GB2187725A - Impregnated activated carbon - Google Patents

Abstract

An adsorbent material capable of chemisorbing hydrogen cyanide from a gaseous mixture which consists of a granular or fibrous activated carbon having impregnated thereon a transition metal salt of a non-chelating carboxylic acid. The transition metal salt preferably consists of cobalt, nickel or zinc acetate. The adsorbent material maybe further impregnated with a silver salt and/or a cyclicamine (e.g. AgNO3, pyridine, triethylenediamine). The adsorbent material may be used in respirators and like devices to remove HCN from the atmosphere.

Description

SPECIFICATION Impregnated activated carbon This invention relates to impregnated activated carbon and to methods for preparing impregnated activated carbon.

In the construction of gas masks (respirators), collective protectors and the like, a large variety of adsorbents have been suggested and used for removing the various types of harmful gases which may be encounte- red in chemical warfare. An adsorbent which has been used in the canisters of gas masksforthe removal of poisonous gases such as hydrogen cyanide (HCN) has been activated charcoal (activated carbon) either containing or having impregnated thereon certain metals or metal compounds. The function of the metals or metal compounds is to break down the HCN by chemical reaction into harmless gaseous products and/or products which are readily physisorbed onto the activated carbon.

In the United Kingdom,the adsorbentwhich has been used for many years forthe removal of HCN in particular is an activated charcoal which was formed by adding a copper containing compound to powdered coal whereafterthe mixture was briquetted, carbonised, activated in high temperature steam, and graded to yield the final product. The charcoal could be and sometimes was subsequently treated with pyridine and silver (as Ag NO3) to provide protection against cyanogen chloride and certain arsenicalagents. The charcoal thus prepared provided elimination of the poisonous gases by chemisorption.There are howeverseveral disadvantages in employing an adsorbent ofthis type. The chemisorption properties ofthe charcoal have been found to deteriorate over long periods oftime. Since relatively large quantities of charcoal must be stored to be held in readiness for use in time of need, this problem has necessitated periodic regeneration of the charcoal, which is an expensive and time consuming process. Afurther serious disadvantage of this charcoal is that its production from a mixture of coal and copper-containing compound results in a great deal of wastage. Thus as much as 40% by weight of the starting material is lost as unusablefineswhich are essentially charcoal of carbon-containing copper.This loss is made all the more serious because the waste fines cannot be recycled and have no use. Their production therefore increases the cost ofthe useful charcoal product.

Many of the problems associated with briquetted carbons may be overcome by providing adsorbents prepared by impregnating activated charcoal with two or more metal salts (especially Cu2+ and Cr6+ salts) from solution, and subsequently drying the charcoal. An important group of adsorbents of thistype, known as the whetlerite charcoals and described in, for example, US Patent Nos 1519470 (Wilson et a/),2920050(Blacet metal), and and 2920051 (Wiig metal), are prepared by impregnating activated charcoal with ammoniacal solutions containing copper carbonate and other metal salts.Although whetlerite charcoals are claimed to remain effective against HCN after long periods of storage in conditions of high temperature and humidity, impregnated charcoals in general are susceptible to loss of effectiveness undertheseconditions and whetlerite charcoals themselves have the disadvantage that they can release a stong ammonia odour which is undesirable for use in respirators.

An additional disadvantage of known impregnated activated charcoals (includingthewhetleritecharcoals) is that normally they must be impregnated with more than one metal or metal salt in orderto adsorb HCN effectively. For example, copper or copper salts are effective in oxidising HCN either two harmless gaseous products orto products which may be readily adsorbed by the charcoal. However, appreciable amounts of HCN are oxidised to cyanogen ((CN)2), a toxic gas which (like HCN) is not readily adsorbed by activated charcoal. Chemisorption of (ON)2 generated by the copper has to be effected by one or more further impregnants, such as chromate or a dichromate.This requirementfor a plurality of metals or metal salts on the charcoal leads to the difficulty that the uptake of each metal salt from solution may vary considerablyfrom one type of charcoal to another. Therefore it is difficultto prepare adsorbents by this technique which contain an optimum balance of metal to provide maximum protection against HCN. This has been found a particular problem when attenpting to i m p reg nate fibrous activated carbons.

