Polysilicon Process [Daily Question] 2011.11.19

Reply 1# yizhuxuan C

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Reply to 1# yizhuxuan C: Dry recovery systems. 1. Conventional recovery technologies and processes: In China, for the recovery of polysilicon exhaust gas or trichlorosilane exhaust gas, the process technologies that have been used or are still being used include the freezing method, the dissolution absorption wet method, and conventional dry methods similar to the CDI recovery process. Under the current pressure to protect the environment and the emphasis on energy conservation and reduced consumption, the freezing method and the dissolution absorption wet method have basically been phased out; the main recovery process nowadays is the American CDI recovery technology, which has been adopted or adapted from abroad. 1.1 Freezing method: The freezing method is used to recover polysilicon exhaust gas. Its basic principle involves taking advantage of the differences in boiling points among the various components in the exhaust gas; by gradually cooling the gas, these components with different boiling points can be separated from each other, thereby achieving recovery. During the cooling process, the chlorosilane components with higher boiling points are first condensed into a liquid state and separated out. The remaining mixed gas (whose main components are H2 and HCl) is then subjected to cryogenic cooling until its temperature reaches around the condensation temperature of HCl; at this point, HCl condenses into a liquid state, thereby allowing it to be separated from H2. After the aforementioned freezing step, the only gas remaining is H2, which is then returned to be used in the original process. The condensed chlorosilane liquid is then sent to a distillation process for separation and purification. The freezing method primarily uses low-temperature cryogenic techniques, which therefore places high demands on the equipment. Special materials are required, and the design and manufacturing of the equipment must meet high standards to prevent leaks; excellent insulation is also necessary. To achieve such a low temperature, a large amount of liquid nitrogen (with a boiling point of -196°C) is required, resulting in high energy consumption. On the one hand, these cryogenic devices are relatively expensive, and on the other hand, they are difficult to repair. Due to the shortcomings of this method, it is rarely used in actual production. 1.2 Solvent absorption wet treatment technology: Water and alkali are used as absorbents to wash polycrystalline silicon or trichlorosilane exhaust gases containing chlorosilanes. In this process, chlorosilanes react with water to produce SiO2 and HCl; HCl dissolves in water to form hydrochloric acid, which then reacts with bases to produce salts. The exhaust gas after washing is either released directly into the atmosphere or further processed using recovery devices to extract hydrogen, but it is understood that release into the atmosphere is the main method used. The main advantage of this technology is its simple equipment and low investment cost. Its drawback is the severe pollution it causes, as it generates large amounts of waste residue, wastewater, and waste gas that require further treatment. Meanwhile, effective components such as SiHCl3 and HCl are completely wasted, and due to contamination by impurities in water and poor washing efficiency, the quality of the recovered hydrogen is low to the point where it cannot be reused, resulting in significant waste of resources and severely hindering the further development of this technology. This technology has been largely phased out abroad, but some silicon trichloride manufacturers in China still use it. 1.3 Conventional dry recovery technologies – The recovery techniques in the modified Siemens process and the CDI process belong to this category. This technology primarily uses methods such as bubble spray absorption and pressurized cryogenic absorption to recover chlorosilanes, and then employs cryogenic chlorosilanes for washing in order to recover HCl. Subsequently, processes such as activated carbon adsorption to remove HCl and the extraction of hydrogen were added. This technology essentially achieves a closed-loop circulation of raw materials, improving their recovery rate. Some domestic polysilicon manufacturers have introduced, adopted, and improved this recycling process. For silicon trichloride producers, due to investment and technical barriers, it is not yet possible to adopt this technology. Taking a polysilicon plant in China as an example, the main steps of its dry recovery process are as follows: The reduction off-gases coming out of the reduction furnace are cooled through heat exchange, and then sent to a bubble scrubber tower where most of the chlorosilanes are washed out and recovered. The non-condensable gases emerging from the bubble tower mainly consist of H2, HCl, and a small amount of chlorosilanes. This mixed gas is compressed and then fed into an HCl absorption tower, where it is washed and recovered using a cold lean solution containing little HCl. The lean solution is cooled multiple times using F22 coolant until its