GB2069988A

GB2069988A - Method of producing filter pads

Info

Publication number: GB2069988A
Application number: GB8103408A
Authority: GB
Original assignee: Schumachersche Fabrik GmbH and Co KG
Current assignee: Schumachersche Fabrik GmbH and Co KG
Priority date: 1980-02-19
Filing date: 1981-02-04
Publication date: 1981-09-03
Also published as: JPS56155608A; GB2069988B; DE3006171B1; FR2475921A1; FR2475921B1

Abstract

A method of producing a filter pad comprises filling a quantity of carbon particles, preferably activated charcoal particles, with liquid, coating the filled carbon particles with a carbonizable binder which is not soluble in the filling liquid or miscible therewith, compressing the filled and coated carbon particles, and heating the thus formed body to remove the filling liquid, and to set and carbonize the binder, wherein at least the carbonization is performed in a non- oxidizing environment.

Description

SPECIFICATION Method of producing filter pads The present invention relates to a method of producing filter pads, in particular for the fine filtration of fluids, in which a quantity of particles consisting of carbon are at least partially mixed with a finely dispersed binder and in which the mixture formed from the carbon particles and binder adhering to the latter is compressed to form a filter pad, wherein by the application of heat a mutual adhesion of the carbon particles is attained, the pores of which remain at least partially open with respect to the environment.

Several methods of producing filter pads, the main constituent of which is carbon, are already known and are described for example in German Offenlegungsschrift No. 15 17 298 and in U.S.

Patent Nos. 3 474 600 and 3 217 715.

In all these methods grains consisting of carbon are joined together with a thermoplastic or a thermosetting binder, and the mixture of such carbon grains and pulverized binder are formed into a self-supporting filter pad by the application of heat and pressure.

This method has the disadvantage that the pores of the carbon grains are clogged to a considerable extent by the film produced by the melted binder during the heat treatment, as a result of which the gases flowing through the filter pad release the gases to be adsorbed with a considerable delay. In addition the clogging of the pores results in a considerable reduction of the total adsorption capacity and the rate of adsorption of the carbon.

In addition, German Auslegeschrift No.

22 27 533 describes a method of producing filter pads in which the pores of the carbon particles are closed to a slight extent with respect to the surrounding substance, so that these filter pads are distinguished by a somewhat higher adsorption capacity than those produced according to other methods.

The above methods of production all only allow the production of rigid or inherently resilient filter pads, the technical characteristics of which are essentially determined by the presence of binders which are ineffective for adsorption and do not even consist of carbon. The range of application of such filter pads is severely restricted by the chemical and thermal resistance.

The object of the present invention, therefore, is to provide a method of producing filter pads from carbon so as to allow both the adsorptive properties and the filtering characteristics of such filter pads to be predetermined and hence filter adsorbents specific to the task to be produced, and such filter pads should also be suitable for use at higher temperatures and in chemical conditions compatible with carbon and be capable of being produced on a large scale.

The present invention provides a method of producing a filter pad, comprising supplying filling liquid to a quantity of carbon particles until the latter have been filled at least predominantly, at least partially coating the surface of the carbon particles filled with filling liquid with a carbonizable binder which is not soluble in the filling liquid or miscible therewith, compressing the carbon particles filled with liquid and coated with binder to form a filter pad, and heating the formed body to remove the filling liquid present in the carbon particles, to set the binder and to carbonize the binder, wherein at least the carbonization is performed in a non-oxidizing environment.

According to the method of the invention, therefore, prior to coating with carbonizable binder, the carbon particles are filled with a liquid which may be organic and/or inorganic, so that when the filter pad formed after the coating of the carbon particles is heated the liquid evaporates and then diffuses through the binder which is simultaneously setting during this heat treatment, so that permeable pores are produced in the binder. In this way both the pores of the carbon particles and their openings on the outer surface of the carbon particles are kept free.Finally, by heating the formed body in a non-oxidizing environment the binder which has become permeable is carbonized, so that the desired monolithic formation of the filter pad entirely of carbon is finally attained, which filter pad is resistant even at temperatures of up to for example 100000 in a non-oxidizing atmosphere.

