GB2265143A - Activated carbon from spent ion-exchangers - Google Patents

Abstract

Spent granular organic ion-exchangers and/or adsorbers are disposed of by carbonizing in a substantially inert atmosphere at 300 DEG C to 900 DEG C followed by activation in an oxidizing atmosphere to convert the ion-exchangers and/or adsorbers into activated carbon spherulets.

Description

METHOD FOR THE DISPOSAL OF USED ION-EXCHANGERS The invention relates to a method for the disposal of used up or spent synthetic resin ion-exchangers, in particular to the disposal of such ion-exchangers in the grain form in which they generally occur.

Synthetic resin ion-exchangers are porous polymers having numerous chemical groups with exchangeable ions. In general, they consist of a copolymer framework of styrene and divinylbenzene or styrene and acrylic acid, said framework carrying acid groups, in particular sulfonic acid groups, for cation-exchangers and basic groups (amines) for anion-exchangers.Organic ion-exchangers of the kind having a polymer resin matrix selected from the group consisting mainly of polystyrene resins, polyacrylic resins, polyalkyl amine resins or phenol-formaldehyde resins, which may be present as cation- or anion-exchanye resins, depending on their functional groups, and as adsorber resins are described in Ullmann's Encyclopedia of Industriai Chemistry, Fifth Edition, Volume A 14, VCH Verlagsgesellschaft mbH, Weinheim, Germany 1989 in the chapter "Ion Exchangers", in particular on pages 394-398, and are commercially available under the trade names Lewatit, Dowex, Kastel, Diaion, Relite, Purolite, Amberlite, Duolite, Imac, Ionac, Wofatit. Numerous applications of the ion-exchange resins are also described on pages 399-448 of the cited chapter.

The main purposes of using ion-exchangers are the exchange of undesirable ions present in water for less noxious ions, and the complete removal of ions. If the ions that produce hardness - basically Ca2+ and Mg2+ - are exchanged for Na+ ions, "hard" water becomes "soft" water.

If cations and anions are removed, one obtains demineralized water. Soft water is necessary, for example in textile industry, and demineralized water in the steam generation, in particular in high-pressure boilers.

In general, ion-exchangers become ineffective by obstruction, i.e., their pores get blocked by suspended particles or inorganic residues, such as iron compounds.

The latter are regularly flushed out, but with time, more pores become progressively blocked and finally the bed has to be replaced. At this point, the problem of disposal arises. As long as no ions that pollute the environment are present, the spent ion-exchangers can be disposed of in waste dumps.

Spent granulous organic ion-exchanger resins are contaminated with large amounts of inorganic or organic foreign matter, such as suspended particles of all kinds, sludge, microorganisms, algae and various cations, e.g.

sodium, potassium, iron, and calcium ions. The amount of these impurities is usually up to 20% by weight, based on the dry substance. The granular ion-exchange resins to be disposed of have in most cases a water content which may amount to up to 50% by weight.

In accordance with the invention, a method for the disposal of spent granular organic ion-exchangers of the aforementioned kind comprises carbonizing the ionexchangers in a substantially inert atmosphere at temperatures of from 300"C to 900"C and subsequently activating the same in an oxidizing atmosphere to convert said ion-exchangers into activated carbon spherulets.

A method is known for converting specifically defined, polysulfonated macroporous cross-linked vinyl aromatic polymers into carbonaceous adsorber particles by heating to temperatures of up to 1200"C, see U.S. Patent 4,957,897.

The sulfonic acid groups are released during pyrolysis, radical sites are generated which lead to strongly crosslinked structures that are not meltable and contain little volatile carbon.

However, it has now unexpectedly been found that high quality abrasion-proof activated carbon spherulets can be produced also from strongly contaminated spent synthetic resin ion-exchangers by pyrolysis and that the various foreign substances do not impair the quality and stability of the adsorber. Surprisingly enough, the macro- and mesopore structure of the feedstock is maintained during the disintegration of the impurities and the carbonization.

The accumulated organic and biological products are destroyed or escape without forming any pronounced carbon residues, in particular if the carbonization is conducted in weakly oxidizing atmosphere.

The ion-exchangers may be partly or completely replaced by granular organic adsorber resins to which the method described herein is equally applicable.

In spent or used up cation-exchanger resins, the cations are usually bonded to sulfonic acid groups and are substantially converted into sulfates at temperatures of up to 400"C. At higher temperatures, they are reduced by carbon, resulting in considerable amounts of sulfides. It is, therefore, advantageous to first convert cationexchanger resins into the H+ form prior to carbonization.

This is preferably done by washing the still moist material with an acid, i.e. prior to drying.

In order to remove the water content, which as already mentioned may amount to up to 50% of the granular organic ion-exchange resin, it is recommended to dry the used up granular resin to be disposed of, preferably in a rotary drier dr in a fluidized bed. Prior to attaining the softening point and usually after drying, the synthetic resin ion-exchangers are preferably powdered with an inert inorganic powder, preferably carbon powder, to prevent agglomeration and to maintain the granular structure during the entire treatment.

Up to a temperature of 400 C, preferably up to about 300"C to 350 C, the inert atmosphere of the carbonization step can contain 0.2 to 4 volume% oxygen. The oxygen content is preferably controlled by the addition of air.

This pre-oxidation is recommended not only for the purpose of the destruction of the organic impurities, but also, together with the powdering and/or slow rise in temperature which are preferred, for the purpose of reduction of volatile carbon by oxygen bridges and radical sites, and of preventing melting or sticking together. In particular, the pre-oxidation is very important when the resins do not contain sulfonic acid groups, e.g. anion-exchanger resins or adsorber resins. It is recommended to process these resin types together with cation-exchanger resins containing sulfonic acid groups.

