JP2698457B2 - Electrolytic cell - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an electrolytic cell used in producing nitrogen trifluoride gas by a molten salt electrolysis method.

[Problems to be solved by conventional technology and invention]

Nitrogen trifluoride (NF ₃ ) gas has been increasing in demand in recent years as a dry etching agent for semiconductors and as a cleansing gas for CVD equipment. In these applications, particularly high purity of carbon tetrafluoride (CF ₄ ) is used. Gas is preferably used.

Conventionally, NF ₃ gas is produced by various methods. Among them, the molten salt electrolysis method has a high yield and is easier to mass-produce than other methods, so that it is advantageously produced as an industrial production method. Furthermore, NF ₃ obtained by a molten salt electrolysis method is considered to be the most promising for obtaining a high-purity gas containing a small amount of CF ₄ as described above, since it is the lowest cost.

Production of NF ₃ gas by the molten salt electrolysis method is based on NH ₄ F-HF system using ammonium acid fluoride or ammonium fluoride and hydrogen fluoride as raw materials, or further using acid potassium fluoride or potassium fluoride as a raw material. the KF-NH ₄ F-HF molten salts plus performed by a method of electrolysis.

In the production of NF ₃ gas by molten salt electrolysis, NF ₃ gas and nitrogen (N ₂ ) gas are generated from the anode, and hydrogen (H ₂ ) gas is generated from the cathode. It is an evolving reaction. When the NF ₃ gas generated from the anode and the H ₂ gas generated from the cathode mix, there is a risk of causing an explosion, and it is necessary to take safety measures so as not to cause an explosion.

Therefore, in order to prevent this explosion, the electrolytic cell is provided with a partition plate for preventing mixing of the gas generated in the anode and the gas generated in the gaseous phase in the gas phase, as shown in FIGS. .

In addition, in order to prevent the corrosion of the partition and the partition itself from becoming an electrode, it is usually preferable to use a fluorine-based resin or to coat the partition with a fluorine-based resin.

The temperature of the molten salt during electrolysis by the molten salt electrolysis method is 100 to 1
The operation at 30 ° C. is most convenient because the operation is easy, the conductivity is good, and the current efficiency is good.

However, when the molten salt is NH ₄ F-HF system, when the temperature of the molten salt reaches 100 to 130 ° C., NH ₄ F · HF (melting point)
126 ° C.) has the disadvantage that it precipitates at lower temperatures than the electrolytic bath. When the present inventors performed continuous use for a long time,
It was found that a part of the NH ₄ F-HF system evaporated into the lid of the electrolytic cell and the outlet of the generated gas became NH ₄ F · HF and precipitated, and the gas outlet was blocked relatively easily.

Therefore, as a preventive measure to prevent the gas outlet from being blocked, the carrier gas was used continuously for a long period of time while flowing the carrier gas.
・ It was found that HF was deposited and clogged. When blocked at the inlet of the carrier gas or the outlet of the generated gas as described above, NF which is the anode generated gas partitioned by the above-described partition plate
_Since a pressure difference occurs between the anode chamber where ₃ is present and the cathode chamber where H ₂ which is a cathode-generated gas is present, a difference occurs between the liquid levels of the two electrode chambers, which causes a major trouble.

For example, if the anode gas outlet is blocked,
Since the NF ₃ gas is continuously generated without the NF ₃ gas being released, the pressure in the anode chamber increases. Since the pressure in the anode chamber increases, the liquid level in the anode chamber is pushed down, and the liquid level in one cathode chamber is pushed up. When the liquid level in the anode chamber falls below the lower end of the diaphragm, the NF ₃ gas in the anode chamber mixes into the anode chamber and becomes an explosive mixed gas, which easily causes an explosion in the cathode chamber. Once reaching the explosive, can lead to even serious accident since you have a strong chemicals very corrosive of further hydrofluoric acid partially destroyed in the electrolyzer equipment is large, no longer of NF ₃ Manufacturing becomes impossible.

Blockage of the anode gas outlet in the anode chamber leads to a major accident as described above, but similar obstruction occurs in the cathode chamber. As described above, blockage at the gas inlet / outlet is indispensable for safety.