It is an object ofthe present invention to provide an impregnated activated carbon and a method forthe preparation thereof in which the above disadvantages are overcome or at least mitigated in part.

According to a first aspect ofthe present invention there is provided an adsorbent material capable of removing HCN from a gaseous mixture, which comprises an activated carbon having impregnated thereon at least one transition metal salt of a non-chelating carboxylic acid.

The at least one metal salt is preferably a salt of a mono- or di-carboxylic acid, particularly a monocarboxylic (for example an alkanoic) acid. The monocarboxylic acid is preferably a C1-C4 alkanoic acid, with acetic acid being most preferred. The cation of the at least one metal salt is preferably selected from ions of elements in the transition series of the first long period and from ions of transition metal elements in Group 2B and 8 of the Periodic Table, and is most preferably selected from Co2+, Ni2+, and Zn2+. The at least one metal salt is most preferably cobalt acetate or nickel acetate, which are found to give the best protection against HCN when impregnated onto activated carbon.

Whilethe metal carboxylate can be supported on the activated carbon in an amount upto about 50% by weight, based on the total weight of the final product, when the final product is used as an agent for removing the above described gases, it is preferably supported in an amount of from about 0.1% by weight to 30% by weight, and preferably from about 1% by weight to 20% by weight. When it is supported in amounts less than 0.1% by weight, the effect as a removal agent is insufficient whereas in amounts greaterthan 30% byweight, a decrease in the absorptive capacity of the carbon is observed, probably due to clogging ofthe pores within the carbon by solid metal carboxylate.The apparent specific surface area of the impregnated activated carbon is preferably 500m2/g or more, more preferably 800m2/g or more. Preferably, the activated carbon has only one metal carboxylate impregnated thereon.

The activated carbon may be in any suitable form prepared by any number of known processes. It may be a granular activated carbon derived from, for example, a suitable coal or nut shell, or it may be fibrous activated carbon derived from, for example, fibres of acrylonitrile-based polymers, natural cellulose (such as cotton), regenerated cellulose (such at viscose rayon), phenolaldehyde resins, or pitch. Afibrous activated carbon is preferably in the form of a tow, felt, fabric (eg cloth) yarn, weave, web etc, because this type of carbon is becoming increasingly importantfor many commercial and military applications in view of its strength and high adsorptive capacity.

The fibrous activated carbon on which the metal carboxylate is to be supported desirably has a specific surface area, as measured by the BET method, offrom about 600m2/g to 2,000 m2/g, and preferablyfrom about 700m2/g to 1 ,500m2/g. On fibrous activated carbons having specific surface areas of less than about 600m2/g, the fibrous activated carbon with the metal carboxylate supported thereon is of insufficient capacity to remove toxic substances effectively. On the other hand, those fibrous activated carbons having specific surface areas of more than about 2,000m2/g are low in strength and are subjectto limitations of usefulness.

Further, it is preferred from the standpoint of handling that the fibre diameter be from 3 to 25 microns.

When the fibre diameter is less than 3 microns the fibre is easily cut during the production of activated carbon, and in molding in a felt form, it is difficult to obtain a web with ease. On the other hand, when thefibre diameter is more than 25 microns the activation thereof can be attained only with difficulty, and even if it is possible to obtain activated carbon having a predetermined specific surface area, it is difficu It to obtain those activated carbons having high strengths because the activation yield is reduced.

