Thermal cracking device and safe preparation system for nitrogen trifluoride preparation

Abstract

The present invention provides a thermal cracking device and a safe preparation system for preparing nitrogen trifluoride. The thermal cracking device for preparing nitrogen trifluoride includes a thermal cracking tower, an air inlet arranged at the bottom of the thermal cracking tower, a filter arranged between the air inlet and the thermal cracking tower, a heating coil wound around the outside of the thermal cracking tower, an air outlet arranged at the top of the thermal cracking tower, and heating steel balls filled in the thermal cracking tower. The thermal cracking device is used to thermally crack NF3 gas to remove fluorine-nitrogen impurity gas impurities. The present invention adds a thermal cracking device during the process of preparing nitrogen trifluoride by electrolysis, which can effectively remove fluorine-nitrogen impurity gas impurities, thereby reducing the risk of explosion caused by the accumulation of fluorine-nitrogen impurity gas impurities and improving the safety performance of production.

Description

[0001] The present invention is a divisional application with application number 202310690484.6, application date 2023-06-12, and name “Steam-water separation device and preparation system for preparation of nitrogen trifluoride”. Technical Field

[0002] The invention relates to a thermal cracking device and a safe preparation system for preparing nitrogen trifluoride. Background Art

[0003] Nitrogen trifluoride is an excellent plasma etching gas in the microelectronics industry. When etching semiconductor materials such as silicon and silicon nitride, especially for semiconductor materials with a thickness of less than 1.5μm, it will not pollute the surface. With the development of nanotechnology and the large-scale development of electronic industry technology, the demand for nitrogen trifluoride will increase day by day. With the improvement of the performance of electronic products, the international semiconductor industry has higher and higher requirements for the preparation process of nitrogen trifluoride. There are generally two methods for preparing nitrogen trifluoride gas: direct fluorine gas method and electrolysis method. The direct fluorine gas method is mainly a direct chemical reaction between fluorine gas and NH3. However, fluorine gas is particularly active, the reaction is highly exothermic and difficult to control, the content of by-products is relatively high, and the yield of nitrogen trifluoride is very low, which is not suitable for industrial production. The electrolysis method directly uses ammonium bifluoride as a raw material, and performs electrolytic fluorination under relatively mild conditions to prepare FH3. However, the existing electrolysis method is prone to produce fluorine-nitrogen impurity products such as N2F2 and N2F4. The fluorine-nitrogen impurities are unstable in nature and are prone to explosion and other dangers when accumulated to a certain extent. Summary of the invention

[0004] The present invention provides a thermal cracking device and a safe preparation system for preparing nitrogen trifluoride, which can effectively solve the above problems.

[0005] The present invention is achieved in that:

[0006] The present invention also provides a thermal cracking device for preparing nitrogen trifluoride, comprising a thermal cracking tower, an air inlet arranged at the bottom of the thermal cracking tower, a filter arranged between the air inlet and the thermal cracking tower, a heating coil wound around the outside of the thermal cracking tower, an air outlet arranged at the top of the thermal cracking tower, and heating steel balls filled in the thermal cracking tower. The thermal cracking device is used to thermally crack NF3 gas to remove fluorine and nitrogen impurities.

[0007] The present invention further provides a safe preparation system for high-purity nitrogen trifluoride, comprising the above-mentioned thermal cracking device.

[0008] The beneficial effect of the present invention is that the present invention adds a thermal cracking device during the process of preparing nitrogen trifluoride by electrolysis, which can effectively remove fluorine-nitrogen impurity gas impurities, thereby reducing the explosion risk caused by the accumulation of fluorine-nitrogen impurity gas impurities and improving the safety performance of production. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without paying creative work.

[0010] Figure 1 It is a system architecture diagram of a high-purity NF3 preparation system provided in an embodiment of the present invention.

[0011] Figure 2 It is a schematic diagram of the structure of the electrolytic cell device in the preparation system of high-purity NF3 provided in an embodiment of the present invention.

[0012] Figure 3 It is a schematic diagram of the structure of a thermal cracking device in a preparation system of high-purity NF3 provided in an embodiment of the present invention.

[0013] Figure 4 It is a structural schematic diagram of a part of the structure of a steam-water separation device in a preparation system of high-purity NF3 provided in an embodiment of the present invention.

