WO1992011408A1

WO1992011408A1 - Wood chip screen

Info

Publication number: WO1992011408A1
Application number: PCT/US1991/009353
Authority: WO
Inventors: Joseph B. Bielagus
Original assignee: Beloit Corporation
Priority date: 1990-12-19
Filing date: 1991-12-13
Publication date: 1992-07-09
Also published as: AU9156091A; PT99846B; ZA919929B; MX9102661A; PT99846A; NZ240941A

Abstract

A method and apparatus for screening particulate material such as wood chips including providing parallel extending bars (13, 14), with the surfaces (13a, 14a) of certain bars being planar at the top with sloping sides (13b, 13c; 14b, 14c), and in one arrangement, engaging the material moving from a receiving end (11) to a discharge end (12) against the direction of movement such as by passing teeth (37) through the material in the direction to act counter to the movement of material, increasing the retention time and orienting the material in a longitudinal direction.

Description

WOOD CHIP SCREEN

BACKGROUND OF THE INVENTION

The present invention relates to improvements in method and apparatus for screening particulate material such as wood chips.

More particularly, the invention pertains to a screening deck defining a screening area, wherein the deck is formed of a series of parallel bars with spaces therebetween, with the bars uniquely shaped, and, in one form, further engaging the particulate material in a direction opposite its movement, to increase its retention time and orient the material in the direction of the slots between the bars.

In the screening of wood chips, the chipping mechanism produces various types of chips which vary considerably in size and shape. For the digesting process in the production of wood pulp, it is desirable that the chips supplied have a uniform thickness in order to achieve optimum yield and quality, that is, to obtain a pulp which contains a low percentage of undigested or overtreated fibers. Under preferred conditions of digesting, the pulping liquid penetrates into chips uniformly. If chips are provided which have too great a thickness, the digester will produce chips with an untreated core of undigested fibers. If chips are provided which are too thin, the digester will produce chips that are overcooked and of low quality. To insure proper digestion of the chips in the production of pulp, the supply should not contain any chips having an excessive thickness which will give rise to lack of adequate penetration during the digestion process.

Apparatus has been provided heretofore for screening chips, and customarily these screening devices are of the disk screen type which have a plurality of generally- circular disks mounted on parallel, rotating shafts. The disks are mounted coaxially on each shaft and spaced for each other, and the disks interleave with the disks of adjacent shafts to form screening gaps between the disks of one shaft and the disks of adjacent shafts. One drawback associated with such disk screening apparatus is the fact that the effective or open screen area in a given screen dimension is necessarily limited, and the number of shafts provided with the disks will, therefore, be large in an industrial installation requiring substantial production capacity. Also, by reason of precision requirements of the gaps between the disks, the manufacturing costs are relatively high. Since the disks of adjacent shafts interleave with each other in the screening area, there is friction on the surfaces interleaved due to the material to be screened becoming lodged between the disks and also by reason of resin deposits on the disks. The counter- rotational relationship between adjacent interleaved surface can force material into the gap, degrading chip quality and further increasing friction. It has been found that friction is one of the main causes of the high power requirements of such screen apparatus. It has also been found that it is difficult to maintain a uniform gap during operation of such apparatus since the disks may not be mounted exactly at right angles, and therefore flutter with respect to each other during operation.

The disk screening apparatus heretofore used is also highly sensitive to sand, stones and scrap, and therefore subject to wear. To reduce such wear, it has been common to plate the disk with hard chromium, further increasing cost.

It is accordingly an object of the present invention to provide a screening apparatus for wood chips or the like that, for a comparable screen size, has substantially higher industrial capacity than structures heretofore available and which avoids the drawbacks associated with disk screening apparatus.

It is a further object of the present invention to provide a screening apparatus which provides a horizontally extending screening bed having simplified operation as contrasted with a disk screening apparatus.

A further object of the invention is to provide a bed type screening apparatus having a large screening area and requiring less operating power than devices heretofore available.

A further object of the invention is to provide an improved screening apparatus which is subject to less clogging and better transportation of the chips over a screening bed having features which orient the chips and provide more consistency in the screening operation, while limiting pinching, wedging or other chip quality degrading effects.

