Production process of trichlorosilane

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The production of trichlorosilane primarily employs the boiling chlorination method, which mainly includes processes such as hydrogen chloride synthesis, trichlorosilane synthesis, and trichlorosilane purification. Chlorine and hydrogen generate hydrogen chloride through a combustion reaction in the hydrogen chloride synthesis furnace. The hydrogen chloride gas is cooled by air, water, cryogenic methods, and acid mist capture for dehydration before entering the hydrogen chloride buffer tank, and then is sent to the trichlorosilane synthesis furnace. After being dried, silicon powder is added to the trichlorosilane synthesis furnace, where it reacts with hydrogen chloride at a high temperature of around 300°C to produce trichlorosilane and silicon tetrachloride. The resulting crude trichlorosilane gas is subjected to cyclone separation and dust removal filtering before entering a shell-and-tube condenser for water cooling and cryogenic cooling. The non-condensable gases are sent to an exhaust gas scrubber via a liquid seal; after treatment, they are discharged in compliance with regulations. The condensate is separated and purified in a distillation tower, usually through a two-tower process: low-boiling substances are removed from the top of the first tower, while high-boiling substances such as silicon tetrachloride are removed from the bottom of the second tower; simultaneously, trichlorosilane product is obtained from the top of the tower. Section 1: Hydrogen Chloride Synthesis Process 1.1 Properties of Hydrogen Chloride Hydrogen chloride is a colorless, irritating gas with a melting point of -114.2°C and a boiling point of 85°C. Its specific heat capacity is 812.24 J/kg·°C, the critical temperature is 51.28°C, and the critical pressure is 8266 kPa. Dry hydrogen chloride gas does not possess acidic properties and is chemically inert; it reacts only at high temperatures. Hydrogen chloride is highly soluble in water. Under standard conditions, 500 volumes of hydrogen chloride can dissolve in 1 volume of water, and hydrochloric acid is obtained after dissolution. Since the production of trichlorosilane primarily requires hydrogen chloride gas, this paper does not conduct an in-depth study on the properties of hydrochloric acid. 1.2 Hydrogen chloride synthesis conditions Hydrogen chloride is synthesized in a specially designed synthesis furnace. To ensure that the product contains no free chlorine, hydrogen is used in an excess of 15%~20% compared to chlorine. The flame temperature in the furnace during actual production is around 200°C. Since the reaction is exothermic, in order to prevent the reaction temperature from rising too high, industrial production controls the outlet temperature of hydrogen chloride to below 350°C by regulating the flow rates of chlorine and hydrogen, as well as by circulating cooling water in the jacket surrounding the furnace. To ensure safe production, the purity of hydrogen required is not less than 98%, with an oxygen content of no more than 0.4% ； The purity of chlorine should be at least 65%, with a hydrogen content of no more than 3%. 1.3 Hydrogen chloride synthesis process The reaction equation for hydrogen chloride synthesis is: Cl2 + H2 → 2HCl. Chlorine gas, with a purity of 97% and a pressure of 0.5 MPa, is measured using a turbine flow meter before entering the chlorine gas buffer tank. Hydrogen is metered using a turbine flow meter; it has a purity of 98% and a pressure of 0.09 MPa. The moisture in this hydrogen is removed via a water separation tank, after which it, along with the recycled hydrogen, enters the hydrogen buffer through a turbine flow meter. The metered chlorine and hydrogen gases are regulated in flow rate so as to maintain a ratio of 1:1.04–1.10 (by volume), and then fed into a two-in-one hydrogen chloride graphite synthesis furnace for reaction. The heat generated during the reaction is removed by the circulating water in the furnace’s jacket. The hydrogen chloride gas produced passes through a 3.6-meter-long graphite sleeve cooler, where its temperature is reduced to below 165°C; it is further cooled using circulating water in the graphite cooler, and after cooling its temperature drops to around 45°C. Finally, it is passed through a pre-cooler filled with cryogenic gas, where it is cooled further to -20°C to -30°C for dehydration. After the frozen hydrogen chloride gas has had the mist droplets removed from it using a demister, it is heated to 15–25°C by an pre-heater before entering the hydrogen chloride compressor, where its pressure is increased to 0.3–0.4 MPa. Subsequently, it passes through a buffer tank (V-103) before entering the hydrogen chloride cryocooler, where it is cooled to -15–-25°C to remove the acidic water present in the gas. After that, it enters V-105 for further buffering and to remove any remaining mist droplets. Finally, the hydrogen chloride gas is heated by heater (E-106) before being fed into the fluidized bed for use in the fluidized bed reaction. 