WO2005061783A1

WO2005061783A1 - Method for grinding water-suspended paper fibers or cellulose fibers

Info

Publication number: WO2005061783A1
Application number: PCT/EP2004/012762
Authority: WO
Inventors: Hans Schnell
Original assignee: Voith Paper Patent Gmbh
Priority date: 2003-12-12
Filing date: 2004-11-11
Publication date: 2005-07-07
Also published as: ATE381637T1; PT1702104E; EP1702104A1; EP1702104B1; US20070090209A1; BRPI0417436A; DE502004005776D1

Abstract

The method is used to grind a suspension of paper fibers (S). Said suspension is ground predominantly as a result of pressure forces between two grinding surfaces (1,2) which are located on grinding tools which are pressed against each other since the grinding tools in the grinding area cannot move or only move very little in relation to each other. The grinding surfaces (1, 2) are porous, enabling part of the water (W) of the suspension of paper fibers (S) to be temporarily received or discharged. A device with a central grinding cylinder (3) and peripherally arranged grinding rollers (4) is particularly suitable as a device for carrying out said method. The grinding surfaces can be embodied as cylinders or provided with toothing.

Description

page 1

METHOD FOR GRINDING ASSRIG SUSPENDED PAPER FIBERS OR CELL FIBER FIBERS

Process for fiber treatment

The invention relates to a method for fiber treatment according to the preamble of claim 1.

It has long been known that pulp fibers have to be ground so that the paper subsequently produced therefrom has the desired properties, in particular strength, formation and surface. By far the most frequently used grinding processes use grinding surfaces which are provided with strips known as knives, which are moved past one another at high speed. The corresponding machines are mostly called knife refiners. For special cases, grinding processes are also used in which at least one of the grinding surfaces is knife-free, so that the grinding work is transmitted by friction or shear forces.

The effect of the method can be controlled over a wide range by changing the grinding parameters, and in addition to the amount of grinding, a distinction is made in particular as to whether a more cutting or more fibrillating grinding is desired. When pulp fibers are ground, their drainage resistance generally increases with the amount of grinding. A common measure of drainage resistance is the Schopper-Riegler degree of grinding.

The increase in the degree of grinding has an unfavorable effect on sheet formation on the paper machine, but is accepted because the quality characteristics of the pulp mentioned above play an outstanding role in its usability. In many cases, the grinding parameters are chosen so that the increase in the degree of grinding to achieve the required fiber quality is as small as possible. However, this influence is very limited. In addition, this can make the grinding less economical.

A grinding method is known from US Pat. No. 4,685,623 which is to use less energy. The paper fiber suspension to be ground is guided into narrow wedges ("narrow nips") which form between a rotating central roller and a plurality of rollers that roll on the outside. The Page 2

Wedge nips are very narrow because the central roller is provided with a large number of circumferential grooves or grooves. The outside rollers are pressed against the central roller with a defined force, so that the fibers are dewatered and squeezed in the wedge gap. Part of the suspension and the water squeezed out in the wedge gaps are carried away transversely to the direction of movement and guided past the actual wedge gap in these grooves in order to be mixed again later with the already ground thickened fibrous material. In this way, problems in the operation of such a machine are to be avoided even with a higher throughput. In operation, the housing of this apparatus is completely filled with suspension, which is pumped through with an adjustable volume flow.

In order to hold the fibers in the wedge gaps during grinding, the US specification requires that the width of the wedge gaps be smaller than the fiber length. However, this means a high outlay in terms of equipment in an industrially used grinding machine.

The invention is based on the object of providing a method for treating pulp with which it is possible to change pulp or paper fibers in such a way that the strengths of the paper produced therefrom are increased. The increase in dewatering resistance that occurs should be as small as possible than in known grinding processes. The process is said to be suitable for industrial milling machines.

This object is achieved by the features mentioned in claim 1.

The grinding process mainly acts through pressure forces and avoids large relative movements of the grinding tools. As will be explained later, with the aid of such a grinding process, significant fiber-technological advantages can be achieved.

