选煤厂介耗分析论文.docx

选煤厂介耗分析论文.docx

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选煤厂介耗分析论文

专业英文翻译

中国矿业大学

毕业设计英语翻译

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矿物加工工程　

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选煤厂介耗分析

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英文原文

AnanalysisofmediumlossesincoalwashingplantsAbstract

Amajoroperatingcostindense-mediumseparationisinreplacementoflostmediumsolids.Thelossofmediumsolids,beingcostly,playsacrucialroleindeterminingtheeconomicsofanypreparationoperation.Coalwasheriesthatemploydense-mediumcyclonesoftenattemptoptimizationoftheprocessesbyvaryingthevortexorthespigotdiameterandthefeedrelativedensity.Whilethesechangeshelpinclosercontroloftheseparationprocess,theyalsoresultinmediumlossesduetochangesinthemediumsplitratio（ratioofthemediumflowrateinoverflowtounderflow）.Sincemediumsolidsarelostbyadhesiontoproductsandasmagneticseparatoreffluent,theeffectofthechangeinmediumsplitratioonthedrain-and-rinsescreensand,hence,themagneticseparatorcircuitneedstobestudied.InTataSteel'scoalwasheries,atJharkhandIndia,whichemploysprimaryandsecondarydense-mediumcyclonesinseriestoproducecleancoal,middlingsandrejects,reducingtherelativedensityoffeedmedium,hadaninsignificanteffectonthemediumsplitratio.Ontheotherhand,changingtheconeratio（ratiooftheoverflowdiametertotheunderflowdiameter）changedtherelativedensityandtheflowratesthroughthecycloneoutlets,thusaffectingtheperformanceofthemagnetiterecoverycircuit.

Asystematicstudythroughlaboratorytestsandadetailedplantsamplingcampaignhelpedinidentifyingthecausesofmagnetiteloss.Uponimplementationoftherecommendations,themagnetitelossesdecreased,resultinginasavingofapproximatelyUS$27,500perannum.Thestudyalsohelpedinevolvingsomecheckpointsforplantoperatorsforidentifyingmagnetitelosses.

Keywords:

dense-mediumcyclone;magnetitelosses;drain-and-rinsescreens;magneticseparators

1.Introduction

Dense-medium（magnetiteinthecaseofcoal）,aslurry/suspensionhavingarelativedensityintermediatetothatofvaluablemineralandgangue,isgenerallyusedasthemediumofseparationinmostcoalpreparationplants.Themedium,beingcostly,playsacrucialroleindeterminingtheeconomicsofanypreparationoperation.Dardis（1987）quotesfiguresof20–40%ofdense-mediumplantoperatingcostbeingattributabletomediumlossforplantsengagedinmineralseparationsemployingferrosiliconasthemedium.Thefigureis10–20%inthecaseofmagnetite.Thereareclearlyincentivestoreducethisloss,anditisoftenpossibletodosowithminimalcapitalexpenditurethroughimprovedoperatingproceduresandminorchangestoplantconfiguration.ThispaperdrawsontheexperiencefromonesuchstudycarriedoutbyR&DTataSteel,Jamshedpur,Jharkhand,Indiatoidentifysomeimportantoperatingissuesinmediumlossincoalwashingplantsandthefactorsinfluencingtheloss.

2.Causesofmediumlossindense-mediumplants

Therearenormallyonlytwopossibleroutesbywhichmediumcanbelostfromtheplant:

•adheredtotheproductsofseparation,afterdrainingandwashingonscreens;and

•presentinthefinaleffluentfromthemediumregenerationprocess,usuallymagneticseparators,settlingconesorothersolid–liquidseparationdevices.

Thecausesoflossfromthesesourcesareasfollows:

•forcesofattractionbetweentheoreandmediumparticles,oreporosity,andinefficientwashing;

•magneticseparationandclassificationinefficiencies;

•corrosionandabrasionofthemedium,reportedforferro-siliconmedium;

•excessivecircuitloadingsduringtheadditionoffreshmedium;

•housekeeping（whenthefloorsarebeingcleanedandwashedoff）;

•plantdowntime（associatedwithhousekeeping）;

•mediumproperties（size,shape,magneticsusceptibility）.

Therehasbeenmuchworkdoneovertheyears,usuallybyoperatingplants,toidentifyandquantifythesourcesofmediumlossandtominimizeconsumption.Thetask,however,iscomplicatedbythedifficultyofdetermininganunequivocalmediumbalanceacrosstheplantbysamplingprocessstreams.Itisrarethatabalancethusestablished,forarelativelyshortoperatingduration,reflectsquantitativelytheactualconsumptionrecordedbytheplantovernormalreportingperiodssuchasamonthorayear.

2.1.Factorsaffectinglossesthroughdrain-and-rinsescreens

Napier-Munnetal.（1995）,duringtheirinvestigationsoftheironorewashingplantsatMountNewmanandTomPrice,foundthatadhesionlossincreaseswithscreenloading.Theeffectwasquitestrong,andevenmoderatelyloadedscreensshowedasignificantincreaseinloss（expresseding/t/mofscreenwidth）overlightlyloadedscreens.

