3910 I . P ~ F cation of

romium Platins Baths

@ 9 4

incorrect to assume that the tech- nology is new. The need for purifica- tion became manifest soon after the

particles in solution was identified even though surface preparation in- cluded electropolishing, rinsing, swab- The most valuable

commercialization of chromium bing. and anodic reverse-cu rrent benefits of purification? plating. cleaning in a separate tank. Magnetic

The earliest methods were aided by ~ ~ a ~ ~ ~ a ~ ~ b ! ~ ~ ~ ~ ~ ~ ~ ~ a c h- strong exhausts and scrubbers, tanks m&w.tcr. t h w m r l e surface and coils that leaked too frequently, @atinCbWcl~ bxhihis and accidental overfilling in the p a d i ~ ~ b ~ U @ ' ~ @ t h @ ' e n d * o f the absence of automatic lev$ controls. bUOrkbecause* Passaga of These "natural" losses significantly convat* the =et bode ntoa reduced impurities but lost useful p a g r f u f dectrbmagnet. The prob- chemicals. wasted energy and polluted lemscreated by magnetic particles are the environment. In spite of this form always aggravated by increased Cur-

that "nothing ever changes." of control, it was found necessary rent densities and increased SOlUtiOn to take additional measures to clean agitation. These normally Useful Con- and purify the very tolerant plating ditions keep the Particles Suspended. solutions. It was unfortunate that the authors

of this otherwise-instructive paper Undisso'ved"solid~whicbpmduce combined their study of filtration with electroplating. Some of the most sig- an equally worthy attempt to oxidize

mWauG partic

trivalent chromium with potassium nificant changes have been hastened demit& Particl plated areas), POOP corrosi

permanganate, according to the fol- tiistanqe,&talfr rough ness and, lowing equation:

ntcolted"periodicallw by-allow- i ~ ~ t h e ~ t ~ ~ s ~ t l ~ f o ~ ~ f e ~ d ~ y ~ . The clari f ied solut ion was carefully pumped to another tank while the

here's a TV commercial that begins with the proposition

It goes on to prove that "everything changes." In no industry have the effects of environmental

T

2KMn0, + Cr'' + H2Cr04 =

2MnOr + K2Cr:07 + H:O olids and a Small volume Of

aining solution were cleaned e bottom of the tank and dis- - When time permitted$ the bath was held in the holding

enough to allow more solids

The need to remove the precipitated MnOr provided additional incentive to develop effective filtration equipment and methods. However, it led to an unfortunate conclusion, from their data. that filtration oroduced a sia-

le and the decanting procedure llowed again when pumping the

to the cleaned tank. . of the more progressive al platers investigated the

ages of filtering chromium ns, prompted, in part, by a

r precision, high-quality, finishes in the late 1940s Os. In 1955. Ledford and

- r - - - _ - _, - - nificant reduction in the electricai resistance of the plating solution. This conclusion resulted from the fact that they only examined solutions that had been treated previously with KMnO, prior to filtration. While the results showed the efficacy of the two-stage process in reducing the resistance of the solut ion, ignored was the probability that most of the decrease

ilbert clearly identified some of the urces of contamination in chromic id solutions and their effects on the posit.' The presence of magnetic

i n resistance resulted from the oxidation of Cr" to Crb*.

In spite of this confusion, the case for filtration of chromium solutions

PLATING AND SURFACE FINISHING

'1 1

\

1' f . ,

oxidation of trivalent to hexavalent

eases the cathode cur-

the purification opera

r temperatures

C e a o n : Dilute solutions are more efficient than very concentrated plating baths.

0 Anodclrbtrrid: Lead alloy suitable Recently, the sources and effects for chromium plating. various contaminants found in hexa-

* Current Density: A s seems valent chromium plating solutions were discussed.' Trivalent chromium, reasonable, a given amount of current

Wi l l reoxidize more Cr'' to Cr" in baths it was noted, was recognized many

with increasingly high concentrations years ago as an important con- of the former. taminant. Electrolysis with lead

anodes was found to the These data, and the conclusions detrimental Cr" ion to the required hexavalent form. In 1948, Seegmi l le rp rawn from them, have proved to be of

nfvdent R c o w i ~ o n

and Lamb published a paper that has served as an excellent text on the removal of Cr" from chromic acid baths by electrolysis.' The study was undertaken to determine the effect of sulfate on the reoxid

reduction efficiency at

conditions frequently and net oxidation efficiency of a cell without a diaphraqm. . -

