[0001] The present invention relates to the sizing particulates and more particularly to
adjusting the size distribution of particulate materials, such as artificial stonesands,
for specific applications, such as for use in concrete and asphalt compositions or
as filter or molding sands.
Background of the invention
[0002] The present invention is applicable to adjusting the particle size distribution of
all kinds, of particulates, including sands, ores, minerals, powdered metals, seeds
and grains. The invention is especially useful in obtaining a controlled gradation
of crushed fine aggregate produced from quarried stone by crushing or grinding. Crushed
fine aggregate is referred to in the art by various terms such as stone sand, crusher
sand, crushed fine aggregate, specification sand or manufactured sand. In this specification,
such crushed fine aggregate is referred to as "stonesand". An accepted standard for
stonesand used in concrete is set forth in Standard Specification C-33 for Concrete
Aggregates as published by the American Society for Testing and Materials (ASTM).
Stonesand may be produced from almost all rock types which are commonly quarried to
make coarse aggregate for roadbeds and the like. As natural sand deposits become depleted
or unavailable through land development, the demand for stonesand has increased in
recent years.
[0003] There are basically two different types of crushes for the rock types yielding stonesand.
Jaw, gyratory and cone crushing are compression types depending upon compression (squeezing),
friction and/or attrition between particles to break down the larger rock particles.
Roll, rod mill, hammer mill and centrifugal are impact types which rely largely upon
impact (hitting) for breakage. Depending on the rock type, the impact crushers generally
produce a more cubical shaped particle than the compression crushers. Only limited
control of particle shape or size can be realized in a communition process, especially
in the smallest sizes produced, because of the tendency of breakage to occur along
the surfaces of weakness dictated by the mineralogy of the material being crushed.
Regardless of the type of crusher used, stonesand tends to be somewhat deficient in
the intermediate particle size classes (No. 30 to No. 100 mesh), relative to sands
which will satisfy the ASTM C-33 specification and to contain more fracture dust or
fines (minus 100 mesh) than natural sands. On the other hand, the fractured cubical
shape of some stonesand is capable of providing a concrete of higher strength and
greater durability (more resistant to freezing and thawing deterioration) than some
natural sands which are more rounded in shape.
[0004] In order to obtain good quality stonesands, it is therefore often necessary to remove
at least a portion of the minus 100 and minus 200 mesh material, as well as some of
the larger sizes near 3/8 inch (9.5 mm) mesh. To accomplish this and improve the overall
gradation of stonesand, some type of classifer is usually employed. Classifiers are
also generally of two types, namely wet classifiers and dry classifiers. Classification,
whether by wet or dry processes, is possibly the single most important step in the
production of a stonesand product of acceptable quality. Although wet classification
systems generally produce more reproducible particle size distributions, such systems
are of relatively low capacity per unit of capital cost and are relatively expensive
to operate. On the other hand, dry classification systems of the prior art require
that the aggregate feed be adequately populated in the particle sizes of interest
and be uniform in moisture content because any significant variations, particularly
in moisture content, will result in an output that does not meet the needed criteria.
Excessive moisture content may also cause blinding of screen classifiers such that
the required degree of passage of undersize particles through the screen is prevented
by partial or complete blockage of the screen apertures.
[0005] Conventional approaches to producing a graded stonesand product often involve separating
the crushed feed material into individual size fractions and then recombining two
or more of those fractions in the proportions necessary to obtain the relative quantities
of each fraction desired in a final product. The multiple processing stages required
by these prior art approaches are time consuming and are not energy efficient. The
necessity for blending two or more fractions often causes problems in handling the
particulates and in adequately mixing the different size fractions to achieve the
required uniformity in the final product.
[0006] Conventional classification of particulates with multiple screens may be in the form
of batch sieving or continuous screening. In batch sieving, a stacked set of sieves
are operated so as to provide particle exposure to the screen for a relatively long
period of time that permits passage of nearly all (typically greater than 99 mass
percent) of the undersize particles, i.e., those of a size capable of passing through
a given screen. This is referred to in this patent specification as operating under
complete separation conditions. A set of sieves operated in this manner will separate
the batch feed into mass fractions corresponding to different size classes, where
each size class consists of all particle sizes between the mesh sizes of two successive
sieves (or screens). Each such mass fraction represents the ratio of mass of particles
in the given size class to the total mass of all particles in the sample of the parent
size distribution. The sieving is carried out for the period of time required to achieve
substantially complete separation of the feed material into preselected size classes.
The mass fractions so separated will not be substantially changed by sieving for longer
periods of time. The mass fractions provided by classifiers employing batch sieving
may then be reblended in the desired proportions to provide a finished product having
the size distribution desired for a given application.
[0007] In continuous screening, the screen sizes and lengths are selected as if each screening
stage were to be carried out in a fashion analogous to batch sieving but assuming
a somewhat lesser degree of complete screening (typically 85 to 95 mass percent).
The mesh size of the screen, the screen length, the screen vibratory rate and values
of other screening parameters are therefore selected to provide the desired product
by assuming a predetermined level of essentially complete screening chosen on the
basis of the estimated characteristics of a constant particle size distribution of
feed material under fixed conditions of screening. The 85 to 95% completion values
for continuous screening typically arise because of the finite length of practical
screens. Very long screens of impractical lengths would usually be required to achieve
operation close to complete screening conditions (greater than 95 mass percent passage
of those particles capable of passing through the screen).
[0008] In conventional continuous screening systems, which often operate relatively near
complete screening conditions, it is desirable to control closely the screening conditions
and the moisture content, size distribution and other characteristics of the feed
because significant variations in feed and/or screening conditions can cause corresponding
variations in the rate of passage of undersize particles through the screen apertures
and result in a product outside the limits of the applicable size distribution specification.
Typically these controls are not used and sometimes it is not even recognized that
they should be used. In addition, conventional screening systems are often tailor-made
for a given feed and set of screening conditions such that product specifications
cannot be maintained with a significantly different feed or under significantly different
screening conditions.
[0009] Prior art classifiers employing continuous screening processes depend upon essentially
complete screening to provide the desired size distribution in the finished product.
An example of one such prior art process is illustrated by US―A―No. 4,032,436 to Johnson,
the entire contents of which are incorporated herein by reference. Such classifiers
may be sensitive to screen blinding where a portion of the open screen area is blocked
by near size particles. Variations in the rate of passage of undersize particles through
the screen because of blinding may cause excessive waste and/or the finished product
to be out of specification.
[0010] A specific application of stonesand, such as in making concrete or asphalt, may require
a closely defined sieve analysis and fineness modulus (F.M.). In other words, the
stonesand must be carefully processed so as to have a consistent gradation and a consistent
F.M. as necessary to meet applicable specifications and achieve a high quality concrete
or asphalt composition with good workability, flowability and finishability.
[0011] ASTM Standard Specification C-33 (ASTM C-33) as applied to stonesand has the following
sieve analysis limits based on the cumulative percentages passing through each sieve
size indicated upon screening substantially to completion: 100% passing 3/8 inch (9.5
mm), 95 to 100% passing No. 4, 80 to 100% passing No. 8, 50 to 85% passing No. 16,
25 to 60% passing No. 30, 10 to 30% passing No. 50, 2 to 15% passing No. 100 and 0
to 7% passing No. 200. ASTM C-33 further requires that not more than 45% of the sample
be retained between any two consecutive sieves, that the F.M. not be less than 2.50
nor more than 3.10 and that the F.M. not vary by more than 0.20 unless suitable adjustments
are made in proportioning the concrete to compensate for the difference in grading.
Thus, once the proportion of stonesand is selected for concrete, it is preferable
that such fluctuations in the stonesand grading be prevented to avoid having to change
this proportion.
[0012] To determine whether a stonesand product meets ASTM C-33, a sample of the product
is subjected to a sieve analysis using batch sieving through a set of test sieves
having the sizes specified above to measure the percent retained or each of the sieves.
The F.M. value is then determined by summing the accumulated weight percentages retained
on the successive sieves and the resulting number which is in excess of 100% is divided
by 100 to produce a number which is the fineness modulus. A more detailed explanation
of the F.M. indicator is set forth in the Johnson patent referenced above.
[0013] The invention is directed at a continuous dry screening process having improved control
of particle size distribution in the product and reducing the need for costly classifying
and reblending systems. An aim of the invention is to provide a differential rate
screening process which continuously alters by a controllably variable amount the
size distributions of practical feed materials so as to obtain directly an output
product with a size distribution adhering closely to preselected proportions.
[0014] Another aim of the invention is to provide a differential rate screening process
in which the degree of completeness of screening a particulate feed material is controlled
so as to selectively alter the relative rates at which undersize particles in different
classes pass through the screen and into an output product.
[0015] In a particular aspect the invention seeks to provide a commercially practicable
dry process for continuously screening crushed fine aggregate so as to minimize the
necessity of blending two or more streams of different particle size distributions
and provide a product having a substantially constant particle size distribution.
In another aspect, the invention is concerned with a continuous dry screening process
capable of being adjusted so as to maintain a substantially constant size distribution
in a particulate product in the presence of significant variations in feed and/or
screening conditions. The invention also contemplates a continuous dry screening process
capable of being periodically or continuously adjusted in response to one or more
measured characteristics of one or more input and/or output streams and/or in response
to one or more measured characteristics of the screening conditions so as to maintain
a substantially constant size distribution in a particulate product in the presence
of different feed and/or screening conditions, such as those causing screening blinding.
[0016] Towards the above ends, the present invention provides a screening process for continuously
screening undersize particles of different size classes to provide a product having
a preselected distribution of particle sizes substantially different from the distribution
of particle sizes in a feed of particular material. The process comprises differential
rate screening, and the steps of introducing a stream of the feed onto at least a
first screening member having apertures of sufficient size to pass a plurality of
size classes in said feed stream; separating said feed stream into at least a first
throughs stream, and one other first stream by causing at least two of said undersize
classes to pass through the apertures of said first screening member and into said
first throughs stream in proportions relative to one another which are substantially
different from the proportions of said at least two undersize classes relative to
one another in said feed stream; and providing in said feed stream a sufficient population
of undersize particles in each of said plurality of undersize classes and controlling
the differential between said proportions of undersize classes in said feed stream
and said proportions of undersize classes passing through said first screening member
and into said first throughs stream so as to provide substantially a preselected distribution
of particle sizes in a particulate product comprised of at least a portion of at least
one of said first throughs stream and said other first stream.
[0017] The process of the invention may be adopted to use two screening members. Such a
process .comprises the steps of introducing a stream of the feed onto at least a first
screening member having apertures of sufficient size to pass a plurality of size classes
in said feed stream; separating said feed stream into at least a first throughs stream
and one other first stream by causing at least two of said undersize classes to pass
through the apertures of said first screening member and into said first throughs
stream in proportions relative to one another which are substantially different from
the proportions of said at least two undersize classes relative to one another in
said feed stream; introducing at least a portion of at least one of said first throughs
stream and said other first stream as an input stream to a second screening means
for providing at least a second throughs stream and one other second stream; and providing
in said feed stream a sufficient population of undersize particles in each of said
plurality of undersize classes and controlling the differential between said proportions
of undersize classes in said feed stream and said proportions of undersize classes
passing through said first screening member and into said first throughs stream so
as to provide substantially a preselected distribution of particle sizes in a particulate
product and comprised of at least a portion of at least one of said second throughs
stream and said other second stream.
[0018] The term "differential rate screening" as used herein connotes a continuous process
in which undersize particles in a feed of particulate material are incompletely screened
and the degree of incomplete screening is so controlled as to provide a particle size
distribution substantially different from the particle size distribution of the feed.
More particularly, undersize particles in different size classes are screened to different
degrees of completion on the same screen in a controlled fashion so that the product
obtained has the desired distribution of different particle sizes.
[0019] The differential rate screening process takes advantage of the fact that particles
in successively smaller size classes pass through a screen having given size openings
at successively higher mass flow rates. The terminology "mass flow rate" as used in
this specification denotes the mass of material per unit time which moves as a complete
stream or as a component of a complete stream of particles. By appropriately biasing
to different degrees the effective retention time of different particle size classes
on the screen, the screen is used as an adjustable component in a continuous size
classification system. One tends to think of one or more "variable" screens rather
than one or more "fixed" screens since the invention causes a given screen to act
as if it were a family of screens rather than a single, fixed screening component.
This system is in marked contrast to the conventional approach of separating the feed
into its individual size fractions and then recombining and remixing those fractions
according to a new blend designed to achieve the desired product. Differential rate
screening involves the implementation of controlled differential screening rates between
substreams of different size classes so as to achieve a preselected size distribution
in the product.
[0020] In the conduct of a differential rate screening process according to the invention
a feed stream of particulate material is fed to a first screening member having apertures
of sufficient size to pass a plurality of size classes in the feed stream. The feed
stream is then separated into at least a first throughs stream and a first overs stream
by causing at least two of the undersize classes in the feed to pass through the apertures
of the screening member and into the throughs stream in proportions relative to one
another which are substantially different from the relative proportions of the at
least two undersize classes in the feed stream. The differential between the mass
flow rate of undersize particles in the feed stream and the mass flow rate of undersize
particles passing through the screening member and into the selected throughs stream
is controlled so as to provide substantially a preselected distribution of particle
sizes in a product stream comprised of at least a portion of the throughs stream and/or
the overs stream. A portion of the particles passing through the screening member
may be intercepted before reaching the "selected" throughs stream and diverted as
a separate stream or combined with the overs stream as a "retained" stream.
[0021] The screening member may comprise a screen of apertures with constant size, shape
and orientation and with uniform spatial distribution of position over the screen
surface. Alternately, it may comprise a screen of apertures whose characteristics
of size, shape, orientation and position may individually or in various combinations
be distributed spatially in some defined manner over the screen surface. In particular,
these characteristics may be spatially distributed along the length of the screen,
where the latter is taken to be in the direction of the normal flow of material over
the screen. The feed means for introducing a stream of particulate feed onto the screening
member may comprise some type of conveyor or a special feeder device. The means for
causing undersize particles to pass through the screening member may comprise inclining
and vibrating the screening member.
[0022] A wide variety of adjustment means may be provided for controlling the differential
between the mass flow rate of undersize particles in the feed stream and the mass
flow rate of undersize particles passing through the screening member and into the
throughs stream. These may include an adjustable chute, an adjustable plate, pan or
tray, or an adjustable conveyor so as to vary the location at which feed is introduced
onto the screening member. Alternately or in combination, an adjustable retention
means may be provided such as an adjustable cover for receiving overs from above the
screen or an adjustable plate, tray or pan for intercepting a portion of the throughs
after they pass through the screen but before they pass into the throughs stream having
a controlled proportion of the respective undersize classes. Each of these several
adjustment schemes can be characterized by a parameter called "open length of the
screen" in this specification. This parameter refers to the actual length of uncovered
screen, including both the apertures and the material in between, which interacts
with the feed stream in the sense of differential rate screening.
[0023] Another adjustment means for controlling the undersize differential between feed
and select throughs is to provide means for adjusting the vibratory motion of the
screening member. The means of vibratory adjustment may include adjusting the frequency
or amplitude of the vibrations imparted to the screen, or the wave form followed by
the screen's vibratory motion, or a combination of these vibratory screening parameters.
The screen inclination, that is the angle between the plane of the screen and a horizontal
plate, may also be adjustable.
[0024] A further adjustment means for controlling the undersize differential between feed
and throughs is the provision of means for adjusting the feed rate, that is the rate
at which the particulate feed material is introduced onto the screening member. Such
means may include an adjustable speed conveyor or a feeder of a type wherein the mass
flow of feed from a bin or the like may be adjusted by changing the vibratory rate
and/or size openings of a feeder component. Another such adjustment means is the provision
of means for adjusting the particle size distribution of the feed, such as by prescreening
an adjustable portion of the feed on a conventional scalping screen, or by prescreening
on another screen operated in accordance with the principles of the present invention,
or by adjusting the particle size reduction provided by a crusher or grinder supplying
feed to the feed means. Yet another way to adjust the particle size distribution of
the feed is to return all or a portion of the overs output from the screening member
with larger particulate material to a crusher or grinder supplying feed to the feed
means.
[0025] The invention also contemplates combinations of two or more screening members employing
differential rate screening to achieve the desired distribution of particle sizes
in the final product. The basic screen combinations include (a) conveying throughs
passing through a first screen to a second screen and taking overs from the second
screen as a product stream, (b) conveying throughs passing through a first screen
to a second screen and taking throughs passing through the second screen as a product
stream, (c) conveying overs from a first screen to a second screen and taking overs
from the second screen as a product stream, and (d) conveying overs from a first screen
to a second screen and taking throughs passing through the second screen as a product
stream. Additional screens for either conventional or differential rate screening
may be used in combination with the two differential rate screens. For example, a
third screen may be operated upstream or downstream of the two differential rate screens.
Thus, a scalping screen may be used upstream of the first differential rate screen
for removing coarse materials of a size near or above the mesh size of the first differential
rate screen, or a fines screen may be used downstream of the second differential rate
screen for removing fines or dust-like material much below the mesh size of the second
differential rate screen. Where more than one screen is employed, a portion of the
feed to a given screen may be diverted to a subsequent screen or a portion of the
output from a given screen may be returned to a preceding screen.
[0026] While the invention will usually avoid the need for any blending with another stream
to achieve a desired particle size distribution in the product, it may sometimes be
desirable to blend one or more output streams from a differential rate screening system
to achieve a particular product from a particular feed material. Thus, all or a portion
of an overs or a throughs stream from any of the screens in the screening system may
be blended with another such stream to form a product. In addition, a portion of the
feed to a given screen may be diverted and blended directly with an output stream
from the same or a different screen of the screening system. As a further alternative,
two separate screening systems with different screen setups may be operated in parallel
and one or more output streams from each screening system may be blended to provide
a product.
[0027] Various setup procedures are described in the detailed description below for selecting
an appropriate mesh size, the optimum values for open screen length, and the values
of other screening parameters depending upon the rate, size distribution and other
characteristics of the feed to be processed. These procedures are based upon estimates
or measurements (or a combination of both) of what are referred to herein as transfer
functions (A). A transfer function may apply either to the total mass flow rate of
undersize particles being screened or to the mass flow rate of a specific size class
of undersize particles, and is defined as the ratio of the mass flow rate of undersize
material passing over the screen to the total mass flow rate of undersize material
that would pass through the screen if the feed to the screen were screened so as to
achieve substantially complete separation.
[0028] In certain embodiments of the invention, one or more screening parameters influencing
the transfer functions may be varied either manually or automatically during the screening
process. Screening parameters that can be varied in this fashion are referred to as
"controllably variable" in this specification. A number of screening parameters are
also "variable" in the sense that they may be changed during shutdown or interruption
of the screening process or apparatus. At least one of the "variable" screening parameters
is selected in accordance with the present invention so that the combination of the
screening parameters operative on the feed stream is such that the "differential rate"
screen does not provide essentially complete screening but instead provides a substantial
degree of "incomplete" screening. For purposes of this specification, the degree of
"incomplete" screening is synonymous with the transfer function, A.
[0029] A particularly important feature of the invention is that means may be provided to
automatically vary one or more of the controllably variable screening parameters in
response to a sensed control function. In this manner, the invention provides means
of achieving automatic control over the size distribution of particles in the product
stream. One objective of automatic control of the adjustable rate screening system
is to assure that the size distribution of the product stream meets the desired specifications,
such as the requirements of the ASTM C-33 specification for stonesand. A further objective
is to minimize the quantities of waste materials that must be disposed of either as
low economic return products or by reprocessing with attendant increases in costs.
It is also desirable to achieve these results with the least effort and expense practicable.
[0030] A number of control schemes are feasible. Quite clearly, if control is to be achieved
in a closed-loop sense, it is essential that some function of the size distribution
be sensed to generate an error signal on which such control can be based. Either the
product size distribution or the feed size distribution can provide this error signal.
The use of product size distribution connotes some form of feedback control, whereas
the use of feed size distribution connotes some form of feed-forward control. Because
of difficulties and expense involved in direct sensing of the size distribution of
either feed or product, a simpler basis for generating an error signal was developed.
It was found that the flow rate of material either through the screen or over the
screen may provide sufficient information for maintaining satisfactory control, either
with or without some intermittent particle size analysis. Intermittent size distribution
information provides a refinement to on-line rate control and constitutes a form of
adaptive or hierarchical control. Three basic types of control systems may therefore
be utilized, namely, feedback control, feedforward control and adaptive control.
[0031] In feedback control, at least one characteristic of an output stream from the screening
system is monitored and compared with a set point. An error signal is then generated
and used to adjust a controllably variable screening parameter and/or a parameter
of the crushing machine to null out the error signal. The feedback signal may also
be used to return a flow of out-of-specification material, either for rescreening
or for recrushing.
