Stripping Ratio

9. Open pit mining

9.1 Introduction

9.1.1 Basicdescriptionofopenpitmining

Openpitminingisappliedtotheextractionofnearsurfacedeposits.Overburdenremoval(stripping)andminingarecarriedoutsystematicallyfromaseriesofbenches(steps)asthepitisprogressivelydeepened.Thebenchlayoutisdesignedtoproduceanoverallslopeanglethatiscompatiblewithslopestabilitysothatanopenpitresemblesaninvertedcone.Asthebaseofthepitisdeepened,theupperbenchesarepushedoutsoastomaintaintherequiredslopeangle.Overburdenisstrippedfrombenchestouncoverthedepositandtransportedtoadumpatsomepointremotefromtheoperationitself.Asthedepthincreasestheratioofoverburdentovolumeoforeextractedsteadilyincreasesand,atacertainpoint,thecostofoverburdenremovalmakestheoperationuneconomic.Theremainderofthedepositmightthenbeworkedbyundergroundmining.

Advantagesofsurfaceminingcomparedwithundergroundmining. HigherProductivitydueto:greaterdegreeofmechanisation,largerequipmentcanbeused

economiesofscale,fewerpersonnelrequired Loweroperatingcostspertonnedueto:higherproductivity,concentrationofproduction,

lessconstraintonproductionlevel(easiermaterialshandling) Lowergradedepositscanbemined Reduceddevelopmenttime(generally).Thereforemorefavourablecashflowandquicker

repaymentofcapitalinvestment. Greatergeologicalcertainty. Saferoperations

Disadvantagesofsurfacemines: Largeproportionofwastetoore. Highlevelofenvironmentalimpact. Affectedbyclimaticconditions. Depthlimit

Fig.9.1:Palaboraopenpitcoppermine,SouthAfrica

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9.1.2 PitLimits

Thepitlimitsaretheverticalandlateralextenttowhichtheopenpitminingmaybeeconomicallyconducted.Establishmentofthepitlimitsisthefirststageinmineplanning(Laurich1990).Theydeterminethe: amountofeconomicallyrecoverableore, metalcontent, volumeofwastetobeexcavatedandmoved, locationofwastedumps,tailingslagoons,processingplant,accessroadsandallothersurface

facilities.

9.2 Stripping ratio

9.2.1 Definitions

Thepitlimitsandsequenceofminingaredeterminedultimatelybyeconomics.Theconceptofstrippingratio(SR)isthemethodofanalysisused.Itisameasureoftheamountofwastethatmustberemovedinordertomineoneunitofore.

Grade Thecontentofvaluablemetal(%,g/torkg/t)inamineral.Cutoffgrade Thegradeatwhichthevalueofthemetalequalsthecostofminingand

processingthemineral.Ore MineralthatisabovethecutoffgradeWaste Mineralthatisbelowthecutoffgrade

TheSRatanylevelofthepitisdefinedas:

SR is alsosometimes expressedas tonnes/tonne (tonnageof ore removedper tonneof ore).However,thefirstdefinitionismoreconvenientasthecostsofwasteremovalaredirectlyrelatedtoitsvolumeandtherevenuefromtheoreisdirectlyrelatedtoitsmass.

ThePitLimitisdefinedbytheeconomicstrippingratioSRecon.Thisvalueisthestrippingratioatwhichthecostsofminingandprocessingtheoreandstrippingthewasteareequaltotherevenuefromtheore.

9.2.2 Simplestrippingratiocalculation

Todeterminethepitlimitforasimpleorebody(Figure9.2) CalculatetheeconomicSR: Multiplybyoredensitytoconverttom3/m3(multiplyingtoplineofSRequationtoobtainnet

valueoforein$/Bm3

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=

tm

recovered ore of tonnesremoved wasteof volume SR

3

)($/Bmcost stripping waste($/t)cost processing ore - ($/t)cost mining ore - ($/t)ore from revenue /t)(Bm SR 3

3econ =

EstimateontheorebodysectionwherethepitlimitmaylieanddrawinlineABCattherequiredslopeangle

TheSRatthispoint=AB/BCm3/m3

ComparewitheconomicSR;iflower,movelinedownto,sayA1B1C1;ifhigher,movebackuptoA2B2C2

ContinueuntilSRrepresentedbylineonsectionequalseconomicSR

Inpractice,theprocedureismuchmorecomplicated;forexample,theoregradewillusuallyvarythroughouttheorebody.AsimplesolutionistoexpresstheeconomicSRasafunctionofgradethendrawastraight line graphof economic SRvsgrade. For anygradevalue intheorebody,thecorrespondingeconomicSRcanbereadoffthegraph.

