United StatesEnvironmental ProtectionAgency

Industrial Environmental ResearchLaboratoryCincinnati OH 45268

&EPATechnology Transfer I

Surnmarv Report

Control and TreatmentTechnology for theMetal Finirhing Industry

In-Plant Changes

WM

Technology Transfer II

Summary Report

Control and TreatmentTechnology for theMetal FinisHing Industry

In- Plant Changes

January 1982

This report was developed by theIndustrial Environmental Rese, rch LaboratoryCincinnati OH 45268

EPA 625/8-82-008

ii

This summary report was prepared for the Industrial Environmental ResearchLaboratory's Nonferrous Metals and Minerals Branch in Cincinnati OH.The Centec Corporation, Reston VA, prepared the report. The EPA ProjectOfficer is George Thompson.

The contact for further information is:

Nonferrous Metals and Minerals BranchIndustrial Environmental Research LaboratoryU.S. Environmental Protection AgencyCincinnati OH 45268

This report has been reviewed by the Industrial Environmental ResearchLaboratory, U.S. Environmental Protection Agency, Cincinnati OH, andapproved for publication. Approval does not signify that the contentsnecessarily reflect the views and policies of the U,S. EnvironmentalProtection Agency, nor does mention of trade names or commercial productsconstitute endorsement or recommendation for use.

COVER PHOTOGRAPH: Automatic rack machine for nickel-chromiumplating process

Overview . The metal finishjn g industry in theUnited S~ates is subject to a varietyof changing business conditions.Two of .the mo.s~ significant factorsare the Increasln? costs of materials,such as plating chernicals andprocess water, arld the environmentalconsiderations, Jvhich includethe need to control the dischargeof effluent wastJ streams andthe disp~sal of ~azardous wastes.The survival of lT)any metal finishingcompanies will depend on howeffectively they ~eal with the impactof these changes and requirements.

The basic Platind operation involvesimmersing parts lin a processs~lut~on a.nd ther rinsing off theclinging film of plating chemicals,which is known ·~s drag-out. Ifperformed ineffidientlv, this opera­tion wastes sevJral pounds(kilograms) per d~y of expensiveplating chemicaJ~ and createsthousands of gajons (liters) per dayof contaminated rinse water.Inefficient opera ion, therefore,significantly affebts the interrelatedfactors of materi~1 costs andpollution control

By January 28, 1984, electroplating[ob.shops that d scharge topublicly owned t eatment worksmust reduce con amination in therinse water and bther processwastewaters to fkderally regulatedlevels. The dispdsal of treatmentresiduals is gove[ned by thehazardous waste regulationspromu.lgated in t e Resource Con­servation and Re overy.Act (RCRA).Details of the 'w~stewaterandsolid wast~ requlations for the~Iectropla.tlng Inlustry are providedIn an earlier U.S. EnvironmentalProtection Agen y (EPA) report.'

Becau~e of risinJ prices andchanging requlations, it is necessaryto reevaluate water pollutioncontrol techniques and costs andto examine methbds for improvingraw material yiel~s. In manycases, changing ~he manufacturingprocess can sign ficantly alterchemical losses a d waterflow rates.These in-plant c anges usually

involve techniques for reducing boththe drag-out removed from processsolutions and the amount ofwater used in the rinsing process.The overall effect is a reduction of:

• Chemical purchases• Water use (resulting in lower

water and sewer costs)• Wastewater treatment needs and

disposal costs

Although Federal law does notrequire compliance with electro­plating pretreatment standards untilJanuary 28, 1984, in-plantchanges should be institutedimmediately. In addition to pro­viding chemical savings andreducing water use costs, in-plantchanges provide a basis for apollution control system design.Waste treatment equipment needs­whether wastewater concentratingtechniques, such as ion exchange,or conventional end-of-pipetreatment systems-often will bereduced significantly by in-plantchanges. In some cases, electro­platers will be able to reduceflows to less than 10,000 galld(38,000 lid), thereby reducing theirpollution control requirementsas prescribed in the EPA pretreat­ment standards." This reportdescribes the first steps a platershould take to comply with eitherwastewater or RCRA regulations.

The EPA publication, Economics ofWastewater Treatment Alternativesfor the Electroplating Industry, 3

addresses the costs of meeting waterpollution control requirements.That report provides information onreducing the costs of wastewatertreatment through in-plant modifica­tions to the plating baths andrinse systems. This summary reportexpands that information throughadditional discussion of wastegeneratio'n phenomena and abate­ment measures involving in-plantchanges.

1

Pollution Sources andCharacteristics

2

Contaminants in the effluent fromelectroplating shops originatein several ways. The most obvioussource of pollution is the drag-out ofvarious processing baths into sub­sequent rinses. The amount ofpollutants contributed by drag-out isa function of such factors as thedesign ofthe racks or barrels carryingthe parts to be plated, the shapeof the parts, plating procedures, andseveral interrelated parametersof the process solution, includingconcentration of toxic chemicals,temperature, viscosity, andsurface tension.

With conventional rinsing tech­niques, drag-out losses fromprocess solutions result in largevolumes of rinse water contaminatedwith relatively dilute concentra­tions of cyanide and metals. Rinsewaters that follow plating solutionstypically contain 15 mg/L to100 mg/L of the metal being plated.

Most job shops operate severalplating lines that contain differenttypes of cleaning and electroplatingbaths, such as zinc, copper, nickel,cadmium, and chromium. Thecombined rinse waters dilute theconcentrations of individual metals,usually to less than 50 mg/L. Theresults of a recent survey of effluentfrom 22 electroplaters in theCleveland area are presented inTable 1.

Another source of effluent con­tamination is discarded processsolutions. These solutions are pri­marily spent 'alkaline and acidcleaners used for surface preparationof parts before electroplating.The solutions are not usually madeup of metals; however, there are afew cleaners that contain cyanide.Plating baths and other processsolutions containing high metalconcentrations, such as chromatesolutions, are rarely discarded.

The amount of pollutants con­tributed to the total pollution load bydiscarded process solutions variesconsiderably among plating shops. It

is not uncommon to find cyanideand heavy metals in concentra­tions of several thousand milligramsper liter in spent solutions. Thiscontamination is caused by drag-infrom previous process cycles andattack of the basic metals by thechemicals in the cleaning solutions.Table 2 presents an analysis ofsome typical process solutions.

Accidental spills, leaks, and drips ofprocess solutions also can con­tribute significantly to effluentcontamination. The plating roomusually is laid out so that theentire area drains on the floor,which-is only an extension of thesewer system leaving the facility. Al­though it is not common for atank to spring a leak that would allowthe entire solution to leak away un­detected, a slow leak amountingto a solution loss of 10 to 20 gal/d(38 to 76 LId) could, go undetectedfor months in many shops. Also,it is not unusual to compensatefor evaporation losses in a processtank by adding water to a processsolution with an unattended hose thatcauses overflow of the solutionto the floor drains.

In some shops, the drippinq of platedparts is a significant source ofpollution. Process solution tanksand rinse tanks often are separatedby a distance of several feet (meters)or more. Carrying the racks of partsbetween tanks will cause platingsolution or drag-out to drip onthe floor and enter the drain system.

Other sources of contaminants fromelectroplating shops' exist; how­ever, they are not as universallypresent as the preceding wastesources. Additional pollution sourcesinclude sludges from the bottoms ofplating baths generated duringchemical purification, backwashfrom plating tank filter systems, andstripping solutions.

Table 1. 1

Effluent Characteristics of 22 Cleveland Electroplating FhopS

The percentage contributed by each tion than thco,J normal draq- .pollution source to the pollutant out. The main contribution toconcentration of the final effluent effluent metal c~ncentration in zinccan vary substantially among or cadmium pla~ing is often theelectroplating shops. For shops zinc or cadmiuni that is eitherwhose primary process is the chrome stripped off the danqlers or rack tipsplate (copper-nickel-chromium), in the acid dip ~tep of the clean-drag-out usually will be the major ing cycle or rem10ved from thecause of metal loss. At facilities work in dichromating 4

that engage in large nickel plating Although some slhOPS' may have aoperations, more nickel is lost fromthe operation of the chemical higher contributlon of pollutantspurification filters and through the from other sourges, in almostsludge bottom dumps after purifica- every case, the Tost significant pol-

Table 2. IAnalysis of Typical Spent Process Solutions That Are Dumped Periodically

I

Efflue+ concentration (mg/L)

3

Electroplating shops should con­centrate on drag-out and rinsewaters during the planning stages ofpollution control. The emphasisof this report, therefore, is on the re­duction of drag-out and rinsewater use. To provide a comprehen­sive approach to in-plant control,however, other sources of contami­nation, such as accidents anddiscarding of process solutions, willbe addressed.

lution problem is drag-out andthe resultant contaminated rinsewater. The size and cost of pollutioncontrol equipment depend pri­marily on wastewater volumetricflow rate. Because the volumes ofrinse water are overwhelminglylarger than the volumes of all otherwaste sources, it follows thatcontaminated rinse water is themajor source of pollution. A recentsurvey in Cleveland showed thatthe average rate of rinse waterdischarged from 22 electroplatingshops was 18,500 gal/d (70,000 Ud),whereas spent process solutionaccounted for only 60 gal/d (230 Ud).

3

14.44.75.7

20.219.3

0.44.3

338.02.80.40.1

10.90.30.7

162.0

2

340.085.5

2.6(a)19.4

0.9(al74.0

95.947.252.2

178.0101.4

3.024.3

I Alkaline cleaner

3h.o

1

2 .50.2

4

10.08.16.94.41.2

<0.10.1

<0.10.10.4

<0.1<0.1

Minimuml Maximum AveragePollutant

Cyanide. total .Copper..•....•.•...................•.............Nickel .Chromium. total ...................•.•......•......Zinc .•...•.............................•.........Lead......................................•...•..Cadmium .

Volume (gal) .Cyanide. total (mg/L) ..........•...........................Cadmium (mg/L) ........................•...•..••..••...•Chromium. total (mg/L) .Copper (mg/L)" .Nickel (mg/L) .....•...•..................................Lead (mg/L) .Zinc (mg/L) .

aSol.ution was not analyzed for particular pollutant.

Pollutant or parameter

Drag-Out IViinimization

4

For the typical electroplating jobshop, the drag-out of processsolutions and the subsequent con­tamination of rinse waters are themajor pollution control problems. Thissection explains the basic prin­ciples of drag-out theory andexplores the function and applica­bility of the various drag-outminimization techniques in usetoday.

Principles

Electroplaters are well aware thatdrag-out varies considerably amongthe various parts plated at theirshops. For example, the volume ofdrag-out in rack plating differsvisibly from that in barrel plating.When a barrel emerges from aprocess tank, it usually carries withit over 10 times more solutionthan does a typical rack. In additionto the obvious effects of rackand barrel design and shape of parts,there are more subtle factors thataffect the volume of drag-out.These parameters include viscosityand chemical concentration,surface tension, and temperature.

The viscosity of a plating processsolution can be described as itsresistance to motion or removal byanother liquid (in this case, rinsewater) because of the attractiveforces of the molecules of the solution.The difference between highand low viscosity can be demon­strated with honey and water. Amuch thicker film will form ona knife dipped in honey than on onedipped in water. Honey, therefore,has the higher viscosity becauseof its adhesive quality. The same effectcan be observed with platingsolutions. If two identical surfacesare immersed in separate chromiumbaths with concentrations of53 oz/gal (397 giL) and 33 oz/qal(247 giL), respectively, the lowerconcentration bath will pro-duce 73 percent less volume ofdraq-out.P

Surface tension is another physicalphenomenon that has a signifi­cant effect in the plating shop. Accord­ing to kinetic theory,moleculesof a liquid attract each other. At thesurface of a solution, such as aplating bath, the molecules are sub­jected to an unbalanced forcebecause the molecules in the gaseousphase are so widely dispersed.As a result, the molecules at the sur­face are under tension and forma thin, skinlike layer that adjusts tocreate a minimum surface area.The property of surface tension causesliquid droplets to assume a spheri­cal shape, water to rise in a capillarytube, and liquids, such as water,to move through porous materialsthat they are capable ofwettinq."

