2 applied processes and techniques

3/24/2015 2APPLIEDPROCESSESANDTECHNIQUES

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2.Appliedprocessesandtechniques

Inthischaptertheappliedprocessesandtechniquesarequalitativelydescribed.Chapter3coversthequantitativeaspectsofconsumptionandemissionlevels.

Thechemical,thermodynamicandkineticprinciplesarenotexplained.Forthis,referenceismadetoliteraturerelevanttothesubject[Ullmanns,1996]or[KirkOthmer,1991].

Thechloralkaliindustryproduceschlorineandcausticsolution(sodiumorpotassiumhydroxide)simultaneouslybymeansofdecompositionofasolutionofsaltinwater.Alongwiththechlorineandthecausticsolution,hydrogenisproduced.Anindustrialchloralkaliproductionunitcomprisesaseriesofoperations,structuredasshowninFigure2.1.

Inthechloralkalielectrolysisprocess,achloridesaltsolutionisdecomposedelectrolyticallybydirectcurrent.MostofthetimesodiumchlorideisusedintheprocessinwesternEuropeandlessfrequentlypotassiumchloride(about34%ofthechlorineproductioncapacity).Otherprocessessuchastheelectrolysisofhydrochloricacidortheelectrolysisofmoltensodiumchlorideareapplied,buttheseonlyaccountforabout3%oftotalchlorineproductioncapacityinEurope.

TheelectrolysisofmoltensodiumsaltswhichisappliedtoobtainsodiumandforwhichchlorineisonlyacoproductisdescribedintheBATReferencedocumentonnonferrousmetals,assodiumisanalkalimetal.

Therearethreebasicprocessesfortheelectrolyticproductionofchlorine,thenatureofthecathodereactiondependingonthespecificprocess.Thesethreeprocessesarethediaphragmcellprocess(Griesheimcell,1885),themercurycellprocess(CastnerKellnercell,1892),andthemembranecellprocess(1970).Eachprocessrepresentsadifferentmethodofkeepingthechlorineproducedattheanodeseparatefromthecausticsodaandhydrogenproduced,directlyorindirectly,atthecathode[Ullmanns,1996].

Thebasicprincipleintheelectrolysisofasodiumchloridesolutionisthefollowing:

Attheanode,chlorideionsareoxidisedandchlorine(Cl2)isformed.

At thecathode: In themercury process a sodium/mercury amalgam is formed and hydrogen (H2) and hydroxide ions (OH) are formed by thereactionofthesodiumintheamalgamwithwaterinthedenuder.Inmembraneanddiaphragmcells,waterdecomposestoformhydrogen(H2)andhydroxideions(OH)atthecathode.

Forallprocessesthedissolvingofsalt,sodiumchloride,is:

NaCl Na++Cl

Theanodereactionforallprocessesis:

2Cl(aq) Cl2(g)+2e

Thecathodereactionis:

2Na+(aq)+2H2O+2e H2(g)+2Na+(aq)+2OH(aq)

Theoverallreactionis:

2Na+(aq)+2Cl(aq)+2H2O 2Na+(aq)+2OH(aq)+Cl2(g)+H2(g)



Figure2.1:Flowdiagramofthethreemainchloralkaliprocesses

basedon[KirkOthmer,1991],[Ullmanns,1996]

ThemaincharacteristicsofthethreeelectrolysisprocessesarepresentedinTable2.1.

Mercury Diaphragm Membrane

Causticquality High,



Figure2.2:Simplifiedschemeofchlorineelectrolysiscells

after[Dutchreport,1998]

2.1Themercurycellprocess

ThemercurycellprocesshasbeeninuseinEuropesince1892andaccountedin1999for58%oftotalproductioninwesternEurope.AsshowninFigure2.3,themercurycellprocessinvolvestwo"cells".Intheprimaryelectrolyser(orbrinecell),purifiedandsaturatedbrinecontainingapproximately25%sodiumchlorideflowsthroughanelongatedtroughthatisslightlyinclinedfromthehorizontal.Inthebottomofthistroughashallowfilmofmercury(Hg)flowsalongthebrinecellcocurrentlywiththebrine.Closelyspacedabovethecathode,ananodeassemblyissuspended.

Electriccurrentflowingthroughthecelldecomposesthebrinepassingthroughthenarrowspacebetweentheelectrodes,liberatingchlorinegas(Cl2)attheanodeandmetallicsodium(Na)atthecathode.Thechlorinegasisaccumulatedabovetheanodeassemblyanddischargedtothepurificationprocess.

Asitisliberatedatthesurfaceofthemercurycathode,thesodiumimmediatelyformsanamalgam[KirkOthmer,1991].Theconcentrationoftheamalgamismaintainedat0.20.4%Na(byweight)sothattheamalgamflowsfreely,0.3%isthereferencefigurein[Gest93/186,1993].Theliquidamalgamflowsfromtheelectrolyticcelltoaseparatereactor,calledthedecomposerordenuder,whereitreactswithwaterinthepresenceofagraphitecatalysttoformsodiumhydroxideandhydrogengas.Thesodiumfreemercuryisfedbackintotheelectrolyserandreused.

Thereactionintheelectrolyseris:2Na++2Cl+2Hg 2NaHg+Cl2(g)

Thereactioninthedecomposeris:2NaHg+2H2O 2Na++2OH+H2(g)+2Hg

Thebrineanolyteleavingthecellissaturatedwithchlorineandmustbedechlorinatedbeforebeingreturnedtothedissolvers.

Thesodiumhydroxideisproducedfromthedenuderataconcentrationofabout50%themaximumvaluereportedis73%[Ullmanns,1996].However,industryreportsstatethatnoplantinEuropeisknowntobeoperatingabove50%.

Thedecomposermayberegardedasashortcircuitedelectricalcellinwhichthegraphitecatalystisthecathodeandsodiumamalgamtheanode.



Foritsoperation,themercurycelldependsuponthehigheroverpotentialofhydrogenversusmercurytoachievethepreferentialreleaseofsodiumratherthanhydrogen.However,impuritiesthatcanappearonthemercurysurfacemaylackthisovervoltageprotectionandcancauselocalisedreleaseofhydrogenintothechlorine(hydrogencanformanexplosivemixture(>4%H2)inchlorineorair).Thepresenceofeventraceamountsofcertainmetals,suchasvanadium,cancausethereleaseofdangerousamountsofhydrogen.

