Sharing of process technology solutions for the improved Siemens process

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Improved Siemens process technical solution: 1. Determination of the technological route. Considering the current status and development trends of the main technologies used in polysilicon production, the improved Siemens process can be applied to both the production of electronic-grade and solar-grade polysilicon. Thanks to its mature technology and suitability for industrial-scale production, it is currently the preferred process for polysilicon manufacturing, and it represents the main technological approach used in polysilicon production in China. 2. Production Methods and Reaction Principles The production methods and reaction principles for the main processes in this project are as follows: 2.1 Hydrogen production and purification Hydrogen is produced by electrolyzing deionized water in an electrolyzer. Electrolysis of H2O → H2 + O2. 2.2 HCl synthesis: In the hydrogen chloride synthesis furnace, a mixture of hydrogen and chlorine gases undergoes a combustion reaction to produce hydrogen chloride gas; after passing through an air cooler, a water cooler, a deep cooler, and a mist separator, this gas is sent to the trichlorosilane synthesis process. H2 + Cl2 → 2HCl 2.3 Synthesis of SiHCl3: In the SiHCl3 synthesis furnace, silicon powder reacts with HCl at a temperature of 280–300°C to produce silicon trichloride and silicon tetrachloride. At the same time, as a side reaction of the formation of high-chloride silicon compounds, polysiloxanes of the SinCl2n+2 series and derivatives of the SinHmCl(2n+2)-m type are formed. Main reactions: Si + 3HCl → SiHCl3 + H2; Si + 4HCl → SiCl4 + 2H2. Side reactions: 2SiHCl3 → SiH2Cl2 + SiCl4; 2Si + 6HCl → Si2Cl6 + 3H2; 2Si + 5HCl → Si2HCl5 + 2H2. 2.4 Dry separation of syngas: A dry dust removal system consisting of three stages of cyclone dust collectors is used to remove some of the silicon powder. The mixture is then washed with liquid chlorosilane at low temperature, resulting in the separation of liquid chlorosilane, hydrogen, and hydrogen chloride gases, which are recycled back to the system for use. 2. Separation and purification of 2.5-chlorosilane: The separation and purification of chlorosilane are achieved based on the principle of pressure distillation, through the use of energy-efficient processes. This process can ensure the production of high-purity trichlorosilane and silicon tetrachloride for polysilicon manufacturing (for hydrogenation). 2.6 SiHCl3 hydrogen reduction to deposit polycrystalline silicon on the original silicon rod. High-purity H2 and purified SiHCl3 enter the reduction furnace, where they react on the surface of the silicon-core heating element at 1050°C. 5SiHCl3 + H2 → 2Si + 2SiCl4 + 5HCl + SiH2Cl2 2.7 Dry separation of reduction off-gases: The principle and process of dry separation of reduction off-gases are similar to those of the dry separation process for silicon oxyhydrogen synthesis gas. 2.8 Hydrogenation of SiCl4: Silicon tetrachloride is formed during the hydrogen reduction of trichlorosilane; after condensing silicon tetrachloride and removing trichlorosilane, thermal hydrogenation is carried out to convert it back into trichlorosilane. Silicon tetrachloride is fed into the hydrogenation reactor, where the SiCl4 conversion reaction takes place at a temperature of 400–500°C and a pressure of 1.3–1.5 Mpa. The main reaction is SiCl4 + H2 → SiHCl3 + HCl; the side reactions are 2SiHCl3 → SiH2Cl2 + SiCl4. 2.9 Dry separation of hydrogenated gas: The hydrogenated gas obtained from the silicon tetrachloride hydrogenation process is separated in this step into silane liquids, hydrogen gas, and hydrogen chloride gas, which are then recycled back to the system for use. The principle and process of dry separation of hydrogenated gas are similar to those of the dry separation process for silicon trichloride synthesis gas. 2.10 Preparation of silicon cores and product finishing (1) Preparation of silicon cores: During the preparation of silicon cores, they are corroded using hydrofluoric acid and nitric acid, then washed with ultrapure water, and finally dried. (2) Product preparation: The bulk polycrystalline silicon is etched using hydrofluoric acid and nitric acid, followed by washing the polycrystalline silicon blocks with ultrapure water, and then the blocks are dried. 2.11 Treatment of waste gases and residues (1) Treatment of process waste gases: These are washed with NaOH solution; the chlorosilanes in the waste gases (taking SiHCl3 as an example) and hydrogen chloride react with NaOH and are thereby removed. SiHCl3 + 3H2O = SiO2·H2O↓ + 3HCl + H2; HCl + NaOH = NaCl + H2O. The waste gas is vented through a liquid-sealed tank. The bottom wash liquid containing NaCl and SiO2 is treated as process waste via pumping. (2) Treatment of distillation residues: The residues removed from the chlorosilane separation and purification process mainly consist of liquids containing silicon tetrachloride and polysiloxane compounds, as well as chlorosilane liquids left over after shutting down the equipment. NaOH solution is added to hydrolyze the chlorosilane and convert it into harmless substances. Hydrolysis and neutralization reactions: SiCl4 + 3H2O = SiO2·H2O↓ + 4HCl; SiHCl3 + 3H2O = SiO2·H2O↓ + 3HCl + H2; SiH2Cl3 + 3H2O = SiO2·H2O↓ + 3HCl + H2. NaOH + HCl = NaCl + H2O. After a specified period of treatment, the liquid containing SiO2 and NaCl is pumped out from the bottom of the tank and sent for treatment as process waste. 2.12 Treatment of pickling off-gases: The hydrogen fluoride and nitrogen oxide gases emitted during product sorting and silicon core corrosion treatment are absorbed by lime slurry, which is used to absorb hydrogen fluoride ； Using ammonia as a reducing agent and non-precious metals as catalysts, NOX is reduced and decomposed into N2 and water. 2HF + Ca(OH)2 = CaF2↓ + H2O
6NO2 + 8NH3 = 7N2↓ + 12H2O
6NO + 4NH3 = 5N2↓ + 6H2O
2.13 Treatment of pickling waste liquids: The pickling waste liquids containing hydrofluoric acid and nitric acid, which are generated during silicon core production and product finishing processes, are neutralized using lime slurry. This results in the formation of solid calcium fluoride and a calcium nitrate solution; after treatment, these liquids are sent for disposal as process waste. 2HF + Ca(OH)2 = CaF2↓ + H2O 2HNO3 + Ca(OH)2 = Ca(NO3)2 + H2O This post was last edited by wangli115 on 2008-11-24 20:14]