FIELD OF THE INVENTION

The invention relates to filtration, and more particularly to a filter element for a filter apparatus.

One of the disadvantages associated with known filter elements is that they are often heavy especially as they have absorbed liquid during use. This may make the filter elements hard to handle during maintenance, for example, and may endanger occupational safety.

BACKGROUND OF THE INVENTION

Filtration is a widely used process whereby a slurry or solid liquid mixture is forced through a media, with the solids retained on the media, as a cake, and the liquid phase passing through. This process is generally well understood in the industry. Examples of filtration types include depth filtration, pressure and vacuum filtration, and gravity and centrifugal filtration.

The most commonly used filter media for vacuum filters are filter cloths and coated media, e.g. the ceramic filter medium.

The use of a cloth filter medium requires heavy duty vacuum pumps, due to vacuum losses through the cloth during cake deliquoring. The ceramic filter medium, when wetted, does not allow air to pass through due to a capillary action. This decreases the necessary vacuum level, enables the use of smaller vacuum pumps and, consequently, yields significant energy savings.

BRIEF DESCRIPTION OF THE INVENTION

Viewed from an aspect, there can be provided a filter element for a filter apparatus, the filter element comprising at least one filter member comprising a permeable membrane layer and having a first filter surface for receiving a pressure and directed towards an internal cavity arranged inside the filter element, and a second filter surface for receiving solid particles filtered from a feed, wherein the filter member forms a capillary filter, and a frame member arranged to support the at least one filter member in such a manner that the internal cavity is formed.

Thereby it is possible to select materials for each part of the filter element based on part-specific requirements and a filter element that is light and durable when vacuum is inside filter element may be achieved.

Viewed from another aspect, there can be provided a method for assembling a filter element for a filter apparatus, the method comprising the following steps:

providing at least one filter member comprising permeable membrane layer and having a first filter surface for receiving a pressure directed towards an internal cavity arranged inside the filter element and a second filter surface for receiving solid particles filtered from a feed,

providing a frame member comprising an edge part comprising a peripheral edge surface, and

mounting the filter member to the frame member in such a manner that the peripheral edge surface is arranged at an angle () in relation to the first filter surface.

Thereby a method for manufacturing filter element that is light and durable when vacuum is inside filter element may be achieved.

Some other embodiments are characterised by what is stated in the other claims. Inventive embodiments are also disclosed in the specification and drawings of this patent application. The inventive content of the patent application may also be defined in other ways than defined in the following claims. The inventive content may also be formed of several separate inventions, especially if the invention is examined in the light of expressed or implicit sub- tasks or in view of obtained benefits or benefit groups. Some of the definitions contained in the following claims may then be unnecessary in view of the separate inventive ideas. Features of the different embodiments of the invention may, within the scope of the basic inventive idea, be applied to other embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail by means of preferred embodiments with reference to the attached [accompany- ing] drawings, in which

Figure 1 illustrates a frame member of a filter element for a filter apparatus in perspective view;

Figure 2 illustrates a frame member of another filter element for a filter apparatus in perspective view;

Figure 3a is a side view of a disc filter element; Figure 3b is a cutaway view of the disc filter element shown in Figure 3a;

Figure 4a is a schematic side view of another disc filter element; Figure 4b is a cutaway view of the disc filter element shown in Fig- ure 4a;

Figure 5a is a schematic side view of a disc filter element; Figure 5b is a cutaway view of a filter arrangement;

Figure 5c is a cutaway view of a detail of the filter arrangement shown in Figure 5b;

Figure 6a is a schematic perspective top side view of a disc filter element;

Figure 6b is a cutaway view of a detail of the filter arrangement shown in Figure 6a;

Figure 7 is a perspective top view illustrating a belt filter element; Figure 8 is a perspective top view illustrating a drum filter element;

Figure 9 is a perspective top view illustrating a disc filter apparatus; Figure 10 is a side view illustrating the disc filter apparatus shown in

Figure 9;

Figure 1 1 illustrates a drum filter apparatus;

Figure 12 is a perspective view of a belt filter apparatus; and

Figure 13 illustrate a method for assembling a filter element for a filter apparatus.

DETAILED DESCRIPTION OF THE INVENTION

Principles of the embodiments can be applied for drying or dewater- ing fluid materials in any industrial processes, particularly in mineral and mining industries. In embodiments described herein, a material to be filtered is referred to as slurry, but embodiments are not intended to be restricted to this type of fluid material. The slurry may have high solids concentration, e.g. base metal concentrates, iron ore, chromite, ferrochrome, copper, gold, cobalt, nick- el, zinc, lead and pyrite.

Figure 1 illustrates a frame member 4 of a filter element 1 for a filter apparatus 2 and Figure 2 illustrates a frame member 4 of another filter element 1 for a filter apparatus 2. Figure 3a is a side view of a disc filter element 1 and Figure 3b is a cutaway view of the disc filter element shown in Figure 3a. Fig- ure 4a is a schematic side view of another disc filter element, and Figure 4b is a cutaway view of the disc filter element shown in Figure 4a.

The filter element 1 may comprise at least one filter member 3. The filter member 3 may comprise a permeable membrane layer and have a first filter surface 9a for receiving a pressure. The pressure may comprise an un- derpressure (negative pressure) during filtering of a feed, whereby the underpressure provides suction on the first filter surface 9a. On the other hand, the pressure may comprise a positive pressure during cleaning and/or maintenance of the filter element 1 , such as backwashing. The first filter surface 9a may be directed towards an internal cavity 12 arranged inside the filter element 1 . According to an embodiment, the internal cavity 12 may be used to collect liquid filtered by the filter element 1 and guide the liquid to further processing. The underpressure during filtering may, thus, be provided inside the internal cavity 12.

The filter member 3 may further comprise a second filter surface 9b for receiving solid particles filtered from a feed.

The filter member 3 may form a capillary filter. A capillary filter refers to a filter, wherein the structure and/or the material of the filter, such as the filter member 3, enables a certain amount of liquid, such as water, to be kept in the filter by a capillary action. The liquid may be kept in micro-pores provided in the filter member 3, for example. Such a capillary filter enables the liquid to be filtered to easily flow through the filter member 3, but when all free liquid has passed through the filter member 3, the remaining liquid kept in the filter by the capillary action prevents flow of gas, such as air, through the wet filter member 3. The capillary action thus does not participate in the dewatering it- self, for instance by sucking water out of the slurry. In other words, in a capillary filter liquid, usually water, may be kept in the micro-pores of the filter member 3 by capillary forces and no flow of gas takes place after the free water in the residue, such as the cake, has been removed. According to an embodiment, the filter member 3 formed as a capillary filter prevents air from en- tering the internal cavity 12.

