&EPA

United States Industrial Environmental ResearchEnvironmental Protection Laboratory

Agency I Cincinnati OH 45268I

Technology Transfer II

Summary Report

Control arid TreatmentTechnolodv for theMetal Finishinq Industry

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Ion Exchange

Technology Transfer

Summary ReportIII

Control and: TreatmentTechnoloqv for theMetal Finishing Industry

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Ion Exchange,

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June 1981

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This report was developed i;ly theIndustrial Environmental Hesearch LaboratoryCincinnati OH 45268 I

EPA 625/8-81-007

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Environmental res~arch and development in the metal finishing industry isthe responsibility of the Nonferrous Metals and Minerals Branch, IndustrialEnvironmental Research Laboratory, Cincinnati OH. The U.S. EnvironmentalProtection Agency ,hired the Centec Corporation, Fort Lauderdale IFL andReston VA, to prepare this report. Roger C.Wilmoth is the EPA Project Officer.

Requests for further information can be addressed to:

Nonferrous Metals and Minerals BranchIERL-USEPACincinnati OH 45298

EPA thanks the following companies and organizations for providinginformation and technical review: American Electroplaters' Society; BestTechnology, lnc., Villa Park IL; Dow Chemical Company, Midland MI; Instituteof Precision Mechanics, Warsaw, Poland; Raytheon Ocean SystemsCompany, Portsmouth RI; and Rohm and Haas Co., Philadelphia PA.

Photographs were supplied by Best Technology, lnc., of Villa Park IL andEco-Tec Limited of Toronto ON.

This report has been reviewed by the Industrial Environmental HesearchLaboratory, U.S. Erivironmental Protection Agency, Cincinnati OH, andapproved for publication. Approval does not signify that the contentsnecessarily reflect the views and policies of the U.S. !Environmental ProtectionAgency, nor does mention of trade names or commercial products constituteeriClorsement or recommendation for use.

COVER PHOTOGRAPH: Reciprocating Flow Ion Exchanger used for chromicacid recovery.

Overview Ion exchange i~ a versatileseparation 'proqess with potentialfor broad appllcation in the metal fin­ishing industry, both for rawmaterial recovery and reuse andfor water pollution control. Threemajor areas of spplication have beendemonstrated: i

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• Wastewater purification andrecycle

• End-of-pipe pollution control.- • Chemical recoverv

Although the ion exchange processhas been commercially availablefor many vears.l widespread interestin its use for m;etal finishing pol­lution control has developedonly recently. !

The main impetus for the interest inion exchange technology is thebroad range of 'resins manufacturedtoday. With pr~per resin selection,ion exchange can provide aneffective and eoonomical solution topollution control requirements.As a further stimulus to the use ofthe process, th~ metal-bearingsludge generat~dby hydroxide treat­ment systems il; considered ahazardous material and must bedisposed of in ~n environmentallysafe manner. The ion exchangeprocess can concentrate the heavymetals in a dilute wastewater intoa concentrated metal solution that ismore amenable' to metal recoverythan is a sludg~, and this abilityshould lead to more widespread useof the technology,

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This summary r1eport is intendedto promote an Gnderstanding of theuse of ion exchanqe in the metalfinishing industry. The sec-tions that follow discuss ion ex­change process theory in generaland evaluate each of the three majorareas of application in terms ofperformance, state of development.cost (in 1980 dollars), and operatingreliability. i

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Water Purification and Recylcle

In the first area ofapplication,mixed rinse solutions are deionizedto permit reuse of the treatedwater. The contaminants in therinses are concentrated in the smallvolume purge streams, and arethereby made more economic:al totreat.

Because ion exchange is efficientin removing dissolved solidsfrom normally dilute spent rinsewaters, it is well suited for usein water purification and recycle.Most of the plating chemicals, acids,and bases used in metal finishingare ionized in water solutions;and can be removed by ion ex­change. Several factors make the ionexchange process effective forthis application:

• Ion exchange can economicallyseparate dilute concentrations ofionic compounds from watersolutions.

• The process can consistentlyprovide high purity water over abroad range of loading conditions.

• The resins used for separationare durable in severe chemical en­vironments.

Application of the ion exchangeprocess in a wastewater purificationand recycle system will signifi- .cantly reduce water consumptionand the volume of wastewaterdischarged, thus reducing water useand sewer fees and the size andcost of the pollution control system.Also, for plants that discharqewastewater directly to waterwaysand that are regulated by mass­based pollutant discharge limits, thereduction of discharge volumewill allow for higher concentrationsof pollutants in the dischargeand facilitate compliance with theselimits.

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Ion exchange acid purification unit used for sulfuric acid anodizing solutions

End-of-Pipe Pollution Control

In the second application. toxicheavy metals and metal cyanidecomplexes are removed selectivelyfrom combined waste streamsbefore discharge. The key to this ap­plication is that the ion' exchangeresins remove only the toxiccompounds and allow the nontoxicdissolved ionic solids to remainin solution.

The ion exchange process can beused in two different forms forend-of-pipe pollution control: it hasbeen demonstrated as a meansof polishing the effluent from

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conventional hydroxide precipitationto lower the metal concentrationin the discharge; it has also been ap­plied as a means of directly treat­ing wastewaters to 'remove heavymetal and metal cyanide pollutants.

Most plating shops can removesufficient metal to comply withwastewater discharge regulationsusing the conventional hydroxideprecipitation process. Whereunusually strict limits are placed onthe effluent metal concentration.however. or where the metalsare complexed with' chemical con­stituents that interfere with theirprecipitation as metal hydroxides.conventional treatment maynot be reliable for compliance with,

the discharge limits. Ion exchangecan be used in such cases topolish the effluent from the conven­tional treatment and reduce themetal concentration further. In thisapplication. the process can providea relatively inexpensive meansof upgrading system performance forcompliance with the dischargeregulations.

Ion exchange has been used to alimited extent to remove toxic pol­lutants selectively from an untreatedwastewater while allowing most ofthe nontoxic ions to pass through.Approaches employed to fac:il-

itate this application includeusing:

• Weak acid cation resin in anapplication of the wastewater­softening type to removeheavy metals and other divalentcations from a' wastewatersolution with a high concentra­tion of sodium ions

• Heavy-metal-selective weak acidor chelating cation resin toremove only the heavy metalions while allowing sodium, cal­cium, and magnesium ions topass through

• A stratified bed of resin containingstrong and weak acid cationand strong base anion resins toremove heavy metal and metal cy­anide complex ions from solu­tion while allowing most ofthe wastewater ionic constituentsto pass through

In each of these approaches,wastewater pretreatment entails pHadjustment, to ensure that pH iswithin the operating range of theresin, and filtration, to re.!TI0vesuspended solids that would foul theresin bed. The pollutants removedfrom the wastewater are con­centrated in the ion exchange regen­erant solutions. The regenerantscan be treated in a small batch treat­ment system using conventionalprocesses. Firms with accessto a centralized treatment system to

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dispose of the r~generantsolu­tions resulting from treatment wouldnot need to:ins~all chemical de­struct systems.lr neither case wouldit be necessary to invest in sophis­ticated pH control systems, floc­culant feed systems, clarifiers,and other process equipment asso­ciated with conventional metal .precipitation systems. And, as afurther advantage, ion exchange

t .units are compact and easy toautomate compared with conven­tional precipitation systems.

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Chemical RecoyeryI

In the chernlcalrecoverv application,segregated plati:ng rinse waters aretreated to concentrate the platingchemicals for recycle to the platingbath. The purified rinse wateris also recycled.!

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Ion exchange, evaporation. reverseosmosis, and electrodialvsishave all been used in the platingindustry to recover chemicals fromrinse solutions. rhese processeshave in commol') the ability toseparate specific compounds froma water solution] yielding a con­centrate of those compoundsand relatively P4re water. The con­centrate is recycled to the plating

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.bath and the purified water isreused for rinsing. Determination ofthe separation process best suitedfor a particular chemical recoveryapplication usually requires evaluat­ing both general and site-specificfactors:

• General factors include rinsewater concentration, volume, andcorrosivitv, among others.

• Site-specific factors include, forexample, availability of floorspace and utilities (steam,chemical reagents, electricity, andso forth) and the degree of .concentration needed to recyclethe chemicals to the bath.

As a rule, ion exchange systems aresuitable for chemical recoveryapplications where the rinsewater feed has a relatively dilute con­centration of plating chemicalsand a relatively low degree ofconcentration is required for recycleof the concentrate. Ion exchangeis well suited for processing cor­rosive solutions. Ion exchange hasbeen demonstrated commerciallyfor recovery of plating chemicalsfrom acid-copper, acid-zinc, nickel,tin, cobalt, and chromium platingbaths. The process has alsobeen used to recover spent ac:idsolutions and to purify plating solu­tions for longer service life.

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R indicates the organic portion ofthe resin and S03 is the immobileportion of the ion active group.Two resin sites are needed fornickel ions with a plus 2 valence(Ni+2). Trivalent ferric ions wouldrequire three resin sites.

The selectivity coefficient expressesthe relative distribution of the ionswhen a resin in the A+ form isplaced in a solution containing B+ions. Table 1 gives the selectivitiesof strong acid and strong base ionexchange resins for various ioniccompounds. It should be pointedout that the selectivity coefficient isnot constant but varies with changesin solution conditions. It doesprovide a means of determiningwhat to expect when various ions

solution phase reactions. Forexample:

NiS04 + Ca(OHh += Ni(OHh+CaS04 (1)

In this reaction, the nickel ions of thenickel sulfate (NiS04) are ex­changed for the calcium ions of thecalcium hydroxide [Ca(OHhl mole­cule. Similarly, a resin with hydrogenions available for exchange willexchange those ions for nickel ionsfrom solution. The reaction can bewritten as follows:

(2)2(R-S03H) + NiS04 +=

(R-S03hNi + H2S04

As shown, the ion exchange reactionis reversible. The degree the reac­tion proceeds to the right willdepend on the resin's' preference, orselectivity, for nickel ions com­pared with its preference forhydrogen ions. The selectivity of aresin for a given ion is measuredby the selectivity coefficient, K,which in its simplest form for thereaction

R-A+ + B+ += R-B+ + A+ (3)

is expressed as: K =(concentrationof B+ in resin/concentration of A+in resin) X (concentration of A+ insolution/concentration of B+ insolution).

Most plating process water is usedto cleanse the surface of the partsafter each process bath. To main­tain quality standards, the levelof dissolved solids in the rinsewater must be regulated. Fresh wateradded to the rinse tank accom­plishes this purpose, and theoverflow water is treated to removepollutants and then discharged. Asthe metal salts, acids, and basesused in metal finishing are pri­marily inorganic compounds, theyare ionized in water and could beremoved by contact with ion ex­change resins. In a,water deioniza­tion process, the resins exchangehydrogen ions (H+) for the posi­tively charged ions, (such as nickel,copper, and sodium), and hydroxylions (OH-) for negatively chargedsulfates, chromates, and chlorides.Because the quantity of H+ and OH­ions is balanced, the result of theion exchange treatment is relativelypure, neutral water~

Ion exchange reactions are stoichi­ometric and reversible, and in thatway they are similar to other

An organic ion exchange resin iscomposed of high-molecular-weightpolyelectrolytes that can exchangetheir mobile ions for ions of similarcharge from the surroundingmedium. Each resin has a distinctnumber of mobile ion sites thatset the maximum quantity of ex­changes per unit of resin.

Ion Exchange Heactions

Ion exchange is a reversiblechemical reaction wherein an ion(an atom or molecule that has lost orgained an electron and thus acquiredan electrical charge) from solutionis exchanged for.a similarly chargedion attached to an immobile solidparticle. These solid ion exchangeparticles are either naturallyoccurring inorganic 'zeolites orsynthetically produced organicresins. The synthetic organic resinsare the predominant type usedtoday because their characteristicscan be tailored to specific appli­cations.

Basic Concepts

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Table 11.

Selectivity of Ion Exchange Resins,in Order of Decreasing Preference

Resins currently available exhibit arange of selectivities and thushave broad application. As an exam­ple, for a strong acid resin, therelative preference for divalentcalcium ions (Ca+2) over divalentcopper ions (Cu+2) is approximately1.5 to 1. For a heavy-metal-selectiveresin, the preference is reversedand favors copper by a ratio of2,300 to 1.

(6)

Strong Base Anion Resins. Likestrong acid resins, strong base resinsare highly ionized and can be usedover the entire pH range. Theseresins are used in the hydroxide (OH)form for water deionization. Theywill react with anions in solutionand can convert an acid solution topure water:

R-NH30H + HCI -+

R-NH3CI+ HOH

Regeneration with concentratedsodium hydroxide (NaOH) convertsthe exhausted resin to the hvdroxldeform.

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Weak acid resins exhibit a muchhigher affinity for hydrogen ions thando strong acid resins. This charac­teristic allows for regeneration tothe hydrogen form with signific:antlyless acid than is required for strongacid resins. Almost completeregeneration can be accomplishedwith stoichiometric amounts of acid.The degree of dissociation of II

weak acid resin is strongly influ­enced by the solution pH. Conse­quently, resin capacity dependsin part on solution pH. Figure 1shows that a typical weak acid resinhas limited capacity below a pHof 6.0, making it unsuitable fordeionizing acidic metal finishingwastewater.

to the sulfonic acid group (S03H)used in strong acid resins. Theseresins behave similarly to weakorganic acids that are weaklydissociated.

In an ion exchange wastewaterdeionization unit, the wastewaterwould pass first through a bed of

Weak Base Anion Resins. Weakbase resins are like weak acid resins,in that the degree of ionization isstrongly influenced by pH. Conse­quently, weak baseresins exhibitminimum exchange capacity above apH of 7.0 (Figure 1). These resinsmerely sorb strong acids; theycannot split salts.

Resin Types

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Ion exchange resins are classifiedas cation exchan1gers, whichhave positively cFarged mobileions available for exchange, andanion exchangers, whose exchange­able ions are negatively charged.Both anion and dation resins areproduced from t~e same basicorganic polymers. They differ inthe ionizable gro:up attached to thehydrocarbon network, It is thisfunctional group Ithat determinesthe chemical behavior of the resin.Resins can be broadlv classifiedas strong or weak acid cation ex­changers or strong or weak baseanion exchangers.

iStrong Acid Catibn Resins. Stronqacid resins are so named becausetheir chemical b~havior is similar tothat of a strong ~cid. The resinsare highly ionize~ in both the acid(R-S03H) and salt (R-S03Na) form.They can convert a metal salt tot~e correspondin'lg acid by the reac­tion:

2(R-S03H) + Ni<];(2 -+

(R-S03hNi + fHCI (5)

The hydrogen and sodium forms ofstrong acid resins are highly dis­sociated and the; exchangeable Na+and H+ are readily available forexchange over t~e entire pH range.Consequently, th~ exchangecapacity of strong acid resins isindependent of solution pH. Theseresins would be used in the hydrogenform for complete deionization;they are used in the sodium form forwater softening {calcium andmagnesium removal). After exhaus­tion, the resin is :eonverted back tothe hydrogen form (regenerated)by contact with astrong acid solu­tion, or the resin can be converted tothe sodium form :with a sodiumchloride solution; For Equation 5,hydrochloric acid, (HCI) regenerationwould result in a:concentratednickel chloride (l'iIiCI2) solution.

Weak Acid Cation Resins. In a weakacid resin, the ionizable group is acarboxylic acid (yOOH) as opposed

IodideNitrateBisulfiteChlorideCyanideBica rbonateHydroxideFluorideSulfate

Strong'base anionexchanger

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BariumLeadCalciumNickelCadmiumCopperZincMagnesiumPotassiumAmmoniaSodiumHydrogen

are involved. As indicated in Table 1,strong acid resins have a preferencefor nickel over hydrogen. < Despitethis preference, the resin can beconverted back to the hydrogen formby contact with a concentratedsolution .of sulfuric acid (H2S04) :

(R-S03hNi + H2S04 -->

2(R-S03H) + NiS04 (4)

This step is known as regeneration.In general terms, the higher thepreference a resin exhibits fora particular ion, the greater theexchange efficiency in terms of resincapacity for removal of that ionfrom solution. Greater preferencefor a particular ion, however, willresult in increased consumption ofchemicals for regeneration.

Strong acid cationexchanger

Figure 1.

Exchange Capacity of Weak Acid Cation and Weak Base Anion Resinsas a Function of Solution pH '

2.8002.3001.200

571715

6.741.21

Metal ion

Hg+2 •••••••••••••.•••• ,••••••.

