Bonded Particle Filter - Pyrotek

BONDED PARTICLE FILTERS FOR MOLTEN ALUMINIUM

METAULLICS SYSTEMS
Metaullics Systems, headquartered in Solon, Ohio, USA is a
division of Pyrotek, Inc., which is a privately owned company with
headquarters in Spokane,Washington, USA.
Pyrotek specialises in the development, manufacture and supply of
high temperature materials, technologies and engineered systems
for liquid metal management and associated metallurgical
applications. Manufacturing units and sales offices are located in
more than 60 worldwide locations and allow Pyrotek to offer
outstanding customer service and supply logistics. Products
include launder systems, distribution bowls, tundishes and
specialised insulating shapes and filters.
Metaullics is a global supplier of molten metal pumps, distribution
systems, alloying systems and scrap and waste recovery systems,
filtration systems for all sectors of the Aluminium industry.
Metaullics Systems Europe, with headquarters in The Netherlands
provides local application engineering, customer service and
warehousing facilities for Europe and The Middle East.
1. SUPERIOR FILTER MEDIA
FROM METAULLICS
It gives you the benefits of clean aluminium – reliably and
economically.
Molten aluminium normally contains various solid-phase
inclusions, such as oxides, borides, carbides and spinels, and
refractory particles from vessel erosion. If allowed to remain,
these contaminants adversely affect the characteristics of the
metal in its final, solidified form.
Impact of Inclusions on Final Product Quality
Reduced extrusion die life
Surface streaks
in bright
automotive
trim
Poor
surface
finish
Flange
cracks in
beverage cans
Inclusion
Generated
Defects
Reduced mechanical strength, ductility, fatige resistance
Effective filtering of molten (liquid) aluminium removes these
contaminants and can also reduce inclusion-nucleated porosity.
This not only improves product quality and lowers the reject rate,
creating additional profit to cover the cost of filtration. In fact,
filtration always adds value—never just cost!
However, with some commercially available filters, problems like
rapid head build-up, sudden contaminant release (dumping), filter
breakage and even ineffective or unreliable filtration can occur.
Precaution requires the use of superior media, plus special know-
how and applications expertise to lower the effective cost of
aluminium filtration and achieve maximum benefit.
Poor machine
ability
Reduced
brightness
in
anodised
products
Pinholes in
light
gauge foil

At Metaullics, we analyse your process in detail. Only then can we
determine the correct filter and specify the proper shape, size and
permeability for your specific application. Once this has been
done, you can expect trouble-free service life from your
Metaullics® filters.The media we use, bonded ceramic particulate,
provides high filtration efficiency and excellent structural strength.
Our media is used in selected wrought casthouse applications to
produce semi-finished shapes, and in aluminium gravity casting
(permanent mold), pressure diecasting, low pressure, squeeze cast
and high-performance sand foundries.
Because filtration of aluminium is a complex subject, we hope the
information in this brochure will prove useful. Let’s begin with a
look at what effective filtration should accomplish for you
1.1 INCLUSIONS TO BE REMOVED
Filtration is the separation of solids from liquids. Effective filtration
should trap and hold the major contaminants that are commonly
found in molten aluminium. Oxides of aluminium and magnesium
are among the inclusions most frequently encountered. Of these
Al2O3 is the most troublesome, because it is the most easily
formed, hence, the most abundant. Furthermore, many inclusions
have an effective specific gravity close to that of molten aluminium
and tend to stay in suspension. Spinel, another kind of
contaminant, is a complex oxide with a specific crystal structure
and the general chemical formula of AB2O4 where A and B are
metals. Common examples of spinels are MgAl2O4 and MnAl2O4.
Other inclusions to be found include carbides (especially from
primary aluminium production), nitrides, and refractory particles
from wear and erosion. In foundry alloys, especially diecasting
metal, intermetallic sludge particles (Fe-Mn-Cr complex) can
cause ‘hardspots’ which cause machining difficulty. Additionally,
recycled metal in any remelt operation always contains substantial
oxide content

