US3623964A - Process for the manufacture of sulfur hexafluoride - Google Patents

Abstract

Elemental sulfur is introduced into a cell containing an electrolyte of the composition KF.nHF (wherein n 0.9 to 3), and electrolysis is carried out with use of a carbon anode to produce sulfur hexafluoride.

Description

United States Patent [72] Inventors l-liroshl Ukihashl Tokyo; Yoshio Oda, Yokohama; Manabu Suhara,

I Yokohama, all of Japan [21 Appl. No. 50,892

[22] Filed June 29, 1970 [45] Patented Nov. 30, 1971 [73] Assignee Asahl Glass Co., Ltd.

Tokyo, Japan [32] Priority July 3, 1969 [33] Japan [54] PROCESS FOR THE MANUFACTURE OF SULFUR [50] Field of Search 204/60, 61. 59. I01

[56] References Cited UNITED STATES PATENTS l ,653,605 12/1927 Ashcroft 204/61 2,717,235 9/l955 Prober 204/59 2,937,123 5/l960 Muetterties 204/59 3,146, I 79 8/1964 Davies 204/60 3,345,277 l0/l967 Ashleyetal.

Primary Examiner.|ohn H. Mack Assistant E.\'aminer--D. R. Valentine Au0rneyKelman and Berman ABSTRACT: Elemental sulfur is introduced into a cell containing an electrolyte of the composition KF'nHF (wherein n=O.9 to 3 and electrolysis is carried out with use of a carbon anode to produce sulfur hexafluoride.

PROCESS FOR THE MANUFACTURE OF SULFUR HEXAFLUORIDE BACKGROUND OF THE INVENTION This invention relates to a process for the production of sulfur hexafluoride. More particularly, the invention relates to a process for producing sulfur hexafluoride by electrolysis.

DESCRIPTION OF THE PRIOR ART It is well known to use sulfur hexafluoride as a gaseous insulating medium for sealed electrical apparatus such as transformers.

In the hitherto-known methods for the manufacture of sulfur hexafluoride, sulfur, sulfur dichloride, sulfur monochloride, carbon disulfide or hydrogen sulfide is introduced into anhydrous hydrogen fluoride and nickel anodes are used to effect the necessary electrolysis (See the specifications ofU.S. Pat. Nos. 3,345,277 and 2,717,235).

Anhydrous hydrogen fluoride has a boiling point of l9 C. and the known methods require low-temperature operation at 30 C. and C.

Furthermore, because of the poor electric conductivity of anhydrous hydrogen fluoride, it is necessary to employ a conductive additive, for example, potassium fluoride. The potassium fluoride causes considerable dissolution of the nickel anode and an increase in current density accelerates this dissolution, causing a substantial loss of nickel.

BRIEFSUMMARY OF THE INVENTION It is an object of this invention to provide an electrolytic process for producing pure sulfur hexafluoride in commercial quantities from elemental sulfur and a KF-nHF electrolyte.

Another object is to provide an electrolytic process for producing sulfur hexafluoride in a high yield without a substantial loss of the anode.

Other objects and advantages of the present invention will become apparent to those skilled in the art from the following description and disclosure.

DETAILED DESCRIPTION OF THE INVENTION, INCLUDING PREFERRED EMBODIMENTS It has been found that electrolysis of an electrolyte of KF-nHF (wherein n equals 0.9 to 3.0) in the presence of elemental sulfur and with a carbon anode yields commercial quantities of sulfur hexafluoride refrigeration equipment and substantially without dissolution of the anode.

In the present invention, a mixture of potassium fluoride and hydrogen fluoride in a mole ratio of HFzKF of 0.9:] to 3: I does not liberate substantial amounts of hydrogen fluoride vapor near its melting point and has such a high electric con ductivity that the voltage necessary for electrolysis may be comparatively low.

A carbon anode is substantially immune to dissolution in the presence of potassium fluoride.

If the HF:KF ratio is less than 0.9:1, the vapor pressure of hydrogen fluoride over the mixture will be too high and the melting point of the mixture will also be too high for a commercially practical process. Exceeding the limit of 3:! will also cause a significant increase in the vapor pressure of hydrogen fluoride over the mixture and loss of hydrogen fluoride.

An electrolyte wherein the ratio of HF to KF is 1.8 to 2.2 has a particularly favorable melting point and vapor pressure.

To carry the invention into practice, elemental sulfur and the electrolyte are introduced into an electrolytic cell. The electrolyte is first fed into the cell, and is followed by elemental sulfur added to the anolyte. Hydrogen fluoride gas is fed to the electrolyte to maintain the original composition.

Elemental sulfur may be used in the solid form, such as colloidal sulfur, powdery sulfur or granular sulfur, or in the molten state. Elemental sulfur is substantially insoluble in the KF'nHF melt. The electrolyte temperature is selected between the melting point of the mixture and the melting point plus 50 C.

