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Final report

The team analyzed gold tailings from Barrick Gold's Goldstrike mine using three concentration techniques: magnetic separation, froth flotation, and gravity separation. Magnetic separation initially seemed promising as the gold grade was improved tenfold, but subsequent calculations showed the drying costs were too high. Froth flotation tests were unreliable. Gravity separation resulted in a lower gold grade. However, Barrick provided information on a wet magnetic separation test with promising economic results, including a net present value of $390 million and internal rate of return of 33%.

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Barrick Goldstrike Team Advisor: Dr. Jon Kellar Team Members: Jordan Dick, Justin Brick, Jack Blaisdell, Devin Rowe Date: March 18, 2015
1 | P a g e Table of Contents Executive Summary ..............................................................................................2 Introduction ........................................................................................................3 Theory. ...............................................................................................................3 Results and Discussion ..........................................................................................9 Drying and sampling........................................................................................................... 9 Base line analysis process................................................................................................ 10 Results............................................................................................................................... 10 Flowsheet and implications............................................................................................. 11 Environmental Context .......................................................................................12 Global and Societal Context.................................................................................13 Conclusion.........................................................................................................13 Future Work ......................................................................................................14 References ........................................................................................................15 Appendix...........................................................................................................18 A........................................................................................................................................ 18 B........................................................................................................................................ 22 C........................................................................................................................................ 25 D....................................................................................................................................... 28 E........................................................................................................................................ 30 F........................................................................................................................................ 33 G....................................................................................................................................... 35 H....................................................................................................................................... 41 Acknowledgements ............................................................................................44
2 | P a g e Executive Summary: Barrick Gold,the largestgoldminingcompanyinthe world,hastaskedthisdesignteamto analyze a reserve of tailingstheyhave atone of theiroldestminestosee if the tailingscanbe profitably concentrated, andif theycan, designaprocessto do so.Barrick believes thatwiththe currentprice of goldand withadvancementsin mineral processing,the tailingsare now aviable source of gold. To determine the amountof goldinthe tailings,the teamhasconducted three concentrationtechniques. Magneticseparation,frothflotation,andgravityseparationwere all usedtoconcentrate the gold. Inthe frothflotationconcentrationmethod,fourtestswere performed.The firsttestperformedshowedthe presence of gold,butthe resultscouldnotbe repeated.Giventhe unreliabilityof thistest,the team decidedthatfrothflotationwouldnotbe aviable optionforconcentratingthe goldinthe tailings.The scope of the projectwasshiftedtowardsmagneticseparationandgravityseparation. The resultsof gravityseparation were notbetteras a fire assaydeterminedthatthe grade of the concentrate wasless than the initial grade of the tailingsmakingitaninaffective method. Magneticseparationtestinitially seemedencouragingasthe grade of the tailingwasimprovedtenfold,puttingitabove the minimum grade value that Barrickis willingtoprocess. Subsequenteconomiccalculationsshowedthattoomuch moneywouldbe spentdryingthe tailingpriortoprocessing,meaningthatmagneticseparationasthe teamperformeditwasuneconomical. Howeverourcontactat Barrack Goldprovidedinformationona wetmagneticseparationtestthathe performedandthe economiccalculationsforthisversionof magneticseparationprovedpromisingasthe NPV andIRR were foundto be about$390 millionand33% respectively.
3 | P a g e Introduction: It isa commonmisconception thatwhenamineral processingcompany isdone withanore bodythat all of the valuable materialiscompletelyextracted. While itispossible toprocessthe ore until nearlyall of the valuable material istaken,thisisalmostnevereconomicallyjustified.Itistypicallythe case that initiallyalarge portionof the desiredmaterial canbe easilyextractedfromthe ore but, as lessmaterial isavailable,the more costlyitbecomestoextractthe remainder,thusmakingituneconomical to continue processingthe ore. However,astime movesforwardanddemandschange,the price forsuch material mayincrease tothe pointthat itis economical tocontinue processingthe depletedore body for the remainingmaterial. Thisis whatBarrick gold wishestofindoutas theyhave 170 milliontonsof tailings fromtheirGoldstrike mine.Barrick believeswiththe risinggoldprice thatthe tailingsmaynow be a viable ore source.The team’sobjective istoanalyze samplesfromthe tailingsto measure the grade of the tailingsandthento designflowsheetsthatwill maximizethe netpresentvalueandinitialrate of return. Thiswill determine if it iseconomicallyfeasible forthe tailingsto be reprocessedforthe remaininggold. Barrickhas requestedthatourgroupto be finishedcollectingandanalyzingdatabyFebruaryof 2015. Theory: The tailingsof anymineral processing operation will retain some amountof desiredmaterial. However if it the value of the material increasesornewertechnologyallowsforeasierrecovery,thenfurther processingmaybe justified. There are manywaysto concentrate ore bodiesandall of themtake advantage of the ore’sphysical andchemical characteristics,thisisgenerallyknownasbeneficiation.For thisdesignproject,the groupwill focusonconcentratinggoldbyemployingthe mostavailable and commerciallyusedmethods,gravityseparation,frothflotation,andmagneticseparation. Gravity Separation The firstprocessusedto beneficiate the goldtailingsthatwill be considered,isgravityordensity separation.Gravityseparationisanenvironmentallyfriendlyprocess,whichutilizessimple equipment withfewmovingparts.(Abols) Goldpanningisasimple formof gravityseparationthathasbeenused for centuries. Throughoutthe historyof mineral processing,manydifferenttypesof gravityseparation deviceshave beenutilized,butthe one the teamwill use isa wave table. Gravityconcentration processesrelyonthe principle thatgoldcontained withinanore bodyishigherinspecificgravitythan the gangue. Elemental goldhasa specificgravityof 19.3, andtypical ore has a specificgravityof about 2.6. (Fast) Movementiscreatedbygravityconcentrationdevices betweenthe goldandhostrock particlessothat the heavyparticlesare separatedfromthe lighterparticles. Onawave table,particles will be fedontothe machine alongwithastreamof water.The bedof the table will begintoshake resultingintwoevents,the formationof awave inthe waterandthe gradual forwardmovementsof the particlestowardsatrough at the endof the table.While the particlesare incontactwiththe surface of the table,the shakingwill cause them tomove forward. Whena wave passesoverthem, lessdense
4 | P a g e particleswill be movedbackwardswhiledensermaterial are eithernotmoved,ormove verylittle resultinginaseparationof the particlesbytheirrespectivedensities. See Figure 1. Figure 1: Wave Table wave velocity profile. Yellow is gold and black is gangue To determine if gravityseparationisafeasibleprocessacommontechnique istocalculate the concentrationcriterion.As shown inEquation1the concentrationcriterioniscalculatedbythe specific gravityof the heavymineral subtracted bythe specificgravityof the fluiddividedbythe specificgravity of the lightermineral subtractedbythe specificgravityof the fluid.Assumingagangue withadensityof 2650 kg/m3 goldhas a concentrationcriterionof 10.3 and hematite hasa criterionof 2.5. The other majorfactor of determiningif gravityseparationispossible isdeterminingthe particle size of the mineral thatisdesired.Below inFigure 2showsconcentrationcriterionversusthe sizeof mineral particlesinmicronsshowingwhere separationispossible andimpossible (Guapta).Table 1alsobacks up the data showedinFigure 2.
5 | P a g e Figure 2: Size limit curve for gravity separation Table 1: Concentration criterion guide for gravity separation Ideallygoldandgoldbearingmaterial will move intothe troughof the table butthe teamwill likely collectall of the material closesttothe top fortesting.One disadvantage of gravityseparationisthat ultrafine clayparticlesare noteasilyrejected (Honaker).Anotherdifficultywillbe the change togolds specificgravitybyironoxide sludgeof maghemiteattachedtothe goldore.Finallythe small particlesize of the gangue createsanotherobstacle todeal withif the teamdecidedtouse gravityseparationasa concentrationmethod. Froth Flotation Froth flotationislargelyconsideredtobe one of the mostwidelyusedmethodsinmineral processing. The processwas inventedbyFrankElmore in1896. Elmore had justpurchasedthe Glasdircoppermine whichhad a notorioushistoryof havingdifficultore toworkwith. The mine hadbeensoldseveral times inits lifetime. Elmore experimentedwithusingoil asa collector.The processwasa huge success.The mine hasable to collectnearly70%of the ore’scoppercontent.To give a basicdescription,froth flotationessentiallyutilizesamineral’saffinityforwaterinorderto create a separation.Due tothe process’ssimplicityandhistorical success,the Barrickdesignteamwill be evaluatingfrothflotationasa
6 | P a g e possible concentrationtechniqueforthe goldstrikemine’stailings. Inorderto give the designgroup and anyother readersa betterunderstandingof the process,the followingsectionswill coverhowthe processworks,andhow itis commonlyperformed. How does it work?-Asstatedabove,frothflotationexploitsthe factthatsome mineralsare hydrophilic (likeswater) andsome are hydrophobic(hateswater). Thisisthe basisof creatinga goodseparation. The ore ismixedintoa slurryandthenintroducedintoaflotationcell where theyare agitated. Airis pumpedintothe bottomof the cell inthe form of small bubbledtypicallyof aboutone totwo millimeters.If amineral particle ishydrophobic,itwill attachonto a bubble andfloattothe surface of the slurrywhere itcan be skimmedoff. Those particlesthatare hydrophilicwill notattachto air bubblesandwill remaininthe slurry. What makesdifferentmineralparticleshydrophobicornotcomesdownto the surface chemistryof the particle. The surface of any particle can be splitintotwolayers:the sternlayerandthe diffuse layer. These twolayersare oftencalledthe electrical double layer.The sternlayerisasmall layerdirectlyon the particle’ssurface thattendstoaccumulate ionsof the opposite charge of the surface.Forinstance a particle withanegative surface charge will accumulate positivelychargedionsinthe sternlayer.The diffuse layerisa muchlargerlayeroutside the sternlayerthatcontainsa mixture of positive and negative ionsandessentiallyzeroesthe total charge tothe surroundings(Figure 3).Twoimportantvalue inflotationisa mineral’szetapotential.The zetapotential isthe charge justoutside the sternlayer. Differentsolutionshave differentpHvaluesandthereforedifferentamountsof positive (H+ ) and negative (OH- ) ions.VaryingpHscan change the zetapotential of a particle.The pHvalue where the zeta potential of aparticle isequal tozero iscalledthe point-of-zero-charge(PZC) (Figure 4).Bymakingthe zetapotential negativeorpositive,one canthenaddchemicalsthatwill adhere tothe surface of select particlesandrenderthemhydrophobicallowingthemtofloat. Many differentchemicalsmaybe addedtoa mineral slurrytochange how theyreact withwater.Those that change the hydrophobicityof aparticle are knownas collectorsorsurfactants.Collectorscanbe eitheranionicorcationicdependinguponwhatthey adhere to. Anioniccollectorswill adhere to positivelychargedparticles,while cationiccollectorswilladhere tonegativelychargedparticles.Inthis way,differentcollectorscanbe usedtotarget particleswithdifferentcharges.Chemicalscanalso be usedto alterthe pH of a solutioninordertoisolate amineral forflotation.Forinstance,if one wanted to floatironoxide (PZC6.7) from silica(PZC2) withan anioniccollector,the pHcouldbe loweredtofive to renderhematite positivelychargedandsilicanegativelycharged. The anioniccollectorwill then attach to the iron oxide,makingithydrophobic,causingittofloatwhile leavingsilicabehind.Additional chemicalscalledfrothersmayalsobe usedinordertocreate a more resilientfrothlayer.
How is flotationperformed-Before flotationcanoccur,ore typicallyhastobe reducedto fine particles usinganynumberof comminutiontechniques.Typicallyflotationisperformedwithparticlesof around 0.1 mm butcan sometimesrequireevensmallerparticle sizes. The groundore isthenmixedwithwater to forma slurrybefore the additionof pHalteringchemicalsandsurfactants. The slurryisthen introducedintoaflotationcell where itisaeratedandallowedtofloat. The frothcan thenbe removed as concentrate.Those particlesleftbehindmaybe pumpedoutof the cell as tailings.One advantage of frothflotationisthatit iseasilyscalable fromsmall laboratorycellstolarge industrialcellsthatmaybe linkedtogethertoobtainthe mostout of the process. The Barrick designteamperformedsome basicflotationtestsonthe ore receivedfromthe Goldstrike mine.Underthe suggestionthatgoldwastrappedwithironoxide fromthe group’sadvisoratBarrick,it was decidedthatanattemptto floatthe ironoxide wouldbe made.Frommineralogyreportsperformed on the tailings,the groupwasable todetermine thatalarge portionwas carbonaceousmaterial.This suggestedthata goodseparationmightbe possible.The grouppreparedfourrepresentative samples fromthe tailing,andutilizedoleicacid(anioniccollector)aswell asanunidentifiedfrothertotest differentflotationcombinations.Fourtrialswouldbe run:anatural floatwithnoadditives,asecond trial withan addedfrother,the thirdwoulduse three drops of oleicacid,the lasttrial usedbotholeic acid anda frother.Waterand the tailingssample werefirstintroducedintothe flotationcelland allowedtoconditionwithaironforone minute.Whatwas collectedwasreturnedtothe cell. Thisinitial step’spurpose istoproperlymix andcreate a uniformslurry.The cell wasthenturnedbackon and allowedtorunfor three minutes.Concentratesweredriedandthenscorificationassayswere runto determine goldgrade. Figure 3: Diagram of a particle showing the electrical double layer Figure 4: Graph of zeta potential versus solution pH. PZC equals 9.5
8 | P a g e MagneticSeparation Magneticseparationisa popularandlow-costmethodof recoveringmetals. Magneticseparationis usedprimarilybyminingindustries,butisalsowidelyusedinthe recyclingindustry.Magnetic separationusesamagneticfieldtoseparate magneticmaterialsfromnon-magneticones. Explanation- Whenmineralsare placedina magneticfield,thereare three reactionsthancanoccur. Attractionto the magneticfield,repulsionfromthe magneticfield,ornoreactionat all. The particles that are attractedto the magneticfieldare calledmagnetic.These particlesare subdividedintotwo classifications;stronglymagneticparticlescalledferromagneticandweakermagneticparticlescalled paramagneticparticles.Ferromagneticparticlesmaybe easilyseparatedwithaseparatorhavinga low intensitymagnetic.However,paramagneticparticlesrequire ahigherintensitymagneticfieldto separate them. Particlesthatare repulsedbyamagneticfieldare calleddiamagnetic.EddyCurrentSeparators,take advantage of the diamagneticparticles,separatingthemfromothermaterial.EddyCurrentSeparators are largelyusedinthe recyclingindustry.Here,wire andmetalsmade fromcopperandaluminumare separatedfromplastics.Whenaproduct,such as aluminum, passesoverthe eddy current,amagnetic fieldformsaroundthe piece of aluminum.The polarityof the magneticfieldof the aluminumisthe same as the rotatingmagnets,causingthe aluminumtobe repelledawayfromthe separator.Plastic, glass,or otherprocessmaterialssimplyfalloff the endof the separator.Aneddycurrentisdefinedas the currentscausedby voltagesinducedbychangingflux,andtendtooppose the change of the flux. Non-magneticparticles,suchasgold,quartz,andpyrite,are not susceptible to magneticseparation,but some magneticmaterial maybe removedfromthe feed.Forinstance,inafew situations,plantsusing gravityconcentrationforrecoveringgold,usedmagneticseparatorstoremove the highconcentration of magnetite thatwasrecoveredwiththe gold,priortofurtherprocessing. Magneticseparationisgenerallyalowcostmethodof recovery,unlesshighintensityseparatorsare required.The electro-magnetichighintensityseparatorsthatproduce 20,000 gauss,tendto be expensive. However,the rare earthmagneticseparatorsare relativelyinexpensive andcanproduce magneticfieldsaround6,000 gauss.So,whenlookingfora processto recovervaluable minerals, magneticseparationshouldnotbe overlooked,if some of the material is magneticorpara-magnetic. (Kinsack) Whenconsideringusingmagneticseparationprocessitisimportanttoevaluate amaterial’smagnetic susceptibility.Magneticsusceptibilityisavalue giventoamaterial basedonhow magneticitis per volume andis measuredbyweighingasample andthenturningonan electromagnettosee how the weightisaltered.A material likepure goldhasa negative magneticsusceptibilitymakingitdiamagnetic so usingmagneticseparationmethodonitwouldnotyieldanyresults. The process-There are differentsortsof magneticseparators,butthe mostcommonis the drum separator.(See figure 5) Essentially,astreamof material isdroppedontothe beltof a magneticdrum.
9 | P a g e Non-magneticorweaklymagneticparticlesbounce orfall off of the drum.The remainingmagnetic particlessticktoit until theyare scrapedoff. The speedof the beltas well of the intensityof the magneticfieldwill bothhave aneffectonhow muchof the magneticmaterial isattractedtothe belt. (Osbourne) Figure 5: Drum Separator In additiontothe drummethod,manyothertypesof separatorsexist.These include,butare not limited to, pulleys,discs,beltsandrings.Magneticseparatorscanbe usedeither"wet"or"dry"and come in countlessshapes andsizes. Magneticseparation for our project-Withthe majorityof the goldbeingtrappedinsome formof iron oxide,magneticseparationwill mostlikely,prove tobe veryuseful. A BarrickGoldrepresentative informedthe teamthatthe entire tailing’smasswasonlycomposedof about3-5% ironoxides,sothe resultsfromthe team’sattemptneededtobe nearthisrange.Ironoxide hasa veryhighmagnetic susceptibilitymakingitparamagnetic.If the goldisimbeddedinthe ironoxidemagneticseparation shouldbe a viable processingoption. The resultsof the magneticseparationperformedbythe teamdidnotmatch that of the data from Barrick. Barrickwas able to performthe magneticseparationusingawetmethod.Giventhe resources available atSouthDakotaSchool of MinesandTechnology,the teamwasunable torun a wetmagnetic separationtest. While there are manywaysof beneficiatingthe gold,these threeprocessesare most readilyavailable. Each methodhas itsownadvantagesanddisadvantages.Usingacombinationof these complimentary processescanimprove the overall recoveryof goldfromthe tailings.Eachmethodworksbestincertain situationsandundercertainconditions.Withthe rightknowledge of the tailingsmineralogyand chemical composition,the recoverycanbe maximized.
