Gas separation – Specific media material – Ceramic or sintered
Reexamination Certificate
2002-07-17
2004-08-10
Spitzer, Robert H. (Department: 1724)
Gas separation
Specific media material
Ceramic or sintered
C055SDIG005, C055SDIG003, C264S044000
Reexamination Certificate
active
06773481
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a porous honeycomb filter and a manufacturing method thereof, and more particularly to a porous honeycomb filter that has a high efficiency in collecting fine particles (particulates) and the like. This porous honeycomb filter can prevent an increase in pressure loss due to the plugging of pores, and it is especially suitable for exploiting the characteristics thereof for diesel engines that use recent high-pressure fuel injection, common rails, etc. The invention also relates to a manufacturing method thereof.
BACKGROUND ART
Porous honeycomb filters, having a structure in which a plurality of through holes opened to the end surface of the exhaust gas flow-in side and to the end surface of the exhaust gas flow-out side are alternately sealed at both the end surfaces, have recently been used as apparatuses for removing particulate in exhaust gas. In these porous honeycomb filters the exhaust gas that flows in at the exhaust gas flow-in side end surface is forced to pass through partition walls (having a plurality of pores) between through holes to thereby collect and remove particulate in exhaust gas.
In this porous honeycomb filter, the pore distribution needs to be controlled because performance figures such as collection efficiency and pressure loss vary depending on the diameter of the pores formed on partition walls between through holes in relation to the size of particulates in the exhaust gas.
Traditionally, a porous honeycomb filter made from cordierite, which is excellent in heat resistance, or from silicon carbide, which is frequently used. For porous honeycomb filters made from silicon carbide, of which pore diameter is easily controlled, a filter with an average pore diameter of 1 to 15 &mgr;m and the pore diameter thereof being controlled with the standard deviation (SD) of as extremely narrow a range as 0.20 or less in the pore distribution, has been disclosed (JP-A-5-23512).
On the other hand, for porous honeycomb filters made from cordierite where the pore diameter is controlled, a honeycomb filter has been disclosed with an average pore diameter of 25 to 40 &mgr;m. It is obtained by a manufacturing method in which the porosity is increased by not causing kaolin and aluminum oxide to be contained in the cordierite-forming raw material and also by using a raw material made by adding a specified organic blowing agent or a flammable substance to a cordierite raw material. The cordierite raw material is composed of aluminium hydroxide (the powders with particle diameters of 0.5 to 3 &mgr;m and of 5 to 15 &mgr;m make up 50 to 100% of the whole of the aluminium hydroxide), fused silica (average particle diameter of 30 to 100 &mgr;m) and talc, of which particle diameter is controlled within a specified range, has been disclosed (JP-A-9-77573).
However, in this honeycomb filter, the pore diameter thereof is primarily controlled by aluminium hydroxide and an organic blowing agent or a flammable substance, and so the average pore diameter was capable of being controlled, but the pore distribution was not capable of being set in a desired narrow range. In addition, the aluminium hydroxide was made to become coarse particles, thereby causing the problem of increasing the coefficient of thermal expansion as well.
To the contrary, honeycomb filters made by a manufacturing method in which a raw material prepared by adding graphite as a pore-forming agent to a cordierite-forming raw material produced by making each component of talc, silica, alumina and kaolin a powder of a specific particle diameter and then mixing them in specific contents, with pore distributions in which <1> the pores with a diameter of 2 &mgr;m or less makes up 7% by volume or less of the total pore volume, and <2> the pores with a diameter of 100 &mgr;m or more makes up 10% by volume or less of the total pore volume have been disclosed, respectively, in Japanese Patent Nos. 2578176 and 2726616.
In these honeycomb filters, however, the difference in easiness of controlling the pore diameter for each component was not taken into consideration, and therefore the lower limit or the upper limit of the pore distribution was only controlled at most and it was impossible to set the pore distribution in a desired narrow range.
To the contrary, a honeycomb filter where a pore with pore diameters of 10 to 50 &mgr;m makes up 52.0 to 74.1% by volume of the total pores, is obtained by a manufacturing method in which, focusing on the difference in easiness of controlling the pore diameter for each component of talc, silica, alumina and kaolin, a cordierite-forming raw material is prepared by setting the powder with a particle diameter of 150 &mgr;m or more to be 3% by weight or less of the whole raw material and also setting the powder with a particle diameter of 45 &mgr;m or less to be 25% by weight or less, for both talc and silica, has been proposed (JP-A-7-38930).
In this honeycomb filter, the pore diameter thereof is controlled in a narrow range of from 10 to 50 &mgr;m for the first time in a honeycomb filter made from cordierite. Compared with a variety of cordierite honeycomb filters mentioned above, the filter can not only increase collection efficiency, but also prevents an increase in pressure loss by the prevention of plugging. In addition, the filter can lower the coefficient of thermal expansion by decreasing the particle diameter of the talc contained in the filter.
However, particulates in exhaust gas have lately been made small and been homogenized (particle diameter of particulates is almost about 1 &mgr;m) with decreasing emission as a result of improved diesel engines (high-pressure fuel injection, common rails, etc. are used), and thus a honeycomb filter in which the pore diameter is extremely highly controlled has been strongly required.
On the contrary, while the aforementioned honeycomb filter has been produced, completely neglecting a close association of kaolin in a cordierite-forming raw material with the formation of a pore of 10 &mgr;m or less, pores with a diameter of 10 to 50 &mgr;m cannot be formed at a high level of 75.0% by volume or more, so that recent demand cannot be satisfied.
The present invention has been made considering the aforementioned problem, and the objects thereof are to provide a porous honeycomb filter that has a high efficiency in collecting fine particles (particulates) and the like and prevents an increase in pressure loss due to the plugging of the pores, especially suitable for exploiting these characteristics for diesel engines that use recent high-pressure fuel injection, common rails, etc., and also to provide a manufacturing method thereof.
DISCLOSURE OF THE INVENTION
The inventors, as a result of studies to solve the aforementioned problem, have found out that the pore size distribution can be highly controlled in a desired range by regulating the particle diameter of the silica component of a cordierite-forming raw material and also lowering the concentration of the kaolin, and have completed the present invention.
In other words, the present invention provides a porous honeycomb filter made from a raw material composed of cordierite as the primary crystalline phase, of which the pore distribution is controlled, characterized in that, in the pore distribution, the volume of a pore with a diameter of less than 10 &mgr;m is 15% or less of the total pore volume, the volume of a pore with a diameter of 10 to 50 &mgr;m is 75% or more of the total pore volume, and the volume of a pore with a diameter of above 50 &mgr;m is 10% or less of the total pore volume.
In a honeycomb filter of the present invention, the porosity of the honeycomb filter is preferably 50 to 75%, more preferably 65 to 75%, and particularly preferably 68 to 75%. In addition, the coefficient of thermal expansion of the honeycomb filter is preferably 1.0×10
−6
/° C. or less at 40 to 800° C.
Further, the present invention provides a method of manufacturing a porous honeycomb filter, using a ceramic raw material primarily com
Nishi Hideaki
Noguchi Yasushi
Suenobu Hiroyuki
NGK Insulators Ltd.
Parkhurst & Wendel L.L.P.
Spitzer Robert H.
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