2. DEPENDABLE METAL CLEANLINESS WITHOUT PROBLEMS
DEMANDS DEPENDABLE FILTER MEDIA
2.1 WHAT FILTERING DOES FOR METAL QUALITY
When effective, filtration substantially improves product unifor-
mity. Figure 1a and 1b show the difference between a filtered and
an unfiltered aluminium foil product.
Figure 1a.
Unfiltered aluminium
foil product metal
at 350X.
Figure 1b.
Foil alloy filtered
through 10 grit
Multicast
MC-6 filter.
Magnification 350X.
Filtering the molten metal results in significantly increased fluidity,
which improves castability and helps to prolong mold life.
Since inclusions, or hard spots, are minimised or eliminated, the
metal is easier to form or machine and tool wear is reduced.
With fewer casting defects, your yield and productivity are higher,
as is overall product quality.
Mechanical properties (tensile, yield, fatigue, creep and rupture
strength) can often be improved with filtration, but elongation
or ductility is especially sensitive to metal cleanliness, i.e. low
inclusion levels achieved with filtration means higher ductility. For
example, in extrusion billet casting, as shown in figure 2, the
percentage of elongation can be greatly increased, even doubled in
some cases.
Improved surface finish results from inclusion removal, important
for bright trim stock, cast wheels, or for any other surface finishing
operations (anodizing, painting and plating). Filtration also reduces
the rate of rejection and rework due to pinholes, linear defects or
tear-out in foil and sheet product production.
Figure 2.
Effect of filtration on
short transverse
elongation of 7079-T6.

Filtration is especially beneficial in removing ‘hardspots’ (oxide and
sludge inclusions) from net and near-net-shape foundry castings,
resulting in significantly increased machinibility, less tool wear
and less tool breakage! Clearly, gains in product quality, productivity
and profitability make filtration a very necessary and desirable
process for all aluminium casting applications.
2.2 COMMERCIALLY AVAILABLE FILTER MEDIA
Four major types of filters for aluminium are found in the
marketplace. Along with their predominant characteristics, they
are the following:
A. Alumina bed filters
Bed filters are comprised of ‘layers’ of tabular and ball alumina to
create ‘depth’ as the metal flows between the ‘balls’ and flakes.
The porosity of the bed is not fixed, however, so channelling of the
aluminium flow can occur, either spontaneously through surge
flow, or inadvertently through mechanical disturbances. This can
lead to dumping of already-trapped inclusions and discharge of
unfiltered metal. While deep-bed filters are usually large and fairly
costly, they do provide excellent filtration if the life is not over-
extended.
B. Ceramic foam filters
Ceramic foam filters are bonded or sintered alumina with
porosity controlled during manufacture to provide a gradation
of pore sizes 20 or 30 ppi, or grades 40, 50, 60, 65 and 80. With
high average porosity (80-85%), fairly rapid metal flow rates can be
achieved, but at the expense of filtration efficiency. With such high
porosity, mechanical strength is relatively low, and handling
damage or break-up may occur during melt flow. Providing the
proper filter grade/flow relationship is maintained for an initial
metal cleanliness, ceramic foam filters are low-cost and find their
best application as a single-use (single-cast) application in the
casthouse or foundry.
C. Bonded ceramic particles
This is the type of filter supplied by Metaullics - a bonded granular
ceramic media, fabricated either from aluminium oxide or silicon
carbide. The proprietary bonding agent employed is both strong
and resistant to chemical attack by molten aluminium.
Substantially lower porosity (nominally 40%) and the inherent
properties of the hard alumina or silicon carbide granules
provides a much stonger filter structure which aids in handling
and durability. Especially with silicon carbide construction, thermal
conductivity is substantially higher which aids in preheat and in
thermal transfer. The complex, fully-interconnected internal pore
structure provides a ‘tortuous path’ for metal flow. The bonding
agent also has a particular affinity for inclusions. This combination
of lower porosity, tortuous flow path and binder chemistry allows
the Metaullics bonded particle filter to exhibit the best balance
between cake-mode and depth-mode filtration.

D. Other filter media
In addition to the preceding filter types, woven fiberglass and
extruded ceramics may be used to filter aluminium.
Non-rigid fiberglass socks or screens provide very coarse
filtration through a sieve-like action. Inexpensive, they are used
primarily as a coarse pre-filter after a taphole or where neces-
sary to remove only the largest contaminants.
Extruded ceramics have direct-channel openings where the outlet
area is the same as the inlet area. Accordingly, they are used more
for flow control and removal of large inclusion particulate, but are
not very effective for many of the smaller but still undesirable
inclusions often existing in aluminium melts.
Figure 3 compares the features of the various filter types:
High
Temperature
Strength
Geometric
Shapes
Available
Coarse
Filtration
Fine
Filtration
Thermal
Conductivity
Resistance
to Dumping
Metaullics
Media
+ + + +
+
(SiC)
+
Bonded
Ceramic - - + - - -
Foam
Alumina
Beds
Woven
Fibers
N/A - + + - -
N/A - + - - -
Figure 3. Characteristics of filter media.
With the Metaullics bonded particle filter, the aluminium caster often
has greater flexibility in achieving filtration efficiency objectives than
with other types of filters.
2.3 HOW FILTRATION WORKS
Filtration involves the science of separating solid particles from a
liquid stream as the metal flows through a porous body. All three-
dimensional filters function to various extents through cake-mode
(inclusion capture on the filter inlet surface) and depth-mode
(filtration within the body of a filter). In Metaullics filter media, fil-
tration takes place by an excellent balance between these two modes.
Figure 5a and b.
Depth filtration
vs cake.
Figure 4.
Illustrates the liquid
flow (main arrow) and
shows solid particles
carried by various
forces to the surfaces
of the grains.

