Stripping Ratio
-
Upload
mirko-cuaquira -
Category
Documents
-
view
178 -
download
21
description
Transcript of 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
59530725.doc 8 8/3/2011
-
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$/Bm3
59530725.doc 9 8/3/2011
=
tm
recovered ore of tonnesremoved wasteof volume SR
3
)($/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.
59530725.doc 10 8/3/2011
-
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)
59530725.doc 12 8/3/2011
-
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.
59530725.doc 13 8/3/2011
-
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
59530725.doc 14 8/3/2011
-
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.
59530725.doc 16 8/3/2011
-
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.
59530725.doc 17 8/3/2011
-
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.
59530725.doc 19 8/3/2011
-
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).
59530725.doc 20 8/3/2011
9.7.1Road base9.7.2 Straight sections9.7.3Curves