According to an embodiment, the bubble point of the filter member 3 is at least 0.2 bar. In this context, the bubble point refers to an effective bubble point. The effective bubble point describes a pressure difference between the first filter surface 9a and the second filter surface 9b, at which 1 liter of air flows through one square meter of the second filter surface 9b during a one minute time. In other words, when a 0.2 bar pressure difference is provided, in such a filter member 3, between the outside of the filter element 1 and the inside of the filter element 1 , such as within the internal cavity 12, a maximum of 1 liter of air should be able to pass through a square meter of the second filter surface of the filter member 3 during a one minute time. If a flow of air through the filter member 3 at 1 liter per minute requires a pressure difference of 0.2 bar or greater, the bubble point of the filter member 3 is thus at least 0.2 bar. Thereby, in embodiments where it is not practical to block the flow of air completely, only a very minor amount of air may be able to flow through the filter member 3 when the cake is being dried. When the cake is being dried, an underpressure is provided within the filter element 1 , such as within the internal cavity 12, which means than the pressure inside the filter element 1 is lower that the pressure outside the filter element 1 .

According to an embodiment, at least 600 liter of water per an hour and per one square meter of the second filter surface 9b may be able to pass through the filter member 3 when a pressure difference of 1 bar is provided between the first filter surface 9a and the second filter surface 9b. Thus, a sufficient amount of water may flow through the filter member 3 to provide efficient filtering of the slurry, especially when the actual filtering takes place. During filtering, an underpressure is provided within the filter element 1 , such as within the internal cavity 12, which means that the pressure inside the filter element 1 is lower than the pressure outside the filter element 1 .

The pressure difference between the inside of the filter element 1 and the outside of the filter element 1 may be greater during the actual filtering than during the drying of the cake. The drying of the cake may take place for instance in a disc filter apparatus 2 when the filter element 1 in question has passed the filtering position, such as the lowest position in the filter 15 and rotated back upwards. In other words, a specific filter element 1 participates in the actual filtering at a different point of time and at a different position in the filter apparatus 2 than in the drying of the cake. Thus, the relevant pressure difference for the actual filtering and the drying of the cake may be different from one another.

The structure of the filter member 3, such as the mean pore size of the filter member 3, affects both the effective bubble point and the flow of water through the filter member 3.

The filter element 1 may further comprise a frame member 4 arranged to support the at least one filter member 3 in such a manner that the internal cavity 12 is formed. The frame member 4 may also be arranged to connect the filter member 3 to the filter apparatus 2.

In embodiments, where the filter member 3 and frame member 4 are provided as separate structural parts, materials of the filter member 3 and the frame member 4 can be selected independently. Thereby, the suitability of the materials for each part of the filter element can be evaluated separately and materials and their properties, such as lightness and degree or permeability, can be selected based on the specific requirements of each part. For instance, a filter element 1 can be provided that is light and at the same time durable to withstand changes in pressure related to vacuum being provided inside the filter element 1 during filtering and positive pressure being provided inside the filter element 1 during cleaning and/or maintenance.

According to an embodiment, the filter member 3 may comprise a material comprising micro-pores and the frame member 4 may comprise a ma- terial that does not comprise micro-pores. The frame member 4 may, thus, comprise a non-porous material. According to such an embodiment, filter elements 1 of more uniform quality may be provided, as this enables more au- tomized work phases to be used instead. For instance manual coating, like painting for example, for closing the micro-pores in the parts of the frame member where the micro-pores are not beneficial or are not desirable can be avoided. According to an embodiment, the frame member 4 may comprise a material that does not let the liquid to be filtered through.

According to an embodiment, the frame member 4 may comprise at least one support part 6 for supporting the filter member 3. This enables providing a more durable structure of the filter element. According to a further embodiment, the frame member 4 may comprise a plurality of support parts 6. In such a filter element 1 , the durability can be further improved and/or the flow of the filtered liquid inside the cavity 12 can be optimized.

According to an embodiment, the frame member 4 may comprise a plurality of support parts 6 spaced from other support parts 6, such that the support parts 6 do not transfer forces to one another. In embodiments, where the frame member 4 and filter member 3 arranged in contact with one another comprise materials with different thermal expansion coefficients, forces, such as torsional forces, may occur. These forces may be accumulated if continu- ous contact surfaces with large cross-section are formed, such as when a single support part 6 with a large cross-section in contact with the filter member 3 is provided. These forces may, for instance, compromise the durability of the filter element 1 and if the support parts 6 are arranged to one another in such a manner that these forces may be transferred between them, failure in one of the support parts 6 may accumulate while transferring to other support parts 6. A plurality of support parts 6 spaced from one another can be provided instead to avoid the transfer of forces between the support parts 6. Thus, problems related to thermal expansion can be avoided and the material(s) of the filter member 3 and the frame member 3, such as the material(s) of the support parts 6, can be selected more freely. In some embodiments, in other hand, the material(s) of the filter member 3 and frame member 4 may be selected to avoid or minimize problems related to thermal expansion instead of or in addition to structural means.

According to another embodiment, the frame member 4 may comprise one support part 6 to support the filter member 3. According to one em- bodiment, such a support part may 6 extend on the outer part of the filter member 3 and support the filter member 3 on the edges of the filter member 3. According to another embodiment, such a support part 6 may be positioned on the middle area of the filter member 3 supporting the filter member 3 substantially at the middle of the filter member 3.

According to an embodiment, at least one support part 6 may comprise a material that is less water-absorbent than the material of the filter member 3. This prevents the liquid from being absorbed in the frame member 4 during use. According to an embodiment, all support parts 6 positioned on the middle area of the filter member 3 supporting the filter member 3 substan- tially at the middle of the filter member 3 may comprise a material that is less water-absorbent than the material of the filter member 3. According to a further embodiment, all support parts 6 of the filter member 3 may comprise a material that is less water-absorbent than the material of the filter member 3.

According to an embodiment, the frame member 4 may comprise at least one support part 6 and the sum of the cross-sectional areas of a filter member end 44 of the support parts 6 may be in the range of 6 per cent to 60 percent, preferably in the range of 10 per cent to 40 per cent, and more preferably in the range of 15 per cent to 25 per cent, of the sum of the areas of the first filter surfaces 9a of the filter members 3 arranged on the same side of the internal cavity 12 and at the filter member end 44 of the support parts 6.

The filter member end 44 of the support part(s) refers to the end of each support part 6 directed towards the filter member 3 in an assembled filter element 1 . The cross-sectional area of a filter member end 44 of each support part 6 refers to the cross-sectional area of the support part 6 that is in contact with the filter member 3 and supporting the filter member 3. In embodiments, where there is only one support part 6, the cross-sectional area of the filter member end 44 of the support part equals the sum of the cross-sectional areas of a filter member end 44 of the support parts 6. In embodiments, where there are two or more support parts 6, the sum of the cross-sectional areas of a filter member end 44 of the support parts 6 refers to the combined cross-sectional area of these cross-sectional areas of the filter member end 44 of the support parts 6.

