WO2003014041A2 - $g(a)-alumina for cordierite ceramics, production method of the $g(a)-alumina and structures of cordierite ceramics using the $g(a)-alumina - Google Patents

$g(a)-alumina for cordierite ceramics, production method of the $g(a)-alumina and structures of cordierite ceramics using the $g(a)-alumina Download PDF

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WO2003014041A2
WO2003014041A2 PCT/JP2002/008111 JP0208111W WO03014041A2 WO 2003014041 A2 WO2003014041 A2 WO 2003014041A2 JP 0208111 W JP0208111 W JP 0208111W WO 03014041 A2 WO03014041 A2 WO 03014041A2
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alumina
less
mass
particle size
cordierite ceramics
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PCT/JP2002/008111
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French (fr)
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WO2003014041A3 (en
Inventor
Susumu Shibusawa
Hirokazu Miyazawa
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Showa Denko K.K.
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Priority to AU2002326154A priority Critical patent/AU2002326154A1/en
Publication of WO2003014041A2 publication Critical patent/WO2003014041A2/en
Publication of WO2003014041A3 publication Critical patent/WO2003014041A3/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
    • B01D53/86—Catalytic processes
    • B01D53/88—Handling or mounting catalysts
    • B01D53/885—Devices in general for catalytic purification of waste gases
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    • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
    • C04B35/01—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics
    • C04B35/16—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on silicates other than clay
    • C04B35/18—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on silicates other than clay rich in aluminium oxide
    • C04B35/195—Alkaline earth aluminosilicates, e.g. cordierite or anorthite
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    • C04B35/622—Forming processes; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
    • C04B35/626—Preparing or treating the powders individually or as batches ; preparing or treating macroscopic reinforcing agents for ceramic products, e.g. fibres; mechanical aspects section B
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    • C04B38/00—Porous mortars, concrete, artificial stone or ceramic ware; Preparation thereof
    • C04B38/0006—Honeycomb structures
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
    • B01J21/02—Boron or aluminium; Oxides or hydroxides thereof
    • B01J21/04—Alumina
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    • C04B2111/00—Mortars, concrete or artificial stone or mixtures to prepare them, characterised by specific function, property or use
    • C04B2111/00474—Uses not provided for elsewhere in C04B2111/00
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    • C04B2111/00—Mortars, concrete or artificial stone or mixtures to prepare them, characterised by specific function, property or use
    • C04B2111/00474—Uses not provided for elsewhere in C04B2111/00
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    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/02—Composition of constituents of the starting material or of secondary phases of the final product
    • C04B2235/30—Constituents and secondary phases not being of a fibrous nature
    • C04B2235/32—Metal oxides, mixed metal oxides, or oxide-forming salts thereof, e.g. carbonates, nitrates, (oxy)hydroxides, chlorides
    • C04B2235/3217—Aluminum oxide or oxide forming salts thereof, e.g. bauxite, alpha-alumina
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    • C04B2235/34—Non-metal oxides, non-metal mixed oxides, or salts thereof that form the non-metal oxides upon heating, e.g. carbonates, nitrates, (oxy)hydroxides, chlorides
    • C04B2235/3427—Silicates other than clay, e.g. water glass
    • C04B2235/3436—Alkaline earth metal silicates, e.g. barium silicate
    • C04B2235/3445—Magnesium silicates, e.g. forsterite
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    • C04B2235/34—Non-metal oxides, non-metal mixed oxides, or salts thereof that form the non-metal oxides upon heating, e.g. carbonates, nitrates, (oxy)hydroxides, chlorides
    • C04B2235/349—Clays, e.g. bentonites, smectites such as montmorillonite, vermiculites or kaolines, e.g. illite, talc or sepiolite
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    • C04B2235/02—Composition of constituents of the starting material or of secondary phases of the final product
    • C04B2235/50—Constituents or additives of the starting mixture chosen for their shape or used because of their shape or their physical appearance
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    • C04B2235/5418—Particle size related information expressed by the size of the particles or aggregates thereof
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    • C04B2235/60—Aspects relating to the preparation, properties or mechanical treatment of green bodies or pre-forms
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    • C04B2235/96—Properties of ceramic products, e.g. mechanical properties such as strength, toughness, wear resistance
    • C04B2235/9607—Thermal properties, e.g. thermal expansion coefficient
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N2330/00—Structure of catalyst support or particle filter
    • F01N2330/06—Ceramic, e.g. monoliths

Definitions

  • the present invention relates to an ⁇ -alumina powder for cordierite ceramics, the ceramics finding use as catalyst carriers, exhaust gas control porous filters and similar products for use in an exhaust gas control system of internal combustion engines including automobile engines. More specifically, the invention relates particularly to an ⁇ - alumina powder for producing cordierite ceramics, which powder can reduce the coefficient of thermal expansion of the cordierite ceramics, to a method for producing the powder and to a structure of cordierite ceramics produced from the ⁇ - alumina powder, such as a catalyst carrier for use in an exhaust gas control apparatus, a deodorization catalyst carrier or an exhaust gas control filter.
  • Background Art :
  • Cordierite ceramics having a negative coefficient of thermal expansion along the c crystallographic axis are widely employed in industrial fields where high thermal shock resistance is required.
  • honeycomb structures of cordierite ceramics are employed as exhaust gas control catalyst carriers, deodorization catalyst carriers, exhaust gas control porous filters, heat-exchanger structures and similar products for use in internal combustion engines including automobile engines.
  • JP-A SHO 53-82822 discloses magnesia sources, such as talc, JP-A SHO 50-75611 kaolin sources and JP-A SHO 61-256965 alumina sources.
  • JP-A SHO 50-75611 discloses that honeycomb structures of cordierite ceramics having a small coefficient of thermal expansion can be produced through extrusion of these sources.