If the mixture is heated to an excessively high temperature, hydrogen fluoride is evaporated. A temperature of C. to I30 C. is preferred when a mixture of 1.8 KF to 2.2 HF is to be melted.

At a voltage of 5 to I5 volts and a current density of 0.5 to 25 a./dm.", sulfur hexafluoride is produced at a current eff ciency of about 40 to 92 percent.

At 6 to 12 volts and a current density of 4 to 12 a./dm. based on the effective anode surface, the process of this invention gives sulfur hexafluoride continuously at a current efficiency of 65 to 92 percent.

At a voltage of less than 5 volts and a current density below 0.5 a./dm.', substantially no sulfur hexafluoride will be produced. The output of lower fluorinated sulfur compounds is increased and a larger anode area will have to be provided. At a voltage above 15 volts and a current density beyond 25 a./dm. there may be formed an insulating fluorinated carbon layer on the surface of the carbon anode and make the electrolysis substantially impossible.

A small amount, for example, I to 3 weight percent of the KF-nHF mixture, of lithium fluoride, aluminum fluoride, nickel fluoride, or sodium fluoride, is effective in preventing the coating of the anode at not more than 25 a./dm.

The present invention will be further described in the following examples which are illustrative and by no means limitative thereof. In those examples, two different electrolytic cells were employed.

Type I Electrolytic Cell This electrolytic cell consists of a cylindrical vessel made of iron, 240 mm. in diameter and 550 mm. in depth, and an iron cover. The vessel serves as a cathode. In the center of the cover, there is provided an anode of amorphous carbon, I00 mm. in diameter, 480 mm. in length and I2 dm. in effective surface area. The cover is also provided, about the carbon anode, with a cylindrical skirt of iron, 160 mm. in diameter, I50 mm. in length and 2 mm. in thickness, for separating the gases evolved from both electrodes.

The cover is further provided with holes through which elemental sulfur is fed into the anode chamber.

Type II The electrolytic cell consists of a cylindrical vessel of iron, I00 mm. in diameter and I50 mm. in depth, and a cell cover of iron. The vessel serves as a cathode. In the center of the cover, there is provided an anode of amorphous carbon, 40 mm. in diameter, I00 mm. in length and LI drn. in effective surface area. The cover is also provided, about the carbon anode, with a cylindrical skirt of iron for separating the gas evolved from the electrodes. The skirt measures 70 mm. in diameter, 60 mm. in length and 1 mm. in thickness. The cover is further provided with a pipe for admitting elemental sulfur to the anode chamber. The cell is provided, at the bottom, with a pipe for admitting an inert gas and for thereby sweeping the sulfur hexafluoride from the cell.

Acid potassium fluoride (KHF and anhydrous fluoride gas were admitted into the cell to prepare an electrolyte. The electrolyte was held at a constant temperature (about C. and in order to remove water from the electrolyte, electrolysis was carried out for 24 hours at a low current density (OJ-0.3 a./dm. After this preliminary electrolysis, elemental sulfur was admitted into the anode chamber, an inert gas was introduced, and the product of electrolysis was withdrawn thereby as a gas from the electrolytic cell. The gas contained a small amount of hydrogen fluoride and was passed through a tube packed with sodium fluoride pellets.

It was washed free of byproducts, SO F SOF etc., with water and, then, with aqueous alkali solutions. The traces of S F S-,F,,,O and other compounds occurring in this gas were thermally decomposed and, finally, the gas was passed through an alumina-filled tube to remove any residual trace impurities.

The final gas was sulfur hexafluoride, which was analyzed by gas chromatography and by infrared and mass spectroscopic methods.

Control Example 1n the cell of type ll, the carbon anode was replaced with a Ni plate anode, and electrolysis was carried out in the following manner. To the anode chamber containing 1.6 kg. of KF-ZHF melt, 15 g. colloidal sulfur was added. With the electrolyte being maintained at a temperature of 90 C., electrolysis was conducted at a current density of 8.14 a./dm. The cell voltage was 5.9 volts. The gas evolved from the anode chamber contained such compounds as SF 6, SOP- SO F S F etc.

After 3 hours of electrolysis, the nickel anode was removed from the cell and examined for weight loss. It was dissolved at a rate of 3.8 g./l ampere-hours. Electrolysis was further continued, whereupon the formation of a sludge in the cell bottom was observed.

EXAMPLE 1 (CELL TYPE I) lnto the electrolytic cell, 41.50 kg. of an electrolyte of KF-Z- ISHF containing 1.1 weight percent of LiF was admitted, and 100 g. of colloidal sulfur was introduced into the anode chamber. Electrolysis was carried out at an electrolyte temperature of 98C. and a current density of 5.0 a./dm.". The cell voltage was 8.93 volts. After 3 hours of electrolysis, the gas evolved in the anode chamber was analyzed. The result was: SF 93%; SO F 1%; SOF +SF 1%; S F 1%; S F D 0.5%; C0 1%; F 1%; CF 0.5%; air, 0.5%. On further electrolysis, the amounts of SOF SO F CO and S F O became traces. After purification, the SP was 99 percent pure, containing 0.5CF and 0.5% air. The current efficiency of sulfur hexafluoride formation was 92 percent. The conversion of hydrogen fluoride to sulfur hexafluoride was in excess of 95 percent.

