SE542757C2 - Rotary disc filter with means for keeping the filter surface clean - Google Patents

Abstract

A rotary disc filter (1) comprises a suspension vessel (10), a rotor (20) and a pressurizing device (40). The rotor has a hollow shaft (22) and at least one hollow disc (30) and is arranged for rotating the hollow disc(s) around a rotation axis (24) within the suspension vessel. The hollow disc(s) have at least one filter surface (32, 34). A hollow portion (36) of each of the hollow disc(s) is fluidly connected to the hollow shaft. The pressurizing device is arranged for providing a higher pressure in the interior of the suspension vessel than a pressure at an outlet (28) from the hollow shaft. The rotary disc filter further comprises a flow arrangement (50), arranged for causing an additional flow of suspension along the filter surface(s). This additional flow is provided at different radial positions of the hollow disc(s).

Description

ROTARY DISC FILTER WITH MEANS FOR KEEPING THE FILTER SURFACE CLEAN TECHNICAL FIELD The present invention relates in general to filter arrangements and in particular to filter arrangements using rotating discs.

BACKGROUND In different stages in a pulp manufacturing process, a slurry comprising particles suspended in a liquid is requested to be at least partially de-watered. To this end, different kinds of filters are used. Either the separated liquid is used for further processes, the de-watered slurry is used for further processes, or both.

One type of filters is referred to as cross-flow filters. The filters have stationary vertical filter elements, where the slurry to be de-watered is applied at the upper part of the filter elements. The slurry is allowed to flow downwards along the surface of the filter element. Due to an overpressure in the filter vessel, a part of the liquid from the slurry passes the surface of the filter and is pumped away. The remaining slurry, reaching the bottom part of the filter elements, becomes somewhat thicker. Optionally, this thickened sluriy is pumped back to the top of the filter elements for repeating the de-watering procedure a couple of times. A slurry that eventually is thick enough is removed to typically be further processed by different sludge handling equipment.

Another popular approach, avoiding some of the energy-consuming pumping, is disc filters. Rotating discs are immersed in a slurry, typically up to the shaft level. The discs have perforated filter surfaces, typically directed with surface normals essentially parallel to the rotation axis. The discs are hollow and are typically connected to a hollow shaft, through which liquids can be exited. The discs are placed in a pressurized volume or are connected to a vacuum pump, in order to force some liquids to pass the filter surfaces into the hollow volume and in to the hollow shaft. The part of the disc that at each moment is positioned above the slurry level can be used for washing and de-watering of the sludge being stuck at the disc surface.

There is a general problem with keeping the filter element surfaces clean enough. If too much sludge is stuck at the filter element surfaces, the filtering effect is severely reduced. This is particularly cumbersome for slurries having a high sludge content.

One approach for solving such problems for disc filters uses disc filter elements coated with a so-called pre coat. This pre coat is successively scraped away together with the sludge that has got stuck to the surface. The pre coat is thereby consumed, which means an additional cost and that the filter surfaces have to be re-coated occasionally, causing loss of operation time.

SUMMARY A general object is thus to provide a filter, intended for filtering slurries, that can maintain an efficient filtering operation over a long operation time without consumed parts.

The above object is achieved by methods and devices according to the independent claims. Preferred embodiments are defined in dependent claims.

In general words, in a first aspect, a rotary disc filter comprises a suspension vessel, a rotor and a pressurizing device. The rotor has a hollow shaft and at least one hollow disc. The rotor is further arranged for rotating the hollow disc(s) around a rotation axis within the suspension vessel. The hollow disc(s) have at least one filter surface each arranged transverse to the rotation axis. A hollow portion of each of the hollow disc(s) is fluidly connected to the hollow shaft of the rotor. The pressurizing device is arranged for providing a higher pressure in the interior of the suspension vessel than a pressure at an outlet from the hollow shaft of the rotor. The rotary disc filter further comprises a flow arrangement. The flow arrangement is arranged for causing an additional flow of suspension along the filter surface(s), in addition to a rotational flow of suspension caused by the rotation of the hollow disc(s). This additional flow is provided at different radial positions of the hollow disc(s).

