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An engineering approach to predict the proportion of finesgenerated by blasting

I. Onederra1, S. Esen1& H.A. Bilgin2

1 Julius Kruttschnitt Mineral Research Centre, The University of Queensland, Brisbane, Qld, Australia2 Department of Mining Engineering, Middle East Technical University, Ankara, Turkey

Italo Onederra BE(Hons) MEngSc.Senior Research Engineer - Mining ResearchJulius Kruttschnitt Mineral Research Centre (JKMRC)Isles Rd. IndooroopillyQLD. Australia 4068Phone: +61 7 3365 5888Fax: +61 7 3365 5999e-mail: I.Onederra@mailbox.uq.edu.au

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An engineering approach to predict the proportion of finesgenerated by blasting

I. Onederra1, S. Esen1& H.A. Bilgin2

1 Julius Kruttschnitt Mineral Research Centre, The University of Queensland, Brisbane, Qld, Australia2 Department of Mining Engineering, Middle East Technical University, Ankara, Turkey

ABSTRACT

A new model to predict the proportion of fines generated during blasting is presentedin this paper. Within the context of this work, fines are defined as the proportion ofmaterial less than 1.18 mm. The model is based on the back-analysis of acomprehensive experimental program that included the fragmentation assessment anddirect measurement of the zone of crushing from 92 blasting tests on concrete blocksusing two commercial explosives. The properties of the concrete blocks varied fromlow, medium to high strength and measured 1.5 m in length, 1.0 m in width and 1.0 min height. Sieve analysis showed that the percentage of fines is directly proportional tothe percentage of the mass of crushed material given by the cylindrical volume ofcrushing around a blasthole. The proposed model combines two specificdevelopments arising from the experimental program, first is the relationship betweenthe assumed 1.18 mm cut off point and the volume of crushing; and secondly butequally important is the development of a model to predict the size of the crushingzone around a blasthole.

1 INTRODUCTION

For a number of years, both researchers and practising engineers have been aware ofthe importance of being able to tailor blast fragmentation to optimise the overallmineral extraction and recovery cycle. It is widely acknowledged that in productionblasting a significant proportion of fines present in a muckpile originates from thezone of crushing produced during blasting. Predicting the proportion of finesgenerated from crushing and the overall breakage process is important to practitionersinterested in the modelling of the complete size distribution of fragments in blasting.

Fines can have a negative or positive impact on the efficiency of downstreamprocesses. For example, the generation of excessive fines in operations adopting insitu leaching as their main ore processing method, may hinder recovery as certainfines tend to affect the permeability of leaching pads. Leaching performance may beaffected if the proportion of material that is less than 150µm exceeds 12 percent in thefeed to the agglomerators (Scott et al. 1998). Similarly, the efficiency of coalprocessing is strongly related to the generation of fines of less than 0.5 mm. Increasedfines content in run of mine (ROM) feed leads to higher handling and processingcosts, low yields, increased product moisture content, and in many cases a reducedproduct value (Djordjevic et al. 1998). There is also evidence (e.g., Grundstrom et al.2001) to suggest that by providing an appropriate size distribution to crushing andgrinding circuits, a measurable increased throughput and/or reduced power draw can

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be obtained. This may entail a requirement to increase the proportion of finer materialin production blasting.

The need to be able to predict the amount of fines from blasting has driven thedevelopment of this new engineering model. The model developed in this study isbased on a comprehensive experimental program conducted between 1996 and 1999as part of a collaborative research project between the Middle East TechnicalUniversity and the BARUTSAN explosives company in Ankara, Turkey (Bilgin et al.1999). One of the principal aims of this research project was to investigate theinfluence of the explosive, rock and blast design parameters on the efficiency ofblasting. Tests were not limited to measuring the extent of crushing but also includedmeasurements of breakage angle, breakage width, size distribution, throw, back-break, face velocity, minimum response time and peak particle velocity. In this study,the data set consisted of 92 model scale blasting tests.

