Fundamentals of Ion Exchange - Dowmsdssearch.dow.com/PublishedLiteratureDOWCOM/dh_0032/0901b... · Fundamentals of Ion Exchange Ion exchange is the reversible interchange of ions

Dow Liquid Separations

Fundamentals of Ion Exchange

Based on a paper by: R. M. Wheaton – Dow Chemical U.S.A.L. J. Lefevre – Dow Chemical U.S.A.

DOWEXIon Exchange Resins

Fundamentals of Ion Exchange

Ion exchange is the reversibleinterchange of ions between a solid(ion exchange material) and a liquidin which there is no permanentchange in the structure of the solid.Ion exchange is used in watertreatment and also provides a methodof separation in many non-waterprocesses. It has special utility inchemical synthesis, medical research,food processing, mining, agricultureand a variety of other areas.

The utility of ion exchange rests withthe ability to use and reuse the ionexchange material. For example, inwater softening:

2RNa+ + Ca2+ $ R2Ca2+ + 2Na+

The exchanger R in the sodium ionform is able to exchange for calciumand thus, to remove calcium fromhard water and replace it with anequivalent quantity of sodium.Subsequently, the calcium loadedresin may be treated with a sodiumchloride solution, regenerating it backto the sodium form, so that it is readyfor another cycle of operation. Theregeneration reaction is reversible;the ion exchanger is not permanentlychanged. Millions of liters of watermay be softened per cubic meter ofresin during an operating period ofmany years.

Ion exchange occurs in a variety ofsubstances and it has been used onan industrial basis since circa 1910with the introduction of watersoftening using natural and later,synthetic zeolites. Sulfonated coal,developed for industrial watertreatment, was the first ion exchangematerial that was stable at low pH.The introduction of synthetic organicion exchange resins in 1935 resultedfrom the synthesis[1] of phenoliccondensation products containingeither sulfonic or amine groups whichcould be used for the reversibleexchange of cations or anions.

A variety of functional groups havebeen added to the condensation oraddition polymers used as thebackbone structures. Porosity andparticle size have been controlled by

conditions of polymerization anduniform particle size manufacturingtechnology. Physical and chemicalstability have been modified andimproved. As a result of theseadvances, the inorganic exchangers(mineral, greensand and zeolites)have been almost completelydisplaced by the resinous typesexcept for some analytical andspecialized applications. Syntheticzeolites are still used as molecularsieves.

Physical Properties of Resins

Conventional ion exchange resinsconsists of a cross-linked polymermatrix with a relatively uniformdistribution of ion-active sitesthroughout the structure. A cationexchange resin with a negativelycharged matrix and exchangeablepositive ions (cations) is shown inFigure 1. Ion exchange materials aresold as spheres or sometimesgranules with a specific size anduniformity to meet the needs of aparticular application. The majorityare prepared in spherical (bead) form,

either as conventional resin with apolydispersed particle size distributionfrom about 0.3 mm to 1.2 mm (50-16mesh) or as uniform particle sized(UPS) resin with all beads in a narrowparticle size range. In the water-swollen state, ion exchange resinstypically show a specific gravity of1.1-1.5. The bulk density as installedin a column includes a normal 35-40percent voids volume for a sphericalproduct. Bulk densities in the range of560-960 g/l (35-60 lb/ft3) are typicalfor wet resinous products.

Chemical Properties of Resins

Capacity. Ion exchange capacity maybe expressed in a number of ways.Total capacity, i.e., the total numberof sites available for exchange, isnormally determined after convertingthe resin by chemical regenerationtechniques to a given ionic form. Theion is then chemically removed from ameasured quantity of the resin andquantitatively determined in solutionby conventional analytical methods.Total capacity is expressed on a dryweight, wet weight or wet volume

Figure 1. Cation Exchange Resin Schematic Showing NegativelyCharged Matrix and Exchangeable Positive Ions

basis. The water uptake of a resinand therefore its wet weight and wetvolume capacities are dependent anthe nature of the polymer backboneas well as an the environment inwhich the sample is placed.Variations of dry weight and wetvolume capacities with cross-linkageare shown in Figure 2 for a sulfonicresin.

