CA1046989A - Angularly adjustable bent screen classifying and dewatering device - Google Patents

Abstract

ANGULARLY ADJUSTABLE BENT
SCREEN CLASSIFYING AND DEWATERING DEVICE

Abstract of the Disclosure Screening apparatus provides improved dewatering of liquid slurries by flowing the slurry across a generally parabolically shaped perforate screening surface inclined at an angle to the horizontal. The angularity of the generally parabolically shaped screening surface is adjustable, which adjustment may be made at any time or at any point in the screening process. Adjustment of the angle of the slurry feed is also provided for, particularly in response to adjust-ments in the angularity of the screening surface. The disclosure includes several different arrangements of screen bars and means for feeding the slurry to the inlet end of the screening surface.

Description

BackcJround of the Invention ~___ _ This inverl-tion relates to screening devices for separating liquids from slurries, and in particular to an improved dewatering screen having a parabolic surface, ~nd a method of using the same.
A great many screening devices of the so-called "Sidehill" type have been used for the gravity separation of solids from liquid slurries, or the thic]~ening of such slurries by introducing the slurry to a perforated, slotted, or ~lesh screening surface disposed at some sloping angle to the horizontal. By introducing the slurries at the top of an inclined screening surface, liquids are induced by gravity to drain vertically away from the solid materials contained in the slurry, with the mass becon~ing progressively thickened as it rolls downward across the screening surface. -The classic wire mesh screen, although useful, tends to provide a slow rate of drainage, in addition to a - tendency to "blind" over. To overcome these problems, some devices have employed a bar-type slotted screen surface.
The screening bars are generally rectangular or trlangular in cross-section, and are arranged on and fixed to backing ; bars to provide a controlled width slot substantially con-tinuous throughout the screening surface in any given direction. These bars have been disposed both paralle], and at angles ranging up to 90, to the direction of flow of the incoming slurry.
' '~ , '.' . ' ' ,

- 2 - ~
:
:

r~
8~

Ano~her approach to overcome the tendency of slurries to "blind" over a screen has been to spray the thickened solids with a liquid from the li~uid discharge side of the screen. This approach is generally self~defea-ting in that the reintroduction of liquids to the thickened material tends to minimize total fluids removal, and results in a thinner material being discharged.
Yet another approach has been screening devices of the consta~t or cylindrical curve type. These are generally premised on the assumption that the drainage characteristics of the slurry solids remain constant as the slurry passes downward on the curved surface, but the assumption has not ;
proved to be true. Introduc-tion of the slurry to the screening surface at a considerable velocity at the top thereof does provide rapid dewatering of the solids in the upper surfaces, `
but very little resistance to downward flow of the solids.
As a result, a reduced dewatering rate occurs in the lower regions of the screen when additional resistance is encountered.
.
Thus, none of the prior art devices or processes provide for the varia~ility of the surface tension o~ the `~
fluids in the slurry, the drainage rates of free water from the surface of the solid materials themselves, or the range of consistencies from incoming slurry to outgoing thickened solids between the various solid materials.
A need therefore exists for a screening apparatus which takes into account the varying drainage rates, fluid ;
surface tensions, and consistency ranges possessed b~v slurries as they are fed to and progress down dewatering screen surfaces.
'; ' -, ~ ' ' , ' '' ' ' ' '~' ' . ' .' .
1 ''~ ` "'"' ' ': : ' 6~
Summary of the Invention The invention is directed to a screening apparatus having a semi-parabolic screening surface. The parabolic shape of the screening surface compensates for the varying ability of liquid to be extracted from a slurry flowing thereon and results in greater forces being applied to the slurry in those areas where maximum advantage of these individual drainage forces can best be suited to the dewatering characteristics of the slurry.
The consequential result is increased dewatering of the slurry at those points in the process when there would ordinarily be a decrease in the dewatering because of the thickened solids. `
The invention, in one aspect, provides screening apparatus of the type described for separating a flowing liquid slurry into fractions by substantially unidirectional flow along a perforate screening surface, comprising a) means defining a perforate screening surface having opposed sides and ends, b) said screening surface means being of rigid structure and concave semi-parabolic shape from end to end thereof such that the curvatur~ thereof varies from a zero point at one end thereof through a series of short radii adjacent said one end to a maximum radii adjacent the other end thereof, c) said screening surface means including an assembly of multiple parallel bars extending in closely ' spaced relation substantially parallel with said ends of ~ -said surface and having exposed flat surface areas which define said semi-parabolic screening surface, ` 30 ~) the spaces between adjacent said bars constituting slots defining the perforations in said screening surface, ~-~.

:~ .

