U.S. patent number 3,783,252 [Application Number 05/241,946] was granted by the patent office on 1974-01-01 for control system and method for a reversed ball mill grinding circuit.
This patent grant is currently assigned to Westinghouse Electric Corporation. Invention is credited to Richard E. J. Putman.
United States Patent |
3,783,252 |
Putman |
January 1, 1974 |
**Please see images for:
( Certificate of Correction ) ** |
CONTROL SYSTEM AND METHOD FOR A REVERSED BALL MILL GRINDING
CIRCUIT
Abstract
The dynamically fast in response and stable control arrangement
for a reversed ball mill grinding circuit, which operates in
accordance with the basic control algorithm that the new material
feed rate to the reversed ball mill grinding circuit is controlled
as a function of the net output solids flow production from the
production cyclones associated with that grinding circuit plus the
integral of the error between the desired solids flow load and the
actual solids flow load of the grinding mill associated with that
circuit.
Inventors: |
Putman; Richard E. J.
(Pittsburgh, PA) |
Assignee: |
Westinghouse Electric
Corporation (Pittsburgh, PA)
|
Family
ID: |
22912838 |
Appl.
No.: |
05/241,946 |
Filed: |
April 7, 1972 |
Current U.S.
Class: |
700/122; 700/8;
700/89; 700/41; 241/34 |
Current CPC
Class: |
B02C
17/1805 (20130101); B02C 25/00 (20130101) |
Current International
Class: |
B02C
25/00 (20060101); B02c 025/00 () |
Field of
Search: |
;241/33,34
;235/151.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Botz; Eugene G.
Attorney, Agent or Firm: F. H. Henson et al.
Claims
What is claimed is:
1. In a control system for a reversed ball mill grinding circuit
including at least one particle size classifier production cyclone
operative with a ball mill grinding device, the combination of
means for establishing the output material solids flow leaving said
production cyclone,
means for sensing the actual load material solids flow passing
through said ball mill grinding device,
means for establishing the desired load material solids flow to
pass through said ball mill grinding device, and
means for establishing the feed rate of new ore material to said
ball mill grinding circuit in accordance with a predetermined
relationship between said output material solids flow and the
difference between said desired load material solids flow and said
actual load material solids flow.
2. The control system of claim 1, with said predetermined
relationship being
W.sub.R = P + .intg. (D-L) dt,
where W.sub.R determines the feed rate of new ore material to said
grinding circuit, P is the output material solids flow, D is the
desired load material solids flow and L is the actual load material
solids flow.
3. The control system of claim 1, with the supply of new ore
material to said grinding circuit being in accordance with the
integral of said difference between the desired load material
solids flow and the actual material solids flow.
4. The control system of claim 1, with said ball mill grinding
circuit being operative in a reversed arrangement such that the new
ore material passes to said production cyclone before passing to
the ball mill grinding device and said actual load material solids
flow is the underflow from said production cyclone.
5. The control system of claim 1, with said control of the new ore
material feed rate being in accordance with the relationship
W.sub.error = W.sub.R -W.sub.A
where W.sub.error is the error in said feed rate to be corrected,
W.sub.R is the desired feed rate established by said predetermined
relationship and W.sub.A is the actual feed rate of new ore
material.
6. A method for controlling a reversed ball mill grinding circuit
including at least one classified production cyclone operative with
a ball mill grinding device, including the steps of
establishing the solids flow of output material leaving said
grinding circuit from said production cyclone,
sensing the solids flow of the actual load material passing to said
ball mill grinding device,
establishing a solids flow of the desired load material to pass
through said ball mill, and
controlling the new ore material feed rate to said grinding circuit
in accordance with a predetermined relationship between the
respective solids flows of said output material, said actual load
material and said desired load material.
7. The method of claim 6 with said step of controlling the new ore
material feed rate being operative to provide a substantially
constant mill load.