The activated carbon may additionally be impregnated with other metallic and/or inorganic impregnants which provide for elimination of a wider range of toxic gases. One example of an additional impregnant is silver or a salt thereof (eg silver nitrate), preferably present on the activated carbon in the %weight rangeof 0.1 to 15%, most preferably 1 to 10%, which has been found to assist in the elimination of certain gaseous arsenical agents.Afurtherexample of an additional impregnant is an organic compound selectedfromthe organic amines, especially the cyclic amines, preferred examples of which are pyridine and (most preferably) triethylenediamine (TEDA).These amines are found to assist in the elimination of toxic gases such as cyanogen chloride, and are preferably present on the activated carbon in the % weight range of 0.1% to 20%, most preferably 0.5% to 10%.

According to a second aspect of the present invention there is provided a method of preparing an adsorbent material which comprises providing an activated carbon, wetting the activated carbon with a solution of at least one transition metal salt of a non-chelating carboxylic acid, and drying the wetted activated carbon to provide an activated carbon impregnated with the at least one salt.

In order to wet the activated carbon with the dispersion or solution, the at least one transition metal carboxylate is preferably dissolved in water. Organic solvents which are liquid at ordinary temperature, have boiling points ofnot more than 100'O, and are easilydriable may be used in place of waterthoughtheseare less preferred. Such solvents include, for example, ketones such as acetone, and alcohols such as methanol and ethanol. The activated carbon may be wetted by soaking it in the solution by immersion or by spraying the solution onto the carbon, and is then dried. The concentration of the solution is generally from about 0.1wt%to to about 30wt%, preferably 1 to 20wt%.The soaking time is preferably from about 10 minutes to about 1 hour the drying is carried out at a temperature below the decomposition temperature of the metal carboxylate and generally at about 200 C or less.

The activated carbon may additionally be impregnated with one or both of a silver salt (preferably silver nitrate) and an organic amine, preferably a cyclic amine such as pyridine, most preferablyTEDA. For either impregnant, impregnation is effected by wetting the activated carbon with a solution of the impregnant, and subsequently drying the carbon. It is essential that impregnation with one or both of these impregnants is carried out prior to wetting with the solution ofthe at least one transition metal carboxylate sothatthese impregnants do not upset the performance ofthe carboxylate.Where both impregnants are used, then the activated carbon is preferably impregnated with the organic amine followed by the silver saltfollowed bythe at least one transition metal carboxylate. The concentration of the organic amine or silver salt in their respect ive solutions will generally befrom 0.01 to 10 weight %, preferablyfrom 0.1 to Sweight%.

Activated carbons usually contain traces of inorganic compounds, especialiy chlorides, left over from their manufacture and the presence of these compounds can have a detrimental effect on the successful impregnation of the carbon with silver salts. Where pre-impregnation with a silver salts is employed, in orderto remove these inorganic compounds the carbon is preferably first treated with an aqueous solution of a strong acid before any impregnation step. The pH of the solution is preferably less than one, and is most preferably less than zero. A preferred acid is nitric acid. Howevertreatmentwith strong acid solutions is found to result in some oxidation and hydroxylation of the surface of the carbon, which has been found to result in increased cyanogen formation on the surface of the carbon when challenged with HCN. On the other hand, treatment of the carbon priorto impregnantwith an aqueous solution of a strong base is also preferred because it is found to enhance the subsequent uptake by the carbon of impregnants from solution, although it has little effect on the presence of inorganic impurities in the carbon. The pH of the base solution is preferably more than 13, most preferably more than 14, and the base is preferably an alkali metal hydroxide, especially NaOH.Most preferably, however, the carbon is pre-treated first with the aqueous solution ofthe strong base and then with the aqueous solution of the strong acid. Treatment with the strong base prior to the strong acid isfound to improve impregnant uptake and to reducethe undesirable effects on the carbon ofthe strong acid. The carbon is preferably washed with waterto remove residual acid or base thereon before impregnation commences.

