Ceramic filter for a dust-containing gas and method for its production

Abstract: A ceramic filter for a dust-containing gas, which comprises a filter base having an average pore size of from 10 to 100 µm, with the pore size ratio at positions of 75 vol% and 25 vol% of the accumulated pore size distribution being at least 1.3, and a filter layer having an average pore size of from 0.2 to 10 µm fixed at least to the filtering side surface of the filter base so that the filter layer fills the pores open on the surface of the filter base. forms pores when burned off.

Description: The present invention relates to a ceramic filter useful for removing a dust from a dust-containing gas, particularly from a high temperature dust-containing gas, and a method for its production. Ceramics are usually excellent in refractoriness and corrosion resistance and suitable as materials for filters for high temperature or corrosive gasses. On the other hand, there is an increasing need for ceramic filters which can be used under severe environmental conditions. Their applications cover not only pollution control or improvement of the quality of products but also saving of energy or development of an energy such as coal. Therefore, it is desired to develop a high performance ceramic filter for a dust-containing gas, which is a key to accomplish these technical developments. When a filter is to be used at a high temperature, rapid heating or quenching is often unavoidable. Therefore, a material having a small thermal expansion coefficient which is strong against thermal shock, is required. Among various types of ceramics, a cordierite ceramics has a particularly small thermal expansion coefficient and is a material strong against thermal shock. By utilizing this properties, a cordierite filter for high temperature use has been developed for a certain specific purpose. For example, JP-A-52133810 discloses a cordierite ceramic filter and a method for its production, wherein a honeycomb type filter is discussed which has a pore size suitable for removing fine particulates in an exhaust gas from a diesel engine, and a method is disclosed for producing a cordierite ceramic filter having a relatively large average pore size. Further, it is disclosed to add an alkaline component such as a lithium oxide component for the purpose of forming relatively large pores. Further, as described in JP-A-63031517, one of the present inventors has previously proposed a tubular cordierite filter having a pore size distribution hardly susceptible to clogging, for the purpose of removing a dust from a high temperature dust-containing gas. As the filter, the smaller the pressure loss, the better, since it is thereby possible to obtain a large capacity filter apparatus for treating the dust-containing gas. For this purpose, the larger the porosity, the better. However, there is a problem that the strength decreases substantially when the porosity is increased. It is therefore common to find out a compromise point and produce a filter on the basis of such a compromise. With respect to alumina and silicon carbide which have been practically used as ceramic filters, they have established applications for abrasives or polishing agents. Therefore, by properly selecting a commercial product having a suitable particle size for the grog, it is possible to relatively easily prepare a filter having a small pressure loss and a practical level of strength. In such a filter, it is common to employ a matrix made of glass or clay. The glass matrix has a problem that the high temperature strength is poor such that the strength at 800°C is as low as at most 1/3 of the strength at room temperature. Whereas, the clay matrix has a problem that even the strength at room temperature can hardly be increased. On the other hand, commercially available synthetic cordierite grogs have a large porosity in most cases. Accordingly, even if such commercially available grogs are used for the preparation of ceramics filters, the resulting filters will have a large porosity and a large pressure loss, and there has been a problem that if the porosity is increased to reduce the pressure loss, it becomes difficult to maintain the strength at a practical level, when cordierite ceramic filters are to be used in a high temperature and corrosive atmosphere for a long period of time, it is necessary to have ceramic filters which have adequate strength within a wide temperature range of from room temperature to a high temperature to secure the reliability of a filter apparatus, and which are excellent in the thermal spalling resistance and hardly susceptible to clogging, and have a small pressure loss, and which, in the case of removing a dust from a gas including such corrosive gas as NO x or SO x , are further excellent in the corrosion resistance, particularly in the acid corrosion resistance. It is an object of the present invention to solve the problems of the cordierite ceramic filters of the conventional techniques and to provide cordierite filters for a dust-containing gas which have a small pressure loss and practical strength and are excellent in the corrosion resistance. The ceramic filter of the present invention comprises at least 50 wt% of a cordierite grog having an apparent porosity of at most 10% and a particle diameter of at least 74 µm, and a matrix, wherein said matrix comprises silica (SiO 2 ) and alumina (Al 