1.4 Schematic diagram of the hydrogen chloride synthesis process: Section 2 Silicon powder purification process 2.1 Properties of silicon powder Silicon has a density of 2.329 kg/cm3, a boiling point of 2355°C, and a melting point of 1480–1500°C; in the production of trichlorosilane, its moisture content is less than 200 ppm. Water facilitates the formation of hydrochloric acid, which corrodes equipment due to the presence of free hydrogen; its lower explosive limit is 160 g/cm3. Silicon is widely distributed in the Earth's crust, accounting for about 1/4 of the total mass of the crust, second only to oxygen. It is mainly located in Heilongjiang and Jilin. Silicon is divided into amorphous silicon and crystalline silicon. Crystalline silicon is a gray, shiny, hard, and brittle solid; its structure is similar to that of diamond, and it is also an atomic crystal. Silicon’s electrical conductivity lies between that of metals and insulators. Single-crystalline silicon is an excellent semiconductor that can be used to manufacture semiconductor devices such as silicon rectifiers, transistors, and integrated circuits. 2.2 Purpose and significance of silicon powder purification The purification of silicon powder involves drying this powder, which contains a certain amount of moisture, in a drying furnace using steam heated by a nitrogen gas flow; this process removes the moisture. The moisture content of the dried silicon powder is a key factor affecting the quality of trichlorosilane. Therefore, strictly controlling the process conditions to ensure the quality of the silicon powder is the main task in its purification. 2.3 Brief description of the silicon powder purification process: Silicon powder is drawn into the silicon powder dryer using a vacuum pump or poured in through a manhole, and then nitrogen gas from a nitrogen heater (at a temperature of 200–250°C) is blown in from the bottom of the drying furnace. Simultaneously open the steam valve to supply steam to the jacket of the drying furnace in order to raise its temperature to between 180 and 220°C. The heating time for each batch is approximately 3 to 4 hours (determined based on the amount of silicon powder fed into each batch). Approximately 1,500 kilograms of silicon powder are added per batch; after being heated and dried, the silicon powder is stored in a silo for use in the synthesis process. 2.4 Schematic diagram of the process flow for silicon powder purification Section 3: Process for synthesizing trichlorosilane 3.1 Properties of trichlorosilane Trichlorosilane is also known as silanochloride, silane, or trichlorosilane ； English names: Trichlorosilane, Silicochloroform. Trichlorosilane has a boiling point of 31.8°C, a melting point of -126.5°C, and an auto-ignition temperature of 185°C. At an air density of 1, its vapor relative density is 4.7, and its explosive limit in air is 1.2–90.5% (by volume). The main uses are as raw material for single-crystalline silicon, for epitaxial growth, in silicon melt and silicone oil, for chemical vapor deposition, in the production of silicon compounds, and in electronic gases. Main preparation method: (1) Si reacts with HCl at high temperature. (2) Reduction of silicon tetrachloride with hydrogen (using an aluminum-containing catalyst). Silicon trichlorohydride is a colorless and transparent liquid that is highly volatile, easy to flow, and has an irritating foul odor at normal temperature and pressure. It burns very easily in air, and there is a risk of ignition at temperatures below -18°C. It burns violently when exposed to an open flame, producing red flames and white smoke, as well as SiO2, HCl, and Cl2 ； The reaction equation is: SiHCl3 + O2 → SiO2 + HCl + Cl2 ； The vapor of trichlorosilane can form explosive mixtures with air over a wide range of concentrations, leading to violent explosions when heated. Its thermal stability is better than that of dichlorosilane; at 900°C, it decomposes to produce toxic chloride fumes (HCl), as well as Cl2 and Si. It emits smoke when exposed to moisture, and reacts violently with water; the reaction equation is: 2SiHCl3 + 3H2O → (HSiO)2O + 6HCl ； Hydrogen is released upon decomposition in an alkaline solution; the reaction equation is: SiHCl3 + 3NaOH + H2O → Si(OH)4 + 3NaCl + H2↑ ； 　　It undergoes an explosive reaction when in contact with oxidizing substances. It reacts with hydrocarbons such as acetylene and hydrocarbons to produce organochlorosilanes. The reaction equations are as follows: SiHCl3 + CH≡CH → CH2CHSiCl3; SiHCl3 + CH2=CH2 → CH3CH2SiCl3. In the presence of lithium aluminum hydride or lithium borohydride, SiHCl3 can be reduced to silanes. The liquid SiHCl3 in the container can catch fire when the container is subjected to a severe impact. It is soluble in benzene, ether, etc. Silicon trichloride does not corrode iron and stainless steel in an anhydrous state, but it corrodes most metals in the presence of moisture. 3.2 Purpose and Significance of Trichlorosilane Synthesis: In this process, dried silicon powder is fed into a fluidized bed. Within the fluidized bed reactor, the silicon powder reacts with hydrogen chloride gas to produce chlorosilane mixtures. These mixed monomers are then subjected to processes such as degassing, purification, cooling, pressurization, and further cooling before being sent to a degassing tower. At the top of the tower, low-boiling hydrogen chloride is removed, and the hydrogen chloride gas is recycled back to the fluidized bed for reuse. The mixed monomers at the bottom of the tower are cooled in a monomer cooler before being stored in a storage tank for use in the distillation process. After the mixed monomers are purified through distillation, the product silicon trichloride and the by-product silicon tetrachloride are obtained. 