In this connection German Auslegeschrift No.

26 24 663 has already disclosed a method by means of which carbon-containing absorbents may be produced as a moulded article having a diameter of up to 3 mm and a length of up to 7 mm. An advantage of this method is said to lie in the fact that particularly thin moulded articles with a diameter of less than 1 mm may be produced.

The possibility of checking the adsorption kinetics by purposeful control of the macropore system is described. The method, however, makes use of carbon-containing raw materials which are only converted into carbon during production in a manner which is difficult to control.

The method according to the invention is made possible by the intended predeterminability of the adsorptive method and the filtering characteristics of filter pads produced according to the invention on account of the fact that the adsorption rate and the adsorption capacity are retained in the end product and the filtering characteristics may be determined by selection of the size distribution of the carbon particles.

The filter pad produced according to the method of the invention is thus composed of primary and secondary carbon, wherein the primary carbon formed by the carbon particles remains unchanged from the beginning, while the secondary carbon is produced by carbonization of the binder, and a porous supporting framework, in which carbon particles coalesce with one another to form a stable monolithic structure, is formed from the binder by the carbonization process.

The characteristics of the carbon formed secondarily by the carbonization, in particular activated charcoal, are not dependent on those of the primary carbon.

The possibility of achieving defined multimodal pore size distributions by the method of the invention provides, for example, the range of application of the stero-specific adsorption of macromolecules with molecular weights of an order of magnitude of over 10,000, in particular in the range of pore sizes between the pure adsorption pores of approximately 20 A and the pure filtration pores over approximately 0.1 ym ( 1 zoo A).

In this way, for example, it is possible to account for the unexpected effectiveness of filter pads produced according to the invention for removing, for example, toxins inter alia of pyrogenes from liquids by adsorption filtration with contact periods of approximately 1 minute on account of this stereo-specificity.

In contrast to the conventionally formed filter rods of carbon or carbon molecular sieves which are only available in dimensions of the order of magnitude of a few mm at most, monoliths may be produced according to the method of the invention from carbon of any cross-sectional shape and dimensions relating to practice with extremely high mechanical strength.

It is possible, therefore, to produce hollow cylindrical filter pads, for example, with an external diameter of up to 300 mm and above and with lengths of over 1000 mm. In the same way monolithic solid cylinders may be produced from carbon particles with such dimensions, as well as parallelepipedal plates or blocks with edge lengths of up to 300 mm or more.

In the case of a monolithic hollow cylinder produced according to the invention having an average diameter of the filtering pores of approximately 3 ,um and having dimensions of 70 mm in external diameter, 30 mm in internal diameter and 1000 mm in length, the bursting strength is over 15 bar.

The invention therefore allows the production of monolithic filter pads from carbon by carbon particles joined to one another by carbon, in particular activated charcoal, newly formed therebetween in sits, in such a way that a carbonjoined agglomerate forming the filter pad results.

The primary carbon may be used in powdered to fine-grained and/or fibrous form and/or as a carbon molecular sieve. The primary carbon used in this form, especially activated charcoal particles, preferably has a particle size of 0.1 to 2000 ym, in particular 0.2 to 200 ym. The filling liquid may be supplied to the mass of carbon particles under a vacuum or at atmospheric pressure. By means of the capillary suction effect of the pores of the carbon particles the pores will soak up the filling liquid into their furthest geometrical surface extremities.

In this connection it has been found to be particularly advantageous for particles consisting of activated charcoal or molecular sieves consisting of the latter to be used, so that monolithic filter pads produced consist entirely of one basic material, provided a binder is used whose fusing temperature or thermosetting temperature is below the boiling point of the filling liquid and may be carbonized to activated charcoal.

A substance in liquid, dry powdered or dissolved fdrm may be used as the binder, it being necessary in the latter case to use a solvent which evaporates when the formed body is heated but which has a lower boiling point than the filling liquid.