EXAMPLE 2 (CELL TYPE ll) lnto the electrolytic cell, 1.6 kg. of an electrolyte of KF'ZHF was admitted, and g. colloidal sulfur was introduced into the anode chamber. With the introduction of nitrogen gas at a rate of 2lN ml./min., electrolysis was conducted at an electrolyte temperature of 98 C. and a current density of 8.9 a./dm. The initial cell voltage was 7.60 volts. The concentration of sulfur hexafluoride in the gas evolved from the anode chamber was 48 percent. The balance of the gas was predominantly nitrogen and, to a minor part, fluorine. The current efficiency of sulfur hexafluoride formation was 85 percent. During 3 hours of electrolysis, the carbon anode suffered substantially no dissolution.

EXAMPLE 3 (CELL TYPE 11) lnto the electrolytic cell, 1.6 kg. of an electrolyte of KF'ZHF was admitted, and 15 g. colloidal sulfur was introduced into the anode chamber. With the introduction of nitrogen gas at a rate of 21N ml./min., electrolysis was carried out at an electrolyte temperature of 108 C. and a current density of 4.1 a./dm.*. The initial cell voltage was 6.75 volts and 6.64 volts after 4 hours. The current efflciency of sulfur hexafluoride formation was 68 percent.

EXAMPLE 4 (CELL TYPE 11) Into the electrolytic cell, 1.6 kg. of an electrolyte of KF'2HF was admitted, and 15 g. sulfur was introduced into the anode chamber. With the introduction of nitrogen gas at a rate of 14.5N ml./min., electrolysis was conducted at an electrolyte temperature of 108 C. and a current density of 0.5 a./dm.. The cell voltage was 5.53 volts at the start of electrolysis, while it was 5.54 after 5 hours. The current efficiency of sulfur hexafluoride formation was 45 percent.

EXAMPLE 5 (CELL TYPE ll) lnto the electrolytic cell, 1.6 kg. of an electrolyte of KF'LBHF was admitted, and 15 g. sulfur was introduced into the anode chamber. With the introduction of nitrogen gas at a rate of 20N ml./min., electrola'sis was conducted at an electrolyte temperature of and a current density of 1.4

a./dm. The cell voltage was 5.85 volts at the start of electrolysis, and 5.81 volts after 5 hours. The current efficiency of sulfur hexafluoride formation increased with time and reached 63 percent in 5 hours.

We claim:

1. In a process of manufacturing sulfur hexafluoride by electrolysis between an anode and a cathode of an electrolyte essentially consisting of potassium and hydrogen fluoride and containing elemental sulfur in the anolyte, and by recovery of the hexafluoride formed from the electrolyte, the improvement which comprises:

a. the mole ratio of HF to KF in said electrolyte being 0.9:1

to 3:1 and b. the effective surface of the anode essentially consisting of carbon.

2. In a process as set forth in claim 1, said carbon being amorphous carbon.

3. In a process as set forth in claim 1, the temperature of said electrolyte being above the melting point thereof by not more than 50 C. and the anode current density being between 0.5 2and 25 a./dm..

4. In a process as set forth in claim 3, the voltage between said anode and said cathode being 5 to 15 volts.

5. In a process as set forth in claim 4, said current density being 4 to 12 a./dm. and said voltage being 6 to 12 volts.

6. In a process as set forth in claim 1, said mole ratio being 1.8:1 to 2.221.

7. [n a process as set forth in claim 6, the temperature of said electrolyte being 80 C. to C., the voltage between said anode and said cathode being 5 to 15 volts, and the anode current density 0.5 to 25 a./dm.

8. In a process as set forth in claim 7, said voltage being 6 to 12 volts, and said current density 4 to 12 a./dm.

Claims (7)

1. 2. In a process as set forth in claim 1, said carbon being amorphous carbon.
2. 3. In a process as set forth in claim 1, the temperature of said electrolyte being above the melting point thereof by not more than 50* C. and the anode current density being between 0.5 2and 25 a./dm.2.
3. 4. In a process as set forth in claim 3, the voltage between said anode and said cathode being 5 to 15 volts.
4. 5. In a process as set forth in claim 4, said current density being 4 to 12 a./dm.2, and said voltage being 6 to 12 volts.
5. 6. In a process as set forth in claim 1, said mole ratio being 1.8:1 to 2.2:1.
6. 7. In a process as set forth in claim 6, the temperature of said electrolyte being 80* C. to 130* C., the voltage between said anode and said cathode being 5 to 15 volts, and the anode current density 0.5 to 25 a./dm.2.
7. 8. In a process as set forth in claim 7, said voltage being 6 to 12 volts, and said current density 4 to 12 a./dm.2.