WO2003013688A1

WO2003013688A1 - Porous refractory body and methods of making the same

Info

Publication number: WO2003013688A1
Application number: PCT/GB2002/003577
Authority: WO
Inventors: Helmut Jaunich; Gerd Trinkl; Andreas Ansorge; Glenn Allan Cowie; Colin Powell; David Alan Bell; Gunter Strauch
Original assignee: Foseco International Limited
Priority date: 2001-08-10
Filing date: 2002-08-02
Publication date: 2003-02-20
Also published as: AR035091A1; EP1317315A1; JP2004520982A; GB0119539D0

Abstract

In one aspect, the invention relates to a method of producing a porous body of refractory material having a predetermined configuration of pores, comprising : - (a) providing a plurality of porous body elements, each element being formed from a plastic refractory material and having a plurality of pores formed therein ; (b) assembling together the porous body elements to form a desired shape of porous body having the desired configuration of pores ; and (c) heating the assembled porous body elements such that they become fused together and lose their plasticity. In another aspect, the invention provides a porous body of refractory material having a predetermined configuration of pores (62) comprising a plurality of refractory body elements (60, 70) which are arranged in a stack and joined together to produce the predetermined configuration of pores (62). At least one of the body elements (70) is non-planar and preferably corrugated.

Description

POROUS REFRACTORY BODY AND METHODS OF MAKING THE SAME

The present invention relates to a porous body of refractory material, for example ceramic material. The porous body may be a filter, especially for use at elevated temperatures, for example for removing impurities from molten metal, or for cleaning hot gases. The invention has particular utility as a filter for molten metal as the metal is poured into a casting mould.

Conventional filters for filtering molten metal are manufactured in a variety of different ways. Ceramic foam filters are generally made by impregnating a polymeric (usually polyurethane) foam with an aqueous slurry of ceramic material containing a binder, the impregnated foam is dried to remove water, and the dried impregnated foam is fired to burn off the polymeric foam, thus leaving a ceramic foam. Cellular ceramic filters, comprising an array of channels (known as cells) in a ceramic body, are produced by extrusion.

Another type of filter for molten metal comprises a stack of filter plates. For example, US patent no. 4,721 ,567 discloses a ceramic pouring filter for use in the casting of molten metal, comprising a number of stacked closely spaced apertured plates defining filtering cavities therebetween, with their apertures staggered so that the metal flowing out of the apertures of one plate passes through a restriction before entering the apertures of the next plate. The apertures may be slots or round holes, and the plates are thicker at their periphery than at their centre, producing a shallow space between neighbouring plates. The plates are formed by injection moulding from a mixture of ceramic particles and an organic binder, followed by heating to remove the binder and sinter the ceramic particles. The filter is formed from a simple stack of such plates, which are kept in their staggered relationship by means of inter-fitting depressions and nubs on the plates.

International patent application, with publication no. WO 91/12062 discloses a similar stacked filter comprising layers of ceramic plates, each of which contains slots, the plates being arranged such that the slots of each plate are offset from those of the neighbouring plates. The stack may be contained in a holder and loosely assembled therein, or the plates of the stack may be joined using a suitable adhesive, or by silica welding of the stack.

British patent application no. 2 282764 A discloses a filter for molten metal comprising a stack of ceramic gratings whose slots are partly obstructed by ribs spaced from the upper and lower faces of the gratings to define tortuous passages through the stack. Adjacent gratings may have their slots aligned or staggered or oriented at right angles. Each grating is moulded as a unitary construction from a ceramic composition by first of all forming a "green" pressing in a suitable mould and then firing the "green" pressing. After firing, the gratings are stacked and connected together by a ceramic welding technique, or bonded together using a suitable adhesive.

US patent no. 5,326,512 discloses a ceramic filter, for example for filtering coal ash from the gas stream of a fluidized bed combustor. The filter is formed by mixing together a ceramic powder, a pore forming agent, and any sintering aids required in the sintering of the ceramic powder, forming a tape from the mixture, and moulding filter plates from the tape. Two types of annular disk filter plates are formed; a first plate type has an opening on the external annular surface, and a second type has an opening on the internal annular surface. The two types of filter plates are stacked in an alternating fashion, end pieces are added to form a filter assembly, and the assembly is sintered to form a filter. The pore forming agent is preferably polystyrene microspheres that are decomposed during the sintering process, leaving porosity that effectively captures particulate impurities when a fluid is passed through the filter in service.