Mercurycellsareusuallyoperatedtomaintaina2122%(byweight)concentrationofsaltinthespentbrinedischargedfromtheelectrolyser.Thiscorrespondstothedecompositionof1516%ofthesaltduringasinglepass.Furthersaltdecompositiontoalowerconcentrationinthebrinewoulddecreasebrineconductivity,withtheattendantlossofelectricalefficiency.

Aportion,orinsomecasesall,ofthedepletedbrineissubsequentlydechlorinated,resaturatedwithsolidsalt,andreturnedtothecellbrinefeed.Somefacilitiespurgesmallamountsofbrinesolutionandusenewbrineasmakeupinordertopreventthebuildupofimpurities,mainlysulphate,inthebrine.Figure2.3showsaflowdiagramofthemercurycell.

Figure2.3:Flowdiagramofmercurycelltechnology

2.1.1Themercurycathodeelectrolyseranddecomposer

Thecellismadeofanelongated,slightlyinclinedtroughandagastightcover.Thetroughismadeofsteel,anditssidesarelinedwithaprotective,nonconductivecoatingtopreventcontactwiththeanolyte,toconfinebrinecathodecontacttothemercurysurface,andtoavoidthecorrosiveactionoftheelectrolyte.Modernelectrolysersare12.5mwideand1025mlong.Asaresult,thecellareatodaycanbegreaterthan30m2.Thesizeofthecellscanbevariedoverabroadrangetogivethedesiredchlorineproductionrate.Atthedesignstage,computerprogramscanbeusedtooptimisethecellsize,numberofcells,andoptimumcurrentdensityasafunctionoftheelectricitycostandcapitalcost[Ullmanns,1996].Thesteelbaseismadeassmoothaspossibletoensuremercuryflowinanunbrokenfilm.Intheeventofabreakinthemercurysurface,causticsodawillbeformedonthebare(steel)cathode,withsimultaneousreleaseofhydrogen,whichwillmixwiththechlorine.Becausehydrogenandchlorinecanformahighlyexplosivemixture,greatcareisnecessarytopreventhydrogenformationinthecell.

Characteristicsofthecathode:Thecathodeismadebyashallowlayerofmercurywhichflowsfromoneextremityofthecelltotheotherbecauseoftheslightinclinationfromthehorizontalofthecell.

Characteristicsoftheanode:Electrolyticcellanodesweremadeofgraphiteuntilthelate1960sinwesternEuropewhenanodesoftitaniumcoatedwithrutheniumoxide(RuO2)andtitaniumoxide(TiO2)weredeveloped.TheuseofRuO2andTiO2coatedmetalanodesreducesenergyconsumptionbyabout10%andtheirlifeexpectancyishigher.Inrecentyearstherehavebeencompetitivedevelopmentsindetailedanodegeometry,allwiththeaimofimprovinggasreleaseinordertoreduceohmiclossesandincreasethehomogeneityofthebrinetoimproveanodecoatinglife.

An"endbox"isattachedtoeachendoftheelectrolyser.Theendboxincorporatescompartmentsforcollectingthechlorinegasandweirsforseparatingthemercuryandbrinestreams,washingthemercuryandpermittingtheremovalofthickmercury"butter"thatisformedbyimpurities.[KirkOthmer,1991]



Thewholeelectrolyserisinsulatedfromthefloortopreventstraygroundcurrents.Usually,severalelectrolysersareplacedinseriesbymeansofelectricallyconnectingthecathodeofoneelectrolysertotheanodesofthenextelectrolyser.Individualcellscanbebypassedformaintenanceandreplacement.

Theelectrolyserisoperatedatatemperatureofapproximately7080C.Atthistemperature,theconductivityofthebrinesolutionandthefluidityofthemercuryarehighercomparedtooperationatambienttemperature.Thetemperaturecanbeachievedbypreheatingthesaturatedbrinewithsteamandisincreasedintheelectrolyserbytheheatofresistance.

Thedecomposeroperatesatatemperatureofapproximately90130C,whichiscausedbythechemicalreactionsinthedecomposerandtheinputofwarmamalgamfromtheelectrolyser.

2.1.2Decompositionoftheamalgam

Theamalgamisdecomposedinhorizontaldecomposers,alongsideorbeneaththecell(Figure2.4)ormoreoften,sinceca.1960,inverticaldecomposers(ordenuders),atoneendofthecell(Figure2.5).Industrialdecomposersareessentiallyshortcircuitedelectrochemicalprimarycells.Themostcommoncatalystisgraphite,usuallyactivatedbyoxidesofiron,nickelorcobaltorbycarbidesofmolybdenumortungsten.

Figure2.4:Mercurycellswithhorizontaldecomposer

[LeChlore,1996]

Themercuryprocesshastheadvantageoverdiaphragmandmembranecellsthatitproducesachlorinegaswithnearlynooxygen,anda50%causticsodasolution.However,mercurycellsoperateatahighervoltagethandiaphragmandmembranecellsand,therefore,usemoreenergy(causticsodaconcentrationexcluded).Theprocessalsorequiresapurebrinesolutionwithlittleornometalcontaminantstoavoidtheriskofexplosionthroughhydrogengenerationinthecell.Theamalgamprocessinherentlygivesrisetoenvironmentalreleasesofmercury.



Figure.5:Mercurycellswithverticaldecomposer

[LeChlore,1996]

2.2Thediaphragmcellprocess

Thediaphragmprocesswasdeveloped in the1880s in theUSAandwas the first commercialprocessused toproducechlorineandcaustic soda frombrine.InNorthAmerica,diaphragmcellsarestill theprimarytechnology,accountingforroughly70%ofallUSAproduction.Theprocessdiffersfromthemercurycellprocessinthatallreactionstakeplacewithinonecellandthecelleffluentcontainsbothsaltandcausticsoda.Adiaphragmisemployedtoseparatethechlorineliberatedattheanode,andthehydrogenandcausticsodaproduceddirectlyatthecathode.Withoutthediaphragmtoisolatethem,the hydrogenand chlorinewould spontaneously ignite and the caustic sodaand chlorinewould react to form sodiumhypochlorite (NaClO), with furtherreactiontoproducesodiumchlorate(NaClO3)[KirkOthmer,1991].

