A recovery process of silicon powder in a cold hydrogenation process and a synthesis process of trichlorosilane

Abstract

The application discloses a silicon powder recovery process in a cold hydrogenation process and a trichlorosilane synthesis process. The silicon powder recovery process in the cold hydrogenation process comprises the following steps: (1) after the residual material of the cold hydrogenation process is subjected to static pressure, a silicon powder mixture containing chlorosilane and a small amount of high-boiling substances is obtained; (2) the silicon powder mixture is dried to obtain silicon residue; (3) after the silicon residue is subjected to neutral high-salt water hydrolysis, solid-liquid separation is performed to obtain solid silicon residue; and (4) the solid silicon residue is subjected to drying and physical screening to remove iron impurities. The silicon powder recovery process in the cold hydrogenation process and the trichlorosilane synthesis process solve the problem of reaction between liquid alkali and silicon powder, improve the silicon content, reduce the content of silicon dioxide impurities, thereby about 60-70% of effective silicon residue in the cold hydrogenation residue slurry is recovered, the silicon residue can be sent to trichlorosilane synthesis, or the silicon residue is mixed with dead ash and then sent to trichlorosilane synthesis, the environmental protection pressure is reduced, and the cost is greatly reduced.

Description

Technical Field

[0001] This invention belongs to the technical field of polycrystalline silicon, specifically relating to a silicon powder recovery process in a cold hydrogenation process and a trichlorosilane synthesis process. Background Technology

[0002] In polysilicon production, the residue from the cold hydrogenation process contains a significant amount of silicon powder, which can be recycled for reuse. However, the current process involves sending the residue from the cold hydrogenation process to a slurry unit. After being settled and pressed in a mixing tank, a mixture of silicon powder containing chlorosilanes and trace amounts of high-boiling-point substances is obtained. This mixture then enters a dryer to evaporate the chlorosilanes. The silicon slag containing trace amounts of high-boiling-point substances is then conveyed via a screw conveyor to a hydrolyzer, where it is hydrolyzed with weak alkaline water. Excess alkali reacts with the silicon powder to produce silicon dioxide and sodium silicate. Therefore, the current process does not effectively recover the silicon powder, resulting in resource waste.

[0003] In view of this, the present invention proposes a new process for recovering silicon powder and synthesizing trichlorosilane in a cold hydrogenation process, which can effectively recover silicon powder and the recovered silicon powder can be used in polysilicon production, thereby reducing production and operating costs. Summary of the Invention

[0004] The purpose of this invention is to provide a silicon powder recovery process in a cold hydrogenation process, which can effectively recover silicon powder and directly use it in the trichlorosilane synthesis process.

[0005] To achieve the above objectives, the technical solution adopted is as follows:

[0006] A silicon powder recycling process in a cold hydrogenation process includes the following steps:

[0007] (1) After the residue from the cold hydrogenation process is left to stand and press, a mixture of silicon powder containing chlorosilane and trace amounts of high-boiling substances is obtained.

[0008] (2) The silicon powder mixture is dried to remove chlorosilanes, resulting in silicon slag;

[0009] (3) The silicon slag is hydrolyzed with neutral high-salt water and then separated into solid and liquid to obtain solid silicon slag.

[0010] (4) The solid silicon slag is dried and physically screened to remove iron impurities.

[0011] Furthermore, in step (2), steam drying is employed.

[0012] Furthermore, the temperature of the steam is 95-105°C.

[0013] Furthermore, the temperature of the steam is 100°C.

[0014] Furthermore, in step (3), after hydrolysis, a solid-liquid mixture with a pH of 2-4 is obtained. Then, solid-liquid separation is carried out by pressure filtration. After adding alkaline substances to the filtrate until it is neutral, it can be used for hydrolysis again.

[0015] Furthermore, in step (3), the temperature during hydrolysis is below 55°C and the time is not less than 1 hour.

[0016] Furthermore, in step (4), the moisture content is dried to less than 5%, and the physical screening is performed by using a magnet to remove iron impurities.

[0017] Furthermore, the silicon slag after physical screening can be used to synthesize trichlorosilane.

[0018] Another objective of this invention is to provide a synthesis process for trichlorosilane, which uses recycled silicon powder and dead ash as raw materials for synthesizing trichlorosilane, thereby significantly reducing operating costs while ensuring the normal operation of the trichlorosilane synthesis process.

