Cullinan Zest

It has been nine years since LSE-listed diamond miner Petra Diamonds acquired the Cullinan diamond mine from De Beers.

A subsequent large-scale investment into expanding the operation has increased its operational lifespan by at least 12 years, with additional benefits gained from a new, technologically advanced, streamlined and efficient processing plant.

Together this is supported by an entirely new power system and equipment delivered by electrical solutions provider Zest WEG Group.

LAURA CORNISH visited the site to learn more about the project’s exciting development and milestone achievements.

This article first appeared in Mining Review Africa Issue 1, 2019

Petra Diamond’s multi-million Rand cash injection into expanding Cullinan has ensured the mine’s lifespan has continued well beyond De Beers’ original closure date in 2014 - now estimated to run until at least 2030.

The project specifically entails the development of a new C-Cut expansion to open up an entirely new and undiluted ore body (to a depth of 839 m) below the existing and largely mined out resource.

Increased tonnages from the new mine will also increase diamond production to ca. 1.7 Mctpa (1.6 from ROM and 0.1 from tailings) – more than double what the mine was producing from its original mining footprint when the mine changed hands in 2008.

“It takes about 10 years to fully establish a new block cave and all the supporting development and infrastructure but the project has tracked well in terms of delivery schedules and will produce the volume of carats we have forecast to date,” says Cullinan general manager Juan Kemp.

He outlines the key milestone mining developments achieved since the project kicked off:

  • April 2014:First blue (kimberlite) development commenced
  • June 2015:First rings in the undercut blasted
  • FY2016: New production levels established at 839 m and shaft to 930 m commissioned
  • FY2017:1 425 m ore-handling system, allowing for future ramp-ups to utilise full C-Cut footprint
  • H1 FY'17:Ore shaft completed and commissioned
  • Nov 2017:New Cullinan plant reaches ROM nameplate capacity
  • FY2019:Steady state ROM production (4 Mtpa)

The potential to open up additional Phase 2 C-Cuts on the same horizon and evaluate the resource further down is also on the cards for Cullinan’s long-term future.

Process plant support

The original Cullinan process plant, 70 years in age, is the result of decades and decades of expansion and alteration over time but became unnecessarily cumbersome in its flow and efficiency.

Petra Diamonds subsequently injected equal attention into building a new process plant equipped to handle the larger volumes of material while also incorporating new technology to ensure optimal recovery of large diamonds.

It also incorporates the first autogenous mill in the diamond industry is South Africa.

The new plant is an unbelievable achievement for the team and represents significant levels of improvement across the entire process by comparison with the former plant.

Cullinan surface manager Louis Jacobs highlights the key features of the new plant which has been operating at its ROM design capacity for a little over a year now – four years after construction began.

“We were able to reduce our process footprint from 28 ha to 4 ha and are now able to recycle > 95% of our total water use. Through the introduction of variable speed drives and high efficiency motors, we have also reduced our power consumption from 51 kwh/t to 41 kwh/t.”

Overall diamond liberation has also improved significantly and the inclusion of X-ray luminescence (determined as the right technology fit for Cullinan) will aid in protecting and recovering large diamonds.

“Our labour efficiency (tons/man hour) has also improved by a massive 300% while our cost efficiency has improved by about 30% as well,” Jacobs continues.

To date the new plant has already recovered a few diamonds in excess of 200 carats.

Building a new plant while the existing facility remained operational did provide challenges, but nothing the team couldn’t manage.

“The handover went smoothly with only a 21-day downtime period for the switch between plants. To ensure we didn’t suffer production losses, a 400 000 t stockpile was prepared in advance.”

Ultimately, the result of all the teams’ hard work since conception of the entire expansion and upgrade has resulted in the best production year in 25 years for Cullinan.

 Additional achievements delivered in the new process plant:

  • The new plant has only 28 conveyor belts for a total length of 4 km compared to 151 belts for a total length of 15 km in the old plant;
  • The number of transfer points has reduced from 179 to 48;
  • Similarly, the number of crushers has been reduced from 11 to four;
  • The number of screens is down from 88 to 25;
  • Pumps are also down from 121 to 16;
  • The feeders were reduced from 21 to 14;
  • The substations are down from 17 to three;
  • The electrical motors down from 589 to 84;
  • Over 95 % of water is recycled – resulting in a “new” water consumption of less than 0.1 m³/t compared to the 0.3 m³/t required previously; and
  • Power consumption has been reduced from 25 MW in the old plant to 20 MW in the new plant.

New power for a new mine

To ensure the steadfast performance of both mine and plant, Petra Diamonds invested in a R200 million electrical infrastructure upgrade project – awarded to the Zest WEG Group and its subsidiary companies.

Over the course of two years between 2015 and 2017 the Zest WEG Group supplied various energy infrastructure and product packages to the project to cater to its energy requirements.

“The work we have done at Cullinan is a true demonstration of our integrated electrical solutions offering and approach and as such is considered a prestigious flagship project for the company,” says Zest WEG Group Integrated Solutions executive Alastair Gerrard.

Beyond this, Zest WEG Group added a new capability to its service offering on the back of Cullinan – supplying and managing the construction of an 88 kV self-build switching station – according to Eskom’s designs and specifications, funded by Petra and then handed over to Eskom upon completion.

“This arrangement does offer value to both Cullinan and Eskom – a lower cost per ton agreement, a quicker delivery end date and guaranteed power supply.”

Along with the self-build switching station, Zest WEG Group’s Integrated Solutions division designed, supplied and constructed a 60MVA, 88kV/11kV bulk outdoor substation, dedicated to the Cullinan Diamond Mine.

“This substation incorporates three 20MVA transformers, which at the time of the project, were the largest transformers manufactured locally by the Group at our Heidelberg-based WEG Transformers Africa facility.”

In total, WEG Transformers Africa supplied 26 transformers of various sizes with voltages ranging between 88 000 and 380 V, and sizes ranging between 20 MVA and 100 kVA.

Sister company Shaw Controls supplied a variety of equipment into the system as well including motor control centres, the medium voltage mill drive packages, low voltage variable speed drives low voltage field isolator stations as well as containerised sub-stations.

“Where there are motor control centres and variable speed drives, there are of course electric motors. 

"Being a well-known and respected supplier of electric motors in the industry, Zest WEG Group were also involved in the supply of the motors for this project.”

In addition to the 88 kV overhead line connecting the Eskom self-build station to the Cullinan bulk substation, Zest WEG Group company ENI delivered another 6.6 kV line to the new 7 dam pump station.

“The contribution this company made in terms of installation and construction services for the mill expansion was an integral component of our overall work,” Gerrard notes.

"This included the installation of transformers and switchgear, MV and LV electrical reticulation, cable racking, cabling and terminations, lighting and a communication system. Barring the transformers and switchgear – the company executed the same scope as part of the C-Cut expansion project underground as well.

“We are proud of the achievements we have celebrated and shared with Cullinan in the delivery of our energy solution and products, working in partnership and collaboration with the mine’s in-house team.

"This truly demonstrates our diverse product and solutions portfolio, which places our Group in a strategically advantageous position. With our integrated solutions approach, we look forward to many such success cases in the future,” Gerrard concludes.

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