[0014] Figure 5 It is a schematic diagram of the structure of a steam-water separation device in a preparation system of high-purity NF3 provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0015] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the invention claimed for protection, but merely represents the selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0016] In the description of the present invention, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0017] Reference Figure 1-2 As shown, an embodiment of the present invention provides a system for preparing high-purity NF3, including an electrolytic cell device 10, a thermal cracking device 20, a redox tower 30, an alkali washing device 40, a steam-water separation device 50, an adsorption device 60 and a distillation device 70 connected in sequence.

[0018] Reference Figure 2 As shown, the electrolytic cell device 10 for NF3 preparation comprises:

[0019] A sealed electrolytic cell 11, wherein the top of the electrolytic cell 11 is provided with a first gas outlet 111, a second gas outlet 112, a feed inlet 113, and a heating element 110 arranged around the electrolytic cell 11;

[0020] A cathode 12, a diaphragm 13, and an anode 14 are arranged in the electrolytic cell 11, wherein the diaphragm 13 is used to separate the cathode 12 and the anode 14;

[0021] A negative pressure storage device 17 is provided at the second gas outlet 112 .

[0022] The structure and shape of the sealed electrolytic cell 11 are not limited, as long as they have anti-corrosion and sealing functions. In one embodiment, the electrolytic cell 11 includes a cell body and a cover body matched with the cell body, and the cell body and the cover body are made of corrosion-resistant alloys, such as monel alloy or carbon steel alloy after passivation treatment, and are not limited here. The feed port 113 is used to feed NH3 / HF raw materials in proportion. In one embodiment, the feed port 113 includes an NH3 feed pipe 1130, an HF feed pipe 1132, and an NH4HF2 feed pipe 1134. The advantage of setting the feed port 113 into three channels is that the NH4HF2 feed pipe 1134 can be used as a feed pipe for the initial raw material, that is, after the NH4HF2 is fully mixed, it is fed from the NH4HF2 feed pipe 1134; and in the process of electrolysis, due to the rapid consumption of HF and NH3, it is necessary to regularly replenish HF and NH3 raw materials to maintain the ratio of NH3 / HF in the system within a predetermined range. The heating element 110 can be a resistance wire or a hydrothermal heater, etc., which is not limited here. The heating element 110 is used to control the temperature of the electrolytic cell 11 at 60-110° C. to melt the NH4HF2. The first gas outlet 111 and the second gas outlet 112 are isolated from each other by the diaphragm 13.

[0023] The number of the cathode 12 and the anode 14 is not limited and can be arranged according to actual needs. The material of the cathode 12 can be carbon steel or monel; the material of the anode 14 can be carbon rods and other materials. The diaphragm is used to isolate the hydrogen and fluorine gases to prevent hydrogen and fluorine gases from interfering with each other and causing explosion. Specifically, the cathode 12 is arranged at the bottom of the electrolytic cell 11 and extends from the bottom of the electrolytic cell 11 into the electrolytic cell 11. The anode 14 is arranged at the top of the electrolytic cell 11 and extends from the top of the electrolytic cell 11 into the electrolytic cell 11.

[0024] In other embodiments, the electrolytic cell device 10 for NF3 preparation further includes: a liquid level sensor 15, which can be arranged on the inner side or the outer side of the diaphragm 13, and is not limited here. In one embodiment, the liquid level sensor 15 is arranged on the outer side of the diaphragm 13, so that the liquid level L2 on the outer side of the diaphragm 13 can be obtained.

[0025] In other embodiments, as a further improvement, the anode 14 is a composite electrode formed by carbon nanotubes and carbon electrodes, and its specific preparation process is as follows:

[0026] S10, dispersing pure carbon nanotube powder in an organic solvent;

[0027] S11, then adding a binder and continuing to stir and disperse to form a mixed solution;

[0028] S12, finally coating the mixed solution on the surface of the carbon electrode and drying it to remove excess solvent and finally form a composite electrode.

[0029] In step S10, the carbon nanotube powder is a carbon nanotube with a length of 100 microns to 1000 microns. The organic solvent can be selected from NMP and its analogs. The concentration of the carbon nanotube powder can be 0.05kg to 0.15kg / L. Too high a concentration is not conducive to the dispersion of the carbon nanotubes.

[0030] In step S11, the binder is selected from an inert binder, such as PVDF. The weight of the binder is about 10% to 50% of the weight of the carbon nanotubes. Preferably, the weight of the binder is about 15% to 25% of the weight of the carbon nanotubes. Too low a binder content will result in reduced adhesion, and too high a content will reduce the conductivity of the carbon nanotube layer.

