This invention provides a filter cloth assembly for use on a filter
plate of a center-feed filter press, which comprises a two-part
filter cloth with fittings for rapid and easy assembly on the filter
plate, one cloth on each side. Methods of manufacture of the novel
assembly and its parts and a new method for bonding otherwise incompatible
plastics and elastomers are disclosed.
1. A filter cloth assembly for use on a center-feed filter plate
of a filter press comprising a first and a second section of filter
cloth disposed on opposite sides of said plate, said cloths including
concentric holes adapted to align with a corresponding hole in said
plate; said first cloth including a tubular stem member having a
flanged base, said base being secured to said cloth and extending
from said cloth in concentric alignment with said hole in said cloth
and adapted for extending through said hole in said plate and mating
with a corresponding member on a second filter cloth disposed on
the opposite side of said plate; said second cloth including a flanged
coupling member having a flanged base secured to said second cloth
in concentric alignment with said hole in said second cloth, the
coupling member of said second cloth adapted to fit closely and
concentrically over the tubular stem of the first cloth extending
from the hole in the plate when the cloths are assembled in place
on opposite sides of the filter plate; and means for effecting a
non-leaking liquid seal between the stem and the coupling member
of said first and second cloths as assembled together on the filter
2. An assembly of claim 1 wherein the filter cloths comprise a
layered composite of at least one layer of textile fabric and a
membrane layer of porous expanded polytetrafluoroethylene.
3. An assembly of claim 2 wherein the textile fabric layer is polypropylene.
4. An assembly of claim 2 wherein the textile fabric layer is polyester.
5. An assembly of claim 2, wherein the textile fabric layer is
6. An assembly of claim 3 wherein the flanged stem of the first
filter cloth comprises molded polypropylene.
7. An assembly of claim 4 wherein the flanged stem of the first
filter cloth comprises molded polypropylene.
8. An assembly of claim 5 wherein the flanged stem of the first
filter cloth comprises polytetrafluoroethylene.
9. An assembly of claim 3 wherein the flanged coupling member of
the second filter cloth comprises silicone rubber.
10. An assembly of claim 4 wherein the flanged coupling member
of the second filter cloth comprises silicone rubber.
11. An assembly of claim 3 wherein the flanged coupling member
of the second filter cloth comprises a fluoroelastomer.
12. An assembly of claim 4 wherein the flanged coupling member
of the second filter cloth comprises a fluoroelastomer.
13. An assembly of claim 5 wherein the flanged coupling member
of the second filter cloth comprises a fluoroelastomer.
14. An assembly of claim 1 wherein the means for effecting the
non-leaking liquid seal comprises securing a band or strap under
effective tension around the coupling member of the second filter
15. An assembly of claim 14 wherein the band or strap is metal.
16. An assembly of claim 14 wherein the band or strap is plastic.
BACKGROUND OF THE INVENTION
This invention relates to a filter cloth assembly for use on a
filter press plate, more particularly a center-feed filter press
plate, which operates with a filter cloth on each side of the plate,
a method for assembling a novel two-part filter cloth onto the plate,
and a method for manufacturing the novel two-part filter cloth.
Filter presses are a well-known type of pressure filter made especially
for liquid/solid separations. Such presses include a series of filter
plates which are compressed together between two heads. Covering
each plate is a filter cloth which covers all the area where filtration
occurs. Each plate typically includes an opening located within
the filtration surface area, usually near the center of the plate,
and the plate has channels in its surface for passage of liquids.
A solid/liquid mixture enters through the opening, passes into the
spaces between the plates, then the liquid portion passes through
the filter cloths into the channels and exits through an exit port
for each plate. As the filtrate passes through the filter cloths,
a cake of solids is built up in the spaces between the plates on
the filter cloths. When the filter press is filled with solid filter
cake material, the filtration is stopped, and the cake is either
washed from the press or, alternatively, the press is opened, the
plates separated, and the solid filter cake is removed. The filter
cloth can then also be removed for washing or replacement.
In the center-hole type press, a conventional filter cloth passes
through the center hole of the plate and covers both sides of the
plate. Most of such filter cloths available for use are comprised
of a single unit having two sides with an interconnected tubular
center hole section, the two sides of filter cloth usually being
of unitary construction, and typically sewn together. This construction,
while typical, is barely manageable if the filter cloth is stiff
in nature or large in size, because one side of the cloth must be
folded together and squeezed through the center hole of the filter
plate, then unfolded on the other side to cover the other side of
the filter plate. This process is often very difficult and time
consuming, particularly if the filter cloth must be changed frequently.
