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Siemens improved production process is as follows: The advantages of this method are significant energy saving and consumption reduction, low cost, good quality, comprehensive utilization technology, no pollution to the environment, and obvious competitive advantages. The main equipment used in the improved Siemens process to produce polysilicon is as follows:: Hydrogen chloride synthesis furnace, trichlorosilane ebullating bed pressurized synthesis furnace, trichlorosilane hydrolysis gel treatment system, trichlorosilane crude distillation and distillation tower purification system, silicon core furnace, power-saving reduction furnace, phosphorus detection furnace, silicon rod cutting machine, corrosion, cleaning, drying, packaging system equipment, reduction tail gas dry recovery equipment ； Others include analysis and testing instruments, control instruments, thermal energy conversion stations, compressed air stations, circulating water stations, power transformation and distribution stations, purification plants, etc. (1) Quartz sand is smelted and purified in an electric arc furnace to 98% and generates industrial silicon. The chemical reaction is SiO2+C→Si+CO2↑ (2) In order to meet the needs of high purity, it must be further purified. Industrial silicon is pulverized and reacted with anhydrous hydrogen chloride (HCl) in a fluidized bed reactor to produce pseudo-dissolved trichlorosilane (SiHCl3). The chemical reaction Si+HCl→SiHCl3+H2↑ has a reaction temperature of 300 degrees and is exothermic. At the same time, a gaseous mixture (Н2, НС1, SiНС13, SiC14, Si) is formed. (3) The gaseous mixture produced in the second step needs further purification and decomposition: filter the silicon powder, condense SiНС13, SiC14, and the gaseous Н2, НС1 are returned to the reaction or discharged to the atmosphere. Then decompose the condensate SiНС13, SiC14 and purify trichlorosilane (multi-stage distillation). (4) The purified trichlorosilane uses a high-temperature reduction process to reduce and deposit high-purity SiHCl3 in an H2 atmosphere to generate polycrystalline silicon. The chemical reaction is SiHCl3+H2→Si+HCl. The polycrystalline silicon reaction vessel is sealed, and the silicon rods (diameter 5-10 mm, length 1.5-2 meters, quantity 80) are heated by electricity. Polycrystalline silicon is grown on the rods at 1050-1100 degrees, and the diameter can reach 150-200 mm. About one-third of the trichlorosilane reacts and forms polysilicon. The remaining part is separated from the reaction vessel together with Н2, НС1, SiНС13, and SiC14. These mixtures are cryogenically separated and either reused or returned to the overall reaction. The separation of gaseous mixtures is complex and energy-consuming, which to some extent determines the cost of polysilicon and the competitiveness of this 3 process. In the Siemens improved production process, my country has not yet mastered some key technologies. During the refining process, more than 70% of the polysilicon is emitted through chlorine. Not only is the refining cost high, but the environmental pollution is also very serious. Hazard classification of each process of polycrystalline silicon production using improved Siemens process. 1. Hydrogen Preparation The dangerous substances present in this link are mainly hydrogen, which has flammable and explosive properties. (1) Sparks are accidentally generated during the electrolysis process and explosions occur. It will mainly cause damage to process equipment, factory buildings, etc., and has basically no impact on the external ambient air. ； (2) Hydrogen transportation pipelines, cooling and separation devices may cause leaks due to problems with components, operations or maintenance, which may cause the surrounding air to explode to a certain extent. 2. Synthesis of trichlorosilane (1) Hydrogen chloride pipelines may burst or leak due to their own weight, maintenance errors, misoperation, etc., resulting in the overflow of hydrogen chloride gas. This accident will have an impact on the external environment air and surface water bodies. ； (2) The internal pressure of the trichlorosilane fixed-bed reactor is 2.76MPa and 500℃. The temperature inside the reactor is relatively high, and it has a certain positive pressure. After a leakage occurs in the reactor and its connected trichlorosilane output pipelines, connections, control valves, etc., a certain amount of trichlorosilane will leak out. When it comes in contact with water, it will quickly react with water and easily affect the external ambient air and surface water bodies. ； (3) Tank leakage accidents may occur during the management, equipment, and operation of trichlorosilane storage tanks. When the accident occurs, it will have the same impact as the above situation, and a certain amount of solid waste will be generated during accident rescue (dry powder, sand and other fire extinguishing and adsorbent materials containing trichlorosilane, silicon oxide, hydrogen chloride and other substances) ； (4) The hydrogen generated by the reaction may also leak or explode. 