[0031] In one embodiment, 0.1 kg of pure carbon nanotube powder is dispersed in 1 L of NMP (N-methylpyrrolidone) by ultrasonic dispersion; then 0.02 kg of polyvinylidene fluoride (PVDF, or polyvinylidene fluoride) is added, and stirring and dissolving are continued to form a mixed solution; the mixed solution is applied to the surface of the carbon electrode to form a coating with a thickness of 0.1 mm to 1 mm, and finally dried at a temperature of 150 to 200 ° C to remove excess NMP. Preferably, the mixed solution is applied to the surface of the carbon electrode to form a coating with a thickness of 0.4 to 0.6 mm, because as the thickness of the coating increases, the electrolysis efficiency decreases and the cost increases significantly. It can be understood that a protective layer is formed by bonding carbon nanotubes to the surface of the carbon electrode. Due to the protective effect of the carbon nanotube layer, the carbon electrode will not be easily oxidized to generate carbon tetrafluoride gas during the electrolysis process, thereby greatly reducing the content of carbon tetrafluoride in the product; and the carbon nanotubes in the carbon nanotube layer are not easily oxidized to generate carbon tetrafluoride due to their relatively stable structure. Further, the efficiency of electrolysis can also be improved.

[0032] The present invention can greatly reduce the content of carbon tetrafluoride in the product by simply modifying the existing carbon electrode, and can also increase the service life of the carbon electrode.

[0033] The embodiment of the present invention further provides a control method of the electrolytic cell device 10 for NF3 preparation, comprising the following steps:

[0034] S1, introducing molten NH4HF2 through the feed port 113;

[0035] S2, control the pressure of the electrolytic cell device 10 to ±1kPa, the electrolysis temperature to 100~130℃, the electrolysis voltage to 5~7V, and during the reaction process, introduce NH3 / HF into the electrolytic cell device 10 through the feed port 113 at a molar ratio of 1:3 to maintain the molten salt ratio in the electrolyte.

[0036] In step S1, molten NH4HF2 is added to provide an initiator for the initial reaction. If ammonia and hydrogen fluoride are directly introduced, side reactions will increase.

[0037] In step S2, the reaction chemical equation is as follows:

[0038] NH4HF2+HF-->NF3+3H2;

[0039] NH3+2HF-->NH4HF2.

[0040] It can be seen from the above reaction equation that during the electrolysis process, the HF introduced reacts with NH4HF2 to generate NF3; on the other hand, the HF introduced continuously reacts with NH3 in the electrolytic cell device 10 to generate NH4HF2 molten salt. Therefore, it is necessary to introduce NH3 / HF into the electrolytic cell device 10 through the feed port 113 at a molar ratio of 1:3 to maintain the molten salt ratio in the electrolyte. During the reaction, side reactions will occur to generate fluorine and nitrogen impurity gases, such as N2F2, N2F4 and other impurity products; carbon tetrafluoride gas, and nitrogen oxides, such as N2O.

[0041] Since fluorine and nitrogen impurity gases are unstable and prone to explosion when accumulated to a certain level, they need to be eliminated through subsequent thermal cracking processes.

[0042] In Example A-1, the pressure of the electrolytic cell device 10 is controlled to be ±1 kPa, the electrolysis temperature is 100±2°C, and the electrolysis voltage is 7V. During the reaction, NH3 / HF is introduced into the electrolytic cell device 10 through the feed port 113 at a molar ratio of 1:3 to maintain the molten salt ratio in the electrolyte (anode surface, 0.1 kg pure carbon nanotube powder / 0.02 kg polyvinylidene fluoride, film thickness is 0.5 mm). After testing, the volume content of each gas is: NF3: about 75.79%; NH3 / HF: about 1.6%; N2+O2: about 21.5%; CF4: 0.01%; fluorine and nitrogen impurity gas: about 0.2%; CO2: 0.1%; N2O: 0.6%; others: about 0.2%. It can be seen from the above volume content that NH3 / HF reacts basically according to the theoretical molar ratio. Furthermore, the anode can operate stably for 60 days at a current of 1500A in the electrolytic cell.

[0043] In Example A-2, the pressure of the electrolytic cell device 10 is controlled to be ±1 kPa, the electrolysis temperature is 100±2°C, and the electrolysis voltage is 7V. During the reaction, NH3 / HF is introduced into the electrolytic cell device 10 through the feed port 113 at a molar ratio of 1:3 to maintain the molten salt ratio in the electrolyte (anode surface, 0.1 kg pure carbon nanotube powder / 0.02 kg polyvinylidene fluoride, film thickness is 0.1 mm). After testing, the volume content of each gas is: NF3: about 75.95%; NH3 / HF: about 1.2%; N2+O2: about 21.48%; CF4: 0.03%; fluorine and nitrogen impurity gases: about 0.4%; CO2: 0.1%; N2O: 0.62%; others: about 0.22%. The anode can operate stably for about 30 days at a current of 1500A in the electrolytic cell. After about 30 days, the carbon tetrafluoride content increases significantly, indicating that the carbon nanotube composite layer may be consumed as the electrolysis time increases.

