DE2033109A1 - Process for the production of sulfur hexafluoride - Google Patents

Description

Patent attorney Dipl.-Phys. Gerhard Liedl 8 Munich 22 Steinsdorfstr. 21-22 Tel:

B 4736

ASAHIGLASSCO., LTD., No. 1-2, Marunouchi 2-chome, Chiyoda-ku, TOKYO / Japan

Process for the production of sulfur hexafluoride

The invention relates to a process for the production of sulfur hexafluoride by electrolytic means. Sulfur hexafluoride is used as a gaseous insulation material for closed electrical devices, z. B. Transformers of the dry type or the steam-cooled type, capacitors, switch mechanisms, reactors, cables and other types of electrical equipment as long as they are housed in sealed footsteps.

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20331D ¹

From U.S. Patents 2,717,235 and 3,345,277 are methods for Manufacture of sulfur hexafluoride known, in which sulfur, Sehwefeidichlorid, Sulfur monochloride, carbon disulfide or hydrogen sulfide can be introduced into anhydrous hydrogen fluoride, with Nikkeianodes used to accomplish the required electrolysis.

P Anhydrous hydrogen fluoride has a low boiling point, namely

• of 19 ^° C. Accordingly, the environment must be kept at a low temperature, generally between -30 ° C. and ⁰ ° C., in the processes used previously.

Furthermore, since anhydrous hydrogen fluoride has a very low electrical conductivity, additives to increase the conductivity, e.g. B. potassium fluoride, are added. Due to the presence of potassium fluoride or lithium fluoride as a conductive additive, there is considerable dissolution of the nickel anode when current flows through it. It is also known that by increasing the current density this task W solution is further accelerated, whereby then a substantial loss per unit time obtained at the nickel anode. For the production of sulfur hexafluoride in accordance with the method according to US Pat. As already mentioned, the dissolution of the nickel anode is also considerable.

The use of sulfur dichloride as a source of sulfur is sees disadvantageous that these compounds contain chlorine atoms, which

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find their way into the end product. The use of carbon disulfide Not only does it require a large amount of electricity, it also produces carbon tetrafluoride and CF, · SF-, which is worth mentioning Way at the expense of the conversion of hydrogen fluoride into sulfur hexafluoride.

According to the present invention, an electrolytic process should now be ί

Ren for the production of sulfur hexafluoride in a high degree of purity and proposed in industrially interesting quantities from elemental sulfur The KF-nHF electrolyte has a special composition Has. In particular, the method should despite high Yield, no significant loss of anode material occurs.

The method according to the invention consists in that the electrolysis using a carbonaceous anode and an electrolyte consisting of a mixture of potassium fluoride and hydrogen fluoride where the ratio of the fluorides KF · nHF is chosen between η = 0.9 to 3.0, in the presence of elemental (| Sulfur is carried out.

The process has the advantage that industrially interesting yields. of sulfur hexafluoride with no cooling device and essentially without Dissolution of the anode can be achieved. ■ * ■

According to the present invention, the use of a mixture of potassium fluoride and hydrogen fluoride (KF · nHF, η = 0.9 to 3) has a very important background. No hydrogen fluoride vapor near the melting point comes from such a mixture in

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free quantities. In the molten state, it shows such a thing high electrical conductivity that even if a high current density it is desirable that the voltage drop due to the electrolysis is quite small. The voltage required for the electrolysis can accordingly be selected to be relatively low.

On the other hand, has the use of a carbonaceous or anode made of carbon is of great importance Carbon anode doesn't essentially dissolve at all, either. if the additive potassium fluoride is added to increase the conductivity.

The KF · nHF electrolyte according to the invention should have a composition in the range η = 0.9 to 3.0. If η is less than 0.9, then the vapor pressure of the hydrogen fluoride above the mixture becomes too high. The melting point of the mixture is also too high for the electrolytic process to be carried out smoothly on an industrial scale. If the limit of 3 is exceeded, there is also a noticeable increase in the vapor pressure of the hydrogen fluoride above the mixture and, accordingly, an increased loss of hydrogen fluoride, which is at the expense of the conversion to sulfur hexafluoride,

In particular, the use of a KF nHF electrolyte is preferred, where η is between 1.8 and 2.2. This ratio gives a melting point and a vapor pressure both of which are satisfactory for the production of the sulfur hexafluoride.

