March 2012 www.Agg-Net.com 13 A Perfect Partnership Collaboration delivers optimized blasting process at Thrislington Quarry By Ian Brown, Lafarge Aggregates; Jo Banner, Banner Contracts; and Dr Rob Farnfield and Martin Cooper, EPC-UK Blast material being loaded into a mobile crusher at Thrislington Quarry T here is sufficient published global research confirming that the use of electronic detonators in quarry blasting results in a reduction in the overall cost of rock breaking, yet the take-up of this technology in the UK has been poor. For the most part, electronic detonator systems have been limited to those sites that require strict control of the environmental impacts of blasting. This article describes the process undertaken at a quarry that relied on the effective collaboration between three independent companies, who each placed the single goal of optimizing the entire process above their own individual interests while maintaining and, where possible, improving safety standards. By observation, discussion and experimentation, the use of electronic detonators to control vibration levels in one area of the quarry evolved, while the same technology was used to optimize the rock- breaking process in another area. The partners The three independent partners involved were quarry operators Lafarge Aggregates; explosives suppliers EPC-UK; and crushing contractors Banner Contracts. Established in France in 1833 and now operating in 70 countries, Lafarge are global producers of cement, aggregates, concrete and asphalt. The company’s key objectives are: zero harm; product innovation; and industrial excellence. Blast optimization is essential to achieving these goals. EPC Groupe were established in 1893, their core activity being the manufacture and supply of commercial explosives and tailor-made services to the mining, cement, aggregates and civil engineering industries. The group has activities in Europe, Africa and the Middle East, and has operated in the UK for more than 100 years. Banner Contracts are a family-run company with more than 30 years’ experience in the construction and quarrying industries. The main activities of the company are: contract crushing, screening and washing of aggregates; and earthmoving and related plant-hire services. Thrislington Quarry Lafarge’s Thrislington Quarry was opened in 1954 to extract magnesian limestone from an area near Ferryhill. The quarry, which covers an area of 112ha, plays an important role in mineral supply in Co. Durham and about a third of the limestone extracted from the site is a high-grade variety (known as industrial dolomite). This is processed in the kilns at the adjoining works to produce burnt dolomite which is of considerable importance to the steel industry. Lafarge have gained planning permission to extend the life of the quarry beyond 2015 and to work an area east of the A1(M). Over a period of 32 years the company will extract 29 million tonnes from this new extension, which will involve the building of a 200m long tunnel under the A1(M). Electronic detonators It is widely recognized that the use of electronic detonators has a number of significant advantages compared with detonators ‰
Transcript
March 2012 www.Agg-Net.com 13
A Perfect PartnershipCollaboration delivers optimized blasting process at Thrislington Quarry
By Ian Brown, Lafarge Aggregates; Jo Banner, Banner Contracts; and Dr Rob Farnfield and Martin Cooper, EPC-UK
Blast material being loaded into a mobile crusher at Thrislington Quarry
There is sufficient published globalresearch confirming that the use ofelectronic detonators in quarry blasting
results in a reduction in the overall cost of rockbreaking, yet the take-up of this technology inthe UK has been poor. For the most part,electronic detonator systems have been limitedto those sites that require strict control of theenvironmental impacts of blasting.This article describes the process
undertaken at a quarry that relied on theeffective collaboration between threeindependent companies, who each placed thesingle goal of optimizing the entire processabove their own individual interests whilemaintaining and, where possible, improvingsafety standards. By observation, discussionand experimentation, the use of electronicdetonators to control vibration levels in one areaof the quarry evolved, while the sametechnology was used to optimize the rock-breaking process in another area.
The partnersThe three independent partners involved were
quarry operators Lafarge Aggregates;explosives suppliers EPC-UK; and crushingcontractors Banner Contracts.Established in France in 1833 and now
operating in 70 countries, Lafarge are globalproducers of cement, aggregates, concrete andasphalt. The company’s key objectives are: zeroharm; product innovation; and industrialexcellence. Blast optimization is essential toachieving these goals.EPC Groupe were established in 1893, their
core activity being the manufacture and supplyof commercial explosives and tailor-madeservices to the mining, cement, aggregates andcivil engineering industries. The group hasactivities in Europe, Africa and the Middle East,and has operated in the UK for more than 100years.Banner Contracts are a family-run company
with more than 30 years’ experience in theconstruction and quarrying industries. Themain activities of the company are: contractcrushing, screening and washing of aggregates;and earthmoving and related plant-hireservices.
Thrislington QuarryLafarge’s Thrislington Quarry was opened in1954 to extract magnesian limestone from anarea near Ferryhill. The quarry, which coversan area of 112ha, plays an important role inmineral supply in Co. Durham and about a thirdof the limestone extracted from the site is ahigh-grade variety (known as industrialdolomite). This is processed in the kilns at theadjoining works to produce burnt dolomitewhich is of considerable importance to the steelindustry.Lafarge have gained planning permission to
extend the life of the quarry beyond 2015 andto work an area east of the A1(M). Over a periodof 32 years the company will extract 29 milliontonnes from this new extension, which willinvolve the building of a 200m long tunnel underthe A1(M).