10 | P a g e Results andDiscussion: Drying and sampling- The sample arrived inaslurryform so,before itcould be processed,itmustbe dried. The sample wasgivenafewdaysto sit,thus allowingthe particlestosettle tothe bottomof the shippingbarrel. Withthe particlessettled,the toplayerof waterwasremovedusingabucketuntil the movementof the watercausedbythe bucketbeganto disturbthe particles. The particleswere then allowedtosettle again. Once settledawet-dry-shop-vacuumwasthenusedtogentlyremove the rest of the surface water. As the sample slurrywasbeingpumpedintothe 55 gallondrumitarrivedin,it can be assumedthatthe slurrywas thoroughlymixedfromthe violentmotionsimpartedonitduringthe time itspentmoving throughthe pipingsystemandthe initial agitationlikelyusedtoloosenupthe tailingsduringthe acquisitionfromthe tailingdam. Therefore,if the slurrywasthoroughlymixedinthe drumitcan be furtherassumedthatthe settlingof the particle inthe waterallowedforarelativelyuniform dispersement,because whileindividual particlesprocesseddifferentsettlingvelocitiesthe initial uniformdispersementinthe fluidshouldsufficientlyoffsetanysettlingtrends. Inthisregard, a sample takenby a coringmethod shouldbe representative of the entire tailingssample. Sothe teamuseda PVCpipe tocore and remove arepresentative sample. Thiscore wasthenplacedinan ovento dry. Once the core was dried,the particlesfusedintolarge agglomerates,necessitating breakingthe agglomerates usingamortarand pestle. Afterthiswascompleted,analysisof the tailingsbegan usinga riffleralongwiththe cone andquarteringtechniquetoseparate representativesamplesforvarious tests. Baseline analysisprocess- A baseline analysis of the tailings todetermine suchthingsasmineralogy and particle size distributionwas conductedsince characteristicssuchasthese will influence how a concentrationmethodwillperform. A sample was processedand/oranalyzedineachof the following ways:  Usedto performa fire assay(see Appendix B) bya memberof the team. Thiswill give aresult regardingthe tailingscurrentgoldgrade thatis representative.  Digestedinaquaregiaor anotheracidsolutiontobringthe goldpresentintosolution. Use AAS to determine the concentrationof goldinthe solutiontothenbackcalculate the grade of the ore. This methodmaynot be as accurate as the fire assaybut can be performed quicklyfor multiple samples,thusthismethodwill be done frequently.  Sentto EngineeringandMiningExperimentStationpersonnelfor SEManalysis.Thisalongwith othermethodswill helptodeterminethe mineralogyof the tailings.  Takento EngineeringandMiningExperimentStationpersonneltohave X-raydiffraction(XRD) performed. Thisisa greattechnique fordeterminingmineralogy. Results- Asof now the teststhat have beenperformedinclude SEMandXRD analysis,fire assay, AAS, magneticseparation andpreliminaryfrothflotation.
11 | P a g e The SEM gave a roughelemental compositionof the sample.Forthe SEManalysis,agrab sample from the top of the sample wasusedinsteadof arepresentative sample.The highpeaksof the graphshow the presence of silicates inthe sample.(SeeAppendix A andD) The XRD showedmanyof the minerals withinthe sample andwasperformedafterdifferentprocesses. ResultsfromXRDdidnot change much evenaftermagneticseparation,XRDshowedthe presenceof quartz, gypsum,dolomite,andcalcite (see Appendix A) butnotmuchelse.The expectationwasthat aftermagneticseparation, ironoxidewouldbe found,butthatwasnot the case. The team later discoveredthatthe ironoxide wasmaghemite,anoxidationstage betweenhematite andmagnetite, and itwas not detectedbecause the XRDequipmentwasnotcalibratedtodetectit. Table 2: Grade and recovery results Fire Assayand AASwere performed before anyconcentration toreveal the initial average value of the goldwithinthe sample. Bothresultswereverysimilarwiththe average valueforfire assaybeing 0.038 t Oz/tonand the average value forAASbeing 0.035 t Oz/ton.These valuesare abouthalf of Barrick’s requirementof aconcentrate grade of 0.08 t Oz/ton. The firstconcentration methodcompleted wasmagneticseparation. The groupwasinformed byBarrick personnel thatthe majorityof the goldleftinthe tailing istrappedinmaghemite.Furthermore, maghemite comprisedabout3-5wt% of the tailing.The team’smagneticseparationreflected this expectation withamagneticconcentration of justunder7 wt% of the tailingssample. ResultsfromAAS showedthatthe magneticconcentratesfromthe separationhad a grade of 0.17 t Oz/ton. Thisvalue is nearlytwice Barrick’s minimumrequiredgrade. Later,a fire assayshowedthatthe actual grade was around0.48 t Oz/ton. Flotationsof the tailingswere performed on500 gram samplesinanattemptto determine how much goldcouldbe floatedoutof the naturallyhydrophobic, carbonaceous material presentinthe tailings by floatingwithnoadditives,oleicacid,afrotherand botha frotherand oleicacid. All floatswhere performedata pH around 8, withabout2.6 litersof waterand anyadditiveswhere addedat0.268 lbs/ton. The frothercollectedthe most witharound16.4wt%collectedwhile oleicacidcollectedthe leastwitharound5.18wt% collected. Howevernone of the flotationsmanagedtoconcentrate any significantamountof gold. The resultsof the fire assaysshowedthatonlythe flotationwithnoadditive had collectedanygoldbutthe goldthat wascollectedwasbelow the lowerlimitof the scale (limitof scale is.00002 grams). Meaningthat the highestpossible grade wouldhave been0.018 t Oz/ton,this beinglessthanthe initial grade of the tailings. Initial Magnetic concentrates Unrepresentative floatation (natural) Unrepresentative flotation(w/oleic acid) Natural flotation Oleic Acid flotation Frother flotation Oleic+frother flotation Gravity FireAssay (t Oz/ton) 0.038 0.48 3.89 LOST >0.018 0 0 0 >0.018 Recovery (%) NA 88 NA NA NA 0 0 0 NA
12 | P a g e The last concentrationmethodwasgravityseparationperformedonawave table. Beingthatthe particle sizes were sofine,the lowestsettingonthe table was used. 2004.6 grams of tailingswere separatedand 91.2 grams were collectedwhichisaround4.55wt%. Unfortunatelythe resultswere not promisingasagainthe goldfrom the fire assaywas below the lowerlimitof the scale makingitshighest possible grade around0.018 t Oz/ton. Flowsheetandimplications- Thiswas the laststepof the design project.Basedonthe resultsof the concentrationmethodsaflowsheetyieldingthe greatestprofit wasconstructed aroundthe magnetic concentrationmethodasthiswasthe onlymethodtoshow promisingresults. Usingthe estimated valuesforcapital,horse power,costperton of concentrated,andvariousotherfigure eitherprovided by the team’scontact at Barrick Goldalongwithotherfigurescalculatedfromresearch(seeAppendix F for all economicinformation) aflowsheetwasconstructed. However,asitispossible toperformthe concentrationeitherwetordrya flowsheetforbothmethodsmustbe done andshownbetweenthe twowhichis more profitable bycalculatingbothflowsheet’sNPV andIRR. Flowsheetsforbothwetand dry processingisshowninfigure 6below while the NPV andIRRforbothare shownintable 3. Figure 6: Flowsheets for process. Top is wet and bottom is dry. Table 3: NPV and IRR NPV IRR Wet process $390 million 33.1% Dry process -$166 billion -5.27x1021 % Tailing dam Dredger Gold recovered AutoclaveMagnetic separator Acid leach New tailings dam Tailings Dredger Gold recovered AutoclaveMagnetic separator Acid leach Tailings New tailings dam Tailing dam Fluidbed dryer
13 | P a g e Environmental Context: One majorhurdle forthe designteamonce a concentrationmethodhasbeendecideduponwill be how to fitthe processingplantintoBarrick’sexistingstructuresatthe Goldstrike site.Ideally,the concentrationsite wouldbe asadjacentaspossible tothe damwhere the tailingsare currentlylocated. Thispart of the designprojectwill cause the teamtoevaluate geographical limitationsof the site fora newconcentrationfacility,andisbeyondthe scope of thisproject.Fromwhatthe designteamhas decidedonfora flowsheet (Figure 6) thoughthe environmentaleffectshouldbe minimal.Justlooking at the wetflowsheetthe onlyone newfacilitywill have tobe constructedtoaccommodate the new process. Anothermajorenvironmental issue wouldbe the effectof runoff from thisnew concentrationfacility. The chemicalsinthe tailingsrightnoware alreadya hazard to the environmentandadditional dangerouschemicals maybe needed toconcentrate the tailings.Thispotentiallycouldcreate arunoff that couldbe exceedinglyharmfultothe environmental.Topreventanyenvironmentdamage the team will have todetermine if the tailingsfromthe concentrationfacilitywill be able tomix withthe tailings Barrick will alreadyhave comingoff theirmainprocess.If notthe teamwill have todetermine awayfor Barrick to safelydisposeof thisrunoff withoutanydamage tothe environment. Whenconsideringthe three concentrationmethodsthe designteamtestedtheyall have theirown environmental effects.Firstfrothflotationprobablyhasthe worstenvironmental effectsof the three. Froth flotationprocessoftenusessome type of chemical tocollectcertainelementsorcause frothing. Froth flotationalsohasahighwater usage. Gravityseparationandmagneticseparationare muchmore kindto the environment.Neitherrequire anychemical additionstothe elementswhichmakesthe disposal of the tailingsverysimple.Inthe processthe designteamdecidedonusingawetmagnetic separatorisfairlyenvironmental friendly.The processusesnoharmful chemicalstohelpconcentrate the gold,and the concentrate will notrequire anyotherprocessesthatBarrickisnotalreadyconducting. The major environmental take awayisthatthe processdesignedbythe teamisnotcreatingany environmental threats. Global and Societal Context: On a global scale thisdesignprojectcouldhave huge implications.The mostnotable one couldbe the change of goldprices.The price of gold, isdetermined bysupplyanddemand.If thisdesignprojectis successful thenthere will be animmediateboosttothe supplyof goldwhichmaycause the price of gold to drop. Anotherglobal implicationthisdesignprojectmayhave or influence isBarrickandothermine companiescould mimicthe processingstrategydevelopedhereinandimplicate it attheirolderminesto extractgoldfrom theirtailings.
14 | P a g e There are some societal effectstobe considered if thisdesignprojectissuccessful.Mineshave afinite amountof ore,and when shutdown, canhave amajor economicblow onthe local community.If this designprojectissuccessful, thenitcouldincrease the lifeof the Goldstrikemine,whichmeansthata great employerforthe communitywill staylongercreatingmore jobs. Conclusion: Barrick initiallyapproachedthe designteamwiththe taskof analyzingthe economicfeasibilityof the 170 milliontonsof tailingsfromtheirGoldstrike mine,locatedinnorth-eastNevada.Barrickprovideda sizable representativesample of approximately200 poundsof solidsina slurry,shippedina55 gallon drum. Arsenic,mercuryandcyanide were listedonthe shipping label asbeinginthe sample prompting a meetingwith Ms. JerilynRobertsof campusEnvironmental HealthandSafetytodiscussthe safe handlingof the sample. The teamsetthe goal of havinginitial labresultscompletedbythe endof the semesterwithconcentrationteststofollow inthe spring. A smallerrepresentative sample wasacquiredbyfirstdecantingthe sample andthentakingacore using a PVCpipe fromthe middle of the drum. This sample wasconedandquarteredtocreate smaller samplesforvariousothertasks,these includedAAS, SEM,XRDand fire assays. The goal of these tasks was to learnpropertiesof the tailings suchasmineralogyand goldore grade. Mineralogywaslargelydeterminedthroughthe use of XRD whichindicatedhighpercentagesof silicates as well ascalciumand magnesiumcarbonatesinthe formof dolomite inadditionto trace amountsof othercompounds. Determinationof currentgoldore grade wassuccessful,utilizingbothAASandfire assaymethods. AASresultscame backwithan ore grade of 0.036 t Oz/ton. Thisissupportedbythe fire assayresultsof 0.038 t Oz/tonmeaningof the 170milliontonsof tailings about6.46 milliontroyounces of goldispresent,thishavinga presentestimatedvalue of $8.4billion. However,inorderforthe tailingstobe profitable toBarrick,the ore grade needstobe nearly doubled. Magneticconcentrationproved tobe the most efficientmethodof beneficiatingthe tailingsincreasing the grade to 0.48 t Oz/tonwithan 88% recovery. Howeverperformingthe processdryisnot economically feasibleasmoneyspentdryingthe tailingsexceedanyprofitmade.Butwithexperimental detailsprovidedbythe team’scontactat Barrick gold,itis possible toturna profitbymagnetically separatingthe tailingwet,thoughthislowers the recoveryfrom88% to 26%, for thisprocess the calculatedNPV andIRR forthisprocesswere about$390 millionand33% respectfully.
15 | P a g e Future Work: The overall projectistechnicallyfinishedbutwiththe limitedtime the teamhad,some of the testing was notdone as thoroughlyastheycouldhave been. The magneticseparationthatthe teamperformed was done drybut performingitwetwasalsoa desiredtaskasthe team’scontactat barrack goldhad done thisveryidea. The flotationsperformedshowedthatverylittlegoldwastrappedincarbonaceous material butflotationscouldhave beendone toisolate the maghemite thatthe goldwastrappedinand as thismethodiscommonlyusedtoconcentrate large amountsof low grade ores,like copperores,this couldhave provide verypromisingresults. If more time wasgiventhese tooideaswouldhave been exploredmore completely.
16 | P a g e References: Abols,J. A. P.M Grady, and P.M Grady. "MaximizingGravity Recovery through the Application of MultipleGravity Devices." (n.d.): n. pag. Gekkos.com. Web. "About Us." Barrick Gold Corporation.N.p., n.d. Web. 05 Dec. 2014. "Arsenic and Gold Mine Tailings FAQ." Arsenic in Historic Gold MineTailings(n.d.):n. pag. Arsenic and Gold Mine Tailings FAQ.Nova Scotia.Web. 5 Dec. 2014. Brennan, Ed. "History of Fire AssayingWhich Determines Precious Metal Content in Ores." History of Fire Assaying Which Determines Precious Metal Content in Ores. Assay One. Web. 21 Nov. 2014. <http://www.assayone.com/fire-assaying.html>. Bugbee, Edward Everett. A Textbook of Fire Assaying,. 3d ed. New York: J. Wiley & Sons;, 1940. Print. "Chemical SamplingInformation | Cyanide (as CN)." Chemical Sampling Information | Cyanide(as CN).N.p., n.d. Web. 05 Dec. 2014. "Cyanide Poisoning:Get Facts About Effects and Treatment."EMedicineHealth. N.p., n.d. Web. 05 Dec. 2014. "Environmental Health and Medicine Education." Arsenic Toxicity Case Study: WhatAre the Standards and Regulation for Arsenic Exposure?N.p., n.d. Web. 05 Dec. 2014. Fastand Associates LLC. "Gold MiningProcess Development." Denver Mineral Engineers. N.p., n.d. Web. 14 Dec. 2014. Gupta, A. Yan, D. S.. (2006). Mineral ProcessingDesign and Operation - An Introduction.Elsevier.3 March 2015 Han, Kenneth, and Yuhong Chen. "UsingFlotation to Separate Carbon Material fromCarlin Ore."(n.d.): n . pag. OneMine.org. Web. Honaker, R.Q. "A Fine Coal Circuitry Study UsingColumn Flotation and Gravity Separation." SciTech (1995): n. pag. Osti.gov. Web. Kellar,Jon. Met/EnvE 220 Mineral Processingand ResourceRecovery text part 15. Web. 9 Dec. 2014. <http://www.hpcnet.org/upload/attachments/425768_20080331055541.pdf>. "REGULATIONS AND ADVISORIES." 7. REGULATIONS AND ADVISORIES (n.d.): n. pag. Web. 5 Dec. 2014. Abols,J. A. P.M Grady, and P.M Grady. "MaximizingGravity Recovery through the Application of MultipleGravity Devices." (n.d.): n. pag. Gekkos.com. Web. Fastand Associates LLC. "Gold MiningProcess Development." Denver Mineral Engineers. N.p., n.d. Web. 14 Dec. 2014. Han, Kenneth, and Yuhong Chen. "UsingFlotation to Separate Carbon Material fromCarlin Ore."(n.d.): n. pag. OneMine.org. Web. Honaker, R.Q. "A Fine Coal Circuitry Study UsingColumn Flotation and Gravity Separation." SciTech (1995): n. pag. Osti.gov. Web. Richard A. Clarke. "Magnetic properties of materials".Info.ee.surrey.ac.uk. Retrieved January 15th,2015.
17 | P a g e Ramsaywok, P. "Influence of SalineWater on Electrostatic/High Tension Separation." Eighth International Heavy Minerals Conference. 2011.263-269.Print. Bittner, James. "TRIBOELECTRIC BELT SEPARATOR FOR BENEFICIATION OF FINE MINERALS." Metsco Proceedings 2013.Needham: Separation Technologies,LLC, 2013.1-10. Print. Oberrauner, A. "NEW DEVELOPMENTS ON ELECTROSTATIC SEPARATION OF FINES." XXVI International Mineral Processing Congress (IMPC) 2012 Proceedings. NEW DELHI: IMPC, 2012.Print. "Electrostatic Separation."Electrostatic Separation. MINE-ENGINEER.com. Web. 30 Jan. 2015. <http://mine-engineer.com/mining/minproc/elstat1.htm>. Weiss,Norman L. "Section 6: Electrostatic and Magnetic Separation." SME Mineral Processing Handbook. Vol. 1. New York, N.Y.: Society of MiningEngineers of the American Institute of Mining,Metallurgical,and Petroleum Engineers, 1985. 6.2-6.10. Print. Young, Courtney, and Gerald Luttrell. "Separation Technologies for Minerals,Coal,and Earth Resources." Google Books. N.p., n.d. Web. 17 Apr. 2015.
18 | P a g e Appendix A Figure 7: Colored image created using EDS showing areas of similar elemental composition. Figure 8: Separate colors representing different elements filtered out of figure 7.
19 | P a g e Figure 9: Overall spectrum of figure 7 image. Figure 10: Spectrum 7, BSE spectrum of a dolomite particle.
20 | P a g e Figure 11: Spectrum 12, BSE spectrum of an iron oxide particle. Figure 12: Spectrum 10, BSE spectrum of an aluminum silicate particle.