In depth filtration, inclusions are removed by a two-step capture
mechanism - transport from the liquid metal flow and attachment
to the grain surfaces. Strong attachment forces are essential to
prevent the suspended solids from being swept away, which could
lead to the dumping of contaminants back in to the filtered metal.
Some of the transport forces that come in to play are illustrated
in figure 6. Inertia, shown at (a), tends to keep solid particles
moving in a straight line wherever the liquid flow turns.When the
direction is downward, the effect of gravity causes sedimentation,
especially at lower fluid velocities.
Direct interception or impingement, diagrammed at (b) occurs
when inclusions collide with protrusions in the grain surfaces.The
effect of fluid dynamics appears at (c). Resistance offered by the
irregular surfaces makes the flow slower at the edges than in the
centre of the liquid stream and solid particles tend to be drawn
into the slower flow and deposited on the surface.
Attachment forces are equally important but largely self-
explanatory. Any or all of the following may come into play in
trapping and retaining particles on the grain surfaces:
- Gravity and/or friction
- Physical entrapment (in pockets or crannies)
- Van der Waals forces (intermolecular attraction)
- Chemical bonds
- Electrostatic forces
The Metaullics filter media contains a proprietary binder which
exhibits a special affinity for inclusion attachment through these
mechanisms.
Figure 6.
Major transport forces in depth filtration.
a. inertia, sedimentation, b. direct interception,
c. fluid dynamics

2.4 HIGH-EFFICIENCY FILTRATION
Higher efficiency filtration is achieved with a lower-porosity, more tortuous path filter body - exactly what the Metaullics bonded
particle filter provides. A tortuous flow path within the filter body permits the fluid stream to thoroughly remix, and each thorough
re-mixing of the stream increases the amount of inclusions captured. This high filtration delivers aluminium clean enough for critical
uses like aerospace, can stock, bright trim, thin foils, fine wire or intricate and highly machined castings.
Deep-bed filters can be very efficient under best operating conditions and 95% average
removal efficiency for all particles has been claimed. Data for ceramic foam filters show
efficiencies greater than 80% for coarse particles (over 50), but only 30-60% for finer
particles (25/50). Filtration efficiency for bonded particle filters compares very
favourably in grade-for-grade comparison.
Figure 7 presents a 6 grit bonded particle filter’s efficiency as measured by matrix
dissolution and Coulter counter particle analysis, compared with data from published
sources on a 30 ppi ceramic foam filter.
In general, the filtration efficiency for the full range of grit sizes of bonded particle filters
available is shown in figure 8 as a function of metal flow rate and of a wide variety of
casting applications.
Figure 7.
Figure 8.

3. SUPERIOR FILTER MEDIA... FORMS AND PROPERTIES
3.1 PROPERTIES AND PARAMETERS
Metaullics filters are manufactured in a wider range of shapes and pore sizes
than any other media. Pore diameter is determined by the size of the grit used
to construct the media, and also determines the flow rate capability, size of
inclusion removed and starting head. Figures 9 to 12 portray these relationships.
3.2 PHYSICAL PROPERTIES OF METAULLICS BONDED
PARTICLE FILTER MEDIA
Composition Al2O3 or SiC
Apparent density 2.2-2.3 grams/cm 3 (Al2O3)
Average porosity Approximately 40%
Available grits 6 - 30
Pore diameters 0.063” 0.012” (1600 - 350)
Max. preheat temperature 890°C (1634°F)
Max. working temperature 815°C (1500°F)
MOR at 750°C 450 psi 20 grit Al2O3
300 psi 8 grit SiC
Figure 11.
Starting-head
requirements for
Metaullics media.
Figure 9.
Bonded partical filter
pore diameter
comparisons.
Figure 10.
Aluminium permeability
for Metaullics filter
media as a function of
grit size.
Figure 12.
Pore diameters of
Metaullics media.

