Title of Invention

CERAMIC HOT-GAS FILTER

Abstract A ceramic hot gas filter, comprising: a porous elongated filter support, said support having an outer surface, an opening at one end into a hollow interior defined in part by an inner surface, a closed end opposite said open end, and an external flange integral with said open end, said support being formed of a plurality of layers of oxide ceramic yarn, each layer being arranged in a crisscrossing relationship with neighboring layers to form a plurality of quadrilateral- shaped openings, said yarn being coated with a first oxide ceramic material, said first oxide ceramic material providing, upon heat treatment, a porous refractory oxide support matrix; and a porous membrane layer contacting the outer surface or inner surface of said support, said membrane layer being less porous than said support and comprising (1) at least one circularly wound continuous filament oxide ceramic yarn, adjacent windings of said ceramic yarn defining a gap therebetween, said yarn being coated with a second oxide ceramic material, and (2) at least one ceramic filler material disposed in said membrane surface and substantially uniformly distributed therein.
Full Text

The present invention relates to ceramic hot-gas filter.
Field of the Invention
The present invention relates to a composite ceramic candle filter for removing
particulates from a hot gas stream, and a method for making said filter.
Description of Related Art
Ceramic filters have been tested in processes such as coal gasification and coa]
combustion to remove particulates from hot flue gases to protect downstream equipment
from corrosion and erosion and 10 comply with EPA NSPS (New 'Source Performance
Standards) regulations. Ceramic filters in a tubular (candle) form, with one end closed and
the other end open have been shown to remove the particulates efficiently. The hot gas to
be filtered typically flows from the outside to the inside of the filter, with particulate-free
gas exiting from the open end. The candle geometry is also suited for removal of the
filtered cake by backpulsing with compressed gases.
Ceramic hot-gas candle filters must withstand exposure to chemically corrosive
gas streams at temperatures in excess of 800°C. In addition, they are subjected to
significant thermal stresses during backpulse cleaning which can cause catastrophic failure of
the ceramic candle filter element.
Ceramic hot-gas candle filters known in the art are generally fabricated from
either porous monolithic materials or porous ceramic fiber-containing composite materials.
Monolithic ceramic candle filters are either weak or can fail catastrophically in use.
Composite filters are less susceptible to catastrophic failure and generally have improved
strength, toughness, and thermal shock resistance versus monolithic ceramic filters.
Candle filters may have relatively uniform porosity throughout the filter or they
may comprise a porous support with a thin layer, or membrane, of fine porosity on the outer
surface of the support. The membrane layer is typically applied to the filter using a variety
of methods such as coating from a dispersion containing finer grains than those used in the
support for smaller membrane pore sizes, bonding randomly arranged chopped ceramic
fibers to the support using colloidal (or sol) materials, or forming a ceramic matrix by
chemical vapor infiltration.

Materials used to fabricate ceramic hot-gas filters generally include oxides such as
aluminosilicates, glass, and alumina, and non-oxides such as silicon carbide and silicon
nitride. Oxide-based ceramic filters have adequate resistance to flue gas atmospheres and
fly-ash for the design life of the filters; however, they generally have low thermal shock
resistance. Non-oxide ceramics generally have good thermal shock resistance, however they
are susceptible to oxidation in the corrosive environment to which they are subjected which
results in a degradation of mechanical properties.
The disadvantages of ceramic candle filters known in the art include failure, often
catastrophic, due to thermally induced stresses caused by backpulse cleaning, chemical
degradation caused by species present in the hot gases being filtered, delamination of the
membrane layer, incomplete removal of the filter cake upon backpulsing, and high cost.
They also tend to be heavy, requiring expensive support structures to hold an array of the
candles in the filter unit.
Summary of the Invention
The present invention is directed to a ceramic hot gas filter comprising a porous
elongated filter support and a porous membrane layer on at least one surface thereof.
Specifically, the porous membrane may be on the outer surface, the inner surface, or both
the outer and inner surface of the porous elongated filter support. The membrane layer(s) is
firmly adherent to the support and therefore does not suffer from delamination problems.
The porosities of the support and membrane are controlled such that the support functions as
a bulk filter and the membrane layer functions as a surface filter. The support has an
opening at one end into a hollow interior, a closed end opposite the open end, and an,
external flange integral with the open end. The support is formed of a plurality of layers of
oxide ceramic support yarn, each layer being arranged in a crisscrossing relationship with
neighboring layers to form a plurality of quadrilateral-shaped openings. The yarn in the
support is coated with a first oxide ceramic material which, upon heat treatment, forms a
porous refractory oxide support matrix. The membrane layer(s) may be formed of an
ordered arrangement of continuous filament oxide ceramic membrane yarn, a uniform
coating of ceramic filler material, or some combination of the two. Any yarn present in the
membrane layer is (preferably prior to winding) coated with a second oxide ceramic material
which, upon heat treatment forms a porous refractory oxide membrane matrix. Preferably,
the support yarn and the continuous filament membrane yarn each contain at least 20 weight
percent alumina (Al2O3) and have softening points above about 750°C. The ceramic'
coating materials are generally particulates of oxides or oxide compounds, or mixtures
thereof and may also include oxide precursor materials. The membrane layer(s) has a
porosity that is less than that of the support. Preferably the quadrilateral-shaped openings

have dimensions of about 100 to about 500 microns after heat treatment so that the support
functions as a bulk filter. The membrane layer(s) preferably has pore diameters of about
0.1 to 50 microns and functions as a surface filter. In a preferred embodiment of the
invention, the support yarn has generally the same composition as the membrane yarn and
the support matrix has generally the same composition as the membrane matrix.
The present invention also provides a method for making a ceramic hot gas filter
involving the steps of fabricating an elongated porous filter support by coating a ceramic
oxide support yarn with a first coating composition, winding the coated support yarn onto a
mandrel to form a plurality of layers of the coated support yarn, each layer being arranged
in a crisscrossing relationship with neighboring layers to form a plurality of quadrilateral-
shaped openings. The mandrel may be contoured to provide an integral external flange
adjacent to one end of the support. Alternatively, a separate collar insert may be slid onto a
uniformly cylindrical mandrel to form the flange portion of the support. The resulting
support has an open end adjacent to the flange, an outside surface, and a second open end
opposite the flanged end.
A membrane layer is then formed on at least one surface of the support. For
example, the membrane layer may be formed on the outer surface by coating a continuous
filament oxide ceramic membrane yarn with a second coating composition and applying the
coated membrane yarn in an ordered arrangement on the outer surface of the support.
Methods for forming the ordered arrangement membrane layer(s) include hoop winding a
single yarn, multiple yarn winding, fabric wrapping and coating with a particulate slurry or
a solution containing ceramic precursor materials. In a preferred embodiment, the ordered
arrangement comprises a circular or hoop winding of the continuous filament oxide ceramic
so as to define a gap of predetermined width between adjacent windings. The gap is then
filled with additional ceramic filler material, preferably an oxide material, which upon
subsequent heat treatment, forms a porous refractory membrane matrix. The width and
uniformity of the gap between adjacent hoops or windings is not particularly critical;
however, uniform filling of the gap with filler material is desirable, both around the
circumference and along the length of the filter. In another embodiment, a membrane layer
is formed by winding the coated continuous filament such that adjacent hoops or windings
are as close to one another as possible and no such filler material is applied. Yet another
embodiment features a membrane layer comprising ceramic filler material but without hoop-
wound filaments; i.e., an infinitely large gap between hoop windings. In this embodiment,
for example, ceramic particulates, preferably of an oxide material and preferably in the form
of a slurry, are applied to the support layer as uniformly as possible, so as to essentially
close off the diamond shaped openings formed by the crisscrossing filaments of the support
layer. A slurry is a convenient form for the filler material because the slurry is amenable to

