U.S. patent number RE32,603 [Application Number 07/029,165] was granted by the patent office on 1988-02-16 for process for preparing a ceramic foam.
This patent grant is currently assigned to Swiss Aluminium Ltd.. Invention is credited to Jerry W. Brockmeyer.
United States Patent |
RE32,603 |
Brockmeyer |
February 16, 1988 |
Process for preparing a ceramic foam
Abstract
A ceramic foam material and process for making same having
superior strength and durability properties wherein the impregnated
foam impregnated with an alumina hydrate binder is fired at an
elevated temperature so as to produce a ceramic body characterized
by a solid state, sintered ceramic bond.
Inventors: |
Brockmeyer; Jerry W.
(Hendersonville, NC) |
Assignee: |
Swiss Aluminium Ltd. (Neuhausen
am Rheinfall, CH)
|
Family
ID: |
24624664 |
Appl.
No.: |
07/029,165 |
Filed: |
March 23, 1987 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
654391 |
Sep 26, 1984 |
04610832 |
Sep 9, 1986 |
|
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Current U.S.
Class: |
264/44;
210/510.1 |
Current CPC
Class: |
B01D
39/2093 (20130101); C22B 9/023 (20130101); C04B
38/0615 (20130101); Y02P 10/20 (20151101); Y02P
10/234 (20151101) |
Current International
Class: |
B01D
39/20 (20060101); C04B 38/06 (20060101); C22B
9/02 (20060101); B29C 071/02 () |
Field of
Search: |
;210/510.1
;264/43,44,344 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Cintins; Ivars
Attorney, Agent or Firm: Bachman & LaPointe
Claims
What is claimed is:
1. A process for preparing a ceramic foam filter for filtering
molten metal which comprises: providing a reticulated, organic
polymer foam; impregnating said foam with an aqueous slurry of a
thixotropic ceramic composition including a gelled alumina hydrate
binder in an amount of from 3 to 15%; drying and heating said
impregnated polymer foam to remove the organic component therefrom;
and firing at an elevated temperature to produce a solid state,
sintered ceramic bond characterized by being substantially binder
free and substantially free of glassy phases.
2. A process according to claim 1 wherein the polymer foam is a
polyurethane foam.
3. A process according to claim 1 wherein boehmite is the alumina
hydrate.
4. A process according to claim 1 wherein a colloidal dispersion of
the hydrate is made with an acid that will volatilize under firing
temperature and a thixotropic slurry is made with the ceramic
material.
5. A process according to claim 4 wherein said acid is nitric
acid.
6. A process according to claim 4 wherein the hydrate to acid ratio
is from 2:1 to 5:1 based on 70% acid.
7. A process according to claim 6 wherein the hydrate to acid ratio
is about 3:1.
8. A process according to claim 6 wherein the hydrate is first
gelled with the acid followed by admixing the gelled material with
the ceramic and water to form the slurry.
9. A process according to claim 8 wherein all components are
admixed together.
10. A process according to claim 1 wherein the firing is at a
temperature of at least 2000.degree. F. for at least 15
minutes.
11. A process according to claim 10 wherein the firing is from 15
minutes to 10 hours.
12. A process according to claim 10 wherein the firing is at a
temperature of at least 2500.degree. F.
13. A process according to claim 10 wherein the firing temperature
is approximately 3000.degree. F.
Description
BACKGROUND OF THE INVENTION
The present invention is drawn to an improved ceramic foam material
and process for making same and, more particularly, a ceramic foam
material having superior mechanical, thermal and chemical
properties than ceramic foam materials heretofore known.
It is known in the art to employ porous ceramic foam materials to
filter molten metal, especially aluminum, as described for example
in U.S. Pat. Nos. 3,893,917, 3,947,363, 3,962,081, 4,024,056,
4,024,212, 4,075,303, 4,265,659, 4,342,644 and 4,343,704. The
production material for these filters comprises primarily a
phosphate bonded refractory material, having certain other
additions, which has been fired to a temperature of about
2000.degree. F. in order to mature the bond. See the process
described in U.S. Pat. No. 3,962,081. While this type of refractory
is suitable for use in the aluminum industry and easily withstands
most aluminum alloys which are typically cast at about 1300.degree.
