CA1123207A - Method and apparatus for the filtration and degassing of molten metal - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The present invention comprises a highly efficient degassing and filtration method and apparatus wherein a plurality of sequentially spaced apart filter type media are provided in a filter chamber with a fluxing gas inlet provided therebetween.
In the preferred embodiment of the present invention the first filter type medium is provided with a preferential path for the fluxing gas thus eliminating the undesirable pressure drop across said first filter type medium thereby increasing metal treatment efficiency.

Description

BACKGROUND OF THE INVENTION
The present invention relates to the degassing of molten metal. Molten metal, particularly molten aluminum in practice, generally contains entrained and dissolved impurities both gaseous and solid which are deleterious to the final cast product. These impurities may affect the final cast product after the molten metal is solidified whereby processing may be hampered or the final product may be less ductile or have poor finishing and anodizing characteristics. The impurities may originate from several sources. For example, the impurities may include metallic impurities such as alXaline and alkaline earth metals and dissolved hydrogen gas and occluded surface oxide films which have become broken up and are entrained in the molten metal. In addition, inclusions may originate as insoluble impurities such as carbides, borides and others or eroded furnace and trou~h refractories.
One process for removing gaseous impurities from molten metals is by degassin~. The physical process involves injecting a fluxing gas into the melt~ The hydrogen enters the purge gas bubbles by diffusing through the melt to the bubble where it adheres to the bubble surface and is adsorbed into the bubble itself.
The hydrogen is then carried out of the melt by the bubble~
It is naturally highly desirable to improve the degassing of molten metals in order to remove or minimize such impurities in the final cast product, particularly with respect to molten aluminum and especially, for example, when the resultant ~etal is to ~e used in a decorative product such as a decorative trim or products bearing ~ CON-136-M
3~)7 critical ~pecifications such as aircraft forgings and extrusions and light gauge foil ~tock. Impur1ties as aforesaid cause 108s of properties such a~ tensile strength and corrosion resistance in the final cast product.
Rigorous metal treatment processes such as gas fluxing or melt filtration have minimized the occurrence of such defects. However~ while such treatments have generally been succes'sful in reducing the occurrence of such defects to satisfactory levels, they have been found to be inefficient and/or uneconomical. Conventiona-lly conducted gas fluxing processes such as general hearth flu~ing have involved the introduction of the fluxing gas to a holding furnace containing a quantity of molten metal. This procedure requires that the molten metal be held in the furn~ce for significant time while the fluxing gas is circulated so that the metal being treated would remain constant and treatment could take place. This procedure has many drawbacks, among them, the reduced efficiency and increased cost resulting from the prolonged idleness of the furnace during the fluxing operation and more importantly, the lack of efficiency of the fluxing operation due to poor coverage of the molten metal by the fluxing gas which is attributable to the large bubble size and poor bubble dispersion within the melt. Further factors comprise the restriction of location to the furnace which permits the re-entry of impurities $o the melt before casting, and the high emissions resulting from both the sheer quantity of flux required and the location of its circula~ion.
As an alternative to the batch-type fluxing operations - ~3z~ CON-136-M

employed as aforesald, certain fluxing operations were employed in an inline manner; that is, t~e operation and associated apparatus were located outside the melting or holding furnace and-often between the melting furn2ce and either the holding furnace or the holding furnace ana the casting station. This helped to alleviate the inefficiency and high cost resulting from furnace idleness when batch fluxing but was not successful in improving - the efficiency of the degassing operation itself, in th~t the large size of the units and the undesirably large quantities of fluxing gas required per unit of molten metal were both costly and detrimental to air purity.
A typical inline gas fluxing technique is disclosed in U.S. Patent 3,~37,304. In the aforenoted Patent, a bed of "stones" is positioned in a housing through which the molten metal will pass. A fluxing gas is introduced beneath the bed and flows up through the spaces between the stones in counter flow relationship with the molten metal. The use of a bed of porous "stones" has an inherent disadvantage. The fact that the stones have their pores so`close together results in the bubbles passing through the stones coalescing on their surfaces and thus creatin~ a relatively small number of large bubbles rather than a large number of small bubbles~ The net effect of the bubbles coalescing is to reduce the surface area of bubble onto which the hydrogen can be adsorbed thus resulting in low aegassing efficiency.
One improvea method and apparatus for the inline degassing and filtrltion of molten metal is disclosed in U.S. Patent 4,052,198 to Yarwood et al. and assi~ned to ~3z~ CON-136-M

the as~ignee of the present lnvention. The discl~ure teache~ an improvement in the degassing and filtration of molten metal using an apparatus which employs a pair of seq~entially placed, removable filter-type elements and at least one fluxing gas inlet positioned there-between. The fluxing gas is introduced into the melt through the inlet and flows through the first of said plates in countercurrent contact with the melt. The filter plate serves to break up the fluxing gas into a fine dispersion to insure extensive contact with the melt.
The filter plates employed are made of porous ceramic foam materials which are useful for the filtration of ~olten metal for a variety of reasons included among which are their excellent filtration eficiencies resulting from their uniform controllable pore size, low cost, as well as ease of use and replaceability. The ceramic foam filters are convenient and inexpensive to prepare and easily employed in an inline degassing and iltration unit.
While the aforenoted U.S. Patent 4,052,198 offers significant improvements over those inline gas fluxing techniques previously Xnown in the art, a number of problems have been encountered. It is desirable for economic advantages and increased productivity to have degassing and filtration systPms which can treat molten metal continuously at a rate commensurate with the casting practices. The employment of known inline degassing units such as aforenoted U.S. Patent 3,737,304 for continuous degassing and filtra ion have been found to be extremely inefficient, thus requiring lar~e multiple - ~23~7 CON-136-M

