Abstrict
The invention relates to a filter cloth intended for a filter which
has a variable volume and is based on diaphragm extrusion, particularly
for a Larox.RTM.-type vertical pressure filter. The filtering properties
of the filter cloth (5) are substantially similar in both directions
through the cloth since slurry containing liquid and solids is alternately
placed on the different sides of the filter cloth. The filter cloth
of the invention comprises a middle layer (15) and protective layers
(16a, 16b; 17a, 17b) provided on both outer surfaces of the middle
layer. The middle layer can have e.g. a woven structure, and the
protective layers may be batt fiber layers attached to the middle
layer by needling. According to a preferred embodiment of the invention,
the protective layers according to the invention are denser than
the middle layer.
Claims
What is claimed is:
1. A solid-liquid filtration filter cloth which is intended for
use in a pressure filter based on diaphragm extrusion which comprises
at least two filter chambers and in which the filter cloth is guided
through the filter chambers arranged one after the other in the
direction of movement of the filter cloth so that in the filter
chambers, the opposite sides of the filter cloth are alternately
against the slurry to be filtered which contains liquid and solids,
the filter cloth being symmetrical in respect of the filtering ability,
the filter cloth comprises a middle layer the both surfaces of which
are provided with protective layers which form the outer surfaces
of the filter cloth and are denser than the middle layer, the protective
layers are batt fibre layers attached to the middle layer by needling,
the middle layer has a woven structure comprising machine direction
threads and transverse threads and withstanding tensile stress thus
allowing the solids separated from the slurry in said filter to
be conveyed out of the filter chambers by means of the filter cloth.
2. A filter cloth according to claim 1, wherein the protective
layers are made to resist wear by using wear-resistant fibres.
3. A filter cloth according to claim 1, wherein the air permeance
of the filter cloth is below 0.2 m.sup.3 /m.sup.2 min, 200 Pa.
Description The invention relates to a filter cloth intended for use in a pressure
filter based on diaphragm extrusion which comprises at least two
filter chambers and in which the filter cloth is guided through
the filter chambers arranged one after the other in the direction
of movement of the cloth so that in the filter chambers, the opposite
sides of the filter cloth are alternately against the slurry to
be filtered which contains liquid and solids, the filter cloth being
symmetrical in respect of the filtering ability, and the solids
separated from the slurry in said filter are conveyed out of the
filter chamber by means of the filter cloth.
Removal of liquid, i.e. solid/liquid separation, is needed in the
mining industry, metal processing, chemical industry and in preparation
processes of foodstuffs and pharmaceuticals, for instance. For this
purpose there are various filtering apparatuses the operating principles
and properties of which differ from one another. One known filter
type used in the treatment of slurry consisting of liquid and solids
is a vertical chamber filter which has a variable volume and utilizes
diaphragm extrusion. A pressure filter known by the product name
Larox.RTM. also works on this principle. The operation and structure
of this filter will be described below in FIGS. 1 to 3f. The filter
comprises several superimposed horizontal filter plates which define
filter chambers. A filter cloth is arranged to travel through the
openable and closable filter plates of each chamber. The slurry
to be filtered is fed into the chambers to the other side of the
filter cloth, after which it is pressed against the filter cloth
by means of an expanding elastic diaphragm. In that case the liquid
in the slurry is squeezed through the cloth and the solids remain
on the other side of the cloth, forming a `cake`. After compression
and the other phases the pack formed by the filter plates is opened
and the filter cloth is moved with respect to the filter chambers
to remove the solids from the chambers. The filter cloth travels,
guided by suitable turning rolls or the like, from one of the superimposed
chambers to another, and thus the slurry is alternately placed on
the different sides of the filter cloth in the filter chambers arranged
one after the other in the direction of movement of the cloth. Consequently,
the filter cloth needs to be symmetrical in respect of its filtering
ability to provide filtrate of equal quality from each filter chamber.