Fig.9.2:Simplepitlimitestimation

9.3 Overburden stripping strategies

Describedbelowarethefourbasictypesofstrippingschedule.Thefirsttwoareextremecasesandwouldnotbeappliedinpractice.(BucyrusErie1979,Fourie&Dohm,1992)

9.3.1 DecliningStrippingRatioMethod(Figure9.3)

Aseachbenchoforeismined,allthewasteonthatbenchisremovedtothepitlimit.

Advantages: goodoperatingspace goodaccessibilitytooreonnextbench, allequipmentworkingonsamelevel, nocontaminationfromwasteblastingabovetheore, equipmentrequirementsaminimumtowardsthedepletionoftheorebody. operatingcoststendtobeconstantinlateryearsastheincreasedminingcostwithdepthis

offsetbythedecreasedstrippingratio.

Disadvantage:overalloperatingcostsaremaximumduringtheinitialyearswhenmaximumprofitsarerequiredtohandleinterestchargesandrepaytheprojectcapitalinvestment.

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Fig.9.3:Reducingstrippingratiomethod

9.3.2 IncreasingStrippingRatioMethod(Figure9.4)

Onlysufficientstrippingrequiredtouncovertheoreiscarriedout.Thismethodallowsformaximumprofitintheinitialyearsofoperationandgreatlyreducestheinvestmentriskinwasteremovalfororetobeminedatalaterdate.Itmaybeappliedwheretheeconomicsoftheoperationandcutoffstrippingratioisliabletochangeonveryshortnotice.Themaindisadvantageistheimpracticabilityofoperatingalargenumberofstackednarrowbenchessimultaneouslytomeetregularproductionrequirements.

Fig.9.4:Increasingstrippingratiomethod

9.3.3 ConstantStrippingRatioMethod(Figure9.5)

Wasteisremovedatarateapproximatelyequaltotheoverallstrippingratio.Themethodisacompromisethatremovestheextremeconditionsoftheformertwomethodsdescribed.Equipmentfleetsizeandlabourrequirementsarerelativelyconstant.

Fig.9.5:Constantstrippingratiomethod59530725.doc 11 8/3/2011

9.3.4 PhasedMiningSequence(Figure9.6)

Inpractice,theoptimumstrippingsequenceforalargedepositwouldfeaturealowstrippingratiointheinitialandfinalyearsofoperation.Thisplanhasthefollowingadvantages.

Ahighlevelofprofitcanbegeneratedattheoutsettoimprovethecashflow. Thelabourandequipmentfleetcanbebuiltuptomaximumsizeoveraperiodoftime.This

approachisalsoadvantageousfromacashflowpointofview. Labourandequipmentrequirementsdecreasegraduallytowardstheendoftheminelife. Distinctminingandstrippingareascanbeoperatedsimultaneously,allowingforflexibilityin

planning. Thenumberofminingandstrippingfacesrequiredisnottoohigh. Inalargeorebody,theminingandstrippingareasaresufficientlywidetocreategood

operationalconditions.

Fig.9.6:Phasedminingsequence

9.4 Bench design

9.4.1 Benchheight

Benchheightisthemostimportantparameterasitlargelydeterminestheotherdimensions.Valuesrangefromabout2.5mforsmallgoldminesto20mforlargeopenpits.Thefinalbenchheightmaybesubdividedforextractionpurposesintoanumberofsubbenchesorflitches.Benchheightisinfluencedby:

1. excavatingequipmentdimensions(reach,operatingheight)2. sizeandgeometryoforebodysmallbenchesusedfornarrowlodesorlensesinorderto

minimisedilutionandfacilitategoodgradecontrol.

Benchheightisnolongerlimitedbydrillingdepth.Theprimedeterminingparameteristhemaximumdiggingheightdimensionofashovel.Table9.1givessomeadvantagesanddisadvantagesofmaximizingthebenchheight.