A metal, such as brass, or a plastic, screw-threaded, reusable
mechanical coupling to hold two filter cloths together on opposite
sides of a filter plate, has also been used. Each half has a flange
which holds the filter cloth against the plate. Thread clogging
with slurry and thread wear upon coupling and uncoupling the filter
assembly can be problems. The amount of filter cloth caught behind
and held by each flange is small, and hard turns of the coupling
to hold it firmly in place under high pressure may damage the filter
cloth. The amount of pressure used is also a variable with each
instance of installation of the cloth by each installer. Further,
threaded fittings may open up the interior of the filter plate/filter
cloth assembly to bypass of the seal through the bottoms of the
thread root channels under liquid pressure.
A filter cloth embodying two filter cloths is disclosed in U.S.
Pat. No. 4,053,416. Two single pieces of filter cloth were used,
each having flexible flanged-based tubular stemmed coupling members
which fitted into each end of a coupling cylinder which passed through
the hole in the plate. A tool was utilized to effect the coupling
which involved eversion of the flexible stem of one of the filter
cloths into place in the coupling cylinder by means of the tool.
No attachment was provided for the two cloths to each other and
each cloth was held in place by friction between the springy elastomeric
stem and the coupling cylinder in which it resided when the filter
cloth was assembled. The lack of attachment of the two cloths to
each other and the varying amount of hold available from friction
may be sources of leakage or provide attachment problems under rigorous
conditions. Known methods for attaching the stem or coupling member
to the filter cloth, such as stitching, may provide a source of
leakage through the stitch holes. Manipulation of the tool to evert
the coupling of one cloth, assembling the cloth on a filter plate,
then withdrawing the tool may not be as rapid or efficient a method
of assembly as desired.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide
a filter cloth assembly for use on a center feed filter plate of
a filter press which avoids the problems of prior art filter cloths.
Another object of the invention is to provide a filter cloth assembly
which is fast and easy to assemble on the filter plate or disassemble
from it and provides a good liquid-tight seal with the plate.
A further object of the present invention is to provide novel methods
for attaching and sealing coupling members, especially those having
flanged bases, to filter cloths, then assembling the cloths into
place on the filter plate.
Briefly, the invention embodies a filter cloth assembly for use
on a center hole filter press plate, a method for making flanged-based
coupling members which are sealed to the filter cloth, and a method
for adhering certain incompatible elastomeric polymers together
which are useful in manufacturing the filter cloths of the invention.
The filter cloth assembly comprises two sections of a filter cloth
disposed on opposite sides of a center-hole type of filter press
plate, the first cloth having secured to it in alignment with a
hole corresponding to that in the filter press plate a flanged-based
tubular stem. The second filter cloth comprises a similar cloth
bearing a short flanged-based tubular coupling. The stem of the
first filter cloth is passed through the hole in the press plate,
the coupling of the second filter cloth fitted over the protruding
end of the stem of the first filter cloth such that the coupling
surrounds the stem in a close-fitting concentric configuration,
and a plastic or metal tension strap or band clamp placed around
the outside of the coupling in a groove provided therein, and the
strap or band clamp closed to a tension sufficient to effect a liquid-tight
seal between the stem and the coupling.
Methods are given for either forming in place or adhering to the
two filter cloths the flanged-based tubular stem and coupling by
known methods or by a novel process to be subsequently described
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic cross-sectional diagram of a filter press
FIG. 2 is an exploded section view taken through the center hole
of a filter press plate, including the plate, the first and second
filter cloths disposed on either side of the plate, and the tension
strap or band clamp to hold the two cloths in place tightly to the
FIG. 3 is a broken sectional view of the filter cloth assembly
in place on the filter plate.
FIG. 4 is a cross section of the second flanged-based tubular coupling,
including circumferential beads molded into the inside surface of
the coupling to aid in achieving a tight liquid seal with the stem
from the other cloth.
FIG. 5 is a cross-section of a thermosetting elastomer into which
a thermoplastic cloth has been pressed prior to fully curing the
FIG. 6 is a cross-section of the elastomer and cloth shown in FIG.
