EFFECT OF EXPLOSIVE ON FRAGMENTATION

S. ESEN & H.A. B LG NDepartment of Mining Engineering, Middle East Technical University, Ankara, Turkey

ABSTRACT: In this study, effect of change of explosive type on fragmentation is investigated.Two blast tests in which ANFO is used in the first and BARANFO 50 is used in the second oneas the main blasting agent were conducted at Lafarge Yibita Lalahan Quarry. Both blasts werecarried out at the same bench of the quarry having the same structural geology and same surfaceblast design parameters. The only varying parameter is the blasting agent. Both tests weremonitored by continuous velocity of detonation recorder. Digital images were acquired from themuckpile after each blast test by using the same image sampling technique. Images wereanalyzed by using SPLIT software and size distributions of muckpiles obtained from both blastswere determined. SPLIT is a digital image processing software developed to compute sizedistribution of rock fragments from digital images. As a result of this research, it was shown thatfiner fragmentation is obtained by utilizing BARANFO 50 as the main blasting agent at aconstant blasthole pattern. It is also shown that it is possible to expand the blasthole pattern as analternative if the previous coarser fragmentation is preferred. Moreover, comparison ofexplosives should not be performed based solely on the purchase price. Since fragmentationaffects all post-blast mining operations (loading, hauling, crushing, grinding, etc.), explosiveperformance should also be assessed by fragmentation evaluation using softwares which areproven to be accurate similar to SPLIT.

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1. INTRODUCTION

It is necessary to couple all the parameters, namely explosive and rock properties and surfaceblast design for an efficient blasting. Minor changes in the controllable parameters which areexplosive type and surface blast design can have a major effect on the resultant fragmentation.Once the blast has been carried out, it is necessary to analyze the obtained results, as itsinterpretation will give hints for the successive modifications of the blast parameters for thefollowing rounds.

The factors that must be considered to evaluate blast results are:- Fragmentation,- Geometry of the muckpile,- State of the remaining rock,- Assessment of the results obtained by blast monitoring systems,- Environmental problems due to blasting (ground vibration, airblast, fly rock and dust).

Fragmentation is one of the most important concepts of Explosives Engineering. Blasting is thefirst step of the size reduction in mining and it is followed by crushing and grinding unitoperations. The efficiency of these unit operations is directly related to the size distribution ofmuckpile. Therefore, reliable evaluation of fragmentation is a critical mining problem.

In this study, two blast tests were conducted at the same rock environment and surface blastdesign parameters. ANFO and BARANFO 50 were utilized as main blasting agents in the firstand second tests respectively. The reason to carry out these tests was to investigate the effect ofexplosive type on fragmentation. Split-Desktop software is used to quantify the size distributionof fragmented rock.

2. METHODS DEVELOPED TO DETERMINE THE SIZE DISTRIBUTION OFFRAGMENTED ROCK

Methods to quantify the size distribution of fragmented rock after blasting are grouped as directand indirect methods. Sieving analysis of fragments is the only technique in direct method.Although it is the most accurate technique among others, it is not practical due to being bothexpensive and time consuming. For this reason, indirect methods which are observational,empirical and digital methods have been developed.

Observational method which depends on expert’s common sense is a widely used technique. Anengineer assesses the fragmentation and other blasting results subjectively. This method is not ascientific method as it does not give any information about the size distribution [1], [2], [3].

Blasting parameters are considered to determine the size distribution in some empirical modelssuch as Larsson’s equation, SveDeFo formula, KUZ-RAM model, etc. [3].

The most popular method to quantify the fragmentation is the determination of the sizedistribution using digital imaging processing techniques. This method being cheap and usefulconsumes less time and does not interrupt the production at the site. Due to these reasons, it is

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preferred widely by explosives engineers. It is the second reliable method after sieve analysis. Inthis method, image acquired from muck pile, haul truck, leach pile, draw point, waste dump,stockpile, conveyor belt, etc. are delineated automatically by using digital image processingtechniques and size distribution of fragmented rocks is determined finally [4], [5].

