MagnesiaCements
FromFormulationtoApplication
MarkAlexanderShand
PremierMagnesia,LLC,Waynesville,NC,UnitedStates
AbirAl-Tabbaa
UniversityofCambridge,Cambridge,UnitedKingdom
JueshiQian
ChongqingUniversity,Chongqing,China
LiwuMo
CollegeofMaterialsScienceandEngineering,Nanjing TechUniversity,Nanjing,Jiangsu,China
StateKeyLaboratoryofMaterials-OrientedChemical Engineering,Nanjing,Jiangsu,China
FeiJin
UniversityofGlasgow,Glasgow,UnitedKingdom
Elsevier
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TypesetbyVTeX
0Introduction–CharacterizationofMgO
MarkAlexanderShand,FeiJin
0.1Introduction1
0.2ImportantcharacteristicsofMgOandtestmethods2
0.3RelationshipsamongthemostimportantpropertiesofMgO5
0.4CategorizationofMgO6 References9 PartOneExistingmagnesiacementtechnologies
1Manufactureofmagnesiumoxideformagnesiacements
MarkAlexanderShand
1.1Magnesiumoxideproducedfrommagnesite13
1.2Formationofmacrocrystallinemagnesite15
1.3Formationofcryptocrystallinemagnesite15
1.4Magnesiumoxidederivedfrombrucite15
1.5Miningandprocessingofmagnesite16
1.6Syntheticmagnesia18
1.7Calcinationofmagnesiumcarbonateandmagnesiumhydroxide22 1.8Furnacesandkilns24 References28
2Magnesiumoxychloridecement
FeiJin
2.1Introduction29
2.2Phasecompositionandmicrostructure30
2.3Reactionmechanismandkinetics35
2.4PropertiesofMOC38
2.5CommondegradationmechanismsforMOCandcounter-measures51
2.6ApplicationsofMOC60
2.7Conclusionsandrecommendationsforfuturework65 References67 3Magnesiumoxysulfatecement
MarkAlexanderShand
3.1Introduction75
3.2Improvementinwaterresistance77
3.3Propertiesofmagnesiumoxysulfatecement79
3.4Magnesiumoxysulfateuses79 References82
4Magnesiumphosphatecement 85 JueshiQian
4.1Reviewonmagnesiumphosphatecements(MPCs)85
4.2FormulationofMPCs88
4.3HydrationandmicrostructureofMPCs96
4.4PropertiesofMPCs114
4.5ApplicationsofMPCs148 References159
5Magnesiumsilicatehydratecements 173 MarkAlexanderShand
5.1Introduction–typesofsilicatebinders173
5.2Magnesiumsilicatehydratecement174
5.3Conclusions180 References180
6Carbonatedmagnesiacements 183 AbirAl-Tabbaa,LiwuMo
6.1Introduction183
6.2Carbonatedmagnesiacementinmasonryblocks185
6.3Carbonatedmagnesiacementingroundimprovement193
6.4CarbonatedMgObinaryandternarycementsystems202 References208
7Magnesiainalkaliactivatedcements 213 FeiJin,AbirAl-Tabbaa
7.1Introduction213
7.2RoleofinherentMgOinAAC214
7.3EffectofadditivereactiveMgOonthepropertiesofAAC217
7.4ApplicationsofMgO-bearingAAC225
7.5Conclusionsandrecommendationsforfuturework237 References237
8Magnesiaasanexpansiveadditive 243 LiwuMo,AbirAl-Tabbaa
8.1HistoryofMgOexpansivecementandconcrete243
8.2HydrationandexpansionofMgOincement-basedmaterials246
8.3PerformanceofconcretewithMgOexpansiveadditive259
8.4ApplicationofMgOinshrinkagecompensationandcracking mitigationofconcrete262
8.5ManufactureofMgOexpansiveadditive265 References268
9Magnesiainself-healingcementandconcrete 275 AbirAl-Tabbaa
9.1Introductionandoverview275
9.2Expansiveadditivesinautogenicself-healingofcementitious systems277
9.3Autogenicself-healingincementusingMgO279
9.4Roleofmagnesiainautonomicself-healingapplicationsincement andconcrete291
9.5Fieldtrialsandapplications307 References310
Index 313
Introduction–Characterizationof MgO
MarkAlexanderShanda ,FeiJinb
a PremierMagnesia,LLC,Waynesville,NC,UnitedStates, b UniversityofGlasgow,Glasgow, UnitedKingdom
Contents
0.1 Introduction 1
0.2 ImportantcharacteristicsofMgOandtestmethods 2
0.2.1 Densityandparticlesizes 2
0.2.2 Chemicalanalysis 3
0.2.3 Crystallitesize 4
0.2.4 Texturalproperties 4
0.2.5 ChemicalreactivityandreactiveMgOcontent 4
0.3 RelationshipsamongthemostimportantpropertiesofMgO 5
0.4 CategorizationofMgO 6
Acknowledgments 9 References 9
0.1Introduction
