EP3874089A1

EP3874089A1 - Mixer for mixing chemicals into pulp

Info

Publication number: EP3874089A1
Application number: EP19877758.3A
Authority: EP
Inventors: Mikael Löfgren; Niklas Eriksson
Original assignee: Valmet Oy; Valmet AB
Current assignee: Valmet Technologies Oy; Valmet AB
Priority date: 2018-10-30
Filing date: 2019-08-29
Publication date: 2021-09-08
Also published as: WO2020091640A1; SE542365C2; SE1851348A1; CN112752880A; EP3874089A4; CN112752880B

Abstract

A mixer (1 ) for mixing chemicals into pulp, comprising: - a chamber (30) with an inlet (32) for pulp and chemicals and an outlet (34) for mixed pulp; said inlet (32) for pulp and chemicals being arranged through a first wall (36) of said chamber (30); - a rotor (10) having a rotor drum (20); said rotor drum (20) being perforated, creating openings (22), and having a cylindrical shape; said rotor (10) being arranged through a second wall (37), opposite to said first wall (36), of said chamber (30) and arranged for rotating said rotor drum (20) around a rotation axis (S) coinciding with an inflow direction (A) of said pulp and chemicals through said inlet (32) for pulp and chemicals; said rotor (10) being arranged against said chamber (30) to counteract flow of material from said inlet (32) for pulp and chemicals to said outlet (34) for mixed pulp except through openings (22) in said rotor drum (20).

Description

Mixer for mixing chemicals into pulp

TECHNICAL FIELD

The present disclosure relates in general to pulp treatment equipment and in particular to a mixer for mixing chemicals into pulp.

BACKGROUND

During different stages of the processing of pulp, chemicals are added to the pulp. The chemicals are typically intended to modify the fibers in the pulp. A typical example of such chemical is bleaching agents. Such chemicals are typically requested to be mixed into the pulp as homogeneously as possible. It is e.g. known that the bleaching agents have to be well mixed for a good bleaching result for all fibers.

Medium Consistency (MC) (about 12%) concentration of pulp is approximately the maximum concentration that is considered as liquid and is possible to be pumped. However, if the pulp is exposed for shear forces, the pulp will gain water-like properties. At a high shearing motion, the turbulence and the water-like pulp can be utilized for mixing the pulp with chemicals. In order to keep the energy consumption for mixing as low as possible, the volume of a high-shear zone in the mixer should be kept small. The pulp will then be exposed for the high shearing during a very short time in order to save energy. This volume, though, has to be large enough for being able to handle e.g. unfavorable conditions in the distribution between the pulp and the chemicals to be mixed.

Today, pulp is often mixed with chemical axially by use of e.g. axial rotors. “Axial mixing” is in this context to be interpreted as that the axis of the rotor is parallel with a flow direction of the pulp. In other words, the mixing is performed during a pulp flow along a rotor axis. Axially mixing machines are often configured such that a rotating axis protrudes into a cavity of flowing pulp and on the rotating axis there is a rotor that by use of the rotation mixes the pulp. There are also mixing machines that uses radially directed slits for accomplish a radial mixing. There are also perpendicularly mixing mixers, where a rotor axis is perpendicular to the pulp flow direction. There are also axial mixers that uses blades or vanes to feed out the pulp through a stator with openings in order to create shearing of the pulp. These vanes are placed at an axis and the mixing occurs mainly radially. There are also axially mixers that mixes radially, which mixers use a slit where a rotating disc mixes the pulp against a stationary disc.

The problem with axially mixing machines and mixing in radial slits is that they are difficult to scale up to larger dimensions. The mixing energy necessary for accomplish the mixing increases dramatically if the dimension of the machine is increased in radial direction. Large machines cost more to produce and the energy consumption, i.e. the operating costs becomes huge. At the same time, the scaling abilities are also limited by process economical and shape-depending reasons. The main reason is that the peripheral speed increases with increasing machine size if a constant rotating speed is maintained. Increases peripheral speed leads to increased centrifugal forces which in turn cause the pulp and the bleaching agents to tend to separate. If it turned around, only a small part of the mixer capacity can be used if the peripheral speed is maintained, i.e. the rotating speed is reduced. For radially mixing mixers, the difference between the inner and outer radius of the mixer means gives an uneven mixing as well as the above mentioned separation.