Table9.1:FeaturesofhighwidebenchesAdvantages DisadvantagesHighproductivityandefficiencyCanuselargescaleequipmentLargerblastsFewerequipmentmovesandsetupsFacilitatesmoreeffectivesupervision

LessselectivityMoredilutionFewerworkingplaces,thereforelessflexibilityFlatterworkingslopes(largershovels)

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Generally,itismoreadvantageous,intermsofdrillingandshovelefficiency,todesignbenchesashighaspossible.

a) DrillingEfficiency

Agreaterbenchheightreducessetuptimepermeterdrilled.Also,foragivenblastdesign,thesubgradedrillingrequiredisindependentofthebenchheight.Thismeansthatthegreaterthebenchheight,thegreaterthetonnageyieldpermeterdrilledorperkgofexplosiveused.Considerbenchheightsof10mand12m,eachtobedrilledona5mx5mpatternwitha1msubgrade.Therespectivedrillingyieldsare:

Drilling Y ield = Burden x S pacing x B ench Heigh t x Densit yHole Depth

Assumingrockdensity=2.5tonnes/m3,fora10mbench:

Drilling Y ield = 5 x 5 x 10 x 2.510 + 1

tonnes / m= 56 8.

Fora12mbench:

Drilling Y ield = 5 x 5 x 12 x 2.512 + 1

tonnes / m= 57 7.

Theyieldforthe12mbenchrepresentsanincreaseindrillingyieldofsome1.6%.Althoughseeminglysmall,for10,000mofdrilling,itwouldresultinanextra9000tonnesproduction.Similarly,drillingcostspertonnearereducedasbenchheightisincreased

b) ShovelEfficiency

Increasedbenchheightalsoimprovesoverallproductivityofshovels,FELs,orexcavators.Thenumberofrowsinablastingpatternisgenerallygovernedbytheholediameterandexplosivetype.Iftheseparametersarefixedforagivenoperation,thetotalvolumeofbenchthatcanbeblastedatoncedependsonthebenchheight.Thegreaterthevolumeofbrokenground,thelowerthenumberoftimesashovelhastobemovedinorderforblastingoperationstobecarriedout.

9.4.2 Benchwidth

Figure9.7ashowsthecommonterminologyforopenpitslopes.Abenchisahorizontalledgefromwhichdrilling,blasting,excavationandloadingoforeorwasteiscarriedout.AWorkingbenchisonethatisintheprocessofbeingmined.Thewidthextractedfromtheworkingbenchiscalledthecut.Theworkingbenchwidthisdeterminedbythedimensionsoftrucksandtherequiredreachofexcavatingequipment.Figure9.7billustratesaslopeprofilecuttingacrossanoperatingbench.Itshowsanarrowbenchwidthofonlysome3m,notsufficientlywidetoaccommodateequipment.However,eachbenchissystematicallyminedfromoneend,givingadequateroomfordrillingrigs,shovelsandtrucks.

Afterthecuthasbeenremovedabenchofwidthtypically2.53.0mislefttocatchandcollectmaterial,whichslidesdownfromupperbenches.Normally,thebenchslopeangleis7580oandabermeverysecondorthirdbenchissufficient.

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9.4.3 Benchangle

Benchfacesarenormallyminedassteeplyaspossible.Thesteeperthebenchangle,thesmallerthestrippingratio.Safeanglesaredeterminedby:

geotechnicalconsiderations,takingintoaccountthecohesiveandfrictionalpropertiesoftherockandthecharacter,spacingandorientationofjointsandbeddingplanes.

thedipoftheorebody.

Therearetwoangleswhichdefineabenchdesign:

Overallslopeangle Theangleconsistentwithslopestabilityoverthefullheight/depthofthemine.Usuallyliesbetween45oand60o.Theoverallangleisafunctionofthebenchfaceangleandthebenchwidth.Notethatahaulroadonapitslopewillflattentheoverallslopeangle.

Benchfaceangle Themaximumangleconsistentwithstabilityofasinglebench(say,5to10minheight).Typicalvaluesliebetween60oand80o.

Theoverallslopeangleislessthanthebenchfaceanglebecausethelargertheslope,themoreplanesofweaknessithas.

Fig.9.7a:Pitslopecrosssection,withtypicaldimensions Fig.9.7bGeometryofworkingbench(Atkinson1992)

9.5 Method of working benches

Operationsoneachbenchareconductedincycles;typically:

1. Gradecontrolmarkoutorezoneswithtapeorsurveystaffs2. Drillblastholes3. Chargeholes4. Fireholes5. Excavateblastedmaterialandloadintotrucksforhaulageoutofpit6. Cleanbenchandpreparefordrilling

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9.5.1 Miningdirection

Orebodylensesmaybeexcavatedineitheratransverseorlongtudinaldirection.Atransverseminingdirection(digginginadirectionnormaltotheorevein)ismoresuitableforthinlenses.Itallowsbettergradecontrolandlessdilution.Withathickerlens,itmaybepossiblewithlongitudinalextractiontoblastandloadtheore,leavingthewastetemporarilyinsitu.Thenumberofworkingfacesisdeterminedbytherequiredproductionrateandequipmentcapacities.Figure9.8illustrateshowexcavationcantakeplacesimultaneouslyandonmultiplelevels.