Commerciallyavailablemagnesiumoxide(magnesiaorMgO)isproducedmainlyby twomethods,thefirstviathecalcinationorheatingofmagnesite,anaturallyoccurringmineralofmagnesiumcarbonate.Thesecondmethodinvolvestheprecipitationof magnesiumhydroxidefromseawater,ormagnesium-richbrines,usinglimeordolime; themagnesiumhydroxidethenbeingcalcinedtoformmagnesiumoxide.LaboratorypreparedMgOcanalsobeobtainedeitherfromthecalcinationofbruciteprecipitated fromvariouschemicalprocessesorfromthecalcinationofbasicmagnesiumcarbonatesprecipitatedbycarbonationofMg-richsolutions.Themagnesiaobtainedfromthe calcinationofsyntheticbruciteorbasicmagnesiumcarbonatesisusuallyreferredtoas syntheticorprecipitatedmagnesia.Magnesiacementuseeitherlight-burntorreactive magnesiumoxide,aswellashard-burntordead-burntproductsthatareconsiderably lessreactivethanalight-burnedoxide.Thedegreeofburn,either‘light’,‘hard’or ‘dead-burnt’referstothethermaltreatmentorcalcinationconditionstheprecursor magnesiteorsyntheticbrucitehasbeensubjectedto.Light-burntconditionstypically involvecalcinationtemperatureslessthan1000°Cwhiledead-burntmaterialwillexperiencetemperatures1400–2000°C.Hard-burntmagnesiumoxidewillbeobtained bycalciningatanintermediatetemperaturesomewherebetweenlightanddead-burn, > 1000°C <1400°C.Finally,fusedmagnesia,producedattemperaturesabovethe MagnesiaCements. https://doi.org/10.1016/B978-0-12-391925-0.00006-6 Copyright©2020ElsevierInc.Allrightsreserved.
fusionpointofmagnesiumoxide(2800°C)istheleastreactive.Residencetimeinthe kilnistypicallytwotothreehours.Inaddition,itisfoundthatthereactivityofreactivegrademagnesiavariesconsiderablycomparedtotheothergrades.Therefore,it isdesirabletofurthercategorizereactivemagnesiausingthemostappropriateindex, whichwillaidtheselectionofsuitablereactivemagnesiaforspecificapplications.
ThischapterwillsummarizethepropertiesofMgO,particularlyreactive,hard-and dead-burntgrades,usedintheMgOcementsdiscussedinthisbookalongwiththerelevanttestingmethods.ConsideringtheparamountimportanceofMgOcharacteristics onthepropertiesofMgOcements,aswellasthevastnumberofdifferenttestsavailableforevaluatingthequalityofMgO,literaturedataweregatheredandcorrelated regardingdifferentcharacteristicsofalargenumberofMgOsamples,sothatreadersmayhaveanideaaboutthereactivityofMgOusedinspecificpaperswhereonly limitedinformationwasgiven.
0.2ImportantcharacteristicsofMgOandtestmethods
InordertoselectthemostsuitableMgOforspecificapplications,itisimportantto assessthequalityofthesample,whichincludesitschemicalcomposition,density, particlesizeandmostimportantlyitsreactivity.ManystudiesshowedthatthereactivityofMgOhadanotableimpactontheperformanceofMgOcement.Thereactivity ofMgOisdeterminedbyitsintrinsicpropertiessuchascrystaldistortionandspecific surfacearea(SSA),whichareinturndependentonthesourceandcalcinationhistoryofthesample(seeChapter 1).Thefollowingsectionswilldescribethevarious physio-chemicalpropertiesofMgOthatareusuallytestedandthepertinenttesting methods.
0.2.1Densityandparticlesizes
ThetruedensityofMgOis3.58g/cm3 andduetothelargeporevolumeinreactive MgOpowders,thebulkdensitycouldbefarlessthanthetheoreticalvalue,whichcan bemeasuredaccordingtoASTMD7481-09[1].
TheoldASTMtestingmethodforMgOspecifiedtheuseofwetsieveanalysiswith a200mesh(75µm)sieve.However,moderntestingequipmenthasmadethismethod obsolete,althoughthewetmethodcanstillbeused.Theparticularmethodologyof particlesizeanalysisisdependentuponthefinenessofthematerial.Coarsermaterialsgreaterthan100mesh(150µm)suchasaggregateandfiller,canbetestedusing ASTMsieves(eightinches)andasieveshakersuchasthatmanufacturedbyW.S. Tyler’sRo-Tapmachine.Finelydividedmagnesiumoxideandsomefillersareparticularlycohesiveinnaturemakingitdifficulttoobtainanaccuratesieveanalysiswhen finerthan100mesh.Useofsieveshakers,inthiscase,isnotrecommended,instead, anairjetsieveshouldbeemployed.Hosokawa-MicronPowderSystemsandRetsch GmbHmanufacturethistypeofsieveanalyzer.Inaddition,thelaserlightscattering methodisalsofrequentlyemployedfordeterminingtheparticlesizedistributionof MgOdispersedinethanolorisopropanol[2].