2 O 3 ) as the main components, and contains lithium oxide (Li 2 O) in an amount of from 0.05 to 1.0 wt%, based on the weight of the entire filter. In a preferred embodiment of said ceramic filter of the present invention, the matrix further contains magnesium oxide (MgO). In another preferred embodiment of said ceramic filter of the present invention, the cordierite grog is a cordierite grog crystallized from a class having approximately a cordierite composition (2MgO·2Al 2 O 3 ·5SiO 2 ). In another preferred embodiment of said ceramic filter of the present invention, the cordierite grog is the one obtained by impregnating to a cordierite grog, alumina sol, silica sol, titanic sol, zirconia sol or a mixture of at least two types of such sols, which presents an apparent porosity of at most 10% when sintered. In another preferred embodiment of said ceramic filter of the present invention, the porosity of the filter material is at least 35%, and the bending strength at 800°C is at least 75 kg/cm 2 . In another preferred embodiment of said ceramic filter of the present invention, the matrix contains from 0.5 to 12 wt%, based on the weight of the entire filter, of TiO 2 and/or ZrO 2 . In another preferred embodiment of said ceramic filter of the present invention, the shape of the filter is a straight tube. The ceramic filter of the present invention comprises a filter base comprising at least 50 wt% of a cordierite grog having an apparent porosity of at most 10% and a particle diameter of from 74 to 590 µm, and a matrix, wherein the matrix comprises silica (SiO 2 ) and alumina (Al 2 O 3 ) as the main components and contains from 0.2 to 0.6 wt%, based on the weight of the entire filter, of lithium oxide (Li 2 O), said filter base having an average pore size of from 10 to 100 µm, with the pore size ratio at positions of 75 vol% and 25 vol% of the accumulated pore size distribution being at least 1.3, and a filter layer having an average pore size of from 0.2 to 10 µm fixed at least to the filtering side surface of the filter base so that the filter layer fills the pores open on the surface of the filter base. In a preferred embodiment of the ceramic filter of the present invention, the cordierite grog is a cordierite grog crystallized from a glass having approximately a cordierite composition (2MgO·2Al 2 O 3 ·5SiO 2 ). In another preferred embodiment of the ceramic filter of the present invention, the filter layer is made of diatomaceous earth or rice hull ash. In another preferred embodiment of the ceramic filter of the present invention, the shape of the filter base is tube shape, and the filter layer is fixed on the inner wall surface of the tube. In a method for producing the ceramic filter of the present invention, a powder material to form a matrix obtained by mixing clay and a powder containing lithium oxide, is mixed to at least 50 wt% of cordierite grog having an apparent porosity of at most 10% and a particle diameter of at least 74 µm, and an organic binder is added to the mixture, followed by molding and sintering. In a preferred embodiment of the method for producing the ceramic filter of the present invention, the cordierite grog having an apparent porosity of at most 10% is the one obtained by mixing materials containing MgO, Al 2 O 3 and SiO 2 to constitute approximately a cordierite composition (2MgO·2Al 2 O 3 ·5SiO 2 ), melting the mixture and cooling it to obtain a glass, which is then crystallized by heat treatment. In another preferred embodiment of the method for producing the ceramic filter of the present invention, a β-spodumene powder is added to the powder material to form the matrix, so that the content of lithium oxide in the filter material will be within a range of from 0.05 to 1.0 wt%. In another preferred embodiment of the method for producing the ceramic filter of the present invention, a cordierite powder having a particle diameter of at most 74 µm, is added to the powder material to form the matrix. In another preferred embodiment of the method for producing the ceramic filter of the present invention, a powder of ZrO 2 and/or TiO 2 is added to the powder material to form the matrix, in an amount of from 0.5 to 12 wt% based on the weight of the filter material. In another preferred embodiment of the method for producing ceramic filter of the present invention, a coke powder is mixed to the starting material, as a pore-forming material which forms pores when burned off. In another preferred embodiment of the method for producing ceramic filter of the present invention, the molding of the filter is conducted by an isostatic press using a metal core mold and an cuter cylindrical rubber mold, to form a straight tube. In another preferred embodiment of the method for producing ceramic filter of the present invention, the sintering is conducted within a temperature range of from 1,310 to 1,380°C. In the method for producing the ceramic filter of the present invention, the apparent porosity of the cordierite grog is adjusted to be at most 10%. This is effective to minimize the pressure loss of the filter. If the apparent porosity exceeds 10%, the pressure loss tends to be large. Further, by controlling the apparent porosity of the cordierite grog to a level of at most 10%, the strength of the cordierite grog will be high. Consequently, the strength of the filter itself will be high. The cordierite grog having a particle