Note: Descriptions are shown in the official language in which they were submitted.
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"A METHOD AND APPARATUS FOR TESTING A SHEARING DEVICE"
FIELD OF THE INVENTION
The present invention relates to separation devices for suspensions and pulps
and in
particular to a method for controlling the application of shear to pulp in a
separation device.
It has been developed primarily for use in thickeners and will be described
hereinafter with
reference to this application. However, it will be appreciated that the
invention is not
limited to this particular field of use.
BACKGROUND OF THE INVENTION
The following discussion of the prior art is intended to present the invention
in an
appropriate technical context and allow its significance to be properly
appreciated. Unless
clearly indicated to the contrary, however, reference to any prior art in this
specification
should not be construed as an admission that such art is widely known or forms
part of
common general knowledge in the field.
Separation devices, such as thickeners, clarifiers and concentrators, are
typically used
for separating solids from suspensions (typically containing solids suspended
in a liquid)
and are often found in the mining, mineral processing, food processing, sugar
refining,
water treatment, sewage treatment, and other such industries. These devices
typically
comprise a tank in which solids are deposited from a suspension or solution
and settle
toward the bottom as pulp or sludge to be drawn off from below and recovered.
A dilute
liquor of lower relative density is thereby displaced toward the top of the
tank, for removal
via an overflow launder. The suspension to be thickened is initially fed
through a feed
pipe, conduit or line into a feedwell disposed within the main tank. A rake
assembly is
conventionally mounted for rotation about a central drive shaft and typically
has at least
two rake arms having scraper blades to move the settled material inwardly for
collection
through an underflow outlet.
In its application to mineral processing, separation and extraction, a finely
ground ore
is suspended as pulp in a suitable liquid medium such as water at a
consistency which
permits flow, and settlement in quiescent conditions. The pulp is settled from
the
suspension by a combination of gravity with or without chemical and/or
mechanical
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processes. Initially, coagulant and/or flocculant can be added into the
suspension to
improve the settling process. The suspension is then carefully mixed into the
separation
device, such as a thickener, to facilitate the clumping together of solid
particles, eventually
forming larger denser "aggregates" of pulp particles that are settled out of
suspension.
Typically, several zones or layers of material having different overall
densities
gradually form within the tank, as illustrated in Figure 1. At the bottom of
the tank 1, the
pulp forms a relatively dense zone 2 of compacted pulp or solids 3 that are
frequently in the
form of networked aggregates (i.e. the pulp aggregates are in continuous
contact with one
another). This zone 2 is typically called a "pulp bed" or networked layer of
pulp. Above
the pulp bed 2, a hindered zone 4 tends to contain solids 5 that have not yet
fully settled or
"compacted". That is, the solids or aggregates 5 are not yet in continuous
contact with one
another (un-networked). Above the hindered zone 4 is a "free settling" zone 6,
where
solids or aggregates 7 are partially suspended in the liquid and descending
toward the
bottom of the tank 1. It will be appreciated that the hindered zone 4 is not
always a distinct
zone between the networked layer 2 and the free settling zone 6. Instead, the
hindered
zone 4 may form a transition or an interface between the networked layer 2 and
the free
settling zone 6 that blends between the two zones. Above the free settling
zone 6 is a
clarified zone 8 of dilute liquor, where little solids are present and the
dilute liquor is
removed from the tank 1 by way of an overflow launder (not shown). Figure 1
also
illustrates the feedwell 9 and underflow outlet 10 for removing the compacted
pulp 3 from
the tank 1.
It has hitherto been conventionally thought that to ensure that an appropriate
underflow density is maintained within the pulp bed 2, it and the hindered
zone 4 should be
undisturbed to permit settling of the dense aggregates of solid particles into
their desirable
compacted arrangement. As a consequence, most developments in separation
device
technology concern the improvement of the settling process, either in the
feedwell or the
free settling zone 6, rather than any processes which may disturb the
compacted
arrangement of the solids particles in the pulp bed 3 or the partially
compacted solids in the
hindered zone 4.
It has also been found that as the pulp bed 2 increases in depth, it becomes
increasingly difficult for released liquid to permeate through the pulp bed 2
and migrate
upwardly into the clarified zone 8. One solution has been to provide
dewatering pickets
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mounted to the rake arms to aid removal of such liquid, thereby increasing the
underflow
density and thus the efficiency of the separation process. These pickets are
typically
arranged at equally spaced intervals to produce dewatering channels in the
pulp bed equally
spaced across the diameter of the tank, and are designed to minimise
disturbance of the
pulp bed.
The inventors discovered unexpectedly and surprisingly that the application of
a
disturbance, preferably in the form of shear, to pulp can result in improved
efficiency in the
separation process, especially the settling process in a thickener. It is
believed that by
causing a disturbance substantially uniformly across a disturbance zone in an
upper region
of the networked layer of pulp, the networked pulp is disrupted, by breaking
up, disturbing,
re-arranging, re-orienting or "shaking" the continuous contact between the
pulp, or
subjecting it to a force. This disruption of the networked pulp enables the
release of
trapped liquid upwardly towards the clarified zone of dilute liquor and
increases the density
of the pulp below the disturbance zone relative to pulp density above the
disturbance zone.
At the same time, the disturbance creates turbulence that inhibits or prevents
the formation
of donuts in the pulp bed. Moreover, the inventors have determined that if the
networked
layer is disturbed too much, fractionation of the networked pulp into smaller
pieces occurs,
resulting in the smaller pieces settling more slowly. Too little disturbance
fails to disrupt
the networked pulp sufficiently to release enough liquid to improve settling
efficiency.
Thus, the inventors have determined that by controlling the amount of the
disturbance to an
optimum level, as distinct from a minimum level, the improved separation
efficiency can be
maintained continuously over the work cycles of the separation device.
Throughout the description and claims, the terms "disrupt", "disrupting",
"disruption"
and its variants are taken to mean breaking up, disturbing, re-arranging, re-
orienting or
"shaking" particles or a substance, as well as applying a force to the
particles or substance.
In the context of the present invention, these terms are taken to mean
breaking up,
disturbing, re-arranging, re-orienting, applying a force to, or "shaking" the
organised
structure of the networked pulp, including but not limited to the continuous
contact between
the networked pulp.
As a result of this unexpected and surprising discovery, the inventors
developed a
method and a separation device for controlling a disturbance, preferably in
the form of
shear, to pulp in the separation device, which for convenience will be
described throughout
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the description as the "disturbance control invention". In the disturbance
control invention,
a suspension comprising pulp is fed into a tank of a separation device at a
flux, the pulp
being allowed to separate from the suspension and a disturbance causing
device, preferably
a shearing device, is submerged for shearing pulp at least partially within a
region of the
tank. In the case of a shearing device that created the disturbance through
mechanical
agitation, one or more shearing parameters for the shearing device were
controlled with
respect to the flux and/or one or more operational parameters to controllably
apply an
optimal shear to pulp passing through the region. These shearing parameters
were selected
from the group consisting essentially of the speed of the shearing device, the
shape of the
shearing device and the depth of the shearing region. The operational
parameters were
selected from the group consisting essentially of the pulp composition, the
pulp particle
size, the pulp flow velocity in the tank, the pulp yield stress, the pulp
viscosity, the
underflow specific gravity, the underflow weight per weight percentage and the
rate at
which flocculant is added to the suspension.
Throughout the description and claims, the term "flux" means the rate of flow
of
solids suspended in a fluid (generally a liquid) suspension and is measured in
tonnes per
square metre hour (t/m2h). In the context of minerals separation, the flux is
used to refer to
the flow of suspended pulp solids in the slurry. Although the solids
concentration or pulp
density of the suspension may change as the pulp moves through the tank, the
flow of
solids may be regarded as independent of the pulp density and so is expressed
as a flux.
It will be appreciated that this method is not directly dependent upon a
specific
configuration of the disturbance causing device or its preferred form of
applying a shear
through a shearing device. Rather, the mechanism by which shear is applied to
cause the
disturbance can take a number of forms. For example, one shearing mechanism is
to use
liquid or gas jets to inject a liquid or gas towards, into or through the
disturbance zone 16 to
apply shear substantially uniformly across the disturbance zone. Similarly, a
fluidiser can
direct fluid flow towards, into or through the disturbance zone 16 to apply
shear
substantially uniformly across the disturbance zone. Other shearing mechanisms
include
subjecting the disturbance zone 16 to mechanical vibration using a suitable
vibratory
apparatus or ultrasonic impulses to apply shear substantially uniformly across
the
disturbance zone. While these shearing mechanisms are suitable for
implementing the
disturbance control invention, the inventors have determined that a preferred
shearing
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mechanism is mechanical agitation. Where the mechanical agitation is performed
using a
shearing device, it is expected that most of these shearing devices would have
at least one
arm carrying a plurality of shearing elements. However, the shearing device
may vary in its
overall shape and dimensions, the number of shearing arms and shearing
elements on each
respective shearing arm, and the arrangement of the shearing arms and shearing
elements.
As a consequence, it is necessary to test the design of any proposed shearing
device to
determine whether it will produce the required optimal shear in accordance
with this
method.
It has been found in practice that for the purposes of testing shearing
devices, it is
preferred to limit the variables involved to accurately measure the shear
produced by a
particular shearing device. Accordingly, the variables for the shearing and
operational
parameters are limited to the shearing device speed and the flux of the
suspension. Thus,
this type of testing can determine whether a particular configuration of the
shearing device
results in an optimal amount of shear, and if so, the optimal shearing device
speed and
frequency for that particular shearing device configuration.
In most conventional testing regimes, a scaled down version of a separation
device,
for example a thickener, is typically used as a continuous process testing
apparatus. A
correspondingly scaled down version of the shearing device is used as a test
shearing
device and is placed inside the test apparatus. The test shearing device is
operated over
various speeds and for a predetermined operational cycle, to simulate
conditions in the full
scale thickener. The underflow density of the settled pulp is then measured to
determine
the effectiveness of the shearing device. Thus, it is important to simulate
the correct
amount of shear, as the effectiveness of the shearing device is determined
indirectly by
measuring the underflow density of the settled pulp in the test apparatus
after being sheared
by the test shearing device. However, where tests are conducted in a
conventional testing
apparatus using a scaled down version of the shearing device, these tests do
not provide
accurate results. The test apparatus does not fully and/or accurately simulate
the operation
of the shearing device in the thickener because the amount of shear that is
applied to the
pulp in the tank is a function of the velocity of the shearing device (the
radial distance of
the shearing device from its axis of rotation), the profile of the shearing
device and the
frequency of the shear events applied by the shearing device. Thus, it is
important to
simulate the correct amount of shear and the frequency (number) of shear
events, as the
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effectiveness of the shearing device is determined indirectly by measuring the
underflow
density of the settled pulp in the test apparatus after being sheared by the
test shearing
device. Consequently, it is difficult to reproduce in a testing apparatus the
correct amount
of shear and the frequency of shear events at each radial distance for the
same
predetermined time period or cycle. In other words, results for shear obtained
at one radial
distance cannot be extrapolated to obtain accurate results for a greater
radial distance over
the same cycle, due to the differing amounts of shear and frequency of shear
events that
would be applied at each radial distance over the common cycle.
As a result, the test results must be manipulated statistically to take into
account the
effect of these scale differences between the test apparatus and test shearing
device and the
full scale thickener and shearing device upon the shear. However, in the case
of the shear
profile and the frequency or number of shear events, it is difficult to adjust
the test data to
accurately take these factors into account. Consequently, this introduces
errors into the test
results that make it difficult to gauge the effectiveness of the shearing
device.
It is an object of the invention to overcome or ameliorate one or more of the
deficiencies of the prior art, or at least to provide a useful alternative.
SUMMARY OF THE INVENTION
According to one aspect, the invention provides a method of testing a shearing
device
for a separation device, wherein the separation device comprises a tank for
receiving a feed
material, wherein feed material settles in the tank and the pulp forms into
aggregates, the
pulp aggregates settling and forming a first networked layer of pulp towards
the bottom of
the tank, and the shearing device is moveable to apply shear substantially
uniformly across
a first disturbance zone in an upper region of the first networked layer, so
as to disrupt the
networked pulp in the first disturbance zone within a predetermined period of
time, the
method comprising the steps of:
providing a test tank for a feed material to settle and pulp to form into
aggregates, the
pulp aggregates settling and forming a second networked layer of pulp towards
the bottom
of the test tank;
submerging a portion of the shearing device at least partially within the
second
networked layer in the test tank to apply shear in a second disturbance zone
in an upper
region of the second networked layer;
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calculating a first speed of the shearing device for a first time period in
which the
shearing device is expected to apply shear in the first disturbance zone at a
first
predetermined distance;
positioning the shearing device portion at a second predetermined distance in
the test
tank;
calculating a second speed for the shearing device portion at the second
predetermined distance that corresponds to the first speed at the first
predetermined
distance;
calculating a second time period for moving the shearing device portion at the
second
speed that corresponds to the first time period at the first predetermined
distance;
calculating a time difference between the first time period and the second
time period;
and
moving the shearing device portion in the second disturbance zone at the
second
speed for the second time period and stopping movement of the shearing device
portion for
the time difference, so as to simulate the application of shear in the first
disturbance zone
by the shearing device at the first predetermined distance in the separation
device over the
first time period.
Unless the context clearly requires otherwise, throughout the description and
the
claims, the words "comprise", "comprising", and the like are to be construed
in an inclusive
sense as opposed to an exclusive or exhaustive sense; that is to say, in the
sense of
"including, but not limited to".
Preferably, the method comprises repeating the moving step after the stopping
step.
More preferably, the method comprises successively repeating the moving and
stopping
steps.
Preferably, the moving step comprises rotating the shearing device portion.
