Note: Descriptions are shown in the official language in which they were submitted.
l(~S9535
Porous ceramic foam materials are kncwn in the art,
for example, having been described in U.S. Patents 3,090,094 and
3,097,930 and are known to be particularly useful in filtering
molten metal, as described more specifically in U,S Patent
3,893,917, issued July 8, 1975.
Molten metal, particularly molten aluminum, in practice
generally contains entrained solids which are deleterious to
the final cast metal product. These entrained solids appear
as inclusions in the final cast product after the molten metal
is solidified and cause the final product to be less ductile
or to have ~oor bright finishing and anodizing characteristics.
The inclusions may originate from several soùrces. For
example, the inclusions may originate from surface oxide films
which become broken up and are entrained in the resultant
molten metal. In addition, the inclusions may originate as
insoluble impurities, such as carbides, borides and others or
eroded furnace and trough refractories.
It is naturally highly desirable to devise an im-
proved filter for use in removing or minimizing entrained solids
from the final cast product, particularly with respect to molten
aluminum and especially, for example, when the resultant metal
is to be used in a decorative product, such as decorative trim
or sheet made from the 5000 series of aluminum alloys, as
aluminum Alloys 5252 and 5657. Other aluminum alloys which
benefit from improved filtration include: aluminum capacitor
foil made from the 1000 series of aluminum alloys, as aluminum
Alloys 1145 and 1188 in order to reduce pinhole defects in
light gage products and maximize productivity in rolling;
high strength extrusion alloys, such as aluminum Alloys 2024
and 7075 in order to obtain high ultrasonic quality; and
.
,. . . .
~V59535
extrusion alloys of the 6000 series of aluminum alloys, as
aluminum Alloy 6061, in order to obtain higher productivity
in extrusion operations: etc,
Inclusions as a~oresaid cause loss of properties in
the finally solidified alloy and lead to degradation of process-
ing efficiency and loss of properties in the final product. For
example, one type of finishing flaw which is particularly
significant in decorative trim or sheet made from aluminum
Alloy ~252 is a stringer defect known as a linear defect.
Rigorous melt treatment processes such as gas fluxing
minimize the occurrence of such defects; however, these are
not successful in reducing them to a satisfactory level for
critical applications. Conventionally, melt filtration is
, utilized in order to decrease the extent of such defects, and
others caused by the presence of inclusions in the melt, The
most common form of me]t filtration involves the use of open
weave glass cloth screens placed in transfer and pouring
:,.,
troughs or in the molten pool of metal in the top of a
solidifying ingot, Such filters have been found to be only
~ii, 20 partially effective since they remove only the larger
inclusions, Another type of filter in common use is a bed
filter made up, for example9 of tabular alumina. Such filters
have many disadvantages, perhaps the most serious of which is
the great dif~iculty experienced in controlling and maintain-
ing the pore size necessary for efficient filtration. Another
difficulty with such filters is their tendency to produce an
initial quantity of metal having poor quality at the start up
of each successive casting run, This behavior results in a
so-called ingot "butt-effect", that~is, ingots having butt
portions of relatively poor quality which must be scrapped
,:
. ~ :
. .
- 2 _
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. . - ' - " ,'` ' ::, ' : ' ' ' ~ .
~059S35
and recycled. Still further, the metal in a bed filter mus-t
be maintained molten even when the filter is not in use.
In comparison thereto porous ceramic filters appear to
be highly desirable. However, the successful use of such
articles in exacting functions such as the filtration of -
molten metal requires that the article possess particular
physical and chemical properties. Specifically, the ceramic
foam article requires a certain permeability and structural
uniformity to efficiently filter molten metal at commercially
acceptable rates and purity levels As a corollary property,
the foam material must withstand chemical attack from the
molten metal to facilitate its extended use as a filter
The prior art suggests various methods for the pre-
paration of ceramic foam materials. Particularly, U.S, Patent
No. 3,111,396 to Ball suggests that an organic polymer foam
impregnated with a refractory material may then be compressed
by passage through preset rollers to effect the removal of
excess refractory. This technique, which is comparable to a
wide variety of conventional expulsion techniques used in the
art suffers from an inherent disadvantage in that the slurry
is not completely uniformly distributed through the body of
the article. ~hus, the outer area of the article tends to be
more thinly coated with slurry than that near the center line,
Such defects are particularly extended at the extremes of the
permeability range found suitable for use in the preparation
of molten metal filters; thus, bodies possessing a high
permeability tend to have undesirably weak surfaces and edges
whereas, bodies possessing relatively low permeability tend
to exhibit undesirable center line blockage. Both of the
aforenoted defects render the resulting foams unsuitable for
-- 3 --
1~5~53S
use in the filtration of molten metal.
It is a principal object of the present invention to
devise a ceramic foam filter which has considerable high
temperature resistance so that it can be used on a variety of
molten metals and so that the structure thereof is resistant
to degradation under the severe conditions of use associated
with filtration of molten metal.
It is a further object of the present invention to
devise a high temperature resistant ceramic foam filter as
aforesaid which is convenient to prepare and is characterized
by reasonable cost.
It is a still further object of the present invention
to provide a high temperature resistant foam filter as afore-
said which overcomes the art disadvantages referred to above, -
does not contaminate the melt and does not result in degradation
of desirable characteristics in the final metal product.
It is a further principal object of the present inven-
tion to provide a method for the preparation of ceramic foam
articles which is both accurate and expedient.
It is a further object of the present invention to
provide a method as aforesaid which yields products possessing ~ `
permeabilities within closely set tolerances,
It is yet a further object of the present invention to
provide ceramic foam articles as aforesaid which exhibit
structural uniformity and freedom from defects such as center
line blockage and outer surface weakness~
It is a still further object of the present invention
to provide a method as aforesaid which lends itself to rapid
commercial-scale production techniques.
-- 4 --
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l~S~535
Other objects and advantages will be apparent from a
careful review of the ensuing description.
In accordance with the present invention it has now
been found that the foregoing objects and advantages may be
readily obtained. -
The present invention provides a highly efficient
ceramic foam material for use in filtering molten metal, espec-
ially molten aluminum. The ceramic foam material of the present
invention is characterized by having an open cell structure
with a plurality of interconnected voids surrounded by a web
of said ceramic material. The filter of the present invention
is further characterized by a combination of critical charac-
-teristics. The filter has an air permeability in the range
of from 400 to 8000 x 10 7 cm2. In addition, the filter of
the present invention has a porosity or void fraction of
0 80 to 0.95 Still further, the filter of the present
invention is characterized by from 2 to 20 pores per linear
cm and an effective range of filter thickness of from 10 to
100 mm. It has been found that in accordance with the
present invention the foregoing filter is particularly useful
in the filtration of molten metal, especially molten aluminum.
Numerous advantages are achieved utilizing the filter of ~the
present invention, some of which are referred to above and as
will be discussed hereinbelow.
In the normal situation one uses a relatively fine
filter of the present invention having an air permeability
of from 400 to 2500 x 10-7 cm2, a porosity or void fraction
of 0 80 to 0 95 and from 8 to 18 pores per linear cm,
especially if one is filtering an aluminum alloy of the 5000
series However, if the input metal is particularly dirty,
.~
-- 5 --
~ S 35
one should preliminarily filter the metal throu~h a relati~ely
coarse ceramic foam filter having a pore size of between 2
and 8 pores per linear cm, air permeabilities of 2500 to
8000 x 10 7 cm2 and porosities or void fractions of between
0,90 and 0,95, This may be accomplished by providing a
single ceramic filter with a gradation of properties or by
using a series of filters of differing porosity,
In addition to the foregoing, therefore, the present
invention provides a method of filtering molten metal through
a ceramic foam filter characterized as aforesaid utilizing a
molten metal flow rate through the filter of from 1.25
to 12,~ dm3 per dm of filter area per minute, In
accordance with an alte~native embodiment of the method of
the presen-t invention, the molten metal may be preliminarily
filtered through a relatively coarse ceramic foam filter of
the present invention, followed by filtration through a
relatively fine, preferred filter of the present invention,
This preliminary filtration step may utilize a series of
ceramic foam filters of decreasing porosity and is particular-
ly useful with especially dirty melts,
In accordance with the present invention, the ceramic
;` foam filter described above has been found to be particularly
useful in filtering molten metal, especially molten aluminum,
~he ceramic foam filter of the present invention is a low cost
material which may be conveniently used on a disposable basis,
As indicated hereinabove, the ceramic foam material ofthe present invention is characterized by having an open cell
~, structure with a plurality of interconnec-ted voids surrounded
3, by a web of said ceramic material, It has been found that
the characteristics of the filter of the present invention
' ~
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~059535
define a filter which is surprisingly effective in the
filtration of ~olten metal, especially aluminum, at a low cost
and achieving surprising filtration efficiency with a
flexibility not available heretoore.