diameter of at least 74 µm (the particle diameter not to pass a sieve of 200 mesh) is incorporated in an amount of at most 50 wt%. The grog having a particle size of at least 74 µm is useful for the formation of pores having a size of at least 10 µm suitable for the filter for a gas. A part of the cordierite grog may be as large as 2 mm in diameter. When such a grog is incorporated in an amount of at least 50 wt%, it is possible to impart at least the necessary amount of pores for providing gas permeability useful as a filter. If the grog having a particle diameter of at least 74 µm is less than 50 wt%, the shrinkage during sintering will be substantial, whereby there will be drawbacks such that the filter undergoes a deformation, and the porosity tends to be small and the pressure loss tends to be large. It is further preferred to incorporate the cordierite grog in an amount of at least 70 wt%, whereby the sintering shrinkage will be small, and the apparent porosity will be large, and the pressure loss will be small. Here, the rest is mixed usually in the form of a fine powder, and in the filter after sintering, it constitutes a matrix for the cordierite grog. This matrix preferably has refractoriness slightly lower than the cordierite grog and a thermal expansion coefficient close to the cordierite grog. Accordingly, it preferably has such a composition that during the sintering it partially forms glass. Clay is incorporated in the powder material to form a matrix, because it contains alumina and silica as the main components, and it is effective for the improvement of the moldability and the sinterability. When the matrix contains lithium oxide in addition to alumina and silica, the matrix will satisfy the above requirements, and in combination with the cordierite grog having a low apparent porosity, a filter having adequate strength not only at room temperature but also at a high temperature of about 800°C, will be obtained. The content of lithium oxide (Li 2 O) in the filter material is from 0.05 to 1 wt% (corresponding to the incorporation of from 1 to 20 wt% of β-spodumene). If the content is less than 0.05%, it becomes difficult to obtain a sintered body having sufficient strength. On the other hand, if the content exceeds 1 wt%, the refractoriness of the cordierite ceramic filter will be impaired, such being undesirable. When both the refractoriness and the strength of the resultant filter are taken into consideration, the content of lithium oxide is preferably within a range of from 0.2 to 0.6 wt%. The reason why a filter maintaining its strength even at a high temperature is obtainable by the matrix containing lithium oxide (Li 2 O) is not clearly understood, as the phase relation between cordierite and β-spodumene (Li 2 O·Al 2 O 3 ·4SiO 2 ) is complex (Yoichi Shiraki, Fine Ceramics p310, 1976, Gihodo). However, it may be explained as follows. In said another ceramic filter of the present invention, the matrix of the filter containing lithium oxide (Li 2 O) and comprising alumina (Al 2 O 3 ) and silica (SiO 2 ) as the main components, is vitrified in a substantial amount during the sintering to form strong bonding among the grogs. In this glass, the majority of lithium oxide is contained, and the glass has a low melting point as compared with the grog. By the subsequent cooling, crystallization of the glass will occur. The formed crystals have a low thermal expansion coefficient, and the matrix as a whole has a thermal expansion coefficient close to the cordierite grog. Thus, there will be presented a cordierite ceramic filter having high room temperature strength and a small residual strain caused by the difference in the thermal expansion from the grog even when cooled to room temperature. β-Spodumene is added to the powder material to form a matrix, because it is convenient to avoid localization of lithium oxide, and β-spodumene is effective to reduce the thermal expansion coefficient of the entire matrix even when it remains as it is in the filter material. The cordierite powder is incorporated, since it serves to bring the thermal expansion coefficient of the matrix close to the thermal expansion coefficient of the grog. The cordierite powder having a particle diameter of less than 74 µm serves mainly for the formation of a matrix in said another ceramic filter of the present invention. This is the reason for the limitation of "finer than 74 µm". Since the resultant matrix contains a relatively large proportion of the crystalline component, the strength is maintained without no substantial softening even at a temperature about 1,000°C. Natural ores containing lithium oxide such as petalite and eucryptite may also be used as the powder material for forming the matrix. In the method for producing said another ceramic filter of the present invention, it is preferred to employ a cordierite grog crystallized from the glass, because it is thereby possible to obtain a dense grog, for example, a grog having an apparent porosity of not higher than 4%, although such a grog may need a cumbersome process step for its preparation. Further, as compared with a usual cordierite grog obtained by sintering, the thermal expansion coefficient is smaller by about 1/2. Accordingly, the filter material will have a small thermal expansion coefficient, and it is thereby possible to obtain a cordierite filter for a gas which is particularly