Note: Descriptions are shown in the official language in which they were submitted.
3084-MAL
~05~7
BACKGROUND OF THE INVENTION
Molten aluminum in practice generally contalns entralned solids
which are deleterlous to the final cast metal product. These - -~
entrained solids usually deri~e from three sources. Some are
partic~es o~ alumlnum oxide which are drawn into the liquid stxeam
from the floating oxide layer on its surface, and some entrained
particles are ~ragments of furnace llning, transfer trough and other
portlons of the molten aluminum handling equipment which are eroded
and entrained in the flowing aluminum stream, and some particles are
precipitates Or insoluble impurities such as intermetallics, borldes,
carbides or precipitates of other aluminum compounds, such as
chlorides. When these inclusions appear in the final cast product
~ter the molten aluminum ls solidifled, they cause such ~inal
- product to be less ductile or ~o have poor ~inlshing characteristics.
Accordingly, it is desirable to remove entralned solids from the
molten aluminum stream before it is cast into a solid body which may
be used as such or sub~e¢ted to forming operatlons such as rolling,
` forging, extruslon, etc.
I
Filtering processes to remo~e entrained solids from l~qulds are ~ -
accomplished by passing the solid-laden liquid through a porous ~llter
medlum that wlll not pass the sol~ds. Filtering molten metal ln
general, and mol~en aluminum in particular, creates special pro~lems
because the llquid is so aggressive that i~ is difficult to find a
filter medium capable of withstanding it. ~
In general, two methods o~ filterlng are used for removing
entrained solids from molten aluminum alloys before casting. The
~ ~ most common fllter medlum ls an open~weave glass cloth screen placed
i in the metal transfer trough, around the spout or even in the molten
metal pool in the top of the solldifyln~ ingot. These qloth screens
are able to remove only the larger sizes of lncluslons from the
'~ metal and are easlly ruptured during use because the glass flbers ~ -
'~ become very weak at the temperature o~ molten~alumlnum. In another ~
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3084-MAL
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prior art procedure, molten alumlnum is flltered through a bed of
loose alumlna particles, for example, of tabular alumina, but lt
often suffers from the drawbacks normally associated with bed
filters in that it passes too many solids, there ls a strong
tendency to channeling which prevents efficient use, and pore size
o~ the f~lter i5 not easily controlled but rather readily changes
under conditions of use 50 that, even when originally o~ proper
dimension, it cannot be efriciently maintained. In additlon, the
metal must be kept constantly molten when the fllter is not ln use.
Accordingly, it is a principal ob~ect of the present invention
to provide an improved molten metal ~ilter and a method for preparing
same and also a method for filtering molten metal therethrough.
It ~s an additlonal ob~ect of the present invention to provide
a filter and method as aforesaid which is inexpensive so that it may
readiiy be used on a throw away basls.
A further ob~ect of the present invention is to obtain a filter
and method as aforesaid which obtains high filtration efficiency.
Further ob~ects and advantages of the present invent~on will
appear hereinafter.
SUMMARY OF THE INVENTION
.
In accordance with the present invention it has now been found -
that the foregoing ob~ects and advantages may be readily achieved.
.
The present invention provides a highly e~ficient method of
filtering molten metal, especially aluminum, through a disposable
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; filter characte~ized by being a ceramic foam material having an open
.
~ cell structure with a plurality o~interconnected voids surrounded
by a web of sald ceramic. The molten metal is poured through the
ceramic foam material at a rate o~ from 5 to 500, and preferably
30 to 100, cubic inches per ~quare inch of filter area per minute, ~-
thereby removing entrained ~ollds from the molten metal. The filterof the present lnvention is prepared by:
(A) providing an open cell,~ hydrophilic flexible organlc foam
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5~67
material having a plurality of interconnected
voids surrounded by a web o~ sald material
( B ) impregnating said material with an aqueous ceramic
slurry so that said web is coated therewith and
said voids are filled therewith;
(C) compressing said material to expel from 25 to 75%
of said slurry therefrom while leaving said web
coated therewith;
(D) releasing the compression so that the web remains
coated with said slurry,
( E ) drying said material, and
(F) heating the dried material to first burn out the
. flexible organic foam and then sinter the ceramic
coating, thereby providing a fused ceramic foam
having a plurality of interconnected voids
surrounded by a web of fused ceramic in the
:. configuration of said flexible foam.
Preferably after step (D) but before step (E) the slurry coated
flexible foam is formed to the configuration re~uired for
filtration and retained in said formed configuration throughout
the drying step (E) and heating step (F), with the forming
restraint released following step (F).
