CA1245857A - Molten metal filter and method of filtering - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An improvement in the filtration of molten metal using a removable filter plate disposed in a filter chamber wherein the peripheral surface of the filter plate mates with the wall surface of the filter chamber. The peripheral surface of the filter plate is provided with an intumescent resilient seal such that the heat from the molten metal being filtered and/or preheating causes the intumescent resilient seal to swell thereby providing a positive seal for the filter plate in the filter chamber so as to prevent metal leakage around and/or floating of the filter plate.

Description

~3-147 BACKG~OUMD OF T~-IE INVENTlON
Molten metal, particularly molten aluminum, in practice generally contains entrained solids which are deleterious to the final cast metal product. These entrained solids appear as incluslons in the final cast product after the molten metal is solidified and cause the final product to be less ductile or to have poor bright finishing and anodizing characteristics.
The inclusions may originate from several sources. For example, the inclusions may originate from surface oxide films which become broken up and are entrained in the molten metal. In addition, the inclusions may originate as insoluble impurities, such as carbides, boridesand others or eroded furnace and trough refractories.
Porous ceramic foam materials are known to be particularly useful in filtering molten metal, as described in U.S. Patent No. 3,893,917 for i'Molten Metal Filter" by Michael J. Pryor and Thomas J. Gray, patented July 8, 1975, and also as described in U.S. Patent No. 3,947,363 for "Ceramic Foam Filter" by Michael J. Pryor and Thomas J. Gray, patented March 30, 1976, U.S.
Patent No.~4,081,371 for "Flltering of Molten Metal" by John C.
Yarwood, James E. Dore and Robert K. Preuss, patented March 28, 1978, and U.S. Patent No. 4,024,056 for "Filterlng of Molten Metal" by John C. Yarwood, James E. Dore and Robert K. Preuss, patented May 17, 1977.

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~33_1L~7 35~7 These ceramic foam materials are particularly useful for filtering molten metal for a variety of reasons lncluded among which are their excellent filtration efficiency, low cost, ease of use and ability to use same on a disposable, throwaway basis.
The fact that these ceramic foam filters are convenient and inexpensive to prepare and may be used on a throwaway basis requires the development of means for easily and conveniently assembling and removing porous, molten metal filters from a filtration unit while providing a highly efficient filtration assembly. Since the filters are designed to be a throwaway item, it is essential to provide an effective means of sealing the filters in place in its holder which is easy to assemble, disassemble and clean up. The holder or fllter chamber itself is normally an integral part of a trough, pouring pan or tundish, etc. and should be constructed of refractory materials resistant to the molten ~etal slmilar to those used in standard trough construction. It is greatly preferred to seal the filter plate in place using a resilient sealing means or gasket type seal peripherally circumscribing the filter plate. In order to effectively seal the filter plate in place, the prior art teaches the provislon of a bevelled peripheral surface on the filter plate onto which the gasket seal is placed. The filter plate is then sealed in place by exerting a vertical pressure downwards on the filter plate in the filter chamber which is provided with a correspond1ng be~:lled surface. It has been found that 9 as . .

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a result of the force exerted on the fllter plate during assembly in the filter chamber, structural damage may result to the filter plate which could result in metal leaka@e and/or ineffective metal filtration. Thus, it is highly desirable to provide a filter which can be sealingly mounted in the filter chamber without the need of undue force. In addition, it is highly desirable to provide an effective seal in the filter chamber which may have irregularities on the peripheral surface thereof due to chips, cracks, etc.
Heretofore, the ceramic foam filters were gasketed with pre-cut strips of low density ceramic fiber having an organic binder. The straight pre-cut strips were simply taped to the bevelled edges of the filters after the filter had been baked and fired. As a result of the foregoing process, a number of disadvantages arose. Firstly, as the gasket is merely taped to the filter the fit is relatively loose and thus the gasket can be easily damaged during handling of the filter. Secondly, since the pre-cut strips are straight and the edges of the filter which are to be sealed are bevelled, a misfit occurs which in some cases results in the molten metal passing between the gasket and filter plate. Finally, as the gasket material is not taped to the filter until after it is fired, the filter must be dressed, that is coating the edges with additional ceramic slurry, in order to insure that the edges of the filter are not directly handled during further processing.