5 adhesively bonded to a filter cloth.
FIG. 7 is a cross-section of an elastomeric filter coupling of
the invention firmly bonded with an adhesive to an otherwise adhesively
incompatible filter cloth.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A detailed description of the invention is best provided with reference
to the accompanying drawings wherein FIG. 1 shows a schematic cross-sectional
diagram of filter press 10 containing conventional filter plates
12. The flow direction of slurry to be filtered and the outflow
of filtrate are indicated by the arrows. The filter plates 12, when
the assembly is completed, retain the filter cloths 14 in place,
and the slurry flows to the cloths which collect the solids and
allow the filtrate to pass through and exit the filter through outlet
ports 15. The flange-based stems 28 in the center holes of each
filter plate 12 are discussed in detail below.
Referring to FIG. 2, there is shown a detailed schematic view of
a filter cloth assembly for use on a center-hole type filter plate
for use in a pressure filter press of the standard type, wherein
a series of filter plates having filter cloths in place on each
side are pressed together face to face such that a seal is formed
at the edges of the plates between the stack of plates. The liquid/solid
mixture to be filtered is pumped through the center hole of the
press, the solids are retained on the filter cloths of the plates
as the liquid flows through the filter cloths, and the filtrate
flows from the press from outlet ports in the channels provided
therefor within the area covered by the filter cloths.
Filter cloth 14 is preferably a polypropylene or polyester textile
felt or woven fabric to which is bonded to a polypropylene stem
28 having flange 30 as shown. The flange 30 and the attached stem
28 are molded from a thermoplastic such as polypropylene, or other
suitable thermoplastic materials, or from polytetrafluoroethylene
(PTFE) material. Flange 30 is fuse-bonded or bonded by means of
a polypropylene hot melt or, in the case of a PTFE stem 28, a fluorinated
ethylene-propylene (FEP) or poly fluoro alkylene (PFA) polymer hot
melt, to the cloth 14, thereby providing a liquid-tight seal which
prevents the liquid-solid mixture from bypassing the filter cloth
Referring to FIG. 3, the bond between flange 30 and filter cloth
14 is indicated 32. Other plastics may be used for the stem 28 and
cloth 14 which will have equivalent properties. Stem 28 is bonded
to cloth 14 on the reverse side from the extended portion of the
stem 28 which will extend through the hole of filter plate 12 to
mate with and be attached to the second filter cloth 14 on the other
side of plate 12. Where stem 28 with molded flange 30 is formed
from PTFE, and the cloth is a PTFE felt or fabric filter cloth,
stem 28 is preferably preformed or premolded and then heat-bonded,
melt-fused, or fuse-bonded with heat and pressure to cloth 14 by
a layer 32 of FEP or PFA heat-fusible fluorocarbon polymer.
Referring to both FIGS. 2 and 3, filter cloth preferably is a composition
of a textile cloth 16 and a membrane 18 of expanded porous PTFE.
Coupling 26 having flange 20 is preferably silicone rubber or Viton.RTM.
fluoro elastomer, and is molded in place on the filter cloth from
liquid rubber or Viton fluoro elastomer and cured by a catalyst
over a specified time and/or is heat cured. Where stem 28 is formed
from PTFE, coupling 26 is formed by molding it in place on the filter
cloth from Viton fluoro elastomer or fluoro silicone rubber.
To assemble the filter cloths on the filter plate, the plate 12
is laid over one cloth 14 as shown such that stem 28 protrudes through
the hole therein. The second cloth 14 is placed cloth-down on plate
12 over stem 28 which closely fits through coupling 26 to close
tolerance. In this configuration, the porous PTFE membrane 18 is
always upstream of the textile layer 16. Tension strap or band clamp
24 is then placed in the groove 21 about coupling 26 and tightened
to the degree necessary to insure a liquid tight seal under the
operating pressure being used with the filter press. The band 24
may be a standard metal band clamp or a plastic strap and lock.
FIG. 3 shows the assembled filter cloths 14 on the filter plate
12. Flange 30 of stem 28 provides a positive clamping effect on
filter cloth 14 as does the flange 20 of coupling 26 when coupling
26 is pressed onto the filter plate 12 and strap 24 is appropriately
tightened and locked. Bond 32 is formed between filter cloth 14
and flange 20.