[0044] In Example A-3, the pressure of the electrolytic cell device 10 is controlled to be ±1 kPa, the electrolysis temperature is 100±2°C, and the electrolysis voltage is 7V. During the reaction, NH3 / HF is introduced into the electrolytic cell device 10 through the feed port 113 at a molar ratio of 1:3 to maintain the molten salt ratio in the electrolyte (anode surface, 0.1 kg pure carbon nanotube powder / 0.02 kg polyvinylidene fluoride, film thickness is 0.3 mm). After testing, the volume content of each gas is: NF3: about 25.86%; NH3 / HF: about 1.4%; N2+O2: about 21.49%; CF4: 0.02%; fluorine and nitrogen impurity gases: about 0.3%; CO2: 0.1%; N2O: 0.62%; others: about 0.21%. The anode can operate stably in the electrolytic cell at a current of 1500A for about 45 days. After about 45 days, the carbon tetrafluoride content increases significantly, which indicates that the carbon nanotube composite layer may be consumed as the electrolysis time increases.

[0045] In Example A-4, the pressure of the electrolytic cell device 10 is controlled to be ±1 kPa, the electrolysis temperature is 100±2°C, and the electrolysis voltage is 7V. During the reaction, NH3 / HF is introduced into the electrolytic cell device 10 through the feed port 113 at a molar ratio of 1:3 to maintain the molten salt ratio in the electrolyte (anode surface, 0.1 kg pure carbon nanotube powder / 0.02 kg polyvinylidene fluoride, film thickness is 1 mm). After testing, the volume content of each gas is: NF3: about 74.65%; NH3 / HF: about 2.1%; N2+O2: about 21.65%; CF4: 0.01%; fluorine and nitrogen impurity gases: about 0.2%; CO2: 0.3%; N2O: 0.91%; others: about 0.18%. The anode can operate stably in the electrolytic cell at a current of 1500A for about 70 days. After about 70 days, the carbon tetrafluoride content increases significantly, which indicates that the carbon nanotube composite layer may be consumed as the electrolysis time increases.

[0046] Furthermore, it can be seen from Examples A-1 to A-4 that as the thickness of the carbon nanotube film increases, the content of NF3 decreases (the electrolysis efficiency decreases significantly), but the content of CF4 also decreases (a certain inhibitory effect on CF4), but when the thickness reaches 0.5 mm, the content of CF4 does not change as the film thickness increases. In addition, as the thickness of the carbon nanotube film increases, the service life of the carbon nanotube composite layer also increases significantly.

[0047] Reference Figure 3 As shown, an embodiment of the present invention provides a thermal cracking device 20 including: a thermal cracking tower 21, an air inlet 23 arranged at the bottom of the thermal cracking tower 21, a filter 22 arranged between the air inlet 23 and the thermal cracking tower 21, a heating coil 24 wound around the outside of the thermal cracking tower 21, an air outlet 25 arranged at the top of the thermal cracking tower 21, and a heating steel ball 26 filled in the thermal cracking tower 21. The thermal cracking device 20 is used to thermally crack NF3 gas to remove fluorine-nitrogen impurity gas impurities. The air inlet 23 is detachably connected to the thermal cracking tower 21 through a flange, so as to facilitate the regular cleaning of the filter 22.

[0048] The heating coil 24 is a high-frequency heating coil, which uses electromagnetic induction to generate eddy current in the metal coil, causing Joule heating of the metal coil due to resistance. The metal coil is evenly wound around the periphery of the thermal cracking tower 21, so that the temperature of the thermal cracking tower 21 is uniform, and there will be no local overheating or local temperature failure to affect the thermal cracking effect. At the same time, due to the use of high-frequency heating, the heating steel ball 26 can also reach the set temperature value, and the heating steel ball 26 can achieve a better effect of thermal cracking oxidative impurities, and the heating steel ball 26 can also achieve a dust removal effect. The particle size of the heating steel ball 26 is 0.5cm~2cm. The heating coil 24 can be divided into multiple heating sections from top to bottom, and each heating section is provided with a temperature measuring instrument, so that the entire thermal cracking tower 21 can be more accurately temperature controlled and heated evenly.

