JPH033876B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for purifying nitrogen fluoride. More specifically, the present invention relates to a method for separating and removing low-boiling components such as oxygen and nitrogen contained in nitrogen fluoride. Nitrogen fluoride, particularly nitrogen trifluoride (NF ₃ ), has recently attracted attention as a cleaning agent and dry etching agent for CVD equipment in the field of electronic materials. However, in recent years, nitrogen fluoride used for these purposes has been required to have even higher purity. [Prior art and problems to be solved by the invention] Nitrogen fluoride is produced by various methods, but
Most of them contain relatively large amounts of impurities such as nitrous oxide (N ₂ O), carbon dioxide (CO ₂ ), oxygen (O ₂ ), and nitrogen (N ₂ ), so they cannot be used for the above purposes. In order to use nitrogen fluoride, it is necessary to remove and purify the various impurities mentioned above to make it highly pure. As a method for removing the above impurities, there is a method of removing them by contacting them with an adsorbent such as zeolite.
It is commonly used as it is the most efficient and simple method, and is also applied to the purification of nitrogen fluoride (crude nitrogen fluoride) [Chem.Eng., 84, 116, (1977)]
etc〕. However, when the above method is applied to the purification of crude nitrogen fluoride, impurities with relatively high boiling points such as N ₂ O and CO ₂ can be efficiently removed, but impurities with low boiling points such as O ₂ and N ₂ can be removed efficiently. Most of the components are not removed, and the method for removing them is not yet known. In addition, distillation is generally effective and commonly used as a means of separating components with different boiling points, but when the present inventors tried this method to remove low-boiling components from nitrogen fluoride, it was found that nitrogen fluoride and Despite the large enough difference in boiling point from low boiling point components, efficient separation is not possible. For example, it was found that at least several thousand ppm of _N2 remains in nitrogen fluoride. For this reason, the purity of currently commercially available nitrogen fluoride is limited to about 99.9% by weight, and the reality is that it does not meet the requirement for ultra-high purity. An object of the present invention is to provide a method for efficiently removing low boiling point components such as O ₂ and N ₂ to obtain ultrapure nitrogen fluoride. [Means and effects for solving the problem] The present inventors have discovered that O ₂ contained in crude nitrogen fluoride,
As a result of extensive research into methods for removing low-boiling components such as _N2 , we discovered that crude nitrogen fluoride can be removed extremely efficiently and economically by cryogenic distillation under specific conditions, and we have developed the present invention. It has come to completion. That is, the present invention provides a method for purifying nitrogen fluoride, which is characterized in that crude nitrogen fluoride is cryogenically distilled in the coexistence of a third component that has a lower boiling point than nitrogen fluoride and is not mutually soluble with nitrogen fluoride. be. The present invention will be explained in detail below. Examples of nitrogen fluoride gas include nitrogen trifluoride (NF ₃ ), dinitrogen difluoride (N ₂ F ₂ ), and dinitrogen tetrafluoride (N ₂ F ₄ ), and the method of the present invention uses these gases. It is effective for purifying any nitrogen fluoride. The present invention is based on deep cold distillation, in which substances with low boiling points are separated by distillation at low temperatures. The distillation operation at this time can be either continuous or batch, but if the amount of myelin is large, the continuous method is preferable in terms of energy cost, and conversely, if the amount to be processed is small, it is preferable in terms of equipment cost and operation. A batch method is preferred. Among batch operations, simple distillation without reflux is effective as a simple method. As described above, the present invention is a method for purifying crude nitrogen fluoride by cryogenic distillation, so it is necessary to first liquefy the crude nitrogen fluoride. However, in the present invention, crude nitrogen fluoride is liquefied using a refrigerant. The refrigerants used for this liquefaction include liquid nitrogen, liquid air, liquid argon, and LNG, which have a boiling point lower than that of nitrogen fluoride. Among these liquefied gases, liquid nitrogen is also the cheapest.
