WO2017209678A1

WO2017209678A1 - Mixer unit for use in a mixing apparatus and a mixing apparatus comprising such a mixing unit

Info

Publication number: WO2017209678A1
Application number: PCT/SE2017/050546
Authority: WO
Inventors: Leif ÖDMARK; Mikael Löfgren; Niklas Eriksson
Original assignee: Valmet Oy; Valmet AB
Current assignee: Valmet Technologies Oy; Valmet AB
Priority date: 2016-05-31
Filing date: 2017-05-23
Publication date: 2017-12-07
Anticipated expiration: 2018-11-30
Also published as: SE539789C2; SE1650756A1

Abstract

A mixing unit (1) for use in a mixing apparatus (2) for mixing treatment media in liquid or gaseous state with a fiber suspension, which apparatus comprises a housing defining a mixing chamber(3), at least one inlet (4a, 4b) for feeding a fiber suspension and treatment media into the mixing chamber, a drive shaft (5) connectable to a drive device for rotation of the drive shaft in operation, and an outlet (6)for discharging the mixture of fiber suspension and treatment media. The mixing unit is adapted to be arranged in the mixing chamber (3) and comprises at least one rotor(7), each rotor being adapted to be connected to the drive shaft to rotate with the drive shaft in a direction of rotation, each rotor comprising at least one radially extending rotor arm(8a-c). A trailing surface (16a) of the at least one rotor arm is at least partially provided with a plurality of recesses(17a1-a3).

Description

MIXING UNIT FOR USE IN A MIXING APPARATUS AND A MIXING

APPARATUS COMPRISING SUCH A MIXING UNIT

TECHNICAL FIELD

The invention relates to a mixing unit for use in a mixing apparatus for mixing treatment media into a fiber suspension, e.g. a lignocellulosic pulp suspension. The invention further relates to such a mixing apparatus.

BACKGROUND

Throughout the fiberline, i.e. the different process steps involved when converting wood chips or other fibrous raw material into pulp, there are several positions where mixing apparatuses are used to mix different kind of media into the pulp suspension. The treatment media added into the fiber suspension may for example be for heating, delignification or bleaching purposes. The treatment media may be in liquid or gaseous state.

When mixing treatment media into a fiber suspension, the issue of even and homogeneous distribution is of high importance for the mixing result achieved.

WO2013/089615 discloses a mixing unit and a mixing apparatus comprising at least two rotor bodies arranged on a rotor shaft in an axially separated manner with a stator body placed between the rotor bodies. Although this mixer achieves high mixing performance, there is a need for an improved mixer with further improved mixing performance.

SUMMARY

An object of the invention is to provide an improved unit for mixing for use in a mixing apparatus. A second object is to provide an improved mixing apparatus.

These and other objects are achieved by the present invention by means of a mixing unit and a mixing apparatus according to the independent claims. Preferred embodiments are defined in the dependent claims. According to a first aspect of the invention, there is provided a mixing unit for use in a mixing apparatus for mixing treatment media in liquid or gaseous state with a fiber suspension, which apparatus comprises a housing defining a mixing chamber, at least one inlet for feeding a fiber suspension and treatment media into the mixing chamber, a drive shaft connectable to a drive device for rotation of the drive shaft in operation, and an outlet for discharging the mixture of fiber suspension and treatment media. The mixing unit is adapted to be arranged in the mixing chamber and comprises at least one rotor, each rotor being adapted to be connected to the drive shaft to rotate with the drive shaft in a direction of rotation, each rotor comprising at least one radially extending rotor arm. A trailing surface of the at least one rotor arm is at least partially provided with a plurality of recesses.

