In 1955, Siemens successfully developed the process technology of using H to reduce SiHCl3 to deposit silicon on a silicon core heating element, and built a factory in 1957 for industrial-scale production, which is commonly referred to as the Siemens method.
Based on the Siemens process, it was further improved later, adding a reduction tail gas dry recovery system and SiCl4 hydrogenation process to achieve a closed loop and form an improved Siemens process that is widely used today. This method reduces the energy consumption per unit product by adopting a large reduction furnace: the SiCl4 hydrogenation and dry exhaust gas recovery process are used to significantly reduce the consumption of raw and auxiliary materials. The polysilicon produced accounts for 70% to 80% of the world’s total production today. .
The advantages of the improved Siemens method over the traditional Siemens method are mainly as follows. ①Energy saving and consumption reduction. The improved Siemens method recycles all the various components in the tail gas, which can greatly reduce the consumption of raw materials. In addition, the improved Siemens method adopts large-diameter reduction furnaces and other measures, which can effectively reduce the power consumption of the reduction furnace and reduce pollution. Since the improved Siemens method is a closed loop system, various materials in polysilicon production are fully utilized and discharged There is very little waste. Compared with the traditional Siemens method, pollution is controlled and the environment is protected.
At present, the modified Siemens method is generally used-the closed-loop trichlorosilane hydrogen reduction method. The toxic liquids and gases emitted by the hydrochlorination of silicon and the reduction of trichlorosilane not only pollute the environment, but also increase the cost of the enterprise. At present, the international Siemens method consumes about 150kW·h per kilogram of polysilicon, while China’s power consumption per kilogram of polysilicon is 200~250kW.h. In terms of cost, the international Siemens method costs 30 US dollars per kilogram of polysilicon. About US$70 for Chinese companies. The modified Siemens method uses chlorine and hydrogen to synthesize hydrogen chloride (or purchased hydrogen chloride). Hydrogen chloride and industrial silicon powder synthesize trichlorosilane at a certain temperature, and then separate and purify the trichlorosilane. The purified trichlorosilane Hydrogen silicon undergoes CVD reaction in an oxygen reduction furnace to produce high-purity polysilicon. Most of the existing polysilicon factories at home and abroad use this method to produce electronic grade and solar grade polysilicon. The raw and auxiliary materials for the production of polysilicon are trichlorosilane, hydrogen chloride, hydrogen, calcium oxide, hydrofluoric acid, nitric acid, and hydroxide. sodium.
The equipment used for the modified Siemens process to produce polysilicon mainly includes: hydrogen chloride synthesis furnace, trichlorosilane fluidized bed pressurized synthesis furnace, trichlorosilane hydrolysis gel treatment system, trichlorosilane crude distillation, distillation tower purification system, silicon Core furnace, energy-saving reduction furnace, phosphorus inspection furnace, silicon rod cutting machine, corrosion, cleaning, drying, packaging system equipment, reduction exhaust gas dry recovery equipment; others include analysis, testing instruments, control instruments, thermal energy conversion stations, compressed air The main reactions of power stations, circulating water stations, substations, purification plants, etc. are as follows.
① The industrial silicon is crushed and reacted with anhydrous hydrogen chloride in a fluidized bed reactor to produce trichlorosilane to be dissolved. Its chemical reaction formula is
The reaction temperature is 300°C, the reaction is exothermic, and a gaseous mixture of hydrogen, hydrogen chloride, trichlorosilane and silicon powder is formed at the same time.
②The gaseous mixture produced needs to be further decomposed, the silicon powder is filtered, the trichlorosilane and the tetrachlorosilane are condensed, and the gaseous hydrogen and hydrogen chloride are returned to the reaction or discharged into the atmosphere, and then the condensate, trichlorosilane and tetrachlorosilane are decomposed. Chlorohydrin silicon, purified trichlorosilane, is also called multi-stage distillation.
③ The purified trichlorosilane uses a high-temperature reduction process to reduce deposition in a hydrogen atmosphere to generate polysilicon. Its chemical reaction formula is
The polysilicon reaction vessel is sealed, and the polysilicon is grown on the rod under the condition of 1050~1100℃, the diameter can reach 150~200mm. In this way, about one-third of the trichlorosilane reacts to produce polysilicon, and the remaining part is separated from the reaction vessel with hydrogen, hydrogen chloride, trichlorosilane, and tetrachlorosilane. These mixtures are separated at low temperature, reused, or returned to the entire reaction. The separation of gaseous mixtures is complicated and energy-intensive, which determines the cost of polysilicon and the competitiveness of the process to some extent. In the refinement process of the improved Siemens method, more than 70% of the polysilicon is discharged through chlorine. Not only is the refining cost high, but the daily environmental pollution is very serious.
The process flow of the improved Siemens method is shown in Figure 1.