temperature drops below -45°C. The non-condensable gases that have been absorbed by the lean solution are heated several times through heat exchange before being sent to an activated carbon adsorption bed; hydrogen is obtained at the outlet of this bed and sent back to the reduction system for reuse. The activated carbon adsorption bed, which has become saturated with impurities, is regenerated by introducing heat transfer oil via coils to release the impurities, after which it can be used again. The HCl absorption tower absorbs HCl to produce a rich liquid, which is then heated through multiple heat exchange processes before being sent to the HCl desorption tower. Low-boiling impurities such as HCl are released from the top of the desorption tower; some chlorosilanes are condensed using F22 before being sent to the trichlorosilane synthesis or acid production units. The lean liquid obtained from the bottom of the desorption tower is cooled through multiple heat exchange processes and then sent back to the HCl absorption tower for reuse. Reduction furnace exhaust gas → Distillation → HCl absorption tower → Activated carbon adsorption tower → Pure hydrogen → Rich liquid → HCl desorption tower (with reboiler) → F22 cooling → Heat transfer oil → Poor liquid → Removal of impurities through discharge → Multi-stage heat exchange for heating → Multi-stage heat exchange for cooling → HCl used for synthesis or acid production → Non-condensable gases → Pressurization of non-condensable gases → F22 cooling → Bubble scrubbing for recovery → F22 cooling → Multi-stage heat exchange for cooling → Pressure pump → Liquid chlorosilane. Figure 1: Schematic diagram of the conventional dry recovery process. Most polysilicon manufacturers in China use processes that are similar to those described above; however, it is reported that their energy consumption is generally higher than that of similar recovery processes abroad. The aforementioned process essentially achieves a closed-loop cycle, thereby improving the recovery rate of raw materials. However, in actual operation, it has significant defects and shortcomings that result in poor and unstable quality of polysilicon, as well as frequent failures in the exhaust gas recovery system; in severe cases, these issues can even affect the proper functioning of the entire polysilicon production facility. The main shortcomings are as follows: (1) High investment costs and a lengthy process. As can be seen from the schematic diagram of this process, it includes bubble scrubbers, absorption towers, desorption towers, adsorption towers, reboilers, pressure pumps, heaters, coolers, F22 delivery systems, heat transfer oil delivery systems, etc. There are not only many different types of equipment but also a large number of them, which leads to a complex process structure, high investment costs, and elevated operational and maintenance expenses. (2) During operation, a large amount of low-temperature F22 is required, and heat transfer oil is used for heating and regeneration, which results in relatively high operating costs and energy consumption. (3) For hydrogen purification, temperature-controlled adsorption using activated carbon is employed, and a coil is used inside the adsorption tower to introduce heat transfer oil for heating. This purification process is prone to equipment deformation and damage due to thermal expansion and contraction caused by fluctuations in temperature; it also compresses the activated carbon, leading to its pulverization and significant loss of activated carbon. Furthermore, during actual operation, inadequate heating or cooling may result in incomplete regeneration of the activated carbon, which affects its adsorption capacity and purification accuracy, as well as the quality of the recovered hydrogen. (4) Temperature fluctuations cause deformation of the equipment and pulverization of the activated carbon, which results in a short service life for the activated carbon adsorption columns. It is known that the service life is usually only about 3 months; in some cases it reaches up to half a year, while in other cases it is less than 1 month. If these columns are not replaced in time, it can lead to cracks in the coils, allowing heat transfer oil to enter the process medium and thereby shutting down the entire reduction system. The subsequent cleaning and waste removal tasks for this process are quite extensive. It has been reported that some manufacturers experienced shutdowns of their polysilicon production systems for several months due to heat transfer oil leaking into the reduction system, which posed serious safety risks and significantly disrupted normal production. Therefore, in order to achieve continuous production as much as possible, some manufacturers are forced to keep several spare activated carbon adsorption columns on hand for immediate replacement. However, due to the large size of the adsorption column equipment and the difficulty in replacing it, the operating and maintenance costs are high. (5) This recycling process is unable to remove impurities such as trace amounts of oxygen, nitrogen, methane, and moisture present in the reduced exhaust gas; as a result, the quality of hydrogen