excellent in the thermal spalling resistance. The grog is highly dense, and therefore makes a filter material excellent also in the corrosion resistance. Accordingly, it is possible to produce a cordierite filter for a gas, which is suitable for use particularly under a severe high temperature atmosphere. As the starting material for the cordierite grog crystallized from the glass, magnesia clinker, bayer alumina and siliceous sand are preferably employed. The production is preferably conducted in such a manner that the melt is once quenched to form a glass, which is then subjected to heat treatment for crystallization, as disclosed, for example, in Japanese Examined Patent Publication No. 20269/1982. As the synthetic cordierite grog, a sintered product prepared by using talc, clay and aluminum hydroxide as the starting material, may preferably be employed. In order to obtain coarser particles of cordierite having an apparent porosity of at most 10%, it is economical to sufficiently sinter the material at the stage of the preparation of the usual synthetic cordierite, and it is effective to employ a starting material having a zircon powder incorporated to improve the sinterability. A commercially available synthetic cordierite is usually has a porosity higher than 10% and can not be used by itself as the grog for the cordierite filter of the present invention. Under the circumstances, it may be sintered again for densification. However, as a result of various studies, it has been found that with a grog having a not-so large apparent porosity, the apparent porosity can be changed to a level of at most 10% by impregnating a colloidal solution such as an alumina sol, a silica sol or a titania sol to the cordierite grog. Once the porosity can be brought to a level of at most 10%, it can be used as the grog for the cordierite filter of the present invention without any problem, and the filter thereby produced will have a small pressure loss and high strength. To sufficiently impregnate the colloidal solution, it is preferred to employ reduced pressure. It is further preferred that the grog is dried, and the impregnation is repeated for further densification. For the purpose of the present invention, after gelling or drying the grog impregnated with the colloidal solution, it may be used by itself as the grog. In another ceramic filter of the present invention, the condition that the apparent porosity of the grog is at most 10%, is required to be achieved with the grog after the sintering. However, in a case where the batch is formed into a tubular filter, and then such a colloidal solution is impregnated, it is likely that pores expected to be used as passages for a gas in the filter, will be clogged and reduced, and the filter will have a large pressure loss, such being undesirable. The reason why the pressure loss can be reduced by adjusting the apparent porosity of the cordierite grog to a level of at most 10%, is believed to be as follows. Firstly, on the assumption that the flow of the gas passing through the ceramic filter can be approximated by the equation (Kozeny-Carman equation) representing the flow of a fluid passing through a packed layer of a powder, the pressure loss ΔP through the filter may be represented by the following equation, provided that the gas passing through the filter is a laminar flow (Fine Particle Measurement, Clyde Orr Jr. et al, Chapter 7, The Macmillan Co., New York, 1959). S v 2 = (g c L/2µuL e )(ΔP/L)[ε 2 (l-ε) 2 ] Here, S v , is a surface area per unit volume of the solid, g c is a gravity constant, L is the thickness of the filter, µ is the viscosity of the gas, u is the apparent velocity of the gas, L e is the length of a capillarly meandering in the filter, ε is the porosity, and as approximation, there is a relation of u c = (u/ε)(L e /L) . Namely, the pressure loss of the gas passing through the filter can be approximated to be proportional to the square of the total surface area of the particles in a unit volume of the filter. Accordingly, to reduce the pressure loss of the filter, the total surface area of the particles in the unit volume of the filter is minimized as far as possible, whereby the pressure loss can be reduced. Further, in a sintered body, if the apparent porosity is at most 10%, the gas permeability will be so small that it can substantially be neglected (Porous Material, compiled by Ren-ichi Kondo, published by Gihodo, 1973). Therefore, pores in the grog with a porosity of at most 10% will not be concerned with the gas permeability and will not be a cause for the pressure loss. Namely, the pressure loss is determined primarily by the surface areas of the grog particles and of the structure among the grog particles, whereby a filter having a small pressure loss is believed to be produced. In order to minimize the pressure loss while improving the gas permeability, it is effective to incorporate to the starting material a pore-forming material capable of forming pores when burned off during the sintering. As such a pore-forming material, a coke powder, particularly a pitch coke powder, is preferably incorporated, since it contains a relatively small amount of impurities such as iron content, whereby coloring can be avoided, and which is readily available and has a proper hardness not to adversely affect the moldability. An organic binder is incorporated in the starting material primarily for