Process Flow And Pollution Generation Analysis Of Polysilicon

Process flow and pollution generation analysis of polysilicon

Hydrogen preparation and purification process

Hydrogen is produced in an electrolytic cell by electrolysis desalination. The hydrogen produced by electrolysis is cooled and separated by liquid, then enters into deaerator, under the action of catalyst, the trace oxygen in the hydrogen gas reacts with the hydrogen gas to generate water and is removed. The deoxygenated hydrogen is dried by a set of adsorption dryers. The purified and dried hydrogen gas is sent to a hydrogen storage tank, and then is sent to the hydrogen chloride synthesis, trichlorohydrogen and silicon hydrogen reduction and silicon tetrachloride hydrogenation processes.

The oxygen produced by electrolysis is sent into an oxygen storage tank after cooling and separating the liquid. Put oxygen out of the tank and put it in the bottle.

The waste adsorbent discharged by the gas-liquid separator, the waste deoxidation catalyst discharged by the hydrogen deoxidizer and the waste adsorbent discharged by the dryer are all recovered and reused by suppliers.

Hydrogen chloride synthesis process

Hydrogen from the hydrogen preparation and purification process and circulating hydrogen returned from the dry separation process of the synthesis gas are respectively entered into the hydrogen buffer tank of the process and mixed in the tank. The hydrogen gas from the hydrogen gas buffer tank is introduced into the combustion gun at the bottom of the hydrogen chloride synthesis furnace. The chlorine gas from the liquid chlorine vaporization process is also introduced into the combustion gun at the bottom of the hydrogen chloride synthesis furnace through the chlorine gas buffer tank. The mixed gas of hydrogen and chlorine is ignited at the exit of the combustion gun, and the hydrogen chloride gas is generated by combustion reaction. The hydrogen chloride gas out of the synthesis furnace is sent to the trichlorosilane synthesis process after passing through the air cooler, the water cooler, the deep cooler and the fog separator.

In order to ensure safety, the device is provided with a hydrogen chloride gas absorption system which is mainly composed of two hydrogen chloride falling film absorbers, two hydrochloric acid circulating grooves and a hydrochloric acid circulating pump, and can absorb the hydrogen chloride gas discharged due to the load adjustment or emergency discharge of the device by water. The system maintains continuous operation and can receive and absorb hydrogen chloride gas discharged by the device at any time.

In order to ensure safety, the working procedure is provided with a set of chlorine-containing waste gas processing system which is mainly composed of an exhaust gas processing tower, an alkaline liquid circulating tank, an alkaline liquid circulating pump and an alkaline liquid circulating cooler. When necessary, the chlorine in the chlorine buffer tank and the pipeline can be sent into the waste gas treatment tower, and the chlorine can be washed and removed by sodium hydroxide aqueous solution. The waste gas treatment system is maintained in continuous operation to ensure that chlorine-containing gases are received and treated at any time.

Trichlorosilane synthesis process

The raw material silicon powder is suspended and discharged into the silicon powder receiving hopper through the silicon powder discharging hopper. The silicon powder is put into the lower middle hopper from the receiving hopper, the gas in the hopper is replaced by the hot hydrogen chloride and is boosted to be balanced with the pressure of the lower hopper, and the silicon powder is put into the lower silicon powder supply hopper. The silicon powder in the feeding hopper is fed into the feeding pipe of the trichlorosilane synthesis furnace by a star feeder installed at the bottom of the feeding hopper.

The hydrogen chloride from the hydrogen chloride synthesis process is mixed with the circulating hydrogen chloride from the circulating hydrogen chloride buffer tank, then the hydrogen chloride is introduced into the feeding pipe of the trichlorosilane synthesis furnace, the silicon powder in the feeding pipe is carried and conveyed from the silicon powder feeding hopper, and the silicon powder enters into the trichlorosilane synthesis furnace from the bottom.

In a trichlorosilane synthesis furnace, silicon powder and hydrogen chloride gas form a boiling bed and react to generate trichlorohydrogen silicon, and at the same time, products such as silicon tetrachloride, dihydrogen chloride silicon, metal chloride, polychlorosilane, hydrogen gas are generated, and the mixed gas is called trichlorohydrogen silicon synthesis gas. The reaction was exothermic. The outer wall of the synthetic furnace is provided with a water jacket, and the heat is taken away by the water in the jacket to maintain the temperature of the furnace wall.

