JPH0432592A - Production of gaseous nitrogen trifluoiride - Google Patents

Abstract

PURPOSE:To prevent the generation of gaseous H2 on an anode when electrolysis is stopped at the time of producing gaseous NF3 from NH4F and HF by molten- salt electrolysis by continuously impressing a voltage between the electrodes when the generation of gaseous NF3 is stopped. CONSTITUTION:An electrolytic cell 1 is divided by a partition wall 5, and the anode 3 and cathode 4 in the respective sections are connected to a power source 16. The molten salt 2 derived from NH4F and/or NH4HF2 and HF is introduced into the cell 1 and kept at about 100-150 deg.C by a heater 10. Under these conditions, a DC current is applied between the anode 3 and cathode 4 from the power source 16, and gaseous NF3 is generated from the cathode 3 and gaseous H2 from the cathode 4. An auxiliary DC power source 17 for constant-voltage control is used when electrolysis is stopped, a voltage signal from a function generator 18 is inputted to the power source 17, and a voltage is applied between the anode 3 and cathode 4 continuously or intermittently. Consequently, the generation of gaseous H2 from the anode 3 is prevented, and there is no danger of explosion when electrolysis is resumed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for producing nitrogen trifluoride gas (NF3).

More specifically, ammonium fluoride (NH4F) or/
and a method for producing NF and gas by electrolyzing N)14F liver molten salt using acidic ammonium fluoride (NHJFt) and anhydrous hydrofluoric acid (HF) as raw materials.

[Problems to be solved by conventional technology and invention] NF3
is a colorless gas with a boiling point of 1129°C and a melting point of 1207°C.

NF3 gas is a dry etching agent for semiconductors and CVD]1
In recent years, Nh gas has been attracting attention as a cleaning gas for various purposes, but high purity Nh gas is required for use in these applications.

However, the produced NF gas is nitrogen (N2),
Nitrogen difluoride (NJz), nitrous oxide (N20), carbon dioxide (Co□), oxygen difluoride (OF), carbon tetrafluoride (cpa), oxygen (0□), unreacted fluoride It contains a relatively large amount of impurities such as hydrogen (liver), and the same is true for the NFs gas produced by the molten salt electrolysis method that is the subject of the present invention.Therefore, high purity NFs gas for the above purpose is obtained. Purification is necessary for this purpose.

As a purification method for removing these impurities from NFz gas, the following method is known.

That is, 1) NJt is KI, NazS, NazSz03
A method of contacting with an aqueous solution such as (J, Massonne
+ Chemie Ingenur Technir (Chew,
Ing, Techn, )41°(12), 695.
(1969) ) or a method of contacting with metal at a temperature of 148.9 to 537.8°C (Special Publication No. 15081 of 1981)
It can be removed with etc. 2) HF can be easily removed by heating it to around 100°C and bringing it into contact with NaF. 3) OF, is NazSzOs, Kl
, Na2SO2, ■, NazS, etc. can be removed by contacting with an aqueous solution. 4) N20 and CO□
Components with relatively high boiling points such as
hew, Eng.

84, 116. (1977) etc.]. 5) Low boiling point components such as N2 and 0□ can be removed by cooling to a temperature of -150°C to -190°C to liquefy Nh.

However, CF4 is not removed by each of the above methods,
An effective method for removing it is not yet known, and CF
has a boiling point of 1128°C, which is very close to the boiling point of NF3, so it is impossible to separate it even by cryogenic distillation of NF3.

Now, in the electrolysis of molten salt, which is the object of the present invention, carbon is the anode material that has the best corrosion resistance. However, when electrolysis is performed using carbon as an anode, a large amount of CFa is generated. As mentioned above, this is extremely inconvenient for producing high purity NFs gas. Therefore, when producing high-purity NF3 gas, nickel (Ni) or an alloy containing nickel as a main component, or platinum (Pt) or an alloy containing platinum as a main component is used as an anode.