[0032] Feed-forward control involves monitoring a characteristic of the crusher output or
other source of feed to the adjustable differential rate screening operation. The
monitored characteristic is then used to generate a signal to adjust the product size
distribution so that it comes within specifications. In this control scheme, the output
of the crusher may be delayed in a holdup bin for a sufficient length of time to complete
the monitoring operation so that an adjustment signal can be sent forward and arrive
at the screen in phase with the corresponding material flow. Although material partitioning
by the screen may be sufficiently accurate to avoid the need for compensating adjustments
on the basis of screen output, such a secondary feedback control loop in combination
with the feed-forward control loop is contemplated by the invention. As a further
alternative, a measured characteristic of the feed may be used to generate a feed-forward
signal to the adjustable screen and/or a feedback signal to the crusher. Many other
options also exist for control by means of either feedback or feed-forward loops or
a combination thereof.
[0033] An adaptive control system employs more than one control loop. In one embodiment
of adaptive control of the differential rate screening process, one loop consists
of a means for continuous monitoring of a particulate stream characteristic, such
as mass flow rate, and a means for comparing this monitored characteristic with a
set point. A second loop monitors a second quantity to be used as a basis for changing
the set point on demand. The set point initially selected assumes that the particle-size
characteristics of the feed, as well as the feed mass flow rate, remains relatively
constant. The set point is used as the basis for making operational adjustments to
the adjustable screen, such as adjustment to open screen length, so as to maintain
the mass flow rate needed to satisfy the size distribution requirements of the product.
However, if there should be a substantial change in the mineralogy of the material
being fed to the crusher, the crusher output could experience a significant change
in particle size distribution. As a result, the open screen length would undergo an
excursion beyond its normal operating range, and this phenomenon would signal the
need for set point adjustment. By monitoring open screen length as well as stream
mass flow rate, the system can be programmed to perform an "on-demand" sampling and
particle size analysis of the monitored particulate stream. Particle size analysis
may be performed either manually by conventional sieve analysis or automatically by
a particle-size analyzer of a type available in the industry. The results of this
analysis can then be used to manually or automatically establish a change in the mass
flow rate set point, against which the signal from the continuous weight monitor is
compared to generate the error signal used for screen adjustment. Thus, the system
"adapts" to significant changes in the character of the incoming feed.
[0034] As indicated above, the sensed (measured) characteristic or control function may
be that of either an input or an output stream from the adjustable screening system
and may comprise the mass flow rate of the stream. A number of other stream characteristics
may be measured and used to generate an input signal to the control system. These
include the actual particle size distribution, the relative proportions of particles
above or below a selected size, the relative mass flow rates of two or more streams
containing different particle size distributions, the mean particle size, fineness
modulus, or some other characteristic proportion to or indicative of particle size
distribution, such as the noise level or impact energy generated by particle momentum
on a conveyor or in free fall. A particularly preferred characteristic which is measured
and used for generating a control signal is a mass flow rate ratio between two or
more output streams or between the input feed stream and one or more output streams,
such as the mass flow rate ratio between the feed stream and the product stream. This
product stream may comprise overs and/or throughs from one or more screens within
the adjustable screening system.
[0035] The signal generated by a measured characteristic of a particulate stream is used
as an input to the control system for the adjustable differential rate screening system.
The output from the control system may be used to adjust any of the controlably variable
screening parameters of the differential rate screening system, namely, feed mass
flow rate (by adjusting feed conveyor and/or other feeder device), feed size distribution
(by adjusting crusher, pre-screening device and/or return mass flow rate to crusher),
effective screen opening size (by adjusting location of feed discharge onto a screen
having different opening sizes spatially distributed along its length), open screen
length which passes throughs into a particular throughs stream of interest (by adjusting
relative position of a screen cover, an interceptor pan beneath screen, and/or a feeder
device), screen inclination (by direct adjustment), vibratory motion (by direct adjustment
of frequency, amplitude and/or wave form), feed diversion rate (by adjusting mass
flow rate of feed diverted to a prior or subsequent screen or to an output stream),
and blending ratios (by adjusting relative mass flow rates of mixed output streams
or parallel screening systems).
[0036] The invention may be further understood by reference to the accompanying drawings
in which:
Figure 1 is a diagrammatic illustration of a process and apparatus for differential
rate screening in accordance with the present invention.
Figure 2 is a fragmentary sectional view along lines 2-2 of Figure 1 illustrating
in more detail the means for controllably varying the vibratory motion of the differential
rate screening apparatus.
Figure 3 is a fragmentary sectional view along lines 3-3 of Figure 1 illustrating
in more detail the means for controllably varying the open screen length and/or the
effective screen aperture size of the differential rate screening apparatus.
Figure 4 is a diagrammatic illustration of a simplifying modification of the differential
rate screening process and apparatus of Figure 1.
Figure 5 is a plot of cumulative size distributions for ASTM C-33 Specification stonesand
and sample feed materials.
Figure 6 is a diagrammatic illustration of another modification of the differential
rate screening process and apparatus of Figure 1.
Figure 7 is a block diagram of the control system for the differential rate screening
process and apparatus of Figure 4.
Figure 8 is a circuit diagram of the manual control-safety interlock component of
Figure 7.
Figure 9 is a wiring diagram for providing power to and interconnecting the control
components of Figure 7 and the remotely adjustable screening components of Figure
4.
Figure 10 is a circuit diagram of the interface circuit for integrating the AIM-65
minicomputer into the control system of Figure 7.
Figure 11 is a circuit diagram for interfacing control of feed flow rate with the
AIM-65 minicomputer.
Figure 12 is a block diagram of the computer program for controlling the process and
apparatus of Figure 4.
Figure 13 is a block diagram of a hierarchical control means for the differential
rate screening system of the invention.
Figure 14 is a block diagram of a feedback control means for the differential rate
screening system of the invention.
Figure 15 is a block diagram of a feedback control means providing a return stream
of oversize material in accordance with the invention.
Figure 16 is a block diagram of a feed-foward control means for the differential rate
screening system of the invention.
Figure 17 is a block diagram of a control means incorporating both feed-forward and
feedback elements for control of the differential rate screening system of the invention.
Figure 18 is a diagrammatic illustration of the mass flow rate balances for operating
a single differential rate screen in accordance with the invention.
Figure 19 is a diagrammatic illustration of the mass flow rate balances for operating
successive differential rate screens in accordance with the invention.
Figure 20 illustrates a static setup procedure for the top screen of the differential
rate screening system of Figure 4.
Figure 21 illustrates a static setup procedure for the bottom screen of the differential
rate screening system of Figure 4.
Figure 22 is a plot of the cumulative size distribution predicted by the static setup
procedures of Figures 20 and 21.
Figure 23 illustrates a dynamic setup procedure for the top screen of the differential
rate screening system of Figure 4.
Figure 24 illustrates a dynamic setup procedure for the bottom screen of the differential
rate screening system of Figure 4.
Figure 25 is a plot of class transfer functions, Aj, for the top screen of the differential rate screening system of Figure 4.
Figure 26 is a plot of the cumulative size distribution predicted by the dynamic setup
procedures of Figures 23 and 24.
Figure 27 is a plot of a cumulative transfer function, As, obtained from laboratory
tests using a 30-mesh differential rate screen in accordance with the invention.
Figure 28 is a plot of class transfer functions, Aj, obtained by laboratory tests using a 30-mesh differential rate screen in accordance
with the invention.
Figure 29 is a class transfer function plot similar to Figure 28 but at a different
feed rate.
Figure 30 is a plot of class transfer functions A;, for a single 30-mesh screen used
in the differential rate screening system of Figure 4.
Figure 31 is a class transfer function plot similar to Figure 30 but at a different
feed rate.
Figure 32 is a class transfer function plot similar to Figures 30 and 31 but at a
different feed rate.
Figures 33 and 34 are diagrammatic illustrations of relationships between class transfer
functions, Aj, and cumulative transfer functions, As, at different feed rates.
Figures 35, 36, 37 and 38 are plots of cumulative size distributions based on actual
test data obtained during experimental operation of the differential rate screening
system illustrated diagrammatically in Figure 4.
[0037] Figure 1 is a diagrammatic illustration of the process and apparatus of the rate
screening system of the present invention. With reference to this figure, relatively
large quarried rocks are fed by conveyor 20 to a centrifugal crusher 22, which may
be of a rotary impact type such as described in U.S. Patent No. 4,061,279 to Sautter
of December 6,1977, the entire disclosure of said patent being incorporated herein
by reference. The mass flow rate of quarried rocks to crusher 22 may be varied by
a variable speed motor 24 which drives belt conveyor 20 in response to a control signal
25.
[0038] The centrifugal crusher includes a variable speed motor 26 for driving the crusher
impeller 28 in response to a control signal 27. Variable speed impeller 28 provides
a means for controllably varying the mean particle size and particle size distribution
of the stonesand 30 produced by crusher 22. It is to be understood that ballmills
and other types of crushers having means for adjusting the particle size distribution
of the crushed output may be used instead of crushers of the centrifugal type illustrated.
[0039] The stonesand produced by crushing the much larger quarried rocks is conveyed to
a feed bin 32 by means of a belt conveyor 34 driven by a variable speed motor 36 in
response to a control signal 37. Motor 36 may be synchronized with motor 24 to equalize
the capacities of conveyor 20 supplying quarried rocks to, and conveyor 34 removing
stonesand from, crusher 22. As the stonesand falls from conveyor 34 into bin 32, a
measurable characteristic of the stonesand, such as the cumulative weight or volume
percentage above or below a preselected size, fineness modulus, and/or mean particle
size may be determined by a measuring device 38 providing an input signal 40 to a
control system, generally designated 45. Feed measuring device 38 may also comprise
a weigh belt of the type described hereinafter for measuring the mass flow rate of
stonesand conveyed to bin 32. Bin 32 is preferably in the shape of an inverted truncated
rectangular pyramid having a square discharge opening at its bottom and four sides
each inclined at about 70° upwardly from the horizontal.
[0040] Mounted under the discharge opening of bin 32 is a bin discharging feeder 52, such
as a live bottom "Siletta" feeder manufactured by Solids Flow Control (SFC) Corporation
of West Caldwell, New Jersey. The Siletta feeder has a "venetian blind" feeder tray
comprised of elongated slats 53 spaced transversely apart and sized to pass crushed
stone in the size range from about 3/8 inch to fines (minus 200 mesh). With a feed
density in the range of about 80 to about 100 pounds per cubic foot, feeder 52 can
provide a controllably variable feed rate in the range of about 2 to about 25 tons
per hour. The feeder tray is vibrated horizontally in a direction perpendicular to
the length of slats 53 by an adjustable amplitude magnetic drive unit 54, such as
that manufactured by Eriez Magnetics of Erie, Pennsylvania. In a preferred embodiment,
drive unit 54 vibrates the feeder tray at a constant frequency of about 60 hertz and
has an adjustable amplitude with a maximum amplitude of about 1 mm. The drive unit
and may also include a controller permitting manual or automatic adjustment of the
size of the slat openings and/or the vibratory amplitude in response to the input
of an external analog signal 55. Since the slat opening size and vibratory amplitude
regulate the mass flow rate of stonesand from bin 32, analog signal 55 can be used
to vary the instantaneous mass feed rate passing through feeder discharge chute 56
and thereby provides one means for achieving relatively precise control over the mass
feed rate. If there is no need for the surge capacity provided by bin 32, both the
bin and its feeder may be omitted and feed rate control provided by variable speed
conveyor motors 24 and 36.
[0041] Beneath feeder 52 is a screening unit, generally designated 60, having multiple screens
or "screen decks". A Siletta feeder is preferably mounted so that the length of the
slats of the feed tray is perpendicular to the lengthwise direction of the underlying
screen deck. In this position, the Siletta feeder discharges particulate material
substantially uniformly over the full width of the screening unit in the longitudinal
direction of the "slats" and discharge chute 56 is preferably of the full-width type
so as to maintain this spread condition as the stonesand is fed onto the underlying
screen deck. Discharge chute 56 is manually or automatically adjustable through an
arc of about 90° in the direction of arrow R for purposes of directing the feed discharge
as explained in more detail below.
[0042] The screening unit 60 receiving stonesand feed 62 from chute 56 may be comprised
of one or more screen decks. In the embodiment shown in Figure 1, the screening unit
has three (3) screen decks, namely, a top screen 64 of 8-mesh size, an intermediate
screen 66 of 4-mesh size and a bottom screen 68 having a 50-mesh size section and
a 30-mesh size section. Screen 64 may extend for almost the full length of the screening
unit, e.g., about 84 inches (213 cms), while screen 66 and each section of screen
68 may extend about one-half that length, e.g., about 42 inches (107 cms). Each of
these screens may be about 46 inches (117 cms) in width. The support grid (not shown)
of each screen may be independent of the others and is preferably built as an open
waffle-like structure with only longitudinal stringer supports for the overlying wire
screens. To aid in screen cleaning and preventing screen "blinding", a coarse under
screen having a mesh of about 3/8 or 1/2 (9.5 or 12.7 mm) inches may be attached to
and underneath each support grid so that individual compartments about 6 inches (15
cms) square and 1s inches (3.8 cms) thick are formed adjacent to the under surface
of each screen. Hard rubber balls may then be loaded into each such compartment to
form a ball cleaning system to help prevent screen blinding.
[0043] An adjustable deflector plate 70 is provided along the upper transverse edge of the
screening unit to direct input feed material onto screen 66, onto an interscreen conveyor
72 or through a feed diverter 74 having a pair of chutes, one extending downward past
each side of conveyor 72. Adjustable chute 56 cooperates with deflector plate 70,
interscreen conveyor 72 and feed diverter 74 so as to direct feed 62 to one or more
of the three screens or to divert all or a portion of feed 62 around one or more screens.
Accordingly, when chute 56 is in position "A" all of the feed 62 falls onto top screen
64. When chute 56 is in position "B", feed 62 is divided between top screen 64 and
intermediate screen 66. When chute 56 is in position "C" and deflector 70 is fully
closed to shut off flow to diverter 74, all of feed 62 is fed onto screen 66. When
chute 56 is in position "C" and deflector 70 is open, feed 62 is divided between screen
66 and diverter 74. When chute 56 is in position "D" and deflector 70 is fully open
or fully closed, all of feed 62 bypasses screens 64 and 66 and is conveyed to screen
68 by interscreen conveyor 72, feed being discharged onto either the 50-mesh section
or the 30-mesh section of screen 68 depending on the position of the adjustable discharge
end of the interscreen conveyor. Both chute 56 and deflector 70 may also have intermediate
positions so as to divide feed 62 between screen 66 and screen 68 and between screen
68 and feed diverter 74.
[0044] Screens 64, 66 and 68 are arranged in the form of screen decks carried by vibratory
frame 80 which is dynamically balanced and resiliently mounted on a fixed frame 82.
An adjustable vibratory unit 84 is driven by a variable speed motor 86 in response
to a control signal 87 for varying the vibratory frequency. With reference to Figure
2, the screen vibratory unit 84 includes means for varying both the vibratory amplitude
and vibratory wave form in addition to the vibratory frequency. A rectangular vibratory
cam or bearing member 88 provides a saw-tooth type of wave form and an eccentric cylindrical
bearing member 90 provides a sinusoidal type of wave form. Alternately, cams of other
shapes could be used to generate a variety of other types of wave forms. Members 88
and 90 are axially mounted for rotation upon a shaft 92 carrying a pulley 94 driven
by a belt of motor 86. Pulley 94 engages a spline portion 96 of shaft 92 so that shaft
92 may be adjusted longitudinally by means of a bearing disc 98 engagable by a slotted
journal member 100 threaded to a shaft 102 mounted for rotation parallel to shaft
92. A reversible electric motor 104 rotatably engages shaft 102 so as to reciprocate
journal member 100 and shaft 92 in the direction of arrow "W" in response to a control
signal 106, disc 98 secured to shaft 92 being free to rotate within the slot of journal
member 100 during adjusting engagement between these two components. Longitudinal
adjustment of shaft 92 causes longitudinal displacement of the vibratory members 88
and 90 which are rigidly secured to shaft 92 for rotation therewith. A change in wave
form is achieved by longitudinally displacing shaft 92 so that cylindrical member
90 engages vibratory frame 80 in place of rectangular member 88. As illustrated in
Figure 2, the longitudinal axis of member 90 is canted relative to the longitudinal
axis of shaft 92 so that longitudinal adjustment of member 90 relative to vibratory
frame 80 will change the amplitude at which frame 80 is vibrated by its engagement
with the eccentric bearing surface provided to either side of the longitudinal position
at which shaft 92 passes through the radial center of member 90. Shaft 92 is mounted
both for rotation and for longitudinal reciprocation by a pair of journal members
108 mounted near opposite edges of fixed frame 82, one such journal member 108 being
shown in Figure 1 but omitted from Figure 2 for purposes of clarity.
[0045] The angle of inclination of the screen decks relative to the horizontal may be varied
since one end of the fixed frame 82 is pivotally mounted upon a foundation 112 by
means of a pivot connection 110. The other end of fixed frame 82 is pivotally connected
to a vertically adjustable shaft 114 which has threads engaged by a reversible electric
motor 116 so that actuation of motor 116 in response to a control signal 120 causes
longitudinal movement of threaded shaft 114. Motor 116 is pivotally connected to foundation
112 by a pivotal mounting 118 similar to pivotal connection 110.
[0046] Each of the screens 64, 66 and 68 is configured so that the open length of the screen
can be varied, either manually or automatically. With respect to top screen 64, a
shroud member 130 is arranged to be movable in the direction of arrow "U" and has
a solid bottom pan 132 underlying screen 64 as illustrated in Figure 3. Also attached
at or near the bottom of shroud member 130 is an elongated rack 134 engaged by a pinion
136 rotatably driven by a reversible electric motor 138. Shroud 130 is mounted on
ball bearing rollers that ride on a track preferably comprised of a pair of angle
iron side rails (not shown) so that shrould pan 132 may be adjusted relative to the
longitudinal length of screen 64 by movement of rack 134 upon rotation of pinion 136
by motor 138 in response to a control signal 140. As an alternative, pan 132 may itself
include a screen or other apertured section 139 arranged to cooperate with the apertures
of screen 64 so as to vary the effective opening size of at least some of the apertures
seen by the particles passing along the screen deck formed by such a parallel structure.
[0047] The open length of screen 66 is varied by means of a longitudinally adjustable interscreen
pan 150 connected by a tether 152 to a counterbalance 154. The tether 152 is preferably
in the form of a chain engaged by a sprocket 156 of a reversible pan positioning motor
158. The interscreen pan 150 is mounted on ball bearing rollers that ride on a track
preferably comprised of a pair of angle iron side rails (not shown) mounted on vibratory
frame 80 so as to be vibrated thereby for the purpose of causing movement of particles
falling thereon toward the lower, discharge end. Actuation of motor 158 in response
to a signal 160 causes pan 150 to move in either of the directions indicated by arrow
"V" depending upon the direction of motor rotation as determined by the signal 160.
Particulates falling past the upper end of pan 150 reach interscreen conveyor 72 as
a first throughs stream for transport to bottom screen 68. The particulates falling
on pan 150 are discharged from its lower end into a collection chute 162 through which
they leave the screening apparatus as a separate stream of throughs and/or overs from
the screen 66 and fall on an overs discharge conveyor 164.
[0048] Pan 150 is preferably arranged for sufficient upward travel to completely cut off
the passage of particles from screen 66 to conveyor 72 and for sufficient downward
travel to permit all particles passing through upper screen 64 to reach conveyor 72
either by passing through the larger mesh of screen 66 or by falling off the lower
end of screen 66 directly onto conveyor 72. Pan 150 may also include an apertured
section (not shown) similar to section 139 of pan 132 and arranged so as to alter
the probability of passage from screens 64 and/or 66 to conveyor 72 for at least a
portion of the particulates intercepted by pan 150.
[0049] The discharge end of interscreen conveyor 72 is adjustable in either of the directions
indicated by arrow "X" by means of a tether 170 connecting the upper end of this conveyor
to a counterbalance 172. Tether 170 is preferably a flexible chain arranged to be
engaged by a sprocket 174 driven by a reversible electric conveyor positioning motor
176. The interscreen conveyor is preferably of the belt type and the upper end of
the conveyor assembly includes a drive roller 178 and a tensioning roller 180. Drive
roller 178 is driven by an adjustable speed motor (not shown) which is preferably
synchronized with the feed rate so as to prevent an excessive build-up of particulates
on or near the discharge end of the conveyor belt. A vertically extending deflector
plate 182 is mounted adjacent to the discharge end 183 of conveyor 72 to ensure that
the particulates are fed to screen 68 in a relatively narrow band extending across
the screen width immediately below this end of the conveyor, instead of being thrown
off the end of the conveyor through an unknown variable distance before impacting
on the apertured surface of the underlying screen.
[0050] The longitudinal position of the discharge end of conveyor 72 preferably is adjustable
from a lower position discharging to a throughs pan 184 to an upper position discharging
to the upper portion of the 50-mesh screen section so as to be able to take advantage
of the full open length of this screen section. The upper end of pan 184 is placed
longitudinally downstream of the upper end of the 30-mesh screen section so that the
discharge end 183 of conveyor 72 may be positioned close enough to the discharge end
of this screen section to provide the degree of incomplete screening desired. Located
between the 50-mesh and 30-mesh screen sections is a side discharge channel 186 with
a hinged door 187. Discharge channel 186 conveys particulates around the 30-mesh section
directly to a chute 190 if door 187 is open. With door 187 closed, the particulates
passing off of the end of the 50-mesh section will also pass over the 30-mesh section
and be screened thereby. The particulates reaching either or both of these screen
sections are separated into a fines component 258 passing through screen 68 and a
bottom overs component 256 passing off of the end of screen 68 and through the chute
190 to a conveyor 192. The fines component 258 falls on a fines pan 194 and is discharged
from the lower end of this pan through a chute 196 to a fines conveyor 198. Fines
conveyor 198 is of the weigh belt type having a weight and conveyor speed sensing
element 200 providing a mass flow rate signal 202 to control system 45.
