Patent Description:
To mechanically produce pulp or fibers from lignocellulosic material, e. g, wood, wood chips are fed into a refiner which refines the prepared, e.g., the steamed wood chips into fiber or pulp. This sounds simple enough but to do it an efficient and continuous manner poses substantial challenges. To make the process as efficient as possible one needs to be able to refine with a stable disc gap and this will in turn require at least two things:.

Designing a refiner is therefore subject to a lot of challenges that need to be fulfilled in order to ensure an efficient feeding and subsequent grinding of e.g., wood chips. When it comes to feeding efficiency it is beneficial if the material can be fed into the grinding area or grinding zone with as small restrictions or disturbances as possible. A common refiner of lignocellulosic material usually comprises a rotor unit and a stator unit that are aligned along a pulp feeding axis facing each other. The refining of the material is performed in a bounded area between the rotor unit and the stator unit. During use of the refiner, material, e.g., pulp, is fed into an area arranged in between, and bounded by, the stator unit and a rotor unit. The rotor unit facing the stator unit may in particular versions be arranged on a rotatable shaft that can be rotated by means of an electrical motor. The purpose of the rotor unit, which in the following will be simply referred to as a rotor, is to grind the pulp between a surface of the stator unit and a surface of the rotor. The rotor and/or stator are often provided with refining segments on their surfaces. The purpose of these refining segments is to improve the grinding action on the material. The refining segments are in turn often provided with additional structures to improve the refining action even further. These structures often comprises refining bars arranged on the surface of the rotor and/or the stator. The refining bars protrudes from the surface of the rotor disc/stator disc and faces the material flow. To ensure an efficient material flow in the area between the stator and rotor these refining bars must be provided in a fashion where they disturb the material flow as little as possible while they at the same time produces an efficient grinding of the material. It is a highly non-trivial challenge to fulfill both these criteria's. A refiner segment according to the preamble of claim <NUM> is disclosed in <CIT>.

The proposed technology aims to overcome at least some of the challenges associated with the design of refining segments for a refiner of e.g., lignocellulosic material.

It is an object to provide a refining segment that enables an efficient material flow while at the same time allowing for an efficient grinding action.

It is a further object to provide a refining segment that allows steam produced during the refining process to stream backwards, toward the material feeding flow, with a reduced influence on the flow.

It is yet another object to provide a rotor or rotor disc comprising such a refining segment. An additional object is to provide a refiner comprising such a rotor.

These and other objects are met by embodiments of the proposed technology.

According to the invention, there is provided a refiner segment as defined by claim <NUM>.

According to a second aspect of the proposed technology there is provided a refiner segment according to the invention, wherein the refiner disc is a rotor refiner disc.

According to a third aspect there is provided a refiner comprising a rotor refiner disc according to the second aspect.

Embodiments of the proposed technology provides refiner segments together with corresponding rotor discs, stator discs and refiners that yield both an efficient material flow within an into the refiners refining area and an efficient refining action on lignocellulosic material such as e.g., wood.

Other advantages will be appreciated when reading the detailed description.

Throughout the drawings, the same reference designations 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 an example of a traditional refiner where the proposed technology may be used. This will then be followed by an analysis of the technical problems and challenges associated with the design of refiner segments.

In order to describe a refiner reference is made to <FIG> which schematically shows an exemplary pulp refiner in a cross-sectional view. The arrangement is housed in a housing <NUM> that represents the outer casing of the refiner device together with all components of the device that is not essential for understanding the present invention. Examples of components not shown are an electrical motor for driving e.g. the rotation shaft, the feeding mechanism for the lignocellulosic material etc. Inside a second housing <NUM> a rotor <NUM>* and a stator <NUM>* is linearly aligned along a shaft. The rotor is attached to a rotation shaft <NUM> arranged on bearings <NUM>. The rotation shaft <NUM> is connected to a motor, not shown, that rotates the shaft <NUM>, and thus the rotor <NUM>. The stator <NUM>* facing the rotor <NUM>* can be provided with a centrally located through hole <NUM> that extends between a feeding channel <NUM> for lignocellulosic material and a refining area <NUM>. The rotor <NUM> can in certain embodiments be provided with a center plate <NUM> having a surface facing the incoming flow of lignocellulosic material. The surface of the center plate <NUM> can be provided with structures that will direct the lignocellulosic material outwards. The rotor <NUM>* and/or the stator <NUM>*, also referred to as rotor (refiner) discs and stator (refiner) discs, respectively, are provided with refining segments to enable steering and grinding of the pulp. These grinding segments are often provided with protrusions on the surfaces intended to enhance the grinding action of the pulp.