Preferably, the first and second speeds are rotational speeds of the shearing
device and the
shearing device portion, respectively. More preferably, the method comprises
calculating
the rotational speed of the shearing device portion so that its linear speed
at the second
predetermined distance is substantially equal to the linear speed of the
shearing device at
the first predetermined distance. In one preferred form, the linear speeds of
the shearing
device portion and the shearing device are average linear speeds. In this
case, the average
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linear speed is the average of linear speeds across the respective widths of
the shearing
device portion or shearing device.
Preferably, the method further comprises adjusting the second speed by a
scaling
factor. More preferably, the scaling factor is calculated according to the
relationship:
E = Ct/Ct
where r, is the scaling factor;
Ct is the circumference of a circle travelled by the shearing device at the
first
predetermined distance in metres; and
Ct is the circumference of the test tank in metres.
Alternatively, the scaling factor is calculated according to the relationship:
C = f/ft
where r, is the scaling factor;
f is the first predetermined distance in metres; and
ft is the second predetermined distance in metres.
In one particularly preferred form, the adjusting step comprises applying the
scaling
factor to the rotational speed of the shearing device to obtain the rotational
speed of the
shearing device portion. In this case, the rotational speed for the shearing
device portion is
calculated according to the relationship:
wt = E=wt
where wt is the rotational speed of the shearing device portion in rpm;
E is the scaling factor; and
wt is the rotational speed of the shearing device in rpm.
Preferably, the first time period is the time for one revolution of the
shearing device
in the tank of the separation device.
Preferably, the second time period is calculated by applying a scaling factor
to the
first time period. More preferably, the second time period is calculated
according to the
relationship:
tct= t /C
where tc is the first time period in seconds;
E is the scaling factor; and
tc is the second time period in seconds.
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More preferably, the scaling factor is calculated according to the
relationship:
E = CE/Ct
where r, is the scaling factor;
Ct is the circumference of a circle travelled by the shearing device at the
first
predetermined distance in metres; and
Ct is the circumference of the test tank in metres.
Alternatively, the scaling factor is calculated according to the relationship:
C = f/ft
where r, is the scaling factor;
f is the first predetermined distance in metres; and
ft is the second predetermined distance in metres.
In a further alternative, the scaling factor is calculated according to the
relationship:
C = wt/wt
where r, is the scaling factor;
wt is the rotational speed of the shearing device portion in rpm; and
cot is the rotational speed of the shearing device in rpm.
Preferably, where the shearing device has N shearing arms, the method further
comprises adjusting the second time period according to the relationship:
tct = N x Ot/O X tct ortct = N X g X tct
where tc is the second time period in seconds;
N is the number of shearing arms of the shearing device;
E is the scaling factor;
cot is the rotational speed of the shearing device portion in rpm;
cot is the rotational speed of the shearing device in rpm; and
tct is the first time period in seconds.
Preferably, the first predetermined distance is proportional to the second
predetermined distance.
Preferably, the method comprises the step of measuring the separation of pulp
from
the fluid in the test tank after completion of the first time period to
determine whether the
shearing device would apply the expected optimal shear at the first
predetermined distance.
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More preferably, the measuring step is performed after a predetermined number
of
successive repetitions of the moving step and the stopping step.
Preferably, the first predetermined distance is a radial distance from a
centre of the
separation device. Preferably, the second predetermined distance is a radial
distance from a
centre of the test tank. More preferably, the second predetermined distance is
a radial
distance from a centre of the test tank to a selected radial point on the
shearing device
portion. In one preferred form, the second predetermined distance is a radial
distance from
a centre of the test tank to an outer edge of the shearing device portion.
Preferably, the method comprising adjusting one or more of the first speed,
the first
time period and the time difference in response to one or more shearing
parameters.
Preferably, the shearing parameters are selected from the group consisting
essentially of the
shape of the shearing device and the depth of the shearing region.
Preferably, the method comprises the step of adjusting one or more of the
first speed,
the first time period and the time difference in response to changes in the
flux.
Preferably, the method comprises the step of adjusting one or more of the
first speed,
the first time period and the time difference in response to changes in one or
more of the
operational parameters.
Preferably, the operational parameters are selected from the group consisting
essentially of the pulp composition, the pulp particle size, the pulp flow
velocity in the tank,
the pulp yield stress, the pulp viscosity, the underflow specific gravity, the
underflow
weight per weight percentage and the rate at which flocculant is added to the
suspension.
Preferably, the method comprises the step of reversibly rotating the shearing
device.
Preferably, the moving step further comprises periodically reversing the
rotation of the
shearing device.
Preferably, the method comprises the step of moving the shearing device
portion to
apply a substantially uniform number of shear events to the networked pulp in
the second
disturbance zone within the second time period.
Preferably, the method comprises the step of moving the shearing device
portion to
apply a substantially uniform cumulative shear to the networked pulp in the
second
disturbance zone within the second time period. Throughout the description and
the claims,
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the term "cumulative shear" means the sum of the shear that is applied to a
typical pulp
aggregate or particle passing through a defined region. In this context, the
cumulative shear
is the total sum of shear that a typical pulp aggregate or particle
experiences between its
entry into and exit from the region, which is determined by the sum of "shear"
events that
have occurred and the magnitude of those shear events; that is, the number of
times the
typical pulp aggregate or particle has been "hit" (a shear force has been
applied to it).
These shear events not only include direct "hits" of the pulp aggregate or
particle by the
shearing device but also disturbances or "shaking" of the pulp aggregate or
particle caught
in the wake of the passage of the shearing device, which the inventors call a
"zone of
turbulence". These zones of turbulence are sufficient to apply a shear force
to the pulp
aggregate or particle, albeit less than the amount of shear directly applied
by the shearing
device.
Alternatively, the moving step further comprises rotating the shearing device
portion
about an axis of rotation that is parallel, eccentric or offset with respect
to a central axis of
the test tank. Preferably, the method comprises the step of moving the axis of
rotation
relative to the central axis. More preferably, the axis of rotation rotates,
revolves or orbits
at least partially around the central axis. In one preferred form, the axis of
rotation at least
partially traverses a regular path around the central axis. Alternatively, the
axis of rotation
at least partially traverses an irregular path around the central axis. In
some embodiments,
the axis of rotation moves in a circular path. In other embodiments, however,
the axis of
rotation moves in a non-circular path, which may be geometrically regular or
irregular.
Preferably, the shearing device portion has a plurality of shearing elements.
Preferably, the method comprises the step of defining a zone of turbulence for
each
shearing element to disturb, re-arrange or break-up the pulp aggregates and/or
to release
liquid.
Preferably, the method further comprises the steps of spacing apart the
shearing
elements along at least one arm of the shearing device portion to define
respective intervals
therebetween and applying a substantially uniform average shear to the
networked pulp in
at least two intervals along a line parallel to or coincident with the at
least one arm. More
preferably, the average shear in all the intervals between the shearing
elements along the
line is substantially uniform or the same.
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Throughout the description and the claims, the term "average shear" means the
average of shear applied to pulp between any two predetermined reference
points. In this
context, the two reference points typically (but not necessarily) coincide
with adjacent
shearing elements disposed on the at least one arm of the shearing device. It
will be
appreciated that the line may be non-linear in whole or part. For example, the
line may
include a portion that is arcuate, angled or offset with respect to a straight
portion of the
line. In one preferred form, the line is a radial line.
In one preferred form, the at least one arm extends outwardly. Preferably, the
at least
one arm extends radially outwardly. More preferably, the at least one arm
extends radially
outward substantially to an outer perimeter of the region.
Preferably, the method comprises the step of applying substantially uniform
average
shear along the length of the arms.
Preferably, the method comprises the step of disposing one or more shearing
elements
on the at least one arm. Preferably, the method comprises the step of
arranging the shearing
elements to apply shear along the at least one arm.
Alternatively, the method comprises the step of disposing one or more shearing
elements along the axis of rotation to extend radially outwardly. Preferably,
the method
comprises the step of removably mounting the one or more shearing elements to
a drive
shaft of the shearing device portion.
According to another aspect, the invention provides an apparatus for testing a
shearing device, wherein the shearing device is for a separation device
comprising a tank
for receiving a feed material, wherein feed material settles in the tank and
the pulp forms
into aggregates, the pulp aggregates settling and forming a first networked
layer of pulp
towards the bottom of the tank, and the shearing device is moveable to apply
shear
substantially uniformly across a first disturbance zone in an upper region of
the first
networked layer, so as to disrupt the networked pulp in the first disturbance
zone within a
predetermined period of time, the apparatus comprising:
a test tank for receiving a feed material to settle and pulp to form into
aggregates, the
pulp aggregates settling and forming a second networked layer of pulp towards
the bottom
of the test tank, and
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a portion of the shearing device submerged at least partially within the
second
networked layer in the test tank to apply shear in a second disturbance zone
in an upper
region of the second networked layer;
wherein the shearing device portion is positioned at a second predetermined
distance
in the test tank;
the shearing device portion is moveable in the second disturbance zone at a
second
speed for a second time period, said second speed corresponding to a first
speed of the
shearing device for a first time period in which the shearing device is
expected to apply
shear in the first disturbance zone at a first predetermined distance, and
said second time
period corresponds to the first time period at the first predetermined
distance; and
the shearing device portion is able to be stopped for a time difference
between the
first time period and the second time period, so as to simulate the
application of shear in the
first disturbance zone by the shearing device at the first predetermined
distance in the
separation device over the first time period.
Preferably, the shearing device portion rotates in the test tank. Preferably,
the first
and second speeds are rotational speeds of the shearing device and the
shearing device
portion, respectively.
Preferably, the shearing device portion applies a substantially uniform number
of
shear events to the networked pulp in the second disturbance zone within the
second time
period.
Preferably, the shearing device portion applies a substantially uniform
cumulative
shear to the networked pulp in the second disturbance zone within the second
time period.
Preferably, the shearing device portion rotates about an axis of rotation that
is
parallel, eccentric or offset with respect to a central axis of the test tank.
Preferably, the
axis of rotation moves relative to the central axis. More preferably, the axis
of rotation
rotates, revolves or orbits at least partially around the central axis. In one
preferred form,
the axis of rotation at least partially traverses a regular path around the
central axis.
Alternatively, the axis of rotation at least partially traverses an irregular
path around the
central axis. In some embodiments, the axis of rotation moves in a circular
path. In other
embodiments, however, the axis of rotation moves in a non-circular path, which
may be
geometrically regular or irregular.
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Preferably, the shearing device portion has a plurality of shearing elements.
Preferably, the shearing elements each define a zone of turbulence to disturb,
re-arrange or
break-up the pulp aggregates and/or to release liquid.
Preferably, the shearing elements are spaced apart along at least one arm of
the
shearing device portion to define respective intervals therebetween and apply
a
substantially uniform average shear to the networked pulp in at least two
intervals along a
line parallel to or coincident with the at least one arm. More preferably, the
average shear
in all the intervals between the shearing elements along the line is
substantially uniform or
the same. In one preferred form, the shearing device portion applies
substantially uniform
average shear along the length of the at least one arm.
Preferably, the shearing device portion has at least one arm that extends
outwardly.
In one preferred form, the at least one arm extends radially outwardly. More
preferably, the
at least one arm extends radially outward substantially to an outer perimeter
of the region.
Preferably, one or more shearing elements are disposed on the at least one
arm.
Preferably, one or more shearing elements are arranged to apply shear along
the at least one
arm. Alternatively, one or more shearing elements are disposed along the axis
of rotation to
extend radially outwardly. Preferably, the one or more shearing elements are
removably
mounted to a drive shaft of the shearing device portion.
The test tank can be configured to be any suitable shape corresponding the
tank of the
separation device. Preferably, the test tank is configured to be at least one
of the following
shapes: a substantially cylindrical in shape. rectangular in shape, a trough
that can be open
or closed, a partially arcuate in shape and an arcuate shape that corresponds
to a
circumferential arc of the separation device.
Preferably, the separation device is a thickener.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention will now be described, by way of
example
only, with reference to the accompanying drawings in which:
Figure 1 is a schematic cross-sectional view of the typical zones of material
within a
separation device;
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Figure 2A is a schematic cross-sectional view illustrating the settling
process in the
separation device of Figure 1;
Figure 2B is a schematic diagram illustrating the inventive concept of causing
a
disturbance substantially uniformly across in an upper region of the networked
layer;
Figure 3 is a schematic diagram view of one example of the disturbance control
invention;
Figure 4 is a schematic diagram of another example of the disturbance control
method
of Figure 3;
Figure 5 is a cross-sectional view of a separation device for according to the
disturbance control invention;
Figures 6A and 6B are respective schematic partial cross-sectional and plan
views of
a shearing device for a separation device according to one embodiment of the
disturbance
control invention;
Figures 7A and 7B are respective schematic partial cross-sectional and plan
views of
a dewatering picket assembly in the prior art;
Figure 8A is a cross-sectional view of a separation device according to a
further
embodiment of the disturbance control invention;
Figure 8B is a cross-sectional view of a separation device according to yet
another
embodiment of the disturbance control invention;
Figure 8C is a cross-sectional view of a separation device according to a
further
embodiment of the disturbance control invention;
Figure 9 is a schematic diagram of one embodiment of the method of the
invention;
Figures 1OA to 1OC are schematic diagrams further illustrating the operation
of the
method of Figure 9;
Figure 11 is a cross-sectional view of test apparatus for implementing the
method of
Figure 5;
Figure 12 is a plan view of the test apparatus of Figure 11;
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Figure 13 is a schematic diagram superimposing the test apparatus of Figures
11 and
12 on the separation device of Figure 5 to illustrate the method of Figure 9;
Figure 14A and 14B are plan views of a cylindrical test tank and a thickener
illustrating the linear velocities of the shearing device portion and the
shearing device,
respectively;
Figure 15 is a plan view of a test apparatus according to another embodiment
of the
invention; and
Figure 16 is a plan view of a test apparatus according to a further embodiment
of the
invention.
