blast fragmentation modelling of the codelco-andina open pit expansion

8/13/2019 Blast Fragmentation Modelling of the Codelco-Andina Open Pit Expansion

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1 INTRODUCTIONBlasting activities in major mining operations

have been placing significant emphasis on the abilityto tailor fragmentation to improve downstream proc-esses. In many of these operations, the impact offines has been clearly identified.

At the conceptual and feasibility stages, fragmen-tation modelling studies which support future Mineto Mill strategies can be conducted through the cali-bration of empirical models using existing data; andif need be, through the implementation of specific

trials. An important pre-requisite is the adequateclassification and characterisation of the blastingdomains of interest. As part of the feasibility studyof the open pit expansion project of CodelcosAndina operation, a comprehensive blast monitoringprogram was conducted in the currently mined sec-ondary ore domains of the Don Luis pit. The ob-

jective of this program was to calibrate and imple-ment a site specific blasting model to enable theprediction of fragmentation trends in the deeper,more competent ore zones

2 DESCRIPTION OF BLASTING DOMAINSThe main geotechnical units forming the core of

the mining environment of current and future opera-tions at Andina have been grouped into PrimaryRock, Secondary Rock and Riolite and Dacite chim-neys. This study is mainly concerned with the cali-bration of an empirical fragmentation model in sec-ondary rock and simulations in primary rock.

Primary rock masses have been described ashard and competent with well healed gypsum orAnhidrite filled fractures, typical RMR values are in

the range of 60 to 80. Secondary rock masses canalso be described as hard, however fractures aregenerally open and hence reduce the competency ofthe rock mass with characteristic RMR values in therange of 42 to 50.

Relevant to blast fragmentation modelling is thedegree of in situ fracturing. As shown in Figure 1,total spacing statistics derived from fracture fre-quency data show that the degree of fracturing isclearly more intense in the secondary rock mass do-mains. Results from the available core logging data

indicated that total fracture spacing may be as wideas 0.4 m in the secondary domain and 0.91 m in theprimary rock domain. The analysis shows that thevariability in fracture intensity appears to be greaterin the primary rock domain.

Blast Fragmentation Modelling of the Codelco-Andina Open Pit Expansion

F. MardonesGeoBlast S. A., Chile

C. ScherpenisseGeoBlast S. A., Chile

I. OnederraThe University of Queensland, Sustainable Minerals Institute, W H Bryan Mining and Geology Research

Centre, Qld 4072 Australia

ABSTRACT: In large scale metalliferous mining, there is significant evidence to suggest that by providing anappropriate size distribution to crushing and grinding circuits, a measurable increased throughput and/or re-duced power draw can be obtained. Tailoring blast designs to suit specific fragmentation requirements is nowcommon place at both the pre-feasibility and feasibility study stages. This is particularly the case when sig-nificant increases in ore production rates are being considered. As part of the feasibility study of the open pitexpansion project of Codelcos Andina operation, a comprehensive blast monitoring program was conductedin the currently mined secondary ore domains of the Don Luis pit. The objective of this program was to

calibrate and implement a site specific blasting model to enable the prediction of fragmentation trends in thedeeper, more competent ore zones, also referred to as primary rock domains. This paper gives a brief descrip-tion of the blast fragmentation monitoring program conducted and discusses the calibration and application ofa stochastic blast fragmentation modelling framework. Results from several simulations have highlighted thekey differences in fragmentation if current blast designs are applied in the more competent primary rock do-mains. A number of blast design options have been evaluated and recommendations made in order to achievespecific ore handling and processing targets.

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From a drilling and blasting perspective the sec-ondary and primary domains can be classified asfractured and blocky rock masses respectively. Frac-ture spacing statistics were used to provide first passestimates of the potential range of the mean size ofin situ blocks. This is a required input parameter thatis further defined through back-analysis or the modelcalibration process.

Figure 1. Total spacing statistics of secondary and primary rockmasses.

The rock types or lithological units of concern tothis study include Granodiorita Cascada (GDCC)and Brecha de Turmalina (BXT). Both rock types arefound in the secondary and primary rock domains. Inboth cases, the degree of alteration appears to be themain factor that affects strength and stiffness. Analy-sis of the geotechnical information provided byAndina allowed the definition of the average intactrock material and rock mass parameters used in thecalibration and modelling of different blasting sce-narios. Table 1 gives a summary of the domain andproperties of the GDCC and BXT rock types.

