WO1998025685A1

WO1998025685A1 - Filter

Info

Publication number: WO1998025685A1
Application number: PCT/FI1997/000687
Authority: WO
Inventors: Jarkko Leivo; Antti-Pekka NIKKILÄ; Erkki LEVÄNEN; Tapio Mäntylä
Original assignee: Ensto-Ceramics Oy
Priority date: 1996-12-11
Filing date: 1997-11-12
Publication date: 1998-06-18
Also published as: FI964946A; FI964946A0; FI103644B; EP0948388A1; AU5052498A; FI103644B1

Abstract

The invention relates to a filter element for fluids, the filter element being resistant to high temperatures, in particular 800...1600 °C, and comprising a support layer and a coating formed thereon, the pore size of the support layer being greater than the pore size of the coating. The invention is characterized in that the support layer is made up of inorganic structural particles bonded together by means of a reaction product produced in a reaction-sintering process between the said structural particles and a binder formed by an inorganic starting component, and that at minimum 60 % by weight of the structural particles used are structural particles having a diameter of 87...1000 νm as determined by the screening method. The invention also relates to a novel support layer and to methods for fabricating the support layer and the filter element.

Description

FILTER

The invention relates to a heat-, corrosion- and creep- resistant filter element for separating solid particles from a gaseous or liquid medium, which filter element is made up of a bearing support layer and a coating formed thereon. The invention also relates to the said support layer and to the fabrication of the support layer and the filter element.

PROBLEMS TO BE SOLVED AND THE STATE OF THE ART

It is generally known that one problem in pressurized combustion and gasification processes in thermal power stations is the removal from gases of particles which may damage gas turbine blades, since high pressure and high temperature prevail in the operating environment. Furthermore, the solid or gaseous compounds present in the gases, for example, alkali metals or compounds thereof, heavy metals, chlorine compounds and sulfur compounds, or gas components such as carbon monoxide and water vapor, produce problematic corrosive conditions . Therefore ceramic materials are primarily used in the hot gas filters.

The filter regarded at present as the best is a rigid ceramic tubular barrier filter, called a "candle," which consists of a bearing support structure having a large pore size and of a filtering coating having a small pore size, the filtration being based on a screening mechanism. The ceramic impact filter construction model regarded as the best is a double structure, described in, for example, patent publications FI 92804, US 4,629,483 and WO 87/01610, wherein a support having a large pore size constitutes the bearing part of the element and a thin surface layer having a small pore size constitutes the filtering part. The surface layer may have an abrupt interface relative to the support, or the coating may penetrate far between the particles of the support. With a double-layer structure, a smaller pressure loss is often achieved than with an uncoated structure having approximately the same pore size throughout the element structure, because in order to achieve the same filtering efficiency as a coated structure, an uncoated structure must have a smaller pore size. Thus the pressure loss across the entire element will be approximately equal in both structures. However, it has been observed that in practice an uncoated structure is clogged more easily, since dirt penetrates deep into the structure, and it is more difficult to clean than a coated structure, in which case the ashes left inside will increase pressure losses.

So-called cross-flow or honeycomb filters have also been used in power plants; the construction of honeycomb filters is illustrated well by the structures disclosed in US patents 4,632,683 and 5,198,007. As compared with "candle filters," the problem with these above-mentioned structures is, however, the difficulty of their cleaning owing to their large filtering surface area and the structure which complicates cleaning. The structure disclosed in US patent 5,454,947, wherein the coating is inside a tubular filtering element, would be poor for pressurized gases in a power plant application, since the cleaning of the element is difficult owing to the uneven distribution of the cleaning pulse as a consequence of the shape of the body. Because of the difficult shape of the body the cleaning pulse may also cause thermal stresses in the coating, and as a result the coating may become detached from the support. Furthermore, the structure of already existing filtering units should be altered radically, since the direction of the gas flow is reverse (from inside out) to that in filter units using "candles."

There also exist so-called combination filters, which simultaneously perform both chemical and physical cleaning. These filters are, however, not suitable for the most demanding conditions, since the catalytic or adsorbing materials with which the filters are impregnated may react with the material being filtered, whereby the resistance of the filters to corrosion and creep is reduced.

The commonly known filters are approx. 1...3 m in length, and their outer diameter is within a range of 60...170 mm and their inner diameter within a range of 30...140 mm. The support of commonly known filters is normally constructed of particles 200..1000 μm in size, in which case the pore size will be quite large and the permeability of the support will be good. The coating is usually made up of fibers or particles so that the pore size will be approx. 10 μm. The thickness of the coating is normally approx. 0.1...1 mm in order for the pressure loss to be as small as possible. The advantages of a thin surface layer also include ease of cleaning, since most of the ashes cannot penetrate deep into the structure. The porosity of the entire element, the surface and the support structure, is approx. 40 %.