Electronic detonatorsIt is widely recognized that the use of electronicdetonators has a number of significantadvantages compared with detonators ‰
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based on shock-tube technology. The followingis a comparison of the key aspects:
TestabilityMost electronic detonator systems are fullytestable with two-way communication betweenthe detonator and the exploder. Because thefunctioning of the entire system can beconfirmed prior to blasting, the possibility ofmisfires is significantly reduced. In contrast,shock-tube systems have no testability and relyon a simple visual check before firing.
AccuracyDelay times in electronic detonators aregenerated by means of a digital chip housedinside the detonator, which provides aninitiation accuracy to a fraction of a millisecond.Shock-tube-based detonators rely on the burntime of a pyrotechnical delay element togenerate the required delay. The reliance on apyrotechnical delay element means that suchdetonators are subject to a significant degreeof so-called ‘scatter’. Trials with shock-tubedetonators show that in a single row ofblastholes, the degree of scatter is generallysufficient for at least one of the holes toinitiate out of sequence.
Flexible initiation patternThe use of electronic detonators results incomplete flexibility in the choice of initiationpattern, with almost any combination of inter-hole and inter-row delay being possible. Incontrast, with shock-tube systems the choiceof delays is limited to those provided by thedetonator manufacturer.
CostWhile electronic systems are generally moreexpensive than shock-tube-based systems,their use provides significant benefits in termsof blast and vibration control. When viewed inthe context of overall process costs, the resultis a lower rock production cost.
Hotshot electronic detonatorsystemThe electronic detonator system used in thework described in this article is the Hotshotsystem manufactured in South Africa by Detnetand supplied in the UK by EPC-UK. TheHotshot system is an auto-programmablesystem similar in operation to a shock-tubesystem by using inter-hole and inter-rowdelays. Compared with a shock-tube system,however, the difference is that the Hotshotsystem is both fully testable and accurate, andprovides complete flexibility in the choice ofdelays employed.
Hybrid initiationIn the UK, EPC-UK have pioneered the use ofa combination of electronic and shock-tubedetonators in quarry blasting with a so-called‘hybrid’ system. With this system, each chargeor deck of explosives has a single electronicdetonator and a single shock-tube (in-hole)detonator. The shock-tube detonator is leftunconnected unless the electronic system
indicates that there is a problem with the singlein-hole electronic detonator. When such aproblem is discovered, a replacement electronicdetonator is connected to the in-hole shock-tube detonator and the firing time adjusted totake into account the delay time of the shock-tube detonator. This system has the advantageof reducing cost while retaining all theadvantages of employing electronic detonators.The hybrid system has been employed in thework outlined in this article.
Crushing processAs with any quarry operation, if safety is heldas the primary concern, efficiency and costsavings will follow as a result. It is often thecase that where a quarry employs contractcrushing, the key driver is to minimize drillingand blasting costs, as the cost of the crushingoperation is then fixed and the quality of theblast is no longer a variable that affects it.This is a very short-term view and often
results in increased costs, not only for thecrushing contractor, but also for the quarryowner.Safety in a quarry is a team effort
demanding joined-up thinking in order to besuccessful. Nowhere is this more apparentthan in the relationship between the crushingoperations and the drilling and blastingoperations. The crushing contractors can doall they can to be safe, but the biggesthazards they encounter are found in theblast pile and at the rock face, ie outputs ofthe drilling and blasting operation. The risksfrom a poor face that requires extensivedressing, the handling of oversize stone in themuckpile and subsequent bridging of theprimary crusher are self-evident and haveresulted in numerous fatalities in the industry.
The use of the Hotshot system atThrislington Quarry has resulted in a muckpilewith improved fragmentation, and cleanerfaces. The benefits in terms of safety aredifficult to quantify in terms of monetaryvalue. Cleaner faces and less oversize meanless time spent dressing faces, less timespent handling and breaking oversize stone,and less bridging in the primary crusher.These are all activities where the risks must be managed; eliminating the activityaltogether clearly being the most preferablesolution.However, in terms of efficiency savings
and improved production, the benefits of theHotshot system are more tangible, as outlinedbelow:
Improved fragmentationImproved fragmentation means the faceexcavator spends less time handling andbreaking oversize stone. While handlingoversize material the excavator is not feedingthe primary crusher. Every minute thecrusher spends running empty carries withit a cost in terms of lost production forBanner Contracts and a cost in terms ofwasted fuel for Lafarge.Prior to the use of electronic detonators, the
pile of shot rock was often difficult to dig dueto poor fragmentation. The face excavatorwould spend time winning the rock to feed thejaw crusher. As a result, the crusher could runempty for periods of time, with the costs interms of lost production and higher fuel usagebeing clearly evident. In addition, the stressesplaced upon the face machine would eventuallyresult in premature failure and excessivewear of components. This was especially thecase on short faces with vibration issues.