During use, lignocellulosic material such as wood chips or prepared wood, e.g., pulp, will be fed by means of a feeding mechanism, not shown, through the feeding channel <NUM>. The material will pass through the hole <NUM> in the stator <NUM>* and enter an area <NUM>. The area <NUM> is essentially defined by the open area between the rotor <NUM>* and the stator <NUM>* and this area can be quite small during operation. The lignocellulosic material flowing into the area <NUM> will be incident on the center plate <NUM> on the rotor <NUM>*. The center plate <NUM> acts to steer the lignocellulosic material out towards the refining segments on the rotor and/stator.

In order to provide a more detailed description of a rotor-stator arrangement in which the proposed technology may be used reference is made to <FIG> illustrates a cross-sectional side view of a rotor - stator arrangement housed in a housing <NUM> in a refiner as e.g., described above. Shown is a rotor <NUM>* that is arranged to rotate around a rotation shaft. The rotor <NUM>* is provided, on the surface facing the stator <NUM>*, with a refining disc <NUM>. The stator <NUM>* is provided, on the surface facing the rotor <NUM>*, with a refining disc <NUM>. The refining discs <NUM>, <NUM> may in certain versions of a refiner be referred to as a segment holders since one of the purposes of the refining discs <NUM>, <NUM> is to carry refining segments. In this rotor-stator arrangements the refining discs of the rotor <NUM>* and the stator <NUM>* are provided with two different types of refining segments, a first type of refining segments <NUM>, <NUM>*, referred to as inlet segments, and a second type of refining segments <NUM>, <NUM>*, referred to as refining zone segments. In certain refiner versions, e.g., in large refiners, these segments are sometimes referred to as center segments or c-segments <NUM>, <NUM>* and peripheral segments, or p-segments <NUM>, <NUM>*, respectively. In what follows the segments will be referred to as c-segments and p-segments but it should be noted that it actually relates to inlet segments and refining zone segments. There is a dual purpose with the first type of segment <NUM>, <NUM>*; it should provide an efficient grinding of lignocellulosic material but it should also enable an efficient material flow towards the p-segments <NUM>, <NUM>*. In the area between p-segments arranged on the rotor and the stator, respectively, the main refining action takes place. The disc gap between these p-segments is normally smaller than the disc gap between the c-segments in order to enhance the refining action. A common disc gap between the p-segments is of order <NUM>. Also illustrated in <FIG> is an inlet <NUM> for the lignocellulosic material subject to refining. The inlet <NUM> is arranged in the central area of the stator <NUM>*. Arranged in the center area of the refining disc <NUM> on the rotor side, opposing the inlet <NUM>, is a center plate <NUM>. The purpose of the center plate <NUM>, which was described above with reference to <FIG>, is to distribute material that falls in from the inlet <NUM> towards the outer sections of the refining disc <NUM>. That is, the center plate <NUM> acts to distribute the material towards the c-segments and the subsequent p-segments arranged on the refiner discs. The proposed technology relates to refining segments of the c-segment type, i.e., the type of refining segments that acts to ensure both an effective refining action and an effective steering of the material flow towards the refining segments of the p-segment type <NUM>.

Having described a potential working mode for a refiner it should be clear that the demands put on a refiner segment are quite high and often contradictory. The refining bars provided on the refiner segment aims to provide an efficient grinding action on the incoming material, a purpose that suggest that they should be given a prominent structure, i.e., they should protrude from the surface of the refiner segments. An efficient and even grinding or refining of the material requires however also that the incoming material is evenly distributed in the refining area. A general configuration of refining bars on a refining segment may however cause areas of varying material concentration. The refining bars should therefore be arranged on the refining segment in such a way that the incoming flow of lignocellulosic material gets evenly distributed and can be steered in a controlled manner towards the outer refining areas, e.g., towards the refining segments of p-segment type. The dual purposes of the refining bars make the design of a refining segment very tricky. One additional and substantial problem that negatively affects the material flow is the impact caused by water steam produced during the refining of the material. Since the material to be refined naturally comprises water, the substantial pressure in the housed rotor and stator arrangement will produce significant amounts of water steam. It should also be noted that there, during use of the refiner, is present a pressure peak in the vicinity of the p-segments, this pressure peak posits a hindrance to the possible motion of the water steam and a lot of the produced water steam will as a consequence move backwards toward the center of the arrangement, i.e., towards the material inlet <NUM>. This backward directed movement of steam will interact with the incoming material flow and make it harder to achieve an even material flow without substantial material concentrations.

