WO2003043749A1

WO2003043749A1 - Screening of wood chips

Info

Publication number: WO2003043749A1
Application number: PCT/FI2002/000917
Authority: WO
Inventors: Timo Kurki
Original assignee: Raumaster Oy
Priority date: 2001-11-23
Filing date: 2002-11-19
Publication date: 2003-05-30
Also published as: SE0401244L; SE527654C2; NO330874B1; NO20042611L; FI111055B; SE0401244D0; AU2002342942A1; FI20012306A0

Abstract

The invention comprises a roller screen (1) for separating two or more finer and coarser material fractions of wood chips from each other, in particular for separating fines fraction and pin chips fraction from the wood chips that are usable as regards chip size. On the surface of one or more of the rollers (4) of the roller screen there are substantially radial (14) and substantially axial (15) grooves, of which the axial grooves (15) are deeper than the radial grooves (14).

Description

Screening of wood chips

The invention relates to wood chip screening used in the pulp and paper industry, in other words to the separating of two or more finer and coarser material fractions from each other, in accordance with the preambles to the independent claims presented below.

It is expedient to separate particles of less than a specified chip size from the wood chips to be used as raw material for the production of mechanical or chemical pulp. The minimum acceptable chip size depends, among other things, on the pulp production process, the mechanical structure of the process equipment and the wood's characteristics.

Typically, the goal is to separate a so-called fines fraction, which is generally understood as particles that pass through a 3-mm wide round hole, and a so- called pin chips fraction, which is generally understood as particles that pass through a 7-mm wide round hole. The fines and pin chips are defined in the standard SCAN-CM 40:94, which is widely used in the pulp and paper industry. Fines and pin chips may, among other things, reduce the pulp strength, increase the consumption of pulping chemicals and cause flow problems in the process.

In practice, fines are generally globular or cube-like in shape, whereas pin chips are generally elongated thin sticks. Separation equipment should therefore be able to separate particles of at least two different shapes from the accept fraction. On the other hand it is important to separate only those sizes that are unwanted in a particular case, so as to be able to utilise as much of the valuable fibre raw material as possible.

Known devices for the separation of unwanted fine material from wood chips include gyratory screens, disc screens and roller screens. A gyratory screen consists of an inclined, perforated screen deck, which is made to vibrate at a high speed in order to generate an oscillating or revolving path of movement, whereby the finer materials fall through the holes in the screen deck. One known structure of a gyratory screen is presented in the Finnish utility model U930458. A deficiency of gyratory screens is their low selectivity, i.e. if the mesh size of the screen deck is so large that fines and particularly pin chips may effectively be separated, a considerable proportion of chips of acceptable size as regards the process requirements also fall through the screen. Furthermore, pin chips are inclined to stick in an upright position in the holes of the screen deck, thus plugging the holes.

A disc screen consists of successive, rotating axles, to which thin discs have been attached at equal intervals so that the discs of two consecutive axles overlap. The spaces between the discs have been determined according to the desired chip size, and finer material passes through the screen via the spaces. Known disc screen structures have been described, for example, in the Finnish patent application 780685 and the US patent publications 4871073 and 4901864. A deficiency of disc screens is the heavy wear on the discs and the high power demand due to the large contact surface between the material to be screened and the discs.

In a roller screen the disc axles have been replaced by rotating rollers. In this case, there are spaces of some kind between the rollers, through which finer material passes through the screen, while coarser material is conveyed on the rollers to the rear end of the screen.

The Finnish patent publication 89082 presents a roller screen structure in which there are pyramid-shaped protrusions on the roller surfaces, which protrusions consist of V grooves intersecting each other and encircling the rollers in the manner of a spiral. The diameter of the rollers is typically 50-60 mm and they are located so that the tips of the pyramid-shaped protrusions of consecutive rollers face each other in the longitudinal direction and that the distance between the tips is no more than the chip size of the fines fraction to be separated. A deficiency of this type of roller screen is the relatively poor capacity to convey chips due to the shape of the roller surfaces and, as a result of this, a low handling capacity and minor movement of the chip mat conveyed on the rollers due to the small roller diameter, as a result of which the chips are not mixed effectively and the separation of fine material is more difficult.