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U.S. Patent No 4,331,621 for "Method for Bonding a Gasket Seal to Surface of Ceramic Foam ~ilter" by Jerry ~J. Brockmeyer, patented May 25, 1982 and assigned to the assignee o~ the instant invention discloses an improved method of integrally bonding a gasket seal to the peripheral surface of a ceramic foam filter plate used in the fil~ration of molten metal. In accordance with the method disclosed, a mold is provided having dimensions corresponding to the desired final filter size which is to be inserted in the filter chamber when used in the filtration of molten metal. A wet slurry impregnated foam material is centered in the mold leaving a gap between the mold and the impregnated foam. The gap is then filled with a ceramic fiber slurry consisting of a ceramic fiber, organic and/or inorganic binders and a suitable vehicle, usually water. If a viscous non-flowing slurry is used it is possible to remove the mold prior to drying. However, if thinner slurries are used, it is necessary to provide a porous mold ~hich would be maintained about the part until dried. The part is then fired uslng the practices disclosed in the aforesaid U.S. Patent Nos.3,893,917, 3,947,363 and 4,081,371 Alternatively, the gasket could be applied to a dried and fired ceramic part in the same manner as outlined above. However, in this case an additional drying operation would be required to integrally bond the gasket to the ceramic body. While the method disclosed ln the '621 patent overcomes some of the disadvantages noted above vis-a-vis taping pre-cut strips, there is still a problem of metal leakage around and/or 3o ....

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floating of the filter plates due to the lack of a positive seal o~ the filter plate in the fllter chamber.
Accordingly, it is a principal ob~ect of the present invention to provi~e an improved filter for and method of ~lltering molten metal wherein a positive seal is provided between the filter plate and the filter chamber wall.
It is a particular object of the present invention to provide the peripheral surface of the filter plate which mates wlth the walls of a filter chamber with an intumescent resilient seal.
It is a still further ob~ect of the present invention to provide an improved filter plate seal which allows for a simplified filter bowl design and compensates for structural irregularities in the filter bowl.
It is a still another further object of the present invention to provide an intumescent resilient seal which swells due to the heat of the molten metal being filtered and/or selective preheatlng thereby providing a positive seal.
Further ob~ects and advantages of the present invention will appear hereinbelow.

SUMMARY OF THE INVENTION
In accordance with the present invention it has been found that the foregoing ob~ects and advantages may be readily obtained.
The present invention resides in the filtration of molten metal wherein a removable filter plate having a perlpheral 3o . .

P~ 3 -- 1 4 7 surface is disposed in a filter chamber such that the peripheral surface of the filter plate mates with the wall surface of the filter chamber. The peripheral surf~ace of the filter plate is provlded with an intumescent resilient seal such that the heat from preheating and/or from the molten metal being filtered causes the intumescent resilient seal to swell thereby providing a positive seal for the filter plate in the filter chamber so as to prevent metal leakage around and/or floating of the filter plate. In accordance with the present invention it has been found that it is possible to effectively seal a removable filter plate in a filter chamber used for the filtration of molten metal without the need of an expensive filter chamber design.

BRIEF DESCRIPTION_OF THE DRAWINGS
Figure 1 is a top view of the filter chamber of the present invention including the filter plate in place therein substantially horizontally disposed;
Figure 2 is a sectional view along the lines II-II of Figure l;
Figure 3 is a partial sectional view of a simplified filter bowl design employing the principles of the present invention;
and Figure 4 is a partial sectional view of a vertically disposed filter plate employing the principles of the present invention 3o . .

DETAILED D~SCRIPTION
A typical filter chamber is i.llustrated in Figures 1 and 2, as in a molten metal transfer system, pouring pans, pouring troughs, transfer troughs, metal treatment bays, or the like.
5 The ~ilter apparatus 2 may, if desired, be constructed in two sections which may be bolted together by any suitable means, such as b~ flanges at the peripheries thereof, not shown. The particular filter apparatus illustrated in Figures 1 and 2 is a transfer trough containing a central filter chamber 3 fed by inlet 4 with the metal passing out of ~le filter chamber via outlet 5. The molten metal may enter the inlet 4 by any suitable means, such as a pouring sprout. The filter chamber 3 is a square shaped chamber, the bottom of which is recessed below the level of the inlet 4 so that the molten metal passing into the filter chamber 3 may travel downwardly through the filter plate of the present invention in place in the filter chamber. Thus, the filter chamber 3 is characterized by a peripheral rim 7 whlch may completely surround the upper portion of the filter chamber.
As shown in Figure 1, the filter chamber rim 7 surrounds the filter chamber on all sides except ad~acent the area of inlet 4.
The filter chamber rim 7 is connected to side wall 8 which extends downwardly to filter chamber floor 9 which has a circumferential wall surface 10 pro~ided with a circumferential proJection 11 adapted to mate with the corresponding perlpheral wall surface 13 and recess 14 of tapered filter seat insert 12 having bevelled portion 15. Filter seat 12 is secured in wall 3o ~3_:1L17 surface 10 by any suitable meang known in the art. The filter plate 16 has a corresponding bevelled peripheral surface 17 adapted to mate with the wall surface 10 of the filter chamber.
In accordance with the present invention, the peripheral surface of filter 17 is provided with an intumescent resilient sealing means 18 thereon resistant to the molten metal, and the filter P1ate 16 and intumescent sealing means 18 are inserted in the filter chamber 3 so that the filter plate-sealing means assembly engages the wall surface of the filter chamber.
A suitable intumescent material used for the manufacture of sealing means 18 is that manufactured by Minnesota Mining and Manufacturing Company and sold under the trademark INTERAM~.
The sheet material is produced from unexpanded vermiculite, hydrobiotite, or water-swelling tetrasilicic fluorine mica using 15 organic and/or inorganic binders to provide a desirable degree of wet stren~th. The sheet material can be produced to desirable thickness from about 0.1 to about 25 mm by paper making technlques as will be described more fully hereinbelow.
Suitable inorganic binders may include tetrasilicic 20 fluorine mica in either the water-swelling unexchanged form or after flocculation as the exchanged salt with a di- or polyvalent cation as well as fibrous material such as asbestos.
Organic binders may include small amounts of various polymers and elastomers, often added in latex form, as for example, 25 natural or synthetic rubber latices.