PREFERRED EMBODIMENTS OF THE INVENTION
To fully understand and appreciate the current invention and place it in an
appropriate
context, the general technical field and the inventors' disturbance control
invention will be
discussed below.
A preferred application of the invention is in the fields of mineral
processing,
separation and extraction, whereby finely ground ore is suspended as pulp in a
suitable
liquid medium, such as water, at a consistency which permits flow, and
settlement in
quiescent conditions. The pulp, which includes both solid particles and
liquid, is settled
from the suspension by a combination of gravity with or without chemical
and/or
mechanical processes. The pulp gradually clumps together to form aggregates of
pulp as it
descends from the feedwell towards the bottom of the tank. This process is
typically
enhanced by the addition of flocculating agents, also known as flocculants,
which bind the
settling solid or pulp particles together. These denser pulp aggregates settle
more rapidly
than the individual particles by virtue of their overall size and density
relative to the
surrounding liquid, gradually forming a networked layer or pulp bed 2, where
the pulp is in
a compacted arrangement and continuous contact with each other, as best shown
in Figure
1.
The settling of pulp as it passes through the zones in a thickening tank 1 is
described
in more detail with reference to Figure 2A, where corresponding features have
been given
the same reference numerals. Within the feedwell 9, flocculant 11 is added and
adsorbs
onto discrete pulp particles 12, as best shown in Figure 2A(a). The flocculant
11 and pulp
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particles 12 grow and loosely bind together into porous pulp aggregates 13
within the
feedwell 9 and/or as the pulp particles 12 flow out of the feedwell 9 into the
free settling
zone 6, as best shown in Figure 2A(b). Due to their porous nature, liquid 14
is trapped
within individual pulp aggregates. As the pulp aggregates 13 further descend
in the tank 1
through the free settling zone 6 and into the hindered zone 4, they become
crowded and
impede settling of each other, as best shown in Figure 2A(c). Gradually, the
pulp
aggregates 14 consolidate and compact together into an organised networked
layer 2 of
pulp, also called a pulp bed, as best shown in Figure 2A(d). Nevertheless,
despite this
compacted arrangement of the networked pulp layer 2, it has been found that
areas occur
within the networked pulp layer where liquid remains trapped within and
between the
aggregates in the networked layer of pulp. As it is difficult for this trapped
liquid to escape
the pulp bed into the clarified zone of dilute liquor, the underflow density
of the pulp is
diminished.
Thus, the inventors have unexpectedly and surprisingly found that the optimal
disturbance for achieving this improved separation efficiency continuously
over the work
cycles of the separation device is obtained by causing the disturbance
substantially
uniformly across a disturbance zone in an upper region of the networked layer,
as best
shown in Figure 2B where corresponding features have been given the same
reference
numerals. As shown in Figures 2B(a) to 2B(d), flocculant is added into the
feedwell 9 to
adsorb onto discrete pulp particles 12 to promote the formation of aggregates
13 that
descend and form a networked layer of pulp. Unlike the conventional settling
process
illustrated in Figure 2A, where the pulp aggregates 13 are left alone during
formation of the
networked layer 2, a disturbance 15 is caused substantially uniformly across
within a
disturbance zone 16 in an upper region 17 of the networked layer 2, as best
shown in Figure
2B(e). As a consequence, a proportion of the networked pulp 3 (being the
networked pulp
that passes through the disturbance zone 16) is disrupted to release liquid 14
trapped within
the networked pulp, thus increasing the relative density of the pulp 18 below
the
disturbance zone 16, as best shown in Figure 2B(f).
The inventors have found that a preferred and convenient way of causing the
disturbance is to apply shear substantially uniformly across the disturbance
zone, although
other forms of disturbance may be used, for example creating turbulence across
the
disturbance zone. The disturbance, preferably the application of shear,
substantially
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uniformly across the disturbance zone 16 results in an increased probability
of the
networked pulp receiving a disturbance that disrupts its generally organised
structure. The
disturbance may also disrupt the continuous contact between the networked
pulp. The
disruption can take the form of shaking or disturbing the networked pulp.
Alternatively, or
cumulatively, the disruption can take the form of re-arranging, re-orienting
or breaking up
the networked pulp. In both cases, the disruption has the effect of releasing
liquid 14
trapped in the networked pulp, either between pulp aggregates or within a pulp
aggregate.
Thus, a substantial proportion of this trapped liquid 14 is released upwardly
out of the
networked pulp bed 2. It is believed that the application of shear to the
networked pulp
"shakes", re-arranges or breaks up its structure and/or continuous contact
between the
networked pulp so that the pulp below the disturbance zone becomes more dense,
which
results in an enhancement of their settling rate and/or their packing density.
Moreover, the
disturbance is not so excessive as to cause fractionation of the networked
pulp into smaller
pieces, which settle more slowly. The relatively denser pulp tends to reform
into a
networked pulp layer below the disturbance zone, due to its own weight
applying
compaction forces to the pulp. As a result, the invention provides the
appropriate amount
of disturbance to increase the settling rate and/or underflow density of the
pulp in the
networked layer or pulp bed 2, thus leading to increased efficiency and
performance of the
separation device.
The inventors have discovered that the disturbance, preferably by way of
shear,
induces a stepwise increase in the density of the pulp below the disturbance
zone. In the
context of the application of the invention to a thickening process, the
inventors have found
that by controlling the disturbance, preferably shear, to an optimal amount
using at least
one or more of three primary options that will be discussed in more detail
below, this
stepwise increase in density of the pulp below the disturbance zone is at
least a 5% increase
compared to the density of pulp above the disturbance zone. In one preferred
form, there is
at least a 10% increase compared to the density of pulp above the disturbance
zone. In
other preferred forms, the density of the pulp below the disturbance zone is
at least 25%,
preferably at least 35% and more preferably at least 50%, greater than the
density of pulp
above the disturbance zone.
It will be appreciated that during operation of the separation device, the
depth of the
networked pulp layer 2 will gradually increase. Alternatively, the separation
device may
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have a relatively low networked pulp layer 2 for operational requirements.
Consequently,
the disturbance zone 16 may initially occupy a larger proportion of the
networked pulp
layer 2, and in such cases the disturbance zone may be within an upper 75% of
the
networked layer of pulp. Where a typical depth of the networked pulp layer is
present in
the tank, the disturbance zone is within an upper half of the networked layer
of pulp.
However, the method of the invention may still be implemented where the
disturbance zone
16 is within an upper 30% of the networked layer of pulp, an upper 10% of the
networked
layer of pulp, or even at or adjacent the top of the networked layer of pulp.
The inventors have also surprisingly discovered that where shear is applied by
way of
mechanical agitation, this improved settling effect is best achieved by
carefully controlling
the shear applied to the pulp bed so as to keep the shear at an optimum level,
as distinct
from a minimum level. If too much shear is applied, fractionation of the
aggregates into
smaller pieces occurs, resulting in the smaller pieces settling more slowly.
Too little shear
fails to disrupt the networked pulp sufficiently to release enough liquid to
improve settling
efficiency. This resulted in the inventors developing the disturbance control
invention and
its particular application to a disturbance created by mechanical agitation
using a shearing
device. Where a shearing device is used in the disturbance control invention,
the optimal
shear for achieving this improved separation efficiency continuously over the
work cycles
of the separation device is obtained by controlling or adjusting one or more
shearing
parameters with respect to the flux (throughput) of the incoming suspension
into the
separation device, one or more operational parameters or a combination of
both. These
shearing parameters comprise the shearing device speed (linear or rotational),
the depth of
the disturbance zone and the (three-dimensional) shearing device shape. This
unexpected
and surprising discovery meant that the amount of shear applied to the pulp
could be
optimally controlled in accordance with operational requirements, especially
variations in
the supply of the suspension, whilst maintaining the improved separation
efficiency in the
separation device. In the case of a thickener, the application of the
disturbance control
invention results in improvements in the recovered underflow density of the
settled pulp
relative to the amount of flux or throughput of the liquid slurry that is
processed by the
thickener.
It will be appreciated by those skilled in the art that the concept of causing
a
disturbance, for example by applying shear, in a disturbance zone in the
networked layer 2,
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which may include at least a portion of the hindered zone 6, is contrary to
conventional
thought and has not been contemplated as such in the prior art. In the prior
art, it was
preferred not to disturb the pulp bed 2 or the hindered zone 4, since most of
the aggregates
are compacted or almost compacted (in the case of the hindered zone 4), and
improvements
were focussed on improving the efficiency of the settling process, either in
the feedwell 9
or in the free settling zone 6 in the tank 1. This was reflected in the design
of thickeners
specifically to minimise motion within the pulp bed 2. For example, equally
spaced
predominantly vertically extending pickets were mounted on the rake arms to
create
vertical dewatering channels to release liquid. However, the configuration and
spacing of
the pickets were designed to ensure that the pickets moved gently through the
pulp bed to
minimise any turbulence created by the pickets or their associated dewatering
channels. A
further advantage of the disturbance control invention is that disturbing the
networked pulp
layer 2 in the disturbance zone 16, for example by the application of shear,
tends to inhibit
the formation of donuts in the networked layer.
One example of the disturbance control invention is schematically illustrated
in
Figure 3. The method 20 for controlling shear applied to pulp within a
separation device
comprises the steps of feeding a suspension comprising pulp into a tank of the
separation
device at a flux (step 21), allowing the pulp to separate out of the
suspension (step 22),
submerging a shearing device for shearing pulp at least partially within a
region of the tank
(step 23), and controlling one or more shearing parameters to controllably
apply an optimal
shear to pulp passing through the "shear" region (step 24). This involves
controlling the
shearing device speed (step 25), the depth of the disturbance zone (step 26),
the three-
dimensional shape of the shearing device (step 27), or any combination of
these shearing
parameters, with respect to the flux and/or one or more operational
parameters.
In particular, it has been discovered that optimal shear is obtained by
controlling one
or more of the shearing parameters in accordance with the following equation:
So= f1(X). f2(y).f3(h).f4(f).f5(p) ... (1)
where So is the optimal shear;
fi(X) is the shear factor function;
f2(y) is the shearing device speed function;
f3(h) is the disturbance zone height or depth function;
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f4(f) is the flux function; and
f5(p) is the operational parameter function.
As the shear factor k is a variable representing the average shear applied to
the
aggregates by the shearing device, it is therefore a function of the three-
dimensional shape
and the shearing device speed. Consequently, the shearing device speed is both
a shearing
parameter and a factor influencing the shear factor k.
Therefore, altering the three-dimensional shape of the shearing device will
influence
the optimal shear applied to pulp passing through the disturbance zone through
the shear
factor k. The shearing device may alter its three-dimensional shape by having
movable
shearing elements that can adjust their angle of incidence with respect to the
direction of
movement of the shearing device. Alternatively, the shearing device may have
shearing
elements that can be added or removed during operation to change its three-
dimensional
shape. However, in most practical commercial applications the shearing device
shape has a
predefined shape for simplicity, as providing a shearing device with an
adjustable shape
adds complexity to the design. It is therefore contemplated that the shearing
device speed
and the disturbance zone depth will be commonly selected to comply with the
above
relationship expressed in equation (1) and obtain optimal shear.
The operational parameter function f5(p) represents the one or more variable
operational parameters of the separation device. These operational parameters
typically
comprise the pulp composition, the pulp particle size, the pulp flow velocity
in the tank, the
pulp yield stress, the pulp viscosity, the underflow specific gravity, the
underflow weight
per weight percentage and the rate at which flocculant is added to the
suspension. As the
operational parameter function f5(p) can represent several variables, it is
usually adjusted
where one or more operational parameters are constant, or are assumed to be
constant. For
example, where the pulp viscosity or the underflow specific gravity is known
and does not
vary significantly over the operation of the separation device, the
operational parameter
function f5(p) is then adjusted to f*5(p) multiplied by a constant
representing the known
pulp viscosity or the underflow specific gravity value.
In one embodiment of the disturbance control invention, the selected shearing
parameter(s) are kept proportional to the flux. That is, equation (1) becomes:
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S 2.x yxhx f5(P) (2)
o f
where So is the optimal shear;
X is the shear factor;
y is the speed of the shearing device;
h is the height or depth of the disturbance zone;
f is the flux; and
f5(p) is the operational parameter function.
Where all the operational parameters remain or are assumed to be constant,
equation
(2) becomes:
Axyxhxk
5 = P (3)
f
where So is the optimal shear;
X is the shear factor;
y is the speed of the shearing device;
h is the height or depth of the disturbance zone;
f is the flux; and
kp is a constant representing the operational parameters.
In the method 20, the shearing device speed is initially set to move at the
required
speed, and both the shearing device shape and the disturbance zone depth are
predetermined, in relation to the flux to ensure that an optimal shear is
applied to the pulp.
The disturbance zone depth is controlled simply and directly by controlling
the submersion
of the shearing device, as this will control the extent to which the shearing
device will apply
shear to the pulp. Alternatively, the disturbance zone depth is controlled by
controlling the
level of the suspension in the tank, and thus the extent in which the shearing
device is
submerged. As discussed above, the shearing device shape is usually
predefined, although
it is possible to have shearing devices with movable or adjustable shearing
elements that
change the shearing device shape.
As the disturbance zone depth and the shearing device speed are controlled
through a
suitable control unit or system of the separation device, it is relatively
straightforward for
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these two shearing parameters to be set at the same time to ensure that an
optimal shear is
applied to the pulp during operation of the separation device.
It will be appreciated from equations (1) to (3) that the optimal shear is a
function of
the shearing parameters, operational parameters and the flux. Accordingly, the
selected
shearing parameter(s) can be controlled with respect to one or more of the
operational
parameters of the separation device instead of the flux. The selected shearing
parameter(s)
can also be controlled with respect to both the flux and the operational
parameters.