The synthetic gas with silicon powder is placed on the top of the synthesis furnace, after the part of the silicon powder is removed by a dry dust removal system composed of a three-stage cyclone dust collector, the silicon powder is sent into a wet dust removal system, and the silicon tetrachloride liquid is washed, and part of the fine silicon dust in the gas is washed off. 

At the same time, wet hydrogen is introduced to contact with the gas, and part of the metal oxide contained in the gas is hydrolyzed and removed. The mixed gas purified by removing the silicon powder is sent to a synthesis gas dry separation process.

Synthetic gas dry separation process

The synthesis gas from the trichlorosilane hydrogen synthesis step is separated into chlorosilane liquid, hydrogen gas and hydrogen chloride gas in the step and is recycled to the device for use.

The synthetic gas stream of trichlorosilane is passed through a mixed gas buffer tank, then enters into a spray washing tower, and is washed by the low temperature chlorosilane liquid under the top flow of the tower. Most of the chlorosilane in the gas is condensed and mixed into the washing liquid. The chlorosilane at the bottom of the tower is pressurized by a pump, most of the chlorosilane is circulated back to the top of the tower after being cooled and cooled for gas washing, and the residual chlorosilane is sent to the hydrogen chloride analysis tower.

Most of the chlorosilane gas is removed from the top of the spraying and washing tower, compressed by a mixed gas compressor, cooled and cooled, is sent to a hydrogen chloride absorption tower, is washed by the chlorosilane liquid which is cooled and cooled and sent from the bottom of the hydrogen chloride analysis tower, most of the hydrogen chloride in the gas is absorbed by the chlorosilane, and most of the chlorosilane remaining in the gas is washed and condensed. The top gas of the tower is hydrogen gas containing trace hydrogen chloride and chlorosilane, and the hydrogen gas with high purity is obtained after the hydrogen chloride and the chlorosilane are further removed by a group of temperature-changing pressure-changing adsorbers. The hydrogen flows through the hydrogen buffer tank, and then returns to the hydrogen chloride synthesis process to participate in the reaction of the hydrogen chloride synthesis. The regenerated waste gas of the adsorber contains hydrogen, hydrogen chloride and chlorosilane, and is sent to the waste gas treatment process for treatment.

The chlorine silane dissolved with hydrogen chloride gas at the bottom of the discharge hydrogen chloride absorption tower is heated, then is combined with the surplus chlorine silane from the bottom of the spray washing tower, then is sent into the middle part of the hydrogen chloride analysis tower, and the purified hydrogen chloride gas is obtained at the top of the tower through the decompression distillation operation. The tower hydrogen chloride gas flows through the hydrogen chloride buffer tank, and then is sent to the circulating hydrogen chloride buffer tank which is arranged in the trichlorosilane synthesis process; The bottom of the tower removes the hydrogen chloride to obtain the regenerated chlorosilane liquid, most of which is sent back to the hydrogen chloride absorption tower after being cooled, frozen and cooled to be used as an absorbent, and the surplus chlorosilane liquid (namely the chlorosilane separated from the trichlorosilane synthesis gas) is sent to the raw material chlorosilane storage tank of the chlorosilane storage step after being cooled.

Separation and purification process of chlorosilane

The chlorosilane liquid separated by the dry separation step of the synthesis gas is fed into a chlorosilane storage tank of the raw material of the chlorosilane storage step. The chlorosilane liquid separated by the dry separation step of the reduction tail gas is sent to a reduction chlorosilane storage tank of the chlorosilane storage step; The chlorosilane liquid separated by the hydride gas dry separation step is fed into a hydrochlorosilane storage tank of the chlorosilane storage step. The raw material chlorosilane liquid, the reducing chlorosilane liquid and the hydrochlorosilane liquid are respectively pumped out by a pump and sent into different rectification towers of the separation and purification process of chlorosilane.

Trichlorohydrogen silicon hydrogen reduction process

The trichlorosilane refined through the chlorosilane separation and purification process is fed into the trichlorosilane vaporizer of the process, and is heated and vaporized by hot water; The circulating hydrogen gas returned from the dry separation process of reducing tail gas flows through a hydrogen buffer tank, and is also fed into a vaporizer to form a mixed gas with trichlorosilane vapor in a certain proportion.

