Method and apparatus for screening particulate materials

A method and apparatus for use in separating oversize from on size particles in a mass of particulate materials passed rapidly through a screening device includes a sloping screen mesh having sizing openings dimensioned to be larger than the desired maximum on size particles intended to pass through the mesh. The screen mesh is supported at a relatively steep slope angle and the cosine of the angle of the slope multiplied times the mesh opening size is set up to be substantially equal to the desired maximum on size particles to be passed through the screen mesh. A particle rebound shield is spaced closely above the upper surface of the screen mesh for reflecting and directing particles that bounce off the screen mesh back towards the openings therein. The material flow rapidly over the mesh because of the steep slope angle and the rebound shield insures that on size particles are directed toward the openings in the mesh a sufficient number of times during their travel through the apparatus to pass through the mesh.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a new and improved method and apparatus 
for separating oversize and on size particles in a mass of particulate 
material which is passed through a screening apparatus. 
2. Description of the Prior Art 
In accordance with the prior art, most vibrating screens are mounted with 
the screen cloth at a slope angle of from 15.degree. to 30.degree. above 
the horizontal. The material is passed over the upper surface of the 
screen from top to bottom usually under the influence of gravity alone and 
the flow is relatively slow because of the relatively shallow angle. The 
particles that are much smaller than the openings in the screen cloth 
easily pass through the screen mesh with little difficulty and similarly, 
the particles that are substantially larger than the screen mesh openings 
are positively rejected and do not pass through the mesh. These oversize 
particles move readily along and are eventually taken off the lower end of 
the screen as oversize. The main problem with prior art screening devices 
of the character described occurs with the particles of a critical 
particle size range wherein the particles are just slightly greater in 
size than the openings in the screen cloth. These critical size particles 
tend to fall into the open mesh, but because they cannot pass through the 
openings they become wedged solidly in the mesh causing the screen cloth 
to become blinded or plugged up. When this occurs, the screening operation 
is highly inefficient and as more material is passed across the screen, 
repeated collisions causes the particles to become wedged even more 
firmly. Cleaning of the screen cloth is then required before operations 
can be efficiently maintained. 
In accordance with the present invention, however, the problem of blinding 
or wedging of critical size particles in the screen cloth openings is 
greatly reduced or entirely eliminated and the screening action is greatly 
speeded up. 
It is an object of the present invention to provide a new and improved 
separating device and method for separating oversize from on size 
particles in a mass of particulate material flowing across a screening 
apparatus. 
More specifically, it is an object of the present invention to provide a 
new and improved screening apparatus wherein blinding or wedging of 
particulate material in the screen openings is not a significant problem. 
Another object of the invention is to provide a new and improved screening 
apparatus wherein particulate material is handled at a rapid rate and in a 
highly efficient manner. 
Still another object of the invention is to provide a new and improved 
method and apparatus for screening material wherein the openings in the 
screen mesh are dimensioned to be considerably larger than the desired 
maximum on size particles with less chance of plugging up or blinding of 
the screen cloth yet with little or no oversize particles passing through 
the screen mesh. 
Still another object of the present invention is to provide a new and 
improved screening apparatus wherein the screen mesh is supported at a 
relatively steep slope angle thus minimizing the possibility of critical 
sized particles tending to wedge or lodge in the slightly smaller sized 
screen openings causing the screen cloth to blind or plug up. 
Yet another object of the invention is to provide a new and improved 
screening apparatus having a shield for rebounding the particles which 
bounce away from the screen cloth back toward the screen so that on size 
particles have numerous chances to pass through the openings in the 
screen. 