The requirements set for the properties of the filter cloths intended
for the above-mentioned use are high. Not only do they function
as the filtering medium, but also as a conveyor belt when conveying
the solids cake during the discharge phase of the filtration process
out of the filter chambers. In the largest filters the solids separated
during one phase and conveyed by the filter cloth may weigh even
20 000 kilos. Particularly in the discharge phase the filter cloth
is thus subjected to a high tensile stress. Furthermore, during
use scrapers and washing devices, for example, subject the filter
cloth to considerable wear. The conditions in which the cloths must
operate are also demanding, i.e. in addition to mechanical stress
the cloths are subjected to high temperatures, temperature changes,
high pressure and varying pH conditions, for example. The filter
cloths need to be replaced from time to time not only because of
wear and tear, but also because of a decreased filtering ability
resulting from clogging and dirtying. As regards the use of the
filtering apparatuses, it would be advantageous if the useful life
of the filter cloth could be increased because replacement of the
cloth causes considerable costs and production breakdowns.
The object of the present invention is to provide a novel filter
cloth which eliminates problems associated with the prior art solutions.
The filter cloth of the invention is characterized in that it comprises
a middle layer both surfaces of which are provided with protective
layers which form the outer surfaces of the filter clothr.
The basic idea of the invention is that the filter cloth intended
for use in a pressure filter based on diaphragm extrusion comprises
at least three layers attached to one another. In that case both
outer surfaces of the filter cloth are provided with separate protective
layers. The protective layers are formed so that the filter cloth
is at least functionally symmetrical, i.e. its fitering ability
and other operating characteristics are substantially similar on
both sides of the cloth. The basic idea of a preferred embodiment
of the invention is that the protective layers are made to resist
wear by using wear-resistant fibres or threads and bond structures
between them. Thus the protective layers function as wear surfaces
which effectively protect the middle layer of the filter cloth.
The basic idea of another preferred embodiment of the invention
is that the protective layers are denser than the middle layer,
thus functioning as the actual filtering layers of the cloth.
An advantage of the invention is that the useful life of the filter
cloth can be increased compared to the prior art cloths. Thanks
to the longer useful life, replacement of the filter cloth will
cause fewer breaks in production and the other costs related to
the replacement will also be lower. A further advantage is that
in addition to durability and density, the other filtering and operating
characteristics of the cloth can be influenced by the selection
and dimensioning of the protective layers, if necessary. According
to an embodiment of the invention, the quality of the resulting
filtrates can be improved by means of a filter cloth which is denser
than the cloths used at the moment. The dense filter cloth is also
easier to wash and clean since the dirt is not absorbed into the
cloth as easily as before. Thanks to the multilayer structure, the
middle layer of the filter cloth can be made looser than before,
and thus the required density of the cloth is provided by means
of the protective layers.
The invention will be described in greater detail in the attached
drawings, in which
FIGS. 1a and 1b are schematic side views of the structure of a
vertical pressure filter apparatus where a filter cloth according
to the invention can be used,
FIG. 2a schematically illustrates how a filter cloth according
to the invention travels in the filtering apparatus of the previous
figure, and
FIG. 2b illustrates the same in another fitering apparatus,
FIGS. 3a to 3f schematically illustrate the operating principle
of a filtering apparatus illustrated in FIGS. 1a to 2a,
FIG. 4 is an enlargement of the cross-sectional structure of a
filter cloth according to the invention, and
FIG. 5 is an enlargement of the cross-sectional structure of a
filter cloth according to the invention.
FIGS. 1a and 1b are considerably simplified side views of the structure
of a Larox.RTM.-type pressure filter. The filter comprises several
horizontally arranged filter plates 1 which form a plate pack 2
comprising superimposed filter chambers. The filter cloth has the
shape of an endless loop and is arranged to travel, guided by turning
rolls 3 or the like, between the superimposed filter plates from
one chamber to another in the direction of movement of the cloth.
FIG. 2a illustrates in greater detail how the filter cloth travels
in the filtering apparatus. The fitering apparatus further comprises
closing means, such as hydraulic cylinders 4, a screw mechanism
or the like for pressing the filter plates of the pack against each
other. In FIG. 1a the filter is open, i.e. in the discharge position
in which the filter cloth can be moved. In FIG. 1b the filter is
in the closed position during which the other phases of the filtering
process occur as shown in FIGS. 3a to 3e.
FIG. 2a illustrates in a simplified manner how the filter cloth
5 travels in a filter according to the previous figures. The filter
cloth is guided through filter chambers (not shown) between the
filter plates by means of turning rolls 3. The apparatus also comprises
necessary rolls or other guiding members for adjusting the tenseness
and lateral position of the filter cloth, and a roll which moves
the filter cloth. In the situation shown in the figure the plate
pack is open, in which case a solids cake 6 is removed from the
filter chambers by moving the filter cloth 5 in direction A. The
material stuck on the filter cloth can be removed using scrapers
7 or the like provided for each turning roll. The filter further
comprises a washing apparatus 8 for washing the filter cloth.
