Environmental Assessment
Volume D-2
Process Plant
Project Description
After leaching, the leach discharge slurry will be pumped to the slurry
neutralization circuit, where a portion of the free acid in the slurry will be
neutralized with limestone in a series of three tanks. Partially neutralized slurry
will be pumped to the countercurrent decantation (CCD) wash circuit where the
dissolved metals in solution will be separated from the leached solids by
washing, in a series of seven thickeners. Washed solids from the final CCD
thickener will be pumped to a tailings neutralization circuit. A small portion of
washed solids will be diverted to the ferric sulphate circuit for production of a
reagent used to oxidize residual H
2
S in the barren liquor from the sulphide
precipitation area.
Washed solids (tailings) will be mixed in the tailings neutralization tanks for
neutralization with limestone and lime. The neutralization step will precipitate
and immobilize metals; this material will be disposed to the tailings pond.
Nickel and cobalt bearing solution from the first CCD thickener will be treated
with hydrogen sulphide gas to reduce chromium (VI) to chromium (III) before
neutralizing the free acid with limestone. The resulting gypsum slurry will be
thickened and a portion of the gypsum thickener underflow slurry will be
recycled to the neutralization tanks to act as seed to improve the solids settling
characteristics. The remainder will be pumped to the CCD for washing and
disposal to the tailings pond.
The clear product solution from the thickeners will be treated with hydrogen
sulphide in the sulphide precipitation circuit to precipitate nickel and cobalt as a
mixed sulphide intermediate product. Copper, zinc and a small portion of iron
will co-precipitate with nickel and cobalt in the mixed sulphide product.
The mixed sulphides product will be batch filtered and washed with process
water, and delivered to the sulphides bagging and storage. The bagged mixed
sulphide product will then be transported to the port for shipment to the nickel
refinery.
2.3
UTILITIES
Power
Electricity for the Toamasina process plant site will be provided on-site by three
coal-fired 25 MW steam turbine generators. The power plant will not be
connected to an external electrical power grid. Diesel generators will provide
backup emergency power to operate critical equipment and to restart one boiler
Ambatovy Project
9
January 2006