Description
BACKGROUND OF THE INVENTION
In the present state of the art there are few ball mill grinding
devices that have any type of load control. Regulation of the rate
of feed of new material to a grinding mill circuit has been tried,
whether the grinding mill circuit is arranged in a normal or a
reversed circuit arrangmeent, at a substantially constant rate. The
result has been that while the grinding mill circuit might for a
time run freely and without plugging, with an ore of a certain
hardness in a certain feed size distribution, should either of the
latter parameters change dramatically, such that the ore becomes
harder or the feed size coarser, additional recycling of ore
material will occur within the grinding mill circuit and the
production rate or the feed rate that was present before now
becomes excessive such that the grinding mill can tend to plug. For
a change of hardness or feed size distribution, becoming coarser or
the hardness increasing, the mill will tend to plug when run at the
maximum mill throughput. It is therefore desirable to run as close
to maximum throughout as is reasonably feasible to do so because
the grinding mill is a bottle neck in the total ore processing
plant, whereas the production cyclones are very seldom the bottle
neck and the available rate of feed of new materials is usually far
in excess of what the grinding mill circuit can accept. It is the
throughput in the ball mill, which can be tolerated without
plugging, that is the limiting parameter for the ore processing
plant. Plugging occurs when there is produced less fines without
beying cycloned off than the new material feed coming in, such that
there occurs a buildup of material in the grinding mill circuit and
the ciruit jams up solid if it is allowed to go on in this manner.
This can occur during transients since it takes approximately three
retention times before the grinding mill circuit will settle down
again and transients can become a cause of very heavy mill
throughput for a short period of time such that a plugging
condition can result.
It has been known in the prior art to measure the inventory of
material in the ball mill in relation to the pressure of the oil at
the bearings, which pressure generally bears a somewhat linear
relationship to the load on the ball mill bearings, and therefore
the load in the grinding mill itself. However, the grinding mill
inventory is not necessarily a measure of material throughput
because the same inventory can be present in the grinding mill even
through the flow rate through the grinding mill is varying and of
course the load on the grinding mill bearings can vary with the
ball charge or bearing wear so several assumptions have to be made
when measuring the bearing oil pressure as compared to measuring
the actual ore material flows in and out of the grinding mill.
It is desirable for the operation of a ball mill grinding device
that any condition of input material change, such as a change in
the slurry containing the ore to be ground, does not result in a
wide variation in the operating level of the ball mill. The ball
mill should be operated to avoid plugging as an undesired condition
of operation and it should be operated at an optimum critical level
of operation to be maintained which is below the throughput of ore
material at which the ball mill is likely to become plugged and is
still high enough that the throughput of material is in effect
maximized. In actual practice, a ball mill grinding device will
malfunction slightly before the plugging condition, and therefore
this optimum critical level of operation should be selected to
maintain a desired operation of the ball mill in consideration of
an optimized or substantially maximum throughput of material to be
ground passing through the ball mill grinding device.
In the operation of a well-known normal grinding circuit the new
material from a supply conveyor is added to the underflow material
from the production cyclone at the input of the ball mill. In the
operation of a well-known reversed circuit ball mill grinding
device, the new material is added from a supply conveyor to the
output of the ball mill and then passes through the production
cyclones before the larger sized portion of the new material is
recycled to enter the input of the ball mill.
For a reversed circuit operation of a ball mill operative with
production cyclones, the new material to be ground is fed into a
collection hopper located at the output of the ball mill such that
new ore material added to the circuit first passes through the
production cycles and is classified relative to particle size
before the larger sized portion of this new material passes through
the ball mill. In this way, the new material having a particle size
smaller for example than 200 mesh if the latter size is the chosen
particle size split of the production cyclones, would be separated
by the production cyclones to pass as overflow to the subsequent
mineral separation process such as flotation cells and not pass
through the ball mill. On the other hand the new ore material
particle sizes larger than 200 mesh would be split away from the
smaller particle size material and pass as the underflow from the
production cyclones to the input of the ball mill.