[0049] In one embodiment, the height of the thermal cracking tower 21 is about 1.6 to 2.4 meters, and the inner diameter is about 0.3 meters. And the heating coil 24 is divided into a first heating zone, a second heating zone, and a third heating zone from top to bottom. Among them, the height of the first heating zone is about 0.8 to 1.2 meters, the height of the second heating zone is about 0.5 to 0.7 meters, and the height of the third heating zone is about 0.3 to 0.5 meters. Among them, the temperature of the first heating zone is about 160 to 180°C, the temperature of the second heating zone is about 140 to 160°C; the temperature of the third heating zone is about 120 to 140°C.

[0050] Embodiment B-1: The gas in Embodiment 1 is processed through the thermal cracking tower 21, wherein the parameters of the thermal cracking tower 21 are as follows: the height of the thermal cracking tower 21 is about 2 meters, and the inner diameter is about 0.3 meters. The heating coil 24 is divided into a first heating zone 260, a second heating zone 262, and a third heating zone 264 from top to bottom. The height of the first heating zone 260 is about 1 meter, the height of the second heating zone 262 is about 0.6 meters, and the height of the third heating zone 264 is about 0.4 meters. The temperature of the first heating zone 260 is about 170°C, the temperature of the second heating zone 262 is about 150°C; the temperature of the third heating zone 264 is about 130°C.

[0051] After testing, the gas content after the cracking reaction is: NF3: about 75.75%; NH3 / HF: about 1.4%; N2+O2: about 21.4%; CF4: 0.01%; fluorine and nitrogen impurity gases: 2 (ppmv); CO2: 0.1%; N2O: 0.6%; others: about 0.74%. Through the thermal cracking control of the above three heating areas, the content of fluorine and nitrogen impurity gases, such as N2F2, N2F4 and other impurity products, can be controlled below 2 (ppmv), thereby greatly reducing the occurrence of explosion and other dangers. Furthermore, by increasing the temperature from low to high layer by layer, the side reaction of NF3 gas caused by the rapid change of temperature can be avoided. From the above data, it can be seen that through the three-stage thermal cracking, its impact on NF3 is almost negligible, but the content of fluorine and nitrogen impurity gases can be significantly reduced.

[0052] Comparative Example C-1: The gas in Example 1 is processed through the thermal cracking tower 21, wherein the parameters of the thermal cracking tower 21 are as follows: the height of the thermal cracking tower 21 is about 2 meters, and the inner diameter is about 0.3 meters. And the heating coil 24 is divided into a first heating zone, a second heating zone, and a third heating zone from top to bottom. Among them, the height of the first heating zone is about 1 meter, the height of the second heating zone is about 0.6 meters, and the height of the third heating zone is about 0.4 meters. Among them, the temperature of the first heating zone is about 130°C, the temperature of the second heating zone is about 130°C; the temperature of the third heating zone is about 130°C.

[0053] After testing: the gas content after cracking reaction is: NF3: about 75.77%; NH3 / HF: about 1.5%; N2+O2: about 21.3%; CF4: 0.01%; fluorine and nitrogen impurity gases: about 0.15%; CO2: 0.1%; N2O: 0.6%; others: about 0.57%.

[0054] From Comparative Example C1, it can be seen that when the thermal cracking temperature is about 130° C., a good cracking reaction cannot be produced on the fluorine-nitrogen impurity gas.

[0055] Comparative Example C-2: The gas in Example 1 is processed through the thermal cracking tower 21, wherein the parameters of the thermal cracking tower 21 are as follows: the height of the thermal cracking tower 21 is about 2 meters, and the inner diameter is about 0.3 meters. And the heating coil 24 is divided into a first heating zone, a second heating zone, and a third heating zone from top to bottom. Among them, the height of the first heating zone is about 1 meter, the height of the second heating zone is about 0.6 meters, and the height of the third heating zone is about 0.4 meters. Among them, the temperature of the first heating zone is about 150°C, the temperature of the second heating zone is about 150°C; the temperature of the third heating zone is about 150°C.

[0056] After testing: the gas content after cracking reaction is: NF3: about 74.48%; NH3 / HF: about 1.4%; N2+O2: about 21.2%; CF4: 0.01%; fluorine and nitrogen impurity gases: about 15 (ppmv); CO2: 0.1%; N2O: 0.6%; others: about 2.21%.

[0057] From the comparative example C2, when the thermal cracking temperature is about 150°C, although it can produce a good thermal cracking effect on the fluorine and nitrogen impurity gases, the rapid temperature change also has a certain effect on NF3, causing the NF3 content to decrease.

