Preparation method of solar polycrystalline silicon

Abstract

The invention provides a preparation method of solar polycrystalline silicon, which comprises the following steps: S1, raw material pretreatment, S2, chlorination process, S3, rectification and purification, S4, reduction reaction, and S5, post-treatment, a tail gas recovery closed-loop system is arranged in a matched manner, and the steps are progressive layer by layer and cooperate with each other. According to the method, starting from a raw material source, the purity and particle size adaptability of the silicon powder are controlled through a three-stage synergistic pretreatment process, the reaction efficiency and the product selectivity are improved through a chlorination process with an optimized structure, deep removal of trace impurities is achieved by means of multi-stage rectification with precise temperature control, and the purity of the silicon powder is improved. The polycrystalline silicon deposition rate and the performance stability are synchronously optimized through reduction reaction, surface impurities and residues are removed through refined aftertreatment, meanwhile, raw material circulation reuse and waste gas emission reduction are achieved through a tail gas recovery closed loop, the problems of unstable purity, low efficiency, high energy consumption, large environmental protection pressure and the like in the prior art are solved, and the method is suitable for industrial production. And the solar grade polycrystalline silicon product with high purity and high performance is prepared.

Description

Technical Field

[0001] This invention relates to the field of polycrystalline silicon preparation technology, specifically to a method for preparing solar-grade polycrystalline silicon. Background Technology

[0002] Polycrystalline silicon, a core raw material in the photovoltaic industry, directly determines the photoelectric conversion efficiency and industrialization cost of solar cells due to its purity, preparation efficiency, and environmental friendliness. Currently, the mainstream polycrystalline silicon preparation method in the industry is the modified Siemens process, supplemented by silane methods, metallurgical methods, and other processes. The core processes all revolve around raw material processing, chlorination, purification, reduction, and post-processing. Among these, the modified Siemens process dominates the market due to its mature technology and high product purity. It involves reacting industrial silicon powder with hydrogen chloride to produce trichlorosilane, which is then purified by distillation and deposited via hydrogen reduction to obtain the polycrystalline silicon product, and is widely used in large-scale production.

[0003] However, existing preparation technologies still have many shortcomings that urgently need to be addressed. In the raw material pretreatment stage, most processes only employ simple screening or single impurity removal methods, resulting in incomplete removal of iron-based impurities. The silicon powder has a wide particle size distribution; excessively fine particles are prone to agglomeration, while excessively coarse particles do not react sufficiently. Furthermore, the removal of moisture and volatile impurities is ineffective, directly affecting the efficiency of subsequent reactions and product purity. In the chlorination process, the fluidization state of silicon powder in the fluidized bed reactor is unstable, with frequent localized accumulation or overheating. This leads to low selectivity for trichlorosilane, a high proportion of silicon tetrachloride as a byproduct, and low recovery rates of unreacted raw materials and byproducts, resulting in resource waste and increased costs. In the distillation purification stage, traditional multi-stage distillation suffers from unreasonable tray configurations, crude reflux ratio control, and insufficient temperature control precision in each column, leading to large temperature fluctuations. This makes it difficult to achieve deep removal of trace impurities such as boron and phosphorus, resulting in unstable trichlorosilane purity and difficulty in consistently meeting solar-grade requirements. During the reduction reaction, parameters such as gas ratio and reaction temperature are poorly controlled, resulting in poor adaptability to silicon core structures, slow polycrystalline silicon deposition rates, and poor resistivity uniformity in the product, posing a risk of crystal defects. Post-processing typically involves simplified alkaline washing and cleaning steps, improper control of alkaline concentration and temperature, and insufficient ultrapure water washing cycles and purity, leading to excessive levels of metallic impurities and salt residues on the polycrystalline silicon surface, affecting photoelectric conversion performance. Furthermore, existing technologies lack closed-loop design for tail gas recovery from chlorination and reduction processes, resulting in significant waste gas emissions and substantial environmental pressure, which contradicts current energy-saving and environmentally friendly industrial policies. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing solar-grade polycrystalline silicon to solve the problems mentioned in the background art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing solar-grade polycrystalline silicon, comprising: S1: Raw material pretreatment: Industrial silicon powder is selected as raw material and undergoes magnetic separation to remove iron, air classification and screening and vacuum drying to obtain refined silicon powder with a purity of ≥99.5% and a particle size of 50-200μm. S2: Chlorination process, refined silicon powder and hydrogen chloride gas are introduced into a fluidized bed reactor and chlorination reaction is carried out under the conditions of temperature 280-320℃ and pressure 0.3-0.5MPa to generate trichlorosilane mixed gas, wherein the molar ratio of refined silicon powder to hydrogen chloride gas is 1:3.2-3.5; S3: Distillation purification, trichlorosilane mixed gas is sequentially passed into the primary distillation column, the rectification column and the deweighting column for multi-stage distillation, the top temperature of the primary distillation column is controlled at 32-35℃, the top temperature of the rectification column is 31-33℃ and the top temperature of the deweighting column is 30-32℃, to obtain high-purity trichlorosilane with a purity ≥99.9999%; S4: Reduction reaction. High-purity trichlorosilane and hydrogen are introduced into the reduction furnace at a volume ratio of 1:10-12. Under the conditions of silicon core temperature of 1050-1150℃ and system pressure of 0.1-0.2MPa, trichlorosilane undergoes a reduction reaction on the surface of the silicon core and is deposited to form a polycrystalline silicon rod. S5: Post-processing. After the reduction reaction is completed, the polycrystalline silicon rod is cooled to room temperature and then mechanically cut, alkali washing to remove impurities, pure water washing and vacuum drying to obtain solar-grade polycrystalline silicon products.

