BLAST HOLE SLOTTING: REDUCING OVER BREAKAGE DURING COAL MINE BLASTING.

Matt Stockwell1 and Dihon Tadic1

ABSTRACT CRCMining’s blast hole slotting system offers a potential means of reducing coal loss and dilution via an innovative method to control fracture propagation during blasting in open cut coal mining. The Centre has developed high-pressure water jet technology to cut a slot into the wall at the base of a blast hole to create a narrow disc-shaped cavity. These slots act to protect the coal seam during blasting by directing fractures radially from the base of each hole. Mine site blasting tests demonstrated that an area of slotted blast holes can reduce energy transfer and subsequent blast damage to the rock below the blast hole toe horizon. Slotting reduced the average depth of fragmentation by 58% when compared to an equivalent set of conventional blast holes. This encouraging result provides a clear indication that slotting has the capability to improve coal recovery whilst maintaining the desired fragmentation in the target zone.

INTRODUCTION Coal loss incurred during cast blasting remains a major issue for mine operators. Kanchibotla et al. (2004) has reported that coal loss for a single mine can be up to 20 percent. Cast blasting is often the major contributor to coal loss. During the blast and dig process, coal loss can occur when sections of the coal block shift towards the low wall zone, the coal edge ejects from the free face mixing with the muck pile and/or top of coal sections are lost to the spoil pile during excavation. Figure 1 provides an illustration of coal block movement and coal edge loss situations. Dilution can also occur, negatively impacting the quality of coal and coal processing. Coal fines are also easily lost during the coal recovery and customer delivery process. Drilling quality is recognised to be a prevalent and ongoing contributor to the coal loss process. Blast holes that unintentionally terminate close to the top of coal or intersect the seam can compromise the blast design and adversely precondition the coal seam in advance of digging.

Figure 1. Examples of coal block movement and coal edge loss situations during cast blasting (Kanchibotla et al 2004).

Reducing coal loss presents a significant revenue opportunity for the Australian coal mining industry. In 2007-08, 146.2 million tonnes of saleable black coal was mined from Queensland Open Cut mines (DME 2010). Investigating the income benefit of a 2 percent coal recovery improvement for a mine site producing 5 Mtpa of metallurgical coal and using the 2007-08

1 CRCMining, The University of Queensland Pinjarra Hills Campus, Brisbane, Australia, 4069.

average price of $121 per tonne, an additional income of $12 million per annum could be generated for the mine operator. Slotting costs for a mine of this size are estimated to be $2-$4 million per annum. With increasing coal prices, the value proposition for reducing coal loss using an effective means such as slotting remains a significant market opportunity.

CRCMining has developed a Blast Hole Slotting System incorporating high-pressure water jet technology. The system has the capability to cut a slot into the wall at the base of a blast hole to create a narrow disc-shaped cavity. In open cut coal mine blasting, these slots act to protect the underlying coal seam by directing fractures radially from the toe of each blast hole. Figure 2 shows the blast hole slotting tool that was developed for blast holes between ~240mm and 310mm in diameter.

Figure 2. The Blast Hole Slotting tool being lowered into a blast hole (Stockwell et al, 2010).

This paper presents a summary of the outcomes from research involving a full-scale field test of the blast hole slotting system in a coal mine blast. Full details of this research are reported by Stockwell et al (2010) in the ACARP Project C18036 Final Report – Blast Hole Slotting Demonstration Trial.

RESEARCH OBJECTIVE The objectives of the research work were to:

• Conduct a field trial to demonstrate the blast hole slotting concept and test the research 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 blast damage (to assess coal loss reduction potential).

METHODOLOGY

Blast Hole Slotting

Work completed for ACARP project C13033 (Doktan 2005) demonstrated slot creation using a high-pressure water jet system and scaled blasting experiments in slotted and un-slotted samples. This provided clear evidence that slots can create preferential fracture planes.

ACARP project C14056 (Stockwell and Tadic 2008) produced the CRCMining research prototype Blast Hole Slotting System. This system was designed to operate for blast holes up to 40m in depth. The system is capable of capturing waste water and slot cuttings and returning them to the surface for controlled disposal / recycling.

Field Trial Site

With the support of New Hope Coal, a field trial site was identified at the New Acland Mine in the Clarence Moreton Basin in South East Queensland. An interburden section of the mine pit was targeted for this trial, providing a consistent and competent siltstone rock body approximately 10m in thickness overlying multiple coal seams. The intended placement of the slots was approximately 0.5m above the target coal seam.