Cu+2 •••••••••••••••••••••••••

Pb+2 •••••••••••••••••••••••••

Ni+2 •••••••••••••••••••••••••+2 'Zn ..•.•.......•.••........•

Cd+2 •••••••••••••••••••••••••

co+2 •••••••••••••••••••••••••

Fe+2 •••••••••••••••••••••••••

Mn+2 ••••••••••••••••••••••••

Ca+2 •••••••••••••••••••••••••

Table 2.

Chelating Cation Resin Selectivitiesfor Metal Ions

·Selectivity coefficient for the metal over cal­cium ionsat a pH of 4.

The high degree of selectivity forheavy metals permits separation ofthese ionic compounds fromsolutions containing high back­ground levels of calcium, magnesium,and sodium ions. A chelating resinexhibits greater selectivity forheavy metals in its sodium form thanin its hydrogen form. Regenerationproperties are similar to thoseof a weak acid resin; the chelatingresin can be converted to thehydrogen form with slightly greaterthan stoichiometric doses of acidbecause of the fortunate tendency ofthe heavy metal complex tobecome less stable under low pHconditions. Potential applications ofthe chelating resin include polish­ing to lower the heavy metalconcentration in the effluent froma hydroxide treatment proCI~SS, ordirectly removing toxic heavymetal cations from wastewaterscontaining a high concentration ofnontoxic, multivalent cations.

Table 2 shows the preference of acommercially available chelatingresin for heavy metal cationsover calcium ions. (The chelatinqresins exhibit a similar magnitude ofselectivity for heavy metals oversodium or magnesium ions.] Theselectivity coefficient definesthe relative preference the resinexhibits for different ions. Thepreference for copper (shown inTable 2) is 2,300 times that

1110

aEthylenediaminetetraagetic acid.

provide hydroxide ions. Lessexpensive weakly basic reagentssuch as ammonia (NH3 ) or sodiumcarbonate can be employed.

Heavy-Metal-Selective ChelatingResins. Chelating resins behavesimilarly to weak acid cation resinsbut exhibit a high degree of selec­tivity for heavy metal cations.Chelating resins are analogous tochelating compounds found inmetal finishing wastewater; that is,they tend to form stable complexeswith the heavy metals. In fact,the functional group used in theseresins is an EDTAa compound. Theresin structure in the sodium form isexpressed as R-EDTA-Na.

Legem;!:_ weak acid cation resin_ _ weak base anion resin

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3

4

-,~,

''\

"I ! -.. ..

5 6 7 8 9

SOLUTION pH

SOURCE: Adapted from Schweitzer, P. A.. Handbook of Separation Techniques forChemical Engineers, New York NY, McGraw-Hili, 1979.

strong acid resin. Replacement ofthe metal cations (Ni+2, Cu+2) withhydrogen ions would lower the solu­tion pH. The anions (50;2, CI-) canthen be removed with a weakbase resin because the enteringwastewater will normally beacidic and weak base resins sorbacids. Weak base resins are pre­ferred over strong base resinsbecause they require less regenerantchemical. A reaction between theresin in the free base form andHCI would proceed as follows:

R-NH2 + HCI -- R-NH3CI (7)

The weak base resin does not havea hydroxide ion form as does thestrong base resin. Consequently,regeneration needs only to neutral­ize the absorbed acid; it need not

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Y=0.0625X =0.9375

Tank 4

Y=0.125X=0.875

If the solution were removed fromTank 1 and added to Tank 2, whichalso contained 1 eq of resin in theX ion form, the solution and resinphase would both contain 0.25 eqof V ion and 0.75 eq of X ion. Re­peating the procedure in a third andfourth tank would reduce thesolution content of V ions to 0.125and 0.0625 eq, respectively.Despite an unfavorable resin prefer­ence. using a sufficient numberof stages could reduce the concen­tration of V ions in solution to anylevel desired.

This analysis simplifies the columntechnique. but it does provideinsights into the process dynamics.Separations are possible despite

tanks each containing 1 equivalent(eq) of resin in the X ion form(see Figure 2). A volume of solutioncontaining 1 eq of V ions is chargedinto the first tank. Assuming theresin to have an equal preference forions X and V, when equilibrium isreached the solution phase willcontain 0.5 eq of X and V. Similarly,the resin phase will contain 0.5eq of X and V. This separation isthe equivalent of that achieved in abatch process.

Tank 3

Y=0.25X=0.75

Tank 2

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IBatch and CohiJmn ExchangeSystems I

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Ion exchange Jrocessing can beaccomplished tly either a batch

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method or a column method. In thefirst method, th'e resin and solutionare mixed in a batch tank. theexchange is all6wed to come toequilibrium, thdn the resin isseparated from.lsolution. The degreeto which the e'1'change takesplace is limiterf by the preferencethe resin exhibits for the ion insolution. Conse!quently, the useof the resin's exchange capacity willbe limited unless the selectivityfor the ion in solution is far greaterthan for the exchangeable ionattached to the! resin. Becausebatch reqeneration of the resin ischemically inefficient, batchprocessing by ibn exchange has lim­ited potential fJr application.

Passing a SOluti!on through a columncontaining a bed of exchangeresin is analogJus to treating thesolution in an ihfinite series ofbatch tanks. Cc!nsider a series of

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costs of the ottier commerciallyavailable resins!

I ~ _. • --;.

Y=0.5X=0.5

50-100100-150150-200150-200200-300

Tank 1

Y=1.0X=O

Solutionfeed (eq):

Strong acid cation .. , .Weak acid cation .Strong base anion .Weak base anion , .Chelating cation .

Resin

Note.-1980 dollars.

for calcium. Therefore, when a solu­tion is treated that contains equalmolar concentrations of copper andcalcium ions, at equilibrium.the molar concentration of copperions on the resin will be 2,300 timesthe concentration of calcium ions.Or, when solution is treated thatcontains a calcium ion' molarconcen­tration 2,300 times that of thecopper ion concentration. atequilibrium. the resin would holdan equal concentration of copperand calcium.

Their high cost is the disadvantageof using the heavy-metal-selectivechelating resins. Table 3 com­pares the cost of these with the

Table 3.

Cost of Commercially AvailableResins

Resin aftermixing (eq):

Y=0.5X=0.5

Y=0.25X=0.75

Y=0.125X=0.875

Y=0.0625X=0.9375

Note.-Resin has equal preference for X and Y ions. Solution feed contains 1 eq of Y ions. Each batch tank initially contains 1 eqof resin in X ion form. I '

Figure 2. IConcentration Profile in a Series of Ion Exchange Bat~h Tanks

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Backwash outlet

SOURCE: Kunin, R. "Ion Exchange for the Metal Products Finishers," (3 pts.), ProductsFinishing, Apr.-May-June 1969.

Graded quartz

Upper manifold

Lower manifold

~-.j--Resin

w;';;;:::::::=~:JlRegenerant

Regeneration of a fixed-bed columnusually requires between 1 and2 h. Frequency depends on the vol­ume of resin in the exchangecolumns and the quantity of heavymetals and other ionized com­pounds in the wastewater.

Resin capacity is usually expressedin terms of equivalents per liter (eq/L)of resin. An equivalent is themolecular weight in grams of thecompound divided by its electricalcharge, or valence. For example,a resin with an exchange capacity of1 eq/L could remove 37.5 g ofdivalent zinc (Zn+2 , molecularweight of 65) from solution. Muchof the experience with ion exchangehas been in the field of watersoftening; therefore, capacities willfrequently be expressed in termsof kilograins of calcium carbonateper cubic foot of resin. This unitcan be converted to equivalents per

Meter

Backwash controller:

Water inlet....c:If~*~~~h=m

Figure 3.

Typical Ion Exchange Resin Column

For resins that experience significantswelling or shrinkage during regen­eration, a second backwashshould be performed after regenera­tion to eliminate channeling orresin compression.

gen form. A slow water rinse thenremoves any residual acid.

3. The bed is brought in contactwith a sodium hydroxide solutionto convert the resin to thesodium form. Aga)n, a slow waterrinse is used to remove residualcaustic. The slow rinse pushesthe last of the regenerant throughthe column. .

4. The resin bed is subjected to afast rinse that removes thelast traces of the regenerantsolution and ensures good flowcharacteristics.

5. The column is returned to service.

poor selectivity for the ion beingremoved.

Regeneration Procedure. After thefeed solution is processed to theextent that the resin becomesexhausted and cannot accomplishany further ion exchange, theresin must be regenerated. In normalcolumn operation, for a cationsystem being converted first to thehydrogen then to the sodiumform, regeneration employs thefollowing basic steps:

1. The column is backwashed toremove suspended solidscollected by the bed during theservice cycle and to eliminatechannels that may have formedduring this cycle. The back­wash flow fluidizes the bed,releases trapped particles, andreorients the resin particlesaccording to size. Duringbackwash the larger, denserparticles will accumulate at thebase and the particle size willdecrease moving up the column.This distribution yields a goodhydraulic flow pattern andresistance to fouling by sus­pended solids.

2. The resin bed is brought in con­tact with the regenerant solution.In the case of the cation resin,acid elutes the collected ions andconverts the bed to the hydro-

Ion Exchange ProcessEquipment and Operation

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Most industrial applications of ionexchange use fixed-bed columnsystems, the basic component ofwhich is the resin column (Figure 3).The column design must:

• Contain and support the ionexchange resin

• Uniformly distribute the serviceand regeneration flow through theresin bed

• Provide space to fluidize theresin during backwash

• Include the piping, valves, andinstruments needed to regulateflow of feed, regenerant, andbackwash solutions

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tion of acid in the spent reqenerant,Further, as the acid dose isincreased incrementally, the GOf!­

centration of acid in the spentregenerant increases. By discardingonly the first part of the spent

Legend:_ weak acid cation resin_ _ weak base anion resin

IIIIIiiIl IIIlIIII strong acid cation resin• _. strong base anion resin

o

2.5

0.5

3.0

2.0

::J"--0-~

~uii:

1.5<l:uwCJZ-c:I:UXw

1.0

.....-4-__....... ...... ---'____...JI

o I 2 REGENERATIO~ LEVEL (lb/ft3)a 6 8

alb NaOH/ft3 for!weak and strong base anion; Ib HCI/ft3 for weak and strong acid cation.

SOURCES: Dowthemical Company, Dower WGR-2 Weakly Basic Anion Exchange Resin,T.D. Index 330. , Midland MI, Dow Chemical Company, undated. Dow ChemicalCompany, "Anio Resins: Selection Criteria for Water Treatment Applications," IdeaExchange 5(2), undated. Rohm and Haas Company, Ambertite" 200, Philadelphia PA,Rohm and Haas!company, Nov. 1976. .

Figure 4. I'Resin ExchangEf Capacities

Iincreases consumption to 2.45times the stoichiometric dose [5 IbHCI/ft3 (80 g Hfl/L)].

The need for aCif doses considerablyhigher than sto1chiometric meansthat there is 8 rn.;C8nt concentra-

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Regenerant Reuse. With strong acidor strong base resin systems,improved chemical efficiency canbe achieved by reusing a partof the spent regenerants. In stronglyionized resin systems, the degreeof column regeneration is themajor factor in determining thechemical efficiency of the regenera­tion process. (See Figure 5.) Torealize 42 percent ofthe resin's theo­retical exchange capacity requires1.4 times the stoichiometric amountof reagent [2 Ib HCI/ft3 (32 g HCI/L)].To increase the exchange capacityavailable to 60 percent oftheoretical

Cocurrent and Countercurrent Re- .generation. Columns are designed touse either occurrent or counter­current regeneration. In cocurrentunits, both feed and regenerantsolutions make contact withthe resin in a downflow mode. Theseunits are the less expensive ofthe two in terms 'of initial equip­ment cost. On the other hand, coeur­rent flow uses regenerant chemicalsless efficiently than countercur­rent flow; it has higher leakageconcentrations (the concentrationof the feed solution ion beingremoved in the column effluent), andcannot achieve as high a productconcentration in the regenerant.

Efficient use of regenerant chemicalsis primarily a concern with strongacid or strong base resins. Theweakly ionized resins require onlyslightly greater than stoichiometricchemical doses for completeregeneration regardless of whethercocurrent or countercurrent flowis used.

liter by rnultiplvlnq by 0.0458.Typical capacities for commerciallyavailable cation and anion resinsare shown in Figure 4. Thecapacities are strongly influencedby the quantity of acid or base usedto regenerate the resin. Weakacid and weak base systems aremore efficiently regenerated; theircapacity increases almost linearlywith regenerant dose.

Figure 5.

Effect of Reusing Acid Regenerant on Chemical Efficiency:

100

80

~:i:a:effi 60

sa:c>­:I:

gz 40oiiia:

~ou

20

10

Legend:_ with acid reuse~ without acid reuse

2 4 6 8

REGENERANT CONSUMED (Ib HCl/ft3)

aBased on strong acid resin in calcium form. :

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regenerant and saving and reusingthe rest, greater exchange capacitycan be realized with equal levels ofregenerant consumption. Forexample, if a regenerant dose of 5 IbHCI/ft3 (80 g HCI/L) were used inthe resin system in Figure 5, thefirst 50 percent of spent regenerantwould contain only 29 percent ofthe original acid concentration. Therest of the acid regenerant wouldcontain 78 percent of the originalacid concentration. If this SE~C-

ond part of the regenerant isreused in the next regenerationcycle before the resin bed makescontact with 5 Ib/ft3 (80 giL) of freshHCI, the exchange capacity wouldincrease to 67 percent of theoretlcalcapacity. The available capacitywould then increase from 60 to 67percent at equal chemical doses.Figure 5 shows the improvedreagent utilization achieved by thismanner of reuse over a range ofregenerant doses.

Regenerant reuse has disadvantagesin that it is higher in initial costfor chemical storage and feeidsystems and regeneration proce­dure is more complicated. Still,where the chemical savings haveprovided justification,· systems havebeen designed to reuse partsof the spent regenerant as many asfive times before discarding them.

Wastewater RecycleSystems

, .

In usual practice, metal finishingwastewater is treated and thendischarged to a iiver or sewer system;as an alternative, the wastewatercan be deionizad by ion exchangeand reused in the plating process.Wastewater deionization willsignificantly reduce water consump­tion and the volume of wastewaterrequiring treatment, with thefollowing prima:ry economicadvantages: !

i• Water use a~d sewer fees are

reduced. :• Although treatmentof pollutants

is not eliminhted, the size andcost of the ~ollution controlsystem is significantly reduced.

I

The volume reduction resulting fromwastewater recycling can alsomake pollution idischarge limitseasier to achieve. For plants dis­charging wastewater to municipaltreatment systems, the nationalpretreatment standards callfor more lenien~ discharge limits ifa plant discharges less than10,000 gal/d (~7,000 LId). Plantsdischarging directly to surface .waters are typically regulated bymass-based pollutant dischargelimits. When translated to aconcentration limit based on avolume of discharqe, these limitsmay be difficult to achieve byconventional pollutant removalsystems. The reductlon of dischargevolumes resultinq from waterrecycle will allo~ for higher concen­trations of pollutants in the dis-charge. :

IInorganic plating chemicals such asacids, bases, and metal salts areionized in water solutions and can be

Iremoved from process waters byion exchange: ~ome dissolvedorganic compounds, oils, and freechlorine are tylilically presentin mixed wast~waters and theirpresence constitutes a potentialfor fouling or deterioration of the ionexchange resin; Electroplatingfacilities using iion exchange onmixed wastewaters have found theresins to be operable and stable,however, when the recycle system

incorporates wastewater pretreat­ment to remove constituentsthat degrade the resins. When ionexchange is used to removechromate and zinc from coolingtower blowdown there is similarpotential for resin deterioration.Nevertheless, the effects have notbeen found severe enough to!preclude the successful use of ionexchange for this application.

Hexavalent chromium (Cr-6) can beremoved if the mixed wastewateris passed through an anion column.Cyanide and metal cyanidecomplexes are ionized and couldalso be removed directly fromthe wastewater by anion exchange.Mixing cyanide wastes with therest of the plant's wastewateris potentially hazardous, however;toxic hydrocyanic gas (HCN) wouldresult from contact with acidicwastes. Therefore, cyanide waste­waters are normally pretreatedbefore they are. blended with therest of the wastewater. In manycases, an integrated chemical wastesystem (Figure 6) can providecyanide pretreatment that is lowin cost and easy to operate.

The usual ion exchange sequenceis cationic exchange followed byanionic- exchange. The reversesequence is avoided because pass­ing the solution first through ananion exchange column wouldincrease pH and could precipitateheavy metal hydroxides.