being painted by brush or spray, or being dip coated, etc. Another useful form for
providing the ceramic filler material to the developing candle filter is as a paste, which may
then be applied using, for example, a spatula-like flexible applicator. Other media for
communicating the filler material to the developing candle filter may occur to an artisan of
ordinary skill and should be considered therefore to be within the scope of the present
invention.
Once the support layer has been wound, the support mandrel removed, and the
membrane layer(s) formed, the second open end (opposite the flange end) is closed using an
oxide ceramic material. The support and membrane layer(s) are heat treated to convert the
first coating composition to a porous refractory oxide support matrix and to convert the
various coating compositions to a porous refractory oxide membrane matrix.
The present invention provides a strong, lightweight ceramic hot-gas candle filter
which has a greater than 99.5% particulate collection efficiency, thus meeting EPA NSPS
regulations. Failure of the filter is generally not catastrophic since if the membrane is
damaged, the support quickly blinds at the location of the damage due to its bulk filtration
properties, thus preventing release of particulates and protecting downstream process
equipment such as gas turbines or sorbent beds. The filter of the present invention is
resistant to chemical degradation due to the oxide compositions used, and at the same time
provides excellent thermal shock resistance which is not generally typical of oxide materials.
The smoothness of the membrane surface(s) results in efficient removal of the filter cake
during backpulse cleaning. In addition to the above-mentioned advantages, the filter of the
current invention is potentially low cost relative to most of the commercially.available
candle filters.
Definitions'
"Ceramic" as used herein means crystalline or partially crystalline materials, or
non-crystalline glasses, which comprise essentially inorganic, nonmetallic substances.
"Continuous fiber or filament" as used herein means a fiber or filament having a
length which is at least 1000 times the diameter of such fiber or filament.
"Filler material" or "membrane filler material" as used herein means those bodies
in the membrane layer other than those bodies making up the yarn or any slurry material
coated on the yard. As such, the filler material may be in the form of powders, particulates,
whiskers, chopped fibers, platelets, flakes, spheres, tubules, pellets, etc.
"Membrane" or "membrane layer" as used herein refers to 'that layer which is
deposited or applied onto at least one surface of the support layer, has a lower porosity than
the support layer, and which provides the majority of the filtering action.

"Oxide" as used herein is meant to include oxides, oxide compounds (e.g. mullite,
spinel), or precursors thereof.
"Support" or "support layer" as used herein refers to the structure formed by
winding single or multiple continuous ceramic fiber or filament around a mandrel in a
crisscrossing arrangement to produce an ordered array of diamond shaped openings. The
function of the support or support layer is to provide a suitably strong foundation to which
the membrane adheres.

Brief Description of the Accompaying Drawings
FIG. 1A is a schematic perspective view of an embodiment of a filter element of
the current invention, including an optional flange collar section.
FIG. 1B is a cross section of the filter element taken on line 1B-1B of FIG. 1 A.
FIG. 1C is a cross section of the flange section taken on line 1C-1C of FIG. 1 A.
FIG. 1D is a cross section of the flange section taken on line 1D-1D of FIG. 1A.
FIG. 1E is a cross section of the closed end taken on line 1E-1E of FIG. 1A.
FIG. 2 shows openings formed by the overlap of two layers of yarn in a support
layer comprising an embodiment of the present invention.
FIG. 3A through 3C are cross-sectional views of the filter wall which illustrate the
construction variations of the membrane layer.
Detailed Description of the Invention
The hot-gas filter of the current invention is of the candle filter type and
comprises a porous ceramic support having a porous ceramic membrane layer on at least one
• surface thereof. Specifically, the porous membrane may be on the outer surface, the inner
surface, or both the outer surface and inner surface of the porous ceramic support. The
membrane is less porous than the support and serve as a surface filter, preventing pollutant
particulates from penetrating therethrough. The support has good filtration capacity for fly-
ash and serves as a bulk filter, capable of trapping particulates between its inner and outer
surface, should membrane leakage occur.
Referring to Figures 1A-1E, the filter 10 comprises a support 12 having a
generally elongated tubular shape with an open end 14 at one end into a hollow interior.
The end 15 of the support opposite the open end is generally closed. The support further
includes an external flange 16 integral with the open end 14 which supports the filter in a
tube sheet in use. The flange may also include an optional collar insert 24, integral with the
flange, and described in more detail below. The membrane layer(s) 18, 23 are formed on
the outer surface 20 of the support and/or the inner surface 22 of the support. End 15 is
generally closed by filling with a ceramic material 26, and the flange section 16 and tip

section of the support adjacent the closed end 15 are made impervious as described below.
The overall porosity of the support layer is determined by. a combination of the
open volume created by the diamond or parallelogram-shaped openings (micropores) and
the porosity of the matrix coating surrounding the individual yarns (micropores). The
porosity of the membrane layer is due primarily to the porosity between adjacent particles
making up the layer (micropores) or due to microcracks.
The macroporosity of the support may be calculated from the volume of the
support (calculated from the measured dimensions of the support), the weight of the support,
and the bulk density of the support (fiber and matrix, including any microporosity). The
bulk density is measured using mercury porosimetry.
The matrix is applied in such a way that the channels in the support are not
substantially closed. The matrix generally imparts integrity and mechanical strength to the
support and also provides an excellent degree of thermal shock resistance because of the
ability of the porous matrix to absorb thermally induced mechanical stresses which might
otherwise fracture the fibers in the filter.
The support is formed of a plurality of layers of continuous ceramic oxide yarns
which are laid down in spaced helical coils in a crisscrossing relationship with neighboring
layers to form a plurality of diamond or quadrilateral-shaped openings having dimensions
between 100 and 500 microns after firing. The openings form channels extending between
the inner 22 and outer 20 surfaces of the support which follow tortuous, curved paths (see
Figure IB). If the filter is damaged, for example by damaging the membrane layer during
installation, it will quickly "self-heal" by functioning as a bulk filter and blinding with
particulates in the hot gas stream. A support containing a significant number of straight
radial channels will not blind as readily, resulting in failure of the filter. Forsythe, U.S.
Patent 5,192,597, incorporated herein by reference, describes filament winding of
reticulated ceramic tubes in a preferred winding pattern. The yarns in adjacent layers of
diamond-like patterns are laid down in such a manner that the yarns forming the walls of the
diamonds of each layer substantially cover the diamond shaped openings of each adjacent
layer. This forms a tubular structure comprising series of interconnected diamond shaped
openings, each layer of which interfere with the direct flow of gas from one layer to the
next.
The winding pattern described above is for the elongated central body section of
the support (i.e. the generally cylindrical section of the filter between the flange and closed
end). Due to the contoured closed end and flange sections of the filter, the described
winding pattern is not achieved at the flange and closed end.
FIG. 2 shows two adjacent layers of yarn in a support prepared according to U.S.
Patent No. 5,192,597 (the matrix layer is not shown in this Figure) which define openings

designated by "x". The size of the openings is controlled by the spacing between the yams
in each layer which is determined by the wind angle and yarn denier in addition to the
amount of matrix material applied to the yarn. The spacing "a" between adjacent yarns is
preferably controlled to provide openings having dimensions "a" of between about 100 and
500 microns in the final support, after high temperature firing. The openings have more of
a square shape near the inner surface of the support, with one of the diagonals gradually
increasing in size, as winding continues, to the outer surface, thereby according the opening
more of a diamond shape. The dimension "a" can be calculated based on the yarn spacing
and the amount of matrix applied to the yarn. Alternately, "a" can be measured visually in
the final support. A support having the described construction and having openings in this
size range will function as a bulk filter which can trap particulates within the wall of the
support while maintaining a pressure drop that is insignificant relative to the pressure drop
across the membrane layer.
The support may be formed by winding a ceramic oxide yarn on a suitably
designed mandrel using a filament winder designed to maintain a constant winding ratio
(rotational speed of the mandrel divided by the speed of the traverse arm). A constant
winding ratio is necessary to maintain the proper size and distribution of channels
throughout the wall. The flange section of the support is formed by using a mandrel that is
wider at one end, the wide end being contoured to give the desired flange geometry.
Filament winding on such a mandrel produces a rube with an external flange section at the
open end and a small hole at the opposite end, which is generally closed in the final support
with a ceramic material 26 as shown in FIG. IE. Alternately, if it is desired that the inside
wall of the support be straight as opposed to contoured at the flange section, a filament
wound collar insert 24, shown in FIG. 1C and FIG. ID, having a composition similar to
that of the support and having an inner diameter approximately equal to the outer diameter
of the mandrel and an outer surface contoured to give the appropriate flange geometry may
be used. The collar is then placed on the mandrel and the support is then wound on the
combined mandrel and collar. When the support is removed from the mandrel, at least a
portion of the collar remains with the support as part of the flange section, as illustrated in
Example 2 below.
Field tests have demonstrated that hot-gas candle filters commonly fail at the
flange section. According to the current invention, the flange and the body of the support
are formed as a single unit to ensure homogeneity of the support material across the entire
filter and to eliminate any stresses or weak spots arising from joining materials. The shape
of the flange is not critical but should be reproducible. The flange should provide a good
seal with the tubesheet that supports the filter in use so that no dust leakage occurs. The
shape of the closed end is generally round, but various shapes are possible by suitably