The present invention seeks to provide a simple and cost-effective method of producing monolithic porous refractory bodies (for example filters for molten metal) which have a pre-determined pore configuration. The invention also seeks to provide monolithic refractory bodies having a complex yet ordered and consistent pore configuration. According to a first aspect, the invention provides a method of producing a porous body of refractory material, comprising:

(a) providing a plurality of porous body elements, each element being formed from a plastic refractory material and having a plurality of pores formed therein;

(b) assembling together the porous body elements to form a desired shape of porous body having a desired configuration of pores; and

(c) heating the assembled porous body elements such that they become fused together and lose their plasticity.

The porous body elements can be in the form of flat (i.e. planar) plates which may, for example, be assembled together by being stacked one on top of the other. Additionally or alternatively, other shapes of porous body elements may be used, for example in the shape of bricks or blocks, or cubes, etc. In the broadest embodiments of this aspect of the invention, substantially any shape or combination of shapes of porous body elements may be used, as long as they can be assembled together in some way to form a porous body.

As used in this specification, the term "plastic" when applied to the refractory material, means, at least in its broadest meaning, "able to have pores stamped therein". The plastic refractory material will normally have commonly understood plastic properties, such as maleability and flexibility. The plastic material will generally have the consistency of dough; it will normally be thixotropic.

The plastic refractory material preferably comprises a mixture of one or more particulate refractory materials and one or more polymeric binders (which may also be referred to as plasticizers). The term "particulate" includes powders, fines, fibrous materials, microspheres (hollow and/or solid) etc. Substantially any refractory material may be used; the skilled person will be able to select the appropriate material or mixture of materials according to the particular use requirements of the porous body. Examples of some of the refractory materials which may be used include: zirconia, silica, alumina, titania, silicon carbide, etc. Examples of some of the polymeric binders which may be used include: polyvinyl alcohol (PVA), glycerine, and cellulose. The first aspect of the invention has the advantage that because the porous body is built up from a plurality of elements which are "pre-formed" with pores, the resulting pore configuration (or pore structure) of the porous body is itself pre-determined. This is in contrast with the ceramic filter disclosed in US patent no. 5,326,512, in which the porosity of the filter is formed by a pore forming agent only as the assembled filter plates are sintered. The ceramic filter of US 5,326,512 consequently has a random pore configuration which cannot be pre-determined; this is also true of ceramic foam filters, for example. The present invention, on the other hand, allows the manufacturer to design and produce exactly the correct, desired type of pore configuration (including pore size) for each particular end use of the porous body. The invention essentially provides the possibility of "intelligent" ceramic filter (or other refractory porous body) manufacture. Furthermore, the first aspect of the invention has the advantage over the manufacturing methods disclosed in US 4,721 ,567, WO 91/12062 and GB 2 282 764 in that because the assembled porous body elements are fused together by heating, a monolithic body is produced, and is produced by a simple and effective method.

In the broadest aspect of the invention, the porous body elements may be formed by any suitable method, or combination of methods, including moulding (e.g. injection moulding), extruding, milling (including rolling and/or pressing, for example), stamping, etc. The most preferred method, however, comprises extruding and/or milling a tape or sheet of the plastic refractory material and stamping (including cutting and/or pressing) the tape or sheet to create the pores.

A second aspect of the invention provides a method of producing a plurality of porous body elements for assembling together in the method according to the first aspect of the invention, comprising:

(a) forming a mass of plastic refractory material;

(b) producing a tape or sheet of the plastic refractory material;

(c) forming a plurality of pores in the tape or sheet by a stamping operation, the pores extending through the thickness of the tape or sheet; and (d) cutting or otherwise separating a plurality of individual porous body elements from the stamped tape or sheet.

In one embodiment of the second aspect of the present invention, the stamping operation comprises forming slits in the tape or sheet, for example using a blade. The pores are then formed by stretching the tape of sheet laterally of the slits to open the slits which then define the pores.

It will be understood that the invention encompasses within its scope the possibility of separating the body elements from the tape or sheet prior to formation of the pores therein.

