Process for screening granules

An apparatus and process for screening a product grade of particulate matter from a feed-stream of particulate matter is provided. Specifically, the apparatus is a vibration screening action machine which vibrates, including motion having a component thereof perpendicular to the plane of the screen deck, at least one screen deck having a mesh defining the lower limit of the product grade. Such vibration screening action machine is optimized for ensuring the throughput of particulate fines through the mesh defining the lower limit of the product grade and thus substantially removing particulate fines from the product grade by setting the screen deck at a relatively low angle, as measured from horizontal, of less than 15 degrees.

TECHNICAL FIELD 
The present invention is related to an apparatus for screening dry 
particulate matter so as to sort such incoming particulate matter into a 
plurality of size grades. More particularly, the present invention 
includes a process and apparatus for optimizing such screening to minimize 
unwanted fine material, hereinafter referred to as "fines", in the product 
grade matter. 
BACKGROUND OF THE INVENTION 
The basic problem to which the present invention is addressed is the 
classifying or sorting of particulate matter into certain size grades. The 
present invention is particularly applicable to mineral particulates, 
hereinafter referred to as granules, which are sorted so that a specific 
product grade granule is removed a feed stream of mineral granules. 
Granules larger than the product grade are sorted out and may be further 
processed and/or recirculated within the feed stream. The fines which are 
smaller than the product grade are also to be removed from the product 
grade granules. 
It is particularly desirable to remove all, or at least as much as 
possible, of the fines from the product grade granules when such product 
grade granules are going to be subjected to further treatments or 
processing. Treating unwanted fines within the product grade granules 
increases the costs of treating and producing the product grade granules 
because such processing or treatments are also applied to the fines which 
are unusable as a product grade granules. 
In one specific technology, namely the production of colored granules for 
use as roofing granules to be applied to roofing, such as shingles, it is 
highly desirable to remove the mineral fines from the product grade 
granules because of the expense in coating each of the granules with a 
specific pigment layer. Such pigments are normally applied within a 
ceramic coating. Moreover, specific grade granules are required for proper 
application of the roofing granules to roofing products, such as shingles, 
as such granules not only provide the aesthetic qualities to the end 
roofing product, but also protect the materials which comprise the roofing 
products, such as the asphalt-base of a shingle. 
Many different methods and apparatuses are known for classifying 
particulate matter including dry sorting and wet sizing. The present 
invention is specifically directed to the field of dry screening processes 
and apparatuses. In a dry screening apparatus, a feed stream of 
particulate matter, such as mineral granules discussed above, is fed to 
the machine at an input end thereof, the particulate matter travels over 
at least one screen having a predetermined opening size (mesh), and the 
particulate matter that falls through the screen openings is collected for 
one purpose, while the material that doesn't fall through the screen is 
collected to be otherwise used. The particulate matter may travel over the 
screen under the influence of gravity, the influence of motion imparted 
thereto by the machine, a combination of such forces, or some other 
externally applied force. 
When utilizing gravity, at least partially, to move the particulate matter 
over the screens, the apparatus must be set to dispose the screen at a 
sufficient angle from horizontal so that the particulate matter flows over 
the screen. Moreover, the screens must be pitched at an angle sufficiently 
steep for thinning the particulate matter after it is fed onto the screen 
so that the particulate matter thins and spreads out over the screen to 
ensure that the smaller particles are given the opportunity to pass 
through the screen openings. In other words, the pitch of the screen 
affects the rate and evenness of the traverse of the particulate matter 
over the screen to ensure such proper sorting. If the screens are too 
flat, the particulate matter becomes sluggish acting and the smaller 
particles are blocked from passing through the mesh of the screen by the 
larger particles lying on the screen, and thus the screening is 
ineffective. 
One type of machine that has been particularly applied in the field of 
classifying mineral particulate matter for use in making roofing granules, 
is a screening machine sold by Rotex Company of Cincinnati, Ohio. Known 
examples includes Series 50 and 70 machines. The Rotex made machines are 
known to include plural screen layers, each screen layer having a 
different mesh size, for use in sorting mineral particulate material and 
specifically to remove roofing granules as product from the screening 
machine. 
Typical roofing granules are known as 11 grade product, which means that 
the highest percentage of granule grade will pass through a 10 mesh 
(Tyler, opening size 0.065 inch, 1.68 mm) screen but will be retained on a 
14 mesh (Tyler, opening size 0.046 inch, 1.19 mm) screen. 