System Description

An ion exchange wastewater recyclesystem is shown in Figure 7. Themajor process components include:

• Wastewater storage• Prefilters• Ion exchange columns• Regeneration system• Batch treatment for reqenerant

solutions• Deionized water storage

Wastewater Storage. A collectionsump or storage tank is neededto provide a surge volume in the

AnionCation

Legend:C = conductivity probe

NC= normally closed

Anion

Prefilters. Activated carbon columnsare commonly used as ion exchangeprefilters. The carbon columnsprovide a versatile pretreatmentsystem; they can:

• Filter out suspended solids thatcould hydraulically foul thecolumns.

• React with free chlorine orother strong oxidants that couldphysically degrade the resin.

• Adsorb organics that wouldotherwise build up in the recir­culated wastewater.

• Adsorb oils that would graduallyfoul the resin.

Regular skimming can then purgethe oil from the recycle system.

The columns are typically back­washed daily to remove collectedsuspended solids. The backwashwater goes either to the waste­water storage tank or to the batchtreatment tank. Carbon replace-

Cation

Deionized waterstorage

Tor~-..• process

1ooIliiii....iiiiiiiiii;iiiiiiiiiiiil

Waterrinse

~~"Rinse

water

To wastewater---'"""i~ purification and

recycle

solids can be pumped out atregular intervals and disposed of.Tank design should allow any free oilto separate and then collecton the surface of the wastewater.

Batch treatmenttank

Chemical rinsewith NaOCIsolution

Solids todisposal

Processconcentrates

Solidsseparation

Collection sump

Cyanide plating bath

Wastowater

Wastowator ___dlschlrgO ----

I-----Ion Exchange Columns -----I

Workpiece

system and allow the exchangers tobe fed at a constant rate. Theunagitated collection tank can alsobe used to settle coarse solidsin the wastewater. The collected

Figure 6.

Integrated Chemical Rinse to Oxidize Cyanide Compounds

Figure 7.

Ion Exchange Wastewater Purification and Recycle System

12

mentfrequency depends primarily onloading of oils or organics. If thecarbon is not replaced, organicimpurities can gradually build up inthe recycle water. Some longchain organic molecules will foulstrong base resins. Oil not removedby pretreatment collects on theresin and reduces its exchange ca­pacity, resulting in more frequentregeneration and higher operatingcosts. Cleaning solutions areavailable from resin manufacturersto restore the performance ofoil-fouled resin beds.

Ion Ey.change Columns. In the mostcommon column configuration,wastewater passes in series througha strong acid cation resin columnand then through either a strong orweak base anion resin column.Weak base resins have higher ex­change capacities and require lessregenerant than do strong baseresins. On the other hand, weak baseresins are not effective in removingweakly ionized bicarbonates,borates, and silicates, nor can theyoperate effectively at high pH.These limitations may not be a con­cern for metal finishing waste­waters, and weak base resins arerecommended. If these anionsare present in significant amounts,an anion bed containing bothstrong and weak base resins can beused. A bed of this kind will approachthe higher exchange capacity andregeneration efficiency of aweak base system but provide com­plete deionization.

To provide uninterrupted systemoperation when column regen­eration is required, two sets of col­umns are frequently installed. Whenone set has been exhausted,flow is switched to the off-streamset and the spent columns are regen­erated.

Regeneration System. The cation ex­change column should be regen­erated with hydrochloric acid afterexhaustion. Despite its highercost, HCI is favored over H2S04 forregeneration if the wastewater con­tains a significant amount ofcalcium.

In such a case,i reqeneration withsulfuric acid cah result in pre­cipitation ofcalcium sulfate andhydraulic foulin;g of the resin bed.Calcium sulfate: precipitation can beavoided by usi~g dilute sulfuric acidsolutions (2 percent by weight).Strong base anion columns areregenerated with sodium hydroxide.Weak base resins can be regen- .erated with sodium hydroxide orless expensive basic reagentssuch as SOdiu~carbonate.

Batch Treatme t for RegenerantSolutions. The ollutants removedby the ion exchange system will Ibe concentrated in the regenerant Iand wash solutions. These solutionsmust undergo donventional treat- Iment before be~ng discharged. TheI

I 'type of pollutants present (Cr+6 andheavy metals ~ould be most com- imon) dictates the treatment Isequence that would be required. I

i IDeionized Water Storage. A storagE!tank is used to lprovide an inventoryof water for process needs. The I

effluent from the ion exchange col-iumn should be lmonitored with aconductivity probe to provide a I

relative index oflthe level of dissolvedsolids in the treated water. When !the water conductlvltv increases to a:certain level, the columns are .switched and the spent columns :are regenerated. Because complete iwater deionizatlon is not needed formost process applications, thecolumns are loaded until the maxi­mum allowable !Ievel of impurityis reached before they are re­generated; regeneration frequencyand system OPerating costs are thus,reduced. I

, !

Ion Exchange C~lumnspeCification!I

Columns are us~ally sized as a func­tion of the ratio: of wastewatervolume to resin\volume. Recom­mended rates vary dependingon the application but as a rulerange from 2 to 4gal/min/ft3 (0.26 to0.52 L/min/L) of resin. Higher rateswill usually result in higherleakage, but Will not affect the

I

quantity of ionic compounds theresin bed can exchange.

For rinse water recovery, leak-age of small concentrations ofionic compounds would not signalthe end of the cycle. Therefore,rates should be selected from thehigher end of the recommendedrange to minimize the initial cost ofthe system. Smaller columns willincrease regeneration frequency andthe associated labor cost. Forcolumns with automated regenera­tion packages, increased re­generation frequency will not sig­nificantly increase operating costs,

Cost

Conventional end-of-pipe treatmentrequires removing pollutantsfrom large volumes of dilutewastewater. When pollutants are.concentrated into small volumeregenei'ant SOlutions, treatment isusually more economical. More­over, recycling the purified waste­water reduces operating costsassociated with water consumptionand sewer fees.

As a rule, treating the concentratedregenerant solutions will con-sume chemicals in quantitiessmaller than are needed to treat thesame mass of pollutants in a dilutewaste stream. Capital costs ofwastewater treatment systemsdepend primarily on the unitoperations required and the volu­metric flowrate of the wastewater.Total investment for an ion ex­change water recycle systemand a simplified batch chemicaldestruct unit to treat concentratedsolutions will often be less thanthat for a conventional chemicaldestruct system designed to treat thetotal volume of water consumed bya plant.

Operating Cost. Operating costs foran ion exchange purification SiYS­tem to treat wastewater containing avariety of heavy metals will include:

• Chemicals for column regen­eration

13

TreatmentItem

Table 4.

Wastewater Characteristics and Ion Exchange Capacity Requirements

40405020

220470

70

30

. 0.921.08

0.00360.00423

30

40405020

150310

10

Ion exchange Conventional

Wastewater characterlstlc:Flowrate (gal/min) .•....•........•••.•..••......•••Constituent (ppm):

Cu+2•••••••••••••••••••••••••••••••••••••••••

Ni+2 •••••••••••••••••••••••••••••••••••••••••

Cr04'2.•....•..'.......•.......•..•...••.......Na+ ••••..••••......•••••.•.•.•••.•.••....•..S04'2 .....••..,..•••...•.....••..•...••......••Total dissolved solids ....••........•....•••....

Alkalinity, as Ca(HCOah (ppm) ....•.........•..•.....Wastewater concentrations to be treated by ion exchange (eq/L):

Cations"•••..•..•.•" .. " ...••..•....•.•..•••......Anions •••....••••.•....••......•••.••....••..•...•

Ion exchange resin capacity needed for 15·fta bed (eq/L):bCation resin" ••.••.•:.•........•....•.•.....••.....•Anion resin •..•..••..•...•.......•.....•..••••••..

• Destruct chemicals for treatmentof concentrated regenerantsolutions and purged wash water

• Disposal of the treatment residue• Labor for column regeneration

and operation of the batch treat­ment system (if not automated)

• Maintenance• Resin and activated carbon

replacement• Utilities

How these costs compare with thecosts of operating a conventionalhydroxide treatment process can bedetermined by evaluating thecosts associated with each systemtreating the same waste stream.To simplify the analysis, equal labor,maintenance, and utility chargesare assumed for both systems.

"Does not include hydrogen ions.

b16-h operating cycle.

Table 5 shows the chemical contentand volumes of the reqenerantsolutions after they are mixed with

rate results from the anionsassociated with the hydrogen ionacidity. Adequate capacity would beobtained in the cation column witha regenerant level of 4 Ib HCI/ft3(64 g HCI/L) of resin. In this: case,however, the combined anion andcation column regenerant mustbe acidified to reduce Cr+6• There­fore, excess acid regenerant [6.5Ib/ft3 (104 giL)] can be usedto balance the excess NaOH in theanion regenerant.

A typical waste stream (Table 4)consisting of rinses after nickel, cop­per, and chromium plating bathsand acid and alkali process bathswill be used in the cost analysis.In a water recycle system, onlynatural alkalinity brought in withmakeup water must be treated;recycled water has already had itsinitial alkalinity removed. Thewastewater used in conventionaltreatment, however, containsall the natural alkalinity brought inwith the fresh water; as a resultmore alkali reagent will be consumedand more solid waste generated.

In light of the foregoing analysis, thenext step is to determine therequired column configurationand size of the ion exchange unit.Either of two column configurationscan. be used: strong acid andstrong base or strong acid and weakbase. In either case, ion exchangecolumn sizing is based on volumetricloading. At the normally recom­mended service flowrate of 2 gallmin/ft3 (0.26 L/min/L) of resin, col­umns containing 15 ft3 (425 L)of resin will be needed. The ion ex­change capacity of these columnswill depend on the quantity ofregenerant (dosage rate).

Using the resin capacities given inFigure 4, the columns will be

regenerated with sufficient acid andbase to provide 1 day's operatingcapacity. The plant is assumedto operate 16 hid. Table 4 includesthe resin capacity needed forcolumns with 15 ft3 (425 L) ofresin.

For the strong acid/strong baseunit, sufficient capacity would beobtained in the anion columnwith a regenerant level of 6.5 IbNaOH/ft3 (104 g NaOH/L) of resin.The anion column would requiregreater capacity than the cation col­umn because the wastewater isacidic; the higher anionic loading

Table 5.

Regenerant Solution Chemical Content

Item

Volume •.•.•.••••....•......•.•••....•.•....•.••.Cation column ••..•.•.••.•••........•..•••••......

Anion column••.••••..•...•.•.•.•••••...•.......••

Strong acidlstrong base

850 gal20.3 Ib CuCI221.1 Ib NiCI212.1 Ib NaCI1.6 Ib CaCI267.0 Ib HCI16.8 Ib Na2Cr0453.3 Ib Na2S0 43.0 Ib Na2COa56.9 Ib NaOH

Strong acidlweak base

800 gal20.3 Ib CUCI221.1 Ib NiCI212.1 Ib NaCI1.6 Ib CaCI229.5 Ib HCI16.8 Ib Na2Cr0453.8 Ib Na2S043.0 Ib Na2COa7.4 Ib NaOH

14

15

0.050.0750.050.200.052.50

Cost ($/Ib)aDescription

Carboys. 32% HCICarboys. 50% NaOH100-lb bags100-lb bagsCarboys. 97% H2S04Dry powder

erant chemicals for this columnconfiguration would cost $16.56 foreach cycle.

a1980 dollars.

Reagent

II

alb mJtals expressed as Ib metal ions.!

bAlkal\nity consumes lime and adds to solids generation rate.I

Table 6. iChemical Prices

I

Hydrochloric acid -1- .Sodium hy.droxide'l' -:...•......Hydrated lime '1' ' .Sodium bisulfite ......•..•..•..•....... ,;•..••.••..Sulfuric acid .....•1 ' .

Polyelectrolyte -1- , , .: .

1

Ifor regeneration should be reducedto the minimum required forcolumn capacity, 4 Ib HCI/ft3 (64 gHCI/L) of resin] Table 5 includesthe volume and chemical content of Based on treatment chemicalthe regenerant!solutions. Regen- consumption factors (Figure 8) and

i

... Reduction (NaHSOa• H2SO4) ; Neutralization [Ca(OHhl

Chromium waste ) 3 Ib NaHSOa/lb Cr+6

~1.7 Ib Ca(OHh/1.OOO gal

(gal/min. Ib Cr+6) 2 Ib 1-l2SOJ Ib Cr+6

"0.3 Ib NaHSOa/1.000 gal0.2 Ib H2SOJ1.000 gal

I

:

U~ Neutralization (Ca(OHhlb : Precipitation [Ca(Olihl Flocculation.. ..

Heavy metals wastes )1.2 Ib Ca(OH)~1.000 gal

-,2.6 Ib Ca(OH)~lb Cr -\ 0.02 Ib/l.000 gal(gal/min. Ib metal"] I

-,I 2.2 Ib Ca(OHh/lb metal r--yI,.

Neutralization [Ca(OHhlb

IPrecipitation

0.1 Ib dry solids generated!

Ib dry solids generatedSolids generation factors Ib Ca(OHh consumed Ib metal precipitated

; Cr 2.24! Ni 1.80

ICu 1.75Cd 1.52

I Fe+2 1.83,Zn 1.74

Legend:AI 3.11

Process step (treatment reagent)

Consumption factor

Regeneration cost can be reduced ifa weak base resin is used in theanion column. A weak base resindownstream of the strong acidcolumn is suited for this applicationbecause the entering wastewaterwould always be acidic. Based on thecapacity shown in Figure 5, suf­ficient resin capacity could beachieved with a sodium hydroxidedose equal to 3.2 Ib/ft3 (51 giL) ofresin. The amount of acid consumed

the volume of wash water [50gal/ft3 (6.5 L/L) of resin] usuallyrequired for the backwash and rinsestages of regeneration. Chemicalcost of each regeneration cycle is$29.83 forthe strong acid and strongbase system (based on Table 6).

Figure 8.

Conventional Treatment Chemical Consumption Facto~s

I

I

I

Table 7.

Daily Treatment Cost Comparison: Ion Exchange and Conventional Systems

"Aslumos 10 gal/min of segregated Cr+6wastewater.

bNot required.

"25% sands by weight at $0.20/gal.

dFor 46.3 Ib dry solids.

"For 62.2 Ib dry solids.

Note.-1980 dollars.

Effect of Pollutant Concentration.The volume of wastewater thatcan be deionized by an ion exchangecolumn is in direct proportionto the ionic concentration of thewastewater and is not influenced bythe volume needing treatment.Consequently, when dilute solutionsare processed, a large volume canbe treated before column capacity isexhausted and regeneration isrequired. On the other hand, conven­tional treatment processes--suchas chromium reduction. cyanideoxidation, and metal precipitation­must adjust the chemistry of thewater solution to achieve thedesired reaction. The chemical con­sumption associated with theseprocesses therefore dependson both the mass of pollutant andthe volume of solution to be treated.Because its cost is independentof solution volume, ion exchangeprocessing is highly efficientin terms of chemical consumptionwhen used to treat dilute con­centrations of ionic contaminants.

3.82"0.635.164.97"

43.38

14.5828.80

Conventional

3.34(b)

4.083.70d

9.367.20

27.681.70

29.38

Strong acidlweak base

Treatment cost ($/d)

3.34(b)

3.763.70d

42.33

40.631.70

15.2114.62

Strong acid/strong base

Component

Total treatment cost .....•...........

Total chemical cost.•................Water and sewer fee at $1/1.000 gal .

Chromium reduction:NaHS03 •••••••••••••••••••.••••••...

H1S0 4 • • • • • • • • • • • • • • • •••••••••••••...Neutralization: Ca(OHb ••••••••.•..•••......Sludgs disposlli c ••••••••••••••••••••••••••Ion exchange regeneration:

Hel .NaOH •••••••••••.•.••.•.............

chemical costs (Table 6), Table 7compares the daily cost to operatethe two ion exchange systemswith the costfor a conventional treat­ment system. Although the chem­ical costs are higher for the ionexchange systems, when thesavings in water and sewer fees(assuming $1/1,000 gal) areconsidered, the total cost is lessthan that of conventional treatment.The data also indicate that a strongacid/weak base column config­uration is considerably less expen­sive to operate than the strongacid/strong base configuration. Theeconomics of the ion exchangesystem could be improved further ifthe strong acid column regen­arant were reused.•

For deionization applications,commercially available resins costbetween $50/ft3 and $200/ft3. Ionexchange resins usually need replac-

ing every 2 to 5 years, dependingon the type of resin and the processapplication. Resins can be dam­aged by exposure to strong oxidants,long chain organic compounds,or oil. With proper selection of resinsand effective pretreatment of thewastewater, the potential forresin deterioration and the costfor replacement will be reduced.