Components of the Hotshot electronic detonator system
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Cleaner facesThe quality of the faces left behind afterblasting with the Hotshot system meansless time is spent dressing faces, resultingin improved production due to reduced timedemands on the face excavator.
Bigger blastsThe ability to drop bigger blasts withconfidence has led to a further saving indowntime. The time taken to pull plant out fora blast and then to set the plant up again canamount to several hours. The savingsattributable to eliminating some of thisdowntime are self-evident. The combined benefit in terms of
production gains can be as much as 100tonnes per day when compared with a poorshot using shock-tube initiation.
Blast OptimizationPrior to the start of the optimization process
in the southern part of Thrislington Quarry,a typical blast consisted of two rows of five125mm diameter holes. Sub-drill of 1.0m wasgenerally employed along with a burden of4.5m and a spacing of 5.0m. The hole loadingconsisted of a small packaged explosivebase charge with a column charge of site-mixed ANFO and 3.0m of stemming. As istypical at most quarries, over the yearsattempts had been made to optimize andimprove the blast design, in this case byincreasing blast size and reducing the degreeof sub-drill. However, although these changesdid make some improvements, the resultswere not consistent and usually resulted inoccasional blasts with poor performance.It is also worth noting that Thrislington
shares a drill rig with two other quarries, soblast size is very important in order tominimize the cost of rig transport betweensites.The blast optimization process followed a
simple approach, progressing in phasesfrom the existing blast design based onshock-tube detonators, to a fully optimizeddesign based on electronic detonators.
Phase 1: Change to electronicdetonatorsApart from the fact that the shock-tubesystem was replaced by an electronic system(hybrid), the blasts fired were identical to theoriginal design in all other aspects. Thedelay pattern used 16ms along the row and 40ms between rows. This simple changeresulted in a blast with greatly enhancedfragmentation, as reported by the crushingcontractor, but a muckpile profile that was notideal for the loader/crusher combinationemployed – too low and too wide a spread.Clearly, the cost per tonne of drilling andblasting had increased due to the use ofelectronic detonators, but the crushingcontractor considered that this was more ‰
Same blast as above but taken from the side showing the muckpile profile
Post-blast muckpile from a blast with seven rows each of 10 holes producing 50,000+ tonnes
Drilling & Blasting
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Drilling & Blasting
than offset by the increased crusherthroughput, reduced downtime and reduedfuel consumption. The quarry also reportedthat the post-blast face was much improved.
Phase 2: Increase burden andspacingInitially, the burden was increased by 0.5mwhile continuing to employ the hybridinitiation system, resulting in a cost perblasted tonne equal to that previouslyobtained. Blasts fired using this design werealso reported to produce enhancedfragmentation and improved post-blast faceconditions. The post-blast muckpile shapewas more suitable for the combination ofexcavator and crusher employed.Subsequent blasts were designed with
an additional 0.5m added to the spacing,resulting in an overall blasting cost thatwas less than the original shock-tube-baseddesign.
Phase 3: Increase blast size To reduce drill rig movements, and thereforecost, blast size is critical at this quarry. It wasdetermined that the number of rows and thenumber of holes per row should, if possible,
be increased. Typically, this resulted in blastswith three rows of 13 holes and included oneblast with seven rows of 10 holes. Thisincrease in blast size did not have an adverseimpact on either fragmentation or post-blast face conditions.
Phase 4: Remove sub-drilland all packaged explosiveAs a final stage in the optimization process,it was decided to remove both the sub-drilland packaged explosives from the design.This change reduced costs even furtherwithout causing any noticeable reduction inblast performance or problems with controlof the quarry floor.
Consistent PerformanceThe quarry continues to use the optimizedblast design in conjunction with electronicdetonators and reports that good, consistentblast performance continues.
Relative costsAn idea of the relative costs for each phaseof the trial is shown in table 1. It should benoted that these costs do not include anyadjustment for reductions in rig movements
due to increasing blast size.
ConclusionThe simple trials carried out at Thrislingtonhave shown that, with good collaborationbetween all three interested parties, it has beenpossible to add value to the rock-breakingprocess by the application of electronicdetonator technology.The trials at Thrislington have resulted in
reductions in the cost of both blasting andcrushing. Equally as important, it is clearthat the use of electronic detonators hasimproved the condition of the post-blast faces,resulting in enhanced safety for site operations.The increase in blast size has also improvedsafety by reducing the total area of new face pertonne of rock blasted.Experience tends to suggest that the
situation at Thrislington is unusual in that costshave been reduced for both the blastingoperation and crushing – a synergy which hasonly been realized as a result of the effectivecollaboration between all parties. However,other trials have shown that while blastingcosts may go up with the application ofelectronic detonators, the overall cost of therock-breaking process reduces. QM