the purpose of imparting strength to the green body to facilitate the molding of the filter and thereby avoiding the breakage of the green body during its handling. However, the incorporation of the organic binder also serves to increase the porosity. As a method for molding the filter, an isostatic press molding method, a semi-wet rumming or an extrusion molding method may be employed. Among these methods, the isostatic press molding has been most commonly employed as a molding method suitable for the production of a highly dense sintered body. It seems that there has been no instance wherein the isostatic press molding is used as a method for forming porous filters. However, it is a molding method preferred also for the production of such a filter since the drying step can be thereby omitted, a large sized tube filter can be easily formed, and the productivity is good. Further, a tube shape ceramic filter formed by isostatic press, has a relatively smooth inner surface, since a metal core mold having a smooth surface is used usually for the molding, and when it is used as a filter base, it is convenient to rub a filter layer thinly into the inner surface. Sintering is conducted in many cases by a tunnel kiln. However, a shuttle kiln or an electric furnace may be employed. The temperature range for sintering is preferably from 1,310 to 1,380°C. If the sintering temperature is lower than 1,310°C, the sintering will be inadequate, and the strength tends to be low. On the other hand, if the sintering temperature exceeds 1,380°C, there will be drawbacks such that the filter undergoes a deformation during the sintering by the weight of the filter itself, and with the excess sintering, the porosity will be small, such being undesirable. The apparent porosity is specified to be at least 35%, because such is preferred in order to obtain practically usable gas permeability for a filter. The bending strength at 800°C of at least 75 kg/cm 2 is a large value, which has not been attained by conventional cordierite ceramic filters having an apparent porosity of at least 35%. However, when the cordierite ceramic filter having high strength even at a high temperature thus obtained, was actually used at a high temperature, and then subjected to examination of the bending strength at room temperature, it was found that the strength has decreased. The reason is not yet clearly understood. However, when the strength decreased, the presence of crystals of β-spodumene was detected in the matrix by the X-ray powder diffraction. Namely, it is considered that the decrease in the strength is attributable to the change in the structure in the matrix. The addition of the ZrO 2 and/or TiO 2 component to this matrix is intended to promote the crystallization of glass in the matrix by adding such a component, so that the glass present in the matrix is fully crystallized during the cooling after the sintering to obtain a stabilized state. By the addition of such a component, after the sintering, the presence of β-spodumene crystals in the matrix of the filter is certainly confirmed by the X-ray powder diffraction. As a result, although the initial strength is sacrificed a little as compared with the case where no such a component has been added, there is no tendency for a decrease of the strength during the use which is observed in the case where no such a component is added. Thus, a cordierite ceramic filter showing stabilized strength, which can be used repeatedly with high reliability, is obtained. Here, the amount of ZrO 2 and/or TiO 2 added, is usually from 0.5 to 12 wt%, preferably from 1 to 10 wt%, as the total amount in the filter material. If the total amount is less than 0.5 wt%, no adequate effects for stabilizing the strength will be obtained. On the other hand, if the total amount exceeds 12 wt%, the thermal expansion coefficient of the filter will be large, whereby the thermal spolling resistance tends to be impaired, such being undesirable. Now, the present invention will be described in further detail with reference to Examples. However, it should be understood that the present invention is by no means restricted by such specific Examples. Now, Examples for a ceramic filter of the present invention will be described. As the cordierite grog, seven types of materials i.e. a , b , c , d , e , f and g , were prepared. a . Magnesia, alumina and siliceous sand were mixed to constitute a cordierite composition of 2MgO·2Al 2 O 3 ·5SiO 2 . The mixture was electrically melted by employing carbon electrodes, and the melt was poured into water and quenched to obtain a glass having the cordierite composition. The glass was taken out and crushed to obtain glass particles having a particle diameter of from 1 to 10 mm. The particles were introduced to a tunnel kiln and heat-treated at about 1,380°C for 10 hours for crystallization. The product was crushed and classified by sieving to obtain particles having a size of from 74 to 590 µm.The average particle diameter of this grog was 270 µm, and the apparent porosity was 3%. b . Fine powders of talc, clay, aluminum hydrooxide and zircon were mixed to constitute a cordierite composition comprising a chemical component of 2MgO·2Al 2 O 3 ·5SiO 2 as the main component and containing 3 wt% of ZrO 2 . To 100 parts by weight of the mixed raw material, 2 parts by weight of methylcellulose and a suitable amount of water were added, and