Moreover, such Rotex made machines rely on an orbital movement of the feed 
end of the screening machine, and specifically the screens therein. The 
orbital movement of each screen is substantially within the plane of each 
screen. Furthermore, such machines are typically set so that the plane of 
each screen is at an angle from horizontal primarily at about four 
degrees. The discharge end of such machines slides reciprocably along a 
substantially horizontal path as the feed end moves orbitally. Since the 
screens are set substantially flat, the orbital movement of each screen is 
responsible for dispersing the granules over the screen and traversing the 
granules along the screen to obtain the necessary throughput of granules. 
Such orbital movement at the feed end of the machine is thus relatively 
very substantial to ensure proper throughput of granules. A typical Series 
50 model Rotex made machine having dimensions of approximately 40 inches 
by 120 inches moves about 3.5 inches at the feed end of the machine. 
Another type of screener for sorting dry particulate materials is that 
using a vibration screening action. Vibration screening action means that 
the screens are not limited to movement substantially within the planes of 
each screen, but also include a component of movement in the direction 
perpendicular to the plane of the screens. That is, the screens are 
rapidly moved into and out of the plane of the screen at rest. Moreover, 
such vibrating screening action requires a much shorter displacement of 
the screens, typically about 0.625 inch, which is in the order of about 
0.2 of the displacement of a Rotex type machine. 
Vibration screening machines, however, require a significantly steeper 
angle of the screens to cause the particulate matter that is fed to the 
machine to be evenly displaced over the screens to ensure proper 
throughput of granules and fines. Typically, such machines are set at 
between 25.degree.-50.degree. from horizontal, although certain very light 
particulate materials such as plastics, for example polypropylene, may be 
as low as 15.degree.-25.degree.. Such machines are known to include one or 
more decks of vibrating screens. Known vibration screening action machines 
are available from Derrick Manufacturing Corporation of Buffalo, N.Y., 
which may be provided with one, two or three screening decks. 
Heretofore, such vibration screening action machines have been found to be 
ineffective in the field of sorting mineral particulates, particularly for 
roofing granules, where it is desirable to substantially eliminate mineral 
fines within the product grade. Preferably, the weight percentage of 
mineral fines within the product grade should be below one percent. The 
combination of the vibrating action and the angle of the machine necessary 
to evenly disperse the mineral particulate generally resulted in too high 
a concentration of mineral fines within the product grade. In other words, 
as the product is taken off of the product grade defining screen deck, a 
substantial amount of the mineral fines was not passing through such 
screen. For example, when sorting 11 grade product from mineral 
particulate in the making of roofing granules, it was found that, in 
general, 1.5% or greater of mineral fines was present within samples of 
the 11 grade product as output of the vibration screening action machine. 
In contrast, the percentage of mineral fines making up such 11 grade 
material as product from a Rotex type machine was found to be, in general, 
below 1%. Of course, such percentages depend greatly on many other 
operating conditions which may affect the percentage of mineral fines 
within the product grade. Such operation conditions include the type of 
mineral ore, the crushing or recrushing techniques of the mineral ore 
before screening and the blinding or blocking of the screen mesh. Such 
results, however, were obtained under similar operating conditions 
comparing a Rotex type screening machine as described above to a vibration 
screening action machine available from Derrick Manufacturing Company, 
noted above, set at an angle of 15.degree. from horizontal. 
In accordance with the conventional knowledge and understanding of 
vibration screening action machines, it follows that in order to improve 
the throughput of the mineral fines through the mesh of the screen 
defining the lower limit of the product grade, a steeper angle of the 
machine would be required to more evenly distribute the mineral 
particulate over each screen so that as a particulate matter traverses the 
screens the layer of particulate matter is sufficiently thin so that the 
mineral fines have ample opportunity to fall through the mesh of the 
relevant screen. The basic problem being that the mineral fines were not 
being given the opportunity to fall through the relevant screen openings 
because they were blocked by the larger particles which ride on the 
screen. 
SUMMARY OF THE PRESENT INVENTION 
The present invention overcomes the shortcomings and disadvantages of prior 
art dry screening machines by providing a vibration screening action 
machine that satisfactorily reduces the concentration of mineral fines 
within product grade granules. Thus, the reduction in mineral fines 
corresponds to a savings in overall granule processing in that mineral 
fines are not unnecessarily treated. Moreover, as a result of 
significantly shorter machine movements, such vibration screening action 
machines require less maintenance than the orbital type screening 
machines, and the vibration screening action machines result in 
significantly lower vibration of the machine surroundings including the 
floor and building housing such machines. 