In contrast to the usual knife refiners with a high relative speed between the grinding tools, which lead to cutting and / or shear stress on the fibers, the grinding tools in the grinding process considered here are not or only very slightly moved in the grinding zone. The actual grinding stress is therefore caused by pressure forces. In many cases, however, it has proven to be problematic to use grinding processes of this type on an industrial scale. When producing paper or paper-like products, namely Page 3 very large quantities of fibrous material are continuously made available for the paper machine. If at least some of the pulp used for paper production (it can also be all of the pulp) is to be ground using the new process, the machines have to be designed for a considerable production volume. The method according to the invention can help to keep the machine size within limits, since the effectiveness of the fiber treatment in the grinding zone can be increased considerably for the reasons mentioned.

The pressure forces in the grinding zone can be used to drain the paper fiber. In the method according to the invention, this primarily means that the water is either temporarily absorbed by the porous grinding surface and released again later, or that water can flow off into other areas of the grinding apparatus through the porous grinding surface. The new process has several advantages:

First of all, the compressive force is transferred to the fiber less damped and therefore used more for its technological change. In addition, the fiber is fixed on the grinding surface, i.e. it is prevented from leaving the grinding zone untreated. This effect occurs essentially regardless of the fiber length. In addition, with the same volume, more fibers can get into the grinding zone because the water that is squeezed out is missing. The size of the grinding machine used can be reduced accordingly. In addition, the fiber attaches itself to a surface with many small unevenness under considerable pressure, which can have a favorable influence on the grinding.

In typical application forms, the grinding surfaces are located on grinding tools which perform a rotational movement in such a way that the liquid absorbed by the pores in the grinding zone is thrown off again after leaving the grinding zone. In particular in the case of open-pore material layers, the pores of which are connected not only to the grinding surface but also to other surfaces, the absorption of liquid and the centrifuging off of the liquid can be carried out particularly effectively.

Particularly suitable materials to create a porous grinding surface for carrying out the process are hard metals, chrome steel, plastics (eg polyethylene, GRP), ceramics or copper alloys. These materials can be sintered. As is well known, can also page 4

Composites are specifically created open-pore workpieces that are suitable for the use considered here. The pore size can be set individually between 5 μm and 0.5 mm according to the requirements.

The advantages of the process in terms of grinding technology are as follows: 1. The fiber length is retained much better. 2. The fiber surface is not or significantly less fibrillated. 3. The specific grinding work to achieve the desired strengths is generally less.

Comparative tests with long fiber pulp have shown that to achieve a tear length of 8 km with a knife grinding a 45 ° SR freeness was achieved and with the new process only 18 ° SR. The specific grinding work required was up to 50% lower.

It can be assumed that the new grinding process changes the surface of the fibers in such a way that they gain improved flexibility and binding ability without the need to remove fibrils from the outer surface of the fibers. The production of fine material, i.e. fiber fragments, is also avoided.

If the process is applied to recycled fibers, the advantages mentioned can play a special role. Recycled fibers have already undergone at least one, often even several grinding processes, so that any further size reduction can be avoided.

The invention and its advantages are explained with reference to drawings. Show:

1 detailed view of a grinding zone with strip-shaped grinding surfaces to explain the method; 2 shows a variant with cylindrical grinding surfaces; Fig. 3 part of a special grinding device; 4 shows a grinding device with a central grinding cylinder and peripherally arranged grinding rollers; Fig. 5 shows a grinding device in which the porous layer of the grinding cylinder is provided with rear chambers. page 5

A particularly suitable device for carrying out the method can have a rotating grinding cylinder 3 to which a number of grinding rollers 4 are pressed from the outside. The grinding surfaces 1 and 2 are located on them. The contact pressure influences the grinding effect and can therefore preferably be adjusted. 1 shows a part of such a grinding device, in particular the point at which a grinding zone is created by the interaction of the grinding cylinder 3 and grinding roller 4. In the example shown, both the grinding cylinder and the grinding roller are provided with grinding strips 5 and 6, of which only a part is shown. They extend essentially at right angles to the direction of movement of the grinding tools and are evenly distributed over the circumference. The grinding strips are dimensioned and arranged so that in the grinding zone the grinding strip 5 of one grinding tool extends into the spaces between the grinding strips 6 of the other grinding tool, as a result of which the volume in between is reduced and in the suspension S therein (indicated by an arrow) ) considerable pressure is built up. Both the grinding strips and the cylindrical outer layer of the corresponding grinding tool connected to them are made of porous material. Under certain circumstances, it is sufficient to produce only the grinding strips from porous material and to fix them on the grinding cylinder or on the grinding roller.