[0019] A process for synthesizing trichlorosilane, using silicon powder and dead ash recovered by the above-mentioned recycling process as raw materials to synthesize trichlorosilane;

[0020] The dead ash mentioned above refers to the dead ash discharged after the trichlorosilane synthesis furnace is shut down.

[0021] Furthermore, the mass ratio of silicon powder to dead ash is 1:1-1.5.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] 1. The technical solution of this invention can send approximately 60-70% of the effective silicon slag in the cold hydrogenation slurry to trichlorosilane synthesis, which can reduce the amount of industrial silicon powder purchased by polysilicon enterprises and reduce production costs. At the same time, compared with the synthesis process of silicon alloys, it reduces the high-temperature smelting process, saving 3.5 million kWh of electricity annually and reducing production and operating costs.

[0024] 2. The technical solution of the present invention controls the neutrality by adjusting the pH value of the hydrolyzed water, thereby solving the problem of reaction between liquid alkali and silicon powder, increasing the silicon content, and reducing the silicon dioxide impurity content.

[0025] 3. The dead ash emitted after the trichlorosilane synthesis furnace is shut down is severely oxidized and has poor activity, so it can only be treated as solid waste. The technical solution of this invention, by mixing the silicon powder recovered in the cold hydrogenation process with the dead ash, can use it to synthesize trichlorosilane, reducing environmental pressure while significantly reducing synthesis and treatment costs. Attached Figure Description

[0026] Figure 1 This is a process flow diagram of the present invention. Detailed Implementation

[0027] To further illustrate the silicon powder recovery process and trichlorosilane synthesis process in the cold hydrogenation process of this invention, and to achieve the intended purpose of the invention, the following detailed description, in conjunction with preferred embodiments, details the specific implementation, structure, features, and effects of the silicon powder recovery process in the cold hydrogenation process proposed by this invention. In the following description, different "embodiments" or "embodiments" do not necessarily refer to the same embodiment. Furthermore, specific features, structures, or characteristics in one or more embodiments can be combined in any suitable manner.

[0028] The following will provide a more detailed description of the silicon powder recovery process and the trichlorosilane synthesis process in a cold hydrogenation process according to the present invention, with reference to specific embodiments:

[0029] In the silicon powder recovery process of the cold hydrogenation process described in this invention, neutral high-salt water is used for hydrolysis. The resulting solid-liquid mixture with a pH of 2-4 is sent to a separate waste filter for solid-liquid separation. This process reduces the silicon dioxide content in the solid silicon slag to below 5%, with a silicon content ≥30% and a moisture content ≤50%, achieving a silicon slag purity that meets recycling standards. The silicon slag is then granulated, dried, and sorted by particle size, bagged, weighed, and then packaged, transported, and stored. The technical solution of this invention is as follows:

[0030] A silicon powder recycling process in a cold hydrogenation process includes the following steps:

[0031] (1) After the residue from the cold hydrogenation process is left to stand and press, a mixture of silicon powder containing chlorosilane and trace amounts of high-boiling substances is obtained.

[0032] (2) The silicon powder mixture is dried to remove chlorosilanes, resulting in silicon slag;

[0033] (3) The silicon slag is hydrolyzed with neutral high-salt water and then separated into solid and liquid to obtain solid silicon slag.

[0034] (4) The solid silicon slag is dried and physically screened to remove iron impurities.

[0035] In the above technical solution, the hydrolysis of cold hydrogenated silicon slag is changed from 1-2% alkaline water to neutral high-salt water, solving the problem of reaction between liquid alkali and silicon powder, increasing silicon content, and reducing silica impurity content. Furthermore, the solid silicon slag obtained by using neutral high-salt water (which easily clumps together and does not easily adhere to the filter cloth) has a better forming effect compared to the solid silicon slag obtained by water hydrolysis (which is loose, easily adheres to the filter cloth, and is difficult to remove). In actual operation, the on-site unloading process is convenient, greatly reducing the labor intensity of personnel.

[0036] Preferably, steam drying is used in step (2).

[0037] Preferably, the temperature of the steam is 95-105°C.