The combination of properties of the filter of the
present invention are critical in obtaining the desirable
results of the present invention. As indicated heretofore,
the ceramic foam filters of the present invention have an air
permeability in the range of 400 to 8000 x 10 7 cm2, and in
the normal and preferred case in the range of 400 to 2500 x
10 7 cm2. The air permeability is determined by blowing air
through the ceramic foam at a measured rate In accordance
with this procedure -the pressure drop is measured by
determining the pressure aifferential between the air entering
the foam and the air leaving the foam for a defined area and
thickness of foam The air permeability is then determined
in accordance with the following formula:
K A~P
wherein K = the air permeability, ~ = the dynamic viscosity of
air, Q = the flow rate of air through the body, ~ = the length,
; that is, the thickness of the ceramic filter material, A = the
area, that is, the defined area of the foam and ~P = the
pressure drop~ In accordance with the present invention one
utilizes an air flow rate of 0.857 m3 per minute and an area
of 73 cm2. The foregoing determination of permeability may be
found in the text Micromeretics by J.M. Dallavalle published
by Pitman 1948 at Page 263 It may be seen, therefore, that
the air permeability is a function of many variables. For
example, the bulk density, the pore size, the surface area and
......
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~059535
the tortuosity of the flow paths. In accordance with the
present invention i-t has been found that permeabilities
greater than 2~00 x 10 7 cm2 give i~adequate filtration unless
the melt is particularly dirty in which case permeabilities up
to 8000 x 10 7 cm2 may be used, while permeabilities of less
than 400 x 10-7 cm2 give impractically high rates of head
build up. A particularly preferred range of permeability has
been found to be from 1000 to 1500 x 10 7 cm2 wherein optimum
filtration and low rates of head build up are obtained.
In addition to the foregoing, the ceramic filters of
the present invention should have a porosity or void fraction of
from 0.80 to 0 g5 This variable defines the amount of pores
or voids in the ceramic body and may be determined in accord-
ance with the following formula:
fp = dt - db = 1 _ db
dt dt
wherein fp = the total porosity or void fraction, dt = -the true
density of the solid ceramic body and db = the bulk density of
the ceramic foam body. The foregoing formula may be found in
the text Introduction to Ceramics by W. D. Kingery, published
1960, by John Wiley at Page 416 It has been found that
optimum results are obtained in accordance with porosity values
of from 0.8~ to 0 90 Naturally, the specific value for dt
will depend upon the par-ticular ceramic body. For example,
for alumina-chromia based ceramics, the foregoing porosity
values correspond to bulk densities of o.65 to 0.25 gm/cm3
and the optimum values correspond to from 0 35 to 0.45 gm/cm3.
As indicated above the relatively coarse, preliminary filter
should have a porosity between 0.90 and 0.95.
1059S:~5
In addition to the foregoing, the ceram~c filter of the
present invention should have an effective range of pore size
or pore density in terms of number o~ pores per linear cm,
namely, ~rom 2 to 20 pores per linear cm, from B to 18 pores
per linear cm in the normal and preferred case, and optimally
from 10 to 14 pores per linear cm.
The foregoing three variables, namely, the permeability,
the porosity and the pore size, are critical in obtaining the
greatly improved characteristics of the present inven-tion.
Namely, these variables influence each other in achieving the
surprising efficiency of the filter of the present invention
They define in effect how many pores or holes are present in
the filter, how they are interconnected and how large they
are, the surface area of the ceramic web and they define a
surprisingly effective ceramic foam filter.
Still further the ceramic foam filter of the present
invention should have an e fec-tive range of filter thickness
of from 10 to 100 mm, that is, a thickness in the direction
of metal flow. The optimum filter thickness is from 35 to 65
mm. It has been found that filters of less than 10 mm in
thickness are not effective in removing the bulk of nonmetal-
iics from the molten metal, whereas, increments of thicknesses
above 100 mm suffer from diminishing filtration rates since
i the most effective region of the filter is the first 25 to 35
mm in thickness.
An additional and significant characteristic of the
effective filters of the present invention is that they should
have substantial structural uniformity. In order to provide
an effective filter for molten metal the ceramic foam body
must have a high degree of etructural uniformity. Therefore,
,.. ~ . .
~059S3~
although some percentage of blocked pores are helpful and
desirable in that they increase flow path tortuosity, these
blockages should be homogeneously distributed throughout the
ceramic body rather than grouped together. Grouping of
blockages will onl~ lead to channeling and inefficient
filtration.
A wide variety of materials may be used in the pre-
paration of the ceramic foam filter of the present invention. It
is an advantage of the present invention that the low cost and
ease of preparation of the filter of the present invention
makes the filter convenient to use on a throwaway basis
The principal component of the ceramic foam material
of the present invention is A1203 in an amount from 40 to 95~0
and preferably from 45 to 55% A1203 is particularly de-
sirable for use as a ceramic filter since it is not attacked
by molten aluminum or molten copper, for example; whereas,
silica is attacked by these materials. ~urthermore, the
alumina has reasonable strength to stand up to chemical
attack and structural and/or mechanical strength to stand up
20 to the particular elevated temperature conditions. In addition ; ;~
to the foregoing, the ceramic material of the present in-
vention may contain from 1 to 25% Cr203 and preferably 10
to 17% Cr203 This componen-t is particularly significant ;
since it has been found that it imparts significantly better
high temperature resistance 9 that lS, resistance to attack by
the molten metal at elevated temperature Furthermore the
ceramic foam material contains the products of the thermal
decomposition of 0 1 to 12% kaolin and/or bentonite and of
2 5 to 25% of an air setting agent which is substantially
30 non reactive to the molten metal, preferably of aluminum -
~ '' '
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iO59535
phosphate.
In accordance with the present invention, -the fore-
going objects and advantages are readily obtained by -the follow-
ing method of preparation: Ceramic foams possessing controlled
permeability and structural uniformity are prepared by a
process comprising providing an open-celled organic polymer
foam material possessing a predetermined permeability and
resilience, impregnating said polymer material with an aqueous
slurry of a thixotropic ceramic composition while shearing
said slurry an amount sufficient to maximize impregnation, and
expelling excess slurry from said material by conducting at
least two passes of said material through preset rollers to
effect a temporary compression ranging from about 50 to 90%
of the thickness of said material for the first pass, and 70
to 90% of said thickness for the second pass. After im-
pregnation and expulsion of excess slurry àre complete, the
resulting foam material is then heated to remove the organic
foam component The resulting article is then ready for use,
or may, if desired, be further heated to sinter the ceramic
20 material
In accordance with the present invention, the air
permeability of the resulting ceramic articles has been found
to depend upon the permeability of the organic polymer foam
employed in its preparation. For example, ceramic foams
having permeabilities in the range of from about 800 to about
2,200 x 10 7 cm2 have been prepared from polyurethane foam ~ -
materials having air permeabilities ranging from 4,~00 to
~,400 x 10 7 cm2. Further, the selection of raw foam
permeability in the range of +2~o facilita-tes the preparation
of a ceramic foam having a permeability predetermined to
-- 11 -- .
'.',, : ' ~ .
1(~5~35
within a range of ~%.
In addition to the control of permeability, the foams
of the present invention must possess structural uniformity and
a particular range of cell size. Structural uniformity has
been found to relate to the resilience of the organic polymer
foam precursor. Particularly, resilience may be determined
with reference to certain standards set forth in the ASTM-D- ~`
1564-71 which refers to the properties of compression set and
resilience as measured by ball rebound. Compression set,
determined by the compression load deflection test, measures
the extent to which the foam returns to its original size or
thickness after compression to a stated reduction such as, for -
example, 50%. Foams found suitable in accordance with the
invention exhibit a compression set of less than 30% at 50%
compression, and thus, return to at least 70% of their original
thickness after compression is released. Resilience,
determined by the ball rebound test, measures the resistance
that the material exhibits to compression by the height of
rebound of a steel ball dropped from a stated distance onto a
foam sample. The percentage of return of the ball to the
original height is noted, and foams suitable in the present
invention have been found to be those possessing ball rebounds
of greater than 25%.