chamber arrangements necessary to sufficiently treat the quantities of molten meta} which are required for continuous castlng oparations. As a result of the large size o~
the treatment units, supplemental heating is required to prevent freeze up of the molten metal as it i8 being treated. While some improvement in the quantity of molten metal which can be treated hafi been achieved by using a smaller system such as that disclosed iD U.s.. Patent 4,~52,198 which ~ utilizes ceramic filters and countercurrent gas flow, such a system has been found to have a limited effectiveness in the quantity of molten metal which can be treated due to the large pressure arops encountered in the simultaneous counter-current flow of gas and metal through the filter body. As a result of the large pressure drop, a large head of molten metal is aeveloped upstream of the filter element thus requiring either an increase in size of t~a trans~er passage-way upstream of the filter element or a decrease in the rate of feeding the molten metal to the treatment unit.
Accordingly, it is a principal object of the present invention to provide an i~proved method and apparatus for the aegassing and filtration of molten metal which employs filter-type plates which are characterized by reducing th&
pre~sure drop encountered in the simultaneous countercurrent flow of gas and metal through the filter~type plates.
It is a particular object of the present inventio~ to provide an improved filter-type plate for reducing the pressure drop encountersd across the filter plate as gas and metal flow in countercurrent relationship through the filter-type plate.
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~2~7 It is still a further object of the present invention to provide a filter-type plate having a pre~erential path for gas flow through the filter plate.
It is still a further object of the present invention to proviae improvements as aforesaid which are convenient and inexpensive to utilize and which result in highly efficient metal ~egassing and filtration.
Further objects and advantages of the present invention will appaar hereinbelow.
SUMMARY OF TNE INVENTION
In accordance with the present invention, the foregoing objects and advantages are readily attained.
The preYent invention comprises a highly efficient degassing and filtration apparatus comprising a chamber having respective metal inlets and outlets, and wall surfaces for the support of at least a first and a second removable filter-type medium in sequential spaced-apart relationship, and at least one conduit providing at least one fluxing gas inlet port positioned between said first and said second medium, wherein said port is so positioned that ~luxing gas issuing therefrom is capable of dispersion and percolation througb said first medium. In a preferred embodiment, filter-type media are provided which possess an open cell structure characteri~ed by a plurality of interconnected voids, and are preferably prepared srom a ceramic foam wherein said voids are surr~unded by ceramic material. ~he ~ilter media may comprise plates having bevelled peripheral sur~aces adapted ~o mate with bevelled wall surfases on said chamber. A resilient sealing means is provided which is resist~nt to said molten metal to 3e~1ably engage the bevelled wall surfaee of said chamber upon installation of th~ filter plates.
In accordance with a preferred embodiment of the apparatus of the present invention, the ilter-type media may be provided with the same pore size and permeability or differ in pore size and per~eability whereby said fir~t medium possesses a relatively coarser pore structure, higher permeability and larger available flow area than said second medium. In accoxdance with the improvement of the present invention, the first filter-type medium is provided with an array of holes substantially larger than the pore size o~
the filter medium itself so as to provide a preferential path for gas flow through said first medium. Tha fluxing gas is introduced below the first filter-type medium through a sparger plate located in the chamber. The sparger plate is provided with a plurality of orifices of particular size and spacing so as to minimize the diffusion distance for gaseous impurities while substantially preventing fluxing gas bubble coalescence.
In accordance with thè method o~ the present invention, degassing and filtration of molten metal i5 conducted by the passage of a melt through a chamber wherein said melt passes through at least two sequentially placed, spaced-apart filter-type media, whereby said melt is brought into counter-current contact with a fluxing gas while within and above ths first of said media, said fluxing gas passing through said first filter-type media via a preferential Fath thus limiting undesirable pressure drop across said first filter-type media thereby increasing metal trea,tment effi~iency. ~he fluxing gas issues from at least one inlet ~t pro~ided with~n said ohamber between said first and se~ond of said ~$'~ 07 media, aispersing and percolating up into contact with said melt within said first medium~
The method of the pre~ent invention may employ a fluxing gas such as an inert gas, preferably carrying a fimall quantity J
of an active gaseous ingreaient such as chlorine or a fully ' halogenated carbon compound. The gas u~ed may be any of the - g~ses or mixtures of gases such as nitrogen, argon, chlorine, carbon monoxide, Freon 12, etc., that are known to give acc~ptable degassing. In the preferred embodiment for the degassing of molten aluminum melts, mixtures of nitxogen-Freon 12 or argon-Freon 12 are used. In addition, a supernatant `
salt cover comprised of alkaline and al~aline earth chlorides and a ~luoride may be located on the surface of the melt to aid in the degassing process by minimizing the readsorption of gaseous impurities at the surface of the melt. Typical salts employed may be molten halides such as sodium chloride, potassium chloride, magnesium chloride, or mixtures thereof and should be selected to minimize erosion of the refractory lining of the degassing chamber. Alternatively, gaseous covers such as argon, nitrogen, etc., may be used as a protective cover over the molten metal to minimize the readsorption of gaseous impurities at the surface of the melt.
The present apparatus and method provide a considerable increase in productivity i~ the degassing of molten metal as degassing is conducted without interruptions of the melting furnace. Further, the design of the apparatus enables its placement near to the casting station, whereby th~ possibility of further impurities entering the melt is substantially eliminated.