The main advantage ofthe present adsorbent is that by employing only single salt impregnation it can provide an effective HCN chemisorbentwithoutthe associated generation of substantial quantities of cyanogen. Since little or no cyanogen is generated, the use of possibly carcinogenic chromium salt impregnants can be avoided if desired. The present impregnated products appear two retain their capacityto remove HCN even after prolonged storage under conditions of relativeiy high temperature and humidity, and since ammoniacal solutions are not necessarily used during their preparation, the unpleasant release of ammonia vapour from their surfaces can be avoided.Furthermore, the subsequent use of transition metal carboxylate impregnants over other impregnants such as siiver salts and amines does not appearto interfere unduly with the ability of these other impregnants to assist in the remove of toxic gases other than HCN.

Although the invention is not in any way limited by this explanation, it is believed that HCN removal is particularly effective using the impregnated adsorbentofthe invention because the bond between transition metal cations and non-chelating carboxylic acid anions are easily ruptured (and so are susceptibleto chemi- cal attack) to form very much more stable transition metal - cyano complexes. This effect appears to be most marked with cobalt acetate and nickel acetate.

Examples of impregnated activated carbons and of methods for their preparation in accordance with the present invention will now be described. In each Example, the carbon used was an activated carbon cloth prepared from woven viscose rayon cloth in accordance with the example given in UK Patent No 1310011.

The carbon cloth had a BET surface area in excess of 600m2g-1.

Impregnation -generalprocedure Strips of dry activated carbon cloth were weighed and were then dipped for 30 to 40 minutes in an aqueous impregnating solution at 15 - 25"C containing a known concentration of metal carboxylate impregnant. The strips were then removed from the solution, lightly pressed between two sheets of clean blottong paper, and dried in airat a particulartemperaturefor at least 12 hours.The drying temperature generally established whether the impregnant was present on the cloth in its hydrated form (eg Co(OOCCH3)2.4H2O),whichwas generally the case when drying was performed at less than 40"C, or in its anhydrous form when drying was performed at highertemperatures. The impregnated cloth was then weighed again to establish the loading ofthe impregnant on the cloth.

Examples 1to25 25 examples of activated carbon cloth were impregnated with various impregnants in accordance with the General Procedure outlined above. The actual conditions of impregnation, and the amount of impregnant loaded onto the cloth after impregnation, are given in Table 1 below.

Of the carboxylates used, the acetates and nickel formate were taken as general purpose reagents, and the other salts prepared by reaction of metal carbonates with the appropriate acid in aqueous solution, with crystallization of the product. All of the samples for which results are quoted were prepared by dipping in aqueous solutions of the salts.

Example 25 is included for the purpose of comparison as an activated carbon impregnated with a carboxylate of a non-complexing metal.

Table 1 Example Carboxylate Solution Drying Impregnant Impregnant Concentr- Temp. Loading on ation%w/v C charcoal cloth (%weight) %Total %Metal 1 Cobalt Acetate 5 100 7.6 2.5 2 CobaltAcetate 10 100 10.1 3.4 3 Cobalt Acetate 10 100 19.2 6.1 4 Cobalt Acetate 10 35 21.3 5.0 5 Cobalt Acetate 15 100 13.4 4.5 6 Cobalt Acetate 20 100 17.3 5.8 7 Cobalt Acetate 20 35 23.8 5.6 8 Cobalt Acetate 25 100 20.0 6.7 9 Cobalt Acetate 30 10Q 27.3 9.1 10 Nickel Acetate 5 100 7.2 2.4 11 Nickel Acetate 10 100 12.1 4.0 12 Nickel Acetate 10 35 18.9 4.5 13 Nickel Acetate 15 100 15.5 5.1 14 Nickel Acetate 20 100 17.2 5.7 15 CobaltFormate 5 35 3.2 1.0 16 Nickel Formate 3 100 0.1 1.0 17 Nickel Formate 3 (sprayed) 35 16.7 5.3 18 Cobalt Propanoate 5 35 14.1 19 CobaltPropanoate 10 100 17.5 20 Nickel Propanoate 5 35 13.3 21 Nickel Propanoate 10 100 20.9 22 Zine Formate 5 35 8.0 2.7 23 Zinc Acetate 5 35 13.4 4.0 24 ZincAcetate 10 35 18.8 5.6 25 Sodium Acetate 10 35 15.4 2.6 Example 26 (comparative) Strips of unimpregnated activated carbon cloth were water-washed and dried.

