Note: Descriptions are shown in the official language in which they were submitted.
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APPARATUS AND METHOD FOR DETECTING THE LEVEL OF A MUD BED
BACKGROUND OF THE INVENTION
THIS invention relates to a method and apparatus for detecting and controlling
the level of a mud bed in a gravity thickener, and in particular for detecting
and
controlling the level of a mud bed in a high compression thickener.
Gravity thickeners are typically used to separate the liquid and particulate
components of a slurry by the dynamic creation and removal of a portion of the
mud bed at the base of the thickener.
A portion of the mud bed is conventionally extracted from the base of the
thickener using an underflow pump, and the level of the mud bed is indirectly
monitored by measuring the underflow density and comparing it with a density
set point value, with the underflow pump speed being adjusted accordingly.
Ideally, the mud bed needs to be kept at a substantially constant level, which
means that the underflow pump needs to extract a portion of the mud bed from
the base thereof at a rate which essentially corresponds to the rate of
formation of the mud bed.
Over the past few years, thickener technology has accelerated rapidly with the
advent and general acceptance in the industry of so-called "deep cone" or
"high compression" thickeners. These devices have the ability to generate
high density underflows which approach paste-like consistencies, and have
allowed the previously unattainable concept of thickened tailings disposal to
become a reality. With the advent of these thickeners, the control of the
level
of the mud bed has become a critical problem due to the relatively high
internal
rise rate (typically above 11 metres per hour) associated with such
thickeners.
CONFIRMAT{ON COPY
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Underflow density measurement as a means of monitoring mud bed level has
some serious shortcomings. There are many factors which influence the
underflow density, including slimes-to-grits ratio in the feed, the degree of
flocculation, settling rate of the flocculated slimes and the like. As a
result,
underflow density is not a uniform or predictable function of mud bed level.
In
the past, the control of the mud bed level on the basis of underflow density
has
led to mishaps in which the internal structures of high compression thickeners
have been damaged and have catastrophically failed under the pressure of an
overflowing mud bed. Failure to accurately detect the level of the mud bed has
also resulted in the entire mud bed and overlying water or diluted slurry
being
drawn down through the underflow pump, resulting in a new mud bed having to
be formed, and considerably reducing the efficiency of the thickener.
SUMMARY OF THE INVENTION
According to a first aspect of the invention there is provided an apparatus
for
detecting and controlling the level of a mud bed within a gravity thickener,
the
apparatus comprising a first vibrating probe which vibrates at a predetermined
amplitude and frequency and first mounting means for mounting the probe
substantially vertically in the thickener at a predetermined position in which
it is
arranged to detect a preset mud bed level on the basis of vibration dampening
and to generate a first control signal in response thereto, the first control
signal
being arranged to adjust the throughflow of mud via an underflow pump so as
to control the level of the mud bed within the column thickener.
Preferably, the first probe is arranged to detect a preset high mud bed level
and the first control signal is a "high" signal, which is arranged to increase
the
throughflow of mud via the underflow pump so as to reduce the level of the
mud bed.
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Advantageously, the apparatus includes a second vibrating probe which
vibrates at a predetermined amplitude and frequency, and second mounting
means for the second probe mounting substantially vertically in the thickener
at
a predetermined position in which it is arranged to detect a preset low mud
bed
level, and to generate a second control signal in response thereto, the second
control signal being arranged to decrease the throughfiow of mud via the
underflow pump to maintain, in conjunction with the first control signal, the
level of the mud bed between the high and low mud bed levels.
The first and second control signals may be arranged respectively to turn the
underflow pump on and off or to increase and decrease the speed of the
pump.
One or more intermediate vibrating probes may be located between the first
upper and second lower probes, the intermediate probes being arranged to
generate control signals to vary the mud bed level by increasing or decreasing
the speed of the underflow pump.
Alternatively, the first vibrating probe may be arranged to deliver an analog
signal which is a function of the mud bed level along the length of the probe,
whereby the signal is arranged to vary the speed of the underflow pump in
response to measured changes in mud bed level.
Advantageously, the vibrating probes are provided with adjustable switching
set points arranged to generate control signals.
Conveniently, the probes are arranged to sense differences in SG of as little
as
0.04, and to switch between a setpoint in response thereto.
The thickener is preferably in the form of a high compression thickener. By
the
term "high compression thickener" is meant a thickener having an internal rise
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rate which is generally greater than 11 metres per hour, under a relatively
high
duty or processing rate of twenty to 50 tons per square metre per day. Such
thickeners have relatively small diameter ranging from 1 to 15 metres, and a
shallow feed well. Flocculant is required, and the mud bed which is created
within the thickener is relatively deep, having a height of up to 10 metres.