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The employment of the first filter-type medium of the present invention in the above apparatus allows ~or a preferential path for the flow of gas through said filter medium thu~ limiting the pressure drop across said mediu~
and thereby contributing to greater efficiency o~ the apparatus.
The employment of the ~parger plate in the present invention in the above apparatus minimizes the bubble size of the purged gas while maximizing the gas bubble density thereby increasing the effective surface area for carrying out the adsorption reaction thus optimizing the aegassing of the molten metal.
In addition, the efficiency of the preqent invention permits degassing to be conducted with a sufficiently lower amount of flux material whereby the level of effluence result-ing from the fluxing operation is greatly reduced.
The present apparatus and method provide considerable increase in productivity in the degassing of~molten metal as degassing is conducted without interruption of alloy and melt processes. Further, the aesign of the apparatus permits its placement near to the castiny. The preæent invention enables the operation of a fluxing and filtering process which achieves significant reduction in the level of effluents normally resulting from processing of this kind.
By virtue of the employment of conveniently re~ovable filter-type media possessing carefully controlled filter properties, th~ apparat~s and method of the prssent invention are capable of achi~ing levels of m21t purity hereto~ore attainable only with the most rigorous of processing. Also, the employ~ent of a relatively coarse first filter-type medium :30 to abstract larger entrained non-metallic particulate before ~3~

the melt reaches the second fine filter greatly extends the useful life of the latter. In addition, these high levels of purity are attained utilizing inexpensively manufactured filter-type media.
In accordance with a particular embodiment of the invention, there is provided, an improved filter plate means for use in the filtration and degassing of molten metal, said filter plate means having an open cell structure comprising a plurality o~ interconnected voids surrounded by a web of ceramic, the improvement which comprises means associated with said filter plaie means for providing a preferential path through said filter pla~e mear.s.
In accordance with a further embodiment of the invention, an apparatus for the degassing and filtration of molten metal comprises: chamber means having inlet me~ns, outlet means, and at least one filter plate means, wall means associated with said chamber means, said wall means being adapted to support said at least one filter plate means;
fluxing gas inlet means positioned in said chamber means with respect to said at least one filter plate means such that fluxing gàs issuing from said fluxing gas inlet means passes through said at least one filter plate means and said filter plate means having an open cell structure comprising a plurality of interconnected voids surrounded by a web of ceramic, the improvement which comprises means associated with said filter plate means for providing a preferential path through said filter plate means~
From a di~ferent aspect, and in accordance with the invention, a method for the filtration and degassing of molten metal by passing said molten metal through at least one filter-type medium and purging said molten metal with a fluxing gas by passing said flùxing gas through said molten metal in countercurrent ~low therewith, the improvement 23~

comprising positioning fluxing gas inlet means such that flux-ing gas issuing from said fluxing gas inlet means passes -through - said at-least one filter-type-~m~dium and providing said at least one filter-type medium with a preferential path for passing said fluxing gas through said at least one filter-type medium.
- In accordance wl~h~a further embodiment of this second aspect, there is provided a method for the filtration and degassing of molten metal by passing said molten metal through at least one filter-type medium and purging said molten metal with a fluxing gas by passing said fluxing gas through said molten metal in countercurrent flow therewith, the improvement comprising positioning fluxing gas inlet means such that fluxing gas issuing from said fluxing gas inlet means passes through said at least one filter-type medium and providing said fluxing gas inlet means with a plurality of orifices of controlled size and spacing so as to minimize fluxing gas bubble size while maximizing fluxing gas bubble dispersion thereby optimizing the degassing of said molten metal.
BRIEF DESCRIPTION OF THE DRA~INGS
Figure 1 is a side sectional view of the apparatus of the present invention in which the first and second filter-type media are disposed in substantially side-by-side relation-ship. ~
Figure 2 is a top view of the sparger plate used in the apparatus of Figure 1.
Figure 3 is a perspective view, partly broken away, of the first filter-type media illustrative of the present invention.
Figure 4 is a graphic illustration of the improved flow rate obtained in accordance with the method and apparatus of the present invention.
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Figure 5 is a side view of an alternate embodiment of the present invention wherein said filter-type media are disposed in a substantially horizontal relationship.