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The sheet material may be formed by standard paper making techniques as described, for example, in U.S. Pakent No. 3,458,329 with respect to ceramic convertible papers. From 30 to 85% by weight of intumescent material, preferably unexpanded beneficiated flakes of vermiculite ore, hydrobiotite, or water-swelling synthetic tetrasilicic fluorine type mlca are incorporated in the sheet either alone or in a combination of any of the three materials. Reinforcing a~ents (from 0 to 60~ but pre~erably from 5 to about 60%), such as chrysotile or amphibole asbestos, soft glass fibers such as available under the tradename chopped E glass, refractory filaments including zirconia-silica fibers as described in U.S. Patent No. 3,709,706, crystalline alumina whiskers and aluminosilicate fibers (available commercially under the tradenames Fiberfrax and Kaowool) or metal filaments are incorporated in order to provide integrity to the sheet material in the green state as well as in the finished intumescent sheet material and in the exfoliated sheet. From 10 to 70%
by weight of inorganic binder is used in preparing the intumescent sheet material such as water-swellable synthetic mica microflakes (U.S. Patent No. 3,001,571), asbestos, montmorillonite (bentonite, hectorite, or saponite) or kaolinite (ball clay). Synthetic mica and asbestos which are also included as components for other properties possess a sufficient ability at interlocking or con~orming to other particles that are present that they can ~ also serve in the second capacity as inorganic binders.

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When synthetic mica is utilized as a blnder~ solutlon ls prepared from unexchanged synthetic tetrasilicic fluorine mica or a suspension of exchanged synthetic mica may also be used.
Concentrations may be varied over wide ranges up to 20% by weight or more. Synthetic tetrasillcic fluorine mica can be used as a binder in one of the other of two forms. If the unexchanged mica is used (i.e., usually the sodium or lithium form), flocculation is initiated by the addition of salts of divalent or trivalent cations, such as aqueous solutions of

3 2 2' 2(SO4)3, Al(N03)3. On the other hand if the exchanged mica (e.g. Ba++, K , Ca , Mg* , Sr~+, Pb Al , etc.) is used~ flocculation is best achieved with non-ionic polyelectrolytes such as polyethylene imine or polyacrylamides such as commercially available Seperan NP10.
Small amounts of organic or inorganic fibrous materials may be added to impart additional green strength to the green sheet material. The intumescent material, one or more reinforcing agents and binder are blended to~ether followed by the flocculating agents. A small amount of surfactants or foaming agents may also be employed in order to improve the dispersion of the intumescent material without going beyond the scope of the invention. In order to avoid the use of asbestos in making the sheet, because of possible health hazards associated wlth this materlal, substitution of glass fiber materials or refractory (glass or crystalline) filaments or whiskers is possible without impairing .` '' , ~3-147 ~2'.~S~7 the quality of the sheet. In general, asbestos fibers are less expensive than other ribers. The sheet is conveniently formed by standard paper making techniques either in a handsheet former on Fourdrinier screen although other methods are discussed below. The resulting green sheet is dried at about 90C to form a handleable, flexible, resilient, intumescent sheet material.
A strip of the dry sheet 2.5 cm wide and about 1.5-2.0 mm thick will support suspended loads up to 4.0 kg or more.
The resiliency of the intumescent sheet materlal is determined on 25 mm square samples by a modification of ASTM F-36-66 test in which the penetrator is replaced with an anvil 2 inches (5.1) square and the major load is reduced from 250 lb. (113 kg) to 27.5 lb. (12.5 kg). No control of humidity is maintained.
The test is first to record the dial reading with no sample (to)~
The sample is inserted and surface irregularities are removed by applying a 5 lb. (2.26 kg) weight on the tray ~or 5 seconds.
The dial reading (tl) is recorded with only the gauge anvil and tray in place (total about 5.2 lb. or 2.36 kg). Then the ma~or load as noted is applied and after 60 seconds the dial reading (t2) is recorded and the load is removed and the dial reading (t3) again recorded after a further 60 seconds.