SUMMARY

A general object is to provide a pulp mixing arrangement and method that can be scaled up without causing separation, inhomogeneous mixing or unreasonable energy consumption. 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 mixer for mixing chemicals into pulp. The mixer comprises a chamber and a rotor. The chamber has an inlet for pulp and chemicals and an outlet for mixed pulp. The inlet for pulp and chemicals is arranged through a first wall of the chamber. The rotor has a rotor drum. The rotor drum is perforated, creating openings, and has a cylindrical shape. The rotor is arranged through a second wall, opposite to the first wall, of the chamber and is arranged for rotating the rotor drum around a rotation axis coinciding with an inflow direction of the pulp and chemicals through the inlet for pulp and chemicals: The rotor is sealed against the chamber to prohibit any flow of material from the inlet for pulp and chemicals to the outlet for mixed pulp except through openings in the rotor drum.

Among the advantage with the proposed technology can be mentioned that the solution is easily scalable and can be used for large productions without demanding enormous energy efforts or that the machine becomes extremely large, at the same time as separation and inhomogeneous mixing is prohibited.

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. 1A schematically illustrates an embodiment of a rotor for use in a mixer for mixing chemicals into pulp;

FIG. IB illustrates a cross-sectional view of an embodiment of a mixer for mixing chemicals into pulp; FIG. 1C illustrates the embodiment of a mixer similar to the one of FIG. IB in an elevation view;

FIG. 2-4 illustrate schematically embodiments of a part of a rotor drum; FIG. 5 illustrates a cross-sectional view of one embodiment of a rotor having a rotor drum;

FIGS. 6-7 illustrates a part of a cross-sectional view of embodiments of a rotor drum perpendicular to the rotational axis;

FIG. 8 illustrates a part of an embodiment of a rotor drum;

FIG. 9A illustrates an embodiment of a mixer in an elevated cross- sectional view;

FIG. 9B illustrates another cross-sectional view of the embodiment of FIG. 9A;

FIG. 10 illustrates another embodiment of a mixer with the stator drum positioned radially inside the rotor drum;

FIG. 1 1 illustrates yet another embodiment of a mixer with two stator drums;

FIG. 12 illustrates yet another embodiment of a mixer, where the rotor is provided with inner protruding portions;

FIG. 13 illustrates another embodiment of a mixer with protruding parts inside the rotor drum

FIG. 14 illustrates yet another embodiment of a mixer, where the rotor is provided with outer protruding portions; and

FIG. 15 illustrates a cross-sectional view of yet an embodiment of a mixer for mixing chemicals into pulp.

DETAILED DESCRIPTION

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

For a better understanding of the proposed technology, it may be useful to begin with a brief overview of mixing conditions in a pulp mixer. In an axial mixing, the pulp and chemicals flow between a rotor and housing. If the agitation of the rotor causes an efficient mixing, the length of the mixing zone can be kept relatively short, and there are no advantages in increasing the mixing zone length. For larger mixers, either the flow velocity of the pulp has to increase or the mixer radius has to be increased. An increase in pulp flow velocity is energy demanding and the conditions in the mixing zone may also deteriorate, which may request an increase of the mixing zone length as well. If the radius of the mixer is increased, the energy requirements to reach the same rotational speed varies approximately as the square of the radius, which means that the required energy increases faster than the radius.

Mixing pulp in an axial mixing at different radial distances can give rise to uneven mixing, since the velocity of any agitating parts varies with the radial distance. In large machines, having a large diameter, there may also be a centrifugal separation behavior, as mentioned in the background, tending to separate pulp and gas in the radial direction. This may also cause an uneven mixing. Such difficulties may to a part be prohibited by limiting the radial extension of the mixing zone. However, mixing at one specific radius reduces the benefit of diameter increase. The increased diameter will in such cases only give rise to approximately linear scalar scaling-up of the cross-sectional area of the mixing zone.