DETAILED DESCRIPTION OF PREFERRED EMBODIME~TS
Referring to Figure 1, the apparatus is illustrated in location with a molten metal transfer system which may include pouring pans, pouring troughs, transfer troughs, metal treatment bays or the like. The apparatus and method of the present invention may be employed in a wide variety of locations occurring intermediate to the melting and casting stations in the metal processing system. Thus, Figure 1 illustrates a refractory fluxing and filtering apparatus 10 which is divided p - lOb -1~232~

by baffle wall 12 into cham~ers 14 and 16. The molten metal enters chamber 14 through inlst launder 18, passes under baffle 12 into cha~ber 16 and ~own outlet 20 for further processing. The apparatu~ 10 maY optionally be prvvided with inlet baffle 22 wh~h serve6 to confine an optional salt layer 24 on the surface of the metal in chamber 14 and prevent it from floating bac~wards ~long the launder 1~.
.In accordance with a pref~rred embodiment of the present invention each of the chambers 14 and 16 is pro~ided with at least one peripheral rim, 26 and 28 respectively, positionea in a substantially side-by-side relationship with respect to each other and at a level which is continuous with the bottom of respective inlet and outlet launders 18 and 20. The first p~ripheral rim 26 and second peripheral rim 28 are illustrated in Figure 1 as defining a downwardly converging bevell~d surface which enables the expeditious installation and replacement of appropriately configured filter-type media. Though rims 26 and 28 are illustrated as having bevelled surfaces, the invention is not limited thereto, as rim9 possessing other means for retaining in place the filter-type media of the pr.esent invention may be employed as will be noted hereinbelow.
Filter-type media 30 and 32 are provided in chambers 14 and 16, respectively, and may possess bevelled peripheral surraces 34 adapted to mate with correspondingly configured peripheral rims 26 ana 28. ~he bevelled peripheral surfaces 34 are provided with resilient sealin~ means 36 thereon which are resistant to molten metal, and the respective f~lter media 30 and 32, including seal means 36 are seguentially inserted into chambers 14 and 16, respectiYely, so that seal means ~ [3 ~

36 in each in~tance engages the respective bevelled surfaces of rims 26 and 28.
The floor of refractory fluxing and filtering apparatus 10 in the area Df chamber 14 i9 provided with a cast ceramic sparger plate 38 having a plurality of orifices 40 for intro-ducing a fluxing gas from an outside source, not shown, from the inlet 42 and plenum chamber 44 into the molten metal as it passes through chamber 14.
In the preferred embodiment of the present invention, the use of a cast ceramic sparger plate has a distinct advantage over conventional methods and apparatuses for introducing fluxing gas into a molten metal. In accordance with the present invention, in order to optimize the efficiency of the degassing process, that is, maximize the efficiencies of the kinetics of the adsorption reaction, the introduction of the fluxing gaq into the melt should be Qptimized so as to provide minimum bubble size and maximum bubble density while eliminatin~ bubble coalescence. Thus, the mean distance between the orifices in the sparger plate should ~e co~trolled so as to prevent fluxing gas bubble coalescence while minimizing the diffusion distance ~hich the gaseous impurities must travel through the melt to a bubble. Maximum adsorption efficiency is obtained by employing a sparger plate as illustrated in Figure 2. The use of discrete orifices 40 in the sparger plate avoids bubble coalescence and allows for control of the bubble size and dlspersion. The size of th~
individual orifices 40 determines th~ size of the bubble.
Accordingly, in order to maximize surface area ~o~ the adsorption reaction, the orifices ar~ made a~ small as possible consiste~t with preYenting plugging of the orlfices wi~h metal o~er several uses. In accordance with the pre~ent invention, an orifice size in the range of o.o05n to 0.50n,prefera~ly 0.010"
to 0.20" has been utilizea for degassing molten aluminum and alu~inum alloys. The inter-orifice spaciDg, A, as i71u~trated in Figure 2, i8 critical for providing maxi~um dispersion of bubbles while maintaining sufficient distance between the bsbbles so as to preYent bubble coalescence. Inter-orifice spacings in the range of 0.25" to 5.00n, preferably 0.75" to

2.00" have been found optimum when degassing molten aluminum and its alloys.
The fluxing gas-which may be employed in the present apparatus and method comprises a wide variety of well Xnown components including chlorine gas and other halogenated gaseous materials,carbon monoxide as well as certain inert gas mixtures derived from and including ni~rogen, asgon, helium and the like. A preferred gas mixture for use in the present invention for de~assing molten aluminum and aluminum alloys comprise5 a mixture of nitrogen or argon with dichlorodifluoro-methane from about 2 to about 20~ by volume, preerably 5 to 15% by volume. In conjunction with this gas mixtuxe, a molten salt mixture 24 may be empl~yed on the surface of the melt resiiding within chamber`14 which would comprise halides such as sodium chloride, potassium chloride, magnesium chloride or mixtures thereof. It 5hould be noted that the molten salt mixture should be selectea to minimize erosion of the refractory iining of the fluxing box, In addition, a gaseou~
pro~ective cover of argon, nitrogen or the like may be used over the molten metal so as to minimize readsorption of gaseous impurities at the ~ur~ace of the melt in the same manner a~ the molten salt. The above-noted and foxegoing ~ 13 -

