WO2005035095A1

WO2005035095A1 - Filter plate, method for the preparation of a filter plate, and filter apparatus

Info

Publication number: WO2005035095A1
Application number: PCT/FI2004/000606
Authority: WO
Inventors: Bjarne Ekberg; Olli HÖGNABBA; Dave B. Eveland; Jeffrey J. Yost
Original assignee: Larox Oyj
Current assignee: Metso Finland Oy
Priority date: 2003-10-13
Filing date: 2004-10-13
Publication date: 2005-04-21
Anticipated expiration: 2006-04-13
Also published as: FI20031488L; FI117273B; PE20060701A1; FI20031488A0

Abstract

The invention concerns a filter plate (2) comprising a filter layer (32). The permeability of the filter layer (32) decreases continuously from the inner periphery (36) towards the outer periphery (37). Thereby a cake with uniform thickness is formed on the plate.

Description

FILTER PLATE, METHOD FOR THE PREPARATION OF A FILTER PLATE, AND FILTER APPARATUS

Field of the invention

The invention relates to a suction filter plate, on which a cake is formed in a filtering process. The plate can be used as an element especially in a filter disc to be rotated in slurry.

Background of the invention

A filter disc apparatus comprises filter discs, the insides of which are connected with radial suction lines to a suction device. The discs are rotated in a basin containing slurry so that the lower discs are in the slurry. Due to the suction, cake is formed on the disc surfaces. After the cake has been rotated up of slurry, the cake is removed. One disc consists of a plurality of sector-shaped filter plates.

In a capillary action filter disc apparatus, the filter plate is saturated with the liquid to be sucked through the plate. This principle is described e.g. in FI-C-77161 (corresponds to WO-88/00742). Such apparatuses have been also commercially available (Ceramec® filters of Outokumpu Corp., Finland).

The capillary filter plates can be made of suitable ceramic or polymeric material. Usually the plate comprises a porous support layer and filtering surfaces on the support layer. E.g. US-4863656, US-4981589, and WO-02/055178 describe such plates with ceramic filtering surfaces.

The surface of slurry in the basin is below the rotating axis of th.e discs. Therefore the outer periphery of a filter plate will reside in the slurry for a longer time than the inner periphery. Thus the cake will become thicker at the outer periphery, assum- of the slurry increases towards the bottom of the basin, which further enhances the uneven cake formation. This effect is considerable especially for heavy minerals like iron ore, chrome ore, and lead concentrate. As a result of the uneven cake formation, the cross section of the cake on the filter surfaces is wedge-shaped by cross section.

The properties of the cake also change in accordance with the thickness. Especially, the moisture content of the cake increases with increasing thickness. The uneven properties cause problems in the further steps of the process. Especially the difference in the moisture content may cause troubles. This is of importance when filtering materials the target moisture of which is difficult to achieve. Uneven moisture content may cause downstream troubles for example in pelletizing processes. Moisture may cause also troubles like freezing during storing and transportation. For many materials a transport moisture limit is set. If thermal drying is available, the extra moisture may be removed in this way, but this of course increases the costs. Therefore it is often necessary to run the filter apparatus with smaller overall cake height to make sure that the moisture limit is achieved. Thus the capacity of the apparatus is decreased.

In order to solve the problem of uneven cake formation, WO-02/45815 describes a filter plate consisting of at least two separate filter parts with different filtering properties.

Summary of the invention

Now a filter plate according to claim 1 has been invented. Some preferable embodiments of the invention are presented in the other claims. In accordance with the invention, the permeability of the filter disc decreases continuously towards the outer edge. This leads to a cake with more uniform thickness and correspondingly with more uniform properties. Preferably, the thickness of the filtering layer increases towards the outer edge of the plate.

Brief description of the drawings

The enclosed drawings form a part of the written description of the invention. The drawings illustrate some examples of the invention.

Fig. 1 shows a disc filter apparatus as seen from the end of the rotor.