shaping the mandrel. The diameter of the opening at the closed end of the tube depends on
the diameter of the shaft that supports the mandrel.
The membrane layer is applied to the outer surface of the support, the inner
surface of the support, or to both the outer and inner surface of the support. The membrane
layer usually comprises an ordered arrangement of continuous filament oxide ceramic yarns.
The membrane layer optionally may also comprise one or more ceramic filler materials to
help fill gaps or plug cracks, voids, etc. between adjacent yarns. In the alternative, the
membrane layer may comprise the ceramic filler material, but no ceramic yarns. The
membrane layer(s) in the final filter, after heat treatment, has pore diameters of between
about 0.1-50 microns, preferably 5-25 microns. Preferably, the average pore size and size
distribution is substantially invariant around the circumference and along the length of the
filter.
For those embodiments in which the membrane layer(s) comprises yarns, the
ordered arrangement of yarns in the membrane layer(s) may be formed by various methods
including circular (hoop) winding, multiple yarn winding, or wrapping with yarns pre-
arranged in two or three dimensional forms such as fabric or braided materials. The
membrane yarns should be arranged so as to obtain a smooth membrane surface. A smooth
membrane surface is desirable because it facilitates complete removal of the filtered material
during backpulse cleaning because the filter cake readily debonds from the smooth surface.
If the surface is rough, the filtered cake tends to be mechanically anchored to the surface
making it difficult to completely remove the cake by backpulse cleaning. The circular
winding produces a smooth membrane surface.
The yarns used to form the support and membrane layer(s) preferably comprise
ceramic fibers having softening points of at least about 750° C, more preferably at least
1000°C. The phrase "softening point" is used herein to mean both the softening point of a
glass ceramic and the melting point of a crystalline ceramic. The yarns used in the
membrane layer(s) may be the same as or different than the yarns used in the support.
Suitable oxide fibers include, for example, certain glass fibers such as S glass
(high tensile strength glass containing about 24-26% alumina(Al2C>3)), "Fiber Frax"
alumina-silicate fiber, and polycrystalline refractory oxide fibers containing at least about
20% by weight of alumina such as the alumina-silica fibers disclosed in U.S. Patent No.
3,503,765 to Blaze and certain of the high alumina content fibers disclosed in U.S. Patent
No. 3,808,015 to Seufert and U.S. Patent No. 3,853,688 to D'Ambrosio. Preferably the
oxide fibers comprise between 20% and 80% by weight of aluminum oxide. Examples of
commercially available aluminosilicate fibers include "Altex" (Sumitomo) and "Nextel"
(3M) fibers. Fibers containing significant levels of glass-forming oxides such as

B2O3 and P2O5 are not desirable because they will flux the entire structure resulting in a
dense, nonporous support.
Fibers of refractory oxide precursors can also be used to form the support. After
winding, the precursor fibers are converted to polycrystalline refractory oxide fibers by
firing to remove volatiles, convert salts to oxides, and crystallize the fiber. The preparation
of refractory oxide fibers and their precursors is disclosed in U.S. Patent Nos. 3,808,015
and 3,853,688.
The oxide fibers generally have diameters in the range of 0.2 to 2.0 mils (0.005-
0.05 mm) and are used in the form of continuous yarns, preferably containing 10-2,000 or
more fibers. The fibers are preferably continuous filaments, however yarns of staple fibers
can be used, especially glass. The yarns are preferably loosely twisted so that any loose or
,broken ends do not interfere during filament winding when the yarn is pulled through small
orifices. The yarns may also be ustd in the form of roving. Bulked, interlaced, or textured
yarns may be used. However, the yarns used in the membrane layer most preferably
comprise continuous filament, untextured yarns so as to obtain a membrane layer having a
smooth outer surface. Glass yarns which crystallize to form refractory oxides upon high-
temperature heat treatment are preferred because they are easier to handle and less likely to
break during filament winding than yarns containing crystalline ceramic fibers.
The refractory oxide matrix components of the support and membrane preferably
have softening points above 1000°C, more preferably above about 1400°C, and most
preferably above 2000°C. Preferably the matrix comprises at least 40 wt% alumina.
The matrix components are generally applied to the support and membrane yarn(s)
in the form of a coating composition which is then fired to form a refractory oxide matrix.
The coating composition used in the support may be the same as or different than the
coating composition used in the membrane. The coating composition generally comprises
an aqueous solution, suspension, dispersion, slurry, emulsion, or the like which contains one
or more oxide particulates or oxide precursors. Preferably the oxide particulates have a
particle size of 1-20 microns, more preferably 1-10 microns, most preferably between 1-5
microns. Particle sizes less than 20 microns are preferred because they are readily dispersed
and penetrate into voids between fibers. Slurries prepared using particle sizes less than 1
micron are generally too viscous at useful solid concentrations. Oxide particulates useful as
matrix materials include alumina, zirconia, magnesia, mullite, spinel, etc. Suitable matrix
precursors include water soluble salts of aluminum, magnesium, zirconium and calcium such
as "Chlorhydrol®" (aluminum chlorohydrate solution sold by Reheis Chemical Co.),
zirconyl acetate, alumina hydrate, basic aluminum chloracetate, aluminum chloride, and
magnesium acetate.

Preferably, drying control additives such as glycerol and formamide may be added
to the coating composition at levels of 1-5 wt% based on the total weight of the coating
composition. The drying control additives reduce drying stresses in the green body and also
eliminate macroscopic cracks on the surface of the high-temperature fired filter. Moreover,
drying stresses can be further reduced by winding the support and membrane layer(s) in an
environment with a relative humidity of at least about 30%.
The coating composition preferably includes a ceramic oxide precursor to increase
the green strength of the wound structure. These soluble oxide precursors which are useful
as matrix precursors also function as binders. A preferred binder is aluminum chlorhydrate.
and in particular, the above-mentioned "Chlorhydrol". Preferably the coating composition
includes between about 10-25 wt% binder, calculated based on the total solids content of the
coating composition. The aluminum chlorohydrate serves to bond the oxide particulates of
the coating together and increases the green strength of the support. The binders are
incorporated into the refractory matrix upon heat treatment.
The coating composition may be applied to the support by drawing the ceramic
oxide yarn through the coating composition prior to winding on a mandrel. Preferably, the
coating composition is uniformly distributed around the fibers of the yarn. The distribution
is affected by the viscosity of the coating composition, the method of application, the
density (or tightness) of the yarn bundle, the nature of the yarn and the amount of the
coating composition. The composition should have a viscosity that is low enough to permit
flow and some penetration into voids in the yarn but high enough to adhere to the yarn
bundle. When the coating composition is a particulate slurry, the solids content is
preferably between 50-75 wt% and the slurry preferably has a viscosity in the range of 100-
300 centipoise. If a coating composition containing both an oxide precursor and particulate
oxide powder is used, the solids content of the slurry should be adjusted to about 60-90 wt%
of the refractory oxide matrix material derived from the oxide particulate and about 10-40
wt% derived from the precursor. It is difficult to obtain sufficient amounts of oxide-
containing materials in the coating composition using levels of precursor greater than about
40 wt%. The amount of matrix material applied to the yarn can be controlled by pulling the
yarn through a suitably sized orifice to remove excess slurry. The coating composition may
be also be applied to the yarn by use of a finish roll, spraying, etc. Further, the matrix
coating composition may be applied to the wound filamentary membrane and support by
dipping the wound support in a slurry, draining off the excess and drying. Additional
dipping steps may be used if necessary to provide the desired weight of matrix relative to the
weight of yarn in the support. In general, it is difficult to apply the matrix coating
composition by dipping without closing a significant portion of the channels in the support,
which is not desirable and results in increased backpressure.