The mean maximum throughput of ore material passing through the
ball mill is the desired operation of the ball mill in that this
will in effect, maximize the amount of ore material passing through
the operation including the ball mill. However, it is desired to
avoid surging or substantial increases in this level of material
throughput in that otherwise it is likely that the ball mill will
pass into an undesired plugging condition of operation on the one
hand or the amount of material passing through the ball mill may
decrease below this mean maximum or optimum desired amount and this
reduces the amount of material that is processed by the reversed
grinding circuit apparatus. For a desired return on equipment
investment, it is desired that a maximum tonnage of ore material be
passed through the ball mill and associated apparatus consistent
with the product having the desired particle size distribution. A
typical ball mill grinding circuit may process in the order of 150
to 200 tons per hour of new ore material so a small percentage
increase in this throughput can be economically very significant. A
typical human operator may consistently operate a ball mill at
about 5 percent below the theoretical desired material throughput
where plugging is likely to occur; the operator desires the
operational throughput as high as possible but he is concerned
about avoiding a plugging condition for the ball mill. One cause of
surging of a ball mill is variation in the hardness of the ore
material passing through the ball mill and this in effect varies
the load on the ball mill and causes its operation to vary in an
undesired surging manner.
The prior art practice for the control of a reversed grinding
circuit is to provide a control system responsive to a first signal
related to the sensed weight of the new ore material as sensed by a
weight responsive device and a second signal related to the
conveyor belt speed for the control of new ore feed rate
substantially constant. The latter control system is grinding mill
is running at less than full optimum capacity in order that an
increase in the hardness or a more coarse feed size distribution
will not cause the grinding mill circuit to plug as an overload
condition in which the grinding circuit becomes choked and
effective grinding ceases.
A sonic type mill load controller has been used which regulates the
new feed rate to the grinding circuit such that the sound emanating
from the ball mill is maintained at a desired level. This has been
found to work fairly well on cement grinding circuits where friable
material is involved. However, it does not work satisfactorily with
hard ores and is affected by the weight of the ball charge and
slurry consistency such that the more viscous slurries may dampen
the resulting noise.
It is generally recognized that grinding mill throughput is the
primary constraint on the grinding circuit capacity such that
maximum production of ground ore and the desirable particle size
distribution can thereby be obtained and an attempt to control the
grinding mill load with precision even under changes in hardness,
feed size distribution or cyclone classifier aperture openings
means that the grinding circuit will run more constantly at this
constraint and so be maximized.
SUMMARY OF THE INVENTION
The present invention provides for the control of new feed rate W
of new ore material, supplied to the hopper at the output of the
ball mill grinding device, to be equal to P the cyclone output
production rate, plus the mill load error (D-L) where D is the
desired or reference ball mill load and L is the actual measured
ball mill load. In relation to the above flow of materials P is the
cyclone total production as measured with flow and density meters,
D is the desired ball mill material load flow, L is the actual ball
mill solids material load flow and W is the controlled flow of new
ore material entering the grinding mill circuit. For steady state
operation the cyclone solids production flow P should equal the new
ore material flow W supplied to the grinding mill circuit. For the
purpose of controlling the new material supply conveyor it is
desired to compare a signal dependent upon the measured flow of new
material W.sub.A with another signal related to the calculated
reference flow of desired new material W.sub.R needed for the
desired operation of the ball mill grinding circuit.
For the control of the material throughput of a grinding mill, it
is desired to optimize and maximize the production rate subject to
the constraints of particle size distribution fraction less than
some desired size such as 200 mesh in size, the overflow density
should be greater than a predetermined desired value and the
underflow density should be less than a predetermined desired value
and it is desired to keep the grinding mill throughput at a
substantially maximum and crictical value. To be able to achieve
the optimum operation and promptly return it after a disturbance
the ability to manipulate various setpoints is required such that
the grinding apparatus will achieve the desired operating setpoints
quickly and without overshoot to satisfy the requirement that the
grinding mill circuit operate as desired and be stable under all
operating conditions.