3.3 Brief description of the synthesis process for trichlorosilane: The reaction equation is: Si + 3HCl → SiHCl3 + H2↑. The chemical equation for the side reaction is: Si + 2HCl → SiH2Cl2 (at 350°C). Hydrogen chloride gas at 50–80°C, coming from the hydrogen chloride heater, is regulated by a control valve to enable the recovery of hydrogen chloride; this gas, along with silicon powder, enters the fluidized bed reactor from the bottom of the bed, where silanes are synthesized in a fluidized state. The temperature of the synthesis reaction is maintained between 280–320°C, while the reaction pressure ranges from 0.2–0.3 MPa. During the reaction, the height of the material layer is determined by monitoring the pressure in the reactor. The reaction temperature at different locations within the reactor is used to determine the reaction point; this is an exothermic reaction, and the heat generated is carried away by the hot water flowing through the elbow-shaped tubes in the reactor, thereby keeping the temperature of the fluidized bed between 280 and 320°C. The hot water in the hot water tank is pumped into the reactor’s elbow pipe by a hot water pump. The hot water tank is connected to the steam main, ensuring that the pressure in the tank remains at 0.4 MPa and the liquid level is around 80%. The hot water then vaporizes into steam (at 0.4 MPa) inside the elbow pipe. The saturated steam at 0.4 MPa generated as steam exits the fluidized bed reactor and enters the hot water tank can be used by other gas-consuming equipment. The steam condensates from the entire system, including the slurry receiving tank, slurry evaporator, steam tail condenser, pre-machine heater, feed heater, silicon powder drying furnace, degassing reboiler, dehydrogenation reboiler, and finished product reboiler, flow into the condensate tank; the make-up water pump then pumps water into V-302 to compensate for the water lost due to vaporization during the reaction. The chlorosilane, hydrogen, hydrogen chloride, and mixed gas produced in the fluidized bed reactor are successively passed through a first spin separator and a second spin separator to remove 80%~90% of the unreacted silicon powder; thereafter, they are cooled by being mixed with chlorosilane pumped from a washing pump inside a dust collector, and then sent to a slurry receiving tank for further removal of silicon powder before entering a condenser for condensation. 3.4 Factors Affecting the Synthesis of Trichlorosilane The main factors affecting the synthesis of trichlorosilane include: temperature (already discussed in the production process), the influence of oxygen and moisture, the control of free chlorine, the particle size of silicon powder, the layer height, and the flow rate of hydrogen chloride. The following is a brief overview of the influencing factors. (1) Effects of oxygen and moisture: Oxygen and moisture cause significant damage to synthetic reactions. Since the Si-O chemical bond is more stable than the Si-Cl bond, any oxygen that enters the system reacts with silicon to form silica gel or siloxane compounds. On one hand, this creates a dense oxide layer on the surface of the silicon powder, hindering the proper progress of the reaction and resulting in a lower content of trichlorosilane in the product. Additionally, these silica gel compounds can clog the pipes, causing failures in the production system. (II) Control of free chlorine: The impact of free chlorine on synthesis furnaces is mainly twofold: firstly, high levels pose an explosion risk, and secondly, it affects the quality of the synthesis. Free chlorine is controlled by maintaining a hydrogen excess of about 4% during the reaction in the hydrogen chloride synthesis furnace, and the quality of hydrogen chloride is continuously monitored using a concentration detector to ensure that the free chlorine level remains below the requirements for production. (III) Silicon powder particle size: The reaction between silicon powder and hydrogen chloride gas occurs on the surface of the silicon. The greater the specific surface area of the silicon powder, the more favorable the reaction is; therefore, it is necessary for the particle size of the silicon powder to be small. However, if the particle size is too small, a polydisperse fluidized bed is likely to form during fluidization, with an abundance of bubbles that inhibit mass transfer and reduce the primary conversion rate of hydrogen chloride. At the same time, the smaller particles react rapidly and quickly reach the particle size range for removal, thereby reducing the utilization efficiency of silicon powder. Therefore, it is important to choose silicon powder with an appropriate particle size. (IV) Bed height and hydrogen chloride flow rate: The bed height of silica powder and the flow rate of hydrogen chloride are important factors affecting the yield and quality of trichlorosilane. A higher bed height leads to an increased pressure drop, which requires an increase in the inlet pressure accordingly. Excessively high pressure drops lead to reduced stability inside the furnace, posing a risk of the formation of \"fountains\" or \"gulfs\", and even more so, the risk of \"pipe failure\". If the layer of material is too thin, uneven boiling occurs, the contact time for the reaction is shortened, and the output decreases. The flow rate of hydrogen chloride determines the fluidization state of the particle bed. The specific layer height and hydrogen chloride flow rate need to be determined through actual production practice. 