F., it is unsuitable for many other potential applications due to
its low strength and poor chemical durability. Naturally, it would
be highly desirable to develop a material which maintains the
favorable properties of the ceramic foam materials heretofore
known, namely, high porosity, low pressure drop, high geometric
surface area and tortuous flow path, but which overcomes the
above-noted difficulties of strength and chemical durability. In
addition, it would be desirable to develope a single material which
could be relatively simply produced and used in a number of
applications, especially for use in high temperature applications,
such as with ferrous metals and especially in filtration
applications.
Accordingly, it is a principal object of the present invention to
provide an improved ceramic foam material and process for making
same.
It is an additional object of the present invention to provide an
improved ceramic foam material characterized by superior strength
and chemical properties as well as improved mechanical and
refractory properties.
It is a further object of the present invention to provide an
improved ceramic foam material which is relatively simple to
produce.
It is a still further object of the present invention to provide a
ceramic foam material suitable for numerous diverse applications,
such as for ferrous filtration.
Further objects and advantages will appear hereinbelow.
SUMMARY OF THE INVENTION
In accordance with the present invention the foregoing objects and
advantages are readily obtained.
The present invention provides an improved ceramic foam material
and process for making same which is characterized by superior
mechanical, thermal and chemical properties.
The improved ceramic foam filter of the present invention is
prepared from a ceramic slurry containing a gelled alumina hydrate
binder and is for use in filtering molten metal especially in high
temperature applications such as with ferrous metals. The filter is
characterized by having an open cell structure with a plurality of
interconnected voids surrounded by a web of said ceramic. The
filter is further characterized by being substantially binder free,
i.e., being substantially free of glassy phases which detract from
the strength of the product. The structure is characterized by
having a solid state, sintered ceramic bond.
In accordance with the process of the present invention a
hydrophobic, reticulated organic polymer foam, preferably
polyurethane foam, is impregnated with an aqueous slurry of a
thixotropic ceramic composition including a gelled alumina hydrate
binder in an amount from 3-15% by weight based on dry materials,
dried and heated to remove the organic component therefrom, and
fired at an elevated temperature to produce a solid state, sintered
ceramic bond. Preferably boehmite, Al.sub.2 O.sub.3.H.sub.2 O, is
the alumina hydrate. A colloidal dispersion of the hydrate is made
with an acid that will volatilize under firing temperature, such as
nitric acid, and a thixotropic slurry is made with the desired
ceramic material. In the preferred embodiment the firing is at a
temperature of about greater than or equal to 2000.degree. F. for
from 15 minutes to 10 hours, and preferably about greater than or
equal to 2500.degree. F., so as to completely volatilize any
organic materials and then sinter the refractory material. The
resultant ceramic foam material is substantially free of phosphate
or other organic materials which are susceptible to chemical
attack. In addition, the resultant material is characterized by an
increase in strength properties and chemical durability. The
preferred refractory material is alumina; however, other refractory
materials such as zirconia, chromia, cordierite, mullite, etc., can
readily be used. Naturally, specific temperatures will vary from
system to system.
DETAILED DESCRIPTION
In accordance with the present invention the ceramic foam is
prepared from an open cell, preferably hydrophobic flexible foam
material having a plurality of interconnected voids surrounded by a
web of said flexible foam material. Typical material which may be
used include the polymeric foams such as polyurethane foams, and
the cellulosic foams. Generally, any combustible organic plastic
foam may be used which has resilience and ability to recover its
original shape. The foam must burn out or volatilize at below the
firing temperature of the ceramic material which is employed.
The aqueous ceramic slurry which is employed should be thixotropic,
have a relatively high degree of fluidity and be comprised of an
aqueous suspension of the ceramic intended for use in the material.
Typical ceramic materials which may be employed include preferably
alumina and also others such as zirconia, cordierite, mullite,
etc.
