PASTE SUPPLEMENT 2019
Paste process stability
Outotec’s ACT Thickener Optimizer system at Yara has
helped maintain process stability
Background of the project
In 2015, Yara awarded Outotec the contract for a
full scale paste tailings plant at Siilinjärvi as an
engineering, procurement and construction
(EPC) project. This paste plant delivered by
Outotec increases tailings solids content,
improves tailings beaching properties and
extends the capacity and lifetime of the existing
tailings storage facility. The paste plant has
been operational since February 2017.
As part of the overall project, Outotec also
designed and delivered an optimising control
system for the two paste thickeners at the
tailings plant. This case study discusses the
control application, its challenges and results
following implementation at Yara.
The new plant’s objective was to achieve a
68-70% solids underflow and at least two
degree (3.5%) beach slopes, extending the
tailings pond lifespan from 2020 to 2035.
Factors such as the plant’s changing mineralogy
and tailings feed fluctuations make it
challenging to reach these solids underflow and
beach slopes targets. It was determined that
many of these challenges could be overcome
with adequate process instrumentation and
state-of-the-art control techniques.
Solids content variability (measured value is
green, target value is blue). Actual Jun-Aug 2017
Challenges for process control
1. Varying mineralogy – Siilinjärvi’s tailings sand
rheology varies significantly on a daily basis.
The overall tailings sand mineralogical
composition is apatite (1%, variation 0.7-
10%), calcite and dolomite (20%, 10-35%
variation), phologopite mica (74%, 60-85%
variation) and other silicates (5%, 1-10%
variation). In particular, the high content of
coarse, flake-like mica strongly influences
both thickening and pumping performance
2. Milling circuits –The mills operate with 24-
hour homogenisation stockpiles, where the
incoming ore type varies from one pile to
P2 International Mining | APRIL 2019 Supplement
another. In addition, each milling section has
to be shut down for 1-2 hour periods each
week to charge the new rods. This reduces
the paste plant input mass flow remarkably,
causing feed disturbances.
3. Paste plant location – The paste plant is 6.6
km from the main concentrator plant.
Currently, Yara is operating the paste plant
unmanned, so that there are no permanent
operators at the paste plant.
Multivariable control algorithm
The thickener control application uses the
Model Predictive Control (MPC) algorithm. MPC
is an advanced process control method which is
considered a standard method for control
processes with a multivariable character and
complex response dynamics. MPC control is
based on the process response models, which
are typically obtained by performing process
trials or simulating the process responses.
System structure
The new system implemented at Yara, the ACT
Thickener Optimizer, is based on Outotec’s proven
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 ACT Thickener
Optimizer system also included a remote
connection to the Outotec network, so that the
controller performance monitoring, tuning, and
support could also be performed remotely.
Controller setup
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 ACT Thickener Optimizer has been in use at
the Yara paste plant since May 2017. 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
controls usage rates have 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 behaviour and the discharged slurry
visuals. As the paste plant is operating very
close to the corner point in the solids content
vs. 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 Yara paste 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 fact the targeted beach slopes are
exceeded already with this operating range, so
no requirement for higher solids content.
Since the paste plant started up in February
2017, it has delivered a consistent performance
level. Now, after two years of operation the plant
availability has been the same 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°.
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 setpoint changes
done 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
reduced overall 10-20% flocculant consumption.
Since overall tonnage is high, this means
remarkable savings in plant operating costs.
Since autumn 2018, Yara has operated the
paste plant unmanned, with no permanent
operators. Operators only visit the plant to do
some routine checks and maintenance. IM