TECHNICALBULLETINTF-151DEC. 1997

Filtration and Plating Quality

Working backwards with filtration to eliminatebarriers to high-quality plaUng.

Did we ever really have "quality" plating? In somecases we did. One can find examples of plated automo-tive brightwork and appliance or plumbing parts done inthe 1950's and 1960's that still look good today.

Was the plating good because of heavy deposits anda lot of polishing and buffing? Was it due to proprietarysolutions or laboratory technicians who sniffed and ana-Iyzed the process and were able to make it work right?

We also know about failed plating. Why did the shinyfenders on our bicycles, or the bathroom faucets, or thosenuts, bolts or screws end up rusty? Were buyers whodemanded low-priced products that looked good at fault?Could manufacturers "get by" by offering marginal qual-ity because the price was right? Perhaps, but today'sconsumers are demanding higher and higher levels ofquality.

How can high-quality plating be achieved? Betterchemicals? Purer raw materials and anodes? Better pro-prietary additives? This certainly is a starting point.

Is a more uniform electrolyte or electroless solution

"-'" the answer? Obviously so. Whatever the requirements,solutions must be maintained day after day, hour by hour,minute by minute. Statistical quality control dictates theneed to know the conditions of the plating solution at theprecise moment that plating will commence and know thatan acceptable condition will be maintained during theentire time the part is in the tank. Platers need to knowthat the parts per million of insolubles (dirt particles) arebeing maintained at the lowest possible level to ensurequality results.

Those involved in highly sophisticated plating appli-cations, such as computer memory disks, seek the ulti-mate in quality. They cannot tolerate co-deposition ofsolids or accumulation of organic impurities. All platerscan benefit from understanding their techniques and dedi-cation to "clean solutions."

Some have described various methods of filtrationand carbon purification as "necessary evils." This cer-tainly is not the case with today's improved equipment.Unattended filtration with minimum media changes ispossible with very little solution loss or labor required.

Gone, forthe most part, are layers of sludge, carbon-ates and super-saturated brighteners lying on the bottomof alkaline cyanide zinc tanks or murky solutions of cop-per, nickel, silver or cadmium. Instead, plating is and canbe done in solutions often clean enough to call heads or

'-' tails on coins lying on the bottom of the tank.Previous articles have stressed the importance of

increased solids-holding capacity brought about byvarious grades of filter media, and the advantages of

increased flow rates in achieving fast particle removalfrom a plating tank. However, this article will stress theadvantages of preventing particles from getting into theplating tank in the first place.

START WITH THE CLEANER. Special attention tothe cleaning cycle is perhaps the best place to start. Evenplastic parts that appear to be clean may have siliconemold release on their surface. Therefore, the propercleaner with vigorous agitation in what was formerly astatic tank, may be appropriate. Filtering the cleaner withan appropriate coarse media will maximize solids hold-ing capacity and lengthen the cleaner's active service life.Should a layer of oil develop, it can be removed by decant-ing or skimming when the solution is not being agitated.Additional oil may be removed with coalescing media thatwill separate the non-dissolved oil from the aqueouscleaner. A prefilter may be required to keep the coalesc-ing element free of solids.

Subsequent electrocleaning solutions followed byvarious rinses can also be clarified in this way with theaddition of a chamber of carbon to adsorb oil. (Manufac-turing process oils should never reach your plating solu-tion.) As a final precaution, pre-rinses may require ionexchange to pick up soluble salts. Reverse osmosis maybe required when troublesome salts might be present inrecycled rinse water.

A skimmer on the pump at each tank in the pretreat-ment cycle will minimize carryover of surface contami-nants to the next tank.

ANODES AND AIR. We now have probably pre-vented 50-60 pct of solids and other impurities fromgetting to your plating tank. What else can be done toprevent solution contamination? Quality of anodes,makeup water and chemicals should all be considered.Even tlie air that passes over the tank to an exhaust ventmay be dropping solids. It is also possible that air usedfor agitation contains insoluble particles that can get intothe tank. The air also might carry vapors from other pro-cess operations. These can be absorbed into the platingsolution with the help of wetting agents.

"AIRLESS"AGITATION. Another method of agitatingthe plating solution uses high-flow centrifugal pumps thatdraw solution from the tank and re-deliver it through asparger system similarto that used for air agitation. Educ-tors strategically placed along the horizontal pipe directplating solution across the bottom of a tank or up a cylin-der or into difficult-to-plate, low-current-density areas.Each eductor creates, without additional horsepower, upto four times the actual pumped liquid being delivered toits orifice.This means instead of one gpm, you will be cir-culating four additional, or up to five gpm with the horse-power requirement of only one gpm. .

This agitation method has a number of advantages,as follows:. Elimination of vapors being introduced into the plat-

ing solution.. Elimination of uncontrolled temperature changes.. Elimination of air bubbles entering the suction lines

of centrifugal pumps that could cause them to cavi-tate and lose prime.

. Minimizing brightener breakdown due to oxidation.

