GB2324482A

GB2324482A - Purifying sulphur hexafluoride

Info

Publication number: GB2324482A
Application number: GB9808849A
Authority: GB
Inventors: Hiromoto Ohno; Tetsuo Nakajo; Toshio Ohi; Kuniomi Marumo
Original assignee: Showa Denko KK; Hitachi Engineering Co Ltd
Current assignee: Hitachi Engineering Co Ltd; Resonac Holdings Corp
Priority date: 1997-04-25
Filing date: 1998-04-24
Publication date: 1998-10-28
Anticipated expiration: 2018-04-24
Also published as: TW375531B; JPH10297910A; GB9808849D0; JP3882018B2; GB2324482B; HK1012850A1; KR19980081619A; DE19818435A1; KR100529748B1

Abstract

Impure SF 6 is contacted with at purifying agent comprising a carbonaceous solid and at least one compound selected from the group consisting of alkali earth metal compounds, alkali metal aluminates and tetraalkylammonium salts. Typical impurities removed from the SF 6 are HF, SO 2 , S 2 F 2 , SF 2 , SF 4 , SOF 2 etc.

Description

1 2324482 PROCESS FOR PURIFYING SULFUR HEXAFLUORIDE

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for purifying sulfur hexafluoride, particularly a process for purifying recovered sulfur hexafluoride containing harmful impurities by contacting the same with a purifying agent.

2. Description of the Related Art

There is increasing demand for sulfur hexafluoride (hereinafter referred to as "SF6") as a gaseous insulating medium for circuit breakers, transformers and so on, due to its excellent electric insulating property. SF6 itself is chemically stable and not toxic or corrosive.

However, for example, if SF6 is used in equipment in a small transformer substation, it is exposed to strong electric fields such as corona discharge and arcs generated at circuit breaks, resulting in decomposition to various impurities, which may degrade the performance of the device. Sulfur tetrafluoride (hereinafter referred to as "SF4"), sulfur difluoride (hereinafter referred to as "SF2"),. sulfur monofluoride (hereinafter referred to as "SZF2") and the like are very reactive substances, which react with water if present in the equipment, even in a minor amount, to form strongly acidic substances such as hydrogen fluoride (hereinafter referred to as "HF"), sulfite gas (hereinafter referred to as "SO2"), thionyl fluoride (hereinafter referred to as "SOF2") and sulfryl fluoride (hereinafter referred to as "S02F2"), and minor amounts of sulfur pentafluoride (hereinafter referred to as "S2F10"). If an SF6 gas containing the above impurities is used, the performance of the equipment will be degraded, for example, metallic

1 parts of the equipment will corrode. Accordingly, SF, gas containing impurities is removed from the equipment and replaced at appropriate times.

Since SF6 gas is expensive, and its effect on the environment, such as global warming, cannot be neglected, there has recently been a demand for recycling SF6 by recovering used SF6 and removing harmful impurities as mentioned above from the recovered SF6. To comply with this demand, some purification methods have been proposed. For example, as dry methods, 1) contacting recovered SF6 gas with activated alumina and x-type synthetic zeolite (Japanese Examined Patent Publication (Kokoku) No. 47-6205) and 2) converting impurity gas components with a converting agent, a solid reagent, to a solid product and non-toxic gaseous reagent which is then absorbed by a gas absorber (Japanese Unexamined Patent Publication (Kokai) No. 60-54723) and, as wet methods, 3) bubbling a recovered SF6 gas into an alkali bath (an aqueous solution of calcium hydroxide) (Hamano et al. "SF6 Gas Recovering Apparatus in a High Electric Power Plant", 1996 Electric Society Conference) and 4) contacting a recovered SF6 gas with a lithiumexchanged macro-reticular ion-exchange resin (Japanese Unexamined Patent Publication (Kokai) No. 06-211506).