  • an exhaust gas control catalyst must be elevated to the corresponding activation temperature so as to fully attain its control performance, and the catalyst must be heated rapidly to the activation temperature upon cold starting.
  • 7 ⁇ mong cordierite honeycomb structures according to conventional techniques, proposed is a honeycomb structure exhibiting a relatively small coefficient of thermal expansion and having an outer diameter of 4.66 inches and a length of 4 inches.
  • this type of honeycomb structure exhibits a thermal shock resistance of approximately 800-900°C, the thermal shock resistance is not satisfactory and thermal shock characteristics are not necessarily uniform throughout among products thereof, raising demand for cordierite ceramics having higher thermal shock resistance, i.e. a lower coefficient of thermal expansion .
  • the present invention has been proposed in order to solve the aforementioned problems, and its object is to provide ⁇ -alumina that enables production of cordierite ceramics having a lower coefficient of thermal expansion as compared with conventional cordierite ceramics and a higher thermal shock resistance and exhibiting small variance in characteristics.
  • /Another object of the invention is to provide a method for producing the ⁇ -alumina.
  • Still another object of the invention is to provide cordierite ceramics produced from the ⁇ -alumina and structures of the cordierite ceramics, such as catalyst carriers and filters.
  • the present invention provides a method for producing ⁇ -alumina for cordierite ceramics, comprising the step of pulverizing, by means of a jet mill, ⁇ -alumina having a secondary particle size of 40-70 ⁇ m, a primary particle size of 1.5-3 ⁇ m, and a BET specific surface area of 0.5-1.5 m 2 /g to thereby yield pulverized ⁇ -alumina that comprises alumina particles of 3 ⁇ m or less in an amount of 30 mass% or less and alumina particles of 15 ⁇ m or less in an amount of at least 75 mass% in relation to a particle size distribution profile of the pulverized alumina, and has a mean particle size of 4-10 ⁇ m and a +90- ⁇ m sieve residue of 50 ppm by mass or less.
  • the jet mill has a nozzle from which air is jetted out at 3 x 10 6 Pa (gauge pressure) or less.
  • the pulverized ⁇ -alumina can comprise alumina particles of 3 ⁇ m or less in an amount of 5-20 mass%, alumina particles of 15 ⁇ m or less in an amount of 80-90 mass% and have a mean particle size falling within a range of 4 to 8 ⁇ m and a +90- ⁇ sieve residue of 25 ppm by mass or less.
  • the invention further provides the ⁇ -alumina for cordierite ceramics produced through the aforementioned method.
  • the invention further provides ⁇ -alumina for cordierite ceramics, which comprises alumina particles of 3 ⁇ m or less in an amount of 30 mass% or less and alumina particles of 15 ⁇ m or less in an amount of at least 75 mass% in relation to a particle-size distribution profile of the pulverized alumina, and has a mean particle size falling within a range of 4 to
  • the invention further provides a method for producing a cordierite ceramic, comprising the steps of molding, through an extrusion method, the ⁇ -alumina for cordierite ceramics and firing the molded ⁇ -alumina.
  • the cordierite ceramic can have a honeycomb structure.
  • the invention further provides a ceramic for use in an exhaust gas control apparatus, a deodorization catalyst carrier and an exhaust gas control filter, each produced by molding the ⁇ -alumina for cordierite ceramics and firing the molded ⁇ -alumina.
  • the present invention makes it possible to produce cordierite ceramics having a lower coefficient of thermal expansion and higher thermal shock resistance and exhibiting small variance in characteristics by specifying the characteristics of ⁇ -alumina for cordierite ceramics assumed before and after being pulverized and the method of pulverizing the ⁇ -alumina, thereby obtaining high-quality structures of cordierite ceramics, such as catalyst carriers for use in a waste gas control apparatus, deodorization catalyst carriers and waste gas control filters.
  • pulverizing by means of a jet mill, ⁇ -alumina having a secondary particle size of 40-70 ⁇ m, a primary particle size of 1.5-3 ⁇ m, and a BET specific surface area of 0.5-1.5 m 2 /g as measured in its powder form, there can be yielded pulverized ⁇ -alumina for cordierite ceramics of the present invention that comprises alumina particles of 3 ⁇ m or less in an amount of 30 mass% or less and alumina particles of 15 ⁇ m or less in an amount of at least 75 mass% in relation to a particle size distribution profile of the pulverized alumina, and has a mean particle size of 4-10 ⁇ m; and a +90- ⁇ m sieve residue of 50 ppm by mass or less.
  • a jet mill for jetting out air from a nozzle thereof at 3 x 10 6 Pa (pascal) or less in terms of gauge pressure (relative pressure) is particularly preferred.
  • pressure of around 6 x 10 6 Pa to 7 x 10 6 Pa is used as air-jetting pressure from a nozzle.
  • ⁇ -alumina is lightly pulverized with the air-jetting pressure reduced, thereby enabling ⁇ -alumina of the present invention to be produced with high efficiency.
  • the pulverized ⁇ -alumina comprises alumina particles of 3 ⁇ m or less in an amount 30 mass% or less, preferably 5-20 mass%, in relation to a particle size distribution profile of the pulverized alumina.
  • alumina microparticles of 3 ⁇ m or less are contained in the amount of more than 30 mass%, the microparticles react with talc serving as a magnesia source at comparatively low temperatures of approximately 1,300°C or lower, thereby inhibiting main reaction between talc and kaolin for forming cordierite having a small coefficient of thermal expansion; i.e., thereby increasing the coefficient of thermal expansion.
  • alumina particles of 3 ⁇ m or less in an amount of 30 mass% or less means that the percentage of the total mass of the alumina particles of 3 ⁇ m or less on the basis of the total mass of all particles is 30 mass% or less.
