A method, system and pretreatment method for improving the purity of silicon tetrafluoride

Abstract

The application discloses a method, a system and a pretreatment method for improving the purity of silicon tetrafluoride, and belongs to the technical field of silicon tetrafluoride purification. The method comprises the following steps: S1, continuously feeding a complexing agent and crude silicon tetrafluoride gas into a complexing tower, so that the crude silicon tetrafluoride gas is subjected to a complexing reaction with the complexing agent to generate a complexing liquid, and impurity gas not participating in the complexing reaction is discharged; the complexing reaction between the complexing agent and the silicon tetrafluoride gas is reversible; S2, the complexing liquid generated in the step S1 is subjected to low-temperature heating decomposition to remove HF impurities in the reactants; S3, the complexing liquid generated in the step S2 is subjected to medium-temperature heating decomposition to remove BF3 impurities in the reactants; S4, the complexing liquid generated in the step S3 is subjected to high-temperature heating decomposition to obtain silicon tetrafluoride purified gas; wherein the temperature of the low-temperature heating decomposition in the step S2 is less than the temperature of the medium-temperature heating decomposition in the step S3, and the temperature of the medium-temperature heating decomposition is less than the temperature of the high-temperature heating decomposition in the step S4. The scheme can realize the purification treatment of the silicon tetrafluoride, and can realize the recycling of the complexing agent.

Description

Technical Field

[0001] This application belongs to the field of silicon tetrafluoride purification technology, and particularly relates to a method, system and pretreatment method for improving the purity of silicon tetrafluoride. Background Technology

[0002] Silicon-28 ( 28 Si), Silicon-29 ( 29 Si) and silicon-30 ( 30 Silicon (Si) consists of three natural isotopes of silicon. Their core difference lies in the number of neutrons in their atomic nuclei, leading to variations in physical properties, natural abundance, and applications. In nature, the abundance of these three isotopes varies considerably, with the following abundance (atomic percentage): Silicon-28: approximately 92.23% (the most abundant isotope, accounting for the vast majority of natural silicon); Silicon-29: approximately 4.67% (the second most abundant); Silicon-30: approximately 3.10% (the least abundant). This difference in abundance directly determines the average relative atomic mass of "natural silicon" (approximately 28.085)—the result of a weighted average of the mass numbers of the three isotopes based on their abundance. Silicon-28, due to its highest abundance, is the main component of natural silicon and a core raw material in the semiconductor industry.

[0003] Silicon tetrafluoride-28 isotope ( 28 Silicon tetrafluoride (SiF4), as a specialty gas for electronics, is mainly used in the electronics and semiconductor fields. High-purity silicon tetrafluoride can reach a purity of 99.999% (5N), which can meet the extremely high purity requirements of the electronics industry. 28 SiF4 (SiF4) electronic specialty gases have various applications in the electronics industry, primarily including: etchants, used to remove material from semiconductor surfaces to achieve precise pattern transfer; dopants, used to introduce impurities into semiconductor materials to modulate their electrical properties; deposition sources, used in chemical vapor deposition (CVD) processes to prepare thin film materials; and ion implantation sources, used in ion implantation processes to implant impurity ions into semiconductor materials.

[0004] Patent application WO2009042415A2 discloses a purification process for silicon tetrafluoride. The method involves purifying a silicon tetrafluoride source gas through one or more purification processes, including: contacting the silicon tetrafluoride source gas with an ion exchange resin to remove acidic contaminants; contacting the silicon tetrafluoride source gas with a catalyst to remove carbon monoxide; removing carbon dioxide using an absorbent; and removing inert compounds by low-temperature distillation; removing carbon dioxide diethers by using an absorbent containing at least one ethylene glycol; and removing inert gases and combinations thereof by low-temperature distillation. While this process can remove various impurities, it is lengthy, requires significant equipment investment, and different impurities need to be treated separately using different chemical or physical principles. The system integration is complex, and operational flexibility and synergistic purification efficiency need improvement.

[0005] Chinese patent application CN119971666A discloses a high-purity silicon tetrafluoride separation and purification device, which effectively removes impurities such as dust, moisture, and hydrogen fluoride from silicon tetrafluoride gaseous products by combining a filter cartridge with an internally filled molecular sieve. However, for impurities with similar physicochemical properties to silicon tetrafluoride (such as BF3, which is also a gaseous fluoride), the adsorption selectivity of the molecular sieve is often limited, which may lead to the target product gas (SiF4) competing with the impurities for adsorption sites, resulting in product loss or premature saturation of the adsorbent. At the same time, frequent adsorbent regeneration affects the stability of continuous production, and the deep purification capacity for certain trace impurities may be insufficient.

[0006] One of the conventional methods for large-scale purification of silicon tetrafluoride is direct cryogenic distillation. Because silicon tetrafluoride has a low boiling point (approximately -86°C) and needs to be separated from impurities with similar boiling points (such as BF3, which has a boiling point of approximately -100°C), the distillation process must be carried out at extremely low temperatures (e.g., below -100°C). This not only places extremely high demands on equipment materials and insulation, but also results in enormous energy consumption and high operating costs for the refrigeration system, posing a serious challenge to the overall economic viability of the process.

[0007] In summary, existing silicon tetrafluoride purification technologies either suffer from complex processes, high equipment investment, and cumbersome operation; or they have drawbacks such as insufficient selective separation of key impurities, leading to product loss; or they are hampered by the high energy consumption of the separation process (especially cryogenic distillation). Therefore, the industry urgently needs to develop a new silicon tetrafluoride purification method and system that is relatively simple in process, highly selective, low in energy consumption, and enables resource recycling, in order to meet the growing demand for ultra-high purity silicon tetrafluoride in fields such as high-end electronics manufacturing and isotope separation. Summary of the Invention

[0008] This application aims to at least partially solve one of the technical problems in the related art. To this end, the method, system and pretreatment method for improving the purity of silicon tetrafluoride provided by this application have high impurity removal selectivity, reasonable process, and are easy to achieve stable control on an industrial scale. They can effectively remove specific impurities such as HF and BF3, realize the purification treatment of silicon tetrafluoride, and enable the recycling of complexing agents.

