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Brief description of polysilicon tail gas dry recovery process

I. Overview


"Traditional Siemens method" reduction tail gas recovery is "wet recovery", that is, unreacted trichlorosilane, hydrogen in the reduction tail gas and hydrogen chloride, silicon tetrachloride, dichlorodihydrosilane and other gases generated during the reaction process, after -80°C cooling device, trichlorosilane and silicon tetrachloride are condensed into liquid and then recycled, non-condensable hydrogen, hydrogen chloride and a small amount of chlorosilane are washed and sprayed with water, hydrogen chloride and a small amount of chlorosilane are dissolved and taken away by water , the hydrogen is reused after purification and drying.


"Improved Siemens method" does not touch water during the recovery process of reducing tail gas. Compared with the "wet method" of "traditional Siemens method", it is called "dry method recovery". Dry recovery mainly uses the difference in physical and chemical properties of hydrogen, hydrogen chloride, trichlorosilane, silicon tetrachloride, dichlorodihydrosilane and other components in the reduction tail gas, through dissolution and absorption, absorption-desorption, adsorption-desorption Additional chemical operations realize the separation of components and reuse them.


Since "wet recovery" uses water washing to remove hydrogen chloride and chlorosilane in hydrogen gas, while losing hydrogen chloride and chlorosilane, the amount of wastewater treatment is increased, and water and impurities in water and air enter the hydrogen gas, polluting the hydrogen gas , it must be purified and dried by a hydrogen purification and drying device before it can be returned to the system for use. The wet recovery process consumes a lot and has a low yield, which increases the cost of the entire polysilicon production.


With the dry recovery process, almost 100% of the components in the reduction tail gas can be separated and recovered. Since the whole system is a closed circuit, no new pollution sources are introduced, so the recovered components are of stable quality and can be directly returned to the system for reuse, thereby improving Increased yield and reduced consumption.


2. The principle of dry recycling process


Dry reduction tail gas recovery mainly includes four main processes: bubbling leaching, pressurized condensation, absorption and desorption, and activated carbon adsorption.


3. Description of dry recycling process


(H2, SiHCl3, SiCl4, HCl, SiH2Cl2) from the reduction workshop, after cooling by the bubbling gas-gas heat exchanger, enter the bubbling elution tower, and use the low-temperature chlorosilane bubbling leaching delivered by the chlorosilane circulating shielded pump After most of the trichlorosilane and silicon tetrachloride in the reduction tail gas are dissolved and absorbed, they are returned to the chlorosilane storage tank for recycling by the chlorosilane circulating shielding pump, and the surplus chlorosilane is sent to the dry process refining through the chlorosilane delivery pump. Distillation process. The unabsorbed gas in the bubbling elution tower is compressed by the reduction tail gas compressor, and the pressure is raised from normal pressure to 0.6MPa. After cooling by the compressed gas-absorbing gas heat exchanger and the compressed gas subcooler, the condensed chlorosilane Return to the chlorosilane storage tank, the non-condensable low-temperature gas enters the absorption tower, and most of SiHCl3, SiCl4, HCl, and low-temperature silicon tetrachloride (poor liquid) in the absorption tower are absorbed, absorbing the tetrachloride of chlorosilane and HCl Silicon is called rich liquid and enters the rich liquid storage tank. Transported by the rich liquid shielded pump, the rich liquid enters the desorption tower after passing through the three-stage lean-rich liquid heat exchanger, the second-stage lean-rich liquid heat exchanger, the first-stage lean-rich liquid heat exchanger and the lean liquid heat exchange, and then enters the desorption tower. At a certain temperature (144°C in the bottom of the tower) and pressure (0.8MPa at the top of the tower), hydrogen chloride enters the buffer tank in a gaseous state and returns to the trichlorosilane synthesis system for reuse. SiH2Cl2 enters the buffer tank in a liquid state, and then it is processed and sold out. The silicon tetrachloride after desorption is called lean liquid, with a temperature of 144°C, passing through the lean liquid water cooler, the first-stage lean-rich liquid heat exchanger, the second-stage lean-rich liquid heat exchanger, and the third-stage lean-rich liquid heat exchanger , The lean liquid subcooler cools down to -50°C and enters the absorption tower for reuse.


The gas that is not absorbed in the absorption tower is mainly hydrogen, which contains a small amount of chlorosilane and hydrogen chloride. In order to further purify the hydrogen, an activated carbon adsorber is used to absorb impurities in the hydrogen; the three activated carbon adsorbers are controlled by a time program, and the adsorption is carried out separately. , regeneration, cooling. During the adsorption process, the activated carbon adsorbs a large amount of chlorosilane and hydrogen chloride impurities in the hydrogen gas, and after the carbon powder is filtered out through the bag filter, it is sent to a 100-square-meter pure hydrogen storage tank for use in reduction and other processes. The regeneration process is to heat the saturated activated carbon adsorber and blow it away with hydrogen gas. Chlorosilane and hydrogen chloride are filtered out of carbon powder with the hydrogen gas through the filter, and then return to the bubbling leaching tower for bubbling leaching. The cooling process is to cool down the high temperature (190°C) activated carbon adsorber by cooling the heat transfer oil after regeneration.


The non-absorbed gas in the absorption tower is mainly hydrogen. Under normal temperature and pressure, hydrogen is an extremely flammable, colorless, transparent, odorless and tasteless gas. So anti-static filter bags are needed here.

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