Designing tailings spigot discharge
systems to maximise beach slope
Ray Martinson of Paterson & Cooke Chile discusses how
steeper slopes in a TSF are dependent on the right
distribution system
A typical high volume TSF, such as those in
Chile, receive in excess of 100,000 t/d of copper
tailings, and will require numerous spigot
discharge points spaced at 50 to 60 m intervals
over a length of up to 1.5 km
major consideration in the evaluation of a
suitable thickened and paste tailings
storage facility (TSF) is the ability of the
deposited material to be placed at a suitable
beach slope. This is necessary to both increase
the storage capacity and operating lifetime of
the facility, as well as to improve the overall
environmental impact of the TSF. As the beach
slope increases the need for high retaining walls
reduces and this potentially yields significant
economic savings, both in terms of capital costs
to establish the impoundment walls, and in
operating costs due to lower or fewer wall raises
over the life of the facility.
A
Obtaining steeper beach slopes
To obtain steeper beach slopes on the TSF it is
necessary to deposit the material in thin layers
across a wide area, as opposed to a single high
volume discharge. This requires that the tailings
distribution system is designed for the
simultaneous operation of multiple discharge
spigots spaced far apart. The hydraulic design of
such a system is complex as stable and
balanced flow is required at each discharge
point over an extended deposition area. As the
TSF is often remote, this must be achieved
without actuated instrumentation or active flow
and pressure control. If a multiple discharge
system operates with an uneven, or unbalanced,
flow at each point, there is an increased
operational risk of pipeline blockage as material
settles out on the pipeline invert. The resulting
tailings beach will have varying layer
thicknesses and beach slopes due to different
deposition rates from each spigot that may
result in uneven consolidation and insufficient
desiccation of layers between deposition cycles
that could compromise the stability of the
impoundment.
A typical high volume TSF, such as those in
Chile, receive in excess of 100,000 t/d of copper
tailings and would require numerous spigot
discharge points spaced at 50 to 60 m intervals
over a length of up to 1.5 km. In these applications,
the thickened tailings sheared yield stress will vary
considerably as solids concentration changes and
therefore the key goals of the hydraulic design are
to ensure that there is turbulent flow in the spigot
pipelines to prevent settling, while providing an
equal flow distribution.
During operation, the upstream spigots that
operate at higher pressures than the
downstream spigots, will discharge higher
volumes, and it is necessary to control the
pressure at each offtake to balance the flow.
There are a variety of valves that require active
control to regulate the pressure but experience
has shown that custom orifice plates at each
discharge have proved to be more robust and
cost effective. The design of the main pipeline
and spigot pipelines, each with a custom orifice,
needs to consider an envelope of operating
conditions as orifice plates generally operate
over a narrow range of flows. In some instances,
it may not be possible to obtain fully uniform
flow distribution for all flow rates, and some
discharge spigots will need to be shut.
The image below shows a system that
comprises a tailings pipeline that operates
multiple spigots to balance the flow across 20
discharge points over 1.1 km. The spigot offtake
pipelines vary from 180 mm to 355 mm with
custom-sized orifices in each spigot. This
system has been in operation since 2014 and
on-site topographic surveys have confirmed that
the deposition is uniform and that the beach
slopes were higher than those achieved with
conventional spigot discharge. This has resulted
in improved tailings management due to a
better understanding of the filling requirements
for higher beach slopes, and the system has
since been extended to operate over a 1.6 km
distribution front.
The successful implementation of such a
project requires a thorough understanding of
the flow properties of the material that can be
obtained by test work and pilot plant trials, as
well as the deposition requirements at the TSF.
Better beach angles can be obtained when
operating with high density thickened tailings if
a multiple discharge system is used that allows
for an even distribution of material over an
extended beach, as opposed to a single point
discharge at a high flow rate.
However, the operation of multiple discharge
systems is more complex when compared to a
single point discharge system, and trained
operators are needed that understand the
hydraulic pipeline operational requirements as
well as the TSF deposition needs. Due to the
orifices, these systems require more energy to
transport the tailings when compared to a single
point discharge system, so they generally are
pumped systems, but could be operated as
gravity flow systems if sufficient head is
available. IM
Simultaneous operation of multiple spigots showing thin layer deposition
JUNE 2017 Supplement | International Mining P7