The above properties have been measured in terms of --
tests run under dry conditions, however, such properties must be ~ -
substantially retained in an aqueous environment as, for
example, during impregnation with the aqueous ceramic slurry
of the present invention. Accordingly, it has been found that
hydrophobic foams perform better and are preferred to
hydrophilic foams, as the latter suffer considerable loss of
- 12 -
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lOS9S35
resilience in aqueous environments, This loss of resilience
is evident in the occurrence of the aforenoted defect of
center line blockage,
With the above noted criteria in mind, organic polymer
foam materials which may be employed in the present invention
include a wide variety of highly resilient, reticulated
hydrophobic materials such as the polyester and polyether
polyurethanes, such as "high resilience" or "cold cure"
; urethane materials which utilize polymeric isocyanates in their
formulation; polyvinyl foams such as polyvinyl chloride,
polyvinyl acetate, and polyvinyl foams of different copolymers;
polyurethanes coated with polyethylene or polysiloxane
polymers and copolymers; and foams prepared from suitable
natural resins such as cellulosic derivatives, The foams
must burn out or volatilize at below the firing temperature
, of the ceramic material with which they are impregnated, As
noted earlier, the dimensions of the foam should correspond
roughly to the dimensions of the desired ceramic article,
Thus, for example, a foam material having a thickness ranging
from about 10 to 100 mm is employed when the resulting ceramic
foam is to function as a molten metal filter,
In addition to the properties of permeability and
uniformity, the above noted polymeric materials must possess
a pore size within defined limits in order to render them
effective in the preparation of molten metal filters, Pore
or cell size has been found to be important to the structural
uniformity of the ceramic foam and should vary within the
ranges of 2 and 20 pores per linear cm,
~ The control of the above noted variables contributes to
; 30 the structural uniformity and permeability of the resulting
'~
13 -
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1059535
filter and directly effects metal flow rate and effectiveness
through the tortuosi-ty of the flow path. Though these factors
are significant, additional factors will be discussed herein-
below which combine to provide further con-trol of the final
ceramic foam article.
The organic foam selected wi-th reference to the above
discussion is then impregnated with a slurry of a thixotropic
ceramic material The property of thixotropy is important to
the present invention as it affects the uniformity of structure
10 and strength of the final ceramic foam article. Thixotropic ~
materials are those which display a high resistance to flow ;-
under low rates of shear and correspondingly, a low resistance
to flow under relatively high rates of shear. As this relates
to the method of the present invention, the ceramic slurry
must possess sufficient fluidity to rapidly enter and fill the
voids of the organic foam material and thereby coat the
surrounding polymer web, while possessing a sufficient
viscosity to resist running out or draining from the foam once
impregnation is complete. It has been found in accordance
20 with the present invention that certain ceramic materials -
prepared in combination with particular air setting agents and
temporary binders display the desired thixotropic character to
successfully conduct impregnation ;
As the ceramic slurry which is employed herein may vary ~
:, .
according to the end use of the foam, a wide variety of
i ceramic materials of varying refractoriness may be employed, ;~
'; Particularly, such materials as alumina, chromia, zirconia,
magnesia, titania, silica and mixtures thereof may be presen~.
Such materials are noted for their relatively high
refractoriness or ability to serve in high temperature
.
- 14 -
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lOS9535
situations. However, other materials of lesser refractoriness
such as mullite, calcined clay and various glasses of high
softening temperature may be employed herein either alone or
in combination with each other and with more refractory
materials to prepare the resulting foam article, for example,
; in an amount up to 1~%. Insofar as the ~tility of the
resulting article as a molten metal filter is concerned, the
only requirement placed on a selection of the particular
ceramic materials is that they provide the article with
sufficient resistance to the chemical attack of the molten
alloys over the exposure times involved in filtration. A
particular cornposition which has been successfully employed
herein comprises a mixture of alumina and chromia.
The above composition also includes a room temperature
binder or air setting agent which provides green strength to
the slurry, particularly during the bake out and the optional
sintering operations where the foam is subjected to thermal
stress.
According to the inventlon one provides from 2 5 to 25%
of an air setting agent which is substantially nonreactive to
the molten metal. The air setting or bonding agent sets up
or hardens the ceramic slurry without the need for heating,
and preferably by drying, normally by a chemical reaction,
while heating to moderate temperatures The preferred air
setting agent is aluminum orthophosphate, preferably in the
form of a 50% aqueous solution. Other air setting agents
which may be employed include, for example, magnesium
orthoborate, aluminum hydroxy chloride, etc. Alkaline metal
silicates such as sodium silicates may be employed at least
in part; however, these are less desirable since melting and
.
- 15 -
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,,.~': ~ .: .
-
l~)S953S
consequent loss of set occurs at temperatures around 1500F
(815,56C). Furthermore, the silicon contents thereof, and
perhaps the sodium content, may become dissolved in the melt,
Similarly, ethyl silicate and other phosphates may be employed
but are less desirable. Aluminum orthophosphate is particu-
larly preferre~ due to its very desirable combination of ~ s
properties, that is, nonreactivity, stability over a wide
range of temperatures and good setting properties.
As indicated hereinabove, the air setting agent is
preferably added as an aqueous suspension including equal
parts of binder and water particularly in the case of aluminum ~
~:
orthophosphate, The binder provides green strength before the :
formation of the ceramic bond, that is, after the burning off
or volatilization of the web of flexible foam material, The
binder material provides sufficien-t strength to hold the
mixture together for formation of the final product, In
fact, the stability and strength of the chemical bond provided
by the preferred air setting agent is sufficient for many i
:
applications to enable use of the product at this stage without
high temperature sintering, This strength is substantial and
exists over a wide temperature range, The preferred embodiment
utilizes from 12 to 17% of aluminum orthophosphate,
In addition to the binder noted above, certain agents
herein referred to as rheological agents are employed which
serve to promote the desired thixotropic property of the slurry, ~-
Several materials are known which may serve as rheological
agents, among them certain organic materials such as :
carboxymethyl cellulose and hydroxyethyl cellulose, and
certain inorganic materials such as bentonite and kaolin, of
: 30 the materials available in this regard, bentonite has been
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. - . . ,
1(~5953S
found to be particularly preferred. Bentonite is a naturally
occurring clay composed primarily of aluminum and various
silicates, usually including quantities of magnesium and iron.
In addition to its promotion of -the thixotropic properties of
the slurry, bentonite performs a small setting or binding
function, as certain glassy phases are produced upon firing
of the article which yield increased strength in the ~inal ~-
foam structure, In addition to bentonite, a small amount of
kaolin may also be employed which provides both binding and
rheological improvement to the final slurry in the same manner
as bentonite. Kaolin is a clay composed of primarily of
alumina and silica. Naturally, one could employ the chemical
equivalents of the aforenoted materials to approximate their
compositions. The general range of addition of the rheological
agents of the present invention is within about 0.1 to about
12% by weight of the slurry. In a preferred embodiment, the
rheological agents are added in an amount ranging from about
0.5-5% by weight.
Though, as indicated above, the thixotropic ceramic
20 material may be prepared in a wide variety of formulations, a -~
particular composition has been determined to be eminently
suitable which comprises alumina in an amount ranging from
about 40-80%, and preferably from about 45-50%, chromia in an
amount ranging up to about 20%9 and preferably from about
10-15%, kaolin in an amount ranging up to about 10%, and
preferably from about 2-5%, bentonite in an amount ranging
from about 0 l-lO~o, and preferably from about 0 5-2%,
colloidal aluminum orthophosphate (50% solution) in an amount
of from about 5-50%, and preferably from about 25-35%.
Additional water may be added to the above formulation in
- 17 -
,~ ,
1~5gS35
amounts ranging up to about 20%, and preferably from about
5-lO~o for the purpose of adjusting viscosity, discussed in
detail hereinbelow. Generally from 10 to 40~0 water is present ;
in the slurry Though the foregoing formula-tion is suggested
in its preferred ranges, it is to be understood that the
invention is not limited thereto, as other formulations may
be prepared from the ingredients recited earlier.
In addition to its thixotropic properties, the ceramic -
~slurry of the present invention must possess a carefully
controlled viscosity at and during the time of impregnation
The character of viscosity has been found to exert a material
effect on the achievement of a reproducibly uniform ceramic
article. The desired range of viscosity has been found to be
from 1 x 103 to 80 x 103 cps (centipoise), and preferably
within the range of 10 x 103 to 40 x 103 cps. Viscosity is
regulated during the formulation of the slurry and must be
within the forenoted ranges at the time the slurry is to
impregnate the organic polymer foam. As noted above, a
convenient way of regulating and thereby controlling viscosity
is through the variation in excess water content within the
ranges specified above. For the purposes of the present
invention, viscosity is measured at 25C with a #6 spindle,
Brookfield RVT Viscometer at 20 rpm after 20 minutes rotation, .
the slurry having previously been mixed in an 80-quart Hobart*
Mixer at 60 rpm for 30 minutes.
Once the ceramic slurry is prepared to within the -`
aforenoted limitation of viscosity, the impregnation of the
organic foam material can be conducted. Thus, slabs of
reticulated polyurethane foam having pore sizes lying between
30 2 and 20 pores per linear cm are immersed in the slurry until ~ ;
* Trademark
- 18 -
,' ' ' , ' " ' .,, ' . .