. Elimination of salt crystal formation in the holes of thedispersion piping.With pumped/eductor agitation, we now are plating

with a solution that is totally self-contained, where mini-mal solids or vapors can get into the solution and tem-perature is more easily controlled. We now can proceedto filter the solution to remove any particles that slippedby the barriers and add the necessary carbon to removeany brightener breakdown. However, the amount of car-bon will begreatly reduced and less usable brightener willbe adsorbed.

REDUCING FILTER MEDIA COST. There are stepsyou can take to reduce filter media consumption.1. Prefilter as much as possible with preventive barri-

ers as pointed out earlier, plus carbon adsorption ifrequired in a separate chamber, then,

2. Use high flow rates with coarsest possible media toachieve maximum dirt-holding capacity. Example:three-micron instead of one-micron, or 30-micron in-stead of 15, but not 1OO-microninstead of one-micron.Increase filter media so that flow rate per cartridgeor square foot is low, thus reducing velocity acrossthe media. (Example: four cartridges instead of onewill reduce media consumption by 55 pct). In otherwords, 12 cartridges instead of three with the samepump will consume 50 pct less filter media annually(Table below).

3. Use pump with eductors to minimize solids introduc-tion to the bath.CHOOSING A FILTER. The choice of a filter to

achieve the final clarification will depend on a number offactors - how much carryover of particles occurs on theproduct to be plated, or what amount of insolubles arebeing introduced from tainted anodes, the atmosphere,chemicals or any other source.

Will the particles be slimy as in an alkaline zinc bath,which would blind off the flow through a surface filter

ECONOMICSOF FILTERCHAMBEROVERSIZING

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media? Or gritty and therefore easy to filter from an acidcopper tank? Or contain precipitated iron from plating onsteel in an acid zinc or nickel bath? .- ~.

Aquick evaluation will at least help you get started in "-"the right direction. Choose 15 to 75-micron retention forthe slimy zinc or precipitated iron, and denser on mostother baths. Depth-type filter media provides forthis rangeof particle retention. Otherwise, if surface media is em-ployed, then an extended area must be considered tomatch the solids-holding capacity to maintain good flowrates.

Note that flow rates across the media will somewhatchange the percentage or efficiency of retention becauseof the different levels of velocity per square foot of sur-face or per cartridge. It is worth considering increasingthe amount of filter media so as to reduce velocity. Re-member, reduced velocity across the filter media will paybig dividends by reducing the actual amount of cartridgesexpended orfrequency of surface cleaning.

Extending the life of filter media requires matchingparticle retention ability of the media to the range of

Choose 15 to 75-micron retention for theslimy zinc or precipitated iron, anddenser on most other baths.

solids present in the liquid. Unfortunately, we usually don'tknow the percentage of particles of each size so we must r '

rely on past experience. If necessary, coarser or denser '--"media can be substituted to achieve the desired results.

Having sufficient solids capacity is the main require-ment of a filter, so that the pressure drop across the me-dia is minimal overthe time between servicing.This is onefactor in favor of depth-type cartridges, because psi dropis usually low over 85 pct of their life, whereas surfacemedia follows a straight line increase in pressure drop.

When pressure increases across the media, flowdecreases (based on the assumption that virtually allpumps used with plating solution filters are centrifugal).A redl;lction in flow is critical to the ability of the filter toremove particles from the plating tank, because we areusing recirculatory filtration on a reservoir (the platingtank) instead of in-line clarification, as might be the caseof a filterori your incoming water line.

TIME CARTRIDGE LABOR COSTNUMBER OF DIRT HOLDING BETWEEN CONSUMPTION DOWNTIMEOVERSIZING CARTRIDGES FACTORS PER CARTRIDGE COST REDUCED SOLUTION LOSSFACTOR IN CHAMBER CARTRIDGE CHANGE BY REDUCED BY

1 0 D T 0 0

2 2xO 1.4D 3T 29 pet 67 pet3 3xO 1.7D 5T 42 pet 80 pet4 4xO 2D 8T 50 pet 87% pet

Recirculation has other benefits. Suppose a certaintype of filter media stops most of the solids you want toretain, but not all. Thereafter, a second, third, or fourthpass through the filter may produce the desired result.

For instance, if a filter media having an efficiency of90 pct retention of five-micron particles is used, it also isremoving some lower percentage of finer particles, let'ssay 50 pct ofthree-micron particles. If the porosity of the

Having sufficient solids capacity is themain requirement of a filter, so that thepressure drop across the media is mini-mal over the time between servicing.

media didn't change you could expect to pick-up an ad-ditional 50 pct of the three-micron particles on the sec-ond pass, now leaving 25 pct ofwhat you started with. Withconstant recirculation, it is possible that essentially allthree-micron particles could be retained in the media. Butit must be pointed out that this clarification only appliesto the solution that passes through the filter, which is whyturnover rates are so important.

