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January 2015
MODERN MINING
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DIAMONDS
Top projects
large diamonds and understand that there was
a large-diamond population outside of the
original resource model estimation. We quan-
tified this by undertaking a bulk sampling
campaign on the South Lobe and we subse-
quently published a new NI43-101 compliant
resource statement reflecting this characteris-
tic of the orebody. To handle the large stones
and ensure that we realise their full value, we
decided that the upgrade project must include
a ‘Large Diamond Recovery’ circuit treating
material up to 60 mm in size. This will permit
Karowe to recover an undamaged diamond of
well over 1 000 carats.”
The upgrade of the comminution circuit,
currently underway, involves a new secondary
gyratory crusher (a Kawasaki machine supplied
by IMS) being installed, with a variable scalp-
ing screen arrangement ahead of the crusher to
maintain a controlled percentile of larger mate-
rial to act as grinding medium in the AG mill.
There will be a bleed circuit ahead of the AG
mill to allow finer material to bypass the mill
and continue on to the downstream processes.
The AG mill has been fitted with Turbo Pulp
Lifters and re-designed discharge grates. The
modifications will allow for flexibility around
the AG mill so that its operation can be opti-
mised to varying degrees depending on the
hardness of the ore.
To cater for the anticipated increase in DMS
yield and assist with the recovery of large
stones, Lucara has elected not to increase the
DMS capacity, but to go for new technology
that represents a ‘first’ in the diamond mining
industry. Says Day: “The +1,5 mm, -8 mm size
ore fraction will continue to report to the DMS
but the other size fractions will report to a new
bulk sorting circuit with a maximum capac-
ity of 415 t/h. This is the most technologically
innovative and exciting part of the upgrade
and consists of five high-capacity X-ray
Transmission (XRT) machines, which are able
to identify the carbon signature of diamonds
(atomic number) and eject them separately
from the gangue material. Each machine treats
a different size fraction, and can sort efficiently
at up to 100 t/h depending on the size fraction
being treated.
“The concentrate from the XRT circuits
is delivered directly to a glove box and hand
sorted in the conventional mode. XRT rejects
either go to tailings or back into the circuit
for milling or crushing and further diamond
liberation. This again is dependent on the
size fraction. There will also be a sixth audit
machine on the tailings stream to check upon
operational efficiency. We believe the system is
unique in the world in terms of primary dia-
mond recovery although XRTs have been used
to treat recovery tailings in some operations.”
The EPCM contractor for the plant upgrade
is DRA. “It makes sense for us to be using DRA
again,” comments Day. “They delivered the
original plant under budget and on schedule,
so – based on the fact that the relationship with
themworked the first time around – we engaged
them directly rather than going out to tender.
It’s going well, though a brownfield expansion
is a very different animal to a ‘new-build’.
“To supply the XRTs, we’ve chosen TOMRA
Sorting Solutions of Germany. We undertook
an extensive suite of testwork with TOMRA
and the other suppliers of XRT equipment. We
A striking shot of the AG
mill, the first of its type in
Southern Africa (with the
exception of some units at
the Catoca diamond mine
in Angola).
“What was not
originally realised
when the plant
was designed
is that Karowe
would become a
prolific producer
of large stones.”
Paul Day, COO, Lucara