CA1053267A - Molten metal filter - Google Patents

Abstract

The present invention resides in an improved molten metal filter and a method of preparing same and a method of filtering molten metal therethrough. The filter is an open cell ceramic foam material having a plurality of interconnected voids surrounded by a web of said ceramic.

Description

~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.
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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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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
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In accordance with the present invention it has now been found -that the foregoing ob~ects and advantages may be readily achieved.
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The present invention provides a highly e~ficient method of filtering molten metal, especially aluminum, through a disposable 1 .
; 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 , .: . . . : , 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 : ~ ~ - 4a -: - - : . . -:

` 1~5;~'~67 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 .~ .
prepare, it is quite feasible to use these filters on throw away basis.

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~ O 5 3 ~ 7 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 ' ` ! 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-~ . , .
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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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 : ~ .
rllter Or the present inventlon.
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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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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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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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Claims (10)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. 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 same size, said ceramic foam material being formed by impregnating an open cell flexible organic foam having a plurality of 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.

2. A method according to claim 1 wherein said molten metal is aluminum or an aluminum base alloy.

3. A method according to claim 1 wherein said molten metal is copper or a copper base alloy.

4. A method according to claim 1 wherein said ceramic is selected from the group consisting of alumina, chromia, zirconia, magnesia, titanium dioxide, silica and mixtures thereof.

5. A method according to claim 1 wherein said ceramic foam material has a thickness of from 1/4" to 4".

6. A method according to claim 1 wherein said molten metal is poured through said ceramic foam material at a rate of from 30 to 100 cubic inches per square inch of filter area per minute.

7. A method according to claim 1 wherein said ceramic foam material is placed in a downspout.

8. A method according to claim 1 wherein said ceramic foam material is placed in a transfer trough between a furnace and a casting mold.

9. A method according to claim 1 wherein said ceramic foam material is prepared by heating said dried coated foam to burn off the web of flexible foam and to sinter the ceramic coating.

10. A method according to claim 9, wherein said heating is carried out in two stages, with the first stage being to slowly heat at a rate less than 10°C per minute to a temperature of from 350 to 700°C 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 being to heat at a rate less than 100°C per minute to a temperature of from 1200 to 1600°C and hold within said temperature range for from 15 minutes to 10 hours to sinter the ceramic.