Model scale blasting experiments using synthetic materials (e.g. Plexiglass, Homolite100, Epon 815 epoxy and concrete) have been used to understand the mechanisms ofrock breakage by explosives and have provided very useful insights into the blastingprocess. This includes work conducted by Rustan & Vutukuri (1983), Dick et al.(1993), Fourney (1993), Aimone-Martin et al. (1998) and Raina et al. (2000). Studiesby Dick et al. (1993) and Stimpson (1970) suggest that cement-based materials suchas concrete can be used to simulate rocks. Applications largely arise from thecheapness, ease of fabrication and reproducibility of samples (Stimpson 1970).

Work by Stagg et al. (1992) demonstrated that the proportion of fines generated byblasting can be considered to be scale independent. This means that the proportion offine material generated in a small scale blast can indicate the tendency to generatefines in full production scale blasting (Scott et al. 1998). However, model scale blasttests conducted in this study do not take into account the influence of rock massdiscontinuities and multiple hole blasts in the generation and liberation of fines.

There are only a handful of models that deal explicitly with the prediction of finesgenerated in blasting, these include the approaches proposed by Djordjevic (1999) andKanchibotla et al. (1999). Both of these approaches are mechanistic in nature as theycombine some predictions of the physical response of the rock mass (i.e.crushing/plastic deformation) to the explosive detonation and link this to observedfragmentation analysis conducted in a limited number of large scale production blastsand model scale blast chamber tests.

The model proposed here is also mechanistic in nature and follows the concept ofrelating the volume of crushed material around a blasthole to the very fine materialpresent in a muckpile (i.e. less than 1.18 mm). This model differs from previouslydocumented methods in that it provides a new and improved engineering model topredict the extent of crushing around a blasthole and through direct sieve analysis itdemonstrates and provides an empirical relationship between the volume of crushingand the proportion of fines generated during blasting.

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2 EXPERIMENTAL WORK

2.1 Sample preparation

As shown in Figure 1, concrete blocks were rectangular in shape and measured 1.5 min length, 1.0 m in width and 1.0 m in height.

Figure 1. Concrete block samples (Bilgin et al. 1999).

Three concrete mix designs were prepared to obtain low, medium and high strengthconcrete types. Table 1 summarises the components used in 1 m3 of concrete for eachmixture. Sica FF was used in order to increase the workability of high strengthconcrete. Mix designs were adjusted after determining the moisture content of theaggregates.

Table 1. Material quantity for 1 m3 of concrete (Bilgin et al. 1999).

Amount, kgMaterialLow strength

concreteMediumstrengthconcrete

Highstrengthconcrete

Cement 200 425 500Water 126 192 950/3 mm Aggregate 1393 807 8975/15 mm Aggregate 587 859 956Sica FF - - 10

Specially prepared mixes were poured into a steel mould arrangement which includeda greased cylindrical steel pipe of a specified diameter. The steel pipe was placed atthe centre of the mould and fixed at the desired burden distance from the front side ofthe mould and later removed to create the blasthole. All concrete blocks were left tocure for at least 28 days before testing.

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Cylindrical concrete samples of 15x30 cm in size were tested to obtain physical andmechanical properties, including unit weight, unconfined compressive strength,splitting tensile strength, P and S-wave velocity, all in accordance with ASTMStandards (ASTM 1992). In addition, non-destructive tests such as Rebound Hardnessand Ultrasonic Pulse Velocity were carried out. A summary of the range of measuredphysical and mechanical properties is given in Table 2.

Table 2. Physical and mechanical properties of concretes (Bilgin et al. 1999).