One of the most important setpoints from this is the mill
throughput since it is a principle constraint on the operation of
the grinding mill circuit. To be able to hold throughput at some
maximum value and prevent choking of the mill under all conditions
further increases the available production of new ore material by
raising the mean throughput in conjunction with a reduction in down
time. Further, the desired control of the grinding mill throughput
also controls the retention time for a given ore hardness and
stabilizes the mill and cyclone product size distribution. Some
typical disturbances include changes in the number of production
cyclones changes in their inlet velocity and/or density and in the
grindability or feed size distribution of newly supplied ore
material.
The present control arrangement resulted from the recognition of a
problem of dynamic stability in reversed ball mill grinding
circuits where the load to the ball mill is attempted to be held
constant. For any circuit normal or reverse, if the feed rate is
held constant there will occur an equilibrium position at which the
grinding mill circuit will settle down in its operation. The
inherent instability of the reversed circuit does not occur when
only the new feed rate is held constant, but rather it occurs when
an attempt is made to hold the mill load constant by varying the
new feed rate. The instability problem is created by the delayed
effect of the new material coming back through the grinding mill.
The present control arrangement is dynamically fast in response and
stable, and operates in accordance with the basic algorithm that
the new feed to the mill is controlled as a function of the net
production from the cyclones plus the integral of the error between
the desired load for the grinding mill and the actual load to the
grinding mill. This means that when the grinding mill circuit is
running under equilibrium conditions, at desired mill load, and
suddenly the new ore material changes in its feed size distribution
to become finer or easier to grind, there occurs a change in
production such as an increase and a corresponding and immediate
reduction in the amount of material reporting to the mill. An
effort at this time to increase the new feed to the mill to
compensate for the loss of ore going into the mill results also in
an increase in the underflow reporting to the mill, but by only a
portion of the new feed change so that the load on the ball mill is
not brought back to where it was before since the change in the new
feed will produce less than the desired change in the feed rate or
in the load on the mill itself because of the varying gain of the
production cyclones.
The speed of response is high so the control will tend to hold the
mill load substantially constant or even raise it slightly. The
delay in the control effect should then start to cut it back to
where the actual load is the same as the desired load. The
proportional effect is achieved in relation to a change in
production and the reset effect is achieved in relation to the
difference between the desired load in the actual load on the
grinding mill.
The gain on the system which gain is the ratio of the change in
ball mill load or the cyclone underflow to the change in new
material feed rate, changes from zero at a low mill throughput,
through unity and up to as much as a gain of two, and this presents
a rather difficult control situation. For this purpose, the
heredisclosed control arrangement regulates the new ore feed rate
as desired to provide a stable operation of the grinding mill
circuit. In relation to the error in mill load, as the difference
between the desired mill load D and the actual mill load L which is
sensed as underflow from the cyclone L, this difference is the
operational error on the grinding mill circuit and goes to a
proportional plus reset controller. The output of which provides
the set point for the new material feed rate to the cyclone. The
error generated between the output of the first controller and the
cyclone feed goes to a second controller, the output from which
sets the new ore feed rate through the ball mill. If an error is
established in the mill load, and the flow rate to the cyclone is
adjusted by regulating the new feed rate to the mill, the feed to
the cyclone relates to the production from the cyclone, with the
gain being defined in terms of the ratio of the change in underflow
or ball mill throughput for a change in new feed, which is the
control system gain of interest. It is the underflow leaving the
cyclones or the material flow to the ball mill that it is desired
to regulate.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic diagram of a control arrangement for a
reversed ball mill grinding circuit in accordance with the present
invention;
FIG. 2 shows an illustrative schematic diagram of a well-known
normal ball mill grinding circuit;
FIG. 3 shows a modification of the FIG. 1 schematic diagram to
include a desired maximum slurry density constraint on the control
system arrangement;
FIG. 4 shows a modification of the FIG. 1 schematic diagram to
icnlude a gain variation for the operation of the reset controller
in accordance with the cyclone operating characteristic; and
FIG. 5 is a curve to illustrate the operating characteristic of a
single cyclone classifier operative with a ball mill grinding
device.