In the case of a dry powdered binder the filling liquid acts as an adhesive between the activated charcoal or other carbon particles and the binder, while, on the other hand, when using a liquid or dissolved binder the filling liquid prevents the binder from penetrating into the pores of the activated charcoal or other carbon particles.

In each case the binder must be such that during the compressing step it imparts to the quantity of material formed from activated charcoal or other carbon particles a strength in the green state which lasts until the formed body emerges as a rigid, self-supporting structure after the subsequent heat treatment for evaporating the filling liquid and for the setting of the binder.

In order to carbonize the organic binder it is advantageous to heat the formed body to a temperature of 900 to 1 5000C in a non-oxidizing atmosphere.

If a preferably powdered thermosetting synthetic resin, in particular with a carbon content of at least 40% by weight, or a pitch, in particular powdered coal-tar pitch, is used as a binder for the mutual joining of the activated charcoal or other carbon particles, it is advantageous to perform the heat treatment of the formed body in dependence upon the fusing or thermosetting temperature of the resin at a temperature of 900C to 2000C or in dependence upon the specific fusing temperature of the pitch used.

The quantity of material formed from activated charcoal or other carbon particles and binder for forming the filter pad may be compressed by means of forming methods known per se, such as jarring, stamping, pressing and extrusion, wherein the forms may be subjected in the mould or after forming out to heat treatment in order to evaporate the filling liquid and/or the solvent.

If a powdered synthetic resin or pitch is used as the binder, filter pads with a maximum carbon content may be produced when very finely powdered activated charcoal up to a maximum of 50% by weight of the quantity of the synthetic resin or the pitch is mixed with the binder.

When using a binder in the form of a liquid substance a fine powder of a carbon-containing organic material, e.g. metal, wood or cellulose, which is volatile over 2000 C, may be suspended in order to produce a viscosity of 10 to 100 mP.s which is particularly advantageous for treatment.

These carbonizable additives advantageously increase the density of the structure consisting of secondary activated charcoal and hold together the particles of primary activated charcoal in the final product and thus increase the adsorption capacity and the mechanical strength of the agglomerate.

The invention will be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a fragment of a filter pad produced by the method according to the invention; and Figure 2 is a portion of the fragment, on a greatly enlarged scale, indicated by a dash-dot circle in Figure 1.

As shown by the fragment of the filter pad illustrated in Figure 1 ,the filter pad consists entirely of carbon, preferably activated charcoal. In this case particles 1 0 consisting of primary activated charcoal are joined together in such a way that they form a rigid monolithic structure.

Conveying pores 12 are situated in these activated charcoal particles 10.

The mutual joining of the activated charcoal particles 10 is attained in that, after they have been filled with a filling liquid, their surface is coated with a carbonizable binder 14 which is not miscible with the filling liquid or soluble in it.

After the activated charcoal particles 10 filled with liquid and coated with the binder 14 have been compressed to form a filter pad, the latter sets. At the same time the liquid evaporates and diffuses through the hardening binder 14, so that permeable pores are produced in the latter.

By carbonization of the hardened filter pad in a non-oxidizing environment the binder 14 is finally converted into secondary activated charcoal, and filtration pores 16, which are predominantly surrounded by the porous binder 14, remain between the primary activated charcoal particles 10.

A porous supporting framework, in which the primary activated charcoal particles 10 coalesce with one another to form a stable monolithic structure, is provided by the binder 14 by the carbonization process.

There is given below the sequence of the individual steps of the method according to the invention.

In order to produce a filter pad which in the finished state consists, for example, entirely of primary and secondary activated charcoal, the following are used: Primary activated charcoal in powdered state 64.5% by wt.

Filling liquid 14.0% by wt.

Carbonizable binder 21.5% by wt.

The method comprises the following steps: 1. Saturation of primary activated charcoal particles 10 with filling liquid or wetting the surface of the carbon particles with filling liquid.

2. Coating the primary activated charcoal particles 10 saturated with filling liquid or the wetted carbon particles with carbonizable binder 1 4.

3. Forming the filter pad by compressing the activated charcoal particles 10 which have been filled or wetted with liquid and the surface of which has been coated with binder 14.