Concrete R σc (MPa) T(MPa)

ρ(kg/m3)

Vp(m/s)

Vs(m/s)

Ed

(GPa)νd

Min 15.9 6.7 0.3 2255 3372 1871 20.2 0.278Lowstrengthconcrete

Max 25.1 10.5 0.8 2271 3752 2064 24.8 0.283

Min 29.6 16.3 1.2 2286 3935 2157 27.3 0.285Mediumstrengthconcrete

Max 44.7 24.6 2.9 2379 4553 2471 37.5 0.291

Min 39.5 42.1 2.2 2340 4341 2363 33.7 0.290Highstrengthconcrete

Max 52.9 56.5 4.3 2456 4891 2642 44.4 0.294

R: Hammer rebound; σc: Uniaxial compressive strength; T: Splitting tensile strength; ρ: Density; Vp:P- wave velocity; Vs: S- wave velocity; Ed: Dynamic Young’s modulus; νd: Poisson’s ratio.

2.2 Explosive properties

Properties of the commercial explosives used in the experimental work aresummarised in Table 3.

Table 3. Properties of explosives (Bilgin et al. 1999).

Gelatin Dynamite Elbar 1 DynamiteDensity, g/cm3 1.5 1.0Heat of reaction, MJ/kg 4.70 3.76Ideal VOD, m/s 7527 5070Ideal Pressure, GPa 23.71 8.29Unconfined VOD of 16 mmcharge, m/s

1292 1985

Because charge lengths were small in these particular tests, the confined velocity ofdetonation (VOD) was not measured directly. However, independent unconfined andconfined VOD tests were conducted for all the commercial explosives (dynamites andANFO type explosives) used in the research project. These measurements were usedto determine the confined VOD for each test by adopting the following relationshipdeveloped by Esen (2001):

ωϕβ

να

+

=d

dunconfinednconfined

EDqD

1(1)

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where α, β, ϕ and ω are constants; Dconfined is the confined VOD (m/s); qn is the heatof reaction for non-ideal detonation (MJ/kg); Dunconfined is the unconfined VOD of anexplosive at a given charge diameter (m/s); Ed is the dynamic Young’s Modulus (GPa)and νd is the dynamic Poisson’s ratio.

2.3 Test parameters and data collection procedures

During the tests, factors such as confinement, explosive type, specific charge, burdendistance, blasthole diameter and decoupling ratio were varied one at a time. Asummary of the range of parameters used in the experimental work is given in Table4.

Table 4. Blast design parameters for fully coupled and decoupled model scale tests.

Parameter Fully Coupled Tests Decoupled TestsExplosive Gelatin dynamite, Elbar 1

dynamiteElbar 1 dynamite

Decoupling Ratio* 1 1.25, 1.50, 1.75, 2.00Blasthole Diameter 16-20 20, 24, 28, 32Burden, cm 22.7-46.2 18.2-31.3Hole Depth, cm 40.4-45.4 39.8-45.0Specific Charge, kg/m3 0.110-0.250 0.150-0.175Explosive Amount, g 8.0-22.8 7.8-16.1Stemming Material 1.18-3 mm aggregate 1.18-3 mm aggregateStemming Length, cm 26.5-40.3 21.0-39.6Stemming Length/Burden 0.67-1.47 0.69-2.18Burden/ Blasthole Diameter 14.2-28.9 6.5-15.4Initiation System Electric detonator Electric detonatorConfined VOD**, m/s 1901-2600 -Borehole Pressure**, GPa 1.002-1.469 0.470-0.940* Decoupling ratio = borehole diameter / charge diameter where charge diameter is 16 mm fordecoupled blast tests.** Detonation velocity and borehole pressure are computed by the non-ideal detonation modeldeveloped by Esen (2001).

Each test blast was instrumented with a triaxial arrangement of high frequencygeophones and monitored with the use of a high-speed video camera. After each test,the overall fragment size distribution, the extent of crushing, breakage angle, breakagewidth, throw, backbreak, face velocity, minimum response time and peak particlevelocity were measured. Details of the monitoring procedures have been previouslydiscussed by Bilgin et al. (1999).

Fragmentation assessment was conducted using the following methodology:Fragments larger than 75mm were measured using steel tape in their three axes andweighed on site. The rest of the broken material (less than 75mm) was taken to thelaboratory for sieve analysis. The sieve sizes used were 1.18mm, 4.75mm, 9.5mm,19mm, 25mm, 37.5mm, 50mm and 75mm. A sample sieve analysis is shown inFigure 2.