NF3 gas is generated from the anode by electrolysis, and at the same time hydrogen (N2) gas is generated from the cathode. The NF3 gas and N2
When gases are mixed, they ignite with a small amount of ignition energy and cause a violent explosion.In the NFs-L-Nz ternary system, the explosion range of the mixed gas is NFj gas at 10% by volume or more,
There is a formula in which N2 gas has a wide range of 5% by volume or more, and if even a small amount of NF3 gas and N2 gas are mixed in an electrolytic cell, an explosion will occur.

Most of the mixing of NFs gas and N2 gas occurs during electrolysis. The main reason for this is that the partition wall is polarized and gas is generated on the partition wall, or the pressure balance between the anode gas phase and the cathode gas phase is disrupted, causing a deviation in the liquid level, and the gas on the high pressure side passes through the lower end of the partition wall. It flows into the low pressure side, etc. However, the above phenomenon cannot occur during electrolysis suspension when NF3 gas and N2 gas are not produced.

It is known that when Ni or PT is immersed in the molten salt, natural corrosion occurs and N2 gas is generated. However, the amount generated is usually very small.

For this reason, NF remaining in the anode side gas phase after stopping electrolysis,
In common sense, it is almost impossible to imagine that gas and N2 gas generated by natural corrosion would mix and cause an explosion.

However, the present inventors encountered a phenomenon in which the hydrogen concentration in the gas phase on the anode side fell within the explosive range while electrolysis was stopped. The cause of this was not that N2 gas in the gas phase on the cathode side entered the anode side, but as a result of investigating the cause, we found that hydrogen was generated due to natural corrosion in nickel or molten salt of the anode mainly composed of nickel. Immediately after stopping electrolysis, the speed (amount generated) is abnormally fast (a lot), and H! It was determined that the gas concentration could be within the explosive range.

There is a method of expelling the gas in the anode gas phase using an inert gas at the same time as electrolysis is stopped, but this method is an effective measure when electrolysis is stopped for a relatively long time. However, since an inert gas is vented into the anode gas phase, electrolysis cannot be restarted until the anode gas phase has been expelled, so if a short electrolysis stop is required, inert gas The method of expelling the electrolyte is not preferable because it lowers the operating rate of the electrolytic cell.

[Means to solve the problem]

In view of this situation, the inventors of the present invention have conducted extensive studies and have determined that the anode potential is set immediately after electrolysis is stopped to a potential at which no NF3 gas is generated in the molten salt and no N2 gas is generated after electrolysis is stopped. By doing so, it was discovered that it was possible to prevent the generation of N2 gas at the anode during electrolysis suspension, and the present invention was completed.

That is, the present invention is a method for producing nitrogen trifluoride gas by molten salt electrolysis using ammonium fluoride or/and acidic ammonium fluoride and anhydrous hydrofluoric acid as raw materials, in which the generation of nitrogen trifluoride gas is stopped. The present invention relates to a method for producing nitrogen trifluoride gas, which comprises applying a voltage continuously or intermittently between an anode and a cathode.

[Detailed disclosure of the invention] The present invention will be explained in detail below.

The present invention is carried out immediately after stopping the generation of nitrogen trifluoride gas ([stopping decomposition]) and before restarting the generation of nitrogen trifluoride gas (resuming decomposition). , it is necessary that electrolysis has been carried out using a predetermined anode in a predetermined electrolytic cell until just before the start of the test.

FIG. 1 is a diagram showing an example of an electrolyzer. During electrolysis, NF and gas are generated from the anode 3 and H2 gas is generated from the cathode 4. However, if NF3 gas and H2 gas are mixed, it will explode. In order to prevent this mixing, a partition wall 5 is provided between the anode 3 and the cathode 4. Further, during electrolysis, an inert gas such as N or gas may be introduced as a carrier gas to the anode 3 side and the anode 4 side, respectively.