In the processing of roofing granules, which are coated with opaque 
pigments in order to shield asphaltic roofing materials from ultraviolet 
light and also to provide an aesthetically pleasing appearance, it is 
critical to maintain the concentration of fines within the product grade 
as low as possible to avoid unnecessary costs associated with granule 
coating. It has been found each one percent of mineral fines results in 
approximately 6% increased usage of expensive coloring pigments. Another 
advantage of reduced mineral fines is that there is less dust associated 
with the processing operations thereby reducing environmental problems. 
The aforementioned benefits and advantages are achieved by a vibration 
screening action machine for sorting product grade granules from a 
feed-stream of product containing particulate matter. The vibration 
screening action machine includes at least one screen deck having a mesh 
defining a lower limit to the size of product granules sorted from the 
feed-stream of particulate matter and a means for imparting vibratory 
motion, that is motion at least having a component of movement 
perpendicular to the plane of the screen, to the screen deck. 
Moreover, the vibration screening action machine is set so that the at 
least one screen deck is maintained at an angle of less than 15.degree. 
from horizontal. By maintaining the screen deck or decks of the subject 
vibration screening action machine below 15.degree., it was unexpectedly 
discovered that less mineral fines were present in the product grade 
granules. Contrary to the conventional procedures and understanding of 
such machines, noted above in the Background section of this application, 
which would suggest increasing the angle of the machine to enhance 
throughput of the mineral fines by improving the opportunity for the 
mineral fines to pass through the screens, it was unexpectedly discovered 
that by reducing the angle of the screens, as measured from horizontal, 
the throughput of mineral fines was improved. 
Such improvement in the percentage of mineral fines present in the product 
grade has proved to be true even though the reduction in angle tends to 
increase the thickness of the layer of particulate matter traversing the 
screen which is known to hinder the passage of the mineral fines through 
the screen because of the blocking of the mineral fines by larger granules 
that ride on the relevant screen. It is believed that even though such 
hindering to the passage of mineral fines must occur, that the increased 
time that the particulate matter is on the screen combined with the 
vibratory motion overcomes such hindering and in fact improves the total 
throughput of mineral fines. Moreover, the vibratory motion is believed to 
cause natural segregation of the particulate matter as it traverses the 
screen over time with the fines stratifying nearest the screen. By 
flattening the screens, the traverse time is increased enough for such 
natural segregation to occur and for the fines to fall through the screen. 
The present invention is also directed to the processing of particulate 
matter by a vibration screening action machine so as to sort product grade 
granules from a feed-stream of particulate matter. The process includes 
the steps of providing a vibration screening action machine having at 
least one screen deck which defines the lower limit of product size of the 
product grade granules and a means for imparting vibratory motion, that is 
motion including at least a component thereof in the direction 
perpendicular to the plane of the screen, to the screen deck, and setting 
the screen deck at angle of less than 15.degree. from horizontal. The 
process further includes supplying a feed-stream of product containing 
particulate matter to the vibration screening action machine, vibrating 
the at least one screen deck by the vibration means, transversing the 
particulate matter containing the product grade granules across the at 
least one screen deck, and collecting the product grade granules from the 
upper surface of the at least one screen deck. Also, the mineral fines 
which pass through the mesh of the at least one screen deck are collected 
as waste. 
It is further contemplated to use such a vibration screening action machine 
together with one or more crushing stations. In this case, plural screen 
decks are provided of decreasing mesh size from the top of the machine to 
the bottom, whereby particulate matter above the upper limit of the 
product grade granules can be conveyed to a crushing station and refed 
with the feed-stream of particulate matter. Otherwise, the larger granules 
could be otherwise used or processed in any other way. Preferably, the 
vibration screening action machine of the present invention comprises 
three screen decks with the largest particulate matter coming off the top 
screen and going to a first crushing station, the particulate matter 
coming off the second screen going to a second crushing station, the 
product grade granules coming off the third screen deck, and the mineral 
fines coming from the machine pan to be collected as waste.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the figures, wherein like numerals are used to designate 
like components throughout each of the several figures, and in particular 
to FIG. 1, a process circuit 10 is schematically illustrated including a 
screening machine 12 which is used for sorting product grade granules from 
a feed-stream of particulate matter. The feed-stream of particulate matter 
is illustrated at 14 running from a feed bin 16 to the screening machine 
12. The feed-stream of particulate matter 14 can be fed to the screening 
machine 12 by any conventional conveying means, such as by conveyor belts, 
through conduits, or the like. The feed bin 16 is supplied with new 
particulate matter by a conveying means showing at 18, which initially 
supplies particulate matter to the feed bin 16 and thereafter introduces 
new particulate matter to the circuit 10 as product grade granules and 
waste are produced. 