3. The risk types and links that may occur in the synthesis gas dry separation process mainly include the following parts:: (1) Cyclone dust collector efficiency failure: This accident will not cause the above three gases to leak out, thereby affecting the external ambient air, but will only affect the efficiency of the next link. In this case, it is only necessary to return the syngas to the cyclone dust collector for multiple dust removals, or to stop the operation of the dust collector, use a backup dust collector, or transfer the syngas from this device to another normally operating dust collector. ； (2) Trichlorosilane scrubber: Since this process is the first stage of synthesis gas treatment, the synthesizer contains a certain amount of trichlorosilane, hydrogen and hydrogen chloride. If a leak occurs in the scrubber, air inlet pipeline, control valve, etc., it is easy to cause the leakage of trichlorosilane, hydrogen and hydrogen chloride, which will have a certain impact on the external ambient air. ； The hydrogen chloride scrubber in the second stage only contains a certain amount of hydrogen, hydrogen chloride and a small amount of trichlorosilane. The main dangerous substances after leakage in this process are hydrogen chloride and hydrogen. ； The gas after the above two washings contains trace amounts of hydrogen chloride and trichlorosilane. A leakage accident may only cause a fire and explosion accident, and will have little impact on the quality of the external ambient air. Hazard classification of each process of polycrystalline silicon production using the improved Siemens process (continued) 4. Chlorosilane separation and purification process. The hazardous substances involved in the process mainly include trichlorosilane, silicon tetrachloride and polychlorosilane. The raw material trichlorosilane is mainly distilled through an 11-stage distillation tower to remove low-boiling point and high-boiling point impurities. The main types of risks that may occur are as follows:: The trichlorosilane raw material sent from the storage tank contains a small amount of silicon tetrachloride and polychlorosilane, both of which exist in liquid form. The 11-stage distillation tower has basically the same process characteristics and relies on its own position difference to promote the step-by-step flow of liquid after distillation. The main distilled substances include silicon tetrachloride and trichlorosilane. Silicon tetrachloride is sent to the hydrochlorination process for the synthesis of trichlorosilane, and the distilled trichlorosilane is sent to the CVD reduction process. Since this process uses many distillation towers, its potential risk factor is relatively large. After accidents such as leakage, cracking, fracture or even explosion of the connection between the distillation tower and the pipeline, the pipeline and the distillation tower, and the control valve, it will cause trichlorosilane distillation. The liquid overflows, and will cause a small amount of silicon tetrachloride to overflow. The rapid volatilization of these two substances will have a certain impact on the external environment air. At the same time, it will also produce adsorbents and fire extinguishing agents used for rescue. Improper handling will also have an impact on the external environment. 5. Trichlorosilane reduction process This process is mainly a process of sending the distilled trichlorosilane and hydrogen into the reduction furnace and depositing polysilicon on a given square silicon mandrel. This process mainly includes two processes: the vaporization of trichlorosilane and the reduction of trichlorosilane to generate polycrystalline silicon. The possible risk accidents are as follows: (1) Leakage of reducing gas hydrogen: The reducing gas in this process mainly comes from the product gas of the electrolysis process and the hydrogen separated from the synthesis gas (unified hydrogen storage tank). A leakage accident in this process will lead to gas leakage, possible explosion accidents, certain