  • the pulverized ⁇ -alumina must contain alumina particles of 15 ⁇ m or less in an amount of at least 75 mass%, preferably in an amount of 80-90 mass%.
  • alumina particles of 15 ⁇ m or less are contained in the amount of less than 75 mass%, cordierite formation reaction temperature is elevated, and a portion of ⁇ -alumina particles remains unreacted, thereby increasing the coefficient of thermal expansion and deteriorating thermal shock resistance.
  • the coefficient of thermal expansion (CTE) is a parameter useful for comparing a variety of articles in terms of relative thermal stress resistance.
  • the pulverized ⁇ -alumina must have a mean particle size of 4-10 ⁇ m, preferably 4-8 ⁇ m.
  • Mean particle sizes less than 4 ⁇ m increase the quantity of microparticles and deteriorate thermal shock resistance.
  • cordierite formation reaction temperature is elevated, and a portion of ⁇ -alumina particles remains unreacted, thereby increasing the coefficient of thermal expansion and deteriorating thermal shock resistance.
  • the pulverized ⁇ -alumina has a +90- ⁇ m sieve residue of 50 ppm by mass or less, preferably 25 ppm by mass or less.
  • the amount of +90- ⁇ m sieve residue is in excess of 50 ppm by mass, problematic clogging of slits occurs during extrusion for forming a honeycomb structure.
  • the ⁇ -alumina before undergoing pulverization treatment must have a secondary particle size of 40-70 ⁇ m.
  • the secondary particle size of the ⁇ -alumina is in excess of 70 ⁇ m, satisfying the aforementioned particle size distribution profile of the pulverized particles requires higher pulverization power, thereby generating ultramicroparticles having a particle size of 1 ⁇ m or less. Since these ultramicroparticles having a particle size of 1 ⁇ m or less are highly reactive, the particles react with talc and kaolin, thereby inhibiting main reaction between talc and kaolin for forming cordierite having a small coefficient of thermal expansion to deteriorate crystal orientation of cordierite crystals and provide variance in thermal shock resistance.
  • the ⁇ -alumina before undergoing pulverization treatment must have a primary particle size of 1.5-3.0 ⁇ m and a BET specific surface area of 0.5-1.5 m 2 /g.
  • the reason for limiting the primary particle size and the BET specific surface area is to ensure enhanced efficiency of reaction to form cordierite.
  • the primary particle size is less than 1.5 ⁇ m or the BET specific surface area is more than 1.5 m 2 /g, main reaction between talc and kaolin for forming cordierite is inhibited, as described above, which is not preferred.
  • BET specific surface area refers to that of particles constituting the secondary particles before pulverization.
  • the method of the present invention is characterized by lightly pulverizing ⁇ -alumina having a primary particle size, a secondary particle size and a BET specific surface area falling within respective specific ranges to thereby yield ⁇ -alumina for cordierite ceramics (source powder for cordierite ceramics) .
  • a preferred pulverization method is pulverization making use of the aforementioned jet mill.
  • the reason for choosing the pulverization method is as follows. Specifically, even though the characteristics of alumina before and after pulverization are limited as described above, highly reactive ultramicroparticles having a particle size of 1 ⁇ m or less are generated due to, for example, pulverization at a high magnitude .
  • the magnitude of pulverization is preferably adjusted to as low a level as possible such that highly reactive ultramicroparticles are not generated; i.e. secondary particles of alumina are crushed to a necessary and sufficient extent.
  • a jet mill By use of a jet mill and by adjusting the pressure for jetting out air from a nozzle to 3 x 10 6 Pa (pascal) or less, secondary particles of alumina can be moderately pulverized without generating ultramicroparticles.
  • the pressure for jetting out air from a nozzle of the jet mill is particularly preferably adjusted to 2 x 10 s Pa (pascal) or less.
  • the airflow quantity and the amount of raw material fed to the jet mill are preset in accordance with pulverization performance of the jet mill.
  • the lower limit of the air-jetting pressure is 1 x 10 s Pa (pascal) and, if the pressure is less than the lower limit, pulverization of ⁇ -alumina will not be performed.
  • ⁇ -Alumina after pulverization does not mean raw material ⁇ -alumina pulverized completely to primary particles, but is preferably pulverized ⁇ -alumina of a level containing agglomerates of some primary particles.
  • the particle size distribution of the pulverized ⁇ -alumina is measured with a laser diffraction particle size distribution analyzer after dispersion of the particles by a dispersant.
  • the particle size distribution of the pulverized ⁇ -alumina be measured, after dispersion of the particles in sodium hexametaphosphate, using a laser diffraction particle size distribution analyzer (Microtrack HRA, product of Nikkiso Co., Ltd., Japan).
  • the alumina employed in the present invention is ⁇ - alumina.
  • Preferred ⁇ -alumina is perfect ⁇ -alumina containing no alumina intermediate, such as ⁇ -alumina, identified as a crystalline phase. Migration of alumina intermediates formed from ⁇ -alumina and aluminum hydroxide; e.g., ⁇ -, K-, ⁇ -, ⁇ -, ⁇ - and ⁇ -alumina, is not preferred, since these intermediates are highly reactive and adversely affect the reaction process for forming cordierite.
  • various types of ⁇ - alumina are produced through the Bayer's process. Among them, sandy ⁇ -alumina is suitable for producing cordierite ceramics.
  • the ⁇ -alumina for cordierite ceramics of the present invention may be ⁇ -alumina produced through the aforementioned production method. However, in the present invention, no particular limitation is imposed on the production method.
