Hazard levels at various stages of polysilicon production

Hazard levels for various steps in the production of polysilicon using the modified Siemens process: 1. Hydrogen production – The main hazardous substance in this stage is hydrogen, which has flammable and explosive properties. (1) Sparks were accidentally generated during the electrolysis process, resulting in a combustion explosion. It mainly causes damage to process equipment, factory buildings, and other structures, with virtually no impact on the ambient air ； (2) Leaks in hydrogen delivery pipelines, cooling, and separation systems, caused by issues with components, operation, or maintenance, can pose a certain risk of combustion and explosion in the surrounding air. 2. Synthesis of trichlorosilane: (1) Due to the weight of the hydrogen chloride pipeline, or issues such as maintenance errors or improper operations, valves and pipelines may burst or leak, resulting in the release of hydrogen chloride gas. This accident will have an impact on the ambient air and surface water bodies ； (2) The pressure in the trichlorosilane fixed-bed reactor is 2.76 MPa at 500°C. The temperature inside the reactor is relatively high, and there is a certain positive pressure. In the event of leaks in the reactor, as well as in the silicon trichloride output pipelines connected to it, at the connection points, and in the control valves, a certain amount of silicon trichloride will leak out. This substance reacts rapidly with water when it comes into contact with it, and can thus have an impact on the surrounding air and surface water bodies ； (3) During the management, operation, and handling of trichlorosilane storage tanks, tank leaks may occur; such incidents will have effects similar to those mentioned above. Moreover, emergency response efforts to these incidents generate a certain amount of solid waste (fire-fighting and adsorbent materials such as dry powder and sand containing trichlorosilane, silica, hydrogen chloride, and other substances) ； (4) Hydrogen generated as a by-product of the reaction may also leak and pose a risk of combustion or explosion. 3. Dry separation process of syngas: The main types of risks and potential issues that may arise during this process include the following: (1) Efficiency failure of the cyclone dust collector: This malfunction will not cause the leakage of the aforementioned three gases, thereby avoiding any impact on the external air environment; it only affects the efficiency of the subsequent stage. To address this issue, it is sufficient to return the syngas to the cyclone dust collector multiple times for dust removal, or else stop operating this collector, use a backup collector, or divert the syngas from this unit to another properly functioning dust collector ； (2) Trichlorosilane washing tower: Since this is the first stage in syngas treatment, the synthesizer contains a certain amount of trichlorosilane, hydrogen, and hydrogen chloride. In the event of leaks in the washing tower, intake pipelines, control valves, etc., it is easy for these gases to escape, thereby having an impact on the surrounding air ； The hydrogen chloride scrubber in the second stage contains only a certain amount of hydrogen, hydrogen chloride, and a small quantity of trichlorosilane; the main hazardous substances in case of a leak during this process are hydrogen chloride and hydrogen ； The gas after the two aforementioned washes contains trace amounts of hydrogen chloride and trichlorosilane; in the event of a leak, it may only cause fires and explosions, with little impact on the air quality of the surrounding environment. 4. Chlorosilane separation and purification process: The hazardous substances involved in this process include trichlorosilane, silicon tetrachloride, and polychlorosilanes. Trichlorosilane, the raw material, is purified using a 11-stage distillation tower to remove impurities with low and high boiling points. The main types of risks that may arise are as follows: The trichlorosilane raw material supplied from the storage tanks contains small amounts of silicon tetrachloride and polysiloxane, both of which exist in liquid form. The distillation column of level 11 has essentially the same operational characteristics; it relies on the difference in elevation to facilitate the gradual flow of the liquid after distillation. The main substances distilled out include silicon tetrachloride and trichlorosilane; silicon tetrachloride is sent to the hydrogen chloridation process for the synthesis of trichlorosilane, while the refined trichlorosilane is sent to the CVD reduction process. Due to the large number of distillation towers used in this process, the potential risk level is high. Accidents such as leaks, cracks, breaks, or even explosions at the connections between distillation towers and pipelines, as well as at control valves, can lead to the leakage of trichlorosilane distillate. A small amount of silicon tetrachloride may also leak out. The rapid vaporization of these two substances can have an impact on the ambient air. Additionally, the adsorbents and fire extinguishing agents used for rescue operations, if not handled properly, can also have an adverse effect on the environment. 