The sum of the areas of the first filter surfaces 9a of the filter members 3 arranged on the same side of the internal cavity 12 and at the filter member end 44 of the support parts 6 refers to one or more filter members 3 forming a filter surface on one side of the internal cavity 12 and the combined area of the first filter surfaces 9a these filter members 3. Thus, the areas of the support part(s) 6 and filter member(s) 3 facing one another and at least partly in contact with one another in an assembled filter element 1 are defined and compared. Depending on the embodiment and the type of the filter apparatus 2 the filter element 1 is designed for, the filter element 1 may comprise filter member(s) 3 on one or two opposite sides of the internal cavity 12, for example. If the filter element 1 comprises filter member(s) 3 on more than one side of the internal cavity 12, comparing may also be done for more than one side, for instance separately for each side.

By comparing the sum of the cross-sectional areas of a filter member end 44 of the support part(s) 6 to the sum of the areas of the first filter surfaces 9a of the filter members 3 arranged on the same side of the internal cavity 12 and at the filter member end 44 of the support parts 6, the percentage of combined area of the first filter surfaces 9a of the filter member(s) 3 being in contact with the support part(s) can be defined. When this percentage is in the range of 6 per cent to 60 percent, preferably in the range of 10 per cent to 40 per cent, and more preferably in the range of 15 per cent to 25 per cent, as described above, an optimal combination of filter element properties can be achieved. For instance, a filter element 1 can be provided that endures a 0.3 bar pressure, preferably a 0.5 bar pressure, inside the internal cavity 12 during cleaning/maintenance and/or where both the flow of the liquid and sufficient support is optimized. According to other embodiments, a filter element 1 can be provided that endures a 2 bar or 5 bar pressure inside the internal cavity 12 during cleaning/maintenance. In other words the filter element 1 may endure a pressure inside the internal cavity 12 of the filter element 1 that is 0.3 bar, 0.5 bar, 2 bar or 5 bar higher than the pressure on the outside the filter element 1 . The pressure outside the filter element 1 may typically be equal to the atmospheric pressure, in other words to a pressure of about 1 atm or 1 bar, but in some embodiments external pressure, which may be positive pressure or in some embodiments even underpressure, may be provided also outside the filter element 1 . The filter element 1 should also endure rapid changes in the pressure, as the pressure affecting in the internal cavity 12 may change quickly for instance from underpressure, such as a 0.9 bar underpressure, to a positive pressure, such as a pressure of 1 .5 to 3.0 bar, for example.

According to an embodiment, the structure of the frame member 4 may be formed to prevent transfer of forces, such as torsional forces, between the support parts 6. Thus, problems caused by thermal expansion can be avoided or decreased by structural means instead of or in addition to by the properties of the materials the filter element 1 comprises. This provides an improved and long-lasting solution for avoiding the effect of forces, for example torsional forces, that might compromise the durability of the filter element 1 . In addition to thermal expansion, these forces may comprise mechanical forces caused by loads, changes in the negative and/or positive pressures inside and outside the filter element 1 or some other features related to the use of the filter element 1 , for example.

According to an embodiment, each support part 6 may be connected to at least one other support part 6 by a connector 8 comprising a nonlinear shape, such as a curved shape. Such a structure comprising support parts connected to one other is easy to handle during assembly, for example, while the non-linear shape of the connectors 8 effectively decreases the trans- fer of forces between the support parts 6.

According to an embodiment, at least one of the support parts 6 may be connected to at least one of the other support parts 6 by a connector 8 that does not transfer forces or at least decreases the transfer of forces between the support parts 6. Such a connector 8 may comprise a connector 8 that is formed as flexible in at least one direction. The flexibility may be provided by selection of the material of the connector 8 and/or by making the con- nector 8 so thin that it cannot transfer considerable forces between the support parts 6.

According to an embodiment, the support parts 6 are not connected to one another, but are only in contact with the filter members 3. Such support parts 6 can be formed to be easy to manufacture, for instance by a robot, and modular, such that similar support parts 6 can be used in different kind of filter element configurations. This can save in number and cost of molds, for example. Still, a filter element 1 that is light and durable to stand both positive and underpressures inside the internal cavity 12 during use and maintenance and/or that works well in different temperatures during manufacturing and use can be provided.

According to an embodiment, the number of support parts 6 on a square meter of first filter surface 9a may be in the range of 50 to 4000 support parts 6. A most suitable number of support parts 6 depend on the embodiment, such as the type of the filter apparatus and the purpose it is used for, and the cross-sectional area of each individual support part 6. For instance, according an embodiment where the support parts 6 comprise a round cross-section, a number of support parts 6 on a square meter of first filter surface 9a may be in the range of 1000 to 4000 support parts 6, preferably in the range of 1500 to 2500 support parts 6. According to an embodiment where the support parts 6 comprise an elongated cross-section, a number of support parts 6 on a square meter of first filter surface 9a may be in the range of 50 to 400 support parts 6, preferably in the range of 100 to 200 support parts 6.

According to an embodiment, the cross-sectional area of each sup- port part 6 is in the range of 0.5 to 3000 square centimeters.

By selection of a suitable number and/or cross-sectional area of the support parts 6 optimal flow of the filtered liquid and sufficient support for the filter member(s) 4 to withstand the pressures during use and maintenance can be enabled.

According to an embodiment, at least one support part 6 may comprise an aperture extending through the support part 6 between the filter member end 44 of the support part 6 and the end of the support part 6 opposite to the filter member end 44 in a direction substantially parallel to the direction of the first filter surface 9a. In other words, the aperture may extend in a trans- verse direction between the ends of the support part. In embodiments, where the filter element 1 may be used in a disc filter apparatus, the support part may have two filter member ends, one towards each one of the filter members arranged at opposite ends of the support part 6. In such embodiments, the end of the support part 6 opposite to the filter member end 44 may, naturally, also comprise a filter member end. Such aperture may enable better flow of the fil- tered liquid within the internal cavity 12.

According to an embodiment, the frame member 4 may comprise an edge part 5 comprising a peripheral edge surface 18 that is arranged at an angle a in relation to the first filter surface 9a. In Figure 3b the angle a is 90 degrees or close to it, but in other embodiments the angle a might differ from a right angle. The edge part 5 refers to a part of an edge of the frame member 4 extending on one or more edges of the frame member 4. In some embodiments, this edge part 5 and the at least one filter member 3 may at least partly define the internal cavity 12. The edge part 5 may comprise a part extending on the connecting member end 10 of the filter element 1 and/or the distal end of the filter element 1 opposite to the connecting member end and/or one of the sides S1 , S2 (shown in Figure 3b) of the filter element 1 extending between the ends of the filter element 1 . This enables supporting the filter member(s) 3 also on the edges of the filter element 1 for a better durability and/or forming the internal cavity 12 within the filter element 1 . Additionally, such an edge part 5 may also be formed of a different type of a material, such as a non-porous material, to avoid leaking of liquids from the internal cavity 12 towards the edges of the filter element 1 .