FEATURES OF THE INVENTION

In accordance with the invention, a screening deck or bed is provided which extends substantially horizontally, providing a large screening area. While the inventive features are susceptible of uses in other industries, for the purposes of description herein, the structure and operation will be described in connection with the screening of wood chips for which the features of the present invention are particularly well adapted.

Chips are distributed across a receiving end of the screening deck which is formed by a series of parallel bars. In a preferred arrangement, both sets of bars are oscillated for effecting screening and moving the chips in a forward direction. The bars having a uniquely shaped upper edge with a central planar portion and laterally tapering sides sloping away from the planar top whereby the screening function is

improved by aiding in material agitation and orientation of the chips and avoiding clogging of the openings.

In one a preferred form, an arrangement is made wherein the advancing chips are engaged by a plurality of teeth moving in a direction opposite the progressive movement of the chips, thereby increasing the retention time of the material and orienting the material in a longitudinal direction.

Other objects, advantages, and features will become more apparent with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiments thereof in the specification, claims and drawings, in which:

DESCRIPTION OF THE DRAWINGS

Figure 1 is a side elevational view shown somewhat in diagrammatic form of a screening device constructed in accordance with the principles of the present invention;

Figure 2 is a fragmentary plan view of driving mechanism for oscillating bars of the screening device,- Figure 3 is another side elevational view shown somewhat in schematic form, similar to Figure 1 but illustrating the arrangement of drive mechanism;

Figures 4 and 5 are schematic elevational illustrations taken along the screening bed showing different positions of the screening bars during screening operation; and

Figure 6 is a top plan view showing the screening bed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As illustrated in Figure 1, the mechanism includes a horizontal upwardly facing screening bed 10 having a receiving end 11 where the wood chips are received and a discharge end 12 where the reject material is discharged. The wood chips to be screened are received at the receiving end 11 and move along the bed from left to right as shown in Figure 1, with the chips of acceptable width passing between screening bars and the chips which are too large and other reject material which is too large continuing to move along the bed to be discharged at the discharge end 12 of the screening apparatus.

As illustrated, the screen is for separating oversize from acceptable material. Properly sized and operated, for some applications, the screen can be used to remove undersized material. In such use, the material falling through the screen would be rejects and that discharged at discharge end 12 would be the acceptable material. Further use herein of the terms reject and accepts or variations thereof are for differentiation in description and are not meant as limitations on the use of the present invention.

The screening bed is formed by a plurality of parallel bars 13 and 14 such as illustrated in Figure 6, with the bars having uniformly wide spaces therebetween. The spaces are of predetermined width such that chips which are too large and which would be too thick to be satisfactorily penetrated by the liquor in a digester are not accepted but will stay on top of the screening bed to move off to the discharge end 12.

To aid in the screening operation, and to aid in the movement of the chips from the receiving end 11 to the discharge end, alternate screening bars are joined in sets, forming two grids which are oscillated by being moved both up and down and forward and back in a manner to advance the chips.

The grids of bars each are mounted on movable frames which are carried on rotors having the movable frame eccentrically connected tnereto. At the discharge end of the screening bed, the movable frames are connected to similar eccentric supports mounted on rotors.

Figures 1, 2, and 3 best illustrate the mounting of the grid of bars 14, wherein a frame 15 is carried on rotors 18 and 19 on the inlet end, eccentrically connected to the rotors at supports 20 and 21 respectively. At the discharge end of the screening bed, the frame 15 is connected to eccentric supports 22 and 23 on rotors 30 and 31. The frame of bars 13 is similarly connected by eccentrically mounted supports on rotors at both the inlet and outlet ends.

As the rotors at each end of the bar frames rotate, namely the rotors 18 and 19 at the receiving end of the screen and 30 and 31 at the discharge end of the screen, the bars will oscillate alternately up and down and alternately forward and back.

For driving the movable bars in oscillation, a main prime mover driver 25 is provided. This drives a chain 40 driving a sprocket 32. The sprocket contains additional sprockets driving chains or belts 26 and 27 which are connected to drive the rotors 19 and 31. These rotors carry sprockets which, through chains or belts 28 and 29, drive the upper rotors 18 and 30.