[0009] To achieve the above objectives, in a first aspect, this application provides a method for improving the purity of silicon tetrafluoride, comprising the following steps:

[0010] S1. A complexing agent and crude silicon tetrafluoride gas are continuously introduced into the complexing tower, so that the crude silicon tetrafluoride gas and the complexing agent undergo a complexing reaction to generate a complexing liquid, and impurity gases that do not participate in the complexing reaction are discharged. The complexing reaction between the complexing agent and the silicon tetrafluoride gas is reversible.

[0011] S2. The complex solution generated in step S1 is subjected to low-temperature heating decomposition to remove HF impurities from the reactants;

[0012] S3. The complex solution generated in step S2 is decomposed by medium-temperature heating to remove BF3 impurities from the reactants.

[0013] S4. The complex liquid generated in step S3 is decomposed by high-temperature heating to obtain purified silicon tetrafluoride gas.

[0014] In step S2, the temperature of low-temperature heating decomposition is lower than the temperature of medium-temperature heating decomposition in step S3, which is lower than the temperature of high-temperature heating decomposition in step S4.

[0015] Preferably, step S4 further includes feeding the complexing liquid after high-temperature heating and decomposition into a purification tower for purification, obtaining a purified complexing agent, and then feeding it into the complexing reaction inlet for recycling.

[0016] Preferably, the decomposition temperature of the low-temperature heating decomposition is greater than or equal to 50°C and less than 80°C, the decomposition temperature of the medium-temperature heating decomposition is greater than or equal to 80°C and less than 120°C, and the decomposition temperature of the high-temperature heating decomposition is greater than or equal to 120°C and less than or equal to 300°C.

[0017] Preferably, the silicon tetrafluoride purified gas obtained after heating and decomposition in step S4 is further processed as follows: vapor impurities are removed, and then adsorption purification is performed to further remove impurities and water vapor, thereby obtaining silicon tetrafluoride electronic special gas.

[0018] Preferably, the complexing agent in step S1 is one or a mixture of several of C1-C7 alcohols, C1-C7 amines, or phosphate esters; when the complexing agent is an alcohol, the chemical reaction formula for the reversible complexation reaction is: .

[0019] Preferably, the complexing agent is pentanol.

[0020] Preferably, the molar flow rate of the complexing agent in step S1 is more than 2.5 times the molar flow rate of silicon tetrafluoride gas.

[0021] Preferably, the reaction conditions in step S1 include: a reaction pressure of 0.1 MPa to 1 MPa and a reaction temperature of 5°C to 50°C.

[0022] Preferably, the impurity gases that do not participate in the complexation reaction in step S1 include carbon dioxide, sulfur dioxide, carbon tetrafluoride, carbon tetrachloride, nitrogen, oxygen, hydrogen, methane, helium, and argon.

[0023] Preferably, the method employs multiple independent purification units in series for continuous purification, with the silicon tetrafluoride purified gas produced by the previous purification unit serving as the feed crude silicon tetrafluoride gas for the next purification unit.

[0024] Secondly, this application provides a method for pretreatment of raw materials in a silicon isotope chemical exchange distillation system, wherein the crude silicon tetrafluoride raw material gas is purified by any of the methods described above for improving the purity of silicon tetrafluoride, and the purified high-purity silicon tetrafluoride is used as the feed material for the silicon isotope chemical exchange distillation device.

[0025] Thirdly, this application provides a method for purifying and producing naturally abundant silicon tetrafluoride electronic specialty gas, wherein the method described in any one of the above-mentioned methods for improving the purity of silicon tetrafluoride is used to continuously purify the industrial crude silicon tetrafluoride raw material gas to prepare naturally abundant silicon tetrafluoride electronic specialty gas products.

[0026] Fourthly, this application provides a method for purifying high-abundance silicon isotope silicon tetrafluoride electronic specialty gas. The method described above for improving the purity of silicon tetrafluoride is used to deeply purify the enriched high-abundance silicon isotope crude silicon tetrafluoride gas. Multiple purification units are connected in series to form a multi-stage relay purification system for preparing high-abundance silicon isotope silicon tetrafluoride electronic specialty gas with a purity of 3~6N.

[0027] Fifthly, this application provides a pretreatment method for a silicon isotope separation process, wherein, before silicon isotope separation, the crude silicon tetrafluoride gas is purified using any of the methods described above for improving the purity of silicon tetrafluoride.

[0028] Sixthly, this application provides a system for improving the purity of silicon tetrafluoride, used to achieve the method for improving the purity of silicon tetrafluoride as described in any of the above claims, comprising:

[0029] The complexing tower is introduced into the lower part with crude silicon tetrafluoride gas and into the top with a complexing agent. The silicon tetrafluoride gas can undergo a reversible complexing reaction with the complexing agent under preset reaction conditions to generate a complexing liquid, while impurity gases that do not participate in the complexing reaction are discharged.

[0030] A low-temperature pyrolysis tower, connected to the bottom of the complexing tower, is used to perform low-temperature heating and decomposition of the complexing liquid transported from the complexing tower to remove HF impurities from the reactants.

[0031] A medium-temperature pyrolysis tower, connected to the bottom of the low-temperature pyrolysis tower, is used to heat and decompose the complex liquid transported from the low-temperature pyrolysis tower at medium temperature to remove BF3 impurities from the reactants.

[0032] A high-temperature pyrolysis tower, connected to the bottom of the medium-temperature pyrolysis tower, is used to heat and decompose the complexed liquid transported from the medium-temperature pyrolysis tower at high temperature to obtain purified silicon tetrafluoride gas.

[0033] Based on the above technical solution, it can be seen that the electronic specialty gas preparation method of this application has at least one of the following beneficial effects compared with the prior art:

[0034] 1. The method for improving the purity of silicon tetrafluoride in this application involves a reversible complexation reaction between crude silicon tetrafluoride gas and a complexing agent, while simultaneously discharging inert impurity gases that do not participate in the complexation reaction, thereby reducing the processing load and energy consumption of subsequent pyrolysis and purification units. Furthermore, by utilizing the principle that adducts formed by different impurities and complexing agents have different thermal stability, a step-by-step pyrolysis with precise temperature control is used to achieve sequential and selective removal of key impurities such as HF and BF3, resulting in high purification efficiency and strong targeting.

