JP2914698B2 - Method for producing nitrogen trifluoride by molten salt electrolysis - Google Patents

Description

The present invention relates to a method for producing nitrogen trifluoride (NF ₃ ).

Nitrogen trifluoride is a colorless gas with a boiling point of -129 ° C,
As the plasma cleaning gas device, also silicon, polysilicon, a Si ₃ N _4, WSi _2, a dry etching gas and excimer laser gas such as MoSi _2, also NF ₃ moderately as less active fluorine source than fluorine It is an industrially important gas used conventionally as a fluorine source in the preparation of fluoroolefins and as an oxidizing agent for high-energy fuels.

Various methods have been proposed as a conventional art method for producing NF ₃ from the problems conventionally. For example, U.S. Pat.
How to obtain NF ₃ discloses by passed through a plasma arc, also introducing a gas fluorine post-arc zone close to the anode at a temperature above.

A method of producing NF ₃ by gas-solid reaction at room temperature or a solid ammonium complex and elemental fluorine of metal fluoride is disclosed in Japanese Patent Publication Sho 62-21724.

Also, Japanese Patent Publication No. 55-8926 discloses that NF ₃ is produced by reacting gas-phase fluorine with a liquid-phase ammonium oxyfluoride maintained at a temperature from a melting temperature to substantially 400 ° F. or less. How to do is described.

The Journal of American Chemical Society (J. Am. Chem. Soc.), Vol. 82 , p. 5301 (1960) contains a molar ratio of ammonia to fluorine, about 1: 1 to 2: 1.
It discloses the synthesis of NF ₃ by the direct vapor phase fluorination of ammonia with. Although the details of the reaction conditions are not described, a yield of NF ₃ of about 10 to 25% based on fluorine is reported.

In addition to the various methods described above, a molten salt electrolysis method of ammonium acid fluoride is known, for example, U.S. Pat.
235,454 and JP-A-1-122910. In this molten salt electrolysis method, carbon such as porous carbon, graphite-free carbon and amorphous carbon is usually used for the anode, and iron is used for the cathode. However, when carbon is used for the anode, carbon tetrafluoride (CF ₄ ) is generated by the reaction between carbon and fluorine, and this is mixed into NF ₃ . The boiling points of CF ₄ and NF ₃ are close (CF ₄ : −128 ° C., NF
₃ : -129 ° C), separation by rectification is not possible. In addition, CF ₄ is chemically very stable, so it is extremely difficult to decompose,
If you try to break it down, NF ₃ will break down first.
Adsorption separation is also difficult.

For these reasons, it is extremely difficult to produce high-purity NF ₃ containing no CF _4, and attempts have been made to use a metal such as nickel for the anode. However, when a metal anode is used, CF ₄ is not generated, but the metal is dissolved in the anode, and in the case of nickel, a salt such as nickel fluoride is generated, which precipitates. If this salt accumulates at the bottom of the electrolytic cell, the electrode and the tank will short-circuit, the tank will exhibit anodic behavior, gas will be generated from the tank, and there is a risk of explosion. Therefore, it is necessary to dismantle the electrolytic cell as needed to remove the precipitate. This operation is complicated and makes continuous operation impossible.

Object This invention purpose is to eliminate the drawbacks of the molten salt electrolysis is to produce efficiently NF _3.

The present invention relates to a method for producing nitrogen trifluoride by molten salt electrolysis using a metal anode, wherein a sludge receiving recess is provided on the bottom surface of the electrolytic cell, and a metal salt generated by melting the metal anode is formed. Let it settle in the sludge receiving recess,
This is a method characterized in that it is continuously removed outside the tank.

A specific example for implementing the present invention is shown in FIGS. FIG. 1 is a longitudinal sectional view of the electrolytic cell, and FIG. 2 is a transverse sectional view of the electrolytic cell.
This bottom part is made into a semicircle, and there is a screw feeder 6
Is attached.

Screw feeder is used to prevent the surface of the bath
The rotation speed should be 10 rpm or less. Considering the sludge removal effect
It is necessary to be 1 rpm or more. Therefore, a preferable rotation speed is 1 to 10 rpm.

The molten salt containing sludge is sent to a sludge separator 10 or 11 by a pump 9, where the sludge is separated and returned to the tank.

By the above method, sludge can be continuously removed, and NF ₃ can be continuously, safely and efficiently produced.

The molten salt used in the present invention is NF ₄ F-HF or KF-NH ₄ F
The composition ratio of NH ₄ F: HF is 1: 0.5 to 5 (molar ratio), and the composition ratio of KF: NH ₄ F: HF is 0.5 to 5: 1: 0.5 to 5 (molar ratio). is there. Iron is used for the cathode.

 The bath temperature may be in the usual range of 115 to 130 ° C.

Examples Hereinafter, the present invention will be described in more detail with reference to Examples.

NF ₃ was produced by molten salt electrolysis using the electrode tank shown in FIGS.

The anode is made of nickel and the cathode is made of iron. In this electrolytic bath, acidic potassium fluoride (KF · HF), ammonium fluoride (NH ₄ F), and hydrogen fluoride (HF) were mixed at a molar ratio of KF: NH ₄ F: HF of 1:
The mixture was charged so as to be 1: 3, and the temperature was increased to 125 ° C. Next, dehydration electrolysis was performed at a low current density for about 70 hours to remove water in the electrolytic bath. After the completion of the dehydration electrolysis, the rotation speed of the screw feeder was set to 2 rpm, and the electrolytic bath containing the sludge component was sent to the filter by a pump to remove the sludge component, and the electrolytic bath substantially free of sludge was returned to the electrolytic cell. The pump is a variable flow type, and the discharge rate is about 200 /
hr.

While introducing nitrogen gas into the anode chamber at a flow rate of 150 ml / min, a current of 80 amperes (A) was passed from the anode to the cathode to perform electrolysis. The gas from the anode chamber is sent to the alkaline washing tower,
Then, the gas amount was measured by a wet gas meter. The gas composition was analyzed using gas chromatography.
As a result, the gas amount was 250 to 270 ml / min, and the average gas composition was as follows.

O ₂ : 1.212 Area percent N ₂ : 58.237 〃 NF ₃ : 40.305 〃 CF ₄ : Not detectable 〃 NF: 0.002 〃 NO: 0.030 〃 Unknown substance: 0.214 〃 During electrolysis operation, anode-tank, anode-cathode The voltage between the anode and the cathode was constantly measured by a voltmeter, but even after 700 hours from the start of electrolysis, the voltage increase between the anode and the cathode was 0.4 volt (V), and there was no problem in the continuation of electrolysis.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of an electrolytic cell used in the present invention, and FIG. 2 is a transverse sectional view thereof. 1: Anode, 2: Cathode 3: NF ₃ gas outlet, 4: Hydrogen gas outlet 5: Sludge receiver 6: Screw conveyor 7: Skirt, 8: Jacket 9: Pump 10, 11: Shows sludge separator.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Nobuhiro Ono 1497, Shibukawa-shi, Gunma Kanto Denka Kogyo Co., Ltd. Inside Shibukawa Plant (56) References JP-A-1-191792 (JP, A) JP-A-63- 274789 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C25B 1/00-9/04 C25B 13/00-15/08

Claims (1)

(57) [Claims] 1. A method for producing nitrogen trifluoride by molten salt electrolysis using a metal anode, wherein a sludge receiving recess is provided on the bottom surface of the electrolytic cell, and the metal salt generated by melting the metal anode is supplied to the sludge. Settle in the receiving recess,
A method characterized in that it is continuously removed outside the tank.