DESCRIPTION OF A PREFERRED EMBODIMENT
In reference to FIG. 1, new ore material is mixed in an output
hopper 12 with the discharge material from the ball mill grinding
device 10 and then is supplied by a pump 14 to the input of a
production cyclone classifier 18, which is operative to separate
the ground ore material below a predetermined particle size such as
200 mesh to the overflow output and to recycle the underflow and
larger size material to an input hopper 22 leading to the ball mill
grinding device 10. A controller 25 is operative with a motor 27
for determining the operation of the new ore material supply
conveyor 24, including a weight measuring device 26 being operative
to provide an actual weight W.sub.A control signal to a summing
junction 23 for determining the operation of the controller 25. A
solids flow determining device 31, including a slurry flow sensing
device and a density sensing device, is provided in the input to
the cyclone classifier 18 for determining the solids flow of the
slurry material leading to the classifier 18. A solids flow sensing
device 28, including a slurry flow sensing device and a density
sensing device, is operative in the underflow of the cyclone
classifier for sensing the input load through the ball mill
grinding device 10. Control signals from the load sensing solids
flowmeter 28 and a desired load reference signal source 17 are
supplied to a summing junction 19 and operative with a controller
21 in relation to the difference between the desired load and the
actual load on the ball mill grinding device 10. The output of the
controller is applied to a summing junction 33 as indicated which
in turn is operative with a summing junction 35 such that the
output of the junction 35 is a reference weight control signal
W.sub.R in accordance with the relationship
W.sub.R = (F-L) + .intg. (D-L) dt (1)
In the control arrangement shown in FIG. 1, the controller 21 is
supplied with an input signal in accordance with the material load
error on the ball mill grinding device 10, such that the desired
load signal from the source 17 is combined with the actual load
signal from the flow sensing device 28 and supplied to the
controller 21. The summing junction 23 provides an output signal in
accordance with the reference or mateiral weight signal W.sub.R in
accordance with the above equation (1) relationship. The controller
25 is supplied a control signal in accordance with the
relationship
W.sub.error = W.sub.R -W.sub.A, (2)
for determining the operation of the new ore material supply
conveyor 24.
It should be understood that the controller 21 shown in FIG. 1 is a
variable speed floating controller which reflects the integral of
the load error (D-L), whereas the controller 25 is a proportional
plus integral controller.
In FIG. 2, there is provided a general illustration of the
well-known normal ball mill grinding circuit, wherein the ball mill
grinding device 10 has its input hopper 22 supplied with the
underflow recycled slurry material from the production cyclone
classifiers 18 as well as the new ore material supply from the
conveyor 24.
In relation to the control system shown in FIG. 1, the output
production P as sensed by the solids flowmeter 20 from the
production cyclones 18 is equal to the input material solids flow F
minus the underflow solids flow material L in accordance with the
relationship
P = (F-L). (3)
thusly, the control operation is such that the ball mill load error
(D-L) plus the cyclone production rate P which is (F-L) can be
utilized to determine the desired or reference feed rate W.sub.R
for the new material supply from the conveyor 24. Thusly, the
control arrangement can be utilized such that the summing point 33
provides an output signal in accordance with the relationship F +
.intg. (D-L) dt and the summing point 35 provides the desired
control signal W.sub.R in accordance with the above equation (1).
The summing point 23 then compares the desired or reference new
material feed rate W.sub.R with the actual material feed rate
W.sub.A from the sensor 26 and provides a W.sub.error signal in
accordance with the above equation (2) relationship to the
controller 25 for determining the operation of the motor 27 and the
new material conveyor 24.