[0038] More preferably, the temperature of the steam is 100°C.

[0039] In the above technical solution, the present invention uses steam heating to separate chlorosilane from silicon slag and carry it away with the steam.

[0040] Preferably, in step (3), after hydrolysis, a solid-liquid mixture with a pH of 2-4 is obtained, and then solid-liquid separation is performed by pressure filtration. After adding an alkaline substance to the filtrate until it is neutral, it can be used for hydrolysis again.

[0041] Preferably, in step (3), the temperature during hydrolysis is below 55°C and the time is not less than 1 hour.

[0042] In the above-mentioned technical solutions, the hydrolyzed solid-liquid mixture becomes a strongly acidic mixture with a pH value of 2-4, which exacerbates the corrosiveness to the filter cloth of the filter press. Furthermore, the solid-liquid ratio of the hydrolyzed mixture is unstable, resulting in poor forming effect of the original filter press and a silica slag moisture content of 55-65%. Therefore, this invention improves the filter plate used in the solid-liquid separation process. The filter plate inlet is changed from φ145 to 125, the material is changed from PVC to polypropylene, the sealing surface is changed from 20mm to 60mm, and the support surface is changed from 55mm to 80mm. This solves the problem of corrosion of the filter cloth by acidic wastewater passing through the filter press, while also solving the silica slag forming problem, controlling the silica slag moisture content to ≤50%, meeting the recycling standard.

[0043] Preferably, in step (4), physical screening involves using a magnet to remove iron impurities.

[0044] Preferably, in step (4), the product is dried until the moisture content is less than 5%.

[0045] In the above technical solution, the specific steps of granulation and drying are as follows: the wet powdered silicon slag with an original moisture content of about 30% is dehydrated into dry granular silicon slag by a belt dryer, and the moisture content is required to be <5%.

[0046] Preferably, the silicon slag after physical screening can be used to synthesize trichlorosilane.

[0047] In the above-mentioned technical solution, the recovered silicon slag accounts for 60-70% of the solid silicon slag, and is acidic, making it unsuitable for producing silicon alloy products. This invention sends the recovered silicon powder to a trichlorosilane synthesis furnace to react with high-purity hydrogen chloride. The resulting chlorosilanes are then purified by a distillation unit. This technology allows for the recycling of waste silicon slag generated during polysilicon production, reducing production and operating costs.

[0048] In the trichlorosilane synthesis process described in this invention, recycled silicon powder and dead ash are used as raw materials for synthesizing trichlorosilane, which significantly reduces operating costs while ensuring the normal operation of the trichlorosilane synthesis process.

[0049] A process for synthesizing trichlorosilane, using silicon powder and dead ash recovered by the above-mentioned recycling process as raw materials to synthesize trichlorosilane;

[0050] The dead ash mentioned above refers to the dead ash discharged after the trichlorosilane synthesis furnace is shut down.

[0051] Preferably, the mass ratio of silicon powder to dead ash is 1:1-1.5.

[0052] In the above technical solution, before mixing silicon powder and dead ash for the synthesis of trichlorosilane, the composition of the silicon powder is first determined. The amount of dead ash used is related to the content of iron and other elements in the silicon powder, and increases with the increase of the content.

[0053] Example 1.

[0054] Combination Figure 1 The specific operating steps are as follows:

[0055] (1) The residue from the cold hydrogenation process is normally sent to the slurry unit. After being pressed by the slurry mixing tank, a mixture of silicon powder containing chlorosilane and trace amounts of high-boiling substances is obtained.

[0056] (2) The silicon powder mixture is fed into a dryer and dried with steam at 100°C to remove chlorosilanes, resulting in silicon slag containing trace amounts of high-boiling substances.

[0057] (3) The silica slag is thoroughly soaked in a neutral high-salt water solution for hydrolysis. The hydrolysis process is carried out at room temperature for 1 hour.

[0058] After hydrolysis, a solid-liquid mixture with a pH of 3 is obtained. This mixture is then sent to a separate waste disposal unit for solid-liquid separation using a filter press to obtain solid silica slag.

[0059] The filter press has a φ125 feed inlet, is made of polypropylene, has a 60mm sealing surface, and an 80mm support surface. After filtration, the solid silica slag forms well, requires less manual removal time, and the filtrate has low corrosiveness to the filter plate.