It has been found that mixing in radial direction instead removes, for instance, the separation problems. In this disclosure“radial mixing” is used to specify mixing processes, where the pulp flows in a radial direction with respect to a rotor during the mixing phase. Even if there would be a tendency of separating pulp and gas in radial direction before mixing, a mixing in the same direction will remove such separation tendencies.

Moreover, if mixing is made at essentially one specific radius, the pulp is exposed for homogeneous speed conditions in the entire mixing zone and an improved homogeneous mixing is achieved. Radial mixing is also easily scalable, not only by increasing the diameter, but also by increasing the axial length. Increasing the axial length of the mixing zone will increase the throughput linearly. The scaling in axial length also increases the required energy approximately linearly. Scaling up the diameter, while keeping the peripheral speed, results in a decrease in rotational speed. The increase in energy in such circumstances scales approximately linearly to the increase in diameter.

Scaling up the diameter, while keeping the rotational speed, results in approximately a square energy increase, but at the same time a higher turbulence is achieved at the mixing zone, which probably improves the mixing. The increase in throughput becomes approximately linear.

Therefore, in one embodiment, a mixer for mixing chemicals into pulp comprises a chamber and a rotor. The chamber has an inlet for pulp and chemicals and an outlet for mixed pulp. The inlet for pulp and chemicals is arranged through a first wall of the chamber. The rotor has a rotor drum that is perforated, creating openings, and has a cylindrical shape. The rotor is arranged through a second wall, opposite to the first wall, of the chamber. The rotor is arranged for rotating the rotor drum around a rotation axis coinciding with an inflow direction of the pulp and chemicals through the inlet for pulp and chemicals. The rotor is sealed against the chamber to prohibit any flow of material from the inlet for pulp and chemicals to the outlet for mixed pulp except through openings in the rotor drum.

Figure 1A illustrates schematically an embodiment of a rotor 10 for use in a mixer for mixing chemicals into pulp. The rotor 10 comprises a shaft 12 and a rotor drum 20. The rotor drum 20 has a number of openings 22, in this embodiment in the shape of slits 23. In other words, the rotor drum 20 defines openings 22. The slits 23 are elongated in an axial direction A of the rotor 10. Pulp and chemicals are intended to be introduced into a first open end 24 of the rotor drum 20 with a flow direction parallel to the axial direction A. A second end 26, opposite to the first end 24, of the rotor drum 20 is closed and attached to the shaft 12. This forces the pulp and chemicals to change their flow direction into a mainly radial flow direction, indicated by the reference r. The pulp and chemicals comes into contact with the rotor drum 20 when it tries to escape through the openings 22. Since the rotor drum is intended to rotate in a rotation direction R, this motion will then shear the pulp so that the properties of the pulp becomes as water, becomes turbulent and is mixed with the chemicals. The mixed pulp passes through the openings 22, i.e. in the present embodiment the slits 23, in a radial direction. The rotor drum 20 has in this embodiment a front end surface 28 intended for sealing purposes.

In one embodiment, the rotor drum 20 defines slits 23. The slits 23 have their main extension direction directed non-perpendicular with respect to the rotation axis of the rotor 10.

In one embodiment, the slits are straight slits. However, as is described further below, also other shapes are feasible in alternative embodiments.

In the embodiment of Figure 1A, the slits are directed parallel to the rotation axis S. Also here, there are alternative embodiments presenting other slit directions.