[0006] Preferably, the vacuum drying conditions in step S1 are a temperature of 120-150℃, a vacuum degree of ≤-0.09MPa, and a drying time of 2-3h.

[0007] Preferably, in step S2, a gas distribution plate is provided in the fluidized bed reactor, the opening ratio of the gas distribution plate is 8-12%, and the fluidization state is adjusted by an inert gas during the chlorination reaction, the inert gas being nitrogen or argon.

[0008] Preferably, in step S3, the number of trays in the multi-stage distillation is 30-40 for the initial distillation column, 60-80 for the rectification column, and 25-35 for the deweighting column. The distillation process uses a tray column structure, and the reflux ratio is controlled at 5-8:1.

[0009] Preferably, in step S4, the silicon core in the reduction furnace is a U-shaped or rod-shaped high-purity silicon core with a diameter of 8-12 mm. During the reduction reaction, the silicon core temperature is monitored in real time by an infrared thermometer, and the flow rate of trichlorosilane and hydrogen is precisely adjusted by a flow controller.

[0010] Preferably, in step S5, the alkaline washing for impurity removal uses a sodium hydroxide solution with a mass concentration of 5-8%, the alkaline washing temperature is 40-50℃, and the washing time is 15-20 min; the pure water washing uses ultrapure water with a resistivity ≥18MΩ・cm, the washing is performed no less than 3 times, and the washing time is 5-8 min each time.

[0011] Preferably, in step S2, the tail gas generated by the chlorination reaction is condensed and recovered, and the unreacted hydrogen chloride gas is separated and recycled to the chlorination process. The separated silicon tetrachloride is transported to the hydrogenation unit and converted into trichlorosilane for reuse.

[0012] Preferably, in step S3, the tail gas generated by the reduction reaction is purified by gas-liquid separation and adsorption, and the hydrogen gas in it is recovered and recycled to the reduction process, and the purity of the recovered hydrogen gas is ≥99.99%.

[0013] Compared with the prior art, the beneficial effects of the present invention are: This invention employs a three-stage synergistic raw material pretreatment process—magnetic separation, airflow-based staged vacuum drying—to specifically remove iron-based impurities and mismatched particle sizes from industrial silicon powder at the source. Following this, precise vacuum drying removes moisture and volatile impurities, ultimately yielding refined silicon powder. This effectively avoids the generation of difficult-to-separate impurity compounds in subsequent chlorination processes, reduces the load on distillation purification, and ensures sufficient contact between the silicon powder and the reactant gases, laying a solid foundation for the efficient and stable operation of the entire preparation process.

[0014] This invention optimizes the reactor structure and process parameters of the chlorination process. By using a distribution plate with an inert regulating gas, the silicon powder is kept in a uniform fluidized state. Combined with precise control of molar ratio, temperature, and pressure, the reaction efficiency between silicon powder and hydrogen chloride gas is significantly improved. The selectivity of trichlorosilane is increased to over 90%, and the proportion of the byproduct silicon tetrachloride is reduced to below 8%, while reducing raw material waste and easing the pressure on subsequent purification processes.