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0.01

0.1

1

10

100

1 10 100 1000

Size (mm)

% P

assi

ng

Figure 2. A sample size distribution curve for a typical model scale test.

Of interest to this particular study was the proportion of fragments less than the 1.18mm size fraction. This particular size fraction is consistent with the 1mm cut off pointdiscussed by Comeau (1991) and assumed by Kanchibotla et al. (1999).

3 DATA ANALYSIS AND MODEL DEVELOPMENT

In this paper, the analysis focuses on the development of a model that would enablethe prediction of the proportion of fines (material less than 1.18mm) generated duringblasting.

The mechanistic approach combines two specific developments. First is thedevelopment of a relationship between the measured proportion of fines (percent lessthan 1.18mm) and the volume of crushing around a blasthole. Secondly is thedevelopment of a model to predict the size of the crushing zone around a blasthole(Esen et al. 2002).

The concept of the proposed model is similar to that adopted by Kanchibotla et al.(1999). It is assumed that the proportion of material less than 1.18mm will mainly begenerated by the crushing around a blasthole. The main difference between thismethod and others is that the relationship between fines and volume of crushing isexplicitly defined from model scale blasts and it is not based on full scale run of minefragmentation calibration. ROM fragmentation has generally gone through manyother forms of degradation which are very difficult if not impossible to quantify.

It is recognised that from a fundamental point of view, fines will also come from othersources such as the creation of new surfaces through breakage and fragmentation,liberation of fines within pre-existing discontinuities, particle collisions etc. However,the contribution to the overall concentration of fines prior to loading and hauling isvery difficult if not impossible to quantify. These factors are therefore ignored in this

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particular approach as we attempt to predict only the proportion of fines that would begenerated during the crushing stage of the blasting process.

3.1 Correlation between generated fines and the crushing zone volume

From experimental observations, the shape of the crushing zone generally follows apear shape form and can approach a cylindrical shape depending on the length ofcharge, explosive type and material properties. A cylindrical approach is commonlyused and this shape is adopted here (Figure 3). Given this assumption, the volume ofthe crushing zone (Vc) can be simply estimated by,

bfc VVV −= (2)

where Vf is the volume of the cylinder of crushed material for the maximum radius ofcrushing (rc) and charge length and Vb is the blasthole volume.

Maximum crushing zone radius (rc)

Blasthole radius (ro)

Explosive column (L)

Figure 3. Approximation of crushing zone volume.

Given the above approximation, a relationship between the measured proportion ofmaterial less than 1.18mm and the volumetric proportion of crushed material has beendeveloped (Figure 4).

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% less than 1.18mm = 0.79 (%Crushed) + 0.026

R2 = 0.909

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.001 0.01 0.1 1 10

% Crushed

% le

ss t

han

1.1

8mm

Figure 4. Relationship between % crushed and measured proportion of fines.

In Figure 4, the percent of crushed material is calculated by,

100% ×=M

mCrushed c (3)

where mc is the mass of crushed material and M the total mass of broken material. mc

is calculated by,

ρcc Vm = (4)

where Vc is the volume of crushing around the blasthole and ρ is the material density.

A good correlation exists between the measured proportion of material less than1.18mm and the volume of the crushed zone. It is important to highlight that a certaincontribution of finer particles could come from the breakage and fragmentationprocess itself, however due to the experimental conditions (i.e. model scale,homogeneous environment) this contribution was considered negligible. Thefollowing equation allows the estimation of the proportion of the fines contributionfrom the blasting process only:

% less than 1.18mm = 0.79 (%Crushed) + 0.026 (5)

The above relationship appears to validate findings by Kanchibotla et al (1999) andComeau (1991) in describing the 1mm cut off point as a critical value observed inproduction blasting. In general, literature acknowledges the contribution of othersources to the final ROM fragmentation being fed to processing equipment(Djordjevic et al. 1998, Scott et al. 1998). Therefore, the authors of this paper