The feed-stream of particulate matter 14 is separated and classified by the 
screening machine 12 into a plurality of outputs, of which there are 
preferably four in accordance with the preferred embodiment of the present 
invention described below. Specifically, line 20 is shown connecting from 
the screening machine 12 to a first crushing mechanism 22. Line 24 
connects from the screening machine 12 to a second crushing mechanism 26. 
A third line 28 connects from the screening machine 12 to a product 
collecting vessel 30, and line 32 is connected from the screening machine 
12 to a waste collecting bin 34. Lines 20, 24, 28 and 32 may comprise any 
conventional conveying means, such as by conveyor belts, through conduits, 
or the like. 
As will be more clearly understood from the detailed description of the 
screening machine 12 below, each of lines 20, 24 and 28 come from one of 
three screen decks within the screening machine 12, and the line 32 
connects from the pan or floor of the screening machine 12. The screen 
decks within the screening machine 12 are arranged with decreasing mesh 
(screen opening) sizes from the top of screening machine 12 to the bottom 
thereof. Thus, the particulate matter passing through line 20 is of a 
larger size than the particulate matter passing through line 24. Likewise, 
the particulate matter within both lines 20 and 24 are larger than the 
product grade granules which pass through line 28 to the product 
collecting vessel 30. The waste fines which collect in the pan of 
screening machine 12 are directed through line 32 to the waste collecting 
bin 34. Such fines comprise particles smaller in size than the product 
grade granules. 
As discussed in the Background section of this application, it is extremely 
beneficial to remove substantially all of such fines from the product 
grade granules, specifically in cases where the product grade granules are 
to be subjected to further treatments. Such is the case in the process of 
preparing pigmented roofing granules which are conventionally known for 
application to roofing materials, particularly asphalt-based roofing 
materials such as shingles. Is important that the roofing granules be 
sized in accordance with set standards so that the appearance of the 
granules on the roofing product can be accurately controlled and so that 
such application can be done effectively. Such granules not only are used 
for aesthetic purposes, they also protect the asphaltic material from 
harmful ultraviolet rays which they would otherwise be subjected to and 
which reduces the lifetime of such roofing products. Moreover, in the case 
of roofing granules, the product grade granules are preferably coated with 
opaque pigments. The pigments are often substantially more expensive than 
the granules themselves. Thus, it is important to keep the pigment portion 
to a minimum but which will provide the desired ultraviolet protection and 
aesthetic appearance. Tests have established that each one percent of 
fines within the product grade granules results in approximately 6% 
increased usage of the expensive coloring pigments. Clearly, there is a 
desire to reduce such fines to reduce overall costs. Moreover, removing 
the fines also results in less dust which means less environmental 
problems. 
Thus, it is an important factor in designing the screening machine 12 for 
use in the process circuit 10 so that the fines are most effectively 
removed from the product grade granules, and to do so as quickly as 
possible. In other words, it is most desirable that as much as possible of 
the fines pass through all of the screen decks within the screening 
machine 12 on a single pass of the particulate matter through the 
screening machine 12. 
Further in accordance with the preferred circuit 10 of the present 
invention, lines 36 and 38 comprise conventional conveying means 
connecting the first and second crushing mechanisms 22 and 24, 
respectively, to the feed bin 16. Thus, with respect to particulate matter 
larger than the product size which exits the screening machine 12 through 
either of lines 20 or 24, the circuit 10 is a closed circuit. Such larger 
particulate matter is crushed by the first and second crushing mechanisms 
22 and 26, respectively, so that as it is fed back into the feed bin 16 
and refed to the screening machine 12 through the feed-stream 14, it will 
again be sorted and product grade granules will be removed. As a result of 
the closed circuit system, once the circuit 10 is up to a chosen running 
capacity, the conveying means 18 should supply as much new particulate 
matter as that which is taken from the system as product grade granules 
and waste. 