Table 1 shows that the intact rock material in theprimary domain is slightly stiffer than in the secon-dary domains. In terms of compressive strength,there are no significant differences between the

GDCC rock in the secondary and primary domainswith mean values of 161 and 156 MPa respectively.A more pronounced difference is observed in theBXT rock with mean values of 135 and 158 MPa re-spectively. From a drilling and blasting perspective,all rock material can be classified as hard and com-petent. In these hard competent conditions incipientdamage defined by peak particle velocity is esti-mated to be in the range of 900 to 1100 mm/s; andbreakage is expected to be in the range of 3600 to4400 mm/s. The overall breakage and fragmentation

potential is expected to be driven by the degree andcondition of fracturing; and therefore differences inthe intermediate and coarse end of the fragmentationdistribution are expected between secondary andprimary rock masses. It should be noted that the rock

material data collected by the geology and geotech-nical department of Andina provided the necessaryinput to reliably implement a stochastic modellingapproach.

Table 1. Summary of intact rock properties in secondary andprimary domains.

3 OVERVIEW OF MONITORED BLASTSThe detailed monitoring of production blasts in

secondary rock masses has been an important andnecessary component of the model calibration andverification process. As summarised in Table 2, a to-tal of four production blasts were monitored in the

Don Luis pit of the Andina operation, three werelocated in the GDCC domain and one in the BXTdomain.

Table 2. Design parameters of monitored production blasts.* The explosive Apex 150 is a Heavy ANFO (50% Emulsion)product supplied by Orica Chile.

4 FRAGMENTATION ASSESSMENTA detailed fragmentation assessment program was

conducted during this study. The detailed programincluded the acquisition of images during the exca-vation of muckpiles as well as the sieving of a lim-ited number of samples taken from selected regions.As illustrated in Figure 2, the assessment procedure


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consisted of sampling lines and profiles taken at dif-ferent stages of extraction. This procedure is consis-tent with best practices in fragmentation assessmentusing image analysis methods.

Figure 2. Example plan view of sampling lines of blast

3724_12.

Detailed analysis included both manual editing

and the definition of site specific fine correction fac-tors. These factors were determined directly by thesampling and sieving of fragments in the areas of in-terest. Blasting literature shows that reliable esti-mates of Run of Mine (ROM) fragmentation can beobtained following procedures similar to those in-corporated in this study (Latham et al 2003 and San-chidrian et al 2005).

The data obtained from the monitored areas wereused in this study to calibrate and verify the blastfragmentation models implemented in this study.

The total number of samples taken in both theGDCC and BXT domain are summarised in Table 3.

Table 3. Summary of fragmentation images samples taken dur-ing the monitoring of blasts in the GDCC and BXT domains.

5 BLAST FRAGMENTATION MODELLINGThe expected distribution of fragments in the

fines and coarse regions is modelled by two separatedistributions based on the recently published Swe-brec function (Ouchterlony, 2005). The Swebrecfunction has recently shown to be far superior in fit-ting fragmented rock in the intermediate and finerend of the fragmentation curve than previous mod-els. The main modelling framework includes theability to consider a range of values to key input pa-

rameters through the explicit definition of distribu-tion functions. In this way stochastic simulations canbe conducted to determine a predictive fragmenta-tion envelope that takes into account the variabilityof rock material, rock mass, blast geometry and ex-plosive performance parameters. The current ap-proach also incorporates modelling parameters thatcan simulate the impact of inter-hole delay timing onfragmentation (Onederra, 2008).

5.1 Calibration results in secondary oreAs has been extensively discussed in the literature,

one of the main limitations of empirical fragmenta-tion models is their requirement for site specificcalibration. This necessary process generally in-volves the back analysis or prediction of fragmenta-tion based on measured data and monitored prac-tices. As mentioned earlier, four production blastscovering GDCC and BXT secondary rock domainswere used to calibrate the proposed fragmentationmodelling framework. The calibration process in-

volved the refinement of estimates associated withrock mass parameters likely to impact on uniformity,mean fragmentation outcomes and the propensity ofthe rock fabric to generate fines during the fracturingprocess.