The membrane matrix coating composition may be applied to the membrane yarn
using methods similar to those described for the support. Preferably the combined weight of
the matrix components of the support and membrane layers comprises about 40-70% of the
final weight of the filter, more preferably about 50-60%. To avoid thermal stresses, it is
preferable that the support yarn has generally the same composition as any membrane yarn
and the support matrix has generally the same composition as the membrane matrix. In
certain applications, however, different compositions may be desirable. For the same
reason, it is preferable to have a weight ratio of fiber to matrix which is essentially the same
in the membrane and the support.
In one embodiment, multiple yarns are combined and wound on the support at
substantially the same wind angle as that of the support to fill the underlying (or the eventual
overlaying) openings in the support. This may be accomplished by feeding the separate
yarns through tensioning devices, dipping in a ceramic matrix particulate slurry, and
combining the yarns just prior to pulling through a larger sizing orifice than that used for
single yarn ends and winding on the support if an outer surface membrane is desired, and on
the mandrel if an inner surface membrane is desired. The diameter of the sizing orifice is
selected as described above for hoop winding.
A membrane comprising a single filament wound layer on the support or on the
inner surface of the support is generally adequate for many filtration applications.
Additional layers of wound yarns may be applied to increase the thickness of the membrane
layer. This usually increases the paniculate collection efficiency and the back pressure of
the filters.
In another embodiment, the membrane layer is formed by wrapping the support or
mandrel with a ceramic fabric. The fabric-is wrapped on the filter support or mandrel and a
matrix slurry similar in composition to that used in the support is brushed on the fabric.
The slurry wets the fabric and the support, and provides bonding to the support. Any
wrinkles in the fabric are removed while still wet. Additional layers of fabric are wrapped
on the support or mandrel as necessary to increase the filtration efficiency. The fabrics
useful for building the membrane layer include tightly woven plain and satin weaves. It
may be necessary to use a matrix slurry containing matrix particulates having a smaller
particle size than the matrix particulates used to wind the support in order to improve the
adhesion between the filter support and fabric membrane layer. This is because the smaller
particles will more readily infiltrate the interstices in the woven fabric. In general, this
method is less preferred because it is more difficult to control the amount of matrix applied
to the membrane layer. In addition, it has been found that the fabric layers tend to be less
strongly adhered to the support than membranes formed using the filament winding
techniques described above.

In still another embodiment, the membrane is formed by hoop winding. The
oxide ceramic membrane yarn is coated with the membrane matrix coating composition, for
example by passing through a bath containing a coating composition, followed by passing
through a sizing orifice to remove excess slurry, and winding at approximately 90 degrees to
the axis of the mandrel. Preferably, the diameter of the sizing orifice is carefully selected to
give a matrix pick-up that yields similar weight ratios of fiber and matrix in the membrane
and support layers. The rate of mandrel rotation relative to the rate of the movement of the
transverse arm controls the spacing between adjacent yarns. Figure 3A illustrates the cross-
sectional view of a filter created in this fashion, where "a" represents the end view of a yarn
in the support body, "b" represents the end view of a yarn in the hoop-wound membrane,
and "c" represents the spacing between adjacent yarns in the hoop-wound membrane. In
one version of this embodiment, the slurry coated membrane fiber or yarn is wound as close
as possible with substantially no overlapping of yarns or intentional gaps between yarns in
the membrane layer (e.g., dimension "c" in Fig. 3A equals zero). Filtering action is
provided by the micro-cracks in the matrix material between adjacent yams. Optionally,
filler material may be applied to the wound membrane to fill in any unintentional gaps
between adjacent yarn hoops.
In another version of this embodiment, an intentional gap is left between adjacent
hoop windings of the slurry' coated yarn. Additional ceramic filler material, (e.g..
particulates) or a precursor to a ceramic filler material, preferably in the form of a slurry1, is
then deposited in this gap. A membrane formed in this way is termed a "combination
membrane". Preferably the gap-filling slurry should contain a suspension agent to maintain
a uniform consistency. The desirable viscosity will depend on the method of application
chosen; low viscosities are best suited for a brushing technique, paste-like high viscosities
are more appropriate when applying with a spatula. Figure 3B illustrates the cross-sectional
view of a filter having a combination membrane, where "a" represents the end view of a
yarn in the support body, "b" represents the end view of a yarn in the hoop-wound
membrane, "c" represents the spacing between adjacent yarns in the hoop-wound
membrane, and "d" represents the filler material used to fill the spaces between adjacent
yarns. This additional ceramic filler material may be of the same chemical composition as
the membrane matrix material coating the membrane yarn, or it may have a different
chemical composition. Typically, constituents used for gap filling are larger (e.g., 25-75
microns) than those particulates used for matrix formation (e.g., 3-5 microns). Further, the
intentional spacing "c" is almost infinitely variable; it may range from substantially zero to
many times the diameter of a yarn.
In yet another embodiment, because there appears to be no upper limit to the size
of the gap between adjacent windings in the membrane layer, it is possible to dispense

completely with the hoop wound slurry coated yarn, leaving the membrane layer to consist
essentially of the ceramic filler material. Figure 3C illustrates the cross-sectional view of a
filter created in this fashion, where "a" represents the end view of a yarn in the support
body and "d" represents the ceramic filler material used to fill the quadrilateral-shaped
openings at the surface of the support body. Again, such material preferably is in the form
of a slurry or solution which can be applied directly to the support layer by brushing,
spraying, dip coating, etc. Also, the preferred size of the constituents making up a "filler
material only" membrane layer is about 25-75 microns in diameter.
The above discussion generally pertains to the form of the invention in which the
membrane layer is applied to the outer surface of the support layer. When the membrane
layer is to be applied to the interior surface of the support layer, the fabrication procedures,
may have to be modified. For example, when a fiber or fabric is to make up the membrane
layer, it may be preferred to wind or wrap such fiber or fabric over the mandrel before
winding the support layer. Also, if the inner membrane layer is "filler material only" or if
an additional slurry or solution is to be deposited onto a hoop-wound filament layer, it may
be preferred to do so once the support layer has been formed and the mandrel has been
removed. Further, it may be impractical to apply a slurry or solution to the interior of a
rube by brushing or spraying. In such a case, slip casting or drain casting should achieve
the desired results.
The flange section and the closed end may be reinforced and made impervious to any
gas streams by saturating with additional ceramic slurry or using a ceramic cement
composition. To avoid reactions with the underlying support material and to match the
thermal expansion of the support, the matrix material used in the support is preferred for
this purpose. After winding and reinforcing one or both ends, the candle filter is dried at
room temperature while on the mandrel until it is strong enough to handle.
After overnight drying (about 12-16 hours) at ambient temperature, the ends of the
developing filter are cut off so that the mandrel may be removed. Specifically, the collar
portion of the filter is sliced such that a section of the original collar remain in the flange
section of the support layer (see Figure 1D).
The developing candle filter may then be fired at temperatures below the softening
point of the ceramic yarn and sufficiently above the boiling point of any volatiles, typically
around 300°C to 800°C, to remove the volatiles and stabilize the filter. This is especially
important when oxide precursors are used.
Closing off the tip may then be accomplished using commercial high temperature
cements or by filling with a high viscosity paste (similar in composition to the matrix
coating slurry) mixed with a small amount of the type of yarn used in the support structure,
or by filling with thickened paste similar to the membrane filler material. The solids in