21 | P a g e Figure 13: XRD scan of representative Barrick tailings Table 4: Mineral names and formulas Mineral Name Chemical Formula Mineral Name Chemical Formula Quartz, silica SiO2 Sodium silicate Na2(SiO2)nO Calcite Ca(CO3) Sodium carbonate Na2CO3 Gypsum CaSO4·2H2O Litharge, Lead(ii)Oxide PbO Dolomite CaMg(CO3)2 Borax, Sodium borate Na2B4O7 · 10H2O Hematite Fe2O3 Magnetite Fe3O4 Maghemite γ-Fe2O3
22 | P a g e Appendix B Fire Assays Thoughthe processof Fire Assayingismanycenturiesold,withoriginsthatcanbe tracedas for back to the bible (Jeremiah6:27-30),it haschangedverylittle butisstill heldasthe mostaccurate methodfor determiningthe amountof goldinan ore sample. Infact, thismethodof analysisisheldwithsuchhigh regardthat it isconsideredthe “GoldStandard”of measuringgoldinoresand alloys. Toput itsimply thismethodisoftenusedbyanyone whowishestodetermine the amountof goldina particulargold baringsample. Because the BarrickGoldstrike teamof 2013-14’s entire designprojectrevolvesaround findingouthowmuchgoldis ina particularsample,itisimportantformembersof the teamto learn howto do thisanalysisina timelymanner. Sothe questionsof,“How doesitwork?How is it performed?”,will be answered,withrespecttohow the teamperformedit,inthe followingpassages. How does it work? Fire Assayingcomesinmanyformsbutthe teamdecidedtogowiththe crucible assaymethodasthisis the most commonform. The general theoryof thisprocessstartswiththe understandingthatthe majorityof ores,forthe mostpart, cannotbe fusedbutif the ore’saverage particle size issufficiently reducedandmixedwiththe rightratiosof appropriate reagents,the mixture will fuse ataneasily obtainable temperature.Next,itisassumedthatthe ore particlesare incontact withone or more particlesof litharge andreducingagent. Asthe temperature of the mix increasestoaround500o C, the litharge will begintobe reducedbythe carbon1 inthe mix and these reducedglobulesof leadcollectthe goldand silverformthe surroundingparticlebyalloyingwiththem. At about560o C,the borax of the charge beginstomeltand to formfusible compoundswithsome of the basesof the flux andore charge. Once thisbeginsthe slagbeginstoformrapidly. Forthe processto workproperly,the temperature of the slagshouldremainlow enoughthatitsdensityisgreaterthan lead, 10.66 grams percubic centimeter,andheldatthistemperature longenoughthatore particlesare thoroughlydecomposedandeveryparticleof goldandsilverhasbeencollectedbyanearbysuspended globule of lead. Once thishasoccurredthe temperature canbe raisedto a pointthat the slag’sdensity islessthan thatof lead,allowingthe denserglobulesof leadtosettle atthe bottomforminga single leadmass,calleda button.Itis presumedthatthe buttoncontainsnearlyall of the preciousmetal inthe ore. To separate the preciousmetalsfromthe lead,the buttonmustundergoaprocesscalledcupellation. The processof cupellationisverysimple,afterthe leadbuttonhasbeenbrokenoff fromthe slagandall of the slaghas beenremovedfromthe surface of the button,the buttonisplacedina furnace along witha preheatedcupel. At327o C the leadwill begintomeltandformleadoxides,mostof the lead oxide isthenabsorbedbythe cupel,whilethe restare vaporized.Thisprocesswill leave behindasmall beadmade of silverandgoldasthese twoelementsare notsoeasilyoxidized. The last twostepsbefore ameasurable amountof goldcan be obtaininvolve partingthe silverfromthe goldand annealingthe gold. Topart the silverfromthe gold,nitricacidisthe preferredchoice assilver 2 Carbon can come in many forms, for our project all-purposeflour was the carbon source
23 | P a g e iseasydigestedinthisacidwhile goldremainsstable. Once the silverhasbeenfullydissolvedandthe remaininggoldflake hasbeenwasheddried,the final stepisannealingthe gold.Thisstepdoesnothing more than make the goldeasiertohandle as the formthat it isinbefore annealingisverybrittle. Withthisfinal goldflake the fire assayiscomplete andthe goldcan be weighedtodetermine the equivalentTroyouncespertonbasedon equationtwobelow: Eq(1) Where: Mgold is the massof the goldfoundafterFire Assaying Msample is the massof the ore thatwas usedinthe Fire Assay. How is it performed? Onlyone fire assayhas beenperformedasof thistime sothisprocedure is onlyrelevanttothe measurementof the initial tailings. To performan intelligentfire assaythe ore’smineralogymustbe understoodtoadegree,inthe team’s case it’sthe tailing’smineralogythatmustbe understood. Because the ultimatesuccessof afire assay dependsonthe formationof slag. Thisreferstothe part mentionedearlierthatthe particlesbecome fusible ata relativelylowtemperatureaswell aspropertiesbasedonthe nature of the slag. A desirable slagwill be neutral innature as opposedtoacidicor basic. To accomplishthisgoal itmustbe first determine if the tailingswerecomposedof acidicorbasiccomponents,thenneutralizethemwithan opposingmix. FromX-rayDiffraction(XRD) analysis,itwasfoundthatthe tailingscontainedalarge portionof dolomite,whichisverybasicinnature. Knowingthis,E.Bugbee’s A TextbookforFire Assays was referencedtofindastartingmix that wouldbe appropriate fora basicore. A startingmix would usuallybe justthat,a mix youstart withand adjustuntil the resultsof the pourgave desiredresultslike a large leadbutton. By chance howeverthe initial mix workedquitewell. The mix calledforthe followingora 1 Assayton(30g ore sample): Figure 14: Charge mix for basic ore. Afteraddinga small beadof silvertothischarge,as the collectionof goldwiththe leadisbetterwith silverpresent, the charge wasallowedtoheatto 800o C for 45 minutes. The temperature wasthen raisedto 1050o C for an hour. The moltenassaywasthenpouredintoa cast iron mold,andallowedto solidify. Once cooledthe buttonwas brokenoff withthe use of a hammer,the hammerwasthenused to poundthe leadbuttonintoa cube so as to remove as muchslag as possible. Nextthis cube of lead was placedincupel thatwas sittingina 925o C furnace. 45-50 minuteswaslongenoughforthe leadto be oxidizedandabsorbedbythe cupel. The silverbeadthatwasleftinthe cupel wasthenallowedto cool before beingaddedtoa crucible of nitricacidfor parting. The acid was changedoutperiodically 29167 gold sample M M
24 | P a g e until the liquidnolongerturnedyellowasa resultof the silver. The final flake of goldwasthenrinsed, driedandannealed. The goldflake thatwasrecoveredfromthe original 154 grams hada massof about 0.0002 grams, whichwouldgive the initial tailingsagrade of 0.038 Troy ouncesperton. (See Appendix A forall mineralsandcompoundsmentioned)
25 | P a g e Appendix C Safety Guidelines Abstract In thisdesignprojectthere isaconcernabout the hazardousmaterial intailingsatthe Goldstrike mine. Thisreportwill firstdiscussthe levelsof arsenic,mercury,andcyanide andhow the levelsof these tailingscompare toOSHA’sacceptable levels. Secondly,thisreportwill coverthose concernsof these hazardousmaterial andpossible waysthatsomeonecouldgetthese hazardouschemicalsintotheir system. Introduction Safetyisan importantissue intoday’sworkplace.If youneedproof youneed notlookfartherthana company’swebsite.Everycompanynowadays hasa safetysectionwhere theypreachtheir commitmenttosafetyandhowtheyare takingcare of theiremployee’s.Forexample,here iswhat Barrick itself saidonitswebsite:“Usingbothqualitativeandquantitative methods,risks,suchasthose impactingthe health,safetyandsecurityof ouremployees,ourneighboringcommunitiesandthe environmentinwhichwe operate,aswell asouroperational andfinancialperformance are assessed and appropriate solutionsimplemented(1).”Soforthisdesignprojecttosucceedeffortsneedtobe made to limitanysafetyincidents.Thatiswhythisreporton the potential risksandhow to avoidthose riskswhenworkingwiththe tailingsfromthe Goldstrike mine. Arsenic Arsenicisa notoriouslypoisonousnaturallyoccurringelement.Arsenicusuallyoccursinthe environmentinalloyswithsulfurandmetalsandasa pure elemental crystal.Groundwater contaminationbyarsenicisa majorproblemthat affectsmillionsof peoplearoundthe world(3). In the Goldstrike tailingsarsenichasthe highestconcentrationof the three hazardouschemicalspresent at approximately800ppm.The arsenicwouldnothave beenaddedduringthe mineral processingsoit can be assumedthatthe ore source at Goldstrike hassome arsenicinthe ore.Thismakessense consideringarseniciscommonwithgolddepositcontainingsulfideminerals. OSHA has specificguidelinesonwhatisanacceptable amountof arsenicinthe work place.Inthe general workplace OSHA says,“The permissible exposure limitforarsenicisnogreaterthan 10 microgramsof inorganicarsenicpercubicmeterof air,averagedoverany 8 hour periodfora 40 hour workweek(2).”The sample of tailingscame in a 55 gallondrumwhichisapproximately0.21m3 . Calculatingitoutthere couldpotentiallybe 10.5 microgramsina lab room, but consideringa considerable amountof arsenicwasremovedduringdrainingthe slurry,andthe designteamisnot workingwith the whole tailingsatone time. There are some concernswhenworkingwitharsenicina labsetting.Arsenicisonlydangeroustoa personwhenitisingested.Inmostcasesof poisoningthe arsenichasfounditswayintothe ground
26 | P a g e waterand poisonedthe people whowere drinkingfromthatwatersource (3). Ina labsettingsomeone workingwitharsenicstill hastobe verycareful.Whenworkingwitharsenicitisimportanttowear gloves,longsleeves,pants,longpants,andabreathingmask.The breathingmaskwill stopanydust particlesof arsenicfrombeingingested.If someone doesgetthe slurryontheirskinthe arsenicwill not be a problem,butitisimportantto washthe area thoroughlybecause if the arsenicgetsonyourhands it can laterbe transferredtofoodwhenyouare eating. Mercury Mercury isanothernatural occurring elementlike arsenicthatcan be justas dangerous.Mercuryis used withcyanide ingoldprocessingtoseparate the goldfromthe otherelementsthatare in the ore. Mercury poisoninghappenedoftenbefore people understoodthe dangersof it.Incurrenttimesmost mercurypoisoningoccursmostlybecause of mercuryinfish. Mercury has one of the smallestconcentrationinthe tailingsat4 ppm. Mercury wasusedfor gold processinginthe earlyyearsof mineral processing,buttodayall mercuryintailingswouldcome from the ore.The concentrationisprobablyaslow as itis because Barrickwouldhave tosafelydispose of so much of theirmercuryto avoidrun off of dangerouschemicalsintothe environment. Like arsenicOSHA has standardsforwhat isan acceptable Mercuryis dangeroustoingest,butunlike arsenicitcan be absorbedthroughthe skin.OSHA onlyallowsaconcentrationof 0.1 mg/m3 (4). Consideringthe areathe teamwill be workinginisaround16 m3 there isa potential riskof 0.25 mg/m3 inthe lab.The actual concentrationof Hg inthe air will be lowerthanthat.The level shouldnotbe in the dangerzone,but toavoidany riskof breathing,ingesting,orabsorptionthe teamwill follow the same skinprotectionasfor arsenicanda face mask. Cyanide Cyanide isa compoundthatlike the othertwoelementsdiscussedinthisreportishazardousand extremelydangerousif notproperlyhandled.Like mercury,cyanide isusedformineral processing specificallypreciousmetalslike goldandsilver.Cyanide iscombinedwitharichore source andthe cyanide anionsattackthe ore creatinga pregnantheapleach. Cyanide’sconcentrationcouldnotbe determinedexactly,but the concentrationislessthan20 ppm. While cyanide poisoningcanoccur throughinhalationitwouldtake alarge amountof cyanide tofeel any immediateeffects.Cyanide isanatural occurringchemical intobacco socigarettesliberate cyanide duringsmoking(6).The mainriskfor cyanide isabsorbingtoomuchof the compoundthroughyourskin, or throughinhalationinthe gaseousform. OSHA doesnot have any standardsforan air mediumforcyanide,buttheydohave an allowable limit for skincontact.The allowablelimitforcyanide is5 mg/m3 (5).Assumingthe concentrationof cyanide is 20 ppm ina 0.21 m3 containerthenthe concentrationwouldbe 95.2 mg/m3 .So withthatpossible concentrationglovesandproperclothingatall timesisamust whenhandlingthe tailings.
27 | P a g e Conclusion Withsafetybeingsucha major concernintoday’ssocietyitisimportantto do everythingtoreduce risk inthe workplace.Inthisreport the three hazardouschemicalsinthe tailingswere examinedand explainedhowtheycanaffecta person.The OSHA safetystandardswere consideredalsoandexamined if the team wouldbe at riskof any of these materials.The guidelinesforthose three materialswere laid. The basics of the safetyguidelinesare toweargloves,appropriateclothing,andabreathingface mask.If these guidelinesinplace the designteamcanruntestssafelyworkingwiththe tailingswithoutworryof personal safety.
28 | P a g e Appendix D Mineralogy of Goldstrike Tailings (see Appendix A for relevant figures) Introduction The mineralogical makeup of the tailings body is extremely important information to have for this project. Depending on what makes up the tailings largely effects the specific concentration techniques used and as well as the parameters of these techniques. The tests that we performed included using a scanning electron microscope (SEM), and x-ray diffraction (XRD). The group has samples prepared for use with micro-focused x-ray computed tomography (MXCT) and is expected to get results early next semester. Most of what has been found thus far is purely qualitative. SEM SEM was the first test the group performed on the tailings. A small amount was dried and then mounted on carbon tape to be examined. SEM uses beamed electrons to gain information about a sample’s topography. It also has the ability to use energy-dispersive x-ray spectroscopy (EDS) to create colored images showing particles of similar elemental composition (figure 7). The image can then be filtered by color layer for ease of viewing. As can be seen in figure 8 and 9, the main elements detected were aluminum, calcium, iron, magnesium, oxygen, potassium, silicon and sulfur. Carbon is skewed due to the carbon tape the sample is mounted on. The carbon tape also nulls the Wt. % measurements given by the spectrums. One thing to note is the two distinct iron particles as well as the large presence of particles with both calciumand magnesium. The EDS scans also indicated fair amounts of gypsum and silicates. As can be seen on figure 3, individual point scans were taken and used to create elemental spectrums (Figures 10-12) using back-scattered electrons (BSE). Spectrums 8, 9 and 11 all hit the carbon tape resulting in a skewed spectrum and were not included on this report. Spectrum 7 was placed one of the particles that was high in calcium, magnesium and oxygen, most likely a dolomite particle (CaMg(CO3)2). Spectrum 10 had high amounts of silicon, aluminum and oxygen suggesting an aluminum silicate (Al2SiO5) particle. Spectrum 12 was placed on the particle that scanned high in iron. SEM is great for qualitative elemental data but fails to provide a true quantitative mineralogical breakdown. The second test we performed for mineralogy is an XRD scan.
29 | P a g e XRD XRD uses similar technology to SEM but instead of electrons being used, x-rays are used. Beams of x-rays are shot at the sample and will diffract in different directions. By measuring the angles of diffractions as well as the intensities, an overall spectrum is obtained and minerals can be identified. An initial grab sample was taken before the tailings could be divided into representative parts. It indicated that quartz was in the largest concentration followed by gypsum, mica and dolomite. The results of the XRD coincide with what was found with the SEM. Later in the semester, when several smaller representative samples were prepared, another XRD scan was performed (figure 13). Dolomite appeared in the largest quantity followed again by quartz and calcite. A new mineral also appeared in this scan, pseudobrookite (Fe2TiO3). Curiously, no iron-oxide particles such as magnetite or hematite were detected, raising questions about the magnetism of the tailings. Challenges One of the group’s current concerns that eludes the project is why the tailings are so magnetic. An initial magnetic separation test returned almost 90% weight of magnetic concentrates. According to Barrick, only 3-5% Wt. % of the tailings should be magnetic. Using SEM the group was able to observe particles that contained iron-oxide particles. These iron-oxide particles did not show up in XRD scans. A theory to this discrepancy is that the iron minerals are amorphous. Due to the lack of a consist shape, XRD is not reporting them in high concentrations. The use of MXCT early next semester will hopefully answer these questions about the iron in the tailings. This will be useful as magnetic separation could be an effective concentration process. Conclusion The mineralogy of the tailings is invaluable data to have when designing a concentration flowchart. Using technologies such as SEM and XRD, as well as MXCT in the future, the group has identified what makes up the majority of the sample body. Most of the data collected thus far has been qualitative in nature. The mineral in the highest concentration is quartz followed by dolomite. Other silicates, carbonates and sulfides make up most of the rest with iron based minerals supposedly in low concentrations. Samples have been prepared for use with MXCT and are scheduled to be run early next semester.
30 | P a g e Appendix E Processing Gold Tailings Introduction The tailings of any mineral processing will retain some amount of desired material, however the amount left over does not hold enough value to justify further recovery, but if it the value of the material increases or newer technology allows for easier recovery, then further processing may be justified. However, the tailings will have a majority of undesired material, and any gold that is left, will be sporadically dispersed throughout the ore, or trapped in another material. For this reason, it becomes necessary to concentrate the desired material, reducing time and money wasted on the portion of the tailings containing little to no gold. There are many ways to concentrate ore bodies and all of them take advantage of the ores physical and chemical characteristics, this is generally known as beneficiation. For this design project, the group will focus on concentrating gold by employing the most available and commercially used methods, gravity separation, froth flotation, and magnetic separation. Gravity Separation The first process used to beneficiate the gold tailings that will be considered, is gravity or density separation. Gravity separation is an environmentally friendly process, which utilizes simple equipment with few moving parts. (Abols) Throughout the history of mineral processing, many different types of gravity separation devices have been utilized, but the one the team will use is a wave table. Gravity concentration processes rely on the principal that gold contained within an ore body is higher in specific gravity than the material it is trapped in. Elemental gold has a specific gravity of 19.3, and typical ore has a specific gravity of about 2.6. (Fast) Movement is created by gravity concentration devices between the gold and host rock particles so that the heavy particles are separated from the lighter particles. On a wave table, particles will be fed onto the machine along with a stream of water. The bed of the table will begin to shake resulting in two events, the formation of wave in the water and the gradual forward movements of the particles towards a trough at the end of the table. While the particles are in contact with the surface of the table, the shaking will cause them to move forward. When a wave passes over them, less dense particles will be moved backwards while denser material are either not moved, or move very little resulting in a separation of the particles by their respective densities. See Figure 1 in report. Ideally gold and gold bearing material will move into the trough of the table but the team will likely collect all of the material closest to the top for testing. One disadvantage of gravity separation is that ultrafine clay particles are not easily rejected. (Honaker)
31 | P a g e Froth Floatation The flotation process consists of producing a mineral concentrate through the use of chemical conditioning agents followed by intense agitation and air sparging of the agitated ore slurry to produce a mineral rich foam concentrate. Specific chemicals are added to either float specific minerals or to depress the flotation of other minerals. Several stages of processing are general involved with rough bulk flotation products being subjected to additional flotation steps to increase product purity. Generally, the flotation process does not float free gold particles, but it is very effective when the gold is associated with sulfide minerals. Highly oxidized ores usually do not respond well to the process. An advantage of the flotation process is that the gold is usually liberated at a coarse particle size, near 28 mesh, reducing the grinding cost. Also, the chemical reagents are generally not toxic allowing for easy disposal. (Fast) A majority of the gold within the tailings received from Barrick are trapped in iron oxide and carbonaceous material. The tailings come from Barrick’s Goldstrike mine which is located on the Carlin Trend in Nevada. With this knowledge, a specialized process can be used with a specific collector and dispersing agent. Dr. Kenneth Han and Dr. Yuhong Chen, performed a study on how to remove the carbonaceous material from Carlin ore. The results of this investigation showed that it is possible to use froth flotation, and that a Philips aromatic oil served to be the best collector. One of the reasons for poor flotation recovery was due to oxidation of the carbonaceous material. Therefore, flotation recovery was significantly improved by subjecting the oxidized ore to reduction before flotation. Sodium silicate proved to be an effective dispersing reagent for the Carlin gold ore. The effect of pH on flotation was insignificant. The flotation response at room temperature was as good as any other temperature between 15° and 60° C. (Han) This study will prove to be very beneficial to the team moving forward. With the knowledge of the specific collector and dispersing agent, the floatation of the tailings can be greatly improved. Magnetic Separation With the majority of the gold being trapped in some form of iron oxide, magnetic separation will most likely, prove to be very useful. The concept of magnetic separation is a very simple process. A feed stream is passed through magnetic field, the magnetic attraction force between the magnet and magnetic particles will then pull these particles, separating them from the rest of the stream. See Figure 3 in report for illustration. Additionally because materials are either very magnetic (ferromagnetic), slightly magnetic (paramagnetic) or non-magnetic (diamagnetic), a feed stream can be effectively beneficiated by either adjusting the strength of the magnetic field, the speed at which the particles pass through the magnetic field or both. However information about the tailings suggested that the entire mass was only composed of about 3-5% iron oxides, so the results from the team’s attempt should be near this range.