4. METAULLICS BONDED PARTICLE FILTER PRODUCTS
1 2 3 4
CASTHOUSE APPLICATIONS
1. CARTRIDGE FILTER MCF
• Recognized as the superior filter
configuration for premium product
wrought application such as canstock,
fine foil, lithographic sheet, electronics
• Originated by Metaullics over 35 years
ago
• Standard tube assemblies available -
7, 11, 14, 18, 22, 28 to meet a spectrum
of casting flow rate requirements
• Filter grades 11, 14, 16, 18, 20, 24 grit
2. TROUGH TUBE FILTER
• Lower cost ‘cartridge’ or tube filtration
specifically designed for continuous
casting
• Capability to 200 lb/min (90 kg/min)
casting flow rate with 20 grit
• Produce 6 micron foil
• Filter change-out and restart in less
than two hours
3. MULTICAST FILTER ELEMENT
• Best quality and cost/ton filter option
for many applications
• Lower cost/ton than ceramic foam filter
• Holds metal ready-at-temperature for
casting between casts
• Integrated handling system for easy
filter change
• Extended filter life (several hundred
tons) and 1 hour filter change-over
4. DUAL VERTICAL GATE FILTER
• Dual usage filter capability , ideal for
wide range for continuous casting
operations
• Vertical filtration mode
• Compact unit, easy filter change-over
• Extended filter life (several hundred
tons) for lowest filter cost/ton
• Heated unit available with overhead
heating or immersion heating
• Integrated filter handling system
FOUNDRY APPLICATIONS
5. VERTICAL GATE FILTER (VGF)
• No capital investment or furnace
modification required
• Easy application for melting and casting
furnaces - no furnace draining required
• Separates dipwell from heating
chamber
• Normal filter life 2 - 3 months, minimal
maintenance
6. METAULLICS BOX FILTER (MBF)
• Designed for many sizes to
accommodate specific manual or
auto-ladling space requirements in
casting furnaces and provide point-ofpour-filtration
for permanent mold or
pressure diecasting
• Useful when melting and pouring from
a single vessel
• Supported by legs or by stud/clamped
spanner bars on furnace sill
• Normal filter life 2 - 4 months
depending on filter grade

5 6 7 8 9
7. FILTER PUMP
• Provides first-stage filtration in central
melting or re-melt furnaces
• Transfer filtered metal between
furnaces
• Fill ladles without entraining re-melt
furnace sludge or dross
• Minimise superheat required to
accommodate tap and transfer time
lags
• Overcome barriers to gravity flow
• Prevent carry-over of sludge and other
re-melt debris
• Filter life dependent on usage and
overall metal cleanliness
8. STALK TUBE FILTER
• Direct filtration each cycle in low
pressure casting
• Significant reduction in inclusion-related
scrap for cast wheels and other high
performance castings
• Many shapes and designs available to
accommodate stalk (riser) tube and LP
furnace designs
• Filter life can be matched to stalk tube
life if cast iron or fused silica is used.
Filter can be changed without damaging
higher performance ceramic stalk tubes
• Resists erosion of stalk tube during
mold filling
9. CYLINDER FILTER
• Used with low pressure casting
furnaces in wheel and other
automotive production
• Surrounds stalk tube, does not require
connection and does not depend on
stalk tube life or change-out
• Easily maintained
• 8-12 week filter life for maximum
filtration economy
10. CRUCIBLE BAFFLE FILTER
• Separates pour-in section from ladling
or dip-out section
• Achieves filtered product quality in
casting crucibles and when melting and
casting from the same crucible if metal
level and temperature are maintained
relatively constant
• Useful when space constraints do not
permit use of MBF
• Filter held in position with fixed lugs on
crucible ID
• Filter change-out and crucible re-use
are attained with minimal effort
11. DOSING FURNACE FILTER
• Creates filtration directly in dosing
furnaces
• Filters installed onto the ceramic
dosing tube with special gasketing
• Nominal 3-5 week life
• Spent filters removable from dosing
tube for dosing tube re-use.
10
11

Metaullics Systems Europe B.V.
Ebweg 14
2991 LT Barendrecht
The Netherlands
Tel.: +31 180 590 890
Fax: +31 180 551 040
metaullics.info@pyrotek-inc.com
www.metaullics.com
Metaullics Systems Division, Pyrotek, Inc.
31935 Aurora Road
Solon, Ohio 44139
United States
Tel: +1 440 349 88 00
Fax: +1 440 248 34 32
metaullics.info@pyrotek-inc.com
www.metaullics.com
© Metaullics 2007
M3000E