. . ~ .

e) means supporting said screening surface means with said zero point end uppermost, f) inlet means for supplying the slurry to said higher end of said screening surface whereby the angular velocity of the slurry is adjacent the maximum end of its range while the liquid content of the slurry is adjacent the maximum end of its range, g) means for collecting the fraction of the slurry which is retained on said screening surface from the lower end thereof, and h) means for separately collecting the fraction ~ -of said slurry which passes through said screening surface.
In one embodiment, the screen comprises semi- ~ -parabolically curved, vertically extending, laterally (or ;
horizontally) and parallel spaced wedge shaped bars, supported by and fixed to laterally extending, vertically spaced bar supports. ;~
The screen bars together form the screening surface. In another ~;
embodiment, the screen comprises straight, laterally extending, vertically and parallel spaced wedge-shaped bars supported by and fixed to vertically extending, semi-parabolically curved, laterally spaced bar supports~
The screens of any of the embodiments may have sidewalls attached to their vertical peripheral sides, which sidewalls aid in keeping the slurry on the screen sur~ace. The : screens can be mounted in conventional screening apparatus comprising an input or feed means, and discharge means for both the thickened solids or oversized fraction and the liquid and/or undersized fraction.

; . .

. ~ .
'::

-;-!`,~ ~, .

':~ ' ' : , - ' . , , ., . , . j ~6~

Further, means are provided to adjust the angularity of the screening surface, so that adjustments can be made for variabilities in the slurry inputs, and dewatering efficiency can be optimized once the dewatering process has begun.
The angularity of the screening surface is the relationship of a tangent to the screen at some point in the curvature thereof to a horizontal line, e.g., the ground. This is true regardless of the orientation of the screening surface.
Thus, if a point on the profile of the surface of the par~Glic curve which describes the shape of the screening apparatus is selected and a tangen~ is drawn through that point, the tangent will intersect a horizontal reference line at some angle. This angle is the angularity of the screen-ing surface. A reori~ntation of the screen will result in a change ox adjustment in the angularity of the screen with respect to the tangent drawn through the previously referred to reference point. Means are also provided so that the angularity of the slurry input can be adjusted in relationship to any adjustment of the angularity o the screening surace.
It is therefore an object of this invention to provide a screening apparatus having a semi-parabolic screening surface.
It is auf~ther object of the invention to provide ; an improved screening apparatus which accommodates for the variability in the draining rate of slurries.

. t ~, !

6~9 It is yet another object of the invention to provide an improved screening apparatus having a screening surface which is angularly adjustable.
It is still a further object of the invention to provide an improved screening apparatus having a slurry input which is angularly adjustable to coordinate i-t with the angularity of the screening surface.
It is an additional object of the invention to provide an improved method of screening wherein the decreasing draining rates of slurries can be accommodated.
Other objects and advantages of the invention ~mll be apparent from the following description, the accompanying drawings and the appended claims~

.

,, . , . ,. , , , ,.... .. , , :,: :, . , -.. - . ,: , , .. .:

~la 46~
Brief ~escription oE the Dra~;ngs In the drawinys -Fig. 1 is a perspective view of dewatering apparatus embodyin~ the present invention;
Figs. 2 and 3 are perspective views, partially broke~ away, of embodiments of dewatering screens in accordance with the invention for use in the apparatus of Fig. l;
Fig. 4 is a partial ~section taken along the line 4--4 in Fig. 2;
Fig. 5 is a partial section taken along the line 5--5 in Fig. 3;
Figs. 6 and 7 are views similar to Fig. 5 showing other embodiments thereof;
Fig. 8 is a view partia~ly in section and partially lS in elevation of a dewatering apparatus similar to that shown in Fig. 1, but embodying a modified input or fe2d means;
Fig. 9 is a perspective view of a further modification in the feed means similar to that shown in Fig. ~; and - Fig. 10 is a graph showing parabolic curves for ; 20 incorporation in the dewatering screens of the invéntion.

, ' ~:''',, .,~ . , ' . . .

",'~ :
: . .
. ~,'. '.' .

~'' '. .
''' ', ' ,, , " .. , , - ~. ,- , ., . , . , , , , , , . , ,, .. i:

6~8~
'Descripti.on o~ the Preferred Embodlment -- - .~ -, The screening apparatus shown in Fig. 1 Gomprises a slurry feed or input section 10, a screening sur~ace 11, and a discharge end or section, such AS clischarge means 12 and 13. The slurry to be screened could be, or example, a wood pulp slurry to be dewatered, with the thickened solids or fibrous pulp leaving screening surface 11 via discharge means 12, while the li~uid or wate,r passes through the screening surface'and out discharge means 13. Alternatively, the screening surface could ~e used to classify wet particles, in which case the overs.ized fraction o particles would leave screening sur~ace or screen 11 via discharge 12, while the under-;sized'frac~ion of particles and the li~uid fraction would pass through the screening surface and out discharge 13. Thus, , ~ust about any kind of particle can constitute the slurry to be screened.
. .. .
Generally, screen 11 is sloped or inclined at anangle to the horizontal which is steep enough to permit the '`' slurry or aggregate to .slide or roll down the screen, with ' 20 no vibrating force being applied to the screen. ,he angle ,' will also be shallow enough that the slurry will not over10w the screen. ~he precise angle will vary wi-th the tvpe of - '~
material being screened. An alternative to gravlty feed is pressure feed wherein the slurry is unifor~ly introduced to the screening surface under pressure and thus at a greatQr velocity than is provided by gra~ity. Pressure feed allo~s practically '~unlimited orientation or the screen, as gravity is no longer relied upon to hold the slurr:y on the screen.
Fig. 1 illustrates one embodiment o slurry feed or '' 30 input section 10, which comprises an input pipe or manifold ', ,' 14, from a slurry source, ~not shown), which distributes the slurry to a plurality of hoses 15 connected to noz21es 16 arranged to feed the slurry to ~creen 11. ~lthougll three hoses -~
- ' . ;': ~'' ~

9~
are illustrated, as many hoses can be used as are needed, or as few as one, as long as the slurry is effecitvely intro-duced along the width of the screening surface.
Nozzles 16 may be conveniently mounted on a rack or the like, such as rack 17 attached to housing 20. Once mounted and aimed, the nozzles will continue to distribute slurry unformly to screening surface 11 and at a fixed angle with relation to the screening surface.
The nozzles may be used to impart a high initial velocity to the slurry, as an aid to dewatering, by uniformly feeding under pressure, or merely to distribute the slurry to the screen such that it is carried down the screen by the force of gravity alone. Further, other pressure feeding and/or distributing means can be employed as an alternative to nozzles, such as a pressure head box or the like.
Screening surface 11 will have a semi-parabolic shape.
; Fig. 10 graphically illustrates several parabolic curves which might be employed for the screening surface. The curves in Fig. 10 are based on the formula Y=n~2, where n varies between 1.5 and 1/20, but the invention is not necessarily limited to that formula so long as the result is that the curvature adjacent one end of the screening surface will be about a series of shorter radii than the curvature along the majority of the screening surface approaching its other end, with the end of minimum radius corresponding with the zero point in the example illustrated in Fig. 10. Additionally, and as required by the nature of the slurries fed to the screening apparatus, a logarithmic curve ~ould be employed, e.g. Y=(log X)n. Further, ~..

. . . . . - . . -.: . . . . : , - ~ . - -- .- . - : ~ . ., 69~
the semi-parabolic screens may be employed in series, as when a plurality of screens are arranged so as to permit cascading from one screening surface to another, and the like.
Generally, the parabolic shape of the screening sur-face seems to p~ovide improved dewatering because as the dewateredslurry or thickened stock passes across the screening surface and approaches the discharge point, the radius of curvature of the screen tightens (or lessens), offering increased resistance to the downward flow of the thickened stock and presenting a continually increasing force component to the fluids contained in the slurry to induce them to pass through the screen and depart from the surface of the solids to be thickened.
Although not intending to be limited to the above theory, it does seem ~o explain why the screening surface of the invention compensates for the slowing of dxainage rates and declining velocity gradients, i.e., declin:ing fluid dewatering forces, found in conventional dewatering screens.
Screening surface 11 may be mounted for operation in any conventional screening apparatus, but adjustments may have to be made depending on the shape and design of the apparatus.
As shown in Figs. 1 and 8, screening surf~ace or screen 11 is mounted in housing 20, by supporting it on surface 21 between discharges 12 and 13 and the top 22 of housing ~0. In this way, screen 11 can be readily removed for the replacement of worn screens or the substitution of different size screens.
Sidewalls 23 are provided on the lateral peripheral edges of screen 11 to confine the slurry flow to the screening surface.
Housing 20 is supported for rotational movement by a hinge pin or rod 25 which passes through sides 26 of housing 20 r and is attached thereto by welding or the like, and through holes in frame 28. By mounting housing 20 for rotation about the axis of rod 25, the angularity of screen 11, seated in . . . . ................. . . .
~ ,, : . . . .

