The present invention relates to a rotary press for dewatering a humid mass, such as a sludge or a pulp.

Background art

From FR 2582573 a liquid-extracting press is known comprising a cylindrical rotor having perforated lateral walls, a perforated stationary casing partially surrounding the rotor and defining a portion of a cylinder having an axis parallel to the shaft of the rotor, wherein the axis of the casing is offset from the shaft of the rotor. The press further comprises blades parallel to the shaft of the rotor and mounted so as to move by radial sliding on the rotor, means for maintaining the blades with their outer edge held against the internal face of the casing, wherein the casing comprises chute where the separation between the casing and the rotor is the greatest so that the blades, while the rotor is rotating, continuously brings the product to be pressed into zones where the separation of the rotor from the casing becomes smaller. The known press is particularly suited for pressing fruits. However, the mechanism for rotating and retracting the blades is susceptible to wear, in particular if the product to be pressed contains abrasive solids as is often the case for sludge produced during treatment of wastewater.

It is an object of the invention to provide a more wear resistant rotary press for dewatering a humid mass, such as a sludge or pulp.

Summary of the invention

To this end, according to a first aspect, the invention provides a rotary press for dewatering a humid mass such as a sludge or pulp, comprising: a rotor comprising a hub and mounted for rotating around a central axis in a direction of rotation, the rotor further comprising a plurality of blades, each blade having a proximal end attached to the hub and a distal end spaced apart from the hub; a screen, at least partially surrounding the rotor and arranged for contacting distal ends of the blades during rotation of the rotor, wherein the screen is adapted for letting liquid pass there through while substantially blocking passage of solids and wherein along the direction of rotation of the rotor and in a plane normal to the central axis the distance of the screen to the central axis decreases over an arc of at least 120 degrees; an inlet comprising a passage for humid mass to a location between two adjacent blades of the plurality of blades; an outlet arranged downstream from the passage along the direction of rotation, for allowing passage of dewatered humid mass away from between two adjacent blades of the plurality of blades; wherein each blade comprises a flexible portion extending between the hub and the distal edge, wherein the flexible portion is adapted for, upon contact of the distal edge with the screen during rotation of the rotor and when seen in projection onto a plane normal to the central axis, bending relative to the proximal edge of the blade over an included bend angle of at least 90 degrees. Preferably, the distance of the screen to the central axis decreases over the full arc of the screen. The screen typically covers an arc of at least 180 degrees, preferably of at least 220 degrees.

The rotary press according to the invention may be constructed free from a mechanism for letting the blades protrude from and retract into the hub or for causing the blades to slide or rotate against the hub in any other way, in this manner substantially reducing wear of the blades. During rotation of the rotor, the change in volume of humid mass held between two adjacent blades depends to a large extent on the degree of bending of one or both of the two blades. As the flexible portions are adapted for bending over at least 90 degrees, such a volume can decrease significantly, resulting in efficient dewatering of the humid matter that is held between the two blades. During operation, stress and force on the blades is not concentrated at the proximal edges thereof, but is partially transferred to the humid mass due to the flexibility of the blades. This further reduces wear on the proximal portions of the blades.

Typically the volume between two blades and the screen, during rotation of the rotor while the edges of the blades contact the screen, varies at least a factor 2. For examples, if during such rotation the maximum volume between the blades and screen equals 30 liters, then the minimum volume equals 15 liters or less. This factor is at least 2,66, and more preferably at least 3.

The screen preferably comprises an inner surface facing the central axis and parallel to the central axis, wherein the inner surface may be formed as a part of a cylinder having its longitudinal axis spaced apart from the central axis. In general the screen will be substantially rigid, e.g. will comprise or be made from a metal. It is highly preferably that the mesh screen is or forms a wedge wire filter, e.g. having narrow openings at the inner surface with a width of 1 mm or less, preferably of 0,5 mm or less, more preferably in the range of 0,4 mm to 0,15 mm, e.g. 0,3 mm. This width has been found to substantially prevent passage of solids through the screen, while allowing liquid to pass through. Alternatively, the screen may comprises a number of perforations which allow passage of water but substantially block passage of solids of the humid mass therethrough. The longitudinal axis of the cylinder preferably is spaced apart from the central axis by at least 10% of the diameter of the cylinder.