Example 27 Strips of activated carbon cloth were dipped in a 0.4% (w/v) aqueous solution oftriethylenediamine (TEDA) for 10 minutes, lightly pressed between two sheets of blotting paper, and dried at 10000 in air for at least 12 hours. TEDA loading on the cloth was measured at 2.5% by weight. The cloth was then impregnated with cobalt acetate in accordance with the General Procedure given above. The acetate solution concentration was 20%, and the air drying temperature 10000. Cobalt acetate loading on the cloth was measured at 16% by weight, equivalent to 5.3% by weight cobalt.

Example 28 The procedure of Example 8was repeated, after which the impregnated cloth was urged in air for 14 days underthe artificially severe conditions of 80% relative humidity (80%R.H.) at 6000.

Example 29 Strips of activated carbon cloth were dipped for 2 hours at 1500 in a 2M aqueous solution of sodium hydroxide. The strips were then thoroughly washed in distillled water and then impregnated with cobalt acetate in accordance with the procedure of Example 5. The loading of cobalt on the cloth was calculated asd 7.9% by weight.

Example 30 The procedure of Example 29 was repeated, after which the impregnated cloth was aged in airfor 14days at 6000,80% relative humidity (80%R.H.) Example 31 Strips of activated carbon cloth were dipped in a 10% (w/v) methanolic solution of cobalt acetatetetrahydrate. Impregnantloading resulting after drying at 1000 was 9.3% total (3.1%cho).

Example 32 Strips of activated carbon cloth were dipped in a 0.5% (w/v) aqueous solution of silver nitrate for 10 minutes, partially dried on blotting paper, and then dried in the air at 35"C for more than 12 hours. The loading of the silver nitrate on the cloth was measured at 6.9% by weight. The cloth was then impregnated with cobalt acetate in accordance with the General Procedure given above, employing a 15% (w/v) impregnating solution concentration and a drying temperature of 10000. Cobalt acetate loading on the cloth was measured at 11.9% by weight, equivalent to 4.0% by weight cobalt.

Example 33 The procedure of example 32 was followed, except that prior to the silver nitrate dipping, the following additional steps were taken. The cloth was first dipped in a 5M solution of nitric acid and agitated frequently over a period of 30 minutes. The cloth was then removed and then thoroughly washed to remove all trace of acid. The cloth was then dipped in a 0.4%(w/v) aqueous solution of TEDAfor 20 minutes, partially dried on blotting paper, and then air dried at 10000. TEDA uptake on the cloth resulted in a loading of 4.3% by weight.

Silver nitrate and cobalt acetate loadings were measured iater in the proceedings at 5.1% and 10.7% by weight respectively.

Example 34 (comparative) An activated carbon cloth impregnated with the conventional impregnants copper and chromium were prepared by the following procedure. Strips of activated carbon cloth were dipped in a 5% (w/v) aqueous solution of copper nitrate for 20 minutes, partially dried on blotting paper, and heated in a vertical furnace under reducing conditions to reducethe impregnantto copper metal. The strips were then dipped in a 7.5% (w/v) solution of sodium dichromate, partially dried on blotting paper, and then dried in air at 35 Cfor at least 12 hours.

Example 35 (comparative) The procedure of example 34was repeated, after which the impregnated cloth was aged for 35 days in air at 60 ,20 - 30% relative humidity.

Samples of impregnated activated carbon cloth prepared in accordance with the Examples quoted above were tested fortheir ability to withstand a standard challenge of gaseous hydrogen cyanide in humid air.