The conclusions drawn from this o&ooked by the work, using a 250 g/L (33 ovgal) CrO> solution in a diaphragm cell. were as % harried plater Or

man.

aff ected.appre&bk~m-- si% when-themticr*d.Ce%W&j%

lrrwer great value in the effort to improve the . effectiveness of electrolysis for purify-

Atq the- anads, Xh$TfSfI&Ra@ C r i ing chromium solutions. They provided the information required to minimize he contamination of these baths with

okaff by the sulfate iorpco

chromium and found that this reaction would be promoted efficientlywithout a diaphragm or porous pot. This does not diminish the value of these Cells for the removal of iron, copper or other heavy-metal contaminants that may also require periodic removal.

A resurrection in the use of elec- trolysis is the subject of a separate section. First, we wi l l address purification by ion exchange.

the article is the further increase in cur e@tsti~&, thaholding-tankpriar to returrr to the little effect. gelectrolysis. I t isalso oneof the platingtanks. Solution ta be used for

tly overlooked conditions by maintenanca will requiraa lower SUI- ciency was attained in a conventional the harried plater or foreman setting fate- concentratiom thaw the. plating Plating cell without a ons bath-Thesulfatemay be reduced with the result of the two do gjaCOJ*Th*BaSO, precipitate should Wses), underthefollo me be removed co";npletely from the

holding tank before processing the next solution to avoid introducing the precipitate to the resin bed. Filtration

tiom: during solution transfer will help keep the bath and the equipment clean.

be

Cathode Cumnt' b

Ion Exchange This process, asapplied to purification in chromium plating operations, con- sists of p-kh

exchan P colruzrrr.~~.'l;hs solution ma)lhemdectan&dilutect in eholdmg

column? taremove the metallic con- taminantstamacceptable level.

Thscation-ange resin isusually chsulfonated'crosslinkecfcopolymer of

macracst icul

diameter) pmvidaa the maximum resistanceto physical breakdown and Oxidation effects. Theuniform physical shape gives less resistance to flow and long resimflte. These advantages are accompanieci by some loss in capacity. Theadditional resin and larger column are usually justified by a reduction in operating costs and resin replacement. The conventional crosslinked gel- phase resins have been used success- fully for many years in chromium so lu t ion pur i f icat ion, and the advantage of increased capacity may still justify their use for some instal- lations.

Although the specific procedures may vary from one manufacturer to another, the general steps involved with the IX procedure are listed in Tablel. To expand on Step 9, & i s essentiakthat the purified solution be analyzed.campletely, and adjustecf in

- AUGUST1986 b.. , , ,,, . . . -.. . . . 27

I

-?

P

. I

rs and the reclaim tank are quite dilute. In small in- s, they may be concentrated

water usage. The water that is evaporated may be condensed and used for final rinsing and for makeup

.. of new solutions. Whether evaporation

gy and capital invest-

ave never felt the need

' tion dragout rate is low and work is ' ' frequently drained and rinsed over the .'*plating tank. However, the need for ' pollution control and reduction in

water usage have encouraged the use

techniques can be utilized to-maintain conmtrate-,?!ch. lar&W voiumes Of

the purity of chromium.'plating solu- .'water are: tions, -mchW$c" ' t?Th&.us,e_of. more dilute- plating remow ~ a ~ T ~ ~ t f l l m ~ ~ t i ~ ~ n y - solutlonsoperatlng'Between..l80 and the"rac1aim't tnsc~~scrubb~r 24(r'.@L(24 e n 6 3 2 dgal l .C lean eftfuentPlt can sometimes eiimin e may" 6e"operated satis-

atloKCr03 concentrations.

impurities by feeding back ed rinsewaters will ac-

the plating bath. This may tend to defeat the major advantage of purifi- cation, which is to obtain uniformly high-quality deposits. The decision to retain impurities and reduce the volume of solution to be purified orto keep the bath as clean as possible and use concentration methods is not clear cut. Every installation requires an evaluation of the pros and cons of each technique.