The above methods however have problems. For example, in the above method 1), adsorption of S02, SOF2f S02F2, S2FIO and the like is not sufficient. In method 2), calcium hydroxide reacts with the impurities to produce hydrogen which is then adsorbed and removed, but the produced hydrogen is flammable and the removal capability of this method is not sufficient. In the method 3), the installation is large in scale because of the wet method. In method 4), the absorber is expensive and the gas will be contaminated with a large amount of water content because of the wet method. Thus, the above methods are not economical or suitable for commercial application.

The present invention, in order to solve the above mentioned problems, aims to provide a method for purifying SF6 which allows simple and effective removal of harmful impurities from impurity-containing SF6.

SUMMARY OF THE INVENTION

The present invention provides a process for purifying SF6, which comprises the step of contacting S-- 6 containing an impurity with a purifying agent comprising a carbonaceous solid and at least one compound selected from the group consisting of alkali earth metal compounds, alkali metal aluminates and tetraalkyl ammonium salts.

The impurity of said SF6 may be at least one selected from the group consisting of HF, SO2. S2F2f SF2.

SF4f S2F10, SOF2, and S02F2.

The alkali earth metal compound of the purifying agent may be preferably selected from the group consist.j-ng of oxides, hydroxides, carbonates and nitrates of calcium, magnesium, barium and strontium.

The alkali metal aluminate of the purifying agent may be preferably selected from the group consisting of lithium aluminate, sodium aluminate, potassium aluminate and cesium aluminate.

The tetraalkylammonium salt of the purifying agent may be preferably selected from the group consisting of tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetramethylammonium carbonate, tetraethylammonium. carbonate, tetramethylammonium. chloride, tetraethylammonium chloride, tetramethylammonium. aluminate and tetraethylammonium aluminate.

The carbonaceous solid of the purifying agent may be preferably at least one selected from the group consisting of coke, coal, char, charcoal, pitch, activated carbon and carbon black.

The purifying agent preferably has a specif ic surface area in the range of 80 mz/g to looo m2/7.

The purifying agent preferably has an average pore size in the range of 7A to 1500A.

It is preferable that the concentration of HF in the purified SF6 be not more than 0. 1 ppm by weight.

It is preferable that a dehydration step is added to said SF6 purifying step. The dehydration step preferably uses zeolite as a dehydration agent. The water content of the purified SF6 is preferably not more than 50 ppm by weight.

A process wherein said SF6 containing an impurity is first contacted with said purifying agent of a pellet comprising a carbonaceous solid and an alkali earth metal compound and then with a purifying agent comprising at least one of an alkali metal aluminate and tetraalkylammonium salt carried on a carbonaceous solid, and after each of said contact steps, a dehydration step is added, is preferable.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The SF6 purifying process of the present invention is applied to SF6 which has been used in gas insulation equipment in gas circuit breakers (GCN) or the like and contains harmful impurities produced by decomposition by corona discharges or electric arcs generated at breaking (hereinafter referred to as "impure SF6"), for removing the harmful impurities in the impure SF6 and recycling the purified SF6.

Such an impure SF6 usually includes corrosive harmful impurities such as HF, SF4f SF2, SZF21 S02, SOF21 S02F21 S2F10, etc.

In accordance with an embodiment of the present invention, a purifying agent comprising a carbonaceous solid and at least one compound selected from the group consisting of alkali earth metal compounds, alkali metal aluminates and tetraalkylammonium salts, is charged, for example, in a reactor tube and said impure SF6 is passed as a gas flow through the reactor tube at room temperature, by which the impurities of the impure SF, are adsorbed by the purifying agent and SF6 which contains substantially no harmful impurities (hereinafter referred to as "purified SF61') can be simply and effectively recovered.

When the obtained purified SF6 is recycled into the gas insulation equipment or the like, the water content of the purified SF6 is preferably not more than 5-0 ppm by weight. This can be accomplished by adding a dehydration step using, e.g., zeolite as a dehydration agent, after the SF6 purifying step.

Each element of the present invention is described more in detail below.