The mixed gas of the trichlorosilane and the hydrogen from the trichlorosilane vaporizer is sent into a reduction furnace. On the surface of the hot silicon core/silicon rod which is electrified in the reduction furnace, hydrogen reduction reaction occurs to the trichlorosilane, and silicon is generated and deposited, so that the diameter of the silicon core/silicon rod gradually becomes larger until reaching the prescribed size. The hydrogen reduction reaction produces dihydrogen chloride silicon, silicon tetrachloride, hydrogen chloride and hydrogen at the same time, and is sent out to a reduction furnace together with the unreacted trihydrogen chloride silicon and hydrogen, and is cooled by the circulating cooling water of the reduction tail gas cooler, and then is directly sent to the dry separation process of the reduction tail gas.

The hot water is introduced into the jacket of the reduction furnace barrel to remove the heat radiated from the hot silicon core in the furnace to the inner wall of the furnace barrel and maintain the temperature of the inner wall of the furnace barrel. The high-temperature hot water of the jacket of the discharge cylinder is sent to the heat energy recovery working procedure, and after the steam is produced by the waste heat boiler and is cooled, the hot water is recycled to be used by each reduction furnace jacket of the working procedure.

After the silicon core is installed in the reduction furnace, the water jet vacuum pump is used to pump the vacuum before driving, the nitrogen is used to replace the air in the furnace, the hydrogen is used to replace the nitrogen in the furnace (the nitrogen is exhausted), and then the heating operation is carried out. Therefore, the nitrogen is discharged into the ambient air in the driving stage, and a small amount of water for the vacuum pump (which can be discharged as clean water) is needed; In the furnace shutdown and opening stage (about 5-7 days for one time), the mixed gas containing chlorosilane, hydrogen chloride and hydrogen in the reducing furnace is first pressed into the reducing tail gas dry recovery system for recovery by hydrogen gas, then the nitrogen gas is replaced and then exhausted, the polysilicon product is taken out, the waste graphite electrode is removed, and the ultra-pure water in the furnace is washed according to the situation, therefore, the nitrogen gas, the waste graphite and the cleaning waste water are generated in the furnace shutdown stage. The nitrogen gas is harmless gas, therefore, under normal circumstances, the reduction furnace is open and the stopping stage has no harmful gas discharge. The waste graphite is recovered from the original production plant, and the cleaning waste water is sent to the treatment system of acid-alkali wastewater containing chloride for treatment.

Dry separation process for reducing tail gas

The reduction tail gas from the trichlorosilane hydrogen reduction step is separated into chlorosilane liquid, hydrogen and hydrogen chloride gas through the step, and the tail gas is respectively recycled to the device for use.

The principle and process of dry separation of reducing tail gas are very similar to that of dry separation of trichlorosilane. The hydrogen with high purity obtained from the outlet of the temperature-changing pressure-changing adsorber flows through the hydrogen buffer tank, and most of the hydrogen is returned to the hydrogen trichloride silicon hydrogen reduction process to participate in the preparation of polysilicon, and the surplus hydrogen is sent to the silicon tetrachloride hydrogenation process to participate in the silicon tetrachloride hydrogenation; The adsorber regenerates waste gas and sends it to the waste gas treatment process for treatment; The purified hydrogen chloride gas is obtained from the top of the hydrogen chloride analysis tower and sent to a circulating hydrogen chloride buffer tank which is arranged in the trichlorine hydrogen silicon synthesis process; The extra chlorosilane liquid (namely chlorosilane separated from the tail gas of hydrogen trichloride reduction) extracted from the bottom of the hydrogen chloride analysis tower is sent into a reduced chlorosilane storage tank of the chlorosilane storage step.

Silicon tetrachloride hydrogenation process

The refined silicon tetrachloride through the separation and purification process of chlorosilane is fed into the silicon tetrachloride vaporizer of the process, and is heated and vaporized by hot water. The hydrogen from the hydrogen preparation and purification process and the surplus hydrogen from the reduction tail gas dry separation process are mixed in a hydrogen buffer tank, and then the hydrogen is also introduced into a vaporizer to form a certain proportion of mixed gas with silicon tetrachloride vapor.