In pressurized combustion and gasification processes of thermal power plants, particles should be removed from the gas flow at as high a temperature as possible in order to maintain sufficient efficiency. The filtration temperature may be 850...1000 °C during combustion. Since in gasification the gas has to be cooled before combustion in order to remove alkalis and sulfur, the filtration temperature is determined according to the sorbent material. With iron-based sorbents , the gas temperature is at its highest 650 °C and with calcium-based sorbents 980 °C. In connection with the development of new experimental combustion techniques, filtration experiments have also been conducted at temperatures of 870...1100 °C.

A combination of materials often used in the support is silicon carbide (SiC) particles bound with a silica-based (Si0₂) high-temperature inorganic binder and, as the coating layer, ceramic particles bonded by a silica-based high- temperature inorganic binder. When the high-temperature binder used is a binder containing a large quantity of silica, a great deal of problems appear in the structure during operation. The alkali oxides present in the ashes being filtered cause a drop in the viscosity of the silica- based binder already at temperatures of 700 °C, in which case the binder may even be transported in the gas flow. The decrease in the viscosity of the binder additionally reduces the creep resistance of the binder, i.e. its mechanical resistance at high temperatures under load. Reduced creep resistance may cause breakage of the element in the operating conditions. Alkali compounds affect silica also through chemical reactions. Typical causative agents of high-temperature corrosion are alkali salts, among them Na₂C0₃ yielding sodium silicates as reaction products. These corrosion products may, for example in consequence to a change in volume, split off, whereby new surface exposed to reactions is produced. The third disadvantage of silica consists of uncontrolled phase changes, particularly in atmospheres containing water vapor. Phase changes cause in the structure internal stress states and cracking, which together or separately weaken the strength of the body significantly.

The problem with silicon carbide serving as a support material is its oxidation to silica at high temperatures. Water vapor in particular promotes this reaction, since the solubility of water vapor in silica is many times that of oxygen, and thus silica formed on the surface of silicon carbide will not protect the silicon carbide under the surface from oxidation. Thus the silica formed as an oxidation product of silicon carbide will further increase the amount of silica in the binder. A support structure such as this, containing a large amount of silica, is mechanically weak in the operating conditions owing to creep, chemical reactions and uncontrolled phase changes. In the operating environment there thus follows breakage of the filter before the end of the maintenance interval of the filter system.

Prior known filters also have the disadvantages of the reacting of the silica present in the inorganic binder of the coating layer with a solid or gaseous ingredient being filtered. Thereupon the viscosity of the binder in the coating decreases and the binder is transported in the gas flow, clogging the surface layer. Clogging is also increased by the bonding by sintering of the solid ingredient being filtered to the coating, which may be a consequence of the drop in the viscosity of the binder of the coating. Owing to the clogging, the pressure difference across the filter increases, since the dirt sintered to the filter will no longer be detached by the cleaning pulse. This reduces the efficiency of the filtering apparatus.

The cause of breakage of a candle filter is often mechanical stress, for example vibrations, a thermal shock, or creep owing to the weight of the candle itself. However, it is to be noted that changes occurring in the filter structure at high temperatures may reduce its strength to a fraction of its strength at room temperature. Thus the load causing the final breakdown may be very small.

Said patent publications FI 92804, US 4,629,483 and

WO 87/01610 disclose a filter with a double-layer structure and the use of ceramic materials as filtering materials in the surface layer. However, there are several indications in the literature, that the inorganic, most commonly silica-based binder for binding the coating fibers or particles, present in the coating layers described in these patents, reacts with the material to be filtered. This may cause ashes to be sintered to the coating, whereupon the coating is clogged. The silica-based phase may also be transported in the gas flow and, as a consequence, the porosity and pore size of the coating may increase. If the binder of the coating becomes removed from the surface it is also probable that the coating will easily detach from the support structure. The reactions of the silica-based binder of the support also reduces the strength of the body and contributes to the breakage of the element in operation.

It has also been noted that the use of fibers in the coating, as in patent publication WO 87/01610, is not desirable owing to the crystallization of the fibers and the subsequent loss of strength, whereupon a cleaning pulse may detach the coating. It has also been noted that for the mechanical strength of the coating it is better if the coating layer is not separate, as it is in patent publication US 4,629,483, but that the coating particles penetrate deep into the pores of the support material.

Patent publications FI 92804, US 4,629,483 and WO 87/01610 describe the use of ceramic materials as filter support materials. However, these patents contain no mention that the structural particles of the filter support structure would react chemically with the inorganic binder used in the support, thereby forming, as a reaction product, a new compound which constitutes part of the binder.