the mixture was kneaded and extruded into a rod having an outer diameter of about 30 mm. The rod was dried and then put on a cart and introduced into a tunnel kiln, and held at a maximum temperature of 1,360°C for 10 hours to obtain a synthetic cordierite.This product was crushed and classified by sieving to obtain particles having a particle size within a range of from 74 to 590 µm.The average particle diameter of this grog was 265 µm, and the apparent porosity was 7%. c . Fine powders of talc, clay and aluminum hydroxide were mixed to constitute a cordierite composition of 2MgO·2Al 2 O 3 ·5SiO 2 . To 100 parts by weight of the mixed raw material, 2 parts by weight of methylcellulose and a suitable amount of water were added, and the mixture was kneaded and extrusion-molded into a rod having an outer diameter of about 30 mm. The rod was dried and then put on a cart and introduced into a tunnel kiln, and maintained at the maximum temperature of 1,360°C for 10 hours to obtain a synthetic cordierite.The product was crushed and classified by sieving to obtain grog having a particle size of from 74 to 590 µm.This grog had an average particle diameter of 250 µm and an apparent porosity of 12%. d . A commercial product synthetic cordierite grog was crushed and classified by sieving to obtain grog having a particle size of from 74 to 590 µm. The grog had an average particle diameter of 230 µm and an apparent porosity of 30%. e . An alumina sol was impregnated to the cordierite grog obtained in c , and dried under reduced pressure. The same operation was repeated once again. After heating this grog at 1,000°C, the apparent porosity was 8%. f and g . A silica sol and a titania sol were repeatedly impregnated, respectively, to the cordierite grog obtained in c , followed by drying. After heating these grogs at 1,000°C, the apparent porosities were 7% and 8%, respectively. A powder of β-spodumene was prepared in the following manner. Powders of pyrophyllite and lithium carbonate were mixed in the proportions of 85 wt% and 15 wt%, respectively, and the mixture was further mixed and milled in a wet system. The filter press cake thus obtained was dried and then sintered at a temperature of 1,150°C in an electric furnace to obtain β-spodumene. This was finely pulverized and sieved with a sieve of 325 mesh. Using seven types of cordierite grogs a to g , tube-shaped cordierite filters for a gas were prepared under the conditions as identified in Tables 1 and 2. The Examples and Comparative Examples in Table 1 are directed to the examination of the effects of the porosity of the grog and the blend ratio of the grog mainly over the filter properties. In these Examples, a mixture comprising 10 wt% of clay, from 10 to 25 wt% of a fine powder of cordierite of the same material with the grog and 5 wt% of a fine β-spodumene powder, was used for the matrix. To 100 parts by weight of this mixture, 25 parts by weight of a coke powder having a particle diameter of from 20 to 100 µm was added as a pore-forming material, and 7 parts by weight of a 40 wt% aqueous solution of a phenol resin was added as an organic binder. The mixture was mixed, dried at 110°C and then crushed to obtain a granular material having a particle diameter of at most 3 mm. This material was filled in a mold constituted by a steel columnar core mold and an outer cylindrical rubber mold and molded by an isostatic press under a pressure of 1,000 kg/cm 2 to obtain a tube having an outer diameter of about 170 mm, an inner diameter of about 140 mm and a length of about 850 mm. This tube-shaped molded body was sintered in an electric furnace at the maximum temperature of 1,320°C for 5 hours to obtain a tube-shaped ceramic filter. When the sintering was conducted in a tunnel kiln, a filter having substantially the same sintered state was obtained as in the case where sintering was conducted at a temperature of about 1,350°C. The properties examined with respect to the respective tube filters are shown in Table 1. In other Examples and Comparative Examples, tube filters were prepared under the same conditions as above except that the conditions as identified in Table 1, and their properties are also shown in Table 1. Here, the apparent porosity is the value measured by a water immersion method, and the average pore size is the value measured by a mercury porosimeter with respect to cube shaped test samples of 1 cm 3 . Further, the bending strength was measured by a three point bending test with a span of 70 mm with respect to a test piece of 14 x 20 x 100 mm. The acid corrosion resistance was examined in such a manner that the test piece was immersed in a 1N H 2 SO 4 aqueous solution for 72 hours at room temperature, whereupon the change in the bending strength was examined. Further, the filtering property was determined in such a manner that a tube filter cut to have a length of 400 mm was mounted on the filtration test apparatus equipped with a reverse cleaning system, a dust of a ferric oxide powder having an average particle diameter of about 50 µm was dispersed in air at a concentration of 40 g/Nm 3 to form a dust-containing gas at a temperature of 300°C, and the filtration test was conducted for 100 hours each at a filtration velocity of the gas of 5 cm/sec. The reason for using the ferric oxide powder is that this powder has a color and can easily be observed when it is penetrated into the