  • any ⁇ -alumina can be employed as the ⁇ - alumina for cordierite ceramics of the present invention so long as the ⁇ -alumina comprises alumina particles of 3 ⁇ m or less in an amount of 30 mass% or less and alumina particles of 15 ⁇ m or less in an amount of at least 75 mass% in relation to a particle size distribution profile of the pulverized alumina, and has a mean particle size falling within a range of 4 to 10 ⁇ m and a +90- ⁇ m sieve residue of 50 ppm by mass or less.
  • talc kaolin, calcined kaolin and a similar material are added to thereby yield raw materials for cordierite ceramics.
  • the compositional proportions are appropriately selected in accordance with use and objects.
  • One example of the raw material used contains 15 mass% of ⁇ - alumina, 40 mass% of talc, 25 mass% of kaolin and 20 mass% of calcined kaolin.
  • Cordierite ceramics can be produced by use of the ⁇ - alumina for cordierite ceramics of the present invention. Specifically, in one mode of production, the aforementioned raw material for cordierite ceramics is molded through extrusion, and the molded product is fired, to thereby produce a cordierite ceramic.
  • ceramic honeycomb structures can be formed from the cordierite ceramics .
  • the honeycomb structures may be employed as catalyst carriers of exhaust gas control apparatus for use in automobiles, deodorization catalyst carriers, filters for controlling exhaust gas, heat-exchanger structures and similar products.
  • These articles can be produced from ⁇ -alumina through any known methods .
  • ⁇ -Alumina samples A to K shown in Table 1 were prepared.
  • the particle size of each pulverized ⁇ -alumina sample was measured by means of a laser diffraction particle size distribution analyzer (Microtrack HRA, product of Nikkiso Co., Ltd., Japan), with sodium hexametaphosphate used as a dispersant.
  • the secondary particle size of each alumina sample before pulverization was measured using Microtrack HRA without use of a dispersant.
  • the primary particle size of each alumina sample before pulverization was obtained from SEM photographs (by means of a scanning electron microscope, as secondary electron photo-images) .
  • the BET specific surface area was measured through the nitrogen absorption method.
  • Pulverization was performed by use of a jet mill at a jetting air nozzle pressure of 2 x 10 6 , 3 x 10 s or 5 x 10 6 Pa (pascal) .
  • Other pulverization conditions including the airflow amount of the jet mill, amount of raw material fed and rotation rate of the classifier provided in the jet mill were appropriately adjusted such that the pulverized ⁇ - alumina samples exhibited corresponding values shown in Table 1.
  • the amount of +90- ⁇ m sieve residue was measured through wet sieve analysis.
  • ⁇ -alumina sample A (15 mass%) , talc (40 mass%) , kaolin (25 mass%) and calcined kaolin (20 mass%) were blended, and the resultant mixture was extruded to thereby yield a cylindrical green honeycomb structure that has an outer diameter of 4.66 inches and a length of 4 inches, forms therein in the lengthwise direction a through hole circular or elliptical in cross section and has a cell density of 6 mil/400 CPI 2 .
  • the green honeycomb structure was fired at 1,410°C for four hours to thereby obtain a cordierite honeycomb.
  • cordierite honeycombs were obtained from respective ⁇ -alumina samples B to K.
  • a test piece (50 mm) was cut from each of the obtained cordierite honeycombs in a direction of extrusion, and the coefficient of thermal expansion (CTE) , as measured from 40°C to 800°C, of each test piece was measured.
  • Thermal shock resistance of each cordierite honeycomb was evaluated by electric furnace spalling resistance thereof.
  • the electric furnace spalling resistance was measured over a range of 40 to 800°C. Specifically, a honeycomb sample was placed in an electric furnace, for example, at 40°C for 20 minutes. The sample was then removed from the furnace into the atmosphere at room temperature and inspected for crack generation or sound generation due to cracking. In the case of no inspection, the temperature of the electric furnace was gradually elevated until generation of cracks or occurrence of a similar phenomenon was confirmed.
  • cordierite honeycombs The temperature of the electric furnace at which crack generation or sound generation due to cracking was confirmed was evaluated as electric furnace spalling resistance.
  • the inspection with the furnace temperature elevated gradually until confirmation of crack generation or similar phenomenon occurrence was performed three times, and the average of electric furnace spalling resistance values are shown in Table 2.
  • general evaluation of cordierite honeycombs was performed on the basis of the following standards, and the results are shown in Table 2. Namely, cordierite honeycombs exhibiting excellent characteristics are assigned rating "0, " those exhibiting slightly poor characteristics are assigned rating
  • cordierite ceramic honeycomb structures exhibiting excellent thermal shock resistance can be produced.
  • ⁇ -alumina for producing cordierite ceramics having a low coefficient of thermal expansion and a high thermal shock resistance and exhibiting small variance in characteristics and a method for producing the ceramics can be provided through employment of a specific pulverization method and characteristics of alumina before and after pulverization.

Abstract

α-Alumina for cordierite ceramics having a secondary particle size of 40-70 µm, a primary particle size of 1.5-3 µm, and a BET specific surface area of 0.5-1.5 m2/g as measured in its powder form is prepared. The prepared α-alumina is pulverized by means of a jet mill to obtain pulverized α-alumina that comprises alumina particles of 3 µm or less in an amount of 30 mass% or less and alumina particles of 15 µm or less in an amount of at least 75 mass% in relation to a particle size distribution profile of the pulverized alumina, and has a mean particle size of 4-10 µm and a +90-µm sieve residue of 50 ppm by mass or less. The thus obtained α-alumina is used to produce cordierite ceramics having a low coefficient of thermal expansion and high thermal shock resistance and exhibiting small variance in characteristics. Structures, such as catalyst carriers for use in a waste gas control apparatus and waste gas control filters, are produced from the cordierite ceramics.

Description

- l -
DESCRIPTION
α-Alumina for Cordierite Ceramics, Production Method of the α-Alumina and Structures of Cordierite Ceramics Using the α- Alumina
Cross Reference to Related Applications:
This application is an application filed under 35 U.S.C.