I

Granular activated carbon must bereplaced when its adsorptioncapacity is spent. For small scaleapplications, regenerating thecarbon is not economically feasible.Replacement frequency for activatedcarbon will depend on the levelof organic compounds in thewastewater. Carbon adsorption is aneconomical means of removingtrace amounts of organic compoundsfrom solution. If high levels oforganics are present, however, thecost becomes excessive and alterna­tive removal techniques should beevaluated.

Figure 9a shows the relative costs ofdeionization and conventionaltreatment techniques as a functionof the concentration of waste­water contaminants for acid-alkaliwaste streams and for hexavalentchromium wastewater. Only chem­ical treatment costs are included,not water and sewer use fees.The treatment steps and assump­tions used to derive the conventionaltreatment cost are presented inTable 6 and Figure 8. Also assumedis removal of natural alkalinityduring treatment.

Ion exchange does not comparefavorably with hydroxide precipita­tion of acid-alkali waste streamsexcept at very dilute concentrations.For treating typical metal finishingwastewater, hydroxide precipi­tation will usually have lower

16

(a) i chemical costs. For hexavalent2.00 Legend: I chromium wastewater, however, ion- hexavalent chromium wastewater (H2Cr04)

exchange has a treatment costIliII!IIIIiii!I acid/alkali wastewater (CuS0 4 )

advantage up to a concentration of440 ppm of chromic acid (H2Cr04}'..

1.50 Ion exchange treatment is effi-"iiicient for this application partly be-C>

0

cause, except for contaminants,0qchromic acid wastewater has;<;

~no cations other than hvdrogen:f-en 1.00 consequently, treatment by ion0

uexchange would not affect cationf-zcolumn operating costs.w

~

~In Figure 9a the cost for con-w• a:: 0.50ventional treatment of acid-alkali

f-

Hydroxide wastes includes the volume-relatedprecipitation

cost for lime to adjust solution pHand to react with naturally occurring

0 bicarbonate alkalinity, and fora 100 200 300 400 500 polyelectrolyte conditioners to aid

CONCENTRATION (ppm) in precipitant settling. The curve for(b) 3.00 chromium reduction using bl-

sulfite includes these cost compo-nents plus costs for acid neededto bring the wastewater to requiredreaction pH and base for subse-

2.50 quent neutralization.

The ion exchange system costsare based on 90 percent water

.c recycle; they include the cost for'i... 2.00column regeneration and treatment"iii

C>

of regenerant solutions by con-0

& ventional techniques. The re9lener-~ ant chemical consumption isf-

1.50 based on a strong acid/weak baseen0

column configuration.uf- Hydroxidez precipitationw

An ion exchange water recycle~

~_.:...-..-.J:::':::!!--

system becomes considerably morea:: 1.00 attractive than conventionalf-

treatment techniques if the credit forsavings in water and sewer feesis included in the analysis. Figure 9b

0.50 compares treatment costs of thesame two waste streams butincludes a cost equal to $1/1,000 galwater consumed.

a Waste Reduction. The wastea 100 200 300 400 500 stream volume reduction achieved

CONCENTRATION (ppm) by a wastewater deionization"1980 dollars.

system relates directly to the 'eon-bWater and sewer fees assumed at $1 /1,000 ga~.

Figure 9.I

Cost Comparisons for Ion Exchange and Conventional *reatment Systems:(a) Chemical Cost Only and (b) Chemical Cost and Wa er Use Fees

I17

0.05

i0.04

6,400

0.03

4,800

0.02

3,2001,600

. 0.01

WASTEWATER CONCENTRATION (ppm CuS04)

oK..__........ ......__....I.........__-'-__---i

8,000

0.1

o

o0.5 _--~..,..---....,----......-._-.,..----...,

0.4

WASTEWATER CONCENTRATION (eq/L)

Figure 10.

Relationship of Waste Volume Reduction to Wastewater IonicConcentration

Consider an ion exchange systemwith a strong acid/weak basecolumn configuration. Assume theresin in each column has a capacityof 1.5 eq/L and that regenerat-ing the columns produces purge(regeneration plus rinsing) inthe amount of 50 gal/ft3 (6.5 L/L)of resin. The maximum concentrationof the ionic solids in the combinedpurge streams from both col-umns, then, would be 0.11 eq/L.Expressed in terms of a typical metalsalt, the maximum concentrationof copper sulfate (CUS04) in thepurge solution would be 1.75 per­cent. Using this relationship, Figure10 presents the volume reductionfor treating wastewater over a rangeof ionic concentrations.

Each cubic foot of resin in a columnsystem can remove a specificquantity of ions; regenerating andwashing that volume of resinwill result in a purge stream of limitedconcentration.

centration level of the dissolvedionic solids in the wastewater.The reduction in volume ofthe wastestream and its favorable effect onboth the initial and operatingcost of wastewater treatment arepart of the justification for using ionexchange.

The relationship developed inFigure 10 is based on normal operat­ing procedures. Concentrationcan be improved by selective recycleof part of the purge stream; how­ever, the poor chemical efficiency ofthe ion exchange process-fortreating concentrated solutionsand the poor degree of concentrationachieved make other methods of 'treatment more suitable.

Capital Cost. In the metal finishingindustry, most of the wastewaterrequiring treatment results fromrinsing operations. Selectivetreatment and reuse of rinse streamsby ion exchange can result in

considerable savings in the invest­ment necessary for end-of-pipetreatment systems. This investmentis usually a function of wastewaterflowrate and the required unitoperations. For flows above 15 to 20gal/min (57 to 76 L/min), auto­mated continuous treatmentsystems are usually recommended.A deionization water reuse sys­tem can result in flow reduction suf­ficient to make a single batchtreatment tank feasible for treatingregenerant solutions and anyconcentrated process dumps.

The cost for ion exchange columnsystems is increased significant­ly when dual cation-anion columnconfigurations are needed for

continued operation during regen­eration. Automation adds con­siderably to the initial cost of the unitbut, in addition to savings in labor,can permit the use of smallercolumns with more frequent regen­eration. Figure 11 compares costsfor two- and four-column ionexchange units, automated andnonautornated, as a function of resinvolume in each column. Tho systemsillustrated are skid mounted andpreengineered; costs includethe columns, an initial supply ofresin, reagent storage, and internalpiping and valves necessary for ser­vice and regeneration flow.

18

60

=

19

The cost of the' auxiliary equipmentdescribed earlier for ion exchangewater recycle will add consider­ably to the total capital cost asso­ciated with using the technol-ogy. The total cost, however. maystill compare favorably with that for aconventional end-of-pipe treat­ment system.

20

i

lonautomated

I

ionautomated

I

Legend: I_ dual cation/anion ¢olumns (4 columns)_ cation/anion columns (2 columns)

!!II

4o 8 12 1 16

RESIN VOLUME PER COLUMN (ft3) !"1980 dollars. iNote.-Skid-mounted, preengineered package unit. Includes acid and baseregenerant storage and all internal pipes ana valves. ISOURCE: Equipment vendor. i

50

10

40

~

0"0q

Eti 300UI-Z:::l

20

I

Figure 11. II

Cost for Deionization Units With and Without AutomJtionIII

I

End-of-Plpe Systems

20

Ion exchange can be used in twodifferent ways for end-of-pipepollution control. Th~ process hasbeen demonstrated as a meansof polishing the effluent from con­ventional hydroxide precipitationto lower the heavy metal concentra­tion further, and it has been usedto process untreated: wastewatersdirectly for removal of heavymetals and other regulated pol­lutants.

Most plating shops can achievesufficient metal removal to complywith discharge regulations byemploying the conventional hydrox­ide precipitation process. Conven­tional treatment may: not be reliable,however, in achieving compliancewith discharge limits in certaincases, including where:

• Unusually strict limits are placedon the effluent metal concen­tration.

• The metals are complexed withchemical constituents thatinterfere with their precipitationas metal hydroxides.

In such cases, the use of ion ex­change to polish the effluent can pro­vide relatively inexpensive up­grading of system performance forcompliance with the regulations.

The development of special chelat­ing resins made ion: exchangefeasible for selective removal oftrace heavy metals from a water solu­tion containing a high concentra­tion of similarly charged, nontoxicions. These resins exhibit a strongselectivity, or preference, forheavy metal ions over sodium,calcium, or magnesium ions. Weakacid cation resins also displaya significant preference for heavymetal ions, and in some applicationsthey are superior to, the chelatingresins in performance character­istics. In a polishing applica-tion, both resins can remove theheavy metal ions from the waste­water while leaving most of thenontoxic ions in solution. Thepreference for heavy metal ionsallows a large volume of water to be

treated per unit of resin volumebefore the resin must be reqener­ated. The regenerant solution,which contains a high concentrationof metal ions, is treated upstreamin the conventional process (Figure12a).

Ion exchange has received limitedcommercial application for selectiveremoval of heavy metal and metalcyanide pollutants from an un­treated wastewater while allowingmost of the nontoxic ions to passthrough. Various approacheshave been employed to facilitate thisapplication:

• A weak acid cation resin hasbeen used in wastewater soften­ing to remove heavy metalsand other divalent cations from awastewater solution with ahigh concentration of sodiumcations.

• Heavy-metal-selective weakacid or chelating cation resin hasbeen used to remove the heavymetal ions while allowingsodium, calcium, and magnesiumions to pass throuah,

• A stratified resin bed, containingstrong and weak acid cationresins and strong base anionresins, has been employed to re­move heavy metal cations andmetal cyanide complex anionswhile allowing other ions to passthrough.

In each of these approaches,wastewater pretreatment require­ments consist of pH adjustment toensure that pH is within the operat­ing range of the resin, and filtration toremove suspended solids thatwould foul the resin bed (Figure 12b).The pollutants removed from thewastewater are concentratedin the ion exchange regenerantsolutions. The regenerants can betreated in a small batch treatmentsystem using conventional proces­ses. Firms with access to a cen­tralized treatment facility thataccepts industrial wastes can use the

I II

Collection;

and ; Ion.FI,ltration exchangepH -

adjustment treatment!

1~I tI

II

RegenerantT,udge chemicals

I,

I II

I

clr I

+ 1+,

I"

I,i

Waste Brtch

storagetmatmenrsystem

1

J II ~~

Discharge

Legend:- service_ regeneration

...__...~ DlscharqeIonexchangepolishing

+Regene,rantchemicals

• Ion exchange polishing toreduce residual metal solubilitybefore the water is discharged

21

For effective metal removal byhydroxide precipitation, pH must becontrolled within. the narrowrange where the metals are leastsoluble. Such narrow controlusually requires sufficient retentiontime within the pH adjustmenttank to ensure minimum variation inneutralizing reagent demand.Multistage neutralizers and sophisti­cated control loops are also usedto minimize deviation from the pH

:fOlidSremoval

!

~etalhydroxideslpdge

:,~etal

~ydroxide

sludge

Conventionalhydroxideprecipitationsystem

To centralizedtreatmentfacility

(a)

(b)

Wastewater

Wastewater ..~

facility to dispose of the regenerantsolutions and need not installchemical destruct systems. In eithercase, no investment is neededfor sophisticated pH control systems,flocculant feed systems, clarifiers,and other process equipmentassociated with conventional con­tinuous treatment systems, andion exchange becomes attractivein terms of cost. And, as a further ad­vantage, ion exchange units arecompact and easy to automatecompared with conventional treat­ment systems.

Figure 12.

Ion Exchange Systems: (a) Polishing and (b) End-of-Pide TreatmentiII

Ion Exchange prliShing Systems

Process Descript(on. Figure 13shows. a treatment system employ-ing: :

• Hydroxide neJtralization tocontrol pH and to precipitateheavy metals as metal hydroxides

• Flocculation t6 agglomeratethe suspended solids

• Clarification a~d deep-bedfiltration to remove the precipi­tated metals a'lnd suspendedsolids

IIII

I!

Wastewater

Feedtank

Service

..__'. Wastewaterdischarge.

, Hel, NaOH

Regeneration

Collectionsump

Ion exchange columns

Figure 13.

Conventional Treatment System With Ion Exchange Polishing

control set-point. With effectivepH control, most of the metalsin the wastewater will precipitate asmetal hydroxides.

To provide effective removal ofprecipitated metals and othersuspended solids, coagulating­flocculating compounds are added tothe neutralized wastewater toagglomerate the solids and facilitatetheir removal. Mostofthe suspendedsolids can be removed by clarifica­tion; however, removal of fineparticles (including precipitatedmetals) requires filtration. Deep-bedfilters, which remove solids bypassing the wastewater through abed of sand and gravel, are usedmost frequently.

For most waste streams, the unitoperation sequence of hydroxide pre­cipitation, fluocculation, clarifi­cation, and filtration will producean effluent with a minimumheavy metal content and achievecompliance with discharge permitregulations. In cases where themetal content exceeds the permitlimit, and the excess is in theform of dissolved metals (as opposedto metal hydroxide particles notremoved during solids separatism), apolishing treatment using ionexchange resins will reduce theeffluent metal concentrations.

The failure of hydroxide precipitationto reduce metal solubility to therequired level can be caused by oneof the following:

• Failure to control pH within thenarrow range necessary forminimal metal solubility

• The presence of chelatingcompounds that combine withmetals to form complexes noteffectively removed by hydroxideprecipitation

• Discharge limits requiring metalconcentrations below thosewhich a hydroxide treatmentsystem can achieve consistentlyand reliably

Resin Selection. Ion exchangeheavy-metal polishing systems willusually use a chelating heavy-metal­selective cation resin. A resin ofthis kind forms an essentially'non-ionized complex with divalentmetal ions. Consequently, once anexchanger group is converted to theheavy metal form, it is relativelyunreactive with other similarlycharged ions in solution. Despitehigh concentrations of non-heavy-

22

I

II

metal cations competing for the ex­change sites, the resin has suffi­cient preference for the heavymetal ions to exhibit a high metal­holding capacity 'per unit of resinvolume. The chelating resinswill effectively remove heavy metalcations from solutions with apH above 4.0.

Weak acid cation resins also havepotential for use in ion exchangepolishing systems. These resins havethe advantage of being lessexpensive than chelating cationresins, and they require less chemi­cals for regeneration. On the otherhand, weak acid resins are not .'effective in acidic solutions;moreover, they are less selective forheavy metal cations over otherdivalent calcium and magnesiumions than are chelating resins.

Polishing System Equipment andAuxiliaries. The ion exchange polish­ing system consists of:

• Column or columns containingthe resin

• Acid regenerant storage• Sodium hydroxide regenerant

storage• Piping and valving to facilitate

on-stream wastewater treatment,and regeneration and back­washing of the resin bed

Three ion exchange column configu­rations for a polishing system are:

• Single column• Series column• Parallel column

Unlike deiohization systems, whichrequire both a cation and an anionresin column, the polishing systemusually requires only one kindof resin. Consequently, a singlecolumn design is feasible for smallflows where the wastewater dis­charge can be interrupted to allowfor column regeneration. Dischargepermits are usually based on adaily composite sample, and thisfactor should be considered inevaluating use of a single column.Often the composite effluentquality of a treatment system that is

;

off stream 10 percent of the time Ifor reqeneration in any given, I

day will achieve the discharge per- ;mit limits. Mechanical failuresand system maintenance are inevi­table conseque9ces of using ~he..process, however, and the reliabifitvof a single colu\nn design is prob­ably inadequate /n most applications.

I!

In none of the polishing systemcolumn configurlations is there asimple means of detecting thebreakthrough ofi metal. ions thatwould indicate aneed for column I

regeneration. M~tal breakthrough is'avoided by loading the columnonly to some fraction of itsexchange capacIty. A series col­umn configuration, where thetotal flow of wastewater passesthr.ough .each c4lumn, is particularly.reliable In ensurrng contact ofthe wastewater \Nith a large volumeof unreacted res'in. After theup-stream column is exhausted, itis taken off stream, regenerated,and returned to service as thedown-stream column. This configu­ration rninimizea the possibilitythat the resin will be exhausted andthat metal breakthrough willoccur. On the other hand, pressuredrop over the system will behigh and each c91umn must be sizedto process the t?tal flow. .

