For efficient blasting practice

Investigation of Firing Patterns on Fragmentation in an Indian Opencast Limestone Mine For efficient blasting practice the parameters incorporated in a blast design can be categorized as: rock parameters, explosive parameters and blast-design parameters. Blasting results are sensitive to the complex interactions between these three parameters 1.At the field scale, there is no scope for the blasting engineer to alter the rock parameters, however the explosive and the blast-design parameters can be modified so that the blast energy is used more efficiently.this in turn helps to improve the fracturing, piling and fragmentation characteristics and thus the overall economy of the blasting operation. One of the most fundamental and important controllable parameters is the firing pattern in Fig. 1. A representative borehole section of the deposit (not to scale) multi-row blast rounds 2. Nominal drilling patterns with a burden B and spacing S can be subject to radical modification depending on the firing sequence, which brings into focus the role of effective burden Be and effective spacing Se.The effective burden and spacing affect the fragmentation, displacement and swelling of the rocks 3, and thus the role of the firing pattern is absolutely crucial for the success of the blasting operation 4,5. It is in this context that the following paper investigates the influence of four different firing patterns on fragmentation and fragment size distribution at an Indian opencast limestone mine. OBJECTIVES OF THE STUDY The main objectives of the study were: 1. To conduct field-scale blasts using four different but prevalent firing patterns. 2. To ascertain the influence of Fig. 2. A representative charged blasthole cross-section (100mm Fig. 3. A representative charged blasthole cross-section (150mm The authors, Dr Piyush Rai and Satyendra Singh Baghel, are senior lecturer and post- graduate student, respectively, at the Department of Mining Engineering, Institute of Technology, Banaras Hindu University, India QM February 2004 www.qmj.co.uk 33

Table 1. Physio-mechanical properties and chemical composition of the limestone Item Colour Structure the four different firing patterns and two different blasthole diameters (150mm and 100mm) on fragmentation. 3. To analyze quantitatively the fragmentation and fragment size distribution in the blasted rock piles as a result of the different firing patterns. Physical Properties Description Purple and grey Fine grained and compact Hardness 4 5 Specific gravity 2.4 Joints Compressive strength Tensile strength Mechanical Properties Unevenly fractured and jointed 62MPa 7MPa Bond work index 13 Hard grove index Chemical Composition 11.5 12.3kWh/mt CaCO 3 70 76% MgCO 3 2 6% SiO 2 5 12% Al 2 O 3 1.5 3% Fe 2 O 3 1 3% BRIEF DESCRIPTION OF THE MINE Full-scale blasts were conducted at an opencast limestone mine operated by Century Cements Ltd in Raipur, India.The total leasehold area of the mine is 237ha, of which the mineralized Table 2. Influence of firing patterns on fragmentation (150mm hole S No. Parameters Blast 1 Blast 2 Blast 3 Blast 4 1 Hole diameter, mm 150 150 150 150 2 S x B, m 6 x 4 6 x 4 6 x 4 6 x 4 3 Bench height, m 8.0 8.0 8.0 8.0 4 Sub-grade drilling, m 0.5 0.5 0.5 0.5 5 Stemming, m 3.8 3.5 3.5 3.6 6 Decking, m 0.7 0.9 0.9 0.9 7 No. of holes 30 30 37 24 8 No. of rows 3 3 3 2 9 Explosive quantity (Q t ), kg 2,160 2,325 2,756 1,695 10 Charge length, m 4.2 4.1 4.1 4.0 11 Initiation system Exel Exel Exel Exel 12 Drilling pattern Staggered Staggered Staggered Staggered 13 Firing pattern Skewed V In-line Diagonal Extended V 14 Volume broken (theoretical), m 3 5,760 5,760 7,104 4,608 15 PF (theoretical), m 3 /kg 2.6 2.4 2.5 2.7 16 MFS (K 50 in m) 0.2206 0.4592 0.2553 0.1728 17 K 95, m 0.3469 0.650 0.4402 0.273 18 K 100, m 0.43 0.793 0.600 0.346 area represents some 126ha.The annual output of the mine is 1.5 million tons. The limestone deposit belongs to the Chattisgarh basin of the Precambrian Vindhyan Group, which is largely made up of horizontal, thickly bedded stromatolitic limestones. Other associated rocks include dolomitic limestones and shales. The overburden comprises hard murrum (loose overburden) and clay with an average thickness of 0.5 3m. Figure 1 shows a representative borehole section through the deposit.the 22 24m thick limestone is worked in 6 9m high benches. Drilling was carried out by down-the-hole drill rigs to provide both 100mm and 150mm diameter blastholes. Prilled ANFO explosive with a booster charge was used for primary blasting, with detonation carried out by means of an Exel shock tube bottom-initiation system. Representative diagrams of the charged blasthole sections for the 100mm and 150mm diameter holes are illustrated in figures 2 and 3 respectively. The salient physio-mechanical properties and chemical composition of the limestone are shown in table 1. METHODOLOGY OF STUDY In order to fulfil the stated research objectives, an imaging technique was used whereby views of the blasted rock pile were captured by high-resolution digital camera.these images were then analysed using suitable computer software to provide a measure of the fragment size distribution in the rock pile.with the widespread use of computer hardware and software, the cost of such imaging techniques is relatively low and the characterization of fragment size can be carried out quickly, precisely and with greater ease 6,7,8. In this study images were captured during the excavation of the entire rock pile from front to back. Depending on the size of each blast, some 20 30 goodquality high-resolution photographs were captured for analysis. For calibration purposes, a red-coloured square scale marker measuring 20cm by 20 cm was placed at the centre of each 34 www.qmj.co.uk QM February 2004