[0051] A top overs stream 252 from screen 64 is discharged to overs-conveyor 164 and transported
to a weigh belt 210 having a weight and conveyor speed sensing element 212 for providing
a mass flow rate signal 214 to control system 45. Intermediate overs and/or throughs
254, which pass through screen 64 and/or over or through screen 66 but do not reach
a subsequent screen because of pan 150, are also discharged to conveyor 164 and transported
to weigh belt 210.
[0052] For purposes of explanation only, but without limitation, the bottom overs 256 from
screen 68 are designated as the product stream in Figure 1. However, any of the output
streams, such as those received by conveyors 164 and 198, may be designated as "product".
Furthermore, the "product" stream may be comprised of an intimate mixture of two or
more output streams or one or more output streams in intimate admixture with unscreened
feed diverted through feed diverter 74 to a weigh belt 220 having a weight and conveyor
speed sensing element 222 for providing a mass flow rate signal 224 to control system
45.
[0053] In the embodiment of Figure 1, the product stream on conveyor 192 is discharged to
a product weigh belt 230 enclosed within a housing 232 having an inlet chute 234 and
a discharge chute 236. The weigh belt includes a weight and conveyor speed sensing
element 238 for providing a mass flow rate signal 240 to control system 45. Heated
air or direct heat may be provided within housing 232 so as to control the moisture
content of the particulate stream at a uniform level for continuous mass flow rate
measurements. Similar housings and heating units may be provided for weigh belts 198,
210 and 220.
[0054] A measuring device 242 may also be employed for measuring the particle size distribution
or some other measurable characteristic of the product stream particulates, such as
the mean particle size, and for providing an input signal 244 corresponding to the
measured characteristic to control system 45. Measuring devices 38 and 242 for automatically
measuring one or more characteristics of the particulates may provide either an intermittent
or continuous input signal and may be a radiant and/or impact energy type as illustrated
by U.S. Patent No. 3,478,597 to Merigold, et al., No. 3,797,319 to Abe and No. 4,084,442
to Kay; a sedimentation rate type as illustrated by U.S. Patent No. 3,208,286 to Richard,
et al., and No. 3,449,567 to Olivier, et al.; a centrifugal air classifier type for
providing a control signal responsive to the proportion of particles above or below
a selected size as illustrated by U.S. Patent No. 2,973,861 to Jager; a sieving type
for automatically measuring fineness modulus as illustrated by U.S. Patent No. 2,782,926
to Saxe; a multiple screen classifying type as illustrated by U.S. Patent No. 3,439,800
to Tonjes and No. 3,545,281 to Johnson; a continuous weight comparison type for providing
a control signal responsive to the relative weights of different particulate streams
as illustrated by U.S. Patent No. 3,136,009, No. 3,126,010, No. 3,143,777, No. 3,151,368,
No. 3,169,108 and No. 3,181,370 to Dietert alone or with others; a fluid elutriator
type as illustrated by U.S. Patent Nos. 3,478,599 and 3,494,217 to Tanaka, et al.;
a piezoelectric type as illustrated by U.S. Patent No. 3,630,090 to Heinemann, No.
3,844,174 to Chabre and No. 4,973,193 to Mastandrea; a volume measuring type for providing
a control signal responsive to the rate of accumulation of one or more size fractions
as illustrated by U.S. Patent No. 3,719,089 to Kelsall, et al.; a radiant energy type
for providing a process control signal as illustrated by U.S. Patent No. 3,719,090
to Hathaway, No. 3,836,850 to Coulter, No. 3,908,465 to Bartlett, No. 4,178,796 to
Zwicker and No. 4,205,384 to Merz, et al.; a particle noise measuring type as illustrated
by U.S. Patent No. 4,024,768 to Leach, et al. and No. 4,179,934 to Svarovsky; a trajectory
type as illustrated by U.S. Patent No. 3,952,207 to Leschonski, et al., and No. 4,213,852
to Etkin; a sequential weight of fraction type as illustrated by U.S. Patent No. 3,943,754
and No. 4,135,388 to Orr; or any other type of prior art measuring device capable
of providing a signal proportional to some scalar function of particle size distribution
such as mean particle size, fineness modulus, or a point on the cumulative size distribution.
The entire contents of each of the above mentioned patents are expressly incorporated
herein by reference.
[0055] As a further example, input signals 40 and/or 244 may be produced manually and have
a value selected on the basis of particle size analyses performed manually on particulate
samples taken either automatically or manually from an input or output stream of the
screening unit. Similarly, in some applications, automatic controls such as control
system 45 may be eliminated entirely and necessary adjustments in one or more variable
screening parameters may be made manually on the basis of either manual or automatic
particle size analyses.
[0056] The total of the mass flow rate on weigh belts 198, 210, 220 and 230 equals the mass
flow rate of the feed. Where a feeder of the Siletta type is employed, continuous
measurement of the mass flow rate in all of the output streams may not be necessary
since the feed flow rate from a Siletta feeder may be calibrated and controlled fairly
accurately in the range of 2 to 25 tons per hour by adjustment of the slats 53 and
the vibratory amplitude provided by the drive unit 54. In this regard, the output
of the Siletta feeder may be calibrated by placing feeder chute 56 in position "D"
and adjusting interscreen conveyor 72 over plate 184 so as to discharge the entire
feed stream into chute 190 leading to product weight belt 230. Alternatively, the
Siletta feeder may be calibrated by placing feeder chute 56 in position "A" and adjusting
interscreen pan 150 so as to discharge the entire feed stream onto conveyor 164 leading
to overs weigh belt 210.
[0057] As illustrated in Figures 1, and 3, the screening apparatus 60 has a number of screening
parameters that may be varied either manually or automatically during the screening
process without stopping the equipment. In this specification, the term "controllably
variable" is used to designate these screening parameters. The following controllably
variable screening parameters may apply to each screen deck or screen section where
a deck includes more than one screen in series: feed flow rate; feed particle size
distribution; open screen length for a given screen width providing a separated throughs
stream; effective screen opening size for each screening having different opening
sizes spatially distributed along its length; screen inclination; screen vibratory
frequency; screen vibratory amplitude; and screen vibratory wave form.
[0058] The foregoing screening parameters are also "variable" in the sense that they may
be changed or varied during shutdown or interruption of the screening process. In
this specification, the term "variable" is used alone as being more generic than "controllably
variable". For example, the screening apparatus may be shut down and the screening
process thereby interrupted to change the screens on one or more screen decks. In
this manner, the aperture size or sizes of the screen component on a given screen
deck may be varied. Similarly, the spatial distribution of screen apertures as well
as the size distribution of apertures may be varied such as where the alternate screen
contains more than one size aperture and the mixture of aperture sizes is either constant
or varies down the length of the screen.
[0059] Each of the foregoing "variable" screening parameters is selected in accordance with
the present invention so that the combination of screening parameters operative on
the feed stream is such that one or more screens do not provide essentially complete
screening but instead provide substantially "incomplete" screening. For purposes of
this specification the degree of complete screening is defined as the ratio of mass
flow rate of the feed passing through a screen relative to the total mass rate that
is capable of passing through the same screen if the feed were screened to completion.
The degree of incomplete screening is defined as one minus the degree of complete
screening.
[0060] In addition, one or more of the screening steps may be set up to operate so that
the degree of incomplete screening is "substantially variable". The degree of incomplete
screening is "substantially variable" when it is at a level that can be varied by
a substantial amount by varying one or more of the foregoing screening parameters.
At these screening conditions, the differential rate of screening undersize particles
(mass of throughs passing into output stream per unit time) is also "substantially
variable", i.e., the differential screening rate can be varied by a substantial amount.
In practicing the present invention, the degree of incomplete screening may be substantially
variable for the entire feed stream or for one or more size fractions of the feed
stream, e.g., -4+8 mesh, -8+16 mesh, -16+30 mesh, -30+50 mesh, -50+100 mesh and/or
-100+200 mesh.
[0061] Depending on the size distribution of the feed, it may be that a single screen deck
employing the incomplete screening principles of the invention may be sufficient to
provide either an overs or a throughs output stream having an altered particle size
distribution meeting the preselected distribution desired in the stonesand product.
Any of the previously noted controllably variable parameters may be used to achieved
incomplete differential rate screening with a single screen. However, the degree to
which the particle size distribution of a feed stream can be altered with such a single
screen is significantly less than that which can be achieved with two or more screens.
Inasmuch as system complexity is expected to increase rapidly with increase in number
of screens, it is believed that a practical system for effective control and flexibility
is attained with the use of two or three successive screen decks of different mesh
sizes. The screen decks are considered to be "successive" when the throughs or overs
from one are fed onto the other.
[0062] The number of successive screens or screen decks is another important and controllably
variable screening parameter of the present invention. The screening apparatus and
process illustrated in Figure 1 provide a number of different flow paths, some providing
successive screenings and some having controllably variable mass flow rates. The flow
paths include without limitation those discussed below.
[0063] With adjustable chute 56 in position "A", feed 62 will fall initially on the open
length of top screen 64 and be separated there and on intermediate screen 66 by incomplete
screening into a throughs stream 250 passing through screen 66 and falling on interscreen
conveyor 72 and an overs stream 252 reaching the solid bottom 132 of shroud 130 without
passing through the openings or apertures of screen 64. In this mode of operation,
interscreen pan 150 may be positioned so as not to intercept any of the particulates
passing through screen 64, and the shroud 130 may be adjusted to vary the open length
of screen 64 and thereby vary the degree of incomplete screening provided by this
screen. Since the mesh size of intermediate screen 66 is larger than that of top screen
64 in the embodiment shown, practically all of the particulates passing through screen
64 will pass even more rapidly through screen 66 and not build up on the latter. However,
when pan 150 is in its lowermost position, its upper end is spaced downwardly beyond
the lower end of screen 66 so that any buildup of particulates may be discharged from
the lower end of screen 66 directly onto conveyor 72. Alternately, the position of
pan 150 may be varied, either alone or in combination with the position of shroud
130, to vary the degree of incomplete screening provided by screen 64 and thereby
generate another throughs stream 254 which may be combined with overs stream 252 on
conveyor 164.
[0064] Throughs stream 250 upon reaching interscreen conveyor 72 is discharged from lower
end 183 of this conveyor onto bottom screen 68 where these throughs are further separated
by incomplete screening into two fractions, namely an overs stream 256 discharged
through chute 190 to conveyor 192 and a throughs stream 258 (fines) discharged through
chute 196 to conveyor 198. The degree of incomplete screening provided by bottom screen
68 may be varied by adjusting the longitudinal position of lower end 183 of interscreen
conveyor 72 and thereby changing the location at which throughs stream 250 falls onto
screen 68. This in effect varies the open length of screen 68 exposed to throughs
250.
[0065] Interscreen conveyor 72 may also be adjusted longitudinally so as to discharge throughs
250 either above or below channel 186 dividing screen 68 into two screening components
of different mesh sizes, namely an upper 50-mesh screen and a lower 30-mesh screen
in series. Adjustable door 187 may either allow overs from the upper screen section
to pass unobstructed to the lower screen section or divert these overs into channel
186 providing a flow path for conveying the upper section overs directly to bottom
overs chute 190. The first of these alternatives illustrates another important feature
of the invention, namely, that one or more of the screen decks may be comprised of
a series of different screens each of a different mesh size or of a different size
distribution and/or spatial distribution of screen openings so as to controllably
vary the effective screen aperture size and/or screen aperture spatial distribution
in response to a characteristic of an input stream to or an output stream from the
screening apparatus and process.
[0066] The effective screen aperture size and/or screen aperture spatial distribution of
the screening means may also be controllably varied by positioning feeder chute 56
in position "B" so that the feed stream 62 is split between top screen 64 and intermediate
screen 66 having different mesh sizes and/or different aperture spatial distributions.
Position "B" represents any chute position between position "A" (entire feed to screen
64) and position "C" (entire feed to screen 66) so that the flow rate of feed to one
of these screens may be varied relative to flow rate of feed to the other.
[0067] As another alternative, if throughs 250 have the desired size distribution without
further screening, these throughs may be discharged as product by positioning the
discharge end 183 of interscreen conveyor 72 over plate 184 leading to chute 190.
As interscreen conveyor 72 is preferably mounted on fixed frame 82 so as not to be
vibrated, stream 250 may also be discharged as product by reversing the direction
of travel of the belt of conveyor 72 and providing means (not shown) for discharging
stream 250 from the upper end of the conveyor, such as to weigh belt 220.
[0068] With chute 56 in position "C", all of the feed 62 falls on intermediate screen 66.
In this mode of operation, the open length of screen 66 and thereby the degree of
incomplete screening provided by this screen is controllably varied by positioning
interscreen pan 150 to intercept more or less of the throughs stream 250. As indicated
above, the throughs stream 250 is defined as those throughs passing through either
or both screen 64 and 66 and reaching interscreen conveyor 72 without being intercepted
by pan 150. Upon reaching the belt of conveyor 72, throughs 250 may be subjected to
a second incomplete screening step upon being discharged to bottom screen 68 in accordance
with the screening alternatives provided by this screen as described above.
[0069] As an alternative to discharging all of the feed to screen 66, chute 56 may be left
in position "C" and hinged deflector plate 70 opened so as to divide feed 62 between
screen 66 and diverter 74. The relative flow rates to screen 66 and diverter 74 are
variable in accordance with the precise positioning of the discharge opening of chute
56 relative to the splitting edge formed by the juncture between the screen and the
diverter passageway. In this mode of operation, the desired size distribution of the
product would be achieved by mixing the diverted feed downstream of weigh belt 220
with one or more of the output streams available from the screening apparatus, namely,
the throughs and/or overs 254 from chute 162, the throughs 250 from plate 184 and
chute 190, the bottom overs 256 from chute 190 and/or the fines 258 from chute 196.
[0070] With chute 56 in position "D" and deflector plate 70 in fully open position 70B,
the entire feed 62 is discharged onto interscreen conveyor 72. In this mode of operation,
the entire feed may be subjected to a single screening step on screen deck 68, this
screening step providing incomplete screening by either the 50-mesh section or the
30-mesh section depending on the position of the interscreen conveyor discharge relative
to these screen sections. When the 50-mesh section is to be used alone, channel door
187 is in the open position shown in Figure 1 to divert overs into the transverse
channel 186. Alternately, door 187 is closed so that screening may take place both
on the 50-mesh section and the 30-mesh section, the 50-mesh screening being substantially
varied in response to the position of the interscreen conveyor discharge while the
30-mesh screening may be carried out essentially to completion by reason of the overs
traversing the entire available length of the 30-mesh section.
[0071] In this mode of operation, interscreen conveyor 72 may be positioned so as to discharge
all of the particulates thereon to chute 190 via fixed plate 184 so as to obtain measurements
of the entire feed stream at different flow rates for purposes of calibrating the
controllably variable feed flow provided by the Siletta feeder 52, or to provide periodic
measurements of feed flow when using a feeding component having a relatively fixed
mass flow rate.
[0072] Yet another alternative is provided by placing chute 56 in position "D" and the diverter
door in position 70A so that feed 62 is divided between diverter 74 and interscreen
conveyor 72. In this mode of operation, screening of the feed portion on conveyor
72 is provided by screen deck 68 in accordance with any one of the screening options
provided thereby as described above. A product may then be provided by combining the
diverted feed with one or more of the screened output streams, namely, bottom overs
256 and/or fines 258.
[0073] A number of other flow options are available within the contemplation of the present
invention and it is not intended to describe all of them here. For example, pan 150
may be used to divide the overs discharged from the lower end of screen 66 and plate
184 may be used to divide the throughs discharged from the lower end of conveyor 72,
such divisions affecting a change in the flow rate of particles reaching lower screen
deck 68 and thereby being capable of changing the particle size distribution in the
overs or throughs stream from the 30-mesh portion of this deck. Additional screening
decks may be utilized or adjustable pan components or adjustable conveyor components
utilized with a different screen than that illustrated in Figure 1. All such variations
may provide incomplete screening of an input feed or one or more intermediate feeds
to a screening surface.
[0074] The particle size distribution of both throughs and overs from a given screen deck
operating under incomplete screening conditions can be altered by changing the particle
size distribution (the relative amounts of particles in different size ranges) of
the feed to the screen or screens of that deck. As indicated above, the size distribution
of feed 62 may be controllably varied by changing the degree or type of size reduction
provided by crusher 22.
[0075] The control system 45 and the input signals thereto and the output signals therefrom
will now be described in more detail. With reference to Figure 1, control system 45
may include input signal 40 responsive to some scalar function of particle size distribution
such as mean particle size, fineness modulus, or a point on the cumulative size distribution
and/or mass flow rate of feed; input signal 202 responsive to mass flow rate of throughs;
input signal 214 responsive to the mass flow rate of overs; input signal 224 responsive
to mass flow rate of diverted feed; input signal 240 responsive to mass flow rate
of product; and/or input signal 244 responsive to some scalar function of particle
size distribution of product. In this context, it is emphasized again that the product
may be comprised of output streams other than overs from the lowest screen or of mixtures
of one or more of the output streams and that the measuring device 242 or other devices
measuring a stream characteristic may be located at positions other than those shown
in Figure 1 as appropriate to measure the characteristics of the stream selected as
product for a given application of the invention.
[0076] Outputs from control system 45 may include, without limitation, output signal 25
for regulating the speed of rock conveyor motor 24; output signal 27 for regulating
the speed of crusher motor 26 and thereby the mean particle size and/or particle size
distribution of the feed 30; output signal 37 for regulating the speed of conveyor
motor 36; output signal 55 for regulating the transverse openings between slats 53
and/or the vibratory amplitude of Siletta feeder 52, thereby regulating the mass flow
rate of feed 62; output signal 57 for regulating the position of chute 56 and thereby
the selection of the screen deck to receive all or a portion of the feed 62; output
signal 87 to regulate the vibratory frequency of the screen decks; output signal 106
to regulate the vibratory wave form and/or amplitude of the screen decks; output signal
120 to regulate the angle of inclination of the screen decks; output signal 140 to
regulate the position of shroud 130 and thereby the open length of screen 64; output
160 to motor 158 to regulate the position of interscreen pan 150 and thereby the open
length of screen 66; and/or output 177 to motor 176 to regulate the position of interscreen
conveyor 72 and thereby the open screen length of bottom screen 68.
[0077] For given ranges of feed rate and feed size distribution, a particular set up of
the apparatus and process of the invention may be required to provide particulate
product of a preselected size distribution or range of size distribution. Accordingly,
set points for control system 45 may include a feed rate set point 270, a feed mean
particle size set point 272 and a product mean particle size set point 274. These
set points provide a null point for generating appropriate signals for controlling
the rate and a particular scalar function of particle size distribution of the feed
within ranges compatible with the equipment set up, and for controlling the particle
size distribution of the product within desired limits by regulating one or more screening
parameters affecting particle size distribution of the product as previously described.
[0078] In crushing a number of rock types with conventional crushing equipment, the particle
size distribution of stonesand provided by such equipment can be maintained relatively
constant without controllably varying a crushing parameter. The rate of feeding these
types of stonesand can also be maintained relatively constant by a feeder of the type
described. Furthermore, in many applications, only one or two screens and one or two
variable screening parameters may be needed to achieve the preselected size distribution
desired in the aggregate or stonesand product. One such simplified apparatus and process
is illustrated in Figure 4 wherein the same numbers are used followed by a prime (')
symbol to designate the same element or component as previously described.
[0079] With reference to Figure 4, a feed material 62' is provided to bin 32' so as to keep
the bin relatively full with a substantially constant depth of particulate material.
In the specific screening examples described below, the particulate feed material
had a cumulative size distribution illustrated by curve F in Figure 5. Also illustrated
in Figure 5 by dotted line curves H, M and L are the high, midpoint and low cumulative
size distributions, respectively, of the ASTM C-33 Standard Specification for Concrete
Aggregates as adapted for stonesand and set forth in "Stonesand for Portland Cement
Concrete", Table C, Stone Products Update 1, National Crushed Stone Association, February
1976. The particulates in the feed were produced by crushing limestone rocks with
a centrifugal crusher of the type described in the Sautter patent referenced above,
the crusher parameters being selected so as to reduce the particle sizes of the aggregate
to less than 3/8 inch (9.5 mm) and the crusher discharge being prescreened to remove
any carry over of 3/8 inch (9.5 mm) or larger material before being discharged to
bin 32'.
[0080] The principal components of the system of Figure 4 include a feed bin 32', bin discharger/feeder
52', a modified two-deck screening unit 60', a weight belt 230', an interscreen conveyor
72' and a control system 45'. The entire two-deck screen is mounted on a support framework
(not shown) which permits manually changing the screen inclination angle above horizontal
over the range from 21° to 36°, in 3° increments.