Operating capacity is a measure ofthe useful performance obtained withthe ion exchange material when it isoperating in a column under aprescribed set of conditions. It isdependent on a number of factorsincluding the inherent (total) capacityof the resin, the level of regeneration,the composition of solution treated,the flow rates through the column,temperature, particle size anddistribution. An example is shown inFigure 3 for the case of watersoftening with a standard sulfonicresin at several regenerant levels.

Swelling. Water swelling of an ionexchanger is primarily a hydration ofthe fixed ionic groups and increaseswith an increase in capacity to thelimits imposed by the polymernetwork. Resin volumes change withconversion to ionic forms of differingdegrees of hydration; thus, for acation exchanger, there is a volumechange with the monovalent ionspecies, Li+ > Na+ > K+ > Cs+ > Ag+.With polyvalent ions, hydration isreduced by the cross-linking action;therefore, Na+ > Ca2+ > Al3+. In moreconcentrated solutions, less water istaken up owing to greater osmoticpressure.

Selectivity. Ion exchange reactionsare reversible. By contacting a resinwith an excess of electrolyte (B+ inthe following reaction), the resin canbe converted entirely to the desiredsalt form:

RA+ + B+ ! RB+ + A+

However, with a limited quantity of B+

in batch contact, a reproducibleequilibrium is established which isdependent an the proportions of A+

and B+ and on the selectivity of the

Figure 2. Total Capacity vs. Cross-Linkage (Percent DVB) PolystyreneSulfonic Acid Resin, H+ Form

Figure 3. Operating Capacity vs. Regenerant Level for Sodium-CycleOperation, Sulfonic Acid Resin

resin. The selectivity coefficient, KBA,

for this reaction is given by:

where m and refer to ionicconcentrations in solution and resinphase, respectively. Resin selectivitycoefficients have been determined fora range of ionic species and relatedto H+ for cations and OH– for anions,which are assigned selectivity valuesof 1.00.

Kinetics. The speed with which ionexchange takes place. The ionexchange process involves diffusionthrough the film of solution that is inclose contact with the resins anddiffusion within the resin particle. Filmdiffusion is rate-controlling at lowconcentrations and particle diffusionis rate-controlling at highconcentrations. Whether film diffusionor particle diffusion is the rate-controlling mechanism, the particlesize of the resin also is a determiningfactor. Uniform particle sized resinsexhibit enhanced kinetic performancecompared to conventionalpolydispersed resins due to theabsence of kinetically slow largerbeads.

Stability. Strong oxidizing agents,such as nitric or chromic acid, rapidlydegrade ion exchange resins. Slowerdegradation with oxygen and chlorinemay be induced catalytically. For thisreason, certain metal ions, forexample, iron, manganese andcopper, should be minimized in anoxidizing solution. With cationexchangers, attack is principally anthe polymer backbone. Highly cross-

linked cation resins have an extendeduseful life because of the greatnumber of sites that must be attackedbefore swelling reduces the usefulvolume based capacity and producesunacceptable physical properties, forexample, crush strength reductionand pressure drop increase. Withanion exchangers, attack first occurson the more susceptible functionalgroups, leading to loss of totalcapacity and/or conversion of strongbase to weak base capacity.

The limits of thermal stability areimposed by the strength of thecarbon-nitrogen bond in the case ofanion resins. This strength issensitive to pH and low pH enhancesstability. A temperature limitation of60°C (140°F) is recommended forhydroxide cycle operations. Cationresin stability also is dependent onpH; the stability to hydrolysis of thecarbon-sulfur bond diminishes with alowering of pH. They are much morestable than anions however and canbe operated up to 150°C (300°F).

Resin Structure and Manufacture

The manufacture of ion exchangeresins involves the preparation of across-linked bead copolymer followedby sulfonation in the case of strongacid cation resins, orchloromethylation and the aminationof the copolymer for anion resins.

Cation Exchange Resins. Weakacid cation exchange resins arebased primarily an acrylic ormethacrylic acid that has been cross-linked with a di-functional monomer

(usually divinylbenzene [DVB]). Themanufacturing process may start withthe ester of the acid in suspensionpolymerization followed by hydrolysisof the resulting product to produce thefunctional acid group.

