Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Ot~97
The invention relates to a screening
machine having a drum body, which is outwardly
covered with a screen cloth, and rotates around an
essentially horizontal axis.
The classification of products, especially
metallic powders, with particles mainly of spherical
shape in the range less than 0.1 mm, causes
considerable difficulties since, on the one hand, the
conventional screening methods, because of stuck
particles, require large screen areas in order to
keep the loading area low and, on the other hand, the
available methods of classification give only
unsatisfactory separation efficiencies and solids
outputs. Such products as well as other products
difficult to screen have been classified for a long
~ time on conventional fine particle screening machines
;~ which, admittedly, have extremely large screen areas
due to the low loading available area. With an
increasing number of particle layers on the screen
cloth, there is an increased tendency to plugging
since the weight of the top layer presses the
particles, which potentially can become stuck into
the meshes of the screen cloth. In addition, a great
~ number of particle layers lead to unsatisfactory
-~ 25 separation efficiencies.
The object of the invention is to provide a
compact screening machine suitable for material which
is difficult to screen, of which the operation is not
impaired by plugging.~ ; , !
The invention is based on the idea of
; preventing the number of particle layers from
exceeding a permissible maximum during the entire
screening operation and thus to arrange the screen
loading area in a predictable way.
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More specifically, the solution consists in
providing a screening machine of the type mentioned
above which comprises, openings uniformly distributed
over the drum body, which in conjunction with the
screen cloth mounted thereon form compartments for
the screen feed, a screen feed inlet which extends
inside the drum below the horizontal axis and a
discharge which extends in the longitudinal direction
inside the drum above the horizontal axis.
The compartments ensure that no more
additional screen feed can reach individual area
sections of the screen cloth during the entire
screening process after the screen feed has been fed
over the complete length of the rotary screen, so
that the screen loading area cannot increase beyond a -maximum which depends on the quantity of feed. In
~; this way, there is no plugging and moreover, a high
separation efficiency is obtained. In addition,
because of their transport function, the compartments
prevent any uncontrolled turbulence of the screen
feed which would have an adverse effect on the
separation efficiency. Particularly when operating -
the drum at a centrifugal number slightly less than
1, the compartments prevent the heavier particles
from not being discharged.
~`~ The screen feed inlet which extends in the
~; longitudinal direction below the horizontal axis
enables the simultaneous feeding of the screen feed
into the indivijdual compartments over the entire
length of the screen.
The discharge, arranged longitudinally ;~
above the horizontal axis, catches the oversize ~-~
particles which loosen from the screen cloth on the
entire length of the drum as a result of the force of
gravity which increases with the increasing distance
from the imaginary horizontal plane through the
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Z~OB~7
horizontal axis and carries them out of the drum.
The arrangement of the inlet and discharge according
to the invention, enables to use circular segments of
the screen cloth on the entire length only for the
screening or discharge of the oversize particles.
This results in an especially efficient use of the
available screen area which fits the compact design
of the screening machine.
A zig-zag shaped overflow edge on a feed
chute focuses the feed stream so that wet screen feed
or suspensions in particular can be properly directed
into the compartments.
A screening aid assists the screening
process in the individual compartments so as to
increase the separation efficiency. Such a screening
aid may include a nozzle plate, extending over a
segment of a circle a short distance from the drum
body, the blow fluid through, e.g. air, in a radial
outward direction. A short distance between the
nozzle plate and the drum body prevents the particles
contained in the individual compartments from
escaping and being mixed by means of the fluid
flowing through the compartments. The selection of
the fluid depends on the screen feed. Air is
acceptable for dry screen feeds which do not react
with air. If the dry screen feed can be expected to
react with air, an inert gas should preferably be
used for flushing. For suspensions and wet screen
: feeds, flushing fluids can also be used. Inside the
nozzle plate, collecting chambers connected to supply
pipes permit a simple supply of all nozzles with the
fluid via a main pipe which extends axially through
the screen drum and into which the individual supply
pipes feed together.
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An inlet which discharges into the gap
between the nozzle plate and the drum body, prevents
any turbulence of the feed material when filling the
compartments.
The drum body can be provided alternatively
with circular or rectangular openings. Circular
openings are preferred for very fine feed material.
Each opening preferably has a radial external section
with an opening of constant cross-section and a
section which radially flares in funnel shape towards
the interior. With such a design, the interior drum
area of the individual webs of the drum body, which
extends between the openings, is reduced, so that the
probability that fed material does not immediately
reach the compartments becomes smaller. -
In order to assist in the discharge of the
oversize particles and the individual particles stuck
~ ~ in the screen cloth in the area of the discharge
-~ trough, a spray nozzle sys~em which is essentially
mounted opposite the discharge trough can be arranged
outside the screening drum. Since this spray nozzle
system can be operated at a considerably higher
~ .
pressure than the nozzle system which acts as
screening aid, the discharge of oversize particles is
possible even at centrifugal numbers greater than 1.