Priorto the test, cloth samples were equilibrated in an atmosphere maintained at 80% R.H. by a saturated salt solution. In each test, 20 layers of 2cm diameter of the humidified impregnated cloth were placed within a brasstube with mesh ends, and challenged with 1 I.min-' (1 =litre) of airat ambienttemperature and pres- sure and 80-85% R.H., containing 2 mg.l-1 hydrogen cyanide. The testing stream was passed through the bed perpendiculartothe plane ofthe cloth layers, the effluent stream sampled every 2 minutes by an auto matic valve, and passed to a gas chromatograph fitted with a flame ionization detector to detect the presence of HCN and (CN)2.For each impregnated carbon, the time taken before at least 2 x 10-69. of these gases appeared in the effluent gas was measured and recorded as the breakthrough or retention time of the bed tested.

For metal acetate impregnated samples, itwas found thatthe HCN concentration rose steadily towardsthe applied 2mg.1-1 level once breakthrough had occurred. (CN)2 production was infrequently observed with these samples, and in such cases levels were generally below 15 ppm, compared with 400-500 ppm for conventional copper-dichromate impregnated cloths. The generation of (ON)2 levels often up to 30 ppm by unimpregnated cloth suggeststhatthecarboxylates have no part in its generation, the cause of which is probably reaction of HCN with charcoal surface oxygen species and/or impurities present.

The results ofthetests described above for the sample preparations are given in Table 2.

Table2 Example Breakthrough (retention)Times (*comparative on HCN Gas Challenge (minutes) examples) HCN (CN)2 1 17 2 20 3 29 4 25 5 28 6 32 7 20 8 31 9 11 10 12 2 11 23 12 29 20 13 23 14 20 4 15 11 44 16 7 2 17 20 18 23 19 27 20 20 21 23 16 22 9 23 8 24 11 *25 9 *26 6 2 27 27 28 24 29 33 30 13 31 18 16 32 28 33 23 *34 18 12 *35 < 2 < 2 The results of Table 2 show:: (a) the advantages of Examples 1 - 24 over comparative Examples 25,26,34 and 35, (b) the effects of increasing dip solution concentration on cloth loading and performance on testing, (c) the advantage of pre-washing the charcoal in sodium hydroxide solution (see Examples 5,29), (d) the effects of incorporating other desirable impregnants on to the charcoal along with the carboxylates, and (e) the advantages of the present invention over other methods of charcoal impregnation (see Examples 34,35).

In addition to the test described above, the product of Example 27 was tested for its ability to withstand a standard challenge of cyanogen chloride. The test procedure was identical to that adopted for the HCN test, exceptthat a 1 I.min-l stream of room temperature air at70-75% relative humidity containing 2 mg.l-l cyanogen chloride was used instead. Effluent air, sampled for 5 seconds every 3 minutes, was analysed with a halogen detectorto detect the presence of cyanogen chloride. Cyanogen chloride breakthrough was defined asthe timed interval after which a level of more than 2 x 10-69.1-1 cyanogen chloride was detected in the effluent gas. For the product of Example 27, this wasfound to be 23 minutes.

Claims (33)

1. An adsorbent material capable of removing HCN from a gaseous mixture, comprising an activated carbon having impregnated thereon at least one transition metal salt of a non-chelating carboxylic acid.

2. An adsorbent material according to claim 1, wherein the transition metal cation of the at least one salt is selected from ions of elements in the transition series ofthe first long period.

3. An adsorbent material according to claim 1 wherein thetransition metal cation of the at least one salt is selected from ions of elements in Groups 2B and 8 ofthe PeriodicTable.

4. An adsorbent material according to either claim 2 or claim 3 wherein the transition metal cation ofthe at least one salt is selected from the group consisting of Co2+, Ni2+ and Zn2+.

5. An adsorbent material according to any one of the preceding claims wherein the at least one salt is a salt of a mono-ordi-carboxylicacid.

6. An adsorbent material according to claim 5 wherein the monocarboxylic acid is an alkanoic acid.

7. An adsorbent material according to claim 6 wherein the alkanoic acid is a 01-04 alkanoic acid.

8. An adsorbent material according to claim 7 wherein the C - C4 alkanoic acid is acetic acid.

9. An adsorbent material according to claim 8 wherein the at least one salt is selected from the acetates of nickel, cobalt and zinc.

