Environmental Assessment
Volume E-2
Tailings Facility
Project Description
Ambatovy Project
10
January 2006
•
The specific gravity of the tailings particles is 3.5, while the estimated
average in-situ dry density upon settling in the tailings facility is
1.0 t/m
3
.
•
Sufficient operational water storage capacity must be provided within
the tailings facility to allow adequate storage time to permit solids
settlement. Rainfall generally occurs all year and the peak rainfall
season is during the summer months, between November to March.
Maximum evaporation also occurs during the summer months. The
reported average precipitation for Toamasina is 3,300 mm/yr and the
average pan evaporation is 800 mm/yr in the area. Therefore there is a
substantial water surplus.
•
Construction of a water return pond to store water before it is removed
from the facility. The purpose of this basin is to minimize the length of
time supernatant water remains on top of the tailings.
•
The excess water (runoff and supernatant) under normal operating
conditions will be discharged to the ocean with a small return flow to
the process plant. The Excess Water removal rate will be set to keep the
water levels in the tailings facility within the normal operating range,
except under extreme conditions.
•
In order to minimize the discharge of water directly to the environment,
the tailings facility will also be capable of temporarily storing a 1 in 50
year storm (environmental design storm, EDS) above the maximum
normal operating level (MNOL). Under this scenario, the chance of a
discharge directly to the environment in a 27-year operating period will
be about 50%.
•
Based on the CDA consequence rating the tailings facility will need to
be able to safely pass an inflow design flood (IDF) resulting from the
probable maximum precipitation (PMP). Therefore, sufficient freeboard
(wet freeboard) must be provided above the EDS level to pass the PMP
through a suitably sized overflow spillway. In addition, freeboard above
the IDF maximum flood level (dry freeboard) will need to be provided
to handle wave run-up during a cyclone event.
2.3
PHASED DEVELOPMENT
A three-phase construction plan of Option 4 is described below followed by a
reclamation plan shown in Section 1 of this Volume, Figure 1-3.
Phase 1
Phase 1 will involve discharging tailings within the west end or upstream extents
of the TA-11 valley. The water basin will be located within the west portion of
TA-10 for years 1 to 14. Phase 1 will require the construction of embankments