In addition, with appropriate high pressure even the
most persistently stuck particles can be removed from
the mesh cloth and discharged from the drum via the
discharge trough.
For classifying bulk materials in the
particle size range about 10 mm, which can be carried
with conveyor belts, the product feed and discharge
can be accomplished only with a conveyor belt which
runs through the drum approximately in the zone of
the horizontal axis and has a guide which diverts the
screen feed to a vibrating chute. The vibrating
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chute delivers the screen feed. The oversize
particles remaining on the screen cloth slip back
onto the conveyor belt behind the guide via an
opening of the discharge according to the invention.
The invention is further illustrated by
means of the example which is shown in the drawings,
and in which: ~
FIGURE 1 is a schematic side view of the ~ ;
screening machine according to the invention; ~ -
FIGURE 2 is a schematic front view of the
screening machine according to the invention;
FIGURE 3 is an enlarged representation of a
section of the screen drum and the inside of the
screen drum;
FIGURE 4 is a schematic side view of a
screening machine with a conveyor belt running
through the drum; and
FIGURE 5 is a front view of the screening
~-~ machine according to Figure 4.
The screening machine according to the
-~ invention, consists essentially of a cylindrical
screen drum 1 which rotates around a horizontal axis,
and a drum body 2 which is outwardly covered with a
fine mesh screen cloth 3, preferably with a mesh size
of 0.1 mm. The drum body 2 has openings 4,5 uniformly
; distributed over its surface which in conjunction
. with the screen cloth 3 mounted thereon form
compartments for the screen feed. The openings 4,5
of the drum body 2 aire circular. Each opening 4 ~!5
has a radial external section 4 with an opening of
constant cross-section and a radial iinternal section
5 which flares inwardly in funnel a shape. The webs
13 between adjacent openings 4,5 or a series of
openings inside the drum are of a small width of -
about 4 mm.
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20~(~8~
Inside the screen drum 1, there is provided
a stationary nozzle plate 6, which extends over a
segment of a circle of about 240 , and which starts
at an angle of about 60 from the lowest point of the
screen drum 1. The gap between nozzle plate 6 and the
drum body 2 is about 2 mm.
A screen feed supply chute 7, which extends
longitudinally of the drum, and discharges the screen
feed by vibrations onto the entire length of the
screen drum 1, empties into the gap between the
nozzle plate 6 and the drum body 2.
The openings 8 of the nozzle plate 6 are
used to blow through air in a radial outward
direction in order to help the screening process in
compartments 4,5. Inwardly of the nozzle plate 6
there are air collecting chambers 9 connected to
supply pipes 10. A11 supply pipes 10 are connected
to a main pipe not shown which extends axially
through the screen drum 1. . ~::
In the area of the highest point of the
screen drum ]. there is a discharge trough 11, which
is open from above and extends axially through the
screen drum, with a pipe 14 located at the bottom of
the trough for discharging the oversize particles
.~ 25 from the screen drum 1.
. . ,
Outside the screen drum 1, oppos.ite the
. discharge trough 11, there is a pipe and spray nozzle
system 12 for blowing out stuck particles, which
still; occasionally become stuck in the fine mesh
screen cloth, and for helping to remove the coarse
~: material from the screen cloth.
The principle of air draft screening which
is used in conjunction with the compartments 4,5
. leads to a reduced discharge velocity through the
screen cloth compared to the air inlet velocity
through the openings 8. The slowed down air flow
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gently and thoroughly mixes the screen feed which is
on the screen cloth 3 whereby stuck particles are
avoided.
A conveyor belt 21 runs through the
screening machine according to Figure 4, with a guide
22 for the feed, arranged approximately in the middle
of the longitudinal axis of the drum 1 above the
conveyor belt 21.
Below the conveyor belt 21 a vibrating
chute 23 is built in for supplying the screen feed.
The discharge trough 24 has, in the direction of
movement of the conveyor belt 21, behind the guide
22, an opening 25 via which the separated oversize
particles are returned to the conveyor belt 21. A
screening machine with additional equipment according
to Figure 4, permits with only one conveyor system,
both the supply of screen feed and the removal of
oversize particles. The arrangement according to
Figures 4 and 5, is less suitable for the
classification of fine feed since the finest
particles would be moved away by the conveyor belt 21
~;~ underneath the guide 22.
- Because of the smaller, effective adhesive
forces, a spray nozzle system 12 can generally be
;~ 25 completely omitted when classifying coarser screen
feeds since, because of the component of the force of ;
gravity of the individual particles which increases
towards the highest point of the screen drum 1, the
- particles loosen by themselves from the screen cloth
3 and fall into the discharge trough 24 so long as
the centrifugal number of the drum 1 is not greater
~- than 1. Therefore, Figure 5 shows spray nozzles 12,
in the first place to ensure a reliable discharge of ~ ;
oversize particles also with fluctuating drum speeds
and thus accompanying changes in the centrifugal
number, and secondly with occasional plugging.
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