Modern Mining January 2015

DIAMONDS

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

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

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

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January 2015  MODERN MINING  53