Such thickeners may or may not be provided with a secondary dewatering
device in the form of a "rake" system. Typically, in cases where the underflow
density is less than or in the region of 1.6 kilograms per litre, no secondary
dewatering device is required. Where the underflow density is greater than 1.7
kilograms per litre, a secondary dewatering device is generally used.
The mud bed level detecting method and apparatus of the invention may also
be used with so-called "high rate" and "conventional" gravity thickeners. A
high
rate thickener typically has an internal rise rate from 4 to 10 metres per
hour,
and has a diameter ranging from 10 to 60 metres. A thickener of this type
requires a flocculant and generally utilizes a deep feed well. A mud bed of up
to 5 metres in height is created, and such thickeners are capable of
processing
from 12 to 19 tons per square metre per day. The underflow density of the
mud from a high rate thickener can typically go up to 1.55 kilograms per
litre.
A conventional thickener typically has an internal rise rate of less than 3
metres
per hour and may be up to 100 metres or more in diameter. Flocculant is not
essential, and a relatively shallow mud bed of up to 2 metres is utilized.
Conventional thickeners have a relatively low processing rate of 5 to 7 tons
per
square metre per day, and are able to handle an underfiow density of 1.3 to
1.4 kilograms per litre.
The invention extends to a method of controlling the level of a mud bed within
a gravity thickener comprising the steps of sensing the level of the mud bed
using a first vibrating probe immersed in the mud bed, generating a first
control
signal based on the extent of vibration dampening of the first vibrating
probe,
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and using the first control signal to adjust the rate of extraction of mud so
as to
control the level of the mud bed within the thickener.
Preferably, the method includes the steps of sensing a high mud level using
the first vibrating probe and using the first control signal to increase the
throughflow of mud via an underflow pump to reduce the level of the mud bed.
The method conveniently includes the steps of sensing a low mud bed level
using a second vibrating probe, generating a second control signal based on
the extend of vibration dampening of the second vibrating probe, and using the
second control signal to decrease the throughflow of mud via the underflow
pump to maintain the level of the mud bed between the high and low mud bed
levels in conjunction with the first vibrating probe.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a schematic cross-sectional side view of a first prototype
embodiment of a mud bed level controller of the invention fitted
to a high compression gravity thickener;
Figure 2 shows a graph of interface density versus time, indicating the
operation of the mud bed level controller of the invention;
Figure 3 shows a more detailed cross-sectional view of the manner in
which the vibrating probes are fitted within the high compression
thickener;
Figure 4 shows a top plan view of the thickener of Figure 3; and
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Figure 5 shows a detail of the manner in which the vibrating probe is
located within a guide pipe within the thickener.
DESCRIPTION OF EMBODIMENTS
As was mentioned in the background of the invention, it has been accepted for
some time that the underflow density measurement-based control philosophy
has serious shortcomings, in particular with respect to high compression
thickeners. Numerous different level detection instruments were investigated
on a laboratory scale before a pilot plant was set up to conduct a more
detailed
evaluation. Measuring instruments for measuring material levels on the basis
of capacitance, conductivity, torque, ultrasound speed, ultrasound dampening,
turbidity and finally amplitude of vibration were eventually shortlisted for
further
evaluation. All of the off-the-shelf instruments selected were specifically
designed to measure liquid levels by detecting an air and liquid interface. In
mud bed level detection, it will be appreciated that the properties of two
different slurry layers of marginally differing densities are far more akin
and
less easily measurable than the disparate properties of air and water, where
numerous different measurement techniques can and have been used.
Initially, all of the instruments were evaluated by a dip-in method in slimes
of
different densities, temperatures and solids slimes-to-grits ratios. The
susceptibility of the respective measurements with regards to these
parameters were investigated. A three-by-three matrix of nine samples were
generated, having SG densities of 1.1, 1.2 and 1.3, and a slimes-to-grits
ratios
including no grits, a 1:2 ratio and a 1:1 ratio, with grits being defined as
having
an average particle size ranging from 300 micrometers to 1 mm, and slimes
having an average particle size of less than 300 micrometers. Temperature
changes were simulated by placing the samples in a water bath controlled at
one of 20 C, 30 C and 40 C.
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Capacitance, conductivity, torque, amplitude of vibration, ultrasound speed,
ultrasound dampening and turbidity measurements were all investigated in
depth. The end result was that amplitude of vibration using an off-the-shelf
VEGAVIB 52 vibrating probe was found to be the most satisfactory level
detection means. This probe vibrates at a fixed frequency, with the amplitude
and the frequency of vibration changing according to the amount of dampening
that occurs. The probe does not have an analog output, but is rather provided
with an adjustable switch point, so that the specific amplitude at which the
switching will take place is adjustable. It was surprisingly found that an
extremely sensitive switch point adjustment was possible with this probe, in
that it recognized a difference in amplitude between respective sample
densities of 1.1 SG, 1.2SG and 1.3SG, as well as between sample densities of
1.14SG and 1.18SG. It was also found that the amplitude of vibration was
primarily SG- and viscosity-dependent, and was not affected by the above
mentioned temperature and size distribution variables.
