EP0260703A1 - Method and device for producing hollow extruded ceramic articles - Google Patents

Abstract

1. Method of producing extruded ceramic moulded bodies with hollow cavities provided therein, whereby a plastic ceramic mass is initially produced, the plastic mass is extruded into a trough, the web is cut to length, and the cut moulded bodies are dired and vitrified, the extruded web being supported by a cushion of air which builds-up beneath the web, characterised in that a web, in the form of a lattice-like body which is suitable for use as a catalyst carrier and which is provided with a plurality of channel-like cavities and an open frontal face of more than 80% with a high coefficient of plasticity for the ceramic mass to produce thin cell walls, is extruded without contact directly into a trough, which surrounds the web at its lower external faces in an equidistant manner and in which a cushion of air builds-up as a result of air emerging from numerous openings in the internal walls of the trough, the cushion of air supporting the lower external faces of the web and of the cut moulded body in a uniform and planar manner, and in that the moulded bodies are displaced and mounted in the trough so as to be suspended on the cushion of air after being cut from the web.

Description

The invention relates to a method for producing extruded ceramic molded articles according to the preamble of claim 1 and a device for carrying out the method according to the preamble of claim 5.

In the production of ceramic moldings with channels by the extrusion process from a plastic mass (DE-OS 24 21 311), the demand is often made to form thin-walled bodies as possible. The limit is determined by the deformation occurring under the weight of the fresh extruded ceramic molded body blank during extrusion and during storage immediately after the extrusion, when the weight of the molded body is transferred to its structure. If the ceramic mass is too soft and / or the walls are too thin, the spatial shape created by the extrusion cannot be maintained. The moldings collapse or deform in such a way that they become unusable. For example, this problem arises in the production of ceramic molded bodies of honeycomb structure which serve as a catalyst support. Such honeycomb bodies have a multiplicity of continuous channels arranged next to one another, in particular running parallel, which are formed from web-like cell walls and are separated from one another by these cell walls.

Such so-called honeycomb catalysts are used in particular to remove nitrogen oxides from exhaust gases with ammonia gas (see, for example, DE-PS 26 58 539). The known honeycomb catalysts are optimized in terms of their spatial shape in such a way that the open frontal surface formed from the channels with the hydraulic diameter of the channels and the exhaust gas velocity corresponds so that clogging of the channels by soot and / or dust is avoided and the denitrification can reach a maximum value. A value of 80% has so far been determined as the limit value for the open frontal area because if this value were exceeded, the cell walls made of the ceramic material between two adjacent channels would no longer have sufficient mechanical strength. The limit value for the open frontal area means a restriction with regard to the exhaust gas speed and the hydraulic diameter of the channels and thus with regard to the performance of the honeycomb catalyst.

According to a new proposal, honeycomb catalysts have been produced, the open frontal area of which is more than 80% of the total frontal area, the mechanical strength of the cell walls being sufficiently high and all the requirements imposed on the moldings being met. An important problem is to ensure that the strand emerging freshly from the die of the extrusion press is treated in such a way that it does not deform. This problem arises not only with honeycomb catalysts, but e.g. also for thin-walled multi-hole pipes, hollow bodies with thin-walled profiles such as special hollow bricks and catalyst supports for chemical processes with e.g. star-shaped or spoke-shaped cross sections.

One can guarantee sufficient shard strength even with very thin walls by firing a ceramic mass and special measures that affect the structural strength of the stiffened and fired ceramic shards, but must be taken into account in the shaping that the flexible ceramic mass under its own weight the molded body can be deformed during its extrusion and storage. In this respect, an optimization of the body strength can be ineffective.

A figurative ceramic mass is usually constructed in such a way that it is optimally plastic or figurative at the time of shaping and stiffens as quickly as possible after shaping, so that the shaped body can be handled after a short time. Before the so-called green stone strength stiffens or is reached, the shaped body has only the low strength which gives it the imaginable soft mass. So that no deformations caused by their own weight occur, dimensions of the shaped body had to be provided so far, which are not required for the shaped body after the stiffening and firing.