' ' ~: ., ' : . '
1059535
the interstices of the foam are fully saturated therewith,
Impregnation may be conducted by one of many techniques, For
example, the foam slab may be totally immersed in t~e slurry
and passed through a pair of rolls likewise immersed therein
to expel air from the pores of the foam on compression whereby
the reexpanding foam emerging from the rolls draws in the
slurry and is thereby filled therewith, Another technique
which may be employed would involve placing the foam over a
bath of the slurry in an enclosed vessel, evacuating the
vessel to a reduced pressure, immersing the foam in -the bath
of slurry and then releasing the vacuum in the vessel, This
method which would comprise a modification of vacuum
impregnation would likewise result in the total saturation of
the foam with slurry, Naturally, other forms of impregnation
including the standard vacuum impregnation technique of merely
exerting the vacuum force on one side of the foam while
drawing slurry through the opposite side, would be applicable
herein and the invention should not be limited to a par-ticular
technique per se,
A preferred impregna-tion technique which has been
employed herein comprises complete immersion of the foam in a
bath of slurry and repeated compression and expansion of the
foam by a mechanical plunger device made from perforated steel
sheet, This process is conducted for from 30 seconds to 1
minute, or, of course, until the interstices of the foam are ~ ;
complètely saturated. In view of the thixotropic nature of
the slurry discussed earlier, it is particularly advantageous
and important that the slurry be continually sheared during -
the impregnation to maintain the desired rate of flow into the
foam material, This shearing may be accomplished in a wide
- 19 -
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:. . . : . .
~ : .
,
- ^
~059535
variety of ways such as the continual high speed agitation of
the slurry. A technique which has been employed in the present
invention comprises the continual vibration of the slurry
during impregnation. It should be noted at this point that ~;
all impregnation techniques previously discussed would require
that the slurry be maintained in its highly flowable state by
some form of shearing action such as vibration or the like~
Upon completion of the impregnation of the foam material, ~-
shearing is stopped; the slurry residing within the foam
becomes resistant to flow and is substantially completely
retained therein, with little loss due to drainage during the
subsequent transfer of the foam from the impregnation area.
Upon completion of the impregnation of the foam with
slurry, the impregnated foam material is then treated to
remove excess slurry therefrom. ~his removal or expulsion of
excess slurry must be closely controlled and uniformly
conducted throughout the body of the foam in order to obtain
a uniform ceramic article. As noted earlier, a wide variety
of conventional methods are known for the removal of slurry
from impregnated organic foam, however, such methods which
include squeezing, blowing out by compressed air, centrifuging,
and even passage through rolls, do not provide satisfactory
results in this respect. Generally, in the case where
passage through rolls is employed, the resulting article
possesses either the defect of center line blockage, wherein
excess slurry remains and agglomerates within the center of
the article, or outer surface weakness, wherein insufficient
ceramic material remains after expulsion and the article is
mechanically weak.
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~0S9535
In accordance with the present invention, it has been
found that expulsion is advantageously conducted which results
in consistently uniform ceramic foam articles by a process
comprising conducting at least two passes of the impregnated
foam material through preset rollers to effect the compression
of said material in the range of about 50 to 90% of its
thickness for one pass and 70 -to 90% of thickness for a
further pass. Thus, multiple rolling passes conducted at the
same or increased percent reductions yield a ceramic article
possessing increased strength and freedom from center line
blockage. Further, the employment of multiple rolling passes
affords the careful control of the permeability of the final
ceramic article which is particularly critical when such
articles are prepared for use as molten metal filters.
An additional aspect of significance relating to the
; expulsion technique of the present invention is the use which
is made of the thixotropic nature of the slurry in the rolling
operation, Thus, because the slurry flows freely Imder high
rates of shear but remains virtually static once shearing
ceases, close control over slurry removal is available
through control of roll gap (percent reduction), roll speed
and/or roll diameter. Specificallyl control over the roll
; gap and roll speed determine the rate of shear experienced
by the slurry andr hence, the extent of its removal and the
geometry of its redistribution on the web of the rolled foam,
The preferred rolling technique of the present invention
utilizes a double pass schedule, although a multiple pass ;
schedule may be desirable in certain instances such as where
the foams possess a thickness greater than 5 cm. As noted
earlier, the respective percent reductions which have been
- 21 -
GON-31-r~
lOS9S35
determined wlth a double pass schedule are 50 to 90% for the
flrst pass and 70 to 90% ~or the second pass. Preferred
percent reductions within the aforenoted ranges are 70 to 80%
for the first pass and 70 ~o 90% ~or the second pass.
Expulsion may be conducted wlth a conventional roll stand
apparatus comprising two cooperating rolls. Thus, the foam
material would be passed through the roll stand a first time,
and thereafter recirculated for the second pass. In the
instance where the second pass is to be conducted at a dif-
ferent percent reduction, a palr of roll stands could be
suitably provided in spaced relation to each other to permit
the foam material to pass throu~h the respective reductions
in a sequential manner. A further variation contemplated
within the present invention to provide successive roll gap
settings contemplates a single pass through a roll forming
mill having three rolls defining the respective successive
roll gap settings. This technique offers the advantages of
a double pass schedule in a single operation using only a
single roll stand.
In addition to the roll stands provided above, the rolls
employed therein-may be advantageously coated with a material
such as grit or the like to increase friction between the
~oam material and the roll and to thereby prevent or minimize
slippage in rolling. A ~urther feature of the apparatus
employed in the expulsion of slurry comprises a moving run-out
table placed at the exit of the roll stands to support and
transfer the newly rolled foam as it emerges therefrom. In
sum, the employment of coated rolls and a movin~ run-out
table would serve to add to the integrity, unlform.ity of
structure ar.d ~hape ^f the ~olled ~roduct, s~rce ~;~ey
-22-
. CON-31-M
~ ~ ~()5S3535
alleviate un;~anted dlstortlon efrects and mlnlmi~e unnecessary
handllng O F' ~ne fcam article which might disturb slurry
distribution.
As noted earller, the provlsion o~ multlple rolling
passes has as its advantage the unexpected increase in the
permeabillty of the final foam artlcle. Thus, in the lnstance
where two passes through the rolls are conducted at the same
percent reduction, the permeabillty of the flnal article has
increased by 30 to 50~. This increase is significant -~hen
it is considered that the pass schedule leads to a more
uniform slurry distribution and ultimately stronger and more
uniform articles than would be obtainable by comparable
single pass reductions. Further, pass schedules conducted
wherein the second or subsequent rolling pass is of a higher
percent reduction than the initial pass, have resulted in
permeaDility increases of greater than 100% over the single
pass reduction schedule.
After expulsion of excess slurry is complete, the
resulting foam structures may be dried and, if desired, fired
to provide a ~used ceramic foam article. The drying sequence
is employed for the prirnary purpose of removing the organic -
polymer foam from the article. Generally, conventional
drying techniques may be emplo~ed in this regard, however, it
should be born in mind that a suitable heating rate for the
removal of the foam should take into account the heat
provided by the oxidation of the foam itself. The effect of
- this phenomenon is particularly noticeable in the heating of
large masses of the foam whe~e a significant volume of the
heating chamber may be occupied by the article. In ~uch
cases, ~ may be r.ecessa~Y ~o ma~ntain 'he mate-lal ~ a
: ,
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CON-31-M
1~)S5~35
temperature ranglng from 200-370C to a~old excessive heat up
resulting from chemical reactlon which may cause the ceramic
filaments to rupture under thermal stress. The exact temper-
ature will be dictated by the particular organic foam base
used and need not be further developed herein.
As indicated above, the ceramic roam may, if desired,
. ~
be further heat treated or fired to fuse the ceramic particles
lnto a highly refractory structure. As noted earller, this
practlce is optlonal, as, for example, in the employment of
10- the foam artlcles of the present invention as filters for
molten aluminum, it has been found that the foam mater~al
need only be heat treated at a temperature of from 540 to
600C to remove the organic component. ~he resulting article
would be suitable as such for use with aluminum alloy mel~s
at temperatures as high as 760C. In such an instance, the
alr setting or binding agent would provide the necessary
strength to the article, and the full sintering treatment
would not be required.