3;~

compositions are presented for purposes of illustration only and do not form a material limitation on the present invention.
A preferred emboaiment of the present in~ention calls ~ for the provision of filter-t~pe media of uniform, close tolerance at a significant reduction in COB~. Accordingly, the filter-type meaium comprises a filter plate such a~ that illustrated in Figure 3. Filter plate 46 possesses an open cell structure, characterized by a plurality of interconnected voids, such that the molten metal ma~ pass therethrough to remove or minimize entrained solids from the final cast product, or to facilitata the exchange impurities between the melt and a fluxing gas. Such a filter may comprise, for example, a solid filter plate made from sintered ceramic aggregate, or a porous carbon plate. In the preferred embodiment, a ceramic foam filter is ~tilized as described in U.S. Patent 3,962,081. In accordance with the teaching of said U.S.
Patent, ceramic foam filters may be prepared which have an open cell structure characterized by a plurality of inter-connected voids surrounded by à web of said ceramic material.
The ceramic oam filter describea in said U.S. Pa~ent is particul~rly suitable in the present ~nvention since it is of low cost and may be readily employed on a throwaway basis.
Furthermore, this filter is effective in the filtration of molten metal, especially aluminum, at a low cost achieYing filtration efficienc~ with considerable flexibility. In accordance with the preferred embodi~ent ~irst filter-type medium 30 may be prepared o~ a relatiYely coarse pore size ranging from 5 to 20 ppi, wh~ch possesses an air permeability ranging from 2500 to 8000 x 10 7 cm2, while second filker medium 32 woula comprise a relatively fine filter possessing 23~

a pore count of from 20 to 45 ppi and an air permeability from 400 to 2500 x 10 cm . The metal flow rate through the filter may range from 5 to 50 cubic inches per square inch of filter per minute. Naturally, as noted earlier, both permeability and pore si~e of the respective filter-type media may be varied to suit the particular material being filtered, and the present invention should not be limited to the afore-no~ed exemplary ranges.
The ceramic foam filter preferably utilized in the present invention i~ prepared from an open cell, flexible foam material having a plurality of interconnected voids surrounded by a web of said flexible foam material, such as polyurethane foams or cellulosic foams. The ceramic foam filter may be prepared in accordance with the general procedure outlined in U.S.
Patent 3,893,917 wherein an aqueous ceramic slurry is prepared and the foam material impregnated therewith so that the web thereof is coated therewith and the voids substantially filled therewith. The impregnated`ma~erial is compressed ~o that a portion of the slurry is expelled therefrom and the balance uniformly distrib~ted throughout the foam material. The coated foam material is then dried and heated to first burn out the flexible organic foam and then ~inter the ceramic coating, thereby providing a fused ceramic foam having a plurality of interconnected voids surrounaed by a web of bonded or fused ceramic in the configuration of the flexible foam. Naturally, a wide variety of ceramic materia~s may be chosen depenaing upon the particular metal to be ~iltered.
Preferably, a ~ixture of alumina and chromia is employed, however, these material~ may naturally be utilized saparately or in co~hination with other ceramic materials. Other typical ?.;~ 7 ceramic materials which may be employed include zirconia, magnesia, tltanium dioxide, silica and m~xtures thereof.
Normally, the slurry contains from about 10 to 40~ of water and one or more rheological agents, bindes~ or air setting agents.
As shown in Figure 2, the filter plate 46 of the preferred embodime~t may have a bevelled peripheral surface 48 adapted to mate with the similarly bevelled rims of the ~ilter chamber illustrated in Figures l ana 4. Naturally, variations in desi~n are con~emplated within the scope of the present invention and thus a w~de variety of geometric configurations may be contemplated within the scope of the apparatus disclosed herein, and the illustration of Figure 2 is not meant to be limiting thereto.
In the instance where the filter plate of the present invention is designed to be a throwaway item, it is essential to provide an effective means of sealing the filter plate in place in its holder which is easy to assemble, disassemble and clean up. The holder or filter chamber itself is normally an integral part of a trough, pourin~ ~an or tundish, etc.
and should be constructed of refractory material~ re~istant to the molten metal similar to those used in st-andara trough construction. It is greatly preferred to seal the filter plate in place using a resilient sealing means or gasket type seal as illustrated ana discussed earliex, which peripherally circumscribes the filter plate at the bevelled portion thereof. The gasket type seals ~nsure a leak frea installation and also provide an effective partiny medium which is ~ssential for e se of ~isassembly. In addition, since the ga~ket6 or ~ealing means prevent ingress of metal to the sealing faces of the holder unit, their use considerably ease~ clea~ up and effectively pxolongs the life of the unit by eliminating problems of metal attack. Furthermora, because of its resiliency, the gasket may provide ~ufficient ~ricti~nal force to hold the ilter ~ody in place in the holder or - filter chamber without resorting to other types of hola-down devices. The resilient sealing means should be non-wetting to the particular molten metal, resist chemical attack th~re-fro~ and be refractory enough to withstand the high operating temp~ratures.
Plate type filter units of the present invention may be sealed by gaskets around their edqes and/or at the periphery of their large faces. The plate type filter units of the present invention are preferably sealed by an edge type seal along the peripheral surface of the filter plate thus providing a positive seal and, in conjunction with the gasket, a mechanical advantage to hold the filter in place. In the event that a simple press fit is insufficient to hold the filter in ~lace, naturally a variety of mechanical devices such as wedges and hold-down weights may ~e employed.
AlternatiVely, apparatus 10 in Figure 1 can be made to be spl~t at rims 17 and 18, in a manner not shown, so that pressure can be applied to the seals by the Yise-like action of closing the split unit. The bevelled angle of the filter chamber and corresponaing bevelled angle of the filter plate tends to form a positive seal and hold the filter in place against buoyancy forces acting thereupon. Naturally, as indicated abo~e, the gasket or seal sho~ld be resistant to the molten metal utilized. Typical seal materials utilized in aluminum processing include fibrous refractory type seals of a v~riety of compositions, as the following illustrati~