A particular advantageous example of the disturbance control invention is
illustrated
in Figure 4, where corresponding features have been given the same reference
numerals.
The method 30 is applied to the settling process in a thickener, where at step
21 a slurry
containing a mixture of liquid and pulp aggregates or particles is fed into a
thickening tank
at an initial flux. At step 22, the pulp is then allowed to settle out of the
slurry. At step 23,
a shearing device is submerged within a region of the tank. It will be
appreciated that the
shearing device may be submerged within the region prior to the settling step
22.
The method 30 further comprises monitoring at step 31 the flux of the
suspension
during the feeding step 21, using a suitable sensor (not shown) in
communication with a
control unit or system (not shown) of the thickener. In response to the
monitoring step, the
control unit controls one or more of the shearing parameters relative to the
flux, to apply an
optimal shear to the pulp at step 24, being the shearing device speed (step
32), the depth of
the disturbance zone (step 33), the three-dimensional shape of the shearing
device (step 34),
or any combination of these shearing parameters, to meet the above
relationship expressed
in equations (1), (2) or (3). At step 35, one or more of the operational
parameters are
monitored with corresponding sensors in communication with the control unit or
system.
In response to any change in an operational parameter, the control unit or
system adjusts
one or more of the shearing parameters at step 36 to maintain the relationship
of equations
(1) or (2). In addition, any changes in the flux detected at step 31 may
prompt adjustment
of one or more of the shearing parameters at step 36. This involves adjusting
the shearing
device speed (step 32), the depth of the disturbance zone (step 33), the three-
dimensional
shape of the shearing device (step 34), or any combination of these shearing
parameters, to
maintain the relationship to the changed flux and/or operational parameter(s)
in equations
(1) or (2). Thus, in this embodiment the shearing parameter(s) are adjusted in
response to
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the flux of the incoming suspension and/or operational parameters so that
optimal shear is
applied to the pulp at all times.
The adjusted shearing parameter need not be the same parameter initially
selected at
step 24. For example, the shearing device speed may be controlled initially at
step 32, but
subsequently, the disturbance zone depth is adjusted at step 33 to maintain
its relationship
to the flux, thus maintaining the optimal shear. It will also be appreciated
that the method
may be implemented by using any one of the shearing parameters while keeping
the
remaining shearing parameters constant. For example, the method can be limited
to control
and/or adjustment of the three-dimensional geometry of the shearing device,
while the
shearing device speed and the disturbance zone depth are preset. In this
example, the
shearing device shape can be controlled and/or adjusted by adding or removing
shearing
arms or elements. Alternatively, it is contemplated that one or more shearing
elements are
movable to adjust their angle of incidence to the direction of rotation of the
shearing device.
In addition, the flux monitoring step 31 or the operational parameter
monitoring step
35 may be omitted where it is desired to only control or adjust the shearing
parameters with
respect to only the operational parameter(s) or the flux. However, those
skilled in the art
will recognise that the most accurate control of the shearing parameters is
obtained by
monitoring both the flux and the selected operational parameter(s). In
addition, only one or
some of the operational parameters can be selected for monitoring at step 35
as desired, or
where the other operational parameters are constant or are assumed to be
constant.
Referring to Figure 5, a separation device in accordance with one embodiment
of the
disturbance control invention is illustrated, where corresponding features
have been given
the same reference numerals. The separation device is in the form of a
thickener 40 and
comprises a tank 1, an inlet 41 for feeding the suspension at a flux into the
tank via a
centrally located feedwell 9, and a disturbance causing device in the form of
a shearing
device 42 causing a disturbance substantially uniformly across a disturbance
zone 16 in an
upper region 17 of the networked layer 2, so as to disrupt the networked pulp
3 in the
disturbance zone 16 within a predetermined period of time, thereby releasing
entrained
liquid 14 from the networked pulp in the disturbance zone 16 and increasing
the relative
density of the pulp 18 below the disturbance zone. One or more shearing
parameters are
controlled with respect to the flux and/or one or more operational parameters,
to
controllably apply an optimal shear to the pulp passing through the
disturbance zone 16.
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In this embodiment, the thickener 40 is configured as a bridge-type thickener,
having
a supporting bridge 44 located diametrically across and above the tank 1 and a
circumferential overflow launder 45. A central drive assembly 46 operates a
central drive
shaft 47 to rotate a rake assembly 48 and the shearing device 42 about a
central axis 49 of
the tank 1. The rake assembly 48 comprises rake arms 50 having scraper blades
51
extending downwardly towards the bottom 52 of the tank 1 to move settled and
compacted
pulp towards an underflow outlet 53. The entire tank 1 is supported by columns
54.
The shearing device 42 comprises two outwardly extending radial arms 81, with
a
plurality of shearing elements in the form of angled linear rods or pickets 82
mounted to
each radial arm. The pickets 82 are inclined at an angle of approximately 45
with respect
to a vertical plane and are spaced at uneven intervals 83 to each other, with
the pickets
progressively decreasing in number from the axis of rotation 49 to respective
outer edges
84 of the radial arms 81. This progressive increase in the intervals 83 is in
proportion to the
distance of their associated pickets 82 from the axis of rotation 49. As a
result, the inner
pickets 82a are densely located relative to each other towards the rotational
axis 49,
compared to the outer pickets 82b near the outer edges 84.
In operation, a suspension of pulp in the form of a slurry is fed into the
feedwell 9
through the inlet 41. The slurry may be fed tangentially into the feedwell 9
to improve the
residence time for mixing and reaction with reagents, such as flocculants,
that help create
the aggregates or "flocs" of higher density pulp solids. Tangential entry also
assists in
dissipating the kinetic energy of the slurry in the feedwell 9, thus promoting
settling within
the tank 1. The suspension then flows downwardly under gravity out of a
restricted outlet
64 into the tank 1, where it settles to form the various zones of material,
including the pulp
bed 2, hindered zone 4, free settling zone 6 and clarified zone 8. The
relatively dense pulp
bed 2 displaces the upper clarified zone 8 of relatively dilute liquor towards
the top of the
tank 1. The thickened pulp is drawn off through the underflow outlet 53, while
the dilute
liquor is progressively drawn off through an overflow launder 45.
As the depth of the pulp bed 2 increases to encompass the disturbance zonel6
as part
of its upper half, the shearing device 42 rotates around the tank 1, causing
the pickets 82 to
apply shear substantially uniformly across the disturbance zone 16 to the pulp
aggregates or
particles descending from the feedwell outlet 64 into the disturbance zone 16.
As discussed
above, the disruption of the networked pulp in the disturbance zone 16 results
in the release
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of trapped liquid or liquor and increases the relative density of the pulp
below the
disturbance zone 16. The denser pulp 18 below the disturbance zone 16 tends to
reform a
substantially higher density relative to the pulp above the disturbance zone,
and thus settle
quickly without excessive fractionation and detrimentally affecting the
settling process.
The shear is applied either as direct "hits" from the pickets 55, 61 and 63 or
as disturbances
in the zones of turbulence associated with the wake of the passage of the
pickets 55 through
the disturbance zone 16.
As previously described in relation to Figures 3 and 4, the shearing device
speed, the
depth of the disturbance zone 16, the three-dimensional shape of the shearing
device 42, or
any combination of these shearing parameters are controlled to maintain the
relationship of
equations (1), (2) or (3) with respect to the flux and/or one or more
operational parameters.
Furthermore, these shearing parameters, individually or in combination, are
adjusted in
response to changes in the flux and/or one or more operational parameters to
ensure optimal
shear is continuously applied to the pulp. This results in shear being applied
in a sufficient
amount to the pulp aggregates or particles before they exit the region, to
release trapped
liquid and disturb, "shake", re-arrange or break-up aggregates into aggregates
that settle
quickly without excessively applying shear that would fractionate the
aggregates and
detrimentally affect the settling process. The optimal shear is applied either
as direct "hits"
from the pickets 82 or as disturbances in the zones of turbulence associated
with the wake
of the passage of the pickets 82 through the disturbance zone 16.
The inventors have found that in the invention the shearing parameters of the
shearing
device speed, disturbance zone depth and shearing device shape tend to
dominate the
relationship with the flux over the other possible operational parameters (for
example, the
pulp composition, the pulp particle size, the pulp flow velocity in the tank,
the pulp yield
stress, the pulp viscosity, the underflow specific gravity, the underflow
weight per weight
percentage and the rate at which flocculant (if any) is added to the
suspension) to achieve
an optimal shear profile. However, it is conceivable to apply the disturbance
control
invention so that the shearing parameters are controlled in relation to the
one or more of the
operational parameters instead of the flux. In practice, the shearing
parameters are
controlled in relation to the flux and the operational parameters, so that
these operational
parameters are used to further adjust the shearing parameters of shearing
device speed,
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disturbance zone depth and shearing device shape, to further enhance the
application of
optimal shear to the pulp.
In addition, the specific configuration of the shearing device does not
directly affect
the optimal shear profile that is obtained from the shearing device speed to
flux ratio,
provided that the shearing device is configured to apply shear substantially
uniformly
across the disturbance zone to the networked pulp. It will be appreciated that
the
disturbance control invention can thus be implemented to any shearing device
employed in
a separation device, and so is not limited to a particular shearing device
configuration. The
inventors have, however, determined that there are several preferred
configurations for the
shearing device as they are generally more efficient in achieving an optimum
shear profile,
which are described below.
Thus, the inventors have discovered that the optimal amount of shear that
results in
improved and optimal thickener performance can be achieved primarily where the
shearing
device configuration results in at least one of, or a combination of, the
following:
(1) a substantially uniform cumulative shear being applied to the networked
pulp in
the disturbance zone within a predetermined period of time;
(2) a substantially uniform average shear is applied to the networked pulp in
at
least two intervals between shearing elements spaced apart along at least one
arm of the shearing device, along a line parallel to or coincident with the at
least
one arm; and
(3) a substantially uniform number of shear events is applied to the networked
pulp
in the disturbance zone within a predetermined period of time.
The separation device 40 of Figure 5 has a shearing device 42 that implements
the
concept of substantially uniform cumulative shear. The concept of
substantially uniform
cumulative shear is discussed in more detail below with reference to Figures
5A and 6A,
where corresponding features have been given the same reference numerals.
If a pulp aggregate or particle is settling at a distance f from the centre at
a rate v m
s and the depth of the disturbance zone is d m, then the time taken by the
particle to move
through the disturbance zone is represented by
0 = d/v seconds ... (4)
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Assuming that the disturbance is caused by the application of shear by a
shearing
device 69 having, for example, four rotating radial arms 70 (carrying angled
pickets 72)
mounted on a centre shaft 47 travelling at a rotational speed of co
revolutions per second,
the number of "passes" in time 0 is represented by:
n = 4.6co ... (5)
This number of passes can also be regarded as the number of shear "events"
experienced by each pulp aggregate 13 or particle 12 as the shearing pickets
72 move past.
In this context, the shear applied by any individual picket not only includes
a direct "hit" of
the pulp aggregate by the picket 72 but the disturbance or "shaking" of the
pulp aggregate
caught in the wake of the passage of the picket, which the inventors call a
"zone of
turbulence". These zones of turbulence are sufficient to apply a shear force
to the aggregate
or pulp particle, albeit less than the amount of shear directly applied by the
pickets 72.
Comparing the shearing picket configurations of Figures 6A and 7A, the
probability
of a pulp aggregate 13 or particle 12 being subjected to varying shear rates
during the n
shearing events is greater for the configuration of Figure 6A than the prior
art configuration
of Figure 7A, assuming that the number of shear events is significantly
greater than 1.
Hence, the total shear applied to a layer of settling pulp aggregates or
particles becomes
more uniform as n increases and the angle of the pickets 4 is increased.
However, the
inventors believe that increasing 4 several degrees beyond 45 is not
beneficial because of
fluid flow considerations, and substantially uniform cumulative shear is
optimally obtained
by inclining the shearing elements, such as the pickets 72, at 45 to the
vertical.
With reference to the embodiment of Figure 5, in operation the shearing device
42 is
rotated about the central axis 49 by the central drive shaft 47 to apply a
substantially
uniform cumulative shear to the pulp passing through disturbance zone 16 of
the pulp bed 2
in accordance with principles described above. That is, shearing device 42
makes several
passes through the disturbance zone 16 and the pickets 82 are angled so that
the pulp
aggregates or particles are subjected to several varying shear events, either
by way of a
direct "hit" or being caught in a zone of turbulence, as indicated by
equations (4) and (5).
Thus, the cumulative shear applied to pulp exiting the region 43 is
substantially uniform or
the same.
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Additionally, the inventors have discovered that where the shearing device
comprises
a plurality of shearing elements spaced apart along at least one arm to define
intervals
therebetween, an optimal amount of shear is obtained by providing a
substantially uniform
average shear in at least two intervals along a line parallel to or coincident
with the at least
one arm, and more preferably all the intervals between the shearing elements
along the line.
In most cases, the shearing device will employ two or more outwardly extending
radial arms and thus the substantially uniform average shear applied in the
intervals
between the shearing elements will be along a radial line in alignment with
the radial arms.
In other words, the line along which the substantially uniform average shear
is applied in
the intervals generally corresponds to the profile of the shearing device when
viewed in
plan. However, it will be appreciated that where the shearing device is
partially or fully
non-linear in cross-section, the line will correspondingly be partially or
fully non-linear in
conformity with that cross-section of the shearing device. For example, the
shearing device
may have arms that are sinuous, partially curved or even zigzag-like in shape,
in which case
the substantially uniform average shear would be applied along a sinuous,
partially curved
or zigzag-like line, respectively.