In other words, each rotor is connectable to the drive shaft by means of for example a rotor shaft forming a part of said rotor, and each rotor comprises one or more rotor arms or bars which extend radially from the rotor shaft. The rotor arms or bars may be described as elongated since they may have a length which is substantially longer than their width. The one or more rotor arms do not necessarily extend only in a radial direction, but may in embodiments also extend in the tangential direction such that the rotor arm(s) or the leading surface(s) thereof, either as a whole or at least a portion thereof, is angled or curved in the direction of rotation (as seen from the center of the rotor). Put differently, the leading surface, either as a whole or portion(s) thereof, may be arranged at positive angle relative a thought radial line and in the direction of rotation. It is understood that the thought radial line refers to an imaginary line which extends straight in the radial direction from the center of the rotor to its circumference. The one or more rotor arms may be angled or curved in the direction of rotation such that a radially outer end of the leading surface is positioned ahead of an inner end of the leading surface in the rotational direction. The trailing surface of the at least one rotor arm may alternatively be described as being provided with a plurality of recesses along at least a portion of its radial extension or along at least a portion of its length. The recesses may, but do not necessarily need to, extend through the whole axial width or thickness of the at least one rotor arm.

In an advantageous embodiment, the mixing unit further comprises at least one stator, each stator being arranged axially adjacent at least one rotor. Each stator comprises at least one radially extending stator arm. A rear surface of the at least one stator arm may be at least partially provided with a plurality of recesses. The stator arms or bars may be described as elongated since they may have a length which is substantially longer than their width. The stator arms do not necessarily extend only in a radial direction, but may in embodiments also extend in the tangential direction such that the stator arm(s) or the front surface(s) thereof, either as a whole or at least a portion thereof, is angled or curved against or opposite the direction of rotation (as seen from the center of the stator). Put differently, the front surface, either as a whole or portion(s) thereof, may be disposed at a positive angle relative a thought radial line and against the direction of rotation. It is understood that the thought radial line refers to an imaginary line which extends straight in the radial direction from the center of the stator to its circumference. The one or more stator arms may be angled or curved in the direction of rotation such that a radially inner end of the front surface is positioned ahead of an outer end of the front surface in the rotational direction. In other embodiments, the stator arms or at least a portion thereof is angled or curved in the direction of rotation. The rear surface of the at least one stator arm may alternatively be described as being provided with a plurality of recesses along at least a portion of its radial extension or along at least a portion of its length. The recesses may, but do not necessarily need to, extend through the whole axial width or thickness of the at least one stator arm.

The above described embodiments, having rotor arm(s) angled or curved in the direction of rotation and/or stator arm(s) angled or curved opposite the direction of rotation, are advantageous since separation between the fiber suspension and gas/liquid is reduced or prevented since the fiber suspension is pressed radially inwardly.

The invention is based on the insight that mixing performance is degraded by the formation of gas bubbles formed at the trailing edge of the rotor arms and/or at the rear edge of the stator arms. The invention is further based on the insight that mixing performance may be improved by reducing the size of the formed gas bubbles, and further that such a reduction in size may be achieved by providing the trailing edge of the rotor arms and/or the rear edge of the stator arms with recesses. In embodiments, the trailing surface is provided with recesses distributed along substantially its whole length or radial extension. In embodiments, the rear surface is provided with recesses distributed along substantially its whole length or radial extension. In embodiments, the recesses are formed as serrations, for example having a toothed profile.

In embodiments, the recesses are formed as grooves extending along the axial width or thickness of the rotor or stator. Alternatively, the grooves extend only along parts of the axial width or thickness of the rotor or stator. The grooves may be axial, i.e. extend straight grooves extending in the axial direction.

It is understood that the axis of rotation is defined by the rotation of the rotor, which rotation may be caused by a connection to a drive shaft of a mixing apparatus. It is furthermore understood that leading surface and trailing surface refer to the surfaces of the rotor which face the direction of rotation and opposite the direction of rotation, respectively. It is also understood that front surface and rear surface refer to the surfaces or edges of the stator which face opposite the direction of rotation and in the direction of rotation, respectively. In a mixing apparatus of the present type, no axial transport is usually desired. It is therefore understood that the leading and front surfaces are normally not curved or angled along their axial extension.