A preferred method of manufacture of flanged coupling 26 is to
preform or premold flanged coupling 26 from silicone rubber or Viton
fluoro elastomer or other suitable thermosetting elastomeric materials
which have equivalent properties.
A method for bonding otherwise adhesively incompatible materials,
for example, a fully cured silicone rubber coupling to a polypropylene
textile felt filter cloth is depicted in FIGS. 5 and 6. Therein
is shown, in FIG. 5, a premolded elastomeric coupling 36 of silicone
rubber, into which a thermoplastic cloth, woven or felt or other
similar material, has been pressed prior to full curing of the elastomer
36. The pressing is done so that the elastomer penetrates into the
cloth but does not penetrate all the way through the cloth 38. Upon
full cure of the elastomer, the cloth 38 is mechanically tightly
bound to the elastomer 36 and presents an adhesively compatible
under surface of the composite coupling/cloth which can be adhesively
bonded to the various filter cloths 16 by adhesive 32 as shown in
FIG. 6. The cloth 38 can be polyester or polypropylene or other
similar plastic material.
When using this method and the coupling 26 of the invention, after
the coupling of silicone rubber is fully cured, a silicone adhesive
32 such as silicone "L" is coated on the surface of flange
20 to be bonded and around the hole area on filter cloth 14, as
depicted in FIG. 7, and heat and pressure are applied so that the
silicone adhesive penetrates both the polypropylene felt 38 bonded
to coupling 26 and the surface of cloth layer 16. As shown in FIG.
7, the expanded PTFE layer 18 has been removed from the bond region
so as to provide the adhesively compatible surface of cloth layer
16 for bonding. When cure is complete, a strong, tenacious and leak-proof
bond 32 is formed.
Alternatively, bond 32 may be formed without using an adhesive
when the cloth 38 and the cloth layer 16 are both thermoplastics
such as polypropylene or polyester. The bond is formed by applying
heat and pressure to melt and fuse the cloths together.
Additionally, the flanged coupling 26 of silicone rubber or fluoro
elastomer could be molded in place onto the filter mat felt around
the hole and cured to solid form, with, again, penetration of the
liquid rubber only partially into the interstices of the textile
filter felt. By this method, silicone rubbers or fluoro elastomers
can be made to form strongly bonded structures with a textile felt
where the interstices of the felt are partially filled by the polymer,
and the fibers of the felt link the two otherwise adhesively incompatible
polymers together. Textile felts or woven fabrics of polymers other
than polypropylene and polyester may be used where they provide
FIG. 4 is a cross-section of the flanged coupling 26 which has
been molded with two beads 22 around the interior circumference,
spaced so as to aid in forming a liquid-tight seal around stem 28
where stem 28 may be uneven or out-of-round and also spaced such
that pressure from strap or ring clamp 24 will center between the
two beads when clamp 24 is tightened.
The most preferred filter cloth material 14 for use in the filter
cloth assembly of the invention is a layered composite sheet of
filter cloth containing an upstream layer 18 of porous expanded
PTFE bonded to a woven or felt textile filter 18 cloth of polypropylene,
polyester, or the equivalent of these in properties. The porous,
expanded PTFE is prepared in accordance with one or more of U.S.
Pat. Nos. 4,187,390; 4,110,392; 4,096,227; 3,962,153; 3,953,566;
and 4,025,679. The PTFE surface minimizes adherence of filtered
solids to the filter cloth and aids in rapid and easy cleaning of
When a filter cloth having a layer of porous expanded PTFE is used
in the assembly of the invention, as mentioned above the PTFE in
the area of the cloth around the hole to be bonded to the flanges
of stem 28 and coupling 26 must be removed so that the underlying
polypropylene or polyester layer can be bonded to the flange. This
can be done by a mechanical abrasion method, such as by using a
wire brush or grit disc. Where the flanged stem 28 is made from
PTFE, the PTFE layer of the composite filter cloth is usually not
removed and the flange and cloth in this instance are joined by
bonding the two parts together with a layer of FEP or PFA hot melted
or fuse-bonded by heat and pressure between them.
While the invention has been disclosed herein in connection with
certain embodiments and detailed descriptions, it will be clear
to one skilled in the art that modifications or variations of such
details can be made without deviating from the gist of this invention,
and such modifications or variations are considered to be within
the scope of the claims hereinbelow.