Thediaphragmisusuallymadeofasbestosandseparatesthefeedbrine(anolyte) fromthecausticcontainingcatholyte.Purifiedbrineenters theanodecompartmentandpercolatesthroughthediaphragmintothecathodechamber.Thepercolationrateiscontrolledbymaintainingahigherliquidlevelintheanodecompartmenttoestablishapositiveandcarefullycontrolledhydrostatichead.Thepercolationrate isdeterminedasacompromisebetweena lowrate thatwould produce a desirably high concentration of caustic soda in the catholyte (which provides the cell effluent) and a high rate to limit backmigrationofhydroxylionsfromcatholytetoanolyte,whichdecreasescathodecurrentefficiency[KirkOthmer,1991].

In the diaphragm cell, saturated brine (about 25% NaCl) is decomposed to approximately 50% of its original concentration in a pass through theelectrolyserascompared toa16%decompositionof salt perpass inmercury cells.Heating causedbypassageof current through the diaphragm cellraisestheoperatingtemperatureoftheelectrolyteto8099C.

When graphite anodes were used, the diaphragm became inoperable after 90100 days due to plugging of the diaphragm by particles of graphite.Nowadays,allplants in theEuropeanUnionusemetalanodesand the lifetimeof thediaphragm isoveroneyear.Theirservice lifehasalso increasedbecause their compositions have changed. At the beginning the diaphragms were made of asbestos only and were rapidly clogged by calcium andmagnesium ions coming from the brine. Asbestos was chosen because of its good chemical stability and because it is a relatively inexpensive andabundantmaterial.Beginningintheearly1970s,asbestosdiaphragmsbegantobereplacedbydiaphragmscontaining75%asbestosand25%of fibrousfluorocarbonpolymer of high chemical resistance. These diaphragms, trade namedModified Diaphragms, are more stable. The polymer stabilises theasbestos,whichinitselflowerscellvoltageandalsoallowsfortheuseoftheexpandableanode[LeChlore,1995][Ullmanns,1996].Chrysotileasbestos("whiteasbestos")istheonlyformofasbestosusedindiaphragmcells.



Figure2.6:Typicaldiaphragmelectrolysiscell

[USEPA,1995]

Bothdiaphragmandmembranecells for theproductionof chlorineand sodiumhydroxideare classifiedas eithermonopolar or bipolar. The designationdoes not refer to the electrochemical reactions that take place, which of course require two poles or electrodes for all cells, but to the electrolyserconstructionorassembly.Therearemanymorechloralkaliproductionfacilitieswithmonopolarcellsthanwithbipolarcells.Themonopolarelectrolyserisassembledsothattheanodesandcathodesarearrangedinparallel.Asaresultofthisconfiguration,allcellshavethesamevoltageofaboutthreetofourvoltsupto200cellscanbeconstructedinonecircuit.Bipolarelectrolysershaveunitassembliesoftheanodeofonecellunitdirectlyconnectedtothecathodeofthenextcellunit,thusminimisingintercellvoltageloss.Theseunitsareassembledinseries.

Alldiaphragmcellsproducecellliquorthatcontainsca.11%causticsodaand18%sodiumchloride.Thissolutionisevaporatedto50%NaOHbyweightatwhichpointallof thesalt,exceptaresidual1.01.5%byweight,precipitatesout.Thesaltgenerated isverypureand is typicallyused tomakemorebrine. This high quality sodium chloride is sometimes used as a raw material for an amalgam or membrane process. A flow diagram of a possibleintegratedsiteisshowninFigure2.7onpage*.

Low concentrations of oxygen in chlorine are formed by electrolytic decomposition of water and hypochlorous acid (from the reaction of chlorine withwater).

Precipitationofmagnesiumandcalciumhydroxidesonthecatholytesideofthediaphragmmayalsocreateblockingproblems.Hydrochloricacidisoftenadded to thebrine to removeCO2 itmayalsobeadded to thebrineentering theanodecompartment to reducebackmigrationofhydroxyl ionsand tosuppressformationofhypochlorousacid.



Figure2.7:Flowdiagramofintegrationofmembraneormercuryanddiaphragmprocesses

Diaphragmcellshavetheadvantageof:

operatingatalowervoltagethanmercurycells

operatingwithlesspurebrinethanrequiredbymembranecells

Whenusingasbestosdiaphragms,thediaphragmprocessinherentlygivesrisetoenvironmentalreleasesofasbestos.

2.2.1Diaphragmwithoutasbestos

Duetothepotentialexposureofemployeestoasbestosandemissionsintheenvironment,effortsarebeingexpandedtoreplacetheasbestoswithotherdiaphragmmaterials.

Developmentofnonasbestosdiaphragmsstartedinthemiddleofthe1980sandsomecompanieshavenowsucceededinoperatingwiththem.Thebasisofthematerialusedisthesameinalldiaphragmsdevelopedfreeofasbestos,i.e.afluorocarbonpolymer,mainlyPTFE(polytetrafluoroethylene).ThedifferenceslieinthefillersusedandthewaythehydrophobicPTFEfibresaretreatedanddepositedinordertoformapermeableandhydrophilicdiaphragm(seeSection4.3.2.).

2.2.2Activatedcathodes

Manydifferenttypesofactivatedcathodiccoatingareunderdevelopment inordertoreducethepowerconsumptionof thecell.Thesehavetoberobustbecausethepowerfulwaterjetusedtoremovethediaphragmfromthecathodemeshcanadverselyaffectthecathode.

An industrial application of "integrated precathode" diaphragm has been conducted (full scale) and has been found to contribute to saving energy byreducingelectricpowerconsumptionandimprovingcurrentefficiency.Thelifetimeofthediaphragmhasalsobeenfoundtobeimprovedbyintroductionoftheprecathode.

2.3Themembranecellprocess

Inthe1970s,thedevelopmentofionexchangemembranesenabledanewtechnologytoproducechlorine:themembraneelectrolysisprocess.Thefirstionexchangemembranesweredevelopedatthebeginningofthe1970sbyDuPont(Nafion),followedbyAsahiGlass(Flemion)whichinstalledthefirstindustrialmembraneplantinJapanin1975duetothepressureofJapaneseenvironmentalregulations.NonchloralkalirelatedmercurypollutioninMinamatadrovetheauthoritiestoprohibitallmercuryprocessesandJapanwasthefirstcountrytoinstallthemembraneprocessonamassivescaleinthemid1980s.