[0035] 2. The method for improving the purity of silicon tetrafluoride in this application purifies and regenerates the complex liquid after high-temperature pyrolysis through a subsequent impurity removal tower, which greatly reduces raw material consumption and waste generation, conforms to the principles of green chemical industry, and is economically significant.

[0036] 3. The method for improving the purity of silicon tetrafluoride in this application has a reasonable reaction and pyrolysis temperature range and moderate pressure, which makes it easy to achieve stable control on an industrial scale. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 This is a connection diagram of a method system for improving the purity of silicon tetrafluoride provided in this application. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of this application clearer, the embodiments of this application will be described in further detail below with reference to the accompanying drawings.

[0040] The terms “first,” “second,” “third,” “fourth,” “fifth,” “sixth,” “seventh,” and “eighth,” etc. (if present), in the specification and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of this application described herein can be implemented in orders other than those illustrated or described herein.

[0041] Furthermore, the terms “comprising” and “having”, and any variations thereof, are intended to cover non-exclusive inclusion, such that a process, method, system, product, or apparatus that includes a series of steps or units is not necessarily limited to those steps or units that are explicitly listed, but may include other steps or units that are not explicitly listed or that are inherent to such process, method, product, or apparatus.

[0042] Example 1

[0043] This application provides a method for improving the purity of silicon tetrafluoride, including the following steps:

[0044] S1. A complexing agent and crude silicon tetrafluoride gas are continuously introduced into the complexing tower, so that the crude silicon tetrafluoride gas and the complexing agent undergo a complexing reaction to generate a complexing liquid, and impurity gases that do not participate in the complexing reaction are discharged. The complexing reaction between the complexing agent and the silicon tetrafluoride gas is reversible.

[0045] Among them, the impurity gases that do not participate in the complexation reaction in step S1 include carbon dioxide, sulfur dioxide, carbon tetrafluoride, carbon tetrachloride, nitrogen, oxygen, hydrogen, methane, helium, and argon.

[0046] Complexation reaction control conditions: reaction pressure 0.1MPa~1MPa (inclusive of endpoint value), reaction temperature 5℃~50℃ (inclusive of endpoint value); preferred operating range 10℃~30℃, 0.15MPa~0.5MPa, balancing the efficiency of the complexation forward reaction with system viscosity, equipment pressure resistance, safety, and cost.

[0047] S2. The complex solution generated in step S1 is subjected to low-temperature heating decomposition to remove HF impurities from the reactants;

[0048] S3. The complex solution generated in step S2 is decomposed by medium-temperature heating to remove BF3 impurities from the reactants.

[0049] S4. The complex liquid generated in step S3 is decomposed by high-temperature heating to obtain purified silicon tetrafluoride gas.

[0050] In step S2, the temperature of low-temperature heating decomposition is lower than the temperature of medium-temperature heating decomposition in step S3, which is lower than the temperature of high-temperature heating decomposition in step S4.

[0051] The method for improving the purity of silicon tetrafluoride provided in this embodiment involves a reversible complexation reaction between crude silicon tetrafluoride gas and a complexing agent, while simultaneously discharging inert impurity gases that do not participate in the complexation reaction, thereby reducing the processing load and energy consumption of subsequent pyrolysis and purification units. Furthermore, by utilizing the principle that adducts formed by different impurities and complexing agents have different thermal stability, a step-by-step pyrolysis with precise temperature control is used to achieve sequential and selective removal of key impurities such as HF and BF3, resulting in high purification efficiency and strong targeting.

[0052] Preferably, the decomposition temperature for low-temperature heating decomposition is greater than or equal to 50°C and less than 80°C, the decomposition temperature for medium-temperature heating decomposition is greater than or equal to 80°C and less than 120°C, and the decomposition temperature for high-temperature heating decomposition is greater than or equal to 120°C and less than or equal to 300°C. It should be noted that this range is an effective core interval verified by experiments. In actual operation, it can be fine-tuned according to the impurity content, the type of complexing agent, and the on-site environment, all of which can achieve the technical effect of stepwise impurity removal.

[0053] Non-condensable gaseous impurities (such as N2, O2, Ar, CO2, etc.) in the crude silicon tetrafluoride gas feedstock input to the complexation tower can be directly removed from the top of the complexation tower. However, some impurities, such as HF and BF3, will participate in the complexation reaction. The thermal stability of HF complexes, BF3 complexes, and SiF4 complexes differs. Therefore, this embodiment sets different pyrolysis environments at different temperatures to accommodate the decomposition temperatures of different impurity complexes. The temperatures are set from low to high as low-temperature, medium-temperature, and high-temperature zones. Since the HF complex has the weakest bond energy, it preferentially decomposes into gaseous HF at low temperatures, with a decomposition temperature greater than or equal to 50°C and less than 80°C. Next, the BF3 complex exhibits moderate stability, decomposing into gaseous BF3 in the medium-temperature zone, with a decomposition temperature greater than or equal to 80°C and less than 120°C. The SiF4 alcohol complex is the most stable, decomposing only in the high-temperature zone, with a decomposition temperature greater than or equal to 120°C and less than or equal to 300°C.

[0054] The method for improving the purity of silicon tetrafluoride in this embodiment has a reasonable reaction and pyrolysis temperature range and moderate pressure, making it easy to achieve stable control on an industrial scale.

[0055] One embodiment provided in this application sets the S2 low-temperature pyrolysis temperature to be greater than or equal to 55°C and less than or equal to 65°C, the S3 medium-temperature pyrolysis temperature to be greater than or equal to 85°C and less than or equal to 95°C, and the S4 high-temperature pyrolysis temperature to be greater than or equal to 130°C and less than or equal to 180°C. This combination is suitable for crude silicon tetrafluoride gas with relatively low initial contents of impurities HF and BF3 (e.g., both <200ppm). Impurities can be efficiently removed at relatively mild temperatures, significantly reducing overall system energy consumption and extending the equipment's operating life under lower thermal stress. It is particularly suitable for production scenarios that are energy-sensitive and require high-quality feed gas.