In the plating process, the volume ofsolution that clings to a workpiecesurface depends largely on the sur­face tension. The force of surfacetension appears to be mosteffective at the bottom edge of thepart as it passes through andleaves the process solution. Thisforce and the resultant volumeof drag-out appear to be greatlyaffected by the orientation of the partrelative to the surface of the liquid.Positioning parts so that onlya small surface area makes contactwith the liquid surface at partingresults in less volume of draq-out,?

The third major factor that influencesdrag-out volume is the tempera­ture of the process solution.Temperature is interrelated withviscosity and surface tension.As the temperature of a plating solu­tion is increased, its viscosity.surface tension, and, therefore, drag­out volume are reduced. As apossible exception, when a part iswithdrawn too rapidly from a hotprocess solution, evaporation mayconcentrate the film and impededrainage." This problem, however,

can be overcome by reducingwithdrawal time and using a fog sprayrinse on the parts .as they emergefrom the plating solution.

Techniques

Many devices and procedures canbe used successfully to reducedrag-out. These techniques usuallyare employed to alter viscosity,chemical concentration, surface ten­sion, velocity of withdrawal,and temperature. Also used aredrag-out tanks for capturinglost plating solution and returningit to the bath.

Most drag-out reduction methodsare inexpensive to implementand are repaid promptly throughsavings in plating chemicals.An additional savings many times thecost of the changes will berealized once a pollution controlsystem is installed. The reduced drag­out will decrease the need fortreatment chemicals and, subse­quently, the volume of sludgeproduced. By reducing sludgevolume, many platers may be ableto meet the RCRA definition of asmall generator and therebytake advantage of reduced regula­tory requirements.

For some process solutions, returnof drag-out may be impractical.For example, in the case of process­ing baths that become steadilydepleted in use, the return of drag­out would simply increase thefrequency of dumping.

Controlling Plating Solutions. Asa rule, as the chemical contentof a solution is increased, its viscosityincreases. The result is a thickeningof the film that clings to the workwithdrawn from the process solution.Increased viscosity contributesnot only to a larger volume of drag­out but also to a higher chemicalconcentration of drag-out. The con­sequent need for more rinse watercreates additional pollution controlproblems.

Often plating baths can be operatedat significantly 19wer concentra­tions than thosexecommended bychemical manufaeturers. Researchon chromium plalling9 indicatesthat chromium deposits from solu­tions containing !chromic acid(erOs) at only 3.3 to 6.6 oz/gal(

I .25 to 50 giL) are acceptable. In the

experiments con~ucted, theoperating conditi~nswere almost thesame as those fdr the standard33.4-0z/gal (250~g/L) bath. Thebright range wasl narrower, how­ever, with lower GrOs concentrations.Chromate films, {.vhich appearedon the surface ot

lcdeposits from the

dilute baths, cou d be removed bydipping for a short period in theplating sOlution.-1

Chemical manufacturers and sup­pliers have become concernedwith the pOllutiot control problemsof their clients-I<he platers. As aresult, research 1nd develop-ment efforts by tre chemical manu­fact~rers have prpduced moreenvironmentally sound plating

solutions. ICyanide plating baths have been amajor target of t1e chemicalmanufacturers. Tpe conventionalcyanide bath ha~ been preferred formany plating applications, such as

. d drni Izrnc an ca mluljTl. Because ofstricter effluent Ii'mitations oncyanide, howev~f, an alternative tohigh-concentration cyanide bathsis being sought. fhe chemicalmanufacturers have experimented

d . Ian , In some cases, have developedalkal~ne noncyanlde or low-cyanide baths and acid baths includ­ing neutral chlori!ne solutions."?

Platers should in~estigate andevaluate the varidus advantages anddisadvantages 0lthe new chemi­cal solutions. As a rule, the acidbath substitutes 0 not offer the easeof control or the overall satisfac­tory operating conditions and depositquali:y that are ~~lva~lable from thecyanide bath. Fo zrnc, cadmium,brass, and preci us metals,the cyanide Platirg bath continuesto be the most c mmonly usedsolution.

Most of the substitute solutions alsoare limited in application. Forinstance, the acid copper bath, whichis not only widely accepted butsometimes preferred over the cyanidecopper bath, cannot be used fordirect application to steel and zincdie castings. A cyanide copper .strike is essential on zinc die.castinqs, and either a cyanidecopper or a nickel strike is necessaryon steel before it enters the acidcopper bath. If these substitutebaths are applicable to a plater'smanufacturing conditions, however,they may be a major factor inthe pollution control strategy.

For years wetting agents have beenused in process solutions to aidin the plating process. These sub­stances are used, for instance, inbright-nickel plating to pro-mote disengagement of hydrogenbubbles at the cathode. Their use hasalso found recent popularity asan aid to drag-out reduction. Awetting agent is a substance, usuallya surfactant, that reduces thesurface tension of a liquid, causing itto spread more readily on asolidsurface. A typical plating bath solutionhas a surface tension close to thatof pure water at room temperature,about 0.0050 Ibf/ft (73 dyn/cm).The addition of very small amountsof surfactants can reduce surfacetension considerably-to aslittle as 0.0017 to 0.0024 Ibf/ft(25 to 35 dvn/cm)."?

Kushner" estimates that the use ofwetting agents will reduce drag-outloss by as much as 50 percent. Herecommends the use of non ionicwetting agents that are not harmedby electrolysis in the plating bath.The safest maximum amount ofwetting agent should be used in thebath. A check of the surface ten­sion of the solution will determinewhether sufficient wetting agent hasbeen added. A stalagmometer or aDeNuoy Tensimeter can be used forthis purpose.

5

Kushner further suggests keepingthe concentration of all dissolved saltsat the minimum needed for properoperation. To follow this recom­mendation, the plater should notpermit substances to build up in theplating bath, if it is possible tocontrol and maintain them at theproper level. For example, cyanidebaths are permitted to build upvery high carbonate concentrationseven though the concentration levelcould be controlled by treatment.Such a buildup could increase drag­out by as much as 50 percent,"

Positioning on Rack. The metalfinisher's primary consideration inthe positioning of workpieceson a rack is proper exposure of theparts to the anodes for optimalcoverage and uniform thicknessof the electrodeposit. Drainage andrinsability are important consid­erations in racking. Damage tothe workpiece surface can be causedby insufficient or inefficient rinsing,and succeeding process solutionscan be contaminated by drag-inof unremoved chemicals from theprevious solution.

Several rules apply to the position ofwork on plating racks for drag-out minimization. The basic principle,however, is that every object canbe positioned in at least one way thatwill produce the minimum of drag­out. This position could be deter­mined by experiment, but unless a I

significant number of similar items areto be plated, it may be advisableto follow the suggestions of Kushner"and Wallace:7

• Tilt all solid objects with plane orsingle-curved surfaces so thatdrainage is consolidated, that is,twist or turn the part so thatthe clinging fluid will flowtogether and off the part by thequickest route.

• Rack all parts so that they areextended more in area thanin depth; this will decrease theaverage depth to which the partsare lowered into a solution and, asproven mathematically, will de­crease the film thickness of thedrag-out.

6

• If possible, avoid racking partsdirectly over one another to pre­vent lengthening the drainagepath of the plating solution.

• Avoid tablelike surfaces bytipping the part, but not at theexpense of forming solution"pockets."

• Orient parts so that only asmall surface area comes in con­tact with the liquid surface asit leaves the plating solution.

Workpiece Withdrawal. The velocityat which work is withdrawn fromthe process tank has a markedeffect on drag-out volume. The fasteran item is pulled out of the tank,the thicker the drag-out layerwill be. The effect is so dramaticthat Kushner" suggests that most ofthe time allowed for withdrawingand draining the item should be usedfor withdrawal.

The velocity of withdrawal ofwork from the process tank usuallycan be adjusted with automatedequipment. If the metal finishingcycle is operated by hand, however,the withdrawal velocity is lesscontrollable. The best control methodis to place a bar or rail above theprocess tank where the rack may besuspended for drainage whileits predecessor is removed from therail and transported to the nextphase of the finishing cycle.

The withdrawal motion also affectsdrag-out volume. When a rack isjerked from a process solution,surface tension forces do not have achance to operate and a muchlarger volume of liquid will cling tothe surface. An automatic machinethat performs smooth, gradualwithdrawal usually will drag out lesssolution per item racked than willmanually operated equipment.

Accurate predictions of the drag-outvolume to be saved by a givenreduction in withdrawal speed or by asmooth withdrawal motion are notpossible. A savings may be expected,

but the degree will be determinedby the specific application,

Draining time over the tank may belimited by the tendency of theplated object to spot when the platingsolution dries on the surface. Afog spray that uses water from thefirst rinse is very effective in keepingthe surface from drying, acceleratingthe drainage process, and maxi­mizing the time available fordraining.

When considering the purchase ofnew equipment, close attentionshould be given to withdrawaland drainage times. These factorsare especially important whenpurchasing barrel plating equip- .ment. Slow barrel rotation duringwithdrawal has reduced drag-outvolumes by as much as 50 percent.Machines may be automatedreadily to accommodate this type ofrotation at the time of deslqn."

Maintenance and Design of Racksand Barrels. As an industry average,maintenance of racks, fixtures,and rack coatings has been poor.Transport of chemicals insideloose-rack coatings from oneprocess to another is'not uncommon.Chromium-bearing solutions, forexample, appear in plant effluent inspite of treatment systems designedto handle the normal chromiumdischarge sources. These solutionshave been traced to rinse tanks andprocess solutions that are locatedsome distance from the chromiumdischarge points. The chromium­bearing solutions reach these remoteareas by way of loose rack coat­ings. Increased attention to rackmaintenance not only will eliminatethis potential hazard but also willcontribute to a welcome reductionin the number of workpiecesrejected because of poor contact.

Rack stripping plays an importantrole in rack maintenance. The plater,therefore, should organize rackstripping as a separate operatingline. A separate rack strip line has anumber of practical advantages.

, t I I I ! I I I: ,I

Workpiece

Concentratedsolution

7

Drip Tank. A drip tank is an ordinaryrinse tank that, instead of beingfilled with water, simply collects thedrips from racked parts and barrelsafter plating and before rinsing. Thedrip tank is useful with work thatinvolves continuous dripping over aperiod of time. Barrel plating,therefore, is a better candidate thanrack plating for drip tanks. Withbarrel plating, the barrel should berotated while it is suspendedover the drip tank to ensure maximumdrainage. When a sizable volumeof solution has been collectedin the drip tank, it can be returned tothe plating bath.

Fog Rinsing. Fog rinsing is used atexit stations of process tanks.A fine fog is sprayed on the work,diluting the drag-out film and causinga runback into the process solution.Fog rinsing is applied whenprocess operating temperatures,

Using a drip tank tends to restrict thepotential use of a rinse tank. Aswill be discussed, an additional rinsetank used as a drag-out tank or in aseries arrangement may be morebeneficial. The determining factorsare the volume of drag-out andthe evaporation in the plating bath.