Japanese patent application JP 94-127210 describes a filter intended for the filtering of water, for example for removing micro-organisms from water, which filter is to be classified as an ultra-filter as regards pore size. The filter is made up of a support layer and a coating formed thereon. The coating, which is the actual filtering layer, is in the main made up of mullite crystals (3 A1₂0₂ • 2 Si0₂) which have a diameter of 0.3...3 μm and give the coating layer a pore size of 0.03...1.0 μm. The support part is made up of alumina (A1₂0 ) and silica, which have been reaction-sintered at a temperature of 600...1600 °C. The pore size of the support part is 0.1...3 μm and the diameter of the support part crystals is 3...50 μm. Owing to the small particle size of the surface layer (and the small pore size due to it), the support layer must, of course, have a rather small pore size. If the pore size of the support layer were too large, the surface layer made up of small crystals would not remain on the support layer but would penetrate deep into the support layer, and would even pass through it. Owing to the poor supporting effect of the support the coating would also detach easily from the base.

The filter described in the said Japanese patent application would, however, be out of the question at least in the purification of hot gases . A filter so dense would in practice cause too high a flow resistance, as well as a gas pressure loss and clogging of the filter due to it. Of course, the pressure loss could in principle be reduced by decreasing the thickness of the layers. The function of the support layer is to give mechanical strength to the filter. If, on the other hand, the thickness selected for the support layer described above were such that the gas pressure loss caused by it were within a tolerable range in terms of gas filtration, the support layer would be so thin that its mechanical strength would be highly insufficient.

US patent 4,678,758 describes a filter which is intended for the filtering of molten metal, such as iron and ferrous alloys. The filter is made up of round, hollow, sintered spheres made up of alumina or some other oxide and having a diameter of 0.2...3 mm. These oxide spheres are reaction- sintered together with the help of a binder component, such as magnesium silicate or calcium silicate, whereby a reaction-sintered product is formed which bonds the spheres. 1400...1700 °C, specifically 1500...1600 °C, is mentioned as a suitable reaction-sintering temperature range. Since this product is made up of very large spheres and yields a product in which the pore size is very large (which is, of course necessary for operation in the filtering of molten metal), the strength of the filter is very low. By using specifically magnesium silicate or calcium silicate as the binder component, however, a relatively high strength can be obtained.

The filter described in US patent 4,678,758 would not be suitable as a support structure for a filter in the filtration of hot gases, since the pore size is far too large. The amount of alumina and silica used as a binder component, in total 10 % by weight, is too low for producing sufficient mechanical strength. The magnesium and calcium silicates used as binder components would cause poor corrosion resistance.

OBJECT OF THE INVENTION

The object of the present invention is to provide a novel ceramic support layer for a heat-, corrosion- and creep- resistant filter element, which support layer withstands the filtration of hot fluids, in particular gases within a temperature range of 800...1600 °C, in particular 800...1000 °C, for a long term of use. The pore size of the support layer must be such that the pressure loss of the fluid being filtered will remain within acceptable limits.

Another object of the invention is to provide said filtration element, which consists of a support layer and a coating formed thereon.

A third object is to provide a novel method for manufacturing said filter element, in which method the support layer and the coating are fired in one step.

It is a particular object of the present invention to inhibit a reduction of the filter strength at the operating temperature, by selecting the filter materials and construction so as to be best suited for the operating conditions . A fourth object of the invention is to provide a coating which does not contain a separate inorganic binder (e.g. silica) which may during operation react with the material being filtered, thereby causing clogging of the filter. Therefore, an option has been developed wherein the coating is made only of a ceramic material which does not contain silica or any other components readily reacting with the material being filtered. No inorganic binder is used for binding the coating particles to each other; instead, the coating particles, made of a chemically resistant material, are in direct contact with each other. Thus the porosity of the coating is produced from the pores remaining between the coating particles attached to each other in a sintering process .

DESCRIPTION OF THE INVENTION

The invention thus relates to a filter element for fluids, the filter element which is resistant to high temperatures, in particular 800...1600 °C, and comprises a support layer and a coating formed thereon, the pore size of the support layer being greater than the pore size of the coating. The support layer is made up of inorganic structural particles which are bonded together by a reaction product produced in a reaction-sintering process between the said structural particles and a binder formed by an inorganic starting component. At least 60 % by weight of the structural particle amount consists of structural particles having a diameter of 87...1000 μm, i.e. -18, +170 mesh, as determined by the screening method. Thus the average particle size (d₅₀) of the structural particles also remains within these limits.