There are several softwares namely SPLIT, WipFrag, GoldSize, FRAGSCAN, TUCIPS, CIAS,PowerSieve, IPACS, KTH, WIEP, etc. that are commercially available to quantify the sizedistribution. The accuracy of these systems varies between 2 % to 20 % [6], [7].

3. THE SPLIT SYSTEM FOR ANALYZING THE SIZE DISTRIBUTION OFFRAGMENTED ROCK

3.1. Description of the Split-Desktop System

SPLIT is an image processing program for determining the size distribution of rock fragments atvarious stages of rock breaking in the mining and processing of mineral resources.

The desktop version of SPLIT refers to the user-assisted version of the program that can be runby mine engineers or technicians at on-site locations. The desktop SPLIT system consists of theSPLIT software, computer, keyboard and monitor. There must be a mechanism (software and/orhardware) for downloading digital or video camera images onto the computer. For digitalcameras the software that is supplied with the camera is required and for video camera images aframe grabber board is necessary. For higher resolution images and for ease of image selection,than is available by most frame grabbers, a digital camera is recommended. Resolution of theimages should be at least 512x512. The first step is for the user to acquire images in the field anddownload these images onto the computer. The source of these images can be a muck pile, haultruck, leach pile, draw point, waste dump, stockpile, conveyor belt, or any other situation whereclear images of rock fragments can be obtained. The SPLIT program first assists the user inproperly scaling the images. SPLIT can then automatically delineate the fragments in each of theimages and determine the size distribution of the rock fragments. SPLIT allows the resulting sizedistributions to be plotted in various forms (linear-linear, log-linear, log-log, and Rosin-Rammler). The size distribution results can also be stored in a tab-delineated file for access inseparate spreadsheet and plotting programs [5].

The desktop version of the SPLIT program has five major parts. The first part of the programconcerns the scaling of images taken in the field. The second part of the program deals with theautomatic delineation of the fragments in each of the images that are processed. The third part ofthe program allows editing of the delineated fragments to ensure high quality results. The fourthpart of the program involves the calculation of the size distribution based on information from thedelineated fragments. Finally, the fifth part of the program concerns the plotting or export of thesize distribution results. Each of the five parts of the program are described below [5], [8].

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3.2. Image Acquisition and Scaling

There are many ways that images can be acquired in the field and scaled. For instance, if imagesare taken along a moving conveyer belt, the scaling of the images is straightforward and can be assimple as measuring the width of the belt. When acquiring images of muck piles, the angle of theslope relative to the axis of the camera needs to be considered. If it is not perpendicular, the scaleas represented in the image varies continuously from the bottom of the slope to the top of theslope. There are several ways to correct the scale in muck pile images [5], [8]. The simplest wayis to place two objects of known size in the image, one near the bottom of the slope and one nearthe top of the slope, as shown in Figure 1. To eliminate side-to-side distortion, all pictures shouldbe taken perpendicular to the line of the toe of the slope.

Three scales of image which are large scale (6x6 m), medium scale (3x3 m) and small scale(0.5x0.5 m) are required. Equal numbers of images at each scale should be acquired. If one is notinterested in the size distribution of the smallest scale of the material and is happy to acceptSchuhmann or Rosin-Rammler curve in this range, taking the small-scale images may be omitted.Total number of images acquired from each blast range from 8 to 20 depending on the size of theblast. Figure 1 and Figure 2 show that images acquired at large scale and medium scalerespectively. While taking the images, lighting is important. Best lighting is provided in overcastdays due to even lighting and fewer shadows.

3.3. Fragment Delineation

Once the images have been acquired and scaled, the next step is for SPLIT to delineate theindividual rock fragments in each of the images. Lighting corrections and auto thresholdingalgorithms are used. After preprocessing and auto-thresholding, the Split-Desktop programautomatically delineates the fragments using a set of algorithms based on the following 4 steps:gradient filter, shadow convexity analysis, Split algorithm, and Watershed algorithm. Details ofthese steps are described by Girdner et al. [8] and Kemeny [9].

