Note: Descriptions are shown in the official language in which they were submitted.
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MOLTEN METAL FILTRATION
This invention relates to the filtration of molten metals, and
especially to the filtration of molten aluminium and molten aluminium alloys,
and
the invention will be particularly described with reference to those metals.
Molten metals, and in particular aluminium have been filtered for
many years by means of a variety of filtration devices, and in the aluminium
industry it is now common practice to filter molten aluminium and molten
aluminium alloys prior to casting. Although various types of filter have been
used
in the past the filters which are now most commonly used are ceramic foams,
which are produced by impregnating polyurethane foam with a slurry containing
refractory material and a binder, and then drying and firing the impregnated
foam,
and fitters of bonded particulate material made by mixing together refractory
material and a binder, forming the mixture to a desired shape by pressing, and
heating the formed shape.
Ceramic foam filters and their use in the filtration of molten metal are
described in a number of patents, for example US 3090094, US 3893917, US
3947363, US 4024056, US 5190897 and US 5520823. US 4024056 describes the
application of a bevelled, removable ceramic foam filter plate having pore
sizes of
approximately 2 - 18 pores per linear centimetre or 5 - 45 pores per linear
inch
(ppi), and this type of filter is used extensively in the filtration of molten
aluminium.
US Patent No. 3524548 describes a rigid, porous filter medium for
aluminium made by bonding together particles of refractory material with a
vitreous material which is resistant to molten aluminium. The filter is used
in the
form of a plate or a tube. US Patent No. 3747765 describes rigid filter tubes
which
are used for filtering molten metal, and which are made from a similar
admixture of
refractory particles bonded together with a prefused vitreous binder.
US Patent No. 4964993 describes molten metal filters which consist
of at least one cylindrical, close-ended porous ceramic filter element in a
vertical
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orientation connected to an essentially horizontal porous ceramic sealing
plate
filter element. The vertical filter elements may be made from bonded
particulate
refractory material or they may be in the form of a ceramic foam.
Ceramic foam filters have been commercially exploited successfully
for many years because of their ease of use, low cost and small unit footprint
relative to bonded particulate filters. However, their pore size is larger
than the
pore size of bonded particulate filters, and consequently filtration
efficiencies are
lower. Ceramic foam filters are extensively used as fine as approximately 24
pores per centimetre or 60 ppi. However as the pore size decreases they become
more difficult to make because it becomes more difficult to impregnate the
finer
pore size polyurethane foam with the slurry of refractory material. An
increased
number of blocked pores are formed and consequently the porosity available for
filtration is reduced. This results in a reduction of flow rate per unit
surface area,
and can also cause application problems during priming or a reduction in
filtration
capacity.
Bonded particulate filters offer a number of advantages over ceramic
foams when used as filters for molten aluminium. They are generally of higher
strength and higher density, and since they have lower porosity, have a more
tortuous flowpath and do not contain blocked pores, they give a greater
filtration
efficiency in terms of removal of oxide and other inclusions from the
aluminium.
However, the high density and low porosity (typically 30 - 45 %) of
bonded particulate filters can be disadvantageous, and have limited their
commercial exploitation. As bonded particulate filters are more dense than
ceramic foams they require significantly more preheating in order to achieve
complete priming of the filters at the start of filtration and optimum
filtration
performance, and it is very difficult to achieve the required preheat quickly
and
uniformly. Some bonded particulate filters also have other disadvantages which
result from their preheating requirements. For example, when used in the form
of
plates in aluminium cast-house filtration equipment, it is necessary to
preheat the
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plates to permit molten aluminium to flow through the plates without
solidifying.
This preheating of the plates is usually done by direct flame impingement on
the
bottom face of the plates and because some bonded particle filter plates have
poor thermal shock resistance, spalling of the filter plates can result.
The high density of bonded particulate filters also makes changing
bonded particulate filter elements a very labour intensive operation. The low
porosity not only limits flow rates per unit of surface area, but it also
increases
interstitial metal velocity within the pores of the bonded particulate filter.