Design of Experiment

A choke blast with a centre tie up configuration provided suitable zones for comparison of slotted and un-slotted blast performance. This blast design facilitated a balanced and controlled approach to the blast, ensuring that blast conditions in the slotted and unslotted zones were closely matched. The test zones were located 1.5 rows away from the centre tie up to avoid the initial confinement in this area and prevent complications during performance analysis. Figure 3 presents a view of the mine blast that has the comparative slotted (yellow) and unslotted (blue) test zones highlighted. The red line is a general marker for the centre tie up path for the blast initiation design.

Figure 3. The blast pattern highlighted with the slotted (yellow) and unslotted (blue) test zones – divided by the

centre tie up (red) of the blast initiation circuit.

Blast design and analysis software was used to calculate and benchmark the three dimensional explosive energy distribution in the target area. The concept of 3D explosive energy distribution was introduced by Kleine (1988). The method may be described as an integration or summation of the energy contribution of all explosive charges at a point in space. The 3D energy distribution approach in its simplest form (also referred to as 3D powder factor) is limited to calculations that only require the pattern geometry, blast hole diameter, charging quantities and the density of the explosive and rock mass; and thus is calculated in units of kg/m3 or kg/t.

Geophysical Seismic Refraction and Dispersive Surface Wave data collection was the technique selected to allow quantitative assessment of blast performance. The seismic refraction method is a well established geophysical method for the investigation of near surface layer and rock strength. The seismic refraction analysis was based on picking first

arrival times to each of the geophones, establishing layer velocities and then calculating depths on the basis of the intercept times at each of the shot points. This method allows mapping of layers provided their velocities increase with depth and provided their thicknesses do not change significantly along each spread of the geophones.

Post-processing of this data was completed using conventional seismic refraction and Multichannel Analysis of Surface Wave (MASW) methods. In addition, a trench was excavated down to the coal seam across the slotted and un-slotted zones to assist with validation of geological conditions and post blast fragmentation data.

Integrated analysis of the trenching, blast modelling and geophysical sensing data was performed to improve the interpretation of the data sets and establish conclusions for the research work.

FIELD TRIAL OUTCOMES

Blast Hole Slotting

Three test holes were prepared for commissioning of the Slotting System prior to slotting in the blast pattern. The use of a borehole camera system incorporating a slot depth measurement device confirmed that slots of ~1m diameter could be created in the interburden material. The approximate thickness of the slots at the hole wall was ~25mm, tapering to the slot perimeter. The commissioning tests indicated that a slotting time of approximately 2 minutes was adequate; however, 4 minutes was allowed per slot in the actual pattern to ensure consistent and sufficient slot creation.

Upon commencement of slotting work in the test area, it was identified that the test site had some inherent geological and drill depth complexities. The major issues were:

1. Minor faulting, seam segmentation and banding thickness variations making identification of the top-of-coal and the target coal seams difficult;

2. The collection of groundwater by natural means within the blast holes. Jointing of the rock was also identified, with water transfer between some holes apparent;

3. Moderate thinning of the interburden from the slotted area to the unslotted test area.

4. Drill depths relative to the targeted top of coal (TOC) were inconsistent and ranged from 0.5m above the TOC to 2m below TOC; and

5. The blast holes had a degree of spiralling. Some targeted blast holes could not be slotted due to tool clearance issues i.e. the downhole tool could not be deployed to the target depth.

The borehole camera system was used for detailed mapping of the TOC to improve the accuracy of slot placement. The hole depths across the test area were also measured. Drill cuttings and stemming material were used to backfill holes as required to a depth that was 0.5m above the TOC.

Thirty-three blast holes were successfully slotted within a standard blast pattern. Airbags were installed in the slotted test zone to protect the slot and reduce the impact of drill cuttings fall-back during the post slotting and the explosive loading phase. Two rows of the blast pattern across the slotted and unslotted test zones were also fitted with air bags as an experimental control measure.

Blast

Orica completed a successful blast of the test area using their Coal 11 product and Powergel™ explosive with non-electric detonators. Powergel™ was used on seven blast holes within the slotted zone due to the small amounts of residual water in these holes.

The 3D explosive energy distribution analysis of the test area identified the following:

1. Relatively higher explosive energy levels were expected to be imparted into the rock mass in the centre region of the blast;

2. With the exception of blast hole L8 and the longer charges in the centre region of the test area, the blast pattern shows a relatively even distribution of explosive energy. The lower energy levels due to shorter charges in the southern end of the pattern are clearly identified; and

3. For comparison purposes, it is clear that at the 442m RL the concentration of energy appears to be higher on the slotted side of the blast pattern.

A high-speed video camera and two standard video cameras were used to capture the blast. Review of the high-speed and standard video footage identified the following:

1. The blast initiation performed as expected;

2. No abnormal stemming ejection was observed within the test zone during the blast. Minor ejection from shorter blast holes was observed on the southern boundary further away from the slotted and un-slotted areas; and

3. The swell of the muck pile - in particular the heave section along the centre tie up and to a lesser degree, along the eastern border - was consistent with the blast design expectations.

