Modern Mining January 2016

DIAMONDS

the technology has been adopted for two dia- mond projects in Botswana – the new Karowe mine near Orapa and the Ghaghoo mine in the Central Kalahari. “As regards HPGR machines, they were first introduced at Cullinan more than 15 years ago – in fact, Cullinan was the first kimberlite mine to make use of the technology. XRF is also well proven, both in concentrating and final recov- ery roles.” The new plant will be extremely energy effi- cient – Petra estimates that it will deliver a 12 % increased energy efficiency per tonne compared to the existing plant. IE3 Top Premium motors, enabling an almost constant efficiency in the 75-100 % load range, will be used throughout the facility while variable speed drives will drive conveyors and pumps. Motor Control Centres (MCCs) will be equipped with multi- step, low voltage power factor correction units, significantly enhancing the ability to manage current load, and all lighting will be energy efficient. In addition, the use of a gravity-feed slimes disposal system will reduce the need

utilise AG milling, which Petra describes as a gentler recovery process that breaks down ore via attrition rather than crushing, and HPGR technology, which employs inter-particle crush- ing. The +25 mm material will only be exposed to AG milling while the -25 mm material will be liberated through the HPGRs, eliminating the need for high impact cone crushing. The plant will also make use of XRF to replace Dense Media Separation with only the -12 mm material reporting to the DMS cir- cuit. The XRF machines are being supplied by Russia’s Bourevestnik, one of the pioneers in the XRF field. While some of the technology being deployed in the new plant is relatively new to Southern Africa, it is nevertheless well proven. Jim Davidson, Petra’s Technical Director, com- ments: “AG milling is new to South Africa – and to Petra – but AG mills have been used by Russia’s Alrosa for many years and their capa- bility in the processing of kimberlite ore is well established. In Africa AG mills are installed at the Catoca mine in Angola and more recently

The existing plant at Cullinan (seen here) covers 26 ha. It will eventually be dismantled, removed and mostly sold for scrap.

Schematic of the new processing plant.

to pump slimes. Total power con- sumption will be 25 MW compared to the 22,5 MW of the existing plant but the new plant will treat 6 Mt/a as opposed to the 5,3 Mt/a handled at present. Water consumption will also improve – by a massive 66 %. Present water consumption is 3,5 m 3 per tonne treated while the new plant will only require 1,2 m 3 per tonne treated. While ‘brownfields’ projects can sometimes be problematic, Davidson says that Petra is confi- dent that construction will proceed smoothly. “There are integration

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36  MODERN MINING  January 2016