Fig. 2 shows a radial cross section of a filter plate that can be used e.g. in the apparatus of Fig. 1.

Detailed description of the invention

In accordance with the invention, the disadvantages of the uneven cake formation can be eliminated by means of a filter plate having continuously decreasing per- meability in the radial direction, i.e. the permeability decreases towards the outer periphery. In this way the filter cake cross section profile is corrected and the result is a cake with more uniform thickness.

The required difference in the permeability may be accomplished e.g. in the fol- lowing ways:

- The thickness of the filter layer of the filter medium increases towards the outer periphery.

- The pore size of the filter layer of the filter medium decreases towards the outer periphery. - The porosity of the filter layer of the filter medium decreases towards the outer periphery. These ways may also be combined. The main advantage of the invention is that the properties, especially the moisture, of the cake formed in the filtration are uniform. Thus the filtering apparatus can e.g. be run at the maximum capacity. A more uniform material is easier to process further, e.g. by cake washing or pelletizing. The even moisture is impor- tant also in storing and transportation. When the cake is uniform, the filter plate is also more evenly loaded by the cake.

When the surface of the plate has no separate areas (like the plate according to WO-02/45815), the manufacturing is simpler. When the filter layer is continuous (i.e. without joints), the risk of leakages is smaller.

The principle of the invention may be used e.g. in

- capillary filters

- conventional vacuum disc filters - pressurized disc filters

- leaf filters.

Materials that can be used to accomplish the non-uniform filter medium include, e.g.: - sintered or silicate bonded ceramic materials, such as AI₂O₃, SiC, TiO₂, ZrO₂, corundum, etc., or combinations thereof

- plastic filter cloth materials, such as polypropylene, polyethylene, polyamide, fluoro plastics, etc., or combinations thereof

- sintered or cast plastic materials, such as polypropylene, polyethylene, polyam- ide, fluoro plastics, etc., or combinations thereof,

- metallic filter cloth materials, such as stainless steel, titanium, nickel, etc., or combinations thereof

- sintered metallic filter materials, such as stainless steel, titanium, nickel, etc., or combinations thereof - carbon based materials, such as graphite, amorphous carbon, etc., or combinations thereof.

The filter plate preferably comprises an inner layer upon which the filter layer is fixed. The inner layer supports the filter layer. Preferably there is a filter layer on both sides of the inner layer. Preferably the inner layer is porous so that the filtrate transfers from the filter layer to the inner layer, from where it is then collected.

The (radial) length of the ceramic plate may be e.g. 200...800 mm, and the width e.g. 200...600 mm. The thickness of the whole plate may be e.g. 20...80 mm, and the thickness of the filter layer e.g. 0.1...10 mm. When the thickness of the filter layer increases from the inner periphery towards the outer periphery, the thicke- ness at the inner periphery may be e.g. 0.1...1 mm, and at the outer periphery e.g. 0.2...2 mm.

When the pore size of the filter medium decreases towards the outer periphery, the permeabilty decreases correspondingly. The pore size at the inner periphery may be e.g. about 1 μm, and at the outer periphery e.g. 0.5...0.8 μm. The porosity is essentially constant.

When the porosity of the filter medium decreases towards the outer periphery, the porosity at the inner periphery may be e.g. about 40 vol-% and e.g. about 30 vol- % at the outer periphery.