In embodiments, at least one stator arm and at least one rotor arm are mutually configured such that at least a portion of a leading surface of said rotor arm passes a front surface of the stator arm at an angle during rotation of said rotor.

In embodiments, one of the leading or front surfaces is provided with at least one concave portion, while the other of the surfaces may be plane or flat. In

embodiments, both the leading surface and the front surface are provided with at least one concave or strictly concave portion, thereby ensuring an even greater engagement length between the rotor and stator arm. The at least one concave portion may be strictly concave. The at least one concave portion may define a curved portion having a radius. The at least one concave portion may be formed by linear segments or a combination of linear and curved segments. The leading and/or front surface may be provided with a single concave or strictly concave portion extending along essentially the entire length of the respective arm.

In embodiments, the at least one rotor arm is provided with a trailing surface facing opposite the direction of rotation and side surfaces connecting the leading and trailing surfaces, wherein at least one of said side surfaces is provided with at least one groove extending between the leading and trailing surfaces. This embodiment is advantageous since the overall shear surface, and thus the size of the mixing zone, is increased without an increase in the projected area in the rotational direction, thus without an increase in inputted energy. The at least one rotor arm may be provided with a plurality of grooves which are distributed along the radial extension of the arm. The at least one groove or the plurality of grooves may be tangential.

In embodiments, the at least one stator arm is provided with a rear surface facing the direction of rotation and side surfaces connecting said front and rear surfaces, wherein at least one of these side surfaces is provided with at least one groove extending between the front and rear surfaces. This embodiment is advantageous since the overall shear surface, and thus the size of the mixing zone, is increased without an increase in the projected area in the rotational direction, thus without an increase in inputted energy. The at least one stator arm may be provided with a plurality of grooves which are distributed along the radial extension of the arm. The at least one groove or the plurality of grooves may be tangential. In an advantageous embodiment, both the rotor and stator are provided with side surfaces with grooves, resulting in an even greater increased mixing performance.

In embodiments, the rotor arms of the rotor extend radially outwardly from a radially innermost center portion or rotor shaft. The stator arms of the stator may extend radially inwardly from an outer peripheral ring portion, preferably to a radial position in close vicinity of the center portion or rotor shaft. In such an embodiment, the rotor arms preferably extend to a radial position in close vicinity of the outer peripheral ring portion. The stator and rotor arms advantageously extend in this manner to ensure a high mixing efficiency by providing the longest possible radial overlap between the leading and front surfaces. Alternatively, the rotor arms may extend radially inwardly, and the stator arms extend may radially outwardly. In embodiments, at least one of rotor comprises flow-restraining means in the form of at least one concentric ring portion interconnecting said rotor arms.

In embodiments, the tangential width of the stator and/or rotor arms decreases along their length, i.e. with the distance from the axis of rotation, i.e. the arms have a greater width at a radial inner end than at an outer radial end.

The rotor and stator arms usually extend in respective planes being perpendicular to the axis of rotation, but may in embodiments be angled in the axial direction. In such embodiments, the above described side surfaces are formed at an angle relative these perpendicular planes.

According to a second aspect of the invention there is provided a mixing unit there is provided a mixing unit for use in a mixing apparatus for mixing treatment media in liquid or gaseous state with a fiber suspension, which apparatus comprises a housing defining a mixing chamber, at least one inlet for feeding a fiber suspension and treatment media into the mixing chamber, a drive shaft connectable to a drive device for rotation of the drive shaft in operation, and an outlet for discharging the mixture of fiber suspension and treatment media. The mixing unit is adapted to be arranged in the mixing chamber and comprises at least one rotor. Each rotor is adapted to be connected to the drive shaft to rotate with the drive shaft in a direction of rotation, and each rotor comprises at least one radially extending rotor arm. The rotor arm is provided with a leading surface facing in the direction of rotation and a trailing surface facing opposite the direction of rotation. Side surfaces connect the leading and trailing surfaces, wherein at least one of said side surfaces is provided with at least one groove extending between the leading and trailing surfaces. The rotor arm may be provided with a plurality of grooves which are distributed along the radial extension of the arm. The at least one groove or the plurality of grooves may be tangential.