In a second aspect, a filtering method comprises rotating of a rotor around a rotation axis in a suspension. The rotor has a hollow shaft and at least one hollow disc. The hollow disc(s) have at least one filter surface each arranged transverse to the rotation axis. A hollow portion of each of the hollow discs is fluidly connected to the hollow shaft of the rotor. A higher pressure is provided in the suspension than a pressure at an outlet from the hollow shaft of the rotor. Liquids from the suspension is collected through the filter surface(s) and is transported through the hollow shaft to the outlet. The filtering method comprises the further step of causing an additional flow of suspension along the filter surface(s), in addition to a rotational flow of suspension caused by the rotation of the hollow disc(s). This additional flow is provided at different radial positions of the hollow disc(s). Thereby, particles at the filter surface(s) are continuously removed.

One advantage with the proposed technology is that an efficiency of a filtering operation of the filter surfaces can be maintained over long operation periods without causing any extensive wear of the filter surfaces. Other advantages will be appreciated when reading the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which: FIG. 1 illustrates schematically one embodiment of a rotary disc filter 1; FIG. 2 illustrates a part of an embodiment of a single hollow disc; FIG. 3 illustrates a part of another embodiment of a single hollow disc; FIG. 4 illustrates an axial cross-section of an embodiment of a rotary disc filter; FIG. 5 illustrates another embodiment of a rotary disc filter; FIG. 6 illustrates yet another embodiment of a rotary disc filter; FIG. 7 illustrates a close-up illustration of an embodiment of a flush nozzle and a filter surface; FIG. 8 illustrates an embodiment of a hollow disc of a rotary disc filter partly suspended in suspension; FIG. 9 illustrates an embodiment of a hollow disc of a rotary disc filter fully suspended in suspension; and FIG. 10 illustrates a flow diagram of steps of an embodiment of a filtering method.

DETAILED DESCRIPTION Throughout the drawings, the same reference numbers are used for similar or corresponding elements.

The basic problem with prior art filters is that the filter surfaces cannot be maintained free from sludge stuck at the surface. Methods using e.g. doctor blades to mechanically remove the sludge cause unwanted wear of the filter surface. Washing the filtering surfaces at periods when no filtering takes place, e.g. when a rotary filter disc surface passes above the level of the slurry has not proved to be efficient enough and furthermore adds more liquids to the slurry.

According to the technology presented here, another approach is used. Instead of removing material that has stuck to the filter, measures are taken to mitigate that the material is stuck in the first place. It has been found that the tendency for a sludge particle to adhere to a filter surface is considerably reduced if the slurry that comes into contact with the filter surface has a nonnegligible velocity. At the same time, the filtering action, i.e. the rate in which the liquid passes the filter surface, is not significantly changed.

A rotating disc filter does indeed inherently cause a relative motion between the filter surface and the slurry, but with the typical dimensions and rotation velocities used, this relative motion has not been found to be sufficient in most cases. Furthermore, the relative motion caused by the rotation itself becomes, due to the geometry, uneven over a disc surface. The relative velocity always becomes high closer to the rim of the disc compared to the areas close to the shaft. Furthermore, a constant rotation velocity of the discs will after a while induce a motion pattern in the sluriy, giving a velocity component in the same direction as the disc closest to the disc surface. This results in that the effective relative motion is reduced.

According to the here presented technology, arrangements for creating an additional flow along the rotating disc are instead used. Flush nozzles or similar equipment provides an additional flow of flush suspension along the filtering surface of the rotating disc. This additional flow is preferably provided at different radial positions in order to interact with all active filters surfaces during one rotation of the rotating filter disc.