A filter plate, the thickness of which increases towards the outer periphery, may be suitably prepared so that an inner layer substrate is dipped with the outer periphery downwards into a mixture of material forming the filter layer. Due to vacuum, adhesion forces, electrostatic forces or magnetic forces, the filter forming material adheres as a layer to the surface of the inner layer substrate, and when the substrate is lifted out of the mixture, a layer, whose thickness increases from the top edge to the bottom edge, is inherently formed. This treatment is repeated, if necessary, so that the desired thickness is achieved. The difference in thickness between the top edge and the bottom edge can also be adjusted with the dipping ve- locity. The lower is the velocity of the substrate in the vertical direction, the bigger is the difference in thickness. After the dipping stage, the filter layer usually has to be cured to form the final filter layer. The edges of the plate are usually covered by a suitable impermeable layer, e.g. by glazing or coating. A suitable ceramic filter plate in accordance with the invention to be used in a capillary filtering apparatus dryer has a microporous surface layer where the pore size is under 5 μm, preferably between 0.2 to 3 μm, and this microporous surface layer is supported by a porous inner layer. The inner layer has means for the removal of liquid from the surface layer. These means preferably comprise at least one recess. The filter plate is also provided with a fitting member fixed at the inner periphery.

When manufacturing the ceramic filter plate, the inner layer is formed of at least one substrate. The substrate is preferably made of a ceramic material in a powder form, such as for instance alumina, silicon carbide, titania, zirconia, corundum, or silica. The substrate can also be made of a metal or a metal alloy, polymer or graphite. In a preferred embodiment, the ceramic material is mixed with binding medium and liquid so that the ceramic mix formed is suitable for further processing. The ceramic mix is first charged into a mold so that the mold is partly filled with the ceramic mix. The core material for at least one desired recess area is then installed on the surface of the ceramic mix in the mold. Finally the rest of the ceramic mix is charged into the mold. When the total amount of the ceramic mix has been charged into the mold, the ceramic mix is pressed into a green body. After pressing, the green body is sintered in the temperature range of 1150 - 1550 °C. During the sintering stage of the green body, the core material for the recess area is burnt out through the porous structure of ceramic mix. The substrate now has recesses corresponding to the shape of the core material. Aϊterna- tively, the substrate can be manufactured of parts which have been processed separately as described above. After the sintering, the parts are assembled to complete filter elements.

After the sintering stage, the substrate is covered by at least one microporous layer. The covering process is preferably carried out so that the substrate is dipped with the outer periphery downwards into a bath of the microporous layer material. Thereby a desired surface layer with a thickness increasing towards the outer periphery is inherently achieved. The covering process can also carried out for instance by spraying or tape casting the ceramic microporous layer material on the surface of the substrate, but in that case special means are required to achieve the required radially changing permeability. The microporous layer mate- rial can also be made of a metal or a metal alloy, polymer, or graphite.

After the covering process the substrate with the microporous ceramic layer material is sintered at the temperature range of 1150 - 1550 °C. If further ceramic layer is needed, the covering and sintering process are repeated respectively. The sintering can also be done in one step for a substrate with a plurality of microporous layers. After the sintering of the microporous layer the filter element is essentially monolithic having a hierarchical structure and the filter element is then ready for mechanical processing. This means that the filter element is drilled if the hole or holes for the fitting member are not formed during the previous forming stage so that a fitting member is possible to install to the filter plate.

The filter apparatus of Fig. 1 comprises a drum 1 with filter discs consisting of a plurality of sector-like filter plates 2. A radial suction pipe 3 leads to the interior of each plate. The pipes of corresponding plates in different discs are connected to a specific filtrate line, which can be subjected via a distributor valve and through line 4, 5, or 6 to vacuum in the cake forming and drying phase, and through line 7 to overpressure in the cleaning phase.

The discs are rotated in basin 8, into which slurry is fed through line 9. When the plates are subjected to the vacuum, liquid is sucked through plates 2, and cake containing solids and liquid is formed on the surfaces of the plates. When the plate rises from the slurry, the vacuum is maintained, whereby more liquid is sucked and the cake dries further. Before final drying, washing liquid can be sprayed on the cake especially for removing salts from the cake. Clean water can be used as the washing liquid. After drying, the cakes are discharged with scrapers 10. The discharged dry cake 11 falls through chutes in the basin front end on conveyor 12. In the discharge phase, the plates are for a while subjected to liquid overpressure through line 7 in order to clean the plates. The filtrate and possibly some additional liquid can be used as a cleaning medium. After the cleaning phase, the plates are again subjected to the vacuum, and a new cake formation phase starts. The different phases are shown outside the circumference of the apparatus in Fig. 1 as follows: cake formation phase 13, cake washing phase 14, drying phase 15, cake discharge and plate cleaning phase 16. If no cake washing is carried out with the filter, the drying phase consists of phases 14 and 15.