The thickness of the rotor drum 20 will define the length of the openings 22, which in turn to some degree determines the width of the mixing zone. A long mixing zone having changing radii may lead to differing mixing conditions at the beginning and end, respectively, of the mixing zone. On the other hand, a too short mixing zone may instead lead to an incomplete mixing. It has been found that a thickness of the rotor drum that is less than 10% of an inner diameter of the rotor drum gives rise to acceptable small mixing differences. However, preferably, a thickness of the rotor drum is less than 6% of an inner diameter of the rotor drum. Moreover, it is also preferred if the thickness of the rotor drum is larger than 1% of an inner diameter of said rotor drum, to ensure a complete mixing. Figure 1B illustrates a cross-sectional view of an embodiment of a mixer 1 for mixing chemicals into pulp having a similar rotor 10. The mixer 1 comprises a chamber 30. The chamber 30 has an inlet 32 for pulp and chemicals and an outlet 34 for mixed pulp. The inlet 32 for pulp and chemicals is arranged through a first wall 36 of the chamber 30. The rotor 10 has a rotor drum 20 that is perforated and the rotor drum has a general cylindrical shape. The rotor 10 is arranged through a second wall 37, opposite to the first wall 36, of the chamber 30. Pulp and chemicals entering the chamber 30 through the inlet 32 in the axial direction A will flow into the interior of the rotor drum 20 through the first open end 24.

The rotor drum 20 is arranged against the chamber 30 so that openings between the rotor drum 20, typically the front-end surface 28, and the chamber 30 are kept small. This is made in order to counteract flow of material from the inlet 32 for pulp and chemicals to the outlet 34 for mixed pulp except through the openings 22 in the rotor drum 20. Typically, a cross-sectional area of the opening between the front-end surface 28 and the chamber 30 should be less than the total area of the openings 22 in the rotor drum 20, preferably by at least one order of magnitude. The arrangement of the rotor drum 20 arranged against the chamber 30 can be considered as a sealing, partial or complete, prohibiting flow of material from the inlet 32 for pulp and chemicals to the outlet 34 for mixed pulp except through the openings 22 in the rotor drum 20.

In one embodiment, the rotor 10 is sealed against the chamber 30 to prohibit any flow of material from the inlet 32 for pulp and chemicals to the outlet 34 for mixed pulp except through openings 22 in the rotor drum 20.

Preferably, an inner radius of the rotor drum at the end facing the inlet for pulp and chemicals is equal to or larger than a radius of the inlet for pulp and chemicals. This ensures a smooth entrance into the rotor drum. Due to the closed second wall 37, the pulp and chemical is, when entered into the rotor drum, changing flow direction into a radially directed flow. The rotor 10 is arranged for rotating the rotor drum 20 around the rotation axis S, which coinciding with an inflow direction of the pulp and chemicals through the inlet 32 for pulp and chemicals. The rotor 10 is sealed against the chamber 30, in this embodiment by the front end surface 28 of the rotor drum 20 and a chamber sealing surface 38. Such a sealing prohibits any flow of material from the inlet 32 for pulp and chemicals to the outlet 34 for mixed pulp except through the openings 22 in the rotor drum 20. A mixing of the pulp occurs in a radial direction r when it passes the openings 22, and the mixed pulp exits the chamber 30 through the outlet 34, in this embodiment in the radial direction r.

In the present embodiment, the rotor drum 20 has a constant inner radius. This ensures that the mixing conditions are as homogeneous as possible for all pulp passing the mixer 1.

In the present embodiment, the outlet 34 for mixed pulp is arranged in a direction transverse to the inflow direction of the pulp and chemicals. However, in alternative embodiments, the output from the chamber 30 may also be provided parallel to the inflow direction.

Figure 1C illustrates the embodiment of a mixer 1 similar to the one of Figure IB in an elevation view.

The mixer thus comprises a rotor body in shape of a rotor drum that mixes in radial direction. In the embodiment above, the rotor drum has slits where the pulp can pass through the rotor drum that rotates with a relatively high velocity. The rotation velocity will then shear the pulp so that the properties of the pulp becomes as water, becomes turbulent and is mixed with the chemicals. The rotor drum is hollow to receive the pulp axially and arranged to change the direction of the pulp to be radially mixed. Since the rotor drum is symmetric, the mixing will be performed around the entire rotor drum. Since the pulp and the gas or liquid have to be transported through the rotor drum openings, all pulp suspension will be exposed for mixing. Since the mixer mixes radially, there will be an increase in pressure due to the addition of energy that will rotate the pulp suspension. This rotation will naturally cause a static pressure increase.