The second aspect of the invention provides a simple and effective method of mass- producing the porous body elements with a pre-determined pore configuration, in contrast with the process of US 5,326,512 in which the pores are produced only as the assembled filter plates are sintered, and in contrast with US 4,721 ,567, and GB

2 282 764 in which the plates or elements are individually moulded.

As already mentioned, the plastic refractory material is preferably prepared by mixing together one or more particulate refractory materials and one or more polymeric binder materials. This may be carried out in substantially any suitable mixing apparatus, for example a planetary mixer (preferred) and/or a twin screw mixer. The plastic refractory material which is produced is preferably milled and/or extruded into a tape or sheet, preferably using a Z-blade extruder or a pressure extruder. If required, the thickness of the resultant sheet or tape can be further reduced by passing through one or more pairs of rollers. The final tape or sheet preferably has a thickness of at least 0.1 mm, more preferably at least 0.2 mm, even more preferably at least 0.3 mm, especially at least 0.5 mm. The tape or sheet preferably has a thickness of no greater than 5 mm, more preferably no greater than

3 mm, even more preferably no greater than 2 mm. A particularly preferred thickness range for the tape or sheet is 0.6 mm - 1.0 mm, for example approximately 0.7 mm. The tape or sheet of plastic refractory material is stamped in a stamping operation, in order to form a plurality of pores extending through the thickness of the tape or sheet, the pores having a desired pre-determined configuration or pattern. Individual porous body elements are then created by cutting or otherwise separating discrete elements from the tape or sheet. Stacks of the porous body elements are then produced, the stacking being carried out in a pre-determined way using the required number of porous body elements in each stack, and stacking the elements with respect to each other in such a way that the desired pre-determined configuration (or structure, or pattern) of pores is produced in the assembled body. The number of porous body elements stacked to produce the assembled porous body is at least two, preferably at least three, more preferably at least four, for example six. Substantially any number of elements may be used, however, depending upon the particular requirements. Where the porous body forms part of a filter for molten metal, the number of elements in the stack generally is preferably no greater than fifty, more preferably no greater than twenty, especially no greater than ten. It will be understood that the exact number of elements chosen for a particular application ("intelligent filter design") will depend on filtration efficiency, metal flow rate, mechanical strength and cost considerations.

The stacked porous body elements are then heated, e.g. fired in a kiln or similar. The heating step fuses the stacked porous body elements together, and causes the refractory material to lose its plasticity (for example by curing and/or pyrolysing the polymeric binder(s), where present). The temperature to which the stacked porous body elements are heated is preferably in the range 1000-1600°C.

In a preferred series of embodiments, at least one of the elements is non-planar, for example corrugated. The non-planarity may be introduced by deforming the (plastic) tape or sheet before it is cut or otherwise separated, or afterwards. The advantage of incorporating non-planar elements is to increase the mechanical strength, which is particularly important where the porous body is intended for use as a filter for molten metal. By way of example, corrugated elements may be stacked on top of each other, the corrugations of one element preferably being disposed perpendicularly to adjacent elements in the stack. Alternatively, the stack may comprise alternating planar and non-planar (eg. corrugated) elements, each non-planar element being sandwiched between two planar elements.

Some preferred pore configurations of the porous body elements and porous bodies according to the first and second aspects of the invention will now be defined and described.

According to a third aspect, the invention provides a porous body element formed from plastic refractory material having an array of indentations in a first surface thereof, and an array of indentations in a second, opposite, surface thereof, each indentation of the first array overlapping with one or more indentations of the second array such that the overlapping indentations break through to each other and form an opening between the indentations, the openings and indentations together forming the pores of the porous body element.

A fourth aspect of the invention provides a porous body of refractory material, comprising a plurality of porous body elements joined (e.g. fused) together, each porous body element comprising refractory material having an array of indentations in a first surface thereof, and an array of indentations in a second, opposite, surface thereof, each indentation of the first array overlapping with one or more indentations of the second array such that the overlapping indentations break through to each other and form an opening between the indentations, the openings and indentations together forming the pores of the porous body.

The arrays of indentations in the porous body elements are preferably regular. The arrays preferably comprise regular rows of indentations parallel to each other. Each array may, for example, comprise regular rows and columns of indentations arranged substantially at right angles to each other. More preferably, however, each array of indentations may comprise regular rows having regular columns arranged at angles of substantially 60° thereto (and at substantially 60° to each other).