SUMMARY OF THE INVENTION 
The foregoing and other objects and advantages of the present invention are 
accomplished in a new and improved screening apparatus for use in 
separating oversize from on size particles in a mass of particulate 
material passed over a mesh. The apparatus comprises a steeply sloping 
screen mesh having sizing openings therein dimensioned to be considerably 
larger than the size of the desired maximum on size particles which are 
intended to pass through the mesh. The slope of the screen is selected so 
that the cosine of the slope angle multiplied times the opening size in 
the screen mesh is substantially equal to the size of the desired maximum 
on size particles. A particle rebound shield is spaced closely above the 
upper surface of the screen mesh and the shield repeatedly directs any 
particles bouncing off the screen mesh back towards the mesh openings in 
the screen. Because of the steep slope angle and because the openings in 
the screen cloth are considerably larger than the on size particles 
passing therethrough, there is little or no tendency for critically sized 
particles, slightly larger than on size, to become wedged or lodged in the 
screen cloth as in the prior art screens. In addition, because of the 
steep slope angle the material moves rapidly down the screen and the 
rebound shield provides assurance that the on size particles are 
repeatedly directed back toward the screen mesh so that the on size 
material passes through the screen and does not flow along with the 
oversize particles without separation.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now more particularly to the drawings, in FIGS. 1 and 2 is 
illustrated a new and improved screening apparatus constructed in 
accordance with the features of the present invention and referred to 
generally by the reference number 10. Particulate material which is to be 
treated in the apparatus is introduced into the upper end portion by means 
of a funnel shaped hopper 12 and flows out of a discharge opening at the 
lower end of the hopper in a relatively steeply sloped downward path 
indicated generally by the dotted arrows"A." The screening apparatus 
includes a rectangular support frame 14 formed of side channels or frame 
members 16 and upper and lower headers or cross members 18. If required, 
several intermediate cross members may be provided. The frame 14 provides 
support for a screen cloth or mesh 20 preferably of the woven type having 
transverse cross strands 22 and longitudinal strands 24 interwoven to form 
generally square or rectangular openings for sizing the particles of 
material. The screen mesh or cloth may be formed of strands of steel, 
plastic fibre or other material and the diameter of the strands is 
selected to provide the needed physical strength for the particular 
application and materials involved. 
On the underside of the screen frame 16 there is provided a removable 
bottom panel 26 for collecting the on size particles of material passing 
through the openings in the screen cloth and containing a flow of this 
material as it moves downwardly as indicated by the arrows "B" along the 
underside of the screen. The on size material eventually reaches the lower 
end of the apparatus and passes out through a discharge opening 26a (as 
shown in FIG. 1) into a discharge chute 28 which is connected to the 
bottom panel 26. 
The particulate material flowing along the upper surface of the screen 
cloth 20 which does not pass through the mesh and becomes oversize 
material eventually passes out the lower end of the apparatus into an 
oversize discharge chute 29 (as indicated by the arrows "C"). 
In accordance with the present invention, the apparatus is provided with a 
rebound shield 30 or cover spaced a short distance "D" above the upper 
level of the screen mesh 20 as best shown in FIG. 2. The distance "D" is 
in the range of approximately 1/8 inch to 1/2 inch and may vary from this 
range if required. The distance "D" is selected to insure that the 
particles of the material which rebound or bounce off the screen cloth 
strands 22 or 24 as shown by the bent arrows "E" will again and again be 
deflected and reflected back towards the sizing openings in the screen 
cloth as indicated by the bent arrows "F" in FIG. 2. Accordingly, as the 
mixture of on size and oversize particles of material flow along the upper 
surface of the screen cloth as indicated by the arrows "A," the particles 
which strike the strands and bounce away repeatedly are deflected back 
towards the screen cloth again and again so that there is little chance of 
any sizable portion of the on size material flowing out the oversize 
discharge chute 29. This is true even though the velocity of the flow is 
relatively high in comparison with customary low angle screen 
applications. To additionally aid in the screening action, the screen 
frame 14 may be provided with one or more vibrator units 32 which cause 
the screen mesh 20 to vibrate and provide a live screening action for the 
particles of material. Preferably, the vibrator units are mounted on the 
side to vibrate the cloth back and forth rather than up and down. 
In accordance with the present invention, the screen apparatus 10 is 
mounted in a position with the screen cloth 20 sloping upwardly above the 
horizon at a relatively steep slope angle indicated by the angle "alpha" 
in FIG. 1. It has been found that a steep slope angle in the range of 
approximately 40.degree. to 60.degree. works well and preferably, a slope 
angle of 45.degree. or greater provides excellent screening action in 
accordance with the principles of the present invention. The relatively 
steep slope angle results in a relatively high velocity of flow of the 
material through the screen apparatus and this is advantageous from a cost 
standpoint in handling materials. 