Geophysical Sensing

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. 2D seismic data was acquired along 10 profile lines collinear with blast hole rows E to N and along 3 lines perpendicular to these. A 24-geophone seismic system was used with vertical component geophones at intervals of 1m deployed in a land streamer cable

Post-processing of this data was completed using conventional seismic refraction and Multichannel Analysis of Surface Wave (MASW) methods. The most useful results were obtained from the seismic refraction analysis. The thickness of the surface layer or Highly Damaged Zone below the blast hole toe horizon is shown using the plan view graphical representation in Figure 4. This figure includes a cross section schematic of the slotted blast holes used during the trial. Blast holes marked with crossed squares had a slot and air bag. The empty squares indicate blast holes that contained air bags only and the small black squares indicate normal blast holes prepared with no enhancement (conventional blast holes).

Analysis of the data illustrated in Figure 4 identified the following significant outcomes:

1. On average, the fragmentation depth (i.e. thickness of the Highly Damaged Zone) in the slotted area was ~0.8m while that of the unslotted area was ~1.9m. Slotting reduced the depth of fragmentation by 58%;

2. In the slotted area, the depth of the Highly Damaged Zone is consistent across the general area, despite some inherent variability with the quality of the blast holes. This suggests that there is a degree of positive interaction between slots and air bagged only blast holes; and

3. The depth of fragmentation within the centre tie up zone has inconsistent damage profiles. This is most likely due to the complexities presented by blast confinement during the initial period of the blast and the variability of blast hole drilling depth.

Figure 4. A graphical representation showing the fragmentation depth below the toe of blast holes using the Seismic Refraction technique (Stockwell et al 2010).

Trench

A trench aligned with row K was excavated down into the coal seams across the slotted and un-slotted zones to assist comparison of blast damage. Machine digging of the rock appeared to be more difficult in the slotted zone when compared to the unslotted zone. More ripping was required on the slotted side and larger blocks of rock were encountered. The competency and fracturing of the rock as observed on the side walls of the trench supported this observation.

Geological evaluation of the trench cross sections identified some geological complexity. The sedimentary layering of the rock, rider (D3) and target coal (D4 and deeper) seams showed minor variations in their depths. A small fault in the northern section of the slotted test zone was also identified. On a macro level, these complexities were not deemed to have significantly influenced the results. In the slotted zone, the fragmentation of the underlying coal seam was found to be blockier and the coal had a greater degree of competency when

compared to the coal in the un-slotted zone. Figure 5 presents the observed results from the top of coal fragmentation analysis. For this analysis, a dozer completed a single drag pass with its tine along the middle section of the completed trench.

Figure 5. Images showing the trench excavated through the slotted and un-slotted test zones and the observed fragmentation at the top of coal (Stockwell et al, 2010).

CONCLUSIONS CRCMining’s prototype Blast Hole Slotting System was successfully trialled on a full-scale mine blast. A 58% reduction in the thickness of the Highly Damaged Zone below the toe of the slotted blast holes was demonstrated (0.8m for slotted holes versus 1.9m for the conventional holes).

Comparison of the blast modelling and Seismic Refraction data reinforces the positive effect of the slotting on blast performance. The results clearly show that the transfer of blast damage below the level of the slots has been significantly reduced when compared to conventional blast holes. Despite having a greater degree of explosive energy in the slotted zone, the depth of fragmentation below the toe of the blast holes in this area is significantly less than the unslotted zone. The observations made on the differences in the digging characteristics during trenching support the findings from the geophysical data analysis.

The outcomes of this project suggest that this technology has the potential to reduce coal loss associated with coal seam damage during overburden blasting. This “protection” offered to the seam during blasting is expected to be particularly valuable during highly damaging cast blasting operations. This technology may also be applicable to other surface and underground mining applications to improve floor conditions, slope stability or to assist preconditioning for optimised blast fragmentation.

FUTURE TECHNOLOGY DEVELOPMENT The path to commercialisation is challenging and can require significant funds and development time. The progress of the technology’s development status relative to commercialisation is illustrated in Figure 6. This figure provides a basic commercialisation model for technologies like the Blast Hole Slotting System. The work to date has progressed from proof of concept through to laboratory testing, construction of a research prototype system and demonstration of the system in a set of full-scale trials. This model suggests that the technology has almost completed its Research, Development and Demonstration (RD&D) phase of a commercialisation process.

Figure 6. Basic commercialisation model – the RD&D phase for the technology is nearing completion.

