PASTE SUPPLEMENT 2019
Automatic flocculants dosing control for mining
Jack Evans of Hawk Measurement discusses how
flocculants dosing control for mining thickeners and
clarifiers is influenced by a number of process factors
he first key issue is the flow rate into the
thickener or clarifier. As the flow rate
increases into the thickener or clarifier,
“retention time” decreases, which causes a
greater affect hydraulically on the settling
solids. So, as the flow rate increases it will
generally mean an increase in flocculants
dosing rate.
Secondly, as the flow rate decreases into the
thickener or clarifier the retention time
increases, which reduces the hydraulic effect on
settling, improving settling rates. This will
generally mean that as the flow rate decreases,
we will decrease the flocculants dose rate.
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Density of solids in suspension
As the solids concentration density increases in
the incoming media, the flocculants dose rate
will increase proportionally to compensate for
the change in suspended solids loading. The
above “flow rate” and “density of suspended
solids” are used as a general look up table in
Example of ORCA diagnostics data
the PLC to provide a “base flocculants dose
rate” to compensate for these changes.
However, in mining many different orebodies are
mined and processed at one site and what must
be understood is that different orebodies have
different settling characteristics and, therefore,
“flow rate” and “suspended solids loading”
process variables are not enough to control
flocculants dosing. The operation needs to
monitor insitu process conditions in the
thickener and clarifier that are being affected by
settling changes caused by different orebodies
and the hydraulic affect that results. We
therefore utilise, in addition the following ORCA
Sonar bed level transmitter control.
ORCA Sonar bed level transmitter
This monitors two different densities in the
process thickener or clarifier. By monitoring the
compact bed level (higher density) and the
hindered/mud layer interface (lighter density),
we are able to provide a supplementary dosing
control to the “base flocculants rate” by
monitoring the deviation distance between the
two densities when they are affected by ore
body changes, flow rate changes, etc. This
supplementary control will increase and
decrease the “base flocculants dose rate” based
on the process conditions seen in the thickener
or clarifier.
Typical automatic flocculants dosing
controls
For mineral process thickeners and clarifiers, we
use flow volume and density measurement of
the incoming liquid media to the process
thickener or clarifier. This information is used to
set up the “base” flocculants dose rate via a
look table in the PLC.
Using the ORCA Sonar bed level transmitter
as a supplementary control for the flocculants
dosing pump to compensate for “real”
process settling changes caused by different
orebody settling characteristics as well as
changes in flocculants/coagulant quality.
Control of tailings disposal pump speed
and water injection using cascaded density
and thickener bed level is done as follows.
Analogue output 1 from the ORCA Sonar
controls the heavy density compact bed
interface. The compact bed level output is
also used as the datum in conjunction with
the hindered/interface layer for providing
additional process feedback for controlling
chemical dosing and alarms.
Optimising an existing mining
thickener
There are a number of components that need to
be addressed, to make the thickener
work more efficiently and also reduce
the amount of downtime costs to
production:
n Automatic control of flocculants and
or coagulant: The use of clarometers,
or automatic jar testing, for
controlling the broad spectrum
flocculants and coagulant can really
only be used as a “base” dose rate
controller. It does not take into
consideration flow rates into the
Thickener and the hydraulic affects to
settling.
n Broad spectrum flocculants dosing should be
A thickener at a coal mining operation
dosed based on a system that automatically
monitors the settling efficiency between the
compact bed and the lighter interface layer.
This takes into account flow rate increase and
decrease, changes in the type of orebodies
that are being processed from underground
and open pit, clay in suspension, etc.
n The underflow pump should be controlled by
a transmitter not affected by density change,
as well as the underflow density transmitter.
This will optimise the underflow density
which can be maintained.
n Thickener design allows for a maximum rate
of inflow and a retention time in the
Thickener for solid particles in suspension to
settle before they overflow into the launders.
thickener design requires 360° laminar flow
from the inside feed well to the launder
around the thickener. When sections of the
launder are blocked and not functioning,
laminar flow 360° around the thickener
ceases, thus retention time diminishes, which
decreases the “design settling capacity” of
the thickener. IM
PLC logic flow diagram for flocculant addition
APRIL 2019 Supplement | International Mining P3