[0081] Bin-discharging feeder 52' is a "Siletta" 30-inch live bottom feeder of the type
previously described. This is a carbon steel unit with a "Venetian blind" type feed
tray sized to pass crushed stone with a density in the range of 80 to 100 Ib/ft3 (1280
to 1600 kg/m
3} and particle sizes 3/8 inch (9.5 mm) and smaller at a feed rate in the range of
approximately 2 to 25 tons per hour (0.56 to 7.1 -kg/s). The feed tray is vibrated
horizontally in a direction perpendicular to the length of slats 53' with an adjustable
amplitude magnetic drive unit 54' manufactured by Eriez Magnetics of Erie, PA. The
drive unit vibrates the feed tray at a constant frequency of 60 Hz and a variable
amplitude up to about 1 mm, and includes a Model FS-75A controller configured to permit
control both manually and in response to an external analog signal 55'. This analog
signal can be used to vary the feed mass flow rate and thereby provides one means
of achieving automatic control over the product particle size distribution. The Siletta
unit is mounted so the length of slats 53' is perpendicular to the lengthwise direction
of underlying screen 64'. Although the cant of these slants may be adjustable, it
is preferably fixed in this embodiment. The mass flow rate of material discharged
from the Siletta is quite uniform from one element of length to the next over the
full length of the feed tray. To maintain this spread condition, the feed material
62' is fed into a full-width discharge chute 56'. Discharge chute 56' is manually
adjustable through an arc R' of about 90° so that feed can be directed to the screen
or to an interscreen conveyor 72', or divided between the screen and conveyor.
[0082] The screening unit 60' is preferably a Model 46-8400, lightweight, two-deck screening
system manufactured by Forsbergs, Inc., of Thief River Falls, MN. Each of the screens
in this system has a screen size of 46x84 inches (117x213 cms). Unit 60' is dynamically
balanced and mounted upon a fixed frame (not shown) by four eccentric bearing assemblies
having a fixed throw of about 3/16-inch (4.8 mm) and a corresponding vibration amplitude
of about 3/32-inch (2.4 mm). An adjustable sheave drive unit permits the screening
unit to operate over the speed range of approximately 800 to 1200 rpm. Each screen
has an independent support grid built as an open waffle-like structure with only longitudinal
stringer supports for the overlying wire screens. A coarse under screen is attached
to each support grid so as to form individual compartments about 6-inches (15 cms)
square by 12 inches (3.8 cms) thick. Hard rubber balls are loaded into each such compartment
to form a ball clearing system for the screens to prevent screen blinding. Separate
discharge chutes 131', 190' and 196' receive the overs 252' from top screen 64', the
overs 256' from bottom screen 68' and the throughs 258' from bottom screen 68', respectively.
[0083] Each screen is configured so that its open length can be changed to vary the degree
of incomplete screening provided by each successive screening stage. This is accomplished
by fitting top screen 64' with a thin overlying adjustable plate 132' placed in such
a manner that the plate and screen sandwich can be tightened down against the support
deck with side screen tensioning screws. This permits manual adjustment of the open
length of the upper screen, preferably over the length range of about 0 to 24 inches
(0 to 61 cms). This open length of top screen 64' is measured from the lip of an overlying
discharge deflector plate 70' at its upper end to the upper edge 133' of cover plate
132' at its lower end. The open screen length range may be extended easily if necessary
by changing the relative lengths of screen 64' and cover plate .132'.
[0084] The open length of bottom screen 68', whose entire length remains uncovered at all
times, is measured from the position where interscreen conveyor 72' dumps material
onto the screen surface to the downstream end of this screen. This effective length
preferably varies from about 0 to about 70 inches (0 to 178 cms). Inasmuch as the
position of the interscreen conveyor can be adjusted by a reversible motor drive unit
176', the effective length of the bottom screen can be controlled automatically during
the screening process. This provides another means for controlling the size distribution
of particles in the output streams of this embodiment.
[0085] Interscreen conveyor unit 72' is preferably a low profile flatbelt type conveyor
with an adjustable DC speed control drive available from Processing Equipment Co.,
Inc. The total thickness of the conveyor may be as little as approximately 3.0 inches
(7.6 cms) and its usable flat belt surface is at least about 12 inches (30 cms) longer
than the screens. A conveyor of relative small thickness may be necessary in order
for it to fit between the two screening components, such as between the central bearing
support shaft and the lower screen of a Forsbergs unit. Rubber bumpers are preferably
located on the screen support frame so that screen wobble transients during start
up and shutdown will not cause the screening unit to impact against the interscreen
conveyor. The entire interscreen conveyor 72' is mounted on ball bearing rollers that
ride on a pair of angleiron side rails (not shown). The rails are end-supported outside
of screening unit 60' and extend down between the screen decks without attachment
to the screening unit. Thus the conveyor does not vibrate and motion of its belt is
required to carry material to the prescribed dump point onto the bottom screen. A
vertical deflector plate 182' is mounted at discharge end 183' of the conveyor to
insure that particles 250' fall onto bottom screen 68' in a relatively narrow band
instead of being thrown off the end of the conveyor through some variable distance.
[0086] The system layout of Figure 4 in combination with a crusher of variable size output
permits the following screening parameters to be varied for control of particle size
distribution in the product: screen opening size(s) and/or size distribution and/or
spatial distribution of screen openings (by manually changing screens on one or both
screen decks), open screen lengths (by manually changing the position of shroud 130'
and/or manually or automatically changing the position of conveyor discharge 183'),
screen inclination (by manual adjustment of frame), screen vibratory frequency (by
manual adjustment of vibrator drive), feed flow rate (by manual or automatic adjustment
of Siletta feeder), feed size distribution (by manual adjustment of crusher), and/or
feed division between top and bottom screens (by manual adjustment of chute 56').
Of these, the open length of screen 68', the inclination and vibratory frequency of
both screens, and the flow rate, size distribution and division of feed 62' are controllably
variable while the process is in operation.
[0087] When conveyor 72' is at its lowest position, material can be fed directly from the
feeder 52' onto the upper end of this conveyor belt, and subsequently conveyed and
discharged without screening to bottom overs discharge chute 190'. This arrangement
permits introducing the entire feed stream to weigh belt unit 230' for calibrating
or periodically checking the input mass flow rate to the screening unit. Likewise,
material which has gone through the top screen alone can be directed to the weigh
belt for periodic mass flow measurements.
[0088] In passing feed material from one screen to another screen in sequence, a screened
product may be taken from four basic sources. The throughs from a first screen may
be passed to a second screen and a product stream may be comprised of either the overs
or the throughs from the second screen. These two operational possibilities are illustrated
by the screening systems of Figures 1 and 4. Alternately, the overs from a first screen
may pass to a second screen and a product stream may be comprised of either the overs
or the throughs from the second screen. These operating alternatives of the rate screening
process of the present invention are illustrated in the simplified apparatus and process
shown in Figure 6 wherein the same numbers are used followed by a double prime (")
symbol to designate similar elements or components as previously described with reference
to Figures 1 and 4. Since the components bearing the same number operate in the same
manner previously indicated, primarily the differences in equipment setup will be
described below.
[0089] The principal components of the system of Figure 6 include a Siletta feeder 52",
a modified screening unit 60" having a first screening deck 64" and second screening
deck 68" arranged so as to receive the overs 252" from the first screening deck, an
interscreen conveyor 72", a product conveyor 192", a product weigh belt 230", and
a control system 45". Since the two screening decks are separated horizontally, they
may be mounted either on the same support framework or on separate support frameworks.
Separate support frameworks for each screen deck provide the option of independent
screen inclinations and independent screen vibratory motions. In other words, each
screen deck may have its own means for controllably varying screen inclination (similar
to elements 114, 116, 118 and 120 of Figure 1) and/or its own means of controllably
varying screen vibratory amplitude, frequency and/or wave form (similar to elements
84, 86 and 87 of Figure 1 and the elements of Figure 2). In addition, adjustable screen
shroud 130" may be either the manually adjustable type of Figure 4 or the automatically
adjustable type of Figures 1 and 3.
[0090] In the embodiment of Figure 6, the throughs of first screen 64" are designated as
first throughs 250" and are collected on a first throughs conveyor 300" having a weight
and conveyor speed sensing element 302" providing a mass flow rate signal 304" to
control system 45". The overs 252" from first screen 64" are retained by the pan portion
of shroud 130" and fall from the lower end of this pan onto interscreen conveyor 72".
Interscreen conveyor 72" has an adjustable discharge location as previously described.
The overs 252" on the interscreen conveyor are then discharged beneath deflector plate
182" onto the second screen 68" which separates this feed into a second throughs component
258" and a second overs component 256". The second throughs component is collected
by a second throughs conveyor 198". The second overs component 256" is collected on
a second overs conveyor 192" from which these overs are discharged as product onto
the product weigh belt system 230".
[0091] Total particulate flow rate from the Siletta feeder may be measured by adjusting
interscreen conveyor 72" so as to bypass screen 68" entirely and discharge directly
to the product weigh belt system. Total mass flow rate is then obtained by adding
the output of weigh belt 300" to that of the product weigh belt 230". The total mass
flow rate so obtained may then be used to calibrate Siletta feeder 52". This particulate
flow path may also be utilized where the first overs stream is already within specification
so that further screening is unnecessary.
[0092] With further reference to Figure 6, second throughs conveyor 198" may be exchanged
with weigh belt system 230" and associated conveyor 192" so that the product comprises
the second throughs stream instead of the second overs stream. In the case where the
second throughs comprise the product, the discharge end 183" may be positioned over
a gap or open area 306 in screen 68" so as to discharge all of the first overs directly
onto throughs pan 194" and thence to the second throughs conveyor which in this option
would discharge to a weigh belt. This option allows the second throughs system to
measure either total first overs flow or to recover all of the first overs stream
where it already meets its specification without further screening.
[0093] Screen 64" and 68" are each configured so that its open length can be changed to
vary the degree of incomplete screening provided by each corresponding screening stage.
The open screen length of screen 64" may be adjusted by fitting this screen either
with a thin overlying adjustable plate (such as plate 132" of Figure 4) or by an automatically
adjustable shroud having a solid bottom pan underlying the screen (such as pan 132
of Figures 1 and 3). Adjustments in the open screen length of the second screen 68"
is accomplished by changing the discharge position of interscreen conveyor 72" with
respect to the length of this screen in the same manner that interscreen conveyor
72' is adjusted in relation to bottom screen 68' as described above in reference to
Figure 4.
[0094] Another advantage of the Figure 6 embodiment over the other embodiments shown is
that the height or thickness of the conveyor unit as a whole is not critical so that
there is greater flexibility in designing and/or selecting the conveyor equipment
for transporting particulates from the first (upstream) screen to the second (downstream)
screen.
[0095] Product material 256' (the overs of bottom screen 68' in the screening unit of Figure
4) and product material 256" (the overs of second screen 68" in the screening unit
of Figure 6) pass onto continuous weigh belts 230' and 230", respectively. These weigh
belt units may be of the type manufactured by Autoweigh Inc., of Modesto, CA. This
weigh belt has a 24-inch (61 cms) wide troughing belt and uses a torsion bar type
weigh unit resting on special strain-gauge load cells. The weigh belt system is preferably
designed to operate over a range of about 2 to 20 tons per hour (0.56 to 5.7 kg/s)
for material with a bulk density or approximately 100 Ib/ft
3 (1600 kg/m'). This system preferably includes a Mark IV integrator unit, which provides
a display of integrated mass flow rate and instantaneous flow rate (which are labelled
"total" and "mass rate", respectively), and an electronics package capable of supplying
a signal in the 0-1.0 volt range proportional to the instantaneous mass flow rate.
This output signal is preferably introduced directly into an analog digital (A/D)
converter, such as is available in a Rockwell AIM-65 minicomputer.
[0096] In the embodiments of Figures 4 and 6, the weigh belt provides the only on-line measurement
signal for controlling the overall screening system. Its calibration, performance
and input to the control system is therefore of prime importance. The interfaces,
circuitry and calibration procedures for this integrated weigh belt system are given
in the manufacturer's hardware manual.
[0097] A key element of the preferred control system is a Rockwell AIM-65 minicomputer which
has a 4,000 bytes of memory, a BASIC language capability, a thermal printer and a
full keyboard. Programs for the AIM-65 can be stored permanently on cassette tape
but must be reloaded any time the AIM-65 loses power. The AIM-65 receives its principal
measured signal as a mass flow rate input from the weigh belt through an analog to
digital (A/D) converter interface and controls the positioning motor of the interscreen
conveyor and/or the drive unit of the Siletta feeder, each through a digital to analog
(D/A) converter. All conversions are quantized at 8 bits, and accept a 10-bolt signal
range.
[0098] The positioning motor unit for interscreen conveyor 72' preferably includes a 1/4
HP, 1750 RPM, permanent magnet, ball bearing, DC motor with a 0-90 VDC armature, and
a Winsmith 300:1 ratio, double-reduction worm gear reducer. This motor unit is preferably
controlled by a Polyspede Electronics Corporation Model RPD2-16 DC regenerative drive.
The complete variable speed capability of this driver may not be necessary in view
of the large speed reduction ratio employed, but the position control feature of this
Polyspede unit is particularly advantageous.
[0099] The conveyor positioning control system essentially operates with its own separate
feedback loop. That is, a position-correction signal is generated by the AIM-65 minicomputer,
either as a result of a program input to set an absolute position or as the result
of a mass flow rate deviation of the weigh belt signal from a set point value. In
either case, this correction signal consists of two parts; namely, a direction component
and a given number of counts. Once the signal appears, the Polyspede driver actuates
the reversible positioning drive motor in the proper direction for the correction.
A set of points on the motor shaft generates a given number of pulses for each shaft
rotation and these pulses are counted by the AIM-65 minicomputer. When the count equals
the preset count the motor stops. For example, the control system may register 22.65
counts per inch (9 per cm) of travel of the interscreen conveyor. In the preferred
configuration, an auto/manual and safety interlock system provides for manual operation
of the positioning system and prevents the interscreen conveyor for overrunning the
ends of its track.
[0100] A block diagram of the control system as integrated with a AIM-65 minicomputer is
shown in Figure 7. With reference to this figure, the mass flow rate measuring component
310 feeds on analog signal 312 to an analog to digital (A-D) converter 314 of the
AIM-65 computer 316. The output of the AIM-65 is used as an input either to the Siletta
control 54 or to the conveyor positioning control 320, each of these alternative output
signals passing through a corresponding digital to analog (D-A) converter. Siletta
control 54 directly regulates the mass flow rate provided by Siletta feeder 52. Conveyor
control 320 directly regulates the position of the discharge end 183" of interscreen
conveyor 72" by controlling movement of conveyor positioning motor 176" as previously
described. Rotational movement of the set of points on the motor shaft is sensed by
a motor rotation sensor 322 which provides an output signal to the AIM-65 through
a Schmitt trigger 324.
[0101] The control system of Figure 7 provides proportional control for either feeder mass
flow rate or interscreen conveyor discharge position, stable control being available
for only one of these functions at a time since only one downstream characteristic
is measured in the embodiments of Figures 4 and 6, namely product mass flow rate.
However, the invention contemplates measuring two or more output characteristics so
that feed flow rate and conveyor discharge position may be controlled simultaneously.
Periodic or continuous regulation of the Siletta feeder is desirable to maintain a
relatively constant mass flow rate in the presence of upstream variations in feed
flow rate and/or feed conditions. Periodic or continuous regulation of the position
of the interscreen conveyor is desirable to control open screen length so as to maintain
the preselected output size distribution in the presence of changes in the feed and/or
screening conditions, such as compensating for screen blinding caused by cohesive
(e.g., moist) feed material. Complete compensation for screen blinding may not be
possible when the blinding is due to moisture. It is expected that the material flow
rate can be compensated for, but this may not make the appropriate compensation in
particle size distribution. Some of the cohesive material would be expected to pass
through the screen as agglomerates rather than as individual particles and the resulting
size distribution may very well differ from the one expected if no agglomerates were
present. Deviations in the output size distribution may also be corrected by changing
the rate of incoming mass flow provided by the Siletta feeder, but the output size
distribution is much more sensitive to changes in open screen length as provided by
changing the discharge position of the interscreen conveyor.
[0102] In a preferred configuration, an auto/manual and safety interlock system 326 provides
for manual operation of the conveyor positioning system and prevents interscreen conveyor
72" from overrunning the ends of its track. A circuit diagram of the auto/manual and
safety interlock system is shown in Figure 8. The interlock system includes a manual
control 334 and upper and lower limit sensors 328 and 330 which actuate an automatic
disabling circuit 332.
[0103] A basic writing diagram of the electrical circuits interconnecting the various components
of the control system is shown in Figure 9. In addition to the components already
described with reference to Figure 7, the diagram of Figure 9 includes a power supply
336 for the AIM-65, a power supply 338 for the conveyor positioning control circuitry,
a master interface board 340 and a flow control board 342. The AIM-65 interface circuits
on interface board 340 are shown in Figure 10 and the flow control circuits on flow
control board 342 are shown in Figure 11.
[0104] In setting up the various measuring and control system components, such as the weigh
belt and integrator components of the Autoweigh unit, the calibration and setup procedures
set out in the manufacturer's equipment manuals should be followed carefully and each
of the equipment set points should be carefully checked and accurately calibrated.
[0105] While the AIM-65 is very versatile and can be programmed to do a wide variety of
tasks, there is a memory limitation of about 100 basic statements. A preferred set
of programs for operating the AIM-65 as part of the control system is listed in Table
1. The Master Control Program is a real-time control program for normal system operation
and includes statements 1 to 155 for inputs and initialization, including flow stabilization,
and statements 200 to 250 for controlling the normal operating cycle. Statements 200
to 250 call upon subroutines 400 to 475 to convert the Autoweigh input, subroutines
601 to 680 to control the interscreen conveyor discharge position, and subroutines
800 to 900 to provide operational data output if desired. Subroutines 2000 to 2060
may also be provided for data runs to calibrate the weigh belt and/or the feeder.
A schematic diagram of the process control program is shown in Figure 12 where the
"low pass filter" is a programmed filter for stabilization of the control signals.
This filter is contained in statements 42 through 44 of Table 1.
Adaptive hierarchical control
[0106] The preferred control scheme described above is a form of adaptive hierarchical control
comprised of both a continuous monitoring system with feed-back control to correct
for minute-to-minute process variations and an on-demand, discrete sampling and analysis
step to update existing set-point values and to handle long term drift or known process
alterations. To avoid the use of expensive and complex continuous monitoring systems
which directly measure particle size distribution, the continuous system is operated
on the basis of monitoring a process parameter which is particle size dependent, namely,
the mass flow rate of the output particle stream relative to the mass flow rate of
the feed.
[0107] The discrete sampling and analysis aspect of the control scheme may be comprised
of an off-line sampling of the product stream and a rapid sieve analysis carried out
either automatically or manually on a periodic basis and as needed to ensure compliance
of the screened product with the preselected specifications. This on-demand scheme
represents a practical standard against which both system performance and final product
may be judged. The hierarchical concept of control is applicable to the control systems
of Figures 1, 4 and 6 and is illustrated more generically in Figure 13. The system
shown in Figure 13 is designed to accommodate material which has excessive fines.
However, a return loop for returning oversize particles to the crusher supplying the
feed (not shown) may be incorporated for controlling both fines and overs, the overs
returned to the crusher being further reduced in size.
[0108] The discrete sampling of the product stream may be performed on demand, either by
manual sampling or by automatic sampling, in response to an appropriate demand signal,
the origin of which is not shown in the figure. This signal may be preprogrammed to
call for a sample at regular intervals of time, or it may be in response to some monitored
operating parameter of the system, such as open screen length. Open screen length
can be monitored by monitoring the position of the screen blocking member, if that
is the device used to vary open screen length, or the position of a feed conveyor,
if that is the means employed to alter the open screen length. The scope of the invention
is not limited to these means for executing on-demand sampling, and those skilled
in the art will see other means for realizing the objectives of the adaptive-control
scheme.
[0109] In the embodiment of Figure 13, the characteristics of the incoming uncrushed stone
and of the crusher output are determined and the crusher and adjustable screen are
set up to provide a basic size distribution range in the feed and product, respectively.
Trimming control of the size distribution within these ranges to maintain a desired
size distribution specification and/or fineness modulus in the product is achieved
by adjustments to the adjustable screen in response to a signal generated by changes
in the mass flow rate of overs coming off the screen. In other words, the mass flow
rate information from the continuous weigh device is compared with a mass flow rate
set point and an error signal is used as the basis for screen adjustment.
[0110] If there is a substantial change in the nature of the feed to the screen, such as
the particulates being of a different size distribution, this change will alter the
overs mass flow rate required to maintain the desired particle size distribution of
the product. The purpose of the on-demand particle size analysis is to detect the
consequences of such a substantial change in the feed so that a new mass flow rate
set point can be implemented to compensate for that change. In this way, the system
"adapts" to changes in the character of the incoming feed to the adjustable screen.