The selection of the material will be most successful when the binder formed by the inorganic starting component is reaction-sintered with the structural particles of the support to a reaction product which will become part of the binder. The binder may also contain a reaction product formed as a result of reaction-sintering within a starting component or between the starting components, usually at lower temperatures .

By an "inorganic starting component" is meant in this context not only inorganic compounds but also organo etallic and organoceramic compounds.

The recommended diameter of the structural particles is above 87 μm and below 200 μm, i.e. -74, +170 mesh, in order for the particle size distribution of the support structure to be sufficiently narrow, whereby a pore size distribution optimal for the filter support properties is obtained, for example for strength and permeability. Thus the average particle size d₅₀ is above 87 μm and below 200 μm.

Measured by, for example, the liquid expulsion method (Gelinas C, Angers R. : Improvement of the dynamic water- expulsion method for pore size distribution measurements, American Ceramic Society Bulletin, vol. 65, No. 9, pp. 1297-1300, 1986), these structural particle sizes yield 10...50 μm as the average pore size diameter in the support.

Preferably 80 % by weight of the structural particle material consists of particles which pass through a 200 μm, i.e. 74 mesh, screen of a screen series.

The structural particles may be of any oxide ceramic, in particular alumina, mullite, aluminum silicates, MgO»Al₂0₃ spinel, or x CaO • y A1₂0₃ structures, where x and y are integers. Furthermore, the capacity of the structural particles to participate in reaction-sintering can be achieved by coating the support particles (by support particles is meant the structural particles of the support layer) of zirconium oxide (Zr0₂) or chromium oxide (Cr₂0₃) with alumina or possibly with silica, or both. In this case the coating layer of the structural particles is reaction- sintered with the binder.

According to an especially preferred embodiment, the structural particles are of alumina (Al₂0₃). The alumina particles of the support layer may be of any -Al₂0₃, e.g. an -Al₂0₃ prepared by the Bayer process, i.e. activated (through dehydration of aluminum hydroxide) or calcined (low sodium-content grades, pure grades, tabular alumina, or alumina prepared through molten state). The alumina used may also be a natural alumina, i.e. corundum or sapphire. It is especially preferable that the support particles are of synthetic alumina made from molten state. The form of the particles may vary from spherical to angular.

In the first embodiment, the starting component of the reaction-product-containing binder according to the invention may be any aluminum silicate mineral, for example kaolinites, halloysite, allophane, chlorite, cyanite, sillimanite, andalusite, ball clays, montmorillonites , illites, or feldspars, but preferably so that the anhydrous composition of the binder is Si0₂ 50...56 % by weight and Al₂0₃ 44...40 % by weight, and additionally other oxides 0...10 % by weight, but so that preferably the amount of CaO and MgO present as impurities would be very small, in total at maximum 5 % by weight, in order that the formation of anorthite and cordierite would be avoided, since these compounds are not sufficiently durable in the operating conditions. A mineral starting component such as this is easier to sinter than, for example, the mixture of alumina and silica powders mentioned in US patent 4,678,758, since at high temperatures the viscosity of a silica-based phase is, owing to the melt materials and lower alumina amount, lower and thus more fluid than the composition presented in Example 4 of US patent 4,678,758. The glass phase properties of the present invention promote the spreading of the melt over the surfaces of the structural particles, improve the reaction-sintering of the silica present in the binder with the alumina serving as structural particles, and thus produce better strength. Furthermore, the steps of the reaction-sintering mechanism are different and better suited for lower temperatures than in US patent 4,678,758, which describes the use of separate powders (alumina and silica) as the starting components for the binder. In the present invention the elements of the silicate mineral starting component of the binder are atomically mixed together, and the use of such a mineral facilitates the preparation process, in particular the reaction-sintering process .

When aluminum silicate minerals are used, the alumina concentration in the binder can be increased up to 70 % by weight of the chemical composition of the binder by adding as starting components for the binder other alumina- containing minerals, e.g. aluminum hydroxide or bauxite, which take part in the reaction-sintering.

According to another embodiment, so-called chemically prepared mullite (hereinafter 'chemical mullite') can be used as the starting component for the binder, wherein the starting materials generally forming mullite at low temperatures can be prepared in many different ways. The most coirunon of such methods for preparing chemical mullite and its starting materials include the sol-gel method

(mixtures prepared from sols or from sols and salts), the deposition method (starting materials for chemical mullite are precipitated out from clear solutions by using certain precipitating agents), hydrolysis (production of alkoxides for use as starting materials for chemical mullite by means of an addition of water), the spray pyrolysis (the starting material solution is sprayed into a kiln, whereupon chemical mullite will form under the effect of the high temperature), chemical gas phase deposition, CVD (vaporized starting materials react in a gas phase to chemical mullite; the support particles can, for example, be coated by depositing in this manner in advance some chemical mullite on the support particles as nucleating sites for the mullite formed during reaction-sintering). A large number of examples of all of these methods can be found in the reference (Schneider H., Okada K. , Pask J.A.: Mullite and Mullite Ceramics, John Wiley & Sons, 1994).