For purposes of describing the operating cycle of the screen, the cycle will be presumed to start from a position wherein the grids are at substantially equal height as illustrated in Figure 4, wherein the tops of all bars define a substantially horizontal plan. From this position, one grid moves upwardly and the other grid moves downwardly. Figure 5 depicts the grids with the bars 14 being at the upper most position in the operating cycle, and the bars 13 being at the lower most position in the operating cycle.

From the position illustrated in Figure 5, the bars 14 begin moving downwardly, and the bars 13 begin moving upwardly. At a point half-way through the operating cycle, all bars will again be at substantially equal heights as illustrated in Figure 4. The bars 13 continue moving upwardly, and the bars 14 continue moving downwardly until a bar position substantially opposite that shown in Figure 5 is reached, wherein the bars 13 are at the upper most position, and the bars 14 are at the lower most position.

Since the grids of bars are mounted on eccentric drives, the vertical movement is accompanied with horizontal movement. Thus, from the position illustrated in Figure 4, as the bars 14 move upwardly they also move forwardly to the upper most position as shown in Figure 4 and continue moving forwardly until mid-way through the cycle when the bars 13 and 14 are again substantially aligned. As the bars 14 move downwardly from the mid point, the bar also moves rearwardly through the lower most position, and continues moving rearwardly as the bar moves upwardly to the mid-way point illustrated in Figure 4. The horizontal movement of bars 13 is the same as that for bars 14. Thus, as a grid of bars moves upwardly from the position illustrated in Figure 4 to the upper most position, and as the grid moves downwardly from the upper most position again to the position illustrated in Figure 4, the grid also moves forwardly. As either grid moves downwardly from the position illustrated in Figure 4 to the lower most position, and as the grid moves upwardly from the lower most position again to the position illustrated in Figure 4, the grid moves rearwardly. Since the grids are 180° out of phase, one grid is moving forwardly as the other grid is moving rearwardly, and one grid is moving upwardly while the other is moving downwardly.

The combined movement of the bars up and down and forward and rearward conveys the oversize chips from the inlet end to the discharge end, and also aids in turning the chips so that the thickness dimension is presented to the space between bars for proper screening.

A feature of the present invention is the unique shape of the upper surface of the bars. This shape aids in material agitation and in material orientation, and promotes unclogging of the openings while inhibiting initial clogging of the openings between the bars.

As illustrated in Figures 4 and 5, the bars have an upper planar surface 13a, 14a which is flat and parallel to the horizontal bed. At each side of the horizontal portion 13a, 14a, are tapered portions which provide planar surfaces 13b, 14b and 13c, 14c facing generally upwardly but sloping away from the top surface. These surfaces have been found to tend to prevent clogging of the gaps between the bars and to aid in material agitation.

For typical wood chip screening, acceptable bar dimensions have been found to be one-half inch in thickness and one and one-half to two inches in height from top to bottom. The top surfaces 13a, 14a are about one-eighth inch, and the side surfaces 13b, 14b and 13d, 14c are disposed at a forty-five degree angle from the top surface, and extend approximately one-quarter inch.

For distributing the wood chips laterally relatively uniformly across the receiving end of the screening bed, distributing auger 34 is mounted for rotation and is driven by a chain 33. Such augers are conventional devices for distributing material along their length and will not be described in greater detail herein.

A further feature of the present invention is the provision of independent means engaging the wood chips in a direction counter to their flow, to increase retention time on the bed and to orient the chips in a longitudinal direction. This is accomplished by fingers 37 which move through the chips on the bars. For this purpose, the fingers are carried on a rotor 35 which is driven by a drive chain 36 in rotation in a clockwise direction as shown in Figure 1 . The fingers 37 pass through the chips against the direction of movement of the chips along the grids. This increases the retention time of the chips on the screen and tends to orient the material in the longitudinal direction, improving the screening operation and improving the efficiency and uniformity by properly aligning the chips for screening.

As shown in Figure 6, two shafts with fingers are used. In some instances, one may be adequate and in others more than two may be desirable. Shafts with evening fingers positioned downstream from the inlet may be provided with fingers spaced more closely than shafts closer to the inlet end. The more closely spaced fingers will properly orient more chips, and, since the volume of chips on the screen downstream from the inlet is reduced from the volume at the inlet end, the closely spaced fingers will not overly retard chip advancement.

