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Chapter5Comminution破碎磨矿5.0Introduction导论Becausemostmineralsarefinelydisseminatedandintimatelyassociatedwiththegangue,theymustbeinitially“unlocked”or“liberated”beforeseparationcanbeundertaken.Thisisachievedbycomminution,inwhichtheparticlesizeoftheoreisprogressivelyreduceduntilthecleanparticlesofmineralcanbeseparatedbysuchmethodsasareavailable.Comminutioninitsearlieststagesiscarriedoutinordertomakethefreshlyexcavatedmaterialeasiertohandlebyscrapers,conveyors,andorecarriers,andinthecaseofquarryproducts,toproducematerialofcontrolledparticlesize.Explosivesareusedinminingtoremoveoresfromtheirnaturalbeds,andblastingcanberegardedasthefirststageincomminution.Comminutioninthemineralprocessingplant,or“mill”,takesplaceasasequenceofcrushingandgrindingprocesses.Crushingreducestheparticlesizeofrun-of-mineoretosuchalevelthatgrindingcanbecarriedoutuntilthemineralandganguearesubstantiallyproducedasseparateparticles.Crushingisaccomplishedbycompressionoftheoreagainstrigidsurfaces,orbyimpactagainstsurfacesinarigidlyconstrainedmotionpath.Thisiscontrastedwithgrindingwhichisaccomplishedbyabrasionandimpactoftheorebythefreemotionofunconnectedmediasuchasrods,balls,orpebbles.Crushingisusuallyadryprocess,andisperformedinseveralstages,withsmallreductionratiosrangingfromthreetosixineachstage.Thereductionratioofacrushingstagecanbedefinedastheratioofmaximumparticlesizeenteringtomaximumparticlesizeleavingthecrusher,althoughotherdefinitionsaresometimesused.Tumblingmillswitheithersteelrodsorballs,orsizedoreasthegrindingmedia,areusedinthelaststagesofcomminution.Grindingisusuallyperformed“wet”toprovideaslurryfeedtotheconcentrationprocess,althoughdrygrindinghaslimitedapplications.Thereisanoverlappingsizeareawhereitispossibletocrushorgrindtheore.IthasbeenconcludedbySteanethatitischeapertogrind,andthecrushingshouldbecarriednofurtherthanisnecessarytoprovidesuitablefeedforthegrindingmills.Disseminate：嵌布unlocked：解锁Liberated：解离scraper：铲运机carrier：运输机械quarry：采石场,石矿naturalbeds：矿床thefirststage：第一段asequenceofcrushingandgrindingprocesses：一系列破碎磨矿作业substantially：大部分compression：受压Impact：冲击arigidlyconstrainedmotionpath：刚性约束的运动路径abrasion：磨蚀,剥蚀；磨剥unconnectedmedia：分散的介质rods,balls,orpebbles：钢棒，钢球或砾石adryprocess：干式作业reductionratios：破碎比Crusher:破碎机Crushing：破碎Grinding：磨矿Tumblingmills：泻落式磨矿机theconcentrationprocess：选矿作业drygrinding：干磨thegrindingmills：磨矿机因为大部分矿物都是细粒嵌布，并于脉石紧密共生，要将这些矿物分离必须先“解锁”，或者用专业术语就是“解离”。解离通过破磨来完成，在破磨过程中，矿石的粒度逐渐减小，直至可以用现行的选矿方法获得纯净的矿物颗粒。矿石最初的几次破碎目的是使新采出矿石易于用铲车、皮带及运输机械处理；如果是采石场，破碎的目的则是要控制石料的大小。炸药用于从矿床上剥离矿石，爆破可以视为破磨的第一段。在选矿厂，破磨是一系列破碎和磨矿过程。破碎将矿石粒度减小到适合给入磨机进行磨矿，[磨矿]直至目的矿物和脉石大部分以分离的颗粒形式存在。5.1principlesofComminution破磨原理Mostmineralsarecrystallinematerialsinwhichtheatomsareregularlyarrangedinthree-dimensionalarrays.Theconfigurationofatomsisdeterminedbythesizeandtypesofphysicalandchemicalbondsholdingthemtogether.Inthecrystallinelatticeofminerals,theseinter-atomicbondsareeffectiveonlyoversmalldistances,andcanbebrokenifextendedbyatensilestress.Suchstressesmaybegeneratedbytensileorcompressiveloading.Evenwhenrocksareuniformlyloaded,theinternalstressesarenotevenlydistributed,astherockconsistsofavarietyofmineralsdispersedasgrainsofvarioussizes.Thedistributionofstressdependsuponthemechanicalpropertiesoftheindividualminerals,butmoreimportantly,uponthepresenceofcracksorflawsinthematrix,whichactassitesforstressconcentration.Ithasbeenshownthattheincreaseinstressatsuchasiteisproportionaltothesquarerootofthecracklengthperpendiculartothestressdirection.Thereforethereisacriticalvalueforthecracklengthatanyparticularlevelofstressatwhichtheincreasedstresslevelatthecracktipissufficienttobreaktheatomicbondatthatpoint.Suchruptureofthebondwillincreasethecracklength,thusincreasingthestressconcentrationandcausingarapidpropagationofthecrackthroughthematrix,thuscausingfracture.Althoughthetheoriesofcomminutionassumethatthematerialisbrittle,crystalscan,infact,storeenergywithoutbreaking,andreleasethisenergywhenthestressisremoved.Suchbehaviourisknownaselastic.Whenfracturedoesoccur,someofthestoredenergyistransformedintofreesurfaceenergy,whichisthepotentialenergyofatomsatthenewlyproducedsurfaces.Duetothisincreaseinsurfaceenergy,newlyformedsurfacesareoftenmorechemicallyactive,andaremoreamenabletotheactionofflotationreagents,etc.,aswellasoxidisingmorereadily.Griffithshowedthatmaterialsfailbycrackpropagationwhenthisisenergeticallyfeasible,i.e.whentheenergyreleasedbyrelaxingthestrainenergyisgreaterthantheenergyofthenewsurfaceproduced.Brittlematerialsrelievethestrainenergymainlybycrackpropagation,whereas“tough”materialscanrelaxstrainenergywithoutcrackpropagation,bythemechanismofplasticflow,wheretheatomsormoleculesslideovereachotherandenergyisconsumedindistortingtheshapeofthematerial.Crackpropagationcanalsobeinhibitedbyencounterswithothercracksorbymeetingcrystalboundaries.Fine-grainedrocks,suchastaconites,arethereforeusuallytougherthancoarse-grainedrocks.Realparticlesareirregularlyshaped,andloadingisnotuniform,butisachievedthroughpoints,orsmallareas,ofcontact.Breakageisachievedmainlybycrushing,impact,a