5. Hydrogen reduction process of trichlorosilane: This process involves feeding the distilled trichlorosilane and hydrogen into a reduction furnace, where polycrystalline silicon is deposited on a designated square silicon core rod. This process mainly includes two steps: the vaporization of trichlorosilane and the reduction of trichlorosilane to produce polysilicon. The potential risk accidents associated with this process are as follows: (1) Leakage of hydrogen gas, the reducing gas used in this process. This gas comes primarily from the product gases generated during the electrolysis process and from the hydrogen separated from syngas (stored in a common hydrogen tank). In the event of a leakage, it can lead to gas release, which may result in fires or explosions, causing damage to the equipment and surrounding structures; however, the impact on the ambient air is relatively minor ； (2) Vaporization of trichlorosilane: The liquid trichlorosilane from the storage tank is sent to a trichlorosilane vaporizer, where it is heated and vaporized before being fed into the reduction furnace. Since trichlorosilane is in a gaseous state at this point and at a relatively high temperature, in the event of a leak, it quickly enters the surrounding air, having an impact on the environment ； (3) Reduction of trichlorosilane: The substances present in the reduction furnace mainly include trichlorosilane gas, hydrogen, and polycrystalline silicon. Additionally, there are dichlorodisilane, silicon tetrachloride, hydrogen chloride, and hydrogen generated as a result of the reduction reaction. In the event of a leak in the reduction furnace, the materials that leak out include all these raw materials and by-products. Due to their small quantities within the furnace, the impact on the surrounding air is relatively minor. 6. Dry separation process for reduced exhaust gases: The unreacted trichlorosilane and hydrogen in the reduction furnace, along with the dichlorodisilane, silicon tetrachloride, hydrogen chloride, and hydrogen produced as a result of reduction, are all sent to a dry separator. Techniques similar to those used in the syngas separation process are employed to separate these exhaust gases. High-purity hydrogen is obtained through pressure swing adsorption; part of it is sent to the raw material storage tank, most of it is used for the reduction of trichlorosilane, and the remainder is used for the hydrogenation of silicon tetrachloride ； The hydrogen chloride in the exhaust gas is then removed using a hydrogen chloride stripping tower, and the resulting gas is sent to a buffer tank used for the synthesis of trichlorosilane ； The remaining chlorosilane liquid is sent to the reduced chlorosilane storage tank in the chlorosilane storage process. The exhaust gas processed in this process contains relatively low levels of toxic and harmful substances; its risk level is much lower than that of the aforementioned processes, and any accidents that occur have a relatively minor impact on the surrounding environment. 7. Silicon tetrachloride hydrogenation process: The hazardous substances involved in this process mainly include silicon tetrachloride, hydrogen, trichlorosilane, hydrogen chloride, etc. The gas after the reaction will be sent to the dry separation process for hydrogenated gases. The main types of risk accidents that may occur in this process are as follows: (1) Vaporization of silicon tetrachloride: In this process, the substance is vaporized by heating with hot water and then sent to the hydrogenation furnace. In the event of a leakage, silicon tetrachloride gas will escape into the surrounding air; the environmental factor directly affected is the ambient air ； (2) Hydrogenation furnace: After the hydrogenation reaction takes place, the hazardous substances present in the hydrogenation furnace mainly include trichlorosilane, hydrogen chloride, unreacted silicon tetrachloride, and hydrogen gas. After a leak occurs in the reactor, it can cause the aforementioned substances to enter the ambient air, having certain effects as a result. 8. Hydrogenated gas dry separation process: The hydrogenated gas derived from the silicon tetrachloride hydrogenation process is separated in this process into silane liquid, hydrogen gas, and hydrogen chloride gas, which are then recycled back to the facility for use. The principle and process of dry separation of hydrogenated gases are very similar to those of the dry separation process for silicon trichloride synthesis gas. This process primarily involves the separation and recovery of process exhaust gases; the toxic and harmful substances involved include silicon tetrachloride, hydrogen, trichlorosilane, etc. After the processes of distilling the raw materials, distilling the products, and treating the exhaust gases, the subsequent steps include the silicon core preparation process, product finishing process, and treatment of exhaust gases and residual liquids. (1) Silicon core preparation process: This process involves acid etching using hydrofluoric acid, nitric acid, etc., which generates small amounts of hydrogen fluoride and nitrogen oxide gases; however, no hazardous incidents occur, and there is no sudden threat to the surrounding environment ； (2) Product processing step: This involves cutting and crushing the polycrystalline silicon rods, followed by acid etching using hydrofluoric acid and nitric acid; the gases released are captured using collection hoods

The risk level during the reduction process is very high; don’t underestimate it

Hydrogenation, synthesis, hydrogen production, treatment of three wastes, reduction, raw materials. .

Well, actually, there is another type of damage that is often overlooked in the process of restoration: electrical radiation and thermal radiation. Normally, I definitely avoid getting close to restoring that building when I have nothing to do