Utilizing the method disclosed above, ceramic foams may
be-prepared which range in thickness from 6 to 100 mm and
may be of an area ranging up to about 1 m2 or more. The
foams would possess, based on the raw foam employed, pore
counts of from about 2 to 20 pores per linear cm with
permeabilities ranging from about 100 x 10-7 cm2 to 10,000
x 10-7 cm2 and bulk densities of from 0.2 gm/cm3 to 1 gm/cm3
In the lnstance where the foam articles of the present
lnventlon are utllized as fllters for molten metal, alr
permeabllities may range from about ~00 to 8,~00 x 10-7 cm2
; and pore counts may range from about 2 to 18 ~ores per linear
cm. Natur 11~, as ~.oted -arlier, ~oth p-rmeabi'~i^s ~r.d
.1 -24-
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CO~-31-M
1(~5~i35
pore sizes may be varied to suit the particular mode of end
use of the artlcle. Thus, ~or example, a relatively fine
fllter may be prepared whlch would possess an alr permeability
of from 400 to 2,500 x 10-7 cm2 and a pore count of from 8 to
18 pores per linear cm. Such an article would be useful in
'the filtration of aluminum alloys of the 5000 series. However,
lr, ln the instance of molten metal filtration, the input ~`
metal is particularly dirty, one should prellminarily filter
the metal through a relatively coarse ceramic foam filter
having a pore size of between 2 and 8 pores per linear cm, and
an air permeability ranging from 2,500 to 8,000 x 10-7 cm2.
This may be accomplished by providing a single filter with a
gradation of properties or by using a series of filters of
differing porosity.
The method of the present invention enables the accurate ~'
control of the permeability of the resulting ceramic foam
article. Further, foams prepared in accordance with the
lnvention exhibit structural uniformity, as neither center
llne blockage nor weak outer surfaces are encountered. When
the articles'of the present invention are employed in the
filtration o~ molten metal, it has been found that they can
' successfully withstand the rigors of the extended exposure to
metal flow without blockage or fracture, and the resulting
metal filtrate possesses unexpectedly improved purity.
- ' The resultant product is a bonded ceramic foam material
having an open cell structure characterized by a pl'urality of
lnterconnected voids surrounded by a web of ceramic, with the
foam material having the characteristics defined herei-.above.
If one desires a single foam filter having a gradation of
properties from. -oarse ~o f ne throu :~out the thi~'~ness ';^er~
.
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' ' '
.
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CON-31-M
~ 105~53S
one may combine two or more slabs of polyurethane foam having
approprlately different pore sizes. Naturally, the ceramic
foam may have any deslred configuration based on the
configuration needed for the particular molten metal
filtration process. Although naturally these configurations
can be many and varied~ particular conflgurations may be
preferred for filtration in a transfer trough between the
furnace and the casting mold in filtering molten aluminum.
A wide variety of suitable configurations may be readily and
convenlently prepared in view of the flexibility afforded by
the preparation process utilized herein. It ls a particular
advantage of the ceramic foam material of the present
invention that said ceramic foam has sufficient strength
property to withstand attack by molten metal and also
advantageously does not require excessive heads of molten
metal in order to start the filtration process.
As lndicated hereinabove, the present invention also
provides a method of filtering molten metal throug~ a
dlsposable ceramic filter characterized as aforesaid by
pouring sa~d molten metal through the ceramic material at a
rate of ~rom 1.25 to 12.5 dm3 per dm2 Of filter area per minute,
and preferably from 2.5 to 7.5 dm3 per dm2 of filter area per
minute for aluminum. Metal flow rates in normal aluminum
casting operations vary from a mInimum of about 90 kg. per
minute to a maximum in excess of 900 kg. of metal per minute,
with a typical bulk metal flow rate being about 225 kg. per
minute. In accordance with the present invention the ceramic
materlals o~ the present invention are well suited to opera'e
success~ully utili~ing bulk metal flow rates as described
above. ~Torma lJ~ ~or ~lum'r.um, the ~artlclllar s~ecific f;ow
-25-
.
CON-31-M
i OS ~ 5 ~ S
rate of metal within the filter should not exceed 35 kg. of
metal per dm2 (square decimeters) of filter cross section per
minute and preferably should be less than 21 kg. per dm2 per
minute. Hlgher flow rates through the filter than indlcated
above results in the filters passing too many undesirable
nonmetalllcs for the production of a high grade sheet product. -
The lower limit is governed by practical size considerations
and would require an impractically large filter to handle bulk
metal flow rates in excess of 450 kg. per minute, that is, a
ceramlc filter greater than 114 cm square or 130 dm2 would be
required. A typical filter of the present inventlon may,
therefore, be defined as being 40 cm square or about 16 dm2 ;
designed to pass 225 kg. of metal per minute at a specific
~low rate of 14 kg. per dm2 per minute.
In addition to the foregoing, the quality of the input
metal is an important variable. If the input metal is -~
particularly dirty, and if the preferred, relatively finer
foam is used, it will quickly block the ceramic fiIter of the
present invention. The output quality, that is, the resultant
filtered metal is a function of input quality. Hence, a
minimum input quality should be provided in order to assure
a good ouput quality. In order to lnsure a good output ;
quality, in accordance with the present invention one may
preliminarily filter the molten metàl through a relatively
coarse ceramic foam filter and optimally utilize a series of
foam filters of decreasing porosity. Thus, as indicated
hereinabove, in accordance with the present invention a
typical preliminary fi'tration step would utilize a
relatively coarse ceramic roam filter having a oore si~e of
between 2 and 8 pores per linear cm, ~ir permeabil~ties of
.. . .. ..
, CON-31-M
S'~5;~5
2500 to 800o x 10-7 cm2, porosities or void fractions of
between 0.90 and 0.95, bulk densities of between 0.20 and 0.35
and thicknesses of 10 to 100 mm. A series of such filters
having decreasing permeability is particularly suitable.
Alternatively, a single preliminary filter or a single fiiter
of the present invention may be utilized having a gradation
o~ properties from coarse (high permeability) to fine (low
permeability) through its thickness may be used.
In accordance with the present invention the specific
features t,hereof will be more readily understandable from a
consideration of the following illustrative examples.
EXAMPLE I
A polyurethane foam was provided having a thickness of
5 cm. An aqueous ceramic slurry was provided having the
following composition: 47% A12O3, 13% Cr2O3, 3-5% kaolin, 1%
bentonite and 14-1/2~ colloidal aluminum orthophosphate added
as an aqueous solution with an equal amount of water. The
slurry contained 82% solids and 18% water. The foam material
was immersed in the slurry and kneaded to remo~e air and
substantially fill voids with the slurry and also to coat the
fibrous webs of the foam with the slurry. The resultant
impregnated foam was removed from the slurry and sub~ected to
compression through preset rolls to squeeze approximately 80~o
of the slurry out of the foam by passing the impregnated foam
through the preset rollers. The f'oam material sprung back to
its original dlmension,after passing through the rollers and
had the fibrous polyurethane filaments coated with a
substanti 211~ uniform re~idue of the ceramic 31urry. The
materia' W2S o~en d~ied at 125C for 1 hour~ heated slowly at
a heat up ra'.e of 0.5C per .~inute to 500C to drIve off the
_28-
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'
CON-31-M
~! lQ5~ S 3S
water and allow the polyurethane flbers to ~olatilize and/or
burn out without collapsing the ceramic and wlthout destroying
the filamentary ceramic configuration. The foam was held at
500C for 1 hour and was subsequently heated to 1350C at a
rate of 1C per minute, held at 1350C for 5 hours to permit '
the ceramic to sinter together and thereby provide an open
cell ceramic foam material having a configuration o~ the
origlnal polyurethane foam material. The characteristics of
the resultant foam were as follows:
Permeabil'ity 1425 x 10-7 cm2
' Porosity 0.87
Pore Size 12 pores per linear cm
Thickness 5 cm
Structural Uniformity Excellent
- EXAMPLE II
A 22,500 kg. charge of aluminum Alloy 5252 contalning
2.3% magnesium, 0.04% silicon, 0.05% iron and 0.06% copper
was melted in a gas fired open hearth furnace and ~luxed with
chlorine gas according to conventional practice. This
unfiltered metal was then cast into three 50 x 135 cm cross
section rolling ingots weighing 4,500 kg. each.
A second charge of the same composition was melted and
prepared for casting in accordance with the same practice
except, however, the metal was passe'd ~ith a rate of 14 kg/
dm2~min. through the ceramic foam filter prepared in Example
I lnstalled in the pouring trough be~ore the metal was cast
into rolling ingots. A metal head of only 15 cm was required
to prime the fllter and during the casting the running head
loss built up from 0.3 - 2.5 cm after 13,500 kg. of metal had
been ~ ered l~.eret'nrough.
~ .
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!s
CON-31-M
i~5~535
Pressure filter tests were run on metal taken from
upstream and downstream of the filter during the castlng of
the filtered metal and from the pourlng trough durlng the
casting of the unfiltered metal. The flltratlon affect of the
ceramic foam filter of the foam of the present invention proved
to be excellent. Cross sectlons of the pressure filter disc
from the unfiltered metal were compared with cross sections
of the pressure filter disc from the filtered metal. These
cross sections clearly showed that little or no residue was
present in the metal filtered with ceramic foam filter of the
present invention; whereas, considerable amounts of residue
e~isted in the cross section of the pressure filter disc from
the unfiltered metal. Similarly, a pressure filter disc was
obtained upstream of the ceramic foam filter and resulted in
the presence of considerable residue therefrom. This clearly
shows the effectiveness of the ceramic foam filter of the
present invention.