t - t % compressability = ~ X 100 % recovery = ~ X 100 t3 ~ 7 5'7 Compresslon of 10-35% and recovery up to about 40% are observed on the dry unexpanded sheet. The intumescent sheet is exfoliated at temperatures of about 200 to 600C. The expanded sheet maintains its integrity and a 2.5 cm wide by 1.5-2.0 mm thick strip supports suspended loads of 2.5 kg or more and maintains its integrity up to 1200C. Thermal expansion of the intumescent sheet material under a constant load of about 155 gm/cm2 produces an expansion in the thickness of the sheet o~ about 3 to 100% or more depending upon the internal bond strength, rate of heating, and composition. In general, the higher the concentration of intumescent material in the sheet or the less load, the greater the expansion.
In accordance with the present invention, the intumescent material can be used in either sheet or a paste-like form depending on the application requirements. The refractory fiber and the organic binder can be varied according to the specific requirements of the application as can the amount of intumescent material incorporated in the seal. When sheet material is used the seal can merely be glued to the peripheral wall surface of the filter plate.
As shown in Figures 1 and 2, the filter 11 is substantially horizontally disposed in a trough. The filter plate 16 is positioned in a recessed section of a filter chamber of filter bowl 3, such as in the floor portion 9 thereof. Molten metal is fed to the filter 16 via inlet 4 into the filter chamber 3.

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5~5;7 The molten metal passes downwardly through fllter 16 into the recess 19 beneath the fllter plate 16. The fllter 16 ls sealed in place by means of lntumescent resilient seal 18 whlch swells as a result of the heat generated by the molten metal being filtered thereby providing a positive seal for the filter plate in the filter chamber so as to prevent metal leakage around and/or floating of the filter plate. In addition to the foregoing, a positive seal may be achleved by preheatlng the seal prior to passing molten metal through the filter plate. This provides some added flexibility in the design of filtration systems. As noted above, the seal would tend to be self-sealing. If any metal leakage started, the surrounding seal would heat and intumesce tending to stop the leak. Even if there were chips or other irregularities on the filter seat 12, the seal 18 would swell and fill the voids thereby preventing leaks. In addition to the foregoing, in accordance with Figure 3 the abillty of the seal 18 to swell allows for a simpler filter chamber design. As noted above the seal is self-sealing which would allow the filter seat 12' to be simply a square seat 15' rather than a tapered seat 12. Likewise, the peripheral surface of the filter plate 16' need not be tapered thereby resulting in lower productlon costs. With reference to Figure 4, the intumescent seal 18 allows for easy vertlcal location of a filter plate 16 " in a filter apparatus. Again, if a metal leakage started, the seal would heat and intumesce tending to stop the leak.

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Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In the filtration of molten metal with a removable filter plate, the method which comprises:
providing a filter chamber having a metal inlet and a metal outlet and having a wall surface adapted to be partitioned by a filter plate;
providing a filter plate having a peripheral surface adapted to mate with the wall surface of the filter chamber, said filter plate having an intumescent resilient sealing means on said peripheral surface of said filter plate;
inserting said filter plate and sealing means in said filter chamber to engage the wall surface of the filter chamber;
and thereafter passing molten metal through said filter plate for discharge through said outlet, wherein the heat from said molten metal causes said intumescent resilient sealing means to swell thereby providing a positive seal for said filter plate in said filter chamber so as to prevent metal leakage around and/or floating of the filter plate.

2. The method according to claim 1 including the steps of pre-heating said filter plate after insertion of said filter plate and sealing means in said filter chamber and prior to passing molten metal through said filter plate.

3. A molten metal filter for use in the filtration of molten metal in a filter chamber having a metal inlet and a metal outlet and having a wall surface adapted to be partitioned by a filter plate having a peripheral surface adapted to mate with the wall surface of the filter chamber, the improvement which comprises an intumescent resilient sealing means secured to the peripheral surface of said filter plate wherein the heat from said molten metal causes said intumescent resilient sealing means to swell thereby providing a positive seal for said filter plate in said filter chamber so as to prevent metal leakage around and/or floating of the filter plate.