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recognise that in order to predict the total proportion of fines (i.e. material less than1.18 mm) being fed downstream, a site specific adjustment factor should beincorporated. This factor would be a function of the following:

• Rock mass characteristics such as degree of fracturing, infilling type andweathering characteristics,

• The creation of new fractures during the blast fragmentation process,• Properties of rock material after the blast, that is, degree of pre-conditioning

making the rock material more susceptible to further degradation,• Impact of digging/loading equipment on the further generation of fines,• Specifically to underground blasting, the impact of attrition before material

reaches the drawpoint and through ore passes prior to feeding primary crushers.

The above is an extensive list of factors that can only be assessed on a site by sitebasis. Back-analysis of ROM fragmentation data would allow for the development ofempirically derived site specific degradation factors.

As discussed earlier, one of the key components of the proposed approach is theability to predict the size of the crushing zone around a blasthole for a given explosiveand rock type. This model is discussed in the following section.

3.2 Modelling the size of the crushing zone around a blasthole

As discussed by Whittaker et al. (1992), the compressive strength and stiffnesscharacteristics of the rock material play a major role in the development of the zone ofcrushing or zone of plastic deformation. As the process of crushing is dynamic, acrushing zone model should include dynamic rock material properties. However,many of these dynamic properties cannot be directly measured and are not readilyavailable, resulting in the use of assumed multipliers. For example, Szuladzinski(1993) assumes the dynamic confined compressive strength to be 8 times the value ofunconfined compressive strength. Mohanty & Prasad (2001) give an insight into thedynamic strength characteristics of rock materials at strain rates similar to thoseexperienced during blasting. Using the Split Hopkinson Bar they suggest that the ratioof the unconfined dynamic strength to static values of unconfined compressivestrength ranged between 2.5 and 4.6 for twelve rock types.

As indicated earlier, the approach adopted in this study was to include both static anddynamic properties that are readily available. Hence the extent or radius of crushingdenoted as rc shown in Figure 5 was assumed to be a function of explosive type,material properties and borehole diameter.

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rc

ro

rc = f (ro,Pb,K,σσσσc)

Figure 5. Parameters influencing the extent of crushing.

As shown by Figure 5, ro is the original borehole radius (mm), Pb is the boreholepressure (Pa) calculated using non-ideal detonation theory, K is the rock stiffness (Pa)and σc is the uniaxial compressive strength (Pa). Rock stiffness K is defined assumingthat the material within the crushing zone is homogeneous and isotropic and is givenby,

d

dEK

υ+=

1(6)

where Ed is the dynamic Young’s modulus and νd is the dynamic Poisson’s ratio. Ifthe dynamic Young’s modulus Ed is not available, it may be estimated fromknowledge of the static value by the following relationship (Eissa & Kazi, 1988),

)(log77.002.0log 1010 dst EE γ+= (7)

where Est is the static Young’s modulus (GPa) and γ is the density (g/cm3).

Borehole pressure is computed from the non-ideal detonation model developed byEsen (2001).

Given the above measurements and calculations and by applying dimensionalanalysis, two dimensionless indices (π1 and π2) were derived:

c

o

r

r=1π (8)

and

( )( ) 2

3

2c

b

K

P

σπ

×= or crushing zone index (CZI) (9)

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where as noted earlier, rc is the crushing zone radius (mm), ro is the borehole radius(mm), Pb is the borehole pressure (Pa), K is the rock stiffness (Pa) and σc is theuniaxial compressive strength (Pa).