As already described, the water of the fiber suspension, which is under increased pressure in the grinding zone, can penetrate into the pores of the grinding surface, thereby thickening the fiber itself. After passing through the grinding zone, the volume between the grinding tools increases again, and the water W (indicated by an arrow) can emerge from the pores again. This effect is strongly supported by the fact that both grinding surfaces rotate, so that a centrifugal force acting perpendicular to the grinding surface is generated. Since the grinding strips form a toothing, the relative speed of the two grinding tools to one another is low within the grinding zone, as a result of which shear movements and shear forces acting on the fibers are largely avoided. The toothing also has an impact in terms of grinding technology, which is due to the fact that the compressive forces acting on the fibers occur in a pulsating manner, which is desirable in many cases in terms of grinding technology. Another application is shown in FIG. 2 with cylindrical grinding surfaces, that is to say those which are not provided with strips and which, in order to carry out the method, roll against one another with little or no slip. It can be seen in this illustration that the Mahi cylinder 1 'is provided with a porous layer 8 ^' and page 6

the grinding rollers 4 ^' with a porous material layer 7'. In another embodiment, not shown, it may also be sufficient for only one of the two grinding tools, which together form a grinding zone, to have a porous grinding surface. This applies both to cylindrical grinding surfaces (see Fig. 2) and to strip-shaped ones (see Fig. 1).

The advantages offered by the method according to the invention are particularly evident when the grinding device is not completely filled with suspension during operation. This principle is explained by way of example in FIG. 3. One can again see a Mahi cylinder 3, to which several grinding rollers 4, which are toothed with it, are pressed, two of which are shown here. The grinding surfaces are porous. Since the housing of the grinding device is filled only to a small extent with suspension, the suspension S is centrifugally thrown out of the interdental spaces 20 into the free space after leaving the grinding zone and reaches the next grinding roller, in the free space the liquid has a much higher density than the air around them. The water that was pressed into the pores due to the porosity of the grinding surfaces when passing through the grinding zone can now also be thrown out and reach the next grinding zone. 3 also shows that the grinding strips can have the shape of involute teeth, which optimizes the rolling conditions in the grinding zone, but is somewhat more complex to manufacture than e.g. the grinding strips shown in Fig. 1. The grinding rollers 4 rotate here in a fixed backdrop 11. The mahizylinder 3 has annular circumferential shoulders 12 on the axial end faces, which axially seal the volume formed between the grinding strips 5, 6 in the grinding zone.

The grinding device shown in FIG. 4 for carrying out the method is only shown schematically. One can see a horizontally lying grinding cylinder 3, on which there are several grinding rollers 4, which are arranged predominantly evenly distributed over the circumference, a gap 9 without a grinding roller being left in the area of the material discharge 13. The suspension S is added through an inlet 14. This can be done via a wide gap or via a headbox, similar to that used in paper machines. The addition point is located next to the material drain 13 and is separated from it by a guide plate 15. In this way, forced delivery over almost 360 ° over the entire circumferential course of the housing 16 is ensured. The suspension flow through this grinding device is defined and can be adjusted easily and safely. For the reasons already mentioned, the housing is preferably not completely included Page 7

Suspension filled. However, a suspension level can form at the material drain 13. The housing 10 can - as drawn here - be cylindrical on the inside or, according to FIG. 3, be provided with a link 11 in which the grinding rollers rotate with a small gap distance.