Figure 1 illustrates schematically one embodiment of a rotary disc filter 1. The rotary disc filter 1 comprises a suspension vessel 10. The suspension vessel 10 has an inlet 12, through which a slurry 14 to be filtered is entered. The suspension vessel 10 also has an outlet 16, through which a filtered slurry 18 is exited. The rotary disc filter 1 further comprises a rotor 20. The rotor 20 has a hollow shaft 22. At least one hollow disc 30, in this embodiment seven hollow discs 30 are attached to the hollow shaft 22. The rotor 20 is arranged for rotating the hollow discs 30 around a rotation axis 24 within the suspension vessel 10. In this embodiment, a motor 26 is connected to the rotor for providing the rotational motion. The hollow discs 30 have at least one filter surface 32, 34 arranged transverse to the rotation axis 24. In the present embodiment, all hollow discs 30 have filter surfaces 32, 34 on both sides. A hollow portion 36 of each of the hollow discs 30 is fluidly connected to the hollow shaft 22 of the rotor 20.

The rotary disc filter 1 further comprises a pressurizing device 40. The pressurizing device 40 is arranged for providing a higher pressure in the interior of the suspension vessel 10 than a pressure at an outlet 28 from the hollow shaft 22 of the rotor 20. This results in that liquids from the slurry 14 to be filtered is sucked through the filter surfaces 32, 34 of the hollow discs 30, into the hollow portion 36 and further into the hollow shaft 22. Such separated liquid 29 is then exited from the hollow shaft 22 through the outlet 28.

The rotary disc filter 1 further comprises a flow arrangement 50. The flow arrangement 50 is arranged for causing an additional flow of suspension along the filter surfaces. This additional flow thus is provided in addition to a rotational flow of suspension caused by the rotation of the hollow discs 30. The additional flow is provided at different radial positions of the hollow discs. In this embodiment, a part 19 of the filtered slurry 18 exiting the suspension vessel 10 is provided to a pump arrangement 54. The pump arrangement 54 provides the slurry through a net of connection lines 52 to flush nozzles 56. The flush nozzles 56 are arranged close to the filter surfaces 32, 34 and are arranged to give the additional flow along the filter surfaces 32, 34. The flush nozzles 56 are provided at different radial distances in order to reach different parts of the filter surfaces 32, 34 at different radial positions.

In an alternative embodiment, the pump arrangement 54 is instead supplied with un-filtered slurry 12, to be used for achieving the additional flow. Also other types of suspensions can be used for this purpose.

Figure 2 illustrates a part of an embodiment of a single hollow disc 30. In this embodiment, the hollow disc 30 has two filter surfaces 32, 34. These filter surfaces comprises in this embodiment a textile layer 37 provided on a perforated sheet 39. In other words, the filter surface 32, 34 comprises a perforated sheet 39 covered by a textile layer 37. Liquid passing the textile layer 37 and the perforated sheet 39 enters into the hollow portion 36 and flows 35 towards the hollow shaft 22 due to the pressure difference.

In an alternative embodiment, the textile layer 37 is omitted. The filter surfaces 32, 34 this comprise a perforated sheet 39 each.

Figure 3 illustrates a part of another embodiment of a single hollow disc 30. In this embodiment, the hollow disc 30 has a tapered shape, being broader closer to the hollow shaft. The filter surface 32, 34 here is constituted by only the perforated sheet 39. The filter surface 32, 34 are thus provided with a small angle relative to the radial direction, but are still provided transverse to the rotation axis 24.

Figure 4 illustrates an axial cross-section of an embodiment of a rotary disc filter 1. Here three flush nozzles 56 are provided on three connection lines 52. The three flush nozzles are provided at different radial distance R1-R3, which results in that all active surfaces of the filter surface 32 will be exposed for an additional flow 55 of suspension, when the hollow disc 30 is rotated one full turn. In other words, the flow arrangement is arranged for providing the additional flow 55 of suspension at all radial positions of the filter surface 32.

In the present embodiment, the flush nozzles 56 provide the additional flow 55 of suspension at different azimuthal positions around the hollow disc 30. It is preferred if the flow arrangement is arranged for providing the additional flow 55 of suspension at at least two azimuthal positions along the hollow disc 30.

It is easily seen that in the present embodiment, the flow arrangement is arranged for directing the additional flow 55 of suspension with a velocity component opposite to the direction of the rotation R of the hollow disc 30. First, such an arrangement will increase the relative velocity between the additional flow 55 and the hollow disc 30 by the velocity of the hollow disc 30 at each point. It also mitigate the build-up of a rotation-induced velocity motion within the slurry to be filtered, which also tends to increase the mean velocity at the filter surface 32.