Fig.9.8:Miningonmultiplebenches(Hustrulid&Kuchta1995)

9.5.2 Selectionofexcavatingequipment

Inamine,certainproductionrequirementshavetobesatisfiedandinacivilconstructionprojecttheoperationwillhavetocomplywiththeprojectschedule.Asequipmentisveryexpensiveintermsofcapitalorcontractandoperatingcosts,itsutilisationshouldbemaximisedinordertominimisetheunitcostsofearthmoving.

Table9.2:GuidelinesforselectionofexcavatingequipmentTypeofmachine ApplicationElectricropeshovelHydraulicexcavator(frontendloader)

Largebenches

Backhoesitsontopofbench,diggingdown Smallbenchesonly(

otherwisestated.Figure9.9showsthetwoalternativemethodsofexcavatingabenchandofspottingtrucks.

a)Parallelcut b)Frontcut

Fig.9.9:Methodsofexcavatingbenches(Hustrulid&Kuchta1995)

9.6 Haul road layout

Theformofhaulroadsmaybespiralorswitchback(zigzag).Theymayalsobeeithertemporaryorpermanent,dependingontheconfigurationoftheorebody.Wherebenchesarebeingsystematicallyworkedallroundthepitasitisdeepened,haulroadswillbeminedthroughandnewonesformedasthepitdevelops.Often,however,itispossibletoconstructpermanenthaulroadsatonesideofthepit.Thiswouldbethecaseforadippingorebody,wherethepermanenthaulroadcouldbelocatedatthefootwallandextendedasthepitdeepened(figure9.10).Notethattheinclusionofahaulroadinapitwallwilllowertheoverallslopeangleandhenceincreasethestrippingratio.Wheretheorebodydipsatashalloweranglethanthestablepitslope,constructingthehaulroadasaswitchbackonthefootwallwilltaketheoverallpitslopeclosetotheorebodydip.Thehangingwallslope,formedwithoutahaulroad,canbemadeassteepaspossible,consistentwithslopestability.

Fig.9.10:Illustrationofhowpermanenthaulroadscanbeestablishedinafootwall

Factorsdeterminingselectionoflayoutincludethefollowing.(Atkinson1992)1. Theswitchbacklayoutallowsapermanenthaulroadtobelocatedatonesideofthepit.2. Inlargepits,aspirallayoutcanresultinahaulagedistancethatistoogreat.3. Areaswherepotentialslopestabilityhazardsexistshouldbeavoided,possiblyeliminatingthe

spiraloption.

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4. Thepitwallsmaybetoosteeptoallowsuitablebendstobeformedforaswitchbacklayoutwithoutgreatlyincreasingthestrippingratio.

5. Tightbendsassociatedwithaswitchbackmaybedetrimentaltotruckandtyrelife.9.7 Haul road Construction

9.7.1 Roadbase

Goodhaulroaddesignandconstructionpromoteslowerhaulagecostsandimprovedsafety.Roadsareconstructedwiththreeorfourlayers(figure9.10):

1. Subgrade2. Subbase(optional)3. Base4. Wearingsurface

Thesubgradeisthefoundationlayer,usuallycomprisingcompactedrockorsoil.Itmustbestrongenoughtobeartheloadsassociatedwithvehicles,whicharetransmittedfromtheroadsurface.

Asubbasemayormaynotbepresent,dependingonlocalconditions.Itisusedwherethereisveryweaksubgradematerialorinareassubjecttoseverefrost.Itisgenerallyconstructedfromaclean,granularmaterial.

Fig.9.11:Haulroadconstruction(Hustrulid&Kuchta1995)

Thebaseisalayerofveryhighstabilityanddensity.Itsmainpurposeistodistributetheloadfromvehicletyres.Italsoservestoinsulatethesubgradefromfrostpenetrationandprotecttheupperwearingsurfacefromanyswellingorsofteningofthesubgrade.

Thetoproadlayeristhewearingsurface,whichshouldprovidetraction,reducerollingresistance,andresistabrasion,ravelingandshear.Itisformedusuallyofcrushedrock.

9.7.2 Straightsections

Thecrosssectionofanopenpithaulroadfeaturesaoneortwowaytravellane,asafetybermandadrainageditch(figure9.11).Fordeterminationoflanewidth,anumberofrulesofthumbcanbeapplied,inwhichthewidestvehiclesdeterminetheroadwidth.Threeoftheserulesare:

Theclearanceoneachsideofatruckshouldbeequaltoabouthalfthetruckwidth. For2waytraffic,thelanewidthshouldbegraterthanorequalto4xthetruckwidth.