The grooves provide an increase in the overall shear surface, and thus the size of the mixing zone is increased, thus providing improved mixing performance. This effect is achieved without an increase in the projected area in the rotational direction, thus without an increase in inputted energy.

In embodiments of the mixing unit according to the second aspect of the invention, the mixing unit further comprises at least one stator arranged axially adjacent at least one rotor, wherein each stator comprises at least one radially extending stator arm provided with a rear surface facing in the direction of rotation and a front surface facing opposite the direction of rotation. Side surfaces connect the front and rear surfaces, wherein at least one of said side surfaces is provided with at least one groove extending between the front and rear surfaces. The at least one stator arm may be provided with a plurality of grooves which are distributed along the radial extension of the arm. The at least one groove or the plurality of grooves may be tangential.

In an advantageous embodiment of the mixing unit according to the second aspect of the invention, both the rotor and stator are provided with side surfaces with grooves, resulting in an even greater increased mixing performance.

According to a third aspect of the invention, there is provided a mixing apparatus for mixing treatment media in liquid or gaseous state with or into a fiber suspension. The apparatus comprises a housing defining a mixing chamber, at least one inlet for feeding a fiber suspension and treatment media into the mixing chamber, a drive shaft connectable to a drive device for rotation of the drive shaft in operation, an outlet for discharging the mixture of fiber suspension and treatment media, and a mixing unit according to the first or second aspect of the invention or embodiments thereof.

The features of the embodiments described above are combinable in any practically realizable way to form embodiments having combinations of these features. Further, all features and advantages of embodiments described in connection with the mixing unit according to the first or second aspects of the invention may be applied in corresponding embodiments of the mixing apparatus according to the third aspect of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be described in more detail with reference to the appended drawings, which show presently preferred

embodiments of the invention, wherein:

fig. 1 shows a schematic illustration in an axial view of a prior art mixing unit having straight radially extending rotor arms;

fig. 2 shows a schematic illustration in an axial view of a mixing unit according to an embodiment of the first or second aspects of the invention, which mixing unit is provided with straight radially extending rotor arms provided with recesses at their trailing surfaces;

fig. 3 shows a schematic illustration in an axial view of a mixing unit according to another embodiment of the first or second aspects of the invention, which mixing unit is provided with forwardly curved rotor arms provided with recesses at their trailing surfaces;

fig. 4 shows a schematic illustration in an axial view of a mixing unit according to another embodiment of the first aspect of the invention, which mixing unit is provided with curved rotor and stator arms, where the rotor arms are provided with recesses at their trailing surfaces;

fig. 5 shows a schematic illustration in an axial view of a mixing unit according to another embodiment of the first aspect of the invention, which mixing unit is provided with curved rotor arms provided with recesses at their trailing surfaces and curved stator arms provided with recesses at their rear surfaces;

fig. 6 shows a schematic illustration in an axial view of a mixing unit according to another embodiment of the first or second aspects of invention, which mixing unit is provided with curved rotor and stator arms provided with grooves on their side surfaces;

fig. 7 shows a perspective view of the rotor of the mixing unit shown in fig. 6; and

fig. 8 shows a schematic illustration in a cross sectional view of a mixing apparatus according to an embodiment of the third aspect of the invention

DETAILED DESCRIPTION

Figure 1 shows a schematic illustration in an axial view of a prior art mixing unit. The mixing unit comprises a rotor 507 having a center portion connectable to for example a drive shaft for rotation of the rotor in a direction of rotation R, and straight rotor arms or bars 508a-c extending radially outwardly from the center portion, and two concentric ring connecting the rotor arms. The rotor arms are distributed with even angular spacing there between. When the treatment media which is to be mixed into the fiber suspension is in gaseous form, large gas wakes are formed at the trailing surfaces of the rotor arms during rotation thereof, as is illustrated in the right part of figure 1 . When the gas wake gets sufficiently large, it is separated from the trailing surface as a large gas bubble. This results in degraded mixing performance.