32 | P a g e Conclusion While there are many ways of beneficiating the gold, these three processes are most readily available. Each method has its own advantages and disadvantages. Using a combination of these complimentary processes can improve the overall recovery of gold from the tailings. Each method works best in certain situations and under certain conditions. With the right knowledge of the tailings mineralogy and chemical composition, the recovery can be maximized.
33 | P a g e Appendix F Table 5: Economic details discountrate (I) 0.05 numberof years 10.78 $/kWh 0.06 operates80% of year 0.8 Tailings(tonage/day) 54,000 Concentrateswt%/100 0.05 kW/hp 0.75 personel 8 ore grade gold(tOz/ton) wet 0.2 goldrecovery%/100 0.75 goldprice $/troy 1,300 Facitity'shp 1,500 Dredgerhp 8,500 autoclave & acid$ /ton 90 personel salaraies/year 150,000 Maintenance %total cost/100 0.05 AC powerkWbasedon square footage 8,900 lightingpowerconsumption 120,000 square footage 2,000,000 dryerhp 5 Dryer PowerBTU 1E+12 BTU/ton coal 24,000,000 $/ton coal 70 ore grade gold(tOz/ton) dry 0.48
34 | P a g e Figure 15: Spreadsheet for wet process Figure 16: Spreadsheet for dry process Initial Costs $ Annual costs $ Annual prophits Facity construction 210000000 Autoclave and acid 70956000 Gold 153738000 Dredger 2000000 Facitity's hp 470327.8233 personel salaries 1200000 Dredger hp 2660958.048 maintenance 983181.1769 Lighting 117761.28 AC 3751392.505 total 212000000 80139620.83 153738000 Wet process NPV ($390,107,433.16) $0.00 IRR 0.331286996 Initial Costs $ Annual costs $ Annual prophits Facity construction 210000000 Autoclave and acid 70956000 Gold 307476000 Dredger 2000000 Facitity's hp 470327.8233 Fluid bed dryer 21875000 personel salaries 1200000 Dredger hp 2660958.048 maintenance 1084629.706 Lighting 117761.28 AC 3751392.505 Dryer hp 39193.98527 Dryer power 20440000000 Total 233875000 20520280263 307476000 Dry process NPV -$165,594,577,635.91 #NUM! IRR -5.26879E+23
35 | P a g e Appendix G Electrostatic Separation Accordingto the SME Mineral ProcessingHandbook,electrostaticseparationisdefinedasthe selective sortingof solidspeciesbymeansof utilizingforcesactingonchargedor polarizedbodiesinanelectric field. Thisdefinition,atface value,sounds alotlike magneticseparation. Infact if someone were to watch bothof these typesof separatorsinactionhe maybelieve theywere the same. Despitethe visual similaritiesthese processingdevicesmaysometimesshare,the onlyreal similaritytheypossessisthat theybothtake advantage of electrons. Whilemagneticseparationusesthe differingmagnetic propertiescreatedbythe numberof spinningunpairedelectronsalreadypresentinthe material to cause a separation,electrostaticseparationusesthe diffingelectrical chargescreatedfromthe number of and/orpositionof electronsonamaterial tocause separation. Howeversince the forcescreated fromthe attractionof opposingelectrical chargesare normallymuchweakerthanthose producedby magnetismthe effectiveparticle size forelectrostaticseparationismuchsmallerthanmagnetic separation2 soitis notnormallyusedasa primaryseparationmethodbutratherasa recoverymethod followingmostmineral processingandendproductproducingsteps. Inthe followingsectionsthe conceptsbehindhowelectrostaticseparationworksalongwithtypical designsandapplicationswillbe discussedingreaterdetail. Theory BehindElectrostatic SeparationandBasic Components The primaryforce usedinelectrostaticseparationsisbasedonCoulomb’sLaw whichstatesthat, 1 2 2 4 o o q q F r r        ur r EQ 2 where:q1 and q2 are magnitude of the twocharges,in coulombs. r the distance betweenthe twocharges,inmeters. or r isa unitvectoralong r r . o is knownasthe permittivityof free space withavalue of 8.85 X 10-12 farads/meter. F ur is expressedinNewtons. Since the force betweentwochargedbodiesdependontheirmagnitude,itbecomesconvenientto define anelectricfieldintensity,E ur withunitsof volt/meters,asthe force perunitcharge exertedbyone charge onanotherand withEQ 2 thiscan be expressedas, 1 2 2 4 o o q rF E q r   rur ur EQ 3 Withthisequationitispossible todefineaunitof work perunitcharge involvedinmovingone movinga charge fromone pointinan electricfieldtoanother. Thisunitof workperunitcharge inknownas the 2 Effective particlesizefor electrostatic separation is 200-40 microns,magnetic separation istypically used for particlesizes of 152-.5 mm or greater.
36 | P a g e difference inelectrostaticpotentialandissymbolizedby V andexpressedaccordingtothe following equation, V E dr   ur r EQ 4 where the negative signmeanthe V isincreasingwhenworkisdone onthe charge. For a conductive material EQ 4 can be expressedas, 2 1 2 0 0 1 2 1 1 4 4 r r q dr q V r r r            EQ 5 But for nonconductinganddielectricmaterialEQ5 can be simplifiedtoeitherof the followingequations dependingonwhatconditionisconsidered. 04 q V r  EQ 6 0 1 4 rq dr V r   EQ 7 Where EQ 6 isvalida reference pointisconsideredinfinitelyfarawayandEQ 7 isvalidif charge q is due to distribution—thatistoa charge rq (C/m3 ). Nowthat the theorybehindelectrostaticseparationhasbeenexplainedlet’sfocusonthe basic componentsof the methodof separation. All electrostaticseparationsystemshave atleastfourbasic components,those beingacharging-dischargingmechanism, anexternalelectrical field,anonelectrical particle trajectoryregulatingdevice andafeeding/productcollectionsystem. A charging-dischargingmechanism Perhapsthe mostimportant component,the mechanismbywhichthisbeneficiationmethodcharges and/ordischargesitsparticleswill resultinone of the followingcategoriesof charge distribution: 1. Particlesof twodifferentspeciesenteranelectricfieldinaseparatingzone bearinganelectric charge of opposite sign. 2. Particlesof twodifferentspeciesenteraseparatingzone where onlyone type of particle bears a significantelectrical charge. 3. Particlesenterthe separatingzone,suchthatparticlesof differentspeciesbearthe same sign charge,but the magnitudesof the electricchargesare significantlydifferent. 4. Particlesof differentspeciesenterthe separatingzone withsignificantlydifferentdipole moments. An external electrical field For thisseparationmethodtoevenbe possible anelectricfieldisrequired. The electrical potentialsof these fieldsvarygreatlybuttypicallyrange from10 to 100kV andare normallyunidirectional.The fields themselvesrange from400kV/mto the breakdownstrengthof airwhichisaround3000kV/m. A nonelectrical particle trajectory regulatingdevice The physical separationof twodifferenttypesof particlesisalwaysmade byadjustingthe forcesand time overwhichthe forcesact on a particle,suchthat at a predeterminedtime the trajectoriesof the
37 | P a g e twoparticle typeswill have differenttrajectories. Howeverinadditiontoelectrical forcesitmaybe advantageoustoutilize otherforcessuchasgravity,andfrictiontoaid inthe selective sorting. A feeding/productcollection system Thisis a feature commontoall separators.A separatormusthave some meansof conveyingthe feed material tothe separationzone inadditiontoa methodof cuttingthe streamof particlesata desired pointso thatdifferentparticletypescanbe collectedandconveyedtoa followingseparationdevice or productstorage tank. Typical Designs andApplications As mentionedbeforethe charginganddischargingmechanismispossiblythe mostimportant componentof an electrostaticseparatorasthiswill dictate the machinesoverall designandapplication. There are manywaysto electricallycharge particlesforseparation,butonlychargingby contactand frictional electrification,ion or electron bombardment andconductiveinduction are usedcommercially. Howeveritshouldbe made clearnowthat these mechanismsare notmutuallyelusive foreachdesign, infact it’smore thanlikelythatatleasttwo of these mechanismsare presentatanygiventime foreach separationdesign,the distinctioncomesfromwhichchargingmethoddominates. Contact electrification Chargingby frictionand/orcontactelectrificationisthe mostfrequentlyusedmechanismforseparating twospeciesof dielectricmaterial.Some of the typical examplesbeingfeldsparfromquartz,quartzfrom apatite andhalite fromsylvite.Butthisis notto sayconductive material wontcharge thisway,because theywill,it’sjustthatsince theyare conductive itismore likelythatthese particlewillrelinquishtheir charge before theyare separatedfromthe feedstreamnegativelyeffectingthe results. The theorybehindexactlyhowchargingaparticle bycontact isverycomplex andis not fully understood,butconceptuallyit’srelativelyeasytoexplain. If yourememberbacktowhenyouwere a childandyou wouldruban inflatedballoonagainstyourheadthenlifteditawaytosee your hairnow standingonendtryingto followthe balloon,wellthat’sbasicallywhatishappeningbetweenthe particlesof thischargingmechanism.Yousee some dielectricparticleswill more readilygive uptheir surface electronstoanotherdielectricparticle once contacthasbeenmade leadingthemnow to become positivelyandnegativelycharged. Thoughthese particle are now oppositelychargedthe coulombicforce betweenisstill weakenoughthatmomentumfromthe initiallycollisionwill separate themand/orthe coulombicforcesbetweenthe particlesandthe electrical fieldgivenoff bythe electrodeswill attractthe oppositelychargedparticle whilesimultaneouslyrepellingthe likecharged particle leadingtothe desiredseparation. A typical designisthe Free-Fall separatorasseeninfigure 16 below.
38 | P a g e Figure 3: Free-Fall Separator As seenfromthe figure,aFree-Fall separatorconsistsof ahopperanda vibratingfeederwheremuchof the change of charge throughcontact will occur.The feedwill thenemptyintoachute thatleadsto an emptycolumnsurroundedbychargedelectrodes. While gravityisthe force movingthe particles throughthe machine the coulombicforcesfromthe chargedelectrodescause the separationbypulling the oppositelychargedparticleduringtheirfall.Howeversince the effectivenessof separationcanvary drasticallydependingonmaterial of feed,particle sizes,temperatureandpressure,aFree-Fall separator isoftencustomizedaccordingtothese variablesbychangingthe feedrate,electricfieldintensityand/or lengthof fall betweenthe electrodes. Charging by corona Chargingby ionor electronbombardmentisoftendone withacoronadischarge. These typesof separatorsare bestsuitedfororeswitha mixture of conductive andinsulatingmaterial. Common applicationincludeseparatingrutileandilmenite frombeachsandbutanothercommonuse isto recovervaluable finesfromvariousmineral processingandfabricationplantsthatescape intothe air. To charge bycorona solidsmustpassthrougha corona discharge froma wire electrode toagrounded surface. A corona discharge occurswhenthe electrical potential of anelectrode israisedtothe point that the electrical fieldinthe immediate areaexceedsthe electrical breakdownstrengthof air,this internscreateschargedionswhere half are repelledbythe electrode andbombardthe feedparticles thuscharging them.The mosttypical designforthistype of electrostaticseparationiselectrodynamic separatoror as it ismore commonlyknownasthe High-Tensionseparatorwhichcanbe seeninFigure 17 below.
39 | P a g e Figure 4: High-tension separator. Diagram of separation in progress is also shown How thistype of machine causesa separationconceptuallyspeakingismoderatelysimple. Thismachine takesadvantage of three forcesgravity,centripetalandcoulombic.The particle are feedontoaroller that iselectricallygroundedwheretheyare chargedbya corona. The conductive material quickly transfertheirelectrical charge tothe groundedrollerandwithnocoulombicforce tokeepingitattached to the rolleristhrownoff ina predictable trajectorywhere itiscollected. The nonconductive material will give theircharge tothe groundedrollertoobutat a much slowerrate meaningthese particle will remainattachedto the rollerwithcolumbicforce until the charge decreasesenoughforgravityand centripetal forcesbreakthe connectionorthe particlesare brushoff at the backof the roller.These particle alsofollowasomewhatpredictabletrajectorysotheyare collectaccordingly. The collectionbin that isdesignatedtocollectbothnonconductive andsmallerconductive material will be processedagain at a differentrate toimprove separation. Charging by conduction Whenseparationisdesiredforamixture of conductive anddielectricmaterialschargingbyconduction isoftenpreferred. Thismethodistypical usedasafollowingseparationstepforthe conductivematerial collectedbythe coronamethodwhile processingbeachsand. As the name suggestmaterials are chargedbyconductionmeaningthatthe particle mustcome in contact witha chargedsurface.Thisis the beginningof the separationprocessusingthismethod. As the particlesare chargedtheyare simultaneouslypassingthroughanelectricfield. The conductive material will charge almostinstantlyandthiswhenthe coulombicforcesof the electrical fieldtake over causingthe separation. The dielectricparticle will charge byconductionbutat a much slowerrate so before theyare sufficientlychargedtheycome outof contact withthe conductingsurface andare thusly unaffectedbythe electricfield. Twoof the more common designsforthismethodare Roll-type separatorand the Tobogganseparatorseeninfigure 18 below.
40 | P a g e a) b) Figure 5: a) Roll-type separator. b) Toboggan separator For the Roll-type separatorthe conductingsurface isthe rollersothisdesigntake advantage of gravity, centripetal andcoulombicforcestocause a separation,while the Toboggandesignhasaconducting plate initscute where the particle descend meaningthatonlygravityandcoulombicforcesare usedto cause a separation. Asyoucan image withthese designalotof chargingby contact ispossible sothe actual chargingmechanismiscomplex anddifficulttopredictbutsince the conductionwasthe only intendedmechanismandthe predominantone these designsare notconsideredacontact electrificationmethod.
41 | P a g e Appendix H Gravity Seperation INTRODUCTION Gravityseparationisa commonextractive technique forfreeinggoldfromitsgangue.Whenconsidering gravityseparationyouhave tofigure outthe specificgravityof the desiredelementandthe surrounding gangue.Gravityseparationwasone of the possible extractive techniquesthatcouldbe usedtoprocess the tailingsatthe Goldstrike Dam.To determine if the goldinthe tailingscanbe recoveredusinggravity separationthe teamhasto determine concentrationcriterionusingthe specificgravityof the goldand the gangue.Thispaperwill alsocoversome of the historyof gravityseparation,the benefitsand disadvantagesof gravityseparationconcentration,andcommonwaystouse gravityseparationfora large scale miningoperation. HISTORY Gravityseparationisone of the oldestformsof ore concentrations.The ancientEgyptiansdepicted gravityseparationin3,000 BC. Theynoticedhow weatheringcausedbywaterthatcreatedalluvial depositsof metal andgemstones.Excavationshave alsounveiledthatmultiple ancientcivilizationshad miningoperationsusinggravityseparation(Guapta).Anothercommontype of gravityseparationisgold panning.Panningwasasimple techniquethatdrove manysettlerstoCaliforniainthe 1849 goldrush. Panningobviouslyisnotusedinlarge scale miningoperationswithmore effective andchemical techniquesbecomingmore available.Gravityseparationhasobviouslyhadamajor effectonhow the worldtodayhas beenshapedbypreviousgenerations. BENEFITSAND DISADVANTAGES Gravityseparationisa concentrationmethodwithmanybenefitsanddisadvantages.Inideal ore sourcesgravityseparationisthe bestmethod.Whenanore source containsfree goldparticleswitha gangue that has a lowerspecificgravitythangold.However,manyof these ore sourceshave been depletedbelow economicfeasiblelevels.There are betterrecoverymethodslike goldleaching,but these concentrationmethodsuse harmful chemicals.Dealingwiththese harmful chemicalslikecyanide are hard andexpensivetogetridof them.Gravityseparationonthe otherhanddoesnot needany chemicalsinorderto separate goldfromthe gangue. LARGE SCALE OPERATIONS
42 | P a g e There have beenrecentdevelopmentsingravityseparationtomake ita more feasible forlarge scale miningoperations.One of these advancementsisthe KelseyCentrifugal Jig(KCJ).KCJcombinesjigging technologywithcentrifugal forcesthatgreatlyimprovesongravityseparation.The KCJusesarotating wedge wire screenwithabedof raggingmaterial onthe screen.The range of feedfora KCJ is10 to 500 micrometerswithdifferentmineralslike gold,tin,chrome,andiron(Luttrell,Young).Another developmentwasthe KnelsonBatchCentrifugalConcentrator(KBCC).The KBCCisan outwardlyinclined, walls-spinningcentrifugebowl withriffles.The spinningbowl spinsthe feedslurryandthenseparates and recoversminerals.The KBCCisable toseparate 10 to 10,000 tonnesperhourdependingonthe feed.The KBCChas beenusedtoseparate gold,platinum,andgold-copperores(Luttrell,Young).With these andmanyother advancementsgravityseparationhascome alongway fromgoldpanningand offersthe optionforlarge scale miningoperations. DETERMING FEASABILITYOFGRAVITY SEPERATION In orderto determine if gravityseparationisapossible concentrationmethodthe designteamneedsto take a fewthingsintoconsideration.The firstthingtoconsideristhe concentrationcriterion.The calculationforconcentrationcriterion(CC) isshownbelow inFigure 19.Giventhe gangue of density 2650 kg/m3 goldhas a CC of 10.3 and an ironore like hematitehasaCC of 2.5 (Guapta).However,the goldmineral inthe tailingsare coveredinanironoxide knownasmaghemite.Sogiventhatfactitis unlikelythatthe goldmineral wouldhave ahighenoughCCthatwouldallow forgravityseparationand withan ironoxide combinedwiththe golditismuchlikelierthatmagneticseparationwill be amore successful concentrationmethod. Figure 19: Calculation for Concentration Criterion (Guapta) To testif gravityseparationwaspossible the teamusedthe wave table inthe MIBuildingatSchool of Mineson a sample of tailingstosee howmuchgoldcouldbe recovered.ShowninFigure 20 isa representationof howawave table usesthe specificgravitytoseparate goldfromthe gangue.Asthe wave table shakesbackand forththe table movesthe goldforwardwhile the watermovesthe gangue back creatinga separationof the two.Whentestingthe teamwasable to recover4.55 wt%of the testedsample of tailings.Howeveronfurtheranalysisusingfire assaythe goldamountwasbelowthe lowerlimitsof the test.
43 | P a g e Figure 60: Representation of wave table. Yellow dots show gold and black shows gangue movement CONCLUSION Gravityseparationisa verysimple concentrationmethod,butunderthe rightcircumstancescouldbe veryeffective.The technique of usingthe specificgravityof a mineral toconcentrate ithasbeenused for hundredsof yearsandhas done a lotto shape the worldwe know today.Overthe years advancementshave beenmade inthe conceptof gravityseparationwhichhasallowedaccesstomore and more mineral sources.Throughsimplethinkingandcommontestproceduresthe designteamhas shownthat gravityseparationtechnique wouldbe difficulttouse forconcentratingthe goldtailings. Therefore,ithasbeendecidedthatdespite possible benefitsof gravityseparation,itwouldnotbe the bestconcentrationmethod.
44 | P a g e Acknowledgments: A special thankstothe followingpeople anddepartments: Dr. Ed Duke for runningthe SEM ontailings. Dr. Jon Kellarforadvisingthe team. Alex Wulff forMXCTanalysisontailingparticles. RussLingenfelterforperformingthe XRDanalysisandteachingmembersof the teamhowto performfire assays.

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Final report

  • 1.
    Barrick Goldstrike Team Advisor: Dr.Jon Kellar Team Members: Jordan Dick, Justin Brick, Jack Blaisdell, Devin Rowe Date: March 18, 2015
  • 2.