FIG. 2b illustrates how the filter cloth 5 according to the invention
travels in another vertical pressure filter. The phases of the filter
cycle may be the same as those illustrated in FIGS. 3a to 3f. The
solution differs from the solution of FIG. 2a in that the filter
cloth does not have the shape of a dosed loop, but the filter cloth
runs through the filter from the first roll 18a to the second roll
18b. The filter cloth 5 can be moved in the direction opposite to
direction A shown in the figure to lead the cloth back to roll 18a.
FIGS. 3a to 3f are simplified schematic views of the phases of
the filter cycle according to the previous figures in one chamber.
The different filtering phases occur simultaneously in all different
chambers of the filter. In FIG. 3a the slurry 9 to be filtered is
fed into the filter chamber between a pressing diaphragm 10 and
a filter cloth 5. Due to gravitation and feeding pressure of the
slurry the liquid included in the slurry starts to penetrate through
the filter cloth into a space 11 on the other side of the cloth
from which it is supplied forward by means of suitable channels.
A layer of moist solids begins to build up on the upper surface
of the filter cloth 5. FIG. 3b illustrates the compression phase
in which pressure, e.g. pressurized air or water, is supplied above
the elastic diaphragm, and consequently the diaphragm 10 presses
the solids cake 6 against the filter cloth, thus making the liquid
included in it to move to the other side of the cloth. FIGS. 3c
and 3d illustrate phases which are related to washing the cake and
may be in use in the apparatus. In the washing phase illustrated
in FIG. 3c, washing liquid 12 is fed between the diaphragm and the
solids cake and the liquid pushes the diaphragm into its upper position
and penetrates the cake, simultaneously washing it. In the second
compression phase the washing liquid contained in the chamber and
solids are pressed through the filter cloth and out of the filter
chamber by means of the diaphragm. The solids cake is finally dried
by blowing pressurized air through it as shown in FIG. 3e. After
this, the plate pack is opened and the filter cloth is moved forwards
in its direction of movement, in which case the dry cake 6 formed
on the surface of the cloth can be led out of the filter chamber.
The cloth is also washed. The discharge phase is illustrated in
FIG. 3f. After this, the phases described above are repeated simultaneously
in all filter chambers.
FIG. 4 is a simplified cross-sectional view of an application of
the filter cloth according to the invention. The filter cloth comprises
a middle layer 15 woven of machine-direction warp threads 13 and
transverse weft threads 14. Some bonds known per se and suitable
threads or thread materials can be used for weaving the middle layer
of the cloth. Thus currently used woven filter cloths are well suited
for the middle layer, but it could be made of non-woven fabrics,
too. The threads typically used for filter cloths are multi-filament
threads made of some of the following plastic materials, for example:
polyethylene terephthalate (PET), polypropene (PP), polyamide (PA),
polyphenylene sulphide (PPS) or polyetheretherketone (PEEK). As
is seen in the figure, both sides of the woven middle layer 15 are
provided with symmetrical protective layers 16a and 16b which form
the outer surfaces of the filter cloth to be arranged against the
slurry to be filtered. Protective layers made separately of the
middle layer, such as batt fibre layers or layers made of staple
fibres e.g. by compression, can be attached to the middle layer
e.g. by needling, but other attachment methods known in the field,
such as gluing and melting bonds, may also be used, depending on
the structure of the protective layers. In the structure illustrated
in the figure, the primary purpose of the protective layers is not
to affect the density of the filter cloth, but to protect the middle
layer. Protective layers which are looser than the middle layer
are preferably made of threads or fibres having good wear-resistance
using bonds or attachment methods with good wear-resistance. The
protective layers receive the strain to which the filter cloth surfaces
are subjected during the compression and discharge of the cake and
by the doctoring and washing of the filter cloth. The fact that
the protective layers, which are clearly looser than the middle
layer, wear in use does not substantially affect the filtering ability
of the filter cloth. The protective layers can also be made to receive
part of the tensile strain directed to the filter cloth, in which
case the middle layer can be made of a fabric which is less resistant
to tensile strain, provided that this is advantageous to the filtering
ability. It should be mentioned that the thickness of the protective
layers and that of the middle layer can be adjusted according to
the need. Furthermore, the protective layers on both sides of the
middle layer of the cloth may consist of more than one layer, provided
that they are arranged so that the filtering ability of the filter
cloth is independent of the fact on which side of the cloth the
material to be filtered is placed. It is advantageous to manufacture
a cloth with a symmetric structure where both sides of the middle
layer are provided with the same number of similar protective layers.