The indentations may in general have substantially any shape, but preferably the indentations are generally conical, frusto-conical, or bowl-shaped, e.g. substantially semi-spheroidal. Most preferably, the indentations are substantially semi-spherical (hemispherical). Each indentation preferably overlaps with at least two, more preferably three (or more, but preferably exactly three) indentations in the opposite surface of the porous body element. Each indentation therefore preferably has at least two, more preferably three (or more, but preferably exactly three) openings by which it "breaks through" to adjacent indentations in the opposite surface.

The porous body elements are preferably joined (e.g. fused) together in such a way that the indentations form cavities in the porous body. Preferably each indentation joins with a corresponding indentation in an adjoining porous body element, to form a cavity defined by both indentations. For embodiments in which the indentations are generally-bowl shaped, the resulting cavities will normally be generally spheroidal in shape. In some particularly preferred embodiments of the invention, the indentations are substantially hemispherical, and therefore the resulting cavities in the porous body are substantially spherical.

The pore configuration in the porous body consequently is preferably made up of a three-dimensional array of overlapping cavities, each cavity breaking-through to one or more, but preferably six, other cavities (for those embodiments in which each indentation breaks through to three other indentations) by means of six openings in the cavity.

According to a fifth aspect, the invention provides a porous body of refractory material having a regular array of cavities therein, each cavity overlapping with at least one adjacent cavity such that the overlapping cavities break through to each other and form an opening between the cavities, the openings and cavities together forming the pores of the porous body.

According to a sixth aspect, the invention provides a porous body element formed from plastic refractory material, the element having an array of indentations in a first surface thereof, the indentations projecting beyond a second, opposite, surface of the element to form an array of protrusions on the second surface, each indentation/protrusion having one or more openings, the openings and indentations together forming the pores of the porous body element.

According to a seventh aspect, the invention provides a porous body of refractory material, comprising a plurality of porous body elements joined (e.g. fused) together, each porous body element comprising refractory material having an array of indentations in a first surface thereof, the indentations projecting beyond a second, opposite, surface of the element to form an array of protrusions on the second surface, each indentation/protrusion having one or more openings, the porous body elements being joined together in one or more pairs, each pair comprising two porous body elements joined together by their second surfaces with each protrusion of one element joined to a corresponding protrusion of the other element.

In embodiments of the seventh aspect of the invention in which there are two or more pairs of porous body elements joined together, each pair is joined to an adjacent pair by respective first surfaces of component elements, with each indentation of an element of one pair joined to a corresponding indentation of an element of the other pair, the openings and indentations of the elements together forming the pores of the porous body element.

The arrangement of indentations and openings results in a pattern of interconnecting webs of refractory material in the porous body element. Particularly for embodiments in which the indentations are generally bowl-shaped, the webs may appear similar in shape and pattern to those of a conventional ceramic foam filter formed from a polymeric foam precursor, but both the shape and pattern of such webs will preferably be significantly more regular in the refractory material according to the present invention, since they result from pre-determined, designed, arrays rather than a random foaming process. The types of patterns of webs and pores produced in accordance with the third, fourth and fifth aspects of the invention consequently have the tremendous advantage over those of ceramic foam filters in that the patterns produced according to the invention can be completely regular, and hence consistent, throughout the porous body. Therefore, the performance (e.g. filtering performance) of the porous body according to the invention can be consistent throughout the entire porous body, whereas this is generally not the case with ceramic foam filters. The invention achieves this consistency and predetermined pore configuration while at the same time providing a complex pore and web structure which is akin to the complexity of a ceramic foam. This means that the filtering performance, produced by the tortuosity of the flow paths of molten metal (for example) through the porous body, is similar to that of ceramic foam filters (and therefore good), but the consistency of performance between refractory bodies (e.g. filters) and between different regions of the same body (e.g. filter) is generally vastly superior to that of ceramic foam filters. Porous bodies according to the present invention have the additional advantage of their pore configurations being pre-determined, which provides the possibility of tailoring the pore configuration to each particular end use.