The object of the invention is to provide a method and a device of the type mentioned at the beginning, with which or with the molded body with even thinner walls than was previously possible and can be produced with exact dimensions.

This object is achieved by the features specified in the characterizing part of claim 1 and 5, respectively. Advantageous embodiments of the invention are characterized in the subclaims.

• Fig. 1 eine perspektivische Ansicht einer Vorrichtung zur Herstellung stranggepreßter Formkörper,
• Fig. 2 einen Schnitt entlang der Linie II-II in Fig. 1,
• Fig. 3 perspektivisch einen Wabenkatalysator,
• Fig. 4 einen Ausschnitt aus einer Frontalfläche des Wabenkatalysators.

Based on the example shown in the drawing, the invention is explained in more detail below.
Show it:

• 1 is a perspective view of an apparatus for producing extruded molded articles,
• 2 shows a section along the line II-II in FIG. 1,
• 3 is a perspective view of a honeycomb catalyst,
• Fig. 4 shows a detail from a front surface of the honeycomb catalyst.

A device for producing extruded ceramic molded articles has, in addition to other known conventional devices, an extrusion press, of which only the outlet 5 with the mouthpiece 6 is shown in FIG. 1. The mouthpiece 6 is rectangular and has slots 7 which, crossing, are arranged in such a way that square island webs 8 remain between the slots. It is essential that the mouthpiece 6 is arranged with one of its tips 9 projecting vertically downwards. A trough 10 adjoins the extrusion press, which has two inner walls 11 and 12 positioned in a V-shape relative to one another and form a longitudinal edge 13 in trough 10, which lies at the lowest point of the trough. The gutter supports e.g. with supports 15 on a plate 14.

The groove 10 is placed in front of the mouthpiece in such a way that the tip 9 of the mouthpiece 6 is positioned somewhat above the longitudinal edge 13 and the groove 10 itself extends in the direction of the strand movement.

The inner walls 11 and 12 of the channel 10 have numerous small openings 16 and are part of a hollow body with outer walls 17 and longitudinal edge walls 18 and transverse edge walls 19. At least one outer wall 17 is equipped with a connection opening 20 for connecting an air line 21.

In the channel 10, a fresh, cut-to-length, cross-sectionally shaped molded body 22 is suspended on an air cushion. The air cushion is formed by compressed air, which is pressed through the pipeline 21 and 21 a into the cavity 23 of the channel hollow body. The arrows 24 illustrate that the air exits from the openings 16 of the inner walls 11, 12 and forms an air cushion under the outer surfaces 25, 26 of the molded body 22, the thickness of which depends on the weight of the molded body and the dynamic Pressure of the air emerging from the openings 16. The shaped body 22 is thereby suspended in the channel.

The spatial shape of the inner walls 11, 12 is expediently designed such that the molded body 22 is surrounded equidistantly on the outer surfaces 25, 26, i.e. that the air cushion has the same thickness everywhere. This also applies to other spatial shapes of the moldings. The invention thus shows a way in which a fresh, ceramic blank with walls that are as thin as possible can be stored without it collapsing during storage immediately after extrusion under the effect of its own weight or being deformed by its own weight.

According to the invention, the reaction forces generated by the weight during storage are distributed over the outer walls of the molded body in such a way that the lowest possible surface pressure occurs. In the case of a shaped body which is round in cross section, the channel would be cylindrical and would surround the shaped body at least on one half of the cylinder. All possible shapes lying between a square and round or oval shaped body can be stored with an appropriately shaped channel on an air cushion which preferably surrounds the shaped body in half, ie if possible acts on half of its outer surface. The shaped body is stored on the air cushion until the mass is sufficiently stiffened and can be handled easily. For this purpose, the channel (10) can be of any length; it can be divided into different segments in which the air can be supplied independently of one another. In addition, the air in the individual segments can be supplied with different quantities, temperatures, pressures and speeds. The strand emerging from the mouthpiece 6 is pushed directly onto the air cushion, on which it is advanced without contact. In this respect, no frictional forces can occur that can lead to deformations.