In operation, wood chips are distributed laterally along the receiving or head end 11 of the screening deck 10. The wood chips move along the screening bed longitudinally toward the discharge end 12, and those which are sufficiently thin will pass through the spaces between the bars. The bars supported on the movable grids will oscillate up and down in the manner shown in Figures 4 and 5. To delay the movement of the chips and to help orient the chips in a longitudinal direction, fingers 37 carried on. rotor 35 are moved against the direction of chip movement. Acceptable chips of the maximum tolerable width and narrower will pass through the spaces between the bars, and other unacceptable chips will continue on down the screening deck toward the discharge end 12.

The stroke of each bar should be only slightly less than the bar height dimension. Thus, if bars two inches high are used, the distance from the upper most elevation of bar movement to the lower most elevation should be only slightly less than two inches. Some vertical overlap should be maintained so that proper proper screen opening size is maintained between adjacent bars, and so that chip wedging does not occur. However, the overlap region should be minimal when the grids are at the extreme positions shown in Figure 5. This opens up the screen below each screen opening, again minimizing chip wedging and allowing "caught" chips to pass through without clogging the screen.

For typical wood chip screening, bar displacements of 2 inches to 3 inches are preferred, with the rotary drives to which the bars are eccentrically connected being driven at 200 to 250 r.p.m. Too slow operation and too shallow of displacements result in chip matting due to insufficient agitation and insufficient chip tipping. Excessive speeds of the drive cause the chips, and particularly smaller acceptable chips, to become suspended above the screen, limiting engagement time for proper sizing.

Thus, it will be seen I have provided an improved chip screening device which meets the objectives and advantages above set forth and provides an improved, simplified screening mechanism and method.

Claims

I CLAIM AS MY INVENTION:

1. A screening apparatus for separating a particulate material comprising in combination, an upwardly facing screening deck (10) defining a screening area with screening openings, said screening deck having a receiving end (11) and a discharge end (12) with material distributed onto the receiving end (11) to move toward the discharge end (12) so that large material moves longitudinally the length of the deck (10) and smaller material passes through the screening deck (10), said deck (10) having screening elements (13,14) defining thickness openings therebetween for passing smaller material, char terized in that: said elements have substantially planar top surfaces (13a,14a) extending from said receiving end (11) to said discharge end (12) , and have generally upwardly facing top lateral planar surfaces (13b,13c,14b, 14c) forming an angle with the top surfaces (13a, 14a) for aiding in material agitation and orientation and unclogging of the openings.

2. A screening apparatus for separating a particulate material such as wood chips constructed in accordance with claim 1: wherein said lateral planar surfaces (13b,13c; 14b,14c) extend downwardly and outwardly from said top planar surfaces (13a,14a) at angles of about 45°.

3. A screening apparatus for separating a particulate material such as wood chips constructed in accordance with claim 1: wherein said elements (13,14) have opposed lateral planar surfaces (13b,13c; 14b,14c) on two sides, each forming an angle of about 45° with the top planar surfaces (13a,14a).

4. A screening apparatus for separating a particulate material such as wood chips constructed in accordance with claim 3: wherein said opposed lateral planar surfaces extend in a longitudinal direction the entire length of said screening elements.

5. A screening apparatus for separating a particulate material such as wood chips constructed in accordance with claim 1: including means (18,19,25,26,27,28,29,30,31,32) for moving some of said elements upwardly in an agitating motion relative to other of said elements.

6. A screening apparatus for separating a particulate material such as wood chips constructed in accordance with claim 1: wherein alternate elements are collectively joined into grids (15), thereby defining two grids, and said grids are mounted for combined substantially vertical and horizontal movement.

7. A screening apparatus for separating a particulate material such as wood chips constructed in accordance with claim 1: wherein said elements comprise individual bars (13,14) extending in the direction from the receiving end (11) to the discharge end (12) and extending parallel to each other.

8. A screening apparatus for separating a particulate material such as wood chips constructed in accordance with claim 7: wherein alternate bars (13,14) are collectively mounted in grids (15) and each grid is eccentrically mounted on rotary drive means (18,19,20,21,22,23, 30,31).