The invention will now be described, by way of example, with reference to the accompanying drawings, of which:

Figure 1 is a schematic diagram of the methods according to the first and second aspects of the invention;

Figure 2 is a schematic illustration of a preferred form of porous body element according to the invention;

Figure 3 (a) to (d) is a schematic illustration of another preferred form of porous body element according to the invention;

Figure 4 is a schematic illustration of a preferred form of porous body according to the invention, formed from a fused stack of porous bodies as illustrated in Figure 2; and

Figures 5 to 7 are schematic illustration of further preferred form of porous bodies according to the invention. Figure 1 schematically illustrates the production of porous body elements and assembled porous bodies, in accordance with the invention. The plastic refractory material is prepared by mixing together one or more particulate refractory materials and one or more polymeric binder materials (indicated by arrow A). This mixing operation is carried out in a Hobart mixer 1 and a twin screw mixer 3. The plastic refractory material which is produced is then milled and/or extruded (indicated by reference numeral 5) into a tape or sheet 7. In an alternative (and preferred) embodiment (not shown), the twin screw mixer is omitted and replaced by a Z-blade mixer/extruder. In addition, further pairs of rollers may be provided to reduce the thickness of the extruded refractory sheet.

The tape or sheet 7 of plastic refractory material is then stamped in a stamping operation (reference numeral 9), in order to form a plurality of pores extending through the thickness of the tape or sheet, the pores having a desired predetermined configuration or pattern (eg. circular). Individual porous body elements are then created by cutting or otherwise separating discrete elements from the tape or sheet (reference numeral 11). Stacks of the porous body elements are then produced (reference numeral 13), the stacking being carried out in a pre-determined way using the required number of porous body elements in each stack, for example six or eight, and stacking the elements with respect to each other in such a way that the desired pre-determined configuration (or structure, or pattern) of pores is produced in the assembled body.

The stacked porous body elements are then heated (e.g. fired) in a kiln 15 or similar. The heating step fuses the stacked porous body elements together, and causes the refractory material to lose its plasticity (for example by curing and/or pyrolizing the polymeric binder(s), where present). The temperature to which the stacked porous body elements are heated is preferably in the range 1000-1600°C. The final fused porous body is then allowed to cool to ambient temperature, ready for shipping.

The above-described process may be modified in a number of ways, for example:- - The stamping operation can be replaced by a cutting operation in which blades are used to cut slits in the tape or sheet, the pores being formed by stretching the plastic refractory sheet laterally relative to the slits, prior to the stacking operation.

- The stamping/cutting-stretching operation can be effected on the body elements after they have been separated from the tape or sheet.

- Prior to stacking, adhesive (eg. colloidal silica solution) can be applied to the porous body elements to aid on the fusion process.

- The tape or sheet can be shaped, either before or after the stamping/slitting operation and before or after the tape is cut to form the discrete porous body elements. In a preferred embodiment, the shaping is effected after the stamping/slitting operation but prior to the cutting operation. In a preferred shaping operation, corrugated rollers are used to form corrugations (or "waves") in the tape or sheet. When the porous body elements formed from the corrugated tape or sheet are stacked, the corrugations in one body element are arranged to be perpendicular to the corrugations in adjacent body elements. In a further variation, the porous body can be made up of a mixture of shaped and un-shaped (i.e. flat) body elements. For example, three or more body elements can be stacked with flat elements alternating with corrugated elements.

Figure 2 is a schematic illustration of a preferred type of porous body element according to the invention, which has been formed by injection moulding. The porous body element 17 is formed from plastic refractory material having an array 19 of substantially hemispherical indentations 21 in a first surface 23 thereof, and an array 25 of substantially hemispherical indentations 21' in a second, opposite, surface 27 thereof. Each indentation 21 of the first array 19 overlaps with three indentations 21 ' of the second array 25 such that the overlapping indentations break through to each other and form openings 29 (three per indentation) between the indentations, the openings 29 and indentations 21 , 21' together forming the pores of the porous body element. The arrays of indentations 21, 21' in the porous body element are regular, comprising regular rows 31 and regular columns 33 arranged at angles of substantially 60° thereto (and at substantially 60° to each other). The porous body element includes a solid periphery 34, which has been formed in the injection moulding process.