Figure 3 summarises the results of comparisonsbetween predicted and measured fragmentation out-comes for one of the blasts in the GDCC domain(3724_10). In this analysis, statistics associated withrock material input parameters, pattern geometry andexplosive performance were included to generate anexpected fragmentation bounded by envelopes ofminimum, maximum and 95% confidence. It is im-portant to note that the fragmentation envelope givenby each simulation is a function of the level of un-

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certainty or variability assigned to the available inputdata.

Figure 3. Summary of calibration results based on monitored

production blasts.

5.2 Fragmentation modelling of primary rockA total of 14 simulations were conducted to quan-

tify relative changes in ROM fragmentation in pri-mary rock. Table 4 gives a summary of the patterngeometries and powder factor ranges investigated.As shown, powder factors reflect the use of patterngeometries similar to those currently implemented atAndina, as well as more aggressive designs whichinclude both reductions in burden, spacing andstemming lengths. All simulations have maintainedthe use of 270 mm diameter blastholes using Apex150 and Apex 165 as the base case explosive prod-ucts. It should also be noted that a single hole firing

mode was assumed with inter-hole delays of 10 ms.As discussed earlier, the adopted stochastic ap-proach has allowed the inclusion of distributionfunctions to rock material and rock mass input pa-rameters as well as design specific parameters suchas hole and charge lengths. The Latin Hypercubesampling technique was used with simulations set to500 iterations.

Table 4. Summary of pattern configurations for GDCC andBXT primary ore domains.* The explosive Apex 165 is a Heavy ANFO (65% Emulsion)product supplied by Orica Chile.

6 RESULTS AND DISCUSSIONFragmentation modelling results for GDCC and

BXT primary ore domains are summarised in Fig-ures 4 and 5 respectively. Note that only the ex-pected size distribution curves are shown for com-parison purposes. Modelling results demonstrate theinfluence that changes in pattern geometry may have

on fragmentation, particularly in the intermediateand finer size fractions. Differences between do-mains and designs are also summarised in Table 5.

For similar pattern geometries and correspondingpowder factors, modelling results suggest that blast-ing in the GDCC domain has the potential to gener-ate more fines than in the BXT domain. Relative dif-ferences may be of the order of 3% to 5% betweenthese two domains. As expected, designs D3 and D6give the finest fragmentation in the GDCC domain;and designs D9 and D12 give the finest fragmenta-tion in the BXT domain. By comparing designs D2

and D2A, modelling results suggest that by decreas-ing stemming lengths by approximately 1 m, a 1%gain is expected in the amount of fines generated inthe GDCC domain. In the BXT domain however thegain is only approximately 0.5%, as shown by com-paring designs D8 and D8A. It is important to notethat model calibrations in the BXT domain wereonly based on a single production blast. More datamay be required to improve the predicted capabili-ties of the model in this particular domain.

Table5. Summary of fragmentation modelling results in pri-

mary ore conditions.

Figure 4. Comparison between designs in GDCC primary ore.


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Figure 5. Comparison between designs in BXT primary ore.

As discussed earlier, single hole firing conditionswere adopted in the modelling calculations, in thiscase, a 10 ms inter hole delay was assumed based onestimations of minimum response time (Onederra,2007). The current modelling framework was usedto investigate potential gains in fines generation by

introducing shorter delays (e.g. 2 ms to 10 ms). De-signs D2 and D8 were used as base cases for theGDCC and BXT domain respectively. Results of theanalysis for the expected values in GDCC are sum-marised in Figure 6. As shown, for the and 1size fractions with the use of inter hole delays of 2ms, gains of approximately 2 % and 3.5 % may beachieved in the GDCC. The use of very short interhole delays (e.g. 2 ms) demonstrates gains in the in-termediate and fine fractions, however as shown inFigure 6, these gains may not be significant if one is

to consider the variation associated with modellingpredictions, and in particular the lower limit predic-tive envelope. It should also be noted that the interhole delay adjustment factors proposed in the currentmodelling framework (Onederra, 2008) are based onlimited data and further validation will still be re-quired in primary rock conditions.

Although fragmentation may be improved, it isimportant to note that high intensity blasting withthe use of short inter hole delays may be counterproductive if the risk of rock mass damage is in-creased and loading productivity is influenced by thelack of muckpile looseness. Preliminary modellingresults have highlighted the need to further quantifythe potential impact of short delays on near fielddamage and downstream loading productivity. Thisshould be considered a priority if short inter hole de-lays are to be used in primary rock production blast-ing.

