Tailings Disposal - Mining and the Environment: From Ore to Metal - page 735

Decant Systems

Decanting is important, not only to maximize re-use of process water, but also to maxi-
mize the area of exposed beaches subject to solar drying, and to maintain a safe free-board.
The system should provide a high capacity for removing storm-water to allow for con-
secutive storm events, ensuring a safe free-board also under unusual but possible meteoro-
logical events. As a guide the decant system should be able to remove storm-water within
2 to 4 weeks, subject of course to prevailing climate conditions. The main types of decant
systems are: (1) l oating decants; (2) decant towers; (3) inclined decant: and (4) perimeter
sump - for central discharge systems.

Floating Decant System
Floating decant systems are probably the most common systems in use. Essentially, this sys-
tem involves a pump mounted on a l oating pontoon, usually secured in place by mooring
cables. The depth of intake and the pontoon location can be varied if necessary. However,
in practice, the location and size of the decant pool are managed by the discharge locations
and the rate of decant pumping.
A l oating decant system requires power to operate the pumps, adding to operating
costs. A constant and reliable power source is required since loss of power will interrupt
the decant system. Standby pumps and diesel generators are good practice to use in emer-
gency or when the decant system cannot cope with unplanned water surge (e.g. in storm
conditions).

Standby pumps and diesel
generators are good practice to
use in emergency or when the
decant system cannot cope with
unplanned water surge.

Decant Tower
Decant towers are permanent installations established at the i rst stage and raised pro-
gressively as the tailings accumulate. Supernatant water overl ows the rim of the tower,
until the tailings level approaches the overl ow level, at which time the tower is raised.
One or more submerged pumps are located at the base of the tower, discharging to the
mill via a buried pipeline. Access to the tower is usually via a causeway connected to the
embankment.
Decant towers are effective at removing ponded water from a TSF but they are also prob-
lematic (Engels et al . 2006). Decant towers are solid structures while tailings around them
continue to settle over time. This differential movement, together with the ever-increasing
weight of the tailings can crack and damage the decant system (Engels and Dixon-Hardy
2004), eventually leading to impoundment failures (ICOLD and UNEP 2001). Failure may
be subtle and hence remain unnoticed over a long period over time. The Stava disaster in
Italy in 1985 serves as a reminder of a decant conduit failure that led to loss of 269 lives. The
rise in the phreatic water table in the embankment due to the failure of the decant system
caused a rotational slip in the upper embankment. Tailings from the upper impoundment
inundated the lower impoundment which eventually overtopped and failed (Penman 2001;
Davies 2001). Tailings escaped down the hillside and l ooded the town of Stava resulting in
one of the world's worst tailings disasters in terms of loss of human life.

Decant towers are effective
at removing ponded water
from a TSF but they are also
problematic.

Inclined Decant
Inclined decants are similar to decant towers except that, instead of a vertical tower, the
conduit follows the slope of the embankment or side of the impoundment. The conduit
contains a series of ports into which the supernatant water l ows. As the level of the settled
tailings rises toward the active port, this port is plugged, and the pool level rises until over-
l ow commences into the next highest port.