December 2015

Improving production cost-efficiency at Middlemount through cast-doze

  • By Warren Fair, General Manager, Action Drill & Blast

Focusing on precision and efficiency to yield the best results and value

The Middlemount coal mine, 90 km north-east of the mining town of Emerald in Queensland’s coal-rich Bowen Basin, is an open cut operation operated by Middlemount Coal Pty Ltd – a joint venture between Peabody Energy and Yancoal. The mine produces low-volatile pulverised and low-phosphorous semi-hard coking coal for export markets. Mining operations consist of truck and excavator strip mining augmented by cast blasting and dozing.

In 2011 Action Drill & Blast (ADB) was awarded the contract for managing all production drilling and blasting activities. This includes the supply of raw materials, explosives, equipment, labour and associated maintenance, and the provision of technical and health, safety, environment and training services.

The contract was extended by three years in 2013.

ADB works with Middlemount’s management and blasting engineers at all levels, from strategic planning and scheduling to optimisation initiatives and blast-by-blast advice.

Overcoming challenging conditions

ADB and Middlemount engineers have been working to improve the project’s production cost efficiency through cast-doze. Cast-doze is a process in strip mining where the blast is optimised to throw as much rock as possible to its final position. Dozers are then used to push as much as they can so only the final wedge requires excavation.

Middlemount is a very challenging bench shape for casting as the coal dips down in excess of ten per cent, the coal rolls up and down along the strike and the bench is twice as wide as it is high. Due to the high focus on coal recovery, the dividing line between fully casted dirt and dirt yet to be dug is further out
than normal.

Prior to implementing improvements, cast blasting in the challenging conditions was achieving cast to final position of under 20 per cent, versus a target outcome of 1/3 casted, 1/3 dozed, 1/3 dug.

In a collaborative effort between ADB, Middlemount engineers and independent consultants, a list of potential improvement opportunities was made and ranked based on marginal cost versus marginal benefit, and a trial program was put in place. A focus was placed on precision and efficiency to yield the best results and value, rather than simply using more explosives.

Adopting precise and innovative drilling and loading practices

Precise drilling and loading practices were used to get the energy exactly where it needed to be.

Accurate drilling is an important factor for optimising the blast. Vipers drills were chosen because of their ability to use different drill bits of 229, 251 or 270 mm diameter. This provides greater flexibility in being able to match the explosive mass and distribution to the requirements of the ground being worked. GPS technology is also being utilised to deliver greater accuracy.

Electronic detonators were then implemented to facilitate more efficient blasting. Many blasters can supply electronic detonators but they are often not used to their full potential. Further, as ADB is an independent contractor, we are not tied to any explosives manufacturers. This meant we were able to trial a number of different electronic detonators in the cast-doze process to select the best product for the job.

Electronic detonators get the most out of the explosive column. They make the holes work together, rather than on their own so the same energy is used more effectively. It means less explosives and less holes are required.

The timing between holes and rows was then tweaked to improve coordination between holes and minimise vibrations.

Improvements realised

The program achieved its cast objectives, with profile, fragmentation and looseness also improved.

As a result, production has increased at Middlemount from 1.8 million tonnes of ROM to in excess of 5 million tonnes a year over a three year period, and with the same level of resources – freeing up a whole digger fleet for more productive work.

ADB is continuing to strive to get more out of the explosives, with further adjustments to timing, in-hole loading practices and product formulation expected to achieve even greater results.

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