Figure 2 shows a schematic illustration in an axial view of a mixing unit according to an embodiment of the first aspect of the invention. The mixing unit is adapted to be arranged in a mixing chamber of a mixing apparatus such as the mixer shown in figure 7. The mixing unit is similar to that shown in figure 1 , but the rotor arms have a different configuration. The trailing surface 16a of each rotor arm is provided with recesses 17a1 -a3 forming a serrated surface portion. The recesses causes a plurality of smaller gas wakes to be formed, as illustrated in the right part of figure 2, thus causing much smaller gas bubbles to be released therefrom. The rotor arms 8a-c are provided with a leading surface facing in the direction of rotation and side surfaces (for example 14a) connecting the leading and trailing surfaces. The side surfaces are each provided with three tangential grooves (for example 15a1 -a3) extending between the leading and trailing surfaces. In other embodiments, more tangential grooves may be provided. The tangential grooves 15a1 -a3 have a width

corresponding to the width of the recesses 17a1 -a3. The rotor arms are of uniform thickness and may thus also be described as rotor bars.

Figure 3 shows a schematic illustration in an axial view of a mixing unit according to another embodiment of the first aspect of the invention, which embodiment corresponds to the embodiment in figure 2 except in that the rotor arms 108a-c are curved forwardly, i.e. in the direction of rotation, and in that four recesses and grooves are provided instead of three. The leading surfaces (for example 1 1 1 a) of the rotor arms are provided with a concave portion in the form of a curved portion 1 13a having a radius. The axis defining the radius is perpendicular to the axis of rotation. The leading surface may also be described as being disposed at positive angle in the direction of rotation relative a thought radial line. In other embodiments, the concave portion may be formed by linear segments or a combination of linear and curved segments. The curved portion extends along the whole length of the rotor arms. In other embodiments, there may be two or more curved portions on the leading edge. In yet other embodiments, the rotor arms, or at least portions thereof, may be angled in the direction of rotation rather than being curved.

Figure 4 shows a schematic illustration in an axial view of a mixing unit according to another embodiment of the first aspect of the invention. The mixing unit is provided with a rotor 207 and a stator 209. The rotor has rotor arms 208a-f which are curved forwardly in the direction of rotation. The leading surfaces (for example 21 1 a) of the rotor arms are provided with two concave portions in the form of radially separated curved portions (for example 213a1 , 213a2) having a radius, where the inner curved portion 213a2 is provided radially inwardly of a concentric ring which connects the rotor arms, and the outer curved portion 213a1 is provided radially outside the concentric ring. The axes defining the radiuses are perpendicular to the axis of rotation. The trailing surfaces (for example 216a) of the rotor arms are provided with recesses 217a1 -a3 forming a serrated surface portion radially outside the concentric ring. The recesses are formed as axially extending grooves extending along the axial thickness of the rotor. The stator 209 is provided with stator arms 210a-g which are curved backwards, i.e. against or opposite the direction of rotation, i.e. the stator arms extend radially outwards being curved against the direction of rotation. The front surfaces (for example 212a) of the stator arms are provided with a concave portion in the form of a curved portion (for example 214a) having a radius. The axis defining the radius is perpendicular to the axis of rotation. The front surface may also be described as being disposed at positive angle and against the direction of rotation relative a thought radial line. The curved portion extends along the whole length of the stator arms.