The screen is stationary relative to the inlet during rotation of the rotor and while distal ends of the blades are in direct contact with the screen. The hub and in particular the blades are preferably impermeable to both liquid and solid.

In an embodiment the screen has an inner surface which faces the central axis and which is shaped as a cylinder segment which is parallel to the central axis and extends over an arc of 180 degrees or more.

In an embodiment the flexible portion of each blade has a length greater than a smallest radial distance between the central axis and the screen. This ensures that the blade can bend over a significant portion of the length of the blade, e.g. over at least 30% of the length of the blade, preferably over at least 50% of the length of the blade. Herein, each blade has a width parallel to the central axis, and a length extending from the proximal end to the distal end. In an embodiment the flexible portion of each blade comprises an elastic material and/or is provided with a spring for biasing the distal end away from the proximal end. Suitable elastic materials include elastomers such as polyurethane, nitrile rubber, (styrene-)butadiene rubber and the like.

The flexible portion may comprise a sheet of elastic material, preferably having a shore hardness of 80 or more, and may be provided with reinforcement elements, e.g. along the lateral edges of the flexible portion.

Alternatively, the flexible portion may be made of such an elastic material. The elastic range of the flexible portion of each blade is preferably such that during a complete rotation of the rotor, the stress exerted on the flexible portion remains within the elastic range.

In an embodiment the flexible portion of each blade is formed as an elastic sheet between the proximal end and the distal end, and adapted for bending in such a manner that the flexible portion remains spaced apart from the screen. During a complete rotation of the rotor the distal end thus contacts the screen while the flexible portion remains spaced apart from the screen.

In a preferred embodiment the flexible portion comprises a sheet of an elastomeric material having a thickness between 0,8 and 25 mm, preferably between 10 and 20 mm. Flexible portions formed as sheets made from shore 90 polyurethane were found to be particularly suitable.

In an embodiment the distal end of each blade comprises a substantially rigid portion for contacting the screen. This helps to avoid deformation of the edge of the blade during rotation, so that contact between the edge and the inner surface of the screencan be maintained over the arc of the screen. The blade’s flexible portion allows rotation of the rigid portion relative to the proximal end when the distal end contacts the screen. Preferably, the rigid portion tapers in a direction from the distal end of the blade towards the flexible portion of the blade. In order to minimize wear of the screen, the rigid portion is preferably made from a material that is that has a considerably lower Mohr hardness than the material of the screen it contacts. For instance, when the screen is made from steel, the rigid portion may comprise or consist of a high-modulus polyethylene (HMPE), polyoxmethylene (POM) or polytetrafluoroethylene, or a metal-alloy such as brass.

In an embodiment the flexible portion of each blade is adapted for allowing the blade to bend during rotation of the rotor between a position at or near the inlet in which its distal end is at a first distance from the central axis, to a position spaced apart from the inlet in which its distal end is at a second distance from the central axis, wherein the first distance is at least 1 ,5 times as large as the first distance. The change in distance of the blade’s distal end, or edge, to the central axis when the blade is at the first as compared to when the blade is at second position, ensures that the distance between two blades neighboring blades changes during rotation of the rotor, with a corresponding change in space for humid mass that is held between the two blades.