It is recognised that further work is required in the RD&D phase to demonstrate the System in a cast blast situation and provide a directly quantifiable assessment of the system’s ability to reduce coal loss. Investigations have commenced to continue development of the technology. It has been proposed that a full-scale test of the slotting in a cast blast be completed with the intention to directly compare the system performance to a selection of competing technologies. Competing technologies that have been adopted by industry - such as baby decking - should provide a relevant and reliable benchmark for the performance of the slotting technology. In addition, further theoretical review of slotting is also necessary to improve understanding of the slotting performance and optimisation potential. It is envisaged that an optimised blast design for slotting will be designed for a future trial and the physical results will be used to validate model results.

The commercial vision of the Blast Hole Slotting System is that the technology could be integrated into a mine drill rig or alternatively onto a truck suitable for mine use and public road travel. Figure 7 shows a high-pressure water sewage cleaner truck system and explosives loading truck. The platform of these types of commercial truck systems could be used for the development of a slotting truck. Determining the appropriate development path will depend on operational demands, usage and flexibility requirements, and economic considerations. As these issues become further understood, this aspect of the technology commercialisation can be progressed.

Figure 7. A blast hole slotting system integrated onto a truck could be suitable for providing a commercial service, similar to the service approach used for the loading of explosives in blast holes.

To complete the verification phase of development, it is recognised that a commercially capable system will need to be manufactured. The CRCMining research prototype Blast Hole Slotting System was adequate for initial demonstration and testing purposes; however, significant refinements are required for extended use and verification in different conditions.

Significant progress has been made in recent years to reduce coal loss and dilution using processes such as hard capping, buffering and baby decking; however, CRCMining’s ongoing consultation with mine operators has provided evidence that a high level of coal loss and high implementation cost continues to be a major issue for mine operators. Slotting presents an alternative and simplistic method to reduce coal loss during blasting. It also has the potential to deliver a greater cost-benefit when compared to existing technologies.

A preliminary economic evaluation of the slotting system has been performed to estimate the cost-benefit of delivering a commercial service. Figure 8 illustrates an estimate of the components of the slotting costs. Almost 90 percent of the total cost of a slotting system is attributable to operating costs, with fuel and labour being the most significant individual components. The outputs of the economic model are sensitive to input parameters and various assumptions (i.e. seam geometry, hole depth, blast pattern, coal price etc); however, a

basic scenario involving a 5 Mtpa mine that reduces coal loss from 8% to 6% with slotting suggests that extra annual revenue of $12M is potentially achievable for a cost of about $7M (~$3M slotting costs plus ~$4M additional coal mining and processing costs). CRCMining is currently refining this economic model and applying it to case studies. This will strengthen the business case for commercialisation of the slotting technology. The immediate aim is to develop and deploy a commercially capable prototype system for extended verification testing.

Figure 8. Blast hole slotting system cost breakdown estimate

ACKNOWLEDGEMENTS This project has been funded by the Australian Coal Association Research Program (ACARP), CRCMining and New Hope Coal’s New Acland Mine. The project team wishes to thank the ACARP project monitor John Brett and the industry monitors Steven Stook, Brett Domrow and Pierre Formosa. The valuable support provided by Industry has contributed to the successful completion of this research.

The authors also thank Prof. Peter Hatherly, Dr. Italio Onderra, Dr Koya Suto, Mr. Joji Quidim, Mr. Samuel Leonard and Dr. Sedat Esen for their assistance to conduct research associated with this project.

REFERENCES

DME: Queensland Government Mining Journal Winter (2009), viewed 7th march 2010.

http://www.dme.qld.gov.au/zone_files/QGMJ/pages_from_qgmj_winter_qld_coal_an_industry_review_part_1.pdf

Doktan, M. (2005), ACARP Project C13033 Final Report – Slotting While Drilling for Reduced Blast Damage to Coal Seam-Final Report. CRCMining.

Kanchibotla, S. Grouhel, p. Tucker, P. Podoliak, K. (2004), ACARP Project C11051 Final Report - Controlling Block Movement of Coal During Overburden Blasting. Dyno Nobel Asia Pacific Limited.

Kleine, T. (1988). A mathematical model of rock breakage by blasting. PhD Thesis. The University of Queensland, Australia.

Stockwell, M. Tadic, D,M. (2007), ACARP Project C14056 Final Report –Slotting While Drilling for Reduced Blast Damage to Coal Seam: Stage 2-Final Report. CRCMining.

Stockwell, M. Tadic, D. (2008), 2008 ACARP Funding Proposal, Blast Hole Slotting Demonstration Trial, CRCMining.

Stockwell, M. Tadic, D,M. Hatherly, P. Onderra, I. Suto, K. (2010), ACARP Project C18036 Final Report – Blast Hole Slotting Demonstration trial, CRCMining.