According to an embodiment, the frame member 4 may comprise a different material or combination of materials than the filter member(s) 3. Ac- cording to an embodiment, the frame member 4 may have a thermal expansion coefficient different from that of the filter member 3.

According to an embodiment, the filter member 3 may comprise a ceramic material or a composition comprising a ceramic material. Using a ceramic material or a composition comprising a ceramic material in the filter member(s) 3, very good filtering properties may be achieved. These materials are also wear resistant and hydrophilic. According to an embodiment, the ceramic material may comprise alumina (AI2O3), aluminium silicates, silicon carbide and/or titania (TiO2).

According to an embodiment, the filter member 3 may comprise at least one of the following: a polymer material, a composition comprising a polymer material and a metal. According to an embodiment, the frame member 4 may comprise a polymer material or a composition comprising a polymer material. This enables making the frame member 4 and, thus, the filter element 1 light and durable, avoiding the frame absorbing water that would increase the weight of the frame member 4 and the filter element 1 in use and/or providing more flexibility in the frame member 4 and, therefore, the filter element 1 . The polymer material may comprise a thermoplastic, for example. The thermoplastic may comprise at least one of the following: polyamide (PA), polysulphone (PSU), polyethersul- phone (PES), polyphenylene oxide (PPO), polyphenylene sulphide (PPS), ac- rylo butadiene styrene (ABS), polybutylene terephthalatate (PBT), polycarbonate (PC), and polyolefins, such as polypropylene (PP), polyethylene (PP), e.g. high density polyethylene (HDPE). The thermoplastics may be particularly suitable for molding such frame member 4 structures. According to another embodiment, the polymer material may comprise a thermoset plastic, for in- stance an epoxy, a polyurethane or a polyester.

According to an embodiment, at least the surface of the support parts 6 comprises the polymer material. A polymer material may be used for instance to provide smooth surfaces minimizing the effect of the support parts 6 on the flow of the filtered liquid.

According to a further embodiment, the frame member may comprise metal.

According to an embodiment, the edge part 5 of the frame member 4 may comprises a material that differs from the material of the support parts 6. In other words, parts of the frame member 4 may comprise different materials or combinations of materials. This enables selection of a material most suitable for each structural part of the frame member 4 from part-specific requirement point of view.

According to another embodiment, the edge part 5 of the frame member 4 may comprise the same material as the material of the support parts 6. This ensures that the parts of the frame member 4 have the same thermal expansion coefficient, which helps avoiding forces being formed between the edge part 5 and the support parts 6.

According to an embodiment, the frame member 4 comprises a material, the water absorbance of which is less than 15 per cent, preferably less than 5 per cent, of the dry weight of the material. In other words, the frame member 4 may comprise a material capable of absorbing a maximum of 15 g of water per 100 g of dry weight of the material. Preferably, the frame member 4 may comprise a material capable of absorbing a maximum of 5 g of water per 100 g of dry weight of the material.

According to an embodiment, at least one filter member 3 may be arranged fixedly to the frame member 4. According to an embodiment, the at least one filter member 3 may be glued or melted fixedly to the frame member 4. These methods of fixedly arranging filter member 3 to the frame member 4 can provide a durable bond, be beneficial from manufacturing point of view and/or provide a filter element 1 that is light and durable when vacuum (= un- derpressure) or pressure (= positive pressure) is provided inside the internal cavity 12. According to an embodiment, the at least one filter member 3 may be arranged fixedly to at least one support part 6 of the frame member 4. This may additionally help avoiding problems related to thermal expansion.

According to an embodiment, at least a part of at least one surface of the filter member may be formed as a rough joining interface 7 comprising a grit number lower than or equal to 180, preferably comprising a grit number in the range of 40-180, more preferably comprising a grit number in the range of 60 to 120. This enables a more durable bond between a ceramic material and the frame member material to be provided. According to an embodiment, at least a part of the surface of the filter member formed as a rough joining interface may be in contact with the frame member. This is beneficial, as a tough joining surface makes the ceramic material easier to join durably to the frame member material.

According to an embodiment, a part of a surface of the filter member 3 outside the rough joining interface 7 may have a smooth surface that is smoother than the rough joining interface 7, i.e. its grit number is larger than 180. According to an embodiment, said smooth surface has a grit number of 600 or more. The smooth surface may cover at least a part of the first filter surface 9a and/or at least a part of the second filter surface 9b. Preferably, the second filter surface 9b as a whole is formed as a smooth surface. In other words, according to an embodiment at least a part of at least one surface 9a, 9b of the filter member 3 may comprise a grit number greater than or equal to 600. This kind of smooth surface may reduce contaminations of the surface as well as provide more effective removal of a residual cake in a backflush wash- ing of the filter element 1 .

According to an embodiment, the rough joining interface 7 may ex- tend over the first filter surface 9a as a whole.

According to an embodiment, the filter element may comprise at least one filter member 3 arranged on each side of the frame member 4 providing two first filter surfaces 9a, one on each side of the frame member 4. Such a filter element 1 may be suitable for a disc filter apparatus and the filtering surface may be doubled. A known type of a filter element may, thus, be provided that is easier to manufacture than the known configurations and enables au- tomized manufacturing process and equipment with less or no need for manual work phases enabling more uniform quality.

According to an embodiment, each end of a support part 6 may be arranged fixedly to one of said filter members 3, such that the support part 6 is arranged fixedly between two filter members 3. This is particularly beneficial in connection for filter members 3 and filter elements 1 suitable for use in connection with a disc filter apparatus. Then, a more durable bond and filter element structure may be provided. Also the risk of the filter element 1 structure failing during back washing of the filter element, in other words cleaning with positive pressure provided on the first filter surface 9a, can be decreased.

According to an embodiment, the strength of the connection between a support part 6 and a filter member 3 is sufficient for the filter element 1 to endure pressure of at least 0.3 bar, preferably 0.5 bar, more preferably 2 bar and most preferably 5 bar, inside the internal cavity 12. In other words the filter element 1 may endure a pressure inside the internal cavity 12 of the filter element 1 that is 0.3 bar, 0.5 bar, 2 bar or 5 bar higher than the pressure on the outside the filter element 1 . The pressure outside the filter element 1 may typi- cally be equal to the atmospheric pressure, in other words to a pressure of about 1 atm or 1 bar, but in some embodiments external pressure, which may be positive pressure or in some embodiments even underpressure, may be provided also outside the filter element 1 . Such a filter element 1 can withstand the pressures during use and maintenance, for example backwashing.