Therefore, the invention provides a low cost method
. of filtering molten metal through a disposable filter which
.: comprises: providing a mass of molten metal; providing a ceramic
; foam material having an open cell structure characterized by a
plurality of interconnected voids surrounded by a web of said
ceramic, said ceramic foam material having a density of less
than 30% of the theoretical density for a ceramic material of the
:30 same size, said ceramic foam material being formed by impregnat~
ing an open cell flexible organic foam having a plurality of
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interconnected voids surrounded by a web of said foam with an
aqueous ceramic slurry so that the web is coated therewith and
the voids are filled therewith, expelling slurry therefrom while
leaving the web coated therewith, drying said coated foam and
heating the dried coated foam to form said ceramic foam ; and
pouring said molten metal through said ceramic foam material at
a rate of from 5 to 500 cubic inches per square inch of filter
area per minute, thereby removing entrained solids from said
molten metal.
The invention also provides a method of preparing a
disposable filter for filtering molten metal which comprises:
(A) providing an open cell flexible organic foam
material having a thickness of from 1/4" to 4"
and having a plurality of interconnected voids
surrounded by a web of said material, whérein
said foam contains from 5 to 100 pores per inch
(B) providing an aqueous ceramic slurry containing
~ from about 10 to 40% water and impregnating said
'~ material therewith by immersing said material in
said aqueous ceramic slurry so that the web is
coated therewith and the voids are filled there- :
: with,
- (C) continuously compressing the impregnated material
and releasing the compression by passing said
impregnated material through preset rollers to
expel from ~5 to 75% of said slurry therefrom
while leaving the web coated therew~th
(D3 drying said material, and
(E) heating the dried mate.rial to burn off the web of
flexible foam and to sinter the ceramic coating
while. avoiding collapse thereof, wherein the dried
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impregnated material is heated in two stages, with
the first stage being to slowly heat at a rate
less than 10C per minute to a temperature of
from 350 to 700C and hold within said temperature
range for from 15 minutes to 6 hours to burn off
the web of flexible foam, and with the second
stage heing to heat at a rate less than 100C per
minute to a temperature of from 1200 to 1600C and
hold within said temperature range for from 15
minutes to 10 hours to sinter the ceramic,
thereby providing a fused ceramic foam having a plurality of
interconnected voids surrounded by a web of fused ceramic in the
configuration of said flexible foam, wherein the resultant fused
ceramic foam has a density of less than 30% of the theoretical
density for a ceramic material of the same size.
In accordance with the present invention it has been
found that it is possible to prepare low cost, porous, ceramic
filtration media for molten metal, especially aluminum, having
densities less than 30% of theoretical and in many cases only
5 to 10% of theoretical. The filter of the present invention
represents an extremely efficient and low cost filter for use in
`~ filtering molten metal. With the extremely high ratios of
available area for filtration at a controlled pore size, a given
projected area of the filter media in inherently much less
; susceptible to clogging by nonmetallic particulate. Since the
filters of the present invention are extremely inexpensive to
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prepare, it is quite feasible to use these filters on throw
away basis.
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. 3084-MAL
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DESCRIPTION OF DRAWINGS
The present invention will be more readily understood from a
consideration of the following illustrative drawlngs in which:
Fig. 1 is a photomicrograph at a magni~ication o~ 400X showing
metal res~due after filtration through the filter of the present
lnvention and after passing the filtered metal through a pressure
~ilter disc; and
Fig. 2 ls a photomicrograph at a magnification of 400X showing
~ metal residue after filtration through a conventional tabular
alumina bed filter and after passing the filtered metal through a
pressure filter disc.
The photomicrographs will be discussed in more detail in the
examples.
DETAILED DESCRIPTION -
In accordance with the present invention the ceramic foam is
prepared ~rom an open cell, hydrophilic flexible foam material
having a ~lurality of interconnected voids surrounded by a web of
said flexible foam materlal. Typical material whlch may be used
include the polymeric foams such as polyurethane Yoams, and the
cellulosic foams. Generally, any combustible organic plastlc foam
may be used wh~ch has resilience and ability to recover its original
shape. The foam must burn out or ~olatillze at below the f~,ing
temperature o~ the ceramic material which is employed. Also, one
should use a foam material having from 5 to 100 pores per inch in
~rder to prov~de the necessary filtration surface. The dimensions
o~ the ~oam material may, of course, be var~ed depending upon the
desired dimenslons of the end fllter ma~erlal. Generally, one
utilizes a foam material havlng a thlckness of from 1/4 to 4", with
~ from 1 to 2" belng preferred. ~
30~ ~- The aqueous ceramlc slurry which is employed depends naturally
on the desired ceramic material for the chosen metal to be flltered.
