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 slub layers attached to the middle layer
by needling. The protective layers according to the invention are
denser than the middle layer.
Claims
1. A filter cloth (5) 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 (5) is guided through the
filter chambers arranged one after the other in the direction (A)
of movement of the filter cloth so that in the filter chambers,
the opposite sides of the filter cloth (5) are alternately against
the slurry to be filtered which contains liquid and solids, the
filter cloth (5) being symmetrical in respect of the filtering ability,
and the solids separated from the slurry in said filter are conveyed
out of the filter chambers by means of the filter cloth (5), characterized
in that the filter cloth (5) comprises a middle layer (15) the both
surfaces of which are provided with protective layers (17a, 17b)
which form the outer surfaces of the filter cloth (5) and are denser
than the middle layer.
2. A filter cloth according to claim 1, characterized in that the
protective layers (17a, 17b) are made to resist wear by using wear-resistant
threads or fibres and a wear-resistant bond structure.
3. A filter cloth according to claim 1 or 2, characterized in that
the middle layer (15) has a woven structure.
4. A filter cloth according to any one of the preceding claims,
characterized in that the protective layers (17a, 17b) are slub
layers.
5. A filter cloth according to claim 4, characterized in that the
protective layers (17a, 17b) are attached to the middle layer (15)
by needling.
6. A filter cloth according to any one of the preceding claims,
characterized in that the air permeance of the filter cloth (5)
is below 0.2 m.sup.3/m.sup.2 min, 200 Pa.
Description [0001] 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.
[0002] 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.
[0003] 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.
[0004] The object of the present invention is to provide a novel
filter cloth which eliminates problems associated with the prior
art solutions.
[0005] 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
cloth and are denser than the middle layer.
[0006] 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 denser than the middle
layer, thus functioning as the actual filtering layers of the cloth.
The protective layers are formed so that the filter cloth is at
least functionally symmetrical, i.e. its filtering 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.
[0007] 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 filter cloth with a dense
surface 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.
[0008] The invention will be described in greater detail in the
attached drawings, in which
[0009] 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,
[0010] 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 filtering apparatus,
[0011] FIGS. 3a to 3f schematically illustrate the operating principle
of a filtering apparatus illustrated in FIGS. 1a to 2a,
[0012] FIG. 4 is an enlargement of the cross-sectional structure
of a filter cloth according to the invention, and
[0013] FIG. 5 is an enlargement of the cross-sectional structure
of a filter cloth according to the invention.
[0014] 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 filtering 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.
[0015] 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, whereby 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.
[0016] 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 closed 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.
[0017] 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 an impermeable diaphragm 10
and a permeable filter cloth 5. Due to gravitation and feeding pressure
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, whereby 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.
[0018] 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 slub 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.
[0019] 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.
[0020] 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. 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. According to the prevalent opinion in the field, filter cloths
as dense as that cannot be used in pressure filters. 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.
[0021] 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.
[0022] As was mentioned above, the protective layers may consist
of slub 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.
[0023] 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. |