Drain board

Pumped back to tank

Platin~ tank

I

Drain Board A .qJrain board is thesimplest method of drag-outrecovery. It can~1apture drips ofplating solution s racks and barrelsare transferred etween tanks(Figure 1). Not qnly do drain boardssave chemicals and reduce rinsewater requiremeTts, they also preventunnecessary floor wetting.

The drain sUrfac~ can be plasticor metal. For acid solutions, the bestmaterials are Vi~IYI chloride, poly­propylene, poly thylene, andTeflonv-Iined st el. Stainless steelshould be used t.or hot alkalinesolutions." The ~rain surfaceshould be positioned at an angle thatallows the Plati~g solution toreturn to the ba ,h.

Figure 1. jSimple Drag-Ou Recovery Devices

simple methOdsjOf drag-out recoverythat require mu h less capitalto im Plement. 1!fter using thesemethods and es ablishing new drag­out conditions, he plater shouldconsider the applicability of additionalrecovery throug~ commerciallyavailable units. A discussion of foursimple drag-out recovery methodsfollows.

It prevents the introduction ofpossible contaminants to the platingline (for example, chrome strip­ping in the soak cleaners and electro­cleaners). A separate rack stripline also eliminates uncontrolledspreading of solutions over the plantfloor and allows for more regular,frequent, and efficient stripping.

This separate rack strip line shouldbe incorporated into the rackingoperation. A racking workflow for allplating should be organized in thefollowing cycle:

• Rack off machine• Rack unloading• Rack stripping• Rack loading• Rack onto machine

The rack strip line should employmultiple counterflow rinses and driptanks for maximum dischargecontrol.

Commercially available equipmentfor the recovery of plating bathchemicals includes types that applysuch principles as ion exchange,reverse osmosis, electrodialysis, andevaporation. These devices usuallyare applied to a single platingbath where they concentrate the saltsin the rinse water, return them tothe plating bath, and recycle thepurified water to rinse tanks.

When new racks and barrels arepurchased, the shape of the rack andthe coating material should beexamined closely. The shape shouldnot hamper the drainage of platingsolution, and the rack coatingmust be a nonwetting material. Thesize and shape of the holes onthe barrels also need to be consideredbecause they affect the rate ofdrainage. Increasing the drainagearea with larger holes, when feasible,can speed drainage and reducedrag-out.

Simple Drag-Out Recovery

Before determining the costs andbenefits of recovery equipment, theplater should consider several

Evaporation

high enough to produce a highevaporation rate, allow replacementwater to be added to the processin this manner. Fog rinsing preventsdry-on patterns by cooling theworkpieces, but it may preclude theuse of a drag-out tank as a recoveryoption. Forfog rinsing to be effective,work must be withdrawn fromthe process tank at a slow rate.

Drag-Out Tank. The drag-out tank(Figure 2) is a rinse tank thatinitially is filled with pure water. Asthe plating line is operated, thedrag-out rinse tank remains stag­nant; the salt concentration increasesas more work passes through therinse tank. Air agitation must beused to aid the rinsing processbecause there is no waterflow withinthe tank to cause turbulence. Thepresence of a wetting agent ishelpful,"

Workpiece_

Plating Recoverybath rinse

Figure 2.

Recovery With a Drag-Out Tank

Finalrinse

.....Water

Towastetreatment

After a period of operation, thediluted plating salts in the drag-outtank can be used to replenishthe losses to the plating bath. If suffi­cient evaporation has taken place,a portion ofthe drag-outtank solutioncan be added directly to the platingbath. Evaporation usually willbe sufficient with baths, such asnickel, that are operated at elevatedtemperatures. Low-temperaturebaths have minimum surfaceevaporation and their temperaturecannot be increased withoutdegrading heat-sensitive additives.Recently, new additives, whichare not as readily degraded by heat,have been developed for many

8

of these plating baths. Theseadditives might make operation oftheplating bath possible at highertemperatures, facilitating drag-outrecovery by recycle techniques.Usually the value of the recoveredchemicals is much greater thanthe increased energy cost associatedwith operating the bath at ahigher temperature.

As a rule, the use of a drag-out tankwill reduce chemical 'losses by50 percent or more. The efficiency ofthe drag-out tank arrangementcan be increased significantly byadding a second drag-out tank. Useof a two-stage drag-out systemusually reduces drag-out losses by70 percent or more.

The applicability and benefits of drag­out tanks are discussed in moredetail in the next section.

Rinsing The major pollu~ion control problemfor electroplate~s is process solu­tion on workpie'ces beingdragged out an~ subsequentlyrinsed with water, Many electroplat­ing shops still Jmploy single,flow-through ri~se tanks to removethe clinging disFolved salts andsolids from worR~Pieces.This methodof rinsing is ext emely ineffi-cient and, for a ypical plating shop,results in the generation of thou­sands of gallon~ (liters) perday of rinse water contaminated

with dilute conCJ~ntrationsof cyanideand metals.

The enforceme t of pollutioncontrol standar s and the risingcosts of water dnd sewer use are

disrupting the ctbnventional rinsingpractices of the plating industry.Traditional rinsi g techniquesare being repla ed by more efficientmethods, such s parallel andseries rinse tan

1karrangements and

drag-out rinses, that reducewater use and t e amount of pollu­tants to be trea ed or discharged.

Principles I

The purpose of ~inSing is to preparethe surface of tie workpiece forthe. next step in the platinq process.A film of proce~s solution thatis picked up in fhe previous platingstep clings to the workpiece. Rinsingmust remove ertouqh of this film toensure that the folution in thenext process ta~k will be effectiveand remain uncontaminated.

To meet this Obl[.ective, the platermust use a rinsing strategy thatincludes:"

• Turbulent mOl ion between work­piece and wa,ter

• Adequate pefn'od of contactbetween wor piece and water

• Presence of ufficient waterduring contadt to reduce the .COIl­centration of the salts that arewashed off t e surface

These three principles apply to allrinsing operations, includingthose using flow-through or stillrinse tanks.

Turbulence. Agitation is needed toimplement the first principle.Agitation can be in the form of flow­ing water, such as in conventional,single, flow-through rinse tanks. Thisform is inefficient, however,because a very high flow velocityis necessary to achieve the requiredturbulence when water flowsinto a rectangular tank.

Direct water flow can be usedefficiently with spray, fog, and floodrinsing. With spray rinsing, theworkpiece is exposed to high­velocity water jets. Spray rinsinguses from one-eighth to one-fourththe amount of water that wouldbe used for equivalent dip rinslnq."but this method has limitedapplication because it is not effectivewith recessed and hidden surfaces.

In both spray rinsing and fog rinsing,water is applied to the workpiecefrom nozzles. With fog rinsing,however, the water is so highlyatomized that it approaches the con­sistency of vapor. The fog rinsingmethod uses less water thanthe regular spray and is used mostoften directly over the platingbath to remove a major portion of thedrag-out before the workpiecegoes to the rinse tank.

With flood rinsing, the workpiece isrinsed under a faucet that is con­nected to an air entrainer or aspirator.The air bubbles improve the effec­tiveness of the water movement byincreasing the agitation anddisplacing some of the plating solu­tion from the workpiece surface.The flood rinse usually is operated bypressure on a foot treadle.

9

Q =0.013(84,626/25)=44.0 gal/min

Sample Problem. A Watts nickelplating solution contains a nickelconcentration of 11.3 oz/ged(84.6 gil). The drag-out rate is0.05 gal (0.19 L) per rack, and theproduction rate is 15 racks per hour.What flow is necessary to main­tain rinse tank nickel concentrationsof 50 mg/L and 25 mg/L?

(1 )

(2)

when turbulence is achieved,R can be determined by:

R =c,«;where

where

Q = rinse tank flow rate()= drag-out rate

Solution. First convert 11.3 oz/galto milligrams per liter using themultiplication factor of 7,489:

11.3 oz/gal X 7,489 == 84,626 mg/L

Calculate the drag-out rate ingallons per minute:

() =0.05(15/60)=0.013 gal/min

Then, apply Equation 2 using anallowable rinse tank concentrationof 50 mg/L:

Q =0.013(84,626/59)=22.0 gal/min

Using an allowable limit of 25 mg/L,the required flow would be exactlytwice as much as for a 50-mg/Lconcentration:

When the volume of drag-outentering the rinse is considered,Equation 1 can be expanded tocalculate the required rinse rate:

The following simple' exampleillustrates the use of Equation 2.

Cp = concentration of salts inprocess solution

C, = allowable concentration inrinse

The maximum allowable concentra­tion becomes a very importantparameter when the other two param­eters are satisfied. In fact, maxi­mum allowable concentration is thegoverning factor with respectto water use. To understandthe importance of this parameter, itwill be helpful to begin the dis­cussion of rinsing equations.

As discussed, the conventionalmethod of rinsing uses the single,flow-through rinse tank. A work­piece covered with a thin filmof process solution enters the rinsetank and the solution is removedto an allowable limit beforeproceeding to the next processtank. The volume of rinse water nec­essary to complete this processdepends mainly on the agitationwithin the tan k, the period of contact.and the maximum allowableconcentration of process solution onthe workpiece.

Equations

To determine the proper water flowand to evaluate the advantages ofrinsing techniques (such asparallel, series, and still tanks),plating managers can use two equa­tions or their equivalent nomo­graphs.5 The relationship betweenthe concentration of salts in theplating bath and the allowable con­centration within the rinse isreferred to as the rinsing criterion, R.Under conditions of completemixing, which are closely approached

Rinse Water Volume. The finalprinciple of good rinsing is the pres­ence of sufficient water duringthe contact period for properreduction of the concentration of saltsthat are washed off the surface.Because water volume is the maincontributor to waste treatment costs,this principle must be studiedclosely.

Of the forced-convection methods,air bubbles usually produce thebest rinsing. Air bubbles createsufficient agitation within the rinsetank to dislodge the plating solu­tion from the workpiece. Air usuallyis filtered and then blown at thebottom of the tank through a pipedistributor. Air also can be forced intothe water by means of an airentrainer on the water feed line.

Contact Time. The second principleof a good rinsing strategy is toallow an adequate period of contactbetween the workpiece and thewater. For any particular instance,this time will depend on the effective­ness of the turbulence in the rinsetanks. With good agitation anda wetting agent, 5 s may be longenough in the rinse water; ifthere is little agitation and thegeometry of the work hinders forcedconvection, even 10 min may notbe enough. Usually, however, whengood agitation is present, a con­tact period of 10-15 s will be suffi­cient. If the agitation is only fair,30-60 s usually is sufflcient.f

The most common and efficientmeans of creating adequateturbulence is to apply forced con­vection within the rinse tank bypumping water, by propeller action,or by blowing air through thewater. The first two methods areused only for special purposes andusually are not as efficient as the airblower for agitating a rinse tank.Pump rinsing, for example, hasbeen satisfactorily applied in wireplating.

Agitation also can be achieved bymoving the workpiece in the water.This method is used on manuallyoperated hand rack lines, andits effectiveness dependson the conscientiousness of theoperator. It is possible to move theworkpiece mechanically, but,in most instances, the bar would haveto be moved so rapidly that thepieces would tend to fall off the racks.

10

11

(3)

n = number of parallel rinse tanks

where

The effect of parallel rinsing onwater use can be evaluated using thefollowing equation:

Q = (CpOn/Cn)1/n

Equation 3 is used in the followingexample. Using the operatingparameters from the sample problemillustrating Equation 2, determinethe water flow necessary to obtain arinsing concentration of 50 mg/Lwith two parallel tanks and withthree parallel tanks.