0.2.2Chemicalanalysis
ChemicalanalysisofcementgrademagnesiumoxideisdetailedbyASTMC245-52 andEN14016-2:2004(E).Thesemethodsareratherlengthyandcomplicatedwet chemicalprocedures,fortunately,modernchemicalanalyticalmethodscanbeused todeterminethechemicalpurityofparticularmagnesiumoxideinamuchmore rapidandaccuratemanner.X-rayfluorescence(XRF)andatomicabsorptionspectroscopy(AAS)orinductivelycoupledplasmaemissionspectroscopy(ICP)canbe usedtoachievethis.XRFanalyticalsamplescanbepreparedusingfinelyground pressedpowderorlithiummetaborate/lithiumtetraboratefluxblenddiscs.Thefluxingmethodwillgivethemostaccurateanalyticaldata.SamplesfortestingbyAAS orICPcanbepreparedbydissolutioninhotacid,typicallynitric,nitric/hydrochloric orperchlorate/nitricmixture.Thetypeofacidusedfordissolutionwilldependupon howwellthesilicaimpuritiesgointosolution.Forrecalcitrantacidinsolubles,lithium boratefusioncanalsobeemployedwhichcanthenbedissolvedinacidsolution. TheuseofAASshouldbeusedwithcautionespeciallywhenmeasuringmagnesium directlysincetheworkingsolutionhastobedilutedmanyfoldstobringitsconcentrationwithinthelinearrangeoftheinstrumentdetector,thusincreasingtheriskof magnesium-richdilutionerrors.Experiencehastaughtthatdeterminationofmagnesiumistypicallymostreliablewhendonebydifference;whatthismeansisthatallthe otherimpuritycomponentsoftheoxide,e.g.Ca,Si,FeandAlaremeasuredandsubtractedfrom100%,thedifferencebeingtheMgconcentration.Theelementalanalysis isconvertedtotheoxidebasisandthechemistryreportedasMgO,CaO,SiO2 ,Fe2 O3 , andAl2 O3 ,etc.
Lossonignition(LOI)measuresthequantityofresidualmagnesiumcarbonateand otherimpuritycarbonatesnotthermallydecomposedduringthecalcinationprocess. Inthecaseofsyntheticmagnesiumoxide,theLOIrepresentsresidualmagnesium hydroxidealsonotdecomposedduringcalcination.Lossonignitionisdeterminedby heatingasampleinacrucibleinalaboratoryfurnacebetween1000–1150 o C.The lossinweightafterheatingisexpressedasapercentageoftheinitialweight.LOIis anindicationofthermaltreatmentandwillreflectthereactivityofmagnesiumoxide. Ingeneral,alowLOIandhencelowresidualcarbonatecontentindicatesthattheoxide hasseenhighertemperaturesorlongercalcinationtimesandwill,therefore,beless reactivethanoxidethathasahigherLOIandhencelargerresidualcarbonatecontent.
Thefree-limecontentcanbemeasuredasdescribedinEN14016-2:2005(E), whichinvolvesusinghotethyleneglycoltoextractlimefromthesample.Theextract solutionisraisedtopH12usingammoniumhydroxidesolution,whichprecipitates magnesiumionfromsolution.Acomplexometrictitrationusingmurexideindicator andEDTAisperformedonthemagnesiumdepletedsolution.Themurexideformsa weakcomplexwithcalciumionwhentitratedwithEDTAwhichisamuchmorepowerfulchelatingagent,calciumionsareremovedfromthemurexide-Cacomplexby EDTAuntilallthecalciumisboundbyEDTA,whichistheend-pointofthetitration. Theend-pointisindicatedbythemurexidechangingfromredtoviolet.
0.2.3Crystallitesize
PowderX-Raydiffraction(XRD)iscommonlyusedtostudythemineralphasescontainedinaMgOsample,whichmayincludequartz,magnesite,calcite,brucite,and talc.XRDisalsousedtocalculatethecrystallitesizeoftheMgO,whichincreases withthecalciningtemperatureandduration,indicatingthesinteringextentduringthe calcinationprocess.Thecrystallitesize,GXRD ,canbecalculatedusingScherrer’sformula,usinganX-raydiffractometerwithCuKα radiation:
GXRD = Kλ/β · cos θ
where λ isthewavelengthofCuKα (0.15405nm), β thefull-widthathalf-maximum intensity(FWHM)ofaBraggreflectionexcludinginstrumentalbroadening, θ the BraggangleandKaconstant(= 0.9).ThemajorcharacteristicpeakofMgOat42.93° (2θ )isusuallyusedinthecalculation.