As an example of the use of a sol-gel method, in this case first a liquid silicate, e.g. tetraethyl orthosilicate, could be mixed together with some aluminum salt, e.g. A1(N0₃)₃, to a mixture with which the support particle is, for example, coated in order to facilitate the reaction- sintering process, or which is mixed with another binder starting component to speed up reaction-sintering and thereby to cause the reaction-sintering to begin at lower temperatures, or just the mixture is contacted with the structural particles and is fired. It is desirable that in all of the options the high-temperature binder contains a slight excess of silica in order to make proper reaction- sintering with the support particles possible at high temperatures. A chemical mullite produced only at low temperatures, i.e. temperatures below 1200 °C, does not alone suffice for achieving sufficient support strength for this application; the binder should also react with the support particles at higher temperatures, which can be regarded as being temperatures above 1500 °C. Of course, for other filtration applications, in which high strength is not required, it is possible to use as the binder chemical mullite alone. Chemically formed mullite may be first so-called 2/1-mullite which, when the temperature is raised, will convert to 3/2-mullite, which is the most common form in which synthetic reaction-sintered mullite occurs .

In a third embodiment, a mixture of a powder containing aluminum or its compounds and a powder containing silicon or its compounds, in particular oxide powders, can be used as a starting component for the binder. In this case, however, the particle size of these powders must be especially small (preferably less than 5 μm, at maximum approximately 50 μm) , in order to produce sufficient strength in the reaction-sintering process. Even though the said starting components are the same materials as stated in US patent 3,959,002, it would not, however, be expedient to fabricate a filter using the particle sizes stated in the patent (e.g. quartz particles of 100 μm...2 mm), since the mechanical strength of the body would be left insufficient.

In a fourth embodiment, combinations of all of the three previous ones can be used as starting components for the binder.

In addition to alumina and silica, it is possible to use in the binder additives (0...50 % by weight, preferably

0...10 % by weight) which are resistant to the corrosion conditions and which may, for example, facilitate the reaction-sintering. Examples of suitable additives include titanium dioxide and potassium oxide.

The reaction-sintering process can also be facilitated by coating the structural particles (alumina) with silica, for example by thermal spraying, whereupon silica will be evenly available.

In the present invention, the creep resistance of the support layer of the filter has been improved by reaction- sintering the structural particles (e.g. Al₂0₃) of the support layer with a binder formed of an inorganic starting component (e.g. aluminum silicate mineral) to a reaction product (e.g. mullite), whereby it has been possible to reduce through chemical reactions the amount of free silica in the binder. The mullite formed as a reaction product is in the operating environment chemically a significantly more stable compound, than the silica present in the binder. By means of the reaction-sintering process the resistance of the support layer of the filter to heat, corrosion and creep is thus increased. Through the use of reaction- sintering, the mechanical strength of the bond between the structural particles of the support layer and the inorganic binder has been improved as compared with the mechanical, fused bond between the mere binder and the support particles presented in patent publication FI 92804.

The coating on the filter support may be either single- layered or multi-layered; in the latter case it contains zones of different particle sizes.

According to a preferred embodiment the coating is made up of inorganic structural particles attached to one another, the particles being chemically of the same material as the structural particles of the support layer.

According to an especially recommended embodiment, the coating is made up of alumina particles sintered directly to one another, their particle size being substantially smaller than that of the alumina particles of the support layer.

The average particle size in the coating is 0.5...50 μm, preferably 3...40 μm, specifically 5...25 μm. In this case, the average pore size in the coating, measured by the water expulsion method mentioned earlier, is 1.0...20 μm.

According to another suitable option, the coating may be of the same material as the reaction product formed in the reaction-sintering between the structural particles of the support layer and the inorganic binder, in which case the thermal expansion coefficients of the coating and the support layer, in general close to each other, enable the coating to adhere well to the support structure. Thus it is also possible to use as the coating some other material having a thermal expansion coefficient close to the thermal expansion coefficient of the support material. The invention also relates to a novel support layer for a filter element resistant to high temperatures, the support layer consisting of inorganic structural particles bonded together by a reaction product produced in a reaction- sintering process between the said structural particles and the binder formed by an inorganic starting component. The invention is characterized in that at least 60 % by weight of the structural particles used are structural particles having a diameter of 87...1000 μm as determined by the screening method, and that the starting component for the binder is an aluminum silicate mineral, mullite prepared from chemical starting materials, a small particle size mixture of a powder containing aluminum or compounds thereof and a powder containing silicon or compounds thereof, in particular oxide powders, or any combination of all of the above alternatives .