The method according to the invention and the device according to the invention are particularly applicable or usable for the production of honeycomb catalysts whose open frontal area is more than 80% of the total frontal area and is preferably between 80 and 90% and the cell wall thicknesses are preferably between 0.6 and 1, 2 mm. These honeycomb bodies can be formed with channels which are square in cross section, the ratio between the wall thickness and the edge length preferably being 1: 7 to 1:14, in particular 1: 8 to 1:12. The honeycomb bodies are preferably rectangular in cross section and are preferably 300 to 1200 mm or even up to 2000 mm long and have an edge length of preferably 100 to 300 mm. For example, the honeycomb bodies consist of cordierite and / or mullite and / or stoneware and / or aluminum oxide and carry on the surfaces and / or contain a catalytically active substance in the body.

In the manufacture of such honeycomb catalysts, the constituents of the ceramic mass are ground to grain sizes below 0.18 mm or correspondingly fine raw materials are used. In addition, an unusually high ductility value of the mass according to peppercorn between 25 and 27 is set. An extruded with such thin walls of 0.4 to 1.2 mm, preferably 0.5 to 0.8 mm thick, relatively large honeycomb body blank with lengths of over 1000 mm and cross-sectional areas over 100 cm² is very difficult and can not without conventional means Risk of deformation, especially caused by dead weight, stored or transported.

According to the invention, an air cushion is used which surrounds a shaped body which is quadrangular in cross-section on two adjacent side surfaces, preferably over the entire surface, the shaped body, with a longitudinal edge pointing vertically downward, being suspended on the air cushion. With a round shaped body preferably an air cushion is used which, viewed in cross-section of the molded body, surrounds at least a quarter-circle arc up to a semicircular arc. The air cushion area is in any case chosen so large that the strand flowing out of the press is carried without deformation on the molded body, ie the weight of the molded body is distributed over the largest possible air cushion surface. To produce the air cushion, a channel which is appropriately dimensioned and dimensionally adapted to the shaped body and which has a smooth, perforated surface, preferably made of metal, is preferably used, air being pressed into the channel through the holes. The channel surface is designed so that it surrounds at least a part of the lower surfaces of the molded body and an air cushion of approximately 0.1 to 3 mm between the wall of the channel and the outer wall of the molded body is formed. The thickness of the air cushion is preferably approximately 1 to 1.5 mm.

If a shaped body with a square cross-section is stored with a longitudinal edge protruding downwards, the dead weight is distributed on the air cushion on two side surfaces, so that shaped bodies can also be stored if they are stored on one surface or their weight on only one surface would be effective, would collapse. The same effect of the weight distribution results from the described support of an unstable honeycomb body which is round or oval in cross-section with an air cushion. It is advantageous to make the outer wall of a shaped body somewhat thicker than the cell walls. A ratio of the thickness of the outer wall to the thickness of the cell walls of 1.05: 2.5 is expedient.

A ceramic molded body that can be produced by the method according to the invention with the device according to the invention is shown in FIGS. 3 and 4.

The quadrangular shaped body 1 shown in FIG. 3 contains a multiplicity of channels 2 of square cross section, which are separated from one another by cell walls 3. The molded body has an outer wall 4. The length l can be up to 2000 mm. The external dimensions m and n of such a shaped body can be 100 to 300 mm. The moldings are provided with a catalyst substance. They can contain a catalytically active substance in the body and / or be coated or soaked after the body has been fired. They are arranged in a larger number next to one another and at a distance one behind the other, for example in a flue gas stream, so that environmentally harmful substances can be converted into harmless substances.

The section shown in FIG. 4 from the front surface of a honeycomb catalytic converter 1 shows the wall thickness k of the walls 3 between the channels 2 and the edge length q of the preferably square channels. According to the invention, the ratio k: q can be 1: 7 to 1:14. The outer walls 4 are thicker than the cell walls 3.