The proposed technology provides a refiner segment whose design has shown to provide a satisfactory refining action while at the same time ensuring an efficient and controlled flow of lignocellulosic material. The proposed technology provides in particular mechanisms that will reduce the negative impact the back-travelling steam have on the material flow. This is accomplished at least in part due to a particular configuration of refining bars that will enable the main material flow to occur on one refining disc side, e.g., on the rotor side of a rotor - stator arrangement while the other refining disc side, e.g., the stator side, can be occupied by back-travelling water steam. This will reduce the interaction between incoming wood chips and back-travelling water steam.

In order to obtain these positive effects the proposed technology provides a refiner segment <NUM> for a refiner <NUM> of lignocellulosic material. The refiner segment <NUM> being part of a refiner disc <NUM> comprising a center area <NUM>, wherein the refiner segment <NUM> comprises a number N, N ≥ <NUM>, of bar zones Zi, i = <NUM>, <NUM>,. N arranged at different radial positions with regard to a radial direction R extending from the center area <NUM> of the refiner disc <NUM> towards the periphery of the refiner segment <NUM>, each of the bar zones Zi being defined by a corresponding set of refining bars ZiRBi, i = <NUM>, <NUM>,. M, distributed angularly and encircling the centrally located through-hole <NUM>, where refining bars ZiRBi belonging to different but neighboring bar zones Zi, Zi+<NUM> are angularly offset.

In other words, there is provided a refiner segment <NUM> that is integrated with a refining disc <NUM>, or is adapted to be attached to a refining disc <NUM>. The refining segment have a surface that comprises a number of refining bars ZiRBi that are arranged in an angular fashion around a common center area <NUM> in such a way that they form distinct bar zones Zi that encircles a common center on the refining disc <NUM>. A particular bar zone Zi is defined as the area on the refining segment that comprises a corresponding set of refining bars ZiRBi. Hence a number of refining bars Z<NUM>RB1 are provided in an innermost area, corresponding to bar zone Z<NUM>, with regard to the center area of the refiner disc <NUM>, a number of refining bars Z<NUM>RB2 are provided in an area, corresponding to bar zone Z<NUM>, that lies outside the innermost area. The directions are in relation to a radial direction having its origin in the center <NUM> of the refiner disc <NUM>. This pattern is repeated so that a number of concentric bar zones are defined along the surface of the refining segment <NUM>. Each bar zone comprises its own refining bars and refining bars belonging to neighboring bar zones may be spatially offset, i.e., arranged in such a way that there is a radial distance between refining bars belonging to neighboring bar zones. This is for example illustrated in <FIG>. A particular feature of the proposed refining segment <NUM> is that refining bars ZiRBi that belong to different but neighboring bar zones Z and Zi+<NUM> are angularly offset. That is, they are arranged in such a fashion that the length direction of specific refining bars belonging to different but neighboring bar zones does not coincide. This can for example be seen in <FIG> and <FIG> where the length directions for refining bars belonging to bar zone Z<NUM> and Z<NUM> has been illustrated by means of dotted arrows, labelled L1 and L2, respectively. This angular offset between refining bars belonging to different but neighboring bar zones creates open bar areas that will allow material sub-flows to move over to the following, i.e., the neighboring, bar zone in a particular manner. This refining bar pattern has proved effective for achieving an even material flow over the refining segment towards the periphery, or towards the p-segments. It has in particular shown to be an efficient counter measure to the problem associated with water steam going backwards. The proposed refining segment ensures that the material flow along a particular refining segment is not forced towards the oppositely arranged refining disc, e.g., towards the stator side, if the refining segment is provided on the rotor side. Due to this fact the oppositely arranged refining disc will display a lot of open area which may be occupied by any water steam travelling backwards. Any residual amount of steam that might end up on e.g., the rotor side would still have space to move towards the center through the openings provided by the open areas between the refining bars. To appreciate this technical effect reference is made to <FIG> which illustrates a rotor disc comprising a refining segment according to the proposed technology. The schematic drawing illustrates both the path the material flow will take and the number of ways that the steam can travel. The refining bars that are angularly offset provides a smooth way for the material to follow while it also provides way for the movement of steam.