Weak acid resins have a high affinityfor the hydrogen ion and are thereforeeasily regenerated with strong acids.The acid-regenerated resin exhibits ahigh capacity for the alkaline earthmetals associated with alkalinity anda more limited capacity for the alkalimetals with alkalinity. No significantsalt splitting occurs with neutral salts.However, when the resin is notprotonated (e.g., if it has beenneutralized with sodium hydroxide),softening can be performed, even inthe presence of a high saltbackground.

Strong acid resins are sulfonatedcopolymers of styrene and DVB.These materials are characterized bytheir ability to exchange cations orsplit neutral salts and are usefulacross the entire pH range.

Anion Exchange Resins. Weakbase resins do not containexchangeable ionic sites and functionas acid adsorbers. These resins arecapable of sorbing strong acids with ahigh capacity and are readilyregenerated with caustic. They aretherefore particularly effective whenused in combination with a strongbase anion by providing an overallhigh operating capacity andregeneration efficiency.

Strong base anion resins are classedas Type 1 and Type 2. Type 1 is a

CH3

CH CH2 + CH CH2

COOR

methacrylate

polymerization

catalyst

CH3

CCH2CHCH2

COOR

hydrolysis

OH–

CHCH2

CH3

CCH2CHCH2

COO–

Na+

CHCH2

• • • • • • • • •• • •

• • • • • • • • • • • •

K = •BA

m mm m

B A

A B m

quaternized amine product made bythe reaction of trimethylamine with thecopolymer after chloromethylation.The Type 1 functional group is themost strongly basic functional groupavailable and has the greatest affinityfor the weak acids such as silicic acidand carbonic acid, that are commonlypresent during a waterdemineralization process. However,the efficiency of regeneration of theresin to the hydroxide form issomewhat lower, particularly whenthe resin is exhausted withmonovalent anions, such as chlorideand nitrate. The regenerationefficiency of a Type 2 resin isconsiderably greater than that of Type1. Type 2 functionality is obtained bythe reaction of the styrene-DVBcopolymer withdimethylethanolamine. Thisquaternary amine has lower basicitythan that of the Type 1 resin, yet it ishigh enough to remove the weak acidanions for most applications. Thechemical stability of the Type 2 resinsis not as good as that of the Type 1resins, the Type 1 resins beingfavored for high temperatureapplications.

Other Functional Groups. Ionexchange resins with specialfunctional groups have been made forspecific applications. Of interest to thehydrometallurgical industry are avariety of resins having chelatingability and which are particularlyapplicable for the selective exchangeof various heavy metals from alkalineearth and alkali metal solutions[2].

Polymer Matrix. The structure andporosity of an ion exchange resin aredetermined principally by theconditions of polymerization of thebackbone polymer. Porositydetermines the size of the species,molecule or ion, that may enter aspecific structure and its rate ofdiffusion and exchange. There also isa strong interrelationship between theequilibrium properties of swelling andionic selectivity.

For example, a conventional gel type,styrenic ion exchanger is built on amatrix prepared by co-polymerizingstyrene and DVB. In these systems,porosity is inversely related to theDVB cross-linking. Gel resins exhibitmicroporosity with pore volumestypically up to 10 or 15 Ångstroms.

Macroporous (macroreticular) ionexchange resins have pores of aconsiderably larger size than those ofthe gel type resins with porediameters up to several hundredÅngstroms. Their surface area mayreach 500 m2/g or higher.Macroporous polymers are generallyhighly cross-linked and thereforeexhibit little volume change (swelling).Because of the high cross-linkage inthe matrix, the apparent oxidationstability of macroporous resins isimproved. However, at similar cross-linkages, macroporous resins havegreater exposure to potential oxidantsthan gel resins due to their greaterporosity and surface area.

Poorer regeneration efficiencies,lower capacities and higherregeneration costs are the penaltiespaid for the use of the macroporousresins. Macroporous resins may beused as catalysts, particularly in non-polar media where gel resins do notperform satisfactorily because of theirinaccessibility to the reactants.