In the case of inner cell walls which intersect approximately at right angles, the extrusion onto the air cushion which forms in the channel under the shaped body takes place through a mouthpiece in a position in which the inner cell walls form an angle of approximately 45 ° with the vertical center plane of the channel. As a result, the forces resulting from the weight of the mass are apparently distributed laterally up to the side walls of the molded body supported by the air cushion and a deformation of both the inner, very thin cell walls and the unsupported upper outer walls is avoided. The arrangement of the slots 7 of the mouthpiece 6 shown in FIG. 1 to form inner cell walls 3 (FIG. 4) is therefore also preferred if the outer shape of the molded body is not square.

The air cushion built up between the lower outer surfaces 25, 26 of the molded body 22 and the inner walls 11, 12 of the channel not only enables contactless advancement and storage of a molded body 22 which has not yet been adequately stiffened, but also favors the drying and cooling of these air-coated outer surfaces.

The length of the channel 10 can be selected such that several molded bodies 22 extruded one after the other can be stored or pushed further. For a gentle delivery of the shaped bodies 22, at least the part or segment of the trough 10 carrying the most stiffened shaped body can be moved by a parallel to the longitudinal edge 13 e.g. arranged on the plate 14 axis 29 can be pivoted. For delivery, a channel-shaped support plate is placed on the upper sides of the molded body 22. By swiveling the channel or the segment downward by approximately 180 °, the still sensitive molded body is transferred to the support plate (not shown) and can then be dried.

Claims (12)

1. A process for the production of extruded, molded hollow bodies, in particular serving as a catalyst support honeycomb body, in which first of all an imaginative ceramic mass is produced, the shapeable mass is extruded, the strand is cut to length and the shaped bodies are fired, characterized in that the strand is immediately on an air cushion is extruded, which builds up in a trough under the strand by air flowing out against the strand and, after being cut off from the strand, the shaped bodies are moved and stored in the trough floating on the air cushion. 2. The method according to claim 1, characterized in that the storage of the moldings on the air cushion takes place until the mass is sufficiently stiffened and the moldings can be handled easily. 3. The method according to claim 1 or 2, characterized in that a honeycomb body with cell walls intersecting approximately at right angles is extruded onto the air cushion in the channel in such a way that the cell walls are at an angle of approximately 45 ° to the vertical central plane of the channel . 4. The method according to any one of claims 1 to 3, characterized in that for the production of a square shaped body, the strand is extruded with a longitudinal edge pointing vertically downward and is stored with this edge facing down on the air cushion. 5. The method according to any one of claims 1 to 4, characterized in that a strand with an open frontal area of more than 80% of the total frontal area is extruded, the square channels with a ratio of wall thickness to the edge length of the channel cross sections from 1: 7 to 1 : 14 has. 6. Device for performing the method according to one of claims 1 to 5 with an extrusion press, characterized in that a groove (10) is arranged immediately after the mouthpiece of the extrusion press, which extends in the direction of extrusion movement, that in the inner walls (11, 12th ) the channel (10) numerous small openings (16) are introduced that the inner walls (11, 12) are part of a channel hollow body with outer walls (17) and longitudinal edge walls (18) and transverse edge walls (19) and that at least one of the outer walls (17 ) is equipped with a connection opening (29) which belongs to a compressed air system (21a). 7. The device according to claim 6, characterized in that the inner walls (11, 12) are designed such that a molded body (22) on its outer surfaces (25, 26) is surrounded equidistantly by the inner walls (11, 12) and in the space there is an air cushion 0.1 to 3 mm thick. 8. The device according to claim 6 or 7, characterized in that the inner walls (11, 12) are positioned V-shaped to each other. 9. Device according to one of claims 6 to 8, characterized in that the channel (10) surrounds the shaped body (22) to a third to half on the lower outer surfaces. 10. Device according to one of claims 6 to 9, characterized in that the channel (10) is divided into different segments in which the air is applied independently of one another. 11. The device according to claim 10, characterized in that the air is given in the individual segments with different temperatures, amounts, pressures and speeds. 12. The apparatus of claim 10 or 11, characterized in that at least one segment of the channel (10) about an axis parallel to its longitudinal edge (13) (29) is pivotable downward by approximately 180 °.

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