[0060] After adding an alkaline substance to neutralize the filtrate, it can be reused for hydrolysis.

[0061] (4) The solid silicon slag is dried to a moisture content of less than 5%, sieved, and iron impurities are removed by using a magnet to obtain the recovered silicon slag, i.e. the recovered silicon powder, which is 69 wt% of the solid silicon slag.

[0062] The specific operation of drying in step (4) is as follows: After the existing solid silicon slag is compressed and filtered, it is found that the iron content is high, so it needs to be removed by iron removal and granulation drying. The original wet solid silicon slag with a moisture content of about 30% is extruded or granulated into granules by disc rolling, and then dehydrated into dry granular silicon slag by belt dryer. The moisture content is required to be <5%. The dried silicon slag is conveyed to the dry material silo by belt.

[0063] Comparative Example 1.

[0064] The operating steps are the same as in Example 1, except that the filter plate inlet of the filter press is φ145, the material is PVC, the sealing surface is 20mm, and the supporting surface is 55mm.

[0065] After pressure filtration, the solid silica slag is in a loose state and easily adheres to the filter cloth, requiring a long time to remove manually; it is also highly corrosive to the filter plate.

[0066] Example 2.

[0067] Combination Figure 1 The silicon slag recovered in Example 1 was added to a trichlorosilane synthesis furnace and reacted with hydrogen chloride. The daily consumption of the recovered acidic silicon powder was 2 tons. The reaction was carried out in synthesis furnace #7 of the trichlorosilane synthesis unit. Synthesis furnace #8 was used for comparison with new silicon powder and other recovered silicon powders.

[0068] The gas phase at the outlet of the synthesis furnace was filtered through a filter bag, condensed with water at 7°C, and then analyzed. Table 1 shows samples #7 (acidic silicon powder) and #8 (new silicon powder and other recycled silicon powder).

[0069] Table 1

[0070] HCl wt% 0 0 DCS wt% 0.31 0.06 TCS wt% 83.16 70.81 TET wt% 16.53 29.13 B μg / ml 0.25 0.2 P μg / ml 0 0.04 Fe μg / ml 6.23 0.38 Al μg / ml 0.681 0.336 Mg μg / ml 0.016 0.003 Cu μg / ml 0.333 0.02 Ca μg / ml 0.111 0.062 Ti μg / ml 870.63 5.0567

[0071] The above data shows that the acidic silicon powder recovered in Example 1 of this invention can meet the production operation requirements and achieve the actual reaction effect. Furthermore, as shown in Table 1, the content of trichlorosilane in synthesis furnace #7 is greater than that in synthesis furnace #8, indicating that the silicon powder recovered in this invention is more conducive to the main reaction in the synthesis of trichlorosilane.

[0072] (2) The material obtained after the reaction in the No. 7 synthesis furnace in step (1) was sent to the distillation process section. The gas phase at the outlet of the distillation synthesis furnace was filtered through a filter bag and condensed with water at 7°C. The outlet sample was then analyzed. A total of 15 samples were taken, with each sample taken 1 day apart. The samples were colorless and transparent liquids. The results are shown in Table 2.

[0073] Table 2

[0074] Unit μg / ml μg / ml μg / ml μg / ml μg / ml μg / ml μg / ml μg / ml μg / ml Standard ≤2.8 ≤0.8 ≤1.0 ≤5.0 ≤0.05 ≤0.05 ≤0.05 ≤0.8 1 0.6543 0.0398 0.4759 1.3956 0.0012 0.0848 0.2053 2.8952 0.46 2 0.4314 0.0086 0.186 0.5714 0.0064 0.034 0.0017 0.0423 0.3176 3 0.2949 0.0095 1.7581 4.1315 0 0.0175 0.009 0.0379 2.2756 4 0.2257 0.0065 0.4698 2.5044 0 0.0226 0.004 0.0141 0.8087 5 0.3341 0.0111 0.9002 2.0365 0.0035 0.0119 0.0103 0.0546 2.0443 6 0.3036 0.016 1.2872 3.6977 0 0.0297 0.0086 0.1906 1.3463 7 0.2985 0.0146 0.6373 2.5325 0.0026 0.021 0.0073 0.0482 1.0147 8 0.3162 0.0245 1.6805 4.6477 0 0.0343 0.0418 0.1751 1.7488 9 0.2572 0.0048 0.182 0.9249 0.0069 0.0445 0.005 0.0368 0.324 10 0.1878 0.0104 0.1801 1.2547 0.0054 0.0755 0.0064 0.02 0.384 11 0.4497 0.0271 0.0805 0.6258 0.004 0.0696 0.0123 0.0222 0.2769 12 0.2132 0.0053 0.3395 1.3172 0 0.0104 0.0143 0.0331 0.7748 13 0.2618 0.0492 5.934 5.8138 0 0.0174 0.0286 0.2124 1.0806 14 0.3082 0.0023 0.4576 3.3938 0.0042 0.001 0.0278 0 3.037 15 0.3134 0.0168 0.2416 1.0472 0 0.1166 0.005 0.0476 0.2077