In order to produce high-purity NFs gas, Ni or a Ni-based alloy; Monel or Inconel is used for the anode. Pt or an alloy mainly composed of pt can also be used, but
The proportion of current used for dissolving platinum (electrode dissolution current efficiency) of the current used for electrolysis reaches 10% or more, making it undesirable to use in industrial electrolysis. For other metals such as (Cu) and titanium (Ti), the t-flow rate of electrode melting is almost 100%, making them completely unsuitable for use. Monel and Inconel are also not impossible to use, but - for boat fishing. It is preferable to use Ni with a purity of 90% or more.There are no particular restrictions on the size and shape of the anode, but the weight per electrode must be considered from the viewpoint of handling. .

Since the cathode 4 is electrically protected from corrosion during electrolysis, the material can be easily selected. Fe is generally available at low cost. Although Fe was used in the implementation of the present invention, this does not limit the use of other materials. Regarding the size and shape of the cathode 4, Fe is used. Although not particularly restricted, in order to minimize the voltage drop at the cathode 4, it is desirable to increase the cathode surface area as much as possible.

The molten salt is NH4HF! Alternatively, NHaF and HF are mixed and prepared in an electrolytic bath. When NH, OF, and HP are used as raw materials,
Mix and prepare at a weight ratio of 57: (10 to 30). The obtained molten salt is preferably maintained at a temperature in the range of 100 to 150°C.

The raw material N11.1 (F2) is hygroscopic and contains water. Also, the molten salt itself is extremely hygroscopic, so the prepared molten salt also contains 1 to 2% water. When a molten salt containing such moisture is electrolyzed, OL gas and H2 gas are generated as by-products due to the influence of this moisture, and these OF2 gas and H2 gas are mixed into the NFs gas generated from the anode, causing an explosion.

Therefore, in the production of NFs gas by molten salt electrolysis, so-called dehydration electrolysis, in which a current density lower than the current density during electrolysis (main electrolysis) is passed in advance, is essential. to move to. When the water content in the molten salt is 1.0% or less, the amount of by-products of OF, gas, and H2 gas is small, and there is no danger of explosion (Patent Application No. 01-3).
34811), dehydration electrolysis is performed until the water content in the molten salt becomes 1.0% or less.

Regarding the fixation of moisture, the Karl Fischer method, which is a simple moisture measurement method, can be applied if the moisture content is up to about 0.2%.

When the dehydration electrolysis is completed, the current density is increased and the main electrolysis begins. By continuing to solve 1, NFS gas is generated from the anode, the molten salt is consumed and reduced, and eventually passes through the lower end of the partition wall 5 between the anode 3 and cathode 4, flows into the low pressure side, and becomes NF3 gas and H2 gas. may mix and cause an explosion.

Therefore, molten salt must be replenished as appropriate.

There are two replenishment methods: one is a batch method in which electrolysis is stopped when the molten salt becomes small, the raw materials are prepared again, and then dehydration electrolysis and main electrolysis are repeated, and the other is intermittently or continuously during electrolysis. There is a way to replenish molten salt. The latter problem is that there is a risk that the water content of the molten salt in the electrolytic cell may exceed 1.0% due to the moisture in the molten salt to be replenished. Continuous supply is fully possible as long as the water content does not exceed 2%.

As a method of supplying the power necessary to carry out the present invention, if the DC power supply used for electrolysis is capable of constant voltage control, the specified voltage may be applied by the DC power supply, or by applying a predetermined voltage separately from the DC power supply. Uses a DC power supply that can be controlled at constant voltage.
etc. are possible.

Application here means applying a voltage from the outside using a power supply device.

To apply a voltage continuously, it is sufficient to carry out the procedure as described above, but to apply a voltage intermittently, it is necessary to further control the DC power supply. A common method is to input a voltage signal from a commercially available function generator 18 to a DC power source and to make it interlock with this voltage signal.

Possible methods for stopping electrolysis include gradually reducing the current or cutting it off instantaneously. In either method, no abnormality occurs the moment electrolysis is stopped, so the method for stopping electrolysis is not particularly limited.