In accordance with the present invention, a particular size screen cloth 20 
is selected so that the horizontal wires 22 are spaced apart by a selected 
distance or opening size "G" (FIG. 2) which is considerably greater than 
the maximum size of on size particles of material which are to pass 
through the screen mesh. The opening size "G" is related to the particular 
slope angle "alpha" that is used and is also related to the maximum size 
selected for the on size particles which are to pass through the screen as 
indicated by the distance "H." This relation is chosen in accordance with 
the equation wherein the cosine of the angle "alpha" multiplied times the 
screen opening distance "G" between the cross strands 22 is substantially 
equal to the maximum size of the on size particles (dimension "H"), which 
should pass through the screen mesh. For example, if the screen mesh 20 is 
supported in a horizontal position wherein the angle "alpha" is 0.degree. 
then the cosine of the angle is equal to 1 and accordingly, the spacing 
between the strands 22 of the mesh and the maximum size of the on size 
particles would be the same. When the screen is then elevated to a 
relatively steep slope angle of 45.degree. or more in accordance with the 
invention, the distance "G" between the strands 22 of the screen may be 
chosen to be considerably larger than the maximum size of the on size 
particles desired (dimension "H"). The steep slope angle decreases the 
effective size of the openings (the horizontal component) and as an 
example, the spacing "G" between the cross strands 22 may be selected to 
be 1.4 to 2.0 times greater than the maximum diameter of an on size 
particle that is to pass through the screen cloth. A screen cloth having a 
mesh spacing sized to pass particles of 610 microns or less can be used in 
accordance with the present invention at a steep angle to effectively 
screen and separate out particles having a dimension maximum of 420 
microns. The steep slope angle of the screen apparatus 10 provides a 
number of advantages, for example, the feed rates for a given material can 
be substantially increased over the feed rate for the same material on a 
screen having a lower slope angle of the same mesh opening size. In 
addition, with a relatively steep slope angle, the screen cloth becomes 
self-cleaning because the tendency of the particles of a critical size 
range (i.e., slightly larger than the maximum size of the on size 
particles) to become wedged in the screen openings or blind the screen 
mesh is eliminated or greatly reduced. In the event, however, that a 
slightly oversize particle does become temporarily wedged in the screen 
mesh, the continuing impact of the steeply flowing material against the 
side of the wedged in particle almost always dislodges the wedged 
particle. The closely spaced rebound shield 30 is positioned near the 
upper surface of the screen cloth 20 to permit free downward flow of the 
particles (arrows "A") and those particles that strike the strands and 
then bounce off, are repeatedly deflected back to again contact the screen 
mesh. This increases the effective contact time of the mass of flowing 
particles against the screen mesh as the particles move from top to bottom 
even though the particles are moving very rapidly along the screen cloth 
on the relatively steep slope thereof. 
Another feature of the present invention resides in the fact that a 
particular size of screen mesh 20 may be utilized for providing screening 
action for several desired particle sizes by changing the angle of slope 
"alpha" to increase or decrease the horizontal effective screen opening 
"G." Thus, it is not necessary to change to different mesh size screen 
cloth each time a different particle size is required. For example, a 30 
mesh screen having an opening size of 610 microns may be supported in a 
45.degree. slope angle and with this particular angle, the effective 
opening size of the screen becomes 420 microns which is equivalent of a 40 
mesh screen. A further increase of the angle "alpha" to approximately 
54.degree. provides an effective screen opening size of 300 microns which 
is the equivalent of a 50 mesh screen. An increase of the slope angle to 
60.degree. further reduces the effective screen opening to 210 microns 
which is the equivalent of a 70 mesh screen. One screen size can be used 
to screen many different sizes of particles. The closely spaced rebound 
shield 30 works in conjunction with the steep slope to provide a greatly 
increased contact between the flowing particles and the screen mesh than 
would otherwise be possible with the high overall flow rate. Referring to 
FIG. 2, even though the effective vertical opening in the screen mesh 20 
(dimensioned as indicated by the letter"I") is somewhat greater than the 
desired maximum size of the on size particles to pass through the screen 
mesh there is little chance of such oversize particles passing through the 
mesh and this is because of the parallel direction of flow generally along 
the screen (as indicated by the arrows "A"). The close spacing of the 
rebound shield 30 and its generally parallel orientation relative to the 
screen causes the particles to rebound in a generally downward direction 
at a relatively steep angle incident to the surface of the screen cloth so 
that only the smaller on size particles pass through the larger size 
screen openings. The problem of wedging or blinding of the screen cloth in 
the apparatus 10 is thus virtually eliminated, high flow rates are 
possible, and a highly efficient screening action results. 
Although the present invention has been described with reference to a 
single illustrated embodiment thereof, it should be understood that 
numerous other modifications and embodiments can be devised by those 
skilled in the art that will fall within the spirit and scope of the 
principles of this invention.