§ 111(a) claiming the benefit pursuant to 35 U.S.C. §
119(e) (1) of the filing date of Provisional Application No.
60/311,365 filed August 13, 2001 pursuant to 35. U.S.C. § 111 (b) .
Technical Field:
The present invention relates to an α-alumina powder for cordierite ceramics, the ceramics finding use as catalyst carriers, exhaust gas control porous filters and similar products for use in an exhaust gas control system of internal combustion engines including automobile engines. More specifically, the invention relates particularly to an α- alumina powder for producing cordierite ceramics, which powder can reduce the coefficient of thermal expansion of the cordierite ceramics, to a method for producing the powder and to a structure of cordierite ceramics produced from the α- alumina powder, such as a catalyst carrier for use in an exhaust gas control apparatus, a deodorization catalyst carrier or an exhaust gas control filter. Background Art:
Cordierite ceramics having a negative coefficient of thermal expansion along the c crystallographic axis are widely employed in industrial fields where high thermal shock resistance is required. Particularly, honeycomb structures of cordierite ceramics are employed as exhaust gas control catalyst carriers, deodorization catalyst carriers, exhaust gas control porous filters, heat-exchanger structures and similar products for use in internal combustion engines including automobile engines.
Materials, such as talc, kaolin, silica, alumina and aluminum hydroxide, are generally employed as source materials for producing cordierite (compositional formula: 2MgO-2Al2θ3'5Si02) . JP-A SHO 53-82822 discloses magnesia sources, such as talc, JP-A SHO 50-75611 kaolin sources and JP-A SHO 61-256965 alumina sources.
JP-A SHO 50-75611 discloses that honeycomb structures of cordierite ceramics having a small coefficient of thermal expansion can be produced through extrusion of these sources.
From the year 2000, exhaust gas has been more strictly controlled from the viewpoint of global environmental issues, and techniques for greatly reducing emission levels are needed in the automobile industry. In connection with these techniques, particularly, an exhaust gas control catalyst must be elevated to the corresponding activation temperature so as to fully attain its control performance, and the catalyst must be heated rapidly to the activation temperature upon cold starting.
To control exhaust gas upon cold starting plays an important role in reducing emission levels. Therefore, instead of the current practice of placing automobile catalytic converters under floors, catalytic converters tend to be placed near exhaust manifolds, which are heated to higher temperature, thereby requiring further enhanced thermal shock resistance of cordierite ceramics. The thermal shock resistance can be evaluated as the difference in temperature between the rapidly elevated temperature and the rapidly cooled temperature, when the difference induces cracks. 7Among characteristics of honeycomb structures, the mentioned difference that is called the index of durability is known to be reciprocally proportional to the coefficient of thermal expansion. Thus, the smaller the coefficient of thermal expansion is, the greater the index of durability.
7Λmong cordierite honeycomb structures according to conventional techniques, proposed is a honeycomb structure exhibiting a relatively small coefficient of thermal expansion and having an outer diameter of 4.66 inches and a length of 4 inches. Although this type of honeycomb structure exhibits a thermal shock resistance of approximately 800-900°C, the thermal shock resistance is not satisfactory and thermal shock characteristics are not necessarily uniform throughout among products thereof, raising demand for cordierite ceramics having higher thermal shock resistance, i.e. a lower coefficient of thermal expansion .
The present invention has been proposed in order to solve the aforementioned problems, and its object is to provide α-alumina that enables production of cordierite ceramics having a lower coefficient of thermal expansion as compared with conventional cordierite ceramics and a higher thermal shock resistance and exhibiting small variance in characteristics. /Another object of the invention is to provide a method for producing the α-alumina. Still another object of the invention is to provide cordierite ceramics produced from the α-alumina and structures of the cordierite ceramics, such as catalyst carriers and filters.
Disclosure of the Invention:
The present invention provides a method for producing α-alumina for cordierite ceramics, comprising the step of pulverizing, by means of a jet mill, α-alumina having a secondary particle size of 40-70 μm, a primary particle size of 1.5-3 μm, and a BET specific surface area of 0.5-1.5 m2/g to thereby yield pulverized α-alumina that comprises alumina particles of 3 μm or less in an amount of 30 mass% or less and alumina particles of 15 μm or less in an amount of at least 75 mass% in relation to a particle size distribution profile of the pulverized alumina, and has a mean particle size of 4-10 μm and a +90-μm sieve residue of 50 ppm by mass or less. The jet mill has a nozzle from which air is jetted out at 3 x 106 Pa (gauge pressure) or less.
The pulverized α-alumina can comprise alumina particles of 3 μm or less in an amount of 5-20 mass%, alumina particles of 15 μm or less in an amount of 80-90 mass% and have a mean particle size falling within a range of 4 to 8 μm and a +90- μ sieve residue of 25 ppm by mass or less.
The invention further provides the α-alumina for cordierite ceramics produced through the aforementioned method.
The invention further provides α-alumina for cordierite ceramics, which comprises alumina particles of 3 μm or less in an amount of 30 mass% or less and alumina particles of 15 μm or less in an amount of at least 75 mass% in relation to a particle-size distribution profile of the pulverized alumina, and has a mean particle size falling within a range of 4 to
10 μm and a +90-μm sieve residue of 50 ppm by mass or less.
The invention further provides a method for producing a cordierite ceramic, comprising the steps of molding, through an extrusion method, the α-alumina for cordierite ceramics and firing the molded α-alumina.
The cordierite ceramic can have a honeycomb structure.