There is a finite retention time period for newly supplied ore
material to pass through the ball mill grinding device 10. Thusly,
at any given point in time, the material leaving the ball mill does
not equal the material entering the ball mill, and to obtain a
desired maximum throughput of ore material passing through the ball
mill grinding device 10, the input to the ball mill grinding device
is varied as required to maintain this desired maximum
throughput.
The operation of the new material feed conveyor 24 which supplies
new ore material to the grinding circuit is controlled as necessary
to maintain the desired optimum throughput loading of the ball mill
grinding device 10. The production cyclones 18 are set to give the
desired particle size split, the grinding device load quantity L is
a measure of the total input material flow entering the ball mill
grinding device 10 and for this reason, it is desired that the load
L be held substantially constant so that the material passing
through the ball mill grinding device 10 is thereby maximized. It
should be understood that the quantity of load material L for a
typical ball mill operation is in the order of three or four times
the new are material flow W.sub.A supplied to the grinding
circuit.
In a typical operation of a grinding circuit each ball mill is
separately controlled with a substantially constant speed drive
motor and is coupled to a bank of one or more production cyclones
as necessary to handle the quantity of ore material that passes
through the ball mill. One flotation cell may be provided for each
ball mill or it could be operative with several ball mills
depending upon the size of the flotation cell. Only part of the new
feed material goes to the ball material in a reversed circuit
operation and this new feed material varies with the inlet velocity
to the production cyclone in the feed size distribution. The inlet
velocity to the production cyclone is varied by the level of
material accumulating in the sump 12 before the pump 14 end by the
supply of new ore material from the conveyor 24. The surging of
input material supplied to the ball mill is what varies the
material throughput of the ball mill. The pulp density to the
production cyclones and the particle size split as well as the
inlet velocity to the production cyclones can also cause this
surging.
A variable gain operation of the ball mill grinding device 10 in
regard to new ore material supplied to the grinding circuit is
inherent since the portion of new material that will pass through
the ball mill is variable depending upon the operation of the
production cyclone 18.
The floating controller 21 shown in FIG. 1 for the load error (D-L)
will provide a fast speed of response because the response of the
ball mill load L to a change in the input flow F to the production
cyclones 18 is relatively fast. By virtue of the sharp response of
the load L to a change in the flow F, a floating controller will
permit desired control system stability. The floating controller
has a reset term and does not include a proportional term, so the
rate of change of the signal is a constant times the error. If a
proportional plus reset controller were to be used, the change
would be a function of the ball mill load error plus the integral
of that ball mill load error and this tends to slow down the
response of the control system. For a very tight control loop, the
floating controller 21 such as shown in FIG. 1 is inherently faster
and tends to be more accurate. There is shown in FIG. 1 a control
arrangement for generating the desired reference weight W.sub.R
which is the feed to the production cyclone 18 minus the underflow
to give the present net feed (F-L) plus the integral of the error
relative to the desired load D for the mill. The load to the mill
has a double effect such that for an increase in W this will result
in an increase in the flow F and the ball mill load L increases to
give a double effect on the flow F from the change in new ore
material supply W. It is desired to hold the input flow F to the
production cyclones at some substantially constant value.
The reversed ball mill grinding circuit is used with ores that are
quite friable like Galena or lead sulfide, which ores are crushed
to produce a lot of fines and these can be initially passed through
the production cyclones to permit separation of the material which
is already a flotation size to thereby reduce the load on the ball
mill grinding device 10.