The flow rate is controlled mosteasily by installing flow regulators onthe fresh water lines feeding eachtank. These devices, which areavailable in a wide range of flowsettings, control the flow ratewithin a narrow limit despite varia-

.tionsin Iinepresslii;e:Flow regulatorsalso eliminate the need to resetthe flow each time the valve is closed.Some are designed to act as siphonbreakers and aerators (by theventuri effect).

t~Drip guards tailored from inexpensive plastic pipe and installed over spacebetween countercurrent rinse tanks

nated overtlow ~om critic, I 0' to obtain the optimal wale,final rinsing operations is reused savings.for intermediate~1rinse steps, such asacid dips and a kaline cleaningsteps. The rins water followinga nickel plating bath can be routed tothe rinse tank fbllowing the acid .dip. This rinse fater, in turn, couldbe routed to thj alkaline cleaner.

Choosing the optirTI81configurationrequires analysis of the particularrinse water needs, Interconnectingrinsing systems] might makeoperations more complicated, butthe cost advantaqe justifies theextra attention teqUired.

Multiple Rinse fankS. The bene­fits from recycli g rinse waterare limited bec use that method ofconservation ca not be appliedto all rinse tanks, Methods exist,however, that elm be applied morewidely and that!result in moredramatic water ravings. The threemost common rrethods are paralleland series rinsimg and the useof still rinse tan s.

A parallel rinse ank arrangementusing three rins tanks is illustratedin Figure 3a. W th the parallel feedsystem, each tank is individually fedwith fresh wate~. The rate of waterflow to each tank should be the same

Techniques

Rinse Water Recycling. Use of asimple method ofwater conservationis becoming more widespread.It involves the reuse of rinse water attwo or more rinse tanks wherethe contaminants in the rinse waterafter a processing step do notdetract from the rinse water qualityat another station. This method isapplied most often to the rinsesfollowing acid dips and alkalinecleaners. For example, instead ofusing 5 gal/min (19 LJmin) ofrinse water in each rinse tank [totalof 10 gal/min (38 LJmin)], the rinsewater used following the aciddip can be reused as rinse waterdirectly after the alkaline cleaner.Th is practice wi II reduce the water usefor these two tanks by 50 percent.In most cases, contamination doesnot appear to be a problem. Infact, the rinsing following the alkalinecleaner appears to improve. Thediffusion part of the mass transferprocess is accelerated as theconcentration of alkaline material atthe interface between the alkalinedrag-out film and the surroundingwater is reduced by the chemicalreaction there. Also, alkalinesolutions usually are more difficultto rinse off than acid solutionsbecause of the higher viscos-ities, so neutralization aids in thisrespect.f

Other recycling arrangements can beemployed where the less contami-

The most effective means of reducingwater use and waste treatmentcosts is to alter rinsing techniques.Changes can range from simplepiping alterations for recycling rinsewater to more complex changes,such as installation of two orthree additional rinse tanks that arearranged to combine the advantagesof series and recovery rinsing.A discussion of current rinsingmethods follows. The discussion isaccompanied by examples ofusing the rinsing equations to eval­uate the various rinsing techniques.

~Air

r.=== ...-.. Rinsewaterfeed

~--~---, ~------~I I I II I I II ~ I~I

k:i=~==::~;= Water

Overflow pipes

r----'I.. _ II II II II III

Drag-out

r-----~.,

I II I

To wastetreatment

Workpiece "'

III

~ ~ ~Rinse Rinse Rinse

To To Towaste waste waste

(a) treatment treatment treatment

(b)

Figure 3.

Three-Stage Rinse Systems: (a) Parallel and (b) Series With Outboard Arrangement

For two parallel rinse tanks:

= [(84,626)(0.013)2]1/2Q 50

= 0.54 gal/min per rinse tank, or1.08 gal/min total flow

For three parallel rinse tanks:

Q= [(84,62~bo.013)3r/3

= 0.16 gal/min per rinse tank, or0.48 gal/min total flow

Using a series, or countercurrent,rinse tank arrangement, the plater canachieve even greater water savingsthan with the parallel system.With the series feed (Figure 3b),water flows into the rinse tankfarthest away from the plating tankand moves toward the rinse tankclosest to the plating tank either bygravity or by pumping. The work-

piece is dipped in the least pure waterfirst and in the cleanest water last.

A conductivity probe can be usedwith a series rinse system to ensureefficient operation. This water­saving device controls a conduc­tivity cell, which measures the levelof dissolved solids in the rinsewater and, when the level reaches apreset minimum, shuts a valveinterrupting thefreshwater feed.

12

Figure 4. IPercentage of Draq-Out Recovery With Rinse-and-Recovery System

0.U....",U

0.2

1.0

0.4

0.6

0.8

a

1086

volume of drag-out. The recycle rinserate in the recovery rinse tanksis equal to the evaporation rate.Evaporation rates can be figuredusing Figure 5. Equation 2 can beused to determine the requiredwater rates for the final rinse oncethe concentration in the final rinse isknown.

13

Figure 4 is used .in the followingexample. Using the operating param­eters from the sample problemfollowing Equation 2 and a surfaceevaporation rate of 3 gal/h (11 Ljh),determine the water flow neces­sary in the free-flowing rinse to obtaina rinsing concentration of 50 mg/Lusing a single drag-out tank (as inFigure 2) and with a two-stagedrag-out tank.

42

oL..-+_.....L. .L...__...l-__--L__~

a

20

100

RECYCLE RINSE RATIO (r)"

80

I­::Jo<b<{ 600:au..o>­0:UJ> 40ouUJ0:

'*

"r = recycle rinse (gal/h) + drag-out (gal/h).Note.-r (reCyple rinse flow) = surface evaporati~n f~om. bath. n = number of counter­flow rinse tanks in recovery use. C, = concentration In final stage of r-ecovery.C = concentr1tion in platinq bath.

P I

salts concentra1ion until a portion isreturned to the plating bath tocompensate fori evaporative losses.The concentrati1on of salts in thedrag-out tank can reach as highas 75 percent qf the plating bathconcentration. Consequently, asignificant wat~r flow in the rinsefollowing the drrg-out tank would benecessary to meet the maximum

allowable C~~Cl~ntration.

Figure 4 can b used to determineflow rates with drag-out recovery.The percentage recovery of drag­out is first defin11d as a function of therecycle ratio, r. which is the volumeof recycled rinse divided by the

(4)

The quantity of chemicals enteringthe final rinse will be significantlysmaller than that entering asingle-tank rinse system. The amountof rinse water required for dilutionwill be reduced the same degree.

The use of drag-out tanks usuallyresults in less water savings than doesparallel or series rinsing. Theoperational procedure used withdrag-out tanks is responsible for thiseffect. The rinse water in the drag­out tank increases in plating

Q =[(84,626/50)1/2 + 1/2]0.013=0.54 gal/min

For two series rinse tanks:

Another equation can be applied tosolve for the required flow withseries rinsing.

A rinse tank arrangement employinga drag-out tank (Figure 2) isanother application of multiplerinse tanks. This arrangement isalmost always associated with therecovery of drag-out solution; there­fore, it is only applicable to rinsingfollowing plating baths wherethe bath is of significant value.

The use of Equation 4 is illustratedin the following example. Usingthe operating parameters from thesample problem following Equation2, determine the water flow ratenecessary to obtain a rinsingconcentration of 50 mg/L with twoseries tanks and with three seriestanks.

For three series rinse tanks:

Q = [(84,626/50)1/3 + 1/3] 0.01 3= 0.16 gal/min

When the concentration of dissolvedsolids builds up to the maximumallowable level, the conductivityprobe opens the valve. The probe isespecially valuable with anirregular or varied work sequenceand probable fluctuations in thelevel of dissolved salts in the rinsesystem.

Note.-Ambient conditions are 75° F. 75% relative humidity. Plating solution is 95% molefraction H20.

Figure 5.

Surface Evaporation Rate From Plating Baths With No Aeration

3 gal/h +0.78 gal/h1'= 0.78 gal/h . =4.8

mg/L for a single drag-out tankandO.05 X 84,626 =4,231 mg/Lfora two-stage system. :

To illustrate the benefits of a drag- in/drag-out system, consider addinga drag-in tank to the recoverysystem just discussed. The recycleratio would become:

0 1 = 0.013(17,772/50)= 4.6 gal/min

O2 = 0.013(4,231/50)= 1.1 gal/min

The drag-in/drag-out system findsapplication with plating baths thathave a low evaporation rate. The re­cycle ratio, which determinesrecovery efficiency, is calculated asthe volume of recycled rinseplus the volume of drag-out dividedby the volume of drag-out. The recycleratio, therefore, is greater witha drag-in/drag-out system than acommon recovery tank. If theevaporation rate is low, the differencebetween the recycle ratios forcommon recovery and drag-in/drag-out systems is significant.When evaporation ratios are high, thedifference is less.

A relatively new application ofmultiple rinse tanks is the drag- in/drag-out configuration (Figure 6).With the drag-in/drag-out system, therinse tank preceding the platingbath (drag-in tank) is 'connected tothe recovery rinse (drag-out tank)following the bath; the recovereddrag-out solution is circulatedby a pump. The concentrations ofsalts in the drag-in and drag-outtanks remain about equal. When arack or barrel is processed, it drags inplating solution to the platingtank, thereby increasing recovery.

Applying Equation 2, using theconcentration of the last drag-outtank as Cp' the required rinse rateswould be:

180160140

claim 78 percent; a two-stage systemwould reclaim 97 percent. Atthese recovery rates, the concentra­tion ratios are 0.21 and 0.05,respectively. Because the plating bathconcentration is 84,626 mg/L, theconcentration entering the finalrinse is 0.21 X 84,626 = 17,772

120

BATH TEMPERATURE (OF)

10080

0.Q1

0.5

0.02

0.2

0.3

~~:=. 0.1'"~w

~Z0

~a:0e,

~ 0.05wwuita::>(/l

0.03

The recycle ratio would be:

3 gal/hr= 0.013 gal/min X 60 min = 3.8

From Figure 4. a one-stage recoveryrinse and recycle system would re-

14

Recirculate

Figure 6.

Drag-In/Drag-Out Recovery Arrangement

Rinse

-!--~

Chemicals are added to the reservoirto provide a controlled excess ofreagent in the solution. The reservoiracts as a combined reaction andsettling tank. Because ofthe presenceof a controlled excess of reagentsin the chemical rinse tank, toxicmaterials and heavy metals areremoved from the metal finishingsequence and are prevented fromentering the subsequent water rinse.At the same time, rinsing isimproved because the diffusion layer,which is present during conven­tional water rinsing, is broken downby the chemical reaction.

Rinsing Recovery Systems. Theinformation provided on drag-outand rinsing principles can be formu­lated into a strategy for simplerecovery systems using multiplerinse tanks and a minimum of addi­tional equipment. Examples willbe presented for various platingapplications and considerations, such

15

flow from the reservoir is pumpedback to the rinse tanks, forminga complete closed-loop system.

Towastetreatment

Recycle

ing from chromium solution, whichis notoriously di!fficult to rinse.By simply makin1g the first rinse afterchromium Platel a stagnant rinsecontaining sodium bisulfite, thedrag-in of hexa~alentchromium wasconverted to tr~ialent chromium.The rinsability f the workpiecein the second rinse was improvedconsiderably by changing the chem­ical nature of thje film on the work­piece in the stagnant rinse and byreducing film c9ncentrations beforeattempting to ripseby diffusion.The same princIples are frequentlyemployed in "neutralizing" dips.