The Finnish patent publication 88118 presents a roller screen structure that has radial and substantially axial grooves on the roller surfaces, which grooves form teeth on the surface of the rollers. The teeth of each roller intermesh with the radial grooves of the adjacent roller, whereby, as the rollers rotate, they form momentarily closing compartments, in which fine material passes through the screen. This type of roller screen is suitable for separating fines from chips, but the separation of pin chips fraction containing elongated particles is not successful because particles of this shape are often too big for the compartments due to their length. Furthermore, intermeshing teeth moving to opposite directions create cutting planes which crush some of the usable chips.

Due to the circumstances described above, it has generally been necessary to make compromises in separation equipment according to the prior art as regards the separation of fines and pin chips.

The purpose of the invention presented here is to create a roller screen that minimises the above mentioned problems relating to the prior art.

The purpose of the invention presented here is in particular to create a roller screen that is able to separate both fines and pin chips from wood chips simultaneously and with good efficiency. The deficiencies mentioned above may be eliminated or decreased and the goals presented above may be achieved with a roller screen, a method used in the roller screen and an apparatus relating to the invention, which are characterised by what is defined in the characterising parts of the independent claims presented below.

Some preferred embodiments of the invention are described in the dependent claims presented below.

On the surfaces of the rollers of a typical roller screen according to the invention there are radial and substantially axial grooves, which form protrusions on the roller surfaces. The substantially axial grooves are deeper than the radial grooves, so that crosswise recesses are formed in the bottom of the radial grooves. Typically rollers rotating around parallel rotational axes rotate to the same direction, so that the movement of the rollers conveys the chip mat to be screened on the rollers.

A typical roller screen according to the invention is suitable for separating both fines and pin chips simultaneously from wood chips. Fines, i.e. cube-shaped or globular particles not more than 3 mm in diameter, pass through the screen mainly through the preferably substantially rectangular openings formed between the roller surface and the bottom of the radial grooves of the adjacent roller. The crosswise recesses in the bottom of the radial grooves improve the conveying capacity of the rollers as regards small particles. The elongated particles of the pin chips settle mainly in the bottom of the axial grooves due to the shape of the roller surface and pass through the screen. The inventive shape of the rollers according to the invention improves the movement of the chip mat being conveyed on the rollers to a suitable extent so that chips are efficiently mixed and the separation of fine material is successful. The sides of radial grooves are preferably perpendicular to the axis of the roller. Such grooves are easy to make and they form exactly suitable spaces for the separation of fines.

The rollers are preferably placed so that the radial protrusions on the surface of a roller are located at the radial groove of the adjacent roller. In this way the fines pass through the screen mainly through the preferably substantially rectangular openings formed between the protrusions of a roller and the bottom of the radial grooves of the adjacent roller.

It is typical of a particularly preferred embodiment of the invention that the sum of the maximum rotational diameters of two adjacent rollers is smaller than the axle spacing of the rollers multiplied by two. In other words, there is a small gap between the rollers in the direction of the material flow. Because the protrusions of this type of roller do not intermesh with the grooves of the adjacent roller no cutting planes are formed and thus no unwanted crushing of usable chips takes place. In one preferred embodiment of the invention the diameter of the rollers is 100-130 mm and the roller spacing is 0,5-1,5 mm.

It is typical of one preferred embodiment of the invention that the axial grooves are shaped like a letter V opening towards the roller surface. Due to the groove shape i.e. relief angle, chips that have fallen between rollers are not forced through the screen, but are lifted up by the rising teeth of the next roller. The opening angle of the said V is preferably 50-90 degrees and very preferably 60-70 degrees.

When required, a roller screen according to the invention may be used together with another or several screening devices. A typical screening plant for wood chips according to the invention includes a roller screen according to the invention combined with a thickness screen and/or deck screen, which is used for separating oversized and/or overthick particles from the chip flow. The thickness screen refers to a screen that separates chips substantially by thickness. A thickness screen may, for example, be a disc screen as to its mechanical structure. The deck screen refers to a gyratory screen which typically has 2 to 4 screen decks, i.e. perforated plates above one another. As opposed to thickness screens, the separation by a deck screen is based substantially on the maximum diameter of a chip.

In the following, the invention is described in more detail by referring to the accompanying schematic drawings, in which Figure 1 represents the side view of a preferred screen according to the invention,

Figure 2 represents the screen in Fig. 1 viewed from the discharge end,

Figure 3 represents rollers according to the invention from above and partially as a sectional view, Figure 4 represents a side view of the roller pair in Fig. 3,

Figure 5 represents a side view of one of the rollers in Fig. 3, and

Figure 6 represents an enlarged view of a part of Fig. 3.

