Mining in focus
Water security is key to the
mining industry in South Africa.
significance of each can be estimated as
follows: While interstitial storage amounts
to between 25% and 35% of the tonnes
processed, this means that there will be about
0.25–0.35 cubic metres of water ‘locked up’
in this way per tonne of tailings deposited on
the TSF. Water lost through seepage could be
as much as 0.1–0.35 cubic metres per tonne
(depending on the nature of the tailings and
the in situ foundation materials below the
tailings dam), while the evaporation could be
0.15–0.45 cubic metres per tonne. In total,
then, about 0.5–1 cubic metre of water per
tonne of tailings is generally lost to the mine
in conventional tailings dam designs.
By addressing each of these three factors,
mines can considerably reduce water loss,
making it more readily available for recycling
and re-use. For instance, achieving a higher
rate of rise on a tailings dam — which can be
done through the use of cyclones — will have
a direct effect on evaporation: an increase in
the rate of rise from 2.5m per year to 5m per
year will halve the evaporation rate.
By the same token, installing a liner can
reduce seepage and allow that water to
be recovered for re-use. South Africa has
followed a worldwide trend to provide liners
under tailings dams, allowing increased
water recovery and reduced risk of polluting
groundwater. National legislation and the
www.miningmirror.co.za
regulatory requirements from the Department
of Water and Sanitation now insist on liners
for tailings dams, unless an acceptable risk
mitigation plan can be presented to argue why
liners would not be required.
Dealing with interstitial storage
To deal with interstitial storage, various
options are available. Taking the route of
paste tailings — using a high-rate thickener
and positive displacement pump — will
not drastically improve water recovery
(perhaps by only 0.05–0.1 cubic metres per
tonne of tailings), although this strategy can
positively affect water management on top
of the dam. To save water more effectively,
it will be necessary to adopt the filtered
tailings approach — which can substantially
improve the recovery of water that is stored
interstitially. Producing filtered tailings
may also obviate the need for a liner, as the
amount of seepage possible after filtering is
insignificant.
While filtered tailings technology has
been employed for many decades on mines
in South Africa and elsewhere, its use has
generally been limited to small portions
of the material stream — often related to
just the final concentrate, for instance. It is
significant that in one recent local project, a
Water security on a mine can be described
as the reliable availability of acceptable
quantities and qualities of water to allow
production and treatment to proceed
according to plan, with the management
of an acceptable level of water-related
risks. Additional available water, achieved
as part of water security strategies, could
mean increased mine production —
which conversely could be limited by less
available water.
Water saving on mines is not without its
side-effects: more recycling means that
the tailings water circuit becomes more
saline over time, and increased salinity has
an impact on the properties of tailings.
High salinity with finer grind reduces
tailings density and therefore reduces shear
strength, while also creating greater risk of
groundwater pollution. This requires a good,
low-cost technology to be developed to
remove the salinity.
Water recovery using filtering technologies
may increase additional recovery of metals
remaining in solution, which normally
would have been disposed of with the
tailings on the TSF.
FEBRUARY 2019 MINING MIRROR [39]