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Improved Siemens method preparation process and precautions

admin on December 9, 2021 0 Comments • Tags: #improvedsiemenspreparationprocess #pointstonotefortheimprovedsiemensmethod #polysiliconpreparationprocess

Improved Siemens method preparation process and precautions

1. Hydrogen preparation
Hydrogen is produced by electrolysis of desalinated water in the electrolytic cell, and the hydrogen is cooled, separated from the liquid, and then enters the deaerator. Under the action of the catalyst, the trace amount of oxygen in the hydrogen reacts with hydrogen to form water and is removed; the deoxygenated hydrogen is dried by a set of adsorption dryers; the purified and dried gas is sent to the hydrogen storage tank, and then sent to the hydrogen chloride Synthesis, trichlorosilane oxygen reduction, silicon tetrachloride hydrogenation process. The oxygen produced by electrolysis is cooled and separated from the liquid, and then sent to the oxygen storage tank
The hazardous substance in this process is mainly hydrogen, which has the characteristics of flammability and accidental sparks during the electrolysis process, which can cause flammability and explosion. In addition, hydrogen transmission pipelines, cooling and separation devices have caused leakage due to components, operation, or overhaul, which makes the surrounding air have a certain possibility of deflagration.

2. Hydrogen chloride synthesis
The hydrogen from the hydrogen production and purification process and the circulating hydrogen returned from the synthesis gas dry separation process enter the hydrogen buffer tank in this process and are mixed in the tank. The oxygen from the 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 passes through the chlorine buffer tank and is also introduced into the combustion gun at the bottom of the hydrogen chloride synthesis furnace. The mixed gas of hydrogen and chlorine is ignited at the exit of the combustion gun, and hydrogen chloride gas is generated through the combustion reaction. The hydrogen chloride gas from the synthesis furnace flows through an air cooler, a water cooler, a deep cooler, and a mist separator, and then is sent to the trichlorosilane synthesis process.
To ensure safety, this device is equipped with a hydrogen chloride gas absorption system consisting of two hydrogen chloride falling film absorbers, two hydrochloric acid circulation tanks, and hydrochloric acid circulation pumps. Water can be used to absorb the hydrogen chloride discharged due to device load adjustment or emergency release. The system maintains continuous operation and can receive and absorb the hydrogen chloride gas discharged from the device at any time. To ensure safety, a set of chlorine-containing waste gas treatment system mainly composed of waste gas treatment tower, lye circulation tank, lye circulation pump and lye circulation cooler is installed in this process. When necessary, the chlorine in the chlorine buffer tank and the pipeline can be sent into the waste gas treatment tower and washed and removed with sodium hydroxide aqueous solution. The waste gas treatment system keeps running continuously to ensure that chlorine-containing gas can be received and processed at any time.

3. Synthesis of trichlorosilane
The raw material silicon powder is lifted and discharged into the silicon powder receiving hopper through the silicon powder lower hopper. The silicon powder is put into the lower intermediate hopper from the receiving hopper. After replacing the gas in the hopper with hot hydrogen chloride and increasing the pressure to balance with the pressure of the lower hopper, the silicon powder is put into the silicon powder supply hopper below to supply the silicon in the hopper. The powder is fed into the feed pipe of the trichlorosilane synthesis furnace by a star feeder installed at the bottom of the hopper. The hydrogen chloride gas from the hydrogen chloride synthesis process is mixed with the circulating hydrogen chloride gas sent from the circulating hydrogen chloride buffer tank, and then introduced into the feed pipe of the trichlorosilane synthesis furnace, and the silicon powder fed into the pipe from the silicon powder supply hopper is entrained and combined Convey, enter the trichlorosilane synthesis furnace from the bottom. In the trichlorosilane synthesis furnace, silicon powder and hydrogen chloride gas form a fluidized bed and react to generate trichlorosilane and simultaneously generate silicon tetrachloride, dichlorodisilane, metal chloride, polychlorosilane, hydrogen and other products , This mixed gas is called trichlorosilane synthesis gas.
This reaction generates a lot of heat. The outer wall of the synthesis furnace is equipped with a water jacket. The water in the jacket takes away the heat to maintain the temperature of the furnace. The dry dust removal system removes part of the silicon powder and then sends it to the wet dust removal system, where it is washed by silicon tetrachloride liquid, and part of the fine silicon dust in the gas is washed; while washing, the wet hydrogen is in contact with the gas, and the gas contains Part of the metal oxide is hydrolyzed and removed. The purified mixed gas after removing the silicon powder is sent to the next step of syngas dry separation process.
The types of risks that may occur during this process are as follows. ① The hydrogen chloride pipeline bursts or leaks due to its own weight or maintenance errors, misoperation and other problems, which leads to the overflow of hydrogen chloride. ②The internal pressure of the trichlorosilane fixed bed reactor is 2.76MPa, 500℃, the temperature in the reactor is relatively high, and it has a certain positive pressure. After a valve leakage accident occurs, a certain amount of trichlorosilane will overflow, and it will quickly react with water when it encounters water, which will easily affect the external ambient air and surface water. ③The accidental leakage of the trichlorosilane storage tank may occur in the management, equipment, and operation process. ④ The hydrogen generated by the reaction may also leak or deflagrate.