CH CH2 CH CH2

styrene

polymerization

catalyst

CH CH2

CHCH2CHCH2

sulfonating acid

swelling agent

CHCH2

CHCH2CHCH2

SO3–H+

CHCH2

SO3–H+

+

• • •

• • •

• • •

• • •

• • •

• • •

• • •

• • •

divinylbenzene

CHCH2CHCH2

CHCH2

CH2Cl + CH3OH

CHCH2CHCH2

CHCH2

(1)

CH2N+(CH3)3Cl–

CHCH2CHCH2

CHCH2

catalyst+ ClCH2OCH3

(1) + N(CH3)3• • •

• • • • • •

• • • • • •

• • • • • •

• • • • • •

• • • • • •

• • •

Ion Exchange RegenerationTechnologies

Ion exchange regenerationtechnology has developed over theyears from the early co-flowregenerated systems to counter-flowblock systems and through to packedbed technology, including the DowUPCORE* process. Counter-currentregeneration systems have reducedchemical costs, improved waterquality and less waste volumescompared to traditional co-flowregenerated systems. They are alsomore productive, utilizing smallervessels, faster regenerations andfewer mechanical failures.

Co-Current Regeneration System.This is the simplest system where aresin is regenerated in the samedirection as the service flow(downwards). The vessel has a largefreeboard to allow expansion of theresin bed when backwashing is

carried out to remove suspendedsolids and resin fines.

Counter-Current RegenerationSystems. In these systems, theregenerant is applied in the oppositedirection to the service flow. This hasthe advantage of providing betterwater quality (lower ionic leakage),higher chemical efficiency andreduced waste water. In order toobtain low leakage levels from acounter-flow regenerated resinsystem, the contaminating ions mustbe kept from the effluent end of thecolumn during re-generation andrinse. This requires avoidance ofconditions that would disrupt the resinbed configuration. Backwashfrequency also must be minimized.

The difference between ion leakagefor co-current and counter-currentregeneration is illustrated in Figure 4.Re-exchange of the contaminatingNa+ ion occurs from the base of theco-flow regenerated bed when the

incoming salt solution (feed) isconverted to the corresponding diluteacid. When this acid solution contactsthe sodium ion, re-exchange ofhydrogen ions for sodium ions occursand the sodium ions exit the columnas leakage (Figure 4a). Leakage incounter-flow regeneration issubstantially reduced as the resin bedis predominantly in the regeneratedform at the bottom of the vesselFigure 4b).

There are two main types of counter-current systems:

Blocked Systems. This includes airhold down, water hold down and inertmass blocked. The service flow isdownwards and regeneration upflow.To avoid disturbance of the resinpolishing zone at the bottom of thevessel, the resin bed is held down(blocked) during regeneration by airpressure, water flow or an inert massin the top part of the vessel. Theregenerant passes up through the

Feed

Na+

H+

Na

Product

Exhaustion

Waste

Na+

H+

Product

Exhaustion

Regenerantand Rinse

H+ Resin

Na+ Resin

Waste

Regenerationand Rinse

Waste

Na+ Resin

H+ Resin

Regenerantand Rinse

a.

b.

Washings

ExpandedResin

ServiceWater

Backwash

Feed

Na+ Resin

H+ Resin

Treated Water

Exhausted Bed

Feed

Na+ Resin

H+ Resin

Treated Water

Exhausted Bed

Figure 4. Ion Leakage: a. Co-flow and b. Counterflow Regenerated Fixed-Bed Column Contractors (H+ FormCation Resin; Na+ Removal)

resin and out of a collector system inthe middle part of the vessel.

Packed Bed Systems. These maybe up-flow service with down-flowregeneration or down-flow servicewith up-flow regeneration, such as theDow UPCORE system.

Semi-Continuous and Continuous.Semi-continuous and continuouscontactors operate as intermittentlymoving packed beds as typified bythe Higgins contactor[3] or as fluidizedstaged (compartmented) columnssuch as the Himsley contactor[4-6].Flow is counter-current and their useis in increased resin utilization andhigh chemical efficiency. Commercialinstallations include those forphosphoric pickle acid recovery,water softening and ammoniumnitrate recovery.

Resin Applications

Water Softening. Water softeningaccounts for the major tonnage ofresin sales. Hard waters, whichcontain principally calcium andmagnesium ions, cause scale inpower plant boilers, water pipes anddomestic cooking utensils. Hardwaters also cause soap precipitationwhich forms an undesirable gray curdand a waste of soap. Water softeninginvolves the interchange of hardnessfor sodium on the resin. Typically,hard water is passed through a bed ofa sodium cation exchange resin andis softened.