In order to
overcome the problem of the limited flow rate it is necessary to utilise
bonded
particulate filter systems having a very high surface area. This is normally
accomplished using bundles of long tubes which require the use of a box or
container with a prohibitively large footprint.
In Japanese Patent Application Laid-Open No. 1-141884 it has been
proposed to use as a filter for molten metal a carbon foam substrate which has
been coated with silicon carbide by chemical vapour deposition. The coated
material has a porosity of 90 to 99 %, and it is stated that when filtering
molten
aluminium blockage of the pores and loss of aluminium are excessive if the
porosity is less than 90 %, and that if the porosity exceeds 99 % efficiency
of
impurity removal is poor, and the strength of the foam is insufficient.
It has now been found that, in contrast to what is taught in Japanese
Application Laid-Open No. 1-141884, superior filters for filtering molten
metal, and
in particular molten aluminium, can be produced by depositing a refractory
metal
or refractory compound on a carbon foam substrate by chemical vapour
deposition, if the filters have a filtration porosity of less than 90 % and a
ligament
solid fraction of at least 0.95.
According to the present invention there is provided a method of
filtering molten metal comprising providing a filter comprising a porous
carbon
foam substrate coated substantially throughout with a refractory metal or
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refractory compound formed by chemical vapour deposition, the filter having a
filtration porosity as hereinafter defined of less than 90% and a ligament
solid
fraction as hereinafter defined of at least 0.95, and causing molten metal to
flow
through the filter so as to remove inclusions contained in the molten metal.
In the context of this invention a ligament is defined as one of the
substantially solid interconnected struts which form the structure of a foam.
Filtration porosity is defined as the percentage of the total volume of
a foam filter body which is open, interconnected space exterior to the
ligaments,
and available for molten metal filtration.
The void fraction is the total volume of void space within a foam filter
body divided by the volume outlined by the foam filter body, and void space in
a
foam filter body is any volume which is not solid. For example for a foam
filter
body having dimensions of 100 mm x 100 mm x 100 mm, and containing 850,000
mm3 of void space, the void fraction is 850,000/(100 x 100 x 100) or 0.85.
Void fraction can be expressed in the following manner
Void fraction - 1 - density of foam filter body
theoretical density of foam ligament
For example, a 100 mm x 100 mm x 100 mm foam filter body having
a mass of 420 g has a density of 0.42 g/cm3. The theoretical density of the
ligament if the foam is an alumina foam is 2.8 g/cm3. Therefore the void
fraction is
1 - (0.42/2.8) or 0.85.
The ligament solid fraction is defined as the volume of solid material
forming the ligament divided by the volume outlined by the ligament.
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For example, a filter with dimensions 100 mm x 100 mm x 100 mm,
having a ligament volume of 150,000 mm3 and a ligament solid volume of 75,000
mm3 has a ligament solid fraction of 0.50.
Preferably the filters used in the method of the invention have a
filtration porosity of at least 75%, more preferably at least 80%.
The carbon foam is preferably a reticulated carbon foam, and the
porous carbon substrate may be formed by pyrolysis of a porous organic
substrate. For example, a polyurethane foam may be pyrolised, and if desired
the
polyurethane foam may first be impregnated with a resin or a similar organic
material. European Patent Application Publication No. 747124A describes a
suitable method for the production of a carbon foam in which polyurethane foam
material is impregnated with a resin, and the resin impregnated foam is then
pyrolised at a temperature of 600 to 1200 °C to convert the
polyurethane and resin
to carbon.
As used herein the term chemical vapour deposition includes both
chemical vapour deposition and chemical vapour infiltration. In chemical
vapour
deposition solid phase refractory materials are nucleated and grown from gas
phase precursors, for example a coating of silicon carbide may be deposited on
a
porous carbon substrate by the decomposition of methyltrichlorosilane. The
chemical vapour deposition may be carried out by known techniques, and
suitable methods of coating carbon substrates by chemical vapour deposition
are
described in US Patents Nos. 5154970, 5283109 and 5372380 and in European
Patent Application Publication No. 747124A.