The purifying agent of the present invention comprises a carbonaceous solid and at least one compound selected from the group consisting of alkali earth metal compounds, alkali metal aluminates and tetraalkylammonium salts.

The carbonaceous solid is generally effective to adsorb most impurities which result from decomposition of SF6 but the impure SF6 contains a considerable amount of HF which may degrade the adsorption capability of the carbonaceous solid. In accordance with the present invention, it was found that all the harmful impurities in the impure SF6 can be effectively removed by contacting a carbonaceous solid combined with at least one of alkali earth metal compounds, alkali metal aluminates and tetraalkylammonium. salts. Although the inventors do not want to be bound to a particular theory, it is believed that at least one of alkali earth metal compounds, alkali metal aluminates and tetraalkylammonium salts is particularly effective to remove a considerable amount of HF, in addition to the other impurities, which facilitates the effective removal of all the harmful impurities of the impure SF6 by the purifying agent of - 5 the present invention as a whole.

At any rate, it was found that a Purifying agent comprising a carbonaceous solid and at least one compound selected from the group consisting of alkali earth metal compounds, alkali metal aluminates and tetraalkyjammonium salts is effective to selectively remove harmful impurities from the impure SF6 and to obtain pure SF6 The alkali earth metal compound includes oxides, hydroxides, carbonates, nitrates, etc. of calcium, magnesium, barium or strontium. These may be used alone or in combination. Calcium oxide, hydroxide, carbonate and nitrate are preferable and, more specifically quick lime, slaked lime and limestone are more preferable since these are cheap and easy to handle.

The alkali metal aluminate includes lithium aluminate, sodium aluminate, potassium aluminate and cesium aluminate. These may be used alone or in combination. Sodium aluminate and potassium aluminate are preferable since they are easily available. Sodium aluminate is the most preferable.

The tetraalkylammonium salt includes tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetramethylammonium carbonate, tetraethylammonium carbonate, tetramethylammonium chloride, tetraethylammonium chloride, tetramethylammonium. aluminate and tetraethylammonium aluminate. These may be used alone or in combination. Tetramethylammonium hydroxide, tetramethylammonium. carbonate, tetramethylammonium chloride and tetramethylammonium aluminate are preferable since they are easily available. Tetramethylammonium carbonate is particularly preferable. Tetramethylammonium. carbonate and tetramethylammonium aluminate can be produced by reacting tetramethylammonium hydroxide with either carbonate gas or aluminum hydroxide.

The carbonaceous solid includes coke, coal, char, - 7 charcoal, pitch, activated carbon and carbon black. These may be used alone or in combination. The form of the carbonaceous solid is preferably in the form of powder, fine particles or pellets. The fine particles may be in a spherical form or a crushed or ground form. The carbonaceous solid preferably has a specific surface area in the range of 100 mz/g to 2500 M2/g, more preferably 500 M2/g to 2000 M2/g. Particularly preferable carbonaceous solids are char in the form of powder and activated carbon in the form of fine particles.

The preparation of the purifying agent may be carried out in many manners. For example, a supportedtype purifying agent may be obtained, for example, by immersing a carbonaceous carbon, for example, activated carbon, in an aqueous solution of sodium aluminate or the like. Alternatively, a pellet-type purifying agent may be obtained, for example, by mixing slaked lime etc. with char powder etc. in a predetermined ratio, kneading the mixture with water, shaping the kneaded mixture into a pellet or granule having a size suitable for handling, and drying it.

When the purifying agent is of a supported-type, for example, the amount of the supported alkali metal aluminate and/or tetraalkylammonium salt is preferably in the range of 1g to 30g per 100 ml of activated carbon as the carbonaceous solid. If the amount of the supported agent is less than 1 g/100 ml, the purifying effect is not sufficient. If the amount of the supported agent is more than 30 g/100 ml, micropores of the carbonaceous solid are filled and the purifying efficiency is reduced. The support preferably has a pore size in a range of 7k to 300A. The supported-type is generally preferable since it can have a large specific surface area for the agent to work.