It is an advantage of the present invention that the use of a
phosphate or other detrimental inorganic binder is unnecessary.
This results in excellent high temperature properties because
additives tend to reduce the high temperature properties. On the
other hand, the process of the present invention produces a solid
state, sintered product.
In accordance with the present invention alumina hydrate,
preferably boehmite, but also mono- and trihydrates, is used as a
temporary binder and rheological agent. First a colloidal
dispersion is made with an acid that substantially volatilizes
under firing temperature, preferably nitric acid but also
hydrochloric, sulfuric or others. As indicated above this is a
considerable advantage as no residual materials are left such as
phosphate that are potentially reactive with molten metals. This is
followed by the preparation of a thixotropic slurry with the
desired ceramic material, preferably alumina.
Generally, one uses from 3-15% alumina hydrate by weight based on
dry materials, and a hydrate:acid ratio of from 2:1 to 5:1,
preferably about 3:1, all based on 70% acid. In the subsequent
preparation of the thixotropic slurry one can use small amounts of
organic additives if desired, for example, rheological agents,
supplemental binders, dispersants and the like. The water content
is not especially critical, for example 10-50% water based on total
weight can readily be used. The water component simply obtains
reasonable fluidity to impregnate the foam and coat the fibers.
Thus, in accordance with the processing of the present invention
one first gels the hydrate with the acid and adds the ceramic
components and water to form the slurry. If desired, one could
admix all components together. The slurry is then used to prepare
the ceramic foam filter.
Detailed procedures for preparing ceramic foams for molten metal
filters are described in U.S. Pat. Nos. 3,962,081, 4,075,303 and
4,024,212, the disclosures of which are hereby incorporated by
reference.
The flexible foam material is impregnated with the aqueous ceramic
slurry so that the fiber-like webs are coated therewith and the
voids are filled therewith. Normally, it is preferred to simply
immerse the foam in the slurry for a short period of time
sufficient to insure complete impregnation of the foam.
The impregnated foam is then compressed to expel a portion of the
slurry while leaving the fiber-like web portion coated therewith
and with a plurality of blocked pores throughout the body to
increase flow path tortuosity, i.e., homogeneously distributed
throughout the ceramic body rather than grouped together. In a
continuous operation one may pass the impregnated foam through a
preset roller to effect the desired expulsion of slurry from the
foam and leave the desired amount impregnated therein. Naturally,
this may be done manually by simply squeezing the flexible foam
material to the desired extent. At this stage, the foam is still
flexible and may if desired be formed into configurations suitable
for specific filtration tasks, i.e., into curved plates, hollow
cylinders, etc. It is necessary to hold the formed foam in position
by conventional means until the organic substrate is decomposed, or
preferably until the ceramic is sintered. The impregnated foam is
then dried by any suitable means, such as air drying, accelerated
drying at a temperature of from 100.degree. to 700.degree. C. for
from 15 minutes to 6 hours, or by microwave drying. Air drying may
be achieved in from 8 to 24 hours. After drying, the material is
heated at an elevated temperature to sinter the ceramic coating on
the fiber-like webs leaving the plurality of blocked pores as
described above.
In accordance with the present invention, the drying procedure
first yields crystallites or alumina, initially gamma-alumina
eventually transformed to alpha-alumina. This provides sufficient
green strength for handling and firing.
The actual firing conditions depend on the ceramic. Generally,
temperatures in excess of 2000.degree. F. and preferably in excess
of 2500.degree. F. for at least 15 minutes and generally at least 1
hour at temperature and generally less than 10 hours in order to
volatilize the web of flexible foam and sinter the ceramic to form
the solid state sintered ceramic bond.
