JPH04160176A - Electrolytic cell - Google Patents

Abstract

PURPOSE:To allow the safe and stable operation of the electrolytic cell for production of NF3 by a fused salt electrolysis method by limiting the distance between the joint parts of electrodes and electrode connecting rods and a liquid surface to a specified range. CONSTITUTION:The joint parts 7c, 7d of the electrodes 5, 6 of the electrolytic cell for production of the NF3 by the fused salt electrolysis method and the electrode connecting rods 7a, 7b which support these electrodes and supply electricity thereto are provided between caps 3, 11 of the electrolytic cell and the surface of an electrolytic bath 4. The distance between the above-mentioned joint parts 7c, 7d and the surface of the electrolytic bath 4 is specified to 20 to 200mm. The corrosion of the above-mentioned joint parts 7c, 7d is thereby decreased and the gaseous NF3 is safely produced over a long period of time.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to an electrolytic cell used for producing nitrogen trifluoride gas by molten salt electrolysis.

[Prior art and problems to be solved by the invention] Demand for nitrogen trifluoride (NFff) gas has been increasing in recent years as a dry etching agent for semiconductors and a cleaning gas for CVD equipment, and it is especially important for these applications. Carbon tetrafluoride (
High purity gas with low CF4) content is used.

Nh gas can be produced by various methods, among which molten salt electrolysis is used as an industrial production method because it has a good yield and is easier to mass produce than other methods. Furthermore, this method is the most convenient manufacturing method because it is the only method that can obtain a high-purity gas with less CF as described above.

The production of NF3 gas by this molten salt electrolysis method is based on the NH, F-HF system, which uses acidic ammonium fluoride or ammonium fluoride and hydrogen fluoride as raw materials, and further uses acidic potassium fluoride or potassium fluoride as raw materials. added KF-NH,
This is carried out by electrolyzing F-HF based molten salt.

In the production of NF3 gas by this molten salt electrolysis method, Nh gas and nitrogen (N2) gas are generated from the anode, and hydrogen (H2) gas is generated from the cathode. Both the so-called picture electrodes undergo a gas generation reaction. be. Therefore, if the NF+ gas generated from the anode and the H2 gas generated from the cathode mix, there is a safety problem that may cause an explosion. Therefore, in order to prevent this explosion, the electrolytic cell is provided with a partition plate to prevent the anode-generated gas and the cathode-generated gas from mixing in the gas phase, as shown in Figures 1 and 2. .

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

Here, the anode is carbon (C) or nickel (Ni)
Although electrodes can be used, only Ni electrodes can be used to obtain high-purity gas with less CF4.However, when using the Ni anode, there is a drawback that the Ni anode is slightly dissolved. have Once the Ni electrode immersed in the electrolytic bath is consumed and reduced by this dissolution, further electrolysis becomes difficult, so the anode must be replaced at regular intervals.

In addition, in industrial terms, the period is from at least one month to several years at most, and it is usually replaced every three months to one year. Therefore, unless the joint part with the electrode rod connected to the electrode has a lifespan equal to or longer than that of the electrode, the lifespan of this part becomes a bottleneck and efficient manufacturing becomes impossible.

On the other hand, any metal can be used for the cathode, and usually inexpensive iron is used. When used as an electrode, there is no consumption due to melting, so there is no need to replace the electrode, and the joint with the electrode connecting rod needs to have a longer lifespan than the joint with the anode and the electrode connecting rod.

Here, since the electrode connecting rod is also used for the purpose of conducting electricity to the electrode, metal is usually used. Further, the electrode and the electrode connecting rod are connected by using a joint such as a bolt/nut or by welding. Here, the temperature of the above-mentioned molten salt during electrolysis is most convenient if it is carried out at 100 to 130[deg.] C. because the operation is easy, the conductivity is good, and the current efficiency is also good.

However, in this NH,F-IP system, 100 to 13
Due to the vapor pressure of IP (at a temperature of 0°C), metals such as the electrodes and electrode connecting rods located between the liquid level of the electrolytic bath and the lid of the electrolytic bath, as well as the joints between the electrodes and electrode connecting rods, but,
It is gradually corroded by the HF vapor that accompanies the generated gas. After long-term use, we found that the most severe corrosion occurred at the joint between the electrode and the electrode connecting rod, and if this joint was located close to the surface of the molten salt, the bolts and welds would corrode faster. In some cases, the electrode became detached from the welded part of the electrode connecting rod, making it impossible to continue electrolysis. In addition, if the electrode does not come off from the welding part, it may short circuit in the middle, and in extreme cases, it may cause an explosion or fire. Thus, it was found that the distance between the joint between the electrode and the electrode connecting rod and the liquid level is an important issue for long-term stable operation and safety.