2RNa+ + Ca2+ ! R2Ca2+ + 2Na+

Regeneration of the exchangerinvolves the passage of a fairlyconcentrated (8-12 percent) solutionof sodium chloride through the resin.

R2Ca2+ + 2Na+ ! 2RNa+ + Ca2+

Dealkalization. Many industrialprocesses require that hardness andalkalinity be removed from a rawwater before the water is used in theprocess. Two main processesinvolving ion exchange are used fordealkalizing:

1. Dissolved solids are removed tothe extent of the alkalinity in theraw water by passing the rawwater through a bed of weak acidcation resin in the hydrogen form.The 100 percent utilization ofregenerant acid that ischaracteristic of this processdecreases operating costs andgreatly minimizes the wastedisposal problem. A weak acidcation resin creates no freemineral acidity in the effluent whenregenerated at a level of not morethan 105-110 percent of thetheoretically required amounts forthe cations picked up.

2. Chloride anion dealkalizinginvolves passing the raw waterthrough a Type 2 anion exchangeresin that is in the chloride form toremove alkalinity.

Demineralization. Ion exchangedemineralization[7] is a two stepprocess involving treatment with bothcation and anion exchange resins.Water is passed first through acolumn of strong acid cationexchange resin that is in thehydrogen form (RH+) to exchange thecation in solution for hydrogen ions:

RH+ + C+ ! RC+ + H+

where C+ represents commoncations, for example, Ca2+, Mg2+ andNa+. This effluent is passed to acolumn of anion exchange resin in thehydroxide form R OH–) to replaceanions in solutions with hydroxide:

ROH– + A– ! RA– + OH–

where A– represents common anions,for example, Cl–, SO4

2– and NO3–.The hydrogen ions from the cationresin neutralize the hydroxide ionsfrom the anion resin:

H+ + OH– ! H2O

The net effect is the removal ofelectrolytes and a yield of purifiedwater.

Alternatively, the impure water maybe passed through an intimatelymixed bed of cation and anion

exchange resins where both types ofexchange occur simultaneously:

RH+ + ROH- + C+A- ! RC+ RA- + H2O

The choice of the ion exchangesystem for demineralization dependsan the water quality desired,operating and capital economics andcomposition of the raw water.

Condensate Polishing. Single ormixed bed ion exchange resins areused in deep bed filter demineralizersfor reduction of particulate matter anddissolved contaminants in utilitypower plant condensates.

Ultra Pure Water. Ultra pure water(UPW) is essential to the properfabrication of integrated circuit boardsin the semiconductor industry. As thedegree of integration becomesincreasingly more complex, thesemiconductor industry requireshigher levels of water purity. Singlebeds, mixed beds and also reverseosmosis are used in the production ofultra pure water.

Nitrate Removal. Ion exchange isused for the removal of nitrates fromnitrate polluted waters[8]. Strong baseanion exchange resins operating inthe chloride ion form (salt solutionregenerated) have been successfullyused for this service.

Waste Treatment. Radioactive.Radiation waste systems in nuclearpower plants include ion exchangesystems for the removal of tracequantities of radioactive nuclides fromwater that will be released to theenvironment. The primary resinsystem used is the mixed bed.

Chemical Processing – Catalysis.Since ion exchange resins are solid,insoluble (but reactive) acids, bases,or salts, they may replace alkalis,acids and metal ion catalysts inhydrolysis, inversion, esterification,hydration or dehydration,polymerization, hydroxylation andepoxidation reactions. Theadvantages of ion exchange resins ascatalysts include easy separationfrom the products of reaction,

repeated reuse, reduction of sidereactions and lack of need for specialalloys or lining of equipment.

Purification. Purification by ionexchange is used to removecontaminating acids, alkalis, salts ormixtures from non-ionized or slightlyionized organic or in-organicsubstances. Examples include formicacid removal from 50 percentformaldehyde solutions, removal ofamines from methanol, removal ofiron from steel pickling operations,purification of aluminum bright dipbaths and removal of iron in thepurification of hydrochloric acid[9].

Metal Extraction, Separation andConcentration. In aqueous or solventmixtures containing large amounts ofcontaminants and small amounts of adesired solute, ion exchange resinscan be used to selectively isolate andconcentrate the desired solute, forexample, the recovery of uraniumfrom sulfuric acid leach solution withstrong base anion resins. Otherspecific chelating resins can be usedfor metals recovery such as copper,nickel, cobalt and precious metals.