9. A screening apparatus for separating a particulate material such as wood chips constructed in accordance with claim 7: wherein said elements have a central planar portion (13a,14a) extending parallel to the screening area, with lateral areas (13b,13c; 14b,14c) on each side of the planar portion extending downwardly and outwardly at an angle to the screening area.

10. A screening apparatus for separating a particulate material such as wood chips constructed in accordance with claim 1: wherein said elements comprise a first set of bars (13) extending longitudinally along said screening deck in the direction from the receiving end (11) toward the discharge end (12) ; a second set of bars (14) interdispersed between the first bars and defining uniform spaced openings between the bars of a predetermined width; said bars having flat planar upper surfaces extending in the screening area; and at least one set of the first and second sets of bars having a central planar portion (13a or 14a) and lateral downwardly and outwardly sloping portions (13b and 13c or 14b and 14c) at each side of the planar portion.

11. A screening apparatus for separating a particulate material such as wood chips constructed in accordance with claim 10: wherein said sets of bars (13,14) are mounted on drive means (18,19,25,26,27,28,29,30,31,32) to be oscillated in substantially vertical and horizontal directions simultaneously, thereby aiding in the screening of the material between the bars and moving the material from the receiving to the discharge end of the deck.

12. A screening apparatus for separating elongate, particulate material such as wood chips, comprising in combination: an upwardly facing screening deck (10) defining a screening area with openings and extending from a receiving end (11) to a discharge end (12); means (34) discharging material onto the receiving end (11) of the deck to move toward the discharge end (12), with accepts passing through the screening deck (10) and rejects moving to the discharge end (12); and independent engaging means (35,37) moving in a direction from the discharge to the receiving end and being in contact with material on the screening deck and increasing retention time of the material and orienting the material in a longitudinal direction extending from the receiving to the delivery end.

13. A screening apparatus for separating a particulate material such as wood chips constructed in accordance with claim 12: wherein said engaging means is carried on a rotor (35) rotating in a direction to move the engaging means in the direction from the discharge end (12) to the receiving end (11).

14. A screening apparatus for separating a particulate material such as wood chips constructed in accordance with claim 12: wherein said engaging means is in the form of a plurality of downwardly extending teeth (37) engaging the material as it moves along the screening deck.

15. A screening apparatus for separating a particulate material such as wood chips constructed in accordance with claim 12: wherein the screening deck (10) is defined by a plurality of parallel bars (13,14), with screening spaces therebetween, extending from the receiving end (11) to the discharge end (12); and said engaging means includes a plurality of teeth (37) extending outwardly from a shaft rotated above the screening deck.

16. The method of screening elongate particulate material such as wood chips comprising the steps: distributing particulate material onto an upwardly facing screening deck (10) leading from a receiving end (11) to a discharge end (12) and formed of parallel bars (13,14) having screening spaces therebetween extending in a longitudinal direction from the receiving end to the discharge end; raising and lowering adjacent of said bars alternately, to agitate and orient material; and simultaneously oscillating adjacent bars alternately forwardly and rearwardly from inlet to outlet for transporting the material from the receiving end to the discharge end.

17. The method of separating particulate material such s wood chips in accordance with the steps of claim 16: including engaging some of said material opposite in direction from said transporting.

18. The method of separating particulate material such s wood chips in accordance with the steps of claim 16: said engaging some of said material is performed by rotating teeth (37) through said material from above the screening deck.

19. The method of screening particulate material such od chips in accordance with the steps: distributing particulate material onto an upwardly facing screening deck (10) having screening openings therethrough with the material being moved from a receiving end (11) to a discharge end (12) of the screening deck; and engaging the chips in a direction opposite the chip movement as the chips move from the receiving end to the discharge end increasing retention time of the material and orienting the material in the longitudinal direction parallel to the movement from the receiving end to the discharge end.

20. The method of screening particulate material such od chips in accordance with the steps of claim 19: wherein the screening deck is formed of parallel bars (13,14) extending from the receiving end (11) to the discharge end (12) and having uniform spaces therebetween and increasing the retention time of the material by passing teeth (37) into engagement with the material in the direction opposite the material movement from the receiving end to the discharge end.