Modern Mining January 2019

DIAMONDS

markedly superior on every measure, with a compact footprint of just 4 ha and far fewer items of equipment. For example, it has only 28 conveyor belts with a total length of 4 km compared to 151 belts with a total length of 15 km in the old plant while the number of transfer points has reduced from 179 to 48. Similarly, the number of crushers has come down from 11 to four, screens from 88 to 25, pumps from 121 to 16, feeders from 21 to 14, substations from 17 to three and electrical motors from 589 to 84. Power consumption has been reduced from 25 MW

in the old plant to 20 MW in the new (despite it having a bigger capacity), reflecting the use of modern technologies such as Variable Speed Drives (VSDs) and energy-efficient motors. Over 95 % of water is recycled – resulting in a ‘new’ water consumption of less than 0,1 m 3 per tonne compared to the 0,3 m 3 per tonne required previously. The new plant utilises ‘gentler’, more diamond-friendly processing methods with all material fed to the plant being exposed to autogenous milling (representing comminution via attrition rather than crushing). The +55 mm particles in the mill discharge are processed via a jaw crusher back to the mills, while the -55+8 mm particles are checked for large dia- monds via the X-ray luminescence (XRL) plant before the +19 mm particles return to the mill for further attritioning. The -19 mm material is being liberated through High Pressure Grinding Rolls (HPGR), thereby moving away from high- impact cone crushing. The two 6,4 MW Polysius autogenous (AG) mills, supplied by thyssenkrupp, are the first AG mills to be used in a South African dia- mond mine although there are AG installations in Angola and Botswana. They each have a diameter (inside shell) of 9,2 m and an effec- tive grinding length of 4,6 m. Also a ‘first’ – at least for Petra – is the use of XRL sorters from Russian company Bourevestnik. XRL technology has been in use within Petra for years but this is the first time the company has opted for Bourevestnik machines. Bourevestnik has supplied 24 sort- ers in total, all of them specifically upgraded on the basis of Petra’s requirements, which

as well as raise bore drilling of 5 786 m to estab- lish an appropriate ventilation system. According to Kemp, Cullinan’s life could potentially extend decades into the future, given that the current C-Cut block operation will only exploit approximately 15 % of the total resource at the mine. “Although it will obviously depend on market conditions and many other factors, one can see at least another 50 years of life at Cullinan,” he said. New block caves are typically ten-year proj- ects but Petra has bettered this. Work on the C-Cut declines started in 2010, the first under- cut rings were drilled and blasted in mid-2015 and first production was recorded in 2016. The project is now essentially complete and in a ramp-up phase with steady-state ROM produc- tion of 4 Mt/a (which includes ore from CC1 East) expected to be achieved in the 2020 finan- cial year (to 30 June 2020). Cullinan can also treat up to 2 Mt/a of tailings. While it is not specifically part of the C-Cut project, the new processing plant does form part of Petra’s overall strategy of totally transform- ing Cullinan. When Petra took over the mine, the plant it inherited was already more than 60 years old (it was commissioned in 1947), extended over a vast area of ‘real estate’ – 27 ha to be precise – and due to its complexity and incorporation of dated technology was increas- ingly costly to maintain and operate. Moreover, it was based on old crushing technology, with the attendant risk of diamond breakage. In his presentation to the media group, Cullinan’s Surface Manager, Louis Jacobs, described the old plant as a “monster” and said the new plant – with a capacity of 6 Mt/a – was

The mill recycling belt in the new Cullinan plant’s compact configuration, which has reduced the length of conveyors from 15 km to just 4 km.

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32 _ MODERN MINING _ January 2019