[0015] This invention constructs a closed-loop tail gas recovery system linking the chlorination and reduction processes. Unreacted hydrogen chloride and hydrogen gases are efficiently recovered, purified, and recycled. The byproduct silicon tetrachloride is converted into trichlorosilane and reintroduced into the production process. This design increases chlorine utilization to over 95%, hydrogen recovery rate to no less than 99%, reduces raw material consumption by 12%-18%, and reduces waste gas emissions by over 80%, achieving green and environmentally friendly production.

[0016] This invention optimizes parameters such as gas ratio, temperature, and pressure in the reduction reaction stage, and combines this with a suitable silicon core structure and intelligent control equipment to construct an optimal reaction system. The polycrystalline silicon deposition rate is increased by more than 25% compared to traditional processes, while effectively avoiding crystal defects and controlling the resistivity uniformity error of the polycrystalline silicon rod to within 5%, thus improving the stability of the product's electrical performance and ensuring efficient photoelectric conversion of solar cells. Attached Figure Description

[0017] Figure 1 This is a flowchart illustrating the polycrystalline silicon preparation process of this invention. Figure 2 This is a pretreatment of raw materials for the present invention; Figure 3 This is a flow chart of the chlorination process of the present invention; Figure 4 This is a flowchart of the distillation and purification process of the present invention; Figure 5 This is a flowchart of the reduction reaction of the present invention; Figure 6 This is a flowchart of the post-processing of the present invention; Figure 7 This is a closed-loop flow diagram of the tail gas of the reduction reaction process in this invention; Figure 8 This is a closed-loop flow diagram of the tail gas from the chlorination process of this invention. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] Example: Please see Figure 1 The present invention provides a technical solution: This embodiment provides a method for preparing high-efficiency, high-purity, and environmentally friendly solar-grade polycrystalline silicon. The specific steps are as follows: S1: Raw material pretreatment. Industrial silicon powder is selected as the core raw material. First, it is subjected to magnetic separation to remove iron by a high gradient magnetic separator, removing more than 90% of the iron-based impurities in the silicon powder. Then, the silicon powder after iron removal is passed into an air classifier to screen out silicon powder particles with a particle size of 50-200μm, and remove particles that are too fine (<50μm) and prone to agglomeration and particles that are too coarse (>200μm) and have insufficient reaction. Finally, the classified silicon powder is sent to a vacuum drying oven and dried for 2-3 hours at a temperature of 120-150℃ and a vacuum degree of ≤-0.09MPa to completely remove the moisture and trace volatile impurities adsorbed on the surface of the silicon powder, and finally obtain refined silicon powder with a purity of ≥99.5%.

[0020] S2: Chlorination process. The refined silicon powder is fed into a fluidized bed reactor equipped with a gas distribution plate (open area ratio 8-12%), while hydrogen chloride gas is introduced into the reactor at a molar ratio of 1:3.2-3.5. During the reaction, nitrogen or argon is introduced as an inert regulating gas to maintain the silicon powder in a stable fluidized state and avoid local accumulation that could lead to uneven reaction. The temperature inside the reactor is controlled at 280-320℃ and the pressure is stabilized at 0.3-0.5MPa to ensure sufficient contact and reaction between the refined silicon powder and the hydrogen chloride gas, generating a mixed gas with trichlorosilane as the main component. The tail gas generated by the reaction is treated by a condensation and recovery device. The unreacted hydrogen chloride gas is recycled back into the chlorination process for reuse, while the separated byproduct silicon tetrachloride is sent to the hydrogenation unit, converted into trichlorosilane, and then reintroduced into the production process.

[0021] S3: Distillation and purification. The trichlorosilane mixed gas generated in the chlorination process is sequentially introduced into a multi-stage distillation system consisting of a primary distillation column, a rectification column, and a heavy removal column. The primary distillation column has 30-40 trays, the rectification column has 60-80 trays, and the heavy removal column has 25-35 trays. All three columns adopt a plate column structure, and the reflux ratio is controlled at 5-8:1. The top temperature of each column is precisely controlled. The primary distillation column is maintained at 32-35℃ to remove low-boiling-point impurities, the rectification column is maintained at 31-33℃ for main purification, and the heavy removal column is maintained at 30-32℃ to remove high-boiling-point impurities. Through multi-stage precise separation, high-purity trichlorosilane with a purity ≥99.9999% is finally obtained.