Moreover, it is most preferable because it is inert. As a method other than the above, for example, a method in combination with an LNG vaporization process is advantageous because it can save energy. Furthermore, a method of directly exchanging heat between the refrigerant of the refrigerator and the crude nitrogen fluoride can also be effectively adopted. The present invention is a method of cryogenic distillation of the crude nitrogen fluoride liquefied in this way, and the purpose of this is to remove and purify low-boiling components such as N ₂ and O ₂ contained therein. In the present invention, it is essential that a third component having a boiling point lower than that of nitrogen fluoride and having no mutual solubility with nitrogen fluoride be present in the cryogenic distillation. Such third components include helium, argon, neon, and the like. This third component may be used alone or in a mixed gas of two or more types. In the present invention, as a specific method for making the third component coexist with the crude nitrogen fluoride, as shown in FIG. 1, the third component is fed together with the crude nitrogen fluoride into the distillation column, and as shown in FIG. In this method, the crude nitrogen fluoride and the third component are fed into the distillation column from separate feed ports.As shown in Figure 3, the inside of the distillation column is replaced with the third component in advance, and then the crude nitrogen fluoride is fed into the distillation column. Various methods can be adopted, such as a method of feeding the nitrogen, and a method of bubbling a gaseous third component into the crude nitrogen fluoride liquefied and staying at the bottom of the distillation column as shown in FIG. Among these methods, the method of bubbling a gaseous third component into the crude nitrogen fluoride liquefied and retained at the bottom of the distillation column is particularly effective in removing low-boiling components such as N ₂ and O ₂ . Highly desirable. The methods shown in Figures 1 to 4 above are all methods of continuous distillation, but in the case of batch distillation, as shown in Figure 5, gaseous nitrogen is distilled into the liquefied crude nitrogen fluoride. A method of bubbling the third component, or a bubble agitation method of adding a stirrer to the method and stirring the crude fluorinated gas as shown in FIG. 6 is preferable because it has a high removal efficiency of low-boiling components. When using a method of bubbling a gaseous third component into liquid nitrogen fluoride, the finer the bubbles are dispersed, the higher the removal efficiency of the low-boiling point component. A method of bubbling the ingredients is preferred. When carrying out cryogenic distillation at normal pressure in each of the above methods, it is necessary to prevent air from entering the distillation system. Fig. 7 is a diagram showing a more detailed embodiment of the method shown in each of the above figures, but when carrying out cryogenic distillation at normal pressure, a water seal tank is used to exhaust the low boiling point components and the third component. The water is bubbled through the water seal tank and then released into the atmosphere, thereby preventing air from entering the distillation system. In the present invention, cryogenic distillation is performed at a temperature at which nitrogen fluoride is in a liquid state, that is, at a temperature range below the liquefaction temperature of nitrogen fluoride and above the solidification temperature, and at any temperature at which the third component coexisting does not liquefy. carried out at a temperature of Therefore, the crude nitrogen fluoride must be in a liquefied state in the distillation column, and this is achieved by cooling and keeping the distillation column cold with a refrigerant such as liquid nitrogen, liquid air, or liquid argon.
The state of the crude nitrogen fluoride when feeding it into the distillation column is not necessarily in a liquefied state and may be in a gaseous state, but in order to prevent loss of nitrogen fluoride as much as possible, it is preferable to feed it in a liquefied state to the distillation column. It is preferable to feed. In the present invention, it is convenient to perform distillation at normal pressure in the distillation column, but in order to further improve the removal efficiency of low-boiling components, distillation under reduced pressure is preferable. The removal efficiency increases as the vacuum increases. To reduce the pressure inside the distillation column, for example, a commonly known pressure reducing device such as a vacuum pump or an aspirator may be used to draw suction from the top of the distillation column. A method such as liquefying a gas mixture with three components using liquid helium or the like is adopted. In the present invention, the amount of the third component coexisting in the crude nitrogen fluoride during cryogenic distillation is not particularly limited, but N ₂ ,
From the viewpoint of the removal effect of low boiling point components such as O ₂ , the larger the amount, the more preferable. However, when this third component is discharged from the top of the distillation column together with the gasified low-boiling components, a portion of the nitrogen fluoride is also entrained in the gaseous state. Therefore, as the coexisting amount of the third component increases, the loss of nitrogen fluoride increases, so in practice, the molar ratio of the gaseous third component to the liquid crude nitrogen fluoride is 1 to 100. A range of % is preferred. The method of supplying the third component to the distillation column is not particularly limited, and may be continuous supply, batch supply, or intermittent supply. As explained in detail above, the method for purifying nitrogen fluoride of the present invention is a method for removing low boiling point components such as N ₂ and O ₂ contained as impurities in crude nitrogen fluoride gas. As mentioned above, the crude nitrogen fluoride contains relatively high boiling point components such as N ₂ O and CO ₂ in addition to the above-mentioned low boiling point components, and it is necessary to remove and purify these relatively high boiling point components as well. However, in the method of the present invention, the above-mentioned relatively high boiling point components are not sufficiently removed. Therefore, in order to obtain high purity nitrogen fluoride,
