PASTE WESTECH_proof 25/05/2016 09:45 Page 2
Paste Supplement
In the lab, movement of the sensor through the
samples provided a measurable response over
a range of solids concentrations that would be
expected in a thickener bed
rake arm at an elevation approximately 600 mm
above the bottom of the tank wall. A circular
disk on a tubular arm transmits the drag force to
the load cell where the output signal is
generated. Wiring from the sensors was routed
up the support structure to the transceiver,
located in a protective enclosure on top of the
support and above the liquid level of the
thickener.
The thickener is a 22.86 m diameter, high
density thickener with a 3.66 m tank sidewall,
producing 40-60 Pa yield stress underflow. The
process dewaters insoluble material from soda
ash production for either underground or
surface deposition. The thickener has a
peripheral walkway that allowed convenient
access to the instrument and provided the
ability to make manual bed level readings at any
angular position around the tank.
Current thickener control is based on the
operator conducting manual bed level reading
every four hours. These bed readings were used
to control the underflow discharge rate.
Additional manual readings were taken at other
times as needed to correlate with instrument
data. The plant procedure for making manual
measurements was to lower a long PVC pipe
into the tank at a shallow angle until the operator
could feel the bed. Then maintaining the feeling
of the bed, the pipe would be transitioned to
vertical along the tank wall. Markings on the pipe
allowed operators to measure the depth of the
bed below the liquid level.
Testing of the prototype instrument started
with the focus of detecting bed level. The
instrument consisted of three sensors spaced
300 mm apart vertically. The output signals from
the sensors as the mud bed level varied, during
normal operation, demonstrated that the bed
level interface could be clearly identified.
Sensor signals were seen to increase from zero
to maximum output before the bed level was
even detected by the next higher sensor, 300
mm above it. This indicates
a very distinct bed level
interface and that the
instrument was able to
detect it. Correlation was
demonstrated to exist
between manual bed level
measurements and
instrument signals.
Sensor signals are
proportional to bed yield
stress. Test data indicates
that as bed level rises above
the level of a sensor, the
yield stress at that sensor also rises. This
indicates that a relationship exists between
yield stress at a position in the settled bed and
the depth of the bed above that position.
Potentially a single sensor may be able to
detect not only when the bed level reaches the
elevation of the sensor but also estimate the
bed level as it rises above the elevation of the
sensor. However, this will need to be confirmed
through site specific operating experience.
Accurate bed level sensing will enhance the
ability to control the thickener.
Data from testing the prototype instrument
detected significant angular variability around
the thickener. The cyclic nature of this data
correlated to the period of rotation of the
mechanism. Plotting the data of one sensor
(one elevation) showed the settled bed had
areas of high yield stress while other areas had
low yield stress, indicating a significant
variation in solids concentration and particle
size distribution in the bed as a function of
angular position. Further investigation revealed
the cause was due to poor distribution from the
feedwell. A current was carrying coarse solids to
one side of the tank. This segregation within the
tank was happening even though the manually
measured bed level showed very little variation
at different locations around the tank. The
observed segregation within a thickener is
known as an island and its presence was
previously unknown. Having this amount of
information about the state of a
thickener bed has important
implications for thickener
design, operation and control.
Instrument potential in
thickening
Developing a bed profile, as was
demonstrated above, has the
potential of detecting other
operating problems within
thickeners that are often difficult
to diagnose. This may be done
by using both vertical and
horizontal sensor arrays. The
ailments that may be possible to observe
include rat-holes, shortcircuiting flows,
doughnuts and rotating beds. In addition, this
instrument can show how the bed becomes
denser with depth and how that changes with
varying operating conditions. A vertical bed
profile that is out of the ordinary may indicate
changes to the flocculating characteristics of the
feed or changes in particle size distribution. The
potential of this instrument as a diagnostic tool
for thickeners is significant.
Testing clearly shows the potential of this
instrument goes far beyond measurement of the
mud bed level. The ability to measure and
observe the dynamics of the thickening process
opens up possibilities in the fields of thickener
control, process optimisation, thickener design
and theoretical modelling of thickeners. The
common thickener theory describing a bed as
radially homogeneous is already in jeopardy.
The data suggest non-homogeneous
distributions and variable angular, horizontal
and vertical density gradients are more likely.
The magnitude of sensor signals was found
to have a linear relationship to the yield stress
of the material. The instrument described
provided data from which the bed level interface
inside a thickener tank can be determined. It
also detected increasing yield stress as the bed
level increased above the elevation of the
sensor. This provides unprecedented
information from within the thickener bed that
could be used for early warning of reduced
thickener performan