[0022] S4: Reduction reaction. High-purity trichlorosilane and hydrogen are mixed at a volume ratio of 1:10-12 and then introduced into a reduction furnace containing a U-shaped or rod-shaped high-purity silicon core (8-12mm in diameter). The system pressure inside the reduction furnace is controlled at 0.1-0.2MPa, and the silicon core temperature is raised to 1050-1150℃ using a heating device. Under these conditions, trichlorosilane undergoes a reduction reaction on the silicon core surface, gradually depositing to form a polycrystalline silicon rod. During the reaction, the silicon core temperature is monitored in real time using an infrared thermometer, and the flow rate of trichlorosilane and hydrogen is dynamically adjusted using a high-precision flow controller to ensure stable reaction conditions. The tail gas generated by the reduction reaction is first treated by a gas-liquid separator to remove liquid impurities, and then further treated by an adsorption purification system to recover the hydrogen (recovered hydrogen purity ≥99.99%), which is then recycled back to the reduction process for reuse.

[0023] S5: Post-processing. After the reduction reaction is complete, the heating device is turned off, and the polycrystalline silicon rod is allowed to cool naturally to room temperature in the reduction furnace. Then, the polycrystalline silicon rod is taken out and mechanically cut to remove the irregular parts at both ends, obtaining a polycrystalline silicon billet of the preset size. The billet is immersed in a sodium hydroxide solution with a mass concentration of 5-8% and alkaline washed at 40-50℃ for 15-20 minutes to remove metal impurities and surface oxide layers introduced during the cutting process. After alkaline washing, the billet is washed at least 3 times with ultrapure water with a resistivity ≥18MΩ・cm, each time for 5-8 minutes, to thoroughly remove residual alkali and impurities on the surface. Finally, the washed polycrystalline silicon billet is sent to a vacuum drying oven for drying to obtain solar-grade polycrystalline silicon products.

[0024] Regarding the S1 raw material pretreatment stage, a three-stage synergistic process of "magnetic separation for iron removal + airflow classification + vacuum drying" is adopted. The core value lies in controlling the purity and compatibility of the raw materials from the source. This process first uses high-gradient magnetic separation to specifically remove the main metallic impurity Fe from industrial silicon powder, avoiding the generation of difficult-to-separate impurity compounds such as FeCl3 in the subsequent chlorination process, thereby significantly reducing the processing load of distillation and purification.

[0025] Subsequently, the silicon powder particle size was strictly limited to 50-200μm through airflow classification. This range can ensure sufficient contact area between silicon powder and hydrogen chloride gas, effectively avoid reaction dead zones caused by the agglomeration of excessively fine particles, and avoid the problem of raw material waste caused by incomplete reaction of excessively coarse particles.

[0026] Finally, with precisely set vacuum drying parameters (120-150℃, ≤-0.09MPa, 2-3h), the moisture and trace volatile impurities adsorbed on the surface of the silicon powder can be completely removed, while avoiding silicon powder oxidation caused by high temperature environment. The result is refined silicon powder with a purity of ≥99.5%, which lays a solid foundation of pure raw materials for the efficient and stable development of subsequent processes.

[0027] Regarding the S2 chlorination process, breakthroughs in both reaction efficiency and resource utilization have been achieved through structural optimization and synergistic design of process parameters. This process utilizes a fluidized bed reactor equipped with a gas distribution plate (8-12% porosity), while simultaneously introducing nitrogen or argon as an inert regulating gas. This ensures that the refined silicon powder remains in a uniform fluidized state, increasing the contact area between the silicon powder and hydrogen chloride gas by more than 25% compared to traditional processes. This not only effectively suppresses side reactions caused by localized overheating but also improves the selectivity of trichlorosilane to over 90%, controlling the proportion of the byproduct silicon tetrachloride to below 8%.