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seals: tl) a saal containing about 45~ alumina, 52~ silica, 1.34 ferric oxide and 1.7~ t~taniaJ ~2) a seal containing about 55~ silica, 40.5~ alumina, 4~ chromia and 0.5~ erric oxide; and (3~ a ~eal containing about 53~ ~ilica, 46~ ~lumina and 1~ ferric oxide.
It iB a primary feature of the present invention, as ~llustrated in Figure 1, to provide the first filter-type medium 30 with an array of holes 50 which are ~ubstantially larger than the pore size of the filter itself. In accordance with the teachings of the present invention, large pressure differences were observea between the gas fluxing treatment zone downstream of first filter-type medium and that area upstream of the first filter-type medium when employing the method and apparatus disclosed in aforesaid U.S. Patent 4,052,198.
The large pressure di~ferential resulted from a ~luxing gas buildup under the filter plate. This head differential placed a physical l;~it on the amount of fluYing gas which can be passed in countercurrent flow with the lten metal flow, thus limiting the quantity of molten metal which might be effectiv~ly and~continuou~ly treated. It ha~ been found, in accordance with the present invention, that the pressure differential and thus the head differential can be effectively controlled and decreased by providing an array of holes in the filter plate which are substantially larger than the individual pore size of the filter thereby providing a preferential fiow path for the fluxing gas through the filter plate while substantially all of the molten metal passes through the filter body. The decreased head differential obtainad in accordance with the present invention allows for an increase in`the amount of fluxing gas which can be introduced into the melt thus incxeasing the quantity of molten me~al which can be effectively treated o~er that of - 18 ~

3:~97 previously known systems. The following example iB illustrative of the present invention.
A plurality of ceramic foam filter-type media were preparea in accordance with the general procedure outlined in U.S. Patent No. i,893,917 for use as the first filter medium in a pilot scale unit similar to the apparatus illustrated in Figure 1. Fach filter size was 5" x 5" ~ 1"
thick and was designed to have an air permeability of 1750 x 10 7 cm2, a porosity of 0.90 and a pore size of 30 pores per linear inch. An array of 25 holes of equal size were then drilled in each of the filters. It should be noted that besides introducing t`he holes into the finished filter product by drilling or the like, the holes may be introduced into the flexible foam material prior to the processing as outlined in aforesaid U.S. Patent 3,893,917. It i~ preferred that the holes are of equal size and uniformly distributed across the surfaoe of the`filter plate so that the fluxing gas will be evenly distributed across the filter plate. The hole 8ize8 in the filters varied from 0.10" to 0.45" in increments of O.OS". The fllters were then incorporated into the pilot scale unit so as to stuay the countercurrent gas-liquid flow characteristic of the filtering and degassing process.
Li~uid was introduced into the filtering and degassing chamber at rates of 924 in. per minute, 1386 in. per minute and 1848 in.3 per minute. The gas flow was varied so as to produce a 1/2 inch head drop between the inlet and outlet launder as illustrated by the letter B in Figure 1. The results are graphically illustrated in Figure 4.
As can be seen from Figure 4 the provision of an array of drilled holes in the filter-type media of the pxesent . CON-136-M
~L~.23~7 invention allows for a greater volume of fluxing gas to be introduced into the melt thu~ allowing for a more efficient, high volume ~ilter and degas6ing apparatus as heretofore known.
By providing a preferential flow path of the fluxing ga~
through the ~ilter plate, the pressure differential across the filter plate is decreased thus resulting in a corresponding decreass in head differential which allows for the use of correspondingly smaller units while increasing aegassing efficiency. It has been found that hole sizes of up to 45"
may be effectively employed without materially diminishing the filtering function of the filter plate.
Figure 5 illustrates an alternate embodiment of the present invention in which the filter-ty~e plates are sub-stantially horizontally disposed. Apparatus 50 is provided with inlet launder 52 leading to chamber 54 wherein the filtration and degassing operations take place. Chamber 54 is illustrated as roughly bowl-shaped and is dispo~ed with its bottom reces5ed below the level of inlet launder 52 so that molten metal traveling thereto will flow downwardly through the filter-type media. Chamber 54 is characterized by the proYision of at least two peripheral rims 56 and 58, comprising, respectively, a first and a second peripheral rim. First peripheral rim 56 is located at the upper portion of chamber 54 and, in Figure 1 is positioned at a level continuous with the bottom of inlet launder 52. Second peripheral rim 58, as depicted in Figure 5, is disposed within chamber 54 so as to effectively divide it into sub-chambers 60 and 62. As with first peripheral rim 56, second peripheral rim 58 is illustrated in the Figure as defini~g a downwardly converging bevell~d surface which enables ~he , CON-136-M
3;; ~