In the embodiment of Figure 13, the advantages of "adaptive control" include keeping
the need for a complete size analysis to a minimum while maintaining a continuous
check on product output. The on-demand checks for particle size distribution can be
made at regular intervals or, alternatively, the need for such a check can be recognized
if it is observed that the screen-blocking member is abnormally displaced from its
customary operating position. To those skilled in the art it will be evident that
other means exist for restoring the system to normal operation, including modification
of the feed size distribution by appropriate adjustment of the crushing operation.
[0111] The objective of holding to a preselected product size distribution can be assured
most evidently by monitoring and evaluating the product stream, either continuously
or intermittently. Nothing is as convincing as a sieve analysis performed on the actual
material to be marketed, e.g., stonesand manufactured in accordance with the ASTM
C-33 Specification. The attractiveness of such an approach, however, does not preclude
control concepts based on direct monitoring of the feed size distribution. The scope
of the present invention encompasses a variety of schemes for controlling the differential
rate screening process, including feedback and feedforward alternatives, with or without
utilization of the adaptive-control principle.
Feedback control alternatives
[0112] In a straightforward application of feedback control, the output of the screen is
monitored through some form of particle size analysis of the product. An error signal
then forms the basis for adjusting a variable screening parameter, such as the open
screen length of the screen and/or the size reduction characteristics of the crushing
machine, to null out the error signal. A return flow of material for either rescreening
or recrushing may also be provided. Because there may be limitations on the transient
capacity of various elements in the system, as well as time lags associated with particle
size analysis (depending on the method used), it may be necessary to incorporate in
the system some form of "capacitance," such as surge bins or other components for
delaying material transfer.
[0113] Figure 14 illustrates a control system employing closed-loop control of the adjustable
differential rate screening operation but open-loop control of the crusher. Ostensibly,
the crusher would be set at a fixed speed and at fixed throughput rate. Closed-loop
control might be used to maintain these operating conditions, but the crusher operates
open-loop so far as information feedback from the product size distribution is concerned.
[0114] The system of Figure 14 presupposes that the crushing machine is set to produce material
which tends to be "overground" that is, material which contains excess fines. The
excess fines are removed by a differential rate-controlled screen which operates according
to the principles discussed elsewhere and the overs from the screen ultimately become
the product. The overs are sampled by means of a sampler or splitting device, and
the sample is fed to a particle size analyzer, which generates size-distribution information
for control purposes.
[0115] The analyzer may be as simple as an accelerated sieve analysis employing a system
capable of sieving a sample to completion in a relatively short length of time or
one of the more complex devices previously described for directly measuring particle
size distribution on a continuous basis. Of course, the time interval for manual sampling
and analysis introduces a time lag so far as adjustment of the screen is concerned
and may allow the passage of some amount of unsatisfactory material into the product
stream before the output can be corrected. For example, if 5 minutes is required to
sieve a sample, as much as a ton or so of material could go downstream during that
time if the system is operating at approximately 10 tons per hour (2.82 kg/s). However,
if this material is fed to a mixer by way of a reservoir or surge bin, as shown, and
if the system is designed with a several-minute holdup capacity, the product stream
can be "smoothed" to eliminate inhomogeneities in particle size distribution.
[0116] The operation of the control system of Figure 14 is as follows. An appropriate set
point is determined as some scalar function of the desired, preselected particle size
distribution. This function can be mean particle size, fineness modulus, a point on
the cumulative size distribution, or other parameter as may occur to those skilled
in the art. A particle-size analyzer operates in conjunction with a sampling unit,
presumed in Figure 14 to be of the intermittent variety. Cooperating with the sampling
unit is a gating element which, during the time the analysis is being performed, diverts
the output from the screen to a surge bin or reservoir where it accumulates until
the analysis is complete. Material from the reservoir is then metered out by the feeder
at a rate which permits it to be intimately mixed with material coming from the adjustable
screen after the error correction has been implemented. It will be evident that if
the particle size analyzer is of the continuously monitoring variety, the mixing system,
including the mixer, feeder, reservoir and associated gating unit may be eliminated.
[0117] In the event that material retained on the screen is too coarse to meet specifications,
a means may be provided to eliminate excessive overs. One option is to screen the
overs on a second screen and take the fines of that screen as the usable product.
The second screen could return overs for recrushing. An alternative scheme and one
which has certain advantages is shown in Figure 15. This figure shows information
from a size analyzer being fed to a logic element or computer (such as an AIM-65 microprocessor).
This arrangement generates control signals for three purposes: (1) control of the
adjustable screen; (2) diverting screen output as a return stream to the crusher;
and (3) control of the rate of feed of unground stone to the crusher. A surge bin
in the overs return loop may be required, but it is omitted here. It is assumed that
the sampling and particle size analysis system is of the continuously monitoring variety,
but it is to be recognized that the scope of the invention is not limited to such
a system.
[0118] The system shown schematically in Figure 15 operates as follows. So long as the crusher
produces material with excess fines, the logic element would call for only screening
control of the size distribution, and no returns would go to the crusher for recrushing.
However, the logic could include a provision for diminishing waste fines by increasing
the rate of feed to the crusher and/or by decreasing the crusher speed. Should excessive
adjustment result in excess overs, this would be detected by the particle size analyzer
as soon as the effects of the adjustment reach the sampling point. The logic element
would then call for a counteracting correction and/or send a signal to the splitter
feeder to direct a portion of the material back to the crusher for further crushing.
Again, a surge bin may be required in the return line, but is omitted here.
[0119] By controlling the rate of returns and rate of feed of uncrushed stone, the system
can be made to maintain a desired rate of throughput to the crusher. One other option
of many would be to do a three-way split, with a return stream going to the screen
as well as to the crusher. If material with excess fines comes off the screen, a portion
may be sent back for additional screening (again with the prospect that a surge bin
may be necessary). If material with excess overs comes off the screen, a portion may
be sent back to the crusher for further crushing.
[0120] Clearly many possibilities for feedback control exist, and it is evident that these
possibilities cover a gamut of degrees of sophistication. It is not the intent here
to be exhaustive, but to disclose additional modes of size distribution control. One
important consideration in selecting a control scheme is the matter of control stability.
It is entirely possible that if control corrections are made at discrete and relatively
long time intervals (possibly governed by the time required for a manual sieve analysis),
the control loop could become unstable. In other words, a correction dictated by a
current size analysis could call for a correction which would be inappropriate at
the time it is applied and could therefore induce oscillations or ever increasing
error signals. A delay line appropriately introduced into the system may therefore
help keep information flow and material flow in time phase. Alternatively, some version
of feedforward control may be employed.
Feedforward control
[0121] An illustration of the principles of feedforward control is provided in Figure 16.
In the figure, it is presumed that a single screen is sufficient to adjust the size
distribution by removing fine particulates from an excessively ground crusher output.
Rather than monitoring the size distribution of the screen output, the size distribution
of the crusher output (i.e., the feed to the screen) is monitored. Knowledge of the
feed size distribution dictates the screening which must be done in order to adjust
the product size distribution so that it comes within specifications. By delaying
the output of the crusher a sufficient length of time to perform sieve analysis, an
adjustment signal can be sent forward to the screen so as to arrive in phase with
the corresponding material flow. Such delay may be accomplished by discharging the
output from the crusher into a holding bin and metering material out of the bin onto
the screen by means of a screw conveyor or other appropriate material handling equipment.
It will be evident that a timing element, not shown in the figure, may be required
to synchronize the throughput of material with information from the particle size
analyzer. In Figure 16 it is presumed that the sampler and particle size analysis
unit is of the continuously monitoring variety and that the delay of material throughput
may be minimal since it is necessary only to compensate for any time lag involved
in the particle size analyzer. The scope of the invention is not limited to this type
of sampling, however, and it will be evident that intermittent sampling and longer
cycle times for particle size analysis can be accommodated by incorporating the mixing
concepts set forth in Figure 14.
[0122] It will be further evident to those skilled in the art that both feedback and feedforward
principles can be incorporated in the control system. If the transfer functions of
the screening operation are sufficiently accurate, feedforward control can be relied
upon to satisfy the particle size distribution in the product. In some cases, however,
it may be necessary to monitor the output of the screen and make compensating adjustments
by means of a secondary control loop. It will be further evident that the use of an
adaptive control concept in conjunction with the feedback and feedforward control
loops is within the scope of the present invention.
[0123] An embodiment which advantageously employs both feedback and feedforward control
is illustrated in Figure 17. Acting on information received from the particle size
analyzer, the logic unit of Figure 17 generates a feedforward signal to the screen
and/or a feedback signal to the crusher. So long as the output from the crusher has
excess fines, the logic calls for screen adjustment to remove those fines. If the
output from the crusher contains excess coarse material, clearly no amount of screening
will bring the product into specifications. Instead, the computer calls for more complete
crushing. Although the controller for this purpose is shown as a generalized element,
its function may be realized by employing a controlled feeder to the crusher or a
speed or other size reduction control for the crusher itself. Though the particle
size analyzer and sampling unit are presumed here to be of the continuous monitoring
variety, the scope of the invention is not limited to continuous sampling.
[0124] Although no recycle stream is shown in Figure 17, a return line may be incorporated
to recycle coarse material to the crusher by means of a splitter feeder, as in Figure
15. The scope of the invention also does not preclude returning material for additional
screening in circumstances in which additional screening would be advantageous. It
is clear that many other options for control by means of feedback, feedforward or
adaptive loops or a combination of these control loops will occur to those skilled
in the art.
Principles of differential rate screening
[0125] In order to select the mesh size and length for each screen, establish operating
values for each effective screening parameter, and set up the adjustable components
of the system so as to achieve and control the alteration in size distribution needed
to convert feed to product, some understanding of the physical processes involved
in screening and of the quantitative equations representing a continuous, differential
rate screening process may be necessary. Consideration is therefore given below to
the formulation of basic relations relative to the differential rate screening process.
These form the bases of practical schemes for setting up and controlling the differential
rate screening apparatuses described above. The invention thus provides a simple quantitative
characterization of differential rate screening sufficient to set up and operate differential
rate screening systems over a wide range of conditions.
[0126] In order to quantify certain features of the differential rate screening process
for purposes of system setup and control it is convenient to indicate relevant mass
flow rate balance relations and introduce generalized mass transfer functions.
[0127] First consider the case of a single screen as shown in Figure 18. The mass flow rate
balance for total flow, Figure 18(a), becomes
where
=mass flow rate of input,
mo=mass flow rate of overs,
mT=mass flow rate of throughs.
[0128] The terminology "input" to the screen is used here rather than the previously used
term "feed" because feed is reserved in the following considerations to apply to the
overall input to the screening system.
[0129] Two mass flow rate ratios f and g are defined by:
[0130] From equations (1), (2) and (3) it follows that:
[0131] Next consider the mass flow rate balance for each individual size class. Following
customary procedure an individual size class of particles is defined as consisting
of all particle sizes between the mesh sizes of two successive classification screens.
Here the index j is used to denote a particular size class. Further, the ratio of
mass of particles in a size class j to the total mass of all particles in the parent
size distribution is defined as the mass fraction of the distribution in size class
j. This mass fraction is designated by C,
j for the input, C
oj for the overs and C
Tj for the throughs.
[0132] Suppose the size distribution of input material has a mass fraction C,
j in size class j. Then the input mass flow rate in size class j is
C
ij. This is balanced by the sum of the mass flow rates for particles in the same size
class which pass over and through the screen. This balance is written:
where C
o; and C
T; are the mass fractions of the overs and throughs, respectively, in the size class
j. It should be noted that the mass fractions for all the size classes j sum to unit
for each separate stream (i.e., input, overs or throughs) consistent with the way
each size distribution is determined by sieve analysis:
[0133] Further, consider the mass flow rate balance for the cumulative size distributions
of the input, overs, and throughs particle streams. The cumulative size distribution
indicates the mass fraction of particles with sizes less than a given screen mesh
size. Equivalently this mass fraction can be expressed as a sum of the mass fractions
of the constituent size classes j smaller than the given mesh size. In particular
if the size classes j are arranged in order of increasing particle size and if the
mesh size of the largest screen used to define size class j=n is the same as the given
screen mesh size, then the summation will run over the size index values j=1 to n.
The given mesh size in this case will be referred to as "the mesh size with (or corresponding
to) index n." The mass flow rate balance expression is then obtained from relation
(5) by forming the following sum:
[0134] Alternately this can be expressed in a form which resembles expression (5), that
is
where the cumulative mass fractions of material in the input, overs and throughs streams
with particle sizes smaller than the mesh size corresponding to index n are designated
by I
n, On and T
n, respectively, and where
[0135] It is also possible to characterize the effect of the screening process on the mass
flow rate within each size class j by introducing a class transfer function A
j. Here A
j is defined mathematically as a function of the screen operating parameters such that
when it is multiplied by the input mass flow rate in size class j, the result is the
mass flow rate of overs in the same size class. Hence, by definition:
[0136] Substituting equation (10) in (5) gives a corresponding expression for the mass flow
rate of material of size class j which passes through the screen:
[0137] Thus the transfer function for the mass flow rate of throughs for size class j is
(1-A). Upon dividing both sides of equations (10) and (11) by
o and m
T, respectively, and using equations (2) and (3), the following alternate forms are
obtained:
[0138] These forms now refer to the mass fractions of the relevant size distributions. In
effect, A
j/g can be thought of as the transfer function which characterizes the action of the
screen is changing the size distribution of the input into the size distribution of
the overs. Likewise, (1 -A
j)/f can be thought of as the transfer function which relates the input distribution
to that of the material which passes through the screen. These transfer functions
can be viewed in an operational sense as shown in Figure 18(b), where A
j/g is the factor which changes C
ij into C
oj, and (1-A
j)/f is the factor which changes C
ij into C
Tj.
[0139] Equations (12) and (13) when rearranged are convenient to use in determining the
transfer function experimentally. They become:
[0140] It is also convenient to introduce another transfer function A
n, called the cumulative transfer function in this specification, to characterize the
effect of the screening process. This function An relates the mass flow rate of the
input to the mass flow rate of the overs in the category of sizes smaller than the
mesh size with index n. In other words, the transfer function An acts on the portion
of the input particle stream consisting of particles smaller than mesh size with index
n (which may be of mesh size less than or equal to that of the screen with index S
actually used for differential rate screening) to give the mass flow rate of particles
in this same size range which remain in the overs stream. Hence, by definition:
[0141] This relation is similar in form to relation (10), but expression (15) applies to
the cumulative size distributions rather than to individual size classes.
[0142] From relations (8) and (15) a corresponding expression is obtained for the mass flow
rate of particles in the same size range which pass through the screen:
[0143] These cumulative transfer functions are shown in an operational sense in Figure 18(c).
[0144] It is noted that the transfer function An is defined relative to a particular differential
rate screen with size index S. If a different screen with size index S' is used as
basis, the value of the transfer function A
n' for the cumulative size of index n will differ from the value of the function An
based on a screen with index S.
[0145] The following rearrangement of equations (15) and (16) are convenient to use in determining
the cumulative transfer function experimentally:
[0146] The following relation also exists between the cumulative transfer function An and
the class transfer function A
j:
as can be readily shown.
[0147] The preceding formulations can be readily extended to the case of two or more screens
as may be used in the differential rate screening systems of the invention. In these
cases, a superscript is introduced to designate which screen is being referred to,
e.g.,
[0148] The configurations shown in Figures 19(a), 19(b) and 19(c) apply. The mass flow rate
balance relations for total flow become:
[0149] In equation (20) the fact has been used that the mass flow rate which passes through
the first screen becomes the input mass flow rate to the second screen. While this
is the case in the configuration of Figure 4, it would not be the case in the configuration
of Figure 6 where the input mass flow rate to the second screen is the mass flow rate
of overs from the first screen. The mass flow rate ratios for Figure 4 are now given
by:
and
[0150] The balance of mass flow rates in a given size class j becomes:
[0151] The transfer functions for each size class j now need to be defined for each screen.
These functions are given by:
[0153] The cumulative transfer functions for mass flow rate of particles smaller than the
mesh size with index n can also be defined for each screen by particularizing the
relations (15) and (16).
[0154] In terms of the cumulative transfer functions A
n(1) and A
n(2), defined relative to screens S
(1) and S(
2) respectively, the particles smaller than the mesh size with index n which pass through
the first screen are:
and those which pass over the second screen are:
[0155] Since the mass flow rate through the first screen in the configuration of Figure
4, is the input to the second screen, the following relations hold:
and
and
[0156] Combining the above gives:
[0157] Hence, the cumulative mass fraction for mesh size with index n for the product (overs
in this case) is given in terms of the corresponding cumulative mass fraction values
of input to the first screen, the cumulative transfer functions for the two screens,
and the mass flow ratios for both screens.
[0158] The complexity of these relations suggests that it would be very difficult to define
precisely the fractional values represented by either type of transfer function as
an explicit function of each of the influential screening parameters. This difficulty
is circumvented by using a combination of offline experimental measurements and simple
approximation procedures to set up the differential rate screening system.
[0159] For purposes of approximating the operational performance of differential rate screens,
two performance representation techniques are used. The first is an exponential model
(which can be applied graphically), and the second is a graphical representation involving
both the class and cumulative transfer functions.
[0160] An explicit model for approximating a class transfer function which is of use because
of its simplicity is the following exponential model:
[0161] This model represents a transfer function for screen i and mass flow rate of particles
in size class j, whose locus of values is a straight line on a semi-log plot of
j versus open screen length L
(i). This straight line locus passes through the "origin" where A
j=1.0 and L=
0. Use of this model is discussed in the following sections in connection with system
setup.
[0162] A second useful representation of screen transfer function characteristics is a graphical
presentation. In this scheme the (approximate) class transfer functions A
j for particles of size classes j=1 to n, which correspond to the components of the
cumulative transfer function An, are plotted as functions of An. This particular plot
is most useful when the concern is with material which passes over a screen, such
as the lower screen of Figure 4. As an alternative form of this second representation,
the transfer functions (1-A
j) for particles of size classes j=1 to n may be plotted as functions of (1-An), where
again the A
j correspond to the components of the cumulative transfer function A". This form is
most useful when the concern is with material which passes through a screen, such
as the top screen in the system of Figure 4. It is particularly convenient in both
representations to take the mesh size with index n of the cumulative transfer function
An equal to the mesh size (of index S) of the screen used for differential rate screening.
In this case the function An is denoted by Aε.
System setup
[0163] In any screening operation the feed to the screen is decomposed into a throughs stream
and an overs stream. In differential rate screening, the screen operates in an adjustable
mode, and its action can be modified in response to one or more measured characteristics
of one or more of these streams. It is evident that if the screen is to be adjusted
controllably so as to produce a preselected particle size distribution in one of the
effluent streams, means must be provided for translating a given screen adjustment
into its corresponding effect on the size distribution of the selected output stream.
Conversely, if a given change in output size distribution is specified, means must
be provided for translating that change into the corresponding screen adjustment required
to produce that change. Establishing the relationship between screen adjustment and
particle size distribution modification and specifying the operating conditions required
to produce a preselected particle size distribution in the product is referred to
herein as the setup problem.
[0164] A first task in setting up a differential rate screening system is to determine the
number of screens to be used and to make a provisional selection of screen mesh sizes.
Though it is possible to envision product particle size specifications and feed size
distributions for which more than two successive screens might be needed, current
experience with practical inputs suggests that a two-screen system will satisfy a
large percentage of practical cases to be encountered. The screen mesh sizes can often
be selected by examination of the feed and the desired specification size distribution.
[0165] An example will suffice to illustrate this point. Figure 5 illustrates the mass-size
distribution limits and the mid- or centerline of the ASTM C-33 Standard Specification
for Concrete Aggregates as adapted for stonesand, together with the size distribution
for a sample of -3/8 inch (-9.5 mm) crushed limestone used in some of the operational
tests to be described below. It is evident that this material, if used as the feed
to a screening process, is too coarse and that size distribution adjustment must consist,
in part, of the removal of excess coarse material.
[0166] By reference to the centerline of the C-33 Specification, it is evident that less
than 3% of the material in the product can be allowed to exceed 4-mesh and that the
percentage of material coarser than 4-mesh must lie within bounds of 0% to 5% even
if the extremes of the C-33 Specification are allowed. Since the feed material contains
about 25% of its mass in sizes greater than 4-mesh, it is evident that a 4-mesh screen
is a likely candidate for removing excess coarse material. It will be further evident,
however, that complete removal of material coarser than 4-mesh will not satisfy the
C-33 Specification and that portions of material in finer size fractions such as -4+8
mesh, -8+16 mesh, and so on will also have to be removed. It is here that the principle
of incomplete screening becomes an evident advantage, because incomplete screening
on a 4-mesh screen is capable of removing material finer than 4-mesh.
[0167] It will be evident to those familiar with the adjustment of particle size distributions
that removal of coarse fractions from a size distribution has the effect of enriching
the finer fractions in the adjusted distribution. To prevent this enrichment process
from proceeding too far is the function of the second or bottom screen, which provides
a means for removing excess fines from the material passing through the top screen.