One advantage with the proposed technology is that the mixing zones will maintain a symmetric mixing energy effort. The solution is easily scalable and can be used for large productions without demanding enormous energy efforts or that the machine becomes extremely large. By extending the rotor body, the time in the mixing zones is influenced. The pressure drop through the mixer is reduced since a part of the energy is used for creating an increase of potential by rotation.

Since the drum is hollow, the pulp comes from the inside and passes outwards. By mixing in radially increasing direction, a natural separation cannot occur since the pulp and gas are forced to pass the mixing zone for mixing. If a difference in inner radius of the drum and outer radius of the drum is small, the difference in speed becomes small. At a small difference in speed, about the same mixing intensities will be present around the entire drum. If the mixing intensity can be kept on an even level over the fluidizing point, the mixer will use low amounts of energy.

The openings in the rotor drum can be designed in many different ways. Figure 2 illustrates schematically a part of a rotor drum 20 having openings 22 in the shape of curved slits. Note that, in order to facilitate the understanding of the figures, the drawing is made in the plane of the rotor drum surface, i.e. the depicted plane illustration is in reality a part of a cylindrical surface. The rotor drum 20 is rotated in the direction of the arrow R. The curved shape will tend to move the pulp somewhat towards the middle, which may be advantageous if the pulp tend to get stuck at the ends of the rotor drum 20.

In different embodiments at least a part of the slits is directed in a direction that is non-parallel to the rotation axis of the rotor drum. This is the condition in Figure 2. Another embodiment of such slits is illustrated schematically in Figure 3. Also here the drawing is made in the plane of the rotor drum surface. The slits 23 are here directed in an angle with respect to the rotor drum 20 rotation axis. The slits are also of a non-constant width. In this embodiment, the width is increased in the inner part of the rotor drum 20, closest to the second end 26. This design may take care of tendencies to build up congestions of pulp in the inner part. However, in alternative embodiments, the width may instead be decreased in the inner part of the rotor drum 20.

In Figure 4, two types of slits 23 are provided. Also here the drawing is made in the plane of the rotor drum surface. A first type of slits covers essentially the full length of the rotor drum 20, whereas shorter slits are provided there between. Such a design increases the dynamic action of the rotor drum 20, thereby avoiding static flow paths through the mixer.

Figure 5 illustrates a cross-sectional view of one embodiment of a rotor 10 having a rotor drum 20 comprising an inner disc as the second end 26 and an annular part (not shown) as the first end. The first and second end 26 are connected by a number of rods 25 extending along the cylindrical shape of rotor drum 20. The openings 22 in the shape of slits 23 are defined by the rods 25. This also leads to that the openings 22 of the rotor drum 20 have different cross-sections at different radial distances.

Similarly, differing cross-sections at different radial distances can be achieved by other means. Referring back to Fig. 1A, it can for instance be noticed that the slits 23 in that embodiment are slightly cone-shaped. However, in alternative embodiments, the slits may be designed to be straight.

Figure 6 illustrates a part of a cross-sectional view of an embodiment of a rotor drum perpendicular to the rotational axis. In this embodiment, the increasing cross-section in the direction of increasing radial distance is enhanced. The additional tilting of the sides of the slits 23 also results in that the surfaces 19 defining the openings 22 of the rotor drum 20 are inclined in relation to the radial direction r. Such changing cross-section and/or inclined opening surfaces 19 may influence the pressure drop over the openings 22.

Figure 7 illustrates a part of a cross-sectional view of an embodiment of a rotor drum perpendicular to the rotational axis. In this embodiment, the cross- section in the direction of increasing radial distance is constant. However, the tilting of the surfaces 19 of the slits 23 results in that the surfaces 19 defining the openings 22 of the rotor drum 20 are inclined in relation to the radial direction r.

The openings of the rotor drum may also be designed in many other ways. Figure 8 illustrates a part of a rotor drum, where the openings 22 are provided in the shape of holes 17.

Also the rotor drum shape can be varied. In the embodiments shown above, the radius of the rotor drum has been constant along the entire axial extension of the rotor drum. However, in alternative embodiments, rotor drums with varying radius may also be used, e.g. rotor drums in the shape of a frustum of a cone.