expeaitious installation and replacement of appropriately configured filter-type media. Thou~h rims 56 and 58 are illustrated a~ having bevelled surface8, the invention i8 not limited thereto, aR rims posse~sing other mean~ ~or retaining in place the ~ilter-type m~dia of the present invention may be employed as will be noted later on. Rim 58 is shown to be reduced in size from first rim 56 to enable the unobstructed manipulation of the filter mediu~ locat~d the~ein.
First sub-chamber 60 comprises the area residing between the first and second filter-type media labeled 64 and 66, respectively. As illustsated herein, filter-type media 64 and 66 may likewiRe possess bevelled peripheral surfaces 68 adapted to mate with corregpondingly configured peripheral rims 56 and 58. The bevelled peripheral surfaces 68 whan employed are provided with resilient sealing means 70 thereon which are resistant to molten metal, and the respective filter media 64 and 66, including sealing means 70 are se~uentially inserted into chamber 54 so that sealing means:70 in each instance engages the respective bevelled surfaces of rims 56 and 58.
As noted earlier, the provision of peripheral rims 56 and 58 supporting respective filter media 64 and 66 effectively divides chamber 54 into sub-chambers 60 and 62. Referri~g again to Figure 5, in acc,ordance with the present invention sub-chamber 60 is Qrovided with at least one inlet port 72 comprising the opening or openings of ~ conduit 74 or manifold through which a fluxing gas may be introduced to the melt from an outside source, not shown.
~ :

, . - 21 -. ~ CON-136-M

In accordance with the teachings of the present invention, the first filter-type medium 64 is provided with an array of holes 76 to thereby form a preferential fl~w path for ths fluxing gas. It should be appreciated that the fluxing gas may issue from a sparger plate as 5hown and dascribed with regard to Figure 1.
A wide variety of instances exist where the apparatus and method of the present invention in all of the above disclosed-variations may be employed. Specifically in the instance of a continuous casting operation~ a pair of flux filtration chambers may-be employed in parallel arrangement.
In such an operation, the great length and associated total flow of metal involvea ~ay require the changing of filter media in mid-run. Such changes may be facilitated by the employment of parallel flow channels each containing a filtra-tion chamber, together with a means for diverting flow from one channel to the other, by valves, dams or the like. F1QW
would thus be restricted to one filtration chamber at a time and would be diverted to an alternate channel once the head drop across the first filtration chamber became excessive.
It can be seen that such a switching procedure could supply an endless stream of filtered metal to a.continuous castinq station.
In addition to the above, the present apparatus and method are capable of several modifications within the s~ope of the invention to accommodate variations in operating procedure. FOr example, in the instance where small, in- :
dividual lots of molten metal are prepared and cas , it is aesirable that the filter-ty~e media remain operable for several such lots~ To this end, the filter ~edia may be . CON-136-M
,-~3~7 recessed somewhat from the levels of the transfer passageway and exit trough whereby, after melt flow has ceasea~ reidual melt remains which iill8 the chamber and covers both filters.
In coni~nction with this modification, at least one cover unit ~ay be employed which would reside above the remaining melt, and would be provided with heating means such as a plurality of radiant heater~ to keep the melt in the liquid state.
Other moaifications contemplated within the scope of the .invention include the provision of a plurality of inlet ports surrounding the respective chambers immediately below the respective first filter-type media, In addition, and likewise not illustratea herein, the inlet ports could be extended to the center of said chambers by the extension of the respective conduits thereinto, whereby fluxing gas may enter the melt from a point centrally located within the chamber. Both tha provision of a manif~ld of fluxing gas inlet ports, and an inlet port or ports centrally located within the chamber, neither of which are illustrated herein, comprise modifications which are, themselve5, 3ubject to alterations of design, etc., and accordingly, the invention 5hould not..be strictly interpreted thereby.
It is to be understood that the invention is not limited to the illustrations described and shown herein, which are deemed to be merely illustrative of the best modes of carrying out the invention, ana which are susceptible of modification of form, size, arrangement of parts and details of operation~
The invent~on rather is intended to encompass all s~ch modifications which are within its spirit and scope as defined ~y the ~laims.

Claims (42)

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

1. An apparatus for the degassing and filtration of molten metal comprising:
chamber means having inlet means, outlet means, and at least one filter plate means;
wall means associated with said chamber means, said wall means being adapted to support said at least one filter plate means;
fluxing gas inlet means positioned in said chamber means with respect to said at least one filter plate means such that fluxing gas issuing from said fluxing gas inlet means passes through said at least one filter plate means; and said filter plate means having an open cell structure comprising a plurality of interconnected voids surrounded by a web of ceramic, the improvement which comprises means assoc-iated with said filter plate means for providing a preferential path through said filter plate means.

2. An apparatus according to claim 1 wherein said passageway means consists of an array of holes in said at least one filter plate means.

3. An apparatus according to claim 2 wherein said holes are of equal size and substantially larger than the pore size of said at least one filter plate means.

4. An apparatus according to claim 3 wherein said hole size is from about 0.10" to about 0.45".

5. An apparatus according to claim 1 wherein said fluxing gas inlet means comprised a sparger plate being provided with a plurality of orifices of controlled size and spacing so a to minimize fluxing gas bubble size while maximizing fluxing gas bubble dispersion thereby optimizing the degassing of said molten metal.