There are a number of advantages that can be obtained with refining segments according to the proposed technology. They provide in particular an energy economical feeding with minimum restrictions. The refining segment of the proposed technology provides a lot of open volume. This open volume can carry the material flow without forcing it towards the opposite side of the rotor-stator arrangement. The proposed refining segment also provide the highly desirable feature that it enables an even feed not only over the spatial disc geometry but also over time, and in particular a uniform flow over time despite the fact that the incoming material feed itself might be non-uniform. The proposed technology enables this feature by having a refining bar pattern that allows a material buffering effect. When sub-flows over the refining segments emerges from one bar zone and reaches a new bar zone, the sub-flows will mix with already existing flows. This mixing of sub-flows paired with potential turbulence and friction caused by the mixing will yield a slight material buffering effect. This buffering effect will in turn ensure a more uniform material flow over time.

Having described the cooperating features of a refining segment <NUM> that enables both an efficient grinding of the material and an efficient material flow, in what follows a number of embodiments of the proposed technology will be described with reference to the accompanying drawings.

A particular embodiment of the proposed technology provides a refiner segment <NUM> wherein the angular offset between all neighboring bar zones Zi are in the same angular direction, the angular direction being the direction opposite to the intended rotational direction of the refiner segment <NUM>. This embodiment provides an improved material flow since at least some of the refining bars belonging to different but neighboring bar zones can cooperate to obtain a uniform flow both spatially and over time. This embodiment is illustrated schematically in <FIG>.

Another embodiment of the proposed technology provides a refiner segment <NUM>, wherein the refining bars of a particular bar zone are distributed angularly in a band of essentially equidistantly spaced refining bars ZiRBi that encircles the center <NUM> of the refiner disc <NUM>. This embodiment is schematically illustrated in, for example, <FIG>. It can be seen in <FIG> how refining bars belonging to the same bar zone are arranged equidistant to each other in an angular pattern. Since they form part of the same band they are also arranged more or less equidistantly from a center of the disc. This particular embodiment ensures a symmetric configuration of refining bars which in turn has shown to lead to an even material flow. There are however alternative embodiments where the shape of the refining bar pattern can be adjusted to improve the material feeding for different radii.

Yet another particular embodiment of the proposed technology provides a refiner segment <NUM>, wherein the different bar zones comprises bands with different number of equidistantly spaced refining bars ZiRBi. Such an embodiment is illustrated in <FIG>. In <FIG>, which illustrates a refining segment with three different bar zones, the innermost bar zone is provided a number of refining bars. In the bar zone that is adjacent to the innermost bar zone the number of refining bars are higher. This pattern may than be repeated for each additional bar zone. Still another particular embodiment of the proposed technology provides a refiner segment <NUM>, wherein the number of equidistantly spaced refining bars ZiRBi increases from the lowest number in the innermost bar zone Z<NUM>, with regard to the center <NUM> of the refiner disc <NUM>, to the highest number in the outermost bar zone ZN adjacent the periphery of the refiner segment <NUM>. According to the invention the number of refining bars provided in the bar zones are doubled for each bar zone going outwards towards the periphery. If the innermost bar zone Z<NUM> is provided with a number X of refining bars, the zone Z<NUM> that is adjacent to the zone Z<NUM> is provide with 2X bars and so on. By providing more bars towards the periphery of the refining segment the difference between the available open volumes in the center compared to the outermost bar zone is reduced. This is in particular true when the size of the refining bars gets smaller in bar zones closer to the periphery of the refining segment. Achieving an even distribution of open volume will enable a more even flow.

By way of example, the proposed technology provides a refiner segment <NUM>, wherein refining bars ZiRBi belonging to different but neighboring bar zones are arranged in such a way that a tangential direction of a particular refining bar ZkRBk belonging to a bar zone Zi, points in a direction towards the mid-point between two refining bars Zk+<NUM>RBk+<NUM> belonging to a neighboring bar zone Zi, Zi+<NUM>. <FIG> provides an illustration of this particular embodiment. The dotted lines illustrates the tangential direction of a refining bar. In the case of essentially straight refining bars, the tangential direction will coincide with the length direction of the refining bar while the tangential direction for a curved refining bar essentially follows the slope of the curvature of the refining bars. The latter case is schematically illustrated in <FIG>, where the dotted lines illustrates the tangential direction for slightly curved refining bars. The embodiments where refining bars belonging to different bar zones or bands are arranged based on the tangential direction of refining bars belonging to the inner bar zone or band ensures an efficient material flow since it provides a lot of open area that can carry the material flow.