Relatively high boiling point components such as N ₂ O and CO ₂ are removed and purified by contacting with an adsorbent such as zeolite using a conventionally known method, while low boiling point components such as N ₂ and O ₂ are removed and purified.
This can be achieved by removal and purification using the method of the present invention. Although either of the above two purifications may be carried out first, it is usually carried out by removing relatively high-boiling components and then removing low-boiling components by the method of the present invention. [Examples] Hereinafter, the present invention will be explained in more detail with reference to Examples and Comparative Examples. In the Examples and Comparative Examples, % and ppm represent capacity standards. Example 1 Using commercially available zeolite (granular product with a pore size of 5 Å and a mesh size of 24 to 48), crude nitrogen trifluoride (crude nitrogen trifluoride) of the quality shown in Table 1 was prepared by adsorbing and removing N ₂ O, CO ₂ , etc. contained in it. NF ₃ ) was purified by cryogenic distillation using the apparatus shown in FIG. That is, after cooling the collection container 1 (inner capacity 1) by filling the cold container 6 with liquid nitrogen, gaseous NF ₃ of the quality shown in Table 1 and helium gas as the third component are added to the collection container. Feed was carried out through lines 2 and 3 at flow rates of 50 ml/min. and 3 ml/min., respectively. NF ₃ was liquefied in the collection container, and the coexisting helium gas and fractionated gaseous low-boiling components (oxygen, nitrogen, etc.) were exhausted through the water seal tank 7 (therefore, the inside of the system was almost completely empty). (maintained at atmospheric pressure). When the feed amount of _NF3 gas reaches 100g,
After closing the valves 12 and 13 to stop the NF ₃ gas and helium gas feeds and closing the valve 16 leading to the water seal tank 7, the valve 17 is opened and the vacuum pump 8 removes gaseous low-boiling components in the system. and helium gas was exhausted. After completion of evacuation, the collection container 1 was returned to room temperature, and the liquefied NF ₃ in the collection container 1 was gasified and analyzed. As shown in Table 2, oxygen and nitrogen were largely removed. Example 2 Using the apparatus used in Example 1 (FIG. 7), instead of exhausting helium gas and gaseous low-boiling components generated by cryogenic distillation through the water seal tank 7, a vacuum pump 8 was used. Cryogenic distillation of crude NF ₃ having the quality shown in Table 1 was carried out under the same conditions and method as in Example 1, except that the pressure inside the collection container 1 was reduced by exhausting air through line 10 using a vacuum cleaner. The pressure inside the system during cryogenic distillation was 10 mmHg abs. The results of gasifying and analyzing NF ₃ in collection container 1 are shown in Table 2.
As shown in Figure 2, the contents of oxygen and nitrogen were significantly reduced compared to Example 1. Example 3 The apparatus shown in FIG. 7 was modified so that the insertion tube 11 was extended to the bottom of the collection container 1 and helium gas could be bubbled into the liquefied NF ₃ . After cooling the collection container 1 by filling the cold container 6 with liquid nitrogen, 100 g of gaseous crude NF ₃ of the quality shown in Table 1 was fed into the collection container 1 and liquefied in the collection container 1. . In this liquefied crude NF ₃ ,
Helium gas was fed and bubbled for 30 minutes at a flow rate of 100 ml/min. Note that the helium gas and the vaporized low-boiling components were exhausted through the water seal tank 7 as in Example 1 (the pressure inside the system was atmospheric pressure). Thereafter, in the same manner as in Example 1, the inside of the system was evacuated using the vacuum pump 8 to remove the coexisting gas. The resulting liquefaction
When NF ₃ was vaporized and analyzed in the same manner as in Example 1, the results are shown in Table 2, and highly pure NF ₃ from which oxygen and nitrogen were largely removed was obtained. Example 4 Example 3 was carried out in the same manner as in Example 3, except that the vacuum pump 8 was used during bubbling of helium gas, and the system pressure during bubbling was set at 10 mmHg abs.
Cryogenic distillation was carried out under the same conditions and method. As shown in Table 2, the analytical values of the obtained NF ₃ gas showed that the content of oxygen and nitrogen was extremely small, and the NF ₃ gas was of extremely high purity. Example 5 Crude NF ₃ was cryogenically distilled under the same conditions and method as in Example 4, except that argon gas was used instead of helium gas as the third component. obtained
As shown in Table 2, the analytical values of the NF ₃ gas showed that extremely high purity NF ₃ gas was obtained with trace amounts of oxygen and nitrogen.