This concept of applying a substantially uniform average shear is discussed in
more
detail below with reference to Figures 5, 6B and 7B. The shearing device 80 of
Figure has
pickets 82 that are spaced at uneven intervals 83 to each other, with the
pickets
progressively decreasing in number from an axis of rotation 49 to respective
outer edges 84
of the radial arms 81. This uneven spacing of the pickets 82 along the radial
arms 81
results in the average shear in the intervals 83 between each pair of pickets
82 being
substantially the same or uniform along a radial line defined by the radial
arms 81. In
particular, the inventors have determined that the shear applied to pulp
aggregates or
particles is generally a function of the linear speed or velocity of the
pickets (or other
shearing elements) and the distance between the picket and the pulp aggregate
or particle.
Since the linear velocity of the picket is also a function of the rotational
speed of the drive
shaft and the distance of the picket from the axis of rotation, the inventors
have determined
that as the distance from the axis of rotation increases, the linear velocity
of the picket
increases proportionately. This relationship between the shear and the
distance between
pickets is described in more detail below with reference to Figures 5B and 6B.
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The shear rate applied to a pulp particle or aggregate by a moving picket is
generally
expressed by:
`2 = k=ux/4 ... (6)
where `is the shear rate in s-',
ut is the linear velocity of the picket in m=s ',
4 is the distance between the picket and the pulp aggregate or particle in
metres, and
k is a constant, which is a function of material properties of the pulp.
Also,
ut = 2irco=f ... (7)
where co is the rotational speed of the shaft in s-1; and
f is the distance from the centre in metres.
Equation (7) can be simplified, by using revolutions per minute (rpm) as the
unit for
the rotational speed co and 2ir being a constant b, to
ue = b-co-f ... (8)
where ut is now expressed in m/min instead of m/s.
Referring to Figures 6A and 6B, equations (6), (7) and (8) indicate that as
the
distance fi, 32, 6, f4, fs, f6 from the axis of rotation 49 increases, the
linear velocity of the
picket 82, ut, increases proportionally as ut (ui, u2, u3, u4, u5, u6) is a
product of 271(0 and tj,
32, 6, f4, fs, 4. For a set of particles (or aggregates 13) between any two
pickets 82, in
order to ensure that the average shear is substantially the same or uniform
along the line
parallel or coincident with the radial arms (ie. along the length of the
radial arm 81), the
spacing (4) between the pickets and the aggregates needs to increase
proportionally to the
linear velocity. That is, the distance or gap (32 - t1, f3 - fz, f4 - t3, f5 -
f4, f6 - f5) between
the pickets 82 is in proportion to their distance fi, 32, f3, f4, fs, f6 from
the axis of rotation
49 along the radial arm 81. Hence, the requirement for a substantially
constant or uniform
average shear can be met by increasing the distance or gap between the pickets
in
proportion to their distance along the radius. By way of contrast, this
substantially constant
or uniform average shear cannot be achieved by means of a set of evenly spaced
pickets or
rods fixed to a radial arm, since the linear speed of any such rod is
proportional to its
distance from the centre, as illustrated in Figures 7A and 7B.
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The configuration of the shearing device 42 results in a substantially uniform
average shear being applied in the intervals 83 between the pickets 42 along a
radial line
defined by the radial arms 81. The outer pickets 82b provide a higher shear
force than the
inner pickets 82a due to the outer pickets 82b having a greater linear
velocity, as indicated
by equations (6), (7) and (8). However, due to the denser distribution of the
inner pickets
82a compared to the outer pickets 82b, aggregates closer towards the axis of
rotation 49 of
the shearing device 80 have a more uniform shear profile over a smaller range
of shear (in
the amount of shear) than that applied to aggregates further from the axis of
rotation 49.
The shear profiles in the intervals 83 toward the outer edges 84 of the radial
arms 81 are
relatively less uniform and extend over a larger range or amplitude of shear
than the shear
profiles closer towards the axis of rotation 49. However, due to the
differential spacing, the
average shear applied to the pulp aggregates in the intervals 83 defined
between the pickets
82 will be substantially uniform across the radial arms 81.
Thus, both the cumulative shear from the total number of shear events and the
average shear between the pickets 82 are each substantially uniform (although
not generally
the same value) due to the arrangement of the angled pickets 82 on the radial
arms 81. This
causes the disruption of pulp aggregates to release trapped liquid, improving
the overall
density of the pulp bed 2, and to create denser aggregates that settle quickly
in the pulp bed
2, thus improving the separation efficiency.
Furthermore, the inventors have unexpectedly discovered that the application
of a
substantially uniform number of shear events across the disturbance zone 16
will also
achieve an optimal shear profile that disrupts the networked pulp, thereby
releasing trapped
liquid 14 and increasing the density of the pulp 18 below the disturbance
zone. The
inventors have discovered that so long as the number of shear events received
by the pulp
passing through the disturbance zone 16 is substantially uniform over a
predetermined
period of time (for example, the period it takes for an x number of
revolutions), then shear
is being applied substantially uniformly across the disturbance zone, as
indicated by
equation (4). Thus, the necessary disruption to the networked pulp is
obtained, along with
the associated release of trapped liquid 14 and increase in the density of the
pulp 18 below
the disturbance zone 16. It follows that a uniform number of shear events does
not require
substantially uniform cumulative shear or substantially uniform average shear
to be applied
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at the same time, since the number of shear events is significant and not the
amount of each
shear event.
Accordingly, Figures 8A, 8B and 8C illustrate shearing devices that achieve a
uniform number of shear events without applying substantially uniform
cumulative shear or
substantially uniform average shear.
In Figure 8A, where corresponding features have been given the same reference
numerals, the shearing device 85 has two outwardly extending radial arms 81,
with a
plurality of shearing elements in the form of angled linear rods or pickets 86
mounted to
each radial arm. The pickets 86 are inclined at an angle of approximately 45
with respect
to a vertical plane and are spaced at even intervals 87 to each other from an
axis of rotation
49 to respective outer edges 84 of the radial arms 81. The shearing device 85
makes
several passes through the disturbance zone 16 and the pickets 86 are angled
so that the
networked pulp aggregates 13 or particles 12 receive the same number of shear
events
between entry and exit of the pulp into and out of the disturbance zone 16.
However, the
even spacing of the pickets 86 means that the average shear in the intervals
87 between
each pair of pickets 86 is not the same or uniform along a radial line defined
by the radial
arms 81. In addition, as the pickets 86 are not arranged to compensate for the
progressive
increase in linear velocity of the pickets 86 towards the outer edges 84 of
the radial arms
81. Thus the amount of shear and therefore the cumulative amount of shear is
not the same
or uniform.
Similarly, in Figure 8B, where corresponding features have been given the same
reference numerals, the shearing device 88 has two outwardly extending radial
shearing
arms 89 that apply shear across their respective lengths, and thus
substantially uniformly
across the disturbance zone 16. As there are no shearing elements other than
the radial
arms 89 that occupy the depth of the disturbance zone 16, there are no
intervals for average
shear nor any way to compensate for the progressive increase in linear
velocity of the
shearing arms 89 towards their respective outer edges 84.
In Figure 8C, where corresponding features have been given the same reference
numerals, the shearing device 90 has two outwardly extending radial arms 81,
with a
plurality of shearing elements in the form of substantially vertical linear
rods or pickets 91
mounted to and equispaced along each radial arm. In this embodiment, the
pickets 91 are
grouped closely together in a tight concentration to increase the area of the
disturbance
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zone 16 to approximately 50% of the cross-sectional area of the upper region
17, and hence
50% of the networked pulp in the upper region, that receives a shear event
during a pass of
the shearing device 90. The shearing device 90 makes several passes through
the
disturbance zone 16 and the concentration of pickets 91 ensures that 50% of
the networked
pulp aggregates 13 or particles 12 receive the same number of shear events
between entry
and exit of the pulp into and out of the disturbance zone 16. As the pickets
91 are
equispaced along the radial arms 81, there is no uniform average shear between
each pair of
pickets 91 along a radial line defined by the radial arms 81. In addition, the
pickets 91 are
not arranged to compensate for the progressive increase in linear velocity of
the pickets 91
towards the outer edges 84 of the radial arms 81, and hence, the amount of
shear.
Consequently, the cumulative amount of shear is not the same or uniform. In
one variation,
another set of radial arms 81 are provided with pickets 91 offset to the
pickets 91 on the
first set of radial arms 81 to apply shear in the intervals and thus
increasing the disturbance
zone 16 to encompass the entire upper region 17 (100%), and thus apply shear
to the entire
(100%) networked pulp passing through the upper region.
The inventors have unexpectedly and surprisingly discovered that it is
particularly
advantageous for the shearing device 42 to be located in the upper half of the
pulp bed 2, as
the liquid is readily able to escape the pulp bed 2 into the clarified zone 8
of dilute liquor.
By way of contrast, applying shear in only the bottom half of the pulp bed 2
will release
liquid upwardly, however, the undisturbed upper layer of the pulp bed tends to
produce a
blanketing effect that hinders or even prevents further upward migration of
the liquid into
the clarified zone 8. Thus, the improved efficiency attained by the shearing
device 42 is not
as effectively achievable in the bottom half of the pulp bed 2, as it is in
the upper portion,
particularly in the upper half. In addition, the shear applied in the
disturbance zone 16 is
not constrained by the need to minimise the rotation speed of the shearing
device 42, as it
has been unexpectedly and surprisingly found that a greater amount of shear
produced by
the increased rotational speed does not adversely affect the compaction of the
pulp solids in
the pulp bed 2. The inventors also contemplate that this advantageous effect
can be
extended to include a portion of the hindered zone 4 above the pulp bed 2,
especially a
lower portion of the hindered zone. This is particularly the case where the
hindered zone 4
is relatively shallow compared to the networked pulp layer 2 or the free
settling zone 6 or
forms an interface between the networked pulp layer 2 and the free settling
zone 6.
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It has also been determined that an optimal shear can be obtained by either
providing
a substantially uniform cumulative shear, a substantially uniform average
shear between the
shearing elements or a substantially uniform number of shear events
independently of each
other, or a combination of any two or all three.
In particular, it has been discovered that an advantageous implementation of
the
disturbance causing step 27 is to apply shear substantially uniformly across
the disturbance
zone 16. The mechanism by which shear is applied in the disturbance causing
step 27 can
take a number of forms. For example, one shearing mechanism is to use liquid
or gas jets
to inject a liquid or gas towards, into or through the disturbance zone 16 to
apply shear
substantially uniformly across the disturbance zone. Similarly, a fluidiser
can direct fluid
flow towards, into or through the disturbance zone 16 to apply shear
substantially
uniformly across the disturbance zone. Other shearing mechanisms include
subjecting the
disturbance zone 16 to mechanical vibration using a suitable vibratory
apparatus or
ultrasonic impulses to apply shear substantially uniformly across the
disturbance zone.
While these shearing mechanisms are suitable for implementing the disturbance
causing
(shearing) step 27 in the method of the disturbance control invention, the
inventors have
determined that a preferred shearing mechanism is mechanical agitation,
advantageously by
way of a shearing device that moves through the disturbance zone 16 to apply
shear
substantially uniformly across the disturbance zone. In one preferred form,
the shearing
device is rotated in the tank in accordance with the disturbance causing
(shearing) step 27.
While the above discussion of the disturbance control invention describes
three
particularly preferred configurations for the shearing device, other shearing
device
configurations can be designed to obtain an optimal shear profile. Design of
these
alternative shearing device configurations involves testing the effectiveness
of a specific
shearing device configuration in a test apparatus, as well as determining the
optimal
shearing device speed and/or flux of the suspension. While this may seem
straightforward,
the inventors have found in practice that conventional testing regimes have
been generally
unsatisfactory in obtaining accurate results for the performance of a specific
shearing
device configuration. In particular, the inventors have found that the shear
applied by a test
shearing device in a test tank does not correlate to the shear that is applied
by a shearing
device in a thickener. This is due to the fact that shear is dependent on the
linear velocity u
of the shearing device and linear velocity u is in turn dependent on the
rotational speed w
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and the distance f from the centre of the thickening tank, as outlined in
equations (6) and
(7) above. This means that to properly simulate the shear applied at a
designated distance
greater than the radius of the test tank, it is necessary to have the test
shearing device move
at the same linear velocity as the linear velocity of the shearing device at
the designated
distance. This would involve increasing the rotational speed w to compensate
for the
reduced distance f in the test tank. This causes a further problem in that
increasing the
rotational speed w of the test shearing device results in a decrease in the
period in which the
test shearing device performs a cycle in the test tank. This decreased time
period does not
equate to the time period in which the full scale shearing device performs a
cycle in the
thickener. While the test results could be manipulated statistically to obtain
approximate
results, this necessarily introduces errors into the data and thus reduces the
accuracy of the
test results. Therefore, the inventors have developed a method for accurately
testing a
shearing device by simulating the expected conditions for optimal shear in a
full scale
thickener for a portion of the shearing device in a test apparatus.
Referring to Figure 9, one embodiment a method of testing a shearing device
for a
separation device according to the present invention is schematically
illustrated. In the
method 100, a test tank 101 is provided for containing the fluid comprising
the pulp (for
example, a feed slurry) at step 102. A portion 103 of the shearing device 42
is submerged
at least partially within the fluid held in the test tank 101 at step 104. As
best shown by a
comparison of Figure 5 with Figure 9, the shearing device portion 103 is a
full scale "slice"
or portion of the shearing device 42 and is not a scaled down version of the
shearing device.
Using a full scale portion 103 of the shearing device 42 is preferred in order
to obtain the
most accurate test results, as it provides the same shear profile as the
shearing device 42.