When the purifying agent is of the pellet-type, the - 8 ratio between the alkali earth metal compound, for example, and the carbonaceous solid is preferably such that the weight of the alkali earth metal compound is equal to or more than that of the carbonaceaus solid, although the ratio depends on the particular composition.

Irrespective of whether it is the supported- or pellet-type, the purifying agent should be sufficiently dried. The drying may be preferably carried out by using an inert gas such as nitrogen, helium or argon as a drying gas.

The purifying agent preferably has a specific surface area in the range of 80 M2/g to 1000 M2 /g, particularly 100 m 2 /g to 700 M2/g - The purifying agent preferably has an average pore size in the range of 7A to 1500A, particularly 7A to 1100A. Outside these ranges, the purifying efficiency is reduced. The specific surface area and average pore size of the purifying agent may be adjusted, for example, by selecting the source materials, composition, preparation method and conditions in preparation of the purifying agent.

When the purifying agent is of the supported-type, the purifying agent preferably has a specific surface area in the range of 300 M2/g to 800 m 2 /g, particularly 400 m2 / g to 600 n12/g - The pore size of the supportedtype purifying agent is preferably 4A to 200A, more preferably 8A to 100A.

It is preferable that the impure SF6 IS treated first with a pellet-type purifying agent and then with a supported-type purifying agent, after each of which a dehydration step is preferably added. This is because a pellet-type purifying agent may remove a considerable amount of HF without deterioration thereof and a supported-type purifying agent is more effective to remove the other impurities of the impure SF6 while it is susceptible to deterioration by HP.

To purify an impure SF6 with the purifying agent, the impure SF6 is made to contact a purifying agent. The concentration of the impurities in the impure SF6 to be subjected to the purifying process are preferably not more than 2.0% by weight. If the concentration of the impurities is more than this amount, a large amount of the purifying agent becomes necessary and the time for purifying becomes longer.

When contacted with the purifying agent, impure SF, may be in the f orm of a gas or liquid, but impure SF6 in the form of gas is preferable. The method for contacting may be any one used in general fluid-solid contact, including fluid bed-type and moving bed-type methods as well as a fixed bed-type method. The fixed bed-type method is preferable. When the gassolid contact method is used, the operating pressure is not limited but a pressure in the range of ambient pressure to 2 MPa is preferable from the economical viewpoint. The temperature during contact is not limited but a temperature in the range of -400C to 1OCC, may be appropriately selected. The ambient temperature is preferable from the economical viewpoint.

In accordance with the above step of contacting an impure SF6 with a purifying agent as described above, the impure SF6 may be purified to such a level that, for example, the concentration of HF is not more than 0.1 ppm by weight (measured by halogen ion analysis or ion chromatography). In the same purifying step as the above, the concentration of S ion becomes not more than. 0.1 ppm as S04 2- by weight (measured by ion chromatography). The other harmful impurities such as S02/ SF2, SF41 SO2F2 and SOF2 may be reduced to a level lower than the detectable limits by gas chromatography (GC) or gas chromatography/mass spectrography (GC/MAS), i.e., not more than 0.1 ppm by weight.

To recycle the purified SF6 obtained in the above contact step as a gas insulation medium, the water - content should be sufficiently reduced since it is one cause of decomposition of SF6. Accordingly, it is preferable that a dehydration step by which the water content can be strictly controlled is added to the purifying process of the present invention. This dehydration step can be preferably carried out by gassolid contact using, for example, synthetic zeolite as a dehydration agent after said contact step. A preferable dehydration agent used in the dehydration step is synthetic zeolite and it is particularly preferable to select synthetic zeolite having an adequate particle size among molecular sieves known as "Molecular Sieves" (tradename).

