US1950924A - Method and apparatus for screening fibers in aqueous suspension - Google Patents

Description

M. W. xmmssma LEEQEM METHOD AND APPARATUS FOR SCREENING FIBERS IN AQUEOUS SUSPENSION mmh L59 Filed Nov. 2, 1952 M MU @EMM mm Nm my www. @fm VWK@ www? mm/WT MLA Patented Mar. 13, 1934 UNITED STATES METHOD AND APPARATUS FOR SCREENING FIBEBS IN AQUEOUS SUSPENSION Henry Wyatt Johnston, Montreal, Quebec, Canada Application November 2, 1932, Serial No. 640,828

6 Claims.

This invention relates to the screening and classifying of fibrous material in aqueous suspension and has particular reference to the screening and classifying of wood and other pulp bers used in the manufacture of paper and paper products.

In the paper industry, screening of pulp suspensions is resorted to for various purposes. In some cases the principal object is to eliminate w undesirable constituents such as fibers or bundles of bers which are too long, or otherwise dimensioned, so that their presence in the finished paper would be liable to impair its desired properties. In other instances, selective screening is resorted to in order to effect a definite classification of the fibers according to their physical dimensions or to determine the proportions in which fibers of different sizes are present in pulps used in the manufacture of different kinds of paper or paper products, this last information being desirable as a basis for removing or adding fibers or reducing the size of the bers by processing to produce a particular kind of pulp.

With the methods and apparatus now employed for screening aqueous suspensions of pulp bers certain difculties are encountered with respect to classifying the fibers according to a fixed standard. This is due, in certain instances, to conditions which prevent the openings in the screen serving their intended purpose of establishing an absolute standard for determining, within limits, the physical dimensions of the bers permitted 4to pass the screens. In the absence of special precaution the fibers retained by the screen form a matte which clogs the screen opening so that the matte, rather than the screen, constitutes the filter medium which determines the nature of the filtered pulp. The characteristics of the matte formed on a screen having openings of definite size varies considerably according to the concentration of the pulp suspension and the nature and dimensions of the bers contained therein. Consequently, when operating under these conditions, it is well known that the character of the filtered pulp, instead of being definitely determined according to the standard established by the dimensions ofthe screen openings, is largely controlled by other factors such as the rate of pulp passing in unit time, and the concentration, nature and dimension of the fibers present in the pulp suspension. Water jets, raking devices and various other expedients have been resorted to in order to prevent the matting of the fibers on the screen surface, but even with these expedients the difficulties mentioned persist to some extent. Moreover, there is another difficulty encountered with the screening methods now employed in that, in each instance, the bers tend to orient themselves in the direction of flow through the screen openings so that the fibers are separated or classified according to variations in cross sectional dimensions rather than in accordance with variations in length. Now, it is well known that fibers used in paper making differ from each other more in the matter of length 35 than they do in the matter of diameter or cross sectional dimensions so that it is more desirable to classify them according to their variations in length rather than according to their variations l in cross sectional dimensions. It is also desirable that the fibers should be thus classified according to an absolute standard established by the dimensions of the screen openings and not according to a variable standard such as that established by the character of the matte formed on the screen by the fibers which are too large to pass the screen openings.