In addition, a multilayer filter has a kind of sandwich structure
which provides the filter cloth with better transverse rigidity,
if necessary. It is advantageous to make both the middle layer and
the protective layers easily cleanable e.g. by using soil-repellent
fibres or fibres treated to be soil-repellent.
FIG. 5 is a cross-sectional view of the structure of a filter cloth
according to the invention. The same fabric as in the solution according
to the previous figure can be used in the middle layer 15. Here
the outer surfaces of the middle layer are provided with protective
layers 17a and 17b which are denser than the middle layer 15. In
that case the protective layers do not only function as the outermost
surface receiving the mechanical stress, but also as layers affecting
the actual filtering process. Since the desired density of the cloth
is adjusted by means of the protective layers, the structure of
the middle layer is not relevant to the filtering ability. The middle
layer is mainly a base to which the protective layers are attached
and provides the cloth with the necessary tensile strength so that
it can function as the conveyor belt in the manner required by the
filtering apparatus. In that case it is possible to use a standard
middle layer which is mechanically strong and the surfaces of which
are provided with different protective layers according to the properties
to be achieved.
Since rather a high pressure is used in pressure filtering apparatuses,
the filter cloths used are relatively dense compared to the filter
cloths used in other filtering apparatuses. Nowadays the air permeance
of the filter cloths is within the range of 0.3 to 15 m.sup.3 /m.sup.2
min, 200 Pa, depending on the filtering quality needed and the slurry
to be filtered. A single-layer woven filter cloth, which is suitable
for the purpose, can be provided with the minimum permeance of 0.2
m.sup.3 /m.sup.2 min, 200 Pa by calendering. The density of the
multilayer cloth according to the invention can be below 0.2, preferably
between 0.02 to 0.15 m.sup.3 /m.sup.2 min, 200 Pa. According to
the prevalent opinion in the field, filter cloths as dense as that
cannot be used in pressure filters. In the tests carried out a filter
cloth with the permeance of 0.03 m.sup.3 /m.sup.2 min, 200 Pa, which
is only about a tenth of the permeance of the currently used cloths,
was found to function well. However, the test runs carried out with
the cloth of the invention, which is denser than the prior art cloths,
showed that, contrary to the preconceptions, it functions particularly
well and the resulting filtrate is clearly cleaner than before and
the solids cake drier. Surprisingly, it was also noted that the
useful life of the cloth had increased even by several times compared
to the previously used filter cloths. The latter results from the
fact that particles of the slurry to be treated cannot penetrate
through the small holes in the filter cloth into the inner structure
of the cloth, but the solids remain on the surface of the cloth
from which they can be removed properly by means of scrapers and
washing devices. Thanks to the high pressure, the denser filter
cloth does not substantially reduce the filtering ability.
It is also possible to influence the dewatering properties of the
filter cloth and its cleanliness and cleanability by adjusting the
hydrophobicity and/or hydrophilicity of the different layers of
the filter cloth in the desired manner. In this way the protective
layers, for example, can be treated to be soil-repellent.
As was mentioned above, the protective layers may consist of batt
fibre layers which are attached to the middle layer e.g. by needling.
The protective layer of the invention can also be provided by attaching
a woven layer, porous permeable coating material or an appropriately
perforated diaphragm to the outer surface of the middle layer. The
materials used in the protective layers include polyethylene terephthalate
(PET), polyethylene (PE), polyamide (PA) and polytetrafluoroethylene
(PTFE). The protective layers can also be attached to the middle
layer by welding, e.g. by ultrasonic welding.
The drawings and the related description are only intended to illustrate
the inventive concept. The details of the invention may vary within
the scope of the claims. Thus the invention is not limited only
to the filters illustrated in FIGS. 1a to 2b, but it can also be
applied to other similar pressure filters where the filter cloth
is arranged to travel through filter chambers arranged one after
the other in the direction of the filter cloth so that the mass
or slurry to be filtered is alternately placed on the different
sides of the filter cloth in successive filter chambers. |