ceramic filter. The particle size distribution of the ferric oxide powder was such that less than 1 µm was 1 wt%, from 1 to 10 µm was 7 wt%, from 10 to 40 µm was 35 wt%, from 40 to 100 µm was 46 wt%, and from 100 to 800 µm was 11 wt%. Table 2 shows the results of the examination of the effects of the change of the composition of the matrix over the properties of the filter material and over the filtration properties. Among the conditions for the production of the filter, those not shown in the Table were the same as mentioned above. Bentonite contains an alkali component and when sintered, it readily vitrifies to form a glass matrix, whereby a filter having high strength at room temperature is obtained. In Table 2, the amount of ZrO 2 and/or TiO 2 is represented by parts by weight as the ratio relative to 100 parts by weight of the total weight of the grog and matrix component. As a method for examining the presence or absence of a decrease of the strength during use, a test piece for bending strength cut out from the ceramic filter is maintained in an electric furnace at 900°C for 24 hours, and then the bending strength at room temperature was measured. As a result, it has been found that ceramics filters having zirconia and/or titania added to the matrix, show a little decrease in the initial strength, but no decrease in the strength after the heat treatment was observed. When the ceramic filter of the present invention is constructed properly with a combination of proper grog material, matrix material, filter layer material and inorganic binder, it is possible to obtain a filter which can be used for the filtration of a dust-containing gas having a corrosive nature and a high temperature stably for a long period of time. By another ceramic filter of the present invention and the method for its production, a ceramic filter suitable particularly for the filtration of a high temperature gas, which is excellent in the thermal spolling resistance and corrosion resistance and has a small pressure loss and high strength even at a high temperature at a level of 800°C, and a method for its production, have been shown. In said another ceramic filter of the present invention, a grog having a low apparent porosity of at most 10%, particularly a cordierite grog having a very low apparent porosity crystallized from a glass, is used and combined with a matrix containing lithium oxide, whereby a filter for a gas having a small pressure loss, high strength even with a large porosity and being capable of maintaining this strength even at a high temperature and at the same time having excellent corrosion resistance, particularly excellent acid corrosion resistance, is obtainable. Since the pressure loss of the filter is small, the treating capacity of the dust-containing gas can be increased even when the same filtration apparatus is employed. Since the filter strength is adequate, breakage of the filter during the installment and the maintenance of the filtration apparatus, is substantially reduced, and the reliability of the apparatus is remarkably improved. Further, a silicon carbide filter which is considered to be prospective presently, has been found to have a problem that it is susceptible to oxidation by steam contained in a combustion gas, whereby pores are likely to be clogged, or the pressure loss tends to be large. Whereas, another ceramic filter according to the present invention has excellent durability in an acidic corrosive gas atmosphere containing steam, NO x and SO x , such as a combustion gas of coal, and thus is useful for the filtration of a combustion gas from a fluidized bed combustion boiler or a pressurized fluidized bed combustion boiler in which coal is burned for power generation, or for the filtration of a synthetic gas produced by a coal gasification furnace. With another ceramic filter of the present invention, there has been a tendency that the strength decreases when used at a high temperature. However, by adding TiO 2 and ZrO 2 to the matrix so that an instable glass phase in the matrix is converted to crystals by sintering, it has been improved to a reliable filter having no tendency for a decrease of the strength. Said another ceramic filter of the present invention is widely useful for the removal of a dust from various high temperature dust-containing gases discharged from industry employing high temperatures, and makes it possible to recover and effectively use a heat energy which used to be difficult to recover and wasted since it contains a dust. Further, with the filtrated high temperature gases, removal of NO x or SO x is facilitated, and the dust is recovered in a dried state, which facilitates effective utilization of the dust. By mounting the ceramic filter for a gas of the present invention on a filtration apparatus, it is possible to obtain a filtration apparatus which is relatively small in size and yet has a large capacity, and which has a wide range of applications. For example, the application field of the filtration apparatus expands to cover the filtration of a gas from a coal gasification process and the filtration of a combustion gas under pressure from a pressurized fluidized bed combustion boiler for power generation in which coal is burned. Thus, the present invention opens a way for efficient utilization of a coal energy without pollution and thus its value for industrial application is significant.