The invention further provides a ceramic for use in an exhaust gas control apparatus, a deodorization catalyst carrier and an exhaust gas control filter, each produced by molding the α-alumina for cordierite ceramics and firing the molded α-alumina. Thus, the present invention makes it possible to produce cordierite ceramics having a lower coefficient of thermal expansion and higher thermal shock resistance and exhibiting small variance in characteristics by specifying the characteristics of α-alumina for cordierite ceramics assumed before and after being pulverized and the method of pulverizing the α-alumina, thereby obtaining high-quality structures of cordierite ceramics, such as catalyst carriers for use in a waste gas control apparatus, deodorization catalyst carriers and waste gas control filters.
Best Mode for Carrying Out the Invention:
By pulverizing, by means of a jet mill, α-alumina having a secondary particle size of 40-70 μm, a primary particle size of 1.5-3 μm, and a BET specific surface area of 0.5-1.5 m2/g as measured in its powder form, there can be yielded pulverized α-alumina for cordierite ceramics of the present invention that comprises alumina particles of 3 μm or less in an amount of 30 mass% or less and alumina particles of 15 μm or less in an amount of at least 75 mass% in relation to a particle size distribution profile of the pulverized alumina, and has a mean particle size of 4-10 μm; and a +90-μm sieve residue of 50 ppm by mass or less.
In the present invention, employment of a jet mill for jetting out air from a nozzle thereof at 3 x 106 Pa (pascal) or less in terms of gauge pressure (relative pressure) is particularly preferred. In general pulverization using a jet mill, pressure of around 6 x 106 Pa to 7 x 106 Pa is used as air-jetting pressure from a nozzle. In the present invention, however, α-alumina is lightly pulverized with the air-jetting pressure reduced, thereby enabling α-alumina of the present invention to be produced with high efficiency. Furthermore, the pulverized α-alumina comprises alumina particles of 3 μm or less in an amount 30 mass% or less, preferably 5-20 mass%, in relation to a particle size distribution profile of the pulverized alumina. When alumina microparticles of 3 μm or less are contained in the amount of more than 30 mass%, the microparticles react with talc serving as a magnesia source at comparatively low temperatures of approximately 1,300°C or lower, thereby inhibiting main reaction between talc and kaolin for forming cordierite having a small coefficient of thermal expansion; i.e., thereby increasing the coefficient of thermal expansion.
In addition, orientation of cordierite crystals is deteriorated, thereby providing variance in thermal shock resistance. The expression "alumina particles of 3 μm or less in an amount of 30 mass% or less" used herein means that the percentage of the total mass of the alumina particles of 3 μm or less on the basis of the total mass of all particles is 30 mass% or less.
In the production method of the present invention, the pulverized α-alumina must contain alumina particles of 15 μm or less in an amount of at least 75 mass%, preferably in an amount of 80-90 mass%. When alumina particles of 15 μm or less are contained in the amount of less than 75 mass%, cordierite formation reaction temperature is elevated, and a portion of α-alumina particles remains unreacted, thereby increasing the coefficient of thermal expansion and deteriorating thermal shock resistance. The coefficient of thermal expansion (CTE) is a parameter useful for comparing a variety of articles in terms of relative thermal stress resistance.
In the production method of the present invention, the pulverized α-alumina must have a mean particle size of 4-10 μm, preferably 4-8 μm. Mean particle sizes less than 4 μm increase the quantity of microparticles and deteriorate thermal shock resistance. When the mean particle size is in excess of 10 μm, cordierite formation reaction temperature is elevated, and a portion of α-alumina particles remains unreacted, thereby increasing the coefficient of thermal expansion and deteriorating thermal shock resistance.
In the present invention, the pulverized α-alumina has a +90-μm sieve residue of 50 ppm by mass or less, preferably 25 ppm by mass or less. When the amount of +90-μm sieve residue is in excess of 50 ppm by mass, problematic clogging of slits occurs during extrusion for forming a honeycomb structure.
In the production method of the present invention, the α-alumina before undergoing pulverization treatment must have a secondary particle size of 40-70 μm. When the secondary particle size of the α-alumina is in excess of 70 μm, satisfying the aforementioned particle size distribution profile of the pulverized particles requires higher pulverization power, thereby generating ultramicroparticles having a particle size of 1 μm or less. Since these ultramicroparticles having a particle size of 1 μm or less are highly reactive, the particles react with talc and kaolin, thereby inhibiting main reaction between talc and kaolin for forming cordierite having a small coefficient of thermal expansion to deteriorate crystal orientation of cordierite crystals and provide variance in thermal shock resistance.
In the production method of the present invention, the α-alumina before undergoing pulverization treatment must have a primary particle size of 1.5-3.0 μm and a BET specific surface area of 0.5-1.5 m2/g. The reason for limiting the primary particle size and the BET specific surface area is to ensure enhanced efficiency of reaction to form cordierite. When the primary particle size is less than 1.5 μm or the BET specific surface area is more than 1.5 m2/g, main reaction between talc and kaolin for forming cordierite is inhibited, as described above, which is not preferred.
When the primary particle size is in excess of 3.0 μm or the BET specific surface area is less than 0.5 m2/g, cordierite formation reaction temperature is elevated, and a portion of α-alumina particles remains unreacted, thereby increasing the coefficient of thermal expansion and deteriorating thermal shock resistance. In the specification, BET specific surface area refers to that of particles constituting the secondary particles before pulverization.
As described above, the method of the present invention is characterized by lightly pulverizing α-alumina having a primary particle size, a secondary particle size and a BET specific surface area falling within respective specific ranges to thereby yield α-alumina for cordierite ceramics (source powder for cordierite ceramics) . A preferred pulverization method is pulverization making use of the aforementioned jet mill. The reason for choosing the pulverization method is as follows. Specifically, even though the characteristics of alumina before and after pulverization are limited as described above, highly reactive ultramicroparticles having a particle size of 1 μm or less are generated due to, for example, pulverization at a high magnitude .