The pressure filter test described above is a method of
concentrating and examining the nonmetallic particulate in a
9 to 11.3 kg. sample of molten aluminum. The molten aluminum
in this test is carefully ladled into a preheated 11.3 kg.
clay graphite crucible into the base of which is set a 30 mm
diameter, 3 mm thick porous silica disc plug. 90% o~ the
metal is forced through the disc by application of air pressure
and the remaining metal solidified in situ. The disc and -
ad~acent metal are then sectioned, polished and examined by
normal metallographic techniques to reveal the quantity of
nonmetallics filtered o~t.
EXAMPLE III
The lngots prepared in E~ample II were all rolled down - -
-3~-
.
:~ . .. . . .
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CON-31-M
105~3S
to 0.75 mm gage and samples o~ the sheet from coil locations
corresponding to the head and butt sections of the lngots were
tested to reveal the incldence of llnear defects per unit area
of sheet. The sheet manufactured from flltered metal was
found to contain about 10 times fewer linear defects than did
the sheet manufactured from unfiltered metal as indicated in
Table I below. Thls is a further strong indication of the
effectiveness of the ceramic foam filter of the present
inventlon.
In addition to the foregoing, the spent ceramic foam
filter utllized in Example II was examined metallographically.
Considerable oxide stringers and fine nonmetallic particulates
were found to be captured in the web of the filter, showing
~urther evidence of the filtration power of the filter of the
present invention.
In addition, mechanlcal properties and composition were
tested in metal filtered in accordance with the present
invention. Good mechanical properties were obtained and no
metal contamination was found to exist in the resultant
product due to the use of the ceramic filter of the present -
invention.
EXAMPLE IV
A further 22,500 kg. melt of aluminum Alloy 5252 was
prepared as ln Example II. In this case, the metal was
filtered through a tabular alumina bed in accordance with
conventional practice prior to casting into rolling ingots
for comparative purposes. The ingot thus cast was rolled
down to 0.75 ~m gage sheet and samples were removed at
locations in the resultant coil corresponding to the ingot
butt~ 2 location 50 cm ~rom the ingot butt and the ingot head.
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CON-31-M
lVS9~35
The samples were then tested to reveal the lncldence of llnear
defects. The resu]ts of thls inspection are shown in Table I
below together wlth slmllar data carrled out on unfiltered
metal and ceramic foam filter metal from Example II. The
results in this table are given as percentages using the
unfiltered metal as basis for comparlson.
TABLE I
Comparative Llnear Defect Counts
Type of Metal Butt 20" From Butt Head
Unfiltered Metal 100% 100% 100%
Bed Flltered Metal 150% 25% 10%
Ceramic Foam Filter Metal 10% 10% 10%
These results clearly show the reduced quality of the butt
reglon of the ingot produced by filtration with the commercial
bed filter as compared with that produced by the filter of
the present invention.
EXAMPLE V
An addltional 22,500 kg. charge of aluminum Alloy 5252
was prepared as in Example II. The metal was passed through a
ceramic foam filter prepared in Example I, with the filter
being installed in the pouring trough in a manner similar to
that described in Example II. The filter used was identical
to that of Example I except that the available area of the
filter was cut by two thirds with a result that the specific
flow rate of metal withln the filter was 42 kg. per dm2 per
minute ~hich is higher than the maximum specified flow rate
of the present invention.
The resultant ingots were all rolled down to 0~75 mm gage
and samples were removed and tested to reveal the incidence
of linear defects in the manner of Example I~. The results of
' -32-
CON-31-M
1055~535
this lnspection are set out ln Table II below and compared to
equivalent inspections carried out on unflltered metal and
metal filtered through the ceramic ~oam as in Example II at a
flow rate of 14 kg. per dm2 per minute. The results are glven
as percentages using the unflltered metal as a basis for
comparison.
TABLE II
Comparative Linear Defect Counts
Type of Metal Butt20" From Butt Head
Un~lltered Metal 100% 100% 100%
Ceramic Foam Filtered Metal
(14 kg./dm2~min.) 10% 10% 10%
Ceramic Foam Filtered Metal
(42 kg./dm2/min.) 150~ 50% 150%
The foregoing clearly demonstrates the reduced product quality
obtained when one exceeds the specified flow rate range of the
present invention.
EXAMPLE VI
:,
A further 22j500 kg. charge of aluminum Alloy 5252 was
prepared as in Example II. The metal was passed through a
ceramic foam filter prepared in accordance with Example I
installed in the pouring trough in a like manner to that
described in Example II. The filter was similar to that of
Example I in composition, thickness and structural uniformity
but di~fered in the following respects:
Permeability 2140 x 10-7 cm2
Porosity 0.92
Pore Slze 8 pores per linear cm
In addition, the specific metal flow rate in the filter was
about 28 kg. per dm2 per minute. Thus, the physical properties
of the filter and metal flow rate are within the present
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,r.~. . . . . .
CON-31-M
lOS~535
lnventlon, but outside the preferred range in terms of
permeability, porosity, pore size and speciflc metal flow
rate in the ~ilter.
The ingots thus prepared were all rolled down to 0.75 mm
gage and samples from the coils at locations corresponding to
the heads and butts of the ingots were tested to reveal the
incidence of linear defects in a like manner to that described
in Example IV. The results of this investigation are set
forth in Table III below and are compared to the results of
equivalent inspections carried out on unfiltered metal and
; metal ~iltered through the pre~erred ceramic foam of Example
I. The results are given as percentages using the unfiltered
metal as a basis for comparison.
TABLE III
Comparative Linear Defect Counts
Type of Metal Butt Head
Unfiltered Metal 100% 100%
Ceramic Foam Filtered Metal
(12 p/cm filter - 14 kg./dm2/min.) 10% 10%
Ceramic Foam Filtered Metal
(8 p/cm filter - 28 kg./dm2/min.) 40% 60%
Thus, the foregoing clearly shows that the use of the filter
within the broad specifics of the present invention resulted
in an approximately 50% decrease in the incidence of linear
- defects as opposed to a 90% increase when operating in
accordance with the preferred speci~ication of the present
invention.
In accordance with the present invention, further
advantages and features of the above method o~ preparing
ceramic ~oams will be more readily understandable from a
consideration of he following illustratire examples.
'
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CON- 31-M
~OS9S35
EXAMPLE_VII
A polyester polyurethane foam material was provided havlng
a thickness of 5 cm, containing 12 pores per llnear cm and
having an air permeability of 4,600 x 10-7 cm2. An aqueous
ceramic slurry comprising 47% alumina, 13% chromia, 3.5%
kaolin, 1% bentonite, 29% of a 50% aqueous solution of
aluminum orthophosphate (= 14.5% aluminum orthophosphate) was
mixed in an 80-quart Hobart mixer at 60 rpm for 1 hour. After
1/2 hour of mixing a sample was removed for viscosity testing.
This sample showed that the slurry had a viscosity of 25.5 x
103 cps at 25C measured with a #6 spindle, Brookfield RVT
Vlscometer at 20 rpm after 20 minutes testing. A sample of
the foam material was immersed in the slurry and repeatedly
compressed and expanded with a plunger device for about 30
seconds while the bath of slurry was vibrated at 50 cycles
- per second in order to fill the voids with slurry. The foam
sample thus impregnated was taken from the slurry and passed
through grit-coated rolls preset to provlde a 70% reduction
in thlckness to expel the excess slurry. The rolls were of
76 mm diameter and were driven at a speed of 12.5 rpm. The
sample exhibited substantlally complete spring-back after
rolling was completed.
The sample was then dried in an oven at 65C for 1 hour
and at 95C for 2 hours. The dried sample was then heated
from 95C to 260C at 56C/hr., then ~o 315C at 11C/hr.,
and to 345C at 56C/hr. and held at this temperature for 4
hours in order to remove the polyurethane fibers without
collapsing the ceramic web. The slow heating rate from 260C
to 315C was needed to prevent a sudden temperature excursion
res~11ting from oxidation of the polyureth2ne.
;
J~
CON-31-M
1C)59S35
The resulting sample was then fi.red in a klln using
heatlng rates of 56C/hr. to 540C and 224C/hr. to 1315C
followed by a ~urnace cool.
The flred sample proved to be sound and the surface was
resistant to spalling. Its permeability was measured as 400
x 10-7 cm2 and its bulk density 0.74/gm/cm3. The body
possessed good physical strength and its modulus of rupture
was found to be 17.6 g/mm2. However, sectioning the body
revealed that it possessed a non-uniform structure, as it
suffered from a center line blockage which would preclude its
use in many applications such as filtration of molten metal.