Figure 5 illustrates another embodiment of a rotary disc filter 1. The flow arrangement is here collected on one side of the hollow shaft 22. The flush nozzle positions, in particular in radial direction, and the number of flush nozzles 56 are adapted in such a way that the entire active part of the filter surface 32 is exposed for the additional flow 55 at least once during the rotation of the hollow disc 30.

Figure 6 illustrates yet another embodiment of a rotary disc filter 1. In this embodiment, the flow arrangement comprises a single flush nozzle 56 connected to a connection line 52. The flush nozzle 56 is designed to give a spread additional flow at all radial positions of the hollow disc 30.

Figure 7 illustrates a close-up illustration of a flush nozzle 56 and a filter surface. Here it is illustrated that the flush nozzle 56 provides the additional flow 55 slight directed towards the filter surface 32. The direction of the additional flow 55 with respect to the filter surface 32 is characterized by the angle A. This angle A should as a minimum be zero degrees. The angle A may be up to 90 degrees, however it is preferred to use smaller angles. A preferred angle interval is 0-45 degrees. Most preferably the angle A is within the range of 0-30 degrees.

As has been illustrated in the figures above, it is preferred to use both sides of the hollow discs 30 for providing a filter surface, and as a consequence the flow arrangement preferably provide the additional flow at both sides of the hollow disc 30. In other words, the hollow disc has a filter surface on both sides transverse to the rotation axis. The flow arrangement is then arranged to providing the additional flow of suspension at both sides of the hollow disc.

In alternative embodiments, one or more of the hollow discs may have a filter surface only on one side. Consequently, the flow arrangement provides the additional flow only on the side having the filter surface for such hollow discs.

Figure 8 illustrates an embodiment of a hollow disc 30 of a rotary disc filter. The hollow disc 30 is in this embodiment suspended up to the shaft in slurry 14 to be filtered. In other words, the level 15 of the suspension does only cover a part of the hollow disc 30.

Figure 9 illustrates another hollow disc 30 of a rotary disc filter. In this embodiment, the rotor is arranged to be fully suspended by suspension in the suspension vessel 10.

In a typical case in pulp production, the slurry to be filtered comprises at least one of green liquor and white liquor. Consequently, the liquid that is separated and exited through the hollow shaft is green liquor and/or white liquor.

The above ideas can also be expressed as a method. Figure 10 illustrates a flow diagram of steps of an embodiment of a filtering method. In step S10, a rotor is rotated in a suspension. The rotation takes place around a rotation axis. The rotor has a hollow shaft and at least one hollow disc. The hollow disc(s) have at least one filter surface arranged transverse to the rotation axis. A hollow portion of each hollow disc is fluidly connected to the hollow shaft of the rotor. In step SI 2, a pressure difference is provided. In other words, a higher pressure is providing in the suspension than a pressure at an outlet from the hollow shaft of the rotor. Thereby, liquids from the suspension is collected through the filter surface(s) and is transported through the hollow shaft to the outlet. In step SI 4, an additional flow of suspension is caused along the filter surface(s), in addition to a rotational flow of suspension caused by the rotation of the hollow disc(s). The additional flow is provided at different radial positions of the hollow disc(s). Thereby, particles at the filter surface(s) are continuously removed.

Preferably, the additional flow of suspension is providing at all radial positions of the filter surface(s).

Preferably, the additional flow of suspension is directed with a velocity component opposite to the direction of the rotation of the hollow disc(s).

The proposed technical solution, presented above, gives an improvement of the operation of a rotary disc filter. The prohibition of particles to get stuck to the filter surface enables to keep the filter surface clean so that the capacity of the filter can be maintained over a long time.

The embodiments described above are to be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible. The scope of the present invention is, however, defined by the appended claims.