Today,itisthemostpromisingandfastdevelopingtechniquefortheproductionofchloralkalianditwillundoubtedlyreplacetheothertwotechniquesin



time.Thiscanbededucedfromthefactthatsince1987practically100%ofthenewchloralkaliplantsworldwideapplythemembraneprocess.Thereplacementofexistingmercuryanddiaphragmcellcapacitywithmembranecellsistakingplaceatamuchslowerratebecauseofthelonglifetimeoftheformerandbecauseofthehighcapitalcostsofreplacement.

Inthisprocess,theanodeandcathodeareseparatedbyawaterimpermeableionconductingmembrane.Brinesolutionflowsthroughtheanodecompartmentwherechlorideionsareoxidisedtochlorinegas.Thesodiumionsmigratethroughthemembranetothecathodecompartmentwhichcontainsflowingcausticsodasolution.Thedemineralizedwateraddedtothecatholytecircuitishydrolysed,releasinghydrogengasandhydroxideions.Thesodiumandhydroxideionscombinetoproducecausticsodawhichistypicallybroughttoaconcentrationof3235%byrecirculatingthesolutionbeforeitisdischargedfromthecell.Themembranepreventsthemigrationofchlorideionsfromtheanodecompartmenttothecathodecompartmenttherefore,thecausticsodasolutionproduceddoesnotcontainsaltasinthediaphragmcellprocess.Depletedbrineisdischargedfromtheanodecompartmentandresaturatedwithsalt.Ifneeded,toreachaconcentrationof50%causticsoda,thecausticliquorproducedhastobeconcentratedbyevaporation(usingsteam).

Thecathodematerialusedinmembranecellsiseitherstainlesssteelornickel.Thecathodesareoftencoatedwithacatalystthatismorestablethanthesubstrateandthatincreasessurfaceareaandreducesovervoltage.CoatingmaterialsincludeNiS,NiAl,andNiNiOmixtures,aswellasmixturesofnickelandplatinumgroupmetals.Theanodesusedaremetal.

Themembranesusedinthechloralkaliindustryarecommonlymadeofperfluorinatedpolymers.Themembranesmayhavefromoneuptothreelayers,butgenerallyconsistoftwolayers.Oneoftheselayersconsistsofperfluorinatedpolymerwithsubstitutedcarboxylicgroupsandisadjacenttothecathodicside.Theotherlayerconsistsofperfluorinatedpolymerwithsubstitutedsulphonicgroupsandisadjacenttotheanodicside.Togivethemembranemechanicalstrength,themembraneisgenerallyreinforcedwithPTFEfibres.Themembranesmustremainstablewhilebeingexposedtochlorineononesideandastrongcausticsolutionontheother.Thegeneraleconomiclifetimeofchloralkalimembranesisapproximatelythreeyears,butrangesbetween25years[EuroChlorreport,1997].

Membranecellshavetheadvantageofproducingaverypurecausticsodasolutionandofusinglesselectricitythantheotherprocesses.Inaddition,themembraneprocessdoesnotusehighlytoxicmaterialssuchasmercuryandasbestos.Disadvantagesofthemembraneprocessarethatthecausticsodaproducedmayneedtobeevaporatedtoincreaseconcentrationand,forsomeapplications,thechlorinegasproducedneedstobeprocessedtoremoveoxygen.Furthermore,thebrineenteringamembranecellmustbeofaveryhighpurity,whichoftenrequirescostlyadditionalpurificationstepspriortoelectrolysis(seeparagraphonbrinepurification).

Figure2.8:Diagramofamembraneprocess

Membranescellscanbeconfiguredeitherasmonopolarorbipolar.Asinthecaseofthediaphragmcellprocess,thebipolarcellshavelessvoltagelossbetweenthecellsthanthemonopolarcells.However,thenumberofcellsconnectedtogetherinthesamecircuitislimited.



Figure2.9:Explodedviewofamonopolarmembraneelectrolyser

[Source:DeNora]



Figure2.10:Viewofamembranecellroomequippedwithbipolarelectrolysers

(Source:HoechstUhde)

2.4Auxiliaryprocesses

Apart from the cells, which remain the heart of the chlorine production line, there are other processing steps or equipment, common to amalgam,diaphragmandmembranetechnologies.Theseare:

saltunloadingandstorage

brinepurificationandresaturation

chlorineprocessing

causticprocessing

hydrogenprocessing.

2.4.1Saltunloadingandstorage

Thebrineusedinthemercurycellandmembraneprocessesisnormallysaturatedwithsolidsalt,althoughsomeinstallationsusesolutionminedbrineonaoncethroughbasis(i.e.nobrinerecirculation).

Brineisgenerallyproducedbythedissolvingoffreshsaltinwaterordepletedbrinefrommercuryandmembraneprocesses.Thebasicrawmaterialisusuallysolidsalt:rocksalt,solarsalt,orvacuumevaporatedsaltfrompurifyingandevaporatingsolutionminedbrine.

Generallythesaltisstoredinasealedareaequippedwitharoof.Becauseofitshighpuritythevacuumsaltinparticularneedstobeprotected.



2.4.2Brinepurificationandresaturation

2.4.2.1Brinepurification

Ascanbeseen inFigure2.1onpage10, thebrinepurificationprocess consistsof aprimary system formercury and diaphragm technologies and anadditionalsecondarysystem formembrane technology.Thisoperation isneeded toavoidanyundesirablecomponents (sulphateanions,cationsofCa,Mg, Ba andmetals) that can affect the electrolytic process. The quality of the rawmaterial and the brine quality requirements for each of the threetechnologiesdeterminethecomplexityofthebrinetreatmentunit.

Primarypurification

Precipitation

The initialstageofpurificationusessodiumcarbonateandsodiumhydroxide toprecipitatecalciumandmagnesium ionsascalciumcarbonate(CaCO3)andmagnesium hydroxide (Mg(OH)2). Metals (iron, titanium, molybdenum, nickel, chromium, vanadium, tungsten) may also precipitate as hydroxideduringthisoperation.Theusualwaytoavoidmetalsistospecifytheirexclusioninthepurchaseandtransportspecificationforthesalt.Sodiumsulphateiscontrolledbyaddingcalciumchloride(CaCl2)orbariumsaltstoremovesulphateanionsbyprecipitationofcalciumsulphate(CaSO4)orbariumsulphate(BaSO4).Precipitationofbariumsulphatecantakeplacesimultaneouslywith theprecipitationofcalciumcarbonateandmagnesiumhydroxide,whereastheprecipitationofcalciumsulphaterequiresaseparatevessel.