cannot be effectively improved, leading to polysilicon of poor quality that lacks market competitiveness. (6) Domestic dry recovery processes generally suffer from a low level of automation; the operating parameters usually require manual intervention, and it is not possible to make intelligent adjustments to parameters such as temperature, pressure, flow rate, and product quality during the operation of computerized systems. In some recovery units, due to the lack of appropriate empirical data and mathematical models, even the main control points can only be managed manually, resulting in significant fluctuations. Therefore, it is not possible to ensure stable hydrogen quality over the long term. (7) It has poor operational flexibility, and capacity expansion and renovation are difficult. 2. New dry-process recovery technology: In response to the current situation regarding the recovery of polysilicon off-gases in China, Sichuan Kaiyuan Technology Co., Ltd. has developed a new dry-process recovery technology through extensive and painstaking experimental research and pilot tests, drawing on its rich experience in gas separation, recovery, and purification; it has also applied for relevant patents. This technology is part of a project supported by innovation funds, and it is completely different from conventional dry recovery systems. It not only enables the separation of the useful gas components in polysilicon reduction exhaust gases, thereby ensuring that the exhaust meets regulatory standards, but also facilitates the recycling of resources through the separate recovery of components such as chlorosilanes, HCl, and H2. Compared with similar foreign technologies and devices, it has obvious advantages in terms of simple process, low energy consumption, high product quality, low investment, and low recovery costs. The successful operation of this device has broken the foreign technological blockade in this field, bringing it to the international leading level in this industry, while also offering more favorable economic and environmental benefits. 2.1 Technical Principles and Process Flow: The new process utilizes specialized and efficient absorbents to separate components such as chlorosilanes, HCl, and H2, thereby producing products like chlorosilanes, HCl, and high-purity hydrogen, which are then sent back to the corresponding processes for reuse. A schematic diagram of this process is shown in Figure 2. The new dry recovery process first uses leaching to recover part of the chlorosilanes, after which the non-condensable gases are pressurized and fed into the recovery stage. This recovery stage consists of several recovery towers, which utilize a variety of specialized adsorbents that offer excellent separation capabilities for chlorosilanes, HCl, as well as hydrogen, oxygen, nitrogen, etc.; these adsorbents form a composite bed used to absorb and separate the non-condensable gases. The non-adsorbed gas mixture is discharged from the top of the recovery tower and sent to the purification process. This purification process consists of several purification towers, which use adsorbents with excellent separation capabilities for hydrogen from gases such as oxygen and nitrogen; a composite bed of such adsorbents is used to absorb and separate the non-adsorbed gas mixture, resulting in high-purity hydrogen gas being obtained from the top of the purification towers. Once the absorption tower becomes saturated with chlorosilane and HCl, the absorption process is terminated. Subsequently, chlorosilane and HCl are released at different stages in the opposite direction to the flow of the absorption gas; chlorosilane is sent for recovery through distillation or washing, while HCl is sent to processes for the synthesis of trichlorosilane or for acid production. After the release of chlorosilane and HCl, the absorption tower is regenerated and ready to begin another cycle of absorption. Once the purification tower becomes saturated with oxygen, nitrogen, and other gases, the absorption process is halted. Oxygen, nitrogen, and other gases are then released in the opposite direction to the flow of the absorbed gases and sent to the waste discharge system. At the same time, the purification tower is regenerated, allowing it to carry out the absorption process again and thus enabling cyclic operation. All the aforementioned absorption and separation operations are carried out at room temperature. Figure 2: Schematic diagram of the process flow for the new dry recovery technology. Pressurization of non-condensable gases; washing for recovery; liquid chlorosilane; cooling with F22; heat exchange for temperature reduction; pressure pump; recovery tower; purification tower; exhaust gas from the reduction furnace; distillation or washing of chlorosilane; removal of impurities such as nitrogen and oxygen through wastewater discharge; high-purity hydrogen; distillation; HCl used for synthesis or acid production. 