Claims (13)

1. A rotary disc filter (1), comprising: - a suspension vessel (10); - a rotor (20), having a hollow shaft (22) and at least one hollow disc (30), and being arranged for rotating said at least one hollow disc (30) around a rotation axis (24) within said suspension vessel (10); said at least one hollow disc (30) having at least one filter surface (32, 34) arranged transverse to said rotation axis (24); wherein, a hollow portion (36) of each said hollow disc (30) is fluidly connected to said hollow shaft (22) of said rotor (20); and - a pressurizing device (40), arranged for providing a higher pressure in the interior of said suspension vessel (10) than a pressure at an outlet (28) from said hollow shaft (22) of said rotor (20), characterized by - a flow arrangement (50), arranged for causing an additional flow (55) of suspension along said at least one filter surface (32, 34), in addition to a rotational flow of suspension caused by said rotation (R) of said at least one hollow disc (30), said additional flow (55) being provided at different radial positions (R1-R3) of said at least one hollow disc (30).

2. The rotary disc filter according to claim 1, characterized in that said flow arrangement (50) is arranged for providing said additional flow (55) of suspension at all radial positions of said at least one filter surface (32, 34).

3. The rotary disc filter according to claim 1 or 2, characterized in that said flow arrangement (50) is arranged for directing said additional flow (55) of suspension with a velocity component opposite to the direction of said rotation (R) of said at least one hollow disc (30).

4. The rotary disc filter according to any of the claims 1 to 3, characterized in that said flow arrangement (50) is arranged for providing said additional flow (55) of suspension at at least two azimuthal positions along said at least one hollow disc (30).

5. The rotary disc filter according to any of the claims 1 to 4, characterized in that said flow arrangement (50) comprises flush nozzles (56) arranged to flush suspension in an angle (A) to said at least one filter surface (32, 34) of said at least one hollow disc (30), said angle (A) being in the range of 0-90 degrees, preferably, in the range of 0-45 degrees and most preferably in the range of 0-30 degrees.

6. The rotary disc filter according to any of the claims 1 to 5, characterized in that said at least one hollow disc (30) has a filter surface (32, 34) on both sides transverse to said rotation axis (24), wherein said flow arrangement (50) is arranged to providing said additional flow (55) of suspension at both sides of said at least one hollow disc (30).

7. The rotary disc filter according to any of the claims 1 to 6, characterized in that said rotor (20) is arranged to be fully suspended by suspension in said suspension vessel (10).

8. The rotary disc filter according to any of the claims 1 to 7, characterized in that said suspension is at least one of green liquor and white liquor.

9. The rotary disc filter according to any of the claims 1 to 8, characterized in that said filter surface (32, 34) comprises a perforated sheet (39).

10. The rotary disc filter according to claim 9, characterized in that said filter surface (32, 349 comprises a perforated sheet (39) covered by a textile layer (37).

11. A filtering method, comprising the steps of: - rotating (S10) a rotor (20) in a suspension around a rotation axis (24); said rotor (20) having a hollow shaft (22) and at least one hollow disc (30); said at least one hollow disc (30) having at least one filter surface (32, 34) arranged transverse to said rotation axis (24); wherein, a hollow portion (36) of each said hollow disc (30) is fluidly connected to said hollow shaft (22) of said rotor (20); and - providing (S12) a higher pressure in said suspension than a pressure at an outlet (28) from said hollow shaft (22) of said rotor (20); whereby liquids from said suspension is collected through said at least one filter surface (32, 34) and is transported through said hollow shaft (22) to said outlet (28), characterized by the further step of: - causing (S14) an additional flow (55) of suspension along said at least one filter surface (32, 34), in addition to a rotational flow of suspension caused by said rotation (R) of said at least one hollow disc (30), said additional flow (55) being provided at different radial positions of said at least one hollow disc (30), whereby particles at said at least one filter surface (32, 34) are continuously removed.

12. The filtering method according to claim 11, characterized in that said additional flow (55) of suspension is providing at all radial positions of said at least one filter surface (32, 34).

13. The filtering method according to claim 11 or 12, characterized in that said additional flow (55) of suspension is directed with a velocity component opposite to the direction of said rotation (R) of said at least one hollow disc (30).