Filtration

Theprecipitatedimpuritiesareremovedbysedimentation,filtrationoracombinationofboth.Theseparatedfiltercakeisgenerallyconcentratedto5060%(althoughafigureof6080%isreportedintheliterature)solidscontentinrotarydrumvacuumfiltersorcentrifugesbeforedisposal.[Ullmanns,1996]

Thepurifiedbrineshouldcontainideally[Ullmanns,1996]:

Ca:



(Source:AsahiGlassCO)

Thesecondarybrinepurificationconsistsofapolishfiltrationstepandbrinesofteninginanionexchangeunit:

Thesecondaryfiltrationgenerallyconsistsofcandletype,plateframeorpressureleaffilters(eitherwithorwithoutcellulosebasedprecoat)inordertosufficientlyreducethesuspensionmatterandprotecttheionexchangeresinfromdamage.

Theionexchangechelatingresintreatmentisdesignedtodecreasethealkalineearthmetalstoppblevel.Table2.2indicatesthespecificationsrequiredformetals,SO4andotherimpurities.Thesespecificationscanvaryiftheuserswanttooperateatalowcurrentdensity(



electrolyticcell.Ifconcentratedinliquidformindownstreamprocesses,NCl3mayexplodewithdisastrousresults.

NitrogencompoundsinthebrineisthemainsourceofNCl3.Rocksalt,inparticularsolutionminedsaltusingsurfacewaters,willcontainvaryinglevelsofammoniumandnitratesalts,whereastheuseofvacuumsaltinthebrinerecyclecircuitwillgiveverylowlevelsofNCl3,exceptwhereferrocyanidesareaddedtoavoidcaking.Also,thewaterqualitymayvary,inparticularifsurfacewaterisused.Thetotalconcentrationofnitrogencompoundsinthebrineshouldbecheckedregularly.ChlorinationatapHhigherthan8.5orhypochloritetreatmentofthebrineishowever,capableofdestroyingalargeproportionoftheammoniumsaltimpurity.[Gest76/55,1990]

MethodstoremoveNCl3fromchlorineafteritisformedaredescribedinSection4.1.6.

Figure2.12:Viewofchelateresintowersinasecondarybrinepurificationsystem

(photographsuppliedbyAsahiGlassCo)



Figure2.13:Viewofpolishingfiltersinasecondarybrinepurificationsystem

(photographsuppliedbyAsahiGlassCo)

2.4.2.2Brineresaturationanddechlorination

Mercuryandmembranesystemsusuallyoperatewithbrinerecirculationandresaturation.Thereare,however,3wastebrinemercuryplantsand1wastebrinemembraneplantoperatinginwesternEurope.

Somediaphragmcelllineshaveaoncethroughbrinecircuit,whilstothersemploybrinesaturationusingthesaltrecoveredfromthecausticevaporators.

Inrecirculationcircuits,thedepletedbrineleavingtheelectrolysersisfirstdechlorinated:

Partiallyforthemercuryprocess(leavingactivechlorineinthebrinekeepsthemercuryinionicformandreducesthepresenceofmetallicmercuryinthepurificationsludge)

Totallyforthemembraneprocess(necessaryherebecausetheactivechlorinecandamagetheionexchangeresinsofthesecondarybrinepurificationunit).

Forthispurpose,thebrineissenttoanairblownpackedcolumnorissprayedintoavacuumsystemtoextractthemajorpartofthedissolvedchlorine.

Nosuchdechlorinationtreatment isrequiredfor thediaphragmsystemsinceanychlorinepassingthroughthediaphragmreactswithcausticsoda in thecatholytecompartmenttoformhypochloriteorchlorate.

Forthemembraneprocess, there isapreliminarystageofhydrochloricacidaddition(to reachpH22.5) inorder toachievebetterchlorineextraction.Afurtherstageisalsonecessarytoeliminatethechlorinecompletelythisisdonebypassingthebrinethroughanactivatedcarbonbedorbyinjectionofareducingagent(e.g.sulphite).

Ifthesaturationismadewithimpuresalt(followedbyaprimarypurificationsteponthetotalbrineflow),thepHofthebrineisthenbroughttoanalkalinevaluewithcausticsoda,toreducethesolubilisationofimpuritiesfromthesalt.Ifthesaturationismadewithpuresalt(withsubsequentprimarypurificationonasmallpartoftheflow),thereisnoalkalisationstepatthatlevel(onlyinthepurificationphase).

Depletedbrinefromthemercuryandmembranecells,withaconcentrationof210250g/l,dependingonthetechnology,currentdensityandheatbalanceofthecell,isresaturatedbycontactwithsolidsalttoachieveasaturatedbrineconcentrationof310315g/l.

Inthecaseofdiaphragmcells,thecatholyteliquor(1012%NaOH,15%NaCl)goesdirectlytothecausticevaporatorswheresolidsaltand50%causticarerecoveredtogether.Freshbrinecanbesaturatedwithrecycledsolidsaltfromthecausticevaporatorsbeforeenteringthediaphragmelectrolysers.



Resaturatorscanbeeitheropenorclosedvessels.

ThepHofthebrinesenttotheelectrolysersmaybeadjustedtoanacidicvalue(pH4)withhydrochloricacidinordertoprotecttheanodecoating,tokeepthe formation of chlorate at a low level and to decrease the oxygen content in the chlorine gas. Hydrochloric acid can also be added in the anodiccompartments of membrane cells to further reduce the content of oxygen in the chlorine, especially for electrolysers with older membranes (poorerperformances)The(bi)carbonatesintroducedwiththesaltaredecomposedbytheseacidadditions,producinggaseouscarbondioxide.

2.4.3Chlorineproduction,storageandhandling

Generally,before the chlorine can be used, it goes through a series of processes for cooling, cleaning, drying, compression and liquefaction. In someapplications,itcanbeusedasadrygaswithoutneedforliquefaction.Veryoccasionallyitcanbeuseddirectlyfromtheelectrolysers.Ageneral flowofchlorinefromtheelectrolyserstostorageispresentedinFigure2.14.

Figure2.14:Theflowofchlorinefromtheelectrolyserstostorage

[EuroChlorreport,1997]

Thechlorineprocessusuallytakeshot,wetcellgasandconvertsittoacold,drygas.Chlorinegasleavingtheelectrolysersisatapproximately8090Candsaturatedwithwatervapour.Italsocontainsbrinemist,impuritiessuchasN2,H2,O2,CO2andtracesofchlorinatedhydrocarbons.Electrolysersareoperatedatessentiallyatmosphericpressurewithonlyafewmilliatmospheresdifferentialpressurebetweentheanolyteandthecatholyte.