In practicing the method according to the invention

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becomes elemental sulfur and the KFnHF electrolyte (n = 0.9 to S) introduced into an electrolytic cell which has suitable dimensions. In particular, the electrolyte is first filled into the cell. Elemental sulfur is then introduced into the anode chamber of the cell. In the case of a continuous run, they are to be traversed to compensate for the used hydrogen fluoride, preferably fresh supplementary batches of hydrogen fluoride gas immediately added to the electrolyte so that the starting mixture is maintained. On the other hand, it is not advisable to compensate for the consumed Hydrogen fluoride es - add fresh mixture as the potassium fluoride in the mixture does not take part in the reaction. It would accordingly accumulate in the electrolyte, so that one outside of the favorable composition of the mixture comes.

elemental sulfur can be used in solid form, e.g. B. as colloidal sulfur, as sulfur powder or as sulfur grain or in the molten state. Elemental sulfur is essentially completely insoluble in the KF · nHF · ä melt. To achieve the desired electrolysis, the KF · nHF mixture is heated until it melts. The heating changes with the n-value of the mixture. In general, it can be said that the heating temperature should be between the melting point of the mixture, which is related to the value η, and the melting point + 50.degree.

When the mixture is heated to an excessively high temperature, the hydrogen fluoride in the mixture evaporates. It is, accordingly, a temperature in the range of 8O ⁰ C to 13O ⁰ C preferably, when a mixture of KP / 1, 8 to 2.2 HF melted

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Into the anode chamber containing such a molten mixture elemental sulfur is then introduced. It will then be with a

Voltage from 5 to 15 volts and with a current density of 0.5 to 25 A / dm Sulfur hexafluoride generated with a power output of about 40 to 92%.

Under the term "power output" as it is still used is to be understood that a current output of 100% is achieved when 1 mol of sulfur hexafluoride with an electrolysis capacity of 6 Faradays is generated. At a voltage of 6 to 12 volts and a current density of 4 to 12 A / dm, the process according to the invention provides an industrially acceptable yield of sulfur hexafluoride with continuous operation with a power output of 65 to 92%.

When electrolysis at a voltage below 5 volts and a current density is carried out below 0.5 A / dm, then substantially no sulfur hexafluoride is generated. Rather, it becomes the fit of lower ones fluorinated sulfur compounds increase, so that a larger anode area must be provided. Accordingly, this practice is economically unattractive. At a voltage above 15 volts and one

Current densities in excess of 25 A / dm can develop on the surface of the carbon anode form an insulating fluorinated carbon layer. Due to such a layer, the so-called anode effect occurs, i. electrolysis is made essentially impossible.

To prevent the anode effect, a metal fluoride from the group Lithium fluoride, aluminum fluoride, nickel fluoride or sodium fluoride in a small amount, e.g., 1 to 3% by weight of the KF · nHF mixture Find use. It should be noted, however, that at a current density

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above 25 A / dm it is essentially no longer possible to occur to prevent the anode effect,

The following examples serve to further illustrate the process according to the invention without however limiting the scope of the invention. Two different types of electrolytic cells were used in these examples. ■ '■ J

Type I of an electrolytic cell.

This electrolytic cell consists of a cylindrical iron vessel with a diameter of 240 mm and a depth of 550 mm and made of an iron cover. The cylindrical iron vessel serves as the cathode. In the middle the iron cover is an amorphous carbon anode with 100 mm

• 2

Diameter, 480 mm length and 12 dm effective surface. The iron is also provided with a cover disposed about the carbon anode around cylindrical iron Schurz, 166 mm diameter, 150 mm long and 2 mm thick, to the trodes at both electron _Λ occurring gases separate. -

The iron lid is also provided with holes through which elemental sulfur is introduced into the anode compartment.

