Note: Descriptions are shown in the official language in which they were submitted.
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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
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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
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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
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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
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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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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
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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
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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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3~
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
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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
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~ [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
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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
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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
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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
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?.;~ 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
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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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32~
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
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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
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~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
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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.
~ :
,
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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
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~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.
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