    1 | Pa g e Table of Contents Executive Summary ..............................................................................................2 Introduction ........................................................................................................3 Theory. ...............................................................................................................3 Results and Discussion ..........................................................................................9 Drying and sampling........................................................................................................... 9 Base line analysis process................................................................................................ 10 Results............................................................................................................................... 10 Flowsheet and implications............................................................................................. 11 Environmental Context .......................................................................................12 Global and Societal Context.................................................................................13 Conclusion.........................................................................................................13 Future Work ......................................................................................................14 References ........................................................................................................15 Appendix...........................................................................................................18 A........................................................................................................................................ 18 B........................................................................................................................................ 22 C........................................................................................................................................ 25 D....................................................................................................................................... 28 E........................................................................................................................................ 30 F........................................................................................................................................ 33 G....................................................................................................................................... 35 H....................................................................................................................................... 41 Acknowledgements ............................................................................................44
  • 3.
    2 | Pa g e Executive Summary: Barrick Gold,the largestgoldminingcompanyinthe world,hastaskedthisdesignteamto analyze a reserve of tailingstheyhave atone of theiroldestminestosee if the tailingscanbe profitably concentrated, andif theycan, designaprocessto do so.Barrick believes thatwiththe currentprice of goldand withadvancementsin mineral processing,the tailingsare now aviable source of gold. To determine the amountof goldinthe tailings,the teamhasconducted three concentrationtechniques. Magneticseparation,frothflotation,andgravityseparationwere all usedtoconcentrate the gold. Inthe frothflotationconcentrationmethod,fourtestswere performed.The firsttestperformedshowedthe presence of gold,butthe resultscouldnotbe repeated.Giventhe unreliabilityof thistest,the team decidedthatfrothflotationwouldnotbe aviable optionforconcentratingthe goldinthe tailings.The scope of the projectwasshiftedtowardsmagneticseparationandgravityseparation. The resultsof gravityseparation were notbetteras a fire assaydeterminedthatthe grade of the concentrate wasless than the initial grade of the tailingsmakingitaninaffective method. Magneticseparationtestinitially seemedencouragingasthe grade of the tailingwasimprovedtenfold,puttingitabove the minimum grade value that Barrickis willingtoprocess. Subsequenteconomiccalculationsshowedthattoomuch moneywouldbe spentdryingthe tailingpriortoprocessing,meaningthatmagneticseparationasthe teamperformeditwasuneconomical. Howeverourcontactat Barrack Goldprovidedinformationona wetmagneticseparationtestthathe performedandthe economiccalculationsforthisversionof magneticseparationprovedpromisingasthe NPV andIRR were foundto be about$390 millionand33% respectively.
  • 4.
    3 | Pa g e Introduction: It isa commonmisconception thatwhenamineral processingcompany isdone withanore bodythat all of the valuable materialiscompletelyextracted. While itispossible toprocessthe ore until nearlyall of the valuable material istaken,thisisalmostnevereconomicallyjustified.Itistypicallythe case that initiallyalarge portionof the desiredmaterial canbe easilyextractedfromthe ore but, as lessmaterial isavailable,the more costlyitbecomestoextractthe remainder,thusmakingituneconomical to continue processingthe ore. However,astime movesforwardanddemandschange,the price forsuch material mayincrease tothe pointthat itis economical tocontinue processingthe depletedore body for the remainingmaterial. Thisis whatBarrick gold wishestofindoutas theyhave 170 milliontonsof tailings fromtheirGoldstrike mine.Barrick believeswiththe risinggoldprice thatthe tailingsmaynow be a viable ore source.The team’sobjective istoanalyze samplesfromthe tailingsto measure the grade of the tailingsandthento designflowsheetsthatwill maximizethe netpresentvalueandinitialrate of return. Thiswill determine if it iseconomicallyfeasible forthe tailingsto be reprocessedforthe remaininggold. Barrickhas requestedthatourgroupto be finishedcollectingandanalyzingdatabyFebruaryof 2015. Theory: The tailingsof anymineral processing operation will retain some amountof desiredmaterial. However if it the value of the material increasesornewertechnologyallowsforeasierrecovery,thenfurther processingmaybe justified. There are manywaysto concentrate ore bodiesandall of themtake advantage of the ore’sphysical andchemical characteristics,thisisgenerallyknownasbeneficiation.For thisdesignproject,the groupwill focusonconcentratinggoldbyemployingthe mostavailable and commerciallyusedmethods,gravityseparation,frothflotation,andmagneticseparation. Gravity Separation The firstprocessusedto beneficiate the goldtailingsthatwill be considered,isgravityordensity separation.Gravityseparationisanenvironmentallyfriendlyprocess,whichutilizessimple equipment withfewmovingparts.(Abols) Goldpanningisasimple formof gravityseparationthathasbeenused for centuries. Throughoutthe historyof mineral processing,manydifferenttypesof gravityseparation deviceshave beenutilized,butthe one the teamwill use isa wave table. Gravityconcentration processesrelyonthe principle thatgoldcontained withinanore bodyishigherinspecificgravitythan the gangue. Elemental goldhasa specificgravityof 19.3, andtypical ore has a specificgravityof about 2.6. (Fast) Movementiscreatedbygravityconcentrationdevices betweenthe goldandhostrock particlessothat the heavyparticlesare separatedfromthe lighterparticles. Onawave table,particles will be fedontothe machine alongwithastreamof water.The bedof the table will begintoshake resultingintwoevents,the formationof awave inthe waterandthe gradual forwardmovementsof the particlestowardsatrough at the endof the table.While the particlesare incontactwiththe surface of the table,the shakingwill cause them tomove forward. Whena wave passesoverthem, lessdense
  • 5.
    4 | Pa g e particleswill be movedbackwardswhiledensermaterial are eithernotmoved,ormove verylittle resultinginaseparationof the particlesbytheirrespectivedensities. See Figure 1. Figure 1: Wave Table wave velocity profile. Yellow is gold and black is gangue To determine if gravityseparationisafeasibleprocessacommontechnique istocalculate the concentrationcriterion.As shown inEquation1the concentrationcriterioniscalculatedbythe specific gravityof the heavymineral subtracted bythe specificgravityof the fluiddividedbythe specificgravity of the lightermineral subtractedbythe specificgravityof the fluid.Assumingagangue withadensityof 2650 kg/m3 goldhas a concentrationcriterionof 10.3 and hematite hasa criterionof 2.5. The other majorfactor of determiningif gravityseparationispossible isdeterminingthe particle size of the mineral thatisdesired.Below inFigure 2showsconcentrationcriterionversusthe sizeof mineral particlesinmicronsshowingwhere separationispossible andimpossible (Guapta).Table 1alsobacks up the data showedinFigure 2.
  • 6.
    5 | Pa g e Figure 2: Size limit curve for gravity separation Table 1: Concentration criterion guide for gravity separation Ideallygoldandgoldbearingmaterial will move intothe troughof the table butthe teamwill likely collectall of the material closesttothe top fortesting.One disadvantage of gravityseparationisthat ultrafine clayparticlesare noteasilyrejected (Honaker).Anotherdifficultywillbe the change togolds specificgravitybyironoxide sludgeof maghemiteattachedtothe goldore.Finallythe small particlesize of the gangue createsanotherobstacle todeal withif the teamdecidedtouse gravityseparationasa concentrationmethod. Froth Flotation Froth flotationislargelyconsideredtobe one of the mostwidelyusedmethodsinmineral processing. The processwas inventedbyFrankElmore in1896. Elmore had justpurchasedthe Glasdircoppermine whichhad a notorioushistoryof havingdifficultore toworkwith. The mine hadbeensoldseveral times inits lifetime. Elmore experimentedwithusingoil asa collector.The processwasa huge success.The mine hasable to collectnearly70%of the ore’scoppercontent.To give a basicdescription,froth flotationessentiallyutilizesamineral’saffinityforwaterinorderto create a separation.Due tothe process’ssimplicityandhistorical success,the Barrickdesignteamwill be evaluatingfrothflotationasa
  • 7.
    6 | Pa g e possible concentrationtechniqueforthe goldstrikemine’stailings. Inorderto give the designgroup and anyother readersa betterunderstandingof the process,the followingsectionswill coverhowthe processworks,andhow itis commonlyperformed. How does it work?-Asstatedabove,frothflotationexploitsthe factthatsome mineralsare hydrophilic (likeswater) andsome are hydrophobic(hateswater). Thisisthe basisof creatinga goodseparation. The ore ismixedintoa slurryandthenintroducedintoaflotationcell where theyare agitated. Airis pumpedintothe bottomof the cell inthe form of small bubbledtypicallyof aboutone totwo millimeters.If amineral particle ishydrophobic,itwill attachonto a bubble andfloattothe surface of the slurrywhere itcan be skimmedoff. Those particlesthatare hydrophilicwill notattachto air bubblesandwill remaininthe slurry. What makesdifferentmineralparticleshydrophobicornotcomesdownto the surface chemistryof the particle. The surface of any particle can be splitintotwolayers:the sternlayerandthe diffuse layer. These twolayersare oftencalledthe electrical double layer.The sternlayerisasmall layerdirectlyon the particle’ssurface thattendstoaccumulate ionsof the opposite charge of the surface.Forinstance a particle withanegative surface charge will accumulate positivelychargedionsinthe sternlayer.The diffuse layerisa muchlargerlayeroutside the sternlayerthatcontainsa mixture of positive and negative ionsandessentiallyzeroesthe total charge tothe surroundings(Figure 3).Twoimportantvalue inflotationisa mineral’szetapotential.The zetapotential isthe charge justoutside the sternlayer. Differentsolutionshave differentpHvaluesandthereforedifferentamountsof positive (H+ ) and negative (OH- ) ions.VaryingpHscan change the zetapotential of a particle.The pHvalue where the zeta potential of aparticle isequal tozero iscalledthe point-of-zero-charge(PZC) (Figure 4).Bymakingthe zetapotential negativeorpositive,one canthenaddchemicalsthatwill adhere tothe surface of select particlesandrenderthemhydrophobicallowingthemtofloat. Many differentchemicalsmaybe addedtoa mineral slurrytochange how theyreact withwater.Those that change the hydrophobicityof aparticle are knownas collectorsorsurfactants.Collectorscanbe eitheranionicorcationicdependinguponwhatthey adhere to. Anioniccollectorswill adhere to positivelychargedparticles,while cationiccollectorswilladhere tonegativelychargedparticles.Inthis way,differentcollectorscanbe usedtotarget particleswithdifferentcharges.Chemicalscanalso be usedto alterthe pH of a solutioninordertoisolate amineral forflotation.Forinstance,if one wanted to floatironoxide (PZC6.7) from silica(PZC2) withan anioniccollector,the pHcouldbe loweredtofive to renderhematite positivelychargedandsilicanegativelycharged. The anioniccollectorwill then attach to the iron oxide,makingithydrophobic,causingittofloatwhile leavingsilicabehind.Additional chemicalscalledfrothersmayalsobe usedinordertocreate a more resilientfrothlayer.
  • 8.
    How is flotationperformed-Beforeflotationcanoccur,ore typicallyhastobe reducedto fine particles usinganynumberof comminutiontechniques.Typicallyflotationisperformedwithparticlesof around 0.1 mm butcan sometimesrequireevensmallerparticle sizes. The groundore isthenmixedwithwater to forma slurrybefore the additionof pHalteringchemicalsandsurfactants. The slurryisthen introducedintoaflotationcell where itisaeratedandallowedtofloat. The frothcan thenbe removed as concentrate.Those particlesleftbehindmaybe pumpedoutof the cell as tailings.One advantage of frothflotationisthatit iseasilyscalable fromsmall laboratorycellstolarge industrialcellsthatmaybe linkedtogethertoobtainthe mostout of the process. The Barrick designteamperformedsome basicflotationtestsonthe ore receivedfromthe Goldstrike mine.Underthe suggestionthatgoldwastrappedwithironoxide fromthe group’sadvisoratBarrick,it was decidedthatanattemptto floatthe ironoxide wouldbe made.Frommineralogyreportsperformed on the tailings,the groupwasable todetermine thatalarge portionwas carbonaceousmaterial.This suggestedthata goodseparationmightbe possible.The grouppreparedfourrepresentative samples fromthe tailing,andutilizedoleicacid(anioniccollector)aswell asanunidentifiedfrothertotest differentflotationcombinations.Fourtrialswouldbe run:anatural floatwithnoadditives,asecond trial withan addedfrother,the thirdwoulduse three drops of oleicacid,the lasttrial usedbotholeic acid anda frother.Waterand the tailingssample werefirstintroducedintothe flotationcelland allowedtoconditionwithaironforone minute.Whatwas collectedwasreturnedtothe cell. Thisinitial step’spurpose istoproperlymix andcreate a uniformslurry.The cell wasthenturnedbackon and allowedtorunfor three minutes.Concentratesweredriedandthenscorificationassayswere runto determine goldgrade. Figure 3: Diagram of a particle showing the electrical double layer Figure 4: Graph of zeta potential versus solution pH. PZC equals 9.5
  • 9.
    8 | Pa g e MagneticSeparation Magneticseparationisa popularandlow-costmethodof recoveringmetals. Magneticseparationis usedprimarilybyminingindustries,butisalsowidelyusedinthe recyclingindustry.Magnetic separationusesamagneticfieldtoseparate magneticmaterialsfromnon-magneticones. Explanation- Whenmineralsare placedina magneticfield,thereare three reactionsthancanoccur. Attractionto the magneticfield,repulsionfromthe magneticfield,ornoreactionat all. The particles that are attractedto the magneticfieldare calledmagnetic.These particlesare subdividedintotwo classifications;stronglymagneticparticlescalledferromagneticandweakermagneticparticlescalled paramagneticparticles.Ferromagneticparticlesmaybe easilyseparatedwithaseparatorhavinga low intensitymagnetic.However,paramagneticparticlesrequire ahigherintensitymagneticfieldto separate them. Particlesthatare repulsedbyamagneticfieldare calleddiamagnetic.EddyCurrentSeparators,take advantage of the diamagneticparticles,separatingthemfromothermaterial.EddyCurrentSeparators are largelyusedinthe recyclingindustry.Here,wire andmetalsmade fromcopperandaluminumare separatedfromplastics.Whenaproduct,such as aluminum, passesoverthe eddy current,amagnetic fieldformsaroundthe piece of aluminum.The polarityof the magneticfieldof the aluminumisthe same as the rotatingmagnets,causingthe aluminumtobe repelledawayfromthe separator.Plastic, glass,or otherprocessmaterialssimplyfalloff the endof the separator.Aneddycurrentisdefinedas the currentscausedby voltagesinducedbychangingflux,andtendtooppose the change of the flux. Non-magneticparticles,suchasgold,quartz,andpyrite,are not susceptible to magneticseparation,but some magneticmaterial maybe removedfromthe feed.Forinstance,inafew situations,plantsusing gravityconcentrationforrecoveringgold,usedmagneticseparatorstoremove the highconcentration of magnetite thatwasrecoveredwiththe gold,priortofurtherprocessing. Magneticseparationisgenerallyalowcostmethodof recovery,unlesshighintensityseparatorsare required.The electro-magnetichighintensityseparatorsthatproduce 20,000 gauss,tendto be expensive. However,the rare earthmagneticseparatorsare relativelyinexpensive andcanproduce magneticfieldsaround6,000 gauss.So,whenlookingfora processto recovervaluable minerals, magneticseparationshouldnotbe overlooked,if some of the material is magneticorpara-magnetic. (Kinsack) Whenconsideringusingmagneticseparationprocessitisimportanttoevaluate amaterial’smagnetic susceptibility.Magneticsusceptibilityisavalue giventoamaterial basedonhow magneticitis per volume andis measuredbyweighingasample andthenturningonan electromagnettosee how the weightisaltered.A material likepure goldhasa negative magneticsusceptibilitymakingitdiamagnetic so usingmagneticseparationmethodonitwouldnotyieldanyresults. The process-There are differentsortsof magneticseparators,butthe mostcommonis the drum separator.(See figure 5) Essentially,astreamof material isdroppedontothe beltof a magneticdrum.
  • 10.
    9 | Pa g e Non-magneticorweaklymagneticparticlesbounce orfall off of the drum.The remainingmagnetic particlessticktoit until theyare scrapedoff. The speedof the beltas well of the intensityof the magneticfieldwill bothhave aneffectonhow muchof the magneticmaterial isattractedtothe belt. (Osbourne) Figure 5: Drum Separator In additiontothe drummethod,manyothertypesof separatorsexist.These include,butare not limited to, pulleys,discs,beltsandrings.Magneticseparatorscanbe usedeither"wet"or"dry"and come in countlessshapes andsizes. Magneticseparation for our project-Withthe majorityof the goldbeingtrappedinsome formof iron oxide,magneticseparationwill mostlikely,prove tobe veryuseful. A BarrickGoldrepresentative informedthe teamthatthe entire tailing’smasswasonlycomposedof about3-5% ironoxides,sothe resultsfromthe team’sattemptneededtobe nearthisrange.Ironoxide hasa veryhighmagnetic susceptibilitymakingitparamagnetic.If the goldisimbeddedinthe ironoxidemagneticseparation shouldbe a viable processingoption. The resultsof the magneticseparationperformedbythe teamdidnotmatch that of the data from Barrick. Barrickwas able to performthe magneticseparationusingawetmethod.Giventhe resources available atSouthDakotaSchool of MinesandTechnology,the teamwasunable torun a wetmagnetic separationtest. While there are manywaysof beneficiatingthe gold,these threeprocessesare most readilyavailable. Each methodhas itsownadvantagesanddisadvantages.Usingacombinationof these complimentary processescanimprove the overall recoveryof goldfromthe tailings.Eachmethodworksbestincertain situationsandundercertainconditions.Withthe rightknowledge of the tailingsmineralogyand chemical composition,the recoverycanbe maximized.
  • 11.
    10 | Pa g e Results andDiscussion: Drying and sampling- The sample arrived inaslurryform so,before itcould be processed,itmustbe dried. The sample wasgivenafewdaysto sit,thus allowingthe particlestosettle tothe bottomof the shippingbarrel. Withthe particlessettled,the toplayerof waterwasremovedusingabucketuntil the movementof the watercausedbythe bucketbeganto disturbthe particles. The particleswere then allowedtosettle again. Once settledawet-dry-shop-vacuumwasthenusedtogentlyremove the rest of the surface water. As the sample slurrywasbeingpumpedintothe 55 gallondrumitarrivedin,it can be assumedthatthe slurrywas thoroughlymixedfromthe violentmotionsimpartedonitduringthe time itspentmoving throughthe pipingsystemandthe initial agitationlikelyusedtoloosenupthe tailingsduringthe acquisitionfromthe tailingdam. Therefore,if the slurrywasthoroughlymixedinthe drumitcan be furtherassumedthatthe settlingof the particle inthe waterallowedforarelativelyuniform dispersement,because whileindividual particlesprocesseddifferentsettlingvelocitiesthe initial uniformdispersementinthe fluidshouldsufficientlyoffsetanysettlingtrends. Inthisregard, a sample takenby a coringmethod shouldbe representative of the entire tailingssample. Sothe teamuseda PVCpipe tocore and remove arepresentative sample. Thiscore wasthenplacedinan ovento dry. Once the core was dried,the particlesfusedintolarge agglomerates,necessitating breakingthe agglomerates usingamortarand pestle. Afterthiswascompleted,analysisof the tailingsbegan usinga riffleralongwiththe cone andquarteringtechniquetoseparate representativesamplesforvarious tests. Baseline analysisprocess- A baseline analysis of the tailings todetermine suchthingsasmineralogy and particle size distributionwas conductedsince characteristicssuchasthese will influence how a concentrationmethodwillperform. A sample was processedand/oranalyzedineachof the following ways:  Usedto performa fire assay(see Appendix B) bya memberof the team. Thiswill give aresult regardingthe tailingscurrentgoldgrade thatis representative.  Digestedinaquaregiaor anotheracidsolutiontobringthe goldpresentintosolution. Use AAS to determine the concentrationof goldinthe solutiontothenbackcalculate the grade of the ore. This methodmaynot be as accurate as the fire assaybut can be performed quicklyfor multiple samples,thusthismethodwill be done frequently.  Sentto EngineeringandMiningExperimentStationpersonnelfor SEManalysis.Thisalongwith othermethodswill helptodeterminethe mineralogyof the tailings.  Takento EngineeringandMiningExperimentStationpersonneltohave X-raydiffraction(XRD) performed. Thisisa greattechnique fordeterminingmineralogy. Results- Asof now the teststhat have beenperformedinclude SEMandXRD analysis,fire assay, AAS, magneticseparation andpreliminaryfrothflotation.