The refractory metal or refractory compound which is coated on to
the porous carbon substrate by chemical vapour deposition to produce the
filter
must be compatible with the molten metal to be filtered, i.e. it must be
resistant to
the molten metal, and it must not contaminate the molten metal. Refractory
metals
which can be applied to porous carbon substrates by chemical vapour deposition
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to produce filters for use in the method of the invention include tungsten and
molybdenum. Suitable refractory compounds include silicon carbide, silicon
nitride, zirconium carbide, zirconium nitride, zirconium boride, niobium
carbide,
niobium nitride, niobium boride, titanium carbide, titanium nitride, titanium
boride,
hafnium carbide, hafnium nitride and hafnium boride. The preferred coating
material is silicon carbide as it has suitable physical properties, such as
relatively
low density and a high thermal conductivity, and enables filters for use in
the
method of the invention to be made economically.
Particularly when the method of the invention is practised using a
reticulated carbon foam, which has been coated with a refractory metal or
metal
compound by chemical vapour deposition, the filter has a number of advantages
over conventional ceramic foam filters made by impregnating an organic foam
with
a slurry of particulate refractory material.
The deposition of coated refractory metal or metal compound is
preferably a minimum of 0.25 g/cm3. The strength of the individual ligaments
and
of the filter body itself are dependent on the amount of solid matter coated
on to
the ligaments. The nature of the chemical vapour deposition process is such
that
small variations in coating density throughout the carbon foam may result, and
if
the intended coating density is less than 0.25 g/cm3 these variations may
cause
the filter to have insufficient strength of corrosion resistance to withstand
the
industrial environment.
Individual ligaments can be broken in handling if they are not coated
with a sufficient amount of refractory metal or refractory compound, and a
filter
body can be broken in handling if it does not have sufficient strength. It has
been
found in practice that filters which are to be used to filter molten aluminium
for
greater than one week need to have a minimum crush strength of 35 kg/cm2 (500
Ib/in2), and the filters used in the method of the invention meet this
requirement.
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An important feature of the filter used in the method of the invention
is that, in the as manufactured state, essentially all the void fraction of
the filter
body is available for molten metal flow, and the void fraction is the same as
the
above defined filtration porosity. Higher accessible porosity results in
higher
molten metal flow rate or lower interstitial velocity at a constant pore size
and
surface area. A lower interstitial velocity is associated with a higher
capture
efficiency of inclusions in molten metal and therefore greater overall
filtration
efficiency. The higher porosity of the filter also results in a greater
storage volume
for captured inclusions, and this means that a single filter is capable of
filtering a
greater throughput of metal before clogging of the filter, and the need to
replace
the filter, occurs.
This is in contrast to traditional ceramic foam filters which have a
number of voids contained in the hollow centre of the ligament, and as
microporosity with the ligament itself. These portions of the void fraction
are
inaccessible to molten metal flow because of the high pressure which would be
required to force molten metal into such fine capillaries, and so are not
included in
the filtration porosity. Additionally, many voids can become inaccessible to
molten
metal flow due to blocked areas being formed in the ceramic foam. This further
decreases the filtration porosity compared with the void fraction.
Figures 1 and 2 of the accompanying drawings show a comparison
of a traditional ceramic foam filter and a chemical vapour deposited
refractory
metal or refractory compound coated carbon foam filter as used in the method
of
the invention.
Figure 1 is a diagrammatic representation of a section of a ligament
of a traditional ceramic foam filter produced by impregnating a polyurethane
foam
with a ceramic slurry and firing to burn out the foam. The filter has a hollow
ligament core(A) and a ceramic ligament (B) containing voids and having a
solid
fraction of 0.50.
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Figure 2 is a diagrammatic representation of a section of a ligament
of a filter for use in the method of the invention produced by coating a
carbon
foam substrate with a refractory metal or refractory compound by chemical
vapour
deposition. The filter has a solid carbon ligament core (C) and an essentially
void
free ligament coating of refractory metal or refractory compound (D) having a
minimum ligament solid fraction of 0.95.