,A parallel column configurationemploying three-or more columnshas advantages, ipertlcularlv forlarger flows. Both equipment costand reliability are intermediatebetween the single and series col­umn configuratidns. In a parallelconfiguration, each column issized based on t:he assumption thatone column is always off streamfor regeneration.1 This designreduces the total resin volume re­quirements compared with those ofa series column ~esign. Using abank of small columns does increaseregeneration frequency; many of theunits are desiqned with automatedregeneration cap;abilities, how- 'ever, and more frequent regenerationdoes not increase the need foroperating labor.

Operating Procedure. Operation ofan ion exchange polishing system iscomplicated by the lack of prac­tical meansfordetermining when thecolumn is exhausted and metalbreakthrough occurs. Unlikedeionization systems, where <I con­ductivity probe will signal theend of a column cycle, polishingsystems have no simple, directtechnique for continuously monitor­ing the levels of heavy metalsin the effluent. To compensate forthis lack, the columns are operatedon either a time or flow cycle.This approach requires determiningthe column exchange capacityand the loading per unit volume ofwastewater. Then, based on theresin volume in the column,the volume of wastewater that canbe processed before exhaustingthe exchange sites can be estimated.As a rule, to provide a factorof safety, a capacity equal to three­quarters of the actual exchangecapacity is used to determine thevolume that can be processedper cycle.

For a constant flow system, thevolume capacity can be convertedto a cycle time. A flow totalizer canbe used for variable flow systemsto monitor the cumulative volumeand indicate when the columnshould be regenerated. Many manu­facturers provide automaticregeneration capabilities withtheir column systems. For such sys­tems, the control mechanismcan be directed to begin regenera­tion by either a timing deviceor a flow totalizer.

The regeneration sequence for achelating and a weak acid cationresin is:

1. Water backwash to remove sus­pended solids from resin bed

2. Acid regeneration3. Water wash to remove residual

acid4. Sodium hydroxide regeneration5. Water backwash to remove

residual caustic and reclassifythe resin particles

6. Cocurrent fast rinse to ensurethat the resin bed's flow charac-

23

Table 8.

Ion Exchange Polishinq System Performance Characteristics;

teristics are adequate and toremove any unused reagents

7. Retum to service

The resin is used in the sodium formeven though it adds extra stepsto the regeneration processand increases the chemical con­sumption. Treating an alkalinewaste stream with a resin in the hy­drogen form would gradually resultin conversion of the resin toeither the sodium or calcium form;however, the exchange for hydro­gen ions would depress the effluentpH below the control limitationsand result in a period of noncom­pliance. Also, the resin exhibitsa greater selectivity for heavymetals in the sodium form.

Ite~

Ion exchange column .•.••..................•

Service:Wastewater to column •......•...•..••...

Discharge•..••.... r ••••••••••••••••••••

Regeneration:Flow to column ..•.•...........•.....••.

Purge streams .•...•.••....•.....••..•.•

Characteristic

Chalatlnq resinWastewater volumeResin capacity

pHNi+2

Cu+2

pHNi+2

Cu+2

Wash water5% NaOH5% HCIVolumeNickelCopper

Value

10 ft3120.000 gal/cycle0.87 Ib N i/ft3

0.03 \b Cu/ft3

8.48.9 ppm0.3 ppm8.40.16 ppm0.02 ppm

500 gal/cycle68 gaI/cyc Ie65 gal/cycle633 gal/cycle0.16%q.006%

Hydrochloric acid is normallyused for acid regeneration althoughit Is more expensive than sulfuricacid. Sulfuric acid regenerationcould result in the precipitation ofmagnesium or calcium sulfateduring regeneration, and the resinbed could thus be hydraulicallyfouled. This effect can be avoided,however, if a dilute (2-percent)sulfuric acid regenerant solution isused.

The final backwash to reclassify theresin bed is critical. "Classlfi­cation" refers to positioning theresin particles so that the largestparticles are at the base of thecolumn and the particle sizegradually decreases as distancefrom the base increases. Thisarrangement results in maximumflowrate per unit of pressure dropanu makes the bed more resistantto fouling from suspended solidsin the column feed.

With strong acid and base resins,an initial backwash before regenera­tion is usually sufficient to ensuregood flow characteristics duringthe service cycle. In the caseof weak acid or chelating resins,however, the resin beads swellconsiderably when convertedto the hydrogen torm and subse-

24

quently shrink when converted to thesodium form, which necessitatesa final backwash before the columnis returned to service.

All regenerant and wash solutionsare sent to the hydroxide treat­ment system for processing.Table 8 presents a typical polishingsystem performance with volumeof wastewater processed and therelative volumes of :the regenerantstreams.

System Performance. The number ofion exchange polishing systemsinstalled to date is limited;however, abundant pilot test dataverify system effectiveness inreducing the soluble metal contentof a neutralized waste stream.The data from these controlledexperiments can lead to a betterunderstanding of how process vari­ables and design factors influenceperformance.

Volumetric loading for ion exchangesystems is usually expressed inbed volumes (bv) of solutiontreated per hour or in gallons perminute per cubic foot (litersper minute per liter) of resin. Bothmeasures describe: loading interms of the volume of solutiontreated per volume of resin in a unit

of time. In essence, they definethe length of time the solution is incontact with the resin.

Figure 14 shows the concentrationprofile of the effluent from a pilottest column containing a chelatinqresin. The feed solution has aninitial cadmium concentration of 50ppm, a calcium chloride (CaCI2)

concentration of 1,000 ppm, and apH of 4.0. Tests were run at twodifferent volumetric loadings:8 bv/h [1 gal/min/ft3 (0.13 L/min/L)]and 16 bv/h [2 gal/min/ft3 (0.26L/min/L)]. The higher loadingresulted in earlier breakthrough.Assuming the column cycle isterminated at-a cadmium concen­tration of 2.0 ppm in the effluent, the8-bv/h system could treat 400 bvbefore regeneration, comparedwith 325 bv for a system operatingat 16 bv/h.

The influence of volumetric loadingon capacity results in a trade-offbetween investment and operatingcost. Specifying a larger, moreexpensive column will resultin greater capacity per unit volumeof resin and less frequent andmore efficient regeneration.

Figure 14.

Influence of Flowrate on Chelating Resin Capacity

Ion Exchange WastewaterTreatment Systems

When the resin is selected for apolishing application both weak acidand chelating cation resins shouldbe tested. The lower initial cost,greater capacity, and more efficientuse of regeneration chemicalsmake weak acid resins the choice forthose applications where theyare effective in metal removal. Manywastewater applications, how-ever, will require the chelatingresins' greater affinity for heavymetals to achieve the necessaryeffluent quality.

25

Pilot evaluations have also beenperformed with actual plating waste­water. Figure 17 shows the feed andeffluent concentrations of copper.and nickel when the effluentfrom a hydroxide precipitationsystem was treated by ion exchangepolishing. After adjustment toa pH of 8.4, the wastewater stillcontained a high level of nickel,although copper was removed to lessthan 1 ppm. Dissolved ammoniacontent was approximately 80ppm. The weak acid cation resin insodium form was ineffective inremoving the nickel and thetest was terminated after 700 bv ofsolution had been treated. Thechelating resin in sodium formconsistently removed the nickel tolevels below 0.5 ppm and thecopper to below 0.1 ppm until '1,600bv of solution had been treated.The equivalent would be processing12,000 gal/ft3 (1,600 L/L) of resinbefore regeneration would beneeded.

The conventional practice of con­verting the heavy metal pollutantsin metal finishing wastewater toa hydroxide sludge was thoughtto bea means of eliminating any envi­ronmental hazard the metals mightpose. In fact, a solid waste stream isgenerated that, although itsvolume is much smaller than that ofthe wastewater, requires furthercontrols to ensure that disposal of

(8)

500

16 bv/h

I,I

stable complexes with the heavymetals. Ammonia, a commonconstituent of m'anyplating waste­waters, tends to! increase metalhydroxide solubility. For example, ina copper solutior containingdissolved ammonia, the ammoniawould compete for copper ionsas follows:

Cu(()Hh +4NH31-= Cu(NH 3)t2+20H- II

IIn the presence lof many chelatingcompounds, a chelatinq resinis more effectlvs in removingheavy metals than a weak acid resinbecause it formJ a less-ionizedcomplex with th~ heavy metal ion.This effect is demonstrated inFigure 16, whicf shows the superi­ority of the chelatinq resin inremoving copper from solutionscontaining ammonia, A similarsituation would be expected forother complexed metal ions.

Breakthrough

100

Legend:O. resin capacity (Ib Cd/ft3)

6

e-o.

E::z 4:::E:::l..J0U

~

,

II

200 300 1400

BED VOLUMES TREATED . INote.-Feed solution: 50 ppm Cd+2, 1,000 ppm CaCI2, pH = 4.0. I

. ISOURCE: Adapted from Rohm and Haas Company, "Ion Exchange In Heavy MetalsRemoval and Recovery," Amber Hilite No.1 62, Philadelphia PA. Ro~m and Haas Company,1979. I

!

-

Figure 15 shows concentrationprofiles of the effluent from twopilot test columns. One columncontained a chelating resin,the other a weak acid resin. Eachcolumn treated a solution with50 ppm cadmium and 1,000 ppmcalcium chloride at pH 2.07,4.0, and8.0. Neither resin is effectiveat a pH of 2.07. The chelatingresin shows approximately equalcapacity at pH 4.0 and 8.0. The weakacid resin shows a capacityincrease when pH is increased from4.0 to 8.0. It is significant thatthe weak acid resin showedgreater capacity than the chelatingresin at pH 4.0 and 8.0. Wherethey are suitable, the less expensiveweak acid resins are the resinsof choice in metal removal applica­tions.

Ion exchange polishing is oftenconsidered because hydroxide pre­cipitation cannot effectivelyreduce metal solubility in the pres­ence of compounds that form

6pH =2.07 pH :k:4.0 pH =8.0

Legend:IiIIlI!I1II!llI chelating resin_. weak acid resin

o resin capacity (Ib Cd/ft3 ) .

5

800700600300 400 500

BED VOLUMES TREATED

200

EQ.

4.9:

!z pH =2.07Ul

:3u.u.Ul

z:: 3:::l-J0U

~

:::::l

Breakthrough~c 2

C3

NotD.-Feed solution: 50 ppm Cd+2, 1,000 ppm CaCI2• 8 bv/h.

SOURCE: Adaptod from Rohm and Haas Company. "Ion Exchange in Heavy Metals Removal and Recovery," Amber Hilite No. 162, PhiladelphiaPA. Rohm and Haas Company. 1979.

Figure 15.

Influence of Solution pH on Chelating and Weak Acid Cation Resin Capacity

the metal residue is environmentallyacceptable.

Ion exchange represents an alterna­tive means of concentrating thepollutants. The metals are concen­trated in the regenerant solutionsand are then in a form moreeasily handled and more amenableto further processing. With theincreasing cost of virgin metalsand the significant cost of heavy

metal waste disposal, the devel­opment of processes that recovermetals from mixed metal wastesis inevitable. When metal recoveryis commercialized on a widescale, the ion exchange regenerantsolutions will represent a byproductof metal finishing operations, nota waste product.

Currently, firms using ion exchangefor end-of-pipe pollution controlmust also install small batch treat­ment systems to treat the regenerantand wash solutions, These systems,

which use conventional destructprocesses, result in a residue withthe same disposal criteria asthe sludge from a conventionaltreatment process. The ion exchangesystem may still present a lesscostly means of complying withpollution control regulations. Thekey to using ion exchange for wastetreatment is to remove only thetoxic pollutants while allowing mostof the nontoxic ions in solution topass through the column. Normally,the toxic compounds represent

26

,-

requirement is substantially dif­ferent from those of hydroxide pre­cipitation systems, which needminimum deviation from the controlset-point.

27

As a rule, filtration to removesuspended solids is the only otherpretreatment required. Suspendedsolids in the feed would hydraulicallyfoul the resin bed. Different filtershave been employed, includingdeep bed, diatomaceous earth pre­coat, and activated carbon filters.In one approach, a filter withfine resin particles is used to trapsuspended solids. Regardless of thefilter type, the resulting purgestream containing the suspendedsolids must be processed anddisposed of.

The specifications of the columnsystem containing the ion exchangeresin depend on the flowrateand the pollutants in the wastewater.Two potential cases emerge withrespect to pollutants:

• Heavy metal cations alone• Heavy metal cations along

with cyanides, and complex metalanions

For waste streams containing bothheavy metals and cyanides, astratified bed of resin has proveneffective. This patented approachuses a bed of resin with successivelayers of strong base anion, weakacid cation, and strong acid cationresins. The wastewaterfirstcomes incontact with the strong base resin,which selectively adsorbs thecomplex metal cyanide ions but

In the case of wastewater containingonly heavy metal cations, a columnwith the sodium form of a weakacid or heavy-metal-selective chelat­ing cation resin would be employed.For a weak acid resin, a pH closeto neutral is recommended. Ifa chelating resin is used, the pH canbe slightly acidic (>4.0). In bothcases, strongly basic conditionsshould be avoided becausesuch conditions favor formationof anionic metal complexes.

500i 400

Legend: I_ weak acid cation resin I

~c~elating resin ' Io resin capacity (Ib Cu/ft3) i

Ii

Ii

200 300

BED VOLUMES TREATED

Breakthrough

100

o ...---...L.. -L...~_........._:;....

o

6

Ea.

; 4:;'E:;)....IoUZ

0:~ 2a..oU

II

Note.-Feed solution: 50 ppm Cu+2 , 1,000 ppm CaCI2 , pH = 4.0, ~ bv/h.j

SOURCE: Adapted from Rohm and Haas Company, "Ion Exchange In Heavy Metals~;~;val and RecO~ery,.. Amber Hilite No. 162, Philadelphia PA, ROh:m and Haas Company,

• Wastewater collection• Wastewater pretreatment• Ion exchange columns

IFigure 16. IInfluence of Ammonia onChelating and Weak Acid dtion Resin Capacity

I .

!i

• Ion exchangel column regenera-tion system

• Batch treatment for regenerants(or waste sto~age if regenerantsare shipped o;ff site for treat­ment or reco~ery)

I

I •Wastewater collection mostfrequently consists of gravity drain­age of rinses to ~ collection sumpbelow ground. The sump provides astorage volume ~o allow the flowto the treatmentlsystem to be .controlled at a constant rate. If theion exchange col~mnsemploy eitherweak acid or weak base resins, thecapacity and performance of theresins will be influenced by pH.Consequently, the collectionsump should include coarse pHadjustment capabilities. The pH ad­justment system Imust only ensurethat the solutionlpl-l does notdeviate from the ibroad operatingrange of the resin. This pH controlI .

Process Description. Wastewatertreatment systems employingion exchange include the followingcomponents:

only a small percentage of the ionicsolids in the wastewater, If theion exchange system is not selectivein the species it removes from thewastewater, the column capacityrequired and the regenerant chem­icals consumed will result in prohibi­tive costs.

Ion exchange has proved successfulin selectively removing many ofthe pollutants encountered inmetal finishing wastes. Proper appli­cation of the process requiresselecting the appropriate resin andregeneration sequence and, usually,some pretreatment of the waste­water before ion exchange.

Figure 17.

Metals Removal Data: (a) Nickel and (b) Copper

0.1 L_...!~~~:t=;;;;:;I~LJ.....!!....L.._.f-~_...L.---Jo 200 400 600 800 1,000 1,200 1,400 1)600 1,800 2,000

BED VOLUMES TREATED

With the stratified bed used for heavymetals and metal cyanide, theresin bed is first backwashed gentlyto remove suspended solids andthe resin bed is fluidized. Becausethe three types of resins have

The wastewater then comes incontact with the weak acid cationresin in the sodium form. The resinemployed exhibits a strong pref­erence for multivalent cations. Con­sequently, cation resin capacityis a function of the concentration ofcalcium, magnesium, and heavymetal cations. Finally, the resinmakes contact with, a layer ofstrong acid cation resin that is pre­dominantly in the hydrogen form.The exchange of the hydrogen ionstends to balance the pH rise thatnormally would occur at thebeginning of the cycle.

allows the rest of the negatively'charged ions to pass through.It should be noted that, although theresin will remove cornplexedmetal cyanides selectively, thepresence of free cyanide will resultin early cyanide breakthrough. .The strong base resin does not showsignificant selectivity for hexavalentchromium or free cyanide overthe sulfate, chloride, and other non­toxic anions in a wastewater. Foreffective use of this type of resinsystem, hexavalent chromiumwastes should be treated to reducethe chromium to the trivalent formbefore they are mixed with therest of the wastewater. ThE! trivalentchromium will be removed selec­tively by the weak acid resin.