It is for this reason that a two-screen system is found to be widely applicable in
practice. In the present example, the product is taken as material which passes through
the top screen and is retained on the bottom screen. Selection of the mesh size of
the bottom screen is not obvious, but bases for its selection will be seen to evolve
from experience with the incomplete screening principle. Often the mesh size of the
bottom screen is advantageously selected to be near the size of the smallest particles
desired in the product but not so fine as to cause screen blinding or other operational
difficulties. In the instance of satisfying the C-33 Specification, the bottom screen
is often advantageously selected as either 30-mesh or 50-mesh.
[0168] Setup of the differential rate screening system also involves appropriate selection
and implementation of values of the various screening parameters so that in operation
the system will convert a feed material with known size distribution into a product
which meets a predetermined size distribution specification. There are, of course,
associated questions concerned with realizability of a solution; maintaining a practical
(generally large) throughput for the system; and operation of the system under conditions
which will require a minimum amount of control to keep the product within suitable
specification boundaries. The scope of the invention encompasses two different but
similar ways to approach the setup problem.
[0169] In one embodiment, the control function for the adjustable screen is temporarily
disabled so that known, discrete changes can be made in the operating values of the
adjustable screen parameter. The corresponding effects on particle size distribution
are observed and, by interpolation, a set-point value is selected for the adjustable
screen parameter, the set point being capable of producing a size distribution in
substantial agreement with the one desired. The control function for the adjustable
screen is then activated to maintain compliance with the selected set point. This
approach can be referred to as the static approach to set-point determination. In
another embodiment, which can be referred to as the dynamic approach to set-point
determination, the control function for the adjustable screen remains active and is
the means by which the operating value of the adjustable screen parameter is determined.
The preferred embodiment will be determined by the nature of the screening application,
as will become evident in the following to one versed in process-control principles.
Static set-point determination
[0170] The technique advanced here for operational setup of the differential rate screening
system employs the simple exponential model for transfer functions together with results
of sieve analysis for selected product samples.
[0171] The scheme can be used in setting up a differential rate screening system whether
or not the system is configured with a capability for measuring mass flow rate or
for automatically controlling flow rate or screen open length. In other words, it
could be effective for use with a system which employed mere manual adjustment of
open screen lengths, and no weighbelt or control system. These procedural alternatives
arise from the fact that the system setup is achieved by use of direct measurement
results. Changes in how the system operates in the vicinity of this set point depend
principally on the mass flow rate ratios rather than the absolute values of mass flow
rates. The needed mass flow rate ratio information can be obtained during setup by
taking an additional selected flow sample for each regular sample and sizing both
by sieve analysis. As confirmation that this approach does work analytically, a setup
sample was carried through without using mass flow rate data provided by the Autoweigh
unit.
[0172] Use of the static technique presupposes that the feed material exhibits a relatively
constant size distribution and that its mass flow rate is relatively constant. The
setup procedure which follows applies specifically to the screening system of Figure
4, but may be readily adapted to other system configurations. The procedure, itself,
treats first the top screen alone and then deals with both the top and bottom screen
as a complete system.
(a) Set the top screen at a trial open screen length L(1)=I1 and close the bottom screen. Set the feed mass flow rate at a desired value, if such
a value is known. If the feed rate must be determined as well, then two flow rate
conditions may need to be run so that a suitable value can ultimately be attained
via interpolation or extrapolation of selected characteristics of the output stream.
In the latter case, set the feed rate to a value that represents a likely lower or
upper bound.
(b) With the system operating, measure the feed mass flow rate and sample the feed
for subsequent analysis of particle size distribution. Shift the feed flow onto the
top screen, measure the mass flow rate of the throughs and sample the throughs for
particle size analysis. If no flow rate measurements are made, the overs must also
be sampled so the mass flow rate ratios which apply to the top screen can be determined
for the run. Without stopping the material flow, reset the bottom screen to a predetermined
value L(2)=I2 of open screen length, measure the mass flow rate of the overs, and sample the overs
for particle size analysis. The overs of the bottom screen forms the product stream
in this case.
(c) This will result in 3 (or 5) samples for sieve analysis. This analysis will lead
to the transfer functions (1-Aj(1)) and Aj(2) for the upper and lower screens for a given feed rate. To obtain information for
establishing feed flow rate, repeat the foregoing steps at the second bounding value
of feed rate. For a constant input size distribution, this will result in an additional
2 (or 4) samples, at the second feed rate for sieve analysis. The resulting data will
allow determination of transfer functions as above at the second feed rate.
(d) Plot the transfer functions on a semi-log plot, with Aj(1) on the log scale against open screen length L(1) on the linear scale. Similarly, plot Aj(2) versus open screen length U2). Construct exponential model approximations to the
transfer functions in each case by connecting the function values for different sizes
j with the "origin" at Aj(')=1.0, L'"=0 using straight lines.
(e) From these straight lines determine approximate transfer function values (1-Aj(1)) and Aj(2) for intermediate screen lengths of L(1)=I1/2 and L(2)=I2/2.
(f) Select two mass-fraction values corresponding to given screen sizes on a particular
(e.g., median) cumulative size distribution curve within the particle size band associated
with the size distribution specification. These two mass fractions, together with
the selected top screen mesh size, effectively constitute three constraints to be
imposed on the product size distribution. Limited experience suggests that one of
the selected mass-fraction values should be near 0.25 and the other near 0.75. If
the small particle size end of the distribution is the most critical, these values
may both need to be lowered somewhat. Since the cumulative size distribution curve
is non,decreasing, a small particle size is associated with the small mass-fraction
value and a larger particle size with the larger mass fraction value. Let "a" refer
to the cumulative mass fraction corresponding to the small particle size, and "b"
refer to one minus the mass fraction corresponding to the larger particle size. Note
how many and which explicit size classes j span the size range less than the small
particle size associated with "a", and those which span the size range greater than
the larger particle size associated with "b". The quantities "a" and "b" each represent
a sum of specific mass fractions Coj(2) of the desired product size distribution. Each such sum can be expressed in terms
of the corresponding values Clj(1) of the feed, together with the transfer functions (1-Aj(1)) and Aj(2) using formula (30).
(g) For example, if the largest two size classes, say j=6, 7 contribute to the value
of "b", then the explicit equation for this constraint is:
[0173] where it is assumed that the lower screen will not pass particles in size classes
j=6 or 7 and therefore that A
6(2)=A
7(2)=1.0.
(h) Likewise, if the smallest three size classes j=1, 2, 3... contribute to the value
of "a", then the explicit equation for this constraint is:
[0174] Both of these equations are exact, and can readily be adapted to alternate conditions
as needed. Although these analytical expressions are known, the values of the transfer
functions, the corresponding open screen lengths and the flow rate ratio r which are
required to satisfy the constraint equations are unknown. A graphical means for obtaining
a solution follows.
(i) Using the transfer function values Aj(1) and Aj(2) measured for the given values of L(1)=I1 and L(2)=I2, approximated for L(1)=I1/2 and L(2)=I2/2, and known analytically to be unity for L(1)=L(2)=0, together with the corresponding measured value of r, and the values C16 and CI7 obtained from the feed size distribution, the right hand side of equations (38) and
(40) can be evaluated. Strictly, the value of r also changes, but these changes can
usually be neglected without serious error. Consider the "b" equation first. Plot
the calculated right hand side values as ordinate and corresponding open screen length
L(1) values as abscissa. Construct a simple smooth curve through these points. Determine
the abscissa corresponding to the point where this curve intersects the line of constant
ordinate whose value equals (b). This gives a solution L(1)=L1. If the curve does not intersect the line, then no exact solution exists for this
combination of parameters. In general a second feed flow value must then be used,
and, in difficult cases, different combinations of other operating parameters as well.
Using the solution L1' approximate the corresponding transfer functions (1-Aj(1)) from the previous semi-log transfer function plots. Next, determine a solution for
L(2)=L2 in a similar manner, utilizing the "a" equation and the approximate (1-Aj(1)) just obtained.
(j) From the approximate solutions L1 and L2, their corresponding approximate transfer function values and the size distribution
of the feed, the predicted mass-size distribution of the product can be evaluated.
[0175] An example is given in Table 2 and in Figures 20, 21 and 22 to illustrate this setup
scheme in detail. In this example, a 4-mesh top screen and 50-mesh bottom screen were
used together with the C-33 size distribution specification. Only a single feed rate
was used; this was independently measured at 9.8 tons per hour for this test. Samples
of the material which passed over the top screen were taken and sized by sieve analysis.
The results of the first sample (for L
(1)=10 inches, L
(2)=0 inches) were used in evaluating the flow rate ratio f
(1). The flow rate ratio f
(1) arises from calculation of the mass-fraction ratio C
oj(1)/C
ij(1) for the size classes j that cannot pass through the top screen. By equation (28)
this ratio is equal to A
j(1)/
g(1), but for the particular size classes used A
j(1)≡1, so the ratio is directly equal to 1/g
(1). The value of f
(1) follows using equation (4). A corresponding scheme is used to evaluate 1/g(
2) and f
(2) using the ratio C
oj(2)/C
Tj(1). In this case an average of the ratios for the several size classes larger than the
screen are used.
[0176] The constraint values adopted were a=.20 and b=.325. The corresponding points on
the C-33 size specification centerline are shown circled in Figure 22. Graphical solutions
give setup lengths of L
(1)=L
1=8.5 inches and L
(2)=L
2=0 inches. These length values represent approximations, since approximate transfer
function values have been used in the graphical solutions. These approximate results
indicate that, for the example shown, one screen (i.e., the top screen) should be
adequate.
[0177] Using the individual class transfer function values and the flow rate ratios corresponding
to these setup values, the predicted product size distribution was calculated and
plotted in Figure 22 together with the centerline C-33 distribution. The predicted
distribution compares favorably with the size specification.
[0178] The setup steps just indicated should generally give a close estimate for values
of open screen lengths and mass flow rate ratios required to produce screened material
close to specifications. If the product size distribution obtained from a confirmatory
run using these approximate setup values is not as close as desired to the size specification,
then the foregoing static setup procedure can be repeated to refine the solution.
In such a case, the setup values obtained above are used as the starting trial values.
Convergence of the results of such successive approximation should be quite rapid
so that no more than a second correction of the setup values should be required.
Dynamic set-point determination
[0179] As in the static technique, this scheme presupposes that the feed material exhibits
a relatively constant particle size distribution and that its mass flow rate to the
screen is very nearly constant. It is assumed that the original size distribution
of the feed has been determined and that the product (at least) can be sampled and
subjected to sieve analysis upon demand.
[0180] The dynamic approach to set-point determination is based on the assumption that mass
flow rate is available as a measured characteristic of an output stream from the screen
and that means exist for monitoring the ratio between this mass flow rate and the
mass flow rate of the feed. The control system is configured so that once a desired
mass flow rate is set, the system adjusts the open screen length to maintain that
mass flow rate ratio. It is therefore not necessary to know explicitly the relation
between transfer function and open screen length, given that the relation between
screen transfer function and mass flow rate ratio is known. The position control system
can be given the burden of increasing or decreasing the open length of the screen
to attain the value of the transfer function required to realize the preselected particle-size
distribution in the product. The setup procedure described below deals first with
the top screen alone and then treats the setup of the overall system.
(a) Establish a trial feed rate and determine the mass flow rate ratio corresponding
to some intermediate value of the cumulative transfer function for the top screen
about midway between the extreme values of zero and one. The required mass flow rate
ratio can be computed directly from the feed rate and the known particle size distribution
of the feed.
[0181] It is to be noted that, alternatively, a trial open length for the screen can be
selected and the corresponding flow rate ratio determined by direct measurement of
the input and output flow rates.
(b) Calculate the transfer functions (1-Aj(1)) for material which passes through the screen for each size class j using equation
(29). For this same sample determine the cumulative transfer function (1-As(1))from equation (17). Plot the values of (1-Aj(1)) as ordinate and (1-As(1)) as abscissa using linear scales. Fair a set of curves from the origin (0,0) through
the sample points and to the point (1,1). In the event that there is considserable
latitude as to how and where the curves should be drawn, repeat the process for a
second intermediate value of the cumulative transfer function.
(c) From the feed distribution Cij(1) and the centerline values (or other chosen locus) of the desired size specification
(denoted by Subscript "Sp') [Coj(2)]Sp for the final product, determine the ratio [Coj(2)]Sp/Cij(1) and renormalize this set of values so the largest value becomes unity. Designate
the renormalized ratios [Coj(2)]Sp/Cij(2) as Aj(2)(1-Aj(1))· M, where M is a normalization constant. Plot the values Aj (2)(1-Aj(1)). M as ordinates on the same scale as that previously used for (1-Aj(1)) versus size class interval j as abscissa. It is convenient to arrange these plots
side by side as shown in Figure 23. Select a particular trial value of As(1) and read the corresponding values of AP) from the several curves. Once these values
are known, the distribution which will result when the feed passes through the top
rate screen for the given conditions can be predicted. If the distribution is not
as desired, a different trial value of As(1) can be employed and a solution approached by iteration of the above procedure.
(d) With the top screen setting determined and the top screen reset to this value,
one proceeds to find corresponding conditions for the bottom screen. This can be done
in either of two ways.
(e) First, the system is run using a preselected value for As(2) as a set point for the position control system. The burden in this case is on the
position control system to extend or close the open length of the screen until the
mass flow ratio g(2) (or r) is attained that corresponds to the preselected value of As(2). When this condition is reached, the output product is sampled and size analyzed.
This product output can be compared directly with the desired size distribution specification
to ascertain agreement. If further adjustment appears necessary, a new value of As(2) must be determined and set into the length control system. In making this determination,
it appears to be convenient to construct a plot of Aj(2) as ordinate versus the corresponding As(s) as abscissa, similar to the plot for the top screen. The measured value of As(2) and the corresponding Aj(2) calculated from the sample size analysis provide coordinate values for points through
which a set of curves can be drawn for the second screen. Using the adjusted value
of As(2), the system is run again and the product sampled and compared against the size specification.
(f) A second way of setting the bottom rate screen within this overall scheme involves
following the same type of procedure used in the case of the top screen. One or two
flow rate ratios are used, samples taken and analyzed and values of Aj(2) versus As(2) plotted.
[0182] Curves are drawn through the origin, the data points and the (1,1) point to obtain
results of the general type shown in Figure 24. A trial value of A
s(2) (or L
2) is then selected and the corresponding values of A
j(2) are read from the curves. Upon combining the values of 1-A
i(1) and A
j(2) for the full set of size classes j, the result for each j can be multiplied by the
appropriate C
ij(1), to obtain an unnormalized C
oj(2). (Generally, values of 1-A
j(1) and A
j(2) may both occur for some of the same size classes j. These must be multiplied together
in that case.) By adding the C
oj(2)'s over all j and renormalizing so the sum equals unity, the predicted Coj
(2) for the selected system settings is obtained.
[0183] The analytical features of this setup procedure are illustrated in a static sense
in the following example. The system dynamics of adjusting the open screen length
to seek out and maintain a mass flow rate ratio set point are not illustrated directly.
However, the dynamic aspects of system behavior corresponds to the indicated analytical
feature of convergence of the sequence of mass flow rate ratio trial values to the
desired set point value.
[0184] The example is given in Tables 3 and 4 and Figures 23, 24, 25 and 26. The dynamic
setup scheme was carried out as indicated using samples taken at two lengths for each
screen. Only one feed rate was used. A 4-mesh top screen and 50-mesh bottom screen
were used together with the C-33 size distribution specification. Since the tests
for setup of the top and bottom screens were run independently, and the feed size
distributions measured for the two runs were not identical a separate feed size distribution
was used in reducing the top screen data. The results of sieve analysis on the sample
taken for the top screen at the open length L
(1) of 6 inches (15 cms) were used as a basis for constructing the curves in Figure 23.
[0185] Estimates of A
j(1) for L
(1)=10 inches were read from the curves of Figure 25 and used in Figure 23 to help establish
the curves. A value of L
(1)=10 inches was adopted as the value to use for setup of the bottom screen. This selection
was based heavily on the results for the largest two size classes j=6 and 7.
[0186] A second test was made and samples of the overs from the bottom screen were taken
for L
(2)=0, 5, 15 and 25 inches (0, 12.7, 38 and 63.5 cms). The L(
2)=0 sample was used to directly determine the input to the second screen. The samples
yielded useful transfer function values only for size classes j=2 and 3. Since the
full system performance was influential in the bottom screen tests, the composite
transfer function was recalculated for this casing using the appropriate feed size
distribution. The setup length L(
2)=2 inches (5.1 cms) was selected with little ambiguity.
[0187] Using the individual class transfer function values and the feed for the bottom screen
tests, the predicted product size distribution was calculated and plotted in Figure
26 together with the centerline C-33 distribution. The predicted product distribution
compares favorably with the size specification.
Examples 1-5
[0188] A key phenomenological aspect of the differential rate screening process is that
the mass fraction of material which passes through a screen under given conditions
changes, often exponentially, with open screen length L. The following examples indicate
the experimental basis for this feature and certain other characteristics of differential
rate screening.
[0189] Figure 27 shows, for different input mass flow rates, how the cumulative transfer
function As decays as a function of open screen length L. Recall that this transfer
function is the ratio of mass flow rate of undersize material which passes over the
screen to the total mass flow rate of material which could pass through the screen.
Although these decay curves do not follow any known simple mathematical expression,
the exponential model has been found to apply approximately to portions of these curves.
As will be seen in the following examples, the exponential model applies somewhat
better to the decay curves for the class transfer functions A
j than to the cumulative transfer functions As.
[0190] The data for the decay curves of Figure 27 were obtained using a single screen, laboratory
scale differential rate screening system similar in concept to the system of Figure
4.
[0191] Figures 28 and 29 show how the class transfer functions which are components of the
cumulative transfer functions of Figure 27 decay as functions of open screen length.
These decay data are for a commercial sand (SAKRETE All Purpose Sand) continuously
screened on a square mesh screen of variable open length made from an experimental
No. 30 stainless steel wire mesh screen (designated No. 30E). In these tests, the
particulates were fed onto the screen with velocities principally in the plane of
the screen.
[0192] Examples of similar class transfer function decay curves as determined using a pilot
scale differential rate screening system similar to Figure 4 are shown in Figures
30, 31 and 32.
[0193] The data points used to construct these plots cover a more restricted range of open
screen lengths than in the three previous figures. The data on which Figures 30, 31
and 32 are based are similar to that given in Table 2 and were obtained using -3/8
inch (-9.5 mm) crushed limestone screened on a square mesh screen of variable open
length made from standard No. 30 stainless steel wire. In these latter tests, the
particulates were fed onto the screen with velocities principally perpendicular to
the plane of the screen.
[0194] The screening decay curves of Figure 29, while for specific screen sizes and types
of material, are believed to be representative of the general type of phenomenological
behavior to be expected in rate screening according to the invention. The decay curves
exhibit three distinct regions: an initial transient region at short open screen lengths,
a central region where the decay is roughly exponential, and a final region of (usually)
rapid decay.
[0195] It was noted during testing that the behavior of the class transfer functions appears
to be influenced somewhat by the nature of the input size distribution of particles
fed to the rate screening system. Small changes in the distribution seemed to have
neglible effects on the transfer functions, and this is important for control considerations.
However, large changes need to be compensated for. Two obvious problems here are,
first, to decide when a distribution change is sufficiently large to require action,
and second, to decide what action to take. These questions are generally circumvented
by the setup and control techniques discussed elsewhere.
[0196] In Figures 33 and 34, the transfer functions A
j for particles in size classes j, which correspond to the components of cumulative
transfer function As, are plotted as functions of As. This figure illustrates the
shape changes in the resulting curves in response to changes in mass flow rate to
the screen.
Examples 6-29
[0197] A series of tests were run using the equipment setup of Figure 4 to demonstrate that
the differential rate screening process of the invention could readily yield screened
products which satisfy the ASTM C-33 Specification for stonesand. The feed was crushed
limestone obtained from a centrifugal crusher. The particle sizes in the feed were
all -3/8 inch (-9.5 mm). The opening size of the upper screen was 4-mesh and that
of the lower screen was 30-mesh. The results of these tests are set forth in Table
5.
[0198] Some explanation of the nomenclature used in Table 5 will be helpful in understanding
this data. The groups of numbers and letters used in designating each test sample
have the following meanings. The first two numbers starting at the left represent
the inclination of the screen, namely 27°, relative to the horizontal. The next two
numbers represent the open length of the top screen (L(')) in inches, namely 6 inches.
The first two numbers following the dash (-) represent the nominal total mass flow
rate of the feed in tons per hour. For example, -04, -10 and -15 represent nominal
mass flow rates of 4, 10 and 15 tons per hour (tph), respectively. The actual measured
or calculated total mass flow rate for each test sample is set forth under Column
I, subcolumn
1. The final group of two numerals represents the open length of the bottom screen
(U2)) in inches. The final letter designations are to be interpreted as follows. B
1 denotes samples of the feed taken at the start of each test series. B
2 denotes samples of the feed taken at the end of each test series. B
3 denotes samples of the feed taken upon restart of a test series which was interrupted
to refill the feed bin. B
4 denotes samples of the feed taken at the end of an interrupted test series. S denotes
a set of samples taken while differential rate screening was occurring on either one
or two screens. S
1 and S
2 designate the set of samples taken with the top screen closed in the first and second
portions of an interrupted test series.