In addition to reducing the filtration porosity, voids in the ligament
also reduce the strength of the ceramic foam filter body.
In order to be useful in an industrial filtration process, a filter body
must be strong and durable in order to withstand handling, installation and
use.
On the other hand to operate efficiently as a filter, the filter body must
have a
sufficiently high filtration porosity to operate at industrial flow rates with
low
pressure drops and a minimum filtration system footprint. All other factors
being
the same, the greater the void fraction of the filter body the lower its
strength.
In the filters used in the method of the invention the strength of the
flter body is maximised at a given void fraction as 100% of the void fraction
is
incorporated in the filtration porosity.
For example, in producing a silicon carbide filter for use in the
method of the invention, a carbon foam substrate with nominally 3% volume
solid
or 97% filtration porosity and essentially void free carbon ligaments is
coated with
a uniformly distributed, essentially void free silicon carbide coating to a
minimum
amount of 0.32 g/cm3. A void free solid volume of chemical vapour deposited
silicon carbide has a density of 3.2 g/cm3, and a silicon carbide coating of
0.32
glcm3 is a 10% solid by volume of silicon carbide. The total volume percent of
solids is the 3% carbon and the 10% silicon carbide. The filter body has a
void
fraction of 0.87 all of which is accessible to molten metal so the filter has
a
filtration porosity of 87%.
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The filter contains no binders, no sintering aids (because no firing or
sintering of the filter is necessary after its production) and no impurities.
The filter
therefore has improved corrosion resistance in contact with molten metal and a
higher strength to weight ratio. Compared with a conventional ceramic foam
filter
the filter is both stronger and has a lower density. As the density of the
filter is
lower the filter is more readily preheated.
As a result of these advantages, when using the method of the
invention, it is possible to utilise a filtration system having higher
filtration
efficiency and longer life, when compared to a system which contains ceramic
foam filters or bonded particulate filters. It may also be possible to design
the
filtration system such that it is smaller and more compact.
The filters used in the method of the invention may be in the form of
plates, but they are preferably in the form of tubes. In the aluminium
industry a
plurality of tubes is commonly referred to as a cartridge. Although plates are
commonly used to filter molten aluminium they are disadvantageous because, in
order to provide sufficient filter surface area to enable large quantities of
molten
aluminium at an acceptable rate of throughput, it is necessary to make the
filter
plates very large, and the furnace or vessel in which the filtration is
carried out,
also has to be large in order to accommodate the filter plates. For this
reason,
there is a desire to use filters whose shape provides a greater surface area,
and
which require relatively less space to accommodate them, and tube or cartridge
filters are advantageous in these respects.
Tube or cartridge filters made from conventional ceramic foam have
achieved little commercial use because such filters are difficult to make,
they have
relatively low strength, they have blocked pores, they are difficult to
preheat and to
wet with the molten metal, and they are only suitable for applications in
which they
are in contact with molten metal for a relatively short time. Therefore, tube
or
cartridge filters, which are used commercially in the aluminium industry, are
usually bonded particulate refractory filters.
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Carbon foams, coated with a refractory metal or refractory
compound by chemical vapour deposition, and having a filtration porosity of
less
than 90 % and a ligament solid fraction of at least 0.95, suffer from none of
these
disadvantages, and that is why they are particularly suitable for use in the
method
of the invention as tubes or cartridges.
The method of filtering molten metal may be practised, for example,
in filtration apparatus comprising a holding vessel having an inlet and an
outlet for
the molten metal, at least one filter housing, the or each housing containing
a filter
comprising a porous carbon foam substrate coated substantially throughout with
a
refractory metal or refractory compound formed by chemical vapour deposition
and having a filtration porosity of less than 90% and a ligament solid
fraction of at
least 0.95, means for holding and sealing the filter or filters in place, and
means
for preheating the holding vessel and the filter or filters prior to using the
apparatus for filtering molten metal.
Therefore, according to a further feature of the invention there is
provided apparatus for filtering molten metal as hereabove described.