Figure 6. Modelling results showing the potential influence of

short inter-hole delay times on fines for design D2 (GDCC

Domain).

It is important to note that simulations are indica-tive of what may be achieved if all measured and as-

sumed modelling conditions are met. Actual meas-

urable results will undoubtedly be influenced by the

field implementation process. For this reason, the

implementation of a Quality Assurance / Quality

Control strategy (QA/QC) was strongly recom-

mended, particularly as improved designs are im-

plemented in both current and future domains (Sec-

ondary and primary rock). The impact on

fragmentation outcomes given by variations in pat-

tern geometry is demonstrated for design D2 in Fig-

ure 7. In this case, a standard deviation of 0.5 m was

assumed for the mean values of burden and spacing.

Results show a widening of the predictive envelope,

which can translate into coarser or more bi-modal

fragmentation outcomes.

Figure 7. Potential impact on fragmentation outcomes given by

simulated variations in pattern geometry


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7 CONCLUSIONSA comprehensive production blast monitoring

program was conducted in secondary ore domains of

the Don Luis pit at the Codelco-Andina operation.

The objective of this program was to calibrate and

implement a site specific blast fragmentation model

to predict fragmentation outcomes in primary rock

domains. The rock types or lithological units of main

concern to this study included Granodiorita Cascada

(GDCC) and Brecha de Turmalina (BXT). A total of

four production blasts were comprehensively moni-

tored in secondary rock, three were located in the

GDCC domain (i.e. 3724_10, 3724_12 and

3724_09) and one in the BXT domain (i.e. 3708_3).

The calibration process allowed the definition and

refinement of estimates associated with key rock

mass indices which impact on the expected uniform-

ity, mean fragment size and the propensity of therock fabric to generate fines.

The calibrated model used in this study can be

best described as a two component model utilising

the Swebrec fragmentation distribution function. The

adopted approach is stochastic and therefore allows

the inclusion of distribution functions to rock mate-

rial and rock mass input parameters as well as design

specific parameters such as hole and charge lengths.

The modelling framework also incorporated model-

ling parameters that can simulate the impact of interhole delay timing on fragmentation.

14 simulations were conducted to quantify relative

changes in ROM fragmentation in primary rock

(GDCC and BXT rock types). A range of pattern ge-

ometries and corresponding powder factors were in-

vestigated. The analysis indicated that for similar

pattern geometries, blasting in the GDCC domain

has the potential to generate more fines than in the

BXT domain. Relative differences may be of the or-

der of 3% to 5% between these two domains.

As expected, designs D3 and D6 gave the finest

fragmentation in the GDCC domain; and designs D9

and D12 gave the finest fragmentation in the BXT

domain. By comparing designs D2 and D2A, model-

ling results suggested that by decreasing stemming

lengths by approximately 1 m, a 1% gain is expected

in the amount of fines generated in the GDCC do-

main. In the BXT domain however the gain was only

approximately 0.5%, as shown by comparing designs

D8 and D8A.

Model calibrations in the BXT domain were only

based on a single production blast. More data may be

required to improve the predicted capabilities of the

model in this particular domain

The current modelling framework was used to in-

vestigate potential gains in fines generation by intro-

ducing shorter delays (e.g. 2 ms to 10 ms). Designs

D2 and D8 were used as base cases for the GDCC

and BXT domain respectively. Results showed that

for the and 1 size fractions with the use of interhole delays of 2 ms, gains of approximately 2 % and

3.5 % may be achieved in the GDCC domain. The

use of very short inter hole delays (e.g. 2 ms) dem-

onstrates gains in the intermediate and fine fractions,

however these gains may not be significant if one is

to consider the variation associated with modelling

predictions, and in particular the lower limit predic-

tive envelope.

Preliminary modelling results have highlighted the

need to further quantify the potential impact of short

delays on near field damage and downstream loading

productivity. This should be considered a priority if

short inter hole delays are to be used in primary rock

production blasting.

It is important to note that proposed changes in

blasthole configurations and geometry (i.e. tighter

patterns) may be restricted by operational matters.These types of constraints should be reviewed and

assessed at the operational level.


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blast fragmentation modelling of the codelco-andina open pit expansion

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