  • 12.
    11 | Pa g e The SEM gave a roughelemental compositionof the sample.Forthe SEManalysis,agrab sample from the top of the sample wasusedinsteadof arepresentative sample.The highpeaksof the graphshow the presence of silicates inthe sample.(SeeAppendix A andD) The XRD showedmanyof the minerals withinthe sample andwasperformedafterdifferentprocesses. ResultsfromXRDdidnot change much evenaftermagneticseparation,XRDshowedthe presenceof quartz, gypsum,dolomite,andcalcite (see Appendix A) butnotmuchelse.The expectationwasthat aftermagneticseparation, ironoxidewouldbe found,butthatwasnot the case. The team later discoveredthatthe ironoxide wasmaghemite,anoxidationstage betweenhematite andmagnetite, and itwas not detectedbecause the XRDequipmentwasnotcalibratedtodetectit. Table 2: Grade and recovery results Fire Assayand AASwere performed before anyconcentration toreveal the initial average value of the goldwithinthe sample. Bothresultswereverysimilarwiththe average valueforfire assaybeing 0.038 t Oz/tonand the average value forAASbeing 0.035 t Oz/ton.These valuesare abouthalf of Barrick’s requirementof aconcentrate grade of 0.08 t Oz/ton. The firstconcentration methodcompleted wasmagneticseparation. The groupwasinformed byBarrick personnel thatthe majorityof the goldleftinthe tailing istrappedinmaghemite.Furthermore, maghemite comprisedabout3-5wt% of the tailing.The team’smagneticseparationreflected this expectation withamagneticconcentration of justunder7 wt% of the tailingssample. ResultsfromAAS showedthatthe magneticconcentratesfromthe separationhad a grade of 0.17 t Oz/ton. Thisvalue is nearlytwice Barrick’s minimumrequiredgrade. Later,a fire assayshowedthatthe actual grade was around0.48 t Oz/ton. Flotationsof the tailingswere performed on500 gram samplesinanattemptto determine how much goldcouldbe floatedoutof the naturallyhydrophobic, carbonaceous material presentinthe tailings by floatingwithnoadditives,oleicacid,afrotherand botha frotherand oleicacid. All floatswhere performedata pH around 8, withabout2.6 litersof waterand anyadditiveswhere addedat0.268 lbs/ton. The frothercollectedthe most witharound16.4wt%collectedwhile oleicacidcollectedthe leastwitharound5.18wt% collected. Howevernone of the flotationsmanagedtoconcentrate any significantamountof gold. The resultsof the fire assaysshowedthatonlythe flotationwithnoadditive had collectedanygoldbutthe goldthat wascollectedwasbelow the lowerlimitof the scale (limitof scale is.00002 grams). Meaningthat the highestpossible grade wouldhave been0.018 t Oz/ton,this beinglessthanthe initial grade of the tailings. Initial Magnetic concentrates Unrepresentative floatation (natural) Unrepresentative flotation(w/oleic acid) Natural flotation Oleic Acid flotation Frother flotation Oleic+frother flotation Gravity FireAssay (t Oz/ton) 0.038 0.48 3.89 LOST >0.018 0 0 0 >0.018 Recovery (%) NA 88 NA NA NA 0 0 0 NA
  • 13.
    12 | Pa g e The last concentrationmethodwasgravityseparationperformedonawave table. Beingthatthe particle sizes were sofine,the lowestsettingonthe table was used. 2004.6 grams of tailingswere separatedand 91.2 grams were collectedwhichisaround4.55wt%. Unfortunatelythe resultswere not promisingasagainthe goldfrom the fire assaywas below the lowerlimitof the scale makingitshighest possible grade around0.018 t Oz/ton. Flowsheetandimplications- Thiswas the laststepof the design project.Basedonthe resultsof the concentrationmethodsaflowsheetyieldingthe greatestprofit wasconstructed aroundthe magnetic concentrationmethodasthiswasthe onlymethodtoshow promisingresults. Usingthe estimated valuesforcapital,horse power,costperton of concentrated,andvariousotherfigure eitherprovided by the team’scontact at Barrick Goldalongwithotherfigurescalculatedfromresearch(seeAppendix F for all economicinformation) aflowsheetwasconstructed. However,asitispossible toperformthe concentrationeitherwetordrya flowsheetforbothmethodsmustbe done andshownbetweenthe twowhichis more profitable bycalculatingbothflowsheet’sNPV andIRR. Flowsheetsforbothwetand dry processingisshowninfigure 6below while the NPV andIRRforbothare shownintable 3. Figure 6: Flowsheets for process. Top is wet and bottom is dry. Table 3: NPV and IRR NPV IRR Wet process $390 million 33.1% Dry process -$166 billion -5.27x1021 % Tailing dam Dredger Gold recovered AutoclaveMagnetic separator Acid leach New tailings dam Tailings Dredger Gold recovered AutoclaveMagnetic separator Acid leach Tailings New tailings dam Tailing dam Fluidbed dryer
  • 14.
    13 | Pa g e Environmental Context: One majorhurdle forthe designteamonce a concentrationmethodhasbeendecideduponwill be how to fitthe processingplantintoBarrick’sexistingstructuresatthe Goldstrike site.Ideally,the concentrationsite wouldbe asadjacentaspossible tothe damwhere the tailingsare currentlylocated. Thispart of the designprojectwill cause the teamtoevaluate geographical limitationsof the site fora newconcentrationfacility,andisbeyondthe scope of thisproject.Fromwhatthe designteamhas decidedonfora flowsheet (Figure 6) thoughthe environmentaleffectshouldbe minimal.Justlooking at the wetflowsheetthe onlyone newfacilitywill have tobe constructedtoaccommodate the new process. Anothermajorenvironmental issue wouldbe the effectof runoff from thisnew concentrationfacility. The chemicalsinthe tailingsrightnoware alreadya hazard to the environmentandadditional dangerouschemicals maybe needed toconcentrate the tailings.Thispotentiallycouldcreate arunoff that couldbe exceedinglyharmfultothe environmental.Topreventanyenvironmentdamage the team will have todetermine if the tailingsfromthe concentrationfacilitywill be able tomix withthe tailings Barrick will alreadyhave comingoff theirmainprocess.If notthe teamwill have todetermine awayfor Barrick to safelydisposeof thisrunoff withoutanydamage tothe environment. Whenconsideringthe three concentrationmethodsthe designteamtestedtheyall have theirown environmental effects.Firstfrothflotationprobablyhasthe worstenvironmental effectsof the three. Froth flotationprocessoftenusessome type of chemical tocollectcertainelementsorcause frothing. Froth flotationalsohasahighwater usage. Gravityseparationandmagneticseparationare muchmore kindto the environment.Neitherrequire anychemical additionstothe elementswhichmakesthe disposal of the tailingsverysimple.Inthe processthe designteamdecidedonusingawetmagnetic separatorisfairlyenvironmental friendly.The processusesnoharmful chemicalstohelpconcentrate the gold,and the concentrate will notrequire anyotherprocessesthatBarrickisnotalreadyconducting. The major environmental take awayisthatthe processdesignedbythe teamisnotcreatingany environmental threats. Global and Societal Context: On a global scale thisdesignprojectcouldhave huge implications.The mostnotable one couldbe the change of goldprices.The price of gold, isdetermined bysupplyanddemand.If thisdesignprojectis successful thenthere will be animmediateboosttothe supplyof goldwhichmaycause the price of gold to drop. Anotherglobal implicationthisdesignprojectmayhave or influence isBarrickandothermine companiescould mimicthe processingstrategydevelopedhereinandimplicate it attheirolderminesto extractgoldfrom theirtailings.
  • 15.
    14 | Pa g e There are some societal effectstobe considered if thisdesignprojectissuccessful.Mineshave afinite amountof ore,and when shutdown, canhave amajor economicblow onthe local community.If this designprojectissuccessful, thenitcouldincrease the lifeof the Goldstrikemine,whichmeansthata great employerforthe communitywill staylongercreatingmore jobs. Conclusion: Barrick initiallyapproachedthe designteamwiththe taskof analyzingthe economicfeasibilityof the 170 milliontonsof tailingsfromtheirGoldstrike mine,locatedinnorth-eastNevada.Barrickprovideda sizable representativesample of approximately200 poundsof solidsina slurry,shippedina55 gallon drum. Arsenic,mercuryandcyanide were listedonthe shipping label asbeinginthe sample prompting a meetingwith Ms. JerilynRobertsof campusEnvironmental HealthandSafetytodiscussthe safe handlingof the sample. The teamsetthe goal of havinginitial labresultscompletedbythe endof the semesterwithconcentrationteststofollow inthe spring. A smallerrepresentative sample wasacquiredbyfirstdecantingthe sample andthentakingacore using a PVCpipe fromthe middle of the drum. This sample wasconedandquarteredtocreate smaller samplesforvariousothertasks,these includedAAS, SEM,XRDand fire assays. The goal of these tasks was to learnpropertiesof the tailings suchasmineralogyand goldore grade. Mineralogywaslargelydeterminedthroughthe use of XRD whichindicatedhighpercentagesof silicates as well ascalciumand magnesiumcarbonatesinthe formof dolomite inadditionto trace amountsof othercompounds. Determinationof currentgoldore grade wassuccessful,utilizingbothAASandfire assaymethods. AASresultscame backwithan ore grade of 0.036 t Oz/ton. Thisissupportedbythe fire assayresultsof 0.038 t Oz/tonmeaningof the 170milliontonsof tailings about6.46 milliontroyounces of goldispresent,thishavinga presentestimatedvalue of $8.4billion. However,inorderforthe tailingstobe profitable toBarrick,the ore grade needstobe nearly doubled. Magneticconcentrationproved tobe the most efficientmethodof beneficiatingthe tailingsincreasing the grade to 0.48 t Oz/tonwithan 88% recovery. Howeverperformingthe processdryisnot economically feasibleasmoneyspentdryingthe tailingsexceedanyprofitmade.Butwithexperimental detailsprovidedbythe team’scontactat Barrick gold,itis possible toturna profitbymagnetically separatingthe tailingwet,thoughthislowers the recoveryfrom88% to 26%, for thisprocess the calculatedNPV andIRR forthisprocesswere about$390 millionand33% respectfully.
  • 16.
    15 | Pa g e Future Work: The overall projectistechnicallyfinishedbutwiththe limitedtime the teamhad,some of the testing was notdone as thoroughlyastheycouldhave been. The magneticseparationthatthe teamperformed was done drybut performingitwetwasalsoa desiredtaskasthe team’scontactat barrack goldhad done thisveryidea. The flotationsperformedshowedthatverylittlegoldwastrappedincarbonaceous material butflotationscouldhave beendone toisolate the maghemite thatthe goldwastrappedinand as thismethodiscommonlyusedtoconcentrate large amountsof low grade ores,like copperores,this couldhave provide verypromisingresults. If more time wasgiventhese tooideaswouldhave been exploredmore completely.
  • 17.
    16 | Pa g e References: Abols,J. A. P.M Grady, and P.M Grady. "MaximizingGravity Recovery through the Application of MultipleGravity Devices." (n.d.): n. pag. Gekkos.com. Web. "About Us." Barrick Gold Corporation.N.p., n.d. Web. 05 Dec. 2014. "Arsenic and Gold Mine Tailings FAQ." Arsenic in Historic Gold MineTailings(n.d.):n. pag. Arsenic and Gold Mine Tailings FAQ.Nova Scotia.Web. 5 Dec. 2014. Brennan, Ed. "History of Fire AssayingWhich Determines Precious Metal Content in Ores." History of Fire Assaying Which Determines Precious Metal Content in Ores. Assay One. Web. 21 Nov. 2014. <http://www.assayone.com/fire-assaying.html>. Bugbee, Edward Everett. A Textbook of Fire Assaying,. 3d ed. New York: J. Wiley & Sons;, 1940. Print. "Chemical SamplingInformation | Cyanide (as CN)." Chemical Sampling Information | Cyanide(as CN).N.p., n.d. Web. 05 Dec. 2014. "Cyanide Poisoning:Get Facts About Effects and Treatment."EMedicineHealth. N.p., n.d. Web. 05 Dec. 2014. "Environmental Health and Medicine Education." Arsenic Toxicity Case Study: WhatAre the Standards and Regulation for Arsenic Exposure?N.p., n.d. Web. 05 Dec. 2014. Fastand Associates LLC. "Gold MiningProcess Development." Denver Mineral Engineers. N.p., n.d. Web. 14 Dec. 2014. Gupta, A. Yan, D. S.. (2006). Mineral ProcessingDesign and Operation - An Introduction.Elsevier.3 March 2015 Han, Kenneth, and Yuhong Chen. "UsingFlotation to Separate Carbon Material fromCarlin Ore."(n.d.): n . pag. OneMine.org. Web. Honaker, R.Q. "A Fine Coal Circuitry Study UsingColumn Flotation and Gravity Separation." SciTech (1995): n. pag. Osti.gov. Web. Kellar,Jon. Met/EnvE 220 Mineral Processingand ResourceRecovery text part 15. Web. 9 Dec. 2014. <http://www.hpcnet.org/upload/attachments/425768_20080331055541.pdf>. "REGULATIONS AND ADVISORIES." 7. REGULATIONS AND ADVISORIES (n.d.): n. pag. Web. 5 Dec. 2014. Abols,J. A. P.M Grady, and P.M Grady. "MaximizingGravity Recovery through the Application of MultipleGravity Devices." (n.d.): n. pag. Gekkos.com. Web. Fastand Associates LLC. "Gold MiningProcess Development." Denver Mineral Engineers. N.p., n.d. Web. 14 Dec. 2014. Han, Kenneth, and Yuhong Chen. "UsingFlotation to Separate Carbon Material fromCarlin Ore."(n.d.): n. pag. OneMine.org. Web. Honaker, R.Q. "A Fine Coal Circuitry Study UsingColumn Flotation and Gravity Separation." SciTech (1995): n. pag. Osti.gov. Web. Richard A. Clarke. "Magnetic properties of materials".Info.ee.surrey.ac.uk. Retrieved January 15th,2015.
  • 18.
    17 | Pa g e Ramsaywok, P. "Influence of SalineWater on Electrostatic/High Tension Separation." Eighth International Heavy Minerals Conference. 2011.263-269.Print. Bittner, James. "TRIBOELECTRIC BELT SEPARATOR FOR BENEFICIATION OF FINE MINERALS." Metsco Proceedings 2013.Needham: Separation Technologies,LLC, 2013.1-10. Print. Oberrauner, A. "NEW DEVELOPMENTS ON ELECTROSTATIC SEPARATION OF FINES." XXVI International Mineral Processing Congress (IMPC) 2012 Proceedings. NEW DELHI: IMPC, 2012.Print. "Electrostatic Separation."Electrostatic Separation. MINE-ENGINEER.com. Web. 30 Jan. 2015. <http://mine-engineer.com/mining/minproc/elstat1.htm>. Weiss,Norman L. "Section 6: Electrostatic and Magnetic Separation." SME Mineral Processing Handbook. Vol. 1. New York, N.Y.: Society of MiningEngineers of the American Institute of Mining,Metallurgical,and Petroleum Engineers, 1985. 6.2-6.10. Print. Young, Courtney, and Gerald Luttrell. "Separation Technologies for Minerals,Coal,and Earth Resources." Google Books. N.p., n.d. Web. 17 Apr. 2015.
  • 19.
    18 | Pa g e Appendix A Figure 7: Colored image created using EDS showing areas of similar elemental composition. Figure 8: Separate colors representing different elements filtered out of figure 7.
  • 20.
    19 | Pa g e Figure 9: Overall spectrum of figure 7 image. Figure 10: Spectrum 7, BSE spectrum of a dolomite particle.
  • 21.
    20 | Pa g e Figure 11: Spectrum 12, BSE spectrum of an iron oxide particle. Figure 12: Spectrum 10, BSE spectrum of an aluminum silicate particle.
  • 22.
    21 | Pa g e Figure 13: XRD scan of representative Barrick tailings Table 4: Mineral names and formulas Mineral Name Chemical Formula Mineral Name Chemical Formula Quartz, silica SiO2 Sodium silicate Na2(SiO2)nO Calcite Ca(CO3) Sodium carbonate Na2CO3 Gypsum CaSO4·2H2O Litharge, Lead(ii)Oxide PbO Dolomite CaMg(CO3)2 Borax, Sodium borate Na2B4O7 · 10H2O Hematite Fe2O3 Magnetite Fe3O4 Maghemite γ-Fe2O3
  • 23.
    22 | Pa g e Appendix B Fire Assays Thoughthe processof Fire Assayingismanycenturiesold,withoriginsthatcanbe tracedas for back to the bible (Jeremiah6:27-30),it haschangedverylittle butisstill heldasthe mostaccurate methodfor determiningthe amountof goldinan ore sample. Infact, thismethodof analysisisheldwithsuchhigh regardthat it isconsideredthe “GoldStandard”of measuringgoldinoresand alloys. Toput itsimply thismethodisoftenusedbyanyone whowishestodetermine the amountof goldina particulargold baringsample. Because the BarrickGoldstrike teamof 2013-14’s entire designprojectrevolvesaround findingouthowmuchgoldis ina particularsample,itisimportantformembersof the teamto learn howto do thisanalysisina timelymanner. Sothe questionsof,“How doesitwork?How is it performed?”,will be answered,withrespecttohow the teamperformedit,inthe followingpassages. How does it work? Fire Assayingcomesinmanyformsbutthe teamdecidedtogowiththe crucible assaymethodasthisis the most commonform. The general theoryof thisprocessstartswiththe understandingthatthe majorityof ores,forthe mostpart, cannotbe fusedbutif the ore’saverage particle size issufficiently reducedandmixedwiththe rightratiosof appropriate reagents,the mixture will fuse ataneasily obtainable temperature.Next,itisassumedthatthe ore particlesare incontact withone or more particlesof litharge andreducingagent. Asthe temperature of the mix increasestoaround500o C, the litharge will begintobe reducedbythe carbon1 inthe mix and these reducedglobulesof leadcollectthe goldand silverformthe surroundingparticlebyalloyingwiththem. At about560o C,the borax of the charge beginstomeltand to formfusible compoundswithsome of the basesof the flux andore charge. Once thisbeginsthe slagbeginstoformrapidly. Forthe processto workproperly,the temperature of the slagshouldremainlow enoughthatitsdensityisgreaterthan lead, 10.66 grams percubic centimeter,andheldatthistemperature longenoughthatore particlesare thoroughlydecomposedandeveryparticleof goldandsilverhasbeencollectedbyanearbysuspended globule of lead. Once thishasoccurredthe temperature canbe raisedto a pointthat the slag’sdensity islessthan thatof lead,allowingthe denserglobulesof leadtosettle atthe bottomforminga single leadmass,calleda button.Itis presumedthatthe buttoncontainsnearlyall of the preciousmetal inthe ore. To separate the preciousmetalsfromthe lead,the buttonmustundergoaprocesscalledcupellation. The processof cupellationisverysimple,afterthe leadbuttonhasbeenbrokenoff fromthe slagandall of the slaghas beenremovedfromthe surface of the button,the buttonisplacedina furnace along witha preheatedcupel. At327o C the leadwill begintomeltandformleadoxides,mostof the lead oxide isthenabsorbedbythe cupel,whilethe restare vaporized.Thisprocesswill leave behindasmall beadmade of silverandgoldasthese twoelementsare notsoeasilyoxidized. The last twostepsbefore ameasurable amountof goldcan be obtaininvolve partingthe silverfromthe goldand annealingthe gold. Topart the silverfromthe gold,nitricacidisthe preferredchoice assilver 2 Carbon can come in many forms, for our project all-purposeflour was the carbon source
  • 24.
