Modern Mining January 2016

DIAMONDS

boost revenue per tonne

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

The bases for the mills under construction.

Simplified process flow diagram of the new Cullinan plant.

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January 2016  MODERN MINING  35