The resultant product is a porous, fused ceramic foam material
which is substantially free of organic components, and is
characterized by superior mechanical, thermal and chemical
properties to ceramic foam materials heretofore known. The ceramic
foam is characterized by having an open cell structure with a
plurality of interconnected voids surrounded by a web of ceramic
with the substantial absence of potentially harmful binders or
glassy or clay phases. The foam is a solid state, sintered product
especially useful for high temperature applications such as ferrous
or steel filtration. The aforesaid structure results in increasing
desirable physical properties, such as increased mechanical,
thermal and chemical properties. Any small amounts of organic
additives would be driven off in the firing process. The small
amounts of inorganic additives that may be used if desired would
not detract from properties, for example, less than 1% by weight
sintering aids, such as zinc oxide, grain growth inhibitors, such
as magnesium oxide, or inorganic rheological aids, such as
clays.
In accordance with the present invention, the specific features
thereof will be more readily understandable from a consideration of
the following data.
EXAMPLE I
A thixotropic ceramic slurry was prepared by first preparing a gel
consisting of the following:
1853 grams of boehmite
476 milliliters of concentrated nitric acid
9060 milliliters of water.
This gel .Iadd.within the ranges described herein, .Iaddend.was
added to a dry powder blend of:
74 kilograms of alumina
79 grams of magnesia.
An additional 300 milliliters of water was added to this mixture
and the entire batch was thoroughly mixed in a high speed intensive
mixer.
The aforesaid thixotropic ceramic slurry was used to impregnate
nominal 20 pore per inch, open cell, flexible polyurethane foam
blocks to a green density of about 10% of theoretical so that the
fiber-like webs of the foam were coated therewith and the voids
filled therewith. Impregnation was accomplished by immersing the
foam in the slurry and using pre-set rolls to compress the foam and
expel a portion of the slurry while leaving the fiber-like web
portion coated therewith and with a plurality of blocked pores
throughout the body to increase flow path tortuosity.
The resultant impregnated foams were dried and heated to remove the
organic component therefrom and fired to produce a solid state,
sintered ceramic bond by heating to approximately 3000.degree. F.
for one (1) hour. The resultant product is a porous, fused ceramic
foam material substantially free of organic components and
characterized by excellent mechanical, thermal and chemical
properties. The linear firing shrinkage was about 15% such that the
resulting fired ceramic foam had a fired nominal pore size of 23
pores per inch and a fired density of about 15% of theoretical.
EXAMPLE II
Ceramic foams prepared in accordance with Example I and having
fired dimensions of 2".times.2".times.1" were used in an investment
casting operation to filter high quality stainless steel castings.
The ceramic filters were placed directly in the mold cavity and
were under poured. The resulting castings were substantially free
of inclusion related defects.
EXAMPLE III
Ceramic foams prepared in accordance with Example I and having
fired dimensions of 4".times.4".times.1" were used to filter
stainless steel fan housings. In this case a top pour arrangement
was employed. High quality castings were obtained. Weld repair,
normally required for such castings and which is generally due to
inclusions, was substantially eliminated.
EXAMPLE IV
Ceramic foams prepared in accordance with Example I and having
fired dimensions of 4".times.4".times.1" were used to filter a high
temperature nickel-aluminum-bronze alloy test casting. A high
quality casting was obtained. This casting would normally require
many hours of weld repair; however, the resultant casting of this
example reduced the weld work substantially.
EXAMPLE V
A thixotropic slurry was prepared as in Example I and was used to
impregnate a nominal 10 pore per inch, open cell, flexible
polyurethane foam block as in Example I. the resultant impregnated
foam was microwave dried and then fired to 3000.degree. F. to
produce a solid state, sintered ceramic bond and a porous, fused
ceramic foam material substantially free of organic components
characterized by excellent mechanical, thermal and chemical
properties. The fired ceramic foam, measuring 4".times.4".times.1"
with a fired pore size of about 12 pores per inch, was used to
filter a low alloy steel casting. Inclusion related defects were
substantially eliminated.
Thus, as can be seen from the foregoing, a ceramic foam material
having superior strength properties and chemical durability is
obtained.
This invention may be embodied in other forms or carried out in
other ways without departing from the spirit or essential
characteristics thereof. The present embodiment is therefore to be
considered as in all respects illustrative and not restrictive, the
scope of the invention being indicated by the appended claims, and
all changes which come within the meaning and range of equivalency
are intended to be embraced therein.
* * * * *