    23 | Pa g e iseasydigestedinthisacidwhile goldremainsstable. Once the silverhasbeenfullydissolvedandthe remaininggoldflake hasbeenwasheddried,the final stepisannealingthe gold.Thisstepdoesnothing more than make the goldeasiertohandle as the formthat it isinbefore annealingisverybrittle. Withthisfinal goldflake the fire assayiscomplete andthe goldcan be weighedtodetermine the equivalentTroyouncespertonbasedon equationtwobelow: Eq(1) Where: Mgold is the massof the goldfoundafterFire Assaying Msample is the massof the ore thatwas usedinthe Fire Assay. How is it performed? Onlyone fire assayhas beenperformedasof thistime sothisprocedure is onlyrelevanttothe measurementof the initial tailings. To performan intelligentfire assaythe ore’smineralogymustbe understoodtoadegree,inthe team’s case it’sthe tailing’smineralogythatmustbe understood. Because the ultimatesuccessof afire assay dependsonthe formationof slag. Thisreferstothe part mentionedearlierthatthe particlesbecome fusible ata relativelylowtemperatureaswell aspropertiesbasedonthe nature of the slag. A desirable slagwill be neutral innature as opposedtoacidicor basic. To accomplishthisgoal itmustbe first determine if the tailingswerecomposedof acidicorbasiccomponents,thenneutralizethemwithan opposingmix. FromX-rayDiffraction(XRD) analysis,itwasfoundthatthe tailingscontainedalarge portionof dolomite,whichisverybasicinnature. Knowingthis,E.Bugbee’s A TextbookforFire Assays was referencedtofindastartingmix that wouldbe appropriate fora basicore. A startingmix would usuallybe justthat,a mix youstart withand adjustuntil the resultsof the pourgave desiredresultslike a large leadbutton. By chance howeverthe initial mix workedquitewell. The mix calledforthe followingora 1 Assayton(30g ore sample): Figure 14: Charge mix for basic ore. Afteraddinga small beadof silvertothischarge,as the collectionof goldwiththe leadisbetterwith silverpresent, the charge wasallowedtoheatto 800o C for 45 minutes. The temperature wasthen raisedto 1050o C for an hour. The moltenassaywasthenpouredintoa cast iron mold,andallowedto solidify. Once cooledthe buttonwas brokenoff withthe use of a hammer,the hammerwasthenused to poundthe leadbuttonintoa cube so as to remove as muchslag as possible. Nextthis cube of lead was placedincupel thatwas sittingina 925o C furnace. 45-50 minuteswaslongenoughforthe leadto be oxidizedandabsorbedbythe cupel. The silverbeadthatwasleftinthe cupel wasthenallowedto cool before beingaddedtoa crucible of nitricacidfor parting. The acid was changedoutperiodically 29167 gold sample M M
  • 25.
    24 | Pa g e until the liquidnolongerturnedyellowasa resultof the silver. The final flake of goldwasthenrinsed, driedandannealed. The goldflake thatwasrecoveredfromthe original 154 grams hada massof about 0.0002 grams, whichwouldgive the initial tailingsagrade of 0.038 Troy ouncesperton. (See Appendix A forall mineralsandcompoundsmentioned)
  • 26.
    25 | Pa g e Appendix C Safety Guidelines Abstract In thisdesignprojectthere isaconcernabout the hazardousmaterial intailingsatthe Goldstrike mine. Thisreportwill firstdiscussthe levelsof arsenic,mercury,andcyanide andhow the levelsof these tailingscompare toOSHA’sacceptable levels. Secondly,thisreportwill coverthose concernsof these hazardousmaterial andpossible waysthatsomeonecouldgetthese hazardouschemicalsintotheir system. Introduction Safetyisan importantissue intoday’sworkplace.If youneedproof youneed notlookfartherthana company’swebsite.Everycompanynowadays hasa safetysectionwhere theypreachtheir commitmenttosafetyandhowtheyare takingcare of theiremployee’s.Forexample,here iswhat Barrick itself saidonitswebsite:“Usingbothqualitativeandquantitative methods,risks,suchasthose impactingthe health,safetyandsecurityof ouremployees,ourneighboringcommunitiesandthe environmentinwhichwe operate,aswell asouroperational andfinancialperformance are assessed and appropriate solutionsimplemented(1).”Soforthisdesignprojecttosucceedeffortsneedtobe made to limitanysafetyincidents.Thatiswhythisreporton the potential risksandhow to avoidthose riskswhenworkingwiththe tailingsfromthe Goldstrike mine. Arsenic Arsenicisa notoriouslypoisonousnaturallyoccurringelement.Arsenicusuallyoccursinthe environmentinalloyswithsulfurandmetalsandasa pure elemental crystal.Groundwater contaminationbyarsenicisa majorproblemthat affectsmillionsof peoplearoundthe world(3). In the Goldstrike tailingsarsenichasthe highestconcentrationof the three hazardouschemicalspresent at approximately800ppm.The arsenicwouldnothave beenaddedduringthe mineral processingsoit can be assumedthatthe ore source at Goldstrike hassome arsenicinthe ore.Thismakessense consideringarseniciscommonwithgolddepositcontainingsulfideminerals. OSHA has specificguidelinesonwhatisanacceptable amountof arsenicinthe work place.Inthe general workplace OSHA says,“The permissible exposure limitforarsenicisnogreaterthan 10 microgramsof inorganicarsenicpercubicmeterof air,averagedoverany 8 hour periodfora 40 hour workweek(2).”The sample of tailingscame in a 55 gallondrumwhichisapproximately0.21m3 . Calculatingitoutthere couldpotentiallybe 10.5 microgramsina lab room, but consideringa considerable amountof arsenicwasremovedduringdrainingthe slurry,andthe designteamisnot workingwith the whole tailingsatone time. There are some concernswhenworkingwitharsenicina labsetting.Arsenicisonlydangeroustoa personwhenitisingested.Inmostcasesof poisoningthe arsenichasfounditswayintothe ground
  • 27.
    26 | Pa g e waterand poisonedthe people whowere drinkingfromthatwatersource (3). Ina labsettingsomeone workingwitharsenicstill hastobe verycareful.Whenworkingwitharsenicitisimportanttowear gloves,longsleeves,pants,longpants,andabreathingmask.The breathingmaskwill stopanydust particlesof arsenicfrombeingingested.If someone doesgetthe slurryontheirskinthe arsenicwill not be a problem,butitisimportantto washthe area thoroughlybecause if the arsenicgetsonyourhands it can laterbe transferredtofoodwhenyouare eating. Mercury Mercury isanothernatural occurring elementlike arsenicthatcan be justas dangerous.Mercuryis used withcyanide ingoldprocessingtoseparate the goldfromthe otherelementsthatare in the ore. Mercury poisoninghappenedoftenbefore people understoodthe dangersof it.Incurrenttimesmost mercurypoisoningoccursmostlybecause of mercuryinfish. Mercury has one of the smallestconcentrationinthe tailingsat4 ppm. Mercury wasusedfor gold processinginthe earlyyearsof mineral processing,buttodayall mercuryintailingswouldcome from the ore.The concentrationisprobablyaslow as itis because Barrickwouldhave tosafelydispose of so much of theirmercuryto avoidrun off of dangerouschemicalsintothe environment. Like arsenicOSHA has standardsforwhat isan acceptable Mercuryis dangeroustoingest,butunlike arsenicitcan be absorbedthroughthe skin.OSHA onlyallowsaconcentrationof 0.1 mg/m3 (4). Consideringthe areathe teamwill be workinginisaround16 m3 there isa potential riskof 0.25 mg/m3 inthe lab.The actual concentrationof Hg inthe air will be lowerthanthat.The level shouldnotbe in the dangerzone,but toavoidany riskof breathing,ingesting,orabsorptionthe teamwill follow the same skinprotectionasfor arsenicanda face mask. Cyanide Cyanide isa compoundthatlike the othertwoelementsdiscussedinthisreportishazardousand extremelydangerousif notproperlyhandled.Like mercury,cyanide isusedformineral processing specificallypreciousmetalslike goldandsilver.Cyanide iscombinedwitharichore source andthe cyanide anionsattackthe ore creatinga pregnantheapleach. Cyanide’sconcentrationcouldnotbe determinedexactly,but the concentrationislessthan20 ppm. While cyanide poisoningcanoccur throughinhalationitwouldtake alarge amountof cyanide tofeel any immediateeffects.Cyanide isanatural occurringchemical intobacco socigarettesliberate cyanide duringsmoking(6).The mainriskfor cyanide isabsorbingtoomuchof the compoundthroughyourskin, or throughinhalationinthe gaseousform. OSHA doesnot have any standardsforan air mediumforcyanide,buttheydohave an allowable limit for skincontact.The allowablelimitforcyanide is5 mg/m3 (5).Assumingthe concentrationof cyanide is 20 ppm ina 0.21 m3 containerthenthe concentrationwouldbe 95.2 mg/m3 .So withthatpossible concentrationglovesandproperclothingatall timesisamust whenhandlingthe tailings.
  • 28.
    27 | Pa g e Conclusion Withsafetybeingsucha major concernintoday’ssocietyitisimportantto do everythingtoreduce risk inthe workplace.Inthisreport the three hazardouschemicalsinthe tailingswere examinedand explainedhowtheycanaffecta person.The OSHA safetystandardswere consideredalsoandexamined if the team wouldbe at riskof any of these materials.The guidelinesforthose three materialswere laid. The basics of the safetyguidelinesare toweargloves,appropriateclothing,andabreathingface mask.If these guidelinesinplace the designteamcanruntestssafelyworkingwiththe tailingswithoutworryof personal safety.
  • 29.
    28 | Pa g e Appendix D Mineralogy of Goldstrike Tailings (see Appendix A for relevant figures) Introduction The mineralogical makeup of the tailings body is extremely important information to have for this project. Depending on what makes up the tailings largely effects the specific concentration techniques used and as well as the parameters of these techniques. The tests that we performed included using a scanning electron microscope (SEM), and x-ray diffraction (XRD). The group has samples prepared for use with micro-focused x-ray computed tomography (MXCT) and is expected to get results early next semester. Most of what has been found thus far is purely qualitative. SEM SEM was the first test the group performed on the tailings. A small amount was dried and then mounted on carbon tape to be examined. SEM uses beamed electrons to gain information about a sample’s topography. It also has the ability to use energy-dispersive x-ray spectroscopy (EDS) to create colored images showing particles of similar elemental composition (figure 7). The image can then be filtered by color layer for ease of viewing. As can be seen in figure 8 and 9, the main elements detected were aluminum, calcium, iron, magnesium, oxygen, potassium, silicon and sulfur. Carbon is skewed due to the carbon tape the sample is mounted on. The carbon tape also nulls the Wt. % measurements given by the spectrums. One thing to note is the two distinct iron particles as well as the large presence of particles with both calciumand magnesium. The EDS scans also indicated fair amounts of gypsum and silicates. As can be seen on figure 3, individual point scans were taken and used to create elemental spectrums (Figures 10-12) using back-scattered electrons (BSE). Spectrums 8, 9 and 11 all hit the carbon tape resulting in a skewed spectrum and were not included on this report. Spectrum 7 was placed one of the particles that was high in calcium, magnesium and oxygen, most likely a dolomite particle (CaMg(CO3)2). Spectrum 10 had high amounts of silicon, aluminum and oxygen suggesting an aluminum silicate (Al2SiO5) particle. Spectrum 12 was placed on the particle that scanned high in iron. SEM is great for qualitative elemental data but fails to provide a true quantitative mineralogical breakdown. The second test we performed for mineralogy is an XRD scan.
  • 30.
    29 | Pa g e XRD XRD uses similar technology to SEM but instead of electrons being used, x-rays are used. Beams of x-rays are shot at the sample and will diffract in different directions. By measuring the angles of diffractions as well as the intensities, an overall spectrum is obtained and minerals can be identified. An initial grab sample was taken before the tailings could be divided into representative parts. It indicated that quartz was in the largest concentration followed by gypsum, mica and dolomite. The results of the XRD coincide with what was found with the SEM. Later in the semester, when several smaller representative samples were prepared, another XRD scan was performed (figure 13). Dolomite appeared in the largest quantity followed again by quartz and calcite. A new mineral also appeared in this scan, pseudobrookite (Fe2TiO3). Curiously, no iron-oxide particles such as magnetite or hematite were detected, raising questions about the magnetism of the tailings. Challenges One of the group’s current concerns that eludes the project is why the tailings are so magnetic. An initial magnetic separation test returned almost 90% weight of magnetic concentrates. According to Barrick, only 3-5% Wt. % of the tailings should be magnetic. Using SEM the group was able to observe particles that contained iron-oxide particles. These iron-oxide particles did not show up in XRD scans. A theory to this discrepancy is that the iron minerals are amorphous. Due to the lack of a consist shape, XRD is not reporting them in high concentrations. The use of MXCT early next semester will hopefully answer these questions about the iron in the tailings. This will be useful as magnetic separation could be an effective concentration process. Conclusion The mineralogy of the tailings is invaluable data to have when designing a concentration flowchart. Using technologies such as SEM and XRD, as well as MXCT in the future, the group has identified what makes up the majority of the sample body. Most of the data collected thus far has been qualitative in nature. The mineral in the highest concentration is quartz followed by dolomite. Other silicates, carbonates and sulfides make up most of the rest with iron based minerals supposedly in low concentrations. Samples have been prepared for use with MXCT and are scheduled to be run early next semester.
  • 31.
    30 | Pa g e Appendix E Processing Gold Tailings Introduction The tailings of any mineral processing will retain some amount of desired material, however the amount left over does not hold enough value to justify further recovery, but if it the value of the material increases or newer technology allows for easier recovery, then further processing may be justified. However, the tailings will have a majority of undesired material, and any gold that is left, will be sporadically dispersed throughout the ore, or trapped in another material. For this reason, it becomes necessary to concentrate the desired material, reducing time and money wasted on the portion of the tailings containing little to no gold. There are many ways to concentrate ore bodies and all of them take advantage of the ores physical and chemical characteristics, this is generally known as beneficiation. For this design project, the group will focus on concentrating gold by employing the most available and commercially used methods, gravity separation, froth flotation, and magnetic separation. Gravity Separation The first process used to beneficiate the gold tailings that will be considered, is gravity or density separation. Gravity separation is an environmentally friendly process, which utilizes simple equipment with few moving parts. (Abols) Throughout the history of mineral processing, many different types of gravity separation devices have been utilized, but the one the team will use is a wave table. Gravity concentration processes rely on the principal that gold contained within an ore body is higher in specific gravity than the material it is trapped in. Elemental gold has a specific gravity of 19.3, and typical ore has a specific gravity of about 2.6. (Fast) Movement is created by gravity concentration devices between the gold and host rock particles so that the heavy particles are separated from the lighter particles. On a wave table, particles will be fed onto the machine along with a stream of water. The bed of the table will begin to shake resulting in two events, the formation of wave in the water and the gradual forward movements of the particles towards a trough at the end of the table. While the particles are in contact with the surface of the table, the shaking will cause them to move forward. When a wave passes over them, less dense particles will be moved backwards while denser material are either not moved, or move very little resulting in a separation of the particles by their respective densities. See Figure 1 in report. Ideally gold and gold bearing material will move into the trough of the table but the team will likely collect all of the material closest to the top for testing. One disadvantage of gravity separation is that ultrafine clay particles are not easily rejected. (Honaker)
  • 32.
    31 | Pa g e Froth Floatation The flotation process consists of producing a mineral concentrate through the use of chemical conditioning agents followed by intense agitation and air sparging of the agitated ore slurry to produce a mineral rich foam concentrate. Specific chemicals are added to either float specific minerals or to depress the flotation of other minerals. Several stages of processing are general involved with rough bulk flotation products being subjected to additional flotation steps to increase product purity. Generally, the flotation process does not float free gold particles, but it is very effective when the gold is associated with sulfide minerals. Highly oxidized ores usually do not respond well to the process. An advantage of the flotation process is that the gold is usually liberated at a coarse particle size, near 28 mesh, reducing the grinding cost. Also, the chemical reagents are generally not toxic allowing for easy disposal. (Fast) A majority of the gold within the tailings received from Barrick are trapped in iron oxide and carbonaceous material. The tailings come from Barrick’s Goldstrike mine which is located on the Carlin Trend in Nevada. With this knowledge, a specialized process can be used with a specific collector and dispersing agent. Dr. Kenneth Han and Dr. Yuhong Chen, performed a study on how to remove the carbonaceous material from Carlin ore. The results of this investigation showed that it is possible to use froth flotation, and that a Philips aromatic oil served to be the best collector. One of the reasons for poor flotation recovery was due to oxidation of the carbonaceous material. Therefore, flotation recovery was significantly improved by subjecting the oxidized ore to reduction before flotation. Sodium silicate proved to be an effective dispersing reagent for the Carlin gold ore. The effect of pH on flotation was insignificant. The flotation response at room temperature was as good as any other temperature between 15° and 60° C. (Han) This study will prove to be very beneficial to the team moving forward. With the knowledge of the specific collector and dispersing agent, the floatation of the tailings can be greatly improved. Magnetic Separation With the majority of the gold being trapped in some form of iron oxide, magnetic separation will most likely, prove to be very useful. The concept of magnetic separation is a very simple process. A feed stream is passed through magnetic field, the magnetic attraction force between the magnet and magnetic particles will then pull these particles, separating them from the rest of the stream. See Figure 3 in report for illustration. Additionally because materials are either very magnetic (ferromagnetic), slightly magnetic (paramagnetic) or non-magnetic (diamagnetic), a feed stream can be effectively beneficiated by either adjusting the strength of the magnetic field, the speed at which the particles pass through the magnetic field or both. However information about the tailings suggested that the entire mass was only composed of about 3-5% iron oxides, so the results from the team’s attempt should be near this range.
  • 33.
    32 | Pa g e Conclusion While there are many ways of beneficiating the gold, these three processes are most readily available. Each method has its own advantages and disadvantages. Using a combination of these complimentary processes can improve the overall recovery of gold from the tailings. Each method works best in certain situations and under certain conditions. With the right knowledge of the tailings mineralogy and chemical composition, the recovery can be maximized.
