Mine excursion
which ensures that the plant actually copes
with what it is fed,” Barnard explains.
The Otjikoto circuit
According to Sion Kamati, plant
metallurgist at Otjikoto, there are
slight differences between the Otjikoto
plant and conventional gold mining
circuits. “The major difference is that
we have installed four Knelson gravity
concentrators, which basically constitutes
our gravity circuit, and that is one of the
most important parts of the Otjikoto
Mine excursion
and oxidising agents. The process takes
about 12–14 hours, which is quicker
than a conventional gold circuit.
However, you probably would not be
able to leach out the coarse gold in a
conventional circuit.
The plant is designed to be able
to treat the three main ore types,
designated as XR1 (oxide), XR2
(transition), and XR3 (fresh). ROM
ore from the open pit is fed to a 750
tonnes per hour (t/h) crushing plant,
which consists of a gyratory crusher and
The processing plant at Otjikoto is one of the main reasons the mine operates so efficiently. Recoveries at
the plant at times exceed 98.7%.
Overview of the area’s geology
The Otjikoto Project is located within the Neoproterozoic Damara Mobile Belt, which
forms part of the pan-African mobile belt system. The deposit falls under the general
classification of an orogenic gold deposit and occurs in a similar stratigraphic position
as the Navachab gold deposit, Namibia’s other operating gold mine.
The deposit is hosted within amphibolite-grade metasediments of the Okanguarri
Formation. The OTC albitite-hornfels unit hosts most of the mineralised vein system
and is underlain by a distinct marker horizon, the un-mineralised OTB calcitic marble.
Gold in the main Otjikoto deposit is hosted by a north-northeast (NNE)-striking
sheeted sulphide (+ magnetite) - quartz+carbonate vein system. The sheeted veins and
related gold mineralisation occur in a series of distinct en-echelon zones. The bulk of
the mineralisation occurs in the Central and West shoots. Vein concentrations range
from one to 30 veins per metre, with higher vein concentration within the shoots. The
mineralised system has been traced over a strike length of 2.3km and to a depth of
475m below surface.
plant. In comparison to other processing
plants, where only a small percentage of
its material is fed into a gravity circuit, at
Otjikoto, 100% goes into the circuit, of
which we recover 60–70% of our gold by
means of gravity,” says Kamati.
A Knelson can best be described as
a big centrifuge — a bowl with several
perforations, which spins and generates
high G-forces within. Because gold is
heavy, it flows to the outside and down
through the bottom and inside the
vortex. Anything light will be drawn
out through the top and enter the leach
circuit. Then coarse gold enters a leach
reactor, where the gold is leached out
with high concentrations of cyanide
www.miningmirror.co.za
a conveyor system that feed the crushed
ore stockpile.
Material is reclaimed from the
stockpile, which has 15 hours’ live
capacity, and treated in a grinding circuit
consisting of a 24ft by 14.5ft-high aspect
ratio semi-autogenous grinding (SAG)
mill and a secondary 16.5ft by 28ft ball
mill. The entire ball mill discharge stream
is treated in a gravity concentration circuit
for recovery of coarse gold and the gravity
concentrate is processed in an intensive
cyanidation leach circuit.
The gravity tailings product is thickened
to 45% solids and treated in a pre-
aeration/cyanide leach circuit. The leach
product stream flows by gravity to a
FEBRUARY 2019 MINING MIRROR [19]
FEBRUARY 2019 MINING MIRROR [19]