Figure 3 (views (a) to (d)) is a schematic illustration of a porous body element 17 according to the invention, which has been formed by the extrusion of a tape of plastic refractory material, stamping to create the pores, and cutting of the tape to form the individual element. View (d) is a generally orthogonal view of the element 17, view (a) is a plan view of the element, and views (b) and (c) are end views in the directions indicated by arrows B and C, respectively. Grid-type contour lines are drawn on the views merely for clarity, to illustrate the shapes of the indentations and openings more clearly. The reference numerals used are the same as those of Figure 2, and they indicate the same features as those of Figure 2. The only significant difference between the element shown in Figure 3 and that shown in Figure 2 is that the Figure 2 element has a solid periphery 34, whereas the Figure 3 element does not, because it has been formed from a continuous tape.

Figure 4 is a schematic illustration of a porous body 35 according to the invention, formed from a stack of porous body elements 17 as illustrated in Figure 3. The porous body elements 17 have been fused together in such a way that the indentations 21 , 21' form substantially spherical cavities 37 in the porous body 35. The pore configuration in the porous body 35 consequently is made up of a three- dimensional array of overlapping substantially spherical cavities 37, each cavity breaking-through to six other cavities by means of six openings 29 in the cavity.

Figure 5 (views (a) to (d)) is a schematic illustration of a further preferred form of porous body according to the seventh aspect of the invention. The porous body 39 comprises a plurality of porous body elements 41 joined (e.g. fused) together, each porous body element 41 comprising refractory material having an array of indentations 43 in a first surface 45 of the element 41. The indentations 43 project beyond a second, opposite, surface 47 of the element to form an array of protrusions 49 on the second surface, each indentation/protrusion 43/49 having one or more openings 51. The porous body elements 41 are joined together in pairs 53, each pair 53 comprising two porous body elements 41 joined together by their second surfaces 47 with each protrusion 49 of one element joined to a corresponding protrusion 49 of the other element. Each pair 53 of porous body elements 41 is joined to an adjacent pair by respective first surfaces 45 of component elements, with each indentation 43 of an element of one pair joined to a corresponding indentation 43 of an element of the other pair, the openings 51 and indentations of the elements together forming the pores of the porous body element.

Figures 6 and 7 are schematic illustrations of further preferred porous bodies in accordance with the present invention. The illustrated bodies are intended for use as molten metal filters. Referring to Figure 6, each element 60 is provided with circular pores 62 and is corrugated. The body is made up of a stack of six such elements 60, with the corrugations of each element being arranged perpendicularly to the corrugations of those elements 60 immediately above and below. It will be understood that the corrugations (and the arrangement of the corrugations) imparts additional strength to the body, and that the number of elements 60 can be increased or decreased according to the filtration efficiency and strength required from the body.

Referring to Figure 7, each element 60,70 is provided with circular pores. The body is made up of a stack of three elements 60,70. The outer elements 70 are flat (planar), whereas the inner element 60 sandwiched between the two outer elements 70 is corrugated. Such a body has greater strength and filtration efficiency than a stack of three planar elements.

Method of filter production

A composition of 62.2% silicon carbide powder (with d₅₀ particle size of about 10 microns and maximum particle size about 100 microns), 14.8% reactive alumina (maximum particle size about 15 microns), 6.2% fumed silica (maximum particle size about 2 microns), 6.6% partially hydrolysed high molecular weight polyvinyl alcohol, 9.3% water and 0.9% glycerol was mixed briefly until substantially homogeneous by visual inspection in a Hobart mixer and then transferred to a water-cooled Z-blade mixer/extruder, where it was mixed under conditions of partial vacuum for a period of twenty minutes. The additional mixing ensures that all the refractory particles are coated with the water and binder. The partial vacuum removes entrained air bubbles.

1 kg of the resultant mixture was extruded from the mixer, then rolled down to a sheet of thickness about 1 mm using hand operated rollers. The rolling process was achieved in three to six steps by successively decreasing the gap between the rollers.

Rectangular filter elements of approximately 51 mm by 48 mm were cut from the large sheet produced by the rollers using a knife. Approximately 156 holes of 3 mm diameter were then punched in each filter element using a hand-operated press. Some of the filter elements were then corrugated using a hand-operated roller machine having corrugated rollers (annular ribs on one roller facing corresponding troughs in the other roller); the pitch (i.e. distance between adjacent "peaks" and "troughs") of the corrugations was approximately 5 mm and the depth (i.e. distance of "peak" or "trough" above or below central plane of element) of the corrugations between 2 mm and 6 mm. All of the filter elements were then part-dried in a drying oven set at between 60°C and 110°C until the free water content fell below 3%. The elements were then "glued" together using colloidal silica solution (containing at least 25% by weight of silica) and by pressing the elements together either by hand or by using a special stacking tool. A minimum of two sheets up to a maximum of six sheets was used to form a single filter. Filters were made up of only corrugated elements, or alternate flat (planar) and corrugated elements. Each filter was then dried at 110°C for at least 10 minutes.