In an embodiment the rotary press comprises a first and a second sidewall which extend transverse to the axis of rotation, wherein a compacting space for the humid mass is formed between the first and second sidewall and two adjacent ones of the flexible blades that have a distal end contacting the screen. Preferably the first and second sidewall are adapted for preventing passage of both liquid and solid matter through the sidewalls. In an embodiment for said two adjacent flexible blades, the volume of the compacting space when the rotor is at a first angular position is at least twice, preferably at least three times, the volume of the compacting space between the same two blades when the rotor is at a different second angular position. It has been found that this allows thickening of sludge by the press from 0,2 - 9,8% wt. dry solid content at the inlet to a 10-20% wt. dry solid content at the outlet. The press can be used for dewatering sludge regardless of whether a polymer has been added to the sludge before it enters the press and/or at the inlet of the press.

In an embodiment the first and/or second sidewall comprise a transparent portion for allowing a view of the lateral edges ofthe flexible portions of the blades. This allows easy monitoring and maintenance of the rotary press. For instance, when an operators looking through the transparent portion sees substantial amounts of humid mass pass beyond the lateral edge of a flexible portion, then the corresponding blade may have to be replaced.

In an embodiment, the central axis is arranged substantially horizontally. Preferably the outlet is arranged above a horizontal plane through the central axis, to prevent humid mass from directly falling from the inlet to the outlet. A plate, e.g. a rounded plate, may be provided between the downstream edge of the screen and the outlet.

In an embodiment the rotary press further comprises a spray cleaning system for cleaning the outer surface of the screen, the spray cleaning system comprising a plurality of spray nozzles mounted on an arm which extends in a plane normal to the central axis, the system further comprising an actuator for moving the arm along a direction parallel to the central axis, and wherein the plurality of spay nozzles is arranged adapted for spraying liquid onto substantially the entire outer surface during movement of the arm. Preferably, the screen comprises a plurality of walls or wedge shaped wires which extend parallel to each other along the arc of the screen, each wall or wire having a maximum width significantly smaller, e.g. at least 4 times smaller, than a distance the wall or wire extends in a radial direction.

The screen can thus be viewed as a plurality of thin walls or wedge shaped wires, which allow liquid to pass through between two walls while substantially preventing solid mass from passing through. By moving the arm which carries the spray nozzles, it is ensured that spray jetted from the spray nozzles can pass between two of the walls or wires in a plane parallel to the thin walls, instead of being deflected by the walls or wires. This is particularly useful when the screen comprises a wedge wire filter with the wedge wires extending along the arc of the screen.

In an embodiment the inlet is provided with an overflow wall having an overflow edge which partitions the inlet into the passage for humid mass to a location between two adjacent blades of the plurality of blades, and a separate bypass that is connected to the outlet. When sludge is provided to the rotary press at a rate faster than can be processed by the press, some of the sludge will simply pass through the press without being dewatered.

In an embodiment the radial press further comprises an actuator for driving rotation of the rotor, and a controller for controlling the actuator to drive rotation of the rotor at a speed of 10 rpm or less, e.g. 5 or 1 rpm or less, and preferably equal to or less than 0,5 rpm, more preferably between 0,25 rpm and 0,1 rpm. Dewatering is thus achieved by compacting the humid mass while letting the liquid escape through the screen. The relatively low rpm ensures that the sludge, which typically acts as a non-Newtonian material, does not clog the screen. Centrifugal force does not play a significant part in the dewatering process.

In an embodiment each blade, when in an extended position in which it is substantially not bent, has a length in the radial direction of the rotor that is at least two times larger than a maximum outer diameter of the hub. Preferably, the flexible portion of each blade has a length that is at least two times larger than the maximum outer diameter of the hub. The flexible portion can thus bend along a relatively large part of its length. In general, the smaller the ratio of the length of the blades to the maximum outer diameter of the hub, the higher speed of rotation of the rotor can be. For instance, when this ratio is 5 or more, the rotor is preferably driven at 1 rpm or less. If this ratio is between 2 and 5, it may be possible to drive rotation of the rotor at higher speeds, e.g. at 2 rpm or more.