The system also employs a novelregeneration sequence for thestratified resin bed. In a conventionalmixed bed system, cationic andanionic resins are separated by beingbackwashed into discrete layers.Each layer is then regeneratedindependently; acid is broughtin contact with the cation resin andsodium hydroxide regeneratesthe anion bed. The bed is then mixedwith air and the resin tyPI~S aredistributed equally throughout thebed.

Legend:__ • column feed

_ weak acid cation resirl column effluent-.. chelating resin column effluent

!zo

~suz8 0.10:wc,e,oU

10.0

E~

s~~uz 1.08.....w~

!:!z

(b) 2.0

0,01 ...._ ......._ ...._ ...._--'_.....1..._"- -"'_.....

o 200 400 600 800 1,000 1,200 1,400 1.600 1,809 2,000

BED VOLUMES TREATED

NOltt.-Feed conditions (average): 0.1 ppm Cr, 275 ppm Ca, 2,200 ppm Na, 0.05 ppm Zn,80 ppm NH4• pH =8.4, 8 bv/h.

(a) 20.0

28

r . I I' ; I

NaCN forchemicalmakeup

legend:_ regeneration

-.. sodium cyanide recovery

NaOH

~<J1-'-'-'-""'"

~"~l

A system treating a combinedwastewater containing both ferrousions and cyanides will have asignificant concentration of ferro­cyanides in the regenerant solution.These difficult-to-treat cyanidecomplexes result from mixingof the cyanide wastewater withacidic streams containing dissolvediron. An additional treatment stepis needed to oxidize the ferro..cyanides. In this step hydrogen per­oxide is added to the wastewater,which is subjected to irradiation byultraviolet light. The strong oxidiz­ing power of this system is effectivein treating the ferrocyanides.

29

treated in a small batch treatmentsystem. Depending on the pollu­tants present, the system may needcapability for cyanide oxidation,chromium reduction, and metal pre­cipitation. The sludge resultingfrom batch treatment can be eithersettled and disposed of or mechan­ically dewatered before disposal.

Airout

HCN and air

Cyaniderecoverytank

by the time it reaches the strong acidresin. After another water wash,the column is rrurned to service.

The stratified bed system also fea­tures cyanide recovery to avoidthe significant dost of treating thecyanide contained in the acidregenerant. Thejacid regenerant andthe subsequentiwater wash arerouted to a closed-top vessel (Figure18) where heat lis supplied toraise the solution temperature to1400 F (600 C) ~nd air is bubbledinto the solution, The result is arapid release oflHCN gas. Theliberated gas is Ibrought in contactwith a caustic soda solution; thecaustic soda absorbs the cyanide,yielding a sodium cyanide (NaCN)solution that ca~ be used for chem­ical makeup in t1he cyanide platingbaths. I

,

Regenerant solutions from the ionexchange column are usually

!

!HeaterCompressed air i

i, I

SOUI'lCE: C. Terrian; Best Technology, lnc., personallcommunication to P. Crampton, Aug. 10. 1980.I

Resincolumn

HCI andwash

different densities, the resin stratifi­cation can be maintained withproper backwashing. The strongbase resin is least dense, the weakacid resin is intermediate, andthe strong acid resin most dense.After backwash, the bed makescocurrent contact with a 20-percentHCI solution. The acid elutesthe metal cyanide complexes fromthe anion resin and replacesthem with chloride ions. The heavymetals and divalent cations areremoved from the weak acidcation resin and replaced withhydrogen ions. The strong acid resinis also converted to the hydrogenform.

After a water wash, the bed iswashed with a 20-percent sodiumhydroxide solution. The sodiumhydroxide converts the anionresin to the hydroxide form andelutes any metal chloride complexesformed during the acid wash. Theweak acid cation resin is convertedto the sodium form. The sodiumhydroxide is essentially depleted

Figure 18.

Sodium Cyanide Recovery

aNa form.

Table 10.

Acid Regenerant Composition

3.2 3,38014 6,944

1.8 1,0031.3 6770.12 670.01 1.70.01 00.01 0.6

2.6 1,5002,440

Zinc Cadmium CalciumpH

Constituent (ppm)

6.0 6002.4 13,0000.4 3,0000.3 2,6000.5 2902.2 3.33.0 0.23.1 0.06

The design of the ion exchangewastewater treatment system is es-

water composition at variouspoints in the treatment system andthe concentration of the purgestreams. Sample points are raw feed,filtered feed, filter backwash,regenerant purge, and treatedeffluent (see Figure 12b).

Total loadingLeakage (ppm)

(gal/ft3 resin) pHZinc Cadmium Calcium

75 10.1 0.01 0.01 1190 10.1 0.01 0.01 3260 8.3 0.01 0.01 53410 7.2 0.01 0.01 303520 6.0 0.16 0.01 338750 7.0 0.1 0.01 385

1,120 6.9 0.13 0.01 4041,200 6.8 0.25 0.D1 4051,230 6.8 0.37 0.D1 4071,300 6.7 0.56 0.D1 4041,420 6.8 0.64 0.01 3951,500 6.8 1.3 0.D1 3941,680 6.8 6 0.01 395

The entire wastewater flow wascollected in a single sump equippedwith a pH control.system to ensurethat cyanide wastes were notsubjected to acidic conditions. Theion exchange columns were strati­fied bed units containing strongbase, weak acid, and strong acidresins. Table 11 gives the waste-

1 ..•.•....•........." .2 , .3 .4 : .5 .6 •....•.............:....•.....•...........7 ' .8 .

Average .•.•.•..•......•..............•

Bed volume"

Bed volume sampled

Note.-Feed characteristics: 391 ppm Ca, 91 ppm Zn, 0.12 ppm ce.350 ppm Mg, 57 ppm Na, 3.5ppm Mn, 0.12 ppm Ni; ~H = 4.7; 8-bv/h (1-gal/min~ft3)flowrate.

SOURCE: Rohm and Haas Company, "Ion Exchange in Heavy Metals Removal and Recovery," AmberHilite No. 162, Philadelphia PA, Rohm and Haas Company, 1979.

Table 9.

Removal of Zinc and Cadmium from Wastewater by Weak Acid Cation Resin"

a3.6% HCI in bv 1 through 4; distilled water in bv 5 through 8.

SOURCE: Rohm and Haas Company, "Ion Exchange in Heavy Metals Removal and Recovery," AmberHilite No. 162, Philadelphia PA, Rohm and Haas Company, 1979.

10 ••••••••.•••••••••.•••..•••..•25 .••.•.•.•.••..•.•..'..•........35 .55 .70 ••••••.•.••.•......•.....•...•100 .' .150••.••••.•.•..•..•.•••........160••••••••.•.••.•....••.....•..165...•••...••..••...••....•..•.175 •••.•••..•.••.••• :,.•••....•..190...•••....••.•.•• : ..•.•.•..•.200..•••....•.•..•.. ; .•...••.•..225•••••••.••••••••• ; •.•.....••.

In one case, treatment was of aslightly acidic heavy metal waste­water containing a moderateconcentration of calcium, magne­sium, and sodium cations. Aweak acid cation resin in the sodiumform was evaluated for removingthe heavy metals. The resin was ableto remove both the zinc and cad­mium selectively while allow-ing most of the calcium ions to passthrough (Table 9). Initially, theresin exchanged its sodium ions forcalcium ions in solution; however,the resin then exchanged thesecalcium ions for heavy metals. After70 bv had been processed, theeffluent contained essentially thesame calcium concentration asthe column feed. The column wasregenerated With 3.6 percent HCIfol­lowed by conversion to .the sodiumform with NaOH. Table 10 showsthe composition of the acidregenerant solution. In this case,ion exchange treatment reducedthe waste volume associated withthe pollutants to less than 5 percentof the original volume.

In a second application, an ion ex­change waste treatment unitwas installed to treat the combinedwaste flow from a plating shopperforming copper, nickel, andassorted cyanide plating processes.

System Performance. Operating datafrom ion exchange wastewatertreatment systems is scarce becauseof the small number of facilitiesemploying the technology; however,the available performance data indi­cate the potential for applicationin metal finishing wastewatertreatment.

When ion exchange column size isdetermined, hydraulic and con­taminant loadings must be consid­ered. Resin manufacturers recom­mend volumetric loading ratesin the range of 1 to 2 gal/min/ft3(0.13 to 0.26 L/min/L) of resin.Unless the contaminant loadingresults in unmanageable regenera­tion frequency, the hydraulicloading should be selected fromthe high end of the range.

30

Weak acid ........................•..

Resin system

Chelating cation " ...........•..

31

To pretreat the wastewater before itpasses through the ion exchangecolumn, suspended solids removaland coarse pH adjustment areneeded; removal of organic com­pounds may also be required. (Foul­ing by organics is primarily aproblem with strong base anionresins.] Organics can be removedusing activated carbon or syntheticadsorbent materials. The syn­thetic materials have the advantageof being regenerable; spent carbonmust be disposed of and replaced,As a rule, filters that remove or­ganics are also effective for remov­ing suspended solids.

Exchange column size and the asso­ciated resin volume specificationsdepend on the wastewater flowrateand the contaminant loadings.Resin manufacturers usually recorn­mend loadings for resin systemsat about 2 gal/min/ft3(O.26 Llrnin/L)of resin. Flowrate indicates theflow of solution related to the aver­age period oftime it is in contac;twiththe entire resin bed. However,the active zone of an ion exchangesystem can be represented asan exchange front proceeding clownthe column (Figure 19). The depth ofthe active front is a function ofthe volumetric loading and thespeed of the ion exchange reactionkinetics. For ion exchange applica­tions where the columns are

,run to exhaustion, the benefits oflow loadings include greatercapacity per unit of resin and moreefficient use of regenerant chern­icals.ln wastewater treatment,however, the lack of direct measure­ment techniques to signal columnbreakthrough precludes load-ing the column to exhaustion, andhigher loading rates are recom-

is applied to treat the wastedirectly, the ion ~xchange unit isbasically the same. The majorequipment cost differences betweenthe two systems is in the auxiliaries.For a polishing application afterconventional treatment, the auxiliaryrequirements are !provided bythe upstream process, In a directtreatment application, however,these items add ~ignificantly to thetotal system cost Determinantsof the total system cost include:

• Feed pretreatment requirements• Volumetric and contaminant

loadings (resin! volume needed)• Regeneration rhode• Equipment needed to process or

store regeneration solutions andpurge streams

flaw feed Filtered Fflter I Regenerant Treatedfeed backwash ; purge effluent

I

Constituent

Equipment Cost

Stratified bed (strong base. weak acid,strong acid) ........•.•..••......•..

Total cyanide. . . . . . . . . . . . . . . 31.4 3Cadmium. . . . . . . . . . . . . . . . . . . 0.8 0.4 0.13 '0.295 0.0001Calcium. . . . . . . . . . . . . . . . . . . . 61.4 28.8 74.8 600 1.637Chromium. . . . . . . . . . . . . . . . . . 1.37 0.52 3.47 13.4 0.356Copper.. .. .. .. .. . .. . .. .. .. . 2.11 0.28 3.8 4.57 0.41Iron 14.2 3.3 65 I 11.5 0.195Nickel 3.14 2.92 31.1 I, 36.4 0.425Zinc....................... 42 23 95 1 251 2.62

SOURCE:C.Terrian, BestTechnology, Inc.•personal communication to P.rrampton. Aug. 10.1980.

1I

Table 12.. . 1.Comrnerciallv Demonstrated Hesin Systems for Wastewater Treatment

sentially uniform over the range ofpollutant removal capabilitiesit exhibits. Proper resin selection,however, is the key to effectiveand efficient pollutant removal.Testing to verify performance of aresin system is essential beforesystem selection. Table 12 presentsvarieties ofresin systems in commer­cial use for pollution control andthe pollutant removal capabilitiesof each.

Table 11. . I'Treatment of Metal Cyanide Wastewater by Ion EXChangr: Pollutant Analysis

Content (ppm) at sa\nple point

Whether ion exchange [s used topolish the effluent of an existingtreatment system or whether it

..-----.... Treated water. A+ ions

"2().mln retention. pH-controlled addition of NaOH. skid-mounted unit.

bOUll1 filters with backwash system and backwash storage. skid-mounted ~nit.

CAgltllted reaction tank. pH-controlled addition of H2S04 and NaOH. ORP;controlied addition ofNIlHSOa• manual operation.

Noto.-1980 dollars.

The columns would typically beloaded to 75 percent of their actualcapacity before regeneration. That isto say, there should usually be aband of unreacted resin left over atthe end of the column on-streamcycle. For both wastewatertreatment and polishing, highervolumetric loading rates. if theystill result in a manageable regenera­tion frequency. offer the advantageof reduced equipment size andcost, Loading rates as high as 20gal/min/ft3 (2.6 Llmin/L) 01' crosssectional area [equal to 5 gal/min/ft3(0.65 Llmin/L) of resin volume,assuming a bed 4 ft (1.2 rn) deep]have been used in some applications.High loading rates for polishingsystems are particularly advanta­geous considering that the contami­nant loading is usually low.

The costs 'for various column config­urations are shown in Figure 20for skid-mounted units that require'only piping and utility connectionsfor installation. The reqenerantsare metered into the units byeductors. Regeneration is manualfor the single- and dual-bod units.The three-bed parallel flow unit issized based on two columns in ser­vice while the third is beingregenerated; costs are with andwithout automated reqeneration.

The regeneration sequence is laborintensive and automation is costeffective except where regenerationis needed infrequently. Regen­eration of a column normally takes1 to 2 h. As a rule, columns inmulticolumn parallel flow arrange­ments are designed to operateat least 4 h before regeneration.

mended. Regeneration is basedeither on time or on cumulativevolume interval, As the interval willbe based on assumed wastewaterconcentration established byearlier testing, a safety factor mustbe used in determining the dura­tion of the cycle.

7.0008.500

10.75012.25013.500

25.00038.00045.00049.000

25.00032.00045.00051.000

Installed cost ($)

Regenerated zone

Exhausted zone

Ion exchangeactive zone

Legend:+ resin containing A+ ionsED resin containing B+ ions

ED EDE9 ED ED e ED ED'ED eED ED ED ED ED ED ED ED ED ED

ED ED ED ED ED ED ED ED ED EDEDEDEDEDEDeEDEDEDED

ED + ED + + ED + + ED ++ee+ee++ED+e++e++e++e+

+ ED + + e+ + ED + + +t + e + +'e + + e +

+ + + + + + + + + + ++ + + + + + + + + +

+ + + + + + + + + + ++ + + + + + + + + +

+ + + +,+ + + + + + ++ + + + + + + + + +

+ + + + + + + + + + +

Influent water. B+ ions: \111 •

Table 13.

Typical Costs for Ion Exchange Equipment Auxiliaries

pH adjustment tank. by Ilowrate in gal/min:"25 .50 .75 •••••••••••••••••••••.••••.•.•••....••....•...............100 .

Coep bed sand filters. by flowrate in gal/min: b

25 .50 •••••.••••••.•...•....•...................... '" .75 ••••••••••••••••••••••••••.••.•..••••...•....•............100•••••••••.•.••.•.•..•....................................

Batch treatment system. by volume in gal:c

250•••••••••.•••..•.•...•....•..............................500••••••••••••.••.•.........•..............................1.000 •••••.•.••.•....•• : .1.500 •••.••••••.•.••..••....................................2.000 .'

Auxiliary

Figure 19.

Ion Exchange Column in Service

32

r 'I I ~ I t

Assuming a three-column parallelflow unit is selected to treat the50-gal/min (190-Llmin) wastestream and that the columnsare operated on a 4-h cycle, thenecessary column size can be deter­mined. It is assumed that the resincapacity is actually 80 percentof the theoretical capacity. Thisadjustment is similar to applying afouling coefficient to a heat transfersurface and accounts for gradual

Determining. exchange capacityrequirements requires analysis of thewastewater feed and columneffluent chemical concentrations.Consider the weak acid resin systemwhose performance for removalof zinc and cadmium was describedin Table 9. Assuming that a concen­tration of 1 ppm' zinc in the efflu­ent signaled the end of the cycle, 175bv of solution could be treatedbefore regeneration. Table 14 givesthe composite feed and effluentconcentrations in milligramsper liter and equivalents per liter ofsolution. The change in the equiva­lents per liter represents the numberof resin exchange sites that wouldbe exhausted if 1 L or solutionwere passed through me exchangecolumn. The test indicated thateach liter of solution treated wouldexhaust 0.0145 eq of resin exchangecapacity. Breakthrough occurredafter 175 bv had been treated,indicating that the resin had a totalcapacity of 2.5 eq/L, which is thesame as the resin manufacturer'sdata indicated.