[0028] The design of a molar ratio of 1:3.2-3.5 between refined silicon powder and hydrogen chloride gas ensures the full reaction of silicon powder while avoiding raw material waste and subsequent separation pressure caused by excessive hydrogen chloride. Furthermore, through the matching design of the tail gas recovery system, unreacted hydrogen chloride gas is recycled and reused, and the by-product silicon tetrachloride is transported to the hydrogenation unit to be converted into trichlorosilane before being returned to the production process, thereby increasing the utilization rate of chlorine to over 95% and reducing raw material consumption and waste emissions.

[0029] By integrating tail gas recovery with the chlorination and reduction processes, a synergistic optimization of environmental protection and cost is achieved. After condensation and recovery, the tail gas from the chlorination process exhibits a ≥95% recovery rate of unreacted HCl, which can be directly recycled back to the chlorination process for reuse. The tail gas from the reduction process, after gas-liquid separation and adsorption purification, achieves a ≥99% hydrogen recovery rate and a purity of ≥99.99%, allowing it to be reintroduced into the reduction furnace for further reaction. This design reduces raw material consumption by 12%-18%, significantly lowering production costs. Simultaneously, the byproduct silicon tetrachloride is recycled through hydrogenation conversion, avoiding the environmental pressure caused by its direct emission. This reduces the overall waste gas emissions of the production process by more than 80% compared to traditional processes, achieving a green production model that aligns with current energy-saving and environmentally friendly industrial policies.

[0030] Regarding the S3 distillation and purification module, to overcome the purification bottleneck of high-purity trichlorosilane, a deeply synergistic solution combining structural design and temperature control strategies is adopted. This module optimizes the tray configuration of the primary distillation column, rectification column, and heavy metal removal column (30-40 trays, 60-80 trays, and 25-35 trays respectively), and uses a reflux ratio of 5-8:1 to extend the residence time of the mixed gas within the columns, achieving deep removal of trace impurities such as boron, phosphorus, and metal chlorides.

[0031] Simultaneously, precise temperature control ranges were set for the three-stage distillation column (32-35℃ for the primary distillation column, 31-33℃ for the rectification column, and 30-32℃ for the heavy removal column), and temperature fluctuations were controlled within ±0.5℃. This effectively avoided the problem of incomplete impurity separation caused by temperature fluctuations in traditional processes, ultimately achieving a stable trichlorosilane purity of ≥99.9999%. This provided a core raw material guarantee for the subsequent reduction reaction to generate high-purity polysilicon, successfully solving the industry pain points of unstable purity and poor impurity removal in traditional distillation processes.

[0032] Regarding the S4 reduction reaction stage, based on the high-purity trichlorosilane raw material obtained by distillation and purification, the simultaneous optimization of polycrystalline silicon deposition quality and production efficiency is achieved through the organic combination of intelligent control and structural design.

[0033] By mixing trichlorosilane and hydrogen at a volume ratio of 1:10-12 and conducting the reaction at a silicon core temperature of 1050-1150℃ and a system pressure of 0.1-0.2MPa, this parameter combination constitutes the optimal reaction system, increasing the polycrystalline silicon deposition rate to 1.0-1.2mm / h, which is more than 25% higher than the traditional process, and significantly shortening the production cycle. At the same time, the use of U-shaped or rod-shaped high-purity silicon cores (diameter 8-12mm) optimizes heat conduction efficiency. Combined with real-time temperature monitoring by an infrared thermometer and precise adjustment by a flow controller, crystal defects caused by excessive local temperature of the silicon core or gas imbalance are effectively avoided. This ensures that the resistivity uniformity error of the polycrystalline silicon rod is ≤5%, significantly improving the electrical performance stability of the product and providing a guarantee for the high-efficiency photoelectric conversion of subsequent solar cells.

[0034] Regarding the S5 post-processing, the polycrystalline silicon rods generated by the reduction reaction need to undergo refined processing to ensure the purity and surface quality of the final product. This step first involves mechanically cutting the polycrystalline silicon rods to remove irregular ends. Then, a 5-8% sodium hydroxide solution is used for alkaline washing at 40-50°C for 15-20 minutes. This condition effectively dissolves metallic impurities and surface oxide layers introduced during the cutting process without corroding the polycrystalline silicon itself.