0.2.4Texturalproperties
Specificsurfacearea(SSA)determinedbythenitrogenadsorptiontestusing Brunauer–Emmett–Teller(BET)modelisauniversalmethodformeasuringtheactivityofceramicoxidessuchasMgO.Theunitsofmeasurementareareapergram ofsample(m2 /g).TheappropriatetesttouseisASTMC1274-10,andBETSSA willdirectlyreflectthethermaltreatmentthattheoxidehasexperienced.Apartfrom SSA,othertexturalpropertiessuchasporevolumeandmeanporeradiuscanalsobe determinedfromnitrogenadsorption-desorptionisotherms.
Analternativemethodforevaluatingthesurfaceareaisbyiodinenumber,which isoftenusedforactivatedcarbons[3].Bycomparingthetwotestvaluesofmagnesia sampleswithSSAfrom1to200m2 /g(determinedfromnitrogenadsorptiontestusing PointBmethod,whichisingoodagreementwiththevaluescalculatedfromtheBET model),ZettlemoyerandWalker[4]reportedanempiricalrelationshipbetweenthe twovalues:
0.2.5ChemicalreactivityandreactiveMgOcontent
ChemicalreactivityiswidelyusedbyindustrytoassessthequalityofMgO,whichis evaluatedbymeasuringthetimedurationrequiredfortheneutralizationofanacidic solutionbyacertainsamplemass.ThetimefromaddingtheMgOtothechangeofthe solutioncolorisrecordedasthereactivityandtheshorterthetime,themorereactive theMgO.
Therearemanyacidreactivitytestsreportedintheliterature[5–10]differingin thetypeandconcentrationofacid(citricacidandaceticacid),temperature,stirring speedandthepHindicatoremployedinthesolution.Generally,thetotalH+ content generatedbydissociationoftheacidismuchlessthantheOH generatedbythe
SSA = 0.95 ∗ iodinenumber.
hydrationofMgO,sothatatacertainhydrationdegreethecolorchangeofthepH indicatorwillbeobserved.Recentlyamoresophisticatedmethodwasproposedin [11]tospecificallyevaluatethereactivityofMgOasanexpansiveadditiveinPC. However,duetothedifferentreactionmechanismsbetweendifferenttypesofacids andMgOcomparedtoitshydrationinwater,itishardtocomparetheresultsbetween differenttests[12].Moreover,thecorrelationsbetweenthehydrationdegreesofMgO inacidswithitshydrationdegreeinwaterhavenotbeenestablishedyet.
TheChinesestandard[10]alsoproposedamethodofmeasuringtheactiveMgO contentinasample.TheMgOsampleishydratedatroomtemperaturefor24hours andthenat100–110 o Cuntildry.TheweightincreaseduetothehydrationofMgOin thisperiodisregardedastheactiveMgOcontent,whichisexpressedin%.Afewother hydrationmethods[13,14]wereproposedwhichdifferinthehydrationtemperature anddryingmethod,renderingvaryingactiveMgOcontentsforagivenMgOsample, whichmadethetestresultsimpossibletocomparedirectly.
Inaddition,itshouldbenotedthatthechemicalreactivityvalueandreactivity contentvaluearetotallydifferentandnotcomparable.Theformeristhetimeneeded foraMgOtoreachacertainhydrationdegree,whilethelatterindicatestheMgO contentthatwillbehydratedunderacertaintemperatureandperiod.
0.3Relationshipsamongthemostimportantproperties ofMgO
Althoughthereisnospecificreactivityrequirementforcementgrademagnesiumoxide,thereareanumberoftestmethodsthatcangaugehowreactiveaMgOis,andthis canbeusefulwhenselectinganappropriateMgO.Althoughthechemicalreactivityis oftenreferredtowhencomparingthereactivityofMgO,amongthevariouscharacteristicsofMgOasshowninthelastsection,SSA,asanintrinsiccharacteristicofMgO powder,isagreedtobethemostappropriateindexforevaluatingthereactivityof MgOfromdifferentproductionprocesses.SmithsonandBakhshi[15]suggestedthat thehydrationrateofMgOisdirectlyproportionaltotheSSAoftheMgOparticles, whichisinlinewithMaryškaandBláha[16],whoclaimedthatadecisiveinfluenceof thehydrationratewasexhibitedbythedegreeofcrystallizationandsinteringofMgO, whosemeasureisitsSSA.TheSSAofMgOisgreatlyinfluencedbythecalcination temperatureandresidencetime.Inaddition,Fig. 0.1 showstheprecursoralsohasa significantimpactonthechangeofSSAduringthecalcinationprocess.Therefore, providinginformationoncalcinationconditionsunderwhichthemagnesiumoxide wasproduceddoesnotnecessarilypredictitschemicalreactivity.Thephysicochemicalpropertiesoftheseprecursorsdiffersignificantly,whichresultedintheirdifferent thermalbehaviorsandthemicrostructuralevolutionduringcalcination(Fig. 0.2A). Similarly,thechemicalreactivitycannotbesolelyreflectedbythecrystallitesizeas theprecursoralsoplaysanimportantrole(Fig. 0.2D).Fig. 0.2Bshowsthecorrelation betweenSSAandLOI.Itcanbeseenthatnoclearrelationshipbetweenthetwovalues canbeobservedwhenMgOsamplesproducedfromdifferentrouteswerecompared. TherelationshipbetweenSSAandchemicalreactivityvaluesfromdifferenttestsis
Figure0.1 EffectofprecursorandcalcinationtemperatureontheSSAofMgO.Theresidencetimeis2 hours.Datawerecollectedfrom[17–21].