Filters were fired in a batch kiln, reaching a maximum temperature of between 1300°C and 1400°C in less than 3 hours, and maintaining that temperature for between 10 and 20 minutes. The final mass of a fired filters was generally between 6g and 30g (dependent on the number of elements used). The dimensions of the fired filters were approximately 49 mm square and between 5 mm and 25 mm tall. Impingement Testing

The filters produced by the above method were tested with molten iron using a direct impingement test in which 50 kg of iron at a temperature of about 1480 °C was poured from a bottom-pour ladle from a height of 250 mm (metal head 750 mm) onto a face of the filter which was supported on all four sides within a resin-bonded sand mould. After the impingement test, the filters were visually inspected for cracks and other damage such as erosion. The test provides a measure of the mechanical strength of the filter from the initial metal impact, thermal shock resistance, mechanical strength at elevated temperatures, and resistance to chemical attack and erosion due to the hot metal.

Example 1

Four three-element filters of the type illustrated in Figure 7 were made in accordance with the above-described method (the corrugation depth of the middle element being about 2.0 to 2.5 mm, the pitch being about 5 mm). Two of the four filters passed the impingement test in that there were no visible cracks and/or breakages to the elements of the filter. It is anticipated that the pass rate will be improved by increasing the thickness of the individual elements and/or decreasing the pitch of the corrugated elements and/or increasing the depth of the corrugations.

Example 2

Four six-element filters of the type illustrated in Figure 6 were made in accordance with the above-described method (the corrugation pitch being about 5 mm, corrugation depth being about 2.0 to 2.5 mm). All four filters passed the impingement test.

Claims

1. A method of producing a porous body of refractory material having a predetermined configuration of pores, comprising:-

(b) assembling together the porous body elements to form a desired shape of porous body having the desired configuration of pores; and

2. A method as claimed in claim 1 , wherein step (a) is effected by one or more of moulding, injection moulding, extruding, milling, rolling and/or pressing, cutting and stamping.

3. A method as claimed in claim 1 , wherein step (a) comprises: (i) forming a mass of plastic refractory material;

(ii) producing a tape or sheet of the plastic refractory material;

(iii) cutting or otherwise separating a plurality of individual body elements from the tape or sheet, and

(iv) forming a plurality of pores in the body elements, the pores extending through the thickness of the body elements.

4. A method as claimed in claim 3, wherein steps (i) and (ii) of step (a) are effected using a planetary mixer and/or a Z-blade extruder.

5. A method as claimed in claim 4, wherein after extrusion, the thickness of the tape or sheet produced is reduced using one or more pairs of rollers.

6. A method as claimed in any one of claims 3 to 5, wherein the tape or sheet produced in step (ii) is from about 0.1 mm to 5 mm thick, preferably about 0.2 mm to 3 mm thick and most preferably about 0.5 mm to 2 mm thick.

7. A method as claimed in any one of claims 3 to 6, wherein step (iv) of step (a) is effected prior to step (iii).

8. A method as claimed in any one of claims 3 to 7, wherein step (iv) of step (a) is effected by a stamping operation.

9. A method as claimed in any one of claims 3 to 7, wherein step (iv) of step (a) is effected by a cutting operation to form slits, followed by a stretching operation to open said slits, said open slits defining the pores.

10. A method as claimed in any preceding claim, wherein at least one of the porous body elements provided in step (a) is planar.

11. A method as claimed in any preceding claim, wherein at least one of the porous body elements provided in step (a) is non-planar.

12. A method as claimed in claim 11 , wherein said at least one non-planar porous body element is formed by shaping a planar porous body element.

13. A method as claimed in claim 12, wherein said shaping is effected prior to step (b).

14. A method as claimed in claim 12 or 13, wherein said shaping involves forming regions generally above and/or below the plane of said planar porous body element.

15. A method as claimed in claim 14, wherein said porous body element is shaped to define corrugations in said porous body element.