The relationship between the two indices given above is shown in Figure 6. Thefunction obtained by non-linear regression is given by:

( ) 219.0231.1 −= CZIr

r

c

o (10)

where CZI is defined as the crushing zone index. This is a dimensionless index thatidentifies the crushing potential of a charged blasthole. The correlation coefficient ofthe relationship given by Equation 10 is R2=0.83. The crushing zone index (CZI)appears to capture the dynamic process taking place in the crushing zone by takinginto account both explosive and rock properties.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 500 1000 1500 2000 2500 3000

CZI

ro/r

c

Fully coupled Decoupled

Figure 6. Relationship between CZI and ro/rc for 92 model scale test blasts.

Because it is physically impossible for the ratio between ro and rc to be greater than 1,the relationship (Equation 10) is constrained to 1 for very small values of CZI, in thiscase for values of CZI of less than 2.6. These small values of CZI will generallycorrespond to small borehole pressures (i.e. decoupled charges). The 92 data pointsshown in Figure 6 clearly cover a wide range of conditions for which ratios of ro/rc

can be obtained.

The new proposed approach is compared against predictions given by a selection ofmodels found in the open literature, namely Szuladzinski's (1993), Kanchibotla et al.(1999), Djordjevic (1999), Il'yushin (1971) and Vovk et al. (1973). This comparisonis conducted for the results obtained in the model scale blasting experiments.Measured versus predicted values are shown in Figure 7. The new model developed inthis study predicts the size of the crushing zone with reasonable accuracy. However,other models could not approximate the conditions of the experimental work.

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Szuladzinski's (1993) predictions are closer to the new model than the approachesproposed by , Kanchibotla et al. (1999), Djordjevic (1999), Il'yushin (1971) and Vovket al. (1973). One of the possible reasons is that these models assume ideal detonationwhich is not valid for the model scale tests. Explosives show a more pronounced non-ideal detonation behaviour under these conditions.

Further details of the proposed model are discussed by Esen et al (2002). Theiranalysis has indicated that this new model follows the expected trends; for examplefor a specific rock environment and explosive type, as the blasthole diameterincreases the crushing zone radius increases. Similarly, the model shows that anexplosive with the capacity to generate higher borehole pressures has the potential toincrease crushing for the same blasthole diameter and rock environment.

0

50

100

150

200

250

300

350

400

450

500

0 5 10 15 20 25 30 35 40 45 50

Measured crushing zone radius (mm)

Pre

dic

ted

cru

shin

g z

on

e ra

diu

s (m

m)

New model Szuladzinski (1993)

Kanchibotla et al. (1999) Djordjevic (1999)

Il'yushin (1971) and Vovk et al. (1973) Perfect fit

Figure 7. Comparison of other models against model scale blasting experiments.

4 APPLICATION OF THE PROPOSED MODEL

The proposed model can be used to predict the likely proportion of fine materialgenerated during the blasting process. This method allows engineers to analysedifferent drilling and blasting strategies to better match explosives to blastingdomains, this is particularly useful where domains are highly susceptible to crushingand continual degradation (e.g., low strength rocks).

To demonstrate the application of the proposed approach, a number of simulationswere conducted for the geotechnical conditions described in Table 5, thesesimulations are based on actual case studies (Esen et al. 2000, Thornton et al. 2001).The analysis included, three different explosive types, namely ANFO1 (density=0.8

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g/cm3), ANFO2 (density=0.928 g/cm3) and Emulsion (density=1.25 g/cm3). Resultsare summarised in Table 6.

Table 5. Physical and mechanical properties of rock types used in the simulations.

Rock σc, MPa T,MPa

ρ,kg/m3

Ed, GPa νd

Coal (Thornton et al. 2001) 20.0 2.0 1440 7.61 0.144Limestone (Esen et al., 2000) 99.0 8.0 2707 71.48 0.325

Table 6. Relative comparison of % fines generated during blasting.