Cooling

Intheprimarycoolingprocess,thetotalvolumeofgastobehandledisreducedandalargeamountofmoistureiscondensed.Coolingisaccomplishedineitheronestagewith chilledwater or in two stageswith chilledwater only in the second stage.Care is taken to avoid excessive cooling because, ataround 10 C, chlorine can combine with water to form a solid material known as chlorine hydrate. Maintaining temperatures above 10 C prevents



blockagesinprocessequipment.

Twomethodsaremostfrequentlyusedtocoolchlorinegas:

1. Indirectcoolingthroughatitaniumsurface(usuallyinasinglepassverticalshellandtubeexchanger).Theresultantcondensateiseitherfedbackintothebrinesystemofthemercuryprocessordechlorinatedbyevaporationinthecaseofthediaphragmprocess.Thismethodcauseslesschlorinetobecondensedorabsorbedandgenerateslesschlorinesaturatedwaterfordisposal.[BrienWhite,1995]

2. Directcontactwithwater.ThechlorinegasiscooledbypassingitdirectlyintothebottomofatowerinwhichthepackingisdividedintotwoSections,for2stagecooling.Waterissprayedintothetopandflowscountercurrenttothechlorine.Thecoolingwatershouldbefreeoftracesofammoniumsaltstoavoidtheformationofnitrogentrichloride.Thismethodhastheadvantageofbettermasstransfercharacteristicsandhigherthermalefficiency.

Closedcircuitdirectcoolingofchlorinecombinestheadvantagesofthetwomethods.Thechlorineladenwaterfromthecoolingtoweriscooledintitaniumplatecoolersandrecycled.Thesurpluscondensateistreatedexactlylikethecondensatefromindirectcooling.

Followingprimarycooling,chlorinegasisdemistedofwaterdropletsandbrineimpurities.Impuritiesareremovedmechanicallybyusingspecialfilters,orbymeansofanelectrostaticprecipitator.Chlorineisthenpassedtothedryingtowers.

Drying

Chlorinefromthecoolingsystemismoreorlesssaturatedwithwatervapour.Thewatercontentistypically13%.Thismustbereducedinordertoavoiddownstreamcorrosionandminimisetheformationofhydrates[BrienWhite,1995].

Dryingofchlorineiscarriedoutalmostexclusivelywithconcentratedsulphuricacid[Ullmanns,1996].Dryingisaccomplishedincountercurrentsulphuricacidcontacttowerswhichreducethemoisturecontenttolessthan20ppm[Stenhammar].Drychlorineleavingthetopofthedryingtowerpassesthroughhighefficiencydemisterstopreventtheentrainmentofsulphuricaciddroplets.Thespentacidusuallybecomesawasteproductorrequiresreprocessingifitisreused.Forexample,ithastobedechlorinatedbyairblowingandmaybereconcentratedbeforebeingsoldorusedforeffluenttreatment.

Compression

Afterdrying,chlorinegasmightbescrubbedwithliquidchlorineortreatedwithultravioletirradiationtoreducelevelsofnitrogentrichlorideandthenitmaybecompressedinavarietyofcompressors:

sulphuricacidliquidringcompressorsatlowpressures(~4bar)

monoormultistagecentrifugalcompressors(5barorhigher)

reciprocatingcompressors(>11bar)

screwcompressors(variouspressures)

Becauseofheatbuildup fromcompression,multistageunitswithcoolersbetweenstagesareusuallynecessary.Compressor sealsaregenerally fittedwithapressurisedpurgetoinhibitleakageofchlorinetotheatmosphere[UKGuidancenote,1993].

Toavoidnoise,whichisquiteimportantevenforlowpressures,chlorinecompressorsshouldbesoundinsulated.

Liquefaction

Liquefactioncanbeaccomplishedat different pressureand temperature levels, at ambient temperature andhighpressure (for example 18 Cand712bar),atlowtemperatureandlowpressure(forexample35Cand1bar)oranyotherintermediatecombinationoftemperatureandpressure.

Thechosenliquefactionpressureandtemperatureinfluencethechoiceofcoolingmediaandthesafetyprecautionsnecessarytooperatesafely.However,theefficiencyof liquefaction is limitedbecausehydrogen isconcentrated in theresidualgasand itsconcentrationneeds tobekeptbelowtheexplosivelimits.

Thechoiceof thecoolingmediuminacertainstageof the liquefactiondependsonthetemperatureof thechlorine.When the temperature issufficientlyhigh,watercanbeusedasan indirect coolingmedium.When the temperature is relatively low, other coolingmedia such asHCFCs orHFCs (indirectcooling),ammonia(indirectcooling)orliquidchlorine(directcooling)areused.

Thetemperatureofthechlorinegasinacertainstagedependsmainlyontheinitialtemperatureandonthepressureincreaseduringcompression.Alargepressureincreasegenerallyenableswatercooling,butimpliesanincreasedhazardrisk.Chlorinetemperaturehastobekeptwellbelowthepointwhereitreactsspontaneouslyanduncontrollablywithiron(approx.120oC).

Constructionmaterialsmustbechosentosuittheconditionsunderwhichchlorineisbeinghandled:

Wetordry



Gasorliquid

Temperature

Pressure

In termsof safety, it is very important to avoid, during compression and liquefaction, any possibility ofmixing chlorinewith oils or greaseswhich arereactiveasregardschlorine.

Table2.3showsthepossibletradeoffbetweendifferenttypesofchlorinegasliquefaction,coolingmethodsappliedandsafetyaspects.

Liquefactionsystem

Coolingmedium

Safetyaspect

Storage

Highpressure(716bar)andhightemperatures Water Highprecautions

Lowestenergycostsbuthighmaterialcosts

Mediumpressure(26bar)andmediumtemperatures(between10and20C)

WaterHCFC/HFCorammonia

Moderateprecautions

Moderateenergyand

materialcosts

Normalpressure(~1bar)andlowtemperatures(below40C)

MainlyHCFC/HFCorammonia

Precautions1

Cryogenicstorageofliquidchlorineispossible.Highenergyandlowermaterialcosts

1.Attentionmustbepaidtotheincreasedsolubilityofothergasesatlowtemperatures,especiallycarbondioxide

Table2.3:Possibletradeoffinchlorinegasliquefaction

basedon[Ullmanns,1996],[Dutchreport,1998]

Handlingandstorage

Chlorineisliquefiedandstoredatambientorlowtemperature.Theliquidchlorinefromthebulktankcanbeusedasafeedstockforonsiteprocessesorloadedintocontainers,roadorrailtankers.Becauseofthehightoxicityofchlorine,thestorageareamustbecarefullymonitoredandspecialcaremustbetakenduringloadingoperations.