According to the present invention these desirable conditions are obtained by the application of principles which, so far as I am aware, have not heretofore been employed in connection with the screening and classifying of pulp fibers in aqueous suspension. When operating according to this invention the pulp suspension is caused to flow over the surface of a screen in a nonsinuous manner so that the velocity of flow parallel or substantially parallel with the screen surface is considerably greater than the velocity of fiow which takes place through the openings of the screen or, in other Words, at right angles to the screen surface. Under these conditions the fibers tend to orient themselves in the direction of the ow parallel with the screen surface and are presented broadside to the screen openings. If a fiber thus presented to one of the screen openings is short enough to pass therethrough Without striking it will do so but if it is too long to pass or strikes one of the edges of the opening, it will be carried away by the current flowing parallel with the screen. Consequently, it will be evident that only those fibers which are less than a certain length will be passed by the screen while the longer bers will be kept in the suspension. Under these conditions each aperture Vof the screen surface constitutes a limit gauge or standard for determining the maximum length of the fibers passed by the screen. By employing a series of such screens, the suspension may be classified*u into groups of fibers, each of a length lying between certain limits determined by the absolute physical dimension of the screen apertures selected irrespective of other considerations such as the nature or concentration of the fibers. Another advantage inherent to this method of screening is that'it is impossible for a matte of the rejected bers to form on the surface of the screen, since the bers which are too long to pass the screen openings are kept in the suspension by the force of the current flowing parallel with the screen surface. Provision is also made for repeated recirculation of the pulp suspension over the screen so that each ber is washed free from smaller fibers adhering thereto and all fibers are eventually presented broadside to the screen openings until all the fibers which are capable of passing the openings when so presented thereto have been removed in the afliuent.

In the accompanying drawing I have shown a View partly in vertical section and partly in elevation of one form of apparatus which I have successfully used in reducing the present invention to practice.

As herein illustrated, said apparatus includes a casing 11 having its top wall 12 provided with a central opening 13 surrounded by a depending neck 14. Said wall 12 is also surmounted by a vertical annular wall 15 forming a tray 16 provided with a lateral inlet 17. The pulp suspension to be screened enters the tray through the inlet 17 and is then delivered through the opening 13 and neck 14 into the chamber 18 of the casing 11 in which the actual screening of the pulp is effected. The screen 19 through which the pulp is iiltered is clamped between the lower portion of the casing 11 and a bottom plate 20. Any suitable means may be provided for fastening the casing and the bottom plate together. In the present instance I have shown a plurality of bolts 21 pivoted to the casing and provided with wing nuts 22 in clamping engagement with the lower surface of the bottom plate. As herein shown, the bottom plate 20 is shaped or recessed to provide a chamber 23 beneath the screen from which the filtered pulp is discharged through a suitable outlet 24.

The screen 19 comprises a metal plate provided with perforations or slots 19a of predetermined dimensions. The central portions thereof is covered by a baille plate 25 positioned directly below an impeller 26 having its blades or vanes 2'7 vertically inclined as shown. This impeller is driven by the shaft 28 of a motor 29 and is arranged to operate in the central opening 30 of a surrounding annular baille 31 of hollow construction. As viewed in cross section this baille presents parallel side walls 32, a rounded or dome shaped top wall 33, and a flat bottom wall 34, the latter being joined to the side walls by rounded corner portions 35. Due to this particular cross section it will be noted that the upper portion of the central opening 30 is relatively large and flaring. Said baille 31 is suitably supported in the casing with its bottom wall 34 spaced above the screen 19 and its outer side walls 32 spaced from the surrounding wall portion of the casing l1.

In the use of this apparatus the impeller 26 imparts a vertical velocity to the pulp contained within the chamber 18, the inclination of the impeller blades 27 serving to direct the pulp as a jet against the baille plate 25 covering the central portion of the screen 19. Batlle 25 serves another purpose in that it prevents the fluid or pulp which has already passed the screen 19 being sucked back into the chamber 18. If this baille is omitted or is not of proper diameter with respect to the size of the openings in the screen, there is a tendency for the suction or siphoning e'ect incident to the flow of pulp over the upper surface of the screen to suck back a portion of the uid ano'. possibly a portion of the fibers whichhave already passed the chamber 23. This slows up the screening action in considerable degree and also gives rise to certain other difficulties. After striking this baille plate the pulp spreads out in nonsinuous flow towards the periphery of the screen and then circulates upwardly over the outer side and top walls of the baille 31 and is drawn into the vortex formed by the action of the impeller for recirculation.