plottedinFig. 0.2C.Itisshownthatregardlessofthereactivitytestmethodandthe precursorused,thereactivityvaluesgenerallydecrease(smallerreactivityvaluesindicateamorereactiveMgO)withanincreaseofSSA.Moreimportantly,aboveacertain valueofSSA(∼10m2 /g),thereactivityvaluehardlychangeswithanincreaseofSSA. TherelationshipalsorevealsadrawbackofthechemicalreactivitytestfortheevaluationofMgOreactivitysinceitisnotcapableofdifferentiatingMgOwithveryhigh reactivities.SinceareactivegradeofMgOusuallyhasSSAof>10m2 /g(Fig. 0.1),it isrecommendedthatSSAshouldbeusedforreactivityevaluationinsteadofchemical reactivity.
0.4CategorizationofMgO
Studiesshowthatthecharacteristicsofreactivemagnesiumoxidesobtainedfromdifferentsourcesandproductionprocessesvarysignificantly,whichislikelytoinfluence theirperformanceindifferentcementsystemsandapplications.Itisthereforedesirabletofurthercategorizethemusingthemostappropriateindexthatcanbeusedfor reactivityevaluationofMgOregardlessoftheirsourcesandcalcinationprocesses. SSAisemployedherebasedonthediscussionspresentedabove.Itisrecommended thatreactiveMgOcanbedividedintothreecategoriesofhigh,mediumandlowreactivityMgOasfollows:
• CategoryI:highreactivityMgO,withSSA>60m2 /g;
• CategoryII:mediumreactivityMgO,with10m2 /g ≤ SSA ≤ 60m2 /gand
• CategoryIII:lowreactivityMgO,withSSA<10m2 /g.
Itshouldbenotedthatthecategorizationisarbitraryanddoesnotconsiderthespecifichydration/reactionmechanismindifferentMgOcementdescribedinthisbook.It helpstodistinguishMgOsampleswithinthereactivegraderegardlessoftheirprecur-
Figure0.2 Correlationsbetween(A)SSAandcrystallitesize;(B)SSAandLOI;(C)SSAandchemical reactivityand(D)crystallitesizeandchemicalreactivityusingdatafromliterature[4,14,21–35].
sorsandcalcinationhistorywhichintendstoaidtheselectionofthemostappropriate MgOfordifferentapplications.ThereadersmayfindthepossiblecorrespondingcharacteristicsoftheMgOintheliteraturebycomparingrelevanttestvaluesshownin Fig. 0.2.Nevertheless,careshouldbetakenwhenselectingreactiveMgOasother properties(e.g.,purityanddensity)ofMgOapartfromreactivitymaybealsoimportantforaspecificcementtype.
Hardburnedmagnesiaiscalcinednear1500 o Cinarotarykiln.Ithasadensityof approximately2.0g cm 3 andcrystalsizeof1to3microns.Atthistemperatureand duetotherotatingactionofthekiln,theMgOcanhaveaparticlesizeupto25mm. Hard-burntMgOisclassifiedasacategoryIIIMgOwithlowreactivity.
Figure0.2 (continued )
Dead-burnmagnesia,frequentlycalledbyitsmineralogicalnameofpericlase,is calcinedinatwo-stagefiringprocesstoasurfacearealessthan1m2 /gandcrystal sizerangingfrom50to150microns.Asthecalcinationtemperatureandresidence timeincreases,thecrystalssintertoformlargercrystalswithlessporosityandsurface areaandhigherdensity.Typicaldensitiesrangefrom3.1to3.45gs/cm3 .Inthefirst stage,themagnesiumhydroxideormagnesiteiscalcinedinamultiplehearthfurnace tomagnesiumoxidethatisthencompressedbyrollcompactorstoagreenbriquetteof suitablesizeanddurabilitytowithstandtheextremesoftheshaftkiln.Thebriquettes arefiredatnearly2100 o Ctosinterthebriquettestotheirfinalsizerangingfrom6to 25mm.Periclaseismostlyusedtomakerefractorybricksandrepairmaterialstoline steelfurnaces.
Acknowledgments
ThefinancialsupportforFeiJinfromtheEPSRC/NSFCgrantMagMats:Magnesia-bearingconstructionmaterialsforfutureenergyinfrastructure(EP/M003159/1)isgratefullyacknowledged.