[0035] After alkaline washing, the polycrystalline silicon product undergoes at least three multi-stage cleanings using ultrapure water with a resistivity ≥18MΩ・cm, with each cleaning session lasting 5-8 minutes, to ensure thorough removal of residual alkaline solution and salt from the surface. Finally, vacuum drying is performed to prevent secondary adsorption of impurities from the air, ultimately ensuring that the total impurity content of the polycrystalline silicon product is ≤0.8ppm, fully meeting the stringent requirements of high-efficiency solar cells for polycrystalline silicon raw materials and filling the gap in traditional post-processing processes where impurity residues exceed the standard.

[0036] All other parts of this invention not described herein are the same as existing technologies, or are known technologies, or can be implemented using existing technologies, and will not be described in detail here.

[0037] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing solar-grade polycrystalline silicon, characterized in that, Includes the following steps: S1: Raw material pretreatment: Industrial silicon powder is selected as raw material and undergoes magnetic separation to remove iron, air classification and screening and vacuum drying to obtain refined silicon powder with a purity of ≥99.5% and a particle size of 50-200μm. S2: Chlorination process, refined silicon powder and hydrogen chloride gas are introduced into a fluidized bed reactor and chlorination reaction is carried out under the conditions of temperature 280-320℃ and pressure 0.3-0.5MPa to generate trichlorosilane mixed gas, wherein the molar ratio of refined silicon powder to hydrogen chloride gas is 1:3.2-3.5; S3: Distillation purification, trichlorosilane mixed gas is sequentially passed into the primary distillation column, the rectification column and the deweighting column for multi-stage distillation, the top temperature of the primary distillation column is controlled at 32-35℃, the top temperature of the rectification column is 31-33℃ and the top temperature of the deweighting column is 30-32℃, to obtain high-purity trichlorosilane with a purity ≥99.9999%; S4: Reduction reaction. High-purity trichlorosilane and hydrogen are introduced into the reduction furnace at a volume ratio of 1:10-12. Under the conditions of silicon core temperature of 1050-1150℃ and system pressure of 0.1-0.2MPa, trichlorosilane undergoes a reduction reaction on the surface of the silicon core and is deposited to form a polycrystalline silicon rod. S5: Post-processing. After the reduction reaction is completed, the polycrystalline silicon rod is cooled to room temperature and then mechanically cut, alkali washing to remove impurities, pure water washing and vacuum drying to obtain solar-grade polycrystalline silicon products.

2. The method for preparing solar-grade polycrystalline silicon according to claim 1, characterized in that, The vacuum drying conditions in step S1 are: temperature 120-150℃, vacuum degree ≤ -0.09MPa, and drying time 2-3h.

3. The method for preparing solar-grade polycrystalline silicon according to claim 1, characterized in that, In step S2, a gas distribution plate is installed inside the fluidized bed reactor. The opening ratio of the gas distribution plate is 8-12%. During the chlorination reaction, the fluidization state is adjusted by an inert gas, which is nitrogen or argon.

4. The method for preparing solar-grade polycrystalline silicon according to claim 1, characterized in that, In step S3, the number of trays in the multi-stage distillation is 30-40 for the initial distillation column, 60-80 for the rectification column, and 25-35 for the removal of heavy components column. The distillation process uses a tray column structure, and the reflux ratio is controlled at 5-8:

1.

5. The method for preparing solar-grade polycrystalline silicon according to claim 1, characterized in that, In step S4, the silicon core in the reduction furnace is a U-shaped or rod-shaped high-purity silicon core with a diameter of 8-12 mm. The silicon core temperature is monitored in real time by an infrared thermometer during the reduction reaction.

6. The method for preparing solar-grade polycrystalline silicon according to claim 1, characterized in that, In step S5, alkaline washing to remove impurities uses a sodium hydroxide solution with a mass concentration of 5-8%, an alkaline washing temperature of 40-50℃, and a washing time of 15-20 minutes; pure water washing uses ultrapure water with a resistivity ≥18MΩ・cm, and each washing session lasts 5-8 minutes.

7. The method for preparing solar-grade polycrystalline silicon according to claim 1, characterized in that, The tail gas generated by the chlorination reaction in step S2 is condensed and recovered, and the unreacted hydrogen chloride gas is separated and recycled to the chlorination process.

8. The method for preparing solar-grade polycrystalline silicon according to claim 1, characterized in that, The tail gas generated by the reduction reaction in step S3 is purified by gas-liquid separation and adsorption, and the hydrogen gas in it is recovered and recycled to the reduction process.

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