Plastic and nonmetallic article shaping or treating: processes – Outside of mold sintering or vitrifying of shaped inorganic... – Shaping by extrusion
Reexamination Certificate
1999-09-30
2001-07-03
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Outside of mold sintering or vitrifying of shaped inorganic...
Shaping by extrusion
C264S681000
Reexamination Certificate
active
06254822
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to chemically durable porous mullite-based ceramic articles and a process for the manufacturing the same. The mullite-based ceramic articles are particularly suitable for use as a porous filtration devices and/or supports in the chemical processing industry.
2. Discussion of the Related Art
In the field of membrane separations, thin porous membranes deposited on porous supports are widely used for microfiltration and ultrafiltration of liquid media and gas separation. The macroporous support functions to provide mechanical strength for the thin porous membrane. Porous support materials include alumina, cordierite, mullite, silica, spinel, zirconia, other refractory oxides and various oxide mixtures, carbon, sintered metals and silicon carbide.
Several considerations and limitations are important in selecting the appropriate material for the porous support. The porous support should preferably exhibit the following characteristics: (1) a total porosity, as measured by Hg intrusion of greater than 30%; (2) a high permeability; and, (3) pores exhibiting good connectivity, a greater than sub-micron average pore size and a narrow size distribution. The combined effect of these properties is that the porous support will exhibit both good filtration efficiency and permeability such that the porous support will be suitable for most microfiltration and ultrafiltration applications. Lastly, for chemical processing applications, the porous support should exhibit a sufficiently high mechanical strength (MOR) and very high resistance to chemical attack. It is this last characteristic, resistance to chemical attack, that makes mullite particularly preferred for use in these chemical processing applications.
It is known to those skilled in the art, that one conventional method for making a sintered mullite structure involves firing, at about 1600° C., a mixed powder of alumina (Al
2
O
3
) and silica (SiO
2
), the constituent components of mullite; i.e. the reaction sintered formation of mullite bodies. Although mullite structures produced in this manner exhibit sufficient chemical resistance and mechanical strength, the mullite structures formed in this conventional manner are dense and exhibit pores of a submicron average pore size.
Mullite formation methods involving the use of pre-reacted mullite powder represent an improvement over the aforementioned reaction sintered methods. Two such reference which disclose the use of pre-reacted mullite powder include U.S. Pat. No. 4,935,390 (Horiuchi et al.) and German Pat. No. 42 26 276 (Levkov).
The Horiuchi reference discloses a method for forming a sintered mullite-based body, having improved flexural strength, involving heat treating a composition of 80 to 99.1%, and 0.1 to 20%, by weight, of a mullite powder and a sintering aid, yttrium oxide, respectively. The Levkov reference discloses a method for the production of a ceramic sintered filter body characterized in that the starting mixture consists of 90-93% mullite, having grains of between 0.63 to 0.1 mm, an opening material, either 4-8% cork scrap or 12-16% rubber scrap, having a grain size of up to a maximum of 0.2 mm, and a binder comprising 5-7% clay and 1-3% Al
2
O
3
; all in weight percent. Although both of these processes result in the production of mullite bodies exhibiting larger pore sizes and improved filtration when compared to reaction sintered mullite bodies, the filtration efficiency of these mullite bodies and their chemical durability are not such that they would be suitable for use in those applications where the porous supports would be exposed to highly basic or highly acidic environments akin to those environments seen in the chemical processing industry.
A recent innovation disclosed in copending, co-assigned application, U.S. Ser. No. 60/102,621 (Brundage et al.) results in mullite bodies which are improved over those disclosed in the Horiuchi and Levkov references. The sintered mullite bodies, possessing improved filtration efficiency and permeability, are produced from a raw material composition comprised of 75 to 99% by weight pre-reacted mullite powder, and 1.0 to 25% by weight of a water-swelling clay. Although these mullite bodies exhibit improved filtration properties, the chemical durability, sufficient for most applications involving the microfiltration and ultrafiltration of liquid media and gas separation, is below that necessary for use in those chemical processing applications involving exposure to highly basic or highly acidic environments.
There is, accordingly, a clear need for a means for producing a porous mullite structure exhibiting an increased chemical durability and possessing both sufficient filtration efficiency and permeability suitable for use as porous supports in the chemical processing industry.
SUMMARY OF THE INVENTION
The object of the present invention is to solve the above problems of the prior art and to provide a method for making a sintered ceramic substrate, having mullite as its primary phase, having improved resistance to chemical attack and sufficient filtration efficiency and permeability.
It has been surprisingly found that when a water swelling clay, in combination with the use of pre-reacted mullite powder and alumina, comprise the raw materials used in the preparation of mullite structures, the resulting ceramic bodies exhibit the above mentioned properties. Specifically, the invention is directed at a composition for use in preparing a sintered substrate, having mullite as its primary phase, comprised of 75 to 99%, by weight, pre-reacted mullite powder, and 1.0 to 25%, by weight, of a water-swelling clay and 6-25%, by weight, of an alumina and/or an alumina-yielding precursor.
This invention also relates to a method for producing a sintered ceramic substrate having mullite as its primary phase, comprising preparing a plasticizable raw material mixture as defined above, adding an organic binder system to the mixture and mixing the mixture to form an extrudable mixture, and extruding the mixture to form a substrate of the desired configuration.
The green body is dried and fired for a time and at temperature sufficient to form a sintered mullite structure exhibiting an acid/base exposure strength loss of less than about 20%.
REFERENCES:
patent: 4295892 (1981-10-01), Matsuhisa et al.
patent: 4601997 (1986-07-01), Speronello
patent: 4608357 (1986-08-01), Silverman et al.
patent: 4628042 (1986-12-01), Speronello
patent: 4826790 (1989-05-01), Jones et al.
patent: 4921616 (1990-05-01), Minjolle
patent: 4935390 (1990-06-01), Horiuchi et al.
patent: 4950628 (1990-08-01), Landon et al.
patent: 42 26 276 (1994-02-01), None
patent: 0 236 249 (1987-01-01), None
Corning Incorporated
Fiorilla Christopher A.
Schaeberle Timothy M.
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