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Collaborative blast optimisation in a limestone mine

 

Boral’s largest mining operation in southern NSW has collaborated with Orica on the testing of bulk explosives.

Boral’s Limestone Mine near Marulan in southern New South Wales is the source of limestone for up to 60 per cent of all cement products used in the state. It has operated since the 1830s, and today it is Boral’s largest mining operation in Australia. 

The mine blasts 3.5 million tonnes of rock a year using modern open-cut hard rock drill and blast techniques. 

Orica provides down the hole (DTH) bulk explosive loading, survey and laser profiling, and boretracking services to site. It also supports the site with technical services and blast design advice, explosive type and requested Orica’s support with measurement technology.

The original aim of the project was to identify the bulk explosive most suitable for the geology and Boral’s blasting objectives.  Boral’s target fragmentation for productivity in mining and processing is between 250mm and 600mm.

Orica provided services, software, and hardware to support the project by measuring these outcomes in three production blasts:

Table 1. Results from three blasts in limestone using Centra Gold bulk explosive @1.25g/cc average in-hole density.

The average sonic velocity of the rock adjacent to each blast, using arrays of Kelunji accelerometers and Orica’s
Cycad software (Table 1).

Fragmentation, using aerial photography and Orica’s Powersieve analysis
software (Figure 1).

The velocity of detonation (VOD) of bulk explosive.

When blasting for fragmentation, it is better to choose an explosive with a VOD higher than the local sonic velocity of the rock.  Under this condition, damage to the rock around each blasthole is increased.

Figure 2. Dig rate comparison.
Figure 3. Fragmentation curves measured using Powersieve showing trial results.

Sonic velocity was measured using an array of precisely synchronised accelerometers adjacent to each blast.

The values shown in Table 1 are therefore “average” values, including the effects of pre-conditioning and structure and are probably lower than the sonic velocity of the limestone rock fabric. The results indicate the average sonic velocity of the limestone is in the range of 3900 to 4380 metres per second (m/s).  This is lower than the VOD of the bulk emulsion blend used in the blasts.

The Boral and Orica teams identified that the high energy Centra Gold 1.25 g/cc bulk explosive is most suitable for the blasting objectives and local conditions. However, further analysis indicated that localised fragmentation surrounding the explosive column was finer than required, while the stemming zones and regions between holes was producing oversize. 

 

TECHNICAL SOLUTIONS, RESULTS

Orica and Boral agreed on an objective to create a more uniform fragmentation distribution by reducing the hole diameter from 165mm to 127mm.  The 127mm pattern was adjusted to reduce the overall energy factor but create a more even energy distribution. If successful, this would produce more uniform fragmentation, reduce costs, and improve productivity. 

Blasting results using both diameters were assessed. The results from the comparison trials were shared between Boral and Orica so both parties could jointly determine potential opportunities for improvement.

Measures used to identify the effectiveness included:

  • Fragmentation analysis using drone imagery and Orica’s Powersieve software.
  • Drilling accuracy using GPS survey and boretracking to measure blasthole deviation.
  • Oversize per blasted volume measured using drone survey and Powersieve™. 
  • Crusher throughput measured by site methods.
  • Instantaneous dig rate measured using Orica’s cyclepro software (Figure 2).
  • The collaborative effort demonstrated that value can be unlocked by applying technology to blast measurements. The design change produced the following improvements in blast outcomes:
  • A 27 per cent reduction in the cost (dollars per tonne) of blasted material.
  • An 11 per cent reduction in instantaneous dig rate (Figure 2).
  • A smaller and more uniform fragmentation distribution (Figure 3), ie
    • P80 size reduced by 12 per cent.
    • P50 reduced by nine per cent.
    • P20 reduced by nine per cent.
  • A 41 per cent decrease in oversize (Figure 4).
  • A crusher throughput increase of three per cent.
  • Minor improvement in drill accuracy.
Figure 4. Measured oversize as a proportion of blast volume.

CUSTOMER TESTIMONIAL

“Orica’s Technical Services team did an exceptional job. They looked at Boral’s problem with an advanced set of skills and tools, which allowed Orica to move the investigation from one originally looking at explosives selection to now focus on explosives distribution. In NSW, Boral and Orica enjoy open and robust technical conversation. This has been the key to collaboration, continuous improvement and project successes such as this one.” 

Alex Nicetin – Boral Drill and Blast Manager Quarries

 

Authors:

Jonathon Keller, Orica Advisor Technical Services 

Cameron Ingles, Orica Blast Technician

ACKNOWLEDGEMENTS

Orica thanks the following people for their contribution to the project, strong collaboration, support, and permission to publish this case study:

Shane Braddy, Boral GM Quarries,

Alex Nicetin, Boral Drill and Blast Manager,

Wayne Beattie, Boral Pit Production Manager,

Darryl Young, Boral Pit Supervisor,

Craig Chapman, Boral Shotfirer.