Figure 5 shows a schematic illustration in an axial view of a mixing unit according to another embodiment of the first aspect of invention. The mixing unit is provided with a rotor 307 and a stator 309. The rotor has rotor arms 308a which are curved forwardly in the direction of rotation in the same manner as in figure 4. The leading surfaces (for example 31 1 a) of the rotor arms are provided with a concave portion in the form of a curved portion (for example 313a) having a radius. The axis defining the radius is perpendicular to the axis of rotation. The trailing surfaces (for example 316a) of the rotor arms are provided with recesses 317a1 -a6 forming a serrated surface portion which extends along most of the length of the respective rotor arm. The recesses are formed as axially extending grooves extending along the axial thickness of the rotor. The stator 309 is provided with stator arms 310a which are curved backwardly , i.e. against or opposite the direction of rotation in the same manner as in figure 4. The rear surfaces (for example 318a) of the stator arms are provided with recesses 319a1 -a5 forming a serrated surface portion which extends along most of the length of the respective stator arm. The recesses are formed as axially extending grooves extending along the axial thickness of the rotor. The front surfaces of the stator arms are provided with a concave portion in the same manner as in figure 4.

Figure 6 shows a schematic illustration in an axial view of a mixing unit according to another embodiment of the first or second aspects of invention, which mixing unit is provided with curved rotor and stator arms. This embodiment is identical to the embodiment shown in figure 5 except that side surfaces of each rotor and stator arm are provided with tangential grooves. The side surfaces (for example 414a) which connect the leading and trailing edges of the rotor arms (for example 408a) are each provided with three tangential grooves (for example 415a1 -a3) extending between the leading and trailing surfaces. Each of the stator arms, such as 410a, are provided with tangential grooves in the same manner. The tangential grooves have a width corresponding to the width of the recesses.

Figure 7 shows a perspective view of the rotor of the mixing unit shown in fig. 6. In this figure it is shown that the rotor 407 has a certain axial thickness, and that the leading and trailing surfaces (for example 41 1 a) are curved or angled only in the direction of rotation. Thus the normal direction of these surfaces have no axial component. Furthermore, the tangential grooves (for example 415a1 -a3) are shown.

Figure 8 shows a schematic illustration in a cross sectional view of a mixing apparatus 2 according to an embodiment of the third aspect of the invention. The apparatus comprises a housing defining a mixing chamber 3. A first inlet 4a for the fiber suspension is provided as well as a second inlet 4b, 4b1 , 4b2 for treatment media. A drive shaft 5 is provided for connection to a drive device for rotation of the drive shaft in operation. The mixture of fiber suspension and treatment media is discharged through the outlet 1 . A mixing unit 1 is arranged in the mixing chamber 3. The mixing unit comprises two rotors 7 of the same type shown in figure 2 and one stator 9. In other embodiments, other types of rotors and stators may however be used. The rotors are connected to the drive or rotor shaft 5 and are axially spaced apart from each other with the stator arranged there between. The stator is connected to the housing of the apparatus.

The description above and the appended drawings are to be considered as non- limiting examples of the invention. The person skilled in the art realizes that several changes and modifications may be made within the scope of the invention. For example, the number of rotors and stators of each mixing unit may be varied.

Furthermore, the number of rotor and stator arms on each rotor and stator may be varied. Furthermore, the absolute and relative dimensions and positions of the recesses and grooves may be varied. The scope of protection is determined by the appended patent claims.

Claims

1 . A mixing unit (1 ) for use in a mixing apparatus (2) for mixing treatment media in liquid or gaseous state with a fiber suspension, which apparatus comprises a housing defining a mixing chamber (3), at least one inlet (4a, 4b) for feeding a fiber suspension and treatment media into the mixing chamber, a drive shaft (5) connectable to a drive device for rotation of the drive shaft in operation, and an outlet (6) for discharging the mixture of fiber suspension and treatment media, said mixing unit being adapted to be arranged in said mixing chamber (3) and comprising:

- at least one rotor (7; 107; 207; 307; 407), each rotor being adapted to be connected to the drive shaft to rotate with said drive shaft in a direction of rotation, each rotor comprising at least one radially extending rotor arm (8a-c; 208a-f; 308a; 408a);

wherein a trailing surface (16a; 216a; 316a) of at least one rotor arm is at least partially provided with a plurality of recesses (17a1 -a3; 217a1 -a3; 317a1 -a6), and wherein said at least one rotor arm (208a; 308a) is angled or curved in the direction of rotation.

