Environmental Engineering Reference
In-Depth Information
FIGURE 18.17
Deep Sea Tailings Disposal Illustrated
Nearshore Littoral Zone
Surface Mixed Zone
Euphotic Zone
The risk associated with DSTP
depends on the discharge depth, and
is determined by three distinct layers
that characterize the ocean.
Tailings
Discharge
Pipeline
Entrapment Depth of
Horizontal Plume of Tailings Fines
Tailings Outfall
Deep-Sea Zone
Path of Tailings
Density Current
Area of Final
Tailings Deposition
It becomes appropriate at this point to mention the practice of marine tailings disposal
by ship rather than by pipeline ('ocean dumping' of tailings). Examples include marine
red mud disposal practised by various alumina plants (three rei neries in Japan; one in
France and one in Greece) or disposal of Jarosite residues from the Risdon Zinc Rei nery
by Pasminco in Tasmania prior to 1994. Ocean dumping of tailings is increasingly chal-
lenged by environmental groups. It is fair to assume that public pressure will eventually
lead to a discontinuation of this practice, irrespective of whether or not there are serious
environmental impacts.
Deep Sea Tailings Placement
Despite the controversy that invariably accompanies any proposal to dispose of mine tail-
ings (or any other waste) in the sea, submarine tailings placement, particularly the most
recent improvement known as deep sea tailings placement (DSTP), warrants serious con-
sideration wherever the required environmental conditions are present. DSTP involves
discharge of tailings slurry near the seabed in water depths of 100 m or more. The tail-
ings slurry l ows down slope under gravity as a coherent, bottom-attached density current.
The tailings solids eventually deposit on the seabed at the base of the slope, in very deep
water - typically in water depths exceeding 1,000 m, well below aquatic zones with high
biological productivity ( Figure 18.17 ). The risk associated with STP depends on the dis-
charge depth and is determined by three distinct layers that characterize the ocean (Jones
and Jones, 2001). The surface mixed layer in the upper ocean layer is well mixed by wind
and waves and is generally of uniform temperature, density and salinity. The bottom of
the surface mixed layer is marked by an abrupt change in density and temperature. The
euphotic layer is dei ned as the depth reached by only 1% of the photosynthetically active
light transmitted from the ocean surface. The euphotic zone with highest biological pro-
ductivity may reach a depth of several tens of metres. The deep-sea layer lies beneath the
euphotic zone. Biological productivity is low.
Conditions required for DSTP include:
Despite the controversy that
invariably accompanies any
proposal to dispose of mine
tailings in the sea, submarine
tailings placement, particularly
the most recent improvement
known as deep sea tailings
placement (DSTP), warrants
serious consideration.
The processing facilities need to be located within reasonable proximity to deep sea.
Of the projects undertaken to date, overland tailings slurry pipelines of up to 25 km
have been used. However, studies for projects not yet constructed have indicated that
pipeline lengths of 200 km or more may be feasible in some cases. (Transportation by
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