most lay people do not realise is that
most people in these industries have,
by and large, been very aware that, to
remain in business, to open new
properties and to sustain modern
living, the way they deal with or plan
to deal with tailings is as paramount to
the success of the venture as the
deposit itself.
The best way for producers to deal
with tailings is to answer the question:
what do you envision as the long-term
solution? This will often depend on the
characteristics of the tailings. If the
tailings are comprised of relatively
coarse particles (+38 µm) then the
answer may be as simple as pumping
them to cyclones for recovery and dam
building, with the fine cyclone
overflow being impounded behind the
dam. This method has, for many
decades, been employed to both collect
and store tailings, as well as to clarify
water for reuse in the process plant.
What could possibly be cheaper
than to dig a hole or build a dam and
create a space for the waste generated
by processing activities? Well, there are
certainly costs associated with this
plan. Two of the most frequent laments
heard in over 30 yr as a mineral
processing engineer are: “we don’t
have enough screens” and “our ponds
are running out of space”. Dredging a
pond to create more room for
incoming tails may not be an everyday
operational cost, but when the time
comes, the piper wants his cut. Of
course an additional pond could be
opened, but space and/or permitting
might not be possible. Additionally,
there is the potential to be covering up
reserves that someone may need to get
to one day, as well as the loss of
process water due to evaporation or
percolation.
Besides the costs associated with
the pond solution, there are also risks
– placing people on or near the water
while maintaining the pond or
building ever higher dam structures.
Impoundment failure is another
possibility that should weigh heavily
into any risk analysis.
Types of thickeners
If the tailings are finer (-38 µm) and if
ponds are still the desired method for
storage, it may be worthwhile
considering a thickener to prepare the
tailings before sending the entire
stream out to a pond. Thickeners
reduce the total volume headed to
ponds, as well as return a majority of
process water to the plant immediately.
In processing operations, there are
three basic styles of thickeners:
n
n
High rate.
n
n
High density.
n
n
Paste.
Each of these thickeners shares a
similar operation as far as feeding and
collection of the settled solids. Feed
enters the unit via a feedwell that is
designed to provide optimum mixing
and residence time for contacting
suspended solids with a flocculant and
allowing flocs to form. The flocculated
suspension then flows out of the
feedwell and is dispersed across the
area of the tank. Flocculated solids fall,
while clarified water rises up over a
peripheral weir for collection in a
process water tank. The flocculated
solids collect in the cone section of the
thickener. To gather this mud and
bring it to a central discharge area,
tanks use slowly rotating rake
mechanisms. Lower yield stress muds,
such as those produced in a high rate
thickener, require less torque to move
these rakes through the mud.
The selection of what thickener
style fits your needs depends heavily
Figure 2. Yield stress continuum for thickeners.
Figure 1. The clean water produced from a thickener after fines and tailings are removed.
76
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World Coal
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June 2015