A first speed of the shearing device 42 is calculated at step 105 for a first
time period
in which the shearing device is expected to apply an optimal shear to the pulp
in the
disturbance zone 16 at a first predetermined distance. In this embodiment, the
first speed is
the rotational speed of the shearing device 42, as this determines the linear
speed or
velocity of the shearing device 42 at the first predetermined distance. The
first time period
is the time in which the shearing device travels the first predetermined
distance at its linear
velocity. The linear speed is frequently determined by controlling the
rotation speed CO of
the shearing device 42. However, it will be appreciated that in other
embodiments, the first
speed is the linear speed or velocity, rather than the rotational speed. The
shearing device
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portion 103 is positioned at a second predetermined distance in the test tank
101 at step
106. For the purposes of the calculation, the second predetermined distance is
taken to be
the distance from the centre of rotation 49 to the outer edge of the shearing
device portion
103. A second speed for the shearing device portion 103 at the second
predetermined
distance is calculated at step 107 that corresponds to the first speed at the
first
predetermined distance. In this embodiment, the second speed is the rotational
speed of the
shearing device portion, and is calculated so that the corresponding linear
velocity of the
shearing device portion 103 is equal to the linear velocity of the shearing
device 42 at the
first predetermined distance. This corresponding relationship is based on
equations (7) and
(8). As shear is proportional to the linear velocity u, if the shearing device
portion 103 and
the shearing device 42 have the same linear velocity, then they will apply the
same shear
rate. This enables the same amount of shear that is applied at the first
predetermined
distance to be applied at the second predetermined distance, since the
shearing device
portion 103 has the same profile as the shearing device 42.
While equation (7) appears to suggest a simple proportional relationship, the
inventors have discovered that the calculation of the rotational speed of the
shearing device
portion 103 has to be adjusted for two primary factors. Firstly, the distance
travelled by the
shearing device portion 103 in one revolution of the test tank 101 at the
linear velocity ut is
equal to the circumference Ct of the test tank, as best shown in Figure 10A.
However, the
distance travelled by the shearing device 42 at the linear velocity ut at the
radial distance f
will be less than, the same as or greater than the circumference Ct of the
test tank 101. The
shearing device 42 would travel along a circumferential arc of a circle Ct
that is equal to Ct
at the radial distance f. Therefore, the inventors have determined that the
rotational speed
for the shearing device portion 103 needs to be adjusted by a scaling factor E
equal to the
ratio of the circumference of the circle Q travelled by the shearing device 42
at the linear
velocity ut to the circumference Ct of the test tank 101 so that their
respective linear
velocities are equal. That is,
E = Ct/Ct ... (9)
where r, is the scaling factor;
Q is the circumference of the circle travelled by the shearing device 42 at
the linear velocity ut in metres; and
Ct is the circumference of the test tank in metres.
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The relationship in equation (9) is derived from the fact that the
circumference Q
forms part of a circle having a circumference Q = 27rf, as best shown in
Figure I OB. Thus,
from equations (7) and (8),
ut = 27rco*=f = 27r=Q=cot = b=Cvw ... (10)
Similarly, the linear velocity of the shearing device portion 103 in the test
tank 101 is
also expressed as
ut = 27rcot'ft = 2i=Ct=cot = b=Ct=cot ... (11)
As discussed above, for the magnitude of shear to be the same at radial
distance ft in
the test tank 101 and at the radial distance f in the thickener, ut = ut. This
means that
ut = ut
b-Ctiwt = b-Ce o
Ct=wt = C;o
wt/we = CeiCt ... (12)
In addition, as Q = 27uf and Ct = 27r-ft, Ce1Ct = 27uf/27r=ft = f/ft. Thus,
for ut = ut,
equation (12) can be rewritten as
wtiwe = CeiCt = f/ft ... (13)
Thus, the scaling factor r, can be calculated from either the ratio of the
respective
circumferences of the test tank and thickener, the ratio of the radial
distances of the
shearing device portion 103 and the shearing device 42 from their respective
axes of
rotation 49 or the ratio of the rotational speeds of the shearing device
portion 103 and the
shearing device 42. In addition, these ratios remain the same whenever the
linear velocities
of the shearing device portion 103 and the shearing device 42 are the same.
From equation (13), it can be seen that this adjustment for the rotational
speed of the
shearing device portion can be calculated by directly applying the scaling
factor r, to the
rotational speed cot of the shearing device 42. Thus, as part of step 107, the
rotational speed
for the shearing device portion 103 is calculated as:
wt = E=wt ... (14)
where cot is the rotational speed of the shearing device portion 103 in the
test tank
101 in rpm;
E is the scaling factor from equation (9); and
cot is the rotational speed of the shearing device 42 in rpm.
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The second adjustment that is required relates to the fact that the rotational
speed cot
of the shearing device portion 103 tends to be greater than the rotational
speed cot of the
shearing device, where the second predetermined distance is less than the
first
predetermined distance. This means that the shearing device portion 103 is
providing a
higher frequency or number of shear events at the second predetermined
distance over the
first time period, resulting in excess shear being applied. The magnitude of
each shear
event ( ) is the same for the shearing device 42 and the shearing device
portion 103
because the latter is a full scale "slice" of the former. However, the
frequency of shear
events for the shearing device 42 and the shearing device portion 103 are not
the same and
are proportional to the scaling factor E. Accordingly, a second time period
for moving the
shearing device portion 103 at the second speed is calculated at step 108 that
corresponds to
the first time period at the first predetermined distance. That is, in step
108 the second time
period for the shearing device portion 103 moving at the second rotational
speed cot is
calculated as a proportion of the first time period in the following manner
set out below in
more detail.
The circumference of the test tank 101, typically being smaller than the
thickener
tank, means that Ct is a proportion of Ct. Thus, there are Ce1Ct sections of
Ct in Ct, as best
shown in Figure I OC. As ut = ut, the time for one shear event to occur in a
circumferential
arc equal to Ct is tct and the time for one shear event to occur in the
thickener having the
circumference Ct is tct.
As the distance travelled by the shearing device 42 is equal to the
circumferential arc
Ct and linear velocity is equal to distance travelled over time, then
ut = Ct/tct
and so
tct = Ct/ut
= 27rft/ut
From equation (11), ut = 2iuot=ft and therefore
tct = 2ift/2icot'ft
= 1/cot ... (15)
Similarly,
tc = 1/cot=N ... (16)
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where N is the number of arms of the shearing device 42.
This N term in equation (16) takes into account where the shearing device 42
has
more than one shearing arm 81.
Dividing equation (15) by equation (16),
tct/tct = N=wt/wt
and so,
tct = N x w,/wt X tct ... (17)
or
tc = N=tc/E ... (18)
Where N = 1 (ie. one shearing arm), then equation (18) simplifies to tc = tct
/E.
The second time period that is calculated at step 108 is the same as the time
tct,
because it is the time period in which the shearing device 42 would apply
shear in a
circumferential arc equal to the circumference Ct of the test tank 101 at the
radial
distance f. The inventors call this second time period tc the shear or "on"
time period, to,,,
as it is the time in which the shearing device portion 103 is moved in the
test tank 101.
The time difference between the first time period and the second time period
is then
calculated at step 109. The inventors call this time difference the "off' time
period, tog,
since it is the time in which movement of the shearing device portion 103 is
stopped. That
is,
toff = time for one cycle or revolution in the test tank - ton
tC-ton ... (19)
The shearing device portion 103 is then moved at the calculated second
rotational
speed for the second time period at step 110. Movement of the shearing device
portion 103
is then stopped for the time difference toff at step 111. The moving step 110
and the
stopping step 111 simulate the application of shear by the shearing device 42
at the first
predetermined distance in the separation device over the first time period.
At step 112, the method determines whether to repeat steps 110 and 111. If so,
then
the method returns to steps 110 and 111 and this may be repeated successively
to acquire
statistically meaningful test results for the shearing device portion 103 and
to allow for
aggregate movement through the disturbance zone 16. Once the required number
of cycles
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of steps 110 and 111 has been completed (which can include a single cycle if
desired), then
the underflow of settled pulp is withdraw from the test tank 101 and measured
at step 113
to determine the effectiveness of the shearing device portion 103 in improving
settling
efficiency at the second predetermined distance. Due to the simulation of
shear in a full
scale thickener, these results can be directly applied for the shearing device
at the first
predetermined distance.
The method 100 can then be repeated for another first predetermined distance,
essentially by returning to step 105 to enable the recalculation of another
linear speed for
the shearing device 42 at a different predetermined distance for the same
first time period or
another time period, at the same distance but with a different rotational
speed, or any
combination of these parameters. In this way, the method 100 can successively
test the
shearing device 42 at various predetermined distances and/or rotational speeds
in the
separation device using a single test tank 101.
However, the inventors have determined that it is not necessary to repeat the
tests of
the shearing device portion 103 at different radial distances because the
amount of shear
applied per pulp particle is substantially the same for both the test tank 101
and the full
scale thickener 40. Calculations performed by the inventors have established
that the
amount of shear applied in the test tank 101, as moves from the inner end of
the shearing
device portion 103 to its outer extremity is equivalent to the amount of shear
applied by the
shearing device 42 in the thickener 40 from its inner end of the shearing
device arm 81 to
its outer end 84. This relationship holds where the linear or "tip" speeds of
the respective
shearing arms are essentially the same and the shearing device profiles are
the same (that is,
the size of the pickets, distance between pickets and overall geometrical
shape of the
shearing device).
Consequently, a series of tests simulating the shear applied by the shearing
device
portion 103, and by extension the shearing device 42, along various radial
distances is
unnecessary due to the shearing device portion 103 being a full scale "slice"
of the shearing
device 42, and thus having the same shearing device profile. Rather, the
inventors
contemplate that testing the shearing device portion 103 at the same tip speed
or "outer
circumferential" (linear) speed as the outer end 84 of the shearing arm 81 of
the shearing
device 42 would obtain the most accurate results. Even so, it is still
necessary to perform
the method 100 and in particular the rotational speed calculation step 107,
the second time
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period calculation step 108 and the time difference calculation step 109 for
the test tank
101.
Thus, the method 100 simulates the operational speeds and time periods in
which the
shearing device 42 applies shear in a separation device, for example, a
thickener. By
moving the shearing device portion 103 at the second rotational speed so that
the linear
velocity of its outer edge is the same as the linear velocity of the shearing
device at a
predetermined distance, and adjusting the second time period to take into
account the
differences in the circumferential arc lengths travelled by the shearing
device portion 103
and the shearing device 42, the method 100 reproduces the shear that would be
applied at
the first predetermined distance in the test tank 101. Furthermore, the
intermittent motion
of the shearing device portion caused by the stopping step 111 ensures that
the shearing
device portion 103 does not apply more shear than would occur at the first
predetermined
distance, thus ensuring that the correct amount of shear is applied to the
pulp at the second
predetermined distance and that the measuring step 113 obtains the correct
value for the
underflow density for the settled pulp. In other words, stopping movement of
the shearing
device portion 103 for the time difference simulates the time that passes
between successive
applications of shear to any one section of pulp by the shearing device 42 at
the first
predetermined distance.
By way of contrast, where scaled down versions of a shearing device and a
thickener
are used as a test apparatus under a conventional test regime, it is clear
from equations (6)
and (7) that the results obtained for the conventional test apparatus cannot
be readily
extrapolated or adjusted to fit a practical in situ shearing device in a full
scale thickener.
This is because the amount of shear that is applied in the conventional test
apparatus is not
the same as the amount of shear that is applied at radial distances greater
than the maximum
radius of the test apparatus. The amount of shear cannot be adjusted, since
what is
measured is the effect of the shear upon the underflow density of the settled
pulp, so it is
difficult to determine how to adjust the measured underflow density values for
the variance
in shear. Moreover, the shearing device in the test apparatus is typically a
scaled down
version of the actual shearing device and therefore does not apply the same
amount of shear
as a full scale portion of the shearing device at the radial distance of the
thickener tank.
As equation (7) can enable one to determine the linear velocity of the
shearing device
at a specific radial distance f from its axis of rotation 49, this enables the
method 100 to
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calculate the rotational speed of the shearing device portion 103 in the test
apparatus to
reproduce the same expected amount of shear at the specific radial distance f.
However,
simply adjusting the rotational speed of the shearing device portion 103 as
described above
is insufficient to replicate the amount of shear at the first predetermined
distance. As
discussed above, the method 100 reduces the time period for moving the
shearing device
portion 103 to take into account its increased rotational speed and the
reduced
circumference Ct of the test tank 101, using equations (9), (13), (14), (18)
and (19). Thus,
by moving the shearing device portion 103 at a different rotational speed to
the rotational
speed of the shearing device 42 at the first predetermined (radial) distance,
but over a
reduced time period, the shearing device portion 103 is able to travel at the
same linear
speed as the shearing device 42 over the first time period, thus reproducing
the same
amount of shear that is applied at the first predetermined distance in the
test apparatus.
It will be appreciated that in the method 100, the second time period reflects
the time
between successive applications of shear to pulp at any particular point on
the
circumference of the tank of the separation device, described by the circle
having the
specified radial distance. That is, for any section of fluid or pulp in the
circumference,
there is an interval between successive applications of shear by the shearing
device 42 as it
travels around the circle. Therefore, to reproduce the correct number of shear
events for a
section of pulp at any particular part of the circumference of the circle that
has a radial
distance f in the test tank 101, movement of the shearing device portion 103
must be
intermittently paused between successive movements where more than one cycle
is
performed.
Furthermore, steps 105 and 106 can performed in reverse order (ie. step 106
before
step 105) or at the same time. In practice and for convenience, it is
preferred to fix the
second predetermined distance of the shearing device portion 103 at the radius
of the test
tank 101.
Referring now to Figures 11 and 12, a test apparatus 120 in accordance with
one
embodiment of the invention is illustrated, where corresponding features have
been given
the same reference numerals. The test apparatus 120 comprises a one metre
diameter tank
121, an inlet 41 for feeding a suspension comprising pulp at a flux into the
tank via a
centrally located feedwell 9 and a shearing device portion 103 for shearing
pulp within the
tank 121. As described earlier, the shearing device portion 103 is a full
scale portion of the
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shearing device 42 of Figure 4 that is installed in a full size thickener.