However, since these problems are not yet well known and there are no effective countermeasures, the present inventors have conducted intensive studies and found that the distance between the lid of the electrolytic tank and the liquid level of the electrolytic bath is large. By controlling within this range, NF ₃ gas can be obtained safely without causing a trouble of clogging due to evaporation of the electrolytic bath for a long period of time.

[Means for solving the problem]

In view of the above situation, the present inventors consider that NF ₃ by molten salt electrolysis
NH easily generated at gas inlet and outlet in production electrolytic cell
₄ As a result of repeated investigations on the blocking problem due to evaporation of FHF, if the distance between the lid of the electrolytic tank and the liquid level of the electrolytic bath is limited to a certain range, the blocking does not occur for a long time,
The inventors have found that NF ₃ gas can be safely produced, and have completed the present invention.

That is, the present invention is an electrolytic cell for producing nitrogen trifluoride gas by a molten salt electrolysis method, in which a molten salt forms an electrolytic bath, and an anode and a cathode are immersed in the electrolytic bath to prevent evaporation of the electrolytic bath. Thus, the present invention relates to an electrolytic cell having an electrolytic cell with a lid, wherein the distance between the lid of the electrolytic cell and the liquid level of the electrolytic bath ranges from 100 to 500 mm.

[Detailed Disclosure of the Invention]

 Hereinafter, the present invention will be described in detail.

The electrolytic cell referred to in the present invention means that NF ₃ gas is safe, and
An electrolytic cell for manufacturing for a long time, wherein a molten salt forms an electrolytic bath, and an anode and a cathode are immersed in the electrolytic bath, and the electrolytic bath is covered so as to prevent evaporation of the electrolytic bath. In the electrolytic cell, the distance between the lid of the electrolytic cell and the liquid level of the electrolytic bath is fixed.

The molten salt electrolysis method is an electrolysis method used in the production of NF ₃ gas, and is usually an NH ₄ F-HF system using ammonium acid fluoride or ammonium fluoride and hydrogen fluoride as raw materials, It is carried out by a method of electrolyzing a KF-NH ₄ F-HF molten salt to which potassium acid fluoride or potassium fluoride is added as a raw material.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a longitudinal sectional view showing an example of an electrolytic cell suitable for carrying out the present invention for producing NF ₃ gas, and FIG. 2 is a view taken along the line AA ′ in FIG.

In the present invention, the distance between the lid 3 of the electrolytic bath (hereinafter, the lid 3 of the electrolytic bath includes the cover plate 11 for fixing the diaphragm) and the liquid surface of the electrolytic bath 4 has a distance of 100 to 500 mm. . The electrolytic bath 4 is NH ₄ F−
HF-based or KF-NH ₄ F-HF-based molten salt may be used.
Electrolyzed at ~ 130 ° C. Then, NF ₃ gas is generated from the anode 5, exits from the anode gas outlet 12, and exits from the cathode 6.
H ₂ is generated and exits from the cathode gas outlet 13.

Although the following description is carried out for the case described above, may want to flow the nitrogen (N ₂₎ an inert gas such as to each of the electrolytic cell in order to flow the gas generated from the poles, N ₂ gas to the flow Etc. will be provided. There is no problem in such a case.

The distance between the lid 3 of the electrolytic cell of the present invention and the liquid level of the electrolytic bath 4 is as described above. When the distance between the lid 3 of the electrolytic bath and the liquid level of the electrolytic bath is shorter than 100 mm, NH ₄ F is discharged at the cathode gas outlet 13 or the cathode gas outlet 12 as a part of the electrolytic bath evaporates.
-HF precipitates and leads to blockage when used for a long time.

For example, when the cathode gas outlet 13 is blocked, the pressure in the cathode chamber is increased for continued H ₂ gas without loss of H ₂ gas from the cathode chamber is generated, and the liquid surface of the cathode chamber is pressed down, The liquid level in one anode chamber will be pushed up. When the liquid level in the cathode chamber falls below the lower end of the partition plate 10, the H ₂ gas in the cathode chamber mixes into the anode chamber and becomes an explosive mixed gas, which easily causes an explosion in the anode chamber. Once an explosion occurs, part of the equipment in the electrolytic cell is destroyed, and the use of a highly corrosive chemical called hydrofluoric acid can lead to serious accidents. _Three
Cannot be manufactured.

