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January 2016
MODERN MINING
35
DIAMONDS
Top projects
boost revenue per tonne
Simplified process flow
diagram of the new Cullinan
plant.
The bases for the mills under
construction.
It is expected to improve the recovery of the
full spectrum of diamonds and improve the
efficiency of the material flow, thereby signifi-
cantly lowering operating cost by as much as
R20 to R25 per tonne. The grade will effectively
be increased by approximately 10 % (ROM and
tailings). The +1 mm unliberated kimberlite
particles in tailings will reduce from the cur-
rent 60 % to 40 % (post AG mill) while the
-1 mm slimes will increase from the current
40 % to 60 % (post AG mill).
One of the guiding principles behind the
design of the facility is that it should reduce
diamond breakage, thereby increasing revenue
from larger/exceptional diamonds. This reflects
the fact that Cullinan – which produced the
3 160-carat Cullinan diamond, the largest gem
diamond ever recovered, more than a century
ago – is known for its prodigious output of
large, high quality stones. At the time it was
taken over by Petra in 2008, it had produced a
quarter of all the world’s diamonds greater than
400 carats. The trend has continued since then
and significant recoveries under Petra’s man-
agement have included the
Cullinan Heritage
,
a 507-carat stone recovered in 2009 (which
was sold for a world record US$35 million); an
exceptional 232-carat white diamond in 2014
(which was subsequently sold for just over
US$15 million); a 29-carat blue diamond which
was sold in the rough for US$25,6 million (a
world record +US$860 000 per carat) and
which was subsequently cut into the perfect
12-carat
Blue Moon of Josephine
and sold for a
world record US$48,5 million; and a 122-carat
blue diamond (which was sold for US$27,6
million, with Petra retaining a 15 % share in
the polished yield).
To prevent breakage, the new plant will