The application If chemical rinsing toplant effluent treatment, knownin the industry Js integrated wastetreatment, has tleen described byLancv" and Pi~~er.12Aside from theenvironmental genefits, this typeof chemical rinsiing also prevents themajority of hea"!Y metal solidsformed in the chemical rinse fromreaching the sUlJbeedingwater rinsesby removing th9se materials in anexternal settlinq vessel. Removalof these solids taccomplished byflowing the che ical rinse solutionto a treatment r servoir. The over-

------'..~~-------------

Platingbath

Workpiece- _

1 gal/h0.78 gal/h = 1 .28

For a drag-in/drag-out system, therecycle ratio would be:

1 gal/h + 0.78 gal/h0.78 gal/h =2.28

Chemical Rinsing. The techniqueof chemical rinsing has been used bythe metal finishing industry formany years. One of its earliestapplications was to eliminate stain-

The percentage recovery, in thiscase, would increase from 51 percentto 68 percent by adding a drag-intank.

If the evaporation rate were only1 gal/h (4 L/h), the recycle ratio for arecovery rinse system would be:

From Figure 4, a one-stage recoveryrinse and recycle system would re­claim 83 percent. The increase inpercentage recovery is only 5percentage points. Considering thecost of an additional tank and pump,this change is not likely to becost effective.

Stationary drain board under rack passing between tanks

as space limitations. Before wastetreatment equipment is installed,the implementation ofthese systemswill generate substantial savingsin plating chemicals and water.After waste treatment equipment isinstalled, the system can continue tooperate and will provide furtherbenefits by reducing waste treatmentcosts.

The tank arrangement in Figure 2,which consists of a drag-out tankfollowed by a flow-through rinse tank,is the simplest recovery system.The drag-out rinse collects a signi­ficant portion of the process solutioncarried on the parts, rack, orbarrel. Periodically, the strongsolution in the drag-out tankis returned to the plating tank. Thevolume returned is limited tothe volume made available in theprocess tank by evaporation.

The efficiency of a drag-out tankrecovery system can be improvedsignificantly by the addition ofa rinse tank. The additionaltank could be used as a secondrecovery tank to decrease chemicallosses further, or it could be usedin a series arrangement by connect­ing it with the overflow rinse(Figure 3b). The latter change wouldprovide a water savings but wouldnot reduce chemical losses.

Various other rinsing configurationscould be developed by addingtanks. The choice of a best arrange­ment is difficult because of the trade­offs involved between furtherreducing chemical losses andfurther reducing the rinse flow rate.Obviously, the value of the lostchemicals is a significant cost.Chemical losses also result in addi­tional rinse water and waste treat­ment chemical requirements andmore sludge.13

Although complex, the evaluationand selection of a multiple rinsetank system can be accomplished byanalyzing each rinsing configura­tion. Such an evaluation involvesusing the equations and graphspresented in this section and com-

16

paring cost factors, such as water,sewer, and waste treatment. Theresults of the evaluation will enablethe plater to determine whether amultiple rinse tank arrangementis beneficial and to identify themost appropriate configuration. It isimportant to find this optimalpoint because the space needed foradditional rinse tanks is limited.

The application of the complexrinsing evaluation will be presentedthrough the use of examples.Two widely used electroplatingbaths-a concentrated chromiumand a Watts nickel bath-will be con­sidered. These baths were selectedbecause they differ with respect totwo important factors affecting theselection of an optimal rinsingconfiguration: operating temperature

and bath concentration. Theconcentrated chromium bath normallyis operated at 1100 F (43 0 C) with achromium concentration of200,000 mg/L. The Watts nickelbath normally is operated at atemperature of 1400 F(60 0 C) with anickel concentration of 85,000mg/L. The higher evaporation rateand lower plating bathmetal concen­tration make nickel a bettercandidate for recovery. A summaryof the operating conditions for thetwo baths is presented in Table 3.

The cost evaluation of complexrinsing systems must- include allsignificant operating and investmentcosts that are affected by theinclusion of additional rinse tanksand either flow or chemical lossreduction. Some site-specific costs,such as plating room rearrange-

Table 3. IOperating Parameters for Watts Nickel and concentratrd ChrofQ)um.,Baths

I Bath

Water and sewer ($/1 ,000 gal wastewater). . . . . . . . . . . . . .. . . . . . . . . . .. . . . . .. . . 2.00Additional rinse tanks, 10 yr depreciation ($/yr/tank)a. . . . . . . . . . . . . . . . . . . . . . . . . 240Treatment chemicals:

Chromium (Cr):b$/lb Cr. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.41$/1,000 gal wastewater. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.28

Nickel (Ni):c$/Ib Ni. . . . . . . .. . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . 0.18$/1,000 gal wastewater. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . .. . . . . . 0.21

Sludge disposal.f$/Ib Cr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.48$/Ib Ni. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1,5

Plating chemicals:$/Ib Cr. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . 4.20$/Ib Ni . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.35

De-ionized water, 5-yr depreciation ($/yr). . . . . . . . . . . . . . . . . . . . . . . . . . . .. . .. . . . 1,080

"Cost is for flanged, open-top, mild steel tank lined with polyvinyl Chl1lride; cost does not in-clude installation, which is highly site specific. '

bTreatment chemicals for chromium: sulfur dioxide, 2 lb/lb Cr; sulfuric rcid, 0.2 Ib/l ,000 galwastewater; sodium hydroxide (NaOH). 1.5 Ib/l,OOO gal wastewater; N

1aOH, 2.3 Ib/lb Cr; floc-

culant, 0.1 Ib/l.000 gal wastewater.., - I "CTreatment chemicals for nickel: NaOH, 1.0 Ib/l,OOO gal wastewater; ]NaOH' 2.0 Ib/lb Ni; floc-culant, 0.1 Ib/l,OOO gal wastewater. ' .

d$0.25/gal at 4% solids by weight.

Note.-AII costs, except those for treatment chemicals, are in 1981 d liars. Costs for treat­ment chemicals. originally in 1979 dollars. were updated to reflect aver~ge 1980 prices using theMonthly Labor Review Producer Price Index for industrial COmmoditiesl . .

SOURCE: U.S. Environmental Protection Agency, Environmental Regul tions and Technology:The Electroplating Industry, EPA 625/10-80-001, Aug. 1980.

The actual cost or individual plantswill differ from hose used in theanalysis. For ins ance, the water andsewer cost($2/~,000gal, in 1981dollars) varies cpnsiderablyamong municipalities. Platers areurged to insert the costs that bestreflect their situ~tions, including

2.75-1-=2.75

recycle rinser =--,-----­

drag-out

17

any additional costs, before proceed­ing with their analyses.

For the two examples, 15 differentrinsing configurations wereconsidered (Figure 7). Configura­tion 1 is the basic single overflowrinse. Configurations 2 through 4 arethe three possible options usingtwo rinse tanks. Configurations 5through 9 use three rinse tanks, andConfigurations 10 through 15employ four rinse tanks.

First, calculate the recycle ratio:

Each of the 15 configurations canbe analyzed using the methods pre­sented earlier in this report.Examples already have been pre­sented for Configurations 1 through7. Such arrangements as Con­figuration 8 are more complex, butthey can be divided into simplerproblems and analyzed using Equa­tions 2 and 4 and Figure 4. Thefollowing example will illustrate thismethod.

A Watts nickel plating solutioncontains a nickel concentration of85,000 mg/L The drag-out rate is1 gal/h (4 Uh), or 0.017 gal/min(0.063 Umin), and the evaporationrate (Figure 5) is 2.75 gal/h(10.41 Uh). Determine the per­centage of nickel solution that issaved and the flow required to meeta maximum allowable nickel con­centration criterion of 50 mg/Lin the final rinse using Configura­tion 8.

Then, using Figure 4, find the per­centage recovery for a single­stage drag-out tank, which is 77percent. At this recovery rate, theconcentration ratio is 0.23. Nowcalculate the concentrationentering the final rinse:

0.23 X 85,000 mg/L = 19,550 mg/L

Cost

200,000110

2535

Concentratedchromium

86,000140

2550

I .wajs nickel

ment, must be considered in theanalysis. Because these costswill vary from shop to shop, however,they will not be included in thisanalysis. Instead, only those coststhat are common to all shops will beconsidered (Table 4).

Cost Variables for Evaluating Rinsing Options

Table 4.

Parameter

Parameter

Concentration (mg/L) .Operating temperature (OF) .•••.........•...............•.Surface area of plating tank (ttl) .Maximum allowable concentration in final rinse (mg/L) .

Configuration 11: Four-staqe series

Configuration 10: Four-stage 'parallel

FOUR RINSE TANKS

•

THREE RINSE TANKS

Configuration 6: Three-stage series

Configuration 5: Three-stage parallel

•Configuration 1:Single overflow

TWO RINSE TANKS

ONE RINSE TANK

Configuration 2: Two-stageparallel

Configuration 7: Two drag-out. one overflow Configuration 12: Three drag-out. one overflow

.~Configuration 3: Two-stageseries

Configuration 8: One drag-out. two-stageseries

Configuration 13: Two drag-out. two-stage series

Configuration 4: One drag­out. one overflow

Configuration 9: Drag-in/drag-out. oneoverflow

Configuration 14: One drag-out. three-stage series

Configuration 15: Drag-in/drag-out. two-stageseries

Note.-Decreasing heights of shading show that metals concentrations decrease.

Figure 7.

Rinsing Configurations

18

Table 5.

Evaluation of Chromium and Nickel Rinsing Systems

l-gal/h ~rag-outI

mined. After nn

l;ng these pararn­

eters, a cost anflysis can beperformed USin1the data in Table 4,and the optimal configurationcan be identifie .

To illustrate thel use of the costanalysis, the 151 configurations havebeen analyzed ~or a concentratedchromium bath and a Watts nickelbath. The opera~ing parameters forthe baths were fhown in Table 3.Two analyses are performed foreach bath. The first assumes a drag­out rate of 1 ga /h (4 I./h) and the

"Pounds of metal (chromium or nickel) per year.

The following conclusions can bedrawn from the cost analysis ofthe chromium and nickel rinsingsystems:

• Single overflow rinses requireextremely high flow rates to meetgood rinsing criteria, even atlow drag-out rates.

19

second assumes a drag-out rate of2 gal/h (8 L/h). The results arepresented in Tables 5 through 7.

2-gal/h drag-out

Water use Plating chemicals lost

gal/min 1,000 gal/yr(Ib/yr)"

188.6 47,074 13,9005.0 1,248 13,9002.5 624 13,900

127.0 31,699 9,3131.5 374 13,9000.3 75 13,900

102.0 25,459 7,5002.1 524 9,313

80.0 19,968 5,8381.0 250 3,9000.1 256 13,900

84.0 21,080 6,1511.9 474 7,5000.6 150 9,3131.6 412 5,838

56.6 14,127 5,9002.7 674 5,9001,4 349 5,900

26.9 6,714 2,4841.6 400 .5,9000.2 50 5,900

15.7 3,918 1,3831.0 250 2,484

17.6 4,383 1,8290.7 176 5,9000.1 25 5,9008.2 2,071 8590.7 174 1,3830.3 75 2,4840.8 195 1,829

2,9502,9502,950

6502,9502.950

177650649

2,9502,950

91177650649

6,9506,9506,9503,3366,9506,9501,9463,3362,0856,9506,9501,2041,9463,3362,085

Plating chemicals lost(Ib/yr)a

INic~el plating

chro1ium plating

7,213349175

1,587150

50500100

1,555100

25225

755075

24,236649324

11,638225

506,789

2257,131

12525

4,133175

75175

1,000 gal/yr

Water use

28.91.40.76.40.60.22.00.46.20.40.10.90.30.20.3

97.12.61.3

46.60.90.2

27.20.9

28.60.50.1

16.50.70.30.7

gal/min

1 .2 .3 .4 .5 .6 .7 .8 .9 .10 .11 .12 .13 .14 .15 .