According to a second aspect, the present invention provides a method for dewatering a humid mass such as a sludge or pulp, using a rotary press that comprises a rotor and a screen at least partially surrounding the rotor along a direction of rotation of the rotor, wherein along said direction of rotation and in a plane normal to a central axis of the rotor the distance of the screen to the central axis decreases along an arc of at least 120 degrees; and wherein the rotor comprises a hub and a plurality of blades attached to the hub, each blade having a proximal end attached to the hub and a distal end spaced apart from the hub, and each blade comprising a flexible portion extending between the hub and the distal end, the method comprising: rotating the rotor at a speed of 5 rpm or less in such a manner that at least one of the blades contacts the screen with its distal end, and such that the flexible portion of said blade, when seen in projection onto said plane, is bend over an included bend angle of at least 90 degrees. The rotary press is preferably a rotary press as described herein.

Short description of drawings

The present invention will be discussed in more detail below, with reference to the attached drawings, in which

Fig. 1A shows a perspective view of a rotary press according to the invention;

Fig. 1 B shows schematically a cross-sectional view of the rotary press of Fig. 1 A along line ll-B;

Fig. 1 C shows schematically a front view of the press of Fig. 1A;

Fig. 2A-2E illustrate how humid matter held between adjacent rotor blades is compressed and dewatered in the press of Fig. 1 A.

Fig. 3 shows a detail of a screen of the rotary press of Fig. 1A, which shows the wedge- shaped wires of the screen. Description of embodiments

Figs. 1 A and 1 B respectively show a perspective view of a rotary press 1 according to the invention, and a cross-section through plane l-B of Fig. 1A. The press 1 comprises an inlet 2 at a top side, for receiving a humid mass to be guided to between two blades of a rotor of the rotary press. The rotor 10, see Fig. 1 B, is mounted for rotating in direction of rotation R around an central axis X and is adapted for moving the blades in such a manner that the two blades 20a, 20b are brought towards each other during rotation of the blades along a wedge wire mesh screen 30 which partially surrounds the rotor. In this manner, as the blades rotate along the wire mesh screen, the humid mass that is held there between, is compacted and dewatered. The humid mass, after having been dewatered to some degree, exits the press at an outlet 3 at a bottom side of the press. The rotary press of the invention is capable of dewatering sludge such that the dry water % wt. at the outlet 3 is about half or less of the dry water %wt. at the inlet 2, for sludge at the inlet having a dry water %wt in the range of 0,2 - 10 %, preferably 3-7%, e.g. 5%.

Referring to Fig. 2A, the wedge wire mesh screen 30 partially surrounds the rotor 10 and extends over an arc Q of 210 degrees. Along a portion of this arc, which portion extends over y degrees, the distance of the inner surface of the screen to the central axis X decreases along the direction of rotation. The screen 30 comprises wedge shaped wires which have their wedge points directed away from the center axis X, each of the wedge wires extending substantially along a plane normal to the central axis X, with the wide parts of the wedges forming contact surfaces for distal ends of the blades. This allows continuous contact between the distal end of a blade and the wires along the cylindrical inner surface of the screen 30.

Referring back to Figs. 1A and 1 B, sidewalls 40 and 50 are arranged on the transverse sides of the screen 30, which sidewalls, together with the hub, the blades and the screen, form spaces for dewatering the sludge or pump. A controller 5 controls an electromotor 5 to rotate the rotor. The controller typically controls the motor 5 to drive rotation of the rotor at a speed of 1 rpm or less.

The rotary press 1 is further provided with a spray cleaning system 60, for cleaning solid matter from the humid mass that may have become clogged in the screen. The spray cleaning system 60 comprises an arm 61 which extends in a plane normal to the central axis X and substantially follows the curvature of the outer surface of the mesh screen 30. The arm is slidably supported on rails 62, 63 which extend parallel to the central axis X and allow the arm to be moved along substantially the entire width of the screen 30. Spray nozzles 64 are supported on the arm with their nozzle openings facing the outer surface. Spray liquid, generally water, can be supplied to the nozzles by opening liquid supply valve 65. A linear actuator 66 attached to sidewall 50 and to the arm 61 , is adapted for driving movement of the arm in a direction parallel to the central axis X.