Fig. 4. Skewed V firing pattern for blast one Fig. 5. In-line firing pattern for blast two Fig. 6. Diagonal firing pattern for blast three image frame. A wide range of computer software is commercially available for the analysis of such images, but Fragalyst Version 2.0, developed by the Central Mining Research Institute, Nagpur, in collaboration with the Wavelet Group, Pune, was used in this study as it represents an indigenous system which is both cheaper and proven under conditions in India. After enhancement and calibration of the captured images, the software performed an edgedetection function to demarcate the boundaries of fragmented rocks as they appear in the rock pile.the edges detected by the software were observed on a computer screen and corrected where necessary by means of edit network functions. On completion of the edge-detection function the images were subjected to further analysis to generate a typical Rosin-Rammler distribution.this provides the entire range of fragment sizes (with percentages) present in a rock pile, from which the mean fragment size (MFS), coarse fragment size (K 95 ), maximum fragment size (K 100 ) etc, and the uniformity index, of the rock pile can be obtained. In this way, fragment size characterization was conducted in a quantitative manner for all the rock piles generated during the study. blasts to investigate their influence on fragmentation.the firing patterns used were: skewed V, in line, diagonal and extended V. Results for blast designs using a 150mm blasthole diameter The field observations and the results of the four blasts, all conducted on the same limestone bench, are tabulated in table 2. The drilling and firing patterns for the four blasts are shown in figures 4 7. Perusal of table 2 reveals that the blast-design parameters for all four blasts were almost identical. Also, since all the blasts were conducted on same limestone bench with similar explosive, the differences in the fragmentation results can clearly be attributed to changes in the firing pattern alone. The results in terms of MFS, K 95 and K 100 were most Fig. 7. Extended V firing pattern for blast four favourable for the extended V firing pattern.the in-line pattern yielded poor results, whereas the skewed V firing pattern appeared to be better than the diagonal pattern. Figure 8 presents the cumulative passing percentages at different screen sizes for the four firing patterns under investigation. These quantitative results can be qualitatively ascertained by some of the images captured in the field, as shown in figures 9 12. Figure 9 shows a fine, uniformly fragmented rock pile created using the extended V firing pattern. Figure 10 shows generally good fragmentation with some large sized boulders in the rock pile produced using the skewed V pattern. On the other hand, figure 11 clearly reveals the occurrence of large to very large fragments in the rock pile created using the inline firing pattern, whereas figure 12 shows the occurrence of a few coarse fragments in the rock pile produced using the diagonal RESULTS AND DISCUSSION As already stated, four different types of firing pattern were implemented in the field-scale Fig. 8. Variations in percentage passing at different screen sizes for blasts one to four (150 mm hole QM February 2004 www.qmj.co.uk 35