4. Dry separation of synthesis gas
The trichlorosilane synthesis gas stream passes through the mixed gas buffer tank, and then enters the spray washing tower, where it is washed by the low-temperature chlorosilane liquid flowing down from the top of the tower. Most of the chlorosilane in the gas is condensed and mixed into the scrubbing liquid. The chlorosilane at the bottom of the tower is pressurized with a pump, and most of the chlorosilane is refrigerated and cooled back to the top of the tower for gas washing, and the excess chlorosilane is sent to the hydrogen chloride desorption tower. Most of the chlorosilane gas is removed from the top of the spray scrubber tower, compressed by a mixed gas compressor and cooled by freezing, then sent to the hydrogen chloride absorption tower, and the frozen and cooled chlorosilane liquid sent from the bottom of the hydrogen chloride desorption tower In washing, most of the hydrogen chloride in the gas is absorbed by chlorosilane, and most of the remaining chlorosilane in the gas is also washed and condensed. The gas exiting the top of the tower is hydrogen containing trace amounts of hydrogen chloride and chlorosilane. After further removing the hydrogen chloride and chlorosilane through a set of temperature swing pressure adsorbers, high-purity hydrogen is obtained. The hydrogen flows through the hydrogen buffer tank, and then returns to the hydrogen chloride synthesis process to participate in the synthesis of hydrogen chloride. The adsorber regeneration waste gas contains hydrogen, hydrogen chloride and chlorosilane, and is sent to the waste gas treatment process for treatment. The chlorosilane with hydrogen chloride gas dissolved at the bottom of the hydrogen chloride absorption tower is heated, and merges with the excess chlorosilane from the bottom of the spray washing tower, and then is sent to the middle of the hydrogen chloride desorption tower, and it is purified at the top of the tower by vacuum distillation. Hydrogen hydride gas. The hydrogen hydride gas exiting the tower flows through the hydrogen hydride buffer tank, and then sent to the circulating hydrogen chloride buffer tank in the trichlorosilane synthesis process; the hydrogen chloride is removed from the bottom of the tower to obtain the regenerated chlorosilane liquid, most of which are cooled, frozen and cooled , Sent back to the hydrogen chloride absorption tower for absorbent, the excess chlorosilane liquid, after cooling, sent to the raw material chlorosilane storage tank of the chlorosilane storage process.
The risk types and links that may occur in this process are as follows: 1 The synthesizer contains a certain amount of trichlorosilane, hydrogen and hydrogen chloride. After the leakage of the scrubber or the intake pipeline, the control valve, etc., it is easy to cause the trichlorohydrin. Leakage of silicon, hydrogen and hydrogen chloride gas; 2 The hydrogen chloride scrubber contains only a certain amount of hydrogen, hydrogen chloride and a small amount of trichlorosilane. The main dangerous substances after this process leaks are hydrogen chloride and hydrogen: the above two washings The gas contains trace amounts of hydrogen chloride and trichlorosilane, which may cause fire and explosion accidents after a leak.