In the gas-solid contact dehydration using molecular sieves, the pressure is preferably from ambient pressure to 2 MPa and the temperature is preferably in the range of -400C to 1000C, particularly at room temperature, for the economical reasons. By this dehydration step, the water content of the purified SF6 can be reduced to not more than 50 ppm by weight, particularly not more than ppm by weight.

In accordance with the present invention, SF6 containing impurities is contacted with a purifying agent comprising a carbonaceous solid and at least one compound selected from the group consisting of alkali earth metal compounds, alkali metal aluminates and tetraalkylammonium salts. Accordingly, the harmful impurities of the impure SF6 can be simply and effectively removed without a large installation. Further, by adding a dehydration step to the purifying process of the present invention, moisture which is one cause of SF6 decomposition, can be reduced, which allows the purif ied SF6 to be recycled as a gas insulation medium.

EXAMPLES

The present invention is now described in more detail with reference to Examples.

is Imvure S, sam l.e:

The following impure SF6 samples were prepared for test purposes.

(Impure SF6 sample No. 1) Sulfur hexafluoride (manufactured by Showa Denko K.K.; purity: 99.999% or more; halogen ion: 1 ppm by weight or less) was charged in a gas insulation switching test apparatus to about 0.5 MPa and the gas, after gas insulation switching tests were repeated, was collected, as impure SF6 sample No. 1, in a cylinder which had been vacuum dried and cooled.

The obtained impure SF6 sample No. 1 was analyzed mainly by gas chromatography using various kind of detector such as electron capture detector (GC/ECD), flame photo- ionization detector, thermal conductivity detector and gas chromatography/mass spectrography (GC/MS). Halogen ion analysis (ion chromatography) and water content analysis method were also adopted (quartz oscillator method).

The results are expressed in the unit of ppm by weight. (Amount of a component compared with the total amount of all components excluding H20. In the case of H20, H20 weight/total weight.) Analyzed amounts of components (ppm by weight):

SF6 99-8430 CF4 0.0620 C2F6 0.0063 C2F4 0.0012 SOF2 0.0512 SO2F2 0.0022 S02 0.0013 HF 0.0328 H20 0.0086 (Impure SF6 sample No. 2) Commercially available sulfur tetrafluoride (SF4) and sulfur pentafluoride (S2F,()) were added to the above impure SF, sample No. 1, to prepare impure SF6 sample No. 2.

The results of analysis of the impure SF6 sample No. 2 are shown in the following. The analysis of S2FIO was made by electron capture-detector gas chromatography (GC/ECD).

Analyzed amounts SF6 C2F6 0.0063 SOF2' 0.0512 S02 0.0013 S2FIO 0.0003 H20 0.0118 Prevaration of vurifvina aaent:

The following three purifying agents were prepared.

(Purifying agent No. 1) ml of particulate activated carbon (specific surface area: 1200 M2/g, particle size: 0.5 mm - 2 mm) was immersed in 40 ml of a 25%-aqueous solution of sodium aluminate and stirred for one hour. After filtration, the supported carbon was dried with a dry nitrogen gas at 2000C for 3 hours in a stainless tube to obtain sodium aluminate-supported activated carbon having no water content (10g of supported NaA1021100 Ml of activated carbon), to obtain purifying agent No. 1.

Purifying agent No. 1 had a specific surface area of about 570 m 2 /g and an average pore size of about 14K.

(Purifying agent No. 2) ml of particulate activated carbon (specific surface area: 1200 m 2 /g, particle size: 0.5 mm - 2 mm) was immersed in 40 ml of a 25%-aqueous solution of tetramethylammonium hydroxide and stirred for an hour. After filtration, the supported carbon was dried with a dry nitrogen gas at 500C for 5 hours to obtain tetramethylammonium hydroxide-supported activated carbon containing no water content (10g of supported (CH3)4NOH/100 ml of activated carbon), to obtain purifying agent No. 2.

Purifying agent No. 2 had a specific surface area of of components (ppm by weight):

CF4 0.0620 99.8422 C2F4 0.0012 S02F2 0.0022 SF4 0.0005 HF 0.0328 about 560 m 2 /g and an average pore size of about 13A.