The invention also relates to a method for the preparation of the novel support layer described above. The method is characterized in that - the inorganic structural particles and the inorganic starting component for the binder are mixed with a liquid medium and a suitable organic temporary binder and possibly other auxiliaries to form a slurry or paste,

- the slurry or paste is formed into the shape of the desired body, and liquid medium is removed in order to obtain a processable body, and

- the dried body is fired within a temperature range of 1500...1820 °C.

According to a recommended embodiment, the temperature range of the firing step is 1660...1780 °C, preferably 1705...1730 °C.

The firing can be carried out in any kiln atmosphere, i.e. for example in an oxidizing, reducing, negative-pressure or inert atmosphere or an atmosphere containing water vapor, nitrogen or hydrogen. The support layer may be fabricated by any known technique, for example by casting processes, dry or wet pressing processes, extrusion processes, the injection molding process or hot pressing processes. In particular the support layer is fabricated by pressing processes known in conventional ceramic art, i.e. by dry or wet pressing processes or extrusion processes . The structural particles of the support layer and the starting component of the inorganic binder are mixed together with a liquid medium (which is, for example, water, trichloroethylene, methyl ether ketone, ethyl or methyl alcohol, toluene, acetone, or a mixture thereof ) , in which there are used suitable temporary organic binders, e.g. PVA, butyrals (PVB), cellulose derivatives such as CMC, MC, HPC, HPMC and HEC, gums, acrylics (PMMA), alginates, starches, PEO, styrenes, resins or latexes, as well as other organic auxiliaries for slurry preparation and forming, for example, plasticizers (e.g. phthalates, glycols, glycerol ) , lubricants (e.g. stearates, oils, fats), dispersing agents (polyelectrolytes and surfactants), and anti-foaming agents. After the forming the body is dried.

The present invention also relates to a method for the fabrication of a novel filter element according to the invention.

By techniques known per se, it is possible to form on a support layer any coating suitable for the end use of the filter. The inorganic structural particles of the coating layer, e.g. fibers or particles, or both, consisting of alumina or mullite, are mixed with a suitable liquid medium, possibly together with suitable organic auxiliaries such as binder, plasticizer, lubricant, dispersing agent, anti-foaming agent and other additives regulating the colloidal and rheological state of the coating slurry, in order to form a spreadable slurry. Some examples of suitable liquid mediums are water, trichloroethylene, methyl ether ketone, ethyl or methyl alcohol, toluene, or acetone, or a mixture thereof. Suitable temporary organic binders include PVA, butyrals (PVB), cellulose derivatives such as CMC, MC, HPC, HPMC and HEC, gums, acrylics (PMMA), alginates, starches, PEO, styrenes, resins, and latexes. Thereafter the slurry is spread, for example, by immersion, spraying or applying on a fired support layer, and is fired. Before firing it is possible, if so desired, to remove the liquid medium in a separate drying step, but this is not necessary. The sintering of the coating particles can be improved by adding to the coating slurry very small amounts, most commonly less than 1.0 % by weight of the mass of the coating particles, of known coating material sintering additives, which are usually added in the oxide form of these additives in order to facilitate the fabrication. Such oxide-form additives for alumina or mullite include MgO, Ag₂0, CaO, Cr₂0₃, NiO, Fe₂0₃, Zr0₂, Mn0₂, Ti0₂, Nb₂0₅, Y₂0₃ and Ta₂Oz, . These additives can also be used simultaneously with one another.

According to an especially recommended embodiment, the filter element is fabricated by mixing the inorganic structural particles of the coating layer with a suitable liquid medium, possibly together with suitable organic auxiliaries such as binder, plasticizer, lubricant, dispersing agent, anti-foaming agent, and other additives regulating the colloidal and rheological state of the coating slurry in order to form a spreadable slurry, the slurry is spread on a dried but unfired support layer, is dried in order to dry the coating layer, and is fired. Inorganic sintering additives may also be added to the slurry.

This method, in which the firing of the support layer and the coating is carried out in one step, is especially usable if the structural particles of the coating layer and the support layer are chemically of the same material. However, the method does not require the structural particles to be of the same material. If, for example, the structural particles of the support layer are of alumina in which the particle diameter is within a range of 87...1000 μm, and the structural particles of the coating are of alumina in which the particle diameter is 0.5...50 μm, the small particles of the coating layer are sintered directly to one another and to the support simultaneously, while the structural particles of the support layer are reaction- sintered to mullite with the aluminum silicate used as the starting component for the binder of the support layer.