Claims: A ceramic filter for a dust-containing gas, which comprises at least 50 wt% of a cordierite grog having an apparent porosity of at most 10% and a particle diameter of at least 74 µm, and a matrix, wherein said matrix comprises silica (SiO 2 ) and alumina (Al 2 O 3 ) as the main components and contains from 0.05 to 1.0 wt%, based on the weight of the entire filter, of lithium oxide (Li 2 O). The ceramic filter according to Claim 1, wherein the matrix contains magnesium oxide (MgO). The ceramic filter according to Claim 1 or 2, wherein the cordierite grog is a cordierite grog crystallized from a glass having approximately the cordierite composition (2MgO 2Al 2 O 3 5SiO 2 ). The ceramic filter according to Claim 1 or 2, wherein the cordierite grog is the one obtained by impregnating alumina sol, silica sol, titania sal, zirconia sol or a mixture of at least two types of these sols, to a cordierite grog, and after sintering the grog has an apparent porosity of at most 10%. The ceramic filter according to any one of Claims 1 to 4, wherein the filter material has an apparent porosity of at least 35% and a bending strength at 800°C of at least 75 kg/cm 2 . The ceramic filter according to any one of Claims 1 to 5, wherein the matrix contains from 0.5 to 12 wt%, based on the weight of the entire filter, of TiO 2 and/or ZrO 2 . A ceramic filter according to Claim 1 for a dust-containing gas, which comprises at least 50 wt.-% of a cordierite grog crystallized from a glass having approximately the cordierite composition (2MgO 2Al 2 0 3 5SiO 2 ) and a particle diameter of at least 74µm, and the filter material has an apparent porosity of at least 35 % and a bending strength at 800°C of at least 75 kg/cm 2 . A method for producing a ceramic filter for a dust-containing gas, which comprises mixing at least 50 wt% of a cordierite grog having an apparent porosity of at most 10% and a particle diameter of at least 74µm and a powder material to form a matrix obtained by mixing a powder containing lithium oxide, clay and a cordierite powder having a particle size of less than 74µm so that lithium oxide would be from 0.05 to 1.0 wt% based on the weight of the entire filter, adding an organic binder to the mixture, followed by molding and sintering. The method for producing a ceramic, filter according to Claim 8, wherein the cordierite grog having an apparent porosity of at most 10% is prepared by mixing magnesia, alumina and siliceous sand to constitute approximately a cordierite composition (2MgO 2Al 2 0 3 5SiO 2 ), melting the mixture and cooling it to obtain a glass, followed by heat treatment to crystallize the glass. The method for producing a ceramic filter according to Claim 8 or 9, wherein a β-spodumene powder is added to the powder material to form the matrix so that the content of lithium oxide in the filter would be from 0.2 to 0.6 wt%. The method for producing a ceramic filter according to Claim 8, 9 or 10, wherein a coke powder is mixed to the starting material, as a pore-forming material which forms pores when burned off. A method for producing a ceramic filter according to Claim 8, wherein molding is molded by an isostatic press using a columnar metal core mold and an outer cylindrical rubber mold, and sintering is sintered at a temperature of from 1310 to 1380°C.

Inventors: ENDO YASUHIKO, MAENO HIROSHI, MIMORI TAKASHI, MORISHITA TOMOHIRO, ENDO, YASUHIKO, MAENO, HIROSHI, MIMORI, TAKASHI, MORISHITA, TOMOHIRO, MAENO, HIROSHI, HAINESU FUNABASHI 204,
Applicants: ASAHI GLASS CO LTD, ASAHI GLASS COMPANY LTD.
Application year: 1990
Main classification: B01D39
IPC codes: C04B38, B01D39, C04B35, C04B41, C03C10