[0056] One embodiment provided in this application sets the S2 low-temperature pyrolysis temperature to be greater than or equal to 70°C and less than 80°C, the S3 medium-temperature pyrolysis temperature to be greater than or equal to 105°C and less than 120°C, and the S4 high-temperature pyrolysis temperature to be greater than or equal to 220°C and less than or equal to 280°C. This combination is suitable for crude silicon tetrafluoride gas with high impurity content (e.g., HF > 1000 ppm, BF3 > 500 ppm) or containing small amounts of more difficult-to-decompose fluoroborate ester impurities. The higher temperatures ensure sufficient decomposition and removal of impurity complexes, enhance the purification depth, can handle lower-quality feed gases, and improve process robustness and feedstock adaptability.

[0057] Preferably, the purified silicon tetrafluoride gas obtained after heating and decomposition in step S4 is further treated as follows: vapor impurities are removed, followed by adsorption purification to further remove impurities and water vapor, thereby obtaining silicon tetrafluoride electronic specialty gas. This further removes any trace amounts of complexing agent vapor and water vapor that may be entrained, ensuring that the product meets the extremely low dew point and impurity content standards required for electronic specialty gases or isotope separation raw materials. Specifically, the process of removing vapor impurities can involve further condensing the gas at -40°C to remove any possible trace amounts of pentanol vapor, followed by passing it through a molecular sieve adsorption tower, activated alumina, or other specialized adsorbents for deep removal of trace amounts of moisture and residual alcohol molecules.

[0058] Preferably, the complexing agent in step S1 is one or a mixture of several of C1-C7 alcohols, C1-C7 amines, or phosphate esters; when the complexing agent is an alcohol, the chemical reaction formula for the reversible complexation reaction is: The complexation reaction between the complexing agent and SiF4 is reversible, which facilitates the release of SiF4 and the regeneration of the complexing agent through heating.

[0059] Alcohols have a strong complexing ability for SiF4, while having virtually no effect on impurity gases such as carbon dioxide, sulfur dioxide, carbon tetrafluoride, carbon tetrachloride, nitrogen, oxygen, hydrogen, methane, helium, and argon, thus achieving the preliminary separation of inert impurity gases. Specifically, pentanol and methanol can be used as complexing agents.

[0060] Alcohols that can undergo reversible chemical reactions with silicon tetrafluoride gas include methanol (CH3OH) and ethanol (C2H5OH). The advantages of these two types are their relatively low price and small molecular weight, allowing for the formation of more SiF4 complexes per unit mass of solvent. The disadvantages are their low boiling points and high volatility, leading to significant losses during the complexation process, increased recovery costs and safety hazards, and relatively poor stability of the resulting complexes.

[0061] Alcohol complexing agents can also include hexanol (C6H4O3). 13 OH), heptanol (C7H) 15The advantages of these two types (OH) are higher boiling points, lower volatility, and less operational loss; they are also more hydrophobic, making them less likely to trigger side reactions. The disadvantages are that viscosity increases with carbon chain length, which may lead to decreased mass transfer efficiency, requiring higher energy consumption for transport, and potentially affecting the complexation reaction rate.

[0062] Preferably, the complexing agent is pentanol. Considering factors such as volatility, viscosity, cost, and complex stability, pentanol is the most preferred option. Pentanol has suitable boiling point and viscosity, which facilitates liquid-phase mass transfer, transport, and subsequent distillation separation within the corresponding temperature and pressure range.

[0063] Preferably, the molar flow rate of the complexing agent in step S1 is more than 2.5 times the molar flow rate of silicon tetrafluoride gas. Since the complexation reaction of SiF4 and ROH is in a 1:2 ratio, in order to ensure the complete reaction and provide sufficient solubility, this scheme uses excess pentanol to promote the reaction to proceed fully. The excess pentanol shifts the reaction equilibrium to the right, maximizing the SiF4 capture rate and reducing losses.

[0064] Preferably, the reaction conditions in step S1 include: a reaction pressure of 0.1 MPa to 1 MPa and a reaction temperature of 5°C to 50°C, which ensures the high efficiency and controllability of the complexation reaction.

[0065] Since the forward direction of the complexation reaction is exothermic, the reaction temperature should not be too high to ensure the forward reaction proceeds completely. Furthermore, to reduce side reactions and suppress HF formation, the reaction temperature also needs to be lowered. However, the reaction temperature cannot be lowered indefinitely, because if the reaction temperature is too low, the viscosity of the complex solution increases sharply, which is detrimental to the flow and mass transfer of the liquid within the column.

[0066] Furthermore, in order to balance reaction efficiency, suppress side reactions and energy consumption, the optimal reaction temperature is 10°C to 30°C.

[0067] Increasing the pressure of the complexation tower appropriately can improve the solubility of SiF4 in the liquid solvent, thereby driving the complexation reaction to the right and increasing the reaction rate. However, excessively high pressure places higher demands on the pressure resistance of equipment (tower body, pumps, valves), increasing costs and safety hazards. Since both SiF4 and pentanol are flammable or toxic substances, the pressure should not be too high for safety reasons.

[0068] Furthermore, to balance reaction rate and safety considerations, the optimal reaction pressure is 0.15 MPa to 0.5 MPa.

[0069] In one embodiment provided in this application, the reaction pressure in step S1 is controlled at 0.15 MPa to 0.3 MPa, the reaction temperature at 10°C to 20°C, and the molar flow rate of the complexing agent is 2.8 to 3.2 times that of SiF4. Therefore, the lower pressure and temperature facilitate a shift in the exothermic complexation reaction equilibrium towards the formation of a complex liquid, increasing the single-pass complexation yield of SiF4. The excess complexing agent ensures sufficient capture of SiF4 molecules in the gas, significantly reducing the loss of SiF4 with non-condensable gases (N2, O2, etc.), resulting in high raw material utilization. This method is particularly suitable for the pretreatment of high-value isotope separation raw materials containing SiF4.

[0070] In one embodiment provided in this application, the reaction pressure in step S1 is controlled at 0.6 MPa to 0.9 MPa, the reaction temperature at 35°C to 45°C, and the molar flow rate of the complexing agent is 2.5 to 2.8 times that of SiF4. Therefore, the higher pressure improves the gas-liquid mass transfer efficiency, and the higher temperature accelerates the reaction kinetics, making it suitable for handling large flow rates of coarse gas and achieving a large processing capacity per unit equipment volume. Although the reaction equilibrium constant may decrease slightly, this is compensated for by a moderate excess of complexing agent, achieving efficient complexation at high throughput, suitable for large-scale industrial production.