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1 Manufactureofmagnesiumoxide formagnesiacements
MarkAlexanderShand PremierMagnesia,LLC,Waynesville,NC,UnitedStates
Contents
1.1 Magnesiumoxideproducedfrommagnesite 13
1.2 Formationofmacrocrystallinemagnesite 15
1.3 Formationofcryptocrystallinemagnesite 15
1.4 Magnesiumoxidederivedfrombrucite 15
1.5 Miningandprocessingofmagnesite 16
1.6 Syntheticmagnesia 18
1.6.1 Precipitationprocess 20
1.6.2 Settlingandcompaction 21
1.6.3 Washing 21
1.6.4 Filtration 22
1.6.5 Generalpropertiesofsyntheticmagnesia 22
1.7 Calcinationofmagnesiumcarbonateandmagnesiumhydroxide 22
1.7.1 Calcinationofmagnesite 22
1.7.2 Calcinationofmagnesiumhydroxide 23
1.8 Furnacesandkilns 24
1.8.1 Introduction 24
1.8.2 Multiplehearthfurnaces(MHF) 24
1.8.3 Horizontalrotarykilns 26
1.8.4 Shaftkilns 28
References 28
1.1Magnesiumoxideproducedfrommagnesite
Magnesiumistheeightmostabundantelementintheearth’scrustandisthethird mostabundantelementinseawateraftersodiumandchlorine,beingpresentatabout 1272ppm.Thevastmajorityofmagnesiumoxideproducedworldwideisderivedfrom anaturallyoccurringmagnesiumcarbonatemineralcalledmagnesite(MgCO3 ).Other magnesiumcarbonatemineralsexist,suchasnesquehonite(MgCO3 ·3H2 O)andhydromagnesite(4MgCO3 Mg(OH)2 4H2 O),however,theiroccurrenceisrarerthanthat ofmagnesiteandaretypicallynotusedformagnesiumoxideproduction.Commercial depositsofmagnesitehavereservesinexcessofseveralmilliontonnesofexploitable
MagnesiaCements. https://doi.org/10.1016/B978-0-12-391925-0.00008-X Copyright©2020ElsevierInc.Allrightsreserved.
mineral.Worldwideproductionofmagnesitein2016wasestimatedat27.3million tonnesandtheestimatedglobalreservebaseisabout13billiontonnes[1].
Naturalmagnesiteoccursintwophysicallydifferentformsnamelycryptocrystallineandmacrocrystalline.ThemacrocrystallineformoccursasfinetocoarsegrainedcrystalsandisotherwiseknownasSparrymagnesite.Thistypeofmagnesite canoccurinavarietyofcolors,fromwhitetodarkgrey,however,thecolorisnot necessarilyagoodpredictorofpurity.Ingeneral,Sparrymagnesiteislesspurethan thecryptocrystallineform.China,Russia,andSlovakiaarethetopthreeproducersof magnesiumoxidederivedfrommacrocrystallinemagnesite.Cryptocrystallinemagnesitehassmallcrystalsize,1–10µm,andisotherwiseknownasamorphousmagnesite. Itisgenerallywhiteincolorandoccursashigh-qualitydepositswithlowFe2 O3 and CaOcontentwithAustralia,Turkey,andGreecebeingthethreetopproducersofmagnesiumoxidederivedfromthecryptocrystallineform.SeeTable 1.1 forabreakdown ofmagnesiumoxideproductionbycountry.Sparrymagnesiteisthemostcommon formandrepresents93%ofglobalmagnesiteresources,whereascryptocrystalline onlyrepresents7%.
Table1.1 Magnesiumoxideproductionbycountry,2009(source: adaptedfrom[7]). Country
Magnesiumoxidederivedfromthetwoformsofmagnesitenotonlyhavedifferinglevelsofpuritybutalsohavedifferentphysicalproperties.Sparrymagnesite tendstohaveahigherdensityduetolowerporosityoftheore,typically2.98g/cm3 , whereasthecryptocrystallineformhasahigheroreporosityandthereforealower density,around2.68g/cm3 .Macrocrystallinemagnesitehasaslightlyhighertemperatureofdecompositionofabout30–40 o C(decompositiontemperature678 o C).Itis notknownwhethermacrocrystallineorcryptocrystallinemagnesitemakesthebetter magnesiumoxideforuseinSorelcement,butbothtypesofmagnesitearesuccessfully usedtomakeoxideforthiscement.
1.2Formationofmacrocrystallinemagnesite
Crystallinemagnesiteisformedthroughhydrothermalreplacementofolderdolomite orlimestone(CaCO3 )formations[2].Theextantrockispenetratedbyhotmagnesiumrichwaterreleasedfrommagmathroughfissureandcracksinthebodyoftherock formation.Thehotwaterbecomesmagnesiumrichasitpassedthroughunderlying dolomite(CaMg(CO3 )2 ),seeReactions(1.1)and(1.2)
Anintermediatelayerofdolomiteisalwayspresentsuchthatlimestoneneverabuts magnesite.