commercial cement should not react with the tube material to reduce the thermal stability of
the filter. It is also preferable to have fired the candle filter, as described above, prior to the
application of a ceramic filler material to the membrane layer(s).
An additional firing at high temperatures is then carried out, typically at 1200° to
1400°C, to form stable crystalline phases. Firing above 1450°C may melt some of the
phases and result in a fused product which is undesirable due to reduced thermo-mechanical
properties. Preferably, the heating rate during the high temperature firing does not exceed
20°C per minute, in order to allow any glass phases to crystallize, and may be as low as
0.1 °C per minute. During high temperature firing glass fibers may devitrify into crystalline
phases, the matrix may convert to stable crystalline phases or the crystalline phases in the
fiber and matrix may react to form new stable crystalline phases. The final phase
composition of the product depends on the amounts of fiber and matrix, the heating profile,
soaking time at intermediate temperatures and the dwell time at the highest firing
temperature. The typical crystalline phases are corundum, mullite, cordierite and
cristobalite. As used herein, the term cordierite is intended to include indialite, a crystalline
material having the same composition as cordierite, but a slightly disordered crystal
structure. Excess cristobalite formation is undesirable since cristobalite undergoes a volume
change at 200-270°C, which contributes to poor thermal shock resistance. The final filter
should contain no more than 10% by weight cristobalite. Preferably the final composition
of the filter is 3-7 parts by weight magnesia, 20-45 parts silica and 45-70 parts alumina.
More preferably the final filter comprises between about 60%-70% alumina.
In a preferred embodiment, the yarn used to prepare both the support and membrane
comprises glass fibers comprising 61-66% SiC>2, 24-26% AI2O3, and 9-10% MgO. A
coating composition consisting essentially of alumina is applied to the yarn prior to winding
in an amount sufficient to provide a refractory oxide matrix comprising 40-70% of the final
weight of the filter. The coating composition contains a binder comprising aluminum
chlorhydrate and alumina matrix particulates having an average particle size of 2-3 microns.
The membrane is applied to the support or mandrel by hoop winding. The as-wound filter
element is heated to remove volatiles and then high temperature fired at temperatures above
about 1350°C, preferably at a temperature of about 1380°C. During high temperature
firing, the glass fiber softens and a portion of the silica and magnesia in the glass combine
with the alumina matrix material to form cordierite and mullite. The final filter comprises
about 20-40% by weight SiO2, about 3-6% by weight MgO and about 50-70% by weight
Al2O3. The final crystalline composition, after heat treatment, is 25-40% cordierite, 5-15%
mullite, 40-60% corundum and 0-10% cristobalite, based on the total crystalline content.
Approximately 50-90 vol % of the material is crystalline with the remainder being
amorphous. The formation of crystals of mullite, cordierite, and corundum, each having

different coefficients of thermal expansion, leads to formation of microcracking in the
structure. The microcracks form along crystalline boundaries as well as within regions
having only a single crystal phase. The microcracks are believed to absorb stresses caused
by thermal shock. After firing, the filter is stable up to 1200°C for extended periods of
time and has excellent thermal shock resistance.
EXAMPLES
All percentages referred to herein are weight percent, unless otherwise indicated.
The filament winder used to wind the support in the Examples below had a chain-
driven traverse of approximately 70 inches (178 cm) (278 teeth of 0.5 inch (1.27 cm) pitch
passing in a narrow loop driven and supported by 11 tooth drive sprockets at each end).
The drive ratio was set such that the spindle rotated at a speed of 50 and 10/111 revolutions
for each complete rotation of the chain loop for winding of the filter support. The mandrel
was a tube having a length of 65 inches (165 cm) and an outer diameter of 1.75 inches (4.45
cm) with end closures at each end. One of the end closures was conical with about a 30
degree taper on each side of the cone with a 0.50 inch (1.27 cm) diameter drive shaft
mounted at its axis. The second end closure was hemispherical (1.75 in (4.45 cm) diameter)
with a 0.25 inch (0.64 cm) drive shaft mounted at its axis. The mandrel was attached to and
driven by the spindle in such a position as to be traversed along its length by the traversing
yarn guide. The mandrel was attached to and driven by the spindle via the 0.50 inch (1.27
cm) shaft and supported in a bearing at the 0.25 inch (0.64 cm) shaft. It was mounted
parallel to the chain-driven traverse guide such that the guide traversed above the mandrel
surface at a distance of about 0.75 inch (1.91 cm) from the surface of the mandrel and the
traverse stroke extended from about 0.75 inch (1.91 cm) past the hemispherical closure onto
the 0.25 inch (0.64 cm) shaft and to about 0.75 inch (1.91 cm) past the conical closure onto
the 0.5 inch (1.27 cm) shaft.
A plastic collar having a 7 mm wall thickness and a 45 degree edge relative to the
axis of the collar was inserted on the mandrel near the conical end to form the flange on the
filter support for Examples 1 and 3.
For Example 2, a separate winder having a 6 inch (15.2 cm) traverse stroke with
means to adjust this stroke to contour the package ends was used to form a collar insert for
the flange section of the filter. The drive ratio was set such that the spindle rotated at a
speed of 4 and 11/180 revolutions for each complete rotation of the traverse cam to provide
the same wind angle in the collar insert as the wind angle in the support. A mandrel
comprising a short piece of 1.75 inch (4.45 cm) outer diameter tube was mounted on the
spindle and wrapped with 2 layers of 0.002 inch (0.005 cm) thick "Mylar" polyester film to

facilitate removal of the wound unit. The mandrel was wrapped with 90 grams of S-glass
(S-2 CG150 1/2 636, available from Owens-Corning Fiberglas Corporation of Toledo, .
Ohio) that was coated with an aqueous A-17 alumina slurry (see Example 1 for composition
of slurry) applied in such a quantity to form a unit having 50-60 wt% ceramic from the
slurry and 40-50 wt% ceramic derived from the feed yarn after drying. The collar insert, as
wound, had the form of a cylinder of approximately 1.75 inch (4.45 cm) inner diameter and
a 3/8 inch (0.95 cm) wall thickness with the ends of the cylinder wall exhibiting a taper of
approximately 45 °. The insert was removed from its mandrel while still wet and transferred
to the mandrel on the main filament winder, described above. The insert was positioned so
as to leave about 57 inches (145 cm) of the straight tube portion of the mandrel exposed
between the insert edge and the junction of the tube with the hemispherical end closure.
The filter support units were wound onto the mandrels with either the collar insert
or plastic collar mounted thereon. Winding was carried out with the spindle set at a
rotational speed of approximately 500-520 revolutions per minute. The final (fired) support
units had diamond-shaped openings on the outer surface having dimensions of about 175-
250 microns.
TEST METHODS
The density and porosity of the membrane layers was determined using mercury
porosimetry. Membrane samples were prepared for porosimetry measurements using either
of two methods. The membrane layer can be readily debonded from the support prior to
firing of the candle assembly. The debonded membrane layer is then high-temperature fired
and submitted for porosimetry measurements. Alternatively, the membrane sample may be
prepared by scraping away the support layer from a sample of a high-temperature fired
candle assembly. The median pore size is reported in microns and the porosity is reported
in volume percent. The median pore size is the value obtained at the maximum intrusion
volume.
The average oxide composition was determined using X-ray Fluorescence
spectroscopy. The samples and standards were fused in a lithium tetraborate flux and the X-
ray emission lines for the elements of interest were measured. The results are reported as
weight percent with the samples being dried at 130°C.
Crystalline phase compositions were determined using X-ray diffraction using a
Scintag Pad X theta-theta diffractometer using Cu K-alpha radiation. The following
conditions were used: copper tube operated at 45 kilovolts, 40 milliamps, goniometer
radius 250 mm, beam divergence 0.24 degree, scatter slit 0.43 degrees, receiving slit 0.2
mm, germanium solid state detector bias 1000V, scan speed 0.2 degrees 2-theta per minute,
chopper increment 0.03 degrees 2-theta, scan range 3 to 112 degrees 2-theta (overnight
scans), samples front packed against filter paper in a 1 inch square aluminum well-type

sample holder, single sample changer. The samples were wet milled in acetone for 5
minutes in a McCrone vibratory mill using corundum grinding elements and dried under a
heat lamp. The percentages of crystalline phases were determined based on a mixture of
standard materials with 20% fluorite as an internal standard. Standard materials used were
NIST (NBS) 674 alpha alumina (corundum); Baikowski high purity cordierite (indialite)
standard, Coors mullite standard, NIST (NBS) 1879 cristobalite, NIST (NBS) 1878 quartz,
and Coors spinel standards. The samples themselves were not mixed with an internal
standard but were normalized to 100% of the crystalline components after dividing each
measured intensity by its respective reference intensity ratios. Analysis lines were: indialite
at 10.4, 18.2 and 29.5 degrees; mullite at 16.5 and 26.1 degrees, corundum at 25.6 and
52.6 degrees, cristobalite at 21.8 degrees (overlap corrected for indialite), and quartz at 20.8
degrees.
Example 1
This example illustrates the fabrication of a ceramic filter according to the current
invention, wherein the membrane layer is applied to the outer surface of the support and is
formed using a woven glass fabric.
An alumina slurry was prepared by charging 7.0 liters of water and 20.0 ml of
formic acid in a mixing vessel. Fumed alumina having an average particle size of 13-15 nm
(manufactured and sold by Degussa Corp., Ridgefield, NJ) (2.0 kg) was added slowly with
stirring. The pH of the dispersion was adjusted to 4.0 to 4.1 using formic acid. After
stabilizing at this pH for two hours, 11.0 kg of Grade A-17 alumina (average particle size 2-
3 microns, manufactured and sold by Alcoa Industrial Chemicals Div., Bauxite, AR) was
added in portions and stirred overnight. Glycerol was added to the slurry at a level of 3
wt% based on the total weight of the slurry. The solids content of the dispersion was 62-65
weight percent and the viscosity was adjusted to 140 centipoise by water addition, measured
with a Brookfield viscometer (Model No. RV1) using the #1 spindle.
A 2-ply glass yarn (150 filaments/ply) comprising 65.2% SiO2, 23.8% Al2O3,
and 10.0% MgO having a hydrophilic sizing to aid wetting by the aqueous coating
composition (S glass, designation S-2 CG150 1/2 636, available from Owens-Corning
Fiberglass Corporation) was fed through a ball tensioner, passed through the alumina slurry,
and pulled out through a 0.017 in diameter(0.043 cm) sizing orifice to remove excess slurry.
The sizing orifice controlled the amount of slurry applied to the yarn so that, after drying,
about 50-60% by weight of ceramic in the support was from the slurry and about 40-50% by
weight was derived from the yarn. The wet yarn was then passed through a guide attached
to the traverse arm of the filament winding machine and wound onto the contoured mandrel
described above wrapped with 2 layers of 0.002 in (0.005 cm) "Mylar" polyester film. The