2.2 Process characteristics and advantages of this technology 2.2.1 Technical support: This technology possesses complete independent intellectual property rights, and relevant patents have been applied for. It is a project that receives significant support from innovation funds. The technical personnel involved in the research, development, and design of this technology have extensive experience in engineering technologies related to the separation, recovery, and purification of various mixed gases, ensuring the technology’s advancement, stability, and reliability. 2.2.2 High quality of hydrogen: With the use of this technology, the hydrogen produced has high quality; its purity remains consistently above 99.999% over time. The dew point temperature of the hydrogen product is below -80°C, and both the flow rate and pressure of the hydrogen product remain stable, which contributes to the safe and stable operation of the production system. 2.2.3 The polysilicon produced using this approach has high quality. After the reduction exhaust gases are purified using new technologies, the hydrogen obtained as a result can be used as a reducing agent; thus, the polysilicon produced possesses high quality, with detection parameters such as impurity content and resistivity being significantly better than those of polysilicon produced through conventional recovery methods. Several polysilicon production lines have already adopted or plan to adopt this technology, and dozens of trichlorosilane manufacturing companies use it to recycle production exhaust gases. 2.2.4 Simple process and low investment: The main equipment for this technology includes recovery towers, purification towers, and associated program-controlled valves. The operating temperature varies little, making the design and manufacturing relatively easy; as a result, it features low investment costs and low maintenance expenses. 2.2.5 Good separation performance of the adsorbent: Adsorbents that have been independently selected and modified exhibit excellent performance. Given the reactive nature and susceptibility to hydrolysis of chlorosilanes, extensive research was conducted on dozens of adsorbents during the development process, and four specialized adsorbents were successfully selected, each showing excellent separation efficiency for chlorosilanes, HCl, as well as oxygen and nitrogen compounds. At the same time, to improve the adsorption and separation performance of the adsorbent, special modification treatments were carried out to increase the distribution of micropores within the adsorbent, thereby effectively increasing the selective active centers and achieving optimal separation of hydrogen, trichlorosilane, and HCl in polysilicon mixed gases. The modified adsorbent features a high separation coefficient, high strength, wear resistance, acid resistance, and a long service life. To meet the requirements for the separation and recovery of polysilicon off-gases under various operating conditions, during the industrial design process, the aforementioned adsorbents are combined in different proportions to form composite beds, based on factors such as the composition of the off-gases to be recovered, pressure, flow rate, and the requirements for the output gas, thereby ensuring optimal separation efficiency and purity. Nearly 3 years of industrial application experience have shown that these adsorbents are fully capable of meeting the requirements for separating the relevant components in polysilicon and trichlorosilane exhaust gases. 2.2.6 The program-controlled valve features good sealing performance and stability. It is a key device that enables the proper completion of this process and the reliable operation of the installation. Considering the characteristics of this technology and the special properties of the process medium, a \"fully enclosed radial-axial self-compensating programmable valve\" was developed to ensure zero leakage of the process gas. This type of programmable valve features a structural design with a seamless connection between the valve body and the cylinder, thereby achieving complete sealing of the valve and preventing any contact between the valve stem and the outside environment ； The valve stem is made of a highly elastic, low-friction, self-lubricating material, and it features a self-sealing structure that enhances the sealing performance of the process medium, improves operational safety, and extends the valve’s service life. Practice has proven that it is fully capable of adapting to application environments containing process media such as trichlorosilane, silicon tetrachloride, and HCl. 2.2.7 High degree of automation: Our company utilizes data from hundreds of industrial units to create a comprehensive engineering database, and develops mathematical models that best reflect the actual operating conditions of such industrial units, both domestically and internationally. This enables a close integration of theory and industrial practice, facilitating intelligent, self-adjusting control of the units. The uniquely developed intelligent self-adjusting control software can automatically monitor the operation of the exhaust gas recovery device based on parameters such as flow rate, pressure, temperature, and composition during its operation, as well as any changes in product quality requirements. It adjusts the relevant operational and process parameters accordingly, ensuring that the device operates in an optimal