1.3Formationofcryptocrystallinemagnesite
Theformationofcryptocrystallinemagnesitecanoccurbyseveralprocesses.Sedimentaryformationoccursinlagoons,freshwaterandsaltwaterlakes.Theconcentrationofsaltswithinthelakebywaterevaporationcausesprecipitationofcalcium carbonatewhenthesolubilityconstantexceeds10 8 32 (Ksp = [Ca2+ ][CO2 3 ]).Since calciumcarbonateislesssolublethanmagnesiumcarbonate,itwillbethefirstto precipitate.Thisresultsinthebodyofwaterbecomingconcentratedwithrespectto magnesiumion.Ifevaporationofwatercontinues,theneventuallythesolubilityconstantofmagnesiumcarbonateisexceeded,anditwillprecipitateoutofsolution.
Cryptocrystallinemagnesitecanalsobeformedviathealterationofserpentinebyhydrothermalprocesses.Depositsofmassivecryptocrystallinemagnesiteoccurinthe serpentizedultrabasicrockthathasundergonehydrothermalleachingofmagnesium fromtheserpentine(H4 Mg3 Si2 O9 ).Thehydrothermalsolutioncontainsdissolvedcarbondioxidethatisnecessaryforthedissolutionprocess,seeReaction(1.3).
Themagnesiteisdepositedintheveins,crackandfissureswithinthehostrockand thesilicaiscarriedawayinsolution.
1.4Magnesiumoxidederivedfrombrucite
Brucite,whichwasnamedafterArchibaldBruce(1777–1818)in1824,whodiscoveredthenaturallyoccurringmineralinHoboken,NewJersey,typicallyoccursas
MgCl2 + CaCO3 → MgCO3 + CaCl2
(1.1)
CaMg
(1.2)
[Mg][CO2 3 ]= Ksp = 10 5
tabularcrystals.Lesscommonlyitcanoccurinacicular,fibrous,andscalyform.Its colorcanrangefromwhite,palegreen,gray,gray-blue,andblue.Itcanalsohavea transparent,pearly,waxy,orvitreousappearance.
Bruciteformsthroughdedolomitizationbythermalmetamorphismatalowpartial pressureofcarbondioxideandhighpartialpressureofwater[3],seeReaction(1.4) and(1.5).
Itcanalsoformthroughthermaldecompositionofmagnesitetoformpericlase(MgO), Reaction(1.6)whichinbothcasesissubsequentlyhydratedtoformthehydroxide, Reaction(1.7).
Bruciteisararemineral,andcommercialdepositsareonlyfoundinChina,Russia, andtheUSA.Bruciteisoftenfoundinassociationwithserpentine,calcite,aragonite, dolomite,magnesite,hydromagnesite,artinite,talcandchrysotile.Itsthermaldecompositiontemperatureisapproximately350 o C,dependentuponthelevelofpurity.
1.5Miningandprocessingofmagnesite
Practicallyallmagnesiteminesareopenpitinnature.Thebasicprocessofmagnesite extractioninvolvesover-burdenremoval,drilling,blasting,loading,hauling,andpostprocessing.Theprocessingofmagnesitecanbedividedintothefollowingoperations: crushing,sizingandbeneficiation,andmostoftheseoperationsarecommontothe productionofsizedaggregatesandores,seeFig. 1.1 forageneralizedprocessingflow diagramformagnesite.
Overburdenisalayerofinterveningmaterial,primarilysoil,androck,between thesurfaceandtheorebody.Thisisremovedfirstinordertoexposetheunderlying ore.Thethicknessoftheoverburdencanvaryconsiderablyfromlessthanonefoot totensoffeet.Iftheoverburdenthicknessistoogreat,thecostofremovalcanbe considerableandmayforcetheuseofsub-surfaceminingmethods.
Oncetheoverburdeniscleareddrillingoperationcancommence.Thepurposeof drillingistwo-foldinnature.Thefirstistoascertainthechemistryofthemagnesite orebody,drillcuttingsaresampledfromaroundthedrillholeastheholeisbeingcut andarechemicallyanalyzed,andsecondly,thedrillholesaresubsequentlyusedto fillwithexplosivetofracturetherocktoasuitablesizesothatitcanbeloadedonto trucksandhauledawayforfurtherprocessing.
MgCO3
Figure1.1 Schematicflowdiagramofmagnesiteprocessing[8].©JohnWiley&Sons2006.
Thetypicalexplosiveusedinthedrill-holeisanammoniumnitrate/fueloilmixture (ANFO).Thepatternandspacingofdrill-holesdependupontheuniformityofthe deposit.Adepositofuniformthicknessandcompositionmayallowwidelyspaced holesof100–500feet,whereasthenon-uniformdepositmayrequirecloselyspaced holesofevery3to20feet.Thebottomoftheholeistypicallyprimedwitha1.0lb. castbooster,whichhasanexplosivevelocityof26000fps,usingnon-electrictype detonators.Theusualroundisabout400to700holesdrilledon10ft.centersand 18ft.deep,andeachholeisfilledwith45lb.ANFO.Thepowderfactoriskeptat between0.35to0.40lb.perton,dependinguponthedensityofANFObeingused. Aftertheblast-holeischargedwithexplosives,thetopoftheholeisstemmed(filled) withfinestone,typicallydrillhole-cuttings.Thisactstocontaintheexplosiveforce withintheholeandreducetheoccurrenceofair-overpressure(noiselevel).