According to an embodiment, the filter element 1 may be a truncated-sector-shaped filter element 1 and comprising second filter surfaces 9b on both sides of the element 1 . Such a filter element is suitable for a disc filter apparatus. Thus, a complicated structure may be manufactured in an easier manner and/or a more uniform quality may be achieved by automized manu- facturing.

According to an embodiment, the filter element 1 may be a filter el- ement 1 of a drum filter apparatus. Such a filter element 1 may comprise a pair of side edge surfaces 18a arranged parallel with each other, and comprising second filter surface(s) 9b only on one side of the filter element 1 . Such a filter element 1 may further comprise a pair of curved end surfaces 18d, wherein the curvature of the curved end surfaces 18d and the curvature of the second filter surface(s) 9b of said filter element 1 coincide with the circumference of the outer surface of a drum filter of the drum filter apparatus 2. Such a filter element 1 may help manufacturing a known, complicated filter element structure more easily, as the filter member can be manufactured separately and ar- ranged to the "box".

According to an embodiment, the filter element (1 ) is a filter element (1 ) of a belt filter apparatus.

According to an embodiment, the total area of second filtering surface^) 9b of one filter element 1 may be greater than or equal to 0.1 square meters. The total area of second filtering surface(s) 9b of one filter element 1 suitable for use in a disc filter apparatus may preferably be in the range of 0.1 square meters to 2 square meters, and more preferably in the range of 0.2 square meters to 1 square meters. The total area of second filtering surface(s) 9b of one filter element 1 suitable for use in a belt filter apparatus may prefera- bly be in the range of 0.5 square meters to 5 square meters, and more preferably in the range of 1 square meter to 3 square meters. Thus, such a filter element 1 can be sufficient for production environment.

According to an embodiment, at the least one frame member 4 may comprise between a connecting member end 10 of the frame member 4 and the filter member 3 a material that is more flexible than the material of the filter member 3, wherein the connecting member end 10 of the frame member is the end at which the filter element 1 is arrangeable to the filter apparatus (2). In other words, the frame member may comprise such a flexible material in the vicinity of the connecting member end, that is at least a part of the frame member 4 extending between the connecting member end 10 and the filter member(s) 3 may comprise such flexible material. This helps enabling the filter element 1 to bend when arranged in contact with a scraper (not shown) of the filter apparatus to avoid damage caused by the scraper to the filter element. This is particularly beneficial in embodiments, where the filter member com- prises a ceramic material, as ceramic material is typically rigid and the flexibility is particularly important in connection with filter member(s) 3 comprising such a rigid material. In this context, flexibility refers to a material's ability to receive external forces applied to it by bending but not breaking. Thus, the more flexible material can receive larger external forces applied to it without breaking than the less flexible material.

According to an embodiment, the more flexible material may revers- ibly flexible. In this context, reversibly flexible material refers to a material that is capable of receiving external forces of the magnitude typical for filter apparatuses in such a manner, that the object formed of the reversibly flexible material may bend but does not deform permanently. Thus, when flexible material is reversible flexible, machine could be used even after hit as the hit would not deform the filter element 1 permanently. According to another embodiment, the more flexible material may be non-reversible flexible. This might be sufficient in some embodiments, as even if the more flexible material would be only non- reversibly flexible, it could still make it possible to avoid a domino effect in filter apparatus breaking also adjacent filter elements even if the certain plate would be damaged. Flexible material may comprise a polymeric material or a composition comprising a polymeric material.

According to an embodiment, the frame member 4 may comprise between the connecting member end 10 of the frame member 4 and the filter element(s) a plastic portion comprising a plastic material. According to an embodiment, the plastic portion may comprise identification information for filter element 1 identification purposes. Such a plastic portion may permanently provide good elastic properties and enable flexibility that decreases the probability of deformations related to hits and other external forces affecting the filter ele- ment 1 .

According to an embodiment, the at least one frame member 4 may form at least the outer edge surface 18c of the filter element 1 . The outer edge surface 18c of the filter element 1 may be located at the opposite end of the filter element 1 compared to the connecting member end 10 of the filter ele- ment 1 at which the filter element 1 is arrangeable to the filter apparatus 2. At least the outer edge surface 18c of the frame member 4 may comprise a non- porous and acid-resistant material. The frame member 4 may, thus, extend from the outer edge surface 18c towards the connecting member end 10.

According to an embodiment, the connecting member end may comprise at least two holes 41 , 42 for arranging the filter element 1 to the filter apparatus 2. According to an embodiment, the openings extend through the material. In other words, the openings are not blind holes.

According to an embodiment, the connecting member end comprises the narrower end of the truncated-sector-shaped filter element 1 .

According to an embodiment, the non-porous and acid-resistant ma- terial of the frame member 4 is additionally tough, durable, resilient and/or shock absorbing.

According to an embodiment, the non-porous and acid-resistant material of the frame member 4 is additionally less water-absorbent than the material of the filter member 3. This helps avoiding acid leaking during mainte- nance and providing lighter elements and, thus, supports more safe maintenance of the filter apparatus.

According to an embodiment, the frame member 4 comprises a material, the water absorbance of which is less than 15 per cent, preferably less than 5 per cent, of the dry weight of the material. In other words, the frame member 4 may comprise a material capable of absorbing a maximum of 15 g of water per 100 g of dry weight of the material. Preferably, the frame member 4 may comprise a material capable of absorbing a maximum of 5 g of water per 100 g of dry weight of the material.

According to an embodiment, the at least one frame member 4 addi- tionally covers the connecting member end 10 of the filter element 1 .

According to an embodiment, the at least one frame member 4 may additionally at least partly cover both side edges 18a of the filter element 1 , wherein the side edges are located at the each side of the filter element 1 ar- rangeable to face towards the adjacent filter element 1 .

According to an embodiment, a filter apparatus 2 may comprise at least one filter element 1 described in this description, wherein the filter apparatus (2) comprises at least one of the following: a disc filter apparatus, a drum filter apparatus and a belt filter apparatus. In such a filter apparatus, support part(s) 6 support the filter member 3 especially during underpressure in the internal cavity 12. The frame member 4 and the filter member(s) being fixedly arranged to one another in some embodiments may further improve keeping the parts together during backwashing, for example.