A specific embodiment of the proposed technology provides a refiner segment <NUM>, wherein the refining bars are provided with geometrical shapes such as straight edged bars, rounded bars, conical bars, arrow-shaped bars with or without chamfers, etc. By way of example, the proposed technology provides a refiner segment <NUM>, wherein at least a subset of the refining bars ZiRBi belonging to a particular bar zone Z; have a geometrical shape that is distinct from the geometrical shape of refining bars ZkRBk belonging to other bar zones Zk.

Another embodiment of the proposed technology provides a refiner segment <NUM>, wherein the length of refining bars belonging to different bar zones decreases from a largest length for refining bars Z<NUM>RB1 belonging to the innermost bar zone Z<NUM>, with regard to the center <NUM> of the refining disc <NUM>, to the smallest length for refining bars ZNRBN belonging to the outermost bar zone ZN adjacent the periphery of the refiner segment <NUM>. By providing refining bars belonging to different bar zones with different length dimensions ensures that the open volume on the bar zone remain sufficiently large. With open volume is here intended the area on the refining segment where the material is allowed to flow freely, without interacting with any refining bars. Since the number of provided refining bars become higher towards the periphery of the refining segment, with a purpose of obtaining an efficient flow, the open volume may decrease. This may be compensated by a stepwise shortening of refining bars, i.e., the farther out from the center of the refining segment or refining disc the refining bars are provided, the shorter they are, this is schematically illustrated in e.g., <FIG>.

According to a particular embodiment of the proposed technology there is provided a refiner segment <NUM>, wherein at least a subset of the refining bars ZiRBi is provided on the surface of the refiner segment <NUM> in such a way that an angle α is formed between the radial direction of the refiner segment <NUM> and the length direction of a refining bar ZiRBi. The lower part of <FIG> illustrates such an embodiment. The length direction of a particular refining bar is denoted L and it can be seen how this length direction forms an angle α with the radial direction.

A particular version of the above mentioned embodiment provides a refiner segment <NUM>, wherein the angle α formed between the radial direction of the refiner segment <NUM> and the length direction of a refining bar ZiRBi defines the bar feeding angle and wherein the angle α takes value in the interval <NUM>° < α ≤ <NUM>°.

Another particular embodiment of the proposed technology provides a refiner segment <NUM>, wherein refining bars ZiRBi belonging to different bar zones Zi have different widths, and wherein the widths decreases from a largest width for refining bars Z<NUM>RB1 belonging to the innermost bar zone Z<NUM>, with regard to the center <NUM> of the refiner disc <NUM>, to the smallest width for refining bars ZNRBN belonging to the outermost bar zone ZN adjacent the periphery of the refiner segment <NUM>. The purpose of this embodiment is the same as in the embodiment described above regarding refining bars having different lengths. That is, it ensures that a satisfactory degree of open volume that can carry the material flow is present on the refining segment even when the number of refining bars increases toward the periphery.

In still another embodiment of the proposed technology the refining segment may be provided on a refining disc <NUM> that also comprises refining segments <NUM>,<NUM>* of p-segment type. <FIG> illustrates such an embodiment. Such an embodiment may in particular comprise a refiner disc <NUM>, <NUM> that comprises the refining segments <NUM> as has been described earlier, here referred to as c-segments <NUM>, <NUM>* and additional refining segments referred to as p-segments <NUM>, <NUM>*. The p-segments <NUM>, <NUM>* are provided with refining bars to enable an efficient grinding of material flowing in from the c-segments <NUM>. The refining disc <NUM>, <NUM> may be a rotor refiner disc <NUM> or a stator refiner disc <NUM>.

It should be noted that the proposed technology may be utilized on both the rotor side of a refiner and on the stator side. The proposed technology may be provided in the form of a refining segment that can be attached a refining disc <NUM> that in turn can be attached to the rotor <NUM>* or stator <NUM>*. The refining disc <NUM> may in this particular case be referred to as a segment holder, see <FIG> for an illustration. The refining segment may however also be provided in the form of complete integrated disc, thus forming part of, or defining, the refining disc in itself. In this case the refining segment <NUM> and the refining disc <NUM> form an integrated structure that can be attached to a rotor <NUM>* or a stator <NUM>*.

A particular embodiment of the proposed technology provides a refiner segment <NUM>, wherein the refiner segment <NUM> comprises the refiner disc <NUM>. That is, the refining segment <NUM> can be provided in the shape of a refiner disc that can be either a rotor refiner disc or a stator refiner disc.