2.4.3.1Dealingwithimpurities

Chlorinegasfromtheelectrolysiscellsmaycontainimpuritiessuchasnitrogentrichloride(NCl3),bromine(Br2),halogenatedhydrocarbons(CXHYXZ),carbondioxide(CO2),oxygen(O2),nitrogen(N2)andhydrogen(H2).

Nitrogentrichloride,bromineandhalogenatedhydrocarbonspredominantlydissolveintheliquidchlorine,whereasthenoncondensablegases(CO2,O2,N2,H2)remaininthegasphaseandincreaseinconcentrationduringchlorineliquefaction.Tracesofsulphuricacid,ferricsulphate,ferricchlorideand/orcarbontetrachloridemightalsobepresentinthegasphaseafterdryingandliquefactionofchlorine.

Specialattentionshouldbepaidtothefollowingimpurities:

Water

Allmetalsareattackedbywetchlorinewiththeexceptionoftitaniumandtantalum.Titaniumcanonlybeusedinwetchlorineconditionsitspontaneouslycombustsindrychlorine.

Hydrogen

Allthreetechnologiesproducehydrogenwhichcanformanexplosivemixture(>4%H2)inchlorineorair.Light,frictionandgasdepressurisationmaybringenoughenergytoinitiatethereactionatambienttemperature.Chlorinegasisanalysedregularlytoensuretheabsenceofanexplosivemixture.

Nitrogentrichloride



Nitrogentrichlorideisformedduringtheelectrolyticproductionofchlorine,duetosidereactionsbetweenthechlorineandvariousnitrogencompoundsinthebrinesolution.1ppmofNH3inbrineisenoughtogive>50ppmNCl3inliquidchlorine.Inplantswhichusedirectcontactwatercoolingofthechlorinegasbeforedryingandcompression,NCl3mayalsobeformedifthewaterispollutedwithnitrogencompounds[Gest76/55,1990].

Nitrogentrichlorideischaracterisedbyitsutmostinstability.ExperimentalresultsshowthataconcentrationofNCl3greaterthan3%byweightatambienttemperatureiscapableofaccelerateddecompositionwhichisstronglyexothermic.

NCl3hasahigherboilingpointthanchlorineandanyNCl3presentinthechlorinegaswillconcentrateintheliquidphaseinachlorineliquefactionprocess.AnyevaporativehandlingofliquidchlorineispotentiallydangerousduetotheselectiveconcentrationofNCl3intheliquidphase.

MethodstoremoveNCl3fromchlorinearedescribedinSection4.1.6.

Bromine

Thequantityofbrominepresentdependsonthequalityofthesaltused.Itsconcentrationisgenerallyhigherifchlorineisobtainedbyelectrolysingpotassiumchloridetoobtainpotassiumhydroxide.Bromine,likewater,canacceleratethecorrosionofthematerials.

Noncondensablegases(CO2,O2,N2,H2)

Thereareseveralwaystodealwiththenoncondensablegases,dependingonthelayoutofthechlorineliquefactionunit.Somearedescribedbelow.

DilutionwithairandproductionofweakNaOCl

Duringchlorinegascompressionandcooling,mostofthechlorinegasiscondensed.However,thenoncondensablegases(H2,CO2,O2,N2)increaseinconcentration.Bydilutingtheremainingchlorinegaswithair,theconcentrationofhydrogencanbekeptbelowtheexplosionlimit.Thisallowsadditionalliquefactionofchlorinegas.Theremaininggasesafterliquefaction(socalled"tailgas")havetobepurgedfromthesystem.Thetailgasstillcontainsasignificantamountofchlorine,andthegasisthereforenormallyledtothechlorinedestruction/absorptionunit.

Productionofhydrochloricacid

Insteadofdilutingtheremaininggasesafterpartialcondensationofthechlorinegas,thehydrogencanberemovedfromthesystembymeansofareactionwithchlorinegasinacolumn.Thisremovesvirtuallyallthehydrogenandyieldsgaseoushydrochloricacid,whichexistsharmlesslywiththechlorinegasandcanberecoveredinahydrochloricacidunit.Theremainingchlorinegascannowsafelybefurthercondensed.Thetailgaseswithsomechlorinegasandtheremainingnoncondensablegases(CO2,N2,O2)willbepassedthroughahydrochloricacidunit.ThissolutioncanbechosenifHClisasaleableproductorifitcanbeusedasafeedstockfordownstreamproduction,suchasferricchloride.

2.4.3.2Thechlorineabsorptionunit

Thepurposeofatypicalchlorineabsorptionsystemistwofold:

1. Tocontinuouslyabsorbchlorinegasarisinginstreamssuchastailgasfromliquefaction,airblownfromwastebrineorchlorinecondensatedechlorinationandalsowetanddrymaintenanceheaders.Upto5%,butnormallylessthan1%,oftheplantproductionisabsorbedinthisway.

2. Toabsorbthefullcellroomproductionduringemergencyforanadequateperiod,usuallynotlessthan15to30minutes,toenablecorrectivemeasurestobetakenortheplanttobeshutdowninasafemanner.Gravityfedheadtanksorpumpssuppliedwithbackuppowersuppliesmaybeusedtogiveincreasedreliabilityandoperationunderpowerfailureconditions.

Theabovefunctionscouldbeundertakeninseparatepurposebuiltunits,providingtheintegrityofthesystemsismaintainedbyhavingbackupscrubbers.All gaseous vents contaminated or potentially contaminated with chlorine thus pass into the atmosphere through caustic scrubbing towers containingpackingirrigatedwithcausticsoda.Heatisgeneratedbytheabsorptionreactionandtemperaturesshouldnotbeallowedtoincreaseaboveabout30Ctoavoid formation of sodium chlorate instead of sodiumhypochlorite solution. To avoid overheating during a fullscale relief, the fresh caustic soda feedsolutionshouldnotbestrongerthanabout12%byweight.Highercausticconcentrationscanbeusedprovidingadequatecooling is installed,butthere isanincreasedriskofsolidsdepositionandblockage.