Here, even if the blockage occurs at the anode chamber outlet 12, there is the same danger as described above. Further, when an inlet for N ₂ gas or the like is provided as described above, the same applies even if the gas inlet is blocked. Therefore, these occlusions are a very serious problem in terms of safety and must be avoided.

On the other hand, the distance between the lid 3 of the electrolytic bath and the liquid level of the electrolytic bath 4 is 500 m.
If the length is longer than m, the volumes of the anodic gas NF ₃ gas and the cathodic gas H ₂ gas present on the liquid surface of the electrolytic cell lid 3 and the electrolytic bath 4 become large, and NF ₃ may become H ₂
If an explosion or the like occurs due to the generation of a gas mixture, it is sufficiently expected that serious damage will be caused. Therefore, since damage such as explosion must be minimized,
Such an electrolytic cell must be avoided.

Further, the distance between the lid 3 of the electrolytic bath and the liquid level of the electrolytic bath 4 is
If the length is longer than 500 mm, the size of the electrolytic cell also becomes large, resulting in excessive equipment and high cost. Especially,
Since this electrolytic bath has a very strong hygroscopic property, it always absorbs moisture in the air at the stage of preparing the raw material. Thus in the production of NF ₃ is essential dehydration electrolysis performed by applying a lower current than the current density of the pre-present electrolysis time, the process proceeds to the electrolysis subsequently dehydrated completion of electrolysis. As a result of repeated studies by the present inventors, if the electrolytic cell is too large, there is a disadvantage that the time for the dehydration electrolysis becomes too long and the dehydration efficiency becomes very poor.

In the electrolytic cell for producing NF ₃ gas by the molten salt electrolysis method, a fluorinated resin plate is usually laid on the bottom plate of the electrolytic cell main body, thereby preventing corrosion of the bottom plate. Also, in the electrolytic cell of the present invention, as shown in FIGS. 1 and 2, a fluorine-based resin plate 2 is provided. In addition, it is preferable to coat (line or coat) the electrolytic bath not only with the bottom plate but also with the molten salt and the gas generated by electrolysis with a fluorine-based resin in order to prevent corrosion of the electrolytic bath.

Here, when the above-mentioned fluorine-based resin is exemplified, for example, polytetrafluoroethylene, polychlorotrifluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-ethylene copolymer Polymers, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers, chlorotrifluoroethylene-ethylene copolymers and the like can be used, any of which are generally known, among which polytetrafluoroethylene and tetrafluoroethylene- Perfluoroalkyl vinyl ether copolymers are particularly preferred because of their excellent heat resistance and acid resistance.

〔Example〕

Example 1 Using a molten salt of NF ₄ F · HF system (HF / NF ₄ F molar ratio = 1.8),
Using an electrolytic cell in which the distance between the lid 3 of the electrolytic cell and the liquid surface of the electrolytic bath 4 shown in FIG. 1 is 150 mm, a current of 50 amperes (A) is passed (average anode current density 2 A). / dm ²⁾ 120
Dehydration electrolysis was started at ℃. When the O ₂ concentration in the gas generated from the anode was analyzed by gas chromatography, it was determined that the dehydration electrolysis was completed after 80 hours after the gradual decrease and became constant at around 2% (hereinafter referred to as volume%).

After 80 hours when dehydration is considered complete, the process proceeds to main electrolysis and the current is 250 A (average anode current density 10 A / dm ² )
While performing electrolysis continuously for 3 months, the flow rate of the gas generated by the anode and the flow rate of the gas generated by the cathode were monitored, and the clogging status of each gas outlet was confirmed from the change over time. NF ₃ could be safely produced over a long period of time without producing any problems.

Example 2 Dehydration electrolysis and main electrolysis were performed in the same manner as in Example 1 except that the distance between the lid 3 of the electrolytic cell and the liquid surface of the electrolytic bath 4 was 400 mm (the molten salt was the same as in Example 1). Was used.)