It is easily possible to produce porous layers in which the capillary cavities are connected to one another. The drainage effect can be significantly improved if the water pressed into the pores is removed after leaving the grinding zone. For this, e.g. Centrifugal forces are used. The water is then not like with filter elements constantly pressed through the pores in one direction. Rather, the pores serve as storage volumes with a flow reversal, which immediately flushes away the adhering or possibly penetrated fines.

It is easy to see that the drainage effect of the porous grinding surface can be particularly effective if the pores can be kept free of blockages. In order to avoid any difficulties in this regard, it may be expedient to provide a chamber on the back of the porous layer (that is to say the side of the layer opposite the grinding surface). If necessary, this can be fed with pressurized water. The water may be mixed with suitable cleaning chemicals. 5 shows a simple possibility of realizing such a chamber 17 using the example of a grinding cylinder 3 ″, which is peripherally surrounded by grinding rollers 4 ^' . In this example, the chambers 17 are formed by the fact that between the continuously porous material layer 8' and the cylinder body of the Mahi cylinder 3 "spacer strips 18 are attached, between which the volumes of the chambers 17 remain free. The rinse water is added through lines 19.

In a further embodiment of the method, the chamber 17 shown in FIG. 5 and the lines 19 connected to it can be used to discharge water which has penetrated through the pores of the material layer 8 ^' . A chronological sequence of drainage and - if necessary - flushing is also conceivable.

Of course, this embodiment shown here on cylindrical grinding surfaces with chambers 17 can also be used on other grinding surface shapes.

Claims

Page 8 Patent claims:

1. A method for grinding water-suspended paper fibers or cellulose fibers, in which the fibrous material is passed through at least one grinding zone which lies between grinding surfaces (1, 2), in which the grinding surfaces (1, 2) lie on grinding tools pressed against one another, ■ whereby mechanical grinding work is transferred to the fibers in such a way that the strengths of the paper produced therefrom change, the grinding surfaces (1, 2) being moved relative to one another in such a way that the relative speed between the fibrous material and the grinding surfaces, viewed in the main direction of movement of the grinding surfaces , at the point at which two grinding surfaces (1, 2) are closest in the grinding zone is at most 10% of the absolute speed of the fastest moving grinding surface, characterized in that at least one of the grinding surfaces (1, 2 ) is porous.

2. The method according to claim 1, characterized in that both grinding surfaces (1, 2) interacting in the grinding zone are porous.

3. The method according to claim 1 or 2, characterized in that the porous grinding surface (1, 2) by at least to the grinding surface (1, 2) open-pore material layer (7, 7 ^' , 8, 8 ^' ) is formed.

4. The method according to claim 3, characterized in that the material layer (7, 7 ^' , 8, 8 ^' ) consists of sintered material.

5. The method according to claim 4, characterized in Page 9 that the material layer (7, 7 ', 8, 8 ^' ) consists mainly of chrome steel.

6. The method according to claim 4, characterized in that the material layer (7, 7 ', 8, 8') consists predominantly of hard metal.

7. The method according to claim 4, characterized in that the material layer (7, 7 ', 8, 8') consists predominantly of a copper alloy.

8. The method according to claim 4, characterized in that the material layer (7, 7 ^' , 8, 8 ^' ) consists predominantly of ceramic.

9. The method according to claim 4, characterized in that the material layer (7, 7 ', 8, 8 ^' ) consists predominantly of plastic.

10. The method according to any one of claims 3 to 9, characterized in that the layer thickness of the material layer (7, 7 ^' , 8, 8 ^' ) is at least 1 mm and at most 30 mm, preferably 10-20 mm.

11. The method according to any one of the preceding claims, characterized in that the average pore size of the porous grinding surface (1, 2) is less than 0.5 mm.

12. The method according to any one of the preceding claims, characterized in that the relative speed between the fibrous material and the grinding surfaces in the main direction of movement of the grinding surfaces (1, 2) seen at the point where two grinding surfaces (1, 2) in the Grinding zone closest, less than 5% of the absolute speed of the am Page 10 fastest moving grinding surface (1, 2).