5. Separation and purification process of chlorosilane
The raw material trichlorosilane is mainly rectified through a multi-stage rectification tower to remove low-boiling and high-boiling impurities. The main types of risks that may occur are: leakage, cracking, fracture and even bursting accidents at the connection between the rectification tower and the pipeline, the pipeline and the rectification tower, and the control valve will cause the trihydrogen silicon rectification. The overflow of liquid will cause a small amount of silicon tetrachloride to overflow. The rapid volatilization of these two substances will affect the external ambient air.

6. Trichlorosilane hydrogen reduction
The separated and purified trichlorosilane is sent to the trichlorosilane vaporizer, and is heated and vaporized by hot water; the circulating oxygen returned from the dry separation process of the reduction tail gas flows through the hydrogen buffer tank, and is also passed into the vaporizer to combine with the trichlorosilane vaporizer. Oxygen and silicon vapor form a certain proportion of mixed gas, which is sent into the reduction furnace. On the surface of the hot silicon core/silicon rod that is energized in the reduction furnace, the trichlorosilane undergoes a hydrogen reduction reaction to produce silicon and deposit it. The diameter of the core/silicon rod gradually increases until it reaches the specified size. The hydrogen reduction reaction simultaneously produces dichlorodisilane, silicon tetrachloride, hydrogen chloride and hydrogen, which are sent out of the reduction furnace together with the unreacted trichlorosilane and hydrogen, and are directly sent to the reduction after being cooled by the reduction tail gas cooler with circulating cooling water. Exhaust gas dry separation process. The jacket of the reduction furnace is filled with hot water to remove the heat radiated from the hot silicon core in the furnace to the inner wall of the furnace and maintain the temperature of the inner wall of the furnace. The high-temperature hot water from the jacket of the furnace is sent to the heat recovery process. After the temperature is cooled by the production of steam from the waste heat boiler, it is recycled back to the jackets of the reduction furnace in this process. In the specific operation, attention should be paid to the reduction furnace after the silicon core is installed, and the hydraulic jet vacuum pump is used to vacuum before the start-up, and then the air in the furnace is replaced with nitrogen, and the ammonia in the furnace is replaced with hydrogen, and then heated and operated. Discharge nitrogen and a small amount of water from the vacuum pump to the ambient air; during the shutdown and start-up phase (once every 5-7 days), the mixed gas containing chlorosilane, hydrogen chloride and hydrogen in the reduction furnace is first pressed into the reduction tail gas with hydrogen for dry recovery The system is recycled, then replaced with nitrogen and emptied. The polysilicon product is taken out, the waste graphite electrode is removed, and the furnace is washed with ultrapure water as the case may be. Therefore, nitrogen, waste graphite and cleaning wastewater will be generated during the shutdown phase. Nitrogen is a harmless gas, so under normal circumstances, there is no harmful gas emission during the start and stop phases of the reduction furnace. The waste graphite is recovered by the original production plant, and the cleaning wastewater is sent to the project’s chloride-containing acid-base wastewater treatment system for treatment.
The possible risk accidents include the leakage of reducing gas hydrogen and thermally vaporized trichlorosilane.

7. Dry separation of reduced exhaust gas
The unreacted silicon oxychloride, hydrogen and dichlorodisilane, silicon tetrachloride, chlorine chloride and hydrogen produced by reduction in the reduction furnace are sent to the dry separator together, and a process similar to the synthesis gas separation process is selected. Technology to separate the exhaust gas. High-purity hydrogen is obtained through pressure swing adsorption, part of which is sent to the raw material storage tank, most of which is sent to the reduction of trichlorosilane, and the rest is sent to the hydrogenation of silicon tetrachloride; the hydrogen chloride in the tail gas is removed through the hydrogen chloride analysis tower, and sent to To the buffer tank used for the synthesis of trichlorosilane: the remaining hydrogen silicon burning liquid is sent to the reduced chlorosilane storage tank in the silane storage process.
The exhaust gas processed in this process contains relatively low levels of toxic and hazardous substances.