The operation of the press will now be described in more detail with reference to Fig. 1 B. Sludge or pulp may enterthe press at inlet 2, in particular at passage 6 of the inlet, which is arranged for passage of the sludge or pulp to a location between two blades 20a, 20b of the rotor 10. An inlet valve 4 that is arranged in the passage 6 can be operated between an open position, as shown, in which it leaves the passage 6 open, and a closed position, shown in dotted lines, in which it prevents humid mass at the upstream end of the passage 6 from reaching the rotor 10. This allows supply of sludge or pulp to the rotor to be temporarily stopped, allowing inspection and/or maintenance of the rotor 10, motor 5 and/or screen 30 to be carried out. To facilitate access to the rotor, the screen is releasably attached to the sidewalls, though during rotation of the rotor the screen is stationary with respect to the inlet 2. When the valve 4 is in the closed position, humid mass supplied to the inlet 2 will eventually reach a level above that of overflow edge 8 of overflow wall 7 of the inlet 2, and any additional humid mass will flow across the overflow edge 8 via bypass passage 9 to the outlet 3, without being transported by the rotor blades. A wall 80 is connected at one end to overflow wall 7 and at another end to the screen 30. The wall 80 is formed as a substantially closed surface, except at an opening 82 in the wall 80 between points 81 a, 82b, which allows passage of humid mass away from between the blades and towards the outlet 3.

When the valve 4 is open, humid mass will however fall onto blade 20a to a position between said blade 20a and blade 20b immediately upstream in the direction of rotation R of the rotor 10. During rotation of the rotor, a space in which the humid mass is held, is formed between the blades, the hub 1 1 , the sidewalls 40,50, and for at least a part of the rotation by the mess screen and/or by wall 80. Each of the rotor’s blades 20a-20h has a proximal end 21 a-21 h attached to a hub 1 1 of the rotor 10, and an opposite distal end 22a-22h arranged for contacting the screen 30 during rotation of the rotor. Between the hub 1 1 and the distal end each blade comprises an elastic flexible portion 23a-23h, which in the present example comprises a sheet of polyurethane having a thickness of about 12 mm.

The blades 20a-20h each comprise a flexible and resilient portion 23a-23h which is adapted for bending in such a manner that a forward facing portion thereof is convex in the direction of rotation. In Fig. 1 B, the flexible portions 23a, 23b of blades 20a, 20b are substantially but not completely straight, i.e. the respective bend angles of the flexible portions are relatively small, i.e. between 3 and 7 degrees. In this position, blade 20a can support humid mass supplied from the inlet 2, without buckling under the weight of said mass. The flexible portions 23c-23h of the blades downstream of blade 20b in the direction of all have bend during contact of the distal edge of each blade with the screen 30. For instance, the flexible portion 23e of blade 20e has bend over an included bend angle a of about 80 degrees, and the flexible portion 23h of blade 20h has bend over an included bend angle b of about 180 degrees. As the flexible portions 23a-23h bend from being substantially straight, or planar, to relatively high included bending angles during rotation of the rotor, significant differences in volume between two blades and the screen are achieved. The high degree of bending permitted by the flexible portions also helps in more evenly distributing among the blades the force that is exerted on the blades during compression of the humid mass. As the blades are flexible along a significant portion between the hub and the distal edge, the risk of breaking of a blade is low.

Though in an alternative embodiment the flexible portions of the blades, when the blades are near the inlet for receiving humid mass between the blades from the inlet, may be substantially straight or at an angle of 0 degrees, it is preferred that the flexible portions remain at at least a relatively small angle to keep them pre-tensioned during filling, so that buckling of the flexible portions during filling is avoided.

