Mining in focus
Richards Bay Minerals makes use of dredging methods to
extract minerals in the shallow water of the Indian Ocean
on the east coast of South Africa.
are looking at deposits in the Indian
Ocean. Nauru, Tonga, Kiribati, the
UK and Belgium intend to explore
poly-metallic nodules in the Pacific
Ocean. China, Korea, India, Russia and
France have made their intent clear
to explore the Indian and Atlantic
Oceans for hydrothermal sulphides. The
International Seabed Authority is in the
process of developing environmental
management regulations for mining
in areas beyond national jurisdiction.
Papa New Guinea was the first
country to sign off on commercial
seafloor mining in its waters. The Pacific
Rim island leases an off-shore site called
Solwara 1 to Canadian based Nautilus
Minerals. Nautilus is gearing up its
equipment fleet in preparation to begin
copper production at Solwara 1 in the
first quarter of 2018. New Zealand’s
Environmental Protection Authority
also approved Trans-Tasman Resources’
application to mine iron sands from the
seabed of South Taranaki Bight, located
22km to 36km offshore from Patea.
Minerals of the sea
Ocean-hosted mineral deposits can
range from heavy mineral placer
deposits to chemical precipitation from
solution and concretions or deposited by
so-called ‘black smokers’ hydrothermal
vents. Seafloor poly-metallic nodules,
ferromanganese crusts and hydrothermal
sulphides are potential sources of
[26] MINING MIRROR FEBRUARY 2018
millions of tonnes of metals such as
copper, nickel, cobalt, manganese and
iron. It estimated that up to 358 000t
of metals can be recovered annually
from marine mining operations.
Unconsolidated deposits include
construction materials such as sand,
gravel and shells. Heavy mineral placers
contain materials like titanium, tin
and gold, while metalliferous muds
and nodules oozes silica and calcium
carbonate. Consolidated deposits include
bedded deposits such as coal and iron
ore; crusts, such as the cobalt-rich
manganese oxides; massive sulphide
deposits in the form of mounds and
stacks occurring at spreading centres; and
essentially tabular veins or mineralised
channels in consolidated host rocks.
Marine mining methods
Marine mining operations employ
various methods of collecting and sorting
materials obtained from the seabed. Bulk
marine sediment mining entails using a
trailing suction hopper dredger, a ship
with powerful suction pumps that run
to the seabed, that is used to dredge the
seabed up to a depth of about 3m to
remove marine sediment. This sediment
is then either transferred to the shore
where minerals are extracted before
excess sediment and water is released
back into the ocean, or it is suctioned
on board the vessel where larger
phosphate-bearing sediment is separated.
Mining kilometres under the ocean surface
requires special automated equipment.
This is the proposed method of marine
mining off the Western Cape coast.
In a dragline dredge operation material
is recovered by large dredge buckets
that scrape slabs and nodules from the
surface of the deposit and feed them
into barges for transportation to shore.
In contrast a crust miner is a vessel
equipped with a hydraulic lift system
with an active bottom miner. The miner
is a self-propelled tractor, controlled
from the surface vessel, capable of
breaking and removing the thin crust
and feeding it to the hydraulic lift system
through a hydro-cyclone to separate
entrapped substrate. The roughly cleaned
ore is pumped to the surface vessel
for further cleaning and transported
to shore. A continuous line bucket
dredge works on a similar operation
principal, but is a continuous operation.
Excavation of seabed materials can
also be done in shallow marine settings.
Clamshell buckets are mechanically
actuated to ‘bite’ into the seabed and
remove material. It is a highly complex
operation due to multiple cables to
actuate the grabs, particularly in heavy
seas where wave compensating devices
are required. A bucket ladder dredge
delivers a virtually water-free product
to the mineral dressing plant on board
the dredge and is mostly used in the
recovery of heavy minerals, such as gold.
Bucket wheel suction dredges use a small
diameter bucket wheel mounted on the
suction ladder to excavate material. Very
high torque is applied to the wheel that
delivers the excavated material directly
into the mouth of the suction pipe for
transportation to the sea’s surface.
Anchored suction dredges can
only be used in very shallow marine
environments and leave pits in the
ocean floor. Cutterhead suction
dredges excavate compacted, granular