2. Mixing unit according to claim 1 , wherein said trailing surface (316a) is

provided with recesses (317a1 -a6) distributed along substantially its whole length in a radial direction.

3. Mixing unit according to claim 1 or 2, further comprising at least one stator (309; 409a), each stator being arranged axially adjacent at least one rotor (307; 407), said stator comprising at least one radially extending stator arm (310a; 410a), wherein a rear surface (316a) of at least one stator arm is at least partially provided with a plurality of recesses (317a-a6).

4. Mixing unit according to claim 3, wherein said rear surface (318a) is provided with recesses (319a1 a5) distributed along substantially its whole length.

5. Mixing unit according to any of the preceding claims, wherein said recesses are formed as serrations (217a1 -a3; 317a1 -a6; 319a1 -a5).

6. Mixing unit according to any of the preceding claims, wherein said recesses are formed as grooves (217a1 -a3; 317a1 -a6; 319a1 -a5) extending along the axial thickness of the rotor or stator.

7. Mixing unit according to any of claims 1 -5, wherein said recesses are formed as grooves (17a1 -a3) extending only partially along the axial height of the rotor or stator.

8. Mixing unit according to claim 6 or 7, wherein said grooves are axial (17a1 -a3;

217a1 -a3; 317a1 -a6).

9. Mixing unit according to any of the preceding claims, wherein said at least one rotor arm is provided with a trailing surface facing opposite the direction of rotation and side surfaces (14a; 414a) connecting said leading and trailing surfaces, wherein at least one of said side surfaces is provided with at least one groove (15a1 -a3; 415a1 -a3) extending between said leading and trailing surfaces.

10. Mixing unit according to any of the preceding claims, wherein said at least one stator arm is provided with a front surface facing opposite the direction of rotation and side surfaces connecting said front and rear surfaces, wherein at least one of said side surfaces is provided with at least one groove extending between said front and rear surfaces.

1 1 . Mixing unit according to claim 9 or 10, wherein said at least one side surface (414a) is provided with a plurality of grooves (415a1 -a3) distributed along the radial extension of the rotor arm (408a).

12. Mixing unit according to any of claims 9-1 1 , wherein said at least one groove (15a1 -a3; 415a1 -a3) is tangential.

13. Mixing unit according to any of the preceding claims as dependent on claim 3, wherein a leading surface (21 1 a; 31 1 a) of said rotor and/or a front surface (212a) of said stator is provided with at least one concave portion (213a, 214a).

14. Mixing unit according to any of the preceding, wherein said rotor arm (208a;

308a) is angled in the direction of rotation such that a radially outer end of said leading surface (21 1 a; 31 1 a) is positioned ahead of an inner end of said leading surface in the rotational direction.

15. Mixing unit according to any of the preceding claims as dependent on claim 3, wherein said at least one stator arm (21 Og; 310a) is angled opposite the direction of rotation such that a radially inner end of said front surface (212a) is positioned ahead of an outer end of said front surface in the rotational direction.

16. Mixing apparatus (2) for mixing treatment media in liquid or gaseous state with a fiber suspension, which apparatus comprises

- a housing defining a mixing chamber (3);

- at least one inlet (4a-b) for feeding a fiber suspension and treatment media into the mixing chamber;

- a drive shaft (5) connectable to a drive device for rotation of the drive shaft in operation;

- an outlet (6) for discharging the mixture of fiber suspension and treatment media; and

- a mixing unit (1 ) according to any of the preceding claims.