In addition, if the bonded part is located far above the liquid level, the proportion of the electrode that is immersed in the electrolyte and actually causes a reaction and is effectively used is reduced, and expensive materials such as Ni are used. Metals have cost problems.

Furthermore, as mentioned earlier, if the NFS gas generated from the anode mixes with the H gas generated from the cathode, there is a safety issue that could cause an explosion. For safety reasons, it is essential to minimize the damage in the event of an explosion by reducing the amount of gas present in the phase.

As mentioned above, in the production of NFS gas by molten salt electrolysis, it has been found that the distance between the joint between the electrode and the electrode connecting rod and the liquid level is very important for safe and stable operation. There has been almost no study on the structure of the liquid, and in particular, there are no known reports on the specific structure of the distance between the joint between the electrode and the electrode connecting rod and the liquid surface.

[Means to solve the problem]

In view of the above situation, the present inventors have developed NFs using molten salt electrolysis method.
As a result of various studies on the distance between the joint between the electrode and electrode connecting rod and the liquid surface in electrolytic cells for production, we found that if the distance is limited to a certain range, NF can be safely and for a long period of time. They discovered that gas can be operated safely and stably, leading to the completion of the present invention.

That is, the present invention provides an electrolytic cell for producing nitrogen trifluoride gas by molten salt electrolysis, in which a joint between an electrode and an electrode connecting rod that supports the electrode and conducts electricity is connected to the lid of the electrolytic cell and the electrolytic cell. The present invention relates to an electrolytic cell characterized in that the distance between the joint portion and the electrolytic bath surface is within a range of 20 to 200 mm.

(Detailed disclosure of the invention) Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

The most important point in the present invention is the distance between the joint between the electrode and the electrode connecting rod and the liquid level in the electrolytic cell for producing NF3 gas safely and over a long period of time.

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

In an electrolytic cell for producing Nh gas using the molten salt electrolysis method, the joint between the electrode and the electrode connecting rod is made by using a joint such as a bolt/nut or by welding. In the electrolytic cell of the present invention, as shown in FIGS. 1 and 2, the parts are joined by welding. Therefore, in this case, the term "joint part" as used in the present invention means the welding surface between the electrode and the electrode connecting rod. In addition, even when a joint such as a bolt nand is used, it means the part where the electrode and the electrode connecting rod come into contact. Although the present invention can be effectively carried out in either case, since the effects are the same, the following explanation will be given only for the welded case shown in FIG.

In the present invention, the joint 7 between the anode 5 and the anode connecting rod 7a
The distance between c and the liquid level of the electrolytic bath 4 is 20 to 200-1. As mentioned above, in this case, the joint portion 7c means the welding surface of the anode 5 and the anode connecting rod 7a shown in FIG. The distance between the joint 7 of the cathode 6 and the cathode connecting rod 7b and the liquid level of the electrolytic bath 4 must similarly be 20 to 200 mm.

In this case, only the anode 5 will be described, but the same applies to the cathode 6.

The distance between the joint between the anode 5 and the anode connecting rod 7a and the liquid level of the electrolytic bath 4 in the electrolytic cell of the present invention is as described above. When the distance of the liquid level becomes shorter than 20sn, the HF generated from the electrolytic bath
Corrosion progresses rapidly due to the atmosphere, and when used for a long period of time, the thickness of the welded portion becomes thinner and gradually becomes unable to withstand the weight of the anode, and the anode 5 comes off from the anode connecting rod 7a. Since the detached anode 5 can no longer function as an electrode, it becomes impossible to continue electrolysis. That is, the production of NF3 gas is stopped. These are serious problems in terms of stable operation.

Furthermore, when the electrode is detached from the anode connecting rod 7a and is falling, a part of the anode may come into contact with the cathode 6, resulting in a short circuit, which in extreme cases may cause an explosion or fire. do.

Including such problems is a very big safety problem and must be avoided at all costs.

On the other hand, the joint 7c between the anode 5 and the anode connecting rod 7a and the electrolytic bath 4
When the distance between the liquid surface of There is. Furthermore, as mentioned earlier, NF and gas generated from the anode 5 and H generated from the cathode 6 are added.
For safety reasons, mixing of the two gases may cause an explosion, so the distance between the joint 7C of the anode 5 and the anode connecting rod 7a and the liquid level of the electrolytic bath 4 should be as short as possible to avoid the gas present in this gas phase. For safety reasons, it is essential to minimize the damage in the event of an explosion by reducing the amount.