Accordingly, the magnitude of pulverization is preferably adjusted to as low a level as possible such that highly reactive ultramicroparticles are not generated; i.e. secondary particles of alumina are crushed to a necessary and sufficient extent. By use of a jet mill and by adjusting the pressure for jetting out air from a nozzle to 3 x 106 Pa (pascal) or less, secondary particles of alumina can be moderately pulverized without generating ultramicroparticles. The pressure for jetting out air from a nozzle of the jet mill is particularly preferably adjusted to 2 x 10s Pa (pascal) or less. The airflow quantity and the amount of raw material fed to the jet mill are preset in accordance with pulverization performance of the jet mill. The lower limit of the air-jetting pressure is 1 x 10s Pa (pascal) and, if the pressure is less than the lower limit, pulverization of α-alumina will not be performed. α-Alumina after pulverization does not mean raw material α-alumina pulverized completely to primary particles, but is preferably pulverized α-alumina of a level containing agglomerates of some primary particles. The particle size distribution of the pulverized α-alumina is measured with a laser diffraction particle size distribution analyzer after dispersion of the particles by a dispersant. For example, it is preferable that the particle size distribution of the pulverized α-alumina be measured, after dispersion of the particles in sodium hexametaphosphate, using a laser diffraction particle size distribution analyzer (Microtrack HRA, product of Nikkiso Co., Ltd., Japan).
The alumina employed in the present invention is α- alumina. Preferred α-alumina is perfect α-alumina containing no alumina intermediate, such as γ-alumina, identified as a crystalline phase. Migration of alumina intermediates formed from α-alumina and aluminum hydroxide; e.g., χ-, K-, γ-, δ-, η- and θ-alumina, is not preferred, since these intermediates are highly reactive and adversely affect the reaction process for forming cordierite. Generally, various types of α- alumina are produced through the Bayer's process. Among them, sandy α-alumina is suitable for producing cordierite ceramics. The α-alumina for cordierite ceramics of the present invention may be α-alumina produced through the aforementioned production method. However, in the present invention, no particular limitation is imposed on the production method.
In other words, any α-alumina can be employed as the α- alumina for cordierite ceramics of the present invention so long as the α-alumina comprises alumina particles of 3 μm or less in an amount of 30 mass% or less and alumina particles of 15 μm or less in an amount of at least 75 mass% in relation to a particle size distribution profile of the pulverized alumina, and has a mean particle size falling within a range of 4 to 10 μm and a +90-μm sieve residue of 50 ppm by mass or less.
To the α-alumina for cordierite ceramics of the present invention, talc, kaolin, calcined kaolin and a similar material are added to thereby yield raw materials for cordierite ceramics. The compositional proportions are appropriately selected in accordance with use and objects.
One example of the raw material used contains 15 mass% of α- alumina, 40 mass% of talc, 25 mass% of kaolin and 20 mass% of calcined kaolin.
Cordierite ceramics can be produced by use of the α- alumina for cordierite ceramics of the present invention. Specifically, in one mode of production, the aforementioned raw material for cordierite ceramics is molded through extrusion, and the molded product is fired, to thereby produce a cordierite ceramic. For example, ceramic honeycomb structures can be formed from the cordierite ceramics . The honeycomb structures may be employed as catalyst carriers of exhaust gas control apparatus for use in automobiles, deodorization catalyst carriers, filters for controlling exhaust gas, heat-exchanger structures and similar products.
These articles can be produced from α-alumina through any known methods .
Examples of the present invention will now be described, but it is to be construed that the present invention is not limited thereto.
Examples : α-Alumina samples A to K shown in Table 1 were prepared. The particle size of each pulverized α-alumina sample was measured by means of a laser diffraction particle size distribution analyzer (Microtrack HRA, product of Nikkiso Co., Ltd., Japan), with sodium hexametaphosphate used as a dispersant. The secondary particle size of each alumina sample before pulverization was measured using Microtrack HRA without use of a dispersant. The primary particle size of each alumina sample before pulverization was obtained from SEM photographs (by means of a scanning electron microscope, as secondary electron photo-images) . The BET specific surface area was measured through the nitrogen absorption method. Pulverization was performed by use of a jet mill at a jetting air nozzle pressure of 2 x 106, 3 x 10s or 5 x 106 Pa (pascal) . Other pulverization conditions including the airflow amount of the jet mill, amount of raw material fed and rotation rate of the classifier provided in the jet mill were appropriately adjusted such that the pulverized α- alumina samples exhibited corresponding values shown in Table 1.
The amount of +90-μm sieve residue was measured through wet sieve analysis.
From the thus-prepared α-alumina samples shown in Table 1, corresponding cordierite ceramics were produced.
Specifically, α-alumina sample A (15 mass%) , talc (40 mass%) , kaolin (25 mass%) and calcined kaolin (20 mass%) were blended, and the resultant mixture was extruded to thereby yield a cylindrical green honeycomb structure that has an outer diameter of 4.66 inches and a length of 4 inches, forms therein in the lengthwise direction a through hole circular or elliptical in cross section and has a cell density of 6 mil/400 CPI2. The green honeycomb structure was fired at 1,410°C for four hours to thereby obtain a cordierite honeycomb. In the same manner as described above, cordierite honeycombs were obtained from respective α-alumina samples B to K.
In the cell density of 6 mil/400 CPI2 used herein, 6 mil
(approximately 150 μm) represents the wal-1 thickness of the cells formed in the presence of the through hole, and 400 CPI2 means the presence of 400 cells per square inch. In other words, 20 cells are arranged in one inch (2.54 cm). Therefore, the total wall thickness per inch is 3 mm (= 150 μm x 20 cells) , and the size of one cell (average diameter) is approximately 1.1 mm {= (25.4 mm - 3 mm) /20 cells}.