3.5 Treatment of off-gases from trichlorosilane synthesis: All chemical manufacturing processes are confronted with the issue of waste generation, and the main problem in trichlorosilane production is the treatment of off-gases. Therefore, this paper only conducts research and provides an overview of exhaust gas treatment. 3.5.1 Exhaust gas treatment methods: The exhaust gas emitted from the trichlorosilane synthesis furnace is compressed to a pressure of 0.7 MPa before being fed into a water cooler for initial cooling. It then enters a condenser where it is further cooled by a coolant at 45°C; as a result, the majority of the trichlorosilane is condensed into a liquid form, separating it from hydrogen chloride and hydrogen. The liquid trichlorosilane, along with the synthesized trichlorosilane, is sent to the intermediate product storage tank. The small amount of chlorosilane, hydrogen chloride, and hydrogen that remain uncondensed can be recovered using one of the following three methods. (1) Comprehensive recovery method: The hydrogen chloride, hydrogen, and chlorine that have not been condensed are returned to the hydrogen chloride synthesis system. Hydrogen and chlorine are mixed in a certain ratio and burned to produce hydrogen chloride, which is then reused in the cycle. By adopting a comprehensive recycling method, the open-loop process flow of the synthesis process is transformed into a closed-loop system, which improves the utilization rate of raw materials, reduces the consumption of these materials, eliminates waste gas emissions, and completely resolves environmental pollution issues. However, the trace amounts of chlorosilanes present in the exhaust gases readily react with the water in hydrogen chloride to form solid silicon dioxide, which blocks the pipes and reduces the yield of trichlorosilane. (2) Scrubbing and neutralization method: Hydrogen chloride, hydrogen, and a small amount of uncondensed chlorosilane are sent to the exhaust gas scrubber tower, where they are absorbed by spraying large amounts of water. Hydrogen chloride dissolves in water, and the resulting hydrogen chloride solution is neutralized with sodium hydroxide to meet the required standards before being discharged. The leaching neutralization treatment method is simple in process, mature in technology, and requires low investment; by controlling the water volume of the spraying system and the amount of sodium hydroxide added to the neutralization tank, it is also possible to achieve compliant discharge. The downside is that hydrogen chloride and hydrogen are not reused, resulting in increased consumption. Furthermore, the chlorosilanes in the exhaust gases react with water to produce insoluble silica and hydrochloric acid; simultaneously, hydrogen chloride also forms hydrochloric acid when it dissolves in water, resulting in a large volume of waste that needs to be treated. (3) Exhaust gas adsorption treatment method: Utilizing the adsorption effect of activated carbon on chlorosilanes. When hydrogen chloride, hydrogen, and a small amount of unliquidified chlorosilane are passed through activated carbon, the chlorosilane is adsorbed by it. Once the activated carbon becomes saturated with adsorbate, steam is used to heat it, thereby releasing the adsorbed chlorosilane, which is then sent out of the system along with the synthetic product for separation. The unadsorbed hydrogen chloride is absorbed by water and converted into hydrochloric acid, a by-product, which is then sold. The remaining hydrogen is sent to the hydrogen chloride synthesis system, where it burns with chlorine in a certain ratio to produce hydrogen chloride for reuse. The adsorption method in the adsorption tower involves recovering a small amount of chlorosilane; meanwhile, the hydrogen chloride in the exhaust gas is absorbed by water to form hydrochloric acid. It not only solves the problem of acidic water discharge but also generates hydrochloric acid as a by-product, thereby increasing economic benefits. However, this method requires high standards for activated carbon, uses a large amount of steam, and is not cost-effective. Taking all the above factors into account, it is believed that a comprehensive approach should be adopted by combining the advantages of the second and third techniques on the basis of the first method; the exhaust gases should be processed in a closed-loop system before being returned to the fluidized bed to participate in the reaction, thereby maximizing the utilization rate of hydrogen chloride and improving economic efficiency.