GB2280898A - Activated carbon - Google Patents

Abstract

Activated carbon is produced by carbonizing granular organic gel ion-exchangers in a substantially inert atmosphere at 600 to 900 DEG C, followed by activation in an oxidising atmosphere at 800 to 900 DEG C. The ion-exchangers may be cation-exchangers or anion-exchangers. The activated carbon so produced has a narrow mesopore distribution of mainly 100 to 300 ANGSTROM and only a few micropores.

Description

ACTIVATED CARBON SPHERULES AND METHOD OF PRODUCTION THEREOF FROM ION-EXCHANGERS Activated carbon is the most commonly used adsorbent. This fact is a result of its quite unspecific adsorptive properties making it the "workhorse" of adsorption processes. Restrictive legislation as well as an increasing awareness of the need for environmental responsibility have resulted in a rising demand for activated carbon. Activated carbon is produced by carbonizing and activating compounds containing carbon atoms. As a matter of fact, those compounds which produce an adequate yield are preferred, given that a considerably high loss in weight occurs, as some components volatilize during the carbonization process and as a result of burning off during activation.

Moreover, the type of active carbon depends on the basic material from which it is produced, which may be fine or coarse pored, solid or fragmentary. Common raw materials are coconut shells, wood cuttings, peat, pit coal, tar but also certain polymers which, inter alia, play a specific role in the production of active carbon fabrics. Active carbon is applied in various forms: pulverulent carbon, granular carbon, moulded carbon and, since the end of the 1970's, also spherical carbon. There is great demand for spherical carbon in specialised fields, such as the production of protective clothing against chemical toxicants and filters for low pollution concentrations in high air quantities, on the one hand due to its special form, and on the other because of its extremely high abrasion resistance.

Even today, most spherical carbon is produced by a multistage method using tar-like distillation residues, which is thus both complicated and expensive, although high quality spherical carbon can be produced by carbonization and activation of polysulfonated, macroporous ion-exchangers on the basis of styrene and divinyl benzene.

It is stated in the relevant patents that macroporous resins should form the basic material and that gel-type resins are not suitable as they cannot be activated since no "inner surface" is formed. In most cases, macroporous ion-exchanger resins have a stronger cross-linked structure than gel-types and normally require a higher proportion of expensive divinyl benzene.

As a result of the considerable weight losses during carbonization and activation, the production costs of the basic materials are of essential importance. This may be the reason that activated carbon spherules produced from macroporous ion-exchangers only have a small market share.

It is amongst the objects of the present invention therefore to find a suitable method for the production of activated carbon spherules from low-priced ion-exchangers, particularly gel-type ion-exchangers.

It has surprisingly been found that activated carbon spherules can be produced from granular organic gel-type ion-exchangers by carbonizing them in predominantly inert atmosphere at temperatures of 600 to 900"C and afterwards activating them in an oxidizing atmosphere at 800 to 900 C. The carbonization temperature preferably is 750 to 875"C.

Both cation- or anion-exchangers may be employed, suitable cation-exchangers particularly consisting of sulfonated styrene-divinyl benzene copolymers or styreneacrylic acid copolymers. Cation-exchangers are preferably employed in the H+ form.

Suitable anion-exchangers are in particular those based on polystyrene resins or polyacrylic resins with tertiary or quaternary ammonium groups.

Prior to carbonization, the gel-type ionexchangers are preferably subjected to oxidation in an oxidizing atmosphere at temperatures of up to 400"C.

As a rule, particularly with ion-exchanger resins, the oxidation phase is preceded by a drying process during which up to 55% of the moisture is driven out. Such drying process can be carried out in air. However, the oxygen content has to be reduced when the temperature is raised in the oxidation step. When the temperature has reached 300"C it should have been reduced to 1 to 5%. The oxidation phase lasts 20 minutes to 6 hours, depending on the aggregate used (fluidized bed/rotary tube). The oxygen obviously plays a decisive part in the oxidation process, as do the sulfonic acid groups of the cationexchangers: it forms oxygen bridges, reactive functional groups or radical sites, a process which reduces the amount of volatile components and increases the softening temperature.

The softening or agglomeration of the ionexchangers during heating can mainly be prevented by sulfonation and preoxidation. However, part of the spherules may nonetheless agglomerate, in particular if an undesired standstill occurs in the rotary tubular furnace.