To carry out the present invention, it is only necessary to apply a voltage in the manner described above after stopping the electrolysis, and the applied voltage is such that the potential of the anode relative to the cathode is in the range of 0 to 2.8 . In an electrolytic cell in which a plurality of anodes and cathodes are arranged, when a plurality of cathodes and anodes are connected in series, the voltage is multiplied by the number of the plurality of electrodes.

If the applied voltage exceeds 2.8■, theoretically NF3
This is disadvantageous because gas can be generated.

In reality, NF3 is applied at 2.8V due to various overvoltages.
Although gas generation does not occur, unnecessary voltage is undesirable because it results in energy loss. If it is less than 0V, it is disadvantageous because the voltage is close to the voltage at which hydrogen generation starts on the anode.

When a voltage is applied intermittently, if the non-current period becomes significantly long, H2 gas will be generated from the anode, which is not preferable. Further, even if the energizing time is shorter than the non-energizing time, H2 gas is generated, which is not preferable.

When applying a voltage intermittently, the non-current time must be less than 10 seconds and the current time must exceed 0.5 seconds.

In parallel with the implementation of the present invention, it is more preferable for safety to purge the gas phase portion of the electrolytic cell with an inert gas.

However, in this case, a carrier gas supply port 6 is required at least in the anode side gas phase section 9.

Inert gases include nitrogen (N2), helium (He),
Although argon (Ar) and the like are easily available, N2 gas is suitable because it is easy to obtain and inexpensive.

In addition, H22 due to natural corrosion not immediately after stopping electrolysis.
It goes without saying that implementation of the present invention also contributes to improved safety with regard to gaseous phenomena.

(Example) Hereinafter, the present invention will be explained in more detail with reference to Examples. Note that in the following, % represents a capacity standard unless otherwise specified.

Example 1 N by molten salt electrolysis using the electrolyzer shown in Figure 1.
Fs gas was produced.

That is, acidic ammonium fluoride (NH4F.H
Molten salt 2 was prepared by charging 24 kg of F) and gradually adding 8.5 kg of anhydrous HF while raising the temperature to 120°C (HF/NH4F molar ratio 2.0, water content 1). .8% by weight).

Thereafter, a current of 20 amperes (A) was passed from the anode 3 to the cathode 4 to perform dehydration electrolysis for 72 hours. Since the water content of molten salt 2 at this point was 0.32% by weight,
The current was increased to 80 A and the main electrolysis started.

As the main electrolysis continues, the molten salt 2 in the electrolytic cell 1 decreases, so the IP/NI1. Molten salt 12 having an F molar ratio of 2.0 and a water content of 1.8% by weight was intermittently replenished into the electrolytic cell 1 in an amount of 0.5 kg every 12 hours, and the main electrolysis was carried out for 300 hours.

As a method for applying voltage while electrolysis is stopped, an auxiliary DC 11t capable of constant voltage control is used separately from the DC power supply 16 for electrolysis.
Ifl17 was used, a voltage signal from a commercially available function generator 18 was input to the DC power supply, and a method was adopted in which the voltage signal was linked to this voltage signal.

In order to clarify the implementation status of the present invention, electrolysis was stopped by cutting off the current instantaneously (within 1 second). Anode 3
The potential difference between the cathode 4 and the cathode 4 is about 4 to 5 μm during electrolysis, but the potential difference decreases rapidly as soon as the current is interrupted. When the potential difference becomes approximately 1V, connect the DC power source to the electrolytic DC power source 16.
Therefore, the switch was made to the auxiliary DC power supply 17.

The applied voltage was set to 1.sup. with a voltmeter 19, and the current was applied continuously for about 2 hours. During this period, gas in the electrolytic cell was sampled every 30 minutes and analyzed by gas chromatography.

In addition, in consideration of the specific gravity of the gas, the gas sampling positions were set at the upper part of the anode side gas phase part and the lower part of the anode side gas phase part (near the liquid level).

As a result, as shown in Table 1, no generation of N2 gas was observed.

Examples 2 to 3 The same procedure as in Example 1 was carried out except that the voltage applied in Example 1 was changed to 0.5V, 2V.