8. Hydrogenated silicon tetrachloride
The silicon tetrachloride refined by the chlorosilane separation and purification process is sent to the silicon tetrachloride vaporizer and heated by hot water to vaporize. The hydrogen sent from the hydrogen preparation and purification process and the excess hydrogen sent from the reduction tail gas dry separation process are mixed in the hydrogen buffer tank, and then passed into the vaporizer to form a certain ratio 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. Near the surface of the hot electrode that is energized in the hydrogenation furnace, the hydrogenation reaction of silicon tetrachloride occurs to generate trichlorosilane and hydrogen chloride at the same time. The mixed gas containing trichlorosilane, hydrogen chloride, unreacted silicon tetrachloride, and hydrogen from the hydrogenation furnace is sent to the hydrogen gas dry separation process.
Risk accidents that may occur during this process include: leakage of silicon tetrachloride, hydrogen, trichlorosilane, and hydrogen chloride.

9. Hydrogen gas dry separation
The principle and process of the dry separation of hydrogen gas are very similar to the dry separation of trichlorosilane synthesis gas. The high-purity hydrogen obtained from the outlet of the temperature swing pressure adsorber flows through the hydrogen buffer tank and then returns to the silicon tetrachloride hydrogenation process to participate in the hydrogenation reaction of silicon tetrachloride: the exhaust gas from adsorption regeneration is sent to the waste gas treatment process for treatment; from hydrogen chloride The purified hydrogen chloride gas obtained at the top of the analysis tower is sent to the circulating hydrogen chloride buffer tank placed in the trichlorosilane synthesis process: excess chlorosilane liquid is drawn from the bottom of the hydrogen chloride analysis tower.
This process mainly separates and recovers process waste gas, and the toxic and harmful substances involved mainly include silicon tetrachloride, hydrogen, and trichlorosilane.

10. Other processes
(1) Silicon core preparation
Using the technology of zone melting furnace drawing and cutting, in the silicon core preparation process, it is necessary to corrode the silicon core with hydrofluoric acid and nitric acid, and then wash the silicon core with ultrapure water, and then dry the silicon core. During the corrosion process, hydrogen fluoride and nitrogen oxide gases escape into the air, so the air containing hydrogen fluoride and nitrogen oxides is sucked by a fan through a wind hood that covers the acid corrosion treatment tank, and then the gas is sent to the exhaust gas The treatment device performs treatment and discharges up to standard.
(2) Product finishing
The polysilicon rods produced in the reduction furnace are taken out of the furnace, cut and broken into chunks of polysilicon. The block polysilicon is corroded with hydrofluoric acid and nitric acid, and then the polysilicon block is washed with ultrapure water, and then the polysilicon block is dried. During the acid etching process, hydrogen fluoride and nitrogen oxide gases escape into the air, so the air containing hydrogen fluoride and nitrogen oxides is sucked by a fan through the air hood that covers the acid corrosion treatment tank, and then the gas is sent to The exhaust gas treatment device is used for treatment and discharges up to the standard. After testing, the bulk polysilicon products that meet the specified quality indicators are sent to the packaging.
(3) Waste gas and residual liquid treatment
After the exhaust gas is continuously washed with 10% NaOH in the leaching tower, the scrubbing liquid at the bottom of the tower is pumped to the process waste treatment process, and the tail gas is discharged through a 15m high exhaust gas stream.
(4) Treatment of waste silicon powder
The silicon powder discharged from the raw material silicon powder feeding dust collector, the cyclone dust collector of the trichlorosilane synthesis workshop and the synthesis reactor is transported to the waste slag funnel through the waste slag conveying tank, and enters the pickling pipe with agitator. 31% hydrochloric acid dealkalizes waste silicon powder (dust) and dissolves impurities such as aluminum, iron and calcium in the waste silicon. After washing, it is filtered by a filter press, and the waste residue is sent to a dryer for drying. The dried silicon powder is returned to the trichlorosilane synthesis cycle for use, and the waste liquid is collected into the waste gas residue treatment system for processing and discharged from the pickling tank and the filtrate tank The HCI-containing room gas is sent to the room gas residual liquid treatment system for processing.

Improved Siemens method preparation process and precautions
Modified Siemens method preparation process

In addition to the technical process, the polysilicon plant of the Siemens method also has a relatively large investment. If the process starts from the production of trichlorosilane from metal silicon to polysilicon, a 1,000-ton plant will need to invest about 1.2 billion to 1.5 billion yuan.

Now polysilicon manufacturers and researchers in various countries are studying new processes for cheap production of solar-grade polysilicon.

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