Anincreaseinoperatingrelativedensityalsoledtosignificantincreasesinlosses.Mostoftheincreaseinlosswasattributedtothepoordrainagecharacteristicsofthehigherviscositymedium（Kitteletal.,1987）.Asmallincreaseinrelativedensityledtoalargeincreaseinviscosityandthuspoorerdrainagecharacteristics.

Thewashingarrangementwasalsofoundtoaffectmediumlossessignificantlythroughdrain-and-rinsescreens.Ofthevariouswashingarrangements,screenswithweirsandavigoroustumblingactionreducedthemagnetitelossesconsiderablycomparedtoslottedspraybarsandscreenswithfloodboxes.

2.2.Lossesthroughthemagneticseparators

Thereisnoconsensusintheliteratureastothecontributionwhichmagneticseparatorlossesmaketototalmediumlossindense-mediumplants.Dardis（1987）,forexample,claimsthatmagneticseparatorsaccountformorethan75%oflosses,whereasMulder（1985）attributesonly18%tothissourcefortheSishenironoredense-mediumcyclones.Kitteletal.（1987）reportedmagneticseparatorlossesbetween2.4%and24%ofthetotalfortheMt.Newmandense-mediumcycloneplant.However,onoccasions,whenveryhighviscositymediawereused,substantialelevationoftheadhesionlosseswasobserved.

Adhesiontocoalandthelossesinthemagneticseparatorarethetwomainroutesthroughwhichmagnetitegetslostinacoalwashingplant.Ingeneral,magneticseparatorsseemtocontribute20–40%ofthisloss,thoughthisproportionwillfallwhereadhesionlossesareabnormallyhigh,forexample,withporousores.Magneticseparatorsarethereforeanimportant,though,notnecessarily,adominantsourceofmediumloss.Sincetheirperformancecandeterioratemarkedlyifnotoperatedcorrectlyorproperlymaintained,theydeservecloseattention.

AnalysisoflossesinmagneticseparatorscollectedinplantsurveysbyRayner（1994）suggeststhatthiscouldbeduetotheseparatorbeingoverloaded,intermsofeitheritsvolumetriccapacityor,lessoften,itsdrysolidscapacity.Hawker（1971）andSealyandHowell（1977）gaveloadinglimitsintermsofdrysolidsfeedrateofmagneticsandvolumetricflowrateoffeedslurry,whichcouldnotbeexceededwithoutlossofperformance.

Dardis（1987）confirmedthattheoperatingvariables,whichaffectmagneticseparatorperformance,includepulpheight,magnetposition（angle）,separationanddischargezonegaps,drumspeed,andmagneticstonon-magneticsratio.Lantto（1977）,writingfromtheperspectiveofahardrockilmeniteconcentrator,explainedthattherecoveryinamagneticseparatorwasfeedqualitydependent.Healsogaverecommendationsforvariousseparatorparameters.

BasedonoperatingexperienceattheIscorironoremines,DeVilliers（1983）observedthatoverloadingofthemagneticseparatorswasthemaincauseofmagneticlosses.HealsogavetheseparatorsettingsusedattheIscorplants.

3.InvestigationsatTataSteel'scoalwasheries

TataSteelatJamshedpur,Jharkhand,Indiaownscaptivecoalwasheries,whichsupply60%ofcokingcoalrequirementsforitsintegratedsteelmakingoperations.Inthewasheries,theROMcoalafterbeingcrushedandscreenedat0.5 mm,the+ 0.5 mmfractionistreatedindense-mediumcyclones（calledthecoarsecircuit）andthe− 0.5 mminaflotationcircuit（calledthefinescircuit）.The+ 0.5 mmcoalisfedtotheprimarycyclones,whichproducecleancoalatalowerrelativedensityofseparation（1.3–1.5）.Theunderflowfromtheprimarycyclonesformthefeedtothesecondarycycloneswhichinturnproducemiddlingsandrejectsatahigherrelativedensityofseparation（1.6–1.9）.ThemagnetiterecoverycircuitisatypicalcircuitthatexistsinanycoalwashingplantandisshowninFig.1.

Fig.1. Schematicmediumrecoverycircuitandthesamplingpoints.

Thedense-mediumandcleancoal（middlingsorrejects,asthecasemaybe）islaunderedtosievebendsandonesetofdrain-and-rinsescreens.Thesievebendsandthefirstsectionofeachdrain-and-rinsescreenareusedtodrainmediumfromthecoal;themediumiscollectedinscreenunder-pansandreturnedtotheprimarycyclonesumpviatheprimarycyclonemediumdistributionbox.Thesecondsectionofthescreensisusedtorinseanddrainthecoalfreeofadheringmedium.Thespraywatercontainingthedense-mediumrinsedfromthecoaliscollectedinthesecondsectionofthescreenunder-pansandreturnedtothedilutemediumsumpforsubsequentmagnetiterecovery.ThelevelofmagnetitewaterslurryinthedilutemediumsumpcanbeadjustedusingthePID（Proportional,Integral,Derivative3-termcontroller）loopprovidedforlevelcontrolandthemodulatingsplitteractuator.Whentheslurrylevelsinthesumprises,thesplitteractuatorwoulddiverttheflowawayfromthesystemtomaintainbalance.Theindicationloopalsogenerateshighandlowalarmlevelswithinthecontrolsystem.

Thedilutemediumthuscollectedinthedilutemediumsumpispumped