refining process is adopted. Oxygen with a purity of 99% is introduced and operated according to the segmented control parameters: before the empty ladle enters, the oxygen is adjusted to 24 Nm³ and the air opening is 30%; after 15 minutes of refining, the oxygen is adjusted to 44 Nm³ and the air opening is 20%; after 1 hour of refining, the oxygen is adjusted to 62 Nm³ and the air opening is 30%; after 30 minutes of ladle pulling (when the silicon water temperature is high), the oxygen is adjusted to 28 Nm³ and the air opening is 20%; after 50 minutes of ladle pulling, the water pouring judgment standard is met, and the air opening is maintained at 20%. The blowing temperature is controlled at 1580℃; the silicon ladle is kept stable and turned during the blowing process. After the blowing is completed, a mixed refining agent of fluorite and soda ash (mass ratio 3:1) is added, with the amount added being 0.9% of the mass of the silicon melt. After standing for 16 minutes, the slag is removed. When casting, oxygen is turned on first. After the core leaks out, the air is turned on to blow the core black.

[0028] 5. Finished product molding: The refined silicon melt is poured into a mold, cooled to room temperature, and then crushed to obtain ultra-low calcium high-purity industrial silicon products.

[0029] During the implementation of the method in this embodiment, the average daily output of a single furnace was 64 tons, and the silicon recovery rate was 85%. The product test results were: Fe 0.28%, Al 0.13%, Ca 0.017%, which met the requirements of Si3202 grade.

[0030] Example 3

[0031] Si4202 and Si3202 have the same aluminum and calcium content, and the refining process in this embodiment is the same as in Example 2.

[0032] The above are merely preferred embodiments of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of this invention.

Claims

1. A method for producing ultra-low calcium high-purity industrial silicon using a 33MVA large-scale submerged arc furnace, characterized in that, Includes the following steps: Raw material pretreatment: The particle size range of semi-coke, silica, caking coal and non-caking coal is controlled, and the semi-coke is dried and preheated to remove moisture and increase porosity. Ingredient mixing: The pretreated raw materials are mixed in proportion to obtain a mixture; Smelting in an electric arc furnace: The mixed material is fed into a 33MVA large electric arc furnace to ensure the stability of the material layer structure of "raw material zone - clinker zone - SiC generation zone - SiC decomposition zone - crucible zone"; current is introduced through three-phase electrodes, and the smelting temperature is provided by electric arc heat and resistance heat, controlling the material surface temperature to not exceed 480℃, and the reaction temperature in the crucible zone to be controlled above 1600℃. Ladle refining: The silicon melt produced by the submerged arc furnace is introduced into the refining furnace, and an oxygen-enriched bottom-blown oxidation refining process is adopted. Oxygen with a purity of ≥98% is introduced, and the refining time and gas ratio are controlled by a segmented oxygen-air control strategy. If the silicon ladle does not turn over, the oxygen supply is gradually increased or decreased until the silicon ladle turns over. When changing the gas source pipe, the air and oxygen supply must be gradually closed, and the air and oxygen supply is gradually opened after replacing it with another completely closed gas pipe. Finished product molding: The refined silicon melt is cast into ingots, cooled, and crushed to obtain ultra-low calcium high-purity industrial silicon products.

2. The method for producing ultra-low calcium high-purity industrial silicon using a 33MVA large-scale submerged arc furnace according to claim 1, characterized in that, The specific requirements for the raw materials used are as follows: Silica: SiO2≥99.4%, Fe2O3≤0.035%, Al2O3≤0.35%, CaO≤0.1%, P2O5≤60ppm; Semi-coke: Volatile matter ≤6% on an air-dried basis, fixed carbon ≥80%, Fe2O3 ≤0.10%, Al2O3 ≤0.5%, CaO ≤2%, TiO2 ≤200ppm, B2O3 ≤500ppm; Caking coal: air-dried volatile matter ≥38%, fixed carbon ≥58%, Fe2O3≤0.15%, Al2O3≤0.4%, CaO≤0.30%, P2O5≤100ppm, TiO2≤300ppm, B2O3≤60ppm, caking index>85; Uncaking coal: Volatile matter ≥38% on an air-dried basis, fixed carbon >57%, Fe2O3 ≤0.15%, Al2O3 ≤0.3%, CaO ≤0.2%, P2O5 ≤40ppm, TiO2 ≤100ppm, B2O3 ≤60ppm.

3. The method for producing ultra-low calcium high-purity industrial silicon using a 33MVA large-scale submerged arc furnace according to claim 1 or 2, characterized in that, The particle size of the raw materials is controlled as follows: semi-coke particle size is controlled at 5-30mm, silica particle size is controlled at 30-200mm, caking silica particle size is controlled at 3-20mm, and non-caking silica particle size is controlled at 3-15mm.

4. The method for producing ultra-low calcium high-purity industrial silicon using a 33MVA large-scale submerged arc furnace according to claim 3, characterized in that, The process of drying and preheating semi-coke is as follows: drying temperature 120-150℃, drying time 2-3h, preheating temperature 200-250℃, preheating time 1-1.5h.

5. The method for producing ultra-low calcium high-purity industrial silicon using a 33MVA large-scale submerged arc furnace according to claim 1, characterized in that, When mixing pretreated raw materials, the proportion of silica is fixed, while the proportions of other raw materials can be adjusted according to the actual fixed carbon content of each raw material.

6. The method for producing ultra-low calcium high-purity industrial silicon using a 33MVA large-scale submerged arc furnace according to claim 1 or 5, characterized in that, Pretreated silica, caking coal, uncaking coal, and semi-coke are mixed in batches of 200kg:20kg:100kg:10kg:40kg.

7. The method for producing ultra-low calcium high-purity industrial silicon using a 33MVA large-scale submerged arc furnace according to claim 1, characterized in that, Control the furnace depth to be no less than 500mm from the material surface to ensure the stability of the material layer structure in the "raw material zone - clinker zone - SiC generation zone - SiC decomposition zone - crucible zone".

8. The method for producing ultra-low calcium high-purity industrial silicon using a 33MVA large-scale submerged arc furnace according to claim 1, characterized in that, The electrode insertion depth during the smelting process in an electric arc furnace is 2000-2800 mm.

9. The method for producing ultra-low calcium high-purity industrial silicon using a 33MVA large-scale submerged arc furnace according to claim 1, characterized in that, The segmented oxygen-air control strategy is as follows: before the empty ladle enters, the oxygen adjustment value is 10-15 Nm³, and the air opening is 20-30%; during the 15-minute refining process, the oxygen adjustment value is 20-35 Nm³, and the air opening is 20-30%; from the 1-hour refining process until the end of the furnace discharge, the oxygen adjustment value is 40-50 Nm³, and the air opening is 10-20%; 30 minutes after the ladle is pulled out, the oxygen adjustment value is 30±5 Nm³, and the air opening is 20-40%.

10. The method for producing ultra-low calcium high-purity industrial silicon using a 33MVA large-scale submerged arc furnace according to claim 1 or 9, characterized in that, During the ladle refining process, the blowing temperature is controlled at 1550-1650℃. The water pouring judgment standard is 70 minutes after the ladle is pulled or the ladle is hung at 50mm. At this time, the air opening is maintained at 10-20%.

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