PASTE & TAILINGS 2020
The controlled variables in the optimising
system are underflow density (kg/m 3 ), overflow
solids content (%), bed mass (%), bed level (m)
and rake torque (%)
Solution architecture
The new solution implemented at Yara, the
Thickener Optimizer, is based on the Outotec
Advanced Control Tools (ACT) platform. This
platform is a dedicated environment developed
especially for advanced process control
solutions.
In the Yara paste plant project, a high level of
system integration was desired. The controller
user interface, alarms, interlockings, history data
and documentation were therefore all
implemented into the customer DCS, giving
operators full functionality from the interface
also used for plant control. The Thickener
Optimizer solution also included a remote
monitoring capability, which enables controller
performance monitoring, tuning, and support.
Controller setup and KPIs
The controlled variables in the optimising system
are underflow density (kg/m 3 ), overflow solids
content (%), bed mass (%), bed level (m) and
rake torque (%). Underflow density was selected
as a primary control variable, which has a precise
target value and the highest priority during
normal operation. Overflow solids is the second
most important variable. Overall, the system
should run with minimal flocculant dosage to
minimise plant operating costs.
Operational results
The Thickener Optimizer has been in use at the
Yara paste plant since May 2017. The experiences
have been good, and operators have adopted the
controls well in their daily toolkit. Since start-up,
the controls have been in continuous use. It is
only during exceptional situations such as
underflow line flushing or maintenance work that
manual operations are needed. Overall, the control
solution’s utilisation rate has been over 92%.
The basic operating principle is to run the
system with the highest possible density that
still ensures trouble-free pumping. In practice,
this means the operator adjusts the density
target value in accordance with the observed
pumping behavior and the discharged slurry
visuals. As the paste plant is operating very close
to the corner point in the solids content vesus
yield stress curve, even a very small 10-20 kg/m 3
change in density makes a remarkable difference
in pumping resistance.
One of the key targets was continuous
operation with 68-70% underflow solids content.
As seen from site trend lines (June-Aug 2017), the
actual operating window of both thickeners is
66-68%. Actual operating values are slightly
lower than original targets due to the targeted
beach slopes having already exceeded this
operating range, so there was no requirement for
higher solids content.
Since the paste plant started up in February
2017, it has delivered a consistent performance
level. One year of operation has shown overall
plant availability at over 97%. Underflow solids
content is kept continuously on the required
operating window and the achieved average
beach slope angles are over 3.5 degrees. Average
solids content in February-November 2017 has
been 67.5% and the overall solids content
operating range 66-69%. The main variation
comes from density set-point changes carried out
by the operator. In the below figure, the solids
content target value calculated from the actual
density target is shown as a blue line, and the
measured value with a green line.
One of control system’s benefits is the 10-20%
overall reduced flocculant consumption. Since
overall tonnage is high, this means remarkable
savings in plant operating costs.
A major long-term target for Yara is remote
paste plant operation from the main concentrator
plant. Currently, the plant is partially manned,
operating both autonomously and remotely from
the main plant during night shifts. The next step
is to gradually move to fully-remote operation.
Conclusion
With Metso Outotec’s Thickener Optimizer
solution, the paste thickeners can consistently
run with high and stable underflow solids
content of 66-68%, regardless of challenging
mineralogy and tailings feed rate variation. This
results in average beach slope angles of 3.5
degrees (6.1%) in the tailings disposal area.
Additionally, the control pushes the system
towards the minimum use of flocculant,
delivering 10-20% savings in flocculant costs.
The optimising controller also helps in underflow
pumping, as it keeps the process in its operation
window, with fewer manual operations required.
The measured and proven outcome has
significantly improved the customer’s
sustainability targets. IM
Solids content variability (measured value is green, target value is blue). Actual Jun-Aug 2017
SEPTEMBER 2020 Supplement | International Mining P13