(Purifying agent No. 3) Charcoal having a particle size of 250 Lm or less and slaked lime having a particle size of 250 im or less were mixed in a weight ratio of 1:3, to which water was added while mixing in a Henschel mixer, and the mixture was granulated, then dried at 1100C for 4 hours, and further heat treated at 8000C in a nitrogen atmosphere to dehydration and calcinate the granules. From the obtained granules, pellets having a size of 2 mm to 4 mm were made, as purifying agent No. 3.

Purifying agent No. 3 comprised carbon (C) and calcium oxide (CaO) as the main components and had a specific surface area of about 120 m 2 /g and an-average pore size of about 1000A.

Purification test:

(Example 1) ml of purifying agent No. 1 was charged in a 75 ml-SUS cylinder, to which impure SF6 sample No. 1 was supplied at a rate of 12 Nl/hr at room temperature.

The gas was sampled at the outlet of the cylinder and analyzed by gas chromatography and halogen ion analysis. The halogen ion analysis was carried out by blowing the sampled outlet gas in ultrapure water for a predetermined time period and the ultrapure water thus treated was analyzed by ion chromatography.

The results are shown below.

Analyzed amounts of components (ppm by weight; stands for "less than the detectable limit',):

SF6 99.9467 CF4 0.0482 C7.F6 0.0051 SOF2 S02 C2F4 S02F2 HF F- <0.001 so, 2- 0.004 H20 0.0129 The above shows that harmful impurities in impure is SF6 sample No. 1 were effectively removed by purifying agent No. 1.

(Example 2) ml of purifying agent, No. 2 was charged in a 75 ml-SUS cylinder, to which impure SF6 sample No. 1 was supplied at a rate of 20 Nl/hr at room temperature.

The gas was sampled at the outlet of the cylinder and analyzed by gas chromatography and halogen ion analysis in the same manner as in Example 1.

The results are shown below.

Analyzed amounts of components (ppm. by weight; stands for "less than the detectable limit,,):

SF6 99.9415 CF4 0.0528 C2F6 0.0057 C2F4 - SOF2 - SO2F2 S02 HF F <0.01 H20 about 150 It is demonstrated by the above that harmful impurities in impure SF6, sample No. 1 were effectively removed by purifying agent No. 2.

(Example 3)

S04 2- 0 - 11 0.01 From the outlet gas sampled in Example 2, water was removed.

ml of commercially available synthetic zeolite (Molecular sieves 3A, manufactured by Union Showa, Ltd.) was charged in a 75 ml-SUS cylinder, in which the outlet gas sampled in Example 2 was introduced and the gas at the outlet of this cylinder was analyzed for the water content.

The water content was not more than 5 ppm by weight.

It is demonstrated by the above that the water content in the sample was effectively removed by the method in Example 3.

(Example 4)

Two 75 ml-SUS cylinders were connected in series.

- 15 ml of purifying agent No. 3 was charged in a first cylinder, and 40 ml of purifying agent No. 1 was charged in a second cylinder. Impure SF6 sample No. 1 was supplied to the first cylinder to the second cylinder at a rate of 20 Nl/hr at room temperature. The gas was sampled at the outlet of the second cylinder and analyzed by gas chromatography and halogen ion analysis in the same manner as in Example 1. The results are shown below. Analyzed amounts of components (ppm by weight stands for "less than the detectable limit"):

SF6 99.9579 CF4 0.0389 CZF6 0.0032 SOF2 S02 F H20 <0.001 0.005 11 11 C2F4 S02F2 HF S04 2- 0.002 It is demonstrated by the above that a combination of purifying agent Nos. 3 and 1 was effective not only in removing harmful impurities but also in removing perfluorocarbons (CF4, C2F6) (Example 5) ml of purifying agent No. 1 was charged in a 75 ml-SUS cylinder, in which impure SF6 sample No. 2 was supplied at a rate of 12 Nl/hr at room temperature.