One must have sufrlcient properties in the flnal product to stand
up to the particular molten metal wlth respect to chemic~l attack
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` ! 105~ 7 3084-MAL
and structural and/or mechanlcal strength to stand up to the
partlcular elevated temperature conditions. In addltion, the s1urry
should have a relatively high degree of fluidity and be comprlsed
of an aqueous suspension of the ceramlc lntended for use ln the
fllter. Typlcal ceramic materials whlch may be employed include
alumina, chromia, zirconia, magnesia, titanium dioxide, silica and
mlxtures thereof. For use wlth molten aluminum and its alloys, an
alumina based slurry is quite satisfactory. For use wlth copper
and lts alloys, elther zirconia or chromia are preferred. Normally,
the slurry contains ~rom about lO to 40~ water. Additives may be
employed ln the slurry such as blnders.
The flexible ~oam material is then lmpregnated wlth the aqueous
ceramlc slurry so that the fiber-like webs are coated therewith and
the volds are filled therewlth. Normally, it ls preferred to slmply
lmmerse the foam ln the slurry for a short perlod of tlme sufficlent
to insure complete impregnation of t~he foam.
The lmpregnated foam is then compressed to expel from 25 to 75%
1 of the slurry whlle leaving the flber-like web portion coated there-
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wlth. In a continuous operation one may pass the impregnated foam
~20 through a preset roller to effect the~desired expulsion of slurry
l! ~ ~from the foam and leave the desired amount impregnated therein.Naturally, this may be done manually by simply squeezlng the
flexible ~oam material to the des,red extent. A~ this stage the
foam is still flexible and may be formed~into configurations
suitable~for speclfic ~iltration tasks, i.e., lnto curved plates,
bollow cyllnders, etc. It 1s neceseary to hold the formed foam ln
posltion by conventional means untll;the organic substrate is
de~composed,~ or pre~ferably until the ceramic is sintered. 'I'he
lmpregnated foam i9 then dried by either alr drying or accelerated
drying at a temperature or~rrom lO0 to 700C for from 15 minutes
to 6 hours. Alr drying may be achieved ln from 8 to 24 hours.
After drylng, the material is heated a~ an elevated temperature
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3084-MAL
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to sinter the ceramic coating on the riber-llke webs. It ls
preferred to heat the dried impregnated materlal in two stages,
with the flrst stage being to slowly heat to a temperature of ~rom
350 to 700C and hold within thls temperature range ror from 15
minutes to 6 hours ln order to burn off or volatilize the web of
~lexible foam. Clearly this step can be part Or the dryin~ cycle,
lf desired. The second stage is to heat to a temperature of from
1200 to 1600C and hold within said temperature range for from 15
minutes to 10 hours in order to sinter the ceramic. It is also
pre~erred to control the heat up rates for each of these stages in
order ~o avoid collapse of the ceramic material. Thus, the heat up
rate ~rom stage one ls preferably less than 10C per minute and the
heat up rate in stage two is preferably less than 100C per minute.
The resultant product is a fused ceramic ~oam having an open
cell 8tructure characterized by a plurality of interconnected volds
surrounded by a we~ of said ceramic, with the ceramic foam material
having a density of less than 30% of the theoretlcal density for a
ceramic materlal Or the same size. Naturally, the ceramic foam may
have any desired configuration based on the configuration needed for
the particular molten metal riltration process. Although, naturally,
these conflgurations can~be many and varied3 semielllptical configu-
ratlon is preferred for ~iltratlon ln a transfer trough between the
~urnace and the casting mold in filtering molten aluminum. A holiow
:1
cylindrlcal configuration is preferred for-~iltering molten aluminum
;
passlng through a down spout. In either case, the height of the
filtration media must exceed that of the mol~en metal to be
~lltered. It is a partlcular advantage Or the ~iltration process
of the present invention that excessive heads o~ molten metal are
- not requlred in order to start the ~iltration process utllizing the
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rllter Or the present inventlon.
;
In accordance with the present invention, the specirlc features
- thereor wlll be more readily understandable ~rom a conslderation Or
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tne followlng lllustrativè examples.
EXAMPLE I
A polyurethane foam material was provlded havlng a thickness Or
1/2" and contalnlng 10 pores per lnch. A ceramlc slurry ln water
wa~ provided containlng 85% alumlna, 15% chromia and 25% water.