The result of the automatic delineation is a binary image (2 gray levels, black and white) thatcontains white particles and a black background. Figure 3 is the binary image that results fromthe delineation of the muck pile image shown in Figure 1 (some editing has also been performed,which is shown in gray, and is described in the next section). The black areas in these imagescontain fine material too small to delineate in addition to the unfilled air space between particles.This black area is very important in estimating the amount of fines.

3.4. Editing of the Delineated Binary Image

In most muck pile images and in many images from other sources such as haul trucks or leachpiles, there are instances when the automatic delineation algorithms in SPLIT will not delineatethe fragments properly. This may be due to situations where the lighting is poor, there is anabundance of fines in the image, the image quality is low or other reasons. In these cases, thebinary file containing the delineated fragments needs to be edited using hand editing tools in theprogram. There are three common cases where minor editing is needed. First of all, if there are

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Figure 1. A large scale muckpile image.

Figure 2. A medium scale muckpile image.

Figure 3. Delineation of the muckpile image shown in Figure 1.

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large patches of fines in the image, SPLIT sometimes mistakes these patches as a single largefragment. Secondly, if there is excessive “noise” on a fragment (due to bedding, rock texture,etc.), the SPLIT program may divide this fragment into a number of smaller fragments. Thirdly,some of the delineated particles are neither rock fragments nor fines and should not be counted inthe final size distribution, such as the balls in Figure 3.

The SPLIT program has built in editing capabilities to handle the situations described above. TheSPLIT program first makes a stack of images, where one file in the stack is the delineated imagepasted over the original grayscale image and the other file in the stack is the original grayscaleimage. The user can quickly toggle between the original and delineated images to determinewhich parts of the image need editing. Three kinds of editing are most common: paint bucketfilling of fines, erasing unwanted delineations, and identifying non-rock features such as scalingobjects. In most cases a skilled user can edit the images in less than 3 minutes [5].

3.5. Calculation of the Size Distribution

Once the individual fragments in the images have been delineated, the next step is to usecharacteristics of the fragments to calculate their size distribution. These characteristics includethe area and dimensions of each fragment and the area of the non-particle regions (black areas).Screen size and volume of each fragment from these characteristics are determined.

The second step is to determine a realistic distribution for the fine material. Two options for thedistribution within the fines are available in SPLIT, a Schumann distribution and a Rosin-Rammler distribution. Each of these distributions has two unknown parameters and theseparameters are determined from two known points in the size distribution, one point at thefinesize and the other at 1.5 times the finesize. Figure 4 shows an example size distributioncalculated from a muck pile image using the Schumann distribution assumption for fines. Boththe full size distribution and various percent passing sizes (P20, P50 and P80) are given.

3.6. Presentation of the Size Distribution Results

Once the size distribution has been calculated, it can be plotted in 4 ways: linear-linear plot, log-linear plot, log-log plot, and Rosin-Rammler plot. Figure 4 is an example of a log-linear plot.Next to each plot, the size distribution data is also printed in one of four formats (ISO standard,British standard, US standard, no standard). The P20, P50, P80 and topsize are also shown. Thesize distribution and percent passing sizes are written to files stored on the hard disk in textformat for further manipulation in separate database or plotting programs.

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Figure 4. Size distribution results of the first test.

3.7. Accuracy of the SPLIT System

Many calibration studies have been conducted of the SPLIT system in the past several years.For calibration studies involving muck piles, images are taken of rock fragments in real fieldsituations and processed by SPLIT and these rock fragments are also screened using traditionalmethods. According to the test results, the error in evaluation of the size distribution by SPLITsystem is much less than 10 % and average error is about 5% [5], [8], [9], [10].

4. CASE STUDY

Two blast tests required for this study were conducted at Lafarge Yibita Lalahan Quarry.Mechanical and physical properties of limestone are given in Table 1. At this quarry currentlyused explosive is regular ANFO. Since the limestone is medium to hard rock, it is intended toprove the benefits to be gained by using another explosive having higher shock energy.