Figures 1 and 2 present side and end views of a preferred roller screen 1 according to the invention. The wood chips to be screened are fed onto the roller screen 1 on the chute 3 of the screen's infeed end 2. Wood chips to be screened generally contain particles of several different sizes, for example the accept fraction, and pin chips and fines to be separated from the accept fraction by a device 1 according to the invention. The roller screen 1 comprises several rollers 4, which rotate in the same direction so that the chips to be separated are conveyed on the rollers towards the screen's discharge end 5. The rotational direction of the rollers 4 is marked in the drawings with arrows. The idea is to remove unwanted pin chip and fines fractions from the chips during the travel between the infeed end and discharge end by conveying these fractions through spaces between the rollers 4 into the space 7 below the rollers, where there is arranged, for example, a belt conveyor, not depicted in the drawing, for transferring the separated material to further processing. The accept fraction exiting from the last roller 4 of the device depicted in Fig. 2 is also passed on for further processing, for example by a belt conveyor, which is not depicted in the drawing.

A belt drive has been arranged for the rollers 4. Fig. 1 shows an electric motor 8 moving a driving belt 9, which is connected to the pulley 10 of the first roller. The pulleys 10 of two adjacent rollers 4 are connected with each other by drive belts 9 so that every other belt drive is always situated at the opposite side of the roller screen 1 as compared with the previous drive.

Fig. 2 further shows a support beam 11 running under the roller screen 1 parallel to the roller 4. A cover 12 covering at least most of the rollers 4 has been arranged on top of the roller screen 1. The sides of the device 1 have also been covered with walls 13 so that dust created by separation is not easily released into the environment. The cover 12 and walls 13 also stop unwanted materials from getting mixed among the wood chips.

The roller 4 in Fig. 2 is illustrated, for the sake of the clarity of the drawing, schematically mainly as a sectional view so that the roller mainly appears to have only radial grooves 14. A roller 4 according to the invention actually incorporates, in addition to the radial grooves 14, also slightly deeper axial grooves 15 as compared with the radial grooves, which axial grooves are discernible in the area 16 not drawn as a sectional view at the left-hand edge of the roller 4. The inventive grooving of the rollers 4 can be seen more clearly in Figures 3, 4, 5 and 6.

Fig. 3 gives a view from above of two adjacent rollers 4 of the roller screen 1. Again, both the radial grooves 14 and axial grooves 15 are only seen in the area 16 not drawn as a sectional view. Fig. 3 and particularly the enlarged part of it, i.e. Fig. 6 show how the radial groove 14 of one roller 4 has been arranged at the radial protrusion 17 formed between the radial grooves 14 of the adjacent roller. The adjacent rollers 4 have been arranged at such a distance from each other that the protrusions 17 do not intermesh. As shown by Fig. 6 and Fig. 4 in particular, a small gap 18 has even been left between the outermost parts of the rollers.

Fig. 5 shows a radial cross-section of the roller 4. The axial grooves 15, the edges 19 of which are formed so as to open at an angle of approx. 60 degrees in relation to each other towards the outer surface of the roller, are clearly visible in the figure. The axial groove 15 has a bottom 20. Between the axial grooves 15 remain the protrusions 17 parallel to the grooves. Fig. 5 also shows the bottom 21 of a radial groove 14 of the roller. Thus it can clearly be seen how the axial groove 15 is slightly deeper than the radial groove 14. The edges 22 of the radial grooves 14 (see Fig. 6) are typically perpendicular to the bottom 21 of the groove. In the roller 4 of one preferred embodiment of the invention, the depth a of the axial groove 15 is approximately 4 mm and the depth b of the radial groove 14 is approximately 3 mm. Thus the width of both grooves at the outer surface of the roller is correspondingly 5 to 6 mm. The diameter d of the roller 4 is typically 120-130 mm and the distance 18 arranged between the rollers is approximately 1 mm. Therefore the axle spacing e between two adjacent rollers 4 (see Fig. 3) is slightly greater than the diameter d of the rollers. A typical roller screen 1 according to the invention is, for example, 1 ,5- 2 metres wide and 2,5-3,5 metres long.