Figure 13 illustrates a method for assembling a filter element 1 for a filter apparatus 2. The method may comprise providing 100 at least one filter member 3 comprising permeable membrane layer and having a first filter surface 9a for receiving a pressure directed towards an internal cavity 12 ar- ranged inside the filter element 1 and a second filter surface 9b for receiving solid particles filtered from a feed. The method may further comprise providing 1 10 a frame member 4 comprising an edge part 5 comprising a peripheral edge surface 18, and mounting 120 the filter member 3 to the frame member 4 in such a manner that the peripheral edge surface 18 is arranged at an angle () in relation to the first filter surface 9a.

According to an embodiment, the frame member 4 may comprise support parts 6 that are arranged to support the filter member 3 when the filter member 3 is mounted to the frame member 4. According to an embodiment, the support parts 6 are spaced from one another, such that the support parts 6 do not transfer forces to one another. Thereby transferring forces, such as torsional forces caused for instance by thermal expansion, between the support parts 6 can be avoided. This decreases problems related to thermal expansion, for example.

According to an embodiment, the method further comprises forming at least a part of at least one surface of the filter member 3 as a rough joining interface 7. In such a method, the filter member 3 may be mounted to the frame member 4 in such a manner that at least a part of the surface of the filter member 3 formed as a rough joining interface 7 is in contact with the frame member 4. The rough joining surface can make the ceramic material easier to join durably to the frame member material. According to an embodiment, the rough joining interface 7 may comprise a grit number in the range of 40-180, preferably in the range of 60 to 120. This can enable more durable bond between a ceramic material and the frame member material.

According to an embodiment, at least part of the peripheral edge surface 18 comprises at least one groove 35 or at least one ridge 36 arranged along the at least part of the peripheral edge surface 18. The filter element shown in Figure 1 comprises two ridges 36 and one groove 35 there between. This kind of structure is very simple and rugged. An alternative structure being simple and rugged comprises one ridge 36 and a groove 35 either on one or both side(s) of the ridge 36.

The groove 35 and the ridges 36 are arranged on the side edge surfaces 18a and the outer edge surface 18c. On the side edge surfaces 18a the length of the groove 35 and the ridges 36 is essentially equal with the length of the filter member 3.

It is to be noted here, that the at least one groove 35 or at least one ridge 36 may have length that is at least essentially equal with the length of the corresponding edge surface 18a, 18b, 18c, or alternatively, the at least one groove 35 or at least one ridge 36 may be essentially shorter than the corresponding edge surface 18a, 18b, 18c. It is possible, of course, to arrange mul- tiple of grooves 35 or ridges 36 on the peripheral edge surface.

In another embodiment, there is at least one groove 35 or at least one ridge 36 on side edge surface 18a, but not on the outer edge surface 18c.

In further embodiment, the filter element 1 comprises at least one groove 35 or at least one ridge 36 on only one of its two side edge surfaces 18a. It means that one of its two side edge surfaces 18a is without the groove 35 and the ridge 36.

In still another embodiment, the inner edge surface 18b also comprises at least one groove 35 or at least one ridge 36.

An advantage of the at least one groove 35 or at least one ridge 36 is that a seal member 37 may be attached to the peripheral edge surface 18, the seal member 37 filling a gap between two adjacent filter elements 1 . Thus the gap may be avoided and the user safety of the apparatus increased. The gap in a rotating filter would be a potential safety risk because there is a possibility that an operator of the filter apparatus push something, e.g. finger, a tool etc., in the gap, resulting an accident the consequence of which is unforeseeable.

The seal member 37 may be manufactured e.g. from an elastic material or elastic material composition, such as natural or synthetic rubber and compositions thereof. The profile of the seal member 37 fits to the at least one groove 35 or at least one ridge 36 such that, preferably, no adhesives or any other attaching means are needed. However, it is also possible to use attaching means in order to attach the seal member 37 to the at least one groove 35 or at least one ridge 36.

According to an embodiment, the at least one groove 35 or at least one ridge 36 is applied to a filter element of a drum filter apparatus. The drum filter element comprises a pair of side edge surfaces arranged parallel with each other, and a second filter surface(s) only on one side of the filter element. Furthermore, the drum filter element comprises a pair of curved end surfaces, the curvature of which and the curvature of the second filter surface(s) of said filter element coincide with the circumference of the outer surface of a drum filter of the drum filter apparatus. The at least one groove 35 or at least one ridge 36 may be arranged on at least one of the side edge surfaces and/or the curved end surfaces.

According to another embodiment, the at least one groove 35 or at least one ridge 36 is applied to a filter element of a belt filter apparatus. The belt filter element comprises a pair of side edge surfaces arranged parallel with each other, and a pair of end surfaces. The at least one groove 35 or at least one ridge 36 may be arranged on at least one of the side edge surfaces and/or end surfaces.

The connecting member end 10 of a disc filter apparatus comprises a connecting member 22 in order to attach the filter element 1 to the filter apparatus 2.

In the embodiment shown in Figure 1 , the connecting member 22 comprises two fastening openings 38 for attaching the filter element 1 to mounting means in the frame of the filter apparatus. The fastening openings 38 are formed by the material of the connecting member end 10.

According to an embodiment, there is a fastening arrangement for fastening the filter element 1 to the filter apparatus. Said arrangement comprises mounting means 39 (shown in Figure 5b) in the frame of the disc filter apparatus 2, and fastening elements 43a, 43b (shown in Fig 2a) that are ar- rangeable in the fastening openings 38 in direct coupling with the material of the connecting member end 10.

Thus there are no glued or sealed components in the structure of the fastening openings 38 that could cause a risk for penetration of washing acid out from the filter element 1 . This way the user safety of the apparatus is increased. Additionally, the dimensional accuracy of the fastening openings, especially in their longitudinal direction, may be improved, since gluing step tends to cause variations in dimensions in said direction.

According to an embodiment, the length of the fastening opening 38 is greater than the distance between the second filter (i.e. outer) surfaces 9b of the filter element 1 . An advantage is that a firm and stabile support for the filter element may be created.

According to an embodiment, the fastening opening 38 is arranged in a hub 40 that is thicker than other parts, especially the connecting member end 10, of the filter element 1 . An advantage is that the firm and stabile support for the filter element can be created without using an excessive amount of material in the connecting member end 10. According to an embodiment, all the fastening openings 38 are equal in length. Thus the fastening means, e.g. bolts, can be standardized in one length.

According to an embodiment, the first end of each of the fastening openings 38 arranged on the first side S1 of the filter element 1 are arranged outer and at a first distance D1 from plane of the second filter surface 9b arranged on said first side S1 of the filter element 1 , and said first distance D1 is equal in each of the fastening openings 38. The first distance D1 is selected so that the filter element 1 automatically sets in its correct position in the filter ap- paratus. An advantage is that the installation work of the filter element may be quickened. According to an embodiment, the first distance is in range of 1 mm - 20 mm, preferably 4 mm - 12 mm, more preferably 6 mm - 10 mm.