13. The method according to any one of claims 1 to 11, characterized in that the relative movement of the grinding surfaces (1, 2) in the grinding zone is a rolling movement.

14. The method according to any one of the preceding claims, characterized in that the mechanical grinding work is transferred by compressing the fibrous material.

15. The method according to any one of the preceding claims, characterized in that at least one grinding surface (1, 2) is provided with grinding strips (5, 5 ^' , 6, 6 ^' ) which run transversely to the main direction of movement of the moving grinding surface.

16. The method according to claim 15, characterized in that the grinding strips (5, 5 ^' , 6, 6 ^' ) have a height of at least 2 mm and a width in the direction of movement of the moving grinding surfaces of at least 2 mm.

17. The method according to claim 15 or 16, characterized in that both grinding surfaces (1, 2) are provided with transverse grinding strips (5, 5 ^' , 6, 6 ^' ).

18. The method according to any one of the preceding claims, characterized in that the absolute speed of at least one grinding surface (1, 2) is kept at a value between 5 and 30 m / sec.

19. The method according to any one of the preceding claims, characterized in that the grinding surfaces (1, 2) are pressed against each other so that in the grinding zone Page 11 Line force between 2 and 10 N / mm arises.

20. The method according to any one of the preceding claims, characterized in that at least one of the grinding tools on the grinding surface (1, 2) has spaces, in particular tooth spaces (20), which are moved in the grinding zone so that they pass through the suspension (S) transport the grinding zone in the direction of movement of the grinding tools.

21. The method according to claim 20, characterized in that the spaces outside the grinding zone are emptied of the fibers.

22. The method according to claim 21, characterized in that the gaps are emptied by centrifugal forces.

23. The method according to claim 20, 21 or 22, characterized in that one of the grinding tools is a grinding cylinder (3, 3 ^' , 3 ") and that the other grinding tools are grinding rollers (4, 4 ^' ) which are arranged in parallel and which are on the Circumferential surface of the Mahi cylinder (3, 3 ^' ) can be set in a rolling motion.

24. The method according to claim 23, characterized in that the grinding cylinder (3, 3 ^' , 3 ^" ) is driven and that the grinding rollers (4, 4') rotate about spatially fixed axes.

25. The method according to claim 23 or 24, characterized in that the Mahi cylinder (3 ^' , 3 ^" ) is provided with a cylindrical porous surface and that the spaces are located on the circumference of the grinding roller. page 12

26. The method according to any one of claims 23 to 25, characterized in that Mahi cylinder (3, 3 ', 3 ^" ) and grinding rollers (4, 4') are substantially horizontal and that the direction of the suspension transport through the grinding device used essentially that is the circumferential movement of the Mahi cylinder.

27. The method according to any one of the preceding claims, characterized in that the compressive forces for grinding the paper or cellulose fibers are transmitted to surfaces on which the grinding tools roll against each other.

28. The method according to any one of claims 23 to 27, characterized in that the diameter of the grinding cylinder (3, 3 ', 3 ") is at least 1 1/2 times, preferably at least twice the diameter of the grinding roller (4, 4 ^' ) is.

29. The method according to claim 28, characterized in that the grinding rollers are arranged closely adjacent on the circumference of the grinding cylinder (3, 3 ^' , 3 ^" ), so that as many grinding zones as possible are formed on a grinding cylinder.

30. The method according to any one of the preceding claims, characterized in that the grinding is carried out on a suspension which is continuously guided by the grinding device used.

31. The method according to any one of the preceding claims, characterized in that the aqueous suspension is fed into the grinding device used with a consistency of 1-6%. Page 13

32. The method according to any one of claims 1 to 30, characterized in that the aqueous suspension is fed into the grinding device used with a consistency of 6-15%.

33. The method according to any one of claims 1 to 30, characterized in that the aqueous suspension is fed into the grinding device used with a consistency of 15-25%.

34. The method according to any one of the preceding claims, characterized in that the force with which the grinding tools forming a grinding zone are pressed together can be adjusted, in particular can be set differently for different grinding zones.