Accordingly, the
shearing device portion 103 provides a more accurate simulation of the shear
that would be
applied by the shearing device 42 than a scaled down version of the shearing
device.
A central drive assembly 46 operates a central drive shaft 47 to rotate a rake
assembly
48, comprising rake arms 50 having scraper blades 51 extending downwardly
towards the
bottom 52 of the tank to move settled and compacted pulp towards an underflow
outlet 53.
The test tank 121 is supported by columns 54. In addition, a feed pipe 122
connects the
feed inlet 41 to a supply of suspension used in the test apparatus 120. A
launder 45 is
provided to collect diluted liquid from the top of the test tank 121.
EXAMPLES
Referring now to Figure 13, operation of the test apparatus 120 and
implementation
of the method 100 according to the embodiment of the invention will now be
described in
relation to several examples, where corresponding features have been given the
same
reference numerals. These examples were conducted as separate tests, in which
the
shearing device portion 103 was used to simulate different segments of the
shearing device
42. Figure 13 schematically illustrates the test apparatus 120 having a
diameter of 1 m
superimposed upon a thickener that has a tank 1 with a diameter of 24 m and
hence a radius
of 12 m.
In the examples, the shearing device 42 has six separate shearing arms 81 that
extend
radially outwardly from the centre of the thickener tank 1. Each shearing arm
81 has a
plurality of shearing elements in the form of pickets 82. The shearing arms 81
are
equispaced from each other at approximately 60 . The shearing device portion
103 is a full
scale slice of one of the shearing arms 81.
For the sake of simplicity and ease of reference, the rotational speeds of the
single
shearing device 42 and the single shearing device portion 103 will be
expressed in
revolutions per minute (rpm). In the following examples, the rotational speed
of the single
arm of the shearing device 42 in the thickener will be taken to be 0.167 rpm,
which
corresponds to a typical speed in a full scale thickener. At this rotational
speed, the
shearing device 42 completes a revolution of the tank 1 in a first time period
or cycle of
5.988 z 6 minutes. This means that the fluid at any radial distance f would
receive six
applications of shear for every six minute cycle.
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As noted above, the test tank 121 has a diameter ~t = 1 m and hence a radius
or radial
distance ft = ~t/2 = 1/z m and a circumference of Ct = #t t = it z 3.14 m. In
addition, the
outer position of the shearing device portion 103 is fixed at the radial
distance ft = t/2 m in
the test tank 121.
Example 1
In accordance with the method 100 and step 105, a radial distance fi = 12 m
for the
shearing device 42 in the full scale thickener tank 1 is selected for testing
at a rotational
speed of 1/6 or 0.167 rpm. Thus, the linear velocity of the shearing device 42
in the
thickener at fi = 12 m would be ui = b=coe=ft = 2ir x 0.167 rpm x 12 m = 247E
x 0.167 m/min
12.59 m/min.
The rotational speed for the shearing device portion 103 at ft = t/2 m to
apply the same
shear as the shearing device 42 at fi = 12 m is then calculated in accordance
with step 107.
As discussed above, this is achieved by equalising the linear velocities of
the outer edge of
the shearing device portion 103 and the shearing device 42 at ti = 12m; that
is, setting ut =
ui. To achieve this, the rotational speed cot is adjusted by the scaling
factor r, relating to the
ratio of the circumference Ct of the test tank 121 to a circumferential arc Ci
traversed by the
shearing device 42 at the linear velocity ui = 24ir x 0.167 m/min z 12.59
m/min. At a
radial distance fi = 12 m, the shearing device 42 would transcribe a
circumference of a
circle Ci =24i z 75.40 m. Therefore, the scaling factor r, = Ct/Ct = 24ir/i z
75.40/3.14 = 24
and the rotational speed for the shearing device portion 103 is:
cot = Ei.(O
=24 x 0.167=4rpm
If the shearing device portion 103 were allowed to rotate at 4 rpm in the same
one
minute cycle as the shearing device 42, then it would perform 4 revolutions in
a one minute
cycle. By way of contrast, the shearing device 42 would only perform 0.167 of
a revolution
in a one minute cycle, assuming a six-arm shearing device would apply one
shear event in
this time. This would result in the shearing device portion 103 applying at
least four times
the number of shear events to the pulp than the shearing device 42 because it
is rotating
faster in the test tank 121 compared to the shearing device 42 in the
thickener.
Consequently, to ensure that the pulp receives the same frequency or number of
applications of shear (or shear events) over the same one minute cycle, at
step 108 a "shear"
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or "on" time period is calculated for the shearing device portion 103 to apply
shear at
ft = 1/2 m in an equivalent frequency of shear events as the shearing device
42 applies at ti =
12 m for 1 minute. This is achieved by reducing the shear or on time period in
proportion
to the increase in the rotational speed so that the same frequency or number
of shear events
is performed by the shearing device portion 103 as the shearing device 42.
Also, the shear
time period needs to be adjusted by the number of arms (N) on the shearing
device 42,
because the shearing device portion 103 is only a slice of one of these arms.
Since cot is
expressed in revolutions/min, then the shear time period ton = N=cot/cot = 6 x
0.167/4 min =
s.
10 In order to properly simulate the time between successive applications of
shear by the
shearing device 42, at step 108, the time difference between the shear time
period ton and
the cycle ti for the shearing device 42 is calculated to obtain a pause or
"off' time period in
which the shearing device portion 103 is stopped and thus prevent the further
application of
shear to the fluid in the cycle ti. For fi = 12 m, the off period for the
shearing device
15 portion 103 is thus calculated as toff = ti - ton = 1 min - 15 s = 45 s.
Thus, in accordance with step 110, the shearing device portion 103 at ft = 1/2
m is
operated at cot = 4 rpm for ton = 15 s to reproduce the same linear velocity
ui = 12.59 m/min
as the shearing device 42 at fi = 12 m rotating at cot = 0.167 rpm for ti = 1
minute. The
shearing device portion 103 is then paused for toff = 45 s in accordance with
step 111. At
step 112, a decision is made whether to repeat the rotation step 110 and the
stopping step
111. Where more than one rotation is required for test purposes (and usually
is to obtain
more statistically meaningful data and to allow for pulp aggregate movement
through the
disturbance zone), rotation of the shearing device portion 103 at step 110 is
resumed,
followed by the required stopping step 111. When the desired number of
"on"/"off" cycles
has been completed, then the underflow of settled pulp is removed from the
test tank 121
and its density is measured at step 113 to determine the effectiveness of the
shearing device
portion 103, and hence the shearing device 42, in applying an optimal shear to
the pulp.
Example 2
In this example, the radial distance fz = 9 m is selected for the shearing
device 42 in
the full scale thickener tank 1 for testing at step 105 of the method 100.
Thus, the linear
velocity of the shearing device 42 is u2 = bio 32= 2ir x 0.167 rpm x 9 m =
187E x 0.167
m/min z 9.44 m/min.
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The rotational speed for the shearing device portion 103 at ft = t/2 m to
apply the same
shear and frequency of shear events as the shearing device 42 at f'2 = 9 m is
then calculated
in accordance with step 107 by equalising the linear velocity ut of the
shearing device
portion 103 with the linear velocity u2. At a radial distance f'2 = 9 m, the
shearing device 42
would transcribe a circumference of a circle C2 = 18i z 56.55 m. Therefore,
the scaling
factor Ez = C2/Ct = 187r /t z 56.55/3.14 = 18 and the rotational speed for the
shearing device
portion 103 is:
COt = 82* (0t
=18X0.167=3rpm.
As discussed in example 1 above, this means that if the shearing device
portion 103
were allowed to rotate at 3 rpm for the same one minute cycle as the shearing
device 42,
then it would perform 3 revolutions compared to the shearing device 42 only
performing
0.167 of a revolution in the one minute cycle. This would result in the
shearing device
portion 103 applying three times the number of shear events to the pulp than
the shearing
device 42 because it is rotating faster in the test tank 121 compared to
shearing device 42 in
the thickener. Consequently, to ensure that the pulp receives the same
frequency or number
of applications of shear (or shear events) over the same one minute cycle, at
step 108 the
"shear" or "on" time period is calculated so that the shearing device portion
103 applies
shear at ft = t/2 m in an equivalent amount of time as the shearing device 42
applies at fz = 9
m for 1 minute. Again, this is achieved by reducing the shear or on time
period in
proportion to the increase in the rotational speed so that the same frequency
of shear events
is performed by the shearing device portion 103 as the shearing device 42.
Also, the shear
time is adjusted for the increased number of arms on the shearing device 42
compared to
the shearing device portion 103. Thus, the shear time period ton = N=cot/cot =
6 x 0.167/3 =
0.334 min = 20 s.
Once again, to properly simulate the time between successive applications of
shear by
the shearing device 42, at step 108, the time difference between the
calculated shear time
period ton and the cycle t2 for the shearing device 42 is calculated to obtain
the pause or
"off' time period in which the shearing device portion 103 is stopped and thus
prevent the
further application of shear to the fluid in the cycle t2. For fz = 9 m, the
shearing device
portion 103 should be stopped for a period toff = t2 - ton = 1 min - 20 s = 40
s. Once again,
steps 110 and 111 may be repeated where more than one rotation is required for
test
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purposes. When the desired number of "on"/"off' cycles have been completed,
then the
underflow of settled pulp is removed from the test tank 121 and its density is
measured at
step 113 to determine the effectiveness of the shearing device portion 103,
and hence the
shearing device 42, in applying an optimal shear to the pulp.
Thus, in accordance with step 110, the shearing device portion 103 at ft = 1/2
m should
be operated at cot = 3 rpm for ton = 20 s to reproduce the same linear
velocity ui = 9.44
m/min as the shearing device 42 at fz = 9 m rotating at cot = 0.167 rpm for t2
= 1 minute.
The shearing device portion 103 is then paused for toff = 40 s in accordance
with step 111.
Again, at step 112, the method 100 determines whether more than one rotation
is required
for test purposes. If so, then rotation of the shearing device portion 103 at
step 110 is
resumed, followed by step 111. When the desired number of "on"/"off' cycles
has been
completed, then the underflow of settled pulp is removed from the test tank
121 and its
density is measured at step 113 to determine the effectiveness of the shearing
device
portion 103, and hence the shearing device 42, in applying an optimal shear to
the pulp at
the rotational speed (02.
Example 3
In this example, the radial distance f3 = 6 m is selected for the shearing
device 42 in
the full scale thickener tank 1 for testing at step 105 of the method 100.
Thus, the linear
velocity of the shearing device 42 is u3 = b=wr= e3= 2ir x 0.167 rpm x 6 m =
12ir x 0.167
m/min z 6.29 m/min.
The rotational speed for the shearing device portion 103 at ft = 1/2 m to
apply the same
shear force as the shearing device 42 at f3 = 6 m is then calculated in
accordance with step
107 by equalising the linear velocity ut of the shearing device portion 103
with the linear
velocity u3 = 6.29 m/min. At a radial distance f3 = 6 m, the shearing device
42 would
transcribe a circumference of a circle C3 = 12ir m z 37.70 m. Therefore, the
scaling factor
E3 = C3/Ct = 127r/7r = 12 and the rotational speed for the shearing device
portion 103 is cot =
E3=(043=12X0.167=2rpm.
Again, if the shearing device portion 103 were allowed to rotate at 2 rpm in a
one
minute cycle, then it would perform 2 revolutions, whereas the shearing device
42 would
only perform 0.167 of a revolution. As the shearing device 42 has six radial
arms 81
applying shear to the pulp, it applies one shear event per minute. This would
result in the
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shearing device portion 103 applying twice the number of shear events to the
pulp in the
fluid than the shearing device 42 because it is rotating faster in the test
tank 121, but has
one sixth of the number of shearing arms, compared to the shearing device 42
in the
thickener.
Consequently, to ensure that the pulp in the fluid receives the same frequency
or
number of applications of shear (or shear events) over the same cycle for each
shear event,
at step 108 the "shear" or "on" time period is calculated so that the shearing
device portion
103 applies shear at ft = 1/2 m in an equivalent amount of time for each
radial shearing arm
81 of the shearing device 42 to apply shear at f3 = 6 m. Again, this is
achieved by reducing
the shear or on time period in proportion to the increase in the rotational
speed so that the
same frequency or number of shear events is performed by the shearing device
portion 103
as the shearing device 42. Also, the shear time is adjusted for the increased
number of arms
on the shearing device 42 compared to the shearing device portion 103. Thus,
the shear
time period ton = N=cot/cot = 6 x 0.167/2 = 0.5 min = 30 s.
In order to properly simulate the time between successive applications of
shear by the
shearing device 42, at step 108, the time difference between the shear time
period ton and
the cycle t3 for the shearing device 42 is calculated to obtain a pause or
"off' time period in
which the shearing device portion 103 is stopped and thus prevent the further
application of
shear to the pulp in the fluid in the cycle t3. For f3 = 6 m, the shearing
device portion 103
should be stopped for a period toff = t3 - ton = 1 min - 30 s = 30 s.
Thus, in accordance with step 110, the shearing device portion 103 at ft = 1/2
m is
operated at (03 = 2 rpm for ton = 30 s to reproduce the same linear velocity
u3 = 127E x 0.167
m/min z 6.29 m/min as the shearing device 42 at f3 = 6 m rotating at cot =
0.167 rpm for t3
= 1 minute. The shearing device portion 103 is then paused for toff = 30 s to
take into
account the six shearing arms 81 of the shearing device 42, in accordance with
step 111.