According to an embodiment, the connecting member end 22 may comprise at least two fastening openings 38 for attaching the filter element 1 to mounting means 39 in a frame of the disc filter apparatus. According to an embodiment, the openings 38 extend through connecting member end 22. In other words, the openings are not blind holes.

According to an embodiment, the fastening openings 38 comprise a fitted hole 41 and an elongated hole 42.

The fitted hole 41 is dimensioned for an accurate tolerance with a first fastening element 43a attaching the fitted hole 41 to said mounting means 39.

The elongated hole 42 is dimensioned for an accurate tolerance in a first direction T1 but for a loose tolerance in a second direction T2 with a sec- ond fastening element 43b attaching the elongated hole 42 to said mounting means 39. The first direction T1 is orthogonal to the second direction T2 and, furthermore, the second direction T2 is directed towards the fitted hole 41 .

The fitted hole 41 takes care of an accurate positioning of the filter element 1 to the apparatus.

Also the elongated hole 42 takes care of an accurate positioning in the first direction T1 but allows dimensional deviations in the second direction T2. Said dimensional deviations may be caused by e.g. inaccuracy at placement of the fastening elements 43a, 43b, manufacturing tolerances of the filter element 1 , different heat expansions in the element 1 and the mounting means 39, etc.

Further advantages may include: decreasing the assembly time due to tolerance allowance, and decreasing the risk of damaging the filter element 1 during its assembly.

According to an embodiment, the fitted hole 41 may have a round shape. Such shape is simple to manufacture. According to another embodi- ment, the fitted hole 41 has a complex shape, e.g. the inner surface of the hole may comprise apexes being situated closer to the middle axis of the hole and depressions being situated farther from the middle axis of the hole, or the hole fitted hole may have a shape of polygon. The complex shape may e.g. reduce friction between the fitted hole 41 and the first fastening element 43a during the assembly work.

According to an embodiment, the elongated hole 42 may have a shape of rectangular with rounded ends. The rounded ends reduce tensions in the material of the connecting member end 10. However and alternatively, the elongated hole 42 may have rectangular ends.

According to an embodiment, in the first direction T1 the elongated hole 42 has equal diameter with the smallest diameter of the fitted hole 41 . Thus the first and the second fastening elements 43a, 43b can be identical in their dimensions and thus they can be used in any of the holes 41 , 42.

According to an embodiment, the dimension of the elongated hole 42 in the second direction T2 is larger by 1 .01 to 1 .20 times the distance between the fitted hole 41 and the elongated hole 42, when compared to the dimension of said elongated hole 42 in the first direction T1 . According to an embodiment, the distance between the holes 41 and 42 is 239 mm, and the dimension in the second direction T2 may be larger by 2.39 mm to 47.8 mm compared to the dimension in the first direction T1 .

According to an embodiment, the dimension of the fitted hole 41 is 12 mm to 30 mm depending e.g. the size and mass of the filter element 1 . According to an embodiment, the dimension of the fitted hole 41 is 20 mm whereas the elongated hole 42 is 20x25mm.

Figure 5a is a schematic side view of a disc filter element, Figure 5b is a cutaway view of a filter arrangement, and Figure 5c is a cutaway view of a detail of the filter arrangement shown in Figure 5b.

The purpose of the arrangement is to attach the filter element 1 described earlier in this description to a mounting means 39 in a frame of a disc filter apparatus.

The arrangement may comprise fastening elements 43a, 43b and fastening nuts 47. The material of the fastening elements 43a, 43b and fastening nuts 47 may be e.g. any suitable metal or alloy.

The fastening element 43a, 43b comprises a thread part 46, and the fastening nut 47 comprises a nut thread 48 matching to the thread part 46 of the fastening element 43a, 43b.

According to an embodiment, the fastening nut 47 further comprises a nut sleeve 49 that sets between the fastening opening 38 and the fastening element 43a, 43b. The length of said nut sleeve 49 is accurately selected such that it abuts to the fastening interface 50 of the mounting means 39 when the fastening nut 47 is in its predetermined tightening torque. An advantage is that together with accurate length fastening opening 38, the correct tightening torque can be found without using special tools, e.g. a torque wrench.

According to an embodiment, the nut sleeve 49 is a component of its own, separate from the fastening nut 47.

According to an embodiment of the disc filter element 1 , it comprises a connector 8 for placing the internal cavity 12 in a fluid communication with a drainage line 13 of the disc filter apparatus 10. The connector 8 may comprise an aperture 1 1 (shown in Figure 1 ) arranged to extend inwards from an orifice 14 arranged on an outer surface of the filter element and which aperture ends inside the filter element 1 . The aperture 1 1 is in fluid contact with said internal cavity 12 of the filter element.

The aperture 1 1 is able to receive detachably a sleeve 16 (shown in Figure 2) connected to said drainage line 13 of the disc filter apparatus. Furthermore, there may be a sealing arrangement arranged between the aperture 1 1 and the sleeve 16 and at a distance from said orifice 14.

According to an embodiment, the sealing arrangement is attached to the aperture 1 1 , i.e. to the filter element 1 . An advantage is that the sealing arrangement is automatically renewed together with renewing the filter element 1 .

According to another embodiment, the sealing arrangement is attached to the sleeve 16. An advantage is that the condition of the sealing arrangement is easy to check by taking the sleeve 16 out from the aperture 1 1 .

According to an embodiment, the sealing arrangement comprises at least one O-ring that is a cheap and reliable type of seal.

There are several advantages in the connector 8 described above.

Firstly, the contact surfaces to be sealed are limited because there is only one orifice 14 in the filter element 1 . Secondly, in case the sealing arrangement 17 fails, leakage or flush of acid out from the filter element would direct mainly to direction of the aperture 1 1 , i.e. in direction towards the inner parts of the filter apparatus, not outwards, i.e. in direction where operators of the filter apparatus are usually working.

According to an embodiment, the orifice 14 is arranged on one of the peripheral edge surfaces 18a, 18b, 18c. In the embodiment shown in Figures 1 and 2, the orifice 14 is arranged on the inner edge surface 18b and between fastening openings 38 arranged for attaching the filter element 1 to the disc filter apparatus. An advantage is that if the sealing arrangement 17 fails, the mounting means 39 (and the hubs 40, if any) arranged in the fastening openings 38 will limit the zone affected by the leakage/flush. Thus the user safety of the filter apparatus can be increased.

As described earlier, the filter element 1 comprises a pair of side edge surfaces 18a which radiate respectively from the inner edge surface18b to the outer edge surface 18c. According to an embodiment, the aperture 1 1 is directed towards the crossing point of imaginary extensions of said side edge surfaces 18a. When this kind of filter element 1 is in its place in the disc filter apparatus, the aperture is directed towards the central shaft of the apparatus. An advantage is that if the sealing arrangement 23 fails, the resulting leakage or flush of washing acid is directed towards inner parts axle of the apparatus.