winding was stopped after about 1000 grams of yarn were wound onto the mandrel, when
the support reached the desired outside diameter (approximately 60 mm). After drying
overnight at room temperature, the filament-wound tube was removed from the mandrel by
cutting through the wound material at about the center of the raised flange section
(indicating the location of the plastic collar insert) and removing the two pieces from the
opposite ends of the mandrel.
The outer membrane layer was attached to the support as follows. S-2 glass fabric
(plain weave, 1.5 oz/square yard) available from Burlington Glass Fabric (Altavista,
Virginia) was cut into pieces of length and width approximately equal to the length and
circumference of the tube respectively. Each piece was wrapped on the body of the tube
and an alumina slurry containing Grade A-16 alumina (manufactured and sold by Alcoa,
average panicle size 0.45 micron) with 55 to 60 weight percent solid content, 3 wt%
glycerol, and 100 to 120 cps viscosity, was brushed on the fabric. The fabric was not
applied to the flange and the bottom end of the tube. Any wrinkles in the fabric were
removed by rubbing with a wet sponge while the fabric was still wet before adding
additional layers of fabric. Two additional layers of fabric were attached in a similar
manner such that the closing of the ends in each layer of fabric fell approximately 120
degrees apart in the final filter. After all fabric layers were applied, the tube was dried
overnight at room temperature. It was then low-temperature fired at 700° C for one hour in
a muffle furnace to remove volatiles and stabilize the structure.
The flange section was reinforced and sealed by dipping one time in an alumina
slurry (fumed alumina/A-17 alumina, described above) and draining off the excess. A wad
of S-2 glass fibers was inserted into the hole in the bottom end of the filter and the bottom
end was then dipped in the A-17 alumina slurry. After thorough drying and firing at 700°
C for one hour, the filter was fired in a high temperature furnace. The temperature was
increased to 800° C in about 40 minutes, held for about 20 minutes, then increased to 1300°
C at a rate of 2° C/minute, held for 2 hours, then heated at a rate of 1 ° C/minute to 1380°
C, held for two hours and cooled to 800° C at a rate of 5° C/minute, followed by
unrestrained cooling of the furnace to 200°C. The filter was then removed from the furnace
and allowed to cool to room temperature in air.
The membrane layer had a bulk density of 1.62 g/cc and a volume porosity of
39% with a median pore diameter of 0.45 micron, measured by mercury porosimetry. The
average oxide composition of the filter, determined by X-ray fluorescence, was 27% silica,
68% alumina and 4% magnesia. The crystalline phase composition, determined by X-ray
diffraction, was 35% cordierite (indialite), 6% mullite, 50% corundum and 9% cristobalite.

Example 2
This example illustrates the fabrication of a ceramic filter of the current invention,
wherein the outer membrane layer is formed by circular winding.
A filter support was prepared in a manner similar to that described in Example 1
except that the filament-wound collar insert was used to form the flange section instead of
the plastic collar. When the support element was cut through for removal from the mandrel,
the wound collar was cut through as well such that a section of the original collar remained '
in the flange section of the support. The mandrel with the support wound thereon was
immediately transferred to a specialized winder for formation of the membrane layer.
The outer membrane was applied to the support by circular (hoop) winding of a glass
yarn (Owens-Corning S-2 CG 150 1/2 636) on the surface of the support. The filament
winder used for formation of the membrane layer had a screw driven traverse, with the
drive ratio set such that the spindle rotated at a speed of 75 complete revolutions for each 1
inch (2.54 cm) travel of the traverse guide so that the yarn was placed at a spacing of 75
yarns per linear inch (30 yarns per linear cm) of tube surface. Adjacent yarn windings were
as close to each other as possible without overlapping. The yarn was soaked in the A-
17/fumed alumina slurry, and pulled through a 0.017 in (0.043 cm) sizing orifice prior to
winding. About 60 grams of yarn were wound on the support surface to form a single layer
of winding over its length. The circular winding was done across the entire length of the
filter, bottom end and flange section. After overnight drying (12-16 hours) at ambient
temperature, the tube was removed from the mandrel as described in Example 1. After
inspection for defects, the filter unit was fired at 700 degrees C for two hours. Then the
bottom hole was then filled with a wad of S-glass yarn. The flange and bottom sections of
the tube were dipped in the A-17/fumed alumina slurry, the excess drained off, and dried
thoroughly. The combined support and membrane was then high-temperature fired as
described in Example 1.
The membrane layer had a bulk density of 1.61 g/cc and a volume porosity of 39%
with a median pore diameter of 0.43 um, as measured by mercury porosimetry. The
average oxide composition of the filter, determined by X-ray fluorescence, was 27% silica,
68% alumina and 4% magnesia. The crystalline phase composition, determined by X-ray
diffraction, was 33% cordierite, 8% mullite, 49% corundum and 10% cristobalite.
Example 3
This example illustrates the fabrication of a ceramic filter of the current invention,
wherein the outer membrane layer is formed by multiple yarn winding.
A filter support element was prepared as described in Example 1.
The outer membrane layer was formed using the same filament winder as was used

to form the support. Yarns from three different bobbins of S-2 CG 150 1/2 636 glass yarn
were combined and fed through a tension device, dipped in the A-17/fumed alumina slurry
described in Example 1, pulled through a 0.025 in (0.64 mm) diameter sizing orifice, and
wound on the support. The same wind angle, mandrel rotation rate, and traverse arm speed
used for the support was used for winding the membrane layer. The winding was continued
until two layers of yarn had been wound onto the mandrel so that the yarn covered the entire
surface of the support. After drying overnight, the bottom end and flange sections were
treated-as described in Example 2. The assembly was then high temperature fired as
described in Example 1.
The membrane layer had a bulk density of 1.75 g/cc and a volume porosity of 37%
with a median pore diameter of 0.64 urn, as measured by mercury porosimetry. The
average oxide composition, determined by X-ray fluorescence, was 27% silica, 68%
alumina and 4% magnesia. The crystalline phase composition, determined by'X-ray
diffraction, was 35% cordierite, 6% mullite, 50% corundum and 9% cristobalite.
Example 4
This example illustrates the fabrication of a ceramic filter of the current invention,
wherein membrane layers are applied to both the inner and outer surfaces of the support and
are formed by circular winding.
The inner membrane was formed by the circular winding of glass yarn, saturated
with a ceramic paniculate slurry, around a plastic-wrapped mandrel. After preparing a
mandrel (as described in Example 1), a filament-wound collar insert (as described in
Example 2) was positioned so as to leave about 57 inches (145 cm) of the straight tube
portion of the mandrel exposed between the insert edge and the junction of the tube with the
hemispherical end closure. The mandrel was then placed on the same circular (hoop)
winder used in Example 2, but with the spindle set to rotate at a speed of about 71.6
complete revolutions for each 1 inch (2.54 cm) travel of the traverse guide so that the yarn
was placed at a spacing of about 71.6 yarns per linear inch (28 yarns per cm) of tube
surface. The Owens-Corning "S-2 CG150 1/2 636" glass yarn was soaked in the A-
17/fumed alumina slurry, and pulled through a 0.015 in (0.038 cm) diameter sizing orifice
prior to winding. Using approximately 60 grams of yarn, the mandrel was wrapped from
the junction of the tube with the hemispherical end closure to the edge of the filament-
wound collar insert.
While the yarn was still wet, a filter support was laid down, essentially as
described in Example 1, using the chain-driven filament winder. The support was wound
over the hemispherical end, the inner membrane and the filament-wound collar insert.