EXAMPLE VIII
Another sample was prepared in accordance with the
practice employed in Example ~II, with the exception that a
variation in the percent reduction in rolling was made. In
the present example, the roll gap was set to give an 86%
reduction, comprising a significant increase over that of
Example VII.
The fired sample appeared to be sound but possessed
rather weak surfaces and edges which made it susceptible to
spalling in rough handling. Its permeability was measured as
1,600 x 10-7 cm2 and its bulk density as 0.39 gm/cm3. The
body possessed adequate physical strength and its modulus of
rupture was found to be 12.7 g/mm2. Sectioning the body
revealed that it was quite uniform although the outer fibers
were somewhat finer than those of the bulk which explained its
characteristic weak surfaces. This reasonably uni~orm body
would be suitable for applications where surface strength is
of lesser importance compared to overall unlformity, and does
not comprise a drawbac~. The ~ragility of the body, however,
-36-
CON-31-M
1~5g535
would render lt unsuitable for use as a fllter, partlcularly
of molten aluminum.
EXAMPLE IX
A further sample prepared in acccrdance with the practice
described in Example VII was sub~ected to a two pass rolling
schedule employing, respectively, roll reductions of 75%
(first pass) and 86% (second pass). The above double pass
schedule was within the purview of this invention.
The fired sample proved to be sound and possessed strong
surfaces and edges. Its permeabllity was measured as 1,700
x 10-7 cm2 and its bulk density as 0.41 gm/cm3. The body
possessed excellent strength and its modulus of rupture was
found to be 19 g/mm2. Sectioning the body revealed that it
was exceptionally uniform having a moderate number of blocked
pores evenly distributed through its bulk. This uniform,
strong sample would be suitable for critical applications such
as filtration of molten metal.
EXAMPLE X
An additional sample was prepared with a polyester
polyurethane foam possessing a thickness of 5 cm, containing
12 pores per linear cm and having an air permeability of
6,ooo x 10-7 cm2. The ceramic slurry employed was of the
same composition as in Example VII, but possessed a viscosity
of 31 x 103 cps due to some water addition.
The foam sample was impregnated in the manner disclosed
in Example VII. Expulsion of slurry was carried out with a
double-pass rolling schedule, wherein the first pass was set
at a reduction of 62%, and the second pass at a reduction of
86%.
The resulting sample was dried ard fired, an~, on
~7
'''' .
:: , . .
CON-31-M
S3~
inspection, was observed to have an air permeability of 1,700
x 10-7 cm2. The sample possessed a uniform structure which
was free from surface fragility and center llne blockage, and
was therefore suitable for use as a molten metal filter.
EXAMPLE XI
.
An additional sample was processed in accordance with
the procedure of Example VII to further lllustrate the
present $nvention. A polyester polyurethane foam was employed
which dlffered from the previous samples in possessing an air
per~.eability of 4,700 x 10-7 cm2. Likewise, the ceramic slurry
was the same with the exception that its viscosity was 25 x
103 cps.
As noted, processlng was identical to Example ~I~, with
the difference that expulsion was conducted with a double-
pass rolllng schedule wherein the first pass was performed to
an 84% reduction, while the second pass was set at 86%.
Upon drying and firing, the resulting sample was likewise
sound and uniform, and free from both surface weakness and
center line blockage. The sample possessed a permeability of
2,650 x 10-7 cm2, and is considered suitable for use as a
filter of molten metal.
Unless otherwlse specified, all percentages expressed
herein are in terms of percent by weight.
; As indicated hereinabove, the present invention provides
considerable advantages in the filtration of molten metal,
especially aluminum. Thus, for example, the present invention
enables one to filter molten metal with a conveniently
removable filter plate which may be easily and quickly
inserted in the filtration apparatus and easily and
conveniently removed therefrom. In accordance with the
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CON-31-M
l~)SS~S35
preferred embodiment of the present lnventlon when a ceramic
foam filter plate is utllized, extremely high flltratlon
- efflciencles are obtained and these efficlencies are obtained
utillzlng a dlsposable ~ilter plate which can be easlly and
convenlently inserted and removed ~rom the filtration
apparatus, as will appear from the accompanying drawings
showing as examples varlous appllcations of the ceramic foam
filter and shapes of said filter.
Figure 1 is a top view of a filter chamber including the -
filter plate ln place therein substantially horizontally
dlsposed;
Figure 2 is a sectional view along the lines II-II of
Figur~ l;
; Figure 3A is a perspective view of the filter plate as
shown in Figures 1 and 2;
Figure 3B is a perspectlve view of a modifled filter
plate similar to the filter plate shown in Figure 3A;
Figure 4 is a top view of another embodiment of the i
fllter chamber wherein the filter plate is substantially
vertically disposed;
Figure 5 is a sectional view along the lines V-V of
Figure 4;
Figure 6 is a perspective view of the filter plate as
shown ln Figures 4 and 5; and
Figure 7 is a sectional view of another embodiment
wherein the filter plate is disposed above an individual
. . .
pouring spout.
' A filter chamber is illustrated in Flgures 1 and 2, as
.. }
in a molten metal transfer system, pouring pans, pouring
troughs, transfer troughs, metal treatment bays, or the l~ke.
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CON-31-M
55~5;~5
The filter apparatus 2 may lf deslred be constructed ln two
sections 2a and 2b which may be bolted together by any
sultable means, such as by flan~es at the peripheries thereof,
not shown. The particular filter apparatus ill~strated in
Figures 1 and 2 is a transfer trough containing a central
filter chamber 3 fed by inlet 4 with the metal passing out of
the filter chamber vla outlet 5. The molten metal may enter
the inlet 4 by any suitable means, such as pouring spout 6.
` The filter chamber 3 is a bowl shaped chamber, the bottom of
which is recessed below the level of the inlet 4 so that the
molten metal passing into the filter chamber 3 may travel
downwardly through the filter plate of the present ~nvention
in place in'the ~ilter chamber. Thus, the filter chamber 3
is characterized by a peripheral rim 7 which may completely
surround the upper portion of the filter chamber. As shown
in Figure 1, the filter chamber rim, 7 surrounds the filter
chamber on all si~des except ad~acent the area of inlet 4. The
filter chamber rim 7 is connected to side wall 8 which~extends
downwardly to filter chamber floor 9 which has a circumfer-
ential bevelled portion or aperture 10 (Figure 2) extending
around the periphery thereof to mate with the bevelled wall
surface of the filter plate. The filter plate 11 has a
corresponding bevelled peripheral surface 12 adapted to mate
with the bevelled wall surface 10 of the filter chamber. The
bevelled peripheral surface of filter 12 is provlded with a
resilient sealing means 13 thereon resistant to the molten
metal, and the filter plate 11 and sealing means 13 are
' inserted in the filter chamber 3 so that the filter plate-
sealing means assembly engages the bevelled wall surface of
the filter chamber.
,
_40_
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CON-31-M
535
Thus, as shown in Figures 1 and 2, the filter 11 is
substantially horizontally disposed in a trough. The filter
as shown has a square configurationj however, any convenlent
shape may be readlly employed for the filter, such as round,
hexagonal or the like. The filter plate ll is positloned ln
a recessed section of a filter chamber or filter bowl 3, such
as in the floor portion 9 thereof. Molten metal is fed to
the filter ll via inlet 4 into the filter chamber 3. The
molten metal passes downwardly through filter ll lnto the
recess 14 beneath the fllter plate 11. The filter ll is
sealed in place by means of resilient seal 13 so that the
filter plate may be readily inserted by pressure vertically
downwards and easily removed by pressure vertically upwards.
Alternatively, as indicated above, the filter chamber may be
:t split and moved laterally for positioning the gasketed or ;
sealed filter plate in the filter chamber, with the filter
v plate held therein with a vise type action. Preferably, the
bevelled peripheral surface of the filter chamber 10 is - -
bevelled at an angle of from 2 to 20 and the filter plate is
preferably provided with a bevelled surface 12 corresponding
thereto at an angle of from 2 to 20. The filter plate ll is
preferably substantially horizontally disposed at an angle of
` from 1 to 5 upwardly sloped towards the metal outlet 5 in
order to prevent entrapment of air against the underside of the
filter. In addition, the floor 15 of recess 14 beneath filter
plate 11 is preferably sloped downwardly at an angle of from
l to 5 sloped towards outlet 5 in order to facilitate drain-
age of metal during operation and at the completion of the
pouring or transfer operation.
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CON-3l-r~
105~S~
Alternatively, ~f desired, the filter chamber may be ~plit
horlzontally, 2S along a horizontal plane beneath floor 9, or
on an angle beneath floor 9, particularly to enable easy
cleaning of recess 14. It may be deslrable to reverse the
direction of the taper of bevelled portion 10, effecting a
positive seal by means of the wall portion in recess 14.
