Blast design parameters and their impact on rock fragmentation

Transcription

1 Blast design parameters and their impact on rock fragmentation By Pradeep K Singh, Chief Scientist & Professor, Academy of Scientific & Innovative Research CSIR Central Institute of Mining and Fuel Research, Dhanbad, India

2 Backdrop Fragmentation control through effective blast design and its effect on productivity appears self-evident. In actual practice it is difficult to achieve. Reasons: In-adequate knowledge of actual explosive energy release in blasthole. The effect of varying initiation practices in blast design and its effect on explosive energy release. Reliable and statistically significant analysis of fragments. Absence of controlled blasts in production scale to generate reproducible results. Site specificity of blast designs.

3 Backdrop Blasting operation at a mine plays a pivotal role in overall economics of any open-cast mine. Blasting subsystem affects all other associated subsystems, i.e. loading, transporting, crushing and milling operations.

4 Backdrop Criteria for a good blast can be varied depending upon results desired (i.e. good heave, loose muck, muck profile angle, ease of digging, uniform fragmentation, or normal fragment size distribution or any combination of these performance parameters) None of the above parameters are currently linked to study their effect on Productivity.

5 Blasting performance Throw Back-break & Wall Control Blast Vibration Blasting Noise Blast Fumes Blasting Performance Degree of Fragmentation Digging and Hauling Efficiency

6 Objective Link Overall Blasting Performance with Productivity

7 Blast Optimisation Pyramid Results Blast output and productivity Execution Blast design compliance and execution Explosive performance Planning Rock mass characterization Conditions at the blasting site Drilling pattern and blast design

8 Rock Fragmentation Fracture and fragmentation behaviour in rock due to explosive and other high strain-rate loads Blasting Time Scale: <~1 ms Drilling and Cutting ~10 ms Crushing and Grinding ~100 ms

9 Rock Fragmentation Essential need in practically all mining and excavation operations. Careful tailoring of explosives properties with rock properties and blast design to achieve desired fragmentation and rock movement. Fragmentation specific to each mining method (e.g. Coarse fragments but large movement in coal mining vs. fine fragments but very limited movement in gold mining). Control and prediction of blast-induced fractures to limit damage.

10 Experimental Sites Nigahi Project, Northern Coalfields Limited (NCL) Sonepur Bazari Project, Eastern Coalfields Limited (ECL)

11 Nigahi Project Stands out as a hilly plateau with elevation of about m above the mean sea level. There are three coal seams namely Turra (thickness: m), Purewa (Bottom, Top and sometimes combined thickness: m & 7-9 m respectively) seams. The block has Mt of coal reserves. The mine is currently producing 14 million tonne of coal per annum.

12 Sonepur Bazari Project Located in the Eastern part of Raniganj Coalfields. Four coal seams viz. R-IV, R-V, R-VI and R-VII. The mine is producing about 4.5 Mt of coal and removal of overburden is about 12 million cubic meters. The total coal reserve of the mine is Mt.

13 Physico-Mechanical Properties of Rocks Name of the project Sonepur Bazari Nigahi Rock type/location Sandstone (dragline bench) Sandstone (shovel bench) Sandstone (dragline bench) Sandstone (shovel bench) Compress ive strength Tensile strength Density Poisson s ratio Young s modulus (MPa) (MPa) (kg/m 3 ) (GPa)

14 Blast Details and Analyses The blast design parameters data collected from 91 blasts from three experimental sites are analyzed to find out its impact on rock fragmentation level. The main important parameters which decide the fragmentation level of particular blast includes: - burden to hole diameter ratio, - spacing to burden ratio, - stemming column length, - stiffness ratio, - explosives amount and its type, - initiation mode and charge/powder factor. The near field blast vibration signatures were also recorded to diagnose the impact of delay timing on rock fragmentation.