BlastingDomain

Explosive Holediameter,

mm

BoreholePressure

Pb,GPa

Crushedradius

rc,mm

Chargelength,

m

Burden xSpacing x

Bench height,m

TotalMass,

kg

Masscrushed,

kg

%fines

-1.18mm

Coal Emulsion 150 7.189 812 5.4 7x7x9.5 670320 15970 1.91Coal ANFO1 150 2.514 408 5.4 7x7x9.5 670320 3929 0.50Limestone ANFO1 89 1.617 57 6.7 2.6x3.6x7.5 190031 72 0.06Limestone ANFO2 89 2.308 72 6.7 2.6x3.6x7.5 190031 183 0.10

The above analysis clearly shows the influence of explosive type and rock propertieson the amount fines likely to be generated by blasting. The proportion of fines isgiven by a cut off point of 1.18 mm.

In general, the proposed model follows the expected trends, for example for a givenrock type and borehole diameter, an explosive with the capacity to generate higherborehole pressures, has the potential to form a larger zone of crushing and produce ahigher proportion of fines.

As shown in Table 6, the proportion of fines generated during coal blasting can besignificantly affected by the type of explosive used. The crushing radius is decreasedwith the use of a low density explosive (ANFO1) by almost one half which translatesto a reduction in the mass of crushed material per hole of approximately four times. Inaddition, the percent of fines (less than 1.18 mm) is decreased by a factor of 3.8. Thistype of simulations give a good insight into the coal loss problem during the blastingprocess.

The above analysis indicating a proportion of fines (-1.18mm) of 1.91% excludes thewell documented degradation occurring during both excavation and handling(Djordjevic et al. 1998, Thornton et al. 2001). Sieved measurements discussed byThornton et al. (2001) indicate that the proportion of fines (-1mm) is approximately17%, combining both blasting, excavation and handling processes. This suggests thatcoal is susceptible to significant downstream degradation after blasting.

A study conducted by Esen et al. (2000) in a limestone quarry, indicated that by theintroduction of a higher density ANFO product (i.e. ANFO2), the overallfragmentation was significantly improved, based on comparisons of the coarser

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fragments of muckpiles (i.e. 50% and 80% passing sizes). However, based on thesimulations (Table 6) the generation of fines (-1.18mm) produced during blastingwould have been increased by a factor of 1.7, which translates to a loss of 5.6 tonnesof material per blast, this is assuming a blast consisting of 50 blastholes and excludesthe influence of degradation from excavation and handling processes.

5 CONCLUSIONS

An engineering approach to predict the proportion of fines generated during blastinghas been presented. The model is based on the back-analysis of a comprehensiveexperimental program that included the fragmentation assessment and directmeasurement of the zone of crushing from 92 blasting tests on concrete blocks usingtwo commercial explosives.

The proposed model follows the expected trends, for example for a given rock typeand borehole diameter, an explosive with the capacity to generate higher boreholepressures, has the potential to form a larger zone of crushing and produce a higherproportion of fines.

Simulations conducted with this model have confirmed the susceptibility of lowstrength rock masses (e.g., coal) to further degradation after blasting. In addition, inquarry operations it was clear that blast optimisation to improve fragmentation mustconsider the whole size distribution for a particular application.

A significant contribution to the proposed approach has been the development of anew model to predict the radius of crushing around a blasthole. This particulardevelopment was required as a number of previously proposed models could notapproximate the conditions measured in the experimental work and there were noteddiscrepancies between the reviewed approaches, particularly in smaller diameter holesand low strength rock conditions. This new model has confirmed that the ratiobetween the crushing zone radius and borehole radius (rc/ro) is a function ofexplosive, rock properties and blasthole diameter.

This engineering approach provides a good insight into the source of fines from theblasting (explosive/rock interaction) process, enabling the further development offragmentation optimisation tools. The proposed approach is currently beingincorporated in the further development of a mechanistic fragmentation model foropen pit and underground blasting applications.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the support of the BARUTSAN ExplosivesCompany, Turkey. Many thanks go to Muharrem Kilic, Nedim Yesil andBARUTSAN Co. staff and technicians. The authors would also like to thank Prof. BillWhiten, Dr. R. Trueman and Dr. G. Chitombo of the Julius Kruttschnitt MineralResearch Centre (JKMRC) for their suggestions.

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