When the support was complete it was put back on the circular (hoop) winder and
the flange and tip ends of the unit were reinforced as follows by the infiltration of alumina
slurry. While the mandrel was rotated at 100 RPM, approximately 20 cc of the A-17/fumed
alumina slurry described in Example 1 was slowly poured onto and absorbed into about 2
inches (5.1 cm) of the tip of the unit. An additional 20 cc of slurry was similarly applied to
the flange region, from the shoulder of the flange to 1.5 inches (3.7 cm) below the collar
insert. When the slurry had been absorbed into the tip and flange by capillary action, and
there was no excess slurry on the surface, the outer membrane was hoop wound onto the
support layer, essentially as described in Example 2. During each of the winding
procedures, a humidity level of at least 30% was maintained.
Heat guns were set up to dry the infiltrated regions, while maintaining rotation,
for at least 20 minutes. Afterward, the mandrel was removed from the winder and placed in
a vertical support rack.
After overnight drying (about 12-16 hours) at ambient.temperature (e.g., about
2Q°C), the tube comprising an inner membrane, a support layer and an outer membrane,
was removed from the mandrel as described in Example 1.
After inspection for defects, the open hole in the tip of the unit was filled with a
paste comprising by weight about 68% of the A-17/fumed alumina slurry described in
Example 1, about 3% short staple S-glass yarn, and about 29% partially-fired alumina-
coated S-glass particulate (e.g., S-glass yarn coated with matrix slurry, as described above,
then heated to approximately 700°C to partially crystallize the glass yarn, then
comminuted).
The filter was fired to a peak temperature of 1380°C following the cycle
described in Example 1.
Example 5
A ceramic filter was prepared in substantially the same manner as the filter
described in Example 4, with the following notable exceptions:
No inner membrane layer was produced.
After winding the outer membrane (as shown in FIG. 3A) and firing to about
700°C to stabilize the structure, as described in Example 1, the membrane was coated with
an aqueous slurry comprising equal weight fractions of 400 grit 38 Alundum® alumina
particulate (23 microns ave. particle size, Norton-St.Gobain, Worcester, MA) and Bluonic®
colloidal alumina (obtained from Wesbond Corp., Wilmington, Delaware). The slurry was
applied by brush to the outer surface of the filament wound support. The liquid component
of the slurry was quickly absorbed by the support, leaving a buildup of particulates on the
surface. The excess alumina particulate was manually removed by gently rubbing the

surface; then the developing filter was dried overnight at room temperature. The developing
filter was then again low-temperature fired at about 700°C for one hour in a muffle furnace
to remove volatiles. The filtering surface created by this addition of filler material such as
particulate on top of a "hoop membrane" is also considered to be a "combination
membrane".
Example 6
This Example demonstrates, among other things, the fabrication of a filament wound
ceramic hot gas filter featuring a "combination" type membrane.
First, a slurry for coating yarns was prepared by charging a mixing vessel with about
90 kg of "Chorhydrol® 50%" aluminum chlorhydrate solution (Reheis. Inc., Berkeley
Heights. NJ). While stirring, about 113 kg of Grade A-17 alumina powder (2-3 microns
ave. particle size, Alcoa Industrial Chemicals Div., Bauxite, AR) and about 1435 g of
hydrochloric acid were added to the solution.
Next, a filament wound collar insert (described previously)-was inserted on the
mandrel near the 57 inch (145 cm) position (described previously) to later form the flange
section of the filter tube.
A 2-ply glass yarn (150 filaments/ply) comprising 65.2% SiC>2, 23.8% AI2O3, and
10.0% MgO having a hydrophilic sizing to aid wetting by the aqueous coating composition
(S glass, designation S-2 CG150 1/2 636, available from Owens-Corning Fiberglass
Corporation) was fed through a ball tensioner, passed through the alumina slurry, and pulled
out through a 0.017 in diameter(0.043 cm) sizing orifice to remove excess slurry. The
sizing orifice controlled the amount of slurry applied to the yarn so that, after drying, about
50-60% by weight of ceramic in the support was from the slurry and about 40-50% by
weight was derived from the yarn. The wet yarn was then passed through a guide attached
to the traverse arm of the filament winding machine and wound onto the contoured mandrel
described above wrapped with 2 layers of 0.002 in (0.005 cm) "Mylar" polyester film. The
winding was stopped after about 1000 grams of yarn were wound onto the mandrel, when
the support reached the desired outside diameter (approximately 60 mm).
The mandrel with the support and collar insert thereon was then transferred to a
specialized winder. The flange end of the developing filter unit was then reinforced as
follows. With the mandrel rotating at about 100 RPM, approximately 20 cc of the above-
described alumina slurry used for yarn coating was slowly poured onto the flange region,
from the shoulder of the flange to about 1.5 inches (3.7 cm) below the collar insert. When
the slurry had been absorbed into the flange by capillary action, and there was no excess
slurry on the surface, construction of the membrane layer commenced.

The yarn portion of the membrane layer was applied by hoop winding. Specifically,
the filament winder used for formation of the membrane layer had a screw driven traverse,
with the drive ratio set such that the spindle rotated at a speed of about 46.8 complete
revolutions for each 1 inch (2.54 cm) travel of the traverse guide so that the yarn was placed
at a spacing of 46.8 yarns per linear inch (18.4 yarns per linear cm) of tube surface. This
spacing is such as to leave a gap between windings roughly equal to the width of the yarn.
The yarn was soaked in the A-17/Chlorhydrol® alumina slurry, and pulled through a 0.017
inch (0.043 cm) sizing orifice prior to winding. The circular winding was done across the
entire length of the filter, bottom end and flange section. During winding, a humidity level
of at least 30 percent was maintained.
Heat guns were set up to dry the reinforced regions, while maintaining rotation, for
at least 20 minutes. The developing filter and mandrel were then removed from the winder
and placed in a vertical support rack.
After overnight drying (about 12-16 hours) at ambient temperature, the ends of the
developing filter were cut off so that the mandrel could be removed. The collar portion of
the filter was sliced such that a section of the original collar remained in the flange section
of the support layer. The developing filter was then heated from ambient to a temperature
of about 700°C in a muffle furnace equipped with a hydrochloric acid scrubber. After
maintaining this low firing temperature for about one hour, the furnace and its contents were
permitted to furnace cool.
Next, a paste for closing the tip end of the tube and for filling in the gap between
windings in the membrane layer was prepared. Specifically, about 980g of de-ionized water
was measured out in an open container. While stirring, about 20g of "Superloid"
ammonium alginate (Kelco Co., San Diego, CA) was added. Stirring of this mixture was
continued until a smooth-flowing solution, free from gel particles, was obtained. Then,
while continuing to stir, about 330g of talc (Grade MP 12-62, manufactured by Minerals
Technologies) was added to the solution. When the talc had been evenly dispersed, an
additional '2700g of 320 grit 38 Alundum® alumina particulate (Norton-St. Gobain,
Worcester, MA, 32 microns ave. particle size) was slowly added. Mixing was continued
until a smooth paste, without apparent lumps or agglommerates, was obtained.
The low fired candle filter was slid back onto a mandrel and put back onto the
winder. The mandrel was rotated at approximately 100 RPM while the particulate paste was
applied to the surface of the filter with a plastic spatula until the entire surface was covered.
Sufficient pressure and "drag" were then applied with a clean spatula to remove most of the *
excess, material. A cross-sectional schematic view of the tube wall is illustrated in Figure
3B.