[0071] In one embodiment provided in this application, the reaction conditions and the optimal molar flow rate ratio of the complexing agent in step S1 vary slightly depending on the complexing agent (e.g., from methanol to hexanol). For example, for methanol with a smaller molecular weight and higher activity, its molar flow rate can be more than 3.5 times higher than that of SiF4, and the reaction temperature can be appropriately reduced to 5°C to 15°C to suppress its volatilization; while for pentanol or hexanol with a larger molecular weight, its molar flow rate is maintained at 2.5 to 3.0 times, and the reaction temperature can be maintained at 20°C to 40°C to maintain suitable viscosity and flowability. Thus, by setting matching reaction conditions according to the characteristics of the complexing agent, high complexing efficiency is ensured while optimizing system energy consumption, material flowability, and the ease of subsequent separation.

[0072] In summary, the range of process parameters disclosed in this invention is not fixed, but rather provides a verified and optimized operating window. Those skilled in the art can combine and fine-tune the aforementioned parameters such as temperature, pressure, and flow rate within the scope disclosed in this invention, based on specific raw material gas composition, product purity requirements, complexing agent type, and production scale. This allows for the achievement of the core technical effects of selective stepwise impurity removal, efficient complexation recovery, and complexing agent recycling, thereby economically and efficiently obtaining ultra-high purity silicon tetrafluoride gas.

[0073] This process can be operated independently as a single unit; alternatively, multiple independent purification units can be connected in series for continuous purification. The purified silicon tetrafluoride gas produced by the previous purification unit is used as the feed crude silicon tetrafluoride gas for the next purification unit, progressively reducing trace impurities and gradually increasing product purity. In this application, 3N to 6N are common purity designations in the electronic specialty gases industry, where N represents the number of 9s in the purity value. 3N corresponds to a purity of 99.9%, 4N to 99.99%, 5N to 99.999%, and 6N to 99.9999%. A single purification unit can stably prepare 3N to 5N silicon tetrafluoride, while multi-unit series purification can produce 5N to 6N ultra-high purity silicon tetrafluoride.

[0074] Example 2

[0075] This application provides a method for pre-treatment of raw materials in a silicon isotope chemical exchange distillation system. The method described in Example 1 for improving the purity of silicon tetrafluoride is used to pre-purify the industrial crude silicon tetrafluoride raw material gas. The purified high-purity silicon tetrafluoride is then directly fed into the silicon isotope chemical exchange distillation unit as feed material.

[0076] Typical components of crude silicon tetrafluoride raw material: main component SiF4 (a mixture of natural abundance silicon isotopes), containing approximately 800 ppm HF, approximately 500 ppm BF3, approximately 0.5% N2, approximately 0.3% O2, trace amounts of CO2 and moisture, designed to process gas with a capacity of 100 standard cubic meters per hour.

[0077] n-Pentanol was selected as the complexing agent, and the molar flow rate of pentanol was 3.0 times that of SiF4. The operating pressure of the complexing tower was 0.4 MPa, and the reaction temperature was 30±2℃. Low-temperature pyrolysis was performed at 68±2℃ to remove HF, medium-temperature pyrolysis at 105±2℃ to remove BF3, and high-temperature pyrolysis at 185℃ to release and purify SiF4. The pentanol after pyrolysis was sent to the impurity removal tower for regeneration and recycling.

[0078] After processing by a single purification unit, HF and BF3 impurities are reduced to below 3 ppm, producing silicon tetrafluoride with a purity of 3-5N, which is directly supplied to the downstream chemical exchange distillation column. This significantly reduces the impurity load in the distillation section and minimizes side reactions and packing contamination during isotope separation. If the downstream distillation section requires ultra-high purity feed, a second purification unit can be added in series for further purification to obtain 6N grade feed gas.

[0079] Example 3

[0080] This application provides a method for purifying and producing naturally abundant silicon tetrafluoride electronic specialty gas. The purification method of Example 1 is used to continuously purify industrial crude silicon tetrafluoride raw material gas, directly producing commercial naturally abundant silicon tetrafluoride electronic specialty gas products.

[0081] The raw material is crude silicon tetrafluoride, an industrial byproduct. Impurities are mainly inert gases, HF, and trace amounts of BF3, with no high-abundance silicon isotope components. Single or multiple sets in series can be selected based on product purity requirements.

[0082] 1) Conventional 4N-grade industrial electronic specialty gases: Only one purification unit is used, with a complexation temperature of 35-45℃ and an operating pressure of 0.6-0.9MPa, which increases the throughput of the unit equipment and is suitable for large-scale mass production;

[0083] 2) 6N grade high-end semiconductor electronic specialty gas: Two sets of purification units are connected in series for continuous purification, and a condensation + composite adsorption purification unit is added at the end. The total impurity content is less than 10ppm, which meets the standards for conventional chip etching and deposition processes.

[0084] The entire system is independently built as a continuous electronic specialty gas production line. The complexing agent is recycled throughout the process, and the single loss of pentanol is less than 0.5%. The production and operating costs are low, and it can continuously and stably produce naturally abundant silicon tetrafluoride electronic specialty gas products.

[0085] Example 4

[0086] This application provides a method for purifying high-abundance silicon tetrafluoride electronic specialty gas. For the deep purification of enriched high-abundance silicon tetrafluoride crude gas, the purification process described in Example 1 is adopted, and multiple purification units are connected in series to form a multi-stage relay purification system to prepare high-abundance silicon tetrafluoride electronic specialty gas with a 3N to 6N gradient purity.

[0087] The feed is high abundance produced by the isotope enrichment device. 28 SiF4 crude gas has a silicon-28 abundance of over 99.9%. The system contains trace amounts of HF, BF3, water vapor, and residual complexing agents, which imposes stringent requirements on product purity.

[0088] Two purification units are connected in series for continuous refining: the first-stage purification unit performs basic impurity removal on high-abundance crude SiF4, producing 5N intermediate; the gas produced in the first stage is directly fed into the complexation tower of the second-stage purification unit as feed gas, repeating the process of complete complexation-low-temperature HF removal-medium-temperature BF3 removal-high-temperature pyrolysis-adsorption refining; at the end of the second stage, pentanol vapor is removed by cold trap and water and impurities are removed by composite adsorption, finally producing 6N ultra-high purity high-abundance silicon isotope silicon tetrafluoride electronic special gas with a total impurity content of less than 0.1 ppm.