Configuration

1 .2 : .3 .4 .5 .6 .7 .8 :9 .10 .11 .12 .13 .14 .15 .

Using a similar approach of break­ing down the complex probleminto two lesser problems, theflow and percentage recovery ofmost rinsing systems can be deter-

Next, apply Equation 4, using19,550 as c,Q = [(19,550/50)1/2 + 1/2](1/60)

= (19.8 + 0.5)(0.017)= (20.3)(0.01 7)=0.35 gal/min

Table 6.

Chromium Rinsing System Costs

Cost ($)

Config-Flow Treatment chemicals at Plating

urationrate Water and Additional

Sludge at chemicals De-ionized(gal/min) sewer at rinse tanks at Total

$0.28/1,000 gal $1.48/lb Cr lost at water$2/1,000 gal $240/tank $0.41/lb Cr

wastewater $4.20/lb Cr

1-gal/h drag-out

1· •••.•. 97.1 48,472 0 2,850 6,786 10,286 29,190 0 97,5842 ...... 2.6 1,298 240 2,850 182 10,286 29,190 0 44,0463 ...... 1.3 648 240 2,850 91 10,286 29,190 0 43,3054· •••••• 46.6 23.276 240 1.368 3,259 4,937 14,011 1,p80 48,1715 ...... 0.9 448 480 2,850 63 10,286 29,190 0 43,3176 ...... 0.2 100 480 2,850 14 10,286 29,190 0 42,9207· ...... 27.2 13.578 480 798 1,901 2,880 8,173 1,080 28,890a ...... 0.9 450 480 1,368 63 4,937 14,011 1,.080 22,3899· ...... 28.6 14.262 480 855 1,997 3,086 8,757 1,.080 30,51710 ..... 0.5 250 720 2,850 35 10,286 29,190 0 43,33111 ..... 0.1 50 720 2,850 7 10,286 29,190 0 43,10312 ..... 16.5 8,250 720 494 1,155 1,782 5,057 1,.080 18,53813 ..... 0.7 350 720 798 49 2,880 8,173 1,080 14,05014 ..... 0.3 150 720 1,368 21 4,937 14,011 1,.080 22,28715 ..... 0.7 350 720 855 49 3,086 8,757 1,080 14.897

2-gal/h drag-out

1·...... 188.6 94.148 0 5.699 13,181 20,572 58,380 0 191,9802 ...... 5.0 2,496 240 5,699 349 20,572 58,380 0 87,7363 ...... 2.5 1,248 240 5,699 175 20,572 58,380 0 86,3144· •••••• 127.0 63.398 240 3,818 8,876 13,783 39,115 1,080 130,3105 ...... 1.5 748 480 5,699 105 20,572 58,380 0 85,9846 ...... 0.3 150 480 5,699 21 20,572 58,380 0 85,3027· ...... 102.0 50.918 480 3.075 7,129 11.100 31.500 1,080 105,2828 ...... 2.1 1.048 480 3,818 147 13,783 39,115 1;080 59,4719· ...... 80.0 39.937 480 2,393 5,591 8,640 24,520 1;080 82,64110 ..... 1.0 500 720 5,699 70 20,572 58,380 0 85,94111 ..... 0.1 50 720 5.699 7 20,572 58,380 0 85,42812·..... 84.0 42,160 720 2.522 5,902 9,103 25,834 1,080 87,32113 ..... 1.9 948 720 3.075 133 11.100 31,500 1.080 48,55614 ..... 0.6 300 720 3.818 42 13,783 39,115 1;080 58,85815 ..... 1.6 842 720 . 2.393 115 8,640 24,520 1;080 38,310

"High rinse rate required with this configuration to meet maximum allowable concentration in the final rinse (35 mg/L en:may ,preclude its use.

NotD.-AIl costs. except those for treatment chemicals, are in 1981 dollars. Costs for treatment chemicals, originally in 1979 dollars, were updatedto refloct avorago 1980 prices using the Monthly Labor Review Producer Price Index for industrial commodities,

• Multiple rinse tank arrangementscan provide significant costsavings.

• With low recycle ratios (1.37 orless), drag-out recovery is imprac­tical because of high waterrequirements, unless three or moredrag-out tanks are used (seeupper half of Table 6. Configura­tion 11) or series rinsing followsdrag-out (see upper half ofTable 6, Configurations 12 and13).

20

• With recycle ratios of 2.75 orgreater, all additional tanks shouldbe used as drag-out tanks ratherthan in parallel or seriesarrangements.

• Drag-in/drag-out systemsusually must include series rinsing.These systems become costeffective with recycle ratios at or

below 1.25 (see lower half ofTable 6, Configuration 15).

Plating baths that operate at lowertemperatures than those describedin the foregoing examples havelower evaporation rates and offerfewer opportunities for rinse.recovery. If the bath can be operatedat an elevated temperature. eventhough it is not required. the cost ofadded energy usually will be more

Table 7.

Nickel Rinsing System Costs /'

Cost ($)

Config-Flow

Treatme!nt chemicals at Platingrate Water and Additional

uration(gallmin) rinse tanks at

$0.1 8/lb NiISludge at chemicals De-ionized

Totalsewer at$0.21/1,000 gal $1.15/lb Ni lost at water

$2/1,000 gal $240/tankwastewater $6.35/lb Ni

l-gall~ drag-out

l a...... 28.9 14,426 0 531 1.515 3,393 18,733 0 38,5982 ...... 1.4 698 240 531 73 3,393 18,733 0 23,6683 ...... 0.7 350 240 531 37 3,393 18,733 0 23,2844 ...... 6.4 3,174 240 117 333 748 4,127 1,080 9,8195 ...... 0.6 300 480 531 32 3,393 18,733 0 23,4696 ...... 0.2 100 480 531 11 3,393 18,733 0 23,2487 ....... 2.0 1,000 480 32 105 204 1,123 1,080 4,0248 ...... 0.4 200 480 117 21 748 4,127 1,080 6,7739 ...... 6.2 3,110 480 117 327 748 4,127 1,080 9,98910 ..... 0.4 200 720 531 21 3,393 18,733 0 23,59811 ..... 0.1 50 720 531 5 3,393 18,733 0 23,43212 ..... 0.9 450 720 16 47 104 578 1,080 2,99513 ..... 0.3 150 720 32 16 204 1,123 1,080 3,32514 ..... 0.2 100 720 117 11 748 4,127 1,080 6,90315 ..... 0.3 150 720 117 16 748 4,127 1,080 6,958

2-gallr drag-out

1"...... 56.6 28,254 0 1,060I

2,967 6,785 37,465 0 76,5312 ...... 2.7 1,294 240 1,060 142 6,785 37,465 0 46,9863 ...... 1.4 698 240 1,060 73 6,785 37,465 0 46,3214" ...... 26.9 13,428 240 447 1,410 2,856 15,773 1,080 35,2345 ...... 1.6 800 480 1,060 84 6,785 37,465 0 46,6746 ...... 0.2 100 480 1,060 11 6,785 37,465 0 45,9017a...... 15.7 7,836 480 249 823 1,590 8,782 1,080 20,8408 ...... 1.0 500 480 447 53 2,856 15,773 1,080 21,1899a...... 17.6 8,766 480 329 920 2,104 11,614 1,080 25,29310 ..... 0.7 348 720 1,060 37 6,785 37,465 0 46,41511 ..... 0.1 50 720 1,060 5 6,785 37,465 0 46,08512 ..... 8.2 4,142 720 155 435 987 5,455 1,080 12,97413 ..... 0.7 348 720 249 37 1,590 8,782 1,080 12,80614 ..... 0.3 150 720 447 16 2,856 15,773 1,080 21,04215 ..... 0.8 390 720 329 41 2,104 11,6"4 1,080 16,278

"High rinse rate required with this configuration to meet maximum allorable concentration in the final rinse (50 mg/L Ni) may preclude its use.

Note.-AII costs, except those for treatment chemicals, are in 1981 dqllars. Costs for treatment chemicals, originally in 1979 dollars, were up-dated to reflect average 1980 prices using the Monthly Labor Review Producer Price Index for industrial commodities.

than offset by the benefits ofrecovery. Drag-in/drag-out systemsoffer perhaps the best situation forimplementing the techniques

presented here hen the recycleratio is low. Ne ertheless, whenthe evaporation rate is very low, themost cost-effec ive solution may beconcentration 0 I the drag-out

rinse itself, such as by an evaporatoror by ion exchange, before replace­ment in the plating tank.!"

21

Plant Assessment

22

Procedures

A plant assessment is the initial stepin a pollution control program. Itinvolves a thorough analysis ofthe operations of a metal finishingplant that relate to pollutant sourcesand water use. The informationgenerated during a plant assessmentis used in evaluating the applica­bility of in-plant changes for reducingchemical loss and water use.

A plant assessment includes thefollowing steps:

• Inspect the plating room layout.• Review plant operating practices.• Examine process water use.• Conduct sampling and laboratory

analysis to characterize wastestreams and to determine drag­out rates.

• Identify the frequency, volume,and characteristics of batchdumps.

A plant assessment can be per­formed by the plater or by a qualifiedengineering consultant. Althoughthe plater has the advantage ofthoroughly knowing the manufactur­ing process, the consultant canfrequently provide a fresh andimpartial view of the plant and oftencan identify overlooked possi­bilities. If the plater performs theassessment, a laboratory can behired to analyze the samples. Mostlaboratories qualified to performthe analyses charge between $12and $20 for each heavy metalparameter that is analyzed andfrom $20 to $40 for cyanideanalysis." When the plant assess­ment is performed by a consultant,the complete service ranges from$4,000 to $10,000, depending onthe size and complexity of the shopand the extent of the survey andsubsequent evaluation.

In nearly every case, the benefits ofa plant assessment will far out­weigh the amount oftime and money

'Costs in this section are given in 1981 dollars,except for treatment costs, which are in1980 dollars.

expended. Usually the assessmentwill be repaid in less than 1 yearthrough savings in chemicals andwater.

Inspect Plating Room Layout. Thefirst step in a plant assessment isrelatively simple; it involves thepreparation of drawings showingthe layout of the plating room(s).For many platers, this task will havebeen performed already.

The drawings should be made toscale showing the location of allrelevant equipment and tanks. Eachtank should be numbered andlabeled with its contents (forexample, Tank 1, soak cleaner; Tank2, rinse). Individual plating linesshould be identified, such as zincbarrel and chrome plate rack. Also,water feed lines, gutters, sumps,and sewer lines should be indicated.On the water lines, all controlvalves and flow regulators shouldbe identified.

Review Plant Operatipns. All opera­tions of the plating room thatrelate to chemical or water use­including the plating .ssquences foreach plating line-should bereviewed and documented. Oftensequences will vary on a particularline because of differences in platingrequirements and specifications.Each major variation should belisted.

Estimates of production for eachline should be developed. Productioncan be measured either by hours ofoperation or by production units,such as number of square feet(square meters) plated or number ofparts, racks, or barrels to passthrough a particular platinq line orsequence.