The first and second crushing mechanisms 22 and 26 can comprise any 
conventionally known crushers, such as cone crushers, roll crushers, or 
the like, which are commercially available. The first and second crushing 
mechanisms 22 and 26 can be similar crushers, or may comprise different 
crushers specific to the size of particulate matter fed thereto. Moreover, 
if only a two deck screening machine 12 is used, only one crushing 
mechanism would be needed. 
In a similar sense, the particulate matter taken from the screening machine 
12 that are larger than the product grade granules can be disposed of or 
otherwise used in any other process if it is not desirable to recrush the 
particulate matter or if there are other intended uses thereof. 
Referring now to FIG. 2, the details of the screening machine 12 will be 
more specifically described. The screening machine 12 basically comprises 
a screen supporting body 40, a support base 42, a vibration generating 
means 44 and a chute system 46. The screen supporting body 40 is movably 
connected to the support base 42 by a plurality of mounting blocks 48, 
preferably provided at the four corners of the screen supporting body 40. 
More specifically, the mounting blocks 48 preferably comprise a resilient 
material such as rubber and are fixed with flanges shown at 50 and 51 
which are further fixed with side beams 52 of the support base 42. The 
resilient mounting blocks 48 are fixed at their other end with the screen 
support body 40. It is necessary that the mounting blocks 48 comprise some 
sort of resilient material so as to permit limited relative movement to 
the degree of vibration generated of the screen support body 40 to the 
support base 42. More or less of such mounting blocks 48 can be provided 
depending on the degree of vibration and movement of the screen supporting 
body 40 relative to the support base 42. It is also understood that other 
resilient connections could be substituted for the mounting blocks 48 
which permit the needed limited movement. 
As best seen in FIG. 3, the vibration generating means 44 preferably 
comprises a pair of electrical three-phase induction vibration motors 54 
that deliver high speed centrifugal force or impact to the screen 
supporting body 40. Such vibration motors 54 are rigidly connected with 
the screen supporting body 40 at both sides thereof by mounting plates 56 
connected with the upper side edges 58 of the screen supporting body 40. 
Thus, as the vibration motors 54 are caused to vibrate by supplying power 
to each of the vibration motors 54 by power lines 59, the vibration 
thereof is transmitted through the mounting plates 56 and to the screen 
supporting body 40. Furthermore, since the screen supporting body 40 is 
movably supported to the support base 42 by way of the resilient mounting 
blocks 48, the vibratory motion of the screen supporting body 40 is 
permitted while the screen supporting body 40 is generally held in place, 
at least with respect to the angle that the screen supporting body 40 is 
disposed, as will be further explained below. 
Referring again to FIG. 2, the screen supporting base 40 supports at least 
one screen deck which extends substantially entirely over the longitudinal 
length of the screen supporting body 40. At least one such screen deck is 
necessary having a mesh defining a lower limit to the size of product 
granules to be sorted from the feed-stream of particulate matter. 
Additional screen decks may be provided as desired for removing other 
sizes of particulate matter for recycling within the machine circuit 10, 
as discussed above, or for other uses. 
Preferably, the screen supporting body 40 supports a first screen deck 60, 
a second screen deck 62, and a third screen deck 64. The first screen deck 
60 is preferably divided into screen deck portions 60a, 60b and 60c; the 
second screen deck 62 is preferably divided into screen deck portions 62a, 
62b and 62c; and the third screen deck 64 is preferably divided into 
screen deck portions 64a, 64b and 64c so that the screen deck portions can 
be more easily placed in and removed from the screen supporting body 40 
through access openings 66 provided at strategic locations of the 
sidewalls on the screen supporting body 40. The access openings 66 are 
covered with removable covers 68 that close off the access openings 66 
during operation of the screening machine 12. One of such access openings 
66 is illustrated in FIG. 2 with its cover 68 removed just above the 
second screen deck 62 at the uphill portion thereof. The covers 68 may 
comprise any type cover that is removably mounted to the screen supporting 
body 40 to cover such access opening 66, including the use of quick 
connect devices or resilient materials which deform and connect over 
flanges or the like. 