[0058] Comparative Example C-3: The gas in Example 1 is processed through the thermal cracking tower 21, wherein the parameters of the thermal cracking tower 21 are as follows: the height of the thermal cracking tower 21 is about 2 meters, and the inner diameter is about 0.3 meters. And the heating coil 24 is divided into a first heating zone, a second heating zone, and a third heating zone from top to bottom. Among them, the height of the first heating zone is about 1 meter, the height of the second heating zone is about 0.6 meters, and the height of the third heating zone is about 0.4 meters. Among them, the temperature of the first heating zone is about 170°C, the temperature of the second heating zone is about 170°C; the temperature of the third heating zone is about 170°C.

[0059] After testing: the gas content after cracking reaction is: NF3: about 71.73%; NH3 / HF: about 1.2%; N2+O2: about 21.6%; CF4: 0.01%; fluorine and nitrogen impurity gases: about 3 (ppmv); CO2: 0.1%; N2O: 0.6%; others: about 4.76%.

[0060] From comparative example C3, when the thermal cracking temperature is about 170°C, although a good thermal cracking effect can be produced on the fluorine-nitrogen impurity gas, the rapid change in temperature also has a certain effect on NF3, causing the NF3 content to be significantly reduced. From the above experiments, it can be seen that the temperature control of thermal cracking will have a great impact on both the fluorine-nitrogen impurity gas and NF3.

[0061] The redox tower 30 is used to remove N2O. Specifically, the redox tower 30 contains a sodium sulfite solution, and the sodium sulfite solution reacts with N2O to remove N2O gas. The specific chemical equation is as follows:

[0062] Na2SO3+2NO2=Na2SO4+N2.

[0063] The sodium sulfite solution in the redox tower 30 may be a dilute solution with a concentration lower than 5 wt %. This has the advantage that it can react with N 2 O to remove N 2 O gas and also play a role in water washing to remove HF gas.

[0064] The alkali cleaning device 40 is used to remove trace acidic gases in the gas, such as HF, CO2, etc. The alkali solution in the alkali cleaning device 40 can be a KOH alkali solution with a concentration of 20% to 40%.

[0065] See also Figure 4-5 The steam-water separation device 50 is used to remove water vapor from the gas. Specifically, the steam-water separation device 50 includes a steam-water separator 51, a cyclone separator 52 and a freeze dryer 53.

[0066] The steam-water separator 51 comprises:

[0067] Container 510;

[0068] A first air inlet 512 disposed at the top of the container 510;

[0069] The water vapor separation pipes 511 are vertically arranged side by side in the middle and upper part of the container 510, wherein each water vapor separation pipe 511 is further provided with a plurality of water vapor separation chambers 5110 along its height direction, and the water vapor separation chambers 5110 are arranged in a staggered manner;

[0070] A water storage portion 518 formed in the lower middle portion of the container 510 and used to receive water from the water vapor separation pipe 511;

[0071] An air outlet pipe 514 is provided from the top of the container 510 and passes through the water vapor separation pipe 511, and the air outlet pipe 514 is connected to the water storage portion 518;

[0072] a first air outlet 515 communicating with the air outlet pipe 514;

[0073] A drain port 516 is disposed at the bottom of the container 510 .

[0074] The water vapor separation chamber 5110 is a cochlear structure and is arranged in two rows in the vertical direction, with the top of one row of cochlear structures facing the middle of the other row of cochlear structures. When the chemical gas flows along the wall of the water vapor separation chamber 5110 with water droplets, the water droplets adhere to the wall of the water vapor separation chamber 5110 due to the adhesion force, forming a liquid film flowing down and separating from the chemical gas. Furthermore, in one embodiment, the water vapor separation chamber 5110 is filled with a metal ball 5112 of condensed water. The material of the metal ball 5112 can be selected from ferritic stainless steel with a carbon content of less than 0.01%, such as 1.4024 (SUS410J1, 1Cr13Mo), 1.4113 (S43400, SUS434), 1.4418 (X4CrNiMo16-5-1, 0Cr16Ni5Mo). The size of the metal ball 5112 can be selected according to actual needs, and its diameter is preferably 1cm~5cm.

