Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B07—SEPARATING SOLIDS FROM SOLIDS; SORTING
  • B07B—SEPARATING SOLIDS FROM SOLIDS BY SIEVING, SCREENING, SIFTING OR BY USING GAS CURRENTS; SEPARATING BY OTHER DRY METHODS APPLICABLE TO BULK MATERIAL, e.g. LOOSE ARTICLES FIT TO BE HANDLED LIKE BULK MATERIAL
  • B07B1/00—Sieving, screening, sifting, or sorting solid materials using networks, gratings, grids, or the like
  • B07B1/12—Apparatus having only parallel elements
  • B07B1/14—Roller screens
  • B07B1/15—Roller screens using corrugated, grooved or ribbed rollers
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21B—FIBROUS RAW MATERIALS OR THEIR MECHANICAL TREATMENT
  • D21B1/00—Fibrous raw materials or their mechanical treatment
  • D21B1/02—Pretreatment of the raw materials by chemical or physical means
  • D21B1/023—Cleaning wood chips or other raw materials

Definitions

• 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.
• 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.
• 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.
• gyratory screens 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.
• 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.
• 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.
• 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.
• intermeshing teeth moving to opposite directions create cutting planes which crush some of the usable 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.
• 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.
• 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.
• 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.
• the diameter of the rollers is 100-130 mm and the roller spacing is 0,5-1,5 mm.
• 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.
• 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.
• 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.
• 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.
• 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.
• the edges 22 of the radial grooves 14 are typically perpendicular to the bottom 21 of the groove.
• 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.
• 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.
• 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.

Landscapes

• Engineering & Computer Science (AREA)
• Life Sciences & Earth Sciences (AREA)
• Wood Science & Technology (AREA)
• Mechanical Engineering (AREA)
• Combined Means For Separation Of Solids (AREA)
• Debarking, Splitting, And Disintegration Of Timber (AREA)
• Chemical And Physical Treatments For Wood And The Like (AREA)

Priority Applications (3)

Applications Claiming Priority (2)

Publications (1)

Family

ID=8562340

Family Applications (1)

Country Status (5)

Cited By (1)

Citations (3)

• 2001
  • 2001-11-23 FI FI20012306A patent/FI111055B/fi not_active IP Right Cessation
• 2002
  • 2002-11-19 AU AU2002342942A patent/AU2002342942A1/en not_active Abandoned
  • 2002-11-19 WO PCT/FI2002/000917 patent/WO2003043749A1/en not_active Application Discontinuation
• 2004
  • 2004-05-14 SE SE0401244A patent/SE527654C2/sv not_active IP Right Cessation
  • 2004-06-22 NO NO20042611A patent/NO330874B1/no not_active IP Right Cessation

Patent Citations (3)

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