As shown in Figure 3A, the filter plate of the present
lnvention 11 has a bevelled peripheral surface 12 adapted to
mate with a bevelled surface 10 of the filter chamber.
Naturally, variations in design are contemplated within the
scope of the present invention, such as shown in Figure 3B
wherein a corresponding flat surface 16 is provlded around
the entire perlphery of filter plate 11 ad~acent bevelled
surface 12. Figures 3A and 3B show filter plates wherein the
bevelled sur~ace extends around t'ne entire periphery of the
plate; however, it may be more convenient to provide that the
bevelled surface extends around less than the entire periphery,
as in Figure 6 wherein the bevelled surface extends around two
faces of the plate.
Thus, it can be seen that the filter plate of the present
invention may be conveniently utilized in a variety of
locations, including pouring pans, pouring troughs, transfer
troughs, pouring spouts and metal treatment bays. The filter
should not be placed in the immediate vicinity of turbulent
molten metal flow, especially where such turbulence results
in oxide formation and entrainment. This is true for the case
- of turbulence both upstream and downstream of the filter.
Turbulence upstream o~ the filter with attendant oxide
entrainment tends to lead to channeling of the ~llter,
inefflcient filtration and in severe cases premature blockage
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CON-31-M
``~ 11)5~S35
of the filter. Turbulence downstream of the fllter will tend
to undo the ~ood rendered by the filter and once agaln load
the molten metal with oxide or other nonmetallics which are
present or are formed on the surface o~ the metal. Frequently
- encountered sources o~ turbulence are Yurnace tap holes,
pouring spouts and other devices whlch cause rapld changes in ;
flow cross section and consequent high velocity gradients. `!
Naturally, the particular filter lnstallation must be chosen
with care to ensure that lt too does not become another source
of turbulence. The foregoing considerations of turbulence are,
of course, particularly relevant to chemically reactive metals,
such as aluminum and magneslum and their alloys which readily
oxidize on contact with air, however, these considerations are
.j, .
also significant for less reactive metals, such as copper and
; its alloys. Naturally, devices are available in the art to
mitigate turbulence~ as, for example, appropriately placed
vanes.
As can be seen from the foregoing, the filter plate of the
present invention may be conveniently disposed substantially
horizontally. In addition, i~ desired, the filter may be
.
disposed substantially vertically or at an intermediate
angle with respect to the flow of molten metal. The vertlcal
disposition of the filter plate of the present invention wlll
: be described in a specific embodiment hereinbelow. The
;,
horizontally disposed filter, however, has two principal
advantages over a vertically disposed filter. First, by
utilizing the significant length of a pouring trough or length
and breadth of a pouring pan a large filter area can be easily
and conveniently accommodated. On the other hand, in order to
accommodate a large filter area in a vertical installation,
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CON-31-M
~os95
one must generally resort to a deep trough, pan or tundish.
Such geometrlc constraints are often of great practical
signiflcance, especially where adaptation of existing casting
facilities is contemplated. A second advantage of a
horizontal filter is the fact that it ensures that all parts
of the filter will prime under substantially the same
metallostatic head; whereas, a vertlcal filter will naturally
prime under a head varying from top to bottom. For this
reason naturally a vertical filter primes in a non-uniform
manner. The upper portlons o~ a vertical filter will not pass
as much metal as the lower portions thereof and in fact may
; not prime at all. Furthermore, changes of head upstream of
the filter will have more effect on a vertical filter than on
a horizonta~ filter and may result ln momentary or significant
loss of prime to the upper portions of the filter and even
freezing off thereof. To minimize loss of primed area in
operation, a filter should remain buried under a minimum head
of molten metal. This is easier to achieve with a horizontal
filter than with a vertical one. Also, exposure of unprimed
areas of a vertical filter above the metal line can result in
` cracking of the filter due to thermal stress from the high
temperature gradients obtaining under such conditions. For
the foregoing reasons, a horizontally or substantially
horizontally disposed filter is preferred in the present
invention.
A disadvantage, however, of horizontally placed filters
is that air can be trapped beneath these filters. This in turn
can lead to oxide formation downstream of the filter and to
channeling of ~low through the ~ilter and, hence, less than
optimum filtration. This type OL entra2ment is obviated by a
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~ON-31-M
lQ5~535
vertical disposltion of the fllter.
In accordance wlth the present inventlon, it has been
found that the foregoing aisadvantages of horlzontally dls! ;~
posed fllters can be greatly minlmized whlle retainlng
essentially all advantages of a horizontally disposed filter
by canting the filter at a small angle of from 1 to 5 to the
horiæontal. Such a dlsposltlon allows escape of the air
during lnitlal primlng of the filter without relinquishlng the
uniform or substantially uniform depth of lmmerslon of the
filter body resultlng from horlzontal placement. Preferably,
as shown in Figure 2, the high point on the horlzontally
dlsposed fllter surface should be at the extreme downstream
end of the filter so that the escape of air is augmented by
the sweeping action of the metal str~am. It can be seen that
the feature of a substantlally horizontally disposed, upwardly
' sloped fllter is highly advantageous so that excellent results
may be obtained u~ing said feature without the use of a
bevelled peripheral surface, as, for example, using a spllt
; filter chamber and holding the filter therein wlth a vise type
; 20 action. Naturally, an appropriate sealing means should also
- be used and also the floor 15 of recess 14 beneath filter
plate 11 ls preferably sloped as described above. -
Figures 4 and 5 show a vertically dlsposed filter
. . .
installation in a transfer trough according to the present
~; inventlon. In the embodiment of Figures 4 and 5, a filter
plate 20 ls held in place by a refractory dam 21 and
positloned ln a slot 22 in a filter chamber 23. Molten metal
is fed to the fllter chamber 23 via inlet trough 24 and passes
horizontally into fllter chamber well 25 and thence through
; 30 filter plate 20 into outlet trough 26. Filter plate 20 is
:
,
~(~5~S~S
sealed into slot 22 by means of a ceramic fiber gasket 27 which
completely circumscribes the filter plate 20, The pregasketed
filter plate 20 and dam 21 are placed into the slot 22 and
sealed in place by means of wedges 2~, A drain hole 29 is
provided to drain well 25 of metal at the completion of
pouring or transfer. In operation, the drain hole 29 may be
closed by a stopper rod or other convenient closure means,
not shown,
The filter plate of the present invention, as filter
plate 20, is a frustum or segment of a solid figure with
sloping sides so that the peripheral surface thereof has a
bevelled configuration, Filter chamber well 25 has a corre-
sponding bevelled wall surface 30 (Figure 4) to mate with the
bevelled peripheral surface 31 of the filter plate (Figure 6),
Filters up to several mm thick and several square meters in
area can be conveniently located in troughs in the foregoing
manner, The dam 21 and the filter chamber 23 may be made of
conventional materials of construction, The filter well 25
and corresponding trough linings may be conveniently prepared
of castable refractory or ceramic tile, The dam 21 and wedges
28 may be made of refractory boards such as MARTINITE if the
metal to be filtered is aluminum or some lower melting alloy,
MARTINITE is a trademark which stands for a refractory material
having the chemical composition ~a5H2(P04)4, Naturally, the
sealing means 27 is preferably adjacent the bevelled filter
plate surface 31; however, às shown in Figures 4-6 where the
filter plate is bevelled on only two (2) peripheral faces
thereof, the sealing means is preferably adjacent all peripheral ;
surfaces of the filter plate including non-bievelled peripheral
surfaces,
Figure 7 shows an example of a horizontally disposed
frustoconical filter plate installation designed to feed a
- 46 -
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.
CO~-31-M
iOS~S35
single feeding pouring spout. In this unit, filter plate 40
ls located in a recess 41 in the refractory base 42 of a
pouring pan or tundish 43. Durlng casting metal ~rom pan 43
flows vertically through filter plate 40 lnto channel 44
beneath filter plate 40 and thence into pourlng spout 45
feeding an ingot or casting below. The fllter plate is
provided wlth a bevelled peripheral surface 46 for matlng wlth
a corresponding bevelled surface 47 in recess 41. A resilient
sealing means 48 is provided between the corresponding
bevelled surfaces so that the presealed filter 40 is positioned
and sealed in place by pressure from above in a manner similar
to the preceding embodiments. Preferably, some means should
be provided for ventlng alr bubbles from the bottom of the
fllter.
This invention may be embodied in other forms or,carrled
out in other ways without departing from the spirit or
essential characteristics thereof. The present embodiments
are therefore to be considered as in all respects illustratlve
and not restrictive, the scope of the lnventlon being indicated
by the appended claims, and all changes which come within the
meaning and range of equivalency are intended to be embraced
therein.
', '
. .
..
,~
, - ~.
; -47-
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