The first, second and third screen deck 60, 62 and 64, respectively, are 
preferably disposed substantially parallel with one another and at an 
angle from horizontal so that particulate matter will traverse over each 
screen deck from the uphill side 70 of the screen supporting body 40 to 
the downhill side 72 thereof. The specific range of suitable angles will 
be described below. The screen deck portions 60a, 60b, 60c, 62a, 62b, 62c, 
64a, 64b and 64c are each preferably mounted independently to the 
sidewalls of the screen supporting body 40 so that each screen deck 
portion is independently removable. Any conventional means can be utilized 
for connecting each screen deck portion to the sidewalls of the screen 
supporting body 40, and such connecting means preferably comprises 
conventional, mechanical connectors, such as bolts which pass through the 
sidewalls of the screen supporting body 40 and screw into side flanges 
integral with the screen deck portions. A plurality of such bolts are 
illustrated for each of the screen deck portions. The screen deck portions 
60 a, b and c, 62 a, b and c, and 64 a, b and c also preferably overlap 
each other at the facing ends thereof so as to create a slightly stepped 
screen deck surface over which the particulate matter will traverse. 
In order to feed particulate matter to the upper surface of the first 
screen deck 60 at or near the uphill side 70 of the screen supporting body 
40, and infeed chute 74 is provided which is mounted to the screen support 
base 42. The uphill side 70 of the screen supporting body 40 includes an 
opening (not shown) through which the particulate matter passes from the 
infeed chute 74 to the top surface of the first screen deck 60. Since the 
infeed chute 74 is fixed with the support base 42, it does not vibrate 
with the screen supporting body 40 under the influence of the vibration 
motors 54. Thus, an appropriate flexible connection is preferably provided 
between the outlet opening (not shown) of the infeed chute 74 and the 
opening through the uphill side 70 of the screen supporting body 40. The 
infeed chute 74 could just as easily be fixed with the conveying means 
(not shown) which brings the particulate matter to the screening machine 
12 to be processed. Alternately, the infeed chute 74 could be fixedly 
mounted to the screen supporting body 40, and a flexible connection could 
be conventionally provided between the inlet opening 76 thereof and the 
conveying means (not shown) that supplies the particulate matter. 
At the downhill side 72 of the screen supporting body 40, the chute system 
46 is provided which is preferably connected with the downhill side 72 of 
the screen supporting body 40 so as to vibrate with the screen supporting 
body 40. Such connection can comprise any conventional connection means. 
The chute system 46 preferably comprises a first chute 78 which is 
positioned to receive particulate matter as it exits the downhillmost edge 
of the screen deck portion 60c of the first screen deck 60. In other 
words, as the particulate matter that doesn't fall through the mesh of the 
first screen deck 60 falls from the downhillmost edge of the screen deck 
portion 60c, it falls into an inlet opening of the first chute 78. Such 
inlet opening is appropriately configured and positioned to receive all of 
such matter. Likewise, a second chute 80 is provided as part of the chute 
system 46 for catching the particulate matter that does not pass through 
the second screen deck 62 which falls from the downhillmost edge of the 
screen deck portion 62c. Furthermore, a third chute 82 is provided which 
catches the product grade granules that do not fall through the third 
screen deck 64 at the downhillmost edge of the screen deck portion 64c. 
The first, second and third chute 78, 80 and 82, respectively, are 
preferably connected with one another by a housing 84 for stability. 
The first chute 78 terminates at an outlet opening 86 thereof from which 
the particulate matter off the first screen deck 60 is discharged. The 
second chute 80 terminates in an outlet opening 88 from which the 
particulate matter of the second screen deck 62 is discharged. The third 
chute 82 likewise terminates in an outlet opening 90 from which product 
grade granules off the third screen deck 64 are discharged. The outlet 
openings 86, 88 and 90 are preferably disposed so that the particulate 
matter and product grade granules discharged therefrom are discharged into 
or onto appropriate conveying means for transferring such materials in 
accordance with the desired process circuit. 
A fourth chute 92 is also provided opening through the floor or pan 94 of 
the screen supporting body 40 at the downhill end thereof. Thus, 
particulate matter which falls through all three of the screen decks 60, 
62 and 64, known as fines, can exit the screen supporting body 40. The 
fourth chute 92 includes an outlet opening 96 from which such fines are 
discharged to an appropriate conveying means for disposal or other use. 
Such fines are collected by the pan 94 and moved downhill to the fourth 
chute 92 and through the outlet opening 96. 