[0075] As shown in Table 2, the content of metal impurities in the material sent for distillation varied, but the B and P indices were all within the control range, confirming that the recycling of acidic silicon powder meets the production requirements and achieves the recycling of waste silicon slag generated during the polysilicon production process.

[0076] Example 3.

[0077] The specific operating steps are as follows:

[0078] (1) The residue from the cold hydrogenation process is normally sent to the slurry unit. After being pressed by the slurry mixing tank, a mixture of silicon powder containing chlorosilane and trace amounts of high-boiling substances is obtained.

[0079] (2) The silicon powder mixture is fed into a dryer and dried with steam at 95°C to remove chlorosilanes, resulting in silicon slag containing trace amounts of high-boiling substances.

[0080] (3) The silicon slag is soaked in neutral high-salt water for hydrolysis. The hydrolysis process is carried out at room temperature for 1.5 hours.

[0081] After hydrolysis, a solid-liquid mixture with a pH of 2 is obtained. This mixture is then sent to a separate waste disposal unit for solid-liquid separation using a filter press to obtain solid silica slag.

[0082] After adding an alkaline substance to neutralize the filtrate, it can be reused for hydrolysis.

[0083] (4) The solid silicon slag is dried to a moisture content of less than 5%, sieved, and iron impurities are removed by using a magnet to obtain the recovered silicon slag, i.e. the recovered silicon powder, which is 60 wt% of the solid silicon slag.

[0084] Example 4.

[0085] The specific operating steps are as follows:

[0086] (1) The residue from the cold hydrogenation process is normally sent to the slurry unit. After being pressed by the slurry mixing tank, a mixture of silicon powder containing chlorosilane and trace amounts of high-boiling substances is obtained.

[0087] (2) The silicon powder mixture is fed into a dryer and dried with steam at 105°C to remove chlorosilanes, resulting in silicon slag containing trace amounts of high-boiling substances.

[0088] (3) The silicon slag is soaked in neutral high-salt water for hydrolysis. The hydrolysis process is carried out at room temperature for 1.2 hours.

[0089] After hydrolysis, a solid-liquid mixture with a pH of 4 is obtained. This mixture is then sent to a separate waste disposal unit for solid-liquid separation using a filter press to obtain solid silica slag.

[0090] After adding an alkaline substance to neutralize the filtrate, it can be reused for hydrolysis.

[0091] (4) The solid silicon slag is dried to a moisture content of less than 5%, sieved, and iron impurities are removed by using a magnet to obtain the recovered silicon slag, i.e. the recovered silicon powder, which is 65 wt% of the solid silicon slag.

[0092] Example 5.

[0093] The silicon powder and dead ash recovered in Example 3 were used as raw materials, with a mass ratio of silicon powder to dead ash of 1:1. These were added to a trichlorosilane synthesis furnace to react with hydrogen chloride. The daily consumption of raw materials was 2 tons. The gas phase at the furnace outlet was filtered through a filter bag, condensed with water at 7°C, and the outlet sample was then analyzed.

[0094] Example 6.

[0095] The recovered silicon powder and dead ash from Example 4 were used as raw materials, with a mass ratio of silicon powder to dead ash of 4:6. These were added to a trichlorosilane synthesis furnace to react with hydrogen chloride. The daily consumption of raw materials was 2 tons. The gas phase at the furnace outlet was filtered through a filter bag, condensed with water at 7°C, and the outlet sample was then analyzed.