Tailgasfromthechlorineliquefactionunitcontainsresidualchlorineandcarbondioxide,whichareabsorbedinthecaustic,andhydrogen,whichisdilutedwithairtolessthan4%byvolumetoavoidflammablemixtures.

Optimum design of scrubbing systemsmust include high reliability, automatic operation in emergencies, and countercurrent flow of liquid and gas toachievelowexitconcentrations.Ifsaleisnotpossible,efficientdecompositionofthesodiumhypochloriteproducedintosodiumchloridecanbeachievedusinganickelcatalyst.

2.4.4Causticproduction,storageandhandling

Sodiumhydroxide(causticsoda)isproducedinafixedratioof1.128tonnes(as100%NaOH)pertonnechlorineproduced.



Figure2.15:Viewofcausticproductionandstorage

[OxyChem,1992]

Thecausticsodasolutionfromthethreetechnologiesistreatedinslightlydifferentwaysduetothedifferenceincompositionandconcentration.

Inthemercurycellprocess,50%causticsodaisobtaineddirectlyfromthedecomposers.Thecausticsodaisnormallypumpedthroughacooler,thenthroughamercuryremovalsystemandthentotheintermediateandfinalstoragesections.Insomecasesthecausticisheatedbeforefiltration.Themostcommonmethodforremovalofmercuryfromcausticsodaisaplate(orleaf)filterwithcarbonprecoat.Undernormaloperatingconditions,mercurycellcausticsoda(as100%NaOH)contains20100ppmofsodiumchlorideand4060gHg/kgNaOH.

Inthecaseofdiaphragmandmembranetechnologiesthecausticsodaisconcentratedbyevaporationbeforefinalstorage.



Figure.16:Theflowtostorageofcausticsodafromthedifferenttechnologies

Basedon[OxyChem,1992]

Steamisusedasthesourceofevaporativeenergy.Thepresenceofsaltinthediaphragmcellliquorrequiresthattheevaporatorisequippedwithscraperbladesorotherdevicestodrawofftheprecipitatedsalt.Thishighqualitysodiumchloridecanthenbeusedtoenrichdepletedbrine,sometimesitisusedasarawmaterialforanamalgamormembraneprocess.Theresiduallevelofsodiumchlorideinsodiumhydroxidefromdiaphragmcellisabout1%andsodiumchlorate0.1%.Forthisreason,itisunsuitableforcertainendapplicationssuchasthemanufactureofrayon.

Saltandsodiumchlorateinthecausticsodafromdiaphragmcellscanbereducedbyammoniaextractiontoincreasemarketability,butatincreasedcost.

Thecausticsodafrommembranecellsisofhighquality,althoughthecausticsodaproduced(usuallyaround33%NaOH)needsconcentrationto50%NaOHforsomeapplications.Thesaltcontentofthemembranecellcausticsodaliesbetween20100ppm(in100%NaOH),butisonaverageslightlyhigherthanmercurycellcaustic(seeTable2.1).

Insomeplantsthecausticsodaisfurtherconcentratedtoa73%solutionandto100%assolidcausticprillsorflakes.

Somechloralkaliproductionfacilitiescancombinethecausticproductionprocessfrommercuryandmembranecellsinordertominimiseenergycosts.Itispossibletofeed33%causticfromthemembranecellstothedecomposertoproduce50%causticwithouttheneedforevaporation.

Storageandhandling

Becauseofitshighlyreactiveandcorrosiveproperties,causticsodamaycorrodecontainersandhandlingequipment.Constructionmaterialsmustbesuitedtothecausticsodahandledandstored.

Sodiumhydroxidesolutionsrequiresteamorelectricalheatingwheretemperaturescanfallbelowtheupperfreezingpoint.Frozenpipelinespresentbothsafetyandenvironmentalriskswhenattemptsaremadetounblockthem.SafetymeasuresaresetoutinChapter4.

Storagetanksmaybelinedinordertominimiseironcontaminationoftheproductortoavoidstresscorrosioncrackingthetank.Tanksareusuallyincludedinprocedurestopreventoverfloworspillageofcausticsoda.Suchproceduresincludecontainmentandmitigation.

Itshouldbenotedthatdissolvedhydrogengascanbereleasedintothevapourspaceabovetheliquidinstoragetanks.Tanksarenormallyventedfromthehighestpoint.Testingforanexplosivemixtureofhydrogeninairnormallyprecedesanymaintenanceactivityinthearea.



2.4.5Hydrogenproduction,storageandhandling

Hydrogen isproduced ina fixed ratioof28kgper tonnechlorineproduced.Hydrogen leaving the cells is highly concentrated (>99.9%by volume) andnormallycooledtoremovewatervapour,sodiumhydroxideandsalt.Thesolutionofcondensedsaltwaterandsodiumhydroxideiseitherrecycledasbrinemakeup or treated with other waste water streams. In the mercury cell process, hydrogen has to be treated to remove mercury. Primary cooling atambienttemperatureiscarriedoutattheelectrolyser,allowingmercuryvapourtocondenseintothemainmercurycircuit.Furthercoolingtakesplaceatalaterstageusinglargeheatexchangersandcondensateissentformercuryrecovery.

Hydrogenmaybedistributedtousersusingboosterfansorfedtothemaincompressionplant.Themainhydrogencompressionplantusuallycomprisesanumberofcompressorsandagasholder (surgechamber).Thehydrogengasholder is incorporated into thesystem tominimise fluctuations in thegaspressurefromtheprimarystage.Thehydrogenproductgasstreamisalwayskeptpressurisedtoavoid ingressofair.Allelectricalequipmenttakenintothehydrogencompressionplantareamustbe"intrinsicallysafe",i.e.theequipmentwillnotproduceaspark.Areliefvalveisnormallyprovidedwithinthesystemtorelievehighpressuretoatmosphere.

Hydrogenisnormallyanalysedforoxygencontentthecompressionwillshutdownautomaticallyincriticalsituations[EuroChlorreport,1997].

The hydrogen is in general used for onsite energy production. It is burnt as a fuel, either by the company operating the chlorine plant or by anothercompany towhom ithasbeensoldasa fuel.Someorall of it canalsobeusedonsite in the caseof integrated sitesor sold toother companiesaschemicalfeedstock(productionofhydroxylamines,hydrochloricacid,hydrogenperoxide,sodiumsulphite,forexample).

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