Other, more specific informationthat is needed on the plating opera­tion can be gathered throughobservation during manufacture.If automatic lines are used in plating,rinsing and draining times shouldbe measured. If manual hoist or hand

Table 8. IProcess Water Survey Sample

I

periodic water te for washingdown floors or ~imply from hosesbeing allowed tb run without regard

to waste. IWater uses other than rinsing alsomay contribute to a difference inmeasured versuk metered flow. Formany plating Sh:OPS, these non­rinsing uses of Fater include fumescrubbers, wate -cooled rectifiers,heat exchanger, boilers, heatingand cooling coi s, air conditioners,and welders. If the second or third method is

employed, the test should be repeatedseveral times and the resultsaveraged. Averaging is recommendedbecause it is difficult to ascertainvisually the exact time at whichthe flow over the overflow darn orweir reaches a stabilized flow condi­tion. It is especially important torepeat the test if the rinse tank has ahigh flow rate or if air spargers areused to create turbulence.

Conduct Sampling and Analysis. Theusual procedure for sampling at aplating shop is to sample the finaleffluent, as local pollution controlauthorities often do. During the plantassessment, the final effluent issampled for the same reason, that is,to determine which pollutants are notin compliance with the regulations.The effluent, however, is only one ofseveral points for sampling and

A third method of measurement in­volves depressing a 5-gal (19··L)bucket into the flowing rinse so thatthe water reaches the top of thebucket but does not enter it. This willcause 5 gal (19 L) of water to bedepleted from the rinse tank immedi­ately. Once the overflow from therinse tank appears normal, theempty bucket should be removed andthe time it takes the water to resumean overflow should be measured.

23

Plating line

AnodizingZinc rack

Hoist Stillautomatic

10.0 24.0 23.6 9.59.5 22.5 22.0 8.9

0.5 1.5 1.6 0.65 6.25 6.8 6.3

4,800 11,520 11,328 4,5604,560 10,800 10,560 4,272

240 720 768 2885 6.25 6.8 6.3

1.57

21.520.0

7207

Barrel

10,3209,600

Gallons per minute:Metered .Measured .Difference:

Number .. 1..•.•...Percent .. l' .

Gallons per 8-h shiftMetered _Measured .Difference:

Number ..Percent ..

Water flow rate measurement atrinse tanks can ?e performed simplyusing one of several methods. First,a bucket with a !predeterminedvolume can be set under the over­flow from a rinsf tank, providedrinse water is discharged through anaccessible vertidal pipe. The time ittakes to fill the bucket is measuredand the flow rata calculated. Oftenthis method canpot be used becauseof the location or construction ofthe discharge Piipe.

An alternative for measuring waterflow rate is to s.~ut off the incomingwater and remoJe a specific amountof water from the rinse tank [5 to10 gal (19 to 38 L)]. The water isthen turned backII on, and the time ittakes the water level in the tankto return to its overflow height ismeasured.

Table 8 shows an example of awater balance in which the plant hadindividual water meters installedon each line. (Whereas manyplants have meters only at the watersource, individual meters shouldbe considered for their value inmonitoring and controlling wateruse.) The actual, or measured, flowrates are within an acceptablelimit (15 percent or less), indicatingthat there are no unforeseen waterlosses.

lines are used, the efficiency of theoperators in rinsing and drainingshould be noted. Also, if the tankarrangement requires that racks orbarrels be transported betweentanks that are not located next to oneanother, the relative amount of drip­ping onto the floor should berecorded.

Examine Process Water Use. Asurvey of water use is a basic step inthe plant assessment because thecapital costs of water pollutionabatement equipment dependprimarily on the volume of waterused. Wherever water is used,therefore, accurate measurementsmust be taken. To increase theaccuracy of these data, the platershould start by reviewing past waterbills to determine an expectedwater use rate in gallons (liters) perday and per minute. The platershould then measure and record theactual water use at each processstep. Because most of the water at aplating shop is used in rinsing,rinse flow rates must be measured.Then, a comparison of actual flowversus metered flow (that is, waterbills) can be made. This compari­son is referred to as a water balance.

Often, however, measured water usediffers considerably from waterbills. One potential source of varianceis faulty water meters. Thisproblem can be resolved by request­ing the local water authority tocertify the water meter. A discrep­ancy is more likely to result from

analysis during the plant assessment.Samples also should be taken of allindividual rinse tanks, overflows,batch dumps, and, to some extent,plating solutions. Plating solutions aresampled when calculations ofdrag-out are needed. These addi­tional samples will be used to isolatesources of pollution, to calculatechemical losses, and to evaluate thepotential benefits of drag-outreduction and flow minimizationtechniques.

Sampling can be performed by takinga single, or grab, sample of theeffluent or rinse water. A grab samplegives an instantaneous reading ofthe conditions of the water. Ifconditions are variable (for instance,if plating is intermittent, causingfluctuations in pollutant concen­trations), the sample is not likely tobe representative. When variability issignificant, it is necessary to com­posite samples over a period of time(usually one sample every 15 to30 min over a single shift or a singleday) and to analyze the com­posite to determine the averageconditions.

Compositing is recommended forsamples taken of final effluentand individual rinse tanks. Grab sam­ples will suffice for batch dumpsand plating solutions.

Electroplaters conducting their ownplant assessments will needsampling containers. Often the con­tainers can be secured through thelaboratory performing theanalysis. If they are not availablefrom that source, appropriate con­tainers can be purchased from ananalytical supply house. Because theanalysis will be limited to pH,metallics, and cyanide, eitherplastic or glass containers can beused. The plastic containers cannotbe used if trace organics are tobe measured. The appropriate plasticcontainers are made of polypropy­lene or polyethylene and should havea volume of approximately 0.26gal (1 L).

24

If metals are to be analyzed, it will benecessary to collect at least 0.13gal (0.5 L). Usually this samplewill be enough to analyze for the sixfederally regulated metallic pollu­tants in the common metals electro­plating subcategory.' If cyanideis to be analyzed, an additional 0.13gal (0.5 L) of sample must be takenand placed in a separate container.Cyanide is relatively unstable andmust be "fixed" once it reachesthe laboratory.

Samples should be delivered to alaboratory as quickly as possible toretain the accuracy of the analy­sis. Speed is especially critical withcyanide samples, which shouldreach the laboratory within 24 hours.

The sample of the final effluentshould be taken at a point where allrinse waters and other platingwastes are combined but before theintroduction of domestic andother process wastes. Often thispoint is not accessible to manualsampling, so it may be necessary torent a compositing device.

The final effluent analysis includespH, all metals being plated,base metals, regulated metals, andcyanide. For shops plating over10,000 gal/d (38,000 L'd), the analy­sis usually includes pH, chromium,copper, lead, cadmium, zinc,nickel, iron (a base metal), andcyanide; the cost of this effluentanalysis usually ranges from $115to $180.

Rinse tanks are sampled to deter­mine whether a smaller overflow rateis possible and to isolate thesources of pollution. Also, samplingof rinses following soak cleanersand acid dips may provide datashowing concentration levels ofpollutants below the standards; suchrinses would not need furthertreatment.

Because pollutant concentrationsin the rinses fluctuate, a compositesample is advisable. A compositesample can be taken by runningplastic tubing from a rinse tank to acollection container and placing aclamp on the tubing. By creatinga siphon with the tube and con­trolling the flow with the clamp, asample can be taken over the timeperiod of a shift without constantattention. Care must be taken, how­ever, to avoid positioning the end ofthe tube in a "dead" zone of therinse tank, such as a corner. If thetank is not aerated and fully turbulent,the end of the tube should beplaced in or near the overflow.

A complete analysis usually is notnecessary for rinse samplesbecause it can be assumed, based onknowledge of the manufacturingprocess, that some metals orcyanide are not present. By review­ing the plating operation, a signi­ficant savings in analytical workcan be achieved. The analysis,however, should include any metalsand cyanide if they are presentanywhere on the plating line wherethe rinse is located, even when therinse precedes the plating of aspecific metal. Plating' solution oftenremains on racks, especially if theyare worn, resulting in' contamina­tion of process solutions, suchas soak cleaners. The concentrationof chromium, for instance, canbuild up to such a level in asoak cleaner that the rinse followingthe soak cleaner hasa chromiumconcentration above 5 mg/L.

Drag-out measurements are per­formed at plating tanks to determinethe amount of metal or cyanidecontributed by these sources. Thisinformation is used to evaluatethe applicability and potential bene­fits of drag-out minimizationtechniques and innovative rinsingsystems.

,Because drag-out volume is affectedby the size and shape of the partsand racks or barrels, measure-ment of drag-out on a particular line

with significant variability ofthese factors will have little meaning.Where variability is extreme, theplater should refer to typicaldrag-out rates found in the literatureinstead of producing meaninglessdata.

The method of drag-out measure­ment described in this reportdiffers slightly from those presentedby Kushner," The first step in thedrag-out measurement is to stop theflow in the rinse tank followingthe plating tank and empty its con­tents. If the rinse tank is a still rinseor drag-out tank, it should be con­sidered as a plating bath and therinse following it should be used.

After filling the rinse tank withclean water, the water should beshut off and a sample should betaken and marked "Time 0." A sampleshould also be taken from theplating tank (or drag-out rinse, ifapplicable). The plating line shouldthen be operated. After three tofive racks have gone through,a second sample of the rinse shouldbe taken and the number or racksand elapsed time recorded. The lineis again operated and severalmore racks are plated and rinsed. Athird sample is taken and thenumber of racks and elapsed timeare recorded.

The plating solution sample and thethree rinse tank samples shouldbe analyzed for the plated metal. Thedrag-out is then calculatedbased on unit production [gallons(liters) per part or rack] or time[gallons (liters) per hour] for the twoincrements and is averaged.

Identify Batch Dump Parameters.Process solutions, such as alkalicleaners and acid dips, become ex­hausted after a period of use.These solutions are routinelydumped to the sewer, usually on aweekly or monthly schedule.They often contain significantconcentrations of heavy metals and,therefore, will require treatmentby January 28, 1984. Also, these

solutions are by hature eithervery basic or acidic and can cause

I

a significant pH fEluctuation inthe total effluent when dischargeddirectly to the sel er. Suchchanges in pH often result in non­compliance with ocal pH standards.

Attention should be given to?atch dumps co.ntCerning theirfutureImpact on waste treatmentchemical require I ents and theircurrent impact 01 compliancewith local pH stal.,dards. In terms ofanalytical paramEjters, batch

~~;~~Is~~~~1sb:hl~a~~~:d:o;~r:~_idetial for accumulatrng in the solution.A separate sample should be taken todetermine pH ad ustment require­ments.

The adjustment of pH on batchdumps is performrd by adding eitheran acidic (usuallYI sulfuric acid) orbasic (usually caustic soda) solu­tion until an acc~Ptable pH isreached. Local regulationsoften require tha~ the pH of dis­charges be in the range of 6.0 to9.0. The Federal ~eneral pre­treatment regulat ons15 require thatthe pH be above 5.0.

By determining thle relative strengthsof the alkaline cleaners and aciddips, the plater often can develop abatch dump sche~ule that usesthese solutions to neutralizeeach other. This practice will providea significant SaVirgS in pH adjust­ment chemicals.

Application. The information gatheredduring a plant aSEessment pro­vides the plating shop with the inputneeded to evaluate the opportun­ities for rinse reclovery. Thesedata are useful i1 assessing otherin-plant change~las well, suchas those designed to reduce .totalwastewater flow to less than10,000 gal/d (3 ,000 LId).

Case Study

The following case study is anexample of the procedures for per­forming a plant assessmentand evaluating optional in-plantchanges. The data are based on anactual plant's operation; however,an incomplete data base necessitatedthe assumption of certain param­eters. The plating shop in theexample performs mostly zinc, .cadmium, and tin plating as well aschromate conversion coatings.The analysis focuses on the shop'sautomatic cyanide zinc barrel line.