[0096] Example 7.

[0097] The silicon powder and dead ash recovered in Example 1 were used as raw materials, with a mass ratio of silicon powder to dead ash of 4.5:5.5. These were added to a trichlorosilane synthesis furnace to react with hydrogen chloride. The daily consumption of raw materials was 2 tons. The gas phase at the furnace outlet was filtered through a filter bag, condensed with water at 7°C, and the outlet sample was then analyzed.

[0098] The analysis results of the samples obtained in Examples 5-7 are shown in Table 3.

[0099] Table 3

[0100] HCl wt% 0 0 0 DCS wt% 0.23 0.25 0.42 TCS wt% 78.28 81.12 79.36 TET wt% 21.49 18.63 20.22 B μg / ml 0.059 0.3088 0.4936 P μg / ml 0 0 0 Fe μg / ml 5.1534 6.08 4.52 Al μg / ml 0.4146 1.34 0.844 Mg μg / ml 0.0114 0.021 0.0148 Cu μg / ml 0.2193 0.066 0.262 Ca μg / ml 0.1038 0.073 0.0546 Ti μg / ml 469.4 218.25 674.91

[0101] As shown in Table 3, the acidic silica powder and dead ash recovered in Examples 5-7 of the present invention can be used as raw materials to meet the production and operation requirements of the trichlorosilane synthesis process and achieve the actual reaction effect.

[0102] This invention discloses a silicon powder recovery process and a trichlorosilane synthesis process in a cold hydrogenation process. By adjusting the hydrolysis water to a neutral pH, the problem of reaction between liquid alkali and silicon powder is solved, increasing the silicon content and reducing the silica impurity content, thereby recovering approximately 60-70% of the effective silicon slag in the cold hydrogenation slurry. This silicon slag is no longer used for silicon alloy production but is instead sent to trichlorosilane synthesis, or mixed with dead ash emitted after the trichlorosilane synthesis furnace is shut down and then sent to trichlorosilane synthesis. This reduces environmental pressure while significantly lowering synthesis and processing costs.

[0103] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A silicon powder recovery process in a cold hydrogenation process, characterized in that, Includes the following steps: (1) After the residue from the cold hydrogenation process is allowed to stand and press, a mixture of silicon powder containing chlorosilane and trace amounts of high-boiling substances is obtained; (2) The silicon powder mixture is dried to remove chlorosilanes, yielding silicon slag; (3) The silicon slag is hydrolyzed with neutral high-salt water and then separated into solid and liquid to obtain solid silicon slag; The solid-liquid separation process is carried out using a filter press plate. The filter plate has a feed inlet of φ125, is made of polypropylene, and has a sealing surface of 60mm. (4) The solid silicon slag is dried and physically screened to remove iron impurities.

2. The recycling process according to claim 1, characterized in that, In step (2), steam drying is used.

3. The recycling process according to claim 2, characterized in that, The temperature of the steam is 95-105℃.

4. The recycling process according to claim 3, characterized in that, The temperature of the steam is 100°C.

5. The recycling process according to claim 1, characterized in that, In step (3), after hydrolysis, a solid-liquid mixture with a pH of 2-4 is obtained. Then, solid-liquid separation is carried out by pressure filtration. After adding alkaline substances to the filtrate until it is neutral, it can be used for hydrolysis again.

6. The recycling process according to claim 1, characterized in that, In step (3), the temperature during hydrolysis is below 55°C and the time is not less than 1 hour.

7. The recycling process according to claim 1, characterized in that, In step (4), the moisture content is dried to less than 5%, and the physical screening is performed by using a magnet to remove iron impurities.

8. The recycling process according to claim 1, characterized in that, The silicon slag after physical screening can be used to synthesize trichlorosilane.

9. A process for synthesizing trichlorosilane, characterized in that, Using the silicon powder and dead ash recovered by the recycling process described in any one of claims 1-8 as raw materials, trichlorosilane is synthesized. The dead ash mentioned above refers to the dead ash discharged after the trichlorosilane synthesis furnace is shut down.

10. The synthesis process according to claim 9, characterized in that, The mass ratio of silicon powder to dead ash is 1:1-1.5.

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