Operations
B2Gold’s processing plant at Otjikoto achieves recoveries of gold as high as 98.6%.
According to the original bankable
feasibility study, the plant should operate
at a recovery of about 96.7%. In addition
to the higher recovery rates, the plant is
processing more material than originally
designed. The mining team is pushing
3.3 million tonnes run of mine (ROM)
through the plant, while the plant was
designed to run at 2.5 million tonnes and
upgraded to 3.1 million tonnes per annum.
The power of gravity
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
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.
28 AFRICAN MINING MARCH - APRIL 2019
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 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 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 carbon in pulp
(CIP) circuit for recovery of gold in solution.
The tailings stream from the CIP circuit is
treated in a cyanide destruction circuit using
the sulphur dioxide/air process, before being
pumped to a lined tailings storage facility.
Gold is recovered in the CIP circuit by
loading it onto carbon, and the loaded carbon
is sent to a split Anglo-American Research
Laboratories (AARL) elution circuit. Gold
solutions from the gravity-intensive leach
circuit and elution circuit are treated in an
electro-winning process, followed by smelting
to produce doré bars.
“The bottom line is that each mine has
to adapt its processing plant according to
what it has in the ground,” says Barnard.
“B2Gold’s plant at Fekola in Mali also runs
at high recoveries, although they have a
grade of about two grams per tonne. The
major difference is that Fekola doesn’t have
a gravity circuit. The plant has a different
design because the ore body it treats is
completely different,” Barnard adds. b
The grade at Otjikoto, nonetheless, remains
low at about 1.6 grams per tonne. To
recover the gold, Otjikoto mines a lot of
waste, and moves on average about 106 000
tonnes of material per day, of which only
approximately 10 000 tonnes are milled
— thus at a high average stripping ratio
of roughly 11:1, excluding the low-grade
ore, and roughly 7:1 if the low-grade ore is
taken into account.
Mark Dawe, country manager and
managing director of B2Gold.
www.africanmining.co.za