Such agglomeration can reliably be avoided by powdering the ion-exchangers particularly ion-exchanger resins with a small amount of a non-softening powder like carbon power, preferably with pit coal powder or activated-carbon powder, usually in a weight of 0,5 to 5%.

The carbon powder is added when filling the rotary tubular furnace. It distributes very quickly and provides the surface of the ion-exchanger spherules with a "dry" coating in case they become sticky. At least at test- or pilot level, no agglomeration was observed in the fluidized bed.

In the subsequent carbonization, which may also be considered as a pyrolysis or smouldering process, CO2, SO2, H2O and CO as well as hydrocarbons and partly oxidised hydrocarbons volatilize first. In the case of polysulfonated ion-exchangers - and such is the normal case - the sulfur content may reach a value above 15%, based on anhydrous material. If one starts from the H+ form, most of the sulfonic acid separates as SO2 and H2O, while a certain percentage of sulfur is incorporated into the material, partly in the form of thio-ether. If one starts from the Na+ form, the sulfate is formed first, before the carbon reduces it to become a sulfide. Quite apart from the considerably unpleasant smell, the high ash content is problematic. It is therefore recommended to convert the Na+ form into an H+ form by means of an acid washing.Furthermore, it is advantageous to effect a partial or light activation during the pyrolysis (smouldering) process, namely by addition of some steam, in order to increase the material's capacity and readiness for burning off" within the subsequent full activation step.

This smouldering leads to a weight loss of 40 to 60%, based on the dry substance. Given the additional loss of more than 10% by weight of sulfur, the weight loss based on carbon amounts to about 30 to 50%. Depending on the respective technical equipment employed, the smouldering process at a temperature of from 350 to 900"C will take from less than one hour up to several hours.

The oxygen introduced in the course of the oxidation process volatilizes again during smouldering, first in the form of CO2, and later, when higher temperatures have been reached, mainly as CO. This process is comparable to a first, or partial, activation which will actually show a positive effect when the real, or full, activation step is carried out.

Normally, activation is conducted at temperatures between 800 and 900 C, which step includes the burning off of CO2, H20, 02 or 2 in the form of air, or respectively diluted with inert gas. The steam activation process is carried out by adding 3 to 50, preferably 3 to 15% of water vapour to the predominantly inert atmosphere. The diffusion of the oxidizing gas into the interior of the spherules has to be faster than the burn-off reaction; otherwise the burn-off would be mainly concentrated on the outer shell of the spherules. This can be achieved by appropriate combinations of temperatures and concentrations, as are known to producers of activated carbon.According to the desired activation degree, 30 to 50% of the carbon present after the pyrolysis is additionally gasified while the sulfur content reduces down to 1 to 2t. The approximate yields, based on the dry substance, are 25 to 30%, at a BET-surface area of 800 m2/g and a benzene adsorption of about 30 to 35% (P/P0 = 0.9).

Inner surfaces of 1,500 m2/g may well be obtained, in which case, however, the yield accordingly goes down to 12 to 15%, based on the dry starting material.

The thermal treatment at first leads to an increase in density. Afterwards, an increasing porosity as well as shrinkage of the material can be observed. The granular size and the granulation distribution depend on the starting material. It must, however, be assumed that the end product's diameter will be by 10 to 20% smaller.

The bulk weight of the end product varies between 430 and 650 g/l.

The different thermal treatments can be carried out either in a rotary tubular furnace or in a fluidized bed. They may be conducted in one and the same aggregate or even in separate aggregates, thus permitting optimum conditions. This is also advantageous with respect to the considerable mass or weight difference between the starting material and the end product. The carbonization and activation process can both be carried out in a fluidized bed, but also alternatively in different stages, the carbonization being conducted in a rotary tubular furnace, whereas the activation is effected in a fluidized bed. As a result of the poor heat transfer and gas exchange, reaction times are considerably longer in a rotary tubular furnace, and this is particularly true for the activation step. However, this has no influence on the quality of the final product.The differences affect the processing time rather than the product quality.

The invention also provides activated carbon spherules of high stability, prepared according to the above described method. A special feature of this spherical activated carbon is the pore distribution structure therefore which exhibits a small mesopore spectrum within the range of 100 to 300 A and only few macropores.