Desiccation. Ion exchange resins,particularly strong acid cationexchange resins in the dry state, areuseful as desiccants[10]. Ion exchangeresins show their greatest capabilityas desiccants in the drying ofhydrophobic solvents, for example,hydrocarbons and chlorinatedhydrocarbons.

Sugar Separations and Purifications.Ion exchange resins are used as anintegral part of corn syrup, high-fructose corn syrup (HFCS)processing and other starch basedsyrups. In sucrose processing, theresins are often used for softeningfeed streams, recovering sugar frommolasses streams, or decolorization.They are also used in the productionof non-nutritive sweeteners such assorbitol or mannitol. Resins andadsorbents are used in four major unitprocesses in corn sweetenerprocessing: deashing,chromatographic separation of

glucose and fructose, mixed bedpolishing and color removal. Indeashing, a bed of strong acid cationresin is typically followed by a bed ofweak base anion resin. The resinsused are macroporous, as their largeporous structure allows syrupcomponents to move freely into thebead.

Chromatographic Separation.Chromatographic separation is amanufacturing process using ionexchange resins to separate onedissolved component from another. Itis applied in the sugar industry for thepurification of compounds such assucrose, glucose, fructose,oligosaccharides, sorbitol andmannitol. It can be used to separatesalt from glycerol and in purifyingamino acids and various organicacids. Most industrial chromatographytoday utilizes simulated moving bed(SMB) technology to minimize solventuse, leading to a significantly reducedcost of operation when compared totraditional batch chromatography.

Pharmaceuticals andFermentation. Ion exchange resinsare useful as carriers for medicinalmaterials and in slow releaseapplications. In some cases, the ionexchange resin has the medicinalaffect desired, for example,Cholestyramine, a dried and groundstrong base anion resin used to bindbile acids for reducing bloodcholesterol. Ion exchange resins alsoare used in a variety of fermentationand biotechnology processes, suchas the isolation and purification oflysine, streptomycin and neomycinand other similar antibiotics.

Bibliography[1] B. A. Adams and E. L. Holmes, J. Soc.

Chem. Ind. 54,1-6T (1935).

[2] D. C. Kennedy, Chem. Eng., (Dune 16,1980).

[3] U.S. Pat. 3,580,842 (May 25, 1971), I.R. Higgins (to Chemical SeparationsCorporation).

[4] Can. Pat. 980,467 (Dec.. 23, 1975), A.Himsley (to Himsley Engineering).

[5] U.S. Pat. 3,549,526 (Dec. 22, 1970), H.Brown.

[6] U.S. Pat. 3,551,118 (Dec. 29,1970), F.L. D. Cloete and M. Streat (to NationalResearch Development Corporation,London).

[7] W. J. Weber, Jr., “PhysicochemicalProcesses for Water Quality Control” inIon Exchange, Wiley Interscience, NewYork, 1972, Chapter 6.

[7] S. B. Applebaum, Demineralixation byIon Exchange, Academic Press, Inc.,New York, 1968.

[8] M. Sheinker and J. Cudoluto, PublicWorks, 71 (Dune 1977).

[9] K. A. Kraüs and G. E. Moore, J Am.Chem. Soc. 71, 3263 (1949).

[10] C. E. Wymore, Ind. Eng. Chem. Prod.Res. Deu. 1, 173 (1962).

*Trademark of The Dow Chemical CompanyForm No. 177-177-01837-600QRP

CH 171-527-E-600

WARNING: Oxidizing agents such as nitric acid attack organic ion exchange resins under certain conditions. This could lead to anything fromslight resin degradation to a violent exothermic reaction (explosion). Before using strong oxidizing agents, consult sources knowledgeable inhandling such materials.

NOTICE: No freedom from any patent owned by Seller or others is to be inferred. Because use conditions and applicable laws may differ fromone location to another and may change with time, Customer is responsible for determining whether products and the information in thisdocument are appropriate for Customer’s use and for ensuring that Customer’s workplace and disposal practices are in compliance withapplicable laws and other governmental enactments. Seller assumes no obligation or liability for the information in this document. NOWARRANTIES ARE GIVEN; ALL IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE AREEXPRESSLY EXCLUDED.

Published June 2000.

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