Data Gathering. The initial step inthe plant assessment or datagathering phase is to inspect theplating room and develop drawingsshowing the location of allrelevant equipment and tanks. Adrawing of the cyanide zinc line ispresented in Figure 8.

After sufficient information isgathered to prepare drawings, theoperating practices of the plantshould be observed. In thisexample, only one work sequence isused. It involves the followingsteps:

• Barrels are filled with parts atthe loading area.

• The barrels proceed through eachprocess and rinse tank (Tanks 1through 22) in a straight-line order.The observed time at eachstation was 2 min; the drainingtime was 15 s.

• Finished parts are unloaded at theend of the line. The observedproduction rate was 12 barrels perhour.

• The empty barrels are trans­ported back to the loading area.

It was observed that once the barrelswere removed from one tank, theywere not allowed to drain fullybefore immersion in the subse­quent tank. Another potentialproblem noted was the lack of airagitation in the rinse tanksfollowing acid dip and zinc plating.

25

Wastewater2 gal/min

Unload barrels

tHot rinse @

tSeries rinse @

A-~-(

"

Series rinse 03 gal/min

(25.0 mg/L)aWastewater

AIf

Chromate @

tChromate @

t®

Series rinse @

Cyanide zinc plating bath t::::\(1.34-gal/h drag-out at 86° F) \2..V

City water

City water

City water

Wastewater

Wastewater

3 gal/min

o 6 gal/min

Acid bath 0.i;:"

Acid bath 01i

Load barrelsII.11..

"'",

JII

Series rinse 0

Rinse(0.1 mg/L)8

Soak cleaner 0

Soak cleaner 0

Series- rinse V,«0.1 mg/L)8 \.V 4 gal/min

Electrocleaner0

Electrocleaner0

'Concentratlon of zinc.

Figure 8.

Cyanide Zinc Line Before Plant Assessment

26

Series

Series

®

side red for recovery. The chemicallosses from drag-out at Tank 1:3are 1.34 gal/h (5.07 L/h) ofplating solution. The replacementcost of this solution is approxi­mately $2,425/yr. b

With the current rinsing arrange­ment, the water use is 5 gal/minfor the rinses following the acid bathand zinc plating. This water userate is high considering the useof a series rinse. It is apparentthat the efficiency of the rinsingsystems is reduced because of poormixing in the rinse tanks. Chemicallosses from the zinc plating opera­tion are 1,280 Ib/yr (581 kg/y~

of zinc and 1,477 Ib/yr(670 kg/yr) ofcyanide, assuming one shift perday and 260 days of operation peryear.

bBas~d· on a value of $0.87/gal and operating2,080 h/yr.

Two alternatives were consideredfor improving the rinsing system(Figure 9). Alternative 1 involves

27

Two-stage recovery

Zinc plate

I Drag-in/drag-out

Zinc plate

(b)

(a)

Evaluation. The rinse water flowrates in Tanks 3 and 6 appearto be excessive, especially in Tank 6,which is part of a two-stage seriesrinse. The flows to these tanks wereregulated by hand valves, whichwere nearly wide open. Flowcontrol devices should be installedon the water feed lines to thesetanks. Tank 3 probably could operateeffectively at 3 gal/min (11 L/min)and Tanks 6 and 7 at 1 gal/min(4 L/min). Control devices arereadily available in these sizes. Twoflow regulators would cost approxi­mately $40 and could save about$1,500/yr in water and sewercosts.

Figure 9. 11Rinsing Alternatives Considered: (a) Alternative 1 and ( ) Alternative 2

A water use surveywas conducted by At lower flow ra es, the concentra­measuring tank flow rates. Samples tion of zinc in the rinse waterswere taken from the rinse tanks would be expected to increase toand the plating bath and were 0.2 mg/L and approximatelyanalyzed for zinc. A drag-out 0.4 mg/L. respedtivelv, for Tanks 3measurement was taken at the plating and 6. At these eoncentrations, thebath. The results from this work rinse waters are ftill beloware shown on Figure 8. Federal pretreatment standards and

will not require fetal removal.(Cyanide is not introduced untilTank 13.) Depending on local regula­tions, however, dH adjustmentstill may be necdssarv,

The major portioh of zinc pollutionfrom the sample Iplating line isfound in the rins1 waters dischargedfrom Tanks 10 and 14. Tank 10follows an acid d~ilp solution, which isdumped periodic lIy. Becauserecovery would nly shorten the lifeof this solution, iit will not be acandidate for dral~-out recovery.Tank 14 follows ~he zinc platingoperation and, therefore, can be con-

Water useRinse system

Table 9.

Evaluation of Case Study Rinsing Systems

Platingchemicalslost (Ib/yr)

Zinc Cyanide

1,280 i,477845 975294 340

1,248824824

53.33.3

gal/min 1,000 gal/yr

Table 10.

Current - .Alternative 1 , .Alternative 2 , .

moving the final rinse tank followingthe acid bath and convertingthis rinse and the three-stage seriesrinse arrangement followingplating to a two-stage recovery rinseand two-stage series rinse. UsingAlternative 1, approximately 34percent of the drag-out could berecovered. Air spargers also would beadded to the rinse tanks to in­crease the mixing and improve theefficiency of rinsing. The addition ofair spargers would lower the wateruse rate after zinc plating to0.3 gaVmin (1.1 L/min). The rinsesystem following the acid dip isunchanged in Alternative 1.

Evaluation and Cost Comparison of Case Study Rinsing Alternatives

'Cost of retrofitting tanks with air and repiping spargers ($200 per tank depreciated over 5 yr).

Note.-AII costs, except those for treatment chemicals, are in 1981 dollars, Costs for treat­ment chemicals, originally in 1979 dollars, were updated to reflect average 1980 pricesusing the Monthly Labor Review Producer Price Index for industrial commodities.

Current

Total cost. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8,843

Rinse system

Alternative 1 Alternative 2

3.3 3.3

1,648 1,6480 0

1,296 452152 53173 173972 338

1,597 5561,080, 1,080

160 280

7,078 4,580

control devices ($80). Consideringthe reduced water and sewercosts, reduced treatment and dis­posal costs, and the savings inplating chemicals, the benefits ofin- plant changes total $7,123/yr.

Parameter

Flow rate (gal/min). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Cost ($):

Water and sewerat$2/1 ,000 gal wastewater. . . . 2,496Additional rinse tanks at $240/tank . . . .• .. . . . 0Treatment chemicals at:

$1.33/lb cyanide. . . . . . . . . . . . . . . . . . . . . . 1,964$0.1 8/lb zinc. . . . . . . . . . . . . . . . . . . . . . .. . 230$0.21/1,000 gal wastewater. . . . . . . . .. . . 262

Sludge at $1.15/lb zinc. . . . . . . . . . . . . . . . . . . . 1,472Plating chemicals lost at $1 .89/lb zinc 2,419De-ionized water , 0Other"........... . 0

Recommended changes to the zincbarrel line are presented in Figure10. The investment cost of thechanges will be approximately $6,840,which includes installation of ade-ionized water unit ($5,400),retrofitting of tanks with air spargersand repiping ($10400), and flow

The drag-in/drag-out rinsing con­figuration provides 77 percentrecovery of plating chemicals. Thisrelatively high rate is primarily aresult of increasing the recycle ratiofrom 0.35 in Alternative 1 to 1.35in Alternative 2.

Tables 9 and 10 summarize thewater use and cost analysis of therinsing options. For this plating line,the best choice is the drag-in/drag­out rinsing system. The majorbenefits include a savings inplating chemicals and waste treat­ment and sludge disposal costs.

Alternative 2 employs the drag-in/drag-out rinsing configuration. Thedrag-in tank (Tank 12) wasoriginally the final rinse followingthe acid dip. To eliminate the needfor additional water in the acid diprinsing system, air spargers areadded to Tanks 10 and 11. Similarly,air spargers are added to therinses in the new zinc plate rinsingsystem.

28

Load barrels Unload barrels

29

Pump

Wastewater

Wastewater

@.

@I~-"Drag-in

Hot rinse @

Chromate @

Strip tank @

Cyanide zinc plating bath

3 gal/min

0.3 gal/min

Wastewater

Rinse

Acid bath 0

Soak cleaner CD

Soak cleaner 0

Electrocleaner 0

3 gal/min

1 gal/min

3 gal/min 4 0_3.....~ Wastewater

Figure 10.

Recommended Rinsing Arrangement for Cyanide Zinc L ne \

References

30

1 U.S. Environmental ProtectionAgency. Environmental Regulationsand Technology: The Electroplat­ing Industry. EPA 625/10-80-001.Aug. 1980.

2U.S. Environmental ProtectionAgency. "Effluent Guidelines andStandards; Electroplating PointSource Category Pretreat-ment Standards for ExistingSources." Federal Register 46(18):9462-9473, Jan. 28, 1981.

3U.S. Environmental ProtectionAgency. Environmental PollutionControl Alternatives: Economics ofWastewater Treatment Alter­natives for the Electroplating In­dustry. EPA 625/5-79-016.June 1979.

4Steward, F. A., and Leslie E. Laney."Minimizing the Generation ofMetal-Containing Waste Sludges."In U.S. Environmental Protec-tion Agency and American Electro­platers' Society, Inc. (cosponsors),First Annual Conference on Ad­vanced Pollution Control forthe Metal Finishing Industry. NTISNo. PB 282-443. Jan. 1978.

SKushner, Joseph B. Water andWaste Control for the Plating Shop.Cincinnati OH, Gardner Publi­cations, 1976.

6Sawyer, Clair N., and Perry L.McCarty. Chemistry for SanitaryEngineers. (2nd ed.) New YorkNY, McGraw-Hili, 1967.

7Wallace, A. J., Jr. "SolutionDragout and Rinsing After ChromiumPlating." Plating and SurfaceFinishing, 66(11 ):47-51, Nov. 1979.

aU.S. Environmental ProtectionAgency. In-Process Pollution Abate­ment: Upgrading Metal-FinishingFacilities to Reduce Pollution.EPA 625/3-73-002. NTIS No.PB 260-546. July 1973.

9Konishi, Saburo, and MitsuakiTadagoshi. "ChromiLlm Plating FromLow Concentration Baths. Part I.Effect of Bath Compositionand Plating Conditions on Appear­ance." Metal Finishing. 7'1 (11):49-52, Nov. 1973.

10Lowenheim, Frederick A.Electroplating-Fundamentalsof Surface Finishing. New YorkNY. McGraw-Hili, 1978.

11 Laney, L. E. Metal Finishing,49(2):56, 1951.

12Pinner, R. Electroplating and MetalFinishing, .Julv-Auq--Sept. 1967.(3-pt. article)

13Roy, Clarence. "Methods andTechnologies for Reducing theGeneration of ElectroplatingSludges." In U.S. EnvironmentalProtection AqencvandAmerican Electroplaters' Sqcietv,Inc. (cosponsors). SecondConference on AdvancedPollution Control for the MetalFinishing Industry. EPA600/8-79-014. NTIS No. PB297-453. Feb. 1979.

14Crampton, Peter. "The Applicationof Separation Processes in theMetal Finishing Industry." In U.S.Environmental Protection Agencyand American Electroplaters'Society, Inc. (cosponsors).Third Conference on AdvancedPollution Control for the MetalFinishing Industry. EPA 600/2-81 -028. Feb. 1981:.

lSU.S. Environmental ProtectionAgency. "General PretreatmentRegulations for Existing and NewSources." Federal Register46(18):9439-9460, Jan. 28.1981.

* u.s.GOVERNMENT PRINTING OFFICE. 560-565 (RI