15 Impact of Hole Diameter on Velocity of Detonation

16 Impact of Booster Placement on VOD of Explosives

17 Impact of Cleaning of mouth of Holes on VOD of Explosives VOD trace when explosives were not contaminated

18 Deck Blasting with Different Initiation System and Resultant Fragmentation 500 ms 450 ms 6.3 m 20 m Explosives- 120 kg Deck-4.5 m Explosives- 415 kg Mean m (dia. of equivalent sphere) Mode m (dia. of equivalent sphere) Index of uniformity 1.91

19 Deck Blasting with Different Initiation System and Resultant Fragmentation 450 ms 450 ms 6.3 m 20 m Explosives- 120 kg Deck-4.5 m Explosives- 415 kg Mean m (dia. Of equivalent sphere) Mode m (dia. Of equivalent sphere) Index of uniformity 2.17

20 Deck Blasting with Different Initiation System and Resultant Fragmentation 450 ms 6.3 m 20 m Explosives- 120 kg Deck-4.5 m D- cord Explosives kg Mean m (dia. Of equivalent sphere) Mode m (dia. Of equivalent sphere) Index of uniformity 2.19

21 Scattering Test Results

22 Nigahi Project Fragmented size analysis - medium hard OB bench Loading cycle of 10 cubic meter shovel

23 Sonepur Bazari Project Fragmented size analysis - hard OB bench Loading cycle of 10 cubic meter shovel

24 Burden to Hole Diameter Ratio Vs Mean Fragment Size Mean fragment size increases with increase in the ratio of burden to hole diameter.

25 Spacing to Burden Ratio Vs Mean Fragment Size As most of the data have little variation in spacing to burden ratio, the outcomes of the graphs are not so significant. However, spacing to burden ratio between 1.1 and 1.3 shows good results except for a few blasts which are having low index of uniformity (n) due to presence of joints and back break of previous blast.

26 Stemming Length to Burden Ratio Vs Mean Fragment Size The data points are relatively scattered but general trend shows that mean fragment size of fragmented rock decreases with the decrease in stemming length to burden ratio

27 Charge/Powder Factor Vs Mean Fragment Size Mean fragment size decreases with increase in charge factor. A few scattered data in this graph are due to the geological discontinuities of rock mass of the blasting patch

28 Stiffness (Bench Height to Burden Ratio) Vs Mean Fragment Size It is observed that stiffness value of less than 2 gives coarser fragmentation and the best optimum value comes between 2 and 3

29 Joint Plane Orientation and Spacing Joint and bedding planes act as natural pre-splits during blasting and if possible, should be used to improve performance. Spacing of joints within a rock mass will have significant impact on the size distribution of the blasted muck. In general, the joint spacing will also improve the fragmentation level.

30 Blast Design and Resulted Muck Profile at Hard OB Bench FREE FACE

31 Blast Design and Resulted Muck Profile at Medium Hard OB Bench FREE FACE

32 Conclusions... Optimum blasting should comprise the generation of fragment size distribution with suitable muck pile optimal for loading, which should improve the downstream operations. This study is confined to the effect of blast design parameters on the fragment size distribution of the blasted muck. The main conclusions of the study are: Mean fragment particle size increases with the increase in the burden to hole diameter ratio. This increase was mainly due to the increase in burden as the hole diameter was kept constant. Mean fragment size and index of uniformity (n) of the blasted muck decreases with the increase in the spacing to burden ratio. The optimum value of spacing to burden ratio in most of the blasts ranges from 1.1 to 1.3 and it resulted into excellent fragmentation.

33 Conclusions Stemming length to burden ratio was plotted against mean fragment size and the general trend shows that mean fragment size of fragmented rock decreases with the decrease of stemming length to burden ratio. As anticipated, the increase in the charge/powder factor will increase the rock fragmentation level i.e. decrease the mean fragment size of the rock. Change in burden with respect to bench height has significant effect on rock fragmentation. Therefore, the stiffness (bench height to burden ratio) value of less than 2 gives coarser fragmentation and the best optimum value was around 3.

34 Perfection is achieved, not when there is nothing more to add, but when nothing left to take away