[0200] Figure 35 illustrates that with the particular feed tested, ASTM C-33 can be met
by a single screen employing the rate screening process of the invention. In this
figure, the dotted lines represent the upper and lower limits of the ASTM C-33 specification.
The curve marked "FEED" is a plot of the cumulative size distribution of test sample
2706-0400B, as given in Table 5. The curve marked "P," is a plot of the cumulative
size distribution of the product from screening test sample 2706-0400S and is obtained
from the data presented in the corresponding line of column VI in Table 5. The solid
curve marked "P2" is a plot of the cumulative size distribution of the product produced
by screening test sample 2706-1500S and is obtained from the data presented in the
corresponding line of column VI in Table 5.
[0201] Figures 36, 37 and 38 each illustrate the change in product size distribution where
the first screen is set at six inches (15 cms) and the second screen is changed from
five inches (12.6 cms) to twenty inches (52 cms) of open length. The data for these
figures is given in Table 5 and was obtained at nominal feed rates of 4.5, 10.1 and
16.7 tons per hour (tph) (1.27, 2.85 and 4.71 kg/s), respectively. In the upper right
corner of each figure, there is also given the mass flow rate of the feed to the lower
screen in tons per hour per inch of lower screen width, the same being .0578, .156
and .244tph/in. (0.0163,0.044 and 0.069 kg/s) for Figures 36, 37 and 38, respectively.
The test samples screened to obtain the data plotted on these figures are identified
on each figure. The corresponding cumulative size distributions of the product streams
were calculated by summing appropriate data lines in Column VI of Table 5. The cumulative
size distribution of the feed stream in each of these figures was obtained from the
appropriate data lines in Column III of Table 2. The specific mass flows for each
sample tested appear in Column I of Table 5.
[0202] A comparison between the sets of curves in each of these figures further illustrates
that for the particular feed tested, the C-33 specification can be met by increasing
the feed rate to about 16 to 18 tph (4.5 to 5.1 kg/s) while maintaining the open lengths
of both the upper and lower screens at the values indicated.
[0203] The examples presented and the screening data incorporated in Table 5 have demonstrated
the feasibility and technical merits of this novel differential rate screening process
and apparatus. In addition, the data not only provide qualitative and quantitative
assurance that the setup and control schemes described in this specification perform
satisfactorily, but also support the claims of this patent with reference to certain
preferred embodiments.
Industrial applicability
[0204] The invention has a wide range of commercial uses as illustrated by the specific
embodiments and examples set forth above. These embodiments and examples are merely
exemplary and the true scope of the invention is not to be limited to those embodiments
and examples but is as defined by the claims at the end of this specification. Additional
embodiments and modifications which may prove to have significant commercial utility
are set forth below.
[0205] The theory of differential rate screening teaches that of all the particles capable
of passing through a screen, the finer particles pass more readily and the coarser
or "near-mesh" particles pass with greater difficulty. Consequently, a size-distribution
gradient exists along the screen from the point at which the feed is first introduced
onto the screen to the point at which the overs exit off of the open apertured screen
length. If one samples the material passing through the screen early in its traverse
along the screen, that material will be found to be rich in fine or "far-mesh" material.
For example if the screen were 30-mesh, an early sample would be-rich in -200 and
-100+200 particles but relatively lean in -30+50 (near-mesh) particles. On the other
hand, if the material passing through the screen is sampled at a position near its
downstream end, that material would be found to be rich in the relatively coarse,
near-mesh particles and relatively deficient in very fine particles. For the 30-mesh
sieve, for example, the late sample might be expected to consist mostly of -30+50
(near-mesh) particles. This postulated behavior is in accordance with the transfer
functions for differential rate screening as previously given in this specification.
[0206] A typical embodiment of this differential rate screening concept is that of screen
64' in Figure 4, in which the lower end of the screen is masked by a plate 132' and
the effective length of the screen is restricted so that something less than essentially
complete screening occurs. Screen 64' avails itself of the size-distribution gradient
cumulatively up to the point of screen obstruction by plate 132', which constitutes
a "cut-off" so far as coarse, near-mesh particles are concerned. The particles deprived
of access to the screen comprise the overs 252' discharged through chute 131', while
the throughs 250' fall onto the interscreen conveyor 72'.
[0207] An alternative approach to limiting the effective open length of the screen is represented
by interscreen pan 150 in Figure 1. Instead of a plate to restrict access of the particles
to the screen, all particles are allowed to pass through screen 66, but a portion
of the throughs is retrieved by interscreen pan 150 and the retrieved or "retained"
part is recombined with the overs coming off of the end of screen 66. These combined
"overs" would be equivalent in size distribution to overs emerging from collection
chute 162 if a masking plate was used over the same portion of screen 66 as is intercepted
by pan 150.
[0208] The principles described above do not exploit all of the flexibility available for
preferentially selecting regions along the length of the screen to be used as the
effective portion of that screen. For example, a catch tray 400 is employed in Figure
39 in a manner similar to pan 150 in Figure 1, but the throughs recovered by catch
tray 400 are treated as a separate stream 405 and are not combined with the overs
stream 407. There can then be employed as at least part of the product stream either
throughs stream 409 or throughs stream 405. If throughs stream 409 is elected, the
result is substantially the same as in the previous embodiments, that is, the effective
length of the screen is simply shortened. If throughs stream 405 is selected, however,
it is possible to take advantage of the coarser end of the size-distribution gradient
and to eliminate from the product an appreciable portion of the very fine material
without having to screen the material on a second, finer mesh screen.
[0209] A similar effect to that achieved by catch tray 400 of Figure 39 can be realized
by the use of a masking plate 410 as shown in Figure 40. Masking plate 410 is movable
in either direction relative to screen 412 as illustrated by the arrow P. By masking
a central portion of the screen 412, there is formed an inlet effective part 414 of
screen 412 which yields a throughs stream 415, and an outlet effective part 418 of
the screen yielding a second throughs stream 419. Throughs stream 419 may be separated
from overs stream 421 by baffle 422 so as to be utilized as a separate throughs stream
similar to throughs stream 405 of Figure 39.
[0210] Many possibilities exist in selecting those portions of a screen along its length
to be used in generating all or a portion of a product stream. A further example of
this is illustrated by Figure 41 in which a catch tray 430 is positioned about midway
between the two ends of a screen 432. Three (3) throughs streams 435,436 and 437,
in addition to an overs stream 438, are generated by this arrangement. Throughs streams
435,436 and 437 each exploit a unique portion of the size-distribution gradient. If
stream 435 were to be used in the product, the material would contain a high percentage
of the finest particles available in the feed. If stream 437 were to constitute the
product, very fine particles would be relatively scarce. If stream 436 were employed
in the product, very fine particles would be present in an amount intermediate between
the amounts of those particles available in streams 435 and 437. It is also evident
that similar selective means could be used to acquire specific portions of the near-size
overs for purposes of tailoring the size distribution of the product in the desired
manner.
[0211] A larger number of additional options can be implemented by varying the position
of catch tray 430 along the length of screen 432 as represented by arrow T. Instead
of varying the position of catch tray 430 relative to screen 432, the effective length
of the catch tray as measured in the direction of particle flow along the screen may
be varied so as to receive throughs from a greater or lesser apertured screen length.
[0212] It is also evident that both the masking plate 410 and the catch tray 430 may be
moved relative to their corresponding screen either by making the plate or tray the
movable component and/or by making the corresponding screen the movable component.
The possibility of still further embodiments exists through the use of more than one
masking plate, more than one catch tray, other configurations of masking plates and/or
catch trays, and/or combinations of such masking plates and catch trays.
1. Siebverfahren zum kontinuierlichen Sieben von Unterkornteilchen verschiedener Größenklassen,
um ein Produkt zu schaffen, das eine vorgewählte Korngrößenverteilung aufweist, die
im wesentlichen verschieden von der Korngrößenverteilung in einer Charge eines teilchenförmigen
Materials ist, das genannte Siebverfahren umfassend das Sieben mit unterschiedlichen
Raten, dadurch gekennzeichnet, daß ein Strom (62, 62', 62") der Charge bzw. Beschickung
auf wenigstens ein erstes Siebelement (64, 64', 64") mit Öffnungen von ausreichender
Größe aufgebracht wird, um eine Vielzahl von Korngrößenklassen im genannten Beschickungsstrom
hindurchtreten zu lassen; daß der genannte Beschickungsstrom (62, 62', 62") in wenigstens
einen ersten Siebdurchlaufstrom (250, 250', 250") und einen anderen ersten Strom (252,
252', 252") geteilt wird, indem bewirkt wird, daß wenigstens zwei der genannten Unterkornklassen
durch die Öffnungen des genannten ersten Siebelementes (64, 64', 64") hindurchtreten
und in den genannten ersten Siebdurchlaufstrom (250, 250', 250") in Mengenverhältnissen
eintreten, die sich in bezug aufeinander wesentlich von den Mengenverhältnissen der
genannten wenigstens zwei Unterkornklassen im genannten Beschickungsstrom unterscheiden;
und daß im genannten Beschickungsstrom (62, 62', 62") eine ausreichende Gesamtzahl
von Unterkornteilchen in jeder der genannten Vielzahl von Unterkornklassen geschaffen
wird und das Differential zwischen den genannten Mengenverhältnissen von Unterkornklassen
im genannten Beschickungsstrom (62, 62', 62") und den genannten Mengenverhältnissen
von Unterkornklassen gesteuert (45, 45', 45") wird, die durch das genannte erste Siebelement
(64, 64', 64") hindurchtreten und in den genannten ersten Siebdurchlaufstrom (250,
250', 250") eintreten, um im wesentlichen eine vorgewählte Korngrößenverteilung in
einem teilchenförmigen Produkt (256, 256', 256") zu schaffen, das wenigstens eine
Teilmenge wenigstens eines der genannten ersten Siebdurchlaufströme (250, 250', 250")
und des genannten anderen ersten Stroms (252, 252', 252") umfaßt.
2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß wenigstens ein Siebparameter
des Siebelementes (64) variabel ist, um die relativen Mengenverhältnisse der genannten
wenigstens zwei Unterkornklassen, die durch das Siebelement hindurchtreten und in
den Siebdurchlaufstrom (250) eintreten, zu variieren.
3. Verfahren nach Anspruch 2, dadurch gekennzeichnet, daß der variable Siebparameter
so gesteuert ist, daß er die genannte im wesentlichen vorgewählte Korngrößenverteilung
im Produktstrom (256) aufrechterhält.
4. Verfahren nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß
der Siebdurchlaufstrom (250) oder der genannte andere Strom (252) ein mittels des
Siebverfahrens erhaltener Strom ist und daß der Produktstrom (256) wenigstens einen
der genannten Ausstoßströme umfaßt.
5. Verfahren nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß
die genannten verschiedenen Korngrößenklassen durch den kumulativen Prozentsatz an
Unterkornteilchen definiert sind, die durch verschiedene Standardsiebgrößen hindurchtreten
und daß das Differential zwischen der Massenflußrate wenigstens einer kumulativen
Unterkornklasse im ersten Durchlaufstrom (250) und der Massenflußrate der genannten
wenigstens einen kumulativen Unterkornklasse im genannten ersten Beschickungsstrom
wenigstens fünf Prozent der Massenflußrate der genannten wenigstens einen kumulativen
Unterkornklasse im genannten Beschickungsstrom (62) beträgt.
6. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß wenigstens ein Siebparameter
des genannten ersten Siebelementes (64) variabel ist, sodaß ein Differential zwischen
einer Massenflußrate der genannten Unterkornteilchen im genannten Beschickungsstrom
und einer Massenflußrate der genannten Unterkornteilchen verändert werden kann, die
durch das genannte erste Siebelement hindurchtreten und in den ersten Siebdurchlaufstrom
(250) eintreten, und daß das genannte Siebverfahren weiters den Schritt der Steuerung
des genannten variablen Siebparameters auf solche Weise umfaßt, daß die genannte im
wesentlichen vorgewählte Korngrößenverteilung im Produktstrom (256) aufrechterhalten
bleibt.
7. Verfahren nach Anspruch 6, dadurch gekennzeichnet, daß der genannte variable Siebparameter
die wirksame Sieblänge des ersten Siebelementes (64) ist und daß die genannte wirksame
Sieblänge die mit Öffnungen versehene Länge des ersten Siebelementes (64) umfaßt,
die die Unterkornteilchen des genannten Beschickungsstromes (62) durch das erste Siebelement
(64) hindurchtreten und in den ersten Siebdurchlaufstrom (250) eintreten läßt.
8. Verfahren nach Anspruch 6, dadurch gekennzeichnet, daß der genannte variable Siebparameter
die Korngrößenverteilung im Beschickungsstrom (62) ist.
9. Verfahren nach Anspruch 8, gekennzeichnet durch den Schritt des Brechens von Steinen,
um die Größen der genannten Steine auf die Korngrößen im genannten Beschickungsstrom
(62) zu verringern, wobei die durch den genannten Brechschritt geschaffene Korngrößenverringerung
steuerbar so variabel ist, daß die genannte im wesentlichen vorgewählte Korngrößenverteilung
im Produktstrom (256) aufrechterhalten wird.
10. Verfahren nach Anspruch 6, dadurch gekennzeichnet, daß der genannte variable Siebparameter
die Neigung des genannten ersten Siebelementes (64) in bezug auf eine horizontale
Ebene ist.
11. Verfahren nach einem der Ansprüche 6 bis 10, dadurch gekennzeichnet, daß der genannte
wenigstens eine variable Siebparameter als Reaktion auf wenigstens ein gemessenes
Merkmal des Produktstromes oder wenigstens eines der Siebdurchlauf- oder Siebüberlaufströme
des genannte Siebverfahrens ist, um die im wesentlichen vorgewählte Korngrößenverteilung
im Produktstrom (256) aufrechtzuerhalten.
12. Verfahren nach Anspruch 11, gekennzeichnet durch den Schritt der gesteuerten Veränderung
des genannten wenigstens einen variablen Siebparameters als Reaktion auf Veränderungen
im genannten wenigstens einen gemessenen Merkmal.
13. Verfahren nach Anspruch 12, dadurch gekennzeichnet, daß wenigstens ein variabler
Siebparameter automatisch als Reaktion auf das genannte wenigstens eine gemessene
Merkmal verändert wird.
14. Verfahren nach Anspruch 12, dadurch gekennzeichnet, daß das genannte wenigstens
eine gemessene Merkmal eine Massenflußrate wenigstens eines der Ströme ist.
15. Verfahren nach Anspruch 12, dadurch gekennzeichnet, daß wenigstens ein gemessenes
Merkmal eine Funktion einer Korngrößenverteilung im genannten Strom ist.
16. Verfahren nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß
es weiter den Schritt des Brechens von Steinen auf die Korngrößen im genannten Beschickungsstrom
(62) umfaßt, wobei wenigstens eine Teilmenge eines Siebüberlaufstromes (252) vom genannten
Siebverfahren zum genannten Brechschritt zurückgeführt wird und auf das genannte erste
Siebelement (64) als Teil des Beschickungsstromes (62) aufgebracht wird.
17. Verfahren nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß
der Beschickungsstrom (62) auf das erste Siebelement (64) so abgegeben wird, daß er
im wesentlichen weniger als der vollen mit Öffnungen versehenen Länge des ersten Siebelementes
(64) ausgesetzt ist, und daß das Verfahren weiters den Schritt der steuerbaren Veränderung
des Ausmaßes der genannten mit Öffnungen versehenen Länge umfaßt, die dem Beschickungsstrom
(62) ausgesetzt ist, um die im wesentlichen vorgewählte Korngrößenverteilung im genannten
Produktstrom (256) aufrechtzuerhalten.
18. Verfahren nach Anspruch 17, dadurch gekennzeichnet, daß die genannte Aussetzungssieblänge
durch ein Sperrelement (130) begrenzt wird, das so angeordnet ist, daß es eine Teilmenge
des Beschickungsstromes (62) abfängt, nachdem die genannte Teilmenge auf die mit Öffnungen
versehene Länge des ersten Siebelementes (64) aufgebracht ist, aber bevor im wesentlichen
alle der genannten Unterkornteilchen in der genannten Teilmenge durch die Öffnungen
des ersten Siebelementes (64) hindurchgetreten sind, und daß die Aussetzungssieblänge
verändert wird, indem die Stellung verändert wird, an der das Sperrelement (130) die
genannte Teilmenge des Beschickungsstromes (62) abfängt.
19. Verfahren nach einem der Ansprüche 1 bis 16, dadurch gekennzeichnet, daß das Differential
zwischen den genannten Mengenverhältnissen von Unterkornklassen im Beschickungsstrom
(62) und den genannten Mengenverhältnissen von Unterkornklassen, die durch das erste
Siebelement (64) hindurchtreten und in den genannten ersten Durchlaufstrom (250) eintreten,
mittels einer Einrichtung gesteuert wird, die einen Fangeinsatz (400, 430) umfaßt,
der unter und zwischen den Enden des Siebelementes (64) angeordnet ist, wobei die
Länge des Einsatzes (400, 430) geringer ist als jene des Siebelementes und der Einsatz
(400, 430) entlang der Länge des Siebelementes (64) bewegbar ist.
20. Siebverfahren zum kontinuierlichen Sieben von Unterkornteilchen von verschiedenen
Korngrößenklassen, um ein Produkt zu schaffen, das eine vorgewählte Korngrößenverteilung
aufweist, die im wesentlichen verschieden von der Korngrößenverteilung in einer Beschickung
von teilchenförmigem Material ist, das genannte Siebverfahren umfassend ein Differential
in der Siebrate, dadurch gekennzeichnet, daß ein Strom (62, 62', 62") der Beschickung
auf wenigstens ein erstes Siebelement (64, 64', 64") aufgebracht wird, das mit Öffnungen
von ausreichender Größe versehen ist, um eine Vielzahl von Korngrößenklassen im genannten
Beschickungsstrom hindurchtreten zu lassen; daß der genannte Beschickungsstrom (62,62',
62") in wenigstens einen ersten Siebdurchlaufstrom (250, 250', 250") und einen anderen
ersten Strom (252, 252', 252") getrennt wird, indem wenigstens zwei der genannten
Unterkornklassen durch die Öffnungen des genannten ersten Siebelementes (64, 64',
64") hindurchtreten und in den genannten ersten Siebdurchlaufstrom (250, 250', 250")
in Mengenverhältnissen in bezug aufeinander eintreten gelassen werden, die im wesentlichen
verschiedenen von den Mengenverhältnissen der genannten wenigstens zwei Unterkornklassen
im genannten Beschickungsstrom sind; daß wenigstens eine Teilmenge wenigstens eines
der genannten ersten Siebdurchlaufströme (250, 250') und des genannten anderen ersten
Stromes (252") als Eingabestrom in eine zweite Siebeinrichtung (68, 68', 68") eingebracht
wird, um wenigstens einen zweiten Siebdurchlaufstrom (258, 258', 258") und einen anderen
zweiten Strom (256, 256', 256") zu schaffen; und daß in dem genannten Beschickungsstrom
(62, 62', 62") eine ausreichende Gesamtzahl von Unterkornteilchen in jeder der genannten
Vielzahl von Unterkornklassen geschaffen wird und das Differential zwischen den genannten
Mengenverhältnissen von Unterkornklassen im genannten Beschickungsstrom und den genannten
Mengenverhältnissen von Unterkornklassen, die durch das genannte erste Siebelement
hindurchtreten und in den genannten ersten Siebdurchlaufstrom (250, 250', 250") eintreten,
so gesteuert (45, 45', 45") wird, daß eine im wesentlichen vorgewählte Korngrößenverteilung
in einem teilchenförmigen Produkt geschaffen wird, das aus wenigstens einer Teilmenge
wenigstens entweder des genannten zweiten Siebdurchlaufstroms (258, 258', 258") oder
des genannten anderen zweiten Stromes (256, 256', 256") besteht.
21. Verfahren nach Anspruch 20, dadurch gekennzeichnet, daß wenigstens ein Siebparameter
des ersten Siebelementes (64) variabel ist, um die relativen Mengenverhältnisse zu
einander der genannten wenigstens zwei Unterkornklassen zu verändern, die durch das
erste Siebelement hindurchtreten und in den ersten Siebdurchlaufstrom (250) eintreten.
22. Verfahren nach Anspruch 21, dadurch gekennzeichnet, daß der variable Siebparameter
so gesteuert wird, daß die genannte im wesentlichen vorgewählte Korngrößenverteilung
im Produktstrom (256) aufrechterhalten wird.