In the embodiment shown, the inner surface of the wedge screen which faces the central axis X forms part of a cylinder having a longitudinal axis C which is offset from the center axis X, and a distance of the inner surface of the wedge screen decreases along the direction of rotation R along an arc of 180 along the cylinder.

It will however be appreciated that the inner surface of the mesh screen 30 may instead be formed as another curved shape which extends parallel to central axis X of the rotor, as long as the distance between said inner surface and the central axis along the direction of rotation of the rotor and in a plane normal to the central axis decreases gradually, preferably over an arc of at least 120 degrees.

Fig. 1 C schematically shows a partially cut-away front view of the radial press of Fig. 1 B, in which of the blades only blade 20 d is shown for reasons of clarity, The figure further shows the inlet 2, sidewalls 40 and 50 and the motor 5 for driving rotation of the rotor. The mesh screen 30 is also shown partially cut-away, the mesh screen having a width walong a direction parallel to the central axis X, and having wires 31 which extend in a plane that is normal to the central axis X. The structural integrity of the mesh screen is improved by rods 32 which extend parallel to the central axis X and are attached to the outer surface of the mesh screen.

The sidewalls 40, 50 close of the space between the two rotor blades is closed at the transverse ends of said blades. Sidewall 40 comprises a portion 41 through which the central axis X extends, and sidewall 50 comprises a similar portion. The sidewall 40 is further provided with transparent portions 42, see Fig. 1A, which allow the interior of the rotary press to be viewed from the side. In particular, the transparent portions allow lateral edges of blades of the rotor to be monitored during rotation of the rotor. These transparent portions are optional, and additionally or alternatively the sidewall 50 may be provided with transparent portions for allowing the interior of the press to be viewed from the other side.

Figs. 2A - 2E illustrate how a humid mass S supplied from inlet 2 is dewatered and compacted during rotation of the rotor in the press 1 . In Fig. 2A, the humid mass is received between blades 20a and 20b, which together with the sidewalls 40,50 the hub 1 1 and walls 7, 80 prevent the humid mass S from falling past blade 20a towards the outlet 3. Fig. 2B shows the same blades 20a, 20b as the rotor has rotated for about 45 degrees, wherein the humid mass has been somewhat compressed between the blades 20a, 20b so that some of the liquid content of the humid mass S has been pushed out through the mesh screen 30. Fig. 2C shows the rotor rotated by another 45 degrees, with a corresponding decrease of the volume between the blades 20a, 20b so that further liquid is pressed out of the mass and through the mesh screen 30. Fig. 2D shows the blades in a position just before the humid mass is falls out of a volume defined between the blades and the mesh screen. The distal edges of blades 20a and 20b are both still in contact with the mesh screen, and the blades 20a, 20n have bent to an extent that the distance between distal end of blade 20a and blade 20b is less than a distance of the distal end of blade 20a to the hub 1 1 . Upon further rotation, as shown in Fig. 2E, contact between the distal end of blade 20b and the mesh screen 30 is lost, and the distal end of the blade 20b has been moved across the opening 82 in the wall 80. The distal edge of blade 20a is still in contact with the mesh screen and the blade 20a pushes the humid mass towards the opening 82 from where it falls due to gravity to outlet 3.

Fig. 3 shows a detail of the a cross-section of the wire mesh screen 30. The mesh screen comprises a number of parallel and substantially wedge shaped wires 31 , the flat ends of the wedges forming the inner surface of the mesh screen which faces the central axis X, and the pointed ends point away from the central axis X. The wires extend such that the distal end of a blade contacting the wire will run substantially perpendicular to said wire. In this manner each distal end of a blade may remain substantially continuously in contact with the wires during rotation of the blade from the inlet towards the outlet.

The present invention has been described above with reference to a number of exemplary embodiments as shown in the drawings. Modifications and alternative implementations of some parts or elements are possible, and are included in the scope of protection as defined in the appended claims.