16. A method as claimed in any preceding claim, wherein step (b) is effected by stacking the porous body elements one on top of another.

17. A porous body element formed from plastic refractory material having an array of indentations in a first surface thereof, and an array of indentations in a second, opposite, surface thereof, each indentation of the first array overlapping with one or more indentations of the second array such that the overlapping indentations break through to each other and form an opening between the indentations, the openings and indentations together forming the pores of the porous body element.

18. A porous body element as claimed in claim 17, wherein the arrays of indentations in the porous body element are regular, preferably comprising regular rows of indentations parallel to each other.

19. A porous body element as claimed in claim 18, wherein each array comprises regular rows having regular columns arranged at angles of substantially 60° thereto and at substantially 60° to each other.

20. A porous body element as claimed in any one of claims 17 to 19, wherein the indentations are generally conical, frusto-conical, or substantially semi-spheroidal, and preferably substantially semi-spherical.

21. A porous body element as claimed in any one of claims 17 to 20, wherein each indentation preferably overlaps with at least two, but preferably exactly three indentations in the opposite surface of the porous body element.

22. A porous body element formed from plastic refractory material, the element having an array of indentations in a first surface thereof, the indentations projecting beyond a second, opposite, surface of the element to form an array of protrusions on the second surface, each indentation/protrusion having one or more openings, the openings and indentations together forming the pores of the porous body element.

23. A porous body element according to any one of claims 17 to 22 suitable for use in a filter for molten metal.

24. A porous body of refractory material, comprising at least two porous body elements joined (e.g. fused) together, each porous body element comprising refractory material having an array of indentations in a first surface thereof, the indentations projecting beyond a second, opposite, surface of the element to form an array of protrusions on the second surface, each indentation/protrusion having one or more openings, the porous body elements being joined together in one or more pairs, each pair comprising two porous body elements joined together by their second surfaces with each protrusion of one element joined to a corresponding protrusion of the other element.

25. A porous body as claimed in claim 24, wherein two or more pairs of porous body elements are provided, each pair being joined to an adjacent pair by respective first surfaces of component elements, with each indentation of an element of one pair joined to a corresponding indentation of an element of the other pair, the openings and indentations of the elements together forming the pores of the porous body element.

26. A porous body of refractory material having a predetermined configuration of pores comprising a plurality of refractory body elements arranged in a stack and joined together whereby to produce said predetermined configuration of pores, characterised in that at least one body element is non-planar.

27. A porous body of refractory material as claimed in claim 26, wherein said at least one non-planar body element is corrugated.

28. A porous body of refractory material as claimed in claim 27, wherein all the body elements are corrugated.

29. A porous body of refractory material as claimed in claim 28, wherein each body element is arranged such that the corrugations of successive body elements in the stack are perpendicular.

30. A porous body of refractory material as claimed in claim 26 or claim 27, wherein said non-planar body element is sandwiched between two planar body elements.

31. A porous body of refractory material, comprising a plurality of porous body elements according to any one of claims 17 to 21 joined together, the openings and indentations together of the body elements forming the pores of the porous body.

32. A porous body as claimed in claim 31 , wherein the porous body elements are joined together in such a way that the indentations form cavities in the porous body.

33. A porous body producible by the method of any one of claims 1 to 16.

34. A porous body of refractory material having a regular array of cavities therein, each cavity overlapping with at least one adjacent cavity such that the overlapping cavities break through to each other and form an opening between the cavities, the openings and cavities together forming the pores of the porous body.

35. A filter for molten metal comprising a porous body as claimed in any one of claims 24 to 33.

36. A method as claimed in any one of claims 1 to 16, a body element as claimed in any one of claims 17 to 23, a body as claimed in any one of claims 24 to 34 or a filter as claimed in claim 35, wherein the plastic refractory material comprises a mixture of one or more particulate refractory materials selected from powders, fines, fibrous materials, and solid or hollow microspheres and one or more polymeric binders.

37. A method, a body element or a body as claimed in claim 36, wherein the particulate refractory material is selected from one or more of zirconia, silica, alumina, titania, and silicon carbide.

38. A method, a body element or a body as claimed in claim 36 or 37, wherein the polymeric binder is selected from one or more of polyvinyl alcohol, glycerine, and cellulose.