The foam material was immersed in the slurry and kneaded to remove
air and fill the volds wlth the 81urry and also to coat the fibrous
webs of the foam with said slurry. The foam thus impregnated was
removed from the slurry and subJected to compresslon to squeeze ~-
approximately 50% of the slurry out o~ the foam by passing the
lmpregnated foam through preset rollers. The foam material sprung
back to lts origlnal dimenslon after passing through the preset
rollers and had the fibrous urethane ~ilaments coated with a
substantially uniform resldue of the ceramic slurry.
Two samples were dried in the following manner. Sample A was
'l air drled for 24 hours and Sample B was oven dried at 125C for
one hour.
~, Both dried samples were heated slowly at a heat up rate of 0.5C
per minute to 500C to boll off the water and then to allow the
~0 polyurethane fibers to volatilize and/or burn out without collapsing
the ceramic and without destroying the filamentary ceramic configu~
ration~. The foam was held at 500C for one~hour and was subsequently
heated~to 13~0C at a rate of 1C per minute, held at 1350~ for~5 ~ ~
hours to permit the ceramlc to sinter together and thereby provide s
an open cell ceramic foam material having a con~iguration o~ the
original polyurethane foam material.
EXAMPLE II
Several ceramic foam~materials were prepared ln a manner after
~, ~ the procedure O~r Example`~I having~the following con~igurations: ,~
6" wlde; 10" long and 1" thicko These mater~als were cemented into
transfer troughs between the ~urnace and the castlng mold for
~: ~ test1ng as a fllter materlal ~or molten alumlnum. Approxlmately
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~ 3084-MAL
lOS3~7
5,400 pounds of aluminum alloy 5252, containing from 2.2 - 2.8%
magnesium, up to 0.0~% sllicon, up to 0.10% iron, up to 0.10%
copper, and up to 0.10% manganese, were transferred through the
rllter at an average rate of about 80 cubic inches per square inch
of rilter per minute. It was surprising that a large head was not
required to start the metal flow. Conventional rigid filtration
medla normally require a head of approximately 1 to 2 ~eet; whereas,
a head of 2-1/2" was required to start the metal flow in the ~rocess
of the present invention.
The filtratlon effect w~s excellent. Fig. 1 sho~s a cross-
section of a pressure filter disc through which had been run
aluminum alloy 5252 after filtration through the filter of Example
I as shown in thls Example II. Fig. 2 shows a similar filter disc
through which had been passed the same volume of alloy 5252 which
had prevlously been filtered through a commercial tabular alumina
bed filter~ The higher the residue in the ~ilter disc shown in
Fig. 1 and Flg. 2, the lower is the efficiency of the previous
- filter. It will be clearly seen that there is more residue ln Fig.
2 than ln Fig. 1, thereby indicating~that the commercial tabular
alumina bed filter is less efficient than the filter Or the present
lnvention.
The pressure filter test is a means of concentrating and
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examining the nonmetallic particulate in a 20-25 lb. sample of
molten aluminum. To this end, molten metal is ladled carefu~ly
lnto a preheated 25 lb. clay graphite crucible into the base of
which is set a 30 mm diameter, 3 mm thick porous silica disc plug.
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90% of the metal is then forced through the disc by appIication of
~` a~r pressure and the remaining metal solidified in situ. The disc
and ad~acent metal are then sectioned, polished, and examined by
~0~ normal metallographic techniques to reveal the quantity of
nonmetallics ~iltered out.
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~OS~7 3084-MAL
EXAMPLE III
A ceramlc foam ~ilter Or the present invention was prepared
in a manner after Example I having the configuration 3-1/2" wlde,
6" long and 1" thick. This filter was cemented.into a transfer
trough between the meltlng ~urnace and the casting mold. Some 1800
lbs. of copper alloy 194, containing from 2.1 - 2.6% iron, from
0.0~ - 0.20% zinc, from 0.01 - 0.04% phosphorus and balance
essentia71y copper, were transferred through the fllter at an average
rate of 35 cubic inches per square inch of filter per minute. A
head o~ 0.75" was all that was required to start metal flow through
the filter of the present invention. Filtration was excellent and
resulted in a lO~ improvement in elongation of the cast metal over
that of un~iltered metal. Tensile strength was not a~fected.
This invention may be embodied in other ~orms or carried out
in other ways without departing from the spirit or essential
.
characterlstics thereof. The present embodiment is therefore to
~ be considered as in all respects il~ustrative and not restrlctlve,
,,,! the scope of the invention being indicated by the appended claims,
,~ and all changes which come within the meaning and range of
equl~alency are intended~to be embraced therein.
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