The invention is described below in greater detail with the help of the following examples .

Example 1

Fabrication of the support layer by dry pressing

10 kg of α-alumina particles (d₅₀ approximately 200 μm) , 1 kg of molochite and 0.3 kg of A1(0H)₃ were mixed dry in a kneader for approximately 30 minutes. 1.2 liters of a premixed solution which contained 12 per cent by weight of a polyvinyl alcohol binder and 4 per cent by weight of a glycerol plasticizer was added to the dry powder mixture. The paste was initially rather moist, but as mixing was continued, a moisture content suitable for pressing was sought; the suitable moisture content was determined according to the tool and the pressing pressure used. Thereafter the paste was pressed into its shape in a die.

The paste was pressed both by means of a piston monoaxially and by means of an isostatic hydraulic medium (oil). The latter method is preferable in order to achieve a better final result. In isostatic pressing the said pressing paste was packed into a mold of suitable shape, which was capable of transmitting the pressure evenly into the powder (rubber mold). The powder was packed evenly into the mold, and there was installed inside the mold a shape-retaining porous core producing a hollow shape. The mold was pressed with a pressure of 50 MPa in order to pack the paste tightly. The pressure was removed and the mold was dried at 80 °C in order for the body to shrink and detach from the mold. When the body had dried somewhat, the core and the mold were removed. Thereafter the body was dried at 95 °C for approximately 24 hours . The dried support was processed before coating by grinding it to the desired dimensions .

Example 2

Fabrication of the support layer by extrusion

In 3.8 kg of distilled water a mixture A was prepared by mixing into it 1.1 kg of molochite, 1 kg of ball clay and 1.1 kg of kaolin. Into this slurry A additionally Darwan C was mixed as a dispersing agent until a suitable fluidity was achieved in order to distribute the clay particles evenly throughout the slurry and to avoid the formation of strength-lowering agglomerates. The slurry was stirred for 2 hours. Simultaneously a solution B was prepared which contained 1.4 kg of distilled water, into which there were mixed first 400 g of carboxymethyl cellulose CMC, or 400 g of methyl cellulose MC, as green-state binder, 250 g of polyethylene glycol as a plasticizer, and a water-based silicon emulsion as an anti-foaming agent in such an amount that foaming stopped. The solution was mixed for approximately 30 minutes with a strong propeller stirrer. Thereafter 20 kg of α-Al₂0₃ particles (d₅₀ = 100 μm), the clay mixture A and the binder solution B were combined to form one paste, which was mixed with a strong propeller stirrer for one hour to achieve homogeneity. Thereafter the paste was extruded through a die by using both a piston- type and a screw-type extruder. The tube which had come through the die was cut and dried until the body kept its shape. The bottom plug and conical flange needed were attached afterwards by pressing them by using a mold and the same paste. The body was re-dried at 95 °C for 24 hours .

Example 3

Two-step firing, immersion coating

A body was fabricated according to Example 1 and was fired at a temperature of 1750 °C. The body was coated by immersing it in a coating slurry prepared by mixing 1 kg of α-alumina (d₅₀ approximately 5 μm) into one liter of distilled water. To adjust the viscosity to a suitable level, approximately 1.5 per cent by weight of CMC or hydroxypropyl cellulose HPMC was used as an organic binder. 0.5 % by weight of Nb₂0₅ was also added as a sintering additive to the coating slurry. After a suitable time, which determined the thickness of the coating, the body was lifted out of the slurry and was dried. A second firing was carried out at a temperature of 1650 °C.

Example 4

Single-step firing, spraying

A body was fabricated according to Example 2. A coating slurry was prepared in the manner of Example 3 except that the coating particles were α-alumina (d₅₀ = 15 μm) . The coating slurry was sprayed onto the dried body. The body was dried and fired in one step at a temperature of 1720 °C.

Figures 1-5 show scanning electron micrographs of a support structure fabricated according to Example 2, in which kaolin was used as the inorganic starting component for the binder. The sample was obtained by cutting, from a body fired at 1705 °C for three hours, a piece, which was pickled in hydrofluoric acid to remove any remaining glass phase and to make visible the mullite and alumina particles. In Figures 1 and 2 there can be seen, deposited on the surface of the alumina particles, mullite formed as a result of the reaction-sintering process between the support particle alumina and the silica of the binder. If the mullite needles had been formed in a reaction-sintering process inside the starting component or between the starting components within the binder with the alumina and silica contained therein, the shape of the mullite would have been long and needle-like, which can be observed in

Figure 3. The mullite in Figures 1 and 2 has grown from the surface of the alumina particles, and the mullite of Figure 3 from the melt.