[0075] In one embodiment, the metal ball 5112 is made of ferritic stainless steel 1.4418. On the one hand, since ferritic stainless steel 1.4418 has an extremely low carbon content, it has the best hydrophilicity (the hydrophilic angle with water is about 35°, while that of ordinary stainless steel is about 50°). On the other hand, ferritic stainless steel 1.4418 contains nickel, which can be further passivated by fluorine / chlorine gas to improve its service life and hydrophilicity. Specifically, the step of passivating ferritic stainless steel 1.4418 with fluorine / chlorine gas includes:

[0076] Place ferritic stainless steel 1.4418 steel balls into the passivation kettle;

[0077] Inert gas is introduced to drive out the air in the passivation kettle, and then vacuum is drawn;

[0078] The passivation kettle is heated to 100-150° C., a fluorine / chlorine mixed gas is introduced, wherein the ratio of fluorine / chlorine is 1-3:1, the gas pressure is controlled to be 2 kPa-5 kPa, and the passivation is performed for 2-10 minutes;

[0079] After the passivation is completed, the temperature is naturally lowered. After testing (a flat stainless steel sheet was actually used), the hydrophilic angle with water dropped from about 35° to about 23°. This may be due to the nanoscale or micron microstructure produced on its surface. According to the Cassie and Wenzel wetting state theory, if the liquid is in direct contact with the concave and convex surface of the solid surface microstructure, the droplet is in the Wenzel state. The Wenzel state determines that when the liquid directly contacts the microstructured surface, the angle θ will be transformed into cos(θ w)=r∗cos(θ) where r is the ratio of the actual area to the projected area (roughness, roughness factor), θ represents the actual contact angle between the droplet and the surface, and θ w is the angle after transformation. Wenzel's equation of state shows that microstructuring a surface will amplify the surface tension. A hydrophobic surface (with a contact angle greater than 90°) will become more hydrophobic after microstructuring, and its new contact angle will be larger than the original. However, a hydrophilic surface (with a contact angle less than 90°) will become more hydrophilic after microstructuring, and its new contact angle will be smaller than the original.

[0080] During maintenance, the metal ball 5112 can be replaced, thereby facilitating the maintenance of the cleaning agent. Compared with replacing the entire water vapor separation pipeline 511, the present invention can be more economical by replacing the metal ball 5112.

[0081] As a further improvement, the drain port 516 is further connected to a water seal pipe 517, and the height H of the water seal pipe 517 is 0.3-0.5 meters. If the height of the water seal pipe 517 is too high, the removal rate of water vapor will be affected. If the water seal pipe 517 is too low, the pressure will be too low, so that chemical gas can be easily discharged through the water seal port, causing pollution and waste. In one embodiment, the height of the water seal in the water seal pipe 517 is 0.4 meters.

[0082] The cyclone separator 52 includes a cyclone separator body 520, a second air inlet 521 and a second air outlet 522 disposed on the cyclone separator body 520, and a second drainage pipe 523 disposed at the bottom of the cyclone separator body 520. As a further improvement, the cyclone separator 52 is disposed on the top of the steam-water separator 51, and the second drainage pipe 523 extends along the air outlet pipe 514 to the water storage portion 518 for liquid sealing.

[0083] The freeze dryer 53 includes a freeze dryer body 530, a third air inlet 531 and a third air outlet 532 arranged on the freeze dryer body 530, and a third drainage pipe 533 arranged at the bottom of the freeze dryer body 530. As a further improvement, the freeze dryer 53 is arranged on the upper part of the gas-water separator 51, and the third drainage pipe 533 is connected to the buffer chamber 519 through the water inlet 513 arranged on the top of the container 510. Since the third drainage pipe 533 is connected to the buffer chamber 519, the droplets frozen by the freeze dryer 53 can be discharged into the buffer chamber 519, and flow from the buffer chamber 519 through the water vapor separation pipe 511 to enter the water storage part 518. The temperature of the droplets frozen by the freeze dryer 53 is relatively low, so the low-temperature liquid enters the gas buffer chamber 519, and can further condense the droplets in the chemical gas, thereby improving the water removal efficiency of the gas-water separation device after water washing. It can be understood that by completely removing the moisture, the effect of water vapor on the adsorbent in the subsequent adsorption device 60 can be greatly reduced.