The screening machine 12, as described above and with exception to the 
specific orientation of the machine as set up for usage, is commercially 
available from Derrick Manufacturing Corporation of Buffalo, N.Y., 
including the vibration motors 54. Such vibration motors 54, as available, 
comprise three-phase induction motors and rotating eccentric bearing 
housings. When two such vibration motors 54 are used, it is preferable to 
rotate the eccentric bearings in opposite rotational directions from one 
another. 
The action of such vibration screening action machines, as driven by the 
vibration motors 54 includes movement of the screen decks 60, 62 and 64 to 
at least some degree in a direction including at least a component of such 
movement in the direction perpendicular to the plane of each screen deck. 
In other words, the vibratory motion is not entirely within the plane of 
the screen decks. Such vibratory motion can be substantially reciprocable, 
as shown by the arrows at point X shown in FIG. 2. Such motion could also 
be elliptical. It is, however, preferable that such vibratory motion 
causes the particulate matter to move downhill across screen decks 60, 62 
and 64. Such movement is controlled by the vibration motors 54, the 
positions thereof with respect to the screen supporting body 40, the speed 
of rotation, and the relative directions of rotation. 
Moreover, the use of such machines, including a three deck screening system 
for sorting mineral particulate matter in the making of roofing granules 
has been previously attempted. Such use, however, proved unsatisfactory in 
that too high a percentage of fines were retained within the product grade 
granules. Such fines within the product grade granules, as amplified 
above, can greatly increase the costs associated with any further 
processing of the product grade granules. In such previous attempt, the 
screen supporting body 40 was disposed relative to the support base 42 
such that the screen decks 60, 62 and 64 were disposed with angle .theta. 
set at 15.degree.. 
Roofing granules, as processed and screened as above, comprise mineral 
particulate matter that is produced from raw mineral ore. Roofing granules 
of mineral ore are characterized by bulk densities in the range of between 
60 and 120 lbs./ft.sup.3. Furthermore, the specific gravity of such 
mineral ore generally ranges between 2.55 and 3.05. Such mineral ore is 
preferably crushed by conventional crushing means to produce particles of 
a suitable size usable as roofing granules, which, as defined above, is 
preferably 11 grade. 
During such previous attempt, representative samples of product grade 
granules were taken as they exited the outlet opening 90 of the third 
chute 82 and the composition make up of particle sizes was determined. 
Table 1 below shows the weight percent of granules retained on screens of 
meshes between 10 mesh and 35 mesh, and the percentage of fines which are 
smaller than 35 mesh and which are noted as "pan". The mesh sizes used 
within Tables 1 and 2 of this application refer to meshes of the Tyler 
scale. The opening sizes for each mesh is as follows: 10 mesh-0.065 inch 
(1.68 mm); 14 mesh-0.046 inch (1.19 mm); 20 mesh-0.0328 inch (0.841 mm); 
28 mesh-0.0232 inch (0.595 mm); and 35 mesh-0.0164 inch (0.420 mm). 
Moreover, 11 grade roofing granule samples, which is the preferred product 
grade granules means the highest percentage of granule grade will pass 
through 10 mesh (Tyler) screen but will be retained on a 14 mesh (Tyler) 
screen. See the second column of Table 1. 
Such representative samples generally show that the percent of pan 
material, fines, is above 1%, which is unacceptable. Of course, during the 
taking of such representative samples, other pan values were obtained both 
higher and lower than those shown, but which were believed adversely 
affected by other operating parameters. Such other operating parameters 
include the blinding or blocking of one or more of the screen decks, the 
processing of abnormally high fine content particulate matters, or the 
effects of other machines or circuits thereof which when operational or 
not affect the quality of particulate matter within the system circuits. 
TABLE 1 
______________________________________ 
Percent (Weight) Retained on Each Screen 
10M 14M 20M 28M 35M PAN 
______________________________________ 
9.5 36.7 30.1 19.6 3.2 1.0 
11.2 35.9 28.9 19.0 3.5 1.5 
11.9 36.9 28.9 18.1 3.1 1.1 
10.3 35.3 30.5 19.7 3.1 0.8 
5.0 36.5 29.7 19.2 2.1 1.1 
10.4 35.7 29.2 14.4 3.7 1.3 
11.6 32.4 29.5 20.7 4.9 2.3 
7.5 32.6 32.9 22.4 4.3 1.1 
10.1 32.9 27.9 18.9 4.6 2.3 
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In accordance with the present invention, and contrary to the conventional 
procedures and understanding of such vibration screening action type 
machines, applicants discovered that by maintaining the screen decks 60, 
62 and 64 at angles less than 15.degree. from horizontal provides 
unexpectedly low percentages of fines within the product grade granules. 