In the subsequently established pilot plant, a commercially available Ultrasep
high compression thickener supplied by Bateman Process Equipment (Pty) Ltd
of Barlett Road, Boksburg North, Gauteng, South Africa, was used.
Unflocculated thickener feed slimes from Premier mine were used in the
operation thereof. As the natural pH of the slimes was relatively high, at
10.25,
the pH was dropped to below 8 by means of adding H2SO4 to the feed slimes
to assist in flocculation. The flocculant used was AD2, which was supplied by
Ore Pro Consultants.
The average density of the feed material to the pilot plant was in the region
of
1.04 to 1.05SG, with the flocculated feed material having an average density
of
approximately 1.1SG. The switch point of the VEGAVIB 52 vibrating probe
was adjusted to switch in densities higher than the flocculated feed density
in
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an attempt to detect the mud bed level, which is typically in the region of
1.2 to
1.3SG.
Referring now to Figure 1, an Ultrasep high compression thickener 10 is
schematically shown fitted with a first VEGAVIB 52 vibrating probe 12. The
Ultrasep thickener includes an uppermost slurry inlet pipe 13A and a
lowermost mud outlet pipe 13B which extends from the base of a conical sump
portion 14 of the thickener. An upper flanged tank insert 15 is inserted
between two main sections of the tank. The insert 15 incorporates a recess 16
within which the first probe 12 was vertically mounted. A sampling point 18
was installed at a position close to the tip of the probe 12. The probe was
initially adjusted not to switch on the flocculated feed material having a
density
of 1.06SG. On switching of the probe, a sample was manually extracted from
the sampling point 18 and the density was measured to be 1.3SG. At this
point, the underflow at a discharge end 20 of an underflow pump 22 was
measured by an underflow density measuring meter 23 as having a density of
1.5SG. The mud bed level was then dropped by manually starting the
underflow pump 22, as a result of which the probe 12 reverted to its original
non-switched state.
As the VEGAVIB probe is arranged to deliver an adjustable switching signal,
and is incapable of delivering a variable analog signal, a control philosophy
was adopted in which the high level probe 12 was supplemented with a low
level probe 24. The low level probe 24 was similarly fitted in a recess 26
within
a second flanged pipe insert 28 located directly below the first pipe insert
15.
When the mud bed reached a level 30, this caused the vibrating probe 12 to
switch, thereby delivering a signal via a control line 32 to switch on the
underflow pump 22. The vibrating probe 24 was used to detect a lower mud
bed level 34. At this level, the vibrating probe 24 was adjusted to transmit a
signal via control line 36, thereby switching the pump 22 off and allowing the
mud bed level to increase.
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Referring now to Figure 2, a graph of density (SG) vs time is shown. A
lowermost broken outline plot 38 indicates the on/off status of the underflow
pump 22, and an intermediate plot 40 indicates the interface density or actual
density measurements of the mud bed at both the high level 42 when the
underflow pump 22 starts and the low level 44 when the underflow pump
stops. It was found that the high and low level density measurements were not
exactly the same, as would theoretically have been expected. This was
probably due to the fact that the mud bed is not uniformly level, with the
sampling points 18 possibly producing a sample that was not fully
representative of the material surrounding either of the vibrating probes 12
and
24. Nevertheless, it was visually observed that the mud bed level never
exceeded the high level position, thereby successfully protecting the internal
structures of the Ultrasep0 unit. By way of reference, an uppermost plot 46
showing underflow density was included. This plot shows a gradual reduction
in density, but is not sufficiently sensitive or responsive to indicate
variations in
mud bed level shown by the plot 40.
In Figures 3 and 4, a large commercial Ultrasep0 high compression unit 50 is
shown. This includes a dissipator cone 52, an inner feed well 54 leading to a
re-circulator cone 56, and an overflow launder 58. High and low level guide
pipes 60 and 62 respectively extend vertically from a walkway 63 at the top of
the unit. As can more clearly be seen in Figure 5, the vibrating probe 12 is
fitted within the guide pipe 62, with the probe head 64 sitting on gusset
plates
66 located at the base of the pipe for centering the probe. The probe cable 68
extends to a probe control box 70. It is clear from Figure 5 how the pipe 60
extends through an opening in the dissipator cone 52.
Certain critical levels have been identified in the Ultrasep0 high compression
thickener unit 50. These include a so-called "high high level" 74, which is
100mm below the re-circulator cone 56, a "high level" 76, which is 100mm
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below the top of a Trioid 78, and a "low level", which is the lowermost
operating level 80 just below the Trioid . A suitable additional guide pipe 82
may also be installed, to which a lowermost vibrating probe is fitted to
detect
the lowermost level 80.