[0089] If only high-abundance intermediate raw materials with 3N to 4N purity are required, a single purification unit can be operated. For advanced processes and isotope target raw materials, two or more units are connected in series to flexibly match different purity production requirements. Each purification unit is equipped with an independent complexing agent regeneration and impurity removal tower, and the solvent circulation systems do not interfere with each other. They can be started and stopped independently, and process parameters can be adjusted independently.

[0090] Example 5

[0091] This application provides a pretreatment method for silicon isotope separation. Before silicon isotope separation, the crude silicon tetrafluoride gas is purified using the method for improving silicon tetrafluoride purity described in Example 1.

[0092] This embodiment clarifies the specific application of this purification method in the field of silicon isotope separation, emphasizes the core value of its produced gas as a high-quality feed, and improves the efficiency and economy of the entire separation process.

[0093] Specifically, this embodiment will elaborate on the method for improving the purity of silicon tetrafluoride in Example 1, and specifically apply it to the pretreatment method of raw gas in a silicon isotope (especially silicon-28) separation production line. The process includes:

[0094] 1. Pretreatment objectives and feed

[0095] The design objective of this pretreatment section is to provide a stable, ultra-high purity silicon tetrafluoride feed gas for the downstream silicon-28 isotope separation unit. The feed is industrially produced crude silicon tetrafluoride gas, typically composed of SiF4 (main component, containing naturally abundant silicon isotopes), and approximately 800 ppm HF, 500 ppm BF3, 0.5% N2, 0.3% O2, and trace amounts of CO2 and moisture. The designed processing capacity is 100 standard cubic meters per hour.

[0096] 2. Detailed description of the pretreatment process flow

[0097] Step S1: Complexation reaction and preliminary impurity removal

[0098] Crude silicon tetrafluoride gas, after being pressurized in a buffer tank, enters the complexation tower (a packed tower made of Hastelloy C-276) from the bottom. Simultaneously, a complexing agent (n-pentanol) preheated to 25°C is sprayed down from the top of the complexation tower at a molar flow rate 3.0 times that of SiF4. The operating pressure inside the tower is controlled at 0.4 MPa, and the temperature in the reaction zone in the middle of the tower is maintained at 30 ± 2°C. Under these conditions, a reversible complexation reaction occurs: The resulting complexed liquid flows downwards, while light impurity gases such as N2, O2, and CO2, which do not participate in the reaction, are discharged from the top of the column and sent to the tail gas treatment system. This step can remove most of the inert gas impurities in the feed, and the complexation collection rate of SiF4 is >99.5%.

[0099] Step S2: Low-temperature pyrolysis to remove HF

[0100] The SiF4-rich pentanol complex solution was discharged from the bottom of the complexation tower and pumped to a low-temperature pyrolysis tower (thin-film evaporator type). This tower operated at a temperature of 68±2℃ and a slight negative pressure (-5kPa). Under these mild thermal conditions, HF impurities, which are relatively weakly bound to pentanol, preferentially dissociated from their adducts and vaporized, then were drawn from the top of the tower, condensed, and collected. After this step, the HF content in the complex solution was reduced to below 5ppm.

[0101] Step S3: Removal of BF3 by intermediate-temperature pyrolysis

[0102] The complex solution after HF removal enters the medium-temperature pyrolysis tower (structure is the same as the low-temperature pyrolysis tower). The operating temperature is raised to 105±2℃. At this temperature, the BF3 impurities are fully pyrolyzed and released from their complexes and removed from the top of the tower. This step can reduce the BF3 concentration to below 3ppm.

[0103] Step S4: High-temperature pyrolysis and gas purification

[0104] The purified complexed liquid finally enters a high-temperature pyrolysis tower (reboiler-type desorption tower), where it undergoes final pyrolysis at 185°C and atmospheric pressure, completely releasing the silicon tetrafluoride gas, which escapes from the top of the tower. This gas first passes through a cryogenic cold trap maintained at -35°C to condense and recover most of the entrained pentanol vapor. Subsequently, the gas passes sequentially through a composite adsorption tower containing 4A molecular sieves and a specialized metal oxide adsorbent to thoroughly remove any trace amounts of moisture and any remaining polar impurities. The resulting high-purity silicon tetrafluoride gas has a purity exceeding 99.9995%, with key impurities HF and BF3 both below 2 ppm and water content below 0.5 ppm, fully meeting the stringent requirements for feed gas in silicon isotope separation.

[0105] Step S5: Complexing agent regeneration and recycling

[0106] The pentanol-rich bottom liquid from the high-temperature cracking tower is sent to a complexing agent purification and impurity removal tower (precision distillation tower). Through distillation, high-purity n-pentanol is obtained at the top of the tower and, after cooling, is returned to the top storage tank of the complexing tower for recycling. A small amount of heavy component residue at the bottom of the tower is periodically discharged. The single-pass loss rate of pentanol is less than 0.5%, achieving efficient solvent recycling and economical operation.

[0107] The ultrapure silicon tetrafluoride raw material gas obtained after the above pretreatment is directly transported to the inlet buffer chamber of the silicon-28 (or silicon-29, silicon-30) isotope separation system through a high-cleanliness pipeline.

[0108] Example 6

[0109] like Figure 1 As shown, this application provides a system for improving the purity of silicon tetrafluoride, used to achieve the method for improving the purity of silicon tetrafluoride in Embodiment 1 above. The system includes:

[0110] The complexing tower is introduced into the lower part with crude silicon tetrafluoride gas and into the top with a complexing agent. The silicon tetrafluoride gas can undergo a reversible complexing reaction with the complexing agent under preset reaction conditions to generate a complexing liquid, while impurity gases that do not participate in the complexing reaction are discharged.

[0111] A low-temperature pyrolysis tower, connected to the bottom of the complexing tower, is used to perform low-temperature heating and decomposition of the complexing liquid transported from the complexing tower to remove HF impurities from the reactants.

[0112] A medium-temperature pyrolysis tower, connected to the bottom of the low-temperature pyrolysis tower, is used to heat and decompose the complex liquid transported from the low-temperature pyrolysis tower at medium temperature to remove BF3 impurities from the reactants.