The motion causing the pulp to become water-like is provided by the rotor drum. However, in order to ensure a high shear action on the pulp suspension, it might in certain applications be advantageous to provide static portions of the mixer in close proximity to the rotor drum. Figure 9A illustrates such an embodiment in an elevated cross-sectional view. Besides the rotor drum 20, a stator drum 40 is arranged concentrically with the rotor drum 20. The stator drum 40 is also perforated. In the present embodiment, the stator drum 40 is positioned radially outside the rotor drum 20. The openings in the stator drum 40 can be of any kind. They can be of the same type as in the rotor drum 20 or different therefrom.

Figure 9B is another cross-sectional view of the embodiment of Figure 9A. Here, it can be seen that the stator drum 40 and rotor drum 20 are concentric. In this particular embodiment, both the stator drum 40 and the rotor drum 20 present straight slits parallel to the rotational axis S of the rotor. In this particular embodiment, the stator drum 40 has more slits than the rotor drum 20, and which stator drum slits are somewhat broader than the rotor drum slits. However, in other embodiments, other relations can be employed.

Figure 10 illustrates another embodiment of a mixer 1. In this embodiment, the stator drum 40 is positioned radially inside the rotor drum 20.

Figure 1 1 illustrates yet another embodiment of a mixer. In this embodiment, there are two stator drums 40. The stator drums 40 are arranged concentrically with the rotor drum 20. The stator drums 40 are as before perforated. One of the two stator drums 40 is positioned radially outside the rotor drum 20 and the other one of the two stator drums 40 is positioned radially inside the rotor drum 20.

The rotor may further be provided with inner protruding portions, protruding into a volume inside the rotor drum. Figure 12 illustrates one such embodiment, where the inner protruding portions 42 protrude inwards from an inner surface 41 of the rotor drum 20. The shape, direction and position in circumferential and axial directions of the inner protruding portions 42 may be adapted according to different applications. The provision of the inner protruding portions 42 may improve e.g. pulp flow, angular distribution of pulp flow and/or pre-mixing of chemicals into the pulp.

Figure 13 illustrates another embodiment with protruding parts inside the rotor drum 20. Here, the inner protruding portions 42 protrude outwards towards an inner surface 41 of the rotor drum 20.

Supporting protruding parts may also be provided outside the rotor drum. In Figure 14, the rotor 10 further comprises outer protruding portions 44, protruding into a volume 46 outside the rotor drum. In this embodiment, the outer protruding portions 44 are attached to an outer surface 45 of the rotor drum 20. By these outer protruding portions 44, flow properties of the pulp outside the rotor drum can be influenced.

Figure 15 illustrates a schematic cross-sectional view of an embodiment of a mixer 1 for mixing chemicals into pulp. In this embodiment, the inner protruding portions 42 comprise a rotationally symmetric flow directing structure 29 provided at the rotational axis S. In such a mixer 1, the incoming pulp and chemical substances, travelling substantially in the axial direction A will be deviated by the flow directing structure 29 to obtain at least a velocity component in the radial direction r. In the illustrated embodiment, the flow directing structure 29 is illustrated as a cone. However, in alternative embodiments also other designs with rotationally symmetric bodies having a successively increasing diameter along the axial direction can be used. The chemicals that are intended to be mixed with the pulp can be of essentially any kind. The design is primarily intended for liquid or gas chemicals, but also powder or fine granular solid chemicals may be used.

In one embodiment, the chemicals comprise bleaching agents.

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

1. A mixer ( 1) for mixing chemicals into pulp, comprising:

- a chamber (30) with an inlet (32) for pulp and chemicals and an outlet (34) for mixed pulp;

said inlet (32) for pulp and chemicals being arranged through a first wall (36) of said chamber (30);

- a rotor ( 10) having a rotor drum (20);

said rotor drum (20) being perforated, creating openings (22), and having a cylindrical shape;

said rotor ( 10) being arranged through a second wall (37), opposite to said first wall (36), of said chamber (30) and arranged for rotating said rotor drum (20) around a rotation axis (S) coinciding with an inflow direction (A) of said pulp and chemicals through said inlet (32) for pulp and chemicals;

said rotor (10) being arranged against said chamber (30) to counteract flow of material from said inlet (32) for pulp and chemicals to said outlet (34) for mixed pulp except through openings (22) in said rotor drum (20).