The mixed gas of silicon tetrachloride and hydrogen from the silicon tetrachloride vaporizer is sent into the hydrogenation furnace. The hydrogenation reaction of silicon tetrachloride takes place near the surface of the hot electrode which is electrified in the hydrogenation furnace, so as to generate silicon trichlorohydride and hydrogen chloride at the same time. The mixed gas containing trichlorosilane, hydrogen chloride, unreacted silicon tetrachloride and hydrogen is sent to the hydrogen dry separation process.

Hot water is introduced into the jacket of the furnace barrel of the hydrogenation furnace to remove the heat radiated from the hot electrode in the furnace to the inner wall of the furnace barrel and maintain the temperature of the inner wall of the furnace barrel. The high-temperature hot water of the jacket of the discharge cylinder is sent to the heat energy recovery working procedure, and after the steam is produced by the waste heat boiler and cooled, the hot water is recycled back to the hydrogenation furnace jacket of the working procedure.

Hydrogenation gas dry separation process

The hydrogen gas from the silicon tetrachloride hydrogenation process is separated into chlorosilane liquid, hydrogen gas and hydrogen chloride gas through the process, and the hydrogen gas is respectively recycled to the device for use.

The principle and process of dry separation of hydrogen gas are very similar to that of dry separation of trichlorosilane. The high-purity hydrogen obtained from the outlet of the temperature-changing pressure-changing adsorber is returned to the silicon tetrachloride hydrogenation process to participate in the silicon tetrachloride hydrogenation after passing through the hydrogen buffer tank; The absorbed regenerated waste gas is sent to the waste gas treatment process for treatment; The purified hydrogen chloride gas is obtained from the top of the hydrogen chloride analysis tower and sent to a circulating hydrogen chloride buffer tank which is arranged in the trichlorine hydrogen silicon synthesis process; The excess chlorosilane liquid (namely chlorosilane separated from the hydride gas) extracted from the bottom of the hydrogen chloride analysis tower is fed into the hydrochlorosilane storage tank of the chlorosilane storage step.

Chlorosilane storage process

The following storage tanks are arranged in the process: 100m1 chlorosilane storage tank, 100m1 industrial grade trichlorosilane storage tank, 100m1 industrial grade tetrachlorosilane storage tank, 100 m1 chlorosilane emergency discharge tank, etc.

The chlorosilane liquid obtained from the dry separation step of the synthesis gas, the dry separation step of the reduction tail gas and the dry separation step of the hydrogenation gas is respectively fed into a raw material, a reduction and a hydrochlorosilane storage tank, and then the chlorosilane liquid is respectively fed as the raw material to different rectification towers of the separation and purification step of the chlorosilane.

The mixed liquid of trichlorosilane and dichlorodihydrosilicon obtained at the top of the first-stage rectifying tower in the separation and purification process of chlorosilane, the trichlorosilane liquid obtained at the bottom of the fourth-stage rectifying tower and the trichlorosilane liquid obtained at the bottom of the sixth-stage rectifying tower and the eighth-stage rectifying tower and the tenth-stage rectifying tower are sent to an industrial-grade trichlorosilane storage tank, and the liquid is mixed in the tank and sold as the industrial-grade trichlorosilane products.

Silicon core preparation process

The technology of zone furnace drawing and cutting is adopted to process and prepare the conductive silicon core which needs to be installed in the furnace during the initial production of the reduction furnace. In the process of preparing silicon core, the silicon core needs to be corroded by hydrofluoric acid and nitric acid, then the silicon core is cleaned by ultrapure water, and then the silicon core is dried. The hydrogen fluoride and the nitrogen oxide gas will escape to the air during the acid corrosion treatment process, so the fan is used for sucking the hydrogen fluoride and the nitrogen oxide air through the wind cover which is covered on the upper part of the acid corrosion treatment tank, and then the gas is sent to the exhaust gas treatment device for treatment, and the exhaust gas is discharged according to the standards.