4. Setting of the binder 1 4 and evaporation of the filling liquid present in the primary activated charcoal particles 10 or on the surface of the carbon particles.

5. Carbonization in a non-oxidizing environment of the binder 14 which has set.

6. Cooling of the formed body in a nonoxidizing environment. Mechanical treatment to finished size.

Claims

1. A method of producing a filter pad, comprising supplying filling liquid to a quantity of carbon particles until the latter have been filled at least predominantly, at least partially coating the surface of the carbon particles filled with filling liquid with a carbonizable binder which is not soluble in the filling liquid or miscible therewith, compressing the carbon particles filled with liquid and coated with binder to form a filter pad, and heating the formed body to remove the filling liquid present in the carbon particles, to set the binder and to carbonize the binder, wherein at least the carbonization is performed in a nonoxidizing environment.

2. A method as claimed in Claim 1, wherein powdered to fine-grained and/or fibrous carbon particles and/or a carbon molecular sieve is or are used.

3. A method as claimed in Claim 1 or 2, wherein particles consisting of activated charcoal or a molecular sieve consisting of the latter is or are used.

4. A method as claimed in any of Claims 1 to 3, wherein activated charcoal particles with a grain size of 0.1 to 2000 ,lim are used.

5. A method as claimed in Claim 4, wherein activated charcoal particles with a grain size of 0.3 to 200 ym are used.

6. A method as claimed in any of Claims 1 to 5, wherein the filling liquid is supplied to the carbon particles under a vacuum or at atmospheric pressure.

7. A method as claimed in any of Claims 1 to 6, wherein a substance in liquid, dry powdered or dissolved form is used as the binder, and in the latter case the binder is dissolved in a solvent which evaporates when heating the formed body to a temperature below the boiling range of the filling liquid.

8. A method as claimed in Claim 7, wherein a binder is used in dry powdered form, the fusing temperature or thermosetting temperature of which is below the boiling range of the filling liquid.

9. A method as claimed in Claim 7 or 8, wherein a synthetic resin is used as the binder.

10. A method as claimed in Claim 9, wherein the synthetic resin has a carbon content of at least 40% by weight.

11. A method as claimed in Claim 7 or 8, wherein pitch is used as the binder.

12. A method as claimed in Claim 1 1, wherein powdered coal-tar pitch is used as the binder.

13. A method as claimed in any of Claims 9 to 12, wherein a synthetic resin or pitch is used as the binder, and wherein the heat treatment of the formed body is performed in dependence upon the fusing or thermosetting temperature of the synthetic resin at a temperature of 900C to 200or or in dependence upon the specific fusing temperature of the pitch used.

14. A method as claimed in any of Claims 9 to 13, wherein a synthetic resin or pitch is used as the binder, and wherein powdered carbon up to a maximum of 50% by weight of the quantity of the synthetic resin or the pitch is mixed with the binder.

15. A method as claimed in Claim 7, wherein a liquid substance is used as the binder, and wherein a fine powder of a carbon-containing organic material which is volatile over 2000C is suspended in order to produce a viscosity of 10 to 100 mP.s which is suitable for treating the liquid substance.

16. A method as claimed in any of Claims 1 to 1 5, wherein the said compression of the particles filled with liquid and coated with binder is carried out at atmospheric temperature.

17. A method according to Claim 1 of producing a filter body, substantially as herein described with reference to the accompanying drawings.

17. A method according to Claim 1 of producing a filter pad, substantially as herein described with reference to the accompanying drawings.

New claims or amendments to claims filed on 28th May 1981.

Superseded claims 1 and 17.

New or amended claims.~

1. A method of producing a filter body, comprising supplying filling liquid to a quantity of carbon particles until the latter have been filled at least predominantly, at least partially coating the surface of the carbon particles filled with filling liquid with a carbonizable binder which is not soluble in the filling liquid or miscible therewith, compressing the carbon particles filled with liquid and coated with binder to form a filter body, and heating the formed body to remove the filling liquid present in the carbon particles, to set the binder and to carbonize the binder, wherein at least the carbonization is performed in a nonoxidizing environment.