PASTE & TAILINGS 2020
Enhanced paste plant control
A Metso Outotec Thickener Optimizer has been in place at
the Yara Siilinjärvi paste plant since May 2017. Veli-Matti
Järvinen, VP, Analyzers and Automation, Minerals, Metso
Outotec, outlines how successful the installation has been
The Outotec paste plant, which has been
operational at Yara Siilinjärvi since February
2017, increases tailings solids content,
improves tailings beaching properties and
extends the capacity and lifetime of the existing
tailings storage facility
The Yara Siilinjärvi plant in Finland is
owned by Yara International ASA, a
Norwegian fertiliser company. Siilinjärvi is
Western Europe’s only apatite mine, producing
apatite concentrate mainly for its Siilinjärvi
phosphoric acid plant. Part of the concentrate is
also dried and shipped to other Yara fertiliser
plants. About 1 Mt/y of apatite concentrate is
produced, creating approximately 10 Mt/y of
tailings.
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.
Process control challenges
Prior to the project, Yara and Metso Outotec
intensively studied the operating environment
and process challenges, as a result a mutual
agreement on the project targets was achieved.
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
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, the paste plant is partly manned, but
the target is to operate and maintain it from the
main plant. The paste plant must therefore be
fully-remote controllable to allow for critical
automated process sequences and efficient
process controls.
Multivariable control algorithm
The thickener control application uses the
Model Predictive Control (MPC) algorithm. MPC
is an advanced process control method
considered a standard method for control
processes with a multivariable character and
complex response dynamics. MPC control is
based on the process response models typically
obtained by performing process trials or
simulating the process responses.
Project background
In 2015, Yara awarded Outotec the contract for a
full-scale paste tailings plant at Siilinjärvi as
part of an engineering, procurement and
construction (EPC) project. The paste plant,
which has been operational since February
2017, increases tailings solids content, improves
tailings beaching properties and extends the
capacity and lifetime of the existing tailings
storage facility.
As part of the overall project, Metso Outotec
designed and delivered an optimising control
solution 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
The new solution implemented at Yara, the Thickener Optimizer, is based on the Outotec Advanced
Control Tools (ACT) platform
P12 International Mining | SEPTEMBER 2020 Supplement