Chemicalcontourmaps(orbenchmap)aremadeforthepurposeofselectiveminingoftheore.Thechemicalcontourmapisformedbycorrelatingthedrill-holecutting chemicalanalysiswiththeuniquelyidentifieddrill-holefromwhichthecuttingsamplecame.TheMuckmapisusedtodeterminewhichareasoftheblastedrockareto beminedorputtowasteandisthe“in-field”map.Astheblastedoredoesnotchange itspositionappreciably,thechemicalcontourmapcanbetranslateddirectlyontothe blastedarea.
Oncetheorehasbeenbrokenbyblastingandthechemicalcontourmapoverlaid ontothebrokenrock,theusableoreismarkedbystakingorbysomeothervisible marker.Unusableoreisremovedandtransportedtoawastesiteanddumped.The usableoreisthentransportedtotheprimarycrusherwhereitisreducedinsize,the crushedorethenbeingscreenedandanyoversizedmaterialreturnedtoasecondary crusher.
Thecrushedandsizedoreisthenstockpiledoverabelttunnelfeedersystemwhich hasnumerousaccessportsthatallowsloadingofmaterialontothebeltfromanywhere inmultiplestockpiles.Thisarrangementallowstheblendingofmagnesitefromdifferentpilestoachievethedesiredchemistryinthefinalproduct.Orethatrequires beneficiationisgenerallyputintoastockpileseparatefromtheorethatisofsufficientpuritytousewithoutfurtherpurification(directhaulore).Thecurrentprocess ofheavymediaseparation,HMS(sometimescalleddensemediaseparation),isthe mostwidelyusedsink-floatprocessusedtodayandisveryefficientatreducingthe acidinsolublecontentofmagnesite;however,itisnotaseffectiveatreducingthelime content.ThismeansthatmagnesiteundergoingHMScanhaveamuchhigherinsolublecontentthanthatoflimeandstillachieveachemicalpurityequivalenttothatof directhaulore.Thepracticallimitforthelimecontentisapproximately6wt.%,while thelimitforacidinsolublematerialcanbeashighas20wt.%.
Heavysolutions,heavyliquids,andferrofluidshaveallbeenusedasthedensemedia.However,themostpopularareaqueoussuspensionsoffineparticlesofmagnetic solidssuchasferrosilicon(density,6.8g/cm3 )ormagnetite(5.2g/cm3 ).Thebasisof theprocessisthepreparationofasuspensionthathasaspecificgravitysomewherein betweenthespecificgravityofthemineraltobebeneficiatedandthatoftheimpurities. Theaqueoussuspensionofdensemediaistypicallyadjustedtohaveaspecificgravity intherange3.0–3.10g/cm3 ,closetothedensityofmagnesiteitself(∼3.0g/cm3 ). Ganguemineralshavingadensitylowerthan3.0–3.10g/cm3 ,suchasquartzandnumeroussilicates,willfloatintheconeseparator,whilethemagnesitewillsink.Cone separatorscanprocessupto300tph,whereasdrumsandtroughordragtankvessels cantreat700–800tph.Thefeedsizetoaconeseparatorunitistypicallyintherange 1/4-in.to5inches.Theorefedintotheheavymediaseparationplantisfirstsized beforeenteringtheconeseparator,thefinesbeingrejectedfromthesystemtoensure thatthesuspensionmediaisnotcontaminatedwiththismaterial.Thefloatsandsinks areremovedfromtheconeandontoseparatescreenbeltsandarethoroughlywashed usingoverheadspraystoremoveanyadheringsuspensionmedia.Thesuspensionmediaisreclaimedfromthewashingsusingawetmagneticdrumseparator.Themediais thenfedintoadewateringunitandtheferrosiliconormagnetiteisthendemagnetized beforebeingrecycledbacktothemediamakeuptank.
Aftercalciningthemagnesite,theresultantmagnesiumoxidemayundergoacombinationofscreeningandgrindingtreatments.Theexacttreatmentwoulddependupon howthemagnesitedecrepitatesinthekiln;macrocrystallinemagnesitetendstodecrepitatemorethanthecryptocrystallinevariety.Thepurposeofthescreeningprocess wouldbetoeitherproduceacoarsesizedproductortoperhapsimprovetheproduct puritybyrejectingcertainsizefractionsofmaterialthatmaycontainahigherlevelof impurities.
1.6Syntheticmagnesia
Syntheticmagnesiaisproducedbyprecipitatingmagnesiumhydroxidefromeither seawaterorsub-surfaceorsurfacehighmagnesiumbrines,seeFig. 1.2.Astrongbase