Minor amounts of cations, such as alkaii metal and alkaline earth metal ions, selected from the group consisting of sodium, potassium etc. or calcium, which were already converted into sulfates at the beginning of the pyrolysis, do not disturb the carbonization and activation, surprisingly enough, they even promote the activation step.

The activation of the carbonized material follows upon the carbonization at about 700 C. As with the carbonization, it can be conducted in a rotary drier or even better in a fluidized bed. To activate the material, steam and/or carbon dioxide is added in an amount of 3 to 50, preferably 3 to 15 volume%, to the substantially inert atmosphere. The activation temperature can be up to 900"C.

To save energy, the activation can be conducted in the same apparatus after the carbonization with the carbon dioxide, if used, being added during carbonation. However, it may be advantageous for specific technical and procedural reasons to conduct the activation in an independent separate step, all the more since the carbonization up to temperatures of about 500"C already entails a considerable shrinkage and a weight loss of the feedstock of from 60-90%. The carbon content of the activated carbon spherulets after activation is more than 90% by weight.

Example 1: 1 kg of a macroporous ion-exchanger consisting mainly of styrene and divinylbenzene, which was present in the H+ form and was used in the synthesis of fuel additives (MTBE) and had become inactive, was dried in a rotary drier at 110 C. The loss in weight, caused by vaporization of hydrocarbons and some moisture, was approximately 13%.

Thereafter, it was heated up to 300"C in an atmosphere consisting of 80% inert gas and 15% air and maintained therein for one hour. The gain in weight was about 8%.

Whereupon the temperature was raised to 700"C in inert atmosphere within 3 hours. 5% steam was added in the range of 700"C to 900 C. The temperature rise from 700"C to 820or lasted for thirty minutes, the 9000C was attained in ten further minutes.

The yield was 28%, based on the feedstock. An agglomeration of the spherulets did not occur at any time.

A shrinkage of diameter of from approximately 0.8 mm to 0.6 to 0.7 mm was observed. The apparent density of the spherulets was 1.08 g/cm3 at a pore volume of more than 0.9 ml/g, of which 0.55 ml/g were micropores. A specific surface of 1088 m2/g was determined by the BET method. A 0.55 mm spherulet could be loaded puncti-form with 300 g without breakage.

Example 2: 1 kg of a gel-type cation-exchanger which had been used for the softening of water and had no sufficient activity was converted into the H+ form by means of a hydrochloric acid solution. After superficial air-drying, the moisture was about 50%. After drying at 710 C, oxidation was carried out in air at 300"C for 6 hours. The procedure of Example 1 was thereafter applied. The yield was 31%, based on the feedstock. The spherulets were partly agglomerated.

Example 3: The process of Example 2 was applied in the same manner and with the same feedstock, however, after oxidation at 300"C, powdering was conducted with 5% carbon powder and the temperature was raised to 700"C within 6 hours. The agglomeration of the spherulets and the formation of a blistered structure could thus be prevented.

The inner surface of the obtained activated carbon particles amounted to approximately 1000m2/g (BET) and the yield was 40% based on the feedstock. The average bursting pressure was 250 g at a diameter of 0.5 mm.

Also macroporous adsorber resins consisting mainly of a divinylbenzene-copolymer were processed in the same manner to yield activated carbon, said resins had been already used up and still contained adsorbed organic substances. Since these products do not contain any sulfonic acid groups, a pre-oxidation is particularly important.

Claims (14)

1. A method for the disposal of spent granular organic ion-exchangers comprising carbonizing said ionexchangers in a substantially inert atmosphere at temperatures of 300"C to 900"C and thereafter activating the same in an oxidizing atmosphere to convert said ionexchangers into activated carbon spherulets.

2. A method as claimed in Claim 1, wherein the spent ion-exchangers include cation-exchangers, in particular those consisting of sulfonated styrene-divinylbenzen copolymers or styrene-acrylic acid copolymers.

3. A method as claimed in Claim 2, wherein the cation-exchangers are in the H+ form.

4. A method as claimed in any preceding Claim, wherein the spent ion-exchangers include anion-exchanger resins, in particular those consisting of polystyrene resins or polyacrylic resins with tertiary or quaternary amine groups.

5. A method as defined in any preceding Claim, wherein the spent ion-exchangers are dried prior to carbonization.

6. A method as defined in any preceding Claim, wherein at a carbonization temperature of up to 400 C, preferably up to 350 C, the inert atmosphere contains 0.2 to 4 volume% oxygen.

7. A method as claimed in any preceding Claim, wherein for activating the carbonized material, at above 700"C, steam is added to the substantially inert atmosphere in an amount of 3 to 50 volume%, preferably 3 to 15 volume%.

8. A method as claimed in any preceding Claim, wherein for activating the carbonized material, at above 700or, carbon dioxide is added to the substantially inert atmosphere.

9. A method as claimed in any preceding Claim, wherein the activation is conducted as a separate step.

10. A method as claimed in any preceding Claim, wherein the granular ion-exchangers are powdered with an inert powder, preferably powdered carbon, prior to attaining the softening point.

11. A method as claimed in any preceding Claim, wherein the steps are conducted in a fluidized bed or in a rotary drier.

12. A method as claimed in any preceding Claim, wherein the ion-exchangers are partly or completely replaced by granular organic adsorber resins.

13. Activated carbon spherulets of high stability prepared by a method as claimed in any one of Claims 1 to 12.

14. A method substantially as hereinbefore described and illustrated in the accompanying drawings.