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Forastraight,evengrade,onelanehaulroad,theminimumroadwidthis2timestruckwidth;fortwolanes,3.5timestruckwidth.Theroadcrosssectionshouldalsoberaisedorcrownedslightly,tofacilitatewaterrunoff.Theheightofthecrownisexpressedinmmpermeterofroadwidth.Afigureof45mm/mistypical.

Fig.9.12:Typical2wayroadsection(Hustrulid&Kuchta1995)

RoadGradeisdeterminedfromthetruckperformancechartswithrespecttospeedandbraking.Gradientsof4.5to6o(8to10%)areusuallyadopted,withreardumptrucksbeingthepreferredhaulageunit.A12%grademaybeusedfortrolleyassisttrucks.(Atkinson1992)

9.7.3 Curves

Forsharpcurves,additionalwidthmustbeincluded,bothonthecurveandthetangenttothecurve,tocoverthefrontandrearoverhangsofthevehicleandthedifficultyofnegotiatingthecurve.Arecommendedadditionalallowanceforareardumptruckona6mradiusis125%anda45mradius118%.(Atkinson1992.)Table9.3givesminimumturningradiiforarangeoftrucks,classifiedaccordingtogrossweight.Theseradiicanthenbeusedintable9.4tofindtherecommendeddesignwidthforsingleanddoublelanecurvesofaparticularminimumradius.

Table9.3:Minimumtruckturningradius(adaptedfromHustrulid&Kuchta1995)Vehicleweightclass Grossvehicleweight(tonnes) Minimumturningradius(m)

1 181 11.9

Table9.4:Designwidthsforcurvesrigidbodytrucks(adaptedfromHustrulid&Kuchta1995)Radiusoninneredgeofroad(m)

SinglelaneroadTruckcategory

DoublelaneroadTruckcategory

1 2 3 4 1 2 3 4Minimum 8.8 10.4 13.7 21.3 15.5 18.3 24.1 37.57.6 8.2 10.4 13.4 20.7 14.6 18.3 23.2 36.315.2 7.6 9.4 12.5 19.2 13.4 16.5 21.9 33.530.5 7.3 8.8 11.9 18.0 12.8 15.5 21.0 31.445.7 7.3 8.8 11.9 17.7 12.5 15.2 20.7 30.861.0 7.0 8.8 11.6 17.4 12.2 14.6 19.8 29.959530725.doc 18 8/3/2011

Tangent 7.0 8.5 11.3 17.1 12.2 14.6 19.8 29.9

Dependingonvehiclespeedsandbendradius,acurvemayalsohavetobebanked(superelevation).Typicalsuperelevationsforminehaulroadsandtrucksarearound40mmpermeterofroadwidth(Hustrulid&Kuchta1995).Thedistancerequiredtomakethetransitionfromthenormalcrossslopesectiontothesuperelevatedsectionandbackagain(superelevationrunout)alsoneedstobeconsidered.

9.8 Equipment

ThefollowingdiagramsillustratesomemodernequipmentcurrentlyoperatinginAustralianmines.

Fig.9.13:Hitachi20m3

hydraulicshovelloadingintoKomatsu240tonnetruck

Fig.9.14:TheKOMATSUDEMAGH655Sistheworldslargestprovenhydraulicshovelatover685tgrossweightand35m3bucket.

Fig.9.15:TheKOMATSUHAULPAK930EisthelargesttruckinminingtodayandwasthefirsttouseACdrive.Over100930Esareoperatingworldwide,providingproductionupto320tpercycle.

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9.9 References

AtkinsonT.Designandlayoutofhaulroads.SMEMiningEngineeringHandbook,Vol2,Chapt13.4,pp13341342.SME(1992).

BucyrusErieCompany.MinePlanning.SurfaceMiningSupervisoryTrainingProgramme,Chapt3.BucyrusErieCo.(1979).

FourieGA,DohmGC.Openpitplanninganddesign.SMEMiningEngineeringHandbook,Vol2,Chapt13.1,pp12741297.SME(1992).

HartmanHL.IntroductoryMiningEngineering.Wiley(1987).

HustrulidW,KuchtaM,(1995),OpenPitMinePlanningandDesign,AABalkema,Rotterdam.

LaurichR.Ultimatepitdefinition.SurfaceMining,2ndEdition,pp465469.SME(1990).

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9.7.1Road base9.7.2 Straight sections9.7.3Curves