Note: Descriptions are shown in the official language in which they were submitted.
BackcJround of the Invention
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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.
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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.
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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.
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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, ~-
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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.
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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.
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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~
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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.
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'Descripti.on o~ the Preferred Embodlment
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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.
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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 -~
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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,
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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
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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
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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
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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.
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