2. The mixer according to claim 1, characterized in that a cross- sectional area of an opening between a front end surface (28) of said rotor and said chamber (30) is less than a total area of said openings (22) in said rotor drum (20).

3. The mixer according to claim 1 or 2, characterized in that said outlet (34) for mixed pulp being arranged in a direction transverse to said inflow direction (A) of said pulp and chemicals.

4. The mixer according to any of the claims 1 to 3 , characterized in that an inner radius of said rotor drum (20) at the end facing said inlet (32) for pulp and chemicals is equal to or larger than a radius of said inlet (32) for pulp and chemicals.

5. The mixer according to claim 4, characterized in that said rotor drum (20) has a constant inner radius.

6. The mixer according to any of the claims 1 to 5, characterized in that said openings (22) of said rotor drum (20) are defined as slits (23) having their main extension direction directed non-perpendicular with respect to said rotation axis (S).

7. The mixer according to claim 6, characterized in that said slits (23) are straight slits.

8. The mixer according to claim 7, characterized in that said slits (23) are directed parallel to said rotation axis (S).

9. The mixer according to claim 6, characterized in that said slits (23) are curved slits.

10. The mixer according to any of the claims 6 or 7, characterized in that at least a part of said slits (23) are directed in direction that is non-parallel to said rotation axis (S).

1 1. The mixer according to any of the claims 6 to 10, characterized in that said slits (23) are defined by rods (25) extending along said cylindrical shape of rotor drum (20).

12. The mixer according to any of the claims 1 to 1 1, characterized in that said openings (22) of said rotor drum (20) have different cross-sections at different radial distances.

13. The mixer according to any of the claims 1 to 12, characterized in that surfaces (19) defining perforations of said rotor drum (20) are inclined in relation to a radial direction.

14. The mixer according to any of the claims 1 to 13, characterized by a stator drum (40), arranged concentrically with said rotor drum (20), said stator drum (40) being perforated.

15. The mixer according to claim 14, characterized in that said stator drum (40) is positioned radially outside said rotor drum (20) .

16. The mixer according to claim 14, characterized in that said stator drum (40) is positioned radially inside said rotor drum (20).

17. The mixer according to any of the claims 1 to 13, characterized by two stator drums (40), arranged concentrically with said rotor drum (20), said stator drums (40) being perforated, whereby one of said two stator drums (40) is positioned radially outside said rotor drum (20) and the other one of said two stator drums (40) is positioned radially inside said rotor drum (20).

18. The mixer according to any of the claims 1 to 17, characterized in that a thickness of said rotor drum (20) is less than 10% of an inner diameter of said rotor drum (20), preferably less than 6% of an inner diameter of said rotor drum (20).

19. The mixer according to any of the claims 1 to 18, characterized in that a thickness of said rotor drum (20) is larger than 1% of an inner diameter of said rotor drum (20) .

20. The mixer according to any of the claims 1 to 19, characterized in that said rotor (10) further comprises inner protruding portions (42), protruding into a volume inside said rotor drum (20).

21. The mixer according to claim 20, characterized in that said inner protruding portions (42) protrude inwards from an inner surface (41) of said rotor drum (20).

22. The mixer according to claim 20, characterized in that said inner protruding portions (42) protrude outwards towards an inner surface (41) of said rotor drum (20) .

23. The mixer according to claim 20, characterized in that said inner protruding portions (42) comprises a rotationally symmetric flow directing structure provided at said rotational axis (S).

24. The mixer according to any of the claims 1 to 23, characterized in that said rotor (10) further comprises outer protruding portions (44), protruding into a volume (46) outside said rotor drum (20).

25. The mixer according to any of the claims 1 to 24, characterized in that said chemicals comprises bleaching agents.