In general, a reversed grinding mill circuit is inherently unstable
when new ore material feed is regulated to control the grinding
mill throughput substantially constant. Although it may achieve a
condition of equilibrium if the new feed rate and hardness and so
forth are maintained substantially constant. This instability in
relation to direct control from the grinding mill throughput is due
to the fact that the slurry flow F leading to the production
controller cyclone 18 is the sum of W + L and a change in W also
produces a change in L of the same sign. Thusly, the gain of the
control system is in accordance with the following relationship
gain equals change in mill throughput divided by change in new
material feed, and is therefore very high. Response to a
disturbance is consequently slow with a good change of overshooting
the critical mill throughput rate, to result in plugging of the
grinding mill during transient conditions. Instability is further
assisted by the delayed effect of the changes in ball mill load L
on the output production from the cyclone classifiers 18 after
grinding due to the mill mixing lag and distance velocity lag and
hence on the ball mill load L in the feedback loop. The full effect
of any step changes such as a new condition of equilibrium even
under otherwise steady state conditions is not reached until
approximately three retention times have elapsed or three passes of
feed material through the ball mill grinding device reporting
initially to the underflow which must be made through the grinding
mill before it can leave in the overflow from the production
cyclones 18. Since the new ore material feed W cannot be acceptably
controlled directly from the ball mill load error (D-L) a more
complex control scheme is required. The control problem is first
complicated by the variable nature of the gain inherent in the
cyclone characteristic, such that the result is a control system
with a high and widely variable gain.
The control system arrangement shown in FIG. 1 is operative with
the following control relationship
W = P + V + .DELTA. T/T .times. (D-L), (4)
where T is the integration time constant and should be in the order
of 1/2 minute. The latter equation (4) relationship indicates the
changes in new ore material feed W are made initially equal to
changes in the production output P in an attempt to maintain total
control circuit inventory substantially constant. Changes P in
production output P will also immediately subtract from ball mill
load L and a part of the new ore material feed W will report to the
ball mill load L leaving a residual error smaller than P. For the
purpose of comparison, a normal controller equation using a
velocity logarithm is in accordance with the relationship
W.sub.N = K.sub.l .times. (D-L) + V + .DELTA. T/T .times. (D-L),
(5)
where V is the past value of the reset term.
The term P in equation (4) is a feedforward term while the second
term V + .DELTA. T/T .times. (D-L) represents a variable speed
floating controller which is acceptable due to the fast feedback of
new material feed W on the ball mill load L. In equation (5), both
terms are only a function of the error (D-L) and at equilibrium
where D is equal to L, V must have change until W at least equals
P. In equation (4), V need not change since W is always forced to
be equal to P. Thusly, in equation (4), V is only adjusted to
integrate out the long-term error (D-L) and not to also replace the
diminution of the proportional term Kl .times. (D-L) which is
returned to zero at equilibrium. It is a residual feedforward
effect of P in equation (4) which causes equation (4) to provide
better stability.
To establish the solids flow rate of slurry material passing
through a pipe or conduit, such as ball mill actual material load
L, both a flowmeter and a density meter are required. As shown in
FIG. 3, a flow sensing device 100 and density sensing device 102
are provided to enable a determination of the ball mill material
load L. In addition, the slurry density from the sensing device 102
is compared with a desired and maximum value of this density
D.sub.s max and combined with the desired or reference load signal
D, as an upper density constraint to modify the desired maximum
ball mill load D, such that the new ore material feed rate will be
controlled accordingly.
The solids flow rate can be determined as the product of flow and
density in accordance with the relationship ##SPC1##
where S is the solids flow rate in pounds per minute, F is the
slurry flow in cubic feet per minute, D is the slurry density in
pounds per cubic feet and .rho. is the specific gravity of
solids.
In FIG. 4 there is included an operational amplifier or function
generator 110 which operates as a function of the sensed solids
flow F to modify the operational gain of controller 21 in
accordance with the slope of the cyclone operating
characteristic.
In FIG. 5 there is shown a curve to illustrate a typical operating
characteristic for a single cyclone classifier operative with a
ball mill grinding device such as shown in the grinding circuit of
FIG. 1.
The following is an illustrative digital computer instruction
program written in Fortran language to set forth a theoretical mill
model with production cyclones in a reversed circuit arrangement,
and is provided to illustrate the control system operation in
accordance with the teachings of the present invention. ##SPC2##
##SPC3## ##SPC4##
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