Modern Mining: Featured News

Having provided two of the largest diamonds in the Crown Jewels, the legacy of large and high-quality stones being unearthed at Cullinan mine near Pretoria continues; little wonder, then, that Petra Diamonds’ new processing plant at Cullinan promises to go gentler on the mine’s over-sized discoveries.

By any standards, the installation of this exciting new and considerably more efficient plant is long overdue – replacing a sprawling 26 ha installation that was first commissioned in 1947 and evolved with multiple alterations and upgrades since then. With the go-ahead for construction granted in April 2015, the project was completed and commissioned in September 2017. The company estimates it will finish the project largely in line with the April 2015 cost estimate of R1,65 billion, adjusted for escalation and currency fluctuations. The main contractor behind the engineering, procurement and construction of the plant was Amec Foster Wheeler’s South African arm, MDM Engineering.

A view of Cullinan diamond mine’s new buildings housing its high pressure grinding rolls (HPGRs), X-ray luminescence (XRL) machines and final recovery phase.

According to Petra, all components of the Cullinan plant are now fully operational, with ramp-up and optimisation activities ongoing; the plant is expected to achieve nameplate capacity of 6 Mt/a during the second half of the 2018 financial year.

Key among the technology advances has been the focus on comminution through attrition, rather than the less forgiving high-impact crushing that characterised these functions in the old plant. This approach employs autogenous (AG) milling for the +25 mm material, and high pressure grinding rolls (HPGRs) which promote inter-particle crushing; this equipment was supplied by thyssenkrupp Industrial Solutions South Africa – formerly thyssen­krupp Polysius. There is also a top-cut of 75 mm to cater for diamonds of over 3 000 carats – an important consideration for the mine that produced, up to Petra’s acquisition in 2008, a quarter of the world’s diamonds over 400 carats in size.

The plant also makes use of X-ray luminescence (XRL) to replace dense media separation – with only the -12 mm material reporting to the DMS circuit. The XRL machines have been supplied by Russia’s Bourevestnik, one of the pioneers in this field.

There are already signs that the new plant is delivering value, according to Johan Dippenaar, Petra Diamonds’ CEO, with the XRL modules of the plant put into operation during September 2017.

“Encouragingly, two plus-200 carat stones have already been recovered, proving the efficacy of the large diamond recovery circuit,” said Dippenaar in his October 2017 trading update. “However, both these stones were of poor quality and low value.”

Since Petra acquired the mine, Cullinan has produced a number of large, high-value stones including a 507,5 carat white diamond – the ‘Cullinan Heritage’ – worth over US$35 million, and a 122,5 carat blue diamond worth almost US$28 million.

Dippenaar expects the plant ramp-up and optimisation into early 2018 to give clarity on the economic level of recovery of the fine diamonds, which are the small stones of minus 5 Diamond Trading Company (DTC) sieve size; they constitute only a small percentage of overall value, however.

The plant’s comminution improvements will contribute to the mine’s strategic aim of improving its recovery rates and better grades. Despite the much-reduced footprint of only 5 ha – just a fifth of its predecessor’s – the new plant will deliver even more punch. Its processing capacity of over 6 Mt/a is an improvement on the old plant’s 5,3 Mt, and will initially comprise 4 Mt ROM and the remainder from tailings. The ROM treatment will deliver 2 Mct a year, with 0,2 Mct coming from the tailings.

The new design, more modern technology and reduced size have significantly improved the energy efficiency of the material flow and lowered operating costs. Overall, a 6 % saving in the maximum electricity demand costs has been achieved, following new 88 kV take-off from Eskom, and the plant as a whole will be 12 % more energy efficient per tonne processed.

There are a range of contributors to this achievement, including an average 5 % efficiency improvement through the use of IE3 rated Top Premium electric motors – which deliver an almost constant efficiency in the 75-100 % load range. Energy savings on all conveyor drives and pumps will be further enhanced through controlling them with variable speed drives (VSDs).

Standardisation on LED lighting in and around the plant is another energy saver, while all motor control centres (MCCs) are equipped with multi-step, low-voltage power factor correction units, enhancing their ability to manage current load and reducing the risk associated with medium-voltage units. Powering the mills with a medium-voltage VSD drive motor combination makes for more efficient control of the load and torque required.

The leaner infrastructure comprising the new plant is well reflected in the drastic reduction in the sheer volume of equipment in operation. The number of conveyor belts has been reduced from 151 in the old plant to just 22 – cutting the distance covered by these belts from 15 km to 3 km. There is now less need to pump slimes due to the new design that allows slimes to be gravity fed to the tailings dams, and the number of pumps is down from 121 to only nine. With the slimming down of conveyors and other equipment, the number of electrical motors required is now only 84 – compared to the 589 motors in the old installation.

Water saving – an imperative of growing concern in South Africa – has also been decisively addressed in the new plant; the amount of water consumed by the plant per tonne of material processed will be 0,06 m3 – a 65 % improvement on the 0,17 m3/t previously.

It is expected that the plant will increase liberation of diamonds across the total spectrum, leading to a grade improvement of about 10 %. The amount of +1 mm unliberated kimberlite particles in the tailings will reduce from 60 % to 40 %, while the -1 mm slimes will do the reverse – rising from 40 % to 60 %.

Security has been enhanced through a totally ‘hands-off’ final recovery system; new final recovery sorting machines now concentrate load reduction from 50 t/h to 25 t/h.

Improved liberation (and hence reduced tonnes in ciruculation), less material reporting to DMS, reduced need for step-wise crushing (due to AG milling) and higher levels of automation are expected to lead to savings in operating expenditure of R20-R25 per tonne.

The new Cullinan plant will certainly be a useful test for AG milling in South Africa; this technology is well proven by Russia’s Alrosa in the processing of kimberlite ore, but has only in recent years been applied in African countries such as Angola and Botswana.

Report by Paul Crankshaw, photos courtesy of Petra.

Contact Modern Mining

Title: Editor
Name: Arthur Tassell
Email: mining@crown.co.za
Phone: +27 11 622-4770
Fax: +27 11 615-6108

Title: Advertising Manager
Name: Bennie Venter
Email: benniev@crown.co.za
Phone: +27 11 622-4770
Fax: +27 11 615-6108

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