If the starting component of the binder contains impurities, they may additionally form a reaction-sintering product of their own. For example, potassium oxide may react with the alumina of the support particles and the silica of the binder, forming potassium aluminum silicate, which is shown in Figure 4 (arrows). From the outer appearance of the crystals it can be judged that they have grown from molten binder through the screw dislocation mechanism.

It is also clear that the mullite crystals were sintered to each other, so that the entire "support structure" of the support layer was formed by mullite needles which were sintered to one another and were also sintered to the mullite formed on the surface of the alumina particles of the support layer, as can be observed in Figure 5.

For a person skilled in the art it is clear that the various embodiments of the invention may vary within the scope of the claims stated hereinafter.

Claims

1. A filter element for fluids, the filter element being resistant to high temperatures, in particular 800 - 1600 °C, and comprising a support layer and a coating formed thereon, the pore size of the support layer being greater than the pore size of the coating, characterized in that

- the support layer is made up of inorganic structural particles which have been bonded together by means of a reaction product produced in a reaction-sintering process between the said structural particles and a binder formed by an inorganic starting component, and that

- at least 60 % by weight of the structural particles used are structural particles having a diameter of 87...1000 μm, i.e.

-18, +170 mesh, as determined by the screening method.

2. The filter element according to Claim 1, characterized in that at minimum 80 % by weight of the structural particles used are structural particles having a diameter above 87 μm and below 200 μm, i.e. -74, +170 mesh.

3. The filter element according to Claim 1 or 2 , characterized in that the starting component for the inorganic binder is an aluminum silicate mineral, a starting material for chemical mullite, a small particle size mixture of a powder containing aluminum or compounds thereof and a powder containing silicon or compounds thereof, in particular oxide powders, or any combination of all of the alternatives mentioned above, and that the reaction-sintering product is mullite.

4. The filter element according to Claim 1, 2 or 3 , characterized in that the coating is made up of inorganic structural particles sintered to one another, the particles being chemically of the same material as i) the structural particles of the support layer, ii) the reaction-sintering product or iii) a material having a thermal expansion coefficient approximately equal to that of the structural particles of the support layer.

5. The filter element according to Claim 4, characterized in that the coating is made up of alumina particles which are sintered directly to one another and have a particle size substantially smaller than that of the alumina particles of the support layer.

6. A method for the fabrication of a filter element according to any of Claims 1-5, characterized in that

- the inorganic structural particles of the coating layer are mixed with a suitable liquid medium, possibly together with suitable auxiliaries and inorganic sintering additives, to form a spreadable slurry,

- the slurry is spread onto a fired support layer, and

- is fired.

7. A method for fabricating a filter element according to any of Claims 1-5, characterized in that

- the slurry is spread on a dried but unfired support layer, and

- is fired.

8. A support layer for a fluid filtering element resistant to high temperatures, in particular 800-1600 °C, the support layer being made up of inorganic structural particles which are bonded together by means of a reaction product produced in a reaction-sintering process between the said structural .particles and a binder formed by an inorganic starting component, characterized in that at minimum 60 % by weight of the structural particles used are structural particles having a diameter of 87...1000 μm, and that the starting component for the inorganic binder is an aluminum silicate mineral, a starting material for chemical mullite, a mixture, having a particle size of at maximum 50 μm, of a powder containing aluminum or compounds thereof and a powder containing silicon or compounds thereof, in particular oxide powders, or a combination of all of the alternatives mentioned above.

9. The support layer according to Claim 8, characterized in that the structural particles are of alumina (Al₂0₃) and the starting component for the binder is aluminum silicate, in which case the reaction-sintering product is mullite.

10. The support layer according to Claim 9, characterized in that the structural particles and possibly also the inorganic binder additionally contain one or more other inorganic materials .

11. The support layer according to any of Claims 8-10, characterized in that at minimum 80 % by weight of the structural particles used are structural particles having a diameter above 87 μm and below 200 μm.

12. A method for the fabrication of the support layer of a filter element according to any of Claims 8-11, characterized in that

- the inorganic structural particles and the inorganic starting component for the binder are mixed with a liquid medium and with a suitable temporary organic binder, and possibly with other auxiliaries, to form a slurry or a paste,

- the slurry or paste is formed into the shape of the desired body and the liquid medium is removed in order to obtain a processable body, and - the dried body is fired within a temperature range of 1500...1820 °C.

13. The method according to Claim 12, characterized in that the temperature range of the firing step is 1660...1780 °C, preferably 1705...1730 °C.