Product sort operation

The polysilicon rod made in the reducing furnace is removed from the furnace, and is cut off and broken into massive polysilicon. The polysilicon block was corroded by hydrofluoric acid and nitric acid, then cleaned by ultrapure water, and then dried. The hydrogen fluoride and the nitrogen oxide gas will escape to the air during the acid corrosion treatment process, so the fan is used for sucking the hydrogen fluoride and the nitrogen oxide air through the wind cover which is covered on the upper part of the acid corrosion treatment tank, and then the gas is sent to the exhaust gas treatment device for treatment, and the exhaust gas is discharged according to the standards. A bulk polysilicon product that has been detected to meet a specified quality index is delivered for packaging.

Waste gas and residue treatment process

1.Purification of waste gas containing hydrogen chloride

The waste gas discharged in the SiHCl1 purification process, the shutdown of the reducing furnace, the accidental discharge of waste gas, the safe discharge gas of the storage process tank of the chlorosilane and the hydrogen chloride, and the CDI adsorption waste gas are all sent to the waste gas leaching tower by pipelines for washing.

After the exhaust gas is continuously washed by 10%NaOH of the leaching tower, the washing liquid at the bottom of the tower is pumped into the processing procedure of technological waste, and the tail gas is discharged by an exhaust cylinder with a height of 15m.

2.Residue treatment

The residue liquid of the reactor mainly containing silicon tetrachloride and polychlorosilane compounds and the residue liquid of the chlorosilane which is discharged from the rectifying tower and is discharged by the stopping device are sent to the process for treatment.

The liquid to be treated is fed into a residue collecting tank. The liquid is then pressed out by nitrogen gas and sent to the residual liquid leaching tower for washing. 10%NaOH lye was used for treatment. Chlorosilane in waste liquid reacts with NaOH and water and is converted into harmless substances (treatment principle is treated with waste gas containing hydrogen chloride and chlorosilane).

3.acidic waste gas

The acid waste gas generated in the silicon core preparation and product finishing process is pumped to the waste gas processing system through a gas collecting cover. The acid waste gas is washed by 10% lime milk in a spraying tower to remove the fluorine-containing waste gas in the gas, and at the same time, ammonia is added into the washing liquid to reduce most NOx to N2 and H2O. After the gas is dehumidified after washing, the residual NOx in the gas is absorbed by the SDG adsorbent through the solid adsorption method (taking the non-noble metal as the catalyst), and then discharged by the exhaust cylinder with a height of 20m.

Waste silicon powder treatment

The silicon powder discharged from a raw material silicon powder charging dust collector, a cyclone dust collector of a silicon trichlorohydride silicon synthesis workshop and a synthesis reactor is transported into a waste slag funnel through a waste slag transporting tank, enters into an acid washing pipe with a stirrer, dealkali the waste silicon powder (dust) through 11% hydrochloric acid, and dissolves impurities such as aluminum, iron and calcium in the waste silicon. After the washing is finished, the waste residue is filtered by a filter press, sent to a dryer for drying, the dried silicon powder is returned to the trichlorosilane synthesis circulation for use, and the waste liquid is collected into the waste gas residue treatment system waste water and treated together.

The HCl-containing waste gas discharged from the pickling tank and the filtrate tank is sent to an exhaust residue treatment system for treatment.

Process waste treatment process

1.Treatment of class Ⅰ waste liquid

The acid-containing waste liquid treated by the washing tower washing liquid and waste silicon powder is mixed, neutralized and settled in the process, and then filtered by a filter press. The filter residue (mainly SiO2) shall be delivered to the Cement Factory to produce cement (see Annex). The sediment liquid and the filtrate are mainly salt-containing wastewater with high concentration, containing NaCl 200 g/L or more, the part of water does not introduce calcium magnesium ion and sulfate ion in the process operation and treatment, and the water quality meets the production requirements of chlor-alkali, so the salt-containing wastewater pipeline is transported to the caustic soda production line of Sichuan Yongxiang Co., Ltd. as the production raw material for recycling (see Annex). The evaporating condensate is reused to configure alkali liquor.

2.Treatment of class Ⅱ waste liquid

The waste hydrofluoric acid, waste nitric acid and acid washing waste water from the silicon core preparation process and the product finishing process are neutralized and settled by 10% lime emulsion, filtered by a filter press, and the filter residue (mainly CaF2) is sent to a cement factory for producing cement (see Annex). The settling solution and the filtrate are mainly calcium nitrate solution, and after evaporation and concentration, the solution is sold as by-products (see Annex). The evaporating condensate is reused to configure alkali liquor.