The aperture 1 1 shown in the Figures has a round profile. However, the aperture may have some alternative profile, e.g. oval, polygon, such as rectangular, etc. Said alternative profiles may have an advantage that the cross-sectional area of the aperture 1 1 can be enlarged without growing the thickness of the frame member 4.

According to an embodiment, the cross-sectional area of the aperture 1 1 is 1 cm ² - 10 cm ² depending e.g. of the size of the filter element 1 . Figure 6a is a schematic perspective top side view of a disc filter element, and Figure 6b is a cutaway view of a detail of the filter arrangement shown in Figure 6a.

According to an embodiment, the aperture 1 1 is connected to the internal cavity 12 by a distributing channel 51 that joins to the internal cavity 12 at least essentially as broad as said internal cavity 12 and that gradually con- verges to the aperture 1 1 . The distributing channel 51 is essentially free from any flow obstacles. An advantage of the distributing channel 51 is very low flow resistance.

According to an embodiment, the distributing channel 51 is created as an integral part of the frame member 4. In the embodiment shown in Figure 6a, the distributing channel 51 is arranged in the connecting member end 10 of the filter element.

According to an embodiment, the aperture 1 1 comprises a first locking shape 27 whereas the sleeve 16 comprises a second locking shape 28. The locking shapes 27, 28 are arranged to lock the sleeve 16 detachable within the aperture 1 1 . Thus there is definite means for attaching the aperture 1 1 into drainage line 13.

According to an embodiment, the first locking shape 27 and the second locking shape 28 are arranged to receive a locking member locking the locking shapes 27, 28 to each other. The locking member may be e.g. a spline 29 as shown in Figure 2. An advantage is that the locking is simple to use and the correctness of the attachment is easy to verify.

According to an embodiment, the connector 8 or the filter element 1 comprises a quick-release arrangement 30, such as shown in Figure 6a. The quick-release arrangement 30 is arranged to release the first locking shape 27 from the second locking shape 28. An advantage is that the locking is easy and simple to do without using any tools.

Figure 7 is a perspective top view illustrating a belt filter element. According to an embodiment, the filter element 1 is a filter element of a belt filter apparatus. The filter element 1 comprises a vacuum box 52 that comprises a bottom, two opposite long sides, two opposite end walls, and a filter member 3.

The vacuum box 52 comprises an internal cavity 12 into which vacuum or underpressure is applied. When the filter element 1 is submerged in the slurry basin, the cake forms onto the filter member of the filter element 1 by influence of the underpressure in the internal cavity 12.

Figure 8 is a perspective top view illustrating a drum filter element.

According to an embodiment, the filter element 1 is a filter element of a drum filter apparatus that comprises a pair of side edge surfaces 18a arranged parallel with each other. The filter member 3 is arranged only on one side of the filter element 1 . The the filter element 1 further comprises a pair of curved end surfaces 18d, wherein the curvature of the curved end surfaces 18d and the curvature of the filter member 3 coincide with the circumference of the outer surface of a drum filter of the drum filter apparatus.

Figure 9 is a perspective top view illustrating a disc filter apparatus, and Figure 10 is a side view illustrating the disc filter apparatus shown in Figure 9.

The disc filter apparatus comprises a filter 15 consisting of several consecutive co-axial filter discs arranged in line co-axially around the central shaft 21 of the filter 15.

The filter 15 is supported by bearings on a frame of the filter apparatus and is rotatable about the longitudinal axis of the central shaft 21 such that the lower portion of the filter 15 is submerged in a slurry basin located below the filter 15. The filter is rotated by e.g. an electric motor.

The number of the filter discs may range from 2 to 20, for example. The filter apparatus shown in Figure 9 comprises twelve (12) filter discs. The outer diameter of the filter 15 may be ranging from 1 .5 m to 4 m, for example. Examples of commercially available disc filters include Ceramec CC filters, models CC-6, CC-15, CC-30, CC-45, CC-60, CC-96 and CC-144 manufactured by Outotec Inc.

All the filter discs can be preferably essentially similar in structure. Each filter disc may be formed of a number of individual sector-shaped filter elements 1 discussed earlier in this description. The filter elements 1 are mounted circumferentially in a radial planar plane around the central shaft 21 to form an essentially continuous and planar disc surface. The number of the filter plates in one filter disc may be 12 or 15, for example.

As the central shaft 21 is fitted so as to revolve, each filter element 1 is, in its turn, displaced into a slurry basin and further, as the central shaft 21 revolves, rises out of the basin. As the filter member 3 is submerged in the slurry basin, the cake forms onto the filter member 3 under the influence of the vacuum. Once the filter element 1 comes out of the basin, pores of the filter member 3 are emptied as the cake is deliquored for a predetermined time which is essentially limited by the rotation speed of the disc. The cake can be discharged by e.g. scraping, after which the cycle begins again.

Operation of the disc filter apparatus may be controlled by a filter control unit, such as a Programmable Logic Controller, PLC.

Figure 1 1 is a perspective top view illustrating another filter appa- ratus 2. The filter apparatus 2 shown here is a drum filter apparatus comprising a drum-like filter 15. It is to be noted that the filter apparatus 2 is shown by dash lines in Figure X in order to clarify the structure of the filter 15.

In the drum filter apparatus the filter element 1 is a part of outer surface of the filter 15. The features of the filter element 1 are described earlier in this description.

The diameter of the filter 15 may be e.g. in range of 1 .8 m - 4.8 m and length in axial direction 1 m - 10 m. The surface area of the filter 15 may be e.g. in range of 2 - 200 m ² .

Examples of commercially available drum filters include CDF-6/1 .8 manufactured by Outotec Inc.

Function of drum filter apparatus has already described in background part of this description.

Figure 12 illustrates in a side view, still another filter apparatus 2. The filter apparatus 2 here is a vacuum belt filter apparatus.

The filter 15 of the vacuum belt filter apparatus comprises an endless belt comprising a multitude of individual filter elements 1 arranged one after another in the longitudinal direction of the belt. The features of the filter element 1 are described earlier in this description. The filter elements 1 follow one after another along the whole length of the belt, but for sake of simplicity, all vacuum boxes have not been illustrated.

The filter element 1 comprises a vacuum box into which vacuum or underpressure is applied. When the filter element 1 is submerged in the slurry basin, the cake forms onto the filter member of the filter element 1 by influence of the underpressure in the vacuum box.

The cake can be discharged by e.g. scraping, after which the cycle begins again.

The capillary filter member 3, when wetted, does not allow air to pass through which decreases the necessary vacuum level, enables the use of smaller vacuum pumps and, consequently, yields significant energy savings.

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.