6~
housing 20 can be adjusted to compensate for variations of input slurry conditions.
In order to hold housing 20 in the selected angular 5 positions, a hole 30 is provided in each of sides 26 of housing 20. A series of holes 31 are provi ded in frame 28 in such location that the loci of their axes describe an arc which co-incides with the arc described by the movements of the axes of holes 30 when housing 20 is rotated about rod 25. Thus once 10 the angular position is selected, the hole 30 on each side 26 should aoincide with one of holes 31, or at least be in proximity theretoO By passing pins 32 through selected holes 31 and holes 30, housing 20 is temporarily locked in the selected angular position. To adjust or readjust the angularity of 15 screen 11, it is a simple matter to remove pins 32 and realign holes 30 with others ~3~ holes 31. Alternatively, more sophisticated means, such as screws, levers, latches, and the like, can be emplo~ed to move and hold the housing in various angular ; positions.
The parabolic shape or curve of the screening surface 11 may be the generatices of either vertically or horizontally longitudinally extending screening bars such as bars 40 in Fig. 2 and bars 44 in Fig. 3. Alternatively, the screening sur-face could comprise a wire mesh or any of the traditional screening 25 materials, as long as they were shaped to have a generally parabolic curve.
As shown in Fig. 2, each bar 40 has a parabolic shape nd is supported by and joined to transversely extending, but vertically spaced, supports 42, by welding or the like. The 30 spacing of the bars i5 not critical, and will depend upon such factors as the material being screened, the kind of classification .~"?.~ desired and the like. Thus the spacing can range :Erom a few thousandths of an inch to a rough grizzly. Sidewalls 23 are ' :

.

8~
joined to the ends of supports 42 Oll either side of the screen-ing surface and confine the slurry to the screening surface.
The embodiment of the screen in Fig. 3 is formed by joining laterally (or horizontally) extending bars 44 to para-bolically shaped supports 45, which are laterally spaced.
Addltional horizontal supports 46 can be employed so that side-walls 23 can be joined to the vertical pexipheral edges of the screen.
Bars 40 and 44 are shown in Figs. 4, 5, 6 and 7 as generally wedge-shaped, i.e., triangularly or trapezoidally shaped bars joined to their supports 42 and 45 at their apexes, i.e., the point or narrow edge of the bar. But the invent~on is not limited to any particular shape or material for or way of joining the screening bars. Rectangular or cylindrical shapes can also be used, and the bars can be made from metal, plastic or the like material. The bars need not be`normal to the direction of flow of the slurry, and thus could be angled to the direction of flow. Further, the bars could be canted in the direction of flow. Bars 51 in Fig. 6 are canted downstream, while bars 52 in Fig. 7 are canted upstream.
An alternative embodiment for the input or feed means 10 is shown in Fig. ~. Instead of nozzles, the slurry is fed via hose 15 to a gravitv flow box 60 supported for ` pivotal movement by cvlindrical bosses or hinge pins 61.
The slurry fills flow box 60 and overflows edge 62 so that it passes down screening surface 11 essentially tangentially thereto as indicated by arrow 63. When adjustments are made in the angularity of screening surface 11, gravity flow ~ .. -.;.................................................................. ~
.~
' -12~
. . , . ~ . . .

~469OEE
box 60 can be pivoted about hinge pin 61, adjusting the angle of discharge of the slurry, so that it will continue to feed tangentially to screening surface 11. Discharge edge 62 can comprise a V-notch weir 64, as shown in Fig. 9. Alter-natively, the weir could be a U-notch or other geometry, since its purpose is merely to provide even distribution of the slurry to the screening surface.
While the methods herein described, and the forms of apparatus for carrying these methods into effect, constitute preferred embodiments of this invention, it is to be under-stood that the invention is not limited to these precise methods and forms of apparatus, and that changes may be made in either without departing from the scope of the inven~ion.

~.; ;t " ' `

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

Screening apparatus of the type described for separating a flowing liquid slurry into fractions by substantially unidirectional flow along a perforate screening surface, comprising a) means defining a perforate screening surface having opposed sides and ends, b) said screening surface means being of rigid structure and concave semi-parabolic shape from end to end thereof such that the curvature thereof varies from a zero point at one end thereof through a series of short radii adjacent said one end to a maximum radii adjacent the other end thereof, c) said screening surface means including an assembly of multiple parallel bars extending in closely spaced relation substantially parallel with said ends of said surface and having exposed flat surface areas which define said semi-parabolic screening surface, d) the spaces between adjacent said bars constituting slots defining the perforations in said screening surface, e) means supporting said screening surface means with said zero point end uppermost, f) inlet means for supplying the slurry to said higher end of said screening surface whereby the angular velocity of the slurry is adjacent the maximum end of its range while the liquid content of the slurry is adjacent the maximum end of its range, g) means for collecting the fraction of the slurry which is retained on said screening surface from the lower end thereof, and h) means for separately collecting the fraction of said slurry which passes through said screening surface.

Screening apparatus as defined in claim 1 further comprising means supporting said screening surface means for pivotal movement about a horizontal axis to adjust the angularity thereof, and means for releasably holding said screening surface means in adjusted angular position.

Screening apparatus as defined in claim 1 wherein said bars are of generally trapezoidal section with their larger base surfaces exposed and defining said generally parabolic screening surface.

Screening apparatus as defined in claim 1 wherein said semi-parabolic shape of said screening surface is based on the formula Y=nX2 where the zero point of said curvature is at the intersection of the X and Y axes, and n varies between 1.5 and 1/12.