The vacuum lines 4, 5, and 6 lead to a filtrate tank 17, in which the vacuum is formed by means of a pump 18. The filtrate is discharged from the tank is discharged through line 19 by means of pump 20 through valve 24. Some filtrate is led as washing liquid into the washing line through valve 21 and pressure control valve 23. Also special washing liquid can be led from tank 25 to the washing line with pump 22 through valve 28. Alternatively, extermal water may be used for washing and it is pumped through line 30.

At the bottom of basin 8 there is a cradle-type mixer 26. The basin can be emptied through bottom valve 27. Inside the basin there are ultrasonic vibrators 29. These are used from time to time for more thoroughly cleaning the plates. For the ultra- sonic treatment, the basin is emptied and filled with desired cleaning liquid.

Plate 2 in Fig. 2 comprises a porous internal layer 31 and a microporous filtering layer 32 on both sides and on the edges of the internal layer. The internal layer comprises at least one radial recess 33 starting from the inner edge of the plate. Here the inner edge is provided with a fitting member 34 with a joint 35, but the plate can also be fitted without such a specific member.

The thickness of the filtering layer 32 on the sides of the plate 2 increases continuously from the inner edge 36 to the outer edge 37. Thereby a cake with a uni- form thickness is formed on the surfaces of plate in the filtering process.

Claims

1. A filter plate (2) comprising a filter layer (32) having permeability to a liquid, the filter layer comprising an inner periphery (36) and an outer periphery (37), characterized in that the permeability of the filter layer (32) decreases continuously from the inner periphery (36) towards the outer periphery (37).

2. A filter plate (2) according to claim 1 , wherein

- the thickness of the filter layer (32) increases continuously from the inner periph- ery (36) towards the outer periphery (37), or

- the pore size of the filter layer (32) decreases continuously from the inner periphery (36) towards the outer periphery (37), or

- the porosity of the filter layer (32) decreases continuously from the inner periphery (36) towards the outer periphery (37).

3. A filter plate (2) according to claim 1 or 2, comprising an inner layer (31 ).

4. A filter plate (2) according to claim 3, wherein the inner layer (31 ) is porous.

5. A filter plate (2) according to claim 3 or 4, wherein there is a filter layer (32) on both surfaces of the inner layer (31 ).

6. A filter plate (2) according to any preceding claim, wherein the filter layer (32) is of ceramic material.

7. A method for the preparation of a filter plate (2) comprising an inner layer (31) and on both sides of it a filter layer (32) with permeability to a liquid, the filter layer comprising an inner periphery (36) and an outer periphery (37), characterized in that a substrate forming the inner layer (31) is prepared, and a filter layer (32), the permeability of which decreases continuously from the inner periphery (36) towards the outer periphery (37), is fixed to the surface of trie inner layer.

8. A method according to claim 7, wherein a filter layer (32) is fixed to both surfaces of the inner layer (31 ).

9. A method according to claim 8, wherein in preparing the filter layer (32), a sub- strate of the form of the inner layer (31 ) is dipped with the outer periphery downwards into and lifted up of a mixture of material from which the filter layer is formed.

10. A filter apparatus, characterized in that it has at least one filter plate (2) ac- cording to any of claims 1 to 6, or at least one filter plate (2) having been prepared according to any of claims 7 to 9.

11. A vacuum disc filter apparatus, a pressurized disc filter apparatus, or a leaf filter apparatus according to claim 10.

12. A capillary disc filter apparatus according to claim 11.