[0084] The adsorption device 60 is mainly used for adsorbing carbon tetrafluoride gas in the gas. Carbon tetrafluoride is one of the most difficult gases to separate from impurity gases in nitrogen trifluoride, because the properties of carbon tetrafluoride and nitrogen trifluoride are relatively similar. The adsorption device 60 described in the embodiment of the present invention includes at least two levels of flat-pressure adsorption tanks. The pressure of each level of flat-pressure adsorption tank is 1-3kPa. The flat-pressure adsorption tank is filled with a carbon tetrafluoride-specific adsorbent, which is mainly used for the special adsorption of carbon tetrafluoride. In one embodiment, the carbon tetrafluoride-specific adsorbent adopts the carbon tetrafluoride-specific adsorbent produced by Ningxia Huahui Environmental Protection Technology Co., Ltd. In one embodiment, it includes 3 levels of flat-pressure adsorption tanks, and each of the flat-pressure adsorption tanks is about 4 meters long and has an inner diameter of about 0.2 meters. It is filled with a carbon tetrafluoride-specific adsorbent, and more than 99.95% of carbon tetrafluoride gas can be removed by the adsorption device 60.

[0085] The distillation device 70 is used to remove other impurity gases. The distillation device 70 and its control are prior art and will not be described here. After the above control, nitrogen trifluoride gas with a purity of more than 99.999% can be obtained.

[0086] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A thermal cracking device for preparing nitrogen trifluoride, characterized in that: It comprises a thermal cracking tower, an air inlet arranged at the bottom of the thermal cracking tower, a filter arranged between the air inlet and the thermal cracking tower, a heating coil wound around the outside of the thermal cracking tower, an air outlet arranged at the top of the thermal cracking tower, and heating steel balls filled in the thermal cracking tower, wherein the thermal cracking device is used to thermally crack NF3 gas to remove fluorine and nitrogen impurities; The heating coil is divided into a plurality of heating sections from top to bottom, and a temperature measuring instrument is arranged in each heating section; the height of the thermal cracking tower is 1.6-2.4 meters, and the inner diameter is 0.3 meters; the heating coil is divided into a first heating zone, a second heating zone, and a third heating zone from top to bottom, wherein the height of the first heating zone is 0.8-1.2 meters, the height of the second heating zone is 0.5-0.7 meters, and the height of the third heating zone is 0.3-0.5 meters; the temperature of the first heating zone is 160-180°C, the temperature of the second heating zone is 140-160°C; the temperature of the third heating zone is 120-140°C.

2. The thermal cracking device for preparing nitrogen trifluoride according to claim 1, characterized in that: The heating coil is a high-frequency heating coil, which utilizes electromagnetic induction to generate eddy current in the metal coil, causing Joule heating of the metal coil due to resistance, and the metal coil is evenly wound around the outer periphery of the thermal cracking tower.

3. The thermal cracking device for preparing nitrogen trifluoride according to claim 1, characterized in that: The particle size of the heating steel balls is 0.5 cm to 2 cm.

4. A safe preparation system for high-purity nitrogen trifluoride, characterized in that: in, It includes an electrolytic cell device, a thermal cracking device, a redox tower, an alkali washing device, a steam-water separation device, an adsorption device and a distillation device connected in sequence. The thermal cracking device comprises a thermal cracking tower, an air inlet arranged at the bottom of the thermal cracking tower, a filter arranged between the air inlet and the thermal cracking tower, a heating coil wound around the outside of the thermal cracking tower, an air outlet arranged at the top of the thermal cracking tower, and a heating steel ball filled in the thermal cracking tower, and the thermal cracking device is used to thermally crack NF3 gas to remove fluorine nitrogen impurity gas impurities; the heating coil is divided into a plurality of heating sections from top to bottom, and each heating section is provided with a temperature measuring instrument; the height of the thermal cracking tower is 1.6-2.4 meters, and the inner diameter is 0.3 meters; the heating coil is divided into a first heating zone, a second heating zone, and a third heating zone from top to bottom, wherein the height of the first heating zone is 0.8-1.2 meters, the height of the second heating zone is 0.5-0.7 meters, and the height of the third heating zone is 0.3-0.5 meters; the temperature of the first heating zone is 160-180°C, the temperature of the second heating zone is 140-160°C; the temperature of the third heating zone is 120-140°C The steam-water separation device includes: a container; a first air inlet arranged at the top of the container; steam separation pipes arranged vertically and side by side in the middle and upper part of the container, wherein each steam separation pipe is further provided with a plurality of steam separation chambers along its height direction, and the steam separation chambers are arranged in a staggered manner; the steam separation chambers are filled with metal balls with condensed water; the material of the metal balls is selected from ferritic stainless steel with a carbon content of less than 0.01%; a water storage part formed in the middle and lower part of the container and used to receive water from the steam separation pipe; an air outlet pipe penetrating the steam separation pipe from the top of the container, and the air outlet pipe is connected to the water storage part; a first air outlet connected to the air outlet pipe; a drain outlet arranged at the bottom of the container.

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