Type II electrolytic cell .

This electrolytic cell consists of a cylindrical iron vessel - 100 mm in diameter, 150 mm in depth - and made of an iron cover. The cylindrical iron vessel serves as the cathode. In the middle

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the cell cover is an amorphous carbon anode - 40 mm diameter

knife, 100 mm length and 1.1 dm effective surface - provided. The iron cover continues with an anode around the carbon arranged cylindrical iron apron to separate the gas developing at the electrodes. The dimensions of the Iron aprons are 70 mm in diameter, 60 mm in length and 1 mm in thickness. The cover is provided with a tube through which more elementary Sulfur can be introduced into the anode chamber. The cell is also provided with a tube at its bottom to hold an inert gas to be introduced into the cell, if necessary, so that the product sulfur hexafluoride can be withdrawn from the cell »

The electrolytic processes described in the examples below were carried out in these electrolytic cells. Certain amounts of acidic potassium fluoride (KHF ₂ ) and anhydrous hydrogen fluoride gas were introduced into the cell to prepare an electrolyte. The electrolyte was kept at a constant temperature (about 90 C). In order to remove impurities and in particular water from the electrolyte, the electrolysis was carried out for 24 hours at a low current density (0.1 to 0.3 A / dm). After this preliminary electrolysis, elemental sulfur was introduced into the anode chamber. The electrolysis was carried out at a certain current density. If necessary, an inert gas was introduced into the anode chamber at a certain rate so that the product could be withdrawn from the electrolytic cell in gaseous form. The gas evolved in the anode compartment contained a small amount of hydrogen fluoride. Accordingly, in order to remove the hydrogen fluoride from the product, the gas obtained was passed through

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a tube filled with sodium fluoride tablets.

The gas produced in this way was washed with water and then with aqueous alkaline solutions to remove by-products such as SOgFg, SOF _{2, etc.} Traces of S ₀ F _1Λ , S _n F _1n O and other-

a JLU Δ XU

ren compounds as they can occur in this gas thermally decomposed. Finally the gas was passed through an i

filled pipe to remove any trace impurities. The product thus produced consisted of sulfur hexafluoride. It was determined by gas chromatography and by infrared method and masr sensespectroscopic methods analyzed.

Control example

In the cell of type U , the carbon anode has been replaced by a plate-shaped Ni anode. The electrolysis was carried out in the following manner: 15 grams of colloidal sulfur were added to the anode chamber, which contained 1.6 kg of KF.2HF melt. The E lek- g

trolyse was performed dm / at a current density of 8.14 A, while the electrolyte was maintained at a temperature of 9O ⁰ C. The cell voltage was 5.9 volts. The Gae developed in the anode chamber contained compounds such as SF _g , SOF ₂ , SO ₂ F ₃ , S ₃ Fj ₀ , etc.

After three hours of electrolysis, the Nidel anode was removed from the cell and tested for its weight loss due to corrosion . The result showed that the nickel anode is rapidly dissolved at a rate of 3.8 g / 100 ampere χ hours. The electrolysis was carried out further, the formation of sludge on the bottom of the cell being observed.

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Example 1 (cell of genus I)

41.50 kg of an electrolyte, which consisted of KF · 2-15HF with 1.1% by weight of LiF, were introduced into the electrolytic cell. In the anode compartment was filled with 100 g of colloidal sulfur. The electrolysis was carried out at an electrolyte temperature of 98 ° C and one

2
Current density of 5.0 A / dm carried out. The cell voltage was 8.83 volts. After three hours of electrolysis, the gas evolved in the anode chamber was analyzed. The result was: SF- 93%, SO ₂ F ₂ 1%, SOF ₂ + SF ₄ <1%, S ₂ F ₁₀ 1%, S ₃ F ₁₀ O <0.5% CO ₂ 1%, F ₂ 1 %, CF ₄ 0.5%, air 0.5%. After further electrolysis, the amounts of SOF ₂ , SO ₂ F ₃ , CO ₃ and S ₃ F ₁₀ O decreased to traces. After cleaning, the SF «was 99% pure, containing 0.5% CF ₄ and 0.5% air. The current output of the sulfur hexafluoride was 92%. The current outputs of hydrogen and SFg show that the conversion of hydrogen fluoride to sulfur hexafluoride was over 95%.

