AS the world’s more accessible copper deposits become depleted, miners are turning to block cave mining: a highly-productive, low-cost underground method by which deep copper deposits are carefully collapsed under their own weight.
While this comparatively new technique is expected to become the most widespread form of copper mining by 2020, the Northparkes gold and copper operation – a joint venture between Rio Tinto (80 per cent) and Sumitomo (20 per cent) – trailblazed block caving in Australia more than 15 years ago. Northparkes is one of Rio Tinto’s smallest copper operations, but its importance to the future of efficient and sustainable underground copper mining outweighs its size. The company has continued to refine and develop its block caving process, and Northparkes remains the only operation in Rio Tinto’s copper portfolio to record a year-on-year production increase.
Northparkes, 27km north of Parkes in NSW, comprises 6000 hectares including a 1630ha mining lease. The operation had produced about 750,000t of copper and 1 million ounces of gold to the end
of 2010 (about 20 per cent from the open pits and 80 per cent from underground) during its life.
Ore is currently sourced from the E48 Lift 1 block cave mine – the third mine on site – and open-cut stockpiles. About 5.8 million tonnes of ore is processed on site each year to produce between 140,000t and 150,000t of copper and gold concentrates that are shipped to smelters in Japan, China and India.
The small, deep and low-grade nature of the Northparkes deposits — which are vertically continuous and in close proximity – have excluded the use of many conventional underground mining methods. While the near-surface deposits were suited to open cut mining, the deeper sections presented more of a challenge.
They could not be economically extracted using conventional underground mining methods such as long hole open stoping so, after much research and study, Rio Tinto adopted the block cave mining technique.
This innovative method allowed the operation to achieve very low mining costs and high productivity, mainly through the efficiency of its automated material handling systems that minimise ore rehandling.
However, Rio Tinto has conceded that block cave mining – still in its relative infancy – remains technically challenging and incorporates a number of unexpected risks. Consequently, the construction lead-time for a block cave mine is significant, typically between three and four years, and significant upfront planning is required.
Northparkes has exploited the full value of the method by continuously improving and innovating during the last 15 years. The block cave layout has evolved significantly, with each subsequent mine representing an improvement on the previous version.
The Step Change project Rio Tinto’s Step Change project at Northparkes could potentially increase output from 8.5 million tonnes per annum to 30mtpa through the construction and expansion of multiple underground block caves, making it one of Australia’s largest copper mines. Northparkes mine manager Stefanie Loader said three more block cave mines could be built at the site in addition to the E48 block cave mine in production, extending mine life beyond 2024.
The project involves the expansion of mining operations including depth extensions to three existing ore bodies (E22, E26 and E48) and the development of one new ore body (GRP 314) through expansion of the Northparkes block cave mining method.
As a result of the expected increase in production, the project also provides for major upgrades to mine infrastructure, including the development of a new processing plant, tailings management facilities, filtration plant and rail loading facility at the Goonumbla rail siding, and the construction of water pipelines from Lachlan Valley Water properties to the mine site. With 520 members, Lachlan Valley Water is the peak body representing river water and groundwater users in the Lachlan Valley.
Rio Tinto, which completed the Step Change Project Order of Magnitude Study in 2010, is currently undertaking a $144 million pre-feasibility study to evaluate the opportunities and risks inherent in the project.
More than 100 people have been employed during the pre-feasibility study phase, including Northparkes employees, contractors and consultants. A key element of the pre-feasibility study – which was due for completion in late 2012 – is more than 155km of predominately underground diamond drilling to define the mineralised areas to allow detailed design of the potential mine layout and associated surface infrastructure, including the ore processing plant and tailings facilities.
In August, Rio Tinto reported that it had delayed completion of its pre-feasibility study after more ore was found. Rio Tinto Copper chief executive Andrew Harding said the two-year pre-feasibility study would take an extrasix to nine months, but gave no indication of the grades being encountered.
“We’ve had a lot of success with the drilling. The exploration phase has been more successful than we thought. It has opened up more options,” Mr Harding told The Australian newspaper.
The pre-feasibility study has focussed strongly on the application of new technologies to reduce water and energy consumption while improving underground tunnel development and safety at Northparkes. Rio Tinto intends to use these innovations as part of the Step Change project.
As part of Rio Tinto’s Mine of the Future program, Northparkes began trialling an Aker Wirth-developed tunnel boring system in August 2012.
“The [Mine of the Future] program is designed to create next generation technologies for mining operations that result in: greater efficiency; lower production costs; improved health, safety and environmental performance; and more attractive working conditions,” Rio Tinto stated on its website.
The Aker Wirth system, which merges the flexibility of a roadheader with the sturdiness of a tunnel boring machine, utilises undercutting technology to move through hard rock up to a maximum 300 megapascals of force.
It also cuts rectangular tunnels or horseshoe cross sections up to 6m in diameter in addition to circular tunnels, removing the need to backfill the lower section of the round cross section.
Although Northparkes contributes a fraction of Rio Tinto’s overall revenue, technological advancements like those utilised in block caving have far-reaching implications for the company.
This technology will be tested at Northparkes, but Rio Tinto hopes to use it to accelerate the development of deep copper deposits such as Oyu Tolgoi.
When the Oyu Tolgoi mine in Mongolia’s South Gobi Desert reaches full production in 2018, it is expectedto be an international top-10 copper producer as well as one of the world’s biggest gold mines.
Training for the future
While block caving is expected to be the dominant form of copper mining by 2020, its techniques are not well represented in university engineering courses. As a result, Rio Tinto, a company eager to staff its five international block cave mines, has been delivering its own training in conjunction with the University of NSW (UNSW).
“This is an exciting partnership that will enable us to develop a world-leading research, teaching and training effort in the growing field of underground mining and block caving,” UNSW dean of engineering Graham Davies said.
Opened in August 2012, Rio Tinto’s $13 million Block Cave Knowledge Centre of Excellence at Northparkes features a Thoroughtec Cybermine4 underground simulator: an efficient and cost-effective way for both new recruits and experienced block cave operators to develop specialised operator skills.
To ensure authentic and accurate training, Rio Tinto has also purchased an ultra-realistic visual recreation of the Northparkes mine site that enables operators to train in the same environment in which they work.
The centre will be used to train miners and engineers from Rio Tinto mines around the world in the creation and operation of block cave mines. Among the first trainees will be teams from Oyu Tolgoi in Mongolia, and the Argyle diamond mine in WA.