[0113] A high-temperature pyrolysis tower, connected to the bottom of the medium-temperature pyrolysis tower, is used to heat and decompose the complexed liquid transported from the medium-temperature pyrolysis tower at high temperature to obtain purified silicon tetrafluoride gas.

[0114] This embodiment constructs a dedicated equipment system for improving the purity of silicon tetrafluoride. The towers are connected in functional order, realizing the device-based and continuous operation of the method.

[0115] Preferably, step S4 further includes feeding the complexed liquid after high-temperature heating and decomposition into a purification tower for purification, obtaining a purified complexing agent, which is then fed back to the complexation reaction inlet for recycling. Thus, by purifying and regenerating the complexed liquid after high-temperature pyrolysis using a subsequent purification tower, raw material consumption and waste generation are significantly reduced, conforming to green chemical principles and demonstrating significant economic benefits.

[0116] The complexing agent liquid (containing high-boiling-point impurities) is obtained by decomposition in the cracking tower and output from the bottom of the cracking tower to the impurity removal tower for purification (i.e., distillation). According to the actual processing requirements, multi-stage purification can be carried out. The complexing agent vapor is output from the top of the impurity removal tower and then condensed to obtain regenerated solvent, which is then transported to the complexing tower for recycling as a lean solvent. The high-boiling-point impurities and residual liquid at the bottom of the impurity removal tower are discharged as waste liquid.

[0117] The system for improving silicon tetrafluoride purity provided in this embodiment reduces the processing load and energy consumption of subsequent pyrolysis and purification units by reversibly complexing crude silicon tetrafluoride gas with a complexing agent and simultaneously discharging inert impurity gases that do not participate in the complexation reaction. Furthermore, by utilizing the principle that adducts formed by different impurities and complexing agents have different thermal stability, the system achieves sequential and selective removal of key impurities such as HF and BF3 through precisely temperature-controlled stepwise pyrolysis, resulting in high purification efficiency and strong targeting.

[0118] A specific implementation plan is provided as follows:

[0119] Complexation Tower: This tower employs a packed tower design, with a diameter of 600mm and a height of 8m. It is constructed of carbon steel lined with PTFE and filled with stainless steel wire mesh corrugated packing to provide a large gas-liquid contact area. A complexing agent distributor is located at the top, and a rich complexing liquid collection section is located at the bottom. Multiple temperature and pressure monitoring points are located in the middle of the tower. Its function is to efficiently complete the reversible complexation reaction between the gas and liquid phases and separate light impurities that do not participate in the reaction.

[0120] Low-temperature pyrolysis tower and medium-temperature pyrolysis tower: Both have similar structures, being vertical falling film evaporators with shells made of 316L stainless steel and heat exchange tubes made of Hastelloy. The low-temperature pyrolysis tower is designed to operate at 70℃ with a heat exchange area of ​​25㎡; the medium-temperature pyrolysis tower is designed to operate at 110℃ with a heat exchange area of ​​20㎡. They utilize built-in reboilers (heat transfer oil as the heat medium) to provide precisely controlled heat, promoting the selective cracking and vaporization of HF and BF3 complexes. The steam line at the top of the tower is connected to a condenser to recover the distillate.

[0121] High-temperature pyrolysis tower: Designed as a desorption tower with a reboiler, operating temperature 200℃, tower height 5m, diameter 500mm, internally packed with trays, its heat source is high-pressure steam, its function is to completely decompose the purified SiF4 complex and release product gas.

[0122] Gas purification unit: Located after the gas outlet at the top of the high-temperature cracking tower, it includes a shell-and-tube condenser (coolant is aqueous ethylene glycol solution) and two vertical adsorption towers connected in series (containing molecular sieves and special adsorbents) for deep removal of solvent vapors and trace moisture from the product gas.

[0123] Complexing agent purification and impurity removal tower: It is a complete precision distillation system, including a distillation tower (packed tower), reboiler, condenser, reflux tank, etc., used to regenerate pentanol from the bottom of the high-temperature cracking tower and restore its purity to the standard of recyclability.

[0124] Control system: A distributed control system (DCS) is adopted to automatically detect and regulate the temperature, pressure, and flow rate of key materials in all towers. In particular, the temperature of the three pyrolysis towers is precisely controlled in cascade to ensure the stable execution of the step-by-step pyrolysis process.

[0125] Specifically, the system operation involves the continuous input of crude SiF4 gas and fresh / recycled complexing agent. By adjusting the feed pumps, reboiler heat load, and system pressure of each column, the complexing liquid flows sequentially through each column. Product gas is continuously produced from the outlet of the gas refining unit, and the regenerated complexing agent is collected from the top of the impurity removal column and pumped back to the storage tank at the top of the complexing column. The entire system achieves continuous, closed-loop, and automated operation from crude gas inlet to pure gas outlet, and from waste liquid to regenerated solvent, demonstrating its high degree of engineering feasibility. The system is compact, consumes significantly less energy than cryogenic distillation units with the same processing capacity, and achieves green production through solvent recycling.

[0126] The system can operate independently; alternatively, multiple purification units can be connected in series, with the purified gas produced at the top of the high-temperature pyrolysis tower of the previous unit connected to the inlet of the complexation tower of the next unit, to construct a multi-stage relay purification system that progressively improves product purity and is suitable for the production needs of silicon tetrafluoride with different purity levels from 3N to 6N.

[0127] The system is equipped with a DCS distributed control system, which precisely controls the temperature, pressure and material flow of each tower in a cascade manner, enabling fully automated and continuous operation. The back-end is equipped with a gas purification unit (low-temperature cold trap, multi-stage adsorption tower) and a complexing agent distillation and regeneration tower to achieve deep gas purification and closed-loop recycling of complexing agents.

[0128] The sequence numbers of the embodiments in this application are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0129] Those skilled in the art will understand that all or part of the steps of the above embodiments can be implemented by hardware or by a program instructing related hardware. The program can be stored in a computer-readable storage medium, such as a read-only memory, a disk, or an optical disk.

[0130] The foregoing has described specific embodiments of the present invention. In some cases, the described actions or steps may be performed in a different order than those shown in the embodiments and the desired results may still be achieved. Furthermore, the processes depicted in the drawings do not necessarily require a specific or sequential order to achieve the desired results. In some embodiments, multitasking and parallel processing are also possible or may be advantageous.