Again, steps 110 and 111 may be repeated at step 112 where more than one
rotation is
required for test purposes. When the desired number of "on"/"off' cycles have
been
completed, then the underflow of settled pulp is removed from the test tank
121 and its
density is measured as per step 113 to determine the effectiveness of the
shearing device
portion 103, and hence the shearing device 42, in applying an optimal shear to
the fluid at
the rotational speed (03.
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Example 4
In yet another example, the radial distance f4 = 3 m is selected for the
shearing device
42 having six radial shearing arms 81 in the full scale thickener tank 1 for
testing at step
105 of the method 100. Thus, the linear velocity of the shearing device 42 is
u4 = b=wr=f4= 2ir x 0.167 rpm x 3 m = 6ir x 0.167 m/min z 3.14 m/min.
The rotational speed for the shearing device portion 103 at ft = 1/2 m to
apply the same
shear as the shearing device 42 at f4 = 3 m is then calculated in accordance
with step 107
by equalising the linear velocity uq of the shearing device portion 103 with
the linear
velocity u4 = it m/min z 3.14 m/min. At a radial distance f4 = 3 m, the
shearing device 42
would transcribe a circumference of a circle C4 = 6ir m z 18.85 m. Therefore,
the scaling
factor E4 = C4/Ct = 67r/7r or 18.85/3.14 = 6 and the rotational speed for the
shearing device
portion 103 is Wt = 84=(O = 6 x 0.167 = 1 rpm. As in example 3 above, the
shearing device
42 has six radial shearing arms 81 and so will provide one shearing event per
minute. This
means that the frequency of shear events at the rotational speed wt for the
shearing device
portion 103 is the same as the frequency of shear events at the rotational
speed wt of the
shearing device 42, despite the differences in radial distance.
The shearing device portion 103 thus applies shear to the pulp in the fluid
the same
number of times as the shearing device 42 having six radial arms. Therefore,
at step 108
the time shear or "on" period is calculated so that the shearing device
portion 103 applies
shear at ft = 1/2 m in an equivalent frequency of shear events as the shearing
device 42
applies at f4= 3 m for 1 minute. Thus, the shear or on time period ton = N-
wt/wt = 6 X
0.167/1 min= 60 s. Thus, there is no time difference between the two periods
as toff = t4 -
ton = 1 - 1 min = 0, meaning that the shearing device portion 103 may be
rotated in the test
tank 121 continuously without stopping and still reproduce the same number of
shear
events and the same amount of shear as would be applied by the shearing device
42 having
six radial shearing arms 81 at the radial distance f4 = 3 m for a 1 minute
cycle.
Consequently, with the data generated from the operation of the test apparatus
120 in
this method 100, various configurations of the shearing device can be designed
so as test
whether an expected optimal shear is applied by each configuration of the
shearing device
at the calculated shearing device speed for selected radial distances. The
intermittent
rotation of the shearing device portion 103 in the test apparatus 120 provides
a more
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accurate set of data for designing the shearing device in a full scale
thickener, as opposed to
extrapolating results for a scaled down version of the shearing device
operated continuously
in the test apparatus 120.
Further tests were conducted using examples 3 and 4. That is, a portion of a
full scale
six arm shearing device was rotated in a 1 m diameter tank to simulate the
performance of
the six arm shearing device in a 24 m diameter thickener. A summary of the
results
obtained with these further tests are set out in Table 1 below.
Table 1: Test Results for Examples
"Full
Scale" "Full Shearing shearing Shearing
shearing Scale" device device Shear time Pause or device
device - shearing portion portion or "On" "Off'
distance device tip 1 revolution rotational period period portion tip
from speed Cycle speed speed
centre
3.14 m
3.0 m 3.14 m/min continuous 1 RPM @ 3.14 None 3.14 m/min
m/min
30s"on" 3.14m 1x3.14m
6.0 m 6.28 m/min 30s off' 2 RPM @ 6.29 @ 6.29 6.29 m/min
m/min m/min
20s "on" 3.14m 2x3.14m
9.0 m 9.42 m/min 40s "off' 3 RPM @ 9.44 @ 9.44 9.44 m/min
m/min m/min
12.0 m 12.56 15s "on" 3.14 m 3 x 3.14m 12.59 12.59
tip m/min 45s "off' 4 RPM m/min m/min m/min
Experiments have been conducted in accordance with Table 1. The various
distances
along the "Full Scale" shearing device of 12 m length (i.e. suitable for a 24
m diameter
thickener tank) operating at a rotational speed of 0.167 rpm and having 6
radial shearing
arms, are simulated by operating the shearing device portion at the speeds and
on:off cycle
times shown.
The results of these tests are tabulated in Table 2, together with an array of
data
simulating other operating conditions.
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Table 2: Experimental Shear Test Results for 6-Arm Shearing Devices
Shear Tests with Scale-up shear mechanism (single-sided) at 0.6 flux
Linear (Tip) 1.57 3.14 6.29 9.44 12.59 24
speed m/min
lm rake 0.5 1 2 3 4 8
speed rpm
Operating
cycle (on:off) continuous continuous continuous continuous continuous
continuous
No. of 0.5 1 2 4 8
passes/min
Underfloor
42.3 39.2 36.3 36.0 33.6
W density %
~n Operating 60s : 60s 30s : 30s 20s : 20s 15s : 15s 7.5s : 7.5s
6' cycle (on:off)
No. of 0.5 1 1.5 2 4
passes/min
Underfloor
c density % 37.5 41.7 42.8 39.4 36.5
c Operating
60s : 120s 30s : 60s 20s : 40s 15s : 30s 7:5s : 1.5s
cycle (on:off)
No. of 0.33 0.66 1.00 1.32 2.64
passes/min.
Underflow 29.2 40.5 39.7
density %
Operating
cycle (on:off) 15s : 45s 7.5s : 22.5s
No. of
1 2
passes/min.
Underflow 39.9 35.9
density %
It can be seen from these experimental tests that optimal underflow densities
are
obtained when varying the operating cycle in accordance with the on/off shear
time periods
described in Examples 1 to 4. From the results in Table 2, it can then be
determined what
shear magnitude and frequency is optimal at different radii within the
thickener, thus
allowing for optimal design of the shearing mechanism across the entire radius
of the
thickener. Thus, the experimental tests demonstrate that the method 100
enables an
accurate measurement of the effectiveness of the shearing device
configuration. As a
consequence of these experimental tests, the inventors conclude that employing
the method
100 of Figure 5 will result in accurate testing for shearing devices in terms
of their
effectiveness in applying an optimal shear to the pulp that is consistently
maintained and
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provide the same level of improvement in the overall performance of a
thickener employing
the shearing devices.
In the preferred embodiments of the invention, the test apparatus 120 employs
a
cylindrical tank 101. This results in the linear velocity ut of the shearing
device portion 103
approaching zero close to the rotational axis 49, as best shown in Figure 14A.
Where the
shearing device 42 is being tested at a radial distance f that is
substantially displaced from
the rotational axis 49 of the thickener tank 1, for example at the outer edge
of the tank
between radial points ri and r2, the linear velocity un closest to the
rotational axis 49 is
unlikely to approach zero. Therefore, even though the shearing device portion
103 has the
same length Wr as the shearing device 42 between radial points ri and r2,
there is a disparity
between the range of linear velocities ui, u2, u3.... uõ of the shearing
device portion 103 and
the linear velocities un to ur2 of the shearing device 42. This disparity
increases as the
radial distance f increases from the rotational axis 49. In the previously
described
embodiments, the inventors have used the respective outermost edges of the
shearing
device 42 and the shearing device portion 103 for the first and second
predetermined
distances, in order to minimise or reduce any error in the test results caused
by this disparity
in linear velocities.
If more precise results are required, then the convergence of the linear
velocities ut in
a cylindrical test tank 101 towards zero is taken into account by using an
average linear
velocity ut(av) for the shearing device portion 103 that is the same as the
average linear
velocity ur(av) of the shearing device 42 over the same length Wr as the
shearing device
portion 103. That is, the average linear velocity ut(av) would be the average
of the linear
velocities ui, u2, u3 ...uõ taken over length Wr of the shearing device
portion 103, as best
shown in Figure 14A. Likewise, the average linear velocity ujav) of the
shearing device
42 would be the average of the linear velocities url, ... ur2 between the
radial points ri and r2,
as best shown in Figure 14B. The method 100 is equally applicable to either
case, whether
the linear velocities are taken as the linear velocities of the respective
outermost edges of
the shearing device 42 and the shearing device portion 103, or the average
linear velocities
edges of the shearing device 42 and the shearing device portion 103 over their
respective
lengths Wr.
Another embodiment of the invention which takes into account this disparity in
linear
velocities is illustrated in Figure 15, where corresponding features have been
given the
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same reference numerals. In this embodiment, the test apparatus 230 has an
arcuate trough
or tank 231 with the shearing device portion 103 extending across its width
Wr. The
shearing device portion 103 is attached to a moveable arm 232, which is
mounted to a
central drive shaft 233 for rotational movement about an axis 49.
The use of an arcuate trough or tank 231 ensures that the linear velocity ui
does not
approach zero close to the rotational axis 49. Instead, the innermost edge of
the tank 231 is
radially displaced from the rotational axis 49 of the shearing device portion
103. In
addition, the average linear velocity is not used, since the linear velocities
ui, u2, u3, ... U11 of
the shearing device portion 103 along the width Wr, would correspond to the
same linear
velocities ui, ... uõ of the shearing device 42 along the same width Wr on the
shearing arm
81. Thus, there is a more accurate simulation of the shear at the radial
distance f. This is
because the linear velocity ui closest to the rotational axis 49 is less
likely to approach zero,
since it is spaced from the rotational axis 49 by a distance of ft = fn - fi,
as best shown in
Figure 15.
A further embodiment of the invention is illustrated in Figure 16, where
corresponding features have been given the same reference numerals. In this
embodiment,
the test apparatus 240 has a rectangular trough or tank 241 having a length d,
with the
shearing device portion 103 extending across its width Wr. The shearing device
portion
103 is attached to a moveable arm 242, which is mounted to a carriage 243 for
linear
movement along a track 244 parallel to the tank 241.
In this embodiment, there is no need to calculate a rotational speed for
either the
shearing device 42 or the shearing device portion 103. Instead, the linear
velocities ut, ut of
the shearing device 42 and the shearing device portion 103 are used to test
the shearing
device portion 103 in the straight tank 241. This results in the method 100
being simplified
in that the calculating step 107 is now reduced to simply equating the linear
velocity ut of
the shearing device 42 to the linear velocity ut of the shearing device
portion 103. In
addition, as Ct is effectively the length d of the tank 241, the second or
"on" time period is
calculated in step 108 according to equations (18) and (19) as
ton=Ct/CtXtct x N
= d/Ct x tct x N ... (20)
The "off' time period is then calculated in accordance with step 109 and the
shearing
device portion 103 is moved for ton along the tank 241 and stopped for tog.
Ideally, the
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shearing device portion 103 would travel the length of the trough 241 during
the on time
period ton and would simply wait during the "off' time period toff to move in
the reverse
direction back to its original starting point at the next on time period to,,.
It will be appreciated that while shearing device portion 103 has been
described as
extending across the respective radii of the tanks 101, 231 and 241, in
further embodiments,
the shearing device portion 103 occupies a fraction of this distance to enable
lateral
adjustment of its position in the test tank 101, 231 and 241. As discussed
above, the
average velocity of the width of the shearing device portion 103 would be
calculated at step
107 to match the corresponding average linear velocity of the shearing device
42 over the
same width and range of radial distances.
In other embodiments, the method 100 employs a variety of shearing device
configurations other than the configuration illustrated in the Figures 4, 6
and 7. For
example, the shearing device portion 103 may have several arms to simulate a
multiple arm
shearing device, as discussed in Examples 3 and 4. The pickets arranged on the
shearing
device may also vary in both orientation and configuration. For example, in
the preferred
embodiment, the shearing device 42 has been described and illustrated with
pickets angled
with respect to a vertical plane that is at right angles to the radial arm.
However, it will be
appreciated that the pickets can be angled with respect to other vertical
planes, such as a
vertical plane parallel to or coplanar with the radial arms so that the
pickets have an angle
of incidence with respect to the direction of rotation of the shearing device.
In other
embodiments, the pickets may be only angled with respect to the vertical plane
parallel to
or coplanar with the radial arms.
Whilst the preferred embodiments of the invention have been described as
employing
shearing elements in the form of linear pickets or rods, it would be
appreciated by one
skilled in the art that other configurations for the shearing elements can be
used, such as V-
shaped angled rods, half or semi-circular tubes or other shearing elements
having different
polygonal cross-sections. In particular, the pickets themselves can be altered
in shape to
produce the desired shear profile. For example, a non-linear picket can be
used, such as a
spiral or helical shape.
It will be appreciated by one skilled in the art that by simulating the same
linear
velocity and shear profile of the shearing device the invention ensures that
the correct
amount of shear for a predetermined cycle can be reproduced in a test tank,
enabling
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accurate results to be obtained directly, without having to manipulate the
test data to take
into account the differences in scale between the test apparatus and the full
scale thickener.
Thus, in the invention, equalisation of the linear velocities of the shearing
device portion
and the full scale shearing device is achieved by adjusting the rotational
speed and the shear
time period to ensure that the simulation accurately reflects the amount of
shear that is
applied at any radial distance in the full scale thickener. By obtaining
accurate test results
for shearing device configurations and varying rotational speeds, the design
of shearing
devices is improved and more efficient, thus reducing development costs
involved with
designing shearing device configurations that achieve an optimal shear in a
separation
device, for example, a thickener. In all these respects, the invention
represents a practical
and commercially significant improvement over the prior art.
Although the invention has been described with reference to specific examples,
it will
be appreciated by those skilled in the art that the invention may be embodied
in many other
forms.