damage to the device and surrounding structures, and relatively little impact on the external ambient air. ； (2) Trichlorosilane vaporization: The trichlorosilane liquid from the raw material tank is sent to the trichlorosilane vaporizer, heated and vaporized, and then sent to the reduction furnace. During this process, since trichlorosilane is in a gaseous state, it is at a relatively high temperature. After a leakage accident, trichlorosilane quickly enters the external air and has a certain impact on the external environment. ； (3) Trichlorosilane reduction: The substances present in the reduction furnace mainly include trichlorosilane gas, hydrogen and polycrystalline silicon, as well as dichlorodisilane, silicon tetrachloride, hydrogen chloride and hydrogen generated by the reduction reaction. After a leakage accident occurs in the reduction furnace, the leaked materials include the above-mentioned various raw materials and by-products. Due to the small amount in the furnace, the impact on the external ambient air is relatively small. 6. In the dry separation process of reduction tail gas, the unreacted trichlorosilane and hydrogen in the reduction furnace and the dichlorodisilane, silicon tetrachloride, hydrogen chloride and hydrogen produced by reduction are sent to the dry separator together, and a technology similar to the synthesis gas separation process is used to separate the tail gas. High-purity hydrogen is obtained through pressure swing adsorption. Part of it is sent to the raw material storage tank, most of it is sent to trichlorosilane for reduction, and the rest is sent to silicon tetrachloride for hydrogenation. ； The hydrogen chloride in the tail gas is then removed through a hydrogen chloride desorption tower and sent to a buffer tank for trichlorosilane synthesis. ； The remaining chlorosilane liquid is sent to the reduced chlorosilane storage tank of the chlorosilane storage process. The content of toxic and harmful substances in the exhaust gas treated in this process is relatively low, the risk value is much smaller than the above process, and the impact of accidents on external environmental factors is also relatively small. 7. Silicon tetrachloride hydrogenation process. The hazardous substances involved in this process mainly include: Silicon tetrachloride, hydrogen, trichlorosilane, hydrogen chloride, etc. The reacted gas will be sent to the hydrogenation gas dry separation process. The main types of risk accidents that may occur in this process include the following aspects:: (1) Silicon tetrachloride vaporization: This process mainly involves heating and vaporizing hot water and then sending it to the hydrogenation furnace. A leakage accident will cause silicon tetrachloride gas to overflow into the external ambient air. The environmental element it directly affects is the ambient air. ； (2) Hydrogenation furnace: After the hydrogenation reaction occurs, the dangerous substances in the hydrogenation furnace mainly include trichlorosilane, hydrogen chloride, unreacted silicon tetrachloride and hydrogen. After the reactor leaks, it will cause the above-mentioned substances to enter the external ambient air, causing certain effects. 8. Hydrogenated gas dry separation process: The hydrogenated gas from the silicon tetrachloride hydrogenation process is separated into chlorosilane liquid, hydrogen gas and hydrogen chloride gas through this process, and each is recycled back to the device for use. The principle and process of dry separation of hydrogenated gas are very similar to the dry separation process of trichlorosilane synthesis gas. This process mainly separates and recycles process waste gas. The toxic and harmful substances involved mainly include silicon tetrachloride, hydrogen, and trichlorosilane 9. Other processes include the above-mentioned raw material distillation, product distillation, and tail gas treatment processes, and then go through the silicon core preparation process, product sorting process, and waste gas and residual liquid treatment processes. (1) Silicon core preparation process: This process will use hydrofluoric acid, nitric acid, etc. for acid etching, and a small amount of hydrogen fluoride and nitrogen oxide gas will be produced, but there will be no risk accidents or sudden harm to the external environment. ； (2) Product finishing process: The product polysilicon rods are mainly cut and broken, and then etched with hydrofluoric acid and nitric acid, and the dissipated gas is collected using a gas collecting hood. This post was last edited by LHY8771 on 2009-3-31 11:42 ]