23. Verfahren nach einem der Ansprüche 20 bis 22, dadurch gekennzeichnet, daß der
Aufgabestrom auf die Oberfläche eines zweiten Siebelementes (68) aufgebracht wird,
das Öffnungen von ausreichender Größe aufweist, um wenigstens eine Unterkornklasse
im Aufgabestrom hindurchtreten zu lassen, daß der Aufgabestrom in wenigstens den genannten
zweiten Siebdurchlaufstrom (258) und den genannten anderen zweiten Strom (256) getrennt
wird, indem die genannten wenigstens eine Unterkornklasse im Aufgabestrom durch die
Öffnungen des genannten zweiten Siebelementes (68) hindurchtreten und in den zweiten
Durchlaufstrom (258) mit einer Massenflußrate eingetreten gelassen wird, der im wesentlichen
verschieden von der Massenflußrate der genannten wenigstens einen Unterkornklasse
im Aufgabestrom (62) ist und im Aufgabestrom ausreichende Massenflußraten von Teilchen
in jeder der genannten Vielzahl von Unterkornklassen und ein ausreichendes Differential
zwischen der Massenflußrate der genannten wenigstens einen Unterkornklasse im Aufgabestrom
(62) und der Massenflußrate der genannten wenigstens einen Unterkornklasse geschaffen
werden, die durch das genannte zweite Siebelement (68) hindurchtritt und in den genannten
zweiten Durchlaufstrom (258) eintritt, um im wesentlichen die vorgewählte Korngrößenverteilung
im Produktstrom (256) zu schaffen.
24. Verfahren nach Anspruch 23, dadurch gekennzeichnet, daß wenigstens ein Siebparameter
des genannten zweiten Siebelementes (68) variabel ist, um die Menge an der genannten
wenigstens einen Unterkornklasse zu verändern, die durch das genannte zweite Siebelement
(68) hindurchtritt und in den genannten zweiten Siebdurchlaufstrom (258) eintritt.
25. Verfahren nach Anspruch 24, in dem der genannte variable Siebparameter so gesteuert
wird, daß die genannte im wesentlichen vorgewählte Kongrößenverteilung im genannten
Produktstrom (256) aufrechterhalten wird.
26. Verfahren nach einem der Ansprüche 20 bis 25, dadurch gekennzeichnet, daß die
zweiten Siebelemente (64, 68) im wesentlichen einheitliche Öffnungen einer genormten
Meshgröße aufweisen und die Meshgröße des genannten ersten Siebelementes (64) sich
von der Meshgröße des genannten zweiten Siebelementes (68) um wenigstens zwei genormte
Meshgrößenstufen unterscheidet.
27. Verfahren nach einem der Ansprüche 20 bis 22, dadurch gekennzeichnet, daß der
Beschickungsstrom (62) auf wenigstens ein zweites Siebelement (68) aufgebracht wird,
das mit Öffnungen von ausreichender Größe versehen ist, um eine Vielzahl von zweiten
Unterkornklassen im Aufgabestrom hindurchtreten zu lassen, daß der Aufgabestrom in
wenigstens den genannten zweiten Siebdurchlaufstrom (258) und den genannten anderen
zweiten Strom (256) getrennt wird, indem wenigstens zwei der genannten zweiten Unterkornklassen
durch die Öffnungen des genannten zweiten Siebelementes (68) hindurchtreten und in
den genannten zweiten Siebdurchlaufstrom (258) in Mengenverhältnissen zueinander eintreten
gelassen werden, die im wesentlichen verschieden von den relativen Mengenverhältnissen
der genannten wenigstens zwei Unterkornklassen zueinander im Aufgabestrom (62) sind
und daß im genannten Aufgabestrom eine ausreichende Gesamtzahl von zweiten Unterkornteilchen
in jeder der genannten Vielzahl von zweiten Unterkornklassen geschaffen und das Differential
zwischen einer Massenflußrate der zweiten Unterkornteilchen im Aufgabestrom und einer
Massenflußrate von zweiten Unterkornteilchen, die durch das genannte zweite Siebelement
(68) hindurchtreten und in den genannten zweiten, Siebdurchlaufstrom (258) eintreten,
so gesteuert wird, daß im wesentlichen die genannte vorgewählte Korngrößenverteilung
im genannten Produktstrom (256) geschaffen wird.
28. Verfahren nach Anspruch 27, dadurch gekennzeichnet, daß wenigstens ein Siebparameter
des ersten Siebelementes (64) variabel ist, um die relativen Mengenverhältnisse zueinander
der genannten wenigstens zwei Unterkornklassen zu verändern, die durch das erste Siebelement
(64) hindurchtreten und in den genannten ersten Durchlaufstrom (250) eintreten.
29. Verfahren nach Anspruch 27 oder 28, dadurch gekennzeichnet, daß wenigstens ein
Siebparameter des zweiten Siebelementes (68) variabel ist, um die relativen Mengenverhältnisse
zueinander der genannten wenigstens zwei zweiten Unterkornklassen zu verändern, die
durch das zweite Siebelement (68) hindurchtreten und in den genannten zweiten Siebdurchlaufstrom
(258) eintreten.
30. Verfahren nach einem der Ansprüche 20 bis 29, dadurch gekennzeichnet, daß der
Beschickungsstrom (62) so auf das erste Siebelement (64) aufgegeben wird, daß er im
wesentlichen weniger als der vollen mit Öffnungen versehenen Länge des ersten Siebelementes
(64) ausgesetzt ist und daß die Stelle der genannten Aufgabe variabel ist, um das
Ausmaß der genannten mit Öffnungen versehenen Länge zu verändern, die dem Beschickungsstrom
(62) ausgesetzt ist.
31. Verfahren nach einem der Ansprüche 20 bis 30, dadurch gekennzeichnet, daß der
Aufgabestrom (62) auf das zweite Siebelement (68) so aufgegeben wird, daß er im wesentlichen
weniger als der vollen mit Öffnungen versehenen Länge des zweiten Siebelementes (68)
ausgesetzt ist und daß die Stelle der genannten Aufgabe variabel ist, um das Ausmaß
der genannten mit Öffnungen versehenen Länge zu verändern, die dem Beschickungsstrom
(62) ausgesetzt ist.
1. Procédé de tamisage pour tamiser en continu des particules sous-dimensionnées de
classes de dimensions différentes pour fournir un produit ayant une distribution présélectionnée
de dimensions des particules sensiblement différente de la distribution des dimensions
de particules dans une alimentation en matériau particulaire, le procédé de tamisage
comprenant un tamisage à des taux différentiels, caractérisé par les étapes consistant
à introduire un courant (62, 62', 62") de l'alimentation sur au moins un premier élément
de tamisage (64, 64', 64") comportant des ouvertures de dimensions suffisantes pour
laisser passer une multitude de classes de dimensions dans le courant d'alimentation,
à séparer le courant d'alimentation (62, 62', 62") en au moins un premier courant
de tout venant (250, 250', 250") et un autre premier courant (252, 252', 252") en
amenant au moins deux des classes sous-dimensionnées à traverser les ouvertures du
premier élément de tamisage (64, 64', 64") et à entrer dans le premier courant de
tout venant (250, 250', 250") dans des proportions l'une par rapport à l'autre qui
sont sensiblement différentes des proportions desdites au moins deux classes sous-dimensionnées
l'une par rapport à l'autre dans le courant d'alimentation; et à fournir dans le courant
d'alimentation (62, 62', 62") une population suffisante de particules sous-dimensionnées
dans chacune de la multitude de classes sous-dimensionnées et à commander (45, 45',
45") le différentiel entre les proportions de classes sous-dimensionnées dans le courant
d'alimentation (62, 62', 62") et les proportions de classes sous-dimensionnées traversant
le premier élément de tamisage (64, 64', 64") et entrant dans le premier courant de
tout venant (250, 250', 250") de manière à fournir pratiquement une distribution présélectionnée
des dimensions des particules dans un produit particulaire (256, 256', 256") constitué
d'au moins une partie d'au moins l'un du premier courant de tout venant (250, 250',
250") et de l'autre premier courant (252, 252', 252").
2. Procédé selon la revendication 1, caractérisé en ce qu'au moins un paramètre de
tamisage de l'élément de tamisage (64) est variable de manière à changer les proportions
relatives de l'une par rapport à l'autre desdites au moins deux classes sous-dimensionnées
traversant l'élément de tamisage et entrant dans le courant de tout venant (250).
3. Procédé selon la revendication 2, caractérisé en ce que le paramètre de tamisage
variable est contrôle de manière à maintenir la distribution pratiquement présélectionnée
de dimensions des particules dans le courant du produit (256).
4. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce
que le courant de tout venant (250) ou l'autre courant (252) est un courant sortant
du procédé de tamisage et le courant du produit (256) es constitué d'au moins l'un
des courants de sortie.
5. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce
que les différentes classes de dimensions sont définies par le pourcentage cumulé
des particules sous-dimensionnées passant par différentes ouvertures de maille de
tamis standard, et en ce que le différentiel entre le débit massique d'au moins une
classe dimensionnée cumulée dans le premier courant de tout venant (250) et le débit
massique de ladite au moins classe sous-dimensionnée cumulée dans le courant d'alimentation
est au moins 5% du débit massique de ladite au moins classe sous-dimensionnée cumulée
dans le courant d'alimentation (62).
6. Procédé selon la revendication 1, caractérisé en ce qu'au moins un paramètre de
tamisage du premier élément de tamisage (64) est variable de manière à modifier un
différentiel entre un débit massique des particules sous-dimensionnées dans le courant
d'alimentation et un débit massique des particules sous-dimensionnées traversant le
premier élément de tamisage et entrant dans le premier courant de tout-venant (250),
et dans lequel le procédé de tamisage comprend en outre l'étape consistant à commander
le paramètre de tamisage variable de manière à maintenir la distribution pratiquement
présélectionnée des dimensions des particules dans le courant de produit (256).
7. Procédé selon la revendication 6, caractérisé en ce que le paramètre de tamisage
variable est la longueur effective de tamisage du premier élément de tamisage (64),
cette longueur effective de tamisage comprenant la longeueur des ouvertures du premier
élément de tamisage (64) laisant passer des particules sous-dimensionnées du courant
d'alimentation (62) dans le premier élément de tamisage (64) et entrant dans le premier
courant de tout-venant (250).
8. Procédé selon la revendication 6, caractérisé en ce que le paramètre de tamisage
variable est la distribution des dimensions des particules dans le courant d'alimentation
(62).
9. Procédé selon la revendication 8, caractérisé en ce qu'il comprend l'étape consistant
à broyer des roches pour réduire les dimensions des roches à celles de particules
dans le courant d'alimentation (62), la réduction des dimensions des particules fournie
par l'étape de broyage étant variable de façon maîtrisée de façon à maintenir la distribution
pratiquement présélectionnée des dimensions des particules dans le courant de produit
(256).
10. Procédé selon la revendication 6, caractérisé en ce que le paramètre de tamisage
variable est l'inclinaison du premier élément de tamisage (64) par rapport à un plan
horizontal.
11. Procédé selon l'une quelconque des revendications 6 à 10, caractérisé en ce que
ledit au moins paramètre de tamisage variable est contrôlé en réponse à au moins une
caractéristique mesurée du courant du produit ou au moins l'un des courants de tout-venant
ou des refus supérieurs du procédé de tamisage, de manière à maintenir la distribution
pratiquement présélectionnée des dimensions des particules dans le courant du produit
(256).
12. Procédé selon la revendication 11, caractérisé en ce qu'il comprend l'étape consistant
à modifier de manière contrôlée ledit au moins paramètre de tamisage variable en réponse
à des changements d'au moins ladite caractéristique mesurée.
13. Procédé selon la revendication 12, caractérisé en ce que ledit au moins paramètre
de tamisage variable est changé automatiquement en réponse à ladite au moins caractéristique
mesurée.
14. Procédé selon la revendication 12, caractérisé en ce que ladite au moins caractéristique
mesurée est le débit massique d'au moins l'un des courants.
15. Procédé selon la revendication 12, caractérisé en ce que ladite caractéristique
mesurée est une fonction d'une distribution des dimensions des particules dans le
courant.
16. Procédé selon l'une quelconque des revendications précédentes, caractérisé en
ce qu'il comprend une autre étape consistant à broyer des roches aux dimensions ou
particules dans le courant d'alimentation (62), au moins une partie d'un courant de
refus supérieurs (252) provenant du procédé de tamisage étant recyclée dans l'étape
de broyage et introduite sur le premier élément de tamisage (64) comme faisant partie
du courant d'alimentation (62).
17. Procédé selon l'une quelconque des revendications précédentes, caractérisé en
ce que le courant d'alimentation (62) est déchargé sur le premier élément de tamisage
(64) de manière à être exposé à pratiquement moins de la longueur totale des ouvertures
du premier élément de tamisage (64), le procédé comportant en outre l'étape consistant
à faire varier de manière contrôlée la valeur de la longueur des ouvertures exposée
au courant d'alimentation (62) de manière à maintenir la distribution pratiquement
présélectionnée des dimensions des particules dans le courant du produit (256).
18. Procédé selon la revendication 17, caractérisé en ce que la longueur exposée du
tamis est définie par un élément de blocage (130) disposé de manière à intercepter
une partie du courant d'alimentation (62) après introduction de ladite partie sur
la longueur à ouvertures du premier élément de tamisage (64) mais avant que la quasitotalité
des particules sous-dimensionnées dans ladite partie ait traversé les ouvertures du
premier élément de tamisage (14), la longueur exposée du tamis étant modifiée en changeant
l'emplacement auquel l'élément de blocage (130) intercepte ladite partie du courant
d'alimentation (62).
19. Procédé selon l'une quelconque des revendications 1 à 16, caractérisé en ce que
le différentiel entre les proportions des classes sous-dimensionnées dans le courant
d'alimentation (62) et les proportions des classes sous-dimensionnées traversant le
premier élément de tamisage (64) et entrant dans le premier courant de tout-venant
(250) est commandé par un moyen comportant un plateau de retenue (400, 430) situé
au-dessous et entre les extrémités de l'élément de tamisage (64), la longueur du plateau
(400, 430) étant inférieure à celle de l'élément de tamisage, et le plateau (400,
430) étant mobile sur la longueur de l'élément de tamisage (64).
20. Procédé de tamisage pour le tamisage en continu de particules sous-dimensionnées
de classes de dimensions différentes pour fournir un produit ayant une distribution
présélectionnée des dimensions des particules sensiblement différente de la distribution
des dimensions des particules dans une alimentation en matériau particulaire, le procédé
de tamisage comprenant le tamisage à des taux différentiels, caractérisé par les étapes
consistant à introduire un courant (62, 62', 62") de l'alimentation sur au moins un
premier élément de tamisage (64, 64', 64") ayant des ouvertures de dimensions suffisantes
pour laisser passer une multitude de classes de dimensions dans le courant d'alimentation;
à séparer le courant d'alimentation (62, 62', 62") en au moins un premier courant
de tout-venant (250, 250', 250") et un autre premier courant (252, 252', 252") en
amenant au moins deux des classes sous-dimensionnées à traverser les ouvertures du
premier élément de tamisage (64, 64', 64") et à entrer dans le premier courant de
tout-venant (250, 250', 250") dans des proportions l'une par rapport à l'autre qui
sont sensiblement différentes des proportions desdites au moins deux classes sous-dimensionnées
l'une par rapport à l'autre dans le courant d'alimentation; à introduire au moins
une partie d'au moins le premier courant de tout-venant (250, 250') ou de l'autre
première courant (252") comme courant d'entrée dans un second moyen de tamisage (68,
68', 68") pour fournir au moins un second courant de tout-venant (258, 258', 258")
et un autre second courant (256, 256', 256"), et à fournir dans le courant d'alimentation
(62, 62', 62") une population suffisante de particules sous-dimensionnées dans chacune
de la multitude de classes sous-dimensionnées et à commander (45, 45', 45") le différentiel
entre les proportions des classes sous-dimensionnées dans le courant d'alimentation
et les proportions des classes sous-dimensionnées traversant le premier élément de
tamisage et entrant dans le premier courant de tout-venant (250, 250', 250") de manière
à fournir une distribution pratiquement présélectionnée de dimensions de particules
dans un produit particulaire constitué d'au moins une partie d'au moins le second
courant de tout-venant (258, 258', 258") ou de l'autre second courant (256, 256',
256").
21. Procédé selon la revendication 20, caractérisé en ce qu'au moins un paramètre
de tamisage du premier élément de tamisage (64) est variable de manière à modifier
les proportions relatives de l'une par rapport à l'autre desdites au moins deux classes
sous-dimensionnées traversant le premier élément de tamisage et entrant dans le premier
courant de tout-venant (250).
22. Procédé selon la revendication 21, caractérisé en ce que le paramètre de tamisage
variable est contrôlé de manière à maintenir la distribution pratiquement présélectionnée
des dimensions des particules dans le courant du produit (256).
23. Procédé selon l'une quelconque des revendications 20 à 22, caractérisé par l'étape
consistant à introduire le courant d'entrée sur la surface d'un second élément de
tamisage (68) ayant des ouvertures de dimensions suffisantes pour laisser passer au
moins une classe sous-dimensionnée dans le courant d'entrée, à séparer le courant
d'entrée en au moins le second courant de tout-venant (258) et l'autre second courant
(256) en amenant ladite au moins classe sous-dimensionnée du courant d'entrée à traverser
les ouvertures du second élément de tamisage (68) et à entrer dans le second courant
de tout-venant (258) à un débit massique sensiblement différent du débit massique
de ladite au moins classe sous-dimensionnée dans le courant d'entrée (62) et à fournir
dans le courant d'entrée des débits massiques suffisants de particules dans chacune
de la multitude de classes sous-dimensionnées et un différentiel suffisant entre le
débit massique de ladite au moins classe sous-dimensionnée dans le courant d'entrée
(62) et le débit massique de ladite au moins classe sous-dimensionnée passant par
le second élément de tamisage (68) et entrant dans le second courant de tout-venant
(258) pour fournir la distribution pratiquement présélectionnée das dimensions des
particules dans le courant du produit (256).
24. Procédé selon la revendication 23, caractérisé en ce qu'au moins un paramètre
de tamisage du second élément de tamisage (68) est variable de manière à changer la
quantité de ladite au moins classe sous-dimensionnée traversant le second élément
de tamisage (68) et entrant dans le second courant de tout-venant (258).
25. Procédé selon la revendication 24, dans lequel le paramètre de tamisage variable
est commandé de manière à maintenir la distribution pratiquement présélectionnée des
dimensions des particules dans le courant du produit (256).
26. Procédé selon l'une quelconque des revendications 20 à 25, caractérisé en ce que
les seconds éléments de tamisage (64, 68) ont des ouvertures sensiblement uniformes
correspondant à une ouverture de maille de tamis standard et l'ouverture de maille
du premier élément de tamisage (64) est différente de l'ouverture de maille du second
élément de tamisage (68) d'au moins deux ouvertures de maille de tamis standard.
27. Procédé selon l'une quelconque des revendications 20, 22, caractérisé en ce que
le courant (62) est introduit sur au moins un second élément de tamisage (68) ayant
des ouvertures de dimensions suffisantes pour laisser passer une multitude de secondes
classes sous-dimensionnées dans le courant d'entrée, séparant le courant d'entrée
en au moins le second courant de tout-venant (258) et l'autre second courant (256)
en amenant au moins deux des secondes classes sous-dimensionnées à traverser les ouvertures
du second élément de tamisage (68) et à entrer dans le second courant de tout-venant
(258) dans des proportions l'une par rapport à l'autre sensiblement différentes des
proportions relatives desdites au moins deux secondes classes sous-dimensionnées l'une
par rapport à l'autre dans le courant d'entrée (62) et fournissant dans le courant
d'entrée une population suffisante de secondes particules sous-dimensionnées dans
chacune de la multitude de secondes classes sous-dimensionnées et commandant le différentiel
entre un débit massique de secondes particules sous-dimensionnées dans le courant
d'entrée et un débit massique de secondes particules sous-dimensionnées traversant
le second élément de tamisage (68) et entrant dans le second courant de tout-venant
(258) de manière à fournir la distribution présélectionnée des dimensions des particules
dans le courant du produit (256).
28. Procédé selon la revendication 27, caractérisé en ce qu'au moins un paramètre
de tamisage du premier élément de tamisage (64) est variable de manière à modifier
les proportions relatives l'une par rapport à l'autre desdites au moins deux classes
sous-dimensionnées traversant le premier élément de tamisage (64) et entrant dans
le premier courant de tout-venant (250).
29. Procédé selon la revendication 27 ou la revendication 28, caractérisé en ce qu'au
moins un paramètre de tamisage du second élément de tamisage (68) est variable de
manière à modifier les proportions relatives l'une par rapport à l'autre desdites
au moins deux secondes classes sous-dimensionnées traversant le second élément de
tamisage (68) et entrant dans le second courant de tout-venant (258).
30. Procédé selon l'une quelconque des revendications 20 à 29, caractérisé en ce que
le courant d'alimentation (62) est déchargé sur le premier élément de tamisage (64)
de manière à être exposé à une longueur sensiblement inférieure à la longueur totale
des ouvertures du premier élément de tamisage (64) et l'emplacement de la décharge
est variable de manière à modifier la valeur de la longueur des ouvertures qui est
exposée au courant d'alimentation (62).
31. Procédé selon l'une quelconque des revendications 20 à 30, caractérisé en ce que
le courant d'entrée (62) est déchargée sur le second élément de tamisage (68) de manière
à être exposé à une longueur sensiblement inférieure à la longueur totale des ouvertures
du second élément de tamisage (68) et l'emplacement de la décharge est variable de
manière à modifier la valeur de la longueur des ouvertures qui est exposée au courant
d'alimentation (62).