Conventional procedures and understanding of such vibration screening 
action machines, as discussed above in the Background section of this 
application, suggests that it would be necessary to increase the angle 
from horizontal of the screen decks in order to enhance throughput of the 
particulate fines by improving the opportunity for the particulate fines 
to pass through the screens, by decreasing the thickness of particulate 
matter on the screens. Thus, the fines would traverse over the screens and 
fall through. 
In contrast, the improvement in the reduction of particulate fines within 
the product grade granules has proved to be true even though the reduction 
in angle tends to increase the thickness of the layer of particulate 
matter that traverses the screen, which tends to hinder the passage of 
particulate fines because of the blocking to the passage of particulate 
fines by the larger granules that don't pass through the mesh of the 
product defining screen. It is believed that even though the passage of 
particulate fines must be hindered by the increased thickness of the 
particulate matter layer, that the increased time that the particulate 
matter traverses the screen combined with the vibratory motion of such 
screening machine 12 overcomes such hindering and in fact improves the 
throughput of the particulate fines. Moreover, the vibratory motion is 
believed to cause natural segregation of the particulate matter as it 
traverses the screen over time with the fines stratifying nearest the 
screen. By flattening the screens, the traverse time is increased enough 
for such natural segregation to occur and for the fines to fall through 
the screen. 
Representative samples obtained in a similar manner as that described above 
with respect to Table 1 were taken during the sorting of product grade 
roofing granules on a screening machine 12 disposed with the angle .theta. 
at 10.degree.. In the same sense as the samples taken for the 15.degree. 
machine, such values are dependent on the other operating conditions which 
occasionally result in values higher and lower than those of Table 2 
below. It is, however, believed that the general trend of a substantially 
reduced percentage of pan particulate fines is established. 
TABLE 2 
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Percent (Weight) Retained on Each Screen 
10M 14M 20M 28M 35M PAN 
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8.5 38.4 28.4 20.7 3.1 0.9 
8.1 39.3 29.4 21.2 1.8 0.3 
7.1 38.6 30.5 22.1 1.4 0.3 
8.1 39.0 30.9 21.2 0.5 0.3 
7.3 38.4 31.3 22.2 0.5 0.3 
8.1 38.7 30.6 21.6 0.7 0.3 
8.9 41.4 28.7 20.2 0.6 0.2 
6.4 37.2 31.1 22.7 2.0 0.6 
7.2 38.5 30.7 22.3 1.0 0.4 
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The same screening machine 12 was also operated for sorting roofing 
granules with the screen angle .theta. at 12.degree.. Similarly, 
representative samples were taken from the product grade granules and the 
composition sizes thereof determined. Again, the results obtained were 
generally better than that of the 15.degree. machine, but were not as good 
as the values obtained from the 10.degree. machine. However, the trend was 
supported that lowering the angle .theta. below 15.degree. actually 
increases the throughput of particulate fines through the product granule 
defining screen deck contrary to the conventional procedures and 
understanding of such vibration screening action machines. 
It is understood that many other modifications or additions could be made 
to the apparatus and process of the present invention without departing 
from the spirit thereof, and that the scope of the present invention 
should not be limited by the specific features and steps of the apparatus 
and process of the present invention. In particular, the positioning of 
the screens of such a vibration screening action machine at angles below 
15 degrees is applicable to other types of vibration screening action 
machines than that specifically disclosed and that utilize similar 
operating principles. Moreover, the trend that decreasing the angle of the 
machine below 15 degrees, as measured from horizontal, results in the 
increase of the throughput of fines is believed applicable to all angles 
below 15 degrees. However, as the machine becomes flatter, greater forces 
must be exerted on the machine to generate movement of the particulate 
matter over the screens. At some point the application of more force 
becomes impractical. Furthermore, the principles of the process and 
apparatus of the present invention are also applicable to other 
particulate matter than mineral particulate or granules since the effects 
of the vibratory motion and the reduced angle should be equally applicable 
to such other particulate matter, whether lighter or heavier, although the 
optimized angle may vary somewhat.