Open Cut » Drilling & Blasting
Coal loss remains a key issue for the mining industry, with some Australian mines reporting up to 20% coal loss during mining. This provides an exciting opportunity for the development of innovative techniques and processes to improve coal recovery. Since 2004, CRCMining has been developing Blast Hole Slotting technology to address this issue. This final project report details the 2009-2010 component of this work, which demonstrated CRCMining's Blast Hole Slotting System at a coal mine and assessed its potential for increasing coal recovery.
The technology aims to reduce coal loss by controlling energy transfer and fragmentation damage during overburden blasting in open cut mining. A disc-shaped cavity (slot) is cut into the wall at the toe of each blast hole to achieve this. The intention is to protect the underlying coal seam from blast damage whilst maintaining the desired fragmentation for digging in overburden or interburden zones.
The specific objectives of this project were to:
· Conduct a field trial to demonstrate the blast hole slotting concept and test the prototype field system - slotting a minimum of 20 blast holes;
· Blast a pattern with slotted and un-slotted holes to assess blast behaviour; and
· Perform post-blast assessment to measure and analyse the fragmentation and comparative blast damage (to assess coal loss reduction potential).
The mining conditions and processes at the field trial site did not allow direct assessment of the system's ability to reduce coal loss. The intention was to infer the coal loss reduction potential of the system by assessing its capability to reduce fragmentation below the toe of blast holes.
A full-scale slotting field trial at the New Acland Coal Mine was completed in July 2009. An interburden section of the mine pit was targeted for this trial, providing a consistent and competent siltstone rock body approximately 10m in thickness. Slotting experimentation in test holes adjacent to the target pattern identified that slots of approximately 1m diameter could be created. Thirty-three blast holes were subsequently slotted in the slotting test area. The intended placement of the slots was approximately 0.5m above the target coal seam.
A choke blast with a centre tie up configuration provided suitable zones for comparison of slotted and un-slotted blast performance. Blasting of the test area was completed successfully. Modelling of the three-dimensional explosive energy distribution in the test area assisted with analysis and validation of test results.
Upon controlled removal of the muck pile to approximately 0.5m above the top-of-coal, Geophysical Seismic Refraction and Dispersive Surface Wave data collection was successfully performed. Post-processing of this data was completed using conventional seismic refraction and Multichannel Analysis of Surface Wave (MASW) methods. A trench was excavated down to the coal seam across the slotted and un-slotted zones to assist comparison. Qualitative assessment of ground conditions and observational analysis of digging performance indicated that blast fragmentation was consistent with the expectations of the project. In the slotted zone, the fragmentation of the underlying coal seam was found to be blockier and had a greater degree of competency when compared to the coal in the un-slotted zone.
Integrated analysis of the trenching, blast modelling and geophysical sensing data has demonstrated a strong correlation of results and a positive indication that slotting has reduced fragmentation below the toe of the blast holes. A Highly Damaged Zone has been identified, defined as a region of relatively severe and consistent damage below the base of the blast holes. This zone is thinner by approximately 1.1m in the slotted area, demonstrating that slotting has had the desired effect of reducing energy transfer and fragmentation damage below the blast bench.
This positive result provides a strong indication that slotting can reduce coal loss. Limiting damage below the blast bench will reduce dilution and loss of the upper coal seam material. It may also, and more importantly, introduce significant opportunities for minimising the extreme losses associated with coal block movement and edge loss during cast blasting.
CRCMining recognises that further field testing is required to build on the success of this initial trial. Testing at a site that is experiencing significant coal loss would allow quantitative assessment of coal recovery improvements attributable to slotting. This would allow a definitive evaluation of the technology's potential and a more accurate assessment of its feasibility in a commercial operation. It is also expected that tailoring and refinements to the blast design to specifically suit slotted holes will further improve performance and/or offer benefits such as increased flexibility, applicability, cost effectiveness, etc.
For immediate consideration by Industry, two options for future technology development are proposed:
· Option 1, Conduct a test program over multiple (5 to 10) full-scale blasts using a commercially-capable Slotting System designed for a production environment; or
· Option 2, Conduct a single full-scale test using an improved version of the current Slotting System to validate conclusions identified in this report and attempt to measure the System's capability to reduce coal loss.
Further development and testing will require a risk mitigation study involving Industry consultation, an engineering investigation and economic modelling. Preliminary economic modelling is purely speculative due to the uncertainty of coal recovery improvement; however, even modest assumptions of 1-2% suggest a significant cost-benefit for this technology (>$5M per annum for a 5 Mtpa operation).