The invention will now be further described by way of the following examples: EXAMPLE 1 4,300 kg of a gel-type ion-exchanger (DOW HCRSE H+) having a diameter of 0.4 - 0.8 mm was dried (weight loss of little more than 50%) and pre-carbonized in a rotary furnace at 400"C and for a period of 12 hours in a nitrogen/air mixture of a 2:1 ratio. The carbonization process was then finished in a nitrogen atmosphere at approximately 900"C (6 hours): After the precarbonization process (400 C), the yield was about 22%, based on the (moist) basic material, but fell to 17% after the 900"C treatment.

The carbonized material had a small inner surface (about 200 m2/g). Afterwards, it was activated at 900"C for 8 hours -in a pilot plant (rotary tube) by addition of water vapour. As a result, an inner surface of 1,300 m2/g could be achieved, with a burn-off percentage of 35.

After precarbonization (400 C), for one hour, the apparent density of the material was 750 g/l. After the second step (900 C), it reached a value of about 900 g/l, but fell to 650 g/l after the activation step. At the same time, the diameter of the spherules diminished by 20%.

EXAMPLE 2 starting Material: 10 m3 (= 7,8 t) gel-type ion-exchanger resin, acid form, commercialized as DOW HCRSE H+, water content 52%.

I. Smouldering Stage 1 (up to 400"C). Addition of approximately 5% oxygen in nitrogen.

Dwell time in the rotary furnace approximately 1 hour.

Yield: 1.975 kg, based on the dry substance: 52,7%.

Stage 2 (up to 850"C). Addition of 20% water vapour in nitrogen.

Dwell time approximately h hour.

Yield: 1.663 kg, related to dry substance: 44,4%.

The smouldered material had a packing density of 938 g/l and a BET-surface of 80 m2/g.

II. Activation Activation was also conducted in a rotary furnace because the fluidized bed was defective.

Temperature: 875"C Addition of 25% water vapour in nitrogen Dwell time: 8 hours Yield: 1.104 kg, based on the dry substance: 27%.

The BET-surface was 1.250 m2/g at an apparent density of 634 g/l.

The ash content was 0,4%.

Similar results in a much shorter period of time would have been obtained if the activation had been carried out in a fluidized bed.

Claims (18)

1. A method for the production of activated carbon, comprising carbonizing granular, organic gel ionexchangers in a substantially inert atmosphere at temperatures of 600 to 900"C and thereafter activating the same in an oxidizing atmosphere at temperatures of 800 to 900"C.

2. A method as claimed in Claim 1, wherein the ionexchangers are cation-exchangers selected from the group consisting of sulfonated styrene-divinyl benzene 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 Claim 1, wherein the ionexchangers are anion-exchangers selected from the group consisting of polystyrene resins or polyacrylic resins with tertiary or quaternary ammonium groups.

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

6. A method as claimed in any preceding Claim, wherein the carbonization temperature is within the range of 750 to 875 C.

7. A method as claimed in any preceding Claim, wherein the gel ion-exchangers are subjected to oxidation in an oxygen containing atmosphere at temperatures up to 400"C, prior to carbonization.

8. A method as claimed in Claim 7, wherein the oxygen content in the atmosphere is progressively reduced and the temperature progressively increased.

9. A method as claimed in any preceding Claim, wherein water vapour in an amount from 3 to 50% by volume is added to the substantially inert atmosphere to activate the carbonized material.

10. A method as claimed in any preceding Claim, wherein carbon dioxide is added during activation in the substantially inert atmosphere.

11. A method as claimed in any preceding Claim, wherein air is added during activation in the substantially inert atmosphere.

12. A method as claimed in any preceding Claim, wherein both carbonization and activation are conducted in a fluidized bed.

13. A method as claimed in any of Claims 1 to 11, wherein carbonization and activation are conducted in separate steps.

14. A method as claimed in Claim 13, wherein the carbonization is effected in a rotary furnace and the activation is carried out in a fluidized bed.

15. A method as claimed in any preceding Claim, wherein the gel ion-exchangers are powdered with a nonsoftening powder before pyrolysis.

16. Activated carbon prepared by the method of any one of Claims 1 to 15.

17. Spherical activated carbon having a narrow mesopore distribution of mainly 100 to 300 A and only few macropores.

18. A method substantially as hereinbefore described and illustrated by the Examples.