According to a particular version of the latter embodiment there is provided a refiner segment <NUM>, wherein the refiner disc <NUM> is a rotor refiner disc. As was mentioned earlier, the refining segment <NUM> according to the proposed technology may form part of a refiner disc <NUM> or be attached to a refiner disc <NUM>. A refining segment may be provided in the shape of a circle, optionally with a removed central area <NUM>, as is shown in e.g. <FIG>, or in the shape of a circle sector as in <FIG>. A refiner disc <NUM> may thus be provided with a number of refiner segments <NUM> whereby it will either be completely covered by refining segments <NUM> or partially covered. The refining segment may in particular form part of a rotor disc or equivalently a rotor refiner disc. In case the refiner segment <NUM> form part of a rotor refiner disc the center area <NUM> of the rotor refiner disc <NUM> may comprise a center plate <NUM>.

An alternative embodiment of the proposed technology provides a refiner segment <NUM> wherein the refiner disc <NUM> is a stator refiner disc <NUM>*. A schematic cross-sectional view from the side of a stator refiner disc <NUM>* is illustrated on the right side of <FIG>. In this particular embodiment the stator refiner disc can be provided with a hole in the center area <NUM>. This hole defines an inlet <NUM> for the refining material.

The proposed technology may however also be used in a rotor-stator arrangement or a refiner <NUM> where the stator disc <NUM>* is adapted to cooperate with a rotor refiner disc <NUM> that comprises a refining segment as has been described earlier. The stator disc may also be provided with refining segments according to what has been described earlier. The stator disc <NUM>* is adapted to face, and cooperate with, the rotor refiner. The stator refiner disc <NUM> is provided in the form of an essentially circular shape having a center area provided with a hole that defines a material inlet <NUM>. The stator disc may also be provided with two different but adjacent surface regions, a first surface region that is arranged adjacent to, and encircling, the inlet <NUM>, and a second surface region that is arranged adjacent to, and encircling, the first surface region. The second surface region is essentially planar while the first surface region is inclined relative the second region where the inclination is in a direction opposite the intended material flow direction during use. The fact that the first surface region is inclined relative the second region provides more open volume closer to the center of the stator disc <NUM>*. This open volume can be occupied by water steam and thus provides ample space for any back-travelling steam. <FIG> and <FIG> provides schematic illustrations of such a stator disc. <FIG> provides a view facing the stator disc while <FIG> illustrates how the stator disc interacts with a rotor disc equipped with a refining segment according to the proposed technology.

Another particular embodiment of the proposed technology provides a refiner <NUM> comprising a rotor refiner disc <NUM> provided with refining segments as described herein.

The proposed technology also provides a refiner <NUM> comprising a rotor refiner disc <NUM> provided with refining segments as described herein and a stator refiner disc <NUM> as described above. <FIG> provides an illustration of a possible refiner where the present invention may be used. To this end the rotor disc <NUM> may comprise a refining segment according to the proposed technology. The rotor disc <NUM> is adapted to cooperate with a stator disc <NUM> according to another aspect of the proposed technology.

Claim 1:
A refiner segment (<NUM>) for a refiner (<NUM>) of lignocellulosic material, the refiner segment (<NUM>) being part of a refiner disc (<NUM>) comprising a center area (<NUM>), wherein the refiner segment (<NUM>) comprises a number N, N ≥ <NUM>, of bar zones (Zi, i = <NUM>, <NUM>,...N) arranged at different radial positions with regard to a radial direction R extending from said center area (<NUM>) of said refiner disc (<NUM>) towards the periphery of said refiner segment (<NUM>), each of said bar zones (Zi) being defined by a corresponding set of refining bars (ZiRBi, i = <NUM>, <NUM>,...M) distributed angularly and encircling the center area (<NUM>), where refining bars (ZiRBi) belonging to different but neighboring bar zones (Zi, Zi+<NUM>) are angularly offset, wherein refining bars (ZiRBi) belonging to different but neighboring bar zones are arranged in such a way that a tangential direction of a particular refining bar (ZkRBk) belonging to a bar zone (Zi,) points in a direction towards the mid-point between two refining bars (Zk+<NUM>RBk+<NUM>) belonging to a neighboring bar zone (Zi, Zi+<NUM>) and wherein the length of refining bars belonging to different bar zones decreases from a largest length for refining bars (Z<NUM>RB1) belonging to the innermost bar zone (Z<NUM>), with regard to the center (<NUM>) of said refining disc (<NUM>), to the smallest length for refining bars (ZNRBN) belonging to the outermost bar zone (ZN) adjacent the periphery of said refiner segment (<NUM>) characterized in that the number of refining bars (ZiRBi) are doubled for each zone going outwards towards the periphery.