IXCwHbtoy The cation-exchange unit installed at the Rock Island Arsenal, Rock Island, IL: was one of the first I X systems for purifying chromium plating solutions. The decision to use IX was based on the need to reduce pollutant dis- charges to the Mississippi River and to improve the quality of chromium plating to meet more rigid specifica- tions.

designedfta treat 1000 t"i&-ttaminated chro-

miumsoiutiorrat5000gal(19,OOOL) of spentchromicacid anodiringsolution. The sulfanated4 styrene diary1 trenzidene resinthat was used required that the platinssolution bediluted.to

0-1 25 g l L C 0 (13-27 ozlgal) rapid oxidation of the, resin.

The installation was reported to be both technically and economically successful. It ellminated the need to decant or dump either the plating or anodizing solutions, solving the pol- lution problem.

The Cpconcentratlon in thsplating bath wasreduced regularly from about

from 1% tcr. 0.6, g/k (0.13 to. 0.08, adgal). Consistently highquality deposits were obtained. It is interesting to note that it was the filtering effect of the sand bed support- ing the resin that led to the arsenal's later work on the effects of filtering chromium solutions.

The saving in.chromic acid alone was reported ta beabout $18,000 in? 1 months. for ani operating; expenditure ak$22OOr. Ths-$l5,800~savlng did not include &s 25.

value attach&

pmh*eir& an& installing"thel~3lp- overed attrac-

tiveness of the system would have been much greater if the cost of any alternative form: of waste treatment had been considered. These additional savings would have more than offset

al) tO: 1. glL (a13

PLATING AND SURFACE FINISHING

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HING

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previouslL availabl

the~qation~'Q,a[esin~ from degradqion. The design alscr. 'iequired an: evaP

doubtedly Would res by eliminating all sources of or the pdf ied savings but r a i d th owever, that

ustive tests conducted on a e ions from the system. Chem- orrosion, promoted by the - - -

of oversized equipment is '?reduced PH and the low cation con- modified cation-exchange resin with a obviously higher Capital investment. I centration Of the purified Solution, is higher crosslinked structure indicated T ~ @ a n t t i g e S . O * ~ ~ ~ @ ~ s ameliorated by repk ina only a por- that the practical life of the resin was

tiOn Of a bath at one timeTThe purified solution may be concentrated and

costs arweduced. 4. There, is increased

mpansiorr of t h e platin purchase an IX unit must be guided by ,,without an additional Capi

iture. for purification. - .co"nY The decision to purchase an IX unit

of a siven size or type must be auided by the requirements, capabilities of each f plier's recommendat given serious consider proven experience.

approach has the the tanks. In this case, the

Id be replaced with new chromic and the sulfate adjusted as

us examples, the chro- was diluted because oxidized quite rapidly

b*~uni for rm schaui tinn -.".I.

I

sufficient to permit direct treatmentof the undiluted bath at 250 UL."

By eliminating the evaporator and the larger tank capacity, the initial cost was reduced by more than two-thirds. The IX column contained 8 ft' of the modified resin and was designed to handle 500 gal of plating solution. Purification was performed weekly when the meta l l i c impur i t ies approached 3 g/L (0.4 oz/gal). Treat- ment costs were based om one cycle that removed 5670 g- (200 02) of metallic impurities. (as CaCOJ equiv- alent). The materials, 10-year depre- ciation, resin replacement, and direct labor costs were estimated to be $16,80/cycla vs. $316.40 for the value of the chromic acid bathmaintained at peak efficiency.

Paulson and Saunders' reported that a similarly modified sulfonated polystyrene resin" showed no physical or chemical breakdown using 400-g/t (53. oz/gal) CrOl solutions. Their tests demonstrated that, within limits, higher concentrations of re- generant acid increased the capacity of. the resin. They also showed that resin regenerated with hydrochloric acid had more capacity than that with sulfuric acid. The higher capacity achieved with HCI can reduce dilution or loss of solution slightly, depending on the startup procedure. (Authors' note: The higher cost of HCI and the risk of introducing CI- ions to the plating solution make it a poor choice. Sulfuric acid should be used for

'Nalcite HRD. Nalco Chemical Co.. Oak Brook.

**Permutit ac. ~ ~ m n r ~ c o : . prvun~. N$ a c m n o milu-resins

may bs obtained from Rohm & HMS k, Philadelphia Oow Chemical, Mkfknd, MI. OT the varloutequipment suppliers.