As a result, as shown in Table 1, no generation of N2 gas was observed.

Example 4 The energization performed in Example 1 was not performed continuously, but the applied voltage was 1■, the energization time was 0.5 seconds, and the non-energization time was 0.
．． The same procedure as in Example 1 was carried out except that energization was repeated intermittently every 5 seconds.

As a result, as shown in Table 1, almost no N2 gas was generated, and it remained within the safe range even after 120 minutes.

Example 5 The energization performed in Example 1 was not carried out continuously, but the applied voltage was 1■, the energization time was 1 second, and the non-energization time was 0.5
The same procedure as in Example 1 was carried out except that energization was repeated intermittently every second.

As a result, as shown in Table 1, almost no N2 gas was generated, and it remained within the safe range even after 120 minutes.

Comparative Example 1 The same procedure as in Example 1 was carried out except that the voltage applied in Example 1 was changed to -1 V.

As a result, as shown in Table 2, an increase in N2 gas generation was observed, and after 30 minutes, the N2 gas concentration in the upper part of the gas phase on the anode side approached the explosive range, so the inert gas supply port 6 More Nt gas was vented to keep it out of the explosion range.

Comparative Example 2 The same procedure as in Example 1 was carried out, except that energization was not carried out continuously in Example 1, but the applied voltage was 1 V, and the energization was carried out intermittently with a energization time of 10 seconds and a non-energization time of 10 seconds.

As a result, as shown in Table 2, an increase in the generation of 82 gas was observed, and after 90 minutes, the N2 gas concentration in the upper part of the gas phase on the anode side approached the explosive range, so the inert gas supply port 6 N2 gas was then vented to remove the area from the explosion range.

Comparative Example 3 In Example 1, the current was not applied continuously and the current application time was 0.
．． The same procedure as in Example 1 was carried out except that the energization was conducted intermittently for 5 seconds and the non-energization time was 1 second.

As a result, as shown in Table 2, an increase in N2 gas generation was observed, and after 90 minutes, the N2 gas concentration in the upper part of the anode side gas phase approached the explosive range, so the inert gas supply port 6 N2 gas was then vented to remove the area from the explosion range.

〔Effect of the invention〕

As explained in detail above, the present invention provides NH4F-IP
When producing NF3 gas by electrolyzing molten salt,
This is an extremely effective method for preventing the mixing and explosion of H2 gas and NF3 gas remaining in the gas phase on the anode side due to natural corrosion that occurs when electrolysis is stopped.

Therefore, when electrolysis is interrupted for a long time or temporarily and then restarted, there is no gas other than Nh gas in the gas phase on the anode side, and there is no risk of explosion even if electrolysis is restarted. This is advantageous because electrolysis can be restarted smoothly in a short time.

Furthermore, the effect is remarkable even when electricity is applied intermittently, and it is expected to reduce power consumption and consumption due to electrode melting.

It goes without saying that the method of the present invention can also be applied to a method for producing NF and gas by molten salt electrolysis using ammonia and anhydrous hydrofluoric acid as raw materials.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an electrolysis device used in Examples and Comparative Examples. In the figure, ■---Electrolytic cell, 2-3-m-anode, 5-m-partition wall, 6---Inert gas supply port 7--NF3 gas outlet pipe, 8-1182 gas outlet pipe, 9 10--1 heating device, 12-m-molten salt, 13 14--1 valve, 15 16--DC power supply for electrolysis 17--auxiliary DC power supply, 18 19-m-voltmeter. Molten salt, cathode, anode side gas phase, raw material tank, m-raw material supply pipe, heating device, function generator

Claims (1)

[Claims] 1) In the method for producing nitrogen trifluoride gas by molten salt electrolysis using ammonium fluoride or/and acidic ammonium fluoride and anhydrous hydrofluoric acid as raw materials, when stopping the generation of nitrogen trifluoride gas, the anode and a method for producing nitrogen trifluoride gas, which comprises continuously or intermittently applying a voltage between electrodes of a cathode.