[Example] Example I NFJF-HP system (HF/NF, F molar ratio = 1.8)
Using a molten salt of
Using an electrolytic bath in which the distance between the cathode connecting rod 7b and the liquid level of the electrolytic bath 4 is 30-30 degrees, long-term continuous electrolysis was carried out at a current of 250 A (anode average current density 10 A/dm). NF and gas could be produced safely over a long period of time without causing an explosion.Then, when the electrode was removed from the electrolytic cell and the corrosion status of the joint 7c was observed, no corrosion had progressed. , and was still available for use.

Example 2 Except that the distance between the joint 7C of the anode 5 and the anode connecting rod 7a and the liquid level of the electrolytic bath 4 and the distance between the negative 8ii6, the cathode connecting rod 7b and the liquid level of the electrolytic bath 4 are 181)*a+. Electrolysis was performed under the same conditions as in Example 1. As in [Example], NF can be safely maintained for a long period of time without any explosion even after 6 months.
, 6 The electrode was taken out from the electrolytic bath and the corrosion status of the joint 7C was observed.
Corrosion had not progressed and the product was still usable.

Comparative Example 1 Electrolysis was performed in the same manner as in Example 1 except that the distance between the liquid levels in 4 was Losm. As in Example 1, main electrolysis was carried out with the aim of long-term continuous electrolysis for 3 months.C. Since the sliding voltage showed an abnormal value that rapidly increased after about 3 weeks, it was judged that it was impossible to continue electrolysis any further. However, the electrolysis was immediately stopped. Therefore, when the electrode 5 was taken out from the electrolytic bath and the corrosion status of the joint 7co) was observed, it was found that the corrosion had progressed and the anode 5 was about to come off from the anode connection FIJ 7a, and there was a poor contact between the anode 5 and the cathode connection rod 7a. It was found that the sliding voltage showed an abnormal value.

It should be noted that the distance between the joint between the anode 5 and the anode connecting rod 7a and the liquid level of the electrolytic bath 4 and the distance between the cathode 6, the cathode connecting rod 7b and the liquid level of the electrolytic bath 4 are greater than 2001 (the numerical value specified in the present invention is As for electrolytic cells, as mentioned above, they are risky and meaningless, so no study was conducted.

〔Effect of the invention〕

As explained in detail above, the present invention is an electrolytic cell for producing NF and gas by the molten salt electrolysis method.
This made it possible to produce gas safely and over a long period of time.

That is, the comparative example in which the distance between the joint between the electrode and the electrode rod and the liquid level is outside the scope of the present invention shows an abnormality in which the sliding voltage rapidly increases around 3 weeks, and When we observed the corrosion situation, we found that the corrosion was progressing. In contrast, in Examples 1 and 2, in which the distance between the joint and the liquid level is within the range of the present invention, NF can be safely maintained over a long period of time.
, was able to produce gas.

Through this invention, the present inventors succeeded in obtaining an optimal value for the distance between the joint between the electrode and the electrode connecting rod and the liquid level in the electrode of an electrolytic cell for NF and gas production. N
This is extremely significant in industrially producing Fs gas.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal sectional view showing an example of an electrolytic cell for producing NF3 gas suitable for carrying out the present invention, and FIG. 2 is a view taken along the line AA' in FIG. In the figure, 1-1--electrolytic cell main body, 2---monofluorine resin plate, 3--mm lid plate, 4--11-electrolytic bath, 5--mm anode, 6-----cathode, 7a --- One anode connecting rod, 7b --- One cathode connecting rod, 7cm --- Joint part with anode connecting rod, 7d --- One joint part with cathode connecting rod, 8a --- One anode insulation. Material, 8b---Cathode insulating material, 9a---One anode connecting rod fixing cap nut, 9b-mm cathode connecting rod fixing bag, T, 10---One partition plate, 11---One corner Plate fixing cover plate, 12----Anode generated gas outlet pipe, 13----Cathode generated gas outlet pipe, 14---Backing, 15----Boltner for cover plate, t, holder. - Bolts for fixing one diaphragm are shown. Patent applicant Mitsui Toatsu Chemical Co., Ltd.

Claims (1)

[Claims] 1) In an electrolytic cell for producing nitrogen trifluoride gas by molten salt electrolysis, the joint between the electrode and the electrode connecting rod that supports the electrode and conducts electricity to it is connected to the lid of the electrolytic cell and the liquid surface of the electrolytic bath. and the distance between the joint and the electrolytic bath liquid level is 20
An electrolytic cell characterized in that it is within the range of ~200 mm.