6. An apparatus according to claim 5 wherein said plurality of said orifices size is in the range of 0.005"
to 0.050".

7. An apparatus according to claim 6 wherein said plurality of said orifices spacing is in the range of 0.25"
to 5.00".

8. An apparatus according to claim 5 wherein said plurality of said orifices size is in the range of 0.010"
to 0.020".

9. An apparatus according to claim 8 wherein said plurality of said orifices spacing is in the range of 0.75"
to 2.00".

10. An apparatus according to claim 5 wherein said passageway means consists of an array of holes in said at least one filter plate means.

11. An apparatus according to claim 10 wherein said holes are of equal size and substantially larger than the pore size of said at least one filter plate means.

12. An apparatus according to claim 11 wherein said hole size is from about 0.10" to about 0.45".

13. An apparatus according to claim 12 wherein said plurality of said orifices size is in the range of 0.005" to 0.050".

14. An apparatus according to claim 13 wherein said plurality of said orifices spacing is in the range of 0.25"
to 5.00".

15. An apparatus according to claim 12 wherein said plurality of said orifices size is in the range of 0.010"
to 0.020".

16. An apparatus according to claim 15 wherein said plurality of said orifices spacing is in the range of 0.75"
to 2.00".

17. A method for the filtration and degassing of molten metal by passing said molten metal through at least one filter-type medium and purging said molten metal with a fluxing gas by passing said fluxing gas through said molten metal in countercurrent flow therewith, the improvement comprising positioning fluxing gas inlet means such that fluxing gas issuing from said fluxing gas inlet means passes through said at least one filter-type medium and providing said at least one filter-type medium with a preferential path for passing said fluxing gas through said at least one filter-type medium.

18. A method according to claim 17 wherein said preferential path consists of an array of holes in said at least one filter-type medium.

19. A method according to claim18 wherein said holes are of equal size and substantially larger than the pore size of said at least one filter-type medium.

20. A method according to claim 19 wherein said hole size is from about 0.10" to about 0.45".

21. A method according to claim 17 wherein said fluxing gas inlet means is provided with a plurality of orifices of controlled size and specing so as to minimize fluxing gas bubble size while maximizing fluxing gas bubble dispersion thereby optimizing the degassing of said molten metal.

22. A method according to claim 21 wherein said plurality of said orifices size is in the range of 0.005" to 0.050".

23. A method according to claim 22 wherein said plurality of said orifices spacing is in the range of 0.25" to 5.00".

24. A method according to claim 21 wherein said plurality of said orifices size is in the range of 0.010"
to 0.020".

25. A method according to claim 24 wherein said plurality of said orifices spacing is in the range of 0.75"
to 2.00".

26. A method according to claim 21 wherein said preferential path consists of an array of holes in said at least one filter-type medium.

27. A method according to claim 26 wherein said holes are of equal size and substantially larger than the pore size of said at least one filter-type medium.

28. A method according to claim 27 wherein said hole size is from about 0.10" to about 0.45".

29. A method according to claim 28 wherein said plurality of said orifices size is in the range of 0.005" to 0.050".

30. A method according to claim 29 wherein said plurality of said orifices spacing is in the range of 0.25" to 5.00".

31. A method according to claim 28 wherein said plurality of said orifices size is in the range of 0.010" to 0.020".

32. A method according to claim 31 wherein said plurality of said orifices spacing is in the range of 0.75"
to 2.00".

33, An improved filter plate means for use in the filtration and degassing of molten metal, said filter plate means having an open cell structure comprising a plurality of interconnected voids surrounded by a web of ceramic, the improvement which comprises means associated with said filter plate means for providing a preferential path through said filter plate means.

34. An improved filter plate means according to claim33 wherein said means comprises an array of holes in said filter plate means.

35. An improved filter plate means according to claim 34 wherein said holes are of equal size and substantially larger than the pore size of said filter plate means.

36. An improved filter plate means according to claim 35 wherein said hole size is from about 0.10" to about 0.45".

37. An improved filter plate means according to claim 36 wherein said filter plate means has an air permeability in the range of 400 to 8000 x 10-7 cm2, a porosity of 0.80 to 0.95 and a pore size of 5 to 45 pores per linear inch.

38. A method for the filtration and degassing of molten metal by passing said molten metal through at least one filter-type medium and purging said molten metal with a flux-ing gas by passing said fluxing gas through said molten metal in countercurrent flow therewith, the improvement comprising positioning fluxing gas inlet means such that fluxing gas issuing from said fluxing gas inlet means passes through said at least one filter-type medium and providing said fluxing gas inlet means with a plurality of orifices of controlled size and spacing so as to minimize fluxing gas bubble size while maximizing fluxing gas bubble dispersion thereby optimizing the degassing of said molten metal.

39. A method according to claim 38 wherein said plurality of said orifices size is in the range of 0.005"
to 0.050".

40. A method according to claim 39 wherein said plurality of said orifices spacing is in the range of 0.25"
to 5.00".

41. A method according to claim 38 wherein said plurality of said orifices size is in the range of 0.010" to 0.020".

42. A method according to claim 41 wherein said plurality of said orifices spacing is in the range of 0.75"
to 2.00".