33

hand, achieves much greaterchemical efficiency per unit ofregenerant at lower regenerantdoses. Consequently, weak acidresin systems can be designed touse the total resin exchangecapacity; this capability reduceseither required resin volume orregeneration frequency. Strongacid systems will realize greaterefficiency if they are designed touse approximately 40 to 60 percentof the total resin exchange capacity.

15

Ishown in Figure 21 for typical strongand weak acid cation resins overa range of acid r~generant doses.The weak acid resin requires signifi­cantly less reqenerant per unitof exchange capacity,

Figure 21 also s~ows that the capa­city of weak acid, resin increases 'almost linearly "",ith the amountof regenerant. Th~t is to say, increas­ing the reqenerant dose 50percent increases the exchangecapacity by an almost equal ratio.The strong acid resin, on the other

I

Legend:_ _ 3-bed parallel flow, automated regenerationIIII!III lIIIlii 3·bed parallel flow, mandai regeneration_ 2-column parallel flow, manual regeneration_ I-column, manual regen~ration

!

3

5

25

. 6 9 : 12RESIN VOLUME PER COLUMN (ft3) i

I

·1980 dollars. Add $200/ft3/column for chelating resin. iNote.-Skid-mounted unit with weak acid cation resin, acid and base regenerl!nt,storage, and all internal piping and valves. i

I

20

5-&EI- 15(fJ

0UI-Z;:)

10

30

-

Figure 20.

Ion Exchange Unit Costs

Operating Cost

The chemical cost of operating anion exchange system relatesdirectly to the quantity of toxiccontaminants removed from thewastewater by the resin bed. Thechemical efficiency of the ionexchange reaction is a function ofthe resin selected and of the per­centage of the resin's exchangecapacity used. This relationship is

Feed ProductConstituent Change (eq/L)

giL eq/L giL eq/L

Calcium ••••••••••••• '" •••••••• 0.39 0.0195 0.3 0.015 0.0045Mognoslum ••••••••••••••••••••• 0.35 0.0292 0.27 0.0225 0.0067Zinc ••••••••••••••••••••••••••• 0.09 0.0028 (al ,(0) 0.0028Sodium••••••••••••••••••••••••• 0.06 0.0026 0.32 0.0139 -0.0113Mongllnese ••••••••••••••••••••• 0.03 0.001 0.02 0.p006 0.0004Cadmium ••••••••••••••••••••••• 0.001 (0) (al ,(OJ 0.0001

Table 14.

Two safety factors, then, havebeen used in sizing the ion exchangesystem; one to compensate for agradual deterioration of resinexchange capacity and one to com­pensate for lack of direct meansof determining column breakthrough.

Table 15 shows regenerationchemical consumption and cost,the purge streams from the unit, andthe waste concentration factor.The purge stream containing the pol­lutants is approximately 7 percentof the original volume of wastewater.Consumption of HCI and NaOH forthe system was assumed at 120percent of the stoichiometric reagentrequirement, based on the theo­retical resin exchange capacity of2.5 eq/L. Sodium hydroxide needsare only slightly above stoichiomet­ric amounts, despite the resin'spreference for being in the hydrogenform, because the product of thecaustic regeneration reaction is notionized. The caustic regenerationreaction is:

deterioration in resin performance.The adjustment yields a resincapacity of 2 eq/L

The resin volume requirementcalculation per column is shown inTable 15. The unit is designedto have two of the three columns onstream at any time. Assumingthat the column is run to exhaustion,5.8 ft3 (164 L) of resin wouldbe needed per column.

No direct indication of columnbreakthrough is available forend-of-pipe process applications.To prevent discharging high concen­trations of regulated pollutants, thecolumns can only be operated tosome fraction oftheir actua I capacltvr75 percent is a reasonable safetyfactor. Required resin volumewould then increase to 7.8 ft3 (220 L)per column.

8

Legend:_ weak acid cation resin.... strong acid cation resin

2 4 6

REGENERANT LEVEL(Ib HCl/ft3)

I I

oo

2.5

0.5

2.0

Figure 21.

Exchange Capacity versus Acid Regenerant Load for Cation Resins

Resin Capacity Based on Test Results

"Negligible.

Note.-Exchange requirements: per liter of feed, 0.0145 eq/L; per 175 bv offeed, 2.53 eq/Lofresin. R-H + NaOH -+ R-Na + H20 (9)

Once the resin's hydrogen ionis exchanged, it combines with a

34

..

=

II,I

iI.

Table 15. IColumn Size Determination for Three-Column Parallel Flow Unit

. i

Evaluating resin capacity by runninga test column to exhaustion (Figure17), is time consuming, particu­larly for a polishing application

Safety features similar to thoseused in the direct treatment analysiswill be applied to the polishingsystem. The theoretical capacity willbe reduced to 80 percent of thecapacity indicated in the testdata to compensate for fouling,and the column will be exhaustedto only 75 percent of its actualcapacity to avoid breakthroughbefore regeneration. These featureswill yield a volume-processingcapability of 960 bv of wastewaterbefore regeneration.

Regeneration frequency is a functionof column size. Table 17 givesregeneration frequencies, costsper regeneration cycle, and annualcosts for units in three sizes, eachoperating 4,000 h/yr. Operatlnqtime for each regeneration cycle wasassumed at 1 h. Operating costsare approximately the same forall three units, and would thereforefavor the smaller unit, which requiresthe least capital outlay. The chern­ical cost for a polishing systemis significantly lower than thatfor the direct treatment system (Table16) because most of the metalsare removed during conventionaltreatment.

resin exhaustion; for the polishingsystem (Figure 17) breakthroughdoes not occur until 1,600 bv ofwastewater have been treated.

35

The longer column cycle associatedwith polishing often eliminatesthe justification for automated

, regeneration. If regenerationis manual, a two-column unit, oper­ated in either parallel or seriesflow with each column sized toprocess the total flow, wouldprobably be most effective in termsof regeneration frequency andreliability. For automated units, athree-column parallel flow unit,designed to have one column offstream for regeneration, would prob­ably be most effective.

2.300230

2.530

Cost

23.000

36.110

2.0001,400

30.180

33.580

. \25 gal/min50 gal/min4h0.0145 eq I

4 X 60 X 25 X r.79 X 0.0145 = 330 eq

[330/(2 ec::L)] X~~/(3.79)(7.48)] = 5.8 ft35.8/0.75 - 7.8

1

.

45 Ib '50 Ib390 gal$15.09 I

(6.000 gal wastewater per cycle)/{400 galpurge per cycle) = 15

I

,I Factor

hydroxide ion to form a non-ionizedwater molecule and no longercompetes f~r exchange sites.

The installed cost of a three-columnparallel flow ion exchange systemwith 7.8 ft3 (220 L) of resinper column, skid-mounted withautomated regeneration, is $23,000(Figure 20). Table 16 shows thetotal annual cost for a systemoperating 4,000 h/yr. Capital andoperating costs of wastewater pre-

Item

Investment ($) '" " 1 .

oper~:i~~r.c~s~):~i~:at $8/h 1. .Maintenance. 6% of investment 1 .

Regenerant chemicals. 4.000 h at 2 h/cycle ..................• i" ••••••••••

Total operating cost ~ .

R~oonW~: I~:~;:c~~~~~~~;a~~~' : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :l::::::::::

Tota I fixed cost \ .

I

Total annual cost ................•....................... \ .

Note.-1980 dollars. Operation 4.000 h/yr. Does not include water pretteatrnent or batch treat-ment system. I

Itreatment and batch treatmentare not included. IA similar analysis! can be performedfor a polishing system using per­formance data frdm Figure 17and a flowrate of \50 gal/min (190L/min). The majo~ differenceis in the large volume of solutionthat can be treated per unit volumeof resin. In the direct treatmentcase, 175 bv could be treated before

III

Annual Cost of Ion Exchange Treatment System

"1980 dollars.

Flowrate:Per column .Total .

Column cycle : .Exchange capacity per liter of feed .Capacity needed per column .Resin volume needed:

Per column .With safety factor .

Regenerant consumption per column per cycle:HCI (based on 100%) .NaOH (based on 100%) .Wash water ; ',' .Cost per cycle" .

Waste concentration factor .

Item

Table '16.

Table 17.

Annual Cost for Ian Exchange Polishing Systems

Item

Column resin volume (ft3)•••••••••••••••••••••••••••

Ion exchenge unit cost IS)•••••••..••..••..•••..•...

Opllrating costs ($/yr):Labor. at $8/hb •••••••••••••••••••••••••••••••

Maintenance. at 6% of unit cost .Regeneration chemicals? .•...•.................

Total operating cost...•.••.....•............

Fixed COlts ($/yr):Depreciation ••••••••..•......................Texas lind Insurance .•...•....... " .

Total fixed cost.•.•.•••....•................

Total annual cost •...•.•....................

"For cheillting resin column. 50-gal/min flowrate.

b1 h labor per regeneration.

cBased on 120% 1heoretical rasin capacity = 1 eq/L.

Noto.-1980 dollars. Systems operating 4,000 h/yr.

36

Regeneration frequency

16 h 24 h 36 h

6.7 10.0 15.015,700 19,000 24,000

2,000 .1,330 890780 900 1,140

1,660 ; 1,660 1,660

4,440 ·3,890 3,690

1.570 1.900 2,400160 190 240

1,730 2,090 2,640

6,170 5,980 6,330

where the resin can process alarge volume of solution beforeexhaustion. It is more expedient topass only sufficient volume throughthe column until the column effluentreaches equilibrium. then analyzethe feed and product for ionicconstituents. The exchange per unitof feed solution will thus be deter­mined and. when compared tothe resin's theoretical exchangecapacity (from manufacturer'sliterature). can be used to predictthe solution volume the..resincan process before exhaustion. Thesafety factors described earliershould be used with this approach.

The foregoing. then. are some of thealternatives and process variablesto be considered in evaluatingion exchange systems. Actual test­ing. decisions regarding systemspecification. and type of resinshould be left to experts 'in lise of thetechnology. An awareness of theflexibility and power of the ionexchange process for wastetreatment apptications, however.can aid the metal finisher in obtainingthe most effective system for theleast total cost.

Chemical Recoveryand Recycle Systems

Pollution controlIlegislation hasaffected industry! by increasingthe economic penalty associatedwith inefficient use of raw materials.In the plating industry, for example,loss of raw material in the waste­water can result ih costs in three dis­tinct areas:

• Replacement 6f the material• Removal of the material from the

wastewater before discharge• Disposal of the solid waste

residue iIn response to thb increased costof raw materials, lplating shopsare modifying th~ir processesto reduce their losses. Recent yearsalso have seen tHe cost-effectiveapplication of various separa-tion processes that reclaim platingchemicals from rinse waters,permitting reuse 9f both the rawmaterial and the fateI'.

I

Ion exchange, evaporation, reverseosmosis, and electrodialysis have allbeen used in the !plating industryto recover chemi~als from rinsesolutions. These processes have incommon the ability to separatespecific compounds from a watersolution, yielding :a concentrateof those compounds and relativelypure water. The concentrate isrecycled to the 'pl~ting bath and thepurified water is reused for rinsing.To determine whi~h separationprocess is best suited for a particularchemical recovery: application, itis usually necessary to evaluate bothgeneral and site-specific factors,for example: !

i• General factors-would include

rinse water concentratlon,volume, and corrosivltv,

• Among site-speciftc factors arefloor space available, utilities(such as steam,' chemicalreagents, electricity) available,and degree of concentratlonneeded to recycle the chemicalsto the bath.

As a rule, ion exchange systems aresuitable for chemical recoverywhen the rinse water feed has arelatively dilute concentration of

plating chemicals and the degree of, concentration needed for recyc:le

is not great. Ion exchange iswell suited for processing corrosivesolutions. The process has beendemonstrated commerciallyfor chemical recovery from acidcopper, acid zinc, nickel, cobalt.rtln,and chromium plating baths. Ithas also been used to recover spentacid solutions and for purifyingplating solutions to prolong theirservice life.

Economic Analysis of RecoverySystems

To evaluate the economic benefit ofinstalling ion exchange or otherrecovery processes, the followingdetermination must be made:

• Quantity and replacementcost of the chemicals and water tobe recovered

• Savings in wastewater treatmentcost expected to result fromrecovery unit installation

• Reduction in solidwaste and costof sludge disposal expected toresult from recovery unit installa­tion

In evaluating a plating cnernlcaldrag-out recovery system, the rinsewater volume and chemical con­centration must first be measured.This step will establish the quantityof chemicals available for recovery.When the relationships of waste­water volume and metal content tothe associated wastewater treatmentand sludge disposal cost havebeen determined, the potentialsavings can be determined. Table 18 ,shows the economic penalties forlosses of typical plating chemicals.

The high investment cost forinstalling an automated recoveryprocess limits application ofthis process to plating operationswith high drag-out rates, as illus­trated for chromic acid recovery inFigure 22. The analysis assumed an

37.

Table 18.

Economic Penalty for Losses of Plating Chemicals

Cost ($/Ib)Chemical

Replacement Treatment" Disposal'' Total

Nickel:As NiS0 4 •••••• • ••••••••••••••••• 0.84 0.31 0.38 1.53As NiCI2•••••••.•••....••........ 1.14 0.34 0.52 2.00

Zinc cyanide, as Zn(CNh:Using CI2 for cyanide oxidation ...... 1.55 0.80 0.50 2.85Using NaOCIfor cyanide oxidation ... 1.55 1.68 0.50 3.73

Chromic acid. as H2Cr04:Using 502 for chromium reduction... 0.98 0.53 0.64 2.15Using NaH503 for chromium reduc-

tlon •••.•••••.•....•........... 0.98 0.76 0.64 2.38Copper cyanide, as CU(CN)2:

Using Cl2 for cyanide oxidation...... 2.05 0.80 0.50 3.35Using NaOCIfor cyanide oxidation ... 2.05 1.68 0.50 4.23

Copper sulfate, as Cu504••••••••••••••• 0.62 0.31 0.34 1.27

-At concentration of 100 mg/L in wastewater.

b4% solids by weight at SO.20/gal.

Note.-1980 dollars.

50

40

~Ul

~a:wIi: 30«!zw

ffi> 20~z0za::;)

~a:10

O .....__-.l--~---'--_--I._---:-l-----'

oDRAG-OUT RATE (Ib Cr03/h)

Note.-Operating 4,000 h/yr. $30.000 investment cost. Tax rate at 48% of profit.

Figure 22.

Return on Investment in Chromic Acid Recovery Unit

38

investment cost for the recoverysystem of $30,000, with theunit depreciated over 10 years.Typical operating, labor, and mainte­nance costs ·for an ion exchangesystem were used to determineoperating costs. Chemical savingswere derived from Table 18,which indicated a total saving of$2.15/lb of H2Cr04 recovered (equalto $2.50/lb of Cr03): From theforegoing, a reasonable rate of returnis achieved for a CrO~ drag-out rateabove 3 Ib/h (1.4 kg/h), for whichpayback equals 2.8 years. Platingoperations with rates signi'Ficantlylower than 3 Ib/h (1.4 kg/h)would not be economically justifiedin installing this recovery system.

Tax credits associated with invest­ments in pollution control hard­ware were not included in theforegoing analysis. The credits wouldimprove the economy of otherwisemarginal investments, but notenough to justify installing an auto­mated recovery system in anoperation with low drag-out rates.

Drag-Out Recoveryby Ion Exchange

The Reciprocating Flow Ion Ex­changer (RFIE) is the kmd of ion ex­change system most widely usedfor chemical recovery from platingrinses. This proprietary unit wasespecially developed for purifying thebleed stream of a large volumesolution such as the overflow from aplating rinse tank. It operateson the principle that, for the shortperiod of time the unit goes offstream for regeneration, the buildupof contaminants in the rinse sys­tem is negligible.

The RFIE units are more attractivethan fixed bed systems for plat-ing chemical recovery bec:ause thecolumns use smaller resin vol­umes and, therefore, capital costsand space requirements are usuallylower. The units incorporateregenerant chemical reuse tech­niques to reduce operating costs and

Spent rinsewater

ed

n

I Reagent measuring PACKAGE

- I tanks ! UNIT COMPONENTS

~-----~-------------~~~:r~Note.-Automated control included with package unit. _ Water

- Air

I Purifi

Irinse

H2SO4 t--- NaOHI

!I

- Ir-----+----~

! I

t i !