Example 2 (cell of genus H)

1.6 kg of an electrolyte, which consisted of KF · 2HF, were placed in an electrolytic cell. 15 grams of colloidal sulfur was added to the anode compartment. The electrolysis was carried out at an electrolyte temperature of 98 ⁰ C and a current density of 8.9 k / to the under introduction of gaseous nitrogen at a rate of 21 N ml / min. At the beginning the cell voltage was 7.6 volts. The concentration of sulfur hexafluoride in the gas evolved in the anode chamber was 48%. In addition, it existed

Gas consists mainly of nitrogen and a smaller part of gas Fluorine. The sulfur hexafluoride power output was 85%. After three hours of electrolysis, the carbon anode showed essentially no resolution yet.

Example 3 (cell of genus II)

In the electrolytic cell, 1.6 kg of an electrolyte, softer consisted of KF · 2HF. There were in the anode chamber Filled with 15 g of colloidal sulfur. The electrolysis was carried out at a Electrolyte temperature of 108 C and a current density of 4.1 A / dm with the introduction of gaseous nitrogen at a rate of 21N ml / min. The cell voltage was at the beginning 6.75 volts. After 4 hours the voltage was 6.64 volts. The sulfur hexafluoride power output was 68%.

Example 4 (cell of genus Π)

1.6 kg of an electrolyte, which *

consisted of KF · 2HF. In the anode chamber 15 g Filled with sulfur. The electrolysis was carried out at an electrolyte temperature of 108 C and a current density of 0.5 A / dm, with gaseous nitrogen at a rate of 14.5N ml / min, was introduced. At the start of the electrolysis, the cell voltage was 5.53 volts. After 5 hours it was 5.54 volts. The current output of sulfur hexafluoride was 45%.

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Example 5 (cell of genus II)

In the electrolytic cell was 1. β kg of an electrolyte, which consisted of KF x 1.8HF. There were 15 g of sulfur in the Anode chamber filled. The electrolysis was carried out with an electrolyte

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temperature of 125 C and a current density of 1.4 A / dm, nitrogen gas being introduced at a rate of 2ON ml / min. At the start of the electrolysis, the Cell voltage 5.85 volts. After 5 hours it was 5.81 volts. The current output of sulfur hexafluoride increases with time and reached after 5 hours 63%.

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Claims (5)

Claims - 1. Process for the preparation of sulfur hexafluoride by electrolytic Ways, characterized in that the electrolysis using a carbonaceous anode and an electrolyte, which consists of a mixture of potassium fluoride and hydrogen fluoride, where the ratio of fluorides KF nHF between N = 0.9 to 3.0 ( is chosen, carried out in the presence of elemental sulfur 2. The method according to claim 1, characterized in that the electrolysis at a temperature in the range from the melting point of the KF · nHF mixture, which is dependent on the factor η, up to the melting temperature + 50 ⁰ C, at a voltage of 5 to .15 volts and at one Current density of 0.5 to 25 A / dm is carried out. 3. The method according to claim 1, characterized in that the electrolysis at a temperature in the range of the melting point of | KF · nHF mixture, which depends on the factor η, up to a temperature of the melting point +50 C, at a voltage of 2 ■ 6 to 12 volts and a current density of 4 to 12 A / dm 4. The method according to claim 1, characterized in that the mixture is chosen so that the factor η is between I ₁ 8 and 2.2. 5. The method according to claim 4, characterized in that the electrolysis is carried out at a temperature between 8O ⁰ C and 130 °, at a voltage of 5 to 15 volts and at a current density of 0.5 to 25 A / dm. ₄₇₃ 0 009885/1936 O O trolysis at a temperature between 80 C and 130 G raid of a chip rairig from 6 to 12 volts at a current density of 4 to 12 A / dm. 009885/1936