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Table 1. Mechanical and physical properties of limestone.Uniaxial Compressive Strength, MPa 99.0Indirect Tensile Strength, MPa 8.0Density, g/cm3 2.707p-wave velocity, m/s 6204s-wave velocity, m/s 3156Laboratory Dynamic Elastic Modulus, GPa 71.475Laboratory Dynamic Poisson’s Ratio 0.325

Surface blast design parameters of the quarry are given below:Hole diameter: 89 mmBench height: 7.5 mHole length: 9.2 mBurden: 2.60 mSpacing: 3.60 mStemming length: 2.5 mIgnition system: ElectricalBlasthole pattern: StaggeredDelay between rows: 30 ms

Above parameters were kept same for both tests. Tests were conducted at the same bench of thequarry. In other words, structural geology of the bench did not vary. The only changingparameter was main blasting agent. It is intended to investigate the effect of explosive onfragmentation at the same rock environment and surface blast design parameters. ANFO andBARANFO 50 were used as main blasting agents in the first and second tests, respectively.Technical properties of these explosives are given in Table 2.

BARANFO 50 differs from ANFO in that BARANFO 50 consists of a mixture of prill porousand crushed ammonium nitrate. However, ANFO contains completely prill porous ammoniumnitrate. BARANFO 50 has different detonation characteristics and energies partitioned duringblasting when compared to ANFO due to the size of the ammonium nitrate.

Table 2. Technical properties of ANFO and BARANFO 50.

ANFO BARANFO 50Appearance Prill Prill+CrushedLoading density, g/cm3 0.820 0.928Average VOD, m/s 3316 3838Detonation pressure at given holediameter and rock properties, GPa

2.617 4.551

Effective shock energy, MJ/kg 0.161 0.355Effective heave energy, MJ/kg 0.529 1.144Total useful shock energy, MJ/kg 0.374 0.839Total useful heave energy, MJ/kg 3.717 5.559Total useful energy, MJ/kg 4.091 6.398

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34 kg of ANFO is charged into each blasthole in the first test. On the other hand, 39 kg ofBARANFO 50 is loaded into each blasthole in the second test due to its higher loading density.Both tests were monitored by continuous velocity of detonation recorder.

Same sampling strategy is employed for the images acquired to compare the size distribution ofthe both muckpiles after blasting. Since the aim of this study is to compare the size distribution ofmuckpiles, images were acquired from the surface of the muckpile. If it is desired to obtain thereal size distribution of the blasted material, loader should advance into the half of the pile andone should acquire the images from this part.

6 large scale and 6 medium scale images were acquired from each test. Totally 24 digital imageswere acquired by a digital camera having a resolution of 1024x1024. Having downloaded allimages from digital camera to computer, they were transferred to SPLIT Engineering LLC byusing FTP software. They were analyzed by using Split-Desktop software and the results returnedback within three days.

5. DISCUSSION OF RESULTS

Figure 4 and 5 show the size distribution results of first and second tests, respectively.Comparison of both shots in terms of resulting fragmentation is given in Table 3. Various percentpassing sizes, P20, P50, P80 and top size values are shown in Table 3.

P20, P50, P80 and top size in Case 2 are 2.52, 1.93, 2.05 and 1.94 times less than that in Case 1.Mean fragment sizes (P50) are 23.08 cm and 11.96 cm for the first and second tests, respectively.Therefore, finer fragmentation is obtained by utilizing BARANFO 50 as the main blasting agent.

The change in main blasting agent affects fragmentation greatly at same rock environment andsurface design parameters. The reason for finer fragmentation in Case 2 is that BARANFO 50has much greater effective shock energy (Table 2). Effective shock energy governsfragmentation. The higher the effective shock energy, the finer the fragmentation is.

It was observed after blasting that first muckpile profile was tighter than the second one whilebackbreak, overbreak and fly rock problems were not observed for both test blasts.

Table 3. Comparison of both shots in terms of resulting fragmentation.

Explosive P20, mm P50 , mm P80, mm Top size, mmCase 1 ANFO 90.81 230.78 480.37 911.95Case 2 BARANFO 50 36.07 119.57 233.87 468.96Size Reduction Ratio 2.52 1.93 2.05 1.94

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Figure 5. Size distribution results of the second test.

6. CONCLUSIONS

Two blasts carried out within this research study were conducted by utilizing two differentblasting agents, namely, ANFO and BARANFO 50 under the same test conditions (rockenvironment and surface design parameters). Images acquired after each shot were analyzed byusing Split-Desktop software.