IL

29

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' .;'. ,'puflrid3!5~~&W,QOQto metals then are precipitated as '? ,' 133,,ah,nb@&&@!!&tip- hydroxides. or sulfides.and filteredto ,: t r e a t m e ~ ~ o n a R e s i n a i M a : w a s collect thesludge. Aldrich' has shown

that the use of sodium sulfide and ferrous chloride at pH 8 can substan-

g/L (1.1 tially reduce the volume of waste lume of sludge. Insomscases, it may be pos- became sibla. ta use the regenerating acid for

pickling, steel or acidifying chromate west& prior ta reducing. it, to the tri- valentstate. The cost of operating the

used for mainte usted cation-exchange unit obviously will be with new chromic acid a6d used to reduced by any additional use for the replace the next contaminated bath. acid. The installation was designed to handle diluted solution, so the addi- tional volume required no new equip- ment.

Visit" rest one installation with capitalinvestment of catiori&-=ge

purification, i f , fhq&+u ipment Is designed to handle lowsconcentration solutions with little orna>dilution. If any dilution is desired, the heavily

am operating pufifidon

the recentlyz introduced

the tank,. diluted to I ) , and batch purified

returrrtothe plating the next cycleuThe

ca~$&tp! the unit is stated as 20,000 & (Z6,OOQ a %sin life of,6000 hr of operation.

It is strongly recommended that anyone considering the purchase of any type of purification equipment try to visit at least one installation with an operating system. The best sites are those that have been in operation for a year or more and that are processing approximately the same volume of solution the potential buyer expects to treat. If it is not possible to arrange such a visit, i t is certainly advisable to ask for the names of responsible per- sonnel at a similar site who can answer pertinent questions.

g2u..th

Elccbotysb ~urrec t ion This very old method of chromium solution purification has been resur- rected and now forms an important segment in the growing variety of systems offered today. Like cation exchange, i t is used primarily to remove (or oxidize, as in the case of Cr'+) undesirable cations from solu- tion. It may be used in combination with other systems to optimize cost and effectiveness.

A simple electrolysis cell or series of same may be set up in the plating tank or in aseparate tank. If the plating tank is used, care must be taken to prevent the current from flowing from the electrolyzing anodes to the outside of work being plated with inside or con- forming anodes. Baffles or com- partments are quite effective for this purpose. A separate rectifier is recom- mended. Maintaining higher tem- peratures in the compartments or separate tank will also improve the efficiency of the oxidation. Such a system using a large (e.g., 30:l) lead- anode-to-cathode area ratio could greatly reduce the required frequency of IX treatment, especially if most of the work were inside (internal diameter) plating. It would also reduce the amount of chromium that would be lost as Cr'+, to the IX unit:

I f a separate tank is used, it should be set up so that all of the solution must pass between the anodes and cathodes before return to the plating tank. If circulation is achieved by a suitable pump and filter, this will pro- vide a relatively inexpensive but effec- tive basis for a more complete system that may be added at a later date.

The addition of a porous pot 01 membrane to the electrolyzing cell can add significantly to the effective ness of the system. By placing the

PLATING AND SURFACE FINISHING

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E c t r Q:

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a F t a L

A

1 ia t * of C- ,

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3r a i n g

Of sto nge e to per- .wer

\ium ?sur- rtant Y of ation lY to se of solu- ation cost

ies of j tank 3 tank

I . ..le ide of r con- com- )r this ecom-

tem- -tts or ve the uch a ) lead- could

luency lost of ternal reduce would

P

ing solution are reoxidized to

combines some of the istics of IX and e k -

and/or carboxylate groups.

than justifies the Op

rified is pumped through the bdiscounted asa cost"ltem.

c

?-

P

h the cation-permeable mem- into the catholyte and the solution with a

designed to be hung on the the Plating tank. One or more

the hexavalent state and remain in

emically very resistant to oxidation d can purify chromic acid solution

to the tank anodes and cathode. The only service

unately, the membranes are percent efficient, so a small of the chromate will leak into

it is the buildup of the anions ting bath that limits the life of

of metals or their hydroxides.

These can be to concentrat describes the and carboxylic acids and th that decompose to harmless in the plating bath.