A test piece (50 mm) was cut from each of the obtained cordierite honeycombs in a direction of extrusion, and the coefficient of thermal expansion (CTE) , as measured from 40°C to 800°C, of each test piece was measured. Thermal shock resistance of each cordierite honeycomb was evaluated by electric furnace spalling resistance thereof. The electric furnace spalling resistance was measured over a range of 40 to 800°C. Specifically, a honeycomb sample was placed in an electric furnace, for example, at 40°C for 20 minutes. The sample was then removed from the furnace into the atmosphere at room temperature and inspected for crack generation or sound generation due to cracking. In the case of no inspection, the temperature of the electric furnace was gradually elevated until generation of cracks or occurrence of a similar phenomenon was confirmed. The temperature of the electric furnace at which crack generation or sound generation due to cracking was confirmed was evaluated as electric furnace spalling resistance. The inspection with the furnace temperature elevated gradually until confirmation of crack generation or similar phenomenon occurrence was performed three times, and the average of electric furnace spalling resistance values are shown in Table 2. In addition, general evaluation of cordierite honeycombs was performed on the basis of the following standards, and the results are shown in Table 2. Namely, cordierite honeycombs exhibiting excellent characteristics are assigned rating "0, " those exhibiting slightly poor characteristics are assigned rating
"Δ, " and those exhibiting poor characteristics are assigned rating "X."
Table 1
Figure imgf000018_0001
Table 2
Figure imgf000019_0001
As is clear from Tables 1 and 2, all α-alumina samples C, D, G, H and J falling within the scope of the present invention provide cordierite ceramics exhibiting excellent characteristics. In contrast, α-alumina samples A, B, E, F, I and K falling outside the scope of the present invention provide cordierite ceramics exhibiting poor thermal shock resistance.
Industrial Applicability:
As described hereinabove, from the α-alumina for cordierite ceramics of the present invention, there can be produced cordierite ceramics having a low coefficient of thermal expansion and a high thermal shock resistance and exhibiting small variance in characteristics. In addition, by using, as a raw material, the α-alumina for cordierite ceramics of the present invention, cordierite ceramic honeycomb structures exhibiting excellent thermal shock resistance can be produced.
In short, according to the present invention, α-alumina for producing cordierite ceramics having a low coefficient of thermal expansion and a high thermal shock resistance and exhibiting small variance in characteristics and a method for producing the ceramics can be provided through employment of a specific pulverization method and characteristics of alumina before and after pulverization.

Claims

1. A method for producing α-alumina for cordierite ceramics, comprising the step of pulverizing, by means of a jet mill, α-alumina having a secondary particle size of 40 μm to 70 μm, a primary particle size of 1.5 μm to 3 μm, and a BET specific surface area of 0.5 m2/g to 1.5 m2/g as measured in its powder form, to thereby yield pulverized α-alumina that comprises alumina particles of 3 μm or less in an amount of 30 mass% or less and alumina particles of 15 μm or less in an amount of at least 75 mass% in relation to a particle size distribution profile of the pulverized alumina, and has a mean particle size of 4 μm to 10 μm and a +90-μm sieve residue of 50 ppm by mass or less.
2. The method according to claim 1, wherein the jet mill has a nozzle from which air is jetted out at 3 x 106 Pa (gauge pressure) or less.
3. The method according to claim 1 or 2, wherein the pulverized α-alumina comprises alumina particles of 3 μm or less in an amount of 5 mass% to 20 mass%.
4. The method according to any one of claims 1 to 3, wherein the pulverized α-alumina comprises alumina particles of 15 μm or less in an amount of 80 mass% 90 mass%.
5. The method according to any one of claims 1 to 4, wherein the pulverized α-alumina has a mean particle size falling within a range of 4 μm to 8 μm.
6. The method according to any one of claims 1 to 5, wherein the pulverized α-alumina has a +90-μm sieve residue of 25 ppm by mass or less.
7. The α-alumina for cordierite ceramics, produced through the method according to any one of claims 1 to 6.
8. α-Alumina for cordierite ceramics, comprising alumina particles of 3 μm or less in an amount of 30 mass% or less and alumina particles of 15 μm or less in an amount of at least 75 mass% in relation to a particle-size distribution profile of the pulverized alumina, and has a mean particle size falling within a range of 4 μm to 10 μm and a +90-μm sieve residue of 50 ppm by mass or less.
9. A method for producing a cordierite ceramic, comprising the steps of molding, through an extrusion method, the α- alumina for cordierite ceramics according to claim 7 or 8 and firing the molded α-alumina.
10. The method according to claim 9, wherein the cordierite ceramics have a honeycomb structure.
11. A ceramic for use in an exhaust gas control apparatus, produced by molding the α-alumina for cordierite ceramics according to claim 7 or 8 and firing the molded α-alumina.
12. A deodorization catalyst carrier produced by molding the α-alumina for cordierite ceramics according to claim 7 or 8 and firing the molded α-alumina.
13. A filter for controlling exhaust gas, produced by molding the α-alumina for cordierite ceramics according to claim 7 or 8 and firing the molded α-alumina.
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US8591800B2 (en) 2004-09-24 2013-11-26 Ngk Insulators, Ltd. Method for producing cordierite-based honeycomb structure
WO2022147407A1 (en) * 2020-12-29 2022-07-07 Saint-Gobain Ceramics & Plastics, Inc. Ceramic article and methods of making the same
CN116648306A (en) * 2020-12-29 2023-08-25 圣戈本陶瓷及塑料股份有限公司 Ceramic article and method of making the same
US11826725B2 (en) 2020-12-29 2023-11-28 Saint-Gobain Ceramics & Plastics, Inc. Ceramic article and methods of making the same

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