The time when the value of the O ₂ concentration by gas chromatography analysis of the anode generated gas judged that the dehydration electrolysis was completed was gradually reduced and became constant at around 2% was 100 hours, which is Example 1.
Although it became slightly longer, while performing continuous electrolysis for 3 months as in Example 1, the flow rate of the positive gas and the flow rate of the cathode generated gas were monitored, and the state of blockage of each gas outlet was confirmed from the change over time. As a result, no change was observed in both poles, and of course, NF ₃ could be safely produced for a long time without explosion.

Comparative Example 1 Dehydration electrolysis and main electrolysis were performed in the same manner as in Example 1 except that the distance between the lid 3 of the electrolytic cell and the liquid level of the electrolytic bath 4 was 50 mm (a value exceeding the value specified in the present invention). (The same molten salt as in Example 1 was used). The time when the value of the O ₂ concentration by gas chromatography analysis of the anode generated gas judged that the dehydration electrolysis was completed was gradually decreased and became constant at around 2% was 80 hours, which was the same as in Example 1. .
However, in the same manner as in Examples 1 and 2, the main electrolysis was carried out for the long-term continuous electrolysis for 3 months, and the flow rate of the anode-generated gas and the flow rate of the cathode-generated gas were monitored. As a result, after about one week, the flow rate from the anode rapidly decreased and approached zero. When the electrolysis was stopped and the anode gas outlet 12 was observed, NH ₄ F
It was found that HF was deposited and blocked the anode gas outlet 12.
Also, precipitation of NH ₄ F ・ HF at the cathode gas outlet 13 was observed,
It soon became apparent that there was a risk of blockage. Thus, it was found that long-term operation was impossible as compared with Examples 1 and 2.

The distance between the electrolytic bath lid 3 and the liquid level of the electrolytic bath 4 is 50.
Greater than 0mm (exceeding the value specified in the present invention)
The electrolyzer was not examined because it is clear from Example 2 that no danger was involved.

〔The invention's effect〕

As described in detail above, the present invention is an electrolytic cell for producing NF ₃ by the molten salt electrolysis method, by specifying the distance between the lid of the electrolytic cell and the liquid level of the electrolytic bath, the NF ₃ gas can be used for a long time. It has made it possible to manufacture safely.

Therefore, as described above, by specifying the distance between the lid of the electrolytic cell and the liquid level of the electrolytic bath, the carrier gas inlet in the electrolytic cell or the generated gas outlet from both electrode chambers and the like are closed even when used for a long time. Things can now be avoided.

As a result, there is no danger of explosion due to the mixture of NF ₃ gas and H ₂ gas as the generated gas, and the safe operation of NF ₃ gas for a long period of time is a factor in industrial production of NF ₃ gas. The implications for the NF ₃ industry are extremely significant.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing an example of an electrolytic cell for producing NF3 gas, which is suitable for carrying out the present invention, and FIG. 2 is a view taken along the line AA 'in FIG. In the drawing, 1 ... the electrolytic cell main body, 2 ... the fluorine resin plate, 3 ... the lid plate, 4 ... the electrolytic bath, 5 ... the anode, 6 ... the cathode, 7a, 7b ... the connecting rod, 8a, 8b: Insulation material, 9a, 9b: Cap nut for fixing the connecting rod, 10: Separator plate, 11: Lid plate for fixing the separator plate, 12: Anode generating gas outlet tube, 13: Cathode generating gas outlet Pipe, 14 packing, 15 bolt nut for lid plate, 16 bolt for fixing plate.

Claims (1)

(57) [Claims] 1. An electrolytic cell for producing nitrogen trifluoride gas by a molten salt electrolysis method, wherein a molten salt forms an electrolytic bath, and an anode and a cathode are immersed in the electrolytic bath to prevent evaporation of the electrolytic bath. An electrolytic cell comprising an electrolytic cell having a lid as described above, wherein the distance between the lid of the electrolytic cell and the liquid level of the electrolytic bath ranges from 100 to 500 mm.