There is, of course, the effect of increased densitycaused by the collected particles on the media, which mayspeed upor increase the percentage of retention. Or theirpresence may hinder the flow and slow down the turnoverrates. This would suggestthattoo-dense media may havebeen used.

Filter media with a broad range of porosities lends"'-"' itselfto recirculation applications. Consider the possibil-

ity of using coarse media instead of fine or two grades ofmedia on the same tank.

A significant benefit of using less dense media toachieve the desired particle retention is the increasedsolids-holding capacity offered by coarser media. Com-pared with fine media, coarse media may provide up tofive times the solids retention before flow is reduced toan unacceptable rate. The media is then replaced withcoarse media and recirculation commences until all oftheliquid is clarified.

Will it work?Yes, it has worked for years. Swimmingpools, hydraulic and lubricating systems, plating andother types of finishing processes usually don't have a"dirty" tank and "clean" tank. They rely on continuousrecirculation filtration to get the desired results. The dif-ference is that these applications allow for some solidsto be present in a limited amount until removed.This pres-ence of solids could not be tolerated in the finished prod-uct such as beer, whiskey, soft drinks, food oils and syr-ups or chemicals, hence the need to either do a good jobof filtering the first time, or recirculate until the desiredclarity is achieved. We are aware of many examples ofsuccess with coarse media. For instance, 3D-micron car-tridges will keep hydraulic oil looking like new,will changea neglected swimming pool from green to clear overnight

"'-- and turn a slimy-oily alkaline zinc solution from milky toclear. It all depends on the number of passes, which dic-tates the flow rate required.

TF-151Page 3

For instance, a 1,ODD-gal.batch being transferred at10 gpm will take one hour and forty minutes, but to turnthe tank over ten times to achieve 100 pct contact withthe filter, a 160gpm pump is required. With one-hour turn-over recirculation a "dirty" tank becomes clean with thesolids in the filter and the desired results are achieved.

Now let us take this approach one step further, keep-ing in mind that high quality plating is your number-oneobjective. Ten turnovers per hour might come close tohaving all of the solution pass through the filter at leastonce. But you're plating every time parts enter your solu-tions. Do you need ten times turnover per minute? Prob-ably not, but the fact is your original intent was to filter outthe particles to achieve high-quality plating. Remember,in the name of Malcolm Baldrige or ISO 9000, no rejects.So, you do have to consider the turnover rate that willachieve your objective.

If organics are a problem due to their decomposition,then separate carbon treatment is required. Some plat-ers still use powdered carbon, citing a need for fast ad-sorption of organic impurities either in a batch processor with the carbon coated on the surface of the filter. How-ever, if uniform purification is necessary, gradual, consis-tent adsorption downstream of the filter works very well,offering some significant advantages that contribute tosolution clarification and desired levels of plating quality.

Aseparate carbon chamber allows the filter to achievemaximum solids holding capacity and maintains a lowlevel of organic impurities without the mess of handling

Filter media with a broad range ofporosities lends itself to recirculationappl ications.

the dusty, black powder. Other benefits include a reduc-tion on manual evaluations in the laboratory and, whentroubleshooting is required, it is comforting to know thatbath contaminants are not the cause.

Statistical quality control will monitor results attain-able from increased filtration and separate carbon purifi-cation; or indicate the further need to increase same un-til the ultimate quality goals are achieved.

Some platers may still be using powdered carbon toadsorb orgqnic impurities. Standard practice was to in-sert a filter aid that would precoat the surface of the filtermedia, making it possible to support or retain a mixtureof powdered carbon and filter aid.This practice requiredfrequent filter servicing because the powdered carbonrestricted the flow, reducing the amount of solids thatcould be retained by the filter before service was neces-sary.

Platers considered using granular carbon because itdidn't reduce the flow rate as much, although the rate ofadsorbency was considered to be less because solidscould plug the pores at the surface of the granule. Itwasn'tuntil platers switched the sequence and filtered first andthen allowed the solution to pass through the unrestrictedpores in the granular carbon, that equivalent adsorptionwas achieved.

...look for ways to prevent solids and oilsfrom getting to the plating tank in the firstplace.

Inconclusion, I'd like to rem.indyou to work backwardsthrough your processing sequence to create a filtrationprogram that will give you clean solutions and high-qual-ity deposits. Start with the pre-plate cleaning and rinsingsteps and look for ways to prevent solids and oils fromgetting to the plating tank in the first place. Move forwardto the plating solutions, recognizing the effect flow rate(turnover) will have on getting the solids to the filter. Con-sider the benefits of two-stage, coarse filter media for theextra solids-holding capacity it can provide. Consider

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"airless" solution agitation. When the program is estab-lished and operating smoothly, you will notice a reductionin laboratory personnel becoming involved in problem'solving. Instead, they will have more time to work on otheraspects of your total quality assurance program.

By Jack BergPresident

SERFILCO, Ltd.Northbrook, IL

Reprinted fromPRODUCTS FINISHING

November 1993

Edited December, 1997

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