From this description it will be evident that the baille 31 and the impeller conjointly provide means whereby the pulp is directed against the central portion of the screen in the form of a jet so that, in accordance with well known principles of hydraulics, the resulting spreading of the pulp over the screen is in the nature of a steady nonsinuous flow. In addition to this, the baille 31 is useful for maintaining a substantially uniform velocity over the surface of the screen and for causing the pulp to follow a circuit which permits its repeated recirculation over the screen by the action of the impeller. The flow of the pulp over the screen is regulated so that the velocity of flow parallel or substantially parallel with the plane of the screen is sufficiently greater than the flow through the screen openings to cause.

the bers to orient themselves in the direction of the ow parallel to the screen and be thereby presented broadside to the screen apertures as indicated by the fibers appearing at A, B and C. The fiber shown at A is of a length that will just permit it to pass broadside through one of the screen openings, as indicated. The fiber shown at B is of the same length but is just approaching one of the screen apertures. The ber shown at C is too long to pass the screen aperture over which it is positioned and is being moved onward by the force of the current flowing parallel to the screen. From this description, it will heapparent that, as previously stated, each fiber that is short enough to pass through one of the openings when presented broadside thereto will do so, while the longer bers Will be moved onward by the current and kept from matting on the screen. Due to the relative arrangement of the battle 31 and the impeller, repeated circulation of the suspension is provided for until all the bers that are capable of being passed through the screen as described have been removed from the suspension.

The allluent draining through the screen into the chamber 23 is discharged through the outlet 24 to any suitable point of delivery.

If it is desired to practice selective screening of this invention, this may be conveniently accomplished by using the screening devices in series, each unit having a properly selected screen. In this case the outlet 24 of one unit will be connected to the inlet 1'7 of the next adjacent unit.

In this way the fibers of the suspension may be pulp is screened in each unit will be substantially uniform.

The casing 11 is preferably provided with a valve controlled air escape vent 3'? since it is found that air, if present in the chamber, is apt to interfere in appreciable degree with the desired non-sinuous flow of the pulp suspension over the pulp screen 19.

In the present instance I have not shown any means for supporting the baille 31 in the chamber 18 since this may be accomplished in various ways without interfering with the required circulation of the pulp. For example, said baille may be suspended by brackets from the side walls of the chamber 18 or it may alternately be suspended from the depending neck 14 by any suitable type of suspension or fastening means.

Having thus described my invention, what I claim isz- 1. A process of screening fibers in aqueous suspension which comprises forcibly directing the suspension against the central portion ot the screen and causing said suspension to flow over the screen in a steady, non-sinuous manner.

2. Screening apparatus of the character described comprising a chamber equipped with a horizontal screen forming the bottom wall of the chamber, an impeller positioned above the screen, and a baille encircling said impeller, said baffle being spaced-from the screen and from the surrounding wall of the chamber.

3. Screening apparatus of the character described comprising a chamber equipped with a screen forming the bottom wall of said chamber, a baille plate covering the central portion of the screen, an impeller spaced above the baiiie plate, and an annular baille surrounding said impeller and spaced from said screen and from the surrounding wall of the chamber.

4. A process of screening fibers in aqueous suspension which comprises masking the central portion of a horizontally disposed screen to prevent flow of the suspension therethrough, forcibly directing the suspension against the masked portion of the screen and causing said suspension to ow over the screen in a steady non-sinuous manner.

5. Screening apparatus of the character described comprising a horizontal screen, a bathe covering the central portion of the screen and means for forcibly directing fibers in aqueous suspension against said baiiie and causing said suspension to ow over the screen in a steady non-sinuous manner.

6. Screening apparatus of the character described comprising a chamber equipped with a horizontal screen forming the bottom wall of the chamber, a baiile covering the openings in the central portion of the screen, means for directing a jet of bers in aqueous suspension against said baille and 'causing said suspension to flow over the screen in a steady non-sinuous manner so that the velocity of flow parallel with the plane of the screen is considerably greater than the velocity of iiow through the screen openings.

, HENRY WYATI JOHNSTON.

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