Petra Diamonds and BME have accomplished technological innovation for underground blasting with the implementation of the new Axxis centralised blasting system (CBS), writes Dineo Phoshoko.
Located in Pretoria, Cullinan Diamond Mine is well known for its rare blue diamonds and is considered the world’s largest diamond resource. The mine was acquired by Petra Diamonds in 2008, which has operations in Botswana and Tanzania. In 2017, the mine managed to produce diamonds that made up two-million mega carats in total.
The mine uses two methods: sublevel caving (SLC) and block cave mining. In the old block cave areas, pillar mining (retreat mining) was used. “Similar to more conventional mines that mine multiple narrow reefs on different elevations simultaneously, Cullinan Diamond Mine also exploit different mining blocks within the subvertical kimberlite pipe intrusion on different horizons across the pipe. Mining blocks are remote and demand effort and time to travel between them, making any localised blast system complex to manage, time consuming, and more prone to blast failures and safety breaches,” explains Johan du Plooy, drill and blast engineer at Cullinan Diamond Mine.
The mine is over a hundred years old, having been established in 1903. The mine has two vertical shafts: one shaft for transporting material and people, and the other for bringing out the rock from the mine.
On average, it produces four million tonnes of diamonds a year, averaging 16 000 tonnes each month, and operates on a five-day work week, which works out to 250 shifts a year. There are 210 vehicles, including a fleet of LHDs (six and nine tonnes), utility vehicles, and dozers.
Expansion and new plant
The Centenary Cut (C-Cut) project is expected to ramp up production and is close to full production. The C-Cut, supplemented by primarily the CC1 East block, will deliver the run-of-mine (ROM) production.
The C-Cut Phase 1 expansion project is currently in production at Cullinan. For this project, a new block cave being was established on the western side of the ore body; therefore, block cave mining methods are being used for the expansion project. The CC1 East block uses SLC mining methods. The C-Cut ore body is average grade, whereas the CC1 East has a higher grade. The C-Cut project is expected to add an additional 1.7Mtpa of diamond production and is expected to reach around 4Mtpa from the 2019 financial year. The CC1 East and the integration of the C-Cut project will extend Cullinan’s life of mine to 2028 and beyond. Cullinan still has a significant source of diamond reserves, with a total of 192.74 million carats at a depth of 1 073m.
The current ROM is 34 carats per 100 tonnes; however, with the mine’s expansion plans, ROM is expected to increase to 42 carats per 100 tonnes. By 2020, the project is expected to reach full production, where 81% of the tonnages will come from the C-Cut expansion project.
In 2017, a new plant was built and completed for Cullinan. The plant was implemented in May 2017 and started operating in June 2017. The old plant had a capacity of 2.8Mtpa and produces 2.5Mtpa tailings. The new plant’s capacity will be ramped up to 6Mtpa ROM, with an initial feed of 4Mtpa ROM. The tailings will be between 2.3 and 2.5Mtpa.
There have been significant changes with the equipment of the new plant, one of which is the reduction of the conveyor belts from 151 to 22. As a result, the total kilometres covered by the conveyor belts decreased from 15km to just 3km. The new plant also has 22 screens and seven pumps — a reduction from the previous plant’s 88 and 121, respectively. Due to these and other changes, the new plant will reduce the processing footprint at Cullinan from 26 hectares to five hectares.
The old Cullinan plant was originally commissioned in 1947 and has been revamped over the years since its initial construction. The plant’s age and operational complexity make it expensive to maintain and costly to operate — especially given the large size of its footprint.
Construction and maintenance of the new plant are much lower compared to the old one. This plant uses less power and water. In addition, the new plant has been designed for a higher extraction percentage, targeting a 97% extraction factor. The latest processing technology implemented at the new plant makes achieving the targeted extraction percentage possible. The technology includes autogenous milling, Bourevestnik X-ray machines, and high-pressure grinding rolls. The new plant is expected to optimise recoveries and improve mining costs.
Benefits of CBS
Neil Alberts, BME’s product manager, explains that the CBS was designed to improve efficiencies and production from a safety and operational point of view. Petra Diamonds became a design and development partner for the CBS. Due to the nature of underground blasting and the risks involved in trialling such a system in a real production environment, finding a mine that would be receptive to the idea was not be easy.
Discussions between BME and Petra about the CBS started around 2016. “In 2017, early March, we started around the design of the system,” says Alberts. The CBS was installed at Petra’s Cullinan Diamond Mine in December 2017, with the first blast taken live on 14 March 2018. Cullinan continues to use the system.
Alberts believes that one of the unique offerings of the underground blasting system is that it takes a leap forward in communication technology. “The cornerstone of the system is that it introduces sophisticated capabilities without clouding or complicating the operation. That’s what made this partnership with Petra so easy at the end of the day.”
The implementation of the CBS at the mine, in December 2017, included extensive engagement with internal and external stakeholders. With safety as a key consideration, the system offered improved control over seismicity (where relevant) and reduced risks of underground employees coming into contact with potentially harmful gases and fumes emitted by explosives. Ease of use within a mining environment is a key feature of the system, as it is compatible with the current infrastructure of the mine. So far, over 80 successful blasts have taken place at Cullinan with the CBS.
With the ability to thoroughly test the complete blast circuit up to each detonator in each blast area prior to the blast, it is expected that blast events will consistently deliver the expected results safely, every time, on time and within a smaller window, allowing for shorter shift changes and optimised value.