The filters used in the above apparatus may be filter plates but in a
preferred embodiment filter tubes are used, and the filters are preferably
substantially horizontally disposed tubular shaped filter elements.
The sealing means should be capable of sealing the filters in place
so as to ensure that all the molten metal passes through the filters without
exerting
excessive force on the filters, and it is desirable that the configuration of
the
housing and the sealing means used are such that the filters may be readily
installed and removed. The heating means may be for example one or more gas
fired burners or electrical heaters, which may be located in a lid which is
fitted to
the top of the holding vessel. It is important that the filters are located in
the
holding vessel in relation to the inlet such that when the holding vessel is
filled
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with molten metal via the inlet the height of the surface of the molten metal
in the
holding vessel above the filters is sufficient to prime the filters, i.e. to
initiate flow of
molten metal through the filters.
Since the inner surface of the vessel must withstand molten metal
such as aluminium and should not contaminate the metal, the vessel should
either
be made from or be lined with a refractory material which meets those
requirements. In order to reduce heat loss through the walls of the vessel a
lining
of refractory heat-insulating material may be installed between the shell of
the
vessel and the refractory lining. Similarly, to reduce heat losses the lid may
also
be lined with a thermally insulating material.
Examples of suitable refractory materials for forming the inner, metal
contacting surface of the vessel are fused silica based castable materials
having a
density of approximately 1.8 g/cm3. Examples of thermally insulating materials
for
the walls of the vessel and for the lid are refractory castable materials
having a
density of below 1.4 g/cm3 and also ceramic fibre containing materials in
block,
blanket or module form.
The invention is illustrated with reference to Figures 3 to 6 of the
accompanying drawings in which :-
Figure 3 is a horizontal section through molten metal filtration
apparatus according to the invention containing a plurality of filter tubes
Figure 4 is a vertical section through the apparatus of Figure 3
Figure 5 is a detail of the molten metal filtration apparatus of Figure
3 showing how the filter tubes are located in the apparatus and held in place
and
Figure 6 is a vertical section through another embodiment of molten
metal filtration apparatus according to the invention.
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Referring to Figures 3 to 5, an apparatus for filtering molten metal
comprises a refractory lined vessel 1, an inlet 2 for the molten metal, and
outlet 3
for the molten metal, and a refractory lined lid 4 having heating means 5
located
therein. The interior of the vessel 1 is divided into an inlet chamber 6 and
an outlet
chamber 7 by a refractory baffle plate 8 extending across and near to the top
of
the vessel 1 and having an aperture 9 connecting the inlet chamber 6 and the
outlet chamber 7. An array of eleven filter tubes 10 in the form of pyrolised
reticulated carbon foam which has been coated with silicon carbide by chemical
vapour deposition, and having a filtration porosity of 87% and a ligament
solid
fraction of 0.995, is located near the base of the vessel 1 so that the filter
tubes
are in three rows, one above the other. One end of each of the filter tubes 10
is
inserted in a recess in a refractory plate 11, and the opposite end of each of
the
filter tubes is located in an apertured recess in a refractory plate 12 so
that the
apertures in the recesses are aligned with the aperture 9 in the baffle plate
8. The
filter tubes 10 are held and sealed in position without exerting excessive
force on
the filter tubes 10 by holding means 13 which fits over the filter tubes 10
near their
end which is adjacent the aperture 9 in the baffle plate 8. The location of
the filter
tubes 10 in the vessel 1 is such that the height of the top of the vessel 1
above top
row of the filter tubes 10 is sufficient for the filters to be primed when the
inlet
chamber 6 is filled with molten metal.
Before the apparatus is used to filter molten metal the interior of the
vessel 1 and the filter tubes 10 are preheated by the heating means 5, which
may
be for example a gas fired burner or an electrical heater. In the case of
molten
aluminium filtration it is necessary for the preheating to be sufficient for
the inner
surface of the filter tubes 10 to be heated to at least 600 °C. The
heating means 5
also maintains the temperature of the molten metal on the vessel 1.