10. An adsorbent material according to any one of the preceding claims wherein the at least one transition metal salt is supported on the activated carbon in an amount of from 0.1% to 30% by weight of said carbon.

11. An adsorbent material according to any one of the preceding claims having an apparent specific surface area, as measured by the BET method, of at least500m2g-1.

12. An adsorbent material according to claim 11 having an apparent specific surface area, as measured by the BET method, of at least 800m2g -1.

13. An adsorbent material according to any one of the preceding claims having one or more compounds, selected from a silver salt and an organic cyclic amine, additionally impregnated thereon.

14. An adsorbent material according to claim 13 wherein the silver salt is silver nitrate and the cyclic amine is pyridine ortriethylenediamine.

15. An adsorbent material according to either claim 13 or claim 14wherein the silver salt is supported on the activated carbon in an amount of from 0.1 % to 15% by weight ofthe carbon.

16. An adsorbent material according to any one of claims 13 to 15wherein the cyclicamine is supported on the activated carbon in an amountoffrom 0.1 % to 20% by weight of the carbon.

17. An adsorbent material capable of removing How from a gaseous mixture substantially as hereinbefore described with reference to any one of Examples 1 to 24 and 27 to 33.

18. A method of preparing an adsorbent material capable of removing HCN from a gaseous mixture, which comprises the steps of (a) providing an activated carbon; (b) wetting the activated carbon with an impregnating solution of at least one transition metal salt of a non-chelating carboxylic acid; and (c) drying the wetted activated carbon to provide an activated carbon impregnated with the at least one salt.

19. A method according to claim 18 wherein the transition metal cation of the at least one salt is selected from ions of elements in the transition series of the first long period.

20. A method according to claim 18 wherein the transition metal cation of the at least one salt is selected from ions of elements in Groups 2B and 8 of the Periodic Table.

21. A method according to claim 19 or 20 wherein the transition metal cation of the at least one salt is selected from the group consisting if Co+2, Ni2+ and Zn2+.

22. A method according to any one of the preceding claims 18 to 21 wherein the at ieast one salt is a salt of a mono-ordi-carboxylicacid.

23. A method according to claim 22 wherein the monocarboxylic acid is an alkanoic acid.

24. A method according to claim 23 wherein the alkanoic acid is a Ci - C4 alkanoic acid.

25. A method according to claim 24 wherein the C, - 04 alkanoic acid is acetic acid.

26. A method according to claim 25 wherein the at least one salt is selected from the acetates of nickel, cobalt and zinc.

27. A method according to any one of the preceding claims 18 to 26 wherein the concentration of the at least one salt in solution is from 0.1 weight % to 30 weight%.

28. A method according to any one of the preceding claims 18 to 27 wherein between step (a) and step (b) the activated carbon is wetted with one or more further impregnating solutions of impregnants selected from silver salts and cyclic amines, and is subsequently dried afterwetting with each of the one or morefurther impregnating solutions.

29. A method according to claim 28 wherein the concentration of the impregnants in the one or more further impregnating solutions is from 0.01 to 10weight%.

30. A method according to claim 28 or 29 wherein between step (a) and step (b) the activated carbon is wetted with a solution of a cyclic amine and is subsequently wetted with a solution of a silver salt.

31. A method according to any one of claims 18to 30 wherein priorto wetting the activated carbon with any impregnating solution, the activated carbon is wetted with one or both of a basic solution having a pH greaterthan 13 and an acidicsolution having a pH less than 1.

32. A method according to claim 31 wherein the activated carbon is wetted with the acidic solution followed by the basic solution.

33. A method of preparing an adsorbent material capable of removing HCN from a gaseous mixture, substantially as hereinbefore described with reference to any one of Examples 1 to 24 and 27 to 33.