【table】

[Table] Comparative Example 1 The same operation as in Example 1 was performed except that the feed of helium gas, which was the third component, was stopped. The contents of oxygen and nitrogen in the obtained NF ₃ gas were as shown in Table - As shown in Figure 3, it was hardly removed. Comparative Example 2 The same operation as in Example 2 was performed except that the helium gas feed was stopped, and the contents of oxygen and nitrogen in the obtained NF ₃ gas were as shown in Table 3. Although the removal rate was not as high as in Comparative Example 1, it was quite insufficient.

〔Effect of the invention〕

As explained in detail above, the present invention is a method of cryogenically distilling crude nitrogen fluoride in the coexistence of nitrogen fluoride and a third component that is not mutually soluble, such as helium or argon. Therefore, it is possible to improve the purity of crude nitrogen fluoride, which was not possible with conventional purification methods.
This makes it possible to remove low boiling point components such as O ₂ and N ₂ . As shown in the examples, if the deep cold distillation is carried out under reduced pressure or by bubbling the third component into the liquefied crude nitrogen fluoride, the low boiling point components can be separated very efficiently.・Can be removed. Note that impurities other than low boiling point components such as N ₂ O and CO ₂ contained in the crude nitrogen fluoride can be removed by combining the method of the present invention with other known purification methods in which they are brought into contact with an adsorbent such as zeolite. Possibly, e.g.
It is possible to obtain nitrogen fluoride with extremely high purity of 99.99% or more, which is suitable for use in electronic materials. Nitrogen fluoride of such high purity has not been obtained to date, and can sufficiently meet recent demands in the field of electronic materials, and the present invention is of great significance.

[Brief explanation of the drawing]

FIGS. 1 to 6 are schematic diagrams showing distillation apparatuses showing each embodiment of the present invention, and FIG. 7 is a flow sheet showing the distillation apparatus used in each example and comparative example. In the figure, 1...distillation column or collection vessel, 2
...crude nitrogen fluoride feed line, 3...third component feed line, 4...low boiling point component and third component exhaust line, 5...purified nitrogen fluoride extraction line, 6...cold container, 7...water seal Tank, 8...
Vacuum pump, 9... Normal pressure exhaust line, 10... Vacuum exhaust line, 11... Insertion tube, 12, 13, 1
4, 15, 16, 17...indicates a valve.

Claims (1)

[Claims] 1. Purification of nitrogen fluoride, characterized by cryogenic distillation of crude nitrogen fluoride in the coexistence of a third component that has a boiling point lower than that of nitrogen fluoride and is not mutually soluble with nitrogen fluoride. Method. 2. Claim 1, wherein the coexistence of a third component that has a lower boiling point than nitrogen fluoride and is not mutually soluble with nitrogen fluoride is bubbling of a gaseous third component into liquid nitrogen fluoride. the method of. 3. The method according to claims 1 and 2, wherein the cryogenic distillation is carried out under reduced pressure. 4 Claim 1 in which the cryogenic distillation is simple distillation
The method described in Items 1 to 3. 5. The method according to claims 1 to 4, wherein the third component having a lower boiling point than nitrogen fluoride and having no mutual solubility with nitrogen fluoride is at least one of helium, argon, and neon.