In use molten metal enters the inlet chamber 6 via inlet 2, is filtered
through filter tubes 10, passes into the outlet chamber 7 through the aperture
9 in
the baffle plate 8, and exits the vessel via outlet 3.
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Referring to Figure 6, an apparatus for filtering molten metal
comprises a refractory lined vessel 21 having an inlet for the molten metal
(not
shown), and an outlet 22 for the molten metal, and a refractory lined lid 23.
The
interior of the vessel 21 is divided into an outlet chamber 24 and an inlet
chamber
25 by a refractory baffle plate 26 extending across the vessel 21 and having
an
aperture 27 connecting the outlet chamber 24 and the inlet chamber 25. Four
filter tubes 28 in the form of pyrolised reticulated carbon foam which has
been
coated with silicon carbide by chemical vapour deposition, and having a
filtration
porosity of 87% and a ligament solid fraction of 0.995, are located in line
near the
base of the vessel 21 in inlet chamber 25. One end of each of the filter tubes
28 is
inserted in a recess in a refractory plate 29, and the opposite end of each of
the
filter tubes is located in an apertured recess in a refractory plate 30 so
that the
apertures in the recesses are aligned with the aperture 27 in the baffle plate
26.
The filter tubes 28 are held and sealed in position without exerting excessive
force
on the filter tubes 28 by holding means 31 which fits over the filter tubes 28
near
their end which is adjacent the aperture 27 in the baffle plate 26. The
location of
the filter tubes 28 in the vessel 21 is such that the height of the top of the
vessel
21 above the filter tubes 28 is sufficient for the filters to be primed when
the inlet
chamber 25 is filled with molten metal.
The refractory lined lid 23 has two apertures 32, 33 which are in line
with apertures 34, 35 in the top of the vessel located above the chambers 24,
25
respectively, and two plugs 36, 37. Before the apparatus is used the plugs 36,
37
are raised so that they close the apertures 32, 33 in the lid 23, and the
interior of
the lid 23 and of the vessel 21 are preheated by means of gas burner 38. After
the
preheating operation the plugs 36, 37 are lowered so as to close the apertures
34,
35 in the top of the vessel.
In use molten metal enters the inlet chamber 25, is filtered through
the filter tubes 28, passes through the aperture 27 in the baffle plate 26
into the
outlet chamber 24, and exits the vessel 21 via the outlet 22. During
filtration
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molten metal in the vessel 21 is maintained at the desired temperature by
means
of immersion heaters 39 located in the inlet chamber 25 and outlet chamber 24.
The following Examples will also serve to illustrate the invention :-
EXAMPLE 1
In practice it has been found that filtration tubes which are to be used
for the filtration of molten aluminium on a commercial basis must have a
minimum
crush strength of greater than 35 kg/cm2 (500 Ib/in2). A series of experiments
was
carried out to establish the relationship between crush strength and
filtration
porosity.
Carbon foam substrates of nominally 97% filtration porosity and
having void free carbon ligaments were coated with various levels of silicon
carbide by chemical vapour deposition, to produce plate filters having a
ligament
solid fraction of 0.995 and a range of filtration porosities. A number of
coated
plates was produced for each level of silicon carbide, and hence each
porosity. A
section was cut from each plate and its crush strength was determined. The
ligament solid fraction was measured by mounting and polishing a section of
the
plates, and inspecting the ligaments visually.
The silicon carbide solid by volume (%), the filtration porosity (%) and
the minimum crush strength for each level of silicon carbide coating are shown
in
the table below.
Silicon Carbide SolidFiltration Porosity Minimum Crush Strength
By Volume (%) (%) (kg/cmz)
2 95 5.6
3 94 9.8
4 93 15.5
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92 21.1
6 _ _ 91 28.1
_. g0 35.2
8 8g 42.9
g - 88 51.3
- 87 59.8
EXAMPLE 2
Two plate filters 30.5 cm x 30.5 cm x 2.5 cm thick were prepared,
one having a pore count of approximately 26 pores per centimetre (ppc) or 65
ppi
and the other having a pore count of approximately 32 ppc or 80 ppi, both
having
filtration porosity of 87% and a ligament solid fraction of 0.995. Reticulated
polyurethane foam was pyrolised to produce carbon foam and the carbon foam
was machined to the required size. The carbon foam was then coated with
silicon
carbide by chemical vapour deposition to produce the filter. A filter was
inserted in
a 50 cm deep filter bowl, and the filter and bowl were preheated using a gas
torch.