Fig. 9. Fine and uniformly fragmented rock pile from blast four Fig. 11. Large fragments present in the rock pile from blast two firing pattern. These quantitative and qualitative results highlight the influence of the firing pattern on the fragmentation. A change in the firing pattern also results in a change to the Se (effective spacing) to Be (effective burden) ratio. For blasts one and three this was the same at 2.16, for blast two it was around 1.5 and for blast four it was 4.0.With an increasing Se to Be ratio the blast energy and the stress developed by the blast is distributed more evenly and uniformly, which improves the degree of fragmentation 9. Another noteworthy result Fig. 10. Good fragmentation with few largesized boulders from blast one Fig. 12. Few coarse-sized fragments from blast three even though the Se to Be ratio was the same for blasts one and three is that blast number one, fired using the skewed V pattern, yielded better results in comparison to blast number three which was fired using the diagonal pattern.this can be explained on the basis that inflight collisions among the broken rock fragments are increased in a V firing pattern in comparison to diagonal firing.these in-flight collisions were seen to be of paramount significance in enhancing the fragmentation results in the hard and strong limestone formation being studied. Table 3. Influence of firing patterns on fragmentation (100mm hole S No. Parameters Blast 5 Blast 6 Blast 7 Blast 8 1 Hole diameter, mm 100 100 100 100 2 S x B, m 3.5 x 2.5 3.5 x 2.5 3.5 x 2.5 3.5 x 2.5 3 Bench height, m 6.0 6.0 6.0 6.0 4 Sub-grade drilling, m 5 Stemming, m 2.4 2.4 2.4 2.5 6 Decking, m 0.5 0.6 0.5 0.5 7 No. of holes 22 24 30 30 8 No. of rows 3 3 3 3 9 Explosive quantity (Q t ), kg 540 594 730 725 10 Charge of length, m 3.1 3.0 3.1 3.0 11 Initiation system Exel Exel Exel Exel 12 Drilling pattern Staggered Staggered Staggered Staggered 13 Firing pattern V In-line Diagonal Extended V 14 Volume broken (theoretical), m 3 1,155 1,260 1,575 1,575 15 PF (theoretical), m 3 /kg 2.13 2.12 2.15 2.17 16 MFS (K 50 in m) 0.1998 0.3507 0.2554 0.1594 17 K 95, m 0.3204 0.5068 0.402 0.2523 18 K 100, m 0.422 0.732 0.523 0.324 Results for blast designs using a 100mm blasthole diameter In this part of the investigation four more blasts were conducted on the same bench with a similar explosive.table 3 shows the salient design parameters for these blasts, while the four types of firing pattern used are illustrated in figures 13 16. Perusal of table 3 reveals that, with the smaller 100mm blasthole diameter, the blast-design parameters have changed for all four blasts.the fragmentation results indicate that, for smallerdiameter blastholes with reduced spacing and burden, fragment sizes are also reduced for all the blasts when compared to largerdiameter (ie 150mm) blastholes with corresponding firing patterns. Figure17 shows the fragment size distribution curves for the four blasts. The fragment size and size distribution follows a similar trend to the larger 150mm diameter hole size results. Once again, the extended V pattern yielded the best results while the in-line pattern yielded poor results. Also, the V type firing yielded better results than the diagonal pattern.these results may again be attributed to the Se to Be ratio, which was almost 4.0 for blast eight and about 2.11 for blasts five and seven. For blast six it was 1.4. Good and uniform fragmentation from blasts eight and five can clearly be seen in figures 18 and 19 respectively. On the other hand, very large sized fragments are quite distinct in the rock pile from blast six (fig. 20). CONCLUSIONS The following main conclusions can be drawn from the study: 1. Imaging techniques coupled with computer software analysis provide a powerful tool for quantitatively characterizing blast fragmentation at the field scale. 2. Firing pattern greatly influences the degree of fragmentation as it affects the effective spacing (Se) to effective burden (Be) ratio, which in turn plays a significant role in altering the fragmentation results. 36 www.qmj.co.uk QM February 2004

Fig. 13. V firing pattern for blast five Fig. 14. In-line firing pattern for blast six Fig. 15. Diagonal firing pattern for blast seven 3. Fragmentation improves with an increase in the Se to Be ratio because of the even and uniform distribution of the explosive energy. 4. In hard, strong formations the in-flight collisions between broken rock fragments are of great significance in improving the degree of fragmentation. ACKNOWLEDGEMENT The authors would like to express their thanks to the management and staff of Century Cements Ltd for permission to carry out the field study and for their full co-operation in conducting the various blasts. Fig. 16. Extended V firing pattern for blast eight Fig. 17. Variations in percentage passing at different screen sizes for blasts five to eight (100mm hole Fig. 18. Good fragmentation from blast eight REFERENCES 1. MAMUREKLI, D.: A user-driven computer model for open-pit blast design, Journal of Min., Met. and Fuels, Expl. & Blasting special issue, 1990, pp 17 23. 2. RUSTAN, P.A.: Automatic image processing and analysis of rock fragmentation-comparison of systems and new guidelines for testing the systems, Fragblast,V.2, 1998, pp 15 23. 3. JIMENO, C.L., JIMENO, E.L., and F.J.A. CARCEDO: Drilling and blasting of rocks, A.A.Balkema, Rotterdam,The Netherlands, 1995. 4. HAGAN,T.N.: The influence of some controllable blast parameters upon muckpile Fig. 19. Good fragmentation from blast five characteristics and open-pit mining costs, Procs. Conf. Large open-pit mining, Aust. Inst. of Min. Met., 1986, pp 123 132. 5. BHANDARI, S.: Engineering rock blasting operations, A.A. Balkema, Rotterdam,The Netherlands, 1997. 6. MAERZ, N.H., FRANKLIN, J.A., ROTHENBURG, L., and D.L. COURSEN: Measurement of rock fragmentation by digital photo analysis, 5th Int. Congr. Int. Soc. Rock Mech., 1987, pp 687 692. 7. MAERZ, N.H.: Photo analysis of rock fabric, Ph.D. thesis, Dept. of Earth Sciences, Univ. of Waterloo, 1990. 8. WANG,W., BERGHOLM, F., and O. STEPHANSSON: Image analysis Fig. 20. Large size boulders in the rock pile from blast six of fragment size and shape, Procs. Fragblast-5, Montreal, 1996, pp 233 243. 9. DOJCAR, O.: Investigation of blast parameters to optimize fragmentation,trans. Inst. Min. Metall., 1991, 100, pp A31 A41. QM February 2004 www.qmj.co.uk 37