I EJ rIAnio

H2Cr04 I i !storage

I tiI

~ ;,

I

(a)

(b)t---- ON STREAM (LOADING) ----i

IWASHING -fREGENERATION

Rinse iwater

IExhaust

H2SO4(to waste Water

!treatment)

Cation

H2SO4 Water

Exhaust(to waste-treatment N,tOH ,NaOH

Figure 23. - IChromic Acid Recovery RFIE System: (a) Hardware Components and (b) Operating Cycle

I

yield higher product concentrationfor recycle. They are sold as skid­mounted package units, whichare automated to minimize operatinglabor requirements. Two basicunits are available for drag-out re­covery: one for chromic acid recoveryand one for metal salt recovery.Another unit is designed to deionizemixed-metal rinse solutions to

recover only the.water and concen­trate the metals before treatment.

IChromic Acid. FIgure 23 showsthe hardware components of an RFIEchromic acid recoverv systemand necessary a~xiliaries and de­scribes the operating cycle. Thesegregated rinse water after a

I

1

chromium plating bath (or baths) ispumped to the ion 'exchangeunit and passes in series through acartridge filter, a strong acidcation resin bed, and a strong baseanion bed. The demineralized wateris returned to -the rinse system.The RFIE unit regenerates itself auto­matically based either on a cycletimer or on the conductivity ofthe treated water. With the, conduc-

39

Table 19.

Three-c~lumn parallel-flow ion exchange package unit ready for installation

tivity controller. the conductivity ofthe treated water is comparedwith that of the unit feed. When theunit is no longer achieving sufficientconductivity reduction. regenera­tion is initiated. Regenerationfrequency is based on the quantity ofchromic acid in the rinse and theunit's resin volume. The unit isoff stream for regeneration for ap­proximately 20 min. The chromateions removed from the rinse areconcentrated in the anion resin bed.They are eluted in the form of asodium chromate solution when thisbed is regenerated with sodiumhydroxide. The sodium chromate so­lution is passed through a secondstrong acid cation resin bed toconvert the sodium chromate tochromic acid. The recovered chromicacid solution is stored and used forchemical makeup in the chromiumplating bath. The product concentra­tion is approximately 10 percentchromic acid. After the resin bedsare washed with water. the unit goesback on stream.

The RFIE units come in severalsizes; higher chromic acid loadingrates require larger resin bedvolume. Ideally. the unit performstwo cycles per hour. Each cyclereclaims a certain amount of chromicacid and consumes a set amountof regenerant chemicals. Table 19shows the chemical savings.reagent cost. and amount of chro­mium recovered per cycle.

Figure 24a presents the purchasecost of RFIE units for chromicacid recovery as a function of theamount of chromic acid the unit canrecover. Including reagent andproduct storage. piping and utilityconnections. startup. and shippingexpenses. the total installed costfora system should be approximately120 percent of the unit cost.

Metal Salts. RFIE units are recover­ing plating drag-out from nickel.copper. zinc. tin. and cobalt platingrinses. The major area of applicationis for nickel plating baths. Twobasic units are used for metal

40

Performance of R~IE Chromic Acid Recovery Unit"

Item

Regenerant solutions:NaOH ........•.......•....•••.•••••..••••.•......H2S04 •••••••• , •.••••••••••••••••••••••••••••••••

Water ...•...•.......•..••.......•••....•...•.•..Spent rinse....................••.•..•••..•.••.•..•.••Purified rinse .....•.......•.•.........•••••••••.••••••Product, CrDa••...•....•....•.....•..................•Purge to waste treatment ...•.•.....•...•..•.....••••..•

Chemical savings ($):b .CrDa, 2 Ib at $2.50/lb•..•....•...•.•....••..••.....NaOH at $0.15/lb .'••.......•......................H2S04 at $0.05/lb., .••....•.•...•••••.••.•.•.•.....

Total saving per cycle .••••••••..•........•.•.....

"0.35 fta anion resin.

b1980 dollars.

Value (per cycle)

3.71b12.21b80 gal1,200 gal/cycle; 200 ppm CrOa1,200 gal/cycle2·lb each at 10% c-o,80 gal

5.00-0.56-0.61========:=

3.83

41

recovery. One employs a cation bedto reclaim the metal ions and ananion bed to remove the counter­ions; the deionized water is recycledto the rinse station. For applicationswhere only recovery of the metalis desired, the anion bed is elim­inated and the metal-free water isdischarged.

The purchase cost for an RFIE metalsalt recovery unit is -presented-inFigure 24b as a function of theamount of metal salts the unitcanrecover. The price is for a unit-with both a cation and an anion !bed;the price is approximately one­third less for a unit with a single cat­ion bed. Including the basic RREunit, reagent and product storage,piping and utility conn-ections, start­up, and shipping, the total installed

Metal salt recovery units also comein various sizes, with unit sizedetermined by the amount of metalsalts in the rinse water. Eachcycle will reclaim a set amount ofmetal salts and consume a setamount of regenerant chemical.Table 20 shows chemical savings,reagent consumption, and theamount of metal recovered per cyclefor nickel plating recovery.

Figure 25 presents RFIE systemhardware and the necessary auxil­iaries for metal salt and rinsewater recovery and describes theoperating cycle. The segregated

. rinse water after the plating bath (orbaths) is pumped to the ion ex­change unit and passed, in series,through a prefilter, a strong acidcation resin bed, and a strong

. base anion bed. The demineralizedwater is returned to the rinsesystem. The metal ions concentrateon the cation resin and are elutedwith either sulfuric or hydrochloricacid. The concentrated salt solution(either the metal sulfate or chlo­ride) is stored and used for chemicalmakeup in the plating bath. Theregenerantfrom the anion bed is sentto waste treatment.

10

50

8

. aBased on 2 cycles per hour.

bDual-bed (cation and anion).

cBased on 7.~ cycles per hour.I,1

I

4 6

DRAG-OUT RATE (Ib erOs/h)

2

Minimumunit size

10 20 30 40I

DRAG-OUT RATE (Ib NiS04 • 6H20/h) I

o0 ....---...L..----4 "'-__.......... .......l

40

40

30

20

10

~ 30

El-en0u!:::z 20:::l

Minimumunit size

10-,

(a) 50

(b) 50

~reU iEquipment Cost of RFIE Units: (a) Chromic Acid Recovetv and(b) Metal Salt Recovery i

I

----,

Spent rinsewater

ed

n

Purifirinse

H2SO4 - NaOH

r-----~----I 'i

+ II II Anio

NiS04 ! •storage

tI!II Reagent measuring

I tanks PACKAGE UNIT COMPONENTS

L--------------------~--T~Note.-Automated control included with package unit. Utilities -

o Watero Air

(a)

Water

WasteWater

t----- WASHING ----I

Caustic

......-- REGENERATION----4Recoverednickelelectrolyte WastePurified water

(b)t---- ON STREAM (LOADING)----i

Acid Acid

Figure 25.

Metal Salt Recovery RFIE System: (a) Hardware Components and (b) Operating Cycle

cost for a recovery system shouldbe 120 percent of the unit cost.

Acid Recovery Systems

Ion exchange is used to purifyconcentrated acids (such as sulfuric,hydrochloric, and nitric) that have

been contaminated by metal salts.The process, called acid retardation,brings an acid solution in contactwith a strong base anion resin. Theresin will sorb the strong acidbut not the metal salts. The acid can

be desorbed with water. Thistechnique has been commercializedusing reciprocating flow methodssimilar to those described forchemical recovery.

The two process steps areshown in Figure 26: In the on­stream step (upstroke), the metal-

42

!(a) I

Compressed ~etal salt solution

.Water air to waste

I li

II

Spent IWater

Imetering acid Resin bedtank metering

!tank

iI

II;

!

(b)

Compressed air Spent acid

l i, II

SpentI

Water,

acid Imeteringmetering I

Resin bedtank

tanki

II ,II

P~rified acid (p~oduct)iI,

Item

Table 20.

Performance of RFIE Metal Salt Recovery Unit"

Investment in an acid purificationsystem is justified by the savings inpurchases of replacement acidand of neutralizing reagents for treat­ing the spent acid. The amountsaved depends on the type of acidto be recovered, the volume andconcentration of the spent acid dis­carded yearly, and the cost oftreating the spent acid.

salt-contaminated acid is meteredinto the bottom of the resin bed.The free acid is sorbed by the resinand the metal salt byproductsolution flows out the top of the bed.In the regeneration step (down­stroke), water elutes the acidfrom the resin, yielding an acid con­centration equal to that of thefeed solution and a lower concentra­tion of metal contaminants.

Two applications are seen for thissystem:

• Purification of strongly acidicprocess baths

• Recovery of excess acid from cat­ion exchange regenerant solutions

Demonstrated uses of ion exchangeacid purification include removingaluminum salts from sulfuricacid anodizing solutions, removingmetals from nitric acid rack-strippingsolutions, and removing metalsfrom sulfuric and hydrochloric acidpickling solutions. The majorarea of application is for aluminumanodizing solutions.

Acid purification systems areavailable in a range of sizes. Size isa function of the volume of acidthat can be purified per unit of time;size requirement is determinedby the rate at which metal saltaccumulates in the acid bath.Table 21 shows the feed, product,and waste stream concentration of apurification system for sulfuric acid

Value (per cycle)

1.38

1.53-0.09-0.06

0.631b1.21b58 gal .250 gal/cycle; 600 ppm NiS04 • 6H20,

150 ppm t-.\iCI • 6H 20250 gal/cycle'1.7..lb each a~ 17% NiS04 • 6H 2058 gal I

Total savings per cycle ..............•....•.

Chemical savings ($):bAnhydrous NiS04, 1 Ib at $1.53/lb •••.•........NaOH at $0.15/lb ., •......••.•....••..•.....H2S04 at $0.05/lb ...•..•...............•....

Purified rinse .......•.............•..•..••.•..••Product, NiS04 • 6H 20 ......•....•..••.....•....•Purge to waste treatment ...••.......•..••.••..•..

Regenerant solutions:NaOH••......•••......•.......•......•.....H2S04 ••••••••.••.•••••.•••••••.•••••••••••

Water ......•............•.................Spent rinse ...............................•....•

·0.35 ft3 cation resin.

b1980 dollars.

Figure 26.i

Acid Recovery System Operation: (a) Upstroke and (b) DownstrokeI

43

-

Table 21. Table 22.

Performance of Acid Recovery Unit Costa and Iron Removal Capacity of Sulfuric Acid Purification Unitfor Purifying Sulfuric Acid PicklingSolution Unit size (ft3 anion resin)

Item

"1980 dollars.

bSkid-mounted package unit. including filter and automated control systems.

cBased on Table 21.

dFor neutralization.

Item

Water ••••••••••••.•••.•••Food, at 11 gal/h:

H2S0 4••••••••••••••••Iron•••••••••.•...•...

Product, at 10.4 gal/h:H2S04••••••••••••••••Iron ..

Purge, at 7.6 ga1/11:H2S0 4" , •••••••••••••Iron •••••••••..•.•...•

Iron removed •.•.•.••.••.•.Acid recovered••.•..•......

Performance

7.0 gallh

0.94lb/gal1.15 Iblgal

0.94lb/gal0.74lb/gal

0.07lb/gal0.65lb/gal4.9lb/h94%

Unit cost ($)b ',' .Acid feed rate (gal/h) .. . . . • . . . • . . . . . . . . . . . . . . . . .Iron removal rate (Ib/h)c .Savings ($/h):c

H2S0 4 , at $0.05/Ib . . . . . . . . . • . • . . . . . . . . . . • . .NaOH. at $0.08/Ibd .

0.40

10.000114.9

0.310.41

2.79

20.0008035.8

2.263.02

14.12

56.000400179

11.3215.09

pickling solution. Based on thequantity of iron in the purge stream,the iron salt removal capacitycan be determined from vol-ume processing capacity. Oncethe rate of iron accumulation in theacid solution has been determined, apurification unit with equal saltremoval capacity can be selectedto control the buildup.

44

Acid purification svsterns areinexpensive and simple to install.Air supply and water are the onlyutilities needed. Piping requires onlyfeed, product, and waste streamconnections. Table 22 givesapproximate costs for units of differ­ent sizes, the volume of sulfuricpickling acid they can process, andthe value of the recovered acid.

In another, well-establishedapplication of ion exchanqe, metalbuildup iri dilute acid solutionsis controlled by passing the solutionthrough a cation exchanqer in thehydrogen form. This approachhas been used for hydrochloric andsulfuric acid etching solutionsand to remove trivalent chromiumand ferrous ions from chromicacid solutions.

Bibliography Abrams, I. M. "Seilective Removal ofHeavy Metals fro~ Wastewatersby Ion Exchange and Absorb-ent Resins." Paper read at SouthCentral Regional ~eetingof NationalAssociation of Corrosion Engineers,1974. I

1

Anderson, R. E. "Some Examplesof the Concentration of TraceHeavy Metals with Ion ExchangeResins." In Proceedinqs: TraceHeavy Metals in yvater, RemovalProcess and Moni~oring.Nov. 15-16,1973. Princeton NJ, Princeton Uni­versity Press, 19~3.

Brown, C. J., D. D~vy, and P. J. Sim­mons. "Nickel Salt Recovery byReciprocating Flow Ion Exchange:'Paper read at 62~d Annual Tech­nical Conference of AmericanElectroplaters' Sotiety, 1975.

Brown, C. J., D. Davy, and P. J. Sim­mons. "Purlflcation of SulfuricAcid Anodizing Solutions." Platingand Surface Finis/:Jing, 66(1 ):54-57,Jan. 1979.

Brown, C. J., and :C. R. McCormick."Pollution Abatement via Re­source Recovery for a Plastics PlatingShop." Paper read at AmericanSociety of Electroplated Plastics12th Annual Meetinq, 1979.

Calmon, C., and M. Gold. IonExchange for Potllition Control.Vol. 1. West Palm Beach FL, CRCPress, 1979. I

Crampton, P. "Application ofSeparation Processes in the MetalFinishing lndustrvl" Paper readatThird Annual EPA/AES Conferenceon Advanced Pollution Control inthe Metal Finishirlg Industry, 1980.

iDorfner, K. Ion Exchangers; Proper-ties and Apptlcetions. Ann ArborMI, Ann Arbor Science, 1977.

Dow Chemical Company. "AnionResins: Selection iCriteria for WaterTreatment Appllcations." IdeaExchange, 5(2), u?dated.

Dow Chemical Company. "ChemicalProcessing by Ion Exchange."Midland MI, Dow ChemicalCompany, undated. '

Dow Chemical Company. "DowerWGR-2 Weakly Basic AnionExchange Resin." T.D. Index 3310.1.Midland MI. Dow Chemical Com­pany, undated.

Dow Chemical Company. "WeakAcid Cation Resins:' Idea Exchange,2(3), undated.

Kunin, R., and R. J. Myers. IonExchange Resins. John Wiley andSons, 1950.

Kunin, R. "Ion Exchange for theMetal Products Finishers:' ProductsFinishing, Apr.-May-June 1969.(3 pt. article)

Rohm and Haas Company. "HelpfulHints in Ion Exchange Tech­nology:' Philadelphia PA, Rohmand Haas Company, May 1972.

Rohm and Haas Company. "Am­berlite" 200." Philadelphia PA,Rohm and Haas Company,Nov. 1976.

Rohm and Haas Company. "IonExchange in Heavy Metals Removaland Recovery:' Amber Hilite No.162. Philadelphia PA, Rohmand Haas Company, 1979.

Rohm and Haas Company. "POrt:lUSPolymers and Absorbents-AReview of Current Practices:' AmberHilite No. 163. Philadelphia PA,Rohm and Haas Company, 1980.

Schweitzer, P. A. Handbook ofSeparation Techniques for ChemicalEngineers. New York NY, McGraw~Hill, 1979.

45

U.S. Environmental ProtectionAgency. Environmental PollutionControlAlternatives: Economics ofWastewater Treatment Alternativesfor the Electroplating Industry.EPA 625/5-79-016, 1979.

46

,

Wing, R. E. "Processes for HeavyMetal Removal from Plating Waste­water." Paper read at First AnnualEPA/AES Conference on AdvancedPollution Control for the MetalFinishing lndustrv, 1978.

Yeats, A. R. "Ion Exchange Selec­tively Removes Heavy Metalsfrom Mixed Plating Wast~ls." Paperread at 32nd Purdue IndustrialWaste Conference, 1977.

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