Technically, the electrodialysl tem is feasible but the cost data fr pilot operations have not encouraged widespread use to date.

b l g n Anlom It is fortunate that foreign anions are not encountered frequently, and rarely in large quantities, in functional chromium plating. The two most commonly faced-chlorides and sul- fates-in excess of their desired concentrations can be removed by precipitation.

Barium carbonate (BaCO,) reacts with chromic acid to form insoluble BaCrO., but, because barium sulfate is less soluble than barium chromate, the sulfate is preferentially precip- itated. The chloride ions can be precipitated with silver oxide but this expensive remedy is rarely needed in functional chromium plating. The high current densities employed usually keep chloride under control by oxidizing the ions to chlorins gas at the anodes.

In decorative plating, where lower anode current densities are normally used, drag-in of CI- ions from the nickel tank frequently become exces- sive. In such cases, the precipitation of silver chloride is a "quick fix" for the problem. There is no such quick fix for most of the other common anions: Nitrates, phosphates, borates and fluorides are difficult to remove from the chromate solution. Consequently, every effort should be made to avoid contamination by these ions!

One procedure applicable to dilute solutions (e.g., rinsewater) involves reducing and partially neutralizing after cation purification. The solution then may be treated with a sodium- regenerated anion-exchange resin. When the regenerant solution con- taining sodium chromate, sodium sulfate, etc., is given a second cation- exchange treatment, the sodium chromate is converted to a more concentrated solution of chromic acid. The cationic resin remains charged with the sodium ions, which may be replaced with hydrogen by regen- erating the resin with sulfuric acid and discarding the sodium sulfate solution.

While this process has been used for small volumes of diluted plating elec-

31

may not b is will probably reduce the acceptable for a moderate volume of cost of purchasing and operating the ,. + ,$ ' ''.= chromium solution at normal con- new system. centrations. e most valuable benefit from

ion purification-and perhaps the "my appreciated-may well be the It has been our intent to provide significant improvement in the quality enough information on the value osits and the attendant reduc- purifying chromic a encourage industrial to consider it seriou tried purification years not satisfied with the re ford and L.O. Gilbert, it is hoped, given enough detail about various systems to j a new, 2. E.C. Knill and H. Chessin, Plat. and better-infbrmed trial.

Even the best purification system 3. R. Seegmiller and V.A. Lamb, Proc. will not be satisfactory unless it is 4 p € S 35th An. Conf. (1 948). properly applied and maintained. A L.O. Gilbert, W.S. Morrison and knowledgeable plater can greatly F.H. Kohler, Proc. AES 39th An. reduce the cost of any purification Conf. (1952). system by eliminating or at least 5. E.B. Tooper, Plating, 42, 1416 minimizing the degree af contamina- (lass). tion. A reduction in scrubber and 6. J.C. Hesler, Proc. AES 42nd An. rinsewater consumption will simplify Conf. (1955). and reduce the cost of purification and 7. C.F. Paulson and G.H. Saunders, waste treatment. Filtration and/or Proc; AES 40th An. Conf. (1953). electrolysis may be adequate to im- 8. J.R. Aldrich, Met. Fin., 82, 51 (Nov. prove the quality of plating in some 1984). cases. If additional purification is 9. R.F. Ehrsam, US. patent 3,909,381 required, the use of filtration and (1975).

Plating, 42, 1151 (1955).

Surf. Fin., 73, 24 (July 1986).

-~

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Platini andhodizing Assemblies Accessories, Parts and Supplies Pumps, Filters and Heates Modular Plating Systems-Manual and Automatic

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AbOUtUUAUthon Edmund C. Knill recently retired as a research associate for MBT Chemicals Inc., Rahway, NJ 07065-0970. H~ joined the company in 1970 after serving in various capacities, includi technical director, for the Chromium Corp. of America over a span 30 years. Mr. Knill holds a EChE degree from Clarkson College and is a member of the AESF Newark Branch.

Dr. Hyman Chessin is a senior. scientist at M&T Chemica R&D on chromium plating and hole more than two dozen patents. ~ r , , a

. . . ~

earnea r l f s riiu uayree in pnysiaa chemistry from Weste University. He holds membership in the AESF Detroit Branch.

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PLATING AND SURFACE FINISHIM 32