[0131] In the description of the embodiments of the present invention, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In the embodiments of the present invention, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in a suitable manner in any one or more embodiments or examples. Furthermore, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in the embodiments of the present invention, as well as the features of the different embodiments or examples.

Claims

1. A method for improving the purity of silicon tetrafluoride, characterized in that, Includes the following steps: S1. A complexing agent and crude silicon tetrafluoride gas are continuously introduced into the complexing tower, so that the crude silicon tetrafluoride gas and the complexing agent undergo a complexing reaction to generate a complexing liquid, and impurity gases that do not participate in the complexing reaction are discharged. The complexing reaction between the complexing agent and the silicon tetrafluoride gas is reversible. S2. The complex solution generated in step S1 is subjected to low-temperature heating decomposition to remove HF impurities from the reactants; S3. The complex solution generated in step S2 is decomposed by medium-temperature heating to remove BF3 impurities from the reactants. S4. The complex liquid generated in step S3 is decomposed by high-temperature heating to obtain purified silicon tetrafluoride gas. In step S2, the temperature of low-temperature heating decomposition is lower than the temperature of medium-temperature heating decomposition in step S3, which is lower than the temperature of high-temperature heating decomposition in step S4.

2. The method for improving the purity of silicon tetrafluoride according to claim 1, characterized in that, Step S4 further includes feeding the complexing liquid after high-temperature heating and decomposition into a purification tower for purification, obtaining a purified complexing agent, and then feeding it into the complexing reaction inlet for recycling.

3. The method for improving the purity of silicon tetrafluoride according to claim 2, characterized in that, The decomposition temperature of the low-temperature heating decomposition is greater than or equal to 50℃ and less than 80℃, the decomposition temperature of the medium-temperature heating decomposition is greater than or equal to 80℃ and less than 120℃, and the decomposition temperature of the high-temperature heating decomposition is greater than or equal to 120℃ and less than or equal to 300℃.

4. The method for improving the purity of silicon tetrafluoride according to claim 1, characterized in that, The silicon tetrafluoride purified gas obtained after heating and decomposition in step S4 is further processed as follows: vapor impurities are removed, and then adsorption purification is performed to further remove impurities and water vapor, thereby obtaining silicon tetrafluoride electronic special gas.

5. The method for improving the purity of silicon tetrafluoride according to claim 1, characterized in that, The complexing agent mentioned in step S1 is one or a mixture of several of the following: C1-C7 alcohols, C1-C7 amines, or phosphate esters; when the complexing agent is an alcohol, the chemical reaction formula for the reversible complexation reaction is: .

6. The method for improving the purity of silicon tetrafluoride according to claim 5, characterized in that, The complexing agent is pentanol.

7. The method for improving the purity of silicon tetrafluoride according to claim 1, characterized in that, In step S1, the molar flow rate of the complexing agent is more than 2.5 times that of the molar flow rate of silicon tetrafluoride gas.

8. The method for improving the purity of silicon tetrafluoride according to claim 1, characterized in that, The reaction conditions in step S1 include: a reaction pressure of 0.1 MPa to 1 MPa and a reaction temperature of 5°C to 50°C.

9. The method for improving the purity of silicon tetrafluoride according to claim 1, characterized in that, Impurity gases that do not participate in the complexation reaction in step S1 include carbon dioxide, sulfur dioxide, carbon tetrafluoride, carbon tetrachloride, nitrogen, oxygen, hydrogen, methane, helium, and argon.

10. The method for improving the purity of silicon tetrafluoride according to claim 1, characterized in that, The method employs multiple independent purification units in series for continuous purification, with the silicon tetrafluoride purified gas produced by the previous purification unit serving as the feed crude silicon tetrafluoride gas for the next purification unit.

11. A method for pretreatment of raw materials in a silicon isotope chemical exchange distillation system, characterized in that, The crude silicon tetrafluoride feed gas is purified using the method for improving silicon tetrafluoride purity as described in any one of claims 1-10, and the purified high-purity silicon tetrafluoride is used as the feed material for a silicon isotope chemical exchange distillation unit.

12. A method for purifying and producing naturally abundant silicon tetrafluoride electronic specialty gases, characterized in that, The method for improving the purity of silicon tetrafluoride according to any one of claims 1-10 is used to continuously purify industrial crude silicon tetrafluoride raw material gas to prepare naturally abundant silicon tetrafluoride electronic specialty gas products.

13. A method for purifying high-abundance silicon isotope silicon tetrafluoride electronic specialty gas, characterized in that, The method for improving the purity of silicon tetrafluoride according to any one of claims 1-10 is used to deeply purify the enriched high-abundance silicon isotope crude silicon tetrafluoride gas. Multiple purification units are connected in series to form a multi-stage relay purification system, which is used to prepare high-abundance silicon isotope silicon tetrafluoride electronic specialty gas with a purity of 3~6N.

14. A pretreatment method for a silicon isotope separation process, characterized in that, Before performing silicon isotope separation, the crude silicon tetrafluoride gas is purified using the method for improving silicon tetrafluoride purity as described in any one of claims 1-10.

15. A system for improving the purity of silicon tetrafluoride, used to implement the method for improving the purity of silicon tetrafluoride according to any one of claims 1-10, characterized in that, include: The complexing tower is introduced into the lower part with crude silicon tetrafluoride gas and into the top with a complexing agent. The silicon tetrafluoride gas can undergo a reversible complexing reaction with the complexing agent under preset reaction conditions to generate a complexing liquid, while impurity gases that do not participate in the complexing reaction are discharged. A low-temperature pyrolysis tower, connected to the bottom of the complexing tower, is used to perform low-temperature heating and decomposition of the complexing liquid transported from the complexing tower to remove HF impurities from the reactants. A medium-temperature pyrolysis tower, connected to the bottom of the low-temperature pyrolysis tower, is used to heat and decompose the complex liquid transported from the low-temperature pyrolysis tower at medium temperature to remove BF3 impurities from the reactants. A high-temperature pyrolysis tower, connected to the bottom of the medium-temperature pyrolysis tower, is used to heat and decompose the complexed liquid transported from the medium-temperature pyrolysis tower at high temperature to obtain purified silicon tetrafluoride gas.

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