According to the results of the software, mean fragment size of the muckpile obtained byutilizing BARANFO 50 as a main blasting agent is 11.96 cm and that by using ANFO as a mainblasting agent is 23.08 cm. By using BARANFO 50 as a main blasting agent, P20 value reducedfrom 9.08 cm to 3.61 cm and, P80 value reduced from 48.04 cm to 23.09 cm.

The most important result is that finer fragmentation is obtained by utilizing BARANFO 50 as analternative blasting agent when two shots at Lalahan Quarry are compared. The reason is thatBARANFO 50 has 55 % higher effective shock energy at this rock environment.

If the quarry management accepts the previous coarser size distribution as satisfactory, in thiscase, it seems possible to expand the blasthole pattern to keep the fragmentation the same. It

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would reduce the drilling cost for quarries where cost of drilling is high. Size distribution shouldalso be determined while trying to expand the blasthole pattern.

This study shows that the minor changes in blast parameters affect fragmentation greatly and sizedistribution should be quantified by using softwares which are proven to be accurate similar toSPLIT.

Since fragmentation affects the efficiencies of post-blast mining operations (loading, hauling,crushing and grinding), fragmentation should be assessed properly which would lower the overallproduction cost of the company significantly.

7. ACKNOWLEDGEMENT

We would like to acknowledge the help provided by the Lafarge Yibita Lalahan Quarry for thecase study, BARUTSAN Company for the supply of explosives, accessories and blast monitoringsystems and SPLIT Engineering LLC for the Split-Net Service.

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[2] Kemeny, J.M., Devgan, A., Hagaman, R.M., and Wu, X., “Analysis of Rock FragmentationUsing Digital Image Processing”, Journal of Geotechnical Engineering, Vol. 119, No. 7, pp.1144-1160, 1993.

[3] Jimeno, C.L., Jimeno, E.L. and Carcedo, F.J.A., Drilling and Blasting of Rocks, A. A.Balkema, Rotterdam, Netherlands, 1995.

[4] Higgins, M., BoBo, T., Girdner, K., Kemeny, J. and Seppala, V., “Integrated Software Toolsand Methodology for Optimization of Blast Fragmentation”, Proceedings of the Twenty-Fifth Annual Conference on Explosives and Blasting Technique, Nashville, Tennessee, USA,Volume II, pp. 355-368, February 1999.

[5] Kemeny, J., Girdner K. and BoBo T., “New Advances in Digital Image Analysis Software toQuantify the Size Distribution of Fragmented Rock”, MINNBLAST 99, pp. 27-43, 1999.

[6] Franklin, J.A., Kemeny, J.M. and Girdner K.K., “Evolution of Measuring Systems: Areview”, Proceedings of the Fragblast-5 Workshop on Measurement of Blast Fragmentation,A.A. Balkema, Montreal, Quebec, Canada, pp. 47-52, August 1996.

[7] Rustan, P.A., “Automatic Image Processing and Analysis of Rock Fragmentation –Comparison of Systems and New Guidelines for Testing the Systems”, FRAGBLAST –International Journal of Blasting and Fragmentation, Vol. 2 No. 1, pp. 15-23, 1998.

[8] Girdner, K., Kemeny, J., Srikant, A., and McGill, R., “The Split System for Analyzing theSize Distribution of Fragmented Rock”, Proceedings of the Fragblast-5 Workshop onMeasurement of Blast Fragmentation, A.A. Balkema, Montreal, Quebec, Canada, pp. 101-108, August 1996.

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[9] Kemeny, J., “A Practical Technique for Determining the Size Distribution of BlastedBenches, Waste Dumps, and Heap-Leach Sites”, Mining Engineering, Vol. 46, No. 11, pp.1281-1284, 1994.

[10] Liu, Q., and Tran, H., “Comparing systems - Validation of Fragscan, WipFrag, and Split”,Proceedings of the Fragblast-5 Workshop on Measurement of Blast Fragmentation, A.A.Balkema, Montreal, Quebec, Canada, pp. 151-155, August 1996.