condition. Under stable conditions in the preceding and subsequent processes, it enables long-term automatic control without human supervision, thus maintaining stability in all process parameters and reducing or eliminating the negative effects caused by human errors or variations in operation. A large number of safety interlocks and control interlocks have been independently developed, including practical functional modules such as product gas quality interlocks, exhaust gas anomaly interlocks, system shutdown interlock protections, pressure interlocks, and control point alarm functions. These enable timely monitoring and control of parameters such as the flow rate and pressure of exhaust gas, product hydrogen, and chlorosilane, as well as the quality of the product gas. It was the first to develop and implement remote monitoring functionality. Once this feature was in use, when operators were unable to resolve issues on their own, they could contact after-sales engineers to remotely monitor the operation of the recycling device via the Internet and conduct real-time diagnostics, thereby enabling quick and efficient resolution of faults. 2.2.8 Expansion is simple and easy. With the adoption of the new dry-type exhaust gas recovery technology, only a small number of identical or similar devices need to be added to significantly increase the production capacity of the facility. Similar to adsorption-based systems, expansion is very convenient, requires minimal investment, and yields quick results; moreover, it has almost no impact on the normal operation of the facility during the process of technical upgrades and capacity expansion. 2.2.9 Advanced analysis and testing methods: Since polysilicon off-gases contain hydrolytically sensitive and highly corrosive components such as chlorosilanes and HCl, high standards are required for analytical instruments and testing methods. During the development of this technology, analysis methods suitable for use with this off-gas recovery technique were developed, taking into account the specific physical and chemical properties of these components. In-depth research was conducted on mixed gases with varying levels of chlorosilanes, covering aspects such as sampling methods, analysis and sample introduction procedures, quantitative calibration, and the quantitative analysis of trace chlorides. During the debugging and operation of the off-gas recovery system, continuous monitoring was carried out at key analysis points, ensuring long-term stability in the quality of the hydrogen product. This approach also helped to reduce the impact of corrosive gases on analytical instruments and extend their service life. 2.2.10 Comparison of various recovery technology parameters The comparison of this technology with similar products at home and abroad is shown in Table 1. As can be seen from Table 1, the new dry recovery technology has clear advantages in terms of polysilicon off-gas recovery. Table 1 Comparison of Parameters for Various Recovery Technologies
Process Type: New dry recovery technology, Wet treatment technology, Conventional dry technology (modified Siemens)
Separation Principle: Adsorption separation, Washing-neutralization, Freezing-temperature swing adsorption
Environmental Pollution: Emissions meet standards; Most severe cases where emissions do not meet standards; Emissions do not meet standards
Purification Precision: Chlorosilanes and HCl can be removed to below 1 ppm, with stable parameter control
Conversion to Waste Acid/Waste Slag: Chlorosilane and HCl levels are reduced to approximately 5–3000 ppm, with unstable parameter control and significant fluctuations
Hydrogen Purity: High purity, 99.99–99.999% or higher; capable of removing elements such as oxygen, nitrogen, and carbon; Unrecovered hydrogen: Unable to remove elements such as oxygen, nitrogen, and carbon
Energy Consumption: Low; Lowest; Highest
Control Level: Simple operation with high control precision; Simple operation; Complex operation with average control precision
Operating Costs: Low; High (large consumption of water and alkali solutions); High (large consumption of steam, heat transfer oil, Freon, and alkali solutions)
Investment: Lower; Low; High
Economic Benefits: High; Negative benefits; Low
Application Scope: Suitable for manufacturers producing chlorosilane- and HCl-containing exhaust gases; Mainly used by trichlorosilane manufacturers; Mainly used by polysilicon manufacturers
3. Conclusion
In summary, compared to conventional dry processes, the new dry exhaust gas recovery technology offers advantages such as higher product quality, lower energy consumption, reduced investment costs, higher degree of automation, longer service life, and lower maintenance expenses. After being treated by this technology, the polysilicon reduction off-gases yield chlorosilane products that are sent back to distillation or bubble scrubbing processes for complete recovery and reuse. The HCl produced can be used to synthesize trichlorosilane or to produce acid, while the high-purity hydrogen gas is returned to the reduction system to manufacture high-quality polysilicon products.

Reply 1# yizhuxuan c

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