A gas was sampled at the outlet of the cylinder and analyzed by gas chromatography and halogen ion analysis in the same manner as in Example 1. S2FIO Was analyzed by electron capture detector gas chromatography (GC/ECD) The results are shown below.

Analyzed amounts of components (ppm by weight; stands for "less than the detectable limit"):

SF6 99.9423 CF4 0.0472 C2F6 0.0050 SOF2 S02 C2F4 S02F2 SF4 16 F S2F10 < 0 0 0 0 1 < 0 0 0 1 0.0124 H20 HF S04 2- 0.003 It is demonstrated by the above that harmful impurities as well as SF4 and S2F10 in impure SF6 sample No. 1 were effectively removed by purifying agent No. 1.

Claims

1. A process for purifying SF, which comprises the step of contacting SF, having an impurity with a purifying agent comprising a carbonaceous solid and at least one compound selected from alkali earth metal compounds, alkali metal aluminates or tetraalkylammonium salts.

2. A process according to claim 1 wherein the impurity comprises at least one of HF, S02. S2F,, SF, SF,, SF,,, SOF,, or SO,F_

3. A process according to claim 1 or 2 wherein the alkali earth metal compound comprises at least one of an oxide, hydroxide, carbonate, or nitrate of calcium, magnesium, barium, or strontium.

4. A process according to any preceding claim wherein the alkali metal aluminate comprises at least one of lithium aluminate, sodium aluminate, potassium aluminate or cesium aluminate.

5. A process according to any preceding claim wherein the tetraalkylammonium salt comprises at least one of tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetramethylammonium carbonate, tetraethylammon-Jum carbonate, tetramethylammonium chloride, tetraethylammonium chloride, tetramethylammonium aluminate, or tetraethylammonium aluminate.

6. A process according to any preceding claim wherein the carbonaceous solid comprises at least one of coke, coal, char, charcoal, pitch, activated carbon or carbon black.

18 -

7. A process according to any preceding claim wherein the carbonaceous solid is in the form of a powder, fine particles, or pellets.

8. A process according to any preceding claim wherein the carbonaceous solid comprises char in the form of a powder or activated carbon in the form of fine particles.

9. A process according to any preceding claim wherein the purifying agent has a specific surface area in the range of 80M2/g to 10OC)M2/g.

10. A process according to any preceding claim wherein the purifying agent has an average pore size in the range of TA to 1500A.

11. A process according to any preceding claim wherein the alkali earth metal compound, alkali metal aluminate or tetraalkylammonium salt is supported by the carbonaceous solid.

12. A process according to any preceding claim wherein the purifying agent is in pellet form.

13. A process according to any preceding claim wherein SF6 purified by the process does not comprise more than 0.1ppm. HF by weight.

14. A process according to any preceding claim further comprising a dehydration step.

15. A process according to any of claims 1 to 13 wherein the SF, having an impurity is first contacted with a purifying agent in the form of a pellet comprising a carbonaceous solid and an alkali earth metal compound and then with a purifying agent comprising at least one of an alkali metal aluminate or tetraalkylammonium salt supported on a carbonaceous solid, and after each of said contact steps, a dehydration step is carried out.

16. of:

a) contacting SF, with a purifying agent in the form of a p=l=- a carb.cnac------ " a n, a a metal compound; b) dehydrating the SF6 obtained from step a) c) contacting the dehydrated SF, with a purifying agent comprising at least one of an alkali metal aluminate or a tet-raalkylammonium salt supported on a carbonaceous solid; and d) dehydrating the SF, obtained from step c).

A process for purifying SF, which comprises the steps

17. A process according to any of claims 14 to 16 wherein the SF6 purified by the process does not comprise more than 50 ppm water by weight.

18. A process according to any of claims 14 to 17 wherein zeolite is used as a dehydrating agent.

19. A process substantially as described with reference to the accompanying examples.