Pin chips and fines are separated from acceptable chips with a roller screen 1 according to the invention as follows. Chips are fed into the infeed end 2 of the screen 1, onto the rollers 4 rotating in the same direction. Fines, i.e. cube- shaped or globular particles not more than 3 mm in diameter, pass through the screen through the preferably substantially rectangular openings formed between the outer surface of the mainly radial protrusion 17 and the bottom 21 of the radial groove 14 of the adjacent roller. The crosswise i.e. axial recesses 15 in the bottom of the radial grooves improve the conveying capacity of the rollers as regards small particles. The elongated particles contained in the pin chip fraction settle, due to the shape of the roller surface, mainly on the bottom 20 of the said axial grooves 15 and thus pass through the screen 1. The particles in the accept fraction of the chips are too big to fit into the said recesses and openings and are thus conveyed on the rollers 4 towards the discharge end 5 of the screen 1.

It is obvious to a person skilled in the art that the invention is not confined merely to the examples presented above, but may vary within the scope of the claims presented below. The size and shape of the roller screen and its rollers may vary according to the need. There may, for example, be fewer or more rollers than in the examples presented in the Figures and the rollers may also be slightly conical in shape, for example. The rollers may also be of different sizes as compared with each other.

Claims

1. A roller screen (1) for separating two or more finer and coarser material fractions of wood chips from each other, in particular for separating fines fraction and pin chips fraction from the wood chips that are usable as regards chip size, the roller screen (1) comprising:

- two or more successive rollers (4) arranged to rotate around parallel rotational axles,

- a drive assembly (8, 9, 10) for rotating said rollers (4), - equipment (3) for the infeed of wood chips to be screened into the infeed end (2) of the screen, and

- equipment for the removal of screened wood chips from the screen (1), whereby on the surface of one or more of the said rollers (4) there are substantially radial (14) and substantially axial (15) grooves, which form protrusions (17) on the roller surface, characterised in that the substantially axial grooves (15) are deeper than the substantially radial grooves (14).

2. A roller screen as claimed in claim 1, characterised in that the rollers (4), rotating around parallel rotational axles, rotate in the same direction.

3. A roller screen as claimed in claim 1 or 2, characterised in that the radial protrusions (17) of the roller (4) are located at the radial groove (14) of the adj acent roller (4) .

4. A roller screen as claimed in any of the claims above, characterised in that the edges (22) of the radial grooves (14) are perpendicular to the longitudinal axis of the roller (4).

5. A roller screen as claimed in any of the claims above, characterised in that the axial grooves (15) are shaped like a letter V opening towards the surface of the roller (4), the opening angle of the said V being preferably 50-90 degrees and most preferably 60-70 degrees.

6. A roller screen as claimed in any of the claims above, characterised in that the sum of the maximum rotational diameters (d) of any, preferably all the adjacent rollers (4), is smaller than the axle spacing (e) of the same rollers multiplied by two.

7. A roller (4) of the roller screen for wood chips, on the surface of which there are substantially radial (14) and substantially axial (15) grooves that form protrusions (17) on the surfaces of the rollers (4), characterised in that the substantially axial grooves (15) are deeper than the substantially radial grooves (14).

8. A roller as claimed in claim 7, characterised in that the radial protrusions (17) of the roller (4) are located at the radial groove (14) of the adjacent roller

(4).

9. Apparatus for screening of wood chips comprising a roller screen (1) as claimed in any of the claims 1-6 above and is additionally combined with a thickness screen and/or deck screen.

10. Method for separating the fines fraction and pin chips fraction from acceptable wood chips in a roller screen (1) consisting of two or more rollers (4), which method comprises the following stages:

- wood chips to be screened are fed onto the rollers (4) at the infeed end (2) of the roller screen (1), - the rollers (4) of the roller screen are rotated in the same direction, - the fines fraction and pin chips fraction are conveyed between the protrusions (17) and recesses (14, 15) on the surfaces of adjacent rollers (4) to the underside (7) of the rollers,

- accepted wood chips are conveyed on the rollers (4) to the discharge end (5) of the roller screen (1), characterised in that the fines fraction is conveyed through the roller screen (1) mainly through the openings formed between the radial protrusions (17) of the roller (4) and the bottom (21) of the radial grooves (14) of the adjacent roller, and the pin chips fraction is conveyed through the roller screen (1) mainly on the bottom (20) of the axial grooves (15) of the roller (4).