After removing the developing candle filter from the mandrel once again, the 1/4
inch (6 mm) diameter opening in the tip of the candle was filled with the above-identified
paste After overnight drying, a 1.25 inch (32 mm) diameter, 4 inch (102 mm) long 40-
watt illuminated light bulb was inserted into the open end of the filter. All room hghts were
extinguished and the surface of the filter was examined. In any location where there were
bright points of light ("pin holes") additional particulate paste was applied.
The candle filter was then high temperature fired as follows, The candle filter was
placed into an air atmosphere furnace at about ambient (e.g., about 20°C) temperature. The
furnace temperature was increased to about 800°C in about 40 minutes, held for about 1
hour then increased to about 1300°C at a rate of about 2° per minute, held for about 2
hours then increased to about 1380°C at a rate of about 1°C per minute, held for about 2
hours' cooled to about 800°C at a rate of about 5°C per minute, and finally furnace cooled
to about 200°C. The furnace was then opened and its contents permitted to cool naturally to
ambient temperature.
A ceramic hot gas filter was produced substantially in accordance with Example 5
except that no circularly wound filaments or yarns made up the membrane layer. The slurry
for the membrane layer was applied by brush, and excess particulates were gently rubbed off
of the filter tube. A cross-sectional schematic view of the tube wall is illustrated m Figure
3C.

We Claim:
1. A ceramic hot gas filter, comprising:
a porous elongated filter support, said support having an outer surface, an
opening at one end into a hollow interior defined in part by an inner surface, a
closed end opposite said open end, and an external flange integral with said
open end, said support being formed of a plurality of layers of oxide ceramic
yarn, each layer being arranged in a crisscrossing relationship with neighboring
layers to form a plurality of quadrilateral-shaped openings, said yarn being
coated with a first oxide ceramic material, said first oxide ceramic material
providing, upon heat treatment, a porous refractory oxide support matrix; and
a porous membrane layer contacting the outer surface or inner surface of said
support, said membrane layer being less porous than said support and
comprising (1) at least one circularly wound continuous filament oxide ceramic
yarn, adjacent windings of said ceramic yarn defining a gap therebetween, said
yarn being coated with a second oxide ceramic material, and (2) at least one
ceramic filler material disposed in said membrane surface and substantially
uniformly distributed therein.
2. The filter as claimed in claim 1, wherein said at least one ceramic filler material
comprises a particulate oxide ceramic material.
3. The filter as claimed in claim 1, wherein said at least one ceramic filler material
comprises a form selected from the group consisting of powders, particulates,
whiskers, chopped fibers, platelets, flakes, spheres, tubules and pellets.

4. The filter as claimed in claim 1, wherein said filter has a crystalline composition
of about 25-40% cordierite, 5-15% mullite, 40-60% corundum, and 0-10%
cristobalite, based on the total crystalline content of the filter.
5. The filter as claimed in claim 1, wherein said quadrilateral-shaped openings
have dimensions of about 100 to about 500 microns after heat treatment.
6. The filter as claimed in claim 1, wherein said membrane layer defines pores
having diameters of about 0.1 to about 50 microns.
7. The filter as claimed in claim 1, wherein said membrane layer defines pores
having diamters of about 5 to about 25 microns.
8. The filter as claimed in claim 7, wherein an average size and size distribution of
said pores is substantially invariant around a circumference and along a
longitudinal extent of said membrane layer.
9. The filter as claimed in claim 1, wherein said second oxide ceramic material
provides, upon heat treatment, a porous refractory oxide membrane matrix.
10. The filter as claimed in claim 9, wherein between about 40 to about 70 percent
of the total weight of said filter is from the combined weight of said support
matrix and said membrane matrix.
11. The filter as claimed in claim 1, wherein said support yarn has generally the
same composition as said continuous filament membrane yarn and wherein said
first oxide ceramic material has generally the same composition as said second
oxide ceramic material.

12. The filter as claimed in claim 1, wherein said first and second oxide ceramic
materials each comprise Al2O3.
13. A ceramic hot gas filter, comprising:
a porous elongated filter support, said support having an outer surface, an
opening at one end into a hollow interior defined in part by an inner surface a
closed end opposite said open end, and an external flange integral with said
open end, said support being formed of a plurality of layers of oxide ceramic
yarn, each layer being arranged in a crisscrossing relationship with neighboring
layers to form a plurality of quadrilateral-shaped openings, said yarn being
coated with an oxide ceramic material, said oxide ceramic material providing,
upon heat treatment, a porous refractory oxide support matrix; and
a porous membrane layer disposed on the outer surface or inner surface of said
support, said membrane layer being less porous than said support and consisting
essentially of at least one ceramic filler material comprising a plurality of
bodies which define interstices therebetween having a size of about 0.1 to about
50 microns.
14. A hot gas filter, substantially as herein described, particularly with reference to
and as illustrated in the accompanying drawings.


A ceramic hot gas filter, comprising:
a porous elongated filter support, said support having an outer surface, an opening at
one end into a hollow interior defined in part by an inner surface, a closed end opposite
said open end, and an external flange integral with said open end, said support being
formed of a plurality of layers of oxide ceramic yarn, each layer being arranged in a
crisscrossing relationship with neighboring layers to form a plurality of quadrilateral-
shaped openings, said yarn being coated with a first oxide ceramic material, said first
oxide ceramic material providing, upon heat treatment, a porous refractory oxide
support matrix; and
a porous membrane layer contacting the outer surface or inner surface of said support,
said membrane layer being less porous than said support and comprising (1) at least
one circularly wound continuous filament oxide ceramic yarn, adjacent windings of
said ceramic yarn defining a gap therebetween, said yarn being coated with a second
oxide ceramic material, and (2) at least one ceramic filler material disposed in said
membrane surface and substantially uniformly distributed therein.

Documents:

747-kol-2004-abstract 1.1.pdf

747-kol-2004-abstract 1.2.pdf

747-kol-2004-abstract.pdf

747-kol-2004-amanded claims 1.1.pdf

747-kol-2004-assignment 1.1.pdf

747-kol-2004-claims.pdf

747-kol-2004-correspondence 1.1.pdf

747-kol-2004-correspondence 1.2.pdf

747-kol-2004-correspondence.pdf

747-kol-2004-description (complete) 1.1.pdf

747-kol-2004-description (complete).pdf

747-kol-2004-drawings.pdf

747-kol-2004-examination report 1.1.pdf

747-kol-2004-form 1 1.1.pdf

747-kol-2004-form 1.pdf

747-kol-2004-form 18 1.1.pdf

747-kol-2004-form 18.pdf

747-kol-2004-form 2 1.1.pdf

747-kol-2004-form 2.pdf

747-kol-2004-form 3 1.1.pdf

747-kol-2004-form 3.pdf

747-kol-2004-form 5 1.1.pdf

747-kol-2004-form 5.pdf

747-kol-2004-granted-abstract.pdf

747-kol-2004-granted-claims.pdf

747-kol-2004-granted-description (complete).pdf

747-kol-2004-granted-form 1.pdf

747-kol-2004-granted-form 2.pdf

747-kol-2004-granted-specification.pdf

747-kol-2004-others 1.1.pdf

747-kol-2004-others 1.2.pdf

747-kol-2004-pa 1.1.pdf

747-kol-2004-pa.pdf

747-kol-2004-pct priority document notification.pdf

747-kol-2004-petition under rule 137.pdf

747-kol-2004-petition under rule 138.pdf

747-kol-2004-reply to examination report 1.1.pdf

747-kol-2004-specification.pdf

747-kol-2004-translated copy of priority document 1.1.pdf


Patent Number 246135
Indian Patent Application Number 747/KOL/2004
PG Journal Number 07/2011
Publication Date 18-Feb-2011
Grant Date 15-Feb-2011
Date of Filing 22-Nov-2004
Name of Patentee ALLIEDSIGNAL COMPOSITES INC.
Applicant Address 1300, MARROWS ROAD, P O BOX 6077, NEWARK, ELAWARE
Inventors:
# Inventor's Name Inventor's Address
1 DANIEL MATTHEW DOMANSKI 1017, OLD FORGE ROAD, NEW CASTLE, DELAWARE 19720
2 JEFFREY ALLEN CHAMBERS 52, KINGS GRANT ROAD, HOCKESSIN, DELAWARE 19707
3 GOVINDASAMY PARAMASIVAM RAJENDRAN 6, CIDER MILL COURT, BOOTHWYN, PENNSYLVANIA 19061
4 ELIZABETH SOKOLINSKI CONNOLLY 20, WINDSOR ROAD, WILMINGTON, DELAWARE 19809
5 GEORGE DANIEL FORSYTHE 326 GLENN RAOD, LANDENBERG, PENNSYLVANIA 19350
PCT International Classification Number C04B 33/32
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 08/896,372 1997-07-18 U.S.A.