Environmental Engineering Reference
In-Depth Information
pipeline, of slurried coal, ore and ore concentrates has been successfully accomplished
for many years. Examples include the 112 km copper concentrate pipeline operated
by Freeport McMoran in Papua, Indonesia, the 134 km ore slurry pipeline being con-
structed for the Ramu Nickel Project in Papua New Guinea and the 440 km long coal
slurry pipeline from the Black Mesa Mine in Arizona to the Mojave Power Sation in
Nevada, USA).
●
Deep water, at least 100 m in depth, needs to be present reasonably close to the coast.
In many volcanic island situations such as Misima and Lihir Islands in PNG, the sea-
bed slopes steeply from the coastline and 100 m water depth occurs within 200 m of the
shore. In other areas, the slopes are less steep. At Batu Hijau, the outfall is at a water
depth of 120 m, 3.2 km offshore, while at Pechiney's DSTP site in the Mediterranean
Sea near Marseilles, the outfall is located at a depth of 320 m, 5.6 km offshore.
●
The seabed slope at the outfall should be at least 10º to ensure that the tailings slurry con-
tinues to l ow downslope as a bottom-attached density current, into very deep water.
●
Flattening of the seabed, where main tailings deposition will occur, should be in very
deep water - 400 m or more.
●
There should be no upwelling currents below the level of the tailings pipeline outfall.
●
The surface mixed layer should not extend to the level of the tailings pipeline outfall.
In many areas the surface mixed layer never exceeds 80 m in thickness. However, in
some areas, particularly those that experience cyclones, hurricanes or typhoons, mixing
during such events may penetrate to a depth of 120 m or more, for short periods.
●
Seabed currents in the deposition areas should not be sufi cient to cause resuspension
of tailings solids.
●
The seabed along the course of the marine pipeline route should be stable. This is a
concern in the case of volcanic islands where very steep slopes of 40º or more may occur
near the shore. Such slopes are frequently subject to submarine landslides. In fact, dur-
ing operation of the DSTP system at Misima in PNG, a submarine landslide occurred
which severed the pipeline. The new discharge point was still sufi ciently deep that the
tailings continued to l ow downslope as before the landslide, and there were no envi-
ronmental consequences from the event.
Other environmental conditions that are favourable but not essential include:
1. the presence of a submarine canyon into which the tailings slurry can be discharged
(e.g. as found at the Batu Hijau DSTP site, see
Case 18.5
). The canyon serves to con-
i ne the tailings density current; and
2. the presence of sediment in the deposition area with particle sizes similar to those
of the tailings. In practice this is common because both tailings and natural sediment
obey the same laws of physics, with the result that similar size fractions will settle at
similar locations on the seabed. This is normally the case for a slope that gradually
l attens. Where deposition occurs in a more or less enclosed depression or basin, all
sediment size fractions will accumulate in more or less the same area. In this case, the
tailings may have quite different characteristics to the natural sediment.
From a variety of marine tailings
disposal projects over the
past 30 years, a considerable
body of experience has been
accumulated, so that it is now
possible to design DSTP systems
and to predict their associated
environmental impacts with
confi dence.
From a variety of marine tailings disposal projects over the past 30 years, a considerable
body of experience has been accumulated, so that it is now possible to design DSTP sys-
tems and to predict their associated environmental impacts with coni dence. The main
features of modern DSTP systems are shown in
Figure 18.18
.
Important components are as follows:
1. Overland Pipeline is usually constructed of welded steel, as shown in
Case 18.5
, and
supported just above the ground surface on concrete stands;
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