  • 34.
    33 | Pa g e Appendix F Table 5: Economic details discountrate (I) 0.05 numberof years 10.78 $/kWh 0.06 operates80% of year 0.8 Tailings(tonage/day) 54,000 Concentrateswt%/100 0.05 kW/hp 0.75 personel 8 ore grade gold(tOz/ton) wet 0.2 goldrecovery%/100 0.75 goldprice $/troy 1,300 Facitity'shp 1,500 Dredgerhp 8,500 autoclave & acid$ /ton 90 personel salaraies/year 150,000 Maintenance %total cost/100 0.05 AC powerkWbasedon square footage 8,900 lightingpowerconsumption 120,000 square footage 2,000,000 dryerhp 5 Dryer PowerBTU 1E+12 BTU/ton coal 24,000,000 $/ton coal 70 ore grade gold(tOz/ton) dry 0.48
  • 35.
    34 | Pa g e Figure 15: Spreadsheet for wet process Figure 16: Spreadsheet for dry process Initial Costs $ Annual costs $ Annual prophits Facity construction 210000000 Autoclave and acid 70956000 Gold 153738000 Dredger 2000000 Facitity's hp 470327.8233 personel salaries 1200000 Dredger hp 2660958.048 maintenance 983181.1769 Lighting 117761.28 AC 3751392.505 total 212000000 80139620.83 153738000 Wet process NPV ($390,107,433.16) $0.00 IRR 0.331286996 Initial Costs $ Annual costs $ Annual prophits Facity construction 210000000 Autoclave and acid 70956000 Gold 307476000 Dredger 2000000 Facitity's hp 470327.8233 Fluid bed dryer 21875000 personel salaries 1200000 Dredger hp 2660958.048 maintenance 1084629.706 Lighting 117761.28 AC 3751392.505 Dryer hp 39193.98527 Dryer power 20440000000 Total 233875000 20520280263 307476000 Dry process NPV -$165,594,577,635.91 #NUM! IRR -5.26879E+23
  • 36.
    35 | Pa g e Appendix G Electrostatic Separation Accordingto the SME Mineral ProcessingHandbook,electrostaticseparationisdefinedasthe selective sortingof solidspeciesbymeansof utilizingforcesactingonchargedor polarizedbodiesinanelectric field. Thisdefinition,atface value,sounds alotlike magneticseparation. Infact if someone were to watch bothof these typesof separatorsinactionhe maybelieve theywere the same. Despitethe visual similaritiesthese processingdevicesmaysometimesshare,the onlyreal similaritytheypossessisthat theybothtake advantage of electrons. Whilemagneticseparationusesthe differingmagnetic propertiescreatedbythe numberof spinningunpairedelectronsalreadypresentinthe material to cause a separation,electrostaticseparationusesthe diffingelectrical chargescreatedfromthe number of and/orpositionof electronsonamaterial tocause separation. Howeversince the forcescreated fromthe attractionof opposingelectrical chargesare normallymuchweakerthanthose producedby magnetismthe effectiveparticle size forelectrostaticseparationismuchsmallerthanmagnetic separation2 soitis notnormallyusedasa primaryseparationmethodbutratherasa recoverymethod followingmostmineral processingandendproductproducingsteps. Inthe followingsectionsthe conceptsbehindhowelectrostaticseparationworksalongwithtypical designsandapplicationswillbe discussedingreaterdetail. Theory BehindElectrostatic SeparationandBasic Components The primaryforce usedinelectrostaticseparationsisbasedonCoulomb’sLaw whichstatesthat, 1 2 2 4 o o q q F r r        ur r EQ 2 where:q1 and q2 are magnitude of the twocharges,in coulombs. r the distance betweenthe twocharges,inmeters. or r isa unitvectoralong r r . o is knownasthe permittivityof free space withavalue of 8.85 X 10-12 farads/meter. F ur is expressedinNewtons. Since the force betweentwochargedbodiesdependontheirmagnitude,itbecomesconvenientto define anelectricfieldintensity,E ur withunitsof volt/meters,asthe force perunitcharge exertedbyone charge onanotherand withEQ 2 thiscan be expressedas, 1 2 2 4 o o q rF E q r   rur ur EQ 3 Withthisequationitispossible todefineaunitof work perunitcharge involvedinmovingone movinga charge fromone pointinan electricfieldtoanother. Thisunitof workperunitcharge inknownas the 2 Effective particlesizefor electrostatic separation is 200-40 microns,magnetic separation istypically used for particlesizes of 152-.5 mm or greater.
  • 37.
    36 | Pa g e difference inelectrostaticpotentialandissymbolizedby V andexpressedaccordingtothe following equation, V E dr   ur r EQ 4 where the negative signmeanthe V isincreasingwhenworkisdone onthe charge. For a conductive material EQ 4 can be expressedas, 2 1 2 0 0 1 2 1 1 4 4 r r q dr q V r r r            EQ 5 But for nonconductinganddielectricmaterialEQ5 can be simplifiedtoeitherof the followingequations dependingonwhatconditionisconsidered. 04 q V r  EQ 6 0 1 4 rq dr V r   EQ 7 Where EQ 6 isvalida reference pointisconsideredinfinitelyfarawayandEQ 7 isvalidif charge q is due to distribution—thatistoa charge rq (C/m3 ). Nowthat the theorybehindelectrostaticseparationhasbeenexplainedlet’sfocusonthe basic componentsof the methodof separation. All electrostaticseparationsystemshave atleastfourbasic components,those beingacharging-dischargingmechanism, anexternalelectrical field,anonelectrical particle trajectoryregulatingdevice andafeeding/productcollectionsystem. A charging-dischargingmechanism Perhapsthe mostimportant component,the mechanismbywhichthisbeneficiationmethodcharges and/ordischargesitsparticleswill resultinone of the followingcategoriesof charge distribution: 1. Particlesof twodifferentspeciesenteranelectricfieldinaseparatingzone bearinganelectric charge of opposite sign. 2. Particlesof twodifferentspeciesenteraseparatingzone where onlyone type of particle bears a significantelectrical charge. 3. Particlesenterthe separatingzone,suchthatparticlesof differentspeciesbearthe same sign charge,but the magnitudesof the electricchargesare significantlydifferent. 4. Particlesof differentspeciesenterthe separatingzone withsignificantlydifferentdipole moments. An external electrical field For thisseparationmethodtoevenbe possible anelectricfieldisrequired. The electrical potentialsof these fieldsvarygreatlybuttypicallyrange from10 to 100kV andare normallyunidirectional.The fields themselvesrange from400kV/mto the breakdownstrengthof airwhichisaround3000kV/m. A nonelectrical particle trajectory regulatingdevice The physical separationof twodifferenttypesof particlesisalwaysmade byadjustingthe forcesand time overwhichthe forcesact on a particle,suchthat at a predeterminedtime the trajectoriesof the
  • 38.
    37 | Pa g e twoparticle typeswill have differenttrajectories. Howeverinadditiontoelectrical forcesitmaybe advantageoustoutilize otherforcessuchasgravity,andfrictiontoaid inthe selective sorting. A feeding/productcollection system Thisis a feature commontoall separators.A separatormusthave some meansof conveyingthe feed material tothe separationzone inadditiontoa methodof cuttingthe streamof particlesata desired pointso thatdifferentparticletypescanbe collectedandconveyedtoa followingseparationdevice or productstorage tank. Typical Designs andApplications As mentionedbeforethe charginganddischargingmechanismispossiblythe mostimportant componentof an electrostaticseparatorasthiswill dictate the machinesoverall designandapplication. There are manywaysto electricallycharge particlesforseparation,butonlychargingby contactand frictional electrification,ion or electron bombardment andconductiveinduction are usedcommercially. Howeveritshouldbe made clearnowthat these mechanismsare notmutuallyelusive foreachdesign, infact it’smore thanlikelythatatleasttwo of these mechanismsare presentatanygiventime foreach separationdesign,the distinctioncomesfromwhichchargingmethoddominates. Contact electrification Chargingby frictionand/orcontactelectrificationisthe mostfrequentlyusedmechanismforseparating twospeciesof dielectricmaterial.Some of the typical examplesbeingfeldsparfromquartz,quartzfrom apatite andhalite fromsylvite.Butthisis notto sayconductive material wontcharge thisway,because theywill,it’sjustthatsince theyare conductive itismore likelythatthese particlewillrelinquishtheir charge before theyare separatedfromthe feedstreamnegativelyeffectingthe results. The theorybehindexactlyhowchargingaparticle bycontact isverycomplex andis not fully understood,butconceptuallyit’srelativelyeasytoexplain. If yourememberbacktowhenyouwere a childandyou wouldruban inflatedballoonagainstyourheadthenlifteditawaytosee your hairnow standingonendtryingto followthe balloon,wellthat’sbasicallywhatishappeningbetweenthe particlesof thischargingmechanism.Yousee some dielectricparticleswill more readilygive uptheir surface electronstoanotherdielectricparticle once contacthasbeenmade leadingthemnow to become positivelyandnegativelycharged. Thoughthese particle are now oppositelychargedthe coulombicforce betweenisstill weakenoughthatmomentumfromthe initiallycollisionwill separate themand/orthe coulombicforcesbetweenthe particlesandthe electrical fieldgivenoff bythe electrodeswill attractthe oppositelychargedparticle whilesimultaneouslyrepellingthe likecharged particle leadingtothe desiredseparation. A typical designisthe Free-Fall separatorasseeninfigure 16 below.
  • 39.
    38 | Pa g e Figure 3: Free-Fall Separator As seenfromthe figure,aFree-Fall separatorconsistsof ahopperanda vibratingfeederwheremuchof the change of charge throughcontact will occur.The feedwill thenemptyintoachute thatleadsto an emptycolumnsurroundedbychargedelectrodes. While gravityisthe force movingthe particles throughthe machine the coulombicforcesfromthe chargedelectrodescause the separationbypulling the oppositelychargedparticleduringtheirfall.Howeversince the effectivenessof separationcanvary drasticallydependingonmaterial of feed,particle sizes,temperatureandpressure,aFree-Fall separator isoftencustomizedaccordingtothese variablesbychangingthe feedrate,electricfieldintensityand/or lengthof fall betweenthe electrodes. Charging by corona Chargingby ionor electronbombardmentisoftendone withacoronadischarge. These typesof separatorsare bestsuitedfororeswitha mixture of conductive andinsulatingmaterial. Common applicationincludeseparatingrutileandilmenite frombeachsandbutanothercommonuse isto recovervaluable finesfromvariousmineral processingandfabricationplantsthatescape intothe air. To charge bycorona solidsmustpassthrougha corona discharge froma wire electrode toagrounded surface. A corona discharge occurswhenthe electrical potential of anelectrode israisedtothe point that the electrical fieldinthe immediate areaexceedsthe electrical breakdownstrengthof air,this internscreateschargedionswhere half are repelledbythe electrode andbombardthe feedparticles thuscharging them.The mosttypical designforthistype of electrostaticseparationiselectrodynamic separatoror as it ismore commonlyknownasthe High-Tensionseparatorwhichcanbe seeninFigure 17 below.
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    39 | Pa g e Figure 4: High-tension separator. Diagram of separation in progress is also shown How thistype of machine causesa separationconceptuallyspeakingismoderatelysimple. Thismachine takesadvantage of three forcesgravity,centripetalandcoulombic.The particle are feedontoaroller that iselectricallygroundedwheretheyare chargedbya corona. The conductive material quickly transfertheirelectrical charge tothe groundedrollerandwithnocoulombicforce tokeepingitattached to the rolleristhrownoff ina predictable trajectorywhere itiscollected. The nonconductive material will give theircharge tothe groundedrollertoobutat a much slowerrate meaningthese particle will remainattachedto the rollerwithcolumbicforce until the charge decreasesenoughforgravityand centripetal forcesbreakthe connectionorthe particlesare brushoff at the backof the roller.These particle alsofollowasomewhatpredictabletrajectorysotheyare collectaccordingly. The collectionbin that isdesignatedtocollectbothnonconductive andsmallerconductive material will be processedagain at a differentrate toimprove separation. Charging by conduction Whenseparationisdesiredforamixture of conductive anddielectricmaterialschargingbyconduction isoftenpreferred. Thismethodistypical usedasafollowingseparationstepforthe conductivematerial collectedbythe coronamethodwhile processingbeachsand. As the name suggestmaterials are chargedbyconductionmeaningthatthe particle mustcome in contact witha chargedsurface.Thisis the beginningof the separationprocessusingthismethod. As the particlesare chargedtheyare simultaneouslypassingthroughanelectricfield. The conductive material will charge almostinstantlyandthiswhenthe coulombicforcesof the electrical fieldtake over causingthe separation. The dielectricparticle will charge byconductionbutat a much slowerrate so before theyare sufficientlychargedtheycome outof contact withthe conductingsurface andare thusly unaffectedbythe electricfield. Twoof the more common designsforthismethodare Roll-type separatorand the Tobogganseparatorseeninfigure 18 below.
  • 41.
    40 | Pa g e a) b) Figure 5: a) Roll-type separator. b) Toboggan separator For the Roll-type separatorthe conductingsurface isthe rollersothisdesigntake advantage of gravity, centripetal andcoulombicforcestocause a separation,while the Toboggandesignhasaconducting plate initscute where the particle descend meaningthatonlygravityandcoulombicforcesare usedto cause a separation. Asyoucan image withthese designalotof chargingby contact ispossible sothe actual chargingmechanismiscomplex anddifficulttopredictbutsince the conductionwasthe only intendedmechanismandthe predominantone these designsare notconsideredacontact electrificationmethod.
  • 42.
    41 | Pa g e Appendix H Gravity Seperation INTRODUCTION Gravityseparationisa commonextractive technique forfreeinggoldfromitsgangue.Whenconsidering gravityseparationyouhave tofigure outthe specificgravityof the desiredelementandthe surrounding gangue.Gravityseparationwasone of the possible extractive techniquesthatcouldbe usedtoprocess the tailingsatthe Goldstrike Dam.To determine if the goldinthe tailingscanbe recoveredusinggravity separationthe teamhasto determine concentrationcriterionusingthe specificgravityof the goldand the gangue.Thispaperwill alsocoversome of the historyof gravityseparation,the benefitsand disadvantagesof gravityseparationconcentration,andcommonwaystouse gravityseparationfora large scale miningoperation. HISTORY Gravityseparationisone of the oldestformsof ore concentrations.The ancientEgyptiansdepicted gravityseparationin3,000 BC. Theynoticedhow weatheringcausedbywaterthatcreatedalluvial depositsof metal andgemstones.Excavationshave alsounveiledthatmultiple ancientcivilizationshad miningoperationsusinggravityseparation(Guapta).Anothercommontype of gravityseparationisgold panning.Panningwasasimple techniquethatdrove manysettlerstoCaliforniainthe 1849 goldrush. Panningobviouslyisnotusedinlarge scale miningoperationswithmore effective andchemical techniquesbecomingmore available.Gravityseparationhasobviouslyhadamajor effectonhow the worldtodayhas beenshapedbypreviousgenerations. BENEFITSAND DISADVANTAGES Gravityseparationisa concentrationmethodwithmanybenefitsanddisadvantages.Inideal ore sourcesgravityseparationisthe bestmethod.Whenanore source containsfree goldparticleswitha gangue that has a lowerspecificgravitythangold.However,manyof these ore sourceshave been depletedbelow economicfeasiblelevels.There are betterrecoverymethodslike goldleaching,but these concentrationmethodsuse harmful chemicals.Dealingwiththese harmful chemicalslikecyanide are hard andexpensivetogetridof them.Gravityseparationonthe otherhanddoesnot needany chemicalsinorderto separate goldfromthe gangue. LARGE SCALE OPERATIONS
  • 43.
    42 | Pa g e There have beenrecentdevelopmentsingravityseparationtomake ita more feasible forlarge scale miningoperations.One of these advancementsisthe KelseyCentrifugal Jig(KCJ).KCJcombinesjigging technologywithcentrifugal forcesthatgreatlyimprovesongravityseparation.The KCJusesarotating wedge wire screenwithabedof raggingmaterial onthe screen.The range of feedfora KCJ is10 to 500 micrometerswithdifferentmineralslike gold,tin,chrome,andiron(Luttrell,Young).Another developmentwasthe KnelsonBatchCentrifugalConcentrator(KBCC).The KBCCisan outwardlyinclined, walls-spinningcentrifugebowl withriffles.The spinningbowl spinsthe feedslurryandthenseparates and recoversminerals.The KBCCisable toseparate 10 to 10,000 tonnesperhourdependingonthe feed.The KBCChas beenusedtoseparate gold,platinum,andgold-copperores(Luttrell,Young).With these andmanyother advancementsgravityseparationhascome alongway fromgoldpanningand offersthe optionforlarge scale miningoperations. DETERMING FEASABILITYOFGRAVITY SEPERATION In orderto determine if gravityseparationisapossible concentrationmethodthe designteamneedsto take a fewthingsintoconsideration.The firstthingtoconsideristhe concentrationcriterion.The calculationforconcentrationcriterion(CC) isshownbelow inFigure 19.Giventhe gangue of density 2650 kg/m3 goldhas a CC of 10.3 and an ironore like hematitehasaCC of 2.5 (Guapta).However,the goldmineral inthe tailingsare coveredinanironoxide knownasmaghemite.Sogiventhatfactitis unlikelythatthe goldmineral wouldhave ahighenoughCCthatwouldallow forgravityseparationand withan ironoxide combinedwiththe golditismuchlikelierthatmagneticseparationwill be amore successful concentrationmethod. Figure 19: Calculation for Concentration Criterion (Guapta) To testif gravityseparationwaspossible the teamusedthe wave table inthe MIBuildingatSchool of Mineson a sample of tailingstosee howmuchgoldcouldbe recovered.ShowninFigure 20 isa representationof howawave table usesthe specificgravitytoseparate goldfromthe gangue.Asthe wave table shakesbackand forththe table movesthe goldforwardwhile the watermovesthe gangue back creatinga separationof the two.Whentestingthe teamwasable to recover4.55 wt%of the testedsample of tailings.Howeveronfurtheranalysisusingfire assaythe goldamountwasbelowthe lowerlimitsof the test.
  • 44.
    43 | Pa g e Figure 60: Representation of wave table. Yellow dots show gold and black shows gangue movement CONCLUSION Gravityseparationisa verysimple concentrationmethod,butunderthe rightcircumstancescouldbe veryeffective.The technique of usingthe specificgravityof a mineral toconcentrate ithasbeenused for hundredsof yearsandhas done a lotto shape the worldwe know today.Overthe years advancementshave beenmade inthe conceptof gravityseparationwhichhasallowedaccesstomore and more mineral sources.Throughsimplethinkingandcommontestproceduresthe designteamhas shownthat gravityseparationtechnique wouldbe difficulttouse forconcentratingthe goldtailings. Therefore,ithasbeendecidedthatdespite possible benefitsof gravityseparation,itwouldnotbe the bestconcentrationmethod.
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    44 | Pa g e Acknowledgments: A special thankstothe followingpeople anddepartments: Dr. Ed Duke for runningthe SEM ontailings. Dr. Jon Kellarforadvisingthe team. Alex Wulff forMXCTanalysisontailingparticles. RussLingenfelterforperformingthe XRDanalysisandteachingmembersof the teamhowto performfire assays.