Animprovedprocessforthepurificationofphosphogypsum

ELSEVIERConstructionandBuildingMaterials,Vol.10,No.8,pp.597~500,1996©1997ElsevierScienceLtdPrintedinGreatBritain.Allrightsreserved0950~618/96$15.00+0.00PH:S0950-0618(96)00019-0AnimprovedprocessforthepurificationofphosphogypsumManjitSingh,MridulGarg,C.L.Verma,S.K.HandaandRakeshKumarCentralBuildingResearchInstitute,Roorkee247667,UP,IndiaReceived23December1995;revised7June1996;accepted10June1996Phosphogypsum,aby-productofthephosphoricacidindustry,iscontaminatedwiththeimpuritiesofP205,F,organicmatteretc.Theseimpuritiesimpairthestrengthdevelopmentofcalcinedproducts.Foreffectiveandproperutilization,apurificationprocessforphosphogypsumbywetsievingthrougha300micronsievehasbeendeveloped.Basedonthewetsieving,thelaboratorytrialswereconductedusinghydrocyclone.Thefindingsofthepurificationtrialsandproposedpilotplantalongwithalistofusesandapplicationsofthepurifiedmaterialarepresentedinthispaper.©1997ElsevierScienceLtd.Allrightsreserved.Keywords:phosphogypsum;hydrocyclone;wetsieveanalysisIntroductionAbout5.0milliontonnesofphosphogypsumperannumisproduced,asthedownstreameffluentby-productinthewetprocessmanufactureofphosphoricacid,inIndia.Amaximumof10.0percentofitisbeingusedbythecement,ammoniumsulphatefertilizer,plasterandsoilrechtmationindustries.ThemajorobstaclesintheeffectiveconsumptionofphosphogypsumarethefreemoistureandpresenceofimpuritiesofP205,Eorganicmatterandalkaliesinit.Theseimpuritiesimpairthesettingandhardeningofcementsandplasters,producedfromphos-phogypsum1.Forthemassutilizationofphosphogypsuminthebuildingsectorandalliedindustries,itisimportanttoreduceorinactivatetheseimpurities.Attemptshavebeenmadeabroad,inJapana,Germany3,France4andRussia5,toreducethedetrimentaleffectoftheimpuritiespresentinphosphogypsum.Washing,filtrationorfloatationstepsareadoptedtogetridoftheimpurities.IntileRussianprocess,thepurificationofphosphogypsumisconductedbyadsorbingtheimpurities,P205andF,usingabedofaluminiumsilicateandlime.ExtensiveR&DstudieshavebeendoneintheCentralBuildingResearchInstitute,Roorkee,aboutthepurificationandutilizationofphosphogypsum.Thepurificationpro-cessesvariedfromwashingtochemicaltechniques.Wetsievingofphosphogypsumusinga150micronsieveandtherejectionoftheresidualcoarsefractionwasreportedbyTanejaandSingh6.Recentstudiesdoneonthewetsievingofphosphogypsumthrough300micronsievearediscussedinthepaper.Basedonthewetsievingprocess,purificationtrialswereconductedforphosphogypsum,usinghydrocyclone.Thefindingsofthelaboratorytrialsandtheproposedpilotplantofthecapacityofonetonnepershiftarereportedinthepaper.Someoftheusesofpurifiedphosphogypsuminthebuildingindustryaredescribed.ExperimentalCharacterizationofphosphogypsumThephosphogypsumsample,procuredfromSouthernPetrochemicalIndustries,Tuticorin,wasanalyzed(Ta-ble1)forvariouschemicalconstituentsfollowingdifferenttestprocedures7-9.PurificationofphosphogypsumWetsieveanalysis.Thephosphogypsumsamplewaswetsievedthrougha300micronISsieve.Thefractionretainedbythesievewasfurtherwashedwithwateranddriedat42°C.Thefractionsretainedbythesieveandthosepassedthroughitwereanalyzedforresidualimpuritiesanddifferentialthermalanalysis(DTA)(StantonRedCroft,UK).ScanningelectronmicroscopyoftheunpurifiedandpurifiedphosphogypsumwasdoneusingaSEM,Model501(Phillips,Holland).Throughhydrocyclone.Phosphogypsumandwater,intheproportion1:3byvolume,wasthoroughlymixedinthemixer/slurrytank(Figure1)tosolubilizetheimpurities,P205,F,organicmatteretc.;andkeepingthebypasslinevalveopen,theslurrypumpwasstarted.Afterattaininguniformityinthegypsumslurry,thecontrolvalveinthefeedlinewasopenedslowlyandgradually.Thesystemwasallowedtoacquireequilibriumfor15minutesjustaftergettingdesiredpressurewasobtained.Theunderflowandoverflowsamplesofthegypsumslurrywerecollectedaftereveryhalfanhour.Thematerialwasthenallowedtosettletothebottomofthecontainers.Thewaterwasremovedbydecantationandfiltration.Thephosphogypsumsampleswerethendriedat42_+2°Cuntilaconstantweightwasobtainedandanalyzedforvariousconstituents.597598Improvedprocessforpurificationofphosphogypsum:M.Singhetal.Table1ChemicalanalysisofphosphogypsumConstituentsPercentageTable2ImpuritiesinphosphogypsumImpurityImpurities(%)inphosphogypsumfractionP2051.28Retainedover300Fractionpassing300F1.80micronsievemicronsieveOrganicmatter1.58SiOz+insolubleinHC11.05P2050.8680.41R203(A1203+Fe203)0.50F1.230.57CaO31.20Organicmatter1.2390.340MgO0.06NazO+K200.180.15SO343.0Na20+K200.33Combinedwater19.2076XbI1(1)Slurrytank(2)Rubber-linedcentrifugalpump(3)Pressuregauge(4)Hydrocyloneinlet(5)Hydrocyclone(6)Controlvalve(7)Bypassvalve(8)Funnelforunderflow(9)Funnelforoverflow(10)Vortexfinder(11)DrainvalveFigure1ExperimentalsetuplinediagramforthepurificationPreparation,testingandevaluationofgypsumplasterPhosphogypsumsamples,bothpurifiedandunpurified,werecalcinedinanovenat150°Cfor5.0hoursandintermittentspatulationwasdonetoobtaingypsumplaster(/3-hemihydrate).Thegypsumplasterthusproducedwascooledandgroundtopassa150micronsieveandtestedforvariousphysicalpropertiest°.ResultsanddiscussionWetsieveanalysisOnwetsievingofphosph