It was found that due to their high thermal conductivity and low thermal mass
the
filters heated up almost instantly. Approximately 3175 kg of molten aluminium
alloy 6061 were then poured into the filter bowl and passed through the
filter. In
the test using the 32 ppc or 80 ppi filter the molten aluminium alloy was
deliberately contaminated by the addition of 5 to 50 micron size alumina. In
the
case of the 26 ppc or 65 ppi filter it took about 1 hour for all the molten
aluminium
alloy to be filtered, corresponding to a flow rate of approximately 0.05
kglmin/cm2.
In the case of the 32 ppc or 80 ppi filter a flow rate of approximately 0.03
kg/min/cm2 was used. In both tests samples of the alloy were taken after
filtration,
and tested for the presence of inclusions using an inclusion concentration
technique. Both samples were completely free of inclusions.
EXAMPLE 3
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A tube filter having an external diameter of 4 inches and an internal
diameter of 2 inches was produced by pyroiising reticulated polyurethane foam
having a pore count of 26 ppc or 65 ppi, and coating the resulting carbon foam
with silicon carbide produced by chemical vapour deposition so as to produce a
filter having a filtration porosity of 87% and a ligament solid fraction of
0.995. The
tube filter was cut to a length of 205 mm and tested as a filter for molten
aluminium in a filter box as shown in vertical section in Figure 7 of the
accompanying drawings.
The filter box 40 which was lined with refractory material had an inlet
chamber 41 and an outlet chamber 42 separated by a refractory baffle plate 43.
The filter tube 44 was positioned horizontally near the bottom 45 of the
filter box
40 with one end inserted in refractory plate 46 and the other end inserted in
a
recess 47 in the baffle plate 43. A fibre gasket (not shown) was placed around
the
ends of the filter tube 44 to seal the filter tube 44 in place, and prevent
bypass of
molten aluminium. The baffle plate 43 had an aperture 48 with which the end of
the filter tube 44 was aligned. The height of the inlet chamber 41 above the
filter
tube 44 was 240 mm. The inside of the filter box 40 and the filter tube 44
were
preheated by means of two gas burners inserted in the inlet chamber 41 and the
outlet chamber 42 until the filter tube glowed red. Care was taken to avoid
direct
flame impingement from the burner in the inlet chamber 41 on to the filter
tube 44.
After preheating the surface of the filter tube 44 showed no evidence of
cracking.
Molten aluminium alloy of purity 99.5 % was poured into the filter box
40 from a tilting furnace of 800 kg capacity at a temperature of 746
°C. The molten
alloy started to prime the filter tube 44 when the height of the molten alloy
above
the filter tube 44 reached approximately 200 mm, as indicated by air trapped
in the
filter tube 44 escaping to the surface, and the appearance of molten alloy in
outlet
chamber 42. Once a steady state was reached the difference in head height of
the
molten alloy in the two chambers was approximately 25 mm.
CA 02311810 2000-OS-26
- WO 99/28273 PCT/GB98/03436
-17-
Three billets were cast in sequence. Each billet was approximately
3.5 m long and 190 mm in diameter. The fill speed was 95 mm/minute which
equates to a flow rate of about 8 kg/minute for the total of 800 kg of molten
alloy.
There was about 15 minutes between casts so the filter tube 24 was immersed in
the molten alloy for a total of 2.5 hours.
After the third billet had been cast the filter tube 24 was removed
from the filter box 20 and inspected. There was no evidence of any form of
degradation in the filter material. The effectiveness of the filter material
in
removing inclusions from the molten aluminium alloy was confirmed by
sectioning
the filter tube, and examining the section microscopically.
